Synthesis of porous monoclinic tungsten oxides and their application in sensors

Semiconducting metal oxide sensors are limited in their usage because of their poor detection selectivity. The current approach to achieve better selectivity in SMO detection uses prefiltering/preconcentration schemes to reduce the number of gases in contact with the sensor in combination with array-based detection. In this thesis we have investigated different materials and approaches for use as elements in an array based detection system.; One approach we have investigated involves the use of porous monoclinic WO₃ to obtain size selectivity in detection within the sensing element itself. In chapter 3 we describe the synthetic protocol used to generate high surface area porous monoclinic tungsten oxide. Mesoporous oxides are produced by a sol-gel polymerization in the presence of a self-assembled surfactant structure. This approach has not been applied to the synthesis of WO ₃ based oxides because the presence of salts leads to mixtures of WO ₃ and tungstates. By minimizing the presence of Na+ ions, it is shown that ordered porous monoclinic WO₃ can be prepared. The sodium tungstate is first passed through an ion exchange resin to remove the sodium and tungstic acid thus formed is then added to solution containing a cationic surfactant, n-cetyltrimethylammonium bromide (CTAB) to template the structure. While a salt is formed with the CTAB cation, it does not lead to stable tungstates because these salts are easily decomposed during the calcination step. It is also shown that the need for ion-exchange can be avoided by using ammonium tungstate as a precursor in place of sodium tungstate.; In Chapter 4, we examine the sensor properties of the various porous WO₃ powders. The sensors were tested to a series of alcohols of various size as well as dimethyl methyl phosphonate (DMMP, a nerve agent stimulant) and it was found that there was a size dependent response signal on the porous WO₃ relative to sensors fabricated with nonporous WO₃ powders. 1R spectroscopic measurements shows that the difference in sensor responses on porous material was due to a size dependent control over the amount of alcohol absorbed on the surface.; In chapter 5 we examined a different approach to achieve selectivity in an array based SMO sensor. Specifically, the approach involves the use of UV illumination to selectively decompose adsorbed molecules from the surface of WO₃. In infrared studies, it is found that adsorbed DMMP decomposes under UV illumination at room temperature to form a stable methyl phosphate species on the surface. (Abstract shortened by UMI.)...