Title Page
Contents
Abstract 25
Chapter 1. INTRODUCTION 28
1.1. Energy challenges facing the world 28
1.2. Conversion of water into hydrogen using solar energy 30
1.3. Relationship between PEC cell and solar spectrum 31
1.4. Photoelectrochemical water splitting 34
1.4.1. Photocathode of Cu-based oxide for HER 35
1.4.2. Issues of Cu-based oxide photocathode for PEC cell 37
1.5. References 39
Chapter 2. Electrochemical deposition of high-quality cuprous oxide photoabsorber with artificially controlled preferred orientation 41
2.1. Control of preferential orientation of cuprous oxides via crystallo-graphically anisotropic functional metal ions 41
2.1.1. Introduction 41
2.1.2. Results and discussion 44
2.1.3. Conclusion 61
2.1.4. Methods 61
2.1.5. References 63
2.2. Bundle-type columnar cuprous oxide photocathodes by pulse electrodeposited metal seeds inducing dense nucleation 66
2.2.1. Introduction 66
2.2.2. Results and discussion 69
2.2.3. Conclusion 81
2.2.4. Methods 82
2.2.5. References 84
Chapter 3. Photoelectrochemical surface treatment and engineering of the buffer layer 88
3.1. Atomically controllable photo-assisted electrochemical oxidation process design for the formation of ultra-thin cupric oxide buffer layer in cuprous oxide photocathodes 88
3.1.1. Introduction 88
3.1.2. Results and discussion 93
3.1.3. Conclusion 121
3.1.4. Methods 122
3.1.5. Reference 124
3.2. Optimal n-type Al-doped ZnO overlayers for charge transport enhancement in p-type Cu₂O photocathodes 129
3.2.1. Introduction 129
3.2.2. Results and discussion 132
3.2.3. Conclusions 143
3.2.4. Methods 143
3.2.5. References 144
Chapter 4. Surface protection layer for stability and durability in the photoelectrodes 148
4.1. Resistive switching via electrochemical forming process of protection layer boosts durability and photoelectrochemical reaction of photocathode 148
4.1.1. Introduction 148
4.1.2. Results and discussion 150
4.1.3. Conclusion 181
4.1.4. Methods 182
4.1.5. References 185
논문요약 192
Chapter 3.1. Atomically controllable photo-assisted electrochemical oxidation process design for the formation of ultra-thin cupric oxide buffer layer in cuprous oxide photocathodes 11
Table 3.1. TRPL fitting data with a bi-exponential fitting model for various photocathodes 114
Chapter 3.2. Optimal n-type Al-doped ZnO overlayers for charge transport enhancement in p-type Cu₂O photocathodes 11
Table 3.2. Types and nomenclature of prepared samples 133
Table 3.3. Band gap and wavelength of prepared overlayers estimated by Tauc plot 136
Table 3.4. Flat band potential and carrier density of overlayers estimated from Mott-Schottky analysis 141
Chapter 4.1. Resistive switching via electrochemical forming process of protection layer boosts durability and photoelectrochemical reaction of photocathode 11
Table 4.1. Compositional distribution (atomic percentage, %) of TiO₂ films determined from XPS measurements 159
Table 4.2. Comparison of the protective layer, photocurrent, and stability of our photocathode with those of previously reported photocathodes 180
Chapter 1. Introduction 12
Figure 1.1. Schematic of various resources and methods of generating hydrogen energy 28
Figure 1.2. Several different photo-driven methods of water splitting and the charge fluxes that occur as a result of light absorption, including (a) PC, (b)... 30
Figure 1.3. Solar energy spectrum of AM 1.5 G one sun condition expressed in terms of the number of photons for each energy level of the photon 32
Figure 1.4. Illustration of the photocurrent generation in a photoelectrochemical cell comprising semiconducting photocathode and photoanode 35
Figure 1.5. (a) Schematic band energy diagrams at equilibrium, including theoretical Jph and STH; (b) performance of recently reported...[이미지참조] 36
Figure 1.6. Mechanism of photo-corrosion by photoelectrodes 37
Chapter 2. Electrochemical deposition of high-quality cuprous oxide photoabsorber with artificially controlled preferred orientation 12
Figure 2.1. On (a) the (200) and (b) the (111) surfaces, there are various amounts of Cu-end. Schematic representations of faceted crystal formation at... 45
Figure 2.2. Pourbaix diagrams of the metal elements (a) Sb and (b) Pb. (c) Basic information that shows the difference between instantaneous and progressive... 49
Figure 2.3. An illustration of the OH¯ ion supply and consumption during the reaction of metal precursors. Schematic showing the mechanisms of... 50
Figure 2.4. XRD data of (a) Cu₂O:Pb films prepared with different Pb mole concentrations and (b) Cu₂O:Sb films prepared with different Sb mole... 52
Figure 2.5. Analyses of the morphology and electrochemistry of each modified Cu₂O film. Cu₂O, Cu₂O:Sb, and Cu₂O:Pb films were electrodeposited, and (a)... 53
Figure 2.6. Analyses of the morphology and electrochemistry used during the synthesis of Cu₂O with embedded nanoparticles. (a) LSV data for the... 56
Figure 2.7. Cu₂O embedded with nanoparticles during TEM analysis. The optimized Cu₂O:Sb&Pb film is shown in (a) low-magnification bright-field TEM... 59
Figure 2.8. Schematic diagrams of photoabsorber: (a) single-crystal, (b) conventional poly-crystal, and (c) bundle-type vertical grain boundary poly-... 70
Figure 2.9. (a) Diagram of Cu₂O nucleation and growth. (b) Electrolyte solution LSV curve showing Cu and Cu₂O deposition potentials. (c) C0 and C001 sample... 72
Figure 2.10. (a)-(d) These tilt-view AFM pictures demonstrate the difference in the morphology of Cu that occurs depending on the strike time. (e)-(h) are... 75
Figure 2.11. The photoelectrochemical linear sweep voltammetry curves of C0 and strike treated C001 photocathodes in 0.5 M Na₂SO₄ with phosphate buffer... 79
Figure 2.12. (a) Dark Mott-schottky plots and (b) Dark and illuminated EIS response for C0 and C001 photocathodes (1 M Na₂SO₄ with H₂SO₄ buffer at pH 5) 80
Chapter 3. Photoelectrochemical surface treatment and engineering of the buffer layer 15
Figure 3.1. Schematic band energy diagrams at equilibrium in the dark for (a) a pristine Cu₂O-based photocathode with surface defects in the depletion zone... 93
Figure 3.2. A Pourbaix diagram showing the fundamental constituents of Cu, including Cu₂O and CuO 98
Figure 3.3. Cu₂O cyclic voltammetry (CV) curves in the dark state (black) and the light state after adding a positive potential at pH 11 (red) 99
Figure 3.4. Work-function and valence band offset analyses of a Cu₂O:Sb/Cu₂O double layer were performed utilizing a UPS spectrum with He I excitation 100
Figure 3.5. (a) Band diagram of Cu₂O created from UPS analysis at the point of contact with the pH 11 electrolyte, where Eg and Vac stand for the band gap and...[이미지참조] 101
Figure 3.6. Current density–time transient curves showing the effect of light illumination on electrochemical oxidation reaction of the Cu₂O 102
Figure 3.7. Following photo-assisted electrochemical oxidation (PAEO), TEM analysis of UT-CuO. Low-magnification bright-field TEM images of the... 104
Figure 3.8. SEM image of Cu₂O:Sb double; (a) Cross section, and (b) Top section exposing {200} of Cu₂O with perfect triangular pyramid 105
Figure 3.9. High resolution XPS spectra of (a) Cu₂O, (b) Cu₂O/UT-CuO, and (c) Cu₂O/A-CuO 106
Figure 3.10. (a) Differently prepared Cu₂O-based photocathodes' photoelectrochemical linear sweep voltammetry (LSV) characteristics; (b) a... 107
Figure 3.11. (a) Stability test for Cu₂O/AZO/TiO₂/Pt, Cu₂O/A-CuO/AZO/TiO₂/Pt, and Cu₂O/UT-CuO/AZO/TiO₂/Pt photocathodes at a fixed... 109
Figure 3.12. ITO/Cu₂O/UT-CuO/electrolyte structure: (a-b) Nyquist plots for various applied voltages from 0.5 to 0 VRHE, (c) Bode modulus plots, (d) Bode...[이미지참조] 110
Figure 3.13. EIS for pure Cu₂O (black) and Cu₂O/UT-CuO (red) in the presence of an AM 1.5 light source 112
Figure 3.14. (a) Time-resolved photoluminescence emission decay spectra of Cu₂O with three different surface conditions. (b) An energetics schematic... 113
Figure 3.15. (a) Measured in a pH 5 electrolyte, the open circuit potential (OCP) degradation of Cu₂O, Cu₂O/A-CuO, and Cu₂O/UT-CuO photocathodes with a Pt... 115
Figure 3.16. J–V response under simulated 1 sun 1.5 G chopped illumination for Cu₂O and Cu₂O/UT-CuO photocathodes with scavenger 119
Figure 3.17. (a) The current variation of a modified Cu₂O/UT-CuO photocathode in pH 5 buffer solution as it is continuously illuminated and stirred (black line),... 120
Figure 3.18. ZnO (AZO) overlayered Cu₂O film, shown in (a) top view, (b), (c), and (d) cross-view SEM images, and (e) X-ray diffraction pattern 134
Figure 3.19. (a) and (b) UV-Vis-NIR absorption spectra. (c) and (d) Tauc plots of pristine and overlayered Cu₂O and ZnO and AZO overlayers 135
Figure 3.20. I-V graphs (a) and resistivity values (b) derived from I-V data for ZnO and AZO overlayers on ITO substrates are shown below 137
Figure 3.21. (a) and (b) Results of electrochemical impedance spectroscopy performed under AM 1.5G illumination on pristine Cu₂O and Cu₂O/overlayer... 139
Figure 3.22. The Mott-Schottky plots (a) were used to estimate the band diagrams of the Cu₂O/ZnO junction in (b) and the Cu₂O/AA:AZO junction in (c) 140
Figure 3.23. (a) LSV properties of prepared Cu₂O/overlayer photocathodes. (b) Long-term stability test of Cu₂O/AA:AZO/TiO₂/Pt photoelectrode at 0 VRHE in...[이미지참조] 142
Chapter 4. Surface protection layer for stability and durability in the photoelectrodes 20
Figure 4.1. A comparison of the PEC performances of pure and multiple-stacked Cu₂O structures, together with charge-conduction process schematic diagrams.... 151
Figure 4.2. SEM images showing the photocorrosion evolution of Cu₂O subjected to photocorrosion for 5, 15, and 30 s. (a–c) Top-view and (d–f) cross-sectional... 153
Figure 4.3. Optical images before and after PEC measurements of the photoelectrodes (Cu₂O/TiO₂) with different thickness of TiO₂: (a) 5 nm, (b) 50... 154
Figure 4.4. XPS at the (a & c) Ti 2p level and (b & d) O 1s level of the (a & b) LRS (with filaments) and (c & d) HRS (without filaments) TiO₂ films 157
Figure 4.5. Schematic diagrams showing the electrochemical formation and reset of the stacked structure of the Cu₂O/TiO₂/electrolyte: (a) initial state, (b)... 160
Figure 4.6. The bipolar resistive switching properties of the MIM structure (ITO/TiO₂/Pt), where the switching voltage is 0.8 V, are examined in (a)... 161
Figure 4.7.(a) Electrochemical filament forming (ECF)-treated samples and untreated ITO/Cu₂O/TiO₂ 100 nm (R-sample) samples were used in stability... 165
Figure 4.8. (a) Top-view SEM images of the Cu₂O/AZO/TiO₂ and the bare Cu₂O (inset image) and (b) its high-resolution image. (c, d) Cross-sectional view... 167
Figure 4.9. Nyquist plots (a) in the dark and (b) under illumination for ITO/Cu₂O/non-treated TiO₂ (with different thickness). TiO₂ thickness varies... 170
Figure 4.10. Nyquist plots (a) in the dark or (b) under illumination for ITO/Cu₂O/ECF-treated TiO₂ (with different thickness). TiO₂ thickness varies... 171
Figure 4.11. (a) Current density potential (J-E) responses of photoelectrodes under simulated 1-sun illumination in a pH 5 buffer electrolyte where the Pt... 173
Figure 4.12. Incident photon-to-current conversion efficiency (IPCE) spectra with respect to wavelength for the ITO/Cu₂O/AZO/TiO₂ 100 nm/Pt photocathode... 174
Figure 4.13. Representative IMPS for Cu₂O/AZO/TiO₂/Pt with various Pt catalyst deposition techniques shown under monochromic light illumination (＝490 nm;... 177
Figure 4.14. Charge transfer efficiencies calculated from IMPS results for Cu₂O/AZO/TiO₂ ECF-CC3 photoelectrodes decorated by different Pt deposition... 179