SURFACE COMPLEXATION OF COLLOIDAL SEMICONDUCTORS STRONGLY ENHANCES INTERFACIAL ELECTRON-TRANSFER RATES

Surface complexation of colloidal titanium dioxide by monodentate and bidentate benzene derivatives, i.e., benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, salicylic acid, and catechol was investigated and was found to obey the Langmuir isotherm. The adsorption constants in water/methanol mixtures (90/10 v/v) at pH 3.6 decrease from 2 x 10(5) for terephthalate to 2 x 10(3) for benzoate. Electrophoretic measurements show the adsorption to be accompanied by a decrease of the point of zero zeta potential (PZZP), the shift observed for salicylate being 0.5 pH unit. At pH 3.6, the influence of the adsorbate on the zeta potential of the TiO2 particles is relatively small. The largest effect is observed with isophthalate, which decreases zeta from 78 to 51 mV. Laser photolysis experiments indicate that surface complexation of TiO2 by these benzene derivatives drastically accelerates the electron transfer from the conduction band of the colloidal oxide to acceptors in solution, i.e., methylviologen (MV2+) and oxygen. The rate enhancement depends greatly on the structure and chemical nature of the adsorbate. At monolayer coverage, isophthalate enhances the rate of interfacial electron transfer to MV2+ 1700 times while terephthalate gives a 133-fold rate increase. Monodentate benzoate accelerates the interfacial electron transfer only by a factor of 3. Similar effects were observed for oxygen as electron acceptor. Trapping of electrons by Ti(IV) surface states and the removal of such traps by complexation with the benzene derivatives is invoked to rationalize these observations.