Effective production of nano-sized graphene via straight-forward exfoliation of microcrystalline graphite

Abstract

We report a facile and effective technique for the large-scale production of nano-sized graphene sheets via subjecting the microcrystalline graphite to ball milling. The products consist of single- or few-layer (≤5 layers) graphene with lateral dimensions concentrated in the range of 100–200 nm. It displays a high intensity of photoluminescence at a wavelength of 339 nm and graphene dispersions at concentrations up to 1.22 mg ml⁻¹.

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Mass production of high-quality graphene has attracted great attention since its discovery.¹ Generally, relatively defect-free graphene can be achieved successfully by breaking up and peeling off natural graphite via a variety of mechanical stripping methods.^1–9 It is noteworthy that most of these methods focus on obtaining single- or multi-layer graphene with a large lateral size and the exfoliation process along the c-axis direction of graphite grain is widely discussed and studied.^10,11 There is no denying that large-sized graphene flakes exhibit a huge specific surface area¹² and many excellent properties, including ultrahigh electrical¹¹ and thermal conductivities,¹³ outstanding mechanical¹⁴ and optical properties,¹⁵ etc. However, the poor film-forming ability and biocompatibility of large-sized graphene restrict the applications in the field of electronics¹⁶ and bio-analysis,¹⁷ etc. The fruitful progress in the research of small lateral sized graphene (less than 500 nm) has been reported in recent years. Jang et al.¹⁸ produced the nano-scaled graphene plates via exfoliating the graphite crystallites in the polymeric carbon using a mechanical attrition treatment, their resultants had at least one dimension of about 200 nm or less. With the help of long-time ultrasonic treatment (up to 400 hours) of natural graphite in the solvent of N-methyl-pyrrolidone (NMP)¹⁹ or in surfactant–water solutions,²⁰ Coleman's group achieved graphene sheets with mean length and width of 1 μm and 300 nm, respectively. Similarly, graphene sheets with lateral size ranging from 100 to 500 nm were obtained via sonicating graphite in ortho-dichlorobenzene and collecting the centrifugal supernatant by Hamilton's team.²¹ Xu et al.²² prepared few-layer graphene products with the lateral dimension in the range of 200–500 nm through sonicating graphite in the solvent with the assistance of hyperbranched polyethylene (HBPE). Moreover, He et al.²³ synthesized disk-shaped graphene with the diameter around 200 nm utilizing carbonized and further heat treated D113 resin through an ion-exchange methodology. Unfortunately, some intractable problems in practical production, esp. huge energy consumption, tedious operation process and low efficiency, seem inevitable. Large scale and effective production of uniform small lateral size of graphene through a facile technique remains a major challenge.

Herein, we provide a novel strategy to prepare uniform nano-sized graphene (NG) by ball milling process. Unlike other reported techniques for achieving NG through natural graphite or polymeric carbon, we chose a unique graphite material, namely microcrystalline graphite (mc-G). The mc-G is made up of microcrystalline graphite particles and the microcrystalline graphite particles are formed by graphite crystals with diameter mainly concentrated in the range of 100–200 nm, which are beneficial to the promotion of the production efficiency. In a typical experiment, 0.1 g mc-G was dispersed homogeneously in 40 ml N-methyl-2-pyrrolidone (NMP) through a low power ultrasonic bath for 10 min. Then the pre-dispersed mc-G dispersions were ball milled for 12 h and collected the 80% of the centrifugal supernatant, it reveals that more than 90% of the nano-sized graphene products are consist of single- or few-layer (≤5 layers) graphene flakes without severe body-defects or oxidation. The lateral dimension of the graphene flakes concentrates in the range of 100–200 nm and the nano-sized graphene have a high intensity of photoluminescence at wavelength of 339 nm. Moreover, we have acquired the graphene dispersions at high concentrations up to 1.22 mg ml⁻¹.

The morphology of the NG sheets was investigated by TEM characterization. As shown in Fig. 1a and b, the NG flakes display homogeneous thin nanometer sized sheet-like structure. It is noted that the edges of the NG flakes are slightly wrinkled and folded. Additionally, a number of rather disordered multilayer frameworks are emerged by fold over or stack together of some flakes. Fig. 1c displays a TEM image of a typical NG flake with lateral size about 100 nm and edge folds. Then we analyze 100 NG flakes to measure the lateral size distribution of the NG flakes. Fig. 1d shows a histogram revealing the distribution of the lateral sizes for the NG flakes. It is clearly that about 90% of the NG flakes are in the range of 100–200 nm, suggesting a narrow distribution of the lateral size for the NG flakes.

Fig. 1 (a), (b) and (c) TEM images of NG sheets deposited onto grids. (d) Statistical flakes size analysis of NG sheets by TEM.

We also use TEM characterization to observe the morphology of the mc-G (see Fig. S1 in the ESI†). Similar to the NG sheets, the mc-G also shows uniform sheet-like structure with scrolled and folded edges. In particular, it is noteworthy to find that the lateral size of the mc-G flakes concentrates in the extent of 100–500 nm, which hints that we can acquire NG sheets readily through subjecting the mc-G flakes to straight-forward ball milling. As illustrated in Scheme 1, compared to nature graphite, the use of mc-G flakes to prepare the NG avoids fracturing mc-G flakes on the plane orientation. The mc-G is made up of microcrystalline graphite particles and the microcrystalline graphite particles are formed by graphite crystals with diameter mainly concentrated in the range of 100–200 nm. As a consequent, it only needs to exfoliate mc-G on c-axis direction to acquire thin NG sheets. This reduces energy consumption during the procedure and obtains homogeneous thin NG sheets, bringing tremendous economic benefits.

Scheme 1 Schematic illustration of the approach for nano-sized graphene compared with exfoliated graphene.

The thickness of the NG flakes can be inspected by AFM images and HRTEM images.²⁴ The AFM image (Fig. 2a) shows the sheet-like structure with a thickness of 0.970 nm (Fig. 2b), which is in accord with the practical thickness of individual single-layer graphene.^25,26 Fig. 2c shows that the lateral size of NG is well-distributed and concentrates in the range of 100–200 nm, which agrees with the results of TEM. The layers of the NG sheets can also be realized easily by counting the dark lines shown in the HRTEM images.²⁴ As shown in Fig. 2d and S2 (see Fig. S2 in the ESI†), it discloses that the NG sheets range from a single layer to seven layers, where single and few layers (≤5 layers) graphene flakes are the dominant products. A statistical analysis (see Fig. S2 in the ESI†) on flake thickness was performed to verify the exfoliation. 100 sheets were investigated from HRTEM images. Evidently, more than 90% of NG sheets comprise thin graphene (≤5 layers), revealing a good exfoliation degree for NG.

Fig. 2 (a) AFM image of graphene sheets deposited on a mica substrate. (b) Height profile along the lines in (a). (c) and (d) High-resolution TEM images of NG sheets embedded in epoxy resin slice.

Then Raman spectroscopy was conducted to identify the defects in the NG flakes.^27,28 The Raman spectroscopy of NG in the range of 500–3000 cm⁻¹ are shown with the spectroscopy of mc-G for comparison in Fig. S3 (see Fig. S3 in the ESI†). The D peak (∼1340 cm⁻¹), the G peak (∼1567 cm⁻¹) and the 2D peak (∼2674 cm⁻¹) are clearly observed. Besides, the peak ratio between the intensity of D and G peaks (I_D/I_G) for NG is 0.48, which is much smaller than the reported value of the chemically or thermally derived graphene obtained by reduction of graphene oxide (∼1.1–1.5),²⁹ indicating a low degree of defects. According to previous reports,^{3,19,20,30–33} the intensity of the D peak in small flakes is resulted primarily from edge defects rather than basal plane defects, because they own more edges per unit mass resulting in an increase in edge defects population.³¹ Thus, we believe that the D peak shown in the NG flakes originates chiefly in edge defects of the NG flakes. Moreover, compared to the broad and overlap D and G peaks of graphene oxide,³⁴ the D and G peaks in NG are fairly split, which further confirms the NG flakes are free of severe body-defects like graphene oxide.

Furthermore, XPS analyses were used to probe the chemical composition and the elemental composition of NG. As shown in Fig. S4b (see Fig. S4b in the ESI†), the C 1s spectrum of NG exhibits three componential peaks at 284.6, 286.3 and 290.7 eV, corresponding to C=C, C=O and C(O)–O groups, respectively. It discloses a C/O ratio of 14.2, which is higher than other graphene,^35–37 demonstrating a low level of oxidation for NG. Briefly, Raman spectrum together with XPS analyses, indicate that the obtained NG sheets are free of severe body-defects and oxidation.

As we know, the raw material mc-G flakes consist of graphite crystal and a few mineral salts made up of SiO₂, Al₂O₃. Ball milling process also remove those impurities with ease (Scheme 1). As shown in Fig. S4c (see Fig. S4c in the ESI†), the powder X-ray diffraction pattern of the NG flakes displays a graphite peak at 26.34° and the impurity peaks observed in mc-G disappear. Further evidence of perfect purity of the NG products can be received by XPS spectrum. Fig. S4d (see Fig. S4d in the ESI†) presents the full scan spectrum of the mc-G sheets together with the NG sheets. As can be seen, the mc-G sample shows six peaks at binding energy of 534.5, 285.6, 158.2, 121.8, 106.2 and 75.6 eV, corresponding to O 1s, C 1s, Si 2s, Al 2s, Si 2p and Al 2p, respectively. In comparison, only C 1s and O 1s peaks can be observed in the spectrum of the NG sample, suggesting the detachment of the impure components.

The uniform NG flakes were successful obtained via ball milling of mc-G. The collected products were then dispersed in NMP by sonication for 10 min. Thus, homogeneous black graphene dispersions were obtained, which are stable for six months without apparent agglomeration (Fig. 3a, inset). Besides, it displays the Tyndall phenomenon revealing the colloidal nature of the dispersion. Coleman's group has demonstrated that the increase in graphene concentration is correlated with the decrease in flake dimensions.¹⁹ To examine the concentrations of the NG dispersion, UV-vis spectra were carried out together with gravimetry. According to the Lambert–Beer law, A = αcl, absorption coefficient of graphene can be determined by preparing a series of dispersions at given concentrations. The absorbance (660 nm) plotted versus concentration discloses an absorption coefficient α = 503 ml mg⁻¹ m⁻¹, we have acquired graphene dispersions at high concentrations up to 1.22 mg ml⁻¹, which will facilitate a wide range of graphene potential applications. The absorption coefficient is much lower than others reported in literature,^3,31,38 our group has proposed the theory that the absorption coefficient is related to the lateral size, the thickness and the functional groups grafted on of graphene flakes.³⁹ The huge low absorption coefficient confirms that the NG sheets possess a narrow lateral size distribution in the range of 100–200 nm and free of functionalization.

Fig. 3 (a) Optical absorbance (λ = 660 nm) divided by cell length (A/I) as a function of concentration for NG graphene in NMP. Inset: photographic image of NG dispersion with Tyndall effect; (b) PLE spectra of the NG sheets in water solution; (c) PL spectra of the NG sheets in water solution.

In addition, the optical properties of graphene sheets may have an important influence on future applications in optoelectronic devices⁴⁰ and biological sensors.⁴¹ Thus, we carried out the photoluminescent excitation (PLE) spectrum and PL emission spectrum of the NG flakes. PL characterization (Fig. 3c) shows that the NG emits strong PL at a near-UV wavelength 339 nm under excitation at 226 nm, which is attributed to the sp² configuration of similar size in nanostructured NG.²³ The PLE spectrum for the NG is shown in Fig. 3b. It is noteworthy to find that the photoluminescent excitation wavelength is in the scope of 200–250 nm, which represents the absorption energies corresponding to emission of near-UV.⁴² The strong PL of NG supports its future applications in optoelectronic devices, biological, etc.

In conclusion, we have demonstrated a facile and effective method to produce substantial homogeneous single- or few-layer (≤5 layers) nano-sized graphene sheets ranged in 100–200 nm and without severe body-defects or oxidation. The NG products display strong PL at near-UV wavelength of 339 nm and can acquire graphene dispersions at high concentrations up to 1.22 mg ml⁻¹. The unique feature of this technique is the application of the mc-G, consisting of microcrystalline graphite particles with lateral size in the range of 100–500 nm. Virtually the microcrystalline graphite particles are made up of graphite crystals with diameter mainly concentrated in the range of 100–200 nm. We believe that this technique can be easily extended to industrial scale-up production of high-quality nano-sized graphene sheets for large-scale applications, such as energy conversion, bio-analysis and sensors.

Acknowledgements

This work was funded by Natural Science Foundation of China (51373059), Science Foundation of Fujian Province (2013H6014), Science Foundation of Xiamen (3502Z20143044) and Science and technology innovation team of Huaqiao University (Z14X0046).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and supporting results. See DOI: 10.1039/c4ra08259a

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