Compositional dependence of the proton conductivity of anodic ZrO ₂-WO₃-SiO₂ nanofilms at intermediate temperatures

Novel proton-conducting ZrO₂-WO₃-SiO₂ nanofilms of various compositions and thicknesses (∼50 to ∼300 nm) have been prepared by anodizing of magnetron-sputtered Zr-W-Si alloys in 0.1 mol dm^-3 phosphoric acid electrolyte at 20°C. All the anodic oxide nanofilms examined reveal efficient proton conductivity after post-annealing at 250°C. Further increase in the post-annealing temperature results in the conductivity degradation for the anodic oxide nanofilms on the alloy containing only 5 at% silicon, while the high conductivity is maintained even after post-annealing at 300°C for those containing 15 at% or more silicon. The proton conductivity is dependent upon tungsten content; the conductivity of 5 × 10^-6 S cm^-1 for the ∼100 nm-thick films on the Zr₃₁W₅₅Si₁₄ at 100°C is approximately 10 times that on the Zr₄₈W₃₇Si₁₅. The anodic oxide nanofilms consist of two layers, comprising a thin outer ZrO₂ layer and an inner ZrO₂-WO₃-SiO₂ layer. Both layers show thickness-dependent conductivity and the proton conductivity of the two-layer anodic films is enhanced one order of magnitude by reducing the film thickness from ∼300 nm to ∼100 nm. Different mechanisms are proposed for the thickness dependence of the conductivity of the outer and inner layers.