Formation and decay of methylviologen radical cation dimers on the surface of colloidal CdS: separation of two- and three-dimensional relaxation

Methylviologen molecules adsorbed to CdS particles can be reduced to radical cations in situ by optical excitation of the semiconductor. Confinement of the viologen to the surface results in a dramatic increase in the rate of radical cation dimer formation, compared with the bulk solution reaction, arising from the reduction of dimensionality of the reaction space. Subsequently, the dimers are destroyed via reoxidation by unreduced viologen from the bulk solution.