Nitrogen dioxide is spewed into the atmosphere by industries, automobiles and various agricultural activities. Inhaling the gas damages the lungs. Long-term exposure can even cause chronic lung disease and asthma. To monitor environmental nitrogen dioxide, many types of sensors have been designed.
Among nitrogen dioxide sensors, metal oxide-based gas sensors are quite popular. When the gas is adsorbed on the metal oxide, the resistance changes which can be easily measured. Metal oxide sensors are easy to make, have simple control electronics and are low cost. But, for practical use, their performance needs further improvement.
The sensitivity of the metal oxides depends on the surface area exposed to nitrogen dioxide. In recent years, the surface area of the metal oxides was increased by converting them into nanostructures, thus improving their performance.
Can we not increase the available surface area even more by converting the metal oxides into linear one-dimensional structures? We may even be able to improve the performance by creating a junction of two different metal oxides – a heterojunction.
Biji Pullithadathil and co-workers from the PSG Institute of Advanced Studies, Coimbatore decided to test the idea.
They used coaxial electrospinning, which makes use of a concentric arrangement of spinneret orifices to create core-sheath structured nanofibres that can then be collected by electrostatic attraction
To make such heterojunction metal oxide nanofibres, the researchers fed the precursor solutions through the coaxial spinneret followed by a high-temperature oxygen environment to create a heterojunction nanofibres having Zinc oxide core and bismuth oxide or indium oxide sheath.
Zinc oxide and indium oxide conduct electricity primarily by the movement of negatively charged electrons. In other words, both are n-type semiconductors. In bismuth oxide, on the other hand, it is primarily the positively charged holes that conduct electricity which makes it a p-type semiconductor. So, in effect, the researchers had both n-p and n-n type heterojunction nanofibres.
They deposited both types of heterojunction nanofibers directly onto a chip made of alumina having interdigitated gold electrodes. These overlaid chips acted as sensors.
The sensors were tested in a gas chamber with controlled atmosphere for their gas sensing performance. Zinc oxide-bismuth oxide nanofibre sensors worked best at a higher temperature while, zinc oxide-indium oxide nanofibre sensors worked best at a lower temperature compared to bare zinc oxide nanofibre sensors.
The sensors were extremely sensitive to nitrogen dioxide and could sense the gas even at parts per billion concentration levels. Both heterojunction nanofibre sensors were about ten times more sensitive to nitrogen dioxide than conventional zinc oxide nanofibre sensors.
The zinc oxide-bismuth oxide sensors were slightly more sensitive than the zinc oxide-indium oxide sensors, but the zinc oxide-indium oxide sensors responded to nitrogen dioxide gas slightly faster.
To understand the high sensitivity, the researchers characterised the heterojunction nanofibers using various techniques. At the junction between the metal oxides, there were nanograins, tiny structures that increased the surface area. This resulted in changing the accumulation and depletion region widths at the heterojunction interface. The increase in surface area and increased chances of charge transfer made the sensor more sensitive to nitrogen dioxide. The results suggest that the high performance of the nitrogen dioxide sensors is probably due to the presence of more active sites and the unique conduction created by the heterojunctions.
The excellent sensitivity and fast response of the nitrogen dioxide sensors made from cost effective materials using a one-step fabrication method – what better boon can environmental monitoring agencies ask for?
DOI: 10.1039/d2ma01095j;
Mater. Adv., 4, 3010–3025 (2023)
Reported by Vishal Baloria
BML Munjal University, Gurugram
This report was written during the fourth online workshop organised by Current Science.
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