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The effects of varying plating variables on the morphology of palladium nanostructures for hydrogen sensing applications
h [electronic resource] /
by Ophir Ortiz.
[Tampa, Fla.] :
University of South Florida,
Thesis (M.S.E.E.)--University of South Florida, 2004.
Includes bibliographical references.
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ABSTRACT: Present state-of-the-art hydrogen sensors are limited by a number of defects such as poisoning effects, slow response, and/or the range of concentrations that can be detected. Thus, hydrogen sensors are currently under investigation. In the search for the ultimate sensor, a variety of materials have been employed as the sensing layer. One of these materials is palladium. Palladium is widely used for hydrogen sensing due to its high selectivity and property of spontaneously absorbing hydrogen. Thin and thick film palladium hydrogen sensors have been reported, as well as palladium nanostructures. Specifically, palladium nanowires for hydrogen sensing have had improved results relative to other types of sensors; these have been reported with a response time down to 75ms and do not suffer from poisoning effects.Additionally, the fabrication of these nanostructures via electrodeposition is simple and cost efficient. For this reason, palladium nanostructures were chosen as the front-end for a novel hydrogen sensor. The nanostructures were to be employed as the sensing front-end of a Surface Acoustic Wave (SAW) sensor. It was theorized that the response time would be vastly improved if these were used as opposed to a thin or thick palladium film due to the decreased hydrogen diffusion distance, which is a result of the structures being one-dimensional. Because it was theorized that the dimensions of the nanostructures play an integral role in the response time to hydrogen, control of the morphology was required. This control was achieved by varying the plating variables in the electrodeposition experiments. The plating variables investigated were deposition potential, time, and counter-electrode area.The dimensions of the resulting nanostructures were measured via Scanning Electron Microscopy (SEM) and correlated to the conditions of the electrodeposition experiments. Nanowires under 40nm were successfully fabricated.
Adviser: Bhansali, Shekhar.
x Electrical Engineering
t USF Electronic Theses and Dissertations.
The Effects Of Varying Plating Variab les On The Morphology Of Palladium Nanostructures For Hydroge n Sensing Applications by Ophir Ortiz A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering Department of Electrical Engineering College of Engineering University of South Florida Major Professor: Shekhar Bhansali, Ph.D. Venkat Bhethanabotla, Ph.D. Andrew Hoff, Ph.D. Arun Kumar, Ph.D. Date of Approval: October 13, 2004 Keywords: electroplating, electrodeposition, gr aphite, template, nanowires, nanoparticles Copyright 2004 Ophir Ortiz
i TABLE OF CONTENTS LIST OF TABLES iii LIST OF FIGURES iv ABSTRACT vii CHAPTER 1 INTRODUCTION 1 1.1 Objective 1 1.2 Overview of Hydrogen Gas Sensors 2 1.2.1 Catalytic Bead Sensors 2 1.2.2 Semiconductor Sensors 3 1.2.3 Electrochemical Sensors 4 1.2.4 Resistive Palladium Alloy Sensors 5 1.2.5 Palladium Mesowire Array (PMA) 6 1.2.6 Surface Acoustic Wave (SAW) Sensors 8 1.3 Standards for Hydrogen Sensors 9 1.4 Summary 9 CHAPTER 2 NANOSTRUCTURE SYNTHESIS OVERVIEW 11 2.1 Introduction 11 2.2 Overview of Nanostructure Synthesis 11 2.2.1 Dip-Pen Nanolithography 12 2.2.2 Alumina Oxide Template 13 2.2.3 Vapor Liquid Solid (VLS) 15 2.2.4 Molecular Beam Epitaxy 17 2.2.5 Electrodeposition on HOPG 18 CHAPTER 3 ELECTRODEPOSITION OVERVIEW 19 3.1 Introduction 19 3.2 Reduction of Palladium 19 3.3 Standard Electrode Potential 21 3.4 Cyclic Voltammetry 22 3.5 Nernst Equation 23 3.6 FaradayÂ’s Law 24 3.7 Mass Transfer Mechanism of Palla dium Electrodeposition on HOPG 25 CHAPTER 4 EXPERIMENTAL 27 4.1 Electrochemical Setup 27 4.2 Experimental 30
ii 4.2.1 General Process for Nanostructure Synthesis 31 CHAPTER 5 RESULTS AND DISCUSSION 33 5.1 Introduction 33 5.2 Results 34 5.3 DiscussionEffect of Increasing Time 34 5.3.1 Nanostructure Morphology at 300s 34 5.3.2 Nanostructure Morphology for 400s 36 5.3.3 Nanostructure Morphology at 500s 38 5.3.4 Nanostructure Morphology at 600s 40 5.4 DiscussionEffect of Increasing Potential 42 5.4.1 Nanostructure Morpho logy at 0.29V vs. SCE 45 5.4.2 Nanostructure Morpho logy at 0.33V vs. SCE 46 5.4.3 Nanostructure Morpho logy at 0.37V vs. SCE 48 5.5 Effect of Counter-Electrode Area 49 5.6 Comparison of Results to Current Literature 50 5.6.1 Current Density 50 5.6.2 Nucleation Phenomena at Inhomogeneities 52 5.6.3 Lateral Growth 54 5.6.4 Width of Nanostructures 54 CHAPTER 6 TESTING 57 6.1 Palladium Nanostructures on SAW Sensor 57 6.1.1 Transferring of Nanostructures 57 6.1.2 Testing of Palladium Nanos tructures on SAW Device 58 6.2 Palladium-plated Porous Silicon 59 6.2.1 Preliminary Experiments 60 6.2.2 Sample 1Aluminum Contacts 60 6.2.3 Sample 2Aluminum Contacts 62 6.2.4 Sample 3Gold Contacts 64 6.2.5 Discussion 65 CHAPTER 7 CONCLUSIONS AND FUTURE WORK 68 7.1 Conclusions 68 7.2 Future Applications of Palladium Nanostructures for Hydrogen Sensing 69 APPENDICES 74 Appendix A: Properties of Palladium 75 Appendix B: Process Steps for Experiments 76 Appendix C: Examples of Current vs. Time Plots from Electrodepositions 79
iii LIST OF TABLES Table 5.1: Description of Experiments fo r Nanostructure Synthesis. 33 Table 5.2: Results of Nanostructure Synthesis Experiments. 34 Table 6.1: Sample 1Response to Hydrogen Gas. 61 Table 6.2: Sample 2Response to Hydrogen Gas. 63 Table 6.3: Sample 3Response to Hydrogen Gas. 65
iv LIST OF FIGURES Figure 1.1: DelphianÂ’s Catalytic Bead Sensor. 3 Figure 1.2: Cross Section of MOS Schottky Diode Hydrogen Sensor. 4 Figure 1.3: Electrochemcial Sensor (DrgerSensor). 5 Figure 1.4: Chemiresistor with Palladium Film as Sensing Layer. 6 Figure 1.5: SEM Image of PMA Sensor. 7 Figure 1.6: Wave Propagation in Surface Acous tic Wave Device. 8 Figure 2.1: Dip-Pen Nanolithography. 12 Figure 2.2: Nanowires Measuring 10nm Formed via Dip-Pen Lithography. 13 Figure 2.3: Alumina Oxide Template Nanowir e Synthesis Method. 14 Figure 2.4: SEM of Nanowires Synthesi zed via Alumina Oxide Template. 15 Figure 2.5: Nanowire Synthesis via VLS Growth. 16 Figure 2.6: SEM of Si Nanowires from VLS Growth. 16 Figure 2.7: Si NWs Measuring 20-40n m in Diameter Synthesized using MBE. 17 Figure 3.1: Schematic of a Three-Electrode Setup. 21 Figure 3.2: Potential vs. Time Sweep Utilized in Cyclic Voltammetry. 22 Figure 3.3: General Cyclic Voltammogram. 23 Figure 3.4: HOPG. 26
v Figure 3.5: Before and After Cleaving. 26 Figure 3.6: Step-Edge Transfer Mechanism. 26 Figure 4.1: Working Electrode-Flat Sp ecimen Holder. 28 Figure 4.2: Flat Specimen Holder with Mach ined O-ring (Teflon). 29 Figure 4.3: Photograph of Electrochemical Setup. 30 Figure 5.1: SEM of Nanostructures Electrodeposited for 300s (scale: 2.0 0m). 35 Figure 5.2: SEM of Nanostructures Electrodeposited for 300s(scale: 500nm). 35 Figure 5.3: SEM of Nanostructures Electrodeposited for 400s (scale: 500nm). 36 Figure 5.4: SEM of Nanostructures Electrodeposited for 400s (scale: 10.0m). 37 Figure 5.5: 3-D Image of Nanostructures El ectrodeposited for 400s (scale: 500nm). 38 Figure 5.6: SEM of Nanostructures El ectrodeposited for 500s (scale: 30m) 39 Figure 5.7: SEM of Nanostructures Electrodeposited for 500s (scale: 1.0 0m). 39 Figure 5.8: 3-D Image of Nanostructures Electr odeposited for 500s (scale: 500nm). 40 Figure 5.9: SEM of Nanostructures Electrodeposited for 600s (scale: 3.00m). 41 Figure 5.10: SEM of Nanostructures Electrodeposited for 600s (scale: 300nm). 41 Figure 5.11: Width (nm) of Nanowires vs. Deposition Time (s). 42 Figure 5.12: Cyclic Voltammogram of HOPG in Palladium Nitrate Solution. 44 Figure 5.13: SEM of Plated HOPG at 0.29V (scale: 10m). 46 Figure 5.14: SEM of Nanoparticles Synthe sized at 0.29V (scale: 100nm). 46 Figure 5.15: SEM Image of Nanowires El ectrodeposited at 0.33V vs. SCE. 47 Figure 5.16: SEM Image of Nanowires El ectrodeposited at 0.33V vs. SCE. 47 Figure 5.17: SEM of Nanostructures at 0.37V vs. SCE (scale: 500nm). 48
vi Figure 5.18: SEM of Nanostructures at 0.37V vs. SCE (scale: 200nm). 49 Figure 5.19: SEM of Nanostructures Electrodeposite d for 400s, wire CE (~36nm). 50 Figure 5.20: SEM of Nanostructures Electrodeposite d for 400s, foil CE (~90nm). 50 Figure 5.21: Pd Nanoparticles Electrodeposited on Etched Alum inum. 52 Figure 5.22: Pd Nanostructures Grown at E= -.100V. 54 Figure 5.23: Plot of Reported Diameter of Pd NWs on HOPG. 55 Figure 6.1: Palladium Nanowires Embedded in Cyanoacrylate Film. 57 Figure 6.2: Pd Deposited Between the IDTs. 59 Figure 6.3: Response of Pd-Plated P.S. with Al Contacts (10%H/N). 61 Figure 6.4: Response of Pd-Plated P.S. with Al Contacts (10% H/N). 63 Figure 6.5: Response of Pd-plated P.S. with Au Contacts (10% H/N). 64 Figure 7.1: Preferential Growth at 90 Step Edges. 69 Figure 7.2: Pd-Plated Po Si. 70 Figure 7.3: EDAX of Pd-Plated Po Si. 70 Figure C.1: Current (A) vs. Elapse d Time (s) from a 400s Deposition. 79 Figure C.2: Current (A) vs. Elapsed Ti me (s) Graph from a 600s Deposition. 80
vii THE EFFECTS OF VARYING PL ATING VARIABLES ON THE MORPHOLOGY OF PALLADIUM NANOSTRUCTURES FOR HYDROGEN SENSING APPLICATIONS Ophir Ortiz ABSTRACT Present state-of-the-art hydrogen sensor s are limited by a number of defects such as poisoning effects, slow re sponse, and/or the range of concentrations that can be detected. Thus, hydrogen sensors are currently under investigation. In the search for the ultimate sensor, a variety of materials have been employed as the sensing layer. One of these materials is palladium. Palladium is widely used for hydrogen sensing due to its high selectivity and property of spontaneously absorbing hydrogen. Thin and thick film palladium hydrogen sensors have been repor ted, as well as palladium nanostructures. Specifically, palladium nanowires for hydr ogen sensing have had improved results relative to other types of sens ors; these have been reported with a response time down to 75ms and do not suffer from poisoning effects. Additionally, the fabrication of these nanostructures via electrodeposition is simp le and cost efficient. For this reason, palladium nanostructures were chosen as th e front-end for a novel hydrogen sensor.
viii The nanostructures were to be employed as the sensing front-end of a Surface Acoustic Wave (SAW) sensor. It was theorized that the response time would be vastly improved if these were used as opposed to a thin or thick palladium film due to the decreased hydrogen diffusion distance, which is a result of the structures being onedimensional. Because it was theorized that th e dimensions of the nanostructures play an integral role in the response time to hydrogen, control of the morphology was required. This control was achieved by varying the pl ating variables in the electrodeposition experiments. The plating variables investig ated were deposition potential, time, and counter-electrode area. The dimensions of th e resulting nanostructures were measured via Scanning Electron Microscopy (SEM) and correlated to the conditions of the electrodeposition experiments. Nanowires under 40nm were successfully fabricated.
1 CHAPTER 1 INTRODUCTION 1.1 Objective Hydrogen (H2) is a clear and odorless gas that is explosive at levels above 4.7% . The use of this gas requires the employme nt of fast and reliable gas sensors selective to hydrogen. The need for such a sensor is in creasing, as this gas is considered a possible alternative to fossil fuel. Up to date, no hydroge n sensor exists that is fast or reliable enough for the widespread use of this gas. Therefore, hydrogen gas sensing technology must be improved. In the search for th e ultimate hydrogen gas sensor, palladium nanostructures were investigated in this study as possible frontends for hydrogen gas sensors. Palladium was chosen due to its property of absorbing hydrogen . Palladium in the form of nanowires was chosen due to a reported fast response, wh ich is a result of a Â“break junctionÂ” mechanism that causes the re sistance of the nanowires to decrease when exposed to hydrogen . Due to this mech anism, these nanostructures were initially going to be implemented as the sensing laye r in a Surface Acoustic Wave (SAW) sensor. It was theorized that a decrease in width of the structures wo uld allow a faster response to hydrogen. Therefore, the effects of pla ting variables on the morphology of the nanostructures were investigated in order to have the ability to synthesize continuous
2 nanowires that were as thin as possible. The plating variables investigated were potential, time, and counter-electrode area. 1.2 Overview of Hydrogen Gas Sensors Palladium nanostructures were chosen as the front end for a hydrogen sensor. This was done after a careful review of stat e-of-the-art hydrogen sensors. These sensors are based on six main technologies: cataly tic bead sensors, semiconductor sensors, electrochemical sensors, resi stive palladium alloy sensor s, and palladium mesowire arrays. Their sensing mechanisms, as well as their advantages and disadvantages, are explained in the following subsections. 1.2.1 Catalytic Bead Sensors Applications: Leak Detection, Explosive Limit Detection The concept of this class of sensors (a lso known as pellistors) is based on the catalytic oxidation of hydrogen gas at the surf ace of a bead containing an electricallyheated platinum wire filament . The sensor works at elevated temperatures and detects gas concentrations ranging from .05-5% by m onitoring changes in the wire resistance resulting from temperature increases pr oduced by combustion. Although the response time varies up to a few seconds, pellistors ha ve become commercially successful due to their robustness and simp licity of fabrication. Figure 1.1 is a photograph of a catalytic bead sensor . This sensor is comprised of two beads arranged as a Wheatstone Bridge. One bead contains the catalyst (palladium) while the other bead serves as the reference. The active bead (with the catalyst) is held at 500C to ensure co mbustion of hydrogen molecules. Once the
3 hydrogen molecules combust, the resistance in the Wheatstone Bridge rises. This is due to the oxidation reaction of the hydrogen at the catalytic bead surface. Therefore, resistance values have been correlated to specific concentrations of hydrogen. Though stable, these sensors suffer from poisoning effects from lead, phosphorus and silicon containing vapors, and high power consumption . Figure 1.1: DelphianÂ’s Catalytic Bead Sensor . 1.2.2 Semiconductor Sensors Application: Leak Detection Metal-Oxide Semiconductors (MOS) that utilize platinum or palladium as a catalyst function as hydrogen detectors. Thei r transient and steady state responses have been studied with hydrogen concentrations . In order to operate, these sensors are first heated to temperatures ranging from 300 to 400 C by a heating coil. Upon exposure to hydrogen, these molecules become adsorbed ont o the surface and the semiconductivity of the device changes. MOS sensors have been re ported that can detect up to 1% H in air Pellistor #1 Pellistor #2
4 . Although commercially available with an ac ceptable lifetime, characteristics of this type of sensor include high operating te mperatures and poisoning effects from oxygen. Figure 1.2: Cross-Section of MOS Sc hottky Diode Hydrogen Sensor . This sensor functions in hydroge n concentrations up to 1% H2 in air. It was found to detect .05% at 300K and is able to detect c oncentrations of H in ai r at temperatures of up to 600K. Its lower limit allows this sensor to be used as a leak detector, but is limited by its high operating temperature and sl ow response time (up to 10s) . 1.2.3 Electrochemical Sensors Application: Leak Detection Electrochemical sensors consist of an elec trolytic material be tween two sides; one side contains the working electrode and the other has the reference and counterelectrodes. Hydrogen gas diffuses through a porous membrane electrode (working electrode), which produces ions from th e electrochemical reaction of the hydrogen molecules at this surface . A current propor tional to gas concentrations is created as the positive ions flow to the cathode and th e negative ions flow to the anode. This reaction is reversible . These sensors have been found to have high sensitivity,
5 selectivity to hydrogen, and low power cons umption. A major disadvantage is a slow response time, ranging from a few seconds to >2 0s for a full scale response . Regular calibration is also required. Figure 1.3: Electrochemcial Sens or (DrgerSensor) . The sensor in Figure 2.3 detects hydroge n by measuring the electrochemical reaction that occurs when exposed. The two reactions that can occur at the working electrode are reduction (gain electrons) or oxidation (lose electrons); in the case of hydrogen exposure only oxidation occurs. A thre e-electrode system is used, and the oxidation reaction is measured at the porous measuring electrode (w orking electrode). This type of sensor is temp erature and pressure dependent and can measure down to 1% hydrogen in air . 1.2.4 Resistive Palladium Alloy Sensors Application: Leak detection Literature indicates that Pd/Ni films ha ve been employed as the sensing element in chemiresistors . The chemiresistor ope rates through a change in resistance caused by hydrogen atoms diffusing into the thin film . When hydrogen atoms come into
6 contact with the Pd/Ni surface, the diatom ic hydrogen catalytically reacts and forms monoatomic hydrogen . The monoatomic hydroge n then occupies in terstitial sites in the lattice Pd, which induces increased scattering of electrons in the metal . This leads to a change in resistance, which in turn i ndicates increased hydroge n concentration. This type of sensor can detect from 0.5 to 100% th eoretically, but is poisoned by gases such as CO and H2S. Above 10% hydrogen, because of the lattice volume increase of the palladium film, the film becomes Â“mechanically unstable,Â” which coul d lead to incorrect responses . Figure 1.4: Chemiresistor with Pallad ium Film as Sensing Layer . 1.2.5 Palladium Mesowire Array (PMA) Application: Leak Detection Arrays of Pd mesoscopic wires have been reported that respond to hydrogen . These wires were electrodeposited onto HOP G, then transferred onto a degreased glass slide. It was required that (at least) ten wires parallel to each other be located in order to have a hydrogen sensor. Silver conductive pa ste was then applied, and the wires were measured for conductance variations with changes in hydrogen concentrations. These H H
7 wire arrays were found to have very low response times, about 75 milliseconds, for hydrogen concentrations ranging from 2-10% . The operation mechanism of these wires differs from other mechanisms. In the majority of sensors that work upon resistance variations, such as in chemiresistors, the resistance increases. The PMA, on the other hand, decreases in resistance. This phenomenon is due to the closing of nanoscopic gaps that are formed from an init ial air-hydrogen cycle . Figure 1.5: SEM Image of PMA Sensor . Although the Palladium Mesowire Array (P MA) has had promising results, the transferring of these wires is problematic. As aforementione d, the wires were transferred onto a degreased slide. Super glue was applied onto the HOPG following the electrodeposition, and th e slide placed on the HOPG surface. Once hardened, the glass slide was peeled off, which transferred wires from the HOPG onto the slide . The slide was then examined under an optical micr oscope for areas with parallel nanowires
8 However, locating nanowires under an optical mi croscope is a painstaking task due to the size of the wires (40-200nm). 1.2.6 Surface Acoustic Wave (SAW) Sensors SAW sensors employ piezoelectric material (e.g. lithium niobate) as the substrate, which allows a surface acoustic wave to propagate between two Interdigitated Transducers (IDTs) . The piezoelectri c material generates acoustic wave on application of ac potential th rough the IDTs (Figure 1.6)  .The characteristics of the acoustic wave are controlled by the geometry of the IDTs.. Changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave . Surface wave velocity variations that result from the absorption of a specific gas species, such as hydrogen, can be detected as either phase shif ts or as frequency shifts . A layer selective to hydrogen can be added in order to make it sensitive to this gas . Figure 1.6: Wave Propagation in Su rface Acoustic Wave Device .
9 1.3 Standards for Hydrogen Sensors As a result of a lack of national standards for hydrogen sensors, certain institutions have come up with their ow n standards; one such institution is the Department of Energy (DOE). The following is a set of criteria for hydrogen sensors developed by the DOE : Detection range .1-10% Response time under 1 sec H2 specific Operating temperature range -30 to 80C Lifetime greater than 5-10 years Reliable and rugged Cost effective Unfortunately, none of the aforementioned hydrogen sensors work according to these specifications. This is why hydroge n sensors are still under investigation. 1.4 Summary The palladium nanostructures fabricated as part of this work were synthesized using electrodeposition on a specific type of graphite called Highly Oriented Pyrolitic Graphite (HOPG), which is one of various types of synthe sis methods discussed in the following chapter. Chapter 3 is an overvi ew of electroplating, and describes the fundamental laws that govern this electroc hemical process. The experimental setup is described in Chapter 4, followed by the resu lts of the electrodeposition experiments
10 (Chapter 5). The methods attempted to test these nanostructures are described in Chapter 6. Chapter 7 states the conclusions and future work.
11 CHAPTER 2 NANOSTRUCTURE SYNTHESIS OVERVIEW 2.1 Introduction The nanostructures investigated in this wo rk were fabricated via electrodeposition on a Highly Oriented Pyrolitic Graphite (HOP G) surface. HOPG is a form of graphite that serves as a template for nanowire growth. Electrodeposition of palladium on HOPG was chosen over the various existing methods of nanostructure synthesis such as alumina oxide templates, Vapor Solid Growth (VLS), and laser ablation due to simplicity and low cost. An overview of nanowire synthesis me thods is discussed in the next section. 2.2 Overview of Nanostructure Synthesis The two most general options for nano structure synthesis are the top-down approach and bottom-up approach . The top-down approach includes methods such as dip-pen nanolithography and plasma assi sted dry etching. The bottom-up approach includes methods that utilize direct chemical synthesis . Nanofabrication techniques that use this approach are alumina oxide templates, Vapor Liquid Solid (VLS) growth, and what is known as step-edge decorati on on Highly Oriented Pyrolitic Graphite (HOPG). The latter utilizes a form of graphite as a template. The bottom-up approach is the more popular method due to simplicity and cost relative to the top-down.
12 2.2.1 Dip-Pen Nanolithography Electrochemical dip-pen lit hography utilizes an Atomic Force Microscope (AFM) to transfer material from the AFM tip to a surface . This method is able to create nanowires down to 1nm (Figure 2.2) . In order to fabri cate nanowires, the AFM tip is dipped in a solution containing the desired mole cule (monomer). The tip is then dried, and can then be used to Â“writeÂ” on the de sired surface . The surface on which the patterning is performed has a native oxide layer. A voltage is applied, and the tip is scanned over the surface. The tip does not come into direct contact with the surface in order to transfer molecules from the AFM tip to the surface; the transfer occurs via a Â“meniscusÂ” that naturall y forms (Figure 2.1) . Figure 2.1: Dip-Pen Nanolithography, Mirkin Group, Northeastern University .
13 Figure 2.2: Nanowires Measuring 10nm Formed via Dip-Pen Lithography . An advantage to the use of this method is it does not require a lithography step to pattern a substrate; the writi ng and patterning are all done in one step . Also, various materials can be used to Â“write;Â” metals semiconductors, polymers, and biomolecules . The results of this method are nanos tructures with well-defined geometries. A disadvantage is this method is the sp eed, as it is quite slow . 2.2.2 Alumina Oxide Template Alumina Oxide (AAO) templates are self-o rdered arrays composed of nano or micro sized pores that can be filled to fabricate nanostructures . The general fabrication procedure is to take a desired area of Aluminum film and apply a potential (anodize) in a bath. Anodization entails the application of a voltage to induce a chemical reaction from Al to Al2O3. This reaction forms pores in the Al2O3 film. The desired material (metal, semiconductor, etc.) can then be deposited into these pores (after step
14 three according to figure 2.3). The Al2O3 is then etched away to free the nanowires from the template. An illustration of this process is as follows : 1Start high puritly Al film: -V +V 2Anodize (Apply V) 3Pores are formed in Aluminum Oxide layer through anodization Figure 2.3: Alumina Oxide Templa te Nanowire Synthesis Method. In order to achieve highly ordered pores, extra steps that precede the general process given above must to be taken. The Al film has to be Â“preannealed to remove mechanical stress and enhance the grain size .Â” Next, the film has to be electropolished in order to make it as planar as possible. Once the template is fabricated, nanowires are formed by depositing the desire d material into the pores of the template. Al Al film Counter electrode Al2O3 Al Pores
15 This can be achieved by various depo sition techniques, two of which are electrodeposition and chemical vapor deposition. The diameter of the nanowire is dependent on pore size . Nanowires fabricat ed using this type of template range from about 5 nm to several hundred nm . Figure 2.4: SEM of Nanowires Synthesized via Alumina Oxide Template . 2.2.3 Vapor Liquid Solid (VLS) Vapor-liquid-solid growth is another bottom-up method that is utilized for nanowire synthesis. Th is method involves the employmen t of a catalyst, usually gold (Au) as a seed . The gold catalyst is deposited on a substr ate such as Si, and heated (one method uses 900C). A vapor containi ng the desired metal or semiconductor is introduced into the chamber containing the Au catalysts. This vapor forms an alloy with the Au, and after some time this alloy beco mes supersaturated. Nanowire growth begins under the catalyst in the liquid state as a result of precipitation. Lastly, the nanowire becomes a solid . The diameter of the na nowire equals the diameter of the cluster . Cluster size is controlled by the pressure of the vapor. Nanowires measuring tens of
16 nanometers have been reported using this me thod . The basic steps are illustrated below: Start bare Silicon (Si) Deposit Au clusters Vapor introduced (precursor) Epitaxial growth at liquid solid interface Figure 2.5: Nanowire Synthesi s via VLS Growth . Figure 2.6: SEM of Si Nanowir es from VLS Growth . Liquid-solid interface
17 2.2.4 Molecular Beam Epitaxy MBE is a top down method for nanowire synthesis that has been widely investigated due to the high quality of depos ition. It deposits materi al on a substrate by employing thermal energy beams of atoms or mo lecules onto a crystalline substrate . This takes place under ultra high vacuum conditions (total pressure <10-10 Torr). One benefit of this method is the thickness of the deposit can be controlled to within one monolayer . Nanowires measuring tens of nanometers in diameter have been fabricated with this method. Silicon (Si) nanowir es fabricated using Titanium (Ti) islands formed by MBE (see figure 1.4) were re ported . A molecular beam Si2H6 was directed at the Si substrate for this to occur. Figure 2.7: Si NWs Measuring 20-40nm in Diameter Synthesized using MBE .
18 2.2.5 Electrodeposition on HOPG Electrodeposition on a particular form of graphite known as Highly Oriented Pyrolitic Graphite (HOPG), is another exam ple of a bottom-up approach. The deposition, or Â“step-edge decorationÂ” of the desired metal onto the HOPG template is achieved through electrochemical synthesis. This synt hesis is performed using a three electrode system, which is discussed in detail in Chap ter 3. Basically, the HOPG surface is held at a negative potential relative to a counter elec trode, which is held at a positive potential. The beaker contains an elec troplating solution composed of ions of the metal to be deposited (also known as electrolytic solution). Once biased, the ions are reduced at the graphite surface. The metal ions are deposited on the steps (defects) of the graphite surface . The length of the nanowires is dependent on the step length of the HOPG, while diameter range is controlled by the plating variables The benefits of this method are it is simple and low-cost. For instance, one HOPG template can be reused up to ~40 times. Additionally, this process does not re quire extraordinary conditions such as ultrahigh vacuum, as required by MBE, or hi gh temperatures, as required byVLS. Due to these benefits, this method was chosen for palladium nanostructure synthesis.
19 CHAPTER 3 ELECTRODEPOSITION OVERVIEW 3.1 Introduction Electrodeposition is interchang eable with the term electro plating. In general, this is a process utilized to coat a surface with a material, usua lly a metal. This process is driven by a current that causes a metal in solu tion to become Â“reducedÂ” (gains electrons). This metal is attracted to the surface to be coated (or decorated) and becomes reduced, causing deposition of metallic ions. The thic kness of this coat can be controlled by varying certain experimental conditions, such as deposit ion time and potential. This process, as well as the basic laws that govern it, will be further discussed in the following sections. 3.2 Reduction of Palladium The palladium (Pd) is contained in an elec trolytic (containing ions) solution. In this solution, the charge of Pd is 2+. A current can cause the palladium to gain electrons, and the 2+ becomes reduced to 0 . The reaction of this process is as follows: Pd2+ (aq) + 2ePd (s) The above reaction states that the ionic palladium is initially contained in an aqueous solution. When the palladium gain s two electrons, the palladium becomes a solid. This reaction occurs on the surface of the working electrode. The mechanism that
20 drives the palladium to become attracted to the surface to be plated is achieved by the application of a negative potenti al to the working electrode. The surface to be plated is known as th e Working Electrode (WE). Besides this electrode, one or two additional electrodes are included to achieve electrodeposition. These electrodes, coupled with the employment of an electrochemical cell, are part of the electrochemical setup. The cell contains the electrolytic so lution, which is necessary for the conductance of a signal. Th e solution is an ionic conduc tive phase between the metal electrode (solid) and the metal in the soluti on (aq) . This signal originates as an electrical signal from a potentiostat, then is transformed into a chemical signal. This chemical signal is generated by a chemical reaction (described below) and is carried by the ions in solution. It was mentioned that two different setups can be utilized fo r electroplating. The difference between a two-electrode and a thr ee-electrode setup is the control of the experiments that can be achieved. With a two electrode setup, there is a cathode and an anode. The negative potential is applied at the cathode (WE). The anode, also referred to as the counter electrode, balances the reacti on that occurs at th e cathode. This is in accordance with the law of c onservation of mass and char ge. The opposite reaction is called oxidation and occurs at the anode. In general X + eReduction XeOxidation
21 where X is a particle or a substance . Thus, a redox reaction can occur with only two electrodes. In a three electode setup there is an additional reference electrode (see Figure 3.1). It is called a Â“referenceÂ” because it is non-polarizable, thus the voltage does not change with a small current . The potenti al applied between th is electrode and the working electrode is controlled by an el ectronic device known as a potentiostat. A potentiostat, in general, uses a voltage as the cont rolled variable, while the current is the measured variable. This current is measured between the counter-electrode and working electrode. Figure 3.1: Schematic of a Three-Electrode Setup. 3.3 Standard Electrode Potential It has been universally agreed that the el ectrode to which all others are compared is the Standard Hydrogen Electrode (SHE) [2 3]. This electrode has the reaction of the simplest element (hydrogen). The standard el ectrode potential (the potential at which reduction will occur) for this type of referen ce elctrode has been chosen as 0V . Reference Counter Working
22 However, its employment is impractical in ma ny instances because it reacts with various substances. Other types of electrodes such as the Ag/AgCl and Saturated Calomel Electrode (SCE) are alternatively used and thei r potentials are taken with respect to the SHE. The standard electrode potential, E0, of AgCl/Ag is +.2221 vs SHE, while that of the SCE is +.242 vs. SHE . It is important to specify the standard electrode potential of the reference electrode employed sin ce this affects the measured potential. 3.4 Cyclic Voltammetry In an electrolytic solutio n, a deposition potential can cause a reduction of a metal to occur. The potentials for this to occur ar e gathered from experiments known as Cyclic Voltammograms (CVs). Cyclic voltammetry is a tool used for the characterization of electrochemical systems. The term Â“volta mmetryÂ” indicates the variables that are graphed; Â“volt-Â” is for voltage, and Â“am-Â” for current . In a CV experiment, a potential is applied to the wo rking electrode. This potential is applied at a gradual rate, and is applied in forward and reverse scans. The basic shape of the applied potential vs. time is as follows (Figure 3.2): Figure 3.2: Potential vs. Time Sweep Utilized in Cyclic Voltammetry. Potential Time
23 There exist two standards for graphing CVsIUPAC and American. The IUPAC convention has the reduction (cathodic) cu rrent negative, and the oxidation (anodic) current positive. The American standard is th e oppositecathodic is positive and anodic is negative . An example of an IUPAC c onvention CV is given below (Figure 3.3): Figure 3.3: General Cyclic Voltammogram. According to Figure 3.3, there are two p eaks (cathodic, anodic). The peaks are a response to a reaction. These two parameters st ate that at these pot entials a reduction or oxidation reaction occurs. The importance of th ese parameters is the desired reaction can be obtained by analyzing the cy clic voltammogram. For exampl e, if the potential where the reduction current peak is obs erved is utilized as the depo sition potential, reduction of the metal will occur. 3.5 Nernst Equation To review, both oxidation and reduction occur in electrodeposition. A system is deemed as reversible in cases where the ki netics of electron transf er is rapid in both Cathodic peak Anodic peak Reduction Oxidation E(V) I(A)
24 directions . A reversible system contai ns a concentration of reduced and oxidized species according to the Nernst equation : E = E0 + RT ln a(Mz+) (3-1) zF where E: potential E0: relative standard electrode potential (reversible potential) R,T,z,F: gas constant, absolute temperature, number of electrons invol ved in the reaction, and FaradayÂ’s constant (96,500 coulombs) a(Mz+): activity of the ion, in moles per liter This equation states that the application of a potential (E) to an electrochemical system will cause a change in the reduced/oxidized spec ies ratio that exists in the solution . 3.6 FaradayÂ’s Law FaradayÂ’s Law states that th e weight of a product of electro lysis, w, is equal to the amount of electrochemical reaction at an el ectrode (Z) multiplied by the quantity of charge (Q) passed thr ough the cell . This yields the formula w = Z*Q (3-2) where Z is the electrochemical equivalent. Th e charge, Q, is also dependent on current (Amps) and time (seconds): Q = It (3-3) w = ZIt (3-4) The Faraday constant, F, represents one mo le of electrons and e is the charge of one electron (1.602 x 10-19 coulombs, C)
25 F = NA e (3-5) where NA is AvogadroÂ’s number (6.0225 x 1023 molecules mol-1) F = (6.0225 x 1023)(1.6021 x 10-19) = 96,487C mol-1 (3-6) The weight equivalent corre sponds to that fraction of a molar unit of reaction that corresponds to the transfer of one electron . This is equal to the atomic weight of the material (Awt) divided by the number of electro ns required for reduction (n, for Pd2+, this number is 2) and the Faraday constant (F). Finally, the amount of material reduced (in grams) at the electrode surface can be calculated from w = Awt /nF Q (3-7) 3.7 Mass Transfer Mechanism of Palla dium Electrodeposition on HOPG Highly Oriented Pyrolitic Graphite (HOPG) is a sp ecial form of high purity carbon which provides a renewable surface for app lications as a template in nanostructure depositions (see Figure 3.4) . This material is chemically in ert, which is beneficial for deposition experiments because it will not inte rfere with the other chemicals involved in the setup. Hence, it is often used as the working electrode. HOPG, in its initial state, is smooth. It is transformed into a template by a simple process known as Â“cleavingÂ”. To cleave an HOP G block, all that is required is scotch tape. The tape is applied to the blockÂ’s surface and then peeled off. This causes a thin layer to be removed from the block, which produces Â“stepsÂ”, or incomplete graphite layers on the HOPG surface (see Figure 3.6) . These steps are arranged in a parallel fashion and some span the length of the grain. They are separated between 500-5000 nm
26 on the surface . A grain is an area on the HOPG in which the steps are all aligned in the same direction. A single grain can span millimeters in length and contain hundreds of step edges . Figure 3.4: HOPG . Figure 3.5: Before and After Cleaving. Metal ions can be reduced at the HOPG to form nanostructures. This occurs through a mass transfer mechanism in which palladium becomes deposited on the surface defects of the HOPG . Ideal surfaces are assumed as exhibiting no surface defects such as vacancies or dislocations. Real surfaces ha ve such defects, as in the case of HOPG. The metal ions are transferred into the meta l lattice of the HOPG at these steps, as illustrated in figure 3.6. Figure 3.6: Step-Edge Transfer Mechanism. Metalion No Steps Steps
27 CHAPTER 4 EXPERIMENTAL 4.1 Electrochemical Setup A three-electrode system was employed fo r all electrodeposition experiments. The electrodes were (see Figure 4.1): Reference electrodeSatuated Ca lomel Electrode (SCE), (PAR). Counter-electrodeTwo CEs were utilized; the first was platinum foil-50mm x 50mm, .025mm thick. The second was a platinum wire 0.25mm (diameter), 25cm in length (Alfa Aesar). Working electrodeHOPG held with flat specimen holder (HOPG from SPI, holder from PAR). Metal Glass O-ring Teflon Opening for exposure to solution
28 According to the above diagram, there is a metal rod inside the flat specimen holder. This is necessary to ensure conduc tivity throughout the electrode. At the top of the holder, the metal rod is enclosed in a gl ass tube, and at the bottom where it meets the holding chamber there is an o-ri ng. The o-ring in this case is used at the glass-Teflon junction so that the glass will not be damaged. The Teflon holding chamber is where the HOPG block is inserted. The back of this specimen holder has a Teflon piece that comes off by twisting, which exposes a metal disk. This metal disk is also twisted off and the chamber is open. The HOPG is inserted in this chamber and the various parts that were removed in the process are re-inserted. The metal disk allows for this part of the device to be conductive. There is also an o-ring that is visible after the Tefl on piece is taken off; this o-ring serves to seal against liquids that could interfere with the specimen inside the cell. Figure 4.1: Working Electro de-Flat Specimen Holder.
29 As illustrated in Figures 4.1 and 4.2, th e specimen holder has an opening to expose the surface to be plated. In order to va ry the area that the specimen is exposed to, Teflon o-rings with the desired area can be m achined to fit inside this holder. The stock flat specimen holder has an opening that measures 1cm2. A size of .3cm2 was desired, therefore, a Teflon ring that fits inside th e holder was machined at the USF Engineering Machine Shop for this purpose. Figure 4.2: Flat Specimen Holder with Machined O-ring (Teflon). The electrodes were inserted into an electrochemical cell. The following is a photograph of the cell with the electrodes connect ed to the potentiost at (see Figure 4.3): O-ring inserted in specimen holder
30 Figure 4.3: Photograph of Electrochemical Setup. The electrodes, as visible in Figure 4.3, were inserted in specific inlets of the cell. The working electrode was inserted in th e middle, while the counter-electrode and reference electrode were at either side. Th e electrode placement wa s constant throughout the experiments. 4.2 Experimental The three plating variables investigat ed for their morphological effects on palladium nanostructures were potential, time, and counter-electrode area. One variable was changed in each experiment, while keeping the rest of the conditions fixed. The fixed conditions included temperature of bath, re ference electrode (SCE ), distance between electrodes, and solution. The so lution utilized for all expe riments had a composition of 0.2mM Pd NO3 (palladium nitrate) and 0.1M HClO4 (perchloric acid). These chemicals were obtained from Alfa Aesar. Scanning Electron Microscope (SEM) images were acquired at the STAR Center (Dave Edwards, Largo, Fl) following the electrodepositions. WE CE RE Potentiosat
31 4.2.1 General Process for Nanostructure Synthesis The first step required cleaning the electro des and electrolytic solution container. The electrodes/cell were rinsed with deionize d (DI) water and either dried with nitrogen or air. Next, the software (PowerSuite) was prepared for the experiments. The electrodes were specified in the software, as well as the deposition potential and length of experiment. The potential and time depended on which variable was under investigation at that time. Next, the electrolytic (containing ions) solution is transferred to the cell. The amount utilized was ~0.5L; this amount was chos en to ensure all of the electrodes were immersed in solution. In order to work with a smaller amount of solution, a smaller cell can be used. Prior to inserting the electro des into the cell, the Saturated Calomel Electrode (SCE) was filled with a layer of KC l particles at the tip. This is necessary for potentiostatic experiments because Â“an SCE not having a thick crust of KCl crystals should be avoided, since its pot ential might not be known  .Â” The electrodes were then placed into the reaction cell and connections vi a alligator clips were made. The software was then initiated. The investigation of the th ree variables (poten tial, time, counter-electrode area) required a change in the process for each type of investigation. The deposition potential was investigated by only changing this variable while keeping the rest of the conditions constant. The time was fixed at 400s, and th e counter-electrode employed was a platinum wire. Next, the time variable was investigated. The time was varied from 300-600s, while the potential was constant at 0.33V vs. SHE. The counter-electrode was platinum foil. Lastly, the counter-electrode area was inve stigated. This was ach ieved by switching the
32 foil to a wire, while keeping the rest of th e conditions fixed. Appendix B lists the step-bystep procedure.
33 CHAPTER 5 RESULTS AND DISCUSSION 5.1 Introduction The three variables investigated for their morphological effects on palladium nanostructures were time, potential, and c ounter-electrode area. One variable was changed in each experiment, while keeping th e rest of the conditions constant. The fixed conditions included temperature of bath, reference electrode (SCE), distance between electrodes, and solution. Sca nning Electron Microscope (SEM) images (Hitachi S4800) were acquired at the STAR Center (Largo, Fl) following the electrodepositions. The following table lists the parameters for each type of experiment: Table 5.1: Description of Experime nts for Nanostructure Synthesis. Variable under Investigation Constant Variables Deposition time 300 Â– 600s, in 100s Increments Temperature of bath: room temp. Pulsing potential: -0.2V for 5ms Deposition potential: 0.33V Counter electrode: foil Deposition potential 0.29 0.33V, in 0.04V increments Temperature of bath: room temp. Pulsing potential: -0.2V for 5ms Deposition time: 400s Counter-electrode: wire Counter-electrode Platinum wire Platinum foil Temperature of bath: room temp. Pulsing potential: -0.2V for 5ms Deposition time: 400s
34 5.2 Results Table 5.2: Results of Nanostr ucture Synthesis Experiments. Experiment Width of Nanowire/particle (nm) Current Density (A cm-2) Deposition time300s 75-90 50 Deposition time400s 90-100 40 Deposition time500s 100-120 40 Deposition time600s 120-140 45 Deposition potential0.29V 24-40 70 Deposition potential0.33V 28-38 70 Deposition potential0.37V 40-50 80 Counter-electrode: Pt wire 30-40 40 Counter-electrode: Pt foil 90-100 70 5.3 DiscussionEffect of Increasing Time The effect of increasing deposition time w ith a fixed potential was investigated. The deposition potential remained fixed at 0.33V vs. SCE and the deposition times were increased from 300 to 600s. The samples were first pulsed for 5 ms at -0.2V. Platinum foil was used as the counter electrode. 5.3.1 Nanostructure Morphology at 300s When plated for 300s, the wires measured on average less than 100 nm. Some were continuous and spanned th e entire grain, but most did not as evident in Figure 5.1. Although the distances between steps differs, th e alignment is very similar. All of the
35 agglomerations in this image (Figure 5.1), except for one instance, are aligned in the same direction. This occurrence is characte ristic of the HOPG surface and the transfer mechanism of palladium. The higher magnification image of a 300s nanowire measured 74nm (Figure 5.2). The morphology is smooth ex cept for some mushrooming; crystallites are well defined. Nanoparticles we re also formed in this pr ocess and are visible in the image. These measured roughly the same width of the nanowires found on the sample. Figure 5.1: SEM of Nanostru ctures Electrodeposited for 300s (scale: 2.00m). Figure 5.2: SEM of Nanostru ctures Electrodeposited for 300s (scale: 500nm).
36 5.3.2 Nanostructure Morphology for 400s Figure 5.3 is a low magnification microgr aph of a sample electrodeposited for 400s. This image is evidence of the depositi on of palladium along the step edges formed from the cleaving. The wires begi n parallel and close to each ot her, and then the graphite steps comes to a region with more space betw een the steps and seems to Â“open up;Â” these steps were decorated according to this sh ape. The region then closes back up and continues with the previous alignment and spans between each other. This occurrence seems to be from the cleaving itself. This shows the dependence of the contour of the steps to the morphology of the wires. Ideally the steps are parallel, but in practice the distance between the steps varies. Figure 5.4 is a high magnification image of continuous nanowires synthesized during the 400s deposition. The average widt h of these nanowires ranged from 90 to 100nm. A large majority of nanowires were smooth, as observed in the wires in figure 5.4. Mushrooming was also observed, but occu rred to a lesser degree than nanowires formed from higher potentials and de position times under the same conditions. Figure 5.3: SEM of Nanostru ctures Electrodeposited for 400s (scale: 500nm).
37 Figure 5.4: SEM of Nanostru ctures Electrodeposited for 400s (scale: 10.0m). A 3-D image of the sample was acquire d in order to inve stigate the morphology and gaps of these wires (figure 5.5). Th e nanowire on the left, although mostly continuous, has a nanoscopic gap. This ga p is the mechanism which produces the increase in conductivity when exposed to hydr ogen. Since the palladium grains expand up to 900 times its own volume, the gap must be within this limit for a connection between both sides of the wire to occur. If the gap is too la rge, the grains cannot connect and the wire behaves as an open circuit.
38 Figure 5.5: 3-D Image of Nanostructures El ectrodeposited for 400s (scale: 500nm). 5.3.3 Nanostructure Morphology at 500s The wires in Figure 5.6 had a deposition tim e of 500s (the scale at the bottom right is for the left half of the composite SEM). The image shows numerous nanowires on this sample. Nanowires up to ~120 nm in diam eter were measured throughout this sample via a SEM. The nanowires are aligned in the sa me direction at the center of the image. A higher magnification of this region was obt ained and is shown in Figure 5.7. It shows that while the nanowires were continuous, they suffered fr om mushrooming effects. A 3D image of nanostructures fabricated during a 500s electrodeposition run was acquired. Two wires grew on adjacent steps and almost fused together. If a longer deposition time would have been used, it is highly probable this w ould have occurred. Therefore, as deposition times are increased, the steps become plated up to a point where
39 the wires formed on adjacent steps fuse together forming nanowires with dimensions wider than those where the steps had la rger distances between each other. Figure 5.6: SEM of Nanostructures El ectrodeposited for 500s (scale: 30m). Figure 5.7: SEM of Nanostru ctures Electrodeposited for 500s (scale: 1.00m).
40 Figure 5.8: 3-D Image of Nanostructures El ectrodeposited for 500s (scale: 500nm). 5.3.4 Nanostructure Morphology at 600s When a 600s deposition time was applied, the wires were still continuous and spanned the length of the grains. The nanowir e in Figure 5.9 was deposited utilizing this time and measured 131nm. Nanowires in the range of 120 to 140nm were found throughout the sample with this deposition time. On the right side of the nanowire (Figure 5.10) there is latera l growth of palladium. This was also routinely found throughout the sample.
41 Figure 5.9: SEM of Nanostru ctures Electrodeposited for 600s (scale: 3.00m). Figure 5.10: SEM of Nanostructures Elect rodeposited for 600 s (scale: 300nm). The dependency of deposition time on width of the structures produced became evident through electrode positing the samples at various tim es. The average widths of the samples and their times were graphed (figur e 5.11), which agrees with this observation. Although there is an increase in width with an increase in ti me, this relationship is nonlinear.
42 Figure 5.11: Width (nm) of Nanowires vs. Deposition Time (s). 5.4 DiscussionEffect of Increasing Potential The effect of potential on plati ng of nanostructures was studied by electrodepositing at various potentials. The potentials investigated were a nucleation pulse of -0.2V with deposition potentials of 0.29V, 0.33V, and 0.37V vs. SCE. The time remained fixed at 400s, while the rest of the plating conditions were kept constant. A Pt wire was employed as the counter electrode. A pulsing potential of -0.2V was required fo r nucleation to occur. This value was obtained from cyclic voltammetry (Figure 5.12), which indicates th e fast reduction of palladium at potentials rangi ng from ~-0.2V to 0.1V vs. SCE. The CV results were repeatable, as the values from the 3 overlays match closely. It should be noted that at the time these CVs was taken, the solution had undergone numerous deposition experiments, which means there is a possibility it is not 100% accurate. The potential range 0.290.33V vs. SCE allowed for a slower deposition rate in order to have better control of the Width (nm) vs. Time0 20 40 60 80 100 120 140 160 180 300 400 500 600 Time (s) Width ( nm )
43 process. Although this range correlates to a negative current in the CV, it was observed during the deposition experiments that a positive current existed for the entirety of each experiment. This indicates that depos ition did indeed occur at this range.
44 Figure 5.12: Cyclic Voltammogram of HOPG in Palladium Nitrate Solution.
45 5.4.1 Nanostructure Morphology at 0.29V vs. SCE The first deposition potential investigated was 0.29V vs. SCE. This resulted in exclusive nanoparticle formati on; no nanowires were formed at this potential. A low magnification view of the palladium-plated surface reveals nucleation of palladium at preferential locations (figure 5.14). Parallel agglomerations w ithin grains are visible. The scale at the bottom right of figure 5.14, 10m, can be used to visualize the span of the agglomerations. These agglomerations span microns and resemble nanowire formation. Upon closer inspection (figure5.15), they are visibly not continuous nanowires. Nanoparticles were measured using a SEM a nd the size ranged from ~24nm to 40nm. Exclusive nanoparticle formation occurre d due to the react ion rate of the palladium ions at this potential. Theoretica lly, a higher potential wi ll cause reduction to occur at a faster rate, thereby increasing th e amount of palladium reduced. It should be noted that the reduction occurs, as previously stated, to the left of the equilibrium potential (IUPAC convention). The increase in electroreduced palladium will cause more palladium to be deposited along the step e dges, thereby increasi ng the width of the nanostructure. At a certain potential ra nge and deposition time, the agglomerated palladium nanoparticles begin to connect and form continuous nanowires. This theory is proved in the following experiments.
46 Figure 5.13: SEM of Plated HOPG at 0.29V (scale: 10m). Figure 5.14: SEM of Nanoparticles S ynthesized at 0.29V (scale: 100nm). 5.4.2 Nanostructure Morphology at 0.33V vs. SCE The second deposition potential investig ated was 0.33V vs. SCE. Figure5.16 is the SEM micrograph of the substr ate plated at this potential. This image is evidence of the step-edge palladium transfer mechanismthe steps are clearly plated, while the basal plane, also plated, does not have the same type of agglomeration. The basal plane has
47 palladium nanoparticles distributed on the su rface. Palladium is reduced on the whole HOPG surface, but agglomeration occurs predomin antly at the step edges. Thus, the basal plane can also be plated, but only nanopartic les will form. There are no steps on the basal plane at which the palladium particles can adhere and grow in a nanowire shape. The diameter of the nanowires in Figure 5.17 ranged from 28 to 38nm. These widths were obtained from SEMs and commonly found throughout this sample. Figure 5.15: SEM Image of Nanowires Electrodeposited at 0.33V vs. SCE. Figure 5.16: SEM Image of Nanowires Electrodeposited at 0.33V vs. SCE.
48 5.4.3 Nanostructure Morphology at 0.37V vs. SCE Figure 5.18 is the SEM micrograph of th e substrate plated at a deposition potential of 0.37V. Deposition at this potential resulted in na nowires that were wider than the nanowires formed from the 0.33V deposition potential. These wires also had a greater number of nanoscopic gaps than those plat ed at 0.33V. The average width of nanowires plated at this potential was 40 to 50nm. Although the palladium mostly agglomerated along the step edges and formed continuous nanowires, the palladium also fuse d along the side the nanowire and formed mushrooming (figure 5.19). The lateral growth, from visual inspection, is consistent with the size of the surrounding nanoparticles on the basal plane. The cause of this lateral growth is theorized to be due to a randoml y deposited nanoparticle which grew directly next to the step-edge agglomeration and fuse d with time. Hence, the nanoparticle was within distance for this to occur. Figure 5.17: SEM of Nanostructures at 0.37V vs. SCE (scale: 500nm).
49 Figure 5.18: SEM of Nanostructures at 0.37V vs. SCE (scale: 200nm). 5.5 Effect of Counter-Electrode Area The effect of counter-ele ctrode area on the morphology of the nanostructures was investigated. The deposition potential was held at 0.33V vs. SCE, and the deposition time was for 400s. The counter-electrode s utilized in this experime nt were a platinum foil and a platinum wire. Both serve th e purpose of a counter-electrode but the two differ in size. The foil measured 50mm x 50mm x 2mm, wh ile the foil measured .25mm in diameter and 25cm in length. The counter-ele ctrode, since it is employed to facilitate current to the working electrode, should change the morphol ogy of the nanostructures if its area is varied. Therefore, the foil, having a larger ar ea, should allow a faster reduction due to a larger current. This proved to be the case; the morphology of the nanowires was affected by the counter-electrode area. The SEM images show different sized-wires as a result of this change in electrode area.
50 Figure 5.19: SEM of Nanostructures Electrodepo sited for 400s, wire CE (width ~36nm). Figure 5.20: SEM of Nanostructures Electrodepo sited for 400s, foil CE (width ~90nm). 5.6 Comparison of Results to Current Literature 5.6.1 Current Density Current density (I/A, units Acm-2) is an important parameter in the growth of nanostructures (refer to Table 5.2). There is a correlation between this parameter and the resulting morphology of the nanostructures. It has been observed through the studies of
51 reduced palladium with HOPG in this thesis that the current density range for nanowire synthesis is higher than for nanoparticle sy nthesis. Continuous na nowires were observed with a current density in the range of 30-90A/cm2. Exclusive nanoparticle synthesis was observed with a lower current density, under 50A/cm2. Current vs. time plots appear in Appendix C; the current density can be gathered from thes e plots by dividing the current value by .3cm2. The range of current density for palladium nanostructure synthesis has been reported in other studies. Bera et al , for instance, reported an Â“optimized current density in the range of 8-20Acm-2Â” for exclusive nanoparticle synthesis. BeraÂ’s results are similar to the range observed for nanopartic le synthesis reported in this thesis . Nanoparticles were synthesized at this range but also above 20Acm-2. This variation in current density can be attributed to the em ployment of a differe nt type of working electrode, as the WE in BeraÂ’s work is alum inum thin films. These nanoparticles were synthesized using a potentiostat at 0.3V for 400s. Nanowires did not form on this template due to its topology. Pits were etch ed on the aluminum films to enable the exclusive growth of particles (Figure 5.21).
52 Figure 5.21: Pd Nanoparticles Electrodeposit ed on Etched Aluminum Films . Penner et. al. also reported a current density range for the synthesis of palladium nanostructures . The electrochemical setup of this report is very similar to the one utilized in this thesis. Penner utilized a th ree-electrode system with the same working electrode (HOPG), counter elect rode (Pt) and reference el ectrode (SCE). The current density range reported by this group is very si milar to the one reporte d in this thesis. One report by this group states that the cu rrent density range was 30 to 60Acm-2 , while in a separate report of the same topic stat es that this range was 20 to 80Acm-2 . The range observed in this thesis more closely matches the second reported range. The difference in density range between this group and the experiments in this thesis could stem from variations in the electrochemical setup. 5.6.2 Nucleation Phenomena at Inhomogeneities The palladium nanostructures deposite d primarily on the defect s, the Â“stepsÂ”, of the HOPG. Growth was also observed on terrace s ites, but to a lesser degree. This finding
53 is consistent with literature reports. Vazquez  grew Ag nanowires on HOPG and concluded that growth is pref erential at steps. The growth mechanism was stated to be diffusion of the metal from its aqueous form in solution to the crystal lattice of the HOPG. These findings are consistent with th e Â“step edgeÂ” mechanism, with nucleation originating at the defects (refe r to Figure 5.22). This is a result of weak HOPGmetal interactions at those sites  Growth at a terrace site is also visible in Figure 5.21, but because the growth was not pr eferential at that location, on ly nanoparticles were formed. One group has attributed this phenomenon to poin t defects on terraces that are not visible to the NC-AFM or STM . PennerÂ’s group ha d similar results, as they also concluded that nanowires and nanoparticles grow on th e step edges . They attributed the preferential growth to the exis tence of an energetic barrier that is lower at the step edges than that the terrace sites . GimenoÂ’s group reiterates the step-edge ion-transfer mechanism . Palladium Â“islandsÂ” we re grown on HOPG under various potentials . A different electrolyt e was utilized for this purpose, which makes a complete comparison of the results questionable. This is due to a different reduction potential of the solution. It should still be noted that at lower potential s Â“islandsÂ” were formed, while at higher potentials wires were observed.
54 Figure 5.22: Pd Nanostructures Grown at E= -.100V . 5.6.3 Lateral Growth Lateral growth was observed in the synthe sis of the palladium nanostructures. The nanostructures have a tendency to agglomer ate, and due to this tendency there are instances where the agglomeration grows crystal lites in a lateral direction. This growth consists of nanocrystallites that fuse with th e neighboring crystals to form a larger mass. In one study, palladium nanoparticles were examined under a high-resolution Transmission Electron Microscope (HR TEM). The report concluded that the nanocrystallites measured between 5-10nm . 5.6.4 Width of Nanostructures The dimensions of electrodeposited na nostructures on HOPG have been reported . The same molarity solution was employed as described in this thesis, as well as types of electrodes in the el ectrochemical setup. The thinne st nanowires obtained were
55 ~55nm in diameter . This is comparable to thinnest wires obtaine d from the results of this thesis; nanowires under 40nm were obser ved. Although these were indeed wires and not mere agglomerations, these were not as continuous as the wider nanostructures that synthesized at other conditions. Figure 5.23: Plot of Reported Diam eter of Pd NWs on HOPG . Figure 5.23 is a graph of the diameters of the electrodesposited nanowires from the aforementioned report . The diameters of interest ar e those formed from the .1M HClO4 solution. According to this graph, the di ameters obtained were under 200nm. This graph can be compared to Figure 5.18. Both graphs synthesized nanowires under 200nm. The smallest diameter obtained, on the other hand, differs. In the experiments described earlier, nanowires under 40nm were obtained. These nanowires were formed using a smaller counter-electrode (platinum wire). The nanowires from Figure 5.22, on the other hand, used a foil CE. The difference in th e ranges obtained can be attributed to differences in electrochemical setup. For exam ple, a simple variation that would change
56 the potential measured is changing the electrode leads (from Al to Cu, for instance). This variation would cause a decrea sed resistance, hence more si gnal would get through to the electrodes. An increased signal could cha nge the amount of reduced palladium and consequently the morphology of the nanostructures.
57 CHAPTER 6 TESTING 6.1 Palladium Nanostructures on SAW Sensor 6.1.1 Transferring of Nanostructures The palladium nanostructures were intended to be employed as the sensing layer in a SAW device for hydrogen detection. In order to accomplish this, the deposited palladium nanostructures were required to be transferred from the HOPG onto the appropriate location on the SA W device (between the IDTs). For the first transfer attempt, a drop of cyanoacrylate (super) glue was dropped on the HOPG. A degreased (rinsed with acetone and meth anol) glass slide was then placed upon the HOPG. The glue was allowed to harden for 8hrs. After this time the HOPG was removed from the glass slide. Figure 6.1: Palladium Nanowires Em bedded in Cyanoacrylate Film. Nanowires
58 This process was not successful for the transfer of the nanowires. First, this method of transfer requires locating the nanowires on th e glass slide with embedded nanostructures. An optical microscope wa s used for this purpose. Because of the limitations of this type of microscope, lo cating the nanostructures was excrutiating. A few were indeed located (Figure 6.1), but not enough to cause a response from hydrogen (at least 10 parallel nanowires are re quired for hydrogen sensing ). A second method of transfe rring the nanostructures was attempted. This method used an ultrasonicator to remove the nanostr uctures from the HOPG. The process for this was following the deposition, the HOPG was placed in a 50mL beaker containing acetone. Acetone was chosen due to its fast ev aporation. The beaker was then placed in the ultrasonicator, which contained water. Th e beaker was placed in this bath, and the timer was set for 5 minutes. After this time, the solution in the beaker was inspected. The solution had turned a grayish color. A sec ond sonication was done, this time with an HOPG block that did not have nanostructure s on the surface. After the 5 minutes, the solution remained clear. It was then conclude d that the grayish color was as a result of palladium nanostructures that had been removed from the surface. 6.1.2 Testing of Palladium Nanostructures on SAW Device After concluding that the acetone solution contained palladium nanostructures, the nanostructures were dropped via a micropi pette between the IDTs on a SAW device (Figure 6.2). The specific de vice tested was a 200MGHz dual delay line SAW, and this was placed in a test bed for testing with hydroge n gas. The test bed consisted of a set of MKS mass flow controllers fo r controlling a range of 1100% hydrogen. The MFCs were
59 controlled with Labview. The SAW device was tested as to mass loading as well as electro-acoustic effects when exposed to hydrogen gas. The expected response was a frequency shift. The frequency output of the device was monitored with the Labview program. Upon exposure to hydrogen gas, the frequency did not shift frequencies. It was determined that the SAW device did not re spond to hydrogen gas. Various amounts of palladium were dropped, but this did not shift the frequency. In creasing hydrogen gas concentrations (1-100%) were flown into the chamber, and this also proved unsuccessful. Figure 6.2: Pd Deposited between the IDTs. 6.2 Palladium-plated Porous Silicon Because the SAW sensor testing was unsu ccessful, Palladium (Pd) nanostructures were tested by electrodepositing Porous Silicon (P.S.). The testing of the samples required a probe station, impedance analyzer nitrogen, and hydrogen. Prior to the testing, the impedance analyzer was calibrated a nd programmed to supply 500mV. The gases were then programmed for the specific c oncentrations using an MKS box. The probes were then placed on the sample, and testing was initiated. Bond pad IDT
60 6.2.1 Preliminary Experiments Two preliminary experiments were done to characterize the behavior of porous silicon under certain conditions. The first c ondition explored was a plated porous silicon sample with a sweeping frequency. A frequenc y was set to start at 5KHz and stop at 100KHz. The sample was tested with and without hydrogen. Although there were slight variations in the resistance, these were attributed to a slight drift that is considered normal in stable signals. A second experiment was performed with bare porous silicon. This was done to investigate any inherent responses por ous silicon might have to hydrogen, as this could essentially conceal a response from th e palladium. The porous silicon was tested under various concentrations ranging from .05-100% hydrogen/nitroge n mixtures, and no resistance variations were observed. 6.2.2 Sample 1Aluminum Contacts The first sample probed was a porous silic on wafer with Aluminum (Al) contacts (Figure 6.3). The probes were placed on the contacts, and th e resistance was allowed to stabilize. The resistance was observed for ~20 minutes befo re a stable signal of 1.74M was observed. At this point, gases were tu rned on. The hydrogen flow was set at 20sccm, and nitrogen set to 200sccm. A drop in the re sistance was observed after 2min. of flowing the gases. The resistance at this point measured 900k The next step was to turn the gases off. This caused the resistance to increase to 1.53 M Following this response, the gases were again turned on (Table 6.1). Va rious concentrations were used, but the resistance did not change.
61 Figure 6.3: Response of Pd-Plated P. S. with Al Contacts (10%H/N). Table 6.1: Sample 1Response to Hydrogen Gas. % GasSccm Resistance variation .05% H/N 1/200 None 1% H/N 2/200 None 4% H/N 8/200 None 10% H/N 20/200 R1=840 R2=630 100% N 200 (N) None The resistance not increasing for a sec ond cycle implies that the hydrogen did not diffuse out of the sample. In this case, an annealing step would have been the R1 R2
62 progression. Certain materials have been repo rted to require an annealing step of 500600C to evolve the hydrogen out of the samp le. Unfortunately, sample heating is not possible in the probe station. 6.2.3 Sample 2Aluminum Contacts A second sample was found to respond to hydrogen, but this sample had a smaller recovery following the first hydrogen cycle. A baseline resistance of 19.5M was established prior to hydrogen exposure (refer to Figure 6.4). The hydrogen was again set to 20sccm, and nitrogen to 200sccm. The sample was exposed to one cycle of hydrogen, and the resistance was observed to decrease to ~18 M A second subsequent hydrogen cycle was then run in order to observe whet her the resistance woul d stay the same or decrease. The resistan ce decreased to 17.94 M so it relatively st ayed unchanged. The hydrogen was then shut off to observe the recovery of the sa mple. The resistance increased, but it increased at a slower rate than the previous sample. A resistance of 18.23 M was observed for the first 100% nitrogen cycle, and for th e second nitrogen cycle a resistance of 18.46 M was observed (Table 6.2). The successive increases in resistance after hydrogen exposure imply that the diffusion of hydrogen is not instantaneous fo r this material. Alt hough the resistance did not increase to the original resistance, an increase of 520kOhms was observed. After the resistance reached 18.46 M the resistance did not shift with any other concentrations of hydrogen/nitrogen. It also did not respond to 100% nitrogen.
63 Figure 6.4: Response of Pd-Plated P. S. with Al Contacts (10% H/N). Table 6.2: Sample 2Response to Hydrogen Gas. % Gassccm Resistance variation .05% H/N 1/200 None 1% H/N 2/200 None 4% H/N 8/200 None 10% H/N 20/200 R1=1.56M R2=290k R2=230k 100% N 200 (N) None Resistance vs Time17.8 18 18.2 18.4 18.6 18.8 19 19.2 19.4 19.6 024681012 Time (minutes)Resistance (MOhm) Series1 R2 R3 R1
64 Five other sets of experiments with the gas range specified in the chart were done with other plated porous silicon samples, but the signal was too noisy to obtain a reliable baseline. At this point the contact mate rial was switched from aluminum to gold. 6.2.4 Sample 3Gold Contacts A second type of samples was plated with palladium and interrogated for a response to hydrogen gas. The contact materi al was switched from aluminum to Gold (Au). A stable baseline resist ance was initially obtained, after which hydrogen exposure measurements were initiated (Figure 6.5). Th e baseline resistance for this sample was 2.13 k The device only responded to 10% hydroge n (Table 6.3). Below is a graph of the response: Resistance vs Time2.05 2.06 2.07 2.08 2.09 2.1 2.11 2.12 2.13 2.14 02468 Time (minutes)Resistance (KOhms) Series1 Figure 6.5: Response of Pd-plated P. S. with Au Contacts (10% H/N). R2 R1
65 Table 6.3: Sample 3Response to Hydrogen Gas. % GasSccm Resistance variation .05% H/N 1/200 None 1% H/N 2/200 None 4% H/N 8/200 None 10% H/N 20/200 R1=700 R2=200 100% N 200 (N) None 6.2.5 Discussion It was observed that the palladium plat ed porous silicon samples did not fully revert to the baseline resistance after respondi ng to one cycle. It has been reported that the response and recovery time of PS-bas ed gas sensors is limited by the diffusion coefficient of the gaseous atoms, a nd molecules along the inner PS surfaces. Investigations into porous silicon based sens ors show that as gas sensors, PS has been observed to have low response and recovery times. These diffusion issues of PS stem from the characteristics of the PS structure. There are various issues which could have hampered the response of the samples. One issue is the reported aging effects of the material, which can affect the measurements. Aging effects refer to sample degradation due to a number of variables, notably a layer of native oxide that forms on the surface if not stored properly. The existence of this native oxide can affect the electrical measurements by passivating the
66 surface, thereby inhibiting conduction. One met hod to alleviate sample degradation is adequate storage of the samples. PS samples should be kept in a nitrogen environment. A second critical issue that has been discussed is cont amination as a result of contacting the porous silicon. Due to its structur e, the probes from the probe station easily damage the material. This occurrence was observed when performing all of the experiments; after one initial placement of the probes on the desired location, the probes became visibly contaminated with the materi al. This contamination can be a cause of response degradation. Another problem that arose as a result of the porous silicon structure was as the probes were being placed, a sma ll amount of the material contacted was scratched off. It is possible that the material was so fragile th at the probes scratched off a part of the bond pad and actually penetrated the sample, which would also affect the response. A third aspect of the testing that can cause inconclusive results is the quality of the evaporated contacts on the porous sili con. The surface of the porous silicon is not smooth, and the evaporated material takes on th is shape. The subjec t of contacting porous silicon has been a challenge w ith the use of this material for some time. One alternative to evaporating bond pads is the fabri cation of IDTs on the surface through a photolithography step. The test bed could be another cause of lack of response. As aforementioned, two gas tanks (nitrogen, hydrogen) we re utilized to control th e concentrations of hydrogen. Although the control box for the MFCs has a readout of the amount of gas, an output
67 sensor was not implemented. Therefore, it is not certain that the programmed concentrations of gases were those ac tually flown into the probe station.
68 CHAPTER 7 CONCLUSIONS AND FUTURE WORK 7.1 Conclusions The electrodeposition experi ments of palladium nanostr uctures on highly oriented pyrolitic graphite show that this nanost ructure growth method has a wide range of applications in the area of hydrogen sensors due to its simplicity and low cost. These experiments allowed for the deposition potential time, and counter-electrode area to be investigated. The resulting morphologies of the nanostructures when each variable was investigated is an indication of their de pendency on electrodeposition conditions. From the deposition experiments, it was concluded that: An increase in potential cau ses an increase in width. Increased deposition time l eads to increased width. A smaller counter-electrode yields thinner wires. At lower potentials (0.29V), nanoparticles are synthesized. Nanowires are synthesized at higher potentials (0.33V, 0.37V). Continuous nanowires can be fabr icated via electrodeposition. Nucleation/growth occurs predominantly at the 90 angle of step edges (Figure 7.1).
69 There exists a relationship between current density and nanostructure morphology. Figure 7.1: Preferential Grow th at 90 Step Edges. 7.2 Future Applications of Palladium Nanostructures for Hydrogen Sensing Future work lies in interrogating the pa lladium plated porous silicon for hydrogen sensing (Figures 7.2, 7.3). Preliminary result s of palladium-plated porous silicon have been obtained and appear in Chapter 6. Although it appears that the devices are responsive to hydrogen, problems with the setup ha ve led to several inconsistencies in the results. The results are therefore regarded as inconclusive. More work is required in areas such as the test bed and measurement me thodology in order to generate repeatable results. Pd growth at 90 step edges
70 Figure 7.2: Pd-Plated Po Si. Fi gure 7.3: EDAX of Pd-plated Po Si. Pd peak at 2.8 Pd
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75 Appendix A: Properties of Palladium The properties presented here were taken from M odern Electroplating, Wiley, 2000.
76 Appendix B: Process Steps for Experiments B.1 Process for Experiments with Potential as the Variable: Clean electrodes and cell Turn on potentiostat Input specifications of experiment into software (PowerSuite) o Solid WE o RE is SCE o Area of WE: .3cm2 o Deposition potential (var ied between .29V,.33V,.37V) o Deposition time: 600s Pour solution into cell Place CE and RE into cell Cleave HOPG o Adhere tape onto surface of HOPG o Peel back Place HOPG into WE holder Place WE into cell Connect electrodes to potentiostat o Green tipWE, Red cable/Red Ti p-CE, White cable/Red tipRE Start experiment B.2 Process for Experiments with Time as the Variable Clean electrodes and cell
77 Appendix B: (Continued) Turn on potentiostat Input specifications of experiment into software (PowerSuite) o Solid WE o RE is SCE o Area of WE: .3cm2 o Deposition potential: .33V o Deposition time varied: 300s, 400s, 500s, 600s Pour solution into cell Place CE and RE into cell Cleave HOPG o Adhere tape onto surface of HOPG o Peel back Place HOPG into WE holder Place WE into cell Connect electrodes to potentiostat o Green tipWE, Red cable/Red Ti p-CE, White cable/Red tipRE Start experiment B.3 Process for Experiments of Count er-Electrode Area as the Variable Clean electrodes and cell Turn on potentiostat Input specifications of experiment into software (PowerSuite)
78 Appendix B: (Continued) o Solid WE o RE is SCE o Area of WE: .3cm2 o Deposition potential: .33V o Deposition time constant: 400s Pour solution into cell Place CE and RE into cell Cleave HOPG o Adhere tape onto surface of HOPG o Peel back Place HOPG into WE holder Place WE into cell Connect electrodes to potentiostat o Green tipWE, Red cable/Red Ti p-CE, White cable/Red tipRE Start experiment
79 Appendix C: Examples of Current vs. Time Plots from Electrodepositions Figure C.1: Current (A) vs. Elapse d Time (s) from a 400s Deposition. Current = ~8 A Current Density = ~27 A cm-2
80 Appendix C: (Continued) Figure C.2: Current (A) vs. Elapsed Time (s) Graph from a 600s Deposition. Current = ~10 A Current Density = ~30 A cm-2