Graphene properties and applications.
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.
- graphite oxide
- Improved Hummers method
- thickness value
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 different allotrope forms . Among them, the softer and the harder materials known in nature are included: graphite (Figure 1a) and diamond (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 . Each atom is bonded by a covalent bond to other three carbon atoms. These carbon atoms are densely packaged in a honeycomb-shape crystal lattice  comprising, in turn, of two superimposed triangular subnets . 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 .
1.1. Graphene chronology
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 . Table 1 shows the extraordinary properties of graphene related to the applications that can be derived from them.
|• High-speed electron mobility||Transistors, lasers, photo detectors||[3, 22]|
|• Large specific surface area|
|• Linear band structure|
(Dirac spectrum for mass less fermions)
|Field effect transistors|||
|• High electrical conductivity|
• High-speed electron mobility
• High optical transmittance
|Transparent conductive film|||
|• High theoretical surface area|
• Electron transfer along 2d surface
|Clean energy devices|||
|• Anomalous quantum hall effect||Ballistic transistors|||
|• Irrelevant spin-orbit coupling||Spin-Valve Devices|||
|• High conductivity||Conductive materials, electrical batteries,|
|• Easy absorption of gases||Contamination control|||
|• Transparency (>99%)|
• High electronic conductivity
|Displays, touch screens|||
|• High mechanical stress (hardness)||Construction|||
1.3. Graphene synthesis
Two different routes can be followed to synthesize graphene:
Among the different
On the other hand, the simultaneous reduction and exfoliation of graphene oxide can be considered, among the different
Next, the most relevant results obtained in the
2. Chemical vapor deposition of graphene layers
Chemical vapor deposition (
Depending on the metal used, two different mechanisms can be differentiated in the
2.1. Optimization of the CVD operational parameters
Several studies have established a close correlation between the
Zhang et al.  and Nie et al.  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 .
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
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 .
Figure 5a shows the experimental installation used for
Methane and hydrogen were actually used as precursor gases. Graphene samples were grown by
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 . For this purpose, a logarithmic scale was considered in the Excel-VBA software design. Thus,
Ferrari et al.  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.  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).
In this study, polycrystalline copper and nickel were chosen as metal catalyst in the synthesis of
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
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
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
Table 2 shows the optimum operating conditions for each metal resulting from this study.
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 . 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 . The relationship between the intensity of D and G peak (
3. Graphite oxide
Graphite oxide (GrO) can be defined as a set of functionalized graphene sheets that are mainly composed of carbon, oxygen, and hydrogen atoms. This material is considered a precursor of graphene itself . The structure of graphite oxide is similar to that of graphite differing only in the oxygenated groups present in GrO, which give rise to a greater distance between the graphene layers . GrO consists of a hexagonal network of sp2- and sp3-hybridized carbon atoms that bear hydroxyl and epoxide functional groups on the ‘basal’ plane and carboxyl and carbonyl groups at the edges .
3.1. Graphite oxide synthesis
Graphite oxide can be synthesized by the Brodie , Staudenmaier , or Hummers and Offeman  methods or by variations of the latter, namely
3.2. Improved Hummers method
In 1958, Hummers reported the most popular procedure to synthesize graphite oxide, which is based on the oxidization of graphite by using KMnO4 and NaNO3 in concentrated H2SO4 . However, this method yields NOx and is dangerous itself due to the constant explosions, which take place during the synthesis . In 2010, Marcano et al.  reported an improved synthesis based on the Hummers method by using graphite flakes as the raw material. Graphite oxide synthesized from graphite flakes can be easily soaked and dispersed in water and could be used as the precursor for different applications due to its hydrophilic character. They detected that an improved graphite oxide with fewer defect in the basal plane can be prepared using KMnO4 as oxidation agent and a 9:1 mixture of concentrated H2SO4 and H3PO4. They also reported that graphite oxide synthesized with this
3.3. Optimization of the Improved Hummers method
The most remarkable results obtained in the optimization study of the synthesis of graphite oxide based on the method proposed by Marcano et al.  (
First, the oxidation time, which is the most time-consuming step of the whole synthesis process, was reduced. This way, the oxidation time was reduced from 12 to 3 h, whereas the other synthesis conditions were kept constant without affecting the quality of the final product. The introduction of functional groups, both at the edges and in the basal plane, was achieved in 3 h instead of the 12 h required in the original method.
Second, it was demonstrated that the coagulation step used by Marcano et al.  did not significantly influence over the quality and characteristic properties of the final product. Consequently, it was removed from the synthesis procedure.
On the other hand, Marcano et al. used three different products twice during the washing step: deionized water, which was used to reach the pH neutralization; HCl, which was required to remove the remaining metal from the graphite oxide, and ethanol, which was used to achieve a faster drying of the final product. We demonstrated that the quality and characteristic of the final product were not affected at all if the treatment of the cake with these three products was or not repeated. In addition, the elimination of H3PO4 in the synthesis procedure was considered. Similarly, this action did not alter the characteristics of the final product.
Finally, a series of tests were conducted in order to increase the amount of graphite that can be treated per batch, without altering the properties of the final product (the raw method considers 3 g of graphite and 9 g of KMnO4 in 400 ml of solution). Here, the KMnO4/graphite ratio (3:1) was maintained in order to not alter the degree of oxidation. This way, the amount of these materials was progressively increased. It was observed that it was possible to use up to 15 g of graphite (and hence 45 g of KMnO4) in 400 ml of solution without altering the characteristics and quality of the product. Figure 8 schematically summarizes the differences between the original method (
Table 4 lists some properties of the graphite oxide samples synthesized by the
The almost similar characterization values of both samples of graphite oxide demonstrated that the optimization process did not affect both quality and structure of the final product.
The present work was performed within the frame of the NANOLEAP project. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 646397.