Functionalization of ceramic tile surface by sol–gel technique F. Bondioli a, * , R. Taurino a , A.M. Ferrari b a Università di Modena e Reggio Emilia, Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Via Vignolese 905, 41100 Modena, Italy b Università di Modena e Reggio Emilia, Dipartimento di Scienze e Metodi dell’Ingegneria, Via Amendola 2, 42100 Reggio Emilia, Italy article info Article history: Received 19 December 2008 Accepted 23 February 2009 Available online 7 April 2009 Keywords: Functionalization Tile Titania Photocatalicity Sol–gel Contact angle Scratch resistance Self-cleaning abstract The aim of this investigation was the surface functionalization of industrial ceramic tiles by sol–gel tech- nique to improve at the same time the cleanability of unglazed surfaces. This objective was pursued through the design and preparation of nanostructured coating that was deposited on polished unglazed tiles by air-brushing. In particular TiO 2 –SiO 2 binary film with 1, 2 or 5 wt% of titania were prepared by using tetraethoxysilane and titania nanoparticles as precursors. The obtained films were characterized by scratch tests to verify the adhesion of the coatings to the polished tiles. To mainly evaluate the effect of the thermal treatment (temperature range 100–600 °C) on the photocatalicity of the coatings, the films were studied under UV exposure by contact angle measurements and cleanability test. Particular atten- tion has been paid to preserve the aesthetical aspect of the final product and the obtained hue variation was evaluated by means of UV–visible spectroscopy and colorimetric analysis. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction The unglazed fine porcelainized stoneware tile, also called por- celainized stoneware or grés, is a product used both for internal and external applications in building field. Low-porosity, high mechanical, abrasion, chemical and stain resistance make this material ideal to flooring areas with elevated public use [1,2]. Its high technical properties are mainly due to an extremely sintered body composed of different crystalline phases (anorthite, mullite and quartz) immersed in a vitreous matrix [3,4]. In the ceramic tiles industrial field, the porcelainized tiles have become more and more important with regard to its spread from very few mar- ket shares limited as to their application fields to more and more diversified ones; the result has been a clear increase in production volumes. This product, which was formerly considered only from a technical standpoint, nowadays shows high aesthetical potentiali- ties allowing its use for over-refined purposes. In particular, the high surface hardness of the unglazed porcelainized tiles allows mirror-polished surfaces that give to this product a high aestheti- cal quality. The main problem is that this material, even if it has a very low open porosity (about 0.1% as to the absorbed water, and 0.5% according to the mercury porosimetry), has an internal closed porosity of around 6%, with pore sizes range from 1 to 10 lm. This porosity appears during the polishing phase, where about 0.5– 1 mm of the superficial layer is removed, causing a superficial microporosity that increases the tendency to the product dirtiness. There are many ways of intervention (some of them have already been widely adopted) as for exemplum using transparent poly- meric layers, however the mechanical properties and chemical durability of these coatings are often very poor [5]. The development of new easy-to-clean or even self-cleaning surfaces has recently been under the focus of nanotechnology, i.e. by investigating different surface structures. Some models of self- cleaning surfaces are available in nature, such as lotus plant leaves [5] and the wings of insects [6]. Recently to improve surface clean- ability properties [7–9] the photocatalicity of TiO 2 nanoparticles has been used. For exemplum, the production of photocatalytically active building materials, allows to obtain self-cleaning and self- sterilizing surface that, moreover, might degrade several organic contaminants in the surrounding environment by UV radiation activation [10]. Moreover, one of the main interesting properties of nanoparti- cles, characterized by a mean diameter below the light wavelength, is their transparency if applied on a substrate or dispersed in a ma- trix. To take advantage of this property, the aim of this study was ori- ented in design and synthesis of silica (SiO 2 ) and titania (TiO 2 ) multifunctional inorganic coating, to apply on the polished unglazed surfaces. The goal was the obtainment of a transparent physical bar- rier to enhance the cleanability of tile exploiting at the same time the photocatalicity of titania in order to obtain a multifunctional coat- ing. This objective was pursued through the design and preparation of nanostructured sol–gel coating that was deposited on polished unglazed tiles by air-brushing. The obtained films were fully charac- 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.02.054 * Corresponding author. Fax: +39 059 205 6243. E-mail address: federica.bondioli@unimore.it (F. Bondioli). Journal of Colloid and Interface Science 334 (2009) 195–201 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis