As main solid-state gas sensors, nano metal oxide semiconductor gas sensors are widely used in industrial production, environmental monitoring, health care and other fields for their high sensitivity, low manufacturing cost and simple signal measurement. At present, research on the improvement of gas sensing properties of nano metal oxide sensing materials mainly focus on the development of nanoscale metal oxides, such as nanostructure and doping modification.

Nano metal oxide semiconductor sensing materials are mainly SnO2, ZnO, Fe2O3,VO2, In2O3, WO3, TiO2, etc. The sensor components are still the most widely used resistive gas sensors, non-resistive gases sensors are also being developed more quickly.

At present, the main research direction is to prepare structured nanomaterials with large specific surface area, such as nanotubes, nanorod arrays, nanoporous membranes, etc.to increase the gas adsorption capacity and the gas diffusion rate, and thus improve sensitivity and speed of response to gas of the materials. The elemental doping of the metal oxide, or the construction of the nanocomposite system, the introduced dopant or composite components can play a catalytic role, and can also become an auxiliary carrier for constructing the nanostructure, thereby improving the overall gas sensing performance of the sensing materials.

1. Gas sensing materials used Nano Tin Oxide(SnO2)

Tin oxide (SnO2) is a kind of general sensitive gas sensitive material. It has good sensitivity to gases such as ethanol, H2S and CO. Its gas sensitivity depends on the particle size and specific surface area. Controlling the size of SnO2 nanopowder is the key to improving gas sensitivity.

Based on mesoporous and macroporous nano tin oxide powders, the researchers prepared thick-film sensors that have higher catalytic activity for CO oxidation, which means higher gas sensing activity. In addition, the nanoporous structure has become a hot spot in the design of gas sensing materials due to its large SSA, rich gas diffusion and mass transfer channels.

2. Gas sensing materials used Nano Iron Oxide(Fe2O3)

Iron oxide (Fe2O3) has two crystal forms: alpha and gamma, both of which can be used as gas sensing materials, but the gas sensing properties of them have large differences. α-Fe2O3 belongs to corundum structure, whose physical properties are stable. Its gas sensing mechanism is surface controlled, and its sensitivity is low. γ-Fe2O3 belongs to spinel structure and is metastable. Its gas sensing mechanism is mainly body resistance control.It has good sensitivity but poor stability, and is easy to change to α-Fe2O3 and reduce gas sensitivity.

The current research focuses on optimizing the synthesis conditions to control the morphology of Fe2O3 nanoparticles, and then screening for suitable gas-sensitive materials, such as α-Fe2O3 nanobeams, porous α-Fe2O3 nanorods, monodisperse α-Fe2O3 nanostructures, mesopores α-Fe2O3 nanomaterials, etc.

3. Gas sensing materials used Nano Zinc Oxide(ZnO)
Zinc oxide (ZnO) is a typical surface-controlled gas-sensitive material. The ZnO-based gas sensor has a high operating temperature and poor selectivity, making it far less widely used than SnO2 and Fe2O3 nanopowders. Therefore, the preparation of new structure of ZnO nanomaterials, doping modification of nano-ZnO to reduce operating temperature and improve selectivity is the focus of research on nano ZnO gas sensing materials.

At present, the development of single crystal nano-ZnO gas sensing element is one of the frontier directions, such as ZnO single crystal nanorod gas sensors.

4. Gas sensing materials used Nano Indium Oxide(In2O3)
Indium oxide (In2O3) is an emerging n-type semiconductor gas sensing material. Compared with SnO2, ZnO, Fe2O3, etc., it has wide band gap, small resistivity and high catalytic activity, and high sensitivity to CO and NO2. Porous nanomaterials represented by nano In2O3 are one of the recent research hotspots. The researchers synthesized ordered mesoporous In2O3 materials by means of mesoporous silica template replication. The obtained materials have good stability in the range of 450-650 °C, so they are suitable for gas sensors with higher operating temperatures. They are sensitive to methane and can be used for concentration-related explosion monitoring.

5. Gas sensing materials used Nano Tungsten Oxide(WO3)
WO3 nanoparticles is a transition metal compound semiconductor material which has been widely studied and applied for its good gas sensing property. Nano WO3 has stable structures such as triclinic, monoclinic and orthorhombic. The researchers prepared WO3 nanoparticles by nano-casting method using mesoporous SiO2 as template. It was found that the monoclinic WO3 nanoparticles with an average size of 5 nm have better gas sensing performance, and the sensor pairs obtained by electrophoretic deposition of WO3 nanoparticles Low concentrations of NO2 have a high response.

The homogeneous distribution of hexagonal phase WO3 nanoclusters was synthesized by ion exchange-hydrothermal method. The gas sensitivity test results show that the WO3 nanoclustered gas sensor has low operating temperature, high sensitivity to acetone and trimethylamine and ideal response recovery time, revealing a good application prospect of the material.

6. Gas sensing materials used Nano Titanium Dioxide(TiO2)
Titanium dioxide (TiO2) gas sensing materials have the advantages of good thermal stability and simple preparation process, and have gradually become another hot material for researchers. At present, the research on nano-TiO2 gas sensor focuses on the nanostructure and functionalization of TiO2 sensing materials by using emerging nanotechnology. For example, researchers have made micro-nano-scale hollow TiO2 fibers by coaxial electrospinning technology. Using the premixed stagnant flame technology, the cross electrode is repeatedly placed in a premixed stagnant flame with titanium tetraisopropoxide as the precursor, and then directly grown to form he porous membrane with TiO2 nanoparticles, which is sensitive response to CO. Simultaneously grows the ordered TiO2 nanotube array by anodization and applies it to the detection of SO2.

7. Nano oxide composites for gas sensing material
The gas sensing properties of nano metal oxides powders sensing materials can be improved by doping, which not only adjusts the electrical conductivity of the material, but also improves the stability and selectivity. Doping of precious metal elements is a common method, and elements such as Au and Ag are often used as dopants to improve the gas sensing performance of nano zinc oxide powder. Nano oxide composite gas sensing materials mainly include Pd doped SnO2, Pt-doped γ-Fe2O3, and multi-element added In2O3 hollow sphere sensing material, which can be realized by controlling additives and sensing temperature to realize elective detection of NH3, H2S and CO. In addition, WO3 nano film is modified with a layer of V2O5 to improve the porous surface structure of WO3 film, thereby improving its sensitivity to NO2.

At present, graphene/nano-metal oxide composites have become a hotspot in gas sensor materials. Graphene/SnO2 nanocomposites have been widely used as ammonia detection and NO2 sensing materials.