**2. Chemical vapor deposition of graphene layers**

Chemical vapor deposition (*CVD*) is a *Bottom-Up* technique, which allows to synthesize waferscale graphene [41, 42]. In the *CVD* procedure, a metal substrate, which is used as the catalyst, is placed into a furnace and heated to high temperatures. The heat anneals the metal, increasing its domain size [43]. Nitrogen, a carbon source (such as methane), and hydrogen are flowed through the furnace during the graphene synthesis. Hydrogen catalyzes the reaction produced between the carbon source and the metal substrate resulting in carbon atoms coming from the carbon source decomposition, which are deposited onto the metal surface through chemical absorption [3]. Hydrogen activates the carbon bounds of the metal surface and controls the size and morphology of graphene domains [44]. After the reaction, the furnace is cooled to keep the deposited carbon atom layer from aggregating into bulk graphite, which crystallizes into a contiguous graphene layer on the metal surface [45]. This method has the advantage of producing large size and high-quality graphene layers, and the ability to synthesize graphene at wafer scale [41]. Moreover, it is considered a low-cost method leading to a high yield if compared to other growth methods. However, the *CVD* graphene tends to wrinkle due to the difference in thermal expansion between graphene and metal layer. This fact could be decreased via proper annealing [43]. Nickel and copper are commonly used in the *CVD* method as the metal substrate material for graphene synthesis [46]. Other transition metals such as Ru, Co, and Pt are alternative transition metals, although they are used less frequently [47, 48].

**Figure 4.** *CVD* growth-mechanism graphene on copper and nickel sheets [50].

*micromechanical cleavage* [5, 22], *exfoliation of graphite intercalation compounds (GICs)* [29], *arc discharge* [30, 31], *unzipping carbon nanotubes (CNTs)* [32, 33], *graphene oxide exfoliation* [27] and

Among the different *Bottom-Up* synthesis methods, *CVD* is considered the most extensively one used to synthesize large amounts of high-quality graphene sheets. This method is simple and easily scalable [38]. It is important to highlight that the quality and the type of graphene (monolayer, bilayer, few layer, or multilayer) can be varied as a function of the catalytic metal

On the other hand, the simultaneous reduction and exfoliation of graphene oxide can be considered, among the different *Top-Down* synthesis methods, the easiest one to synthesize graphene-based powder materials. However, the synthesis of graphite oxide (GrO) is first required since it is the intermediate product leading to graphene oxide from graphite. Graphite oxide synthesis is an exothermic process that involves the use of strong acid solutions. In addition, it can be considered as a tedious procedure because many steps are required before

Next, the most relevant results obtained in the *CVD* synthesis of graphene are summarized using nickel and copper as catalytic metals, with particular emphasis on the optimization of the main operating variables (synthesis time, temperature, and amount of gases involved during the synthesis). Similarly, the synthesis of graphite oxide is also described. In the latter case, the optimization study here reported pursued the reduction of the time of preparation and the amount of chemical oxidants used during the synthesis of this intermediate product.

Chemical vapor deposition (*CVD*) is a *Bottom-Up* technique, which allows to synthesize waferscale graphene [41, 42]. In the *CVD* procedure, a metal substrate, which is used as the catalyst,

*solvent-base exfoliation* [34–37] comprise the *Top-Down* route.

1164 Recent Advances in Graphene Research

**Figure 3.** *Bottom-Up* and *Top-Down* routes to synthesize graphene.

**2. Chemical vapor deposition of graphene layers**

used [3, 39, 40].

the ultimate product is obtained.

Depending on the metal used, two different mechanisms can be differentiated in the *CVD*graphene synthesis (**Figure 4**). The first one, called *Carbon atoms surface deposition*, which is the growth mechanism observed over copper sheets, is described like the direct deposition of carbon atoms on the catalyst surface. In the second one, called *Carbon atoms surface segrega‐ tion*, which is the growth mechanism observed when nickel is used as the catalyst, carbon atoms decomposed from the carbonaceous source are diffused onto the catalyst bulk during the annealing step at high temperatures. Then, they precipitate on the catalyst during the cooling period [49].

#### **2.1. Optimization of the CVD operational parameters**

Several studies have established a close correlation between the *CVD* growth parameters, the quality of the graphene obtained, and the number of graphene layers, leading to the formation of different types of graphene (named as monolayer, bilayer, few layer, and multilayer) on the metal substrate [51–55].

Zhang et al. [51] and Nie et al. [52] found that the graphene quality improved at higher temperatures of reaction. On the contrary, lower temperatures gave rise to graphene with a number of defects. Rybin et al. reported that the larger the temperature of reaction, the higher the amount of atoms dissolved into the metal layer, leading to the production of more and more graphene layers [53].

Recent studies have showed that the concentration of hydrogen, which is obviously related with the CH4/H2 flow rate ratio and the total flow rate of CH4 and H2 during the reaction step, also plays an important role in providing quality to *CVD* graphene. Gao et al. found for an atmospheric pressure *CVD* system that high hydrogen concentrations contributes to the degradation of the graphene quality as a result of the occurrence of defects and wrinkles [54]. In a similar way, Vlassiouk et al. detected the presence of a critical value of the partial hydrogen that determines the occurrence of graphene growth (<2 Torr with 30 ppm of CH4). No graphene nucleation was observed below this value, whereas higher hydrogen concentrations caused degradation in graphene quality [44]. Finally, several groups have shown that the growth of bilayer and few-layer graphene depends on the concentration of active carbon species [55], which was in turn related with the CH4/H2 flow ratio and the total flow rate of CH4 and H2 used during the reaction step.

On the other hand, monocrystalline metals favor the formation of superficial and uniform monolayer, and bilayer graphene, being hindered the formation of multilayer graphene due to graphene, is grown over smooth and free defect surfaces (**Figure 4**). However, the industrial production of graphene strongly recommends to use polycrystalline metals, since it is much lower than that of monocrystalline one [56].

**Figure 5a** shows the experimental installation used for *CVD*-graphene synthesis over poly‐ crystalline metals (Ni and Cu). **Figure 5b** shows the stages followed during the graphene synthesis as well the duration, temperature, and gases used in each of them.

Methane and hydrogen were actually used as precursor gases. Graphene samples were grown by *CVD* at atmospheric pressure on 25-μm-thick polycrystalline metal foils in a 40-inch quartz tube heated in a furnace. Firstly, the reduction step was carried out by heating the furnace to 900°C, passing through the tube a flow of N2 (400 sccm) and H2 (100 sccm) to prevent metal sheet oxidation. The annealing step was carried out by maintaining the furnace at this temperature for 45 min. Later, the reaction step was started and carried out at different operational conditions in order to improve the quality of the obtained graphene by decreasing the amount of multilayer graphene formed over the metal. The temperature set point was increased and varied in the range of 900–1050°C. A ratio of methane to hydrogen in the range 0.4–0.07 v/v was introduced into the reactor for different times (15 min to 30 s) to complete the reaction step. The total flow of gases involved during the reaction step ranged from 80 to 100 Nml/min. Finally, the system was cooled down (10°C min−1) by flowing 400 sccm of nitrogen.

**2.1. Optimization of the CVD operational parameters**

metal substrate [51–55].

1186 Recent Advances in Graphene Research

more graphene layers [53].

used during the reaction step.

lower than that of monocrystalline one [56].

Several studies have established a close correlation between the *CVD* growth parameters, the quality of the graphene obtained, and the number of graphene layers, leading to the formation of different types of graphene (named as monolayer, bilayer, few layer, and multilayer) on the

Zhang et al. [51] and Nie et al. [52] found that the graphene quality improved at higher temperatures of reaction. On the contrary, lower temperatures gave rise to graphene with a number of defects. Rybin et al. reported that the larger the temperature of reaction, the higher the amount of atoms dissolved into the metal layer, leading to the production of more and

Recent studies have showed that the concentration of hydrogen, which is obviously related with the CH4/H2 flow rate ratio and the total flow rate of CH4 and H2 during the reaction step, also plays an important role in providing quality to *CVD* graphene. Gao et al. found for an atmospheric pressure *CVD* system that high hydrogen concentrations contributes to the degradation of the graphene quality as a result of the occurrence of defects and wrinkles [54]. In a similar way, Vlassiouk et al. detected the presence of a critical value of the partial hydrogen that determines the occurrence of graphene growth (<2 Torr with 30 ppm of CH4). No graphene nucleation was observed below this value, whereas higher hydrogen concentrations caused degradation in graphene quality [44]. Finally, several groups have shown that the growth of bilayer and few-layer graphene depends on the concentration of active carbon species [55], which was in turn related with the CH4/H2 flow ratio and the total flow rate of CH4 and H2

On the other hand, monocrystalline metals favor the formation of superficial and uniform monolayer, and bilayer graphene, being hindered the formation of multilayer graphene due to graphene, is grown over smooth and free defect surfaces (**Figure 4**). However, the industrial production of graphene strongly recommends to use polycrystalline metals, since it is much

**Figure 5a** shows the experimental installation used for *CVD*-graphene synthesis over poly‐ crystalline metals (Ni and Cu). **Figure 5b** shows the stages followed during the graphene

Methane and hydrogen were actually used as precursor gases. Graphene samples were grown by *CVD* at atmospheric pressure on 25-μm-thick polycrystalline metal foils in a 40-inch quartz tube heated in a furnace. Firstly, the reduction step was carried out by heating the furnace to 900°C, passing through the tube a flow of N2 (400 sccm) and H2 (100 sccm) to prevent metal sheet oxidation. The annealing step was carried out by maintaining the furnace at this temperature for 45 min. Later, the reaction step was started and carried out at different operational conditions in order to improve the quality of the obtained graphene by decreasing the amount of multilayer graphene formed over the metal. The temperature set point was increased and varied in the range of 900–1050°C. A ratio of methane to hydrogen in the range 0.4–0.07 v/v was introduced into the reactor for different times (15 min to 30 s) to complete the

synthesis as well the duration, temperature, and gases used in each of them.

**Figure 5.** (a) Experimental installation for *CVD* graphene synthesis. (b) Summary of the *CVD*-graphene synthesis steps and conditions.

To control the graphene thickness and determine the percentage of each type of graphene (monolayer, bilayer, few layer, and multilayer) deposited over the polycrystalline metal foils, a homemade Excel-VBA application was designed. This software used the different colors present in a digitalized optical microscope image to evaluate the percentage of the different types of graphene deposited over the metal sheets. By using Raman spectroscopy, the relationship between the different colors present in optical images has been demonstrated with each type of graphene [3]. For this purpose, a logarithmic scale was considered in the Excel-VBA software design. Thus, *thickness values* 1, 10, 100, and 1000 were assigned when the 100% of the sample was covered by multilayer, few-layer, bilayer, and monolayer graphene, respectively. The *thickness value* was calculated as an average of the percentage obtained for each type of graphene calculated by the Excel-VBA application.

Ferrari et al. [57] demonstrated that using the second-order 2D feature obtained in the Raman spectra, it was possible to know the number of graphene layers. Based on that study, Malard et al. [58] investigated the theoretical background associated with the double-resonance Raman-scattering mechanism that gives rise to the main feature in the Raman spectra of the different types of graphene. Thus, the deconvolution of the 2D peak, corresponding to each type of graphene, showed that in the case of monolayer graphene the 2D peak could be fitted with a symmetric single peak only; in the case of bilayer graphene, the 2D peak could be deconvoluted in four different contributions; in the case of few-layer and multilayer graphene, the 2D peak could be deconvoluted in two contributions, which is characteristic of graphite (material consisting of many layers of graphene) (**Figure 6**).

**Figure 6.** Relationship between optical microscope image and typical Raman spectra of monolayer [48, 59–61], bilayer [47, 59, 60, 62], few layer [63–65], and multilayer [63, 64] graphene.

In this study, polycrystalline copper and nickel were chosen as metal catalyst in the synthesis of *CVD* graphene.

Regarding polycrystalline Cu, 1050°C was required to maximize the amount of monolayer graphene over the metal, whereas 980°C was selected as the optimum reaction temperature in the case of using polycrystalline nickel. In the former case, the *thickness value* was found to be 4.2, the proportion of each type of graphene being the following one: 81% multilayer graphene, 17% few-layer graphene, and 2% bilayer graphene; the presence of monolayer graphene was considered negligible. In the latter case, the *thickness value* was found to be 397, in turn the proportion of each type of graphene being the following one: 0.9% multilayer graphene, 40% few-layer graphene, 22% bilayer grapheme, and 37% monolayer graphene.

Regarding the CH4/H2 flow rate ratio, an optimal value of 0.07 v/v was obtained when both metals were used as catalysts. A *thickness value* of 34.7 was obtained using Cu (20% multilayer graphene, 20% few-layer graphene, and 51% bilayer graphene), whereas a *thickness value* of 536 was obtained using Ni (0.5% multilayer graphene, 27% few-layer graphene, 20% bilayer graphene, and 52% monolayer graphene).

Finally, regarding the study of the influence of the total flow of gases (CH4+ H2) and reaction time, it could be concluded that the best results in the case of using Cu as the catalyst were obtained for a total gas flow of 60 Nml/min and a reaction time of 10 min, leading to an increased *thickness value* of 60, 56 and 11% of the resulting sample being covered by bilayer graphene and multilayer graphene, respectively. In the case of using Ni as the catalyst, the best results were obtained for a total gas flow of 80 Nml/min and a reaction time of 1 min (*thickness value* of 810). At these conditions, just 0.3% of the sample was covered by multilayer graphene, 11% by few-layer graphene, 9% by bilayer graphene, and 80% by monolayer graphene.


**Table 2.** Optimum synthesis conditions.

relationship between the different colors present in optical images has been demonstrated with each type of graphene [3]. For this purpose, a logarithmic scale was considered in the Excel-VBA software design. Thus, *thickness values* 1, 10, 100, and 1000 were assigned when the 100% of the sample was covered by multilayer, few-layer, bilayer, and monolayer graphene, respectively. The *thickness value* was calculated as an average of the percentage obtained for

Ferrari et al. [57] demonstrated that using the second-order 2D feature obtained in the Raman spectra, it was possible to know the number of graphene layers. Based on that study, Malard et al. [58] investigated the theoretical background associated with the double-resonance Raman-scattering mechanism that gives rise to the main feature in the Raman spectra of the different types of graphene. Thus, the deconvolution of the 2D peak, corresponding to each type of graphene, showed that in the case of monolayer graphene the 2D peak could be fitted with a symmetric single peak only; in the case of bilayer graphene, the 2D peak could be deconvoluted in four different contributions; in the case of few-layer and multilayer graphene, the 2D peak could be deconvoluted in two contributions, which is characteristic of graphite

**Figure 6.** Relationship between optical microscope image and typical Raman spectra of monolayer [48, 59–61], bilayer

In this study, polycrystalline copper and nickel were chosen as metal catalyst in the synthesis

Regarding polycrystalline Cu, 1050°C was required to maximize the amount of monolayer graphene over the metal, whereas 980°C was selected as the optimum reaction temperature in the case of using polycrystalline nickel. In the former case, the *thickness value* was found to be

each type of graphene calculated by the Excel-VBA application.

1208 Recent Advances in Graphene Research

(material consisting of many layers of graphene) (**Figure 6**).

[47, 59, 60, 62], few layer [63–65], and multilayer [63, 64] graphene.

of *CVD* graphene.

Summarizing, the *thickness value* was increasing at each stage of the optimization study regardless of the metal used. However, it was not possible to detect monolayer graphene on polycrystalline copper foil. In the case of using polycrystalline nickel, monolayer graphene covered 80% of the foil for the optimal conditions of synthesis.

**Table 2** shows the optimum operating conditions for each metal resulting from this study.

**Figure 7.** Raman spectra corresponding to graphene samples synthesized using (a) copper and (b) nickel.

**Figure 7** shows the Raman spectra corresponding to the graphene samples obtained at the optimal conditions. D peak (1350 cm−1) is related to the presence of defects in graphitic materials [66]. G peak (∼1560 cm−1) denotes the symmetry of graphite band and is a way of checking the vibration of sp2−hybridized carbon atoms in the same plane. Finally, 2D peak, visible around 2700 cm−1, is the hallmark of graphene layers [67]. The relationship between the intensity of D and G peak (*I*D/*I*G) is a way to check the amount of defects present in the graphene sheet. The number of graphene layer is also related to the ratio between the intensity of 2D and G peak (*I*2D/*IG*). 2D peak position in graphene sample should be displaced to lower Raman shift values in comparison with that of graphite. Finally, full width at hall maximum (FWHM) is related to the lifetime of the excited states and is calculated as the Raman shift difference to the half average height of the 2D peak [39]. For both metal catalysts, the *I*D/*I*<sup>G</sup> ratio values were low, demonstrating that graphene samples had low amount of defects, whereas the *I*2D/*I*<sup>G</sup> ratio values increased, as expected, from multilayer to monolayer graphene [68]. The contrary effect could be observed for the FWHM parameter, which decreased from multilayer to monolayer graphene. Finally, 2D peak position was, in all cases, located at around 2700 cm−1, which is characteristic of graphene materials [3, 39].
