Atomic Layer Deposition of Aluminum Oxide in Mesoporous Silica Gel

Abstract

Silica gel is a mesoporous, granular material with a high specific surface area that is widely used as a support for heterogeneous catalysts. This study explores the atomic layer deposition (ALD) of aluminum oxide (Al2O3) in silica gel. The coated materials were analyzed using weight-gain measurements, nitrogen adsorption surface-area analysis, energy dispersive X-ray analysis, and scanning electron microscopy. In addition, the individual ALD surface reactions on the silica gel powder were monitored in situ using quadrupole mass spectrometry. This study confirmed that the silica gel could be conformally coated using reactant exposure times of ∼90 s, and the surface area remained relatively unchanged even after 20 Al2O3 ALD cycles. EDAX measurements performed on silica gel specimens prepared using subsaturating trimethyl aluminum (TMA) exposures demonstrated that the spherical particles become infiltrated with the Al2O3 ALD progressively from the outside of the particles to the core, and the kinetics of this process are dictated by the rapid consumption of TMA by the high surface area silica gel. The Al2O3 ALD is inhibited on the silica gel surface for the first ∼5 ALD cycles, and this leads to a lower initial weight gain as well as a reduced proportion of methane released during the initial TMA exposures. This study provides useful insights into ALD processes on high surface area supports that will be valuable for the future development of nanostructured catalysts using ALD.