Methanol gas-sensing properties of CeO2–Fe2O3 thin films

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Abstract

Ce-doped iron oxide thin films have been prepared by a liquid-phase method (LPD) and tested as methanol gas sensors. Sensing tests have shown that the addition of Ce to Fe2O3 increases the response to methanol at low temperature (<350 °C). The optimum loading of Ce was 50% in moles. At operating temperatures > 400 °C, the response of CeO2-doped Fe2O3 sensors is instead lower than that of pure Fe2O3.

To find properties–sensitivity relationships, the microstructure, acid–base properties and reactivity of CeO2–Fe2O3 powders were investigated by X-ray diffraction (XRD), temperature-programmed desorption of adsorbed CO2 (TPD-CO2) and gas-phase oxidation of methanol. Ce-doped iron oxides powders have shown the formation of a FexCe1−xO2 solid solution, smaller grain size and an increase of the number of basic sites compared to pure Fe2O3. Moreover, the addition of Ce to Fe2O3 was found to promote the adsorption of methanol and its subsequent oxidation to CO2.

Correlation between the amount of basic sites, oxidation activity and the response to methanol were reported. According to the obtained results, the role played by Ce on modifying the methanol-sensing properties of iron oxide thin films was related to its capability to: (a) form a solid solution and reduce the grain size; (b) enhance the number of basic sites and methanol adsorption; (c) promote the methanol oxidation activity of the sensing layer. This latter was found to be the factor limiting the sensors response at higher temperatures.

Introduction

Metal oxide-based semiconductors gas sensors are currently used as safety devices in massive scale in both domestic and industrial application for monitoring a variety of combustible and/or toxic gases. Such devices show increasing interest for monitoring alcohol vapors in many practical applications, such as the detection of ethanol in the human breath (i.e. for control of drinking drivers) or of methanol leaks in industrial distribution lines [1], [2].

Several pure or mixed metal oxides have been proposed as sensing materials for alcohols detection, but the mechanism of gas sensing is still not fully clarified [1], [3], [4], [5], [6], [7], [8]. On SnO2-based sensors, it was proposed that the preferential formation of intermediate products of oxidative dehydrogenation is a key factor to obtain high sensitivity to alcohols [7]. Under the usual operating conditions of gas sensors (150–500 °C), alcohols undergo, in fact, transformation into a variety of products. The selectivity to these products depends on the acid–base characteristics of the sensing material, with the basic sites promoting the dehydrogenation rather than dehydration paths [7]. Accordingly, the increase of sensitivity was found to be favored by the addition of basic promoters [7], [8].

In previous studies, we have extensively investigated the sensing characteristics of Fe2O3 thin films prepared by liquid-phase deposition (LPD) [9], [10], [11]. Fe2O3 is a basic oxide showing also good properties for alcohols detection as reported by many authors using doped Fe2O3 films prepared by different techniques [12], [13], [14]. Here, we focused our attention on Ce-doped iron oxide thin films. CeO2 is more basic than Fe2O3 and could act as a promoter improving the sensing characteristics of the pure iron oxide.

In a previous paper, we have reported a preliminary study on the alcohols-sensing properties of Fe–Ce mixed oxides [15]. In this study, we describe in details the properties of CeO2–Fe2O3 thin films of different composition in the methanol gas sensing. A study on the microstructural and acid–base characteristics of CeO2–Fe2O3 powders has been undertaken in order to find correlation with the sensing properties of these materials towards methanol. The reactivity in the gas-phase methanol oxidation was also investigated.

Section snippets

Preparation and characterization of CeO2–Fe2O3 powders

CeO2–Fe2O3 powders for characterization studies were prepared by coprecipitation by adding a solution containing the suitable amount of the metal oxide precursors, CeCl3·7H2O and Fe(NO3)3·9H2O (Fluka), to an aqueous solution of NH4OH 1 M under vigorous stirring (500 rpm) at 7.5 ml/min rate and at a temperature of 80 °C. The precipitate was digested overnight at room temperature, washed several times with water, dried under vacuum (P = 10−2 mbar) at 80 °C and then calcined at the temperature of 400 °C

Microstructural analysis

The microstructure of CeO2–Fe2O3 mixed oxides powders was investigated by XRD (Fig. 1). Previously, we reported that phases composition depend on the Ce–Fe ratio and calcination temperature [15]. Diffraction pattern registered on the sample of pure iron oxide, FC00, after calcination at 400 °C shows that hematite (α-Fe2O3) is the only phase present. Ce-doped iron oxide powders show instead no reflections related to hematite, indicating that Ce inhibits the crystallization of iron oxide and/or

Conclusions

Iron oxide thin films with different Ce loading have been prepared and investigated with respect to their methanol-sensing properties. Ce affects the microstructure, grain size and acid–base characteristics of the sensing layer. Moreover, it influences the reactivity and products distribution in the gas-phase methanol oxidation. Ce-doped sensors with an intermediate Ce loading, between 20 and 50%, show high response to methanol at low operating temperature (<350 °C). A linear correlation between

Giovanni Neri was born in 1956 and received his degree in chemistry from the University of Messina in 1980. He is full professor of chemistry and Director of the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity, documented by more than 100 papers on international journals and books, cover many aspects of the synthesis, characterization and chemical-physics of solids with particular emphasis to catalytic and sensing properties. In

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    Giovanni Neri was born in 1956 and received his degree in chemistry from the University of Messina in 1980. He is full professor of chemistry and Director of the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity, documented by more than 100 papers on international journals and books, cover many aspects of the synthesis, characterization and chemical-physics of solids with particular emphasis to catalytic and sensing properties. In the latter research area, his work has been focused on the preparation of metal oxide thick and thin films and their application in gas sensors.

    Anna Bonavita was born in 1972. She received her degree in materials engineering from the University of Messina in 1997. At present time, she is at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. Her research activity concerns with the preparation, characterization and development of semiconductor films for gas-sensing applications.

    Giuseppe Rizzo was born in 1975 and received his degree in chemistry from the University of Messina in 1999. Actually, he works at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity is focused on the synthesis and characterization of materials by sol–gel method both for catalytic and optical applications.

    Signorino Galvagno was born in 1950. He received his degree in industrial chemistry from the University of Catania. Since 1994, he is full professor of chemistry at the Department of Industrial Chemistry and Materials Engineering of the University of Messina where he is involved in research projects on the catalytic and electrical properties of highly porous materials.

    Simonetta Capone was born in 1970 and graduated cum laude in physics at the University of Lecce in 1996 with a thesis on semiconductor gas sensors. In January 2001, she received the PhD degree in physics by discussing a thesis on metal oxide gas sensors for applications in electronic nose. At present, she works as a researcher at the Institute for Microelectronic and Microsystems of the Council National of Research (IMM-CNR) in Lecce (Italy). Research topics are the characterization of metal oxide-based gas sensors to be used in electronic nose and gas sensors based on metal oxide hetero-junctions.

    Dr. Pietro Siciliano, physicist, senior researcher, received his degree in physics in 1985 from the University of Lecce. He took his PhD in physics in 1989 at the University of Bari. During the first years of activities, he was involved in research in the field of electrical characterization of semiconductors devices. He is currently a senior member of the National Council of Research in Lecce, where he has been working from many years in the field of preparation and characterization of thin film for gas sensor, being incharge of the Sensors Group. He is author of about 180 scientific papers, on national and international journals and a lot of communication, some invited, to International Conferences. He is responsible for several national and international projects at IMM-CNR in field of sensors and microsystems, mainly for environmental, automotive and agro-food applications. He has been organiser and chairman of International Conferences and director of International Schools on Sensors and Microsystems. He is member of the Steering Committee of AISEM, the Italian Association on Sensors and Microsystems. At the moment, he is director of IMM-CNR in the Department of Lecce.

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