USF Libraries
USF Digital Collections

Effects of Zn doping and high energy ball milling on the photocatalytic properties of TiO₂

MISSING IMAGE

Material Information

Title:
Effects of Zn doping and high energy ball milling on the photocatalytic properties of TiO₂
Physical Description:
Book
Language:
English
Creator:
Algarin, Paula C
Publisher:
University of South Florida
Place of Publication:
Tampa, Fla
Publication Date:

Subjects

Subjects / Keywords:
Photocatalysis
Methyl orange
Surface
Band gap
Sol-gel process
Dissertations, Academic -- Electrical Engineering -- Masters -- USF   ( lcsh )
Genre:
non-fiction   ( marcgt )

Notes

Abstract:
ABSTRACT: TiO₂ photocatalysis is been widely studied for air and water purification applications; titanium dioxide is the most used semiconductor principally because its low cost, stability and chemical properties. However it only utilizes the UV portion of the solar spectrum as an energy source (less than 4% of the total sunlight energy). This behavior is due to its high band gap value of 3.2 eV. The modification of light harvesting properties of TiO₂ by doping has become an important research topic to achieve an efficient operating range under UV and visible light. In addition, the structure and surface properties of photocatalysts play an important role. This thesis explores the effects of Zn doped TiO₂, prepared by the sol-gel method, on its photocatalytic activity to decompose organics and the characterization of the doped samples. Since this study is part of a collaborative initiative, the samples were synthesized and provided by Dr. A. R. Phani from the Department of Physics, University of L'Aquila. Preliminary examination revealed a relatively low photocatalytic efficiency of the samples. The objective is to modify/improve its properties by high energy ball milling which is expected to generate accumulations of defects, particle size reduction and an increase in the active surface area. The characterization of doped and mechanochemically treated materials will be analyzed by optical diffuse reflectance measurements and optical absorption calculations using the Kubelka-Munk approach. The phase structure and particle size of the materials will be determined using X-ray diffraction (XRD). The BET surface area of the samples will be obtained using an Autosorb instrument. The photocatalytic properties will be studied by the analysis of decomposition of Methyl Orange in an aqueous solution. An aqueous photocatalytic tubular reactor with capability of operation using UV and/or fluorescent light will be designed and built.
Thesis:
Thesis (M.S.E.E.)--University of South Florida, 2008.
Bibliography:
Includes bibliographical references.
System Details:
Mode of access: World Wide Web.
System Details:
System requirements: World Wide Web browser and PDF reader.
Statement of Responsibility:
by Paula C. Algarin.
General Note:
Title from PDF of title page.
General Note:
Document formatted into pages; contains 87 pages.

Record Information

Source Institution:
University of South Florida Library
Holding Location:
University of South Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 001999311
oclc - 318216012
usfldc doi - E14-SFE0002462
usfldc handle - e14.2462
System ID:
SFS0026779:00001


This item is only available as the following downloads:


Full Text

PAGE 1

Effects of Zn Doping and High Energy Ball Milling on the Photocatalytic Properties of TiO 2 by Paula C. Algarn A thesis submitted in partial fulfillment of the requirement s for the degree of Master of Science in Electrical Engineering Department of Electrical Engineering College of Engineering University of South Florida Major Professor: E.K. Stefanakos, Ph.D. D. Yogi Goswami, Ph.D. Nikolai Kislov, Ph.D. Date of Approval March 26,, 2008 Keywords: photocatalysis, methyl orange, surface, band gap, sol-gel process Copyright 2008, Paula C. Algarn

PAGE 2

DEDICATION I dedicate this thesis to my husband, for his love and support in every step of this long way, for a brand new beginni ng. I hope to honor him, for every night I thought I could not do it and he held me tight and told me I could. I want to share this work with my mother and my family back home, for their love and support that always keep me going.

PAGE 3

ACKNOWLEDGEMENTS I would like to thank my advisor, Dr Elias Stefanakos, for the opportunity to work with him and the members of the Clean Energy Research Center. I would like to thank Matt Smith, I greatly appreciate the trust and confidence he placed in me in that first interview when I first came to this country. I would also like to especially thank Dr. Nikolai Kislov fo r his wisdom, guidance, patience and enthusiasm; Dr Sesha Srinivasan for all his help, guidance and for teaching me during this process. Their help wi ll never be forgotten and always be remembered. I would also like to thank Dr. Yogi Goswami for his guidance and insights. I would also like to thank the staff and faculty of CERC. I would especially like to recognize the guidance and assistanc e given to me by Dr. Nikhil Kothurkar and Mr. Chuck Garretson. I would like to thank the staff of NNRC for the assistance and support during this work, especially Dr. Yusuf Emirov. Last but not least, I would like to thank all my fellow students associ ated with CERC for their help and support with my projec t, especially Mr. Mark Schmidt.

PAGE 4

NOTE TO READER The original of this document cont ains color that is necessary for understanding the data. The or iginal thesis is on file with the USF library in Tampa, Florida.

PAGE 5

i TABLE OF CONTENTS LIST OF TABLES .................................................................................................iii LIST OF FIGURES ...............................................................................................iv CHAPTER 1: INTRODUCTION ............................................................................1 CHAPTER 2: TITANIUM DIOXIDE .......................................................................3 2.1 Titanium Dioxide Lattice Structure ...............................................................4 2.2 TiO 2 Applications .........................................................................................5 2.3 Degussa P-25 TiO 2 .....................................................................................7 CHAPTER 3: TiO 2 PHOTOCATALYSIS ...............................................................8 3.1 Photocatalysis .............................................................................................8 3.2 Basic Concepts of a Semiconductor ............................................................9 3.2.1 Band Gap ..............................................................................................9 3.2.1 Electron Hole Pair Trapping and Recombination ...............................10 3.3 TiO 2 in Photocatalysis ...............................................................................14 3.4 Modification to TiO 2 ...................................................................................15 CHAPTER 4: EXPERIMENTAL METHODS AND PROCEDURES ....................17 4.1 XRD: X-ray Diffractometer .........................................................................17 4.2 BET Surface Area and Po re Size Distribution ...........................................20 4.3 SEM: Scanning Electron Microscope ........................................................22 4.4 Energy Dispersive X-ray Spectroscopy (EDS) ..........................................24 4.5 Optical Absorption .....................................................................................25

PAGE 6

ii 4.6 Photocatalytic Reactor ..............................................................................26 4.7 Experimental Procedures for Photocatalytic Measurements .....................28 CHAPTER 5: METHYL ORANGE AS POLLUTANT ...........................................32 5.2 TiO 2 Photocatalytic degradation ................................................................32 5.3 Catalyst loading .........................................................................................36 5.3.1 UV Light Source ..................................................................................36 5.3.2 Fluorescence Light Source .................................................................38 CHAPTER 6: STUDY OF Zn DOPED TiO 2 NANOPOWDERS ...........................42 6.1 Sol gel process on Zn doped TiO 2 .............................................................42 6.2 Characterization of TiO 2 -Xwt.% Zn Nanopowders .....................................45 6.2.1 X-ray Diffraction Charac terization and SEM Measurements ...............46 6.2.2 BET Surface Area Measurements ......................................................50 6.3 Photocatalytic Activity of TiO 2 Nanopowders .............................................53 CHAPTER 7: BALL MILL INDUCED TRANSFORMATIONS ..............................59 7.1 High Energy Ball Mill .................................................................................60 7.2 Ball Milling Transformation of TiO 2 -X wt% Zn ............................................62 CHAPTER 8: SUMMARY AND CONCLUSIONS ...............................................81 LIST OF REFERENCES ....................................................................................83

PAGE 7

iii LIST OF TABLES Table 1 BET Surface Area Result s ....................................................................51 Table 2 EDS values for TiO2 Xwt% Zn 500C After Ball Milli ng...........................79 Table 3 EDS values for TiO2 Xwt% Zn 500C After Ball Milli ng...........................79

PAGE 8

iv LIST OF FIGURES Figure 1 Stick and Ball M odel Structures of TiO 2 (a) Anatase (b) Rutile.............5 Figure 2 Semiconductor Band Gap Stru cture...................................................10 Figure 3 Electron-hole Generation [4]...............................................................11 Figure 4 Schematic Phot oexcitation in a Solid followed by Deexcitation Event s [16]..........................................................................................12 Figure 5 Surface and Bulk El ectron Carrier Tr apping [16] ................................13 Figure 6 Photoinduced Processes in TiO 2 [4]...................................................14 Figure 7 X-ray Tube Components [35].............................................................18 Figure 8 Constructive Interfer ence of Reflected Waves [34]............................19 Figure 9 Basic Geometry of an X-ray Diffracto meter [ 34].................................20 Figure 10 Electron Beam and Specimen Interaction Sig nals [37] .......................23 Figure 11 Schematic Working Princi ple Diagram for a SEM [38]........................24 Figure 12 Kubelka-Munk Theory Ba sics.............................................................25 Figure 13 Tubular Reactor for P hotocatalytic Ex periments................................27 Figure 14 Reactor Casing and UV Light Irradi ation............................................28 Figure 15 Tubular Reactor Experimental Set Up................................................30 Figure 16 Methyl Orange Optical Absorption Ca libration....................................33 Figure 17 Spectra of 20 ppm Meth yl Orange Soluti on [6]...................................34 Figure 18 Concentration as a Function of Time for Methyl Orange in the Presence of Degussa P-25 TiO 2 without an Irradiation Source....35 Figure 19 Effects of Catalyst Loading (Grams per Liter) on the Rate of Discoloration for Untreated Degussa P-25 TiO 2 Under UV Irradiation37

PAGE 9

v Figure 20 Aparent Rate Constant for Ca talyst Loading (Grams per Liter) for Untreated Degussa P-25 TiO 2 Under UV Irr adiation...........................38 Figure 21 Effects of Catalyst Loading (G rams per Liter) on the Rate of Discoloration for Untreated Degussa P-25 TiO 2 Under Fluorescence Irr adiation .....................................................................39 Figure 22 Aparent Rate Constant for Ca talyst Loading (Grams per Liter) for Untreated Degussa P-25 TiO 2 Under Fluorescence Irradiation..........40 Figure 23 Preparation of Zn Doped TiO 2 Nanopowders by Sol Gel Proce ss [40]........................................................................................45 Figure 24 XRD Spectra of TiO2 Zn Annealing at 500C at 3h [30].....................46 Figure 25 XRD Spectra of TiO2 Zn Annealing at 600C at 3h [30].....................47 Figure 26 SEM Images of Different Conc entrations of Zn Doping of TiO 2 [30]...49 Figure 27 Pore Volume vs Pore Diameter of Zn Doped TiO 2 Annealed at 500 C 3h.............................................................................................52 Figure 28 Pore Volume vs Pore Diameter of Zn Doped TiO 2 Annealed at 500 C 3h.............................................................................................52 Figure 29. MO Photodegradation of TiO 2 Zn 500C 3h........................................53 Figure 30 Comparision of Photocatalytic of TiO2 Degussa and TiO2 Zn 500C 3h..............................................................................................54 Figure 31 MO Photodegradation of TiO 2 Zn 600C 3h.........................................54 Figure 32 Comparision of Photocatalytic of TiO 2 Degussa and TiO 2 Zn 600C 3h..............................................................................................55 Figure 33 Apparent Rate Constant for Zn Doped TiO 2 500C 3h using a Tubular Reactor and a UV Light Irradi ation......................................................55 Figure 34 Apparent Rate Constant for Zn Doped TiO 2 600C 3h using a Tubular Reactor and a UV Light Irradi ation......................................................56

PAGE 10

vi Figure 35 Plot of Optical Absorption (F(R)*h )1/2 vs. Incident Photon Energy, h for the Sol Gel Sample s Annealed at 500C..................................57 Figure 36 Plot of Optical Absorption (F(R)*h )1/2 vs. Incident Photon Energy, h for the Sol Gel Sample s Annealed at 600C..................................58 Figure 37 Schematic Cross-section of a Planetary Ba ll Mill [ 52].........................61 Figure 38 MO Photodegradation of TiO 2 Zn 500C 3h After Ball Milling for 2h....63 Figure 39 Comparision of Photoc atalytic Activity of TiO 2 Degussa and TiO 2 Zn 500C 3h After Ball Mil ling for 2h.........................................................63 Figure 40 MO Photodegradation of TiO 2 Zn 600C 3h After Ball Milling for 2h....64 Figure 41 Comparision of Photoc atalytic Activity of TiO 2 Degussa and TiO 2 Zn 600C 3h After Ball Mil ling for 2h.........................................................64 Figure 42 Apparent Rate Constant for Zn doped TiO 2 500C 3h Before and After Ba ll Milling, using a Tubular Reacto r and a UV Light Irradiation.........65 Figure 43 Apparent Rate Constant for Zn doped TiO 2 600C 3h Before and After Ba ll Milling, using a Tubular Reacto r and a UV Light Irradiation.........65 Figure 44 XRD Spectra of TiO2 Zn Annea ling at 500C 3h After Ball Milling.....66 Figure 45 XRD Spectra of TiO2 Zn Annea ling at 600C 3h After Ball Milling.....67 Figure 46 Crystall Size Concentration for 500C and 600C After Ball Milling......67 Figure 47 Comparision BET Surface Area Measurement Before and After Ball Milling for 500C 3h ..............................................................................68 Figure 48 Comparision BET Surface Area Measurement Before and After Ball Milling for 500C 3h ..............................................................................69 Figure 49 Pore Volume vs Pore Diameter of Zn Doped TiO 2 Annealed After Ball Mill at 500C 3h ..................................................................................69 Figure 50 Pore Volume vs Pore Diameter of Zn Doped TiO 2 Annealed After Ball Mill at 600C 3h ..................................................................................70

PAGE 11

vii Figure 51 Plot of Optical Absorption (F(R)*h )1/2 vs Incident Photon Energy, h fo r the Sol Gel Samples Annealed at 500C.and Ball Milled for 2h......71 Figure 52 Plot of Optical Absorption (F(R)*h )1/2 vs Incident Photon Energy, h fo r the Sol Gel Samples Annealed at 600C.and Ball Milled for 2h......71 Figure 53 SEM Image of TiO 2 Annealed 500C 3h Before and After Ball Mill......72 Figure 54 SEM Image of TiO2 Zn Doped at 1.3 wt% 500C 3h Before and After Ball Mill...............................................................................................72 Figure 55 SEM Image of TiO2 Zn Doped at 2.2 wt% 500C 3h Before and After Ball Mill...............................................................................................73 Figure 56 SEM Image of TiO2 Zn Doped at 3.1 wt% 500C 3h Before and After Ball Mill...............................................................................................73 Figure 57 SEM Image of TiO2 Zn Doped at 4.0 wt% 500C 3h Before and After Ball Mill...............................................................................................73 Figure 58 SEM Image of TiO2 Zn Doped at 4.9 wt% 500C 3h Before and After Ball Mill...............................................................................................74 Figure 59 SEM Image of TiO2 Annealed 600C 3h Before and After Ball Mill.....74 Figure 60 Image of TiO2 Zn Doped at 1.3 wt% 600C 3h Before and After Ball Mill...............................................................................................74 Figure 61 SEM Image of TiO2 Zn Doped at 2.2 wt% 600C 3h Before and After Ball Mill...............................................................................................75 Figure 62 SEM Image of TiO2 Zn Doped at 3.1 wt% 600C 3h Before and After Ball Mill...............................................................................................75 Figure 63 SEM Image of TiO2 Zn Doped at 4.0 wt% 600C 3h Before and After Ball Mill...............................................................................................75 Figure 64 SEM Image ofTiO2 Zn Doped at 500C 3h After Ball Mill at 20000X St arting Left: 0wt%, 1.3wt%, 2.2 wt%, 3.1 wt%, 4.0 wt% and 4.9 wt%76 Figure 65 SEM Image ofTiO2 Zn Doped at 600C 3h After Ball Mill at 20000X

PAGE 12

viii Starting Left: 0wt%, 1.3wt%, 2.2 wt%, 3.1 wt% and 4.0 wt%..............77 Figure 66 EDS Spectra for 500C 3h Zn Doped TiO2 After Ba ll Milling...............78 Figure 67 EDS Spectra for 600C 3h Zn Doped TiO2 After Ba ll Milling...............78

PAGE 13

ix EFFECTS OF ZN DOPING AND HIGH ENERGY BALL MILLING ON THE PHOTOCATALYTIC PROPERTIES OF TIO 2 Paula C. Algarn ABSTRACT TiO 2 photocatalysis is been widely studied for air and water purification applications; titanium dioxide is the most used semiconductor principally because its low cost, stability and chemical proper ties. However it only utilizes the UV portion of the solar spectrum as an ener gy source (less than 4% of the total sunlight energy). This behavior is due to its high band gap value of 3.2 eV. The modification of light har vesting properties of TiO 2 by doping has become an important research topic to achieve an efficient op erating range under UV and visible light. In addition, the structure and surface properties of photocatalysts play an important role. This thesis explores the effects of Zn doped TiO 2, prepared by the sol-gel method, on its photocatalytic acti vity to decompose organics and the characterization of the doped samples. Since this study is part of a collaborative initiative, the samples were synthesized a nd provided by Dr. A. R. Phani from the

PAGE 14

x Department of Physics, University of LA quila. Preliminary examination revealed a relatively low photocatalytic efficiency of the samples. The objective is to modify/improve its properties by high energy ball milling which is expected to generate accumulations of defects, particl e size reduction and an increase in the active surface area. The characterization of doped and me chanochemically treated materials will be analyzed by optic al diffuse reflectance measurements and optical absorption calculations using the Kubelka-Munk approach. The phase structure and particle size of the materials will be determined using X-ray diffraction (XRD). The BET surface area of the sample s will be obtained using an Autosorb instrument. The photocatalyt ic properties will be stud ied by the analysis of decomposition of Methyl Orange in an aqueous solution. An aqueous photocatalytic tubular reac tor with capability of operation using UV and/or fluorescent light will be designed and built.

PAGE 15

1 CHAPTER 1: INTRODUCTION Water pollution is one of the main problems affect ing the environment due to waste products generated by industrie s and households. Detoxification and purification of water, to achieve drinking water quality for human use, has become the main focus of todays scientific research. The main causes of surface and groundwater contam ination are the industrial effluents (even in small amounts), excessive use of pesticides, fertilizers (agrochemicals) and domestic waste landfills. The pollution is caused mainly by non-degradable organic pollutants that are not treatable by conv entional techniques due to their high chemical stability and/or low biodegradability [1]. Today, one of the greatest health thr eats to humans is t he lack of potable water because without it life on Earth would be non-existent. This has lead to research on methods to disinfect pollu ted water at low cost by using cheap sources of energy, i.e. the sun. Solar photochemical processes have been proposed over the years and ar e currently being employed to destroy toxins in water by decomposing organic pollutants using sun light and semiconductors. Solar photochemical technology can be defined as the technology that efficiently collects solar photons and uses them to promote specific chemical reactions [2]. In other words, this tec hnology will allow us to take advantage of

PAGE 16

2 the energy from the sun by means of a photochemical process called photocatalysis [1]. The titanium dioxide (TiO 2 ) photocatalyst is being widely studied for air and water purification applications and has emerged as an excellent photocatalytic material for environmental purification because of its high stability, low cost, non toxicity, high oxidation potential and chemically favorable properties. [3]. However it only utilizes the UV portion of the solar spectrum as an energy source (less than 4% of the total sunlight energy). This behavior is due to its high band gap value of 3.2 eV. The modification of light harvesting properties of TiO 2 by doping has become an important research topic to achieve an efficient operation range under UV and visible light. In addition, structural and surface properties of photocatalysts play an important role. This thesis explores the effects of Zn doped TiO 2 by prepared by the solgel method on its photocatalytic ac tivity to decompose organics and the characterization of the doped samples. The objective is to mo dify/improve their properties by high energy ball milli ng which is expected to generate accumulations of defects, particle size reduction and an increase in the active surface area.

PAGE 17

3 CHAPTER 2: TITANIUM DIOXIDE Titanium dioxide (TiO 2 ) is a wide-band gap semiconductor used in solar and chemical processes that has emer ged as an excellen t material for environmental purification [3, 4]. Titanium dioxide is an n-type semiconductor with electrons as the majority carriers and exists in three different polymorphic phases: anatase, rutile, and brookite. Anatase and rutile are the most common polymorphs that crystallize in a tetragonal lattice and their structure is described in chains of TiO 6 octahedra with different physical and chemical behavior [5, 6]. TiO 2 is non-toxic, chemically stable and low cost material that has a positive impact on the envir onment [4]. It has attracted more attention in environmental studies and applicat ions due to its applicability to the treatment of pollutants and waste using photocatalysi s. The term photocatalysis can be explained simply as a reac tion where light and a subst ance (the catalyst, in this case TiO 2 ) are necessary entities to influence a reaction [7]. This definition will be explained further in the next chapter. When TiO 2 is irradiated with light (photons), the chemical result can be applied to chemical processes to cr eate or degrade specific compounds [4]. Investigations have concluded that t he anatase particles with a large surface area are very efficient for the decompositi on of pollutants in air and water [8].

PAGE 18

4 2.1 Titanium Dioxid e Lattice Structure TiO 2 occurs in nature in three forms, has a stable phase called rutile and two metastable phases called anatase and brookite. Only anatase and rutile have characteristics that can be used as photocatalysts. The lattice structure for anatase and rutile is described in terms of distorted TiO 6 octahedra, this configuration consists of Ti 4+ ions surrounded by six O 2 ions [5, 6], and in both structures, each titanium ion is at the centre of an oxygen octahedron and each of the oxygen has three coplanar near neighbour titanium cations. In rutile, the oxygen ions form a slightly distort ed hexagonal compact lattice; the three Ti-O-Ti angles are roughl y equal to 120. In anatase, the oxygen forms a fcc lattice and one Ti-O-Ti angl e is about 180 while the two others are close to 90 [9], both structur es are shown in Fig. 1. According to band theory, semic onductors are characterized by two energetic bands: the valence band, VB, co mpletely filled with electrons and a conduction band, CB, with higher energy and no electrons. The energetic distance between them (0.7-3.5 eV ) is called the band gap and determines electronic and electric properties of the solid [10]. The band gap of anatase is 3.2 eV whereas the band gap of rutile is 3.0 eV. The difference in the bandgap value for these two phases is explained by small structural differences which affect electronic band properties [10]. Despite th e higher energy required for electronhole pair creation, anatase is reported to be more active than rutile for solar applications [6].

PAGE 19

Band gap values also determine the color of the semiconductor, because they absorb light having energy equal to or higher than the band energy. The energy of visible light lies in the region of 1.5 (red) 3.0 eV (violet). Thus, the semiconductors having a narrow band gap of about 1.5 eV are black, those having a band gap of about 3.0 eV are white [10]. (a) ( b ) Figure 1. Stick and Ball Model Structures of TiO 2 (a) Anatase (b) Rutile. [Red balls are oxygen atoms and white balls are titanium]. 2.2 TiO 2 Applications Titanium dioxide (TiO 2 ) is a multifaceted compound that has a high number of industrial applications. TiO 2 is also a potent photocatalyst that can break down almost any organic compound when exposed to sunlight, and a wide 5

PAGE 20

6 range of environmentally beneficial products are being designed such as selfcleaning fabrics, auto body finishes, and ceramic tiles. It remains to be seen, however, whether the formation of undesirable intermediate products during these processes outweigh the benefits offered by TiO 2 's photocatalytic properties [4]. TiO 2 is used in the production of hydrogen and electric energy, as a gas sensor, as white pigment (e.g. in paint s and cosmetic products), for corrosionprotective coatings, as an optical coati ng, in ceramics, and in electric devices such as varistors. Because of its non-toxicity, it plays a role in the biocompatibility of bone implants and is being discussed as a gate insulator for the new generation of MOSFETS [11] By far, the most actively pursued applied research on TiO 2 is its use for photo-assisted degradati on of organics. TiO 2 is a semiconductor and the electronhole pairs that ar e created upon irradiation wit h sunlight can separate and the resulting charge carriers migrate to the surface where they react with adsorbed water and oxygen to produce radica l species. These can attack any adsorbed organic molecules and, ultimately, lead to the complete decomposition into CO 2 and H 2 O [11]. Applications of TiO 2 range from purification of wastewater (e.g. in operating rooms in hospitals) [12]; in t he use of self-cleaning coatings on car windshields [13], to protect coatings of marble (preservation of ancient Greek

PAGE 21

7 statues against environment al damage). Research is been done and future applications for TiO 2 are growing exponentially. 2.3 Degussa P-25 TiO 2 Degussa P-25 is generally consider ed the most photoactive commercially available form of TiO 2 structure which contains both separately, anatase and rutile phases in a ratio of about 3:1. P25 is usually 20-25% rutile phase and 7580% anatase. The average sizes of the a natase and rutile particles are 25nm and 85 nm, respectively [8]. Degussa P-25 has a surface ar ea of approximately 50 m 2 /g with anatase particles having a larger surface area than rutile particles. Ohno et al. concluded t hat the larger surface area of anatase particles improves the efficiency of decomposition of the pollutant in air and water. Bickley et al. concluded that the increased photocatalytic activity was, in part, due to this anatase-rutile particle confi guration. Anatase phase and rutile phase particles exist completely separate fr om one another [8, 14]; a well developed crystallinity is responsible for the high efficiency because of the low density of recombination centers, which will be explored in the following chapters [14].

PAGE 22

8 CHAPTER 3: TiO 2 PHOTOCATALYSIS 3.1 Photocatalysis A catalyst can be defined as a substanc e that facilitates an increase in the rate of reaction of a chemical process, which otherwise is thermodynamically favored but kinetically slow, maintaining th e catalyst unaltered after the reaction. The process can be described as: A B (1) A + Catalyst B + Catalyst (2) where A and B represent the reac tants and products respectively. The term photocatalysis is used when photons are involved as the mechanism to accelerate the catalyst reaction, combining the concepts of photochemistry and catalysis. Photocatalysis can be defined as a reaction where a substance (the catalyst) and a source of light (sun or an artificial light) are needed to influence a response in a reactant where the chemical structures of the reactants are modified and the catalyst remains unaltered [7] The process can be described as: A + h +Catalyst B + Catalyst (3)

PAGE 23

9 where h is a quantum of energy from t he incident photons that cause the reaction. In recent years, applications to environmental clean up have been one of the most active areas in heterogeneous ph otocatalysis. It has become a potential technology for the treatment of organic po llutants in water such as aromatic compounds products of industrial waste t hat present a potential hazard to the environment and can not be treatrd by conventional techniques [15]. 3.2 Basic Concepts of a Semiconductor 3.2.1 Band Gap Semiconductors are solids whose electrical conductivity is determined by the amount of energy that is required to mo ve electrons from the valence band to the conduction band, whereas metals hav e a sea of electrons available for conduction and insulators have no electron s. The conduction band is separated from the valence band by the defined energy gap E g The energy gap E g generally refers to the energy di fference between the top of the valence band and the bottom of the conduction band as shown in Figure 2 [16]. The energy gap varies from 0.7-3. 5 eV and determines the electronic properties of the solid such as conducti vity. When there is light absorption with sufficient energy, an electron is transferred from the valence band to the conduction band. The energy of visible light varies from 1.5 eV (red) to 3.0 eV

PAGE 24

(violet), and can define the color of the material; a band gap of 1.5 eV is black and 3.0 eV is white [10]. In a heterogeneous photocatalysis system, after the initial excitation takes place, the generated electron-hole pairs lead to chemical reactions and molecular transformations that take place at the surface of a catalyst. The term heterogeneous photocatalysis is used to describe the charge transfer to the adsorbed species if the semiconductor catalyst remains intact during a continuous exothermic process [16]. Figure 2 Semiconductor Band Gap Structure [48] 3.2.1 Electron Hole Pair, Trapping and Recombination The most important process of photocatalysis is the photo-induced charge separation. Absorption of a photon with an energy h greater than or equal to the bandgap energy Eg (eV) excites an electron from the valence band to the conduction band, leading to the formation in the semiconductor of an electron/hole pair as shown in Figure. 3 [17]. 10

PAGE 25

Figure 3 Electron-hole Generation [4] For photocatalytic processes to occur, these photogenerated electrons and holes must migrate to the surface of the catalyst where they can be transferred to the adsorbed organic or inorganic pollutants. However, migration to the surface is not the only pathway the electron-hole pairs can follow as shown in Fig. 4. Once excitation occurs across the band gap the life time of the electron-hole pair is on the order of nanoseconds, which is sufficient time for them to undergo charge transfer to adsorbed species (organic or inorganic pollutants) on the semiconductor surface [16-18]. 11

PAGE 26

Figure 4 Schematic Photoexcitation in a Solid followed by Deexcitation Events [16]. Both surface and volume recombination can also occur, as denoted by pathways A and B. If the electron-hole recombination rate is too high it can degrade or even halt photocatalysis [16]. The concept of charge separation, by any number of means, is an important idea as it relates to doped semiconductor catalysts, and is explored further in this study. While at the surface, the semiconductor can donate electrons to reduce an electron acceptor (usually oxygen in an aerated solution, pathway C); also a hole can migrate to the surface where an electron from a donor species can combine with the surface hole oxidizing the donor species (pathway D) [16]. The efficiency of the photocatalyst in degrading the pollutant is based on different factors such as the diffusion of the electron-hole pair in the surface and, as a quantum yield, the number of events occurring per photon absorbed. In an ideal case the diffusion of the products into the solution occurs quickly without 12

PAGE 27

the reverse reaction of electrons recombining with donors and holes recombining with acceptors. However, in a real system recombination does occur and the concentration of both holes and electrons is not equal. Charge separation or charge trapping is used to increase the photocatalytic effect and maintain an efficient process [16]. Since the crystal structure of the photocatalyst is not pure, but instead has both surface and bulk defects, it is expected that surface states (or charges) exist across the surface. These surface states, which differ in energy from the bulk, serve as charge carrier traps. The carrier lifetimes of the electrons and holes are therefore increased since these traps stop the recombination of electrons and holes as shown in Figure 5 [16]. Figure 5 Surface and Bulk Electron Carrier Trapping [16]. Modifications to semiconductor surfaces such as addition of metals, dopants, or combinations with other semiconductors are beneficial in decreasing the electron-hole recombination rate and thereby increasing the quantum yield of the photocatalytic process [16]. 13

PAGE 28

3.3 TiO 2 in Photocatalysis Photocatalytic reactions on TiO 2 powders have attracted much attention because of their applicability to the treatment of a variety of organic (dyes, phenols, etc.) and biological pollutants (viruses, bacteria, fungi, algae, and cancer cells), which can be totally degraded to CO 2 H 2 O, and other harmless inorganic anions, eliminating their toxicity [21]. Whenever different semiconductor materials have been tested under comparable conditions for the degradation of the same compounds, TiO 2 has generally been demonstrated to be the most active [20]. When a photon is absorbed and produces energy greater than the band gap then an electron/hole pair is formed and creates an active surface site where the valence band holes can oxidize an organic compound to CO 2 H 2 O and mineral acid [19]. When in contact with water, hydroxyl radicals (OH) are created and help retarding the recombination of the electron hole pairs. Figure 6 Photoinduced Processes in TiO 2 [4] 14

PAGE 29

15 The photoelectrochemical properties of different structures of TiO 2 rutile and anatase, have been repor ted [22, 23]. The anatase form appears to be the most photoactive and the most practical for environmental applications [24]. The band gap of the anatase is 3.2 eV and is larger than the rutile band gap. The photocatalytic activity of TiO 2 is influenced by many factors such as the preparation method, particle size, crystal microstructure, specific surface area, porosity and so on. In order to obtain a TiO 2 powder with highly photocatalytic activity for a practical purification system, these factors must be taken into consideration. 3.4 Modification to TiO 2 As mentioned previously, modificati ons of semiconductor surfaces by the addition of metals, dopants, or combinatio ns with other semiconductors can play an important role in science and technolog y. The modified materials with their unique optical, electrical, magnetic, cataly tic, and chemical properties are widely used in fields such as photoluminesc ence, photocatalysis, and nanoelectronics [25]. Particularly, people have tried to im prove the photocatalytic activity of TiO 2 through a number of modification methods, such as noble metal doping, composite semiconductors and transition metal doping. Many researches have focused in enhancing the photocatalytic activity of TiO 2 in the whole spectrum. Among the mo st widely used modification methods, transition metal doping has aroused great interest since this method can

PAGE 30

16 enhance the activities of a TiO 2 photocatalyst in many types of photocatalytic reactions [26]. Another reason for the surface modification of TiO 2 is to inhibit recombination of photogenerat ed electrons and holes by increasing the charge separation and therefor e enhancing the efficiency of t he photocatalytic process. Zhao et. al. found that the photodegradati on activity can be enhanced by doping by an appropriate amount of Zn which enhances the elec tron injection into the conduction band of TiO 2 by capturing electrons and promoting the formation of reactive oxygen species. Hence, the enhanced photodegradati on of dyes under visible irradiation can be realized [32]. The modification of TiO 2 by doping with metal i ons and coupling with other semiconductors can significantly influ ence the process of photodegradation. Marci et. al. [33] found that Zn can considerably enhance the photocatalytic performance of TiO 2 under UV irradiation, due to an increase in the separation rate of photoinduced charges because of the difference in the energy band position. The dynamic processes of pho toinduced charges are affected by oxygen vacancies, and can be determined by the metal ions present on the surface of TiO 2 nanoparticles. For this study, TiO 2 has been modified by Zn doping to improve the visible-light absorption, prevent or delay charge carrier recombination and improve its surface properties.

PAGE 31

17 CHAPTER 4: EXPERIMENTAL METHODS AND PROCEDURES 4.1 XRD: X-ray Diffractometer X-ray Powder Diffraction (XRD) is an efficient analytical technique used to characterize and identify unknown cryst alline materials. The most widespread use of XRD is the identification of comp ounds by their diffraction pattern [34]. Monochromatic x-rays, electromagnetic radi ation similar to light, but with a much shorter wavelength is used to determine t he interplanar spacing of the unknown materials. X-rays are produced when electrica lly charged particles of sufficient energy are deccelerated; these charges ar e electrons that are formed when the filament of a cathode ra y tube is heated. These electrons are accelerated by means of a high voltage that draws them to a metal target. The points of impact of the electrons (anode), produce the X-rays that are radiated in all directions. Figure 7 shows the X-ray tube co mponent mentioned earlier [35].

PAGE 32

Figure 7 X-ray Tube Components [35] For XRD analysis the X-ray beam interacts with the planes of atoms of the sample; part of the beam is transmitted, part is absorbed by the sample, part is refracted and scattered, and part is diffracted. Diffraction of an X-ray beam by a crystalline solid is analogous to diffraction of light by droplets of water, producing the familiar rainbow. X-rays are diffracted by each mineral differently, depending on what atoms make up the crystal [34] With this technique the samples are analyzed as powders with grains in random orientations to insure that all crystallographic directions are "sampled" by the beam. The basic principle of operation of the XRD spectrometer is based on Braggs law. When the Bragg conditions for constructive interference are obtained, a "reflection" is produced, and the relative peak height is generally proportional to the number of grains in a preferred orientation. 18

PAGE 33

According to Braggs law, to obtain constructive interference the path difference between the incident and the scattered waves, which is 2.d.sin, has to be a multiple of the wavelength For this case, the Bragg law gives the relation between interplanar distance d and diffraction angle [34]: 2.d.sin = n. (4) Where n is an integer, is the wavelength of X-rays, d is the spacing between the planes in the atomic lattice, and is the angle between the incident ray and the scattering planes; Since is known, can be measured, the d-spacing can be calculated using the Braggs equation. Figure 8 shows the basic principles of constructive interference of the scattered X-rays. Figure 8 Constructive Interference of Reflected Waves [34] 19 The basic geometry of an X-ray diffractometer is shown in Figure.9; it has a source of monochromatic radiation and an X-ray detector situated on the circumference of a graduated circle centered on the powder specimen. The detector and specimen holder are mechanically coupled with a goniometer so that a rotation of the detector through 2 degrees occurs in conjunction with the rotation of the specimen through degrees, a fixed 2:1 ratio. Divergent slits,

PAGE 34

located between the X-ray source and the specimen, and between the specimen and the detector, limit scattered (non-diffracted) radiation, reduce background noise, and collimate the radiation. Figure 9 Basic Geometry of an X-ray Diffractometer [34] The phase structure of the TiO 2 samples was characterized by X-ray diffraction (XRD). A Philips Xpert pro PreFix powder X-ray diffractomerter with CuK radiation (=1.54060 ) was employed for this purpose. The incident and diffraction slit width used for all the experiments are 1 and 2, respectively, and the incident beam mask used corresponds to 10 mm. The sample preparations for the XRD measurement are strictly followed to obtain maximum signal to noise ratio. 4.2 BET Surface Area and Pore Size Distribution Catalysts and photocatalysts are often characterized by their interaction with gases. The tendency of all solid surfaces to attract surrounding gas molecules gives rise to a process called gas sorption. Monitoring the gas sorption process provides a wealth of useful information about the characteristics of solids 20

PAGE 35

21 such as surface area and pore size. At low temperatures, non-reactive gases (nitrogen, argon, krypton, etc.) are physisorbed by t he surface. Through gas physisorption the total surface area of t he sample can be calculated by the BET method [36]. Before performing a surface area anal ysis or pore size measurement, solid surfaces must be freed from cont aminants such as water and oils. The process is call outgassing and is carried out by placing a sample in a glass cell and heating it under vacuum, or a flow of dry, inert gas Once clean, the sample is brought to a constant te mperature by means of an external bath, typically a Dewar flask containing a cryogen like li quid nitrogen. Then, small amounts of a gas (the absorbate) are adm itted in steps into the evacuated sample chamber [36]. Absorbate molecules quickly find their way to the surface of every pore in the solid (the adsorbent). These molecule s can either bounce o ff or stick to the surface. Gas molecules that stick to the surface are said to be adsorbed. The strength with which adsorbed molecules in teract with the surface determines if the adsorption process is to be considered physical (weak) or chemical (strong). In the present study, Autosorb-1C from Quantachrome Instruments has been employed to determine the surface ar ea and pore size distribution of the samples. Each of them was placed in a glass tube and was outgassed at 300 C for 3 hours. The external bath for the sa mple was liquid nitrogen (77 K) and a

PAGE 36

22 multi point BET method using nitrogen as the adsorbate gas was used to analyze these samples [36]. 4.3 SEM: Scanning Electron Microscope The Scanning Electron Microscope ( SEM) is a type of microscope capable of producing high resolution images of a sample surface using electrons rather than light to form an image. Electron microscopy takes advantage of the wave nature of rapidly moving electrons. Wher e visible light has wavelengths from 4,000 to 7,000 Angstroms, electrons accelera ted to 10,000 KeV have wavelengths of 0.12 Angstrom s. Optical microscopes have their resolution limited by the diffraction of light to about 1000 diameters magnification. The Hitachi S800 scanning electron microscope, in the present study, is limited to magnifications of around 3,000,000 [34]. The SEM uses secondary electrons when a focused electron beam is incident on the specimen to form t he image. The secondary electron signal provides information about the surface of a specimen. Since secondary electrons do not diffuse much inside the specimen, they are most suitable for observing fine structure of the specimen surface. Fi gure 10 shows the signals generated in an electron beam and specimen interaction.

PAGE 37

Figure 10 Electron Beam and Specimen Interaction Signals [37] The basic diagram of the operation of the Hitachi S800 SEM is shown in Figure 11. Electrons from a filament in an electron gun are beamed at the specimen inside a vacuum chamber. The beam is collimated by electromagnetic condenser lenses, focused by an objective lens and then swept across the specimen at high speed. The secondary electrons are detected by a scintillation material that produces flashes of light from the electrons. The light flashes are then detected and amplified by a photomultiplier tube. The microstructures of the samples in the different stages were observed by Hitachi S800 scanning electron microscope (SEM) and local phase composition was determined in the energy dispersive X-ray spectrometry (EDS) mode using the same instrument. A fixed voltage of 25 KV and resolution of 1100X and 20000X were used. Genesis software was used to analyze the SEM images and EDS mappings. 23

PAGE 38

Figure 11. Schematic Working Principle Diagram for a SEM [38] 4.4 Energy Dispersive X-ray Spectroscopy (EDS) Another important signal that can be analyzed by the Hitachi S800 SEM, when the electron beamspecimen interaction occurs is the x-ray emission. EDS identifies the elemental composition of materials imaged in a Scanning Electron Microscope (SEM) for all elements with an atomic number greater than boron (B). Most elements are detected at concentrations of the order of 0.1% excluding hydrogen. When the electron beam of the SEM hits the sample surface, it generates x-ray fluorescence from the atoms in its path. The energy of each x-ray photon is characteristic of the element which produced it. The EDS microanalysis system collects the x-rays, sorts and plots them by energy, and automatically identifies and labels the elements responsible for the peaks in this energy distribution. The liquid nitrogen cooled detector is used to capture and map the x-ray counts continuously [34]. 24

PAGE 39

4.5 Optical Absorption The optical absorption data was deduced from the KubelkaMunk function. The Kubelka Munk method is a diffuse reflectance technique that uses a salt, NaCl in this case, mixed with the powder being measured. This technique accounts for the difference in transmission and reflectance measurements due to absorption of certain wavelengths by powders. The material was diluted to about 1% by weight in NaCl and ground using a mortar and pestle. Transmission measurements were made of the powders, which were lightly packed into small sample holders, using a spectrometer. A Kubelka-Munk conversion was applied to a diffuse reflectance spectrum to compensate for any differences. Figure 12 shows the basic concept of the technique. The Kubelka-Munk equation is: skRRRf212 (5) where R is the absolute diffuse reflectance of the sampled layer, k is the molar absorption coefficient, and s is the scattering coefficient [6]. Figure 12 Kubelka-Munk Theory Basics 25

PAGE 40

A linear relationship is created between the spectral intensity and the sample concentration. This equation assumes that the diluting salt is non-absorbing, that the scattering coefficient of the salt is constant, and that the sample thickness is infinite. These assumptions can be made for samples greater than 1 millimeter of highly diluted small particles. Given that we used nanoparticles and had a sample thickness of 3mm, our packing technique was the only variable that could affect the scattering coefficient. The optical band gap, E g of the material can be calculated on the basis of the optical spectral absorption using the well-known formula, hEhAkmg)( (6) where A is a constant, h is the incident photon energy, and m depends on the nature of band transition, m = for direct and m = 2 for indirect allowed transitions. Taking into account Equation. 6, for the purpose of determining the bandgap, extrapolating the f(R d ) 2 and f(R d ) 1/2 versus energy curve to f(R d ) 2 = 0 is equivalent to carrying out the same procedure with the k 2 and k 1/2 respectively. 4.6 Photocatalytic Reactor The photocatalytic experiments were performed using a tubular reactor that was fabricated by Mr. Chuck Garretson, who is the CERCs Project Coordinator. Figures 13 and 14 show the general configuration of the system. A one liter Pyrex beaker is used to hold the solution that was mixed by a magnetic 26

PAGE 41

stirrer that also suspended the photocatalytic particles. The solution flow through the reactor casing was pum ped by a peristaltic system An aerating stone was used to diffuse either compressed air or oxygen into the suspension. Experiments were conducted using breathi ng quality compressed air, which was metered through gas specific flow meters. Six UV lamps or Fluore scence lights were mounted in the reactor casing. The lights used were RPR 35 00 UV fluorescent lamps that emit a gaussian distribution of light from 300 to 400 nm with max = 350 nm with a nominal power of 14W and 1.5W of UV Radiation. A thermocouple, also mounted through the top plate, m onitored the temperatur e of the solution. Figure 13 Tubular Reactor for Photocatalytic Experiments 27

PAGE 42

Figure 14 Reactor Casing and UV Light Irradiation 4.7 Experimental Procedures fo r Photocatalytic Measurements Photocatalytic experiments were per formed using a glass batch reactor system, which was described in detail a bove. A 20 ppm methyl orange solution using de-ionized water wa s prepared using (A.C.S. Reagent) MO from SigmaAldrich. Methyl orange was dissolved into the solution using a magnetic stirrer for 15 minutes. Samples were drawn and the initial concentration was measured and calculated in accordanc e with Beers Law using an Ocean Optics USB2000 spectrometer. 28

PAGE 43

29 Once the 20 ppm solution was prepar ed, 100 milliliters of the MO solution was reserved in a beaker. The prepared ca talyst was ground in a crucible with 2 milliliters of MO solution to de-agglomerate the material. Portions of the reserved MO solutions were added to dilute the catalyst paste that was then poured back into the beaker. This process was r epeated with the remaining MO solution until the maximum amount of the catalyst was recovered from the crucible. To further de-agglomerate the particles and achiev e Langmuir equilibrium, the MO-catalyst solution was then sonica ted using a Fisher-Scientific Sonic Dismembrator Model 100. Sonication was performed at 5 watts (RMS) for 15 minutes while being magnetically stirred. It is important to note that during this time, adsorption of the pollut ant onto the surface of the catalyst also took place altering the initial concentration of the MO solution. The solution was then moved to the experimental area w here it again was placed on a magnetic stirrer [6]. Figures 15 shows the experimental set up.

PAGE 44

Figure 15 Tubular Reactor Experimental Set Up. A thermocouple was placed where? to monitor and control the solution temperature. The photocatalytic experiments were conducted in a range from 22C to 38C. Breathing Quality Air was used as an oxygen source. The flow rate was controlled by flow meters and bubbled into the solution by aerating stones at a rate of 0.5 liters per minute. 2.0 milliliter samples, representing zero time, were drawn using Micromate 5cc glass syringes and placed into micro-centrifuge tubes. The solution was then irradiated using the light sources detailed above. Spectrum experiments were conducted with duration of three hours. Samples were drawn at 30 minute intervals for the first hour, followed by one hour intervals until the 3 hours were completed. 30

PAGE 45

31 At the completion of the exper iment, the samples collected were centrifuged using an Eppendorf 5414C centrifuge at 8,000 rpm for 10 minutes. The samples were then syringed to new tubes then centrif uged again at 8000 rpm for 10 minutes. The concentrations of the samples were then calculated by measuring the absorbance of the samples using a spectr ometer. The results were compiled and the rate constant for the material was calculated [6].

PAGE 46

32 CHAPTER 5: METHYL ORANGE AS POLLUTANT Methyl Orange (MO) is a common industria l dye that is categorized as an azo-dye. Azo compounds are synthetic inorganic chemical compounds that account for about 50-70% of the worlds production of dyes. In the textile industry, it is estimated that 10% of the dye is lo st during the dyeing process and released as effluent waste causing co ntamination [27]. Th e main problem is that dye wastes can also generate dangerous by-products through oxidation chemical reactions taking place in the wa stewater phase or through generation of hazard products in the cleaning process. The chemical formula and molecular composition for methyl orange is C 14 H 14 N 3 SO 3 Na. It is a very stable compound due to the large proportion of aromatics in the dye and the presence of the benzene rings, which keep this pollutant from decomposing easily by chemic al or biological methods [6]. In this study, methyl orange was us ed because of its visible co lor that allows optical measurements for the evaluat ion of its degradat ion. This compound is an acidbase indicator showing orange in basic medium and red in acidic medium. 5.2 TiO 2 Photocatalytic degradation For the purpose of this t hesis, titanium dioxide (TiO 2 ) was used as a photocatalyst for the detoxif ication of water containi ng methyl orange (MO) as a

PAGE 47

model compound. The experimental technique for evaluating MO degradation is similar to the experimental technique described by M. Schmidt in his thesis work [6]. The experiments were conducted using de-ionized water as the solvent, to determine the optical absorption spectra of methyl orange. To establish the calibration curves, aqueous solutions of methyl orange were prepared in varying concentrations from 0.3125 ppm to 20 ppm as shown in Figure. 16. 00.20.40.60.811.21.4240340440540640Wavelength (nm)Absorban c 0.3125 0.625 1.25 2.5 5 10 20 Figure 16. Methyl Orange Optical Absorption Calibration. For this work the 20 ppm solution was chosen and its spectrum is shown in Figure 17. This chart shows two absorption peak maxima, one at approximately 272 nm and a second with a higher absorption magnitude at 451 nm. Consistent with published work in this area, the second maximum peak at 451 nm was used to calculate the concentration changes as a function of time for 33

PAGE 48

methyl orange [6,15,27]. These peaks and corresponding spectra are in line with published results depicting absorption peaks at 270 nm and 458 nm [20]. UV and fluorescence lights were used as an irradiation source in the experimental set up. An additional test was made to measure the degradation Mo due to TiO 2 without light sources. 00.20.40.60.811.21.4240290340390440490540590Wavelength (nm)Absorbance MO Init Figure 17. Spectra of 20 ppm Methyl Orange Solution [6] The test was conducted using a 20 ppm MO solution prepared in a glass beaker loaded with 1 g/L of TiO 2 and stirred in the dark with samples taken after 30 minute intervals. The results are depicted in Fig. 18 and showed that there was no degradation for the MO in the presence of TiO 2 34

PAGE 49

Consistent with the literature, there was no degradation of the MO in the presence of TiO 2 without UV lights [15, 27] and the complete disappearance of the dye is only observed in the presence of both UV light source (320nm 400nm) and catalyst (TiO 2 ). Consistent with Guettai et. al. [27] as well as other published data on the degradation of methyl orange by TiO 2 the peak at 451nm was used as the evaluation point for the absorption measurements of the MO solution. 0.80.850.90.9511.05050100150200Time (min)C/Co 11-13-07 MO test 6UV 20 ppm V7 Figure 18. Concentration as a Function of Time for Methyl Orange in the Presence of Degussa P25 TiO 2 without an Irradiation Source The value of 451 nm was derived using a box car smoothing method for values between 449-453 nm to account for the fluctuation of data points recorded by the spectrometer at the moment of the measurements. The ratio of the concentration versus initial concentration was then plotted as a function of time. 35

PAGE 50

36 For the development of the plot of concentration versus initial concentration a rate constant for first or der kinetics was calculated. The method used here is similar to the technique employed by both Guettai et al. and AlQaradawi et al. for determining rate constants for first order kinetics of the decolorization of MO [15, 27]. 5.3 Catalyst loading The effect of catalyst loading was tested to det ermine the optimal loading for both UV light and florescence lights fo r the tubular reactor designed for this process which will be expl ained in the following chapt ers. MO photodegration for different TiO 2 loading was conducted for this study in a natural pH solution. It should also be noted that significant adsorption of the pollutant occurs during the initial 15 minutes of the loading of the MO solution wi th the catalyst. Therefore, to allow adsorption-desorption equilibration, t he solution irradiation by a light source was started with a delay at least 15 minut es in the all experiments described in this work. 5.3.1 UV Light Source MO photodegradation of TiO 2 was performed using UV lights and a loading in the range of 0.01g/L to 1g/L. The test was conducted using a 20 ppm MO solution prepared in a glass beaker loaded with differ ent amounts of TiO 2 and stirred; samples were taken at 15 minute intervals the first hour and then 30 minutes intervals. The results are depicted in Figure 19.

PAGE 51

y = 1.001e-0.0035xR2 = 0.9995 y = 0.964e-0.017xR2 = 0.99790.3 0.4 0.5 0.6 0.7 0.8 0.9 1 10 20 30 40 50 60 70 Time (min)C/Co 0.01g/L 0.025g/L 0.05g/L 0.1g/L 0.25g/L 0.5g/L 1 g/L Exponencial (001g/L) Exponencial (0.25g/L) Figure 19. Effects of Catalyst Loading (Grams per Liter) on the Rate of Discoloration for Untreated Degussa P-25 TiO2 Under UV Irradiation. The results for the experiments were conducted to find the apparent rate constants for different catalyst loadings for Degussa P-25 TiO2 under UV irradiation are shown in Figure. 20. From this curve one can see that the degradation rate increases insignificantly as the catalyst loading increases from 0.25 g/L to 1.0 g/L. The optimum cata lyst loading found in this study was determined to be 0.25 g/L, with no fu rther increase or decrease in the performance due to increased loading. Choosi ng an efficient loading is important because an increase in the loading limits the light penetration through the TiO2 suspension. 37

PAGE 52

0.00000.00400.00800.01200.01600.02000.024000.20.40.60.81Loading (g/L)Apparent Rate Constant (1/min ) ErRate Er+ Figure 20. Aparent Rate Constant for Catalyst Loading (Grams per Liter) for Untreated Degussa P-25 TiO 2 Under UV Irradiation 5.3.2 Fluorescence Light Source MO photodegradation of TiO 2 was also performed using fluorescent light with loading in the range of 0.25 g/L to 4 g/L. The test is similar to the one described in the previous section. A 20 ppm MO solution was prepared in a glass beaker loaded with different amounts of TiO 2 and stirred; samples were taken at 30 minute intervals for the first 2 hours and then at 60 minute intervals. The results are depicted in Figure. 21. 38

PAGE 53

y = 1.0171e-0.0026xR2 = 0.998 y = 0.9894e-0.0011xR2 = 0.990.60.650.70.750.80.850.90.95120406080100120140160180200Time (min)C/Co 0.25g/L 0.5g/L 1g/L 2g/L 4g/L Exponencial (2g/L) Exponencial (0.25g/L) Figure 21. Effects of Catalyst Loading (Grams per Liter) on the Rate of Discoloration for Untreated Degussa P-25 TiO 2 Under Fluorescence Irradiation. Apparent rate constants for differing catalyst loadings for Degussa P-25 TiO 2 under Fluorescent irradiation are shown in Figure 22. The degradation rate for visible-light increases significantly as the catalyst loading increases from 0.25 g/L to 2 g/L with a further decrease at 4g/L. The results above indicate that the catalyst loading concentration is a function of the type of reactor used, its geometry, type of source and the incident irradiation. 39

PAGE 54

00.00050.0010.00150.0020.00250.0030.003501234Loading (g/L)Apparent Rate Constant (1/min) ErRate Er+ Figure 22 Aparent Rate Constant for Catalyst Loading (Grams per Liter) for Untreated Degussa P-25 TiO 2 Under Fluorescent Irradiation. Initially, an increase in the concentration of MO increases the probability of a reaction between the pollutant and the oxidizing species. This in turn results in an increase in the discoloration rate. As the concentration of MO increases, the active sites on the catalyst surface are more fully covered reducing the photogeneration of holes or hydroxyl radicals. It has been concluded that the hydroxyl radical is responsible for most heterogeneous photocatalytic oxidations. The hydroxyl radical is formed by the reduction reactions of holes with either water or hydroxide ions [22]. 40

PAGE 55

41 There are many factors that affect the photocatalytic activity of TiO 2 These include the lattice structure and phas e of the material, specific surface area, adsorption of the po llutant, electronhole generation and recombination, carrier lifetime and trapping, solution pH, method of synthesis, catalyst loading, and initial pollutant concentration, among many others. M.Schimdt [6] research showed the profound difference that the design and materials of the reactor itself can have on the photocatalytic effect or rate. The general conclusion is that the optimum catalyst loading is a function of the active surface area and the pollutant concentration, and therefore is not necessar ily static [27]. Similarly, Al-Qaradawi et al. also found that an increase in in itial catalyst concentration ultimately decreases the overall degradation efficiency [15].

PAGE 56

42 CHAPTER 6: STUDY OF Zn DOPED TiO 2 NANOPOWDERS The first part of this thesis work explores the effects of Zn doped TiO 2 by the sol-gel method on the p hotocatalytic activity and the characterization of the doped samples. Since this study is part of a collaborative initiative, the samples have been synthesized and are provided by Dr A. R. Phani from the Department of Physics, University of LAquila. The characterization of the doped and mechano-chemically treated materials will be analyzed by optical diffuse reflectance measurements and optical absor ption calculations using the KubelkaMunk approach. The photocatal ytic properties will be studied by the analysis of decomposition of Methyl Orange in an aqueous solution. An aqueous photocatalytic tubular reac tor with capability of oper ation under UV and/or fluorescent light was design ed, UV light was used for the Photocatalytic portion of this study. 6.1 Sol gel process on Zn doped TiO 2 Untreated TiO 2 can utilize less than 4% of the available solar energy for photocatalysis. Given the exceptional photocatalytic properties of TiO 2 a great deal of study has gone into decreasi ng the band gap to allow visible-light activated photocatalysis. Doping TiO 2 by metals or transition metals and anion doping have dominated this area of resear ch. One of the pr imary goals of this

PAGE 57

43 research was to investigate the characteristics of treated TiO 2 with zinc. If successful it would account for a relatively inexpensive way to lower the band gap of Degussa P-25 TiO 2 and improving it s photocatalytic efficiency. Doping TiO 2 can alter the conductivity and optical properties by creating new surface states that are believed to lie near the conduction band or valence band of TiO 2 [5]. Also, doping of TiO 2 with transitional ions offers a way to trap charge carriers and extend the lifetime of both charge carriers. Consequently, dopants enhance the photocatalytic activity [30] In the present inve stigation Zn doped TiO 2 nanopowders supplied by Dr. A.R. Phani, have been prepared by simple and cost effective sol-gel process. The sol-gel process has been used to obta in particles with higher purity and homogeneity at lower processing temperatures. Recently, the solgel process has become a novel technique for t he preparation of nanocrystalline TiO 2 It has been demonstrated that through the solgel process, the physico-chemical and electrochemical properties of TiO 2 can be modified to improve its efficiency. It provides a simple and easy means of synthesizing nanoparti cles at ambient temperature under atmospheric pressure and this tec hnique does not require a complicated set-up. Since this method is a solution process, it has all the advantages over other preparation techniq ues in terms of purity, homogeneity and flexibility in introducin g dopants in large concen trations, stoichiometry control, ease of processing and composition control [39].

PAGE 58

44 Phani et. al. prepared the samples used for this study using a calculated quantity of titanium dissolved in ethanol (99.98%) solvent along with acetylacetone as a chelating agent and cetylt rimethyl ammonium bromide (as surfactant to contro l the growth of TiO 2 crystallites). This had been dissolved in ethanol solvent in a beaker and the contents were added drop-wise to the titanium butoxide sol under vi gorous stirring for 6 hour s at room temperature. Calculated quantities of HCl a nd deionized water were added to increase the rate of reaction [40]. In addition, the contents were reflux ed at 60 C for 4 h to complete the reaction and cooled down to room temper ature. The contents were filtered using Whatman filter paper in order to remo ve any particulates formed during the reaction and then dried at 100 C for 1h. At the end, the powders were calcined at different temperatures ranging from 500C to 800C for 3 h in air in order to enhance the photocatalytic activity. Zn was used to dope (Zn acetate as a source) the TiO 2 base material. Figure 23 shows the flow-chart of the sol gel process [40, 41].

PAGE 59

Titaniumbutoxide( 0.1M)(in ethanolsolvent) Mix and stirring(6 h in air ambient) Refluxingand heating(60 oCfor 4 h) Filtration Dryingat100C for 1 h Characterization Cetyltrimethylammonium bromide(in ethanol) Annealing(500C 800C for 3 h) stabilisingagent(in ethanol) Zinc acetate(in ethanol) DI H2O HCl Figure 23 Preparation of Zn Doped TiO 2 Nanopowders by Sol Gel Process [40] Doping of TiO 2 with transitional ions offers a way to trap charge carriers and extend the lifetime of both charge carriers. Consequently, dopants enhance the photocatalytic activity. Zn doped TiO 2 was prepared with different concentrations of Zn. Table 1 shows the list of the nanopowders received. 6.2 Characterization of TiO 2 -Xwt.% Zn Nanopowders The as-prepared and annealed nanopowders from the process explained above have been examined for their phase identification and surface morphology by employing the X-ray diffraction and scanning electron microscopy (SEM) techniques. Photocatalytic activity studies have been conducted in collaboration 45

PAGE 60

with the information of the samples already sent. The samples received are: TiO 2 nanopowders (sol-gel method) annealed at 500 C and 600 o C for 3h with a Zn concentration of 1.3 wt%, 2.2 wt%, 3.1 wt%, 4.0 wt%, 4.9 wt%. 6.2.1 X-ray Diffraction Characterization and SEM Measurements XRD and SEM measurements of the samples were carried out in Italy by Dr. A.R. Phani and are reproduced in Figures 24, 25 and 26. Figure 24. XRD Spectra of TiO2 Zn Annealing at 500C at 3h [30] 46

PAGE 61

Figure 25. XRD Spectra of TiO2 Zn Annealing at 600 C at 3h [30] The study of XRD and SEM results by Phani et. al. [30] concluded that pure TiO2 and Zn doped TiO2 powders annealed at 500 C for 3 hours have shown the formation of cryst alline phase of anatase TiO2, independent of Zn concentration. In addition, the variation in the peaks intensity in dicates a variation in the anatase phase proportion. The XR D of the nanopowders annealed at 600C for 3 hours also showed the formation of crystalline phase of anatase TiO2 accompanied with the formation of rutile phas e, the content of which is increased with increasing Zn concentration. In additi on, at 3.1wt% and hi gher,,there exists the formation of ZnTiO3. Phani et. al. [30] conclud ed that by increasing the Zn concentration (1.3 wt% to 4.9 wt %) the crystallite size of TiO2 was drastically 47

PAGE 62

48 reduced from 40 nm to 25 nm and had an effect on t he formation of pores with diameter dimensions of 100 -120 nm for the 500C annealed samples. For the 600 o C annealed samples the crystallite size of TiO 2 was drastically reduced from 55 nm to 30 nm and was accompanied with the formation of pores with diameter dimensions of 80 -100 nm. Figure 26 represents the SEM microstructures of TiO 2 doped with different concentrations of Zn. It is clearly seen fr om these microstructu res that particle fragmentation, homogenization and agglomeration effects, at different loading concentrations of Zn, the TiO 2 matrix nanopowder.

PAGE 63

49 TTiiOO 22 ZZnn 33..11%% ZZnn 11..33%% ZZnn 44..00%% ZZnn 22..22%% ZZnn 44..99%% Figure 26. SEM Images of Different Concentrations of Zn Doping of TiO 2 [30]

PAGE 64

6.2.2 BET Surface Area Measurements Autosorb-1C from Quantachrome Instruments has been employed to determine the surface area and pore size di stribution of the Zn doped samples. Each of these samples was placed in a glass tube and was out gassed at 300 C for 3 hours. The external bath for the sa mple was liquid nitrogen (77 K) and a multi point BET method using nitrogen as the adsorbate gas were used to analyze these samples [36]. Autosorb-1C uses the Brunauer-Emmet-Teller (BET) method for the determination of the surface area of solid materials using the BET equation: Po P CW C CW P Po Wm m11 1 1 (7) where W is he weight of gas adsorbed at a relative pressure, P/Po, and Wm is the weight of the adsorbate constituting a monolayer of surface coverage. The term C is a constant related to the energy of absorption in the first adsorbed layer and its value is an indication of the magnitude of the adsor bent/adsorbate interactions [42]. Table 1, shows the surface area of the data collected for the Zn doped samples. The total pore volume is derived fr om the amount of vapor adsorbed at a relative pressure close to unity, by assuming that pores are then filled using liquid adsorbate. Figure 27 and Figure 28 show the relationship between pore diameter and total pore volume of the samples. The pore volume of TiO2-Xwt.% Zn varies 50

PAGE 65

51 with the value of X. For X= 1.3 wt.% (lower doping leve l of Zn), the pore volume increases two times when compared to pure TiO 2 however, it decreases with an increase in concentration of Zn. The pore diameter of around 18-20 was uniformly obtained irrespective of t he Zn concentration in the sample. Table 1 BET Surface Area Results Sample # Description Surface Area (m 2 /g) 8 Nanopowders (sol-gel) 500C 3h 33.27 9 Nanopowders (sol-gel) 1.3% 500C 3h 73.14 10 Nanopowders (sol-gel) 2.2% 500C 3h 40.80 11 Nanopowders (sol-gel) 3.1% 500C 3h 37 12 Nanopowders (sol-gel) 4.0% 500C 3h 53.98 13 Nanopowders (sol-gel) 4.9% 500C 3h 35.18 14 Nanopowders (sol-gel) 600C 3h 12.48 15 Nanopowders (sol-gel) 1.3% 600C 3h 26.41 16 Nanopowders (sol-gel) 2.2% 600C 3h 15.81 17 Nanopowders (sol-gel) 3.1% 600C 3h 13.68 18 Nanopowders (sol-gel) 4.0% 600C 3h 17.80 19 Nanopowders (sol-gel) 4.9% 600C 3h 13.14

PAGE 66

Figure 27. Pore Volume vs Pore Diameter of Zn Doped TiO 2 Annealed at 500C 3h Figure 28. Pore Volume vs Pore Diameter of Zn Doped TiO 2 Annealed at 500C 3h 52

PAGE 67

6.3 Photocatalytic Activity of TiO 2 Nanopowders Photocatalytic activity for this study was performed using a Tubular Reactor and 6 UV lights, the experimental set up will be explained in the following chapters. Methyl Orange is the pollutant of choice; its degradation in the suspension with the photocatalyst is shown in the initial stages of the reaction (during the first two hours). Figures 29 to 31 show the plots for photocatalytic degradation of methyl orange using TiO 2 with different Zn doping concentration. Both 500C and 600C annealed photocatalysts revealed relatively low photocatalytic efficiency as compared to the industrial TiO 2 P-25 Degussa catalyst (see Figures. 30 and 32). This may be due to the different preparation conditions and the obtained phase proportion of anatase TiO 2 structure. 0.80.850.90.951020406080100120Time (min)C/Co 0 wt% 1.3 wt% 2.2 wt% 3.1 wt% 4 wt% 4.9 wt% 500C 3h 53 Figure 29 MO Photodegradation of TiO 2 Zn 500C 3h

PAGE 68

0 0.2 0.4 0.6 0.8 1 0 20 40 60 80100120 Time (min)C/Co 0 wt% 1.3 wt% 2.2 wt% 3.1 wt% 4 wt% 4.9 wt% 500C 3h Figure 30. Comparision of Photocatalytic of TiO2 Degussa and TiO2 Zn 500C 3h 0.8 0.85 0.9 0.95 10 20406080100120 Time (min)C/Co 0 wt% 1.3 wt% 2.2 wt% 3.1 wt% 4 wt% 4.9 wt% 600C 3h Figure 31 MO Photodegradation of TiO2 Zn 600C 3h 54

PAGE 69

0 0.2 0.4 0.6 0.8 10 20406080100120 Time (min)C/Co 0 wt% 1.3 wt% 2.2 wt% 3.1 wt% 4 wt% 4.9 wt% TiO2 Degussa 600C 3h Figure 32. Comparision of Photocatalytic of TiO2 Degussa and TiO2 Zn 600C 3h 55 0.0000 0.0004 0.0008 0.0012 0.0016 012345Doping (TiO2 Zn x-wt%)Apparent Rate Constant (1/min) ErRate Er+500C 3h Figure 33. Apparent Rate Constant for Zn Doped TiO2 500C 3h using a Tubular Reactor and a UV Light Irradiation

PAGE 70

0.00070.00090.00110.00130.00150.001701234Doping (TiO2 Zn x-wt%)Apparent Rate Constant (1/min) 5 ErRate Er+600C 3h Figure 34 Apparent Rate Constant for Zn Doped TiO 2 600C 3h using a Tubular Reactor and a UV Light Irradiation The results above (Figures 30 and 32) show that the TiO 2 -Zn doped photocatalysts prepared by the sol gel method show lower performance compared to TiO 2 Degussa. There is no significant variation in the photocatalytic materials prepared with the increased Zn correlation. In addition, there is no significant difference in the performance of the photocatalysts prepared by and annealed at 500C and 600C; nevertheless the samples have different phase content (anatase at 500C and anatase+rutile at 600C). In addition, we can notice that the optical absorption measurements of 3.1 wt.% Zn 500C material do not exhibit any changes in the bandgap as compared to the undoped TiO 2 (Figure 35). However, for 600C, 3.1 wt% Zn TiO 2 there is a 56

PAGE 71

red shift in the band gap of the doped material, which can be seen in Figure 36. Such a difference in the effect of the annealing temperature can be explained by the presence of the rutile phase in the samples annealed at 600C. 00.511.5233.23.43.63.844.2Energy (eV)(F(R')*hn)1/2 500C 3h 4.0 wt% 1.3 wt% 2.2 wt% 3.1 wt% TiO2 4.9 wt% Figure 35. Plot of Optical Absorption (F(R)*h)1/2 vs. Incident Photon Energy, h, for the Sol Gel Samples Annealed at 500C. 57

PAGE 72

00.511.5233.23.43.63.844.2Energy (eV)(F(R')*hn)1/2 600C 3h 4.0 wt% 1.3 wt% 2.2 wt% 3.1 wt% TiO2 4.9 wt% Figure 36. Plot of Optical Absorption (F(R)*h)1/2 vs. Incident Photon Energy, h, for the Sol Gel Samples Annealed at 600C. The results described in this chapter indicate that there is no significant change in the photocatalytic efficiency for TiO 2 -X wt.% Zn at two different calcination temperatures (500 and 600 o C). The reason for this may be due to the large crystallite sizes and the lower surface area of these initial samples observed from XRD and BET analysis. In the next chapter, we will describe our strategy to improve photocatalytic properties of sol gel Zn doped TiO 2 materials. 58

PAGE 73

59 CHAPTER 7: BALL MILL INDUCED TRANSFORMATIONS Researchers are interested in t he application of nanomaterials to environmental applications. Nanocrystalline materials exhibit a variety of unique properties, such as high surface area, short interface migration distance and visible light activity that can enhance photocatalytic perform ance [39]. The sol gel process is considered to be a nov el technique for the preparation of nanocrystalline TiO 2 and it has been demonstrated that by using this technique the physico-chemical and electr ochemical properties of TiO 2 can be modified to improve photocatalytic efficiency [39]. In this study, an attempt has been made to enhance the photocatalytic activity of Zn doped TiO 2 by variying the calcinatio n temperature at different concentrations. However, as it was shown in the previous chapter, Zn doping has not demonstrated a significant change in the photocatalytic efficiency of TiO 2 -X wt% Zn compared to the regular TiO 2 In this chapter, we discuss a. new strategy for the modification of the catalysts, namely the mechano-chemical milling of TiO 2 -X wt% Zn materials in a high energy planet ary mill. As it will be seen below, this procedure can optimiz e the micropore size distribution and increase the surface area of the photoc atalytic material.

PAGE 74

60 7.1 High Energy Ball Mill McCormick et. al. [43], explains t hat Ball Mill can be defined as a mechano-chemical process (MCP) that us es mechanical energy to activate chemical reactions and structural changes. It has been demonstrated that the activation of chemical reactions by mechanical energy can lead to many interesting applications, like waste proce ssing. High energy ball milling is an alternative technique to obtain nanosized materials through the application of high pressure. The objectives of the ball milling pr ocess include particle size reduction, mixing or blending, and par ticle shape changes. The most common mill used for these purposes has been a plan etary ball mill; a bowl sits on a grinding platform and rotates in a direction opposite to the di rection of the base fixture. This action is a lot like the "teacup and saucer" rides commonly found in amusement parks. In planetary action, centrifugal forces alternately add and subtract. The grinding balls roll halfway around the bowls and then are thrown across the bowls, impacting on the opposite walls at high speed. Grinding is further intensified by interaction of the balls and sample. Pl anetary action gives up to 10g acceleration [34]. The schematic cross-section of the planetary ball mill principle is illustrated in Figure 37,

PAGE 75

Figure 37. Schematic Cross-section of a Planetary Ball Mill [52] The mechano-chemical milling of semiconductor oxide materials, particularly TiO 2 and its photocatalytic behavior has been extensively investigated in the past years. Mechanochemical milling is known to generate accumulations of defects, particle size reduction and local temperature increases which contribute to the activation of solid compounds so that they store additional energy which facilitates chemical reactions or transformations [43]. Another important factor in the success of this instrument is its economy; it is an inexpensive and rapid process when compared with other synthesizing methods such as induction melting, quenching, sintering, etc. The present study shows the effect of the ball-mill process on the photocatalytic activity of TiO 2 -X wt% Zn samples, which was evaluated by the photocatalytic oxidation of methyl orange as a model organic compound. Pore size, surface area, crystallite size and phase formation are parameters that were studied in this work. 61

PAGE 76

62 7.2 Ball Milling Transformation of TiO 2 -X wt% Zn Many studies have reported t he relation between crystallographic structure and surface properties and the effect of these properties on the catalytic properties of TiO 2 [45]. We noticed that Zn doped TiO 2 composite materials prepared by sol-gel met hod and annealed at 500C and 600C were agglomerated in relatively large (crude) dense particles Therefore, we treated these samples by the ball milling process for 2h. Our preliminary results indicate that wet mechano-chemical synthesis in creases the surface area as well as improves the photocatalytic activity of the Zn doped TiO 2 samples, as it is seen from the results below. Figure 38-41 show plots of photocatalyt ic degradation of methyl orange using TiO 2 with different Zn doping concentration. Both 500C and 600C annealed photocatalysts revealed lower p hotocatalytic efficiency as compared to the industrial TiO2 P-25 Degussa cata lyst (see Fig. 39 and 41). However, from Figs. 42 and 43, one can see that the rate of degradation of methyl orange in an aqueous soluti on is more than 2 times higher as compared with the unmodified catalyst. The ball milling process results show a two fold increase in the aver age improvement of the photocatalytic activity, which can be related to an increas e in the surface area and a decrease in the crystal size by the same proportion.

PAGE 77

0.650.70.750.80.850.90.951020406080100120Time (min)C/Co 0 wt% 1.3 wt% 2.2 wt% 3.1 wt% 4 wt% 4.9 wt% Figure 38. MO Photodegradation of TiO 2 Zn 500C 3h After Ball Milling for 2h. 00.10.20.30.40.50.60.70.80.910204060801001Time (min)C/Co 20 0 wt% 1.3 wt% 2.2 wt% 3.1 wt% 4 wt% 4.9 wt% TiO2 Figure 39 Comparision of Photocatalytic activity of TiO 2 Degussa and TiO 2 Zn 500C 3h After Ball Milling for 2h 63

PAGE 78

y = 0.9998e-0.0018xR2 = 0.9995y = 1.0067e-0.0033xR2 = 0.99870.60.650.70.750.80.850.90.951020406080100120Time (min)C/Co 0 wt% 1.3 wt% 2.2 wt% 3.1 wt% 4 wt% 4.9 wt%600C 3h BM Figure 40. MO Photodegradation of TiO 2 Zn 600C 3h After Ball Milling for 2h. 00.10.20.30.40.50.60.70.80.910204060801001Time (min)C/Co 20 0 wt% 1.3 wt% 2.2 wt% 3.1 wt% 4 wt% 4.9 wt% TiO2 Degussa600C 3h BM Figure 41. Comparision of Photocatalytic Activity of TiO 2 Degussa and TiO 2 Zn 600C 3h After Ball Milling for 2h 64

PAGE 79

0.00000.00050.00100.00150.00200.00250.00300.0035012345Doping (TiO2 Zn x-wt%)Apparent Rate Constant (1/min) TiO2 Zn 500C 3h BM 2h TiO2 Zn 500C 3h AverageAverage Figure 42. Apparent Rate Constant for Zn doped TiO 2 500C 3h Before and After Ball Milling, using a Tubular Reactor and a UV Light Irradiation. 00.00050.0010.00150.0020.00250.0030.0035012345Doping (TiO2 Zn x-wt%)Apparent Rate Constant (1/min) TiO2 Zn 600C 3h BM 2h TiO2 Zn 600C 3h A verage A verage Figure 43 Apparent Rate Constant for Zn doped TiO 2 600C 3h Before and After Ball Milling, using a Tubular Reactor and a UV Light Irradiation. 65

PAGE 80

These results are supported by the decrease of the crystalline particle size obtained by XRD measurements (see Figures. 44-45) that correlate with the apparent rate constants of Figures. 42 and 43. For an increase in surface area there is a decrease of particle size as can be seen in Figure. 46 when compared with Figures. 42 and 43, respectively. 20253035404550556065702 (Degrees)Intensity (arb. units)TiO2 TiO2 -1.3wt.% ZnTiO2 2.2wt.% ZnTiO2 3.0wt.% ZnTiO2 4.0wt.% ZnTiO2 4.9wt.% ZnAnnealing at 500 oC/3h, Ball Mill 2h(101)(004)(200)(105)(211)(204) Figure 44. XRD Spectra of TiO2 Zn Annealing at 500C 3h After Ball Milling 66

PAGE 81

20253035404550556065702 Idegrees)Intensity (arb. Units)Annealing at 600 oC/3h, Ball Mill 2h* (101)* (004)* (200)* (105)* (211)* (204) (110)* TiO2 (Anatase) TiO2 (Rutile)TiO2 TiO2 -1.3wt.% ZnTiO2 2.2wt.% ZnTiO2 3.0wt.% ZnTiO2 4.0wt.% Zn Figure 45. XRD Spectra of TiO 2 Zn Anneling at 600C 3h After Ball Milling 0510152025303540455001234Concentration (X-wt% Zn)Crystal size (nm) 5 500C 3h BM 2h 600C 3h BM 2h Figure 46 Crystall Size Concentration for 500C and 600C After Ball Milling. 67

PAGE 82

These results seems to be a good agreement between increasing BET surface area and pore size distribution of ball milled samples as shown in Fig. 47-50. 020406080100120012345Zn Concentration (wt %)Surface Area (m2/g)TiO2 Zn 500C 3h, Ball Mill 2h Ti O2 Z n 500C Figure 47. Comparision BET Surface Area Measurement Before and After Ball Milling for 500C 3h 68

PAGE 83

051015202530354001234Zn Concentration (wt%)Surface Area (m2/g)TiO2 Zn 600C 3h TiO2 Zn 600C 3h, Ball Mill 2h 5 Figure 48. Comparision BET Surface Area Measurement Before and After Ball Milling for 600C 3h Figure 49 Pore Volume vs Pore Diameter of Zn Doped TiO 2 Annealed After Ball Mill at 500C 3h 69

PAGE 84

Figure 50. Pore Volume vs Pore Diameter of Zn Doped TiO 2 Annealed After Ball Mill at 600C 3h In addition, we can see that the optical absorption measurements of samples prepared by annealing at 500C do not present any variation in the bandgap with the increase of Zn dopant concentration (see Figure 51). However, for Zn doped TiO 2 annealed at 600C, the highest red shift in the band gap of the doped material is observed for 3.1 wt.% Zn doping concentration, which can be seen in Figure 52. Again, such a difference in the effect of the annealing temperature can be explained by the presence of the rutile phase in the samples annealed at 600C (compare Figures 48 and 49). By comparing the optical absorption spectra in Figures. 35 and 51 and Figures. 36 and 52, we can see a blue shift as high as 0.1 0.15 eV in the optical absorption, which may be explained by the presence of the quantum size effect. 70

PAGE 85

0 0.5 1 1.5 2 2.5 3 3.5 4 3 3.2 3.4 3.6 3.8 4 4.2Energy (eV)(F(R')*hn)1/2500C 3h BM 4.0 wt% 1.3 wt% 2.2 wt% 3.1 wt% TiO2 4.9 wt% Figure 51 Plot of Optical Absorption (F(R)*h )1/2 vs Incident Photon Energy, h for the Sol Gel Samples Annealed at 500C. and Ball Milled for 2 h. 0 0.5 1 1.5 2 2.5 3 3.5 4 2.52.72.93.13.33.53.73.94.1Energy (eV)(F(R')*hn)1/2 TiO2 1.3 wt% 2.2 wt% 3.1 wt% 4.0 wt%600C 3h BM Figure 52. Plot of Optical Absorption (F(R)*h )1/2 vs Incident Photon Energy, h for the Sol Gel Samples Annealed at 600C.and Ball Milled for 2h 71

PAGE 86

Finally, the SEM microstructural investigations reveal the variation in the surface morphology with different Zn doping concentrations in the TiO2-Xwt.% Zn nanoparticulates (Figs. 53-65). EDS spectra of these samples confirm the stoichiometric concentration of Zn. Images of SEM are shown in the Figures. 66 and 67. Figure 53. SEM Image of TiO 2 Annealed 500C 3h Before and After Ball Mill. Figure 54 SEM Image of TiO2 Zn Doped at 1.3 wt% 500C 3h Before and After Ball Mill. 72

PAGE 87

Figure 55 SEM Image of TiO2 Zn Doped at 2.2 wt% 500C 3h Before and After Ball Mill Figure 56 SEM Image of TiO2 Zn Doped at 3.1 wt% 500C 3h Before and After Ball Mill Figure 571 SEM Image of TiO2 Zn Doped at 4.0 wt% 500C 3h Before and After Ball Mill 73

PAGE 88

Figure 58 SEM Image of TiO2 Zn Doped at 4.9 wt% 500C 3h Before and After Ball Mill Figure 59 SEM Image of TiO2 Annealed 600C 3h Before and After Ball Mill Figure 60 SEM Image of TiO2 Zn Doped at 1.3 wt% 600C 3h Before and After Ball Mill 74

PAGE 89

Figure 61. SEM Image of TiO2 Zn Doped at 2.2 wt% 600C 3h Before and After Ball Mill Figure 62. SEM Image of TiO2 Zn Doped at 3.1 wt% 600C 3h Before and After Ball Mill Figure 63. SEM Image of TiO2 Zn Doped at 4.0 wt% 600C 3h Before and After Ball Mill 75

PAGE 90

Figure 64. SEM Image ofTiO2 Zn Doped at 500C 3h After Ball Mill at 20000X. Starting Left: 0wt%, 1.3wt%, 2.2 wt%, 3.1 wt%, 4.0 wt% and 4.9 wt% 76

PAGE 91

Figure 65. SEM Image ofTiO2 Zn Doped at 600C 3h After Ball Mill at 20000X. Starting Left: 0wt%, 1.3wt%, 2.2 wt%, 3.1 wt% and 4.0 wt% 77

PAGE 92

01234567891Energy (KeV)Intensity (arb. Units)O KTi KTi KZn KZn KC KZn LTiO2 TiO2 -1.3wt.% ZnTiO2 2.2wt.% ZnTiO2 3.0wt.% ZnTiO2 4.0wt.% ZnTiO2 4.9wt.% ZnAnnealing at 500 oC/3h, Ball Mill 2h 0 Figure 66. EDS Spectra for 500C 3h Zn Doped TiO2 After Ball Milling 01234567891Energy (KeV)Intensity (arb. Units)C KO KZn LTi KTi KZn KZn KTiO2 TiO2 -1.3wt.% ZnTiO2 2.2wt.% ZnTiO2 3.0wt.% ZnTiO2 4.0wt.% ZnAnnealing at 600 oC/3h, Ball Mill 2hSi K 0 Figure 67. EDS Spectra for 600C 3h Zn Doped TiO2 After Ball Milling 78

PAGE 93

79 Table 2. EDS values for TiO2 Xwt% Zn 500C After Ball Milling 1100X 20000X Sample # (Theorical wt%) Element Wt% At% Wt% At% 8 (0wt%) OK 35.97 62.71 33.64 60.49 TiK 64.03 37.29 64.19 38.55 9 (1.3 wt%) OK 29.11 55.33 31.68 58.27 TiK 68.9 43.74 66.91 41.1 ZnK 1.99 0.92 1.41 0.63 10 (2.2 wt%) OK 35.69 62.8 33.95 61.07 TiK 60.51 1.64 4.77 2.1 ZnK 3.8 35.56 61.28 36.83 11 (3.1 wt%) OK 35.1 62.11 34.9 61.9 TiK 61.91 36.59 2.98 1.3 ZnK 2.99 1.29 62.12 36.8 12 (4.0 wt%) CK 5.17 11.4 5.11 11.3 OK 33.08 54.75 32.86 0.95 ZnK 2.03 0.82 2.34 33.14 TiK 59.73 33.02 59.7 54.61 13 (4.9 wt%) CK 6.53 13.35 5.36 10.66 OK 38.34 58.87 42.77 63.86 ZnK 3.54 26.46 2.9 24.42 TiK 51.59 1.33 48.97 1.06 Table 3. EDS values for TiO2 Xwt% Zn 500C After Ball Milling 1100X 20000X Sample (Theorical Zn wt%) Element Wt% At% Wt% At% 14 (0wt%) OK 28.98 54.99 28.91 54.9 TiK 71.02 45.01 71.09 45.1 15 (1.3 wt%) OK 34.62 61.42 32.7 59.39 TiK 64.34 38.13 66.03 40.05 ZnK 1.04 0.45 1.26 0.56 16 (2.2 wt%) OK 40.28 67.03 SiK 0.64 0.6 36.84 63.91 TiK 55.92 31.08 59.86 34.69 ZnK 3.17 1.29 3.3 1.4 17 (3.1 wt%) OK 35.36 62.19 32.54 59.2 SiK 0.65 0.66 0.71 0.74 TiK 61.21 35.95 63.69 38.7 ZnK 2.78 1.19 3.06 1.36 18 (4.0 wt%) TiL 26.47 16.6 27.44 17.4 OK 30.25 56.79 29.34 55.72 ZnL 1.79 0.82 1.85 0.86 TiK 40.13 25.16 40.07 25.42 ZnK 1.36 0.62 1.3 0.6

PAGE 94

80 Comparing Figures 46 and 51 one can see that Zn concentration of 1.3 wt % is an optimum for obtaining higher photocat alytic efficiency for the ball milled TiO2-Xwt.% Zn samples calcinated at 500C with an optimum por e volume size. The correlation between the increase of photocatalytic activity and the BET surface area implies that the liquid-solid interface of ballmilled samples plays an important role in the improv ement of the photocatalytic activity for the Zn doped TiO 2 For the samples calcinated at 600C, the maximum photoc atalytic activity was observed for the samples with 4.9 wt.% Zn doping. Thus we can conclude that the proposed new strategy to reduce the average crystallite size and optimize the micropore size distribution by mechanochemically milling TiO2-Xwt.% Zn in a high energy pla netary mill has resulted in average improvement of the phot ocatalytic activity by a two fold increase as it can be seen in Figures 42 and 43.

PAGE 95

81 CHAPTER 8: SUMMARY AND CONCLUSIONS The Results of this study are as follows: 1. Attempts were made to improve the photocatalytic behavior of TiO2 by doping with various concentrations of Zn (0, 1.3, 2.2, 3.1, 4.0 and 4.9) in a sol-gel process. 2. There is no significant change in the photocatalytic efficiency for TiO2Xwt.% Zn processed at two different calcination temperatures (500 and 600 C). 3. The reason for this may be due to the large crystallite sizes and the lower surface area of these initial samples obse rved from XRD and BET analysis. 4. Pursued a new strategy to reduce t he average crystallit e size and optimize the micropore size distribution by me chano-chemically milling TiO2-Xwt.% Zn in high energy planetary mill. 5. This approach resulted in average improvement of the photocatalytic activity by a two fold increase which can be explained by: a) increase in the surface area by two times and b) decrease in average cyrstallite size by two times.

PAGE 96

82 6. Zn concentration of 1.3 wt.% is optimum for obtaining higher photocatalytic efficiency for the ball milled TiO2-Xwt.% Zn samples calcined at 500C. 7. For the case of samples calcined at 600C, the maximum photocatalytic behavior was observed for TiO2 4.9 wt.% Zn. 8. The above photocatalytic enhancem ent of TiO2-Xwt.% Zn can be explained by the increase in the surface area and the optimized size of the particles. 9. Comparing the Kubelka-Munk spec tra of pristine and ball milled samples revealed a blue shift (increase in Eg) from 3.2 eV to 3.35 eV, which may be because of the presence of quantum size effects. 10. SEM microstructural investigations revealed variations in the surface morphology with different Zn doping conc entrations in the TiO2-Xwt.% Zn nanoparticulates. EDS spectra of these samples confirmed the stoichiometric concentration of Zn. Experimental observations also sugges t that the average crystallite size measurements obtained from the X RD analysis and BET surface area calculations are in good agreement with each other.

PAGE 97

83 LIST OF REFERENCES [1] S. Malato, J. Blanco, D. Alarcon, M. Maldonado, P. Fernndez-Ibez, W. Gernjak, Photocatalytic deconta mination and disinfection of water with solar collectors, Catalysis Today vol. 122, pp.137, 2007 [2] M. Kositzi, I. Poulios, S. Mala to, J. Caceres, A. Campos, "Solar photocatalytic treatment of sy nthetic municipal wastewater," Water Research, vol. 38, pp. 1147-1154, 2004 [3] A. Fujishima, X. Zhang, "Titanium dioxide photocatalysis: present situation and future approaches," C.R. Chimie, vol. 9, pp. 750-760, 2006 [4] O. Carp, C. L. Huisman, and A. Relle r, "Photoinduced reactivity of titanium dioxide," Progress in Solid State Chemistry, vol. 32, pp. 33-177, 2004 [5] K. Madhusudan Reddy, B. Baruwati, M. Jayalakshmi, M. Mohan Rao, and S. V. Manorama, "S-, Nand Cdoped titanium dioxide nanoparticles: Synthesis, characterization and redox charge transfer study," Journal of Solid State Chemistry, vol. 178, pp. 3352-3358, 2005 [6] M. Schmidt, Thermochemical Treatment of TiO 2 Nanoparticles for Photocatalytic Applications," University of South Florida, 2007 [7] N. A. Serpone, Emeline, A.V., "Suggested terms and definitions in photocatalysis and radiocatalysis," International Journal of Photoenergy, vol. 4, pp. 91-131, 2002 [8] T. Ohno, K. Sarukawa, K. Tokied a, and M. Matsumura, "Morphology of a TiO 2 Photocatalyst (Degussa, P-25) Consisting of Anatase and Rutile Crystalline Phases," Journal of Catalysis, vol. 203, pp. 82-86, 2001 [9] M. Schiavello, Heterogeneous Photocat alysis, Volume 3, First ed. Baffins Lane, Chichester: Wiley, 1997 [10] A. Sobczynski, A. Dobosz, "Wat er Purification by Photocatalysis on Semiconductors," Polish Journal of Environmental Studies, vol. 10, pp. 195-205, 2001

PAGE 98

84 [11] U. Diebold, "The surface sci ence of Titanium dioxide," Surface Science Reports, vol. 48, pp. 53-229, 2003 [12] A. Mills, R. Davies, D. Worsley, "Water Purification by Semiconductor Photocatalysis," Chemical Society Reviews, vol. 22, pp.417-425, 1993 [13] Y. Paz, Z. Luo, L. Rabenberg, A. Heller, "Phot ooxidative self-cleaning transparent titanium dioxide films on glass," Journal of Materials Research, vol. 10, pp. 2842, 1995 [14] A. K. Datye, G. Riegel, J. R. Bolton, M. Huang, an d M. R. Prairie, "Microstructural Characterizati on of a Fumed Ti tanium Dioxide Photocatalyst," Journal of Solid State Chemistry, vol. 115, pp. 236-239, 1995 [15] S. Al-Qaradawi, S. R. Salman, "Photocatalytic degradation of methyl orange as a model compound," Journal of Photochemistry and Photobiology A: Chemistry, vol. 148, pp. 161-168, 2002 [16] A. L. Linsebigler, G. Lu, and J. T. Yates, "Photocatalysis on TiO 2 Surfaces: Principles, Mechanisms, and Selected Results," Chemical Reviews vol. 95, pp. 735-758, 1995 [17] D. Hufschmidt, L. Liu, V. Selzer and D. Bahnemann, "Photocatalytic water treatment: fundamental kn owledgerequired for its pr actical application," Water Science and Technology, vol. 49, pp. 135, 2004 [18] I. A. Shkrob and M.C. Sauer, "H ole Scavenging and Photo-Stimulated Recombination of Electron-Hole Pair s in Aqueous TiO2 Nanoparticles," Journal of Physics and Chemistry, vol. 108, pp. 12497-12511, 2004 [19] D. Bahnemann, Photocatalytic water treatment: solar energy applications," Solar Energy, vol. 77, pp. 445 459, 2004 [20] S. Rodr guez, J. Blanco, M. Fernandez, D. Alarcon, M. Collares, J. Farinha, Engineering of sola r photocatalytic collectors, Solar Energy vol. 77, pp. 513-524, 2004 [21] T.Ohno, K. Sarukawa and M. Matsum ura, "Crystal faces of rutile and anatase TiO2 particles and their rolesin photocatalytic reactions," The Royal Society of Chemistry and t he Centre National de la Recherche Scientifique, vol. 26, pp. 1167-1170, 2002

PAGE 99

85 [22] A. Tsujiko, T. Kisumi, Y. M agari, K. Murakoshi and Y. Nakato, Selective Formation of Nanoholes with (100 )-Face Walls by Photoetching of n -TiO2 (Rutile) Electrodes, Accompanied by Increases in Water-Oxidation Photocurrent," Journal Physics and Chemistry. B, vol. 104, pp. 4873-4879, 2000 [23] T. Sugiura, S. Itoh, T. Ooi, T. Yoshida, K. Kuroda and H. Minoura," Evolution of a skeleton structured Ti O2 surface consisting of grain boundaries," J. Electroanal. Chem. vol. 473, pp. 204-208, 1999 [24] W. C. Hao, S. K. Zheng, C. Wang, "Comparis on of the phot ocatalytic activity of TiO2 powder with different particle size," Journal of Materials Science Letters. vol. 21, 1627-1629, 2002 [25] L. Jing, B. Xin, F. Yuan, L. Xue, B. Wang, and H. Fu, "Effects of Surface Oxygen Vacancies on Photophysical an d Photochemical Processes of ZnDoped TiO2 Nanoparticles and Their Relationships," J. Phys. Chem. B, vol. 110, pp. 17860-17865, 2006 [26] W. Zhanga, S. Zhub, Y. Lib, F. Wang, "Photocatalytic Zn-doped TiO2 films prepared by DC reactive magnetron sputtering," Vacuum, vol. 82, pp. 328, 2008 [27] N. Guettai and H. Ait Amar, "Photocat alytic oxidation of methyl orange in presence of titanium dioxide in aque ous suspension. Part I: Parametric study," Desalination, vol. 185, pp. 427-437, 2005 [28] K.T. Chung, G.E. Fu lk and A.W. Andres, "Mutagenicity testing of some commonly used dyes," Applied and Environmental Microbiology vol. 42 pp. 641, 1981 [29] S. Doh, C. Kim, S. G. Lee, S. J. Lee, H Kim, "Development of photocatalytic TiO2 nanofibers by el ectrospinning and its application to degradation of dye pollutants," Journal of Hazardous Materials 2007 [30] A.R. Phani, "Structu ral evolution and its effect on photocatalytic properties of pure TiO 2 and Zn doped TiO 2 nanopowders," NANO-Center for Advanced Nanotechnologies Presentation, Nov. 2007 [31] M.N. Rashed, A.A. El -Amin, Photocatalytic degradation of methyl orange in aqueous TiO2 under different sola r irradiation sources, International Journal of Physical Science, vol. 2, pp. 73-81, 2007 [32] Y. Zhao, C. Li, X. Liu, F. Gu, H.L. Du, L Sh i, "Zn-doped TiO2 nanoparticles with high photocatalytic activity syn thesized by hydr ogen-oxygen diffusion flame," Applied Catalysis B: Environmental, vol. 79, pp. 208, 2008

PAGE 100

86 [33] G. Marc V. Augugliaro, M. Lopez-Munoz, C. Mart n, L. Palmisano, V. Rives, M. Schiavello, R. J.Tilley, A. Venezia, "Preparation Characterization and Photocatalytic Activity of Po lycrystalline ZnO/TiO2 Systems. 2. Surface, Bulk Characterization, and 4-Nitrophenol P hotodegradation in Liquid-Solid Regime," J. Phys. Chem. B, vol. 105, pp. 1033-1040, 2001 [34] D. Escobar, Investigation of ZrNi, ZrMn 2 and Zn(BH 4 ) 2 Metal/Complex Hydrides for Hydrogen Storage," University of South Florida, 2007 [35] NNRC, Training Material for X-Ray Diffraction, University of South Florida 2007 [36] On line post: http://www.quantachrome.com/gassorption/index.html [37] New Mexico tech. FESEM types of signals, online posting, http://infohost.nmt.edu/~maximi no/fesem_types_of_signals.htm. [38] ETH Zrich website. SEM technology, online posting, www.microscopy.ethz.ch/sem.htm. [39] N. Venkatachalam, M. Palanichamy, V. Murugesan, "S olgel preparation and characterization of nanosize TiO2 : Its photocatalytic performance," Materials Chemistry and Physics vol. 104, pp. 454, 2007 [40] D. Di Claudio, A.R. Phani, S. Santucci, "Enhanced optical properties of solgel derived TiO2 films us ing microwave irradiation," Optical Materials vol. 30, pp. 279, 2007 [41] A.R. Phani, "Structu ral evolution and its effect on photocatalytic properties structural evolution and its effect on photocatalytic proper ties of of pure TiO pure TiO2 and Zn doped Ti O and Zn doped TiO2 nanopowders," Department of Physics, University of LAquila Nov. 2007 [42] Quantachrome Instrument, Autosorb 1, Operation Manual, pp 68-70, 2005 [43] P.G. McCormick, F.H. Froes, "T he Fundamentals of Mechanochemical Processing," JOM pp. 6165, 1998 [44] The Glassware Gallery. Planetary ball milling, online posting, www.ilpi.com/inorganic/gl assware/ballmill.html [45] C. Wu, L. Feng, Y. Kuo, C. Shu, "Enhancem ent of the photocatalytic activity of TiO2 film via surf ace modification of the substrate," Applied Catalysis A, vol. 226, pp. 199-211, 2002

PAGE 101

87 [46] S. Jane, S Cheng "Effect of TiO: crystalline st ructure in photocatalytic degradation of phenolic contaminant s," vol. 33, pp. 227-237, 1997 [47] S. Darrin, J L. Falconer,Catal yst Design to Change Selectivity of Photocatalytic Oxidation," Journal of Catalysis, vol.175, pp 213-219, 1998 [48] Online posting:http://www.rpi.edu/ dept/phys/ScIT/InformationProcessing


xml version 1.0 encoding UTF-8 standalone no
record xmlns http:www.loc.govMARC21slim xmlns:xsi http:www.w3.org2001XMLSchema-instance xsi:schemaLocation http:www.loc.govstandardsmarcxmlschemaMARC21slim.xsd
leader nam 2200385Ka 4500
controlfield tag 001 001999311
005 20090410163104.0
007 cr mnu|||uuuuu
008 090410s2008 flu s 000 0 eng d
datafield ind1 8 ind2 024
subfield code a E14-SFE0002462
035
(OCoLC)318216012
040
FHM
c FHM
049
FHMM
090
TK145 (Online)
1 100
Algarin, Paula C.
0 245
Effects of Zn doping and high energy ball milling on the photocatalytic properties of TiO
h [electronic resource] /
by Paula C. Algarin.
260
[Tampa, Fla] :
b University of South Florida,
2008.
500
Title from PDF of title page.
Document formatted into pages; contains 87 pages.
502
Thesis (M.S.E.E.)--University of South Florida, 2008.
504
Includes bibliographical references.
516
Text (Electronic thesis) in PDF format.
3 520
ABSTRACT: TiO photocatalysis is been widely studied for air and water purification applications; titanium dioxide is the most used semiconductor principally because its low cost, stability and chemical properties. However it only utilizes the UV portion of the solar spectrum as an energy source (less than 4% of the total sunlight energy). This behavior is due to its high band gap value of 3.2 eV. The modification of light harvesting properties of TiO by doping has become an important research topic to achieve an efficient operating range under UV and visible light. In addition, the structure and surface properties of photocatalysts play an important role. This thesis explores the effects of Zn doped TiO, prepared by the sol-gel method, on its photocatalytic activity to decompose organics and the characterization of the doped samples. Since this study is part of a collaborative initiative, the samples were synthesized and provided by Dr. A. R. Phani from the Department of Physics, University of L'Aquila. Preliminary examination revealed a relatively low photocatalytic efficiency of the samples. The objective is to modify/improve its properties by high energy ball milling which is expected to generate accumulations of defects, particle size reduction and an increase in the active surface area. The characterization of doped and mechanochemically treated materials will be analyzed by optical diffuse reflectance measurements and optical absorption calculations using the Kubelka-Munk approach. The phase structure and particle size of the materials will be determined using X-ray diffraction (XRD). The BET surface area of the samples will be obtained using an Autosorb instrument. The photocatalytic properties will be studied by the analysis of decomposition of Methyl Orange in an aqueous solution. An aqueous photocatalytic tubular reactor with capability of operation using UV and/or fluorescent light will be designed and built.
538
Mode of access: World Wide Web.
System requirements: World Wide Web browser and PDF reader.
590
Advisor: E.K. Stefanakos, Ph.D.
653
Photocatalysis
Methyl orange
Surface
Band gap
Sol-gel process
690
Dissertations, Academic
z USF
x Electrical Engineering
Masters.
773
t USF Electronic Theses and Dissertations.
4 856
u http://digital.lib.usf.edu/?e14.2462