Photoactive and self-cleaning TiO 2 –SiO 2 thin films on 316L stainless steel Behrooz Shayegh Boroujeny, A. Afshar ⁎, A. Dolati Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran abstract article info Article history: Received 12 July 2011 Received in revised form 12 March 2012 Accepted 15 March 2012 Available online 27 March 2012 Keywords: Self-cleaning properties Photocatalysis Photo-generated hydrophilicity Titanium dioxide Silicon dioxide In this study, TiO 2 –SiO 2 nanocomposite films with different amounts of SiO 2 were prepared by sol–gel pro- cess and were coated onto stainless steel 316L. The effect of addition of various amount of SiO 2 in the precur- sor solution on the photocatalysis, photo-generated hydrophilicity and self-cleaning property of TiO 2 thin films was investigated by X-ray diffraction, Fourier transform infrared spectroscopy, water contact angle measurements and UV spectroscopy. In the tested ranges of SiO 2 content and sintering temperature, the highest photocatalytic activity and self-cleaning property were observed in the 15 mol% SiO 2 sample sintered at 750 °C. Addition of less than 30 mol% SiO 2 had a suppressive effect on the transformation of anatase to ru- tile and on the crystal growth of anatase in the sintering heat treatment. Phase separation occurred in the composite films for SiO 2 content of 30 mol% or more. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Titanium dioxide (titania, TiO 2 ) has become well-known by the introduction of photocatalytic products [1–5]. Transparent and thin TiO 2 films on glass substrates have a high potential for practical appli- cations such as in mirrors, window glasses, etc. while the thicker ones are the best choice for photocatalysis purposes [6–10]. Self-cleaning windows have been produced by the famous glass companies since the last decade. Pilkington Activ™, Saint-Gobain Bioclean™ and the Pittsburg Plate Company's Sunclean™ use a nanocrystalline thin film of titania [11,12]. This film cleans the glass in sunlight through two different properties: photocatalysis induces the coating to de- compose organic dirt adsorbed onto the glass, while hydrophilicity reduces water contact angles to very low values in sunlight (UV light) and washes away dirt [13]. Depending on the composition and the processing, the surface can have more photocatalytic activity and less superhydrophilic activity or vice versa because the photo- induced hydrophilic conversion is a different process from the photo- catalytic decomposition of organic dirts. Self-cleaning is possible only if these two processes take place simultaneously, even though they are basically different in nature. TiO 2 can create both of these two dis- tinct photo-induced phenomena leading to one of the unique charac- teristics of this composition and making it ideal to be used as a self- cleaning coating [1,5,14]. Unfortunately, anatase TiO 2 , which is the most photoactive phase of TiO 2 , only absorbs ultraviolet radiation with the wavelengths of shorter than 380 nm [15–17]. In addition, it has been already observed that the water contact angle of TiO 2 films goes up quickly in the darkness and remains relatively constant as long as they are subjected to the darkness [14]. It is desirable for the contact angle to rise slowly in the darkness and to stay low for a long time because in actual applications like sunlight in rainy or cloudy days, the surface is not always irradiated by UV radiation [18]. It was observed that not only the hydrophilic property but also the photocatalysis was improved in TiO 2 –SiO 2 composite films by adding SiO 2 [18–23]. In most previous studies, glass and tile have been used as substrate while stainless steel appears to be an ideal substrate for architectural self-cleaning applications, anti-microbial coatings and many environ- mental applications including water purification [24–26]. Furthermore, the relationship between the photocatalytic, hydrophilic and self- cleaning effect has not been focused in other studies. In the present study, TiO 2 –SiO 2 nanocomposite films with different amounts of SiO 2 content were prepared by sol–gel process and were coated on stainless steel 316L. The phase structure and the photocatalytic, hydrophilic and self-cleaning properties of TiO 2 –SiO 2 films were measured. The relation- ship between the photocatalysis, the photo-generated hydrophilicity and the self-cleaning effect were discussed to obtain an optimum condition. 2. Experimental details 2.1. Solution preparation The sol–gel method was used to prepare the TiO 2 –SiO 2 mixed films containing 0–25 mol% SiO 2 from the mixtures of TiO 2 and TiO 2 precursor sols. A SiO 2 polymeric sol was prepared by diluting 1 mol tetraethoxysilane (TEOS) in 2.68 ml ethanol, deionized water and nitric acid (HNO 3 ). TEOS concentration in concentrated sol was Thin Solid Films 520 (2012) 6355–6360 ⁎ Corresponding author. Tel.: +98 21 6616 5204; fax: +98 21 6600 5717. E-mail address: afshar@sharif.edu (A. Afshar). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.03.051 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf