Generating highly reflective and conductive metal layers through a light-assisted synthesis and assembling of silver nanoparticles in a polymer matrix

www.nature.com/scientificreports OPEN Received: 20 June 2017 Accepted: 7 September 2017 Published: xx xx xxxx Generating highly reflective and conductive metal layers through a light-assisted synthesis and assembling of silver nanoparticles in a polymer matrix Mohamed Zaier, Loïc Vidal, Samar Hajjar-Garreau & Lavinia Balan The development of metalized surfaces exhibiting mirror properties and/or electric conductivity without heavy equipments and with low metal charge is a big challenge in view of many industrial applications. We report herein on the photo-assembling of silver nanoparticles (AgNPs) in a polymer matrix, carried out within minutes from an acrylate monomer and silver nitrate at room temperature, under air and without any solvents. The top surface of the material gets converted into a continuous silver thin film and a depthwise concentration gradient of AgNPs is created in the polymer, which images the absorption profile of the actinic UV light in the reactive formulation. This specific assembling of the silver@polymer coating induces excellent reflective and conductive properties. The conductance was observed to strongly increase with increasing the exposure from 3 to 30 min due to the formation of a more and more compact metal film. This coating strategy works with a variety of substrates (textile, paper, glass, wood, plastic and stainless steel). Moreover, on flexible surfaces such as textile, the flexibility was preserved. The possibility to use this kind of nanomaterial as a printing ink, with a much lower metal concentration (3 to 5 wt.%) than concurrent inks, was also demonstrated. In the past decades, the deposition of metal nanoparticles (MNPs) at the surface of various substrates for applications such as highly reflective coatings or printing techniques has gained wide interest. For example, Ag films can be used as decorative elements and reflector concentrators for solar power generation1, as contacts in microelectronics2, as antibacterial surfaces3,4, and for their reflective and conductive properties5–8. Various methods were developed to engineer highly reflective Ag mirrors. The early works of the German chemist Justus von Liebig, based on spraying a glass surface with a solution of Ag+ and sugar9 is still used in the manufacture of common household mirrors. In 1988, Yogev and Efrima introduced an approach for generating Ag mirrors at a liquid-liquid interface via multilayered metal liquid films (MELLFs) and Ag aggregates were formed at a water/ dichloromethane interface10. Southward et al. developed a thermally cured silver-polyimide films via the in situ reduction of silver(I) acetate for a few hours and at 300 °C11,12. Electron-beam gun evaporation13 can also be used to generate metal mirrors but the method requires severe deposition conditions such as high temperature and high electron gun intensity. Recently, an electrochemically switchable stable and bistable silver mirror was prepared by introducing a thiol-modified indium tin oxide (ITO) electrode in ionic liquids to improve the stabilization of the metallic film14. However, the mirror state requires the continuous application of a reductive voltage to avoid the dissolution of the Ag film into the electrolyte. Another chemical method developed recently to generate Ag mirrors is the fluoride-induced reduction of Ag+ cations. It relies on the reduction of a Ag+ Lewis acid into Ag(0) by a F-, thus generating a Ag mirror15. However, this method suffers some drawbacks, e.g. use of only aprotic solvents, additional washing step to remove the excess of reagents for the mirrors, and noteworthy is also that the deposition is feasible only on the surface of reaction containers. CNRS, Institut de Science des Matériaux de Mulhouse, UMR 7361, 15 rue Jean Starcky, 68057, Mulhouse, France. Correspondence and requests for materials should be addressed to L.B. (email: ) Scientific Reports | 7: 12410 | DOI:10.1038/s41598-017-12617-8 1 www.nature.com/scientificreports/ Inkjet printing has also attracted increasing attention due to its potential applications in photovoltaics16, light-emitting diodes (LEDs)17, sensors18, batteries19 or smart clothing20. Among inkjet printings, the deposition of conductive patterns on substrates like paper, plastic or textile is of high interest for the fabrication of electronic devices such as chemical sensors, field effect transistors (FETs), electrical circuits or for radio frequency identification (RFID)21,22. Because the conductivity of materials like polymers, carbon or graphene (10 to 102 S.cm−1) is generally lower than that of metals (104 to 105 S.cm−1), the use of MNPs, and especially of AgNPs, was recently investigated intensively. The interest in AgNPs is mainly motivated by their unique electric conductive properties, their ease of production compared to AuNPs, high stability and low cost23,24. Recent reports demonstrated that AgNPs inks can be prepared using preformed poly(acrylic acid) or poly(vinylpyrrolidinone) while AgNPs obtained by reduction of an Ag+ salt using ethylene glycol at high temperature or monoethanolamine as reducing agents or via a modified Tollens’ process require ammonium hydroxide and formic acid25–29. However, the need to synthesize AgNPs in advance and/or the use of hazardous chemicals or organic solvents restricts the use of these methods. During recent years, our research group developed MNPs and metal-polymer nanocomposites (silver, gold or palladium) using a photo-induced approach. Because it allows activation of chemical reactions at ambient temperature, light acts without inducing collateral damages due to heating of the surrounding media. This approach offers the advantage over concurrent thermally activated processes to generate MNPs in situ and in a photosensitive formulation or in a polymer matrix. Therefore, it has become highly valuable for elaborating metal-polymer nanocomposites containing homogenously dispersed MNPs30–32. The self-assembly of MNPs has also recently emerged as a promising way of generating tunable optical or plasmoniques devices33,34. However, the perfect control of the spacial distribution of MNPs and their assembling is clearly a challenge for the synthesis of 3-dimensionally (3D) shaped nanomaterials. In this paper, we report on an efficient, in situ, one-step and all-photoinduced approach to produce metal mirrors and conductive coatings at room temperature and under air. This is conducted by spatially controlling and assembling MNPs in a 3D polymer network. Indeed, the depthwise arrangement of AgNPs, i.e. tuning their density from the surface to the depths of the coating, is obtained through a kinetic key, which is activated by the absorption of the actinic light. In this way and under particular experimental conditions, it is possible to generate a continuous thin metal film at the coating top surface exhibiting excellent electric conductivity and light reflectivity i.e. a mirror with a sub-wav (...truncated)