Smart sensors can help deliver India’s net zero targets

Safeguarding the environment is the biggest problem of this century. As countries become increasingly industrialised, tons of potentially harmful gases are continuously released into the atmosphere. Controlling and monitoring gases such as NO, NO₂, NH₃, CO, H₂S, SO₂ and hydrocarbons that can be harmful to the environment and human health can help to reduce their impact. Tracking emissions of greenhouse gases is also a key component of achieving the goal of net zero emissions.

Sensors that can detect these gases are therefore needed in three key areas: for safety monitoring in the environment; to measure emissions from automobiles and industry; and to detect fires.

Sensor solutions

In automobiles, chemical sensors placed in the emission stream of the engine can provide information on the gases being emitted. However, the ability to monitor the type and quantity of emission being generated is very important to judge the status of the engine. In vehicles and industries, monitoring of leakage of fuels like hydrogen, NO_x etc. is important for safe operation. To perform this function, a chemical sensor needs to have special characteristics such as high surface area, stability in harsh environments, low power consumption and high sensitivity. In addition, the gas sensor should be simple and cost-effective to make.

Hybrid inorganic oxide–conducting polymer nanocomposites can help to solve these challenges

Nanoscience and nanotechnology can provide solutions to these problems. In particular, nano-structured materials’ large surface to volume ratios make them attractive candidates for use in gas sensing. Though gas sensors based on nanomaterials have been investigated for the past decade, there are still challenges that need to be addressed. Some of the challenges are: making gas sensors selective enough to detect the target species even in the presence of other gases; ensuring that the sensor is sensitive enough to detect levels well below the threshold limit value (TLV) of a gas so that detection occurs before harmful levels are reached; and to fabricate the gas sensor so that it works at room temperature so that no unnecessary heating is required to activate the sensor.

Unfortunately, most sensors either have a complicated synthesis and fabrication, or only work at high temperatures requiring a heater to be added to the fabrication or result in detecting gases above the TLV limit. Moreover, the complicated adsorption mechanisms involved in gas sensing mean that selecting a material is typically based on ‘trial and error’ procedures. A more systematic approach to discover materials is needed.

Hybrid complex inorganic oxide–conducting polymer nanocomposite heterostructure sensors can help to solve these challenges. Here, a conducting polymer acts as a filler dispersed throughout a metal oxide matrix. The complex metal oxide is robust, semiconducting and can work nearer to room temperatures, and the conducting polymer has an inherent porous structure with large surface area, which act as channels for charge transport to help lower the operating temperature. By controlling the matrix to filler ratio, micro/nanostructures band gap, surface area and interface characteristics can be tuned.

Combining metal oxides with polymers in an organic–inorganic nanocomposites can deliver a selective, sensitive gas sensor

Metal oxide sensors (MOSs) have emerged as one of the most important advances in gas sensing in recent years, exemplified by their use in the ‘electronic nose’ – a device designed to recognise and classify odours. These devices detect volatile organic compounds and generate an electric signal in response that represents chemical information. MOSs show excellent potential for lab- and industry-scale design and development to obtain a close to ideal gas sensor with desirable characteristics.

At the laboratory scale, these sensors are prepared by a simple process of heating components at 250°C and using a hydrothermal autoclave. A gas sensing set up with mass flow controllers and a gas sensing chamber monitors the sensor response with respect to the TLV limit. A Keithley resistivity setup with a thermocouple can be used to measure the resistance of the gas at different temperatures. Once the small scale set up is optimised, then a larger scale gas sensing station can be initiated.

Next-gen nanotech

Studies have found that combining MOSs with polymers in an organic–inorganic hybrid nanocomposites can deliver a selective, sensitive gas sensor that can be tuned to work at room temperature. In this context, conducting polymers have emerged as promising materials that help to improve the mechanical stability and conducting properties of the sensor. They also improve sensitivity and selectivity to a particular gas by enabling specific binding sites and they have a large surface area thanks to their inherently porous structure. They have exceptional electrical properties due to the delocalisation of π-electrons throughout the polymer chain and act as channels for charge transport to deliver the sensor’s electrical response.

In this context, the laboratory synthesis and design of hybrid ternary oxide-conducting polymer nanocomposite heterostructure sensors would be benefical. Various compositions of a ternary metal oxide system with differing electronic structures can be combined together to provide varied composition, structure and work function, offering improved performance of sensor materials. Thus smart sensor systems can be tailored to measure a range of chemical species and help in monitoring the air quality. This would help India to be closer to realising its net zero emissions target.