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

GeO2 thick (> 10 mu m) 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 degrees C because of the formation of soluble Ge5O112-. With the addition of sulfuric acid, the alkaline GeO2-aqueous ammonia was neutralized, leading to the transformation of Ge5O112- into GeO2 precipitates. When the H2SO4/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 H2SO4/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 H2SO4/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 mu m 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/NH4OH 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.