A new method uses gold to assemble multi-element nanoparticles under mild conditions, offering improvements in catalytic activity and long-term stability.
(Nanowerk Spotlight) The drive to engineer nanoparticles composed of multiple metal elements has been motivated by the promise of exceptional performance in catalysis, energy conversion, electronics, and biotechnology. Traditionally, single-metal nanoparticles dominate practical use, but incorporating several different metals into a single particle offers the possibility of tuning chemical and physical properties at an atomic level.
However, creating homogeneous multi-element nanoparticles has posed serious challenges. The problem lies in the differing atomic sizes, melting points, and chemical affinities between metals, often leading to phase separation or uneven distribution during synthesis.
Previous efforts to overcome these barriers included techniques such as high-temperature annealing, carbothermal shock, laser-assisted reduction, and in situ chemical reduction. While these methods have produced advances, they typically require extreme conditions—high temperatures, pressures, or potent reducing agents—which limit scalability and control.
Studies of noble metals such as gold have revealed a curious property relevant to this challenge. During synthesis, gold atoms can catalyze their own reduction from precursor salts without external intervention. This self-initiated reduction, known as autocatalysis, hinted at an alternative path toward creating complex nanoparticles under more benign conditions.
Gold’s electronic structure—marked by high electronegativity and strong electron affinity—makes it behave in some respects like a reactive nonmetal, stabilizing electrons and mediating chemical transformations. Although previous observations of gold autocatalysis were limited to specific and tightly constrained reaction systems, the broader synthetic potential of this phenomenon remained unexplored.
Building on this foundation, researchers at Northwestern Polytechnical University and associated institutions have developed a strategy that exploits gold’s autocatalytic behavior to synthesize multi-element nanoparticles under mild, ambient conditions.
Their study, published in Advanced Materials (“Gold-Autocatalyzed Synthesis of Multi-Element Nanoparticles”), introduces a gold-autocatalyzed synthesis method that enables the incorporation of diverse metals, including immiscible combinations, into single-phase nanoparticles. This process proceeds without the need for high temperature, pressure, or strong reducing agents.
Diagram illustrating how gold initiates the formation of multi-element nanoparticles. Gold atoms first form small clusters, then attract other metals through electrostatic forces, leading to the growth of uniform, complex nanoparticles without external energy input.
The method relies on the spontaneous electron transfer from selected “electron donor” metal chlorides to gold ions. Using computational modeling grounded in frontier molecular orbital theory, the team identified several metal chlorides capable of serving as effective electron donors. When gold chloride (AuCl₃) and an electron donor such as molybdenum chloride (MoCl₅) are mixed, the donor reduces gold ions to metallic gold atoms. These atoms then initiate the nucleation of nanoparticles while simultaneously incorporating additional metal species from the surrounding solution.
The synthesis follows a three-step process. First, electron donors reduce gold ions, forming gold-rich molecular complexes referred to as “monomers.” These monomers acquire a slight negative charge, enabling them to attract positively charged metal cations through electrostatic forces. Finally, the growing clusters undergo nucleation and coalescence into fully formed nanoparticles. The entire sequence occurs spontaneously, driven by the inherent chemical properties of the precursors, and requires no external energy input beyond simple mixing.
Using this method, the researchers synthesized nanoparticles incorporating up to five different metals. The combinations included elements traditionally considered immiscible, such as molybdenum, copper, chromium, vanadium, tantalum, tungsten, zinc, iridium, platinum, rhenium, ruthenium, rhodium, palladium, niobium, zirconium, scandium, iron, manganese, yttrium, nickel, and cobalt.
Electron microscopy and energy-dispersive X-ray spectroscopy confirmed uniform distribution of elements within the nanoparticles, while X-ray diffraction measurements revealed characteristic shifts in lattice parameters consistent with atomic-level mixing and lattice distortion.
Beyond successful synthesis, the team demonstrated precise control over nanoparticle size and composition. By adjusting the concentrations of gold chloride, electron donors, and additional metal chlorides, they could tune particle diameters from tens to hundreds of nanometers. The relative ratios of incorporated elements could also be modulated, offering the ability to tailor material properties systematically.
To highlight practical applications, the researchers fabricated a catalyst composed of gold, iridium, platinum, palladium, and ruthenium nanoparticles and tested it for the hydrogen evolution reaction (HER) in alkaline electrolytes. Efficient hydrogen production in alkaline media poses challenges because competing interactions with hydroxide ions reduce catalyst performance.
The multi-element catalyst produced using gold-autocatalyzed synthesis delivered strong performance, achieving a current density of 10 mA/cm² at an overpotential of just 24 millivolts and 100 mA/cm² at 42 millivolts. These values surpass the performance of commercial platinum-on-carbon catalysts, which require significantly higher overpotentials.
The enhanced catalytic behavior was attributed to modifications in the electronic structure of the platinum atoms within the nanoparticles. In the multi-element environment, the d-band center of platinum shifted downward, which weakens the binding of hydrogen intermediates and facilitates hydrogen gas release during HER. Electrochemical impedance spectroscopy revealed that the multi-element nanoparticles had lower charge transfer resistance compared to platinum-on-carbon, indicating faster reaction kinetics. Furthermore, the electrochemical surface area, measured by double-layer capacitance, was larger for the multi-element catalyst, suggesting more available active sites for the reaction.
Structural stability during operation was also confirmed. After 72 hours of continuous hydrogen evolution, the catalyst exhibited minimal structural degradation, with only minor changes in lattice parameters and morphology observed through X-ray diffraction and electron microscopy. Additional durability tests under varying conditions further reinforced the robustness of the multi-element nanoparticles.
The theoretical framework based on frontier molecular orbital theory not only guided the initial synthesis but also suggests broader possibilities for rational design of multi-element nanoparticles. The ability to predict which metal precursors will successfully undergo electron transfer reactions with gold offers a path forward for creating even more complex nanoparticle systems with engineered functionalities.
This study introduces a flexible, accessible method for synthesizing complex multi-element nanoparticles under mild conditions. By exploiting the unique autocatalytic properties of gold, the researchers have opened new avenues for material design across energy, catalysis, and nanotechnology. The approach expands both the practical and theoretical toolkit available for engineering nanoscale materials that incorporate diverse chemical species into uniform, stable structures.
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