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Atmospheric Plasma Nanotexturing of Organic - Inorganic Nanocomposite Coatings for Multifunctional Surface Fabrication Panagiotis Dimitrakellis, Anastasios Patsidis, Athanasios Smyrnakis, Georgios Psarras, and Evangelos Gogolides ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00381 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019
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Atmospheric Plasma Nanotexturing of Organic Inorganic Nanocomposite Coatings for Multifunctional Surface Fabrication P Dimitrakellisa, A C Patsidisb, A Smyrnakisa, G C Psarrasb and E Gogolidesa aInstitute
of Nanoscience and Nanotechnology, NCSR “Demokritos”, Aghia Paraskevi, 15341
Attica, Greece bSmart
Materials & Nanodielectrics Laboratory, Department of Materials Science, University of
Patras, 26504 Rio, Patras, Greece E-mails:
[email protected],
[email protected] KEYWORDS: atmospheric plasma selective etching, nanotexturing, nanocomposite coatings, superhydrophobic, antireflective
ABSTRACT. We present the concept of the combined synthesis of organic-inorganic nanocomposite coatings and atmospheric pressure plasma etching / nanotexturing for tailoring the surface topography and fabricate multifunctional surfaces. As demonstration we fabricated superhydrophobic ZnO / PMMA nanocomposite coatings. Different in ZnO content composite coatings were synthesized and plasma etched in a dielectric barrier discharge operating in He / O2 in open-air environment. The phase selective plasma etching of organic over inorganic matter
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resulted in the gradual revealing of the inorganic ZnO particles, which were multi-sized due to agglomeration during synthesis and plasma etching process. The creation of hierarchical topography led to the fabrication of roll-off superhydrophobic surfaces with water contact angle ~158° and sliding angle ~3° after the application of a low-pressure plasma deposited Teflon-like film. Moreover, we studied the optical properties of the superhydrophobic, atmospheric plasma nanotextured surfaces in terms of reflectance measurements (total, diffuse and specular) to evaluate their possible use as antireflective surfaces.
1. INTRODUCTION Organic - inorganic nanocomposite materials are widely studied the last decades due to their exceptional properties1 and the numerous potential applications such as active packaging,2,3 antibacterial surfaces,4-6 advanced nanodielectrics,7,8 anti-slip surfaces9 and scaffolds for tissue engineering10-12. The addition of inorganic nanomaterials (nanoparticles, nanofibers etc) in a polymer matrix targets the formation of a new material that combines the intrinsic properties of the bulk polymer, such as chemical inertness, thermomechanical stability and high dielectric breakdown strength, with the properties of the inorganic nanomaterial, such as dielectric or conductive response and antibacterial activity. The advantage of nanocomposite materials compared to their microstructured counterparts is the much higher effective surface area of the nanomaterials that augments the desired behavior. Most of current research on nanocomposites is focused on the formation of new bulk materials or the improvement of already existing ones, combining different polymer resins with different inorganic materials in the nanoscale. Those materials range from metals to ceramics and vary
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also in shape and size, e.g. metal nanoparticles, graphene nanosheets and carbon nanotubes. For most of the applications, the desired material performance is determined by the synthesis of the bulk nanocomposite, which in turn is determined by process parameters such as the kind of polymer resin, the kind and size of nanomaterial and the dispersion homogeneity. However, there are several applications where the final performance of the material is not driven by the bulk material structure, but is based on the meticulous ‘tailoring’ of its surface properties. For example, the increase of polymer surface energy through alteration of its surface chemistry can enhance its wettability and adhesion13 and a proper surface texturing (roughening) can lead to enhanced friction,14 anti-reflection15 and enhanced cancer cell isolation in polymeric substrates.16 The manipulation of both surface chemistry and topography can be especially important for the fabrication of bioinspired superhydrophobic surfaces, with various applications such as self-cleaning, anti-icing and anti-biofouling.17-21 The key technology that is widely used to tailor the surface properties of polymers without affecting their bulk is plasma treatment. Low pressure plasma technology has proven its efficiency in polymer thin film deposition and, most important, in polymer etching / nanotexturing for many applications ranging from controlled bio-adhesion to superhydrophobic and anti-reflective surface fabrication.22-28 Low pressure plasma treatment is a ‘batch’ process and requires expensive vacuum equipment, thus it is rather difficult for such systems to be implemented in already existing industrial lines. Atmospheric pressure plasmas, on the contrary, are much more promising means of surface treatments as they offer the possibility for roll-to-roll continuous processing.29-32 In our recent review we have discussed in detail the recent advances and the potential of atmospheric pressure plasma technology for fabrication of superhydrophobic surfaces.33 Such
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surfaces require a low surface energy ultra-thin layer combined with appropriate hierarchical micro-nano-roughness (e.g. dual-scale topography). Atmospheric pressure plasma etching / nanotexturing shows a great potential to induce the proper hierarchical topography that will lead to superhydrophobic behavior after coating with a low surface energy thin film or monolayer.34,35 We have previously proved this potential using paper as substrate, which is a composite-like material composed of cellulose fibers and inorganic (calcite) particles. After oxygen-rich plasma etching in atmospheric pressure we realized an enhanced surface topography and superhydrophobicity, due to the selective etching of organic over inorganic phase followed by deposition of a Teflon-like ultra-thin film.36 The phenomenon of phase selective etching is based on different etching rates among different materials and is widely applied for the creation of nanostructures and nanodevices for semiconductor industry. For example, polymeric and silicon substrates can be etched in oxygen and fluorine-rich plasma chemistry respectively, but a metal mask is etching resistant. High etching selectivity can be achieved in a dual-phase material as well, if the one phase is etched (volatilized) by the plasma and the other is etching resistant (unetchable). Organic - inorganic composite materials are of particular interest due to the very high resistance of inorganic nanomaterials in oxygen-based plasma, which volatilizes the organic phase. Several works have reported on the significant enhancement of surface roughness of composites after treatment in low pressure, oxygen-based plasmas.37-42 However, there are no reports so far studying the combined synthesis of composite systems in terms of filler content and plasma etching / nanotexturing in order to manipulate the surface topography and create hierarchical structures. Atmospheric pressure plasmas in open-air environment, particularly important for large-area and outdoor applications, have been barely
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investigated for selective plasma etching of composites. Finally, the possibility to induce additional properties to the composite material, such as extreme anti-wetting states, by creating hierarchical topography using top-down atmospheric plasma treatment has never been reported. Herein we propose a combination of organic – inorganic nanocomposite material synthesis and subsequent atmospheric pressure plasma etching / nanotexturing for tailoring its final surface topography towards fabrication of multifunctional surfaces such as superhydrophobic and antireflective. Our approach integrates the synthesis of novel nanocomposite coatings, the application of large-area, uniform atmospheric plasma etching / nanotexturing in open-air environment to create hierarchical structures and the evaluation of materials multifunctional behavior as a function of the surface topography evolution. As model system we used Zinc Oxide / Poly(methyl)methacrylate nanocomposites. ZnO is a multifunctional material with unique properties such as piezoelectric, semiconducting, chemical sensing etc.43,44 With our approach, already functional nanometerials such as ZnO / polymer composite are able to acquire additional properties due to atmospheric plasma surface treatment without affecting bulk properties.
2. EXPERIMENTAL SECTION 2.1 Materials. The nanocomposite samples were prepared employing commercially available products. Poly(methyl)methacrylate (PMMA) polymer with Mw = 120 kDa was bought from Sigma Aldrich and dissolved in Propylene Glycol Methyl Ether Acetate (PGMEA) solvent. Ceramic ZnO nanopowder with a particle diameter
