Instability of Gas‐Phase‐Assembled Pd–Fe Alloy Nanoparticles Under Ambient Conditions: Pd/FeO x Core–Shell Formation and Magnetic and Hydrogen‐Sensing Consequences

Pd‐based nanomaterials are widely studied for catalysis and hydrogen sensing and storage; however, the internal structure of bimetallic Pd systems and its impact on performance in these applications are often overlooked. Here we show that exposure to ambient conditions fundamentally alters small (∼4 nm), gas‐phase‐assembled Pd‐rich Pd–Fe alloy nanoparticles (7–28 at.% Fe) by driving Fe segregation toward the particle surfaces and, hence, the formation of a Pd/FeO x core–shell morphology. This finding is based on structural, spectroscopic, magnetic, and hydrogen‐absorption studies of highly porous films of these surfactant‐free nanoparticles. The FeO x shells—ultrathin (<1 nm) and likely amorphous—give rise to glassy magnetic behavior in the films. The segregation‐oxidation is suppressed by embedding the pristine (Pd–Fe alloy) nanoparticles in a metallic Nb matrix, forming continuous nanogranular films that exhibit conventional superparamagnetic blocking. Electrical resistance measurements of the oxidized films during cyclic hydrogen exposure reveal an irreversible decaying background, which we attribute to progressive reduction of the FeO x shells. These results demonstrate that oxidation‐driven segregation can govern the magnetic and hydrogen‐sensing properties of Pd–Fe nanoparticles.