Improvement of H2S sensing properties of SnO2-based thick film gas sensors promoted with MoO3 and NiO.

The effects of the SnO2 pore size and metal oxide promoters on the sensing properties of SnO2-based thick film gas sensors were investigated to improve the detection of very low H2S concentrations (<1 ppm). SnO2 sensors and SnO2-based thick-film gas sensors promoted with NiO, ZnO, MoO3, CuO or Fe2O3 were prepared, and their sensing properties were examined in a flow system. The SnO2 materials were prepared by calcining SnO2 at 600, 800, 1,000 and 1,200 °C to give materials identified as SnO2(600), SnO2(800), SnO2(1000), and SnO2(1200), respectively. The Sn(12)Mo5Ni3 sensor, which was prepared by physically mixing 5 wt% MoO3 (Mo5), 3 wt% NiO (Ni3) and SnO2(1200) with a large pore size of 312 nm, exhibited a high sensor response of approximately 75% for the detection of 1 ppm H2S at 350 °C with excellent recovery properties. Unlike the SnO2 sensors, its response was maintained during multiple cycles without deactivation. This was attributed to the promoter effect of MoO3. In particular, the Sn(12)Mo5Ni3 sensor developed in this study showed twice the response of the Sn(6)Mo5Ni3 sensor, which was prepared by SnO2(600) with the smaller pore size than SnO2(1200). The excellent sensor response and recovery properties of Sn(12)Mo5Ni3 are believed to be due to the combined promoter effects of MoO3 and NiO and the diffusion effect of H2S as a result of the large pore size of SnO2.

ZnO nanorods, nanotubes and nanorings: controlled synthesis and structural properties.

Controlled synthesis of ZnO nanorods (ZNRDs), nanotubes (ZNTs) and nanorings (ZNRs) has been carried out by a two-step sonochemical/chemical process at room temperature without any catalyst, template or seed layer. The crystallinity, structure and morphology of ZNRDs, ZNRs and ZNTs were examined by X-ray diffraction (XRD) analysis, scanning electron micrographs (SEM), high resolution transmission electron microscope (HR-TEM) and selected area electron diffraction (SAED). The as-prepared ZnO nanostructures were single crystalline with hexagonal cross-section and uniform size. The effect of precursor concentration on the growth and that of the etching duration on the hollow formation were analyzed, and the obtained results revealed that the precursor concentration and etching time play an important role in determining final morphologies of the samples. By tuning the etching time, the precise size control of ZNTs and ZNRs was achieved. Possible formation mechanisms of these nanostructures are proposed based on the experimental results.

Effects of textural properties on the response of a SnO2-based gas sensor for the detection of chemical warfare agents.

The sensing behavior of SnO(2)-based thick film gas sensors in a flow system in the presence of a very low concentration (ppb level) of chemical agent simulants such as acetonitrile, dipropylene glycol methyl ether (DPGME), dimethyl methylphosphonate (DMMP), and dichloromethane (DCM) was investigated. Commercial SnO(2) [SnO(2)(C)] and nano-SnO(2) prepared by the precipitation method [SnO(2)(P)] were used to prepare the SnO(2) sensor in this study. In the case of DCM and acetonitrile, the SnO(2)(P) sensor showed higher sensor response as compared with the SnO(2)(C) sensors. In the case of DMMP and DPGME, however, the SnO(2)(C) sensor showed higher responses than those of the SnO(2)(P) sensors. In particular, the response of the SnO(2)(P) sensor increased as the calcination temperature increased from 400 °C to 800 °C. These results can be explained by the fact that the response of the SnO(2)-based gas sensor depends on the textural properties of tin oxide and the molecular size of the chemical agent simulant in the detection of the simulant gases (0.1-0.5 ppm).