Analysis of the noble metal catalytic additives introduced by impregnation of as obtained SnO2 sol–gel nanocrystals for gas sensors
Introduction
The effect of the addition of many different metallic particles on the gas sensing properties of metal oxides has been widely studied [1], [2], [3], [4], [5]. This influence has formally been classified as chemical or electronic according to two basic sensitisation mechanisms. In the electronic mechanism, the reaction with the gas molecules takes place on the surface of the introduced clusters and not in the metallic oxide. These clusters change their charge state, which induces a variation of the surface barrier height and therefore leads to a conductance change on the base oxide. In this case, the base semiconductor has only a transducer role of the changes induced in the additive particles by their interaction with the gas.
In the chemical mechanism, the semiconductor itself acts as a chemical catalyst. Now, the additive role is to increase the reaction rate of the gas molecules, which are firstly adsorbed on the metallic cluster and, later, moved to the oxide surface in a, so-called, spill-over process. As metallic surface additive, palladium, like silver, is typically considered to be related with an electronic mechanism, whereas Pt is supposed to lead to the chemical one [6], [7].
The two mechanisms have to be closely dependent on the electronic interaction between both the gas-additive and the additive-oxide systems, which at this point is basic for the not yet clearly understood role played by these additives. This interaction strongly depends on the location and chemical state of the additive, which it is accepted to be determined by the used metallic addition method. Nevertheless, this information is scarcely reported. The above models assume the picture that noble metal additives form isolated metallic nanoparticles of a few nanometers, or clusters, which are distributed on the surface of the tin oxide nanograin. However, there are not many evidences to corroborate this hypothesis for all the experimental cases. In a previous work [8], [9] about the influence on the gas sensor performances of the additive chemical states introduced by impregnation of previously calcined SnO2 sol–gel nanocrystals, it was pointed out that not many metallic clusters were formed and that the main contribution of the noble metals was as oxidised states. It corroborates the importance of the introduced catalytic element as the used technological steps. Therefore, the knowledge and understanding of the involved steps related to the impregnation procedure become a determining factor for the accurate control of the thick film technology and the achievement of its reliability. Moreover, it should be noticed that the additive introduction prior to the calcination treatment allows to reduce the number of technological steps involved in the gas sensor fabrication.
It is the aim of this work to analyse the impregnation method and posterior calcination treatment, which determine the oxide structure, its nanograin size and its surface coverage with the different chemical states of the metallic additives. The performed analyses show the dependence on the thermal treatment of the additives chemical states, their localisation and distribution. Moreover, the lattice distortions and electronic states introduced by these additives, as well as their influence on the grain growth mechanisms, are discussed. Finally, sensor sensitivities to CO and CH4 are shown and correlated with the above results. All these features become keystones for an accurate knowledge of the gas sensing mechanisms.
Section snippets
Experimental
Catalytically modified tin oxide nano-powders were obtained by a wet chemical method from a SnCl4 solution. The precursor chloride was precipitated, by adding an ammonia solution, to hydrated SnO2. In this step, three different noble metals, Pt, Pd and Au, were introduced, with nominal weight concentrations of 0.2% and 2%, in the chloride forms PdCl2, PtCl4 and AuCl3. The resulting solutions were then washed with bi-distilled water and let to dry, obtaining finally the hydrated SnO2 precursors.
Grain size
Fig. 1 shows the evolution of the average crystallite size of SnO2 with the calcination temperature. These values and trends have been deduced, using the Scherrer's equation, from the X-ray Diffraction (XRD) spectra, considering different crystallographic directions [12]. As it can be seen, the grain size increases as the treatment temperature increases, showing an important change from 400°C to 450°C. It is worth to observe in this figure, that the presence of metallic additives modifies the
Conclusions
Until now, many works have reported on phenomenological data about the effects produced by the introduction of noble metal additives in the gas sensor characteristics and performances. These effects on the sensitivity can be summarised as: (i) to decrease the sensitivity maximum temperature and (ii) to increase its value.
In this paper, a detailed analysis of the noble metal impregnation of as obtained SnO2 sol–gel has been reported. XRD, TEM, Raman and XPS data confirm the existence of
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Unfortunately deceased on 14 of June 1999.