Disclosing the True Atomic Structure of {001} Facets in Shape-Engineered TiO2 Anatase Nanoparticles

In the last decade shape-engineering of TiO2 anatase nanoparticles (NPs) has attracted increasing attention owing to the possibility to maximize the presence of {001} facets, which have been reported to show peculiar adsorption, electronic, and (photo)catalytic properties. It is well-known that the anatase (001) surface is prone to reconstruction and several models have been proposed and validated by DFT calculations and single crystal studies. However, its true atomic structure in shape-engineered TiO2 anatase nanoparticles often remains elusive. In this study we shed light on this issue combining IR spectroscopy of CO adsorbed at very low temperature and thorough DFT modelling. Our results show that the thermal treatment in oxygen, performed to remove the capping agents (i.e., fluorides) employed in the synthesis of shape-controlled NPs, leads to a reconstruction of the {001} facets which is compatible with an "add-oxygen" model (AOM) and not with the most commonly reported "add-molecule" model (ADM). These findings can guide future experimental and computational studies highlighting that the AOM reconstruction is the most appropriate model to describe the properties and reactivity of the {001} facets in shape-controlled TiO2 nanoparticles after thermal removal of fluorides.