Optical and structural properties of nanocrystalline anatase (TiO 2 ) thin films prepared by non-aqueous sol-gel dip-coating E. Haimi a, ⁎, H. Lipsonen a , J. Larismaa a , M. Kapulainen b , J. Krzak-Ros c , S.-P. Hannula a a Department of Materials Science and Engineering, Aalto University, P.O. Box 16200, FI-00076 Aalto, Finland b VTT, Technical Research Centre of Finland, Tietotie 3, Espoo P.O. Box 1000, FI-02044 VTT, Finland c Institute of Materials Science and Applied Mechanics, Wroclaw University of Technology, PL-50371, Wroclaw, Poland abstract article info Article history: Received 26 August 2010 Received in revised form 16 February 2011 Accepted 23 February 2011 Available online 11 March 2011 Keywords: Titanium dioxide Anatase Thin film Non-aqueous sol-gel Optical properties Wemple–DiDomenico model Anatase (TiO 2 ) thin films were grown by non-aqueous sol-gel dip-coating using titanium (IV) n-butoxide as precursor and 1-butanol as solvent. High withdrawal speed of 4.7 mm/s in dip-coating resulted in defect free films of 100 nm average film thickness after subsequent heat treatments. According to scanning electron microscope and X-ray diffraction measurements, the films consisted of nanocrystalline anatase with 30 nm mean crystallite size. Refractive index n(λ) and extinction coefficient k(λ) were determined over the wavelength range from 200 to 1650 nm. The optical band gap of the film material was approximately 3.2 eV. The results showed very similar optical characteristics to those that are accomplished with chemically more reactive aqueous sol-gel processes. Furthermore, it was found that in addition to porosity, coordination number of Ti atoms to nearest oxygen neighbors is likely to have a significant role in explaining differences of optical properties between bulk anatase and thin film materials of the present work. © 2011 Elsevier B.V. All rights reserved. 1. Introduction TiO 2 is a wide band-gap semiconductor that is of interest for various optical applications, such as photocatalysts, dye sensitized photovoltaic cells, optical spacers in polymer photovoltaic cells, and optical sensors [1–6]. At atmospheric pressures TiO 2 exists in three crystalline polymorphs that are rutile, anatase, and brookite. In addition to the fully crystalline polymorphs, partially crystalline or non-crystalline TiO 2 is frequently encountered especially in thin films. These materials can be classified in terms of decreasing nearest- neighbor atomic order as nanocrystalline or amorphous. In a number of applications, nanocrystalline anatase is technologically the pre- ferred form of TiO 2 . Optical properties of materials are described by refractive index n(λ) and extinction coefficient k(λ) that comprises the complex index of refraction. Since anatase has a tetragonal lattice, it is optically anisotropic. Consequently, two sets of refractive indices and extinction coefficients are required to fully describe optical proper- ties of anatase at each wavelength. After development of single crystal growth of anatase by chemical transport reactions [7], well- defined optical single crystal data has become available [8–12]. Furthermore, a large number of optical measurements on polycrys- talline anatase thin films have been published [13–33]. Optical properties of the material can be summarized as follows. Fully dense polycrystalline anatase has high average refractive index that gradually decreases toward the infra-red wavelength region. At a reference wavelength of 550 nm, the refractive index is approxi- mately 2.5. Moreover, pure anatase is transparent in the visible light. In ultra-violet region, anatase has an absorption edge relating to inter band transitions with characteristic band gap energy. The average optical band gap energy for polycrystalline material with large crystallite size is around 3.2 eV. Slightly higher values are obtained for nanocrystalline anatase, where an increase of band gap has been observed up to 0.2 eV for crystallite sizes in the range of 5–10 nm [5,28]. Photon energies above the band gap result in high absorption. The absorption peak structures have been clarified in the literature based on theoretical calculations [35–37]. Near the absorption edge, the results show significant optical anisotropy between parallel and perpendicular measurements to the c axis. In order to grow polycrystalline TiO 2 thin films for optical applications, a number of deposition techniques have been used. These techniques include radio frequency sputtering [9,13–15], pulsed laser deposition [16,17], plasma enhanced chemical vapor deposition [18,19], ion beam induced chemical vapor deposition [20], and sol-gel methods [20–34]. Optical characterization shows that significant differences in the properties of the films are obtained by using different processing techniques and conditions. The differences have been attributed to variations in such factors as impurity and doping elements, stoichiometry, phase relations, crystallite size, texture between parallel and perpendicular orientations to the Thin Solid Films 519 (2011) 5882–5886 ⁎ Corresponding author. Tel.: +358 9 470 22672; fax: +358 9 470 22677. E-mail address: eero.haimi@tkk.fi (E. Haimi). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.02.091 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf