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 co mplex using second-harmonic spectroscopy. The second-harmonic spectrum of c atechol 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 Save an excellent fit to the Langmuir adsorpti on model. From this, we infer the free energy of the adsorption to be Delta G degrees = -6.8 kcal/mol.