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Description
Bimetallic AuxAg1-x clusters with various compositions (x = 0.9, 0.7, 0.4 and 0.2) grown in the gas phase and deposited on amorphous SiO2 wafers and TEM grids were characterized by a combination of aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and grazing-incidence small-angle X-ray scattering (GISAXS). Clusters with an average diameter of 3.0 ± 0.6 to 3.7 ± 0.7 nm were produced using dual-laser ablation of pure Ag and Au metal targets under UHV conditions1,2. Control over the bimetallic clusters composition was achieved in the gas-phase prior to deposition by fine tuning of the laser energy densities and the time delay between the laser pulses. Composition was monitored by reflectron time of flight mass spectrometry using a newly developed binomial combination model. Clusters were then deposited in soft landing mode with a low coverage of 0.1 atomic monolayer (ML) to minimize aggregation on SiO2 wafers and TEM grids for GISAXS and HAADF-STEM measurements respectively. HAADF-STEM and GISAXS data analysis3,4 show, in excellent agreement, core-shell structures for all the investigated compositions with a systematic composition-dependent inversion of the element in the core and the shell. Less abundant species always form the core of the clusters: a gold rich core is observed in Au0.9Ag0.1 and Au0.7Ag0.3 clusters while a silver rich core is observed in Au0.4Ag0.6 and Au0.2Ag0.8 clusters. Despite the low surface coverage fraction, GISAXS proved to be extremely sensitive to measure the cluster size and structural organization. The composition-dependent core-shell structure of bimetallic AuxAg1-x clusters is expected to affect significantly their catalytic properties that are currently being tested for CO oxidation reactions.
References
1 Bouwen, W. et al. Production of bimetallic clusters by a dual-target dual-laser vaporization source. Rev. Sci. Instrum. 71, 54-58 (2000).
2 Verschoren, G. et al. Electron scattering in Au films containing Co clusters. Thin Solid Films 516, 8232-8239, doi:10.1016/j.tsf.2008.02.055 (2008).
3 Li, Z. et al. Structures and optical properties of 4–5 nm bimetallic AgAu nanoparticles. Faraday Discuss. 138, 363-373 (2008).
4 Lazzari, R. IsGISAXS: a program for grazing-incidence small-angle X-ray scattering analysis of supported islands. J. Appl. Crystallogr. 35, 406-421, doi:Doi 10.1107/S0021889802006088 (2002).
