Quantum dynamics simulations of interfacial electron transfer in sensitized TiO2 semiconductors

Ab initio DFT molecular dynamics simulations are combined with quantum dynamics calculations of electronic relaxation to investigate the interfacial electron transfer in catechol/TiO2-anatase nanostructures under vacuum conditions. It is found that the primary process in the interfacial electron-transfer dynamics involves an ultrafast (tau(1) similar or equal to 6 fs) electron-injection event that localizes the charge in the Ti4+ surface ions next to the catechol adsorbate. The primary event is followed by charge delocalization (i.e., carrier diffusion) through the TiO2-anatase crystal, an anisotropic diffusional process that can be up to an order of magnitude slower along the [-101] direction than carrier relaxation along the [010] and [101] directions in the anatase crystal. It is shown that both the mechanism of electron injection and the time scales for interfacial electron transfer are quite sensitive to the symmetry of the electronic state initially populated in the adsorbate molecule. The results are particularly relevant to the understanding of surface charge separation in efficient mechanisms of molecular-based photovoltaic devices.