Unveiling the relationship between graphene formation and carbonaceous species (C, C2 and C2H2) via CH4 cracking in Ar/CH4/H2 microwave plasma: A global model investigation

• The highest C 2 H 2 density (∼10 18 m −3 ) is obtained at the atmospheric pressure. • Electron density (∼10 18 m −3 ) and electron temperature (∼1.3 eV) are calculated. • High ratios of C 2 /C, C 2 H 2 /C, C 2 H 2 /C 2 are the key factors for high-quality graphene. • Optimum power are (200 W), flow rate (5 slm), CH4 (16-22%) and H2 (2-5%) percentage. A global model on microwave Ar/CH 4 /H 2 plasma is developed. The number density of C, C 2 and C 2 H 2 at low pressure, reduced pressure and atmospheric pressure are compared. Atmospheric pressure is favorable for graphene growth, and our simulation shows that high C 2 /C and C 2 H 2 /C ratios are achieved under this condition. Ratio analysis incorporating C 2 /C, C 2 H 2 /C and C 2 H 2 /C 2 and selectivity analysis are introduced for conducting parametric studies including microwave power (200 to 600 W), flow rate (1 to 5 slm), CH 4 percentage (1 to 35 %) and H 2 percentage (1 to 30 %) systematically. C 2 /C, C 2 H 2 /C, and C 2 H 2 /C 2 ratios decrease with increasing microwave power; 200 W is chosen ideal microwave power. High selectivity of C, C 2 , and C 2 H 2 is observed at 400 W, corresponding to a gas temperature of 3000 K. A flow rate of 5 slm ensures sufficient residence time for sp² growth, as indicated by elevated C 2 /C and C 2 H 2 /C ratios. The analysis shows that while higher CH 4 levels enhance C 2 /C, C 2 H 2 /C, and C 2 H 2 /C 2 ratios, the sharp decline in the selectivity of C, C 2 , and C 2 H 2 increases the risk of amorphous carbon. The analysis indicates that 10% H 2 offers balanced C 2 /C, C 2 H 2 /C, and C 2 H 2 /C 2 ratios while 1 to 10% H 2 achieves the highest C 2 H 2 selectivity. Experimental validation confirmed 2 to 5% H 2 and 16 to 22% CH 4 as optimal, which is in agreement with simulated values. As such, this work casts a robust framework for optimizing reactor design and operation through ratio and selectivity analyses, enabling efficient and scalable graphene production.