Preparation and Characterization of Niobium Pentoxide Thin Films for Catalytic Gas Sensor Applications

Thin films of niobium pentoxide were fabricated by spin-coating quartz plates with a solution of niobium ethoxide and ethanol. The niobium alkoxide was prepared from the reaction of niobium pentachloride with ethanol following the ammonia method. The composition of the precursor solution was characterized with x-ray fluorescence and the structural evolution during heating with thermal analysis (TGA and DTA). X-ray diffraction and SEM of the films calcined at 400^circC revealed amorphous and porous structures about 0.3 mu m thick. Calcination at 600^circ C, 800^circC and 1000 ^circC resulted in the crystallinization of niobia into mostly single phases. Infrared spectroscopy of adsorbed pyridine on xerogels calcined at 400 ^circC revealed the existence of both Lewis and Bronsted acidity. The potential application of thin films of niobia as ammonia gas sensors was explored by measuring the changes in conductance of the films when exposed to inert atmospheres and to ammonia, at different temperatures of operation. It was found that the preparation route of the film, which determines its crystalline structure and morphology, influenced its sensitivity and response time toward ammonia. The increase in conductance of the films upon ammonia exposure was explained in terms of an electron-donor mechanism from the ammonia to the coordinated unsaturated sites of niobia. Electrical conduction properties of thin films and compressed powders of niobium pentoxide were characterized with solid-state impedance spectroscopy. The real and imaginary components of the impedance data were fitted to a Voigt electrical circuit model with two and three circuit elements. The results show that the films conductance is determined by surface processes such as polarization, chemisorption and gas-solid chemical reactions.