Amorphous alumina nanoparticles: Structure, surface energy, and thermodynamic phase stability

To provide a complete picture of the energy landscape of Al ₂O₃ at the nanoscale, we directed this study toward understanding the energetics of amorphous alumina (a-Al₂O ₃). a-Al₂O₃ nanoparticles were obtained by condensation from gas phase generated through laser evaporation of α-Al₂O₃ targets in pure oxygen at25 Pa. As-deposited nanopowders were heat-treated at different temperatures up to 600 C to provide powders with surface areas of 670-340 m²/g. The structure of the samples was characterized by powder X-ray diffraction, transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. The results indicate that the microstructure consists of aggregated 3-5 nm nanoparticles that remain amorphous to temperatures as high as 600 C. The structure consists of a network of AlO₄, AlO₅, and AlO₆ polyhedra, with AlO₅ being the most abundant species. The presence of water molecules on the surfaces was confirmed by mass spectrometry of the gases evolved on heating the samples under vacuum. A combination of BET surface-area measurements, water adsorption calorimetry, and high-temperature oxide melt solution calorimetry was employed for thermodynamic analysis. By linear fit of the measured excess enthalpy of the nanoparticles as a function of surface area, the surface energy of a-Al₂O₃ was determined to be 0.97 ± 0.04 J/m². We conclude that the lower surface energy of a-Al₂O₃ compared with crystalline polymorphs γ- and α-Al₂O₃ makes this phase the most energetically stable phase at surface areas greater than 370 m²/g.