Thermal/Electrochemical Growth and Characterization of One-Dimensional ZnO/TiO2 Hybrid Nanoelectrodes for Solar Fuel Production

One-dimensional core-shell nanostructured electrodes made of aligned TiO2 nanotubes (NTs)/ZnO were fabricated via the thermal sputtering of ZnO layers on anodically fabricated titania NTs. Increasing the ZnO sputtering time resulted in the increase in the shell thickness, which in turn showed different morphological, structural, and optical characteristics. The crystallinity of the core nanotubes was found to be a determinant factor in the formation of the NTs/ZnO heterojunctions as revealed by field emission scanning electron microscopy, glancing angle X-ray diffraction, X-ray photoelectron spectroscopy, and Raman analyses. Used as a photoanode to photoelectrochemically split water, the TiO2/ZnO heterojunction sputtered for 53 min showed an almost 80% increase in the photoconversion efficiency (7.3%) compared to that of pure TiO2 NTs (4.1%) under light illumination (320-400 nm, 100 mW/cm(2), 0.5 M Na2SO4). This was confirmed via the incident photon conversion efficiency (IPCE) measurements, which showed an enhancement in the charge carriers' collection efficiency in the heterojunction electrodes compared to pure titania nanotube films. The decrease in the band gap (3.06 eV), the high surface area, and the difference in the band positions between ZnO and TiO2 are thought to be the main reasons for the observed enhancement in the photoactivity.