Chemi-resistive Gas Sensors

Industrialization and increasing vehicular emissions have led to the widespread release of toxic gases into the atmosphere, posing severe environmental and health risks. Among these pollutants, nitrogen dioxide (NO₂) is particularly hazardous due to its high reactivity and harmful effects on human health. Long-term exposure to NO₂ has been linked to respiratory issues such as lung inflammation, reduced pulmonary function, and an increased risk of asthma, especially in children and the elderly. Additionally, NO₂ plays a key role in the formation of acid rain and secondary pollutants like ground-level ozone further deteriorating air quality. Therefore, the development of high-performance NO₂ sensors with rapid response, high selectivity, and low power consumption is crucial for continuous air quality monitoring.

Fig 1. Schematic representation of proposed sensing mechanism of Fe mediated α-MoO3 Nano Lamella

In this context, we developed NO₂ gas sensors for both room temperature and high temperature applications using metal oxide-based and 2D layered materials sensing platforms. To enhance performance, we employed various strategies including heterostructure formation, metal doping, noble metal decoration, integration with carbon-based materials and liquid exfoliation. For instance, we developed Fe-Mediated α-MoO₃ Nano Lamella for NO₂ Gas detection under low operating temperatures. The enhanced performance of the sensor under these conditions can be attributed to reduced activation energy and the higher bending (Small Methods, 2401214).

We have also developed a WO₃ based NO₂ gas sensor by doping metal atoms such as Al, Co. Among them, the Co-doped sensor demonstrated the highest sensitivity of 20776 % with fast adsorption and desorption kinetics of 15 s/23 s at an operating temperature of 200 °C. (Sensors and Actuators B: Chemical, 421, 136477, ACS Applied Nano Materials, 8). In this continuum, the impact of facet-controlled Ag decoration on the gas-sensing redox characteristics of WO₃ was analyzed. The incorporation of Ag led to a reduction in work function, favorable band bending, and a decrease in barrier height, thereby enhancing the redox properties of the sensor (Langmuir, 2025). 

Fig 2. Schematic representation of proposed sensing mechanism of Ag nanoprism on WO₃ gas sensing

In addition, a mesoporous Co₃O₄/SnO₂Sensor showed a maximum response at a low operating temperature of 150 °C. The enhanced performance is attributed to depletion region formation at the interface of Co₃O₄/SnO₂, high surface area, and a large number of oxygen vacancies (ACS Applied Nano Materials, 6). Furthermore, we developed room temperature NO₂ gas sensor based on CuO engineered with ZnO and rGO interfaces. The CuO/ZnO exhibited a response of 337% towards 5 ppm of NO₂, which is due to the enhanced oxygen vacancies and surface area of the prepared sensor. Additionally, the CuO/rGO demonstrated an exceptional high sensitivity of 1004 % towards 5 ppm of NO₂. (Journal of Environmental Chemical Engineering, 11, 110056; Applied Surface Science, 657, 159604).

Further, room temperature gas sensors were developed by 2D layered materials such as MoS₂, SnS₂, WS₂ and g-C₃N₄. For instance, uultrathin layered MoS₂/N-doped graphene quantum dots (NGQDs) heterostructures were designed for highly sensitive room temperature sensor. (Sensors and Actuators B: Chemical, 403, 135083). Additionally, the SnS2/mesoporous TiO2 heterostructure showed an enhanced sensing response of 245.4% at room temperature, attributed to the combined electronic and geometric effect (Chemosphere, 346, 140486). Moreover, the confined oxidation of 2D WS₂ nanosheets, forming  WO₃ /WS₂ nanocomposites resulted in a maximum response of 123% with short response/recovery time of 11 s/ 163 s at RT (Applied Surface Science, 642, 158554). 

Fig 3. Schematic representation of proposed sensing mechanism of WO₃/WS2 nanocomposites

Thin film-based gas sensor

Our research further explored the development of thin film-based gas sensors using the chemical vapor deposition technique to enhance the sensing performance. Vertically oriented SnS₂ flakes were successfully grown on a SiO₂/Si substrate via CVD, providing increased reactive sites that resulted in an impressive sensing response of 98% at room temperature (Applied Surface Science, 661, 159991). Furthermore, by modifying the growth temperature, an in-situ SnS₂/SnS heterostructure was fabricated, which exhibited an enhanced sensing response of 119% with a rapid response time of 32s (Journal of Alloys and Compounds, 1001, 175002).

Fig 4. Schematic representation of proposed sensing mechanism of SnS2/SnS sensor

Further, we have prepared SnS₂/Si, achieving a sensing response 302% against 40 ppm of NO₂ at RT.  The vertical nanoflakes provides more gas adsorption sites which leads to increase the gas sensing performance. Further, the formation of heterostructure resulted in an even higher response of 671% at room temperature (Sensors and Actuators: B. Chemical, 428, 137165).

Fig 5. Schematic representation of proposed sensing mechanism of SnS2/Si sensor