Can reduced graphene oxide look like few-layer pristine graphene?

https://doi.org/10.1016/j.diamond.2021.108616Get rights and content

Highlights

  • Fine control in graphene oxide production

  • Position and type of functional groups proven by s-SNOM characterization

  • rGO with defect distances close to 20 nm

  • rGO showing characteristics very similar to few-layer pristine graphene

Abstract

The discovery of graphene brought several similar structures, highlighting its main derivative, reduced graphene oxide (rGO). Such structures can be obtained through several simple synthesis methods, mainly from variations of the Hummers method followed by chemical reduction using the most different reducing agents. This has helped to increase its applicability, but there always has been a gap related to the structural quality of the final material, which can still be filled. Therefore, to overcome the density of defects in the Hummers method, instead of creating another variation, we seek to improve existing methodologies, by controlling the KMnO4 addition rate and the temperature of the system. This control leads to a shorter oxidation stage compared to the literature, favoring the graphite oxidation reaction in a controlled and selective way, and producing a graphene oxide (GO) with excellent structural quality. After, through the reduction using hydrazine vapor, a remarkably high quality (HQ) material, the rGO HQ, could be produced. Raman results showed that the rGO HQ has a very low defect density with an average defect distance around 20 nm, which to the best of our knowledge is among the best values ascribed for this kind of graphene derivatives. Four probe measurements revealed low sheet resistance values with high transmittance, additionally, synchrotron infrared nanospectroscopy and density functional theory revealed the preferential formation of epoxy groups on GO HQ flakes as the main reason for its high structural quality. These impressive results allow us to state that our improved methodology produced a rGO HQ with characteristics very similar to those of few-layer pristine graphene without long-term procedures, new steps, or equipment, with the possibility of obtaining large scale production.

Introduction

Since the discovery of graphene, some derivatives of its structure have increasingly attracted attention, e.g., reduced graphene oxide (rGO). This graphene sheet has some oxygenated groups bonded to its structure, as well as point defects. It is obtained from the chemical reduction of graphene oxide (GO), which can be synthesized through the chemical oxidation of graphite, commonly denominated as Hummers method [1], [2].

Nowadays, it is well known that the Hummers method is the most used methodology for the exfoliation of graphite and subsequent production of the GO. Many changes in this procedure have been reported, whether in the type and amount of oxidizing agents used, temperature, reaction time, and even the sum of two or more conditions [3], [4], [5], [6]. In the same vein, many studies have already reported different routes for the production of rGO, using a wide variety methods of reduction [7]. However, the vast majority of the electrical properties of the reported materials, such as sheet resistance, for example, are still far from pristine graphene [8], [9].

This barrier can be mitigated using two different strategies: i. lower oxidations to minimize structural defects arising from this step, which after reduction would generate fewer defective structures with improved electrical properties, or ii. reduction reactions that eliminate the oxygenated groups and still restore the defects generated in the oxidation stage.

It is notorious that some authors have already reported methodologies to solve these problems, such as Pei et al. [10], who used HI, a strong and toxic acid, under high temperature, to reduce GO to obtain high-quality rGO. On the other way, Butz et al. [11] also showed variations on the graphite oxidation method to obtain GO with reduced number of defects. However, despite the success obtained in their works, the high oxidation time and the low reaction yield hampers this methodology's application in industrial processes. Chhowalla group [12], in a very well elaborated work, used a two steps reduction process using a chemical reduction, followed by microwave-assisted reduction, producing an extremely conductive material. However, the need for new equipment or the addition of steps in the production process can also make this methodology unfeasible for large-scale production.

Even with the high number of studies, it is evident that there is still room for new ones to improve rGO structural quality further. The non-stoichiometric characteristics in the oxidation stage open an opportunity for small procedure changes resulting in a significant impact on the final structure of the obtained material. Temperature, structural quality of the graphite precursor, chemicals purity, as well as the reaction's own chemical potential are factors that can change the oxidation kinetics. In this sense, the variation in some oxidation parameter can shift the reaction to provide more selective oxidation with the insertion of oxygenated groups at the edges and/or in the basal plane, as well as preventing the formation of CO2 molecules that leads to the loss of carbon atoms from the structure to the environment. Studies regarding rGO's structure are critical, and the control of the oxygen content, functional groups, and density of point defects directly influence its application, as demonstrated by Jovanovic et al. [13], who attested the difference in the capacitance of graphene derivatives when functionalized mainly with ketone or epoxy groups.

Based on that, and inspired by the work of Eigler group [14], [15], [16], we carried out a thorough control on two crucial parameters for rGO production: the oxidant addition rate and the temperature of the entire oxidation stage. We aimed at obtaining a high amount of few layers GO with lower specific defects, named GO HQ, followed by a chemical reduction stage, as described in the literature [17], thus obtaining few layers rGO with high structural quality, named rGO HQ. These parameters choice seek to improve the literature related to this topic, by demonstrating a frank and clear evolution on the reported methodologies, highlighting the absence of the increment of more steps or equipment and increasing the yield of produced materials. We also demonstrated that these materials can be applied as transparent conductive films, in some cases, with characteristics not so far from other graphene films. Thus, the produced rGO HQ was widely characterized by different experimental techniques and theoretical studies. Raman spectroscopy pointed out a GO HQ with high structural quality and low quantity of defects. Synchrotron Infrared NanoSpectroscopy and DFT calculations revealed a higher epoxy content than other oxygenated functional groups. These results show that the fine control during the oxidation generated few defects on the GO structure, which were suppressed after chemical reduction. By producing transparent films with very low sheet resistance values, we highlight the structural quality of the rGO HQ film that can allow its use in larger-scale processes since no extra step or equipment was required in its production.

Section snippets

Synthesis of high-quality reduced graphene oxide – rGO HQ

The synthesis of rGO HQ was divided into two main steps: to obtain high-quality few layers graphene oxide (GO HQ) and to reduce this material to form rGO HQ.

The first step was performed using a fine control on some reaction parameters during the graphite oxidation process to obtain the GO HQ. For this purpose, 1.00 g of graphite (Nacional de Grafite 99580) was added to a round bottom flask that was placed in an ice bath at 5 °C. Then, 60 mL of sulfuric acid (ACS reagent >99.0%) was added to the

Results and discussion

The effectiveness of our methodology has been attested through the analysis of several GO STD and GO HQ isolated flakes, using Raman mapping to monitor their ID/IG ratio. This ratio is a very powerful tool for assessing how defective the material is, ID/IG values close to or greater than 1.0 indicate the high intensity of the band related to structural defects, the D band, suggesting very functionalized or defective structures. Lower values, ≥ 0.5, indicate that the material structure is more

Conclusions

In summary, our work showed an elaborated study of the oxidation process of graphite in Hummers method. Using principles previously demonstrated by other researchers, we managed to improve the synthesizing process of GO, generating a material with high structural quality that was proven by several characterization techniques. Also, we showed the possibility of decreasing the total reaction time, thus allowing an increase of production. Through the s-SNOM analysis, we determined the quality and

CRediT authorship contribution statement

D. A. Nagaoka - Collaborated in carrying out all the experiments and in the discussion of the article.

D. Grasseschi - Collaborated in the s-SNOM and SINS experiments, as well as in the discussion and writing of the article.

S. H. Domingues - Coordinated all activities, participating in all of them. The main writing of the article was in charge of him.

Declaration of competing interest

Authors declare no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Acknowledgements

Authors acknowledge the financial support from FAPESP (2017/21988-5), Mackenzie Presbyterian University (151019), CNPq (403544/2016-5) and (306808/2020-0), CAPES (CAPES-PRInt, 88887.310281/2018-00) and National Institute of Science and Technology of Carbon Nanomaterials (INCT-Nanocarbon), D. A. N also thanks CAPES for the fellowship. The authors thank the Brazilian National Laboratory of Synchrotron Light (proposal ID 20190026) for providing beam time to this project and LNNano (proposal ID

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