Electronic core levels of hydroxyls at the surface of chromia related to their XPS O 1s signature: A DFT + U study

摘要

The Cr 2p and O 1s binding energy (BE) levels have been calculated by first principles methods for different models of hydroxylated (0001)-Cr_2O_3 surfaces. Several surface terminations have been considered. The calculations allow us to reproduce the O 1s shifts between O in oxide and OH groups. It is found that two main effects account for the OH binding energy shifts. On the one hand, the increased covalency of the O-H bond with respect to the Cr-O bond, lowers the electronic O (1s and 2p) energy, and in consequence the BE of the core levels (O 1s) are higher. On the other hand, the lower the OH coordination number, the higher the valence and core levels energy, and the lower the BE. Consequently, mono-coordinated hydroxyls have a binding energy near that of O~(2-) in the oxide (△BE_(OH-O) = -0.2-0.0 eV). Two-fold coordinated hydroxyls have a slightly higher BE (△BE_(OH-O)= +0.3 eV). Three-fold coordinated OH groups have a higher binding energy (△BE_(OH-O)= +0.6-0.7 eV), corresponding to that experimentally measured for OH groups. Finally, water adsorbed above OH groups exhibits a still higher BE (△BE_(HOH-O)= + 0.9-1.0 eV). The △BE are slightly under-estimated under the initial state approximation, and overestimated under the final state (Z+1) approximation.