A study of water influence on CO response on gold-doped iron oxide sensors
Introduction
Noble metal-doped semiconductor oxides are widely used as sensing materials for CO gas sensors [1]. The sensing mechanism is related to change in conductivity caused by adsorption of the target gas, reaction on the semiconductor surface with adsorbed oxygen mediated by the presence of the catalytic dopant, and subsequent desorption of reaction products. These surface processes are influenced by the presence of the gas environment of interferent gases, mainly water vapor, which may alter the response to the target gas. The role played by H2O molecules in the sensing mechanism is very complex depending on numerous factors, including the nature of metal oxide, promoters, operating temperature, thermal treatments, etc. [2], [3], [4], [5]. Water can be adsorbed in the molecular form or react with the surface of n-type semiconductor oxides to form hydroxyl groups, changing the space charge region in an opposite way [6]. It increases the conductance acting as a donor specie when adsorbed in the molecular form. It decreases the conductance when interacting with the surface of the metal oxide, it gives an extensive surface hydroxylation.
Moreover, water can influence CO oxidation. CO oxidation catalyzed by noble metals was reported to play a fundamental role on CO sensing [7], [8], [9]. However, the effect of humidity on CO oxidation over gold/metal oxides catalysts is still controversial and not yet fully understood [10], [11].
Studies so far reported in literature on this topic refer mainly to SnO2 [2], [3], [4], [5]. Little is reported on others sensing metal oxides such as α-Fe2O3. Cantalini et al. have shown that water vapor interferes with CO, increasing the response [12]. No study, at best of our knowledge, deals with the humidity influence on the CO sensing mechanism over gold-doped Fe2O3. Recently, we have investigated thin films of Au-doped iron oxides for CO gas sensors reporting a detailed physico-chemical and electrical characterization [13], [14]. We have noted a strong influence of water vapor 3 of 17 on the CO response of these sensors. With the aim to better understand the interaction of these species with the sensor surface, here we present a study by temperature programmed techniques such as Temperature Programmed Desorption (TPD) and Temperature Programmed Reaction (TPReaction) [15]. TPD is a powerful technique for studying: (i) adsorbate–solid surface interactions; (ii) adsorbate–coadsorbate interactions. TPReaction allows to study the reactivity of species of interest on the surface. These techniques offer then a unique and versatile tool to investigate the effect of water on the CO sensing mechanism over the Au/Fe2O3 sensors.
Section snippets
Sensors preparation
The sensors for the electrical test were made by depositing Au-doped Fe2O3 thin films on alumina substrates ( mm) provided with gold interdigited contacts and, on the backside, with a Pt heater. The thin film was deposited by a liquid phase method as follows. An aqueous solution of Fe(NO3)3·9H2O and HAuCl4 in the appropriate amount in order to have 5 mol% of Au, was prepared. Then, 10 μl of the solution was dropped on the alumina substrate by micropipetting. The sample was placed in the
Temperature programmed desorption
Water vapor is the main interferent gas for CO sensing on metal oxides [2], [3], [4], [5], [6]. However, the data reported in literature on the influence of humidity on the sensitivity to CO are often contradictory. This depends on the nature of the sensing material and the experimental conditions used. In order to have more information on the CO- and H2O-surface interaction, the adsorption of CO and H2O on the Au–Fe2O3 powder has been investigated by TPD. To avoid interference in the mass
Conclusions
Water influence the CO sensing characteristics of Au-doped iron oxide sensors. In the presence of water, the maximum of response to CO shifts to higher temperature. To explain this behavior TPD and TPReaction studies of CO and H2O adsorbed have been carried out. They have provided evidence that the presence of water as coadsorbate favors the adsorption of CO and its successive oxidation, likely through the formation of oxygenate intermediates. On the basis of the results presented, it was
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 at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity, documented by more than 80 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
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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 at the Department of Industrial Chemistry and Materials Engineering of the University of Messina. His research activity, documented by more than 80 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 is 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 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.
Giuseppe Rizzo was born in 1975 and received his degree in chemistry from the University of Messina in 1999 and the PhD degree in 2003. 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.
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.
Lorenzo Caputi was born in 1957, and received is degree in physics in 1980. His research activity regards the surface of the surface properties of solids. He is Associate Professor of Physics in the Sciences Faculty of the University of Calabria.