Synthesis of graphene nanosheet powder with layer number control via a soluble salt-assisted route

Abstract

The principle features in this study lay on the synthesis of graphene nanosheet powders with layer number control from pristine graphite powder via a soluble salt-assisted route and on the high thermal stability and good dispersibility of our products in ethanol.

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1. Introduction

The discovery of individual graphene sheets in 2004 ignited a gold rush of graphene-based nanomaterial research due to the superiority of graphene over traditional nanomaterials.¹ In theory, graphene has extraordinary mechanical, electrical, and thermal properties, and many versatile promising applications. For example, its surface area, modulus, fracture strength and thermal conductivity have been estimated to be up to 2600 m² g⁻¹, ∼1 TPa, ∼130 Gpa, and 5300 W mK⁻¹, respectively. Furthermore, its electron mobility is as high as 15 000 cm² (V⁻¹ s⁻¹). Hence, potential applications of graphene have been anticipated in nanoelectronics, sensors, batteries, supercapacitors, and for hydrogen storage.^2–6 The low cost and large scale preparation of high-quality graphene crystals is the first and most crucial step, both for fundamental research and for device applications.

So far, many methods have been developed for the preparation of graphene nanomaterials. The typical technique routes usually include mechanical peeling by adhesive tape, vapor synthesis, and reduction–oxidation of graphite powder. The manual Scotch-tape technique has some advantages such as simple labour, low cost, and high quality of product, but it is often accompanied by many disadvantages such as low yield, lack of controllability and inevitable contamination from glue tape.¹ Vapor synthesis, including chemical vapor deposition, pyrolysis of SiC and epitaxial growth, has also been used to prepare high quality few-layer graphene, which is usually deposited on a single-crystal substrate, but it is not fit for the large-scale preparation of graphene powder.^7–9 Reduction–oxidation of graphite powder is an ideal route to prepare cost-effective graphene powder on a large scale.¹⁰ However, the strong oxidation and reduction processes lead to the presence of numerous defects, for instance, topological defects emerging from five- or seven-membered rings and structural defects from hydroxyl groups. Other methods such as the solvothermal route, chemical peeling and microwave processes have also been tentatively attempted in the lab in order to synthesize graphene powder, but have usually led to many problems associated with environment unfriendliness, high energy-consumption, structural corruption of the product and so on.^11–13 Through a comprehensive comparison, mechanical peeling would seem the most promising route to graphene powder as far as the quality of the prepared powder is concerned, once the problems for the scalable preparation and contamination from the glue tape are avoided.

As it is widely known, the main component in mandrel pencil is graphite due to its layer-like structure. Scoring the mandrel on a paper sheet imprints a light black line, which consists of thin graphene nanosheets. However, paper is insoluble in water. Therefore, it is difficult to collect graphene nanosheet powder imprinted on paper. If graphene nanosheets could be uniformly dispersed on the surface of soluble salts through ball-milling of graphite, the above difficulty would be easily overcome. After a simple washing process, the residue would be what we expected, graphene powder. In our previous reports, we have developed a soluble salt-assisted route to prepare GaN and ZnO nanopowders by uniformly dispersing metallic Ga or Zn on the surface of Na₂SO₄ powder, followed by a nitridation or oxidation process and a final washing.^14,15 Herein, we would like to extend the soluble salt-assisted route again to fabricate graphene powder. Through simple mechanical peeling and washing, graphene nanosheet powder has been harvested conveniently and economically.

2. Experimental section

Graphene nanosheet (GN) powder was prepared by the soluble salt-assisted route as shown in Fig. 1. In detail, 50 g of Na₂SO₄ powder and some graphite powder (99.99% in purity and 100–200 mesh in diameter), with a certain weight ratio, were added to a stainless steel jar with a volume of 300 ml, and ball-milled for 24 h with a rotary speed of 150 R min⁻¹. The compact graphite–Na₂SO₄ mixture was loosened every 2 h by opening the jar. Then, the milled mixture was washed with distilled water several times until SO₄²⁻ ions could not be detected in the filtrate by a BaCl₂ solution. Finally, the filtered product was dried in an oven at 90 °C for 3 h and collected for characterization. Note: the products have been designated according to the weight ratio of Na₂SO₄ powder to graphite powder. For example, when the weight ratio of Na₂SO₄ powder to graphite powder is 200 : 1, the final product is called GN 200. The rest may be deduced by analogy; the products were named GN 300, GN 400, GN 600, GN 700 and GN 1000, respectively.

Fig. 1 Schematic of the soluble salt-assisted ball-milled route to graphene nanosheet powder. The inset is the SEM image of GN 600.

The products were characterized by X-ray diffraction (XRD; Philips X'pert Pro diffractometer), micro-Raman spectroscopy (excited with an Ar⁺ laser at 488 nm), scanning electron microscopy (SEM; JEOL JSM-6300), high-resolution transmission electron microscopy (HRTEM; JEM-40001X), X-ray photoelectron spectroscopy (XPS, VG ESCALAB MKII), Fourier transform infrared (FT-IR) spectroscopy (Nicolet Magna 560 FTIR spectrometer at a resolution of 2 cm⁻¹) and thermogravimetry (TG, STA-499C thermal analyzer, Netzsch).

3. Results and discussion

The spatial structure and defects of various products were examined by XRD and Raman, as shown in Fig. 2. Compared to the diffraction peaks of graphite, the (002) peak intensity for all the products was sharply reduced and dramatically broadened (Fig. 2a), and the (004) peak gradually disappeared. Furthermore, the obtained products showed smaller and wider (002) peaks in comparison with graphite. This phenomenon is a result of the following: on one hand, the layers of the products became thinner and thinner when increasing the weight ratio of Na₂SO₄ to graphite, although their areas did not change greatly in comparison with the area of pristine graphite; hence the corresponding X-ray diffractions peaks became gradually weaker and wider.^16,17 On the other hand, the thin graphene nanosheets present more defects, such as suspending bonds, which also contribute to the broadening and decrease in intensity of the diffraction peaks. Fig. 2b shows the Raman spectra of pristine natural graphite and a series of GN powders, exhibiting D, G and 2D bands around 1350 cm⁻¹, 1580 cm⁻¹ and 2700 cm⁻¹. The D band appears due to the disorder in the atomic arrangement or to the edge effect of the carbon-based material; the higher the band intensity is, the lower the crystalline degree of the carbon-based material. The G band arises from the plane vibration of the sp² carbon atoms in carbon-based materials; the higher the band intensity is, the higher the crystalline degree of the carbon-based material. The 2D band is at almost double the frequency of the D band, originating from second order Raman scattering processes. It is observed that the ratio of the intensity between the disorder (I_D) and the graphitization (I_G) bands increases in the series of GN powders when increasing the ratio of the Na₂SO₄ powder, which arises from the fact that more and more suspending bonds are present in the products when the graphene nanosheets become thinner and thinner. However, the ratio of I_D to I_G in our products is greatly lower than that of nanosheets prepared by reduction–oxidation of graphite powder,¹⁰ but equal to that of powders synthesized by mechanical peeling.^17,18 This means that few structural and topological defects like five- or seven-membered rings exist in our products. It is reasonably expected that our graphene powder has better thermal stability and electric properties than those obtained by reduction–oxidation of graphite powder.^10,19 Fig. 2c shows a zoomed region of the Raman spectra. The blue shift of the 2D band of the products indicates fewer layers of graphene nanosheets compared to the number of layers of pristine graphite.^20–26 Moreover, the wider 2D band of the products is due to the turbostratic graphite structure in the graphene nanosheet powder.^22–26 Additionally, both XRD and Raman did not show other peaks except for carbon material, demonstrating the purity of the product.

Fig. 2 XRD patterns and Raman spectra of pristine natural graphite and graphene nanosheets. Panel c shows a zoomed section of the Raman spectra.

The products were further characterized by HRTEM and selected-area electron diffraction (SAED). A little powder sample was dispersed in ethanol under ultrasonic conditions, and a droplet of the suspension was dripped onto a copper grid (200 mesh) for observation. The results are shown in Fig. 3. The products have sheet-like shape with multilayer sheets corrugated together with sizes of hundreds of square nanometers (Fig. 3a, b, e and f). The number of layers of GN decreased quickly from tens to double layers when increasing the ratio of Na₂SO₄ (Fig. 3c, d, g and h). Accordingly, the SAED patterns changed from spot rings to weak diffuse rings (insets in Fig. 3a, b, e and f), associated with a turbostratic graphite structure pattern, and revealing disordered A–B stacking^22–26 as thin graphene sheets lead to a large inner stress thereby forming corrugations. In previous reports, turbostratic carbon was also experimentally observed in ball-milled graphite.¹⁷ The SAED images of the products change consistently with the Raman patterns (Fig. 2b and c).^21–26

Fig. 3 Low magnification TEM (a, b, e and f) and HRTEM (c, d, g and h) images of the as-prepared graphene nanosheets (GN 300, GN 400, GN 700 and GN 1000). Insets in panels a, b, e and f are the corresponding SAED patterns.

XPS analysis is a powerful instrument usually applied to detect trace impurities and learn related chemical information. The binding energies of the examined elements are calibrated by using the C 1s peak (284.6 eV) as the reference. The survey XPS spectrum (Fig. 4a) shows C and O peaks, suggesting that the as-synthesized product is pure GN. The non-detection of Na and S elements by XPS is in accordance with the observations by XRD and Raman. Furthermore, compared to pristine graphite, GN 1000 had a larger oxygen content. This is because more suspending bonds exist in GN 1000 than in graphite powder. The C 1s core level peaks of both graphite and GN 1000 have a main peak located at 284.6 eV and a shoulder peak at 286.7 eV, the former arising from C–C bonds and the latter from C–O bonds. Moreover, the amount of C–O bonds in GN 1000 is greater than in graphite. It is imaginable that our products have good dispersion in water or in ethanol due to the large amount of C–O bonds present in GN. However, the absence of a shoulder peak at 288.3 eV in the C 1s high-resolution XPS spectrum indicates the non-existence of C=O bonds in GN, revealing that our products were not deeply oxidized during ball-milling and will therefore have a better thermal stability than those synthesized by reduction–oxidation of graphite powder.^10,20

Fig. 4 XPS curves of obtained graphene nanosheets (GN 1000) and the used pristine graphite powder.

Thermal stability is an important property for material applications. Fig. 5 exhibits the TGA curves of the obtained graphene nanosheets (GN 400, GN 600 and GN 1000) and pristine graphite powder, which were obtained by heating the targeted powders in air at a heating rate of 20 °C min⁻¹. Compared to graphite powder having an excellent thermal stability up to 700 °C in air, our GN products showed an inferior thermal stability (340 °C in air), as the layer number of our GN products is much smaller than in graphite, and a large amount of suspending bonds (C–O) is present in the GN powder (Fig. 4b). On the other hand, the TG curves also show that most of the graphite powder was transformed into very thin graphene nanosheets after a long period of ball-milling. However, the thermal stability of our products is superior to the reported GN powder by reduction–oxidation of graphite,¹⁰ indicating few topological defects like five- or seven-membered rings in the obtained GN powder. This is in accordance with the detection by Raman spectroscopy (Fig. 2b).

Fig. 5 TGA curves of the obtained graphene nanosheets (GN 400, GN 600 and GN 1000) and the used pristine graphite powder, obtained by heating the targeted powders in air.

FT-IR could provide some information on the functional groups on the surface of the product, and the as-obtained results are shown in Fig. 6a. The band at 1210 cm⁻¹ is usually ascribed to the C–O stretching vibration of the C–OH groups of hydroxybenzene, suggesting that the suspending bonds at the edge of GN absorb plenty of molecular water. The band at 1570 cm⁻¹ arises from the aromatic rings (C=C) in the basal plane of GN, revealing that GN is so thin that it is permeable to infrared light.²⁷ In contrast, graphite powder did not show these feature peaks located at ca. 1200 or 1570 cm⁻¹ except for some absorbed water peaks. The absence of the characteristic band at ca. 1730 cm⁻¹ (carbonyl C=O stretching vibration) suggests the lack of product oxidation during ball-milling, in accordance with the examination by XPS. Due to the presence of plenty of suspending bonds easily binding the oxygen atoms in the ethanol molecules, the as-prepared GN powder can be dispersed in ethanol for long periods of time (see Fig. 6b). Compared to GN powders, the graphite powder settles very quickly once the ultrasound treatment is stopped. However, the GN powder could not be kept in a dispersed state in water for one hour due to the absence of C=O bonds in the GN products and the strong hydrogen bonding in water molecules.²⁸

Fig. 6 (a) FTIR spectra and (b) the dispersion of GN 600 and graphite in ethanol. Note: the dispersion tests were completed by dropping 5 mg of the sample into 20 ml of solvents and ultrasonicating for 10 min, then leaving to rest.

4. Conclusions

A soluble salt-assisted route has been applied to the low-cost and promisingly scalable preparation of pure graphene nanosheet powder via simple ball-milling of graphite–Na₂SO₄ mixtures, followed by washings with water. Systematic characterization showed that the as-prepared graphene nanosheets have a size in the range of hundreds square nanometers and ripple-like corrugations; the number of layers is also controllable from two layers to tens of layers by changing the weight ratio of graphite powder to soluble salt. Their thermal stability is greatly reduced whereas their dispersibility in ethanol is increased, when compared to pristine graphite powder, due to the existence of a large amount of suspending bonds in the products and their smaller size. The harvested graphene nanosheets present a higher thermal stability than those prepared by the reduction–oxidation of graphite powder. The simple and low-cost preparation of graphene nanosheet powder indicates the wide versatility of the soluble salt-assisted route for the prospective industrial preparation of graphene powder.

Acknowledgements

This work was supported by the NSFC (21161016) and the Foundations from the Environmental Protection Department [2003]370 and the Educational Department (GJJ13710) of Jiangxi Province.

Notes and references

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