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The morphology and adhesion energy of nanosized metal particles supported on dielectrics are a puzzling issue since, due to the increasing contribution of surfaces and interfaces in their energetics, their equilibrium shape escapes the rules established for large objects. The evolution of wetting during Volmer–Weber growth of nanoparticles is herein studied by in situ ultraviolet/visible surface differential reflectivity spectroscopy (SDRS). The integrated s-polarized SDR signal is shown to be proportional to the oscillator strength of the optically excited plasmon resonances parallel to the surface. Dielectric modelings show that this quantity, which is marginally affected by the size and density of the objects, depends mainly on the aspect ratio of the particles from which adhesion energy can be derived. Applied to noble (Ag, Au) or transition metals (Cr, Ni) and Zn on weakly interacting dielectric (Al2O3, SiO2, KBr) and semiconducting (TiO2, ZnO) substrates, this plasmonic approach evidences a robust U-shaped variation of the aspect ratio with film thickness and therefore size. In line with the thorough study of the Ag/Al2O3(0001) growth and linear elasticity predictions of the equilibrium shape of strained epitaxial particles, the first branch of the "U" is assigned to a size-dependent equilibrium shape related to surface/interface stress effects. A significant decrease in adhesion energy parallels a rounding of the particles. The second branch partly stems from flattening due to incomplete coalescence. The common behavior of poorly wetting supported metal nanoparticles that is revealed herein, with strong changes in shape and adhesion as a function of particle size, had not been evidenced so far. Both the proposed optical methodology and the final findings about adhesion at the nanoscale are of interest in the wide field of application of supported metal nanoparticles that involves heterogeneous catalysis and thin film growth. [ABSTRACT FROM AUTHOR]

The wealth of properties of titanium dioxide relies on its various polymorphs and on their mixtures coupled with a sensitivity to crystallographic orientations. It is therefore pivotal to set out methods that allow surface structural identification. We demonstrate herein the ability of photoemission spectroscopy to provide Ti LMV (V = valence) Auger templates to quantitatively analyze TiO2 polymorphs. The Ti LMV decay reflects Ti 4sp-O 2p hybridizations that are intrinsic properties of TiO2 phases and orientations. Ti LMV templates collected on rutile (110), anatase (101), and (100) single crystals allow for the quantitative analysis of mixed nanosized powders, which bridges the gap between surfaces of reference and complex materials. As a test bed, the anatase/rutile P25 is studied both as received and during the anatase-to-rutile transformation upon annealing. The agreement with X-ray diffraction measurements proves the reliability of the Auger analysis and highlights its ability to detect surface orientations.

Surface stress and energy are basic quantities in the Gibbsian formulation of the thermodynamic description of surfaces which is central in the formation and long-term behavior of materials at the nanoscale. However, their size dependence is a puzzling issue. It is even unclear whether they decrease or increase with decreasing particle size. In addition, for a given metal, estimates often span over an order of magnitude, far apart from bulk data, which, in the absence of any explicit size-dependence rule, escapes understanding. Here, we combine X-ray absorption and nanoplasmonics data with atomistic simulation to describe α-Al2O3(0001)-supported silver particles. By comparison to MgO(001)-supported and embedded silver, we distinguish epitaxial and surface stress. The latter is shown to dominate above 3 nm in size. Since the observation mostly relies on surface/bulk ratio, a metal-independent picture emerges that is expected to have far-reaching consequences for the understanding of the energetics of nanoparticles.