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RSC Advances Paper (the top two-thirds of the dispersion from S3U samples) of second centrifuged suspension on silicon wafer substrates. The microscopic size and morphology of graphite/graphene were characterized by optical microscopy (Leica DM2700 M, Ger- many) and SEM (FE-SEM, LEO Supra VP-55, Germany). SEM images were taken aer coating a very thin layer of gold–palla- dium alloy to observe the physical morphology and thickness of existing layers of the graphene. AFM measurements of graphene were made under ambient conditions at 60% relative humidity and 22 C with a Digital Instruments Bruker Multimode 8 in tapping mode. The characterization was obtained using a NanoAndMore tip with a bending spring constant of 40 N m1, resonance frequency of 50–200 kHz, and tip radius of 10– 20 nm. UV-visible measurements were conducted on the samples in disposable cuvettes using a double-beam device (Varian Cary 5000 UV/Vis-NIR spectrometer) in the range of 200–800 nm. Raman spectroscopy was performed on a Renishaw inVia Reex with the laser frequency of 532 nm as an excitation source. Raman spectra were obtained and normalized from at least 15 different spots on each sample. The size distribution of akes aer the specic cycles was deter- mined using the dynamic light scattering (DLS) method. In this method, 1 mL of each sample was characterized in disposal cuvettes. The experiment was carried out with a Zetasizer Nano ZS (Malvern Instruments) device equipped with a He/Ne laser operating at 633 nm as a light source. 3. Results and discussion Graphene nanosheets were produced in the hydrodynamic cavitation reactor system (Fig. 1a), where a top-down approach was adapted, and natural graphite akes were exfoliated by energy released from the collapse of the cavitation bubbles. It is worth noting that the hydrodynamic cavitation-assisted production of graphene nanosheets in water is a green and sustainable process since it does not use any kind of chemicals such as surfactants and/or stabilizers. In this process, graphite particles act as a solid interface in the working uid and facil- itate the heterogeneous bubble nucleation so that the process had low input energy for cavitation generation. The method is a fast and energy-efficient production method, where the aqueous dispersions of graphite are treated through the cavi- tation setup, and the process lasts just a few seconds. The current hydrodynamic cavitation reactor system relying on a single nozzle microreactor is able to produce $3.125 mg of graphene in a day, however, the production may be scaled up from milligrams to kilograms by engineering parallel multi- channel chips with multi-nozzle microreactors. 3.1. Hydrodynamic cavitation and ow patterns Under cavitating ow conditions, the static pressure at the nozzle area drops to a critical value due to a sudden change in the ow geometry. The high-speed camera system captures cavitating ows at the beginning of the nozzle area. The upstream pressure (Pi) corresponding to cavitation inception is 350 psi. The corresponding ow velocity is 68.2 m s1, while the constructed in similar lines with our previous studies.47 The working uid (the starting graphite dispersion) was kept in a stainless-steel container (1 gallon), which was connected to a high-pressure pure nitrogen tank, was introduced to the system via proper ttings and stainless-steel tubing. The microuidic device was installed and sandwiched into a home- made aluminum package, which facilitated ow visualization and prevented any leakage. The sandwich holder consists of one inlet connected to the uid container and one outlet, where the uid leaves the reactor. The pressure sensors (Omega, Man- chester, UK) were also installed on the package to measure the static pressures at three different locations of the reactor. A double-shutter CMOS high-speed camera (Phantom v310) along with a macro camera lens with a focal length of 50 mm was used to record the ow patterns during the experiments, while the volumetric owrate of the system was measured at different upstream pressures. The prepared solution was introduced to the tubing system by applying the upstream pressure supplied by the nitrogen tank. The solution was propelled to the hydrodynamic cavita- tion reactor, where the exfoliation process happened in the nozzle and extension regions. The increase in the upstream pressure leads to a faster uid ow in the system. One of the major parameters, Reynolds number, is expressed as: Re 1⁄4 rVDh (1) m where r and m are the uid density and dynamic viscosity, respectively. The density of water at 20 C is 998.2 kg m3, and the dynamic viscosity is 1 cP in this study. Since the concen- tration of the graphite suspension is low, its effect on the density and viscosity of this working uid is neglected. The velocity of the system, on the other hand, is calculated from the measured volumetric ow rate and cross-sectional area. Dh is the hydraulic diameter of the nozzle. Cavitating ow charac- terization is of great importance to assess the intensity and ow pattern formation. For this purpose, the cavitation number is used and dened as: s1⁄4 PiPvap 0:5rV 2 (2) where Pi is the upstream pressure, Pvap is the saturation vapor pressure of the working uid, V is the characteristic velocity of the uid in the reactor, which is calculated at the beginning of the nozzle based on the volumetric ow rate of the system (ow rate/cross-sectional area). 2.4. Characterization methods Aer different cycles of hydrodynamic cavitation, the collected samples were subjected to sequential centrifugations (Allegra X- 15R, Beckman Coulter, Fullerton, CA, USA) to remove any unexfoliated material. The procedure for the sequential centri- fugations is given in the Discussion section. The optical microscopy, Raman spectroscopy, scanning electron micros- copy (SEM), and atomic force microscopy (AFM) for the samples were performed by transferring several drops of the supernatant 17968 | RSC Adv., 2021, 11, 17965–17975 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Open Access Article. Published on 18 May 2021. Downloaded on 4/3/2023 11:27:08 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.PDF Image | graphene exfoliation hydrodynamic cavitation on a chip 2021
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