  Nanoparticles of very small size (below 10 nm) of TiO2 material are nowadays
the functional building blocks of many developing technological applications.
Nano is clearly different from bulk or extended systems as regards surface
area, molecular binding properties, charge separation efficiency, electron/hole
transport, photochemical conversion properties, etc. In this work, we
investigate the life path of energy (excitons) and charge (electrons and holes)
carriers in anatase TiO2 nanoparticles of different size (2-3 nm) and shape
(faceted vs spherical), by means of a wide set of hybrid density functional
theory calculations. The attention is focused on the exciton/charge carriers
formation, separation, recombination, self-trapping processes, which are
analyzed in terms of structural deformations, energy gain or cost, charge
localization/delocalization and electronic transitions involved. The
computational models are corroborated by an extensive comparison with available
experimental data based on photoluminescence measurements, electron
paramagnetic resonance and transient absorption spectroscopies. Peculiar
differences are observed for spherical nanoparticles with respect to faceted
ones because of the higher disorder and larger diversity of coordination sites
present on the surface. For example, charge delocalization on several lattice
sites is more competitive with self-trapping processes in faceted than in
spherical nanoparticles. This relates to the fact that selective compression or
elongation of Ti-O bonds play a key role in determining the effectiveness of
trapping sites, with spherical nanoparticles being more flexible. Moreover,
hydroxyl groups on surface five-fold coordinated Ti sites are also found to be
good hole trapping sites.
