## 4 Reliable and Highly Sensitive Gas-Sensitive Materials for Intelligent Monitoring
### 2D (Two-Dimensional) Material Sensor Material
Metal oxide (MOX)-based gas sensors are inexpensive and miniaturized and have ease of integration and real-time monitoring. [PERSON] et al. [83] prepared electro-spun ZnO-SnO\({}_{2}\) composite nanofiber materials to study their sensing of H\({}_{2}\)S. The sensor is capable of detecting H\({}_{2}\)S at 0.5 ppmv at an optimal temperature of approximately 250 \({}^{\circ}\)C at a 50 ppm concentration of H\({}_{2}\)S gas. [PERSON] et al. [84] made NiO-modified ZnO nano-flower materials to analyze their gas sensing performance. When the concentrations of SO\({}_{2}\), SOF\({}_{2}\), and SO\({}_{2}\)F\({}_{2}\) gases are 100 ppmv, the optimal operating temperature for SO\({}_{2}\) detection is 220 \({}^{\circ}\)C with a response value of 84.26, and the optimal operating temperature for SO\({}_{2}\)F\({}_{2}\) and SOF\({}_{2}\) detection are both 260 \({}^{\circ}\)C. The modified NiO/ZnO composite is about twice as sensitive as pure zinc oxide nano-rods. [PERSON] et al. [85] hydrothermally synthesized cerium oxide nanoparticles (CeO\({}_{2}\) NPs) and doped Au, Ag, and Pd as sensing materials. The optimal operating temperature of Au-CeO\({}_{2}\) for 50 ppmv SO\({}_{2}\) is about 100 \({}^{\circ}\)C and Pd-CeO\({}_{2}\) has the best response to 50 ppmv SO\({}_{2}\)F\({}_{2}\) at 250 \({}^{\circ}\)C. These studies show that the traditional metal oxide-based gas sensors have problems such as high optimal operating temperature, complex synthesis process, and low sensitivity; so, they are not conducive to online monitoring. The transition metal (oxygen) hydroxides of Co, Ni, and Fe have attracted great interest in gas sensing [86; 87] due to their good electrochemical stability. [PERSON] et al. [88] synthesized octahedral Co\({}_{3}\)O\({}_{4}\)-modified NiSnO\({}_{3}\) nanofiber materials through a two-step synthesis process. At 50 \({}^{\circ}\)C, the optimal sensing response value of the composite is 22.5 when the SO\({}_{2}\)F\({}_{2}\) concentration is 100 ppmv. This indicates that the material has a high sensing response to SO\({}_{2}\)F\({}_{2}\) at a relatively low temperature. Cobalt oxyhydroxide (CoOOH) is a mineral called heterogeneous rock that serves as a precursor for electrode materials such as energy storage, gas sensors, and catalysis [89]. The hydroxyl group on the surface makes the CoOOH more conductive, which is suitable for gas sensing [90]. [PERSON] et al. [91] performed a functional calculation of dispersion-corrected density and studied the adsorption performance of CoOOH on SF\({}_{6}\) decomposition products. The results indicate that the band gap of the gas adsorption systems was all reduced; so, the conductivity and gas sensing performance are improved, among which CoOOH has higher charge transfer and adsorption energy for SOF\({}_{2}\). CoOOH flakes can sense 10 ppmv SF\({}_{6}\) decomposition gases. [PERSON] [92] incorporated Ni into a ZnO (Ni-ZnO) monolayer to  
Figure 7: **(a)** Band structure of BN (i) and Rh-BN (ii). (**b**) Sensitivity (changes of resistance before and after gas adsorption, S) at different temperatures. **(c)** The adsorption geometry, adsorption energy, charge transfer, and band structure of H\({}_{2}\)S (I), SO\({}_{2}\) (II), SOF\({}_{2}\) (III), and SO\({}_{2}\)F\({}_{2}\) (IV). Adapted with permission from [81]. detect SO2 and SOF2 based on density functional theory. The Ni-ZnO sensing material has a strong chemical adsorption force on the two gases. A significant increase in the band gap of Ni-ZnO monolayer results in a significant decrease in its conductivity, which is sufficient to be detected by electrochemical devices. In addition to the above, many high-frequency low-dimensional materials have emerged for gas-sensitive materials, such as monolayer tellurium [93], Ru-InN monolayer [94], phosphorene [95], InP3 monolayer [96], monolayer SbP [97], stone-Wales (SW) defective Sb [98], and so on.