Structure and Disorder in Amorphous Alumina Thin Films: Insights from High-Resolution Solid-State NMR

Revealing the extent of disorder in amorphous oxides is one of the remaining puzzles in physical chemistry, glass sciences, and geochemistry. Here, we report the Al-27 NMR results for amorphous Al2O3 thin films obtained from two different deposition methods (i.e., physical vapor-deposition and atomic layer-deposition), revealing two distinct amorphous states defined by a fraction of five-coordinated Al (Al-[5]). The fractions of Al-[4] and Al-[5] are dominant (similar to 92-95%) in both films. While the overall similarity between these two states suggests a narrow stability of available amorphous states, the fraction of Al-[5] in atomic layer-deposited thin films is apparently larger and thus more disordered than that in physical vapor-deposited films. Such results require that varying extents of disorder exist in the amorphous oxides prepared under different processing conditions. As the Al-[5] site (<1%) in crystalline Al2O3 is known to control its catalytic ability over Al-[4] and Al-[6], the significant fractions (similar to 40%) of Al-[5] in our amorphous thin films suggest that amorphous Al2O3 may be potentially useful as a new class of catalysts.