Study of Interfacial Charge-Transfer Complex on TiO₂ Particles in Aqueous Suspension by Second-Harmonic Generation

We report the first direct observation of an interfacial charge-transfer complex using second-harmonic spectroscopy. The second-harmonic spectrum of catechol adsorbed on 0.4 micron-sized TiO2 (anatase) colloidal particles in aqueous suspension reveals a charge-transfer band centered at 2.72 eV (456 nm). In addition, the adsorption isotherm of catechol on the colloidal TiO2 suspension was obtained and gave an excellent fit to the Langmuir adsorption model. From this, we infer the free energy of the adsorption to be ¢G ° )-6.8 kcal/mol. 1.