Band gap tuning in gold nanoparticle decorated TiO₂ films: effect of Au nanoparticle concentration

Highly crystalline titanium dioxide (TiO₂) and gold nanoparticle (Au NP) incorporated TiO₂ films (with varying Au NP concentration), prepared by sol-gel route have been compared as regards their optical absorption spectra. Results indicated an increase in absorbance as well as systematic shift of the absorption edge towards the visible region, with increase in Au NP concentration, upto a critical concentration, beyond which a decrease in absorbance was observed. The optical band gap, calculated from the UV-Vis spectra, shows significant decrease ($△=0.14$ eV) with respect to pristine TiO₂, for this critical concentration (10% Au NP in TiO₂ by volume), above which the decrease ($△$) was found to be less. The morphology of the composite film at this critical concentration revealed a homogeneous microstructure comprising particles of  ∼50 nm. The optimum band gap decrease was found to be associated with a structural change from rutile for the pristine to anatase-rutile mixed phase for the composite film. X-ray diffraction and transmission electron microscopy results indicated a reasonably good match of the lattice spacing for the optimum film. Preliminary current–voltage measurements indicated decrease in electrical resistivity of the optimum Au–TiO₂ film, compared to that of its pristine counterpart. (paper)