Size-scaling of proton conductivity in amorphous aluminosilicate acid thin films

Amorphous aluminosilicate nanofilms, a-Al_0.1Si _0.9O_x, exhibit unique size-enhancement of the proton conductivity along the thickness direction because of the presence of the zeolite-like, acid site network with the mesoscopically sized dimension inside glass matrix. The dense films with the thickness of 22-1400 nm were uniformly formed over the electrode substrate in nanometer thickness precision by multiple spincoating with a mixed precursor sol. XANES measurements indicated that the basic framework of a-Al_0.1Si_0.9O_x films was similar to the zeolitic one, consisting of the corner-linkage of SiO₄ and AlO₄ tetrahedral units. These films revealed the complex temperature- and humidity-dependency of proton conductivity by the existence of two kinds of protonic carriers: Brønsted acidic protons and Lewis acidic protons. The Brønsted acidic protons could be persistent in amorphous films at around 500°C, as checked by thermal desorption spectroscopy, so that the film exhibited the humidity-independent proton conductivity at temperatures above 300°C. Furthermore, the conductivity across the film σ increased in a power low by reduction of the film thickness d to less than 120 nm as σ ∝ d^-τ, and it was saturated when the thickness become less than 40 nm. The observed scaling index τ was 2.2 in agreement with the value of the theoretical index (2.3) of cluster size scaling in a three-dimensional percolation system. This conduction behavior is explicable by finite size-scaling of the highly conductive pathway based on the interconnected Brønsted acid centers in the range of a few tens to hundreds of nanometers.