The {010} and {110} crystal facets of monoclinic bismuth vanadate (m-BiVO4) has been demonstrated to be the active reduction and oxidation sites, respectively. Here, we show using dual-faceted m-BiVO4 with distinctly different dominant exposed facets, one which is {010}-dominant and the other {110}-dominant, contrary to prediction, the former m-BiVO4 exhibits superior photooxidation activities. The population of photogenerated electrons and holes on the surface are revealed to be proportional to the respective surface areas of {010} and {110} exposed on m-BiVO4, as evidenced by steady-state photoluminescence (PL) measurements in the presence of charge scavengers. The better photoactivity of {010}-dominant m-BiVO4 is attributed to prompt electron transfer facilitated by the presence of more photogenerated electrons on the larger {010} surface. Additionally, the greater extent of electron trapping in {110}-dominant m-BiVO4 also deteriorates its photoactivity by inducing electron-hole pair recombination.

This review summarises the recent advances of various strategies in improving the performances of BiVO4 in photocatalytic and photoelectrochemical systems.

Photocatalytic and photoelectrochemical processes are two key systems in harvesting sunlight for energy and environmental applications. As both systems are employing photoactive semiconductors as the major active component, strategies have been formulated to improve the properties of the semiconductors for better performances. However, requirements to yield excellent performances are different in these two distinctive systems. Although there are universal strategies applicable to improve the performance of photoactive semiconductors, similarities and differences exist when the semiconductors are to be used differently. Here, considerations on selected typical factors governing the performances in photocatalytic and photoelectrochemical systems, even though the same type of semiconductor is used, are provided. Understanding of the underlying mechanisms in relation to their photoactivities is of fundamental importance for rational design of high‐performing photoactive materials, which may serve as a general guideline for the fabrication of good photocatalysts or photoelectrodes toward sustainable solar fuel generation.

This review summarizes current experimental techniques, including the conventional and the state-of-the-art tools, to examine the key aspects of photocatalysts.

Thermal annealing of metal oxides in oxygen‐deficient atmosphere, particularly reducing hydrogen gas, has been demonstrated to induce oxygen vacancy formation for enhanced photoactivity of the materials. Here, it is demonstrated that argon annealing (another prevalently used oxygen‐deficient gas) in the temperature range of 300–700 °C greatly affects the activity of dual‐faceted BiVO4 microcrystals for photocatalytic O2 generation and photocurrent generation. While treatment at 300 °C has little to no effect, higher temperatures of 500 and 700 °C significantly improve the crystallinity, alter the local structure distortion, and reduce the bandgap energy of the treated BiVO4. The higher temperature treatment also favors formation of new subgap states attributed to oxygen vacancies, as supported by surface photovoltage and electron paramagnetic resonance spectroscopies. Despite the most profound improvements in structural, optical, and electronic properties displayed by the 700 °C‐treated BiVO4, the sample annealed at 500 °C exhibits the highest photoactivity. The lower activity of the 700 °C‐treated BiVO4 is ascribed to the creation of bismuth vacancies and the loss of well‐defined crystal facets, contributing to impeded electron transport and poor charge separation.

Efficient interfacial charge transfer is essential in graphene-based semiconductors to realize their superior photoactivity. However, little is known about the factors (for example, semiconductor morphology) governing the charge interaction. Here, it is demonstrated that the electron transfer efficacy in reduced graphene oxide-bismuth oxide (RGO/BiVO ) composite is improved as the relative exposure extent of {010}/{110} facets on BiVO increases, indicated by the greater extent of photocurrent enhancement. The dependence of charge transfer ability on the exposure degree of {010} relative to {110} is revealed to arise due to the difference in electronic structures of the graphene/BiVO {010} and graphene/BiVO {110} interfaces, as evidenced by the density functional theory calculations. The former interface is found to be metallic with higher binding energy and smaller Schottky barrier than that of the latter semiconducting interface. The facet-dependent charge interaction elucidated in this study provides new aspect for design of graphene-based semiconductor photocatalyst useful in manifold applications.

Electronic Supplementary Information (ESI) available: [XRD patterns of FTO substrate, CdS film and CdS-ZnO film from pulsed electrodeposition, SEM images, PEC of CdS-ZnO from non-pulsed electrodeposition, absorption spectra of pristine ZnO, pure CdS and CdS-ZnO and onset potential of CdS and CdS-ZnO are available]. SeeAn ultrathin layer of CdS (~8 nm) was successfully coated on an array of vertically aligned ZnO nanorods using pulsed electrodeposition. The pulse was essential during the deposition of CdS to ensure equilibrium between diffusion and nucleation of CdS precursors. These ZnO nanorods functioned as a large contact base for the deposition of CdS. This enlarged interface between CdS and ZnO together with its close intimacy facilitated efficient charge transfer from the excited CdS to ZnO upon visible illumination. Owing to the high electron mobility of ZnO, it shuttled the electrons efficiently for enhanced photocurrent generation. Compared to bare CdS film, the CdS-ZnO photoelectrode yielded a doubled anodic visible photocurrent density of 6 mA/cm 2 at 0 V vs. Ag/AgCl. Photoluminescence spectroscopy and photoelectrochemical characterizations showed that the charge recombination within CdS was suppressed and the proper band alignment favored the electron transfers.

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