Spontaneous solution-sol-gel process for preparing tin oxide monolith
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Spontaneous solution-sol-gel process for preparing tin oxide monolith Nae-Lih Wu, Lih-Fu Wu, Ya-Chin Yang, and Shu-Jiuan Huang Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, Republic of China (Received 10 July 1995; accepted 5 January 1996)
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.
I. INTRODUCTION
Tin oxide is a material of versatile applications. It has been used, for example, as catalysts for oxidation of organics,1–3 as solid-state sensors to reducing gases,4–6 and to make indium tin oxide (ITO) transparent conductive glasses.7,8 The success in many of these applications relies on a nano-size pore or grain structure. Such microstructures, in general, can be achieved by the sol-gel method, as demonstrated in many other ceramic materials.9,10 Either alkoxides or inorganic salts, particularly chloride, of tin are suitable starting reagents for the sol-gel processing. Nevertheless, the use of inorganic salts allows more generalized applications of the process, because alkoxides are more difficult to produce11,12 and expensive. The current sol-gel processes for making SnO2 13–16 from tin chloride solutions typically introduce a precipitating reagent, mostly ammonia hydroxide, into the acidic salt solution to induce condensation producing oxyhydroxide or oxide precipitates. The precipitate is then peptized into a sol solution, from which a gel and then a monolith is obtained by removing the solvent. Kistler13 reported the preparation of SnO2 gel by the hydrolysis of tin chloride and the peptization of the precipitates with ammonia hydroxide, but did not give experimental detail. By employing a similar approach, Giesekke et al.14 reported gel formation only after the NH4 1 and Cl2 ions were eliminated by washing in bidistilled water for a period of 30 days. Hiratsuka et al.15 studied the colloidal stability and sol-gel formation in the same system. They suggested that the presence of NH4 1 favored the sol stability and that the ratio between NH4 1 J. Mater. Res., Vol. 11, No
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