  Functionalized TiO2 nanoparticles with intense fluorescent dyes is a
promising tool for several technological applications ranging from
photochemistry, photocatalysis, photovoltaics, photodynamic therapy or
bioimaging. Here, we present the case study of the Alizarin adsorption on TiO2
nanoparticles (NPs) of different shape and increasing size up to 2.2 nm (700
atoms), by means of density functional theory (DFT) calculations. We find that
Alizarin can bind in three different ways, depending on the number and type of
bonds between Alizarin and TiO2: "tridented", "bidented" and "chelated". Next,
we investigate the optical properties of these systems by time-dependent
density functional theory (TDDFT) calculations. Based on the absorption spectra
and the Kohn-Sham orbitals analysis, we discovered that the mechanism of
electron injection depends on the Alizarin binding mode to the TiO2 surface.
While for bidented and chelated adsorption modes a direct charge transfer is
observed, for the tridented one an indirect mechanism governs the charge
transfer process following the excitation. Our results are in good agreement
with existing experimental data and suggests that by tailoring the shape of the
TiO2 NPs and, thus, determining the type of undercoordinated Ti atoms
prevalently exposed at the surface, it is possible to control the predominant
injection mechanism.
