Gas sensing with nanosized oxide materials is attracting much attention because of its promising capability of detecting various toxic gases at very low concentrations. In this study, using clustered SnO2 nanoparticles formed by controlled particle aggregation, we fabricated highly sensitive gas sensing films to detect large gas molecules such as toluene. A hydrothermal method using stanic acid (SnO2·nH2O) gel as a precursor produced monodispersed SnO2 nanoparticles of ca. 5 nm at pH 10.6. Decreasing the solution pH to 9.3 formed SnO2 clusters of ca. 45 nm that were assemblies of the monodispersed nanoparticles, as determined by dynamic light scattering, X-ray diffraction, and transmission electron microscopy analyses. Porous gas sensing films were successfully fabricated by a spin-coating method using the clustered nanoparticles due to the loose packing of the larger aggregated particles. The sensor devices using the porous films showed improved sensor responses (sensitivities) to H2 and CO at 300 °C. The enhanced sensitivity resulted from an increase in the film's porosity, which promoted the gas diffusivity of the sensing films. Pd loading onto the clustered nanoparticles further upgraded the sensor response due to catalytic and electrical sensitization effects of Pd. In particular, the Pd-loaded SnO2 nanoparticle clusters showed excellent sensitivity to toluene, able to detect it at down to low ppb levels.

The effect of water vapor on Pd-loaded SnO2 sensor was investigated through the oxygen adsorption behavior and sensing properties toward hydrogen and CO under different humidity conditions. On the basis of the theoretical model reported previously, it was found that the mainly adsorbed oxygen species on the SnO2 surface in humid atmosphere was changed by loading Pd, more specifically, for neat SnO2 was O(-), while for 0.7% Pd-SnO2 was O(2-). The water vapor poisoning effect on electric resistance and sensor response was reduced by loading Pd. Moreover the sensor response in wet atmosphere was greatly enhanced by loading Pd. It seems that the electron depletion layer by p-n junction of PdO-SnO2 may impede OH(-) adsorption.

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Tungsten trioxide (WO3) is one of the important multifunctional materials used for photocatalytic, photoelectrochemical, battery, and gas sensor applications. Nanostructured WO3 holds great potential for enhancing the performance of these applications. Here, we report highly sensitive NO2 sensors using WO3 nanolamellae and their sensitivity improvement by morphology control using SnO2 nanoparticles. WO3 nanolamellae were synthesized by an acidification method starting from Na2WO4 and H2SO4 and subsequent calcination at 300 °C. The lamellae were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), which clearly showed the formation of single-crystalline nanolamellae with a c-axis orientation. The stacking of each nanolamella to form larger lamellae that were 50-250 nm in lateral size and 15-25 nm in thickness was also revealed. From pore size distribution measurements, we found that introducing monodisperse SnO2 nanoparticles (ca. 4 nm) into WO3 lamella-based films improved their porosity, most likely because of effective insertion of nanoparticles into lamella stacks or in between assemblies of lamella stacks. In contrast, the crystallite size was not significantly changed, even by introducing SnO2. Because of the improvement in porosity, the composites of WO3 nanolamellae and SnO2 nanoparticles displayed enhanced sensitivity (sensor response) to NO2 at dilute concentrations of 20-1000 ppb in air, demonstrating the effectiveness of microstructure control of WO3 lamella-based films for highly sensitive NO2 detection. Electrical sensitization by SnO2 nanoparticles was also considered.

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Gas sensing is an important application of metal oxides. The gas sensor response of metal oxide films is greatly influenced by particle size, pore size, thickness, and surface states. To study the effects of particle and pore sizes of sensing films on sensitivity, we fabricated SnO2-based films with different particle and pore sizes and studied sensor responses to three different gases: H2, CO, and H2S with different Knudsen diffusion coefficients. The pore size radii of the gas sensing films were successfully controlled from 2.8 to 5.5 nm using SnO2 nanoparticles of different sizes (4–17 nm diameter) that were synthesized by seed-mediated growth under hydrothermal conditions. Sensor response to H2 increased with decreasing particle size because of the formation of an electron depletion layer within the nanosized crystals. In contrast, the response to CO and H2S increased with increasing particle size and the resultant pore size. Using the Knudsen diffusion-surface reaction equation, we simulated a gas concentration profile within the films, which revealed that the diffusion of CO and H2S is limited by small pores because of their lower diffusion rates compared with H2. We show that controlling the pore size of the sensing films produces ultrasensitive films, and a large resistance change by 4 orders of magnitude is achieved in response to a low concentration of H2S (5 ppm).

The type and amounts of oxygen adsorption species at various atmospheric humidity levels are important factors in improving the sensitivity to combustible gases and stability to humidity changes of SnO 2 -based resistive-type gas sensors. We investigated the effect of antimony (Sb) doping of SnO 2 nanoparticles on the stability of the sensitivity to humidity changes and oxygen adsorption species under humid atmosphere. No significant degradation of the sensitivity to hydrogen of Sb-SnO 2 sensors was observed between 16 and 96 RH%, while an undoped SnO 2 sensor showed gradually decreasing responses with increasing humidity. An evaluation of oxygen adsorption species under humid atmosphere showed a transition from O 2− to O − with increasing humidity from 16 to 96 RH%. However, the O 2− adsorption sites were maintained on the surfaces of the SbSnO 2 , even as the humidity increased. Moreover, the extent of oxygen adsorption on the Sb-SnO 2 was not obviously changed with increasing humidity. These results indicate that Sb atoms function as hydroxyl absorbers and also generate O 2− adsorption sites in their vicinity. Additionally, Pd loading on the Sb-SnO 2 further enhanced the sensor response under humid atmosphere, while maintaining the stability to humidity changes. Therefore, we successfully imparted stability to the sensitivity of SnO 2 nanoparticles during humidity changes, representing an important improvement with applications to the development of high performance, practical, resistive-type gas sensors.

To explore oxygen permeable materials, oxygen permeation properties of partially A-site substituted BaFenormalO3−δ perovskites were investigated. Ba sites in BaFenormalO3−δ were substituted with cations such as Na, Rb, Ca, Y, and La by 5%. The partial substitution with Ca, Y, and La, whose ionic radii are smaller than that of Ba, succeeded in stabilizing a cubic perovskite structure that is a highly oxygen permeable phase, as revealed by X-ray diffraction analysis. This can be explained in terms of a decrease in the tolerance factor (t) . Among the normalBa0.95normalM0.05FenormalO3−δ (M = Na, Rb, Ca, Y, and La) membranes tested, normalBa0.95normalLa0.05FenormalO3−δ showed the highest oxygen permeability at 600–930°C, owing to the stabilization of the cubic phase without the formation of impurity phases. From chemical analysis, the oxygen permeability of normalBa1−xnormalLaxFenormalO3−δ membranes was correlated with the amount of oxygen defects (δ) in the lattice. The oxygen permeation flux of normalBa0.95normalLa0.05FenormalO3−δ membrane was significantly increased by reducing its thickness. Furthermore, a normalBa0.975normalLa0.025FenormalO3−δ membrane exhibited good phase stability under He flow at elevated temperatures. The obtained results indicate the promising properties of normalBa1−xnormalLaxFenormalO3−δ membranes as a cobalt-free material that has a high oxygen permeability, good phase stability, and low cost.