Sensors and Actuators B 174 (2012) 527–534 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb Hydrogen sensors on the basis of SnO 2 –TiO 2 systems D. Shaposhnik a,∗ , R. Pavelko b , E. Llobet a , F. Gispert-Guirado a , X. Vilanova a a Minos-EMAS, Department of Electronic Engineering, University Rovira i Virgili, Tarragona, Spain b Kyushu University, Department of Energy and Material Sciences, Kasuga-Koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan a r t i c l e i n f o Article history: Received 28 December 2011 Received in revised form 28 April 2012 Accepted 7 May 2012 Available online 14 May 2012 Keywords: Semiconductor gas sensor Hydrogen Tin oxide Titanium oxide Crystallite growth a b s t r a c t In this study we compare two types of materials for gas sensor applications: co-precipitated SnO 2 and TiO 2 and their mechanical mixtures. TEM, FTIR, and TXRD analyses were used to compare the synthesized materials. It was found that co-precipitation leads to the formation at low temperatures of a rutile phase only, which does not undergo any phase transformation upon heating. In contrast, separately synthe- sized TiO 2 with anatase-type structure induces crystallite growth in SnO 2 mechanically mixed with the former oxide. Sensing properties of the materials in question were analyzed in a broad range of working temperatures and H 2 concentrations. Higher signals of co-precipitated materials are discussed regarding their electrical properties, thermal stability and surface hydroxyls. © 2012 Published by Elsevier B.V. 1. Introduction Fast development of hydrogen-based technologies, including promising reports on hydrogen vehicles and fuel cells, give rise to a need for inexpensive and sensitive detectors of hydrogen leak- ages. Importance of hydrogen sensors was also sadly proved in atomic industry: both Chernobyl and Fukushima accidents were aggravated by hydrogen explosions. Together with application in early fire alarms, hydrogen sensors seem to become one of the most abundant gas detectors in the near future [1]. Up to now the most suitable technologies for hydrogen sensors mass production include electrochemical and metal oxide (MOx) types. Even though MOx sensors are known to be highly dependent on humidity and this disadvantage (together with slightly higher power consumption) differs them greatly from the electrochem- ical type, the MOx sensors are still promising for the market of hydrogen detectors [1]. Being robust, compact, energy efficient (especially MEMS type) and low-cost, this type of H 2 detectors should overcome the problem of high cross sensitivity under real ambient conditions, which in general, is one of the most crucial shortcomings for many hydrogen detectors [2]. One of the ways to increase selectivity in the presence of water vapors is to modify the sensing material, without technological complication of sen- sor design. Many R&D efforts have been focused on this problem [3–9]. SnO 2 doped with TiO 2 is among the prospective materials. The material has demonstrated rather low cross sensitivity towards ∗ Corresponding author. Tel.: +34 977 25 65 71; fax: +34 977 55 96 05. E-mail address: dmitry.shaposhnik@urv.cat (D. Shaposhnik). water vapors [3,8,10]. It seems therefore important for practical applications to study the SnO 2 –TiO 2 system in more detail. Namely, it is important to understand: what is the role of the doping, is there any difference between bulk doping and simple mechanical mixture of the phases, and finally how stable are the doped mate- rials at elevated temperatures. This article partially answers these questions. It is well known that tin and titanium dioxides (cassiterite and rutile phases) possess iso-structural crystalline modification – both crystallize in tetragonal structure P 4(2)/mnm. Similarity in the crystalline structure ensures formation of solid solutions as well as decreases electron scattering on the interphases between con- tacting crystallites [10,11]. But in spite of structural similarity, the oxides differ remarkably in their electronic properties. N-type conductivity of both materials is mainly determined by under- stoichiometric amount of oxygen atoms in the crystalline lattice. The latter gives rise to numerous donor states within the wide band gaps. For SnO 2 the surface donor states are located at ca. 114 meV below conduction band, while for TiO 2 – ca. 800 meV below Fermi level [12,13]. The difference in shallow levels position explains high resistance of TiO 2 based materials at sensor working temperatures. It also suggests that in SnO 2 –TiO 2 systems, electrons generated either due to temperature or surface reaction will migrate towards TiO 2 , causing electron depletion in SnO 2 phase, likewise in the case of PdO and Ag 2 O [12]. Another important observation for gas sensors is thermal sta- bility of surface and bulk composition in the dispersed SnO 2 –TiO 2 system. At 1430 ◦ C the phases are known to form solid solutions in all ranges of SnO 2 /TiO 2 ratio. However, upon cooling, the solid solution undergoes spinoidal decomposition if the ratio is within 0925-4005/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.snb.2012.05.028