Here, we show that we can synthesize free-standing palladium nanoparticles with a size of about 5 nm embedded in a fluorinated polymer matrix using magnetron codeposition and a subsequent annealing step. Indeed, we deposit with magnetron sputtering at the same time PTFE and Pd, and a subsequent thermal annealing step under a hydrogen atmosphere ensures agglomeration of the Pd atoms into small nanoparticles. This scalable vapor-based method allows deposition on all kinds of surfaces, including substrates and optical fibers. Using a combination of transmission electron microscopy, grazing-incidence diffraction, neutron and X-ray reflectometry, and X-ray photoelectron spectroscopy, we characterize the nanocomposite films and the palladium particles inside. These palladium nanoparticles could have a variety of applications in catalysis, hydrogen compressors, and optical hydrogen sensors. For the later application, we show using optical transmission measurements that the nanoparticles can reversibly absorb hydrogen, having well-defined steps in optical transmission when the hydrogen pressure is changed. Owing to their small size, the polymer matrix, and high surface-to-volume ratio, the nanoparticles show subsecond response times to changes in hydrogen concentration.

Using in situ scanning transmission electron microscopy (STEM) and low-loss plasmon electron energy-loss spectroscopy (EELS), we reveal asymmetric transformation mechanisms during the hydrogenation and dehydrogenation of Mg thin films. Remarkably, during hydrogenation, the MgH2 phase can nucleate from either the bottom or top interface of a Mg thin film while symmetrically sandwiched between two Ti layers. This unexpected behavior, occurring under identical external conditions, highlights the critical role of nucleation barriers in the phase transformation process, challenging conventional diffusion-driven paradigms. In contrast, dehydrogenation proceeds exclusively via an H2 diffusion-controlled frontal growth originating from the top interface. These insights underscore the importance of understanding metal-to-metal hydride phase transformations for advancing hydrogen storage technologies and applications such as hydrogen sensing.

Magnesium thin films covered with a layer of Pd absorb hydrogen at much higher pressures than bulk Mg. Such an effect was originally explained as a consequence of elastic clamping on Mg by the capping Pd layer. An alternative interpretation later suggested that the pressure increase could originate from simple alloying between Mg and Pd. Here we resolve this controversy by measuring the hydrogenation and dehydrogenation isotherms of Mg-Pd thin film alloys over a wide range of compositions. Our results disentangle the effects of elastic clamping and alloying and highlight the role of plastic deformations.