Size dependent gas sensing properties of spinel iron oxide nanoparticles

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Abstract

Spinel iron oxide nanoparticles of sizes from 12 to 60 nm have been prepared via a hydrothermal synthesis. Thick films of the Fe3−xO4 nanoparticles deposited on interdigital electrodes were applied as resistive gas sensors. The electrical and gas sensing properties were characterized by impedance spectroscopy using multielectrode substrates. The reducing analytes CH4, NO, H2 and NH3 in N2 as a reference were applied in order to investigate the sensor response, the selectivity and the recovery behavior at temperature between 50 °C and 300 °C. The materials exhibit good sensor responses towards NH3 with low cross sensitivities towards H2 and NO at 250 °C. A linearly increasing sensor response towards NH3 and H2 with decreasing particle size was found.

Introduction

Due to the ever increasing standard of living and environmental regulations the worldwide sensor technology market is constantly expanding and is meanwhile related to nearly all areas of everybody's life. Nowadays, the application of gas sensors has become indispensable in the private as well as the industrial sector (e.g. food industry, medical care and emission controlling). A special field of application is the monitoring of reducing gases in oxygen free atmospheres (inert gas techniques) which are required for e.g. biotechnology, fermentation processes or metal processing at elevated temperature.

Solid state gas sensors, among which chemiresistors are the most extensively applied type, enable in situ and real time measurements at various locations and conditions [1], [2], [3], [4]. The chemiresistors typically base on semiconducting metal oxides of sufficient thermal and chemical stability even under harsh conditions. Due to the material history the most frequently applied and published metal oxides are in decreasing order of frequency: SnO2, ZnO, TiO2, WO3, In2O3, Nb2O5, Ga2O3, and Fe2O3 besides ternary metal oxides and mixtures of metal oxides [5], [6], [7], [8]. Regarding the binary n-type semiconducting Fe2O3 the most stable corundum-type α modification (hematite) [9], [10] as well as the cubic-type γ modification (maghemite) [11] are commonly used as gas sensor materials in contrast to the metastable β-Fe2O3. In previous studies the p-type semiconducting perovskites LnMeO3 (Ln = lanthanide series, Me = Cr and Fe) exhibited high sensitivity towards C2H5OH and C3H6 at high temperatures [12], [13]. Different publications identified the spinel ferrite family MeFe2O4 (Me = Cd, Ni, Mg, Cu, Zn) as well to be sensitive towards both reducing and oxidizing gases (e.g. C2H5OH, LPG, CH3COCH3, CO, H2, CH4 and N2O) [14], [15], [16], [17].

In contrast to the iron oxides mentioned above the cubic-type Fe3O4 (magnetite) has rarely been reported as resistive gas sensor until now. Fe3O4 exhibits an inverse spinel structure with iron cations in both the valence states Fe2+ and Fe3+. The structure is symbolized by [Fe3+]A[Fe2+Fe3+]BO4, in which the tetrahedral sites are occupied by Fe3+ ions and the octahedral sites by equal numbers of Fe2+ and Fe3+ ions. Due to its outstanding magnetic properties, Fe3O4 nanoparticles are synthesized via many different methods [18], [19], [20] and widely used in magnetic storage devices, ferrofluids, sensors, spintronics, separation processes and biomedicine [21].

For potential gas sensing application one approach combines Fe3O4 nanoparticles with conductive polymers as for example polypyrrole (PPY) or polyaniline (PANI) to yield organic–inorganic hybrid (nano)composites [22]. Composites of various Fe3O4 particles and mainly lower PPY contents between 0 and 50% were reported as humidity and gas sensors (N2, O2, CH4, CO2, and NH3) [23], [24], [25]. Another approach enhances the gas permeability of the sensing material by using anisotropic nanostructures. Resistive measurements on single-crystalline (Mn-doped) Fe3O4 nanowire arrays as network films exhibited a reversible change in (surface) p-type conductivity towards various gases (e.g. H2O, C2H5OH, N2O, NH3, H2S and NO2) at room temperature. For non-hydrogen-bonding di/triatomic gas molecules the found sensitivities correlate with the respective bond strength and were significantly enhanced by the Mn-doping most towards H2O and C2H5OH [26]. Furthermore, Fe3O4 “nanoroses”, synthesized by a microwave-assisted approach in the presence of a block copolymer, were reported to show enhanced sensitivity and reversibly towards C2H5OH at room temperature attributed to the porous superstructure and the small grain size [27].

Since these iron oxides are reported to be sensitive towards mainly reducing analyte gases the pristine nanoparticulate Fe3O4 might be a potential gas sensor material towards NH3. Due to the low oxidation stability of Fe3O4 the application is restricted to oxygen and humidity free atmospheres even at elevated temperature as required in the field of metal processing and fermentation processes. Nowadays, NH3 sensors are industrially applied in selective catalytic reduction (SCR) reactions of automobiles and coal power plants. Restricted to the harsh prevailing conditions thick films of H-form zeolites and metal oxides are mainly used by different sensor concepts [28], [29], [30], [31]. Due to the obvious lack of studies on the gas sensing properties of Fe3O4 the reported NH3 sensing properties make Fe3O4 to a potential candidate for applications under oxygen free conditions at higher temperatures.

In this study the NH3 sensing properties of different sized Fe3−xO4 nanoparticles (12–60 nm) were systematically investigated by means of impedance spectroscopy. In addition to the sensor response towards NH3, the selectivity, i.e. cross sensitivity towards other reducing analyte gases such as CH4, NO and H2, and the recovery behavior were in the main focus by varying the operation temperature in a range from 50 to 300 °C. The application of multielectrode substrates (MES) enabled comparable measurements under equal conditions. Thus, the influence of the incremental mean particle size on the sensor response towards NH3 and the selectivity could be determined to reveal the best sensor material and working temperature.

Section snippets

Experimental

All syntheses were performed in degassed aqueous solutions in absence of any oxygen. Therefore, argon was bubbled through distilled water as solvent for half an hour and was also used as protecting gas throughout the hydrothermal synthesis to prevent the oxidation of Fe2+ in the system. For thick film deposition the used solvents were degassed by dry nitrogen as mentioned above. All processes at elevated temperature as the drying and calcination of thick films as well as the impedance

Results and discussion

Via the hydrothermal synthesis ten different iron oxide sample powders with different mean particle diameters from about 12 to 60 nm have been prepared (Table 1).

Conclusions

In this study nanoparticulate powders of magnetite (Fe3−xO4) have been synthesized via a hydrothermal method which differs in their mean particle sizes from 12 to 60 nm. The characterization of the powders by XRD and SEM revealed that the sample materials are made of a magnetic core surrounded by an oxidized layer maghemite (γ-Fe2O3) and that they consist of sponge-like porous networks of interconnected spherical particles.

The sensor response, the selectivity and the recovery behavior towards

Acknowledgement

We thank CONACYT-MEXICO for the financial support (postdoctoral grant J. Santoyo-Salazar).

Clemens Julius Belle studied chemistry at the RWTH Aachen University (Germany) with research periods at the NTNU Trondheim (Norway) and the University of Cambridge (United Kingdom). After receiving his diploma in chemistry in 2007 he is presently studying for his Ph.D. in the group of Prof. Dr. U. Simon at the RWTH. His research interests focus on the impedometric characterization of nano- and microparticulate metal oxides which are suitable as gas sensors as well as heterogeneous catalysts.

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  • Cited by (0)

    Clemens Julius Belle studied chemistry at the RWTH Aachen University (Germany) with research periods at the NTNU Trondheim (Norway) and the University of Cambridge (United Kingdom). After receiving his diploma in chemistry in 2007 he is presently studying for his Ph.D. in the group of Prof. Dr. U. Simon at the RWTH. His research interests focus on the impedometric characterization of nano- and microparticulate metal oxides which are suitable as gas sensors as well as heterogeneous catalysts.

    Alberto Bonamin studied Industrial Chemistry at the University of Padova (Italy) and obtained his diploma with a thesis on perovskite elecrolyte materials for solid oxide fuel cell in 2008. After a research period in the group of Prof. U. Simon at the RWTH Aachen University (Germany) in 2009, he is presently working for the company UniTec s.r.l. (Italy) that designs and produces solid state sensors based on micro- and nanoparticulate metal oxides for the application in environmental monitoring.

    Ulrich Simon studied chemistry at the University of Essen (Germany) and obtained his diploma in 1990, his doctorate in 1992 and his habilitation in 1999. Since 2000 he holds the chair of Inorganic Chemistry and Electrochemistry at the RWTH Aachen University (Germany). The main interest of his current research includes the synthesis and the electrical properties of metal and semiconducting nanoparticles and of nanoporous materials, as well as their application in molecular electronics and chemical sensing.

    Jaime Santoyo-Salazar studied materials science at the University of Mexico (UNAM), where he obtained his M.Sc. in 2001 and Ph.D. in 2006. After a period of research in microscopy area at UNAM, he worked as postdoc in the group of Prof. Dr. G. Pourroy at Institut de Physique et Chimie des Matériaux in Strasbourg in 2008. Since 2010 he is researcher at the Physics Department of CINVESTAV-Mexico. The main interest areas are synthesis and physical properties of nanostructured systems, and microscopy (SEM, HRTEM and AFM).

    Matthias Pauly studied chemistry and materials science at the European School for Chemistry, Polymers and Materials Science in Strasbourg (France). He obtained his Ph.D. in 2010, after 3 years spent at the Institute for Physics and Chemistry of Materials, where he worked on the synthesis and organization of iron oxide nanoparticles. He is currently holding a postdoctoral position at the Max Planck Institute for Solid State Science in Stuttgart (Germany).

    Sylvie Bégin-Colin completed her Ph.D. in materials chemistry at the University of Nancy (France) in 1992. Afterwards, she has integrated the CNRS at the Mining Engineering School of Nancy and worked on physico-chemical modifications induced in oxides by ball-milling. Since 2003 she is professor and currently deputy director of the ECPM at the University of Strasbourg. Her research interests at the IPCMS of Strasbourg focus on the synthesis, functionalization and organization of oxide nanoparticles (titania, ferrites) for biomedical, catalytic and spintronic applications.

    Geneviève Pourroy obtained her diploma in 1979 and her Ph.D. in 1982 at the Ecole Nationale Supérieure de Chimie in Strasbourg. She is director of research at the CNRS since 2001 and leader of the Department of Chemistry of Inorganic Materials at the IPCMS since 2003. The main interest of her research is the elaboration of controlled morphologies of oxides (nanopowders or thin films) and hybrid materials with magnetic or photoelectronic properties.

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