Communication pubs.acs.org/cm
Graphene Quantum Dots Produced by Microfluidization Matat Buzaglo,§,† Michael Shtein,§,‡ and Oren Regev*,†,‡ †
Department of Chemical Engineering and ‡Ilse Katz Institute for Meso and Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel S Supporting Information *
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raphene quantum dots (GQDs), sheets of graphene with less than 10 layers and lateral dimensions smaller than 100 nm, possess strong quantum confinement and edge effects.1,2 Thus, they possess unique physical properties such as strong wavelength-dependent down- and up-conversion photoluminescence (PL),3−7 which can be tailored for specific applications by controlling their size,8 shape, defects, and functionality.9 In contrast to classic PL nanoparticles, such as metal or silicon quantum dots, GQDs are biocompatible,10,11 photostable,12,13 and inherit superior thermal, electrical, and mechanical properties from the graphene.14 However, although these features can greatly contribute to various state-of-the-art applications (e.g., flexible photovoltaics,15 flash memory devices,16 bioimaging,17 antibacterial systems,18 artificial peroxidase,19 light-emitting diodes,20 or sensors21), GQDs currently have a price tag of ∼2 million USD/kg,22 which limits their commercialization to more affordable biological applications that require only low concentrations (few g/L) of GQDs, such as cellular imaging, molecular tracking in live cells, biosensing, or drug delivery.10,11 GQDs can be synthesized and fabricated by either a bottomup or a top-down approach. In a bottom-up approach, GQDs are manufactured by stepwise oxidative condensation reactions,5 pyrolysis or carbonization of glucose,23 cage opening of C60 molecules,24 or nitration of pyrene.25 In a top-down approach, assemblies of graphene sheets are fragmented into zero-dimensional GQDs with physical, chemical, or electrochemical techniques, including hydrothermal26 or electrochemical27 graphene cutting, ultrasonicating a graphene−acid solution,28 or acid-treating coal,29 carbon nanotubes,30 carbon fibers,31 or graphite.32 Some of these fabrication methods are impractical for bulk quantities production due to their high cost, complexity (e.g., the use of acids and high temperatures), scalability, and environmental issues. In the current study, we employ microfluidization (Figure 1a) to fabricate nonfunctionalized GQDs (free of chemically bound species onto the surface, see Figure S8 and X-ray photoelectron spectroscopy (XPS) section in Supporting Information for details). Graphite−aqueous suspensions are powered by a high-pressure pump (up to 30 kpsi) through microsized Z-shaped channels (Figure 1b and methods section in the Supporting Information). As a result, the millimeter-sized graphite flakes are exfoliated into graphene sheets and are further fragmented into nanosized GQDs (Figure 1c). To the best of our knowledge, this is the first report of a purely mechanical fabrication of GQDs. The commonly used disintegration of carbon precursors by acid treatments26−30 is thereby avoided, and the utilization of an aqueous-based © XXXX American Chemical Society
Figure 1. (a) Photograph of typical microfluidizer. The graphite aqueous suspension is powered by 30 kpsi pump from the feed tank through microsized channels. Schematics of (b) Z-shaped channels with diameters ranging from 400 μm down to 87 μm, and (c) typical flow profile within the channel with maximal flow speed of 400 m/s. The graphite flakes are exfoliated into graphene sheets and further fragmented into nanosized GQDs.
medium makes this method environmentally and user-friendly. Microfluidization is a dynamic high-pressure homogenization process, which generates liquid velocities of 400 m/s and applies high shear rates (>107 s−1) on the solid particles. These shear rates are several orders of magnitude higher than those of conventional rotor-based or other homogenization techniques.33 It is easily scalable through addition of microchannels in parallel and commonly used in advanced pharmaceutical and food industries to produce fine and stable emulsions,34−36 or to form carbon nanotubes dispersion.37 GQDs synthesized in this technique are 2.7 ± 0.7 nm in diameter (Figure 2); they have a hexagonal symmetry in the real and reciprocal spaces (Figure 2b and inset, respectively), and a graphitic in-plane lattice spacing of 0.20 nm, indicative of graphene.10 An AFM topography reveals that the GQDs thickness is 2−4 nm (Figure 2d), namely, between two and four layers of graphene sheets.38 The microscopic characterization demonstrates that the proposed method can readily produce graphene particles that are only a few nanometers wide. To explore further the optical properties of the GQDs, a detailed PL studies were conducted (see Figure S2, S3 and photoluminescence section in Supporting Information for details). The Raman spectrum of the GQDs (Figure S1), which provides information on the Received: August 25, 2015 Revised: December 23, 2015
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DOI: 10.1021/acs.chemmater.5b03301 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials
dotted lines in Figure 3b) with emission at ∼400 nm (purple in the visible region), due to the small size of the nonfunctionalized GQDs. This PL behavior is in line with recent studies showing that a higher amount of sp2 domains (i.e., a lower amount of functional groups and defects) of a very similar diameter (2.9 nm GQDs) results in a shift of the PL spectra from blue (441 nm) to purple (407 nm).42 The PL of GQDs in the blue-purple range, as we obtained, is highly attractive for OLED devices, and short-wavelength diode lasers.43−45 We postulate that the preparation of GQDs by microfluidization is not limited to a size of 2.7 nm, and hence to the PL spectra reported here. In principle, the excitation wavelength of GQDs can be red-shifted29 by increasing their lateral size, which can be controlled by varying the shear rates applied to the graphitic material, namely, by changing the microfluidizer inlet pressure, channel diameters, or number of passes. Furthermore, the nonfunctionalized edges are highly chemical-reactive and can therefore be employed in consequent chemical functionalization. For example, the functionalization of GQDs by −OH or −COOH,46 amine,47 or F48 functional groups will red-shift the emission peaks, whereas N-doping will increase the quantum yield and blue-shift the emission peaks.10,49 Finally, the graphite flakes used in this study contain millimeter-sized crystalline domains, which are challenging to fragment and, therefore, represent an extreme production case leading to efficiency of 0.3% in this current procedure (see Figure S9 and GQDs concentration determination section in the Supporting Information). Utilizing smaller carbon-based precursor, such as coal containing nanometer-sized crystalline carbon domains,29 or carbon fibers with graphitic submicrometer domains,31 could further facilitate the production process and increase its efficiency. The quantum yield of the GQDs (with quinine as reference; see Figures S4−S7 and Quantum Yield calculation section in the Supporting Information) is 1.32%, similar to previously measured GQDs.29 The relatively low quantum yield may be a result of an aggregation quenching effect50 of the stacked polyaromatic structures (GQDs and TX100). In conclusion, we have developed a simple and environmentally friendly purely mechanical method to fabricate nonfunctionalized GQDs. Microfluidization fabrication is easily scalable through the addition of more microchannels in parallel. This approach ensures that the entire product stream experiences an identical shear, resulting in consistent product quality independent of the volume, from 1 to 60 L-per-minute (M-700 series51,51). The proposed method allows utilization of various organic solvents and carbon-based raw materials, and provides an alternative route for large-scale production and commercialization of GQDs.
Figure 2. TEM (a) and HRTEM (b) micrographs of GQDs produced by microfluidization. The FFT (b, inset) reveals a crystalline hexagonal structure. The dashed circles in panel b indicate lattice imaging. (c) Size distribution of GQDs, based on over 200 HRTEM-imaged particles. (d) AFM image and analysis (inset) of GQDs, indicating a thickness of 2−4 nm (inset).
defect density, is characterized by D and G bands at 1331 and 1589 cm−1, respectively, which are similar to those of blue luminescent hydrothermally synthesized GQDs.27 However, the D-to-G peak intensity ratio is significantly lower (
