Revealing the Mechanism of Graphene Oxide Reduction by

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Revealing the Mechanism of Graphene Oxide Reduction by Supercritical Ethanol with First-Principles Calculations Jin-Song Kim,† Chol-Jun Yu,*,† Kum-Chol Ri,† Song-Hyok Choe,† and Jin-Chon Ri‡ Chair of Computational Materials Design, Faculty of Materials Science, and ‡Faculty of Chemistry, Kim Il Sung University, Ryongnam-Dong, Taesong District, PO BOX 76, Pyongyang, Democratic People’s Republic of Korea

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ABSTRACT: Recently, graphene and graphene oxide (GO) have become renowned for versatile applications in cleanenergy technology, motivating extensive research for a largescale production of graphene by GO chemical reduction. In this work, we investigate the mechanism of GO reduction by the supercritical liquid of ethanol using first-principles calculations. The supercell and cluster models for graphene sheet with an epoxy group are built, and the pseudopotential plane-wave and Gaussian-type atomic orbital methods are applied to the supercell and cluster models, respectively. After careful identification of intermediate states for GO + CH3CH2OH complexes along three different routes, we determine the reaction pathways and activation barriers, revealing which route has the fastest reaction velocity. We calculate the reaction enthalpies, demonstrating that the hydrogen donations from ethanol are endothermic reactions, but the GO reductions by ethanol accompanied with its hydrogen donation are exothermic reactions. We further highlight the solvent and entropy effects, which can significantly enhance the GO reduction, especially above the supercritical temperature. Our work may reveal the mechanism of GO reduction with supercritical alcohol and also contribute to opening an alternative way of graphene synthesis.

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production of GO.19 As an important intermediate phase between graphite and graphene, GO includes various oxygencontaining functional groups (epoxy, hydroxyl, carbonyl, and carboxyl), which make GO electrically insulating because of broken sp2 bonding network and a variety of defects on one hand and make it easy for chemical functionalization on the other hand.20 The electrical conductivity can be (partially) recovered by restoring the π-network through the reduction of GO, which is also regarded as the most important functionalization of graphene.21−23 Moreover, it has been reported in recent years that, when controlling the reduction degree, reduced GO (rGO) exhibits enhanced in-plane proton conductivity absent in graphene by the help of epoxy groups (hydrophilic agents) and water molecules attached to them, which is an essential property for electrode materials and solidstate electrolytes for energy-storage and generating devices.6−9 The central aim of GO reduction is to remove the oxygen radicals and structural defects/holes and thus restore the πconjugation of graphene sheet. To date, many approaches have been developed for reducing GO, which can be grouped into two categories; chemical reduction24−34 and thermal reduction.35−37 With regard to mass production and cost effectiveness, the former methods are preferable to the latter because most of the chemical reduction methods are simple to

INTRODUCTION Graphene, graphene oxide (GO), and their complexes draw a great deal of attention for various applications in the field of clean-energy technology.1,2 In fact, they can be used as electrode materials in lithium- or sodium-ion batteries,3−5 solid electrolyte in fuel cells,6−9 and electron or hole conductor in photovoltaic solar cells.10 This is due to their unique material properties such as extremely large surface area, high intrinsic electronic and ionic mobility, high stability, and environmental friendliness.11−14 That is why a number of methods to produce graphene have been developed so far. In particular, the exfoliation of graphite has been established as the most promising method, which utilizes different ways to split graphite into graphene sheets that are bound to each other through the weak van der Waals (vdW) interaction. Typically, micromechanical exfoliation, peeling off graphite by one layer using the so-called “Scotch tape”,15,16 can produce graphene of higher quality, but the output is lower and the production cost is higher. Such a method is beneficial for micro- and nanoelectronics but unsuitable for energy applications that require a massive quantity of graphene. To achieve a large-scale production of graphene with an affordable synthesis cost, liquid exfoliation method has been developed.17,18 In the liquid-phase exfoliation, graphite is oxidized using a strong oxidant (e.g., H2SO4, HNO3, KMnO4, and KClO3) into graphite oxide, which is subsequently exfoliated in a polar organic solvent or aqueous media under moderate ultrasonication or mechanical stirring, leading to a © XXXX American Chemical Society

Received: December 21, 2018 Revised: February 12, 2019

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DOI: 10.1021/acs.jpcc.8b12330 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

byproducts yielded after the reduction, they have not been fully verified by either further experiment or theory. Therefore, it is essential to study the reaction mechanism of GO reduction by ethanol on the atomic scale. In this work, we aim to clarify the mechanism of GO chemical reduction by ethanol with first-principles DFT calculations. We make two kinds of atomistic modeling for graphene sheet with one epoxy group, supercell and cluster models, and the plane-wave (PW) and atomic orbital (AO) methods are applied to the former and latter models, respectively. The reaction pathways and the corresponding activation energies along three different routes are calculated after a careful identification of intermediate states (MS) in every route. We estimate the reaction enthalpies, for which the solvent and entropy effects are considered to take into account the supercritical condition. The atomic charge analysis is performed to get an insight of charge transfer and chemical bonding.

perform, and the cost of the reducing agents is relatively low, whereas thermal reduction requires heating to higher temperature. In these chemical methods, GO can be reduced using a variety of reductants in a liquid phase, resulting in rGO with a tunable reduction degree (measured by C/O ratio). The reducing agents for GO reduction developed so far can be divided into two groups: (1) strong reductants and (2) “green” reductants. The strong reducing agents include hydrazine (N2H4),27,28,36 sodium borohydride (NaBH4),38 and hydriodic acid with acetic acid (HI−AcOH).33 Although high reduction degrees with a C/O ratio from 8 to 15 and high electronic conductivity of about 50−300 S/cm were achieved using these strong reductants, they have critical problems of highly toxicity or carcinogenicity, explosiveness, and high cost. On the contrary, green reducing agents are free of toxicity, which include sodium/potassium hydroxide (NaOH/KOH),26 supercritical alcohol,24,39 phenol,31 vitamin C and amino acid,29,30 hydroxylamine,32 and phenylene diamine.34 In spite of problems such as insufficient reduction and relatively long reaction time, the green chemical reduction is recognized to be more desirable. In particular, the supercritical alcohol route has been demonstrated to produce high reduction degree and to be very fast in addition to green properties of nontoxicity and low cost.24,39 However, a detailed mechanism of reduction, including the role of supercritical alcohol and reaction paths with their corresponding activation energies, is yet to be cleared. As in other applications of materials science, first-principles modeling and simulation based on the density functional theory (DFT) play a crucial role in elucidating the mechanisms of material processes of graphene-related materials.40−42 In that respect, there have been numerous DFT works to make clear the chemical reduction mechanisms of GO by hydrazine,43,44 sulfur compounds,45 calcium,46 and ethanol.47 Although Gong et al.47 reported the combined work of experiment and theory emphasizing the critical role of etch holes in rGO for the reduction and graphitization of GO by alcohol (methanol and ethanol) during thermal annealing, their work did not give the answer to how the epoxy or hydroxyl groups on the GO basal plane can be removed by supercritical alcohol. On the other hand, Seo et al.24 performed the systematic experimental research work to evaluate the effect of different supercritical alcohols on the reduction of GO, finding the superior reduction ability of supercritical ethanol and suggesting the de-epoxidation mechanism with plausible three routes as follows

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COMPUTATIONAL METHODS Supercell and Cluster Models. We first made an atomistic modeling of the GO sheet to carry out first-principles calculations for finding out a viable route of GO reduction by using ethanol as a reductant. Although several atomistic models of GO have been proposed based on the analysis of C/ O ratio, functional groups, and surface defects of experimentally synthesized GO, we here considered only a monolayer graphene sheet containing one oxygen atom (i.e., one epoxide group) in relation to the aforementioned main task of this work. With respect to the modeling of graphene sheet, two different scenarios are possible: (1) two-dimensional infinite plane, in which a supercell including a surface unit cell and a vacuum layer with a certain thickness should be used to hire the three-dimensional (3D) periodic boundary conditions44,46,48,49 and (2) finite-sized graphene sheet, of which the edge carbon atoms are chemically saturated with H atoms, forming a kind of polycyclic aromatic hydrocarbon.41,45 The choice of an atomistic modeling of graphene depends on the DFT method adopted in this work; the PW method is more adequate to use the periodic supercell model, whereas the AO method is more appropriate for the finite model and moreover for taking into account the solvent effect. In this work, we adopted both of the models of graphene sheet to assure the accuracy of calculations by comparing the results. Figure 1 shows the atomistic structures of a graphene sheet with an epoxide group in two different ways of modeling. For the first case of supercell model of graphene sheet, an orthogonal (6 × 3) cell containing 72 carbon atoms was used as the surface unit cell, and one oxygen atom was attached at the center of the cell to form an epoxide group, giving the chemical formula of C72O, as shown in Figure 1a. The optimized lateral lattice constant of 2.46 Å and the vacuum layer of 15 Å thickness were used to construct the 3D supercells and guarantee the negligibility of the artificial interaction between the neighboring graphene sheets, as already used in our previous work.9 For an isolated ethanol molecule, the supercell with a cubic phase and a lattice constant of 15 Å was used to simulate its optimal atomistic structure and calculate the DFT total energy. As shown in Figure 1b, meanwhile, the rhombic cluster for polycyclic aromatic hydrocarbon with a chemical formula of C48H18O was built for the finite-sized graphene modeling.

GO + CH3CH 2OH → G−2OH + CH 2 2H+ CH 2( ↑ ) ⎯⎯⎯⎯⎯→ G−2H 2O+ + 2e → G + 2H 2O

(1)

GO + CH3CH 2OH → HO−G−OCH 2CH3 → G + H 2O + CH3CHO

(2)

GO + CH3CH 2OH → HO−G−H + CH3CH 2O( ↑ ) → G + H 2O

(3)

where G means graphene. Although these routes were proposed based on the experimental characterization of B


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distance between neighboring NEB images was less than 1 Å. We used both the image energies and their derivatives to gain the interpolated path energy profile, as implemented in NEB code included in the QE package. Gaussian Calculations for Cluster Models. To perform the DFT calculations of C48H18O polycyclic aromatic hydrocarbons with a consideration of the solvent effect, we employed the all-electron AO method using the Northwest Computational Chemistry Package (NWChem) 6.3.56 The Gaussiantype AO basis sets of 6-31g**(d, p) were adopted for all of the elements, and the B3LYP hybrid functional57,58 was used for the XC interaction. We applied the conductor-like screening model (COSMO)59 approach as implemented in NWChem 6.3 to take into account the solvent effect with the experimentally measured dielectric constant of ethanol at room temperature as 24.3.60,61 For self-consistent field calculations, the convergence criteria of 10−7 atomic unit (a.u.) and 10−5 a.u. were used for the total energy and electronic density, respectively. To perform geometry optimization, the DRIVER module was selected, in which the convergence criteria was set to “tight” (10−7 a.u.). For numerical integration of the XC potential, the grid parameter was set to “fine” (10−7 a.u.). We also carry out the vibrational frequency calculations to evaluate the reaction-free energies of three routes at a supercritical temperature of 514 K. Also, the NEB method was used to evaluate the reaction pathways and the corresponding activation energies with the same computational parameters to the PW method.

Figure 1. Ball-and-stick view of (a) orthogonal (6 × 3) cell of graphene with one epoxide group (C72O) and (b) polycyclic aromatic hydrocarbon for graphene with one epoxide group (C48H18O). C−O bond lengths are determined to be 1.50 and 1.46 Å in C72O and C48H18O, respectively.

PW Calculations for Supercell Models. For the 3D periodic supercell models, the DFT calculations were performed with the pseudopotential PW method using the QUANTUM ESPRESSO package (QE) 6.2.50 The Vanderbilttype ultrasoft pseudopotentials for all of the elements were used as provided in the package to describe the interaction between ions and valence electrons. For the exchange− correlation (XC) interaction among valence electrons, the Perdew−Burke−Ernzerhof (PBE) functional51 within the generalized gradient approximation (GGA) was used. In addition, the weak vdW interaction energy between the graphene sheet and organic molecules was added to the PBE−GGA total energy by applying the density functional OB86 method (vdw-df-ob86).52 By performing the convergence test, the PW kinetic cutoff energies were selected to be 40 Ry for the wave function and 400 Ry for the electron density, and the Monkhorst−Pack special k-points53 were set to be (2 × 2 × 1), ensuring the total energy convergence of 5 meV per carbon atom. The self-consistency was achieved by using the Methfessel−Paxton first-order smearing approach54 with a 0.02 Ry spreading factor and a 10−9 Ry total energy threshold. In the atomic relaxations, all of the atomic positions were relaxed until residual forces on atoms became less than 0.02 eV/Å. The reaction pathways and activation barriers were determined by using the nudged elastic band (NEB) method55 with an appropriate number of intermediate images so that the

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RESULTS AND DISCUSSION Physical Adsorption of Ethanol on GO. We first performed atomic relaxations of two GO models, that is, orthogonal (6 × 3) supercell (C72O) and polycyclic aromatic hydrocarbon cluster (C48H18O), by using PW and AO methods. Figure 1 shows their optimized structures, where the C−O bond lengths were calculated to be 1.50 and 1.46 Å in C72O supercell and C48H18O cluster, respectively. Then, supercell models with various configurations were constructed for the GO complex with an ethanol molecule adsorbed physically on the GO sheet, and their structures were optimized by performing full atomic relaxations. The lowest energy configuration of GO + ethanol complex was determined by comparing the total energies of these different configurations, as shown in Figure 2. Using the lowest energy configuration determined by the PW method with the PBE + vdW-DF functional as the starting

Figure 2. Various configurations for GO + ethanol complex in the supercell model (C72O + C2H5OH), with their total energy differences with reference to the lowest energy configuration, calculated by the PW method with PBE + vdW-DF functional. C


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bond with the H−O bond lengths of 1.88 and 1.87 Å in the C72O + C2H5OH supercell and the C48H18O + C2H5OH cluster models. Because of the attraction of the adsorbed ethanol molecule, the bond lengths between the C atom of graphene and O atom of epoxide group were confirmed to be slightly stretched like from 1.50 to 1.51 Å in the supercell and from 1.46 to 1.54 Å in the cluster. With regard to the morphology, the ethanol molecular plane containing the C− C−O backbone was found to be almost parallel to the graphene sheet in two models. Using the DFT total energies of GO + ethanol complex (EGO+eth), the GO sheet (EGO), and the isolated ethanol molecule (Eeth), the following formula was

configuration (F configuration in Figure 2), the AO (6-31g** basis sets) method with the B3LYP hybrid functional was applied to obtain the optimized structure for the C48H18O cluster model. Figure 3 shows the optimized structures of GO

Ead = EGO + eth − (EGO + Eeth)

(4)

where the adsorption energies (or binding energies) were calculated to be 0.96 and 0.68 eV (when considered COSMO, 0.65 eV) in the supercell and cluster models. Structures and Activation Energies during GO Reduction. Route 1 in a Supercell Model. The optimized structure of GO complex with a physically adsorbed ethanol molecule was considered as that of reactants in initial state (IS). We constructed the supercells for products in MS and final state (FS) and performed the atomic relaxations to get the optimized structures, prior to NEB calculations. As can be seen in eq 1, the MS state in Route 1 is characterized by two

Figure 3. Optimized structures of GO + ethanol complex in (a) supercell (C72O−C2H5OH) and (b) cluster (C48H18O−C2H5OH) models. Hydrogen H−O and C−O bond lengths in Å unit are shown.

+ ethanol complexes in the supercell and cluster models with the lowest energy configuration. The ethanol molecule was found to be bound to the epoxide group of GO by hydrogen

Figure 4. (a) Optimized supercell structures of IS, MS, FS, and transition state (TS) between them along the ortho-pathway (upper line) and the para-pathway (lower line) in Route 1 (eq 1), together with relevant bond lengths in Å unit, obtained by PW method with PBE + vdW-DF functional. The moving hydrogen atom is denoted. The total energy difference along the path for (b) IS → MS and (c) MS → FS reactions. D


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Figure 5. (a) Optimized supercell structures along the ortho-pathway (upper line) and para-pathway (lower line) in Route 2 (eq 2), together with relevant bond lengths in Å unit, obtained by PW method with PBE + vdW-DF functional. The moving hydrogen atom is denoted as H1. Total energy difference along the path for (b) IS → MS and (c) MS → FS reactions.

ethylene molecule C2H4 was found to be weakly bound to the GO−2OH complex. In the next reaction step MS → FS, the ethylene molecule was removed, and the GO−2OH complex was converted by the addition of two hydrogen atom into the final products, graphene and two water molecules. As can be seen in Figure 4a, the two water molecules in the FS were found to be sufficiently far away from the graphene sheet (3.23 Å), indicating a real separation of graphene sheet and water molecules. The activation barriers for this reaction were determined to be 0.93 and 0.65 eV for ortho- and parapathways, respectively, (Figure 4c). For the whole reaction, the para-pathway had the activation energy of 1.59 eV, which is smaller than that for the ortho-pathway (1.81 eV), indicating that the para-pathway is more favorable for GO reduction in Route 1. Route 2 in Supercell Model. This route follows the mechanism proposed by Ross and Blessing,62 in which ethanol molecule can donate a proton from its OH group to the GO sheet and thus convert it into the alkoxyl group (C2H5O). As shown in eq 2, therefore, the MS state in Route 2 contains one hydroxyl group and one alkoxyl group that can be located on the ortho- or meta- or para-position, and two different pathways (ortho- and para-pathways, excluding the metaposition) were also constructed for the NEB calculations. Figure 5a shows the optimized supercell structures in the IS, MS, TS, and FS along the ortho- and para-pathways in Route 2.

hydroxyl groups bound to the graphene sheet and a desorbed ethylene molecule (C2H4). If one hydroxyl group is assumed to be located on the C1 position, another hydroxyl group can be bound to the C atom in the ortho-position (C2 or C6) or meta-position (C3 or C5) or para-position (C4) (see Figure 3a for the C1−C6 positions). As already revealed for the polycyclic hydrocarbon cluster model in the previous work,45 the meta-position was confirmed to be energetically much higher than the ortho- and para-positions, and thus, it was not considered any more in this work. Therefore, we constructed two different pathways from the reactants in the IS (GO + ethanol) to the products in the FS (G + 2H2O) through the MSs (G−2OH) in ortho- and para-positions, namely the ortho-pathway and the para-pathway, as shown in Figure 4. In these pathways, GO reduction can be divided into two steps; (1) reactant → intermediate (IS → MS) and (2) intermediate → product (MS → FS). For every reaction step, the NEB calculations were carried out to determine the TS and the activation barrier. In the first reaction step, the ethanol molecule can be dehydrated, CH3CH2OH → C2H4 + H+ + OH−, and the proton can be transferred to the epoxide group, whereas the hydroxyl group can be bound to the C atom of the GO sheet, producing the GO−2OH complex, of which the activation barriers were estimated to be 1.81 and 0.94 eV in the ortho- and para-pathways, respectively (Figure 4b). It should be noted that the GO−2OH complex in the para-position is energetically 0.31 eV lower than that in the ortho-position, which is consistent with the previous DFT result.45 The E


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Figure 6. (a) Optimized supercell structures along the reaction pathways in Route 3 (eq 3), obtained by PW method with PBE + vdW-DF functional. The total energy difference along the path for (b) IS → G−OH or MS state of H−G−OH, and (c) MS → FS reactions.

In the first-step reaction IS → MS, the proton was transferred from the ethanol to the epoxy group of GO, yielding the intermediate product HO−G−OCH2CH3, for which the activation energies were estimated to be 0.56 and 0.44 eV along the ortho- and para-pathways, respectively (Figure 5b). In the MS product HO−G−OCH2CH3, the C−O bond lengths between the hydroxyl OH group and graphene were found to be 1.49 Å in both of the positions, whereas those between the alkoxyl CH3CH2O group and graphene were to be 1.48 and 1.57 Å in the ortho- and para-positions, respectively. As in the case of Route 1, the MS state in the para-position was found to be energetically 0.14 eV lower than that in the orthoposition. In the following reaction step MS → FS, one hydrogen atom of the alkoxyl group could be transferred to the hydroxyl group, resulting in the formation of final products, graphene, acetaldehyde, and water molecules, (G + CH3CHO + H2O). As shown in Figure 5a, the alkoxyl and hydroxyl groups were separated (3.09 and 3.17 Å away) from the graphene sheet in the TS structures, accompanied with the hydrogen transfer. The activation energies were determined to be 0.46 and 0.45 eV for the ortho- and para-pathways (Figure 5c). In the final products G + CH3CHO + H2O, the acetaldehyde and water molecules were found to be 3.07 Å away from the graphene sheet. For the whole reactions MS → FS, the para-pathway has lower activation energy (0.45 eV) than the ortho-pathway (0.56 eV), indicating that the parapathway is more beneficial for GO reduction than the orthopathway. Route 3 in the Supercell Model. According to the suggested mechanism,24 as shown in eq 3, the ethanol molecule can be converted into the alkoxyl group by donating a proton to the epoxy group of GO sheet in Route 3. This reaction is similar to the first-step reaction IS → MS in Route 2, but the alkoxyl group in Route 3 goes away from the G−OH sheet unlike in

Route 2. As shown in Figure 6a,b, the activation barrier for this reaction, GO + CH3CH2OH (ethanol) → G−OH + CH3CH2O (alkoxyl group), was determined to be 1.52 eV by the NEB calculation, where the intermediate structure of G−OH···OC2H5 had the lowest energy. The resulting product of G−OH can accept a hydrogen atom, producing the MS state of H−G−OH, for which the reaction was found to occur exothermically; using the DFT total energies of H−G−OH complex (EH−G−OH), G−OH complex (EG−OH), and isolated H atom (EH), the following formula is Ead = E H−G−OH − (EG−OH + E H)

(5)

where the adsorption energies were calculated to be −1.74 and −1.62 eV for the H adsorption on the ortho- and parapositions, respectively. That is, unlike the MS states in Route 1 (G−2OH) and Route 2 (OH−G−C2H5O), the ortho-position structure was found to be 0.12 eV lower than the para-position. The bond lengths of C−H and C−O(H) were determined by optimization to be 1.12 (1.13) and 1.50 (1.52) Å in the ortho (para) position structure. When adding the H atom, the reactions from G−OH + H to H−G−OH in MS occurred with the reaction energies of −1.74 and −1.62 eV for the ortho- and para-pathways. For the next-step reaction from the H−G−OH complex in MS to the final products of graphene and water, MS → FS, we assumed three different pathways, that is, ortho- and parapathways as in the above considerations, and another pathway in which the H atom can directly move to the hydroxyl group of GO, resulting in the dissociation of OH and the formation of H2O, namely the OH-dissociation pathway. As shown in Figure 6c, the activation energies were determined to be 0.55, 0.39, and 0.32 eV for the ortho-, para-, and OH dissociation pathways. For the whole reaction from IS → FS, the activation barriers can be said to be 1.52 eV for the ortho- and paraF


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Figure 7. (a) Optimized cluster structures along the ortho-pathways in the three routes, obtained by the AO (6-31g**) method with the B3LYP functional. The total energy difference along the path for (b) IS → MS and (c) MS → FS reactions.

reactions were calculated to be 3.11 eV (Route 1), 1.06 eV (Route 1), and 1.90 eV (Route 3) (Figure 7b), and the MS → FS reactions could take place without the activation barrier (exothermic reaction) in Route 1 (exothermic energy of −8.94 eV) and Route 3 (−2.56 eV), whereas in Route 2, the activation energy was calculated to be 2.35 eV (Figure 7c). Therefore, for the whole reaction IS → FS, the activation energies can be said to be 3.11 eV in Route 1, 2.35 eV in Route 2, and 1.90 eV in Route 3, respectively. Table 1 summarizes the activation energies along the ortho-, para-, and optionally dissociative pathways for the GO reductions, IS → MS, MS → FS, and IS → FS, using the supercell and cluster models. It was found that for the supercell models, the para-pathway in Route 2 had the lowest activation energy (0.45 eV) by PW calculations, whereas for the cluster models, the ortho-pathway in Route 3 had the lowest one (1.90 eV) by AO calculations. To clarify the reason for such different results, we enumerated different points between two methods; (1) basis set: PW versus AO, (2) model: supercell versus cluster, (3) solvent effect: no consider versus COSMO,

pathways, and 1.84 eV for the OH dissociative pathway. Therefore, it can be concluded that the ortho- or para-pathway can be accepted as a plausible reaction pathway for the GO reduction in Route 3. Cluster Models. We then performed Gaussian-type AO (631g**) calculations using the polycyclic aromatic hydrocarbon cluster models, taking into account the solvent effect with the COSMO approach. The same scenario discussed above was applied to this task, and the most energetic efficient paths were selected for three different routes; the atomic structures in the IS, MS, TS, and FS with the ortho- and para-configurations were optimized by performing atomic relaxations in consideration of COSMO, and then the NEB calculations were carried out. Figure 7 shows the optimized cluster structures in these states along the lowest energy profile in the three routes. It was found that unlike the supercell models where the parapathways had the lowest activation barriers, the orthopathways could have the most plausible activation barriers in the cluster models when considering the solvent effect. Along these reaction pathways, the activation energies for IS → MS G


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enthalpies were calculated to be positive, indicating that the hydrogen donation from ethanol is endothermic. It was found that for the cases of α-hydrogen donation, the reaction enthalpies in consideration of the solvent effect with the COSMO approach were lower than that in the gas phase, indicating that the hydrogen donation ability of ethanol can be enhanced in solution phase. When compared, the reaction enthalpies according to the mechanisms between Nakagawa et al.64 and Ross and Blessing,62 the former can occur more readily than the latter because of the lower reaction enthalpy. For the case of dehydration of ethanol, although the reaction enthalpy at room temperature for CH3CH2+ → C2H4 + H+ in solution phase (8.93 eV) was higher than that in the gas phase (6.10 eV), the total reaction enthalpy for CH3CH2OH → C2H4 + H+ in solution phase (14.91 eV) was again lower than that in the gas phase (18.78 eV). It is worth noting that when the GO sheet participates in these hydrogen donation reaction, the reaction enthalpy can be significantly reduced because of the catalysis of GO sheet, as will be discussed below. We calculated the reaction energies for GO reduction by using supercritical ethanol accompanied with the hydrogen donation of ethanol itself, as listed in Table 3. Figure 8 shows a visual summary of the reaction energetics. It was found that the GO reductions in the suggested three routes are exothermic processes with both the PW and AO calculations though their activation barriers, as discussed above. From the viewpoint of ethanol, the hydrogen donation reactions switch from endothermic (Table 2) to exothermic because of the catalysis of GO sheet. The internal reaction energy (ΔE) calculated by the PW method decreases in magnitude on the order of Route 1 > Route 3 > Route 2. Although for the case of AO calculation with the gas phase, the reaction enthalpy (ΔHgas) decreases on the different order of Route 1 > Route 2 > Route 3, considering the solvent effect (ΔHsol) and the temperature effect (ΔG) at 514 K for supercritical ethanol24 recovering the same decreasing order. Considering that the increased temperature up to 673 K in the real synthesis can sufficiently overcome the aforementioned activation barriers, it can be concluded that Route 1 with the largest Gibbs reaction energy is most likely to occur, which is in agreement with the experimental evidence of a large yield of ethylene byproduct.24 Finally, we performed the Mulliken population analysis to get an insight of the electronic charge transfer and chemical bond during GO reduction. Figure 9 shows the atomic charges of GO complexes around the important part of reactions along three different routes, calculated by the AO method. For the IS → TS process in Route 1, GO + CH3CH2OH → G−OH + C2H4 + OH− (Figure 9a), the atomic charge −0.37|e| of −C− O−C− in IS decreases to −0.17|e| of −C−OH, indicating that the electron is transferred from the epoxy group of GO sheet to the ethanol molecule, producing negatively charged OH−

Table 1. Activation Energies along the Ortho-, Para- and Dissociative Pathways for GO Reduction by Ethanol with the IS, MS, and FS, Determined by NEB Calculations Using the Supercell (SC) Models with PW Basis Sets and PBE + vdW-DF Functional and the Cluster (CL) Models with AO (6-31g**) Basis Sets and B3LYP Functional (Unit: eV) Route 1

Route 2

Route 3

model

pathway

IS → MS

MS → FS

IS → FS

SC SC CL SC SC CL SC SC SC CL

ortho para ortho ortho para ortho ortho para diss ortho

1.81 0.94 3.11 0.56 0.44 1.06 1.52 1.52 1.52 1.90

0.93 0.65

1.81 1.59 3.11 0.56 0.45 2.35 1.52 1.52 1.84 1.90

0.46 0.45 2.35 0.55 0.39 0.32

and (4) XC functional: PBE + vdw-df-ob86 versus B3LYP. All of these different points act synergetically, and among these, the most important point is that the liquid phase was considered indirectly by taking the dielectric constant of ethanol with COSMO approach in the cluster model, whereas the gas phase was used in the supercell model. Therefore, we would prefer the result from the cluster model calculation to the supercell calculation. Reaction Energies and Charge Transfer. To estimate the required or emitted energy during the chemical reaction, we evaluated the reaction energies by calculating the Gibbs free-energy difference between the products (Gpr) and reactants (Gre), using the following formula ΔG = Gpr − Gre = ΔH − T ΔS

(6)

Here, ΔH is the difference of enthalpy calculated as the sum of the DFT ground-state energy and a thermal correction to the enthalpy, and ΔS is the entropy difference calculated by estimating the molecular vibrational frequencies through the Hessian matrix diagonalization with zero-point and thermal corrections to the energy. For reference, the reaction internal energies were also calculated using only the DFT energies by the PW method because of some heavy computational load. We first calculated the reaction enthalpies for hydrogen donation from the ethanol molecule, which is the initial process of GO reduction by supercritical ethanol. In Route 1, the ethanol molecule should be converted into ethylene with a donation of H+ and OH−, namely dehydration of ethanol, whereas in Route 2 and Route 3, the ethanol can donate αhydrogen (H−) together with proton to become an alkoxide, according to the mechanism proposed by Nakagawa et al.64 and Ross and Blessing.62 As shown in Table 2, these reaction

Table 2. Reaction Enthalpies for Hydrogen Donation from Ethanol, Calculated by AO Method with B3LYP Functional in Gas Phase (Gas) and Solution Phase (Sol), Together with Available Experimental Data (Unit: eV) ΔE reaction −

CH3CH2OH → CH3CH2 + OH CH3CH2+ → C2H4 + H+ CH3CH2OH → CH3CHOH+ + H− CH3CH2OH → CH3CH2O− + H+ CH3CH2O− → CH3CHO + H− +

ΔH (298 K)

gas

sol

gas

sol

exp.63

13.98 5.77 11.86 17.47 2.91

7.25 8.60 4.54 14.64 1.09

12.68 6.10 23.43 16.05 4.77

5.98 8.93 20.74 13.22 2.95

10.36 7.03

H

16.23


Article


Table 3. Reaction Energy and Activation Barrier (Ebar) for GO Reduction by Using Supercritical Ethanol in Three Different Routes, Calculated by PW Method with Supercell Model and PBE + vdW-DF Functional, and AO (6-31g**) with Cluster Model and B3LYP Functional (Unit: eV) PW

AO

Route

reaction

ΔE

Ebar

ΔHgas

ΔHsol

ΔG (514 K)

Ebar

1 2 3

GO + CH3CH2OH + 2H → G + 2H2O + C2H4 GO + CH3CH2OH → G + H2O + CH3CHO GO + CH3CH2OH + H− → G + H2O + CH3CH2O−

−6.87 −2.21 −2.52

1.59 0.45 1.52

−8.58 −2.56 −0.66

−9.89 −3.24 −1.96

−11.75 −6.86 −7.01

3.11 2.35 1.90

Figure 9. Mulliken atomic charges of GO complexes in the IS, TS, and MS along (a) Route 1, (b) Route 2, and (c) Route 3, calculated by AO (6-31g**) method with the cluster model and the B3LYP functional. Figure 8. (a) Schematic view of reaction routes for GO reduction by ethanol and (b) Gibbs energy differences corresponding to the routes, calculated by the AO (6-31g**) method with the cluster model and the B3LYP functional.

approach was employed to estimate the reaction pathways and the activation energies in both methods, together with the careful identification of MSs in every route, and the reaction energies and enthalpies were calculated in consideration with the solvent effect with COSMO approach and of finite temperature effect. The activation energies were determined to be 1.59 eV (3.11 eV) for Route 1, GO + CH3CH2OH + 2H → G + 2H2O + C2H4; 0.45 eV (2.35 eV) for Route 2, GO + CH3CH2OH → G + H2O + CH3CHO; and 1.52 eV (1.90 eV) for Route 3, GO + CH3CH2OH + H− → G + H2O + CH3CH2O−, with the PW (AO) method, indicating that Route 2 (Route 3) has the fastest reaction velocity for GO reduction. The calculated reaction enthalpies indicate that the hydrogen donation of ethanol is endothermic, whereas the GO reduction is exothermic, and when considering the solvent and entropy effects, the reaction enthalpies can be significantly reduced. When increasing the temperature up to 514 K (supercritical temperature), the activation barriers can be sufficiently overcome, and thus Route 1 with the largest Gibbs reaction energy can be accepted to be most likely to occur. The Mulliken atomic charge analysis was performed to get an insight of electron and proton transfer during the reactions, revealing that the weak electrostatic attraction between O atom of GO and H atom of ethanol exists in addition to the hydrogen bonding interaction, which can facilitate the surface

(−0.37|e|), whereas the proton is transferred conversely. In the case of MS → FS process in Route 2, HO−G−OCH2CH3 → G−OH− + CH3CHOH+ (Figure 9b), the atomic charges −0.16|e| of −C−OH and −0.13|e| of −C−OCH2CH3 change to −0.31|e| of −C−OH and 0.42|e| of OHCHCH3+, indicating that the electron is transferred from the alkoxyl group to the hydroxyl group of the GO sheet. Meanwhile, for the case of MS1 → MS process in Route 3 (Figure 9c), the atomic charge −0.29|e| of −C−OH changes a little to −0.22|e|, and the atomic charge of H−C− is 0.24, implying the acceptance of proton.

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CONCLUSIONS In this work, we have investigated the reaction pathways and energies and their activation barriers for GO reduction by ethanol in three types of routes using the DFT calculations. For graphene sheet with one epoxy group, the (6 × 3) supercell models (C72O) and the polycyclic aromatic hydrocarbon cluster models (C48H18O) were constructed, and the PW method with the PBE + vdW-DF functional and the AO (6-31g**) method with the B3LYP functional were applied to the supercell and cluster models, respectively. The NEB I


Article


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reaction of GO plane and ethanol molecule mediated by proton or hydroxyl group transfer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chol-Jun Yu: 0000-0001-9523-4325 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported as part of the fundamental research project “Design of Innovative Functional Materials for Energy and Environmental Application” (no. 2016-20) funded by the State Committee of Science and Technology, DPR Korea. Computation was done on the HP Blade System C7000 (HP BL460c) that is owned by the Faculty of Materials Science, Kim Il Sung University.

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