Current gas sensors
Since the discovery of gas sensors in the mid-1920s, significant progress has been made in developing innovative sensors for gas detection.
Some of the more prominent types of gas sensors include electrochemical, optical, and chemical resistance sensors.
Due to their short life, the application of electrochemical sensors is limited, which reduces their versatility, especially in gas sensing. In contrast, optical sensors have high sensitivity and selectivity, as well as fast response time and sufficient life, but they also have limitations due to their large size and high cost.
Although the chemical resistance sensor is not as selective as the optical sensor, its production cost is much lower, and it can be manufactured by a simple method.
Promote the development of QD chemical gas sensors
In order to overcome some of the limitations associated with traditional chemical gas sensors, people have made some different attempts to study how certain QDs can create more sensitive and stable chemical gas sensors.
Colloidal quantum dots (CQD)
Like traditional quantum dots, colloidal quantum dots (CQD) are also semiconductor nanoparticles. However, the uniqueness of CQDs lies in their suspension in the solution phase. This suspension of CQDs provides a powerful quantum confinement effect, which subsequently enhances the electronic and optical properties of these QDs, and improves their absorption, emission, and quantum size tunability.
So far, CQDs have been integrated into photodetectors, environmental sensors, and various light-emitting devices, some of which include LEDs or photoluminescent elements.
Due to the advantageous properties associated with CQDs as sensing materials, a recent study investigated the use of these QDs in the detection of environmental nitrogen oxide (NO 2) levels.
In their work, lead sulfide (PbS) QDs are used because they have been used to detect hydrogen sulfide (H2S), methane (CH4) and ammonia (NH3) gases. It should be noted that in addition to PBS, several other types of CQD have been investigated for their usefulness as gas sensors. Some of these materials include zinc oxide (ZnO) QD, for example, it has been found to be sensitive to both NO2 and H2S.
Sensors based on tin (II) oxide (SnO) QD have also been found to be sensitive to NO2 and H2S, as well as ethanol and liquefied petroleum gas (LPG).
Research on PbS QD in 2020 found that the sensor developed with this material can detect the content of NO2 at room temperature with a detection limit of about 0.15 parts per billion (ppb).
Compared with some of the more advanced NO2 sensors currently on the market, the researchers in this study proposed that their PbS QD-based sensors may have better sensitivity capabilities.
Metal oxide QD
Some of the most widely studied conductivity sensor materials are metal oxides because of their ideal chemical stability and can be used for this application.
For this reason, the development of new nano-scale metal oxide structures, especially metal oxide QDs, has attracted more and more attention. For example, tin (IV) oxide (SnO2) QDs in the size range of 3-4 nm have been found to exhibit up to 3 times the sensing response compared to traditional SnO2 sensors.
This enhanced sensing response is believed to be the result of the reduction in the size of the SnO2 QD structure, as well as the increase in the surface-to-volume ratio, with excellent surface reactivity. When used to detect carbon monoxide (CO), SnO2 QDs have also been found to have enhanced adsorption to this chemical because they have a larger surface area, allowing more surface defects and oxygen vacancies.
Metal Chalcogenide QD
Metal chalcogenides are metal materials containing one or more chalcogen elements, such as sulfur (S), tellurium (Te) or selenium (Se), which are often used for their ability to enhance the electrical and sensing properties of composite structures. Designed as QD.
An example of metal chalcogenide QD can be found in recent discoveries, that is, by forming a depletion layer between p-type PbS QD and n-type TiO 2 nanotubes, the concentration of active sites in the sensing structure increases, It can better adsorb gaseous substances on its surface. It was found that the composite material selectively senses NH 3 at a concentration of up to 100 parts per million (ppm) at room temperature.
It has also been found that chalcogenide materials composed of cadmium sulfide (CdS) quantum dots and cobalt oxide (Co 3 O 4) microspheres can provide enhanced sensing response and recovery time when detecting H 2 S.
The unique chemical stability of CdS QD has allowed it to be incorporated into other metal oxides, including ZnO, SnO 2 and indium oxide (In 2 O 3).
When green light was irradiated on each of these composite structures, it was found that their sensing capabilities were significantly improved.
It should be noted that chalcogenides are relatively new materials for incorporation into gas sensing devices. Therefore, further research should be conducted to evaluate its performance under various environmental conditions.
Author: Benedette Cuffari
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