Multi-bandgap-sensitized ZnO nanorod photoelectrode arrays for water splitting: An x-ray absorption spectroscopy approach for the electronic evolution under solar illumination

This investigation demonstrates an environmentally friendly inorganic light-harvesting nanostructure. This system provides a stable photoelectrochemical platform for the photolysis of water. The device is constructed by first building up an array of ZnO nanowires and then incorporating indium phosphide (InP) nanocrystals into them. A different-sized quantum dots (QDs) sensitization of the ZnO nanowire array for splitting water with a substantially enhanced photocurrent was demonstrated. InP QDs of various sizes are utilized as simultaneous sensitizers of the array of ZnO nanowires, and this multi-bandgap sensitization layer of InP QDs can harvest complementary solar light in the visible region while the ZnO nanostructures absorb the UV part of solar light. A photocurrent of 1.2 mA/cm2 at +1.0 V was observed; it was more than 108% greater than the photocurrent achieved by bare ZnO nanowires. Solar illumination measurements investigated the contribution from photoelectrochemical response and effect in unoccupied states of conduction band. ZnO decorated with single/three-sized InP QDs had a significant increase in photogenerating electrons in 4p orbital, which indicated this increase of photogenerating electrons could be attributable to the absorption of InP QDs in visible region and the photogenerating electrons transfer from conduction band of InP to that of ZnO. The photogenerating electron in conduction band can significantly response to the photoactivity collected in photoelectrochemical measurement, and the contribution of photoresponse from ZnO nanowire or InP quantum dots can be distinguished by comparing the spectra collected under dark/illumination condition.