GRAPHENE OXIDE OPTIMIZATION SYNTHESIS FOR APPLICATION ON LABORATORY OF UNIVERSIDADE FRANCISCANA1 OTIMIZAÇÃO DE SÍNTESE DE ÓXIDO DE GRAFENO PARA APLICAÇÃO EM LABORATÓRIO DA UNIVERSIDADE FRANCISCANA

Graphene oxide (GO) is a 2D material derived from graphene, having a hexagonal arrangement crystal structure together with the addition of various oxygenated functional groups, epoxides, alcohols, ketones, carbonyls and carboxyls. Hummers and Offeman, reported in 1958 a synthesis method that is still employed to the present day, only with few adaptations. Thus, graphite is oxidized by treatment with potassium permanganate and sodium nitrate in concentrated sulfuric acid. Recently our research group evaluated several methodologies described in the literature, about the current synthesis of GO and the respective adaptations and improvements to the Hummers method. And based on these literary accounts, we developed a synthesis mechanism that could be applied in standard laboratory conditions attending demands for further research using the GO, which demonstrated good yield with simple purification method and relatively short time to synthesis.


INTRODUCTION
Graphene oxide (GO) consists of the derivation of graphene material, in an oxidized format, functionalized by carboxyl, hydroxyl, carbonyl and epoxy groups, which provide the processability of the material in aqueous solution. The unique properties of GO have attracted attention for its usefulness as an additive and performance upgrade for composites, structural reinforcement in fibers, energy storage devices, molecular sieves, liquid crystal optical materials (DONG et al., 2017). Due to its 2D structure functionalized with oxygen-containing groups, GO can be superimposed layer by layer, to form macroscopic films (DONG et al., 2017). In biotechnology, carbon allotropes derivatives besides its applications have demonstrated to be a large field for studies regarding bioactivity (VIANA et al., 2019).
In relation to production costs (use of equipment and energy for an excess of time) the most critical step to produce GO is purification. This stage has many warehouses for the synthesis of a final quality product and depends essentially on the capacities of each GO production and the equipment available, to carry out the processes (DIMIEV, 2016).
Purification can be carried out by long processes of washing with water and using acid (HCl) to remove metal ions . After each wash cycle, GO is separated by generally centrifuging processes. When washing and removing impurities and decreasing the acid concentration, the product exfoliates in single layers, characteristic for the formation of GO, generating a stable and bulky colloid solution. Dispersions of GO tend to obtain this gelatinous aspect whereas the value of pH increases during washing procedures. Solvents such as acetone can be used in this step to suppress the formation of this gelatinous aspect (KRISHNAN et al., 2012). Alternatively, dialysis can be used for purification, involving specific equipment. This protocol step increases the purification time of the material, also raises costs, so it is recommended only for the synthesis of small quantities (DIMIEV, 2016).
In this study, experiments were carried out to determine the GO synthesis using graphite flakes -100 mesh -graphite in this size range can be considered large, and therefore favors the formation of GO with an extensive size (> 10 µm), promising for the manufacture of 2D layer structures, and 3D graphene-based networks. In those cases, GO sheets induce less interaction between them, thus favoring better mechanical properties. GO films also have better electrical and thermal conductivities, when compared to their small size (DONG, L. et al., 2017;. The purification step was based on minimalist methods both regarding to equipment and the complexity of techniques to be employed. This method shows to be able to result in high separation and washing efficiency. The Charpy-Hummers method, used in this research context, together with the graphite size limited to 100 mesh, with improvements in the synthesis preparation and purification protocols according to the operating conditions of the laboratories provided by the Universidade Franciscana (UFN), characterize the synthesis as an efficient approach to produce GO in the laboratories of the University, without high costs and encouraging the local research. The main objective was to achieve a trouble-free, low-cost GO preparation protocol with considerable yield of oxidized material, less damage to the crystalline structure and consuming minimal time, energy and easy purification. The second oxidation step (MnO 4 -) takes place during the addition of distilled water to the reaction system, the step is identified by increasing the thermal stability of the GO. The extension of this stage provides an increase in hydroxyl groups, which are thermally more stable than epoxy groups that make up the GO (KANG et al., 2016). However, there is a selective formation of carboxylic groups (~ 4.1%) and an increase in defects in the GO structure with an increase in temperature above 70 to 95 °C (LI et al., 2018;. At temperatures below 45 °C, there are no significant changes in the structure of GO sheets (KANG et al., 2016).

GO SYNTHESIS
All experiments had the addition of graphite in flakes (1 g), sulfuric acid (60 mL) and potassium permanganate (6 g). From the syntheses carried out (alternating variables such as temperature, time, amount of solvent and purification) it was possible to predict the most effective method to produce GO. In operational matters, the temperature used was 40 °C, an adequate temperature to prevent potential deformation of the material's crystalline structure (KANG et al., 2016).
Sequentially, potassium permanganate (6 g) was added slowly, over a period of 20 minutes, maintaining the temperature of the suspension at 20 °C, and stirred for another 10 minutes (B). Afterwards, the reaction was heated to 40 °C for different time intervals for all four experiments (C) ( Table 1).
In the experiment 2, the heating was turned off after 5h and 20 mL of 98% sulfuric acid was added (there was the formation of a very dense liquid, the acid was added to aid in the stirring), and the solution was kept under stirring at room temperature for additional 1 h (D).
In the experiment 3, after 4-5 hours of stirring, it was observed the formation of a very dense solution, making it difficult the stirring of the system, however no amount of acid was added in the next 22 h under heating (E). Then 180 ml of distilled water were dripped into the reaction system and maintained for an additional 1 h at 40 °C (F).
For the experiments 1 and 2, 300 mL of distilled water was used to maintain the 1:3 ratio (CHEN, Ji et al., 2016).
For the experiment 3, stirring was continued at room temperature for an extended time after adding 180 mL of distilled water (23 h), before heating (Table 1).
Finally, for the experiment 4, 180 mL of distilled water was added, and the temperature was maintained at 40 ºC for 2 h (G). All The experimental reactions were completed by filling it up to 500 mL of the beaker with distilled water at 20 °C and adding 10 mL of H 2 O 2 to reduce Mn (VII) permanganate species.

GO PURIFICATION
All the reactions were decanted with 2 L of distilled water 2 times to reduce the concentration of sulfuric acid, and then decanted with 2 L of 1:10 v/v HCl solution for removing side products, metal ions mostly. Reactions products 1 and 2 were again decanted with 2 L of distilled water 2-3 times and centrifugated, about 2-3 times at 6 hours intervals. For reactions 3 and 4, only decantation with distilled water (5 times) was used. This process was carried out only with the intention of correcting the pH approaching to 7 (PENG et al., 2015). Afterwards, all reaction products were dried in an oven at Bruker Optics D2 Advence USA equipment was used for the characterization using X-ray diffraction (XRD), in order to determine the crystalline phases of the samples and assist in proving that the synthesis of this study was efficient for the formation of GO.

AVERAGE SIZE OF CRYSTALLITE AND DEGREE OF CRYSTALLINITY
The average size of the crystallite (D) is related to the width of the half height of the diffracted peaks and the mesh parameter associated with the position of the peaks is given by Equation (1) by Scherrer (1939) (SCHERRER, 1939): (1) Where, D is the average crystallite size, K is the constant that depends on the shape of the particles, the wavelength of the electromagnetic radiation, θ the diffraction angle and β the width of the

GO SYNTHESIS
The volume of sulfuric acid (60 mL) was assigned in order to ensure sufficient heat and mass transfer for the complete oxidation of graphite in flakes, considering 100 mesh as a large size graphite (CHEN et al., 2016. A). The amount of 6 g of KMnO4 was attributed to guarantee a complete oxidation of the precursor material in the requested time, the 3 g usually used by other bibliographies, for the case of this study, was consumed completely before the conversion of graphite into PGO, therefore, the amount of 6 g was necessary (LI et al., 2018). Was applied for hydrolysis of sulfur species contained in the reaction medium (mainly organosulfates). GO sheets can be covalently linked by organosulfates, leading to incomplete exfoliation of graphite oxide . The temperature of 40-50 °C was attributed to not compromise the GO's crystalline structure, based on the temperature used by CHEN et al., 2019 and also taking into account that temperatures close to 50 °C are a safe operating range for GO manufacturing, enabling high yield with excellent quality of the material (LI et al., 2018). The addition of water by dropwise for the start of the second oxidation step is essential, the sudden addition of water excessively increases the temperature of the reaction system, which compromises the crystalline structure of the product (CHEN et al., 2019.   Santa Maria, v. 21, n. 3, p. 15-26, 2020. 22 Figure 2 -Infrared spectrum of expirments.

GO PURIFICATION
An important observation in the literature refers to the variability and complexity of the methods for purifying the synthesized GO (DIMIEV, 2016). In this work, the aim was to use low equipment complexity with consequently low costs.
For all syntheses, the decantation procedure was applied. However with the use of centrifugation in experiments 1 and 2, and only decantation by gravity for experiments 3 and 4. The syntheses in which the centrifugation was used, required many hours (about 6 h) for separation and became less practicable for use on the laboratory. The performance of synthesis 1 and 2 may also have been compromised by the use of the centrifuge, needing several material overflows in centrifuge tubes for each centrifugation, and eventual loss of residual material at every step. For syntheses 3 and 4, only the decantation procedure was used, this method, although simple, proved to be quite viable, requiring no monitoring, and relatively little separation time (average 12 h). Decantation also avoids loss of synthesized product, and several GO replacements are not required. The decanting method, however, implies that the material must be fully oxidized and exfoliated, since through decantation no further auxiliary exfoliation method is no longer possible -unlike the purification processes (agitation, Disciplinarum Scientia. Série: Naturais e Tecnológicas, Santa Maria, v. 21, n. 3, p. 15-26, 2020. 23 centrifugation) where it is still possible to convert GrO to GO, as well as their separation . Thus, for experiments 3 and 4 the second oxidation step (hydrolysis) F and G was extended, assisting in the complete formation of GrO in GO, without requiring more complex procedures, such as the use of centrifugation, or agitation during purification processes, or even the use of ultrasound, which can also compromise the material's crystalline structure and compromise the final product DIMIEV, 2016).

GO YIELD DISCUSSION
The yield of the syntheses performed (ratio of the mass of GO obtained by the mass of graphite used) was 145.7% for the synthesis 2, which does not necessarily correspond to the integral mass of GO, considering the interferences observed in the x-ray diffractogram. For the synthesis 3 a total of 1.747 g of GO was obtained, corresponding to 175% yield (attributed by the exaggerated mass of reagents and complete oxidation of the material). For the synthesis 4 a total of 1,698 g of GO was obtained, corresponding to 169% of yield, which corresponds to the complete oxidation of the material and the large number of reagents used. For the synthesis 1 the yield was not considered because there was a lot of interference, in addition to residual graphite demonstrated in the analysis of the diffractogram, not certifying fidelity in the result.
The synthesis also showed relatively higher yields compared to several methods reported in the literature, for example, considering the synthesis of CHEN et al., 2016. A., (the highest yield of the referenced articles) which presented a value of 152% in its synthesis using small size high quality graphite.

AVERAGE SIZE OF CRYSTALLITE AND DEGREE OF CRYSTALLINITY
The degree of crystallinity of the experiments decreased as the oxidation steps lasted longer (mainly the second oxidation step), this fact is explained by the temperature used (40ºC), since it proved to be safe for the production of high quality GOs, however causing damages in the crystalline structure of the materials (CHEN et al, 2019), see Table 2. It is important to point out that GO crystallinity also influenced by exposure time to different oxidative species (MnO 3 + and MnO 4 -) in a concentrated sulfuric acid solution (JALILI et al., 2014).