Esplorazione del processo di ossidazione di nanocluster metallici selezionati in massa su grafene e ossidi

The core of my PhD research activity has been the investigation of the oxidation of size-selected metal nanoclusters, supported on various substrates, highlighting the differences with the bulk counterparts. Using ENAC (Exact Number of Atoms in each Cluster), the size-selected clusters source developed by the Nanoscale Materials Laboratory of Elettra, I deposited size-selected Fe, W, Mo, Pt and Ta clusters on graphene epitaxially grown on Ru(0001) and Ir(111) and on Fe3O4(001),and investigated their oxidation process. In addition, I explored the carbon monoxide adsorption on Mo nanoclusters and the diffusion of Pt single atoms. A unique characteristic of this source is the possibility of connecting it to the experimental chamber of SuperESCA beamline, at the Elettra synchrotron facility, allowing for in situ deposition of nanoclusters, avoiding air contaminations. All the systems were characterized by means of High-Resolution X-ray Photoelectron Spectroscopy, supported by Density Functional Theory calculations which were performed by collaborating groups. The oxidation of Fen clusters, with n = 11, 12, 13, 15 and 20, led to the growth of spectral com ponents attributed to Fe2+ oxidation state, suggesting a metal-to-oxygen ratio close to 1:1. The size-dependent trend of the electron binding energy core level shifts between bulk iron and clusters has been related to the clusters stability. However, assigning a precise oxidation state to metal nanoclusters can be very challenging. This has been demonstrated in other investigations, dedicated to the oxidation of Wn (n = 13, 25) and Mon (n = 6, 13) nanoclusters. Indeed, oxidation state can not be interpreted in terms of metal-oxygen coordination due to the superposition of core level binding energies of atoms with different oxygen coordinations. Moving to the carbon monoxide adsorption on Mon (n = 10, 13) clusters, the appearance of new components in the clusters core level spectra was attributed to CO molecules adsorbed on the clusters. Theoretical calculations confirmed the dissociation of the molecules upon deposition, and attributed the measured core level shifts to the adsorption of oxygen atoms. Fragmentations of W, Mo and Ta clusters, with a size in the range 1-25, occurs upon their deposition on Fe3O4(001), as evidenced by the absence of a metallic feature in the clusters core level spectra. Theoretical calculations corroborated the fragmentation upon deposition for W1, W3 and W5 nanoclusters, and will be extended to Ta and Mo nanoclusters. Concerning Pt, by combining synchrotron-based XPS with DFT simulations, we monitored the diffusion of Pt monomers on graphene/Ir(111) at 45 K in real-time. Pt 4f7/2 core level spectra were deconvoluted into components attributed to the formation of monomers, dimers, larger clusters and intercalated Pt atoms. The diffusion barrier for Pt atoms, as obtained using a kinetic model, closely matches the value computed using DFT. On the other hand, the oxidation of Ptn (n = 12, 13) nanoclusters on the same substrate led to an inhomogeneous oxidation of Pt clusters, with oxygen atoms confined in the topmost layers, facilitated by the clusters pinning on graphene. Eventually, part of my research activity has been devoted to the search of new potential substrates for anchoring clusters. Specifically, we investigated graphene grown on Ir(311). This carbon layer exhibits a coexistence of one-dimensional ripples and a two-dimensional periodic pattern, with strong C-Ir interactions and significant electronic charge overlap in the flatter region between ripples. The computed curvature of these two corrugations suggests that graphene/Ir(311) system can be a promising also for hydrogen storage and release applications.