8. Noble-Metal-Decorated Metal Oxide Semiconductor Gas Sensors
Gas-sensing properties can be improved by loading MOSs with small amounts of appropriately chosen noble metals. Noble metals such as Au, Pt, Pd, Ag, and Ru are widely used for the decoration or functionalization of the MOS surfaces to improve the overall performance of the resulting gas sensors.
It should be noted that the noble metals need to be dispersed as finely as possible on the surface of the MOSs. Agglomeration of noble metals on the surface of the sensing layer can lead to a poor response of the gas sensor. Generally, noble metals affect the gas-sensing performance via two well-known mechanisms: The chemical sensitization mechanism, which occurs via the spillover effect, is a commonly known phenomenon in catalytic chemistry. In this case, the noble metal activates the target gas to facilitate catalytic oxidation of gas. Noble metals increase the gas sensitivity as they increase the rate of chemical processes
[58]. In electronic sensitization, because noble metals generally have a larger work function than MOSs, electrons are transferred from the MOS to the noble metals, leading to the formation of Schottky barriers and contraction of the EDL or HAL inside the MOS. In the target gas atmosphere, the width of the EDL and the HAL changes, leading to significant modulation of the sensor resistance. Among noble metals, Pd is well known for H
2 gas detection
[59], owing to its excellent catalytic activation ability for hydrogen through the hydrogen spillover effect
[60]. When a sensor decorated with Pd NPs is placed in an atmosphere containing H
2 molecules, hydrogen can easily dissociate into hydrogen atoms on the Pd surface; in the spillover effect, hydrogen atoms move to the neighboring MOS surfaces, leading to additional reactions between atomic hydrogen and adsorbed oxygen
[61]. Other noble metals also exhibit good catalytic activities toward different gases. For example, Ag is often used for H
2S detection because of the generation of Ag
2S upon exposure to H
2S gas
[62]. Pt and Pd also have good catalytic activities toward C
7H
8 and C
6H
6 gases, respectively
[63][64].
9. Hybrid Metal Oxide Semiconductor Gas Sensors
A good strategy to enhance the gas-sensing characteristics of MOS-based gas sensors is using another material in combination with the MOS. Hybrid MOS nanocomposites are either (i) composites of MOS and carbon materials such as graphene, graphene oxide (GO), reduced graphene oxide (rGO), and carbon nanotubes, or (ii) composites of MOS and conductive polymers (CPs). Graphene, which comprises sp
2 carbon atoms, is utilized in gas sensors due to its high charge carrier mobility (200,000 cm
2 V
−1 s
−1) and large surface area (2630 m
2 g
−1)
[65]. The first ever graphene gas sensor was introduced in 2007
[66]. Owing to its single-layer or few-layer nature, graphene can even interact with a single molecule. Pristine graphene can easily agglomerate owing to the surface interactions. Furthermore, graphene has no band gap, hindering its gas-sensing usage
[67]. Thus, rGO, with its many functional groups—such as -OH and –O—as well as defects, is a better choice for gas-sensing studies. Although GO can be used for sensing applications, it has a very high resistance owing to the presence of oxygen-based functional groups, limiting its use in sensing studies
[68]. rGO can be synthesized from GO using chemical reduction, thermal reduction, and UV light reduction methods
[69][70]. Many papers on gas sensors with hybrids of MOSs and carbon allotropes have been reported. However, for sensing studies, rGO is the most effective allotrope of carbon in its hybrid form. This is because of its large surface area, presence of many defects, and high concentrations of charge carriers with high mobility—all of which are beneficial for sensing. Accordingly, several studies have been reported on hybrid MOS–rGO composites with the ability to operate at room temperature
[71][72].
CPs have tunable conductivity and high flexibility in synthesis and processing. However, because of the high affinity of CPs for moisture, they are unstable, and generally exhibit poor sensitivity and selectivity for different gases in their pristine form. The use of hybrid nanocomposites with CPs and MOSs could result in the development of room-temperature gas sensors
[73]. Accordingly, hybrids of CPs and MOSs have been used to enhance the sensitivity of nanostructured sensors at low or room temperature
[74][75]. One application of hybrids of MOSs and CPs is the development of flexible and wearable gas sensors. Flexible gas sensors are generally deposited on flexible substrates, and need to work at low temperatures, e.g., room temperature. Since composites of MOs and CPs can function at low temperatures, they are good candidates for such applications.
10. Comparison of Performance of Gas Sensors with Different Morphologies
Table 1 summarizes the performance of gas sensors with various morphologies. Different morphologies and compositions have been used for the detection of various gases at different temperatures. Moreover, pristine gas sensors show lower responses and higher sensing temperatures, while composite or decorated sensing materials show better performance at lower temperatures.
Table 1. Gas-sensing properties of different sensors with various morphologies.