Magnesium (Mg) has been widely investigated for solid-state hydrogen storage. It is able to absorb up to 7.6 wt % of hydrogen gas, making it one of the most promising candidates for hydrogen storage and also a model system for other energy storage materials. Upon hydrogenation the metallic Mg forms non-metallic magnesium hydride (MgH2), thus its electronic and thereby optical properties are drastically altered. This allows for monitoring the kinetics of the hydrogenation process by easily accessible optical measurements. This technique, termed hydrogenography, has been used to observe the nucleation and growth of MgH2 domains in Mg films. Like all far-field optical methods, however, these investigations suffer from the diffraction limit and thus possess only limited spatial resolution, therefore preventing the direct observation of the hydrogenation process on the nanoscale. Here, we overcome this limitation by employing scattering-type scanning near-field optical microscopy and atomic force microscopy. This technique enables us to map the local dielectric properties at different stages of hydrogenation and dehydrogenation, probing thus the electronic properties and hence the local material composition with a spatial resolution on the order of 10 nm. Upon monitoring the kinetics of hydrogen absorption and desorption in such polycrystalline nanoparticles we reveal that the nucleation of this process progresses within individual crystallites. Our combined measurement techniques additionally corroborate a correlation between structural and electronic properties during this dynamic process. To validate this novel technique we additionally monitor in parallel the far-field scattering spectra of individual nanoparticles, which exhibit plasmonic resonances in the visible spectral range.
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