Fabrication of conductive, transparent and superhydrophobic thin films consisting of multi-walled carbon nanotubes

Abstract

Polydimethylsiloxane (PDMS) was coated on multi-walled carbon nanotubes (MWCNTs) using a chemical vapour deposition method, and the PDMS-coated MWCNTs were well dispersed in various solvents without additional dispersants. Spin casting of the MWCNT-containing solution on a substrate pre-treated with PDMS-SiO₂ nanoparticles resulted in the formation of a uniform thin film. The resulting thin film containing MWCNTs showed high optical transparency, conductivity and superhydrophobicity. We demonstrated that such multifunctional thin films can also be prepared on flexible substrates.

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Introduction

Due to their outstanding mechanical and electrical properties, carbon nanotubes (CNTs) have attracted great attention in various materials science and engineering fields since their discovery in 1991.^1–3 For the past two decades, attempts have been made to fabricate CNT-based thin films with diverse functions, such as superhydrophobicity, transparency and conductivity.^4–8 However, CNTs generally have low dispersity in organic solvents because they are stabilized by π–π interactions (van der Waal's interactions) between adjacent tube-walls. This property is a considerable hurdle impeding the preparation of CNT-based thin films using a solution-based process. To overcome the low dispersity of CNTs in organic solvents, low-surface-energy polymers are coated onto CNT surfaces pre-treated with oxidizing agents such as H₂O₂, HCl, or plasma. This pre-treatment is essential for covalent attachment of polymer functional groups onto CNT walls.^4,9–12 In a previous study by Han et al. conducting and superhydrophobic multi-walled CNT (MWCNT) films was fabricated using a silane sol–gel solution; in the study, CNTs were pre-treated using H₂O₂ to attach hydroxyl groups onto their walls.⁴ Another approach is the physical modification of CNT walls via non-covalent interactions between CNT surfaces and proper dispersants.^13–15 However, this method includes relatively complicated multi-step processes and electrical conductivity of resulting CNT-based film may be impaired due to the existence of remaining dispersants.^16–18 Hence, in order to enhance the electrical conductivity of CNT film, additional treatment for the removal of dispersants covered on CNT surface is required.

To fabricate uniform films using wet-chemical methods such as spin-casting, high dispersity of coating material, surface roughness of substrate and interactions between substrate and coating materials are important.^19–22 To produce uniform films using CNT, enhanced interaction between the substrate and CNTs can avoid flocculation caused by van der Waal's interactions between nanotubes. Diverse strategies such as mechanical roughening, plasma treatment and electrochemical anodization of substrates have been explored to enhance the adhesion of thin films with substrates.^19,22

Superhydrophobicity is a term used to describe a surface with a water contact angle higher than 150°. Recently, superhydrophobicity has attracted attention due to its potential application in self-cleaning, anti-fogging and anti-adhesive coatings.^23–25 Electronic devices with superhydrophobic coatings show enhanced stability under atmospheric conditions with high humidity.²⁶ In general, both low surface energy and dual surface roughness (nano- and micro-structure) are concurrently required to achieve superhydrophobicity of solid surfaces.^27–30 Layers composed of functionalized-CNTs can provide superhydrophobicity when CNTs are coated by hydrophobic functional groups.^7,24

In this study, hydrophobic MWCNTs were fabricated via a facile and cost-effective process based on dry coating polydimethylsiloxane (PDMS) on MWCNTs. No chemical pre-treatment of CNTs by oxidizing agents was needed before PDMS-coating on MWCNTs. Moreover, the PDMS-coated MWCNTs fabricated by this process dispersed well in various solvents. Solutions containing PDMS-coated MWCNTs were used to fabricate conductive, transparent, and superhydrophobic thin films on flexible and rigid substrates with the aid of PDMS-coated SiO₂ nanoparticles (NP).

Experimental

PDMS-coating on particles

The procedure for PDMS coating (Sylgard 184, Dow corning, USA) on MWCNTs (CM-100, HANWHA nanotech Corp.) and SiO₂ NPs (Sigma Aldrich, purity 99.8%, mean particle size = 12 nm) is displayed in Fig. 1.²⁵ MWCNTs and fluidic PDMS precursor were placed in a stainless steel chamber in a 1 : 1 weight ratio. The chamber temperature was maintained at 300 °C for 12 hours. During this process, the chamber was sealed from the outside and PDMS-thin layers were formed on MWCNTs by PDMS precursor vaporization. SiO₂ NPs were coated with PDMS using the same procedure. In this article, bare-MWCNTs, bare-SiO₂, PDMS-coated MWCNTs and PDMS-coated SiO₂ were denoted as B-MWCNTs, B-SiO₂, P-MWCNTs and P-SiO₂, respectively.

Fig. 1 Schematic procedure of PDMS-coating and fabrication of a MWCNT film on substrate.

Dispersion of MWCNTs in various solvents

B- and P-MWCNTs were dispersed in benzene, toluene, acetone, chloroform, tetrahydrofuran (THF) and dimethylformamide (DMF) with a concentration of 1 mg ml⁻¹. An ultra-sonicatior (SONICS & MATERIALS, VCX-500) was used (30 min) to disperse MWCNT solvents. MWCNTs dispersed in each solvent were centrifuged to separate non-dispersed MWCNTs from solution (2000 rpm/10 min).

Fabrication of MWCNT films

The procedure for fabricating MWCNT films on substrate is displayed in Fig. 1. Glass and polypropylene (PP)-plastic film (2.5 × 2.5 cm²) used as transparent substrates were ultra-sonicated in isopropyl alcohol (IPA) and dried at room temperature. B- and P-SiO₂ NPs (1.5 g) were added to 30 ml ethanol in vials. The mixed solution was dispersed using a bath-type sonicator (Saehan tech, SH-D1050) at 120 W for 30 min. The B- or P-SiO₂–ethanol solutions (500 μl) were spin-casted on cleaned substrate at room temperature (1500 rpm/30 seconds) and the spin-casted substrates were subsequently dried at room temperature. After drying, B- or P-MWCNT–DMF solutions were deposited on substrate treated with B- or P-SiO₂ using the drop-cast method. During drop-casting, the substrate was heated to 55 °C using a hot plate. The film was then dried on a hot plate at 55 °C under atmosphere conditions.

Measurement of water contact angle

The water contact angles of MWCNTs were measured using a Theta Optical Tensiometer (KSV instruments, Ltd) and electro-optics comprising a CCD camera connected to a computer. KSV bundle software (software Attension Theta) was used, and Young–Laplace curves were employed as a fitting method. A 3 μl droplet of distilled water was deposited on the sample surface. For each sample, the water contact angle was measured three times and an average value was recorded.

Stability test of P-MWCNT films on substrate under UV irradiation

The stability test of P-MWCNT films on a glass substrate with P-SiO₂ was carried out under irradiation with UV light (Vilber Lourmat/VL-4.LC) at 365 nm for 10 hours (ambient pressure conditions). Film stability was evaluated by measuring changes in the water contact angle.

Characterization methods

XPS and FT-IR (BRUKER, Optics/vertex 70) spectroscopy were used to characterize MWCNTs. FT-IR spectra were obtained by a reflectance mode and background of spectra was corrected using BRUKER bundled software, OPUS 6.5, employing concave rubber band correction method. The surface morphology and structure of MWCNT films were examined using FE-SEM (JEOL, JSM-7100F) and HR-TEM (JEOL, JEM-2100F). The optical properties (absorbance and transparency) of MWCNT-containing samples (solutions and films) were measured with a UV-vis spectrometer (OPTIZEN, 3220 UV). Sheet resistance was determined using a four-point probe measurement (ChangMin Tech, CMT-SR 1000N). After measuring the sheet resistance of nine different locations on the same film sample, an average value was used.

Result and discussion

Characterization of bare and PDMS-coated MWCNTs

The chemical compositions and functional groups of B- and P-MWCNT surfaces were characterized by Fourier-transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) (Fig. 2). The peak at 1545 cm⁻¹ in the FT-IR spectra of B-MWCNTs is assigned to a C=C stretching vibration originating from the fundamental structure of MWCNTs.^31,32 After PDMS-coating on MWCNTs, a new peak at 2975 cm⁻¹ assigned to the sp³ C–H stretching mode was detected.^25,31,33 In addition, vibrational features assigned to C–Si symmetric bending (1272 cm⁻¹), Si–O–Si asymmetric stretching (1125 cm⁻¹) and CH₃ rocking in Si–CH₃ (858 and 818 cm⁻¹) were observed after PDMS-coating.^25,33 These results clearly show that the siloxane frameworks (C–H, C–Si and Si–O–Si) exist on P-MWCNTs. Fig. 2(b–d) display XPS C 1s, Si 2p and O 1s core-level spectra of MWCNTs before and after PDMS-coating, respectively. All core-level spectra were normalized by the intensity of their respective C 1s spectrum. In both C 1s spectra, the main peaks centred at 284.5 eV correspond to the sp² carbon species of MWCNT.³⁴ A shoulder at lower binding energy with respect to the main peak in the C 1s spectrum was detected after PDMS-coating. This shoulder is attributed to the CH₃–Si species of PDMS frameworks.^35,36 In the Si 2p spectrum of P-MWCNTs, a peak at 102.3 eV appeared which was not visible for B-MWCNTs. The binding energy of the Si 2p peak of P-MWCNTs was significantly lower than that of Si(IV), suggesting that Si was not fully oxidized, but existed within PDMS frameworks (alkylated siloxane) on the surface of MWCNTs.^34,37 In the O 1s spectrum, oxygen species in siloxane were confirmed by the appearance of a peak centred at 532 eV upon PDMS-coating.

Fig. 2 (a) FT-IR spectra and XPS spectra of (b) C 1s, (c) O 1s and (d) Si 2p for B- and P-MWCNTs.

The geometric structures of B- and P-MWCNTs were characterized by high-resolution transmission electron microscopy (HR-TEM), as shown in Fig. 3. In both images, multi-wall arrays with 0.34 nm wall-spacing were observed, in agreement with previous studies.^1,38 After PDMS-coating, the PDMS layer on the MWCNT was clearly revealed, as shown in the inset of Fig. 3(b). The PDMS layer was estimated to be ∼1 nm thick. The PDMS-coating layer homogeneously covered the MWCNT surface.

Fig. 3 HR-TEM images of (a) B- and (b) P-MWCNTs. Inset of each figure shows a magnified view of each image.

In order to verify the water repellent properties of WMCNTs after PDMS-coating, dispersion test in water and water contact angle measurements were carried out (Fig. 4). After B- and P-MWCNTs were placed in vials filled with water, B-MWCNTs partly mixed with water, whereas P-MWCNTs floated perfectly on water. Fig. 4(b) and (c) show the water contact angles on pellets consisting of B- and P-MWCNTs, respectively. When water was dropped on the B-MWCNT sample, water droplets were immediately absorbed and gradually disappeared with increasing time, as shown in Fig. 4(b). In contrast to the B-MWCNT results, the water contact angle of P-MWCNTs was 163.2° (Fig. 4(c)), indicating that the PDMS-coating generated the superhydrophobic property on the MWCNT surface. P-MWCNTs are quite hydrophobic due to the Si–CH₃ surface termination of the PDMS framework identified in the IR spectrum. The surfaces of pelletized P-MWCNTs should show dual surface roughness in micro- and nano-scales, a pre-requisite of superhydrophobicity.^29,30

Fig. 4 (a) Images of B- and P-MWCNTs in vials filled with water, and water contact angles on pelletized (b) B- and (c) P-MWCNTs.

To illuminate changes in the electrical conductivity of MWCNTs caused by PDMS-coating, the sheet resistances of two different pelletized MWCNT samples with and without PDMS-coating were measured. The sheet resistances of pelletized B- and P-MWCNTs were ∼0.8 and ∼1.2 Ω sq⁻¹, respectively. It is remarkable that no significant change in MWCNT sheet resistance was observed, even though a slight increase was seen after PDMS-coating. The PDMS-coating presented in this work is highly effective at providing hydrophobic functionality to MWCNTs while maintaining the electrically conductive property of MWCNTs. Since the PDMS-layer is thin enough to allow electron hopping, defects in the PDMS-layer can provide conducting points between adjacent tube walls.

Fig. 5 shows the optical absorbance spectra of various solvents (benzene, toluene, acetone, chloroform, THF and DMF) saturated by B and P-MWCNTs to compare the relative dispersion of two different MWCNTs. After centrifugation of each MWCNT dispersed solvent prepared via the ultra-sonication, the supernatants were collected for absorbance measurement. Non-dissolved MWCNTs can be completely removed by this process. Therefore, the absorbance features of collected supernatants are involved with well-dispersed MWCNTs and the dispersity of MWCNTs in each solvent can be relatively compared by absorbance measurement. P-MWCNTs showed better dispersion in various solvents than B-MWCNTs. The absorbance of P-MWCNTs in various solutions was 1.4–1.9 times higher than that of the respective B-MWCNT solutions. Note that solution absorbance in the UV/vis range increased with an increasing amount of MWCNTs dispersed in solution. The higher dispersity of P-MWCNTs than B-MWCNTs in various solvents is due to the decreased inter-tube interactions caused by the PDMS thin layer on MWCNTs. In this preparation method, MWCNT dispersion in solvents was enhanced without using any dispersants or additional MWCNT surface treatment with oxidizing agents such as H₂O₂, HCl or plasma treatments. It is also worth mentioning that P-MWCNTs in DMF solution showed high stability, i.e., maintained its dispersity over 9 month, which is essential for the practical use (ESI 1†).

Fig. 5 Absorbance spectra of B- and P-MWCNTs saturated in various solvents of (a) benzene, (b) toluene, (c) acetone, (d) chloroform, (e) THF and (f) DMF.

Fabrication of multi-functional thin films based on MWCNTs

Attempts were made to fabricate transparent, conductive and superhydrophobic multi-functional films using MWCNTs and SiO₂ NPs. Fig. 6 shows B- and P-MWCNT films drop-casted on three different glass substrates (glass without any treatment and glass with B- and P-SiO₂ treatments, respectively). For B-MWCNTs (Fig. 6(a)–(c)), non-uniform MWCNT films were obtained on all three different substrates, although film uniformity increased slightly in order as: glass substrate without SiO₂ < spin-coated with B-SiO₂, < spin-coated with P-SiO₂. With P-MWCNTs, the film on the glass substrate pre-treated with B-SiO₂ NPs (Fig. 6(e)) exhibited higher uniformity than that on the substrate without any treatment (Fig. 6(d)). However, film uniformity was still not good enough. A highly uniform P-MWCNT film was obtained when the glass substrate was treated by P-SiO₂ NPs, as shown in Fig. 6(f). The superior uniformity of the thin film shown in Fig. 6(f) compared to the other thin films in Fig. 6 is explained by the following:

Fig. 6 Images of B- and P-MWCNT films on glass substrate treated with and without B- and P-SiO₂ NPs. (a) B-MWCNT/glass, (b) B-MWCNT/B-SiO₂/glass, (c) B-MWCNT/P-SiO₂/glass, (d) P-MWCNT/glass, (e) P-MWCNT/B-SiO₂/glass, and (f) P-MWCNT/P-SiO₂/glass (drop-volume: 0.4 ml).

(1) P-MWCNTs generally show higher film uniformity than B-MWCNTs using drop-casting due to better dispersion of P-MWCNTs in solvents as a consequence of lowered van der Waal's interactions between neighboring tubes by PDMS-coating.

(2) Pre-deposition of SiO₂ NPs on glass substrate can enhance the adhesion of MWCNTs on surfaces due to the increased surface roughness of glass caused by SiO₂ NPs.

(3) P-SiO₂ and P-MWCNTs are easily miscible since both species are covered by hydrophobic PDMS-layers. Both species have relatively strong interactions with each other therefore, drop-casted P-MWCNTs can be evenly distributed on the surface of P-SiO₂-treated glass.

The structure of the P-MWCNT film on P-SiO₂ NP-treated glass substrate was identified by field-emission scanning electron microscopy (FE-SEM). Well-entangled networks of P-MWCNTs with a mean tube diameter of 20–30 nm were formed on the substrate, as shown in Fig. 7. This network exhibited a random structure with micro-level roughness constructed by aggregation of some P-MWCNTs (Fig. 7(a)).

Fig. 7 (a) FE-SEM image of P-MWCNT film fabricated with drop-volume of 0.4 ml (P-MWCNT/P-SiO₂/glass) and (b) magnified SEM image of (a).

Fig. 8 shows optical pictures of uniform P-MWCNT films (top) and their water contact angles (bottom). Here, the mean thickness of P-MWCNT films on P-SiO₂ NP-treated glass was varied by changing the drop-volume of P-MWCNT/DMF. The optical transparency of P-MWCNT films decreased with increasing drop-volume of P-MWCNT/DMF, whereas the superhydrophobic property with water contact angles over ∼160° was shown for all films. The dual roughness of the film surface was formed by random aggregation of P-MWCNTs at the micro-level while nanoscale roughness was due to the intrinsic size of each MWCNT, as shown in Fig. 7. The dual surface roughness of P-MWCNT/P-SiO₂/glass in combination with the hydrophobic surface structure of P-MWCNTs can lead to superhydrophobicity, as shown in Fig. 8.

Fig. 8 Images of P-MWCNT films (P-MWCNT/P-SiO₂/glass) and water contact angles as a function of P-MWCNT/DMF drop-volume.

The transmittance spectra and sheet resistances of P-MWCNT/P-SiO₂/glass samples mentioned in Fig. 8 are summarized in Fig. 9. Sheet resistance and transmittance depend strongly on the amount of P-MWCNT/DMF drop-volume. An increase in the drop-volume led to decreased transmittance at a broad range of wavelengths and sheet resistance. When the P-MWCNT film was fabricated by a drop-volume of 0.3 ml, transmittance of 80% and sheet resistance of 13.9 kΩ sq⁻¹ were observed, indicating that this film had good performance in both transparency and conductivity.^4,8,24 Based on the results of Fig. 8 and 9, we suggest that the electrical and optical properties of P-MWCNT films can be controlled without deteriorating the superhydrophobicity of the film.

Fig. 9 (a) Transmittance spectra in a range of 350–900 nm, and (b) plots of sheet resistance and transmittance at 700 nm as a function of MWCNT/DMF drop-volume for preparing P-MWCNT/P-SiO₂/glass described in Fig. 8.

To test the stability of P-MWCNTs under UV lights, changes in the water contact angle of P-MWCNT/P-SiO₂/glass were measured as a function of UV (365 nm)-irradiation time (Fig. 10). P-MWCNTs exhibited an initial water contact angle of 167°. After UV-irradiation for 10 hours, the film kept its superhydrophobic nature, demonstrating high stability.

Fig. 10 Change in water contact angle of P-MWCNT/P-SiO₂/glass as a function of UV-irradiation (365 nm) time.

P-MWCNT/P-SiO₂ thin films were also fabricated on flexible substrates. The films were flexible (Fig. 11) and maintained optical transparency and superhydrophobicity after repeated bending motions.

Fig. 11 Images of P-MWCNT film deposited on flexible plastic substrate (P-MWCNT/P-SiO₂/plastic substrate). Left: water droplet on flexible substrates, centre: bending-down, and right: bending-up.

Conclusion

A stable and uniform multi-functional film was fabricated using MWCNTs and SiO₂ NPs. A facile coating method using PDMS precursor was used to provide a hydrophobic nature to the surface of MWCNTs. PDMS-thin layers about ∼1 nm thick were homogeneously coated on MWCNTs. PDMS-coating significantly enhanced the water repellent property of MWCNTs without reducing its electrical conductivity. Moreover, P-MWCNTs were dispersed better in various solvents than B-MWCNTs. The fabrication procedure of multi-functional films was optimized using well-dispersed P-MWCNT/DMF solution and substrate pre-treated by P-SiO₂ NPs. The electrical and optical properties of P-MWCNT films could be altered by changing the drop-volume of solution without diminishing the superhydrophobicity of the film. Uniform and superhydrophobic MWCNT films with transmittance of ∼80% of visible light and sheet resistance of 13.9 kΩ sq⁻¹ were fabricated. The P-MWCNTs thin films prepared here were relatively stable in the presence of UV irradiation and applicable on flexible substrates.

Acknowledgements

This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (2012R1A1B3000992).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra04272g

‡ These authors contributed equally to this work.

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