Dynamics of electron injection and recombination of dye-sensitized TiO2 particles

Electron injection and recombination dynamics of the dye fluorescein 27 adsorbed on a titanium dioxide (TiO2, anatase) surface were studied, following femtosecond pulse excitation of the 0-0 transition of the dye molecule. Nanosized colloidal particles (average diameter similar to 2.4 nm) were employed as electron accepters in optically transparent aqueous suspensions. Large changes of the dye absorption spectrum in the presence of TiO2 colloids confirmed strong adsorption of the dye to the TiO2 surface. The strong quenching of steady-state fluorescence under such conditions is correlated to the electron injection process. Transient absorption spectra and kinetics were recorded in the spectral region from 410 to 950 nm. While the transient absorption signal in the interval 410-600 nm originates primarily from the dye molecule, the observed signal in the range 600-950 nm is assigned to intra-band transitions or free carrier absorption of the electron injected into the semiconductor. The decay time of the stimulated emission of the dye and the rise time of the absorption of the injected electron reveal that electron injection into the TiO2 colloid occurs with a characteristic time constant of 300 fs. In contrast, the analysis of the overall signal decay resulted in multiphasic recombination times, ranging from similar to 10 picoseconds up to nano- or even microseconds. A red shift of the absorption maximum of the semioxidized radical anion (the product of electron injection) within similar to 25 ps suggests vibrational cooling, solvation, or structural relaxation of the initially hot product. This provides strong evidence for a dynamic process involving relaxation of the potential energy of the radical on this time scale, leading to a dramatic decrease of the recombination reaction rate with time.