Structural Evolution of Hydrothermally Derived Reduced Graphene Oxide

www.nature.com/scientificreports OPEN Received: 20 November 2017 Accepted: 12 April 2018 Published: xx xx xxxx Structural Evolution of Hydrothermally Derived Reduced Graphene Oxide Hsin-Hui Huang 1 , K. Kanishka H. De Silva1, G. R. A. Kumara2 & Masamichi Yoshimura1 Hydrothermal reduction is a promising approach for graphene oxide (GO) reduction since it is environmentally friendly, simple, and cost effective. We present a detailed study of structural changes occurring in graphene oxide during the reduction process. The correlations between the interlayer spacing, chemical states, work functions, surface morphology, level of disorders, the number of layers, and processing time are elucidated. The results reveal that GO flakes remain in the early stage of the reduction process and that they are fully reduced after a 4-h hydrothermal treatment. With an increase in the reduction time, the resulting product, i.e., reduced graphene oxide, has a low oxygen content, small interlayer spacing, and crumbled and wrinkled structures. We are convinced that these properties can be tuned to a desired level for various applications. Graphene, a two-dimensional monolayer of carbon atoms in a hexagonal lattice with an sp2 bonding hybridization, has come to the forefront in the field of materials science and nanotechnology since the early 2000 s in view of its outstanding electrical and thermal properties combined with excellent mechanical strength1–3. These superior properties lead to graphene making a significant impact on the field of materials science and nanotechnology, with graphene now being considered to replace other materials used in existing applications. To date, the most common methods used to fabricate graphene are micromechanical exfoliation, chemical vapour deposition, and chemical oxidation and reduction of graphite3–5. However, each of these methods has certain problems and limitations, e.g., in terms of yields, defect contents, costs, steps, or production time6. Chemical oxidation and reduction of exfoliated graphite is the best solution among all the other methods due to the relative ease of creating sufficient quantities of products at a desired quality level. However, the chemical reduction involves the use of hazardous reducing agents, such as hydrazine or sulfonate, and its residues might have a significant effect on the structures and properties of the final products. On the other hand, hydrothermal reduction is a simple, fast, and environmentally friendly route which involves water only. The experimental setup is rather cost effective and easy and it only requires an autoclave with a Teflon-lined container along with a furnace. It has been reported that a closed system with certain temperature and internal pressure promotes the restoration of the aromatic structure which is favourable for minimizing the defects7. Moreover, it has a good scalability and suits industrial large-scale production. Hydrothermal process was first introduced in the late nineteenth century, and it was mainly used in the production of synthetic minerals8,9. Since then, many studies have been performed across a wide range of the field, and the attention on the process is still growing. For instance, a graphene/silicon composite can be hydrothermally pruced for use as an anode in lithium ion batteries. A hydrothermally fabricated molybdenum disulphide (MoS2)/graphene composite shows a good onset potential in the hydrogen evolution reaction, giving one of the best performances among MoS2-based catalysts10,11. Reduced graphene oxide (rGO) decorated with titanium dioxide synthesized via a hydrothermal approach exhibits improved photocatalytic properties and would, therefore, be a promising material for future photovoltaic applications12. Additionally, composites of graphene and V2O5 have been developed for enhanced electrochemical energy storage13. Recently, a new form of self-assembled hydrogel rGO developed through a hydrothermal route has received a great deal of attention owing to the high mechanical strength (storage modulus of 450–490 kPa) of 1–3 orders of magnitude higher than conventional self-assembled hydrogels14, high compressive strength of 6 orders higher than conventional graphite products15, and an electrical conductivity as high as 5 × 10−3 S/cm14. Moreover, a three-dimensional NixCo1-xS2 particle/ 1 Graduate School of Engineering, Toyota Technological Institute, Nagoya, 468-8511, Japan. 2National Institute of Fundamental Studies, Kandy, 20000, Sri Lanka. Correspondence and requests for materials should be addressed to M.Y. (email: ) ScIeNtIfIc REPOrTS | (2018) 8:6849 | DOI:10.1038/s41598-018-25194-1 1 www.nature.com/scientificreports/ Figure 1. XRD patterns of graphite, GO, and rGO samples treated at different hours. Figure 2. SEM images of (a) GO, and its deoxygenated samples treated under (b) 1 h, (c) 4 h and (d) 10 h. The corresponding TEM images are shown in (e–h), respectively. The insets of TEM images showing the sheets in low magnification. graphene composite hydrogel was shown to have an interconnected porous network with pore sizes in the range of several micrometres giving high performance as the active material in supercapacitors16. While hydrothermal treatment is a unique synthetic approach to graphene oxide reduction and even though it has been used for years, it is faced with the challenges of firmly understanding the deoxygenation activity, preparing graphene with high quality and dispersibility, and precisely controlling the structure and morphology. Moreover, the structures and properties of the resulting products might vary depending on the control parameters used in the reduction process. Thus, it is necessary to clarify the reduction mechanisms and structural changes throughout the process, since the structure is strongly related to the final properties and hence the performance of the devices. Here, we present a detailed study on the structural, morphological, and electrical changes during the reduction process from the initial 30 min up to 10 h at 200 °C; hereafter, we refer to the individual samples as rGO 0.5 h, 1 h, 2 h, 4 h, 8 h, and 10 h. The gradual changes in the morphology and dramatic drop in the interlayer spacing were elucidated using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force ScIeNtIfIc REPOrTS | (2018) 8:6849 | DOI:10.1038/s41598-018-25194-1 2 www.nature.com/scientificreports/ Figure 3. AFM images of the rGO samples treated at (a) GO, rGO (b) 30 min, (c) 1 h, (d) 2 h, (e) 4 h, (f) 6 h, (g) 8 h, (h) 10 h. microscopy (AFM), and transmission electron microscopy (TEM). The degree of oxidation and reduction and the defects such as vacancies were confirmed using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrical properties of the reduced graphene oxide at different stages were revealed through investigation of the strong correlation between oxygen content and contact poten (...truncated)