Thickness Dependence of Proton Conductivity of Amorphous Aluminosilicate Nanofilm

Abstract

Al x Si 1−x O n films exhibit a drastic change of proton conductivity across the film by reducing their thickness to less than 100 nm. The temperature and humidity dependence of conductivity of the sub-100 nm films is quite different from those of the thicker films. Furthermore, in the former thickness range, the value of conductivity markedly increases with reducing the film thickness, and its thickness dependence follows a power law with a fixed index of −2.1. This size-scaling effect can be explained by the percolation conductivity model that the probability for percolating of the conductive moiety in Al x Si 1−x O n films increases with decreasing the thickness. Proton-and oxide ion-conducting metal oxides have attracted much attention as electrolyte material for solid oxide fuel cells. In particular, nanometer-thick membranes of these oxides are promising for developing next-generation intermediate-temperature fuel cells thanks to diminished area specific resistance ͑ASR͒ enabled by reduction in electrolyte thickness.1-3 Amorphous metal oxides have intrinsic advantages for their application to thin-film electrolyte due to their nongranular covalent networks. Recently, we discovered that silica-based, nanometer-thick films gave high proton conductivity at 100-400°C under nonhumidified atmosphere due to formation of stable Brønsted acid sites when doped with some other metal ions. 4,5 Especially, the aluminosilicate, Al x Si 1−x O n , nanofilm showed the highest proton conductivity and revealed practically usable levels of ASR at around 300°C.5 Several other research groups reported that the ion conductivity of multilayered materials increased with decreasing layer thickness. 6,7 These studies prompted us to study the effect of thickness for our Al x Si 1−x O n film. In fact, it is found that the proton conductivity of amorphous Al x Si 1−x O n films is exponentially enhanced by reduction of thickness into the sub-100 nm regime.The Al x Si 1−x O n film was prepared on an indium-tin-oxide ͑ITO͒ substrate ͑Aldrich͒ by multiple spin-coating of precursor solutions of tetraethoxysilane ͑TEOS͒ ͑Kanto͒ and aluminum sec-butoxide ͓Al͑O s Bu͒ 3 ͔ ͑Kanto͒ at the Al/Si atomic ratio of 5/95. The details of the procedure were described elsewhere. 6 The metal concentration ͑Al + Si͒ in the precursor mixture sol was adjusted in 30 mM for the film of Ͻ100 nm thickness and 100 mM for the film of Ͼ100 nm thickness. A film with thickness of 100 nm was prepared from both of these sols. The precursor sols were spin-coated onto the ITO substrate at 3000 rpm for 40 s by a Mikasa 1H-D7 spin coater. The deposited gel layer was hydrolyzed by blowing hot air for 30 s ͑Iuchi hot gun͒, and the substrate was cooled to room temperature by blowing cold air for 20 s. These cycles of spin-coating, hydrolysis, and cooling were repeated 10-20 times, and the gel film thus obtained was calcined at 400°C for 15 min. The combination of deposition and calcination was repeated more than three times, and the final calcination was performed at 450°C for...

Ion-conducting, sub-100 nm-thick film of amorphous hafnium silicate

“…It is important to note that the amount of H atom in the film increases in accordance with the height of conductivity in the order of Hf5Si95, Hf3Si97 N Hf20Si80. We can conclude from the results of GDOES that amorphous M n Si 1 − n O x nanomembranes contain sufficient acidic sites throughout the membrane interior as in the case of aluminosilicate film [7,9].…”

“…Thus, the ionic conduction in amorphous Hf n Si 1 − n O x films at elevated temperature is not attributed to the water-mediated proton transport which occurs in a xerogel compound [15,16]. These behaviors of Hf n Si 1 − n O x film is very similar to the proton conductivity of Al n Si 1 − n O x film which reveal the conduction of a native proton on Brønsted acid sites of aluminosilica framework [7][8][9]. The depth profile of H atom in the sample as-prepared at 450°C for 1 h in air was measured by glow discharge optical emission spectroscopy (GDOES) [10] (Fig.…”

“…4. The corresponding data of 60 nm-thick Al 0.1 Si 0.9 O x film [7,9] is also shown as a reference. The conductivity of Al 0.1 Si 0.9 O x film is not dependent on the humidity in the measured temperature range.…”

“…Amorphous silicate materials are advantageous for formation of gas-tight, thin films, as they are made of the random network of Si-O bonding and can form a continuous texture. Recently, we discovered that nanometer-thick films of amorphous silica-based oxides gave enhanced ionic conduction at 100-400°C in dry air, when doped with 13 other metal ions [7][8][9]. Some of these double oxide films showed practically-usable levels of area-specific-resistance at 300-400°C, and the determination of electromotive force indicated that the Al-, Zr-and Ce-doped membranes gave predominant proton conductivity without permeation of H 2 gas.…”

Highly Enhanced Corrosion Resistance of Stainless Steel by Sol-Gel Layer-by-Layer Aluminosilicate Thin Coatings

“…40 The result indicated that the aluminosilicate film was gastight even though the film was as thin as ∼100 nm. Therefore, this inorganic thin film is also promising as a corrosion-resistant coating.…”