**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 [69]. 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 [69]. 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 [70].

#### **3.1. Graphite oxide synthesis**

Graphite oxide can be synthesized by the Brodie [7], Staudenmaier [8], or Hummers and Offeman [9] methods or by variations of the latter, namely *Modified Hummers method* or *Improved Hummers method* [71]. The main differences between the abovementioned methods are summarized in **Table 3**, with particular emphasis on the nature of the oxidant, the toxicity, and the main advantages or disadvantages of each approach.

#### **3.2. Improved Hummers method**

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

characteristic of graphene materials [3, 39].

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**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

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 [9]. However, this method yields NOx and is dangerous itself due to the constant explosions, which take place during the synthesis [71]. In 2010, Marcano et al. [71] 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 *Improved Hummers method* provided a greater amount of hydrophilic-oxidized graphite, likewise having a more regular structure with a greater amount of basal plane framework retained. Raman and infrared spectroscopy results indicated that graphite oxide samples obtained through both methods were almost similar, both of them showing the characteristics D and G peaks that confirmed the lattice distortion in Raman spectroscopy. FTIR-ATR spectra also confirmed the presence of functional groups. In addition, atomic force microscopy (AFM) showed that the thickness of both graphite oxides was around 1.1 nm. They confirmed with a large variety of methods, such as thermog‐ ravimetry analysis (TGA), solid-state colossal magnetoresistance (CMR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), that graphite oxide synthesized with the *Improved Hummers method* was, if it is compared to that produced from the Hummers one, a more oxidized material, presented a more organized structure, and contained both more epoxide functionalities, and more regular carbon framework.

**Table 3.** Synthesis methods of graphite oxide.

#### **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. [71] (*Improved Hummers method)* are summarized below. Thus, the objective was to reach the same quality product but using a lesser time-consuming experimental procedure and conducting lower production costs. With this purpose, the different stages of the *Improved Hummers method* used in the synthesis of graphite oxide were optimized. The method consists of the oxidation, in the presence of H2SO4 and H3PO4, of 3 g of graphite per 9 g of KMnO4 used as the oxidizing agent in 400 ml of solution. The oxidation step is maintained at 50°C for 12 h. Later, the reaction mixture is washed twice with 200 ml of deionized water, HCl, and ethanol. Finally, the product is coagulated with 200 ml of dry diethyl ether and dried at 100°C [71].

**Figure 8.** Differences between the *Improved Hummers* and *Optimized Improved Hummers methods*.

**Table 3.** Synthesis methods of graphite oxide.

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**3.3. Optimization of the Improved Hummers method**

coagulated with 200 ml of dry diethyl ether and dried at 100°C [71].

The most remarkable results obtained in the optimization study of the synthesis of graphite oxide based on the method proposed by Marcano et al. [71] (*Improved Hummers method)* are summarized below. Thus, the objective was to reach the same quality product but using a lesser time-consuming experimental procedure and conducting lower production costs. With this purpose, the different stages of the *Improved Hummers method* used in the synthesis of graphite oxide were optimized. The method consists of the oxidation, in the presence of H2SO4 and H3PO4, of 3 g of graphite per 9 g of KMnO4 used as the oxidizing agent in 400 ml of solution. The oxidation step is maintained at 50°C for 12 h. Later, the reaction mixture is washed twice with 200 ml of deionized water, HCl, and ethanol. Finally, the product is

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. [71] 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 consid‐ ered. 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 (*Improved Hummers method*) and the optimized one (*Optimized Improved Hummers method*).


**SEM**

**La:** crystal dimension described by layer sized; **Lc**: stack height; **Nc:** number of layers in the stacking structure; **C:** carbon, **O:** oxygen, **S:** sulfur, **Cl:** chlorine; **Mn:** manganese.

**Table 4.** Characterization results of graphite oxide synthesized using both the *Improved Hummers* and *Optimized Improved Hummers methods*.

**Table 4** lists some properties of the graphite oxide samples synthesized by the *Optimized Improved Hummers method* and those prepared from the parent one. *I*D/*I*<sup>G</sup> ratio, related with the structural disorder in the graphite network and inversely proportional to the sp2 cluster average sized [72], considerably increased after graphite oxidation. Crystal dimension (La value) decreased after the incorporation of oxygenated groups, which agree with the increase in the structural disorder. In the same way, the number of graphene layers in the stacking structure (Nc) decreased after the oxidation process [73]. Elemental analysis showed an increase in the percentage of oxygen from graphite to graphite oxide due to the oxidation process. Comparing both graphite oxides, the percentage of each compound (C, O, S, Cl, and Mn) was quite similar [74]. In addition, scanning electron microscope (SEM) images showed an agglomeration of the product after the oxidation process, being several microns in size. Finally, a significant increase in the surface area was observed after graphite oxidation, because of the expansion of graphene layers [75].

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.
