Correlations between photocatalytic activity and chemical structure of Cu-modified $\mathrm{TiO_{2}–SiO_{2}}$ nanoparticle composites

Copper-modified TiO$_{2}$–SiO$_{2}$ photocatalysts were prepared by sol–gel method based on organic copper, silicon and titanium precursors. Copper concentration varied from 0.1 to 3.0 mol%. A widely applied model reaction of photocatalytic oxidation of terephtalic acid (TPA) in water solution was used in order to evaluate the catalytic activities of elaborated samples. The crystal structures of the titania components of all tested titania–silica species were studied using XRD analysis. The influence of Cu$^{2+}$ cation incorporation on the crystal structure of titania, as well as the chemical states and the neighbouring structures of copper cations, have been examined by means of Cu K-edge EXAFS and XANES analysis. The experimental data show that there is a ten times increase in photocatalytic activity when TiO$_{2}$–SiO$_{2}$ matrix is modified with 0.1 mol% of Cu. It can be supposed that an enhancement of photocatalytic activity of low-concentrated copper-modified titania–silica nanocomposites is probably due to a close attachment of Cu$^{2+}$ cations to the surfaces of photocatalytically active TiO$_2$ nanoparticles. In this case, Cu$^{2+}$ cations may possibly act as free electron traps reducing the intensity of recombination between opposite free charge carriers (electrons, holes) available at the photocatalyst’s surface