Sensors and Actuators B 171–172 (2012) 165–171 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Self-aligned TiO 2 thin films with remarkable hydrogen sensing functionality Meilan Guo a , Xiaohong Xia a,b , Yun Gao a,∗∗ , Guowen Jiang a , Quanrong Deng a,b , Guosheng Shao b,c,∗ a Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, PR China b Institute for Renewable Energy and Environmental Technologies, University of Bolton, Bolton BL3 5AB, UK c School of Materials Science and Engineering, Zhengzhou University, 100 Science Road, Zhengzhou 450001, PR China a r t i c l e i n f o Article history: Received 23 November 2011 Received in revised form 20 February 2012 Accepted 22 February 2012 Available online 2 March 2012 Keywords: Self-assembly TiO2 thin films Surface energy Hydrogen sensing Hydrothermal synthesis a b s t r a c t Self-aligned titania thin films of either the rutile or the anatase phase were successfully grown on FTO (F-doped SnO 2 ) substrates with hydrothermal methods. The selection of the targeted phase, either rutile or anatase, in the thin films was controlled by adjusting the content and type of acidic anions such as Cl − and SO 4 2− . The growth kinetics depended on the synthesis temperature and the types of alcohol in the solvent. The self-alignment in both the anatase and rutile thin films was attributed to the minimization of surface energies during growth. The hydrogen sensing characteristics of dense films of both rutile and anatase phases were investigated and there was remarkable improvement of sensitivity response over reported data. It was found that rutile films had higher sensitivity while anatase films had faster response. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Titanium dioxide (TiO 2 ), or titania, has been extensively exploited as low-cost and multi-functional materials for a wide range of applications such as photocatalysis [1,2], anti-fogging [3], dye sensitized solar cells [4,5], transparent conducting oxides (TCO) [6], microelectronic devices [7], and gas sensing [8–10]. It is obviously advantageous to synthesize titania phases into dense thin films, so that the material can be readily incorpo- rated into functional devices. For most functional devices relying on the semiconducting, photonic or electrical properties of tita- nia, it is desirable to fabricate thin films in the forms of either single crystals or well-aligned columnar grains, so that they can provide electrical pathways free of grain boundaries along the normal direction and thus enable enhanced device perfor- mance. Efforts have been made to synthesize titania films on supporting substrates, with hydrothermal methods having been developed to grow self-assembled rutile nanorods on TCO substrates for low-cost fabrication of titania thin films for functional applications [5,11,12]. We have demonstrated recently that the hydrothermal growth of self-assembled rutile nano-rods can be well controlled in terms of ∗ Corresponding author at: Institute for Renewable Energy and Environmental Technologies, University of Bolton, Bolton BL3 5AB, UK. Tel.: +44 1204 903592. ∗∗ Co-corresponding author at: Faculty of Physics and Electronics, Hubei Univer- sity, Wuhan 430006, China. E-mail addresses: gaoyun@hubu.edu.cn (Y. Gao), gs6@bolton.ac.uk (G. Shao). growth direction, growth rate, and density of population, and even fully dense rutile thin films can be grown on the FTO substrates with the preferred growth direction along the c-axis of well aligned sin- gle nanocrystals [12]. This offers wide choices for microstructural engineering of rutile nano-rods, so that such a titania phase can be used in tailored material microstructures to deliver optimized objective functionalities. While the anatase phase has attracted greater attention than rutile for photo-catalysis [1] and gas sensing [10], no attempt has been reported on the growth of anatase thin films on a solid substrate using the hydrothermal method. The well known anodic oxidation method by Gong et al. results in aligned hol- low nano-scale columns, with each being made of polycrystalline anatase phase after annealing [13]. Polycrystalline anatase coat- ings prepared by sol dip coating [14,15] tend to have problems such as poor adherence to substrates, cracking during calcina- tion and non-uniformity in thickness. In this work we attempted for the first time to synthesize anatase thin films using the hydrothermal method. The work was based on the success in hydrothermal growth of self-assembled rutile nano-rods on FTO coated glass, and the recent progress in studying phase selection mechanisms for hydrothermal synthesis of titania nano-powders [2]. Dense anatase thin films have been successfully grown on FTO coated glass substrates, with the columnar titania grains in the films exhibiting strong self-alignment along the [0 0 1] axis of anatase. The hydrogen sensing characteristics of dense anatase and rutile films were compared, and it was found that rutile films had a higher sensitivity while anatase films had faster response. 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.02.072