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

Amorphous aluminosilicate nanofilms, a-Al(0.1)Si(0.9)O(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 spin-coating with a mixed precursor sol. XANES measurements indicated that the basic framework of a-Al(0.1)Si(0.9)O(x) films was similar to the zeolitic one, consisting of the corner-linkage of SiO(4) and AlO(4) 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 degrees C, as checked by thermal desorption spectroscopy, so that the film exhibited the humidity-independent proton conductivity at temperatures above 300 degrees C. Furthermore, the conductivity across the film sigma increased in a power low by reduction of the film thickness d to less than 120 nm as sigma proportional, variant d(-tau), and it was saturated when the thickness become less than 40 nm. The observed scaling index tau 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.