(Nanowerk Spotlight) Chemical reactions power everything from industrial manufacturing to clean energy production, but their efficiency depends critically on catalysts – materials that speed up reactions without being consumed. The key to a catalyst’s performance lies in its microscopic architecture, which determines how easily molecules can reach active sites where reactions occur.
Traditional catalysts often consist of metal nanoparticles deposited on a support material, but many of their potential reaction sites remain inaccessible or poorly connected. This limitation has driven researchers to seek materials with more optimized structures. Metal aerogels offer a promising solution through their unique combination of metallic properties with an open, organized structure that allows efficient movement of molecules, electrons, and heat.
Aerogels are ultra-light materials that begin as solutions containing metal compounds that transform into gels through a process called sol-gel synthesis. While researchers have developed various ways to control this process using chemical additives and external conditions, understanding how different metals interact during formation has been largely overlooked.
Metal aerogels are part of the broader aerogel family that includes materials made from silica, polymers, and carbon. What makes metal aerogels distinctive is their interconnected pathways that serve multiple functions simultaneously: they conduct electrons like a microscopic electrical grid, transfer heat efficiently throughout the material, and allow molecules to move freely through the network.
Creating these precise architectures has proven particularly challenging because metals naturally tend to clump together rather than forming open networks. Previous attempts either applied specific ligands or initiators to impact microstructures, but these approaches introduced more impurities and provided limited control over the final structure. Scientists needed a deeper understanding of how different metals interact during the formation process itself.
Researchers at the Beijing Institute of Technology, led by Prof. Ran Du at the School of Material Science and Engineering, have now uncovered fundamental principles that govern how different metals combine to form these intricate structures. Their findings build upon years of systematic research exploring various synthesis methods using chemical additives, different reducing agents, and external fields to control aerogel formation fields. We reported on these works in previous Nanowerk Spotlights: “Freeze-thaw made noble metal aerogels: Clean and hierarchical materials for photoelectrocatalysis” and Manipulating noble metal aerogels by reductant chemistry.
Their new work, reported in Matter (“Manipulating multimetallic effects: Programming size-tailored metal aerogels as self-standing electrocatalysts”), reveals that the size difference between metal atoms acts like a molecular-scale construction rule. When metals with similar atomic sizes combine, they pack together efficiently into thick strands through a process called Frank−van der Merwe growth, where new atoms layer smoothly onto the existing surface. However, when metals with different atomic sizes mix—for example, gold atoms that are 13.9% larger than nickel atoms—they create stress in the crystal structure. This stress forces the metals to switch to a different growth pattern, Volmer−Weber growth, where atoms add in clusters or islands rather than smooth layers.
The impact of this atomic size mismatch is dramatic: adding just 1% of a differently-sized metal can reduce the thickness of connecting strands (called ligaments) by up to 78%. While single-metal aerogels typically form relatively thick ligaments, the team found they could precisely control ligament size between 3 and 10 nanometers by carefully selecting combinations of metals with different atomic sizes. This fine control occurs because the mismatched atoms promote growth along the length of existing strands while inhibiting thickening, much like building a network of thin bridges rather than wide highways.
The team also discovered that the average density of the metal mixture controls how particles settle during formation. They observed a consistent precipitation-like process where metal particles first form small aggregates that gradually settle to the bottom of the container. Heavy metal combinations settle quickly, while lighter ones descend more slowly. This settling speed determines whether particles have time to form optimal connections before becoming fixed in place. This is in line with their previous discovery reported in Science Advances (“Specific ion effects directed noble metal aerogels: Versatile manipulation for electrocatalysis and beyond”).
The team observed that the formation process begins with the solution turning black as metal particles initially form, followed by these particles settling and connecting into a gel network. Taking advantage of this natural settling behavior, the researchers developed a new method to create catalyst materials. By placing a piece of carbon paper at the bottom of the reaction vessel, they allow metal particles to settle directly onto the electrode surface, where they form an intact network. This approach preserves the delicate three-dimensional structure that previous methods destroyed when they used sonication to break up and redeposit the material.
The significance of structural control becomes clear when considering how these materials function in fuel cells. Fuel cells generate electricity by breaking down fuels like methanol or ethanol on a catalyst surface. Each catalyst strand must provide sites where fuel molecules can react, conduct the released electrons to an external circuit, and maintain open channels for fresh fuel to reach reaction sites while allowing products to escape.
The importance of maintaining this network structure becomes clear in the performance results. Under standardized testing conditions, a gold-platinum aerogel created using this method achieved record-setting efficiencies for methanol and ethanol oxidation reactions. It converted methanol to electricity with 11.57 amperes per milligram of platinum—21.8 times better than conventional platinum catalysts. For ethanol conversion, the efficiency reached 14.07 amperes per milligram of platinum, a 17-fold improvement.
These dramatic improvements stem from three factors: more accessible reaction sites in the fine network structure, better electrical connections to the electrode, and enhanced interaction between the gold and platinum atoms that makes each site more reactive. The direct formation method ensures these benefits are preserved rather than being compromised by the traditional process of breaking up and rebuilding the network.
The current synthesis process requires approximately 24 hours to produce just milligrams of material, highlighting the gap between laboratory demonstration and industrial needs. The process involves careful control of metal reduction, gelation, and solvent exchange steps, each of which must be optimized for larger-scale production while maintaining precise structural control.
The principles discovered in this work extend beyond metal aerogels to other gel-based systems where three-dimensional connectivity matters. The researchers’ understanding of how multiple metals interact during network formation could inform the design of other materials that require precise structural control, from energy storage materials to chemical sensors.
This research establishes a systematic framework for controlling material structure based on fundamental atomic properties. Rather than relying on trial and error, engineers can now select specific combinations of materials that will produce desired network architectures. This ability to design three-dimensional connected structures from the atomic scale up, combined with methods to preserve these structures during device fabrication, represents an important advance in our ability to create more efficient catalysts for clean energy and chemical production.
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