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Branched nanowires (NWs) are a novel class of composite materials with increased surface area relative to their pristine one-dimensional counterparts. Accordingly, they are good choice for gas sensing studies. In this study, p-n, TeO2-branched SnO2 NWs were produced by a two-step catalyst-assisted vapor-liquid-solid (VLS) growth technique for gas sensing studies. First, SnO2 NWs were synthesized from highly pure Sn powders, and TeO2 branches were subsequently added. The fabricated samples were well characterized in terms of morphology, crystallinity, and chemical composition. Gas sensing results exhibited the enhanced NO2 sensing capability of TeO2 branched SnO2 NW sensors relative to pristine SnO2 NWs. In particular, the maximum responses (Rg/Ra) of pristine and TeO2 branched SnO2 sensors to 10 ppm NO2 were 6.34 and 10.25, respectively. Furthermore, dynamics of TeO2 branched sensor at the optimal temperature was faster. Superior sensing properties of TeO2 branched SnO2 NWs were related to the high surface area of the branched sensors and creation of p-n heterojunctions on the surfaces of this sensor. We believe that branching is a good way to realize gas sensors for practical usages.

In this study, we synthesized porous (porosity: ~16%, average pore size: ~60 nm) ZnO nanosheets (thickness: ~80 nm) using a conventional solvothermal method to investigate NO2 gas sensing properties. Porous ZnO nanosheets triggered the detection of NO2 gas with high sensitivity. Responses of 2.93 – 0.5 ppm and 74.68 – 10 ppm NO2 gas at 200 °C were observed in the porous ZnO nanosheet-based gas sensor. In addition, improved sensing properties with high selectivity to NO2 gas, reasonable stability, and high response even in the presence of water vapor molecules were obtained. We found that the enhanced NO2 gas response of the porous ZnO nanosheet-based gas sensor was due to the synergetic effects of the high surface area, ZnO/ZnO homojunctions, and structural defects. We developed a highly sensitive NO2 gas sensor with improved reliability using morphologically engineered ZnO, which was prepared via a simple and scalable chemical-synthesis route.

Pristine and Al-doped SnO2 nanoparticles with different amounts of Al dopant were synthesized using a conventional hydrothermal process. The existing SnO2 exhibited simple interstitial physical bonding (SnO2–Al (1:0.16)) with the Al dopant; however, with an increase in the Al concentration, the bonding changed to substitutional chemical bonding (SnO2–Al (1:0.33)). We found that this crystal structural change is strongly interrelated with surface reactivity; the optimized Al-doped SnO2 nanoparticles-based sensor exhibited a significantly improved response of 17.38 – 20 ppm H2S gas with a response time of 35 s. The enhanced gas response was related to the high surface area of the optimal gas sensor (BET surface area = 78.087 m3/g) as well as the beneficial effects of Al doping. It is highlighted that this simple technique of engineering the bonding characteristics can be widely applied to other semiconducting metal oxides.

In this study, a peculiar configuration for a gas sensor consisting of vertically aligned silicon nanowires (VA-Si NWs) synthesized by metal-assisted chemical etching (MACE) is reported. Si NWs were prepared via a facile MACE method and subsequent thermal annealing. Etching was performed by generation of silver nanoparticles (Ag NPs) and subsequent etching in HF/H2O2 aqueous solution; the growth conditions were optimized by changing the process parameters. Highly vertically oriented arrays of Si NWs with a straight-line morphology were obtained, and a top–top electrode configuration was applied. The VA-Si NW gas sensor showed good sensing performance, and the VA-Si NWs exhibited a remarkable response (Rg/Ra = 11.5 ∼ 17.1) to H2 gas (10–50 ppm) at 100 °C which was the optimal working temperature. The formation mechanism and gas sensing mechanism of VA-Si NWs are described. The obtained results can suggest new approaches to making inexpensive, versatile, and portable sensors based on Si NWs having a novel top–top electrode structure that are fully compatible with well-developed Si technologies.

In this work, ZnO-decorated multi-walled carbon nanotube (MWCNT) nanocomposites prepared using an in-situ method involving the thermal evaporation of Zn powders in the presence of MWCNTs. The gas sensing characteristics of the MWCNT/ZnO nanocomposites are studied, and results for the material characterizations for the synthesized nanocomposites confirm the formation of well-distributed ZnO nanoparticles onto MWCNTs, creating MWCNT/ZnO nanocomposites. The gas sensing properties of the MWCNT/ZnO nanocomposite gas sensor, such as response, cross-sensitivity, and response-recovery time, are investigated and compared with a bare MWCNT sensor. The decoration of ZnO nanoparticles greatly improves the gas sensing properties of bare MWCNTs. We discussed the possible mechanisms for the enhancement of sensing capabilities. The results suggest that decoration of n-type semiconducting oxide materials, such as ZnO in the form of nanoparticles, is a promising strategy for improvement of gas sensing properties of p-MWCNTs.