Percolation-dominated superhydrophobicity and conductivity for nanocomposite coatings from the mixtures of a commercial aqueous silica sol and functionalized carbon nanotubes

https://doi.org/10.1016/j.jcis.2011.10.029Get rights and content

Abstract

Superhydrophobic conductive nanocomposite coatings are prepared for the first time from the simple mixture of a commercial aqueous silica sol and functionalized multiwalled carbon nanotubes (MWNTs) by air-spraying at ambient conditions followed by fluorosilane treatment. The relationship between MWNT content and the structure and properties of the nanocomposite coatings is investigated systematically. An ultra-low threshold (<5 vol.%) for superhydrophobicity is observed, which suggests that MWNTs are superior to any other spherical fillers for the construction of superhydrophobic nanocomposite coatings. When the content of nanotubes is below the threshold, the surface roughness mainly caused by the silica nanoparticles is not enough for creating superhydrophobic surfaces. Only above the threshold, the multiscale hierarchical structure is enough for both high water contact angles (>165°) and extremely low sliding angles (<2°). The conductivity is also percolation dominated, while the threshold for conductivity is much higher than that for superhydrophobicity, which can be ascribed to the encapsulated structure and the agglomeration of nanotubes in the composite coatings during air-spraying. Moreover, the aqueous silica sols hold merits of great film-forming capability at relatively low calcination temperatures, and being free of organic solvents.

Highlights

► We prepare superhydrophobic conductive coatings from aqueous silica sol and MWCNTs. ► MWCNTs are hydroxylated or physically functionalized by a polysoap. ► The coatings are prepared by air-spraying and are easy to repair. ► The superhydrophobicity and conductivity of the coatings are percolation dominated. ► The percolation threshold for superhydrophobicity is extremely low.

Introduction

Wettability is an important characteristic of solid surfaces. In recent years, superhydrophobic surfaces with a water contact angle (WCA) above 150° and sliding angle (SA) below 10° have been continuously attracting wide research interests [1], [2], [3], [4], [5], owning to their widespread potential applications such as microfluidic system with low resistance [6], [7], self-cleaning [8], [9], anti-adhesion [10], [11], corrosion-resistant [12], [13], or antifogging surfaces [14]. Studies on natural materials, such as lotus leaves and the legs of water striders have shown that superhydrophobic surfaces combine hierarchical surface roughness at both micro- and nanoscale with low-surface-energy materials [15], [16]. Furthermore, electrically conductive superhydrophobic surfaces are of great importance because of their potential applications as electrostatic dissipative [17] or electromagnetic interference (EMI) shielding materials [18]. Due to the excellent conductivity and mechanical properties, carbon nanotubes (CNTs) have been used as the building blocks for superhydrophobic conductive coatings, including non-aligned CNT coatings [19], [20], well-aligned CNT forests [21], [22], [23], as well as CNT composites [24], [25], [26] and CNTs/silane mixtures [27]. However, the relationship between the CNT content and the structure, superhydrophobicity, and conductivity of the composite coatings has not been adequately illuminated.

On the other hand, many other materials have also been employed for the construction of artificial superhydrophobic surfaces, including polymers [28], [29], [30], [31], [32], inorganic materials [33], [34], [35], metals [36], [37], and their composites [38], [39]. Silica and its composites are also typical examples [40], [41], [42]. In most previous studies, silica coatings are prepared by either the well-known Stöber method, i.e., the hydrolysis of tetraethyl orthosilicate (TEOS) in the ethanolic solution of ammonia, or the sol–gel reaction of TEOS using acidic catalysts. Our previous study [42] also demonstrated that superhydrophobic coatings could be obtained from the nanocomposites of silica and functionalized MWNTs using TEOS as the precursor of silica. However, the relationship between the content of MWNTs and the structure and properties of the nanocomposite coatings was not illuminated. Furthermore, the stability of the mixture of functionalized MWNTs and TEOS in the existence of ammonia is poor, and large amount of ethanol was consumed in that process, which is obviously negative for large-scale applications. Therefore, it remains a great challenge to construct large-area superhydrophobic surfaces with high speed, high efficiency, and low environmental cost.

Herein, we present a novel process for the preparation of superhydrophobic conductive coatings from the simple mixture of a commercially available aqueous colloidal silica sol and functionalized MWNTs and systematically investigated the effect of MWNT content on the structure and properties of the nanocomposite coatings. The silica sols are aqueous dispersion of ultrafine colloidal silica nanoparticles containing large amount of silanol groups on the nanoparticle surfaces, therefore exhibit excellent film-forming capability at relatively low temperatures, and have been used as binders in diverse applications, such as foundry production, ceramics and catalysts [43], [44], or to improve the mechanical, adhesive, and wetting properties of polymer coatings [45]. The aqueous silica sols are produced by the neutralization with demetalization or deionization and subsequent concentration of sodium silicate, therefore is inexpensive and more environmentally friendly than TEOS as the silica source. Although aqueous silica sols have a long history that can be traced back to as early as 1941 [46] and are commercially available nowadays in large quantities at low price, it is strange that they have not been used to prepare superhydrophobic surfaces, to the best of our knowledge. In this study, we demonstrate that superhydrophobic conductive coatings can be readily prepared from the mixture of aqueous silica sols and functionalized MWNTs by air-spraying, followed by fluorosilane treatment. To disperse MWNTs uniformly and stably in the diluted aqueous silica sols, two types of functionalized MWNTs are used. One is the noncovalently modified MWNTs wrapped by a mixture of an amphiphilic copolymer of styrene and maleic acid and a silane coupling agent (denoted as w-MWNTs). The other one is hydroxylated MWNTs (MWNTs-OH). We prepared the coatings by air-spraying because of its merit of simplicity, repairability, and being applicable to large-area superhydrophobic surfaces [47], [48], [49], in comparison with other methods for the preparation of superhydrophobic nanocomposite coatings, such as the classic layer-by-layer assembly and sol–gel processes. Moreover, the relationships between the thickness, composition, morphology, and properties of the nanocomposite coatings are investigated.

Section snippets

Materials

MWNTs with an average diameter of 10–20 nm, a length of 5–15 μm, and purity above 95 wt.% were purchased from Shenzhen Nanotech Port Co. Ltd. The commercial silica sol with a solid content of 30 wt.% was purchased from Xiagang Chem. (Jiangyin, China). Styrene and maleic anhydride (MAn) were purchased from Aldrich and are of chemical grade. Styrene was purified by washing with an aqueous solution of sodium hydroxide and deionized water, dried over anhydrous magnesium sulfate, and finally distilled

Interactions between silica sols and functionalized MWNTs

Pristine MWNTs tend to agglomerate in water due to the large surface area and hydrophobic or Van der Waals attraction. To prepare superhydrophobic conductive surfaces from the aqueous or ethanolic aqueous mixture of MWNTs and the silica sols by air-spraying, surface modification of the MWNTs is necessary in order to obtain uniform and stable MWNTs/silica mixtures in water or water/ethanol mixture. Noncovalent modification of MWNTs by amphiphilic copolymer is very facile, and the original

Conclusions

In this study, w-MWNTs and MWNTs-OH were combined with commercial aqueous silica sols for the preparation of superhydrophobic conductive nanocomposite coatings by simple air-spraying. The relationship between nanotube content and the structure and properties of the nanocomposite coatings was investigated in detail. MWNTs play the crucial role for the percolation-dominated superhydrophobicity and conductivity of the nanocomposite coatings. When the contents of MWNTs are above the threshold, the

Acknowledgments

We greatly appreciate financial support from the National Natural Science Foundation of China (50773066), the Natural Science Foundation of Zhejiang Province (Y4110069), the Program for Zhejiang Provincial Innovative Research Team (2009R50004), and the Fundamental Research Funds for the Central Universities (2011QNA4025).

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