Reversible transformation of hydrophobicity and hydrophilicity of aligned carbon nanotube arrays and buckypapers by dry processes

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Reversible transformation of hydrophobicity and hydrophilicity of aligned carbon nanotube arrays and buckypapers by dry processes H.Z. Wang a, Z.P. Huang b, Q.J. Cai c, K. Kulkarni b, C.-L. Chen c, D. Carnahan b, Z.F. Ren a b c

a,*

Department of Physics, Boston College, Chestnut Hill, MA 02467, USA NanoLab Inc., Newton, MA 02458, USA Teledyne Scientific & Imaging, LLC, Thousand Oaks, CA 91360, USA

A R T I C L E I N F O

A B S T R A C T

Article history:

Dry treatment using a combination of UV and ozone can readily change the surface of ver-

Received 20 July 2009

tically aligned carbon nanotubes from superhydrophobic to superhydrophilic. This treat-

Accepted 27 October 2009

ment is also effective for buckypapers. Heating in a vacuum at an elevated temperature

Available online 1 November 2009

(650–750 C) can reverse the surface state from superhydrophilic to superhydrophobic. The UV & ozone treatment causes the least amount of damage to the stripe-like carbon nanotube patterns. The effect of rough surface on apparent contact angles of CNT forests was discussed to explain the origin of superhydrophilicity and superhydrophobicity.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Surface modification of carbon nanotubes (CNTs) has recently attracted a great deal of attention [1–6], because the surface characteristics considerably affect applications of CNTs in the fields of biomedical applications [7–10], biosensors [11– 13], catalysts supports [14], and composites [15–22]. Wettability of CNTs by liquids is one of the most important surface properties, which normally expressed by contact angle [4– 6,23,24]. It is believed that the contact angle of CNTs is determined by chemical composition and surface roughness [6,24– 26]. Due to the nanosized diameter of CNTs, microscopically, almost all of the pristine CNTs should inevitably provide a rough surface. Accordingly, most of the surface modifications of CNTs have been focused on tailoring the chemical compositions [1,27–29]. Normally, as-grown CNTs by chemical vapor deposition (CVD) are insoluble in most solvents, which seriously hinders their applications [28]. There are several methods to induce the transition of CNTs surface from superhydrophobic (contact angle >150) [2,4] or hydrophobic (contact angle >90) to

hydrophilic (contact angle <90) or superhydrophilic (contact angle <5) [3], such as acid treatments [28,30], microwave treatment [6], oxygen plasma etching [5], and incorporation of heteroatoms on the surface of CNTs [3,29,31,32]. However, in general, strong acid treatments can significantly make oxidative damages to the tips and sidewalls of CNTs, introduce new oxygenated groups to the CNTs [33], and decrease the electrical and thermal conductivities and mechanical strength [31]. Although the microwave treatment can modify the wettability in dry conditions, it may seriously weaken the adhesion of CNTs to the substrates on which CNTs are grown [1]. Due to the fact that most of the nanosized catalyst (Fe) particles remain in the interfacial region between the bottom of CNTs and the substrates [34,35], the microwave radiation may melt or oxidize the catalyst nanoparticles. Oxygen plasma etching may also modify the wettability of CNTs, but the hydrophobic-to-hydrophilic transition can only occur on the upper portion (top layer) of the CNTs film [5]. On the other hand, all of the above mentioned treatments are concentrated on the transition from hydrophobic to hydrophilic, only a few studies could be found regarding CNTs surface transition

* Corresponding author: Fax: +1 617 552 8478. E-mail address: [email protected] (Z.F. Ren). 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.10.041

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from hydrophilic to hydrophobic, which is required in some cases for CNT applications. Recently, ultraviolet assisted ozone (UV & ozone) treatment has been employed to functionalize single-walled CNTs [36,37] and multi-walled CNTs [38,39] , or to induce superhydrophobic to superhydrophilic transition on biomimetic nanostructured surfaces [40] and a-Fe2O3 nanoflakes film [41]. In this paper, we report for the first time the transition from superhydrophobicity to superhydrophilicity and the mechanism of vertically aligned CNTs forest by UV & ozone treatment and the reverse by heating-in-vacuum. Furthermore, the method is also applicable to buckypapers [2,42]. The controllable wettability of CNTs will significantly facilitate the fabrication of composites containing CNTs.

2.

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Experimental

A polished silicon wafer, with orientation (100), thickness 0.4 mm, p-type (boron doped), and thermal oxide thickness 2 lm on the surface, was cut into 10 · 10 mm2 pieces, followed by (1) cleaning (immersed in acetone with ultra-sonication for 5 min) and blowing dry with a nitrogen gas; (2) deposition of catalyst layers of Fe 3 nm/Al 3 nm/Fe 3 nm by sputtering; (3) growth of vertically aligned multi-walled CNTs by CVD with gases of ethylene (C2H4, 110 sccm) and hydrogen (H2, 100 sccm) at 745 C and 760 Torr for 30 min [43]. To obtain stripe-like CNTs patterns (width  100 lm and spacing  50 lm), two additional steps were needed before and after the catalyst deposition, respectively. One was the fabrication of stripe-like photoresist (Shipley 1818) pattern on cleaned silicon substrate by photolithography, and another was the removal of the photoresist with acetone. The asgrown CNTs were examined by scanning electron microscope (SEM, JEOL JSM-6340F) and transmission electron microscope (TEM, JEOL JSM-2010F). Two kinds of dry surface treatments were employed for the reversible superhydrophobic and superhydrophilic transition. In order to convert the CNT film from superhydrophobic to superhydrophilic, an UV & ozone Dry Stripper (Samco, Model UV-1) was employed with oxygen gas supply at 0.2 L/min, working at 50 C. For the transition from superhydrophilic back to superhydrophobic, a tube furnace (Thermolyne, 21100) was used to heat the samples from 500 C to 750 C for 10 min. In this study, the wettability of CNTs, which directly related to superhydrophobicity or superhydrophilicity, was revealed by contact angles (with water droplets on CNTs film) and recorded by a digital camera.

3.

Results and discussion

3.1.

Effect of rough surface on apparent contact angles

Microscopically, the top surface of vertically aligned CNTs (length  500 lm and diameter  15 nm) can be regarded as extremely rough, with many tips (ends) oriented vertically. It has been reported that microscaled roughness plays an important role on the wettability of the surface [44]. When liquid can fill into the grooves of the protrutions on the surface [45], Wenzel model [46] may be applied as the following,

cos hw a ¼ r  cos h

ð1Þ

hw a

and h are apparent contact angle on rough surface where and ideal contact angle on flat surface, respectively, and the surface roughness factor r (P1) is the ratio of actual surface area to the projected (flat) area. It is noted that the r in Eq. (1) is an amplification factor of the cosh, which can either make hw a  h in the case of a wetting surface (h < 90), or hw a  h in the case of an unwetting surface (h > 90). If the liquid can not penetrate into the grooves of the protrutions on the surface [45], it has been supposed that a droplet on the surface is supported by both the protrutions and air pocket [47]. In this case, Cassie–Baxter model [48] can be used as the following, cos hca ¼ 1 þ /s ðcos h þ 1Þ

ð2Þ

hca

and h are apparent contact angle on rough surface where and ideal contact angle on flat surface, respectively, and /s is the solid fraction on the whole supporting surface. Some researchers [49,50] pointed out that Eq. (2) should hold for substrates either superhydrophobic or very large roughness factor, whereas Eq. (1) can hold for all kinds of hydrophilic and slight hydrophobic surfaces (h just above 90) [47]. Based on theoretical analysis and experimental results, however, Bico et al. [51] believed that Wenzel model can be used in the hydrophilic regions (h < 90) and Cassie–Baxter model can be used in hydrophobic region (h > 90). In order to obtain more details from the above two classical models, a simplified rough surface was proposed in the inset in Fig. 1a, with spacing (D), height (H), and diameter (d) of the protrutions. It is assumed that each protrution locates in the center of the square region (edge length D + d). Based on these assumptions, Eq. (1) can be modified as, " # pH=d cos h ð3Þ ¼ 1 þ cos hw a ð1 þ D=dÞ2 That is; the roughness factor r ¼ 1 þ

pH=d ð1 þ D=dÞ2

ð4Þ

Supposed the ratio H/d = 5, 20, and 100, respectively, the effect of D/d ratio on the roughness factor is illustrated in Fig. 1a. It is shown that ratio H/d plays an important role on the roughness factor when the ratio D/d is less than 5. For hydrophobic surface, Eq. (2) can be rewritten as, cos hca ¼ 1 þ

pð1 þ cos hÞ 4ð1 þ D=dÞ2

ð5Þ

It is reasonable that Eq. (5) is independent of the height (H) of the protrutions, since a droplet may completely hold on the top tips of the protrutions. Fig. 1b reveals that the ideal contact angles (h = 91, 95, 100, see the inset in Fig. 1b) is insensitive to hca in all the range of the ratio (D/d). Once the ratio (D/ d) reaches more than 5, it should be easy to obtain a superhydrophobic surface no matter how much the ideal contact angle (h > 90) is. It is known that the ideal contact angle for flat graphite is h = 84–86 [52,53], which is close to the critical angle (h = 90) [54]. By surface modifications [54,55], accordingly, it is not difficult to change the surface from hydrophilic to hydrophobic. Regarding the effect of surface roughness on the apparent contact angles (see Fig. 1), it is understandable that the wetta-

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Fig. 1 – Diagrams illustrating simplified models of rough surfaces and their applications: (a) effect of D/d ratio on roughness factor r for Wenzel model, and (b) effect of D/d ratio on the value of cos hca for Cassie–Baxter model.

bility of CNT surface can be switched between superhydrophilic and superhydrophobic due to the nanoscaled diameter of CNTs.

3.2.

UV & ozone treatment for superhydrophilicity

When a drop of water (10–15 lL) was dripped on the as-grown CNTs film, due to its pristine superhydrophobic property, the water droplet can be completely supported on the surface without any wetting, with a contact angle around 158, as shown by the image (0 s) in Fig. 2a. It is shown in Fig. 2a that the UV & ozone treatment can very rapidly change the chemical composition and the CNTs surface from superhydrophobic to superhydrophilic in less than 10 s. During the UV & ozone treatment, the internally generated ozone (O3) can decompose into oxygen gas (O2) and highly reactive oxygen atom (O) under the radiation of UV (wavelength 200–300 nm) light. Simultaneously, on the surface (tips and sidewalls) of each CNT, discontinuous spots, imperfections, and dangling bonds such as –H, –OH, –COOH, @O, and @CO [27,33,38,56] are exposed to the UV, and then excited or dissociated by the UV radiation. The extremely reactive O atoms and fresh O2 molecules can readily react or combine with the defects and dangling bonds on CNTs. Moreover, it is reported that

Fig. 2 – Contact angle dependence of UV & ozone treatment time (a) and of heating-in-vacuum treatment temperature. The SEM image in (a) shows the vertically aligned CNTs and TEM image in (b) shows the diameter of the CNTs.

some undecomposed O3 molecules can directly bind to the carbon atoms on the outer shell of CNTs [36,37], which is similar to the ozonolysis treatment [2,33]. Herein, the oxidation process can quickly finish a sigmoidal wetting transition [40] on the CNTs surface. In Fig. 2a, the contact angle did not change too much at the first 6 s, which suggested that there is an incubation period for the surface oxidation. Once the surface returns to hydrophilic state (h changes from 90+ to 90) in the next 3 s (6–9 s in Fig. 2a), the apparent contact angle quickly decreases to around 5 due to the effect of rough surface. It is suggested that Wenzel model should hold on this  superhydrophilic CNT surface (hw a ¼ 5 ). In this case the roughness factor is around r1 = 11.43 by Eq. (1) with h = 85

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[52,53]. However, based on the inset in Fig. 1a, H = 500 lm, d = 15 nm (TEM image in Fig. 2b), and D = 50 nm (D + d = 30– 100 from the CNT site density 1010–1011 cm2 [57]), the roughness factor r2 = 5577.80 by Eq. (4). The extremely big difference between r1 and r2 could be explained by the following two major reasons. Firstly, there is a limit to the roughness factor in  Wenzel model, which is rmax ¼ cos hw a = cos h ¼ cos 0 = cos 85 ¼ 11:47. It is noted that r1 is already close to the rmax. When r > rmax, the roughness factor can not enhance the reduction of the hw a any more (just like r2) since the contact an gle has reached the limit (hw a ¼ 0 ). Secondly, the CNTs in the aligned arrays are very much tilted, entangled, and overlapped with each other (see the SEM image in Fig. 2a), so the effective height of CNTs should be much smaller than the value H = 500 lm which directly resulted in the huge value of r2 by Eq. (4). Regarding the D/d = 3.33 and r1 = 11.43, in Fig. 1a the effective height of CNTs should be Heff < 100d = 1500 nm. In addition, at present we are not clear about the accuracy of Wenzel model at the study of nanoscaled wettability since the model was originally obtained from microscaled characteristics of wetting.

3.3.

Heating-in-vacuum treatment for superhydrophobicity

Reversibly, a facile and controllable method on the transition was developed to modify the surface property from superhydrophilic to superhydrophobic. When the superhydrophilic CNTs film was heated in a CVD tube furnace [43] with a pressure 0.02 Torr for 10 min, at 500–650 C, the contact angle increased from 5 (superhydrophilic) to >150 (superhydrophobic). It was also observed that further heating at higher temperature (>650 C) could not significantly increase the magnitude of the apparent contact angle, supported by the fact that the angle at 650 C is very close to that at 750 C, as shown in Fig. 2b. Heating-in-vacuum at less than 500 C will not significantly affect the superhydrophilicity of the CNTs. CNTs grown by CVD are often heated in air at around 600 C to remove adsorbates from the surface [58]. This cleaning process is also a method to change the CNTs film from superhydrophobic to superhydrophilic [27]. Apart from the in situ TEM work at 1652 C [58], little literature has been found on heating CNTs in vacuum at high temperature. As shown in Fig. 2b, when the superhydrophilic sample was heated in vacuum (0.02 Torr) at 500 C for 10 min, the apparent contact angle increased from 5 to 18. It is suggested that the heating can evaporate most physically adsorbed molecules, such as O, O2, O3 (from UV & ozone treatment), H2O and CO2 from air, which causes the contact angle increases. When heating at 600–750 C for 10 min, it was reported that the chemically bonded O3 can be removed [36], and most organic composition be burned out. It is supposed that the ozone removal is one of the dominant processes in the increment of contact angle at the heating-in-vacuum. Once the heating makes the contact angle (h) passing the critical value (h = 90 [54]), Cassie–Baxter model can be used to reveal the effect of rough surface. As illustrated in Fig. 1b, at D/d = 3.33, the rough surface can significantly reduce the value of cos hca and produce a superhydrophobic surface regardless of how much the h passing through the critical value (90).

3.4.

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Treatments for buckypapers

In addition, it was also found that the UV & ozone treatment is also effective to modify buckypapers from superhydrophobic to superhydrophilic. When the buckypaper (20 mm in diameter, and 100 lm in thickness) was treated by UV & ozone for 5 min (5 min instead of 10 s is explained below) at 50 C, superhydrophilic surface was achieved. As shown in Fig. 3a, a water droplet can completely spread on the whole top surface of the buckypaper with a contact angle <5 (superhydrophilic). When the treated buckypaper is put on water, it rapidly sinks into water owing to its excellent wettability, as shown in Fig. 3b. It is known that the surface of buckypaper (Fig. 4a) has much fewer tips (ends) than that of the as-grown CNTs film (Fig. 4b), and the tips of CNTs are very reactive and easy to be modified due to the high density of dangling bonds and defects. Accordingly, it is reasonable that the surface modification of buckypapers is much more difficult than that of the asgrown CNTs film. That’s why it takes 5 min (rather than 10 s in Fig. 2a) for UV & ozone treatment to complete the superhydrophobic to superhydrophilic transition for buckypaper (Fig. 3a). However, the above heating-in-vacuum operation can not fulfill the surface modification of buckypapers from superhydrophilic to superhydrophobic at 745 C for 30 min. It is interesting to find that a CVD CNTs growth operation (C2H4  110 sccm, H2  100 sccm, 745 C, and 760 Torr) [43] for 10 min can fulfill the reverse transformation from superhydrophilic to superhydrophobic, as shown in Fig. 3c. This phenomenon can be explained by the following two reasons. One is that the 5 min UV & ozone treatment may have introduced much more structural defects on the CNTs walls than the 10 s operation, and X-ray photoelectron spectroscopy (XPS) and Raman spectra confirmed that UV & ozone treatment could functionalize CNTs surface more aggressively than the microwave irradiation [39]. Another reason is that the mending of structural defects on CNTs surface. With enough carbon source provided by thermal CVD operation at 745 C, it is suggested that some carbon atoms, which decomposed from ethylene (C2H4), can fix the defect sites by covalent bonding to restore the original surface state of the CNTs. It is noted that such a growth procedure does not yield any new CNTs on buckypapers since there is no catalyst. If the treated buckypaper was put into the bottom of the water, it can immediately float up to the water surface (Fig. 3d) due to its superhydrophobicity.

3.5.

Applications of UV & ozone treatment

Apart from being facile, rapid, and dry, the UV & ozone treatment is nondestructive. For a stripe-like CNTs pattern with width of 100 lm, spacing 50 lm, and depth of trench (length of CNTs) 300 lm (see Fig. 5a), UV & ozone treatment can readily modify the pristine CNTs from superhydrophobic to superhydrophilic, and keep the pattern intact without contamination. As a comparison, Fig. 5b shows that the liquid treatment (soaked into acid solution, rinsed by water, and then dried in air) resulted in (1) collapse or tilt of some CNTs fins; (2) breakdown of a few CNTs fins (inset in Fig. 5b); and

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Fig. 3 – Wettability of buckypapers. (a) Hydrophilic: water spreading on buckypaper, (b) hydrophilic: buckypaper sinking into water, (c) hydrophobic: a water droplet on buckypaper, (d) hydrophobic: buckypaper floating on water.

Fig. 4 – SEM surface micrographs of CNTs. (a) as-prepared buckypapers, (b) as-grown CNTs films (top view).

(3) contaminations (many debris in a trench in Fig. 5b). It is believed that the acid treatment will inevitably cause imperfections to the CNTs pattern.

Fig. 5 – SEM micrographs of stripe-like CNTs patterns after superhydrophobic-to-superhydrophilic treatments. (a) UV & ozone treatment (50 C for 4 min); and (b) acid treatment (immerged in HNO3 26% for 5 min, rinsed with deionized water, and dried in air).

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For large area CNTs film (without pattern), UV & ozone treatment is also better, proved by the results shown in Fig. 6. The same CNTs samples were treated by baking-in-air (600 C for 30 min, Fig. 6a), acid treatment (immerged into 2% HCl solution for 9 min, and then rinsed by deionized water, Fig. 6b), and UV

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& ozone treatment (50 C for 4 min, Fig. 6c). After the superhydrophilic treatments, all of the samples were dripped with a few water drops from the top surface till the sample is soaked with water, and then dried in air. SEM micrographs shown in Fig. 6 revealed the morphologies of the dried CNTs films. Due to the effect of surface tension of water during the drying, a lot of CNTs were peeled off the substrate and aggregated in separate domains, which resulted in many huge cracks shown in Fig. 6a. It is proposed that the baking-in-air operation damaged the weak adhesion of CNTs to the substrate, and made most of CNTs easily lift off. As for the acid treatment, Fig. 6b shows a few small cracks. For UV & ozone treated samples, Fig. 5c displays that the whole surface was preserved much better than Fig. 6a and b with very limited cracks.

4.

Conclusions

Under UV radiation at 50 C, ozone (O3) and its decomposed components (O2, O) vigorously react with CNTs by oxidation and ozonolysis in dry condition, and rapidly transform the surface state of CNTs from superhydrophobicity to superhydrophilicity. This treatment also works for buckypapers. A reverse transformation, from superhydrophilic to superhydrophobic, was also achieved by heating the CNTs in vacuum at high temperature. It is believed that the rough surface of CNT forest plays a very important role in all the surface wettability transitions. However, buckypaper needs CVD CNTs growth conditions to reverse its transformation from superhydrophilicity to superhydrophobicity due to the requirement of mending the structural defects on CNT walls. Compared with acid treatment and baking-in-air method, UV & ozone treatment is much better in the sense that it is a facile, fast, clean, and nondestructive.

Acknowledgment This research is sponsored by DARPA/MTO under contract #N66001-08-C-2009 with Dr. Tom Kenny as the program manager. The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense.

R E F E R E N C E S

Fig. 6 – SEM micrographs of CNTs film after different superhydrophobic to superhydrophilic treatments. (a) baked in air at 600 C for 30 min, dripped with water drops on the surface, and then dried in air; (b) acid treatment (immerged into HCl 2% for 9 min), rinsed with deionized water, and then dried in air; (c) UV & ozone treatment for 4 min, dripped with water drops on the surface, and then dried in air.

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