Article pubs.acs.org/Langmuir
Wettability on Inner and Outer Surface of Single Carbon Nanotubes Yutaka Yamada,*,† Koji Takahashi,‡,∥,⊥ Yasuyuki Takata,§,∥,⊥ and Khellil Sefiane#,% †
Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan Department of Aeronautics and Astronautics, Graduate School of Engineering, §Department of Mechanical Engineering, Graduate School of Engineering, ∥International Institute for Carbon-Neutral Energy Research, and ⊥Japan Science and Technology Agency (JST), CREST, Kyushu University, Fukuoka 819-0395, Japan # School of Engineering, The University of Edinburgh, King’s Buildings, Robert Stevenson Road, Edinburgh EH9 3FB, U.K. % Tianjin Key Lab of Refrigeration Technology, Tianjin University of Commerce, Tianjin City 300134, P. R. China ‡
S Supporting Information *
ABSTRACT: The surface wettability of a liquid on the inner and outer surface of single carbon nanotubes (CNTs) was experimentally investigated. Although these contact angles on both surfaces were previously studied separately, the available data are of limited help to elucidate the effect of curvature orientation (concave or convex) on wettability due to the difference in surface structure. Here, we report on the three-phase contact region and wettability on the outer surface of CNT during the dipping and withdrawing experiment of CNT into an ionic liquid. Furthermore, the wettability on the inner surface was measured using a liquid within the same CNT. Our results show that the contact angle on the outer surface of the CNT is larger than that on the flat surface and that on the inner surface is smaller than that on the flat one. These findings suggest that the surface curvature orientation has a noticeable effect on the contact angle at the nanoscale because both inner and outer surfaces expose the same graphite wall structure and the contact line tension will be negligible in this situation. The presented results are rationalized using the free energy balance of liquid on curved surfaces.
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systems.19 However, due to the difficulty of experimentally investigating these issues, reports on the subject are rather limited. Concerning the inner surface studies, Kim et al. observed liquid behavior in a CNT with ∼500 nm in diameter and 15 nm thick walls using an optical microscope.20 They showed a liquid front movement to the empty side due to condensation and capillary forces. Water condensation behavior in the CNT and the detail of meniscus shape were also observed by Rossi et al.21 The contact angle was reported as less than 15°. In addition, Gogotsi et al. synthesized water contained CNTs using the hydrothermal method,22 and then they observed a meniscus shape deformation due to heating via electron irradiation using the transmission electron microscope.23,24 The wettability of the outer surface of single fiber on the other hand has been predicted by previous theoretical work which shows that there is a discrepancy in wettability when fiber diameter is of nanoscale dimensions.25 Experimental work was mainly conducted through the force measurement using the atomic force microscope (AFM). The test fiber was fixed to the AFM cantilever tip, and the force during the penetration and retraction of fiber from a liquid surface was measured. The
INTRODUCTION Carbon nanotubes (CNTs) consisting of hydrophobic graphite structure with nanoscale dimensions have attracted attention in a wide range of engineering fields because of their thermal, electrical, and mechanical properties,1−3 and there have been a plethora of investigations and potential applications. For instance, CNTs are added to polymers to enhance their physical properties.4−6 These composite materials have a contact between the outer surface of CNTs and a liquid polymer. The interaction between the polymer melt and the outer surface of the CNTs can be paramount in dictation of the properties of the composite material. On the other hand, the interaction with the inner surface of the CNTs is important for fluids manipulation and material engineering.7,8 In particular, due to the smooth inner surface of CNTs, liquid flow is enhanced several orders of magnitude compared to the prediction of the no-slip Hagen−Poiseuille relation;7 this unexpected and surprising result was confirmed by other experimental and theoretical investigations.9−11 Many other investigations exploring the potential of CNTs interaction with liquids include water boiling,12 pumping by electric fields,13 and solid−liquid critical behavior.14 Other attempts are made to use CNTs for purposes of the desalination as membranes15,16 and in other cases as nanoreactor.17,18 In all applications mentioned above the wettability of CNT surfaces is a key factor in understanding and optimizing those © 2016 American Chemical Society
Received: April 8, 2016 Revised: June 27, 2016 Published: June 28, 2016 7064
DOI: 10.1021/acs.langmuir.6b01366 Langmuir 2016, 32, 7064−7069
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ensures the connection between the CNT and the probe, as shown in Figure 1b. Then, the meniscus profile on the outer surface of the CNT was observed as follows. One drop of ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate, Kanto Chemical Co. Inc., Japan) is placed on the clean Si substrate, and it is mounted vertically in the SEM chamber (shown in Figure 1a). Because this liquid has a quite low vapor pressure,31 it is possible to maintain a liquid phase in high vacuum conditions, as used in the SEM. Before starting the observations, vacuum condition within the chamber was maintained for more than 1 h in order to evaporate the dissolved air and water molecules in an ionic liquid. The tip of the CNT selected in the previous step was dipped into the ionic liquid, as shown in Figure 1c. To clearly observe a liquid meniscus shape, the CNT tip was directed to approach the liquid at the apex of the droplet. As the CNT tip was pushing into the liquid, a three-phase contact line advances from the tip side to the tungsten probe side. Following this step, the CNT was withdrawn by moving the probe backward, and the contact line recedes to the tip side. During this operation, a small amount of liquid was introduced into the CNT. The meniscus shape was observed at the end of the manipulation. SEM observations were operated at 10 kV and ∼10 pA as the acceleration voltage and current, respectively. After the experiment, the CNT was fixed to the microgrid by EBID. The electron beam current during every EBID process was set at 93 nA. In order to detach the probe from the CNT mechanically, the connection at the microgrid side should be stronger than that at the CNT side. The remaining liquid inside the CNT and the graphitic wall structure were observed by transmission electron microscopy (TEM, JEM-3200FSK, JEOL Ltd., Japan), which was operated at 300 kV as the acceleration voltage. Contact angles were estimated using observation results from both the SEM and TEM. There are several reports which evaluate the static contact angle of droplet on fibers.29,30,32,33 However, CNTs used in this experiment are quite small compared to the droplet of the ionic liquid. Therefore, most of liquid surface behaves as a flat surface shown in Figure 1c, except at the meniscus region near the CNT. The contact angle is evaluated using the meniscus profile, as shown in Figure 2a.
result can be correlated to the contact angle, and it is estimated by the Wilhelmy method.26−28 The other way is the direct observation of a droplet on fibers.29,30 Nuriel et al. observed a polymer liquid wetting on a CNT and discussed the surface energy of solid material.29 Although there are several works that reported about the wettability of inner and outer surface of CNT separately, as mentioned above, the effect of curvature orientation (whether concave or convex) on wettability is still not fully elucidated. This is caused by the lack of experimental studies of both surfaces (inner and outer) using identical CNTs. In the present work, a single CNT tip was dipped into a liquid, and the meniscus shape on the outer surface was observed by scanning electron microscopy. The liquid remaining in the CNT was investigated using transmission electron microscopy to analyze the contact region profile on the inner surface. Contact angles were estimated using the meniscus profile, and various trends are discussed.
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EXPERIMENTAL METHODS
CNT powder was purchased from US Research Nanomaterials Inc. (Houston, TX), with inner diameters larger than 20 nm and wall thickness less than 50 nm. One end of these CNTs is open without any additional treatment. This structure is chosen as it is suitable to introduce liquid inside the CNT. A single CNT was prepared as follows. 1 mm3 of CNT powder was put into 20 mL of ethanol (purity 99.5%) and dispersed by ultrasonication for 10 min. Then several droplets were deposited on the copper microgrid (Gilder Cu G2000HS, Ouken Shoji, Inc., Japan) which was then placed in the cleanroom for more than 12 h to completely evaporate a residual solvent. An electrochemically polished tungsten probe attached to the manipulator (KleinDiek, Germany) was used to pick up individual CNTs; as shown in Figure 1a, it was fitted in the scanning electron microscope (SEM, Versa 3D, FEI, The Netherlands). After a probe tip touches the CNT, an electron beam was irradiated around the contact region, and a precursor gas was injected to deposit an amorphous carbon through electron-beam-induced deposition (EBID). This
Figure 2. Schematics of the contact angle on the (a) outer surface and (b) inner surface of CNT.
The interface in the CNT can be seen as the contrast difference by TEM observations. The contact angle on the inner surface was evaluated, as shown in Figure 2b. In this experiment, we used five different CNTs. The dimensions of each CNT are shown in Table 1.
Table 1. Dimensions of Each CNT Used in This Experiment
CNT1 CNT2 CNT3 CNT4 CNT5
Figure 1. (a) Schematics of the experimental setup in the scanning electron microscope. (b) CNT attached to the tungsten probe and (c) the tip of CNT is dipped into the ionic liquid. Scale bars in (b) and (c) show 5 μm. 7065
outer diameter [nm]
inner diameter [nm]
133 97 192 135 128
53 54 102 49 58 DOI: 10.1021/acs.langmuir.6b01366 Langmuir 2016, 32, 7064−7069
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RESULTS AND DISCUSSION The liquid wetting behavior on the outer surface of a CNT was observed using SEM. During this experiment, the three-phase contact region advanced on the surface. Figure 3a shows the tip
Figure 4. Measured advancing and receding contact angle on each CNT.
is shown in Figure 5a. The inset of Figure 5a shows a magnified image of the liquid-filled part. It can be noted that there are Figure 3. (a) CNT tip before it is dipped into the ionic liquid. (b−g) CNT tip dipped into liquid. (b), (d), and (g) show images after pushing phase. (c), (e), and (g) shows images after withdrawal phase. (h) CNT tip after it is withdrew from the ionic liquid. The scale bar on all panels is 1 μm.
of the CNT1. Figures 3b and 3c show contact regions after CNT tip was dipped into and withdrawn from an ionic liquid. This meniscus shape did not change during SEM observation. This finding means that the contact angle was maintained after movement. The advancing and receding contact angles were estimated from these images. This operation was repeated 2−3 times as shown in Figure 3d−g. Each CNT tip stayed less than 600 s in the liquid. Although the CNT was fitted almost perpendicular to the gravitational direction, liquid should be drawn into the CNT by capillary forces.19 However, a liquid droplet did not appear from the opposite end of the CNT. In this experiment, to prevent the contact angle change due to the remaining thin liquid film on the CNT, the length of the dipped region of this latter was gradually increasing, as shown in Figure 3b,d,f. The maximum dipped length was estimated from a comparison of Figures 3a and 3f; it was estimated as ∼1.7 μm. It is worth noting that the ionic liquid reservoir mounted on the SEM stage was stationary during the experiment (Figure 3b− g). This implies that the level of liquid interface has changed by ca. 200 nm between the pushing and withdrawal phases. Figure 3h shows the CNT tip after detached completely from the liquid. Figure 4 shows the results of the estimated wetting contact angle on the CNTs. Figure 3b−g represents CNT1 in this figure, and “p” and “w” of x-axis indicate after pushing and withdrawal phase, respectively. The result shows 72 ± 3° and 57 ± 3° for advancing and receding contact angles, respectively. It means that the contact angle hysteresis is ca. 15°, which is smaller than that of flat graphite surface.34 The equilibrium contact angle θe is estimated using eq 135
Figure 5. (a) A coupled TEM image of CNT filled with ionic liquid. The scale bar is 1 μm. (b) The three-phase contact region inside the CNT. The scale bar is 100 nm. (c) Side wall of CNT. Layers of graphite can be seen as lines. The scale bar is 10 nm.
void pockets along the CNT filled with liquid. These voids are capsule-like shape in the longitudinal direction. Figure 5b shows the magnified image of the three-phase contact region in the CNT. The cross section of the meniscus was shown in this observation. The contact angle on the inner wall of the CNT was measured following the schematic shown in Figure 2b; it was estimated as ca. 15°. The result is much smaller than the one observed on the outer surface. Furthermore, the contact region in the CNT is observed at ca. 4.7 μm from the tip. As mentioned above, the dipped length into the ionic liquid is ca. 1.7 μm by SEM observation. This implies that the liquid introduced into the CNT is driven by capillary forces, and it also means that the inner wall of CNT shows some degree of hydrophilicity, as shown in Figure 5b. However, according to the capillary action, the liquid-filled length could be predicted using eq 2
cos θa + cos θr (1) 2 where θa and θr are the advancing and receding contact angle, respectively. The result obtained was 64.5° as the θe. Following the above experiments, a small volume of the ionic liquid remained inside the CNT and was monitored by TEM. A coupled low magnified image of the liquid remaining in CNT1 cos θe =
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h=
2γLV cos θ ρgr
line tension is not avoidable when droplet dimension becomes less than 1 μm, because it is assumed to be on the order of 10−6−10−8 N.41 It means that this comparison is unsuited to discuss the effect of nanoscale curvature (further details are given in the Supporting Information). Here, we will propose an alternative explanation from the viewpoint of the free energy of liquid. Figure 6a shows the front
(2)
where h is the resulting height due to capillary force; g, r, ρ, and γLV represent the gravitational acceleration, the inner radius of the capillary, the density of the liquid and the surface tension of liquid, respectively. Here, we assume 1.36 g/cm3 and 44.1 mN/ m for ρ and γLV, respectively; this equation predicts more than 100 m as h considering 50 nm as the inner radius.36,37 And this value will be much larger when the capillary is inclined with respect to the gravitational direction, as in our experiment. Although the theory predicts much longer length, actual filling length was significantly short. This result suggests that the three-phase contact line receded in the CNT during the withdrawal phase, and it showed a receding contact angle as a result. Figure 5c shows the high magnified image of a side wall of the CNT. Graphite layers making the CNT wall can be seen in this image and those inclined from the longitudinal direction of the CNT. This means that the graphite edges are exposed on the outer and inner surfaces of the CNT, and both surfaces show similar structure. In addition, there were no covalent bonding found on graphite edges. This fact suggests that the intrinsic wettability of both surfaces (inner and outer) will be similar except for the effect of surface curvature. Furthermore, shrinking direction of graphite layers might affect the contact angle; however, there were no obvious differences found in this experiment. Here, the contact angle on a flat surface with inclined graphite layers is estimated using that of graphite edge and terrace surface. The results are found to be 27° and 38° for the edge and terrace surface, respectively. The equilibrium contact angle of flat surfaces with inclined graphite layers was assumed to be between these results. The contact angle hysteresis on the flat surface is assumed to be comparable to that obtained from the outer surface; thus, the receding contact angle on the flat surface is estimated as being greater than 19°. However, this value is larger than that obtained on the inner surface. It means that the contact angle of the convex surface (outer wall) shows larger angles than that on the flat surface, and the result from concave surface (inner wall) shows smaller values than the flat surface. Because the experimental dimensions are of nanoscale, the contact line tension can be considered as being the origin of contact angle difference.38 This tension refers to the excess free energy at the vicinity of the three-phase contact region, and it works perpendicular to the contact line when that line has the curvature on the surface.39 Here, we have to consider the curved surface of the CNT. Because of the cross section of the CNT, the contact line should adopt a circular line around the surface. However, when the CNT wall is cut and opened to flatten the surface, the contact line is expected to have no curvature. In addition, the line tension on the flat surface works at the contact line with parallel direction to the surface. On the other hand, that on the CNT surface will act in a perpendicular direction to the CNT surface. The force working in this direction is not taken into account for the estimation of the energy balance at the three-phase junction, i.e., Young’s equation.40 As a result, the line tension is not a plausible explanation for contact angle discrepancies between flat, concave, and convex surfaces. Additionally, although to compare directly with the three-phase contact region morphology of nanoscale droplet on flat surface is the proper procedure to discuss the effect of curvature, the effect of the
Figure 6. Schematics of (a) front view and (b) cross-sectional view of three-phase contact region. (i), (ii), and (iii) mean convex, flat (or macroscale curvature), and concave surface, respectively.
view of the three-phase contact region on the convex, flat (or macroscale curvature), and concave surface, respectively. Morphologies of those region are determined by the energy balance of the system. Before the discussion, we assume the same amount of liquid on each surface, and it wets the same area on each surface. This means that there are the same volume free energy. However, the liquid−gas interface area will have some difference due to surface curvature. This means that a liquid on a convex surface will have a larger surface area than that on a concave surface, which in turn has a smaller surface area than that on a flat surface. These situations imply that the amount of surface free energy on each surface is different. This fact means that the energy balance is not satisfied. Therefore, the surface area must change to obey the energy balance depending on surface curvature. As a result, the contact line on the convex surface would recede to reduce the liquid surface area, while that on the concave surface would advance to increase the area, as shown in Figure 6b, which shows the crosssectional schematics of the three-phase contact region on each surface. Consequently, following the energy balance of the system, the contact angle on a curved surface showed different values compared to the flat surface. In addition, this phenomenon might be observed when the solid surface has nanoscale curvature, whereas this effect will be negligible when the dimensions of the solid substrate is of macroscale dimensions. It is believed that the reported effect is noticeable at the nanoscale because of the relative molecular size of the wetting liquid. Indeed at the macroscale, the size of liquid molecules are many orders of magnitude smaller, hence negligible, in comparison with the scale of curvature. One last outlook from this work is the consideration of a mechanism proposed by Sefiane and Ward involving interfacial adsorption because of curvature.42 This mechanism for altering wettability on curved surfaces is worth further considerations in future works. Concerning the potential effect of line tension and the curvature of the three-phase contact line on flat surfaces, this has been extensively studied using molecular dynamics (MD) modeling techniques. Clearly the wetting behavior of nanodroplets on flat surfaces can be capture using MD. 7067
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Furthermore, it is shown that an amended Young’s equation can still accurately describe the wetting characteristics of a nanodroplet on flat uniform substrates.43
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CONCLUSIONS In the present work, surface wettability on inner and outer surfaces of single CNTs was investigated. The three-phase contact region on the outer surface was observed when the CNT tip was dipped into an ionic liquid surface and withdrawn from it. The contact region moved forward and backward on the CNT surface depending on the experiment phase and exhibited advancing and receding contact angles, respectively. Because of the capillary force, a small amount of liquid penetrated inside the CNT. The three-phase contact region in the CNT was observed using TEM. These observations allowed the quantification of contact angles on the inner and outer surfaces of the CNT and proceed with appropriate comparison. The results indicate that the contact angle on the outer surface is large, and that on the inner surface is small compared to that on a flat surface. These results indicate that the surface nanoscale curvature induces noticeable effects of wettability. This is explained in terms of the free energy balance of liquid on a convex and concave surface.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01366. SEM image and information on the effect of line tension on the nanoscale droplet and three-phase contact region on the flat surface (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(Y.Y.) Tel +81 86 251 8046; e-mail
[email protected]. jp. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by JST-CREST. We are grateful to the research laboratory for High Voltage Electron Microscopy at Kyushu University for use of the TEM.
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