Capturing Two Fruits with One Stone

What if you had two problems, and there was a single solution to tackle both? Such a ‘miracle-cure’ has been discovered to fight pollution. We are all very much aware as to how pollution is a bane, causing global warming and climate change. The various air pollutants such as carbon dioxide, sulphur dioxide, and nitrogen dioxide (NO2) are major threats to human health and the environment.

Of all the air pollutants, NO2 is the most toxic and harmful. It is therefore very important to have gas sensors to detect gas analytes in real time, along with a good sensing performance.

For this purpose, metal oxide semiconductor (MOS)-based gas sensors are used. These gas sensors exhibit good performance due to their surface chemistry, structural morphology, and good physical properties. The unique characteristic of the MOS material is its wide electronic bandgap – that is, it can be ‘programmed’ to function as an insulator or a semiconductor, or in-between, by varying the size of the nanoparticles.

The metal oxides’ inherent characteristics also provide high sensitivity and good stability in high-temperature environments, such as those found in automobiles and various other applications.

However, although metal oxide nanoparticles show high affinity towards gas molecules, the major drawback of the MOS is its poor electrical conductivity which hinders the electron transport in the gas sensor device. To combat this problem, conductive carbon can be used to enhance electron transport. Thus, for these sensors, the presence of conductive carbon support material is as important as that of active metal oxide nanoparticles.

To obtain this carbon source, usually the earth’s crust is mined for coal, graphite, lignite, etc. But scientists are constantly looking for sources that are more environment friendly and sustainable. Thus, natural sources such as agricultural waste, animal waste, and plant parts are good alternatives.

Another aspect many researchers are focusing on nowadays, is the retention of biodiversity for fulfilling the United Nation’s sustainable development goals. The environment is threatened by various invasive alien plant species spread over the earth. These include plants like Prosopis juliflora, Lantana camara, and Mikania micrantha, which grow in both terrestrial and aquatic habitat, and adversely affect socio-economic growth. This is because plants like Prosopis juliflora have deep roots that search and abstract (remove) groundwater which causes surface runoff and groundwater recharge problems. This wasted water could be used to irrigate lakhs of sugarcane and cotton hectares valued at USD 470 million and USD 320 million per growing session, respectively.

In this study, carried out at the Indian Institute of Technology (IIT) Madras, Chennai, India, by Mr. Vetrivel Sankar and Prof. Ramaprabhu Sundara from the Alternative Energy and Nanotechnology Laboratory, Nano Functional Materials Technology Centre, Department of Physics, and Prof. Krishnan Balasubramaniam from the Centre for Non-Destructive Evaluation, Department of Mechanical Engineering,  Prosopis juliflora was converted into a biomass carbon material and in turn used for the detection of NO2 gas. Mr. Vetrivel Sankar also works at the Centre for Non-Destructive Evaluation, Department of Mechanical Engineering at IIT Madras.

The biomass carbon from Prosopis juliflora leaves (PJBMC), was combined with semiconductive tin dioxide (SnO2) nanoparticles, and the resulting nanocomposite was tested for gas sensor applications.

The sensor from Prosopis juliflora was found to have a sensitivity of 31.0 +/- 0.9% for 90 parts per million (ppm) of target gas molecules. The prototype PJBMC/SnO2 gas sensor was demonstrated as a real-time NO2 gas leak detector with an LED flashlight warning system.

This innovative and pioneering work, where an invasive alien plant species, Prosopis juliflora was used as a source of carbon material for gas-sensing application, reinforces the fact that sustainable development of electronic materials is possible through creative research.

Thus, Prosopis juliflora a harmful plant, was used both as a biomass carbon material, as well as a gas sensor which was also an effective NO2 gas detector.

Prof. Wolfgang Bacsa, Professor from the Center for Materials Elaboration and Structural Studies, University of Toulouse,  France, gave his analysis of the work done by the authors, complimenting their efforts with the following comments: ”The publication by V. Sankar et al. on the fabrication and demonstration of NO2 sensors based on a novel metal oxide-biomass carbon composite is an excellent example of how materials research can contribute to address the growing concerns on environmental pollution, loss of biodiversity and climate change. In this work, the authors successfully demonstrate a metal oxide semiconductor/conductive carbon nanocomposite-based sensor for NO2 gas, a common pollutant in the atmosphere which is a greenhouse gas in addition to being hazardous to human health. A novel process is described to source the conductive carbon support from the plant Proposis Juliflora, which is ubiquitous and threatens both the biodiversity and ground water resources. The active sensor comprises SnO2 (n-type) nanoparticles uniformly distributed on the conducting carbon (p-type) support using a sol-gel process. The sensor shows high selectivity to NO2 and a linear response with concentration is also observed. The gas sensing mechanism is elucidated and the high sensitivity of the sensor is demonstrated by using it as a NO2 gas leak detector. ” 

Article by Akshay Anantharaman
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