PNNL

Abstract

Particle morphology is an important property affecting the reactivity of solid phases. The aluminum oxyhydroxide boehmite (?-AlOOH) is one example for which particle morphology has relevance to geochemistry, environmental management, and catalysis. Morphologies can be predicted from surface and attachment energies obtained from electronic structure calculations but the dependence of these predictions on the level of theory and complexity of the physics involved has been overlooked. Density functional theory calculations were therefore performed for a wide range of exchange-correlation (XC) functionals, including a hybrid functional, to calculate the energetics of the four main low-index surfaces of boehmite both in vacuum and with a water monolayer adsorbed. The approximation used for describing core electrons, the size of the plane-wave basis set, and the need for dispersion corrections were also investigated. The results highlight the critical need for converged plane-wave basis sets to obtain accurate surface energies as well as the strong influence the XC functional can have on surface energies and water adsorption energies. The predicted morphologies were compared to particle morphologies obtained from a variety of synthesis routes both carried out in this work and published in the literature. The comparison showed that, at basic pH, growth morphologies dominated at low temperatures and short time periods whereas equilibrium morphologies were achieved as the temperature and/or aging time increased, which could be explained by the slow stacking growth mode of boehmite along the [010] direction. Finally, calorimetry experiments are often used to derive surface energies, but excess water on hydrophilic surfaces can add significant uncertainty. An approach for correcting the surface energetics obtained by calorimetry using electronic structure calculations is presented.