Among the CVD conditions, hydrogen (H 2 ) has been commonly adopted in graphene synthesis for its beneficial effects on several aspects during synthesis. One of the major functions of hydrogen is to clean and reduce the substrate surface by removing oxides or impurities that may form barriers to the growth of graphene. In metals like Cu and Ni, there is a natural oxide formation on the surface, wherein hydrogen reacts with the oxides to reduce them into volatile compounds such as water (H 2 O) which get removed from the system, thereby giving a clean and active surface. This is an important prerequisite for graphene formation . Besides this, hydrogen also controls the dissociation of the carbon precursor-usually methane CH 4 which controls the rate at which carbon atoms are released for graphene formation. Without hydrogen, the methane CH 4 would decompose too fast, leading to a carbon deposition rate that is too high and which would form multilayer graphene or even amorphous carbon. In this way, hydrogen has controlled the formation of high-quality monolayer graphene through this dissociation. This is because hydrogen maintains the active sites on the metal substrate. For metals like Ni, hydrogen diffusion and desorption enhance surface activity. In this case, increasing hydrogen flow increases the number of active sites for CH 4 dissociation, thus facilitating graphene growth with higher efficiency. In contrast, hydrogen can block active sites on Cu at higher flow rates; this reduces the rate of graphene deposition . Hydrogen also acts as a chemical etchant to prevent undue accumulation of carbon through reaction with it in the form of volatile hydrocarbon species, such as CH 4 , by reducing the likelihood of forming amorphous carbon. Such an etching mechanism participates and ensures that graphene gets deposited uniformly with high crystallinity, without defects, and thereby enhances electrical and mechanical properties.