Spontaneous solution-sol-gel process for preparing tin oxide monolith

A sol-gel process for preparing SnO₂ monolith of high specific surface area and transparency from chloride solution is described. Without introducing any alkaline precipitating reagent to induce condensation, this new process employs tin chloride (or its hydrate), water, and, optionally, alcohols as the only process reagents. Spontaneous solution-to-sol and sol-to-gel transitions take place upon mixing these reagents under appropriate conditions, and the entire transition processes are carried out under acidic conditions (typically pH ≤ 4.0). The rate of condensation has been found to increase with decreasing SnCl₄ concentration, which corresponds to decreasing solution acidity, and with increasing temperature. For fixed starting salt concentration and temperature, there exists an optimum amount of ethanol addition for the fastest condensation. Good performance of thus derived SnO₂ monolith has been demonstrated in two applications, including catalytic oxidation and solid-state gas-sensing for carbon monoxide.