Structural and optical properties of silver-indium and silver-aluminium nanoalloys: stability against oxidation

Abstract

Bimetallic nanoparticles (BNPs) are promising candidates for fundamental research and applications as their physico-chemical properties can in many cases be tuned continuously or enhanced with respect to the mono-metallic particles. Here we investigate the possibility of fabricating silver-indium and silver-aluminium BNPs in the range of 4-5 nm diameter and of varying stoichiometry by laser vaporization and gas condensation in the gas phase. We analyse their crystalline structure and segregation behaviour using transmission electron microscopy and probe their oxidation state in optical absorption measurements by tracking spectral changes of the localized surface plasmon resonance (LSPR). These complementary techniques show that, despite the small size and the high reactivity especially of aluminium, the BNPs form a silver-rich alloyed core surrounded by an oxide shell. Exposure to air leads to consecutive oxidation, whereas annealing the BNPs in a reducing atmosphere stabilizes the alloyed particle cores, as demonstrated by a narrow and blue-shifted LSPR. This is a first step towards the stabilization of non-oxidized bimetallic nanoparticles combining a noble and a trivalent metal.

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

“…This has previously motivated our study of silver–indium and silver–aluminium BNPs exhibiting LSPR in the UV range and for which silver-rich alloyed phases are fairly stable towards oxidation. 14–16 In line with these previous works, we also investigated the optical and structural properties of gold-based Au–Al and Au–In BNPs, which are detailed further in this paper. The main difference when turning from silver to gold is a red-shift and broadening of the LSPR.…”

“…BNPs consisting of silver-rich indium cores surrounded by an indium oxide shell). 16 This suggests that the partially oxidized BNPs retain metallic indium for the three compositions, but even though the silica matrix effectively slows down the oxidation, it does not inhibit it entirely. Moreover, the silica-embedded BNPs appeared not to be stable with prolonged air-exposure and the LSPRs shift towards lower energies because of gradual indium oxidation.…”

“…Comparison with theoretical calculations permits to attribute this reduced blue-shift for Ag x Al 1-x to a higher degree of oxidation from the first absorption measurements. 16 This means that the proportion of metallic aluminium is probably lower than that of indium, just after Please do not adjust margins Please do not adjust margins air-exposure, and that the oxidation of aluminium is faster than that of indium. Nevertheless, the absorption spectra do not evolve significantly with time, the BNPs after being rapidly partially oxidized are probably protected by the surrounding alumina shell.…”

“…Table 1 shows the crystallographic characteristics of some of these alloys (a similar table is given in Ref. 16 for In, Ag and Al). A remarkable feature of noble metal/trivalent metal systems involving gold is the existence of an equi-atomic compound (AuAl and AuIn, but also AuGa).…”

“…We also present the results of TEM and optical characterizations of the four systems (Ag–In, Ag–Al, Au–In and Au–In), by more specifically focusing on Au–Al and Au–In BNPs and referring when necessary to previous results published in ref. 16 for Ag–In and Ag–Al BNPs.…”