One-step green synthesis of graphene nanomesh by fluid-based method Shuaishuai Liang,a Min Yi,a Zhigang Shen,*ab Lei Liu,ab Xiaojing Zhanga and Shulin Maa A fluid-based method is demonstrated for preparing graphene nanomeshes (GNMs) directly from pristine graphite flakes by a one-step process. The high efficiency is attributed to the combination of fluid- assisted exfoliation and perforation of the graphene sheets. Atomic force microscopy shows that the as-produced GNMs are less than 1.5 nm thick. The total area of the pores within 1 mm2 of the GNM sheet is estimated as 0.15 mm2 and the pore density as 22 mmÀ2, The yield of GNMs from pristine graphite powder and the power consumption for per gram GNM synthesis are evaluated as 5 wt% and 120 kW h, respectively. X-ray photoelectron spectroscopy, infrared spectroscopy, elemental analysis and Raman spectroscopy results indicate the purity of the GNMs and thus it is a green efficient method. The present work is expected to facilitate the production of GNMs in large scale. Introduction solution-processable GNMs by reuxing reduced graphene oxide (rGO) sheets in concentrated nitric acid solution. Despite graphene's exceptional properties and vast potential in a However, the acid treatment leads to further oxidation, which wide array of applications including electronic devices,1,2 introduces much more functional groups such as –C]O and sensors,3–5 catalysis6,7 and reinforced composites,8–10 its appli- –COOH to the rGO sheets. cation as a eld-effect transistor (FET) working at room Herein, to the best of our knowledge, we for the rst time temperature has been hindered due to its intrinsic semi-metallic report a one-step preparation of GNMs from pristine graphite behavior with zero band gap.11 In order to overcome this akes by using a uid-based method. In this method, bulk problem, researchers have tried to fabricate new graphene-based graphite particles were exfoliated into single- or few-layer gra- nanostructures with suitable band gaps, among which graphene phene and were simultaneously physically punched by the nanoribbon (GNR)12–15 and nanomesh (GNM)11,16 are much more cavitation-induced micro jets to form pore structures. The attractive. Although both of them can open the band gap to a whole procedure causes little oxidation and is green, low cost, level appropriate for transistor operation, GNM performs better efficient and readily scalable. than GNR. In fact, it has been demonstrated that the driving current or transconductance in GNM-based FET could be 100 times higher than that in individual GNR-based FET.11 Addi- Experimental tionally, the pore structure in GNM leads to an increase of its 21 The schematic of the designed device used for production of specic surface area and transparency, thus making GNM suit- GNMs is illustrated in Fig. 1. The critical part of the system is able in many applications such as catalysis, composite mate- rials, etc. Therefore, GNM is emerging as a new fascinating nanostructure and attracting more and more attention. However, the preparation of GNM faces challenges. So far, GNM is mainly prepared by plasma oxidation of graphene,11,17 chemical vapor deposition,18 or UV-assisted photodegradation of graphene oxide (GO) sheets with ZnO nanorods as the pho- tocatalyst.19 All the aforementioned approaches are not without drawbacks, suffering from low throughput, complexity, high cost, etc. Recently, Wang et al.20 reported the preparation of aBeijing Key Lab. for Powder Technology Research & Development, Beijing University of Aeronautics & Astronautics, Beijing, 100191, China. E-mail: [email protected]; Fax: +86-10-82338794; Tel: +86-10-82317516 bSchool of Materials Science & Engineering, Beijing University of Aeronautics & Fig. 1 Schematic of the fluid-based device used for preparing GNMs. Astronautics, Beijing, 100191, China The internal configuration of the nozzle is shown for clarity. Received 12th February 2014 Accepted 24th March 2014 DOI: 10.1039/c4ra01250j www.rsc.org/advances This journal is © The Royal Society of Chemistry 2014 RSC Adv.,2014,4,16127–16131 | 16127 the nozzle, which is equipped with a variable cross-section ow Elemental analysis (EA) was carried out on a vario EL cube channel for inducing cavitation and turbulence ow, as sche- elemental analyzer. Raman spectroscopy was captured with a matized in Fig. 1. Fluid-carried graphite akes can be pressur- Renishaw Rm2000 using a 514 nm laser. The AFM samples were ized by an axial piston pump in the inlet, and released to prepared by dropping the GNM dispersion onto freshly cleaved ambient pressure in the outlet. In our study, natural graphite mica wafer and dried in ambient temperature. Samples for TEM akes were purchased from Alfa Aesar (product number 43209) were made by pipetting several drops onto holey carbon mesh and used as received. The solvent used for production of GNMs grid. For Raman spectroscopy, the dispersions were made into is the mixture of isopropanol and de-ionized water with a mass thin lms by vacuum ltration through porous mixed cellulose ratio of 1 : 1.22 We prepared 10 L graphite dispersion by membranes (pore size: 450 nm) and dried in ambient temper- blending the natural graphite akes in the mixed solvent at ature. GNM powder for BET, XPS, FTIR, and EA study was À 1mgmL 1 and added the particle-laden uid in the designed carefully collected from the ltered lms. device to be processed for 2 h. The pressure of the inlet uid was 30 MPa according to the pressure gauge. The resulting disper- Results and discussion sions were centrifuged at 2000 rpm (Xiangyi L600, Changsha, China) for 1 h, and the supernatant was carefully extracted to For the GNMs produced by uid-based process, two critical obtain the GNM dispersion and retained for further use. issues should be addressed, i.e. the exfoliation state of graphene Optical absorbance of the GNM dispersion was measured and the pore structure. With this in mind, we characterized the using a Purkinje General TU1901 UV-vis spectrometer at a surface morphology of the GNMs by utilizing AFM. Fig. 2a and b wavelength of 660 nm.23 Atomic force microscope (AFM) images show typical AFM images of the as-produced GNMs. Some pores were collected by a Bruker MultiMode 8 scanning probe can be clearly seen on the graphene sheets, indicating the ideal microscope in ScanAsyst Air mode. Bright-eld transmission GNM structure. The height prole diagrams of Fig. 2a and b electron microscope (TEM) images were taken with a JEOL 2100 show that the thickness of the sheets was 1–1.5 nm, which can operating at 200 kV. Surface area measurement of dried gra- be identied as single- or bi-layer because the typical AFM- phene was performed with a Quantachrome Autosorb-IQ-MP measured thickness for monolayer graphene reported by pub- surface and pore size analyzer using the Brunauer–Emmett– lished literatures is taken as 1 nm.24,25 Moreover, by con- Teller (BET) method with nitrogen gas adsorption. X-ray ducting statistical study on 100 akes collected from several photoelectron spectroscopy (XPS) was recorded on a Thermo representative AFM images, we obtained the thickness distri- Fisher Scientic ESCALAB-250 spectrometer equipped with a bution of the GNMs, as shown in Fig. 2d. It can be seen that over monochromatic Al Ka X-rays excitation source (1486.6 eV). 70% of the akes are 1–1.5 nm thick, with others less than 1 nm. Fourier transformer infrared (FTIR) spectrum of the GNM Few akes thicker than 1.5 nm were observed. Therefore, the as- powder (collected from ltered lms) was measured by a Nicolet produced GNMs are proved to be highly exfoliated according to Nexus 870 spectrometer using the KBr pellet technique. our AFM analysis. Nevertheless, the BET surface area of the Fig. 2 (a and b) Typical AFM images of as-produced GNMs. The height profiles of corresponding lines are shown beneath the images. (c) TEM image of a GNM flake produced by the fluid-based method. (d and e) Histograms showing the distributions of thickness and pore size of the GNMs, respectively. (f) A photograph of the supernatant obtained after centrifugation, the GNM concentration was estimated as 50 mgmLÀ1. À dried GNMs was measured to be 45 m2 g 1, which is signi- Another important issue we must concern is the oxidation or cantly lower than the theoretical predictions for isolated gra- defect level. In the GO- or rGO-based preparation of GNM, it is À phene sheets (2600 m2 g 1). This indicates that the as-produced of great challenge to remove the attached functional groups. For GNMs are inclined to aggregate by nature when dried down,26 instance, according to ref. 19, GO sheets were partially reduced due to the absence of functional groups and wrinkle structures by the photodegradation in the presence of ZnO nanorods and in its basal plane, which oen exist in rGO and are critical for further reduced by hydrazine, but the reduction was limited as the separation of adjacent graphene sheets in dry state.27 indicated by the notable oxygen-containing carbonaceous Meanwhile, the distribution of the pore sizes in GNMs was also bands in XPS results. In the most recent work done by Wang evaluated, as presented in Fig. 2e. It can be seen that in Fig. 2e et al.20 the basal-plane-related groups such as C–N, C–O in rGO that the small pores with diameter less than 100 nm cover a sheets were reduced upon the acid treatment, but the procedure large percent (62%). Based on the statistical analysis, the total simultaneously led to a rapid increase of the –C]O and –COOH area of the pores within 1 mm2 of GNM sheet can be estimated as groups which are mainly located at the edges.