Hierarchical porous tin oxide from the sol-gel route

Hierarchical structured micro/nano-sized materials are especially attractive for applications in chemical sensors because it can provide high surface reactions and fast diffusion allowing easy gas penetration inner the sensing... [ view full abstract ]

Hierarchical structured micro/nano-sized materials are especially attractive for applications in chemical sensors because it can provide high surface reactions and fast diffusion allowing easy gas penetration inner the sensing device. Tin dioxide is well-known as the best-understood prototype of oxide-based gas sensors, but highly specific and sensitive SnO₂ sensors are not yet available. In this work sulfated tin oxide suspensions were produced by a direct method that combines the formation of oxide nano crystalline particles from sol-gel process simultaneously to the surface modification by sulfate anchoring. Different SnO₂:SO₄^2- molar ratios were evaluated in the SnO₂ characteristics. Oxide-sulfate bonds formed by uni- and bidentate complexes were observed by FTIR in the nano crystalline powders which present the cassiterite tetragonal phase independent of the sulfate amount. The average crystallites size (13.5 nm) and radius of gyration (7.7 nm) determined from XRPD and SAXS for the sample free of sulfate decrease as the sulfate amount increases to around 7 nm for the crystallite size and 3 nm for the radius of gyration. These sizes decrease is due to the reduction of the surface energy by sulfate groups avoiding the agglomeration of the tin oxide particles. The porous structure generated by the sulfated SnO₂ xerogel network consists in elongated pores with main diameter around 3.3 nm and surface area up to 135.5 m²/g. The sulfate groups allowed obtaining smaller particles size and mesopores with narrow pore size distribution than the free of sulfate tin oxide. The larger particle sizes and agglomerates in the sulfate free SnO₂ xerogel evidence the positive effect of sulfation in the pores features.