Humidity effects on Pd/Au-based all-optical hydrogen sensors

The hydrogen sensing performance of Pd (60 at%)/Au (40 at%) thin films has been examined under room temperature conditions and at variable humidity levels using either air or nitrogen as a carrier gas. Two humidity effects on the sensing properties of these films have been observed: (1) the baseline optical transmittance of the Pd/Au thin film progressively and reversibly increases with an increase in humidity for both air and N2 carrier gases; (2) the transmittance signal change becomes reduced and eventually inverts with an increase in humidity while using an air carrier gas for the hydrogen exposure cycles. Optical microscopy and H2/H2O vapor exposure experiments reveal that the baseline signal increase is induced by the blistering of the Pd/Au thin film due to the adsorption of H2O. The transmittance signal change reduction/inversion during H2/humidified air exposures is caused by Pd surface-catalyzed reactions followed by H2O desorption. Specifically, the sensing characteristics as a function of carrier gas and humidity levels can be explained by the following surface reactions: H2O is dissociatively adsorbed on the film surface forming OH molecules with assistance of surface chemisorbed atomic oxygen. Once H2 is admitted and dissociatively adsorbed as atomic H, a ready reaction between H and OH dominates the consumption of H2 into the formation of H2O, thereby reducing the formation of PdHx. The overall desorption of the surface formed H2O causes a decrease in the optical transmittance observed for a given hydrogen exposure level, thus desensitizing the Pd/Au thin film towards the detection of hydrogen. The addition of Ru or YSZ as a buffer layer between the glass substrate and the Pd/Au thin film demonstrates a reduced or a completely removed baseline shifting, but H2 desensitization in the presence of humidified air is still present.