One Dimensional Core-Shell ZnO/TiO2 Nanowire Arrays for Visible Light Driven Photoelectrochemical Water Splitting

Photo-electrochemical (PEC) water-splitting offers a promising way for clean, low-cost and environmentally friendly production of H2 by solar energy. Wide-band-gap semiconductor materials such as zinc oxide (ZnO) and titanium dioxide (TiO2) have attracted considerable research interest in the past few decades as photocatalysts due to their unique properties: abundance, low cost and possibility to create nanostructures to improve their transport properties. In addition, ZnO nanowires (NWs) is one of the semiconductors with a high electronic mobility (1000 cm2Vs-1), which gives rise to fast electron transport and lower recombination of charge carriers. However, due to their large band gaps, they are active only under UV irradiation, and ZnO has the drawback of a low photo-corrosion resistance in aqueous media, that reduces their practical application. In this work, we present for the first time a fast and low-cost synthesis procedure for preparation of TiO2/ZnO core-shell heterostructures, in order to combine the merit of these two materials and improve their photocatalytic performances, with high efficiency and durability. In a first step, ZnO NWs were grown on glass electrodes covered with a Fluorine-doped Tin Oxide (FTO) conductive film by hydrothermal route. Subsequently, a shell of TiO2 nanoparticles was deposited in the ZnO NWs by in-situ sol-gel synthesis in a non-acidic solution. The resulting core-shell TiO2-ZnO structures were annealed in Air or N2 flow at 450oC and characterized by X-ray diffraction (Philips X’Pert, Cu Kα, λ = 1.54059 Å), Energy Dispersive Spectroscopy (EDS), Field Emission Scanning Electron Microscopy (FESEM, ZEISS Auriga) and Transmission Electron Microscopy (TEM, FEI Tecnai F20ST operating at 200 kV), clearly showing the formation of a crystalline anatase TiO2 shell completely covering the crystalline structures of wurtzite ZnO NWs, with a thickness dependent on the impregnation time in the titania synthesis bath. Moreover, optical properties and the surface properties of the TiO2/ZnO heterojunction have been further investigated by UV-Vis spectra and X-ray Photoelectron Spectroscopy (XPS), that evidence an increase of absorbance over the entire visible light region and a reduction of the band gap, with respect to the pristine ZnONWs treated under the same annealing conditions. PEC activity, action spectra and carriers dynamics of the samples were studied in NaOH (0.1M) electrolyte, using the prepared materials as working electrode, a Pt foil as counter electrode and a Ag/AgCl reference electrode, under dark and simulated solar light irradiation (using a 450W Xe lamp with a AM 1.5 filter) and employing monochromatic light in all the UV-Visible range.