Theoretical study of structural, mechanical and spectroscopic properties of boehmite (γ-AlOOH)

Understanding Pore Formation in ALD Alumina Overcoats

AlO_X thin films deposited by atomic layer deposition (ALD) have previously been used to increase both stability and selectivity of supported palladium catalysts and are known to develop nanoscale porosity upon heating. Understanding the factors that affect ALD thin-film porosity enables future design of layered catalytic structures with tunable nanoscale features on industrially-relevant high-surface-area materials. In this study, porous and nonporous aluminum oxide supports with and without palladium nanoparticles were overcoated with thin films of 2-7 nm AlO_X by ALD deposited at temperatures of 100, 200, and 300 °C. Hydroxyl loss and changes in surface chemistry were observed upon heating the films, and changes in surface area and pore volume of the annealed films were correlated to AlO_X deposition temperature and the presence of Pd. Crystallization of the overcoat to γ-Al₂O₃ is shown to occur separately from hydroxyl loss and pore formation. A mechanistic understanding of pore formation in AlO_X ALD films is obtained by reference to studies of the structural transformations accompanying the formation of transition aluminas from hydroxide precursors. Additionally, a direct and tunable correlation is established between pore development and the overall hydroxyl content of AlO_X ALD coatings.

A computational study of energy barriers of structural transformations and hydrogen transfer in boehmite

The crystal structure of boehmite (γ-AlOOH) contains a large amount of hydrogen bonds that are joined into chains by sharing hydrogen-bond donor and acceptor oxygen atoms. The hydrogen ions in the hydrogen-bond chains are highly mobile and have complicated structural characterizations, and this feature may well be utilized for proton-conducting applications, but the mechanism is unknown without the dynamic parameters of the hydrogen-transfer processes. We propose probable hydrogen-transfer paths and compute their energy barriers using density functional theory with van der Waals density functionals, on both perfect and vacancy-containing crystal structures. It is revealed that the energy barriers are generally below 21 kJ mol^-1 in a perfect crystal, and 14 kJ mol^-1 in a vacancy-containing structure. The low energy barriers are indicators of the high proton conductivity of boehmite even at room temperature.