Sensors and Actuators B 161 (2012) 740–747 Contents lists available at SciVerse ScienceDirect Sensors and Actuators B: Chemical j o ur nal homep a ge: www.elsevier.com/locate/snb Quenched, nanocrystalline In 4 Sn 3 O 12 high temperature phase for gas sensing applications Jens A. Kemmler a,1 , Suman Pokhrel b,1 , Johannes Birkenstock c , Marco Schowalter d , Andreas Rosenauer d , Nicolae Bârsan a , Udo Weimar a , Lutz Mädler b,∗ a Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 15, 72076 Tübingen, Germany b Foundation Institute of Materials Science (IWT), Department of Production Engineering, University of Bremen, Badgasteiner Str. 328359 Bremen, Germany c Central Laboratory for Crystallography and Applied Materials, University of Bremen, Germany d Institute of Solid State Physics, University of Bremen, Germany a r t i c l e i n f o Article history: Received 21 June 2011 Received in revised form 9 November 2011 Accepted 12 November 2011 Available online 26 November 2011 Keywords: Flame spray pyrolysis In4Sn3O12 Quenched metastable phase Facile sensor assembly Formaldehyde sensing a b s t r a c t Flame spray pyrolysis (FSP) allowed quenching the high temperature phase In 4 Sn 3 O 12 in the form of highly single crystalline particles of about 6 nm. These nanoparticles were in situ deposited to form stable porous films on interdigitated electrodes. The resulting gas sensors were tested for formaldehyde sensing in the low ppb range. Comparing systematic composition of the In–Sn–oxide system ranging from pure In 2 O 3 , In 1.9 Sn 0.1 O 3 (ITO), In 4 Sn 3 O 12 and SnO 2 with their corresponding mixtures showed by far the best sensor performance at 250 ◦ C for 43% of (Sn/(Sn + In), corresponding to the In 4 Sn 3 O 12 phase. The sensors tested using this phase outperformed state of the art metal oxide devices. The In 4 Sn 3 O 12 phase was stable beyond the operation time and temperature used here, demonstrating its enormous but largely undiscovered potential in the future. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Gas sensors based on semiconducting metal oxides (SMOX) did find many practical applications due to their good sensitivity to various classes of gases, simplicity of use and low cost [1]. They are intensively studied since more than 50 years and to date sensors based on the only two materials that have shown good performance outside the laboratory, SnO 2 and WO 3 , are commercialized in many millions of devices per year [2,3]. There are two main issues that hinder SMOXs even broader applicability: limited sensitivity, when target gases are in the sub-ppm concentration range and selectiv- ity between different target gases or in complex environments. Combination between different SMOXs, their sensitization with noble metals additives, exploration of new materials and the design of specific crystalline phases are the most popular approaches to increase the sensing performance. The gas sensing properties of In 2 O 3 , alone and in combination with Sn, were thoroughly investigated and the best results were obtained for the detection of oxidizing gases [4]. Introducing Sn in the lattice of In 2 O 3 up to a concentration of 10% is a well-known ∗ Corresponding author. E-mail address: lmaedler@iwt.uni-bremen.de (L. Mädler). 1 These authors contributed equally. approach for increasing the conductivity. The as-obtained material (ITO) is widely used as a transparent conductor. When the concen- tration of Sn is further increased SnO 2 segregates. At temperatures above 1600 K it is possible to obtain a new phase (In 4 Sn 3 O 12 ) which segregates into ITO and SnO 2 under equilibrium conditions when the temperature decreases [5]. Formaldehyde (methanal, HCHO) is a volatile organic com- pound that is widely used to manufacture building materials and numerous household products as well as surface disinfectants and cosmetics. It is considered as one of the most harmful indoor pol- lutants, causing the sick building syndrome (SBS), due to its release into the gas phase and therefore the ease of intake by inhalation [6,7]. At high concentrations it is considered to be carcinogenic [8]. Therefore concentrations of 10–500 ppb are required to be mon- itored in the gas phase. As of to date, there are no methods to monitor, in real time, the HCHO content in buildings. Large efforts were undertaken to utilize SMOX based gas sensors, but no break- through was reported or patented which meets the sensitivity and stability criteria. The most promising results were obtained with ZnO nanowires [9]. Here, we employ Flame Spray Pyrolysis (FSP) to produce sensor materials and direct in situ deposition on the sen- sor substrates – an approach that we successfully applied for the realization of SMOX based gas sensing devices [10]. We were able to quench a metastable nanoscale In 4 Sn 3 O 12 as macro-porous film on the sensor substrate. Our measurements demonstrate that this 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.11.026