Molecular-Level Insights Into the Stability of Aqueous Graphene Oxide

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Molecular-Level Insights Into the Stability of Aqueous Graphene Oxide Dispersions Prerna Bansal, Ajay Singh Panwar, and Dhirendra Bahadur J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00464 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

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The Journal of Physical Chemistry

Molecular-level Insights Into the Stability of Aqueous Graphene Oxide Dispersions Prerna Bansala, Ajay Singh Panwar*b and Dhirendra Bahadur*c

[a] Prerna Bansal Department of Metallurgical Engineering and Materials Science Indian Institute of Technology Bombay, Mumbai, 400076, India E-Mail: [email protected] [b] Ajay Singh Panwar* Department of Metallurgical Engineering and Materials Science Indian Institute of Technology Bombay, Mumbai, 400076, India E-Mail: [email protected] [c] Dhirendra Bahadur* Department of Metallurgical Engineering and Materials Science Indian Institute of Technology Bombay, Mumbai, 400076, India E-Mail: [email protected]

*Corresponding authors: [email protected], [email protected]

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Abstract We present a comparative experimental and molecular dynamics (MD) simulation study to elucidate the role of oxygen functionalization on aqueous dispersibility of graphene oxide (GO). Our experimental results indicated better aqueous dispersibility of hydroxyl-rich GO due to distribution of hydroxyl groups on GO basal plane, covering more GO surface area than the edge carboxyl groups. Photo-luminescence and UV absorbance results indicated that hydroxylrich GO consists of maximum number of well exfoliated GO layers, which leads to the formation of more stable GO dispersions. Further, MD simulations and thermodynamic calculations clearly indicated that the potential of mean force is most repulsive for hydroxyl-modified GO sheets, in comparison to epoxy and carboxyl-modified GO sheets. An increase in the number of hydrogen bonds between GO and water molecules was observed with increased functionalization. The present experimental study, underpinned by MD simulations, suggests that concentration, distribution and chemical nature of individual oxygen functional groups present on GO surface determine the strength and nature of interfacial interactions between GO and water, which in turn decides the stability of aqueous GO dispersions. Thus, our results provide a mechanistic insight into the stability of GO dispersions, and a guide for controlling GO aqueous dispersibility by custom functionalization.

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1. Introduction Graphene-composites are an active area of research due to enhancements in optical1, mechanical2, thermal3, catalytic4 and electrical5 properties of the final composite material6. In order to achieve improvements in composite properties, graphene should be homogeneously dispersed in the matrix phase of the composite7. Thus, the final properties of a graphene composite depend strongly on the extent of interfacial interaction between the graphene (dispersed phase) and the matrix phase (which could comprise of diverse materials such as polymers8 or oxides9 or metals10). The hydrophobic nature of graphene, very large surface area, and consequently high van der Waals attraction energies make dispersion of graphene extremely difficult within the majority of solvents. Therefore, the use of covalently functionalized graphene oxide (GO) sheets which are obtained by oxidation of graphite by Hummer’s method11 is a better approach for the preparation of graphene composites. GO is an atom thick layer of graphene, covalently decorated with various oxygen functional groups, which make it hydrophilic. According to the well-known Lerf-Klinowski model12, hydroxyl and epoxy groups are randomly distributed on the GO basal plane while the carboxyl group is present mostly on the GO sheet edges12-13. The possibilities to tune the structure of GO by varying the concentration and distribution of these oxygen functional groups on the GO sheet edges and basal plane makes GO an appropriate material for various applications14-15. In the recent years, the role of oxygen functional groups on the surface behavior and roughness of GO has been studied using 3D-atomic force microscopy16. The effect of functional groups in GO on its ability to reinforce an epoxy resin has been investigated17. Also, the role of functionalization has been studied for sensing application18 and carbon nanotubes19. A recent 3



study has reported the effect of functional groups in GO on the electrochemical performance of GO based lithium-ion batteries20. In another research highlighting biological applications of GO21, the oxygenated functional group density in GO was found responsible in determining cellular toxicity. Also, the effect of chemical functionalization on mechanical properties of graphene polymer composite has been studied

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. The impact of oxygen functional groups on

electrochemical capacitive behavior and on supercapacitor application of different carbon-based materials has also been investigated

23-24

. Such reports demonstrate that over the years several

experimental investigations have been conducted to understand the importance of oxygen functional groups in GO on its various physical properties. Understanding of dispersion stability of graphene-based materials is important while dispersing them in different media during composite preparation and for applications which would require material in suspension form. Also, several studies have been conducted to understand the dispersion of graphene25-27, GO28, reduced graphene oxide (RGO)29 and carbon nanotubes30 in different organic media25-28, 30-31. In a recent molecular simulation study, authors 32

showed that non-covalent surface modification of graphene sheets by anionic modifier

molecules (sodium salt of 6-amino hexanoic acid) leads to an electrostatic long-range repulsion between the graphene sheets. In another research, the aggregation behavior of GO in pure water and in presence of electrolyte was studied using laser light scattering (LLS)33. In addition to experimental studies, there exist in the literature, several molecular simulations based studies, which offer molecular level insight on the role of chemical functionalization of graphene. For instance, first-principles density functional theory (DFT) calculations have been employed to study the role of major functional groups on the electronic34 4


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and optical35 properties of GO. Molecular dynamics (MD) simulations have been used to study the functionalization mediated interaction of GO with lipid membranes in the context of the use of GO as a carrier for biomedical applications36. The deformation behavior and plasticity of GO were found to be controlled by interaction between oxygen functional groups present on GO surface using MD37. Temperature driven phase transformations in GO were studied through reactive force field (ReaxFF) based MD simulations, indicating diffusion of oxygen functional groups, which finally affected the optical properties of GO38. Edge-functionalized graphene sheets embedded in poly-ethylene matrix were simulated to study spatial and dynamical heterogeneities of this system39. The effect of functionalization on the quantum capacitance of graphene has been explored by first principle study40. Recently, functionalization dependent heat transfer in graphene nanoflakes is studied using ReaxFF based MD simulations41. In another study, the chemical changes of oxygen-containing functional groups upon annealing were simulated through MD42. Annealing was found to lead towards more stable GO structures, preventing the complete reduction of GO to graphene42. Hence, a wide range of both experimental and computational studies have focused on the role of oxygen functional groups on sensing18, electrochemical20, biological21, supercapacitor23, electronic34, optical35, mechanical37 and thermal41 properties of GO.

However, the role of

individual oxygen functional groups on the aqueous dispersibility of GO has not been attempted till date by any experimental or computational method to the best of our knowledge. Understanding the role of individual oxygen functional groups on dispersion stability of GO is of utmost importance for the design of improved solution processing techniques for the preparation of more stable GO suspensions43 and for synthesis of GO composites through wet chemical routes. By the current experimental procedures for oxidation of graphite44, it is not 5



possible to synthesize GO with individual oxygen functionalizations45. Hence, conducting the experimental investigations on the role of individual oxygen functional groups in GO is not possible45. The dispersion of GO in aqueous media can be strongly influenced by the type of oxygen functionalization, the extent of functionalization and the arrangement of oxygen functionalities on the GO surface. Thus, a combination of a detailed experimental study which investigates dispersion stability, and complementary MD simulations which investigate hypothetical chemically distinct oxygen functionalized GO surfaces, can help ascertain the role of oxygen functionalization on the dispersion stability of GO in a systematic manner. In this study, we present direct experimental measurements and complementary MD simulations to examine the role of the type, concentration and distribution of oxygen functionalization on the dispersion of GO in aqueous media. In order to understand the role of individual oxygen functional groups, we synthesized hydroxyl-rich, epoxy-rich and carboxylrich GO samples by oxidation of graphite. The presence of different oxygen functional groups in GO was confirmed by FTIR and XPS measurements. We carried out UV absorbance and photoluminescence (PL) measurements to study the dispersion stability of as-synthesized GO samples. Further, MD simulations were utilized to evaluate the extent of dispersion of differently functionalized GO samples in water by calculating the potential of mean force (PMF) between GO sheets. In addition, hydrogen bonding analysis was performed based on the MD data to comment upon the extent of interaction between functional groups of GO and surrounding water molecules in the simulation box.

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2. Materials and Method 2.1 Materials synthesis and characterization An improved Hummer’s method46 was used for oxidation of pure graphite flakes (SigmaAldrich, 45μm, 99.99%). For this, a mixture of graphite flakes (500mg, 1 wt equivalent) and KMnO4 (4.0 g, 8 wt equivalent) was added to a 9:1 mixture of concentrated H2SO4 and H3PO4 (60:6.66 ml). This reaction produced a slight exotherm to 45-55 °C. The reaction mixture was first cooled to room temperature and then again heated at 55 °C with continuous stirring for 12 h. Further, it was kept in an ice bath with 30% H2O2 and filtered using nylon filter paper (0.2μm pore size), using vacuum filtration assembly. The residue was purified by washing with water, 30% HCl and ethanol alternately for several times and later, it was coagulated with di-ethyl ether. The obtained material was filtered and vacuum dried at 60 °C overnight, giving approximately 1gram of GO powder. Further, to understand the role of oxygen functionalization on the dispersion behaviour of GO, we prepared five different GO samples by varying the graphite:KMnO4 wt ratio from 1:6, 1:3 , 1:2, 1:1 to 1:0.5. As-synthesized GO samples were taken for different characterizations to check the presence of different oxygen functional groups. Based on our FTIR and XPS measurements we observed that samples prepared with 1:6, 1:3 and 1:2 graphite:KMnO4 wt ratio indicated the presence of most prominent hydroxyl, epoxy and carboxyl functional groups, respectively. While GO samples synthesized with 1:1 and 1:0.5 graphite:KMnO4 wt ratio showed relatively very less functionalization. This way by varying the ratios of the precursors (graphite: KMnO4), we could tune the functionalization in GO samples. In this study, GO samples prepared

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with 1:3, 1:6 , 1:2, 1:1 and 1:0.5 precursor concentrations are referred as GO(A) to GO(E), respectively.

2.2 Simulation Details Molecular dynamics simulations were carried out using the open-source package LAMMPS47 with periodic boundary conditions applied in all three directions. The well-known CHARMM force field was employed to describe all inter-atomic interactions in the system

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,

and the standard TIP3P model was used to describe water molecules in the system. All simulations were conducted in an NPT ensemble at a temperature of 300 K and a pressure of 1 atm. A Langevin thermostat was used for maintaining the system temperature with a damping time constant of 100 fs. To maintain the pressure in the system, a Berendsen barostat was used with a time constant of 1000 fs. A simulation time step of 1 fs was used for all simulations. The thermodynamic perturbation method, which has been previously used to calculate the PMF between various nanoscopic solutes

49-52

, was used to calculate the PMF between the two GO

sheets as a function of their separation distance, d. The details of the simulation methodology can be found in our recent article

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and in the Supporting Information. The separation distance, d,

between GO sheets was varied from 3 – 21 Å in steps of 0.2 Å. Thus, separate simulations were carried out corresponding to a particular value of d, where the system was equilibrated for a total of 1 ns before the perturbation calculations. It is important to note that the GO sheets (including the attached functional groups) are held fixed during the simulation (rendered immobile by zeroing the net force on the GO sheets). The PMF was chosen to be zero at the largest GO sheet separation.

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3. Results and discussion Figure 1(a) is a schematic of the simulation setup with two GO sheets of size 34×30 Å2 separated by a distance ‘d’ along the Z-axis. These were solvated in a water box of dimensions 60×60×60 Å3 with nearly 7000 water molecules (Figure S1 in the Supporting Information presents the solvated GO in the simulation box). Three different types of oxygen functional groups, namely hydroxyl (-OH), epoxy (-O-), and carboxyl (-COOH), were considered to investigate the role of oxygen functionalization on the interaction between GO sheets in water (Figure S2 in the Supporting Information presents the simulation snapshots of GO structures with different oxygen functionalization, water molecules are removed for better clarity of GO sheets). The functional groups were distributed randomly only on one side of the GO surface such that the two parallel functionalized GO surfaces faced each other as presented in Fig. 1(a). For epoxy and hydroxyl groups, three different concentrations of functional groups corresponding to ~2.5%, 5.8% and 9.9% oxygen functionalization per GO sheet were considered. Consequently, n = 14, 32, and 54 functional groups (corresponding to these concentrations) were attached to GO basal plane to understand the role of increasing functionalization on GO basal plane. Only two concentrations were considered for the case of the carboxyl-functionalized GO sheets (n = 16 (~2.9%) and n = 32 (~5.8%)) because fewer numbers of carbon atoms are present at the sheet edges and carboxyl-functionalization is usually present on the sheet edges as per reported models for structure of GO12. This way by varying n, we could model the effect of varying the degree of oxidation in GO. These functionalization concentrations are in good agreement with the values reported in the available literature for functionalized GO structures12,

53-54

. Though, in the present study we have chosen only three

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functionalization concentrations to mimic the GO structures, our MD simulations can be scaled up to higher degrees of functionalization by considering larger system sizes in the simulation.

Figure 1 (a) Simulation snapshot of two hydroxyl functionalized GO sheets arranged parallel to each other at a separation d = 10Å. Water molecules are not shown for better clarity of GO sheets. Carbon, hydrogen and oxygen are colored in blue, red and yellow, respectively (Snapshot is captured in visual molecular dynamics (VMD)) (b) Partial charge assignment of covalently functionalized graphene oxide (GO) structures chosen for present study. Here, each 10


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functionalized GO sheet is charge neutral. The charge is distributed on the oxidized carbon of the GO sheet and on the oxygen, carbon and hydrogen atoms of the functional groups. The charges on the carbon atoms not bonded to any oxygen functional group were set to zero. The covalently functionalized GO sheets considered in our simulations are charge neutral on the whole. However, partial charges are assigned to the oxidized carbon atoms of GO sheets and on the oxygen, carbon and hydrogen atoms of the covalently attached functional groups55. The charges on the carbon atoms not bonded to any oxygen functional group were set to zero. Figure 1(b) shows the partial charge assignments for the three types of functionalized GO sheets used in present study. To understand the effect of oxygen functionalization on dispersion behaviour of GO, we prepared five GO samples GO (A – E). Figure 2 presents Fourier-transform infrared (FTIR) spectra of GO samples (A-E), recorded using JASCO spectrometer (6100 type-A) with KBr as reference. A broad peak corresponding to –OH stretching is observed at nearly 3400 cm-1. The band at 1730 cm−1 is assigned to the carbonyl (C=O) stretching mode of the undissociated carboxylic group. The band at 1618 cm−1 is due to C=O of carboxylic group, whereas the band at 1115 cm−1 is due to the C−O stretch, and the band at ~1050 cm−1 is due to the C−O−C stretch of the epoxy group46. In all the samples, out of plane ring bending of aromatic C-C was found between ~550 – 900 cm–14.

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Figure 2 FTIR spectra of GO (A – E) with different chemical functionalizations. These GO samples indicate varying degrees of oxidation. On the basis of the intensity of the most prominent peak, samples GO(A) – GO(C) are referred as hydroxyl-rich, epoxy-rich and carboxyl-rich GO, respectively. From the FTIR spectra we observe that sample GO(A) shows a sharp peak of –OH at ~3400 cm–1. Whereas, GO(B) shows a prominent peak at 1060 cm−1 corrosponding to C-O-C group. Sample GO(C) is rich with –COOH functional group and contains very less C–OH and C–O-C functionalization. GO(D), on the other hand, contains much reduced –OH and C–O-C groups and GO(E) is the least functionalized GO sample. We observe that C-C (aromatic stretch), C-H and C-O were slightly shifted in different GO samples (A – E), due to the change in 12


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hydrophobic environment present in the GO with different functionalization. From FTIR spectra, we deduce that on the basis of the intensity of the most prominent peak, sample GO(A), GO (B) and GO(C) may be classified as hydroxyl-rich, epoxy-rich and carboxyl-rich GO, respectively. Figure S3 (see Supporting Information) presents the Raman spectra of GO samples (A-E). In our study Raman spectra of GO (A-E) showed D peak at ~1349 cm-1 and G peak at ~1581 cm-1. The width of the 2D band (>2500cm-1), which is an overtone of defect-related D-band was found to be dependent on the degree of exfoliation of GO samples (A-E). Nearly similar 2D-band in Raman spectra of samples GO(A – E) suggests multilayered structures presumably with similar number of layers. It may be mentioned that in this work, GO(A – E) were synthesized by the oxidation of the same precursor material (pure graphite flakes purchased from Sigma-Aldrich, 45μm, 99.99%) under identical conditions with different Graphite:KMnO4 for GO(A – E). Figure S4 (see Supporting Information) presents the HRTEM micrographs and the respective diffraction patterns of GO samples (A-E). HRTEM micrographs show the multilayered sheet-like structure of all GO samples (A-E) with sample dimensions upto few microns. Further, the percentages of carbon and oxygen were calculated by energy dispersive X-ray spectroscopy (EDX) analysis (see Supporting Information). X-ray diffraction (XRD) patterns for bare graphite and GO(A-E) are presented in Fig. S5 (see Supporting Information). For bare graphite, a sharp [002] diffraction peak is observed at ~26°. After oxidation process this [002] diffraction peak of graphite shifts towards lower 2θ angle (~10°), indicating an increase in the interlayer spacing for GO(A-E), due to insertion of various oxygen functional groups on surface of graphite, post-oxidation. We observed a broad peak evolving at ~ 20-25° for samples GO(C), GO(D) and GO(E), indicating an increase in the graphitic character of these GO samples. Such changes in the XRD spectra 13



(crystalline nature) of GO(A-E) suggest an increased graphitic nature of GO(C), GO(D) and GO(E) than samples GO(A) and GO(B). Further, X-ray photoelectron spectroscopy (XPS) was used to analyze the molecular bonding of as-synthesized GO using an electron spectroscopy chemical analysis probe (MULTILAB, Thermo VG Scientific) with a monochromatic Al Kα radiation of energy ~1486.6 eV. Qualitative analysis of oxygen functionalization in GO was done by recording C1s core level XPS spectra to understand the surface composition and variation of oxygen functional groups in GO.

Figure 3 (a) – (e) C1s XPS spectra of GO(A – E) with different chemical functionalizations. The C1s spectra is deconvoluted as a superposition of four peaks at ~284.5, ~285.6, ~286.2 and

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~288.8 eV56. These peaks correspond to C–C carbon, hydroxyl carbon (C-OH), epoxy carbon (C-O-C) and carboxyl carbon (HO–C=O) respectively. Figures 3(a) – (e) present C1s spectra of different GO samples, which are Gaussian fitted (using software XPS Peak 4.1 with shirley background substraction) and deconvoluted in four peaks at ~284.5, ~285.6, ~286.2 and ~288.8 eV56-57. These peaks represent the C–C carbon, hydroxyl carbon (C-OH), epoxy carbon (C-O-C) and carboxyl carbon (HO–C=O) respectively58 59

, indicating a considerable degree of variation in oxidation of GO. C1s XPS spectra of all GO

samples (A – E) indicate the presence of a XPS peak at ~284.4 eV corrospond to C-C aromatic carbon. In addition to this, sample GO(A) shows a major peak at ~285.31eV and a minor peak at 288.33 eV, indicating the presence of mainly the hydroxyl group and much less carboxyl functional groups. GO(B) shows a major XPS peak at 286.61 eV indicating prominence of epoxy functionalization and two small peaks at 285.09 eV and at 288.34 eV corresponding to weak hydroxyl and carboxyl functional groups. XPS spectrum of GO(C) indicates a major peak at 288.38 eV of carboxyl group and other small peaks at 285.69 eV as hydroxyl and at 286.56 eV as epoxy group. On the other hand, samples GO(D) and GO (E) are much less functionalized. GO(D) shows two weak peaks at 285.52 and 286.21 eV representing much reduced hydroxyl and epoxy functionalization. Sample GO(E) indicated a single XPS peak corrospond to C-C aromatic carbon (~284.4 eV) and the peaks related to specific major oxygen functional groups were not present in sample GO(E). The FTIR spectra of GO(E) indicated the similar behavior with the presence of C-C and C-H vibrations while C-O binding was not seen. Hence, XPS results indicate that GO(A), GO(B) and GO(C) are well- oxidized GO, indicating the presence of major oxygen functional groups. For GO(A), GO(B) and GO(C) peak intensities corrorponding to the hydroxyl, epoxy and carboxyl functional groups in C1s XPS spectra dominate, 15



respectively. These results are in accordance with our FTIR results and thus we consider GO(A), GO(B) and GO(C) to be hydroxyl-rich, epoxy-rich and carboxyl-rich GO samples, respectively. The qualitative analysis of the peak area ratio of four components of C1s spectra of GO samples suggest that for GO(A), GO(B), GO(C) and GO(D) nearly 35, 55, 18 and 15% GO are oxydized with various oxygen functional groups, respectively. For 1:3 graphite: KMnO4 conc. more number of hydroxyl groups were formed on graphene basal plane than epoxy and carboxyl group, as indicated by the most prominent peak of hydroxyl group in the FTIR and XPS spectra of sample GO(A). While for 1:6 and 1:2 graphite: KMnO4 conc., more number of epoxy and carboxyl functional groups were formed on graphene sheet edges and basal plane than the surface hydroxyl groups. For lower values of graphite: KMnO4 conc. (1:1 and 1:0.5), no prominent oxygen functional groups were seen in XPS and FTIR spectra of GO samples. To understand the dispersion behavior of GO, aqueous GO suspensions (1mg/ml) were prepared by sonication of GO(A – E) in miliQ water for 1hr. The aqueous dispersions of all GO samples (A – E) were found to be uniform immediately after sonication process. These GO samples were kept over a period of one week for visual inspection to observe their dispersion stability with time. Figures 4(a) – (d) present photographs of GO(A – E), taken at different time intervals (immediately after sonication (0hrs), 4hrs, 24hrs and one week).

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Figure 4(a) – (d) Photographs of aqueous GO(A – E) dispersions. These pictures were taken at the following time intervals: immediately after sonication (0hrs), after 4hrs, 24hrs and one week post-sonication. Here, hydroxyl-rich GO (A) and epoxy-rich GO (B) indicate the formation of well-dispersed stable aqueous suspension even after one week post-sonication. Carboxyl-rich GO sample (C) showed intermediate dispersion stability while sample GO (D) and (E) indicate the formation of aggregates after one week post-sonication process. We observed that suspension of GO(A) and GO(B) are highly uniform and stable even after a week post-sonication. However, the dispersion of GO(C) showed intermediate stability. For GO(D) and GO(E), on the other hand, significant decrease in the suspension stability was noticed with time, indicated by aggregation of these GO samples at the bottom of the vials with time. 17



From visual observation, it was not possible to estimate the exact dispersion behavior of GO. Such visible changes in the stability of aqueous GO dispersions over time were determined more precisely by recording UV absorbance spectra of GO dispersions60. The UV absorbance of the dispersion is directly proportional to the quantity of the individual GO sheets dispersed in water. Higher value of UV absorbance suggests the well-dispersed nature of GO while lower value of absorbance is an indication of formation of GO aggregates.

Figure 5(a) – (e) UV absorbance spectra of aqueous GO(A – E) dispersions recorded over a period of one week. Here, GO(A)-GO(C) refers to hydroxyl-rich, epoxy-rich and carboxyl-rich GO, respectively. Hydroxyl-rich GO (A) and epoxy-rich GO (B) indicate the formation of welldispersed stable aqueous suspension even after one week post-sonication. Carboxyl-rich GO sample (C) showed intermediate dispersion stability while sample GO (D) and (E) indicate the formation of aggregates within one week post sonication process.

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Figures 5(a) – (e) show the UV absorbance spectra of aqueous GO(A – E) in the wavelength range of 200 – 800 nm. It is well reported that UV absorbance spectra for GO shows an absorption peak between ~ 210 – 240 nm, which is attributed to the π→π* transition of the CC bonds46. The other shoulder peak at higher wavelengths (~ 300 – 350 nm) may be present which is related to n→π* transitions, indicating the presence of defects and vacancies in GO38, 61. The small differences in energy gaps between GO(A – E) is due to the change in the environment of the localized sp2 carbons in GO due to different functional groups present on GO sheet edges and basal plane in these samples. A high absorbance value (~ 2.0) was observed for all GO samples immediately after sonication. This was followed by the measurement of absorbance after certain time period. The difference between these two absorbance values determines the stability of the prepared dispersion through UV measurements. It was observed that for samples GO(D) and GO(E), the absorbance values decreased significantly to ~0.6 and ~0.2 after one week, indicating the aggregation of these GO samples with time. Visual inspection of aqueous GO(D – E) dispersions (Fig. 5) showed similar aggregation behavior of these two GO samples with time. For hydroxyl-rich GO(A), the absorbance value was almost same as the initial absorbance even after one week (~ 2.0), which is an indication of the formation of a uniform stable dispersion. For epoxy-rich GO(B), the absorbance value after one week was found to be ~ 1.8, which was not much different than the initial absorbance (~ 2.0). However, for carboxyl-rich GO(C) the absorbance value after one week was ~1.2, indicating some aggregation of GO sheets with time. The higher absorbance observed for hydroxyl-rich GO(A) and epoxy-rich GO( B) even after one week post-sonication indicated that these GO dispersions are highly stable,

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leading to their better aqueous dispersibility. We observed a similar behavior of the stability of GO (A-C) dispersions after visual observation presented in Fig. 4. Photoluminescence (PL) emission spectra of GO were recorded using Horiba Fluoromax4 spectrophotometer at 220 nm excitation wavelength. PL emission spectrum is a direct consequence of dangling bonds and defect states in any material. From Fig. 6, it was observed that the PL spectra for highly functionalized well exfoliated samples GO(A), GO(B) and GO(C) exhibit two PL bands at ~400nm and ~455nm, corresponding to intra- and inter-layer electronic transitions62. On the other hand, for multi-layered aggregated GO(D) and GO(E), the band at ~400nm diminishes with time and only the band at higher wavelength is observed due to interlayer electronic relaxation62-63.

Figure 6 Photo-luminescence (PL) spectra of GO(A – E) dispersions with different chemical functionalizations. PL results indicate that the number of well-exfoliated GO layers is reduced significantly from sample GO(A) to GO(E). Here, GO(A) – GO(C) refers to hydroxyl-rich, epoxy-rich and carboxyl-rich GO, respectively. 20


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According to well-known Lerf-Klinowski model12, most of the carbon on GO basal plane is sp3 hybridized and is covalently bonded to epoxy and hydroxyl functional groups. The remaining carbon is sp2 hybridized and bonded with neighboring carbon atoms or with the carboxyl group on GO sheet edges. Thus, GO possess an electronic structure consisting of sp2/sp3 in various ratios64. For our aqueous GO dispersions, we observed blue emission originating from the electron-hole pair recombination65. PL results indicated that hydroxyl-rich GO(A) and epoxy-rich GO(B) consist of the maximum number of well exfoliated GO layers, which in turn leads to superior aqueous dispersibility of these two GO samples. For GO(E), a single weak PL band at higher wavelengths ~460nm is an indication of the presence of aggregated GO layers. These results are in accordance with our UV absorbance results, indicating that functional groups present on GO sheet basal plane in sample GO(A) and GO(B) lead to emergence of well-exfoliated GO layers in water. The chemically synthesized GO samples (A – E) indicated varying degrees of oxidation with different relative concentration of functional groups decorated on GO sheet edges and basal plane. In samples GO(A) and GO(B), the majority of the oxygen functional groups were present on GO basal plane while for GO(C), the prominent carboxyl group was attached to GO sheet edges. The UV absorbance and PL spectral results, along with visual inspection indicated that hydroxyl-rich GO(A) and epoxy-rich GO(B) exhibit excellent aqueous dispersibility. Sample GO(C) showed intermediate aqueous dispersibility with carboxyl group present mostly on the sheet edges. GO(D) and GO(E) had very less oxygen functionalization and got aggregated in water with time. Thus in terms of attached functional groups on GO surface, the order of dispersion stability is Hydroxyl > Epoxy > Carboxyl functional group. The analysis of 2D-band (stacking order) in Raman spectra of GO(A-E) (see supporting information) indicated that 21



GO(A-E) are multilayed structures with a broad 2D-band for GO(A-E) that can be fitted with multiple peaks due to the splitting of electronic band structure of multi-layer graphene samples. The changes in the XRD spectra (crystalline nature) of GO(A-E) about an increased graphitic nature of GO(C), GO(D) and GO(E) than samples GO(A) and GO(B) were found to be in good agreement with our Raman, FTIR and XPS results. The same was found to be correlated with our UV and PL measurements through a decrease in the colloidal stability of samples GO(C), GO(D) and GO(E) due to their higher graphitic nature than samples GO(A) and GO(B). From the above discussion, based on the experimental results, it is inferred that it is primarily the chemical nature of attached oxygen functional groups and their distribution (basal plane and edges) on GO surface, which decides the stability of aqueous GO dispersions. Based on our experimental results, we propose a hypothesis that the presence of hydroxyl and epoxy groups on the basal plane in GO(A) and GO(B) renders the GO surfaces increasingly hydrophilic, leading to net repulsive interactions between GO sheets. This results in more stable and well-exfoliated aqueous dispersions corresponding to samples GO(A) and GO(B). We believe that for the least functionalized sample GO(E), strong van der Waal’s attractive interactions between GO layers lead to its aggregation in water. Further, we assume that hydrogen bonding between GO layers and water molecules might be responsible for attracting more number of water molecules in neighborhood of hydrophilic oxygen functional groups in GO and thus could be playing a role in determining stability of GO dispersions in water. To support our hypothesis of change in the nature of interactions between GO surfaces in aqueous media (attractive to repulsive) as a function of oxygen functionalization , we conducted extensive MD simulations to understand the influence of individual oxygen functional groups 22


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(covalently attached to GO) on the nature of interaction between two GO layers. By current experimental procedures for graphite oxidation (Hummer’s method11), it is not possible to synthesize GO with individual oxygen functionalization. Consequently, it is not possible to systematically study the role of specific individual functional groups in GO through an experimental route. In such scenario, MD simulations provide a way to uncover the role of individual functionalization in GO distributed specifically on GO edges and basal plane. Next, we will discuss results from our MD simulations and related thermodynamic calculations of covalently functionalized GO structures to study how individual functional groups (epoxy, hydroxyl and carboxyl) contribute towards the stability of aqueous GO dispersions. Figures 7(a) – (c) show the potential of mean force (PMF) between two GO sheets as a function of sheet separation for (a) epoxy (-O-), (b) hydroxyl (-OH) and (c) carboxyl (-COOH) functionalized GO. For bare graphene sheets with no chemical functionalization (n = 0), the potential well is the deepest with a contact minimum interaction value of -580 kcal/mol at ~3.5Å separation indicating a strong van der Waal’s attraction between bare graphene sheets. This interaction potential approaches to zero at a distance ~12Å indicating a short-range attractive interaction between the GO sheets. The minima of the PMF curve should correspond to the equilibrium separation between the two GO sheets and the maximum attractive force between them49. With increasing covalent functionalization, the value of this minima for epoxy functionalized GO increases from ≈ -510 kcal/mol for n = 14 to ≈ -40 kcal/mol for n = 54 (Fig. 7(a)). In addition, PMF values are found to consistently increase for all sheet separations with increasing epoxy functionalization. This indicates a qualitative change in the interaction between GO sheets from attraction to repulsion with increasing functionalization. 23



Figure 7 The total calculated PMF as a function of inter-sheet distance ‘d’ between two GO sheets for the four cases of bare graphene sheets (n = 0) and GO with n = 14, n = 32 and n = 54 respectively. Here, ‘n’ is number of attached functional group to each GO sheet. For (a) epoxy(O-), (b) hydroxyl(-OH) and (c) carboxyl(-COOH) functionalized GO. Contribution to the total PMF due to interactions between GO and water molecules for (d) epoxy(-O-), (e) hydroxyl(-OH) and (f) carboxyl(-COOH) functionalized GO. Contribution to the total PMF due to interactions between two GO sheets for (g) epoxy(-O-), (h) hydroxyl(-OH) and (i) carboxyl(-COOH) functionalized GO, respectively.

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A small positive peak is observed at ~5Å. This peak

become more positive with

increasing functionalization with values of -60 kcal/mol, -5 kcal/mol, 70 kcal/mol and 290 kcal/mol for n = 0, n = 14, n = 32 and n = 54, respectively. This peak can be interpreted as a kinetic barrier to aggregation suggesting slower aggregation with increasing functionalization. Both the minima in the PMF and the peak corresponding to the “kinetic barrier” show shifts to higher sheet separations with increasing functionalization. Figures 7(b) and (c) also show similar trends in the PMF between GO sheets, namely, a change from a strong attraction for the case of bare graphene to an increasingly repulsive interaction with addition of hydroxyl and carboxyl groups on the GO surfaces, respectively. For the same functionalization concentration of n = 14, the PMF value was ≈ -510 kcal/mol for the epoxy functionalized GO and ≈ -280 kcal/mol for the hydroxyl functionalized GO. Thus, between hydroxyl and epoxy groups present on the GO basal plane, the hydroxyl groups seem to provide better dispersion stability. A similar behavior was observed for all three functionalization concentrations. For the same number of functional groups (n = 32), the interaction energy between carboxyl functionalized GO sheets was found to be less repulsive (-490 kcal/mol) in comparison to that for the cases of both epoxy functionalized GO (-265 kcal/mol) and hydroxyl functionalized GO sheets (-48 kcal/mol). This suggests that functionalization on GO sheet edges (carboxyl groups) was not as effective as functionalization on the GO basal plane (epoxy/ hydroxyl) in enhancing dispersion stability of GO. Thus, the distribution (edge/basal plane) of functional groups on GO surfaces plays a significant role in deciding the stability of GO dispersions. Figure S8 in the Supporting Information shows the contribution of the non-bonded

25



pair interactions to total PMF which indicates that the total PMF is mainly due to non-bonded pair interactions in the system. In Figures 7(d) – (i), we plot the important contributions to the total PMF of the system as functions of the GO sheet separation. The contributions to the total PMF were calculated on the basis of the pair interaction energies between different groups of atoms; corresponding to GO and water (Figs. 7(d) – (f)), and between the two GO sheets (Figs. 7(g) – (i)). From Figure 7(d) we observe that the GO-water interaction energy turns to be increasingly repulsive from a value 20kcal/mol for bare graphene to 690 kcal/mol for GO with n = 54 epoxy functional groups attached per GO sheet. Similar behaviour of GO-water interaction energy was observed for hydroxyl (Fig. 7(e)) and for carboxyl functionalized GO (Fig. 7(f)). Hence, an increased functionalization of graphene makes the graphene surface more hydrophilic, arising from an increased proximity of water molecules to the GO surfaces66. This lead to a corresponding increase in the GO-water PMF plots as shown in Figs. 7(a) – (c). An increase in the strength of the repulsive PMF with n would be a direct result of a decrease in the average separation distance between GO-water pairs that are used to compute the PMF. These results suggest that functionalization of graphene makes the GO surface more hydrophilic, improving its contact with water molecules, which leads to a repulsive interaction between GO sheets. Further, the repulsion increases with increasing concentration of functional groups on the GO surfaces. A similar effect was seen in a recent study

32

where the graphene-water interaction potential

became progressively more repulsive upon increasing the concentration of non-covalent surface modifier molecules. An increase in the concentration of surface modifier molecules (sodium salt of 6-amino hexanoic acid) lead to an increased adsorption of modifier molecules at the graphenewater interface, which rendered the graphene surfaces increasingly hydrophilic. 26


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Figures 7(g) – (i) show the contribution to the total PMF due to interactions between two GO sheets. The non-bonded pair interaction energy between GO-GO pairs was used for this calculation. It is found to be strongly attractive over short ranges. The PMF shows a deep minimum of -580 kcal/mol for bare graphene sheets at ~3.5Å which rapidly approaches to zero at around 8Å sheet separation. From this we conclude that this interaction is the major contributor to the total calculated PMF for the bare graphene sheets. From the curve we observe a shift in the position of minima of interaction energy curve towards greater sheet separation with increased functionalization in comparison to bare graphene sheets. This shift toward larger sheet separations is due to an increase in the van der Waals thickness of the GO sheets with increasing functionalization. The minima in the PMF curves also shift upward to more positive values with increasing functionalization of GO sheets, indicating an increased steric repulsion between the GO sheets due to addition of functional groups on the graphene surfaces. Figures 8(a) – (f) show the distribution of water molecules in the simulation box along the Z-axis (axis of GO sheet separation) for epoxy groups at (a) d = 10 Å (b) d = 20 Å, for hydroxyl groups at (c) d = 10 Å, (d) d = 20 Å, and for carboxyl groups at (e) d = 10 Å, (f) d = 20 Å, respectively. The number of water molecules were calculated for volumes that were sliced in the X −Y plane with thicknesses of z = 0.1Å. For the two values of d considered here, the GO sheets were located at z = ±5 Å and, z = ±10 Å, respectively. Since the functional groups are added only to the inner surfaces of the GO sheets (opposing surfaces), changes in the water density distribution, as a function of functional group concentrations, are observed most significantly in the space between the GO sheets. In the space between the sheets, water molecules are interacting mostly with the other water molecules. We note that the water density reduces slightly in the immediate vicinity of the GO sheets (within approximately 3.5 Å of a GO 27



sheet) with increasing functional group concentration. As more functional groups are added to the GO surface, they displace some of the water molecules in the region next to the surface (roughly the van der Waals thickness of the sheet). However, a systematic increase in water density with functional group concentration is observed at a distance of approximately 5 Å from either GO sheets (in the region between the two sheets). The effect can be most easily observed in the water density distribution plots in Figs. 8(b) – (d) for the cases of the epoxy and hydroxyl functionalization. Thus, the change in water density distributions leads to a decrease in the average separation for water-GO pairs leading to an overall increase in the non-bonded pair interaction energy with increase in the concentration of functional groups (as shown in Figs. 7(d) – (f)). For the case of the carboxyl functionalization, we do not observe any significant changes in water density distributions since functional groups are present on the sheet edges.

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Figure 8 Plots of number density distribution of water molecules along the Z-axis (separation axis) for epoxy group at (a) d = 10 Å (b) d = 20 Å, for hydroxyl group at (c) d = 10 Å (d) d = 20 Å and for carboxyl group at (e) d = 10 Å (f) d = 20 Å, respectively.

Study of the hydrogen bonds (H-bonds) is a key requirement to evaluate the water-water and water-solute interactions in the system. H-bonds analysis was carried out on the MD data using the Hydrogen Bonds analysis plug-in of the Visual Molecular Dynamics (VMD) software67. To calculate the number of hydrogen bonds between GO and the surrounding water molecules, a standard geometrical criterion for hydrogen bonds was used68-69. According to this criterion, we consider a bond to be a hydrogen bond only if the donor-acceptor distance is 29



smaller than 3.5Å and the D–H∙∙∙A angle is larger than 30° (where D is a hydrogen donor, H is a hydrogen atom, and A is a hydrogen acceptor). The position of the oxygen atom in a water molecule was considered as the position of the water molecule for this calculation.

Figure 9 Average numbers of hydrogen bonds (H-bonds) between the GO and water as a function of inter-plate distance for different functionalization concentrations per GO sheet (n) for (a) epoxy (b) hydroxyl and (c) carboxyl functionalized GO. (d) Number of H-bonds with functionalization concentration for epoxy, carboxyl and hydroxyl functionalized GO.

Based on the above criteria, we calculated the average numbers of H-bonds between the GO and water molecules as functions of sheet separation. The results are presented in Fig. 9. As 30


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we can see, the number of H-bonds formed between GO and water in Figs. 9(a) – (c) are consistent with the behavior of water-mediated potential energy change from Figs. 7(d) – (f). We observe that with increased functionalization there is an increase in the number of H-bonds between water and functional groups of GO sheets, indicating increased hydrophilicity of individual GO sheets with functionalization. Figure 9(d) indicates that a larger number of Hbonds are formed between hydroxyl functionalized GO basal plane and water molecules than the epoxy functionalized GO basal plane. These results further support our other results that functional groups on the GO basal plane (hydroxyl and epoxy) are more effective in forming stable GO aqueous suspensions than the carboxyl groups occurring on the GO edges. Also, the hydroxyl group appears to be a better candidate in terms of providing better stability to GO dispersions in water than the epoxy group. It should be noted that this analysis was performed for the entire simulation box, and not only for the confined region between two functionalized GO layers. Hence, for carboxyl group which is present on GO sheet edges a significant number of Hbonds were formed between the functional group and the surrounding water molecules, not limited to space between two functionalized GO layers.

On the basis of the above discussion, we observe that the dispersion of GO in aqueous media is strongly dependent on the type of oxygen functional groups, and their concentrations and distribution on the GO surface. With increasing degree of oxygen functionalization, the interaction changes from strongly attractive between bare graphene sheets to an increasingly repulsive interaction , indicating an increase in dispersion and stabilization. For instance, in the case of ~9.9% hydroxyl functionalized GO sheets, the highly attractive interaction energy for bare graphene (-580kcal/mol) shifts to a more repulsive value of -40kcal/mol. This quantitative 31



potential may be used to evaluate the formation of aggregates during dispersing GO sheets in water and while making any GO composites.

4. Conclusions In summary, a comparative experimental and MD simulation study is carried out to understand the stability of aqueous GO dispersions. Through systematic experimental observations on the GO samples rich in either hydroxyl, epoxy, or carboxyl groups, respectively, we were able to establish a clear dependence of the extent of GO dispersion in aqueous media on the type of functional groups, their concentrations and their distribution on the GO surface. A significant decrease in the UV absorbance intensity was observed with decreasing oxygen functionalization, which in turn lead to agglomeration of less functionalized GO dispersions with time. PL results indicated that hydroxyl-rich GO consists of the maximum number of wellexfoliated GO layers. This was followed by epoxy-rich and carboxyl-rich samples. However, samples deficient in oxygen functionalization showed greater aggregation of graphene sheets. Thus, experimental results strongly suggest that basal plane modification of graphene (with either hydroxyl or epoxy groups) is more effective than edge functionalization of graphene with carboxyl groups for preparing stable GO dispersions. Further, hydroxyl functionalization seems to be more effective in comparison to epoxy functionalization. In order to explore the dispersion mechanisms for GO, model GO surfaces (with either hydroxyl, epoxy or carboxyl functionalization) were simulated using MD to estimate the strength and nature (attractive or repulsive) of interaction between two GO sheets in water. The extent of interaction between two functionalized GO sheets was evaluated by calculating the PMF as a function of inter-sheet distance using the thermodynamic perturbation method. The simulations 32


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clearly showed that the presence of functional groups on two parallel GO basal planes stabilize the GO layers through strong repulsive forces between them and prevent their aggregation in water. The strength of the repulsive interaction between the sheets increases with increasing concentrations of functional groups for both hydroxyl and epoxy groups. However, for carboxyl groups present on GO sheet edges, the entire GO basal plane acts as a graphitic surface leading to interaction energies that are nearly the same as for bare graphene sheets. Analysis of hydrogen bonding data showed that both hydroxyl and epoxy-functionalized GO surfaces are stabilized through intra-layer and inter-layer H-bond formation mediated by water molecules. Further, the extent of stabilization is the largest for the case of hydroxyl functionalized GO, both in terms of the most repulsive PMF values and the largest number of hydrogen bonds present. Thus, our simulation results are in direct correspondence with experimental findings and provide a clear mechanism for the aqueous dispersion of a hydrophilic solute based on its structure and chemical functionalization. On the basis of our simulations results, supported by corresponding experimental observations, we conclude that GO with functional groups on its basal plane is indeed a better dispersed solute in water than GO with functional groups available only at its edges. Thereby, the stability of GO dispersions sturdily relies on the distribution of oxygen functional groups on the GO surface. Through a series of systematic experiments and MD simulations, this study clearly establishes a link between the strength of interfacial interactions of GO in water, and the concentration, distribution and chemical nature of individual oxygen functional groups present on a GO surface. In addition, we find that graphene sheets deprived of oxygen functional groups are not well dispersed in water, and form aggregates due to strong attractive forces between 33



them. Covalent oxygen functionalization renders the GO sheets hydrophilic due to enhanced Hbonding between functional groups and water molecules. Thus, the present detailed molecularlevel study can guide the synthesis of more uniform GO dispersions with better understanding of the interfacial chemistry between GO and water molecules.

Supporting Information Thermodynamic perturbation method, Schematic of hydrated GO system chosen for MD simulations, Schematic of GO with different covalent oxygen functionalizations, Raman Spectra of GO samples, HRTEM micrographs of GO samples, XRD Spectra of GO samples, Elemental analysis of GO samples, Contribution to total PMF due to non-bonded pair interactions in the system, Minimum interaction energy from PMF calculations.

Acknowledgements The authors would like to thank Nanomission-DST, Government of India, for providing financial support to conduct this research. Also, they thank the central characterization facilities at SAIF, IIT Bombay. We greatly acknowledge the computational resources in MEMS, IIT Bombay, India.

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Molecular-Level Insights Into the Stability of Aqueous Graphene Oxide

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