Role of the Au oxidation state in the CO sensing mechanism of Au/iron oxide-based gas sensors
Introduction
Gas sensors based on Au-promoted transition metal oxides have been the subject of recent investigations [1], [2], [3], [4], [5], [6], [7], [8], [9]. Thick films of coprecipitated powders of Au/iron oxide have been reported to be sensitive to CO at room temperature [1], [9]. Kobayashi et al. first reported the use of thick films of Au-doped Fe2O3 for monitoring CO at room temperature showing that the preparation method of the sensitive layer strongly affects the sensing properties [1]. The preparation of gold/iron oxide powders by coprecipitation provided the best performance. This preparation procedure ensures a strong interaction between gold and the transition metal oxide leading to the formation of small particles of gold and the semiconductor oxide, whose effects on the sensing properties are well known [10]. Neri and coworkers reported that the presence of Cl− ions can also influence the gas sensing properties of Au/Fe2O3 films [8], [9]. Films containing no chloride ions were found to be sensitive at room temperature whereas those having a high content of Cl− ions were sensitive only at operating temperatures above 150 °C.
In this paper, we report the effect of the temperature of calcination on the response of sensors based on thick films of coprecipitated powders of Au/Fe2O3. Numerous papers refers of the microstructure modification of both the noble metal and semiconductor oxide with the temperature [11], [12], [13], [14], [15]. The nature of gold species (Aun+ or Au0) as well as their size strongly depend on the pretreatment temperature. The microstructural characteristics of the iron oxide undergo a remarkable change, too. So far no detailed study has been reported on the influence of these parameters on the sensing properties. Therefore, a detailed microstructural characterization by XRD, TEM and XPS was carried out with the purpose to gain information on the CO sensing mechanism.
Section snippets
Powder preparation
Au/iron oxide powders were prepared by the coprecipitation method. An aqueous solution of Fe(NO3)3·9H2O (Fluka) and HAuCl4, in the appropriate amount was added drop-wise at a rate of 7.5 ml/min, to an aqueous solution of Na2CO3 (pH 11.9) under vigorous stirring (500 rpm) and at a temperature of 80 °C. The precipitate was kept digesting overnight at room temperature, then it was washed with hot water and dried at 80 °C under vacuum. Heat treatments were carried out in air at temperatures up to 400
Au/Fe2O3 powders characterization
Au-doped iron oxide powders have been widely characterized [11], [12], [13], [14], [15]. However, due to the complexity of the preparation of these systems, data reported in literature are difficult to compare. This highlights the fundamental importance of the knowledge of the morphological, microstructural and textural characteristics of the material prepared.
In previous studies, we reported a detailed characterization by XRD, XPS, BET surface area measurements, Mössbauer spectroscopy,
Conclusions
Au/Fe2O3 thick films have been investigated as CO gas sensors at room temperature. The response to CO has shown a strong dependence on the annealing temperature of the sensing layer. It was suggested that the nature of gold particles (Aun+ or Au0) is responsible of the sensing behavior observed. Over the sensing film calcined at the temperature of 100 °C amorphous gold particles with gold in a positive oxidation state prevail; the formation, in the presence of CO, of the redox couple
G. Neri was born in 1956 and received his degree in Chemistry from the University of Messina in 1980. Since 2002, he is full 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, characterization 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 2002, he is full 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, characterization 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.
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