RESUMO
Single-walled carbon nanotubes (SWCNTs) have unique thermal and electrical properties. Coating them with a thin metal layer can provide promising materials for many applications. This study presents a bio-inspired, environmentally friendly technique for CNT metallization using polydopamine (PDA) as an adhesion promoter, followed by electroless plating with nickel. To improve the dispersion in the aqueous reaction baths, part of the SWCNTs was oxidized prior to PDA coating. The SWCNTs were studied before and after PDA deposition and metallization by scanning and transmission electron microscopy, scanning force microscopy, and X-ray photoelectron spectroscopy. These methods verified the successful coating and revealed that the distribution of PDA and nickel was significantly improved by the prior oxidation step. Thermoelectric characterization showed that the PDA layer acted as a p-dopant, increasing the Seebeck coefficient S of the SWCNTs. The subsequent metallization decreased S, but no negative S-values were reached. Both coatings affected the volume conductivity and the power factor, too. Thus, electroless metallization of oxidized and PDA-coated SWCNTs is a suitable method to create a homogeneous metal layer and to adjust their conduction type, but more work is necessary to optimize the thermoelectric properties.
RESUMO
Metallization is a common method to produce functional or decorative coatings on plastic surfaces. State-of-the-art technologies require energy-intensive process steps and the use of organic solvents or hazardous substances to achieve sufficient adhesion between the polymer and the metal layer. The present study introduces a facile bio-inspired "green" approach to improve this technology: the use of dopamine, a small-molecule mimic of the main structural component of adhesive mussel proteins, as an adhesion promoter. To understand dopamine adhesion and identify conditions for successful metallization, polyethylene surfaces were dip-coated with dopamine and metallized with nickel by electroless metallization; essential parameters such as temperature, pH value, concentration of dopamine and buffer, and the deposition time were systematically varied. Effects of adding oxidants to the dopamine bath, cross-linking, thermal and UV post-treatment of the polydopamine film, and plasma pretreatment of the substrate were investigated. The properties of the polydopamine layer and the quality of the metal film were studied by physico-chemical, optical, and mechanical techniques. It was shown that simple dip-coating of the substrate with dopamine under optimal conditions is sufficient to support metal layers with a good optical quality. Technologically relevant metal layer quality and adhesion were obtained with annealed and UV-treated polydopamine films and enhanced by plasma pretreatment of the substrate. The study shows that dopamine provides a new interfacial design for plastic metallization that can reduce energy consumption, use of hazardous substances, and reject rate during manufacturing. The results are essential findings for further technological developments of a universal platform to promote adhesion between plastics and metal or potentially also other material classes, enabling economic material development and more eco-friendly applications.
