Figure 7.

Hydrogen sensing mechanism when using Pd NPs on Si-based substrates as sensing material: (a) In absence of hydrogen, the surface and the interface between two Pd NPs are covered by a layer of spilled over charged oxygen species; (b) During initial hydrogen exposure, H₂ molecules interact with oxygen species resulting in water formation. Therefore, oxygen species are removed from the surface and vacant adsorption sites are formed; (c) The vacant surface sites are then gradually filled by hydrogen that will diffuse in the Pd matrix; (d) Formation of palladium hydride leads to a volume expansion of Pd NPs that reduces the gap between two particles until they touch each other. Reprinted with permission from [6]. Copyright 2014 Sensors and Actuators B: Chemical.