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Multi-functional composite materials for catalysis and chemical mechanical planarization

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Title:
Multi-functional composite materials for catalysis and chemical mechanical planarization
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Language:
English
Creator:
Coutinho, Cecil A
Publisher:
University of South Florida
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Tampa, Fla
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Subjects / Keywords:
Titania
Composites
Ceria
PNIPAM
Chemical mechanical polishing
Dissertations, Academic -- Chemical Engineering -- Doctoral -- USF   ( lcsh )
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non-fiction   ( marcgt )

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Summary:
ABSTRACT: Composite materials formed from two or more functionally different materials offer a versatile avenue to create a tailored material with well defined traits. Within this dissertation research, multi-functional composites were synthesized based on organic and inorganic materials. The functionally of these composites was experimentally tested and a semi-empirical model describing the sedimentation behavior of these particles was developed. This first objective involved the fabrication of microcomposites consisting of titanium dioxide (TiO₂) nanoparticles confined within porous, microgels of a thermo-responsive polymer for use in the photocatalytic treatment of wastewater. TiO₂ has been shown to be an excellent photocatalyst with potential applications in advanced oxidative processes such as wastewater remediation. Upon UV irradiation, short-lived electron-hole pairs are generated, which produce oxidative species that degrade simple organic contaminants.The rapid sedimentation of these microcomposites provided an easy gravimetric separation after remediation. Methyl orange was used as a model organic contaminant to investigate the kinetics of photodegradation under a range of concentrations and pH conditions. Although after prolonged periods of UV irradiation (~8-13 hrs), the titania-microgels also degrade, regeneration of the microcomposites was straightforward via the addition of polymer microgels with no loss in photocatalytic activity of the reformed microcomposites. The second objective within this dissertation involved the systematic development of abrasive microcomposite particles containing well dispersed nanoparticles of ceria in an organic/inorganic hybrid polymeric particle for use in chemical mechanical polishing/planarization (CMP). A challenge in IC fabrication involves the defect-free planarization of silicon oxide films for successful multi-layer deposition.Planarization studies conducted with the microcomposites prepared in this research, yield very smooth, planar surfaces with removal rates that rival those of inorganic oxides slurries typically used in industry. The density and size of these ceria-microgel particles could be controlled by varying the temperature or composition during synthesis, leading to softer or harder polishing when desired.
Thesis:
Dissertation (Ph.D.)--University of South Florida, 2009.
Bibliography:
Includes bibliographical references.
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Statement of Responsibility:
by Cecil A. Coutinho.
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Title from PDF of title page.
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Document formatted into pages; contains 163 pages.
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Includes vita.

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oclc - 495362012
usfldc doi - E14-SFE0002980
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ABSTRACT: Composite materials formed from two or more functionally different materials offer a versatile avenue to create a tailored material with well defined traits. Within this dissertation research, multi-functional composites were synthesized based on organic and inorganic materials. The functionally of these composites was experimentally tested and a semi-empirical model describing the sedimentation behavior of these particles was developed. This first objective involved the fabrication of microcomposites consisting of titanium dioxide (TiO) nanoparticles confined within porous, microgels of a thermo-responsive polymer for use in the photocatalytic treatment of wastewater. TiO has been shown to be an excellent photocatalyst with potential applications in advanced oxidative processes such as wastewater remediation. Upon UV irradiation, short-lived electron-hole pairs are generated, which produce oxidative species that degrade simple organic contaminants.The rapid sedimentation of these microcomposites provided an easy gravimetric separation after remediation. Methyl orange was used as a model organic contaminant to investigate the kinetics of photodegradation under a range of concentrations and pH conditions. Although after prolonged periods of UV irradiation (~8-13 hrs), the titania-microgels also degrade, regeneration of the microcomposites was straightforward via the addition of polymer microgels with no loss in photocatalytic activity of the reformed microcomposites. The second objective within this dissertation involved the systematic development of abrasive microcomposite particles containing well dispersed nanoparticles of ceria in an organic/inorganic hybrid polymeric particle for use in chemical mechanical polishing/planarization (CMP). A challenge in IC fabrication involves the defect-free planarization of silicon oxide films for successful multi-layer deposition.Planarization studies conducted with the microcomposites prepared in this research, yield very smooth, planar surfaces with removal rates that rival those of inorganic oxides slurries typically used in industry. The density and size of these ceria-microgel particles could be controlled by varying the temperature or composition during synthesis, leading to softer or harder polishing when desired.
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Multi-Functional Composite Materials for Catalysis and Chemical Mechanical Planarization by Cecil A. Coutinho A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemical a nd Biomedical Engineering College of Engineering University of South Florida Major Professor: Vinay Gupta, Ph.D. Norma Alcantar, Ph.D. John Wolan, Ph.D. Maya Trotz, Ph.D. Hariharan Srikanth, Ph.D. Date of Approval: February 23, 2009 Keywords: Titania, composites, ceria, PNIPAM, chemical mechanical polishing, remediation, microgels, planar ization, slurry, methyl orange photocatalytic degradation, sedimentation, turbidometry, CMP, De gussa P25, free radical polymerization Copyright 2009 Cecil A. Coutinho

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DEDICATION This dissertation is dedicated to my family, especially… My parents, Agnel and Irene Coutinho, for infusing in me the importance of hard work My sister, Savia Coutinho, for always being my role model and confidant It is through your words of encouragement and push for tena city that I have completed this work. Thank you for all of your love support and sacrific e throughout my life. This dissertation is dedicated to you. I could have not done it without you. With Love, Cecil

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ACKNOWLEDGEMENTS I would like to thank my re search advisor Professor Gupta, for his guidance and creative insights during the cour se of this work. My learning experience with him has been very prolific and enabled me to grow as an engineer and as a researcher. I will always appreciate the time he took to trai n me and to enhance my knowledge on a wide array of topics vital to the success of my graduate studies. I need to extend a heartfelt thanks to my family and friends for thei r tremendous support thr oughout the years. Their unwavering love and confidence in me, have helped me through every hardship. This dissertation would not be po ssible without the intellectua l support of my committee members: Dr. John Wolan, Dr. Norma Alcanta r, Dr. Maya Trotz and Dr. Hariharan Srikanth. I am also grateful for the useful insights that came through discussions with classmates Raghu Mudhivarthi, David Walker Bijith Mankidy, Jonathan Mbah, Chhavi Manocha, Dayling Chapparo and Mark Sugimo to all of whom I now consider close friends. A special thanks to the members of the Interfacial Phenomena and Polymeric Materials labgroup (Dr. Shim, Kristina Tran, Alicia Peterson, Adrian Defante, Justine Molas, Fedena Fanord, Claire Osborn, Violetta Yevstigneyeva) all of whom have become a second family. Finally, I would like to th ank Betty Loraamm for TEM training and endless assistance with microscopy during the c ourse of these projects. This research was conducted with the financial support of the National Science Foundation and the Department of Chemical & Biomedical Engine ering at the University of South Florida.

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i TABLE OF CONTENTS LIST OF TABLES................................................................................................................. iv LIST OF FIGURES...............................................................................................................v ABSTRACT....................................................................................................................... ...xii CHAPTER 1: INTRODUCTION..........................................................................................1 1.1. Introduction to Composite Materials..............................................................................1 1.2. Motivation of Research Projects.....................................................................................2 1.2.1. Microcomposite Particles Used for Photocatalysis......................................................4 1.2.2. Microcomposite and Hybrid Particles Used for CMP.................................................6 1.3. Dissertation Description.................................................................................................. 8 CHAPTER 2: SYNTHESIS OF NOV EL POLYMER BASED PARTICLES...................16 2.1. Thermally Responsive Polymers and Their Networks.................................................16 2.2. Experimental Details and Material Synthesis..............................................................18 2.2.1. Synthesis of Polymer Particles..................................................................................18 2.2.2. Hybrid Particle Synthesis..........................................................................................20 2.2.3. Microcomposite Particle Synthesis...........................................................................22 2.3. Summary................................................................................................................... ...23 CHAPTER 3: PHYSICAL CHARACTERIZATION OF MICROGELS AND THEIR COMPOSITES WITH TITANIA OR CERIA NANOPARTICLES…..........31 3.1. Introduction.............................................................................................................. ....31

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ii 3.2. Material Characterization....................................................................................................31 3.3. Results and Discussion................................................................................................35 3.3.1. DLS, TEM and TN Analysis of Polymer Microgels................................................35 3.3.2. TEM, UV-Vis and TGA Study of T itania-Microgel Particles..................................41 3.3.3. Investigation of Hybrid Microgels Using FTIR, TEM and DLS...........................44 3.3.4. Examination of Ceria-Microgel Particles Using TEM and TGA............................47 3.4. Summary................................................................................................................... ...48 CHAPTER 4: SEDIMENTATION BE HAVIOR OF TITANIA-MICROGEL COMPOSITE PARTICLES..........................................................................................66 4.1. Background: Sedimenting Systems.............................................................................66 4.2. Description of Experimental Apparatus.......................................................................68 4.3. Analytical Model for Sedime ntation Using Turbidometry..........................................69 4.4. Results and Discussions...............................................................................................72 4.4.1. Settling Using Turbidity Measurements : Validation with Silica Spheres................72 4.4.2. Settling Measurements for TitaniaMicrogel Composite Particles...........................73 4.4.3. Semi-Empirical Model De scribing Sedimentation of the Microcomposites............77 4.5. Summary................................................................................................................... ...85 CHAPTER 5: PHOTOCATALYTIC DEGRADATION OF METHYL ORANGE USING TITANIA-MICROGELS.................................................................................94 5.1. Introduction.............................................................................................................. ....94 5.2. Experimental Details....................................................................................................95 5.3. Chemical Kinetics and Pathways Gove rning Photocatalytic Degradation..................97 5.4. Results and Discussion..............................................................................................100

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iii 5.4.1. Photocatalytic Performance of th e Titania-Microgel Composites..........................100 5.4.2. Impact of Irradiation on the Stability of Polymeric Microgels...............................105 5.5. Summary................................................................................................................... .108 CHAPTER 6: CHEMICAL MECHANICAL POLISHING USING MICROCOMPOSITE AND HYBRID PARTICLES................................................118 6.1. Introduction to CMP..................................................................................................118 6.2. Experimental Details for Polishing Studies...............................................................120 6.3. Results and Discussion..............................................................................................122 6.4. Summary................................................................................................................... .127 CHAPTER 7: SUMMARY AND CONCLUSIONS........................................................136 7.1. Future Prospects and Recommendations...................................................................137 7.2. Doping of Titania to Shift the Band-Gap...................................................................138 7.3. Abrasive Pads for CMP.............................................................................................139 REFERENCES.................................................................................................................141 ABOUT THE AUTHOR

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iv LIST OF TABLES Table 3.1: Comparison of TiO2 loading in the microcomposites......................................42 Table 4.1: Comparison of settling velocities.....................................................................73 Table 6.1: Process conditions for polishing oxide wafers...............................................122 Table 6.2: COF and removal rate for slu rry polishing with different particles...............123

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v LIST OF FIGURES Figure 1.1: Schematic of the proposed microcomposite and hybrid materials..................12 Figure 1.2: UV illumination resulting in electron-hole pairs at the TiO2 surface causing redox reactions...................................................................................13 Figure 1.3: Schematic of se tup for photocatalytic study....................................................14 Figure 1.4: Pad-wafer interface, showi ng high points on silica wafer being reduced during the CMP process....................................................................15 Figure 2.1: Schematic of (A) swollen PNIP AM microgel particle at below LCST and (B) collapsed microgel particles above the LCST dispersed in water................................................................................................................25 Figure 2.2: Synthesis setup for (A) PNIPAM microgels and (B) PNIPAM nanogels. In both images, the insets s how a sketch of a representative particle.............................................................................................................26 Figure 2.3: Synthesis setup for (A) pe ripheral penetrating microgel and (B) interpenetrating microgel where the insets show a schematic of a typical particle.................................................................................................27 Figure 2.4: Synthesis setup for (A) s iloxane-microgel hybrid, (B) siloxanemicrogel core-shell, (C) siloxane-n anogel hybrid and inset shows a sketch of particle morphology.........................................................................28

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vi Figure 2.5: Synthesis setup for siloxane -microgel IP-hybri d, where the inset shows the particle morphology, such that PAAc (red), PNIPAM (black) and MPS (blue) co-exist......................................................................29 Figure 2.6: Schematic representing the preparation of (A) titania-microgel particles and (B) the ceria-si loxane-microgel particles...................................30 Figure 3.1: TEM images of (A) PP-microgel that has been stained with uranyl acetate (B) IP-microgel that has been stained with uranyl acetate..................49 Figure 3.2: Variation in size of PNIPAM, interpenetrati ng (IP), and peripherally penetrating (PP) microgels from 40 to 25C measured using DLS where the dashed lines are draw n only as a guide to the eye..........................50 Figure 3.3: Total nitrogen an alysis of IP-microgel............................................................51 Figure 3.4: The digital image shows IP-mic rogels dispersed in DI water at 25C (left) and 40C (right)......................................................................................52 Figure 3.5: DLS measurements of (A) PN IPAM microgels cycled increasing and then decreasing in temperature and (B) IP-microgels cycled increasing and then decreasing in temperature.................................................................53 Figure 3.6: FTIR spectra of PNIPAM micr ogels (green), IP-microgels (black), TiO2 (red) and titania-m icrogel microcomposites (blue)................................54 Figure 3.7: Absorbance spectra of titania-mic rogels with various loadings of TiO2.........55 Figure 3.8: TGA analysis of sample C10S........................................................................56 Figure 3.9: TEM images of microcomposite s made using IP-microgel and titania (A) C10: 10wt% DegussaTM P25 TiO2, (B) C25: 25wt% DegussaTM

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vii P25 TiO2, (C) C75: 75wt% DegussaTM P25 TiO2, (D) C10S: 10wt% sol-gel synthesized TiO2 nanoparticles...........................................................57 Figure 3.10: FTIR spectra of the siloxa ne-microgel hybrid particles made by varying the MPS ratio from 0 to 40wt%.........................................................58 Figure 3.11: DLS of the siloxane-microge l hybrid particles ma de by varying the MPS ratio from 0 to 40wt%............................................................................59 Figure 3.12: TEM images of (A) hybrid pa rticle with an MPS/NIPAM synthesis ratio of 0.4/1.0 (B) hybrid particle with an MPS/NIPAM synthesis ratio of 0.25/1.0...............................................................................................60 Figure 3.13: TEM images of (A) siloxane -microgel core-shell particle (B) siloxane-nanogel hybrid particle.....................................................................61 Figure 3.14: FTIR of hybrid microgels /nanogels and core -shell particles synthesized for CMP applications...................................................................62 Figure 3.15: DLS of siloxane nanogels, microgels and core-shell particles......................63 Figure 3.16: TEM images of siloxane-mic rogel IP-hybrid with (A) 10wt% ceria (B) 50wt% ceria..............................................................................................64 Figure 3.17: TGA analysis of IP-hybrid microgels (blue) and the ceria (IP) microgel microcomposites (orange)................................................................65 Figure 4.1: Depiction represen ting the titania-microgels that were used for the sedimentation studies where: (top ) microcomposite particle heavily loaded with titania and (bottom) microcomposite particle sparsely loaded with titania...........................................................................................86

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viii Figure 4.2: Schematic of the arrangemen t for optical measurement of settling behavior...........................................................................................................87 Figure 4.3: Schematic of the idealized settling of a uniform, monodisperse suspension and the normalized turbidity signal that will be expected as a function of time. The measuremen t window is indicated between the two horizontal dashed lines.............................................................................88 Figure 4.4: From turbidity: (A) Evoluti on in the normalized turbidity signal during sedimentation of both large silica spheres (D=3.21 m) and small silica spheres (D=420nm) a nd (B) distribution of settling velocities correspond ing to the fit shown in (A). The symbols are the experimental data and the solid line is the fitted curve...................................89 Figure 4.5: (A) Changes in turbidity due to sedimentation of the microcomposites (blue) and TiO2 nanoparticles (red) at a pH of 2 (squares) and (B) distribution of settling velocity of freely suspended titania and the microcomposites at three different pH values. The solid line is the fit to a mathematical model.................................................................................90 Figure 4.6: (A) Settling be havior of microcompos ites with different TiO2 loading measured using a turbidometer at 25C and (B) distribution of settling velocities correspondi ng to the fits in (A). Th e lines are the results of the fitting procedure and sparse ma rkers have been used for clarity with one marker for every 10 points...............................................................91

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ix Figure 4.7: TEM images of the C50 sample showing (A) a large floc on the TEM grid and (B) several single micr ocomposite particles with a small aggregate.........................................................................................................92 Figure 4.8: (A) Mean settling velocity for each sample from figure 4.6B plotted as a function of the mass fraction of tita nia. The deviations are obtained from the half-widths of the dist ributions in figure 4.5B. The pink dotted line is a fit using equation 4.19 in the text with a value of K= 0.034 while the dashed lines repres ent fits using K= 0.066 (top) and K= 0.018 (bottom). (B) Predicted value of K (equation 4.20) for a range of and Lf.............................................................................................93 Figure 5.1: TEM images of (A) freely suspended TiO2 nanoparticles in aqueous media and (B) titania-microgel particles.......................................................110 Figure 5.2: (A) Absorbance spectra of MO degradation in solutions containing titania-microgels (200ppm TiO2) at a pH of 2 and (B) absorbance spectra of MO degradation in solu tions containing titania-microgels (200ppm TiO2) at a pH of 6.5.......................................................................111 Figure 5.3: Normalized absorbance using UV-Vis spectroscopy for the photocatalytic degradation of MO using titania-microgels as the photocatalyst source at (A) pH of 2 and (B) pH of 6.5.................................112 Figure 5.4: Rate constants for the photocat alytic degradation of MO using freely suspended titania (squares) titania-mi crogels (circles) at a pH of 2 (A) and a pH of 6.5 (B)........................................................................................113

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x Figure 5.5: Turbidity measurement as a f unction of time reflecting sedimentation in a solution of the titania-microge ls at (A) pH 6.5 and (B) pH 2.................114 Figure 5.6: Optical absorption from the oligomeric species present in the supernatant solution after titania-mi crogels have been irradiated for different durations.........................................................................................115 Figure 5.7: The optical signal is shown as a percentage of the plateau value (dashed lines) obtained at long times............................................................116 Figure 5.8: Rate constants of the reformed and original microge l-titania particles at pH2............................................................................................................117 Figure 6.1: Schematic of the CMP apparatus..................................................................128 Figure 6.2: Digital image of the bench-top CMP tester and other necessary inputs.......129 Figure 6.3: FTIR characterization of s ilica removal from the wafer surface...................130 Figure 6.4: Quantitative ellipsometric charac terization of silica removal from the wafer surface where (A) wafer polished using the ceria-microgel particles (B) blank wafer...............................................................................131 Figure 6.5: (A) COF variation with time and (B) distribution of the COF between 100-150s such that the solid line in dicates the Gaussian fit to the actual COF values (circles)...........................................................................132 Figure 6.6: Optical microscopy images of wa fers polished with slurries containing (A) 0.5wt% ceria nanoparticles (B) 0.25wt% ceria nanoparticles (C) 0.5wt% siloxane-ceria-microgel (50wt% CeO2)...........................................133

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xi Figure 6.7: AFM images of wafers polishe d with slurries containing (A) 0.5wt% ceria nanoparticles (B) 0.25wt% ceria nanoparticles (C) 0.5wt% siloxane-ceria-microgels...............................................................................134 Figure 6.8: Variational surface roug hness of the polished wafers...................................135

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xii Multi-Functional Composite Materials fo r Catalysis and Chemical Mechanical Planarization Cecil A. Coutinho ABSTRACT Composite materials formed from two or more functionally different materials offer a versatile avenue to create a tailored material with well defined traits. Within this dissertation research, multi-functional compos ites were synthesized based on organic and inorganic materials. The functionally of thes e composites was experimentally tested and a semi-empirical model describing the sedime ntation behavior of these particles was developed. This first objective involved the fabrication of microcomposites consisting of titanium dioxide (TiO2) nanoparticles confined within porous, microgels of a thermoresponsive polymer for use in the photo catalytic treatment of wastewater. TiO2 has been shown to be an excellent photocatalyst with potential applications in advanced oxidative processes such as wastewater remediation. Upon UV irradiati on, short-lived electron-hole pairs are generated, which produce oxidative species that degrade simple organic

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xiii contaminants. The rapid sedimentation of these microcomposite s provided an easy gravimetric separation after remediation. Meth yl orange was used as a model organic contaminant to investigate the kineti cs of photodegradation under a range of concentrations and pH conditions. Although after prolonged periods of UV irradiation (~8-13 hrs), the titania-microge ls also degrade, regenerati on of the microcomposites was straightforward via the addition of polymer microgels with no loss in photocatalytic activity of the reformed microcomposites. The second objective within th is dissertation involved th e systematic development of abrasive microcomposite particles contai ning well dispersed nanopa rticles of ceria in an organic/inorganic hybrid polymeric part icle for use in chemical mechanical polishing/planarization (CMP). A challenge in IC fabrica tion involves the defect-free planarization of silicon oxide films for su ccessful multi-layer depos ition. Planarization studies conducted with the microcomposites prep ared in this resear ch, yield very smooth, planar surfaces with removal rates that rival those of inorganic oxid es slurries typically used in industry. The density and size of these ceria-microgel particles could be controlled by varying the temperature or com position during synthesis, leading to softer or harder polishing when desired.

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1 CHAPTER 1: INTRODUCTION 1.1. Introduction to Composite Materials Composite materials (or composites for s hort) are broadly classified as those materials that consist of two or more cons tituent materials with different physical and chemical characteristics1. Generally, these characteristics remain separate and distinct within the resulting composite while yieldi ng enhanced properties when compared with the individual components. Composites can be synthetic or natural. For example wood is a well known (natural) composite of cellulose fibers that are bound together by lignin2. Concrete is another well known (synthetic) com posite material. It consists of aggregates (limestone or gravel) that are bound together via a cement matrix that forms chemical bonds with itself and the aggregates upon hydration3. In fact, the most primitive composite materials were bricks used for bui lding construction that were comprised of straw and mud4. Typically, composite materials consist of two categories of constituent materials: a matrix and reinforcement. The primary function of the matrix is to encompass and support the reinforcement materials. The reinfo rcement materials in turn impart superior mechanical and physical properties to the entire co mposite material3. Hence by careful selection of the matrix, the reinforcement ma terials and their respective ratios within the final structure, composite materials can be eas ily engineered to possess certain desirable

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2 properties. Nowadays, many commercially produced composite materials contain a polymer matrix that is frequen tly known as a resin solution. Polymers are widely used as they are inexpensive, easily available and a wi de variety of structur ally different resins are available. Typical resins include polyesters, epoxies, and polyamides, amongst others. The reinforcement materials ar e often fibers but also many mineral oxides are frequently used. The strength of the product is greatly dependent on the resin to reinforcement material ratio3. Over recent decades many new composites have been developed, some with very valuable properties. By carefully choosing the reinforcement material, matrix, and the manufacturing process that brings bot h of them together, composite materials can be easily tailored to meet specific requirements. 1.2. Motivation of Research Projects In this research, the synthesis of compos ite materials was pursued with the broad aim of combining the physical and chemical properties of inorgani c oxides and organic materials within a final structure. Furthermore, when the composite brings together constituent materials at different length-scale s (nanometer and micrometer), the ability to tailor the functional properties of materials becomes even more powerful. The study of these new materials has generated intense inte rdisciplinary efforts in engineering and the physical sciences because applications of th ese composite materials range from sensing5 and drug delivery6 to wastewater remediation7. A variety of novel systems will be explored within this dissertation, and all entail the coupling of two or more functionally differe nt materials to yield a new composite that has different physical attributes from the individual components. Cross-linked,

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3 microspherical particles or “microgels” of s timuli responsive polymers are of particular interest. When coupled with co-polymers cont aining inorganic segmen ts, or loaded with nanoparticles of inorganic oxides, interesting new materials ar ise. In this dissertation, a hybrid particle is defined as a microgel consisting of an organic polymeric network chemically bonded to an inorganic or a meta llic component. Similarly, a microcomposite particle is a microgel (hybrid or nonhybrid) th at contains nanopartic les of an inorganic oxide embedded within the polymer matrix vi a a physical entrapment and electrostatic interactions, not a covalent bond. Figure 1.1A illustrates a titania-microge l microcomposite, where titanium dioxide nanoparticles are confined within a res ponsive microgel network of a polymer for photocatalytic applications in wastewater remediation. Figure 1.1B shows a schematic of a hybrid particle, where condens ed silica (originating from a siloxane co-polymer) coexist with the organic polymeri c microgel, to create a micr on-sized hybrid particle used in chemical mechanical polishing (CMP). Lastly, figure 1.1C shows a microcomposite particle of a hybrid microgel w ith nanoparticles of ceria embedded within the matrix of a hybrid microgel. This permits the tailori ng of the hardness and softness of the microcomposite particle (termed siloxa ne-ceria-microgel) for CMP processes. The main goal of this research project is, therefore, to establish the principles for the design of novel microcomposites by combina tion of different materials that possess unique optical, electronic, and mechanical attr ibutes and to demonstrate the use of these new materials in tec hnological applications.

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4 1.2.1. Microcomposite Particles Used for Photocatalysis Titanium dioxide is a common and wide ly studied photocatalyst due to its appealing attributes such as non-toxicity, chemical inertn ess and high photocatalytic activity8-10. Since the first report11 of photocatalytic purificati on of water using titania in 1977, use of TiO2 has been widely investigated in literature12-14. The large band gap of TiO2 (~3.2eV) permits it to absorb photons in the UV region, which results in the production of electron-hole pairs that migrate to the catalyst surface and participate in redox reactions with organic species15 as shown in figure 1.2 Upon doping with nitrogen or metal ions, TiO2 shows photocatalytic activity unde r visible radiation, which can potentially increase its commercial viability enormously16, 17. In recent years, there has been increased interest in the use of nanosized titania powders due to enhancements in photocatalytic activity. This enhancement results from changes in properties such as crystallinit y, surface area for reaction, and density of surface groups like OH that accompany the fine size18-20. Because separation of suspended fine particles from water has been a major obstacle, use of nanoparticles of titania in applications such as wastewater treatment has been limited. Strategies that have been investigated to address this obstacle in clude immobilization of titania particles onto planar substrates or reactor walls18-20. However, the reduction in available surface area of the catalyst and the transport limitations to the surface can lead to diminished photocatalytic activity, thereby limiting the usefulness of these strategies. Much attention has been given to the fabrication of polymer-inorganic microcomposites as a means of overcomi ng the ensuing aggregation and separation difficulties that have been frequently enc ountered. Towards this end, a few researchers

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5 have explored using supercritical fluids as a means to incorporate insoluble inorganic nanoparticles into the organic network21, 22. One drawback of this approach is that the nanoparticles often aggregate w ithin the polymer thereby re ducing the effective surface area23. Other approaches have involved us ing polymer synthesized by emulsion polymerization to encapsulate i norganic or metallic nanoparticles23. However the organic-aqueous interface required for polymeri zation frequently requires toxic organic solvents, surfactants, and stabilizers that can be difficult to remove and can create environmental problems. Therefore, approaches that do not require organic solvents or stabilizers and that are easy to load with na noparticles in poly mers to create microcomposites can be quite useful. Our goal is to explore an alternative a pproach that involves using responsive polymer microgels to entrap the TiO2 nanoparticles. In recent years the fabrication of stimuli responsive polymeric microgels has generated much interest due to its ease of synthesis in aqueous media and their technological appl ication. Stimuli responsive microgels can respond in shape and size to ex ternal stimuli like te mperature, pH, ionic strength etc. The porosity of the gel facilitat es the loading of the polymer network with titania while still exploiting the la rge available surface area of the TiO2 nanoparticles and maintaining the transparency of the solutio n for photocatalysis. The microcomposite has a sufficiently high density that it can be se parated from solutions by a simple gravity settling technique. In this doctoral research, commercially available DegussaTM P25 TiO2 nanoparticles entrapped within the polymer microgels were used for photocatalytic experiments for reasons such as easy ava ilability, known remediation properties, and

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6 known crystalline structure24, 25. The synthesis, characterization, and photocatalytic performance of these microcomposites has been investigated. The results reported here are critical and necessary inputs in the deve lopment of processes that can use the novel titania-microgel microcomposites in photodegr adation of chemical contaminants in aqueous streams. 1.2.2. Microcomposite and Hybrid Particles Used for CMP A fundamental process during integrat ed circuit manuf acturing is the planarization of the wafer be fore any patterning or deposition on that wafer surface. To produce multilevel metal interconnects the wafe r surface needs to be optically smooth, devoid of pits, scratches or organic depositi ons. However, as device dimensions approach micrometer and ultimately nanometer si zes, the challenge arises to conduct photolithography and successfully manufactur e multilevel metal interconnects. To produce a working device nowadays, the topmost layer of the previous metallization level has to be microscopically smooth with minimal surface roughness. Any residual roughness that exists at the previous layer wi ll eventually get compounded as the layers increase. First developed by IBM in 1983, chem ical mechanical polishing/planarization has emerged as the method of choice amongst pl anarization techniques. This is due to its ability to achieve excellent local and global planarization across the wafer surface in relatively short time-scales coupled with the relatively low cost of the CMP process. It has been shown that it is not possible to proceed with further processing steps until a smooth wafer surface is first achieved. Uneven wafer surfaces will often lead to errors during photolithography and seve ral other processing challenge s such as voids within

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7 interconnects and compounded roughness tends to arise. Thus, CMP becomes a crucial processing step in device fabrication26. To achieve a planar wafer, two types of CMP processes can be conducted: (i) conventional CMP that cons ists of abrasive particles dispersed in aqueous slurry an d (ii) fixed abrasive CMP where the abrasive particles are firmly lodged in a polymer polishing pad against which th e wafer rubs in a process likened to sandblasting a rough surface. Th e work presented here has focused upon conventional CMP for reasons explained in chapter 6 of the text. Within a CMP process, the chemical and mechanical interactions act in synergy between the wafer material, slurry particle s and polishing pad to planarize the wafer surface27. Abrasive particles in the slurry ac hieve the mechanical action needed for polishing but the performance of these abrasive particles de pends on variables such as particle type, size and concentration27, 28. Colloidal nanoparticles of SiO2, CeO2 or Al2O3 are typically the abrasive part icles used to polish silicon oxide wafers industrially. The slurry achieves chemical isotropic etching of the silicon oxide surface by typically using hydroxides of alkali metals. The high pH chemi cal etching is necessary to enhance the mechanical abrasion of the slurry. The mech anical polishing prefer entially grinds the material at high points on the surface thereby producing a planar wafer as can be seen in figure 1.4. Thus, both chemical and mechani cal actions are needed for the global planarization of the wafer surf ace. However, the abrasive pa rticles produce defects such as scratching, irregular polishing and particle deposition on the wafer surface29. All these defects produce local roughness that blocks lithography and the subsequent addition of metal layers for integrat ed circuit manufacturing.

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8 To remedy some of these drawbacks, the synthesis of slurri es comprising of abrasive particles containing either hybrid microgels or (siloxane-ceria-microgel) microcomposites were pursued in this doctora l research. The synthesis, characterization and performance of a variety of abrasive particles used for planarization studies will be detailed within this dissertation. Some of th ese particles include: (A ) hybrid microgel (B) hybrid nanogel (C) a core -shell abrasive particle of micron dimensions with a silica core and a shell consisting of a responsive polymer, and lastly (D) a (siloxane-ceria-microgel) microcomposite that contains ceria nanopa rticles embedded within a hybrid microgel. Ultimately, the goal is to tailor the abrasive action of the particles to influence the etch rate of the oxide surface and reduce the local r oughness thereby creating a smoother wafer. 1.3. Dissertation Description This dissertation outlines the synthesi s, characterization and technological applications that are represented by novel microcomposites of inorganic oxide nanoparticles encased within polymer microge ls. Successful accomplis hment of two main goals is detailed: (A) fabrication of phot ocatalytic microcomposite particles for wastewater remediation, and (B) developmen t of abrasive particles on micron/nanometer length-scales for the chemical mechanical pl anarization of silicon oxide wafers for IC manufacturing. This dissertation is organized as follo ws. Chapter 2 will discuss the various synthesis strategies that were empl oyed for the polymerization of PNIPAM microgels/nanogels as well as microge ls/nanogels based on copolymers and

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9 interpenetrating networks. All of the polym erization was done in aqueous media using free radical polymerization techniques. Here, the hydrophi lic-hydrophobic in teractions responsible for phase transiti on behavior of the polymer pa rticles are also discussed. Several underlying themes that interconnect the above stated objectives of this dissertation will become evident as many of the formulations used in this research were similar, where slight modifications resulted in functionally and st ructurally different particles. In chapter 3, the extensively characterization techniques that have been employed in this work will be presented. This includes but is not limited to spectroscopic characterization such as Fourie r transform infrared spectrosc opy (FTIR) that was used to ascertain the increase in silo xane in the polymer hybrid microgels. Also ultravioletvisible light (UV-Vis) spectro scopy was used extensively in photocatalytic studies and to determine the amount of titani a within the microcomposites. Multi-angle monochromatic ellipsometry was used to determine the thickness of oxide films before and after planarization. Microscopy was employed extensively in this research via atomic force microscopy (AFM) to examine the planarity of oxide wafer surfaces after polishing and transmission electron microscopy (TEM) to visualize the extent of inorganic oxide dispersion within the polymer microspheres. Light scattering techniques such as dynamic light scattering (DLS) were also utilized to describe the responsive nature of the polymer microgels in solution. Additionally, sedime ntation studies of the microcomposite particles were conducted via turbidity measurements. Othe r characterization techniques such as thermal gravimetric analysis (TGA) and total nitrogen/total organic carbon (TN/TOC) analysis will be presented in this chapter.

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10 The settling behavior of titania-microgel microcomposite particles is presented in chapter 4. The microcomposites showed rapid sedimentation (~minutes) that was ~100 times faster than the titania nanoparticle s alone. This sedimentation behavior was captured by monitoring the decline in the turbidity of the solution. The settling microcomposite particles traveled through a pre-determined scattering zone from which the mean count rate of the scattered light was translated into a normalized turbidity signal. A semi-empirical model was developed and used to characterize the loading of the titania within the microcomposite particle. It was shown that as the content of TiO2 increased within the particles from 10% to 75%, a large enhancement in the settling velocities of the microcomposites was observed experimentally. In chapter 5, the titania-microgel microc omposites were employed to examine the photodegradation of an organic dye. Degrada tion kinetics of the rapidly sedimenting microcomposites were compared against fr eely suspended titania nanoparticles at a variety of photocatalyst concentrations and solution pH conditions. Both were shown to follow pseudo first order reaction kineti cs, although the rate constants of the microcomposite particles can be easily tuned. The eventual degradation of the microcomposite particles under UV-illuminatio n will be presented, in addition to other techniques used to monitor the degrada tion of the titania-microgel microcomposite particles with time. Chapter 6 presents the development of slurries containing novel hybrid microgel particles and siloxane-ceria-microgel microcom posites that are less abrasive as compared to conventional ceramic particles used in silicon oxide CMP. The goal here was to obtain improved surface finish on the wafer surface wi thout compromising oxide removal rates.

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11 This is of significance to the current stringe nt polishing requirements required during IC fabrication. In this context, hybrid a nd microcomposite particles based on polymerinorganic oxide materials have shown to be promising for next generation slurries. A number of slurries will be briefly discussed, but this dissertation will focus on two slurries in particular due to their ease of synthesis and superior performance during planarization. The first slurry co nsisted of hybrid microgels part icles that resulted in very planar surfaces but minimal oxide removal. The second sl urry comprised of siloxaneceria-microgels for improved oxide removal and smooth wafer surfaces. Extensive microscopy characterization depicts th e smoothness of these surfaces, while spectroscopic measurements confirm similar re moval rates of the microcomposite slurries versus slurries comprising of only inorganic oxide nanoparticles. Chapter 7 will provide an overall summary of the research performed in this dissertation and outline future steps that can build upon the new materials synthesized to make them more robust and versatile.

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12 Figure 1.1: Schematic of the proposed microcomposite and hybrid materials. titania nanoparticles titania-microgel (microcomposite) siloxane segments siloxane-ceria-microgel (microcomposite) ceria nanoparticles siloxane segments siloxane-microgel (hybrid particle) (A) (B) (C)

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13 Figure 1.2: UV illumination resulting in electron-hole pairs at the TiO2 surface causing redox reactions. Eg conduction band valence band h o x i d a t i o nre d u c t i o n Eg conduction band valence band h o x i d a t i o nre d u c t i o n Eg conduction band valence band h o x i d a t i o nre d u c t i o n

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14 Figure 1.3: Schematic of se tup for photocatalytic study. 6.4cm 5.7cm ~1cm UVA Irradiation photocatalyst 6.4cm 5.7cm ~1cm UVA Irradiation photocatalyst

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15 Figure 1.4: Pad-wafer interface, showing high points on silica wafer being reduced during the CMP process. polishing pad SiO2 abrasive particles in slurry metal interconnects polishing pad SiO2 polishing pad SiO2 abrasive particles in slurry metal interconnects

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16 CHAPTER 2: SYNTHESIS OF NO VEL POLYMER BASED PARTICLES 2.1. Thermally Responsive Polymers and Their Networks Stimuli sensitive polymers are known to respond to changes in the external environment such as temperature, ionic st rength, solvent concentration and pH. These polymers have been extensively studied for the past two decades30-33 and have found many interesting technological applications34-38. Modifying the properties of these ‘smart’ materials, has led to enhancements in a number of areas such as drug delivery39 and sensors33. One of the current challenges lies in understanding the mechanisms that govern this stimuli responsive transition behavior and fabricate new novel materials that can exploit the “smart” feature of these materials. Since the first synthesis in 198640 by Pelton, microgels of poly(Nisopropylacrylamide) (PNIPAM) have become the most widely studied temperature sensitive responsive polymer particles32, 40. PNIPAM is a nonionic po lymer that is usually prepared by free radical precipitatio n polymerization in aqueous media41-43. Linear chains of PNIPAM have been shown to displays a reversible and c ontinuous volume phase transition behavior at 32C. Below this critical temper ature PNIPAM is hydrophilic and exists as a coiled polymer chain. Above this critical temperature it becomes hydrophobic and transitions to form a denser globular st ructure. It has been hypothesized that the polymer chain folds in on itself and releases hydrogen bonded water molecules44 with the

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17 exact mechanism of this collapse still under investigation45. The lower critical solution temperature (LCST) exits due to a ba lance between the hydrophobic carbon backbone and isopropyl side chain, with the hydr ophilic amide side chain. Increasing the temperature past the LCST has an intere sting effect. It disrupts the hydrogen bonding between the amide side chains and the wate r molecules. As a result, this causes a decrease in the entropy of the system due to the reduction in the degrees of freedom of the swollen polymer chains. Dissolution of th e polymer chains is now thermodynamically unfavorable due to the increase of the Gibbs free energy of the system44. Microgels of PNIPAM are typically synthe sized using a divinyl cross-linker and a free radical initiator to co-polymerize the polymer chains into a polymer chain network. Figure 2.1 displays a schematic of these s pherical microgels that have a hydrodynamic radius around ~0.8 m when swollen and ~0.4 m when shrunk (sizes determined by DLS and TEM imaging). These microgels can be eas ily dispersed in aqueous environments to form transparent solutions when the solution temperature is below the LCST and turbid solutions when the solution temperature is above the LCST. The time taken for these microgels to collapse or swell with change s in temperature is in the order of a few minutes. Tanaka and co-workers have shown that the time taken for the gels to shrink/swell is directly propor tional to the square of the radius of gyration of the microgel46. Hence gels on micron length-scales have very quick responses and this quick, thermal responsive capability of microgels of PNIPAM makes it suitab le for a variety of applications ranging from dr ug delivery to catalysis.

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18 2.2. Experimental Details and Material Synthesis All chemicals were purchased from Si gma-Aldrich and used without further purification, unless othe rwise noted. The monomer NIPAM (TCI, Japan) was recrystallized from hexane once before use. Water utili zed in all microcomposite synthesis was purified using an EasyPu re UV system (Barnstead, IA). A 0.2 m filter in this system was used to remove particulate matter. 2.2.1. Synthesis of Polymer Particles PNIPAM microgel synthesis A typical schematic of the reaction procedure is shown in figure 2.2A. As a first step, NIPAM (~1.0g) was recrystallized fr om hexane and dried under vacuum prior to use to remove impurities and inhibitors. The PNIPAM microgels were synthesized via surfactant fr ee precipitation polymerization41, 42, 47. Polymerization was carried out in a sealed round-bottom flask equipped with a magnetic stirrer and an oil bath to control the reaction temperature. Free radical polymeri zation of NIPAM using N,N’-methylenebisacrylamide (MBAA) (~0.04g) as a cross-linker wa s initiated using potassium persulfate (KPS) (~0.02g) in an aqueous medium at 75C. The solution was purged with nitrogen gas for ~45min before polymerization. After polymerization for 5hr, the final product was centrifuged at 7500rpm The supernatant (that contained some unused reagents and oligomers) was removed and replaced with fresh deionized water. The microgels were then re-dispersed vi a sonication and vortex mixing and later recentrifuged. This washing procedure was repeated three times to try and ensure as pure a product of PNIPAM microgels as possibl e for subsequent characterization.

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19 PNIPAM nanogel synthesis NIPAM (~1.0g) and MBAA (~0.04g) were dissolved in aqueous solution containing Sodium dodecy lsulfate (SDS) (Merck; ~0.13g) using a recipe established by Andersson and co-workers48 (figure 2.2B) The solution (70mL) was purged with nitrogen and stirred at r oom temperature for 45min. The nitrogen inlet and outlet were removed and the flask was pl aced into a preheated oil bath at 75C. Polymerization was initiated by injecting KPS (~0.02g) to the reaction mixture (dissolved in 2mL of water). The reaction was allowed to proceed for 5hr under vigorous agitation using a magnetic stirrer. The polymerizati on was stopped by the addition of 20mL of room temperature deionized water (to in troduce dissolved oxygen that acts as a free radical scavenger) and cooling the produc t to room temperature. Snakeskin pleated dialysis tubing with nominal molecular weight cut off 10000g/mol were used as membranes in nanogel purification. All polym er nanogels synthesized were purified by dialysis for 7days against distilled water that was refreshed daily (t wice daily during the first 3days). Peripheral penetrating (PP) microgel synthesis The PNIPAM microgels (~1.0g) formed above were mixed with Acrylic Acid (~1.0g) and dispersed in deionized water at 0C as shown in figure 2.3A. The incorporati on of the peripheral penetrating chains of acrylic acid in the microgels was performed by adapting a procedure previously reported by Xia and Hu49. The solution was bubbled with (indus trial grade) n itrogen gas for ~45min, to displace dissolved oxygen50. The initiator potassium persulfate (0.04g) and the accelerator N,N,N ,N -tetramethylethylenediamine ( TEMED) (0.1mL) was added and the reaction allowed to proceed for 5hr in an ice bath. The product was centrifuged

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20 (7500rpm), the supernatant decanted and the PP-microgels (figure 2.3A) re-dispersed in fresh DI water followed by sonication and vortex mixing for ~1h each. This washing procedure was repeated three times to ensure a homogeneous pure product that was later characterized. Interpenetrating (IP) microgel synthesis Interpenetrating microgels were formed by the surfactant free precipitation polymerization of NIPAM (~1.0g) in an aqueous solution (~200mL) containing polyacrylic acid sodium salt35 (~1.5g, MW 15000g/mol) as shown in figure 2.3B. The PNIPAM microgel wa s cross-linked using MBAA (~0.04g) while KPS (~0.02g) served as the ionic free radi cal initiator. The solution was purged with nitrogen gas for 45min before polymerization, the reaction mixture plac ed in an oil bath at 75C and the initiator a dded. After polymerization fo r 5hr, the final product was centrifuged at 7500rpm, re-dispersed, and sonicated for 1hr. This washing process was also repeated three times. 2.2.2. Hybrid Particle Synthesis Siloxane-microgel hybrid particle Recrystallized NIPAM (~5.0g) was dissolved in 800mL of deionized water and MBAA (~0.2g) was added (figure 2.4A). The solution was bubbled with nitrogen gas for 45min at ro om temperature, the reaction flask heated to 75C and ~0.1g of KPS was added to init iate the reaction. Two hours later 1.0mL of MPS was added and polymerization was allowe d to continue for a nother 1hr 45min. The product was centrifuged at 7500rpm and washed with deionized water three times in a similar cleaning procedure outlined above.

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21 Core(silica)-shell(polymer) particles were prep ared using a similar procedure. When the reaction was allowed to continue for 12hr (rath er than 1hr 45min) after the addition of MPS, then the particle morphology changed from a hybrid particle to a core-shell particle (figure 2.4B). This product was cleaned in a similar fashion to the siloxane-microgel hybrid particles. Siloxane-nanogel hybrid particle Recrystallized NIPAM (~5.0g) was dissolved in 340mL of deionized water to which ~0.3g of MBAA and ~0.64g of SDS was added. The typical reagents and resulting particle morphology are shown in figure 2.4C. The solution was slowly bubbled with N2 gas for 45min at room temper ature. The reaction flask was heated to 75C and 0.2g of KPS (dissolved in deionized water) was ad ded to initiate the reaction. Two hours later 1.0mL of MPS was added and polymerization was allowed to continue for another 1hr 45min, after wh ich ~50mL of deionized water at room temperature was added to stop the polymer ization. The product was cooled to room temperature and used without any pur ification for planarization studies. Siloxane-microgel IP-hybrid particle Microgels were formed by the surfactant free precipitation polymerization of NIPAM (~5.0g) in aqueous media (800mL) using MBAA (0.2g)36, 51, 52. Interpenetrating chains of poly(acry lic acid) (PAAc) were introduced to form IP-hybrid microgels by adding the sodium salt of PAAc (~10g, MW ~15000g/mol) in the initial reaction mixture (f igure 2.5). Following purging with N2 for 45min, the reaction mixture was heated in an oil bath to 75C and KPS (0.1g) added to initiate polymerization. MPS was added as the co-m onomer to the reacti on mixture 2hr past

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22 initiation to introduce siloxa ne functional groups and the polymerization was continued for another 1hr 45min. The particles were purified by centrifuga tion (7500rpm, 30min) and re-dispersed (via soni cation and vortex mixing) in fresh deionized water. The washing procedure was conducted three times. 2.2.3. Microcomposite Pa rticle Synthesis Titania-PP-microgel microcomposite synthesis Nanoparticles of TiO2 powder (DegussaTM P25 grade or synthesi zed nanoparticles of TiO2) were suspended in deionized water and the suspension adjusted to pH 2 using HCl. The TiO2 solution was centrifuged (8500rpm, 30min) to remove aggregates and the resulting monodisperse suspension was used for microcomposite formation. The PPmicrogel solution was mixed with the TiO2 suspension and stirred for 1 hr. The resulting microcomposite that formed (a fluffy white solid) was collected by centr ifugation (8500 rpm, 30min). These microcomposites were subsequently re-dispersed in fresh DI water an d the pH of the soluti on adjusted to neutral conditions. Ultrafine TiO2 was synthesized using a sol-gel technique where titanium tetraisopropoxide (3mL) together with 37% v/v HCL(0.5mL) were added to 200mL of absolute ethanol at 0C22. The resulting suspension was peptized by stirring for an additional 4 hr. The solvent was removed w ith a rotary evaporator and the resulting nanoparticles of TiO2 were re-suspended in deionized water. Titania-microgel microcomposite synthesis A diagrammatic representation of the titania-microgel microcomposite synthesis is shown in figure 2.6A. A monodisperse suspension of TiO2 (DegussaTM P25 grade or synthesized nanoparticles of TiO2) was

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23 mixed with the IP-microgels under vigor ous stirring at a pH ~2. Titania-microgel composite particles formed instantaneously and using sodium hydroxide, the pH of the resulting mixture was raised to ~6. These microcomposite particle s were optimum for photocatalysis (see chapte r 3) and will be referred to as titania-microgel microcomposites (rather than titania-IP-microgel microcomposites). After formation of the microcomposites and pH adjustment, the com posite particles were allowed to sediment and the supernatant was removed. This wash ing process was repeated three times. Siloxane-ceria-mic rogel microcomposite Nanoparticles of CeO2 were suspended in deionized water and sonicated for 2 hr to obtain a homogeneous suspension. This CeO2 solution was then mixed with the siloxane-mic rogel hybrid particles in a desired loading ratio at a pH ~2. The resulting microcomposite settled to the bottom and the supernatant removed. The microcomposite solution was wash ed three times with deionized water and the pH adjusted to a value of 5 for subsequent polishing studies. 2.3. Summary In summary, several polymer, hybrid and organic-inorganic microcomposite particles were prepared. These include PNIPAM microgels/na nogels (polymer-only particles), siloxane-microgel/na nogel (hybrid particle s), core-shell (hyb rid particles) and inorganic-organic microcomposite particles c ontaining either titani a or ceria dispersed within the polymer matrix. Microgels of PNIPAM cont aining interpenetrating or peripherally penetrating linear ch ains of PAAc were also pr epared that were used for microcomposite fabrication. The PAAc helped to stabilize CeO2/TiO2 nanoparticles

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24 within the microgels. The approach of ma king microcomposites usin g interpenetrating PAAc described here is simple and the loading of CeO2/TiO2 within the colloidal particles was easily manipulated by contro lling the mixing ratios of the polymer and inorganic oxide solutions.

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25 (A) (B) Diameter of microgel below LCST (~32C) dispersed in water is approximately 800nm Diameter of microgel above LCST dispersed in water approximately 400nm Figure 2.1: Schematic of (A) sw ollen PNIPAM microgel partic le at below LCST and (B) collapsed microgel particles above the LCST dispersed in water.

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26 Figure 2.2: Synthesis setup fo r (A) PNIPAM microgels a nd (B) PNIPAM nanogels. In both images, the insets show a sket ch of a representative particle. Cross-linker Initiator Monomers N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 ~500nm Cross-linker Initiator Monomers N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 ~500nm ~500nm Cross-linker Initiator Monomers N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 ~100nm C H3CH2CH2OS O O ONa+ 10Surfactant sodium dodecylsulfate Cross-linker Initiator Monomers N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 ~100nm ~100nm C H3CH2CH2OS O O ONa+ 10Surfactant sodium dodecylsulfate (A) (B)

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27 Accelerator Initiator Polymer N,N,N ,N -tetramethylethylenediamine Poly(N-Isopropylacrylamide) Potassium Persulfate S O O S O O O-O O OK+K+ Acrylic Acid (AAc)C H2CH C O H O CH2CH C NH O C H CH3CH3C H3CH3 n ~500nm C H3N CH3CH2CH2N CH3CH3 Monomer Accelerator Initiator Polymer N,N,N ,N -tetramethylethylenediamine Poly(N-Isopropylacrylamide) Potassium Persulfate S O O S O O O-O O OK+K+ Acrylic Acid (AAc)C H2CH C O H O CH2CH C NH O C H CH3CH3C H3CH3 n ~500nm ~500nm C H3N CH3CH2CH2N CH3CH3 Monomer Cross-linker Initiator Monomer N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 ~500nm Poly(AcrylicAcid) PAAc CH2CH C O-OR R Na+ nPolymer Cross-linker Initiator Monomer N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 ~500nm Poly(AcrylicAcid) PAAc CH2CH C O-OR R Na+ nPolymer ( A ) ( B ) Figure 2.3: Synthesis setup for (A) pe ripheral penetrating microgel and (B) interpenetrating microgel where the insets show a schematic of a typical particle.

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28 Cross-linker Initiator Co-monomer N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 3-trimethoxysilypropylmethacrylateSi CH2CH2O O C H3C H3O C H3CH2O C C O CH3CH2 ~500nm (A) (B) Cross-linker Initiator Co-monomer N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 3-trimethoxysilypropylmethacrylateSi CH2CH2O O C H3C H3O C H3CH2O C C O CH3CH2 ~500nm (A) (B) ~100nm Cross-linker Initiator Co-monomer N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 3-trimethoxysilypropylmethacrylate Co-monomerSi CH2CH2O O C H3C H3O C H3CH2O C C O CH3CH2 C H3CH2CH2OS O O ONa+ 10 Surfactant sodium dodecylsulfate ~100nm ~100nm Cross-linker Initiator Co-monomer N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 3-trimethoxysilypropylmethacrylate Co-monomerSi CH2CH2O O C H3C H3O C H3CH2O C C O CH3CH2 C H3CH2CH2OS O O ONa+ 10 Surfactant sodium dodecylsulfate (C) Figure 2.4: Synthesis setup for (A) siloxane-microgel hybrid, (B) siloxane-microgel coreshell, (C) siloxane-nanogel hybrid and inse t shows a sketch of particle morphology.

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29 ~500nm Cross-linker Initiator Co-monomer N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 3-trimethoxysilypropylmethacrylate Co-monomerSi CH2CH2O O C H3C H3O C H3CH2O C C O CH3CH2 Poly(AcrylicAcid) PAAcCH2CH C O-OR R Na+ nPolymer ~500nm Cross-linker Initiator Co-monomer N,N’-methylenebisacrylamide N-Isopropylacrylamide Potassium Persulfate C H2CH C NH O C H CH3CH3 S O O S O O O-O O OK+K+CH2C H C NH O CH2NH C O CH CH2 3-trimethoxysilypropylmethacrylate Co-monomerSi CH2CH2O O C H3C H3O C H3CH2O C C O CH3CH2 Poly(AcrylicAcid) PAAcCH2CH C O-OR R Na+ nPolymer Figure 2.5: Synthesis setup for siloxane-mic rogel IP-hybrid, where the inset shows the particle morphology, such that PAAc (red), PNIPAM (black) and MPS (blue) co-exist.

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30 IP-Microgel TiO2nanoparticles titania-microgel microcomposite IP-Microgel TiO2nanoparticles titania-microgel microcomposite siloxane-microgel IP-Hybrid CeO2nanoparticles siloxane-ceria-microgel microcomposite siloxane-microgel IP-Hybrid CeO2nanoparticles siloxane-ceria-microgel microcomposite (A) (B) Figure 2.6: Schematic representi ng the preparation of (A) tit ania-microgel particles and (B) the ceria-siloxane-microgel particles.

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31 CHAPTER 3: PHYSICAL CHARACT ERIZATION OF MICROGELS AND THEIR COMPOSITES WITH TITANIA OR CERIA NANOPARTICLES 3.1. Introduction To completely understand the morphology, chemical composition and bonding that existed within the synthesized polymer microgels and microcomposites, a host of characterization tools were utilized. Thes e included microscopy, spectroscopy, thermal gravimetric analysis, total nitrogen (TN) analysis and light scattering. Extensive characterization was conducted of the microgels, hybrid microgels, and the microcomposites of ceria and titania. A simp le and quick procedure was developed to analyze the titania content within the titania-mi crogel composite particles. In this chapter, the methods of characterization used during th e doctoral research ar e listed. In addition the basic characterization of materials that have been synthesized in chapter 2 is discussed. 3.2. Material Characterization Transmission Electron Microscopy (TEM) : TEM was used extensively within this research to achieve a number of objectiv es. Dispersion of the inorganic oxides (TiO2/CeO2) within the microgels was qualitatively examined using TEM, as was the structural differences in the hybrid particle s used for planarization studies. The sample preparation was typically achieved by placi ng a drop of the solution on a Formvar-coated

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32 Cu TEM grid that was dried under a lamp in ambient conditions. The grid was then placed within the TEM (model FEI Morgagni 268D) vacuum chamber and examined. Atomic Force Microscopy (AFM) and Optical Microscopy (AFM) : Polished and unpolished silicon oxide surfaces used for CMP were imaged using a Digital Instruments Dimension 3100 Atomic Force Microscope th at provided an ove rview of the pits, scratches and waviness of polished wafer surface. Surface roughness analysis of the wafer surface was also conducted using the images obtained from the AFM analysis. Optical microscopy of the wafer surfaces was conducted using a Leitz Ergolux Optical Microscope. This was done to confirm/refute the presence of surface defects on the polished wafer surface. Both, AFM and OM im ages were obtained in collaboration with Subrahmanya Mudhivarti (Research Group of Dr. Ashok Kumar, Department of Mechanical Engineering, USF). Fourier Transform Infrared (FTIR) Spectroscopy : FTIR was used to identify bonding peaks within the synthesized microgels/na nogels. This was conducted by pelletizing a small amount of dried gel with KBr, and analyzing the pellet with the spectrometer. Typically, the chamber was purged with dry N2 gas, and then the spectrum obtained using 256 scans at a resolution of 2cm-1. In addition, FTIR was also used to characterize the silicon oxide coated wafer surface before a nd after CMP to determine organic particle residue from the microgels or degrading pol ymer polishing pad onto the wafer surface. The chamber was purged with N2 gas, and the spectrum was obtained using 64 scans at a resolution of 2cm-1. The FTIR spectrum obtained a bove was used to qualitatively

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33 characterize the oxide thickness by analyzing the peak corresponding to silica absorbance and will be described in more detail in chap ter 6 that focuses on the CMP polishing using microcomposite and hybrid partic les. In all infrar ed spectroscopy tests, a Nicolet MagnaIR 860 spectrometer was utilized. UV-Vis Spectroscopy : The concentration of titania (e ither in solution or within the polymer microgels) was determined quantit atively by measuring the UV-Vis absorbance of the titanyl ion. This peak absorb ance was analyzed using a V-530 UV-Vis spectrophotometer (Jasco, MD). UV-Vis spectr oscopy was also used extensively for the photocatalytic degradation studies. The degrad ation of the methyl orange contaminant was monitored by recording the peak absorbance decline at regular time intervals. These results will be further discussed in chapter 5 within the context of the photocatalytic studies. Multi-angle Ellipsometry : Removal rates of the oxide film were measured by characterizing the thickness of unpolished and polished wafers using a home-built ellipsometer. Light of wavelength of 633 nm from a helium-neon laser (05LHP073, Melles Griot, CA) was polar ized by a Glan-Thompson polarizer (03PTH109/A, Melles Griot). The polarization of the light was s ubsequently modulated using a liquid-crystal (LC) variable phase retarder (LRC200, Mead owlark Optics, CO) and directed onto the silicon dioxide film of the wafer surface. Th e laser spot measured approximately 1mm 2mm at the surface. Reflected light was analyzed using a second Glan-Thompson polarizer, and the intensity was measured using a Si photodiode (DET100, Thor Labs,

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34 NJ). Control of the variable LC retarder a nd the data acquisition from the detector was performed using a program written in HP-VEE (version 4.0). Thickness measurements were made at a minimum of four different spot s on each substrate at an incident angles of 75, 70, 65 and 60. Typically, the average value of the rem oval rate with its standard deviation obtained from two different samples was used. Thermal Gravimetric Analysis (TGA) : To determine the inorganic-organic composition of the composite microparticle s, a TA SDT Q600 thermal gravimetric analyzer (TGA) was used. Samples were heated in air at a rate of 2C/min from room temperature to 500C. TGA experimentation was conducted with the help of Gregory McManus (Dr. Michael Zaworotko Gr oup, Department of Chemistry, USF). Dynamic Light Scattering (DLS) : The sizes and polydisper sities of all microgels, nanogels, titania and ceria were determined via dynamic light scatte ring (DLS) using a Malvern Nano-S Zetasizer. Samples were so nicated prior to analysis. The sample solution was placed into a polystyrene cuvette and allowed to thermally equilibrate to a pre-determined temperature for 10 min before each set of measurements. Data fitting was done using a multi-modal algorithm supplie d by Malvern with the instrument. The collected correlelograms were fit to di ffusion co-efficients and converted to a hydrodynamic diameter by the instrument software. Turbidometry : In turbidometric studies, a HF scientific, model DRT 1000 was utilized, where the light scattered at 90 to the incident beam was measured as a voltage

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35 signal from the photodetector. Typically, 1000 po ints were acquired at an A/D sampling frequency of 1 kHz and the mean was recorded as a function of time with a 30s time delay between readings. Turbidometry served two purposes within th is dissertation work. Firstly, turbidometry was used to determine th e rate at which the t itania nanoparticles and titania-microgels settled. These sedimentation results are described in greater detail in chapter 4. Secondly, turbidometry was used to analyze the rate of degradation of the titania-microgels under UV-illumination. Th ese results are part of chapter 5. Total Organic Carbon/Total Nitrogen (TOC/TN) Analysis : The ratio of the polyacrylic acid to PNIPAM within the IP-m icrogels was determined using a Shimadzu TOC-VCSH with TNM-1 TN unit total nitrogen analyzer. Here the to tal mass of nitrogen detected was attributed to the PNIPAM of the PNIPAM/PAAc microgels. 25ml of the IPmicrogel solution (at a pre-determined concentration) was injected into the TOC/TN analyzer, which was burnt at ~600 C from which the mass of nitrogen was detected. TOC/TN experimentation was conducted with the help of Ana Prieto (Dr. Daniel Yeh Group, Department of Civil & E nvironmental Engineering, USF). 3.3. Results and Discussion 3.3.1. DLS, TEM and TN Analysis of Polymer Microgels When microgels polymerized with only NIPAM were used to incorporate TiO2 nanoparticles, the photocatalyst rinsed out immediately during the washing of the microcomposites. This indicated that the por ous framework of the microgel did not retain the nanoparticles in the absence of any speci fic interaction with the polymeric matrix. To

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36 remedy this, the incorporation of acrylic acid groups within the microgel to enhance the retention of TiO2 within the polymeric matrix was pur sued. Prior studies have shown that deprotonated sulfonic and car boxylic acid groups can f unctionalize inorganic oxide surfaces18, 53. However, it is also well known that the incorporation of acrylic acid as a co-monomer with NIPAM to form random P( NIPAM-AAc) copolymers disrupts the hydrophilic/hydrophobic balance with the solvent39, 54. As a result, this not only results in a shifting of the volume phase transition of the microgel but more importantly, the amount of AAc in these random copolymer sy stems has to be limited. Large amounts of AAc present as co-polymers do not yield micr ogels by free radical polymerization, but rather leads to the formation of macrosc opic gels. To circumvent these problems, microgels with interpenetrating linear chains of PAAc were prepared to maintain the temperature responsive behavior and phase transition nature of PNIPAM microgels. Additionally, it serves to introduce significant proportions of carboxylic acid moieties that are known to interact with the oxide surface of inor ganic nanoparticles. Figure 3.1 shows transmission electron microscopy (TEM) im ages of both the IPmicrogels and PP-microgels. To clearly visual ize the interpenetrating chains of PAAc, the sample was stained using uranyl acetate th at has been shown to selectively stain the polyacrylic acid within the PNIPAM-PAAc paritcle557. Both images strongly suggest that PAAc constitutes a significant fraction of the polymeric particle. However, the dark spots on the periphery of the PP-microgels indicate a significant localization of the PAAc within this region. Figure 3.1 also shows that the IP-microgels were synthesized with the NIPAM polymerizing around the deprotonated P AAc chains. These IP-microgels have a more even distribution of the PAAc across the entire microgel. This difference in

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37 structure arises due to the grad ient in cross-linking density th at exists within the PNIPAM microgel. It has been well established that the interior of the PNIPAM microgel is densely cross-linked and the outer regions are sparsely cross-linked31. It is for this reason that the polymerization of th e AAc monomer in a dispersion of the PNIPAM microgels (PP-microgel) leads to greater incorporatio n of PAAc towards the exterior of the microgel. The DLS data shown in figure 3.2 clearly shows that there is no change in the hydrodynamic diameter of the PP-microgels when compared with the original PNIPAM microgel. The average collapsed size of the PP-microgel is approximately 280 ( 38) nm while that of the PNIPAM microgel is 270 ( 38) nm. However, the IP-microgel displays a significant increase in size as shown in figure 3.2 with the collapsed diameter measuring roughly 330 ( 18) nm. This increase has been at tributed to the polymerization of the NIPAM monomer around de protonated chains of PAAc. The resulting electrostatic repulsion from the carboxylic acid moieties creates a slighter larger microgel during the synthesis. The size of the PP-microgels was first established during the initial polymerization of PNIPAM with no AAc pr esent. No appreciable increase in size occurred during the second pol ymerization during which the polymerization of AAc was conducted in solution and within the sw ollen PNIPAM microgels. While hydrogen bonding interactions are possible between pr otonated PAAc chains and the PNIPAM amide side chains, literature has shown that such interactions are negligible56. The two synthesis routes used for the pr eparation of the IP and PP microgels and the measurements by TEM and DLS show important differences from recent reports by Xia49 and Das35. Xia and coworkers performed the pol ymerization of AAc monomer in a

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38 dispersion of PNIPAM microgels to get interp enetrating networks of PAAc, rather than interpenetrating linear chains of PAAC w ithin the PNIPAM matrix (that has been described in this dissertation). Although a few similarities exist be tween their procedure and the techniques used to produce the PP-mi crogels here, two im portant differences exist. First, Xia and co-w orkers used much smaller PNIPAM microgels (~120nm) compared to the experiments described he re (~700nm). Secondly, the temperature at which the polymerization of AAc was performe d was 21C as compared to 0C in this study (that was chosen to allow maximum sw elling of the PNIPAM for maximum PAAc integration within the porous PNIPAM mi crogel). The study performed by the Xia group resulted in a substantial (~75 %) increase in size after the PA Ac formation. This led them to infer that the particles had a core-shell st ructure with a core of PNIPAM and a shell of PAAc. This morphology had significant impact on the aggregation state of the particles while also leading to a much smaller ratio between the swollen and the shrunken state because the PAAc in the shell regions does not respond to temperature. In contrast to the studies by Xia and co-workers, the TEM results detailed in this study (figure 3.1) suggest that there is some tendency for localization of the PAAc at the periphery in the case of the PP-microgels but it is not as extreme as a core-shell structure. This occurs because the starting point for the synthesis of the PP-micr ogels is a larger PNIPAM microgel that is more swollen with sparsely cross-linked outer regions. As a result, there is no appreciable increase in size after the PAAc polymeriza tion (figure 3.2) and the swelling ratio remains practically the same as that observed fo r the PNIPAM microgels before the PAAc incorporation.

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39 Das and coworkers35 have prepared interpenetrating networks of PAAc with microgels formed from copolymers of NIPAM and N -isopropylmethacrylamide (NIPMAM). Their procedure was similar to the steps used in this dissertation research for preparation of IP-microgels. In both in stances, the microgels were formed in a solution containing PAAc. Das et al. reported a decrease in microgel size due to the presence of interpenetrating chains of PAAc. They speculated that th is could be due to hydrophobic interactions between short PAAc ch ains and the methacrylamide segments. However, in figure 3.2 the IP-mic rogels that have been prepar ed here show an increase in size. Additionally, Das and coworkers also re port a significant decrease in the swelling ratio of the interpenetrating network compar ed to the non-IPN case whereas figure 3.2 clearly shows that the IP-microgels have si milar swelling ratio to the PNIPAM microgels. However, even though Das and coworkers us ed a copolymer system of NIPAM with NIPMAM (while only NIPAM was used in th is study), the differences between the results reported here and the past study can be attributed to the other variations in experimental details. In expe riments reported in this dissert ation, a high molecular weight of PAAc (Mw~15000) was used and a larger portion of PAAc (f~50%) was incorporated into the PNIPAM microgel as comp ared to that by Das et al. (Mw~2000; f~2%). The longer, hydrophilic PAAc chains and the highe r fraction can likely, lead to increased electrostatic repulsion and rete ntion of water. This would a ccount for the slight increase in size as well as the pres ervation of the swelling ratio. To incorporate TiO2 within the microgels, this doctoral study has primarily focused on using IP-microgels for the fabrica tion of the titania-microgel particles. This was due to the ease of synthesis (one step pol ymerization) and even distribution of PAAc

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40 within the microgel. As shown in figure 3.3, it was ascertained that the IP-microgels contained ~48% by mass of Poly(Acrylic Acid ) using the TN analysis. Even though IPmicrogels contained a signifi cant fraction of the very hydr ophilic polyacryl ic acid, the distinct and spherical nature of the or iginal PNIPAM microgel was maintained. Additionally, figure 3.4 shows the appearance of turbidity due to phase separation of the IP-microgels when the sample was heated above the transition region. At room temperature the dispersion of the IP-microgel is transparent, similar to that of the PNIPAM microgels. It is known that P(NIPAM-AAc) co-polymer with large amounts of AAc as a comonomer will not yield microspherical gels Additionally, microgels of P(NIPAM-AAc) that contain significant amounts of AAc undergo a large charge repulsion between carboxylic acid segments. This charge repulsion can become sufficiently high that the microgels no longer exhibit thermally res ponsive behavior or shrinking at high temperatures31, 56-58. Figure 3.2 clearly indicates that by using interpenetrating chains of PAAc, the temperature responsiveness and pha se transition behavior of the PNIPAM microgels was successfully maintained, while introducing significant (~48%) proportions of carboxylic acid moieties to interact with TiO2 nanoparticles. Als o, the presence of the PAAc does not significantly affect the LCST be havior of PNIPAM, as all three polymeric systems shown in figure 3.2 phase transition at approximately 32C. The change in size of the PNIPAM and IP-microgels with temperat ure is reversible with temperature and the microgels can be cycled between swollen and collapsed states as shown in figure 3.5. The hydrodynamic diameter of both microgels was analyzed by increa sing the temperature from 25 C to 40 C and then cooling this mi crogel solution down from 40 C to 25 C. The

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41 scatter from the data is not significant, indi cating that both the PNIPAM and IP-microgels are quite monodisperse. Also, it should be noted that the IP-microgels are substantially larger than the PNIPAM microge ls in both temperature cycles. 3.3.2. TEM, UV-Vis and TGA Study of Titania-Microgel Particles The broad infrared absorbance of titania from 400-1000cm-1 is shown in figure 3.6. This absorbance peak is also seen in th e titania-microgel composite but absent in the spectra of the IP or PNIPAM microgels, ther eby confirming the presence of titania within the microcomposite. Absorption peaks at 1650cm-1 and 1550cm-1 due to bending vibrations are indicative of the amide I and II absorption bands that originate from the PNIPAM polymer. Also the twin peaks at 1385cm-1 and 1365cm-1 both confirm the presence of the isopropyl side chain within the final struct ure of the PNIPAM microgels, IP-microgels and titania-microgels. The loading of the TiO2 within the microcomposite material was be easily tailored by careful selection of th e mixing ratios of the TiO2 and IP-microgel stock solutions. Table 3.1 shows the different mass ratios for IP-microgel and TiO2 that was used in this work. Sample ‘C10S’ refers to the micr ocomposite containing sol-gel synthesized ultrafine TiO2 whereas the rest of the titania-microgel samples were synthesized using the commercial available, well-studied DegussaTM P25. For a quantitativ e determination of TiO2 loaded in the microgels, the microcompos ite was first dried under vacuum and then treated with boiling concentrated sulfuric ac id and ammonium sulfate in a 4:1 mass ratio. A few drops of 30 wt% H2O2 were then added to the cooled solution to oxidize the degraded polymer and then the solution was reheated. An additional amount of H2O2 was

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42 Sample ID % (mass) of TiO2 mixed% (mass) of TiO2 measured by UV-Vis analysis C10 11.3 10.3 C25 27.2 24.8 C75 80.1 74.6 C10S 9.6 9.2 added to develop the colorless titanyl ion into the intensely yellow-red colored peroxotitanyl ion TiO2 2+. The peroxotitanyl ion formed shows an absorbance at 405nm that was measured using UV-Vis spectrosc opy. A linear calibration curve that was previously developed in our la b that related the mass of TiO2 and spectral absorbance, was used to assay the TiO2 content in the microcomposite18. Figure 3.7 shows the UVVis results that provide a simple colorimetric quantification of the TiO2 content in microcomposites. A strong characteristic ab sorbance at 405nm is observed due to the complex between the titanyl ion (TiO2 +) and hydrogen peroxide as shown in figure 3.7 where Mie scattering of the solution is at a minimum. The intensity of the absorbance increases with TiO2 content. Table 3.1: Comparison of TiO2 loading in the microcomposites. When the IP-microgel and TiO2 stock solutions were mixed, the high proportion of poly(acrylic acid) within the IP-microgel resulted in a large transfer of TiO2 from the surrounding solution into the IP-microgels. Th is is evident from table 3.1 where a good agreement exists between the mixing ratios and the TiO2 content assayed from the UVVis absorbance approach. Only at the highest loading, a discrepancy (~5%) occurs since not all of the TiO2 can be loaded within the microgels. We believe that 75% loading of titania appears to be an uppe r bound for the microcomposite synthesis. In perspective,

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43 this is a very high mass loading of titania and can be likened to a la rge, yet very porous titania particle of approximately half a micr on in diameter. This fractal aggregate allows easy entrainment of fluid within itself, cr eating an optimum photocatalytic system for wastewater remediation. Sample C10S was al so analyzed by TGA as shown in figure 3.8 to confirm the accuracy of the simple co lorimetric quantification from the UV-Vis technique. TGA showed that the microcomposite C10S had 9.5 wt% TiO2 which agrees well with result from the UV-VIS assay. The UV-Vi s analysis is particularly useful since it quick, but more importantly specific to titania. Thus, it can be extended to other inorganic-organic microcomposites that contain a mixture of oxides with TiO2 as one of the constituents. TEM characterization further reveals the unaggregated and dispersed state of the titania nanoparticles within th e polymer matrix. TEM images in figures 3.9A-D show the microcomposites made from IP-microgels and TiO2 with different loading of the TiO2 from different mixing ratios. It is clear ly shown that the nanoparticles of TiO2 are quite well-dispersed and largely unaggregated with in the microgels. From the figures 3.9A-C, it is observed that the DegussaTM P25 is ~70nm while the ultrafine synthesized TiO2 in figure 3.4D is ~10nm. These results are also confirmed using DLS wherein the Degussa sample’s size distribution showed a major p eak at 71 (17) nm and a minor peak at 310 (62) nm while the synthesized TiO2 sample showed a size distri bution centered at 4 (1) nm. Neither the IP-microgel nor the TiO2 settle when present as separate entities. However, the resulting microcomposite settles readily due to the significant increase in the density of the titania-microge ls and significant porosity that exists within the particles

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44 and the fractal aggregates overall. Sedime ntation studies were conducted by studying the decrease in turbidity from a solution of the microcomposites as a function of time measured using a turbidometer. This will be discussed in further detail in chapter 4. 3.3.3. Investigation of Hybrid Micr ogels Using FTIR, TEM and DLS The incorporation of functional groups in to polymer networks to form new hybrid materials represents an emer ging discipline for the synthe sis of novel materials that comprise of diverse architectures51, 52, 59-63. One area of application is the preparation of abrasive particles with controllable texture and surface hardness that can significantly improve the surface finish in the CMP process. Towards this end, this study has focused on using hybrid microgels of soft polymeric networks based on PNIPAM with hard inorganic components. This was achieved by preparing hybrid microgels that contained siloxane functional groups incorporated in to the polymeric network by co-polymerizing NIPAM with MPS. The starting mass ratio M PS/NIPAM was varied during synthesis (up to 40%) and the bulk FTIR spectra of the hybr id microgel particles that were produced are shown in figure 3.10. For comparison, spec tra are shown for microgels containing only PNIPAM (blue). The shoulder peak at 1727cm-1 corresponds to the carbonyl stretching from the methacrylat e functionality in MPS. Increasing the MPS to NIPAM ratio in the polymerization mixtur e, results in an increase in the intensity of the peak at 1727cm-1 that indicates greater incorporation of the co-monomer in the hybrid microgel. The black spectrum corresponding to 40%-MPS -NIPAM hybrid microg el has the most pronounced peak due to the ca rbonyl of the methacrylate gr oup and the green spectrum for the 10%-MPS-NIPAM hybrid microgel has the weakest peak at 1727cm-1.

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45 PNIPAM-based materials have been of tremendous interest because they are thermally responsive due to a delicate hydr ophilic–hydrophobic bala nce that exists between the amide and isopropyl side chains35, 44, 54. Figure 3.11 shows the change in hydrodynamic diameter of the hybrid microgels with temperature as characterized by DLS. The decrease in the microgel size with increasing temperature is due to the well known volume phase transition40. As shown in figure 3.11, mi crogels prepared using a high MPS to NIPAM ratio of 40% are not temper ature responsive. This occurs due to the formation of a thin silica shell encapsulati ng the polymeric core of the hybrid particle. TEM images in figure 3.12A confirm this hypot hesis, where the roundness of the particle can be attributed to the silica shell, while the transparency occurs due to the thinness of the shell that allows for penetration of the electrons through the interior of the particle that contains mostly polymer. It should be noted that a polymerization mixture with a MPS to NIPAM ratio of 10% re sults in particles that are thermally responsive and the swelling ratio is same as the microgels th at contain no MPS. The material remains temperature responsive when the MPS to NIPA M ratio is 25% in th e reaction mixture but the swelling ratio decreases. Figure 3.12B s hows TEM images of hybrid particles that contain only 25% MPS, and the segments of silica as opposed to a complete shell are clearly seen. Various other abrasive particles were pursued for the polishing of oxide wafers. Core-shell particles that were synthesized contained a core of si lica and a shell of PNIPAM. Since PNIPAM is thermally respon sive, the thickness of the shell could be manipulated by changing the temperature of the slurry solution. Below the LCST (~32C), these particles posse ssed thick, expanded shells, while above the LCST, the

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46 polymer shell collapses onto the silica co re. TEM images confirm the core-shell morphology of these particle s as shown in figure 3.13A Siloxane-nanogel hybrid particles were also synthesized for polishing applications. These part icles tend to possess a higher surface area to volume ratio that would be favorable for increased wafer-particle contact, thereby leading to increased abrasion. TEM images of these hybrid nanogels are shown in figure 3.13B. Due to increased magnification, the resolution of the image tends to be not as sharp as the previous images of siloxane-microgel hybr id particles, making it slightly more difficult to view the sili ca fragments from the PNIPAM. The synthesis techniques of both the hybrid nanogels and the core-shell particle s are available in chapter 2 of this dissertation. FTIR spectra shown in figure 3.14 were used to further characterize the presen ce of siloxane within the hybrid nanogels/microgels and core-shell particles. The shoulder peak due to the car bonyl of the methacrylat e group (of MPS) at 1727cm-1 is clearly seen and absent in both the IP and PNIPAM microgels. Since the size of the shoulder peak is similar in the core-s hell particles, siloxane-nanogels, and siloxanemicrogels, it can be speculate d that all particles have fa irly similar amounts of silica fragments incorporated within the final struct ure. The temperature re sponsive behavior of these particles is shown in figure 3.15 and it re veals some interesting behavior. As expected, the hybrid nanogels are the most responsive particles with the core-shell particles being the least responsive. With th e silica core firmly established, the PNIPAM shell can only extend beyond the unresponsive core rather than the en tire particle being responsive as is the case with the hybrid nanogels/microgels. Plausible arguments for the greater responsive behavior of the hybrid nanogels compared to the hybrid microgel is that the nanogel is not as cross-linked as the microgel, and the presence of the SDS

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47 surfactant may hinder the complete incorp oration of the MPS within the nanogel structure. Due to the poor CMP results that resulted with these hybrid particles, further characterization was not pursued. 3.3.4. Examination of Ceria-Microgel Particles Using TEM and TGA To promote the incorporation of ceria nanoparticles into the hybrid microgels, a similar strategy of the synthesis of the titania-microgels was pursued36, 60. Interpenetrating chains of PAAc within the hybr id microgel led to significant fractions of carboxylic acid moieties in the hybrid microgel, which facilitated th e incorporation of ceria nanoparticles within the microgels. By simply controlling the mixing ratios of the IP-hybrid microgel and ceria nanoparticles so lution, the mass fraction of ceria within the siloxane-ceria-microgel particles was easily tailored. Figure 3.16 shows two types of ceria-microgels, one containing approximately 10wt% ceria and the other 50wt% ceria. In both TEM images, the dark spots correspon d to the ceria nanopar ticles (~20nm). It is evident that the ceria is well-dispersed and largely unaggregated with in the microgel in both images. In the present study for CMP a pplications, the ceria-m icrogel suspension with approximately 50 wt% ceria was used so as to maximize oxide removal rates as detailed extensively in chapter 6 of th is dissertation. Figure 3.17 shows the TGA characterization of the IP-hybr id microgels and the ceria-mi crogel composite particles. From the TGA analysis, it was determined that the inorganic fraction (silica) is approximately 5 wt% in the IP-hybrid micr ogel while the ceria-microgels contained ~50wt% inorganic.

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48 3.4. Summary In summary, several organic-inorganic microcomposites and hybrid particles were prepared and extensively characterized. Mi crogels of PNIPAM with interpenetrating linear chains of PAAc were prepared that ha d an AAc content of n early 50% by weight but showed a volume phase transition at a temp erature similar to that of the original PNIPAM microgels. The PAAc helped to stabilize CeO2/TiO2 nanoparticles within the microgels and the loading of CeO2/TiO2 within the colloidal particles was easily manipulated from a low value of 10% (weight) to a value as high as 75% for titania and 50wt% for ceria. In all case s, the inorganic nanoparticles were observed to be in a dispersed state within the microgels and the supernatant was devoid of inorganic nanoparticles, as shown by TEM imaging. Ot her particles including siloxane-nanogels and core-shell particles we re characterized for subsequent use in CMP.

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49 500 nm A Figure 3.1: TEM images of (A) PP-microgel that has been stained with uranyl acetate (B) IP-microgel that has been st ained with uranyl acetate. 500 nm B

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50 800 700 600 500 400 300 200Hydrodynamic Diameter (nm) 40 38 36 34 32 30 28 26 24 Temperature C PNIPAM Microgel PP-Microgel IP-Microgel Figure 3.2: Variation in size of PNIPAM, interpenetrati ng (IP), and peripherally penetrating (PP) microgels from 40 to 25C measured using DLS where the dashed lines are drawn only as a guide to the eye.

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51 Figure 3.3: Total nitrogen an alysis of IP-microgel.

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52 25C40C Figure 3.4: The digital image shows IP-microgels dispersed in DI water at 25C (left) and 40C (right).

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53 800 700 600 500 400 300DH (nm) 40 38 36 34 32 30 28 26 Temperature (C) PNIPAM Microgel (decreasing temperature) PNIPAM Microgel (increasing temperature) 800 700 600 500 400 300DH (nm) 40 38 36 34 32 30 28 26 Temperature (C) IP Microgels (decreasing temperature) IP Microgels (increasing temperature) (A) (B) Figure 3.5: DLS measurements of (A) PNIPAM microgels cycled increasing and then decreasing in temperature and (B) IP-microgels cycled increasing and then decreasing in temperature.

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54 0.07 0.06 0.05 0.04 0.03 0.02Absorbance 2000 1800 1600 1400 1200 1000 800 600 400 Wavenumber (cm-1) IP-Microgel TiO2 Microcomposite PNIPAM Figure 3.6: FTIR spectra of PNIPAM microge ls (green), IP-microgels (black), TiO2 (red) and titania-microgel microcomposites (blue).

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55 3.0 2.5 2.0 1.5 1.0 0.5 0.0Absorbance (a.u.) 600 550 500 450 400 350 Wavenumber (nm) C10 C25 C75 C10S Figure 3.7: Absorbance spectra of titania-mic rogels with various loadings of TiO2.

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56 1.0 0.8 0.6 0.4 0.2 0.0Weight Fraction 500 400 300 200 100 0 Temperature (C) 90.5% wt loss Figure 3.8: TGA analysis of sample C10S.

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57 Figure 3.9: TEM images of microcomposites made using IP-microgel and titania (A) C10: 10wt% DegussaTM P25 TiO2, (B) C25: 25wt% DegussaTM P25 TiO2, (C) C75: 75wt% DegussaTM P25 TiO2, (D) C10S: 10wt% sol-gel synthesized TiO2 nanoparticles. 1 m A 1 m B 1 m C 1 m D

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58 Absorbance 1800 1600 1400 1200 1000 Wavenumber (cm-1) 0.40/1.0 MPS/NIPAM 0.25/1.0 MPS/NIPAM 0.20/1.0 MPS/NIPAM (with PAAc) 0.10/1.0 MPS/NIPAM PNIPAM-Only Microgel Figure 3.10: FTIR spectra of the siloxane-mic rogel hybrid particles made by varying the MPS ratio from 0 to 40wt%.

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59 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 DH/D25C 38 36 34 32 30 28 26 Temperature (degC) 0.4/1.0 MPS/NIPAM 0.25/1.0 MPS/NIPAM 0.1/1.0 MPS/NIPAM Figure 3.11: DLS of the siloxane-microgel hybrid particles made by varying the MPS ratio from 0 to 40wt%.

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60 200 nm 200 nm A B Figure 3.12: TEM images of (A) hybrid particle with an MPS/NIPAM synthesis ratio of 0.4/1.0 (B) hybrid particle with an MPS/NIPAM synthesis ratio of 0.25/1.0.

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61 500 nm A 100 nm B Figure 3.13: TEM images of (A) siloxane-mic rogel core-shell particle (B) siloxanenanogel hybrid particle.

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62 Absorbance 1800 1600 1400 1200 1000 Wavenumber (cm-1) Hybrid Nanogel Core-Shell Hybrid Microgel Hybrid Microgel (25wt MPS) PNIPAM Microgel IP Microgel Figure 3.14: FTIR of hybrid microgels/nanogels and core-shell particles synthesized for CMP applications.

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63 1.0 0.9 0.8 0.7 0.6 0.5 0.4 DH/D25C 40 38 36 34 32 30 28 26 Temperature (C) Hybrid Microgel Core Shell Microgel Hybrid Nanogel Figure 3.15: DLS of siloxane nanogels, microgels and core-shell particles.

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64 Figure 3.16: TEM images of siloxane-micr ogel IP-hybrid with (A) 10wt% ceria (B) 50wt% ceria. 200 nm 200 nm A B

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65 1.2 1.0 0.8 0.6 0.4 0.2 0.0Weight Fraction 500 450 400 350 300 250 200 Temperature (C) Siloxane-microgel Ceria-microgel Figure 3.17: TGA analysis of IP-hybrid micr ogels (blue) and the ceria (IP) microgel microcomposites (orange).

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66 CHAPTER 4: SEDIMENTATION BEHAVIOR OF TITANIA-MICROGEL COMPOSITE PARTICLES 4.1. Background: Sedimenting Systems Monitoring sedimentation of suspensi ons and measuring fall velocities of particles is of practical signi ficance in areas such as ma rine geology, coastal and ocean science, hydraulic engineering and solid–liquid separation technology64-68. Also, the hydrodynamics of particle settling is a subject of considerable scientific interest as is evident from the range of literature that stre tches from the pioneering studies of Stokes to more sophisticated theories and simulati ons that have emerged in recent years69-74. However, many aspects of settling behavior rema in to be completely understood. This is especially evident in complex systems such as highly concentrated suspensions, solutions of polydisperse materials, aggregating and fl occulating dispersions, and natural sediments under turbulent flow conditions. In applications involving microcomposite s, the phenomenon of sedimentation is significant in the manufacturing process, pr oduct homogeneity, and material application. Within the past two decades there has been an increasing focus on microcomposites containing polymeric and inorganic units fo r use in medicine, paint, and specialty chemical industries37, 75, 76. For instance, zinc oxide part icles coated with fluoropolymers are an important constituent of cosmetic foundation creams37. In these applications,

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67 sedimentation of the particle s can lead to undesirable resu lts. For example, non-uniform formulations in the case of cosmetics are costly errors that occur due to the sedimentation of particles. In the specific case of titania-microgel particles that have been investigated in this dissertation for applications such as photocatalytic remediation of wastewater streams, sedimentation is beneficial as it has the potential for facilitating the recovery of the clean water and nanometer sized TiO2. Optical turbidity is used to monitor part icle concentration at a fixed height in a settling column during the sedimentation and a simple analytical model can be used to describe the sedimentation behavior in term s of the distribution of settling velocities. Figure 4.1 shows (titania-microgel) microcomposites that were used for the sedimentation studies. The loading of the titania within the microgel can be easily manipulated as shown in figure 4.1. However, first both the tec hnique and the model needed to be validated by using solid, impe rmeable silica spheres whose sedimentation behavior follows the well established Stokes law. Additionally, the settling behavior of the silica spheres provides a comparison to the sedimentation behavior of the highly porous, titania-microgel particles. Both, the se ttling of the titania-microgels and the silica spheres are measured under dilute conditions (particle volume fracti on < 0.01) to ensure that hindered settling does not play a large role. Samples with different TiO2 loading are examined to demonstrate the increase in settlin g velocity as the effective density of the particle increases. Additionally, the sedimentation of both the titania-microgels and freely suspended titania were studied at two diffe rent pH values. Since the microgel, and thereby the microcomposites, are thermally responsive materials, the influence of temperature on the sedimentation rate was al so examined. The results presented here

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68 demonstrate that in addition to the general usefulness of the sedimentation of titaniamicrogel particles for remediation purposes, these particles can provide a suitable experimental system to gain insights into sedimentation behavior of complex systems such as permeable particles and flocs of fine particles held by organic matter. 4.2. Description of Experimental Apparatus The titania-microgel samples discussed here are designated C10, C25, C50, and C75 to indicate titania ma ss fraction of 10%, 25%, 50%, and 75%, respectively. The settling features of the microcomposites were obtained using a turbidometer (model DRT 1000, HF instruments). The intensity of light sc attered at 90 to the incident beam was recorded as a voltage signal as a function of time. Fi gure 4.2 shows a schematic of the arrangement where the top edge of the aperture was at 3.9cm (h1) from the sample meniscus and the bottom edge at 5.0cm (h2). This was the area from which the scattered light intensity was measured. The sampli ng cylindrical tubes were 12mm75mm and closed at the top to prevent water loss from evaporation. Suspensions of the titaniamicrogels were prepared by diluting a concentr ated stock solution with DI water to total volume of 5cm3 and mass concentration such that, Cs =0.5 mg/cm3. The temperature of the sample was maintained by circulating wa ter through the apparatu s using a water bath. A typical experiment consisted of first equilib rating the metallic sample holder at a given temperature for ~30min. The sample tube c ontaining the suspension was placed in the holder for 10–15min, which was determined to be an adequate amount of time for the sample to reach the required temperature. At this point a sedimentation run was conducted by taking the cylindrical glass samp le tube out, inverting it a few times to

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69 create a uniform suspension and then placin g it quickly back in the holder for data acquisition. Using an acquisition program written in HP-VEE, 1000 points were acquired at an A/D sampling frequency of 1 kH z and the mean was recorded as a function of time. For the rapid settling (heavily lo aded) titania-microgels, each data point was recorded every 5s. To improve the signal-noise ratio for each sample, typically five runs were performed and the measurements at corresponding times were averaged. For the titania-microgels that settled in less than a few hundred seconds, the initial measurements contain some degree of settling during the 2–3s taken for placing the sample tube back in the holder and the decay of any bulk conv ection effects from the shaking. In the measurements done at 15C, the outside of th e tube formed a thin condensate layer that had to be wiped prior to each run to minimize multiple scattering effects. For comparison and calibration purposes, sedimentation was also performed at 25C with two types of silica spheres. One type was purchased from Bangs Laboratories (Indiana) with an average diameter of 3.21 m. The manufacturer specified a standard deviation in size of 0.35 m. A second, finer silica particle was synthesized (courtesy: Dr. J-Y Shim) via the sol–gel hyd rolysis of tetraethylorthos ilicate that produced silica particles with a nominal diameter of 0.45 m and a standard deviation of 0.03 m determined by TEM imaging. 4.3. Analytical Model for Sedimentation Using Turbidometry Optical techniques that are derived from the scattering of light by particulate matter in suspensions have formed the basis of simple yet useful measurements. Turbidity has been used to examine phenomena such as the flocculation of yeast and

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70 other microorganisms, sedimentation of fine particles (sand grains, powders), and settling of sludges77-79. Using well-established theory of photo-sediment ation, the attenuation (It/Io) in the intensity of light following the transmission through a suspension of monodisperse, spherical partic les can be shown to be: 2 P P P o tD )N LK(D 4 exp I I Equation 4.1 where Dp is the particle diameter, K(Dp) is the extinction co-efficient for the sphere, L being the optical path length, and Np represents the number of particles per unit volume. It has been shown that K(Dp) reaches a limiting value of 2 for large particles (>2.5 m) but is a strong function of particle diameter for finer material. For non-absorbing particles that do not vary in size, a simple turbidity parameter ( ) can be used and related to the number concentration as shown below: P t oN I I ln L 1 Equation 4.2 Equation 4.2 indicates that using a normaliz ed turbidity signal can directly provide information on the evolution of particle concentration due to sedimentation. For a uniform, dilute suspension of monodisperse s pherical particles that settle with a single settling Vs, it can be expected that the turbidity signal measured through an optical aperture of height H(=h2 -h1) should remain constant until the particles reach the aperture

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71 and then decrease linearly until all particles ha ve traversed the aperture. This scenario is graphically described in figur e 4.3 with the analytical re presentation shown below in equation 4.3: 0 ) V h t ( H V 1 1 N ) V t ( Ns 1 s o s P s 2 s 2 s 1 s 1V h t for V h t V h for V h t for Equation 4.3 However, in the case of a suspension of the titania-microgels, several classes of particles that have different settling velocities with the same sample are possible. For the titaniamicrogels and dilute suspensi ons, equations 4.2 and 4.3 can be extended to analyze the turbidity signal normalized by its value at initial time to obtain: ) V ( Y ) V ( N ) V t ( N ) t (si si oi si pi 0 Equation 4.4 where Npi(0, Vsi) represents the number of particles in class “i” at initial time that have a settling velocity of Vsi, Npi(t, Vsi) represents the number of pa rticles in class “i” with a settling velocity of Vsi at time t, and Y(Vsi) represents the fraction of total particles that are in class “i”. Following a common practice in the analysis of light-extinction data, a log-normal distribution of the settling velocities that was fit using the software IGOR

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72 PRO, from which one can determine the fraction Y(Vsi) at the various se tting velocities. The experimental evolution of the turbidity signal in time can be fitted to equation 4.4 with mean and standard deviat ion of the log-normal distri bution as the only adjustable parameters of the fit. 4.4. Results and Discussions 4.4.1. Settling Using Turbidity Measurem ents: Validation with Silica Spheres Figure 4.4A shows the turbidity data measured for a suspension of silica spheres. Since the large silica spheres are expected to follow Stokes law, one can calculate the theoretical fall velocity in water ( w, w) using the Stokes equation shown below: 2 P w w sp sD 18 g V Equation 4.5 where sp is the density of the settling particle. Figure 4.4A shows that the fitting procedure outlined above matches the experimental data. Table 4.1, shows the comparison between the settling velocities of the silica particles ( sp = 1.96 g/cm3), first when calculated by Stokes law, and then when measured using turbidity. The narrow width of the distribution as s een in figure 4.4B indicates th at the silica spheres do not have significant polydispersity.

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73 Average settling velocity of silica spheres (cm/s) 0.45 0.03 m 3.21 0.35 m Stokes Law 1.1 x 10-5 5.4 x 10-4 Turbidity 1.3 x 10-5 5.4 x 10-4 Table 4.1: Comparison of settling velocities. For the finer silica particles (Dp ~450nm), Stokes law predicts a settling velocity of approximately 1.1 x 10-5cm/s. This indicates that the change in the turbidity signal should occur extremely slowly. As shown in figure 4.4 this is indeed the case and the settling occurs over a period of days with the normalized signal goi ng to zero in approximately 5days. In this case, due to th e extended period of measurements, an automated timer was used to turn the turbidometer on and off at pre-determined intervals. This automation made the measurements more susceptible to dr ifts in the voltage signal. However, the fit to the data is acceptable and the velocity dist ribution shown in figure 4.4B is centered at 1.3 x 10-5cm/s, which is in reasonable agreement with the Stokes law prediction. Using the results with solid silica sp heres, it can be safely conc luded that the experimental procedure and the analysis of the data cons titute a valid approach to study the titaniamicrogels and other dilu te settling particles. 4.4.2. Settling Measurements for Titani a-Microgel Composite Particles Even though the polymeric IP-microgels do no t settle in solution, this is not the case for the titania-microgel composites. Figure 4.5A shows the charac teristic changes in normalized turbidity with time for a sample C 65 (~65wt% titania) at pH 2 indicating that

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74 the microcomposites settle w ithin a few hundred seconds. Fi gure 4.5A also shows that the freely suspended titania does not sedime nt at pH 2 but remains well-dispersed as indicated by the constant turbidity signal ove r several hours. This dispersion of the freely suspended titania can be attri buted to the strong electrostatic repulsion that exists between the positively charged titania nanoparticles in acidic solutions. Rapid sedimentation of the titania-microgels produces a favorable gr avimetric separation that has significant potential for use in wastewater remediation as it can facilitate the recovery of the titania nanoparticles. For the C65 sample, the decline in the measured turbidity signal shown in figure 4.5A can transformed to yield a distribution of settling velocities using equation 4.4. This is shown in figure 4.5B, where the microcompo sites show a settling velocity distribution centered at ~0.1cm/s for a pH of 2. At this acidic pH, no settling ve locity of the freely suspended titania, can be detected over a period of a few days. An increase in pH to 6.5 results in reduced electrostatic repulsion of the free titania nanopart icles that in turns cause the titania to sediment with an averag e settling velocity centered at 0.001cm/s. The microcomposite particles still show a velocity distribution centered at ~0.1cm/s. Thus, the microcomposite particles settle nearly a hundred times faster than the free titania particles at near neutral solution conditions, which cl early demonstrates the enhancement in the separation and recovery of phot ocatalyst using these novel tita nia-microgel particles. The bottom panel in figure 4.5B shows that when the pH of the solution is strongly basic (~10.5), the settling velocity obtained from a solution of microcomposite particles is almost identical to that of the freely suspende d titania. The turbidity signal is indicative of the release of titania nanoparticles from the IP-microgels at basic conditions, which

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75 was readily observable. The release of TiO2 nanoparticles at a pH of 10.5 can be attributed to the electrostatic repulsion orig inating from the negativ ely charged titania and deprotonated carboxylic aci d groups of PAAc. Even though the IP-microgel and the TiO2 nanoparticles (under acidic conditions) do not settle when present as separate entities, the turbidity from a solution of the titaniamicrogel particles at 25C decreases with time, which is clear evidence of the rapid settling of these microcomposites. This can be attributed to the change in effective density of the titania-microgels and th e highly porous natu re of the fractal microcomposites. The density of the microcomposite particle ( p) in the dry state can be calculated as: ) )( f ( ) )( f 1 ( ) f (2 TiO 2 TiO 2 TiO pol p Equation 4.6 where TiO2 is the density of titania (~4.16 g/cm3), f is the mass fraction of the titania per particle, and pol is the density of the dried polymer (~1.07 g/mL80). Equation 4.6 suggests that as the fraction of titania changes from 0.1 (sample C10) to 0.75 (sample C75), the density of a dry microcomposite pa rticle will more than double from 1.16 g/cm3 to 2.42 g/cm3. The rate of settling of the titania-micr ogels increases with the loading of TiO2 within the polymer as shown in figure 4.6A. The higher the TiO2 content, the faster the microcomposite particles settle. The normalized turbidity value drops by approximately 95% within a period of 100 s for the sample C75 and within 2200 s for sample C10. The

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76 settling curves in figur e 4.6A can be fit to the log-normal distribution described above to provide a distribution of settling velocities fo r the four different microcomposite samples. These are shown in figure 4.6B. Sample C10 is found to have slightly higher distribution in velocities than the samples with a higher mass fraction of titania. This is consistent with the TEM images in previous study81 that clearly show less uniform loading of TiO2 for sample C10. The turbidity results shown in figure 4. 5 and 4.6 correspond to dilute solution conditions, which minimizes hindered se ttling. The number concentration (Np) of the titania-microgel particles in solution can be estimated using the balance: ) f 1 ( M C N Npol S Avo p Equation 4.7 where Mpol is the molar mass of the microgel in g/mol and NAvo is Avogadro number. In a recent study82 on the structure of PNIPAM microgels Saunders used small angle neutron scattering (SANS) to estimate the molar mass of a microgel particle to be 6109g/mol. The volume fraction of the titania-microgel part icles in solution can now be described as: ) T ( D 6 M ) f 1 ( C N ) T ( D 6 N3 P pol S Avo 3 P P Equation 4.8 where the diameter of the particle Dp(T) is a function of the temperature. Equation 4.8 can be used to estimate the uppe r bound of the volume fraction ( using Dp(T) to equal

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77 the swollen particle size of 0.75 m, f as 10%, and Cs =0.5 mg/cm3. This yields an upper bound volume fraction of the part icles in the settli ng experiments to be approximately 0.01. In comparison, the theoretical limit for the volume fraction in the dilute region has been estimated as 0.03 by Batchelor and Wen83 while Davis and Birdsell78 have shown experimentally that ~0.08 is acceptable for dilute regime behavior. Hence, the dilute condition requirement of the experiments has been met. Thus, the settling velocity is negligibly hindered by watercurrents caused from displace ment of the fluid by other settling particles. It should also be noted th at the small refractive index contrast of the porous microcomposites relative to water, re duces the likelihood that the optical signals are affected by multiple scattering. 4.4.3. Semi-Empirical Model Describing Se dimentation of th e Microcomposites The most significant observat ion from figure 4.6 is that the time it takes for all the particles to settle is significan tly faster than the solid, impene trable silica spheres and this time decreases with an increase in titania load ing. From the experimental data in figure 4.6, it can be quickly concluded that Stokes la w is not suitable for predicting the settling behavior of these titania-microgel particles. As an alternative to Stokes law, one can draw on literature studies on settling of porous sphe res and flocs since the structure of the microcomposites is somewhat analogous to other fractal systems found in nature84-89. This rather large literature database, which dates back over a century, includes both empirical and theoretical re lationships, but no single equati on or analysis exists that is universally accepted. All these studies agre e that the drag co-efficient of a porous sphere should be smaller than the drag co-effi cient of a solid sphere. Thus, this leads to

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78 the conclusion that a permeable particle set tles faster. This inference is qualitatively consistent with the re sults in figure 4.6. For the settling behavior of titania-microgel particles, two factors appear to play an important role. First, each microcomposite particle is a permeab le particle with a typical size that corresponds to that of the cr oss-linked IP-microgel. This is clearly shown in the TEM images in chapter 3 for the C10, C25, and C75 samples. The second factor that influences the settling behavior arises from aggregation of the porous microcomposite particles into larger-size fl ocs. These flocs are quite delicate with a tendency to break apart easily under agitation. In order to visualize the structure of the flocs using TEM, the sample was prepared by dipping the carbon coated TEM grid in a dilute solution of the microcomposites follo wed by drying. Based on the TEM results in figure 4.7, it is shown that the characteristic size (Lf) of the flocs should be 10–100 m. Additionally, since each titania-microgel particle is highly porous, th e flocs of the titaniamicrogel particles tend to be very permeable. The images in figure 4.7 show that the interpretation of the settling experiments within the context of a purely theoretical model is difficult due to the effect of permeability of each microcomposite particle and their aggregates that has to be accounted for in the hydrodynamic drag resist ance during settling. Mo reover, any fractallike aggregation of the microcomposite particles introduces additional complexity. Significant departures between theoretical mode ls and settling of fractal aggregates have been found in past studies90. It has been speculated that the discrepancies arise from factors such as improper accounting of non-ho mogeneous distribution of permeability, an underestimation of the permeability of aggregates and effects such as particle clusters

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79 sheltering each other from the fluid flow. Hence, the goal in the following discussion is to simply assess if the experime nts are qualitatively consistent with the theoretical framework available in literature and develop a semi-empirical model to describe the settling microcomposite particles. Past hydrodyn amic theories have attempted to account for the decreased drag resistance of perm eable spheres by using a correction factor This correction factor is the ratio of drag resistance for a permeable sphere to the drag resistance for solid sphere of same radius and bulk density69, 71-74, 85, 86, 88. Thus, the drag force on a settling porous part icle can be written as: f 2 s w D pA V C 2 1 F Equation 4.9 where CD is the drag co-efficient of an impermeable particle and Af represents the projected area experiencing the drag for ce. Under creeping flow conditions, the Reynolds’s number is very small (NRe<<<1) such that CD can be written as 24/NRe. Recently, Wu and Lee91, 92 have used numerical mode ling of flow through porous particles and have demonstrat ed that for a highly porous spherical or non-spherical particle, a Stokes-law like co rrelation can be used for CD beyond the creeping flow region. They have expressed the drag force as follows: f 2 s w Re pA V N 2 1 F Equation 4.10

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80 where is a parameter that depends on only the characteristic size and permeability of the particle. Wu and Lee show that equation 4.10 is applicable to highly porous particles for NRe<40 and that this accounts for the successful use of CD = /NRe by several different experimental studies in interpreting results on settling of porous particles and flocs. Using a conventional force balance for the mi crocomposite particles as follows: g ) ( V N A V N 2 1w sp sp pf f 2 s w Re Equation 4.11 where sp is the density of one por ous microcomposite particle with its included water, Vsp represents the volume of each microcomposite particle, and Npf is the number of particles in the floc. Many different approaches exist for relating Npf and Af in aggregated systems using concepts such as fractal dimens ion, shape factor, and particle interactions. One of these approaches yields: 2 f fL 4 A Equation 4.12 3 p spD 6 V Equation 4.13 p f pfD L N Equation 4.14

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81 where is defined as the fractal dimension of the flocs. Using a volume and mass balance, the density difference of single micr ocomposite particle can be related in terms of its porosity, such that: } ) f ( ){ 1 ( ) (w p w sp Equation 4.15 where p(f) is given by equation 4.6. Co mbining equations 4.11–4.14 gives: w p w f 3 p p f s) f ( 3 g 4 L D D L ) 1 ( V Equation 4.16 In equation 4.16, the first rati o indicates that even though th e porous particle experiences an increase in settling velocity from reducti on in drag resistance, this increase is moderated by an increase in porosity. It shoul d be noted that equation 4.16 reduces to the Stokes relation (equation 4.5) for the case of a single, solid particle ( =0, Lf =Dp, = 24, and p(f) = sp). Now, for a microgel particle alone, the porosity can be related to the particle diameter by the relation: Avo pol pol 3 PN M ) T ( D 6 1 Equation 4.17 Applying equation 4.17 to the microgel in a collapsed state and setting Dp equal to Dmc (0.33 m) allows us to estimate the minimum porosity as 0.51. This is consistent with the

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82 SANS and DLS data from the study by Saunders82 that reveals a significant amount of water is retained by the outer regions of a microgel particle even above the volume phase transition temperature. However, when the mi crogel is swollen at 20–25C, the value of Dp is approximately 0.75 m and this results in a maximu m porosity close to 0.96. The high porosity indicates that the microgel partic les have a high void fraction as expected. For a microgel-titania particle, the volume fract ion of solids in a part icle needs to account for both the polymer and the titania nanoparticles. In this case, equation 4.17 can be modified to yield: TiO2 pol Avo pol pol 3 P f 1 f 1 N M (T) D 6 1 Equation 4.18 where the latter term corrects for the volum e of titania nanoparticles. Application of equation 4.16 at 25C shows that the value of ranges from 0.925 for C75, 0.947 for C50, 0.954 for C25, and 0.957 for C10. Compari ng this with the poro sity of 0.96 for the microgel with no titania, clearly indicates that the titania nanoparticles contribute only slightly to solids volume fraction and their pr incipal impact is on the effective density of the microcomposite particle. Combining equations 4.16 and 4.18 yields the final semiempirical relation for the functional dependen ce of the settling velocity on the mass fraction of titania: ) f ( K ) )( f ( ) f 1 ( f 1 f 1 K ) f ( Vw pol 2 TiO pol 2 TiO 2 TiO pol S Equation 4.19

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83 where Avo pol pol w f p fN M g 8 L 1 D L 1 K Equation 4.20 Using known values for densities of the polymer titania, and water, one can fit the mean settling velocities from the distributions s hown in figure 4.6B using equation 4.19. Figure 4.8A shows that equation 4.19 captures the trend in mean settling velocity of the different titania-microgel pa rticles with a value of K= 0.034 cm4g-1s-1. The value of K obtained from the fit to experimental data can be compared to the prediction of equation 4.20. However, a pre-requisite for this prediction relies on an accurate estimation of the internal permeability, Several theoretical relations have been attempted to estimate the permeability of a single porous sphere and/or their aggregates. These include early models such as the Carmen–Kozeny permeability model, Darcy’s law and Brinkman’s extension of Darcy’s law, and as well as mo re recent approaches that focus on detailed consideration of internal st ructure of porous aggregates69-71, 73, 74, 85, 86, 88, 93. Happel’s model70 has proven to be extremely useful in th at it takes into account the internal flows for aggregates of fine particles. Math ematically, this can be represented as: 3 5 2 3 5 3 1 21 2 3 1 1 3 1 2 9 1 2 9 3 1 a Equation 4.21

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84 Using porosity of =0.96 and with ‘a’ being the size of the titania clusters (~100 nm), the predicted permeability for a microcomposite particle is ~10-9 cm2. The value can now be used to estimate the Brinkman69 parameter such that ( =Dp/2v ; ~1.2). This is in good agreement with other experiment al studies. Assuming this for and using the correlation by developed by Wu and L ee for flow through porous particles: 2 6 (58 1 5 0 ) Equation 4.22 Now, K can be estimated by equation 4.20. Fi gure 4.8B shows the predicted value of K for three values of between 2.5 and 3 an d floc sizes from 10 m to 100 m. The predicted values lie in the neighborhood of K calculated by fitting the experimentally determined settling. This anal ysis also suggests that when the fractal dimension is less than 3 the value of K is relatively insensitive to the floc size and that the titania loading will play a large role in the variation of settling velocity. One of the interesting aspects of the titan ia-microgels is that these particles are temperature responsive. Qualitatively, as temperature is increased from 15C towards 35C the settling of the particles becomes faster. Changes in the sedimentation behavior of the microcomposite particles with temperat ure can be attributed to the temperature responsive nature of the microge ls and changes in the fluid viscosity with temperature. Further details of this behavior can be f ound in the master thesis (USF) of Reshma Harrinauth94.

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854.5. Summary The results presented here demonstrate the general usefulness of settling as a simple characterization of organic–inorga nic microcomposites and other sedimenting systems of an intricate nature. The sedimentation behavior of titania-microgels and freely suspended titania were studied at a range of pH values and titania loadings. The titaniamicrogels showed rapid sedimentation in a queous dispersions, nearly 100 times faster than the freely suspended titania. It was found that the settling time decreased as the content TiO2 increased within the particles. A semi-empirical model was developed that related the settling behavior in terms of the changes in the effective density. The settling behavior of these microcomposites provide s not only a simple probe of particles characteristics but also insi ght into fundamental issues regarding settling of porous spheres, flocs of inorganic particles w ithin organic material and sedimentation phenomena in marine environments. Lastly, th e microcomposites characterized in this dissertation are promising candidates for app lications such as wastewater remediation where uses of nanoparticles of TiO2 are advantageous for photo catalysis but separation is relatively difficult. In this context, the grav ity settling behavior of the microcomposites can be a promising characteristic in wastewat er remediation as it allows for an easy recovery mechanism.

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86 Figure 4.1: Depiction repres enting the titania-microgels that were used for the sedimentation studies where: (top) microcomposite particle heavily loaded with titania and (bottom) microcomposite particle sparsely loaded with titania.

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87 h1h2Water in from circulating bath Water out Sample tube Apertures (90 ) Incident Light Detected Light Metal Sample Holder h1h2Water in from circulating bath Water out Sample tube Apertures (90 ) Incident Light Detected Light Metal Sample Holder Figure 4.2: Schematic of the arrangement fo r optical measurement of settling behavior.

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88 t=t0t=t1t=t2t=t3t=t5t=t6 t=t4 h2 h1 1 0 t t to Time Figure 4.3: Schematic of the idealized set tling of a uniform, monodisperse suspension and the normalized turbidity signal that will be expected as a function of time. The measurement window is indicated between the two horizontal dashed lines.

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89 1.0 0.8 0.6 0.4 0.2 0.0Normalized Turbidity 6 5 4 3 2 1 Time (days) 10000 8000 6000 4000 2000 0 Time (seconds) 0.45m Silica 3.21m Silica (A) Normalized Frequency (a.u.) 4 5 6 10-5 2 3 4 5 6 10-4 2 3 4 5 6 Velocity (cm/s) 0.45m Silica 3.21m Silica (B) Figure 4.4: From turbidity: (A) Evolution in the normalized turbidity signal during sedimentation of both large silica spheres (D=3.21 m) and small silica spheres (D=420nm) and (B) distribution of settling velocities corres ponding to the fit shown in (A). The symbols are the experimental data and the solid line is the fitted curve.

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90 1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 10-5 10-4 10-3 10-2 10-1 100Velocity (cm/s) 1.0 0.8 0.6 0.4 0.2 0.0Normalized Frequency (B) pH 2 pH 6.5 pH 10 1.0 0.8 0.6 0.4 0.2 0.0Normalized Turbidity 101 102 103 104Time (s) (A) Figure 4.5: (A) Changes in turbidity due to sedimentation of the microcomposites (blue) and TiO2 nanoparticles (red) at a pH of 2 (squares) and (B ) distribution of settling velocity of freely suspended ti tania and the microcomposites at three different pH values. The solid line is the fit to a mathematical model.

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91 Figure 4.6: (A) Settling be havior of microcompos ites with different TiO2 loading measured using a turbidometer at 25C and (B) distribution of settling velocities corresponding to the fits in (A). The lines are the results of the fitting procedure and sparse markers have been used for clarity with one marker for every 10 points. ( A ) 5 6 7 0.1 2 3 4 5 6 7 1Normalized Turbidity 2500 2000 1500 1000 500 0 Time (s) C10 C25 C50 C75 ( B ) Normalized Frequency 0.0001 0.001 0.01 0.1 1 Velocity (cm/s) C10 C25 C50 C75

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92 Figure 4.7: TEM images of the C50 sample showing (A) a large floc on the TEM grid and (B) several single microcomposite particles with a small aggregate. 20 micron A 800 nm B

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93 0.001 2 4 6 8 0.01 2 4 6 8 0.1 2 4 Vs (cm/s) 1.0 0.8 0.6 0.4 0.2 0.0 f K=0.066 K=0.034 K=0.018 (A) Figure 4.8: (A) Mean settling velocity for each sample from figure 4.6B plotted as a function of the mass fraction of titania. The deviations are obtained from the half-widths of the distributions in figure 4.5B. The pink dotted line is a fit usi ng equation 4.19 in the text with a value of K= 0.034 while the dash ed lines represent fits using K= 0.066 (top) and K= 0.018 (bottom). (B) Predicted value of K (equation 4.20) for a range of and Lf. (B) 0.08 0.06 0.04 0.02 0.00Kpredicted 100 80 60 40 20 Lf( m) =3.00 =2.75 =2.50

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94 CHAPTER 5: PHOTOCATALYTIC DEGRADATION OF METHYL ORANGE USING TITANIA-MICROGELS 5.1. Introduction The titania-microgel particles synthesized in this research have the potential for use in photocatalytic degradation of organi c contaminants in industrial settings for wastewater remediation. De pending on the source (househol d, agriculture industry), wastewater can contain a wide variety of both bi ological and chemical contaminants. Typically, wastewater treatment includes many processes such as solids removal, treatment of biodegradable organics, rem oval of heavy metals, neutralization, and degradation of organics. Adva nced oxidative processes using nanoparticles of titania are best suited for remediation of the orga nic components and typically considered downstream of steps such as solids removal or heavy metal removal. Since the results presented within this doctoral research clearly demonstrate the enhancement in separation that occurs when using the titania-micr ogel composites versus freely suspended nanoparticle s of titania, the photocatal ytic performance of the microcomposites is of great inte rest. Therefore, as a first st ep, this chapter explores the phtodegradation of a simplified system that contains only one model organic contaminant. At this stage, role of other factors such as ionic st rength of the solution,

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95 presence of interfering contaminants, and de gradation of two or more organics is not studied. Towards an assessment of the photo catalytic performance of these novel materials, investigation of the photodegradation of a model organic dye, methyl orange (MO) was conducted using both the novel titania -microgels and freely suspended titania. Kinetics of the photodegradation of MO wa s evaluated using UV-Vis spectroscopy to assay the MO concentration over various interval s of irradiation. The influence of pH was monitored, as this influences the interplay between the poly(acrylic acid) in the polymer microgels, the titania surface and the methyl orange adsorbate. Degradation of methyl orange using freely suspended titania was al so conducted for comparison with the titaniamicrogel particles. Lastly, the impact of pr olonged irradiation on th e degradation of the polymeric component of the microcomposites is studied using UVVis spectroscopy and the resultant release of tita nia is characterized by sedime ntation studies. The results reported here are critical and necessary inputs for the development of processes that can use the novel titania-microgels in photodegrad ation of organic contaminants in aqueous streams. 5.2. Experimental Details Methyl orange was used as a model c ontaminant to examine the photocatalytic behavior of the synthesized titania-microgels Aqueous solutions with a concentration of 5ppm MO were evaluated for degradati on. Titania-microgels or free TiO2 nanoparticles were added such that the titania content in the solution was 50, 100, 150 or 200ppm. The pH was adjusted using 0.1M HCL or 0.1M NaOH when needed and degradation kinetics

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96 were evaluated at a pH of 2 (0.1) and 6. 5 (0.2). Photocatalysis was conducted under illumination using two commercially av ailable 15W Philips F15T8 black-light fluorescent bulbs (model 392233) that have spectral energy distri bution centered at 352nm. The intensity of the radiation reaching the solution surface (3.5mJ/cm2) was detected via a Chromaline UV Minder radiom eter (UVM226) connected to a remote probe (UVM226S). The apparatus was kept in side a vacuum hood such that a slight negative air pressure prev ented any upsurge of CO2 or N2 above the reaction vessel that could occur due to the decomposition of the organics. Othe r researchers have shown that much of the irradiation occurr ed within a few centimeters of the liquid surface (even at very low catalyst loadings)95, 96. Consequently, the reac tion was conducted under vigorous agitation to ensure uniform distribution throughout the reacting medium and prevent sedimentation of the C65 particle s. Control experiments were performed by conducting UV irradiation without the addition of any photocatalyst in the MO solution. Negligible decolorization (<1%) was observed confirming that the degradation of MO predominately occurs by photocatalysis usin g titania rather than photolysis. Dark adsorption was conducted for at least three h ours before irradiation to allow for the adsorption of MO onto the TiO2 surface. Aliquots of 1.5mL of the suspension were collected at regular intervals during the de gradation experiments. These samples were then centrifuged (10000g, 30mins) to complete ly remove any particles, and the peak absorbance was analyzed using a V530 UV-Vis spectrophotometer (Jasco).

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975.3. Chemical Kinetics and Pathways G overning Photocatalytic Degradation Photocatalysis can be typically divi ded into three stages: (A) production of electron/hole (e-/h+) pairs by irradiation with light havi ng photonic energy greater or equal to the existing band gap (~3.2eV for titania), (B) migr ation of the charge carriers from the crystalline interior to the surface, and (C) redox interactio ns at the particle surface between the contaminant and the free e-/h+ pairs that survived the migration15. Titania has shown the best photocatalytic ability, when compared with other available inorganic oxides t ypically used like ZnO, Al2O3 and Fe2O3. In particular, mixed phase titania such as DegussaTM P25 (80% anatase and 20% rutile phase) has shown enhanced photocatalytic activity upon illumi nation compared to single crystalline phases97, 98. This enhancement is attributed to th e ideal size of the titania nanoparticle, trace amounts of Fe3+ dopant (that acts as a charge separator) and the synergistic combination of the anatase and rutile phase s that provides prolonge d separation of the photogenerated electrons99. Equation 5.1 shows the eCB-h+ VB pair separation within the conduction and valence bands upon irradiatio n. It is broadly accepted that hydroxy radicals (OH•) are produced from the direct oxidation of H2O, OHions (bulk solution) or terminal hydroxyl groups (catalys t surface) by photogenerated holes (h+ VB) as shown in equations 5.2 and 5.3100, 101. Superoxide radicals ofte n result from the interplay between the photogenerated electrons (eCB) and molecular oxygen. The addition of peroxides increase the occurrence of reaction 5.4 and the presence of hydroxyl radicals, thereby increasing the degradati on kinetics as widely reported102-104. VB CB 2 2h e TiO h TiO Equation 5.1

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98 OH OH hVB Equation 5.2 H OH O H h2 VB Equation 5.3 2 2 CBO O e Equation 5.4 Examining the adsorption characteristics of MO onto the TiO2 surface as a function of pH variations are pertinent to fully understanding the mechanism surrounding the degradation of organic contaminants. Gene rally the observed pH dependence of the reaction rates of MO decompos ition can be understood in term s of the adsorption of both the substrate molecule and hydroxyl ions onto the charged TiO2 surface as much of the degradation occurs on or n ear the titania surface rather than in the bulk medium105, 106. It is well known, that in the presence of water, the surfaces of the metal or semi-conductor oxides are hydroxylated107-109. Depending on the pH, these surface groups may add or abstract protons. The correspond ing acid-base equilibria for titania can be written as follows: 2TiOH H TiOH Equation 5.5 TiOH H TiO Equation 5.6

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99 Degradation of the MO can be explained in terms of the elementary mechanisms shown in equations 5.7-5.9. It can involve the di rect reaction of the dye with photogenerated holes in a process similar to the photo-Ko lbe reaction or oxidat ion through successive attacks by hydroxyl radicals or superoxide species110. The hydroxy radical in particular is an extremely strong nonselective oxidant that has shown to lead to the partial or complete oxidation of many organic chemicals111. product colorless MO OH Equation 5.7 product colorless MO hVB Equation 5.8 product colorless MO O2 Equation 5.9 Termination of active species can also occur by the mechanisms detailed below. Very small particle sizes (~10nm) tend to lead to higher electron-hole recombination (equation 5.10) reducing the photocatalytic activity. However, the optimum size of DegussaTM P25 titania and its efficient electron transfer fr om the rutile to an atase phase, leads to increased charge separation that causes efficient photocatalyt ic reaction at the particle surface98. heat h eVB CB Equation 5.10

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1002 2 2O O H OH HO Equation 5.11 In this study, both the freely suspended ti tania and titania-micr ogels consist of only DegussaTM P25 titania. The degradation products of methyl orange using DegussaTM P25 titania has been studied extensively112-114 with many of the intermediates during the MO degradation process already identified115. Thus, the focus of this research was on the photocatalytic performance of the titania-microg els that could serve as a prototype for the potential optimization of existing advanced oxidative processes. 5.4. Results and Discussion The TEM images of the titania-micr ogel (C65) and freely suspended DegussaTM P25 nanoparticles are shown in figure 5.1. The contrast between the freely suspended titania and titania-microgels is evident. The freely suspended titania is randomly displaced throughout the medi um with no well defined a rrangements, while the C65 particles contain titania that is well localized onto the largely spherical IP-microgels. As described in chapter 4, the titania-microge ls showed rapid sedi mentation on the timescale of minutes, which can be useful fo r gravity separation of these particles. 5.4.1. Photocatalytic Performance of the Titania-Microgel Composites For photodegradation experiments, the tw o different pH conditions (2 and 6.5) were chosen because sedimentation studies described in chapter 4 revealed that the titania-microgels released titania nanoparticles under basic conditions due to the

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101 electrostatic repulsion betw een the negatively charged titania and de-protonated carboxylic acid groups. As a first step, it was imperative to unde rstand the effect of pH of the MO solution. The initial spectra (at 0min) in fi gures 5.2 show the UV-Vis absorbance of MO above and below its pKa (~3.8). A significant peak shift is seen from 506nm (pH2) in figure 5.2A to 464nm (pH6.5) in figure 5.2B. The change in structure of MO was induced by the transition from a high to low pH and vice versa. This change in structure arises due to the increased delocalization of lone pair electrons on the azo group116. The inset in figure 5.2 best describes this delocalization in the form of canonical structures of MO at acidic pH values. In addition to the peak shif t seen in figure 5.2, there is also a 45% increase in the peak height of the MO absorb ance when the pH is lowered from 6.5 to 2. It is well known that the absorbance or irra diation by a solution is readily described by the Beer-Lambert law. Since the concentra tion and absorbance path length are same, it can be deduced that the canoni cal structures of MO, under acidic conditions, result in a larger molar extinction co-efficient when compared to the negatively charged MO molecule at near neutral conditions. Figures 5.2A and B also show that the peak absorbance of MO decreases as a function of time. The rate of change of absorbance of MO caused by photodegradation with either the titania-microgels or the fr eely suspended titania is well described by a mono-exponential curve. This c onstrues that a pseudo-firstorder reaction model can be used for describing the kinetic behavior of the photocatalysis. In this study, the MO concentration was held constant at 5ppm. Using a modified Langmuir-Hinshelwood model, the rate of decolorization can be expressed:

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102 KC 1 kKC r dt dC Equation 5.12 Here ‘k’ refers to the rate constant for the reaction, while ‘K’ refers to the equilibrium adsorption co-efficient. Due to the initial lo w concentration of MO (KC<<1), the ‘KC’ term in the denominator is typically neglected117, 118. Integration of the above equation with the initial condition C=C0 at initial time, and defining th e apparent rate constant as kapp=kK leads to the expected fi rst order reaction equation: t k C C lnapp 0 Equation 5.13 The apparent rate constant (kapp) was obtained directly via regression of the experimentally observed decline in the peak height of MO absorbance (as a function of time). This is shown in figure 5.3, where the fits to the normalized absorbance at regular time intervals using equation 5.13, yields the apparent rate constants for the reaction. In figure 5.3A the apparent rate constants for MO degradation using the titania-microgels over a range of concentrations is depicted at pH 2. At a more neutral pH of 6.5, the rate of MO degradation is reta rded as seen in figure 5.3B. The control experiment, where MO in solution was irradiated in the absence of titania under similar conditions (pH 6.5, UV=3.5mJ/cm2), yielded a null rate constant and th ereby confirmed that MO degradation was achieved via photocatalysis alone. In figure 5.4, the fitted rate constants are compared for both the freely suspended titania and the C65 particles in experiment s performed with different amount of titania

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103 photocatalyst in the solution. First, examining the rate kinetics of the freely suspended titania shows that increasing the concentra tion of the titania re sulted in a faster decolorization of the MO at both pH values tested. This effect is well documented and attributed to the increased active site s on the titania surface available for MO degradation119, 120. Regarding the pH variation the re sults presented here using freely suspended DegussaTM P25 titania correlate well with that of Kansal and co-workers121, who also reported faster degradation kineti cs under basic and ne utral conditions. In general, the degradation of MO is faster at a near neutral pH and slower in an acidic medium. This effect can be explained as fo llows: at a high pH, bot h the surface of the titania (ISP ~ 6.5) and the MO are negatively charged. However, the presence of large quantities of hydroxyl ions on the particle surface (as well as in the reaction medium) favors the formation of the oxidizing OH• ra dicals. Near a neutral pH, even though there is a reduction in the concentration of OHions in bulk solution, the electrostatic repulsion between the titania surface (now relatively uncharged) and the MO is reduced, thereby favoring adsorption of the dye and its degrad ation. Lastly, in an acidic medium the significant reduction in the concentration of th e OHions leads to a decrease in the rate constant as demonstrated e xperimentally at pH 2. A number of significant observations can be made when comparing the photocatalytic performance of the freely su spended titania with the titania-microgels. Under acidic conditions, the photodegradation by the titania-microgels and the free titania show nearly identical rate constants over a range of titania concentrations. In contrast, near a neutral pH the photocatalytic degrada tion of MO using the titania-microgels has a smaller rate constant and this remains largel y unaffected even when the concentration of

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104 titania is increased. A closer look at the stru cture of the titania-microgels can give some insight into these differences. The PAAc ch ains that interpenetrate the IP-microgel contain carboxylic acid groups that are mo stly deprotonated a bove pH4 and known to functionalize inorganic oxide surfaces60, 122, 123. Near a neutral pH, photocatalytic degradation of MO using the titania-microgels is minimal since the modification of the oxide surface by deprotonated negatively ch arged carboxyl groups can result in the disruption of the adsorption of negatively charged hydroxyl species onto the titania surface due to electrostatic repulsion. With th e number of oxidative species generated by the titania diminished, the photocatalytic oxidati on reaction can be expected to be retarded and this is experimentally manifested as a lower reaction rate constant. However, at pH2 the PAAc is protonate d and the titania su rface within the titania-microgels remains primarily unhindered and available for the photocatalysis. As a result, the photocatalytic performance of both the titani a-microgels and the freely suspended titania were comparable. Hence a trade-off exists wh en using the titania-microgels, in that the photodegradation is slower at near neutral c onditions but the settling is nearly a hundred times faster than the free titania particles. Additionally, by a simple change in pH, the rate constant of the titania-microgels can be eas ily manipulated, with the degradation rate ramped up or down when desired. Increasing the photocatalyst concentrati on from 50 to 200 ppm requires increasing the concentration of titania-microgels. At acidic conditions, the concomitant increase in polymer fraction has little infl uence and the effect from in creased titania surface sites dominates, which leads to faster photodegrada tion. At neutral pH conditions, the increase in the PAAc fraction balances the increase in t itania and little change in the reaction rate

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105 constant can be observed. Because the reaction rate constant at neutral pH conditions is already small (~0.005 min-1) when using the microcomposites containing 65wt% content of titania, experiments with titania-microgels possessing less than 65% loading of titania were not pursued as it would further redu ce the rate of photodegradation, and the sedimentation of sparsely loaded titani a-microgel composites is not as quick. 5.4.2. Impact of Irradiation on the Stability of Polymeric Microgels Since the photocatalysis with titania gives rise to oxidative species that can degrade the polymer chains, it is to be expected that the titania-microgels will also degrade (via photocatalysis) over time. Cons equently, the degradation of the titaniamicrogels was studied using two separate a pproaches (in both cases with no MO present in the solution). First, the se dimentation behavior of the C 65 titania-microgels after predetermined periods of UV irra diation was analyzed using turbidometry at the two pH conditions. Since the settling ve locity of the microcomposites is a function of the titania loading in the microgels, any release of the titania nanoparticles from the polymer mesh should be expected to lead to reduced settling velocities The second technique used involved analyzing the scission of the pol ymer chains via photocatalysis. As the oligomeric segments present in the solu tion increased, it was measured via UV-Vis spectroscopy. By performing a thorough analysis of these results, useful insights were gained into the process of degradation of the microcomposites. Figure 5.5A shows the voltage signal during sedimentation of the titaniamicrogels after various irradiation times at pH 6.5 using the turbidom eter. It is evident that little change is observed in the sedi mentation behavior even after 4.5 hours of UV

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106 illumination and as expected, most of the pa rticles settled rapidly within 1000 seconds. This suggests that the titania loading within the microgels is largely constant with no significant loss of the nanoparticles occurri ng during the irradiation window. However, after 6 hours of irradiation, even though settling can be ob served by the decrease in turbidity of the solution, the steady upward shift in the residual signal after 1000s indicates that the solution contains a suspen ded particulate concentration. This is a contribution from the slowly settling titania nanoparticles that have been released and their fraction in the solution is growing with tim e of irradiation. Figure 5.5B shows that at pH 2, a similar trend is observed with the release of titania being manifested in the sedimentation behavior of the titania-microge ls after only 4.5 hours of irradiation. It should be noted that the supernatant liquid remaining after 3 hours of UV irradiation of solutions at both pH 2 and 6.5 was also an alyzed for titania using the quantitative spectroscopic technique81 and no measurable amounts were detected. This supports the interpretation of the results from sediment ation, and the hypothesi s that photocatalysis was conducted by the titania localized onto the polymer microgels, rather than released titania nanoparticles. Figure 5.5B shows that after approximately 6-7.5 hours of continuous irradiation at pH 2, most of the titania has been released and no settling is observed since the positively charged tit ania nanoparticles do not settle. Figure 5.6 reflects the increase in th e oligomeric segments present in the supernatant that is obtained af ter centrifugation of samples dr awn at regular time intervals during UV irradiation of a solution of the t itania-microgels alone. Absorption in the UVC region (190-400 nm) is typical of organic moieti es of the fragmented polymeric segments that result from the cleavage of the IP-microgels during the photodegradati on. Figure 5.6

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107 shows that integrated area of the absorption peak increases with time and becomes fairly constant after several hours of irradiation. The data is agai n consistent with a physical picture of slow microcomposite degradation th at is also governed by the pH conditions of the testing. At a pH 6.5, the measured abso rbance reaches a plateau at ~13hrs and at pH 2, the plateau value is reached ~8hrs. Both of these degradation times are consistent with the turbidity results in figures 5.5 that s how few changes in settling after 10-12 hours (pH6.5) and 6 hours (pH2). Comparison of the results in figures 5.5 with figure 5.6 also indicates that signifi cant changes in settling only occur at times that correspond to a substantial percentage (~50-60% ) of oligomeric absorption. The integrated areas for oligomeric abso rption are shown in figure 5.7. and also support the conclusions drawn form the phot odegradation of MO. At a pH 2, the degradation kinetics were much faster for MO and the same is true when the titaniamicrogels are being degraded as shown by the comparison of sedimentation or the comparison of oligomers in solutions after de gradation. The results in figure 5.7 also indicate that the time for the degradation of the titania-microgels is significantly longer than the time required for MO degradation (~3 hrs). One of the advantages of these novel titani a-microgels is their ease of formation. Following the long time degradation of the tita nia-microgels, the titania nanoparticles are released into the aqueous solution. However, addition of fresh IP -microgels under acidic conditions (pH2) quickly reformed the micr ocomposites particles w ith a 65wt% titania loading. Furthermore, photodegradation of th e organic dye could be repeated using the reformed titania-microgels. When the photode gradation kinetics of MO using the reformed titania-microgels was compared with the original results of the freshly prepared

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108 titania-microgels (before exposure to UV irradi ation), the regression analysis yields the same rate constant for both samples as show n in figure 5.8. Thus, the titania-microgels are excellent candidates for remediation proces ses that can re-use and recycle the titania photocatalyst efficiently. The results reporte d in this study demonstrate that the UVA irradiation (~350nm) can be used for the photodegradation of chem ical contaminants with the titania-microgels w ithout rapid decomposition of th e polymeric matrix and that these novel materials enable large scale separations usi ng gravity thickeners and centrifugal clarifiers. 5.5. Summary The photodegradation of a methyl orange was investigated using both rapidly settling titania-microgel particles and freely su spended titania. Under acidic conditions, the reaction rate constants were found to be identical while the fr eely suspended titania showed faster rate kinetics near neutral c onditions. However, the ra pid sedimentation of the titania-microgels (~100X faster than fr eely suspended titania) makes them promising candidates for applications such as wa stewater remediation where the use of nanoparticles of titania is advantageous for photocatalysis but separation of the nanoparticles is difficult. Even though the cros s-linked matrix of the colloidal polymeric particles showed degradation over several hours, the titania that was released due to the microcomposite degradation could be easily re -captured via the addition of fresh polymer IP-microgels. These reformed titania-microgels showed similar photocatalytic behavior as the original titania-microgels that indicated that these materials can be easily

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109 regenerated and the photocatalyst can be recy cled without significan t aggregation within the reformed microcomposites.

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110 Figure 5.1: TEM images of (A) freely suspended TiO2 nanoparticles in aqueous media and (B) titania-microgel particles. 800 nm A 800 nm B

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111 0.30 0.25 0.20 0.15 0.10 0.05 0.00Absorbance 700 650 600 550 500 450 400 350 Wavelength (nm) 506nm 0 min 30 min 60 min 90 min 120 min 150 min 180 min (A) 0.30 0.25 0.20 0.15 0.10 0.05 0.00Absorbance 700 650 600 550 500 450 400 350 Wavelenth (nm) 464nm 0 min 30 min 60 min 90 min 120 min 150 min 180 min (B) Figure 5.2: (A) Absorbance spec tra of MO degradation in solutions containing titaniamicrogels (200ppm TiO2) at a pH of 2 and (B) absorban ce spectra of MO degradation in solutions containing titania-microgels (200ppm TiO2) at a pH of 6.5. Insets: canonical structures of MO at the corresponding pH.

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112 3 4 5 6 7 8 9 1Normalized Absorbance 200 150 100 50 0 Time (min) (A) 50 PPM 100 PPM 150 PPM 200 PPM 5 6 7 8 9 1Normalized Absorbance 150 100 50 0 Time (min) (B) 50PPM 100PPM 150PPM 200PPM Figure 5.3: Normalized absorbance using UV-Vis spectroscopy for the photocatalytic degradation of MO using titania -microgels as the photocatalys t source at (A) pH of 2 and (B) pH of 6.5. Symbols repr esent the normalized absorbance at pre-determined time intervals and the lines are draw n as fits using equation 5.13.

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113 0.020 0.010 0.000Rate Constant (min-1) 200 150 100 50 0 Time(min) 0.020 0.010 0.000 pH2 pH6.5 Figure 5.4: Rate constants for the photo catalytic degradation of MO using freely suspended titania (squares) tita nia-microgels (circles) at a pH of 2 (A) and a pH of 6.5 (B). Lines are drawn as a guide to the eye.

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114 0.8 0.6 0.4 0.2 0.0Voltage 1000 800 600 400 200 0 Time (s) pH 20hrs 1.5hrs 3hrs 4.5hrs 6hrs 7.5hrs(B) 0.7 0.6 0.5 0.4 0.3 0.2 0.1Absorbance 1000 800 600 400 200 0 Time(s) pH 6.50hrs 15hrs 3hrs 13.5hrs 6hrs 7.5hrs 9hrs 10.5hrs 12hrs(A) Figure 5.5: Turbidity measurement as a func tion of time reflecting sedimentation in a solution of the titania-microgels at (A) pH 6.5 and (B) pH 2.

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115 Figure 5.6: Optical absorption from the oli gomeric species present in the supernatant solution after titania-microgels have been ir radiated for different durations. The curves correspond to (A) pH2 and (B) pH6.5. (B) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0Absorbance 250 240 230 220 210 200 190 Wavelength (nm) pH 6.5 DI_Water 0Hr 2Hr 4Hr 6Hr 8HR 10Hr 12Hr 14Hr 16Hr (A) 2.0 1.5 1.0 0.5 0.0Absorbance 250 240 230 220 210 200 190 Wavelength (nm) pH2 Hr0 Hr1 Hr2 Hr3 Hr4 Hr5 Hr6 Hr7 Hr8 Hr9 Hr10

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116 100 80 60 40 20 0Normalized Abs (190-400nm) 15 10 5 0 Time (hr) Figure 5.7: The optical signal is shown as a percentage of th e plateau value (dashed lines) obtained at long times. The solid lines are draw n as a guide to the eye. Arrow indicates the typical times at which significant changes in settling were observed in figure 5.4 (A) and (B).

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117 4 5 6 7 8 9 1C/Co 100 80 60 40 20 0 Time(min) Reformed Titania-Microgels Original Titania-Microgels Figure 5.8: Rate constants of the reformed a nd original microgel-titania particles at pH2.

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118 CHAPTER 6: CHEMICAL ME CHANICAL POLISHING USING MICROCOMPOSITE AND HYBRID PARTICLES 6.1. Introduction to CMP CMP has become a critical processing step in the fabrication of next generation electronic devices as the semiconductor i ndustry advances to sub-45nm technology nodes124-126. The reduction in device dimensions has led to stringent requirements for the post-chemical mechanical polishing surface qual ity. In addition to global planarization and high polish rate, the CMP process also need s to achieve high material selectivity and a superior surface finish. For silicon oxide CMP, achieving a superior surface quality includes fewer scratches with minimal oxide dishing and nitr ide erosion. This is important for shallow trench isolation (STI) CMP, which is used extensively for logic device fabrication. However, the advantages of using CMP as a global planarizatio n technique can be quickly nullified by contamination from slurry chemicals, particle contamination (residue) and scratches during polishing due to agglomerated particles. Also, other pattern-related defects like dishing and erosion, delamina tion, and dielectric crushing hamper the device yield and quickly negate the advantages of using CMP29, 127. Therefore, making improvements in the polishing process to reduce the surface defects is an important engineering challenge.

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119 The quality of the post-CMP wafer surface depends on a wide variety of factors such as the characteristics of the abrasi ve particles, proce ss conditions, particle concentration in the slurry, hardness of the pad, and chemistry of the slurry128-132. Controlling the characteristics of the abrasi ve particles is particularly significant for improvements in the CMP process. For example, controlling the particle agglomeration in the slurry can greatly aid in reducing su rface scratches and the need of post-processing techniques to remove defects28, 133-135. Commonly used ceramic abrasive particles are much harder than the low dielectric constant materials like copper and silica that are used to blanket the wafers during lithography. This abrasion often results in permanent scratch defects and can lead to problems such as delamination or formation of puddles in subsequent layers of metallization that cause electrical short circuits136, 137. Therefore, tailoring the particle hardness or softness is important. Particle characteristics that result in low friction at the interf ace are beneficial to the CMP process since lower friction helps reduce surface damage during CMP138. Similarly, particles that leave minimal residue on the wafer surface after CMP also im prove the yield and effectiveness of the polish while enhancing further li thography on nanometer dimensions. The research in this dissertation has focused on both hybrid and microcomposite abrasive particles as a route to addressing the challenges in the CMP process. Recent advances in slurry development have involved mixed or modified abra sive particles that reduce defects during CMP139-141. These studies have focused on abrasives of different inorganic oxides, different size s of inorganic oxides, and th e use of micelles of surfaceactive compounds. The surface scratches and part icle residue have not been addressed using these methods since the inherent material characteristics of the abrasive particle

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120 that interacts with wafer surface remain uncha nged. Polymer-silica particles prepared by other researchers have show n fewer surface defects as compared with commercial slurries but these particles appear to aggreg ate at the water-air interphase due to poor dispersion28, 61. In this research, a promising approach for reducing surface defects during polishing by using slu rries of (A) hybrid abrasive pa rticles and (B) microcomposites of siloxane-ceria-microgels was pursued. 6.2. Experimental Details for Polishing Studies Slurries were used to polish thermally grown silicon oxide wafers using a benchtop CMP tester (CETR CP-4). Figure 6.1 show s a schematic of this apparatus where the slurry, typically containing ab rasive particles flows, ont o the polymer polishing pad. Some of these abrasive particles are sandwic hed between the polishing pad and the wafer, thereby resulting in the removal of material from the wafer surface. Using this bench-top CMP tester, the programmable forces, speeds, and slurry flow rates can be chosen to simulate CMP processes and to understand the process in greater detail. Figure 6.2 shows a digital image of this bench-top tester that includes a 6-inch polishing pad and can hold up to a 2-inch wafer to be polished. A dual fo rce sensor allows for continuous monitoring of lateral and normal forces in situ at a to tal sampling rate of 20 kHz. All the slurries were well agitated during experimentation to reduce sedimentation of the abrasive particles. Conditioning of the IC1000 perforat ed polishing pad and suba500 sub-pads was conducted for 10min using deionized water an d a commercial diamond grid conditioner from 3M with a 400 grit size. The planariz ation of 1.5 in. square oxide wafers was

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121 carried out on the CMP bench-top polishe r using slurries of the hybrid and microcomposite particles. Aqueous slurries of the siloxane-micr ogel hybrids, core-shell particles and siloxane-nanogel hybrids were prepared with the pH adjusted to ~12 using sodium hydroxide, since basic solutions have been shown to aid in the removal of silica from the wafer surface142. No additives were used to ascertain the contribution of particle morphology to the abrasive action that occurs during CMP processes. For the microcomposite particles, slurries consisti ng of 0.5wt% siloxane-ceria-microgels with approximately 50 wt% ceria were used to ma ximize silicon oxide removal rates from the wafer surface. For comparison, slurries with commercial ceri a particles with two different weight percentages were also used. A slurry that containe d 0.25 wt% ceria particles was used to keep the content of ceria the same between the slurries of the microcomposite particles and the commercial ceria particles. Additionally, slurry with 0.5 wt% ceria particles was also used to keep the total wei ght percent of particles the same between the slurries of the microcomposites and the commer cial ceria particles. All slurries were dispersed in deionized water at a pH of 5 to maintain a slight positive charge on the CeO2 (ISP of CeO2 occurs at pH~7.5) that helps disperse the ceria more evenly and aids in abrading the negatively charged sili ca surface (ISP of silica pH~2.3).

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122 Parameter Value Pressure 7psi Pad RMP 200 RPM Slurry flow rate 75ml/min Slider velocity 3mm/s Slider stroke 7mm Table 6.1: Process conditions for polishing oxide wafers. The planarization was conducted for 3min at room temperature and all CMP experiments were repeated to ensure reproducibility The process conditions for the polishing experiments are tabulated in table 6.1. The pad rotation speed is comparable to other reports in the literature143-145. To examine the removal rates of the silica from the wafer surface and to test for organic residue on th e polished surface, infrared spectroscopy was used. 6.3. Results and Discussion Although the hybrid and core-shell particles produced a superior surface finish (resulting in very planar surfaces) compar ed to commercial inorganic slurries, the removal rates of oxide from the wafer surface were low (<15nm/min) with considerable values for the co-efficient of friction (~0.25)59. This was a major drawback for any commercial CMP applications. To overcome this removal rate limitation, incorporation of inorganic nanoparticles such as ceria within the microgels was pursued. While

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123 Slurry COF Removal rate (nm/min) 0.5wt% ceria-polymer composites 0.155 0.013 98 0.25 0.5wt% CeO2 0.215 0.011 236 2.54 0.25wt% CeO2 0.108 0.010 111 1.34 nanoparticles of ceria are well known for their selectivity and removal of oxide from a wafer surface, prior studies have shown that they can also produce major and minor scratches146-150. Therefore, microcomposites of ceri a nanoparticles and siloxane-microgel hybrids with interpenetrating PAAc were inves tigated to as a rout e towards significant improvements in the surface finish while achi eving practical rates for oxide removal. FTIR characterization of the wafer surface before and after CMP using the microcomposites of ceria nanoparticles and s iloxane-microgel hybrids revealed that there was no polymer deposition onto the wafer surface. This is evident in figure 6.3 by the lack of amide or carbonyl absorption. The inte nsity of the absorption peak of Si–O–Si at 1075cm-1 decreased after polishing (inset in fi gure 6.3), which indicates substantial removal of the oxide layer by the ceria a nd ceria-microgel microc omposite slurries). Qualitatively, the reductio n in absorption at 1075cm-1 in figure 6.3 shows that the 0.25wt% ceria slurry and 0. 5wt% microcomposite slurry achieves nearly identical removal of the oxide while the slurry contai ning 0.5 wt% ceria removes nearly twice the amount of oxide. Table 6.2: COF and removal rate for slu rry polishing with different particles.

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124 Quantitative thickness measurements of the oxide film on the wafer were performed using ellipsometry at multiple angles. The ellipsometry results in table 6.2 are consistent with the qualitative conclusions from the FTIR re sults in figure 6.3. Figure 6.4, shows the ellipsometric analysis that was conducted to quantitatively determine the amount of oxide on the wafer surface. Scans were done at four different angles (60, 65, 70 and 75) on the blank wafer and wafers polished with th e ceria (0.25wt% and 0.5wt%) and the ceriamicrogels (0.5wt%). Each wafer was polished fo r three minutes using the slurry. In figure 6.4, the solid lines indicate the fits to the experimental data (symbols). The removal rates that are shown in tabl e 6.2 for slurries containing ceria alone are comparable to other litera ture reports. Kim and co-workers151 have reported a removal rate of ~250 nm/min using slurries containing 0.5 wt% of ceria and an anionic acrylic polymer dispersant wh ile Manivannan and Ramanathan152 have reported removal rates of ~75 nm/min using 0.25 wt.% ceria dispersed in an aqueous solution at pH 5. The increase in removal rate using the microc omposites over that when using the hybrid microgel without any ceria makes it feasible to use these microcom posite particles for polishing in the final stages of CMP proce ss where only moderate amounts of material needs to be removed but superior surface quality is required. Since the slurries used here contain no additives or accelerants and also have a low weight fraction of particles, further optimization of the removal rates using chemicals and changing the particle concentration remains an option. Table 6.2 also lists the co-efficient of friction (COF) data measured during polishing. The COF was obtained from the ratio of lateral and normal forces measured in situ using a dual force sensor installed to the upper carriage of th e machine carrying the

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125 wafer carrier. The average co-efficient of fric tion after the process has reached the steady state is shown. As shown in figure 6.5A, the COF reaches an average value from 100150s. The distribution of these COF values fo r both the ceria and mi crocomposite slurry is plotted in figure 6.5B. This can be best described using a normal Gaussian plot, which gives the mean values for the COF (Table 6.2) along with the standard deviations. Average values of COF reveal that the slurry containing 0.25 wt% ce ria particles resulted in lower co-efficient of friction compared to the slurry containing 0.5 wt% of ceria particles. This is plausibly due to the lo wer concentration of th e ceria nanoparticles. Interestingly, the siloxane-ceria-microgels lead to reduced friction at the polishing interface even though 0.5 wt% slurry of these particles should contain a higher number concentration given the lower mass density of the organic polymer. Thus, the lower COF supports the expectation that the microcompos ite particles result in a milder abrasive interaction with the surface. It is crucial to study the post-CMP surface characteristics to draw any conclusions regarding the performance of microcomposites. Optical microscopy images of the postCMP oxide surface are shown in figure 6.6. It is evident that slurries with commercial ceria particles resulted in severe scratches and pitting on the wafer surface. In contrast the surfaces polished with slurries consisting of the microcomposites resulted in few surface defects and other related surface defects. Th e reduction in surface scratches and damage can be attributed to reduced abrasive acti on due to the deformable polymeric component of particles. AFM images in figure 6.7 shows that particle contamination occurs on wafer surfaces polished with only ceria nanopartic les. The wafer surface polished with the microcomposite particles is devoid of pitting and minor sc ratches. The generation of

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126 defects in surfaces polished with ceria alone is consistent with results reported in past studies that use slurri es containing inorganic particles as abrasives146-150. Quantitative assessment of the roughness of surfaces from data obtained using atomic force microscopy has been studied in the past and several statistical parameters have emerged153. One common measure of the roughne ss is the root-mean-square (Rrms) that details the deviation of the heights of the various features imaged by AFM. However, it is well known that Rrms is scale-dependent and a single value can be misleading as it provides little information on the width or sp acing of surface features that correspond to Rrms. Bonnell and co-workers153 have evaluated the merits of several statistical parameters and have shown that it is more appropriate to determine the variational roughness at different lateral length-scales. Be yond a characteristic le ngth of the surface, the value of roughness becomes scale-independent and can be specified as a characteristic of the surface. Implementing a surface rou ghness analysis algorithm described by Bonnell and co-workers153 using the AFM images for the polished surfaces gives the variational roughness plotted in figure 6.8. For the surfaces polished with the commercial ceria nanoparticles, the surface roughness begi ns to become somewhat independent beyond a spatial scale of 10 m while this occurs at approximately 2 m for the surfaces polished with the siloxane-ceria-microgels. This helps add weight to the argument that the surfaces polished with the microcomposite s have better planarization than those polished with just the ceria nanoparticles. It is also evident that the macroscopic roughness beyond the 10 m is much larger for the polis hing with commercial ceria and that there is greater variability in the r oughness results. The results presented above clearly indicate that the microcomposite particles with controlled softness/hardness can

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127 be beneficial and can be successfully implem ented for polishing in the final stage of CMP process where only moderate amounts of mate rial need to be removed but superior surface quality is required. Fewer surface defe cts and particle residue will aid in the elimination of rigorous post-CMP cleaning stages and, consequently, will help in achieving environmentally-benign CMP processes. 6.4. Summary A variety of abrasive particles such as core-shell particles, siloxane-ceriamicrogels microcomposites, and hybrid microge l/nanogels were studied for the CMP of silicon oxide wafers. However, the siloxane-ceria-microgels were the most efficient and can form the basis of novel slurries for CMP. These siloxane-ceria-microgels contained an average mass fraction of approximately 50wt% ceria. Planarization of silicon dioxide wafers in bench-top CMP tester and subseque nt characterization of the polished wafers revealed that slurries formed from the siloxa ne-ceria-microgels lead to removal rates of the oxide from the surface at ~100nm/min. More importantly, the polished surfaces showed lower topographical variations and surface roughness than when polished by slurries of only ceria nanoparticles. Polishing with these novel sil oxane-ceria-microgels yielded surfaces devoid of scratches and partic le deposition, which makes these particles suitable for next generation slurries in CMP.

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128 Figure 6.1: Schematic of the CMP apparatus. Top view of CMP apparatus slurry feed conditioner polymer pad wafer holder platen polymer pad slurry carrier wafer pad conditioner Side view of CMP apparatus

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129 DI water Slurry Peristaltic Pump Conditioner Wafer Slider Stir Plate Polishing Pad COF Sensor Figure 6.2: Digital image of the bench-t op CMP tester and other necessary inputs.

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130 1.00 0.95 0.90 0.85 0.80 0.75 0.70 1120 1080 1040 1.0 0.8 0.6 0.4 0.2 0.0Normalized Absorbance 2000 1800 1600 1400 1200 1000 800 600 400 Wavenumber (cm-1) Blank CeO2 0.25wt% siloxane-ceria-microgel CeO2 0.5wt% Figure 6.3: FTIR characterization of s ilica removal from the wafer surface. Inset: peak of Si-O-Si ab sorbance centered at 1075cm-1.

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131 1.0 0.8 0.6 0.4 0.2 0.0Photodetector voltage (V) 6000 5000 4000 3000 2000 Driving voltage from LCD modulator (mV) 75 70 65 60 2.0 1.5 1.0 0.5 0.0Photodetector voltage (V) 6000 5000 4000 3000 2000 Driving voltage from LCD modulator (mV) 75 70 65 60 (A) (B) Figure 6.4: Quantitative ellipsometric characterization of silica removal from the wafer surface where (A) wafer polished using the ce ria-microgel particles (B) blank wafer.

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132 0.25 0.20 0.15 0.10 0.05 0.00COF 150 100 50 0 Time (s) 140 120 100 80 60 40 20 0Frequency (a.u.) 0.3 0.2 0.1 0.0 COF CeO2 0.25wt CeO2 0.5wt ceria-microgels 0.5wt (A) (B) Figure 6.5: (A) COF variation with time a nd (B) distribution of the COF between 100150s such that the solid line in dicates the Gaussian fit to the actual COF values (circles).

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133 Figure 6.6: Optical microscopy images of wa fers polished with slurries containing (A) 0.5wt% ceria nanoparticles (B ) 0.25wt% ceria nanoparticles (C) 0.5wt% siloxane-ceriamicrogel (50wt% CeO2). 10 micron 10 micron 10 micron A B C

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134 Figure 6.7: AFM images of wafers polished with slurries containing (A) 0.5wt% ceria nanoparticles (B) 0.25wt% ceria nanoparticles (C) 0.5wt% siloxane-ceria-microgels. 40 30 20 10 0 micron 40 30 20 10 0 micron 40 30 20 10 0 micron 40 30 20 10 0 micron 40 30 20 10 0 micron 40 30 20 10 0 micron A B C

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135 8 6 4 2 0 Rq(nm) 40 30 20 10 0 Lateral Length (microns) 0.5 wt% Ceria 0.25 wt% Ceria 0.5 wt% Composite Particles Figure 6.8: Variational surface r oughness of the polished wafers.

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136 CHAPTER 7: SUMMARY AND CONCLUSIONS In this doctoral research, novel hybrid and microcomposite particles were synthesized. In particular, the appr oach of making microcomposites using interpenetrating microgels of PAAc and PNIPAM with CeO2 or TiO2 nanoparticles was demonstrated experimentally. Extensive characterization was conducted using light scattering, spectroscopy, microscopy and ther mal gravimetric analys is to understand the morphology, responsive behavior and structur e of the microcomposites. Additionally, the sedimentation behavior of the titania-microgels was examined and the results presented within this dissertation demonstrate the general usefulness of settling as a simple characterization of complex composite particle s. The sedimentation rate could also be used as a technique to estimate the load ing of inorganic nanoparticles within the polymeric framework. The photodegradation of a model organic compound, methyl orange, was investigated using both the ra pidly settling titania-m icrogel particles and freely suspended titania as a control. Under acidic conditions, the reaction rate constants were found to be identical while the freely su spended titania showed faster rate kinetics near neutral conditions. However, the nearly hundred times faster sedimentation of the titania-microgels makes them promising candidates for applications such as wastewater remediation. Additionally, the rate kinetics of the process could be easily tuned with a simple change in the acidity of the wastew ater when using the titania-microgels. The interplay between the titania, PAAc and adsorbate has been detailed within this

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137 dissertation. Lastly, siloxane-ceria-microgels that contained an average mass fraction of 50wt% ceria were found to be useful for the CMP of oxide wafers. Planarization studies using a bench-top CMP tester and subsequent characteriza tion of the polished wafers revealed that slurries formed from the sil oxane-ceria-microgels led to practical removal rates of oxide from the surface that makes these particles commercially viable. More importantly, the polished surfaces showed lower topographical variations and surface roughness than when polished by slurries containing only ceria nanoparticles. 7.1. Future Prospects and Recommendations There are several aspects of this resear ch that can be extended to enhance the commercial viability of microcomposites a nd to provide additional insights for engineering microcomposite particles in the futu re. In this context, two areas of interest are outlined below. Since only about 7% of sunlight contains UV radiation, one of the areas described below focuses on shrinking the fa irly large band gap of titania into visible regions for economically attr active remediation processes154. This can also be extended from simple remediation process to self-cleani ng sprays or paints that already have been seen commercially but to a limited extent. On e of the drawbacks is that even though other researchers have shown that the band gap of titania can be easily shifted via doping, the resulting photocatalysts are not very active, with a decr ease in the rate constants155-157. The second area of discussion focuses on CM P. Even though the novel microcomposites prepared and tested here provided good removal rates with excellent surface characteristics, ceria based slurries can be quite expensive due to the large slurry

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138 consumption during CMP. Therefore, the sy nthesis of abrasive polishing pads that contain ceria nanoparticles is discussed. 7.2. Doping of Titania to Shift the Band-Gap Although nanoparticles of titania have proven to be extremely photocatalytically active, its band gap (3.2eV) can be limitin g for wastewater remediation and other applications. Since ultraviolet radiation onl y accounts for only a small fraction of the sun's energy as compared to visible light (4 5%), any shift in the optical response of titania from the UV to the visible spectra l range will have a profound effect on the photocatalytic efficiency and marketabilit y of titania as a next generation ‘green’ technology. Recent research has shown that th is band gap can be shifted via doping with nitrogen, chromium and other heavy metal ions. Th e shifting of the op tical response of TiO2 from the UV to the visible spectral range has been first investigated by doping TiO2 with transition metal elements158-160. However, it has been s hown that metal doping has several drawbacks; namely, the doped photocat alyst suffers from thermal instability, and the metal centers act as electron traps, there by reducing the photocatal ytic efficiency. To circumvent these difficulties, considerable efforts have been undertaken to dope TiO2 powders with nitrogen, due to the closer proximity of nitrog en to oxygen in the periodic table 161, 162. There are a number of tec hniques that have been used to incorporate nitrogen within the titania lattice structure. Thes e include ball-milling, sol-gel synthesis, sputtering, ion implantation, sintering of titania in an ammonia atmosphere and plasma processes156, 163-166. Among these, the sol gel process is the most adopted method for the synthesis N-doped TiO2 nanoparticles because doping leve ls as well as the size of

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139 nanoparticles can be easily controlled depe nding on the reaction conditions such as the solvents used, pH, temperature, and hydrolysis rate. 7.3. Abrasive Pads for CMP The microcomposite particles pursued in this doctoral research represented an interesting system for CMP since soft polym eric segments were coupled with harder inorganic nanoparticles. This concept has th e potential for the s ynthesis of organicinorganic abrasive pads. In literature report s, interactions between a soft pad, hard abrasive particles, and the wafer surface have been considered using a variety of approaches that extend from modeling167, 168 to experiments169. In an experimental study, Castillo-Mejia and coworkers found that reduc ing the elastic modulus of an IC 1000 pad surface led to lower polishing rates. In an earlier study by Stavreva et al131, the use of stacked pads with a hard top (IC 1000) a nd a compressible bottom layer (SUBA IV) was found to give better uniformity during polis hing with an aluminabased slurry. More recently, Yongguang Wang et al.21 developed a nonlinear relationship between the indentation depth and the particle/soft-pad microcontact force for the CMP process in case of a single particle. Based on this mode l, they suggest that as the numbers of particles embedded in a soft pad increase, a higher material removal rate and global planarization should be achieved. Based on a ll these past studies, the development of composite films for abrasive CMP polishing pads coupled with abrasive free slurries can be an attractive pursuit. This also helps elim inate problems such as particle sedimentation and agglomeration that frequently cause nonuniform planarization. Additionally, the use of responsive polymers to fabricate the po lishing pads has the pot ential to tune the

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140 polishing from harder to softer abrasion when desired during the manufacturing process by simply changing external stimuli like temp erature or pH of the slurry solution.

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ABOUT THE AUTHOR Cecil Coutinho was born in Mumbai, India. He obtained his Bachelor’s degree in Chemical Engineering at the University of Nebraska Lincoln in May 2005. He began pursuing his Ph.D. degree in the Chemical E ngineering Department at University of South Florida in August 2005. During the course of his study at USF, he presented his work in many conferences and published seve ral papers. He was aw arded the outstanding researcher award (2007-2008), and the outst anding teaching assistant award (2005-2006 and 2007-2008) by the Chemical & Biomedical Engineering department at USF. He was also presented with the outstanding gradua te student achievement award in 2008 for excellence in research and teaching.