Interfacial electron transfer in colloidal TiO2 accelerated by surface adsorption and the electric double layer

Abstract

Interfacial electron transfer from colloidal TiO₂ to methyl viologen (MV²⁺) has been studied for a wide range of ionic strengths and pH values. As in previous work, the rate increases with increasing pH as a result of a combination of electrostatic and thermodynamic factors. The slopes of log k vs. pH plots range from 0.3 to 1.1 as the electrolyte concentration is decreased from 1.0 to 0.001 mol dm^–3. Also the slope for a neutral viologen (PVS) was only 0.18. This shows that electrostatics are the dominant term. By a comparision with conventional electrode kinetics, the dielectric constant in the Helmholtz layer was found to be 63, which is close to the value of 78 for pure water. This situation applies when there is no evidence of ion adsorption, i.e. within 3 pH units of the PZZP and at salt concentrations < 1.0 mol dm^–3. It was also found that the electrostatic behaviour can be described equally well by considering the primary kinetic salt effect. Evidence for ion adsorption was found for NaCl or MgCl₂ electrolytes at high pH (> pH 9) and for Na₂SO₄ at low and high pH. This can result in significant changes in the reaction rates. At pH 9 NaCl and MgCl₂ reduced the rate constants by a factor of 280 and 800, respectively. At pH 3, 0.02 mol dm^–3 Na₂SO₄ was sufficient to increase the rate by a factor of 120, whereas 0.1 mol dm^–3 NaCl did not appear to adsorb on the colloid and only increased the rate by a factor of 9. The yield of MV^˙+ can be used as a direct probe for surface-oxidation reactions in TiO₂ colloids. The net yield of MV^˙+ after a 25 ns laser flash depends on the efficiency with which the photogenerated holes can react with electron donors. This was studied in the absence of poly(vinyl alcohol) at high (pH 11) and low (pH 3) pH. In both cases the efficiency was in the order glycerol > ethylene glycol, formaldehyde ≫ CH₃O[CH₂]₂OCH₃, methanol, isopropanol and suggests that the polyalcohol functional group helps in the adsorption of the alcohol on the TiO₂ particles. This may explain in part the success of PVA as a polymer support in work with TiO₂ colloids.