Structural and optical characterization of nanoalloys mixing gold or silver with aluminium or indium: evolution under various reactive environments.

In this study, the atomic and chemical structure and the optical response of AxB1-x bimetallic nanoparticles (BNPs) combining gold or silver (A) with aluminium or indium (B) were investigated at various stoichiometries in order to examine if stable alloyed phases could exist and promote the emergence of localized surface plasmon resonance (LSPR) in the UV range. The structure and morphology of BNPs of a few nanometres, produced by laser vaporization, were analysed by transmission electron microscopy (TEM) and optical absorption measurements were performed on matrix-embedded BNPs. Information about the oxidation state of the BNPs can be inferred from a comparison between experimental optical spectra and Mie calculations in the dipolar approximation. The BNPs' internal structures were further investigated by additional characterization techniques. Firstly, in situ X-ray photoelectron spectroscopy provided information about the chemical state of the constituent elements and their evolution with time. Secondly, synchrotron-based X-ray scattering techniques were performed on Ag-Al BNPs in a wide-angle configuration under grazing incidence, giving complementary information about structural and morphological heterogeneities in the BNPs. Finally, the restructuring of the partially oxidized Au0.33Al0.67 BNPs annealed in a reducing atmosphere was also attempted by environmental TEM. The complementary techniques of characterization show that silver-based Ag-In and Ag-Al BNPs form metallic silver-rich alloyed cores surrounded by an indium or aluminium oxide shell. The initial LSPR is in the UV range for both systems, but the difference in the kinetics of oxidation between indium and aluminium involves less blue-shifted LSPR for Ag-Al BNPs. In the case of gold-based BNPs, we show evidence of ordered nanoalloys just after air exposure and the appearance of gold and indium (or aluminium) demixing during oxidation. The initial LSPR of Au-In BNPs is the one the most in the UV range among the four systems, with an LSPR peak centred at 254 nm, which may be a sign of the formation of the Au0.33In0.67 alloy. Nevertheless, strategies to preserve BNPs from oxidation have to be developed.