WO3/BiVO4: 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 WO3/BiVO4 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 (mu s-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-mu s electron transfer from BiVO4 to WO3. The interface formed between BiVO4 and WO3 is shown to overcome the dead-layer effect encountered in BiVO4 alone. Moreover, our study sheds light on the role of the WO3/BiVO4 junction in enhancing the efficiency of the water oxidation reaction, where charge separation across the WO3/BiVO4 junction improves both the yield and lifetime of holes present in the BiVO4 layer over timescales relevant to water oxidation.