Preparation of Highly Stable Dispersion of Graphene in Water

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Materials and Interfaces

Preparation of Highly Stable Dispersion of Graphene in Water with Aid of Graphene Oxide Hanyang Gao, Guoxin Hu, and Haijun Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03771 • Publication Date (Web): 31 Aug 2019 Downloaded from pubs.acs.org on August 31, 2019

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Industrial & Engineering Chemistry Research

Preparation of Highly Stable Dispersion of Graphene in Water with Aid of Graphene Oxide Hanyang Gaoa,*, Guoxin Hub, Haijun Liua a

School of Mechanical Engineering, Hangzhou Dianzi University, Xiasha Higher Education Zone, 310018 Hangzhou, Zhejiang Province, China

b

School of Mechanical and Power Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, 200240 Shanghai, China *[email protected]

Abstract Utilization

of

graphene

(GE)

in

water

is

physically

limited

by

its

lack of functional groups, high hydrophobicity, and large interlayer van der Waals forces. Easy agglomeration of GE makes the preparation of GE-filled composites in aqueous environment very difficult. Conventional methods solve this problem by introducing surfactant to the system. However, the addition of a large amount of surfactant may hinder the performance of composite. In this study, highly stable GE aqueous suspensions (remain stable for more than 30 days) with high concentration (1.5 mg/ml) were conveniently prepared by using graphene oxide (GO) as stabilizer. GE and GO filled rubber composite prepared by this GO-GE water suspension in aqueous environment was also prepared and analyzed. It was found that a dual-network with dual functions was formed in the composite matrix: the GE network enhanced the electrical conductivity of the composite while the GO network enhanced its mechanical properties. Our method is green, simple and high-efficiency, providing an alternative to improve GE's dispersion in water and synthesize GE-filled composites in aqueous environment. Key words: Dispersion in water; Aqueous dispersion; Water soluble graphene; Interfacial behavior; Self-assembly

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1. INTRODUCTION In recent years, graphene sheet (GE) has attracted enormous attention from academia and industry due to its extremely high mechanical strength, carrier mobility, electrical conductivity, and thermal conductivity1, 2. Due to the strong van der Waals interaction between layers and the hydrophobicity of carbon surface, it is quite difficult to disperse GE in water and synthesize GE-based composites in aqueous environment3, 4, leading a great limitation in the industrial application of GE. Researchers have proposed a variety of solutions to solve this problem. These methods can be generally divided into two categories: covalent modification and non-covalent functionalization5-7. Covalent modification enhances the dispersion of GE by destroying its stable π orbital and improving the surface activity8. However, the destruction of structure will impair their performance in thermal and electrical conductivities. Non-covalent functionalization refers to the adsorbing of modifiers on GE surface by non-covalent bonds (such as hydrogen bonding, π-π interaction, polar interaction, etc.)9. Since this modification does not damage the structure and impair performance, it is receiving more and more attention now10.

In most non-covalent functionalization approaches, metal ions or oxides materials are loaded as modifiers10; surface-active agents (such as sodium dodecylbenzene sulfonate, polyvinyl alcohol, polyacetylene, polyvinylpyrrolidone, etc.) are used for π-conjugated molecular stacking11. These methods can help to obtain a stable GE aqueous suspension, however, irrelevant agents are passively introduced into in the system and usually remains since they are difficult to remove completely even with additional cleaning steps.

Graphene oxide (GO) is kind of chemically modified GE. It is decorated with a large number of hydrophilic oxygen functional groups (such as epoxides, alcohols, carboxylic acids, etc.)12, and can be stably dispersed in water13. According to previous reports, GO is an amphiphilic substance with hydrophilic edges and hydrophobic planar14, and therefore can adsorb on interfaces like surfactants to reduce surface tension15. In previous studies16, 17, GO has successfully been used as surfactant to enhance the dispersion of carbon nanotubes or graphite particles in water. However, as far as we know, there are currently few reports on using GO to promote pristine 2 ACS Paragon Plus Environment

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GE’s dispersion in water. This may be hindered by an operational difficulty: GE sheets agglomerate immediately once they were added into water, leaving GO no chance to attach on.

In this research, we propose a simple and fast method to prepare water-dispersible pristine GE by using GO as stabilizer and NMP as intermediary agent. GO acts as surfactant to non-covalent attached on surface of GE. Experiments have shown that this GO modified pristine GE can stably remain in water for more than 30 days with high concentration (up to 1.5 mg/ml). Our research also demonstrated that to a GO-GE-rubber composite prepared by adding GO-modified GE to waterborne rubber emulsion, a mechanically enhanced (by GO) and electrically enhanced (by GE) dual-network was formed in the matrix.

Compared with ordinary non-covalent method, this method does not introduce any useless impurities into the system, avoiding the effect of irrelevant agents on the performance of composite. This method provides an alternative method for the preparation of GE-based composites in aqueous environment.

2. EXPERIMENTAL SSECTION 2.1 Materials. The natural graphite powder used in the study was supplied by Shandong Qingdao Graphite Company (Qingdao, China). Analytically pure N-methylpyrrolidone (NMP) was purchased from Aladdin Reagent Co., Ltd. (Shanghai, China), and used without further purification. Graphite oxide (model SE2430W) was supplied by Changzhou Sixth Element Materials Technology Co., Ltd. (Changzhou, China). The natural rubber latex was purchased from the Thai Rubber Latex Co., Ltd. (Thailand) and stored in ammonia (0.4 v/v %) at room temperature.

2.2 Preparation of NMP-coated pristine Graphene. Pristine GE sheets (mono- or multi-layer) was prepared by sonication assisted liquid-phase exfoliation18, 19. In a typical preparation process, graphite powder (2.5 g) was added in NMP (500 mL), and the mixture was sonicated in ultrasonic water bath for 16 h (180 W, 20 KHz, continuous mode) at room temperature. The solution was then placed in a centrifuge tube and centrifuged at 3000 rpm for 10 min. The supernatant liquid was then 3 ACS Paragon Plus Environment


vacuum-filtered and NMP-coated GE slurry was obtained. Stored the slurry in a sealed container for further use.

2.3 Preparation of GO suspension. Before exfoliation, NaOH was added in graphite oxide suspension to neutralize the residual acid. The filtered graphite oxide was then washed repeatedly with a large amount of deionized water to remove the ionic species. After this cleaning step, graphite oxide was re-added into a water-containing beaker for ultrasonic exfoliation. After 1.5 h (180 W, 20 KHz, pulse mode, on/off 3 s/1 s) ultrasonic treatment, the suspension was vacuum filtered and dried at 80 °C for 24 h. The dried GO powder was then redispersed into water to prepare GO water suspension with desired concentration.

2.4 Preparation of GO-GE water suspension. A desired amount of graphene was added into a beaker containing a certain amount of GO. Then, the mixture was ultrasonicated (180 W, 20 KHz, pulse mode, on/off 1 s/1 s) at room temperature for 1 h.

2.5 Preparation of GO-GE natural rubber. The composite material was prepared by aqueous-phase emulsion blending and mechanical blending. In a typical procedure, GO-GE aqueous suspension was added into vigorously stirred natural rubber emulsion. Then, the mixture was poured into a mold and dried to obtain masterbatch. The masterbatch, conventional additives (accelerators, antioxidants, etc.), and sulfur powder were then subjected to two-roll milling, hot-pressing (150 °C, 12 min), and vulcanization process in turn to obtain the composites.

2.6 Material characterization. The microstructures of the GO-GE suspension were characterized using a 120 kv transmission electron microscopy (TEM, Talos L120C G2, Thermo Fisher Scientific, USA). To prepare the sample for TEM testing, a few drops of aqueous GO-GE dispersion was dropped onto an ultrathin carbon film coated copper grid and dried at room temperature for 24 h. Lateral force microscopy (LFM) image was obtained by atomic force microscopy (AFM, Hitachi Nanonavi E-Sweep, Japan). UV-visible spectroscopy was conducted on a UV-Vis spectrophotometer (UV/EV300, Thermo, USA). The dispersion samples were all 4 ACS Paragon Plus Environment

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diluted before testing. Fourier-transform infrared spectroscopy (FTIR) was recorded on a FTIR spectrometer (Nicolet 6700, Thermo, USA) with a resolution of 4 cm-1. The stress-strain curves of the sample films were tested on a universal electronic tensile machine (Model 4465, Instron, USA) with speed of 500 mm/min at 22 °C. The electrical conductivities of the samples were measured in real time by an electrochemical workstation (CHI 660D, Chenhua, China). For the conductivity measurement, the rubber was cut into 5 mm ×30 mm×2 mm block with silver paste coated on the both ends.

3. RESULTS AND DISCUSSION 3.1 Characterization of GO-GE aqueous dispersion and formation mechanism. We prepared a series of GO-GE aqueous dispersion with different concentrations to investigate the effect of GO addition and the ratio of GO to GE on the dispersion stability of GE in water. It was found that when dispersing NMP-coated GE in pure water (without GO), the length of the stabilization time was negatively correlated to GE concentration. When the concentration was low (0.1 mg/ml), GE could be uniformly dispersed in water for about 20 min; when the concentration was as high as 2 mg/ml, it quickly agglomerated after less than 5 min (Figure 1a). Moreover, shaking and temperature was found to hurt the stabilization of GE. Shaking, stirring, or high storage temperature would accelerate the agglomeration and precipitation of graphene. It was also found that no matter how long the ultrasonic treatment, no NMP-coated GE could not be dispersed in water at all and would directly precipitate.

Figure 1. (a) NMP-coated GE (2 mg/ml) quickly agglomerated in water; (b) stable GO aqueous dispersion with high concentration; (c) stable GO-GE mixture made by adding GE into GO suspension. (d) GO-GE mixture in which the addition time of GO was 10 minutes later than that of GE. (e) NMP-free GE agglomerated in GO suspension. 5 ACS Paragon Plus Environment


It can be reasonably inferred from these phenomena that, the coating of NMP, which is miscible with water20, was the decisive factor for the temporarily stable dispersion of GE in water. Shaking and high temperatures would accelerate the migration of NMP into water and therefore caused a rapid agglomeration of GE. Owing to the strong hydrophobicity and large specific surface area, GE surface which was gradually exposed due to the migration of NMP was agglomerated and precipitated in water (Figure 2). This kind of agglomeration could not be reversed even a long time ultrasonication treatment was applied.

Figure 2. The migration of NMP from the GE surface into water causes the explosion, agglomeration, and precipitation of GE.

Due to the presence of abundant hydrophilic groups on surface and edges, GO can stably and uniformly suspend in water at high concentration (Figure 1b). After adding NMP-coated GE into GO aqueous suspension, the mixture turned into black from brown. The mixture was very stable at room temperature with a certain GO: GE ratio -- no significant agglomeration occurred after 1 month standing and a following centrifugation at 3000 rpm for 10 min (Figure 1c). This phenomenon indicated that the presence of GO significantly improved the dispersion of GE in water. The influence of GO: GE ratio and their concentration on the stability of GE in water was shown in Figure 3 and summarized as follows: 

The increase of GO concentration is conducive to GE stability;



With a certain concentration of GO, the concentration of GE is inversely proportional to its stability; 6 ACS Paragon Plus Environment

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

Increasing the concentration of GO, the highest concentration of GE that can be stabilized by it will also increase.

Figure 3. Effect of the concentration of GO and GE on the stability of GO-GE mixtures. The concentration of GE is (a) 0 mg/ml, (b) 0.5mg/ml, (c) 1mg/ml, (d) 1.5mg/ml, and (e) 2 mg/ml.

It was found that the addition timing of GO was also important. When GO was added to water before GE (that is, adding GE in GO suspension), the stability of the mixture was quite good (Figure 1c), and no visible aggregation was formed over 1 month. When GE was added before GO, the later the GO was added, the poorer the stability of the mixture became. Figure 1d is a picture of the GO-GE mixture, to which the GO was added 10 minutes later than the GE. After 3 days of standing, precipitate gradually appeared at the bottom. No NMP-coated GE was also dispersed in GO suspension with a 1:1 ratio. However, after 30 minutes standing, almost all of the GE precipitated at the bottom of the beaker (Figure 1e). UV-vis testing was conducted on GO, GE and GO-GE dispersions to make a further analysis (Figure 4). NMP curve was also shown in the figure to avoid the interference. The characteristic absorption peak of GE was located at ~266 nm, corresponding to the π→π* transition of the aromatic C-C bond21-23. For GO, the absorption peaks in spectra at 230 nm and the small broaden shoulder at 300 nm were assigned to the π→π* transition of aromatic C–C bonds and the n→π* transition of C=O bonds, respectively, in accordance with previous studies22, 24, 25. For the GO-GE mixture, the shoulder peak at 300 nm had no significant shift compared to the spectrum of GO, while the absorption peak at 230 nm was red-shifted to ~247 nm, indicating that a large π-conjugation was formed between GO and GE. It is reasonable 7 ACS Paragon Plus Environment


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to deduce that the GO aromatic region and the surface of GE combined together by π-stacking interactions16.

Figure 4. UV-vis spectra of GO, GE, and the GO-GE mixture.

To further verify this conclusion, the FTIR spectra of GE, GO, and GE-GO was analyzed (Figure 5). The characteristic absorption peak of GO was consistent with the reported results. The broad peaks at 3412 cm-1 were assigned to the stretching vibration band of OH groups. The bands at 1721 cm-1, 1616 cm-1, 1405 cm-1, and 1058 cm-1 belonged to the carboxyl C=O stretching band, C=C band, O-H deformation vibration band, and C-O stretching vibration band, respectively24,

26

.

These characteristic bands were relatively weaker in the FTIR spectra of GO-GE. Combined with the above results, it is reasonable to deduce that the weakening of bands should be caused by the coverage of GE.


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Figure 5. FTIR spectra of GO, GE, and the GO-GE mixture.

TEM images can provide more intuitive information. Figure 6a showed that, after 30 days standing, no obvious agglomeration in the suspension could be seen in the GO-GE aqueous dispersion with high concentration (GE: 1.5 mg/ml; GO: 2 mg/ml). However, when GE sheet is flatly attached to the GO, these two kinds of thin sheets are difficult to distinguish. Luckily, when multilayer GE is attached on GO surface in a folded way, its folded edge will form a face parallel to the incident beam of electrons (Figure 6d)27-29, making the stacked carbon layers of GE distinguishable from the GO planar in image. It can be clearly seen from Figure 6b, c that, many folded multilayer GE sheets with a relatively small size were stacked on the large surface of GO sheets. The fringe spacing in Figure 6c, which is also the d value in Figure 6d, corresponds to the interlayer spacing of GE27. The unfolded or monolayer GE are difficult to distinguish from GO sheets in the TEM images. The images in a lateral force microscopy (LFM) mode can be used as a high contrast visualization tool to distinguish between GE and GO. In the LFM image (Figure 6e, right), brighter pixels correspond to lower frictional force. Since pristine GE has reduced frictional interaction with the AFM tip relative to GO, the brighter region is GE and the dark region is GO30, confirming the combination of GO with GE.



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a

c

b

d

e

LFM image GO GE

3.2 nm

1 μm

0 nm


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Figure 6. TEM images of (a) GO-GE aqueous dispersion with high concentration (GE:1.5 mg/ml; GO:2 mg/ml) after 30 days standing, and (b) multilayer GE stacked on the surface of the GO sheet. Folded multi-layer GEs are shown within the white dashed frame. The sheets outside the frame were unfolded or monolayer GE or GO. (c) The enlarged picture of the selected area (blue dashed frame) in b. (d) The drawing of the folded edge of the multilayer GE shows how graphene planes become parallel to the incident beam of electrons and appear in the high-resolution TEM image. The d value corresponds to the interlayer spacing of GE. (e) LFM image of GO-GE.

Combined with the above information, the process of preparation of highly stable aqueous dispersions of GE nanosheets with aid of GO is inferred as follows (Figure 7). After adding NMP-coated GE to GO aqueous suspension, GE surface is gradually exposed to water due to the transfer of NMP molecules into water. Owing to the presence of a large amount of GO, GE sheets contact with GO in time and attach to its surface. The oxygen-containing functional groups on GO that are not covered by GE keep GO-GE stably dispersed in the water, preventing the occurrence of agglomeration.

Figure 7. Process of preparation of highly stable aqueous dispersions of GE with the aid of GO: With the migration of NMP, the gradually exposed GE sheets make contact with the GO and become attached to its surface, which prevents the agglomeration.

3.2 Electrical and mechanical properties of GO-GE-rubber composites and the dual-network mechanism. 11 ACS Paragon Plus Environment


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GO, GE and GO-GE were added to the waterborne rubber latex to prepared GO-GE-rubber. The electrical conductivities of the composites were tested after drying. Since the composites were synthesized in an aqueous environment, the stability of the components in water would greatly affect the uniformity of their dispersion in solid product31, 32. Figure 8 shows that, when only GE (0-2%) was added to the rubber matrix, the conductivity of the samples was very low and hardly changed. According to our previous stability experiments, this should be attributed to the agglomeration of GE in the matrix. When GO was added alone, the conductivity of the sample did not change much. In contrast, the addition of GO-GE increased the electrical conductivity of the composites by about five orders of magnitude. These results indicate that the addition of GO can increase the degree of dispersion of GE in the matrix, enabling the full utilization of its excellent electrical conductivity.

Figure 8. Effect of the amount of added GE, GO, and GO-GE on the electrical conductivity of the composites.

Next, the effect of the concentration and ratio of GO to GE on the conductivities of rubbers composite (Figure 9) was investigated. 5 ml GO-GE suspensions with different concentration and ratio were added to 500 ml natural rubber emulsion with a solid

content

of

60%.

After

stirring

(30


min),

drying,

two-roll

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milling, hot-pressing, and vulcanization, the GO-GE rubber composites were obtained. Results showed that electrical conductivities of the composites are related to the content of both GO and GE. The obtained results of the experiment are as follows:  When

the GO addition concentration was a certain value in range of 0 % to 2 %,

if the addition amount of GE was low, the conductivity of the sample increased as GE concentration increased. However, when the addition concentration of GE was beyond a certain amount, the conductivity of sample was no longer increased or even decreased slightly. This indicated that, in this case, the addition amount of GO was insufficient to fully disperse GE. The GE which attached to GO may agglomerate with those left in water, resulting in a decrease in stability. This phenomenon is consistent with the result observed in standing experiments.  The

conductivity of the composite material was further improved when the

addition amount of GO was increased, since the amount of GE that was carried by GO was also increased (see data in the light green circle). In this case, more uniformly distributed GE sheets formed a better conductive network in composite.  When

the concentration of GE was low (e.g. 0.5 mg/ml), under the premise of

stability, less GO addition would give the composite a better conductivity (see data in the light red circle). This may be due to the fact that excessive GO will enclose the GE, and had a negative effect on the formation of conductive path.  When

resistance was low, the variance of the currents was also large, indirectly

indicating the unevenness of the mixing. When the conductivity was high (such as the current at GO=2 mg/ml, GE=1~2 mg/ml), the variance of the current was small, indicating a higher degree of uniformity.



Figure 9.

Effect of GE concentration on the electrical conductivity of the composites

when the loading concentrations of GO are different.


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a

b

c

d

Figure 10. Effect of the amount of GO on the (a) modulus at 100 % elongation, (b) modulus at 300 % elongation, (c) tear strength, and (d) stress-strain curves of the GO-GE/rubber composites. The amount of added GE was fixed at 1%.

We also found that the addition of GO not only promoted the dispersion of GE, but also enhanced the mechanical properties of composites. When the addition amount of GE was constant (1 weight percent), the effect of GO amount on mechanical properties of the GO-GE/rubber composite is shown in Figure 10. As the addition amount of GO increased, the modulus at 100 % or 300 % elongation of the composites significantly increased (Figure 10a, b). The tear stress of the samples also showed an increasing trend with the increase of GO (Figure 10c). Compared with the GO-free sample, the tear stress of the sample with 6 % GO increased from 20.13 MPa to 26.42 MPa, increasing by about 31.2 %. Corresponding stress-strain curves (Figure 10d) showed that, for a given strain level, the stress increased with the increase of GO content. It can be concluded from Figure 10 that, the addition of GO is beneficial to the improvement of the mechanical properties of the composites. This result is 15 ACS Paragon Plus Environment


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consistent with previous reports33, 34. The rich oxygen-containing groups anchored in the edges or on the planar of GO, which combined with the natural rubber chain, contribute to the improvement of the mechanical properties of the composite35, 36.

The above experiments show that the addition of GO-GE can improve not only the electrical conductivities of the composites, but also their mechanical properties: the uniform dispersion of GE in the matrix plays a decisive role in improving the electrical properties; while the enhancement of mechanical properties depends on the addition

of

GO.

GE

and

GO

form

a

uniformly

dispersed

electrical-mechanical-enhancement dual-network in the matrix (Figure 11). This also showed that, unlike using ordinary surfactants to promote aqueous dispersion, our method does not introduce any impurities, avoiding interference from any irrelevant additive.

GO

rubber matrix

GE

GO network for GE network for mechanical enhancement conductivity enhancement

Dual network

Figure 11 Electrical-mechanical-enhancement dual network formed by GE and GO in the matrix.

4. CONCLUSION In this study, we propose a green, simple, but effective method to prepare highly stable aqueous dispersions of GE nanosheets with the aid of GO. In the preparation 16 ACS Paragon Plus Environment

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process, when NMP-coated GE is added in the GO aqueous suspension, with the leave of NMP, the gradually exposed GE sheets contact with GO in time and attach to its surface, preventing the agglomeration in water and forming a stabilized GO-GE complex aqueous suspension. It is also confirmed that the increased dispersibility of GE in water contributes to its improved dispersion in the composite prepared in the aqueous phase, resulting in an obvious improvement in electrical conductivity of the composite. Furthermore, the GO in GO-GE can also play an important role in the increase of mechanical strengthening of the GO-GE-rubber composite, suggesting a uniformly dispersed electrical-mechanical-enhancement dual-network in the rubber matrix. Unlike the conventional method of using surfactants as the stabilizer, our method does not introduce impurities, which might otherwise affect the material properties, into the system.

Acknowledgments We gratefully acknowledge the financial support from National Natural Science Foundation of China (51876052). We also thank the Instrumental Analysis Center of Shanghai Jiao Tong University for material characterization.

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Preparation of Highly Stable Dispersion of Graphene in Water

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