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Fabrication of palladium nanoparticles and nanoporous alumina templates

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Fabrication of palladium nanoparticles and nanoporous alumina templates
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Chennapragada, Pavani
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University of South Florida
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Electrodeposition
Anodization
Pore ordering
Pore nucleation
Potentiostat
Dissertations, Academic -- Chemical Engineering -- Masters -- USF   ( lcsh )
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government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
theses   ( marcgt )
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Abstract:
ABSTRACT: Nanostructured materials have potential technological applications due to their characteristic dimensions. The material performance will depend on the atomic structure, and composition of these materials. This thesis focuses on proposing a reliable method for fabricating nanoporous alumina and palladium nanoparticles inside the templates.Palladium nanoparticles were synthesized in commercial porous alumina templates using electrodeposition. Pores within these nanoporous membranes act as templates for the synthesis of nanostructures of the desired material. Electrodeposition is achieved using a three-terminal set-up and a potentiostat. Different types of deposition techniques were investigated to improve the distribution of the deposit. The nanoparticles were characterized by SEM/EDX for composition. The commercial templates have high aspect ratio, but are not hexagonally ordered. Hence porous alumina was fabricated in the laboratory by anodization of aluminum.
Thesis:
Thesis (M.S.Ch.)--University of South Florida, 2005.
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Includes bibliographical references.
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by Pavani Chennapragada.
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Fabrication of Palladium Nanoparticles and Nanoporous Alumina Templates By Pavani Chennapragada A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering Department of Chemical Engineering College of Engineering University of South Florida Major Professor: Venkat R Bhethanabotla, Ph.D. Babu Joseph, Ph.D. Vinay Gupta, Ph.D. Date of Approval March 31, 2005 Keywords: electrodeposition, anodization, pore ordering, pore nucleation, potentiostat Copyright 2005, Pavani Chennapragada

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Acknowledgments I thank Dr.Venkat Bhethanabotla, Dr.Babu Joseph and Dr.Vinay.K.Gupta for having served on my committee. I would like to extend my heartfelt thanks to Dr.Bhethanabotla for having guided me on this project. Also I would like to acknowledge Dr.Latika Menon and Dr.Jianyu Liang for having helped me move in the right direction and providing me with specimen samples. My sincere thanks to all my lab mates Dr.Deepak Srinivasagupta, Stefan Cular, Amol Chaudari and Subbu Krishnan who helped me troubleshoot problems in the lab. Finally I would like to thank David. K. Edwards, MEMS-Metrology specialist at the USF-Centre for Ocean Technology for the SEM imaging and Daron Westly at the Cornell Nanotechnology facility for the silicon mask.

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i Table of Contents List of Tables................................................................................................................. iii List of Figures................................................................................................................ iv 1 Introduction.............................................................................................................1 1.1 Nanostructures.................................................................................................2 1.2 Template synthesis of nanomaterials................................................................3 1.3 Scope of the present thesis................................................................................4 2 Fabrication and Characterization of Anodic Alumina Templates..............................5 2.1 Overview.........................................................................................................5 2.2 Anodization of aluminum.................................................................................7 2.3 Preparation of ordered alumina templates.........................................................8 2.4 Chemical reactions involved in porous oxide growth [9]..................................11 2.5 Barrier and porous type alumina.....................................................................13 2.6 Influence of various conditions on the formation of porous alumina [11]..........14 2.7 Results and discussion....................................................................................15 3 Fabrication and Characterization of Nanoparticles.................................................29 3.1 Overview.......................................................................................................29 3.2 Synthesis of nanostructured materials using electrodeposition........................29 3.3 Experimental set up for DC deposition...........................................................31 3.4 Electrodeposition of nanoparticles in anodic alumina templates......................35 3.5 DC deposition results and discussion...........................................................37 3.6 Experimental set up for AC deposition...........................................................47 3.7 AC deposition – results and discussion...........................................................48 4 Conclusions...........................................................................................................50 5 Future Perspectives................................................................................................51 5.1 Device applications........................................................................................51

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ii References..................................................................................................................... 54

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iii List of Tables Table 1.1 [1] Typical nanomaterials with their characteristic dimensions..........................3 Table 2.1 [4] Optimized parameters for the anodic aluminum oxide template anodization................................................................................................ .11

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iv List of Figures Figure 2.1 Top view of commercial empty porous alumina template............................5 Figure 2.2 Cross section of commercial empty porous alumina template......................6 Figure 2.3 [7] Pore growth mechanism assisted by the electric field...............................7 Figure 2.4 Electromet-4 polisher..................................................................................9 Figure 2.5 Anodization set up.......................................................................................9 Figure 2.6 [8] Electromet-4..........................................................................................10 Figure 2.7 [9] Anodization current as a function of time..............................................12 Figure 2.8 [9] Structural characteristics of porous alumina film...................................13 Figure 2.9 Anodization current vs. time at an applied voltage of 40V.........................15 Figure 2.10 Electropolished sample surface, at 20V; Etching dominates polishing.......16 Figure 2.11 Top view of porous alumina anodized at 40V and 150C (linear array pore structure)........................................................................................... 17 Figure 2.12 Top view of the sample, which shows an average pore diameter of 48nm and an interpore distance of 70.5 nm.........................................................18 Figure 2.13 Cross section of the sample.......................................................................19 Figure 2.14 Bottom view of the anodized alumina obtained using 0.3M oxalic acid at 40V on 200nm scal e................................................................................... 20 Figure 2.15 Bottom view of the anodized alumina obtained using 0.3M oxalic acid at 40V on 2m scale.......................................................................................21 Figure 2.16 Anodization current vs. time at an applied voltage of 40V and electropolishing voltage of 48V, multiple runs–first step of anodization....22 Figure 2.17 Anodization current vs. time at an applied voltage of 40V and electropolishing voltage of 48V-second step of anodization.......................23 Figure 2.18 Top view of porous alumina electropolished at 48V for 30 secs and obtained using two step anodization ;oxalic acid 40V................................24 Figure 2.19 Top view of the sample at higher magnification with 37nm pore diameter and the interpore distance is 86.4nm (Hexagonal structure).........25

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v Figure 2.20 Anodization current vs. time at an applied voltage of 40V, single run–first step of anodization............................................................26 Figure 2.21 Anodization current vs. time at an applied voltage of 40V-second step anodization................................................................................................ .27 Figure 2.22 Cross sectional view of the sample indicating 70nm uniform pore channel................................................................................................... ...28 Figure 2.23 Top view of the sample indicating pore diameter of 44.5nm......................28 Figure 3.1 [11] Stages of electrocrystallization.............................................................30 Figure 3.2 [11] Pulse current waveform........................................................................30 Figure 3.3 Three-electrode set up for DC deposition...................................................32 Figure 3.4 Pote ntiostat 273A ...................................................................................... 33 Figure 3.5 Schematic of the Potentiostat interfaced with PC.......................................34 Figure 3.6 Schematic of nanoparticle synthesis..........................................................35 Figure 3.7 AFM image Au for back electrode, which indicates complete covering of pores thus providing a conductive layer for deposition when commercial templates were used for deposition.........................................36 Figure 3.8 Chronopotentiometric technique for palladium particles deposition particle nucleation.....................................................................................37 Figure 3.9 Chronopotentiometric technique for palladium particles deposition particle deposition.....................................................................................38 Figure 3.10 Top view of palladium filled pores............................................................39 Figure 3.11 Palladium particles in pores.......................................................................40 Figure 3.12 EDAX analysis on the template, which confirms the presence of palladium................................................................................................. .41 Figure 3.13 I vs. t curve for the palladium particles deposition.....................................42 Figure 3.14 Surface deposition of palladium................................................................43 Figure 3.15 Cross sectional SEM, which indicates the presence of palladium inside the pores of alumina template....................................................................44 Figure 3.16 Palladium deposition onto the walls of porous alumina..............................45 Figure 3.17 Palladium deposition to a depth of 5m inside the pores............................45 Figure 3.18 EDAX of the sample that confirms the presence of palladium to a depth of 5m.................................................................................................... ..46 Figure 3.19 EDAX – area scan of the sample to a depth of 5m...................................46

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vi Figure 3.20 [15]Potentiostatic measurement set up .......................................................47 Figure 3.21 SEM Top view – nickel deposition............................................................48 Figure 3.22 EDAX for the Ni deposition, which indicates the presence of nickel on the porous alumina surface........................................................................48 Figure 5.1 Mask design in AutoCAD for IDT structures comprising of finger pairs of various thickness (courtesy Stefan cular)...............................................51 Figure 5.2 IDTs on the porous alumina template........................................................52 Figure 5.3 EDAX that indicates the presence of palladium on the template................52

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vii Fabrication of Palladium Nanoparticles and Nanoporous Alumina Templates Pavani Chennapragada ABSTRACT Nanostructured materials have potential technological applications due to their characteristic dimensions. The material performance will depend on the atomic structure, and composition of these materials. This thesis focuses on proposing a reliable method for fabricating nanoporous alumina and palladium nanoparticles inside the templates. Palladium nanoparticles were synthesized in commercial porous alumina templates using electrodeposition. Pores within these nanoporous membranes act as templates for the synthesis of nanostructures of the desired material. Electrodeposition is achieved using a three-terminal set-up and a potentiostat. Different types of deposition techniques were investigated to improve the distribution of the deposit. The nanoparticles were characterized by SEM/EDX for composition. The commercial templates have high aspect ratio, but are not hexagonally ordered. Hence porous alumina was fabricated in the laboratory by anodization of aluminum. A two-step anodization process was employed to fabricate the nanoporous alumina. The pore formation, influence of the experimental conditions on the pore formation, the structural characteristics of the pore and the oxide chemical reactions involved in the pore growth were discussed.

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1 1 Introduction Palladium based hydrogen sensors were constructed by Penner et al.,[1] using mesowire arrays of palladium electrodeposited onto the step edges of highly oriented pyrolytic graphite. These palladium wires form nanoscopic junctions or gaps when they are exposed to hydrogen during an initial run and then these gaps would open and close depending on the amount of hydrogen absorbed or desorbed. The sensor response depends on the amount of palladium lattice expansion and contraction. The rate at which palladium equilibrates with the hydrogen determines the response time of the sensor. Hence by reducing the dimensions of the sensing element the time required for diffusion of hydrogen into palladium atoms can be decreased and the response time of the sensor could be accelerated. As the template synthesis allows for controlled deposition of particles dictated by the size of the pore, synthesis of palladium nanoparticles in nanoporous alumina templates would hence provide an improved sensing layer for a hydrogen sensor. This thesis is focused on making porous alumina templates through electro deposition of palladium nanoparticles. An optimized procedure is proposed for making stabilized and uniform nanoporous alumina templates and also a reliable method of deposition to obtain the palladium nanoparticles inside the pores is investigated. The first chapter gives a brief overview into the various types of nanostructures presently available and also discusses the template synthesis of nanomaterials, the method that is investigated in this thesis. The second chapter deals with the fabrication of nanoporous alumina templates, their process conditions, effect of experimental conditions on the process and discussion of the results obtained. Fabrication of palladium nanoparticles in the porous alumina templates forms the third chapter. In this chapter an effort is made to optimize a deposition method, which would yield consistent results. Hence different deposition techniques are tried and the results compared. An attempt made to use these palladium deposited alumina templates

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2 in the form of a resistive sensor is discussed in the fourth chapter. Author’s conclusions based on the experiments performed and the processes involved are discussed in the fifth chapter. A brief introduction to nanostructures will provide a better insight into the materials being discussed. 1.1 Nanostructures Materials with structural features between those of atoms and bulk materials are basically classified as nanomaterials. Atoms assembled in the size range ~1-200nm constitute nanostructures. Table 1.1[2] indicates some typical nanomaterials with their characteristic dimensions. These structures exhibit new and unique phenomena, which have potential technological applications in chemical, energy, electronics and space industries. Also research is being carried out on the use of nanotechnology in gene and drug delivery. Making self-organized structures is not new but ability to control the output of the synthesis processes using the new physical properties at the nano-level is where engineering is involved. Methods like E-beam lithography and X-ray lithography are expensive and hence the need to develop a cost-effective and efficient synthesis method to produce nanostructured materials. The performance of the nanodevices would depend on the performance of the materials involved in the device fabrication. The material performance will in turn depend on the atomic structure, composition, microstructure, defects, and interfaces, which are controlled by the thermodynamics and kinetics of synthesis [3]. Nanostructured materials can be made by top-down approaches like laser etching from macro scale to nanoscale, or conversely, by assembly of atoms or particles using bottom-up approach. In this thesis we have tried the bottom-up approach using the template synthesis method, which has been discussed in detail in the following section.

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3 Table 1.1 [1] Typical nanomaterials with their characteristic dimensions Nanostructures Size Materials Nanocrystals and clusters (quantum dots) Diameter 110nm Metals,semiconductors, magnetic materials Other nanoparticles Diameter 1100nm Ceramic oxides Nanowires Diameter 1100nm Metals,semiconductors,oxides, sulfides,nitrides Nanotubes Diameter 1100nm Carbon, layered metal chalcogenides 2-dimensional arrays of nanoparticles Several nm2m2 Metals, semiconductors, magnetic materials Surfaces and thin films Thickness 11000nm Various materials 1.2 Template synthesis of nanomaterials Templates can be defined as a network of porous structures formed in a material due to the electrochemical action on that material. Upon the removal of the template, the cavities would be filled with the desired material and hence different morphological structures would be obtained. “Template synthesis” will allow for ordered nanostructures, whose morphology would be dictated by the template structure. Various nanomaterials can be synthesized using the template approach like Polymeric nanostructures, nanometals, carbon nanotubes and other composite tubular semiconductor-conductor structures. These templates have been made in-house at Sensors Research Laboratory consisting of uniform and hexagonal array pore structures with uniform pore cross section and of an average pore diameter of ~40nm in comparison to the commercial templates. The scope of the project is discussed in the next section, which details the choice of materials for deposition and fabrication.

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4 1.3 Scope of the present thesis The scope of the present thesis is to fabricate a uniform cross-sectioned nanoporous alumina template and to deposit palladium nanoparticles inside those pores. Aluminum is a soft metal which is capable of easy oxidation. The passive oxide layer it forms in the presence of air prevents the corrosive nature in the metal. This oxide layer prevents further degradation of the metal and also aids in the formation of uniform pore structure. The balance between the oxide dissolution and attack of hydrogen ions on the oxide layer is the basis for the formation of pores. Hence aluminum is preferred as opposed to other metals for making nanoporous templates. Palladium is an important metal used in developing hydrogen sensors. Nanoparticles of palladium will help in increasing the response time of the resistive device as the reduced dimensions of the sensing element affect the response time of a device. Instead of the thin film approach towards the sensing layer, the palladium nanoparticles are tried as they aid in the timely diffusion of hydrogen atoms into the palladium lattice maintaining the equilibrium of palladium with hydrogen in the contacting gas phase. Thus faster response times can be observed. The particles that can be deposited using the commercial templates have larger dimensions as the minimum pore diameter of the porous templates obtained commercially is~100nm. Hence the templates were fabricated at the laboratory with an average pore diameter of ~40nm. Then the particle deposition inside the templates is discussed with respect to various deposition techniques and a reliable method has been suggested based on the results obtained. An attempt is made to test the deposited template as a resistive sensor by passing hydrogen through it.

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5 2 Fabrication and Characterization of Anodic Alumina Templates 2.1 Overview As discussed earlier a template is defined as a network of porous structures formed in a material. The commercial templates obtained in the market are mostly used for filtration purposes and hence have larger pore dimensions (Figure 2.1). Figure 2.1 Top view of commercial empty porous alumina template Also the cross sections of these pores reveal tortuous channels (Figure 2.2) with nonuniform diameter. These are through pore templates, very brittle in nature, making it very difficult to handle them in course of different actions to be performed on them for electrodeposition.

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6 Hence the nanoporous templates are fabricated in the laboratory with a layer of bulk aluminum remaining on them, thus providing the template with the hardness and also retaining the electrical conductive nature of the substrate. Figure 2.2 Cross section of commercial empty porous alumina template The process of making a nanoporous template involves pre annealing of bulk aluminum, electropolishing it, pre treating it with certain acids for pore nucleation and anodizing it in a stepwise procedure. Depending on the type of acid used, the voltage applied and the experimental conditions that are maintained two types of porous alumina can be obtained namely the barrier type and the porous type alumina. The pore formation, influence of the experimental conditions on the pore formation, the structural characteristics of the pore and the oxide chemical reactions involved in the pore growth are discussed in this chapter. The data obtained from the experimental runs is logged using Lab views VIs developed (courtesy Dr. Deepak Srinivasagupta) which integrate the DC power supply and the computer.

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7 2.2 Anodization of aluminum Nanoporous alumina is formed by partially dissolving aluminum oxide in strong acids by anodizing it. In the anodization process, an electrical circuit is established between the cathode and aluminum foil (99.997% pure, Alfa Aesar). according to the following reaction [4]: 2Al+3H2O Al2O3 + 3H2 G0 = -864.6KJ (1) Where G0 is the standard Gibbs free energy. When neutral oxides or basic solutions are used nonporous barrier oxide type film (BTF) is formed. Acidic solutions assist the growth of pore type film (PTF) [5]. Pore diameter depends on the PH, Anodization voltage and the choice of acid. Thompson [6] et al. explained the pore growth mechanism (Figure 2.3). Electrolyte Aluminum Alumina Figure 2.3 [7] Pore growth mechanism assisted by the electric field. Pore nucleation is due to the cracking and self-healing of the oxide layer and pore growth is due the field assisted hydrogen-ion attack on the oxide layer. Electropolishing the aluminum will from etch pits and bumps, which act as seeds for pore nucleation. As anodization started pores would start at cracks and imperfections in the surface leaving electric field to concentrate below the regions where the oxide film was thinner. At these

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8 points local dissolution of oxide would take place. The new pore formed deepens with the major pore forming at the expense of shallow pore. At the metal/oxide interface the average field across the barrier layer determines the barrier film growth rate while at the oxide/electrolyte interface the local field at the pore bottom assisted by heating determines the oxide dissolution rate. The film growth rate is approximately constant and independent of pore bottom curvature, while the dissolution rate increased as the pore base radius of curvature decreased. As the pore radius of curvature decreases, the film dissolution rate increases, to enlarge the pores and if the pore radius became too big, the dissolution slows and the pores tend to fill. Thus, these two competing processes keep the pore radius constant. Basically, pore formation is an acid-catalyzed reaction. Aluminum reacts with oxygen in air when exposed and forms aluminum oxide or alumina. Hence the metal/oxide and oxide/electrolyte interfaces are locally curved. When an electric field is applied, it concentrates at the pore bottoms and at surfaces, where oxide is the thinnest pore nucleation occurs. At pore bottoms, acid catalyzed oxide dissolution occurs. The growth becomes self-catalyzing with the acid and electrolyte penetrating into the pores that are formed initially. 2.3 Preparation of ordered alumina templates 99.997% pure aluminum sample (Alfa Aesar), 13mm in diameter and 0.1mm thickness is taken and degreased using 5% NaOH at 600C for 30 sec. Then, the sample is annealed in air at 3500C for one hour. Then, the sample is polished using an Electromet-4 Polisher in an acidic solution of 95-vol% H3PO4, 5-vol% H2SO4 and 20g/l CrO3 at a voltage of 20V for 3 minutes. The current density is maintained at 1-2 amp/cm2 and the temperature of the solution is maintained at 84200C. Pre treating of aluminum plays a very important role in obtaining ordered pore arrays. When the specimens are electrochemically polished and annealed, stresses within the film arising from surface roughness and residual stresses were minimized. Preannealing of aluminum substrate before anodic oxidation enhances the grain size in the metal substrate. It was also found that in electropolished samples the pores started forming after 5 minutes whereas in non-electropolished samples

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9 the pore formation started after an hour. Also due to the high surface roughness present on the samples faster formation of barrier oxide took place and the pores formed at depressions on the surface than at other locations. The oxide layer is dissolved using 3.5 vol% H3PO4 and 45g/l CrO3 at a temperature of 900C for 10 minutes. Then two-step anodization is performed which involves the pore nucleation and poreordering step. The pore nucleation is done using 4% H3PO4 acid at room temperature and passing a current density of 5mA/cm2 through the sample. To obtain ordered pores the sample is subjected to anodization at 40V in 3% H2C2O4 acid solution at 150C for a period of 12 hours. Figure 2.4 and Figure 2.5 show the electromet-4 polisher and anodization set up respectively used in fabricating the porous templates. The electromet-4 polisher comprises of a power source, polishing cell, electrolyte tank, masks, cathodes and an etching cell. It is also equipped with the cooling system, which helps in maintaining the temperature of the process and also protects the instrument from extreme heating. Figure 2.4 Electromet-4 polisher Figure 2.5 Anodization set up The stepwise procedure [8] described below has been followed in operating the instrument. Various parts described in the instrument can be seen in Figure 2.6. The electrolyte tank is filled completely so that all the cooling coils are submerged. The

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10 impeller in the tank is positioned using acid resistant tongs. The pump assembly is placed in the tank such that the lower pump housing encloses the impeller. A mask of the desired aperture size should be selected and positioned on the cathode assembly. The speed of the pump is adjusted so that the electrolyte will rise to the mask opening and overflow slightly. The anode arm is slid down so that it makes electrical contact with the back of the specimen. Now the time polish button is pressed and the time for polish is set. After setting all these parameters the instrument is run for the set time. Anode arm Power source Mask Impeller Electrolyte cell Pump assembly Figure 2.6 [8] Electromet-4 The anodization set-up consists of a HP6633A DC power supply capable of reading an output voltage and output current in the range of 0-50V and 0-2A respectively. The virtual instruments programmed using LabView7.0 operated this power supply remotely.

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11 The RM6 Lauda Brinkmann temperature bath was used to maintain the temperature of the anodization cell at 150C. The coolant from the temperature bath circulated into the bath holding the anodization cell. The anodization cell consisted of the acidic electrolyte, the reference electrode, the anode and the cathode. Anode and cathode electrodes used were both aluminum foils obtained from Alfa Aesar. For anodization large cathode to anode surface area was preferred as then the galvanic attack would be concentrated in small areas and the penetration of anode thickness will hasten. From the literature, it is understood that the pore diameter depends on various factors like the concentration of the acidic electrolyte, the temperature at which the experiment is conducted and also the voltage that is applied. 2.4 Chemical reactions involved in porous oxide growth [ 9 ] 2Al + 3H20 = Al2O3 + 6H+ + 6e– at the anode (2) 6H+ + 6e= 3H2 – at the cathode (3) Al2O3 + 6H+ = 2Al3+ + 3H2O – dissolution of alumina (4) Reaction 1 is the sum of reactions that occur at cathode and anode in an acidic electrolyte. At the anode the pure aluminum foil reacts with the aqueous electrolyte (reaction 2) and forms a passive aluminum oxide layer, thus leaving the hydrogen ions in the solution. Depending on the potential applied to the system part of the hydrogen ions travel towards the cathode, take up electrons thus liberating hydrogen gas (reaction 3) and the rest react with the aluminum oxide layer at the surface of anode dissolving alumina into the electrolyte solution (reaction 4). Table 2.1 [4] Optimized parameters for the anodic aluminum oxide template anodization Electrolyte (acid) Concentration Temperature (0C) Voltage (V) Pore diameter (nm) Oxalic Acid 0.3M 10 40 45 Phosphoric acid 1M 0 160 400 Sulphuric acid 0.5M 0 25 30

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12 A balance between these two reactions helps in forming the porous layer in the aluminum. Hence care needs to be taken that the potential applied does not aid in the production of hydrogen gas as opposed to the reaction of hydrogen ions with alumina. The anodization current, which flows due to the electrical circuit established between cathode and aluminum, follows the behavior as shown in Figure 2.7 [9] as a function of time. A B C D JpJbJ = Jp+JbCurrent density, JAnodization time, t Figure 2.7 [9] Anodization current as a function of time. During the first few seconds the net current rapidly decreases until a minimum is reached. The net current, J, is a combination of the current due to the barrier layer, Jb and the current due to the pore formation, Jp. In the first few seconds the current due to the formation of barrier layer dominates and hence the rapid decrease in the net current is observed. Then the current due to the pore formation is more prominent and hence the net current increases and reaches a steady state value. When the steady state is reached the pore structure is stabilized. The regions A, B, C, D in the Figure 2.7 correspond to the various stages in pore growth such as growth of aluminum oxide, development of fine-featured pores, enhanced pore growth and steady state pore structure.

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13 2.5 Barrier and porous type alumina Main structural characteristics of the porous alumina film are as explained in Figure 2.8[10]. Here a section of hexagonal cell of an oxide is depicted where total oxide film of thickness h, the porous layer, the barrier layer, the pore wall oxide, the cell boundaries, the central pore and the pore axis can be seen The diameter of the oxide layer and the barrier layer are indicated as Do and Db respectively. It was found that the pore growth was due to the field assisted hydrogen ion attack on the oxide layer. The non-porous barrier type alumina film formed either in neutral or basic solutions had a PH>5[5]. The pore type film formed in acidic solutions had a thin barrier layer of 10-100nm. Here the pore forming mechanism has been explained in detail. Aluminum ions migrate from the metal across the metal/oxide boundary into the barrier oxide and the oxygen ions formed from water at the oxide/electrolyte interface migrate into barrier layer. This migration of ions occurs because of the large potential drop across the barrier layer at the pore bottoms, due to which the barrier layer acts as a dielectric. D/2 D0/2 Porous layer h Barrier layer x Al2O3Al3+Db/2 Al3+H+Al metal x Figure 2.8 [9] Structural characteristics of porous alumina film This leads to the dissolution of oxidized aluminum during anodization and the pores are hence formed. The ion migration and oxide dissolution at the pore bottom depend strongly on the applied potential. Self-organization of the pores has been attributed to the repulsive interaction between the pores during their growth. These repulsive forces were found to originate due to the

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14 mechanical stresses formed during the volume expansion of aluminum oxidation. The oxidized aluminum doesn’t contribute to further oxide formation, as the aluminum ions are mobile in the oxide due to the electric field applied. Also no oxide formation takes place at the oxide/electrolyte interface during the porous oxide growth. All the ions reaching the interface were ejected into the electrolyte, which has been attributed to the relative variation of the transport numbers of the aluminum and oxygen ions with respect to the voltage and electrolyte composition. 2.6 Influence of various conditions on the formation of porous alumina [ 11 ] From the literature, various experimental conditions like pre-annealing of the substrate, stirring of electrolyte and electropolishing of the sample before the anodization process have been found to affect the formation of porous alumina. These are discussed briefly here as they have been taken into account while performing the experiments in the laboratory to obtain ordered pore arrays of uniform cross section. Pre-annealing of aluminum substrate helps in obtaining an ordered pore array structure. It enhances the grain size in the metal substrate. Grain boundaries in non-annealed aluminum disturb the process of self-organization of pores during the formation period, as the conditions for oxidation are different at the grain boundaries than at the perfectly crystalline areas. Stirring of the electrolyte is found to be a necessary condition for obtaining ordered pores. Concentration of the dissolution products at pore mouths is equal to the concentration in the bulk electrolyte, while stirring, since the electrolyte above and below the alumina surface is continuously exchanged. Also concentration of aluminum at the pore bottom is a function of the production rate of aluminum at the pore bottom as well as the pore depth, since the diffusion rate of the aluminum is determined by the concentration gradient. When the stirrer is switched off, the concentration gradient inside the pores decreases which results in a lower diffusion rate of aluminum and a higher equilibrium concentration at the pore bottom which leads to the formation of non-ordered pores.

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15 Also electropolishing the surface exhibited formation of ordered pores. The high surface rough nesses lead to a faster formation of barrier oxide and pores at depressions of the surface. This surface roughness then transfers to the etching front at the interface between the aluminum and oxide layer hence preventing the pore self-organization. 2.7 Results and discussion Figure 2.9 indicates the change in current as a voltage of 40V is applied onto aluminum (99.997% pure, Alfa Aesar) after electropolishing it for 2-3 minutes at 20V using the Electromet-4 polisher. Figure 2.9 Anodization current vs. time at an applied voltage of 40V It can be observed that there is a steady increase in the current indicating the prominent formation of the porous layer on the surface.

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16 This linear array could be due to the nucleation of the pores on the etching sites of the substrate, as etching dominates polishing (Figure 2.10) Figure 2.10 Electropolished sample surface, at 20V; Etching dominates polishing The sample was subjected to electropolishing at a voltage of 40V. Stirrer is not included in the experimental set-up during this run.

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17 The run was limited only for a short amount of time and only a single step of anodization was performed thus producing a linear array pore structures (Figure 2.11). Figure 2.11 Top view of porous alumina anodized at 40V and 150C (linear array pore structure) The white colored material that is seen in the figure is the oxide material that is present on the sample. Since the sample was not subjected to the oxide dissolution process, it remained on the sample.

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18 A template of average pore diameter 48nm with an interpore distance of 70.5nm (Figure 2.12) is obtained during this run. Figure 2.12 Top view of the sample, which shows an average pore diameter of 48nm and an interpore distance of 70.5 nm It can be observed from Figure 2.10 and Figure 2.12, that the pores preferentially form in the etched sites on the sample and hence the linear arrayed pore formation.

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19 The cross section of this template reveals a pore channel of 36.8nm (Figure 2.13) indicating pore narrowing from the top to the bottom side of the template. Figure 2.13 Cross section of the sample It can be observed that the pore channels that are formed are irregular in shape. This is because of the exposure of the sample to the acid from both sides, as an electrode holder was not used in this run.

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20 This is corroborated by Figure 2.14, which indicates a bottom pore diameter of 34.4nm and irregularly formed pores. Figure 2.14 Bottom view of the anodized alumina obtained using 0.3M oxalic acid at 40V on 200nm scale This template is a through template that requires an electrically conductive layer to be sputtered onto the bottom side to perform the electrodeposition.

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21 Figure 2.15 shows the irregular pores formed on a 2-m scale. The white lines observed in this SEM are due to the oxide formation at the creases present on the sample due to the stresses that occur during the course of experiment. Figure 2.15 Bottom view of the anodized alumina obtained using 0.3M oxalic acid at 40V on 2m scale This method was improvised to make a uniform and stabilized porous template by including multiple runs in the two-step anodization method and also by applying a voltage of 48V for electropolishing so that polishing would dominate the etching effects.

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22 The first anodization step was conducted in multiple runs that are indicated in the current vs. time graph (Figure 2.16). This run was conducted for a period of 6 hours with a new run being initiated approximately every 2 hours. Graph shows both decreases in current as well as increase in current indicating the formation of pores and the formation of barrier layer. Only when these two reactions are balanced, the formation of a uniform porous structure can be obtained. Figure 2.16 Anodization current vs. time at an applied voltage of 40V and electropolishing voltage of 48V, multiple runs–first step of anodization The decrease in current indicates the formation of barrier layer and the increase in current indicates the pore formation. The run has been repeated three times on the same sample to improve the results and then the sample was taken out and treated with chromic acid to dissolve the oxide layer on it.

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23 Then the second step of anodization was performed (Figure 2.17) which indicates the sudden increase in current and then the steady state indicating the pore stabilization. Figure 2.17Anodization current vs. time at an applied voltage of 40V and electropolishing voltage of 48Vsecond step of anodization During this run a stirrer was included too in the anodization set up thus preventing the higher equilibrium concentration of aluminum at the pore bottom and hence facilitating good mixing of the ions. Also the temperature of the bath was maintained at 150C through out the experimental run.

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24 Figure 2.18 shows an improved template with no oxide layer on top of it and also uniform pores that are formed in a hexagonal pattern that could be observed with more clarity in Figure 2.19. Figure 2.18 Top view of porous alumina electropolished at 48V for 30 secs and obtained using two step anodization ;oxalic acid 40V The average pore diameter obtained is 37nm with an increased interpore distance of 86.4nm. The small particles seen in Figure 2.18 are dust particles present on the sample.

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25 Also the distortion effects observed on the pore edges in Figure 2.19 are due to the charging of the sample when it is viewed under the SEM due to the non-conductive nature of porous alumina. Figure 2.19 Top view of the sample at higher magnification with 37nm pore diameter and the interpore distance is 86.4nm (Hexagonal structure) To obtain a uniform cross sectioned template another run was performed with a single run and two-step anodization process.

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26 Figure 2.20 indicates an alternative increase and decrease of current explaining the formation of pores as shown in Figure 2.23. Figure 2.20 Anodization current vs. time at an applied voltage of 40V, single run–first step of anodization In this step the sample was subjected to a single run, which means the voltage was applied continuously through out the time period of the first step of anodization.

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27 The second step of anodization (Figure 2.21) indicates a decrease in current at a steady state indicating the domination of formation of barrier layer on the surface. Figure 2.21 Anodization current vs. time at an applied voltage of 40V-second step anodization Hence the presence of oxide layer could be observed in the sample (Figure 2.23). This is confirmed by the EDAX (Figure 5.3).

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28 A uniform pore channel (Figure 2.22) is observed to be formed though this method of an average diameter of 70nm. Figure 2.22 Cross sectional view of the sample indicating 70nm uniform pore channel Figure 2.23 Top view of the sample indicating pore diameter of 44.5nm A hexagonal structure of pores is not formed in this case (Figure 2.23) and also the pores formed are irregularly shaped.

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29 3 Fabrication and Characterization of Nanoparticles 3.1 Overview Electrodeposition of palladium particles into the nanoporous templates is the concentration of this chapter. Introduction to electrodeposition, factors affecting electrodeposition and various types of electrodeposition are discussed. The results, based on different methods employed to obtain the nanoparticles are discussed. Electrodeposition involves reduction of metals from aqueous, organic and fused salt electrolytes. There are several parameters that affect this process which have been discussed in detail in the following section. 3.2 Synthesis of nanostructured materials using electrodeposition Nanostructured materials are synthesized to achieve certain physical, chemical and mechanical properties. Electrodeposition parameters are bath composition, PH, temperature, over potential, bath additives etc., and important features of the substrate include grain size, crystallographic texture, dislocation density, internal stress etc., When electrodeposition is carried out the formation of new crystals takes place which involves the process of electrocrystallization. Electrocrystallization occurs by build up of existing crystals or the formation of new ones. The major rate-determining steps for nanocrystal formation are charge transfer at the electrode surface and surface diffusion of adsorbed ions on the crystal surface. Figure 3.1 explains the two stages of electrocrystallization according to Bockris, et al. [12] The foremost important factor in the evolution of electrodeposition in terms of grain size and shape is inhibition resulting from reduced surface diffusion of adions by adsorption of foreign species on the growing surface. The second most important factor during the nanocrystal formation is the overpotential. Grain

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30 growth is favored at low overpotential and high surface diffusion rates. High overpotential and low diffusion rates promote the formation of new nuclei. These conditions can be experimentally achieved by using Pulse electrodeposition conditions, where the peak current density can be considerably higher than the limiting current density attained for the same electrolyte during direct current plating. Cation Concentration Cathode Nernst Diffusion layer Convection and migration in bulk CbDeposition stage mass transfer in th e Nernst diffusion layer CsDistance from electrode Figure 3.1 [12] Stages of electrocrystallization Figure 3.2 indicates the generalized pulse current waveform where T is the period of the waveform, in is the current density and tn is the pulse duration. Cathodic i3 t1 t2 T t3 i1i2idc Figure 3.2 [12] Pulse current waveform

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31 While pulsing current seems to be one form of alternating current, (AC) deposition, there is another method of deposition namely, the DC deposition. Direct current (DC) deposition also contained two other techniques namely chronopotentiometric technique and chronoamperometric technique. Potential is monitored with time while applying a standard current in the former, while in the latter current is monitored with time applying standard potential. In case of templates made in the lab the presence of non-conducting barrier layer of alumina between the aluminum and the porous layer is significant and hence DC deposition cannot be used. To remove this layer the bottom layer of aluminum should be removed and the template soaked in phosphoric acid for a certain period of time. The length of time is decided by the thickness of barrier layer and a longer soaking time will result in the dissolution of the porous layer as well. Hence DC deposition was tried out in commercial templates, which have through pores and a thinner barrier layer. 3.3 Experimental set up for DC deposition A potentiostat (model 273A from Princeton applied research) and an electrochemical polarization cell, which consists of a flat specimen holder, were used for the experiment. The polarization cell consists to three-electrode set up required for DC deposition (Figure 3.3). The potentiostat acts as the current source for the electro deposition. Porous alumina membranes are prepared as described previously. The polarization cell used here is the standardized design approved by the ASTM. The working electrode is centrally located in the cell with a pair of auxiliary electrodes on either side for better current distribution. The reference electrode is placed outside the cell, and the potential of the working electrode is measured through the luggin probe and solution bridge with respect to the reference electrode. The reference electrode in this case is the saturated calomel electrode. The potential of working electrode is measured with respect to the reference electrode and the current to the working electrode is measured through the auxiliary/counter electrode. The reference electrode is placed in a separate cell so that it is not polarized. The counter electrode is placed in a separate compartment to prevent the

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32 products formation. Salt bridge and luggin Capillary are provided to prevent the IR drop and also liquid junction potential. The flat specimen holder attaches conveniently to the electrode mounting rod and the electrode holder so that it can be used with the corrosion flask. Figure 3.3 Three-electrode set up for DC deposition The potentiostat acts as a power source (Figure 3.4). The model that is being used here is the 273A front panel from Princeton Applied Research. There are different sections on the panel of the potentiostat. The function of each of the section is briefly explained below. [13] A.SCAN SETUP Section: The keypad in this section lets us define a staircase or a pulse waveform. Setup: Select the parameter by pressing the appropriate pushbutton, key in the desired number, and press ENTER. B.CONTROL Section: This section lets us start, stop, hold, or continue our measurements. The ADVANCE key lets us skip to the next part of our experiment. Other pushbuttons let us select half-cycle, full-cycle, or continuous-cycle measurements. Pressing E/I APPLIED applies a programmed potential or current to the cell. C. CELL Section: To provide an extra margin of safety, both switches in this section must be set to apply the programmed potential or current to the cell.

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33 The CELL ENABLE switch allows us to override computer control of the cell at any time. The CELL ON/OFF button controls a solid state, transient-free switch without the contact bounce of mechanical switches. Figure 3.4 Potentiostat 273A D. INPUT Section: For rapid-scan cyclic voltammetry, connect an external waveform programmer to the BNC connector in this section. E. CURRENT RANGE Section: This section lets us select from among eight current ranges from 1 A to 100 nA full-scale as well as auto ranging. LEDs indicate the option currently selected. F. DISPLAY: A graphic LED display gives us an instant picture of the progress of a measurement. Current and potential overload indicators warn us of any overload conditions. The alphanumeric LCD displays a continuous readout of current, potential, or charge. It also displays help and error messages, and we can use it to view or change experimental settings. G. INTERFACE Section: When the Model 273A is connected to a computer through the GPIB (IEEE-488) interface, the LED indicators in this section let us monitor communications with the computer. The LOCAL pushbutton lets us switch back to entering parameters from the front panel of the Model 273A. H. MODE Section: This section allows us to choose between potentiostatic, galvanostatic, or measure-only modes I. FILTER Section: If high-frequency noise becomes a problem, this section lets us apply

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34 either of two available lowpass filters. J. iR COMPENSATION Section: If we have high uncompensated solution resistance in our cell, this section lets us select either positive feedback or current interrupt iR compensation. The SET IR pushbutton allows adjustment of the positive feedback iR compensation level. K. E MONITOR Section: The BNC connector in this section lets you send an analog voltage to an X-Y recorder. L. OUTPUT Section: The RESET INTEGRAL and SET I OFFSET pushbuttons are used for integrating current. This section also lets us send an analog output to another device. A pushbutton lets us specify the output as current, log current, or coulombs. Figure 3.5 Schematic of the Potentiostat interfaced with PC Figure 3.5 indicates the interfacing of the potentiostat with the PC using a GPIB-PCI card. Also the powersuite software supplied by PAR can be used to remotely control the instrument for power step, power pulse and power CV modules.

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35 3.4 Electrodeposition of nanoparticles in anodic alumina templates The electrodeposition of palladium particles into the porous alumina substrate is done by a systematic procedure as indicated in Figure 3.6. The electrolyte used for the electrodeposition of Palladium nanoparticles is 2mMPd (NO3)2 and 0.1MHClO4. Before deposition the membranes need to be soaked for 1-2 hours at room temperature in 1M nitric acid solution in order to widen the pores. Theory indicates that in case of a chronoamperometric measurement the first stage consists of steady current where electrodeposition of metal into the pores takes place, second stage will be rapid increase in current indicating complete filling up of pores and the third stage will be constant current stage again which is due to the formation of planar and contiguous metallic layer which results from the coalescence of hemispherical caps which would be formed by the over filling up of pores with the nanoparticles. Electrochemical Anodization Electrodeposition Metal layer deposition through pore template Electrodeposition Figure 3.6 Schematic of nanoparticle synthesis Then depending on the technique used either current or potential was applied to the template and the other parameter was monitored with time.

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36 A 500m layer of gold (Figure 3.7) is sputtered on the bottom side of the template to increase its conductivity. Here commercial templates are used for deposition purpose. Figure 3.7 AFM image Au for back electrode, which indicates complete covering of pores thus providing a conductive layer for deposition when commercial templates were used for deposition The two sides of the commercial templates are characterized by shiny and non-shiny surfaces. The shiny surface has regular uniform pores and therefore care has to be taken that the gold is sputtered on the dull side of the template. Figure 3.7 indicates the complete covering of the pores bye the sputtered gold on the nonshiny side of the commercial template.

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37 3.5 DC deposition results and discussion A chronopotentiometric technique (Figure 3.8) has been applied here wherein a standard current of certain amperes is applied to the electrodes and the potential is recorded with time. The graph below indicates the current applied and the potential changes observed in two different runs performed on the same template. Figure 3.8 Chronopotentiometric technique for palladium particles deposition-particle nucleation Here a commercial template was used which had an average pore size of 100nm and a gold layer of ~500 m thickness to serve as back electrode using the sputtering technique. An AFM of the gold back electrode is as indicated in Figure 3.7.

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38 This run was conducted for a period of ~15secs with a current of 0.03A for 2.4secs and a current of 0.07A for a period of 12secs. The runs here were limited to few seconds as the template got destroyed quickly upon applying current to it. Figure 3.9 Chronopotentiometric technique for palladium particles deposition-particle deposition Figure 3.9 indicates the increase in current and also steady state in current indicating the presence of particles inside the pores.

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39 Figure 3.10 shows the top view of the sample where the pores are filled up but it can be observed that the palladium doesn’t coalesce on the sample surface. Figure 3.10 Top view of palladium filled pores It can be observed that the pores in the commercial template are not uniform in shape and size; hence the palladium particle deposition inside these pores is not uniform through out. This can be confirmed by an empty hole that is seen in Figure 3.10.

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40 Figure 3.11 shows the cross section of the sample indicating the presence of the palladium particles inside the pores. Figure 3.11 Palladium particles in pores The particles deposit on the walls of the pores as seen in the cross section. But it is known that the pore walls consist of alumina which is non-conductive. Hence it is not clear as to what contributes to the deposition of the particles on the pore walls.

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41 The EDAX (Figure 3.12) on this sample shows relatively less percentage of palladium on the surface of the substrate. Figure 3.12 EDAX analysis on the template, which confirms the presence of palladium This can be attributed to the short time of deposition, which was due to the technique of chronopotentiometry. When current is pulsed through the template it is observed that the template gets destroyed quickly due to which the run time is limited.

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42 After observing the results of the previous run and analyzing that in cases where ions are involved it is always better to control the potential and observe the changes in current rather than vice-versa, chronoamperometric technique was applied to the next template. Also the time period was extended this time as it was observed that the template did not get easily destroyed when potential was applied to it. SEMs below indicate the palladium electrodeposition results when a voltage of 0.3V was applied between the reference electrode and working electrode in a electrolyte solution of 2.0mM Pd (NO3)2 and 0.1M HClO4 for a period of 1500 secs. Figure 3.13 I vs. t curve for the palladium particles deposition Figure 3.13 shows the scanned on-screen printout of the power step module in the powersuite software. It shows a sudden increase in current followed by the decrease in current and then a steady state. This behavior might be due the low diffusion of the electrolyte at the surface and low overpotential.

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43 It can be observed in Figure 3.14 that the pores are getting clogged at the surface and thus only the solution that is able to reach the electrode surface is participating in the deposition process. Figure 3.14 Surface deposition of palladium Also uniform deposition does not take place on the entire surface of the sample. This can be seen in the above figure, where only a portion of the pores are covered by the palladium crystals.

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44 Figure 3.15 indicates the palladium deposition inside the pores but the entire pore does not participate in the process. Figure 3.15 Cross sectional SEM, which indicates the presence of palladium inside the pores of alumina template According to theory the deposition is a bottom-up process which in ideal case should indicate palladium deposited at the bottom side of the template. Here this has not been observed, in fact a reverse process of deposition at the top side of the template is indicated.

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45 From Figure 3.16 it can be seen that the pore surfaces are clogged and the pore channels are deposited with palladium up to a depth, till the process is inhibited by the blocked surface. Figure 3.16 Palladium deposition onto the walls of porous alumina Figure 3.17 Palladium deposition to a depth of 5m inside the pores Figure 3.17 indicates the depth to which the deposition occurs inside the template. The deposition occurs unto a depth of 5m in a template of 60m thickness.

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46 Also from the SEMs it has been observed that this led to the formation of palladium particles instead of wires on the pore walls of alumina. Figure 3.18 EDAX of the sample that confirms the presence of palladium to a depth of 5m Figure 3.19 EDAX – area scan of the sample to a depth of 5m EDAX (Figure 3.18 and Figure 3.19) confirms the presence of palladium inside the pores and also on the surface.

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47 From the literature review [14], it was found that a portion of potential applied to the template dropped across the barrier layer and hence the thickness of the barrier layer in each pore affects the deposition. This explains the reason for obtaining only particles inside the pores of commercial templates. 3.6 Experimental set up for AC deposition An alternative method of AC deposition is tried to determine if there would be any better results of deposition, with respect to the coverage area inside the template pores. Based on the literature survey nickel is used for this method as nickel ions respond more quickly to the frequency changes in the current than the palladium ions. AC deposition involved the usage of an external waveform generator (which is indicated in the Figure 3.20 as function generator) to supply the necessary pulses of required frequency to the working electrode that contained the porous alumina sample. In this case the electrolyte solution was 150g of NiSO4.6H2O, 22.5g of NiCl2 6H2O and 22.5g of boric acid which formed an acidic solution of 4.5pH. The AC frequency applied was 750Hz for 40minutes and the sine current waveform was used to achieve better results. Potentiostat Function generator e2 i(t) measured E controlled Figure 3.20 [15] Potentiostatic measurement set up The schematic (Figure 3.20) indicates the measurement set up for AC deposition. An oscilloscope was also used to monitor the applied frequency changes.

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48 3.7 AC deposition – results and discussion The SEM below (Figure 3.21) and the corresponding EDAX (Figure 3.22) show that the Ni has deposited only as a film on the surface and also on portions of the porous template and no diffusion has occurred into the template. Figure 3.21 SEM Top view – nickel deposition Figure 3.22 EDAX for the Ni depos ition, which indicates the presence of nickel on the porous alumina surface There are several factors, which influence the deposition of nickel inside the pores like the throwing power, electrical conductivity of the solution and the temperature. The

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49 throwing power is the complex relation between the factors that influence the current distribution and hence the metal distribution. In this case it might be presumed the current density and frequency with which it is applied might have contributed to the nondeposition of nickel into the pores. Also it is found that AC deposition has several disadvantages like maintaining the waveform of the current and frequency on a constant basis. Control of these parameters need to be highly accurate which makes it highly sensitive to conduct the experiment. Also in this case too, the experiment could not be conducted for a longer period as the template got destroyed in the process.

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50 4 Conclusions Based on the experiments conducted and results obtained the following is inferred. In making the nanoporous alumina templates a two-step anodization process that includes a multiple run in the first step is advisable. This results in uniform hexagonally arranged stabilized pore structures. The pore cross section obtained is also uniform during this process. The electropolishing voltage range also plays an important role in determining the pore formation. Care is to be taken that the electropolishing voltage facilitates polishing instead of etching. In case of electrodeposition, DC method of deposition with the chronoamperometric method is preferred. This technique is better for control of formation of particles and also helps in maintaining the stability of the porous template. The AC and chronopotentiometric methods have found to damage the template and also limit the time of deposition that hinders in the filling up of pores. The deposition time depends on the pore aspect ratio and since the aspect ratio is high in the case of templates used here, longer deposition times are required.

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51 5 Future Perspectives 5.1 Device applications This chapter deals with an attempt made to test the palladium particles deposited in template as a resistive sensor. For this purpose aluminum IDTs are evaporated onto the substrate (Figure 5.2) using ebeam evaporation with the help of a silicon mask (Figure 5.1). Figure 5.1 Mask design in AutoCAD for IDT structures comprising of finger pairs of various thickness (courtesy Stefan cular)

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52 The mask is fabricated at the Cornell nanotech facility (courtesy Daron Westly) and it has different finger pair structures of various thicknesses. Figure 5.2 IDTs on the porous alumina template Figure 5.3 EDAX that indicates the presence of palladium on the template Due to physical absorption of hydrogen onto the pore walls of alumina and also due to absorption of hydrogen into the palladium nanoparticles the device was supposed to act as a resistive sensor.

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53 When hydrogen is absorbed, it decreases the conductivity of the material; hence there is an increase in the resistance of the device. But no significant response has been obtained from the device, even though the EDAX (Figure 5.3) indicates the presence of palladium particles inside the pores. Upon close observation, it can be seen from Figure 5.2 that the IDTs on the structures are not uniformly evaporated and the non-continuity of those structures might be one of the reasons for not obtaining a proper device response.

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54 References 1 E.C.Walter, F.Favier and R.M.Penner, “Palladium mesowire arrays for fast hydrogen sensors and hydrogen-actuated switches”, Anal.Chem 74, 1546-1553, (2002). 2 P.S.Fodor, “Study of magnetic nanostructures in hexagonally ordered porous alumina”, PhD dissertation, Wayne State University, UMI dissertation services, Ann Arbor, Michigan (2002). 3 C.C.Koch, “Nanostructured materials, Processing, Properties and Potential applications”, William Andrew Publishing, Norwich, NY, (2002). 4 J.Liang, H.Chik and J.Xu, “Nonlithographic Fabrication of Lateral Superlattices for Nanometric Electromagnetic-Optic Applications”, Invited Paper, IEEE Journal of selected topics in quantum electronics Vol 8, No. 5, September/October, 998-1008, (2002). 5 F.Li, L.Zhang and R.M.Metzger, “On the growth of highly ordered pores in anodized aluminum oxide”, Chem.Mater Vol 10, 2470-2480, (1998). 6 G.E.Thompson, “Porous anodic alumina: fabrication, characterization and applications”, Thin solid films Vol 297, 192-201, (1997). 7 C.Hennesthal, “Anodization of aluminum: New applications for a common technology”, Application report nanowizard, JPK instruments AG, http://www.jpk.com (2003). 8 Buehler Electromet-4 polisher, Buehler Ltd, Lake Bluff, IL 60044. 9 L.Menon, “Synthesis of nanowires using porous alumina”, Edited by S.Bandyopadhyay and H.S.Nalwa, Quantum dots and Nanowires American Scientific Publishers, Chapter four, 141-191, (2003). 10 G.Patermarakis, “Development of a theory for the determination of the composition of the anodizing solution inside the pores during the growth of porous anodic Al2O3 films on aluminium by a transport phenomena analysis”, J.Elec.Anal.Chem Vol 447, 25-41, (1998). 11 O.Jessensky, F.Muller and U.Gosele, J.Elec.Chem.Soc Vol 145, No.11, November, (1998).

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55 12 J.O.M.Bockris and G.A.Razumney, “Fundamental aspects of electrocrystallization”, Plenum Press, NY, (1967). 13 Potentiostat 273A, Ametek Princeton Applied research, Paoli, PA 19301. 14 K.Nielsch, F.Miller, A.Li and U.Gosele, “ Uniform Nickel deposition into ordered alumina pores by pulsed electrodeposition”, Adv.Mater Vol 12, No.8, 582-586, (2000). 15 M.Paunovic and M.Schlesinger, “Fundamentals of electrodeposition”, WileyInterscience Publication, NY 10158, (1998).


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Fabrication of palladium nanoparticles and nanoporous alumina templates
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ABSTRACT: Nanostructured materials have potential technological applications due to their characteristic dimensions. The material performance will depend on the atomic structure, and composition of these materials. This thesis focuses on proposing a reliable method for fabricating nanoporous alumina and palladium nanoparticles inside the templates.Palladium nanoparticles were synthesized in commercial porous alumina templates using electrodeposition. Pores within these nanoporous membranes act as templates for the synthesis of nanostructures of the desired material. Electrodeposition is achieved using a three-terminal set-up and a potentiostat. Different types of deposition techniques were investigated to improve the distribution of the deposit. The nanoparticles were characterized by SEM/EDX for composition. The commercial templates have high aspect ratio, but are not hexagonally ordered. Hence porous alumina was fabricated in the laboratory by anodization of aluminum.
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