Physicochemical properties and growth kinetics of multilayer graphene by chemical vapor deposition for gas sensing application

This research has employed CVD to obtain high quality and large surface area MLG films on Co-Ni/Al2O3 substrate for gas sensing applications. The effect of process conditions on the yield of the MLG films grown on the Co-Ni/Al2O3 substrate was investigated using RSM. The employed parameters were reaction temperature (700-800°C), nominal catalyst (Co/Ni) composition (0.3-0.7), and ethanol flowrate (9-11ml/min) at a constant pressure. A total of 20 experimental runs were performed for the optimum growth condition of 77% yield of the MLG film. The optimal results show that the 800°C reaction temperature, a catalyst ratio of 0.3/0.7 with an ethanol flow rate of 11 ml/min were the best conditions for a scalable yield of large-area and high-quality MLG for gas sensing applications. The experimental test results show a correlation between the RSM predicted and experimental responses. The obtained MLG films was systematically characterized by using FESEM, EDX, HRTEM, RS, XRD, TGA and DTG, TEM analysis, FT-IR analysis and XPS analysis. All these characterizations confirm the excellent quality and number of layers of the MLG. Furthermore, the growth kinetics of MLG was investigated by varying the reaction temperature and monitoring the partial pressure of the ethanol (C2H5OH) as well as that of hydrogen. The data obtained were fitted to the Langmuir-Hinshelwood kinetic model for the estimation of the reaction rate constants at different temperatures. The results showed that the reaction rate constant increased with temperature and the apparent activation energy of 13.72 kJ mol-1 was obtained indicating a relatively fast rate of MLG growth. The parity plot obtained for the comparison of the predicted and observed rate of C2H5OH consumptions showed an excellent agreement. This study is important for understanding the growth kinetics of MLG in order to develop appropriate measures that can control the production of MLG thin films for use in the electronic industries. Finally, the use of MLG thin films as a sensor material for gas sensing device has been demonstrated. The gas sensing characteristics of MLG films was investigated by measuring the resistance across the MLG film at different time while passing the gas mixtures. When different gases are introduced to the test chamber at a steady flowrate, the resistance increased and reached to a saturation level. The findings showed that the MLG-based sensor device was most sensitive to NH3 gas and H2 gases whereas it shows the least sensitivity to CH4 gas. This study has demonstrated the suitability of the MLG as a material that can be employed as sensor device for gas sensing applications most especially NH3 and H2.

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