FT-EPR STUDY OF PHOTOINDUCED ELECTRON-TRANSFER AT THE SURFACE OF TIO2NANOPARTICLES

Photoinduced electron transfer at the surface of TiO2 nanoparticles in ethanol has been studied with Fourier transform EPR (FT-EPR). Measure ments were performed on three systems: (1) a solution of coumarin 343 (C343) dye and methyl viologen (MV2+) in ethanol, (2) a colloidal solu tion of TiO2 in ethanol containing MV2+, and (3) a colloidal solution of TiO2 in ethanol containing both C343 and MV2+. In the C343/MV2+ sys tem, pulsed-laser (355 nm) excitation of the dye results in MV2+ reduc tion. The rate constant of the photoinduced electron-transfer reaction , derived from the time profile of the FT-EPR spectrum of the MV+ radi cal, was found to be similar to 5 x 10(9) M-1 s(-1), consistent with a near diffusion controlled reaction. Bandgap excitation (308 nm) of th e semiconductor particles in the TiO2/MV2+ system also gives rise to M V+ formation. In this case the FT-EPR signal growth can be described b y a single exponential with rate constant k(f) = 0.41 x 10(6) s(-1). T he kinetics indicates that MV2+ reduction involves photogenerated elec trons trapped an the TiO2 particles and is controlled by interfacial c harge transfer rather than diffusive encounters of acceptor molecules with TiO2 particles, Dye-sensitized formation of MV+ radicals is obser ved as well following laser (355 nm) excitation of the C343/MV2+/TiO2 system. However, whereas the MV+ spectrum produced by the C343/MV2+ sy stem reaches its maximum intensity around 100 ns after laser excitatio n, in the presence of TiO2 the maximum is found tens of microseconds a fter the laser pulse. The time profile of MV+ formation in this case f ollows first-order kinetics with a time dependent rate constant, k(f) = k(0)f(alpha-1), where both k(0) and alpha are a function of the degr ee of dye adsorption onto the TiO2 particles and the pH of the solutio n, In this system, the trapped electrons responsible for MV2+ reductio n are generated by excitation of adsorbed dye molecules, which leads t o electron injection into the conduction band of the semiconductor. Dy e-sensitization of the TiO2 particles strongly increases the free radi cal yield. However, as the surface coverage by dye molecules approache s saturation level, the rate of electron transfer from semiconductor p articles to acceptor molecules in solution is strongly attenuated.