Electrical characterization of Fe2O3 humidity sensors doped with Li+, Zn2+ and Au3+ ions

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Abstract

Li-, Zn- and Au-doped Fe2O3 thin films have been investigated as humidity sensing materials. Their electrical properties have been studied in the 0–100% relative humidity (RH) range by means of electrochemical impedance spectroscopy (EIS). This has allowed to drawn an equivalent circuit which well fit experimental data obtained from the doped films. The resistive and capacitive elements of the equivalent circuit have been also calculated by a modeling procedure. Results indicate that the humidity characteristics of the doped sensing films are correlated to the charge density and surface concentration of the dopants.

Introduction

Ceramic humidity sensors are widely used because of their advantages with respect to the polymeric ones principally arising from an improved thermal, chemical and mechanical stability [1], [2]. This allows a longer life-time for the sensors and the capability of thermal cleaning of the sensors without compromising the sensing layer properties. Moreover, the ceramic humidity sensors response is generally fast and the humidity range is quite large.

Ceramic materials used for humidity sensor application are mainly metal oxides with a porous microstructure consisting of grains, grain boundaries and pores. A fundamental step related to humidity sensing on these materials is the water adsorption on the sensing layer surface [1], [2], [3]. This depends on the nature and specificity of the active sites present on the sensors surface. During humidity interaction, at low relative humidity (RH), chemisorption of water on these surface sites comports the formation of hydroxyl ions. In the presence of an incomplete water layer, H3O+ diffusion on hydroxyls dominate together with H+ proton between adjacent water clusters.

By increasing the RH content, physisorption causes the formation of mono or multilayer depending on water vapor pressure. When water layer is complete Grotthuss chain reaction became active. Due to the porous structure, the capillary condensation take place in all the pores with radii up to rK, at a given temperature and water vapor pressure according the Kelvin equation [3].

It appears obvious that for the activation of the transport mechanisms, the microstructure and the number of water adsorption sites play a fundamental role. These characteristics can be modified by adding suitable dopants to the sensing metal oxide-based material.

Various Fe2O3-based humidity sensing materials in the presence of additives [4], [5], [6] with different characteristics such as charges, ionic radii and charge/ionic volume ratios (see Table 1) have been widely investigated in our laboratory. Li ions were found very effective, increasing the sensitivity at low RH [4]. This was related to the high polarizing effects of Li+ ions, at which correspond an increase of the chemisorption of the water molecules on the iron oxide surface. Zn and Au were found to act as structural promoters changing the microstructure of the iron oxide and improving stability [5], [6].

Electrochemical impedance spectroscopy (EIS) is a powerful technique to investigate in details the electrical properties of humidity sensing materials [7], [8], [9], [10]. This work therefore aims to compare the humidity sensing properties of the Li-, Zn- and Au-doped Fe2O3 thin films evaluating their impedance/frequency characteristics by EIS. The microstructural characteristics of the sensing layer (grain bulk, grain surface and grain boundaries) have been represented as single blocks of circuital elements (resistance, capacitance, or Waburg elements) to drawn related equivalent circuits. Then, an electrical modeling procedure has been approached to calculate the circuital parameter values. The data obtained were correlated to the chemico-physical (charge, ionic radius, charge/ionic volume ratio) characteristics of dopants.

Section snippets

Thin film preparation and sensor assembling

Thin films were prepared by a liquid-phase deposition (LPD) method on a disk shaped ceramic substrate (Ø  12 mm) provided with gold interdigited contacts previously evaporated under high vacuum. The chemical precursors used were Fe(NO3)3, Zn(NO3)2, HAuCl4 and LiNO3 dissolved in water (0.01 M) and mixed in the opportune volumetric ratios. The precursors solution was dropped on the substrate and successively exposed to a controlled atmosphere (5% NH3/He). The deposited film was then dried at 80 °C in

Thin film preparation and characterization

Main steps involved in the deposition of Fe2O3 films by LPD has been previously reported in detail elsewhere [11]. Here, they are briefly recalled. In the first step of the LPD process, the precursors solution is dropped on the substrate. The concentration of the precursors solution is fundamental in determining the sensing layers thickness. At a 0.01 M concentration in Fe(NO3)3, a film thickness around 1–10 μm can be obtained [11]. The pH of the solution is acid due to the precursors hydrolysis.

Conclusions

Fe2O3 thin films show strong modification of their sensing characteristics towards humidity when properly doped with Li, Zn or Au ions. Their impedance–frequency characteristics have been studied in the whole RH range. This has allowed to drawn an equivalent circuit which well fit experimental data from these Fe2O3-based films. The resistive and capacitive elements of the equivalent circuit have also been calculated by a modeling procedure. The effect of dopants is related to their charge/ionic

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 and the PhD degree from University of Reggio Calabria in 2001. 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.

Nicola Donato was born in Messina, Italy, in 1971. He received the laurea degree in electronic engineering from the University of Messina in 1997 and the PhD degree from University of Palermo in 2002. His current research interests temperature-dependent linear/noise characterization techniques for solid-state devices, implementation of software procedures for automated instrumentation control, characterization and modeling of thin-film sensors.

Alina Caddemi received the degree in electronic engineering and the PhD degree from the University of Palermo in 1982 and in 1987, respectively. From 1990 to 1998, she was with the Department of Electrical Engineering, University of Palermo, as assistant professor. In 1998 she joined the University of Messina, Messina, Italy, as an associate professor of optoelectronics. Her current research interests are in the field of temperature-dependent linear and noise characterization techniques for solid-state devices, noisy circuit modeling of bipolar and field-effect transistors, neural network modeling of devices, design and realization of hybrid low-noise circuits based on traditional and super-conductive materials, characterization and modeling of thin-film sensors.

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.

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