AueAg nanoparticles as red pigment in ceramic inks for digital decoration M. Blosi a, * , S. Albonetti b, c , F. Gatti b , G. Baldi d , M. Dondi a a Institute of Science and Technology for Ceramics (ISTEC-CNR), Via Granarolo 64, 48018 Faenza, Italy b Department of Industrial Chemistry and Materials, University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy c INSTM, Research Unit of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy d CERICOL, Colorobbia Research Centre, Via Pietramarina 123, 50053 Sovigliana Vinci, Italy article info Article history: Received 29 August 2011 Received in revised form 20 December 2011 Accepted 11 January 2012 Available online 24 January 2012 Keywords: Ag Au Ceramic pigment Ink Nanoparticles abstract Novel pigments, consisting of AueAg mixed nanoparticles, were developed for digital decoration by ink- jet printing of ceramic wares. Special attention was paid to set up a microwave assisted synthesis route, with a low environmental impact, easily transferable to large-scale production. Several suspensions, based on Au, Ag and AueAg mixed nanoparticles were prepared, trying to get a core-shell assemblage, and the synthesis parameters like metal concentration, Ag/Au ratio, time, temperature and chelating agent amount were optimized. The suspensions are stable over many months and a total reaction yield, assessed by ICP-AES analysis, was achieved. Particle size, shape, composition and optical properties were measured by DLS, TEM-EDS, XRD and UVeVIS spectroscopy. The so-prepared inks were applied on ceramic tiles simulating the ceramic process and the colour performance, assessed by colourimetry, were expressed in the CIELab parameters. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Metal nanoparticles represent an efficient way to bestow yellow to red coloration on glass and transparent glazes [1e3]. Colour is developed through the mechanism of surface plasmon resonance (SPR) e although it depends also on particle size, particles inter- distance, alternating dielectric and metal particles-bearing layers [1] e which is due to charge-density oscillations confined to metal nanoparticles [4,5]. The excitation of surface plasmons by an elec- tric field at an incident wavelength, where resonance occurs, results in strong light scattering, appearance of intense SPR absorption bands, and enhancement of the local electromagnetic field. The frequency (i.e. absorption maxima and therefore colour) and intensity of SPR bands are characteristic of each metal and highly sensitive to particle size and shape [4e7]. As a matter of fact, nanodispersions of Au, Ag and Cu were used to colour glass and lustre glassy coatings since Roman times [8], and particularly during the Middle Age [9e11] and the Renaissance [12e14]. This property of noble metals has been recently exploited by the ceramic industry through digital decoration techniques, primarily ink-jet printing, which make use of gold nanopigments [15,16]. Current technology of metal inks for ceramics encompasses various approaches: (i) application of gold precursors then in-situ nucleation and growth of Au crystals inside the glassy matrix [17,18]; (ii) synthesis of gold nanoparticles by polyol method and application as ceramic pigment [15,19]. These Au nanopigments proved to be particularly stable in a wide range of firing tempera- tures (up to 1200  C) and chemical environments (different glazes, glassy coatings and porcelain stoneware bodies) [16]. However, a strong limitation to gold inks stems from their high cost, even if efficient colouring performance is achieved with very low Au concentrations [15,16]. From this standpoint, industry can take advantage by using silver instead of gold, as Ag is less expensive than Au by a factor of w50. In addition, inks containing both silver and gold are expected to exhibit different optical properties, as the energy of SPR bands changes with the Au/Ag ratio [1e4]. Moreover, these features may have a beneficial effect on the technological behaviour of inks, e.g. improving their colouring performance. In this paper, we present a simple microwave-assisted route for producing gold, silver and AueAg structures in the form of stable nanosuspensions [20] assessing their suitability as ceramic colorant. Control of particle size and colloidal stability was pursued through accurate reaction optimization combined with microwave heating, in order to get a significant process intensification, on a large scale production too. In fact, both low environmental impact and long time stability of suspensions can turn into a breakthrough for industrial scale up. * Corresponding author. Tel.: þ39 0546699718. E-mail address: magda.blosi@istec.cnr.it (M. Blosi). Contents lists available at SciVerse ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2012.01.006 Dyes and Pigments 94 (2012) 355e362