silicon carbide nanowire nanocomposites

Materials and Design 87 (2015) 198–204

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Superhydrophobic carbon nanotube/silicon carbide nanowire nanocomposites Hailing Yu a,b, Jiaqi Zhu a,⁎, Lei Yang a, Bing Dai a, Larysa Baraban b, Gianaurelio Cuniberti b, Jiecai Han a a b

Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, PR China Institute for Materials Science and Max Bergmann Center of Biomaterials, Dresden University of Technology, Dresden 01069, Germany

a r t i c l e

i n f o

Article history: Received 17 April 2015 Received in revised form 1 August 2015 Accepted 4 August 2015 Available online 11 August 2015 Keywords: Composite film Superhydrophobic Self-cleaning Pillar structure model

a b s t r a c t The composite film of carbon nanotubes and silicon carbide nanowires was synthesized directly on the silicon substrate by the catalyst-assisted method. The carbon nanotubes crimped together decorated with silicon carbide nanowires covering the whole substrate. The appropriate amount of aluminum powders is a crucial factor to achieve the composite film. The composite film exhibited excellent intrinsic superhydrophobicity without any further functionalization. By using the nano/micropillar composite structure model, the presence of silicon carbide nanowires is found to be the key factor that results in the superhydrophobicity of the films. The feasible synthesis of the superhydrophobic coating could have potential application in water-repelling devices, like biochemical sensors and microfluidic systems. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction The superhydrophobic surface of lotus leaves with a micro/nanoscale hierarchical surface structure has attracted a lot of scientists' interest [1–4], because the superhydrophobicity has a large application potential in opto-electronics, biochemical sensors, and microfluidic systems. In order to meet the requirement in a wide range of application, from window glass and cement to textiles, the demand for the surface is not only superhydrophobicity but also self-cleaning property [5]. Self-cleaning property means that the surface is superhydrophobic and the droplets can be easily removed by slightly tilting the surface to take the dirt away [2]. As a commercial product, the rapidly developed technology of self-cleaning coatings has been divided into two categories: chemical functional surface [6] and artificial superhydrophobic surface [7,8], both of which can increase the hydrophobicity of the coating. High surface roughness can increase the amount of airtrapping pores [9], while chemical coating lowers the free energy [10]. One-dimensional nanomaterials with high surface area and unique morphology make them particular appealing for fabrication of selfcleaning coating applications with high efficiency [11–13]. The unique morphology of one-dimensional nanomaterials enhances the roughness of the surface and increases the amount of the air-trapping pores. As have been demonstrated previously, the surface coated with 1D nanostructure e.g. aligned nanotubes [14], nanofibers [15], nanorods [16] and nanowires [17], has exhibited excellent superhydrophobicity, with a CA larger than 150°. A vertically aligned carbon nanotube forest ⁎ Corresponding author at: Post-Box 3010, YiKuang Street 2, Harbin 150080, PR China. E-mail address: [email protected] (J. Zhu).

http://dx.doi.org/10.1016/j.matdes.2015.08.025 0264-1275/© 2015 Elsevier Ltd. All rights reserved.

functionalized by poly(tetrafluoroethylene) with a superhydrophobic surface using the nanoscale roughness has been reported by Lau and co-authors [18]. The aligned silicon carbide nanowires also show excellent superhydrophobic property after functionalized with perfluoroalkysilane [19]. Thus, the specific chemical functionalization is a crucial factor to achieve superhydrophobicity [20–25], since the chemical coating decreases the free energy. However, such strategy inevitably increases the cost of the production, makes the coating process more complex and is degradable with time. Since the observation of carbon nanotubes by Iijima in 1991 [26], the synthesis and application of this type of 1D nanostructure have been studied extensively. Because of potential applications in high temperature, high power, and high frequency conditions [27,28], SiC nanowires have attracted much attention and can be synthesized by various methods such as arc discharge [29], laser ablation [30], vapor–liquid– solid reaction [31], and hydrothermal method [32]. Herein, we focus on the combination of the structure of carbon nanotubes and SiC nanowires. This combination has never been reported, i.e., synthesis of nanocomposite film of carbon nanotubes and silicon carbide nanowires which may lead to novel nanomaterials with important application in nanotechnology. Here we present a feasible method to fabricate nanocomposite film with superhydrophobic properties, consisting of the carbon nanotubes/silicon carbide nanowires, directly on silicon substrate that does not require any further functionalization. Nanocomposite film of carbon nanotubes (CNTs) and silicon carbide nanowires (SiCNWs) is obtained by using nickel as a catalyst to assist the growth. Hydrophobic properties of the film were determined, measuring a contact angle (CA) directly at the surface. Obtained results substantially exceed the

H. Yu et al. / Materials and Design 87 (2015) 198–204

hydrophobicity of the e.g. pure CNTs films (120 ± 2°) and pure silicon substrate (86 ± 2°). The superhydrophobicity of the composite film can be attributed to the presence of SiCNWs as revealed by pillar structure model. 2. Experimental method The composite films of CNTs and SiCNWs were produced by vapor– liquid–solid process. The silicon substrates were coated by nickel and amorphous carbon films in the high vacuum magnetron sputtering system. Then the samples were put into a tube furnace and heated with aluminum powders at the temperature of 1000 °C for 3 h. The amount of aluminum powders is 0.01 g, 0.02 g, and 0.04 g, respectively. The atmosphere pressure was 0.001 Pa and the samples were naturally cooled to room temperature in the furnace. Finally, large quantities of nanostructures were obtained directly on the silicon substrate. The pure carbon nanotubes were obtained using nickel and amorphous carbon as precursors and heated in 1000 °C for 6 h, then cooled to room temperature in the furnace. The morphology and microstructure of the composite film were studied in detail by electron microscope techniques including transmission electron microscope (Tecnai G2 F30, operated at 300 kV) equipped with an X-ray energy dispersive spectrometer (EDS) and scanning electron microscopy (HELIOS Nanolab 660i, operated at 20 kV). The samples for TEM were dispersed using ultrasonic dispersion. The water contact angles were measured with 6 μL of deionized water by a contact angle system (OCA 20, Dataphysics, Germany). All of the CAs were measured in ambient atmosphere at room temperature. The value of the CA presented in this paper is an average value after measuring five spots in one surface. 3. Results and discussion 3.1. Structure and morphology characterization Fig. 1(a) is the SEM image of the sample obtained by adding 0.02 g aluminum powders, and it shows that a film composed of much disordered nanowires cover the whole surface. The sample consists of two different wire-like nanostructures, as shown in Fig. 1. One is the curl nanostructure and the other is the straight nanowire. It is observed that the curl nanowires crimped seriously in the images, while the straight nanowires stacked on the top of the curl nanotubes. The straight nanowires with clean surface are attached with a metallic droplet, that is obvious evidence to vapor–liquid–solid process, and the nanowires have the length up to several micrometers. The TEM images of the sample are shown in Fig. 1(b) and (c). The curly nanostructures are twined together and it is clearly seen that the curly nanostructures are hollow. The diameters of nanotubes are in the

199

range of 40–60 nm, while the thickness of the wall is around 15 nm. Moreover, it is clear that the straight nanostructures are solid. The corresponding scattered diffraction pattern and smaller rings are shown in the electronic diffraction analysis through TEM in the inset of Fig. 1(b). The diffraction ring marked by an arrow corresponds to the (200) plane of carbon. In addition, the diffraction spots marked by circle could be regarded as the β-SiC. Fig. 2 shows that the XRD pattern of the composite film reveals that the sample is predominantly consisted of cubic SiC. Besides the peaks from cubic SiC, other peaks can be indexed to Si substrate. The corresponding EDS analysis indicated that the nanostructures were composed of silicon, carbon and copper, while copper comes from the grid. In addition, the crystal quality of the structures of single curl nanotube indicated in Fig. 1(c) is poor and the end of the nanotube is sealed. Therefore, it is revealed that the curl nanostructures are carbon nanotubes and the straight nanostructures are 3C-SiC nanowires. 3.2. The growth mechanism The growth mechanism of the composite film of CNTs and SiCNWs has been studied in detail. To understand the effect of aluminum powders during the growth process, different amounts of aluminum powders (0.01 g and 0.04 g) were added to the heating process and the other parameters were the same. Fig. 3(a) is the SEM image of obtained sample by adding a small amount (0.01 g) of aluminum powders. The wire-like nanostructures covered the whole surface and the length of the nanostructure is up to several micrometers. Besides the long and thin wire-like nanostructures, there are also some floccule amorphous nanostructures on the silicon substrate. Fig. 3(b) is the SEM image of the sample fabricated by adding a lot (0.04 g) of aluminum powders. It is also observed that the silicon substrate is coated by the wire-like nanostructures with many residual metal particles. The corresponding XRD patterns are shown in Fig. 3(c) and (d), which reveal that both of the samples are predominantly consisted of cubic SiC. Due to the different amounts of aluminum powders, the residues are various. When the aluminum powders are less, the residues are amorphous and floccule. Then, the residues are metal particles while the aluminum powders are a lot. Only when the aluminum powders are appropriate (0.02 g), the composite films of CNTs and SiCNWs are fabricated. The composite film of CNTs and SiCNWs was produced by using nickel and aluminum as catalysts, while the formation mechanism is shown in Fig. 4. The silicon substrates were coated by nickel and amorphous carbon films in the high vacuum magnetron sputtering system before heating process. Then the samples were put into a tube furnace and heated with aluminum powders at the temperature of 1000 °C for 3 h. We already fabricated pure SiCNWs by a Ni/C/Si sandwich structure in the previous paper [33], while the nickel droplet acted as a catalyst for

Fig. 1. (a) SEM image of the products obtained at 1000 °C for 3 h duration by adding 0.02 g aluminum powders. (b) TEM image of the as-synthesis samples and the inset is SAED pattern corresponding to the TEM. (c) TEM image of single carbon nanotube.

200

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Fig. 2. XRD pattern of the composite film (The squares is corresponding to the peak of silicon substrate and those of silicon carbide are marked by triangles.).

the SiCNWs and also a path for the silicon atoms. When the temperature increased, the aluminum evaporated and formed small droplets that absorbed the residual oxygen inside the furnace and oxidized to form of alumina. Al2O3 droplets not only provided the support for the Ni particles acting as active catalyst for the growth of CNTs, but also isolated Ni particles from the silicon substrate to avoid dissolving of Si atoms. Therefore, the Ni particles supported by the Al2O3 were the catalysts for the growth of the CNTs, and SiCNWs were fabricated by the catalysis of the Ni particles on the silicon substrate. When the aluminum powders are less, the Al2O3 islands are too tiny that cannot support the nickel droplets that are as a catalyst for CNTs. Instead, when the

Fig. 4. The schematic formation of the composite film on silicon substrate.

amount of the aluminum is quantity, the Al2O3 islands are so large covered the whole surface that most of nickel droplets are covered and cannot act as active catalyst. Only when the amount of aluminum powders is appropriate, the nanocomposite of CNTs and SiCNWs was directly formed on the silicon substrate.

Fig. 3. The SEM image and XRD patterns of samples fabricated by addition of a small (0.01 g) (a, c) and a great (0.04 g) amount (b, d) of aluminum powders.

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201

Fig. 5. The CA of as-synthesis composite film (a) and pure CNTs (b). The tilted SEM images of the composite film (c) and pure CNTs (d). The CA of the samples fabricated by addition of small (e) and great amount (f) of aluminum powders. The SEM images of the samples fabricated by addition of small (e) and great amount (f) of aluminum powders.

3.3. Superhydrophobicity Wettability of the samples was examined by a water contact angle analysis, using the sessile drop method. The CA of 157 ± 2° (average value for several spots) (Fig. 5) of the composite film was measured from a direct image analysis of the liquid drop profiles in sideview, indicating a superhydrophobic surface. It is noted that the surface of the composite film is chemically stable in air for months. However,

water droplets transiently stabilized on either the original CNTs' films or pure SiCNWs give a slightly greater water contact angle than that of pure silicon substrate but smaller than that of the composite film. As a reference, the original CNTs only showed a low CA of 120 ± 2°, compared to the CA of pure silicon wafer (86 ± 2°). The CA of the SiCNWs with amorphous residue is 130 ± 2° and that of less SiCNWs with metal particles is 105 ± 2°, which are presented in Fig. 5(e) and (f). The CA of SiCNWs is influenced by the morphology and composition.

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Table 1 The results calculated by Eq. (2). b1/a1

0

1

2

3

4

5

6

8

10

cos θtrue

0.069

−0.732

−0.881

−0.933

−0.957

−0.970

−0.978

−0.987

−0.991

Based on the previous results the average CA of pure SiCNWs is 117 ± 2°. Original SiCNWs are usually not superhydrophobic because of the existing polar OH group, so superhydrophobic SiCNWs usually are synthesized by surface functionalization that the water CA of SiCNWs films can change from hydrophilic to superhydrophobic state by functionalized surfaces with perfluroalkysilane [9]. However, in our experiment the as-prepared composite films show superhydrophobicity without requiring any further modification. Therefore, the origin of the superhydrophobicity in our composite film deserves a detailed study. As discussed later, the morphology brought about by the unique combination of CNTs and SiCNWs is likely the main factors that dominate the surface wettability. Cassie and Baxter's equation [9] is always used to estimate the contact area fractions of air-trapping action in nanowires by calculating the measure contact angle. The equation is expressed as follows: cos θtrue ¼ f 1 cos θ− f 2 :

ð1Þ

Herein, f1 and f2 are the area fractions of solid and vapor on the surface respectively, and obey the equation of f1 + f2 = 1. θtrue and θ are the water CAs of the composite film and pure silicon wafer, respectively. In the present experiment, the θtrue is 157° and the θ is supposed to be 86°. Therefore the values of f1 = 0.052 and f2 = 0.948 can be obtained according to Eq. (1). When θtrue is valued as 120°, that is the CA of the pure CNTs, the results are f1 = 0.159 and f2 = 0.841. The contact area between droplet and solid decreased with the CA increasing [34]. It proves that the air-trapping process plays an important role for enhancing roughness of the surface and lowering surface free energy. The obtained excellent superhydrophobicity of the composite film is unique as neither CNTs are aligned nor the superhydrophobicity of SiCNWs is good, deviating from the conventional approaches to obtain superhydrophobic coating. To identify the composite structure of CNTs and SiCNWs, we also get some SEM images in sliding mode with the titled angle of 45°, as shown in Fig. 5(c) and (d). The crimped surface with high roughness can be clearly seen from the figure. From the tilted SEM view, it shows that the SiCNWs form some angle with the silicon substrate and are much higher than CNTs. The CNTs are continuous on the whole surface and the SiCNWs cross inward the CNTs' films. Moreover, it is already studied that the direction of SiCNWs is dictated by the crystallographic orientation of the substrates [35]. The SiCNWs can grow along the particular direction and prefer to form some certain angles with the silicon substrate. It can be clearly seen that the asprepared film also formed particular angles with the silicon substrates and some nanowires crossed with each other. There are plenty of airs in the interspace between individual nanowires, which can further enhance the hydrophobicity of the films. The presence of SiCNWs enhances the surface roughness and decreases the contact area between the droplet and the films. Surface roughness is known to be beneficial for amplifying hydrophobicity and the roughness effect is usually analyzed by nano/ micropillar composite structure model [36] proposed by Patanker et al. that is developed from the Cassie and Wenzel models. According to the model, the as-obtained composite film can divide into two structures: the first class is composed of CNTs having nanopillar-like

morphology, with size of a1 × a1 and interspace of b1. The second class is regarded as straight SiCNWs with micropillar-like structure, in which the size a2 × a2, height H, and spacing b2 are arranged in a regular array. When the Cassie model is considered, the function can be described as follows: cos θtrue ¼ ∅s ð1 þ cosθÞ−1

ð2Þ

1 ∅s ¼ h
 i2 bi ai þ 1

ð3Þ

i ¼ 1; 2

where θtrue is the apparent contact angle and θ is the intrinsic contact angle (here is the CA of the pure silicon substrate). ∅ s is the area fraction of the liquid–solid contact. From Eq. (2) it is seen that the CA is independent with the pillars' height. After considering the nature of lotus leaf and one-dimensional nanomaterial coating, the CNTs are considered as the first class which is called nanopillars and θ is valued as 86° and we can obtain the data shown in Table 1. According to the size and interspace of CNTs observed from TEM and SEM images, the average diameter of the CNTs is 30 nm and the interspace is 60 nm. Thus, a CA of 127° is calculated. In this paper, the SiCNWs are considered as the second-class micropillars with small diameters and long lengths, which cannot be ignored in the calculation process. When the effect of the height of micropillars is considered, Wenzel model was described as follows: cos θtrue ¼

  4∅s H cos θ: 1þ a2

ð4Þ

From Eq. (4), the CA is dependent on micropillars' height. The average diameter of the nanowires is nearly 30 nm (a2) and the interspace is estimated to be 100 nm (b2), which can be clearly observed in the TEM and SEM images. Where θ is the CA of pure CNTs and valued as 120° and the results are shown in Table. 2. It indicated that the composite film has a good superhydrophobicity even with a small value of H/a. In addition, the as-obtained SiCNWs have an average diameter of nearly 30 nm and a length of up to micrometer, so the value of H/a can be thought to be greater than 10. Therefore, cos θtrue is smaller than − 1 according to Eq. (4). The results clearly reveal that the presence of SiCNWs is the key factor that determines the hydrophobicity of the film. Therefore, we believe that the SiCNWs has crucial effect on the superhydrophobicity. Moreover, the unique nanostructure of the composite film amplifies the natural non-wetting character of the surface, leading to very large contact angles–close to 160°–for a liquid drop on the surface. The self-cleaning surface needs not only a high CA but also a low sliding angle, so that the water droplet can easily roll off and the dirt can be removed. Sliding-angle measurements reveal that 8-μL water droplets will not slide along the surface of pure CNT films, even when the surface is vertically oriented. For composite film, a sliding angel of 5 ± 2° is measured with 8-μL water droplets. The superhydrophobic

Table 2 The results calculated by Eq. (4). H/a2

0

1

2

4

8

10

20

30

40

cos θtrue

−0.5

−0.606

−0.713

−0.926

−1.352

−1.565

−2.630

−3.695

−4.760

H. Yu et al. / Materials and Design 87 (2015) 198–204

203

Fig. 6. The CA of the composite film after immersing in water for 1(a) and 2 (b) weeks.

Acknowledgment

Table 3 The CAs of the immersed samples. The immersed time in water (week)

0

1

2

Contact angle (±2°)

157

148

141

surface displayed a low sliding angle that means dirt and debris can be easy to be move off the surface by rain droplets. The lifetime is also a key factor in the application of superhydrophobic surface. The chemical stability of the composite film is tested by immersing the samples in water for weeks. After immersing in water for a week, the CA only decreased to 148°, as shown in Fig. 6. After immersing in water for 2 weeks, the composite film also retained a high CA of 141°. The CA of composite film decreased with the immersing time as shown in Table 3. However, the weak reduction of superhydrophobicity is supposed to be related to Si–O–Si hydrolysis removal due to the wetting environment [37]. It indicated that a slight decrease of 8° per week was observed and the composite film possesses the high chemical stability. Niu et al. present that the self-cleaning glass coating SiCNWs in tetraethyl orthosilicate (TEOS) solution also have the high chemical stability that is resulted from chemical reaction of TEOS [37]. The lifetime test confirms that the composite film possesses high chemical stability even in the water without extra protective coating.

4. Conclusion The composite film of CNTs and SiCNWs possessing two different wire-like nanostructures is obtained directly on the silicon substrate by the catalyst-assisted method. The CNTs crimp on the surface of silicon substrate and the SiCNWs grow along particular angles with the substrate. The appropriate addition of aluminum determines the formation of nanocomposites. Moreover, the composite film of CNTs and SiCNWs without any functionalization on surface possesses the superhydrophobicity with a high CA close to 160°. The superhydrophobicity of the self-cleaning composite film is attributed to enhanced water/air interfacial area due to the presence of SiCNWs. In addition, the superhydrophobic structures are analyzed using the composite model with micro/nanopillar structure. The self-functionalized superhydrophobic composite film is believed to be applied in various self-cleaning fields. Furthermore, the superhydrophobicity of the composite film could be tunable by controlling the SiCNWs/CNTs' ratio and different morphology. However, the surface coating nanostructures is not suitable for the direct application due to the risk for human health. As to this point, it still needs to be studied further.

This paper is supported by the National Natural Science Foundation of China (Grant Nos. 51372053 and 51222205), the Ph.D. Programs Foundation of the Ministry of Education of China (20122302110065), the Natural Science Funds of Heilongjiang Province for Distinguished Young Scholar (JC201305), and the Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIP.2015040).

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