A facile single-step fabrication of robust super-hydrophobic carbon

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Materials and Interfaces

A facile single-step fabrication of robust super-hydrophobic carbon nanotube films on different porous supports Hang Yin, Xinbo Wang, Guan Sheng, Wei Chen, and Zhiping Lai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05212 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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Industrial & Engineering Chemistry Research

A facile single-step fabrication of robust super-hydrophobic carbon nanotube films on different porous supports

Hang Yina,†, Xinbo Wanga,†, Guan Shenga, Wei Chenb, Zhiping Laia,*

a

Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal, Saudi

Arabia b

CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute,

Chinese Academy of Sciences, Shanghai, China

* Corresponding author e-mail: [email protected]

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

Super-hydrophobic carbon nanotube (CNT) films have attracted extensive research attentions because of the superior CNT properties that have a significant impact on mass/energy transfer at the CNT/liquid interface. Until now, most reported methods for producing super-hydrophobic CNT films require multiple steps, making the fabrication process very complicated and time-consuming, and may deteriorate the CNT intrinsic properties. In this study, superhydrophobic CNT films composed of short CNT strands were fabricated through a single-step chemical vapour deposition (CVD) method on various types of supports, including hollow fibre and disc supports. Attributed to its firm structure, this durable super-hydrophobic film could withstand water invasion and show promising stability and renewability during a 4-week wettability test. Benefiting from the preservation of CNT properties without post modifications, the film exhibited excellent chemical robustness to corrosive liquids from pH = 0 – 14. In addition, the application of the super-hydrophobicity was demonstrated for effective oil/water separation.


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1. Introduction Carbon nanotubes (CNTs) have attracted sustained and wide research interest since their discovery in 1991 [1] because of their unique folded graphene structures, which exhibit striking mechanical, electrical and chemical properties. CNTs offer excellent performance in many applications, such as batteries, supercapacitors, drug carriers, catalyst supports and separation films [2-5]. The wettability of the CNT layer plays a very important role during mass/energy transfer at the interface between the CNT film and the surrounding environment. Thus, the wettability of the CNT layer and its potential to prepare super-hydrophobic films for applications, such as membrane distillation and oil/water separation, have also received increasing attentions recently [6-16], particularly because of the capability of CNT films to perform in harsh chemical environments [17-19].

However, due to the inherent low hydrophobicity of graphite, the water contact angle (CA) of pure CNT falls in the range of 84-95°, while for a surface to be considered super-hydrophobic, this angle should be greater than 150° [9, 20]. To achieve a robust super-hydrophobic CNT film, almost all reported methods to date require at least two steps: artificializing the surface structure, followed by functionalizing the CNTs with hydrophobic chemicals [10, 11, 21]. Generally, the artificial structures are fabricated by either spray-coating ready-made CNTs or directly growing aligned CNT forests with a micro/nanoscale hierarchical appearance. Meanwhile, the functionalized hydrophobic chemicals are mainly fluorinated chemicals or polymers, such as polytetrafluoroethylene (PTFE) [7, 10-17]. However, the multistep fabrication approaches face several practical problems. Apart from being procedurally complex and time consuming [9], many of the fabricated CNT film structures are unstable due to their water imbuing effect, leading to the irreversible loss of super-hydrophobicity [10, 22]. Although post chemical modifications can enhance superhydrophobicity and, in some cases, structural stability as well, these modifications will also deteriorate many of the abovementioned superior CNT intrinsic properties and may cause secondary contamination [17]. Some of the superhydrophobic CNT films were fabricated without post-functionalization step [9, 12, 18, 19]. The super-hydrophobicity of these CNT films mainly attributed to their hierarchical structures and the specific micro-patterns of the CNT pillars, which were achieved by changing the gas flows during chemical vapour deposition (CVD) process [9] or depositing the catalysts in desired patterns through specially designed accessories [12], respectively. These fabrication processes, however, requires complex experimental producers, and more importantly, they are only applicable to the specific



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supports. Hence, a more practical method is highly demanded for the widespread applications of the superhydrophobic CNT films.

Here, we report a single-step fabrication of robust CNT films on different porous supports to have durable superhydrophobicity. Apart from its water repellency behaviour, the CNT film features the low aspect ratio of large strand nanotubes (diameter ~ 200 nm) with a short length (~ 2 m). Rigid carbon nanotubes were found to be strong enough to avoid structural degradation from capillary induced coalescence and, consequently to stabilize the air pockets between the water/CNT interfaces, resulting in promising stability during a 4-week long test. Additionally, the superhydrophobicity was able to be maintained over a wide pH range from 0 to 14 by preserving the CNT properties without post surface functionalization. Oil/water separation was performed with a super-hydrophobic CNT film fabricated on a commercial AAO support, and the mixture could be effectively separated.

2. Experimental 2.1 Materials CR-6 α-alumina powder was purchased from Baikowski. Yttrium-stabilized zirconia (YSZ) powder was purchased from Inframat Advanced Materials Co. AAO supports with a pore size of 20 nm were purchased from Whatman®. The nickel target in the sputtering process was purchased from Plasmaterials, Inc. Dichloromethane (DCM), methylene blue (MB), toluene and Span 80 were purchased from Sigma-Aldrich. Hydrogen, argon and acetylene were ordered from Air Liquide. All chemicals were analytical grade and used without further treatment.

2.2 Preparation of supports Home-made porous alumina disc supports were prepared by hydraulic pressing, followed by sintering with CR-6 αalumina powder as previously reported [22]. Home-made YSZ hollow fibre supports were prepared through phaseinversion spinning and sintering with YSZ nanoparticles of 30-60 nm as previously reported [23].

2.3 Fabrication of CNT Films


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In a typical process, a thin layer Ni film was first deposited onto the top surface of the disc supports or the outer surface of the hollow fibre supports under rotating plasma magnetic sputtering under optimized conditions. Then, the Ni catalysts were reduced at the target temperature under a hydrogen environment. Subsequently, the CNT film was grown through CVD by introducing hydrogen and acetylene at a volume ratio of 1:10 into the sealed CVD chamber. Lastly, the as-fabricated samples were collected and dried in an oven at 65 °C for 24 hours. The key fabrication parameters of the CNT films are illustrated in Table 1. Table 1 Fabrication conditions of the CNT films Conditions Sputtering time (min)

This work 2

Reference 1

Temperature (°C) / duration (min) in reduction process

700 / 60

600 / -

Total gas flow rate (ml/min) in CVD growth process

335

220

Temperature (°C) / Duration (min) in CVD growth process

700 / 2

650 / 4

2.4 Oil/water separation The oil/water mixture was prepared by mixing DI water (labelled by MB) and dichloromethane (DCM). Water/oil emulsions were prepared by mixing water, toluene and Span 80 at a mass ratio of 1 : 100 : 0.2 and vigorously stirring for 20 hours. A customized separation device was employed, and filtration was conducted under the driving force of gravity or an approximate vacuum of 0.05 bar.

2.5 Characterization methods SEM images were collected from a Zeiss Merlin scanning electron microscope. TEM images were obtained from a Titan ST microscope (FEI Co.). Raman spectroscopy measurements were conducted on a Horiba Aramis confocal microprobe Raman instrument with a He-Ne laser of λ = 632.8 nm. Wettability was measured with the EasyDrop contact angle measurement instrument under ambient atmosphere with a water volume of 5 µL. Optical microscopy images were taken on a Nikon Ti-S* system. The water content of the oil/water mixture was determined using a Mettler Toledo C30 Karl Fischer Titrator.

3. Results and Discussion 3.1 CNT films supported on home-made disc supports



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Figure. 1 shows the CNT film fabricated on a home-made porous alumina disc through the CVD method with all of the key fabrication parameters listed in Table 1. The employed porous alumina disc has a diameter of 22 mm and thickness of approximately 2 mm. From the top view SEM image of the bare disc shown in Figure. 1(a), it is observed that the support is made of -alumina powder that forms interstitial gaps with an average pore size of 200 nm and porosity of 40%. Nickel was used as a catalyst to grow carbon nanotubes and was introduced by magnetic sputtering coating on top of the alumina disc. As shown in Figure. 1(b), numbers of small nickel protrusions were deposited and partially covered the porous surfaces of the alumina disc after sputtering. The nickel coated alumina disc was then reduced at 700 °C for 1 hour under hydrogen atmosphere. The SEM image shown in Figure. 1(c) indicates that the dispersed nickel protrusions fused together and formed evenly distributed islands with a size of approximately 200 nm during the reduction. This nickel agglomeration was attributed to the increased mobility of nickel at high temperature during the reduction process [24, 25]. Figure. 1(d) and (e) show top-view and cross-section SEM images of the CNT film after the CVD process. It is clearly observed that the nickel particles were lifted by the growing carbon nanotubes, whereas these nanotubes were slightly curved at the tip, forming a lawn-like film structure. The thickness of the film was approximately 2 µm, and the diameters of the carbon nanotubes were approximately 200 nm, resulting in a small aspect ratio. The diameters of the CNTs were observed to be about the same size as the nickel particles because the CNT dimension is primarily determined by the catalyst size, a concept that has been widely accepted [25-27]. Figure. 1(f) shows a TEM image of an individual CNT. As expected, the CNT had an inner channel of approximately 12 nm surrounded by multiple graphite layers. From the inserted magnified TEM image, the interlayer distance was measured to be approximately 0.35 nm, which matches the reported value for multi-wall carbon nanotubes. The Raman spectrum shown in Figure. S1 shows all of the characteristic D, G and D’ bands of carbon nanotubes, centred at 1320 cm-1, 1587 cm-1 and 1610 cm-1, respectively. The integrated intensity ratio Id/Ig was 1.04, indicating a high graphitization degree of the CNT film.


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(a)

(b)

(c)

(d)

(e)

(f)

Figure. 1 SEM images of (a) the bare alumina support; (b) Ni deposited support after sputtering; (c) distribution of the Ni islands on the support after reduction; (d) top surface, and (e) cross section of the CNT film; (f) TEM image of the CNT

3.2 Wettability properties of the CNT film surface The wettability of the CNT films was investigated by measuring water contact angles (CAs) under ambient atmosphere. As shown in Figure. 2(a), a perfect spherical water droplet with initial CA over 165° on the CNT film was observed. This super-hydrophobicity was well maintained for over 20 minutes, with a CA of 166.0° at 2 minutes, 156.3° at 15 minutes and 155.1° at 20 minutes, even though the water droplet became smaller due to evaporation during the test.



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The super-hydrophobicity of the CNT film was attributed to the hierarchical structure of the rough CNT surfaces, as shown in Figure. 1 (d) and (e), enabling the initial formation of air pockets when a water droplet started to contact the CNT layer, meanwhile the trapped air between the water and CNTs interfaces lifted the water from underneath, which is referred to as the lotus-leaf effect. The super-hydrophobicity of this CNT film was first found to be applicable to different sizes, including very small water droplets, as shown in Figure. 2(b). Apart from that, weak water hysteresis was identified because the water did not stick to the film surface but rather rolled off when the droplet was deformed upon exposure to the pressure between the needle and lifting support, as shown in Figure. S2. Dynamic water bouncing behaviour was observed when water was dropped onto the CNT film surface with a tilted angle, as shown in Figure. S3, which depicts a water droplet bouncing up and rolling away after contacting the film surface. In light of the large CA and its slippery effect, we classified the water repellent surface of this CNT surface as the CassieBaxter mode, which often applies to rough surfaces that are composed of micro- and nanoscale hierarchical structures [28]. By substituting the reported inherent CA of CNT (93º) [6] and measured CA (163.5º) of this CNT surface into the Cassie-Baxter equation, as shown below, we calculated the fraction of the water/air contact area to be 95.7%, suggesting a large fraction of the air pocket was trapped between the water and CNTs. cos 𝜃𝑟 = (1 − ∅) × cos θ − ∅ where 𝜃𝑟 and θ are the apparent CA and inherent CA of CNT, respectively; 𝜙 and 1- 𝜙 represent the fractions of the water/air and water/solid contact areas, respectively.

(b)

(a)

Figure. 2 (a) Water CAs of the CNT films tested over 20 minutes; (b) photo of different size water droplets standing on the CNT film.

As shown in Figure. 3(a), the CAs were all over 150° over the full pH range of 0-14, indicating the excellent chemical robustness of the CNT film to acid and alkaline liquids, which is attributed to the preservation of the CNT properties without post chemical modifications during the film fabrication process. Moreover, a long-term stability test was performed over a 4-week period, and super-hydrophobicity was well maintained with a CA of over 155° throughout


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the entire period, as shown in Figure. 3(b). The measured liquid entry pressure (LEP) of water through the CNT film was about 0.5 bar. The super-hydrophobicity of the penetrated film could be regenerated after drying, confirming the excellent structural stability of the CNT film.

(a)

(b)

Figure. 3 (a) Super-hydrophobicity of the CNT film over a wide pH range of 0-14 and (b) during a long-term test over a period of 4 weeks

To understand why the CNT film fabricated in this study shows durable super-hydrophobicity, while those reported in the literature using similar CVD processes cannot do the same without post chemical modification [10, 21, 29], a reference CNT film was prepared under conditions similar to those in other reports [10], as listed in Table 1. This reference sample was firstly observed to have a CNT film structure with a high aspect ratio that is believed to promote hydrophobicity [10]. As shown in Figure. 4(a), the heights and dimensions of the CNTs were over 10 µm and less than 100 nm, respectively, while the inter-fibers’ gaps was approximately 0.5 µm. The yield of these thinner and longer CNTs in the reference sample was attributed to the shorter catalyst sputtering duration and lower catalyst reduction temperature but longer CVD growth duration [27] compared to that of the CNT film shown in this work. The wettability of this reference CNT film surface is presented in Figure. 4(b). Unlike the rigid and durable superhydrophobicity shown in Figure. 2(a), an initial contact angle of 157.4° was observed, but it reduced rapidly to approximately 128.2° over 3 minutes. Tuteja et al. defined a dimensionless robustness parameter of A* for correlating the structure robustness with the surface texture parameters [30]. Specifically, a large A* value (>>1) indicates the better robustness of the film and it could be calculated from the equations as below:



𝐴∗ =

𝑅 𝑙𝑐𝑎𝑝

(1−cos 𝜃)

𝐷2

(1+2(𝐷) sin 𝜃)

𝑅

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, where 𝑙𝑐𝑎𝑝 = √𝛾𝑙𝑣 /𝜌𝑔 ,

in which R, D, lcap , γlv ,ρ and g represent the fiber radius, half of the inter-fiber gaps, capillary length of the fluid, the surface tension of the liquid phase, the liquid density and the gravity acceleration, respectively. The structure robustness of the reference and our CNT films were quantitatively analysed and the A* values was found about 3000 and 6500, respectively, suggesting the theoretical structure robustness of both films. However, the reference CNT film though initially exhibited super-hydrophobic behaviour and also owned theoretical structure robustness regarding of the surface texture parameters, as analysed in Figure. 4(c), the flexible and weak CNT bundles with high aspect ratios were unable to provide sufficient mechanical strength against the coalescence induced by the capillary force of water that primarily seeped into the film structure. As a result, the air-pockets were expelled from the voids between the CNTs by the invaded water, and consequently, more water started to penetrate into the spaces in the film structure, ending with an irreversible loss of hydrophobicity because of the successive bundle aggregations and structure deformation after the water fully spreading on the support [29]. This phenomenon has been observed not only for these CNT films but also for many nanoarrays formed with different materials, such as silicon nanorod arrays and polyacrylonitrile nanofiber arrays [31-33], for which the nanostructure size and aspect ratio of the individual fibres are believed to have significant effects on the structure deformation process. By contrast, as shown in Figure. 4(d), the structure robustness and durable CNT film with a low aspect ratio and structure robustness carbon nanotubes fabricated in our study was strong enough to avoid the initial structure degradation from primary invaded water and effectively maintain the air pockets between the CNTs, thus obstructing further water penetration and capillaryinduced coalescence in the film structure. Based on these results, we conclude that the individual fibre morphology and, as a result, the mechanical strength, in addition to the surface hydrophobicity [10], fibre density [9] and the film texture structure [30], also play the significant roles in the stability of the nanostructure arrays and films’ superhydrophobicity.


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(a)

(b)

(c)

(d)

Figure. 4 (a) SEM image of the reference CNT film; (b) water CAs of the reference CNT film. Schematic diagrams of (c) the dynamic wetting process of the unstable reference CNT film, and (d) the durability of the super-hydrophobic CNT film

3.3 CNT films fabricated on different supports Attractively, this single-step fabrication procedure can be easily extended to other types of supports. Figure. 5 shows CNT films prepared on a YSZ hollow fibre support and a commercial AAO support following a similar procedure. The average pore size of the YSZ hollow fibre is approximately 100 nm, with a porosity of 40%, while those characteristics of the AAO support are approximately 20 nm and 50%, respectively. As shown in Figure. 5(a), the super-hydrophobicity repelled water from direct contact with the submerged CNT film on the YSZ hollow fibre supports, yielding a very clear gas interlayer between water and the black CNT layer with a measured CA of 146 º. Similarly, the upper image shown in Figure. 5(b) reveals that many water droplets were statically standing on the CNT film fabricated on the AAO support with measured CA of 156º. A gas interlayer (light reflection area) is also observed between the CNT film and water in the lower image of Figure. 5(b). Herein, the debris rather than the whole CNT



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film on the AAO support was employed to better observe the gas interlayer under the water. Similar to Figure. 1(e), the analogous CNT film structures were duplicated on both the YSZ hollow fibre and AAO supports, as shown in Figure. 5(c) and (d).

(a)

(b)

(c)

(d)

Figure. 5 Water repellence ability of (a) the YSZ hollow fibre-supported CNT film and (b) the commercial AAO supported CNT film; (c) and (d) are the SEM images of the CNT films on a YSZ hollow fibre support and commercial AAO support, respectively.

The CNT film supported on AAO support was found highly permeable that could be used for oil/water separation under gravity force. As shown in Figure. 6(a), methylene blue labelled water and dichloromethane (DCM) were mixed at a volume ratio of 1:1, and the DCM layer settled at the bottom of the mixture due to its higher density. This mixture


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was then poured into the devices fitted with supported CNT film, and shortly thereafter, DCM started to penetrate through the AAO supported CNT film with a flux of 425 L·m-2·h-1·bar-1. After a while, colourless DCM with a purity of 99.7% was collected at the permeation side, while blue water was retained at the feed side due to the superhydrophobicity of the CNT film. Figure. 6(b) shows the performance of the CNT film on AAO support for removing water from a water/toluene emulsion. Before the filtration process, a large amount of stabilized water spheres were found to be well dispersed in toluene under optical microscopy, resulting in a cloudy appearance. By contrast, after the emulsion was passed through the CNT film at a flux of 334 L·m-2·h-1·bar-1, a transparent liquid of over 99.5% pure toluene was collected, and only small water residues were observed under the microscope, whereas most of the stabilized water was retained. The oil flux through the AAO supported CNT film was well maintained at around 300 L·m-2·h-1·bar-1 with consistent toluene purity of 99.5% after 20 runs of oil/water separation experiment, as shown in Figure. 7. The stable performance of the oil/water separation is consistent with long term stability of the superhydrophobicity discussed in Figure.3 (b).



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Figure. 6 Oil/water separation with the super-hydrophobic CNT film fabricated on an AAO support. (a) Water / Dichloromethane Mixture Separation; (b) Water / Toluene Emulsion Separation

Figure. 7 Oil/water separation performance in 20 runs.

4. Conclusions In this study, robust super-hydrophobic CNT films were fabricated through a single-step CVD method on different types of supports. Owing to the rigid and firm individual CNT strands, the structure of the fabricated film gained enhanced mechanical strength, which effectively prevented water capillary induced coalescence, resulting in durable and stable super-hydrophobicity. The CNT films have also exhibited excellent chemical resistance to corrosive etching under a wide pH range of 0-14, which is attributed to the preservation of the excellent inherent properties of the CNTs without post chemical modification. This robust and durable film structure was successfully duplicated onto different porous supports including commercial AAO supports, home-made -alumina discs and YSZ hollow fibers. The CNT film on AAO support showed excellent oil/water separation under gravity force with the oil flux as high as 334 L·m2

·h-1·bar-1, purity more than 99.5% and durability of more than 4 weeks. Although the home-made -alumina discs

and YSZ hollow fibers are not suitable for oil/water application due to lower liquid permeability, they have potentials to be used in applications such as gas separation [18], water desalination [6] and de-icing [34]. Hence in conclusion, the single-step fabrication method reported in this study provides a simple and practical solution for making durable super-hydrophobic CNT films for various applications with significant interfacial phenomena.

ASSOCIATE CONTENT 14 ACS Paragon Plus Environment

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Supporting Information Raman Spectra of the robust and durable super-hydrophobic carbon nanotube film; the images of dynamic process of a water droplet contacting with and removing away from the super-hydrophobic carbon nanotube film; and the bouncing behaviour of the water droplet on the super-hydrophobic carbon nanotube film

AUTHOR INFORMATION Author contributions † These authors contribute equally to this work

ORCID Zhiping Lai: 0000-0001-9555-6009 Hang Yin: 0000-0002-2061-8911 Xinbo Wang: 0000-0001-9607-1396

Funding Sources This work was supported by the financial support of funding baseline BAS/1/1375 from King Abdullah University of Science and Technology.

Competing financial interests The authors declare no competing financial interest.

Notes The new address for Dr. H. Yin is MacDiarmid Institute for Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University, Palmerston North 4442, New Zealand.

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A facile single-step fabrication of robust super-hydrophobic carbon

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