Fabrication of thick (>10 μm) GeO₂ ceramic films by a facile liquid phase deposition process

GeO₂ thick (>10 μm) ceramic films were fabricated by facile liquid-phase deposition. The solubility of hexagonal GeO₂ in water was increased ten times on adding aqueous ammonia at 70°C because of the formation of soluble Ge₅O₁₁^2-. With the addition of sulfuric acid, the alkaline GeO₂-aqueous ammonia was neutralized, leading to the transformation of Ge₅O₁₁ ^2- into GeO₂ precipitates. When the H₂SO ₄/NH₄OH molar ratio was higher than 11.7, immediate precipitation of GeO₂ particles took place in the solution with no GeO₂ particles deposited on the substrate. When the H ₂SO₄/NH₄OH molar ratio was decreased from 5.85 to 0.29, the precipitation process was retarded, leading to the gradual growth of uniform hexagonal GeO₂ thick films on the substrates. The compactness of the film increased with a decrease in the H₂SO ₄/NH₄OH molar ratio. The growth of the GeO₂ crystal film followed the Bravais-Freidel-Donnay-Harker and Hartman-Perdok models, and the cubic GeO₂ particles with a size of about 10 μm were finally formed on the film. No GeO₂ precipitate was produced on the substrate when the GeO₂-aqueous ammonia had an excessively low H₂SO₄/NH₄OH molar ratio (≤0.15) because the GeO₂ solute did not reach the saturation level and the nucleation process could not take place on the substrate. The particles on the film surface deposited from the solution with an H₂SO₄/NH ₄OH molar ratio of 0.29 exhibited a nanopore structure. An initial study using this film as a support for KNO₃ catalyst was carried out and the result revealed that the catalyst showed good catalytic activity of diesel soot oxidation.