*3.1.2 Annealing*

An important step prior to the CVD synthesis of graphene is the thermal treatment of the micrometer-thick Cu foils. Cu substrates undergo an annealing process that consists of heating them at high temperatures close to the Cu melting point, but lower than it (<1,085°C) for a certain period of time [25]. The surface of copper has some roughness, grain boundaries, surface defects, and impurity particles. This process is carried out in order to smooth the copper surface, reduce surface oxides, remove volatile impurities and surface contaminants, and favor the formation of graphene on the surface [26]. In addition, a reorganization of the copper atoms on the surface is promoted, which produces a release of internal stresses and an increase in the size of the Cu microcrystalline structure [25].

Regarding graphene nucleation, flat Cu regions, or terraces, favor the growth of SLG or BLG domains. In contrast, slope regions on the Cu surface, or ledges, promote MLG growth. Additionally, grain boundaries, impurities, and surface defects contribute to the formation of MLG domains [26].

#### **3.2 Precursors**

The synthesis of graphene using the CVD technique can be carried out with a large catalogue of carbon-based compounds used as precursors. These precursors can be categorized by their physical state or by their chemical structure. By physical state, they can be classified as gaseous, liquid, and solid. While based on its chemical structure, they are separated as aliphatic and aromatic compounds. The use of an appropriate precursor with the right conditions can improve the efficiency of the production process and the quality of the final product [15].

#### *3.2.1 Gases*

Gaseous carbon precursors are the main source used for graphene synthesis by CVD. The most used are hydrocarbons such as methane (CH4) and acetylene (C2H2), followed by ethylene (C2H4) [27–29]. A gaseous precursor occupies less space than a liquid or solid one, because they can be stored in certain specialized containers. Some precursors are produced as a by-product of industrial processes, such as biogas, which is constituted essentially of methane and carbon dioxide [15].

#### *3.2.2 Liquids*

Liquid carbon precursors are less commonly used in CVD methods. The most common precursors used in CVD are hexane (C6H14), ethanol (C2H5OH), and benzene (C6H6) [30–32]. These liquid carbon precursors are easy to use and relatively inexpensive compared with gaseous ones [15, 32].

In contrast to the use of gaseous materials, when the precursor used is in a liquid state, a previous stage is needed to transform the precursor from its liquid form to a vapor in order to be deposited on the metal substrate. Various CVD configurations have been used to work with liquid precursors. An approach constitutes the use of an external source of heat to vaporize liquids with high boiling points. Ultrasonic baths can be used to form aerosols from the liquids [15]. Unlike gaseous precursors that have a certain flow, an external constant gas flow is needed to carry the post-treatment precursor to the reaction chamber [15, 16]. Some of these precursors could be harmful to human health, since some may be volatile organic compounds or carcinogenic in nature [15].

#### *3.2.3 Solids*

Solid carbon precursors are equally rarely used. They are more complex in terms of chemical and biological structure. Any organic material that works as a carbon source can be used, including plant waste, plastics, animal waste, insect parts, even food. Therefore, solid precursors occupy more space in the reaction chamber than gaseous or liquid precursors [15, 33].

As with liquid precursors, prior to the synthesis process, an extra step is required to convert the solid material into a gas phase before acting on the metal catalyst. More energy is required to carry out the process, which can increase costs. Solid carbon precursors can be placed directly on top of the catalyst metal so that graphene forms on the back of the catalyst. Polymers have been spin coated directly on copper foils and used as carbon precursor [34]. Most of the precursors used are solid wastes or biomass; using these products to form graphene would positively impact waste

recycling for the production of a high-value product and may reduce the overall cost of synthesis [15].

**Table 1** presents different carbon precursors that have been used for the synthesis of graphene, divided by their previously discussed physical states, along with a detailed example of the synthesis process using one of those precursors.


#### **Table 1.**

*Carbon precursors used for the synthesis of graphene organized by their physical state. An example of the synthesis performed is shown in each case.*
