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Abstract: Three different compositions of ZnO–In2O3 composite nanofibers were prepared by electrospinning, and their gas sensing characteristics were compared to those of pure ZnO and In2O3 nanofibers. All ZnO–In2O3 composite nanofibers showed high response to trimethylamine (TMA), relatively low cross-response to C2H5OH, and negligible cross-responses to CO and H2; pure ZnO or In2O3 nanofibers did not show selective detection of TMA. The maximum responses to 5ppm TMA of the ZnO–In2O3 composite nanofibers with the compositions of [Zn]:[In]=67:33, 50:50, and 33:67 by at% were 133.9 at 300°C, 82.9 at 350°C, and 119.4 at 375°C, respectively. Considering all the sensing characteristics such as gas response, selectivity, and sensing/recovering kinetics together, the operation of the ZnO–In2O3 nanofiber sensor with the composition of [Zn]:[In]=33:67 by at% at 375°C was determined to be in optimal condition to detect TMA. The significant enhancement of gas response and selectivity by the formation of composite nanofibers is discussed in relation to the variation of particle size, the formation of hetero-interfaces between ZnO and In2O3, and the combination of two sensing materials with different catalytic properties, gas adsorption behaviors, and acid–base properties. [Copyright &y& Elsevier]

Abstract: Gas sensors are employed in many applications including detection of toxic and combustible gases, monitoring emissions from vehicles and other combustion processes, breath analysis for medical diagnosis, and quality control in the chemicals, food and cosmetics industries. Many of these applications employ miniaturized solid-state devices, whose electrical properties change in response to the introduction of chemical analytes into the surrounding gas phase. Key challenges remain as to how to optimize sensor sensitivity, selectivity, speed of response and stability. The principles of operation of such devices vary and a brief review of operating principles based on potentiometric/amperometric, chemisorptive, redox, field effect and nanobalance approaches is presented. Due to simplicity of design and ability to stand up to harsh environments, metal oxide-based chemoresistive devices are commonly selected for these purposes and are therefore the focus of this review. While many studies have been published on the operation of such devices, an understanding of the underlying physicochemical principles behind their operation have trailed behind their technological development. In this article, a detailed review is provided which serves to update progress made along these lines. The introduction of nanodimensioned materials has had a particularly striking impact on the field over the past decade. Advances in materials processing has enabled the fabrication of tailored structures and morphologies offering, at times, orders of magnitude improvements in sensitivity, while high-resolution analytical methods have enabled a much improved examination of the structure and chemistry of these materials. Selected examples, illustrating the type of nanostructured devices being fabricated and tested, are discussed. This review concludes by highlighting trends suggesting directions for future progress. [Copyright &y& Elsevier]

Abstract: Chemical and physical synthesis routes were combined to prepare macroporous TiO2 thin films by RF sputtering onto poly methyl methacrylate (PMMA) microsphere templated substrates. The organic template was removed by thermal decomposition at 450°C leaving a thin layer of TiO2 anatase hollow hemispheres with diameter commensurate with that of the original PMMA microspheres (800nm). This unique morphology increased active surface area while reducing the deleterious interfacial effects between film and substrate leading to substantially enhanced NO2 sensitivity. [Copyright &y& Elsevier]

Abstract: An ultraselective and ultrasensitive trimethylamine (TMA) sensor was achieved using MoO3 nanoplates-prepared by ultrasonic spray pyrolysis followed by a heat treatment at 450°C. The small and thin MoO3 nanoplates with gas-accessible structures showed an unusually high response to 5ppm TMA (ratio of resistance to air and gas=373.74) at 300°C with detection limit as low as 45ppb. Moreover, the ratios of the cross-responses to interfering gases (i.e., 5ppm C2H5OH, CO, CH4, C3H8, H2, and NO2) to the response to 5ppm TMA were extremely low (0.008–0.016). The source of the ultraselective and highly sensitive detection of TMA with negligible interference from other gases is discussed with respect to the acid/base properties, size, and morphology of the MoO3 sensing materials. [Copyright &y& Elsevier]

• The non-noble metals are introduced on graphene-based fiber as a sensitizer for the high-performance chemiresistor. • Metal oxides on graphene fibers naturally are converted into metals at a relatively low temperature annealing (under 700 °C) due to catalytic effect of graphene. • The graphene fiber acts as a flexible conductive support as well as a catalyst for the deoxidation of metal oxide. • The metal/metal oxide/graphene fibers exhibited a 16 times higher response to NO 2 gas and recovery than metal oxide/graphene fiber without metal. • The proposed strategy opens a new avenue for the realization of high-performance devices, such as chemical catalysts, photoactive devices. [Display omitted] Noble metal/metal-oxide-based hybrid gas sensors exhibit a low operating temperature, remarkable sensitivity, and fast recovery. As additives, noble metals induce a catalytic sensitization effect, which promotes charge transfer from the metal oxide to the analyte molecules, the so-called spillover mechanism. This suggests that metal catalysts can improve gas sensing performance. Herein, for the first time, non-noble metals are introduced on hybrid metal oxide/graphene fibers as sensitizers to fabricate high-performance chemiresistive sensors. The formation of metal components can be effectively controlled by annealing the metal oxide on graphene. Remarkably, compared with the corresponding metal oxide/graphene fiber sensors without metal components, the metal/metal oxide/graphene fiber sensors exhibit over a 16-fold higher response to NO 2 gas as well as effective recovery characteristics. Specifically, the Cu/Cu 2 O/graphene and Ni/NiO/graphene fiber sensors operating at 150 °C exhibit sensitivities of 18.90 % and 0.82 %, respectively, for 5 ppm NO 2 gas. The proposed strategy to achieve flexible graphene fiber chemiresistors by decorating them with non-noble metal and metal oxide nanoparticles opens a new avenue for realizing high-performance devices, such as photovoltaic devices, photocatalysts, and chemical catalysts. [ABSTRACT FROM AUTHOR]

• Porosity control of nanofibers: The high porosity of the metal oxide nanofibers is the key to achieving the dual sensitization owing to their high gas permeability and numerous active sites. Both saponin and water-ethanol co-solvent play critical roles in modifying morphology of nanofibers into elliptical nanofibers during electrospinning. Additionally, saponin forms numerous pores in the nanofibers when being burnt out during calcination. These porous, elliptical nanofibers possess a larger surface area and porosity.Saponin and water-ethanol co-solvent enabled porous, elliptical nanofibers to emerge. • Bifunctional Ga doping: Ga dopants enhanced sensing performance via conductivity improvement and grain growth inhibition. • Functionalization of Pt catalysts : Spill-over effects and chemical sensitization of remarkably improved C 2 H 2 sensitivity. Herein, we demonstrate a sensitization strategy via solid-solution doping and catalytic metal nanoparticles (NPs) onto porous metal oxide nanofibers (NFs) for superior acetylene (C 2 H 2) sensing capabilities. The introduction of dopants and catalysts not only increases the inherent reactivity of metal oxides, but also induces the spillover of C 2 H 2 onto metal oxides, significantly improving the sensing properties. We elucidate that the high porosity of the metal oxide NFs, prepared by electrospinning and utilizing saponin as a sacrificial template, is the key to achieving high gas permeability and numerous active sites. Consequently, Ga-doped ZnO porous NFs functionalized with Pt catalysts exhibited the highest C 2 H 2 sensing responses among the current state-of-the-art C 2 H 2 sensors (R a /R g = 26.2 at 400 °C) with a fast response time (10.2 s), at a particularly low concentration of 5 ppm. Moreover, these NFs showed excellent selectivity against the interfering gas (H 2 , C 7 H 8 , CO, CH 4 , C 8 H 10 , CH 3 COCH 3 , HCHO and C 2 H 5 OH) and outstanding stability up to 20 sensing cycles to 0.4 ppm of C 2 H 2. These results provide an efficient strategy to tailor composites of metal oxide nanostructures, dopants, and catalysts for highly sensitive gas sensors. [ABSTRACT FROM AUTHOR]