Many seashells, minerals, and semiconductor nanomaterials are made up of smaller crystals, which are assembled together like the pieces of a puzzle. Now, researchers have measured the forces that cause the crystals to assemble, revealing an orchestra of competing factors that researchers might be able to control.

The work has a variety of implications in both discovery and applied science. In addition to providing insights into the formation of minerals and semiconductor nanomaterials, it might also help scientists understand soil as it expands and contracts through wetting and drying cycles. In the applied realm, researchers might use the principles to develop new materials with unique properties for energy needs.

The results, published in the Proceedings of the National Academy of Sciences in July, describe how the arrangement of the atoms in the crystals creates forces that pull them together and align them for docking. The study reveals how the attraction becomes stronger or weaker as water is heated or salt is added, both of which are common processes in the natural world.

The multinational team, led by chemists Dongsheng Li and Jaehun Chun from the Department of Energy’s Pacific Northwest National Laboratory, explored the attractive forces between two crystal particles made from mica. A flaky mineral that is commonly used in electrical insulation, this silicon-based mineral is well-studied and easy to work with because it chips off in flat pieces with nearly-perfect crystal surfaces.

Forces and faces

Crystallization often occurs through assembly of multi-faceted building blocks: some faces on these smaller crystals line up better with others, like Lego blocks do. Li and Chun have been studying a specific crystallization process called oriented attachment. Among other distinguishing characteristics, oriented attachment occurs when smaller subunits of fledgling crystals align their best matching faces before clicking together.

The process creates various nonlinear forms: nanowires with branches, lattices that look like complicated honeycombs, and tetrapods — tiny structures that look like four-armed toy jacks. The molecular forces that contribute to this self-assembly are not well understood.

Molecular forces that come into play can attract or repel the tiny crystal building blocks to or from each other. These include a variety of textbook forces such as van der Waals, hydrogen bonding, and electrostatic, among others.

To explore the forces, Li, Chun and colleagues milled flat faces on tiny slabs of mica and put them on a device that measures the attraction between two pieces. Then they measured the attraction while twisting the faces relative to each other. The experiment allowed the mica to be bathed in a liquid that includes different salts, letting them test real-world scenarios.

The difference in this work was the liquid setup. Similar experiments by other researchers have been done dry under vacuum; in this work, the liquid created conditions that better simulate how real crystals form in nature and in large industrial methods. The team performed some of these experiments at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility at PNNL.

Twist and salt

One of the first things the team found was that the attraction between two pieces of mica rose and fell as the faces twisted relative to each other, like when trying to make a sandwich out of two flat refrigerator magnets (go on, try it). In fact, the attraction rose and fell every 60 degrees, corresponding with the internal architecture of the mineral, which is almost hexagonal like a honeycomb cell.

Although other researchers more than a decade ago had predicted this cyclical attraction would happen, this is the first time scientists had measured the forces. Knowing the strength of the forces is key to manipulating crystallization in a research or industrial setting.

But other things were abuzz in the mica face-off as well. Between the two surfaces, the liquid environment housed electrically charged ions from salts, normal elements found during crystallization in nature. The water and the ions formed a somewhat stable layer between the surfaces that partly kept them separated. And as they moved toward each other, the two mica surfaces paused there, balanced between molecular attraction and repulsion by water and ions.

The team also found they could manipulate the strength of that attraction by changing the type of ions, their concentration, and the temperature. Different types of ions and their concentrations changed electrostatic repulsion between the mica surfaces. The size of the ions and how many charges they carried also created more or less space within the meddling layer.

Lastly, higher temperatures increased the strength of the attraction, contrary to how temperature behaves in simpler, less complex scenarios. The researchers built a model of the competing forces that included van der Waals, electrostatic, and hydration forces.

In the future, the researchers say, the principles gleaned from this study can be applied to other materials, which would be calculated for the material of interest. For example, manipulating the attraction might allow researchers to custom-build crystals of desired sizes and shapes and with unique properties. Overall, the work provides insights into crystal growth through nanoparticle assembly in synthetic, biological, and geochemical environments.

A powdery mix of metal nanocrystals wrapped in single-layer sheets of carbon atoms, developed at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), shows promise for safely storing hydrogen for use with fuel cells for passenger vehicles and other uses. And now, a new study provides insight into the atomic details of the crystals’ ultrathin coating and how it serves as selective shielding while enhancing their performance in hydrogen storage.

The study, led by Berkeley Lab researchers, drew upon a range of Lab expertise and capabilities to synthesize and coat the magnesium crystals, which measure only 3-4 nanometers (billionths of a meter) across; study their nanoscale chemical composition with X-rays; and develop computer simulations and supporting theories to better understand how the crystals and their carbon coating function together.

The science team’s findings could help researchers understand how similar coatings could also enhance the performance and stability of other materials that show promise for hydrogen storage applications. The research project is one of several efforts within a multi-lab R&D effort known as the Hydrogen Materials — Advanced Research Consortium (HyMARC) established as part of the Energy Materials Network by the U.S. Department of Energy’s Fuel Cell Technologies Office in the Office of Energy Efficiency and Renewable Energy.

Reduced graphene oxide (or rGO), which resembles the more famous graphene (an extended sheet of carbon, only one atom thick, arrayed in a honeycomb pattern), has nanoscale holes that permit hydrogen to pass through while keeping larger molecules at bay.

This carbon wrapping was intended to prevent the magnesium — which is used as a hydrogen storage material — from reacting with its environment, including oxygen, water vapor and carbon dioxide. Such exposures could produce a thick coating of oxidation that would prevent the incoming hydrogen from accessing the magnesium surfaces.

But the latest study suggests that an atomically thin layer of oxidation did form on the crystals during their preparation. And, even more surprisingly, this oxide layer doesn’t seem to degrade the material’s performance.

“Previously, we thought the material was very well-protected,” said Liwen Wan, a postdoctoral researcher at Berkeley Lab’s Molecular Foundry, a DOE Nanoscale Science Research Center, who served as the study’s lead author. The study was published in the Nano Letters journal. “From our detailed analysis, we saw some evidence of oxidation.”

Wan added, “Most people would suspect that the oxide layer is bad news for hydrogen storage, which it turns out may not be true in this case. Without this oxide layer, the reduced graphene oxide would have a fairly weak interaction with the magnesium, but with the oxide layer the carbon-magnesium binding seems to be stronger.

“That’s a benefit that ultimately enhances the protection provided by the carbon coating,” she noted. “There doesn’t seem to be any downside.”

David Prendergast, director of the Molecular Foundry’s Theory Facility and a participant in the study, noted that the current generation of hydrogen-fueled vehicles power their fuel cell engines using compressed hydrogen gas. “This requires bulky, heavy cylindrical tanks that limit the driving efficiency of such cars,” he said, and the nanocrystals offer one possibility for eliminating these bulky tanks by storing hydrogen within other materials.

The study also helped to show that the thin oxide layer doesn’t necessarily hinder the rate at which this material can take up hydrogen, which is important when you need to refuel quickly. This finding was also unexpected based on the conventional understanding of the blocking role oxidation typically plays in these hydrogen-storage materials.

That means the wrapped nanocrystals, in a fuel storage and supply context, would chemically absorb pumped-in hydrogen gas at a much higher density than possible in a compressed hydrogen gas fuel tank at the same pressures.

The models that Wan developed to explain the experimental data suggest that the oxidation layer that forms around the crystals is atomically thin and is stable over time, suggesting that the oxidation does not progress.

The analysis was based, in part, around experiments performed at Berkeley Lab’s Advanced Light Source (ALS), an X-ray source called a synchrotron that was earlier used to explore how the nanocrystals interact with hydrogen gas in real time.

Wan said that a key to the study was interpreting the ALS X-ray data by simulating X-ray measurements for hypothetical atomic models of the oxidized layer, and then selecting those models that best fit the data. “From that we know what the material actually looks like,” she said.

While many simulations are based around very pure materials with clean surfaces, Wan said, in this case the simulations were intended to be more representative of the real-world imperfections of the nanocrystals.

A next step, in both experiments and simulations, is to use materials that are more ideal for real-world hydrogen storage applications, Wan said, such as complex metal hydrides (hydrogen-metal compounds) that would also be wrapped in a protective sheet of graphene.

“By going to complex metal hydrides, you get intrinsically higher hydrogen storage capacity and our goal is to enable hydrogen uptake and release at reasonable temperatures and pressures,” Wan said.

Some of these complex metal hydride materials are fairly time-consuming to simulate, and the research team plans to use the supercomputers at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC) for this work.

“Now that we have a good understanding of magnesium nanocrystals, we know that we can transfer this capability to look at other materials to speed up the discovery process,” Wan said.