A dimethyl disulfide gas sensor based on nanosized Pt-loaded tetrakaidecahedralα-Fe2O3nanocrystals.

Surface modification by employing precious metals is one of the most effective ways to improve the gas-sensing performance of metal oxide semiconductors. Pureα-Fe2O3nanoparticles and Pt-modifiedα-Fe2O3nanoparticles were prepared sequentially using a rather simple hydrothermal synthesis and impregnation method. Compared with the originalα-Fe2O3nanomaterials, the Pt-α-Fe2O3nanocomposite sensor shows a higher response value (Ra/Rg = 58.6) and a shorter response/recovery time (1 s/168 s) to 100 ppm dimethyl disulfide (DMDS) gas at 375 °C. In addition, it has better selectivity to DMDS gas with the value of more than 9 times higher than the other target gases at 375 °C. This study indicates that the Pt-α-Fe2O3nanoparticle sensor has good prospects and can be used as a low-cost and effective DMDS gas sensor.

Mesoporous Ti0.5Cr0.5N for trace H2S detection with excellent long-term stability.

Efficient, accurate and reliable detection and monitoring of H2S is of significance in a wide range of areas: industrial production, medical diagnosis, environmental monitoring, and health screening. However the rapid corrosion of commercial platinum-on-carbon (Pt/C) sensing electrodes in the presence of H2S presents a fundamental challenge for fuel cell gas sensors. Herein we report a solution to the issue through the design of a sensing electrode, which is based on Pt supported on mesoporous titanium chromium nitrides (Pt/Ti0.5Cr0.5N). Its desirable characteristics are due to its high electrochemical stability and strong metal-support interactions. The Pt/Ti0.5Cr0.5N-based sensors exhibit a much smaller attenuation (1.3%) in response to H2S than Pt/C-sensor (40%), after 2 months sensing test. Furthermore, the Pt/Ti0.5Cr0.5N-based sensors exhibit negligible cross response to other interfering gases compared with hydrogen sulfide. Results of density functional theory calculation also verify the excellent long-term stability and selectivity of the gas sensor. Our work hence points to a new sensing electrode system that offers a combination of high performance and stability for fuel-cell gas sensors.

Integrated sensing array of the perovskite-type LnFeO3 (LnË­La, Pr, Nd, Sm) to discriminate detection of volatile sulfur compounds.

Distinguishing toxic gases among the various volatile sulfur compounds (VSCs) is of significant practical value for atmospheric and environmental pollution monitoring, industrial monitoring, and even for medical diagnostics (where VSCs are indicators of diseases). The particular challenge lies in the detection and discrimination of sulfur-containing gases such as dimethyl disulfide (DMDS), methyl sulfide (DMS), hydrogen sulfide (H2S), and carbon disulfide (CS2) is of value. Herein, single-phase perovskite-type LnFeO3 nanoparticles were prepared by the citrate sol-gel method. Their gas sensing characteristics regard to the four typical VSCs were investigated. We found that the gas response of the p-type semiconductor LnFeO3 gas sensors to the four typical VSCs are significantly different. In addition, the sensors offer high performance, good tolerance to environmental changes and long-term stability for detecting VSCs gas at an operating temperature of 210 °C. A new design of sensor array was realized by integrating a series of LnFeO3 materials, which revealed excellent recognition ability for various VSCs, showing promise for real time monitoring.

Mesoporous WO3 modified by Au nanoparticles for enhanced trimethylamine gas sensing properties.

In this paper, KIT-6 is used as a template to prepare ordered mesoporous materials WO3 and Au-loaded WO3 (Au-WO3). The pristine WO3 sensor and the Au-WO3 sensor are fabricated for the detection of 19 important gases, such as trimethylamine, formaldehyde and CS2. The results show that the Au-WO3 sensor has better selectivity and higher response to TMA. At a working temperature of 268 °C, the response (Ra/Rg) of the Au-WO3 sensor to 100 ppm of TMA is 41.56 and the response time is 1 s. In addition, the sensor has excellent response/recovery capabilities and stability. These high sensing performances are mainly attributed to the electronic and chemical sensitization of the noble metal Au and the presence of a high specific surface area supported by the mesoporous structure. Therefore, Au-doped mesoporous WO3 should be a promising material for a high performance TMA gas sensor.

Surface Functionalized Sensors for Humidity-Independent Gas Detection.

Semiconducting metal oxides (SMOXs) are used widely for gas sensors. However, the effect of ambient humidity on the baseline and sensitivity of the chemiresistors is still a largely unsolved problem, reducing sensor accuracy and causing complications for sensor calibrations. Presented here is a general strategy to overcome water-sensitivity issues by coating SMOXs with a hydrophobic polymer separated by a metal-organic framework (MOF) layer that preserves the SMOX surface and serves a gas-selective function. Sensor devices using these nanoparticles display near-constant responses even when humidity is varied across a wide range [0-90 % relative humidity (RH)]. Furthermore, the sensor delivers notable performance below 20 % RH whereas other water-resistance strategies typically fail. Selectivity enhancement and humidity-independent sensitivity are concomitantly achieved using this approach. The reported tandem coating strategy is expected to be relevant for a wide range of SMOXs, leading to a new generation of gas sensors with excellent humidity-resistant performance.

ZnO nanoflowers modified with RuO2 for enhancing acetone sensing performance.

Surface modification is a simple and effective means to promote the sensing performance of metal oxide semiconductor-based gas sensors. Marigold-shaped ZnO nanoflowers are fabricated via a simple precipitation reaction and subsequently catalytically modified with RuO2 on the surface through an ethylene glycol solvothermal treatment. The experimental results have proven that a very low content of Ru on the surface of ZnO exists in an oxidized state. However, the gas response of the sensor based on RuO2-modified ZnO is remarkably improved by 17 times to 100 ppm acetone with the decrease of optimal operating temperature from 219 °C-172 °C and reduction in recovery time from 79-52 s. The sensing enhancement mechanism of surface modification can be attributed to the formation of massive small heterostructure between p-type RuO2 ultrasmall nanoparticles and n-type ZnO as well as the catalytic effect of Ru4+ and a rougher surface.

Oxygen-Defective Ultrathin BiVO4 Nanosheets for Enhanced Gas Sensing.

BiVO4 nanomaterials are potentially applicable in gas sensing, but the sensing performance is limited by the less active sites on the BiVO4 surface. In this work, we propose a strategy to improve the gas-sensing performance of BiVO4 by forming ultrathin nanosheets and introducing oxygen vacancies, which increase the surface active sites. Two-dimensional (2D) BiVO4 nanosheets with oxygen vacancies are prepared through a colloidal method with the assistance of nitric acid. Gas sensors based on the oxygen-defective 2D ultrathin BiVO4 nanosheets exhibit an enhanced sensing response, which is 3.4 times higher than those of the sensors based on oxygen-abundant BiVO4 nanosheets. The density functional theory calculation is employed to uncover the promoting effects of oxygen vacancies on enhancing the O2 adsorption capability of BiVO4 nanosheets. This work is not only expected to build a wide range of 2D metal oxide semiconductors with a high gas-sensing performance but also gives an insight into the mechanism of the enhanced response induced by the oxygen vacancies, which will be a guideline for further designing high-performance sensing materials.

Hierarchical Co3O4@NiMoO4 core-shell nanowires for chemiresistive sensing of xylene vapor.

Hierarchical Co3O4@NiMoO4 core-shell nanowires (NWs) were synthesized utilizing a two-step hydrothermal method. The NWs show a high chemiresistive response (at a temperature of 255 °C) to xylene, with an Rgas/Rair ratio of 24.6 at 100 ppm xylene, while the response towards toluene, benzene, ethanol, and acetone, CO, H2S and NO2 is much weaker. In contrast, pure Co3O4 nanowires exhibit weak responses to all the vapors/gases and poor selectivity. The new NW sensor displays an almost linear response (1-100 ppm) to xylene and a lower detection limit of 424 ppb. The remarkable gas sensing characteristics are attributed to the synergistic catalytic effect and the formation of a heterostructure between Co3O4 and NiMoO4. Graphical abstract Schematic presentation of a xylene vapor chemiresistive sensor based on Co3O4@NiMoO4 core-shell nanowires. The Co3O4@NiMoO4 core-shell nanowires-based sensor exhibits a high response (24.6) to 100 ppm xylene at 255 °C and high response/recovery speed (13-15 and 25-29 s).

A dual emission nanocomposite prepared from copper nanoclusters and carbon dots as a ratiometric fluorescent probe for sulfide and gaseous H2S.

A series of dual-emission fluorescent probes was prepared from copper nanoclusters (Cu NCs) and carbon dots (CDs). They show two emission peaks (blue at 469 nm and red at 622 nm) when photoexcited at 365 nm. Upon exposure to sulfide, the Cu NCs will be deteriorated because they react with sulfide to form CuS. This results in the quenching of the red fluorescence of the Cu NCs, while the blue fluorescence of the CDs remains constant. Thus, the color of the nanocomposite changes from red to blue. The ratio of the fluorescences at the two wavelengths decreases linearly in the 2-10 ppb (26-128 nM) sulfide concentration range, and the limit of detection is 0.33 ppb (4.3 nM). The nanocomposite also was placed in an agar gel and then incorporated into a paper strip for fluorometric monitoring of gaseous hydrogen sulfide. Graphical abstract Schematic presentation of the synthesis of Cu NCs (copper nanoclusters)-CDs (carbon dots) dual-emission nano-assembly, Cu NCs-CDs-agar fluorescent film and their application for the detection of sulfide and H2S.

Manganese-doped zinc oxide hollow balls for chemiresistive sensing of acetone vapors.

Both pure and Mn(II)-doped ZnO hollow structures were synthesized by a solvothermal reaction, and their phase structures, morphologies and elemental composition were characterized. SEM and TEM observations show the pure ZnO and the Mn(II)-doped ZnO balls to possess similar hollow structure with a particle size of about 1.5 µm. Their sensing properties were investigated, and the composite containing 1 atom% of Mn(II) (1% Mn-ZnO) is found be display the highest selectivity for acetone. The detection limit is 100 ppm acetone at 234 °C which is 4.6 times lower than that of the pure ZnO. In addition, the response time is shorter. Graphical abstract ZnO and Mn-doped ZnO hollow balls were prepared by a hydrothermal method, and their gas-sensing properties were investigated. Zinc(II) oxide doped with 1 atom% Mn(II) demonstrated an outstanding sensing behavior towards acetone vapors.

Aliovalent Fe(iii)-doped NiO microspheres for enhanced butanol gas sensing properties.

Fe-Doped NiO multi-shelled microspheres have been synthesized via a facile hydrothermal reaction. Various characterization techniques were introduced to investigate the structure and morphology of the as-prepared Fe-doped NiO multi-shelled microspheres. SEM and TEM observations showed that NiO microspheres are about 500 nm in diameter and with three shells. The Fe-doped NiO multi-shelled microspheres were investigated systematically as gas sensing materials for chemiresistive semiconductor-based gas sensors. The results showed that the 1.92 at% Fe-doped NiO (1.92Fe-NiO) multi-shelled microspheres exhibited enhanced gas sensing performance compared to the pure NiO multi-shelled microspheres. The gas response of 1.92Fe-NiO multi-shelled microspheres to 100 ppm butanol was 45.1 at 140 °C, which revealed a remarkable improvement over the pure NiO multi-shelled microspheres (6.80). The increased response of 1.92Fe-NiO multi-shelled microspheres may be attributed to the incorporation of Fe ions into NiO nanocrystals, which adjusted the carrier concentration and caused an increase in the oxygen species on the adsorbed surface. Therefore, the Fe-doped NiO multi-shelled microspheres should be a promising material for high performance butanol gas sensors.

Coordination Polymer-Derived Multishelled Mixed Ni-Co Oxide Microspheres for Robust and Selective Detection of Xylene.

Multishell, stable, porous metal-oxide microspheres (Ni-Co oxides, Co3O4 and NiO) have been synthesized through the amorphous coordination polymer-based self-templated method. Both oxides of Ni and Co show poor selectivity to xylene, but the composite phase has substantial selectivity (e.g., Sxylene/ Sethanol = 2.69) and remarkable sensitivity (11.5-5 ppm xylene at 255 °C). The short response and recovery times (6 and 9 s), excellent humidity-resistance performance (with coefficient of variation = 11.4%), good cyclability, and long-term stability (sensitivity attenuation of ∼9.5% after 30 days and stable sensitivity thereafter) all show that this composite is a competitive solution to the problem of xylene sensing. The sensing performances are evidently due to the high specific surface area and the nano-heterostructure in the composite phase.

Designed formation through a metal organic framework route of ZnO/ZnCo2O4 hollow core-shell nanocages with enhanced gas sensing properties.

The rational design of nanoscale metal oxides with hollow structures and tunable porosity has stimulated tremendous attention due to their vital importance for practical applications. Here, we report the designed synthesis of ZnO/ZnCo2O4 hollow core-shell nanocages (HCSNCs) through a metal-organic framework (MOF) route. The strategy includes the synthesis of a zeolite imidazolate framework-8 (ZIF-8)/Co-Zn hydroxide core-shell nanostructure precursor and subsequent transformation to ZnO/ZnCo2O4 HCSNCs by thermal annealing of the as-prepared precursor in air. Various techniques were employed for characterization of the structure and morphology of the as-prepared ZnO/ZnCo2O4 HCSNCs. When applied as a gas sensing material, the ZnO/ZnCo2O4 HCSNCs show enhanced sensitivity to xylene when compared with ZnCo2O4 shells as well as ZnO nanocages (NCs). In addition, excellent reversibility and superior selectivity of the sensor were observed. The remarkable enhancement in the gas-sensing properties of the ZnO/ZnCo2O4 HCSNCs is attributed to their unique structure and a synergistic effect of ZnO and ZnCo2O4.

Mesoporous InN/In2O3 heterojunction with improved sensitivity and selectivity for room temperature NO2 gas sensing.

Establishing heterostructures is a good strategy to improve gas sensing performance, and has been studied extensively. In this work, mesoporous InN/In2O3 composite (InNOCs) heterostructures were prepared through a simple two-step strategy involving hydrothermal synthesis of In2O3 and subsequent nitriding into InN-composite In2O3 heterostructures. We found that the InN content has great influence on the resistance of InNOCs, and thus, the gas sensing performance. In particular, InNOC-36.9 (with InN content of 36.9% in the composites) shows an excellent sensing response towards different concentrations of NO2, as well as good stability after one week of exposure to 200 ppb NO2 at room temperature. The highest sensing response (ΔR/R0 ) is up to 1.8 for the low NO2 concentration of 5 ppb. Even more significantly, the theoretical limit of detection (LOD) of the InNOC-36.9 sensor is 31.7 ppt based on a signal-to-noise ratio of 3 (the measured LOD is 5 ppb), which is far below the US NAAQS  value (NO2: 53 ppb). In addition, a rational band structure model combined with a surface reaction model is proposed to explain the sensing mechanism.

Low Working-Temperature Acetone Vapor Sensor Based on Zinc Nitride and Oxide Hybrid Composites.

Transition-metal nitride and oxide composites are a significant class of emerging materials that have attracted great interest for their potential in combining the advantages of nitrides and oxides. Here, a novel class of gas sensing materials based on hybrid Zn3 N2 and ZnO composites is presented. The Zn3 N2 /ZnO (ZnNO) composites-based sensor exhibits selectivity and high sensitivity toward acetone vapor, and the sensitivity is dependent on the nitrogen content of the composites. The ZnNO-11.7 described herein possesses a low working temperature of 200 °C. The detection limit (0.07 ppm) is below the diabetes diagnosis threshold (1.8 ppm). In addition, the sensor shows high reproducibility and long-term stability.

Preparation and ethanol sensing properties of In2O3 nanotubes.

In this paper, In2O3 nanotubes were prepared by an electrospinning method combined with an oriented-contract calcinations scheme, and characterized by differential thermal and thermal gravimetric analyzer (DTA-TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Ethanol sensing properties of the as-prepared nanotubes were investigated. The results showed that the In2O3 nanotubes were obtained at a fast heating rate of 100 degrees C/min. Ethanol sensing properties indicated that the nanotubes exhibit a high response of 87.5 to 500 ppm ethanol, fast response (20 s) and recovery (18 s) rate at the optimal operating temperature of 260 degrees C. Moreover, the nanotubes also exhibit good selectivity.