Sputter deposition of iron oxide and tin oxide based films

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

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

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

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

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 sputter-deposited under variable conditions Dopants were incorporated via co-sputtering in an attempt to enhance photocurrent response and overall film stability in basic media, 33 %

containing 2 % oxygen in argon at 100 W at 400 °C for 110 min 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 5 % oxygen in argon at

250 °C for 135 min 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

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

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 1

Section 1-2 Experimental 12 A Preparation for the Deposition of n-type Fe2O3 and 12

F-SnO2 Thin Films by RF Sputter Deposition B Fabrication of n-type Fe2O3 Thin Films With and 15

Without Incorporation of Metal Dopants C Fabrication of n-type F-SnO2 Thin Films 16 Section 1-3 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

Section 1-4 Results and Discussion 22

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 0 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…… 5

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……… 8

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 min 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

α-Fe2O3 thin films deposited on (a) Tec-15 glass and (b) ITO All samples represented were sputtered with a target power of 100 W with 2 % 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 min were more stable, but not very conductive As the deposition time was increased to 135 min the film stability decreased by a small amount, and the film conductivity

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 min 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 min 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 min at 400 °C with a

Fe2O3 r.f deposition power of 100 W in a chamber atmosphere containing 2 % oxygen in argon ambient at a pressure of 6 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 5 s intervals………35

1.22 Photocurrent density (jP, µA/cm2) versus applied potential (mV) of an In-Fe2O3

thin film electrode annealed up to 6 hours in an inert argon atmosphere at

550 °C.……….… 35

1.23 UV-vis spectroscopic measurement of an In-Fe2O3 thin film electrode deposited

with an indium power of 20 W at 200 °C having a thickness of 980 nm by Tauc calculations A Dektak3ST surface profiler was also used to measure film

thickness and similar values were obtained……… 36

1.24 Tauc plot for In-Fe2O3 sample deposited for 120 min at 200 °C with an indium r.f

deposition power of 20 W and a Fe2O3 rf deposition power of 100 W The chamber atmosphere contained 5 % oxygen in argon ambient at a pressure of 6

mTorr The band gap was determined to be 2.6 eV……… 37

1.25 X-ray diffraction measurements of In-Fe2O3 thin film electrodes deposited at

200 °C and with varying indium target deposition powers (5 to 20 W) Peak intensities are greater for higher indium target powers Peaks indicate the presence of (a) α-Fe2O3 and indium and iron oxide compounds such as (b) InFeO3

and (c) InFe2O4.……… … 37

1.26 X-ray diffraction measurements of In-Fe2O3 thin film electrodes deposited at

varying substrate temperatures with an indium target deposition power of 10 W Peak intensities are greater for higher temperatures, indicating greater crystallinity Film composition includes (a) Fe2O3, (b) InFeO3, and (c) InFe2O4……… 38

1.27 AFM images of indium doped α-Fe2O3 deposited with an indium target power of

5 W (a, b) and 20 W (c, d) Both films were deposited at 200 °C with 5 % oxygen

in argon ambient for 120 min Films deposited at higher temperatures demonstrated greater photocurrents due to greater active surface areas The scale

for both images is 5000 ¯ 5000 nm……… 40

1.28 Photocurrent density (jP, mA/cm2) versus applied potential (mV) of a Sb-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 5 s intervals……….… ….41

1.29 Photocurrent density (jP, µA/cm2) versus applied potential (mV) for a F-doped

SnO2 thin film electrode deposited on standard ITO Light and dark currents were measured using a light chopping method by manually blocking the light source

and then illuminating the electrode in 5 s intervals……… 42

1.30 UV-vis transmission spectra for F-SnO2 thin film deposited at 50 W for 135 min

at 250 °C The film transparency was found to be near 90% in the visible portion

of the spectrum, and a band gap of 3.38 was determined from tauc plot

calculations………42

1.31 Tauc plot for a F-SnO2 thin film deposited at 50 W for 135 min at 250 °C The

value of the band gap, E (eV), was found to be 3.37 eV……… 43

1.32 XRD spectra of a F-SnO2 thin film All peaks correspond to SnO2 The amount

of fluorine present is too small to be detected using the available

instrumentation……… 43

2.1 Concentration calibration curve for the platinum electroplating solution……….52

2.2 The stainless steel box used for sintering the metal powders There were four

levels labeled from A to D from the bottom to the top On each level, up to six electrodes could be placed for sintering, each labeled from a to f The box was

open on the front and back sides so that the levels could be accessed………… 56

2.3 8-chamber electrolyzer with H2 and O2 gas collection capabilities Each chamber

was divided into two compartments separated by a nylon membrane The

displacement of water was used to measure gas flow rates……… 56

2.4 H2 generation rates (mL/min) measured over time (min) for selected samples;

sputtered CrN (▲), and sputtered Ni-Co-Mo from (●)……… 62

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……….….63

2.6 XRD spectra of nickel powders of various purities No difference in composition

had been observed All peaks correspond to nickel……… 65

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……….….66

2.8 Current density (j, mA/cm2) as a function of time (h) for various sintered

electrodes, measured at a potential of 1.8 V Electrodes run continuously over an

extended period of time suffered from degradation of performance……….67

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

List of Tables

1.1 Crystal size (nm) based on α-Fe2O3 deposition conditions Crystal size increases

as film thickness is increased and as deposition temperature increases……… 28

1.2 Crystal size based on In-Fe2O3 deposition conditions Crystal sizes increase as In

power is increased (with the Fe2O3 power held at 100 W) and as deposition

temperature is increased……….……… ….38

2.1 List of materials studied for use as cathodes and anodes for water electrolysis

Most materials that were studied were nickel based……… ……50

2.2 List of various aluminum and nickel powders investigated for use in producing

2.3 Sample heating sequence program for sintering The first input variable c01 is the

starting temperature inside the furnace Input t01 is the time it takes to reach temperature c02, and so on Input -121 stops the program at the end of the

sequence……….55

2.4 Current densities (j, mA/cm2) for various anode and cathode combinations

measured at an applied potential of 1.8V Initially, the combination of platinum coated nickel sheet as an anode and sputter coated porous nickel as the cathode demonstrated the greatest performance, determined by comparing current density

values……… … 61

2.5 H2 and O2 evolution rates (ml/min) measured at an applied potential of 1.8 V for

various cathodes with a 3N nickel anode The volume of H2 generated should be exactly double the value of the volume of O2 generated based on water

electrolysis reaction stoichiometry……… ….61

2.6 List of sintered electrodes, from Figure 2.8, subjected to accelerated long-term

testing in 5.9 M potassium hydroxide at a potential of 1.8 V……… …… 67

List of Equations

Equation 1: 4H2O → 4H+ + 4OH-……… 3

Equation 2: 4OH- → 2H2O + O2……….3

Equation 3: 4H+ + 4e- → 2H2……… 3

Equation 4: 2H2O → 2H2 + O2 3

Equation 5: Si + 2OH- + 2H2O → SiO2(OH22-) + 2H2……… 5

Equation 6: hν ≥ Eg ……….……6

Equation 7: c = λν……… 6

Equation 8: photoanode (α-Fe2O3, etc.) + sunlight → 4h+ + 4e-……….8

Equation 9: 4OH- + 4h+ → O2 + 2H2O……… 8

Equation 10: H2O + photocatalyst + sunlight → H2 + ½O2……… 8

Equation 11: ε = [(jP ¯ E°rev) / Io] ¯ 100 % 19

Equation 12: D = 0.9λ / (βcosθ)……… 20

Equation 13: Fe + 4Fe2O3 → 3Fe3O4……… 23

Equation 14: η = 1.23 / V……… … 58

Equation 15: χ = P / H2……… 58

1-1 Introduction

Hydrogen (H2) as an alternative fuel with respect to fossil fuels is of great interest due to the fact that it is very attractive as an environmentally friendly energy carrier and source; assuming that it is produced in an environmentally friendly way Hydrogen may

be utilized either by combustion, producing work, or by running it through a fuel cell to generate electricity Both systems produce only water (H2O) as a byproduct and minimal pollution in the form of nitrogen oxides.1-5

It is very expensive and time consuming to harvest hydrogen in any appreciable amount from air because it makes up only 0.00005 % of the earth’s atmosphere by volume.6 Currently steam reformation of natural gas is the primary means of producing hydrogen, but unfortunately this process is heavily dependant upon the use of fossil fuels which generate pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx) and carbon dioxide (CO2), as well as others.1-7 These pollutants are responsible for undesirable phenomena such as acid rain and smog, and some would argue that the emission of carbon dioxide as well as other greenhouse gases contributes to global warming All of these phenomena affect a negative impact upon the environment and human health.3-9 Fossil fuels are also non-renewable resources and as supplies diminish energy prices associated with their use will continue to rise, especially with world-wide energy consumption continually increasing.4-5

Another method utilized for producing hydrogen is water electrolysis, which is the process of breaking a water molecule into its constituent parts by applying a voltage across two electrodes immersed in an aqueous electrolyte The electrolyte completes the circuit allowing current to flow through the solution, and in so doing, if the applied potential is of a sufficient magnitude, the water chemically breaks down and forms hydrogen (H2) at the cathode and oxygen (O2) at the anode (Figure 1.1) Theoretically,

the minimum voltage required to split water is 1.23 V at 25 °C, based on the Nernst equation, although realistically a voltage greater than 1.5 V is necessary due to electrode overpotentials.4-5, 10 Overpotential is a potential barrier that must be overcome before current can flow through the electrode / solution interface Water electrolysis is ideal due

to the great abundance of water available on this planet However, the electricity required to operate this type of system is still generated using conventional methods such

as the burning of fossil fuels.4

Ultimately, current processes of generating such a clean fuel as hydrogen require methods which are not environmentally friendly However, clean energy sources such as hydroelectric, wind, and solar may be applied to water electrolysis technology.1-5, 7, 10-14

Hydroelectric and wind power are limited in that there are only so many places where one may build a dam or wind farm, but solar energy has shown great promise in providing solutions to the world’s energy problems and for producing clean sustainable energy in the near future

Figure 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 (4)

The process of photoelectrochemical decomposition of water using semiconductor photoelectrodes was first reported in 1972 by Fujishima and Honda.15 Titanium dioxide (TiO2) was used to absorb ultraviolet (UV) radiation and internally generate an electric potential due to the separation of charges within the material The potential was not very large, but when it was enhanced by applying a small outside bias it was observed that water could be electrochemically split into hydrogen and oxygen gases

Titanium dioxide may absorb UV light up to 414 nm, which makes up only a

small portion of the total solar spectrum (Figure 1.2), because its band gap is relatively

Vapp

Cathode (-)Anode (+)

4H2O → 4H+ + 4OH- (1) 4OH- → 2H2O + O2 (2)

4H+ + 4e- → 2H2 (3) _ 2H2O → 2H2 + O2 (4)

is unable to produce sufficient voltage required to split water; this is the reason why Fujishima and Honda had to apply an outside potential bias to their electrode in order for their system to work This limitation indicates that titanium dioxide cannot itself stand

up to the task of photoelectrochemically generating hydrogen

Figure 1.2: Standard, AM 0 (upper) and AM 1.5 (lower), solar spectrum UV 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

A great deal of effort has been invested into devising photoelectrochemical cell (PEC) systems that will generate hydrogen efficiently and at low cost.1-2, 5, 10-13, 15, 18-23 One proposed system involves submersing a multijunction PEC electrode, coated with a transparent conductive oxide (TCO) layer, into an electrolyte which would then

spontaneously generate the gas upon illumination with sunlight (Figure 1.3).1, 20-21 An alternative approach utilizes hybrid multijunction PEC electrodes having photoactive semiconductor (PAS)-electrolyte junctions.21-23 The photoelectrodes utilized are triple junction amorphous silicon (a-Si) solar cells.20-25 The advantage of using these devices is

that they may be fabricated at low cost and they absorb in the visible portion of the solar spectrum which contains the greatest amount of radiation with sufficient energy for water splitting that is able to reach the surface of the earth, allowing for the most efficient use

of the solar spectrum.11 Unfortunately, the silicon alone will deteriorate in the electrolyte

(Equation 5), so a protective layer is necessary

Figure 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

The first PEC system described requires the TCO as corrosion protection Examples of possible TCO materials for use in basic media include indium-tin oxide

Si + 2OH- + 2H2O → SiO2(OH22-) + 2H2 (5)

alternative PEC system does not need to be coated with a TCO Instead the topmost junction of the multijunction is replaced with a photoactive semiconductor material

which must be chemically stable (Figure 1.4) An example of a possible photoactive

semiconductor material for use in basic media is iron (III) oxide (hematite, α-Fe2O3).21-23

Figure 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

When a semicounductor electrode becomes illuminated, it absorbs photons with energies (hν) greater than its band gap energy (Eg),

where h is Planck’s constant (4.14 x 10-15 eV·s) and ν is the frequency of the light The frequency may be obtained from the wavelength (λ) by the relationship,

c = λν (7)

where c is the speed of light (3 x 108 m/s)

The band gap is the energy difference between the material’s valence and conduction bands Materials with band gaps greater than 4 eV are considered insulators; those with little or no band gap are considered metals Semiconductors lie in between When photon energy is absorbed, electrons from the valence band are excited up into the conduction band and a positively charged hole is left in the valence band Semiconductors possessing n-type characteristics (α-Fe2O3, F-SnO2) have upward band bending; the excited electrons move to the back of the electrode and the holes move to

the surface (Figure 1.5) The reaction that takes place in solution (Figure 1.6) is similar

to that for standard electrolysis However, with a self-driven PEC no outside voltage bias needs to be applied All of the voltage required to run the reaction is generated within the device itself and the oxygen and hydrogen gases are generated on the front and back surfaces, respectively

FTO is a good candidate as a TCO for self-driven PECs because it acts as an optical window allowing photons of interest to pass through to be absorbed within the multijunction on which it is deposited.28-36 The band gap of FTO is near 3.3 eV, and as with titanium dioxide it will only absorb photons in the UV region of the spectrum.28FTO is sufficiently conductive, and it has been found to be stable in aqueous alkaline solutions FTO thin films have been fabricated by various methods including chemical vapor deposition (CVD), evaporation, spray pyrolysis, sol-gel, and sputtering.28-36

Figure 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

Figure 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

The overall reaction is given as:

H2O + photocatalyst + sunlight → H2+ ½O2 (10)

Iron (III) oxide, among other iron oxides, is commonly found as rust which is formed from iron (Fe) in the presence of water However, it may be produced as the anhydrous form which has been found to be a cheap and useful semiconductor material α-Fe2O3 has a band gap of 2.0 to 2.2 eV and may absorb electromagnetic radiation with wavelengths up to 621 nm which is in the visible portion of the solar spectrum Its band gap makes it a perfect candidate as the top junction for hybrid multijunction PECs because it will absorb in the same range as the portion of the multijunction it replaces Also, α-Fe2O has been found to demonstrate photoactivity which is necessary for this application; a photocurrent of at least 7.5 mA/cm2 is required in order to effectively current match with the middle and bottom component solar cells of the multijunction Current matching is important because the cell with the lowest current will limit the performance of the entire device A protective TCO layer is not necessary for this system due to the stability of α-Fe2O3 in aqueous alkaline solutions As with FTO films, hematite thin films may be fabricated by a number of methods including CVD, sol-gel, spray pyrolysis, laser ablation deposition, wet chemical deposition, and sputtering.37-53 The problem with pure α-Fe2O3 is that it is a resistive semiconductor, and therefore the introduction of dopants of higher valence to make it more n-type, or lower valence to make it more p-type, has also been investigated to enhance its electrical characteristics.54-

59

Generally, optimized α-Fe2O3 and FTO thin films have been deposited or annealed at temperatures exceeding 400 °C However, this is undesirable because if the amorphous silicon multijunctions are subjected to temperatures greater than 250 °C then

they significantly degrade Therefore methods of producing these materials at low temperatures are being investigated.50, 59

The thin films described in this work were fabricated using a process called radio frequency (r.f.) sputter deposition In the process, the substrates onto which the films are

to be deposited are placed into a vacuum chamber, also known as the sputter chamber, and all of the air is pumped out of the chamber until sufficient vacuum (10-5 torr or less)

is obtained Argon (Ar) gas is then passed into the chamber Within the chamber there may be one or multiple sputter guns onto which targets are placed; targets are disks made out of the materials that are to be deposited, or their metal counterparts that will be oxidized during the deposition A r.f power supply is connected to the sputter gun and when the power is turned on an electric field is generated between the target and the substrate The current is alternating, so free electrons are alternately attracted to and repelled from the target As the electrons move between the target and the substrate they collide with argon atoms and knock off more electrons This process continues exponentially until a cloud of argon ions (Ar+) and electrons, called plasma, is generated The argon ions bombard the surface of the target and physically knock off pieces which

are ballistically ejected and deposited onto the substrate (Figure 1.7) The same process

would occur at the surface of the substrate, however magnets placed within the sputter gun confine the plasma to a region immediately above the target In this way, small pieces of target material are sputtered onto the substrate and over time a thin film is developed

Figure 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

Ar+

Ar+

α-Fe2O3, SnF2/SnO2, etc

sputter gun (cathode)cap (anode)

target substrate

substrate holder

a gun power of 60 W at a temperature of 200 °C for 9.25 min The glass slides were cleaned in a de-ionized water / acetone bath placed in a sonicator (Branson 8510), and then dried under a stream of nitrogen (N2) gas (99.999 %, Linde) Thin films were also deposited on Tec-15 3.2, FTO coated glass (3.2 ¯ 25 ¯ 75 mm, Pilkington) The Tec-15 glass was cut by hand from a 1 ft2 sheet, and the sized pieces were cleaned in the same manner as the microscope slides Four to five glass substrates were placed in a 4 ¯ 4

inch stainless steel substrate holder (Figure 1.8) Strips of stainless steel were used to

cover parts of the substrates allowing for bare electrical contacts (ITO, FTO) necessary for photocurrent and stability measurements

The substrate holder was placed into the deposition chamber through a removable flange on the side of the chamber Steel tracks held the substrates over the target(s); up to

three targets (2 inch diameter) placed on three individual sputter guns could be utilized simultaneously Once the substrates were centered over the target(s), the flange was replaced and a rotary vane vacuum (roughing) pump (DUO 10C, PK D62 727 B, Pfeiffer) was used to pump the chamber down to about 100 mTorr The roughing pump was then isolated from the chamber and a turbomolecular (turbo) pump (TMU 261P DN 100 CF-F 3P, PM P02 826H, Pfeiffer) was used to pump down to less than 10-5 torr Chamber pressure was monitored with a vacuum gauge measurement and control system (type 146C, MKS)

A convection gauge (MKS) was used to measure pressure from atmospheric to about 100 mTorr, a Baratron capacitance manometer (MKS) was used to measure pressures in an operating range of 1 to 100 mTorr, and an ion gauge (MKS) was used to

Figure 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

measure pressure in ultra high vacuum (UHV) Once the chamber pressure was below

10-5 torr, gas flow, substrate heating, and substrate rotation was initiated

The gases utilized in the deposition of the thin films were argon (99.998 %, Linde) and an oxygen / argon mixture (20 % O2 and Ar balance, Linde) Gas flow was controlled with nitrogen mass flow controllers (MKS) connected to a four-channel readout (type 247, MKS) Two transformers, an isolation transformer (115 V, 8.7 A, Stancor GIS 1000) in line with a variable transformer (140 V, 10 A, Staco Energy Products Company), were connected to two halogen bulbs (J120V-500W/FCL, USHIO) wired in parallel The bulbs were housed above the substrate holder; reflectors directed the radiation down onto the substrate holder allowing for substrate heating up to 450 °C Temperatures were set via the variable transformer, but the units were of an arbitrary scale Therefore a multimeter (OmegaetteTM, Omega) was connected to the transformer

and a temperature versus voltage calibration was done prior to depositions (Figure 1.9)

During the calibration the thermocouple (K-type, Omega) was attached directly onto the substrate holder, but during deposition the thermocouple could not be directly attached to the substrate holder due to its rotation; it was attached to part of the lamp housing in a location that would not obstruct the deposition process Heat transfer in vacuum is slow and therefore the temperature gauge (Omega) reading was lower than the actual temperature of the substrate; during depositions the temperature gauge reading was taken only as a relative value of ancillary importance A sample rotator (K J Lesker) ensured uniform heating and deposition

0 10 20 30 40 50 60 70 80 90 100 110 0

50 100 150 200 250 300 350 400 450

Figure 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

B Fabrication of n-type α-Fe 2 O 3 With and Without Incorporation of Metal

Dopants

When the substrate temperature had reached the desired value, the r.f generator (ACG-3B, ENI) was turned on and set to the desired value Oxide targets (e.g α-Fe2O3) are brittle and the power had to be increased at a slow rate (1 W per second) to prevent the target from cracking from an increase of vibrations and temperature within the target After the plasma was ignited it was allowed to sit for 10 to 15 minutes in order to reach a steady state for deposition A shield located over the target prevented the substrate from getting coated during the pre-sputtering After pre-sputtering, the shield was raised and the sputter deposition was initiated

Pure α-Fe O thin films were directly deposited from a 0.25 inch thick hematite

varied were rf power (50 to 100 W), substrate temperature (200 to 475 °C), oxygen concentration in the chamber atmosphere (0 to 10 % by volume in Ar ambient), and deposition time (90 to 150 min) The chamber pressure, controlled by a gate valve located just above the turbo pump inlet, was held at 6 mTorr during all depositions

Doped α-Fe2O3 was done by co-sputtering The process was the same as that for the deposition of pure α-Fe2O3, however two targets (hematite and the dopant material) were sputtered simultaneously For the deposition of tantalum (Ta) doped α-Fe2O3, the α-Fe2O3 power was held at 80 W while a 0.25 inch thick tantalum (99.9 wt%, K J Lesker) target power was varied from 5 to 30 W; temperature was varied from 200 to

300 °C Indium (In) doping was done with a 0.25 inch thick indium target (99.995 wt%,

K J Lesker) The α-Fe2O3 power was held at 100 W while the indium power was varied from 5 to 20 W, and the temperature was varied from 150 to 250 °C The oxygen concentration in the chamber atmosphere was also varied from 0 to 10 % by volume in argon ambient The chamber pressure was held at 6 mTorr for all doped sample depositions

C Fabrication of n-type F-SnO 2 (FTO)

FTO thin films were deposited in the same manner as the α-Fe2O3 thin film depositions A 0.25 inch thick tin fluoride (SnF2) / tin dioxide (SnO2) target (25 wt% and

75 wt% respectively, Cerac, Inc.) was used as the sputter source Films were deposited at temperatures of 250 and 300 °C Power was varied from 40 to 60 W, the oxygen concentration was varied from 10 to 20 % by volume in argon ambient, chamber pressure

was varied from 6 to 15 mTorr, and deposition time was varied from 60 to 165 min

Figure 1.10 shows the actual sputter chamber used for all thin film depositions

Figure 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 r.f 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)

1

2

3 4

5

6

7

1-3 Thin Film Characterization

–0.5 to +1 V was used at a scan rate of 20 mV/s The thin film electrodes were illuminated with 5 s chopped light

The total photoconversion efficiency for a self-driven PEC (ε, %), may be calculated by using the following equation given as,57-58

ε = [(jP ¯ E°rev) / Io] ¯ 100 % (11)

where jP is the measured photocurrent, E°rev is the standard state reversible potential (1.23

V for splitting water) and Io is the light intensity

C UV-vis Spectroscopic Measurements

UV-vis spectra of n-type α-Fe2O3 and FTO thin films were recorded using a UV/VIS/NR-Cary 5 Diode Array (HP8452A) Samples deposited on plain glass

microscope slides were used, with a pure glass substrate as a baseline The spectra of all samples were measured in the wavelength range between 300 and 2000 nm

Film thickness was calculated using an interference method, a procedure developed for calculating the thickness of thin films.60 Band gap values were determined from tauc plot calculations.61

D X-ray Diffraction Spectra Measurements

X-ray diffraction (XRD) spectra were collected on a X-ray powder difractometer (X’Pert Pro, PANalytical) with a Dell Optiplex PC utilizing X’Pert Data Collector software The scans were collected via a glazing angle in the range from 25 to 75 ° (2θ) using copper Kα radiation with a wavelength of 0.15405 nm Scans were analyzed with X’Pert High Score Plus software (PANalytical) and matched against the International Center for Diffraction Data (ICDD) database

The crystal size of the thin film may be determined by applying the XRD data to Scherer’s equation,62

where D is the average crystal size, λ is the wavelength, β is the peak width at half maximum, and θ is the diffraction peak angle

E Thin Film Thickness Measurements

The thickness of the n-type α-Fe2O3 and the FTO thin films was measured using a Dektak3ST surface profiler, as well as by the interference method using UV-vis spectroscopy.60-61

F Atomic Force Microscopy Measurements

Atomic Force Microscopy (AFM) measurements were done with a PicoSPMII

system (Molecular Imaging) The AC-AFM non-contact mode was utilized

G Annealing

Selected films were annealed in a high temperature box furnace (ST-1700-666, Sentro Tech Corporation) A flow of industrial grade argon gas (99+ %, Linde) was directed down into the furnace through a feed-through located at the top of the furnace to create an inert atmosphere Samples that were annealed were cut from larger samples so that trends could be observed as annealing conditions were changed

1-4 Results and Discussion

A n-type α-Fe 2 O 3

The results of photocurrent-potential dependence were optimized with respect to several parameters including r.f power, substrate temperature, oxygen concentration in the argon ambient and deposition time α-Fe2O3 films were deposited on both ITO coated glass and on Tec-15 Depositions on the Tec-15 produced films demonstrating generally higher photocurrents following clearer trends due to consistent film quality

The Fe2O3 targets used for the thin film depositions contained small amounts of pure iron (Fe) which the manufacturer added in order for the targets to be pressed more easily during fabrication Also, during the sputtering process an oxygen deficiency may occur in an inert atmosphere contributing to the reduction of the Fe2O3 target to Fe3O4, then FeO, and then to pure Fe.42, 63 Film deposition in an inert argon atmosphere resulted

in poor quality films that were often times magnetite, not hematite Magnetite is black in color, conductive and magnetic, though not photoactive Addition of oxygen to the chamber atmosphere during deposition alleviated the problem and was found to enhance the stability and photoactivity of the films because the free iron from the target was oxidized to Fe2O3 and reduction of the target was minimized (Figure 1.11) The

optimum oxygen concentration was determined to be 2 % in argon ambient The

reduction of hematite to magnetite is expressed in Equation 13

0 2 4 6 8 10 0

20 40 60 80 100 120 140 160

Fe + 4Fe2O3 → 3Fe3O4 (13)

Over a range of deposition temperatures from 250 to 475 °C, n-type α-Fe2O3 thin films were found to exhibit greater photoactivity as substrate temperature was increased

(Figure 1.12) However, when depositions were carried out at the higher end of the

temperature range, the halogen bulbs tended to degrade quickly because the power required to attain those temperatures was near the bulbs maximum power rating After running depositions at high temperatures, and therefore high voltages, the actual temperature would be lower the next time a deposition would be done at a given voltage

As the bulbs degraded, more power was required to reach the same temperature over

subsequent runs; this was not recognized initially All temperature data was normalized

to correct values using calibrations

0 50 100 150 200 250

n-a

b

The α-Fe2O3 films were found to be stable in 5.9 M potassium hydroxide (KOH)

up to 3 V versus SCE at all deposition temperatures investigated once oxygen had been added to the chamber atmosphere Films that were deposited for 90 min exhibited the greatest degree of stability and as deposition time and therefore film thickness increased,

some stability was lost (Figure 1.13), although conductivity increased as the films became thicker Photocurrent density versus film deposition time was plotted (Figure

1.14) and an optimum deposition time of 110 min was obtained

0 50 100 150 200 250 300

Figure 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 min were more stable, but not very conductive As the deposition time was increased to 135 min the film stability decreased by a small amount, and the film conductivity increased

90 min

105 min

110 min

135 min

80 100 120 140 160 0

50 100 150 200 250

Deposition Time (min)

Figure 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 min with the Fe2O3 deposition power set to 100 W UV-vis transmission spectra were used to calculate an average film thickness of 280 nm

Most n-type α-Fe2O3 thin films demonstrated photoactivity upon illumination, but only to a small degree The greatest photocurrent observed, ~ 0.34 mA/cm2 under 0.75 sun illumination, occurred for a sample deposited with an r.f power of 100 W, at 400 °C

a

b