Marco Calizzi

Self-assembly of gas-phase synthesized magnesium nanoparticles on room temperature substrates

Magnesium nanoparticles (NPs) with initial size in the 10–50 nm range were synthesized by inert gas condensation under helium flow and deposited on room temperature substrates. The morphology and crystal structure of the NPs ensemble were investigated as a function of the deposition time by complementary electron microscopy techniques, including high resolution imaging and chemical mapping. With increasing amount of material, strong coarsening phenomena were observed at room temperature: small NPs disappeared while large faceted NPs developed, leading to a 5-fold increase of the average NPs size within a few minutes. The extent of coarsening and the final morphology depended also on the nature of the substrate. Furthermore, large single-crystal NPs were seen to arise from the self-organization of primary NPs units, providing a mechanism for crystal growth. The dynamics of the self-assembly process involves the basic steps of NPs sticking, diffusion on substrate, coordinated rotation and attachment/coalescence. Key features are the surface energy anisotropy, reflected by the faceted shape of the NPs, and the low melting point of the material. The observed phenomena have strong implications in relation to the synthesis and stability of nanostructures based on Mg or other elements with similar features.

Element-specific channels in photo-excitation of V-doped TiO2 nanoparticles

Refined X-ray spectroscopies can be crucial in elucidating charge transfer phenomena which play a key-role in photo-catalysis and other processes relevant for clean energy production. A deep understanding of electron photo-dynamics is, in fact, essential to develop efficient knowledge-based devices. We developed a differential illumination RIXS and HERFD-XAS [1], method on ID26 @ ESRF to investigate charge transfer phenomena with chemical sensitivity; specifically, we studied V-doped TiO2 nanoparticles, a promising materials system for photo-catalysis, performing measurements around both the V Kα and Ti Kβ emissions. We found that visible light absorption induces the transfer of electrons from the V dopants to the host matrix cations in defective sites. With a steady state model, it was also possible to estimate the lifetime of the excited state. The value we obtained (around 1 ms) suggests that dopant-injected electrons can remain trapped near Ti atoms for a very long time. The procedure we used is completely general and can be successfully applied to detect any kind of long-living charge transfer phenomena in a wide range of possible devices [2].