Percolative proton conductivity of sol–gel derived amorphous aluminosilicate thin films

Abstract

The finite size effect of proton conductivity of amorphous aluminosilicate thin films, a-Al_nSi_1−nO_x (n = 0.07, 0.1, 0.2, 0.3 and 0.45), prepared by a sol–gel process was investigated by experimental and numerical techniques. High-resolution TEM clarified that a-Al_nSi_1−nO_x films had the heterogeneous nanoscale microstructures comprised of the ion-conducting, condensed glass microdomain and the poor-conductive, uncondensed glass microdomain. σ of the films with n ≤ 0.1 exponentially increased upon decreasing thickness in the sub-100 nm range because the volume fraction of conductive domains was less than the percolation threshold and cluster size scaling of the conductive domain was operative. The numerical simulation suggested that conductance of the condensed domain was higher than that of the uncondensed domain by 2 orders of magnitude. Volume fractions of the condensed domain increased with increasing Al/Si molar ratio and were over the percolation threshold (24.5%) with n ≥ 0.2. However, conductance of the condensed domain decreased with increasing Al/Si ratio with n ≥ 0.2 because the aluminosilicate glass framework made of 4-fold-connected MO₄ tetrahedra was deformed by forming the octahedral AlO₆ moieties, as checked by Al K-edge XAS. It was found that the optimal Al/Si composition in terms of the conductance of the condensed domain is not in coincidence with that in terms of the average conductivity of the films.