Graphite Oxide: A Simple and Reproducible Synthesis Route

The synthesis of graphite oxide (GrO) by oxidation of graphite has been carried out by different procedures. In this chapter, we describe a simple synthesis route based on Hummers’ method without the usage of NaNO 3 achieving nearly the same outcomes, and this methodology is directed toward high-quality scale production of GrO with similar properties compared with GrO obtained with traditional and improved Hummers’ methods. The GrO was obtained in a series of batch reactions and characterized by different techniques, and the results showed identical interlayer d-space , type and content of oxygen functionalities, and I D /I G ratio. The high reproducibility of this methodology offers an efficient alternative for the large-scale production of graphene oxide.


Introduction
Graphenic materials have been one of the most studied materials in the history of humanity due to their outstanding properties such high thermal, electrical, mechanical, and permeability properties, among others [1]. For this reason, many potential applications have been proposed and demonstrated in scientific reports and patents. It has even been estimated that the global graphene market size will increase up to 38% from the years 2017 to 2025 [2], taking into an account its potential use in applications as automotive lightweight materials, aeronautics and energy, Li batteries, paints, functional coatings, solar cells, biosensors, membranes, and electronics, just to mention some of them [3][4][5][6][7]. One of the main technological challenges that engineers and scientific community face is the lack of new methods of large-scale production of graphene and its derivative. The graphite is inexpensive and available in large quantity and unfortunately does not readily exfoliate to yield individual graphene sheets. Graphite oxide (GrO) is a layered material produced by the oxidation of graphite. In contrast to pristine graphite, the GrO sheets, known as graphene oxide, are heavily oxygenated, bearing hydroxyl and epoxide functional groups on their basal planes, in addition to carbonyl and carboxyl groups presumable located at the sheet edges, nevertheless, there are certain features that still remain unknown among which stands out the chemical structure [8]. Particularly,

Synthetic approaches
Graphene oxide can be synthesized via chemical oxidation of graphite, predominantly. Nevertheless, there are a few reports with an alternative electrochemical oxidation [11,12]. Brodie's method, reported in 1859, was the first one in utilizing potassium chlorate to the mixture of graphite and nitric acid as the oxidant and intercalating agents, respectively. However, this technique has important flaws, such as the reaction time is about 4 days, low yield of the GO, the evolution of toxic acid vapors and NO 2 /N 2 O 4 gases, and the generation of highly explosive ClO 2 when chlorate mixed with strong acids [13]. Nearly 40 years later, Staudenmaier proposed the use of H 2 SO 4 with HNO 3 , but the explosive ClO 2 gas still remained as long as the prolonged reaction time. Based on Staudenmaier's work, Hummers and Offeman developed an alternative method that has been widely used for the synthesis of graphite oxide [14]. The chemicals used in this case were H 2 SO 4 to intercalate graphite with the assistance of NaNO 3 and KMnO 4 as oxidant agents. The main reasons that this procedure is a reference in this matter are the use of KMnO 4 (strong oxidant) guarantees the completion of reaction within several hours, and the safety issue, in which there is no production of explosive ClO 2 due  to the absence of KClO 3 , and there is no generation of acid fog due to the replacement of HNO 3 with NaNO 3 . Despite of its high efficiency and the safety matter, it still has some drawbacks: (1) the toxic gas generation (NO 2 /N 2 O 4 ), (2) residual Na + and NO 3 − ions are difficult to be removed after GrO synthesis and purification, and (3) incomplete oxidation resulting in the formation of graphite/GrO mixture [15,16]. These problems have led to made several modifications to Hummers' method, and the main strategies includes the removal of NaNO 3 . One of them is reported by Tour et al. [15] by increasing the amount of KMnO 4 and a 9:1 mixture of concentrated H 2 SO 4 /H 3 PO 4 with a reaction time higher than 12 h. The GrO obtained by this methodology was highly oxidized with fewer defects in the basal plane and higher yield (77%), compared to GrO prepared by Hummers' method (40%). Shi et al. [16] removed the NaNO 3 from traditional Hummers method; with this simple modification, it was possible to produce GrO without affecting the yield and still had a high C/O ratio (2.36). Yu et al. [10] also reported a further improvement for NaNO 3 -free Hummers' method by partly replacing KMnO 4 with K 2 FeO 4 ; in addition, the amount of sulfuric acid was considerably reduced. This procedure resulted in a high yield (84%) compared to the Hummers traditional method.
The synthesis yield is normally estimated considering the mass ratio of graphite and graphite oxide. Methods aforementioned reported high yield by increasing the amount of oxidant agent and/or reaction time or by adding another reactant (acid or intercalating agent). These modifications may imply important disadvantages such as high cost, poor scalability, and practical applications. Based on NaNO 3 -free Hummers' method, authors of the present chapter propose some changes in order to obtain oxidized sheets but keeping the graphenic properties and also to get these two themes in a scalable way. Figure 2 illustrates the methodology in which the mixture of graphite-H 2 SO 4 was previously sonicated to improve the intercalation of the acid between graphite galleries. Also, controlling the addition time of KMnO 4 (t ≤ 30 min) and increasing the stirring time would enhance the diffusion of the KMnO 4 in the interlayer space. The obtained GrO in this procedure with the reaction time of 4 h has similar properties with those reported in the literature [17]. Table 1 shows noteworthy aspects of different methods to produce GrO, with respect to the chemicals involved, reaction time, C/O atomic ratio, and yield. The most important thing is the fact that has been proven to be a promising scalable method for obtaining graphite oxide, which was possible to demonstrate with the final features studied from several GrO samples synthesized.

GrO properties
In order to evaluate the reproducibility of the method proposed by the authors of the present chapter, GrO was synthesized in a total of 10 batch reactions, and the graphite oxide obtained was labeled GrO 1, GrO 2, …, GrO n, where n corresponds to the reaction number. All GrO n samples were characterized by different analytical techniques and were compared to each other, including the precursor graphite, labeled as GT. The structural, chemical, thermal, and morphological properties are presented below.

Structural properties
The samples were analyzed by XRD to evaluate the crystalline structure of GT and different synthesized GrO. Figure 3 shows the comparison among GT and three of the GrO samples (GrO 3, GrO 4, and GrO 7). XRD pattern of GT shows a characteristic diffraction peak (d 002 ) at 26.5° that corresponds to a d-space of 0.33 nm. After oxidation, this peak is no longer observable, instead a broad peak at a range of 11.1-11.6° can be assigned to d 001 , which oscillates from 0.76 to 0.79 nm, and this increase in d spacing is attributed to the intercalation of water molecules and to the presence of the functional groups at the basal plane [8,18,19]. The XRD experimental data of each sample are presented in Table 2, and it is observed that the average position peak of all GrOs at 2θ = 11.37 ± 0.18° has an average interlayer space d 001 = 0.78 ± 0.1 nm. The Full Width at Half Maximum (FWHM) was used to estimate the thickness, L c , by the Scherrer equation [20], whose results vary between 9.97 and 15 nm. The thickness, L c , was used to calculate the average number of layers, being of 16.65 ± 2 for GrO, which is significantly low compared with the ~75 layers estimated for GT, and this indicates that periodic structure of graphite has been disrupt, and it has partially exfoliated forming small stacks of few layers. The structural changes caused by oxidation process were also monitored by Raman spectroscopy. The Raman spectra of GT and GrO 2, GrO 4, and GrO 6 are compared in Figure 4. The GT exhibits a sharp and strong G band at around 1580 cm −1 , associated with bond stretching of the sp 2 carbon pairs in both rings and chains, and a weak and broad D band at 1350 cm −1 , associated with the presence of defects in graphite materials such as bond-angle disorder, bond-length disorder, vacancies, and etch defects [21]. The blue shift of the G band and the significantly  increase in the width and intensity of D band for all GrO samples with respect to GT can be associated with the defects induced in the hexagonal carbon network by the formation of oxygen functionalities and the parallel incorporation of sp 3 bonds during the oxidation of graphite (see Table 3). In addition, the notable increase in the intensity ratio I D /I G from 0.17 for GT to 0.96 ± 0.02 for GrO reveals a drastic decrease in the size of carbon sp 2 domains [22,23] and can be corroborated by calculating the crystallite size, La, which is considerably less for GrO (~20 nm) than GT (110 nm) [24].

Thermal properties
Thermal stability of all GrO samples was evaluated by TGA, and some of them are presented in Figure 5a and were compared with GT that remains thermally stable to a temperature of above 700°C, whereas the thermal degradation of GrO presents several weight losses, the first at around 10% below 100°C is associated with the vaporization of adsorbed water molecules onto GO sheets, and the second weight loss of 30% is observed from 150 to 280°C, which is attributed to the thermal decomposition of labile oxygen functionalities, and it is also observed a small weight loss (~10%) from 270°C to 600°C, which is attributed to the removal of more thermally stable oxygen functional groups such as carbonyl groups [21,25,26]. The derivative weight loss curve of GrO presented in Figure 5b displays a maximum at 217-220°C related to the degradation of functional groups, and Table 4 presents the weight loss at this temperature for all samples, whose average value is 30% with a degradation temperature of 218°C.

Chemical characterization
The FTIR and XPS analysis reveal significant chemical changes of GrO samples owing to oxidation process. FT-IR spectra in Figure 6 compare results from GrO 6, GrO 7, and GrO 8 with GT. In all cases, GrO exhibited a broad peak at 3000-3700 cm −1 , which is attributed to O▬H stretch vibration of hydroxyl, carboxyl, and intercalated free water. The vibrational peak at 1725 cm −1 is associated with the C〓O stretch of both carboxyl and carbonyl groups, and the vibrational peak at 1623 cm −1 is assigned to the overlapped frequencies of bending modes of trapped water molecules and C〓C stretch of unoxidized sp 2 carbon domains [27]. The O▬H deformations of the C▬OH groups appear at 1400 cm −1 . The peaks at 1220 cm −1 and 1057 cm −1 are associated with C▬O stretching of epoxy and alkoxy groups, respectively [28].
On the other hand, the elemental chemical information of GrO 2 and GrO 7 samples was obtained by XPS analysis. The XPS survey spectra presented in Figure 7 show the C 1s peak at 284.4 eV and O 1s peak at 533.5 eV, with an atomic content of oxygen of 34.6% and 32.1% for GrO 2 and GrO 7, respectively, being similar to that calculated by TGA. The atomic ratio C/O was 1.89 for GrO 2 and 2.08 for GrO 7, and these values coincide with those obtained for different GrOs synthesized by other techniques, as described in Table 1. High-resolution XPS spectra of GrO in the C 1s region, shown in Figure 8, present the signals corresponding to nonoxygenated carbon rings (C〓C/C▬C, 284.7 eV), hydroxyl (C▬OH, 286.3 eV), epoxy (C▬O▬C, 286.9 eV), carbonyl (C〓O, 287.4 eV), and carboxyl groups (O〓C▬OH, 289.4 eV), which are consistent with the signals of the FT-IR spectra [16,17,23,29].
Owing to the fact that several factors such as the nonstoichiometry nature of GrO, the size of the sheets, and no-homogeneity distribution of functional groups over the sheets and the fact that XPS is a technique of surface analysis, the concentration of oxygen functionalities among the specimens is not 100% reproducible, which has been consistent with previous reports; nevertheless, through other analysis techniques, it is demonstrated that the procedure for obtaining GrO is reproducible. FT-IR spectra of GT, GrO 6,GrO 7,and GrO 8 [17].

Morphological properties
GrO powders were exfoliated in aqueous media via ultrasonic vibration, since it is one of the most common methods to exfoliate graphene oxide sheets [8], and the obtained samples were analyzed by TEM and AFM to monitor the morphology and the structure. The nanosheets and pure GT as well were studied by TEM in conventional mode (CTEM) and selected area electron diffraction mode (SAED), and Figure 9a shows a general view of GT, and it is clearly observed the plate-like shape, the borders are shown with different contrast, caused most likely for the presence of several plates randomly accommodated, which is not the case of graphene oxide, and these sheets tend to wrinkle and fold [30] and have a "silky" appearance. SAED pattern of GT (inset in Figure 9a) confirmed the polycrystalline nature of graphite, and the incident beam is surrounded for several rings with distinctive diffraction dots that correspond with some of its crystallographic planes, such as (201) [17].
The SAED pattern of GO (inset in Figure 9b) confirmed the disordered nature and shows a diffraction rings that are not well defined and unresolved dots, which is consistent with amorphous materials. Nevertheless, the measurements of these rings confirm the planes (100) and (110), which suggest the presence of regions with graphitic nature [23].
Atomic force microscopy (AFM) analysis was carried out to verify the number of layers of graphene oxide. The sample is collected from the dispersion prepared in deionized water, and this demonstrates that sonication promotes near-complete exfoliation of the GO; Figure 10 exhibits an example of GO sheets with an estimated number of ~4 layers. Numerous nanosheets were detected in tapping mode, and the thickness profile showed around 1.45-6.42 nm, taking into consideration the d-space and the individual graphene sheet thickness (0.34 nm), leading to a conclusion that it was obtained GO with 1-6 layers, approximately [31].

Conclusions
In summary, the protocol used to synthesize GrO based on Hummers' modified method proved to be a successful procedure. As has been demonstrated in early reports, the use of NaNO 3 is not a variable that influences the final product, and it is possible to achieve GrO with similar properties and characteristics by using a simple, small reaction time and more safe methodology. Besides the removal of NaNO 3 , changes such as (a) enabling the sonication of graphite with sulfuric acid, (b) the slow addition of KMnO 4 , and (c) the two-hour stirring of the mixture KMnO 4 /graphite/sulfuric acid at 0°C were key factors that contributed to ensure reproducibility. This affirmation was supported by several analyses carried out to the 10 GrO samples synthesized. Oxygen functionality content, determined by TGA and XPS, was estimated of ~30 and ~33%, respectively; the type of these functional groups was identify by FTIR and corresponds to carboxyl, carbonyl, epoxy, and hydroxyl groups. Raman confirms that the layers of GrO have sp 2 /sp 3 domains, suggesting that even though the oxidation has occurred, the sheets still possess graphenic characteristics; analyses performed by TEM pointed out that since SAED results showed graphenic ordering. This means that the material has the advantages of having functional groups in order to accomplish important chemical reactions (functionalization and reduction) while still possessing graphenic properties.
This proposed method can be used to synthesize GrO and other graphenic materials in an economical and large-scale way.