Interaction of the surface of BeO with water: In connection with the two-dimensional condensation of water

The adsorption isotherm of water was measured on BeO, and it was found that BeO reveals a step in the adsorption isotherm when the sample is subjected to hydrothermal treatment. The q_st curve shows a weak maximum over the coverage region from 0.5 to 1.0, which manifests a characteristic lateral interaction between adsorbed water molecules. These phenomena can be explained in terms of the two-dimensional condensation of water which occurs on the hydroxylated surface of the (101̄0) plane of BeO as in the case of ZnO on which the step has been reported to appear. The preparation of a homogeneous surface of BeO which exhibits this phenomenon is rather difficult compared to that of ZnO, and the surface once prepared is apt to be damaged by heat treatment. This fact was considered to be due to a larger surface-ionic polarization of BeO because of a very small radius of the Be²⁺ ion compared to that of the Zn²⁺ ion. The infrared spectra substantiated that the peak at 3518 cm^-1 is due to the OH stretching vibration of surface hydroxyls responsible for the appearance of the step.