Dependence of photocatalysis on charge carrier separation in Ag-doped and decorated TiO2 nanocomposites

Two model Ag-TiO2 nanocomposite systems have been investigated with the aim of understanding the relationship between the photocatalytic efficiency and the contributions of various structures resulting in charge carrier separation, charge transfer and extension of light absorption to the visible range. The first model system is based on a composite scheme consisting of TiO2 decorated with Ag nanoparticles (NPs). The second composite scheme consists of TiO2 simultaneously doped and decorated with Ag NPs. Ag-TiO2 nanocomposites containing various concentrations (0.5 to 6 at%) of Ag nanoparticles were prepared, and the photocatalytic degradation of p-nitrophenol (PNP) and methylene blue (MB) were investigated. Compared to the Ag-decorated samples, the Ag-doped and decorated samples show enhanced activity due to the synergistic effects of the Schottky barrier, doping and the anatase + brookite mixed phase. It was observed from PL and fluorescence lifetime studies that the charge carrier lifetime correlates with the relative photonic efficiency of the photocatalytic activity. An optimum Ag concentration is required to obtain the maximum lifetime of the photo-generated charges. The photocatalytic activity does not show any correlation to the amount of visible light absorption, to plasmonic effects, or to band-gap reduction. Decorating with silver nanoparticles aids electron transfer and creates electron traps in the form of oxygen vacancies on the surface of TiO2. We show that for the Ag-TiO2 system, the photocatalytic activity is directly proportional to the lifetime of the photogenerated charge carrier.