Of SnO2 Nanocrystallites

Two approaches have been reported to control the growth in the size of the nanocrystallites during the high-temperature exposure [16, 78]. The first approach [78] is based on replacing the surface OH group with another functional group that does not condense as OH groups and that could eventually form the "pins" at the grain boundaries during high-temperature exposure. Hexamethyldisilazane (HMDS; [Si(CH3)3]2NH) is a OH scavenging reagent because it replaces the OH groups in the SnO2 gel to form noncon-densing methyl siloxyl group, and when decomposed in air above 300 °C, it transforms to extremely small SiO2 particles, which could serve as the "pinning" for the SnO2 particles. When heated at 150 °C for 1 h, HMDS vaporizes and reacts with the surface OH groups by following reaction:

The methyl siloxyl group decomposes after firing at 200500 °C for 1 h in air, producing Sn—O—Si bond on the surface, which is not condensing as the surface OH groups. Moreover, the process produces extremely tiny SiO2 particles over SnO2 nanoparticles, which act as "pinning sites" for the grain boundary movement during densification, thus avoiding excessive growth at higher temperature. In the second method [16], the alkaline Sn(OH)2 sol is subjected to the hydrothermal treatment in an autoclave at 200 °C for 3 h. Such hydrothermally treated sol particulates are more resistant to grain growth as compared to untreated sol.

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