A subzero ice recrystallization route creates multimetal high entropy aerogels and coatings with uniform element mixing, offering a low temperature path to porous macroscopic alloy structures.
(Nanowerk Spotlight) High entropy alloys promise metals that do not behave like ordinary alloys. Instead of relying on one dominant element, they combine several metals in nearly equal amounts. This design creates many possible atomic arrangements, and that disorder can stabilize single phase structures with unusual combinations of strength, corrosion resistance and catalytic activity. Despite the interest in these materials, producing extended porous networks with multimetal uniformity has remained out of reach because high-temperature alloying methods cause severe sintering that collapses any developing architecture.
Early attempts to turn this idea into practical materials depended on intense heat. Some methods used rapid heating and cooling to force metals to mix, but such conditions required complex equipment and often produced particles that grew too large or separated into different phases. Other methods tried to use simple solution chemistry at low temperatures, yet they faced a timing problem. The different metal salts in solution could not mix quickly enough before atoms formed and clumped together. This mismatch produced uneven particles instead of the uniform mixtures that high entropy alloys require and prevented the creation of stable macroscopic assemblies.
At the same time, progress in ice-based processing reshaped how scientists think about structure formation. Researchers who studied aerogels, ceramics and polymers found that ice can guide matter with surprising precision. When water freezes, a very thin layer of liquid water can remain on the surface of ice, even at temperatures below 0 °C. These thin films, called premelted layers, create narrow channels where molecules can move in a controlled way.
Freeze casting and related techniques used this effect to shape porous materials with fine structural detail. Metal aerogels, which are highly porous networks of nanoscale metals, became one of the beneficiaries of this approach. These advances hinted that ice could control not only structure but also chemical reactions if used carefully.
The method produces high entropy nanoparticles, metal aerogels and ultrathin coatings that contain up to eleven elements.
HAADF-STEM image and STEM-EDX maps of the high-entropy aerogel made from five metals. The microscope image shows the porous network of nanoscale ligaments, and the accompanying color maps indicate that gold, silver, platinum, copper and indium are evenly distributed throughout the structure. (Image: Courtesy of the researchers)
The process begins with two simple solutions. One contains a blend of metal salts. The other contains sodium borohydride, a strong reducing agent. Each solution is rapidly frozen by dropping it onto a surface held at 77 K. Water freezes so quickly under these conditions that it forms glassy ice, which has no crystal structure. The two solid layers are then placed together. At this stage no reaction occurs because the reactants remain locked inside the solid ice.
The chemistry starts when the stacked layers warm to a chosen temperature below 0 °C. As glassy ice warms, it reorganizes into many small ice crystals. Thin liquid films form between the crystals. These films are the premelted channels that allow metal ions and borohydride molecules to move. Movement is slow, which is essential. The reducing agent meets only small amounts of metal ions at any moment. This limited and gradual contact produces atoms in small batches instead of large bursts. That timing prevents the large swings in composition that undermine typical low temperature syntheses of multimetal particles.
Inside the premelted channels, the newly formed atoms cluster into tiny mixed metal seeds. As the ice crystals grow, more ions are pushed into the channels. This keeps the reaction confined to narrow regions where mixing remains steady. Microscopy shows that the resulting nanoparticles contain the different metals in nearly uniform proportions across each particle. This level of uniform mixing is central to the identity of a high entropy alloy. In this method the nanoparticles function primarily as intermediates; their uniform composition is preserved as they merge into the larger porous networks.
The method also influences how the particles assemble. The small seeds attach to the surfaces of the growing ice crystals. They slow the growth of specific crystal faces in a way similar to antifreeze proteins. This behavior changes the shape of the ice crystals and creates stable interfaces where the nanoparticles collect.
Simulations support this explanation. When an ice front moves through a mixture that contains water and metal atoms, the atoms concentrate in the premelted films. Within these confined layers, they form clusters with high mixing entropy. The calculated values approach those expected for ideal high entropy systems. These clusters then gather on the ice surfaces and form dense, flat layers.
As more particles reach the interface, they merge into continuous networks. The cold environment limits atomic motion inside each particle. This restricted motion prevents the metals from separating into different phases. The uniform distribution created during nucleation remains locked in place. When the ice melts and the material is freeze dried, the assembled network becomes a metal aerogel that maintains both its porosity and multimetal uniformity because no high-temperature sintering occurs.
By adjusting the recrystallization temperature and duration, the researchers produce aerogels with five, seven, nine and eleven metal elements. One example contains Pt, Au, Ag, Cu, In, Pd, Rh, Ru, Co, Bi and Ni. It has a density of 61.8 mg cm^−3 and ligament widths near 4.3 nm. The structure shows a single-phase face centered cubic lattice with even elemental distributions.
Simulations provide additional support for this even mixing. They evaluate the Warren–Cowley parameter, which measures whether certain pairs of atoms prefer to group together or stay apart. Values close to zero indicate random mixing. The calculated values for all metal pairs remain near zero, which aligns with the definition of a well-mixed high entropy alloy.
The approach also creates high entropy coatings on existing metal aerogels. A gold aerogel made by the same ice-based process can act as a scaffold. After freezing it, the researchers place a frozen layer containing ions for a ten-element alloy on its surface. Controlled warming opens premelted channels once more. The new metals prefer to nucleate on the gold surface because that path needs less energy than forming separate particles. The product is an Au@PtAgCuInPdRhRuCoBiNi aerogel with a shell roughly 1 nm thick. Similar coatings form on silver, copper and alloy aerogels. These structures use less high entropy material yet place it at locations where catalytic reactions occur.
This study shows that controlled freezing can manage how metals mix and assemble at every stage, from atoms to macroscopic structures. By using ice to slow reaction rates and guide particle attachment, the method avoids the high temperatures that once seemed necessary for high entropy alloy synthesis.
With this study, the researchers demonstrate a route that could extend to other multicomponent systems, including oxides, sulfides and layered hydroxides. It broadens the set of tools available for building complex multimetal structures with tightly controlled composition.
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