Molecular-Level Recognition of Interaction Mechanism between

Feb 5, 2018 - Molecular-Level Recognition of Interaction Mechanism between Graphene Oxides in Solvent Media. Yezi Jin, Zhijun Xu , Yanan Guo, and Xiao...
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Molecular-Level Recognition of Interaction Mechanism between Graphene Oxides in Solvent Media Yezi Jin, Zhijun Xu,* Yanan Guo, and Xiaoning Yang* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China ABSTRACT: The interaction between graphene oxide (GO) in solvent is a fundamental basis in its colloidal suspensible stabilization, which is important in the solution processing technique for the preparation of processable graphene sheets. In this work, the potential of mean force (PMF) between two GO nanosheets in solvents was simulated to quantify the interaction mechanism. It was observed that GO sheets in water with various oxidization levels demonstrate diverse interacting performances. The neutral GO sheets generally show weak attractive interaction with kinetic reversible aggregating/dispersing stability. However, the deprotonated GO sheets with negative charges have strong colloidal stability, which is due to the long-range electrostatic repulsion arising from the charged functional groups. The interaction of GO sheets is a delicate balance of the interacting force between GOs themselves and the corresponding solvation force. The detailed PMF analysis identifies the distinct roles of water in the contribution to GO interactions. For neutral GO sheets, the solvation force provides repulsive action, aiding the GO dispersion. However, for the negatively charged GO sheets, the solvation force contrarily exhibits attractive hydrophilic interaction due to the strong water affinity of deprotonated carboxyl groups. In the nonpolar benzene solvent, the PMF profile displays strong aggregation trend compared with the water solvent. The solvation force in benzene solvent could not afford sufficient repulsive interaction to overcome the attractive interaction between GO sheets. This behavior reflects the specific effect of benzene solvent on functional groups. Our simulation results present new a molecular-level understanding of GO interactions in solvents.

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INTRODUCTION Graphene oxide (GO) due to its superior physicochemical properties has been viewed as promising nanoscale material in many applications.1 In this aspect, the solution manipulation of GO nanosheet can offer scaled-up volume process, as well as convenient chemical modification and transfer, which have attracted significant attention for use in various material preparations and applications.2−6 The fundamental understanding of solution dispersion and stability behavior is important to control these solution manipulation processes. This will improve the design of graphene-based materials and further explore potential applications. In the solution processing of GO-based materials, the colloidal dispersion stability7 of GO nanosheets is critical for determining the performance and quality of final materials. Experiments8−10 have been conducted to characterize the dispersion behavior of GOs suspended in various solvents, including water and organic solvents. It was found that water and some polar solvents could provide stable dispersion of GO sheets, whereas nonpolar solvents generally show low dispersibility for GOs. The pH-dependent stability behavior of GO in aqueous solution has also been investigated both experimentally and theoretically by Shih et al.11 They reported that at low pH, large-scale visible flocculent aggregation occurs, while at high pH condition the GOs prefer to homogeneously dissolve in water. © XXXX American Chemical Society

At present, the efficient use of solvents is still based on the semiempirical rule. For example, it is generally accepted that solubility parameters or surface tension between solvent and GO should be matched11,12 in order to meet the minimization of mixing free energy. This rough semiempirical criterion merely considers the thermodynamic equilibrium solubility. It should be noted that the colloidal stability is largely determined by the interaction between GO nanosheets in solvent media. Classical DLVO (Derjaguin−Landau−Verwey−Overbeek) theory13−15 has long been used to describe the colloidal interaction of charged particles, in which the repulsive electrostatic and attractive vdW interactions cooperatively regulate the colloidal stability. As a mean-field theory, DLVO approach often neglects the molecularity of the contacting molecules in the interfacial region of GO surfaces. Moreover, the function of solvent in the DLVO theory is simply described as the dielectric constant, which is used to screen the electrostatic interaction. As a result, DLVO fails to describe the detailed solvation interaction, which is often critical in compensating the attractive vdW interaction between GO sheets. It has been speculated that the ionization of functional groups on GO sheets occurs very weakly in water under low Received: December 6, 2017 Revised: February 2, 2018 Published: February 5, 2018 A

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Figure 1. (a) The model system used for MD simulations, in which two parallel GO sheets perpendicular to the X-axis were separated with surrounding solvent molecules. (b) Side view of a typical GO sheet. (c) Top view of the GO sheet. Each sphere represents an atom using the following color scheme: white, hydrogen; red, oxygen; cyan, carbon.

under different solution conditions will be helpful to manipulate GO-based solution self-assembly in the formation of GO film20,21 and liquid crystal structures.12,22 With all the above in mind, in this study, molecular dynamics (MD) simulations have been carried out to study the microscopic mechanism of dispersion and stabilization of GO solution by calculating the PMF between two GO sheets. We study the effect of oxidation concentration and functional groups on the PMF profiles, from which the specific role of each functional component can be clarified. The main goal of this work is to develop a comprehensive understanding of the role of solvation force in controlling the colloidal stability for GO suspended in aqueous solution. Meanwhile, for comparison, the PMF in the nonpolar benzene solvent has also been simulated with the purpose of revealing the role of the organic solvent.

pH conditions or in organic solvents. In this case, the electrostatic repulsion between GOs becomes less effective with respect to the solvation force in controlling the GO stabilization. Molecular simulation has been utilized to investigate the thermodynamic interaction between GO sheets in solvent media, from which the mechanism of dispersion and stabilization could be elucidated. Recently, potential of mean force (PMF) was used to quantify the interaction between GO sheets modified with functional groups in aqueous solution.16 However, the established GO model cannot represent actual GO structures because only single functional group on one side of GO surface was considered. PMF profiles have also been simulated to study the stability mechanism of pristine and oxidized nanocarbons, such as carbon nanotubes and graphene nanosheets in various solvents.17 Molecular simulation18 was furthermore performed to study the dynamical aggregation process of GO sheets in water, wherein GO sheets could directly contact with each other to form aggregate. However, a very recent simulation study19 by the same authors with the same models contrarily suggested that GO sheets can aggregate to form water-mediated complexes in water. The confused results might imply GO sheets may agglomerate in multiple forms. Although these available molecular simulations provide valuable insights into the thermodynamics and kinetics behavior for the aggregation or dispersion of GO nanosheets, the stability mechanism of GOs in solvent media remains largely unexplored and further studies are still necessary. For example, under different pH conditions, the contribution of solvent-induced solvation force to the interaction between GO sheets has not been explicitly determined. In particular, at low pH value, the stability and final forms of GO sheets in water are ambiguous. In addition, in order to improve the solution dispersion of GO sheets, it is greatly required to distinguish the effect of individual types of functional groups on the solvation interaction. For the thermodynamic interaction of GO in solvent, what is missing in the aspect is whether or not the GO aggregation only happens in the direct contacting state without confined solvent molecules trapped between GO sheets. For GO sheets with hydrophilic nature, there appears large possibility to form water-mediated hydration agglomerate. Understanding the structure and interaction of GO sheets

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SIMULATION METHOD Models and Potentials. In this work, two parallel 20 Å × 20 Å GO sheets were solvated in the cubic box with the dimensions of 60 × 60 × 60 Å3, and the periodic boundary condition was applied in all three directions, as shown in Figure 1a. The side and top views of GO sheet with functional groups on both sides are presented in Figure 1b,c. The model of GO follows the widely accepted Lerf−Klinowski model, which can represent the typical GO structure.23 For each GO model, hydroxyl and epoxy groups are randomly distributed on the basal plane of GO sheets and carboxyl groups are located at the sheet edges. Various types of GO models were considered here: GO surfaces functionalized only by hydroxyl and epoxy groups with oxidization concentrations 20% (C10O1(OH)1) and 40% (C5O1(OH)1); the addition of carboxyl groups on the edge of GO model (C10O1(OH)1) with oxidization concentration of 30%, including the protonated (C10O1(OH)1(COOH)0.5), partly deprotonated (C10O1(OH)1(COOH)0.25(COO−)0.25), and fully deprotonated (C10O1(OH)1(COO−)0.5). The oxidization concentration is defined as the number of oxygencontaining groups divided by the carbon atoms on graphene sheet, and the choice of oxidization concentration in this work was based on the actual oxidation degrees of GOs in experimental measurements.24,25 The simulated GO models with different oxidization concentrations and functional groups are listed in Table 1. B

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oxidation concn (%)

PMF values were normalized by dividing the surface area of GO sheet (in units of kcal/(mol nm2)). A similar method has been used in the previous research.41

20 40 30 30

RESULTS AND DISCUSSION Figure 2a shows the PMF profiles in water solvent between GO sheets as a function of sheet separation (d). For comparison,

Table 1. GO Models and Their Abbreviations composition

abbrev

C10O1(OH)1 C5O1(OH)1 C10O1(OH)1(COOH)0.5 C10O1(OH)1(COOH)0.25(COO−)0.25

GO1 GO2 GO1-COOH GO1-COOHCOO− GO1-COO−

C10O1(OH)1(COO−)0.5

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30

All the simulations were performed by using the open-source package LAMMPS.26 All the sp2 carbon atoms in GO were frozen11,17 and treated as uncharged Lennard-Jones (L-J) spheres.27 The functional groups on GO were flexible, and the optimized potentials for liquid simulations−all atom (OPLSAA) force field was employed to describe the functional groups.28,29 Because our work is concerned with the effect of functional groups, this rigid treatment of GO skeleton could reduce the noise interference in the PMF simulation, which has been observed in previous work.16 The classical SPC/E model30 was used for water molecules, and the rigid six-site Lennard-Jones potential model31 was utilized for the benzene molecules. For the interaction between unlike particles, the L-J interactions were treated by the Lorentz−Berthelot mixing rule. However, for the L-J interaction between the sp2 carbon atoms and the oxygen atoms of water molecules, the parameters ε = 0.392 kJ mol−1 and σ = 0.319 nm were adopted, which can satisfactorily reproduce experimentally measured water contact angles on graphite surfaces.32,33 The vdW interactions of water and benzene were treated with a cutoff distance of 10 and 15 Å,34 respectively. The particle−particle particle-mesh35 method was applied to compute the long-range electrostatic interaction with a tolerance of 10−5. Simulation Detail. The initial configurations were generated by placing two parallel GO sheets at different intersheet separations (along the z-axis), followed by randomly filling the simulation box with sufficient solvent molecules. The energy minimization was carried out first by the steepest descent method and then followed by 1 ns MD simulation using the NPT ensemble with a time step of 1 fs. The Langevin thermostat36 and Berendsen pressure coupling37 were used to maintain the temperature 300 K and the pressure 1 atm, respectively. Subsequently, all MD simulations were performed in an NVT ensemble at 300 K. The PMF profiles were obtained by the force integrating method38 using the time-averaged force acting on two GO sheets, where the reaction coordinate was defined as the distance between the center of mass (COM) of GO sheets, which varies from 4.5 to 20 Å. To be more specific, the PMF can be calculated using the equation38−40 W (r ) = W (r0) −

∫r

0

Figure 2. (a) Comparison of PMF profiles per unit area (in nm2) between two of GO sheets with different oxidation concentrations (20% GO1 and 40% GO2) and pristine graphene in aqueous solution. (b) Density distribution profiles of water molecules near the GO1 sheets at different COM distances (d).

the result of pristine graphene was also shown. These PMF profiles can be applied to characterize the thermodynamic resistance or tendency against dispersion or aggregation for GO nanosheets in solvent media. As shown in Figure 2a, the PMF profiles display relatively shallow PMF wells42 (2.5−4.5 kcal/ (mol nm2)) at ∼5−6 Å separation in comparison with the deep PMF trap (∼34 kcal/(mol nm2)) at 3.5 Å separation for pristine graphene. The separation distance corresponding to the PMF well (minimum) represents the directly contacting equilibrium distance between two GO sheets. The depth of PMF well reflects the thermodynamic resistance (against dispersion) for the GO sheets to escape from the contacting equilibrium position. The different PMF behavior between GO and pristine graphene displays weak aggregating tendency for GO nanosheets and strong agglomeration state for pristine graphene, consistent with their distinct aggregation/dispersion performances in aqueous environment.16 In Figure 2a, with an increase of surface oxidation concentration in GO sheets, the PMF well becomes shallow, and the corresponding equilibrium position shifts toward larger separation. This means that the functional groups can offer strong repelling action, largely compensating the attractive interaction, weakening the aggregation degree, and promoting the solution dispersion of GO sheets. The effect of oxidation concentration on the GO interaction is consistent with the experimental observation9 that reduced GOs with lower oxidation degree are more prone to aggregate. Meanwhile, the enlarged equilibrium separation between GO sheets in our simulation is in perfect accord with the measured XRD spectra of GO sheets,43 in which the interlayer spacing of GO materials increases with the degree of oxidation.

r

Fc(r ) dr

(1)

where W(r0) is the reference PMF value, which was chosen to be zero at largest separation (20 Å in this work) between two GO sheets. Fc(r) is the constraining force on a sheet, which was calculated as an average over the different separation distances. For each calculation, the interacting force was monitored for long times to reach equilibrium. Each simulated system was equilibrated for 6 ns, and only the last 4 ns of simulation was used for data sampling and analysis. In order to eliminate or reduce the effect of GO size on the simulation, the calculated C

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Figure 3. (a) Comparison of PMF profiles per unit area (in nm2) between two GO sheets with different functional groups (GO1, GO1-COOH) in aqueous solution. (b) Dynamical evolution of the inter-graphene separation as a function of the simulation time with the four different initial separation distances of GO1-COOH. (c) Total PMF profiles of GO1 and GO1-COOH are decomposed into the GO−GO contribution and the GO−H2O contribution.

In order to understand the effect of functional groups on the GO interaction, we further simulated other forms of GOs, including GO1-COOH, GO1-COOH-COO−, and GO1COO−. Figure 3a presents a comparison of PMF profiles between the GO nanosheets decorated with simple hydroxyl/ epoxy groups (GO1) and the one with the additional carboxyl groups (GO1-COOH). It is found that the PMF barrier of GO1-COOH becomes weak as compared with that of GO1. This indicates water molecules can easily get directly accumulated or form stable hydration layer structure between the GO sheets with edge carboxyl groups. However, the overall PMF behavior of GO1 and GO1-COOH is equivalent, suggesting the similar aggregating/dispersing performance. The dynamic aggregation of the GO sheet (GO1-COOH) in aqueous environment can be monitored by a series of unconstrained free MD simulations. Figure 3b shows the time evolution of average separation distance between two GO sheets with different initial COM distances. Each typical figure was derived from several qualitatively consistent results of unconstrained simulations with different random initial configurations. It is found that the two GO sheets demonstrate various contacting actions starting different initial separations. For example, the GO sheets tend to separate from the initial distance of 5.5 Å to a larger separation of 7 Å. In contrast, for the initial separation of 6 Å, the two GO sheets can become aggregated with the close separation of 5 Å, corresponding to the minimum well position in the PMF profile. These results are reasonable by considering the lower free energy barrier and shallower PMF well in the PMF profile. The dynamic simulation offers additional support that the type of GO sheets in water can easily transfer between aggregating and dispersing states. It is noted that the PMF profiles generally exhibit a second minimum point, which suggests that the aggregation of neutral GO sheets might include two structure modes: (i) the direct contacting aggregate without water molecules between GO sheets; (ii) the indirect hydration GO aggregate with single-layer water molecules between GO sheets. The two aggregation modes could occur mutual transformation, which is different from previous works11,18,19 and represents a complete aggregate structure of neutral GO sheets in water. Nevertheless, the coexistence of the two types of GO aggregate structures needs further experimental clarification. In Figure 3c, we decomposed the total PMFs into the direct GO−GO interacting contribution and the GO−water con-

There appears obvious oscillation with the low separation (d < 9 Å) in the PMF profiles (Figure 2a), wherein the free energy barrier (against aggregation) can hinder the GO sheet falling into the deepest PMF trap. Generally, the oscillation behavior is associated with the solvation structure near the GO sheets. Figure 2b presents the corresponding density distributions of confined water molecules between the GO sheets (GO1) in several critical separations. As the two GO sheets approach each other, the confined water molecules successively form multilayer structure (d > 11 Å), two-layer structure (8 Å < d < 11 Å), and single-layer structure (6.5 Å < d < 8 Å) as well as water-depleted region (d < 5 Å) in the intersheet regions. The confined water structures have also been discussed in previous works.44−47 The PMF barrier can be considered as the energetic penalty for the removal of the confined water molecules to the bulk phase, generally identified as the solvation effect,48 which will be further discussed in the following section. As shown in Figure 2a, the small PMF barrier implies that the GO sheets decorated only with hydroxyl and epoxy groups might not produce sufficient hindrance for the GOs contacting with each other.42 According to our simulation, the average kinetic energy of GO1 nanosheet in water is ∼5.04 kcal/(mol nm2), which is greater than the free energy barrier between the GO sheets. The result illustrates this type of GO sheet has possibility to fall into the potential trap by overcoming the energetic barrier, leading to direct aggregation. This behavior has been observed by previous MD simulation18 on aggregation process of two GO sheets in water. However, the kinetic energy of GO sheet is numerically compatible with the depth of PMF well, which also implies that the GO has a tendency to escape from the PMF well. Therefore, our simulation result suggests that the GO sheets in water exhibit metastable stability with low free energy barrier and shallow potential trap in the PMF profile, thus representing that the GO sheets can easily transfer their states between dispersion and aggregation. This further indicates the state of GO sheets in water could be present in the combination of both the dispersed sheet and the aggregate form. This simulated behavior can be applied to explain previous experimental phenomena11,49 that GO aggregates without deprotonation at low-pH conditions do not settle down as a precipitate but instead remain stable suspended and flocculated dispersion. D

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Figure 4. (a) Comparison of PMF profiles per unit area (in nm2) between two GO sheets with different degree of ionization (GO1-COOH-COO−, GO1-COOH) in aqueous solution. The inset shows the water-mediated aggregate structure with single-layer water molecules trapped between GO sheets. (b) Dynamical evolution of the inter-graphene separation as a function of the simulation time with the different initial separation distances for two forms GO sheets. (c) Total PMF of every system is decomposed into the GO−GO contribution and the GO−H2O contribution.

Figure 5. Decomposition of the direct GO−GO term and the GO−water term into the electrostatic interacting contribution and the vdW interacting contribution for (a) GO1-COOH and (b) GO1-COO−.

group attached on the edges of GO sheets at high pH condition can be deprotonated to yield negatively charged COO− groups. Thus, the negatively charged COO− groups provide favorable repulsive interaction, improving stable dispersion of GO sheets. For GO1-COOH-COO− with partly deprotonated, the PMF well depth becomes enhanced with the first PMF minimum corresponding to direct aggregation between the GO sheets. This indicates that with carboxyl ionization degree decreasing GO nanosheets will show reduced repulsive interaction, possibly leading to aggregation of GOs. The dynamic simulation (Figure 4b) shows no direct aggregating state can be formed between the deprotonated GO sheets during the simulation period, which can be ascribed to the strong repulsive interaction, as shown in the PMF profiles. However, the two GO sheets sometimes can aggregate with each other with the separation of 6.5−7 Å, signifying the formation of GO−water−GO structure. This hydration aggregate structure can represent the assembly of liquid crystal by GO nanosheets in which the individual GO platelets are interlinked by functional groups and solvent molecules.12,22 Contrary to the neutral GOs, the decomposed PMF profile of

tribution. The GO−GO interaction has the deepest potential trap at the 4.5 Å separation, indicating strong attractive force, whereas the GO−water contribution instead shows considerable repulsive contribution, meaning water molecules could play a hindering role against the aggregation of GO sheets. The GO−water contribution is consistent with the interacting behavior of carbon-based nanoparticles in aqueous environment17,48,50 and qualitatively agrees well with the repulsive hydration forces in biological tissues.51 Figure 4a shows the PMF results for the GO1-COO− and GO1-COOH-COO−. The PMF profiles of GO1-COO− with full deprotonation generally shows long-range repulsive interaction, which indicates that the deprotonated GO sheets with the negative charge has strong colloidal stability in water. The first PMF minimum corresponds to a formation of metastable hydration structure with single-layer water molecules between the two GO sheets (inset snapshot in Figure 4a). The simulation result is in good agreement with the previous experimental observation11 that GO nanosheets show homogeneous dispersion at pH = 14. At present, it is generally recognized that when suspended in water solvent, carboxyl E

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Figure 6. (a) Average numbers of H-bonds between the single GO sheet and the surrounding water molecules as a function of the separation distance for GO1, GO1-COOH, and GO1-COO−. (b) RDFs between the carboxyl oxygen atom of GO1-COOH and GO1-COO− and the water oxygen atom. (c) Continuous correlation function SHB(t) for the H-bonds between the carboxyl groups of GO1-COOH and GO1-COO− and the surrounding water molecules.

electrostatic GO−GO interaction. Thus, the surface negative charge on GO sheets is the controlling factor determining the interaction between the GO sheets. Our simulation result provides straight theoretical evidence that the electrostatic repulsive interaction between the negatively charged GO sheets is the driving force for keeping the long-term colloidal stability of GO nanosheets in aqueous solutions. At present, the surface zeta potential7,11,49,52 has extensively been measured to characterize the surface charge of suspended GO sheets in aqueous medium. Zeta potential measurements showed that above pH = 4 the GO sheet could keep stable dispersion behavior by developing sufficient negative charge mainly due to ionization of COOH group, which is expected to provide necessary electrostatic repulsive interaction. At low pH value, slight or no deprotonation of COOH is not able to provide sufficient electrostatic repulsive interaction, and in this case the attractive GO−GO interaction is only partially repelled by repulsive GO−water solvation forces. This has been indirectly confirmed by previous experimental phenomenon11,49 that stable flocculation of GO sheet was obtained with pH of 2.0. The pH-dependent GO colloidal stability has been exploited to fractionate GO sheets based on size dimension53 and to regulate the microstructure modes of GO sheets deposited on electrode surfaces.49 According to the preceding results, the solvation force between neutral GO sheet and the surrounding water provides the repulsive action, aiding the GO dispersion. However, in the negatively charged GO sheet (GO1-COO−), the solvation force contrarily demonstrates attractive force, impeding the GO dispersion. It is believed that the solvation force is originated from the interfacial interaction between GO and water molecules. Because of the presence of oxygen-containing groups, GO surfaces possesses high hydrophilic nature. Thus, the hydrogen bonds (H-bonds) can be used to evaluate the

the deprotonated GO sheets shows different behavior (Figure 4c), which was expressed as the repulsive contribution of GO− GO interacting term and the attractive contribution of GO− water term. Although the direct GO−GO interaction provides the repulsive interaction between the ionized GO sheets, the attractive GO−water interaction is the possible origin to form the hydration GO structure. Figure 5 shows the further decompositions of the direct GO−GO PMF term and the GO−water term into the electrostatic interacting contribution and the vdW interacting contribution. From Figure 5a, for the neutral GO1-COOH, both the electrostatic and the vdW interactions in the GO−GO term remain attractive. This is a little different from the computational result based on the DLVO theory,13−15 in which GO sheets exhibit electrostatic repulsion and vdW attraction. The difference is due to the prerequisite of negative charge on GO surfaces in the classical DLVO theory, which fails to predict the interaction between neutral GO sheets. In addition, for the neutral GO nanosheet, the vdW interaction provides larger attraction than the electrostatic interaction in the direct GO− GO term. The result agrees well with the previous molecular simulation18 on the interaction between neutral GOs in water medium. In the repulsive GO−water term, the electrostatic interaction between water molecules and GO sheet offers the repulsive action, which is larger than the attractive vdW GO− water interaction. However, for the GO1-COO − sheet, the GO−GO interaction exhibits a strong and overwhelming electrostatic repulsion along with a very weak vdW attraction, which can be ignored compared to the electrostatic repulsive interaction. The decomposition of the GO−water term shows that both the electrostatic and the vdW interaction provide the attractive action. However, the overlying of two types of attractive solvation forces cannot counteract the strong repulsive F

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Figure 7. Decomposition of the PMF profile of GO1-COOH in the aqueous environment. (a) The GO−GO PMF contribution is divided into the C−C contribution and the functional groups (FG)-induced contribution. The FG-induced contribution is subdivided into the contribution of epoxy, hydroxyl, and carboxyl groups (see inset). (b) The GO−H2O PMF contribution is divided into the C−H2O potential contribution and the FG−H2O contribution. The FG−H2O contribution is subdivided into the contribution of epoxy, hydroxyl, and carboxyl (see inset).

Figure 8. (a) PMF profiles of GO1-COOH in the C6H6 solvent. It is decomposed into the GO−GO potential contribution and the GO−C6H6 contribution. (b) Dynamical evolution of the inter-graphene separation as a function of the simulation time with four different initial separation distances of GO1-COOH. The four snapshots illustrate the aggregation process of two GO sheets by excluding benzene molecules. To make it clearer, the short time scale is enlarged in the inset.

COOH. This indicates that the deprotonated carboxyl groups cause stronger attraction to the surrounding water molecules, leading to the enhanced water accumulation near the GO1COO− sheet. In addition, Figure 6c presents the continuous correlation function SHB(t)56,57 for the H-bonds between the carboxyl groups (deprotonated and protonated) and the surrounding water molecules, which describes the stability of the H-bonds over a period of time. It was observed that the decay rate of the SHB(t) function of COO− is slower than that of COOH, further suggesting more steady H-bond is formed by the deprotonated carboxyl groups. As a result, the attractive solvation force between the GO sheets (Figure 4c) is attributed to the stable and enhanced H-bonds interaction in the GO1COO− sheet. Therefore, for the negatively charged GO sheets, the H-bond-induced solvation interaction and the electrostatic repulsion between GO sheets cooperatively control the GO interaction in the aqueous solvent. Our simulation provides the support to the previous conclusion12 that in the self-assembly of GO liquid crystals the thermodynamic free energy change can be represented as the sum of hydrogen bonding contribution and the electrostatic interaction. In order to characterize the different contributing degrees among these functional groups, in Figure 7, we have decomposed PMF profiles of GO1-COOH into several terms. For the GO−GO interaction, the carbon atom (C−C) on the GO sheet provides the weak effect, and the contribution by the functional groups (including the carbon atoms which the functional groups attached to) produces the main attractive force, in which the epoxy groups (−14 kcal/(mol nm2)) and hydroxyl groups (−7.5 kcal/(mol nm2)) on the GO plane

GO−water interaction. Figure 6a shows the average numbers of H-bonds54 between one single GO sheet and the surrounding water molecules as functions of separation distance. The geometric definition55 of H-bonds is the donor−acceptor distance is smaller than 0.35 nm and the hydrogen donor− acceptor angle is less than 30°. In the H-bond computation, the epoxy is counted as the H-bond acceptor. The hydroxyl and carboxyl may simultaneously act as donor and acceptor. This treatment has taken into account all possibilities that each functional group might play roles in the formation of H-bond. In Figure 6a, there are obvious differences in the total Hbonds number for the three types of GOs. With the separation decreasing (d < 7 Å), the sudden decrease in the number of Hbonds clearly results from the depletion of confined water molecules between two GO sheets. We further decomposed the numbers of H-bonds into the donations from various functional groups. It is observed that the epoxy and hydroxyl groups usually produce the identical H-bonds numbers among the three GO sheets. The main difference in the H-bond number for these GO sheets is due to the H-bond interaction arising from the carboxyl groups. In particular, the deprotonated carboxyl groups in the GO1-COO− sheet can form more Hbonds number compared with the COOH group in the GO1COOH sheet. The different H-bond behavior is also confirmed by the RDFs (radial distribution function) between the oxygen atom of carboxyl group and the water oxygen atom, when the intersheet separation distance is 12 Å. The RDF (Figure 6b) for the GO1-COO− displays higher and sharper peaks in the first and second solvation layers as compared with those in GO1G

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Figure 9. Decomposition of the PMF profile of GO1-COOH in the C6H6 environment. (a) The GO−GO contribution is divided into the C−C contribution and the functional groups (FG)-induced contribution. The FG-induced contribution is subdivided into the contribution of epoxy, hydroxyl, and carboxyl groups (see inset). (b) The GO−C6H6 PMF contribution is divided into the C−C6H6 potential contribution and the FG− C6H6 contribution. The FG−C6H6 contribution is subdivided into the contribution of epoxy, hydroxyl, and carboxyl (see inset).

short time scale is enlarged in the inserted figure. It can be observed that during the simulation process benzene molecules can be rapidly and easily squeezed out from confined space between the two GO sheets, finally achieving aggregation. The thermodynamic PMF profile and dynamic behavior are consistent with the experimental observation9 that GO shows low and transient stable dispersibility in the nonpolar toluene solvent, which exhibits the similar properties to benzene. This experimental observation indirectly supports our simulation results. In short, our result provides the thermodynamic basis why GO sheets are not stable and have the trend of aggregation in the nonpolar benzene solvent, which is very different from that observed in water. Figure 9 shows the respective functional group contributions to the GO−GO term and the GO−benzene term. The performance is qualitatively similar to that in water solvent. However, in benzene solvent, the functional groups induced term has larger contribution to the GO−GO interaction, in comparison with water solvent. This is probably because the benzene solvent has different effect on the stretching structure of functional groups. For the GO−benzene PMF term, the C− benzene interaction becomes larger due to the hydrophobic nature of benzene. The reduced interacting contribution between functional groups and benzene solvent can be ascribed to the attractive interaction between hydroxyl groups and benzene solvent. Currently, the interaction of GO nanosheets in organic solvent is scarcely studied. Our simulation provides preliminary molecular-level picture of the interaction mechanism, which will be valuable in the solution dispersion using organic solvent.

constitute the main portions. Figure 7b shows the decomposed contributions of functional groups to the GO−water PMF term. As expected, the contribution of functional groups is larger than that of C atom, and moreover the epoxy group plays a main role, followed by the hydroxyl group, and the last one is the carboxyl group. Considering the net effects on the attractive and repulsive interactions among these functional groups, it is found that the hydroxyl groups seem to produce large effect on the dispersion stability for the neutral GO sheet in aqueous environment. Comparatively, the carboxyl groups are not effective due to its low amount. The result is consistent with the conclusion in previous work.16 We also simulated the PMF interaction in nonpolar solvent benzene, in which the GO1-COOH sheet was considered. Figure 8a shows the corresponding total PMF profiles along with the decomposition of the direct GO−GO term and solvent-induced term. Compared to water, benzene solvent produces a stronger PMF aggregating well and a higher PMF barrier. The larger PMF trap (−17.4 kcal/(mol nm2)) at the separation of 5 Å means the GO sheets can keep stable aggregation. The free energy barrier (∼4.32 kcal/(mol nm2)) probably provides a hindrance to some extent for the GO sheet falling into the PMF well. The corresponding decomposition shows, compared to water solvent, the GO−GO interaction in the benzene-solvated PMF profile has stronger GO−GO PMF trap and weaker GO− benzene repulsive interaction. In the nonpolar benzene, the solvation force could not afford sufficient repulsive interaction to overcome the attractive interaction between GO sheets, thereby leading to an obvious contacting aggregation state of GO sheets. In addition, although in benzene solvent the PMF barrier against aggregation is larger than that in water solvent, the GO sheets still have larger chance to cross the energy barrier into the PMF trap. This is because in the low-viscosity benzene solvent GO possesses higher kinetic energy. According to our calculation, the average kinetic energy of single GO sheet in benzene is ∼13.5 kcal/(mol nm2), which is higher than the free energy barrier and lower than the PMF trap. As a result, the GO sheet can be stably trapped in this PMF well with less chance to escape. Therefore, GO sheets generally aggregate in the benzene medium by providing the relatively low energy barrier and deep potential trap. Figure 8b shows the time evolution of the average separation between two GO sheets during the dynamic simulation. It can be found that the equilibrium has been achieved after 0.1 ns and continued until the end of 4 ns. To make clear, the figure in

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CONCLUSION Molecular dynamics simulations have been performed to study the interaction mechanism of GO nanosheets in water and benzene solvents. The PMF was used to quantify the interaction strength, which could characterize the thermodynamic resistance and trend of dispersion and stabilization of GOs in solvents. Various factors, including oxidization concentration, type of functional groups, and ionization of functional groups, were considered. The simulation results showed that increasing oxidization level could lead to shallower PMF well and larger equilibrium distance. For neutral GO sheets without deprotonation, the PMF profile shows lower free energy barrier and shallower potential trap, displaying metastable aggregating/dispersing behavior in water. The solvation force for neutral GO sheets provides certain repulsive H

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action against the attractive GO interaction, aiding the GO dispersion. The dynamic simulation provides direct theoretical proof that the type of GO sheets in water can easily transfer between aggregating and dispersing states. Our simulation results suggest that the GO flocculation is the combination of indirect water-mediated structure and direct contacting aggregates. However, for the ionized GO nanosheets with negatively charged COO− groups, the strong and overwhelming electrostatic repulsive interaction between GO sheets is responsible for the stable dispersion of GO sheets. Distinctive solvation interaction between ionized GO and the surrounding water molecules was revealed. The attractive hydrophilic force for the negatively charged GO sheets can be ascribed to the enhanced H-bonding formation between deprotonated carboxyl groups and water molecules. In the benzene solvent, the solvation force could not afford sufficient repulsive interaction to overcome the attractive interaction between GO sheets, thereby leading to an obvious contacting aggregation state of GO sheets. The simulation results further elucidate the role of each functional group on the GO interaction under various solvent environments. In short, our simulation study provides new insights into the GO interaction mechanism, which will be valuable in improving the GO solution dispersion processes. It is worth mentioning that in this work we did not conduct the entropic/ enthalpic-based thermodynamic analysis. The thermodynamic analysis with entropic contribution can enhance the understanding of interaction mechanism between GOs in solvent media. This work can be the direction of future efforts in the study.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Y.). *E-mail: [email protected] (Z.X.). ORCID

Zhijun Xu: 0000-0003-2433-2264 Xiaoning Yang: 0000-0003-0811-3774 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Grants 21676136 and 21606122, Research funding from State Key Laboratory of MaterialsOriented Chemical Engineering (ZK201404), and A PAPD Project of Jiangsu Higher Education Institution. The computational resources generously provided by High Performance Computing Center of Nanjing Tech University are greatly appreciated.

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