Synthesis and Characterization of Electrochemically Exfoliated Graphene with High Conductivity and Thermal Stability after Reducing Treatment

Abstract

Graphene is a nanomaterial of great technological interest due to its properties such as high conductivity, lightweight, and excellent mechanical flexibility with potential applications in the most diverse areas as batteries, supercapacitors and electrochemical sensors. The electrochemical exfoliation of graphite technique has great prominence compared to other techniques to produce graphene because of its relative ease, high yield, high quality products and scalability. However, this technique generates products with a high degree of oxidation. To restore the intrinsic properties of graphene, these oxygenated groups must be removed using reduction methods. This work demonstrates the production of high quality graphene derivative with excellent conductivity, low defects density and excellent thermal stability produced by electrochemical exfoliation with reduction treatment (rEEG) using ascorbic acid (AA) and sodium borohydride (SB) as reducing agents. Raman spectroscopy is an excellent tool for the characterization of graphene quality. The ID/IG ratio is an important parameter used as a criterion to identify the degree of disorder in the graphene structure. This technique was employed to quantify the defects in the lyophilized graphene derivatives. Raman spectra of the different graphene produced (Figure 1) shows that freeze-drying process provides graphene with low defects density, since the drying process is conducted under vacuum atmosphere and in very low temperatures (~ -100°C), favorable conditions to preserve the quality of graphene sheets and the degree of oxidation. SB-rEEG stands out for presenting lower defect density which is evidence of great removal of oxygenated groups, confirmed by the C:O ratio observed in Figure 2 (A). On the other hand, the reduction with ascorbic acid resulted in a more oxygenated graphene and with a greater number of defects. Figure 3 shows the AFM image of exfoliated graphene and its corresponding height profile. AFM measurement is a suitable technique to investigate the number of layers and morphology of graphene. Height profile image can be used to get information about the thickness of the graphene layers. Considering the thickness of 0.35 nm for each graphene layer, obtained from X-ray diffraction (XRD) spectrum (Figure 4). AFM height profile of the derivatives graphene obtained shows profiles varying from 4 - 10 nm thickness and from 40 to a few hundred nanometers wide, characterizing them as few layer graphene. Analyzing the conductivity results in Figure 2 (B), SB-rEEG showed a higher conductivity (~2.6 x 105 S m-1) compared to EEG (~7x 103 S m-1) and AA- rEEG (7 x 104 S m-1), demonstrating that sodium borohydride was an effective reducing agent for electrochemically exfoliated graphene (EEG). According to Figure 5 (a) , TGA analysis in an atmosphere of nitrogenshows that EEG presented three main phenomena: (i) water elimination, at temperatures below 100ºC (approximately 10%); (ii) removal of oxygenated groups, between 150ºC to ~250ºC and (iii) pyrolysis of the carbonic chain of the materials accompanied by structural rearrangements of the polyarene network, between 360ºC and 1000ºC. While for SB-rEEG (Figure 5(b)) the same phenomena were observed, but at a lower intensity. The SB-rEEG showed high thermal stability with a weight loss at 1000ºC close to 14%, while the EEG showed a weight loss of 45%. It was possible to restore the conductivity and thermal stability of the electrochemically exfoliated graphene subjected to reduction process, producing graphene derivatives with different chemical and physical properties for different applications. Figure 1