Differentiating surface titanium chemical states of anatase TiO₂ functionalized with various groups

As the chemical state of titanium on the surface of TiO₂ can be tuned by varying its host facet and surface adsorbate, improved performance has been achieved in fields such as heterogeneous (photo)catalysis, lithium batteries, dye-sensitized solar cells, etc. However, at present, no acceptable surface technique can provide information about the chemical state and distribution of surface cations among facets, making it difficult to unambiguously correlate facet-dependent properties. Even though X-ray photoelectron spectroscopy (XPS) is regarded as a sensitive surface technique, it collects data from the top few layers of the sample, instead of a specific facet, and hence fails to distinguish small changes in the chemical state of Ti imposed by adsorbates on a facet. Herein, based on experimental (chemical probe-assisted NMR) and theoretical (DFT) studies, the true surface Ti chemical states associated with surface modification using -O-, -F, -OH and -SO₄ functional groups on the (001) and (101) facets of anatase TiO₂ are clearly distinguished. It is also demonstrated, for the first time, that the local electronic effects on surface Ti imposed by adsorbates vary depending on the facet, due to different intrinsic electronic structures.