Beyond Shape Engineering of TiO2 Nanoparticles: Post-Synthesis Treatment Dependence of Surface Hydration, Hydroxylation, Lewis Acidity and Photocatalytic Activity of TiO2 Anatase Nanoparticles with Dominant 001 or 101 Facets

TiO2 anatase nanoparticles are among the relevant players in the field of light-responsive semiconductor nanomaterials used to face environmental and energy issues. In particular, shape-engineered TiO2 anatase nano-sheets with dominant {001} basal facets gained momentum because of the possibility to exploit different and/or improved functional behaviors with respect to usual bipyramidal TiO2 anatase nanoparticles, mainly exposing {101} facets. Nevertheless, such behavior depends in a significant extent on the physico-chemical features of surfaces exposed by nano-sheets. They can vary in dependence of the presence or removal of capping agents, namely fluorides, used for shape-engineering, and experimental investigations in this respect are still a few. Here we report on the evolution of interfacial/surface features of TiO2 anatase nano sheets with dominant {001} facets from pristine nanoparticles fluorinated both in the bulk and at their surface, to nanoparticles with F free surfaces by treatment in a basic solution and to totally F free nanoparticles by calcination at 873 K. The nanoparticles fluorine content and its subsequent evolution is determined by complementary techniques (ion chromatography, ToF-SIMS, XPS, AES, SEM-EDX), probing different depths. In parallel the evolution of the electronic properties and the Ti valence state is monitored by UV-Vis spectroscopy and XPS. The calcination treatment results in {001} facets poorly hydroxylated, hydrated and hydrophilic, which appear as surface features consequent to the expected (1x4) reconstruction. Moreover, IR spectroscopy of CO adsorbed as probe molecule indicates that the Lewis acidity of Ti+4 sites exposed on (1x4) reconstructed {001} facets of calcined TiO2 nano sheets is weaker than that of cationic centers on {101} facets of bipyramidal TiO2 anatase nanoparticles. The samples have also been tested in phenol photodegradation highlighting that differences in surface hydration, hydroxylation and Lewis acidity between TiO2 nanoparticles with nano sheet (freed by F by calcination at 873 K) and bipyramidal shape have a strong impact on the photocatalytic activity, that is found to be quite limited for the nanoparticles mainly exposing (1x4) reconstructed {001} facets.