## 4 Reliable and Highly Sensitive Gas-Sensitive Materials for Intelligent Monitoring
### 2D (Two-Dimensional) Material Sensor Material  
As the main component of gas sensors, gas-sensitive materials are currently rich and diverse. After the emergence of graphene, a variety of two-dimensional materials have appeared one after another, such as metal oxides, tellurium, graphene, phosphorene, transition metal disulfides (TMDs), etc., due to the ultra-thin structure, strong surface activity, large specific surface area [68; 69], desirable size, power consumption, and optimal operating temperature of the gas sensor being greatly reduced. The researchers also focused on these two-dimensional materials to study their gas sensing properties, as shown in Figure 6.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Technique** & **Advantages** & **Defaults** & **Gas** & \begin{tabular}{c} **Response** \\ **Time** \\ \end{tabular}  & **LOD** & **Ref. ** \\ \hline \multirow{4}{*}{TIR} & High resolution, fast response, large & \multirow{4}{*}{ \begin{tabular}{c} SO\({}_{2}\) \\ CO\({}_{2}\)F\({}_{2}\) \\ \end{tabular}  } & \multirow{4}{*}{\ \begin{tabular}{c} - \\ CO\({}_{2}\)F\({}_{2}\) \\ \end{tabular}  } & \multirow{4}{*}{
\begin{tabular}{c} 58.03 \\  
Graphene materials have a unique single atomic layer structure and electronic band structure. Its excellent properties, such as high electron mobility, high thermal conductivity, and large specific surface area, make it an excellent candidate material for gas sensors [70; 71; 72; 73]. Pd\({}_{3}\) doped with graphene (Pd\({}_{3}\)-graphene) can chemically adsorb H\({}_{2}\)S, SO\({}_{2}\), SOF\({}_{2}\), and SO\({}_{2}\)F\({}_{2}\) and has the best sensitivity to SOF\({}_{2}\) gas. The authors [74] added N\({}_{3}\) and Ni to graphene for modification, and they found that the conductivity increased significantly after modifications were made for detecting H\({}_{2}\)S and SO\({}_{2}\) gases. Conversely, conductivity decreases significantly when SOF\({}_{2}\) and SO\({}_{2}\)F\({}_{2}\) gases are absorbed. More researchers are setting their sights on graphene-like structures in gas-sensitive materials [75]. h-BN (BN), known as white graphene, is a graphene-like structural material that has better thermal conductivity and thermal stability compared to graphene [76; 77]. Moreover, doping metal atoms in BN has higher reaction properties, and the gas greatly enhances sensing capability [78; 79]. For example, the Pd metal-doped h-BN monolayer can be used as a nano-sensing material [75]. Another example is the rhodium (Rh) atoms, which are popular doped metal in terms of improving the surface properties of 2D materials, such as graphene, carbon nanotubes, and molybdenum disulfide monolayers due to their high carrier mobility and catalytic interaction with gases [80; 81]. [PERSON] et al. [82] studied Rh-doped h-BN (Rh-BN) monolayers to analyze the adsorption of SF\({}_{6}\) decomposition gas. In Figure 7a, it can be seen that Rh-BN has a smaller band gap than the original material, electrons are more easily excited, and electrical conductivity is significantly improved. As shown in Figure 7b, under the different temperatures, SOF\({}_{2}\), SO\({}_{2}\)F\({}_{2}\), SO\({}_{2}\), and H\({}_{2}\)S sensitivity difference is bigger and is advantageous for selective detection. In Figure 7c, the Rh-BN monolayer has good adsorption performance for the four decomposed components, and the negative value of E\({}_{\text{ad}}\) represents the exothermic process. The higher the absolute value, the more easily gas adsorption occurs and the more stable the system. The positive and negative charge transfer corresponds to the characteristics of gas molecules, which will affect the change in electrical conductivity. Compared to Figure 7a, the band gap changes in H\({}_{2}\)S and SO\({}_{2}\) are greater than those of SOF\({}_{2}\) and SO\({}_{2}\)F\({}_{2}\), indicating that the conductivity changes in H\({}_{2}\)S and SO\({}_{2}\) after adsorption are greater and the gas sensitivity is higher. Figure 6: Two-dimensional gas-sensitive materials and their geometric structures.