In the field of solar water splitting, searching for and modifying bulk compositions have been the conventional approaches to enhancing visible-light activity. In this work, manipulation of heterointerfaces in ZnS−GaP multilayer films is demonstrated as a successful alternative approach to achieving visible-light-active photoelectrodes. The photocurrent measured under visible light increases with the increasing number of interfaces for ZnS−GaP multilayer films with the same total thickness, indicating it to be a predominantly interface-driven effect. The activity extends to long wavelengths (650 nm), much longer than those expected for pure ZnS and also longer than those previously reported for GaP. Density functional theory calculations of ZnS−GaP multilayers predict the presence of electronic states associated with atoms at the interfaces between ZnS and GaP that are different from those found within the layers away from the interfaces; these states, formed due to unique bonding environments found at the interfaces, lead to a lowering of the band gap and hence the observed visible-light activity. The presence of these electronic states attributed to the interfaces is confirmed by depth-resolved X-ray photoelectron spectroscopy. Thus, we show that interface engineering is a promising route for overcoming common deficiencies of individual bulk materials caused by both wide band gaps and indirect band gaps and hence enhancing visible-light absorption and photoelectrochemical performance.

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A high photocurrent, particularly under visible-light wavelengths, is critical for developing a semiconductor photoelectrode for efficient solar-to-hydrogen conversion. Here, we demonstrate a ZnS-GaP solid solution thin film grown on a silicon substrate by pulsed laser deposition, where the growth conditions are tailored to promote intermixing throughout the entire film thickness. The photocurrent density of this solid solution film reaches a maximum of ∼27 μA/cm2 at ∼0.9 V bias, which is ∼3 times higher than that of a comparable multilayered ZnS-GaP film, where ZnS and GaP form distinct layers. In addition, the solid solution film shows up to 50 times stronger photosensitivity compared to the multilayered film. Examination of the local atomic structure and nanoscale chemistry of the solid solution thin film using transmission electron microscopy and energy-dispersive X-ray spectroscopy techniques revealed the formation of quaternary solid solution (Ga,Zn)­(P,S) and ternary (Ga,Zn)­S b phases, as well as some trace amounts of binary GaS y . These phases have previously been shown to have a direct band gap in the energy range of visible light. We thus attribute the enhanced photocurrent and photosensitivity in the solid solution film to the presence of the aforementioned phases as well as defects.