**Optimization of the Synthesis Procedures of Graphene and Graphite Oxide**

María del Prado Lavín López, José Luis Valverde Palomino, María Luz Sánchez Silva and Amaya Romero Izquierdo

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63752

#### **Abstract**

The optimization of both the *chemical vapor deposition (CVD)* synthesis method to prepare graphene and the *Improved Hummers method* to prepare graphite oxide is reported. Copper and nickel were used as catalysts in the *CVD*-graphene synthesis, CH4 and H2being used as precursor gases. Synthesis variables were optimized according to a *thickness value*, calculated using a homemade Excel-VBA application. In the case of copper, the maximum *thickness value* was obtained for those samples synthesized at 1050°C, a CH4/H2 flow rate ratio of 0.07 v/v, a total flow of 60 Nml/min, and a time on stream of 10 min. In the case of nickel, a reaction temperature of 980°C, a CH4/H2 flow rate ratio of 0.07 v/v, a total flow of 80 Nml/min, and a time on stream of 1 min were required to obtain a high *thickness value*. On the other hand, the *Improved Hummers method* used in the synthesis of graphite oxide was optimized. The resultant product was similar to that reported in literature in terms of quality and characteristics but both time and cost of the synthesis procedure were considerably decreased.

**Keywords:** graphene, graphite oxide, CVD, Improved Hummers method, thickness value

#### **1. Introduction**

Carbon (C) is a chemical element with atomic number 6 and solid at room temperature. Depending on the synthesis conditions during its growth, carbon can be found in nature with

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different allotrope forms [1]. Among them, the softer and the harder materials known in nature are included: graphite (**Figure 1a**) anddiamond(**Figure 1b**),respectively.Recently, new carbon allotropes have been discovered such as fullerenes (**Figure 1c**), carbon nanotubes (**Figure 1d**), carbon nanofibers (**Figure 1e**), and carbon nanospheres. To date, the last carbon allotrope that has been appended is graphene (**Figure 1f**). It consists of a two-dimensional (2D) carbon atom network with sp2 hybridization and only one atom thick [2]. Each atom is bonded by a cova‐ lentbondtootherthreecarbonatoms.Thesecarbonatomsaredenselypackagedinahoneycombshapecrystallattice[3]comprising,inturn,oftwosuperimposedtriangularsubnets[4].Although graphene has been known since 1960, it was not until 2004 when Andre Geim and Konstantin Novoselov achieved to obtain an isolated graphene sheet using the Scotch® tape method [5].

**Figure 1.** Structure of (a) graphite, (b) diamond, (c) fullerene, (d) carbon nanotube (CNT), (e) carbon nanofiber (CNF), and (f) graphene [6].

**Figure 2.** Roadmap of graphene [5, 7–21].

#### **1.1. Graphene chronology**

different allotrope forms [1]. Among them, the softer and the harder materials known in nature are included: graphite (**Figure 1a**) anddiamond(**Figure 1b**),respectively.Recently, new carbon allotropes have been discovered such as fullerenes (**Figure 1c**), carbon nanotubes (**Figure 1d**), carbon nanofibers (**Figure 1e**), and carbon nanospheres. To date, the last carbon allotrope that has been appended is graphene (**Figure 1f**). It consists of a two-dimensional (2D) carbon atom

lentbondtootherthreecarbonatoms.Thesecarbonatomsaredenselypackagedinahoneycombshapecrystallattice[3]comprising,inturn,oftwosuperimposedtriangularsubnets[4].Although graphene has been known since 1960, it was not until 2004 when Andre Geim and Konstantin Novoselov achieved to obtain an isolated graphene sheet using the Scotch® tape method [5].

**Figure 1.** Structure of (a) graphite, (b) diamond, (c) fullerene, (d) carbon nanotube (CNT), (e) carbon nanofiber (CNF),

hybridization and only one atom thick [2]. Each atom is bonded by a cova‐

network with sp2

1142 Recent Advances in Graphene Research

and (f) graphene [6].

**Figure 2.** Roadmap of graphene [5, 7–21].

Graphene is one of the most extensively researched materials and is currently regarded as a fascinating material [2]. **Figure 2** shows the chronology of graphene, from 1840 to nowadays.

#### **1.2. Properties and applications**

Since 2004, many researchers have been focused on the synthesis of high-yield and high-quality graphene as well as on the search of an easily scalable process to manufacture it [6]. **Table 1** shows the extraordinary properties of graphene related to the applications that can be derived from them.


**Table 1.** Graphene properties and applications.

#### **1.3. Graphene synthesis**

Two different routes can be followed to synthesize graphene: *Bottom Up* and *Top Down* (**Figure 3**). *Bottom-Up* route comprised those methods, which use a carbonaceous gas source to produce graphene. The most relevant ones are *epitaxial growth on Silicon Carbide (SiC)* [27] and *chemical vapor deposition (CVD)* [3]. *Top-Down* route is based on the attack of graphite (used as raw material) to break its layers forming graphene sheets [28]. Methods such as

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

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

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 used [3, 39, 40].

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 the ultimate product is obtained.

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.
