Fabrication of Ketjen black-high density polyethylene superhydrophobic conductive surfaces

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Fabrication of Ketjen black-high density polyethylene superhydrophobic conductive surfaces Lie Shen

a,* ,

Hongliang Ding a, Qinghua Cao a, Weican Jia a, Wen Wang a, Qipeng Guo

b

a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China b Polymers Research Group, Institute for Frontier Materials, Deakin University, Locked Bag 2000, Geelong, Victoria 3220, Australia

A R T I C L E I N F O

A B S T R A C T

Article history:

A Ketjen black-high density polyethylene (HDPE) superhydrophobic conductive surface was

Received 15 January 2012

prepared by a single-step pressing method in which the Ketjen black was pressed into the

Accepted 10 May 2012

HDPE leaving it partially exposed to form a superhydrophobic coating with high static

Available online 18 May 2012

water contact angle (160o), low sliding angle (2.5o), and low sheet resistance (100–102 X/sq). The preparation conditions such as the pressing time and the amount of Ketjen black greatly influence the superhydrophobicity and conductivity. The simple pressing of Ketjen black onto the HDPE substrate provides hierarchically structured roughness, leading to the superhydrophobicity of the surface. The superhydrophobic conductive surface also can be obtained with other carbonaceous materials, such as other carbon blacks and nanotubes, and the superhydrophobicity is decided by whether it can produce hierarchically structure roughness.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Superhydrophobic surfaces (surfaces with water contact angle (WCA) larger than 150o and sliding angle (SA) smaller than 5o) have attracted considerable research interest over the past decade in both scientific research and industrial applications [1–3], such as self-cleaning [4], anti-sticking [5], oil–water separation [6,7]. It has been understood that surface roughness at both the micro- and nano-scaled combined with a low surface energy of the material is essential for superhydrophobicity [8–11]. On the other hand, superhydrophobic conductive surfaces [12–16] are of particular interest because of their conductivity and capability to remove static charges accumulated on the surfaces. They have potential applications in gas sensor devices, for non-wetting electromagnetic interference shielding materials and ice-preventive materials. Ketjen black is a kind of highly conductive carbon with extremely high specific surface area and branched structure. It

is widely used as carbon support in fuel cells and electrochemical capacitors [17–20]. To the best of our knowledge, no work has been reported on combining Ketjen black with a polymer to prepare superhydrophobic conductive composite coatings. Herein, we report a simple but effective approach to fabricate superhydrophobic conductive Ketjen black-high density polyethylene (KB600-HDPE) composite coatings by simply pressing a layer of Ketjen black (KB600) onto the surface of HDPE melts. The melts were infiltrated into the KB600 microchannels and pore space on the surface of the substrate under pressure. Under appropriate conditions, the composite coatings showed rough surfaces possessing both hierarchical micro- and nano-scaled structures, imparting superhydrophobicity and conductivity to the substrates. The WCA values of the composite coatings were about 160o and the SA values were about 2.5o. The surfaces were also highly conductive with a sheet resistance of 101 X/sq. The superhydrophobic

* Corresponding author: Fax: +86 571 87953712. E-mail address: [email protected] (L. Shen). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.05.018

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conductive surface also can be obtained with other carbonaceous materials, and the differences were discussed.

2.

Experimental

2.1.

Materials

Ketjen black EC-600JD and Ketjen black EC-300J were obtained from Akzo Nobel and used as received. Carbon black (N660) was supplied by Degussa. Carbon nanotubes were supplied by Shenzhen Nano Technologies Port Co., Ltd. and graphene nanoplatelets were purchased from Xiamen Knano Graphene Technology Co., Ltd. High density polyethylene (HXM50100) was purchased from Chevron Phillips Chemical Company, Texas, USA. The HDPE substrates were made into rectangular plates with a size of 35 · 25 mm and a thickness of 3.5 mm.

2.2. Preparation composite coatings

of

superhydrophobic

conductive

The procedure to prepare composite coatings with different carbonaceous fillers is the same (Table 1). The steps for fabricating KB600-HDPE composite coatings are schematically presented in Fig. 1 as an example. First, the HDPE plate was placed in a mold. Then the powder of KB600 was uniformly distributed on the surface of the HDPE plate and pressed at a pressure of 8 MPa. The mold was heated to 180 C so that the HDPE melts were infiltrated into the pore space of KB600, and KB600 particles were embedded in the coating surface. All samples were prepared by pressing at 180 C for 5–120 min. After cooling to room temperature, the samples were demolded, soaked in ethanol, ultrasonically cleaned in a sonicator (KQ-100B, Kunsan, China) for 10 min, and rinsed with ethanol to remove the KB600 which was not firmly embedded in the substrate. The sonication and rinsing procedures were repeated for at least three times until no visible KB600 appeared in ethanol after sonication. At last, the samples were dried in a vacuum oven at 60 C until they reached constant weights.

2.3.

The water contact angle and sliding angle values were measured with a DATA Physics System OCA20 instrument (Germany) at room temperature. The volume of the water droplets for WCA measurements were fixed to 4 lL. More than

Table 1 – Preparation of composite coatings with different carbonaceous fillers.

KB600

KB300 N660 Carbon nanotube Graphene nanoplatelet

ten measurements on different positions were conducted for each sample. The SA values were measured by dropping a water droplet (10 lL) onto the tilted surfaces from 4 mm height, and determined as the tilting angles at which the water droplets rolled off the surfaces. Sheet resistances were measured by the standard four-probe technique using a RTS-4 four-probe conductive meter (Guangzhou 4 Probes Tech., China). The morphologies of KB600-HDPE composite coatings were observed by field emission scanning electron microscopy (Hitachi S-4800, Japan) at an accelerating voltage of 5 kV. Prior to the observation, the samples were coated with a thin layer of gold. The surface area measurements were carried out using the Brunauer–Emmett–Teller (BET) nitrogen adsorption method (Quantachrome Instruments, USA). Transmission electron microscopy (TEM) images were obtained using a JEM-1230 instrument at an acceleration voltage of 120 kV.

3.

Characterization

Carbonaceous fillers

Fig. 1 – Schematic of the preparation of KB600-HDPE composite coatings by pressing KB600 onto HDPE substrate and TEM image of KB600.

Filler amount (g) 0.01 0.05 0.10 0.15 0.05 0.05 0.05 0.05

Pressing time (min) 5–120

5–120 5–120 5–120 5–120

Results and discussion

3.1. Superhydrophobic conductive KB600-HDPE composite coating Fig. 2 shows optical photographs of water droplets on a KB600-HDPE composite coating (0.05 g KB600). The behavior of the water droplets in Fig. 2a clearly reveals the superhydrophobic nature of the composite coating. Fig. 2b shows a uniform and black coating on the surface of HDPE substrate; it is the composite coating that imparts the superhydrophobicity to HDPE. As shown in Fig. 2c, a water droplet (4 lL) is formed at high WCA of 169 ± 2o due to the water repellency of the coating. It is impossible to measure the WCA if the water droplets are not kept bound to the dispensing needle. Beside WCA, SA is another important criterion for superhydrophobic surface; Fig. 2d shows a water droplet (10 lL) rapidly rolling off on the slightly tilted (2.5o) substrate. On the other hand, the composite coatings also exhibit excellent conductivity.

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Fig. 2 – (a) Optical photograph of a superhydrophobic HDPE rectangular plate with a dimension of about 35 · 25 · 2.5 mm. (b) Optical image of a water droplet placed on a KB600-HDPE composite coating. (c) A water droplet (4 lL) on the surface of KB600-HDPE composite coating showing a WCA of 169 ± 2o. (d) A water droplet (10 lL) rolling off the substrate showing a SA of 2.5o. Fig. 3 shows the surface and cross-sectional morphology of KB600-HDPE composite coating (0.05 g KB600). At a low magnification (Fig. 3a and b), the composite coating was quite uniform. However, at a higher magnification (Fig. 3c and e), it can be observed that the composite coating exhibits a rough surface possessing heterogeneous structures at both micro- and nano-scales. The HDPE melts were infiltrated into the microchannels or pore space that produced microscale roughness. Nanospheres appeared in each microscale region; a large number of KB600 were exposed on the substrate surface, which looks like the coral reefs in the sea, showing a coralloid morphology. Fig. 3d shows the cross-sectional morphology of the top edge region, it clearly displays the microisland and nano-protruded morphology. Such a structure dramatically increased the surface roughness. Furthermore, KB600 had a high specific surface area due to its highly branched and porous structure, leading to a high porosity and surface roughness, so that a large amount of air could be trapped at coating surface. We believe that this unique hierarchical or coralloid structure was responsible for the superhydrophobic nature of the composite coating. Fig. 3f presents a typical cross-sectional SEM image of the interface region of the KB600-HDPE composite coating, which confirms that KB600 was mechanically embedded in the HDPE matrix. During the pressure infiltration process, the HDPE melts were infiltrated into the microchannels and pore space and the KB600 was wrapped by the HDPE continuous phase, leading to a stable composite coating.

3.2.

Influence of KB600 amount and infiltration time

The morphology, superhydrophobicity, and conductivity of the composite coatings are determined by several factors

including pressure, temperature and thus the viscosity of the polymer melt, the KB600 amount and the time for infiltration. In this work, we held a constant temperature and pressure for the pressing to investigate the influence of the infiltration time and the KB600 amount. As shown in Fig. 4, at the low KB600 amount (0.01 g) the infiltration time has little influence on the WCA values of composite coatings and the WCA values is about 120o. As the KB600 amount is very low, the HDPE melts easily get to the top surface and fuse into a continuous film. When the KB600 amount is high (0.10 and 0.15 g), the influence of infiltration time is not a significant parameter. The WCA values are about 160o and the SA values are all below 5o. Because the infiltration between HDPE and KB600 is limited, the KB600 cannot be completely infiltrated into HDPE at the very high KB600 amount. At intermediate loadings (0.05 g), however, the influence of infiltration time becomes remarkable. When the infiltration time is below 40 min, the WCA values remain above 155o. However, the WCA drops rapidly to 145 ± 4o when the infiltration time is prolonged to 50 min and further decreases to 122 ± 3o at the infiltration time of 120 min. The SEM images in Fig. 5 clearly show the effect of the infiltration time on the surface morphology and superhydrophobicity of the KB600-HDPE composite coatings (0.05 g KB600). When the infiltration time is shorter than 40 min, the surface is rough and a large amount of KB600 is exposed outside the substrate surface which can trap more air at the coating surface. However, with increase of the infiltration time, HDPE occurs on the top surface and bind with KB600, leading to a decrease in surface roughness and porosity. For the infiltration time of 120 min, KB600 was closely packed and HDPE fused into a continuous flat film, leading to a decrease of WCA to 122 ± 3o. Therefore, the superhydrophobicity of the composite coatings is strongly dependent on the composition and morphology of the surfaces. Fig. 6 presents the sheet resistances of KB600-HDPE composite coatings. In general, the conductivity of all the composite coatings is high (low sheet resistance on the order of 100–102 X/sq). At any infiltration time, the more amount of KB600, the higher conductivity. The sheet resistances are only slightly decreased with the increase of infiltration time for the KB600-HDPE composite coatings. The reason is that the amount of KB600 in the composite coatings is high enough so that KB600 forms a continuous conductive network on the surface even for a short infiltration time.

3.3.

Comparison with different carbonaceous fillers

Fig. 7a shows the influence of infiltration time on the WCA values of N660, KB300, carbon nanotube and graphene nanoplatelet-HDPE composite coatings, the amount of the carbonaceous fillers are 0.05 g. when the infiltration time is below 20 min, the WCA values of KB300 and carbon nanotube beyond 150o, which exceeded the superhydrophobicity threshold. While when the infiltration time is prolonged to 30 min and further, the hydrophobicity decreases. For N660 and graphene nanoplatelet, the WCA values all below 150o during whole the infiltration time.

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Fig. 3 – SEM images of (a) the top-view surface and (b) the cross sectional morphology of KB600-HDPE composite coating at low magnifications. (c) The top-view surface morphology at a high magnification. (d) The top edge of KB600-HDPE composite coating. (e) The top-view surface morphology at a higher magnification. (f) The interface region of the composite coatings.

Fig. 4 – Influence of infiltration time on the WCA values of K B600-HDPE composite coatings with different amount of KB 600.

Table 2 show the BET specific surface area (SBET), water contact angle (WCA), sliding angle (SA), and sheet resistance

(Rs) of composite coatings with different carbonaceous fillers when the infiltration time is 10 min. The N660, KB300, KB600 are all spheres carbon black. N660 is solid with a specific surface area of only 28.5 m2/g. Ketjen black KB300 and KB600 are hollow spheres and have a specific surface area of 774.4 and 1456.3 m2/g, respectively. The difference between KB300 and KB600 is that KB600 has more branched structure, which reduced to that more air could be trapped in. The hydrophobicity of N660-HDPE composite coating is not as well as KB300 and KB600-HDPE, because the surface morphology is smooth, it is lack of hierarchically structured roughness even it posses nano-structure (the nanospheres in Fig. 8a). Fig. 8b shows the surface morphology of KB300-HDPE composite coating, which is similar to the KB600-HDPE composite coating. However, the superhydrophobicity of KB300-HDPE composite coating is somewhat worse than KB600 composite coating. We believe that the extremely high specific surface area play an important role. For KB600, more air was trapped in the pore space which reduced the contact area between droplet and coating surface. It is the reason that the KB600HDPE composite coating shows better surperhydrophobicity. We can see from Fig. 8c that the carbon nanotubes are exposed outside the substrate surface, and the curved carbon

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Fig. 5 – SEM images of the KB600-HDPE composite coatings prepared by pressure infiltration at 180 C for (a) 10 min, (b) 20 min, (c) 40 min, (d) 60 min, (e) 90 min, and (f) 120 min. The insets are the optical images of water droplets (4 lL) showing WCA values.

sheet resistances, the composite coatings tended to stabilize with the increase of infiltration time. It is because HDPE fused into a continuous flat film and the carbonaceous fillers forms a continuous conductive network on the surface.

3.4.

Fig. 6 – Influence of infiltration time on the sheet resistances of KB600-HDPE composite coatings with different amount of KB600.

nanotubes showing a carpet-like morphology. This structure leads to a high surface roughness, which is responsible for the superhydrophobicity. However, the graphene nanoplatelet-HDPE composite coating (Fig. 8d) do not show better hydrophobicity than the plain HDPE (95 ± 3o). That is because of the flat graphene nanoplatelets cannot produce hierarchically structured roughness. Fig. 7b shows the sheet resistances of N660, KB300, carbon nanotube and graphene nanoplatelet-HDPE composite coatings. When the infiltration time is below 40 min, the sheet resistances of KB300-HDPE composite coatings decrease with the increase of infiltration time. The sheet resistances stabilize at 20 X/sq when the infiltration time is prolonged to 120 min. In general, the infiltration time slightly affect the

Mechanism of superhydrophobicity

We have demonstrated the fabrication of superhydrophobic conductive KB600-HDPE composite coatings. By controlling the KB600 amount and the pressing time, the superhydrophobicity and conductivity of the composite coatings can be changed. The dramatic changes in the surface properties can be attributed to the surface morphology and the property of KB600 (mainly the branched structure and high specific surface area). The composite coatings show mainly three types of surface morphology, as shown in Fig. 9. The KB600 partially exposed outside the rough and flat HDPE substrate, contributing more significantly to the superhydrophobicity (a and b). These two morphologies are mainly existed in the KB600-HDPE composite coatings when the infiltration time is below 40 min. When the KB600 is all embedded inside the flat HDPE substrate (c), just as the infiltration time is long, the WCA value decreases remarkably. The composite coatings have abundant hierarchical micro and nano-structures, which is regarded as the critical feature of superhydrophobic surface. The composite coatings display not only microisland (composed of strings of nanospheres and HDPE melts) structure, but also rough surface of nanospheres in nanometer scale. The hierarchically structured roughness can trap air efficiently. When a water droplet sits on the top of composite coatings, the contact area of water with coatings is only the part of protruding microislands. Furthermore, the protruding microislands composed of nanospheres can further reduce the real contact area.

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Fig. 7 – Influence of infiltration time on the WCA values (a) and sheet resistances (b) of N660, KB300, carbon nanotube, graphene nanoplatelet-HDPE composite coatings.

Table 2 – BET specific surface area (SBET), water contact angle, sliding angle, and sheet resistance (Rs) of composite coatings with different carbonaceous fillers. Carbonaceous fillers

SBET (m2/g)

WCA (deg)

SA (deg)

Rs (X/sq)

Pure HDPE KB600 KB300 N660 Carbon nanotube Graphene nanoplatelet

– 1456.3 774.4 28.5 175.3 35.5

95 ± 3 163 ± 3 161 ± 3 141 ± 2 156 ± 2 126 ± 2

– 2.5 3.5 – 2.5 –

– 66.5 ± 3.5 33.4 ± 2.3 11.2 ± 0.8 42.0 ± 1.8 4.1 ± 0. 5

Fig. 8 – SEM images of the top-view surface morphology of N660 (a), KB300 (b), carbon nanotube (c) and graphene nanoplatelet-HDPE (d) composite coatings.

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[6]

[7]

[8]

Fig. 9 – Schematic illustration of the surface morphology of KB600-HDPE composite coatings with KB600: (a) partially exposed outside the rough HDPE substrate, (b) partially exposed outside the flat HDPE substrate, and (c) all embedded inside the flat HDPE substrate.

4.

Conclusions

We have demonstrated the fabrication of superhydrophobic conductive KB600-HDPE composite coatings by a single-step pressing method. Ketjen black has been used to achieve superhydrophobicity of a polymer surface for the first time. Under the appropriate conditions, the KB600 can be partially embedded on the surface of HDPE melts, offering both superhydrophobicity and high conductivity of the surface. This method is convenient, inexpensive, and easy to scale up. The composite coatings obtained through this way are useful in many applications such as electrodes for chemical sensors, biosensors, batteries, microwave absorption, catalyst carriers and so on. It also provides the new insights for utilizing Ketjen black in the field of superhydrophobicity.

[9]

[10]

[11]

[12]

[13]

[14]

[15]

Acknowledgement We appreciate the financial support from the National Natural Science Foundation of China (Grant No. 50873091).

[16]

R E F E R E N C E S

[17] [1] Li XM, Reinhoudt D, Crego-Calama M. What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces. Chem Soc Rev 2007;36(8):1350–68. [2] Erbil HY, Demirel AL, Avci Y, Mert O. Transformation of a simple plastic into a superhydrophobic surface. Science 2003;299(5611):1377–80. [3] Upadhyayula V, Gadhamshetty V. Appreciating the role of carbon nanotube composites in preventing biofouling and promoting biofilms on material surfaces in environmental engineering: a review. Biotechnol Adv 2010;28(6):802–16. [4] Parkin IP, Palgrave RG. Self-cleaning coatings. J Mater Chem 2005;15(17):1689–95. [5] Mishchenko L, Hatton B, Bahadur V, Taylor JA, Krupenkin T, Aizenberg J. Design of ice-free nanostructured surfaces based

[18]

[19]

[20]

on repulsion of impacting water droplets. ACS Nano 2010;4(12):7699–707. Lee CH, Johnson N, Drelich J, Yap YK. The performance of superhydrophobic and superoleophilic carbon nanotube meshes in water-oil filtration. Carbon 2011;49(2):669–76. Feng L, Zhang ZY, Mai ZH, Ma YM, Liu BQ, Jiang L, et al. A super-hydrophobic and super-oleophilic coating mesh film for the separation of oil and water. Angew Chem Int Edit 2004;43(15):2012–4. Jiang L, Zhao Y, Zhai J. A lotus-leaf-like superhydrophobic surface. A porous microsphere/nanofiber composite film prepared by electrohydrodynamics. Angew Chem Int Edit 2004;43(33):4338–41. Zha DA, Mei SL, Wang ZY, Li HJ, Shi ZJ, Jin ZX. Superhydrophobic polyvinylidene fluoride/graphene porous materials. Carbon 2011;49(15):5166–72. Wang KW, Hu NX, Xu G, Qi Y. Stable superhydrophobic composite coatings made from an aqueous dispersion of carbon nanotubes and a fluoropolymer. Carbon 2011;49(5):1769–74. Hsieh CT, Chen WY, Wu FL. Fabrication and superhydrophobicity of fluorinated carbon fabrics with micro/nanoscaled two-tier roughness. Carbon 2008;46(9):1218–24. Sansotera M, Navarrini W, Resnati G, Metrangolo P, Famulari A, Bianchi CL, et al. Preparation and characterization of superhydrophobic conductive fluorinated carbon blacks. Carbon 2010;48(15):4382–90. Peng M, Liao ZJ, Qi J, Zhou Z. Nonaligned carbon nanotubes partially embedded in polymer matrixes: a novel route to superhydrophobic conductive surfaces. Langmuir 2010;26(16):13572–8. Peng M, Qi J, Zhou Z, Liao ZJ, Zhu ZM, Guo HL. Carbon nanotubes noncovalently functionalized by an organicinorganic hybrid: new building blocks for constructing superhydrophobic conductive coatings. Langmuir 2010;26(16):13062–4. Peng M, Guo HL, Liao ZJ, Qi J, Zhou Z, Fang ZP, et al. Percolation-dominated superhydrophobicity and conductivity for nanocomposite coatings from the mixtures of a commercial aqueous silica sol and functionalized carbon nanotubes. J Colloid Interf Sci 2012;367(1):225–33. Sansotera M, Navarrini W, Magagnin L, Bianchi CL, Sanguineti A, Metrangolo P, et al. Hydrophobic carbonaceous materials obtained by covalent bonding of perfluorocarbon and perfluoropolyether chains. J Mater Chem 2010;20(39):8607–16. Onodera T, Suzuki S, Mizukami T, Kanzaki H. Enhancement of oxygen reduction activity with addition of carbon support for non-precious metal nitrogen doped carbon catalyst. J Power Sources 2011;196(19):7994–9. Qi J, Yan SY, Jiang Q, Liu Y, Sun GQ. Improving the activity and stability of a Pt/C electrocatalyst for direct methanol fuel cells. Carbon 2010;48(1):163–9. Bakenov Z, Taniguchi I. Physical and electrochemical properties of LiMnPO(4)/C composite cathode prepared with different conductive carbons. J Power Sources 2010;195(21SI):7445–51. Choi JY, Hsu RS, Chen ZW. Highly active porous carbonsupported nonprecious metal-n electrocatalyst for oxygen reduction reaction in PEM fuel cells. J Phys Chem C 2010;114(17):8048–53.