**3. Synthesis of graphene** *via* **CVD**

Chemical vapor deposition has proven to be a very efficient method for graphene film synthesis [15]. It is possible to obtain films with few defects, good uniformity, and good control in the number of layers, acquiring films between one and few layers [16, 17].

Regardless of the CVD equipment configuration, almost all graphene synthesis procedures share the following requirements: a transition metal sheet that acts as a catalyst for carbon dissociation and as a substrate for graphene growth, which is placed inside the reaction chamber; a carbon-based precursor material [18]; temperatures around 1,000°C capable of decomposing the precursor material and dissociating the carbon [19, 20], at such temperatures a borosilicate tube is not convenient, the regular choice is a quartz tube, its transparency allows direct monitoring of the interior of the reaction chamber; a flow of hydrogen (H2) that provides a reducing atmosphere and influences growth behavior; and a flow of inert or carrier gas, argon (Ar) being the privileged option [18]. A diagram of a typical LPCVD setup for the synthesis of graphene is depicted in **Figure 1**. A characteristic of CVD-produced graphene films is the polycrystalline nature of the atom-thick membrane, as will be detailed below.

#### **3.1 Copper foils as substrates**

Although the deposition of graphene on metal substrates by CVD was reported for the first time on other metals such as Ni and platinum (Pt), Cu is currently the most widely used metal as a catalyst substrate for the growth of graphene by CVD [16].

#### *Chemical Vapor Deposition Synthesis of Graphene on Copper Foils DOI: http://dx.doi.org/10.5772/intechopen.106058*

This preference can be explained by the growth mechanism of graphene on these substrates. The difference in mechanisms was elucidated by the group of R.S. Ruoff in 2009 [21] using carbon isotope labeling and Raman spectroscopy mapping of graphene films grown on Ni and Cu. The difference is due to the carbon solubility of each metal. For metals with high solubility (9,000 ppm at 900°C in Ni, and 11,000 ppm at 1,000°C in Pt) [22], carbon atoms infiltrate the metal matrix at high temperatures, and this solubility tends to increase with temperature. When the temperature drops, the solute is precipitated from the matrix, leading to the formation of graphene. This saturation and precipitation process results in the formation of MLG regions [17, 22]. In contrast, for metals with low carbon solubility (7.4 ppm at 1,020°C in Cu) [22], the solubility is relatively low despite high temperatures. Therefore, the process of carbon saturation and precipitation does not occur. Graphene is formed on the surface of Cu, limited to it; hence, the size of the graphene films will be correlated with the copper films in the reaction chamber. This process promotes the formation of SLG [17, 22]. Common Cu foil thicknesses for the growth of graphene films are 25 and 50 micrometers; moreover, the purity of the foil varies among commercial suppliers.

#### *3.1.1 Growth mechanism*

In general, the growth mechanism of graphene by CVD on a copper substrate can be divided into three stages: (1) The precursor molecules collide with the surface of the Cu substrate. As a result, they can be absorbed on the surface, disperse back to the gas phase, or go to the next reaction step. (2) The molecules dehydrogenate totally or partially, or eliminate any element other than carbon (denitrogenation, deoxygenation), and active carbon species are formed. (3) The active species diffuse over the surface, cluster, generate nucleation sites, and begin to grow on graphene islands on the Cu substrate [15, 23]. These graphene islands are grains of honeycomb arranged carbons, they possess a particular orientation with respect to each other, when the grains are large enough and meet carbon atoms from a nearby grain, they coalesce, forming a grain boundary where the mismatch orientation is overcome by the presence of 5-, 6-, 7-, and 8-membered rings [24].
