Mechanism of Gold–Silver Alloy Nanoparticle Formation by Laser Coreduction of Gold and Silver Ions in Solution

Abstract

Photochemical reduction of aqueous Ag+ and [AuCl4]− into alloy Au–Ag nanoparticles (Au–Ag NPs) with intense laser pulses is a green synthesis approach that requires no toxic chemical reducing agents or stabilizers; however size control without capping agents still remains a challenge. Hydrated electrons produced in the laser plasma can reduce both [AuCl4]− and Ag+ to form NPs, but hydroxyl radicals (OH·) in the plasma inhibit Ag NP formation by promoting the back-oxidation of Ag0 into Ag+. In this work, femtosecond laser reduction is used to synthesize Au–Ag NPs with controlled compositions by adding the OH· scavenger isopropyl alcohol (IPA) to precursor solutions containing KAuCl4 and AgClO4. With sufficient IPA concentration, varying the precursor ratio enabled control over the Au–Ag NP composition and produced alloy NPs with average sizes less than 10 nm and homogeneous molar compositions of Au and Ag. By investigating the kinetics of Ag+ and [AuCl4]− coreduction, we find that the reduction of [AuCl4]− into Au–Ag NPs occurs before most of the Ag+ is incorporated, giving us insight into the mechanism of Au–Ag NP formation.

Synthesis of Air-Stable Cu Nanoparticles Using Laser Reduction in Liquid

“…The formation of substantial quantities of sub-10 nm Cu NPs in our LRL synthesis is similar to previous LRL results for Au and Ag NPs using well-controlled solvent chemistry [35,45], whereas the large Cu NPs up to ∼100 nm in diameter resemble the large Fe oxide NPs obtained from LRL of ferrocene in hexane [46]. The formation of two distinct size distributions of Cu NPs strongly suggests the participation of two different reaction mechanisms [33], although specific identification of these mechanisms is beyond the scope of this work.…”

“…The production of Cu metal NPs using LRL requires both an organic solvent and air-free conditions, as no laser-induced conversion of Cu(acac) 2 precursor was observed when the solvent was water or when the precursor in IPA/methanol mixture was irradiated under ambient conditions. This result underscores the importance of minimizing reactive oxygen species formation during LRL, which was also needed to effectively control the sizes of Au NPs [35] and enable formation of Ag-containing NPs [41,44,45]. The important role of reactive oxygen species has also been noted in PLAL synthesis of Cu NPs, where air removal completely eliminated formation of the highly oxidized CuO phase during PLAL in water [28].…”

“…A current limitation to the wide use of LRL is that the vast majority of studies reported to date focus on the easily reduced noble metals Au [34][35][36][37][38], Ag [39][40][41], Pt [42], and their alloys [43][44][45] because these metals are resistant to oxidation. Unlike in PLAL where the zero-valent metal is present in the initial target, metal NP formation in LRL requires chemical reactions between the molecular precursor(s) and the reactive species in LRL plasma.…”

“…Whereas eaq are exceptionally strong reducing agents towards metal ions, OH • radicals and their recombination product H 2 O 2 can back-oxidize zero-valent metal atoms. Although Au is resistant to back-oxidation and HAuCl 4 precursor can be reduced by H 2 O 2 [37], effective production of Ag-containing NPs by LRL requires the addition of OH • scavengers, such as ammonia or isopropyl alcohol, to prevent backoxidation of Ag [40,41,45]. Moreover, the few LRL studies on the non-noble metal Fe report production of only Fe oxides [46,47].…”

Synthesis of Au50Ag50 Alloy Nanoparticles From Metal Ions and Colloidal Nanoparticles Through Photochemical Reduction Methods Using Femtosecond Laser

“…In this method, Au-Ag nanoalloys could be synthesized by only one step. Au and Ag ion obtained from diluting metal salts were mixed together into a glass cuvette and irradiated with laser femtosecond in only 5 minutes to form Au-Ag nanoalloys [19,20]. The prolonged irradiation time in this method would decrease particles size that have been described in our previously experiment [21].…”