Mesoporous catalytic filters for semiconductor gas sensors

 An effective way to improve sensor selectivity and stability is the use of catalytic filters to block interfering and poisoning gas molecules from reaching the sensor surface.

Mesoporous silica with high resistivity and hugh surface areas are ideally suited as a base material for this application. When impregnated with proper catalysts, mesoporous silica has a great potential to eliminate responses to undesired gases even of thin-film and or micro-machined sensors. In this paper, we report our initial results on thick film SnO -based gas sensors covered with a catalytic filter consisting of Pd and Pt loaded mesoporous silica. Results indicate that selective oxidation of CO in the catalytic filter leads to the elimination of CO interference to a CH sensor with no perceptible deterioration in sensing performance.

While semiconductor gas sensors exhibit high sensitivity(small change in gas composition causes dramatic change in resistance), their stability and selectivity still remain unsatisfactory for many applications. In general, electrical properties of semiconducting sensing elements are influenced not only by the gaseous species to be detected but also by other gas molecules in the sample gas mixture, especially those having similar physico-chemical properties as the target gas. Moreover, undesired gas molecules may be irreversibly adsorbed on the oxide surface, leading to sensor response drift.

Selectivity enhancement is usually achieved by the following three approaches: an improvement of the sensing material properties, the adaptation of the sensor working conditions to the target gas and the assembly of different sensors on arrays involving posterior signal treatments. The improvement of the sensing material properties and optimisation of the working conditions mainly take profit of the different activation energy of the gas reaction on the sensing element surface w1–3x.

In micro-machined substrates a modulation of the temperature is also considered in order to obtain a more complete set of data for the different target interfering gases. The same technique is used to prevent poisoning by cyclic cleaning of the oxide surface at higher temperature than that of operation. Gas sensor arrays allow a much more complete sketch of the atmosphere composition to be built by means of a larger amount of parameters, which require more or less complicated signal treatments ideally incorporated in the same chip w4x.

Another effective way of improving selectivity is to take advantage of selective gas diffusion process by using filters w1,5–10x. In this way, influence of interfering gases can be avoided by blocking these species from reaching sensor surface. While filters may improve selectivity and stability of semiconductor gas sensors, it may lower the sensor detection limit w9x.

Until now, the most used filters are passive membranes having different diffusion parameters according to the adsorption affinity of the gas molecules on the sieve material and the pore-molecule size relationship.These kind of adsorbent filters may become saturated for large interfering gas concentrations if no mechanism of gas reaction or desorption are anticipated w10x.

Selective catalytic reaction mechanism in the filtering membrane are expected to overcome these limitations,by means of catalytic conversion of the interfering species into innocuous molecules. Catalytic sieves have already been employed in the form of dielectric oxides, thin metal layers or of dispersed catalytic elements on oxide semiconductor materials. The use of high resistive oxides without the distribution of highly active catalytic elements requires high operation temperatures, which may not always fit the most appropriate conditions to optimize the sensor response to the target gas. Low catalytic activity will also require fabrication of thick enough filters to eliminate the interfering gases, which could lead to longer response times.

Metallic membranes are reported to show exceptional selectivity properties w11x. Nevertheless, on the one hand, in semiconductor devices this may short circuit the system or influence the base sensor resistance if not correctly electrically isolated from the film. Furthermore, metal atoms may diffuse and consequently affect the stability of the device w5,8x. On the other hand, catalytic properties of the noble metals are not fully exploited in such a continuous structure; however, as extensively
reported in catalytic literature, a high dispersion of supported active phases would increase its catalytic efficiency w12x.

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