Interfacial electron transfer between Fe(II)(CN)₆^4- and TiO₂ nanoparticles: direct electron injection and nonexponential recombination

Abstract

Photoinduced electron-transfer (ET) dynamics in Fe(II)(CN)₆^4- sensitized TiO₂ nanoparticles in D₂O solution are studied by subpicosecond tunable laser spectroscopy in the mid-infrared and visible region. The dynamics of the injected electrons are monitored by the mid-IR absorption of electrons in the semiconductor, and the corresponding dynamics of the adsorbate are monitored by the vibrational spectra of the CN stretching mode region and electronic absorption in the visible. After 400 nm excitation, the forward electron injection time from Fe(II)(CN)₆^4- to TiO₂ occurs in <50 fs, indicating a direct photoinduced charge-transfer process. The back ET from TiO₂ to Fe(III)(CN)₆^3- in the <1 ns time scale is found to be a non-single-exponential process. The best three-exponential fit to the data yields back ET time constants of 3 ps (35%), 40 ps (30%), and >1 ns (35%). Combining with previous measurements in the nanosecond to microsecond time scale (Lu et al. J. Am. Chem. Soc. 1993, 115, 4927 and Vrachnou et al. J. Chem. Soc., Chem. Commun. 1987, 868), the total back ET process is found to be highly nonexponential with time constants ranging from 3 ps to 3 μs.