FT-EPR Study of Photoinduced Electron Transfer at the surface of TiO2 nanoparticles

D.M. MARTINO; H. VAN WILLIGEN; M.T. SPITLER

JOURNAL OF PHYSICAL CHEMISTRY B

Año: 1997 vol. 101 p. 8914 - 8919

1089-5647

Photoinduced electron transfer at the surface of TiO2 nanoparticles in ethanol has been studied with Fourier transform EPR (FT-EPR). Measurements were performed on three systems: (1) a solution of coumarin 343 (C343) dye and methyl viologen (MV2+) in ethanol, (2) a colloidal solution of TiO2 in ethanol containing MV2+, and (3) a colloidal solution of TiO2 in ethanol containing both C343 and MV2+. In the C343/MV2+ system, pulsed-laser (355 nm) excitation of the dye results in MV2+ reduction. The rate constant of the photoinduced electron-transfer reaction, derived from the time profile of the FT-EPR spectrum of the MV+ radical, was found to be  5  109 M-1 s-1, consistent with a near diffusion controlled reaction. Bandgapexcitation (308 nm) of the semiconductor particles in the TiO2/MV2+ system also gives rise to MV+ formation.In this case the FT-EPR signal growth can be described by a single exponential with rate constant kf ) 0.41  106 s-1. The kinetics indicates that MV2+ reduction involves photogenerated electrons trapped on the TiO2 particles and is controlled by interfacial charge transfer rather than diffusive encounters of acceptor molecules with TiO2 particles. Dye-sensitized formation of MV+ radicals is observed as well following laser (355 nm) excitation of the C343/MV2+/TiO2 system. However, whereas the MV+ spectrum produced by theC343/MV2+ system reaches its maximum intensity around 100 ns after laser excitation, in the presence of TiO2 the maximum is found tens of microseconds after the laser pulse. The time profile of MV+ formation in this case follows first-order kinetics with a time dependent rate constant, kf ) k0tR-1, where both k0 and R are a function of the degree of dye adsorption onto the TiO2 particles and the pH of the solution. In this system, the trapped electrons responsible for MV2+ reduction are generated by excitation of adsorbed dye molecules, which leads to electron injection into the conduction band of the semiconductor. Dye-sensitization of the TiO2 particles strongly increases the free radical yield. However, as the surface coverage by dyemolecules approaches saturation level, the rate of electron transfer from semiconductor particles to acceptor molecules in solution is strongly attenuated.