A comparison of the reactivity at the solid-solution interface of nano- and micro-crystalline TiO2 phases

The acid–base surface reactivity of rutile ($-TiO2), with the (110) crystal face predominant, has been studied extensively. Specifically, theoretical simulations, X-ray techniques and potentiometric titration studies have provided molecular-scale and macroscopic details on the ion adsorption behaviour of rutile. Results of the collective studies have been integrated into the multisite complexation (MUSIC) model. Similar detailed studies examining the surface charging properties of nano-crystalline anatase samples are now being conducted. Primary surface charging curves of rutile in LiCl, NaCl, KCl, and RbCl electrolyte media are compared with similar titration data for nanoparticle anatase samples. The effect of electrolyte media and ionic strength on the primary charging behaviour of anatase was studied as a function of nanoparticle size (3–40 nm diameter). The primary charging curves of rutile and all nano-anatase samples are generally analogous, when normalized to their respective pHznpc values. At low ionic strength (0.03 m) the development of negative surface charge was similar for all electrolyte cations. However, with increasing ionic strength negative surface charge development was enhanced as the bare crystallographic radii of the cations decreased. Subtle differences in the macroscopic charging behaviour of the 3 nm diameter anatase sample were noted, particularly below the pHznpc value were Cl- counterions screen the surface from bulk solution. For rutile, X-ray data and MD simulations show that electrolyte cations are adsorbed as inner-sphere complexes, principally in tetradentate geometry. Similarly, DFT-MD simulations show inner-sphere sorption of monovalent cations onto anatase; however, bidentate sorption predominates. For both anatase and rutile, a CD-MUSIC model, coupled with a Basic Stern layer description of the electric double (EDL), successfully integrates all microscopic information with the macroscopic experimental results. Though for the smallest anatase particles, it is necessary to account for some curvature of the EDL.