Investigations of conduction mechanism in Cr2O3 gas sensing thick films by ac impedance spectroscopy and work function changes measurements

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Abstract

This paper investigates the conduction mechanism in Cr2O3 gas sensing thick films. A citrate combustion method was used for the preparation of the metal oxide and ethanol vapour as a test gas. The manner in which surface reactions induced electrical changes are affecting the sensor signals inputs was explored by simultaneous dc and work function changes (Kelvin probe method). The identification of the contributions to conduction of the different sensing layer elements was made possible by ac impedance spectroscopy measurements. A conduction model, which qualitatively explains the experimental findings, was elaborated on the basis of the acquired experimental data and the information provided in literature. The model validity should apply, besides Cr2O3, to all p-type metal oxides used as gas sensitive materials.

Section snippets

Experimental details

Cr2O3 is an interesting material for many relevant applications. Amorphous and polycrystalline Cr2O3 (band gap 3.4 eV) powders have been used in a wide variety of catalytic reactions including SO2 oxidation, alcohol dehydration and dehydrogenation, and methanol synthesis [1], [2]. Materials based on Cr2O3 are also used as gas sensors for H2, O2 and NO2 [3], [4]. The ammonia sensing properties of the most studied one – Ti4+ doped Cr2O3 (CTO) – were firstly reported by Moseley and Williams [5].

Experimental details

Micron-sized chromium oxide powder was obtained by citrate combustion technique. Stoichiometric amounts (250 cm3) of [Cr(NO3)3·9H2O] (0.1 M), and conc. HNO3 (0.5 cm3) were mixed and stirred for 10 min at 80 °C to obtain a clear green solution. Ethylene glycol (0.9 cm3) was added to a citric acid solution (99.5%, 4.0 g dissolved in 2 cm3 hot water), followed by heating at 100 °C to obtain a viscous gel. The green solution mixture was then added to the gel and heated at 100 °C for 0.5 h. A resin-like solid

Results and discussion

Fig. 1 shows the results of the simultaneously performed work function and resistance change measurements. The first thing to observe is that under exposure to a reducing gas, the sensor resistance increases, which indicates typical p-type behaviour [20]. This also fits to a mechanism in which ionosorbed oxygen plays the role of surface acceptor [13], [14], [15], [16], [17]; its presence on the surface determines the appearance of an accumulation layer for holes that has a lower resistivity if

Conclusion and outlook

A conduction model, which validity should go beyond the investigated material, was established on the basis of experimental findings and information provided in the literature. The main finding is that huge surface changes induced by gas exposure are not translated into large sensor signals and the reason has to do with the choice of the chosen sensor signal, namely the electrical resistance of the sensitive layer, which is not appropriate for p-type materials.

The next steps in the follow-up

Acknowledgements

Suman Pokhrel would like to thank Alexander von Humboldt Foundation, Germany for the support of this work. Cristan Simion would like to acknowledge the financial support of the European Network of Excellence Gospel.

Suman Pokhrel received his MSc (Physical chemistry in 1996) and PhD (in 2005) from Tribhuwan University, Nepal and University of Madras, India. He was a Jawaharlal Nehru Research Fellow (2000–2002) during his doctoral work. He was a postdoctoral fellow (2005–2006, Overseas Scholars of Heilongjiang Province) in the School of Chemistry and Materials Science, Heilongjiang University, Harbin, China. Recently, he is a Georg Forster Fellow of Alexander von Humboldt Foundation working in the Institute

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    Suman Pokhrel received his MSc (Physical chemistry in 1996) and PhD (in 2005) from Tribhuwan University, Nepal and University of Madras, India. He was a Jawaharlal Nehru Research Fellow (2000–2002) during his doctoral work. He was a postdoctoral fellow (2005–2006, Overseas Scholars of Heilongjiang Province) in the School of Chemistry and Materials Science, Heilongjiang University, Harbin, China. Recently, he is a Georg Forster Fellow of Alexander von Humboldt Foundation working in the Institute of Physical Chemistry, Tübingen University, Germany. His research interests include oxide nano-structured materials for various applications such as chemical, electrochemical and optical sensors, fuel cells and surface science.

    C.E. Simion received his BS (in 2003) and MS (in 2005) degrees in Theoretical and condensed matter Physics from the University of Bucharest. Presently he is a PhD student working in the joint collaboration between the gas sensor group at the National Institute of Materials Physics, Bucharest, Romania and Institute of Physical Chemistry, Tübingen University. His field of interest is metal oxides solid state gas sensors.

    V. Quemener is a graduate student working for his Master degree in Materials Sciences at the University of Rennes 1.

    N. Bârsan received in 1982 his diploma in Physics from the Faculty of Physics of the Bucharest University and in 1993 his PhD in Solid State Physics from the Institute of Atomic Physics, Bucharest, Romania. He was a senior researcher at the Institute of Physics and Technnology of Materials, Bucharest between 1984 and 1995. Since 1995 he is a senior researcher at the Institute of Physical Chemistry of the University of Tübingen. The research interest of Nicolae Barsan focuses on chemical sensors and chemical sensor systems.

    U. Weimar received his diploma in physics 1989, his PhD in chemistry 1993 and his Habilitation 2002 from the University of Tübingen. He is currently the head of Gas Sensors Group at the University of Tübingen. The research interest of Udo Weimar focuses on chemical sensors as well as on multicomponent analysis and pattern recognition.

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