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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 27 Al NMR results for amorphous Al 2 O 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 Al 2 O 3 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 2 O 3 may be potentially useful as a new class of catalysts.
Lee et al. (Wed,) studied this question.