Photoelectrochemical properties of nanocrystalline ZnS discrete versus continuous coating of ZnO nanorods prepared by electrodeposition

We developed nanostructured photoanodes for photoelectrochemical (PEC) water splitting and hydrogen generation. They are based on ZnO nanorods electrodeposited on conductive ITO glass on which ZnO@ZnS heterojunctions were formed using two different approaches. In the first case, the ZnO nanorods were sulfided by a prolonged contact with Na2S aqueous solution, while in the second one, they were immersed in an alcoholic solution of 2 nm sized polyol-made ZnS quantum dots (QDs). Transmission electron microscopy showed that a continuous thin layer of ZnS is formed around ZnO leading to a core@shell structure in the first case, while discrete QD aggregates were grafted at the surface of these rods leading to a kind of tologyin, in the second case. PEC properties of both composite films were measured, using a home-made electrochemical cell and illuminating the anodes with a Xenon lamp. A net enhancement of the photocurrent was observed when the ZnS coating was processed, suggesting a low carrier recombination rate, a higher efficiency toward water oxidation, and then electron transfer to the used cathode (Pt wire) for H+ reduction and H-2 generation. Interestingly, the performances of the two composite films were found to be comparable, suggesting that a discrete coating of the ZnO nanorods by a small amount of preformed ZnS QDs is enough to improve their properties for the desired application.