Nanoparticles (NPs) with well-defined sizes and shapes were synthesized via inverse micelle encapsulation methods. For 2-4 nm Pt and Au NPs supported on TiO2(110), the shape was resolved by scanning tunneling microscopy (STM). Geometrical information on smaller Pt NPs supported on nanocrystalline γ-Al2O3 was extracted by a combination of transmission electron microscopy (TEM) and extended x-ray absorption fine-structure spectroscopy (EXAFS) measurements. It will be shown that the size, interparticle distance, and the geometry (2D vs 3D) of the NPs can be tuned via our micellar synthesis.

Pt NPs in the size range of 2-4 nm supported on TiO2(110) were studied by STM after heating in UHV at high temperature (>1000°C). This thermal treatment facilitates the melting of the NPs and the formation of NP-support epitaxial interfaces. High resolution STM images allowed us to determine the shape of the NPs as well as facet orientations. Three different shape types were observed, and each category of shapes was found to appear within a particular NP size regime. In addition, the epitaxial relationship between the NPs and the TiO2(110) surface was investigated in order to explain the specific orientation of the NPs observed in our study. It was also found that due to interface-induced strain, the NP shapes obtained do not follow the Wulff theorem, namely {100}/{111} facet area ratios deviating from the value obtained for support-free clusters.

Pt NPs in the size range of 0.8-1.5 nm supported on γ-Al2O3 were studied by EXAFS, and the nearest neighbor coordination numbers up to the 4th shell were obtained following multiple scattering analysis. These coordination numbers, together with the NP diameter obtained by TEM were examined against a theoretically generated database of possible NPs shapes to determine the most representative shape of Pt NPs in each of the samples. Correlations between the reactivity and the shape of the Pt NPs were established.