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Small perturbation techniques such as intensity-modulated photocurrent spectroscopy (IMPS) have become an essential tool to unravel the complex, interrelated processes that govern the charge carrier dynamics in photoelectrochemically active materials for solar water splitting. We have fabricated CuWO4-based photoelectrodes by chemical modification of a WO3 nanorod array as a partially sacrificial template. The electrodes have been characterized by photoelectrochemical techniques including IMPS as a function of the annealing temperature, transforming WO3 either partially or completely to CuWO4. The optical properties illustrate the transformation with the absorbance onset moving from the typical onset for WO3 (about 2.5 eV) to that of CuWO4 (about 2.1 eV). In pure CuWO4 photoelectrodes bulk recombination dominates the photoelectrochemical performance, while for FTO|WO3@CuWO4 heterojunction photoelectrodes much larger charge separation and external quantum efficiencies are obtained. In addition, CuWO4 serves as a protective layer for the WO3 material that is not generally stable in neutral aqueous solutions. The FTO|WO3@CuWO4 heterojunction material was further explored as an underlayer substrate, using a thin BiVO4 film as overlayer. The advantages of this configuration include improved light harvesting as BiVO4 is a direct semiconductor with a much larger absorption coefficient than CuWO4, which is characterized by an indirect gap. It is found that the FTO|WO3@CuWO4 underlayer efficiently extracts the photogenerated electrons from the BiVO4 overlayer, hence, the inclusion of a second heterojunction plays an essential role in improving the charge separation and internal quantum efficiency, minimizing both bulk and surface recombination.

Metal oxide (MO) semiconductors based on abundant, low-cost materials are attractive for application in photoelectrochemical water splitting systems related to their inherent stability, specifically against oxidation. In addition, these materials provide a wide-ranging versatility in terms of composition, physical and chemical properties, and configuration of multi-component architectures. The semiconducting MO must at least comply with three requirements: (i) absorb sunlight efficiently; (ii) the valence and conduction band edges must be situated at the appropriate energies; and (iii) the electrochemical reaction must have favorable kinetics compared with charge separation, transport and recombination kinetics. Many systems have been evaluated for photo-oxidation of water in the past 10 years, with some of the more popular materials being Fe2O3, WO3, BiVO4, TiO2, etc. In order to improve the efficiency of a solar water splitting system, the specific loss mechanisms need to be elucidated; based on these insights strategies may then be developed to improve performance. However, it is not straightforward to determine the precise loss mechanisms and generally non-steady state methods need to be applied in combination with steady-state techniques. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful technique to study the carrier dynamics in a photoelectrochemical cell: the photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge separation, charge transfer and surface recombination may be deduced. The kinetic model applicable to the specific system determines the interpretation of the results. On the other hand, theoretical modeling is a very powerful method to validate the interpretation of the results, and to complement the experimental studies in order to obtain a clear picture of the dominant processes and effects. In this presentation, the results from IMPS studies on a variety of systems will be discussed and strategies to improve performance are presented and tested, both experimentally and theoretically: (i) planar, compact WO3 / BiVO4 heterojunction systems are compared with nanorod array WO3 / BiVO4 heterojunctions, with and without a hole scavenger; (ii) CuBi2O4 with and without catalysts and an alternative electron acceptor; and (iii) specific examples of other materials to highlight the proposed interpretations. The results are compared with calculations of the IMPS response using a classic kinetic model and a continuity equation approach in order to incorporate the effects of charge transport and electric field within the electrode. The authors gratefully acknowledge funding from CONACYT, SENER, and CICY through the Renewable Energy Laboratory of South East Mexico (LENERSE; Project 254667; SP-4), and CONACYT under the Basic Sciences (CB) project A1-S-28734. We also thank the Ministerio de Ciencia e Innovación, the Agencia Estatal de Investigación (AEI), and the European Regional Development Fund of the European Union through Project PID2019-110430GB-C22. AR thanks the Spanish Ministry of Education, Culture and Sports via a PhD grant (FPU2017-03684). Finally, we would like thank Dr. Antonio Zapien and the NANOMXCN initiative to foment scientific collaboration between Mexico and China for support.

Cu2BaSnS4 thin films have been successfully deposited for the first time onto ultra clean glass substrates using spray pyrolysis, a simple and low-cost technique. Elemental, structural, optical and photoelectrochemical properties of the annealed (in sulphur rich atmosphere at 450 °C for 30 min) Cu2BaSnS4 thin films have been studied. XRD reveals that the CBTS film is a single phase favorably crystalline. XPS and SEM-EDX confirmed elements distribution on the CBTS surface with minor organic impurities. Raman analysis showed that the films are dominated by the presence of trigonal CBTS. Optical measurements confirmed high absorption ability of the CBTS.

Surface plasmon resonance (SPR) of metallic nanoparticles has become an attractive strategy for increasing the efficiency of solar water splitting. However, the metal/semiconductor junction may introduce unwanted interfaces or surface species that reduce the SPR effect as well as compromising efficient charge transport. The processes of separation, transport, and transfer of charges in metal-based plasmonic photoelectrodes are highly sensitive to the nature of the coupling between metal/semiconductor/electrolyte and a comprehensive understanding of these interfaces is still lacking. In this work, we proposed the construction of hematite photoanodes modified with gold nanoparticles (AuNPs) and aluminum oxide with different arrangements, whose optimized coupling between the interfaces led to enhanced photoelectrochemical (PEC) performance. Using a combination of finite-difference time-domain (FDTD) simulations, well-established materials synthesis and x-ray spectroscopy, electron microscopy, and PEC characterization techniques, selected architecture design strategies are evaluated. The experimental results reveal that the direct contact between semiconductors and metals coated by the dielectric leads to an improvement in localized electric field at the interface upon the formation of hot electrons, boosting the generation and separation efficiencies of electron–hole pairs. The main role of the dielectric coating, which led to an ineffective surface state passivation, is to prevent the photooxidation of AuNPs. FDTD calculations are employed to investigate the spatial distribution of the electric-field intensity around the AuNPs deposited onto the hematite surfaces and to corroborate the local field enhancement effect. The outcome of this combined experimental-theoretical study reveals that engineering plasmonic interfaces is a powerful tool to design efficient photoanodes for plasmon-driven PEC water splitting.

Synergistic effects as a result of combining different photoactive materials provide a promising pathway to improve the photoelectrochemical performance in heterojunction devices for solar water splitting. The photoelectrochemical characteristics of tungsten trioxide (WO3) and bismuth vanadate (BiVO4) have been studied, comparing the single-phase materials with two heterojunction systems based on a WO3 underlayer and a thin, spin-coated BiVO4 top layer, using two types of underlayer: (i) a nanorod-based WO3 substrate prepared by hydrothermal methods (nr-WO3/BiVO4); and (ii) a spin-coated thin, compact WO3 substrate (pl-WO3/BiVO4). Intensity-modulated photocurrent spectroscopy (IMPS) was used to determine the charge separation efficiency, internal and external quantum efficiencies, and process time constants for the four systems in a phosphate buffer at pH 7, both without and with an added hole scavenger, Na2SO3. The heterojunction systems show excellent performance, which is ascribed to the superior capability of WO3 to extract photogenerated electrons from the BiVO4 film that acts as the main absorber. IMPS convincingly shows that the heterojunction configuration prevents surface recombination at the BiVO4 /electrolyte interface, as well as the detrimental surface modification generally observed for WO3 photoelectrodes. The improved electron transport properties of the nr-WO3 substrate and the large area of the heterojunction interface result in better performance for nr-WO3/BiVO4, where at 455 nm water photo-oxidation is quantitative. For the single-phase BiVO4 photoelectrodes, an interesting photocurrent switching phenomenon is observed in the presence of the hole scavenger, indicating the intricate interplay between electron trapping and hole charge transfer; this phenomenon is prevented by the heterojunction structure because of the rapid electron extraction by WO3. These results show that IMPS gives detailed information on the reasons for the excellent performance of heterojunction systems, providing opportunities to design new, more efficient solar water splitting systems.