Editorial Feature

How Nanostructured Tin-Gas Sensors Could Tackle Climate Change

British physicist and engineer Lord Kelvin, after whom the absolute unit of temperature is named, once said: “If you can not measure it, you can not improve it.” One gas we will certainly need to measure in tackling climate change is nitrogen dioxide (NO2), which is important not just for its effects on human health but also for its environmental impact.

nitrogen dioxide, sensors

Image Credit: Rokas Tenys/Shutterstock.com

NO2 can be combined with water, oxygen, or other atmospheric chemicals to form acid rain, and also plays a critical role in determining the atmospheric concentration of important greenhouse gas: ozone. NO2 absorbs radiation in the visible spectra, meaning high concentrations can also reduce atmospheric visibility, and also gives smog its reddish-brown hue. High levels of NO2 in the atmosphere have also been linked to respiratory damage and respiratory diseases such as asthma.

Nitrogen oxides in the atmosphere have origins in both natural sources, such as bacterial and volcanic action and lightning, and man-made sources: predominantly in the combustion of fossil fuels. It’s generally accepted that natural sources are by far the biggest contributors to atmospheric levels of nitrogen oxide, although estimates of natural emissions vary considerably between 20 and 90 million tons per year. Human emissions are estimated at 24 million tons per year.

Measuring Emissions with Gas Sensors

Government-set targets for nitrogen oxide (and other pollutant) emissions have driven the development of gas sensors that can be integrated into combustion environments. Early commercial sensors first appeared in gasoline and diesel passenger cars in the early 2000s and continue to advance.

Typically, these sensors are used to measure nitrogen oxide emissions and as part of a feedback system that can further minimize emissions. For example, in exhaust gas treatment systems a sensor will be placed ‘upstream’ for combustion optimization, and a second sensor will be used ‘downstream’ to measure the final emissions output and to prove legislated emission targets have been met.

Since the 1990s, gas sensors using various materials and configurations have been tested and developed. Metal oxide semiconductor (MOS) based sensors are one variation that has become popular due to their low cost, simple design, and their high sensitivity to oxidizing agents while maintaining great selectivity.

MOS-based sensors use a sensor element that is heated to hundreds of degrees Celsius using a small resistive heater and a semiconductor which can be broken down into three parts: the surface, which interacts with the gas; the bulk, which does not; and the particle boundary, which is the region between the two. Another important variable in designing MOS-based sensors is the Debye length which is the distance over which charge separation can occur inside the semiconductor.

Researchers are finding ways to improve MOS-based sensors so they perform better in harsh conditions such as in a combustion environment. Many factors impact gas sensor response, including film thickness, grain size, and the number of ‘grain boundaries’ (grain-grain junctions) in the device. In particular, multiple grain-grain junctions complicate the calculation of the Debye length, which can make an analysis of the performance of a MOS-based sensor difficult.

A Step Forward for MOS-Based Sensors

One potential way to improve MOS-based sensors is by eliminating the negative impact of multiple grain-grain junctions on performance analysis – particularly with regards to Debye length. This can be achieved by using a single-element device composed of a single nanostructure.

In a paper published in the Physical Chemistry Chemical Physics journal, researchers from the University of Surrey and São Paulo State University (UNESP) contrasted the use of a single-element nanostructured device with a multi-device configuration – both were constructed from tin-oxide systems.

Their research showed that the sensor signal is directly related to the materials’ Debye length, which was successfully obtained using the single-element approach. The researchers were also able to learn that the most stable phase of the tin-oxide system gave the highest sensor signal with a “remarkable” limit of detection, even at a temperature considered not ideal for MOS-based sensors. Finally, using the single-element approach, the researchers proposed the ideal gas-solid interaction mechanisms between nitrogen dioxide and the semiconductor surface.

References and Further Reading

Global Nitrous Oxide Budget (2020) [Online]. Global Carbon Project. Available at: https://www.globalcarbonproject.org/nitrousoxidebudget/20/hl-compact.htm (Accessed on 11th June 2021)

Chapter 7.1: Nitrogen Dioxide [Online]. World Health Organisation. Available at: https://www.euro.who.int/__data/assets/pdf_file/0017/123083/AQG2ndEd_7_1nitrogendioxide.pdf (Accessed on 11th June 2021)

Nitrogen Oxides [Online]. UCAR. Available at: https://scied.ucar.edu/learning-zone/air-quality/nitrogen-oxides (Accessed on 11th June 2021).

Nitrogen oxide sensor [Online]. Wikipedia. Availabe at: https://en.wikipedia.org/wiki/Nitrogen_oxide_sensor (Accessed on 11th June 2021).

Yogendra K. Gautam et al. Nanostructured metal oxide semiconductor-based sensors for greenhouse gas detection: progress and challenges. The Royal Society. Available at: https://doi.org/10.1098/rsos.201324

Philip JD Peterson et al. Practical Use of Metal Oxide Semiconductor Gas Sensors for Measuring Nitrogen Dioxide and Ozonein Urban Environments. MDPI. Available at: https://doi.org/10.3390/s17071653

Mateus G Masteghin et al. The role of surface stoichiometry in NO2 gas sensing using single and multiple nanobelts of tin oxide. Physical Chemistry Chemical Physics. Available at: https://doi.org/10.1039/D1CP00662B

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Written by

Andrew Messios

Andrew is a freelance writer specializing in science, technology and current affairs.


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