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

TiO 2 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 TiO 2 anatase nanosheets with dominant {001} basal facets gained momentum because of the possibility to exploit different and/or improved functional behaviors with respect to usual bipyramidal TiO 2 anatase nanoparticles, mainly exposing {101} facets. Nevertheless, such behavior depends in a significant extent on the physicochemical features of surfaces exposed by nanosheets. They can vary in dependence on the presence or removal degree 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 TiO 2 anatase nanosheets 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 (1 × 4) reconstruction. Moreover, IR spectroscopy of CO adsorbed as probe molecule indicates that the Lewis acidity of Ti 4+ sites exposed on (1 × 4) reconstructed {001} facets of calcined TiO 2 nanosheets is weaker than that of cationic centers on {101} facets of bipyramidal TiO 2 anatase nanoparticles. The samples have also been tested in phenol photodegradation highlighting that differences in surface hydration, hydroxylation, and Lewis acidity between TiO 2 nanoparticles with nanosheet (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 (1 × 4) reconstructed {001} facets.