Zeolite-based gas sensors

A number of review papers on zeoUte-based gas sensors are available in the literature (Moos et al. 2006, 2009 Xu et al. 2006 Walcarius 2008 Sahner et al. 2008 Zheng et al. 2012). They focus on various aspects of this materials class and mostly classify devices according to the analytes to be detected or the type of sensor transduction. When used in sensor elements, zeolites can take various roles, which fall into two major categories (see Fig. 8.7). 

In a great number of cases, zeolites are used as auxiliary elements. They may act either as a framework to stabilize the sensor material, as filter layers (either catalytic or size restrictive) to enhance selectivity of a sensitive film, or as a preconcentrator of specific analytes from diluted solutions. For example, due to excellent chemical and thermal stability, zeolites can be used as a substrate to prepare compounds and devices with desirable fundamental physical and chemical properties (Xu et al. 2006). For example, inorganic or organic compounds, metal and metal-organic compounds, and their clusters can be assembled into the pores and cages in zeohtes. Some nanosized metal or metal oxide particles have been successfully inserted into the caves and the pores or highly dispersed on the external surface of zeohtes. 

The second group encompasses devices in which the zeolite itself is the main functional material leading to a sensor effect. Such detection principles rely directly on adsorptive, catalytic, or conductive properties of one specific zeoUte that are subject to well-defined changes depending on the composition of the gaseous surroundings (Alberti and Fetting 1994 Xu et al. 2(X)6 Sahner et al. 2008). 

However, zeolites do have a strong tendency to adsorb water preferentially relative to analyte, and this appears to be hindering their application in environments with elevated moisture levels. Moreover, due to the above-mentioned effect, thermal treatment for zeolites activation is required. We also need to take into account that, due to small-pore size diffusion, a limitation 

Typical molecules moderately, rapidly or slow Typical molecules rapidly occluded at RT or below occluded at RT or above 

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Fig. 8.7 Oasstfication of zeolite-based gas sensors (Adapted with permission from Sahner et al. (2008). Copyright 2008 Elsevier)...

In particular, Alberti et al. (1991) proposed zeolite-based sensors for detection of hydrocarbons such as butane and Balkus et al. (1997) used thin film aluminophosphate (AlPO)-5 molecular sieve as the dielectric phase in a capacitance-type chemical sensor for CO and CO. AlPO-n is a family of phosphorus molecular sieves which, similar to zeolites, have ordered molecular-sized pores. The AlPO-5 structure used for the dielectric layer consists of four- and six-membered rings of alternating phosphate and aluminum ions bridged by oxygen. These rings are arranged to produce one-dimensional channels 0.73 nm in diameter. The properties of AlPO-n are reviewed in detail by Ishihara and Takita (1996), and one of the attractive properties of these materials is their heat stability. The properties of zeolites as they relate to zeolite-based gas sensors are discussed in a special section in Vol. 2. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

FIGURE 10.30 Scanning electron microscopy (SEM) images of cross section of a commercial optical fiber coated with a NaA zeoUte thick layer (a) total cross section and (b) magnification view of the NaA zeolite layer. (From Lopez, J., Pina, M.P., Coronas, J., Pelayo, J., and Santamaria, J., A novel optical device for gas sensor applications based on zeolitic materials. Books of abstracts of the 1st NanoSpain Workshop, San Sebastian, 2004.)... 

Lopez J, Pina MP, Coronas J, Pelayo J, and Santamaria J. A novel optical device for gas sensor applications based on zeolite materials. Books of abstracts of the 1st NanoSpain Workshop, San Sebastian, 2004. 

Fukui K and Nishida S. CO gas sensor based on Au-La203 added Sn02 ceramics with siliceous zeolite coat. Sens Actuators B 1997 45(2) 101-106. 

In contrast to equilibrium-based sensing such as described above, it is also possible to use the zeolite film as a membrane controlling molecular access to an appropriate transduction mechanism. In this case, Pd-doped semiconductor gas sensors were used as a fairly non-selective sensor platform. After coating these sensors with a thin film of MFI-type or LTA-type zeolites, they were examined with respect to gas phase sensing of different analytes such as methane, propane and ethanol, at different humidity levels (Fig. 14).[121] The response of a zeolite-coated sensor towards the paraffins was strongly reduced compared to the non-coatcd sensor device, thus resulting in an increase of the sensor selectivity towards ethanol. 

Fischeraner, A., Fischeraner, G., Hagen, G. and Moos, R. (2011) Integrated impedance based hydro-carbon gas sensors with Na-zeolite/CrjOj thin-film interfaces From physical modehng to devices. Physica Status Solidi (A) Applications and Materials 208,404-15. 

Fong, Y. Y., Abdnllah, A. Z., Ahmad, A. L. and Bhatia, S. (2007) Zeolite membrane based selective gas sensors for monitoring and control of gas emissions. Sensor Letters 5,485-99. 

Table 8.5 Types of zeolite-based materials for gas sensors...

Since the adsorption of a gas is able to modify the dielectric constant of zeolites, chemical sensors based on interdigital capacitors (IDCs) using zeolites layers as sensitive coatings offer a wide field of applications depending on the type, modification, and working temperature of the coated IDC sensor. 

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