Design of mesoporous oxides as semiconductor gas sensor materials

Abstract

Preparation, characterization and gas-sensing properties of mesoporous SnO2 powders have been described. SnO2 powders prepared from sodium stannate by utilizing a self-assembly of n-cetylpyridinium chloride were characterized to have hexagonally ordered mesoporous structure (d100≈4.4 nm). Treatment of the as-prepared powders with phosphoric acid was effective for depressing the growth of SnO2 crystallites and then maintaining the ordered mesoporous structure (d100≈3.1 nm, mean pore diameter≈1.5-2.5 nm) up to elevated temperatures, while the pore volume decreased. H2 sensitivity of mesoporous SnO2 was largely dependent on its specific surface area, the sensitivity being highest (Rg/Ra≈337 to 1000 ppm H2 at 400°C) with the highest specific surface area (374 m2 g-1 after calcination at 600°C). Mesoporous SnO2 powder with larger mesopores (mean pore diameter: ca. 5.2 nm, surface area: 198 m2 g-1 after calcination at 600°C), which was prepared by utilizing a block copolymer with a large molecular weight (ca. 5800) as a surfactant, showed higher H2 sensitivity (Rg/Ra≈77 to 1000 ppm H2 at 400°C) than that of n-cetylpyridinium chloride-derived SnO2 with the same surface area but smaller mesopores. However, electrical resistance levels in air of the monolith sensors based on the mesoporous materials were close to the limit for practical measurement. Then, the mesoporous SnO2 was utilized as a surface coating layer of conventional SnO2 powder, which was prepared by pyrolysis of tin oxalate. The coating was effective for improving the gas-sensing properties, while maintaining the sensor resistance in air at a practical level. For example, NOx sensitivity of conventional SnO2 markedly increased with the coating, while H2 sensitivity of the coated sensor was lower than that of mesoporous SnO2.