ZnO based Gas Sensor

Figure 5.10 showed the electrical conductivity changes of the ZnO nanosheets in the presence and absence of carbon monoxide (CO) analyte gas. The CO gas concentration was varied from 10 ppm to 200 ppm and the temperature was kept constant at 250⁰C throughout the analysis. In the presence of the CO gas in the surface of ZnO, the electrical conductivity of the ZnO increased. ZnO is a stable n-type semiconductor and upon exposure to a reducing gas such as CO the electrical conductivity increases, which explains the increase in the observed electrical conductivity [15]. However, the electrical conductivity of ZnO did not revert to the initial value when the CO gas was removed, suggesting that the ZnO nanomaterial was not given enough time to fully recover before exposure to the next gas concentration. This is typical of most nanomaterials as they require a relatively long time to fully recover, and for this study, the focus was only on the response time and not the recovery time.

Figure 5.10: Electrical conductivity changes of ZnO exposed to different CO concentration The sensitivity of the gas is defined as the sensor response to different analyte gas concentrations. It is influenced by the interaction between the material surface and the gas. This interaction is directly proportional to the surface area of the material and the concentration of the analyte gas [18]. The sensitivity (S) of the sensor was determined by measuring the change in the electrical resistance of the ZnO and is expressed as

𝑆(%) = [𝑅𝑎− 𝑅𝑔

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Where, Ra is the resistance of a sensor in the reference gas, which was air and Rg is the

resistance of a gas sensor in the presence of a target gas. In this study, a voltage of 5 V was used for the analysis as it was generated by the system, it was also used to convert electrical current into electrical resistance to calculate the sensitivity. The sensitivity of the sensor was calculated to be it 9.7, 9.8 and 9.9 % at different gas concentrations of 120 ppm, 160 ppm and 200 ppm respectively. The sensor sensitivity depends on the interaction between the target gas and the chemisorbed oxygen species on the surface of the semiconductor, which then leads to a change in electrical conductivity. Therefore, the difference in the sensitivity is attributed to the difference in the interaction between the gas and the surface of the sensor as the gas concentration increases. The more chemisorbed oxygen molecules the better the sensitivity [16]. This trend was expected because the sensitivity of the sensor mainly depends on the removal of the absorbed oxygen species, which generate electrons. Hence at high concentrations of the target gas, there was enough gas to cover the materials resulting in relatively high sensitivity. The TEM (Figure 5.7) also showed a high surface area at ZnO nanosheets synthesized from 0.3 M NaOH, which correspond with high sensitivity. These findings are consistent with what is found in the literature, by Kim et al [17] and Ahlers et al [18].

Another parameter that was investigated was the response time and is defined as the time required to reach 90% of a stable electrical current after the gas exposure. The response time was calculated at the CO concentration of 200 ppm because this concentration produced relatively higher sensitivity. The response time was calculated to be 114 seconds. Response time of 114 seconds for CO concentration of 200 ppm is not ideal, as by the time the sensor responds, the CO gas would have caused disorientation, unconsciousness or even possible death to living organisms, especially in the mining industry whereby high levels CO gas are produced. A need to improve the response time of ZnO nanosheets is crucial possibly through doping or making a composite with another nanomaterial [18]. Shingange et al, [19] investigated the effect of doping on the response time, the findings revealed that the response time was improved by doping the ZnO with AuNPs for NO2 detection. This shows the need

and importance of doping or making a composite with other nanomaterials such as Au and CNTs in order to improve the electrical conductivity and surface area thus improving gas sensing performance [19].

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5.4 Conclusions

Different morphologies of ZnO have been successfully synthesised using a microwave-assisted hydrothermal method. Hexagonal, flowerlike and sheets like structures were obtained. The different morphologies depended on the concentration of the reducing agent (NaOH). The NaOH concentrations affected the pH, which in turn determined the morphology and formation of ZnO. A concentration of 0.3 M NaOH, yielded sheet-like nanostructures. These nanostructures were considered to be optimum for gas sensing as it is was phase pure and had the highest surface area of 102 m2_{/g, which is crucial for gas sensing. The ZnO nanosheets}

showed an increasing trend in the sensitivity as the concentration of CO gas increased. The response time was calculated to be 114 seconds at a concentration of 200 ppm. However, there is a need to improve the rapid responsiveness of the ZnO nanosheets. This can be done through making of ZnO nanosheets composites with various nanomaterials in order to improve their response time.

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5.5 References

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[2] Govardhan, K. and Grace, A. (2016). Metal/metal oxide doped semiconductor-based metal oxide gas sensors - A Review. Sensor Letters, 14(8), pp.741-750.

[3] Liu, C., Chen, C. and Leu, J. (2009). Fabrication and CO sensing properties of mesostructured ZnO gas sensors. Journal of The Electrochemical Society, 156(1), pp.16-16. [4] Leonardi, S. (2017). Two-dimensional zinc oxide nanostructures for gas sensor applications. Chemisensors, 5(2), pp.17-18.

[5] Yadav, B.C., Srivastava, R. and Yadav, A. (2009). Nanostructured Zinc Oxide Synthesized via Hydroxide Route as Liquid Petroleum Gas Sensor (S & M 0750). Sensors and Materials, 21(2), pp.87-87.

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[8] Abdullah, M., Shibamoto, S. and Okuyama, K. (2004). Synthesis of ZnO/SiO2

nanocomposites emitting specific luminescence colors. Optical Materials, 26(1), pp.95-100. [9] Al-Hada, N., Saion, E., Shaari, A., Kamarudin, M. and Gene, S. (2013). The influence of calcination temperature on the formation of zinc oxide nanoparticles by thermal- treatment. Applied Mechanics and Materials, 446-447, pp.181-184.

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nanostructures and its antimicrobial activity. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 104, pp.171-174.

[12] Krishnakumar, T., Jayaprakash, R., Pinna, N., Singh, V., Mehta, B. and Phani, A. (2009). Microwave-assisted synthesis and characterization of flower shaped zinc oxide nanostructures. Materials Letters, 63(2), pp.242-245.

[13] Kołodziejczak-Radzimska, A. and Jesionowski, T. (2014). Zinc oxide—from synthesis to application: A review. Materials, 7(4), pp.2833-2881.

[14] Moezzi, A., Cortie, M. and McDonagh, A. (2011). Aqueous pathways for the formation of zinc oxide nanoparticles. Dalton Transactions, 40(18), p.4871.

[15] Beniwal, A. (2016). A study & real time monitoring of metal oxide semiconductor (MOS) gas sensor. IOSR Journal of Electronics and Communication Engineering, 01(01), pp.07-12. [16] Hwang, I. and Lee, J. (2012). Highly sensitive and selective gas sensors using catalyst- loaded SnO2 nanowires. Journal of Sensor Science and Technology, 21(3), pp.167-171.

[17] Kim, H. and Lee, J. (2014). Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview. Sensors and Actuators B: Chemical, 192, pp.607-627.

[18] Ahlers, S., Müller, G. and Doll, T. (2005). A rate equation approach to the gas sensitivity of thin film metal oxide materials. Sensors and Actuators B: Chemical, 107(2), pp.587-599. [19] Shingange, K., Swart, H. and Mhlongo, G. (2018). Au functionalized ZnO rose-like hierarchical structures and their enhanced NO2 sensing performance. Physica B: Condensed

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CHAPTER 6