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Materials and Interfaces
Exfoliation of graphite into graphene by a rotorstator in supercritical CO2: Experiment and simulation Yanzhe Gai, Wucong Wang, Ding Xiao, Huijun Tan, Minyan Lin, and Yaping Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01726 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018
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Exfoliation of graphite into graphene by a rotor-stator in supercritical CO2: Experiment and simulation Yanzhe Gai, Wucong Wang, Ding Xiao, Huijun Tan, Minyan Lin, Yaping Zhao* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University Shanghai 200240, P. R. China Corresponding Author: Tel.: +86-21-54743274, fax: +86-21-54741297. E-mail:
[email protected]. Abstract: High-shear induced exfoliation of graphite into graphene using a rotor–stator mixer in supercritical CO2 is a promising approach to massively produce high-quality graphene. The exfoliation mechanism of the rotor-stator and its geometry influence on the exfoliation yield were investigated in this work. The results showed that the active region of peeling off graphite to graphene was located between the rotor (including the rotating fluid) and the stator, in which the velocity gradient was the highest. The exfoliation time was valid only when the graphite particles fell in the active region. The volume of the effective area and the active exfoliation time affected the graphene yield significantly. The optimal ratio of the wall area of the stator is about 80%. Both of the lengthened rotor-stator and the multi-wall stators increased the yield by 40%. Also, the similar results were obtained in other solvents like water and NMP by the optimal structure of the rotor-stator in terms of exfoliation efficiency. The findings pave the way to scale up the
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approach of a rotor–stator mixer in supercritical CO2 for the industrial-scale production of graphene. Keywords: supercritical carbon dioxide; rotor-stator; exfoliation; graphene; CFD simulation
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1. Introduction: Graphene, a two-dimensional carbon material, has garnered attention because of its excellent electronic, mechanical, optical and thermal properties [1-3] and potential application in numerous fields [4-6]. Various methods have been proposed for the preparation of graphenes, such as micromechanical exfoliation [1-3], chemical vapor deposition [7, 8], reduction of graphene oxide [9, 10] and liquid-phase exfoliation [11-14, 17-24]. Liquid exfoliation was considered to be a scale-up and low-cost method in which ultrasound probe and high-shear mixer were often applied. Liquid exfoliation via the fluid shear stress induced by a high-shear mixer can produce large quantities of defect-free graphene [18-24]. A four-blade impeller with high shear rate causing strong turbulence was applied to create graphene [18]. A kitchen blender was reported to exfoliate graphite into graphene too [19].
Coleman et al. and Liu et al. reported the
large-scale production of the graphene in N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) solvent, respectively, using a high-shear rotor-stator mixer [20,21] and the minimum shear rate required was 104s-1. The exfoliation reason was attributed to the high-shear force induced by the large velocity gradient generated by the high-speed fluid when the high-speed blades expelled the solvents to flow in the narrow gap between the stator and the rotor. The cavitation and collision effects caused by the mixer were also factors to exfoliate graphite into graphene. Similarly, Xu et al. used a conical tube as a stator to prepare the graphene in NMP solvent [22]. Most recently, supercritical CO2 as a green solvent was used to assist in exfoliating graphene [23-26].
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Our group developed a scalable approach to exfoliate graphite into graphene via fluid dynamic force in supercritical CO2 using a rotor-stator mixer [27]. The supercritical CO2-based method has more advantages over the liquid organic solvent and is expected to have industrial applications. In this approach, the geometry of the mixer plays an essential role because it can decide the flow pattern and energy dissipation rate of the fluid [28]. However, the study on the effects of the mixer geometry on the exfoliation is scarce so far. It is highly desirable to study the influence of the geometry of the rotor and stator on the exfoliation yield for scaling up the rotor-stator-based technology of the exfoliation to industrial application. The purpose of this work is to investigate the exfoliation mechanism of a rotor–stator mixer in supercritical CO2 by a combination of the experiment and CFD simulation and to make the optimal design of the rotor-stator mixer in terms of exfoliation efficiency for the potential industrial application. The influences of the structure of the rotor and stator on the exfoliation yield were studied through the fluid flow pattern, turbulent dissipation rate and the distribution of graphite particles. It has been demonstrated that the proper construction of the rotor-stator increased the graphene yield significantly. 2. Experiment section 2.1. Preparation of graphene and analysis The procedure of the peeling of graphite into graphene was similar to the published paper [27]. Briefly, a certain amount of graphite powder (99.5%, Sinopharm Chemical Reagent Co., Ltd. China) was pretreated with sodium dodecyl benzene sulfonate (SDBS, 88.00%,
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Sinopharm Chemical Reagent Co., Ltd. China) at the fixed ratio 3:1 in a planetary ball mill for two hours. Then, a certain amount of the pretreated samples were put into a reactor installed with a rotor-stator mixer and an electric heating sleeve. The CO2 (99.9%, Shanghai High-tech Co., Ltd. China.) was pumped into the reactor by a manual pump from a cylinder. When the pressure and temperature of the reactor reached the preset conditions (12MPa and 40℃), the rotor-stator mixer started to work at the desired speed. After stopping the mixer, the graphene powders were collected in 500ml of SDBS aqueous solution (1mg/ml) via opening the valve of the reactor. After standing for 24h, the supernatant was collected as the exfoliated graphene for analysis later. The graphene content of the dispersion was measured by a UV−vis spectrophotometer (PC 756, Shanghai Spectrum Instruments Co., Ltd., China) [11, 29-30], and calculated according to Lambert−Beer law, A/l = αC, where α was taken as 1100 mg/mL/m. The exfoliation yield was calculated accordingly. The lay number and the morphology of the as-prepared graphene
sheets
were
characterized
by
atomic
force
microscopy
(AFM,
NanoNavi/E-Sweep, SII NanoTechnology, Inc.), transmission electron microscopy (TEM, JEOL JEM-2100F), Raman spectra (Renishaw in Via Reflex Raman System using a 532 nm laser excitation) and X-ray diffraction (XRD, Rigaku Co., Japan). As a comparison, N-methyl pyrrolidone (NMP, 99.5%, Sinopharm Chemical Reagent Co., Ltd. China) and water were selected as the solvent instead of supercritical CO2. 2.2. Numerical simulations
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Five 3D physical models of the reactor were built. A Eulerian-Eulerian two-fluid model which contains the kinetic theory of a granular flow was used to describe liquid-solid two-phase flow in the reactor. 2.2.1 Eulerian-Eulerian two-fluid equations Different phases were treated as interpenetration continuum. The conservation equations were solved simultaneously for each phase in the Eulerian framework. Then, the continuity equations for phase n (n=l for the liquid phase, s for the solid phase) can be expressed by: ∇ ∙ = 0
1
∇ ∙ = 0
2
The momentum balance equations for the liquid and solid phases can be written by: ∇ ∙ = − ∇ + ∇ ∙ ̿ + − + g
3
̿ = ∇ + ∇
4
∇ ∙ = − ∇ − ∇ + ∇ ∙ ̿ + − + g
5
̿ = + + !λ −
2.2.2
"
$ ∇ ∙ % #
̿
6
Kinetic theory of granular flow
A dynamic method of granular flow (KTGF) was employed to describe the rheology and characterize the motion of particles. Constitutive equations were used to describe the particulate phase viscosity and the particulate phase pressure gradient in the two-fluid model. Constituent relations for the solid-phase stress and solids bulk viscosity based on the kinetic theory concepts [31] were expressed by:
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' = ( + 2) 1 + * " g+, ( . =
/
" 0 g 1 + * # 1,
23
7
5 6
! $ 4
8
At the same time, a granular temperature was introduced into the model: # "
::: + ∇ ∙ ;23 ∇( − ? ,@AB ,CD
] (
