Pis'ma v ZhETF, vol. 99, iss. 11, pp. 754-759 © 2014 June 10
Neutron scattering study of reduced graphene oxide of natural origin
E. F. Sheka+1\ I. Natkaniec*x, N. N. Rozhkova0, K. Holderna-Natkaniecx
+ Peoples' Friendship University of Russia, 117198 Moscow, Russia
* Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, 141980 Dubna, Russia
x Department of Physics, Adam Mickiewicz University, 61-712 Poznah, Poland
°Institute of Geology, Karelian Research Centre of the RAS, 185610 Petrozavodsk, Russia
Submitted 31 March 2014 Resubmitted 5 May 2014
The current paper presents a direct confirmation of graphene-like configuration and first suggests the chemical composition of basic structural elements of shungite attributing the latter to reduced graphene oxide nanosize sheets with an average 11:1:3 (C:0:H) atomic content ratio.
DOI: 10.7868/S0370274X14110083
High-yield production of few-layer graphene flakes from graphite is important for the scalable synthesis and industrial application of graphene. Graphene-based sheets show promise for a variety of potential applications, and researchers in many scientific disciplines are interested in these materials. Although many ways of generating single atomic layer carbon sheets have been developed, chemical exfoliation of graphite powders to graphene oxide (GO) sheets followed by deoxygenation to form chemically modified reduced graphene oxide (rGO) has been so far the only promising route for bulk scale production. However, available technologies face a lot of problems among which there are low yield, the potential fire risk of GO and rGO when alkaline salt byproducts are not completely removed, a great tendency to aggregation, a large variety of chemical composition, and so forth (see the latest exhaustive reviews [1,2] and references therein). In light of this, the existence of natural rGO is of utmost importance due to not only lower costs but unprecedented chemical stability provided by the geological long-term stabilization. As shown recently [3], shungite carbon from the deposits of carbon-rich rocks of Karelia (Russia) [4] presents a multilevel fractal structure based on nanoscale rGO sheets. This suggestion was the result of a careful analysis of physico-chemical properties of shungite widely studied to date as well as was initiated by knowledge accumulated by the current graphene science. Since then, two new direct justifications of the suggestion have been obtained. The first one is related to the study of photoluminescence (PL) of shungite aqueous and organic
-^e-mail: sheka@icp.ac.ru
dispersions [5] that exhibits properties similar to those of synthetic graphene quantum dots of the rGO origin of nanometer size [6]. The second was obtained in the course of the neutron scattering study presented in the current paper.
Neutron scattering study was performed at the high flux pulsed IBR-2 reactor of the Frank Laboratory of Neutron Physics of JINR by using the NERA spectrometer [7]. The investigated samples are illuminated by white neutron spectrum analyzed by time-of-flight method on the 110 m flight path from the IBR-2 moderator. The inverted-geometry spectrometer NERA allows simultaneously recording both Neutron Powder Diffraction (NPD) and Inelastic Neutron Scattering (INS) spectra. The INS spectra are registered at final energy of scattered neutrons fixed by beryllium filters and crystal analyzers at Ef = 4.65 meV. Three powdered shungite samples were subjected to the study. The first pristine shungite Shi presents natural material of the highest carbon content from the Shun'ga deposit (see a detailed description of stability of the material properties in [4]). The second shungite Sh2 is a modified Shi after lengthy drying at 110 °C. The third shungite Sh3 is a solid condensate of colloids of the shungite Shi aqueous dispersions. Powdered spectral graphite was used to register both the reference NPD and INS spectra from pure carbon material.
As shown in Fig. 1, the general NPD patterns of all shungite samples are identical and similar to that of spectral graphite while drastically differ from the latter by shape: all Gr(hkl) peaks are upshifted and considerably broadened pointing to irregular structure of the shungites. As seen in the figure, the narrow peak
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Fig. 1. NPD of spectral graphite (Gr) and shungites Shi (1), Sh2 (2), and Sh3 (3) recorded at T = 20K. Scattering angle 2© = 117.4". The data are normalized per neutron flux intensity $(A) at each neutron wave length A, next, intensity of both shungite and graphite peaks in the Gr(002) region are normalized to 1000 arbitrary units; Gr{hkl) and A\(hM) denote characteristic diffraction peaks of spectral graphite and cryostat aluminum at different Miller indexes, respectively
Gr(002) of graphite, the shape and width of which correspond to the resolution function of spectrometer and whose position determines ¿002 interfacial distance between the neighboring graphite layers, is substituted with broad peaks. The slight upshift of the peaks convincingly evidences a conservation of the graphite-like structure of all the shungite samples while the peaks wide broadening tells about a considerable space restriction.
A well known Scherrer equation, due to which the length Lcsr of the coherent scattering region (CSR) is inversely connected: B = kX/LcsR sin©, where A and © are of diffracting flux wave length and scattering angle, has been used when studying nanostructured graphites for a long tome [8]. Applying to shungites, it is possible to determine Lcsr by using the reference Lcsr ~ 20 nm for spectral graphite alongside with the known ratio of FWHMs of the relevant diffraction peaks. The obtained value constitutes 1.3 nm, accuracy of which depends on how accurate are the reference data that may be quite widely spread due to which the obtained Lcsr value can be considered as a certain evaluation. Nevertheless, the obtained Lcsr value is quite coherent with those of 2.18 and 2.30 nm for Shi and Sh3, respectively, ob-
tained by A"-Ray diffraction [9]. Within the framework of shungite multilevel fractal structure [3], there might be two size limitations that correlate with the obtained Lcsr values: these are: 1) the linear dimension of individual rGO sheets 1 nm) and 2) the thickness of the sheet graphite-like stacks that form the first level of the shungite structure. Summarizing NPD and A"-Ray data, Lcsr of ~ 1.5—2 nm can be suggested as a reasonable average estimation. The latter implies 6 layers of ~ 1 nm rGO fragments in the stacks.
INS spectra of Shi and Sh2 are given in Fig. 2. The spectrum of Sh3, scaled in accordance with the mass ratio, is quite identical to that of Shi. Two pristine spectra presented in the figure clearly exhibit strong scattering from both samples in contrast with that from graphite thus indicating that both shungites are evidently hydrogen-enriched. At the same time, the two spectra differ in both intensity and shape. Since Sh2 was produced from Shi by lengthy heating, which was followed by removing water previously retained in Shi, the difference spectrum 1-2 in the figure evidently presents the spectrum of released water. Actually, well known characteristic features of the INS water spectrum are clearly observed in the spectrum of Shi while no traces
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Fig. 2. Time-of-flight INS spectra of shungites Shi (1) and Sh2 (2) at 20 K after extraction of the INS spectrum of spectral graphite that is indistinguishable from the background. Spectra are normalized per 106 monitor counts of incident neutrons. Curve 1, 2 presents the difference between spectra 1 and 2. The intensity of elastic peaks is 66-fold reduced
of such structure are seen in the spectrum of Sh2. At the same time, the latter is quite intense, which undoubtedly points to the presence of hydrogen atoms incorporated in the carbon structure of shungite. The presence of the hydrogen atoms in the core of dried shungite has been directly observed for the first time.
To proceed with a detailed analysis of the obtained spectra, let us move on to the consideration of the density of vibrational states. The relevant G(w) spectra of one-phonon amplitude-weighted density of vibrational states (AWDVS) are shown in Fig. 3 (see details of G(w) spectra obtaining in [10]). Three main features follow from the general overview of the spectra. The first concerns spectrum 1-2 in Fig. 3b that can be evidently considered as the spectrum Gwat(w) of retained water. The spectrum presents the contribution of 4 wt % water in the spectrum Gbh(w) of the pristine shungite and is pretty similar to those well known for retained water in silica gels [11,12], Gelsil glasses [13], oxygenated graphite [14], and various zeolites [15]. The second feature is related to the evident absence of the retained water crystallization so that the Gwat(w) spectrum represents the bound water [14], molecules of which are connected with the solid shungite ground via hydrogen
bonds and are located within the first adlayer. The third feature is related to the spectrum 2 in Fig. 3a that evidently exhibits the presence of hydrogen in the shungite core. The relevant Gcore(oj) spectrum differs drastically from the water one and is identical to the INS spectrum of a synthetic rGO [16] thus directly confirming the rGO nature of shungite. The spectrum is characterized by a considerable flattening up to 500 cm-1 and reveals a pronounced structure in the region of 600-1200 cm-1. The total intensity of Gcore(oj) spectrum constitutes approximately 1/3 of the Gwat(w) one. Taking into account that the latter is provided by 4 wt % water, it is possible to evaluate 1.8 wt % mass content of hydrogen in the shungite core. The data well correlate with 1.5±0
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