New method makes high-entropy oxides in a flash

In their quest for more-efficient catalysts and better battery components, scientists are exploring a new category of materials known as high-entropy oxides. These oxides, made from five or more elements in near-equal proportion, can potentially have more catalytically active sites and last longer than other materials. A group of researchers has come up with a quick, inexpensive way to synthesize new oxides for testing (ACS Nano 2025, DOI: 10.1021/acsnano.4c18277).

Several methods for making high-energy oxides already exist, but they all have drawbacks, says Jihyun Baek, a postdoctoral fellow in Xaolin Zheng’s Z-Energy Lab at Stanford University who performed the study with colleagues at Stanford and Lawrence Berkeley National Laboratory. Some methods take hours or days and require large amounts of energy. Others take mere minutes but give researchers little control over the output. And some take only fractions of a second but require equipment that costs thousands of dollars.

The photoflash method developed at Stanford, in contrast, takes only 10–100 ms and uses a Xenon flash lamp that costs about $400. The synthesis can be done on a wide variety of substrates, even printer paper, in open air on a benchtop.

To demonstrate their method, the researchers mixed equal amounts of metal salts of cobalt, nickel, iron, chromium, and manganese in ethanol. They then dipped a thin film of graphene oxide into the solution and allowed it to dry. The Xenon lamp delivered a brief, intense flash of light, which was absorbed by the graphene oxide, heating it to between 2,000 and 3,000 K. The graphene oxide transferred that heat to the metals, which formed into high-entropy oxide nanoparticles then rapidly cooled. Flashing the lamp two or three times produced smaller, more uniform nanoparticles.

To synthesize the nanoparticles on substrates other than the thin film, the researchers coated different substrates, including fluoride-tin-oxide (FTO) glass, carbon paper, and printer paper, with graphene oxide. Baek says scientists can choose their substrate based on their application. In this case, the researchers used FTO glass for its conductive properties because they wanted to test a catalyst for an oxygen evolution reaction, which requires a conductive substrate.

Other light-absorbing materials might also be used in place of graphene oxide, Baek says. In some applications, the graphene would be considered a contaminant that would have to be removed, while in others, perhaps battery anodes, its presence would not be an issue.

“It’s a neat idea,” says Stefano Curtarolo, director of the Center for Extreme Materials at Duke University. He thinks the photoflash method could be useful for making oxides that are hard to make using other procedures. He says that the high rate of heating and cooling will lead to a lot of disorder in the oxygen sublattice of the nanoparticles, which can affect the conductivity of the materials.