CO₂ Sensing Behavior of Calcium-Doped ZnO Thin Film: A Study To Address the Cross-Sensitivity of CO₂ in H₂ and CO Environment

Abstract

CO2, CO, and H2 are the major pollutants in the environment, which are primarily generated during the combustion of the organic compounds. Development of a smart sensor that could address CO2 sensing and its cross-sensitivity toward CO and H2 gases is highly advantageous. In this work, an advanced gas sensor is proposed and developed to address the cross-sensitivity of CO2 sensing in CO and H2 environment, which is based on calcium-doped ZnO (Ca_ZnO) thin films. The proposed sensors were successfully synthesized using wet chemical synthesis. We have reported that a 5 atomic weight % calcium-doped ZnO thin film exhibits an efficient sensing performance with a wide range of CO2 gas concentration (75 000–5000 ppm) at 350 °C. Additionally, mixed gas sensing characteristics have been carried out for various concentrations of CO (500–25 ppm) with CO2 and H2 (500–100 ppm) with CO2 gas at 350 °C. The response (%) was estimated as 53, 83, and 74% for 50 000 ppm CO2, 50 000 ppm CO2 + 500 ppm H2, and 50 000 ppm CO2 + CO gases, respectively. Furthermore, the enhancement of CO2 sensing was investigated using the Raman spectrum of calcium-doped ZnO thin films. Consequently, the cross-sensitivity of pure and mixed gases has been accomplished using principal component analyses, which shows a distinct cluster formation for each test gas. The proposed gas sensor can be potentially applied to biomass sensing, fuel combustion monitoring, and chemical industries.

Carbon Dioxide Sensing—Biomedical Applications to Human Subjects

“…The resulting expected behavior would be an increase in the metal oxide resistance with an increase in pCO . While this assertion seems to hold in the simple ZnO case [ 160 , 162 , 163 ], opposite behaviors were observed when doping ZnO with lanthanum [ 164 ] or calcium/aluminium [ 165 , 166 ]. Consequently, we believe that further research is needed to gain a deeper insight into the working principle of thin-film metal oxide sensors.…”

“…Several elements were used as doping agents for CuO or ZnO thin film such as the above mentioned lanthanum and calcium/aluminium, but sodium [ 169 ], germanium/neodymium/tungsten [ 160 ], manganese [ 170 ], gold [ 161 ], silver [ 155 , 171 ], and indium [ 172 ] were also explored. While the manganese-, gold-, and indium-doped ZnO-based sensors reported in the literature performed worse than their pure ZnO counterpart, all the other above-mentioned doping elements greatly enhanced the sensitivity of the sensors—more than doubling it in certain cases [ 166 ].…”

Discriminable Sensing Response Behavior to Homogeneous Gases Based on n-ZnO/p-NiO Composites

“…Some strategies have been employed to overcome the low selectivity of this kind of gas sensor, including regulation of the optimal operating temperature [25][26][27]; doping or loading of a noble metal [28][29][30][31][32]; modification of chemical composition, morphology and microstructure of the sensor materials [33][34][35]; and construction of n-n [36][37][38][39] or p-n junction [40][41][42][43][44][45][46][47]. Among them, the construction of p-n junction is expected to be a promising strategy due to the fact that the active sites of the p-n composites are different for the target gases.…”

Effect of Ni and Al doping on structural, optical, and CO2 gas sensing properties of 1D ZnO nanorods produced by hydrothermal method

“…It is just this last finding that enables us to detect the concentration level of CO 2 gas in the environment, being one of the most striking points deduced from this work. The response time of NZ5 and AZ5 samples are found to be 85 and 94 s, respectively while the recovery times are calculated to be 37 and 53 s. According to the experimental findings in Cho et al (2015) and Ghosh et al (2019), the local p–n junctions appear when the Ni +2 ions do not replace with the Zn +2 ions in the wurtzite crystal structure. As a result, the response of sensor may be determined by the p–n junctions rather than Schottky barriers as stated in Cheng et al (2012), Deore et al (2010), and Rambu et al (2013).…”