Microscopic Structure of Naked Au Nanoparticles Synthesized in Typical Ionic Liquids by Sputter Deposition

Au nanoparticles (AuNPs) of varied sizes were synthesized in typical ionic liquids using the sputter deposition technique. Because AuNPs are dispersed freely in each ionic liquid without bonding to particular stabilizing agents, they are sometimes called “naked AuNPs”. To characterize the structure of naked AuNPs, we obtained a size distribution curve from small-angle X-ray scattering (SAXS) experiments and extracted the value of the peak position as the size of the most abundant AuNPs in each ionic liquid. The size was controlled by the type of ionic liquid and preparation temperature. To elucidate the local structure of the AuNPs in relation to their particle size, we measured the X-ray absorption fine structures (XAFS) at the Au L3-edges. The analyses of the extended XAFS revealed that the Au–Au bond lengths for AuNPs ∼1 nm in size are 2.76 to 2.81 A, in contrast with the 2.88 A bond lengths for bulk Au. Comparing our SAXS and XAFS results with previously reported theoretical studies, we conclude that ...