New dealloying method produces sponge-like metal materials

Scientists have developed a new approach for making metal-metal composites and porous metals with a 3D, interconnected ‘bicontinuous’ structure in thin films at size scales ranging from tens of nanometers to micrometers. Metallic materials with this sponge-like morphology – characterized by two co-existing phases that form interpenetrating networks continuing over space – could be useful in catalysis, biomedical sensing, and energy generation and storage.

Called thin-film solid-state interfacial dealloying (SSID), the novel approach uses heat to drive a self-organizing process in which metals mix or de-mix to form a new structure. The scientists used multiple electron- and X-ray-based techniques (‘multimodal analysis’) to visualize and characterize the formation of the bicontinuous structure.

"Heating gives the metals some energy so that they can interdiffuse and form a self-supported thermodynamically stable structure," explained Karen Chen-Wiegart, an assistant professor in Stony Brook University's (SBU) Materials Science and Chemical Engineering Department and a scientist at Brookhaven National Laboratory’s National Synchrotron Light Source II (NSLS-II). "SSID has been previously demonstrated in bulk samples (tens of microns and thicker) but results in a size gradient, with a larger structure on one side of the sample and a smaller structure on the other side. Here, for the first time, we successfully demonstrated SSID in a fully integrated thin-film processing, resulting in a homogenous size distribution across the sample. This homogeneity is needed to create functional nanostructures."

Chen-Wiegart is the corresponding author of a paper on this work in Materials Horizons. The other authors come from Brookhaven Lab’s Center for Functional Nanomaterials (CFN) and the US National Institute of Standards and Technology (NIST).

To demonstrate their process, the scientists prepared thin films of a magnesium (Mg), iron (Fe) and nickel (Ni) alloy on silicon (Si) wafer substrates in the CFN Nanofabrication Facility. They heated the samples to a high temperature (860°F) for 30 minutes and then rapidly cooled them down to room temperature.

"We found that Mg diffuses into the Fe-Ni layer, where it combines only with Ni, while Fe separates from Ni," said first author Chonghang Zhao, a PhD student in Chen-Wiegart’s research group. "This phase separation is based on enthalpy, an energy measurement that determines whether the materials are ‘happily’ mixing or not, depending on properties such as their crystal structure and bonding configurations. The nanocomposite can be further treated to generate a nanoporous structure through chemically removing one of the phases."

Nanoporous structures have many applications, including photocatalysis. For example, these structures could be used to accelerate the reaction in which water is split into oxygen and hydrogen – a clean-burning fuel. Because catalytic reactions happen on material surfaces, the high surface area of the pores would improve reaction efficiency. In addition, because the nanosized ‘ligaments’ are inherently interconnected, they do not need any support to hold them together. These connections could provide electrically conductive pathways.

The team identified the dealloyed bicontinuous structure of Fe and Ni-Mg through complementary electron microscopy techniques at the CFN and X-ray synchrotron techniques at two NSLS-II beamlines – the Hard X-ray Nanoprobe (HXN) and the Beamline for Materials Measurement (BMM).

"Using the scanning mode in a transmission electron microscope (TEM), we rastered the electron beam over the sample in specific locations to generate 2D elemental maps showing the spatial distribution of elements," explained Kim Kisslinger, a technical associate in the CFN electron microscopy research group and the point of contact for the instrument. The team also used TEM to obtain electron diffraction patterns capturing the crystal structure and a scanning electron microscope (SEM) to study surface morphology.

This initial analysis provided evidence for the formation of a bicontinuous structure locally in 2D at high resolution. To further confirm that the bicontinuous structure was representative of the entire sample, the team turned to the HXN beamline, which can provide 3D information over a much larger region.

"With HXN, we can focus hard, or high-energy, X-rays to a very tiny spot of about 12nm," said co-author and HXN physicist Xiaojing Huang. "The world-leading spatial resolution of hard X-ray microscopy at HXN is sufficient to see the sample's smallest structures, which range in size from 20nm to 30nm. Though TEM provides higher resolution, the field of view is limited. With the X-ray microscope, we were able to observe the 3D element distributions within a bigger area so that we could confirm the homogeneity."

Measurements at HXN were conducted in a multimodality manner, with the simultaneous collection of X-ray scattering signals that reveal 3D structure and fluorescence signals that are element-sensitive. Atoms emit fluorescence when they jump back to their lowest-energy (ground) state after being excited to an unstable higher-energy state in response to the X-ray energy. By detecting this characteristic fluorescence, scientists can determine the type and relative abundance of elements present at specific locations.

Co-author and NIST Synchrotron Science Group physicist Bruce Ravel confirmed the sample's chemical composition and obtained the precise chemical forms (oxidation states) of the elements with BMM, which is funded and operated by NIST. The x-ray absorption near-edge structure (XANES) spectra also showed the presence of pure Fe.

Now the scientists have shown that SSID works in thin films, their next step is to address the ‘parasitic’ events they identified in the course of this study. For example, they discovered that Ni diffuses into the Si substrate, leading to voids, a kind of structural defect. They will also make pore structures from the metal-metal composites to demonstrate applications such as photocatalysis, and apply their approach to other metal systems, including titanium-based ones.

This story is adapted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.