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Abstract: Tungsten oxide nanorods assembled microspheres were synthesized by a facile hydrothermal process at 180°C using ammonium metatungstate and oxalic acid as starting materials. The morphology and structural properties were investigated using scanning electron microscopy, powder X-ray diffraction, and transmission electron microscopy. The as-synthesized microspheres are composed of orthorhombic WO3·xH2O nanorods with diameter less than 100nm. These microspheres lose water gradually during annealing and transfer to monoclinic WO3 when annealed at 550°C. The gas sensing properties of the microspheres annealed at different temperatures were studied by exposing the gas sensors made from microspheres to NO2 gas. The results indicated that the crystalline phase of the microspheres has no obvious effect on the gas sensing performance. The microspheres annealed at 350°C showed fast and the highest response to NO2 gas due to the three-dimensional network based on the nanorods and the high effective surface area. [Copyright &y& Elsevier]

Abstract: Gallium oxide nanowires were synthesized by a chemical thermal evaporation method using gallium metal as a source material. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy characterizations indicate that the obtained nanowires are well-crystallized single phase monoclinic Ga2O3. Multiple nanowire gas sensors were fabricated by dispensing the Ga2O3 nanowires on an interdigitated Pt-electrode. The Ga2O3 nanowire gas sensors show reversible response to O2 and CO gases in a working temperature range of 100–500°C. A peak response is found at 300°C for O2 gas and the peak response appears at 200°C for CO gas. For both kinds of gases, the sensor response increases empirically with an increase of gas concentration. The results demonstrate the possibility of using the Ga2O3-based gas sensor at low working temperature field. [Copyright &y& Elsevier]

Abstract: The influences of thermal annealing on the microstructure and NO2-sensing properties of sputtered WO3 thin films were investigated. The WO3 films as-deposited at room temperature were amorphous and would crystallize to monoclinic structure when annealed at 350°C or above. The grains and pores in the WO3 films grew larger with an increase in annealing temperature. The effective surface area and pore volume changed non-monotonously with increasing the annealing temperature. The film annealed at 350°C showed the largest effective surface area. The film annealed at 500°C had the largest pore volume. Among the films investigated in this study, the WO3 thin film annealed at 500°C showed the quickest response/recovery and the highest response to 10ppm NO2. These results indicate the importance of achieving porous structure on improving the gas sensing performance. [Copyright &y& Elsevier]

Abstract: Tin oxide (SnO2) nanowires with a tetragonal structure were synthesized by thermal evaporation of tin grains at 900°C. The obtained nanowires were doped with palladium. The morphology, crystal structure, and H2 gas sensing properties of undoped and Pd-doped SnO2 nanowires were investigated. SnO2 nanowires were approximately 30–200nm in diameter and several tens of micrometers in length. Gas sensors based on undoped, 0.8wt% Pd-doped, and 2wt% Pd-doped SnO2 nanowires were fabricated. These SnO2 nanowire gas sensors showed a reversible response to H2 gas at an operating temperature of RT—300°C. The sensor response increased with increasing Pd concentration. The 2wt% Pd-doped SnO2 nanowire sensor showed a response as high as 253 for 1000ppm H2 gas at 100°C. The results demonstrated that Pd doping improved the sensor response and lowered the operating temperature at which the sensor response was maximized. [Copyright &y& Elsevier]

A photo-activated semiconductor gas sensor presents a promising strategy for detecting highly toxic nitrogen dioxide (NO 2) gases at room temperature. Near-infrared enhanced gas sensor may offer a new strategy for room-temperature NO 2 detection. In this work, MOF-based SnO 2 /SnSe 2 heterostructures prepared by a solvothermal method were used to probe NO 2 gas at room temperature. Under near-infrared irradiation, the SnO 2 /SnSe 2 sensor exhibits a response/recovery time of 23/7 s and a sensitivity value of 1500 for NO 2 at room temperature. In addition, the adsorption behavior between SnO 2 /SnSe 2 and gas molecules was further studied by first-principles calculations. The optimization of the gas-sensing performance was explored by considering the photoexcited electrons and the heterostructure. This work highlights the potential of SnO 2 /SnSe 2 heterostructures as high-performance NO 2 sensors operating at room temperature, as well as the enhanced activation effect of near-infrared irradiation on semiconductor gas sensors. [Display omitted] • MOF-based SnO 2 /SnSe 2 heterojunction were prepared by solvothermal methods. • NIR excitation optimizes the gas sensing performance of the SnO 2 /SnSe 2 sensor. • First principles calculation verifies the excellent selectivity of SnO 2 /SnSe 2. [ABSTRACT FROM AUTHOR]

This work presents the development of a highly selective wireless NO 2 gas sensor based on a resonant antenna circuit using a low temperature co-fired ceramic (LTCC) technique. The wireless LTCC based gas sensor consists of a planar inductor-interdigital capacitor (LC) resonant antenna, which incorporates two dimensional (2D) SnS 2 nanoflakes as a model NO 2 gas selective material. The fabrication of the LTCC platform is fully described, a brief description on the characterization of the gas sensitive material is presented and eventually the operation of the system for the sensing of NO 2 gas is investigated. The response of the LTCC template was associated to the changes of its resistance and capacitance due to the alterations in the properties of 2D SnS 2 as a result of physical adsorption of NO 2 gas molecules onto its surface. The wireless gas sensing performance under different operating temperatures and NO 2 gas concentrations are presented. The obtained LC gas sensor is successfully used for the wireless sensing of NO 2 in the background of atmospheric gas at concentrations lower than 0.6 ppm. The developed technology has a great promise for establishing wireless gas sensors for harsh environments [ABSTRACT FROM AUTHOR]

• Composite of tin oxide-reduced MXene was hydrothermally synthesized. • The obtained n-type response composite based sensor exhibits a higher and more stable response to NH 3 than pure MXene at room temperature. • Formation of heterojunction could introduce extra electrons yielding out more active sites for NH 3 adsorptions. • The use of as-synthesized MXene/SnO 2 heterojunctions in wireless LC sensors have greatly promoted the response/recovery to NH 3. Two dimensional (2D) MXene decorated by the SnO 2 nanoparticles has been successfully synthesized by the hydrothermal method. The formed MXene/SnO 2 heterojunction based chemirestistive-type sensors exhibit an excellent sensitivity and selectivity to different concentrations of NH 3 from 0.5 to 100 ppm at room temperature. The enhanced sensing properties can be attributed to two major factors. On one hand, 2D MXene provides a matrix which has a unique, selective adsorption ability to ammonia and good conductivity that makes the room-temperature sensing of the heterojunction based sensor feasible. On the other hand, the difference in the Fermi level of the 2D MXene and SnO 2 drives the charge transfer at the interface of the heterojunctions enriching the electrons on the surface region of SnO 2 and consequently enhancing the sensitivity. Furthermore, a wireless sensor made from an inductor-capacitor (LC) antenna and the as-synthesized MXene/SnO 2 heterojunctions fabricated by the low-temperature-cofired ceramic technique (LTCC) was preliminary investigated. The wireless sensor shows a even faster response/recovery speeds (<30 s) and more stable response to different concentrations of ammonia when the response was expressed in the mode of resonant frequency indicating a promising practical application. [ABSTRACT FROM AUTHOR]