The influence of Schottky barrier height onto visible-light triggered photocatalytic activity of TiO2 + Au composites

Composites of different TiO2 supports (anatase nanoparticles (TNP) and nanorods (TNR)) and 1 wt% of Au were synthesized upon wet impregnation preparation procedure. The size of formed Au ensembles in the composites was influenced by the difference in the zeta potential value (pHPZC) of TiO2 supports. The positive surface charge of TNR nanorods during the wet impregnation synthesis and its higher specific surface area positively influenced the formation of Ti-s-O-Au(III) complex. In turn, the average Au cluster size in the TNR + Au composite was larger (9.4 nm) than in the TNP + Au composite (2.4 nm) where the surface of the TNP nanoparticles was negatively charged. In spite of the different size of Au ensembles the UV-Vis DR spectra of composites exhibited a broad absorption peak at 550 nm, which is typical for the plasmonic behaviour of Au clusters. A detailed XPS analysis of the valence band maxima (VBM) showed that the value of Schottky barrier height (SBH) in the TNP + Au composite (0.31 eV) was almost double compared to the one in the TNR + Au composite (0.16 eV). The visible-light generated "hot electrons" in Au clusters of the TNP + Au composite need more energy and longer time to overcome the SB when injected into the TNP, compared to "hot electrons" generated in the TNR + Au composite exhibiting lower SBH. Due to this obstacle TNP + Au "hot electrons" have a higher potential to recombine with the generated holes in Au clusters than "hot electrons" injected into the TNR support. The higher specific surface area of the TNR support presents an additional advantage, which prolongs the "lifetime" of electrons in the TNR + Au composite. The electrons can use a larger area to generate reactive oxygen species (ROS) or oxidize adsorbed substrates compared to electrons in the TNP + Au composite. This was proved by: (i) the results of electrochemical impendance spectroscopy (EIS) measurements, (ii) photoluminescence (PL) emission spectra of catalysts, and (iii) measurements with the use of ROS scavengers. The higher photocatalytic activity of the TNR + Au composite was also well expressed by the visible-light assisted photocatalytic oxidation of aqueous solutions of bisphenols.