  Despite most of the applications of anatase nanostructures rely on
photoexcited charge processes, yet profound theoretical understanding of
fundamental related properties is lacking. Here, by means of ab initio ground
and excited-state calculations, we reveal, in an unambiguous way, the role of
quantum confinement effect and of the surface orientation, on the electronic
and optical properties of anatase nanosheets (NSs). The presence of bound
excitons extremely localized along the (001) direction, whose existence has
been recently proven also in anatase bulk, explains the different optical
behavior found for the two orientations (001) and (101) when the NS thickness
increases. We suggest also that the almost two-dimensional nature of these
excitons can be related to the improved photoconversion efficiency observed
when a high percentage of (001) facet is present in anatase nanocrystals.
