ZnO nanorods covered with a TiO2 layer: simple sol-gel preparation, and optical, photocatalytic and photoelectrochemical properties

In this work, composite core-shell ZnO/TiO2 materials were fabricated by deposition of TiO2 layers via a sol-gel method onto ZnO nanorods hydrothermally grown on an ITO electrode. Two approaches to the sol-gel procedure resulted in strongly different morphologies and thicknesses of the deposited TiO2 layer, as shown in electron microscopy studies. The decrease of the optical band gap energies of the ZnO/TiO2 composites by about 0.2-0.3 eV with respect to the TiO2 nanoparticles and ZnO nanorods was determined from UV-Vis diffuse reflectance spectra. The photocatalytic activities of the systems were determined by investigation of the decolorization of Methylene Blue (MB) in aqueous solution, irradiated with monochromatic light (LED, 400 nm). Depending on the TiO2 layer morphology, and therefore the accessibility of the ZnO/TiO2 heterojunction, the photocatalytic activities of the ZnO/TiO2 composite systems showed 2-fold and 6-fold higher efficiency than that of the non-covered ZnO system. Addition of a small amount of methanol (3% v/v) to the MB test solution resulted in a significant drop in performance of all the ZnO/TiO2 samples, suggesting that the photodegradation mechanism proceeds mostly via photogenerated holes and less likely via photoexcited electrons. Photoelectrochemical tests of ITO/ZnO/TiO2 electrodes carried out in an acetonitrile solution containing ferrocene as a redox probe showed an increase in photocurrents in comparison to ITO/ZnO electrodes upon illumination with a xenon arc lamp.