Molecular Dynamics Study of the Aggregation Process of Graphene

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Molecular Dynamics Study of the Aggregation Process of Graphene Oxide in Water Huan Tang,†,‡ Dongmei Liu,†,‡ Ying Zhao,*,†,‡ Xiaonan Yang,†,‡ Jing Lu,†,‡ and Fuyi Cui*,†,‡ †

State Key Laboratory of Urban Water Resource and Environment, Harbin 150090, China School of Environmental and Municipal Engineering, Harbin Institute of Technology, Harbin 150090, China

‡

J. Phys. Chem. C 2015.119:26712-26718. Downloaded from pubs.acs.org by OPEN UNIV OF HONG KONG on 01/23/19. For personal use only.

S Supporting Information *

ABSTRACT: Molecular dynamics (MD) simulations were performed to provide molecular insight into the aggregation process of graphene oxide (GO) in water. The aggregation was found to be a point−line−plane process. Five forces were involved during the process: van der Waals attraction, electrostatic interaction, hydrogen-bond interaction, π−π stacking, and the collision of water molecules. The dominant forces were different in the three stages. The connection “line” was important to the aggregation process and the final overlapping area of the GO aggregate. To study the effect of oxygen content and functional group on the aggregation of GO, four different GOs were used: C10O1(OH)1(COOH)0.5, C30O1(OH)1(COOH)0.5, C10O1(COOH)0.5, and C10O1(OH)1 (termed OGO, RGO, GO-COOH, and GO-OH, respectively). RGO aggregated faster than OGO, and GO-OH aggregated faster than GO-COOH. A quantitative analysis showed the difference in aggregation rate of these four GOs should be attributed to the hydrogen bonds. Additionally, the closer GOs were to each other initially, the faster they aggregated. This study reveals the aggregation process of GO and will be helpful in understanding its behavior in water.

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membranes with MD and discussed several flow enhancement mechanisms through the porous microstructures of GO membranes.20 Shih et al. studied the pH-dependent behavior of GO in water by MD.21 Unfortunately, the detailed microscopic aggregation process of GO is still unknown. It has been reported that the π−π stacking,22 hydrogen bond (H-bond),23 electrostatic interaction,24 and van der Waals (vdW)25 attraction accelerate the interaction between graphitic nanomaterials. However, how these forces drive the aggregation process of GO is unclear. Medhekar et al. proved the functional groups of individual GO platelets play a critical role in establishing the mechanical properties of GO composite; a higher density of functional groups leads to a corresponding increase in stiffness.26 Accordingly, it can be envisioned that oxygen content and the type of functional groups may have an effect on the aggregation process of GO. With all of the above in mind, in this article we report on a series of MD simulation studies to understand the microscopic aggregation process of GO in water. Specifically, the LennardJones (L-J) potential energy, electrostatic potential energy and distance between two GOs, the amount of H-bond between GO and water, and the H-bond between two GOs were calculated to illustrate the aggregation process. Dominant forces in different stages of aggregation were explored. The effects of oxygen content, type of functional groups, and initial

INTRODUCTION Graphene oxide sheet, abbreviated as GO, is receiving increasing attention because of its large surface area1 and extraordinary electronic and thermal properties.2,3 GO is usually formed by the chemical exfoliation of graphite oxide, which is operated in water.4 Additionally, GO has been employed in a wide range of potential applications in recent years, including optics, cosmetics, nanocomposites, and pharmaceuticals.5 Its extensive use will inevitably lead to its release into the water environment.6 Thus, a fundamental understanding of its behavior in water is urgently required to control the processes of exfoliation and evaluate its environmental risks. Because the aggregation of GO is one of the most important factors that ultimately controls its behavior in water,7,8 it is necessary to understand the aggregation of GO in water. Previous investigations on the aggregation of GO mainly focused on macroscopic experiments, elucidating the roles of pH, ionic composition, ionic strength, presence of natural organic matter, etc.9−12 However, limited efforts have been devoted to illustrate the aggregation of GO from a microscopic perspective. Atomic-scale investigations with molecular dynamics (MD) simulations could contribute significantly to understanding the microscopic process and furnish many details that are not accessible experimentally.13−16 Recently, MD has been used extensively in exploring the properties of GO with water.17,18 For example, Wei et al. characterized the wetting properties and calculated the water contact angle of GO by performing MD.19 Wei et al. explored water permeation in GO © 2015 American Chemical Society

Received: July 29, 2015 Revised: November 1, 2015 Published: November 2, 2015 26712

DOI: 10.1021/acs.jpcc.5b07345 J. Phys. Chem. C 2015, 119, 26712−26718

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The Journal of Physical Chemistry C

were computed using the particle mesh Ewald (PME) method.43 Initially, the two GOs were well separated (Figure 1 a), and the separation was defined as the distance between the geometric center of GO. The combined systems were then solvated in a cubic periodic box with the distance between the solutes and box boundary at least 10 Å, and the total number of WMs in all systems were almost the same (33 946 ± 15). Before the MD began, energy minimization was carried out. Then the system was equilibrated for 100 ps at a constant pressure of 1 bar and a temperature of 300 K using a modified Berendsen thermostat. 44 During the minimization and equilibration processes, the basal plane of the GO sheets were constrained. Then, the GOs were released and MD simulations were performed in a NVT ensemble at 300 K. Periodic boundary conditions were applied in all three directions. The equations of motion were integrated with a time step of 2 fs using the leapfrog algorithm,45 and data were collected every 2 ps. The distance, L-J potential energy, electrostatic potential energy, and the amount of H-bond between two GO platelets as a function of time were calculated. The criteria for the formation of a H-bond are as follows: the donor−acceptor distance is smaller than 0.35 nm and the hydrogen−donor−acceptor angle is less than 30°. This criterion for H-bond has been used widely by other researchers.46−48

distance between GOs were investigated. The results presented here will provide molecular-level insights into the aggregation behavior of GO in water.

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COMPUTATIONAL METHODS In our models of GO, both hydroxyl and epoxy groups were considered, following the Lerf−Klinowski model, which represents typical outcomes from the standard oxidation process.26,27 The epoxy and hydroxyl groups were located on the basal plane (both sides), and the carboxyl groups were attached to the carbon atoms on the edge. The distribution of oxidized groups in GO was set to be either random or regular in our models, as will be specified in the following discussion. GOs with different oxygen content and different functional groups were used (Table 1). The oxygen content c is defined as Table 1. GO Models and Their Abbreviations composition

abbreviation

C10O1(OH)1(COOH)0.5 C30O1(OH)1(COOH)0.5 C10O1(COOH)0.5 C10O1(OH)1

OGO35 RGO GO-COOH GO-OH26

nO/nC, where nO and nC are the number densities of oxygencontaining groups and carbon atoms, respectively. The c varies depending on the degree of oxidation in the preparation processes;28 a typical value of c is ∼20% for GO,29,30 and further reduction could yield a lower c.31 OGO stands for the original GO, and the c of OGO is ∼20% (shown in Figure 1b).

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RESULTS AND DISCUSSION First, the microscopic aggregation process of OGO was investigated. The H-bond, L-J potential energy, electrostatic potential, and distance between two OGOs are shown in Figure 2. There were five specific time points: (1) The L-J potential became constant negative after ∼1098 ps (indicated by dotted lines). (2) The electrostatic potential became constant negative after ∼1464 ps. (3) The first H-bond formed at 1494 ps. (4) The slope of these curves became larger after ∼3024 ps (indicated by dotted lines). (5) Maximum or minimum values of these curves were achieved at ∼3580 ps (indicated by dotted lines). Based on these time points, the aggregation can be divided into a point−line−plane process (Figure 3, Supporting Information video 1). (Because MD is a random-walk process, these specific time points will change in different simulations. However, in all simulations, we can find these special time points. The method of utilizing these special times points to divide the aggregation into a point−line−plane process can be applied to all GO aggregation processes.) When the simulation began, OGOs moved randomly and adjusted their motion direction. During the adjustment, the fluctuation margin of the distance curve was high (Figure 2b). It can be speculated from the distance−time curve that the OGOs were first separated from each other and the maximum distance between OGOs was 2.4 nm. As the maximum distance was beyond the scope of vdWs (1.0 nm), it can be speculated that the force driving the OGOs’ approach toward each other was from the WMs. Then OGOs began to come closer together. When the two OGOs were close enough, the OGOs continuously diffused and made transient contacts at many points. Because these contacts were transient, the L-J potential and electrostatic potential energy occasionally became negative during the adjustment. After 1098 ps, the L-J potential became constant negative. OGOs were firmly connected by vdWs attraction at a certain point. This is the point process (0−1098 ps). After ∼1464 ps, the electrostatic potential became constant

Figure 1. Setup of the simulation system: (a) sideview of one example system (hydrogen in white, oxygen in red, carbon in black); (b) the dimension of OGO (the distribution of oxidized groups was regular).

RGO stands for the GO that has been further reduced. To illustrate the effect of functional groups on the aggregation of GO, GO-COOH and GO-OH were used. Hamad et al. and Picaud et al. employed similar GO-COOH and GO-OH models to study the interaction between water molecules and a soot surface.32−34 The optimized potentials for liquid simulations-all atoms (OPLS-AA)36 force field implemented in the GROMACS37,38 software package was used for all simulations. The force field parameters are given in Supporting Information (Tables S1− S4). All of the sp2 carbon atoms in GO were treated as uncharged L-J spheres.39 The water molecules (WMs) were simulated using the standard SPC/E model.40 Bond lengths were constrained with LINCS,41 and water geometries were constrained with SETTLE.42 The cut off for the van der Waals (vdWs) interaction was set to 10 Å. Long-range electrostatics 26713

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negative and electrostatic attraction began to contribute to the aggregation. Because the electrostatic potential value was lower than the L-J potential (Figure 2), the vdWs attraction played a more dominant role than electrostatic interaction. At 1494 ps, the first H-bond formed. By utilizing the vdWs attraction, electrostatic attraction, and H-bond, two OGO platelets approached each other quickly and more atom pairs were connected. At ∼3024 ps, a series of atom pairs connected and a connecting line was formed. This is the line process (1098 ps ∼3024 ps). After 3024 ps, both the potential and the distance decreased quickly (the slope of curves in Figure 2 and Figure 3 became larger), which means the OGOs began to approach each other more quickly. Therefore, the formation of the connecting line indicated the beginning of rapid aggregation. After the line process, OGOs connected to each other in a larger region, and π−π stacking began to form between OGOs. More and more WMs between the OGOs were squeezed out. The amount of WMs between OGOs can be speculated based on the amount of H-bond between OGOs and water (Figure 4). Initially, OGO platelets were surrounded by WMs and

Figure 2. (a) L-J potential energy and electrostatic potential energy between OGOs. (b) Amount of H-bond and distance between OGOs. The initial distance between OGO was 2.0 nm. The L-J potential implied the vdWs attractive interaction energy between OGOs. If the potential value is 0, there is no interaction between the two OGO platelets. If the potential value is negative, there is vdWs attraction between OGOs. The electrostatic potential implied the electrostatic interaction energy between OGOs. If the value is negative, there is electrostatic attraction between OGOs. Figure 4. Amount of H-bonds between GOs and water.

many H-bonds were formed. When OGOs began to came closer, more and more WMs were squeezed out and the amount of H-bond decreased. At ∼3580 ps, both distance and L-J potential reduces to the minimum, OGOs were firmly “connected” in a plane, and a stable OGO aggregate was formed. This is the plane process (3024 to ∼3580 ps). After OGOs aggregated plane-to-plane, there were still several WMs between OGOs (Figure S1). At ∼3690 ps, the amount of Hbond between OGOs and WMs reduced to a minimum, which means all the WMs between OGOs were squeezed out (discussed in detail below). In the point process, vdWs and the collision of WMs were the dominant driving force. The formation of a point indicated the beginning of aggregation. In the line process, vdWs attraction and H-bonds were the dominant driving forces. The formation of a line indicated a rapid aggregation. In the plane process, vdWs, H-bonds, and π−π stacking all played key roles. In the above discussion, oxidized groups in GO were distributed regularly and the initial configuration of GO was parallel. To make the point−line−plane process more convincing, GOs with random distribution of oxidized groups and different initial configurations were used (discussed in Supporting Information). The aggregation of these GOs were all found to be a point−line−plane process. The “line” process was crucial to the overlapping area of the GO aggregate. The longer the connecting line, the larger the overlapping area (discussed in Supporting Information).

Figure 3. Representative trajectories of the point−line−plane process. First, two GO platelets were firmly connected by two atoms at a certain point. Then more and more atoms were connected and a connecting line was formed between GOs. Finally, GOs connected to each other in a plane.

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The Journal of Physical Chemistry C The formation of H-bond is critical in the line and plane process; therefore, H-bond played a key role in the aggregation of GO. There were three kinds of H-bond formed between GOs during the aggregation (shown in Figure 5): H-bonds with water molecules and intralayer and interlayer H-bonds between function groups. These three configurations of H-bonds were also observed by Medhekar et al.26

Figure 6. (a) Distribution of H-bonds. (b) GO−water−GO structure: I, initial decrease; II, distance between GOs decreased from 1.2 to 0.6 nm; III, when the distance between GOs was less than 0.6 nm, all WMs were squeezed out. The blue lines indicate H-bonds.

Figure 5. Three configurations of H-bonds: (a) H-bonds with water molecules, (b) interlayer H-bonds between function groups, and (c) intralayer H-bonds between function groups. The horizontal black lines in panels a and b denote the GO sheet. Interlayer interactions means H-bonds within different GO layers, and the intralayer interactions means H-bonds within the same layer. The black rectangle box in panel c denotes the GO sheet.

To understand the distribution of H-bonds, the number of H-bonds between GOs and the surrounding WMs as a function of the intersheet distance was calculated (Figure 6a). As the distance between GO decreased, the amount of H-bonds showed a two-step decrease (indicated by dashed line). A minor initial decrease (when the distance was ∼1.2 nm) followed by a subsequent major decrease (when the distance was ∼0.6 nm) process. Before the initial decrease, there were several layers of WMs between GOs; two layers of WMs were fixed by the H-bonds formed between GOs and WMs, and other WMs were free (Figure 6b). When the distance decreased from 2.5 to 1.2 nm (initial decrease), only the free WMs were squeezed out. When the distance was 1.2 nm, there were two layers of WMs between GOs. As the distance decreased further, there was only one layer of WMs. When the distance was ∼0.6 nm, all the WMs were squeezed out. Shih et al.21 suggested that GO−single-layer water−GO and GO−two-layer water−GO structures are global energy-minimized configurations by calculating the free energy changes and potential of mean force. Based on the study of the aggregation process, the effects of oxygen content, functional groups, and initial separation distance were studied. The oxygen content had an effect on the aggregation velocity of GO. As is illustrated in Figure 7, RGO possessed an aggregation faster than that of OGO. This can be explained from two perspectives. On the one hand, the amount of Hbonds between OGO and water was larger than that of RGO (Figure 4). We proved that in the plane process, WMs between GO platelets must be squeezed out. The more H-bonds that

Figure 7. Distance−time curves of OGO and RGO with initial distances of 1.2 and 2.0 nm. RGO aggregated faster than OGO, and the closer GOs were to each other initially, the faster they aggregated.

formed between GO and WMs, the longer time that was needed to squeeze out the WMs. On the other hand, the pristine graphene is hydrophobic, but −COOH and −OH are hydrophilic. GO with less −COOH or −OH is less hydrophilic; therefore, the aggregation of RGO is easier. The final interlayer spacing between OGO and RGO platelets was ∼0.5 and 0.43 nm. Marcano et al.49 proved the interlayer spacing of pristine graphene without oxygen functional groups is 0.37 nm, and the spacing of the materials is proportional to the degree of oxidation. Shih et al.21 suggested that the interlayer spacing of OGO should be smaller than 0.6 nm if there is no water between OGOs. The 0.5 nm was between 0.37 and 0.6 nm, and 0.43 nm was smaller than 0.5 nm, which were all reasonable. The aggregation of GOs with different functional groups were also studied (Figure 8). Obviously, G-OH aggregated more quickly than G-COOH. This difference in velocity should be attributed to the driving force in the aggregation process. 26715

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preferential adsorption of water on graphite surfaces containing COOH rather than OH sites.33 On this occasion, it is more difficult for G-COOH to aggregate. Except for oxygen content and functional group, the initial distance between the GOs also had effects on the aggregation process: the closer GOs were to each other, the faster they aggregated (Figures 7 and 8). As we mentioned in the introduction, physiochemical conditions had effects on the aggregation of GO. For example, because of electrical double-layer compression, the presence of electrolytes will screen the electrostatic surface charge on GO and will accelerate the aggregation of GO.50 We performed several simulations with sodium chloride (40 mM) and the aggregation process was also found to be a point−line−plane process (Supporting Information). However, the chargescreening process is complicated and hard to simulate by molecular dynamics. In our future contributions, we will combine the experiment with molecular dynamics simulation and explain the effect of physiochemical conditions from a new perspective.

Figure 8. (a) Distance−time curves of GO-COOHs and GO-OHs when the initial distance was 1.2 nm. (b) Distance−time curves of GO-COOHs and GO-OHs when the initial distance was 1.5 nm. (c) Distance−time curves of GO-COOHs and GO-OHs when the initial distance was 2.0 nm. GO-OH aggregated faster than GO-COOH, and the closer GOs were to each other initially, the faster they aggregated.

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CONCLUSION The aggregation of GO in water was explored by MD. The microscopic process was found to be a point−line−plane process. First, two GO platelets were firmly connected by two atoms at a certain point. Then more and more atoms were connected and a connecting line was formed between GOs. Finally, GOs connected to each other in a plane. In the point process, vdWs and the collision of WMs were the dominant driving forces. The formation of the “point” indicated the beginning of aggregation. In the line process, vdWs and Hbonds were the dominant driving force. The formation of the “line” indicated a rapid aggregation. The last stage is the plane process; vdWs, H-bonds, and π−π stacking all played key roles in this stage. The formation of the line was crucial to the overlapping area of the GO aggregate. Oxygen content, type of functional groups, and the initial distance between GOs all had effects on the aggregation velocity. GO with higher oxygen content was more difficult to aggregate. The interlayer spacings of OGO and GGO were 0.5 and 0.43 nm, respectively. Compared with G-COOH, G-OH was easier to aggregate. The closer GOs were from each other initially, the faster they aggregated. The formation of the H-bond is critical in the line and plane processes, and the effect of oxygen content and functional group should all be attributed to the H-bond. Therefore, H-bond played a key role in the aggregation of GO. We hope that the study presented here provides deeper insights into understanding the aggregation behavior of GO in water.

When two GOs approached each other, four forces were involved: the collision of WMs, electrostatic interaction, vdWs, and H-bond interaction. Because the amounts of WMs in these systems were almost the same, the collision from WMs should be the same. The vdWs interaction is related to molecular weight, but the molecular weight of G-COOH is larger than that of G-OH. The electrostatic interaction was weak. Therefore, the differences in aggregation rate were mainly caused by the H-bond interaction. It can be seen in Figure 9

Figure 9. Amount of H-bonds between GO-COOHs and GO-OHs.

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that the amount of H-bond formed between G-OH was larger than that of G-COOH. The −OH in G-COOH is alcoholic hydroxyl, and the −OH in G-OH is phenolic hydroxyl. The O in phenolic hydroxyl is sp2 hybridization, but the O in alcoholic hydroxyl is sp3 hybridization. Compared to the O in phenolic hydroxyl with higher electronegativity, the O in alcoholic hydroxyl imposes a weaker restriction on its valence electrons. The valence electrons have a larger range of motion and have a higher occurrence probability around H atoms. The H atoms are surrounded by valence electrons and are not liable to leave. Therefore, it is more difficult for the H atoms in alcoholic hydroxyl to form H-bonds, and it is more difficult for G-COOH to aggregate. On the other hand, the easier the WMs were adsorbed on GO, the longer was the time needed to squeeze out the WMs between GOs. Picaud et al. suggested a

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07345. Video illustrating the point−line−plane aggregation process of GOs in water (MPG) (MPG) (AVI) Equilibrated structures of GO aggregates and the aggregation process of GOs with different initial configurations (PDF) 26716

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 86-13144510517. *E-mail: [email protected]. Tel: 86-13904503191. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This paper is supported by National Natural Science Foundation of China (Grant 51278147), the Funds for Creative Research Groups of China (Grant 51121062), and State Key Laboratory of Urban Water Resource and Environment (Grant 2013DX03).

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