Surfactant-Free Colloidal Syntheses of Gold-Based Nanomaterials in Alkaline Water and Mono-alcohol Mixtures

Abstract

Gold nanoparticles (Au NPs) and gold-based nanomaterials combine unique properties relevant for medicine, imaging, optics, sensing, catalysis, and energy conversion. While the Turkevich-Frens and Brust-Schiffrin methods remain the state-of-the-art colloidal syntheses of Au NPs, there is a need for more sustainable and tractable synthetic strategies leading to new model systems. In particular, stabilizers are almost systematically used in colloidal syntheses, but they can be detrimental for fundamental and applied studies. Here, a surfactant-free synthesis of size-controlled colloidal Au NPs stable for months is achieved by the simple reduction of HAuCl₄ at room temperature in alkaline solutions of low-viscosity mono-alcohols such as ethanol or methanol and water, without the need for any other additives. Palladium (Pd) and bimetallic Au _x Pd _y NPs, nanocomposites and multimetallic samples, are also obtained and are readily active (electro)catalysts. The multiple benefits over the state-of-the-art syntheses that this simple synthesis bears for fundamental and applied research are highlighted.

Figure 1

Synthesis and characterization of Au NPs obtained at RT. (a) Schematic representation of the surfactant-free synthesis of Au NPs at RT under ambient light. The bottom image is a collection of pictures of solutions in a UV–vis quartz cuvette with a 1 cm width, for aliquots from a 39 mL solution taken at different times of the synthesis, as indicated, without stirring. (b) Time-resolved UV–vis spectra during Au NP synthesis (min, minutes; h, hours; D, days; M, months). (c) UV–vis spectra of the colloidal Au NPs obtained using NaOH for a total volume of (black) 6.5 mL or (red) 1 L of solution. (d and e) TEM micrographs of Au NPs obtained using ethanol and methanol, respectively. (f) Size retrieved by TEM analysis using different amounts of methanol and ethanol mixed with water. In the experiments depicted in panels a–f, unless specified otherwise, the total volume fraction of alcohol was 30 vol % in water for a total volume of 13 mL, with 2 mM LiOH and HAuCl₄ added last for a final concentration of 0.5 mM. The synthesis was performed at RT for at least 24 h with stirring. (g) Time-resolved PDF obtained during the formation of Au NPs using 50 mM HAuCl₄ and 150 mM LiOH in 30 vol % ethanol and for the alcohol added last. (h) Time-resolved peak intensity related to the Au–Cl interatomic distance in HAuCl₄ and the Au–Au interatomic distance in fcc Au NPs as a function of reaction time retrieved from PDF analysis.

Figure 2

Synthesis and characterization of Au_xPd_y NPs. (a) Schematic representation of the surfactant-free synthesis of Au_xPd_y NPs using alkaline mixtures of ethanol and HAuCl₄ and PdCl₂ as precursors at RT or 50 °C. (b) Illustrative HRTEM micrographs of Au and Pd NPs obtained at RT. (c) Diameter of the Au_xPd_y NPs evaluated by TEM as a function of their composition evaluated by EDS. (d) XRTS data and PDF refinements using the fcc structural model of Au_xPd_y NPs obtained at RT. The NPs were obtained in 30 vol % ethanol in water using 2 mM LiOH and 0.5 mM {HAuCl₄ + PdCl₂} for a total volume of 13 mL and RT (blue) for a synthesis time of at least 24 h or after low-temperature synthesis (50 °C, red) for 1 h, with stirring.

Figure 3

Toward multimetallic NM samples. (a) Schematic representation of the surfactant-free synthesis of Au_xPd_yPt_zOs_uIr_vRu_w NP samples using alkaline mixtures of 30 vol % ethanol and 0.5 mM HAuCl₄, 0.5 mM H₂PtCl₆, 0.5 mM PdCl₂, 0.5 mM OsCl₃, 0.5 mM H₂IrCl₆, and 0.5 mM RuCl₃ at RT or 50 °C. (b) TEM micrograph and (c) PDF refinement using the fcc structural model for multimetallic samples obtained at RT using LiOH and 30 vol % ethanol where ethanol was added last. The data are colored blue. The fit is colored red. The difference between the data and fit is colored green.

Figure 4

(a) Cyclic voltammograms of Au NPs prepared by the Turkevich–Frens method (Na₃Ct) or without a surfactant (Surfactant-free) for the EOR in alkaline media in 1 M EtOH and 1 M KOH recorded at RT at a scan rate of 50 mV s^–1. The 10th scan is displayed. (b) Chronoamperometry of the EGOR performed at 1.27 V_RHE for 1 h in 1 M EG and 1 M KOH. NPs prepared with PVP did not show any measurable electrocatalytic activity.

Figure 5

Electrochemical properties of Au_xPd_y NPs as a function of NP composition. Case study of the EOR. (a) Cyclic voltammogram (50th scan) of Au, Au₈₃Pd₁₇, and Pd NPs in 1 M KOH and 1 M ethanol recorded at 50 mV s^–1, as indicated. The scales used are different to best identify important features. The arrows indicate the forward (F) and backward (B) scans, and the peaks of maximum intensity corresponding to Au (subscript A) and Pd (subscript B) sites are marked by vertical dashed lines. The NPs were characterized following protocol A detailed in panel b. (b) Different characterization protocols used. (c) ECSA estimated following protocol A. (d and e) Mass activity for the EOR estimated following protocols A and B, respectively, detailed in panel b. The NPs were obtained using 2 mM LiOH, 30 vol % ethanol, and different amount of HAuCl₄ and PdCl₂ for a total concentration of 0.5 mM.

Figure 6

Comparison of different surfactant-free NPs and strategies for electrocatalyst design for the EOR. Au NPs, Pd NPs, the most active Au_xPd_y NPs, i.e., Au₆₅Pd₃₅, the most active nanocomposite, i.e., [40Au + 60Pd], and Au₃₃Pd₁₉Pt₁₈Os₂₀Ir₁Ru₉ are compared, as indicated. All materials were obtained at RT using LiOH as the base and 30 vol % ethanol. (a and b) Mass activity for the EOR evaluated following protocols A and B, respectively, detailed in Figure 5b. The related materials are schematized in panel c with (i) Au NPs, (ii) Pd NPs, (iii) Au_xPd_y NPs, (iv) nanocomposite [xAu + yPd], and (v) multimetallic samples. (d) Ratio of the forward and backward scan intensity for the highest peak observed in the characterization of the different materials. I(F_B)/I(B_B) was used for all materials expect for Au NPs, for which I(F_A)/I(B_A) was used. The related peaks are illustrated in Figure 5a.