An alternative method to modify the sensitivity of p-type NiFe 2 O 4 gas sensor A. Sutka • M. Stingaciu • G. Mezinskis • A. Lusis Received: 30 June 2011 / Accepted: 8 November 2011 / Published online: 19 November 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Our objective was to determine the role of cooling rate on gas-sensing properties of annealed nano- grained nickel ferrite (NiFe 2 O 4 ). The sol–gel auto com- bustion method was used for the preparation of NiFe 2 O 4 . To estimate structural and microstructural features, X-ray diffraction, and scanning electron microscopy techniques were used. For gas-sensing measurements different volatile organic compounds (VOCs) were used as testing gases. To identify the contribution of the different sensing layer elements to the conduction, ac impedance spectroscopy (IS) measurements were performed. It was found that the sensors cooled with lower rate exhibited better sensing performance, due to increase of resistance. Overall, this article covers an alternative method for modifying nickel ferrite gas sensor sensitivity. Introduction Spinel ferrites are unique materials that exhibit ferrimag- netic and semiconducting properties and can be considered as magnetic semiconductors. These materials can be used as antenna rods [1], microwave absorbing materials [2], and devices [3], low- and high-frequency transformer cores [4], high-density information storage materials [5], ion selective electrodes [6], magnetorheological devices [7], medical diagnostic tools [8], component of multiferroic composites [9], etc. During last decade spinel ferrites have been intensively studied and developed as semiconducting gas sensors. Different spinel ferrite compounds have shown sensitivity to certain gases (Table 1). At the same time there is still limited information about important ferrite compound compositions, as well as their gas-sensing improvement methods through modification of the electric properties. Sensing properties of spinel ferrite gas sensors are improved by varying synthesis conditions [17, 34, 35] and ferrite composition [18, 30], incorporation of precious metal catalysts [17, 19, 35], synthesis of advanced struc- tures (nano-rods [36], nano-tubes [10], nano-cubes [20], hollow spheres [13]), as well as changing architecture of sensor element [21]. As per our literature survey, annealing condition influence on ferrite gas-sensing properties was not investigated so far. It is known that electrical properties of ferrites can be varied not only by changing stoichiom- etry, composition and microstructure but also by varying cooling rate after sintering [37–39]. For example, NiFe 2 O 4 resistance can be changed by altering cooling rate due to ongoing oxidation–reduction processes. Conduction, in spinel ferrites, is possible when a metal ion is present in more than one valence state. In NiFe 2 O 4 the charge transfer occurs through a hole exchange between Ni 3? and Ni 2? . Oxidation of Ni 2? to Ni 3? or reduction of Ni 3? to Ni 2? tends to decrease or increase conductivity. Herein, for the first time we report on cooling rate influence of NiFe 2 O 4 sensing properties. As testing gases we chose different VOCs. The most VOCs are toxic, harmful, flammable, and explosive thus, it A. Sutka (&)  G. Mezinskis Institute of Silicate Materials, Riga Technical University, Azenes 14/24, Riga LV-1048, Latvia e-mail: andris.sutka@rtu.lv M. Stingaciu Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden A. Lusis Institute of Solid State Physics, University of Latvia, Kengaraga 8, Riga LV-1063, Latvia 123 J Mater Sci (2012) 47:2856–2863 DOI 10.1007/s10853-011-6115-2