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RSC Advances Paper materials were produced by the use of liquid phase exfoliation and microreactor, where Yi et al.46 introduced the uid dynamics method for scalable and efficient production. They performed the experiments with the help of a high-pressure plunger pump, and the suspensions were under the effect of N,N-dimethylformamide (DMF) as a dispersion medium. They treated the working uid in 5-cycles. Motivated by the emerging studies on LPE in microuidic devices, herein, a sustainable hydrodynamic cavitation reactor system with a nozzle, which lead to a sudden decrease in the cross-sectional area of the uid path and an increase in the velocity of the working uid, was designed. This system was shown to be highly efficient in the large-scale preparation of stable graphene solutions from natural graphite powder in water. Accordingly, we developed an eco-friendly hydrodynamic cavitation induced microreactor, which could exfoliate gra- phene with the use of just pure water instead of harmful and expensive solvents and chemicals. aqueous based medium.29,30 Generally, there are three main steps in LPE synthesis: (i) dispersion of graphite in a suitable solvent, (ii) exfoliation, and (iii) purication of the nal prod- ucts.31 In the second step, the formation and collapse of bubbles on the ake surfaces instantly result in a compressive stress wave propagation throughout the particle. Based on the theory of stress waves, the particle is also exposed to a reected tensile stress wave. The cycle of creation and collisions of bubbles leads to intensive tensile stress in the akes. Additionally, the other potential scenario is the exertion of unbalanced lateral compressive stress. This kind of stress can also break down adjacent layers by the shear effect.32 As a result, it is an efficient and fast approach to develop nano-sized particles, where the prominent role belongs to cavitation bubbles.33 Cavitation is a phase change phenomenon involving the nucleation, growth, and collapse of gas or vapor-lled bubbles in liquids.34,35 The collapsing bubbles (cavities) in the liquid provide the energy source to initiate and enhance a wide range of chemical processes and introduce physical effects to break down graphite layers into graphene.36 The resulting bubble collapse could generate very high energy densities (energy per unit volume), which causes a rise in the local temperature and pressure as large as 5000 K and 500 atm, respectively, over an extremely short period of time.37 In general, acoustic-based exfoliation is carried out with an ultrasonic water bath or probe-tip sonicator, which can be scaled up to no more than a few hundred milliliters.38 Indeed, because of the inefficient energy transfer from the source to the liquid medium, the increase in the volume will exacerbate the production rate. Thus, exfoliation of graphite to graphene by ultrasonication is not a suitable way for large scale graphene production.39 Due to the signicance of hydrodynamic cavitation in uidic systems, many studies have been dedicated to provide an understanding about the effects of major parameters such as thermophysical properties of the working uid, geometry of the reactor, and surface roughness elements.40,41 Recently, the generation of hydrodynamic cavitating ows in microuidic devices has gained much attention because of the scalability, cost-effectiveness, and energy-efficiency. Furthermore, facile ow generation processes besides the stationary section of hydrodynamic reactors make them even more popular and effective.42,43 Some studies on liquid exfoliation inside a microreactor are capable of generating hydrodynamic cavitation. For example, Liu et al.44 attempted to prepare single and few-layered gra- phene akes in a cavitation reactor by employing a water– acetone mixture. Their process yield was 4%, and they intro- duced this method as a promising mass production tool with advantages of low cost and green process. In one of the recent studies conducted by Qiu et al.,45 a 50 g L1 graphite suspension with a sufficient amount of surfactant (sodium cholate) was processed by passing around 2000 times through a microreactor ($3 hours). The hydraulic power and relative energy consump- tion of their system were about 5 W and 2 MJ L1, respectively. They reported that the surfactant might undergo destruction under intense cavitation, which can prevent the increase in the yield of process. In another study, graphene and its analogues 2. 2.1. Materials and methods Chemicals and materials 17966 | RSC Adv., 2021, 11, 17965–17975 © 2021 The Author(s). Published by the Royal Society of Chemistry Natural graphite powder was purchased from Alfa Aesar (graphite ake, natural, 10 mesh, LOT: U24E068). The graphite solution with 25 mg L1 solid concentration was prepared using de-ionized water without the use of any surfac- tant or dispersant agent. In a typical experiment, graphite akes in water were sonicated using an ultrasonic bath sonicator (Bandelin Sonorex, Rangendingen, Germany) for 30 min. The resulting graphite solution was kept on a side for 15 min to precipitate out the unstable large graphite akes, and the supernatant (so-called as ‘the starting graphite dispersion’) was separated to be used in the hydrodynamic cavitation reactor, where it was passed through the reactor. 2.2. Microuidic device geometry and fabrication The microuidic device (hydrodynamic cavitation reactor) used in this study was fabricated using the semiconductor micro- fabrication techniques on silicon and was bonded to a glass cover to make sure that the reactors are leakproof. Thus, a xed upstream pressure can lead to a stable owrate in the reactors. The fabricated reactor consists of three main regions, namely inlet, nozzle, and extension zone. The widths of the inlet and extension are identical, while the width of the nozzle is smaller so that a sudden decrease in the ow cross-sectional area can be achieved. According to the Bernoulli's principle, velocity and static pressure are inversely related. Hence, the increase in the uid velocity as a result of the change in geometry of the ow restrictive element in the reactor leads to a decrease in the static pressure, which triggers the formation of cavitating ows. Since the energy released from the collapsing bubbles provides the input of our system, it is vital to make sure that the majority of the bubble collapse occurs inside our reactor. For this purpose, the nozzle length in our reactor is signicantly longer than the available studies in the literature, which View Article Online Open Access Article. Published on 18 May 2021. Downloaded on 4/3/2023 11:27:08 PM. 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