  Donor-modified TiO 2 nanoparticles are interesting hybrid systems shifting
the absorption edge of this semiconductor from the ultra-violet to the visible
or infrared light spectrum, which is a benefit for several applications ranging
from photochemistry, photocatalysis, photovoltaics, or photodynamic therapy.
Here, we investigate the absorption properties of two catechol-like molecules,
i.e. dopamine and DOPAC ligands, when anchored to a spherical anatase TiO 2
nanoparticle of realistic size (2.2 nm), by means of time-dependent density
functional theory calculations. By the differential absorbance spectra with the
bare nanoparticle, we show how it is possible to determine the injection
mechanism. Since new low-energy absorption peaks are observed, we infer a
direct charge transfer injection, which, unexpectedly, does not involve the
lowest energy conduction band states. We also find that the more perpendicular
the molecular benzene ring is to the surface, the more intense is the
absorption, which suggests aiming at high molecular packing in the synthesis.
Through a comparative investigation with a flat TiO 2 surface model, we unravel
both the curvature and coverage effects.
