Sensors and Actuators B 155 (2011) 568–576 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Synthesis and characterisation of nanosized TiO 2 –ZrO 2 binary system prepared by an aqueous sol–gel process: Physical and sensing properties M.R. Mohammadi a,∗ , D.J. Fray b a Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., Tehran, Iran b Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK article info Article history: Received 28 August 2010 Received in revised form 6 January 2011 Accepted 9 January 2011 Available online 28 January 2011 Keywords: Titanium dioxide Zirconium oxide Nanostructure Aqueous sol–gel abstract Nanostructured TiO 2 –ZrO 2 thin films and powders were prepared by a straightforward aqueous particu- late sol–gel route. Titanium (IV) isopropoxide and zirconium (IV) acetate hydrate were used as precursors, and hydroxypropyl cellulose was used as a polymeric fugitive agent in order to increase the specific sur- face area. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy revealed that the powder were crystallised at the low temperature of 500 ◦ C, containing anatase-TiO 2 and tetragonal-ZrO 2 phases. Furthermore, it was found that ZrO 2 retarded the anatase-to-rutile transformation up to 900 ◦ C. The activation energies for crystallite growth of TiO 2 and ZrO 2 components in the binary system were calculated 10.16 and 3.12 kJ/mol, respectively. Transmission electron microscope (TEM) image showed that one of the smallest crystallite sizes was obtained for TiO 2 –ZrO 2 binary mixed oxide, being 5 nm at 500 ◦ C. Field emission scanning electron microscope (FESEM) analysis revealed that the deposited thin films had nanostructured morphology with the average grain size of 20 nm at 500 ◦ C and 36 nm at 900 ◦ C. Thin films produced under optimised conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO and NO 2 gases at low operating temperature of 150 ◦ C, resulted in an increase of thermal stability of sensing films as well as a decrease in the power consumption. Furthermore, calibration curves revealed that TiO 2 –ZrO 2 sensor follows the power law, S = A[gas] B (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration) for the two types of gases, and it has excellent capability for the detection of low gas concentrations. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Titanium dioxide has a widespread range of traditional and new applications, as a white pigment for paints or cosmetics [1], sup- port for catalysts [2], coating for self-cleaning surfaces [3], energy resource for solar cells [4], electrode material for lithium batter- ies [5], antireflection coating for dye sensitized photovoltaic cells [6] and sensing film gas sensors [7]. In developing gas sensors it is critical to use a titanium dioxide film with the highest possi- ble specific surface area. There is also evidence in favor of anatase as the most promising phase for gas detection due to its higher surface reactivity to gases [8,9]. Recently, many efforts have been aimed to improve the gas sensing performance by improvements in selectivity, sensitivity and durability. In order to improve these properties, microstructure control by preparing porous, high spe- ∗ Corresponding author at: Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., Tehran, Iran. Tel.: +98 21 6616 5211. E-mail addresses: mrm41@cam.ac.uk, mohammadi@sharif.edu (M.R. Mohammadi). cific surface area films and doping with hetero components (such as Sn, V, Cr, W, Co, Cu, Fe, Nb, Ta, Ga and Mo) are known to be effec- tive, because active sites for particular gas species can be produced [10–14]. Another method to improve gas sensing performance of metal oxide semiconductors is to employ binary metal oxide semi- conductors. This novel alternative has the potential to form tailored film morphologies, which facilitates gas–film interaction by alter- ing atomic ratio of each element. Furthermore, it is possible to increase the current single metal oxide surface-to-volume ratio and to fabricate stable nano-sized grain morphologies for high per- formance gas sensing thin films [15]. Sensing properties of binary oxides based on TiO 2 such as TiO 2 –MoO 3 [15], TiO 2 –WO 3 [16], TiO 2 –Cr 2 O 3 [17], TiO 2 –V 2 O 5 [13] and TiO 2 –CeO 2 [18] have been reported previously. Biju and Jain [19] reported humidity sens- ing properties of nanostructured multilayered TiO 2 :ZrO 2 thin films prepared by sol–gel processing. An enhancement in the sensitivity was observed for both TiO 2 /ZrO 2 (top layer of TiO 2 on ZrO 2 film) and ZrO 2 /TiO 2 (top layer of ZrO 2 on TiO 2 film) films. The empirical exploration of mixing TiO 2 and ZrO 2 may lead to new gas sensing properties or may simply lead to a material composed of charac- teristics similar to TiO 2 and ZrO 2 . Therefore, in the present work a 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.009