The main role of a cobalt phosphate catalyst on BiVO ₄ is to improve the photocurrent by passivating the surface and suppressing recombination.

The functionality of a photoactive semiconductor (i.e., photocatalysts, photoelectrodes, etc.) is largely dictated by three key aspects: (i) band gap; (ii) absolute potentials of the conduction band minimum and the valence band maximum; and (iii) bulk and surface charge carrier dynamics. Their relevance to governing the energetics and the photo(electro)chemical mechanisms of the semiconductor has prompted development of a multitude of characterization tools to probe the specific characteristic of the material. This review aims to summarize the current experimental techniques, including the conventional and the state-of-the-art tools, directed at examining the key aspects (i), (ii), and (iii) of semiconductors. Although not being exhaustive, this didactic review can be useful to apprise the research community of the sophisticated research tools currently available for characterization of photo(electro)catalyst semiconductors as well as to bridge the multidisciplinary knowledge.

Several application fields can benefit from solar-hydrogen technologies via specific short-term and long-term pathways.

Metal oxide semiconductors are promising photoelectrode materials for solar water splitting due to their robustness in aqueous solutions and low cost. Yet, their solar-to-hydrogen conversion efficiencies are still not high enough for practical applications. Here we present a strategy to enhance the efficiency of metal oxides, hetero-type dual photoelectrodes, in which two photoanodes of different bandgaps are connected in parallel for extended light harvesting. Thus, a photoelectrochemical device made of modified BiVO₄and α-Fe₂O₃as dual photoanodes utilizes visible light up to 610 nm for water splitting, and shows stable photocurrents of 7.0±0.2 mA cm⁻²at 1.23 V_RHEunder 1 sun irradiation. A tandem cell composed with the dual photoanodes–silicon solar cell demonstrates unbiased water splitting efficiency of 7.7%. These results and concept represent a significant step forwarden routeto the goal of >10% efficiency required for practical solar hydrogen production.