Conductive polymers, metal oxides, and carbon nanotubes are examples of semiconducting materials used in gas sensors. Typically, metal oxides are the primary option because they are inexpensive, have good sensitivity and stability.
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Detecting Gases with Sensors
It is common to employ gas sensors to monitor environmental contamination and detect low amounts of combustible or explosive gases. Monitoring gases, humidity, and moisture in medical environments, industrial processes, and agriculture is a critical part of their function.
Gas sensors can be classified as optical, acoustic, chromatographic, and calorimetric systems. The creation of reliable and low-cost gas sensors, particularly at room temperature, is a significant scientific and technical problem; however, advancements in semiconductor materials are helping to overcome these challenges.
Metal Oxides
Metal oxides are the commonly known semiconducting oxides and are the most often utilized material to make sensors. Gas-sensitive effects on germanium were initially reported in 1952 by Brattain and Bardeen; it was the first semiconductor to be employed as a material sensitive to gas. Some examples are Sm-doped CoFe2O4, Sr-doped Fe2O3, and Nb-doped TiO2, which are all metal oxides with enhanced sensing properties.
Using semiconductors made up of metal oxides as the gas sensors has advantages, including low cost, ease of use, and the ability to detect many gases, including those that can be flammable or hazardous.
Metal oxide-based gas sensors cannot detect lower gas concentrations due to low sensitivity or the detection limit and poor selectivity, while their high power consumption and high working temperatures make them unsuitable for specific applications.
Conducting Polymers
For gas sensors, conducting polymers, sometimes referred to as intrinsic conducting polymers, have been continuously employed as active layers and remain under investigation for potential gas sensing applications. These include polypyrrole (PPy), polyaniline (PANI), and poly (3,4-ethylene edioxythiopene).
At room temperature and with good mechanical qualities, conducting polymers can be used as gas sensing materials.
Long-term instability, irreversibility, and limited selectivity are the main drawbacks of widespread practical implementation. Since the sensor response declines over time, a process that cannot be reversed, conducting polymer sensors have a limited lifespan.
Carbon Nanotubes
Carbon nanotubes have frequently been used in gas sensors structure. Single-walled carbon nanotubes (SWCNTs), for example, have been utilized in the formation of single sheet sensor design. Sometimes multi-walled carbon nanotubes (MWCNTs) make multiple sheets where only the outermost atoms are responsible for the sensor response.
Nanotubes feature a high ratio between surface and volume, which makes them ideal for gas sensing applications and exceptional chemical and mechanical stability.
However, chemically reactive gases such as NH3 and NO2 affect CNTs, and their commercialization is hindered by a lack of technological progress. These nanotubes are both expensive and complex since it is tough to produce them constantly without any defects.
Uses of Semiconducting Materials in Gas Sensors and Applications
Many sensors are categorized based on their substance; for example, oxide sensors are often used in automobiles.
Combustion-related chemicals are the most common types of monitored gases. As a result, damper systems are equipped with CO and NO2 sensors that measure the "outside air quality level." In addition, a sensing system installed within the passenger vehicle cabin may detect gases like food odor, cigarette smoke, or bio-effluents to define the "inside air quality level," necessitating more advanced climate management ideas.
Metal oxide sensors are frequently used in automobile gas sensors. A commercially available metal oxide-based sensor is promoted as the "smallest selective gas sensor" appropriate for air quality monitoring in transportation, aerospace, and automotive applications.
The large structural porosity and specific surface area of carbon nanotubes (CNTs) make them an attractive candidate for gas sensors in fossil fuel combustion. The CNT sidewalls and end caps include defects that enable gas adsorption.
Various chemical groups are formed on the surface of carbon nanotubes due to the purification and dispersion operations used to manufacture them, increasing their ability to absorb gas molecules and, thus, their sensitivity to gas detection methods.
Challenges in Designing Semiconductor Gas Sensors
To advance semiconductor gas sensor innovation, several research topics are worth tackling. Sensors made up of metal oxides semiconductors can be operated in both temperature modulate and isothermal regimes, where temperature control circuitry becomes more difficult. The latter provides vision into a temporal reply pattern, distinguishing between different analytes, making the temperature control circuitry more difficult.
Interface circuit design is one of the most demanding components because it must manage gas sensing element accuracy and dynamic range and adjust baseline resistance drift of sensing material.
For optimal operation, separate heating elements are used to heat most gas sensors to work in temperatures much above the ambient. This final requirement is particularly problematic.
There are several extra challenges for wearable gas sensors compared to their industrial equivalents, such as tiny form factor and lightweight, low working temperature, mechanical durability consumption upon various skin deformations, and low energy.
Continue reading: Nanosensors for Gas Analysis and Monitoring
References and Further Reading
Ananya Dey. (2018). Semiconductor metal oxide gas sensors: A review. Materials Science and Engineering. https://doi.org/10.1016/j.mseb.2017.12.036
Lu Zhang, K. K. (2019). Recent Advances in Emerging 2D Material-Based Gas Sensors: Potential in Disease Diagnosis. ADVANCED MATERIALS INTERFACES. https://doi.org/10.1002/admi.201901329
M.Mittal, A. (2014). Carbon nanotube (CNT) gas sensors for emissions from fossil fuel burning. Sensors and Actuators B: Chemical. https://doi.org/10.1016/j.snb.2014.05.080
Maria Vesna Nikolic, V. M. (2020). Semiconductor Gas Sensors: Materials, Technology, Design, and Application. Sensors. https://doi.org/10.3390/s20226694
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