Adsorption of Water Molecule on Calcium Fluoride and Magnesium Fluoride Surfaces: A Combined Theoretical and Experimental Study
Jie Cen, Liang Feng, De‐Li Chen, Lulu Zhang, Ning Yang, Weidong Zhu
Abstract
The adsorption and dissociation of H2O molecule on various surfaces of MgF2 and CaF2 have been investigated theoretically by using a dispersion-corrected density functional theory based method. The calculated adsorption energy of H2O molecule on the low index CaF2(110) and CaF2(111) surfaces ranges from −0.20 to −0.50 eV, smaller than those on the low index MgF2(001) and MgF2(110) with values from −0.80 to −1.06 eV. The modeling shows that the dissociation of water molecule is possible on the high index CaF2(210) surface and the low index MgF2(001) and MgF2(110) surfaces as revealed by the calculations that the dissociated states of H2O are thermodynamically more stable. Furthermore, for MgF2(001) and MgF2(110) surfaces, the dissociation process is almost barrierless, suggesting that the OH groups on the surfaces could be formed easily. These theoretical predictions suggest that MgF2 is superior to CaF2 in terms of water adsorption affinity, and high temperatures may be needed to remove the adsorbed water molecules and the OH groups on the surfaces, which were confirmed by the in situ FT-IR spectra of the synthesized MgF2 and CaF2 samples. The broad peak for CaF2 ranging from 3100 to 3600 cm–1 can be completely removed with a temperature of 573 K, while a higher temperature of 923 K is required for MgF2.