Title Page

Contents

Abstract 10

Chapter 1. INTRODUCTION 12

1.1. Global Warming and CO₂ emission 12

1.2. Hydrogen for Energy Source 15

1.3. Photoelectrochemical Water Splitting for Green Hydrogen Production 18

1.4. Metal Oxide for Photoanode 21

1.5. Tungsten Trioxide (WO₃) for Photoanode 23

1.6. Metal Ion Doping on Transition Metal Oxide 25

1.7. General Doping Method for Metal Oxide Semiconductors 27

1.8. Sol-Flame Method for Transition Metal Doping 29

Chapter 2. Experimental 33

2.1. Preparation of WO₃ 2D nanosheet electrode (Bare WO₃) 33

2.2. Preparation of Sol-Flame Fe-doped WO₃ electrode (Synthesizing Flame Fe-WO₃ from Bare WO₃) 35

2.3. Preparation of Sol-Furnace Fe-doped WO₃ electrode (Synthesizing Furnace Fe-WO₃ from Bare WO₃) 37

2.4. Characterization 38

2.5. Photoelectrochemical Measurements 39

Chapter 3. Results and Discussion 40

Chapter 4. Conclusion 69

References 70

논문요약 76

Table 1.1. Volumetric and gravimetric energy densities of common. 16

Table 3.1. Weight-percent (wt%) and Atomic ratio in EDS mapping for Tail of Flame Fe-WO₃ 52

Table 3.2. Fitting results of Nyquist plots for WO₃ samples with Fe-WO₃ electrodes. 60

Fig 1.1. (a) Graph of global energy consumption by source and (b) percent of each energy source. 14

Fig 1.2. Schematic illustration of common differentiation of hydrogen production pathways into conventional, low-CO₂, CO₂-free, and carbon-free production... 17

Fig 1.3. a) Energy diagrams of water splitting based on one-step excitation, and PEC water splitting using b) a photoanode. 20

Fig 1.4. Bandgap and position of CB and VB of several metal oxide. 22

Fig 1.5. Theoretical maximal photocurrent STH (right axis) and photocurrent density (left axis) as a function of the material band gap. The calculation of... 24

Fig 1.6. Schematic illustration of doping method in metal oxide. (a) Thermal diffusion doping (by J. Fu et al. Small(2017)) and (b) Ion implantation... 28

Fig 1.7. (a) Schematic illustration of sol-flame doping in metal oxide (TiO₂). (by J. Kim et al. Advanced Energy Materials (2018)). (b) Actual picture about... 32

Fig 1.8. Schematic illustration of the Fe-WO₃ electrodes in this paper. 32

Fig 2.1. Schematic illustration of WO₃ hydrothermal synthesis. 34

Fig 2.2. Schematic illustration of Fe sol spin-coating process. 36

Fig 2.3. Schematic illustration of sol-flame doping process. 36

Fig 2.4. Schematic illustration of sol-furnace doping process. 37

Fig 3.1. FE-SEM images of Fe-WO₃ electrodes. (a) Bare WO₃, (b) Flame Fe-WO₃, (c) Furnace Fe-WO₃, cross-sectional view of (d) Bare WO₃, (e)... 41

Fig 3.2. HR-TEM images of Fe-WO₃ electrodes. Lower magnitude images of (a) Bare WO₃, (b) Flame Fe-WO₃, (c) Furnace Fe-WO₃, higher magnitude... 42

Fig 3.3. (a) XRD patterns of Bare WO₃, Flame Fe-WO₃ and Furnace Fe-WO₃ electrodes and (b) the same patterns on short range degrees (20~32°) 44

Fig 3.4. High resolution XPS spectra of C 1s for the Fe-WO₃ electrodes. 47

Fig 3.5. High resolution XPS spectra of (a) O 1s, (b) W 4f, (c) Fe 2p and (d) Sn 3d for the Fe-WO₃ electrodes. 47

Fig 3.6. XPS survey spectra of the Fe-WO₃ electrodes. 48

Fig 3.7. Raman spectra of the Fe-WO₃ electrodes. 50

Fig 3.8. (a) Position of head and tail of Flame Fe-WO₃ with cross-view FE-SEM and HR-TEM, EDS line profile for (b) Center of Flame Fe-WO₃, (c) Head... 52

Fig 3.9. HADDF image and EDS mapping for Tail of Flame Fe-WO₃ 52

Fig 3.10. PEC performance of Fe-WO₃ electrodes. J-V curves of front-side illumination for Fe-WO₃ electrodes under (a) 0.1 M phosphate buffer solution... 55

Fig 3.11. Photocurrent density-potential curves of dark condition for Fe-WO₃ electrodes at the range of 1.5~4.2 V (0.1 M phosphate buffer solution (pH 7.00). 56

Fig 3.12. ABPE (Applied Bias Photon-to-Current Efficiency) of Fe-WO₃ photoanodes. 58

Fig 3.13. EIS (Electrochemical Impedance Spectroscopy) measurement of the Fe-WO₃ electrodes (at 1.23 V vs RHE, 106 Hz ~ 0.01 Hz). 60

Fig 3.14. Stability test of Flame Fe-WO₃ electrode (potential on ABPE max.) 61

Fig 3.15. (a) UV-vis absorptance spectra and (b) Kubelka-Munk plots of of Fe-WO₃ electrodes. 64

Fig 3.16. XPS valence band spectra of Fe-WO₃ electrodes. 65

Fig 3.17. Mott-Schottky plots of Fe-WO₃ electrodes. 68

Fig 3.18. Band diagram of Fe-WO₃ electrodes. 68