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Cite this: Nanoscale, 2018, 10, 17015

Received 14th June 2018,Accepted 24th August 2018

DOI: 10.1039/c8nr04831b

rsc.li/nanoscale

A surface transition of nanoparticle-decoratedgraphene films from water-adhesive to water-repellent†

Shu Wan,a Hao Wan,a Hengchang Bi,*a,b Jingfang Zhu, a Longbing He,a Kuibo Yin,a

Shi Sua,c and Litao Sun *a,b,c

Herein, a facile strategy is introduced to realize the transition of

graphene films from a water-adhesion surface (adhesive pressure

of 541.5 Pa) to a water-repellent surface (adhesive pressure of ∼0Pa) via decoration of carbon nanoparticles. Cassie impregnating

wetting state and Cassie state are used to explain highly adhesive

effect and strong repelling effect, respectively. Droplet impacting

experiments demonstrate that the as-prepared graphene films

have a stable structure, which is beneficial for their applications.

Wettability attracts wide research interest due to its greatimpact in various human activities from daily applications tohigh technology such as microelectronics, Micro-Electro-Mechanical Systems (MEMS), and biomedical devices.1,2

Several reports have demonstrated that numerous applications(e.g., oriented transportation of droplets,3 specific patternprinting,4,5 and superhydrophobic droplet-logic-baseddevices6) can be realized via designing proper wettability ofsurfaces. In these studies, super-anti-wetting surfaces with pro-perties of droplet anti-adhesion and self-cleaning show greatpotential in fluidic devices4 as well as smart coating7 and havebeen broadly investigated in recent years. Materials with super-hydrophobicity have also been widely studied. For instance, ablend containing colloidal graphite and Teflon particles hasbeen reported.8 A superhydrophobic and electrically conduc-tive surface can be achieved through simply drop casting theas-prepared blend on the target substrate. This coating tech-nique is attractive for applications such as electromagneticshielding and anti-wetting electrode fabrication.

Owing to its excellent chemical stability and intrinsic hydro-phobicity,9 graphene can be a possible candidate material todesign an anti-wetting substrate. Despite these advantages,graphene, especially graphene-carbon hybrid structure (all-carbon structure), exhibits superior thermal stability comparedwith a hydrophobic polymer-based structure.10 However, thehigh water adhesive (i.e., water-sticking) property of graphenewas also reported.11–13 This property is ascribed to the stronginteraction between the graphene surface and water droplets.11

Therefore, the graphene-based substrate is generally water-sticking. In this situation, water droplets get pinned in thegrooves of the substrate and cannot roll off smoothly. As theconsequence, this hinders the transportation of droplets influidic devices. Recently, many efforts have been made toreduce the high adhesive strength. By mixing with nano-materials, such as carbon nanotubes,14 the fabrication of asurface structure with sufficient roughness in both microscaleand nanoscale can be a possible solution. However, this strat-egy can also complicate the fabrication process and increasethe cost, which may not be suitable for the mass production ofgraphene-based hydrophobic anti-wetting surfaces. It is thushighly desirable to develop a new facile and low-cost strategyto tackle this challenging but very significant problem.

The wettability properties are dictated by the intrinsic lowsurface energy and roughness of the solid surface.15

Experimental reports16 show that graphene sheets are hydro-phobic and the surface energy is 46.7 mJ m−2, which is lowerthan that of natural graphite flakes (54.8 mJ m−2). In addition,the contact angle (CA) of intrinsic graphene is 127°, which isobtained from an experimental measurement.16 To design anappropriate structure with sufficient roughness possessing theextraordinary water repelling properties and superhydrophobi-city of the graphene surface, we have investigated several exist-ing rough material surfaces, and we were particularlyimpressed by the strong water repelling behavior of lotus leaf’shierarchical structure.17 Lotus leaf is famous for its self-clean-ing phenomenon, and it has different scales of roughness andlow contact angle hysteresis, which indicate that the water

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr04831b

aSEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education,

Collaborative Innovation Center for Micro/Nano Fabrication, Device and System,

Southeast University, Nanjing 210096, P. R. China. E-mail: honesty1983@163.com,

slt@seu.edu.cnbCenter for Advanced Carbon Materials, Southeast University and Jiangnan

Graphene Research Institute, Changzhou 213100, P. R. ChinacCenter for Advanced Materials and Manufacture, Joint Research Institute of

Southeast University and Monash University, Suzhou 215123, P. R. China

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droplet cannot wet the microstructure spaces between thespikes. This allows air to remain inside the texture, resultingin a heterogeneous surface composed of both air and solid. Asa result, the adhesive force between the water and the solidsurface is extremely low, allowing the water to roll off easily.18

We thus surmise that the reduced graphene oxide surface candeliver remarkable water repelling properties if it can be hier-archically structured in a similar manner as the lotus leaf.

In the present study, we demonstrate that a reduced gra-phene oxide film (rGOF) with a hierarchical structure texturedwith different scales of roughness can be readily fabricated byspray coating plus heating followed by flame soot decoration(carbon nanoparticles). As a result, an all-carbon structure canbe synthesised. A transition from a strong water-adhesionsurface (adhesive pressure of 541.5 Pa) to water repellency ofgraphene films (adhesive pressure of ∼0 Pa) can be readily rea-lized. The resulting modified rGOF (MrGOF) exhibits a lotusleaf-like structure, super hydrophobicity, low contact angle hys-teresis and weak adhesive force.

The fabrication process of rGOF is schematically demon-strated in Fig. 1a. In this typical experiment, graphene oxide(GO) was first obtained through a modified Hummersmethod.19 Then, 300 mg of GO was dispersed in 600 mL ofethanol, followed by sonication for an hour. The spray coatingtechnique was adopted to deposit the GO sheets onto glassplates located on a heating stage at 70 °C. After that, the GO

films were placed in an autoclave and then, hydrazine mono-hydrate was added as a reducing agent into the autoclave, fol-lowed by increasing the temperature to 95 °C for 10 h. Inaddition, nanoscale carbon nanoparticles were decorated onthe graphene films, obtaining MrGOF. The as-prepared rGOFand MrGOF showed negligible difference in photographs(Fig. S1†). Scanning electron microscopy (SEM) was adopted todescribe the details of rGOF and MrGOF morphologies.However, negligible difference could be found between rGOFand MrGOF in the low-magnification images (ESI, Fig. S2†).Therefore, high-magnification images were obtained toanalyze the morphology differences between rGOF andMrGOF, as shown in Fig. 1b and c, respectively. From the SEMimage, both rGOF and MrGOF were composed of crumpledgraphene sheets with sizes ranging from 20 to 40 μm. Clearly,large quantities of nanoparticles were adhered to the crumpledgraphene sheets in MrGOF, forming fluffy structures (Fig. 1c).In fact, MrGOF exhibited a similar structure to that of thelotus leaf with a self-cleaning function.20–22 Due to the similarstructure, MrGOF is expected to exhibit the same function asthat of the surface of lotus. To depict the fluffy structure indetail, transmission electron microscopy (TEM) was adoptedto obtain its TEM images (Fig. 1d and e). It was clearlyobserved from the low-resolution images that a large numberof soot particles adhered to the copper mesh (Fig. 1d). High-magnification TEM images (Fig. 1e) showed that a single sootparticle contained small chain-liked spherical particles withsizes ranging from 20 nm to 50 nm.23–25 In addition, from thehigh-resolution TEM images (Fig. 1e), it was found that thesenanospherical particles contained many graphitic fragmentswith a typical d-spacing of 0.34 nm.23 Unlike the perfect onion-like graphitic structure, where the graphitic layers are continu-ous and closed circles, the graphitic layers in these nano-spheres were broken into small fragments. Therefore, an all-carbon lotus-like structure was successfully fabricated, whichcontained a graphene microstructure and a nanostructurecomposed of carbon particles. To describe the surface pro-perties of rGOF and MrGOF, wettability tests were employed toinvestigate the surface properties, as shown in Fig. 1f and g.During testing, a drop (5 μL) of distilled water was droppedonto the surfaces of rGOF and MrGOF. It was clearly observedthat both rGOF and MrGOF are hydrophobic. The contactangle (CA) was 137° for rGOF (Fig. 1f), demonstrating highhydrophobicity, whereas CA for MrGOF was surprisingly 155°(Fig. 1g), indicating the superhydrophobic property of MrGO.

To test the differences in adhesion for oil and water, aseries of adsorption experiments have been conducted. Due tothe microstructure along with the hydrophobic property, bothrGOF and MrGOF exhibited novel oil adhesion abilities on thewater surface, as shown in Fig. 2a. Dodecane floating on artifi-cial seawater was completely adsorbed within 30s (rGOF upperpanel, MrGOF lower panel; dodecane was stained with Sudanred 5B). However, the under-water adhesion performanceexhibited many differences between rGOF and MrGOF (Fig. 2b,CCl4 was stained with Sudan red 5B). For rGOF (Fig. 2b upperpanel), when a CCl4 drop contacted the surface of rGOF under

Fig. 1 (a) Schematic drawings illustrating the spray coating plus heatingprocess to prepare reduced graphene oxide films and modified gra-phene oxide films. (b) The surface morphology of rGOF from high-mag-nification SEM image. Scale bar: 5 μm. (c) The surface morphology ofMrGOF from high-magnification SEM image. It is covered with a fluffystructure. Scale bar: 5 μm. (d) TEM image of flame soot. This sootadhered to the copper mesh and crosslinked each other to form a fluffystructure. Scale bar: 50 nm. (e) High-resolution TEM images of soot. Ithas an onion-like structure and its size ranges from 20 nm to 50 nm.Scale bar: 10 nm. (f ) Wettability test for rGOF. The CA is ∼137°, and itindicates highly hydrophobic property. (g) Wettability test for MrGOF.The CA is ∼155°, and it indicates superhydrophobic property.

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water, it hardly spread on the surface, and it still remainedspherical in shape. It seemed that there was an obstacle forCCl4 adhesion under water. On the contrary, the CCl4 dropsuccessfully spread and adhered on the MrGOF surface, whichis shown in Fig. 2b, lower panel. Moreover, a clear mirrorreflection could be observed, indicating that some airremained in MrGOF. This phenomenon together with theenhancement of underwater adhesion for CCl4 might beascribed to the difference in microstructures between rGOFand MrGOF.

To investigate the different performances between rGOFand MrGOF, various kinds of experiments were adopted. Thecohesive attractions between water molecules and graphenesheets were crucial to detect the nature of wettability for rGOFand MrGOF. A droplet rolling experiment was performed bytilting the sample. Fig. 3a shows the shapes of the water dro-plets with different volumes from 50 to 400 μL on the surfaceof rGOF when it is tilted vertically. The water droplet shapetransformed from spherical to elliptical, but it still remainedpinned to the surface and did not roll off. In addition, todepict the strong adhesive performance quantitatively forrGOF, the surface was tilted to 180°. After that, the 400 μLwater droplet was initially brought in contact with the rGOFsurface, and the volume of the droplet was gradually increasedwith a step of 50 μL using a pipette. From Fig. 3b, it can beclearly observed that the shape of the droplet was elongateddue to gravity. The adhesive force between the water dropletand the surface of rGOF is equal to the gravity of the dropletuntil it falls down from the rGOF surface. The contact areabetween the water droplet and surface was ∼12.56 mm2.

Therefore, the adhesive pressure was up to 518 Pa when thevolume of the droplet increased to 650 μL. After that, morewater was injected into the droplet until the surface tension ofdroplet could not balance its own gravity (total volume:∼680 μL, responding adhesive pressure: 541.5 Pa), whichcaused the droplet to break into two parts. One part fell downfrom the surface, whereas the other part was still adhered tothe surface. It is noteworthy that the broken location occurredat the upper part of the droplet instead of the boundarybetween the droplet and surface, indicating that the adhesiveforce was not up to its maximum value at that moment. Thereis a qualitative relationship: surface tension < gravity <maximum adhesive force. Although the exact value needsspecial apparatus to measure, this experiment has alreadyproven that rGOF possesses very adhesive property, which

Fig. 2 (a) Dodecane adhesion of rGOF (upper panel) and MrGOF (lowerpanel) at an interval of 10 s. Dodecane was stained with Sudan red 5B.(b) CCl4 adhesion of rGOF (upper panel) and MrGOF (lower panel) at aninterval of 5 s. CCl4 was stained with Sudan red 5B.

Fig. 3 (a) The shapes of water droplets with different volumes from 50to 400 μL on the surface of rGOF when it is tilted vertically. (b) Adhesivepressure–volume of droplet curves. The contact area between surfaceand droplet is ∼12.56 mm2. When the volume of the droplet increasedto 650 μL, the droplet still adhered to the surface. (c) The roll-offprocess of MrGOF for the water droplet (50 μL). The droplet rolledsmoothly and the whole process took <5 s. (d) Advanced liquid front forrGOF with an advancing contact angle of ∼146°. (e) Receding liquidfront for rGOF, indicating a receding contact angle of ∼41°. (f )Corresponding advancing contact angle on MrGOF measured to be∼157° and corresponding receding contact angle on MrGOF measuredto be ∼147°.

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endows it with great applications such as micro/nanofluidhandling, devices (lab-on-chip), and biomaterials. In contrast,the droplet (50 μL) on MrGOF rolled off more smoothly, asproven by a series of images in Fig. 3c. MrGOF with glass sub-strate was initially tilted at an angle of ∼3° (I). Once the waterdrop landed on the surface, it exhibited a pancake shape dueto gravity (II). Then, it recovered immediately and began to rolloff on the surface (III). The water drop remained spherical inshape during the whole process (IV–VI), indicating that theroll-off behaviour can be dramatically improved by modifyingrGOF using carbon nanoparticles. Besides glass, differentkinds of substrates (Cu foil, Si wafer, and epoxy resins) can bemodified to exhibit similar roll-off behaviours through thesame method (ESI, Fig. S3†).

To quantify the adhesive behaviours of rGOF and MrGOF,advancing and receding contact angle measurements wereobtained using a goniometer for images. The low-bond axisym-metric drop shape analysis technique was also used to deter-mine the contact angle. For this, a ∼5 μL water drop wasinitially brought in contact with the sample surface.Subsequently, the volume of the drop was increased and thendecreased to advance and retract the liquid front. This wasrepeated several times independently to verify the reproducibil-ity of the experimental process. Fig. 3d–f show the advancingand receding conditions created by dispensing and retractingwater. rGOF exhibits the water contact angle values of ∼146°and ∼41° for advancing (Fig. 3d) and receding (Fig. 3e),respectively (ESI, Movie S1 and S2†). This is in agreement withits static contact angle (137°), which should lie between theadvancing and receding angle values. In comparison to rGOF,MrGOF is much more hydrophobic and far less adhesive tothe water droplet and shows advancing and receding watercontact angles of ∼157° and ∼147° (Fig. 3f), respectively. Bothof them show contact angle hysteresis. Moreover, MrGOFexhibited a lower contact angle hysteresis of ∼10° as comparedto rGOF (∼105°). This demonstrated that the water droplet canroll much more smoothly on the MrGOF surface, and rGOFexhibits much stronger adhesive performance than MrGOF;however, MrGOF showed much stronger water repelling pro-perties than rGOF. The contact angle hysteresis can explain thephenomenon well, as shown in Fig. 3a and b. The behaviour ofrGOF is similar to that of rose petals,15,26 but a similar mecha-nism from the lotus leaf can be found in MrGOF.20–22

Dynamic droplet impact tests were performed to gauge theability of rGOF and MrGOF to repel impacting water drops.Droplet rebound is beneficial in some lab-on-chip appli-cations, where droplet motion is preferred and drop pinning isundesirable. Similarly, self-cleaning and anti-fouling surfacesrequire mobile drops. Droplets as large as 150 μL weredropped onto rGOF and MrGOF from a fixed height using amicrosyringe to impact the surface at a velocity of ∼73 cm s−1.The objective of the impacting drop caused the droplet topenetrate into the pores and become pinned to the rGOFsurface. A high-speed camera operating at 2000 frames persecond was used in this study. Time-lapse images for the rele-vant advancing and receding stages of the droplets at an

impact velocity of ∼73 cm s−1 are shown in Fig. 4. For rGOF(Fig. 4a), the droplet spreads but does not rebound off thesurface. It appears to become strongly pinned to the rGOFsurface and pulsates for a prolonged time period until thedrop’s vibrational energy is dissipated through viscosity. Bycontrast, for MrGOF (Fig. 4b), the droplet first deforms andflattens into a pancake shape and then retracts and finallyrebounds off the surface. Moreover, the droplet remains com-pletely intact during the collision. The droplet impactphenomenon was studied in further detail by plotting thecontact line diameter against time, as shown in Fig. 4c. For thefirst ∼5 ms, the droplet deforms from a spherical to a pancakeshape with the contact line diameter increasing with timeduring this phase. There is no significant difference in thecontact line diameter responses of rGOF and MrGOF duringthe advancing phase. However, after ∼5 ms, the contact line

Fig. 4 (a) Snapshots of a water droplet impacting the surface of rGOF.The impact velocity just prior to the droplet striking the surface was∼73 cm s−1. The sequence of snapshots shows the deformation timehistory of the droplet upon impact. The droplet spreads and retracts butdoes not rebound off the surface. The droplet becomes pinned to thesubstrate. Scale bar: 1 cm. (b) Corresponding snapshots of a waterdroplet impacting the surface of MrGOF. The impact velocity was∼73 cm s−1. For this case, the droplet spreads, retracts, and finallyrebounds off the surface. Scale bar: 1 cm. (c) Contact line diameterversus time for droplets impacting on the rGOF and MrGOF surfaceswith a velocity ∼73 cm s−1. At first, the contact line diameter is similarfor the two cases; however, during the retraction phase, the contact linediameter for rGOF becomes pinned, whereas the droplet on MrGOFretracts and then successfully rebounds off the surface. In addition, thedroplet impacting experiment demonstrates that both rGOF and MrGOFhave a very stable structure.

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diameter for the droplet on rGOF remains fairly constant anddoes not decrease significantly with time. In contrast, thecontact line diameter for MrGOF decreases monotonically tozero as the droplet contracts and then lifts off the surface. Thisresult demonstrates that the contact line diameter for rGOFbecomes pinned to the surfaces after the advancing (or spread-ing) phase of the droplet is complete.

To explain the difference in the behaviours of rGOF andMrGOF, two distinct models, which were developed indepen-dently by Wenzel27 and Cassie (henceforth, the Cassiemodel),28 are commonly used to explain the effect of rough-ness on the apparent contact angle of liquid drops.29 Usually,the Wenzel state corresponds to a high contact angle hyster-esis, whereas a low contact angle hysteresis can be explainedby the Cassie state.14 Considering the previous literature30 andthat the contact angle for rGOF is 137°, it is suggested that thewetting regime of rGOF is the Cassie impregnating wettingstate rather than the Wenzel state (Fig. 5a).15 On the contrary,the wetting regime of MrGOF is clearly in the Cassie state. Inthe Cassie impregnating wetting state, the grooves of the solidare wetted with liquid, and solid plateaus are dry-filled withair. As for rGOF, the sizes of hierarchical micro- and nano-structures are both larger than that of MrGOF. Water dropletsare expected to impregnate into the larger scale grooves ofrGOF but not into the small ones, resulting in the Cassieimpregnating wetting regime. The water droplet sealed inmicropapillae can cling to the rGOF surface, showing highcontact hysteresis in a certain range of volume when thesurface is tilted to any angle or even turned upside down. Inaddition, the capillary force from nano-gaps and the chemicalaffinity from the graphene defects and residual hydrophilicfunctional groups such as hydroxide groups (ESI: Fig. S4–S6†)

are other important factors ascribed to the high adhesionsurface.26 For MrGOF, the triple contact lines on a randomlyrough surface are expected to be contorted and extremelyunstable, preventing water from entering into the microstruc-ture spaces. This allows air to remain inside the texture (ESI,Fig. S7†), which can result in a heterogeneous surface com-posed of both air and solid (Fig. 5a). Thus, the adhesive forcebetween the water and the solid surface is extremely low, allow-ing the droplet to advance and recede at different contact linepoints and to roll off smoothly (self-cleaning phenomenon). Asfor dynamic droplet impact tests, the pinning phenomenonfor rGOF suggests that the momentum of the droplet duringthe initial impact enables it to penetrate into the intersticesand transition into the Cassie impregnating wetting state. Theresulting enlargement in the contact area between rGOF andthe droplet significantly increases adhesion of the droplet tothe substrate and prevents the contact line from retracting.The droplet therefore cannot recover sufficient energy torebound off the surface. Consequently, the droplet becomespinned to the surface and simply vibrates on the rGOF surfaceuntil the energy of the droplet is dissipated. On the otherhand, MrGOF is superhydrophobic, which prevents the tran-sition of the impacting droplet to the sticky Cassie impregnat-ing wetting state. Also, it reduces energy dissipation during thespreading and retraction of the droplet, thereby enabling it torecover sufficient energy to rebound off the surface. An expla-nation can be obtained for the poor ability of rGOF for oiladhesion under water (schematically shown in Fig. 5b). Due tothe poor water solubility of CCl4, the droplet cannot dissolvein water and remains spherical in shape. In the Cassie impreg-nating wetting state, rGOF is adhesive with a water film, whichhinders the contact between rGOF and the CCl4 droplet. As aresult, the adhesion of rGOF is hampered by the adhesivewater film. On the other hand, the enhanced adhesion abilityfor MrGOF is the result of the Cassie state. Unlike that forrGOF, water will not stick to the MrGOF surface. Instead, it willbe repelled due to extremely low adhesive force between thewater and the MrGOF surface. When a CCl4 droplet comes incontact with the MrGOF surface, both squeeze water betweenthem, consequently contacting with each other directly due towhich the CCl4 droplet can adhere successfully.

Conclusions

In summary, two kinds of graphene films, i.e., one with highhydrophobicity and strong adhesive performance (rGOF) andthe other with superhydrophobicity and strong repelling effectfor water (MrGOF) have been successfully fabricated in facilesteps. A transition of surface property from rose petal effect tolotus effect can be realized through simple modification usingflame soot. The hierarchical nanostructures are responsible forthese remarkable characteristics. Cassie impregnating wettingstate and Cassie state are used to explain highly adhesive per-formance and strong repelling effect, respectively. Dropletimpacting experiments demonstrate that the as-prepared gra-

Fig. 5 (a) Schematic illustrations of a drop of water in contact withrGOF and MrGOF. The former is in the Cassie impregnating wettingstate, and the latter is in the Cassie state. (b) The CCl4 adhesion abilityfor rGOF and MrGOF. The former is the adhesion behaviour of rGOF,which is hindered by the water film adhesive to its surface, and the latteris that of MrGOF, which can directly contact the CCl4 droplet, thusadhering CCl4 successfully.

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phene films have a very stable structure, which is very ben-eficial for their applications.

During the test, we found that the superhydrophobicity (CA> 150°) of MrGOF can be maintained for at least 1 month. Thedurability of the superhydrophobic surface is very importantfor practical applications in complicated environments,especially in some extreme cases such as salt fog, heavy dustsand acidic atmosphere. With respect to the practical appli-cations, the durability and robustness of these surfaces stillneed further investigation and improvement in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program ofChina (Grant No. 2017YFA0403600 and 2017YFA0305500), andthe National Natural Science Foundation of China (No.11525415, 51302037, 61274114, 11774050, 51420105003,11327901 and 11204034).

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east

Uni

vers

ity -

Jiu

long

hu C

ampu

s on

12/

28/2

018

3:13

:08

AM

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