Probing the Cation Distribution in Gamma-alumina Enabled by O-K Edge Artifact Suppression Using Cryo-EELS

Abstract

Gamma-alumina (γ-Al2O3) is one of several transitional phases of Al2O3 with valuable properties that render it highly useful in a number of applications including catalysis and absorbents. Yet despite the technological importance of γ-Al2O3, its crystal structure is still not fully understood. While γ-Al2O3 was originally described as having a spinel-like structure [1], many different spinel-based and nonspinel structures have since been proposed. In our previous work, we verified that the cubic spinel-based model is indeed the most accurate for γ-Al2O3 [2]. However, the distribution of Al cations in the lattice remains unclear. Since cation vacancies must be added to the normal spinel structure to achieve the correct stoichiometry of Al2O3, the placement of the vacancies in either tetrahedral or octahedral sites determines the Al cation distribution. Prior studies have reached contradictory conclusions, with some claiming all vacancies are on octahedral sites [3], some suggesting all vacancies are on tetrahedral sites [4], and some claiming a mixed distribution [5]. Determining the cation distribution is key to accurately modeling γ-Al2O3 for various theoretical simulations, including catalytic property simulations which are of great interest to the chemical engineering community. Electron energy-loss spectroscopy (EELS) is a distinctly suitable tool for this study due to the sensitivity of the energy-loss near-edge fine structure (ELNES) to local atomic coordination. We therefore employed scanning transmission electron microscopy (STEM) and monochromated EELS combined with multiple scattering ELNES simulations to investigate the Al atom distribution in the spinel γ-Al2O3 structure. A major factor contributing to the uncertainty around the structure of γ-Al2O3 is the heterogeneity of commercially available γ-Al2O3. To address this, we synthesized single-crystalline γ-Al2O3 thin films through thermal oxidation of single-crystal NiAl (110). From the highly crystalline ~80 nm γ-Al2O3 thin films, cross-sectional TEM samples were prepared using focused ion beam (FIB) for the STEM-EELS experiments. One of the requirements to collect accurate, high-quality STEM-EELS data is the use of sufficiently high electron beam currents. However, like many other low-Z oxide materials, γ-Al2O3 is highly susceptible to structural changes by the highly focused STEM electron beam. Knock-on damage as well as radiolysis effects could occur, easily observed as holes in the sample (Figure 1a) or as changes in the pre-edge fine structure in the O-K EELS edge (Figure 1b). Therefore, we first employed a carefully systematic study investigating the origin of beam effects on γ-Al2O3, in order to find optimum experimental conditions minimizing these beam effects. We found that employing cryogenic conditions-using a liquid nitrogen-cooled Gatan cryo-holder-was most effective in mitigating the impact of beam effects on EELS spectra;