Regular ArticleComparison of hydrophobicity and durability of functionalized aluminium oxide nanoparticle coatings with magnetite nanoparticles–links between morphology and wettability
Graphical abstract
Introduction
Fabrication of robust, easy to apply, low wettability coatings still remains a major challenge within the research community. Thin films created using functionalized nanoparticles have been shown to possess rough surface textures that show superhydrophobic behaviour [1], [2], [3], [4], [5]. Deposition of the nanoparticles onto surfaces has been readily achieved through dipping [3], [6] and spraying [2], [5], which could make it suitable for use within a variety of different industrial and commercial sectors. For example, low wettability nanoparticle coatings could assist the removal of corrosive ions in aqueous solutions from metals, prevent cardboard getting wet during transport or storage, and facilitate the quick-drying of plastics. A large variety of nanoparticles have been shown to display this behaviour, such as metal oxides like ZnO [4] and Al2O3 [2], [5], SiO2 [1], [3], and amorphous carbon [7]. In these reports, the surface energy of metal oxide and SiO2 nanoparticles is lowered through adsorption of suitable carboxylic acids [2], [4], [5] or alkylsilanes [1], [3], [8].
Despite their potential utility, these coatings possess relatively poor durability and can be easily removed through abrasion. Methods have been reported where nanoparticle adhesion has been improved through partial thermal embedding of the particles into materials such as glass [9] and graphene [10]. Alternatively, it has been shown that composite coatings can be prepared through embedding nanoparticles in materials such as silica [11] and polymers [7], [12], [13], [14], [15]. These coatings show greatly enhanced durability relative to films formed solely from the nanoparticles. For example, Manca et al. created a robust superhydrophobic coating through thermally embedding trimethylsiloxane functionalised SiO2 nanoparticles into an organosilica matrix [11]. The organosilica binder was prepared using a sol-gel method and the resulting composite material was observed to maintain low wettability in outdoor exposure tests [11]. Similarly, Bayer et al. fabricated a robust water-repellent coating through annealing alternating layers of SiO2 nanoparticles and fluoroalkyl methacrylic copolymer [13]. Coatings were prepared that remained superhydrophobic after pencil hardness, tape adhesion and linear abrasion tests [13].
Although these coatings showed good durability, many of these approaches require relatively high temperatures in order to thermally embed the particles in the matrix materials. Lower temperature routes for creating superhydrophobic nanoparticle/composite coatings have been achieved using commercially available adhesives [16], [17], [18], [19]. In these reports, superhydrophobic coatings were prepared at room temperature, or at temperatures of less than 80 °C. By comparison, embedding functionalised SiO2 nanoparticles into organosilica was carried out at 350 °C [11]. In addition to their low temperatures, approaches utilising resin are also attractive since it has been shown that the resin and the nanoparticles can be combined and applied onto substrates in one step [17], [19]. However, the proportions of the different materials need to be carefully controlled so that the resin does not engulf the particles. For example, Ebert and Bhushan demonstrated that highly durable, superhydrophobic coatings could be created using suspensions containing phosphonate functionalised nanoparticles and methylphenyl silicone resin [19]. It was observed that the coatings retained their superhydrophobic properties following water jet impact tests, indicating that they could function well as self-cleaning surfaces [19].
We have investigated whether this promising methodology can be used to improve the adhesion of isostearate functionalised Al2O3 and magnetite (Fe3O4) nanoparticles. Isostearic acid is a highly branched carboxylic acid that possesses alkyl moieties terminated with several methyl groups [2]. Methyl groups have lower surface energy than methylene groups [2]. Consequently, this acid could be better suited for use in superhydrophobic coatings since the CH3:CH2 ratio per alkyl chain is larger than carboxylic acids with linear alkyl chains2. Previously, we have reported superhydrophobic coatings formed from isostearate functionalised Al2O3 nanoparticles [2]. However, to the best of our knowledge, the wetting properties of carboxylate functionalised magnetite nanoparticles have not previously been reported. Magnetite nanoparticles have been used to remove harmful metallic species from waste water [20], [21], [22] and Fe (II and III) compounds have been shown to immobilise bacteria and viruses [23], [24], [25]. Consequently, the use of magnetite nanoparticles in self-cleaning coatings would be desirable since they could remove pathogens and harmful materials from the environment.
Section snippets
Materials and reagents
Al2O3 nanoparticles (d = 13 nm, 99.8%) and Fe3O4 nanoparticles (d = 15–20 nm, 99.5%) were purchased from Sigma-Aldrich and US Research Nanomaterials respectively. Isostearic acid was kindly provided by Nissan Chemical Industries and was used without further purification. Toluene and isopropanol were supplied by VWR Chemicals. SP106 Multi-Purpose Epoxy Resin System Slow Hardener was purchased from MB Fibreglass. Plastic film (75 µm thickness, 5-ply ethylene-vinyl acetate/ethylene-vinyl
Characterization of the isostearate functionalized magnetite nanoparticles
XPS and IR spectroscopy were carried out to investigate whether isostearic acid had adsorbed onto the surface of the magnetite nanoparticles. The characterisation of the isostearate functionalised Al2O3 nanoparticles has been reported previously [2]. In our previous study, IR spectroscopy showed that isostearic acid had chemisorbed onto the surface of the Al2O3 nanoparticles as a carboxylate. TGA showed that the chemisorbed isostearate on the Al2O3 had a grafting density of 2.0 nm−2, whilst SEM
Conclusions
Durability and wetting behaviour of coatings formed from carboxylate functionalized Al2O3 and Fe3O4 nanoparticles and epoxy resin was thoroughly studied and compared using SEM, Contact angle, XPS and AFM measurements. Although previous works [16], [17], [18], [19] have shown that commercially available adhesives can improve the wear resistance of hydrophobic nanoparticle coatings, none of these approaches have studied the affect of changes in surface morphology and chemistry on the wettability
Acknowledgment
Financial support was provided by the Welsh Government Sêr Cymru Programme through Sêr Cymru II Welsh Fellowship part funded by the European Regional Development Fund (ERDF), European Union, through the Welsh Government, UK (80761-SU-021) (S.A.) and the Sêr Cymru Chair for Low Carbon Energy and Environment (A.R.B.). The Robert A. Welch Foundation, USA (C-0002) is acknowledged for additional support (A.R.B.). Salts Healthcare Ltd., UK is also thanked for financial support (D.H). We would like to
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