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of H2 treatment temperature on the sheet resistance (R) and the donor density (ND) of TiO2 films. The R was measured by a four‐point probe. The ND was measured by the Mott–Schottky analysis. (c) Effect of H2 treatment temperature on the jphoto of TiO2 films in 0.1 mol l−1 H2SO4 (pH = 1). The jphoto for water oxidation was obtained in LSV measurement at 0.87 V vs. RHE under UV irradiation (λ > 300 nm). Source: Adapted with permission Amano et al. [38]. Copyright 2016, American Chemical Society.Figure 7.16 Schematic illustration of the PEC water oxidation process at the photoanode with cocatalyst layer: (a) conductive substrate, (b) n‐type semiconductor as a photoabsorber, (c) cocatalyst layer for OER, and (d) aqueous electrolyte solution. The number indicates the order of the PEC reaction process: (i) the generation of photoexcited e−–h+ pairs; (ii) charge separation, carrier diffusion, and carrier transport; and (iii) h+ transfer from semiconductor to water.Figure 7.17 Band alignments in three types of semiconductor heterojunctions: (a) straddling gap (type I), (b) staggered gap (type II), and (c) broken gap (type III).Figure 7.18 Schematic illustrations of (a) PEC cell for solar H2 production using water vapor from the air over the sea and (b) vapor‐fed PEC water splitting system using a gas‐diffusion HER cathode, a proton exchange membrane (PEM), and a gas‐diffusion photoanode for OER. The photoanode is composed of a TiO2 nanotube array decorated on porous Ti felt. The surface of TiO2 nanotubes is coated with Nafion ionomer thin film for the gas‐phase operation. Source: (a) Adapted by permission of Wiley‐VCH Verlag. Amano et al. [53]; (b) Based on Amano et al. [54].Figure 7.19 Vapor‐fed water photoelectrolysis by porous SrTiO3 photoanode |PEM| Pt‐carbon black cathode under humidified argon (3 vol% water vapor). The response of (a) photocurrent density and (b) the formation rate of H2 evolved in the cathode compartment and O2 evolved in the photoanode compartment at ΔEapp = 0.3 V under 365‐nm UV irradiation (I0 = 42 mW cm−2, photoanode area 2 cm2). The porous SrTiO3 photoanode was coated by Nafion ionomer thin films for the gas‐phase operation. The evolved gas in each compartment was separated by a PEM and analyzed by on‐line gas chromatographs. Source: (a, b) Based on Amano et al. [54].
7 Chapter 8Figure 8.1 Different types of photocatalytic overall water splitting systems including (a) one‐step photoexcitation, (b) first‐generation Z‐scheme, (c) second‐generation Z‐scheme, and (d) third‐generation Z‐scheme. HEP: hydrogen evolution photocatalyst; OEP: oxygen evolution photocatalyst; HEC: hydrogen evolution cocatalyst; OEC: oxygen evolution cocatalyst; CB: conduction band; VB: valence band; Eg: energy band gap; NHE: normal hydrogen electrode; Ox: oxidant; Red: reductant.Figure 8.2 Representative material types for one‐step photocatalytic overall water splitting systems. (A) SrTiO3 with 18 facets and selective deposition of HER and OER cocatalysts. Source: Mu et al. [68]. Copyright 2016, the Royal Society of Chemistry.Figure 8.3 Representative Z‐scheme types for photocatalytic overall water splitting. (a) First‐generation Z‐scheme coupling ZrO2/TaON(HEP) and BiVO4(OEP) with Fe3+/Fe2+ redox pairs as aqueous electron mediator and corresponding performance for 12 hours. Source: From Qi et al. [237]. © 2018 Elsevier.Figure 8.4 Different types of PEC for water splitting comprising (a) photoanode–cathode, (b) photocathode–anode, (c) photoanode–photocathode tandem cell, and (d) photovoltaic–photoanode (PV–PEC). Source: From Kim et al. [19]. © 2019 RCS.Figure 8.5 Photoanode–photocathode tandem cell for overall water splitting systems using (a) metal oxide‐based photoelectrodes (Cu2O as HEP and Mo:BiVO4 as OEP) and produced photocurrent density with stoichiometric H2 and O2 evolution. Source: Reproduced with permission. Pan et al. [368]. Copyright 2018, Macmillan Publishers Limited, part of Springer Nature.Figure 8.6 Photoanode–photocathode tandem cell for overall water splitting systems using (A) PEC system composed of IrOx/n‐GaAs (photoanode) and Pt/Ti/Pt/Au/p‐GaAS (photocathode) and representative J–E curve. Source: Reproduced with permission. Kang et al. [395]. Copyright 2017, Macmillan Publishers Limited, part of Springer Nature.Figure 8.7 Representative PV–PEC devices for overall water splitting. (a) Inverted metamorphic multi‐junction (IMM) device structure with GaInP/GaInAs tandem PV cells coupled with RuOx–Pt and J–V measurements. Source: From Young et al. [403]. © 2017 Springer Nature.Figure 8.8 Representative PV–PEC devices for overall water splitting. (a) System composed of single‐junction perovskite solar cell and a nanocone/Mo‐doped BiVO4/Fe(Ni)OOH photoanode, and respective J–V curve for the tandem device. Source: From Qiu et al. [325]. © 2016 Yongcai Qiu.Figure 8.9 Representative artificial (wireless) PV–PEC devices for overall water splitting. (a) Tandem CH3NH3PbI3 perovskite single‐junction solar cell with Co‐Ci incorporated H‐doped Mo/BiVO4 photoanode and corresponding gas evolution with calculated STH efficiency. Source: From Kim et al. [421]. © 2015 American Chemical Society.
8 Chapter 9Figure 9.1 Schematic illustration of the natural photosynthesis (a) and the artificial photosynthesis (b) using semiconductor as the photocatalyst.Figure 9.2 Band positions of some semiconductor photocatalysts and the redox potentials of CO2 reduction at pH 7 in aqueous solution. Source: Li et al. [12]. © 2014, Springer Nature.Figure 9.3 Schematic diagram of the slurry reactor (a) and the fixed bed reactor (b). Source: (a) Ola and Maroto‐Valer [17]. Licensed under CC BY 4.0. (b) Bakherad et al. [19]. © 2020, Royal Society of ChemistryFigure 9.4 (a) Schematic illustration of optical‐fiber photo reactor. (b) Light propagation in a TiO2‐coated fiber reactor. Source: Wu et al. [20]. © 2008, Springer Nature.Figure 9.5 Dye‐sensitized photocatalytic reaction.Figure 9.6 Band alignment of bulk CdSe, CdSe QDs with a diameter of 2.5 nm, and TiO2, as well as relevant redox potentials of CO2 and H2O. Source: Wang et al. [25]. © 2010, American Chemical Society.Figure 9.7 Schematic diagram of the reduction of CO2 and water vapor into hydrocarbon fuels through a nanotube–catalyst array. Source: Varghese et al. [28]. © 2009, American Chemical Society..Figure 9.8 Semiconductor heterojunctions in the form of type I (a), type II (b), and type III (c).Figure 9.9 Schematic diagram of the photoexcitation process of AgBr/TiO2 composite under visible‐light irradiation.Figure 9.10 Schematic illustration of TiO2 with {001} and {101} surface heterojunction. Source: Yu et al. [31]. © 2014, American Chemical Society.Figure 9.11 Schematic illustration of the spatial separation of redox sites on the TiO2 photocatalysts prepared without HF (a), by adding a moderate amount of HF (b), and by adding a high amount of HF (c).Figure 9.12 Schematic diagram of charge transfer in a Z‐scheme semiconductor–semiconductor composite.Figure 9.13 Schematic diagram of Z‐scheme BiVO4/C/Cu2O nanowire arrays. Source: Reproduced with permission. Kim et al. [32]. © 2018, American Chemical Society.Figure 9.14 Comparison of the energy band positions and the related redox reaction potentials of g‐C3N4 and ZnO. Source: Yu et al. [33]. © 2015, Royal Society of Chemistry.Figure 9.15 Schematic diagram of the solid–liquid interface structure.Figure 9.16 Potential comparison about the reduction of CO2, H2CO3, and CO32− ions into methanol. Source: Pan and Chen [38]. © 2007, Elsevier.Figure 9.17 Possible adsorption mechanism of CO2 on TiO2 surface. Source: Wu and Huang [40]. © 2010, Springer Nature.Figure 9.18 Schematic illustration of the charge transfer and separation in RGO–CdS nanorod system under visible‐light irradiation. Source: Yu et al. [41]. © 2014, Royal Society of Chemistry.Figure 9.19 Two proposed mechanisms for the photoreduction of CO2 to methane: formaldehyde (a) and carbine (b) pathways. Source: Izumi [42]. © 2015, American Chemical Society.Figure 9.20 Photocatalytic reduction of CO2 (86.65 kPa) by TiO2 (a) and Pd(1%)‐TiO2 (b). Source: Xiong et al. [44]. © 2017, Elsevier.Figure 9.21 Schematic mechanism of photocatalytic CO2 reduction over CQDs/OCN‐x under visible‐light irradiation. Source: Li et al. [45]. © 2019, Springer Nature.
9 Chapter 10Figure 10.1 Schematic diagrams for PEC CO2 reduction in water using a semiconductor as (a) photocathode, (b) photoanode, and (c) both photoanode and photocathode. (d) Schematic diagram for the device combining a photovoltaic cell with an efficient electrochemical catalyst for CO2 reduction. Source: Zhang et al. [18].