Enhanced Charge Collection for Splitting of Water Enabled by an Engineered Three-Dimensional Nanospike Array

Photoelectrochemical (PEC) water splitting is a promising method of converting solar energy to hydrogen fuel from water using photocatalysts. Despite much effort in preparing mesoporous thin films on planar substrates, relatively little attention has been paid to their deposition on three-dimensional (3D) substrates, which could improve electron collection and enhance light-trapping. Here, we report the first synthesis of hierarchically branched anatase TiO2 nanotetrapods, achieved by dissolution and nucleation processes on a ZnO nanotetrapods template. When used as a photoanode for efficient PEC water splitting, the unique branched anatase TiO2 nanotetrapods yielded a photocurrent density of 0.54 mA cm(2) at applied potential of 0.35 V vs RHE, much higher than that of commercial TiO2 nanoparticles under otherwise identical conditions. Moreover, when the nanotetrapods were deposited on an ordered, purposely engineered 3D F-doped tin oxide (FTO) nanospike array, the photocurrent density was upgraded to 0.72 mA cm(2). This large photocurrent enhancement can be attributed to the ultrahigh contact surface area with the electrolyte, which is bequeathed by the hierarchically branched TiO2 nanotetrapods with a skin layer of vertically aligned ultrathin nanospines, as well as the short charge transport distance and enhanced light-trapping due to the peculiar 3D FTO nanospike array we have engineered by design.