Pd/SiO2 and AuPd/SiO2 nanocomposite-based optical fiber sensors for H2 sensing applications

The ability to accurately and safely monitor hydrogen concentration is of significant importance for a broad range of energy, defense, aviation, and aerospace applications with one of the most notable applications being leak detection for hydrogen above the lower explosive limit. Optical-based approaches offer significant safety advantages as compared to electrical-based sensors and Pd or AuPd-alloys are commonly utilized as the functional sensor layer due to a well-known, characteristic, and selective interaction with H2. In this work, optical fiber-based sensors comprised of Pd and AuPd alloy nanoparticle incorporated SiO2 thin films deposited onto unclad multimode silica-based optical fiber evanescent wave absorption spectroscopy sensing elements have been investigated. Selective, sensitive, and monotonic H2 sensing responses have been demonstrated at levels significantly greater than the lower explosive limit in the presence of CO and O2 near room temperature. A tendency for partial oxidation of the noble metal nanoparticles upon exposure to oxidizing atmospheres is confirmed directly through X-ray photoelectron spectroscopy, particularly at elevated temperatures. Monotonic H2 sensing responses are also observed at elevated temperatures in cases where oxygen is not introduced into the atmosphere. However, more complex sensing responses in multi-component elevated temperature gas streams containing oxidizing and reducing species can be observed which likely result from oxidation and reduction of noble metal nanoparticles. These results demonstrate that the incorporation of noble metals such as Pd and Pd-alloy nanoparticles into inert dielectric matrices such as SiO2 can impart new optical sensing functionality potentially useful for H2 sensing applications.