Porous graphene network from graphene oxide: Facile self-assembly and temperature dependent structural evolution

https://doi.org/10.1016/j.mtcomm.2020.101930Get rights and content

Abstract

There has been increasing interest to synthesize porous graphene structures for various applications, viz. super-capacitors, photovoltaic cells and sensors due to their unique structure with interconnected network, high surface area, excellent electrical conductivity and good thermal stability. Herein, a facile and highly effective scalable method has been developed to synthesize porous graphene network (PGN) from reduced graphene oxide. PNG was synthesized by a facile process that combines the synthesis of graphene oxide (GO) by modified Hummers method and optimization of reduction temperature in ambient conditions. The morphology of the prepared samples was investigated by scanning electron microscopy. Structural transformation during reduction of GO under ambient conditions was investigated using X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. In comparison to already existing methods, these results demonstrate a very convenient and general protocol to synthesize the PGN from GO for various applications.

Introduction

Graphene family materials such as, graphene oxide (GO), reduced graphene oxide (rGO) and graphene quantum dots (GQD) are highly studied now a days due to their properties such as high electronic conductivity, large surface area and unique optical and mechanical properties [[1], [2], [3]]. The sp2 hybridized hexagonal carbon honeycomb structure is the elementary skeletal structure of graphene family materials that can be modified and functionalized relevant to the extensive properties required [4]. Graphene synthesized from chemical vapour deposition (CVD) and mechanical exfoliation are free of functional groups and they render exceptional high Young’s modulus (1.0 TPa), thermal conductivity (5000 W/m.K) and chemical stability [[5], [6], [7]]. However, modifying graphene into a hydrophilic material by introducing oxygenated functional groups either chemically or thermally will make it more suitable for applications such as medical [8,9], environmental sensing [10], storage [11] and photovoltaic [12]. Graphene oxide has a much similar structure like graphene with hexagonal carbon lattice and differs by the number of stacking layers accompanied by the defects. It also possesses active functional groups such as hydroxyl and epoxy formed at their basal plane in addition to carboxyl and carbonyl groups formed at their edges [[13], [14], [15]]. Functional groups decorated GO increases the possibility of surface functionalization and it has many fascinating applications than graphene due to its high solubility. The insertion of oxygenated functional groups in between the crystalline plane during the synthesis of GO modifies the electronic bandgap. This changes the local charge carrier mobility and becomes responsible for the new electronic and optical behaviour of the material. Furthermore, the reduction of GO assisted with distinct methods such as chemical, electrochemical, thermal, laser and microwave methods led to an evolution of graphene like structure called reduced graphene oxide [[16], [17], [18], [19], [20], [21], [22], [23]].

In layered graphene like structures, experimentally obtainable surface area is far below the theoretical value due to the strong π-π stacking and van der Waals interactions between graphene sheets. To overcome this problem and achieve higher surface area, graphene porous structures have received increasing interest due to their unique structure with interconnected networks, high surface area and high pore volume [24]. These unique properties have highly promising applications in sensing, energy storage and conversion applications [[25], [26], [27], [28], [29]]

Template dependent and template free protocols are the conventional ways to synthesize porous graphene. The three dimensional macroporous graphene films have been synthesized by Chen and co-workers using uniform polymethyl methacrylate (PMMA) spheres as hard templates [30]. Similarly, Zhao et al. has utilized hydrophobic interactions with a hard template method to synthesize porous graphene foams [31]. Huh et al. has utilized polystyrene as sacrificial template with chemically modified graphene to synthesize a 3D macro-porous graphene structure [32]. Yadav and co-workers have used polymer pyrolysis single step catalyst-free process to synthesize single layer graphene assembled porous carbon [33]. Fan and co-workers has demonstrated microwave irradiation using KMnO4 as oxidizer to synthesize porous graphene [34]. There are many other methods such as templating, chemical etching, doping, chemical vapour deposition and ion bombardment reported by different groups to synthesize the porous graphene [35]. The current techniques reported are very expensive, low yielding and template residue after etching. These are not viable for practical applications. Therefore, it still remains a major challenge to develop low cost, scalable and template free method to synthesize porous graphene by simple technique.

In this study, a facile cost-effective and simple synthesis route is developed to synthesize porous graphene network (PGN) based on thermal reduction of GO. GO was synthesized from synthetic graphite powder by the modified Hummers method. GO was further reduced thermally at different annealing temperatures in ambient conditions to obtain the reduced graphene oxide (rGO). During this reduction process, the reduction temperature and reduction time was optimized to get PNG. The morphologies and structural transformation of GO and rGO was investigated by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. Raman spectroscopy [[36], [37], [38], [39], [40], [41], [42], [43]] reveals the structural transformation during reduction process of GO which have been correlated with the other experimental results. In comparison with the already existing methods as discussed above, reported results demonstrate a very simple and low-cost protocol to synthesize the PGN from GO that further benefit the various applications.

Section snippets

Experimental section

Initially, GO was synthesized from synthetic graphite powder by the modified Hummers method. In this synthesis, 2 g of graphite powder and 2 g of NaNO3 were dissolved in 50 ml of H2SO4 with stirring for 2 h under ice bath and the black suspension was obtained. Subsequently, 6 g of KMnO4 which acted as an oxidizing agent was added very slowly with the continuous stirring for 24 h, while the reaction temperature was slowly increased to 35 °C. Brown colour viscous slurry was obtained due to the

Results and discussion

Fig. 1 shows the XRD pattern of GO sample at room temperature and GO annealed at 200 °C, 300 °C, 400 °C and 500 °C in the range of 10° to 80°. It clearly exhibits the difference in the XRD pattern of GO and rGO samples. The crystallographic parameter from the XRD characteristic peaks of each sample such as interplanar distance (d), crystalline stack height (Lc), number of average graphene layers in graphene stack (nc) are analysed using Bragg’s law (d = λ/2 sinθ) and Scherrer’s formula (Lc =

Conclusion

Here, a facile and scalable fabrication approach has been developed to synthesize the PGN. Initially, GO was synthesized from synthetic graphite powder by modified Hummers method. A detailed structural (microscopic and diffraction) studies establish that porous graphene network can be synthesized from the GO obtained by modifying the synthetic graphite. Structural and spectroscopic investigations show that GO starts reducing at the temperature of 200 °C that is explicitly clear from the change

CRediT authorship contribution statement

Anusuya T.: Methodology, Validation, Investigation, Visualization, Writing - original draft. Prakash J.: Investigation, Validation. Devesh K. Pathak: Investigation. Kapil Saxena: Visualization. Rajesh Kumar: Methodology, Resources, Validation. Vivek Kumar: Conceptualization, Supervision, Validation, Project administration, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

Authors (AT & VK) are thankful to IIITDM Kancheepuram for providing financial support and facilities. Author (VK) acknowledges financial support from Science and Engineering Research Board, Department of Science and Technology (SERB-DST), Govt. of India (grant no. ECR/2016/001715). One of the authors (R.K.) thank funding received from the SERB, Govt. of India (grant no. CRG/2019/000371). One of the authors (D.K.P.) acknowledges the Council of Scientific and Industrial Research (CSIR) for

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