Elsevier

Applied Surface Science

Volume 422, 15 November 2017, Pages 1022-1029
Applied Surface Science

Full length article
Superhydrophobic properties induced by sol-gel routes on copper surfaces

https://doi.org/10.1016/j.apsusc.2017.05.257Get rights and content

Highlights

  • Superhydrophobic (SH) Cu surfaces by sol-gel routes.

  • Surface’s morphology, composition and wettability vs alcoholic- or water-based coating suspensions.

  • Durability of SH Cu in chemical aggressive conditions supported by XPS and FE-SEM analyses.

  • Advantages induced by SH Cu surfaces in industrial applications and working environments.

Abstract

Superhydrophobic surfaces are attracting increasing attention in different fields such as energy, transportation, building industry and electronics, as they exhibit many interesting properties such as high water repellence, anti-fogging, anti-corrosion, anti-fouling and self-cleaning abilities. Here, superhydrophobic nanostructured hybrid materials obtained by depositing alumina nanoparticles on copper surfaces via dip coating in Al2O3 sol are presented. Two different preparation routes were explored, based on either an alcoholic or an aqueous Al2O3 sol, and the resulting wetting properties were compared. Wettability measurements showed that when the alcoholic sol is used superhydrophobicity is attained, with values of water contact angle very close to the upper limit of 180°, while highly hydrophobic coatings are obtained with the aqueous sol. These findings were further supported by electron microscopy and X-ray photoelectron spectroscopy, which revealed that the surface layer deposited on Cu is more homogenous and richer in alumina nanoparticles when the alcoholic sol was used. Durability of the superhydrophobic coating was assessed by performing ageing tests in chemically aggressive environments. A remarkable resistance is displayed by the superhydrophobic coating in acid environment, while alkaline conditions severely affect its properties. Such behaviors were investigated by XPS and FE-SEM measurements, which disclosed the nature of the surface reactions under the different conditions tested. The present results underline that a thorough investigation of surface morphology, chemical composition and wetting properties reveals their strongly connection and helps optimizing the combination of substrate nanostructuring and suitable chemical coating for an improved durability in different aggressive environments.

Introduction

Superhydrophobicity is a property of surfaces displaying enhanced repellence to water, as expressed by large static water contact angles (WCA  150°) and small contact angle hysteresis (CAH < 10°). The discovery of a great number of natural superhydrophobic surfaces (SHS), working according to different mechanisms, has inspired scientists to produce artificial SHS mimicking their physicochemical features [1], [2], [3]. Synthetic SHS have proved to trigger great advantages in many applications because of the relevance of the induced properties, e.g. self-cleaning [4], prevention of icing phenomena [5], drag reduction [6], anti-fouling [7], and anti-corrosion [8].

Copper and Cu-based alloys are widely used in energy conversion devices (HVAC systems, vehicular coolers and radiators, heat sinks for electronic equipment cooling, etc.), which are often employed in corrosive environments [8], [9], [10]. Controlling Cu wettability is a key issue in such applications. So far, two main approaches to modify copper wetting properties have been reported in the literature. One deals with the deposition of either a single or multiple functional layers directly on the metallic surface, while a second one requires preliminary heat or wet chemical treatments of copper surface in order to promote peculiar morphologies and chemical modifications, mainly connected to the formation of Cu oxides. Examples of the first case are copper SHS produced by chemical bath deposition (CBD) of multiple layers [11], [12], [13], [14], [15] or by deposition of self-assembled monolayers (SAM) [16], [17], as well as by electrodeposition [18], [19], [20] or by a treatment with a suspension of hydrophobic functionalized nanoparticles [21]. The second approach is the most commonly reported in the literature to obtain highly performing copper SHS. It generally involves a preliminary etching or, alternatively, wet or dry oxidation steps followed by deposition of low-surface energy compounds like silanes, fluoroalkylsilanes or long-chain organic acids [16], [22], [23]. Surface activation promotes the formation of Cu oxides/hydroxides, while deposition of organic layers lowers the wettability towards water.

Surface scientists frequently present novel SHS preparations or syntheses combining many approaches and strategies. The route suggested in the present work consists in the deposition on Cu of a nanostructured, chemically hybrid thin film made of a ceramic oxide (Al2O3) obtained by sol-gel, coupled with a fluoroalkylsilane (FAS) moiety. The application of sol-gel coatings to obtain copper SHS has not yet been investigated in depth. This process makes neither use of oxidizing or acidic chemical reagents (as instead required by the etching method), nor of severe or complex thermal treatments. All deposition processes are carried out via dip coating, a fast and simple technique that can be easily transferred to an industrial scale. In order to broaden the range of investigations, copper SHS were produced using either a water-based or an isopropanol-based sol. The characterization here reported of the coated surfaces in terms of chemical composition and morphology allowed for a deeper comprehension of the observed wetting behavior.

Section snippets

Preparation of the alumina sol

Alcohol-based Al2O3 sol (referred to as A) was prepared according to the literature [24]: aluminum-tri-sec-butoxide (Al(O-sec-Bu)3, 97%, Sigma–Aldrich) was stirred in isopropyl alcohol (i-PrOH, 99%, Sigma–Aldrich) for 1 h at room temperature. Then, ethyl acetoacetate (EAcAc, >99%, Sigma–Aldrich) as the chelating agent was added and the solution was stirred for 3 h. Finally, water was gradually poured into the solution as a 1:1 V/V mixture with i-PrOH to promote hydrolysis of the alkoxide. The molar

Results and discussion

Wettability performance and surface morphologies of the coated surfaces are presented and commented, followed by ageing simulations and by XPS analysis.

Conclusions

A hybrid organic-inorganic coating was deposited on roughened Cu surfaces, which allowed to achieve high repellence against water. The inorganic component of coating consists of alumina nanoparticles obtained via sol-gel routes in two different media (isopropanol or water), deposited on the surface via dip coating and provided with a flower-like nanostructure resulting from immersion in boiling water. Such structure allows for the establishment of a Cassie-Baxter wetting state and the related

References (41)

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