Detailed experimental studies have been undertaken to quantitatively examine particle-size effects on proton-induced surface charge and accompanying ion adsorption phenomena on a suite of nanocrystalline anatase (TiO2) phases. Commercially available, crystalline, monodispersed anatase particles ranging in diameter from 3 to 40 nm were used in the study. Extensive characterization of these particles revealed that the  face predominates. Bulk surface titrations were completed in LiCl, NaCl, KCl, RbCl and NaCF3SO3 (NaTr) electrolyte solutions, over a wide range of ionic strengths (0.0005 to 0.3 m). Additionally, the specific adsorption of divalent ions (e.g., Sr2+) has been investigated as a function of pH and loading in NaCl media. Molecular simulation, DFT-MD, calculations were completed to complement the surface adsorption studies. Specifically, bonding geometries on the anatase  surface were predicted for the adsorption of cations and anions used in the experimental studies. The DFT-MD simulations show inner-sphere binding for all cations, with bidentate geometries predominating. Conversely, monovalent anions form outer-sphere complexes. To integrate the experimental results with molecular-level insights gained from the simulation studies, surface complexation modelling (SCM) was performed. The CD-MUSIC model, coupled with a Basic Stern layer description of the electric double layer, was used to successfully provide a surface structural description of the ion adsorption data. All fitting parameters within the SCMs were constrained by the DFT simulation results. The resulting SCMs rationalize successfully the subtle differences observed in the surface reactivity of the anatase particles as a function of particle size. For example, the small decrease in pHznpc values with increasing particle diameter are accounted for by slight differences in protonation constants, KH. Similarly, binding constants for adsorbed ions and capacitance values for the EDL vary with particle size.
|Original language||English (US)|
|Title of host publication||Mineralogical Magazine|
|State||Published - 2013|
Ridley, M., Kubicki, J., & Machesky, M. L. (2013). Ion adsorption on nanocrystalline anatase surfaces: integrating experimental and theoretical studies through surface complexation modeling. In Mineralogical Magazine (Vol. 76, pp. 2285)