Response modulation of PdNi nano-film hydrogen sensors by thickness control

Resistive hydrogen sensors based on PdNi nano-film with different thicknesses were fabricated by Micro-ElectroMechanical System technology. It demonstrates that the response of the PdNi nano-film hydrogen sensors continuously increases as the film thickness decreases within the experimental range. The response to 2.5 ppm hydrogen of 12 nm PdNi sensor is 4 times higher than that of 145 nm, and the detected limit of 12 nm is low to 50 ppb hydrogen. Reducing the film thickness doesn't affect the crystal structure, but reduces the crystallinity and grain size and increases the grain boundary density significantly. Thus, the thinner film possesses a higher volume fraction of the distorted part around the grain boundary to react with hydrogen. Meanwhile the thinner film has fewer barriers for hydrogen to transfer to the inside. When the hydrogen concentration is below 50 ppm, the relationship between the sensing response, hydrogen concentration and the film thickness is well fitted through linear regression method. The results agree with the Sieverts' law that the response is proportional to the one-half of the gas concentration, and the Sieverts' coefficient is a function of film thickness.

Automated summary

It was found that the response of PdNi nano-film hydrogen sensors increases as the film thickness decreases, with the 12 nm film showing significant advantages and a detection limit as low as 50 ppb hydrogen. Film thickness only affects crystallinity and grain size, not crystal structure.