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Robust Superhydrophobic Graphene-Based Composite Coatings with Self-Cleaning and Corrosion Barrier Properties Md J. Nine, Martin A. Cole, Lucas Johnson, Diana N. H. Tran, and Dusan Losic* School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia S Supporting Information *
ABSTRACT: Superhydrophobic surfaces for self-cleaning applications often suffer from mechanical instability and do not function well after abrasion/scratching. To address this problem, we present a method to prepare graphene-based superhydrophobic composite coatings with robust mechanical strength, self-cleaning, and barrier properties. A suspension has been formulated that contains a mixture of reduced graphene oxide (rGO) and diatomaceous earth (DE) modified with polydimethylsiloxane (PDMS) that can be applied on any surface using common coating methods such as spraying, brush painting, and dip coating. Inclusion of TiO2 nanoparticles to the formulation shows further increase in water contact angle (WCA) from 159 ± 2° to 170 ± 2° due to the structural improvement with hierarchical surface roughness. Mechanical stability and durability of the coatings has been achieved by using a commercial adhesive to bond the superhydrophobic “paint” to various substrates. Excellent retention of superhydrophobicity was observed even after sandpaper abrasion and crosscut scratching. A potentiodynamic polarization study revealed excellent corrosion resistance (96.78%) properties, and an acid was used to provide further insight into coating barrier properties. The ease of application and remarkable properties of this graphene-based composite coating show considerable potential for broad application as a self-cleaning and protective layer. KEYWORDS: graphene, graphene coatings, superhydrophobic surface, self-cleaning, barrier coating, diatomaceous earth Introduction of different surface textures such as porous,13 hierarchical structure,14 or microsized replicated pillars18 on PDMS with different textured materials creates a suitable chemistry/topography combination for facile fabrication of superhydrophobic surfaces. To date, PDMS has been used as a biomimicking template,19 embossing platform,18 precursor of CVD,20 and composite matrix21 and as a hydrophobic modifier and binding agent of different types of materials, such as oligomeric silsesquioxane (POSS) nanoparticles,11 polytetrafluoroethylene (PTFE) particles,13 tetraethyl orthosilicate (TEOS),16 candle soot,20 ZnO,21 SiO2/TiO2 composite,22 and CaCO3/SiO2 composite.23 Many of these methods and techniques shows very good results in terms of superhydrophobicity and self-cleaning properties, but their scalability, robustness, or cost limit their potential for industrial application. In this regard, durable, cost-effective, easy to apply superhydrophobic coatings with multifunctional properties are required in order to realize benefits across many industries where protective coatings can improve function.
1. INTRODUCTION Engineered superhydrophobic surfaces (water contact angle >150°), combining dual-scale roughness and hydrophobic chemistry, have been studied for self-cleaning applications since the group at Kao corporation1 first demonstrated artificial superhydrophobic surfaces.1−3 Self-cleaning surfaces allow water droplets to pick up and remove dust, viruses, and bacteria during bouncing and rolling contact with the surface. Superhydrophobic properties are desirable for many industrial and biological applications including deicing4 and self-cleaning of antennas, windows, automobile windshields, and outdoor textiles,5 as well as for enhanced antibacterial6 and antibiofouling7 surfaces for medical and marine industries. However, a common weakness of these surfaces is susceptibility to mechanical abrasion that eliminates the self-cleaning function due to destruction of roughness and removal of the layer.8 Furthermore, existing deposition methods limit the size, shape, and materials of targeted substrates. These fabrication methods of superhydrophobic surfaces include template-based extrusion,9 chemical vapor deposition (CVD),10 electrospinning,11 extrusion,12 plasma etching,13 self-assembly,14 layer by layer deposition,15 sol−gel process,16 and lithographic methods.17 Polydimethylsiloxane (PDMS) is one of the most frequently used surface modifiers to create superhydrophobic surfaces. © 2015 American Chemical Society
Received: October 9, 2015 Accepted: December 3, 2015 Published: December 3, 2015 28482
DOI: 10.1021/acsami.5b09611 ACS Appl. Mater. Interfaces 2015, 7, 28482−28493
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic of prepared multifunctional superhydrophobic surface composed of graphene sheets and DE silica particles with nano- and microdimensions coated with PDMS.
To address the aforementioned challenges, we present coating formulations that can be applied using simple, low cost, and scalable techniques to fabricate superhydrophobic coatings with multifunctional properties to apply as robust and self-cleaning barrier layers. To create these properties, our concept was to combine graphene sheets with nano- and microscale diatomaceous earth (DE) particles and PDMS. We hypothesized that crushed DE would provide suitable roughness; PDMS would impart low-surface-energy properties; and the rGO sheets, with high surface area, would improve barrier properties to prevent aggressive molecules from reaching the underneath substrate. The scheme of the composite film is presented in Figure 1. We used natural silica material from DE as a low cost source of micro- and nanoparticles that have been extensively studied by our group and others in the past decade for applications including nanofabrication,24 drug delivery,25−29 water purifications,30 and solar cells.31 DE material is composed of hollow silica microparticles that are fossilized unicellular algae found in both fresh water and seawater sources. This material with attractive structural, chemical, and optical properties32,33 has been used to fabricate the superhydrophobic surface after fluorination.34 However, their large microscale (cylindrical >10 μm, and circular >20 μm) dimensions and inner porous structure are two drawbacks of using DE frustules alone to fabricate a superhydrophobic protective layer.35 To overcome these limitations, we have mechanically crushed DE microparticles to provide both nano- to microscale dimensions with irregular shapes to produce extremely rough surface structure. It was expected that the further inclusion of graphene to this material will improve the film integrity and barrier properties of the coating layer.36 The concept based on a combination of inorganic/organic materials with dual-scale roughness was expected to provide mechanical stability, self-cleaning, and barrier characteristics that are useful for protective applications. The suitability of the prepared formulations for different coating methods was tested by performing spray coating, brush painting, and dip coating techniques (Figure S1). Water contact angle and bouncing phenomena were recorded to analyze superhydrophobic properties of the coatings. In addition, the coatings were tested
for mechanical robustness by abrasion/scratching, for selfcleaning using graphite “dirt”, and for barrier properties by exposure to hydrochloric acid (2 M HCl). The corrosion resistance potential of coatings was studied via a potentiodynamic polarization study.
2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Diatomaceous earth (DE) in the form of diatomite rocks was received from Mount Sylvia Pty Ltd. (Queensland, Australia). Graphite flakes were obtained from Valence Industries Ltd. Australia. TiO2 (P25) nanoparticles were supplied by Degussa, USA, and used as received. Other chemicals including 99.95% tetrahydrofuran (THF) (Chem-Supply), 78% hydrazine (N2H4, Sigma-Aldrich), potassium permanganate (KMnO4, SigmaAldrich), 98% sulfuric acid (H2SO4, Sigma-Aldrich), 85% phosphoric acid (H3PO4, Chem-Supply), 30% hydrogen peroxide (H2O2, ChemSupply), 35% hydrochloric acid (HCl, Merck), and ethanol (ChemSupply) were used as received. Milli-Q water (Purelab option-Q, 18.2 MΩ-cm) was used in all aqueous solutions. Polydimethylsiloxane (PDMS) prepolymer (Sylgard 184) and curing agent were obtained from Dow Corning Corporation (USA). 2.2. Preparation of Diatomaceous Earth (DE). Diatomite rocks were broken up using a hammer and then ground using a benchtop ring mill (Rocklab, New Zealand) to form crushed DE mix. To purify, the resulting coarse powder was dispersed in water (100 g/L) under stirring followed by sonication for 20 min. The supernatant was collected by siphoning, and the remaining sediment was disposed. The supernatant was left to stand for a further 30 min and siphoned to separate it from any further sediment. This process was repeated four times to remove all coarse sediment. The supernatant was left to stand overnight, and the resulting clear solution was removed by siphoning. The remaining solids were dried in an oven at 70 °C for several days and prepared for further crush in a ring mill to obtain fine DE powder. 2.3. Synthesis of Reduced Graphene Oxide (rGO). Graphite flakes (
