Optical fiber evanescent wave absorption spectrometry of nanocrystalline tin oxide thin films for selective hydrogen sensing in high temperature gas samples.

SnO(2) nanocrystalline material was prepared with a sol-gel process and thin films of the nanocrystalline SnO(2) were coated on the surface of bent optical fiber cores for gas sensing. The UV/vis absorption spectrometry of the porous SnO(2) coating on the surface of the bent optical fiber core exposed to reducing gases was investigated with a fiber optical spectrometric method. The SnO(2) film causes optical absorption signal in UV region with peak absorption wavelength at around 320 nm when contacting H(2)-N(2) samples at high temperatures. This SnO(2) thin film does not respond to other reducing gases, such as CO, CH(4) and other hydrocarbons, at high temperatures within the tested temperature range from 300 degrees C to 800 degrees C. The response of the sensing probe is fast (within seconds). Replenishing of the oxygen in tin oxide was demonstrated by switching the gas flow from H(2)-N(2) mixture to pure nitrogen and compressed air. It takes about 20 min for the absorption signal to decrease to the baseline after the gas sample was switched to pure nitrogen, while the absorption signal decreased quickly (in 5 min) to the baseline after switching to compressed air. The adhesion of tin oxide thin films is found to be improved by pre-coating a thin layer of silica gel on the optical fiber. Adhesion increases due to increase interaction of optical fiber surface and the coated silica gel and tin oxide film. Optical absorption spectra of SnO(2) coating doped with 5 wt% MoO(3) were observed to change and red-shifted from 320 nm to 600 nm. SnO(2) thin film promoted with 1 wt% Pt was found to be sensitive to CH(4) containing gas.