Nanoscale roughness and morphology affect the IsoElectric Point of titania surfaces

TL;DR

This study systematically investigates how nanoscale roughness and morphology influence the IEP of titania surfaces in aqueous solutions, revealing that increased roughness lowers the IEP due to double layer self-overlap effects.

Contribution

It provides a quantitative analysis of the impact of nanoscale surface features on the electrostatic properties of titania, using controlled nanostructuring and atomic force microscopy measurements.

Findings

Rough nanostructured titania surfaces exhibit significantly lower IEP than flat surfaces.

Nanoscale roughness induces double layer self-overlap, affecting surface charging behavior.

Surface morphological parameters correlate with shifts in IEP.

Abstract

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.