Colloidal gold-palladium-platinum alloy nanospheres with tunable compositions and defined numbers of atoms

Abstract

The combination of different metals into a discrete colloidal nanocrystal (NC) lattice to form solid solutions can result in synergetic and non-additive effects, leading to physicochemical properties distinct from those observed in monometallic NCs. However, these features are influenced by parameters that are challenging to control simultaneously using conventional synthesis methods, including composition, morphology, size, and elemental distribution. In this study, we present a methodology that exploits seed-mediated growth routes and pulsed laser-induced ultrafast heating to synthesize bimetallic and trimetallic colloidal alloy NCs with tailored compositions, well-defined spherical morphologies, and precise control over the number of atoms per NC lattice. Initially, core-shell heterostructures with adjustable compositions and ca. 10⁷ atoms per NC are formed, using Au as the core material and Pd and Pt as the shell metals. In the subsequent stage, ultrafast heating of the heterostructure lattice via nanosecond pulsed laser irradiation facilitates the formation of colloidal AuPd, AuPt and AuPdPt alloy nanospheres. The ability of the proposed synthesis route to produce multimetallic NCs with distinct compositions, consistent morphology, and a fixed number of atoms provides exciting opportunities to investigate how multimetallic NC composition influences catalytic properties. Accordingly, using the catalytic reduction of nitrophenol as a reaction model, we observed a significantly enhanced catalytic performance for AuPdPt NCs compared to AuPd and AuPt NCs.

Fig. 1. Schematic view of the synthesis of AuPdPt alloy NCs. After the synthesis of the Au core (1), Pd is first deposited to form a cubic Pd shell (2) followed by the reduction of Pt ions, leading to the growth of a dendritic Pt shell (3). The resulting core–shell–shell heterostructures are then rapidly heated with ns-pulsed laser irradiation to induce the formation of alloy NCs.

Fig. 2. Synthesis of Au@Pt, Au@Pd and Au@Pd@Pt NCs. (A) Low-magnification TEM image of Au nanospheres used as seeds during the coating process. (B–E) TEM images of Au@Pd@Pt NCs with different compositions of metals: Au₇@Pd₉₃ NCs (B), Au₇@Pt₉₃ NCs (C), Au₇@Pd₆₃@Pt₃₀ NCs (D), and Au₇@Pd₃₃@Pt₆₀ NCs (E). Scale bars: 100 nm.

Fig. 3. Synthesis of bimetallic alloy NCs via excitation of bimetallic core–shell heterostructures using 30 min of excitation with a 532 nm, 5 ns-pulsed laser irradiation from a Nd:YAG laser at a fluence of 64 J m⁻². HAADF-STEM images (A–D) and EDX maps (E–H) of Au₇@Pd₉₃ (A and E) and Au₇@Pt₉₃ (C and G) heterostructures, along with the resulting Au₇Pd₉₃ (B and F) and Au₇Pt₉₃ (D and H) alloy NCs obtained after laser irradiation. Scale bar: 50 nm.

Fig. 4. Synthesis of trimetallic alloy NCs via excitation of trimetallic core–shell–shell heterostructures using 30 min of excitation with a 532 nm, 5 ns-pulsed laser irradiation from a Nd:YAG laser at a fluence of 64 J m⁻². HAADF-STEM images (A–D) and EDX maps (E–H) of Au₇@Pd₃₃@Pt₆₀ NCs (A and E) and Au₇@Pd₆₃@Pt₃₀ NCs (C and G) heterostructures, along with the resulting Au₇Pd₃₃Pt₆₀ NCs (B and F) and Au₇Pd₆₃Pt₃₀ (D and H) alloy NCs obtained after laser irradiation. Scale bar: 50 nm.

Fig. 5. Investigation of the catalytic activity of synthesized alloy NCs in the reduction of 4-NP with sodium borohydride. (A) Evolution of the absorption maxima of 4-NP (at 400 nm) with time during the reduction reaction catalysed by Au₇Pd₉₃ (black), Au₇Pt₉₃ (yellow), Au₇Pd₃₃Pt₆₀ (cyan), and Au₇Pd₆₃Pt₃₀ (red) NCs. Reaction conditions: [4-NP] = 0.05 mM, [metal atoms] = 0.029 mM and [NaBH₄] = 0.08 M, T = 25 °C. (B) Time evolution of the absorbance at 400 nm during the reduction of 4-NP catalysed by Au₇Pd₆₃Pt₃₀ NCs and linearized data for first-order analysis corresponding to (A). (C) Comparison of the different k_nor obtained for the investigated alloy NCs.

Guillermo González-Rubio