Sputter deposition of iron oxide and tin oxide based films

A Thesis Entitled Sputter Deposition of Iron Oxide and Tin Oxide Based Films and the Fabrication of Metal Alloy Based Electrodes for Solar Hydrogen Production By Daniel Sporar Submitted as partial fulfillment of the requirements for The Master of Science degree in Chemical Engineering Advisor: Dr Xunming Deng College of Graduate Studies The University of Toledo May 2007 The University of Toledo College of Engineering I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY SUPERVISION BY Daniel Sporar ENTITLED Sputter Deposition of Iron Oxide and Tin Oxide Based Films and the Fabrication of Metal Alloy Based Electrodes for Solar Hydrogen Production BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master of Science in Chemical Engineering Thesis Advisor: Dr Xunming Deng Recommendation concurred by Committee Dr G Glenn Lipscomb Dr Steven E LeBlanc On Final Examination Dean, College of Engineering An Abstract of Sputter Deposition of Iron Oxide and Tin Oxide Based Films and the Fabrication of Metal Alloy Based Electrodes for Solar Hydrogen Production Daniel Sporar Submitted as Partial Fulfillment of the Requirement for The Master of Science in Chemical Engineering The University of Toledo May 2007 This M.S Thesis describes the fabrication and characterization of n-type iron (III) oxide thin film semiconductors as well as n-type fluorine doped tin dioxide thin film semiconductors for use as the top oxide layer of hybrid multijunction PEC electrodes and as a transparent conductive corrosion resistant layer, respectively Also described is the fabrication and characterization of various anode and cathode materials in an attempt to devise high quality, cost-effective electrocatalysts for the electrolytic evolution of hydrogen and oxygen gases Iron (III) oxide thin films were radio frequency sputterdeposited under variable conditions Dopants were incorporated via co-sputtering in an attempt to enhance photocurrent response and overall film stability in basic media, 33 % potassium hydroxide Iron (III) oxide thin films deposited with a chamber atmosphere iii containing % oxygen in argon at 100 W at 400 °C for 110 demonstrated the highest observed photocurrents of 0.34 mA/cm2 under 0.75 sun illumination; efficiency was 0.56 % at a potential of 0.38 V The films were also stable Fluorine doped tin dioxide thin films were fabricated in the same fashion as the iron (III) oxide thin films; samples deposited at 50 W with the chamber atmosphere containing % oxygen in argon at 250 °C for 135 demonstrated photocurrents of 0.2 mA/cm2, although they lacked stability Iron (III) oxide was deposited onto the top of a triple-junction amorphous silicon solar cell to investigate its usefulness as a protective oxide layer Anodes and cathodes that were investigated for enhanced electrocatalytic properties consisted of various materials produced by various methods Current densities and hydrogen evolution rates were measured Electrodes demonstrating the greatest performance were made by mixing nickel, aluminum, and molybdenum powders in nickel trays at a ratio of 88:5:7, and then sintering them for four hours in a furnace at 900 °C The electrodes were then soaked in 33 % potassium hydroxide in order to leach out the aluminum, thus creating porous structures of high surface area Current densities near 40 mA/cm2 measured at 1.8 V have been demonstrated after 1000 hours of accelerated continuous long-term testing at a potential of 2.2 V iv Foreword I would like to give my sincere thanks to my thesis advisor, Dr Xunming Deng, for his support during both my undergraduate and graduate careers at The University of Toledo Due to his generosity, I have been able to expand my education beyond the scope of a degree based solely in chemical engineering, allowing me to develop a more complete understanding of the link between science, engineering, and technology I would also like to thank the other members of my thesis committee, Dr Glenn G Lipscomb and Dr Steven E LeBlanc, for their patience as I completed the requirements for my Master of Science in Chemical Engineering degree I would like to give my sincere thanks to my co-worker, mentor, and friend, Dr William B Ingler, whose competence and guidance have allowed me to become a more successful graduate student I found his experience working with thin film semiconductors to be invaluable as I developed this body of work I would also like to thank my co-worker and friend Dr Mahabala Adiga for his knowledge and support concerning my electrocatalyst work It has been an honor to have had the privilege of working with both individuals I would like to thank the faculty and staff of the Department of Chemical and Environmental Engineering for providing an excellent academic and professional education and overall positive experience during both my undergraduate and graduate careers at The University of Toledo; especially Dr Arunan Nadarajah who personally recruited me into the graduate studies program I would also like to thank the faculty and staff of the Department of Physics and Astronomy for their support throughout my graduate career I would like to thank Dr Pannee Burckel, the Chemical Instrumentation Specialist at the College of Arts and Sciences Instrumentation Center, for her training on and assistance with x-ray diffraction measurements and SEM imaging I would also like to thank the graduate students Xinmin Cao for his assistance with thin film thickness and band gap calculations, Xiesen Yang for his assistance with AFM imaging, and Dinesh Attygalle for his assistance with work done on both the thin film semiconductor research and the electrocatalyst research I would like to acknowledge the efforts of Anupam Dighe, Amrutha Asthana, Madhu Kondapi, and Puneeta Bhadsavle, all part-time graduate students, for their assistance with work done on sintered electrodes v Table of Contents Abstract iii Foreword v Table of Contents vi List of Figures viii List of Tables xiii List of Equations xiv Part 1: Thin Film Semiconductors Section 1-1 Introduction Section 1-2 Experimental 12 A Preparation for the Deposition of n-type Fe2O3 and F-SnO2 Thin Films by RF Sputter Deposition 12 B Fabrication of n-type Fe2O3 Thin Films With and Without Incorporation of Metal Dopants 15 C Fabrication of n-type F-SnO2 Thin Films 16 Thin Film Characterization 18 A Photocurrent Measurements 18 B Film Stability Measurements 19 C UV-vis Spectroscopic Measurements 19 D X-ray Diffraction Spectra Measurements 20 E Film Thickness Measurements 21 F Atomic Force Microscopy Measurements 21 G Annealing 21 Section 1-3 vi Section 1-4 Results and Discussion 22 A n-type Fe2O3 22 B Tantalum Doped n-type Fe2O3 33 C Zirconium Doped n-type Fe2O3 33 D Indium Doped n-type Fe2O3 34 E Antimony Doped n-type Fe2O3 39 F n-type F-SnO2 41 Summary 45 Section 1-5 Part 2: Electrodes Exhibiting Enhanced Electrocatalytic Performance Section 2-1 Introduction 47 Section 2-2 Experimental 49 A Basic Electrode Materials 49 B Sputter-Deposited Electrodes 49 C Electroplated Electrodes 51 D Raney Nickel 53 E Sintered Electrodes 53 Section 2-3 Electrode Characterization 57 Section 2-4 Results and Discussion 60 Section 2-5 Summary 70 Future Work 71 References 72 vii List of Figures 1.1 Water electrolysis by conventional means When a potential is applied to the electrodes, the electrolyte completes the circuit and allows current to flow If the applied potential is high enough to overcome the water splitting potential (1.23 V) and the electrode overpotentials, then water molecules dissociate into hydronium ions (H+) and hydroxyl ions (OH-) (1) The hydroxyl ions are oxidized at the anode and form oxygen molecules (2) The hydrogen ions are reduced at the cathode and form hydrogen molecules (3) Because there are two hydrogen atoms to every oxygen atom in the water molecule, twice as much hydrogen gas is produced with respect to oxygen………………………………………………….3 1.2 Standard, AM and AM 1.5, solar spectrum Ultraviolet range is from 115 to 400 nm; visible range is from about 400 to 800 nm The area under the UV portion of the curve is much less than the area under the visible portion of the curve……………………………………………………………………………….4 1.3 PEC system utilizing a TCO protective layer deposited upon a multijunction solar cell.14 From left to right, the H2 catalyst could be platinum islets or a molybdenum compound, the multijunction is a-Si, the transparent protective film could be ITO or TiO2, and the O2 catalyst could be a cobalt compound.20…… 1.4 General design for a hybrid multijunction PEC.21 α-Fe2O3 would act as the photoactive semiconductor, replacing the top cell of the solid-state multijunction (a-Si) The interface layer is a very thin layer of ITO and is used to reduce the series resistance between the solid-state multijunction and the photoactive semiconductor The metal substrate is generally stainless steel and the hydrogen evolution reaction (HER) catalyst is usually platinum islets, or a molybdenum compound………………………………….………………………………………6 1.5 When light with energy hν hits the semiconductor electrode, electrons may become excited up to the conduction band (EC) from the valence band (EV) The electrons (e-) then move to the back of the electrode while holes (+) accumulate at the front surface EF, the Fermi level, is the highest energy state which electrons may occupy at absolute zero In p-type semiconductors it is located closer to the valence band, and in n-type semiconductors it is located closer to the conduction band…………………………………………………………………………….….8 1.6 General reaction mechanism for the evolution of hydrogen and oxygen using a self-driven PEC system utilizing thin film semiconductors Water adsorbs onto the photoanode and dissociates into hydronium (H+) and hydroxyl ions (OH-) Formation of electron and hole pairs occurs at the photoanode where O2 is formed and H2 is formed at the cathode………………………………………………… viii 1.7 Argon ions (Ar+) from the plasma cloud, confined just above the target by a magnetic field generated by magnets located within the sputter gun, bombard the target physically removing small amounts of the target material upon impact The particles, which are neutrally charged, are ejected ballistically and deposit on a substrate Over time a very thin film of target material builds up on the surface of the substrate The substrate is rotated in order to ensure a uniform substrate temperature and film deposition…………………………………………………11 1.8 Diagram of standard substrate orientation in the substrate holder, viewed from the deposition side One piece of Tec-15 glass and one piece of ITO coated glass were placed in the center of the substrate holder and plain glass microscope slides were used to fill in the rest of the spaces Thin pieces of stainless steel were used to block part of the thin film deposition in order to leave bare electrical contacts used for measuring photocurrent and film stability The deposition on the plain glass was used for transmission measurements………………………………….13 1.9 Temperature calibrations for the vacuum deposition chamber After several runs, spot checks were done to make sure the bulbs were maintaining the same power density Full calibrations were done after every change of the halogen bulbs………………………………………………………………………… …15 1.10 All of the thin films were fabricated in this sputter chamber The flange / door is located on the left-hand side of the chamber (1) near the RF generators (2) The sample rotator is located at the top of the chamber (3) along with the power input for the lamps (4) and the thermocouple (5) The vacuum gauges are located on the right side of the chamber (6), above the pressure and gas flow control panel (7)……………………………………………………………………………… 17 1.11 Photocurrent density (jP, µA/cm2) as a function of oxygen concentration (%) in argon ambient of n-type α-Fe2O3 thin films deposited at 400 °C for 120 with 100 W deposition power Adding oxygen to the chamber atmosphere allowed any free iron to oxidize (reactive sputtering) resulting in higher quality films that were more stable in solution during electrochemical testing………………………… 23 1.12 Photocurrent density (jP, µA/cm2) versus substrate temperature (°C) for n-type αFe2O3 thin films deposited on (a) Tec-15 glass and (b) ITO All samples represented were sputtered with a target power of 100 W with % oxygen in argon ambient for 110 min………….…………………………………… …… 24 1.13 Stability scans for n-type α-Fe2O3 thin films deposited with a power of 100 W at 400 °C for various amounts of time Films deposited for 90 were more stable, but not very conductive As the deposition time was increased to 135 the film stability decreased by a small amount, and the film conductivity increased.……………………………………… ………………………… .25 ix 1.14 Photocurrent density (jP, µA/cm2) as a function of film deposition time (min) for α-Fe2O3 thin films deposited on (a) T-15 glass and (b) ITO The optimum deposition time was found to be 110 with the Fe2O3 deposition power set to 100 W UV-vis transmission spectra were used to calculate an average film thickness of 280 nm…………………………………………………………… 26 1.15 Chopped scan of n-type α-Fe2O3 under alternating 0.75 sun illumination and ambient room lighting (dark) When the film surface was illuminated, photocurrent was generated, demonstrated by a sharp increase in current density (mA/cm2) When the light source was then blocked, the current density sharply droped back to the dark current density value ………… ……………… … 27 1.16 XRD spectra for n-type α-Fe2O3 thin films measured from 25° to 65° (2θ) As film thickness increases, the peaks become more intense and defined indicating increased crystallinity (a) As deposition temperature increases, the peaks become more intense and defined indicating increased crystallinity (b) All peaks correspond to only α-Fe2O3.………………………………………… … …….29 1.17 Film thickness as a function of deposition time All films were deposited under similar conditions The error bars are one standard deviation………………… 30 1.18 UV-vis transmission spectra of n-type α-Fe2O3 thin film deposited at 400 °C for 110 Due to the film being very thin (285 nm) there are very few interference fringes A tauc plot was used to determine a band gap of 2.04 eV.…………………………………………………………………………….….31 1.19 Tauc plot for an n-type α-Fe2O3 thin film deposited for 110 at 400 °C with a Fe2O3 r.f deposition power of 100 W in a chamber atmosphere containing % oxygen in argon ambient at a pressure of mTorr The band gap was determined to be about 2.04 eV.………………………………………………………… ….32 1.20 AFM images of n-type α-Fe2O3 thin films deposited at, from left to right, 300, 350, and 400 °C The dimensions of each image are 5000 5000 nm Films deposited with higher substrate temperatures have rougher surfaces and demonstrate greater photoactivity…………………… …………………… …32 1.21 Photocurrent density (jP, µA/cm2) versus applied potential (mV) of an In-Fe2O3 thin film electrode Light and dark currents were measured using a light chopping method by manually blocking the light source and then illuminating the electrode in s intervals……………………………………………………………………35 1.22 Photocurrent density (jP, µA/cm2) versus applied potential (mV) of an In-Fe2O3 thin film electrode annealed up to hours in an inert argon atmosphere at 550 °C.……………………………………………………………………….… 35 x Current Density (mA/cm ) 130 120 110 100 90 80 70 60 50 40 30 20 10 0 100 200 300 400 500 600 700 800 Time (h) Figure 2.5: Accelerated long-term testing of various nickel cathodes Current densities (j, mA/cm2) were measured at 1.8 V over a period of several hundred hours of continuous operation at 2.2 V Degradation of performance occurred over time until equilibrium was obtained The sintered electrode 123005D (▲) demonstrated the greatest performance due to its porosity and large surface area The electroplated nickel electrode (○) performed well initially, but after 200 hours its performance had degraded considerably The platinum coated nickel sponge (□) demonstrated the lowest performance due to the platinum coating being too thin, as well as having less active surface area Sintered electrode 123005D was not reproducible At the time when it was fabricated, the powder trays were not being placed inside of a stainless steel box The trays filled with powder were placed directly onto the floor of the furnace with the argon purge blowing directly down on top of them Nickel oxide formed over the top of the samples and had to be removed by hand before testing When the oxide layer of sample 123005D was removed, it was done in such a way as to create a very gravelly surface underneath It is hypothesized that the random structure was responsible for the electrodes high performance After the electrodes were sintered in the stainless steel box, results could be reproduced and trends were observed A sintering temperature between 850 and 900 °C 63 was found to be optimal If the temperature was too low then the diffusion bonding between the nickel particles was not strong enough and the electrodes would fall apart If the temperature was too high then the diffusion bonding was too great and the pore sizes were too small The reduced surface area resulted in lower current density values Nickel-aluminum electrodes sintered within the optimal range consistently demonstrated initial current densities near 25 mA/cm2 Electrodes sintered with a one hour hold time tended to not be very durable, and they could not be removed from the trays in which they were placed Extending the hold time to four hours resulted in hard, durable bricks that could be removed from the trays However, even with extended hold times, in order for the powders to hold together well it was observed that the nickel powder purity had to be 99.99% or greater X-ray diffraction measurements were performed on samples of nickel powder of varying purities and no difference was observed in the spectra (Figure 2.6) The amount of impurities in the different powders was too small to be observed It was also observed that the various powders that were investigated were composed of differing particle sizes The dependence of electrode quality on sintering temperature and powder particle size and shape needs to be more thoroughly investigated Addition of molybdenum enhanced the performance of the sintered nickel electrodes (cathodes) The optimal ratio of nickel, aluminum, and molybdenum was found to be 88:5:7 based on accelerated continuous long-term testing After over 1000 hours of continuous testing, current densities near 40 mA/cm2 have been demonstrated The addition of cobalt oxide has enhanced nickel electrode (anode) performance 64 Electrodes demonstrating current densities exceeding 20 mA/cm2 have been consistently produced 30000 25000 Counts 20000 99+ % 15000 99.7 % 10000 99.9 % 5000 99.99 % 40 60 80 100 120 2θ (deg) Figure 2.6: XRD spectra of nickel powders of various purities No difference in composition had been observed All peaks correspond to nickel Three Ni-Al-Mo electrodes fabricated under identical conditions were submersed in 5.9 M potassium hydroxide with varying two-dimensional areas exposed, and values of current density were measured at a potential of 1.8 V (Figure 2.7) The composition of all three electrodes, by weight, was 80:10:10 of Ni, Al, and Mo respectively Each was sintered at 900 °C for four hours, and the nickel powder purity of each electrode was 99.99 wt% Two of the electrodes were tested by submersing each at one half of its full area, three quarters of its full area, and then completely The third sintered electrode, which had an initial two dimensional surface area much smaller than the other two, was completely submersed in electrolyte and tested at the same potential Electrode performance, based on current density measurements, tended to be greater for smaller 65 electrodes The reason for the observed trend has not been conclusively identified and is being investigated Current Density (j, mA/cm ) 40 35 30 25 20 15 10 10 15 20 25 30 35 40 45 50 Area (cm ) Figure 2.7: Current density (j, mA/cm2), measured at a potential of 1.8 V, as a function of electrode size Three electrodes fabricated under identical conditions were tested It was observed that as electrode size was increased, values of current density dropped All of the electrodes that were tested long-term suffered degradation of performance over time (Figure 2.8 and Table 2.6) The sintered electrodes were periodically removed from the potassium hydroxide and rinsed off in de-ionized water and then immediately re-tested The cleaned electrodes showed enhanced initial performance, and then the current density values dropped over several hours of continuous operation The electrodes were also agitated in solution during electrolysis in order to remove any gas bubbles that may have adsorbed onto the electrode surfaces, resulting in enhanced performance and then degradation over time When performance was recovered, it was never recovered to the initial testing value 66 40 60 35 30 50 j (mA/cm ) 45 70 j (mA/cm ) 80 40 30 20 10 20 15 10 a 25 200 400 600 800 1000 1200 b 200 Time (h) 600 800 1000 1200 Time (h) 30 50 25 40 20 j (mA/cm2) 60 j (mA/cm ) 400 30 20 10 15 10 c 200 400 600 800 1000 1200 d 100 Time (h) 200 300 400 500 Time (h) Figure 2.8: Current density (j, mA/cm2) as a function of time (h) for various sintered cathodes tested against sintered Ni-Al-CoO anodes, measured at a potential of 1.8 V Electrodes run continuously over an extended period of time suffered from degradation of performance Table 2.6: List of sintered electrodes, from Figure 2.8, subjected to accelerated longterm testing in 5.9 M potassium hydroxide at a potential of 1.8 V ELECTRODE COMPOSITION Ni PURITY (wt%) AREA (cm2) a 88:5:7 99.99 7.02 b 80:10:10 99 7.56 c 80:10:10 99 8.1 d 80:10:10 99.99 28.8 67 Temporary degradation of electrode performance is believed to be due to adsorption of hydrogen gas or hydroxyl ions at the surface of the electrodes, blocking potential reaction sites The adsorbed species reduced the active surface area of the electrodes resulting in lower current densities Permanent degradation of electrode performance is believed to be due to electrochemical passivation of the anode under operating conditions At the pH and potential of the system, the nickel theoretically will develop a thin protective oxide coating which decreases the electrodes conductivity in solution (Figure 2.9).5 However, no evidence supporting the passivation theory has been observed from x-ray diffraction scans due to the oxide layer being very thin Also, physical degredation of the electrode surfaces is believed to contribute to a drop in performance Over time the gas evolution physically wore down the surface of the electrodes reducing the active surface area Evidence of this was observed as black particles settled at the bottom of the electrolyzer during operation Overall, sintered nickel-molybdenum electrodes demonstrated the greatest performance based on current density measurements, electrochemical stability, and physical durability An optimum metal powder mixing ratio of 88:5:7 for nickel, aluminum, and molybdenum respectively produced the best electrodes (cathodes) based on accelerated long-term testing data A sintering temperature of 900 °C, held for a period of four hours, produced hard bricks that handled well Addition of cobalt to the nickel electrodes, instead of molybdenum, was investigated for the production of high quality anodes Electroplated nickel electrodes, fabricated by another individual within the lab, have shown promise, but their physical durability is an issue Both types of 68 nickel electrodes, sintered and electroplated, were porous in structure, having very large active surface areas Figure 2.9: Pourbaix diagrams for the nickel – water system at 25 °C At high pH and high potential, a passive oxide layer forms at the electrode surface 69 2-5 Summary The major contributions of the electrocatalyst work are summarized below: 1) Sintered nickel based cathodes exhibiting a high degree of electrocatalytic activity, electrochemical stability, and physical durability have been produced The optimum mixed metal powder weight ratio was found to be 88:5:7 for the nickel, aluminum, and molybdenum powders respectively Sintering at a temperature of 900 °C for a period of hours produced electrodes that were hard and durable After accelerated long-term continuous testing for over 1000 hours, current densities near 40 mA/cm2 have been demonstrated at a potential of 1.8 V 2) Sintered nickel based anodes exhibiting a high degree of electrocatalytic activity and physical durability have been produced The optimum mixed metal powder weight ratio was found to be 78:10:12 for the nickel, aluminum, and cobalt powders respectively Sintering at a temperature of 900 °C for a period of hours produced electrodes that were hard and durable 70 Future Work In the future more work needs to be done on doping n-type α-Fe2O3 with materials that will allow high quality films demonstrating photocurrent densities of at least mA/cm2 to be deposited at low temperatures (< 250 °C) Sputtering with a Fe2O3 target doesn’t seem to be an effective route to achieving the stated goals so reactive sputtering with an iron target in an oxygenated atmosphere is being considered as an alternative Possible new doping materials include strontium (Sr) and zinc oxide (ZnO) When films demonstrating the desired performance are producible on a consistent basis then a-Si solar cells will have to be coated in order to gauge hybrid PEC system performance In order to gain a better understanding of the r.f sputter deposition of iron (III) oxide thin films, more samples should be fabricated under conditions that will allow for more definite conclusions concerning deposition time, temperature, film thickness, and crystal size Many more sintered nickel electrodes need to be tested long 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