Elena F Sheka
Russian Peoples’ Friendship University of Russia, Moscow, 117198 Russia
Received: 16 December, 2016; Accepted: 10 January, 2017; Published: 11 January, 2017
Elena F. Sheka, Russian Peoples’ Friendship University of Russia, Moscow, 117198 Russia, E-mail:
Sheka EF (2017) Reduced Graphene Oxide and Its Natural Counterpart Shungite Carbon. Int J Nanomater Nanotechnol Nanomed 3(1): 007-014. DOI: 10.17352/2455-3492.000014
Â© 2017 Sheka EF. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Reduced graphene oxide; Technical grapheme; Shungite carbon; Graphene oxide reduction; Quantum-chemical approach; Molecular theory
Large variety of structure and chemical-composition of reduced graphene oxide (RGO) is explained from a quantum-chemical standpoint. The related molecular theory of graphene oxide, supported by large experience gained by the modern graphene science, has led the foundation of the concept of a multi-stage graphene oxide reduction. This microscopic approach has found a definite confirmation when analyzing the available empirical data concerning both synthetic and natural RGO products, the latter in view of shungite carbon, suggesting the atomic-microscopic model for its structure.
According to the judgment of competent experts , the modern graphene technology can be divided into two independent domains, namely, low-performance (LP) and high-performance (HP) ones. The first includes a wide spectrum of practical applications based on graphene nanomaterials. The characteristic products of this domain are modified polymer and other composites, sensors and sensor screens, roll-up electron paper, organic light-emitting diodes, and so forth. The products of the second domain are based on micro- or larger sized one- or multilayer graphene sheets and represent electron devices, such as high-frequency, logic, and thin film field-effect transistors. This de facto division of the graphene technology into two types results from the molecular–crystalline dualism of the graphene nature and the technical implementation of its unique chemical and physical properties rather than from the simplification of operation with complex technologies .
The objective reasons of postponing the graphene HP technology [1,3] up to 2030 are the serious problems related to the development of technologies intended for mass production of micro and macrosized crystalline graphene sheets, which is complicated by the high cost of this material . The implementation of the LP technology is more successful. The active efforts of numerous chemist teams solved the problem of mass production of the required technological material, namely, technical graphene. This material is the end product of a complex redox technological cycle, involving fragmentation of graphite to nanoparticles followed with the particle oxidation and formation of graphene oxide (GO) and completed with the GO reduction. In all the cases, structural analysis demonstrates well-pronounced non-flatness of GO molecules and almost entire restoration of the flatness of the basal plane of reduced graphene oxides (RGO). Therefore, RGO is mentioned as graphene in many works. However, in contrast to the technological materials used to date (which are usually rigorously standardized in chemical composition and structure), the standardization of technological graphene seems to be impossible, since this term covers a very wide set of substances, which represents various oxyhydride polyderivatives of graphene nano- and micro-molecular sheets and-or-molecules. All the substances of this class are characterized by the flatness of their carbon skeletons but differ by the chemical groups that terminate dangling bonds along their perimeter [2,5]. Evidently, the structure and chemical composition of RGO can change at each of the three stages of chemical synthesis mentioned above. The latter results in many versions of the chemical composition as well as shape and structure of synthesized RGOs, which is being actively discussed . For example, the residual oxygen concentration, which is a very important parameter of the material, can differ 20 times in different productions.
Reasons for an intrinsic variability of RGO
Graphene chemistry drastically differs from the conventional molecular one and presents a very large and complicated domain related to substances with spatially distributed targets (see review  and references therein). Nevertheless, despite a great variety, morphologically, graphene-based (derivative) molecules can be divided into three groups: (i) verily graphene molecules (VGMs) presenting pieces of flat honeycomb sheets with non-saturated dangling bonds of edge atoms; (ii) framed graphene molecules that are the above VGMs with saturated dangling bonds in the circumference area (FGMs or CFGMs); and bulk graphene molecules (BGMs) with chemical addends enveloping the whole body of the carbon skeleton. Particular examples of these three groups can be easily found among the extended collection of graphene chemicals [6,8-10]. The first group should be attributed to the pristine molecules while two other are related to the VGMs polyderivatives. Evidently, the division is quite expected and just reveals the unique two-zone feature of the chemical activity of pristine VGMs that governs the formation of any of their derivatives [2,5,7].
While VGMs and BGMs are completely different, including both chemical compositions and the carbon skeleton structure, FGMs show not only the difference with the two groups, but a commonality as well. With respect to VGMs, the FGMs conserve chemically untouched basal plane that even though disturbed maintains some flatness thus keeping graphene-like style. On the other hand, a polyderivative origin joins FGMs and BGMs, which makes them both to be different from VGMs.
Accordingly to this three-group division, the available graphene materials are evidently of three categories. Materials of the first category present graphene crystals in the form of macro- micrometer-size perfect one-atom thick sheets and can be attributed to the VGM group. Actually, real graphene sheets are FGMs however their large size and relatively small number of edge carbon atoms allow for neglecting the frame influence since the main actions concerning the material occur far from the sheet edges. Materials of the second category are related to ‘technical graphene’. Nowadays they, originally produced from GOs, are presented by highly variable RGOs and should be attributed to micrometer- and/or nanometer-size FGMs. Since FGM formation is always connected with edge carbon atoms, each of which are characterized by high chemical reactivity of more than 1 e per atom [2,5,7], the material stabilization under ambient conditions imperatively requires the activity inhibition due to which framing of edge atoms by chemical addends must be completed, not leaving even a single edge atom not attached. There are some nuances related to if one- or two-valence addend is attached to the relevant carbon atoms. The issue was considered in [5, 11] on the examples of graphene oxidation and hydrogenation thus explaining a large variety of produced RGO products with respect to the chemical composition of their framing. Micrometer- and/or nanometer-size sheets of graphene oxides, hydrides, fluorides, and so forth present materials of the third category and can be marked as ‘modified graphene’. Since they represent BGMs, the extent of the pristine VGMs modification in this case is high. It must exceed the saturation level characteristic to FGMs but should not be imperatively completed, once possessing a large variety./p>
Besides the names, the three graphene materials drastically differ by their behavior because of deeply rooted discrepancy between them. Thus, the transformation of perfect graphene into materials of categories 2 and 3 is well mastered and developed using a large spectrum of different chemical technologies but absolutely irreversible. This is mainly due to the loss of the integrity of pristine graphene sheets caused by their cracking under chemical treatment. This is one of more other reasons why any device made of perfect graphene must be reliably protected from the environmental chemical attacks. As for technical and modified graphenes, they are much more resistant and even a reversible transformation between them is partially possible. However, each cycle of such transformation is followed by reducing size of the relevant sheets. Energy-lite deposition on and/or removing of chemicals from the basal atoms of the carbon skeleton greatly favors the action to be performed.
Since the reduction of massively produced GOs is the main way of producing technical graphene (RGO) now, we shall begin the RGO consideration by looking the inherent connection between GO and RGO from the points of the molecular theory of graphene. Starting the heralded consideration, let us try to answer the following questions:
1. Is it possible to liberate any GO from oxygen containing groups (OCGs) completely and if not, to what extent?
2. Is it possible to return a planar honeycomb structure to drastically deformed and curved carbon skeleton structure of GOs?
Obviously, RGO chemical composition and morphology are tightly connected with those of pristine GO ones thus relating to the latter when removing all OCGs from the sheet basal plane. On this background, let us address the (5, 5) GO molecule (GO X) described in details in  and reproduced in Figure 1a. It was computationally synthesized in the course of the stepwise oxidation of the pristine (5, 5) nanographene (NGr) molecule in the presence of three oxidants, such as O, OH, and COOH. Since the removing of each of these oxidants as well as their attachment are characterized by the same coupling energy, it is evident that the per-step coupling energy (PCE) distribution presented in Figure 1b, describes not only the attachment of the groups to either basal plane atoms (curves 1 and 2 in Figure 1b) or edge atoms in the circumference area (curve 3) on the way from the pristine (5, 5) NGr molecule to GO X, but removing the oxidants from the latter towards (5, 5) RGO. As evidences from the Figure, since the PCE values are very different, the reduction should be obviously expected as multistage or multimode one. Actually, the oxygen atoms located in the basal plane of GO X molecule and forming mainly epoxy groups with carbon atoms (within the rose shading) should be removed first. The corresponding (5, 5) RGO molecule is shown in Figure 1c. This apparently happens at the first stage of the real reduction and may present the final state of the reduction procedure when the latter is either short-time or not very efficient, once to be attributed to a mild one. The corresponding mass content of the obtained (5,5) RGO molecule is given in Table 1.
When the reduction occurs during long time or under action of strong reductants, it may concern OCGs located at the RGO circumference. Such two-step reduction of a pristine GO has been convincingly fixed in practice . The second-step reduction faces the following peculiarities. First, due to a waving character of the PCE dependence with large amplitude from -90 kcal/mole to -170 kcal/mole, the second step reduction could be highly variable due to the fact that in practice the applied reduction protocol usually concerns the liberation of atoms with coupling energy that is restricted by the protocol conditions. Thus, restricting the energy interval to 30 kcal/mol (removing oxidants covered by blue shading in Figure 1b) results in remaining only 9 oxygen atoms instead of 22 in the first (5, 5) RGO sheet shown in Figure 1c. Second, since the liberation concerns edge carbon atoms, a release of one of them from oxygen or hydroxyl makes the atom highly chemically active, which imperatively requires the inhibition of its chemical activity. This work can be done by hydrogen atoms thus transforming the reduction into deoxygenation/hydrogenation procedure. The suggestion is well consistent with, first, a scrupulous analysis of C:O content of differently produced RGOs [6,9] which convincingly evidences a strong dependence of the ratio from the reduction protocol in use, and, second, a pronounced hydrogen content detected in real RGO samples .
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