Identifying Facet-Dependent CO₂ Adsorption Sites on Anatase TiO₂ Nanocrystals by Solid-State NMR Spectroscopy

Facet-dependent surface oxygen environments play a central role in adsorption and catalytic processes on oxide nanocrystals, but their direct experimental identification remains challenging. Here, we employ ¹⁷O and ¹³C magic-angle-spinning solid-state NMR spectroscopy to investigate three anatase TiO₂ nanocrystals predominantly exposing (001), (101), and (111) facets. The ¹⁷O NMR spectra reveal distinct facet-dependent surface oxygen environments, including low-coordinated surface O_2c species, near-surface O_3c species, hydroxyls, and adsorbed water. DFT calculations further support the assignment of the oxygen species and indicate that the hydrated A-TiO₂-111 surface is best described by a model containing both dissociatively and molecularly adsorbed water. Upon CO₂ adsorption, combined ¹⁷O and ¹³C NMR results show that surface O_2c species are the dominant adsorption sites on all three facets, whereas differences in the involvement of hydroxyl and water species lead to distinct carbonate-to-bicarbonate distributions. These results provide an atomic-level picture of facet-dependent CO₂ adsorption on anatase TiO₂ nanocrystals.