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Processable aqueous dispersions of graphene stabilized by graphene quantum dots Peng He, Jing Sun, Suyun Tian, Siwei Yang, Shengju Ding, Guqiao Ding, Xiaoming Xie, and Mianheng Jiang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503782p • Publication Date (Web): 03 Dec 2014 Downloaded from http://pubs.acs.org on December 10, 2014

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Chemistry of Materials

Processable aqueous dispersions of graphene stabilized by graphene quantum dots Peng He,† Jing Sun,† Suyun Tian,‡ Siwei Yang,† Shengju Ding,† Guqiao Ding,†,* Xiaoming Xie† and Mianheng Jiang† †

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, P. R. China. ‡

School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, P. R. China. KEYWORDS: graphene, graphene quantum dot, aqueous dispersion, graphene paper.

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ABSTRACT: Dispersing of graphene in various solvents is one of the key technology towards the practical applications of graphene. Herein, using graphene quantum dots (GQDs) as stabilizer, aqueous dispersions of graphene with good stability were demonstrated by directly dispersing commercialized graphene powder into water. Amazingly, 100 mg graphene powder could be stabilized by an average of merely 7.8 mg GQDs to form aqueous dispersions with a maximum concentration of up to 0.4 mg/mL and stability at least 3 months. The introduction of a small amount of GQDs also allowed for the fabrication of water-redispersible graphene slurry and powder, which would largely facilitate the transportation and applications of graphene. The mechanism of the GQDs stabilized graphene in water was proposed and experimentally verified through ultraviolet-visible (UV-Vis) spectroscopy and zeta potential measurements. Moreover, flexible graphene papers directly assembled from the water-dispersible graphene exhibited controllable thickness, good conductivity and acceptable strength. With properties not compromised by GQDs, the water-dispersible graphene are expected to find wide applications in electrical and electrochemical device fields.

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INTRODUCTION Graphene, due to its remarkable properties, has attracted a tremendous amount of attention from both fundamental and applied science communities. The fascinating properties, including remarkable electronic character, exceptional thermal conductivity, superior mechanical properties and high specific surface area, make graphene the next wonder material with wide potential applications.1 Large-scale fabrication of graphene powders for the industrial application has been realized based on both the oxidation-reduction method2 and the liquid exfoliation route3 from expanded graphite or natural graphite flake. The unique properties, as well as large-scale production, endows graphene powder great promise to construct high-performance materials and devices, such as nanocomposites4, batteries5, supercapacitors6 and hydrogen storage7. Graphene dispersions in various solvents are often required for the preparation of macroscopic graphene-based materials, such as coatings8, films9,10, tapes11 and composites4, 12. Unfortunately, dispersing of graphene is challenging due to strong intersheet van der Waals attractions (π-π stacking), leading to aggregate or precipitate. To address this challenge, careful selection of solvents to induce repulsive solvation forces and hence achieve stable dispersing of graphene with high concentration was systematically studied.13 Both simulation and experimental results showed that solvents with surface energy close to that of graphene (~70-80 mJ/m2) contribute to stable dispersing. Good solvents (e.g., benzyl benzoate, N-methyl-2-pyrrolidone (NMP) and Dimethyl sulfoxide (DMSO)) are basically organic agents with high boiling point (> 150 °C), and very hard to remove in subsequent applications. Therefore, low-boiling solvents such as water and alcohol are more desirable owing to their low cost, easy processing and little environmental effects.14,

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Whereas for aqueous system where surface energy cannot get

matched, attractive hydrophobic forces, instead of repulsive solvation forces, arise and further strengthen the tendency of sheet aggregation in alliance with the van der Waals forces.16 To 3



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obtain water-dispersible graphene, various methods were proposed to introduce steric or/and electrostatic repulsion among adjacent graphene sheets. These approaches can be categorized into three strategies: 1) ionization of the carboxylic acid groups retained on the chemical reduced graphene oxide (GO) by pH controlling;10 2) grafting of specific hydrophilic groups on GO through chemical reactions;17 3) surface absorption of stabilizers such as surfactants15,

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polymers19 or small molecules20, 21 on graphene sheets. While the former two methods are based on the chemical reduction of GO and normally involve the use of toxic reducing agents or tedious chemical routes, surface absorption of stabilizers could directly disperse graphene into water and seems to be the most facile method to obtain aqueous graphene dispersions. However, most additives adhering to the graphene surface are very difficult to remove and may severely compromise the graphene properties.22, 23 Given this, we propose the utilization of graphene quantum dots (GQDs) as a stabilizer to realize aqueous dispersions of graphene sheets. This strategy exploits both the special atomic structure and surface chemistry of GQDs. GQDs are one- or few-layered graphene sheets with lateral dimension smaller than 100 nm and well known for their unique photoluminescent performance.24 It is anticipated that GQDs with 2D sp2 carbon structure would strongly attach to the basal plane of graphene sheets via van der Waals attractions (π-π stacking). On the other hand, GQDs fabricated via chemical oxidation of graphene25 or other carbon materials26 often possess the same hydrophilic surface groups (including carboxyl, hydroxyl and epoxy groups) as GO, due to the similar preparation procedure. These hydrophilic groups, if ionized in water, would bring excellent solubility to GQDs and are possible to disperse the intrinsically hydrophobic graphene sheets. More importantly, presence of the GQDs (i.e. small graphene sheets) is expected not to deteriorate the properties of the graphene sheets. In this contribution, graphene derived GQDs was used to disperse commercially available graphene powder in water. The preparation procedure was described in detailed along with the characterization of the water4


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dispersible graphene, and the mechanism for GQDs enhanced dispersing ability of graphene sheets was also investigated. Finally, to demonstrate the processability, flexible and highly conductive graphene papers were fabricated from the graphene dispersions via a facile vacuum filtration technique.

EXPERIMENTAL SECTION Chemicals Sulfuric acid (H2SO4, 95.0-98.9 %), nitric acid (HNO3, 65.0-68.0 %), sodium hydroxide (NaOH, ≥ 96.0 %), hydrochloric acid (HCl, 36.0-38.0 %), and ammonium hydroxide (NH3·H2O, 25-28 %) was purchased from Shanghai Lingfeng chemical reagent. Co., Ltd. Sodium chlorate (NaClO3, 99.0 %) was supplied by Aladdin Reagents (Shanghai, China) Co., Ltd. All chemicals are analytically pure and used as received without further purification. Deionized water (resistivity ~18.2 MΩ cm, 25 °C) obtained through a Milli-Q system was used throughout all experiments. Graphene powders (G-100) were purchased from SIBAT (Shanghai, China). Synthesis of GQDs GQDs were prepared from graphene powders via an acidic oxidation route. In a typical procedure, mixture of H2SO4 (150 mL) and HNO3 (80 mL) in a reaction vessel was firstly cooled to 5 °C by immersion in an ice bath. 4 g of graphene was subsequently added into the cooled mixture under vigorous stirring motion. 2 h later, 66 g of NaClO3 was slowly added to the mixture (over 1 hour period) in order to avoid a sudden increment in temperature and the formation of explosive chlorine dioxide gas. Upon the complete dissolution of NaClO3, the mixture was vigorously stirred for 48 h at room temperature, with the reaction vessel loosely capped to allow the escape of the generated gas. After that, the mixture was kept undisturbed overnight, and the top two thirds was centrifuged at 5000 rpm for 90 min. The upper layer solution was decanted into a beaker containing 100 ml ice, followed by adding NH3·H2O drop by drop until the pH was adjusted to 6-7. Then, the obtained solution was further purified with 5



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deionized water in a dialysis bag (8000-14000 Da) for 5 days, followed by filtering through a 20nm membrane (AAO, Waterman, Germany) to remove large sheets. Finally, the filtrate was concentrated using a rotary evaporation and lyophilized in a refrigerant drier (SCIENTZ-30F, Ningbo, China) to obtain GQDs powders. Preparation of aqueous dispersion of graphene The aqueous dispersion of graphene was prepared through two-step route comprising the dispersing of graphene in GQDs solution and the removal of freely dispersed GQDs from the complex dispersion. Typically, 250 mg of graphene was added to 100 mL of 1 mg/mL GQDs solution, followed by sonication at room temperature for 2 h in a low power ultrasonic bath (SCIENTZ SB-5200DTN bath sonicator). Then the dispersion was filtrated through a 100-nm membrane (AAO, Waterman, Germany) under continuous stirring, followed by the re-dispersing the residue with deionized water. To remove the free GQDs, re-dispersing and filtration were repeated until no fluorescent signal can be detected by the fluorescence spectrophotometer, and the aqueous slurry of graphene was obtained. Graphene dispersions was prepared by dispersing the graphene slurry with deionized water via bath sonication at room temperature for 30 min. Concentration of the dispersions was controlled by varying the final volume of the dispersions, since the mass of the graphene in every batch of slurry is known. Fabrication of graphene papers All papers prepared in this study were fabricated through vacuum filtration technique. Typically, the dispersions were filtrated through a 100-nm membrane (AAO, Waterman, Germany), leading to free-standing papers that could easily be peeled off from the membranes after drying. Prior to characterization, all papers were dried overnight under 60 °C to remove the remaining water. Characterization Scanning electron microscopy (SEM S4700, Hitachi Inc.) and transmission electron microscopy (TEM G2 20, FEI Tecnai) were used to image the morphology of the samples. Raman 6


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Spectroscopy with the excitation laser line of 532 nm were performed using a ThermoFisher DXR Raman Microscope. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo ESCALAB 250Xi spectrometer. Atomic Force Microscopy (AFM) data were obtained in a Bruker Dimension Icon with a Nanoscope 8.15 in tapping mode. Steady state Fluorescence

emission

spectra

were

obtained

in

a

Carry

Eclipse

Fluorescence

Spectrophotometer. Fourier Transform Infrared Spectroscopy (FTIR) was recorded by a Bruker Vertex 70v FT-IR spectrometer under vacuum (< 100Pa) in KBr pellet. Ultraviolet-visible (UVVis) spectra were recorded on a Carry-100 UV-Vis Spectrophotometer. Malvern Zetasizer NanoZS90 was used to measure zeta potential of the nanosheets in the dispersions. pH value of the dispersions was obtained with a digital pH meters (PHS-SC, INESA Inc.). NaOH (1 M) and HCl (1 M) solutions were used to alter the pH of the graphene dispersions. X-ray diffraction (XRD) patterns were obtained from a X-Ray Diffractometer (Bruker D8 ADVANCE) with a monochromatized source of Cu Kα1 radiation (λ= 0.15405 nm) at 1.6 kW (40 kV, 40 mA). The conductance of papers was characterized by four point probe method using HALL 5500PC. In all cases, the measurements were taken 5 times, and the conductivity was averaged. Tensile tests were performed at a computer controlled electronic universal testing machine (HY-0580, Shanghai) with a controlled strain ramp rate of 2 % min-1.

RESULTS AND DISCUSSION Graphene powder and GQDs Few-layered graphene powder with fluffy appearance (Figure S1a) was purchased from SIMBATT (Shanghai, China). According to the product specifications, the graphene powder was synthesized through chemical intercalation of graphite and subsequent rapid thermal shock exfoliation. This typical powdered graphene can be fabricated in bulk but is inherently aggregated, which severely limits its practical applications. Therefore, solutions to the dispersing of the aggregated graphene powder are in urgent demand and expected to provide guidance for 7



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other graphene materials. Detailed structure characterizations were presented in Figure S1b-f. Similar to previously reported graphene powders through chemical oxidation-thermal reduction route,27 heavily agglomerated and wrinkled nanosheets were observed (Figure S1b and S1c). Transmission Electron Microscope (TEM) and Selected Area Electron Diffraction (SAED) revealed that these sheets consist of 2-8 atomic layers with good crystalline structure (Figure S1d and inset). The lateral size and thickness were determined by Atomic Force Microscopy (AFM) to be 0.5-10 µm and 1-3 nm (Figure S1e), respectively. Note that for TEM and AFM measurements of the graphene powder, ethanol was used as solvent to temporarily disperse graphene under ultrasonication. Raman spectrum (Figure S1f) further confirmed the sp2 carbon crystal structure (G peak at 1578 cm-1) and the presence of defects (D peak at 1347 cm-1) in the graphene sheets.28 GQDs (Figure S2a) were prepared by cutting the graphene sheets into nano-sized dots through a modified acidic oxidation route (see the Methods section). Normally, the yield can achieve 5060% and about 3-4 g GQDs could be obtained every batch in our laboratory. TEM images (Figure S2b and S2c) revealed that the obtained GQDs have a relatively broad size distribution ranging from 1 to 7 nm and an average size of 3.95 nm (Figure S2e), which was summarized in Figure S2e. AFM (Figure S2d) confirmed the thickness to be less than 2 nm, corresponding to 13 atomic layers. The High-resolution TEM (HR-TEM) image (Figure S2c) also indicated highly crystal domains with a lattice parameter of 0.24 nm, corresponding to the (1120) lattice fringes of graphene. These results suggest that GQDs derived from the graphene powder retained the crystal sp2 carbon structure in basal planes during the oxidation process. In addition, Figure S3 shows the excitation wavelength-dependent fluorescence emission-spectra of the GQDs solution (0.5 mg/mL), further verifying the synthesis of typical GQDs.

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The dispersing ability of graphene powder and GQDs in deionized water was respectively investigated. As expected, GQDs exhibited excellent solubility in water. 10-minute ultrasonication of GQDs generated a uniform brown solution (0.5 mg/mL) that exhibited obvious Tyndall effect when a laser beam passed through (Figure 1a). Moreover, zeta potential of this solution was measured to be ca. -61 mV in average, indicating the formation of highly negative charged surface and well dispersed GQD colloids. By contrast, it was quite difficult for graphene powder to disperse in the deionized water. As shown in Figure 1b-d, even prolonged ultrasonication (2 hours) of 3 mg graphene powder in 30 mL water can hardly unloosen the agglomerate sheets, leading to obvious precipitation and flotage after one day of storing.

Figure 1. Dispersing behavior of GQDs and graphene powder in water. (a) Digital image of GQDs dispersed in water (0.5 mg/mL) with obvious Tyndall effect when a red laser beam passes though, suggesting the formation of uniform dispersion. (b) Digital image of graphene powder precipitate out from water after 2-hour ultrasonication and 1 day standing. (c, d) SEM images of the precipitated graphene powder revealing the agglomerated graphene sheets. C1s XPS spectra of GQDs. (e) and graphene (f) revealed the different surface chemistry in combination their FTIR spectra (g). It is well established that graphene without surface modification cannot well dispersed in water due to the surface energy mismatch.16 The distinct behavior of graphene powder and 9



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GQDs in water can be ascribed to the different surface chemistry. As determined by X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FT-IR) in Figure 1e-f, high content of oxygen (27.8 %) and considerable presence of oxygen-containing (including carboxylic acid and hydroxyl groups) were detected in GQDs (Figure 1e, g). Therefore, the GQDs can be regarded as nanosheets composed of crystal sp2 carbon in basal planes and oxygen-containing groups at edges. After ionized in water, these negatively charged groups afforded the GQDs strong electrostatic repulsion to overcome the van der Waals forces as well as hydrophobic forces, leading to excellent dispersibility. By comparison, graphene powder contained much less oxygen (10.7 %) and lower content of oxygen-containing groups (Figure 1f, g). Consequently, they cannot generate enough surface charge to repulse each other. Upon exposure to water, graphene sheets remain agglomerated together even with prolonged ultrasonication, as a manner to minimize the graphene-water interface area and the total interfacial energy of the system. In an attempt to obtain well-dispersed graphene in water, GQDs were introduced for the first time. Scheme 1 illustrates the detailed preparation process, in which graphene dispersions were prepared through a three-step route including dispersing of graphene in GQDs solution, removal of excess GQDs and re-dispersing of graphene slurry. Detailed procedures are sequentially described in the following sections.

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Scheme 1. A schematic presenting the fabrication process of aqueous graphene dispersion and graphene paper. ① Dispersing of graphene powders and GQDs in water to obtain graphene/GQDs composite dispersion; ② Removal of excess GQDs through vacuum filtration to get graphene aqueous slurry; ③Re-dispersing of the graphene slurry for aqueous graphene dispersion. Dispersion of graphene in GQDs solution As expected, introduction of GQDs significantly improved the dispersion ability of graphene in water. 2-hour ultrasonication of 100 mg graphene in 100 mL GQDs aqueous solution (0.5 mg/mL) resulted in the gradual elimination of the aggregates and finally a homogeneous black dispersion without obvious macroscopic agglomeration (Figure 2a). Surprisingly, no precipitation was observed after one-month storing, indicating the formation of stable dispersion of graphene/GQDs in water.

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Figure 2. Characterization of the graphene/GQDs complex dispersions. (a) Digital image of the graphene/GQDs complex dispersion kept undisturbed for one month. (b, c) Low- and highresolution SEM of deposited paper of the graphene/GQDs complex dispersion on silicon wafer exhibiting the outstretched dispersing state of graphene sheets. To further understand the dispersing behavior of graphene, morphology of the dispersed sheets was investigated by performing SEM measurement on deposited sample of the composite dispersion. As the SEM images show (Figure 2b, c), isolated and stretched sheets were clearly observed, quite different from those aggregated sheets before dispersing (Figure S1b) and dispersed in water without GQDs (Figure 1c, d). This evidently suggested that, with the aid of GQDs, agglomerated graphene sheets can be effectively unfolded in water under ultrasonication. It can also be inferred that all sheets dispersed in water are freely outstretched, which is totally different from the case of crumpled graphene dispersions reported by previous reports.29 To further confirm this, complete filtration of the complex dispersions was performed through a 100-nm membrane (AAO, Waterman, Germany). A free-standing paper (Figure S4a) composed of graphene sheets well aligned in the plane of the paper (Figure S4b) was obtained. As a control, filtration of graphene powder suspensions immediately after 2-hour ultrasonication was also carried out but failed to form paper on the filtration membranes (Figure S4c), indicating the changed dispersing state of graphene sheets after introduction of GQDs. Given aggregated or crumpled graphene sheets can hardly be assembled into well-aligned layered structure,29 isolated and outstretched graphene sheets in the complex dispersion could be speculated. 12


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Undoubtedly, GQDs played a crucial role in stabilizing graphene in water. We speculate that two mechanisms may contribute to the enhanced dispersing ability of graphene. The first one is the surface modification of the graphene by absorption of GQDs, rending the graphene sheets negatively charged and electrostatically stabilized in water. Alternatively, GQDs may also act as steric hindrances that freely distribute around the graphene sheets and block the neighboring sheets from agglomerating together, resulting in the co-dispersing of graphene and GQDs. Considering the surface energy mismatch between graphene and water, surface modification is more acceptable to explain the outstretched graphene sheet in water. However, it is not reasonable to exclude the role that freely dispersed GQDs (FD-GQDs) may play in the dispersing of graphene. So, removal of the FD-GQDs in the complex dispersions is crucial for the understanding of the actual dispersing mechanisms at play. On the other hand, large use of GQDs is not cost-effective for the scalable fabrication of aqueous solution. Removal of the unnecessary GQDs to obtain graphene slurry To separate the FD-GQDs from the micron-sized graphene sheets, the complex dispersions were vacuum filtrated through 100 nm AAO membranes under continuous mechanical stirring (Figure 3a). Before complete filtration, black and thick slurry was obtained (Figure 3b) on the membrane. At the same time, dark-brown filtrate containing ca. 82-90 % of the GQDs could be collected in the first round of filtration, allowing for recycle in the following dispersing. To thoroughly remove the FD-GQDs in water, the slurry was repeatedly washed with large amount of water and vacuum filtrated under stirring. Figure S5 presents the photographs of the filtrates after every washing cycle. To ensure the complete removal of the FD-GQDs, fluorescence spectra of the filtrates were monitored after every washing cycle. As shown in Figure 3c, after 6 times of washing, no fluorescence signal could be detected from the filtrate, indicative of the absence of FD-GQDs in the slurry.

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Figure 3. Removal process of the excess GQDs in graphene/GQDs complex dispersions. (a) Vacuum filtration that extracts GQDs from the complex dispersions. (b) Thick graphene slurry formed after extraction of excess GQDs. (c) Fluorescence spectra of the filtrates obtained after every washing cycle. To analyze the slurry composition, the slurry was dried at 100 °C overnight in a vacuum drying oven to completely remove any trace of water. Given neglectable graphene loss during filtration, the content of GQDs remained in the dried slurry could be obtained, by subtracting the total mass of the dried slurry with that of the initial graphene powder. The mass of the remained GQDs was calculated to be 7.8 mg on average in 5 batches with the same dispersing recipe.

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Therefore, it could be inferred that 7.8 mg GQDs strongly attach to 100 mg graphene powder and cannot be eliminated by repeated washing and filtration. Aqueous dispersion of graphene The resulting aqueous slurry with definite composition could be easily re-dispersed in water by 30 min of sonication and diluted to dispersion with specific graphene concentrations. Figure 4a showed the photograph of 30 mL 0.2 mg/mL dispersion irradiated by a laser beam. Obvious Tyndall effect indicates colloidal nature and homogenous dispersing of the graphene in water.

Figure 4. Characterization of the graphene dispersions. (a) Photograph of aqueous graphene dispersion (0.2 mg/mL) irradiated by a red laser beam displaying evident Tyndall effect. (b) UVVis absorbance spectra of GQDs, GQDs/Graphene and graphene dispersions in water. Insets are 15



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corresponding dispersions in 1 mm quartz cuvettes. (c) Sedimentation behavior of graphene dispersions with different concentrations (0.4-0.8 mg/mL) determining the concentration to be 0.4 mg/mL. (d, e) Low- and highresolution SEM images of deposited graphene sheets of graphene dispersion on silicon wafer depicting the outstretched sheets and confirm the absence of aggregates. (f) AFM image determining the thickness of the graphene sheets to be less than 3 nm. (g) TEM and HRTEM (inset) images displaying GQDs distribution on wrinkled graphene sheets. The concentration could be controlled by varying the final volume of the dispersions. Dispersions with concentration ranging from 0.1 mg/mL to 0.8 mg/mL were prepared. No macroscopic aggregates were observed in as-prepared samples of all concentrations. However, after standing for one day, evident sediments emerged on the bottom of the dispersions with concentration exceeding 0.4 mg/mL. The stability of the dispersions was further monitored using ultraviolet-visible (UV-Vis) spectroscopy. Figure 4b presented the typical UV-Vis absorption spectrum of the graphene dispersion along with that of GQDs solution and graphene/GQDs complex dispersion. Obviously, graphene dispersions showed absorption peak at 276 nm, suggesting the maintenance of sp2 carbon structure during dispersing processes. Compared with the absorption peaks of GQDs (227 nm) and graphene/GQDs (a broad peak between 227 nm and 276 nm), a sharp peak at 276 nm also indicated the removal of the freely dispersed GQDs. According to previous report,22 the absorbance at 660 nm was chosen as an indicator to reflect the change of graphene content against storing time in the dispersions. The stability of three dispersions with different concentrations (0.4-0.8 mg/mL) was studied. As shown in Figure 4c, dispersions with a concentration of 0.4 mg/mL showed no significant loss in absorbance during the initial 60 hours of storing, while significant reduction of absorbance was observed in the 0.50.8 mg/mL samples. These results are in accord with our experimental observations. For dispersions with concentration less than 0.4 mg/mL, no sediment was observed after 3-month storing, indicating an excellent stability of at least 3 months. Therefore, we determine the

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maximum graphene concentration to be 0.4 mg/mL, which is comparable to that of chemical converted graphene dispersions.10 SEM, TEM and AFM were performed to characterize the dispersed graphene sheets. Samples were prepared directly from the diluted graphene dispersion, without addition of any other solvent. Interestingly, deposition of the graphene dispersion gave a thin layer of uniform film on the silicon wafer after complete drying. As clearly shown in SEM images of the deposited graphene film (Figure 4d, e), graphene sheets were completely disengaged and freely outstretched. Figure 4f and g jointly confirmed the dispersed graphene to be wrinkled sheets with thickness less than 3 nm, suggesting the inhibition of graphene sheet restacking in water with the presence of GQDs. Compared with the initial graphene temporarily dispersed by ethanol, the lateral size of the water-dispersible graphene is increased at the first glimpse of the AFM image, which could be ascribed to outstretching of the sheets. Uniform distribution of GQDs on wrinkled graphene sheets can be clearly seen in TEM images (Figure 4g). This can be regarded as a direct evidence for the absorption of GQDs on graphene, rather than deposition of freely dispersed GQDs since we can hardly find any trace of GQDs on the neighboring amorphous carbon grid. As described above, the whole process is quite suitable for the scalable preparation of aqueous graphene dispersion. To further simplify the process, we have tried to eliminate the second step (removal of GQDs) by precisely controlling the initial dosage of GQDs. It was found that 100 mg graphene powder can be dispersed with 7.8 mg GQDs in 250 mL water without obvious agglomerates, which would further facilitate the preparation of the graphene dispersions described above. As alternatives to the vacuum filtration, centrifugation and vacuum-rotary evaporation were used to prepare the graphene slurry and proven to be feasible. A centrifugation rate of 10000 rpm for 15 min was required to completely precipitate the large graphene sheets 17



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dispersed in the water. In addition, we also noted that graphene powder prepared by lyophilizing the graphene dispersions can re-disperse in water via mild ultrasonication. If commercialized, the water-dispersible graphene slurry and powder are believed to largely facilitate the transportation. Dispersing mechanism The re-dispersing of graphene slurry in water after FD-GQDs removal and the good stability of graphene dispersions with proper concentrations strongly support the GQDs absorption mechanism we proposed before. As illustrated in Scheme 2, this mechanism is based on the absorption of GQDs onto graphene surface in aqueous system. We can firstly understand this from the interfacial energy point of view. Possessing both hydrophobic basal plane and hydrophilic edges with abundant oxygen-containing groups, GQDs are commonly regarded to be amphiphilic.25 Therefore, absorption of GQDs would help to reduce the hydrophobic graphene surface exposed to water and thus minimize the total interfacial energy of the mixture system. More importantly, the absorbed GQDs simultaneously rendered the local graphene domain negatively charged. As a consequence, the graphene sheets can be electrostatically stabilized in water over a long period of time when the electrostatic repulsion between graphene sheets surpassed the van der Waals attraction and hydrophobic forces. In other words, the graphene was rendered water-soluble after GQDs absorbing.

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Scheme 2. A schematic illustration presenting the graphene sheets partially covered by GQDs in water. To further verify the dispersing mechanism, we investigated the surface charge of the graphene stabilized by GQDs (briefed as G-GQDs) through zeta potential measurement. As Figure 5a shown, well-dispersed graphene sheets were indeed negatively charged in water and showed similar variation trend to GQDs dispersion of the same concentration (0.2 mg/mL) when pH value changed from 1 to 13. Whereas zeta potential of the initial graphene aggregates in water cannot be normally detected, indicating the sheet surface with few or no charge. Hence, the charged graphene surface must be originated from the absorbed GQDs and the dispersed graphene could also be regarded as electrostatically stabilized colloidal particles in water. Deteriorated stability in dispersions with concentration higher than 0.4 mg/mL is presumably due to the increased possibility of collision and aggregation between graphene sheets, from the perspective of colloidal theory. Given a zeta potential of more negative than -30 mV is necessary for the stable dispersing of general colloidal system,

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we can determine 4-12 as the proper pH

range for our graphene dispersions. Figure 5b depicts the effect of pH value on the stability of 19



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the graphene dispersions. When the dispersion was adjusted to be strong acidic (pH=1) or alkaline (pH=13), graphene sheets completely precipitated out after one-day storing, which is in consistent with the zeta potential results. For GQDs, gradual protonation of the oxygencontaining groups is responsible for reduced zeta potential (absolute value) as pH declining in low pH region.32 In high pH region, the declined zeta potential with increasing pH could be ascribed to the compression of the electric double layer, as a result of the increased ionic strength. The same chemistry processes occurred in the G-GQDs case since zeta potential and stabilization of G-GQDs were originated from GQDs. Accompanying the gradual reduction of the zeta potential, aggregation was observed when pH13, because the electrostatic repulsion provided by surface charge are not enough to resist attractive hydrophobic forces and van der Waals attractions. Interestingly in each case, we found the collected aggregates could be stably re-dispersed in fresh water after mild bath sonication. This experiment clearly suggested that, GQDs can hardly be detached from graphene surface even by altering pH value of the dispersions, which is in contrast to the adhesion between GO and oxidative debris.33

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Figure 5. Experimental evidence of the dispersing mechanism of graphene in water. (a) Zeta potential of G-GQDs and GQDs of the same concentration (0.2 mg/mL) in water as a function of the pH value. (b) Photograph showing the effects of pH value on the stability of the graphene dispersions. We believe it is the π-π interactions between graphene and GQDs that ensure the strong absorption, as they share the same carbon structure (namely domains of conjugated carbon in basal planes as discussed before). Evident red-shift of the characteristic absorption peaks in UVVis spectrum (Figure S6) from 271 nm for surfactant (sodium dodecyl sulfate) dispersed graphene to 276 nm for GQDs dispersed graphene validated the existence of π-π interactions.14 However, apparently for individual graphene sheet in Figure 4g, large fraction of surface was not covered by GQDs. The partially coverage by GQDs is distinct from the cases where surfactants, polymers or small molecules were used as dispersants. This could be ascribed to the structure 21



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defects (such as microcorrugated surface, quasi-amorphous sp2-bonded areas and topological defects) of graphene30, which is not accessible to the GQDs or unable to form firm π-π interactions with the intact graphene area of GQDs. Incomplete surface coverage may adversely affect the stability and maximum concentration of the graphene dispersions, but it also reduce the use of GQDs and is expected to retain the pristine properties (e.g., electrical conductivity) of the graphene. Graphene papers As discussed in the introduction, graphene dispersions have advantage in the fabrication of macroscopic materials compared with their powder counterparts. To demonstrate this, the graphene aqueous dispersions were used to prepare graphene papers via the routine filtration technique, as depicted in Scheme 1. Typically, 100 ml of 0.2 mg/mL graphene dispersion was filtrated through a 100-nm membranes (AAO, Waterman, Germany), followed by drying overnight at 60 °C. Interestingly, the filtration of graphene dispersion could be completed within 90 min, much faster than the filtration of equal volume GO dispersion.34 This may be explained by the fast flow of water molecules on the graphene surface.35

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Figure 6. Characterization of a typical graphene papers. (a) Photographs showing the flexible graphene paper with metallic luster. (b, c) Low- and highresolution cross-section SEM of the graphene paper showing the uniform thickness (12.5 µm) and layered structure, respectively. (d) Small peak in XRD pattern of the graphene paper comparing with that of graphene powder and GQDs powder indicates the π-π stacking of the graphene sheets during paper fabrication process. A free-standing and bendable paper with a diameter of ca. 3.8 cm (Figure 6a) was obtained after peeling off the membrane. Figure 6b and 6c present the low- and high-resolution cross section SEM images. Evidently, well-aligned graphene sheets in the plane of the paper were observed, indicating the parallel deposition of graphene sheets on the filter membrane, just as GO and chemical converted graphene behave during filtration.34 Again, the layered structure and uniform thickness (appr. 12.5 µm) demonstrated the outstretched and homogeneously dispersed graphene sheets in the aqueous dispersions. Additionally, the thickness can be easily controlled from a few micrometers to dozens of micrometer (Figure S7) through varying the volume and 23



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concentration of the dispersions (Table S1). After complete drying, the apparent density of the papers was evaluated to be 1.22-1.51 g/cm3, by dividing the mass with volume. It is worth noting that although composed of water-dispersible graphene nanosheets, the paper exhibited good stability against water and showed no obvious damage even after vigorous shaking in water. This could be ascribed to the partially π-π stacking between the graphene sheets, as revealed by the XRD pattern in Figure 6d. In contrast to graphene powder and GQDs, a relatively strong peak at 23.7°, corresponding to a d002 distance of 0.37 nm, was detected in graphene paper. This peak suggested π-π stacking between the graphene sheets in the paper samples, presumably induced by capillary pressure during water evaporation. As a consequence of interactions, re-dispersing of the graphene sheets in water was hindered, which would allow for application of the graphene paper in aqueous environments. The electric conductivity of graphene paper is measured to be 7240 S/m at room temperature, which is comparable to that of chemical converted graphene based paper.10 Of particular significance is that, unlike insulating polymers or surfactants that commonly used to disperse graphene23, the presence of GQDs showed no significant adverse effect on the conductivity. We further investigated the effects of GQDs content on the conductivity of the graphene papers. Detailed experimental conditions and conductivity data are summarized in Table S2. As presented in Table S2, the average conductivity decreased from 7240 S/m to 2506 S/m when the content of GQDs increased from 7.0% to 28.8%. These results indicated large amount of GQDs in the paper would degrade the 24


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electrical properties, which highlighted the importance of precise control of the GQD amount. We believe that the partially coverage of graphene surface by GQDs, as discussed above, was responsible for the retained high conductivity. The mechanical property was also characterized by directly performing the tensile test on the graphene papers. Figure S8 presents a typical stressstrain of an 18.0-µm-thick graphene paper. The tensile strength, ultimate tensile strain and Young’s modulus were determined to be 91.2 MPa, 1.7 % and 5.3 GPa, respectively. We attribute the comparable tensile strength while nearly doubled tensile strain compared with the chemical converted graphene paper (150 MPa, 0.8 %)36 to the more corrugated graphene sheets. Therefore, directly from graphene powders, our work establish a new route to fabricate bendable graphene papers with controllable thickness. More importantly, the good conductivity and acceptable strength confirmed that no significant adverse effects was caused by GQDs, which endow the water-soluble graphene great promising to be applied in electrochemical fields, such as supercapacitors37 and flexible electronic products38.

CONCLUSION To conclude, we have demonstrated for the first time the feasibility of GQDs as dispersant to disperse commercialized graphene powder in water system. Stable aqueous graphene dispersion with concentration up to 0.4 mg/mL was prepared directly from graphene powder without the use of toxic and hard-to-move chemical agents that are indispensable for conventional ways of preparing aqueous graphene dispersions. The dispersing mechanism that strong π-π interactions induced absorption of hydrophilic GQDs on graphene sheet affords graphene good stability in 25



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water was proposed and experimentally evidenced. Moreover, the flexible graphene papers fabricated from the aqueous graphene dispersion exhibit excellent conductivity and acceptable mechanical strength. The retained properties after GQDs absorption would enable the watersoluble graphene to be applied in electrical and electrochemistry field. Although we demonstrated only the preparation of water-soluble graphene from commercialized graphene powder, the feasibility of utilizing GQDs as stabilizer of graphene presented herein may also give inspirations to the dispersing of other functionalized graphene and the direct liquidexfoliation of graphite for high-quality graphene, which are beyond the scope of this work and will be presented in our future publications.

ASSOCIATED CONTENT Supporting Information Available Characterization of the graphene powder and GQDs. Photoluminescence spectra of the GQDs aqueous solution. Digital image and cross-section SEM image of the graphene paper prepared by filtration of the graphene/GQDs complex dispersion. Digital images of the filtrates after every washing cycle of the graphene/GQDs dispersion. A comparison between the UV-Vis spectra of the GQDs dispersed graphene and the surfactant dispersed graphene. Photos and cross-section SEM images of the graphene papers fabricated with different volumes and concentrations of graphene dispersions. Table summarizing the characteristics of graphene papers and corresponding preparation parameters. Table presenting the properties of graphene papers containing different amounts of GQDs. Typical stress-strain curve of the graphene paper. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Corresponding Author: [email protected] 26


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Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was supported by projects from the National Science and Technology Major Project (2011ZX02707), the National Natural Science Foundation of China (11104303), the Chinese Academy of Sciences (KGZD-EW-303 and XDA02040000).

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Processable Aqueous Dispersions of Graphene ... - ACS Publications

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