Periodic DFT and Atomistic Thermodynamic Modeling of the Surface Hydration Equilibria and Morphology of Monoclinic ZrO2 Nanocrystals

A comprehensive DFT study of water sorption (0.25 < theta < 1) on monoclinic ZrO2 (P2(1)/c) nanocrystals was performed by means of plane wave periodic DFT/PW91 calculations jointly with statistical thermodynamics. All planes, (001), (110), (011), (-101), (111), and (-111), exposed by m-ZrO2 crystallites of the equilibrium morphology were taken into account, and their atomic structure, reconstruction, and stabilization upon water adsorption were elucidated and discussed in detail. Using the calculated surface energy values, the truncated hexagonal bipyramidal shapes of the monoclinic zirconia nanocrystallites in dry and wet states were predicted by means of the Wulff construction. The results compare very well with the experimental TEM images. The computed changes in the free enthalpy of adsorption under different hydration conditions were used to construct two-dimensional surface coverage versus temperature and pressure diagrams, theta(hkl) = f(T,p(H2O)), for each of the exposed planes. It was shown that full coverage of water on the more stable (-111) and (111) planes was achieved in a bimodal way (1 -> 2 and 2 -> 2, respectively), whereas for less stable (011) and (001) terminations it occurs via a trimodal 1 -> 1 -> 2 and a tetramodal 1 -> 1 -> 1 -> 1 process. In addition, to illustrate water adsorption processes in a more concise way, multisite Langmuir and Fowler-Guggenheim isotherms were calculated and interpreted in terms of molecular interactions within the adsorbate layer and with the surface.