Bandgap determination and charge separation in Ag@TiO2 core shell nanoparticle films

The photocatalytic activity of TiO2 under sunlight irradiation depends on the bandgap energy. Due to the relatively low solar intensity in the UV region (<10%) and the fact that the bandgap of TiO2 is usually greater than 3 eV (below 400 nm), many attempts have been made to shift the bandgap towards lower energies. Here, we investigate the structure, chemical composition, bandgap shift and charge transfer processes of Ag@TiO2 core-shell nanoparticle thin films by field emission scanning electron microscopy, atomic force microscopy, XPS, and UV-Vis spectroscopy. As a solid support, Au-coated Si wafers and Si surface covered with a native oxide were used and homogenously covered by Ag@TiO2 core-shell nanoparticles with overall film thicknesses of 80-100 nm and size distributions between 8 and 15 nm. The shell thickness of the adsorbed Ag@TiO2 particles was estimated to be 1.5-2.0 nm. The effect of the Ag core on the bandgap of TiO2 and photoinduced charge separation of Ag@TiO2 nanoparticle films was studied by UV-Vis reflectance spectroscopy using the Kubelka-Munk formalism. Films of Ag@TiO2 core-shell nanoparticles revealed a substantially reduced bandgap of 2.75 eV (corresponding to 450 nm), and an electron charge transfer to the Ag core occurring upon UV irradiation on nonconductive surfaces. These features make Ag@TiO2 particulate films a promising candidate for photocatalytic surfaces under sunlight irradiation. Copyright (C) 2010 John Wiley & Sons, Ltd.