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 ²⁷Al NMR results for amorphous Al₂O ₃ 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 ( ^[5]Al). The fractions of ^[4]Al and ^[5]Al are dominant (∼92-95%) in both films. While the overall similarity between these two states suggests a narrow stability of available amorphous states, the fraction of ^[5]Al 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 ^[5]Al site (<1%) in crystalline Al2O3 is known to control its catalytic ability over ^[4]Al and ^[6]Al, the significant fractions (∼40%) of ^[5]Al in our amorphous thin films suggest that amorphous Al ₂O₃ may be potentially useful as a new class of catalysts.