Synthesis of mixed-phase titania films by low-temperature ultrasonic spray pyrolysis A. Nakaruk a , G. Kavei a,b , C.C. Sorrell a, ⁎ a School of Materials Science and Engineering, University of New South Wales Sydney, NSW 2052, Australia b Material and Energy Research Centre, P. O. Box 14155-4777, Tehran, Iran abstract article info Article history: Received 10 December 2009 Accepted 9 March 2010 Available online 19 March 2010 Keywords: Titanium dioxide Ultrasonic spray pyrolysis Thin film Glancing angle X-ray diffraction Fully dense TiO 2 films with (1) mixed-phase rutile and anatase and (2) anatase (sole phase) were deposited on fused quartz substrates by ultrasonic spray pyrolysis at nominally 400 °C. The presence and absence of insulation around the entrainment pathway traversing 20 cm above the substrate/hot plate were investigated (174 °C vs 122 °C). The thick films were assessed in terms of mineralogies (qualitative and quantitative), microstructures (topography, thickness and grain size), and visible light transmission (optical and microstructural considera- tions). With insulation, opaque mixed anatase (∼30 vol.%; b 50 nm) and rutile (∼70 vol.%; ∼1 μm) were observed; without insulation, only transparent anatase (b 50 nm) was observed. © 2010 Elsevier B.V. All rights reserved. 1. Introduction In the past decade, interest in titanium dioxide (titania, TiO 2 ) has increased rapidly and significantly due to the material's potential applications in photovoltaics [1–3] and photocatalytic processes. For the synthesis of titania films, numerous methods have been reported, including sol–gel [4], dip-coating [5], gel oxidation [6], anodic oxidation [7], electrophoretic deposition [8], and spray pyrolysis [9]. Spray pyrolysis is one of the most attractive techniques because it involves simple methods and materials, operation in air, and associated low costs. In general, anatase films are known to have high transparencies (allowing coatings on glass and solar cells) [10] and small grain sizes (providing higher surface areas for photocata- lysis) [11]. However, anatase is a wide band gap semiconductor (3.2– 3.5 eV [4,12]), which is disadvantageous for photocatalytic processes since only UV radiation wavelengths are suitable. Conversely, rutile films have lower band gaps (3.0–3.2 eV [4,12]), which allows them to utilise both UV and visible light in photocatalytic processes. However, owing to the high temperatures associated with some processing routes, the anatase → rutile phase transformation occurs. For bulk materials, this is normally considered to occur at ∼ 600 °C [13] but, for thin and thick films, it has been reported to vary between 415 °C and 900 °C [4,12]. Although rutile can be considered desirable from the optical perspective, the grain growth associated with the high temperatures [12,14] usually results in larger grain sizes and associated lower transparencies in the visible region; the accompa- nying lower surface area also is a disadvantage. From the preceding comments, it is clear that the capacity to produce rutile films at low temperatures would be advantageous since this would improve both optical and photocatalytic aspects. However, the kinetics of the phase transformation typically preclude this. Many authors have reported promising results with mixed-phase films of anatase + rutile, which represent a transition stage in the kinetics of the anatase → rutile phase transformation. However, the intention of the present work was to investigate thermal effects of processing on the ultrasonic spray pyrolysis of titania thick films at low temperatures in order to minimise phase transformation and grain growth effects. 2. Experimental procedure The precursor solution was prepared by dissolving titanium butoxide in methanol at 0.5 M Ti concentration. The ultrasonic spray pyrolysis system variants are shown in Fig. 1. These consisted of an ultrasonic generator (1.7 MHz), air compressor, air flow meter, conduit tube, hot plate and controller, and insulator. Further details of the experimental setup are described elsewhere [12]. The presence or absence of the insulator was the sole variable used to assess the effects of temperature on the film properties. The procedure consisted of placing a fused quartz substrate on a hot plate at the maximal setting of 400 °C. However, during pyrolysis, the temperatures at the top of the insulated volume (point A, Fig. 1) were measured by a thermocouple to be 174°± 2 °C and 122° ±2 °C (ΔT = 52 °C) for the insulated and uninsulated experimental setups, respectively. The temperatures at the substrate surface (point B, Fig. 1) were 396° ± 2 °C and 383° ± 2 °C (ΔT = 13 °C) for the insulated and uninsulated experimental setups, respectively. The mineralogies of the titania films were examined by glancing angle X-ray diffraction (GAXRD, Philips X'pert Materials Research Diffrac- tometer). The film thicknesses were determined using single-beam Materials Letters 64 (2010) 1365–1368 ⁎ Corresponding author. Tel.: + 61 2 9385 4421; fax: + 61 2 9385 5956. E-mail address: C.Sorrell@unsw.edu.au (C.C. Sorrell). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.03.016 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet