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Green, fast, and scalable production of reduced graphene oxide via Taylor vortex flow
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Green, fast, and scalable production of reduced graphene oxide via Taylor vortex flow Ki-Ho Nama, Ui Jung Kima, Myeong Hee Jeonb, Tae-Rin Leec, Jaesang Yua, Nam-Ho Youa, ⁎ Young-Kwan Kima, Ji Won Sukb,d, Bon-Cheol Kua, a
Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeonbuk 565-902, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do 16419, Republic of Korea c Advanced Institutes of Convergence Technology, Seoul National University, Suwon, Gyeonggi-do 16229, Republic of Korea d SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon, Gyeonggi-do 16419, Republic of Korea b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
fast, and bulk-scale process • Aforgreen, the production of RGO was developed.
mixing of GO and reductant • Efficient was achieved via TVF, which remarkably shorten the reduction time.
RGO exhibits high water-dis• The persibility and excellent electrical and mechanical properties.
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene oxide Reduced graphene oxide Mechanical properties Electrical conductivity Exfoliation Couette–Taylor reactor
Developing a cost-effective and bulk-scale process for graphene synthesis is essential for its commercialization in a wide range of industrial applications. In this study, for the first time, we used a Couette–Taylor fluid structure with axial flow as a green, rapid, and scalable protocol to synthesize reduced graphene oxide (RGO) flakes. We have determined five different flow characteristics in the laminar, transitional, and turbulent regimes and systematically investigated the effect of flow structure on RGO production. The toroidal vortices ensure the reactants are efficiently mixed, shortening the reduction time of graphene oxide (GO) from several hours to minutes. The results showed that the degree of RGO reduction significantly increased in the Taylor vortex flow (TVF) structure, and decreased in the wavy vortex flow (WVF) regime, because of the secondary instability of the fluid structure. More importantly, the TVF regime results in the synthesis of highly exfoliated and readily waterdispersible RGO products. Finally, the resulting RGO exhibited higher electrical conductivity and mechanical strength than conventional RGO synthesized under circular Couette flow (CCF). Thus, the proposed fluid dynamic protocol may open an effective, potentially cost-competitive, and industrially accessible pathway for producing few-layered RGO flakes for various applications.
⁎
Corresponding author. E-mail address: [email protected] (B.-C. Ku).
https://doi.org/10.1016/j.cej.2019.123482 Received 24 June 2019; Received in revised form 8 October 2019; Accepted 11 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Ki-Ho Nam, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123482
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1. Introduction
enhanced mass transfer and short reaction times. In addition, Couette–Taylor flow can generate high fluid-wall shear stress, preventing the undesired stacking of RGO sheets during the GO sheet chemical reduction process. We determined the different flow regimes in an aqueous reaction mixture to produce RGO flake using the dimensionless driving control parameters of the Reynolds (Re) and the Taylor numbers (Ta). This study demonstrates that the efficiency of reducing GO sheets strongly depends on the fluid flow structure, and stable secondary vortices result in the synthesis of structurally homogeneous RGO flakes.
Graphene, owing to its single-atomic thickness, two-dimensional honeycomb lattices, and its extraordinary physicochemical properties, has attracted massive attention since its discovery [1,2]. Following the discovery of these advantageous characteristics, it has been an urgent issue to produce large quantities of individual graphene sheets for industrial applications in many fields, ranging from conductive inks to electronic devices [3,4]. Some commercial applications, such as smart coatings [5–8], supercapacitors [9], organic electronics [10], sensors [11,12], batteries [13,14], solar cells [15–17], fuel cells [18], and composite fillers [19–23], require the low-cost and scalable production of defect-free graphene to an industrial level. Nevertheless, commercial applications of graphene are still limited because of the expensive, low production rates of current manufacturing processes. Synthetic routes of graphene production include direct mechanical exfoliation from natural graphite [1], bottom-up approaches using the chemical vapor deposition (CVD) reaction of carbon precursors on catalytic metal substrates [24–28], and top-down approaches based on the chemical oxidation/exfoliation/reduction of natural graphite [29–32]. The original method of mechanical exfoliation can yield high quality graphene sheets, but this approach is not scalable for bulk production. Although large-area and single to few-layered graphene films can be fabricated by the CVD method, the method is rather complicated, high-cost, and a difficult process for producing graphene films for practical composite applications. On the other hand, the chemical oxidation/exfoliation/reduction methods can produce solution-processible graphene analogues (reduced graphene oxide, RGO) in large quantities, and as a result, there is considerable interest in investigating this approach from a long-term perspective [33–35]. This method essentially produces a graphene oxide (GO) intermediate from the oxidation of natural graphite, and then finally, RGO flakes through a deoxygenation reaction. However, until now, the synthesis of RGO derivatives by chemical processes has raised environmental concerns because most of the processes require the use of toxic oxidizing and reducing agents, surfactants, and organic solvents [36]. The development of eco-friendly chemical additives to solve the above problems have been steadily pursued, but so far, they have remained at the level of small-scale production of relatively low quality RGO flakes. Therefore, development of a scalable route for the mass production of RGO flakes is urgently required for practical applications. To achieve the mass production of high-quality RGO flakes, it is important to induce a continuous chemical reduction reaction between the GO sheets and reducing agents without inducing aggregation, because once GO sheets become aggregated due to their strong inter-sheet van der Waals interactions, the reduction reaction is terminated. Therefore, to provide high yield and high quality RGO flakes in a short reaction time an efficient reduction process is needed to maintain the colloidal stability of the GO sheets in aqueous media throughout the reduction reaction. Herein, we report a facile method of producing RGO flakes using a continuous Couette–Taylor flow reactor system, which can facilitate scale-up, save cost, and alleviate environmental issues. Quite recently, Amamer and colleagues [37] employed a continuous Couette–Taylor flow reactor to transform the oxidation of graphite flakes, which is based on the revolutionized study on bulk-scale GO production of Park and co-workers [38,39]. Tran et al. also used Couette–Taylor flow reactor to study the effect of high shear stress on the exfoliation of graphite [40]. The Couette–Taylor flow reactor is comprised of an internal rotating cylinder and a fixed outer coaxial cylinder. If the rotation speed of the inner cylinder is high enough, doughnut-shaped toroidal vortices stacked in the annulus are generated, which rotate in reverse directions with constant arrays along the axis of the cylinders [41,42]. This Toroidal vortex induces highly efficient radial mixing as well as uniform fluidic structure within each cellular vortex structure, enabling
2. Experimental section 2.1. Experimental setup The Couette–Taylor flow reactor (Laminar Co. Ltd., Korea) consisted of two concentric cylinders (length: 265 mm), a rotating solid inner one (diameter: 52.1 mm) and a fixed hollow outer one (diameter: 62.5 mm). The gap between the two cylinders was d = 5.2 mm, thus the radii ratio of the cylinders η = 0.8 and the aspect ratio was Γ (L/d) = 50.9. The Couette–Taylor flow reactor system is driven by the inner cylinder rotation, which is quantified as the angular velocity of the inner cylinder (dimensional form) and in dimensionless parameters by the Reynolds and the Taylor numbers. The axial Reynolds number Re is defined as follows.
Re =
ri ωi d ν
where ri is the radii of the inner cylinder, ωi is the rotational angular velocity of the inner cylinder, d is the distance between the outer and inner cylinders, and ν is the kinematic viscosity of the working fluid between the two cylinders. The Taylor number Ta, which relates the centrifugal force to the viscous force, is often referred to as the rotating Reynolds number.
Ta =
ri ωi 2d3 ν2
Taylor [41] reported that the critical Taylor number (Tac) as d/ri approaches 0 is 41.3, and Kataoka et al. [42] classified a map of flow regimes based on a Taylor–Reynolds numbers when d/ri is 0.62 without axial flow. This study investigated the variations in hydrodynamic flow and reduction efficiency of GO in a Couette–Taylor flow reactor according to changes in the rotational speed of the inner cylinder. In these experiments, we used a colloidal suspension of GO in deionized water for which ν = 22.7 mm2 s−1 and ρ = 0.971 g mL−1. 2.2. Shear exfoliation and reduction of GO using Couette–Taylor flow In this work, L-ascorbic acid (AA), also known as vitamin C, was used as a green reducing agent for GO reduction. The mechanisms for the reduction of epoxide and di-hydroxyl groups in GO with AA was reported by Pumera and co-workers [43]. In a typical experiment, a colloidal suspension of GO (obtained from Grapheneall Co. Ltd., Republic of Korea) was adjusted to a concentration of 2 mg mL−1. The pH of the medium suspension (300 mL) was adjusted to ~10 by adding 25% NH3 solution to promote the colloidal stability of the GO sheets via electrostatic repulsion. 0.6 g of AA (99%, Sigma-Aldrich) was subsequently added to the suspension at room temperature. The mixture was injected into the gap between the two concentric cylinders of the reactor. The mixing vessel had a volume capacity of 260 mL and was connected to a circulating antifreeze liquid to maintain the 95 °C condition. The shear mixing of the suspension involved the rotation of the inner cylinder at speeds from 100 to 2500 rpm for reaction times of 1 to 60 min. The resulting black precipitates were simply filtered using cellulose membrane filter paper, washed with copious amounts of distilled water to neutral pH, and freeze-dried. 2
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shear rate within each cellular vortex structure, along with high fluidwall shear stress. This is likely responsible for the efficient mass transfer of the reducing agents to the GO sheets while preventing their aggregation during the chemical reduction process. The reduction reaction of GO was monitored as a function of reaction time by UV–Vis spectroscopic analysis for all the investigated operative conditions (Fig. 1b and c). The λmax of the GO aqueous solution was located 229 nm from the π-π* transition of the aromatic C]C bond but it was gradually red-shifted as deoxygenation occurred, and the aromatic C]C bonds were extended (see Supplementary, Fig. S1) [31,48]. The maximum red-shift that can be obtained depends on the operating conditions of the reactor. From the plots, it is possible to observe that in the CCF condition with an inner cylinder rotational speed of 100 rpm or less, the reduction was not completed, even after 60 min, as indicated by the relatively small red-shift from 229 to 268 nm. When the rotational speed exceeded 250 rpm, the π-π* transition peak was red-shifted to over 268 nm within 15 min. It should be noted that the efficiency and reduction rate of the GO in the TVF regime were greatly higher than those of GO in the unstable flow regimes (above WVF) due to the presence of inhomogeneity and nonaxisymmetric instabilities in the fluid flow characteristics. This confirms that the uniform fluidic motion structure of the TVF regime significantly increases the mixing and reaction efficiency of reducing agents with the GO sheets, within the vortices and between adjacent vortices. The progress of reduction in the Couette–Taylor flow reactor was spectroscopically examined, as shown in Fig. 2. Fig. 2a shows XRD patterns of the RGO flakes prepared using the Couette–Taylor flow reactor as a function of the inner cylinder’s rotating speeds, which induce different flow structures. The characteristic peak of GO at 2θ = 10.2° (interplanar spacing 8.7 Å) was clearly observed in the diffractogram. Reduction of the GO left agglomerated and randomly packed RGO flakes with a broad diffraction peak centered at 2θ = 24.2° (RGO@CCF), which is close to the typical (0 0 2) diffraction peak of graphite (interplanar spacing 3.4 Å at 2θ = 26.5°). In comparison to RGO@CCF, the XRD peaks of the samples prepared in the toroidal fields were shifted towards a lower 2θ angle. In particular, the TVF produced RGO sheets had the largest interplanar spacing of 3.9 Å among the tested fluid structures. This indicates that restacking of the RGO sheets was suppressed when the GO was reduced under high fluid-wall shear. Raman spectroscopy was used to monitor structural transformations that occurred during the fluid mechanistic reduction process (Fig. 2b). GO has a prominent peak at ~1350 cm−1 (D-band) corresponding to the defect sp2 carbon domains, and a weak peak at ~1590 cm−1 (Gband) corresponding to sp2 carbon domains. The Raman D/G band intensity ratio (ID/IG) is an indicator of the quality of graphene derivatives based on the degree of defects in their graphitic structure. Reduction in the Couette–Taylor flow resulted in a material that exhibited a distinctly increased ID/IG ratio, and concomitantly stronger and sharper D-band characteristics relative to the G-band compared to GO. This result indicates the formation of small sized sp2 carbon domains, or an increased in the proportion of edges due to fragmentation. RGO defects were further analyzed by Raman spectroscopy at different excitation energies (EL) because the ratio of D to G intensity changes inversely with the crystallite size La. Fig. S5 shows the Raman spectra of the RGO@TVF with La = 12.7 nm obtained using the three different laser energies, EL = 1.58 eV (λL = 785 nm), 2.41 eV (λL = 514 nm), and 2.54 eV (λL = 488 nm), respectively. We note that ID/IG ratio changes considerably with the EL (see Supplementary) [49,50]. The C 1s XPS spectra (see Supplementary, Fig. S2) of the GO showed typical peaks at 284.8, 285.6, 286.4, 287.2, 288.3, and 289.3 eV from the graphitic carbon structure, and the CeOH, CeOeC, C]O, and CeOOH bonds, respectively [51]. The calculated atomic percentages from the analysis of area under the C1s spectra are summarized in Table S2. The relative intensities of peaks from the oxygen containing functional groups slightly decreased for RGO@CCF, but there were still oxygen containing functional groups. The atomic ratio of carbon and
2.3. Preparation of RGO papers 150 mL of RGO suspension was diluted with 3 L of distilled water and filtered through cellulose membrane and dried. 3 mg of RGO powder was dispersed in 40 mL of DMF by bath sonication (150 W) for 6 h and filtered through an anodic aluminum oxide (AAO) membrane filter (0.2 μm pore size, Whatman, Pittsburgh, PA, USA). After completely drying, the fabricated RGO paper was separated from the membrane filter. 2.4. Characterization Ultraviolet–visible (UV–vis) spectra were recorded on a JASCO V670 spectrometer in absorption mode over a wavelength range of 200–800 nm, resolution of 1 nm, and scanning rate of 400 nm/min. Atomic force microscopy (AFM) measurements were performed in tapping mode on a Multimode atomic force microscope (Park NX-10, Park systems) using a silicon (Si) tip. XRD patterns were obtained using a SmartLab diffractometer equipped with a Cu Kα (λ = 0.15405 nm) radiation source operating at 40 kV and 30 mA. Raman spectra were collected on a LabRAM Raman spectrometer with excitation by an incident laser at 785, 514, and 488 nm. XPS measurements were performed using a K-Alpha spectrometer equipped with an Al Kα microfocused monochromator. Thermogravimetric analysis (TGA) was conducted with a Q50 TA Instruments analyser under a N2 flow at a heating rate of 3 °C min−1. Transmission electron microscopy (TEM) was conducted on a Tecnai G2 F20 (Thermo Fisher Scientific™) operating at 200 kV. For the TEM sample preparation, graphene powder was dispersed in deionized water, and subsequently the dispersion was dropped onto a copper grid and air-dried. Scanning electron microscopy (SEM; JEOL-7800F, JEOL Ltd.) was performed at an acceleration voltage of 10.0 kV. The average hydrodynamic diameter was measured using a Zetasizer Nano-ZS90 (Malvern Inc.). Turbiscan spectra were collected using a Turbiscan LAB Expert at a rate of one scan per hour. The mechanical properties of the RGO papers were evaluated by tensile tests after cutting the papers into rectangular strips [44]. The RGO paper strips were stretched by a universal tensile machine and data obtained from samples having fractures at the center of the strips were used for further analysis. The sheet resistance of the rGO papers was characterized using the four-probe van der Pauw method [45]. The electrical conductivity of the papers was determined from the sheet resistance by considering the measured thickness [46]. 3. Results and discussion As shown in Fig. 1a, a Couette–Taylor flow reactor can generate a secondary flow from the high shear initiated by the rotation of the inner cylinder. An aqueous colloidal suspension of GO with AA as a green reducing agent was fed into the reactor consisting of two concentric cylinders. In the Couette–Taylor flow reactor, the hydrodynamic conditions of the viscous fluid depend on the kinematic viscosity of the working fluid and the rotational speed of the inner cylinder. To test the effect of shear and mixing parameters on the quality of RGO, we determined the rotational speed of the inner cylinder using dimensionless driving control parameters at various angular velocity ratios [47]: (i) circular Couette flow (CCF), (ii) Taylor vortex flow (TVF), (iii) wavy vortex flow (WVF), (iv) modulated wavy vortex flow (MWVF), and (v) turbulent Taylor vortex flow (TTVF). The hydrodynamic flow regimes investigated at each operating condition are shown in Supplementary Table S1. Finally, we performed the reduction of GO in the Couette–Taylor flow reactor with increasing rotational speeds from 100 to 2500 rpm, at reaction times of 1 to 60 min. As the rotational speed of the inner cylinder reaches a threshold, a secondary flow appears in the rotating flow, where toroidal vortices known as TVF regularly form along the axis of the cylinders [41,42]. These TVF can generate highly effective radial mixing and a high local 3
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Fig. 1. a) Schematic of the shear induced-reduction and exfoliation mechanism of GO flakes driven by Couette–Taylor flow fluid dynamics. Conceptual diagram of the Taylor vortex structure in the Couette–Taylor flow reactor. b) UV–vis spectra of GO and RGO produced in the Couette–Taylor flow reactor after 60 min. c) Changes in λmax are shown as a function of reduction time.
hybridized carbons. The XPS results match well with the char yield of the TGA curve (see Supplementary, Fig. S3). It should be noted that despite the higher rotating speed of other hydrodynamic flow cases than TVF regime, and thus increased overall energy consumption, RGO synthesized in inconsistent wavy and turbulent vortex boundaries exhibited lower degree of reduction, indicating the importance of the
oxygen (C/O), which is a widely accepted criterion for evaluating the degree of reduction, significantly increased from 1.6 in GO to 6.2 in the RGO@TVF, verifying a successful reduction process (Fig. 2c). The results imply that under the high fluid-wall shear force, the reduction process more efficiently removed oxygen containing functional groups from the GO, and effectively converted sp3-to sp2-
Fig. 2. Spectroscopic characterizations of the GO and RGO prepared using the Couette–Taylor flow reactor as a function of the inner cylinder’s rotating speed. a) XRD patterns, b) Raman spectra measured with EL = 2.41 eV (λL = 514 nm), and c) XPS-based atomic composition analysis. All results are average values obtained from repeated measurements (N = 5). 4
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Fig. 3. TEM images and SAED patterns (inset) of the a) GO and b) RGO@TVF. c) HR-TEM image of the RGO@TVF. d) The particle size distribution of the GO and RGO dispersions. e) Turbiscan spectra and digital images (inset) showing GO (top), RGO@CCF (middle), and RGO@TVF (bottom) solutions (0.2 mg mL−1) for 24 h.
Fig. 4. a) Van der Pauw arrangement of the 4-point probes for sheet resistance measurements of the RGO paper. b) Sheet resistance and electrical conductivity of free-standing RGO papers. The inset shows a photograph of the RGO@CCF and RGO@TVF papers. c) Representative stress-strain curves and d) tensile properties of the RGO papers. Table 1 Electrical and mechanical properties of the RGO papers. Sample
Sheet resistance (Ω/sq)
Electrical conductivity (S/m)
Tensile strength (MPa)
Elastic modulus (GPa)
RGO@CCF RGO@TVF
453.5 ± 153.8 295.3 ± 103.3
478.9 ± 286.9 711.7 ± 209.5
5.5 ± 0.6 13.5 ± 0.6
0.67 ± 0.06 0.93 ± 0.02
changed significantly with the rotational speed, as shown in Fig. 3d. At a rotational speed of 100 rpm, the RGO@CCF particle size was broadly distributed, and several highly aggregated RGO sheets with large hydrodynamic radii of 3–5.5 μm were observed. However, under the vortex flow, particles larger than 3 μm completely disappeared and the average hydrodynamic diameter of the RGO sheet also gradually decreased. Thus, it should be noted that the continuous high fluid-wall shear produced by fluidic motion resulted in the fragmentation of the RGO sheets. The morphology and thickness of the as-synthesized RGO@TVF sheets were further characterized by SEM and AFM, respectively. Fig. S4 presents the images of a 0.1 mg mL−1 deionized
optimized hydrodynamic condition in Couette–Taylor reactor to maximize reduction efficiency with a homogeneity in the resulting RGO flakes. TEM micrographs of the GO and RGO are shown in Fig. 3a and b, respectively, which show a clean and wrinkled surface. In the selective area electron diffraction (SAED) results, a sharp single set of hexagonal diffraction patterns from the RGO can be seen (Fig. 2b inset) while a polycrystalline structure was observed from the GO. Furthermore, HRTEM analysis revealed that the RGO had clear cut, few-layered hexagonal crystalline structure (Fig. 3c). The particle size distribution of the RGO dispersion at a steady state
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mechanically strong RGO products. The reported protocol serves as an advantageous method for the bulk-scale production of few-layered RGO flakes for potential use in practical top-down processing.
water/RGO@TVF dispersion deposited onto a Si wafer. The SEM image (Fig. S4a) illustrates that the RGO@TVF sheets are well-distributed across the Si wafer with an average size of 17 μm. Fig. S4b shows a 1–2 nm step height range from the surface of the Si wafer to the RGO@TVF sheet. Since the theoretical step height for a single graphene layer is 0.34 nm, the RGO@TVF shows few-layered structure with highaspect-ratio. In addition, we speculate that the increased step height of RGO@TVF may be attributed to residual oxygen functionality on the surface of the sheet, causing some corrugation in the surface of the sheet. The long-term floatation and sedimentation of the RGO sheets in aqueous dispersions were examined quantitatively using Turbiscan, by tracking changes in transmittance throughout the height of a 20 mL vial at fixed time intervals (Fig. 3e). The RGO@CCF remained stable in water for only a few hours, rapidly precipitating to the bottom of the vial, and the transmittance increased by more than 6.7% after 1 day. The aqueous dispersibility of RGO is generally poor with aggregation occurring rapidly by intra-sheet π–π stacking [52]. In comparison, RGO@TVF formed a homogeneous and stable colloidal suspension for 1 day with only a small amount of precipitates, which is comparable to GO. The change in transmittance at different heights over time was less than 1% throughout the entire characterization period. It is noteworthy that the RGO@TVF formed more homogeneous and stable colloidal suspension than that of RGO@CCF, meaning that combination of high fluid-wall shear exfoliation with about 14% of oxygen derivatives on the RGO surface can lead to the efficient dispersion of RGO lattice in water [34,53]. Electrical conductivity behavior is considered to be a strong indicator of the extent to which the networked sp2 carbon structures are restored by the chemical reduction of GO. The electrical conductivity of the RGO papers was measured at room temperature using the 4-point probe Van der Pauw method, with a homemade measurement system (Fig. 4a). Typically, GO paper is not conducting, because its surface oxidation disrupts the networked sp2 carbon structure. Consistent with the results discussed in Fig. 4b and Table 1, the Couette–Taylor flow system with vortex flow was highly effective in restoring the networked sp2 carbon structure of the RGO. The RGO@CCF paper showed conductivities lower than that of the RGO@TVF paper, which was attributed to the lower chemical reduction efficiency. Fig. 4c show representative stress-strain curves of the RGO papers fabricated with and without the formation of toroidal vortices, respectively. As shown in Table 1, the RGO paper derived from CCF exhibited a tensile strength of 5.5 MPa and an elastic modulus of 0.68 GPa, and a low strain at break of around 0.6%. Remarkably, when the RGO was prepared in a toroidal field, the tensile strength and elastic modulus increased to 13.5 MPa and 0.93 GPa, which are 59.3% and 26.9% higher than those of the RGO@CCF paper. This can be caused by the insufficient dispersion of RGO@CCF, evidenced by the fact that the RGO sheets tended to agglomerate and form small clumps in the aqueous suspension after the reduction process. This makes it difficult for the RGO sheet to be reconstituted into a well-packed uniform thin film structure.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by grants from Open Research Program (ORP) of Korea Institute of Science and Technology (KIST). This work was also supported by 2016M3A7B4027223 and 2019R1A5A8080326 through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123482. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field in atomically thin carbon films, Science 306 (2004) 666–669. [2] Y. Zhang, Y.-W. Tan, H.L. Stormer, P. Kim, Experimental observation of the quantum Hall effect and Berry’s phase in graphene, Nature 438 (2005) 201–204. [3] K.S. Novoselov, V.I. Fal'Ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (2012) 192–200. [4] A.P. Kauling, A.T. Seefeldt, D.P. Pisoni, R.C. Pradeep, R. Bentini, R.V.B. Oliveria, K.S. Novoselov, A.H. Castro Neto, The worldwide graphene flake production, Adv. Mater. 30 (2018) 1803784. [5] S.-G. Kim, N.-H. You, W. Lee, J.Y. Hwang, M.J. Kim, D. Hui, B.-C. Ku, J.H. Lee, Effects of the functionalized graphene oxide on the oxygen barrier and mechanical properties of layer-by-layer assembled films, Compos. Pt. B-Eng. 92 (2016) 307–314. [6] J.-U. Jin, D.-H. Lee, K.-H. Nam, J. Yu, Y.-K. Kim, M. Goh, S.G. Kim, H.S. Lee, B.C. Ku, N.-H. You, Methylpiperidine-functionalized graphene oxide for efficient curing acceleration and gas barrier of polymer nanocomposites, Appl. Surf. Sci. 464 (2019) 509–515. [7] N. Selvkumar, A. Biswas, S.B. Krupanidhi, H.C. Barshilia, Enhanced optical absorption of graphene-based heat mirror with tunable spectral selectivity, Sol. Energy Mater. Sol. Cells 186 (2018) 149–153. [8] E. Šest, G. Dražič, B. Genorio, I. Jerman, Graphene nanoplatelets as an anticorrosion additive for solar absorber coatings, Sol. Energy Mater. Sol. Cells 176 (2018) 19–29. [9] M.D. Stoller, S. Park, Z. Yanwu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498–3502. [10] Y. Gim, B. Kang, B. Kim, S.-G. Kim, J.-H. Lee, K. Cho, B.-C. Ku, J.H. Cho, Atomicallythin molecular layers for electrode modification of organic transistors, Nanoscale 7 (2015) 14100–14108. [11] G.P. Keeley, A. O'Neill, N. McEvoy, N. Peltekis, J.N. Coleman, G.S. Duesberg, Electrochemical ascorbic acid sensor based on DMF-exfoliated graphene, J. Mater. Chem. 20 (2010) 7864–7869. [12] M.W. Jung, S.M. Kang, K.-H. Nam, K.-S. An, B.-C. Ku, Highly transparent and flexible NO2 gas sensor film based on MoS2 /rGO composites using soft lithographic patterning, Appl. Surf. Sci. 456 (2018) 7–12. [13] P. Martin, Electrochemistry of graphene: new horizons for sensing and energy storage, Chem. Rec. 9 (2009) 211–223. [14] Y. Li, J. Wang, X. Li, D. Geng, R. Li, X. Sun, Superior energy capacity of graphene nanosheets for a nonaqueous lithium-oxygen battery, Chem. Commun. 47 (2011) 9438–9440. [15] L. Kavan, J.H. Yum, M. Grätzel, Optically transparent cathode for dye-sensitized solar cells based on graphene nanoplatelets, ACS Nano 5 (2011) 165–172. [16] X. Wang, L. Zhi, K. Müllen, Transparent, conductive graphene electrodes for dyesensitized solar cells, Nano Lett. 8 (2008) 323–327. [17] L. Liu, K. Zheng, Y. Yan, Z. Cai, S. Lin, X. Hu, Graphene aerogels enhanced phase change materials prepared by one-pot method with high thermal conductivity and large latent energy storage, Sol. Energy Mater. Sol. Cells 185 (2018) 487–493. [18] L. Qu, Y. Liu, J.-B. Baek, L. Dai, Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells, ACS Nano 4 (2010) 1321–1326. [19] K.-H. Nam, J. Yu, N.-H. You, H. Han, B.-C. Ku, Synergistic toughening of polymer nanocomposites by hydrogen-bond assisted three-dimensional network of functionalized graphene oxide and carbon nanotubes, Compos. Sci. Technol. 149 (2017) 228–234. [20] T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, M. Herrera-Alonso,
4. Conclusion We report a green, rapid, cost-competitive, and industrially scalable protocol for producing few-layered RGO using a high shear-induced reduction method. This method employed a Couette–Taylor reactor system and demonstrated the effect of the flow regime and inhomogeneity in the fluid kinetic structure on the degree of reduction and morphology of RGO sheets. The current study highlights the importance of Taylor vortex flow with axisymmetric instability, which leads to high fluid-wall shear stresses that are sufficient to accelerate the reduction of GO, while preventing aggregation. Moreover, the TVF regime resulted in highly water-dispersible RGO flakes, and thus they can be simply used to produce electrically conductive, and 6
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Green, fast, and scalable production of reduced graphene oxide via Taylor vortex flow Ki-Ho Nama, Ui Jung Kima, Myeong Hee Jeonb, Tae-Rin Leec, Jaesang Yua, Nam-Ho Youa, ⁎ Young-Kwan Kima, Ji Won Sukb,d, Bon-Cheol Kua, a
Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeonbuk 565-902, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do 16419, Republic of Korea c Advanced Institutes of Convergence Technology, Seoul National University, Suwon, Gyeonggi-do 16229, Republic of Korea d SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon, Gyeonggi-do 16419, Republic of Korea b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
fast, and bulk-scale process • Aforgreen, the production of RGO was developed.
mixing of GO and reductant • Efficient was achieved via TVF, which remarkably shorten the reduction time.
RGO exhibits high water-dis• The persibility and excellent electrical and mechanical properties.
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene oxide Reduced graphene oxide Mechanical properties Electrical conductivity Exfoliation Couette–Taylor reactor
Developing a cost-effective and bulk-scale process for graphene synthesis is essential for its commercialization in a wide range of industrial applications. In this study, for the first time, we used a Couette–Taylor fluid structure with axial flow as a green, rapid, and scalable protocol to synthesize reduced graphene oxide (RGO) flakes. We have determined five different flow characteristics in the laminar, transitional, and turbulent regimes and systematically investigated the effect of flow structure on RGO production. The toroidal vortices ensure the reactants are efficiently mixed, shortening the reduction time of graphene oxide (GO) from several hours to minutes. The results showed that the degree of RGO reduction significantly increased in the Taylor vortex flow (TVF) structure, and decreased in the wavy vortex flow (WVF) regime, because of the secondary instability of the fluid structure. More importantly, the TVF regime results in the synthesis of highly exfoliated and readily waterdispersible RGO products. Finally, the resulting RGO exhibited higher electrical conductivity and mechanical strength than conventional RGO synthesized under circular Couette flow (CCF). Thus, the proposed fluid dynamic protocol may open an effective, potentially cost-competitive, and industrially accessible pathway for producing few-layered RGO flakes for various applications.
⁎
Corresponding author. E-mail address: [email protected] (B.-C. Ku).
https://doi.org/10.1016/j.cej.2019.123482 Received 24 June 2019; Received in revised form 8 October 2019; Accepted 11 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Ki-Ho Nam, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123482
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1. Introduction
enhanced mass transfer and short reaction times. In addition, Couette–Taylor flow can generate high fluid-wall shear stress, preventing the undesired stacking of RGO sheets during the GO sheet chemical reduction process. We determined the different flow regimes in an aqueous reaction mixture to produce RGO flake using the dimensionless driving control parameters of the Reynolds (Re) and the Taylor numbers (Ta). This study demonstrates that the efficiency of reducing GO sheets strongly depends on the fluid flow structure, and stable secondary vortices result in the synthesis of structurally homogeneous RGO flakes.
Graphene, owing to its single-atomic thickness, two-dimensional honeycomb lattices, and its extraordinary physicochemical properties, has attracted massive attention since its discovery [1,2]. Following the discovery of these advantageous characteristics, it has been an urgent issue to produce large quantities of individual graphene sheets for industrial applications in many fields, ranging from conductive inks to electronic devices [3,4]. Some commercial applications, such as smart coatings [5–8], supercapacitors [9], organic electronics [10], sensors [11,12], batteries [13,14], solar cells [15–17], fuel cells [18], and composite fillers [19–23], require the low-cost and scalable production of defect-free graphene to an industrial level. Nevertheless, commercial applications of graphene are still limited because of the expensive, low production rates of current manufacturing processes. Synthetic routes of graphene production include direct mechanical exfoliation from natural graphite [1], bottom-up approaches using the chemical vapor deposition (CVD) reaction of carbon precursors on catalytic metal substrates [24–28], and top-down approaches based on the chemical oxidation/exfoliation/reduction of natural graphite [29–32]. The original method of mechanical exfoliation can yield high quality graphene sheets, but this approach is not scalable for bulk production. Although large-area and single to few-layered graphene films can be fabricated by the CVD method, the method is rather complicated, high-cost, and a difficult process for producing graphene films for practical composite applications. On the other hand, the chemical oxidation/exfoliation/reduction methods can produce solution-processible graphene analogues (reduced graphene oxide, RGO) in large quantities, and as a result, there is considerable interest in investigating this approach from a long-term perspective [33–35]. This method essentially produces a graphene oxide (GO) intermediate from the oxidation of natural graphite, and then finally, RGO flakes through a deoxygenation reaction. However, until now, the synthesis of RGO derivatives by chemical processes has raised environmental concerns because most of the processes require the use of toxic oxidizing and reducing agents, surfactants, and organic solvents [36]. The development of eco-friendly chemical additives to solve the above problems have been steadily pursued, but so far, they have remained at the level of small-scale production of relatively low quality RGO flakes. Therefore, development of a scalable route for the mass production of RGO flakes is urgently required for practical applications. To achieve the mass production of high-quality RGO flakes, it is important to induce a continuous chemical reduction reaction between the GO sheets and reducing agents without inducing aggregation, because once GO sheets become aggregated due to their strong inter-sheet van der Waals interactions, the reduction reaction is terminated. Therefore, to provide high yield and high quality RGO flakes in a short reaction time an efficient reduction process is needed to maintain the colloidal stability of the GO sheets in aqueous media throughout the reduction reaction. Herein, we report a facile method of producing RGO flakes using a continuous Couette–Taylor flow reactor system, which can facilitate scale-up, save cost, and alleviate environmental issues. Quite recently, Amamer and colleagues [37] employed a continuous Couette–Taylor flow reactor to transform the oxidation of graphite flakes, which is based on the revolutionized study on bulk-scale GO production of Park and co-workers [38,39]. Tran et al. also used Couette–Taylor flow reactor to study the effect of high shear stress on the exfoliation of graphite [40]. The Couette–Taylor flow reactor is comprised of an internal rotating cylinder and a fixed outer coaxial cylinder. If the rotation speed of the inner cylinder is high enough, doughnut-shaped toroidal vortices stacked in the annulus are generated, which rotate in reverse directions with constant arrays along the axis of the cylinders [41,42]. This Toroidal vortex induces highly efficient radial mixing as well as uniform fluidic structure within each cellular vortex structure, enabling
2. Experimental section 2.1. Experimental setup The Couette–Taylor flow reactor (Laminar Co. Ltd., Korea) consisted of two concentric cylinders (length: 265 mm), a rotating solid inner one (diameter: 52.1 mm) and a fixed hollow outer one (diameter: 62.5 mm). The gap between the two cylinders was d = 5.2 mm, thus the radii ratio of the cylinders η = 0.8 and the aspect ratio was Γ (L/d) = 50.9. The Couette–Taylor flow reactor system is driven by the inner cylinder rotation, which is quantified as the angular velocity of the inner cylinder (dimensional form) and in dimensionless parameters by the Reynolds and the Taylor numbers. The axial Reynolds number Re is defined as follows.
Re =
ri ωi d ν
where ri is the radii of the inner cylinder, ωi is the rotational angular velocity of the inner cylinder, d is the distance between the outer and inner cylinders, and ν is the kinematic viscosity of the working fluid between the two cylinders. The Taylor number Ta, which relates the centrifugal force to the viscous force, is often referred to as the rotating Reynolds number.
Ta =
ri ωi 2d3 ν2
Taylor [41] reported that the critical Taylor number (Tac) as d/ri approaches 0 is 41.3, and Kataoka et al. [42] classified a map of flow regimes based on a Taylor–Reynolds numbers when d/ri is 0.62 without axial flow. This study investigated the variations in hydrodynamic flow and reduction efficiency of GO in a Couette–Taylor flow reactor according to changes in the rotational speed of the inner cylinder. In these experiments, we used a colloidal suspension of GO in deionized water for which ν = 22.7 mm2 s−1 and ρ = 0.971 g mL−1. 2.2. Shear exfoliation and reduction of GO using Couette–Taylor flow In this work, L-ascorbic acid (AA), also known as vitamin C, was used as a green reducing agent for GO reduction. The mechanisms for the reduction of epoxide and di-hydroxyl groups in GO with AA was reported by Pumera and co-workers [43]. In a typical experiment, a colloidal suspension of GO (obtained from Grapheneall Co. Ltd., Republic of Korea) was adjusted to a concentration of 2 mg mL−1. The pH of the medium suspension (300 mL) was adjusted to ~10 by adding 25% NH3 solution to promote the colloidal stability of the GO sheets via electrostatic repulsion. 0.6 g of AA (99%, Sigma-Aldrich) was subsequently added to the suspension at room temperature. The mixture was injected into the gap between the two concentric cylinders of the reactor. The mixing vessel had a volume capacity of 260 mL and was connected to a circulating antifreeze liquid to maintain the 95 °C condition. The shear mixing of the suspension involved the rotation of the inner cylinder at speeds from 100 to 2500 rpm for reaction times of 1 to 60 min. The resulting black precipitates were simply filtered using cellulose membrane filter paper, washed with copious amounts of distilled water to neutral pH, and freeze-dried. 2
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shear rate within each cellular vortex structure, along with high fluidwall shear stress. This is likely responsible for the efficient mass transfer of the reducing agents to the GO sheets while preventing their aggregation during the chemical reduction process. The reduction reaction of GO was monitored as a function of reaction time by UV–Vis spectroscopic analysis for all the investigated operative conditions (Fig. 1b and c). The λmax of the GO aqueous solution was located 229 nm from the π-π* transition of the aromatic C]C bond but it was gradually red-shifted as deoxygenation occurred, and the aromatic C]C bonds were extended (see Supplementary, Fig. S1) [31,48]. The maximum red-shift that can be obtained depends on the operating conditions of the reactor. From the plots, it is possible to observe that in the CCF condition with an inner cylinder rotational speed of 100 rpm or less, the reduction was not completed, even after 60 min, as indicated by the relatively small red-shift from 229 to 268 nm. When the rotational speed exceeded 250 rpm, the π-π* transition peak was red-shifted to over 268 nm within 15 min. It should be noted that the efficiency and reduction rate of the GO in the TVF regime were greatly higher than those of GO in the unstable flow regimes (above WVF) due to the presence of inhomogeneity and nonaxisymmetric instabilities in the fluid flow characteristics. This confirms that the uniform fluidic motion structure of the TVF regime significantly increases the mixing and reaction efficiency of reducing agents with the GO sheets, within the vortices and between adjacent vortices. The progress of reduction in the Couette–Taylor flow reactor was spectroscopically examined, as shown in Fig. 2. Fig. 2a shows XRD patterns of the RGO flakes prepared using the Couette–Taylor flow reactor as a function of the inner cylinder’s rotating speeds, which induce different flow structures. The characteristic peak of GO at 2θ = 10.2° (interplanar spacing 8.7 Å) was clearly observed in the diffractogram. Reduction of the GO left agglomerated and randomly packed RGO flakes with a broad diffraction peak centered at 2θ = 24.2° (RGO@CCF), which is close to the typical (0 0 2) diffraction peak of graphite (interplanar spacing 3.4 Å at 2θ = 26.5°). In comparison to RGO@CCF, the XRD peaks of the samples prepared in the toroidal fields were shifted towards a lower 2θ angle. In particular, the TVF produced RGO sheets had the largest interplanar spacing of 3.9 Å among the tested fluid structures. This indicates that restacking of the RGO sheets was suppressed when the GO was reduced under high fluid-wall shear. Raman spectroscopy was used to monitor structural transformations that occurred during the fluid mechanistic reduction process (Fig. 2b). GO has a prominent peak at ~1350 cm−1 (D-band) corresponding to the defect sp2 carbon domains, and a weak peak at ~1590 cm−1 (Gband) corresponding to sp2 carbon domains. The Raman D/G band intensity ratio (ID/IG) is an indicator of the quality of graphene derivatives based on the degree of defects in their graphitic structure. Reduction in the Couette–Taylor flow resulted in a material that exhibited a distinctly increased ID/IG ratio, and concomitantly stronger and sharper D-band characteristics relative to the G-band compared to GO. This result indicates the formation of small sized sp2 carbon domains, or an increased in the proportion of edges due to fragmentation. RGO defects were further analyzed by Raman spectroscopy at different excitation energies (EL) because the ratio of D to G intensity changes inversely with the crystallite size La. Fig. S5 shows the Raman spectra of the RGO@TVF with La = 12.7 nm obtained using the three different laser energies, EL = 1.58 eV (λL = 785 nm), 2.41 eV (λL = 514 nm), and 2.54 eV (λL = 488 nm), respectively. We note that ID/IG ratio changes considerably with the EL (see Supplementary) [49,50]. The C 1s XPS spectra (see Supplementary, Fig. S2) of the GO showed typical peaks at 284.8, 285.6, 286.4, 287.2, 288.3, and 289.3 eV from the graphitic carbon structure, and the CeOH, CeOeC, C]O, and CeOOH bonds, respectively [51]. The calculated atomic percentages from the analysis of area under the C1s spectra are summarized in Table S2. The relative intensities of peaks from the oxygen containing functional groups slightly decreased for RGO@CCF, but there were still oxygen containing functional groups. The atomic ratio of carbon and
2.3. Preparation of RGO papers 150 mL of RGO suspension was diluted with 3 L of distilled water and filtered through cellulose membrane and dried. 3 mg of RGO powder was dispersed in 40 mL of DMF by bath sonication (150 W) for 6 h and filtered through an anodic aluminum oxide (AAO) membrane filter (0.2 μm pore size, Whatman, Pittsburgh, PA, USA). After completely drying, the fabricated RGO paper was separated from the membrane filter. 2.4. Characterization Ultraviolet–visible (UV–vis) spectra were recorded on a JASCO V670 spectrometer in absorption mode over a wavelength range of 200–800 nm, resolution of 1 nm, and scanning rate of 400 nm/min. Atomic force microscopy (AFM) measurements were performed in tapping mode on a Multimode atomic force microscope (Park NX-10, Park systems) using a silicon (Si) tip. XRD patterns were obtained using a SmartLab diffractometer equipped with a Cu Kα (λ = 0.15405 nm) radiation source operating at 40 kV and 30 mA. Raman spectra were collected on a LabRAM Raman spectrometer with excitation by an incident laser at 785, 514, and 488 nm. XPS measurements were performed using a K-Alpha spectrometer equipped with an Al Kα microfocused monochromator. Thermogravimetric analysis (TGA) was conducted with a Q50 TA Instruments analyser under a N2 flow at a heating rate of 3 °C min−1. Transmission electron microscopy (TEM) was conducted on a Tecnai G2 F20 (Thermo Fisher Scientific™) operating at 200 kV. For the TEM sample preparation, graphene powder was dispersed in deionized water, and subsequently the dispersion was dropped onto a copper grid and air-dried. Scanning electron microscopy (SEM; JEOL-7800F, JEOL Ltd.) was performed at an acceleration voltage of 10.0 kV. The average hydrodynamic diameter was measured using a Zetasizer Nano-ZS90 (Malvern Inc.). Turbiscan spectra were collected using a Turbiscan LAB Expert at a rate of one scan per hour. The mechanical properties of the RGO papers were evaluated by tensile tests after cutting the papers into rectangular strips [44]. The RGO paper strips were stretched by a universal tensile machine and data obtained from samples having fractures at the center of the strips were used for further analysis. The sheet resistance of the rGO papers was characterized using the four-probe van der Pauw method [45]. The electrical conductivity of the papers was determined from the sheet resistance by considering the measured thickness [46]. 3. Results and discussion As shown in Fig. 1a, a Couette–Taylor flow reactor can generate a secondary flow from the high shear initiated by the rotation of the inner cylinder. An aqueous colloidal suspension of GO with AA as a green reducing agent was fed into the reactor consisting of two concentric cylinders. In the Couette–Taylor flow reactor, the hydrodynamic conditions of the viscous fluid depend on the kinematic viscosity of the working fluid and the rotational speed of the inner cylinder. To test the effect of shear and mixing parameters on the quality of RGO, we determined the rotational speed of the inner cylinder using dimensionless driving control parameters at various angular velocity ratios [47]: (i) circular Couette flow (CCF), (ii) Taylor vortex flow (TVF), (iii) wavy vortex flow (WVF), (iv) modulated wavy vortex flow (MWVF), and (v) turbulent Taylor vortex flow (TTVF). The hydrodynamic flow regimes investigated at each operating condition are shown in Supplementary Table S1. Finally, we performed the reduction of GO in the Couette–Taylor flow reactor with increasing rotational speeds from 100 to 2500 rpm, at reaction times of 1 to 60 min. As the rotational speed of the inner cylinder reaches a threshold, a secondary flow appears in the rotating flow, where toroidal vortices known as TVF regularly form along the axis of the cylinders [41,42]. These TVF can generate highly effective radial mixing and a high local 3
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Fig. 1. a) Schematic of the shear induced-reduction and exfoliation mechanism of GO flakes driven by Couette–Taylor flow fluid dynamics. Conceptual diagram of the Taylor vortex structure in the Couette–Taylor flow reactor. b) UV–vis spectra of GO and RGO produced in the Couette–Taylor flow reactor after 60 min. c) Changes in λmax are shown as a function of reduction time.
hybridized carbons. The XPS results match well with the char yield of the TGA curve (see Supplementary, Fig. S3). It should be noted that despite the higher rotating speed of other hydrodynamic flow cases than TVF regime, and thus increased overall energy consumption, RGO synthesized in inconsistent wavy and turbulent vortex boundaries exhibited lower degree of reduction, indicating the importance of the
oxygen (C/O), which is a widely accepted criterion for evaluating the degree of reduction, significantly increased from 1.6 in GO to 6.2 in the RGO@TVF, verifying a successful reduction process (Fig. 2c). The results imply that under the high fluid-wall shear force, the reduction process more efficiently removed oxygen containing functional groups from the GO, and effectively converted sp3-to sp2-
Fig. 2. Spectroscopic characterizations of the GO and RGO prepared using the Couette–Taylor flow reactor as a function of the inner cylinder’s rotating speed. a) XRD patterns, b) Raman spectra measured with EL = 2.41 eV (λL = 514 nm), and c) XPS-based atomic composition analysis. All results are average values obtained from repeated measurements (N = 5). 4
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Fig. 3. TEM images and SAED patterns (inset) of the a) GO and b) RGO@TVF. c) HR-TEM image of the RGO@TVF. d) The particle size distribution of the GO and RGO dispersions. e) Turbiscan spectra and digital images (inset) showing GO (top), RGO@CCF (middle), and RGO@TVF (bottom) solutions (0.2 mg mL−1) for 24 h.
Fig. 4. a) Van der Pauw arrangement of the 4-point probes for sheet resistance measurements of the RGO paper. b) Sheet resistance and electrical conductivity of free-standing RGO papers. The inset shows a photograph of the RGO@CCF and RGO@TVF papers. c) Representative stress-strain curves and d) tensile properties of the RGO papers. Table 1 Electrical and mechanical properties of the RGO papers. Sample
Sheet resistance (Ω/sq)
Electrical conductivity (S/m)
Tensile strength (MPa)
Elastic modulus (GPa)
RGO@CCF RGO@TVF
453.5 ± 153.8 295.3 ± 103.3
478.9 ± 286.9 711.7 ± 209.5
5.5 ± 0.6 13.5 ± 0.6
0.67 ± 0.06 0.93 ± 0.02
changed significantly with the rotational speed, as shown in Fig. 3d. At a rotational speed of 100 rpm, the RGO@CCF particle size was broadly distributed, and several highly aggregated RGO sheets with large hydrodynamic radii of 3–5.5 μm were observed. However, under the vortex flow, particles larger than 3 μm completely disappeared and the average hydrodynamic diameter of the RGO sheet also gradually decreased. Thus, it should be noted that the continuous high fluid-wall shear produced by fluidic motion resulted in the fragmentation of the RGO sheets. The morphology and thickness of the as-synthesized RGO@TVF sheets were further characterized by SEM and AFM, respectively. Fig. S4 presents the images of a 0.1 mg mL−1 deionized
optimized hydrodynamic condition in Couette–Taylor reactor to maximize reduction efficiency with a homogeneity in the resulting RGO flakes. TEM micrographs of the GO and RGO are shown in Fig. 3a and b, respectively, which show a clean and wrinkled surface. In the selective area electron diffraction (SAED) results, a sharp single set of hexagonal diffraction patterns from the RGO can be seen (Fig. 2b inset) while a polycrystalline structure was observed from the GO. Furthermore, HRTEM analysis revealed that the RGO had clear cut, few-layered hexagonal crystalline structure (Fig. 3c). The particle size distribution of the RGO dispersion at a steady state
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mechanically strong RGO products. The reported protocol serves as an advantageous method for the bulk-scale production of few-layered RGO flakes for potential use in practical top-down processing.
water/RGO@TVF dispersion deposited onto a Si wafer. The SEM image (Fig. S4a) illustrates that the RGO@TVF sheets are well-distributed across the Si wafer with an average size of 17 μm. Fig. S4b shows a 1–2 nm step height range from the surface of the Si wafer to the RGO@TVF sheet. Since the theoretical step height for a single graphene layer is 0.34 nm, the RGO@TVF shows few-layered structure with highaspect-ratio. In addition, we speculate that the increased step height of RGO@TVF may be attributed to residual oxygen functionality on the surface of the sheet, causing some corrugation in the surface of the sheet. The long-term floatation and sedimentation of the RGO sheets in aqueous dispersions were examined quantitatively using Turbiscan, by tracking changes in transmittance throughout the height of a 20 mL vial at fixed time intervals (Fig. 3e). The RGO@CCF remained stable in water for only a few hours, rapidly precipitating to the bottom of the vial, and the transmittance increased by more than 6.7% after 1 day. The aqueous dispersibility of RGO is generally poor with aggregation occurring rapidly by intra-sheet π–π stacking [52]. In comparison, RGO@TVF formed a homogeneous and stable colloidal suspension for 1 day with only a small amount of precipitates, which is comparable to GO. The change in transmittance at different heights over time was less than 1% throughout the entire characterization period. It is noteworthy that the RGO@TVF formed more homogeneous and stable colloidal suspension than that of RGO@CCF, meaning that combination of high fluid-wall shear exfoliation with about 14% of oxygen derivatives on the RGO surface can lead to the efficient dispersion of RGO lattice in water [34,53]. Electrical conductivity behavior is considered to be a strong indicator of the extent to which the networked sp2 carbon structures are restored by the chemical reduction of GO. The electrical conductivity of the RGO papers was measured at room temperature using the 4-point probe Van der Pauw method, with a homemade measurement system (Fig. 4a). Typically, GO paper is not conducting, because its surface oxidation disrupts the networked sp2 carbon structure. Consistent with the results discussed in Fig. 4b and Table 1, the Couette–Taylor flow system with vortex flow was highly effective in restoring the networked sp2 carbon structure of the RGO. The RGO@CCF paper showed conductivities lower than that of the RGO@TVF paper, which was attributed to the lower chemical reduction efficiency. Fig. 4c show representative stress-strain curves of the RGO papers fabricated with and without the formation of toroidal vortices, respectively. As shown in Table 1, the RGO paper derived from CCF exhibited a tensile strength of 5.5 MPa and an elastic modulus of 0.68 GPa, and a low strain at break of around 0.6%. Remarkably, when the RGO was prepared in a toroidal field, the tensile strength and elastic modulus increased to 13.5 MPa and 0.93 GPa, which are 59.3% and 26.9% higher than those of the RGO@CCF paper. This can be caused by the insufficient dispersion of RGO@CCF, evidenced by the fact that the RGO sheets tended to agglomerate and form small clumps in the aqueous suspension after the reduction process. This makes it difficult for the RGO sheet to be reconstituted into a well-packed uniform thin film structure.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by grants from Open Research Program (ORP) of Korea Institute of Science and Technology (KIST). This work was also supported by 2016M3A7B4027223 and 2019R1A5A8080326 through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123482. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field in atomically thin carbon films, Science 306 (2004) 666–669. [2] Y. Zhang, Y.-W. Tan, H.L. Stormer, P. Kim, Experimental observation of the quantum Hall effect and Berry’s phase in graphene, Nature 438 (2005) 201–204. [3] K.S. Novoselov, V.I. Fal'Ko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (2012) 192–200. [4] A.P. Kauling, A.T. Seefeldt, D.P. Pisoni, R.C. Pradeep, R. Bentini, R.V.B. Oliveria, K.S. Novoselov, A.H. Castro Neto, The worldwide graphene flake production, Adv. Mater. 30 (2018) 1803784. [5] S.-G. Kim, N.-H. You, W. Lee, J.Y. Hwang, M.J. Kim, D. Hui, B.-C. Ku, J.H. Lee, Effects of the functionalized graphene oxide on the oxygen barrier and mechanical properties of layer-by-layer assembled films, Compos. Pt. B-Eng. 92 (2016) 307–314. [6] J.-U. Jin, D.-H. Lee, K.-H. Nam, J. Yu, Y.-K. Kim, M. Goh, S.G. Kim, H.S. Lee, B.C. Ku, N.-H. You, Methylpiperidine-functionalized graphene oxide for efficient curing acceleration and gas barrier of polymer nanocomposites, Appl. Surf. Sci. 464 (2019) 509–515. [7] N. Selvkumar, A. Biswas, S.B. Krupanidhi, H.C. Barshilia, Enhanced optical absorption of graphene-based heat mirror with tunable spectral selectivity, Sol. Energy Mater. Sol. Cells 186 (2018) 149–153. [8] E. Šest, G. Dražič, B. Genorio, I. Jerman, Graphene nanoplatelets as an anticorrosion additive for solar absorber coatings, Sol. Energy Mater. Sol. Cells 176 (2018) 19–29. [9] M.D. Stoller, S. Park, Z. Yanwu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498–3502. [10] Y. Gim, B. Kang, B. Kim, S.-G. Kim, J.-H. Lee, K. Cho, B.-C. Ku, J.H. Cho, Atomicallythin molecular layers for electrode modification of organic transistors, Nanoscale 7 (2015) 14100–14108. [11] G.P. Keeley, A. O'Neill, N. McEvoy, N. Peltekis, J.N. Coleman, G.S. Duesberg, Electrochemical ascorbic acid sensor based on DMF-exfoliated graphene, J. Mater. Chem. 20 (2010) 7864–7869. [12] M.W. Jung, S.M. Kang, K.-H. Nam, K.-S. An, B.-C. Ku, Highly transparent and flexible NO2 gas sensor film based on MoS2 /rGO composites using soft lithographic patterning, Appl. Surf. Sci. 456 (2018) 7–12. [13] P. Martin, Electrochemistry of graphene: new horizons for sensing and energy storage, Chem. Rec. 9 (2009) 211–223. [14] Y. Li, J. Wang, X. Li, D. Geng, R. Li, X. Sun, Superior energy capacity of graphene nanosheets for a nonaqueous lithium-oxygen battery, Chem. Commun. 47 (2011) 9438–9440. [15] L. Kavan, J.H. Yum, M. Grätzel, Optically transparent cathode for dye-sensitized solar cells based on graphene nanoplatelets, ACS Nano 5 (2011) 165–172. [16] X. Wang, L. Zhi, K. Müllen, Transparent, conductive graphene electrodes for dyesensitized solar cells, Nano Lett. 8 (2008) 323–327. [17] L. Liu, K. Zheng, Y. Yan, Z. Cai, S. Lin, X. Hu, Graphene aerogels enhanced phase change materials prepared by one-pot method with high thermal conductivity and large latent energy storage, Sol. Energy Mater. Sol. Cells 185 (2018) 487–493. [18] L. Qu, Y. Liu, J.-B. Baek, L. Dai, Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells, ACS Nano 4 (2010) 1321–1326. [19] K.-H. Nam, J. Yu, N.-H. You, H. Han, B.-C. Ku, Synergistic toughening of polymer nanocomposites by hydrogen-bond assisted three-dimensional network of functionalized graphene oxide and carbon nanotubes, Compos. Sci. Technol. 149 (2017) 228–234. [20] T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, M. Herrera-Alonso,
4. Conclusion We report a green, rapid, cost-competitive, and industrially scalable protocol for producing few-layered RGO using a high shear-induced reduction method. This method employed a Couette–Taylor reactor system and demonstrated the effect of the flow regime and inhomogeneity in the fluid kinetic structure on the degree of reduction and morphology of RGO sheets. The current study highlights the importance of Taylor vortex flow with axisymmetric instability, which leads to high fluid-wall shear stresses that are sufficient to accelerate the reduction of GO, while preventing aggregation. Moreover, the TVF regime resulted in highly water-dispersible RGO flakes, and thus they can be simply used to produce electrically conductive, and 6
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