Photoelectrochemical water splitting strongly enhanced in fast-grown ZnO nanotree and nanocluster structures

Abstract

We demonstrate selective growth of ZnO branched nanostructures: from nanorod clusters (with branches parallel to parent rods) to nanotrees (with branches perpendicular to parent rods). The growth of these structures was realized using a three-step approach: electrodeposition of nanorods (NRs), followed by the sputtering of ZnO seed layers, followed by the growth of branched arms using hydrothermal growth. The density, size and direction of the branches were tailored by tuning the deposition parameters. To our knowledge, this is the first report of control of branch direction. The photoelectrochemical (PEC) performance of the ZnO nanostructures follows the order: nanotrees (NTs) > nanorod clusters (NCs) > parent NRs. The NT structure with the best PEC performance also possesses the shortest fabrication period which had never been reported before. The photocurrent of the NT and NC photoelectrodes is 0.67 and 0.56 mA cm$^{−2}$ at 1 V vs. Ag/AgCl, respectively, an enhancement of 139% and 100% when compared to the ZnO NR structures. The key reason for the improved performance is shown to be the very large surface-to-volume ratios in the branched nanostructures, which gives rise to enhanced light absorption, improved charge transfer across the nanostructure/electrolyte interfaces to the electrolyte and efficient charge transport within the material.