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A 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 J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07345 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 3, 2015

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The Journal of Physical Chemistry

A Molecular Dynamics Study of the Aggregation Process of Graphene Oxide in Water Huan Tang ,‡, Dongmei Liu ,‡, Ying Zhao* ,‡, Xiaonan Yang ,‡, Jing Lu ,‡, 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. Ying Zhao: [email protected]. Tel: 86-13144510517 Fuyi Cui: [email protected]. Tel: 86-13904503191

ACS Paragon Plus Environment


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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 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 these 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 (short for 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 from 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.

KEYWORDS: point-line-plane process, aggregation mechanism, dominant force, hydrogen bond


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INTRODUCTION Graphene oxide sheet, abbreviated as GO, is receiving increasing attention due to its large surface area, 1 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 had been employed in a wide range of potential applications in recent years, including optics, cosmetics, nano-composite, and pharmaceuticals.5 The extensive use will inevitably lead to its release into the water environment.6 Thus a fundamental understanding of its behaviors in water is urgently required to control the processes of exfoliation and evaluate its environmental risks. Since the aggregation of GO is one of the most important factors that ultimately control 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 had 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 microscopic process and furnish many details those are not accessible experimentally.13-16 Recently, MD had 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 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



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of GO is still unknown. It had 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. But how these forces driving 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 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 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 distance between GOs were investigated. The results presented here will provide molecular-level insights into the aggregation behavior of GO in water. COMPUTATIONAL METHODS In our models of GO, both hydroxyl and epoxy groups were considered, following the Lerf−Klinowski model that 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


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regular in our models, as will be specified in following discussions. GOs with different oxygen content and different functional groups were used (Table 1). The oxygen content c is defined as nO/nC, where nO and nC are the number densities of oxygen-containing 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 1(b)). RGO stands for the GO that had 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 Table 1. GO models and their abbreviations Composition Abbreviation C10O1(OH)1(COOH)0.5 OGO35 C30O1(OH)1(COOH)0.5 RGO C10O1(COOH)0.5 GO-COOH C10O1(OH)1 GO-OH26 The optimized potentials for liquid simulations-all atoms (OPLS-AA)36 force field implemented in the GROMACS 37 , 38 software package was used for all simulations. The force field parameters were given in Supporting Information (Table S1 to Table S4). All of the sp2 carbon atoms in GO were treated as uncharged Lennard-Jones (LJ) spheres.39 The water molecules (WMs) were simulated using the standard SPC/E model.40 Bond lengths were constrained with LINCS41 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 were computed using the particle mesh Ewald (PME) method.43



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

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 as 33946±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 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 an NVT ensemble at 300K. Periodic boundary conditions were applied in all three directions. The equations of motion were integrated with a time step of 2 fs using the leap-frog algorithm45 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 palates as a function of time were calculated. The criteria for the formation of a H-bond: the donor-acceptor distance is smaller than 0.35 nm and the hydrogen-donor-acceptor angle is less than 30 degree.


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This criterion for H-bond had been used widely by other researchers.46-48 RESULT 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 were shown in Figure 2. There were five specific time points: the L-J potential became constant negative after ~1098 ps (indicated by dotted lines), the electrostatic potential became constant negative after ~ 1464ps, the first H-bond formed at 1494 ps，the slope of these curves became larger after ~3024 ps (indicated by dotted lines), and 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)(Since MD is a random-walk process, these specific time points will change in different simulations. But 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 is applicative to all GO aggregation processes.).



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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 palates. 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.


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Figure 3. Representative trajectories of the point-line-plane process. First, two GO palates 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.

When the simulation began, OGOs moved randomly and adjusted their motion direction. During the adjustment, the fluctuation margin of distance curve was high (Figure 2(b)). It can be speculated from the distance-time curve that the OGOs were firstly separated from each other and the maximum distance between OGOs was 2.4 nm. As the maximum distance was beyond the scope of vdWs (1.0nm), it can be speculated that the force drove the OGOs approaching to each other was from the WMs. Then OGOs began to come closer. When the two OGOs were close enough, the OGOs continuously diffused and made transient contacts at many points. Since these contacts were transient, the L-J potential and electrostatic potential energy became negative occasionally during the adjustment. After 1098ps, the L-J potential became constant negative. OGOs were firmly connected by vdWs attraction at a certain point.



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This is the point-process (0~1098ps). After ~ 1464ps, the electrostatic potential became constant negative and electrostatic attraction began to contribute to the aggregation. Since the electrostatic potential value was lower than 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 palates approached each other quickly and more atom pairs were connected. At ~3024ps, a serials of atom pairs was connected and a connecting line was formed. This is the line-process (1098ps~3024ps). After 3024ps, both the potential and the distance reduced quickly (the slope of curves in Figure 2 and Figure 3 became larger), which means the OGOs began to approach to each other more quickly. Therefore, the formation of connecting line indicated the beginning of rapid aggregation. After 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 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 palates were surrounded by WMs and 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 ~3580ps, both distance and L-J potential reduces to the minimum, OGOs were firmly "connected" in a plane, and stable OGO aggregate was formed. This is the plane-process (3024ps~3580ps). After OGOs aggregated plane to plane, there were still several WMs between OGOs (Supporting Information Figure S1). At ~ 3690 ps, the amount of H-bond between OGOs and


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WMs reduced to a minimum, which means all the WMs between OGOs were squeezed out (discussed in detail later). In the point-process, vdWs and the collision of WMs were the dominant driving force. The formation of "point" indicated the beginning of aggregation. In the line-process, vdWs attraction and H-bonds were the dominant driving force. The formation of "line" indicated a rapid aggregation. In the plane-process, vdWs , H-bonds and π-π stacking all played key roles.

Figure 4. Amount of H- bonds between GOs and water.

In the above discussion, oxidized groups in GO were distributed regular 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 SI3). And 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 SI3).



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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, intra-layer and inter-layer H-bonds between function groups. These three configuration of H-bonds were also observed by Nikhil et al.26

Figure 5. Three configurations of H-bonds. (a) H-bonds with water molecules. (b) Inter-layer H-bonds between function groups. (c) Intra-layer H-bonds between function groups. The horizontal black lines in (a) and (b) denote the GO sheet. The inter-layer interactions means H-bonds within different GO layers, and the intra-layer interactions means H-bonds within the same layer. The black rectangle box in (c) denote the GO sheet.

To understand the distribution of H-bond , the number of H-bonds between GOs


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and the surrounding WMs as a function of the intersheet distance was calculated (Figure 6 (a)). As the distance between GO decreased, the amount of H-bond showed a two-step decrease (indicated by dashed line). A minor initial decrease (when the distance was ~1.2nm) followed by a subsequent major decrease (when the distance was ~0.6nm) 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 6 (b)). When the distance decreased from 2.5nm to 1.2nm (initial decrease), only the free WMs were squeezed out. When the distance was 1.2nm, there were two layers of WMs between GOs. As the distance decreased further, there were only one layer of WMs. When the distance was ~0.6nm, 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.



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

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


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Based on the study of aggregation process, the effect 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 a faster aggregation than OGO. This can be explained from two perspectives. On the one hand, the amount of H-bond between OGO and water was larger than that of RGO (Figure 4). We had proved that in the plane-process, WMs between GO palates must be squeezed out. The more H-bonds formed between GO and WMs, the longer time was needed to squeezed 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 palates was ~0.5 nm and 0.43 nm. Daniela 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 nm and 0.6 nm, 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. When two GOs approached to each other, four forces were involved: the collision of WMs,



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electrostatic interaction, vdWs and H-bond interaction. Since the amount of WMs in these systems were almost the same, the collision from WMs should be the same. vdWs is related to molecular weight, but the molecular weight of G-COOH is larger than G-OH. The electrostatic interaction was weak. Therefore, the differences in aggregation rate were mainly caused by H-bond. It can be seen in Figure 9 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 WMs adsorbed on GO, the longer time was needed to squeezed out the WMs between GOs. Picaud et al. suggested a preferential adsorption of water on graphite surfaces containing COOH rather than OH sites33. On this occasion, it is more difficult for G-COOH to aggregate.


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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 from each other initially, the faster they aggregated.

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



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Except for oxygen content and functional group, the initial distance between the GOs also had effects on the aggregation process: the closer GOs from each other, the faster they aggregated (Figure 7 and Figure 8). As we mentioned in the introduction, physiochemical conditions had effect on the aggregation of GO. For example, due to 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 (40mM) and the aggregation process was also found to be a point-line-plane process (Supporting Information Video 2). But the charge screening process is complicated and hard to simulate by molecular dynamics. In our future study, we will combine the experiment with molecular dynamics simulation and explain the effect of physiochemical conditions from a new perspective. CONCLUSION The aggregation of GO in water were explored by MD. The microscopic process was found to be a point-line-plane process. First, two GO palates 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 force. The formation of "point" indicated the beginning of aggregation. In the line-process, vdWs and H-bonds were the dominant driving force. The formation of "line" indicated a rapid aggregation. The last stage is plane-process, vdWs , H-bonds and π-π stacking all played key roles in this stage. The formation of “line” was crucial


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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 spacing of OGO and GGO was 0.5 nm and 0.43 nm, respectively. Compared with G-COOH, G-OH was more easier to aggregate. And the closer GOs from each other initially, the faster they aggregated. The formation of H-bond is critical in the line and plane process, 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 aggregation behavior of GO in water.

ACKNOWLEDGMENT This paper is supported by National Natural Science Foundation of China (Grant No. 51278147), the Funds for Creative Research Groups of China (Grant No. 51121062) and State Key Laboratory of Urban Water Resource and Environment (Grant No. 2013DX03). Supporting Information Available Videos are included to illustrate the point-line-plane aggregation process of GOs in water. Supporting figures illustrate equilibrated structures of GO aggregates and the aggregation process of GOs with different initial configurations. This information is available free of charge via the Internet at http://pubs.acs.org



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The aggregation of graphene oxide in water was found to be a point-line-plane process.


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Molecular Dynamics Study of the Aggregation ... - ACS Publications

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