Among the most promising photocatalysts for solar energy conversion into hydrogen as a clean fuel via photoelectrochemical (PEC) water splitting, two ternary metal oxides able to absorb a relatively large portion of the solar spectrum, i.e., BiVO₄ and CuWO₄, represent good candidates to be employed as photoanodes in the demanding water oxidation reaction [1].

In particular, BiVO₄, having a 2.4 eV band gap energy, has emerged as the leading photocatalyst for this application despite its poor electron transfer ability and slow water oxidation kinetics. Two main strategies have thus been pursued to improve the PEC performance of BiVO₄, consisting in either i) coupling it with another suitable semiconductor oxide in a heterojunction, or ii) doping it with hexavalent metal ions such as Mo⁶⁺.

Indeed, significantly higher values of Incident Photon to Current Efficiency (IPCE) have been attained at wavelengths shorter than 500 nm when BiVO₄ was coupled with WO₃ in a WO₃/BiVO₄ heterojunction, by exploiting the excellent visible light harvesting properties of BiVO₄ combined with the superior conductivity of photoexcited electrons, typical of WO₃. Due to the favourable band alignment between the two oxides, photopromoted electrons in BiVO₄ are expected to migrate into WO₃ and then rapidly to the external circuit, while photoproduced holes may accumulate in BiVO₄. Selected case studies dealing with the performances exhibited under different irradiation configurations by home-made thin coupled electrodes, prepared through variable deposition techniques, will be presented [2-4]. At the same time, the multifaceted role of Mo⁶⁺ doping onto both the bulk and surface properties of BiVO₄ will be clarified by means of a unique combination of morphological and PEC analyses [5, 6].

On the other hand, an efficient use of CuWO₄ as photoanode material requires to overcome its severe internal charge recombination due to intra-gap states, acting as electron traps, as revealed through a PEC investigation coupled with ultrafast transient absorption analyses [7]. This issue has been mitigated by the 50 at.% molybdenum for tungsten substitution [8], with the development of CuW_0.5Mo_0.5O₄ photoanodes, exhibiting a 4-fold increase of the bulk charge carrier separation efficiency compared to pristine CuWO4, as clearly evidenced by intensity modulated photocurrent spectroscopy (IMPS) analysis [9], in full agreement with the results of PEC measurements performed in the presence of sacrificial agents or cocatalysts [10]. Optimized CuW_0.5Mo_0.5O₄ combined with BiVO₄ in a heterojunction finally exhibited a definitely superior PEC performance compared to the individual components, with a synergistic charge separation improvement in the 350-480 nm range under frontside irradiation through a BiVO₄ layer thick enough to absorb most of the incident light [9].