Sensor technology has an important effect on many aspects in our society, and has gained much progress, propelled by the development of nanoscience and nanotechnology. Current research efforts are directed toward developing high-performance gas sensors with low operating temperature at low fabrication costs. A gas sensor working at room temperature is very appealing as it provides very low power consumption and does not require a heater for high-temperature operation, and hence simplifies the fabrication of sensor devices and reduces the operating cost. Nanostructured materials are at the core of the development of any room-temperature sensing platform. The most important advances with regard to fundamental research, sensing mechanisms, and application of nanostructured materials for room-temperature conductometric sensor devices are reviewed here. Particular emphasis is given to the relation between the nanostructure and sensor properties in an attempt to address structure-property correlations. Finally, some future research perspectives and new challenges that the field of room-temperature sensors will have to address are also discussed.

Nanostructured Materials for Room-Temperature Gas Sensors

Because of the interesting and multifunctional properties, recently, ZnO nanostructures are considered as excellent material for fabrication of highly sensitive and selective gas sensors. Thus, ZnO nanomaterials are widely used to fabricate efficient gas sensors for the detection of various hazardous and toxic gases. The presented review article is focusing on the recent developments of NO2 gas sensors based on ZnO nanomaterials. The review presents the general introduction of some metal oxide nanomaterials for gas sensing application and finally focusing on the structure of ZnO and its gas sensing mechanisms. Basic gas sensing characteristics such as gas response, response time, recovery time, selectivity, detection limit, stability and recyclability, etc are also discussed in this article. Further, the utilization of various ZnO nanomaterials such as nanorods, nanowires, nano-micro flowers, quantum dots, thin films and nanosheets, etc for the fabrication of NO2 gas sensors are also presented. Moreover, various factors such as NO2 concentrations, annealing temperature, ZnO morphologies and particle sizes, relative humidity, operating temperatures which are affecting the NO2 gas sensing properties are discussed in this review. Finally, the review article is concluded and future directions are presented.

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Zinc Oxide Nanostructures for NO2 Gas–Sensor Applications: A Review

The development of advanced energy storage devices is at the forefront of research geared towards a sustainable future. Nanostructured materials are advantageous in offering huge surface to volume ratios, favorable transport features, and attractive physicochemical properties. They have been extensively explored in various fields of energy storage and conversion. This review is focused largely on the recent progress in nanostructured Mo-based electrode materials including molybdenum oxides (MoOx, 2 ≤ x ≤ 3), dichalconides (MoX2, X = S, Se), and oxysalts for rechargeable lithium/sodium-ion batteries, Mg batteries, and supercapacitors. Mo-based compounds including MoO2, MoO3, MoO3−y (0 < y < 1), MMoxOy (M = Fe, Co, Ni, Ca, Mn, Zn, Mg, or Cd; x = 1, y = 4; x = 3, y = 8), MoS2, MoSe2, (MoO2)2P2O7, LiMoO2, Li2MoO3, etc. possess multiple valence states and exhibit rich chemistry. They are very attractive candidates for efficient electrochemical energy storage systems because of their unique physicochemical properties, such as conductivity, mechanical and thermal stability, and cyclability. In this review, we aim to provide a systematic summary of the synthesis, modification, and electrochemical performance of nanostructured Mo-based compounds, as well as their energy storage applications in lithium/sodium-ion batteries, Mg batteries, and pseudocapacitors. The relationship between nanoarchitectures and electrochemical performances as well as the related charge-storage mechanism is discussed. Moreover, remarks on the challenges and perspectives of Mo-containing compounds for further development in electrochemical energy storage applications are proposed. This review sheds light on the sustainable development of advanced rechargeable batteries and supercapacitors with nanostructured Mo-based electrode materials.

Nanostructured Mo-based electrode materials for electrochemical energy storage

Shape-controllable porous, hollow metal oxide cages are attracting more and more attention due to their widespread applications. In this paper, octahedral, truncated octahedral and cubic Cu2O/CuO cages were successfully fabricated by the thermal decomposition of the polyhedral crystals of Cu-based metal–organic frameworks (Cu-MOFs) as self-sacrificial templates at 300 °C. The morphology of the Cu-MOF polyhedral precursors was well tuned by using lauric acid as the growth modulator under solvothermal conditions. Gas-sensing measurements revealed that the octahedral Cu2O/CuO cages exhibited a gas-sensing performance far better than those exhibited by truncated octahedral and cubic cages, which is attributed to the cooperative effect of the large specific surface area (150.3 m2 g−1) and high capacity of surface-adsorbed oxygen of the octahedral Cu2O/CuO cages.

Synthesis of porous Cu2O/CuO cages using Cu-based metal–organic frameworks as templates and their gas-sensing properties

Resistance-based H2S gas sensors using metal oxide nanostructures: A review of recent advances.

Cu 2 O/CuO sub-microspheres were synthesized via a one-step reduction approach using copper (II) acetate as precursor in triethylene glycol solution at reaction temperature ranging from 100 to 180 °C. It is found that the reaction temperature plays a very important role in the size, the Cu 2 O content, and morphology of the products. The sub-microspheres obtained at the reaction temperature below 140 °C are assembled by smaller particles with a diameter of about 10 nm. As the reaction temperature is increased to above 160 °C, uneven samples with larger size will be synthesized. The sensing performances of the sub-microspheres are related to the Cu 2 O content, porous characteristics and their surface areas. The sensor response of the sub-microspheres obtained at 120 °C is up to 2.1–50 ppb H 2 S gas at 95 °C, and the recovery time is only about 76 s. Thus, the sub-microspheres are promising for H 2 S sensors.

Ppb H2S gas sensing characteristics of Cu2O/CuO sub-microspheres at low-temperature

In the paper, we developed an in situ diffusion growth method to fabricate porous Fe2(MoO4)3 nanorods. The average diameter and the length of the porous nanorods were 200 nm and 1.2–4 μm, respectively. Moreover, many micropores existed along axial direction of the Fe2(MoO4)3 nanorods. In terms of nitrogen adsorption–desorption isotherms, calculated pore size was in the range of 4–115 nm, agreeing well with the transmission electron microscope observations. Because of the uniquely porous characteristics and catalytic ability at low temperatures, the porous Fe2(MoO4)3 nanorods exhibited very good H2S sensing properties, including high sensitivity at a low working temperature (80 °C), relatively fast response and recovery times, good selectivity, and long-term stability. Thus, the porous Fe2(MoO4)3 nanorods are very promising for the fabrication of high-performance H2S gas sensors. Furthermore, the strategy presented here could be expended as a general method to synthesize other hollow/porous-type transition...

Porous iron molybdate nanorods: in situ diffusion synthesis and low-temperature H2S gas sensing.

Hierarchical nanoarchitectures constructed by MoO2 nanocrystal-functionalized graphene were fabricated through an in situ reduction process. As an anode, even at a 10C (0.1 h per half cycle) charging–discharging rate, the reversible capacity is much higher than the theoretical capacity of graphite. Thus, rapid charging–discharging of the anodes is achieved by the nanoarchitectures.

Graphene–MoO2 hierarchical nanoarchitectures: in situ reduction synthesis and high rate cycling performance as lithium-ion battery anodes

G/Ag2S composites were synthesized for the first time by a hydrothermal method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analysis demonstrated that Ag2S nanoparticles with a diameter of about 130 nm uniformly covered the graphene surfaces. G/Ag2S composites were dispersed in methyl methacrylate (MMA), polymerized at 75 °C for 30-35 min, and finally dried at 45 °C for 10 h, to afford G/Ag2S/PMMA organic glasses. The nonlinear absorption (NLA) properties of the G/Ag2S/PMMA organic glasses with different amounts of G/Ag2S were investigated by an open-aperture Z-scan technique. The experimental results showed that the G/Ag2S/PMMA organic glass with an appropriate amount of G/Ag2S exhibited enhanced reverse saturable absorption (RSA) properties compared to G/PMMA and Ag2S/PMMA organic glasses, which was attributed to the notable synergistic effects between graphene and Ag2S. Both one-photon absorption (OPA) in Ag2S and two-photon absorption (TPA) in graphene played important roles in RSA processes of the G/Ag2S/PMMA organic glasses. The effective NLA coefficient βeff of the G/Ag2S/PMMA organic glasses was in the order of 10(3) cm GW(-1). Thus this kind of organic glasses have great promise in optical limiter and optical shutter applications.

Xin-Peng Di

Papers

Ppb H2S gas sensing characteristics of Cu2O/CuO sub-microspheres at low-temperature

Porous iron molybdate nanorods: in situ diffusion synthesis and low-temperature H2S gas sensing.

Graphene–MoO2 hierarchical nanoarchitectures: in situ reduction synthesis and high rate cycling performance as lithium-ion battery anodes

Enhanced reverse saturable absorption in graphene/Ag2S organic glasses.