Photoelectric detector could be a low-cost route to detecting atmospheric pollutants
“Climate change is widespread, rapid, and intensifying”, wrote the Intergovernmental Panel on Climate Change (IPCC) in August 2021. The only way to limit its impact is through “strong and sustained reductions” in emissions of greenhouse gases (GHGs); particularly, carbon dioxide (CO2) and methane (CH4). The amount of CO2 in our atmosphere has increased globally by about 40 % since the industrial revolution. For methane, the increase is closer to 150 %. To date, monitoring GHGs has been a specialist pursuit, carried out using very expensive, advanced measuring equipment. Implementing real-world mitigation strategies will require a more fine-grained approach – ideally, a dense, low-cost, global network of sensors that can continuously and accurately monitor GHGs in real-time. And given that more than 70% of carbon emissions come from urban areas, they’d be a good place to start.
A group of South Korean researchers may have a solution. They’ve designed a graphene-based photoelectric detector that can measure changes in incident light energy caused by the presence of GHGs in the atmosphere. In a recent issue of Carbon [DOI: 10.1016/j.carbon.2022.06.044], they report that this is the first time a device like this has been used to study GHGs, and say that its low fabrication costs could see it adopted widely.
At the heart of the device is graphene – a material known for its excellent electrical properties. To make the detector structure, the authors exposed vertically arranged graphite cylinders to high temperatures and pressures (under a mixed gas environment) for two weeks. This caused the growth of tens to hundreds of vertically stacked, overlapping graphene nanosheets between the cylinders, which filled the space. The composite structure was then exposed to 1000 °C under 100 atm for one week, along with an electric field – in this environment, an additional layer of 2D graphene sheets formed along the top of the cylinders. The structure was then insulated with boron, placed into a stainless steel vessel, with a quartz window installed on its upper surface, forming a vacuum. When in use, light entering the detector material would pass through a gas medium outside the quartz.
Initial tests proved that the electrical conductivity of the device increased as light intensity increased, and that the number of emitted photoelectrons is proportional to the intensity of the incident light. This confirmed that it was successfully operating as a photoelectric device, and further tests showed that it responded to fluctuations of light within 1 second. The authors then moved on to exploring the effect of GHGs on photocurrent. The device showed no significant change in responsivity when low-intensity incident light reached it after passing through nitrogen and oxygen gas. However, once GHGs were included in the medium, e.g. mixed methane gas (CO2 + CH4), there was an evident change in the device’s conductivity.
To reflect a typical urban environment, the authors chose to test with ‘high concentration standard gases’ (5x average atmospheric concentration), and measured the device’s response. They then adjusted GHG concentrations to 1/2, 1/5, and 1/10 of standard concentrations, and repeated the measurements. In all cases, they used a single intensity of incident light. They found that responsivity decreased as the corresponding gas concentration decreased. They write that “at a relatively high concentration, the energy absorbed by the GHGs is significant and highly reactive, supporting the response mechanism of the graphene photoelectric device.”
The GHG detector accurate responded to concentrations of CO2 between 2159 to 216 ppm, 0.6 to 0.06 ppm of carbon monoxide, and 8.79 to 0.88 ppm of methane mixed gas in the air. The authors say that this “approached the detection limits of high-performance GHG measuring equipment,” and concluded that their “photodetectors will provide a basis for building high-density GHG monitoring network systems in cities.”
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Young Suk Oh, Hun-Seong Kim, Nicole Bassous, Dong Won Kim, Chang Kee Lee, Sangwon Joo, Haeyoung Lee, Chu Yong Chung, Yeon Hee Kim, Sung Mi Jung, Su Ryon Shin, Hyun Young Jung. “Non-contact, low-cost regional greenhouse gases detection via 3D laminated graphene-based photoelectric construct,” Carbon 197 (2022) 246-252. DOI: 10.1016/j.carbon.2022.06.044