Photo-Electron Injection into TiO2: Quantum Dot vs. Graphene

We presented a detailed comparison on the similalaries and differences of the ultrafast photoinduced electron transfer (ET) from two kinds of donor species, namely PbSe quantum dot (QD) and graphene, into the acceptor TiO$_{2}$ surface via \textit{ab initio} time domain density functional theory simulations. The main diffrences stem from the size and dimensionality of the donor species and donor-acceptor bonding characteristics. For exmaple, the QD is localized species and composed by heavy atoms that connected to TiO$_{2}$ surface via chemical bonds. In contrast, the graphene layer is delocalized two-dimensional object that attached to TiO$_{2}$ substrate by van der Waals interaction and partial chemical bonds. The ET mechanism depends on the dimensionality of the donor and donor-acceptor chemical bonding. The injection from the localized donor states of the QD is dominatly adiabatic. In contrast, the injection from the two-dimensional graphene into TiO$_{2}$ exhibits prominently nonadiabatic (NA) component. The NA mechanism is efficient for the graphene/TiO$_{2}$ composites because it is delocalized over two dimensions and is able to couple with a dense manifold of delocalized TiO$_{2}$ conduction band states and weak coupling as well. The high density of acceptor states in this case favors the NA mechanism.