Atomic-Scale Insights into the Oxidation of Aluminum.

Benjamin Berkels,Lan Nguyen,Sarah J Haigh,Tim L Burnett,Teruo Hashimoto,Eric A Stach,Aidan P Rooney,George E Thompson,Dmitri N Zakharov

doi:10.1021/acsami.7b17224

Abstract

The surface oxidation of aluminum is still poorly understood despite its vital role as an insulator in electronics, in aluminum-air batteries, and in protecting the metal against corrosion. Here we use atomic resolution imaging in an environmental transmission electron microscope (TEM) to investigate the mechanism of aluminum oxide formation. Harnessing electron beam sputtering we prepare a pristine, oxide-free metal surface in the TEM. This allows us to study, as a function of crystallographic orientation and oxygen gas pressure, the full oxide growth regime from the first oxide nucleation to a complete saturated, few-nanometers-thick surface film.

Highlights

The ability of aluminum to spontaneously form a self-healing surface oxide is crucial for the metal’s function in a broad range of applications, yet the full oxide formation process remains poorly understood
The early stages of oxygen molecule absorption have been effectively elucidated via surface science techniques,[1,2] whereas the overall growth kinetics have been widely studied with macroscopically averaging surface characterization techniques such as X-ray photoemission spectroscopy (XPS).[3−5] In contrast, the intermediate stages of oxide development, including the mechanism of oxide nucleation and growth remains elusive due to the lack of atomically resolved techniques able to directly observe dynamic atomic rearrangements for increasing oxide thicknesses at relevant pressures
We demonstrate how state-of-the-art atomic resolution aberration corrected environmental transmission electron microscope (E-TEM)[7] imaging can address this knowledge gap

Summary

■ INTRODUCTION

The ability of aluminum to spontaneously form a self-healing surface oxide is crucial for the metal’s function in a broad range of applications, yet the full oxide formation process remains poorly understood. The critical thickness of oxide at saturation is dependent on the oxygen pressure, with higher pressures resulting in quicker initial rates of oxidation and higher critical oxide thickness.[28] At pressures below 3 × 10−5 Torr, we observe similar island growth at surface steps to that illustrated in Figures 2 and 3 (both observed at 3 × 10−5 Torr), but at lower pressure the process occurs over longer time scales This relationship between critical oxide thickness and oxygen pressure has been previously reported in XPS studies,[3,28,29] and is attributed to higher pressures producing a larger concentration of oxygen molecules on the surface of the metal which are available for oxidation and growth, according to Langmuir isotherm behavior.[28,29] The higher concentration of adsorbed oxygen at higher pressures increases the oxide growth rate by increasing the electric field and Letter the rate of ionic migration through the oxide.[29] At the lowest pressure tested (3 × 10−7 Torr), we do not observe any oxide formation even after 1 h on the Al(100) facet, the Al(111) surface reached close to its critical thickness ∼0.9 nm after 600 s. Our measurements of the lattice spacings of this irradiation induced oxide phase show that it has a crystal spacing of 0.240 nm for Al(111), a good match to the irradiation induced intermediate γ-Al2O3, oxide observed previously in oxidized aluminum.[32,33] it differs significantly from the intermediate oxide layers we observe during our in situ experiments, supporting our understanding that the electron beam has a negligible influence on the reaction we observe

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES