We report on the systematic investigation by Atomic Force Microscopy of the role of surface nanoscale roughness and morphology on the charging behaviour of nanostructured zirconium dioxide (ZrOx, x≤2) surfaces in aqueous solutions, and in particular on the influence of nanoscale morphology on the isoelectric point of these surfaces. By using supersonic cluster beam deposition to fabricate nanostructured zirconia films, we achieved a quantitative control over the surface morphological properties of ns-ZrOx films, and in particular we were able to control the root-mean square roughness across a wide range of values matching those of other characteristic lengths of electrostatic double-layer interactions in electrolytic solutions, such as the Debye length, and the typical size of nano-colloids (proteins, enzymes, nano-catalysts). We have characterized by direct AFM measurements the interaction forces between rough zirconia surfaces and a micrometer-sized spherical silica probe in NaCl aqueous electrolyte. A method of incorporating surface roughness into theoretical calculations of hydrodynamic and DLVO forces is presented, where the surface roughness is modeled by a Gaussian distribution of surface heights. We performed a systematic exploration of the electrical double layer properties in different interaction regimes characterized by different ratios of characteristic nanoscale lengths of the system: the surface rms roughness Rq, the correlation length ξ and the Debye length λD. We observed a remarkable reduction by more than one pH unit of the Isoelectric Point (IEP) on very rough nanostructured surfaces (Rq∼26nm), with respect to the flat amorphous ZrOx. A possible explanation for the behavior of IEP, previously characterized also for titanium dioxide nanostructured thin films1, is the roughness-induced self-overlap of the electrical double layers, as a potential source of deviation from the trend expected for flat surfaces.
1 F. Borghi, V. Vyas, A. Podestà, and P. Milani, PLoS ONE 8, e68655 (2013).