  We report on the systematic investigation of the role of surface nanoscale
roughness and morphology on the charging behaviour of nanostructured titania
(TiO2) surfaces in aqueous solutions. IsoElectric Points (IEPs) of surfaces
have been characterized by direct measurement of the electrostatic double layer
interactions between titania surfaces and the micrometer-sized spherical silica
probe of an atomic force microscope in NaCl aqueous electrolyte. The use of a
colloidal probe provides well-defined interaction geometry and allows
effectively probing the overall effect of nanoscale morphology. By using
supersonic cluster beam deposition to fabricate nanostructured titania films,
we achieved a quantitative control over the surface morphological parameters.
We performed a systematical exploration of the electrical double layer
properties in different interaction regimes characterized by different ratios
of characteristic nanometric lengths of the system: the surface rms roughness
Rq, the correlation length {\xi} and the Debye length {\lambda}D. We observed a
remarkable reduction by several pH units of IEP on rough nanostructured
surfaces, with respect to flat crystalline rutile TiO2. In order to explain the
observed behavior of IEP, we consider the roughness-induced self-overlap of the
electrical double layers as a potential source of deviation from the trend
expected for flat surfaces.
