Photoactivity of Molecule-TiO2 Clusters with Time-Dependent Density-Functional Theory

The interaction of molecules with titanium oxide substrates may lead to substantial modifications of their optical properties, in particular a red shift of the absorption spectrum compared to that of bare titania. In the present paper we discuss the role of the interface between two molecules, catechol and dopamine, with gas-phase (TiO2)(N) clusters (N = 2, 4, 6). We studied, for the interface, the bidentate modes (the molecule bonded to two Ti sites via its two oxygen sites), which was the most energetically favorable, followed by the chelated modes (the molecule bonded to one Ti site via its two oxygen sites), and the monodentate mode (the molecule bonded to one Ti site via one oxygen site). The absorption spectra were calculated with time-dependent functional-theory with CAM-B3LYP for the description of charge-transfer excitations. We observe a red shift of the molecule/cluster systems with respect to the molecules and clusters alone. Moreover, the chelated mode was found to present bands at lower energies than the other modes, making it the most interesting mode to tune the absorption edge of these systems.