In recent years, gas sensors have been increasingly used in industrial production and daily life. Metal oxide semiconductor gas sensing materials are favoured for their outstanding physical and chemical properties, low cost and simple preparation methods. However, the gas sensing mechanisms of metal oxide semiconductors have not been considered by researchers, resulting in omissions and errors in the interpretation of gas sensing mechanisms in many articles. This review organizes and introduces several common gas sensing mechanisms of metal oxide semiconductors in detail and classifies them into two categories. The scope and relationship of these mechanisms are clarified. In addition, this review selects four strategies for enhancing the gas sensing properties of metal oxide semiconductors and analyses the gas sensing mechanisms to highlight the importance of the gas sensing mechanism. Finally, some perspectives for future investigations on the gas sensing mechanisms of metal oxide semiconductors are discussed as well.

Gas sensing mechanisms of metal oxide semiconductors: a focus review

Recent progress in metal-doped TiO2, non-metal doped/codoped TiO2 and TiO2 nanostructured hybrids for enhanced photocatalysis

Ni-based metal organic frameworks (Ni-MOFs) with unique hierarchical hollow ball-in-ball nanostructure were synthesized by solvothermal reactions. After successive carbonization and oxidation treatments, hierarchical NiO/Ni nanocrystals covered with a graphene shell were obtained with the hollow ball-in-ball nanostructure intact. The resulting materials exhibited superior performance as the anode in lithium ion batteries (LIBs): they provide high reversible specific capacity (1144 mAh/g), excellent cyclability (nearly no capacity loss after 1000 cycles) and rate performance (805 mAh/g at 15 A/g). In addition, the hierarchical NiO/Ni/Graphene composites demonstrated promising performance as anode materials for sodium-ion batteries (SIBs). Such a superior lithium and sodium storage performance is derived from the well-designed hierarchical hollow ball-in-ball structure of NiO/Ni/Graphene composites, which not only mitigates the volume expansion of NiO during the cycles but also provides a continuous highly conductive graphene matrix to facilitate the fast charge transfer and form a stable SEI layer.

Metal Organic Frameworks Derived Hierarchical Hollow NiO/Ni/Graphene Composites for Lithium and Sodium Storage

SnS2 nanocrystals with adjustable sizes were synthesized via a hydrothermal method from the aqueous solution of common and inexpensive SnCl4·5H2O, thioacetamide and citric acid, simply by varying the reaction temperature and reaction time. The structures, Brunauer–Emmett–Teller (BET) specific surface areas and optical properties of the resultant SnS2 nanocrystals were characterized by X-ray diffraction, transmission electron microscopy, N2 adsorption/desorption isotherms, and UV–vis diffuse reflectance spectra. Besides, their photocatalytic properties were tested for the reduction of aqueous Cr(VI) under visible light (λ > 420 nm) irradiation. It was found that the photocatalytic activities of SnS2 nanocrystals in aqueous suspension depended on their synthesis conditions. The product synthesized under suitable hydrothermal conditions (for example, at 150 °C for 12 h) not only showed high visible light-driven photocatalytic activity in the reduction of aqueous Cr(VI), but also showed good photocatalytic st...

Size-tunable hydrothermal synthesis of SnS2 nanocrystals with high performance in visible light-driven photocatalytic reduction of aqueous Cr(VI).

Metal oxide resistive-type nano-scale gas sensors have been investigated for their low cost, high sensitivity, and environmentally friendly fabrication. In these sensors, electrical resistance measurements are used to detect the presence of gas. In n-type metal oxides, resistance is increased by coverage of adsorbed oxygen and lowered by removal of adsorbed oxygen through reactions with reducing gasses. The sensitivity and selectivity of these sensors have been improved by incorporation of heterostructures. Heterostructures may improve sensor performance through facilitating catalytic activity, increasing adsorption, and creating a charge carrier depletion layer that produces a larger modulation in resistance. Synergistic effects in these gas sensors describe the improved sensor signal due to these combined effects which act to amplify the reception and transduction of the sensor signal. Receptive mechanisms may be improved by increasing adsorption and reactivity. Transduction mechanisms may be improved by restriction of the major charge conduction channels which helps to maximize resistance modulation. In this review, the synergistic effect achieved by combining these two mechanisms are examined. Fundamental properties of the metal oxide surface are used to provide insight for the large body of experimental evidence available for metal oxide resistive-type gas sensors. This review aims to connect experimental evidence to conceptual mechanistic descriptions by examining adsorption processes, charge transfer, reaction mechanisms, morphology, and ambient gas interactions.

Synergistic effects in gas sensing semiconducting oxide nano-heterostructures: A review

Hierarchical structures (HSs) of metal oxide semiconductor (MOS) have stimulated tremendous research attentions owing to their fascinating properties. Here, a HS of ZnO/ZnCo2O4 with varied ZnO contents (0–8 wt%) has been successfully prepared via a surfactant-free hydrothermal method combined with subsequent calcination. The prepared ZnO/ZnCo2O4 HS features an ordered assembly of two dimensional (2D) porous nanosheets with the thickness about 15 nm and evenly distributed ZnCo2O4-ZnO p-n junctions. The gas sensing properties of the ZnO/ZnCo2O4 HS were investigated and compared with pure ZnCo2O4 with similar morphology. It was found that after decorating with ZnO, the gas sensing properties of ZnCo2O4 to triethylamine (TEA) were remarkably improved, such as lowered optimum working temperature (decreased from 260 to 220 °C), faster response/recover speed, and higher sensitivity (0.0377/ppm) within a linear concentration range of 5–300 ppm. The improved gas sensing properties can be attributed to the sensitization effect of n-type ZnO on p-type ZnCo2O4 as well as the unique porous nanosheets-assembled hierarchical structure.

Synthesis of porous nanosheets-assembled ZnO/ZnCo2O4 hierarchical structure for TEA detection

Bimetallic nanoparticles (NPs) usually exhibit some novel properties due to the synergistic effects of the two distinct metals, which is expected to play an important role in the field of gas sensing. PdPt bimetal NPs with Pd-rich shell and Pt-rich core were successfully synthesized and used to modify SnO2 nanosheets. The 1P-PdPt/SnO2-A sensor obtained by self-assemblies of PdPt NPs exhibited temperature-dependent dual selectivity to CO at 100 °C and CH4 at 320 °C. Furthermore, the sensor possessed good long term stability and antihumidity interference. The activation energy of adsorption for CO and CH4 were estimated by the temperature-dependent response process modeled using Langmuir adsorption kinetics, which proved that the lower activation energy of adsorption corresponded to better sensing performance. The gas-sensing mechanism based on the diffusion depth of the tested gas in the sensing layer was discussed. The dramatically improved sensing performance could be ascribed to the high catalytic activity of PdPt bimetal, the electron sensitization of PdO, and Schottky barrier-type junctions at the interface between SnO2 and PdPt NPs. Our present results demonstrate that bimetal NPs with special structure and components can significantly improve the gas-sensing performance of metal oxide semiconductor and the obtained sensor has great potential in monitoring coal mine gas.

PdPt Bimetal-Functionalized SnO2 Nanosheets: Controllable Synthesis and its Dual Selectivity for Detection of Carbon Monoxide and Methane.

Synthesis and triethylamine sensing properties of mesoporous α-Fe2O3 microrods

Pt-doped flower-like SnO2 nanocomposites were successfully synthesized by hydrothermal process, followed by a simple thermal reduction method. And, their morphology, composition and structure were thoroughly studied. The gas sensing performance of undoped and Pt-doped SnO2 gas sensors for methane were systematically investigated. Results indicate that Pt-doped SnO2 nanoflowers (NFs) not only significantly improve the sensitivity, but also reduce the optimal operating temperature compared to undoped SnO2 sensors. Moreover, it improves the stability and selectivity of the SnO2 gas sensor. Combined with adsorption energy and density of states of undoped SnO2 and Pt-doped SnO2 supercells for O2 and CH4 molecules calculated using first principles based on density functional theory (DFT), it is revealed that the excellent methane gas sensing performance of Pt-doped SnO2 sensor is mainly attributed to the chemical sensitization of Pt dopant to methane gas. Simultaneously, the electronic sensitization of the Schottky junction formed between Pt and SnO2, and the unique Pt-doped SnO2 NFs structure is also beneficial for gas sensing performance.

Enhanced methane sensing property of flower-like SnO2 doped by Pt nanoparticles: A combined experimental and first-principle study

Aiming to improve the gas sensing property of SnO 2 by utilizing the advantages of both porous structure and p-n junctions, NiO-decorated SnO 2 microflowers assembled with porous nanorods were successfully prepared via a sacrificial template-assisted synthesis method by using SnC 2 O 4 as sacrificial template for porous SnO 2 and Ni(NO 3 ) 2 as NiO source. Through this method, different amounts of NiO can be homogenously dispersed in porous SnO 2 to form p-NiO/n-SnO 2 junctions without changing the original flower-like morphology of SnC 2 O 4 . In the as-prepared flower-like NiO/SnO 2 architecture, the porous nanorods are about 2 μm in length and 100–200 nm in diameter and are constructed by numerous loosely stacked nanoparticles with the size about 20 nm. Gas sensing tests indicate that the NiO/SnO 2 microflowers exhibit a remarkably improved gas sensing performance compared with pristine SnO 2 . The response of 1NTO (with the optimal NiO content of 1 mol%) to 1000 ppm ethanol is as high as 576.5, which is much higher than that of the pristine SnO 2 (315.5). Besides of high sensitivity, the 1NTO sensor also exhibits excellent linearity in a wide range of ethanol concentration (50–1000 ppm) as well as good repeatability and durability. The p-NiO/n-SnO 2 heterojunction related gas sensing mechanism was discussed.

Zhanying Zhang

Papers

Synthesis of porous nanosheets-assembled ZnO/ZnCo2O4 hierarchical structure for TEA detection

PdPt Bimetal-Functionalized SnO2 Nanosheets: Controllable Synthesis and its Dual Selectivity for Detection of Carbon Monoxide and Methane.

Synthesis and triethylamine sensing properties of mesoporous α-Fe2O3 microrods

Enhanced methane sensing property of flower-like SnO2 doped by Pt nanoparticles: A combined experimental and first-principle study

Synthesis and improved gas sensing properties of NiO-decorated SnO2 microflowers assembled with porous nanorods