Enhancement of visible light photoelectrocatalytic activity of ZnO(core)/TiO2(shell) composite by N-doping and decorating with Au0 nanoparticles

The composites consisting of ITO-supported ZnO nanorods covered with TiO2 shell were doped with nitrogen and decorated with gold nanoparticles in order to improve their photocatalytic activity under visible light. N-doped TiO2 (TiO2(N)) was prepared under mild conditions through a simple sol-gel synthesis in the presence of NH4Cl. Such procedure results in formation of a highly porous shell of TiO2(N) on the ZnO nanorods. The gold nanoparticles (AuNPs) of the size 7–25 nm were grafted onto the surface of TiO2 as well as TiO2(N) by a photodeposition method from aqueous solution of [AuCl(4-x)(OH)x]− precursor at pH 6.7. The composition and microstructure of the prepared samples were characterized by SEM, HR-TEM, HAADF-STEM, XRD, XPS and UV–vis spectroscopy. The photocatalytic properties of ZnO/TiO2/Au and ZnO/TiO2(N)/Au composites were studied under illumination with Xe lamp equipped with a 400 nm cut-off filter. It was found that low AuNPs loading (0.37% at.) resulted in 60% enhancement of photocatalytic decolorization of methylene blue (MB) under visible light with respect to Au-free sample owing to Au-plasmonic effect. Also, a simultaneous N-doping and AuNPs-decoration allows to multiply by three the photocurrent in photoelectrochemical water oxidation at the potential of 0.8 V vs. Ag/AgCl. We also showed that the elaborated Au-loaded composites possess a strong catalytic activity in O2 reduction under a small negative bias (−0.25 V vs. Ag/AgCl) in dark. We proved that illumination of the polarized sample with visible light enhances oxygen reduction and the produced active oxygen species (O2 −, H2O2 and OH) improve significantly degradation of MB, which was confirmed by total organic carbon (TOC) measurements. We demonstrated that rapid decomposition of MB is possible even in the presence of Na2SO4 which completely inhibits the process on the non-polarized catalyst.