A study of a fe2o3 thin films for the oxygen evolution reaction in water photolysis

A STUDY OF α-Fe2O3 THIN FILMS FOR THE OXYGEN EVOLUTION REACTION IN WATER PHOTOLYSIS BAO JI (B. E., ZHEJIANG UNIVERSITY) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2010 I Acknowledgement ACKNOWLEDGEMENT I am sincerely grateful to everyone who has helped me in my master research study at the National University of Singapore. First of all, I would give my greatest gratitude to my supervisor, Professor Lee Jim Yang, for his unrelenting positivism and guidance throughout the course of this research project. His meticulous attention to detail, constructive criticisms and insightful comments have helped me to shape the research direction and define the specific topics for in-depth investigations. I also thank him for providing me a good opportunity to work with a talented team of students and research staffs. At the same time, I am very thankful to Dr. Liu Bin and Dr. Wang Qing, who provided indispensable guidance to this thesis work. Also, I would like to express my sincere thanks to all of my friends and colleagues in the laboratory, especially Dr. Deng Da, Dr. Liu Bo, Dr. Xue Yanhong, Dr. Yang Jinhua, Dr. Zhang Chao, Dr. Zhang Qingbo, Mr. Chia Zhi Wen, Mr. Cheng Chin Hsien, Mr. David Julius, Ms. Yu Yue, Ms. Fang Chunliu, Ms. Ji Ge, Ms. Lu Meihua, and Mr Ma Yue. Without their collaboration, I could not have completed this work. I Acknowledgement Mr. Boey Kok Hong, Ms. Lee Chai Keng, Mr. Chia Phai Ann, Mr. Liu Zhicheng, Dr. Yuan Zeliang, Ms. Siew Woon Chee, and Madam Koh Li Yong are the unsung heroes who provided the technical support for this thesis work. I am indebted to them for all the services rendered. I acknowledge National University of Singapore for its research scholarship during the last two years. Finally, I would like to give my deepest gratitude to my family. I would like to dedicate this thesis to all my family members. Without their understanding and support, I could not finish my master study. II Table of contents TABLE OF CONTENTS ACKNOWLEDGEMENT I TABLE OF CONTENTS .III SUMMARY VI ABBREVIATIONS .VIII LIST OF FIGURES . IX LIST OF TABLES XIII 1. INTRODUCTION .1 1.1 Background .1 1.2 Objectives and Scope 2. LITERATURE REVIEW 2.1 Fundamentals of Photoelectrochemical Water Splitting and Solar Energy .6 2.2 Photoelectrochemical Water Splitting Systems 2.2.1 Powder-based systems 2.2.2 Electrode-based system 2.3 Semiconductor Photocatalysts for Photoelectrochemical Water Splitting .12 2.3.1 Fundamentals of semiconductors 12 2.3.2 TiO2 .16 2.3.3 Fe2O3 .18 3. SYNTHESIS OF α-Fe2O3 ELECTRODES FOR PHOTOELECTROCHEMICAL III Table of contents WATER OXIDIZATION 23 3.1 Introduction 23 3.2 Experimental 27 3.2.1 Electrodeposition of iron thin films 27 3.2.2 Electrodeposition of FeOOH by the Ryan method 28 3.2.3 Electrodeposition of FeOOH by the Schrebler method .28 3.2.4 Calcination .28 3.2.5 Photoelectrochemical performances measurements 29 3.2.6 Characterizations 29 3.3 Results and Discussion .30 3.3.1 Characterizations of the as-deposited and calcined iron thin films 30 3.3.2 Effects of anions 41 3.3.3 Effects of pH in electrodeposition 43 3.3.4 Effects of calcination time 47 3.3.5 Effects of calcination temperature 49 3.3.6 Comparisons with α-Fe2O3 thin films prepared from other acidic baths .52 4. CONCLUSIONS AND SUGGESTIONS 58 4.1 Conclusions of this study 58 4.2 Suggestions for future work 59 4.2.1 Hematite thin films with more impurities .59 4.2.2 Doping .62 IV Table of contents 5. References .63 V Summary SUMMARY The thesis reports a refined method to synthesize effective photocatalysts for the oxygen evolution reaction (OER) in photoelectrochemical water splitting under visible light. It also attempts to seek some basic understandings of the relationships between surface structure and photocatalytic activity through a combination of analytical techniques including field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoelectrochemical measurements. α-Fe2O3 was chosen as the photocatalyst of interest because of its suitable band gap, low cost and its photochemical stability in neutral to basic solutions. A new two-step procedure consisting of electrodeposition of iron under acidic conditions and post-synthesis calcination in air at 650oC was used to synthesize α-Fe2O3 thin films. This method has the advantages of being simple, low cost, environmental friendly and potentials for further modifications by metal doping. The α-Fe2O3 thin films showed appreciable photoelectrochemical performance compared with the results of others. Some optimizations of the preparative conditions have also been carried out including the types of anions in the plating bath, plating pH; calcination time and calcination temperature. The α-Fe2O3 thin films with better photoelectrochemical performance have a non-porous compact morphology favorable for charge carrier mobility and VI Summary high crystallinity which supports the diffusion of electrons and holes through certain highly conducting crystal planes. We also compared our method with methods of others and attributed the improved photoelectrochemical performance observed here to the effects of intrinsic impurities such as Fe(0) and Fe(II) on charge carrier conduction. Topically, this thesis is divided into chapters. Chapter introduces the background and the scope of work. Chapter reviews the literature most relevant to this thesis study. Chapter is the report of major findings. Chapter is the conclusion of this study with suggestions for further work in the future. VII Abbreviations ABBREVIATIONS AM1.5G Air mass 1.5 global EDTA Ethylenediaminetetraacetic acid FESEM Field emission scanning electron microscopy FTO Fluorine doped tin oxide HER Hydrogen evolution reaction IPCE Incident photon to current efficiency NHE Normal hydrogen electrode OER Oxygen evolution reaction RHE Reversible hydrogen electrode PV Photovoltaic XPS X-ray photoelectron spectroscopy XRD X-ray diffraction UV Ultraviolet VIII List of figures LIST OF FIGURES Fig. 1.1 World Renewable energy consumption of 2008. Fig. 2.1 A simplified sketch of photoelectrochemical water splitting .7 Fig. 2.2 The Solar radiation spectrum. Fig. 2.3 Sketch of a visible light powder-based photoelectrochemical water splitting system. Fig. 2.4 Sketch of an electrode-based photoelectrochemical water splitting system under visible light. 10 Fig. 2.5 Mechanism of dye-sensitized photoelectrochemical water splitting under visible light .11 Fig. 2.6 The semiconductor band gap. 13 Fig. 2.7 Band structures of some common semiconductors and the redox potentials of water splitting. 14 Fig. 2.8 Photocurrent density (left) and photoconversion efficiency (right) as a function of potentials applied to the carbon doped n-TiO2 (flame-made) and the reference n-TiO2 (electric tube furnace or oven-made) photoelectrodes under xenon lamp illumination at an intensity of 40 mW cm-2 .17 Fig. 2.9 Photocurrent densities of (a) Si-doped Fe2O3 (b) Si-doped Fe2O3 after Co treatment in darkness and in AM 1.5 respectively 19 Fig. 2.10 Pourbaix (potential-pH) diagram of iron. .20 Fig. 2.11 Photoelectrochemical behavior of double anodized iron oxide film annealed in acetylene at 550℃for 10 min. 21 Fig. 2.12 Photocurrent density -potential curve for the annealed Fe2O3/FTO electrode in 0.1 M NaOH + 0.05 M KI solution 22 Fig. 3.1 XRD patterns of (a) as-deposited iron thin films, (b) α-Fe2O3 thin films obtained from the as-deposited iron thin films after two hours of calcination in air at 650 oC. (c) shows the absence of peaks from crystalline Fe (44.7°), FeO (42.2°) and Fe3O4 (62.7°) in the sample. Peaks from the FTO glass are marked by * in (a) .31 IX Chapter We can use this explanation to interpret the difference in photoelectrochemical performance between our α-Fe2O3 thin films, which were made by the calcination of electrodeposited iron films, and α-Fe2O3 thin films made by the calcination of electrodeposited FeOOH films. The calcination of iron thin films is expected to have a higher probability of generating impurities in the form of Fe(0) and Fe(Ⅱ), through the incomplete oxidation of iron. The calcination of FeOOH, by comparison, is simple and cleaner as it is a simple dehydration process without valence changes; and a higher purity hematite may be expected. Although the amounts of Fe(0) and Fe(Ⅱ) may be too low to be detected by XRD or XPS, the trace presence of these impurities can play important roles in the photoelectrochemical performance of hematite thin films. These impurities can function as electron and hole traps to reduce the electron-hole recombination rate and increase the charge carrier density via the following processes: (Choi et al., 1994) Mn+ + ecb- → M(n-1)+ electron trap Mn+ + hvb+ hole trap → M(n+1)+ Using Fig. 3.23 and the Mott-Schottky equation, we estimated the charge carrier densities of α-Fe2O3 thin films prepared by the method of Ryan and the method of Schrebler to be 4.04×1020 cm-3 and 4.24×1020 cm-3 respectively under illumination. These values are smaller than the charge carrier density of α-Fe2O3 thin films made by our method, which was 6.14×1020 cm-3, attesting to the influence of electron and hole traps. 54 Chapter Hence the holes in the α-Fe2O3 thin films prepared by our method are more available for water oxidation in the midst of recombination with electrons. Compared with processes which make use of extraneous impurities as dopants, the dopants in our case are internally generated, and are hence more economical and easier to realize. Fig. 3.20 FE-SEM images of α-Fe2O3 thin films prepared by (a) the method of Ryan and (b) the method of Schrebler. 55 Chapter 5000 (110) 4500 Intensity (a.u.) 4000 (104) 3500 * 3000 The method of Ryan 2500 The method of Schrebler 2000 1500 Calcinated bare FTO glass 1000 33 34 35 36 37 2θ (Degrees) Fig. 3.21 XRD patterns of α-Fe2O3 thin films prepared by the method of Ryan and the method of Schrebler. Peaks from FTO glass are indicated by *. 35000 Fe 2p 1/2 30000 Fe 2p 3/2 Intensity (a.u.) 25000 20000 15000 10000 The method of Ryan 5000 The method of Schrebler 740 730 720 710 700 690 Binding Energy (eV) Fig. 3.22 XPS spectra of α-Fe2O3 thin films prepared by the method of Ryan and the method of Schrebler. 56 Chapter 2.50E+010 The method of Ryan → 1.50E+010 1/C (cm /F ) 2.00E+010 The method of Schrebler ↙ 1.00E+010 ↖ Our method 5.00E+009 0.00E+000 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 Voltage (Ag/AgCl vs. V) Fig. 3.23 Mott-Schottky plots of α-Fe2O3 thin films prepared by the methods of Ryan et al., Schrebler et al. and the method of this study. Measurement conditions: 1M NaOH aqueous solution at a frequency of 1000Hz under illumination by simulated sunlight (AM 1.5G, 100mW/cm2) 57 Chapter CHAPTER CONCLUSIONS AND SUGGESTIONS 4.1 Conclusions of this study A refined method for synthesizing α-Fe2O3 thin films for photoelectrochemical water oxidization under visible light was proposed. This method involves the electrodeposition of iron thin films in acidic plating baths followed by calcination in air at 650℃ to oxidize the electrodeposited Fe thin films to hematite thin films. The sample showed a photocurrent density of 0.486mA/cm2 at 0.2V vs. Ag/AgCl in 1M NaOH aqueous solution under with illumination by simulated sunlight (AM 1.5), which is more than double the value of α-Fe2O3 thin films prepared by other commonly used electrodeposition methods in acidic baths. The satisfactory photoelectrochemical performance can be attributed to a number of factors: a compact, non-porous film morphology favorable for charge carrier mobility, high crystallinity which supports the diffusion of electrons and holes through certain highly conducting crystal planes; and the effects of intrinsic impurities such as Fe(0) and Fe(II) on electron conduction. 58 Chapter Standard (highest performance) Other plating bath Plating bath Plating pH Calcination time Calcination temperature FeCl2 4.6 2h 650oC FeSO4 4.6 2h Lower Plating pH FeCl2 3.6 2h Longer calcination time FeCl2 4.6 4h Lower calcination temperature Lower calcination temperature FeCl2 FeCl2 4.6 4.6 2h 2h o 650 C Influences Photocurrent density at 0.2V vs. Ag/AgCl) (mA/cm2) 0.486 Lower crystallinity 0.188 650 C Loose and porous morphology 0.246 650oC Sintering 0.393 Loose and porous morphology 0.301 Loose and porous morphology 0.12 o o 520 C o 400 C Table 4.1 Effects of different factors on photochemical performance 4.2 Suggestions for future work 4.2.1 Hematite thin films with more impurities We believe that the trace presence of impurities (although they cannot be detected by XRD and XPS) could significantly affect the photoelectrochemical performance of 59 Chapter hematite thin films. The amount of impurities could in principle be altered by changing the calcination atmosphere. We have produced a sample with more impurities by calcining an iron thin film in nitrogen for 45 minutes and in air for 15 minutes in sequence. However, this film was not produced under the optimal synthesis conditions. After calcination, the sample was found to contain both orange-red color and black color components. The black component was impurities, which could be Fe or Fe3O4. While these films were made with five minutes of plating instead of two minutes of plating, the sample was still too thin to show any XRD peak to reveal the composition of impurities. The calcination temperature for the sample was 520oC. The particular sample, which was not optimally produced, still achieved an appreciable photocurrent density as shown in Fig. 4.1. The photocurrent density at 0.2V vs. Ag/AgCl was 0.349mA/cm2, lower than the sample calcined in air at 650oC (0.448mA/cm2) but higher than the sample calcined in air at 520oC (0.316mA/cm2). When the bias potential increased, the increase in the photocurrent density for the sample with more impurities was faster than the other samples. At the dark current onset potential (0.55V vs. Ag/AgCl), the photocurrent density was 0.904mA/cm2, higher than the other two samples. 60 Chapter Photocurrent Density (mA/cm ) 2.5 2.0 ←a b→ 1.5 1.0 c ↘ 0.5 0.0 Dark -0.2 0.0 0.2 0.4 0.6 0.8 Voltage (Ag/AgCl vs. V) Fig. 4.1 Photocurrent densities of α-Fe2O3 thin films prepared by different fabrication conditions. (a) Electrodeposition time: five minutes; calcination conditions: 45 minutes in nitrogen and 15 minutes in air at 520oC, cooling in air. (b) Electrodeposition time: two minutes; calcination conditions: one hour in air at 520oC. (c) Electrodeposition time: two minutes; calcination conditions: one hour in air at 650oC. Measurement conditions: 1M NaOH aqueous solution. Scan rate: 0.04V/s under illumination by simulated sunlight (AM 1.5G, 100mW/cm2). For future work, it is suggested that we can optimize the preparation conditions for producing hematite thin films with more but controlled impurities. Shorter electrodeposition time and higher calcination temperature should be explored. Since the oxidization of iron thin film is much faster under these conditions, we need more careful control of the calcination times in air and in nitrogen in case of complete oxidization. Furthermore, thinner films may also overcome a heterogeneous film structure, as shown by the film formed by five minutes of electroplating (the 61 Chapter organ-red and black components). Uniform heating is also more possible with thinner films to homogenize the film composition. In addition, Rangaraju et al. have synthesized mixed hematite-maghemite thin films with high photocurrent density, which is another proof for the value of controlled impurity addition to the hematite films (Rangaraju et al., 2009). 4.2.2 Doping Many elements have been used for doping the hematite thin films to improve their photoelectrochemical performance. Since we have developed a new method to fabricate hematite thin films with higher intrinsic photocurrent density than other commonly used methods, we have established a solid foundation for further improvements through doping with other elements. Till now, the most effective dopants are Ti and Si. However, Si cannot be applied as dopants through electrodeposition by conventional methods since it is a non-metal. Fortunately, a new method for electrodeposition of Si from ionic liquid has recently been established (Al-Salman et al., 2008) which may be explored for the electrodeposition of Si on the hematite thin films. 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Mater. 2008, 20, 2629–2636. 69 [...]... project includes the synthesis of α -Fe2O3 thin films, tests of their photoelectrochemical performance; and investigations of associated scientific issues The detailed research activities include the following: 1 Synthesis of α -Fe2O3 thin films The synthesis of α -Fe2O3 thin films by a two-step process comprising the electrodeposition of thin iron films and the calcination of the iron films in air at 650℃... which is a half reaction in water splitting The OER is a four-electron process which is kinetically more hindered than the two-electron hydrogen evolution reaction, the other half reaction in water splitting (Youngblood et al., 2009) We have chosen α -Fe2O3 as the candidate photocatalyst because of its appropriate band gap, its stability in neutral or alkaline solution; and its low cost The scope of this... RHE) The good performance has been attributed to a dendritic nanostructure and the presence of a thin insulating SiO2 interfacial layer between the FTO (fluorine doped tin oxide) glass substrate and the Fe2O3 thin film Further treatment of the film in Co(NO3)2 solution increases the 18 Chapter 2 photocurrent density to 2.7mA/cm2 at 1.23V vs RHE due to hole trapping and water oxidization in the surface...List of figures Fig 3.2 XPS spectrum of α -Fe2O3 thin film formed by calcination of the as-deposited iron thin film for two hours at 650oC in air 32 Fig 3.3 FE-SEM images of (a) the as-deposited iron thin film, (b) cross-sectional view of α -Fe2O3 thin film formed by calcining the as-deposited iron thin film in air for two hours at 650oC, (c) and (d) the top views of the α -Fe2O3 thin film ... are intermediate in electrical conductivities between those of conductors and insulators They are the most commonly used artificial photocatalysts for water splitting because of the existence of a band gap In semiconductors, electrons are initially confined to the valence band The energy gap between the valance band edge and the conduction band edge is known as the “band gap” Absorption of photons with... to form hematite 2 Optimization of the synthesis conditions for improved photoelectrochemical performance In this project we have improved the photoelectrochemical performances of α -Fe2O3 thin films by optimizing the calcination time, calcination temperature, plating pH as 4 Chapter 1 well as the plating bath composition Additionally, we also benchmarked our method with other methods of α -Fe2O3 formation... XRD patterns of α -Fe2O3 thin films prepared by the method of Ryan and the method of Schrebler Peaks from FTO glass are indicated by * 56 XI List of figures Fig 3.22 XPS spectra of α -Fe2O3 thin films prepared by the method of Ryan and the method of Schrebler .56 Fig 3.23 Mott-Schottky plots of α -Fe2O3 thin films prepared by the methods of Ryan et al., Schrebler et al and the method of this study. .. free and trapped holes are of the order of 10-11 and 10-6s respectively (Rothenberger et al., 1985) For photocatalytic reactions, trapped charge carrier diffusion is as important as charge carrier trapping because the trapped charge carriers must diffuse to the surface to react Charge carrier diffusion can be difficult in cases of heavy doping Since the trapped electrons and holes can hardly move to the. .. (3.0eV for rutile and 3.2eV for anatase) is the cause of its ineffective absorption of visible light Significant efforts have been made to narrow the band gap of TiO2 by doping Quite a number of metal dopants have been evaluated such as Fe (Sclafani et al., 1993), Cr 16 Chapter 2 (Borgarello et al., 1982), Mo and V (Luo et al., 1982) Thus far the results have been dismal On the other hand, non-metal doping... conduction band are useful for the hydrogen evolution reaction (HER) and the holes in the valence band are useful for the oxygen evolution reaction (OER) The oxygen evolution reaction generally attracts greater research interest because as a four-electron process, it is kinetically more challenging than the hydrogen evolution reaction (Youngblood et al., 2009) 6 Chapter 2 Fig 2.1 A simplified sketch of photoelectrochemical . the preparative conditions have also been carried out including the types of anions in the plating bath, plating pH; calcination time and calcination temperature. The α-Fe 2 O 3 thin films with. films The synthesis of α-Fe 2 O 3 thin films by a two-step process comprising the electrodeposition of thin iron films and the calcination of the iron films in air at 650℃ to form hematite generated. The electrons in the conduction band are useful for the hydrogen evolution reaction (HER) and the holes in the valence band are useful for the oxygen evolution reaction (OER). The oxygen