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

Yoshitaka Aoki, Hiroki Habazaki, Toyoki Kunitake

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Abstract

Amorphous aluminosilicate nanofilms, a-Al0.1Si 0.9Ox, 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-Al0.1Si0.9Ox 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, 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.

Original languageEnglish
Pages (from-to)14399-14406
Number of pages8
JournalJournal of the American Chemical Society
Volume131
Issue number40
DOIs
Publication statusPublished - Oct 14 2009
Externally publishedYes

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All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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