Directional enhancement of refractive index and tunable wettability of polymeric coatings due to preferential dispersion of colloidal TiO 2 nanorods towards their surface Francesca Pignatelli a, ⁎, Riccardo Carzino a , Marco Salerno a , Marco Scotto a , Claudio Canale a , Monica Distaso a,1 , Francesco Rizzi a , Gianvito Caputo b , Pantaleo Davide Cozzoli b,c , Roberto Cingolani a,b , Athanassia Athanassiou a,b, ⁎ a IIT, Italian Institute of Technology, Via Morego 30, 16163 Genova, Italy b NNL, National Nanotechnology Laboratory, INFM, CNR, IIT Research Unit, Via Arnesano Km 5, 73100 Lecce, Italy c Scuola Superiore ISUFI, Distretto Tecnologico, University of Salento, Via Arnesano Km 5, 73100 Lecce, Italy abstract article info Article history: Received 4 March 2009 Received in revised form 18 November 2009 Accepted 26 January 2010 Available online 2 February 2010 Keywords: Polymer matrix nanocomposites Titanium oxide Thin films Optical properties Wettability Laser irradiation We demonstrate the fabrication of nanocomposite coatings, of organic-capped colloidal TiO 2 nanorods dispersed into a poly(methyl methacrylate) matrix, with rising value of refractive index from the bottom to the top layers, and UV-induced surface wettability alteration, in a reversible manner. This behaviour is attributable to preferential dispersion of the TiO 2 nanoparticles towards the superficial layers of the coatings. Above a critical TiO 2 loading, the nanorods at the surface form aggregates deteriorating the optical and the surface properties of the nanocomposites. The optimal conditions for nanocomposite films preparation in terms of optimized nanorods dispersion, optical clarity, and surface smoothness are determined. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Embedding inorganic nanoparticles in a polymeric matrix can lead to unique nanocomposite systems exhibiting a combination and/or synergistic enhancement of the properties of the individual materials. This potentially allows detailed tailoring of the chemical–physical char- acteristics of any polymer so as to target specific applications by pro- perly choosing and tuning the properties and quantity of the nanofillers. In particular, large band gap metal oxide nanofillers in a polymeric coating are expected to improve the quality of ophthalmic and optical components, enhancing the transmitted light and reducing the reflec- tions. The main characteristics that nanocomposite coatings should provide are high transparency in the visible range and high refractive index [1,2]. These properties combined with the controlled wettability of some nanocomposite films make them ideal coatings for many optical devices (from windows to optical components) [1,2]. Among different polymers the poly(methyl methacrylate) (PMMA) is ideal for optical applications such as optical fibers, lenses and protective coatings. Its widespread use is mainly due to its high transparency, contamination resistance and easy processability [3]. On the other side, titanium dioxide (TiO 2 ) is a well-known white pigment, used since ancient times [4,5]. Some of the properties that made TiO 2 so widespread are biocompatibility, chemical stability, photodurability, mechanical robustness and cheapness. Among others, the capability of TiO 2 to photocatalyze hazardous chemical wastes found in air, soil and water is possibly the most attractive property and it has been largely investigated [5,6]. Moreover, due to its high refractive index TiO 2 is a suitable material for coatings of optical components or for waveguide implementations [5]. Another important property of TiO 2 films or coatings is the capability to change their surface wettability under UV irradiation [7]. This property together with its photocatalytic activity can be properly exploited in functional coatings, producing self cleaning and antifogging surfaces. UV irradiation of titanium dioxide, with energy larger than the band gap E g , induces the excitation of electrons from the valence band to the conduction band. The excited charge carriers can recombine radiatively or nonradiatively. The photogenerated electron–hole pairs may also migrate to the surface of the material, get trapped and react with donor or acceptor molecules absorbed at the surface. In the latter Thin Solid Films 518 (2010) 4425–4431 ⁎ Corresponding authors. IIT, Italian Institute of Technology, Via Morego 30, 16163 Genova, Italy. E-mail addresses: francesca.pignatelli@iit.it (F. Pignatelli), athanassia.athanassiou@unile.it (A. Athanassiou). 1 Current address: Institute of Particle Technology, Friedrich-Alexander-Universität Erlangen-Nürnberg, Cauerstr. 4, D-91058 Erlangen, Germany. 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.01.041 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf