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Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

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Crystal Facet Engineering of Photoelectrodes for Photoelectrochemical Water Splitting Songcan Wang,† Gang Liu,*,‡,§ and Lianzhou Wang*,†

Chem. Rev. Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 03/16/19. For personal use only.

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Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Queensland 4072, Australia ‡ Shenyang National Laboratory for Materials Science, Institute of Metal Research Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China § School of Materials Science and Engineering, University of Science and Technology of China, 72 Wenhua Road, Shenyang 110016, China ABSTRACT: Photoelectrochemical (PEC) water splitting is a promising approach for solar-driven hydrogen production with zero emissions, and it has been intensively studied over the past decades. However, the solar-to-hydrogen (STH) efficiencies of the current PEC systems are still far from the 10% target needed for practical application. The development of efficient photoelectrodes in PEC systems holds the key to achieving high STH efficiencies. In recent years, crystal facet engineering has emerged as an important strategy in designing efficient photoelectrodes for PEC water splitting, which has yet to be comprehensively reviewed and is the main focus of this article. After the Introduction, the second section of this review concisely introduces the mechanisms of crystal facet engineering. The subsequent section provides a snapshot of the unique facet-dependent properties of some semiconductor crystals including surface electronic structures, redox reaction sites, surface built-in electric fields, molecular adsorption, photoreaction activity, photocorrosion resistance, and electrical conductivity. Then, the methods for fabricating photoelectrodes with faceted semiconductor crystals are reviewed, with a focus on the preparation processes. In addition, the notable advantages of the crystal facet engineering of photoelectrodes in terms of light harvesting, charge separation and transfer, and surface reactions are critically discussed. This is followed by a systematic overview of the modification strategies of faceted photoelectrodes to further enhance the PEC performance. The last section summarizes the major challenges and some invigorating perspectives for future research on crystal facet engineered photoelectrodes, which are believed to play a vital role in promoting the development of this important research field.

CONTENTS 1. Introduction 2. Mechanisms of Crystal Facet Engineering 3. Anisotropic Properties of Faceted Crystals 3.1. Surface Electronic Structures 3.2. Redox Reaction Sites 3.3. Surface Built-In Electric Fields 3.4. Molecular Adsorption 3.5. Photoreaction Activity 3.6. Photocorrosion Resistance 3.7. Electrical Conductivity 4. Synthesis of Crystal Facet Engineered Photoelectrodes 4.1. Immobilizing Faceted Crystals on Conductive Substrates 4.1.1. Drop Casting and Spin-Coating 4.1.2. Electrophoretic Deposition 4.1.3. Doctor Blade and Screen-Printing 4.1.4. Vacuum Filter-Transfer 4.1.5. Particle Transfer 4.1.6. Finger Rubbing

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4.2. Directly Growing Faceted Photoelectrode Films 4.2.1. TiO2 4.2.2. ZnO 4.2.3. WO3 4.2.4. α-Fe2O3 4.2.5. BiVO4 4.2.6. BiOX (X = Cl, Br, I) 4.2.7. Cu2O 4.2.8. Other Faceted Semiconductor Films 5. Advantages of Crystal Facet Engineered Photoelectrodes 5.1. Light Harvesting 5.2. Charge Separation and Transfer 5.3. Surface Reactions 6. Modification of Faceted Photoelectrodes with Enhanced PEC Performance 6.1. Nanostructure Engineering 6.2. Heterojunction Construction

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Received: September 24, 2018

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DOI: 10.1021/acs.chemrev.8b00584 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 6.3. Heteroatom Doping 6.4. Intrinsic Defect Generation 7. Conclusion and Outlook Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

Review

Finally, the holes on the surface of the photoanode and electrons on the surface of the counter electrode are injected into the electrolyte for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively (surface reaction process). During the “long journey” of the photogenerated charge carriers traveling to the electrolyte for OER and HER, charge recombination may take place in the bulk of the semiconductor, the semiconductor/electrolyte interface and the semiconductor/substrate interface, resulting in a low STH efficiency of the PEC cell.20 Therefore, a significant suppression of the above-mentioned charge recombination is essential to achieve efficient PEC water splitting. Nanostructure engineering is effective for reducing bulk recombination, while the rational modification of the semiconductor/electrolyte interface and the semiconductor/substrate interface is essential for overcoming the interfacial charge recombination. In particular, semiconductor photoelectrodes with a controlled nanocrystal facet orientation can drastically affect the charge transfer/separation process because a semiconductor with the selective exposure of facets exhibits distinctive electronic and optical properties due to the different atomic coordination and configurations in different facets.21,22 At the semiconductor/substrate interface of a photoanode, charge transfer takes place between the n-type semiconductor and the conductive substrate due to their different Fermi levels.23,24 If the work function (i.e., the energy difference between the Fermi level and the vacuum level) of the conductive substrate is smaller than that of the contacted semiconductor (ϕm < ϕs), electrons will be transferred from the conductive substrate to the semiconductor until equilibrium, causing downward band bending (Figure 1c, left). In this case, photogenerated electrons in the CB of the semiconductor can be transferred to the conductive substrate. If ϕm > ϕs, electrons are transferred from the semiconductor to the conductive substrate to equilibrate the Fermi levels, resulting in upward band bending and creating a Schottky barrier (Figure 1c, right), which may suppress the extraction and transfer of photogenerated electrons from the semiconductor to the substrate.25 The formation of an appropriate interface between the semiconductor and the substrate by controlling the semiconductor surface can reduce and even eliminate the barrier, subsequently promoting charge transfer at the semiconductor/substrate interface. Charge transfer also takes place at the semiconductor/ electrolyte interface until equilibrium is established, where a positively charged depletion layer (i.e., space charge region, SCR) with width “W” is formed in the semiconductor and a negatively charged Helmholtz layer is formed in the electrolyte, causing upward band bending (Figure 1d, left). Under steadystate light illumination (Figure 1d, right), the gradient in the quasi-Fermi level forms an electric field near the semiconductor surface, namely, a “photovoltage” or “open-circuit voltage (VOC)”, functioning as a driving force for the injection of photogenerated holes into the electrolyte for the OER. The surface states are critically important for the reaction kinetics of the surface reaction process. The exposure of highly reactive facets can accelerate the surface kinetics, thereby reducing the charge recombination at the semiconductor/electrolyte interface. Since the breakthrough in the facet-controlled synthesis of TiO2, the crystal facet engineering of TiO2 and other semiconductors has been intensively studied for photocatalysis and a number of other applications.26 However, the number of

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1. INTRODUCTION With the increasing consumption of fossil fuels and the emission of greenhouse gases, the exploitation of novel technologies for sustainable energy production has been regarded as one of the top priorities for humanity. Photoelectrochemical (PEC) water splitting, a process to convert and store solar energy in chemical bonds, offers a promising strategy for renewable hydrogen fuel production and environmental remediation.1 Since the pioneering work of PEC water splitting using TiO2 as the photoanode in 1972,2 PEC hydrogen production technology has attracted great attention, and other applications such as CO2 reduction, organic pollutant degradation, value-added chemical production, and electricity generation have also been developed. The progress in other applications is beyond the scope of this review, and readers may refer to some recent reviews and book chapters for more information.3−10 A typical PEC cell is composed of two conductively connected electrodes immersed in an electrolyte with at least one electrode containing semiconductors (i.e., a photoelectrode) for light harvesting.11 Thermodynamically, the valence band (VB) and conduction band (CB) positions of a semiconductor used for spontaneous PEC water splitting should straddle the water oxidation potential (1.23 V vs normal hydrogen electrode, NHE) and proton reduction potential (0 V vs NHE) to achieve sufficient redox ability for the photogenerated electron−hole pairs.12,13 Considering the overpotential requirements (0.4−0.6 eV) and energy losses (0.3−0.4 eV) during PEC water splitting, an ideal bandgap should be ∼2.0 eV, corresponding to a light absorption edge of ∼620 nm.14−17 As shown in Figure 1a, a semiconductor with a bandgap of 2.0 eV can achieve a maximum photocurrent density of 14.5 mA cm−2 and a theoretical solar-to-hydrogen (STH) conversion efficiency of 17.9% under AM 1.5 G illumination (100 mW cm−2), which can meet the 10% STH efficiency threshold for potential commercialization.18,19 However, no semiconductors hitherto can meet the above requirements, and thus an external bias is generally required to achieve PEC water splitting. In addition to the bandgap energy and band edge positions, efficient charge separation during the PEC reactions is equally important to achieve high STH efficiencies. Figure 1b illustrates a photoanode/counter electrode configuration. Under solar light illumination, photons with sufficient energy (hν ≥ bandgap, Eg) are first absorbed by the photoanode composed of n-type semiconductors (light harvesting process). The electrons in the VB of the semiconductor are excited to the CB, from where they are transferred to the conductive substrate and then travel to the counter electrode through the external circuit, while the holes in the VB diffuse to the surface of the photoanode (charge separation and transfer process). B


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Figure 1. (a) Theoretical maximum STH efficiency and photocurrent density of photoelectrodes based on the band gap under the illumination of standard AM 1.5 G solar spectrum (100 mW cm−2). (b) Schematic of a photoanode/counter electrode configuration for PEC water splitting (inset: charge transfer and separation in the photoanode with possibility of I, bulk recombination, II, semiconductor/electrolyte interface recombination, and III, semiconductor/substrate recombination). (c) Band bending at the semiconductor/substrate interface in the case of ϕm < ϕs and ϕm > ϕs. (d) Band bending at the semiconductor/electrolyte interface with equilibrium under dark and under light illumination (c,d). Adapted with permission from ref 24. Copyright 2017 The Royal Society of Chemistry.

water splitting is still lacking. Considering the increasing interest in crystal facet engineered photoelectrodes for PEC water splitting, a timely overview of the recent progress and outlooks of future directions in this important topic is urgently needed. In this review, we start by concisely introducing the mechanisms of crystal facet engineering and the unique facetdependent properties of semiconductor crystals. Subsequently, the common methods for the fabrication of crystal facet engineered photoelectrodes are presented. The advantages of crystal facet engineered photoelectrodes for PEC water splitting in terms of light harvesting, charge separation and transfer, and surface reactions are discussed in detail. Moreover, modification strategies such as nanostructure engineering, heterojunction construction, heteroatom doping, and intrinsic defect generation for crystal facet engineered photoelectrodes with enhanced PEC performance are discussed. The review concludes with a brief summary of the major challenges, and some invigorating perspectives are also presented. We hope to shed light on crystal facet engineering for the design of efficient photoelectrodes for solar energy conversion.

reports of crystal facet engineered photoelectrodes for PEC water splitting is much smaller compared to those of powdery materials. Compared to photocatalysis, which only requires powder-based photocatalysts suspended in aqueous solutions, the crystal facet engineering of photoelectrodes to achieve efficient PEC performance is more challenging. For instance, in addition to tuning the exposed facets, the proper contact of the semiconductor/substrate interface is also important for efficient charge transfer and separation, while it is not required in photocatalysis. Moreover, both the photogenerated electrons and holes migrate to the surface of the photocatalyst, driven by solely the built-in electric fields in the photocatalyst during photocatalysis, whereas only the holes are transferred to the surface of the photoanode for the oxidation reactions in a PEC cell due to the external bias. Therefore, compared with crystal facet engineering in particle-based photocatalysis, the criteria for photoelectrodes share some similarities yet have apparent differences. Crystal facet engineering has been intensively studied on various metal oxides for suspension photocatalytic systems, and some excellent review articles have systematically discussed the synthesis mechanisms and unique properties of particle-based semiconductors such as TiO2 and Cu2O with tailored facets.21,22,27−34 On the other hand, crystal facet engineering for the design of efficient photoelectrodes for PEC water splitting has a shorter history.35−39 Some recent reviews have briefly mentioned some of the crystal facet engineered photoelectrodes in the field,21,31,40 but a comprehensive review focusing on the unique properties, preparation, and modification of crystal facet engineered photoelectrodes for PEC

2. MECHANISMS OF CRYSTAL FACET ENGINEERING Generally, exposed facets can be produced in semiconductors by selectively controlling the nucleation and growth rates in different directions during the growth of the crystals, which is also known as the bottom-up route. It is well-known that a crystal tends to grow by continuously reducing the total surface energy. According to the Gibbs−Wulff theorem, the facets with higher surface energies always grow rapidly and usually C


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Figure 2. Schematic of crystal facet engineering using organic or inorganic additives.

account for a very small fraction of the surface or even vanish from the final crystals, whereas the facets with lower surface energies grow slowly and are preserved, constructing the shape of the final crystals.22 As a result, the semiconductor crystals obtained under natural or equilibrium conditions normally expose less reactive facets, which is not favorable for solar energy conversion. Fortunately, some organic or inorganic additives can serve as capping agents and tend to selectively cover the facets with higher surface energies and reduce the surface energies of the adsorbed facets.41−43 As a result, the exposure of different facets can be delicately tailored, which is called crystal facet engineering (Figure 2). For example, anatase TiO2 crystals are generally exposed with thermodynamically stable {101} facets under natural growth conditions,44 while the much more reactive {001} facets are more desirable for solar energy conversion.21 Lu’s group reported that fluorine ions can selectively attach to the {001} surfaces of anatase TiO2 crystals to change the surface energy order, and anatase TiO2 crystals with 47% {001} facets were successfully synthesized.26 Subsequently, other groups further modified the conditions, and anatase TiO2 with nearly 100% {001} facets was obtained.45−51 Interestingly, the synergetic effect of both F− and ammonia as capping agents led to the exposure of {111} facets with a much higher surface energy than the {001} facets (1.61 J m−2 vs 0.90 J m−2),52 while TiO2 single crystals with a curved surface (facet with continuous Miller index) were synthesized using HF and citric acid as synergistic capping agents.53 Crystal facet engineering for different semiconductors may require various capping agents, and the exposed facets of a semiconductor crystal are sensitive to the concentration of the capping agents. By changing the amount of the capping agents, polyvinylpyrrolidone (PVP)54 or NH2OH·HCl,55 during the synthesis process, Cu2O crystals with different facets can be prepared; further, BiVO4 crystals with various ratios of {040}/ {110} facets can be achieved by adjusting the concentration of the TiCl3 capping agent and the pH value.42 Interestingly, 30faceted BiVO4 polyhedra with exposed high-index facets of {132}, {321}, and {121} can be synthesized using a trace amount of Au nanoparticles as the capping agent.56

Selectively etching the undesirable facets of the obtained crystals is another strategy to achieve semiconductors with selective exposed facets, which is also known as the top-down route. The mechanism of the top-down route is to choose proper molecules or ions that can be selectively adsorbed on the favorable facets, protecting them from attack by etching reagents. As a result, the undesirable facets exposed to the etching reagent will be dissolved. During the etching process, surface recrystallization on the residual facets of the mother crystals may occur.22 Moreover, different facets of a crystal may exhibit various corrosion resistances to an etching reagent. Thus, some facets will be selectively etched while other facets will survive. For example, concentrated HF can selectively etch the {001} facets of anatase TiO2 while still maintaining the {101} facets.57 Theoretical calculations reveal that etching is energetically permitted solely on the {001} facets through surface−TiOF2 dissolution under high HF concentrations. By precisely adjusting the amount of NH3 solution in the mixture of Ag2O nanocrystals and NaOH, novel Ag2O cubic nanoframes and rhombicuboctahedra with square depressions on all the {100} facets can be synthesized.58 Hydroxide ions can interact strongly with the terminal silver atoms on the {111} facets of Ag2O to protect these facets, while less hydroxide ions are adsorbed on the {100} facets with terminal oxygen atoms, resulting in the selective etching of the {100} facets. Uniform and monodisperse urchin-like Cu2O architectures were successfully fabricated by selectively etching the {100} facets.59 Because of surface Cu atoms with dangling bonds, the {111} facets are positively charged and can be protected by negatively charged molecules or ions. However, the {100} facets are electrically neutral, making it difficult for them to adsorb protective molecules or ions, and thus, they can be selectively etched. Likewise, Cu2O nanoframes were prepared by selectively etching the {110} facets (Figure 3a).60 Owing to the great instability of the {110} facets in the presence of HCl, the {110} facets are preferentially etched until the whole interior space becomes hollow, resulting in nanoframe structures with more stable {100} facets. Interestingly, Cu2O nanoframes constructed of hexagonal {111} skeletons were synthesized by exposing the reaction mixture (after the growth D


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photoreaction activity, anisotropic photocorrosion resistance, and anisotropic electrical conductivity, which will be discussed in detail. 3.1. Surface Electronic Structures

The surface atomic arrangements vary in different crystal facets, leading to differences in the surface electronic structures. Therefore, a semiconductor crystal with different dominant facets may exhibit different bandgap energies and band edge positions, resulting in varying light-harvesting abilities and redox powers of the photogenerated electron− hole pairs. For example, anatase TiO2, with mainly {101} exposed facets (82%), showed a blue-shift of 9 nm in the UV− vis light absorption spectrum when compared to its counterpart with mainly {001} exposed facets (72%).64 By systematically investigating the bandgaps and band edge positions of micrometer-sized anatase TiO2 with different predominant low index facets, {101} and {010} facets were found to have similar electronic structures with larger bandgaps and more negative CBs than the {001} facets.65 This work indicates that even though anatase TiO2 with a dominant exposure of {101} or {010} facets results in reduced light absorption (due to the larger bandgap) compared to the {001}-dominated anatase TiO2, the reduction ability of the photogenerated electrons is stronger (due to the more negative CB). Thus, the change in both the bandgap and the band edge positions caused by crystal facet engineering should be carefully considered for the design of efficient semiconductor photocatalysts. In some extreme cases, TiO2 semiconductors composed of only one facet can be obtained, and they exhibit very different electronic structures compared to their bulk counterparts. For example, monolayer titania nanosheets (Ti0.91O2) composed of a twodimensional lamellar arrangement of TiO6 octahedra possess a bandgap of approximately 3.8 eV,66 which is 0.6 eV larger than that of conventional anatase TiO2 (3.2 eV). Quasi-cubic-like WO3 (QC-WO3) crystals enclosed by almost equal percentages of {002}, {020}, and {200} facets exhibit a smaller bandgap with respect to rectangular sheet-like WO3 (RS-WO3) crystals with dominant {002} facets (2.71 vs 2.79 eV).67 A further electronic structure analysis suggests that the CB and VB of RS-WO3 crystals are elevated by 0.3 and 0.22 eV, respectively, compared to those of QC-WO3 crystals. As a result, the RS-WO3 crystals possess a sufficient reduction potential for photocatalytic CO2 reduction to produce CH4, whereas no such activity has been observed from the QC-WO3 crystals. However, the normalized oxygen evolution rate of the QC-WO3 crystals by excluding the effect of the specific surface area is over 8-fold higher than that of the RS-WO3 crystals. The interesting phenomenon of crystal facet dependent electronic structures has been observed in many other semiconductors, such as ZnO,68−70 SnO2,71 NaNbO3,72 Bi25GaO39,73 BiTaO4,74 BiOCl,75−77 BiOI,78−80 BiOBr,81,82 Ag2O,83 AgBr,84,85 α-Ag2WO4,86 and Ag3PO4.87

Figure 3. (a) Schematics of various Cu2O crystals and SEM images of the Cu2O nanoframes with the absence of {110} facets. Reproduced with permission from ref 60. Copyright 2013 American Chemical Society. (b) Low-magnification and (c) High-magnification SEM images of Cu2O nanoframes with the absence of {100} facets. Reproduced with permission from ref 61. Copyright 2010 John Wiley and Sons.

of Cu2O crystals) to the air at room temperature (Figure 3b,c).61 Further studies reveal that the final morphologies of Cu2O can be attributed to the dual functions of PVP. During the crystal growth process, PVP acts as a capping agent that is preferentially adsorbed on the {111} facets to obtain polyhedral Cu2O crystals. During the etching process in the presence of O2, the selective adsorption of PVP on the {111} facets can prevent the {111} facets from oxidative etching, facilitating the formation of hollow structures. Moreover, by heating cubic Cu2O nanoparticles in a thiourea solution, hollow Cu7S4 nanocages with an 18-facet polyhedron structure were synthesized.62 Similarly, the strong affinity of phosphate ions to the {110} facets of α-Fe2O3 crystals can control the etching along the [001] direction in the presence of oxalic acid.63 To summarize, the selection of proper capping agents that are preferentially adsorbed on the targeted facets to change the surface properties (e.g., surface energy/surface corrosion resistance) is critical to achieve the crystal facet engineering of semiconductor crystals by either the bottom-up or top-down routes. Therefore, the facet-dependent performance of various semiconductor crystals can be further investigated experimentally.

3.2. Redox Reaction Sites

Because of the anisotropic surface band edge positions of a semiconductor crystal, band alignment can reasonably form between different facets, namely, surface heterojunctions, in which photogenerated electrons and holes are separated in different facets for the subsequent redox reactions.88,89 On the basis of the reduction of Pt4+ by photogenerated electrons to form Pt and the oxidation of Pb2+ by photogenerated holes to form PbO2, an early work experimentally confirmed the separated reduction and oxidation sites on different facets of

3. ANISOTROPIC PROPERTIES OF FACETED CRYSTALS Owing to the different atomic arrangements in various facets, semiconductor crystals exhibit unique facet-dependent properties including anisotropic surface electronic structures, anisotropic redox reaction sites, anisotropic surface built-in electric fields, anisotropic molecular adsorption, anisotropic E


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Figure 4. SEM images of (a) a rutile particle and (b) an anatase particle with selective deposition of PbO2 and Pt particles on different facets. Reproduced with permission from ref 90. Copyright 2002 The Royal Society of Chemistry. (c) Density of states (DOS) curves for {101} and {001} facets of anatase TiO2. (d) Surface heterojunction between {001} and {101} facets. Reproduced with permission from ref 88. Copyright 2014 American Chemical Society.

rutile and anatase TiO2 particles.90 As shown in Figure 4a, Pt particles are only observed on the {110} facet of the rutile TiO2 crystal, while PbO2 particles are mainly observed on the {011} facets, which clearly suggests that {110} and {011} facets are the reduction and oxidation sites for rutile TiO2, respectively. Similarly, the {101} facets are more reductive, while the {001} facets are more oxidative for anatase TiO2 (Figure 4b). This work provides new insight into crystal facet engineering for photocatalytic applications. A semiconductor crystal is generally composed of various facets rather than only one isolated facet. Thus, the synergistic effects between the coexisting facets should not be ignored. Anatase TiO2 microspheres composed of nanodecahedrons with 45% {001} facets exhibited much higher photocatalytic activity than their nanosheet counterparts with 82% {001} facets.91 Density functional theory (DFT) calculations reveal that the VB of TiO2 {001} is higher than that of {101}, while the CBs of these two facets are located at similar positions, resulting in the separation of the electrons and holes to the {101} and {001} facets, respectively. The proper coexposed {101} and {001} facets are beneficial for the efficient separation of photogenerated electron−hole pairs. Thus, enhanced photocatalytic activity is observed. Recently, Yu et al. also observed different photocatalytic activities of anatase TiO2 with coexposed {001} and {101} facets and proposed a novel term of a “surface heterojunction” based on DFT

calculations to explain this phenomenon (Figure 4c,d).88 Subsequently, surface heterojunctions have attracted increasing attention in photocatalysis.89 Very recently, the trapping and dynamics of excess electrons at the {101} and {001} facets of anatase TiO2 in a vacuum and in an aqueous solution were systematically investigated using first-principles simulations to further understand the anisotropic redox reaction sites.92 The behavior of the excess electrons is highly dependent on the exposed anatase facet, the environment and the characteristics of the electron donor. In a vacuum, no electron trapping occurs on the {101} facets, while excess electrons at the aqueous {101} facets induce water dissociation and are then trapped into stable surface Ti3+bridging OH complexes. Trapping reduces the redox potential of the electrons but favors the interaction between electrons and adsorbates. Therefore, reduction reactions are promoted on the {101} facets. However, the {001} facets are strongly repulsive for electrons and attractive for holes and are thus favorable for oxidation reactions. Further studies on anatase TiO2 photoelectrodes dominated by {110}, {101}, and {001} facets found that the {001} facets tend to form radical species for advanced oxidation processes, whereas the {110} facets are favorable for the conventional four-electron water to oxygen oxidation.93 Likewise, the {121} facets of brookite TiO2 are beneficial for oxidation reactions, while the {211} facets are favorable for reduction reactions.94 F


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Figure 5. (a) Schematic of charge distributions on different facets of a BiVO4 single crystal with different area ratios of {010}/{011}. (b) Schematic of surface built-in electric fields on {011} and {010} facets of a BiVO4 single crystal. Reproduced with permission from ref 115. Copyright 2015 John Wiley and Sons. (c) SPVM image of a MnOx/{011}BVO{010}/Pt particle. (d) Schematic of surface built-in electric fields of a MnOx/ {011}BVO particle (green line) and a MnOx/{011}BVO{010}/Pt particle (dashed red line). Reproduced with permission from ref 116. Copyright 2017 American Chemical Society.

In addition to numerous studies on TiO2,95−98 surface heterojunctions between reduction and oxidation functional facets have been observed in other semiconductors, such as the {001}/{110} facets of SrTiO3,99 {100}/{111} facets of CeO2,100 {010}/{110} facets of BiVO4,101−106 {100}/{101} facets of a BiVO4:YVO4 solid solution,107 {001}/{101} facets of Cu2WS4,108 {110}/{111} facets of ZnGa2O4,109 {001}/ {110} facets of BiOCl,110 {010}/{100} facets of BiOIO3,111 and {001}/{110} facets of Bi3TiNbO9.112 Interestingly, the separation of the photogenerated electrons and holes among three different facets was observed in Cu2O crystals.113 Because of the band alignment between the {111}, {100}, and {110} facets, the photogenerated holes will be transferred from the {111} facets to the {100} and {110} facets, whereas the electrons will be transferred from the {100} and {110} facets to the {111} facets. As a result, the {100} and {110} facets are favorable for oxidation reactions, while reduction reactions occur on the {111} facets. Efficient charge separation and transfer in a semiconductor crystal may be achieved by tailoring the exposure of the different facets. On the other hand, selective oxidation and reduction reactions on different facets of a semiconductor can also be achieved due to the anisotropic redox reaction sites, which may be beneficial for the design of bifunctional photocatalytic energy conversion systems.8

the underlying mechanisms are still not well understood. The application of spatially resolved surface photovoltage spectroscopy (SRSPS) has provided new insights into the surface charge dynamics.114 For example, SRSPS studies on a single BiVO4 crystal enclosed by {010} and {011} facets revealed that the {011} facet exhibits a much stronger surface photovoltage (SPV) signal intensity than the {010} facet, indicating that the surface band bending in the SCR of the {011} facet is significantly different than that of the {010} facet.115 Consequently, the band bending induced built-in electric fields between these facets are different, leading to the spatial transfer of photogenerated electrons and holes to different facets in a single semiconductor crystal. By increasing the area ratio of {010}/{011} (Figure 5a), the difference in the SPV signal intensities between the {011} and {010} facets can even reach a factor of 70, suggesting a larger difference in their surface built-in electric fields (Figure 5b). Subsequently, SRSPS and SPV microscopy (SPVM) studies on BiVO4 single crystals with MnOx and Pt NPs selectively deposited on the {011} and {010} facets (denoted as MnOx/ {011}BVO{010}/Pt), respectively, demonstrate that cocatalysts also function to align the surface built-in electric fields, providing an additional driving force for the migration of photogenerated electrons and holes to active surface sites beyond their diffusion lengths.116 In particular, the SPVM image of a MnOx/{011}BVO{010}/Pt particle illustrates the separation of photogenerated electrons and holes on the {010} and {011} facets, respectively (Figure 5c), providing direct evidence for facet-dependent redox reaction sites in a BiVO4

3.3. Surface Built-In Electric Fields

Despite the observations of facet-dependent redox reaction sites in various semiconductor crystals with well-defined facets, G


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Figure 6. Preferential adsorption of additives for crystal facet engineering (left) and for selective metal deposition on different facets (right). Reproduced with permission from ref 125. Copyright 2009 American Chemical Society.

low coverage, while a mixed phase composed of dissociated and molecular water is observed at higher coverage. Practically, most of the as-prepared faceted semiconductors contain surface defects, which also affects the adsorption behaviors of water molecules.121,122 In addition to water molecules, the anisotropic adsorption of other molecules on the different facets of TiO2 has been observed. For example, anatase TiO2 with exposed {001} facets showed a higher toluene adsorption ability than its counterpart with dominant {101} facets.123 The different adsorption ability is primarily due to the different distributions of the 5c-Ti and 6c-Ti bonding modes on the {001} and {101} facets. The formation of the terminal Ti-OH species is more favorable on the {001} facets, offering active sites for toluene adsorption. The anisotropic adsorption of molecules has also been reported in other semiconductors. For example, BiOCl nanosheets with exposed {010} facets exhibit a higher methyl orange (MO) adsorption capacity than their counterparts with exposed {001} facets.75 The {001} facets are more negatively charged because of their surface terminal oxygen atoms compared to the {010} facets, which repulsively interact with the anionic MO dyes, resulting in the poor dye adsorption ability. Moreover, the surface atomic structure of the {010} facets exhibits open channel characteristics, providing more active sites as well as a larger space to adsorb MO molecules. In another case, the adsorption capacity of MO to the different-shaped Cu2O nanocrystals decreases in the order of octahedrons (100% of {111} facets) > cuboctahedrons (coexisting {111} and {100} facets) > cubes (100% of {100} facets).54 The {111} facets contain active Cu atoms that can interact with negatively charged MO. As a result, the Cu2O nanocrystals with a larger exposed area of {111} facets would adsorb more MO molecules. Recently, combined experimental and theoretical studies have reported that more Cr(VI) species could be adsorbed on the {110} facet of α-Fe2O3 than on its {001} counterpart.124 The dominant Cr(VI) species of HCrO4− is adsorbed on the {001} and {110} facets in innersphere monodentate mononuclear and bidentate binuclear configurations, respectively. The chromate complexes formed on the hematite facets also strongly affect the Cr(VI) adsorption ability on different hematite facets. Some inorganic ions or organic molecules tend to be selectively adsorbed on the high-energy facets of a semiconductor to lower the surface energy, which is a common bottom-up route for crystal facet engineering, as discussed in section 2. In addition, the selective adsorption of species on different facets of a semiconductor crystal in an etching

single crystal. Systematic studies have revealed that the selective deposition of MnOx on the {011} facets can increase the upward band bending and downward band bending on the {011} facets (65 mV) and {010} facets (−35 mV), respectively. Moreover, the upward band bending on the {011} facets and the downward band bending on the {010} facets can be further expanded to 95 and −79 mV, respectively, by selectively depositing another Pt cocatalyst on the {010} facets, as shown in Figure 5d. Compared with the bare BiVO4 particle with an upward band bending of approximately 7.2 and 3.4 mV on the {011} and {010} facets, respectively, the driving force for charge separation and transfer is significantly increased from 3.8 to 174 mV after the selective deposition of the dual cocatalysts. The discovery of anisotropic surface built-in electric fields on different facets of semiconductor crystals has unveiled the underlying mechanisms of facet-dependent redox reaction sites. In addition, the selective deposition of cocatalysts on the proper facets of a photocatalyst can further strengthen surface built-in electric fields, thereby promoting charge separation and transfer. These findings are critical for the design of efficient photocatalysts through crystal facet engineering. 3.4. Molecular Adsorption

In a general PEC water splitting reaction, water molecules should be adsorbed on the surface of the semiconductor for subsequent redox reactions with the photogenerated electrons or holes. Thus, the effective adsorption of water molecules on the surface of a semiconductor is essential for the PEC water splitting reactions. Because the surface atomic arrangement and coordination vary in different crystal facets, the adsorption energy and states of the water molecule should be different on various facets. Various theoretical calculations have been performed to understand the interaction between water molecules and TiO2 facets.117 It is generally accepted that unsaturated Ti atoms can adsorb H2O through Ti−O bonding, whereas undercoordinated oxygen atoms can interact with water hydrogen to form O−H bonds. For example, both theoretical and experimental studies have confirmed that water molecules are adsorbed on the defect free {101} facets of anatase TiO2 by interacting with the surface 5-fold-coordinated (5c) Ti atoms and 2-foldcoordinated (2c) bridging oxygen atoms.118,119 However, most of the theoretical calculations support the dissociative adsorption of water molecules on the defect-free {001} and {100} facets at low coverage.117 The coverage-dependent adsorption of water on the {001} facets of anatase TiO2 has also been investigated experimentally.120 Water molecules dissociate on the {001} facets in the form of hydroxyl groups at H


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Figure 7. (a) Bulk atomic structural model of an anatase TiO2 crystal. (b) Schematics of electron carrier trapping on the surface and in the bulk of intact- and etched-TiO2, respectively. (c) Hydrogen evolution rates of intact- and etched etched-TiO2 samples. Reproduced with permission from ref 127. Copyright 2016 American Chemical Society.

The chemical activity for the water splitting reaction decreases in the order of (001) > (103)f > (100) > (110) > (103)s > (101). Interestingly, the photocatalytic activity of anatase TiO2 single crystals could be tuned by selectively etching the {001} facets.127 As shown in Figure 7a,b, the rapid recombination of the photogenerated electrons and holes take place on the TiO2 crystals with intact {001} and {101} facets, whereas the proper etching of the {001} facets leads to efficient charge separation; in that instance, the holes and electrons are transferred to titanium and oxygen species, respectively. However, severe charge recombination occurs if the {001} facets are completely etched into a hollow structure. As a result, the anatase TiO2 crystals with appropriately etched {001} facets exhibit a hydrogen generation rate 7 times greater than their counterparts without etching (Figure 7c). This work provides a new strategy for tuning the coexisting facets of a semiconductor crystal toward efficient photocatalysis. By comparing the photodegradation rates of methylene blue (MB) under light illumination with different wavelengths, the photocatalytic activities of anatase TiO2 with dominant {100} facets and its counterpart with dominant {001} facets were investigated.128 The photocatalytic activity of TiO2 nanocrystals depends strongly on the density of the surface active sites and the electronic structures in the different facets. In terms of the MB adsorption, the {100} facets exhibit an approximately 10 times higher active site density than that on the {001} facets. Owing to the high surface-energy-induced surface reconstruction, the {001} facets tend to lose some of the reactive sites to minimize the surface energy and thus become more passive than the {100} facets. Interestingly, the {100} facets possess higher photocatalytic activity than the {001} facets under 254 nm UV light illumination, whereas the reverse is observed under 365 nm UV light irradiation. Similarly, SrTiO3 crystals with exposed high-index facets of {120} and {121} showed significantly enhanced activities for photocatalytic water splitting.129 The different energy barriers for surface reactions on various facets also affect the photoreaction activity. For example, DFT calculations revealed that the energy barriers of water splitting on the {110} facets of Ta3N5 are close to zero, leading to good PEC activities of Ta3N5

environment can selectively protect the adsorbed facets, which is a classic top-down route for controlling the exposed facets. Readers may refer to section 2 for more information. Interestingly, the selectively adsorbed additives can also function as masks to prevent the growth of metal particles, thereby achieving the selective deposition of metal particles on different facets of a semiconductor. For example, the preferential adsorption of sodium dodecyl sulfate (SDS) on the {111} facets of Cu2O crystals could effectively inhibit the nucleation of gold, leading to the selective deposition of gold nanoparticles exclusively on the {100} facets (Figure 6).125 A similar strategy for the selective gold deposition on ZnO with exposed {0001} and {011̅0} facets was also reported,125 and different Au precursors were found to affect the selective deposition of Au particles on the ZnO surfaces as well. Gold particles were deposited on both {0001} and {011̅0} facets when using Au(CH3COO)3 as the precursor. However, Au particles were only observed on the {011̅0} facets when using AuCl3 as the precursor. The preferential adsorption of Cl− ions on the {0001} facets is attributed to the repulsive deposition of Au particles on the {0001} facets. The preferential adsorption of reactant molecules on different facets of a semiconductor may provide new opportunities for enhancing the selectivity of photocatalysis such as in photocatalytic CO2 reduction and photocatalytic organic pollutant degradation. On the other hand, the selective deposition of metal particles on different facets of a semiconductor may inspire the design of efficient photocatalysts by accurately tailoring the selective semiconductor facets/cocatalyst interfaces, even between the selective facets of both the semiconductor and cocatalyst, rather than only roughly adjusting the semiconductor/cocatalyst interfaces. 3.5. Photoreaction Activity

Generally, facets with a higher surface energy are more reactive than those with a lower surface energy. The water splitting reaction pathways on different low-index anatase TiO2 facets were systematically studied by DFT calculations.126 In terms of the low-index facets of (101), (100), (001), (103)f, (103)s, and (110), the water splitting reactions are found to be a structuresensitive reaction. The surface activity is determined by not only the surface energy but also the surface atomic structure. I


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Figure 8. (a) Photodegradation rates of MO by Cu2O with different facets: (A) 26-facet polyhedral Cu2O octahedrons, (B) 26-facet Cu2O cubes, (C) 50-facet polyhedral Cu2O with high-index {211} facets, (D) 50-facet polyhedral Cu2O with high-index {522} facets. Reproduced with permission from ref 131. Copyright 2012 The Royal Society of Chemistry. (b) Apparent reaction rate constants for MO degradation over the BiOCl photocatalysts with exposed {001} (BOC-001) and {010} facets (BOC-010) under (left) UV (λ = 254 nm) and (right) visible-light (λ > 420 nm) illumination. Reproduced with permission from ref 75. Copyright 2012 American Chemical Society.

Figure 9. (a) Energy levels of {101} and {001} facets for a Cu2WS4 crystal. (b) Atomic models for the shape evolutions of a Cu2WS4 crystal in the presence of (i) Na2SO3 and (ii) Na2S/Na2SO3 under light illumination, respectively. Reproduced with permission from ref 108. Copyright 2014 The Royal Society of Chemistry.

activity of the truncated dodecahedral crystals is attributed to the higher density of surface-exposed Fe cations on the {101̅2} facets. Interestingly, CdS crystals with dominantly exposed {0001} and {101̅1} facets were prepared by simply employing a syringe pump to control the synthesis kinetics.133 The photocatalytic hydrogen evolution rate of the {0001} faceted CdS is 2.3 times that of its {101̅1} faceted counterpart. Firstprinciples calculations reveal that both the CB and VB positions of the {0001} facets are lower than those of the {101̅1} facets, forming a type II surface heterojunction. As a result, the photogenerated electrons will be transferred to the {0001} facets for hydrogen evolution reactions. Moreover, the {0001} facets are more favorable for hydrogen atoms adsorption and disproportion due to the smaller Gibbs free energy changes. On the basis of these factors, CdS with a larger percentage of {0001} facets is believed to exhibit a higher photocatalytic activity for hydrogen evolution.

photoanodes with exposed {110} facets, as observed experimentally.130 The crystal facet dependent photocatalytic activity of polyhedral Cu2O was also observed by comparing the photodegradation rates of MO using 50-facet polyhedral Cu2O with high-index {522} facets, 50-facet polyhedral Cu2O with high-index {211} facets, 26-facet Cu2O cubes, and 26-facet polyhedral Cu2O octahedrons.131 The exposure of high-index facets in the novel 50-facet polyhedral architectures provides more unsaturated Cu dangling bonds and surface oxygen vacancies, which can accelerate the formation of highly oxidative •OH radicals, resulting in the enhanced decomposition of MO dyes (Figure 8a). Single crystalline BiOCl nanosheets with exposed facets of {001} and {010} exhibited different photocatalytic activities.75 As shown in Figure 8b, due to the synergistic effect of the surface atomic structure and suitable internal electric fields, BiOCl nanosheets with exposed {001} facets show higher activity for photocatalytic pollutant degradation under UV light illumination. Nevertheless, their counterparts with exposed {010} facets exhibit a much higher activity for indirect dye photosensitization degradation under visible light owing to their larger surface area and open channel characteristics. Truncated dodecahedral α-Fe2O3 crystals with predominant {101̅2} facets exhibited higher photocatalytic activity than rhombohedral crystals with exposed {0001} and {101̅0} facets in the photo-Fenton reaction.132 The higher photocatalytic

3.6. Photocorrosion Resistance

Because of the anisotropic corrosion resistance of faceted semiconductor crystals in a given corrosion environment, some facets are more susceptible to being dissolved. Therefore, hollow crystal structures with the absence of certain facets can be achieved, as was discussed in section 2. This section will mainly focus on the anisotropic photocorrosion resistance of semiconductors with different exposed facets for photocatalysis. J


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Figure 10. (a) Schematic and the corresponding SEM image of an anatase TiO2 single crystal particle exposed with {001}, {010}, and {101} facets. Schematics and SEM images of the measurement setup for the I−V curves of an anatase TiO2 single crystal particle along the (b) [001] and (c) [010] directions, respectively. (d) Corresponding I−V curves. The scale bars in (a) 500 nm, (b) 5 μm, and (c) 3 μm. Reproduced with permission from ref 136. Copyright 2015 John Wiley and Sons.

3.7. Electrical Conductivity

Photocorrosion generally happens in some semiconductors during the photocatalytic reactions due to the strong redox environment. Interestingly, the corrosion resistance of a semiconductor crystal varies for different facets. For example, during the photodegradation of MO dye, the {100} and {110} facets of Cu2O gradually disappear, while the {111} facets survive, resulting in final Cu2O nanosheets with exposed {111} facets.113 These nanosheets exhibited stable photocatalytic activity for MO degradation. A similar facet-dependent photostability of Cu2O was also observed in PEC water splitting.134 Because of the formation of surface heterojunctions, photogenerated holes are accumulated on the {100} and {110} facets, leading to the selective corrosion of these two types of facets. However, the {111} facets enriched with electrons are relatively stable. Interestingly, photocorrosion was observed only on the {101} facets of Cu2WS4 with coexisting {101} and {001} facets during photocatalytic hydrogen production.108 Theoretical calculations reveal that the formation of surface heterojunctions between the {101} and {001} facets promotes the accumulation of holes on the {101} facets (Figure 9a), resulting in the oxidation of S2− and Cu+ from Cu−S4 tetrahedrons in the {101} facets, even in the presence of SO−3 as the sacrificial agent (Figure 10b, situation (i)). However, the photocorrosion of Cu2WS4 can be avoided if the solution contains S2−/SO2− 3 , because the surface-adsorbed S2− ions can quickly consume the photogenerated holes (Figure 9b, situation (ii)). Similarly, wet-chemical etching with the assistance of light illumination on GaN demonstrated that the N-facets are obviously etched while the Ga-facets can survive under the same conditions.135 The anisotropic photocorrosion resistance of different facets offers new strategies for efficient photocatalyst design with enhanced stability. However, it is very challenging to improve the long-term stability of semiconductors that suffer from intrinsic photocorrosion (e.g., CdS, Cu2O, or WO3) using crystal facet engineering alone. The combination of crystal facet engineering with other strategies such as surface protection, heterojunction construction, and cocatalyst modification may be more feasible to further enhance the long-term stability.

Interestingly, different crystal facets of a semiconductor crystal also exhibit anisotropic electrical conductivity. Because electron transfer is an essential process for PEC water splitting, the selective exposure of highly conductive facets is also important for the design of efficient photoelectrodes. As shown in Figure 10, the I−V curves of an anatase TiO2 single crystal particle are obtained by contacting two tungsten probes with a {001} facet and a {010} facet, respectively.136 Under applied potentials of −2 to 2 V, the current along the [001] direction is significantly higher than that along the [010] direction. At an applied potential of 2 V, the [001] direction delivers a current of 11.08 μA, which is approximately 29 times higher than that of the [010] direction (0.38 μA). Taking the lengths of the two directions into account, the conductivity along the [001] direction is approximately 60 times that of the [010] direction. A similar measurement was performed on a Cu2O single crystal particle with nanocube and octahedron structures, respectively.137 Surprisingly, the conductivity of the Cu2O octahedron enclosed by {111} facets is 1100 times higher than the counterpart nanocube bounded by {100} facets. Owing to the symmetric crystal structures of the Cu2O nanocube and octahedron, there is no preferred direction or pathway for carrier transport in a single crystal particle. Therefore, the observed huge electrical conductivity difference is a facet-dependent property. Likewise, the conductivity along the [110] direction is found to be 4 orders of magnitude higher than that of the orthogonal directions in a hematite (α-Fe2O3) single crystal,138 which is believed to be important for charge transport in PEC water splitting.

4. SYNTHESIS OF CRYSTAL FACET ENGINEERED PHOTOELECTRODES Photoelectrodes with the exposure of different crystal facets can be fabricated by either immobilizing faceted semiconductor crystals in powder form on conductive substrates, that is, ex situ preparation, or the direct growth of semiconductor crystals with tailored facets on conductive substrates, that is, in situ preparation. In this section, we will focus on the introduction of these two categories for crystal facet engineered semiconductor film preparations that are currently or that will be potentially used for PEC water K


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be ignored. Along with the concentration of the suspension, the film thickness can also be controlled by adjusting the rotation speed and repeating the spin-coating cycles. Generally, smaller nanoparticles are beneficial for the formation of homogeneous films, and the film thickness can be controlled at the nanoscale. Although drop casting and spin-coating can immobilize faceted crystals on conductive substrates to form photoelectrodes relatively easily, the facet orientations of the resultant films cannot be well controlled. 4.1.2. Electrophoretic Deposition. Because of the surface’s charge-bearing nature, semiconductor particles can be deposited on a conductive substrate by an electrophoretic deposition method (EPD). For instance, by applying an external potential of 20 V between an FTO substrate and a Pt electrode, faceted BiVO4 particles can be deposited on the FTO substrate.103,169 The film thickness and coverage ratio can be simply controlled by tuning the EPD time and repeating the EPD cycles. The interaction between the conductive substrate and the semiconductor particles is important to achieve highquality photoelectrodes. To obtain better contact between the semiconductor particles and the conductive substrate, a “necking treatment” process is applied.170 Specifically, photoelectrodes obtained via EPD are treated with solvents containing proper metal ions, followed by calcination under appropriate conditions. During the calcination process, the metal ions that fill in the vacancies of the photoelectrodes are converted to the corresponding metal oxides or (oxy)nitrides, bridging the semiconductor particles and the conductive substrates. The necking treatment is also applicable to enhancing the interfacial contact of photoelectrodes prepared by other ex situ methods. Importantly, the stability of the semiconductor particles and the conductive substrates under the calcination conditions (e.g., temperature and atmosphere) should be considered when performing necking treatment. 4.1.3. Doctor Blade and Screen-Printing. The doctor blade is a common method to fabricate photoelectrodes, especially for dye-sensitized solar cells (DSSCs).171−173 Generally, nanoparticles are dispersed in a solvent to make a slurry, followed by blading on a conductive substrate. After evaporating the solvent, a semiconductor film can be achieved. Scotch tape is normally used as a spacer to control the film thickness. The thickness of the films fabricated by the doctor blade method is normally on the micrometer scale, and it is very challenging to control the film thickness to less than one micrometer. Therefore, the doctor blade is not suitable to prepare photoelectrodes using semiconductors with a short charge diffusion length (e.g., α-Fe2O3),174 as severe charge recombination may occur, resulting in poor PEC performance. Sometimes, Nafion solution or another conductive polymer is used to increase the conductivity.73 Moreover, the slurry can be used as an ink that can be printed on substrates by a screenprinting method.160,161 The advantage of the doctor blade and screen-printing methods is the feasibility of scale-up. 4.1.4. Vacuum Filter-Transfer. Photoelectrodes can also be prepared by a vacuum filter-transfer method. Typically, semiconductor particles are dispersed in a proper solvent or water, followed by vacuum filtration over a cellulose membrane. The obtained planar film is then transferred to a conductive substrate. By etching or calcination to remove the cellulose membrane, a photoelectrode can be achieved. The film thickness can be controlled by tuning the concentration of the suspension. For example, ultrathin WO3 nanosheets with

splitting to serve as a guideline for researchers who are interested in the relevant fields. 4.1. Immobilizing Faceted Crystals on Conductive Substrates

For this preparation strategy (ex situ preparation), it is important to synthesize semiconductor crystals with highly reactive facets first. Generally, faceted semiconductor crystals can be prepared by the bottom-up and top-down routes introduced in section 2. In a special case, 2D materials with atomic or molecular thickness exfoliated from layered host compounds, so-called nanosheets, are exposed with only two planar facets, which can serve as building blocks for the construction of faceted photoelectrodes.139−141 The preparation and unique photocatalytic properties of 2D materials have been comprehensively summarized in some recent excellent review articles.142−149 The selection of suitable methods to immobilize the faceted semiconductor crystals on a conductive substrate is also crucial for PEC water splitting. The ideal method should not only form excellent contact between the semiconductor crystals and the conductive substrates but also maintain the facet orientation of the semiconductor crystals. So far, drop casting,150,151 spin-coating,35,152,153 electrophoretic deposition (EPD),103,154,155 doctor blade,156−159 screenprinting,160,161 vacuum filter-transfer,162 particle transfer,163 and finger rubbing methods164−168 have been developed, and they will be briefly introduced in this section. 4.1.1. Drop Casting and Spin-Coating. Drop casting is the simplest method to prepare photoelectrodes from semiconductor particle suspensions and does not require any special equipment. Uniform suspensions are prepared by dispersing semiconductor particles in a solvent or water with the assistance of ultrasonication, followed by dropping a certain amount of suspension on a conductive substrate. By evaporating the solvent, a semiconductor film can be achieved. Sometimes, conductive additives are added to the suspension to enhance the conductivity of the film. For example, a {110} facet enriched α-Fe2O3 photoanode can be prepared by dropping a suspension of {110} α-Fe2O3 powder dispersed in a water−alcohol solvent with Nafion solution on an indium−tin oxide (ITO) glass, followed by drying in air.151 The loading amount of the semiconductor particles can be controlled by adjusting the concentration and the volume of the suspension. However, it is challenging to prepare a homogeneous film by drop casting owing to the difficulty of dispersing the suspension evenly on the substrate. Larger particles and higher concentrations generally result in inhomogeneous films, and the detachment of the film may occur after evaporating the solvent. Moreover, the properties of the solvent may affect the quality of the films. Thus, a proper particle size, concentration, and solvent are important for obtaining high-quality films. Post calcination is generally required to improve the contact between the semiconductor particles and the substrates. Alternatively, the prepared suspensions can be dropped on a conductive substrate fixed on a spin-coater by vacuum, that is, spin-coating. Owing to the centripetal force, the solvent will be removed and the semiconductor particles will be dispersed on the substrate homogeneously. For the preparation of BiVO4 photoanodes with exposed {001} facets, an ethanol suspension with {001}-enriched BiVO4 nanoplates was dropped onto a piece of fluorine tin oxide (FTO) glass, followed by spinning at high speed.35 To enhance the interfacial contact for better PEC performance, post calcination at high temperature should not L


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Figure 11. Schematic illustration of the particle transfer method for photoelectrode fabrication. Reproduced with permission from ref. 163. Copyright 2013 The Royal Society of Chemistry.

4.1.6. Finger Rubbing. Although the above-mentioned methods are facile to prepare photoelectrodes based on a suspension with facet-oriented semiconductor powders, the particles are generally dispersed on the substrates randomly. Hence, the advantages of crystal facet engineering for improving the photocatalytic activity and selectivity are not well displayed. To prepare a photoelectrode with crystal facet oriented semiconductors arrayed with good order, a finger rubbing method has been developed.164−168 Typically, a polyethylenimine (PEI) solution in ethanol is first coated on a conductive substrate by spin-coating and then the dry powder of a crystal facet oriented semiconductor is rubbed on the PEI-coated substrate by a finger. After calcination, a photoelectrode with uniformly facet oriented crystals is achieved. For example, an orthorhombic phase {020} facet oriented WO3·H2O nanoplate film was prepared by fingerrubbing the hydrothermally synthesized nanoplate powder on a PEI-coated FTO glass substrate.167 By annealing the film at 500 °C to remove the organic layer and to promote the phase transformation, {002}-oriented monoclinic phase WO3 films were achieved. The obtained thin layer can serve as a seed layer for the subsequent growth of facet-oriented crystals, which will be discussed in section 4.2.

dominant {001} facets were initially dispersed in water, which was then vacuum-filtrated over a cellulose membrane to form a homogeneous thin film.162 A photoelectrode was then obtained by transferring the thin film onto an ITO substrate. Although this method is a simple way to fabricate photoelectrodes, the poor contact between the filtrated film and the conductive substrate may be an issue that constrains the PEC performance. Thus, appropriate post treatment is important to promote the charge transfer between the semiconductor particles and the conductive substrate. 4.1.5. Particle Transfer. The contact between the semiconductor powders and substrates is the main issue in ex situ preparation. To overcome this drawback and achieve high PEC performance, a particle transfer method has been developed.163 As shown in Figure 11, the synthesized semiconductor powders are distributed on a glass substrate, and a thin metal layer (approximately 100−300 nm) is sputtered on the top surface to form ohmic contacts with the semiconductor powders. To achieve sufficient conductivity and mechanical strength, a thick metal layer (∼7 μm) is then deposited on the contact layer to serve as a conductor layer. Then, another glass substrate is bonded to the top surface of the conductor layer by epoxy resin, and the bottom glass substrate is peeled off. The obtained film is ultrasonically treated in water to remove the excess particles on the top surface. Thereby, a monoparticle layer that covers the metal conductor film with excellent contact is finally achieved. Although this method has been reported for the fabrication of LaTiO2N photoelectrodes with irregular powders, it is applicable for the preparation of crystal facet engineered photoelectrodes when the powders are exposed with welldefined crystal facets. Moreover, no high-temperature treatment is required for the obtained photoelectrodes, and thus this method can be applied for other thermally unstable semiconductor particles. However, the equipment and metal targets used in the radio frequency (RF) magnetron sputtering may increase the fabrication cost.

4.2. Directly Growing Faceted Photoelectrode Films

The interfacial contact between the conductive substrate and the semiconductor material is essential to obtain high PEC performance. Although crystalline semiconductors with highly reactive facets are synthesized, the PEC performance of the photoelectrodes prepared using an ex situ method is still not high, possibly due to the poor contact between the semiconductor materials and conductive substrate. For interfacial contact, the in situ preparation method by directly growing semiconductor crystals with various facet orientations on a conductive substrate is more promising to achieve efficient PEC water splitting. Moreover, the orientation M


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Figure 12. SEM images of anatase TiO2 films: (a) With oriented {001} facets deposited on FTO substrates. Inset: a schematic of the truncated pyramidal crystal with {001} and {101} facets. Reproduced with permission from ref 190. Copyright 2011 The Royal Society of Chemistry. (b) With {001} facets vertically on FTO substrates. Reproduced with permission from ref 191. Copyright 2011 John Wiley and Sons. (c) With {001} facets vertically on flexible CFs. Reproduced with permission from ref 192. Copyright 2012 John Wiley and Sons. (d) With oriented {001} facets on a gold substrate. Reproduced with permission from ref 194. Copyright 2012 American Chemical Society.

p-type semiconductors with poor electron/hole transport properties. Moreover, such nontransparent photoelectrodes cannot be used as the front photoelectrode in a tandem PEC system illuminated by one light source. For the wet-chemical process, a proper organic or inorganic capping agent must be chosen that can selectively anchor on the target facets to reduce the surface energy. Therefore, the target facets can be exposed when the growth of the crystal terminates. Note that the selective capping agent should not destroy the substrates as well. Different semiconductor materials may require different capping agents. The compatibility of the substrate and the semiconductor is also important to achieve the successful growth of semiconductor crystals on substrates. For example, TiO2 and α-Fe2O3 can be directly grown on FTO substrates by hydrothermal or solvothermal methods with subsequent calcination,180,181 whereas a seed layer is generally needed for the growth of WO3, BiVO4, and ZnO.182−184 To achieve high PEC performance, postthermal treatment to totally remove the adsorbed capping agents is generally required.185,186 In the following subsections, various semiconductor films with different exposed facets prepared by in situ methods will be discussed in detail. 4.2.1. TiO2. It is widely accepted that anatase TiO2 with a bandgap of ∼3.2 eV exhibits higher photocatalytic activities than either rutile or brookite.187,188 The fabrication of anatase

distribution of the faceted crystals can be well controlled by the in situ routes. According to the growth mechanism, in situ preparation can be divided into bottom-up and top-down routes. The bottomup route is defined as directly growing semiconductor crystals from vapor, i.e., vapor−solid growth (e.g., thermal evaporation,175 chemical vapor deposition,176 or laser ablation177) or solution, i.e., a wet-chemical process (e.g., hydrothermal/ solvothermal process178 or electrodeposition179) on a conductive substrate. Even though vapor−solid growth can achieve semiconductor films with high crystallinity, the requirements of high vacuum and high temperature with special equipment generally boost the cost, which is not favorable for the scale-up application of PEC water splitting. Moreover, the conductive glass substrates such as FTO and ITO that are commonly used for PEC water splitting are fragile and cannot tolerate high temperature. In contrast, the wetchemical process is promising for photoelectrode fabrication. A top-down route for fabricating crystal facet oriented semiconductor films is selectively etching and oxidizing the corresponding metal foils to metal oxide semiconductors with tailored facets. Because of the direct formation of facetoriented semiconductor crystals on the metal foils as photoelectrodes, the interfacial contact is generally excellent. However, an opaque metal substrate requires front illumination for PEC water splitting, which is not favorable for n-type/ N


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Figure 13. (a) SEM image of anatase TiO2 truncated tetrahedrons with exposed {001} facets on Ti foils. Reproduced with permission from ref 196. Copyright 2011 The Royal Society of Chemistry. (b) SEM image of flower-like anatase TiO2 microspheres on Ti foils. (c) XRD patterns of (i) Ti foils and (ii) flower-like anatase TiO2 films. Reproduced with permission from ref 197. Copyright 2011 The Royal Society of Chemistry.

solution containing fluorine ions. Likewise, {001}-faceted anatase TiO2 can also be grown on carbon aerogel substrates.193 Anatase films with ∼100% {001} facets were grown on a gold substrate by immersing the substrate in a solution of TiF4 and NaF by a hydrothermal process (Figure 12d).194 The morphologies and facet orientations of the films are similar when using different fluorine salts. All the anatase films exhibit a strong (004) peak in the XRD patterns, resulting from a strong c-axis preferred orientation of the anatase crystals with nearly 100% exposure of the {001} facets. In addition to the nanosheets structures, anatase nanorods with exposed {001} facets can also be prepared by hydrothermal reactions in the presence of sulfate/chloride ions.195 The phase purity and anisotropic crystal growth are sensitive to the pH of the solution and the ratio of the sulfate/chloride ions, as indicated by the investigation of TiO2 films obtained at different H+ concentrations and Cl−/SO42− ratios. Anatase TiO2 films with oriented {001} facets were also prepared by keeping Ti foils in an aqueous solution containing HF during a hydrothermal process.196 The use of HF not only partially dissolves the Ti foil into titanium complexes for the growth of TiO2 crystals but also acts as a capping agent to lower the surface energy of the {001} facets. As a result, the exposed surface of the film is composed of truncated tetrahedrons with major {001} surfaces and minor lateral {101} surfaces (Figure 13a). By modifying the hydrothermal process, flower-like TiO2 microsphere films with exposed {001} facets were directly synthesized on the Ti foil.197 As shown in Figure 13b, the flower-like TiO2 microspheres consist of agglomerated anatase single crystals with a square surface of {001} facets and four isosceles trapezoidal surfaces of {101} facets. The percentage of exposed {001} facets is calculated to be approximately 30%. In addition to the main anatase phase, a small amount of rutile TiO2 is also generated, which is evident

TiO2 films with exposed reactive {001} facets has been actively pursued for solar energy conversion.189 Fluorine ions are an effective capping agent for the growth of anatase TiO2 nanosheets with {001}-exposed facets.26 The successful growth of {001}-faceted TiO2 films on FTO substrates was achieved by hydrothermally treating the substrates in an aqueous solution of TiF4 (Figure 12a).190 The in situ generation of HF through the hydrolysis of TiF4 provides a capping agent to stabilize the {001} surface of the TiO2. The addition of NaCl in the hydrothermal process is important for obtaining homogeneous and transparent films. The films strongly adhere to the FTO substrate and even cannot be detached by ultrasonication or adhesive tape. However, this procedure cannot be applied to silicon, ITO or glass substrates due to their instability in HF solution. By carefully adjusting the hydrothermal reaction parameters, anatase TiO2 tetragonal nanosheets arrays with {001} facets were vertically grown on the FTO substrates (Figure 12b).191 The reaction temperature, deionized water/hydrochloric acid ratio, reaction time, and addition of (NH4)2TiF6 play pivotal roles in the growth of {001}-faceted anatase TiO2 tetragonal nanosheets arrays. For example, TiO2 cannot grow if the temperature is lower than 150 °C or without the presence of hydrochloric acid. In the absence of (NH4)2TiF6, only square TiO2 nanorod arrays can be obtained. In addition to traditional stiff substrates, anatase TiO2 nanosheets with dominant {001} facets were also grown on flexible carbon fibers (CFs) by a facile hydrothermal process in the presence of HF (Figure 12c).192 The percentage of {001} facets in the anatase TiO2 can be tuned between 40% and 92% by simply changing the concentration of fluorine ions in the hydrothermal process. Nevertheless, such a high percentage of {001}-faceted TiO2 cannot be achieved using FTO glass substrates due to the poor stability of FTO in a hydrothermal O


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Figure 14. SEM images of (a) randomly oriented TiO2 nanosheets and (b) aligned TiO2 nanosheets. Reproduced with permission from ref 212. Copyright 2012 American Chemical Society. (c) Schematic of a DCFTMS configuration. (d) Atomic structures of the {001} and {101} facets of an anatase TiO2 crystal. (e) Projection of the {001} and {101} facets. The gray solid and blue hollow spheres in (d) and (e) refer to Ti and O, respectively. Reproduced with permission from ref 207. Copyright 2014 American Chemical Society.

adsorbed onto the (101) surfaces, promoting the growth of the {001} facets. Consequently, the obtained TiO2 nanotubes exhibit approximately 77% exposed facets of {001}, as evidenced by significant enhancement of the relative intensity of the (004) peak from 20% to 100% in the XRD patterns. In addition, a recent report demonstrated that the presence of TiF62− ions in the electrolyte can also effectively tailor the preferential orientation of anodic TiO2 nanotube arrays from {101} to {001}.204 Interestingly, the heating rate of the anodic TiO2 nanotube arrays from room temperature to 450 °C also affects the orientation of the {001} facets.205 Owing to their advantages of low cost and simplicity, the above-mentioned wet-chemical processes have been intensively investigated for the growth of anatase TiO2 films with exposed reactive {001} facets. Vapor processes including chemical vapor deposition (CVD), molecular beam epitaxy (MBE), laser ablation (LA), and direct current facing-target magnetron sputtering (DCFTMS) have also been reported for the epitaxial growth of anatase TiO2 films with a majority of {001} facets on SrTiO3 or LaAlO3 substrates.27,206−209 Anatase TiO2 films with a {001} facet orientation were epitaxial grown on a (001) single crystal SrTiO3 substrate by a CVD method.210 The mechanism for the successful growth is attributed to the similar lattice constants of SrTiO3 (0.3904 nm) and anatase TiO2 (0.378 nm). However, it is challenging to achieve high-quality epitaxial films of anatase TiO2 using SrTiO3 substrates due to the considerable lattice mismatch (∼3.1%). In contrast, LaAlO3 (001) substrates are a better option because of the smaller amount of lattice mismatches (∼0.2%). Anatase TiO2 films with {001} facets were achieved on a LaAlO3 (001) substrate using an MBE process.211 Further investigation shows that the crystal quality of the anatase TiO2 film grown on a LaAlO3 (001) substrate is equivalent to that of

in the XRD patterns (Figure 13c). The {001} facets can be selectively etched by increasing the reaction time due to their higher reactivity and higher surface energy compared to the {101} facets. Anatase TiO2 films composed of nanotube arrays have been the subject of intensive studies on solar energy conversion owing to their unique structure that can potentially reduce charge recombination and promote PEC reactions.198,199 The electrochemical anodization of Ti foils in an electrolyte containing fluorine ions with subsequent calcination is an effective method to obtain anatase TiO2 nanotube arrays.200 Unfortunately, this method generally results in polycrystalline nanotubes exposing the most stable facets of {101}, which is unfavorable for solar energy conversion. The thermal annealing of the anodic TiO2 nanotube arrays in ambient fluorine could achieve a high percentage of highly reactive {001} facets.201 However, the nanotube structure is also destroyed, and only truncated pyramid-like-shaped nanoparticles can be observed. Interestingly, a solid-state phase transformation from TiO2 {101} to {001} facets was obtained by hydrothermally treating the amorphous anodic TiO2 nanotube arrays with HF vapor at a low temperature of 130 °C.202 The new reaction route between the gas-phase HF and solid amorphous anodic TiO2 nanotube arrays is essential for the solid-state transformation process from amorphous to anatase TiO2 while keeping the nanotube structures. The {001} facets become energetically favored when the surface dangling bonds are terminated with F− ions. After annealing the films at 600 °C, the surface F− ions can be totally removed. Encouragingly, single-crystal-like anatase TiO2 nanotube arrays with mainly exposed {001} facets were directly achieved by introducing acetic acid and PVP into the electrolyte during electrochemical anodization.203 The PVP is preferentially P


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Figure 15. (a) HRTEM image of a rutile TiO2 rectangular parallelepiped. Reproduced with permission from ref 217. Copyright 2004 American Chemical Society. (b) Cross-sectional SEM image of rutile TiO2 nanowires. Reproduced with permission from ref 218. Copyright 2008 American Chemical Society. (c) Surface SEM image of the pyramid-shaped rutile TiO2 film. Reproduced with permission from ref 36. Copyright 2012 Springer Nature. (d) Cross-sectional SEM image of rutile TiO2 pillars. Inset: schematics of the cross-sectional view and top-view shapes of a rutile TiO2 crystal. Reproduced with permission from ref 37. Copyright 2012 The Royal Society of Chemistry.

bulk single crystals, which is promising not only for optical devices but also for basic studies on the electric, optical, and photocatalytic properties of TiO2. Interestingly, {001}-faceted anatase TiO2 films can be grown on other substrates without using the epitaxial growth mechanism that requires a suitable lattice match between the desired facets and the surfaces of the substrates. Crystallinedefect-free anatase TiO2 nanosheets with exposed {001} facets were deposited on silicon and silicon-coated substrates via a CVD process with H2 flow in the high-temperature region.212 By adjusting the H2 flow, randomly oriented nanosheets (Figure 14a) and aligned nanosheets (Figure 14b) were synthesized. Keeping the substrate temperature at 450 °C can prevent the transformation of anatase to a rutile phase. Because of the high temperature of the H2 flame (2000−2400 °C), Si vapor is generated from the Si substrates during the CVD process, which acts as a capping agent to stabilize the {001} facets. Similar to other capping agents, the subsequent removal of the terminated Si may be necessary for PEC applications. Moreover, Si doping is likely to take place at a similar high temperature. Recently, anatase TiO2 crystals with dominant {001} facets were prepared on silicon and glass substrates by DCFTMS at room temperature without the use of capping agents (Figure 14c).207 The relative stability of each facet of a crystal is dependent on the competition between thermodynamics and ion impinging. It is well-known that TiO2 films naturally expose {101} facets due to their having the lowest surface free energy. However, the loose atomic arrangement in the {001} facets can avoid the impinging of high-energy ions

during the DCFTMS process (Figure 14d,e). As a result, the amount of radiation damage to the {001} facets is less than that to the {101} facets, leading to the preferentially growth of the {001} facets. Obviously, the extremely high cost of vapor processes is not favorable for scale-up applications. However, the high-quality films obtained are good for obtaining a fundamental understanding of the real PEC activity of the different facets of a semiconductor. In addition to the widely studied {001} facets, anatase TiO2 films with exposed {100} facets are also achievable. TiO2 nanosheet arrays were synthesized by hydrothermally treated Ti foils in NaOH solution.213 Interestingly, the subsequent hydrothermal treatment of the obtained TiO2 nanosheet arrays in the presence of NaCl led to anatase TiO2 nanorods with exposed {100} and {101} facets.214 The ratio of {100}/{101} can be adjusted by changing the concentration of the NaCl solution. The exposed {100} facets are attributed to the slow reaction rate of hydrogen titanate nanosheet arrays with water in the presence of Na+ and the preferential adsorption of Cl− ions on the {100} facets as capping agents. Moreover, FTO substrates coated with a {100}-oriented anatase TiO2 seed layer were prepared by a finger rubbing method, and a subsequent hydrothermal treatment led to the successful growth of {100}-oriented anatase TiO2 films.215 With a smaller bandgap energy of 3.0 eV, rutile TiO2 is another commonly studied phase in solar energy conversion applications.216 Rutile TiO2 films are generally composed of nanorods or nanowires with top {111}/{001} facets and lateral {110} facets because the growth along the {110} facets with a Q


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Figure 16. SEM images of (a) ZnO nanorod and (b) ZnO nanotube arrays. (c) Schematic of the growth of ZnO nanorod arrays on ZnO coated substrates. Reproduced with permission from ref 226. Copyright 2005 American Chemical Society. (d) Schematic of facet oriented ZnO films grown with different facet oriented seed layers. Reproduced with permission from ref 168. Copyright 2015 Elsevier.

titanium precursors and the added surfactants. For example, using titanium isopropoxide or titanium tetrachloride can obtain similar nanorod structures, and the additives also have little effect on the morphology of the TiO2 nanorods. However, adding saturated aqueous NaCl solution to the growth solution can change the density, alignment, and diameter of the nanorods. Pure rutile TiO2 films with 100% exposed pyramid-shaped {111} facets were prepared on FTO substrates via a facile onepot hydrothermal method (Figure 15c).36,220,221 The presence of H2O2 during the hydrothermal process is important for achieving high-quality films. Without H2O2, only films with irregular surface morphologies and imperfect pyramidal crystal facets can be obtained, and the adhesion to the substrate is weak. DFT calculations reveal that the {111} facets of rutile TiO 2 with high surface energy can be stabilized by hydroxylation, which supports the experimental observation that the {111} facets are formed at the solid−solution interfaces.222 Similarly, H2O2 is also important to obtaining high-quality rutile TiO2 films with exposed pyramid-shaped {111} facets on Ti foils prepared by a vapor-phase hydrothermal method.223 Single-crystal rutile TiO2 pillars with exposed high-energy facets of {011} and {111} were fabricated by thermally treating a compact amorphous film derived from an anodic TiO2 film by removing the top nanotube arrays.37 Systematic studies found that the synergistic effects of the lattice matching between rutile and α-Ti, the crystallization temperature and the pressure of the oxygen atmosphere contributed to the final morphology of the films. By carefully controlling the pressure of the oxygen atmosphere at 103 Pa, rutile TiO2 pillars with well-developed facets are achieved (Figure 15d). 4.2.2. ZnO. Zinc oxide (ZnO) is another UV-active n-type semiconductor with a direct bandgap of approximately 3.37 eV and a hexagonal wurtzite crystal structure. The typical electron mobility in ZnO is 10−100 times greater than that of TiO2,

low surface energy is much slower than that on the top {111} or {001} facets with a high surface energy.21 The crystal facet engineering for rutile TiO2 films can be used to control the exposure of high-energy facets such as {001}, {111}, and {011}. Similar to F− for the control of exposed {001} facets in anatase TiO2, Cl− is an efficient capping agent for the growth of rutile TiO2 nanorod or nanowire arrays with lateral {110} facets. For example, submicrometer-scale rectangular parallelepiped rutile TiO2 films with {001} facets were prepared by subjecting borosilicate glass slides in an aqueous TiCl3 solution containing a large amount of NaCl to a hydrothermal process (Figure 15a).217 Both TiCl3 and NaCl are essential to obtain the faceted nanorod structure. The film cannot grow in aqueous TiCl4 solutions under the same experimental conditions, and only TiO2 powders can be observed in the absence of NaCl. The presence of Cl− ions can not only retard the formation of TiO2 by changing the composition or coordination structure of the growing unit but also passivates the {110} facets, accelerating the growth in the [001] direction. Rutile TiO2 nanowires with a {001} orientation were achieved by using toluene as a nonpolar solvent in the presence of hydrochloric acid by a solvothermal reaction (Figure 15b).218 Hydrochloric acid is important to obtaining the anisotropic growth of the nanowire structure. The hydrochloric acid provides a dual function, not only retarding the hydrolysis of the precursor at low temperatures, but also acting as a capping agent to prevent the growth of the crystal plane sidewall of the {110} facets. Upon replacing the hydrochloric acid with the same amount of deionized (DI) water, only TiO2 nanoparticles with no obvious crystal facet orientation are obtained. Single-crystalline rutile TiO2 nanorods with exposed {001} facets were grown on FTO substrates from hydrothermally treated titanium butoxide in a hydrochloric acid solution.219 The morphology and crystal facet orientation of the rutile TiO2 nanorods are insensitive to the R


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Figure 17. TEM images of (a) a WO3 nanowire and (b) a WO3 nanoflake. Insets: the corresponding selected area electron diffraction (SAED) patterns. Reproduced with permission from ref 235. Copyright 2011 American Chemical Society. (c) SEM image of 3D branched (NH4)xWO3 nanowires. Reproduced with permission from ref 237. Copyright 2015 American Chemical Society. (d) SEM image of WO3 nanomultilayers. Reproduced with permission from ref 238. Copyright 2014 Elsevier. XRD patterns of nanoplate-like WO3 films prepared by hydrothermal reactions for (e) 1-step 16 h, (f) 1-step 8 h, and (g) 2-step 16 h. Insets: the schematics of crystal facet orientations of the corresponding films. Reproduced with permission from ref 38. Copyright 2016 Elsevier.

However, the growth rate is much faster than that of diffusion near the zinc foil, and the zinc concentrations are largely decreased near the top area of the 1D ZnO nanostructures. Thus, the growth of nanorods is suppressed. The 1D ZnO nanostructures formed on the Zn foil are dominated by an etching process. Thus, the {002} facets with a higher surface energy are selectively dissolved to enlarge the lateral areas with the most stable low-index nonpolar surfaces. ZnO nanorod arrays with a {002} facets orientation were also prepared by a hydrothermal method.227,228 Interestingly, the morphology of the ZnO films can be tuned from nanorods to nanosheets by changing the concentration of ZnCl2 in the hydrothermal reaction.229 Furthermore, ZnO nanotube arrays can be prepared by selectively etching the polar {001} facets of the ZnO nanorods in a KOH solution. By immersing ITO substrates coated with different facet orientations of nanocrystalline ZnO seeds in a zinc precursor solution, ZnO films with {002}, {100}, and {101} crystal facet exposures were achieved (Figure 16d).168 4.2.3. WO3. Tungsten trioxide (WO3) is an n-type semiconductor with a smaller band gap of 2.6−2.8 eV than those of TiO2 and ZnO, exhibiting better light harvesting.230 The hole mobility (10 cm2 V−1 s−1) and diffusion length (150 nm) of WO3 are much better than those of α-Fe2O3.231 Moreover, WO3 is one of the rare metal oxide semiconductors that is stable in acidic aqueous solutions (pH < 4) under light illumination.232 The surface energy order of WO3 is {002} (1.56 J m−2) > {020} (1.54 J m−2) > {200} (1.43 J m−2),67 indicating that {002} is the most reactive facet. Therefore, the crystal facet engineering for WO3 films is mainly focused on the exposure of the {002} facets. Monoclinic WO3 nanorods with a highly {001} orientation were prepared on FTO substrates with a rubbing seed layer by a hydrothermal process.233 The rubbing seed layer is prepared

which is favorable for electron transport during PEC water splitting.224 Moreover, the unique polar surfaces of ZnO consisting of positively charged Zn-(0001) and negatively charged O-(0001̅) facets exhibit higher surface energy than other nonpolar facets.225 It should be mentioned that the {0001} facets of ZnO are simplified as {001} or {002} facets in some literature. The selective exposure of the polar facets does affect the charge transfer properties and the photocatalytic activity.68 Therefore, the crystal facet engineering of ZnO films with highly exposed polar facets is important to achieving high PEC performance. ZnO films composed of 1D structures along the [0001] direction are commonly used for PEC water splitting due to the large surface area and short migration distance for photogenerated charge carriers. Highly oriented ZnO nanorods and nanotubes were prepared by immersing one {002}oriented ZnO film-coated substrate and two zinc foils in formamide aqueous solution at 65 °C.226 The ZnO nanorod arrays are grown along the c-axis on the seeded substrates, with the preferential exposure of the {002} facets (Figure 16a), whereas the ZnO nanotubes are formed on the Zn foil (Figure 16b). The precoated ZnO film may lower the lattice mismatch between the deposited ZnO crystals and the substrate for the successful growth of the ZnO nanorods. No ZnO nanorods can be found on the bare substrate. As shown in Figure 16c, zinc precursors are continuously supplied from the metal zinc foils to the ZnO seed coated substrates, forming a gradient in the concentrations of the zinc precursors from the Zn foil to the ZnO seed coated substrate. The diffusion rate of the zinc precursors is relatively fast compared to that of the crystal growth in the region around the ZnO seed layer. Because of the higher surface energy of the {002} facets, the ZnO crystals tend to grow along the [001] direction to reduce the total surface energy under a sufficient supply of zinc precursors. S


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Figure 18. (a) TEM image of an orthorhombic WO3·H2O nanoplate. Inset: the corresponding SAED pattern. Reproduced with permission from ref 241. Copyright 2012 The Royal Society of Chemistry. (b) SEM image (inset: cross-sectional view) and (c) TEM image of WO3 films (inset: the SAED pattern). Reproduced with permission from ref 242. Copyright 2016 Elsevier. (d) TEM image of a WO3 nanorod. Insets: the corresponding SAED pattern and HRTEM image. Reproduced with permission from ref 243. Copyright 2013 The Royal Society of Chemistry.

by a finger rubbing method; readers may refer to section 4.1.6 for more information on that method. The advantage of the seed layer prepared by this method is the high crystallinity and highly oriented {002} facets, which enables the subsequent hydrothermal growth of WO3 nanorods along the [001] orientation using citric acid as a capping agent. Porous hexagonal WO3 nanowire array films were grown on WO3 seed coated FTO substrates by a hydrothermal method in the presence of ammonium sulfate.234 An appropriate amount of ammonium sulfate in the hydrothermal process is essential to growing the 1D single crystal nanowires of hex-WO3 along the [0001] direction, as sulfate ions tend to adsorb on the facets parallel to the c-axis of the WO3 nanocrystals. Without ammonium sulfate, only microbricks can be formed. Vertically aligned WO3 nanowire and nanoflake arrays were grown on FTO substrates coated with a seed layer by a solvothermal process.235 Either nanowire or nanoflake arrays can be selectively grown by tuning the ratio of acetonitrile/ water for solvothermal reactions. Interestingly, the hexagonal nanowires grow along the [001] direction, while the monoclinic nanoplates grow along the [020] direction, as evidenced by the HRTEM images (Figure 17a,b). The urea in the solvothermal process acts as both a hydrogen-bond donor through its two NH protons and a hydrogen-bond acceptor through the CO group, which is essential for the growth of WO3 nanoflake films. Interestingly, the further modification of the solvothermal method without the use of HCl led to the formation of WO3 nanorolls elongated along [001] with [100] parallel to the nanoroll surfaces.236 A 3D branched (NH4)xWO3 nanowire array film was prepared by a facile one-step hydrothermal process without any template.237 Interestingly, the HRTEM images reveal that both trunks and branches are grown along the [001] direction.

It is believed that lactic acid might play an important role in promoting the growth of branches in all six directions of the {100} crystal facets of the trunk, forming the unique 3D structure (Figure 17c). After calcination at 550 °C, porous monoclinic WO3 nanorod array films with a {002} facet orientation were achieved. Monoclinic WO3 nanomultilayers with preferentially exposed {002} facets were grown on FTO substrates by a solvothermal process.238 It is proposed that the Cl− ions function as a capping agent that is preferably adsorbed on the {002} facets to lower the surface energy, resulting in the increased exposure of the {002} facets. The differences in the growth rate and capping effect during the anisotropic growth lead to a step-like morphology (Figure 17d). WO 3 nanostructures with highly reactive {002} facets were prepared by a hydrothermal method in the presence of boric acid.239 During the hydrothermal reaction, B(OH)4− ions act as a capping agent to selectively anchor on the {002} facets, inhibiting the crystal growth along this direction. As a result, the WO3 crystals expose more {002} facets. Nanoplate-like WO3 films enriched in {002} facets was grown on FTO substrates by a hydrothermal method.38 Interestingly, the thermodynamically less stable {002} facets are dissolved and recrystallized to form more stable {200} facets upon elongating the hydrothermal reaction time from 8 to 16 h, as evidenced by the XRD patterns (Figure 17e,f). However, a two-step hydrothermal reaction that is performed by repeating the 8 h hydrothermal reaction with a fresh solution can keep the {002} dominant facets (Figure 17g). Vertically aligned hierarchical WO3 nanoarchitecture arrays were grown on FTO substrates by a template-free solvothermal method.240 Interestingly, both the trunk and branches are single crystalline monoclinic WO3 with a preferential growth direction of [001]. Various T


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Figure 19. (a) TEM image of the nanoporous WO3 prepared by a two-step anodization process. (b) XRD patterns of WO3 prepared by (i) onestep and (ii) two-step anodization. Reproduced with permission from ref 245. Copyright 2007 American Chemical Society. (c) SEM image of WO3 nanotrees. (d) TEM image of a WO3 nanotree. Insets: corresponding SAED patterns. Reproduced with permission from ref 246. Copyright 2009 John Wiley and Sons.

be grown. The reason has not been investigated, but it is possibly due to the different surface properties of the various substrates. In the presence of thiourea (SC(NH2)2) and maleic acid (C4H4O4), WO3 nanowire networks were directly grown on FTO substrates without the assistance of a WO3 seed layer via a solvothermal method.244 C4H4O4 is believed to assist in the formation of small WO3 crystalline nuclei adhering on the FTO substrates, while SC(NH2)2 is preferentially adsorbed parallel to the c-axis of the crystal facets of the WO3 nanocrystals, promoting the formation of c-axis oriented nanowires. By adjusting the C4H4O4 and SC(NH2)2 additives in the growth solution, WO3 films with different morphologies can be achieved. The nanostructure is sensitive to the pH of the solution, as evidenced by investigating the morphologies using different concentrations of HCl. Faceted WO3 films can also be prepared on metal tungsten foils by a hydrothermal etching process or electrochemical anodization. Self-assembled nanoporous WO3 with a preferential orientation of {002} facets were prepared by the anodization of a tungsten foil in a fluorine-containing electrolyte.245 The two-step anodization process of treating the tungsten foil at 60 V for 60 min followed by subsequent anodization at 40 V for 30 min is essential to achieving a nanoporous structure with a {002} facet orientation. In the first step of the anodization, the fluoride-induced dissolution rate exceeds the potential-induced growth rate of the oxide layers, forming irregular pores. In the second step of the anodization, the potential-induced growth rate is higher than the fluoride-induced dissolution rate of the oxide layer, until they reach an equilibrium, which will automatically eliminate any irregularities in the nanostructure, resulting in a uniform nanoporous WO3 structure with a {002} facet orientation (Figure 19a). However, only {020}-oriented WO3 film can be

morphologies of radial WO3 nanowires, branched WO3 nanotree arrays, and well-aligned WO3 nanowire arrays can be achieved by changing the amount of HCl in the growth solution. Most WO3 films are grown on substrates coated with nanocrystalline WO3 seed layers. However, numerous grain boundaries in the seed layers may behave as charge recombination centers, which will reduce the PEC performance. Vertically aligned WO3 plate-like arrays were hydrothermally grown on bare FTO substrates directly without the coating of a WO3 seed layer.241 The presence of (NH4)2C2O4 in the solution plays an important role as a capping agent that is preferentially adsorbed on the {010} facets of the orthorhombic WO3·H2O, restricting the growth along the [010] direction. Consequently, vertically aligned plate-like arrays with the same orientation are grown on the FTO substrate. As shown in Figure 18a, the orthorhombic WO3· H2O nanoplate exhibits a large tetragonal surface of the {020} facet. The orthorhombic WO3·H2O phase can be completely converted to crystalline monoclinic WO3 with a preferential orientation of the {002} facets after calcination. Recently, a facile, controllable and scalable method based on the peroxotungstate reduction reaction was developed for the preparation of WO3 nanoplate array films with dominant {002} facets (Figure 18b,c).242 Impressively, uniform WO3· H2O films with a size of up to 20 cm × 20 cm can also be prepared, indicating the feasibility of this method. In addition, aligned WO3 nanorods with a {002} facet orientation were directly grown on FTO substrates by hydrothermally treating the substrates in a peroxopolytungstic acid (PTA) solution without the assistance of a seed layer or structure-directing agent (Figure 18d).243 FTO is essential for the nucleation and growth of WO3 nanorods. Upon replacing FTO with other substrates such as silicon, ITO, or glass substrates, no films can U


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Figure 20. (a) TEM image, (b) HRTEM image, and (c) schematic of the α-Fe2O3 nanobelts. (d) TEM and (e) HRTEM images of the α-Fe2O3 nanowires. Insets in (b,d): the corresponding SAED patterns. Reproduced with permission from ref 248. Copyright 2005 American Chemical Society. (f) Schematic illustration of the α-Fe2O3 nanowires grown via a self-catalyst vapor−solid process. Reproduced with permission from ref 249. Copyright 2008 American Chemical Society.

Figure 21. (a) XRD pattern of (i) standard powder pattern of α-Fe2O3 (black lines) and SnO2 (blue lines), and(ii) undoped and (iii) Si-doped αFe2O3 films. Reproduced with permission from ref 138. Copyright 2006 American Chemical Society. SEM images of (b) SP and (c) USP α-Fe2O3 samples. Reproduced with permission from ref 252. Copyright 2005 American Chemical Society. (d) XRD patterns of α-Fe2O3 films prepared by different modes. (e) Schematics of the α-Fe2O3 crystal lattice with (104) and (110) oriented facets, respectively. Reproduced with permission from ref 257. Copyright 2015 American Chemical Society.

with a thin oxidized layer in an aqueous solution containing oxalic acid (H2C2O4), rubidium sulfate (Rb2SO4), and nitric acid (HNO3).246 The hexagonal WO3 nanotrees composed of “trunks” and “branches” are single crystals, with the long-length

achieved by a one-step anodization at 20 V for 30 min, as evidenced by the XRD patterns (Figure 19b). Interestingly, aligned WO3 nanotree films (Figure 19c) were prepared by hydrothermally treating a metal tungsten plate V


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pyrolysis (SP) that obtains spherical particles with a wide size distribution (Figure 21b), the α-Fe2O3 film obtained by USP features mesoscopic leaflets with a much smaller thickness of 5−10 nm (Figure 21c). Considering the extremely short hole diffusion length (2−4 nm) of α-Fe2O3,254 the small crystal sizes achieved by USP should be effective to reduce charge recombination. The heteroepitaxial growth of (001, c-axis)-, (110, a-axis)-, and (100, m-axis)-oriented Sn-doped α-Fe2O3 films on transparent Nb-doped SnO2 (NTO) substrates were achieved by pulsed laser deposition (PLD).255 Prior to depositing the αFe2O3 films, NTO substrates with different facet orientations were deposited on sapphire c-, a-, m-, and r-plane oriented substrates by PLD, which is essential for the subsequent heteroepitaxial growth of the Sn-doped α-Fe2O3 films. Interestingly, the α-Fe2O3 orientation is (110) for films grown on both a- and r-plane sapphire, whereas the films grown on c- and m-plane sapphire exhibit (001) and (100) orientations, respectively. Similarly, α-Fe2O3 films with exposed (0001) facets can also be grown on SrTiO3 (111) and α-Al2O3 (0001) substrates via PLD.256 By carefully controlling the conditions of the magnetron sputtering process, α-Fe2O3 films highly oriented along the [110] direction were achieved.257 The energy of heavy ions impinging on the surface of the substrate is essential to tailoring the oriented growth of the films. As shown in Figure 21d,e, the highest-energy bombarding particles generated in the high impulse power magnetron sputtering mode (HIPIMS) can achieve [110]oriented α-Fe2O3 films, which cannot be achieved by the significantly lower-energy particles with negligible bombardment produced in other modes such as pulsing sputtering mode (PS) and medium frequency sputtering mode (MFS). Although high crystallinity can be achieved by vapor−solid growth processes, the high fabrication cost is not favorable for PEC applications. Fortunately, faceted α-Fe2O3 films can be prepared by annealing wet-chemical fabricated FeOOH precursor films. For example, FeOOH nanorod arrays were grown on FTO substrates by a low-temperature (100 °C) hydrothermal method.258 After the subsequent calcination, αFe2O3 nanorod arrays with a (110) orientation were achieved. Highly ordered [211]-oriented FeOOH nanowire arrays were prepared by a solvothermal method in an aqueous acetonitrile solution.259 The growth process is successful on various substrates including a seed coated Pt substrate and bare W, Ti, and FTO substrates. Acetonitrile plays a key role in the growth of FeOOH nanowire array films. Only a densely packed film can be obtained in the absence of acetonitrile. Different pH values lead to differences in the acid-catalyzed hydrolysis of acetonitrile under solvothermal conditions, resulting in various lengths of the nanowires. By annealing the obtained films, αFe2O3 nanowires with their [110] direction perpendicular to the substrate can be achieved. In addition to 1D structures, other 2D or even 3D structures can also be achieved by adjusting the hydrothermal conditions. For example, highly oriented Ge-doped α-Fe2O3 nanosheet arrays vertically aligned on FTO substrates were prepared by hydrothermally treating β-FeOOH nanorod arrays in a Ge colloidal solution.260 The Ge-doped α-Fe2O3 nanosheet arrays are highly preferentially grown along the [110] direction with {001} basal facets vertical to the substrates. The ultrathin nanosheets, proper Ge doping, and [110] orientation are beneficial for enhancing the PEC performance. Interestingly, hydrothermally treating a pristine α-Fe2O3 thin film in an

axes oriented toward the [001] direction (Figure 19d). Rb2SO4 is essential for the growth of WO3 nanotrees, as evidenced by the morphologies of the products in the presence of other metal salts. Although the unique WO3 nanotree structure may provide a large surface area for PEC water splitting, the photocatalytic activity of the hexagonal phase is poor, and thus subsequent calcination for the transformation to the monoclinic phase is necessary. However, post calcination may destroy the nanotree structure if the temperature is too high. 4.2.4. α-Fe2O3. Hematite (α-Fe2O3) is highly attractive as a promising photoanode material due to its low cost, nontoxicity, and excellent stability as well as suitable band gap (2.1 eV) and valence band edge positions.247 Faceted α-Fe2O3 films can be synthesized by thermally oxidizing Fe substrates. For example, vertically aligned bicrystalline α-Fe2O3 nanobelt and nanowire arrays were prepared on a large-area surface by directly thermally oxidizing Fe substrates under a flow of O2.248 The nanobelt and nanowire structures can be controlled by adjusting the thermal oxidizing temperature. As shown in Figure 20a−e, both nanobelts and nanowires are grown uniquely along the [110] direction through diffusing Fe atoms or ions to the tips of the nanostructures. Similarly, single crystalline α-Fe2O3 nanoflakes and nanowires were controllably fabricated by heating iron foils in a box oven under ambient conditions.249 Nanoflakes and nanowire structures grown along the [110] direction can be achieved at different temperatures. In a low temperature range of 300−400 °C, the surface diffusion of iron atoms and iron oxide molecules driven by the oxygen-rich and iron-deficient conditions along the [110] direction lead to the nanoflake structure. On the other hand, at a high temperature of 750−800 °C, which is higher than the saturation vapor pressure point of Fe (above 700 °C), a small amount of iron vapor can be produced; this may induce the continuous growth of nanowires within a selfcatalyzed vapor−solid process (Figure 20f). Faceted α-Fe2O3 films with different morphologies can also be prepared by vapor−solid growth processes. For example, pure α-Fe2O3 films with a strong preferential orientation of the [110] axis vertical to the FTO substrate were fabricated via an atmospheric pressure chemical vapor deposition (APCVD) process.138 The much higher conductive {001} facets in the [110] direction vertical to the FTO substrates should facilitate the collection of photogenerated electrons, which is beneficial for PEC water splitting. The presence of tetraethoxysilane (TEOS) during APCVD not only induces the oriented growth of the α-Fe2O3 films but also provides a Si source to achieve Si doping. Without TEOS, the obtained α-Fe2O3 film only exhibits a stronger (104) reflection and smaller (110) and (116) peaks in the XRD patterns (Figure 21a). Likewise, faceted α-Fe2O3 nanoparticles with Ti dopants were grown along the [110] direction on FTO substrates via an APCVD method.250 Needle-like α-Fe2O3 structures were grown on FTO substrates along the [110] direction by an aerosolassisted chemical vapor deposition (AACVD) method.251 The dimensions of the obtained α-Fe2O3 crystallites can be tuned by using an appropriate precursor for AACVD. In addition to CVD methods, other vapor−solid growth processes have also been developed for the growth of faceted α-Fe2O3 films. For example, mesoscopic leaflet-like α-Fe2O3 films with exposed {001} facets were vertically grown on FTO substrates along the [110] direction by ultrasonic spray pyrolysis (USP).252,253 Compared to conventional spray W


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Figure 22. (a) SEM image of pure α-Fe2O3 nanosheet arrays. Inset: corresponding cross-sectional SEM image. (b) XRD pattern of Ag nanoparticles modified α-Fe2O3 nanosheet arrays. Reproduced with permission from ref 263. Copyright 2016 John Wiley and Sons.

Figure 23. (a) SEM image of a BiVO4 nanowall film. Reproduced with permission from ref 268. Copyright 2010 John Wiley and Sons. (b) Schematic and (c) cross-sectional TEM image of a crystal facet engineered BiVO4 film. (d) TEM image of facet engineered BiVO4 crystals. Insets: corresponding schematic, SAED pattern, and HRTEM image. Reproduced with permission from ref 183. Copyright 2015 John Wiley and Sons. SEM images of BiVO4 crystals with {040} facets grown (e) vertical on the FTO substrate and (f) parallel to the FTO substrate. Reproduced with permission from ref 39. Copyright 2017 John Wiley and Sons.

aqueous solution containing iron(III) chloride (FeCl3·6H2O) and L-arginine led to flower-shaped superstructures composed of nanorods with a highly [110] orientation.261 The α-Fe2O3 nanoparticles of the pristine film function as seeds for the formation of the nanorods, while the functional groups of −NH2 and −COOH in L-arginine act as structure-directing agents for the growth of flower-like structures with a good orientation. The ion source of FeCl3·6H2O in the growth solution is also important for the formation of the flower-like structure. The absence of any of these three components (pristine α-Fe2O3 thin film, L-arginine, and FeCl3·6H2O) results in other morphologies. Electrochemical anodization is another wet-chemical process for the fabrication of facet-oriented α-Fe2O3 films. Smooth α-

Fe2O3 nanotubes were synthesized on a Fe foil by an electrochemical anodization method in an electrolyte containing fluorine ions with the assistance of ultrasonication.262 The mechanism for the formation of α-Fe2O3 nanotubes is similar to that of anodic TiO2 nanotubes, as detailed in section 4.2.1. Ultrasonic waves facilitate the homogeneous chemical dissolution, resulting in the formation of uniform nanotubes on the Fe foil. After annealing under a hydrogen atmosphere, pure α-Fe2O3 nanotubes with a strong preferential orientation of the [110] axis vertical to the substrate is achieved. If the film is annealed in an oxygen atmosphere, other impurities such as Fe3O4 can be observed. Likewise, pure α-Fe2O3 nanosheet arrays were grown on iron foils by a modified electrochemical anodization method followed by subsequent calcination in a X


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mixed nitrogen and hydrogen atmosphere (Figure 22a).263 The films are grown along the [110] direction, as evidenced by the strong (110) peak in the XRD patterns (Figure 22b). 4.2.5. BiVO4. BiVO4 is one of the most attractive photoanode materials for PEC water splitting owing to its intrinsic advantages such as a small band gap (∼2.4 eV), favorable band edge positions, low onset potential, and good aqueous stability under light illumination.264 In addition, the carrier lifetime and hole diffusion length of BiVO4 are four and one orders of magnitude superior to those of α-Fe2O3, respectively.265,266 The interesting hole−electron separation on the oxidation functional facets of {110} and reduction functional facets of {040} has attracted increasing attention39,101−103,183,267 and may open up new opportunities for the design of efficient BiVO4 photoanodes for PEC water splitting. C-oriented and {010} facets exposed BiVO4 nanowall films were prepared by treating ITO substrates in an aqueous solution containing trifluoroacetic acid (CF3COOH) by a template-free hydrothermal method (Figure 23a).268 The lattice mismatch between the {020} of BiVO4 and the {040} of ITO is only 0.59%, while the lattice mismatch between the {200} of BiVO4 and the {004} of ITO is 2.6%. Thus, the BiVO4 films can grow along the c-orientation with the exposure of highly active {010} facets based on an epitaxial growth mechanism. CF3COOH plays an important role in the growth of the nanowall structures. The morphology of the BiVO4 film is sensitive to the concentration of CF3COOH. Only at the pH value of 3−4, produced by adjusting the concentration of CF3COOH in the growth solution, can the normal nanowall structure be obtained. Moreover, the CF3COO− species serve as strong electron donors to adsorb Bi3+ ions, which is essential to changing the migration rate of the freely mobile Bi3+ ions for the growth of the nanowall structure. Upon replacing CF3COOH with other types of acids such as HNO3, CH3COOH, or citric acid at the same pH value, the nanowall structure cannot be achieved. By using TiCl3 as a structure-directing agent, BiVO4 films with exposed {040} facets were hydrothermally grown on FTO substrates with the assistance of a crystalline BiVO4 seed layer.183 The film is grown vertically along the [121] direction, corresponding to the c-axis from the BiVO4 seed layer, leading to the preferential exposure of the {040} facets (Figure 23b− d). The percentage of the {040} facets can be tuned by changing the concentration of the TiCl3 in the growth solution. Such similarly faceted BiVO4 films can also be achieved by adding EDTA to the precursor solution, followed by hydrothermal treatment.269 Moreover, BiVO4 crystals with exposed {010} and {110} facets were grown on FTO substrates coated with a BiVO4 seed layer by a hydrothermal process without any structuredirecting agent.270 The pH value of the growth solution is important to obtaining the truncated pyramidal crystal structures.105 Similarly, crystalline BiVO4 nanoplates with well-defined crystal facets were grown vertically on FTO substrates via a seed-assisted hydrothermal process.39 As shown in Figure 23e, the BiVO4 crystal mainly has exposed oxidation functional facets of {110} and {121}, along with reduction functional facets of {040}, grown vertically on the FTO substrate, as evidenced by the selective photoassisted electrodeposition of cobalt borate (Co−Bi) on the {110} and {121} facets. Interestingly, introducing PVP into the hydrothermal process results in the {040} facets being grown parallel

to the FTO substrates (Figure 23f). The underlying mechanism still needs further investigation. PVP may function as a capping agent to control the growth direction of the BiVO4 crystals. Encouragingly, BiVO4 films with exposed {040} facets can also be prepared by spin-coating the precursor solution in the presence of NaCl, followed by a heat treatment process.271 In addition to wet-chemical methods, faceted BiVO4 films can also be prepared by vapor−solid growth methods based on an epitaxial growth mechanism. The selection of substrates with suitable lattice mismatches compared to the BiVO4 lattices is essential to controlling the preferred growth direction. For example, preferentially [001]-oriented BiVO4 films with exposed (001) facets were deposited on FTO substrates via a laser ablation method.272 The lattice mismatches for [010] BiVO 4//[010]FTO and [100]BiVO 4// [−101]FTO can meet the conventional lattice-matching epitaxy criterion (mismatch BiOBr (∼2.7 eV) > BiOI (∼1.7 eV).274 Thus, it is possible to adjust the bandgap of BiOX in a large range by changing the content of “X”. Moreover, the CB position of BiOX can be tuned by changing the content of bismuth because the CB of bismuth-based semiconductors is mainly composed of Bi 6p orbits.276 Compared to TiO2, the development of BiOX for PEC water splitting is only at an early stage. Even though BiOX crystals with exposed {001}, {010}, {102}, and {111} facets have been reported,75,79,81,277−283 crystal facet engineered BiOX photoelectrodes are still rare. BiOX films with selective facet orientation can be achieved from Bi or Bi2O3 precursor films. For example, BiOCl films with flower-like hierarchical structures composed of ultrathin nanosheets grown along the [110] direction were synthesized by immerging Bi spherical films in an aqueous solution containing H2O2 and HCl at room temperature.284 The Bi spherical films on silicon substrates were fabricated by RF magnetron sputtering. The flower-like hierarchical structure is formed through a nucleation− dissolution−recrystallization process rather than a selfassembly process. Likewise, by immerging a Bi2O3-coated Ti foil in an acidic NaCl solution, BiOCl nanosheets arrays grown along the {110} direction with the {00l} facets exposed were achieved (Figure 24a,b).285 BiOCl thin films with highly Y


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Figure 24. SEM images of (a) a Bi2O3 film, (b) a BiOCl film, (c) a BiOBr film (inset: cross-sectional view), and (d) a BiOI film. Reproduced with permission from refs 285, 288, 289. Copyrights 2013 Elsevier and 2016 The Royal Society of Chemistry.

4.2.7. Cu2O. Cuprous oxide (Cu2O) is one of the most intensively studied p-type semiconductors as a photocathode for PEC water splitting. With a direct bandgap of 1.9−2.2 eV and a CB position of ca. −0.7 V vs NHE, Cu2O can harvest sufficient light for water reduction.291 However, Cu2O suffers photocorrosion (e.g., Cu2O + 2H+ + 2e− → 2Cu + H2O) during PEC water splitting,292,293 which is the main limitation for practical applications. It has been reported that the selective exposure of facets in the Cu2O photoelectrodes does affect the PEC activity and the photostability.294−298 Electrodeposition is an effective method to prepare facetoriented Cu2O films. The {111} facets with a surface interstitial Cu atom are calculated to be the most stable lowindex surfaces for Cu2O, and thus {111} facets should be the main exposed facets for Cu2O in a natural growth process.299 However, Cu2O films with a favorable exposure of {111} or {100} facets can be achieved by adjusting the conditions such as the deposition rates, temperatures, pH values, and additives during the electrodeposition.134,294,300−302 For example, Cu2O films electrodeposited at a pH of 8 preferably contained {200} facets exposed, while adjusting the pH values to 10, 12, or 14 led to the exposure of {111} facets (Figure 25a,b).303 Similarly, Cu2O photoelectrodes consisting of crystals with the [111] direction vertical to the substrate were electrodeposited at a pH of 1.25, whereas the vertical growth in the [100] direction was achieved at a pH of 9.5.295 Interestingly, the concentration of PVP in the electrolyte is essential to directing the morphological evolution of Cu2O with different preferentially exposed facets.302 By adjusting the concentration of PVP, Cu2O films with morphologies of cubes, cuboctahedra, truncated octahedra, and octahedra can be achieved (Figure

exposed {110} facets can also be synthesized by an electrochemical method involving a cathodic electrodeposition of Bi films and an anodic oxidation of the Bi films in the presence of NaCl at room temperature.286 The anode oxidation voltage of 2.0 V is essential to obtaining a pure BiOCl thin film with prominent {110} facets; lower oxidation voltages cannot achieve the complete conversion of Bi to BiOCl. In addition, BiOCl nanosheets were grown vertically on FTO substrates along the [110] direction by a solvothermal method.287 The solvothermal temperature and reaction time are important parameters for controlling the nucleation and growth of the BiOCl nanosheets. Higher temperatures can facilitate the nucleation of BiOCl on the FTO substrates, while longer reaction times lead to the sufficient growth of BiOCl nanosheets distributed on FTO substrates. As shown in Figure 24c, BiOBr thin films with highly exposed {110} facets were prepared by treating FTO substrates in ethylene glycol (EG) containing Bi(NO3)3 and CTAB at 130−180 °C.288 The thickness and morphology of the BiOBr films are sensitive to the reaction temperature. A high reaction temperature is favorable for the growth of BiOBr films with highly exposed {110} facets. Similarly, BiOI nanosheets with exposed {001} facets were directly grown on FTO substrates through a onestep solvothermal process (Figure 24d).289 In addition, crossed flake-like BiOI arrays were fabricated on TiO2-coated FTO substrates at room temperature by a successive ionic layer adsorption and reaction (SILAR) method.290 A proper concentration (5 mM) of the Bi(NO3)3 solution is important for the SILAR process; a higher concentration leads to a turbid solution, which cannot be used for SILAR. Z


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Figure 25. SEM images of Cu2O films electrodeposited (a) at pH = 8, (b) at pH = 14 (insets: schematics of facets), and (c) in the presence of different concentrations of PVP (insets: high magnification views). (d) Schematic of in situ hydrothermal treatment preparing Cu2O films with different facet orientation using different anions. Reproduced with permission from refs 302, 303, 306. Copyrights 2015 Elsevier, 2017 Elsevier, and 2014 American Chemical Society.

illumination, both {100}- and {111}-faceted Cu2O films are unstable in aqueous solutions.294,304,308 Thus, other strategies such as surface protection to prevent photocorrosion should be considered for faceted Cu2O films used as photoelectrodes for PEC water splitting.291,309,310 4.2.8. Other Faceted Semiconductor Films. In addition to the intensively studied photoelectrodes discussed above, other faceted semiconductor films have also been developed through in situ preparation, which may open up new opportunities for efficient PEC water splitting. Bismuth molybdate (Bi2MoO6) is an n-type semiconductor with a bandgap of 2.6 eV and proper band edge positions that can be potentially used as a photoanode material for PEC water splitting.311,312 As shown in Figure 26A−C, Bi2MoO6 nanosheet arrays with exposed {010} facets were grown on FTO substrates in the presence of polyethylene glycol 600 (PEG-600) via a solvothermal process.313 The packing density of the nanosheets can be tailored by changing the precursor concentration in the growth solution. Likewise, {010}-faceted Bi2MoO6 films with a nanowall structure were directly grown on ITO substrates via a solvothermal method without any capping agent.314 Indium oxide (In2O3) is another visible light responsive semiconductor with a bandgap of 2.8 eV. 315 In 2 O 3 nanostructures with different facet orientations were fabricated

25c). Moreover, the particle size and distribution density can be controlled by tuning the electrodeposition time. Faceted Cu2O films can also be synthesized by annealing Cu(OH) 2 nanowire films at 500 °C under an Ar atmosphere.304 The crystal size and ratio of the preferentially exposed {111} facets can be tuned by changing the annealing time. Likewise, Cu2O nanowire arrays with a {111} orientation were prepared by annealing electrodeposited Cu(OH) 2 nanowire arrays on a Cu mesh under a N2 atmosphere.305 Moreover, Cu2O films with different exposed facets and morphologies were prepared by hydrothermally treating Cu foils in a Cu2+ solution.306 The exposed facets and morphologies of the Cu2O crystals are sensitive to the anions of the Cu2+ salts in the precursor solution. In particular, Cl−, NO3−, and SO42− leads to {111}-oriented rod arrays, {111}exposed cross-linked octahedrons, and {111}/{100}-exposed truncated octahedrons, respectively (Figure 25d). In addition, Cu2O films were produced from Cu films in the presence of concentrated NaOH solutions.307 By changing the alkaline treating conditions, Cu2O films with preferentially exposed {111} or {100} facets are obtained. In terms of PEC water splitting, Cu2O films with exposed {100} facets can usually produce higher photocurrent densities but exhibit poorer photostability than their counterparts with exposed {111} facets.294,296 However, under long-term light AA


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Figure 26. (A) TEM image, (B) HRTEM image, and (C) SEM image of the Bi2MoO6 nanosheet arrays. Reproduced with permission from ref 313. Copyright 2017 Elsevier. (D) Morphological evolution of In2O3 crystals with different exposed ratios of the {001} facets. Scale bar: (a) 50, (b) 300, (c) 500 nm, and (d−j) 1 μm. Reproduced with permission from ref 317. Copyright 2014 American Chemical Society.

Other in situ grown faceted semiconductor films such as MoO3 films with a {110} facet orientation,321 Ag3PO4 films with cubic (exposed {100} facets) and rhombic dodecahedral (exposed {110} facets) structures,322 Co3O4 films with {121̅} facets,323 GaP films with exposed (114)A facets,324 ZnWO4 films with exposed (011) facets,325 Cd1−xZnxSe films with a preferential orientation along the (002) direction, 326 (GaN)1−x(ZnO)x solid solution nanorod arrays with tunable {101̅0}, {101̅1}, and {0001̅} facets,327 and InN nanopyramid arrays with well-defined exposed facets of (0001), (112̅2̅), (12̅12), and (2̅112) on ZnO substrate328 have also been reported. Although the above faceted semiconductor films have been reported, the facet-dependent properties, in particular the PEC water splitting performances, have not been fully explored. More works are still necessary to be done with these faceted films. In addition, in situ methods for the fabrication of nonmetal-based faceted films (e.g., g-C3N4) are still lacking. Thus, the mechanism of growing nonmetal-based faceted films should be well understood, providing fundamental knowledge for the preparation of these faceted films. On the other hand, the crystal facet engineering of organic metal halide perovskite (OMHP) films has also been reported in recent years.329−331 However, the application of faceted OMHP films for PEC water splitting is a great challenge due to the extreme instability of OMHP in water. The encapsulation of OMHP films to avoid the direct contact of OMHP with water is generally required for PEC water splitting applications.332−338

on Si substrates by a CVD method.316 In2O3 films composed of {100}-truncated octahedrons or octahedrons can be achieved by keeping the reaction temperature at 950 or 1000 °C. By adjusting the reaction time, regular cubic In2O3 crystals with 100% exposed {001} facets were grown on Si substrates via a CVD method (Figure 26D(a−f)).317 Interestingly, the exposure ratio of the {001} facets of the In2O3 films can be tuned from 100% to 0% by reducing the reaction temperature from 1000 to 920 °C (Figure 26D(f−j)). In addition, cylindrical or square (surrounded by {001} facets) In2O3 nanowire arrays grown along the [100] direction were prepared by changing the pressure in the growth chamber during the CVD process.318 Although highly crystalline In2O3 films with selectively exposed facets can be prepared, the high cost of indium as well as the CVD method may be an issue for cost-effective PEC water splitting. Nickel oxide (NiO) is a p-type semiconductor with a wide band gap of 3.6−4.0 eV that can be used as a photocathode material for PEC water splitting.164 Owing to the high p-type concentration and high hole mobility, NiO is also commonly used as an oxygen evolution cocatalyst (OEC).319 Singlecrystalline NiO films with {110} and {111} orientations were epitaxially grown on LaAlO3 (110) and LaAlO3 (111) substrates by a pulsed laser deposition process.320 However, only polycrystalline NiO films can be achieved on LaAlO3 (100) substrates, possibly due to the lattice mismatch between the grown NiO film and the LaAlO3 (100) substrate. AB


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5. ADVANTAGES OF CRYSTAL FACET ENGINEERED PHOTOELECTRODES Generally, photoelectrodes with more reactive facets exposed exhibit better PEC performance than their counterparts without reactive facets. For example, photoanodes composed of anatase TiO2 microspheres with exposed {001} reactive facets demonstrated a 1.5 times higher photocatalytic activity than anatase TiO2 microspheres with {101} facets for PEC water splitting under UV light illumination.339 Likewise, highly {002}-oriented WO3 photoanodes exhibited an over twice as high photocurrent density than their counterparts with randomly oriented facets.167 A similar enhancement was observed by comparing the photocurrent densities of photoelectrodes composed of Nb2O5 rectangular nanosheet crystals with dominantly exposed {010} facets and Nb2O5 spheres without a preferential facet orientation.340 In addition, α-Fe2O3 photoanodes with high-index {116} facets and {104} facets exposed also exhibit higher photocurrent densities than their counterparts without the exposed facets controlled.341,342 ZnO photoanodes with exposed polar {002} facets demonstrate the highest photocurrent densities compared to their counterparts with exposed {100} and {101} facets, respectively, which is attributed to the Zn-rich surface, low bandgap energy, and high surface energy.168 Recently, a Cu2O photocathode composed of concave octahedrons bound by 24 {344} high-index facets was prepared via a cyclic scanning electrodeposition process, which demonstrates an enhanced catalytic activity and robust durability during PEC water splitting.343 Likewise, Cu2S photocathodes composed of 14-facet polyhedra chalcocite Cu2S nanocrystals bounded by {110} facets exhibit much higher photocurrent densities than the Cu2S hexagonal plates with only {001} facets.344 Interestingly, GaN photoanodes with Ga-{0001} facets exhibit a much more negative onset potential compared to their counterparts with N-{0001̅} facets, while the N-{0001̅} facets show higher photocurrent densities at more positive applied potentials.345 The crystal facet engineering of graphitic carbon nitride (g-C3N4) films346 and Bi2Fe4O9 films347 also result in enhanced PEC performance. Consider that the PEC water splitting performance of a photoelectrode is determined by the efficiencies of light harvesting (ηabs), charge separation (ηsep), and surface charge injection (ηinj). Assuming that 100% of the light with energy larger than the bandgap of the photoelectrode can be absorbed and totally converted to photocurrent (Jmax), the practical PEC water splitting photocurrent density (JPEC) can be expressed as348 JPEC = Jmax × ηobs × ηsep × ηinj

The light absorbance (A) measured by UV−vis spectroscopy is generally used to characterize the light harvesting of a semiconductor. The obtained light absorbance curves can be converted to the light absorption efficiency (ηabs) to characterize the light-harvesting ability of a photoelectrode:349 ηabs = (1 − 10−A) × 100%

Assuming a 100% absorbed photon-to-current conversion efficiency (APCE), the ηabs values will be equal to the incident photon-to-current conversion efficiency (IPCE) values of a photoelectrode.349 Thus, the unity converted photocurrent density (Jabs) can be calculated by integrating ηabs over the standard solar spectrum:247 Jabs =

(1)

5.1. Light Harvesting

The light harvesting of photoelectrode materials is the first step for PEC water splitting. The theoretical maximum light absorption edge (λmax) of a photoelectrode material is determined by its bandgap (Eg):12 1240 Eg

λmax

∫300

λ × ηabs(λ) × E(λ) 1240

d(λ)

(4)

where λmax (nm) is the maximum light absorption edge of a photoelectrode, λ (nm) is the light wavelength, and E(λ) is the power density (mW cm−2) at a specific wavelength (λ) of the standard solar spectrum. Equation 4 is very useful for estimating the actual photocurrent density maximum of a photoelectrode based on its measured light absorbance curve. Under light illumination, only photons with an energy larger than the bandgap of the photoelectrode material will be absorbed to generate electron−hole pairs for the subsequent redox reactions. Thus, the bandgap of a photoelectrode material determines the theoretical maximum solar energy conversion. As discussed in section 3.1, a semiconductor material exhibits facet-dependent surface bandgaps. Therefore, the crystal facet engineering of the photoelectrodes can tune the light harvesting by changing λmax (eq 2). The surface bandgap difference of most crystal facet engineered photoelectrodes is only 0.01−0.1 eV.109,164,245,273,304 For example, the bandgap of a TiO2/carbon aerogel electrode can be tuned from 3.22 to 3.19 eV by changing the {001} facet content from 15% to 90%.350 On the other hand, a larger bandgap difference in some photoelectrode materials has also been observed. Depending on the crystal facet exposure, the bandgap of αFe2O3 films can be tailored between 1.9 and 2.3 eV.351 Interestingly, a two-dimensional pure TiO2 phase with a quasihexagonal structure similar to the cubic (111) plane was prepared on the surface of a rutile TiO2 (011) substrate by the oxidation of bulk titanium interstitials, exhibiting a narrow bandgap of only ∼2.1 eV.352 Compared to conventional rutile TiO2 with a bandgap of 3.0 eV, the λmax of the TiO2 film is significantly expanded from 0.1 M), the thickness of the Gouy−Chapman layer is negligible and indistinguishable from the Helmholtz layer.366 The Helmholtz layer is determined by the adsorption/desorption of ions and the SCR in the semiconductor to make the net rate of electron transfer equal to zero between the semiconductor and the redox couple in the electrolyte, which significantly affects the band bending in the semiconductor when electronic equilibrium is achieved.364 Owing to the high charge density and small width of the Helmholtz layer (only 0.1%−1% the thickness of the SCR), the Helmholtz layer is insensitive to electron transfer. When an external potential is applied to the photoelectrode, almost all of the applied potential appears

ηsep =

JSO2− 3

Jabs

× 100% (9)

As discussed in section 3, semiconductors possess unique facet-dependent properties. In particular, the anisotropic builtin electric fields on different facets of a semiconductor can AE


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provide an intrinsic driving force for charge separation and transfer in a photoelectrode. As shown in Figure 27B(a), a spherical photocatalyst without a facet orientation generates symmetric built-in electric fields, resulting in little net driving force for charge separation and transfer. By crystal facet engineering, an obvious net driving force can be generated due to the differences between the built-in electric fields on various facets (Figure 27B(b)), as evidenced by SPSPS observations on a BiVO4 single crystal.115 In addition, the selective deposition of cocatalysts on a faceted semiconductor crystal can further improve the net driving force (Figure 27B(c)).116 On the basis of the above discussions, it is possible to form the anisotropic built-in electric fields to induce an additional driving force on the photoelectrodes for charge separation and transfer through crystal facet engineering, and thus the required external bias for PEC water splitting may be reduced. On the other hand, other facet-dependent properties such as surface electronic structures, molecular adsorption, and electrical conductivity will also affect the light absorption coefficient, SCR width, band bending in SCR, carrier mobility, and Helmholtz layer potential drop (eqs 5−8), leading to different charge separation and transfer properties. For example, the dielectric constants of the epitaxial BiVO4 (010) film and the epitaxial BiVO4 (001) film are 85 and 55, respectively.273 Assuming that their Nd and VBB values are the same, the SCR width of the epitaxial BiVO4 (010) film is ∼1.24 times that of the epitaxial BiVO4 (001) film (eq 5). Moreover, the hole diffusion length of the epitaxial BiVO4 (010) film is ∼5 times that of the epitaxial BiVO4 (001) film.273 As a result, the epitaxial BiVO4 (010) film exhibits much better charge separation and transfer performance compared to the epitaxial BiVO4 (001) film. A similar facetdependent hole diffusion length was reported in WO3 (74.8 nm on {110} vs 53.4 nm on {001}).369 In addition, Ta3N5 photoanodes with exposed {001} facets exhibit excellent PEC performance, which is attributable to the inherently high surface band bending on the {001} facets and the small effective mass of holes along the [001] axis; this leads to spatial electron and hole separation on the {010} and {001} facets, respectively (Figure 28a).370,371 A transient photocurrent decay scan revealed that the NiGa2O4 octahedron photoelectrode with exposed {111} facets exhibited a transient decay time twice as high as that of the NiGa2O4 nanorod photoelectrode with a [100] growth direction, indicating the facet-dependent charge-transfer lifetime in NiGa2O4.372 Interestingly, facet-dependent chemisorption also affects band bending. Taking ZnO as an example,373 the negatively charged O-(0001̅) facets tend to adsorb H+ in the solution, leading to a downward band bending, which increases the electron diffusion toward the O-(0001̅ ) facets. On the positively charged Zn-(0001) facets, however, OH− species are adsorbed, resulting in an upward band bending, which enhances hole diffusion to the Zn-(0001) facets. Because ZnO is generally used as a photoanode material for OER, efficient charge separation is expected by growing the ZnO crystals with O-(0001̅) facets on the conductive substrates while exposing the Zn-(0001) facets to the electrolytes. Because the ion adsorption on the semiconductor surface affects VH in the Helmholtz layer,374 facet-dependent chemisorption is likely to change the band bending by tuning VH, which has been confirmed by investigating the facet-dependent Helmholtz potentials on a germanium semiconductor.375

In addition, the facet-dependent electrical conductivity significantly affects the charge transport properties, facilitating charge separation and transfer. It is widely accepted that {001} is the most reactive facet of anatase TiO2. Surprisingly, the photocurrent densities of the {101}-faceted TiO2 photoanode were found to be higher than those of the {100}- and {001}faceted photoanodes.152 Further investigation reveals that the electrical conductivity, interfacial resistance, and diffusivity of charge along the c-axis in the {101} photoanode are better than those of the others, resulting in a higher PEC performance. A rutile TiO2 film with exposed {111} facets exhibited almost an identical VB position to the anatase TiO2 nanotube film, suggesting a similar oxidation capability.223 However, the rutile TiO2 film demonstrates more than 3.5 times higher photocatalytic activity for PEC water splitting than the anatase TiO2 nanotube film, which is attributed to the lower intrinsic resistance to photoelectron transport in the {111}-faceted rutile TiO2 film. Likewise, {110}-oriented α-Fe2O3 photoanodes exhibit a photocurrent density over 30 times higher than their counterparts with a {104} orientation (Figure 28b), because the electrical conductivity along the {110} facet is 4 orders of magnitude higher.257 For ferroelectric materials, the facet-dependent dipolar field can induce band bending, promoting the charge separation performance.376 As shown in Figure 28c, the spontaneous polarization in the ferroelectric crystal with directions pointing from the bulk to the surface will generate positive charges on the surface (C+ domain), while the polarization pointing away from the surface to the bulk will produce negative charges (C− domain), forming an internal depolarization field.377 In this case, electrons in the ferroelectric crystal will be driven to accumulate at the surface of the C+ domain, leading to downward band bending in the C+ domain and upward band bending in the C− domain (Figure 28d). Therefore, photogenerated electrons and holes can be separated on the C+ domain and C− domain facets, respectively. Interestingly, other nonferroelectric semiconductors such as ZnO, Bi3O4Cl, Cu2O, and Cu2Se also possess polar facets that can induce internal electric fields to promote charge separation. For example, Bi3O4Cl single-crystalline nanosheets with more exposed {001} facets can induce the generation of stronger internal electric fields, which facilitates the photogenerated charge separation, resulting in enhanced photocurrent densities.156 Similarly, the {001} facets of BiOCl also show superior charge separation compared to the {010} facets, due to the strong internal electric fields along the [001] direction.378 A Cu2O film with polar {100} exposed facets shows much higher photocurrent densities under visible light illumination than its counterpart with nonpolar {111} facets, which is attributed to the higher flat-band potential and carrier concentration in the {100} faceted Cu2O film.297 However, owing to the increased reactivity of the additional O2−terminations in the polar facets, the {100}-faceted Cu2O film suffers from severe photocorrosion, whereas the nonpolar {111}-faceted Cu2O film with only Cu+-termination is stable. A similar charge separation between the {111} and {1̅1̅1̅} polar facets of Cu2Se was also observed.379 Interestingly, the polar facets of different contacted semiconductors can also promote charge separation. For example, an internal electric field generated between Cu−Cu2Se (111) and O−Cu2O (1̅1̅1̅) polar facets was believed to drive the separation of photogenerated electrons and holes in Cu2O/Cu2Se multilayer heterostructure nanowires with exposed {111} facets.380 AF


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Figure 29. (a) Photocurrent density vs applied potential and (b) photocurrent density vs time curves of the 1-step 16 h ({200} facet enriched film), 1-step 8 h ({002} facet enriched film), and 2-step 16 h (modified {002} facet enriched film) WO3 photoanodes. Reproduced with permission from ref 38. Copyright 2016 Elsevier. (c) Photocurrent densities of the obtained In2O3 films with different exposed ratios of the {001} facets measured at 0.22 V vs Ag/AgCl. (d) Photocurrent density vs time (I−t) curves of the In2O3 films with 100% (cube) and 0% (octahedron) exposed {001} facets. Two sloping curves show the corresponding O2 gas evolution of the In2O3 films with 100% (cube) exposed {001} facets. Reproduced with permission from ref 317. Copyright 2014 American Chemical Society.

Typically, the surface reaction process involves reactant adsorption at the active sites of the photoelectrode surfaces, charge exchange between the reactant species and photoelectrode surfaces, and products desorption from the photoelectrode surfaces. In PEC water splitting, the surface charge injection efficiency (ηinj) is generally used to characterize the surface reaction properties of a photoanode, reflecting the yield of holes that are actually involved in the water oxidation reaction after reaching the electrode/electrolyte interfaces. As discussed in section 5.2, JSO2− originates from the total holes 3 separated in the photoanode that reach the photoelectrode surfaces. Therefore, ηinj can be expressed as368

In addition, the formation of surface heterojunctions due to the facet-dependent electronic structures can also promote the separation and transfer of photogenerated electron−hole pairs on different facets, as discussed in section 3.3. For example, a nanoflower-like WO3 single-crystal thin film photoanode with edge dislocations induced the mismatch of the {002} and {020} facets, leading to enhanced photocurrent densities.381 DOS calculation revealed that the photogenerated electrons and holes can be separated and transferred by the dislocation interface due to the energy difference between the {002} and {020} facets. 5.3. Surface Reactions

After the charge separation and transfer process, the electrons are collected on the surface of the (photo)cathode and injected into the electrolyte for the HER, while the holes are collected on the surface of the (photo)anode and injected into the electrolyte for the OER. This is the last step for PEC water splitting. It should be pointed out that both thermodynamics and kinetics should be taken into consideration to describe the surface reactions. For example, even though many photoanode materials are thermodynamically favorable for OER if only considering their VB positions, low photocurrent densities are generally obtained due to the sluggish surface reaction kinetics that cause severe charge recombination before the OER is complete, which is the bottleneck for PEC water splitting.382 Generally, the deposition of OECs on the photoanode surface can function as both a sink for holes from the photoanode and catalytic sites for the OER, resulting in enhanced OER performance.

ηinj =

JH O 2

JSO2− 3

× 100% (10)

It is clear that the photoelectrode surfaces are very important for the surface reactions. As discussed in section 3, the unique facet-dependent properties including the redox reaction sites and molecular adsorption of a semiconductor will lead to facetdependent surface reactions. Therefore, the crystal facet engineering of the photoelectrodes can also tune the surface reactions. For example, DFT calculations reveal that the free energy change of removal of the first proton from water on the Ag3PO4 (111) facet is much lower than that on the (100) and (110) facets, leading to the experimentally observed higher catalytic activity of the (111) facet.383 By tailoring the {040} facets of BiVO4 nanoplate films with {040} facets vertically grown on FTO substrates, an optimized photocurrent density was achieved.183 Detailed characterizations reveal that the {040} facets provide a more favorable local bonding geometry for water molecule adsorption and exhibit a lower energy AG


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Figure 30. (a) Photocurrent density vs applied potential of BiVO4 nanoplates and nanorods. Reproduced with permission from ref 35. Copyright 2010 The Royal Society of Chemistry. (b) Photocurrent density vs applied potential of WO3 nanomultilayers and nanorods. Reproduced with permission from ref 238. Copyright 2015 Elsevier. (c) Photocurrent density vs applied potential of α-Fe2O3 photoanodes with different nanopores. Reproduced with permission from ref 155. Copyright 2015 American Chemical Society. (d) Photocurrent density vs applied potential of the annealed [100]-oriented TaON films exfoliated from the front- (red curve) and back-side surface of the LiTaO3 single crystal substrate. Reproduced with permission from ref 395. Copyright 2018 John Wiley and Sons.

barrier for charge transfer through the photoelectrode/ electrolyte interfaces, resulting in a higher ηinj. Likewise, the improved ηinj and reduced surface charge injection resistance in the {010}-dominated BiOBr films are believed to benefit the enhancement of the PEC performance compared to the {001}dominated BiOBr films.82 By systematically investigating the Nyquist plots of TiO2 films with exposed {101}, {100}, and {001} facets, the {101} facet oriented TiO2 film was found to possess the lowest surface charge injection resistance at the photoelectrode/electrolyte interface.152 Similarly, the low carrier injection resistance of Cu2O photocathodes enclosed by four {111} facets is believed to promote the PEC performance.304 Recently, the PEC performances of WO3 nanoplate arrays with dominant {200} and {002} facets were investigated in a 0.1 M H2SO4 electrolyte under AM 1.5 G illumination.38 With the same film thickness, the photoanode with exposed highly reactive {002} facets exhibits a much higher photocurrent density than that of its counterpart with exposed {200} facets (Figure 29a). By optimizing the synthesis conditions, the WO3 photoanode with exposed {002} facets obtained via a two-step hydrothermal method exhibits a remarkable photocurrent density of 3.7 mA cm−2 at 1.23 V vs RHE without any OECs (Figure 29a). DFT calculations reveal that the complete water oxidation can more easily to take place on the {002} facets compared to the {200} facets. Moreover, the intermediate product of hydroxyl groups tends to accumulate on the {200} facets due to the higher energy barrier for water oxidation. Consequently, the photostability of the {200} facets is poorer than that of the {002} facets (Figure 29b). The higher PEC

performance of the {002}-faceted WO3 photoanodes may also be attributed to the oxidation active sites on the {002} facets, as is evidenced by the selective deposition of Pt on the (200)/ (020) facets and PbOx on the (002) facets of the WO3 nanoplates, respectively.384 Moreover, the exposed surfaces of a photoelectrode for PEC water splitting are only for either the hole-induced OER (photoanode) or electron-induced HER (photocathode). The interaction of the exposed facets with water molecules to efficiently trigger the OER or HER is also essential to achieving a high PEC performance. As shown in Figure 29c, the photocurrent densities of the In2O3 films are sensitive to the ratio of the exposed {001} facets. Cubic In2O3 (100% {001} facets) exhibits a high photocurrent density of 2.1 mA cm−2, while octahedral In2O3 (0% {001} facets) shows a very low photocurrent density of only 0.05 mA cm−2. Moreover, the cubic In2O3 film can achieve stable PEC water splitting for over 2 h, with a Faraday efficiency of approximately 92% (Figure 29d). The excellent PEC performance of the cubic In2O3 film is attributed to the unique surface structure and electronic band structure of the {001} facets. Theoretical calculations reveal that the polar {001} facets of In2O3 can dissociate the adsorbed water molecules to H+ and OH−, while the {111} facets cannot.316 Therefore, the higher ratio of exposed {001} facets is important to driving the PEC water splitting reactions. Likewise, both DFT calculations and X-ray photoelectron spectroscopy (XPS) revealed that the {012} facets of α-Fe2O3 films have higher OH coverage compared to the {001} facets, acting as active sites to mediate the OER.385 Interestingly, these surface OH groups also induce charge recombination. AH


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Figure 31. SEM images of (A) 2D, (B) E-2D, and (C) F-2D. (D) HRTEM image of the backbone plate of E-2D after ion exchange and calcination. (E) Photocurrent density vs applied potential of 2D, E-2D, and F-2D after ion exchange and calcination. Reproduced with permission from ref 396. Copyright 2018 John Wiley and Sons.

stronger light scattering and may induce a slow photon effect to enhance light harvesting. In addition, the thin shell of hollow structures significantly reduces the migration distance of photogenerated charge carriers, which can promote charge separation. The high surface area of hollow structures is also beneficial for surface reactions. Readers may refer to a recent review article for more information.389 Thus, the nanostructure engineering of faceted photoelectrodes is effective to further improving the PEC performance. BiVO4 films composed of nanoplates with highly reactive {001} facets exposed exhibit a photocurrent density more than five times higher than that of nanorods grown along the [100] direction (Figure 30a).35 Similarly, WO3 nanomultilayers with highly exposed {002} facets (60%) exhibited much better PEC performance than WO3 nanorods with less exposed {002} facets (20%), as shown in Figure 30b.238 The unique multilayered nanostructure and the preferential exposure of highly reactive {002} facets are attributed to the enhanced PEC performance. Interestingly, WO3 nanowires with a (200) orientation exhibited a photocurrent density ∼21 times higher than WO3·H2O nanoplates with (002) orientation at 1.0 V vs a saturated calomel electrode (SCE).390 This result seems opposite to the theoretical understanding that the (002) facets of WO3 with higher surface energy should be more active than the (200) facets. However, the WO3·H2O nanoplates reported here are of an orthorhombic phase with a lower photocatalytic activity, while the WO3 nanowires are of a monoclinic phase with a higher photocatalytic activity. Thus, the crystal phase should also be taken into account when comparing the PEC performances of faceted photoelectrodes. With the same growth direction of [0001], the special lateral surface atomic structure of zigzag (GaN)1−x(ZnO)x nanowires is beneficial for charge separation compared to the straight nanowire structure.391 Although both anatase TiO2 mesocrystals and single crystals have {101} facets exposed, the TiO2 photoanode composed of mesocrystals shows 191% and 274% enhanced photocurrent densities compared to their counterparts consisting of TiO2 single crystals and commercial P25, respectively.150 The mesostructured TiO2 is composed of

Another study demonstrated that a photoanode composed of rutile pillars with oxidative {111} and {011} facets exposed exhibits significantly enhanced photocurrent densities for PEC water splitting compared to its counterpart composed of anatase nanotubes with major reductive {101} facets.37 Owing to the presence of oxidative {111} and {011} facets as active sites for the OER, the photocatalytic activity of the rutile TiO2 photoanode is much better than that of its anatase counterpart, even though anatase is generally considered to be more active than rutile for photocatalytic applications. A very recent work revealed that BiVO4 films with {040} facets grown vertically and parallel to the FTO substrates exhibit different performances.39 Because {040} is the reduction functional facet while {110} is the oxidation functional facet, the BiVO4 films with {040} facets grown vertically to the FTO substrate should provide more active sites for the OER, leading to higher photocurrent densities. Likewise, the PEC performance of the Bi2MoO6 nanowall films was increased directly with a higher exposed ratio of {010} facets, which is attributed to the larger number of reactive sites in {010} facets for water oxidation compared to that in other facets.314

6. MODIFICATION OF FACETED PHOTOELECTRODES WITH ENHANCED PEC PERFORMANCE Although the exposed facets of a photoelectrode do affect the PEC performance, the obtained photocurrent densities of most faceted semiconductor films are still far from their theoretical maxima, mainly due to severe charge recombination. Thus, the modification of the faceted photoelectrodes to significantly reduce charge recombination is critical to further enhancing their PEC performance. In this section, promising modification methods for tuning faceted photoelectrodes to achieve enhanced PEC performance will be reviewed. 6.1. Nanostructure Engineering

Nanostructure tailoring is an effective manner to modify the light harvesting, charge transport, surface reaction kinetics, and energetic parameters of photoelectrodes for PEC water splitting.386−388 For example, hollow structures exhibit AI


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Figure 32. (a) Schematic of BiVO4/Ag3PO4 composite photoanodes prepared by electrodeposition and photodeposition. Photocurrent density vs applied potential curves of the obtained photoanodes for (b) water oxidation and (c) sulphite oxidation. Reproduced with permission from ref 269. Copyright 2018 John Wiley and Sons.

2.84 mA cm−2 at 1.23 V vs RHE under AM 1.5 G illumination (Figure 30d).395 The high PEC performance is attributable to the oriented TaON porous nanostructure that promotes electron transfer across the film, thereby enhancing the charge separation. 2D porous single-crystalline TiO2 nanostructures with exposed {001} facets were grown on FTO substrates by a one-pot hydrothermal treatment of a mixture solution of diethylene glycol (DEG) and water containing potassium titanium oxalate, followed by an ion exchange and thermal treatment process.396 By simply changing the volume ratio of the DEG and water in the hydrothermal process, the potassium titanate nanostructures can be tailored from 2D platelike nanostructures (denoted as 2D, Figure 31A) to 2D platelike nanostructures with branches selectively grown on the edges (denoted as E-2D, Figure 31B) and fully covering the entire plates (denoted as F-2D, Figure 31C). Interestingly, the subsequent ion exchange process can generate numerous nanosized pores with an average diameter of 2−6 nm in the single-crystalline plate (Figure 31D). F-2D exhibits a high photocurrent density of 1.02 mA cm−2 at a very low potential of 0.4 V vs RHE for PEC water splitting (Figure 32E), which is close to the theoretical value of TiO2 (1.12 mA cm−2). Systematic studies demonstrate that the vertically aligned, 2D porous single-crystalline anatase TiO2 nanostructures with branches fully covering the entire plates not only benefit charge separation and transfer but also provide a highly accessible

highly ordered nanocrystal subunits, leading to few grain boundaries, nanopores, and a short transport distance, which facilitates charge transfer and separation during PEC water splitting. By creating nanopores in the α-Fe2O3 crystals with exposed {001} and {101} facets to form mesoporous single crystal structures, the photocurrent density is increased by nearly 20 times when compared to their counterparts without nanopores (Figure 30c).155 The impressive improvement in the PEC performance is attributed to the enhanced light harvesting and reduced charge recombination resulting from the generation of nanopores in the α-Fe2O3 single crystals. Likewise, porous Cu2O films with exposed{100} facets,301 porous rutile TiO2 films with exposed {111} and {110} facets,392 and porous ZnO nanosheet arrays with a high (110) orientation393 also show enhanced photocurrent densities compared to their faceted counterparts without pores. The epitaxial growth of ZnO nanodisks with large exposed polar facets on nanowire arrays not only significantly enhances the light scattering capacity but also promotes charge separation, resulting in a 4 times higher PEC water splitting efficiency compared to the ZnO nanowire arrays.394 Very recently, TaON photoanodes composed of interconnected worm-like TaON nanoparticles aligned along the [100] direction vertical to the film surface were prepared by nitriding the (012) surface of single-crystal LiTaO3 under a NH3/CCl4 mixed gas flow, which exhibited a high photocurrent density of AJ


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GaN counterpart.406 The enhanced PEC performance is attributable to the internal spontaneous and piezoelectric polarization fields generated in the dodecagon-faceted LGAC AlGaN/n-GaN heterostructure photoanode. In addition, crystal facet combination between two contacted semiconductors can also affect the PEC performance of the heterojunction. For example, BiOI/BiOCl heterojunctions with contacted facets of BiOI (001)/BiOCl (010) exhibit a higher charge injection efficiency than their counterparts with contacted facets of BiOI (001)/BiOCl (001) due to the optimized electron transfer pathway.407 Other faceted heterojunctions such as {001}TiO2/{001}SrTiO3 epitaxial films,206 coaxial-nanocoupled SrTiO3/TiO2 {001} nanotube arrays,408−413 ultrathin g-C3N4 nanosheets decorated Bi2MoO6 nanosheet arrays with exposed {010} facets,313 αFe2O3 modified TiO2 {101} pyramids,414 {001}BiOI@ Bi12O17Cl2 p−n junctions,415 {002} dominant WO3/BiVO4 junctions,416 and {001}BiOBr/α-Fe2O3 p−n junctions417 also show enhanced PEC performance. Depositing another narrow bandgap semiconductor, quantum dots (QDs), or plasmonic metal nanoparticles (NPs) on the faceted photoelectrodes not only improves the charge separation but also broadens the light adsorption range. For example, Cu2O can be selectively deposited on the {101} facets of anatase TiO2 nanosheet arrays by electrodeposition, resulting in enhanced PEC performance.418,419 With a narrow bandgap of ∼2.0 eV, Cu2O extends the light absorption from the UV to visible light range when compared to bare TiO2. Owing to the different band structures and band edge positions between the {001} and {101} facets, the band offset value of the Cu2O and {101} facets of TiO2 is higher, thus forming a larger driving force to increase the charge transfer efficiency. Moreover, the heterojunction formed between the Cu2O and the {101} facets of TiO2 shortens the migration length of the electrons because the photogenerated electrons are transferred directly from the {001} to the {101} facets, which can effectively reduce the charge recombination. In addition, the deposition of Cu2O NPs on the entire surfaces of anatase TiO2 nanosheet arrays with exposed {001} facets also results in enhanced PEC performance.420 Very recently, a red-color polymerized carbon nitride (RPCN) with a significantly narrowed bandgap of 1.9 eV was prepared, which is efficient to sensitize rutile TiO2 nanorods grown along the (001) direction, resulting in a high photocurrent density of 2.33 mA cm−1 at 1.23 V vs RHE under AM 1.5 G illumination.421 Likewise, the photocurrent densities of {001}-faceted anatase TiO2 films are greatly increased by loading CdS NPs on the {001}/{101} facets.196,422−426 Anatase TiO2 nanocuboids with exposed {111} facets were decorated on TiO2 nanotube arrays, which exhibited a photocurrent density and a photoconversion efficiency of 0.97 mA cm−2 and 0.49%, respectively.427 By depositing CdS QDs, a photocurrent density and a photoconversion efficiency of 3.95 mA cm−2 and 2.05% can be achieved, respectively. A three-dimensional (3D) hierarchical heterojunction composed of one-dimensional (1D) ZnO nanorods deposited on vertically grown twodimensional (2D) anatase TiO2 {001} nanosheets exhibited a photocurrent density approximately 5 times that of the bare TiO2 {001} nanosheet arrays.428 By further loading CdS as a sensitizer, the photocatalytic activity can be significantly improved, which is attributed to the enhanced light absorption and efficient electron transport through the 3D hierarchical structure. Moreover, PbS/CdS QDs cosensitized TiO2 nano-

surface area for the OER, resulting in a high performance of PEC water splitting. 6.2. Heterojunction Construction

Coupling a semiconductor with another material (e.g., semiconductors, cocatalysts, or quantum dots) to construct heterojunctions is an effective approach to improving the charge transfer and separation.397−400 In addition to the properties of the coupled materials, the facets for forming the interfaces in the heterojunction also have an important impact on the performance.401 Therefore, constructing heterojunctions based on the faceted photoelectrodes provides great opportunities to synergistically enhance the PEC performance. The formation of type II heterojunctions is efficient for separating photogenerated electrons and holes in the two constructed semiconductors, reducing charge recombination. A hierarchical {001} facet anatase/rutile TiO2 heterojunction photoanode was prepared by growing anatase TiO2 nanosheets with exposed {001} facets on rutile TiO2 nanorod arrays, which exhibits a remarkable enhancement of the photocurrent density compared to planar anatase/rutile TiO2 and anatase TiO2 films.402 The construction of anatase/rutile TiO2 heterojunctions and the exposure of highly reactive {001} facets synergistically enhance the photogenerated electron lifetime and carrier concentration, resulting in significantly enhanced photocurrent densities. Interestingly, [101]-oriented rutile TiO2 nanorod arrays in a heterojunction with anatase on the FTO substrate can be prepared by a facile single-step hydrothermal process in the presence of NaCl, resulting in a enhanced PEC performance.403 TiO2 heterojunctions composed of [100]-oriented rutile nanorods array and {001}faceted anatase microsheets can also be prepared by a one-pot hydrothermal method in the presence of HF.404 The deposition of another semiconductor on selected facets of the faceted films also affects the PEC performance. For example, Ag3PO4 NPs can be selectively deposited on the side {040} facets or the top {121} facets of BiVO4 films via photodeposition or electrodeposition (Figure 32a), and their PEC performances are very different.269 The BiVO4/Ag3PO4 heterojunction constructed on the {121} facets (holedominant facets) of BiVO4 exhibits a much better PEC performance than its counterpart constructed on the {040} facets (electron-dominant facets) of BiVO4. It is believed that the hole-dominant facets facilitate the hole transfer from BiVO4 to Ag3PO4 in the heterojunction, resulting in a high photocurrent density of 3.78 mA cm−2 at 1.23 V vs RHE under AM 1.5 G illumination, which is almost triple that of the pure BiVO4 films (Figure 32b). In the presence of Na2SO3 to remove surface charge recombination, the photocurrent densities can be further improved (Figure 32c). Owing to the epitaxial relationship between the ZnO (101̅0) and In2O3 {2̅11} facets, In2O3/ZnO heteroepitaxial-junction photoanodes were prepared by epitaxially growing In2O3 nanoparticles on the lateral surface of ZnO NRs, which increases the photocurrent density by ∼1.8 and ∼3.4 times compared to those of the ZnO NRs and In2O3 photoanodes, respectively.405 The formation of a heterojunction and the exposure of {001} and {111} facets with lower surface energy in the In2O3 particles contribute to the enhanced PEC performance. By depositing an AlGaN layer with a linear gradient Al composition (LGAC) on the dodecagon-faceted n-GaN photoanode, the zero-bias photocurrent density is approximately 6 times higher than that of the dodecagon-faceted nAK


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Figure 33. (A) Photocurrent density vs applied potential curves of PbS/CdS cosensitized TiO2 nanosheet arrays with exposed {001} facets. Reproduced with permission from ref 429. Copyright 2015 Elsevier. (B) Charge transfer and separation in an Au/pseudocubic α-Fe2O3 photoelectrode. Reproduced with permission from ref 432. Copyright 2016 Elsevier. (C) Photocurrent density vs applied potential curves of {110} oriented α-Fe2O3 and the Ag and Co-Pi/Ag modified photoelectrodes. (D) Long-term stability test of the Co-Pi/Ag modified α-Fe2O3 photoelectrode with {110} orientation. Reproduced with permission from ref 263. Copyright 2016 John Wiley and Sons.

density over 8 h of consecutive light illumination. Similarly, Au NPs deposited on the {001} facets of epitaxial BiVO4 films,433 Au nanowires deposited on the side facets of ZnO nanorod arrays to form a cross-linked heterostructure,434 and Au/Pt NPs codeposited on TiO2 nanosheets with exposed {001} facets435 also show enhanced photocurrent compared to their bare counterparts. For practical applications, the utilization of Au/Pt is not favorable, and the development of low-cost plasmonic metal NPs is necessary. Bi NPs decorated on the {001} facets of BiOCl nanosheet arrays showed drastically enhanced PEC performance.436 The optimal Bi/BiOCl photocathode connected with a TiO2 photoanode can achieve a photocurrent density of 1.6 mA cm−2 at 0.8 V vs Ag/AgCl under AM 1.5 G illumination, which is approximately 8 times that of the TiO2/ Pt configuration. The metallic Bi NPs with noble metal-like behavior not only facilitate the charge transfer/separation but also induce LSPR effects, resulting in enhanced photocurrent densities. Similarly, Bi NP modified BiVO4 photoanodes with exposed {040} facets also show enhanced PEC performance.437 Ag NPs are another alternative plasmonic metal that can enhance the PEC performance of faceted photoelectrodes. For example, Ag NPs were selectively deposited on the {−110} facets of the H-Nb2O5 nanobelts through a photoreduction process, which generates a photocurrent density over 500 times higher than that of the pure H-Nb2O5 nanobelts under

sheet arrays with exposed {001} facets showed an impressive photocurrent density of 6.12 mA cm−2 under AM 1.5 G illumination (Figure 33A).429 Likewise, CdS/CdSe cosensitized [110]-oriented ZnO nanosheet array films with exposed {0001} facets also show significantly enhanced photocurrent densities.430 Although CdS/PbS/CdSe QDs can significantly improve the photocurrent densities in a heterojunction photoelectrode, their toxic nature is an issue when applied in PEC water splitting. In contrast, environmentally friendly carbon quantum dots (CQDs) are more promising. For example, loading CQDs on the exposed {001} facets of an anatase TiO2 photoelectrode can enhance the visible light PEC performance, which is attributed to the decreased PEC reaction resistance and the greatly increased number of photogenerated carriers.431 Moreover, the outstanding upconversion effect of the CQDs can convert visible light into UV light to excite the {001}faceted TiO2 photoanodes, which also contributes to the enhanced PEC performance. Loading plasmonic metal NPs on the faceted photoelectrodes can also increase the light harvesting. Depositing Au NPs on the surface of α-Fe2O3 films composed of a pseudocubic polyhedral structure with exposed {012} and {112} facets can enhance the photocurrent density by 27% due to the LSPR effect (Figure 33B).432 Moreover, the Au NP modified α-Fe2O3 films also possess excellent long-term stability, with a decay of less than 5% in the photocurrent AL


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Figure 34. (a) Work functions and (b) flatband potentials of doped α-Fe2O3 films grown on Nb-doped SnO2 coated single crystal sapphire substrates different facet orientations. Reproduced with permission from ref 255. Copyright 2016 American Chemical Society. (c) SEM image (inset: cross-sectional view) and (d) photocurrent density vs applied potential curves of Ge-doped α-Fe2O3 nanosheet arrays with exposed {001} facets. Reproduced with permission from ref 260. Copyright 2014 Elsevier.

simulated sunlight illumination.438 Impressively, by modifying the surfaces of {110}-oriented α-Fe2O3 nanosheet arrays with plasmonic Ag NPs and Co-Pi, the light harvesting, charge transfer, and surface charge separation are drastically improved, resulting in an excellent photocurrent density of 4.68 mA cm−2 at 1.23 V vs RHE under AM 1.5 G illumination (Figure 33C).263 Moreover, the obtained Co-Pi/Ag/α-Fe2O3 photoanode is very stable, and no photocurrent decay can be observed during 5 h of consecutive light illumination (Figure 33D). In addition, Ag NPs modified {040}-faceted BiVO4 photoanodes439 and {001}-faceted TiO2 photoanodes440 also show enhanced photocurrent densities. Interestingly, the selective decoration of Ag@AgCl QDs solely on the {040} facets of BiVO4 formed a hierarchical Z-scheme system that facilitates the efficient spatial separation of the photogenerated electron−hole pairs, resulting in enhanced photocurrent densities.441,442 A similar Z-scheme system with enhanced PEC performance was formed by depositing Fe2O3 NPs on BiVO4 with Ag NPs decorated on the {010} facets.443 The deposition of QDs can also affect the surface reaction kinetics. For example, the deposition of WS2-QDs on WO3 nanorod arrays with oriented (200) facets provided efficient relatively electrochemically active surface areas for water oxidation, and thus higher photocurrent densities were observed.444 On the other hand, introducing a conductive material or cocatalyst into the faceted photoelectrodes to promote charge transfer and separation can also improve the PEC performance.

For example, photoanodes composed of a RGO-{001} faceted TiO2 composite exhibited a ∼10-fold increase in the photocurrent density compared to their pure {001} faceted TiO2 counterparts, which is attributed to rapid electron transport and the prolonged lifetime of photogenerated charge carriers resulting from the improved ionic interaction between titanium and carbon.445 Likewise, the interfacial engineering of a {040} and {110} facet exposed BiVO4/Fe-based LDH layer core/shell nanostructure is efficient for enhancing the PEC water oxidation performance.270 Interestingly, the selective deposition of cocatalysts on different facets of a semiconductor also affects the PEC performance, which has been confirmed in BiVO4 photoanodes.103 The reduction and oxidation cocatalysts are selectively deposited on the reductive {010} and oxidative {110} facets of BiVO4 photoanodes, respectively, resulting in a much higher activity for PEC water splitting compared with that of their counterparts with randomly distributed cocatalysts. Interestingly, OECs such as IrO2 and RuO2 also exhibit facet-dependent OER activities.446 Thus, tuning both the facets of the semiconductors and the OECs in the photoelectrode may open up more opportunities to further enhance the PEC performance. 6.3. Heteroatom Doping

Heteroatom doping is efficient for modifying the electronic structures of a semiconductor and promoting light harvesting and charge separation. Introducing heteroatoms into the AM


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Figure 35. (a) Photocurrent density vs applied potential and (b) Mott−Schottky curves of rutile TiO2 film with 100% exposed {111} facets calcination in Ar (TiO2−Ar) or air (TiO2−air). Reproduced with permission from ref 36. Copyright 2012 Springer Nature. (c) Photocurrent density vs applied potential and (d) the corresponding ABPE curves of I, BiVO4 films with {040} facets vertically to FTO, II, electrochemically treated BiVO4 films, and III, Co−Bi modified electrochemically treated BiVO4 films. Reproduced with permission from ref 39. Copyright 2017 John Wiley and Sons.

treating β-FeOOH nanorod arrays in a Ge colloidal solution, resulting in a photocurrent density over 50 times higher than that of the undoped counterpart (Figure 34c,d).260 The annealing temperature also significantly affects the majority carrier density and optical absorption properties of the Gedoped α-Fe2O3 nanosheet arrays (Figure 34d). Similarly, Bidoped (002)-oriented WO3 films, Ti-doped WO3 films with equally exposed (200), (020), and (200) facets, N-doped Nb2O5 nanorod films with exposed {001} facets, and Sn-doped FeS2 films with exposed {200} facets also exhibited improved photocurrent densities.44,449−451 Interestingly, B/N-doped anatase TiO2 photoanodes with exposed dominant {001} facets can significantly reduce the bandgap from 3.22 to 1.9 eV, resulting in enhanced PEC performance.452 Similarly, Rh/F codoped at the {101} facets of anatase TiO2 can also significantly reduce the bandgap to 2.14 eV.453

faceted photoelectrodes can also improve the PEC performance. However, additives in the growth solution during the in situ preparation of the faceted photoelectrodes will change the growth environment, which may affect the exposed facets of the final photoelectrode. How to introduce dopants into the photoelectrodes without changing the exposed facets is very important yet highly challenging. Although the electrical conductivity of α-Fe2O3 is up to 4 orders of magnitude higher along the (001) basal plane (the [110] direction) than the vertical plane, the electron transport properties of α-Fe2O3 grown along the [110] direction are still not good enough, and thus heteroatom doping is generally required to achieve a high PEC performance.253,447,448 For example, introducing a Ti or Sn dopant into the onedimensional [110]-oriented α-Fe2O3 nanowire arrays is effective for enhancing the electron transport properties, resulting in an increased photocurrent density up to 1.3 mA cm−2 under AM 1.5 G illumination for PEC water splitting.259 Moreover, Sn-doped α-Fe2O3 photoanodes with {100} and {110} orientations showed negative shifts in the onset potential of ∼170 and ∼100 mV compared to their {001}oriented counterpart, which is attributed to the different work functions and flat-band potentials of the facets (Figure 34a,b).255 Highly oriented Ge-doped α-Fe2O3 nanosheet arrays with exposed {001} facets were synthesized by hydrothermally

6.4. Intrinsic Defect Generation

Similar to heteroatom doping, creating intrinsic defects such as oxygen vacancies can also tune the electronic structures of metal oxides. Oxygen vacancies function as shallow donors to improve the electrical conductivity of the semiconductors, which promotes charge transfer at the semiconductor/ substrate interface as well as the semiconductor/electrolyte interface.454−456 Oxygen vacancies can be achieved by posttreating the photoelectrodes in a reductive or oxygen-deficient environment,457−465 which usually does not affect the exposed AN


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Because the pioneering work on anatase TiO2 single crystals with enriched {001} facets exposed using HF as a morphologycontrolling agent in 2008,26 an exponentially increasing amount of research work on the crystal facet engineering of anatase TiO2 and a library of other semiconductors was witnessed in the past 10 years. Compared to suspension photocatalysis systems, the history of crystal facet oriented photoelectrodes for PEC water splitting is still very short, yet an increasing number of research works have been published, particularly in the past few years. There is no doubt that crystal facet engineering is an important strategy to modify photoelectrodes for efficient PEC water splitting that will attract increasing attention in the near future. The challenges for the preparation of crystal facet oriented photoelectrodes should not be ignored. Compared to the suspension systems that only require powder photocatalysts, PEC water splitting systems require the loading of photocatalysts on conductive substrates. How to keep the faceted photocatalysts in order on the conductive substrates with excellent interfacial contact is very challenging. The direct growth of faceted photocatalysts on conductive substrates is essential to achieving excellent contact between the photocatalysts and the substrates. However, the durability of the substrates in the growth solution may be an issue. For example, HF is an effective capping agent for the synthesis of {001}faceted anatase TiO2, but HF can destroy the FTO and ITO glass substrates in the hydrothermal conditions. Because of the lattice mismatch between the substrates and the semiconductor crystals, the direct growth of some faceted semiconductor crystals is also a challenging task. Although depositing a crystalline semiconductor layer as seeds can solve this issue to some extent, the numerous grain boundaries in the seed layer generally cause charge recombination.467 Moreover, as the facet-dependent properties of a semiconductor crystal may synergistically affect the final PEC performance, it is challenging to keep the balance between different properties for the rational design of the exposed facets. From a material synthesis point of view, most of the theoretical and experimental studies for crystal facet engineering are focused on tailoring the low-index {001}, {101} and {010} facets of anatase TiO2 and understanding their corresponding photocatalytic behaviors. However, the controllable growth of other anatase TiO2 facets, including {110} and other high-index facets, is still lacking. For PEC water splitting, TiO2 with a large bandgap of ∼3.2 eV can only harvest UV light, which is unlikely to achieve high STH efficiencies. Therefore, more studies should be focused on visible light responsive semiconductors. However, efficient strategies for the controllable formation of the targeted facets for visible light responsive semiconductors have rarely been explored. Therefore, a deeper understanding of the control of various facets for different semiconductor crystals is still critically important. It should be mentioned that crystal facet engineering cannot solve all of the intrinsic drawbacks of a semiconductor crystal. For example, even though the bandgap of TiO2 can be tuned by controlling the preferentially exposed facets, the change in the bandgap is generally less than 0.1 eV. Thus, the enhancement of the light harvesting is limited. To maximize the PEC performance of the faceted photoelectrodes, further modifications are also indispensable. So far, nanostructure engineering, heterojunction construction, heteroatom doping, and intrinsic defect generation have been developed for the modification of the faceted photoelectrodes. Modifying the

facets of the photoelectrodes. Therefore, introducing defects into faceted photoelectrodes is feasible for further improving the PEC performance. By annealing a rutile TiO2 film with 100% exposed {111} facets in an Ar atmosphere, Ti3+ doping resulted in the bulk, which increases the photocurrent density by approximately 1.5 times compared to its counterpart calcined in air (Figure 35a).36 As shown in Figure 35b, Ti3+ doping leads to a higher carrier concentration in the film, and thus more photogenerated electrons can be effectively transferred to the external circuit during PEC water splitting, resulting in higher photocurrent densities. Similarly, the photocurrent density of {001}-oriented anatase TiO2 nanorods can be increased from 0.6 to 1.2 mA cm−2 at 1.25 V vs RHE under AM 1.5 G illumination by self-doping with Ti3+ via electrochemical reduction.195 Moreover, coupling {001}-faceted BiOCl nanosheets with TiO2−x nanoparticles containing Ti3+, Ti2+, and oxygen vacancies can extend light absorption to the visible region and promote charge transfer and separation, resulting in significantly enhanced photocurrent densities when compared to pure TiO2 and BiOCl.160 A very recent work revealed that the photocurrent density of BiVO4 films with {040} facets grown vertically to FTO substrates can be significantly enhanced by a facile electrochemical treatment.39 By loading Co−Bi as the oxygen evolution cocatalyst, the photocurrent density can be further increased to 3.2 mA cm−2, resulting in an impressive 1.1% PEC water splitting efficiency (Figure 35c,d). The partial reduction of the Bi3+ and V5+ ions in BiVO4 generates oxygen vacancy shallow donors, enhancing both the bulk and surface charge separation. Moreover, Co−Bi as an efficient oxygen evolution cocatalyst can accelerate the surface kinetics, facilitating efficient charge separation and resulting in high PEC activity and good stability.

7. CONCLUSION AND OUTLOOK PEC water splitting provides a “green” technique for hydrogen production using abundant solar energy, which can potentially satisfy the increasing energy demand for humanity with a lessadverse impact on the environment. For practical applications, PEC water splitting should achieve the target of a 10% STH efficiency,466 which requires the deliberate tailoring of the light harvesting, charge separation and transfer, and surface reactions of the PEC performing photoelectrodes. As discussed, no photoelectrode materials so far can meet the above requirements, thus requiring urgent research efforts on the modification of potential PEC materials. On the other hand, the photogenerated electrons and holes should be transferred to the conductive substrate/semiconductor and semiconductor/electrolyte interfaces for the subsequent PEC water splitting reactions. The rational engineering of the conductive substrate/semiconductor and semiconductor/electrolyte interfaces are as important as the development of efficient photoelectrode materials. Owing to the interesting facet-dependent properties of semiconductor crystals described in this review, it is possible to modify photoelectrodes through crystal facet engineering toward efficient PEC water splitting. Although the interaction of water molecules with the {101} and {001} facets of anatase TiO2 was predicted through DFT calculations in the 1990s,118 experimental works on the photocatalytic water splitting properties on different facets of anatase TiO2 have been lacking for decades due to the material synthesis challenge. AO


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addition to the high performance of each faceted photoelectrodes. Recently, an unassisted PEC water splitting system composed of a Cu2O nanowire photocathode and BiVO4 photoanode achieved a record STH efficiency of ∼3% using low-cost oxide-only materials;471 however, this still is a considerable distance from the 10% target. Although higher STH efficiencies of 6.2−8.1% can be achieved by coupling the photoelectrode with photovoltaic (PV) devices,472−475 the cost and durability are still issues for practical applications. To further improve the efficiency of water splitting devices, what is the next possible research direction? Interdisciplinary studies may open up new opportunities to achieve efficient water splitting in a cost-effective manner. Other exciting opportunities such as the combination of photoelectrochemistry with bacteriology,476 thermoelectricity,477 or magnetics,478 may also lead to breakthroughs. We look forward to the exciting development of efficient and low-cost PEC-based hybrid devices for practical hydrogen production in the near future.

surface of the faceted photoelectrodes with cocatalysts or other semiconductors to form heterojunctions results in enhanced PEC performance. However, the advantages of the crystal facet orientation may not be fully displayed in the heterojunction. The selective deposition of another material on the different facets of the faceted photoelectrode should be important to combine the advantages of crystal facet engineering and heterojunction construction, but it should be practically challenging. For heteroatom doping, it is important to control the doping concentration without changing the growth environment of the faceted photoelectrodes. In a special case, silicon functions as both a capping agent and dopant for the fabrication of [110]-oriented α-Fe2O3 films, resulting in enhanced PEC performance. 138 Dropping the dopant precursor solution on a faceted photoelectrode followed by subsequent calcination may be another way to introduce dopants without changing the exposed facets. However, the excessive solution may form corresponding oxides as impurities on the surface of the faceted photoelectrode, which should be removed in a proper manner. Creating defects in a faceted photoelectrode by a mild electrochemical treatment is also efficient to further improving the PEC performance without destroying the exposed facets.39 In addition, the exploration of other strategies to further improve the PEC performance of faceted photoelectrodes without sacrificing the advantages of the exposed facets is still necessary for attaining an efficient solar energy conversion. In addition to the preparation and modification of the faceted photoelectrodes, the development of cutting-edge microscopy and spectroscopy technologies to provide direct evidence of the PEC water splitting reaction mechanisms (e.g., surface adsorptions, surface charge separation, and surface reactions) on faceted photoelectrodes remains highly challenging. Considering that the three main PEC reaction processes (i.e., light harvesting, charge separation and transfer, and surface reactions) take place on different facets on a time scale between 10−12 and 100 s,468 a high spatial and temporal resolution are generally required to understand the underlying mechanisms. Very recently, a combination of atomic-scale microscopy and spectroscopy was used to understand the surface adsorptions on rutile TiO2 surfaces in the dark, and it was found that rutile TiO2 (110) surfaces in ambient environments are terminated by a well-ordered carboxylate monolayer, which may block the undercoordinated surface cation sites typically indicated in photocatalysis.469 By using surface photovoltage microscopy, the surface photovoltage difference between the illuminated and shadow facets of cubic Cu2O particles grown along the (111) orientation was observed.470 It was revealed that the driving force induced by the charge mobility difference and asymmetric light illumination is competitive or even stronger than the conventional built-in electric fields, leading to the efficient spatial separation of the photogenerated electrons and holes. These studies are critical to understanding the underlying PEC reaction mechanisms on different facets, which is essential to further optimizing the PEC performance of faceted photoelectrodes. Last but not least, integration devices composed of faceted photoelectrodes to achieve unassisted solar-driven water splitting with high STH efficiencies (e.g., 10%) is the ultimate goal for PEC water splitting. However, the light absorption ranges, band gaps, and band edge positions of the coupled faceted photoelectrodes should also be carefully considered in

AUTHOR INFORMATION Corresponding Authors

*L.W.: phone, 61-7-33654218; E-mail, [email protected]. *G.L. e-mail: [email protected]. ORCID

Songcan Wang: 0000-0002-3848-1191 Gang Liu: 0000-0002-6946-7552 Lianzhou Wang: 0000-0002-5947-306X Notes

The authors declare no competing financial interest. Biographies Songcan Wang received his B.Eng. (2011) and M.Eng. (2014) degrees from Central South University (CSU), China. After obtaining his Ph.D. degree in Chemical Engineering from the University of Queensland (UQ), Australia, in 2018, he worked in Prof. Lianzhou Wang’s group at UQ as a postdoctoral research fellow. His research interests focus on the design of efficient photoelectrodes for solar fuel production. He was the recipient of an IPRS scholarship (2014) and a Chinese government award for outstanding self-finance students abroad (2019). Gang Liu is a Professor of Materials Science at the Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), and the Deputy Director of IMR, CAS. He received his Bachelor’s degree in Materials Physics from Jilin University in 2003. He obtained his Ph,D, degree in Materials Science from IMR, CAS, in 2009. His main research interests focus on solar-driven photocatalytic and photoelectrochemical materials for renewable energy. Lianzhou Wang is a Professor at the School of Chemical Engineering and the Director of Nanomaterials Centre, the University of Queensland (UQ), Australia. He received his Ph.D. degree from the Chinese Academy of Sciences in 1999. Before joining UQ in 2004, he worked at two national institutes (NIMS and AIST) of Japan for five years. Wang’s research interests include the design and application of semiconductor nanomaterials in solar energy conversion/storage systems, including photocatalysts and photoelectrochemical devices.

ACKNOWLEDGMENTS We acknowledge the financial support from Australian Research Council through its DP programs. The financial support from the National Natural Science Foundation of AP


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China (nos. 51629201, 51825204) and the Key Research Program of Frontier Sciences CAS (QYZDBSSW-JSC039) is also appreciated.

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