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6.4 Illustration of the band positions of common semiconductor materials and the energy level toward water splitting.Figure 6.5 Comparison of electronic DOS of (a) 3D bulk semiconductor materials and LD semiconductor materials including (b) 2D, (c) 1D, and (d) 0D. The inset arrows on related models indicate the quantum confinement direction and dimensionalities. (e) Quantum size effect on band structure. (f) Schematic illustration of surface plasmon resonance excitation on metallic NPs.Figure 6.6 (a) TEM image of WO_2.83 NRs. (b) LSPR in 1D WO_2.83 NRs (experimental measurement and theoretical simulation). Source: Manthiram and Paul Alivisatos [55].Figure 6.7 (a–c) TEM images and (d) size distributions of Co₃O₄ NPs with different diameters (3, 10, and 40 nm). (e) The relationship between particle diameter and surface area. Inset: Images of NPs in the form of powder or dispersed in water. (f) Visible‐light‐ driven oxygen evolution of Co₃O₄ NPs with different diameters using Ru(bpy)₃²⁺ as the sensitization system. (g) Visible‐light‐driven oxygen evolution from various cobalt compounds placed on SBA‐15. Source: Reproduced with permission. Grzelczak et al. [69]. Copyright 2013, American Chemical Society.Figure 6.8 (a) TEM image and (b) Mott–Schottky plots for CoO NPs and micropowders. The flat band potentials are obtained via extrapolation method. (c) Band positions of CoO NPs and micropowders according to the band gaps and flat band potentials. Source: Reproduced with permission Liao et al. [77]. Copyright 2014, Nature Publishing Group.Figure 6.9 (a) TEM image quantum‐sized BiVO₄. (b) Comparison of the band position of nanoscale BiVO₄ and quantum‐sized BiVO₄. Source: Reproduced with permission. Sun et al. [89]. Copyright 2014, American Chemical Society.Figure 6.10 (a) Tungsten oxide single‐crystal nanosheets. (b,c) HRTEM images of WO₃, WO_3−x‐VT, and WO_3−x‐HT nanosheets. (d) Band position regulation of WO₃ nanosheets with oxygen vacancies. Source: From Yan et al. [58]. © 2015 John Wiley & Sons.Figure 6.11 (a) Representation of charge transfer between (001) and (110) facets. (b) Detachment and transfer of photoexcited charges in the BiOCl and defect‐rich BiOCl. (c) TEM image of the defect‐rich BiOCl ultrathin nanosheets. (d) Photocatalytic OER for BiOCl and defect‐rich BiOCl. Source: Reproduced with permission. Di et al. [107]. Copyright 2017, the Royal Society of Chemistry.Figure 6.12 The photocatalytic properties of LD materials including light absorption and charge transfer can be improved by doping engineering, active facet exposure, defect or vacancy creation, and LSPR. Hybrid LD materials with heterojunction can also be constructed in the form of 0D/1D, 1D/1D, 0D/2D, and 2D/2D configurations.Figure 6.13 (a,b) TEM images. (c) Time dependence of the O₂ evolution. (d) Proposed energy diagram for Co(OH)₂/TiO₂. Source: Reproduced with permission. Maeda et al. [110]. Copyright 2016, Wiley‐VCH.

      6 Chapter 7Figure 7.1 PEC water splitting by n‐type TiO₂ photoanode for oxygen evolution reaction (OER) and Pt cathode for hydrogen evolution reaction (HER). The TiO₂ with band‐gap energy (E_g) of 3.0 eV is photoexcited under ultraviolet light irradiation. External voltage (ΔE_app) is applied between the electrodes to induce water photoelectrolysis.Figure 7.2 Current–potential curves for conventional water electrolysis in the dark. Over potentials (η) are required for HER and OER over each electrocatalyst electrode (η_HER + η_OER).Figure 7.3 Current–potential curves for water photoelectrolysis using photoanodes for OER and a cathode for HER at the current density (j) of 2 mA cm⁻². (a) Unbiased PEC water splitting and (b) PEC water splitting with externally applied voltage (ΔE_app). The ΔE_photo is the shift of the potential by using photoanodes in comparison with (c) anode electrocatalyst for OER. The absolute value of the current is equal between the series‐connected electrodes.Figure 7.4 (a) Current–potential curves of WO₃ photoanode in 0.1 mol l⁻¹ Na₂SO₄ (pH = 7) with 10 vol% methanol under UV irradiation (monochromatic light, λ = 365 nm) with different intensities of incident light. The light intensity (I₀) was controlled by using a neutral density filter. (b) Effect of the I₀ on the photocurrent density (j_photo) of the WO₃ photoanode in the linear sweep voltammetry. The j_photo was linearly increased with I₀ at each applied potential.Figure 7.5 (a) Flat band state and (b) band bending state of n‐type semiconductor. The recombination of the photoexcited e⁻ and h⁺ pair easily occurs in the flat band state. In contrast, the e⁻ and h⁺ pair is efficiently separated in the space charge layer (SCL). In the absence of surface states, the potential drop in the SCL (Δφ) is linear to the anodic shift of the applied potential from the flat band potential (E_fb).Figure 7.6 The pH dependence of the electrode potentials at 25 °C. The potentials of SHE and Ag/AgCl reference electrode are constant, but the potentials of RHE and the potentials for HER and OER are linearly increased by an increase in pH value with a slope of 59 mV (ln 10 × RT/F) at 25 °C.Figure 7.7 IPCE action spectra of rutile and anatase TiO₂ electrodes in 0.2 mol l⁻¹ Na₂SO₄ with phosphate buffer (pH = 7) at 0.9 V vs. RHE. The inset shows their diffuse reflectance UV–vis spectra using Kubelka–Munk function, F(R). Source: From Amano et al. [21]. © 2018 The Electrochemical Society.Figure 7.8 Mott–Schottky plots of WO₃ electrode in 0.1 mol l⁻¹ H₂SO₄ (pH = 1) in the dark to estimate the flat band potential (E_fb) and the donor density (N_D). The capacitance of the space charge layer (C_sc) was measured at 1 kHz with a sinusoidal amplitude of 10 mV in the dark. The inset shows the SEM image of the WO₃ electrode surface. Source: Adapted with permission Amano et al. [22]. Copyright 2011, Springer‐Verlag.Figure 7.9 Linear sweep voltammograms of the WO₃ electrode for (a) water oxidation in 0.1 mol l⁻¹ H₂SO₄ and (b) methanol oxidation in the solution with 10 vol% methanol under photoirradiation (solid j–E curves) and in the dark (dashed j–E curves). The applied potential where photocurrent response starts is denoted as E_onset. The E_fb is the value measured in Figure 7.8. Source: Based on Amano et al. [22].Figure 7.10 Band diagrams of n‐type semiconductors for PEC oxygen evolution: SrTiO₃, rutile TiO₂, monoclinic WO₃, α‐Fe₂O₃ (hematite), monoclinic scheelite‐type BiVO₄ (clinobisvanite) [28], and TaON [29] with E°(H⁺/H₂) and E°(O₂/H₂O). Here, the reported E_fb is considered as the conduction band (CB) minimum, although the CB minimum is more negative than E_fb by 0.1–0.3 eV. The valence band (VB) maximum is determined from the CB minimum and the optical E_g.Figure 7.11 Pourbaix diagram, which is a potential–pH equilibrium diagram, for (a) tungsten–water system and (b) iron–water system at 25 °C. The molar concentration of WO₄²⁻, Fe²⁺, and Fe³⁺ is 10⁻⁶ mol l⁻¹. The dashed lines show the potentials of HER and OER relative to pH value in water. Source: From Pourbaix [31]. © 1974, NACE.Figure 7.12 Schematic illustrations of photoexcited electron transport in photoanodes composed of (a) nanocrystalline particles with grain boundaries and (b) single‐crystalline materials with anisotropic nanostructures. Source: From Amano et al. [35]. © 2011 The Electrochemical Society.Figure 7.13 Crystal structure of WO₃·H₂O and cross‐sectional sideview SEM image of vertically aligned WO₃ flakes deposited on transparent conductive oxide (TCO) glass substrate for PEC water oxidation under visible‐light irradiation. The seed layer, which is WO₃ nanoparticles, is essential for heterogeneous nucleation of WO₃·H₂O flakes. The WO₃·H₂O flakes with two‐dimensional nanostructure were converted to monoclinic WO₃ by heat treatment. Source: From Amano et al. [36]. © 2010, RCS.Figure 7.14 Current–potential curves of (a) vertically aligned WO₃ flakes (blue dashed) and (b) horizontally stacked WO₃ flakes (black solid) deposited on TCO glass substrate in 0.1 mol l⁻¹ Na₂SO₄ (pH = 7) under visible‐light irradiation (λ > 400 nm). (A) Back‐side and (B) front‐side illumination were performed through the TCO glass substrate and the WO₃ layer, respectively. Chopped illumination was used to show the transient photocurrent response for water oxidation. Source: Based on Amano et al. [35].Figure 7.15 (a) Pictures of the thermally oxidized TiO₂

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