The standard of living of people has increased compared to the previous decades due to the industrial revolution. This industrialization has negative aspects, such as the release of gases that pollute the environment and endanger the health of living beings and humans. The production source of each of these pollutants are industries, houses and cars. Among the consequences of the existence of these gases and pollutants in the environment, we can mention acid rain, destruction of the ozone layer, and greenhouse effects. Therefore, in order to control the release of these industrial and automotive pollutants, home security and environmental monitoring, it is very important to detect gases such as H2S, H2, O2, CO2, CO, NH3. This problem has multiplied the growth, development and evolution of gas sensors with high sensitivity in recent years. A gas sensor is an analytical and analytical tool that converts the response of the transducer into a measurable signal upon contact with a gas. Such sensors should be able to detect and measure selectively and quantitatively the density of a specific gas in the environment continuously. The figure below shows the internal components of a gas sensor.

Figure 1: A view of the internal components of a gas sensor

Nano Gas Sensors

With the progress of science and technology in the world, the need for sensors with more accuracy and capabilities and smaller dimensions was felt more. On the other hand, with the introduction of science and nanotechnology into various fields, including electronics, it became possible to make electrodes on a small scale, which made it possible to make nanometer sensors. One of the important features of nanosensors is higher sensitivity and selectivity; Because it has improved the detection limit up to the nano scale. The miniaturization of sensors reduces their weight and minimizes their power consumption, thus reducing the cost of consumption.

Gas detection methods that were used until 1995 were normal methods. The main problem of these methods was the high time response and the need for high temperature to detect gases. As a result, to solve this problem, a tool with a low time response was needed. For this purpose, suggestions have been made to improve structural parameters, including crystal structure and direction. Nano-structured materials can be suitable materials for use in new generation sensors due to the reduction of the working temperature of the gas sensor, lower power consumption and greater safety in operation. One of the most important characteristics of nanostructured materials is their high surface-to-volume ratio. For this reason, the absorption of gases on the sensor is better, and also the sensitivity of the sensor is higher due to the increased interaction between the analyte and the sensing part. Among all nanostructured materials, the use of nanoparticles , nanotubes and nanorods in this field has been receiving a lot of attention for some time. The use of semi-conductor sensors has been common for a long time, and the use of nanostructures of these materials is also being considered today. In addition to these materials, the use of carbon nanotubes has increased significantly due to their interesting and unique properties. Creating or increasing electrical conductivity, increasing the effective surface area, increasing the detection range of gases, reducing gas temperature, creating new interfaces sensitive to surface reaction and reaction with some molecules are some of the advantages of using carbon nanotubes .
The general classification of gas sensors based on sensor technology is as follows: semiconductor, metal oxide, solid state semiconductor/semiconductor, photoionization detectors, electrochemical, infrared, catalytic and… Customers around the world prefer gas sensors that are based on semiconductor and electrochemical technologies; Because they are cost-effective, both in terms of accuracy and in terms of cost savings, in order to detect toxic or combustible gases, and they are also capable of detecting a wide range of gases.

Characteristics of the gas sensor

An ideal sensor should mainly have the following characteristics:

•  Have a high response speed.
•  Chemical stability and long life.
•  have high resolution and selectivity.
•  be reversible.
•  Environmental factors such as humidity, temperature, etc. do not affect its sensing process so that it can show predictable behavior in any situation.
•  It has small dimensions and its transportation and use is simple and inexpensive.
•  have high sensitivity.

Applications of gas sensors

Gas detection has found wide applications in various other industries and fields. The applications of gas sensors include environmental protection, oil and gas industries, automotive industries, defense industries, mines, factory gas output control, breath alcohol testing, fermentation process control, ventilation control for homes and agricultural and poultry industries. Detection of ammonia leakage in the refrigerator and detection of household gases (methane, butane and propane), fire detectors, warning devices for the presence of dangerous gases in the environment and detection of volatile organic compounds.

Barriers to commercialization

Today, the main efforts in the field of sensors are focused on optimizing the parameters of sensitivity, selectivity, stability and response time. For this reason, it is vital to pay attention to basic materials and issues related to processing processes to achieve a gas sensor with proper efficiency and performance. Among the mentioned parameters, sensitivity of gas measurement (detection of gas concentration in ppm) and selectivity of gas (detection of a specific gas in a mixture of gases) are two important issues in the investigation of gas sensors. Most gas sensors, especially semiconductor gas sensors, are poor in selectivity of gases. Therefore, in order to achieve high selectivity and improve the sensitivity of sensors, nanotechnology and the use of nanostructured materials present new opportunities to the activists of this field. However, the industrial production and commercialization of these sensors are still not successful and the need for improvements in this technology is felt. The first challenge in this direction is to find a cheap and low-energy way to mass produce gas sensors. Current research is solving this problem and it seems that it will take some time to reach the stage of mass production. However, several processing schemes have been successfully tested in laboratory scales, but the processing techniques and processes are desirable to have the minimum number of processing steps. For this, steam-based processes seem to have a more promising approach. Although chemical deposition processes are more cost-effective than sputtering and chemical vapor deposition (CVD) , the quality of films produced by vapor deposition processes is superior to that of deposition processes based on chemical solutions.
As we mentioned above, sensitivity and selectivity of gases are two important and challenging issues for the mass production of sensors.
In general, the sensitivity increases by doping impurities that change the concentration of carriers and change the mobility or by reducing the size of the particles to the nanometer scale. In recent years, the sensitivity of oxidized semiconductor materials has been greatly improved by reducing the particle size in the range of 50-50 nm; But it is still not clear how reducing the size of the particles affects the measurement of the sensor and its sensitivity.
To increase the selectivity of nanomaterials, we can use volume impurities, oxide catalysts, adding metal clusters or surface modification methods. For example, the selectivity of chemical sensors is strongly influenced by the addition of metal clusters such as platinum and palladium, which results in increasing the selectivity of sensors to gases such as carbon monoxide. But there is a fundamental problem for the mass production of such sensors; All these methods have been carried out on a laboratory scale and progress in this field is not possible without understanding surface processes and phenomena at the atomic level.