Size fractionation of graphene oxide sheets by the polar solvent-selective natural deposition method

Abstract

Graphene oxide sheets (GOSs) of homogeneous size are highly desirable for applications and fundamental research. Herein, we developed a novel, simple and economic method to prepare GOSs of homogeneous size by the modified Hummers method. The method is based on the different dispersibility and stability of varisized GOSs in a polar solvent. The homogeneity in the size of the GOSs obtained can be improved. We found that the dispersibility and stability of GOSs were strongly correlated with the C–O content of GOSs and the lateral dimensions of GOSs decreased while the degree of oxidation increased, which enabled the separation of varisized GOSs. GOSs were separated into four groups, namely GOSs1 (d > 25 μm), GOSs1 (15 μm < d < 25 μm), GOSs3 (5 μm < d < 15 μm) and GOSs4 (<5 μm). The percentages of the size distributions of GOSs1, GOSs2, GOSs3 and GOSs4 reached up to 83.16%, 83.92%, 90.42% and 92.32%, respectively. Furthermore, we also found that the smaller GOSs possessed stronger Rhodamine-B luminescence quenching abilities than the larger GOSs. The more efficient fluorescence resonance energy transfer was deemed to be the major reason for their distinct quenching abilities.

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1. Introduction

Graphene, as is well known, is a kind of one atom-thick network of sp²-bonded carbon atoms.^1,2 The first report for free-standing graphene at room temperature featured the scotch tape method. It has great potential applications in various fields, such as drug delivery,^3–5 biosensors,^6,7 solar cells,⁸ and luminescent materials,^9,10 because of its unique structure and superior electronic, optical, thermal, and mechanical properties. A variety of methods have been developed to prepare high-quality graphene of homogeneous size, such as micromechanical cleavage,¹¹ chemical vapor deposition,^1,12,13 epitaxial growth,¹⁴ and the oxidation–reduction method.¹⁵ In these methods, oxidation–reduction method is deemed to be the most promising route due to its simplicity and reliability for large-scale production, and relatively low material cost. However, it is difficult to produce graphene of homogeneous size.¹⁶ A large number of studies have illustrated that the lateral dimensions of graphene play a key role in its properties and applications.^13,16–19 In the process, graphene oxide sheets (GOSs) are the precursors of graphene sheets (GSs). The sizes of GSs are decided by those of their GOS precursors. Thus, developing a method to control the size of GOSs has attracted tremendous attention. The centrifugation method,^20,21 density gradient ultracentrifugation (DGU) technique,²² dialysis method¹⁹ and pH-assisted selective sedimentation method¹⁷ were developed to control the size of GOSs. However, these methods have some drawbacks, such as requiring special equipment, complicated operating procedures, high cost, heterogeneous size of the obtained graphene, etc.

It has been well documented that (i) a large amount of oxygen-containing functional groups are coated on the GOSs,^{15–17,25,26} (ii) varisized GOSs possess different degrees of oxidation,^16,17,21 (iii) the dispersibility of GOSs in a polar solvent is decided by the oxygen-containing functional groups,^9,10,17,27 and (iv) the lateral dimensions of GOSs decrease while the degree of oxidation increases.^{16–19,21,23,28} According to the aforementioned observations, herein we developed a novel, simple and economical method for preparing GOSs with size homogeneity based on the dispersibility and stability of GOSs in the polar solvent. The synthesized GOSs derived using the classical Hummers’ method can be easily divided into series of products with size homogeneity by consecutive dispersion and deposition procedures, namely GOSs1 (d > 25 μm), GOSs2 (15 μm < d < 25 μm), GOSs3 (5 μm < d < 15 μm) and GOS4 (<5 μm).

2. Experimental section

2.1 Synthesis of graphite oxide (GO)

GO was obtained from natural flake graphite by the modified Hummers method.²⁴ In an ice-water bath, a Teflon-coated magnetic stir bar, 1 g of natural flake graphite, 0.5 g of NaNO₃ and 23 mL of concentrated H₂SO₄ were placed in a 250 mL round-bottom flask. After stirring for 30 min, 3 g of KMnO₄ was added slowly. Sequentially, the system was heated for 30 min at 35 °C, 50 mL of distilled water was added gradually, and the mixture was stirred for 15 min at 98 °C. At the end of the reaction, the mixture was diluted with 140 mL of distilled water and approximately 10 mL of 30% H₂O₂ solution, after which the color of the mixture changed to luminous yellow. The mixture was centrifuged and washed with 500 mL of 5% HCl aqueous solution 5 times and distilled water several times to remove metal ions and the acid, respectively. The product was dried at 50 °C for 24 h.

2.2 Preparation of cGO

cGO was prepared from the obtained GO by ultrasonic stripping.¹⁰ 0.1 g of GO was dissolved in 150 mL of ethanol and sonicated for 1 h. The dispersion was concentrated and centrifuged. Finally, the obtained brown precipitate was dried at 35 °C for 12 h.

2.3 Size fractionation of graphene oxide sheets (GOSs)

The process of preparation sketch is given in Scheme 1. Step 1: 150 mL anhydrous ethanol was added to 0.1 g of GO, after ultrasonication for 1 h. The colloidal dispersion let stand for 5.5 h. Step 2: the mixture was separated into the supernatant (S₂) and precipitate (P₁) by decantation. Step 3: 20 mL of ethanol was added to P₁. The mixture was shaken thoroughly, allowed to stand for 2 h, and separated into supernatant and precipitate by decantation. The precipitate underwent the procedure above four times with the allowed standing times of 1 h, 1 h, 1 h and 0.5 h, respectively. The supernatant (S₃) was obtained by combining all of the supernatants. The precipitate that remained was GOSs1. Step 4: GOSs2 were prepared using the supernatant (S₃). S₃ was allowed to stand for 12 h, and the solvent was removed by decantation. The precipitate was washed using ethanol (3 × 20 mL) and dried. Step 5: the S₂ was divided into the supernatant (GOSs4) and precipitate (GOSs3) by centrifugation at 2000 rpm for 10 min. GOSs3 were washed using ethanol (5 × 20 mL) and dried. GOSs4 were obtained by removing the solvent and drying.

Scheme 1 The process of preparing the GOSs.

2.4 Physical measurements

Fourier transform infrared spectroscopy (FT-IR) was performed on KBr disks using a Bruker Tensor 27 spectrometer (Germany) from 4000 to 400 cm⁻¹. Scanning electron microscopy (SEM) measurements were carried out using a FEI NanoSEM 450. A Bruker AX diffractometer (Germany) was used to characterize the X-ray diffraction (XRD) patterns in the range of 5–90°. The degree of oxidation of the GOSs was measured using X-ray photoelectron energy spectroscopy (XPS) (K-alpha) with Al Ka radiation. UV-vis spectroscopy was performed using the Phenix UV1900 in the wavelength range of 260–600 nm. Luminescence emission spectra were obtained using the Hitachi F-4600.

3. Results and discussion

3.1 Mechanism of the dispersion and separation of GOSs

In the colloidal GOSs, negative charges are formed by the oxygen-containing groups on the edges and planar surfaces of GOSs in the aqueous solvent.¹⁷ The electrostatic repulsion effect between GOSs maintains the stability of the colloid, while van der Waals attraction results in the deposition of the colloid.³⁰ When the graphite is oxidised to graphite oxide (GO), its surface will obtain a large number of oxygen-containing groups, which can make its surface present a negative charge. There are many factors that can influence the electrostatic repulsion effect and van der Waals attraction, which change the stability of the GOS colloid. Xiluan Wang et al.¹⁷ utilized HCl to adjust the pH of the colloidal GOSs. Protons reduce the negative charges of the surfaces and edges of GOSs, thereby weakening the electrostatic repulsion effect. The larger GOSs were deposited at the appropriate pH. In this paper, we have investigated three main parameters that affect the properties of the surfaces and edges of GOSs (solvent, concentration and the height of the solution).

Fig. 1 presents UV-vis absorption spectra of the GOSs recorded at different times in various solvents (distilled water (a), ethylene glycol (b), isopropanol (c), ethanol (d), methanol (e), and tetrahydrofuran (f)). Fig. 1a shows that the absorbance of the colloidal GOSs stays at a constant value, which illustrates that the concentration is not changed. This proves that the colloidal GOSs have good stability in the aqueous solvent. Nevertheless, the electrostatic repulsion interactions may be changed by dispersing the GOSs in organic solvents. UV-vis absorption spectra of GOSs in the organic solvents confirm this point (Fig. 1b–f), as the absorbance changes over time. In the organic polar solvent, deprotonation can weaken or eliminate the effects of the carboxyl groups on the GOSs, which form the negative charges that maintain their stability. Therefore, the weaker electrostatic repulsion effects can’t prevent the van der Waals attraction between GOSs, which leads to the agglomeration.

Fig. 1 UV-vis spectra of GOSs in various solvents (distilled water (a), ethylene glycol (b), isopropanol (c), ethanol (d), methanol (e), and tetrahydrofuran (f)). 5 mg of the GOSs was dispersed in 50 mL of isopropanol (with methanol, tetrahydrofuran, water, ethylene glycol or ethanol) through sonication for 1 h to obtain the dispersion. After that, 4 mL of the dispersion was placed in a quartz cuvette and was measured for 2 h at 5 min intervals to obtain the UV-vis spectra in the wavelength range of 260–600 nm.

The UV-vis absorption spectra of the various concentrations of the GOSs at different times after dispersion in ethanol are presented in Fig. 2. They show that the higher the concentration of the GOSs colloid, the faster the speed of the sedimentation of GOSs. It’s reasonable that increasing the concentration can shorten the distance between GOSs, leading to agglomeration, which increases the sedimentation rate of the GOSs.

Fig. 2 UV-vis spectra of 0.04 g L⁻¹ (a), 0.12 g L⁻¹ (b) and 0.2 g L⁻¹ (c) of the GOSs dispersed in ethanol. 2 mg, 6 mg or 10 mg of GOSs was dispersed into 50 mL of ethanol by sonication for 1 h to obtain the dispersion. After that, 4 mL of the dispersion was placed in a quartz cuvette and was measured for 2 h at 5 min intervals to obtain the UV-vis spectra in the wavelength range of 260–600 nm.

Fig. 3 shows the UV-vis absorption spectra of the GOSs at different heights in the ethanol dispersion over the same standing time. It indicates that the settling velocity of the GOSs in the precipitate is faster than that of the GOSs in the upper dispersion. The photographs prove that the sizes of the GOSs in the precipitate are larger than those in the upper dispersion, as is shown in Fig. 4.

Fig. 3 UV-vis spectra of the upper dispersion (a) and the precipitate (b). 25 mg of GO was dispersed into 50 mL of ethanol by sonication for 1 h to obtain the dispersion and allowed to stand for 30 min. After that, 2 mL of the upper dispersion obtained using a burette was added to another 2 mL of ethanol to get the dispersion represented in (a); the remaining dispersion was removed by decantation, and the precipitate was dispersed into 4 mL of ethanol again to obtain the dispersion represented in (b). UV-vis spectra of the upper dispersion (a) and the precipitate (b) were measured over 2 h at 5 min intervals in the wavelength range of 260–600 nm.

Fig. 4 Photographs of the precipitate (a) and the upper dispersion (b).

In summary, the height gradient distribution in a dispersion of GOSs in ethanol is influenced by the size and density of the GOSs, with larger ones at the bottom and smaller ones at the top due to gravity, and the larger sized GOSs have a faster sedimentation rate. Thus, when GOSs were dispersed in the organic polar solvent, varisized sheets could be separated at the appropriate time.

3.2 Separation of varisized GOSs

As shown in Scheme 2, when the GOSs dispersed in ethanol are allowed to stand for a while, the larger sized GOSs are deposited first and the smaller sized GOSs are still dispersed in the solvent. The details of the processes of the separation are shown in Scheme 1. The sizes of the as-obtained GOSs are shown in Fig. 5 and S1.† From the image of the cGO, it can be seen that the sizes of the cGO sheets were disorderly and unsystematic. However, it was found that GOSs prepared by this method were highly homogeneous in size, and the sizes of the GOSs decrease gradually throughout the procedure. The histograms show that the percentages of the size distributions of GOSs1, GOSs2, GOSs3 and GOSs4 reached 83.16%, 83.92%, 90.42% and 92.32%, respectively. Similar results were observed in the photographs provided in Fig. S2.† The good transparency of the GOSs demonstrates that they possess a low number of layers. From the AFM image of the GOSs, it can be seen that the method has produced single layer GOSs (Fig. S3†).

Scheme 2 Mechanism of the separation.

Fig. 5 SEM images of cGO (a), GOSs1 (b), GOSs2 (c), GOSs3 (d) and GOSs4 (e). The histograms of the size distributions of the GOSs were obtained by measuring the lengths of more than 1200 sheets in each sample.

Compared with other separation methods, such as the centrifugation method^20,21 and DGU method,²² centrifugation equipment (operating at speeds of more than 4000 rpm) was not used in this method. Though the DGU method can achieve a good separation result, the particular separation solution needs to be prepared, and a more complicated procedure is involved. Similarly, the dialysis method¹⁹ can separate GOSs well, though the costs of production are relatively high and conditions involved are more complex. In addition, the pH-assisted selective method¹⁷ makes the procedure become quite simple, but varisized GOSs are only separated into two parts. Apparently, the method we have developed presents advantages in terms of simplicity, economy and efficiency, as well as having great potential to achieve the mass production GOSs of homogeneous size.

Additionally, other polar solvents, such as tetrahydrofuran and methanol, may also be selected to separate the GOSs. Not only are the results similar to those already obtained, but the procedures can also be simplified. When tetrahydrofuran was used to separate GOSs3 and GOSs4, the procedure of centrifugation could be replaced and the separation time could be shortened to 2 h. When methanol was utilized to prepare GOSs1 and GOSs2, the separation time could be shortened to 35 min. It’s believed that if we want to obtain GOSs with yet improved size homogeneity (for instance, so that the size in a sample varies by less than 5 μm), different polar solvents or their mixtures may be needed to separate the varisized GOSs.

3.3 Characterization of the obtained varisized GOSs

To analyze their structures, FT-IR spectra were recorded for cGO, GOSs1, GOSs2, GOSs3 and GOSs4 in the spectral region of 4000–400 cm⁻¹, and these are shown in Fig. 6. In the FTIR spectrum of cGO, the characteristic peaks at 3449 cm⁻¹ (O–H), 1732 cm⁻¹ (C=O), 1627 cm⁻¹ (C=C), 1434 cm⁻¹ (C–OH) and 1077 cm⁻¹ (C–O–C) are presented.^20,29 The existence of a large number of oxygen-containing groups on the GOSs has been illustrated by some researchers.^25,27 Similarly, the characteristic peaks of O–H, C=O, C=C, C–OH and C–O–C were found in the FTIR spectra of GOSs1, GOSs2, GOSs3 and GOSs4. Herein, the structures of the obtained GOSs are the same as those of cGO. In order to investigate the relationship between size and the number of oxygen-containing groups, XRD and XPS were employed.

Fig. 6 FT-IR spectra of cGO (a), GOSs1 (b), GOSs2 (c), GOSs3 (d) and GOSs4 (e).

The XRD patterns of GOSs1, GOSs2, GOSs3 and GOSs4 showed peaks at 11.69, 11.01, 10.77 and 10.73°, respectively, and their d-spaces were calculated to be 0.76, 0.80, 0.82 and 0.82 nm, correspondingly (Fig. 7). This demonstrates that the interplanar spacing of these GOSs increases from GOSs1 to GOSs4, in sequence.^16,17 Researchers have identified that a large number of oxygen-containing functional groups are coated on the GOSs, which was similarly proved by the FT-IR spectroscopy. More oxygen-containing groups can make GOSs adsorb more solvent molecules. This leads to an increase in the interplanar spacing of GOSs under the same humidity.^16,17 In addition, van der Waals attractions between larger GOSs are stronger than those between smaller GOSs, which further reduces the interplanar spacing.

Fig. 7 XRD patterns of GOSs1 (b), GOSs2 (c), GOSs3 (d) and GOSs4 (e).

XPS of GOSs1, GOSs2, GOSs3 and GOSs4 shows that the degree of oxidation appears to fluctuate very little (Fig. 8 and S4†). This illustrates that the differences between the varisized GOSs are small. The C/O atom ratios of GOSs1, GOSs2, GOSs3 and GOSs4 were calculated to be 2.69, 2.68, 2.59 and 2.53, respectively (Table 1). This indicates that more oxygen-containing groups are coated on the smaller GOSs. Thus, smaller GOSs can trap more solvent molecules and possess weaker van der Waals attraction, leading to a decrease in the interplanar spacing of the GOSs.^16,17 It also shows that the smaller GOSs have better dispersibility and stability in the polar solvent than the larger GOSs.

Fig. 8 C1s XPS of GOSs1 (b), GOSs2 (c), GOSs3 (d) and GOSs4 (e).

Table 1 The atom ratios of C/O of the GOSs samples

Samples	GOSs1	GOSs2	GOSs3	GOSs4
C/O	2.69	2.68	2.59	2.53

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3.4 Luminescence quenching effect of the varisized GOSs

It is widely recognized that varisized GOSs or reduced GOSs have different characteristics, and these have been demonstrated by many researchers. For instance, the conductivity of the larger sized rGOSs is better than that of the smaller sized rGOSs.^17,18 Varisized graphene quantum dots (GQDs) have different luminescence properties.¹⁹ GOSs possess good Rhodamine-B (RB) luminescence quenching abilities, as illustrated by the literature.²⁶ It is estimated that the luminescence quenching abilities may also be determined by the sizes of GOSs. This was proved by the luminescence emission spectra of RB, RB/GOSs1, RB/GOSs2, RB/GOSs3 and RB/GOSs4 (Fig. 9). They show that GOSs can strongly quench the luminescence of RB, especially the smaller GOSs. This may stem from the fluorescence resonance energy transfer from RB to the GOSs. The smaller GOSs possess more oxygen-containing groups such as hydroxyl, called the high-energy oscillator, which are beneficial to the energy transfer. The high-energy oscillator always has an intense luminescence quenching effect with luminescent materials, as discovered in previous studies.³¹

Fig. 9 Luminescence emission spectra of RB, RB/GOSs1, RB/GOSs2, RB/GOSs3 and RB/GOSs4. 2 mg of GOSs1, GOSs2, GOSs3 or GOSs4 was dispersed into 20 mL of ethanol by sonication for 1 h, and 5 mL of 0.1 g L⁻¹ RB ethanol solution was added. The luminescence emission spectrum of every sample was obtained under 365 nm excitation.

4. Conclusions

In conclusion, we have successfully developed a novel method based on the dispersibility of varisized GOSs in polar solvents. GOSs were divided into four groups, namely GOSs1 (d > 25 μm), GOSs2 (15 μm < d < 25 μm), GOSs3 (5 μm < d < 15 μm) and GOSs4 (<5 μm). The dispersibility, stability and sizes of the GOSs are related to the degree of oxidation, which has also been illustrated. Smaller GOSs possessing more oxygen-containing groups have better dispersibility and stability in the polar solvent. Utilizing this method, GOSs with improved size homogeneity that present good transparency were obtained. Various polar solvents selected to separate the GOSs showed that (i) the procedure was simplified and the efficient separation time was shortened, and (ii) a better separation result could be obtained. Herein, the method based on polar solvents has great potential for the mass-production of GOSs of homogeneous size. Moreover, we found that the smaller GOSs have stronger luminescence quenching abilities than the larger GOSs.

Acknowledgements

We thank Prof. Xiaoqing Wang (School of Environmental and Chemical Engineering, Tianjin Polytechnic University) for providing the XPS measurements for our study.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: SEM images, photographs and XPS spectra. See DOI: 10.1039/c4ra08516g

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