WO/BiVO: impact of charge separation at the timescale of water oxidation.

The four hole oxidation of water has long been considered the kinetic bottleneck for overall solar-driven water splitting, and thus requires the formation of long-lived photogenerated holes to overcome this kinetic barrier. However, photogenerated charges are prone to recombination unless they can be spatially separated. This can be achieved by coupling materials with staggered conduction and valence band positions, providing a thermodynamic driving force for charge separation. This has most aptly been demonstrated in the WO/BiVO junction, in which quantum efficiencies for the water oxidation reaction can approach near unity. However, the charge carrier dynamics in this system remain elusive over timescales relevant to water oxidation (μs-s). In this work, the effect of charge separation on carrier lifetime, and the voltage dependence of this process, is probed using transient absorption spectroscopy and transient photocurrent measurements, revealing sub-μs electron transfer from BiVO to WO. The interface formed between BiVO and WO is shown to overcome the "dead-layer effect" encountered in BiVO alone. Moreover, our study sheds light on the role of the WO/BiVO junction in enhancing the efficiency of the water oxidation reaction, where charge separation across the WO/BiVO junction improves both the yield and lifetime of holes present in the BiVO layer over timescales relevant to water oxidation.