CO and NO2 sensing properties of doped-Fe2O3 thin films prepared by LPD

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Abstract

The CO and NO2 sensing properties of iron oxide thin films doped with Au and Zn and prepared by a liquid-phase deposition method (LPD) have been investigated. The undoped Fe2O3 sensor was found to be sensitive to NO2 but not to CO. The addition of Zn increases the sensitivity at low temperature towards NO2 and decreases strongly the intrinsic resistance of the iron oxide film. The addition of gold was necessary to obtain a detectable response to CO, the Au-doped ZnO sensor was found the most sensitive to CO. By operating at an appropriate temperature, these sensors could be able to detect selectively CO and NO2, with negligible humidity cross-sensitivity.

Introduction

During the past years, different semiconducting materials have been proposed as sensing elements for solid-state sensor devices for the detection of toxic gases. Their integration in portable systems for applications on-line and in situ requires small and preferably cheap sensor devices. To this aim, the realisastion of such devices should be fully compatible with the Si-microelectronic technology and gas sensors based on thin films are good candidate to fulfil this requirement. Moreover, the development of thin film deposition techniques based on chemical processes are arising a great interest in the field of gas sensors as promising deposition methods alternative to the physical deposition ones (like evaporation, sputtering, etc.) [1].

In recent studies, we have reported the preparation of sensor devices based on metal oxides thin films by a simple liquid-phase deposition method (LPD). By this chemical route, we have prepared undoped and Li-doped iron oxide thin films with good sensing properties towards humidity [2].

In this work, we have focused our attention on the realisation of gas sensor devices, based on pure and Au-doped F2O3, ZnO and mixed Fe2O3–ZnO thin films prepared by LPD for the detection of two air contaminants (CO and NO2). In particular, gold has been selected as dopant to promote the CO sensing, while zinc has been considered as promoter in NO2 sensing. Moreover, as the high resistance of pure iron oxide films limits its practical use, the addition of zinc was investigated as a possible way to decrease the films resistance. The effect of working temperature of the sensors and the influence of the humidity on the response to CO and NO2 were also investigated.

The monitoring of CO and NO2 is of utmost importance. CO is a gas produced during incomplete combustion and it is toxic at very low concentrations. NO2 is another environmental pollutant that, together with NO (hence named NOx), arises from combustion facilities. Currently, their concentrations in air or exhausts are determined by means of spectroscopic analyzers based on infra-red absorption and chemical luminescence. These techniques are however very expensive and difficult to handling. Studies have been carried out to develop solid-state CO and NOx sensors using semiconducting oxides, solid electrolytes and organic semiconductors [3]. In this work, we demonstrated the possibility to use a combination of two of our sensors to detect selectively CO and NOx without humidity interference for the control systems of combustion exhausts from cars and stationary plants.

Section snippets

Experimental

Sensor devices were realised by depositing thin films onto alumina substrates 3mm×3 mm sized and provided, on the front side, with gold interdigitated contacts and, on the backside, with a Pt heater. Different samples have been prepared as following: aqueous solutions containing the suitable amount of Fe(NO3)3, Zn(NO3)2 and HAuCl4 were first prepared. Then 10 μl of each solutions was deposited on the alumina substrate by micropipetting. The samples were then introduced in the deposition apparatus

Film morphology

The morphology of the films prepared by LPD is shown in Fig. 1. SEM micrographs at low magnification show the presence of holes and microcracks on the film surface probably due to the roughness of the ceramic substrate, to the evaporation of the solvent and to stresses arising during thermal treatments. The microporous structure of these films is more evident by the SEM images at higher magnification (Fig. 2). On the iron oxide-based films metal oxide, particles showing a round-like morphology

Conclusions

The simultaneous and selective monitoring of CO and NO2 is of utmost technological importance for applications in the control systems of combustion exhausts from mobile (i.e. cars) and stationary plants. A way is to make use of a multifunction sensor based on a only sensing material with different optimum working temperatures for CO and NO2, so that a certain selectivity can be obtained by varying the temperature of the sensor accordingly to the requirement of detection of CO or NO2. This

G. Neri was born in 1956 and received his degree in chemistry from the University of Messina in 1980. Since 1998, he is Associate Professor of chemistry at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity covers many aspects of the synthesis, characterisation and chemical-physics of solids with particular emphasis to catalytic and electrical properties. In the latter research area his work has been focused on the preparation of

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G. Neri was born in 1956 and received his degree in chemistry from the University of Messina in 1980. Since 1998, he is Associate Professor of chemistry at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity covers many aspects of the synthesis, characterisation and chemical-physics of solids with particular emphasis to catalytic and electrical properties. In the latter research area his work has been focused on the preparation of ceramic thin films and their application in gas sensors.

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

S. 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.

P. Siciliano received his degree in physics in 1985 from the University of Lecce. He took his PhD in physics in 1989 from the University of Bari. During the first years of activities, he was involved in research in the field of electrical characterisation of semiconductor devices. He is currently a senior member of the National Council of Research at the Institute for the study of new Materials for Electronics (IME-CNR), where he has been working for many years in the field of preparation and characterisation of thin film for gas sensors. His interest is now devoted to manufacturing artificial intelligence systems, like electronic nose.

S. Capone was born in 1970 and graduated cum laude in physics from the University of Lecce in 1996 with a thesis on semiconductor gas sensors. She obtained PhD in physics at the University of Lecce in January 2001. At present she is involved in the characterisation of metal oxide-based gas sensor array to be used in electronic nose and on data analysis from gas sensor arrays.

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