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Structure of nanographite synthesised by electrochemical oxidation and exfoliation of polycrystalline graphite

2017; Institution of Engineering and Technology; Volume: 12; Issue: 12 Linguagem: Inglês

10.1049/mnl.2017.0339

1750-0443

Adrian Radoń, Dariusz Łukowiec,

Graphene and Nanomaterials Applications

Micro & Nano LettersVolume 12, Issue 12 p. 955-959 ArticleFree Access Structure of nanographite synthesised by electrochemical oxidation and exfoliation of polycrystalline graphite Adrian Radoń, Adrian Radoń Faculty of Mechanical Engineering, Silesian University of Technology, Konarskiego 18 a St.,, 44-100 Gliwice, PolandSearch for more papers by this authorDariusz Łukowiec, Corresponding Author Dariusz Łukowiec dariusz.lukowiec@polsl.pl Faculty of Mechanical Engineering, Silesian University of Technology, Konarskiego 18 a St.,, 44-100 Gliwice, PolandSearch for more papers by this author Adrian Radoń, Adrian Radoń Faculty of Mechanical Engineering, Silesian University of Technology, Konarskiego 18 a St.,, 44-100 Gliwice, PolandSearch for more papers by this authorDariusz Łukowiec, Corresponding Author Dariusz Łukowiec dariusz.lukowiec@polsl.pl Faculty of Mechanical Engineering, Silesian University of Technology, Konarskiego 18 a St.,, 44-100 Gliwice, PolandSearch for more papers by this author First published: 01 December 2017 https://doi.org/10.1049/mnl.2017.0339Citations: 5AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract The Letter presents the results of characterisation of intercalated and exfoliated graphite. The electrochemically exfoliated graphite (EEG) was synthesised in solution containing sulphuric acid and potassium chlorate(V). The number of graphene layers, degree of disordering and the presence of functional groups were determined using X-ray diffraction method, Raman spectroscopy, Fourier transform infrared spectroscopy and UV–visible spectroscopy. High-resolution transmission electron microscopy and scanning electron microscopy were used to imaging morphology and structure of the synthesised material. The obtained micrographs indicate the presence of highly disordered areas near the edge of flakes and in-plane ordered lattice structure in the flat zone between disordered areas. X-ray diffraction patterns clearly indicate the presence of both nanographite and graphene oxide. The EEG was also characterised by relatively high thermal stability, which was confirmed using thermogravimetric analysis. 1 Introduction Many different kinds of graphite-based materials were synthesised and tested [[1]]. Interest in them is due to their properties: high electrical conductivity [[2]], low density [[3], [4]], high surface area [[5]] and the possibility of their functionalisation [[6], [7]]. The use of electrochemical exfoliation of graphite is one of the methods used to synthesise graphene oxide (GO). The ability to use this method for the production of GO results from the high conductivity of the graphite, which is used as the electrode material, and the possibility of intercalating the molecules between the graphene layers [[8]]. The use of different electrolytes allows for obtaining multi-layered and few-layered structures (even one- and two-layered). Depending on the electrolyte used and polarity of the electrodes, it is possible to intercalate anions such as sulphate anions, fluoride anions, metal halides or magnesium and lithium cations [[9]]. Depending on the type of intercalation and exfoliation, it is possible to obtain graphene with low degree of disordering and low degree of oxidation (in the case of intercalation of cations) or strongly oxidised material (anodic exfoliation) [[10]]. The most commonly used electrolytes include sulphuric acid [[11]]. In addition, the form of graphite material plays an important role in exfoliation. Munuera et al. [[10]] have presented in their work the influence of starting graphite material to oxygen and defect content of graphene obtained by electrochemical exfoliation of graphite. They showed that graphene nanosheets with high structural quality and low concentrate of functional groups can be obtained from graphite foil. X-ray diffraction methods are the basic methods for identifying the number of layers in graphite-based materials. Characteristic for GO diffraction peak appears at about 10° [[12]-[14]] and corresponds to the interplanar distance of 0.86 nm. In addition, the results of Morimoto et al. [[15]] indicate that graphite is present in about 50% oxygen in the structure of GO, and its total exfoliation into the monolayer is only possible after this threshold, which is manifested by the disappearance of the graphite peak. This work presents the structure of electrochemical exfoliated graphite. Electron microscopy was used to determine structure and morphology of the synthesised material. In addition, the effect of synthesis method on the structure was determined using X-ray diffraction (XRD) method, thermogravimetric analysis (TGA), UV–visible (UV–vis) spectroscopy, Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. 2 Experimental section 2.1 Preparation of electrochemically exfoliated graphite (EEG) The exfoliated graphite was synthesised by electrochemical oxidation and exfoliation of polycrystalline graphite rod. For this purpose, 100 cm3 solution containing 0.5 g H2SO4 (95%) and 0.3 g KClO3 was prepared. The distance between the electrodes was selected so that the current flowing between the cathode and the anode at 2.16 A is obtained at a voltage of 8 V. Electrolysis was conducted by 1 h and obtained material was then sonicated by 1 h in post-reaction solution, filtered and washed five times by water and ethanol. Synthesised black powder with a mass of 0.5 g was drying in 60°C. 2.2 Characterisation methods The number of graphene layers in the synthesised material was determined using a Rigaku MiniFlex 600 with a copper tube Cu (Cu Kα, λ = 0.15406 nm). The measurements were performed in the Bragg–Brentano geometry in an angular range 2θ 5°–80°. Spectroscopic analysis of the samples was carried out using Raman spectroscopy and FTIR spectroscopy. Renishaw's Raman in Via Reflex spectrometer, equipped with the Leica Research Grade confocal microscope, with the ability to observe reflected and passing light samples was used in this work. Excitations were made by ion-argon laser with a beam with a wavelength λ = 514 nm and with plasma filter for 514 nm. Measurements were recorded using a Long Working Distance (LWD) lens with a magnification of ×20. The synthesised material was studied in a wide range of wavelengths of 50–3500 cm−1. Analysis of the absorption spectra of infrared samples was performed using a FTIR Nicolet 6700/8700 spectrometer. Sample measurements were made in the 4000–400 cm−1 basic infrared transmission mode, where the number of scans was 128, and scanning resolutions were 4 cm−1. The transmission electron microscopy (TEM) measurements were carried out in the high-resolution transmission electron microscope S/TEM TITAN 80. Differential scanning calorimetry (DSC) and TGA curves for synthesised materials were determined using a thermal analyser NETZSCH Jupiter STA 449 F3. The study was conducted in the temperature range of 50–750°C at a heating rate of 10°C/s. UV–vis spectra were obtained in the wavelength range 190–600 nm using Thermo Scientific Evolution 220 spectrophotometer. The measurements were performed at room temperature using 1 cm quartz cell and deionised (DI) water as a blank. The samples were prepared by dispersing 5 mg of powders in 50 ml DI water and sonicated for 5 min to make a homogenous suspension solution. 3 Results and discussion Fig. 1 shows the XRD pattern for EEG. Five characteristic for carbon diffraction reflexes: (002), (100), (101), (004) and (110) were observed. The positions and intensity of the diffraction peaks were satisfactory identified and described by the DB card number: 9012230 of carbon (space group: P63mc). Additional reflex at 2θ = 10.54° (GO (002)) is associated with GO structure obtained also by electrochemical oxidation and exfoliation of polycrystalline graphite. Fig. 1Open in figure viewerPowerPoint XRD pattern of nanographite synthesised by electrochemical exfoliation of polycrystalline graphite rod The distance between graphene layers in nanographite and GO layers was calculated according to Bragg's equation (1). The Sherrer's equation (2) was applied for evaluation the average height of stacking layers (H). In addition, the number of layers in synthesised material was calculated according to formula (3). The obtained results were presented in Table 1 (1) (2) (3) where d002– the distance between graphene layers, λ – the X-ray wavelength, θ – the position of diffraction reflex, H – the average crystallite height, K – the shape factor (for graphite materials K = 0.94), β – the full width at half-maximum of the diffraction reflex. Table 1. Structural parameters (average height of the crystallite H, interlayer distance d002, the number of layers n) of EEG resulting from the XRD patterns 2θ, deg. β, deg. H, nm d002, nm n Peak (002) 26.34 1.181 7.22 0.839 21–22 Peak GO (002) 10.54 8.13 1 0.338 1–2 Disordering structure of the exfoliated graphite was confirmed by Raman spectroscopy. Fig. 2a shows Raman spectrum for synthesised material. Four characteristic bands for disordering carbon materials was observed: D, G, 2D and D + G. In addition, existence of D ′, D ″ and D * bands was confirmed by fitting the experimental data to a sum of five functions [[16]]. The best results were obtained when D and G bands were fitting by pseudo-Voigt functions while, for D ′, D * and D ″ bands Gaussian functions render the best fit (Fig. 2b). Fig. 2Open in figure viewerPowerPoint Disordering structure of the exfoliated graphite a Raman spectrum for EEG b A five function (D *, D, D ″, G and D ′ bands) deconvolution for EEG Band related to the E2g mode and associated with in-plane stretching of the C–C bond in graphitic materials (G band) appears at 1583 cm−1. The broadening of this band is associated with existence of D ′ band and the epoxy and hydroxyl groups in the structure. The 2D band (second order of the D peak) sensitive to the π band in the electronic structure also appears at Raman spectrum for synthesised material. The high degree of disordering is confirmed by the presence of D, D ′, D ″ and D + G bands. The D band at 1352 cm−1 is related to the A1g breathing mode and is associated with basal plane defects in the structure. The D ′ band at 1615 cm−1 is due to crystal defects in the structure (as pentagonal and octagonal rings). Existence of sp2 –sp3 bonds at the edges of graphitic networks confirming D * band with low intensity. Finite size crystals of graphite or amorphous structure in sample confirming D ″ band. This band can be also associated with existence of organic molecules, fragments or functional groups. In addition, appearance of D + G band at 2944 cm−1 confirming high degree of disordering [[16]-[18]]. The FTIR spectrum for exfoliated graphite is shown in Fig. 3. The identified peaks confirming the presence of the functional groups in the structure. The highest, broad peak at 3421 cm−1 is associated with the presence of the H2 O and other functional groups as COH or COOH. Peak at 1581 cm−1 is typical for sp2 carbon. Peaks associated with C–O and C=O vibrations confirming the existence of the carbonyl, carboxyl, epoxy and hydroxyl groups (respectively, 1728 and 1630 cm−1 for C=O and 1395, 1225, 1170 and 1042 cm−1 for C–O) [[19]-[23]]. In addition, peak at 1131 cm−1 can be interpreted as vibration from 5-membered-ring lactol at the edges [[24]]. Three peaks at 2964, 2920 and 2850 cm−1 are associated with the existence of CH and CH2 occurring also most likely at the edges of graphite flakes [[24], [25]]. Fig. 3Open in figure viewerPowerPoint FTIR spectrum for EEG with marked identified vibrations Fig. 4 shows the UV–vis absorption spectrum of exfoliated graphite. Characteristic peak at 270 nm is associated with π –π * transitions originating from sp2 domains. This peak is characteristic for fully reduced GO. For example UV–vis spectrum of GO has one peak at 231 nm, which shifts to higher wavelength (redshift) after reduction [[10], [26]]. Therefore, the existence of the peak at 270 nm for EEG is associated with the structure of obtained material, which is not strongly oxidised in the base plane. Identified on FTIR spectrum functional groups probably are presented at edges of graphitic network or in GO structure identified on XRD pattern. Fig. 4Open in figure viewerPowerPoint UV-vis spectrum of EEG obtained for water dispersion of EEG As shown in Fig. 5, the TGA were performed on EEG in the temperature range 50–750°C. It was observed that EEG is thermally unstable and start to lose mass upon heating above 150°C. The first significant drop in mass around 173°C is associated with decomposition of the labile oxygen-containing functional groups, yielding CO and CO2. This reaction is exothermic, what was confirmed by DSC. The later weight loss is attributed to pyrolysis of the carbon skeleton of EEG. Additionally weight loss around 20% at 750°C is much lower than for graphite oxide and GO [[27]-[29]]. Fig. 5Open in figure viewerPowerPoint TGA curve of EEG; inset: DSC curve in temperature range of the highest weight loss The SEM, TEM and HRTEM images characterising the exfoliated graphite morphologies are shown in Fig. 6. The synthesised material consisted of folded and wrinkled sheets randomly aggregated to form a disordered solid. Observed morphology is associated with electrochemical oxidation and exfoliation of graphite. The precipitated on the graphitic electrode oxygen penetrates into graphite structure, causing not only the formation of functional groups, but also increase in the interlayer spacing and consequent exfoliation of graphite to multi-layered structure with high specific surface area (Figs. 6a and b). In addition, in-plane ordered lattice structures in the flat zone between the microfolding areas are also observed (Fig. 6e). Oxidation and exfoliation of graphite introduce a significant number of defects which disrupts the structure of the honeycomb lattice structure characteristic for graphene. It is especially visible on the edges of the flakes, which are most exposed to the oxygen generated by electrolysis (Figs. 6f–h). Fig. 6Open in figure viewerPowerPoint SEM, TEM and HRTEM images a, b SEM c TEM and d–h HRTEM micrographs of electrochemically synthesised graphite, e represents in-plane ordered lattice structures in the flat zone in area marked in d, g reveals honeycomb structure of graphene performed at the area denoted by red line in f, h represents highly disordered structure at edge of EEG at the area denoted by blue line in f 4 Conclusion Nanographite powder containing 21 graphene layers was successfully synthesised by electrochemical exfoliation of polycrystalline graphite. In addition, used method allows to obtain one layered and double layered GO. 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