Figure 1. Surface structure of Ag-covered Au NCs from synchrotron Ag K-edge XAS.

(a) Schematic illustration of synchrotron X-rays used to obtain Ag K-edge EXAFS spectra from the Ag on the NC surfaces, from which three-layer Ag–Cl, Ag–Ag and Ag–Au surface structures were determined. (b) Experimental EXAFS R-space spectra of the Au NCs with refinements based on theoretical bonding paths generated by FEFF. The refinements were conducted over R-space ranges of 1.7–3.3 Å, hence the disagreement of the refinements to the experimental data outside of that range. (c) Experimentally determined Ag–Cl, Ag–Au and Ag–Ag bond lengths and (d) coordination numbers of each NC surface. The error bars in c and d represent uncertainties in the refinement, and details of their calculation can be found in the Methods section. (e) Top-down view of ideal monolayer Ag coverage on Au with {111}, {110}, {310} and {720} facets, giving average Ag–Ag/Ag–Au coordination numbers of 6/3, 2/5, 2/5 and 3/4.8, respectively.