Recent developments in superhydrophobic graphene and graphene-related materials: from preparation to potential applications.

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Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/

Recent developments in superhydrophobic graphene and graphene-related materials: from preparation to potential applications Jian-Nan Wang,a Yong-Lai Zhang,a,* Yan Liu,c Wanhua Zheng, d Luke P. Lee b and Hong-Bo Suna,* In the past decade, graphene has revealed a cornucopia of both fundamental science and potential applications due to its exceptional electrical, mechanical, thermal, and optical properties. Recently, increasing efforts have been devoted to exploit its new features, for example, wetting properties. Benefit from its inherent material character, graphene shows great potential for the fabrication of superhydrophobic surfaces that could be potentially used for various anti-water applications. In this review, we summarize the recent developments in superhydrophobic graphene and graphene-related materials. Preparation strategies using pure graphene, graphene oxide, and graphene/polymer hybrids are presented; their potential applications are discussed. Finally, the perspective of this dynamic field including both current challenges and future demands has been discussed based on our own opinion. It is anticipated that the cooperation of the numerous merits of graphene and superhydrophobicity will bring new opportunities for high-performance multifunctional devices.

Introduction Recent years have witnessed the increasing research interests in graphene and the graphene-related materials, since they have exhibited many fantabulous properties in a mere decade, including ultra-high carrier mobility, 1-3 optical transmittance,4,5 high mechanical strength,6,7 high thermal conductivity,8 flexibility,9 chemical stability10 and biocompatibiliy.11,12 Thus exceptional characters have made them superb choices in a variety of applications, for example, transistors,13-16 electrodes,9,17,18 microsensors,19-21 supercapacitors,22-26 and optoelectronic devices. 27-31 Beyond the studies that mainly focus on their electronic properties, great efforts have also been devoted to the surface wettability control of graphene and its related materials, as a comprehensive understanding of the surface property of graphene is highly desired in both fundamental science and practical applications, especially for exploiting new functionalities and utilizations. Wetting behaviors of solid substrates have long been investigated under the continued stimulation of the broad applications both in daily life and commercial production.32,33 Generally, the surface wettability is mainly governed by the physical architecture and the chemical composition, so solid surfaces with extreme hydrophobicity can be rationally created by combining hierarchical roughness and low-surface-energy materials.34-36 According to this line, superhydrophobic graphene surfaces have been successfully obtained in recent

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years. To construct binary micro/nanostructures from planar graphene, a series of technologies have been employed, such as self-assembly, electrospinning, freeze-dry; and structured graphene surfaces with various two-dimensional or stereo configurations have been realized, remarkably increasing the surface roughness. Additionally, graphene and its related materials can work as nice building blocks to construct complicated layered assemblies, for instance through layer-bylayer assembly;37 and the organized structures may contribute to their surface topology control.38 On the other hand, to generate low-surface-energy coatings, several kinds of strategies have been generally used, for example, direct utilization of low-surface-energy materials, modification of low-surface-energy layers on the prepared rough surface using molecular grafting, and post removal of surface hydrophilic functional groups. In view of the processing capability and the modification flexibility both brought by the abundant oxygencontaining groups (OCGs), chemically exfoliated graphene oxide (GO) has been firstly chosen as a preferred starting material to achieve superhydrophobic graphene surfaces. With the rapid progress of graphene science, high quality graphene could be acquired by multiple means, such as chemical vapor deposition (CVD), laser treatment, or even commercially available in different sizes and states, which enriches the approaches to the super-nonwetting graphene surfaces. Gradually, investigations on the dewetting properties of

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FEATURE ARTICLE

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graphene and graphene-related materials have emerged as an active research field, which is evidenced by the evergrowing published literatures. In the process of summarizing the pertinent researches, we found that superhydrophobic graphene surfaces inspired from nature creatures have attracted numerous curious eyes of scientists from different realms including chemistry, physics, biology, electronics, and tissue engineering. It has been revealed that the cooperation of graphene and superhydrophobicity will promote the development of nonwetting researches and provide opportunities to develop novel graphene-based devices. In this review, we focus on the recent developments in the superhydrophobic graphene and graphene-related materials. The article starts with a brief presentation of the wettability of graphene and graphene oxide. Then, a comprehensive review of the preparation methods is summarized in the second section, grouped by the material categories. In the following part, potential applications of these functional surfaces are presented one by one. Finally, a brief discussion of the current state and the future challenges in this area is provided in our own perspective.

Fundamentals of superhydrophobic materials Basic principle for the construction of superhydrophobic surfaces Superhydrophopbic surfaces are solid surfaces that are extremely difficult to wet; the contact angle (CA) of a water droplet on such a surface is larger than 150°, and roll-off angle/contact angle hysteresis is less than 10°. For an absolutely flat surface, wettability is determined by the surface free energy of the solid, which can be described by the Young’s equation (Fig. 1a).39

cos Y 

 sv   sl  lv

(1)

where γsv, γsl, γlv are the surface free energy of solid-gas, solidliquid, liquid-gas interfaces involved in the system, respectively. According to this equation, a solid material with low-surfaceenergy is necessary to achieve high CA. But it is not the only factor. For a real surface that is usually rough, topological roughness plays a crucial role in the wetting behavior of solid surfaces. To explain the relationship between surface roughness and wettability, two classical empirical models, Wenzel regime and Cassie-baxter regime, have been extracted from various experimental data. In Wenzel’s model,40 it is assumed that liquid completely fills the grooves of the rough surface (Figure 1b), so the relationship can be explained by the following equation.

cos W  r cos 0

(2)

where r is the surface roughness factor defined as the ratio of the true surface contact area to the projection area, θW is the apparent contact angle in the Wenzel model, and θ0 is the

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contact angle for liquid droplet on an ideal flat surface. The surface roughness could amplify the original wetting nature of the surface, either hydrophobic or hydrophilic. In contrast, for Cassie’s case, liquid is assumed to sit on the top of surface protrusions with air pockets strapped below (Figure 1c). The effect of the solid/air composite interface on the CA of surface can be derived by the following equation: 41

cos CB  f1 cos 1  f 2 cos 2

(3)

where θCB is the apparent contact angle in the Cassie-Baxter model, f1 and f2 are the fractional areas of the solid and air of the surface, respectively, θ1 and θ2 are the CAs for flat solid surface and air, respectively. And we have f1+f2=1, θ2=180°as air is considered as a completely non-wetting material. Then equation 3 can be derived as:

cosCB  f1 cos1  1  1

(4)

Therefore, a high CA may be achieved by increasing the surface roughness or introducing more buried air bubbles. The above theories have been successfully adopted to explain the underlying mechanism of the famous “lotus effect”. It has been found that both the hierarchical micronanostructures (randomly distributed micropapillaes and fine branch-like nanostructures) and the epicuticular wax materials (low surface energy coating) are responsible for the selfcleaning property, in agreement with above theories.

Fig. 1 Schematic illustration of the wetting behavior of a water droplet on different solid substrates. (a) Young’s model. (b) Wenzel’s model. (c) Cassie’s model.

Surface wettability of graphene and graphene oxides Surface wettability of graphene sheets plays a vital role in determining the compatibility with the desired environment. So a deep-understanding the wettability of graphene and its related materials is fundamentally important for the development of graphene-based devices. Recently, several experimental and simulation studies have been focused on the surface property and wettability of graphene films. Based on density functional theory, Leenaerts et al.42 indicated that graphene is strongly hydrophobic. They concluded that the binding energies between water molecules are larger than the associated adsorption energies on the graphene surface, such that water molecules form clusters on the graphene sheet. Quantum molecular dynamic simulations afford fundamental insight into the wetting behavior of water nanodroplet on a free-standing single-layer graphene sheet, and also allow incorporation of the polarization interaction, main-body effect and hydrogen-bond




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Journal Name interactions into the prediction of wettability. The simulation results show that the graphene sheet yields a CA of 87°.43 Compared with the theoretical results, graphene grown by CVD gives a CA of 89.4°.44 Besides, Shin et al.45 reported that epitaxial graphene has a static contact angle (CA) of 92°and is slightly hydrophobic regardless of its thickness, close to that of highly ordered pyrolytic graphite (91°), while graphene reduced by hydrazine has a CA of 127°and a small interfacial energy with water. Proved both theoretically and experimentally, graphene has a natural hydrophobicity, which is essential to understand the wetting behavior of water on the surface of such material. In addition to pristine graphene, another important graphene-related material that has been widely used for the fabrication of graphene-based devices is GO. GO is prepared by chemical exfoliation of natural graphite, and could be considered as a derivative of graphene with oxygen functional groups on their basal planes and edges. Due to the presence of OCGs, GO could be well dispersed in polar solvents such as water. The ease of synthesis and the solution processing capability make GO a promising precursor for the preparation of superhydrophobic graphene surfaces. The wettability of GO film is influenced by the contents of OCGs on the surface. With various levels of oxidization, GO films exhibit different wetting properties. For instance, GO film has a CA 67.4°46 compared to that of 30.7°.47 Invariably, GO shows hydrophilicity owing to the chemical nature of the groups, such as carboxyl, hydroxyl and epoxy. To further increase its hydrophobicity, proper reduction treatments (e.g., hydrazine reduction; thermal annealing; photoreduction) would be implemented, drastically or partially removal of the hydrophilic OCGs would lower its surface energy, giving rise to much higher water CAs of the reduced GO (rGO).

Preparation methods Superhydrophobic surfaces prepared from GO The presence of abundant OCGs endows the GO sheets with tractable solution-processing capability and possibility for further functionalization, therefore, many graphene-based superhydrophobic materials have been fabricated using GO as starting materials. Following the basic principle, artificial superhydrophobic surfaces have been successfully prepared by combining rough surface structures and low-surface-energy material functionalization. In the following section, some typical approaches to superhydrophobic GO have been summarized. Surface functionalization. Surface functionalization, which can alter the surface characteristics, is an effective way to obtain water-repellent surface; and the brought ease of imparting superhydrophobicity to various surfaces greatly extends the utilization of such functional surfaces. As a starting material, GO is naturally hydrophilic, since most of the functionalized OCGs are hydrophilic. So in order to drastically change its surface wettability, post-functionalization of GO


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ARTICLE sheets with hydrophobic groups seems a simple but effective method. As a typical example, Xue et al, obtained superhydrophobic GO surface by functionalization GO with polyhedral oligomeric silsesquioxane (POSS), a unique cagelike molecular structure with eight isobutyl groups surrounding the cubic core. 47 The amine-POSS can be covalently grafted onto GO sheets via amide formation, and chemical functionalization could be simply proved by the phenomenon that POSS-modified GO moved from water phase into CHCl3 phase, as shown in Fig. 2a. The grafting leads to the introduction of hydrophobic moities of isobutyl groups, and the grafted graphene (POSS-graphene) becomes hydrophobic with a water CA of 111.2° (Fig. 2b). To further increase the hydrophobicity, the POSS-graphene was grounded into particles and redispersed in ethanol, the regenerated film exhibited hierarchical asperity and an enhanced CA of 157° (Fig. 2c). Generally, functionalization of GO sheets with large groups would physically separate the graphene layers from each other. As a result, the obtained GO hybrid becomes soluble in various solvents. Such solution-processing capability will facilitate the structuring process, making it possible to prepare surperhydrophobic graphene through various routes. The variety of OCGs on the GO sheets makes the molecular functionalization flexible. Initiated from the neucleophilic substitution between epoxy groups on GO sheets and amine of ODA, Lin et al, also prepared superhydrophobic GO films by octadylamine (ODA) bonding.46 The hydrophobic nature of ODA significantly lowers the surface energy of grafted GO. A rough ODA-grafted GO film could be easily prepared by a simple filtration process. The obtained surface exhibited a high CA of 163.2° and a low hysteresis of 3.1°. Through functionalizing GO with phenylisocynate (C6H5NCO), Wang et al changed hydrophilic GO into hydrophobic nanosheets via solvothermal synthesis.48 Besides, (Heptadecafluoro-1, 1, 2, 2 tetradecyl) trimethoxysilane (HFTMS) was also employed as a surface reactive molecule to create superhydrophobic GO surface.49

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Journal Name decanol, and so on) at the same centrifugal speeds, though there is no clear difference observed among the surface morphologies of the graphene films, they showed different water CAs. Among them, films treated with ethanol and isopropanol exhibit static CAs got close to or exceed 140°, and all films showed CAs above 90°. Interestingly, the film showed reversible transition from hydrophobicity to hydrophilicity under the alternation of ultraviolet irradiation and air storage, deriving from the adsorption and desorption of oxygen molecules on the nanosheets surface. The solvent modification is a simple, low-cost and flexible method for controllable modulation of the surface chemical compositions of graphene, providing an effective approach to tune the wettability of graphene in a large range, from superhydrophobicity to superhydrophilicity.

Fig. 2 (a) Surface modification of GO with POSS, and photographs of original GO in water and the synthesized POSS-graphene in CHCl3. (b) SEM image of the POSS-grafted GO film, the inset is the image of water droplet CA, ~111.2°. (c) SEM image of a rough POSS-graphene film prepared from POSS-graphene particles, the corresponding water CA is 157°. Reproduced with permission. 47 Copyright 2012, American Chemical Society.

Solvent modification. Owning to the large specific surface area and the presence of OCGs, GO can adsorb large amounts of solvent molecules when dispersed in solvents. Even after the formation of graphene films, the adsorbed molecules may also affect its surface energy, and change its wettability significantly. Recently, thermally rGO with tunable wettability has been obtained by solvent adjustment. 50 SEM images of graphene films deposited on an aluminum substrate using pure water as the solvent (graphene-W film) and using pure acetone as the solvent (graphene-A film) were shown in Fig. 3a and b, respectively. As compared to the naked substrate, the graphenecoated samples show one to two orders of magnitude in the average surface roughness due to the irregular stacking of graphene sheets. Interestingly, totally different wetting performances were observed for the graphene-W film which was complete wetting and graphene-A film which showed a water CA of ~160°. The opposite effect was ascribed to the fact that the chemisorption of acetone on the defective graphene introduces hydrophobic carbon-hydrogen bond, while immersion in water leads to the formation of hydrophilic carbon-oxygen bond. Additionally, the surface roughness amplified the wettability according to Wenzel’s theory. Based on these results, the graphene films can be tailored from superhydrophilic to superhydrophobic with a water CA changing from 0° to 160° by using acetone/water mixed solvents with different proportions (Fig. 3c). Zhang et al, also observed solvent-dependent hydrophobic phenomenon in diverse pure organic reagents, as they prepared highly hydrophobic films from chemically rGO via suction filtration.51 In their work, graphene were dispersed in different pure organic solvents (methanol, ethanol, dimethylformamide, toluene,

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Fig. 3 SEM images of a graphene film deposited on an Al substrate: (a) with pure acetone as the solvent (Graphene-A film), (b) with pure water as the solvent (Graphene-W film). (c) Tunable control of the CA of a graphene film on an Al substrate by using a water-acetone mixture as the solvent. Increasing the water proportion in the mixture results in more hydrophilic behavior, whereas higher acetone content yields a more hydrophobic response. (d) Contact angles of the chemically treated and untreated graphene aerogel surface vary with concentration. (e) SEM image of the FDTS modified graphene aerogel. Reproduced with permission. 50,52 Copyright 2010, Wiley-VCH and 2011, American Chemical Society, respectively.

Freeze-dry. Freeze-dry technique has been widely used to generate porous structures. Recently, in the preparation of superhydrophobic graphene, freeze-dry technique has been successfully adopted as a simple and inexpensive method for building rough graphene surfaces. For instance, Lin et al, fabricated a serial of graphene aerogels using freeze-dry technique, and all of them exhibited a water CA larger than 150°, even reaching 160° (Figure 3d). 52 By freeze-drying the GO aqueous dispersion, a GO aerogels have been successfully prepared. The surface roughness could be varied using GO solution with concentrations increasing from 3 to 15 mg/ml, which directly leads to the variation of water CA (Fig. 3d). In this case, the tradeoff between aerogel density and structural




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Journal Name porosity could be taken into account. The graphene aerogels become superhydrophobic when the OCGs were thermally removed at high temperature. In this work, both the formation of porous structure and the removal of OCGs contribute to the hydrophobic natural of the resultant graphene aerogel. (Fig. 3e) To further strengthen the hydrophobicity of the samples, 1H, 1H-2H, 2H perfluorodecyl-trichlorosilane (FDTS) was applied to the surface, which further decrease their surface energy. Template method. In addition to the above-mentioned methods that construct rough graphene surface by irregular stacking, hierarchical graphene structures could also be made in a controlled manner. Lee et al reported the preparation of superhydrophobic and transparent graphene films that consist hollow graphene spheres using amine-functionalized silica particles (NH2-SiO2) as a template. 53 The electrostatic interactions between the negatively charged groups (-COOH, O-, -OH) of GO and positively charged groups (-NH2) of NH2SiO2 have been adopted as driving force to induced such assembly. The GO layer coated on the surface of the SiO 2 particles was thermally reduced and the silica cores were etched away in a HF solution, in this way, a structured graphene film was successfully prepared. To further lower the surface energy, the structure was treated with silane surface, and the CA of the surface increased to 157°, reaching superhydrophobic range. Moreover, the transmittance spectrum showed that the hollow graphene structured surface show possibilities of transparent device for flexible electronics, since the transmittance of modified graphene spheres on the PET film is around 72.7±1.19% at 550 nm. Biomimetic laser processing. Recently, photo-induced deoxygenization of GO have become an appealing GO reduction strategy, considering the controllable reduction degree and flexible patterning. 54-56 Inspired from natural butterfly wings that exhibit both superhyophobicity and beautiful structural color due to the presence of ordered photonic crystal structure, graphene surfaces with superhydrophobicity and iridescence have been demonstrated by photo-reduction.57,58 Two beam laser interference was used for synchronous construction of periodic structure and modulation of surface chemistry (Fig. 4a). Typically, periodic distribution of laser intensity would lead to the formation of grating structures, while the strong laser-matter interaction would result in partial ablation of GO film and the drastically removal the OCGs on the surface. The one-step procedure endowed the graphene surface with both hierarchical roughness and decreased surface energy, both of the two effects contribute to the formation of a superhydrophobic surface. As shown in the Fig. 4b, the tested water CA could reach 153° for the graphene film with a period of 2 μm. Considering the fact that the residual oxygen content was subject to the laser intensity, the surface wettability would be further tuned by the laser power.57 Moreover, the surface topology can be adjusted by the grating period, and thus incident angle of the two laser beams provided another way to tune the wettability. 58 What’s more, the laser intensity distribution could also be tuned by multi-


ARTICLE exposure of two beam laser interference. 2D micro-pillar arrays were fabricated by dual laser interference with 90°rotation (Fig. 4c), and the CA of the prepared film is 155°; this would effectively avoid the anisotropic superhydrophobicity. Interestingly, the prepared graphene films with periodic structures took on a wonderful iridescence, as observed by the naked eyes.

Fig. 4 (a) Fabrication scheme of superhydrophobic graphene surfaces by TBLI and dual TBLI with 90° rotation. (b) SEM image of the grating structure of the graphene surface with a period of 2 μm. The layered nanostructure can be clearly seen. The water CA is 153° vertical to the grating. (c) SEM image of the graphene film with 2D grating-like structures. The period is 2μm and the measured water CA is 155°. Reproduced with permission. 58 Copyright 2014, Wiley-VCH

Superhydrophobic surfaces prepared from graphene Generally, GO featured tractable water processing capability and the flexibility for functionalization, so various superhydrophobic graphene have been successfully developed using GO as starting material. However, despite various reduction protocols have been employed to restore the sp2 region of GO, the residual OCGs and the generated defects dramatically alter the structure of the carbon plane, making the rGO a substantially different material. Nowadays, with the rapid progress of graphene preparation strategies, graphene can be produced by micro-mechanical exfoliation of highly ordered pyrolytic graphite (HOPG),59,60 chemical vapor deposition (CVD)61-65, epitaxial growth66,67 and solvents exfoliation.68-70 As a result, superhydrophobic graphene have been successfully developed based on real graphene. CVD growth on structured templates. CVD method becomes a promising strategy to produce graphene, since it enables the preparation of single layer or few layer graphene with highquality (e.g., large area, low defects, and high carrier mobility). However, the CVD graphene films were usually flat. Despite it

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is compatible with device fabrication, for instance, with the help of substrates transferring technique, the flat surface set obstacles to control its wettability. To introduce structure into the prepared samples, template assisted CVD growth method have been inventively proposed. 44,71-73 Dong et al, demonstrated superhydrophobic graphene films by microwave plasma CVD (MPCVD) using a patterned Si substrate. 71 In their research, vertically aligned few-layer graphene nanosheets were first prepared on usual planar Si substrate. Though rough surface with sharp edges were obtained, the water CA was 132.9°, even when the surface was modified with ODA, the CA slightly increased to 133.7°. To achieve highly nonwetting performance, graphene sheets were grown on Si substrate with micropillar arrays. SEM images in Fig. 5a and b show that graphene sheets were evenly distributed on the substrate including the sides and bottom of the pillars. With a composite structure consisted of the microtexture inherited from the substrate and a nanostructure originated from the sharp edges, the graphene surface got a CA of 149.8°. After ODA treatment, water droplet on the graphene surface presented a quasi-spherical shape (Fig. 5c) and the measured CA increased to 152.0°. As shown in the optical microscope image (Fig. 5d), there is air trapped between the droplet and the silicon pillars, and therefore an ideal Cassie’s mode was used to earn the theoretical CA in this case. Later, similar strategy was employed to create superhydrophobic flower-like graphene surface grown on Si nanocone arrays by hot filament CVD (HFCVD).72 Additionally, 3D hollow molds, such as foamed nickel could also be used to prepare structured graphene.44,73 Graphene could directly grow on Ni templates by CVD technique.73 After chemical etching of the metal sacrificial scaffold, free-standing graphene foam was produced. The imparted porous network and subsequent Teflon coating leaded to the unique superhydrophobicity. Moreover, the graphene foam exhibited superior elasticity and mechanically robustness, as confirmed by the completely rebounding of the impacting droplet. Conveniently, the structure of the graphene foam could be flexibly tuned by selecting appropriate templates. To further increase the surface roughness, carbon nanotubes can then be grown onto the graphene/Ni scaffold by a two-step CVD.44 After the removal of nickel template, an superhydrophobic carbon foam was prepared free of surface modification. The superhydrophobic graphene foam holds great promise for various functional applications such as non-wetting electrodes and oil/water separation.

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Fig. 5 (a) SEM image of graphene grown on silicon microstructures, the micropillars were 15 μm × 15 μm with a special interval of 35 μm. (b) Magnified SEM image of the graphene uniformly distributed even on the sides and bottom of the silicon structure. (c) Photo of water droplet on the ODA modified graphene surface on microstructure. (d) Optical image of the contact line on the water/solid interface where gas gap can be apparently seen. Reproduced with permission. 71 Copyright 2013, Nature Publishing Group.

Dip coating. Using a structured carrier, dip coating is a rapid and active alternative to establish superhydrophobic graphene, as reported by Nguyen et al. 74 A common sponge was employed as a frame for graphene coating. Benefited from the mechanical flexibility of graphene and strong Van der Waals forces between the nanosheets and sponge, graphene was physically adhered to the sponge surface. Through repeated dipping-drying process, controllable amounts of graphene could be loaded onto the porous skeleton. The hydrophobic coating made the powerful water-absorbing media water-proof. As the graphene loading increased to 7.3% (defined as the weight ratio of anchored graphene to the sponge), the graphene sponge obtained a saturated water CA of about 162°. When it was forced to immerse in the water, the graphene sponge was wrapped by an air layer and exhibited a mirror-like look; while released, it was floated on the water surface and kept dry due to the dewetting-induced buoyancy and its low density. To avoid the detachment of graphene and ensure the superhydrophobicity stability, a dimethylsiloxane (PDMS) protective layer was made in surface which is thin enough not to erase the rough feature formed by the protrusions of the nanosheets. The as-treated sponge can go through the squeezing test, indicating the mechanical stiffness and the possibility for recycling use. It was observed that the CAs of the graphene spongs were still larger than 150° after having floated on aqueous solutions with different pH values (2-14) for days, and thus the as-prepared superhydrophobic foam can be applied in corrosive environments with a long working period.




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Fig. 6 (a) Schematic illustration of the strategy for the crumpled graphene induced from the pre-stretched substrate. SEM images of patterns developed on the graphene sheet: (b) wrinkles form and (c) delaminated buckles as the substrate is uniaxially relaxed, (d) crumples as the substrate is biaxially relaxed, (e) unfolded pattern as the substrate is biaxially stretched back. (f) Atomistic modeling results of (from left to right) crumpling of a single-layer graphene under uniaxial compression, biaxial compression, and the Mises stress distribution. Reproduced with permission. 75 Copyright 2013, Nature Publishing Group.

Crumpling and unfolding. Crumpling-unfolding process offers another simple and effective route towards the formation of surface roughness. Zang et al developed a large-area functional graphene with wrinkle structures by harnessing the elongation of the supporting substrate. 75 The fabricating scheme was schematically illustrated in Fig. 6a. With the help of a PDMS stamp, CVD grown few-layer graphene was transferred to a pre-stretched elastomer whose length is three times to its original size (pre-strained by 200%). Then the substrate was relaxed along two directions and reset to its initial size. In this way, the adhered graphene contracted to the same degree, and hierarchical roughness formed. As shown in Fig. 6b and c, the graphene film developed wrinkles and delaminated buckles when the substrate was relaxed in one direction, and then grew crumples (Fig. 6d) as the substrate is relaxed biaxially. These self-organized structures can also spread out and become relatively flat (Fig. 6e), once the relaxed substrate was stretched back. In this way, the crumpling and unfolding of the graphene film can be irreversibly tuned. The microscopic procedure


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ARTICLE triggered by the stress can be reproduced by the atomistic simulation (Fig. 6f), from which a single crumple with ridges and vertices, as well as the corresponding stress distribution can be clearly observed. If the substrate simultaneously relaxed in the two directions, a different irregular pattern would appear on the graphene surface. As the rough structure of the wrinkled graphene is crucial for water-repellency, the wettability of the surface can be tailored by tuning the relaxing manner (e.g., uniaxially or biaxially) and pre-strain level (e.g., 100% or 250%). It is worth mentioning that the graphene film can maintain its integrity over multiple crumpling-unfolding cycles (>50) with only a few unconnected cracks. Doping. Arc discharge technique has been used for the preparation of graphene ever since 2009,76-79 it is a simple and efficient approach to prepare few layer graphene nanosheets. Besides, this technique is very suitable for graphene doping and modification.76,79 As mentioned previously, both the surface roughness and the chemical composition influent the surface wettability. Recently, in addition to the creation of rough graphene structure, as discussed in previous sections, efforts have been devoted to tailoring the surface chemistry of graphene for the purpose of tuning its wetting behavior. As a typical example, Shen et al, prepared superhydrophobic fluorine-doped graphene using direct current arc discharge method.80 In their synthesis, a hollow graphite rod filled with graphite fluoride was used as the anode instead of a conventional pure graphite rod. During the discharge process, the graphite fluoride evaporated and the dissociative F atoms and/or ions would react with carbon, so that F-doped graphene was grown gradually. Morphology observation showed that the F-doped graphene are multi-layers and there are wrinkles or ripples on the edges of the nanosheets. According to the XPS spectra, the content of the fluorine in the prepared graphene was 10 wt% and doped F atoms had an ionic character. With the difference in surface chemical composition, F-doped graphene showed a CA of 158°. Superhydrophobic surfaces prepared from graphene-based hybrids The hybridization of graphene has received significant attention in the sense that the synergistic effect of components leads to the improvement of performance in multiple facets and the creation of new functionalities. In addition to the fabrication of superhydrophobic surfaces using solo graphene and its related materials, guest materials, for instance, polymers, have been mixed with graphene, forming various hybrids for the development of superhydrophobic coatings. The introduction of other materials may provide feasibilities of controlling both the surface morphology and the surface free energy, which facilitates the fabrication of superhydrophobic surfaces. Solubility modulation. Solubility modulation, which can induce precipitation and crystallization, is a simple method to control the morphology of various materials. By solvent replacement, Zhang et al, established a polymer/graphene hybrid surface composed of uniformly packed microspheres

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structure, which exhibited an average water CA of 151.6±1.4° (Fig. 7a).81 In their work, N, N-dimethylformamide (DMF) has been selected as a solvent to dissolve poly (vinylidene fluoridehexafluoropropylene) (PVDF-HFP), and graphene sheets could suspended stably in the solution by ultrasonication. Then water used as a non-solvent was adsorbed by the suspension. After solvent leaching in water and freeze-drying, DMF was removed and a porous materials was obtained. Fig. 7b showed the formation mechanism of PVDF-HFP/graphene microsphere. Upon contact with the poor solvent, polymer tended to separate from the liquid and immobilize on the graphene nanosheets. The entanglements of the graphene sheets intercalated with the crystallized PVDF-HFP chains finally induced the formation of microparticle, and the surface of these particles were covered with irregular spikes and nets in nanometer (Fig. 7c and 7d). Thus multi-level surface structure was regarded as a critical feature of superhydrophobic surfaces. Besides, several other superhydrophobic polymer/graphene surfaces were constructed through solvent replacement, 82-85 demonstrating the versatility of this approach. Generally, in these studies, graphene nanosheets having a large surface area provided abundant immobilization sites for the precipitated polymer and facilitated their crystallization. The polymer-graphene attachment can be further strengthened by multiple forces between carbon surface and polymer chains such as hydrophobic affinity, 81,82 π-π interaction,83,84 dispersive, polar and hydrogen-bond interactions.85 In addition to the change of surface morphology and wettability of graphene, polymer may bring other developments. For example, the deposited polymer on the carbon surfaces may work as a spacer, preventing graphene nanosheets from re-aggregation and improving their dispersion stability.84 In return, Graphene may enhance the thermal stability of the polymer as the release of volatile components was limited by the nanosheets.83,84

Fig. 7 (a) SEM image of PVDF-HFP/Graphene microspheres. The CA of water on the rough surface is 151.6±1.4°. (b) The formation of the graphene based hybrid microsphere based on the entanglement and crystallization process. (c)

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Journal Name Magnified SEM image of a single rough microsphere. (d) Further magnified SEM 81 image of the nanostructures on the microsphere. Reproduced with permission. Copyright 2011, American Chemical Society.

Self-assembly. As a potent nanofabrication method, selfassembly enable flexible construction of stereoscopic topology from nanoscale structure units. 37 Recently, it has been demonstrated that soft 2D graphene materials can be assembled into robust 3D interconnected networks with the aid of nanoparticles, building a novel superhydrophobic graphene based materials.86 Typically, GO was mixed with FeSO 4 and kept at 90 °C for 6 hours, then a black hydrogel containing about 95.8 wt% water was naturally formed. The formation mechanism can be explained as the fact that Fe2+ ions tended to move towards GO sheets due to electrostatic attraction and then oxidized into Fe3+, while GO sheets were in situ reduced and assembled into 3D networks driven by π-π stacking interaction between graphene sheets using the anchored iron oxide nanoparticles as powerful support. Additionally, the as-formed hydrogel exhibited a strong adsorption of heavy ions such as Cr (VI) and Pb (II), which may be arisen from the synergistic effects of the electrostatic interaction, ion exchange, and surface complexation. After freeze-dry, the hybrid hydrogel becomes to superhydrophobic graphene aerogel. Interestingly, the ion oxide component of the graphene hybrid hydrogel can be controlled by the pH values from α-FeOOH to Fe3O4. Magnetic investigations revealed that the graphene/Fe 3O4 aerogel showed ferromagnetic property which can produce strong magnetic signals at a small magnetic field. Moreover, this method was workable to induce the self-assembly of reduced graphene sheets with other metal oxides such as Mn 2O3 and CeO2, assisted with Mn2+ and Ce3+, respectively. This method provides a facile way for building multifunctional and manifold graphene derivatives. Structural reconformation. Due to the presence of OCGs and the water-processing capability, GO could be mixed with various functional polymers, providing the feasibility of fabricating diverse hybrid structures. Recently, Choi et al demonstrated the fabrication of superhydrophobic graphene/Nafion hybrid films through conformational rearrangement.87 As shown in Fig. 8 (top panel), Nafion, a fluorated polymer, was blended with GO, then chemical reduction in hydrazine solution was implemented. The rGO can be uniformly coated with Nafion driven by the hydrophobic interaction between the conjugated graphene sheets and polymer backbone. What’s more, the hydrophilic sulfonic groups of Nafion imparted high dispersion and long-term stability over 2 months to the rGO/Nafion (denoted as CMGN) hybrid. To obtain thin films, the hybrid solution was then vacuum filtrated. SEM images (Fig. 8, bottom panel) showed that a rough surface with petal-like microstructure gradually emerged as the ratio of Nafion component increased. The roughness was contributed to the structural reorientation of the hybrid sheets, that is, the favorable interactions between graphene and Nafion cause the conformational rearrangement of the Nafion and the wrinkle and protrusion of sheet edges.




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Journal Name With the change of surface morphology, the CAs of the films increased from ~97°to ~161°, indicating a tunable wettability with respect to the hybrid composition. It is worth mentioning that the electronic properties of graphene were carefully retained in the hybrid, as the coating of individual graphene sheet by Nafion was at molecular level, and the supramolecular interactions made graphene sheets closely packed. 88,89

Fig. 8 Illustration of the formation of Nafion/Graphene hybrid through intermolecular hydrophobic interaction (top panel) and the conformational rearrangement-induced morphological evolution of the films as the concentration of Nafion increased from 2 wt % to 30 wt % (bottom panel). Scale bars are 1 μm. The inset of the bottom panel is the CA photographs of the hybrid films. Reproduced with permission.87 Copyright 2012, American Chemical Society.

Electrospinning. Electrospinning is a straightforward and costeffective method to provide surface roughness, it has been widely adopted for the fabrication of superhydrophobic surfaces. Recently, superhydrophobic graphene nanocomposite fibers were one-step fabricated by electrospinning.90 In this work, graphene nanoparticles worked as an additive to enhance the surface roughness was added to the polymeric solution. Without the particles, pure electrospun poly (vinyl chloride) (PVC) fibers showed a water CA below 140°. In comparison, the PVC hybrid polymer electrospun in the presence of graphene nanoflakes achieved a high CA of 151.5°even 166.3°, presenting a realization of superhydrophobicity. Considering the small amount (1wt%) needed to introduce sufficient hydrophobic character, graphene can be utilized as an effective superhydrophobic media.

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ARTICLE As a remediation of accidents, the cleanup of leaked chemicals or oil is a global environment issue. Generally, porous materials that have high porosity and high surface areas show great potential in oil/chemicals pickup, however, these materials usually shows poor selectivity towards target oil, thus easily saturated with water. To address this issue, nanoporous polydivinylbenzene (PDVB) with superhydrophobicity has been successfully developed for the high-capacity and high selectivity adsorption of organic compounds. Recently, graphene based materials with both superhydrophobic and superoleophilic properties44,74,91 emerge as a promising candidate for selective absorption of oil and organic solvents without water pickup. Fig. 9a showed the complete removal of gasoline labeled with Sudan III from the water using a graphene aerogel.86 The oil-saturated samples can be easily regenerated after dried in an oven at 100 °C for 30 minutes. Notably, the graphene hybrid aerogel kept a high adsorption capacity after eight absorbing-drying periods for gasoline (Fig. 9b), the excellent recyclability was a promising feature for adsorbent. In addition to gasoline, a variety of oil and solvents such as cyclohexane, toluene, paraffin oil, vegetable oil, phenoxin can be sucked up (Fig. 9c). For another example, a superhydrophobic graphene sponge also show eminent absorption capacities ranging from 54 to 165 times their weight for a wide range of solvents.74 The sponges could also be renewed by a simple distillation or squeezing process, showing a superior mechanical strength. In addition, the light weight, chemical inertness, low cost and environmental friendliness make the graphene sponge a powerful candidate for practical application. Moreover, graphene-based hydrogel was reported to show high adsorption rates of heavy ions which are extremely toxic in water resources of heavy ions.86 The calculated maximum adsorption capacities for Cr (VI) and Pb (II) were 139.2 and 373.8 mg/g, respectively, which are much higher than active carbon for Cr (VI) (69 mg/g). Therefore, functional graphene based materials are regarded as ideal choices for water purification.

Potential applications The combination of superhydrophobicity with graphene has triggered a number of new functionalities and utilizations. From the material point of view, graphene features high carrier mobility, good conductivity, transparency, excellent mechanical strength, and biocompatibility; while in the view point of unique dewetting property, additional functionalities such as self-cleaning, anti-fogging, oil/water separation could be well imparted to graphene and graphene-based devices. Currently, despite the fact that the fabricative strategies for superhydrophobic graphene surfaces have been well developed, functional applications is still limited to laboratory tests. In this regard, we briefly summarized a series of typical applications in this section. Water treatment


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Fig. 9 (a) Photographs of the removal process of gasoline (dyed with Sudan III for clear observation) from water using the graphene/iron oxide aerogel. (b) Regeneration capacity of the hybrid for adsorbing gasoline. The graphene aerogel can be reused after heated in the oven at 100 °C. (c) Adsorption capacities of the aerogel for a range of organic solvents and oils in terms of its 86 weigh gain. Reproduced with permission. Copyright 2012, American Chemical Society.

Wettability switching The manipulation of surface wettability is of great importance in many research areas such as cell culture, microfluidics, and water transportation. To this end, graphene surfaces with tunable wettability may exactly meet the needs of various applications requiring functional coatings. Several strategies have been proposed to investigate the wettability control of graphene.50,51,57,58 Typically, Rafiee et al, reported a simple route which can dramatically alter graphene surface from superhydrophilic to superhydrophobic using solvent modification method, as introduced in previous section. 50 In their work, by combining water and acetone in a suitable proportion as the solvent, graphene film with desired wetting behavior would be achieved. Besides, reversible wettability control of hydrophobic graphene surface has also been realized through ultraviolet light irradiation, as reported by Zhang et al. 51 When the graphene film was exposed to ultraviolet light for 12h, the film show presented a water CA of 40°; while kept in air storage for 12h, the film displayed a high water CA of 140°. The interesting phenomenon was bound up with the adsorption of oxygen molecules which may result in the production of hydrophilic groups on the graphene surface. The observed tunable wettability could be reproduced for more than five cycles.

Journal Name superhydrophobic graphene have exhibited attractive potential in artificial muscles.75 When a voltage of 3000V was applied between the graphene films, the elastomer among them developed an electric field and deformed under the induced Maxwell stress. The pre-crumpled graphene films were unfolded as the elastomer stretched over 100%, and the transmission changed between 40-60%. The acting was fast responsive and the laminate would return to the unfolded state upon withdrawing the voltage (Fig. 10a). The procedure mimicked the behavior of an artificial actuator with tunable transparency. The stretchable graphene ensured the multiple manipulation of the actuator without fracture. This work presents a smart graphene device with dedicate design and brings light into the possible uses of graphene-based systems.

Transparent coatings and electrodes Transparency is an essential requirement for many optical devices, such as windshields, OLEDs, photovoltaic cells. To enhance light transmission and prevent moisture permeation, contaminant-proof or self-cleaning property is also useful or even necessary for them. In this regard, graphene film with transparency and superhydrophobicity seems an ideal candidate for optical and optoelectronic materials. 53,75,87,90 Lee et al demonstrated a simple route towards superhydrophobic graphene film with transparency and flexibility, which exhibits a water CA of 157° and an optical transmittance of 72.7%. 53 Cooperated with high conductivity, functional graphene film shows great potential for transparent electrodes. 71,80 By tailoring the surface roughness, graphene surfaces with variable transmittance, resistance and wettability were obtained. 71 The prepared graphene film manifested high toughness and deformability compared with usual electrode material of gold. The combination of stretchability, tunability and transparency will endow graphene electrodes with sorts of potential utilizations. Smart actuator Man-made actuators are helpful in many applications such as energy harvesters and artificial intelligence. Crumpled

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Fig. 10 (a) Schematic illustration (upper images) and photographs (bottom images) of the voltage-induced working process of the graphene-elastomer actuator. As a voltage is applied, the actuator reduces its thickness and expands its area; and the transmittance increases. (b) Structural color of the graphene film with 2D grating-like structures. The rose patterns are derived from pristine photographs of the graphene film viewed from different angles by computer design. Reproduced with permission. 58,75 Copyright 2013, Nature Publishing Group and 2014, Wiley-VCH, respectively.

Other applications Due to the ability to transfer charge and resist water, conductive superhydrophobic graphene surface may also find broad applications in electronics. For instance, it could serve as a functional protective layer for electrostatic discharge and EMI shielding83,84,92 to improve the operating safety and signal stability of electronic devices; it may also prevent electronic failures caused by water contact. In the light of these capabilities, graphene based materials with unique superhydrophobicity would hold great promise for practical uses. Recently, Fang et al, reported a graphene/polymer hybrid film holding a high conductivity of 6560 S/m and a total EMI shielding effectiveness value of 43.7 dBcm 3/g, which is 4 times



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Journal Name greater than that of traditional metal shielding materials. 84 Besides, superhydrophobic graphene could be employed as functional papers93 and advanced textile94, which may be promised for wearable electronics. In addition to the potential applications in electronic, many superhydrophobic graphene surfaces show high adhesive, which is responsible for water transportation and cell culture, revealing great potential in tissue engineering. 72,49,57 Moreover, due to the water repellency and good dispersion in polymer solutions, polymer/graphene hybrids are considered as good candidates for catalyst supporter. 81 Instead of black looking appearance, superhydrophobic surface with periodic microstructures exhibit brilliant iridescence (Fig. 10b),58 in this regard, the colorful superhydrophobic graphene show potential for embellishment coatings. Additionally, hydrophobic graphene could also be used as a functional coating towards depth filter.95 Droplet coated with superhydrophobic graphene particles may serve as a miniaturized reactor standing on either hydrophobic or hydrophilic surfaces.47 Currently, the research topic “superhydrophobic graphene” is still in their infancy. Nevertheless, we still deem that, with the progress of superhydrophobic graphene and its related materials, it may find more and more promising applications, both for fundamental research and for practical usage.

Conclusions and outlook Due to the outstanding physical/chemical properties, graphene has been intensively investigated by diverse disciplines; in the past decade, it has been considered as a rising star over the horizon of material science. 95 Moreover, as numerous wellcontrolled structures have been triumphantly constructed from graphene-related materials, the development of this family has been vigorously propelled.38 In recent years, to investigate and utilize the surface property of graphene, different routes have been proposed to fabricate superhydrophobic graphene surfaces by controlling surface roughness and chemical composition of graphene. In this review, we discussed the fabrication methods and potential applications of superhydrophobic graphene and related materials. In the beginning, owing to the ease of functionalization and the tractable water-processing capability, GO solution is regarded as a reservoir for the fabrication of various graphene-based non-wetting surfaces. Later, with the rapid progress of graphene production methods, superhydrophobic graphene has been successfully prepared based on pristine graphene. For instance, CVD method provides an effective way for the fabrication of functional graphene surfaces. In addition to the superhydrophobic surfaces fabricated by solo graphene or graphene-related materials, with the aid of polymer and inorganic nanoparticles, manifest graphene derivatives can be also developed by hybridization. All of these graphene-based superhydrophobic materials exhibit great potential in a wide range of applications, such as water treatment, wettability switching, transparent electrodes and smart actuators. To make an overview of the state-of-the-art of this dynamic field, we summarized the recent developments of


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ARTICLE superhydrophobic graphene and its related materials in Table 1. Despite a short history, graphene has become a promising newcomer in superhydrophobicity research. As compared with conventional materials such as metals (including semimetals and metal alloys), oxides, and polymers, graphene and its related materials exhibit many superior features. To make a full comparison, some representative properties of these superhydrophobic materials have been briefly summarized in Table S1 (see supporting information). Currently, as the research of superhydrophobic graphene is still at an early state, there are still lots of challenges that need to be addressed. For example, a majority of the fabrication methods are limited to laboratory research and not suitable for large scale production. Besides, most of the superhydrophobic graphene films generally face the problem of poor mechanical stability. Mechanical durability is crucial to fulfill the need of real-world applications. The third challenge is how to make full use of the superhydrophobic graphene and its related materials. Besides the hydrophobic nature, graphene embraces kinds of merits, including mechanical strength, optical transparency, high conductivity, large specific surface area, flexibility, and so on. It could be expected that the combination of these superior properties of graphene and superhydrophobicity would lead to novel functions in the field of electronic devices, optoelectronics, microfluidics and tissue engineering. Overall, we believe that an exciting future for superhydrophobic graphene surfaces will be witnessed since multidisciplinary scientists and engineers have been contributing to the understanding and the design of such surfaces.

Acknowledgements The authors would like to acknowledge the National Basic Research Program of China under grants No. 2011CB013000 and 2014CB921302, and the NSFC under grant No. 61376123, 51335008 and 61435005) for support.

Notes and references a

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China. Fax: XX XXXX XXXX; Tel: XX XXXX XXXX; E-mail: [email protected]; [email protected] b Institute of Quantitative Biosciences & Biophysics, Departments of Bioengineering, Electrical Engineering & Computer Science, UC Berkeley. c Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun 130022, China. d Nano-Optoelectronic Lab, Institute of Semiconductors, Chinese Academy of Sciences, P. O. Box 912, Beijing, China. † Footnotes should appear here. These might include comments relevant to but not central to the matter under discussion, limited experimental and spectral data, and crystallographic data. Electronic Supplementary Information (ESI) available: Supporting Table S1. See DOI: 10.1039/b000000x/ 1

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Page 13 of 15

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Page 14 of 15

DOI: 10.1039/C5NR00719D

ARTICLE

Journal Name

Category Surface functionalization

Method Covalent bond GO with ODA Covalent bond GO with POSS

Material ODA-GO a POSS-GO c

Solvent modulation

Covalent bond GO with phenyl isocynate Covalent bond GO with HFTMS Solvent adjustment Suction filtration

Phenyl isocynate-GO HFTMS-GO b rGO rGO

Freeze-dry Template method Biomimetic laser processing CVD growth Two-step CVD on Ni foam using structured MPCVD on patterned Si templates HFCVD on Si nanocone CVD on Ni foam Dip coating Crumpling and unfolding

Nonsolvent induced precipitation and crystallization

Others

Arc discharge Solvent replacement Solvent replacement Solvent leaching

FDTS-rGOd Silane-treated rGO rGO Graphene/CNT ODA-Graphene Graphene Teflon-Graphene Graphene Graphene Fluorine-doped graphene Graphene/PVDF-HFP f Graphene/PVDF e Graphene/MEH-PPV h

Solvent replacement Solubility modulation Self-assembly Structural reconformation Electrospinning Covalent graft Solvothermal reduction

Graphene/P3HTg Graphene/PC i Graphene/iron oxide Graphene/Nafion Graphene/PVC j Graphene/PVDF rGO/CNTs k

Microwave reduction Ascorbic acid reduction Spray

Fluro-rGO Silane-rGO Phenyl isocynate-GO

Potential application Superhydrophobic coating Miniaturized reactor Interfacial reinforcement Superhydrophobic coating Oil/water separation, Adhesive Wettability switching Adhesive, Photoresponsive, Reversible wettability Transparent flexible electrode Structural color, Tunable wettability Oil/water separation Adhesive Flexible, Mechanical strong Oil/water separation Transparent electrode Artificial-muscle actuator Batteries, Wide-band gap secmiconductor Catalyst supporter Oil/water separation EMI shielding Luminescence, Thermal stability EMI shielding Removal of heavy metal Transparent conductive flexible Photovoltaic cells Oil/water separation EMI shielding Stable hydrophobicity Paper Textile Depth filter media

Ref. 46 47 48 49 50 51 52 53 57,58 44 71 72 73 74 75 80 81 82 83 84 85 86 87 90 91 92 93 94 95

a

, ODA, octadecylamine; b, HFTMS, (heptadecafluoro-1,1,2,2-tetradecyl) trimethoxysilane; c, POSS, polyhedral oligomeric silsesquioxane; d, FDTS, 1H,1H-2H,2H perfluorodecyl-trichlorosilane; e, PVDF, polyvinylidene fluoride; f, PVDF-HFP, poly(vinylidene fluoride-hexafluoropropylene); g, P3HT, poly(3-hexyl thiophene); h, MEH-PPV, poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]; i, PC, polycarbonate; j, PVC, polyvinyl chloride; k, CNTs, carbon nanotubes

14 | J. Name., 2012, 00, 1-3




Table 1. Summary of the methods of fabricating superhydrophobic graphene and their potential applications

Page 15 of 15

Nanoscale

ARTICLE Yong-Lai Zhang received his BS (2004) and PhD (2009) from Jilin University, China. In 2008, he had worked in Fritz-Haber-Institut of the Max-Planck-Gesellschaft as an exchanged PhD student. After received his Ph.D, he joined the faculty in the State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University. In 2011, he worked as a research professor in Yonsei University, Korea. After that, he was awarded a “Hong Kong Scholar” postdoctoral fellow and worked at Center of Super Diamond and Advanced Films (COSDAF), City University of Hong Kong. His research interests include laser micronanofabrication of biomimetic surfaces, graphene-based microdevices and Lab-on-a-Chip system.

Hong-Bo Sun received his B.S. and Ph.D. degrees from Jilin University, China, in 1992 and 1996, respectively. He worked as a postdoctoral researcher in the University of Tokushima, Japan, from 1996 to 2000, and then as an assistant professor in Department of Applied Physics, Osaka University, Japan. In 2005, he was promoted as a full professor (Changjiang Scholar) in Jilin University, China. His research interests have been focused on ultrafast optoelectronics, particularly on laser nanofabrication and ultrafast spectroscopy. So far, he has published over 200 scientific papers, which have been cited by SCI journals more than 7000 times.


J. Name., 2012, 00, 1-3 | 15



Journal Name

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DOI: 10.1039/C5NR00719D

Superhydrophobic nanocoatings: from materials to fabrications and to applications.

2D and 3D graphene materials: Preparation and bioelectrochemical applications.

Facile preparation of super durable superhydrophobic materials.

Recent advances in the mechanical durability of superhydrophobic materials.

Recent NMR developments applied to organic-inorganic materials.

Graphene Hybrid Materials in Gas Sensing Applications.

Graphene and graphene-based materials for energy storage applications.

Recent developments in sweat analysis and its applications.

Recent developments in nanowires for bio-applications from molecular to cellular levels.

Acoustic black holes: recent developments in the theory and applications.

From structure to catalysis: recent developments in the biotechnological applications of lipases.

Recent developments in the chemistry and biological applications of benzoxaboroles.

Recent developments in β-C-glycosides: synthesis and applications.

Recent developments in the structural science of materials.

Graphene and Carbon Quantum Dot-Based Materials in Photovoltaic Devices: From Synthesis to Applications.

Graphene-Based Materials for Stem Cell Applications.

Recent developments on sequence spaces and compact operators with applications.

Mass-spectrometry-based microbial metabolomics: recent developments and applications.

Recent developments and applications of the Cadiot-Chodkiewicz reaction.

Split-luciferase complementary assay: applications, recent developments, and future perspectives.

Recent developments on potential new applications of emetine as anti-cancer agent.

Recent Developments of Graphene Oxide-Based Membranes: A Review.

Recent Developments of Magnetoresistive Sensors for Industrial Applications.

Recent Developments of Liposomes as Nanocarriers for Theranostic Applications.