*3.2.6 Greener options*

In order to develop ecological and environmentally friendly CVD methods, green and bio-renewable carbon sources that are easily obtained at low cost are sought [40, 42, 44].

With the implementation of these precursors, it is attempted to reduce the use of greenhouse gases, the exploitation of toxic materials, and the development of ecological and sustainable technologies [44].

It has been possible to synthesize graphene using various ecological precursors such as foods, for example, cookies, chocolate, honey, sugar, butter, milk, cheese; waste such as plastics, grass, bones, eggshells, dog and cow feces, wood, leaf, bagasse, fruit, tea tree extracts; and derivatives of insects [44].

In particular, the use of solid botanical camphor (C10H16O) for the synthesis of SLG and BLG graphene sheets has been studied. Camphor is a natural solid botanical hydrocarbon source, which is regenerative, low-cost, and environmentally friendly [42]. Similarly, the use of palm oil as a carbon precursor for the synthesis of MLG has been studied. Palm oil is a natural oil source that has a unique chemical composition with long-chain carbon. It has high potential as a green and renewable carbon precursor in the large-scale production of graphene [40].

#### **3.3 Gases**

Molecular hydrogen has a fundamental role in the synthesis of graphene by CVD since it acts as an activating gas. It acts by cleaning the surface of the substrate, reducing impurities and defects during the annealing process, as well as reducing surface oxides [17, 23]. It behaves as a co-catalyst, along with the substrate, promoting the growth of graphene, as well as an etchant to control the growth and its properties [23, 45]. It controls the adsorption, stability, thickness or number of layers, the population on the catalyst surface, the configuration of the edges, and the morphology of the domains of the grown graphene [17, 23, 45].

The flow of H2 has been considered as an important factor for the quality of graphene during synthesis. By using a gaseous precursor, the radius of H2 with respect to the precursor gas can be studied [17]. It has been observed that when the H2/CH4 volume ratio in a process is greater than 0.5, the graphene remains in small separate islands and does not coalesce to form a large film. This may happen because excess H2 can limit graphene growth by removing weak carbon-carbon bonds despite acting as a co-catalyst [46]. If the ratio of H2 to CH4 is too high, the etching of carbon species becomes much faster than the formation of graphene layers [47], destroying the integrity of the network and reducing the quality of graphene [17].

Regarding the flow of the precursor gas, it has been reported that, for methane, the number of layers does not depend on the CH4 flow rate, although an increase in defects was observed with the increase in flow [17].

Similarly, the use of an inert gas can help the synthesis process. The inert gas can be used to dilute the carbon feedstock to achieve high H2 to precursor ratios, to increase the total pressure of the reaction chamber, or to dilute flammable or explosive material below its lower explosive limit [18, 46]. The concentration of each gas can range between 0 and 100%. The explosive limits of H2 in air range between 18 and 60%, and the flammable limits between 4 and 75%. Pure H2 and H2 mixture beyond the flammable limit are dangerous [46]. Furthermore, it can be used as a carrier gas for precursor molecules in certain cases [16]. Gases such as Ar and nitrogen (N2) have been used to fulfill this function [16, 18, 46]. Ar is chemically inert under the conditions used during the synthesis [18, 46]. N2 is inert as well, although more abundant and cheaper than Ar. Since it exists as a diatomic molecule, there is a risk that N2 will dissociate during synthesis. The dissociation product could act as a substituent in the graphene film, causing a doping or interruption of the network. However, the triple

bond present has a very large binding energy, so the rate of N2 dissociation should be extremely small at the temperatures used. Furthermore, Cu is an inefficient catalyst for the dissociation of N2 [18].
