The measure of TiO2 photocatalytic efficiency and the comparison of different photocatalytic titania

The dependence of hydroxyl radical yield on the substrate concentration, pH, oxygen concentration, and light intensity, using different TiO2 preparations, was investigated. The quantum yields of formaldehyde and carbon dioxide, obtained with the aid of an integrating sphere in the methanol and formate systems, respectively, were used to derive the primary yield of hydroxyl radicals. The limiting yield of (OHads)-O-., achieved at high scavenger concentrations, is independent of the nature of the scavenger (methanol or formate) and is nearly constant in the range 1 < pH < 12. While the presence of air induces a 15-fold increase in product yield, compared to air-free systems, further increase of oxygen concentration has only a small effect. The effect of changing the (OH)-O-. scavenger concentration, pH, and light intensity corresponds to simple competition between pseudo first- and second-order reactions. Under the conditions of the present work the scavengers CH3OH, HC0O(2)(-) and HCO2H react only with the hydroxyl radical, not with its precursor hole. Hydrogen peroxide, which is produced in the above systems upon reduction of oxygen by both TiO2 electrons and the organic free radicals, is further reduced to (OH)-O-. radicals, doubling the products yield. A rigorous treatment of the effect of absorbed photon density on the quantum yield, based on the simple square root dependency, enables the derivation of a constant, K-d, which is related to the efficiency of surface water oxidation to hydroxyl radicals, relative to the electron hole recombination at the given pH and oxygen concentration. This parameter is a property of the TiO2, and is not affected by the experimental conditions such as wavelength and layer thickness. Comparison of K-d using different types of TiO2 shows that in contrast to commonly assumed, preparation methods and sizes of aged titanium dioxide nanoparticles have only a relatively small effect on the basic photocatalytic efficiency. Comparison of the quantum yield in TiO2 layers to results in aqueous suspensions using similar photon flux shows moderate differences. The results are discussed in view of the much larger absorbed photon density in the suspended nanoparticles combined with much larger volume in the bulk of the solution.