Article pubs.acs.org/JPCC
Topology Influence on the Process of Graphene Functionalization by Epoxy and Hydroxyl Groups Vladislav V. Shunaev and Olga E. Glukhova* Saratov State University, Saratov, 410012 Russia ABSTRACT: For the first time the problem of chemical functionalization of mono- and bilayer graphene by epoxy and hydroxyl groups was considered taking into account a real geometry profile of synthesized carbon objects. Such energy characteristics of graphene oxidization as the enthalpy of reaction, binding energy, and the energy of activation barrier were found by means of mathematical modeling. The application of obtained results may help experimenters to decrease energy costs in the synthesis of materials on the base of functionalized graphene.
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INTRODUCTION From the beginning of the XXI century graphene and its derivatives, porous graphene, graphene with defects, and graphene oxide (GO), are actively studied. It was shown that oxidized graphene can be applied in electronic, spintronic, and optoelectronic devices;1−3 conductive films; electrode materials; and composites.4−6 Synthesis of GO is accompanied by binding of hydroxyl (−OH), carbonyl (CO), dioxide (−2O), epoxy (>O), and carboxyl (−COOH) functional groups to graphene. There are several experimental ways of GO synthesis; the most popular of them are Hummers method,7 oxidation in UHV,8 the application of an O plasma,9 and the application of ozone generated by ultraviolet light in atmosphere.10 However, detailed mechanism of oxidation is not explored experimentally that expands horizons for the theoretical investigation. By DFT method Nasenhia et al.11 established that the energy gap of GO increases with the oxygen atoms concentration. Based on firstprinciples calculations, Cheng et al.12 simulated the process of first atoms’ adsorption on graphene and calculated its binding energies. DFT study13 found that the binding energy of oxygen functional groups on defected graphene is higher by modulus than on pristine one. Nguyen et al.14 considered the effect of tensile deformation on energy features of graphene oxide. In particular, it was established that with the deformation growth the binding energy increases while activation energy decreases. In all above-mentioned papers authors considered just flat graphene though it was experimentally proved that ideally flat graphene does not exist−under external factors it either corrugates15 or folds.16 Moreover, the majority of theoretical works consider graphene’s functionalization just by the epoxy group, though the TEM’s images17 and 13C NMR measurements18,19 show that the concentration of hydroxyl groups in GO has a comparable value. In addition it is proven that epoxy and hydroxyl groups are the only functional groups that can be adsorbed on pure defectless graphene.20 © XXXX American Chemical Society
It is known that mechanical and electron properties of bilayer graphene differ from the monolayer one.21 Experiments have shown that monolayer and bilayer graphene have different energy features of chemical functionalization. However, there are few works devoted to oxidation of bilayer graphene. Lawson et al.22 argues that binding energy of bilayer graphene exceeds the monolayer’s one only on 0.02 eV. But the authors of this paper are focused on the binding energy dependence on the amount of oxygen atoms without estimation of such important energy characteristics as the enthalpy reaction energy and the activation energy barrier. It should be also noted that the problem of graphene’s energy characteristics improvement while oxidization due to the atomic structure modification has not been considered before. The goal of this paper is manipulation of the mono- and bilayer graphene functionalization by epoxy and hydroxyl groups due to the change of the considered objects topology.
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THEORETICAL METHODS The goal was achieved within the framework of the SCC DFTB 2 method 23 which was successfully applied both for investigation of electron and mechanical properties of graphene24 and carbon nanoclusters behavior simulation.25 To take into account the van der Waals interaction we have added the term of dispersion energy to the total energy calculation formula. Dispersion energy was determined within the model of Slater−Kirkwood type.26 Registration of the van der Waals interaction is very important for this study since it determines interaction between graphene layers and allows to find the activation barrier energy. All the numerous experiments were carried out using the original software KVAZAR.27 The Received: December 25, 2015 Revised: February 5, 2016
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DOI: 10.1021/acs.jpcc.5b12616 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
atoms bonded to oxygen atoms but their neighbors of the second order too (Figure 1). The results of calculations show that the major part of activation energy was spent on the geometry change of graphene area located near the attached oxygen group. Some energy characteristics of the functional groups
