The size fractionation of graphene oxide induced by Brownian motion Current Research in Materials Chemistry

The size fractionation of graphene oxide (GO) sheets is essential for its performances. Based on the classical “Brownian motion” theory of microscopic particles suspended in a fluid, a novel, cost-effective and eco-friendly method was developed for the size fractionation of graphene oxide (GO) sheets. Owing to the more intensive Brownian motion of small-size sheets compared to the large-size ones, the small-size GO sheets passed through a self-designed wire-mesh nylon filter and were separated successfully. The size of separated GO sheets were mostly (>90%) smaller than 6 μm, and they showed narrower size distribution compared with original GO sheets before size fractionation.


Introduction
Graphene, a monolayer of sp2-bonded carbon atoms with twodimensional (2D) honeycomb lattice [1], has been widely applied in various fields, such as composites, electronics, transistors and biosensors due to its excellent electrical, optical, thermal and mechanical properties [2][3][4][5][6]. The lateral dimensions of graphene sheets play a key role in controlling their properties and applications [7][8][9][10]. Therefore, it is essential to produce graphene with homogeneous size to meet practical applications. It's known that the most widely used methods to massively prepare graphene with homogeneous size are the reduction of graphene oxide (GO) [11,12]. The size of graphene sheets are mainly depended on the GO sheets. However, the prepared GO sheets always possess wide size distribution inevitably. Hence, the size fractionation of GO sheets is necessary for the reduced graphene oxide (RGO). In recent years, tremendous efforts have been devoted to investigate the size fractionation of GO sheets [13][14][15][16][17]. For example, pH-assisted selective sedimentation [9], the density gradient ultracentrifugation technique (DGU) [13], centrifugation [14], the polar solvent-selective natural deposition method [11], the filtration through using track-etched membranes [15], electrophoresis [16], and directional freezing [17]. However, these methods are difficult to scale up in controlling the size of GO sheets for practical applications, owing to the complicated operating procedures, high centrifugation rate and high cost. Therefore, it is highly desired to develop a facile, scalable and effective method for the size fractionation of GO sheets.
To address aforementioned issues, we originally reported a facile and economical method for separating the small-size GO sheets utilizing the Brownian motion of GO sheets in aqueous solution. GO sheets were prepared by modified Hummer's method. A wire-mesh nylon setup was self-designed for the size fractionation. The GO sheets (<10 μm) evidently passed through the filter into the pure water, while the GO sheets (>10 μm) were kept in the original solution. Such can be supported by the Tyndall effect of the upper solution of GO samples that underwent 12 h stirring the bottom solution. The separated GO sheets (<6 μm) shows narrow size distribution. The size-fractionation mechanism of this method was described in detail in view of Brownian motion theory.
We designed a setup to separate GO sheets, and the small-size GO sheets can be observed by a transparent optical path appeared when we using the laser irradiation treated the upper solution. Interestingly, after fractionation, the small-size GO sheets possessed uniform size distribution. Furthermore, this method can be used easily to produce GO sheets with size mostly smaller than 6 μm.

Materials
Graphite powder with a mesh of 8000 and a purity of 99.9% was obtained from Xia'men Kano Graphene Technology Co., Ltd. Sulfuric acid (H 2 SO 4 , 98%), hydrochloric acid (HCl), hydrogen peroxide (H 2 O 2 , 30%), potassium permanganate (KMnO 4 ), were all analytical grade from Sinopharm Chemical Reagent Co., Ltd. All these materials and chemicals were used as it is without further purifications.

Experiment
The GO was prepared by modified Hummers' method. As-prepared GO was kept as a honey-like colloidal without being dried. The Brownian diffusion device was self-designed as shown in Figure. 2. One glass column was connected vertically to a glass container sealing by two silicone O-rings. A wire-mesh nylon net (≤18 μm) was put between the two O-rings carefully. Experimentally, GO aqueous colloidal (1 ml) was poured into the bottom glass container and stirred magnetically. Then, 150 ml deionized water was gradually added, followed by 12 h stirring. After that, the separated GO sheet were obtained by collecting the upper solution using pipet, and remained to be studied.

Characterization
The size of nylon net was observed by using an optical microscope (59 XD-PC, Shanghai optical instrument factory). Powder X-ray diffraction (XRD) patterns were collected on a Rigaku SmartLab 3KW diffractometer using Cu Kα radiation. A Field emission scanning electron microscope (FESEM) was used to characterize the size and morphology of GO sheets (JSM-6700, operating voltage is 5 kV). showed a sharp [002] peak at 26.6º as shown in Figure. 1, which is attributed to the highly organized lay structure. A new diffraction [001] peak appeared at around 10º in Figure. 1, which was caused by the insertion of oxygen functional groups. It showed that GO was synthesized by a modified Hummers' method successfully. Figure. 1 XRD patterns of graphite flake and GO. Figure. 2 described the size fractionation principle based on the Brownian motion of GO sheets. According to the Stokes-Einstein equation [18] (1), where D is diffusion coefficient, K B is Boltzmann's constant, T is the temperature, β=3πμd, μ is fluid viscosity, d is Brownian particle diameter, Brownian diffusion is more extensive as the particle size increases under condition of constant temperature.  Figure. 2 Principle of the size fractionation of GO sheets As is known, GO sheets possess abundant hydroxyl (-OH) and epoxy groups on the layer planes, together with carboxylic groups at the edges of the lamellae. Therefore, GO sheets have the good dispensability in aqueous solution. In the GO solution, random Brownian motion of GO sheets naturally takes place. Based on the equation 1, the Brownian motion of small-size GO sheets is more extensive than that of large-size ones. In the self-designed size fractionation setup, it's easier for the small-size GO sheets to diffuse through the wire-mesh nylon net into the upper pure water than the large-size ones, when stirring the bottom solution. Consequently In Figure. 3, the size fractionation of GO sheets was carried out by a self-designed wire-mesh nylon filter. GO solution was formed by stirring in the bottom ( Figure. 3a). At first, Tyndall effect was not shown when the upper water was irradiated by a laser pen ( Figure. 3b). After 12 h stirring, a transparent optical path appeared in the upper solution when the solution was irradiated using a laser pen at room temperature ( Figure. 3c). This phenomenon was a typical Tyndall effect. It indicated that some GO sheets passed through the nylon net into the upper water. The size of GO sheets in the upper solution and original GO solution were studied by the Field-emission scanning electron microscope (FESEM). The SEM images and corresponding size fractionation histograms of GO sheets were showed in Figure. 4a and b. Before fractionation, the size of synthesized GO sheets ranged from 1 to 35 μm and the distribution was wide ( Figure. 4a). After 12 h stirring, the size of GO sheets diffused into the upper water mainly ranged from 1 to 10 μm ( Figure. 4b). Above 90% of them were smaller than 6 μm. The size distribution was narrower than that before the size fractionation. Furthermore, the Gauss fits of the GO sheets before and after fractionation indicated that their maximum distributions of GO sizes were 4 μm and 3 μm, respectively. These results indicated that the GO sheets were separated successfully according to Brownian motion of GO sheets. It was consistent with the principle discussed above. The successful size fractionation was attributed to the following reasons: (i) the good dispensability of GO sheets; (ii) the different ability of Brownian motion between small GO sheets and large GO sheets; (iii) the pore selectivity of wire-mesh nylon net.

Conclusions
In summary, we have demonstrated a facile, low cost method for the size fractionation of GO sheets based on Brownian motion. Smallsize GO sheets have stronger Brownian diffusion than large-size ones, when the temperature is constant. Hence, the diffusion ability of GO sheets varied from their size dimension in a solution. Owing to this phenomenon, the small-size of GO sheets priorly diffused into the upper water zone and was separated instead of large-size ones in a self-designed separator. The results showed that the smallsize GO sheets collected from the upper solution had narrow size distribution, which was mostly smaller than 6 μm (>90%). The novel size fractionation method in this work shows potential prospect on the preparation of GO with different lateral size.