Extended Data Fig. 8: Reactivity of the hydroxylated surface.
From: Direct assessment of the acidity of individual surface hydroxyls
a, Atom-resolved PDOS of the four symmetry-inequivalent three-fold coordinated O surface sites on the fully hydroxylated In2O3(111) surface with three OWH and three OSH(β) per unit cell. The VBM is at 0 eV. b, Calculated adsorption energies \({E}_{{\rm{ads}}}^{{\rm{H}}}\) for the remaining three unprotonated O(3c) sites (with respect to the H2 gas phase molecule). The adsorption of water and the formation of the hydroxylated surface structure slightly modifies the reactivity of the unprotonated O(α), O(γ) and O(δ) sites. The saturation of the O(β) sites by protons leads to a strong downward shift of the O(β) 2p states. The VBM is now formed by the O(δ) 2p states, followed by the O 2p states from the O(α) and O(γ) sites. The O(δ), which are the second-most-reactive O species on the uncovered surface, are thus expected to be the most reactive sites on the hydroxylated In2O3(111) surface. This is confirmed by the calculated H adsorption energies (b). Whereas for O(γ) the H adsorption energies are rather similar on the clean and on the hydroxylated surface, the local relaxations upon water adsorption make the O(α) sites slightly more and the O(δ) slightly less reactive (see b and inset in Fig. 1b). As for the clean surface, the pronounced peak in the PDOS of the three-fold coordinated OS surface atoms at the VBM also leads to an upward shift of their p-band centre. The shift of the p-band centre has been taken as an indirect measure of surface reactivity24 and confirms the expected trend in the PA of the different surface sites.
