Solution-growth and optoelectronic performance of ZnO:Cl/TiO2 and ZnO:Cl/ZnxTiOy/TiO2 core-shell nanowires with tunable shell thickness

Arrays of vertically aligned ZnO:Cl/TiO2 and ZnO:Cl/ZnxTiOy/TiO2 core-shell nanowires (NWs) were prepared by means of the combination of two solution-growth processes. First, single-crystal ZnO NWs with controlled n-type doping were grown on conducting substrates by a low-cost, high-yield and seed-free electrochemical route. These NWs were covered by a titanium oxide shell of tunable thickness mediating successive adsorption-hydrolysis-condensation steps. Using this atomic-layer growth procedure, titania shells with controlled thickness and the anatase TiO2 phase were obtained after sintering at 450 °C. Higher sintering temperatures resulted in the formation of ZnO:Cl/ZnxTiOy/TiO2 core-shell NWs by the interdiffusion of Zn and Ti ions at the ZnO-TiO2 interface. The performance of ZnO:Cl/TiO2 and ZnO:Cl/ZnxTiOy/TiO2 core-shell NWs towards photoelectrochemical (PEC) water splitting was investigated as a function of the titania shell thickness. Furthermore, the performance of such core-shell NWs as photoelectrodes in dye-sensitized solar cells was also characterized. The TiO2 presence at the ZnO:Cl surface promoted a two-fold increase on the produced photocurrent densities, probing their potential for PEC and optoelectronic applications. Electrochemical impedance spectroscopy was used to corroborate the lower resistance for charge transfer between the NWs and the electrolyte in the presence of the TiO2 shell.