Journal of Hazardous Materials 147 (2007) 213–218 Synthesis of photocatalytic TiO 2 thin films via the high-pressure crystallization process at low temperatures Chung-Hsin Lu ∗ , Wei-Hong Wu, R.B. Kale Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC Received 2 November 2006; received in revised form 21 December 2006; accepted 28 December 2006 Available online 4 January 2007 Abstract TiO 2 thin films with a monophasic anatase structure were synthesized via a high-pressure crystallization (HPC) process which successfully lowered the crystallization temperature of TiO 2 films from 350 to 150 ◦ C. The thermal budget and energy consumption during the crystallization process were markedly reduced and dense films without cracks were obtained. During the HPC process, crystallization took place throughout the films and TiO 2 films with uniform crystallinity were obtained. The HPC process also led to an enhancement in the wettability of TiO 2 thin films. The hydrophilicity of the films increased with heating temperatures via high-pressure annealing. In comparison with the conventional annealing, the HPC process not only produced TiO 2 films with superior photo-induced super-hydrophilicity, but also led to higher photocatalytic activity of the films. The HPC process was confirmed to provide a new route for synthesizing well-crystallized anatase TiO 2 thin films with high photocatalytic activity and good wettability at low temperatures. © 2007 Elsevier B.V. All rights reserved. Keywords: Thin films; Titanium dioxide; High-pressure crystallization; Photocatalytic activity 1. Introduction Titanium dioxide (TiO 2 ), a promising compound in terms of technological applications, has been extensively studied because of its excellent chemical stability, mechanical hardness, high oxidation affinity and optical transmittance with high refractive index [1]. Recently, nanocrystalline TiO 2 has been the focus of investigation for been utilized as a non-toxic, low cost material in various applications including dye sensitized solar cells [2], gate oxides in transistors (MOS-FETs) [3,4], thin film capacitors [5], gas sensors [6], photocatalysis and pigments [7–14]. The amphiphilic TiO 2 surface induced by UV light is expected to be applicable to windshields and mirrors for vehicles [15]. Most semiconductors investigated as photocatalysts in the present days belong to n-type semiconductors, including TiO 2 , ZnO, CdSe, WO 3 , Fe 2 O 3 , PbS, SnO 2 , In 2 O 3 , ZnS, SrTiO 3 , SiO 2 , etc. [16]. These semiconductors have certain degree of photocatalytic activities in breaking down organic compounds. But most of them are chemically or photochemically corrosive ∗ Corresponding author. Tel.: +886 22 23651428. E-mail address: chlu@ntu.edu.tw (C.-H. Lu). indicating that they cannot be applied to water purification. TiO 2 thin films as a photocatalyst (mostly in anatase phase) has the advantages of acid resistance, base resistance, good chemical and photochemical stability, non-toxicity, low cost, and high photocatalytic activity. These advantages have attracted inten- sive research to be focused on TiO 2 . TiO 2 thin films can be prepared using various chemical meth- ods such as chemical bath deposition, electrodeposition, ionic layer adsorption and reaction, and sol–gel process [15,17–20]. In these chemical methods, the as-synthesized TiO 2 films are amorphous and high-temperature heating at or above 350 ◦ C is required to transform the amorphous phases into crystalline phases. Recently, a novel high-pressure crystallization (HPC) process developed by our group [21] has been used to success- fully crystallize ceramic thin films at a temperature considerably lower than that required in the commonly adopted atmospheric- pressure annealing process [22,23]. In the present study, the HPC process was successfully utilized to lower the crystallization temperature of TiO 2 thin films. Various aspects such as crystal quality, crystallite size, crystallization degree, super-hydrophilicity, microstructure, and photocatalytic activity of TiO 2 thin films were investigated as a function of annealing temperature in the HPC process, and the 0304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2006.12.068