#nanocrystals

Understanding the forces that regulate crystallization by particle attachment

A complex interplay of energetics and dynamics governs the behavior of nanocrystals in solution. These dynamics are usually interpreted in terms of the theory developed by Derjaguin, Landau, Verwey, and Overbeek (DVLO), and understanding these forces is particularly important for controlling oriented attachment (OA), where individual nanocrystals fuse together in specific alignments. In a new study published in ACS Nano, researchers explored the effects of forces not accounted for in DLVO theory on a zinc oxide (ZnO) model system undergoing OA. They found that the driving forces behind the attachment are dipole–dipole forces that are not considered in DLVO theory. The dipole forces lead to faster attachment in less polar solutions, validated by calculations that account for non-DLVO forces. The researchers also showed that the short range, repulsive forces that slow attachment depend on the nature of the solvent, particularly its molecular packing and intermolecular interactions.

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.

If you’re heading to the beach on a hot summer day, you don’t want to forget the cooler full of drinks. You might load that cooler with ice. However, ice on its own won’t keep things cold for long. That’s why a cooler packs insulation in its walls. The best insulators have long been plastic-based foams, such as Styrofoam. But a new type of foam made from wood pulp works even better. And it’s friendlier to the environment.

Plastic foam is both incredibly useful and popular. Filled with millions of tiny air pockets, its frothy structure is both lightweight and strong. This material protects fragile packages during shipping. And when used as an insulator, plastic foam’s tiny bubbles help keep heat in — or out — for hours. That’s why people have relied on it for everything from cups and coolers to packaging and home insulation.

Scientists Say: Microplastic

These foams have a few drawbacks, however. They are made from petroleum, a non-renewable material. And when someone has finished using these foam-based products, they’re difficult to recycle. Plastic foam doesn’t biodegrade (break down naturally). Instead, it tends to break apart into tiny little beads that can scatter on the wind, spreading pollution near and far.

That’s why researchers have been trying to find an alternative. They want something that’s “green” — better for the environment, both in how it’s made and how we get rid of it. Xiao Zhang and Amir Ameli are materials scientists who work at Washington State University in Richland. And they think the answer may lie in trees.

Turning to trees

Cellulose is the very sturdy material that makes up a plant’s cell walls. Other scientists had found ways to make foam from the cellulose in trees. They liked that the starting ingredient not only is a renewable resource, but also can break down completely in the environment.

However, straight from the plant, cellulose isn’t foamable. Earlier researchers had found they needed to first dissolve wood pulp with acid and then filter it. What remained would be tiny crystals of cellulose. They are so small that you need 500 of the crystals, side-by-side, to match the width of a human hair. These nanocrystals are the reason tree trunks are so strong.

To turn them into a foam, researchers dissolved the nanocrystals in harsh solvents. (Solvents are liquids that dissolve other substances.) Then they froze it to create a foam and dried it out to remove the solvent. But the resulting material was weaker than traditional plastic foams and didn’t insulate as well as Styrofoam. The cellulose products also broke down in hot and humid conditions.

Zhang and Ameli wanted to make something that not only worked better than plastic foams but also was friendlier to the environment. For instance, Ameli explains, “We wanted to avoid any harmful or expensive solvents.”

“Cellulose is soluble in water,” he notes. His team then chose other ingredients that would dissolved in water. Now, when they removed water from the solution, the team hoped to end up with a strong cellulose foam.

Explainer: What are polymers?

The researchers prepared different recipes. Some were just a mix of cellulose nanocrystals and water. For stretchiness, other recipes contained a polymer known as polyvinyl alcohol. Still others used all of those ingredients, plus an acid. That acid helped the molecules of cellulose and polyvinyl alcohol bond. Those bonds “hold the nanoscale ingredients together and make the material stronger,” Ameli explains. Stronger materials create smaller bubbles inside the foam, he adds, which should produce a higher-quality foam.

3 Feb 2018 // 2:10 am

Today was too long of a day for me. Woke up at 6 am - took a test, did some cardio, ran errands, and went out with friends. I’m beat and doing nothing this weekend as a treat-yo-self. 

 Credit: Zhang Xiaopu. Perspective view of the STM topography of nanocrystalline copper film, which shows a valley with dissociated dislocations and a ridge with recombined dislocations. The size of the image is 50nm by 50nm.

What’s new?

Until now, grain boundaries in nanocrystalline copper films have been thought to be perpendicular to the material’s surface. But new research shows that these grains are often rotated, forming ridges that cause surface roughness.

Who is involved?

This discovery is detailed in Nanocrystalline copper films are never flat by a team of researchers from AMBER, the Science Foundation Ireland-funded materials science centre based in Trinity College Dublin, Ireland, the University of Pennsylvania, USA, Imperial College London, UK, and the Intel Corporation Components Research Group, USA.

What did the research involve?

The researchers used scanning tunnelling microscopy to study the grain boundaries of copper, finding that the presence of these boundaries creates a misaligned surface, with ridges and valleys formed by grain rotation. The rotation is created by reduced grain boundary energy.

How is it novel?

While the team focused on copper films, Professor John Boland, Investigator at AMBER and corresponding author of the paper, claims the findings will also apply to silver, gold and potentially nickel. As nanocrystalline copper and other nanocrystalline metals are used in integrated circuits as electrical contacts and interconnects, this improved understanding of grain structure can aid the development of more efficient circuits and devices that last longer.  

Nanocrystals set new hydrogen production activity record under visible and near-infrared irradiation

The sunlight received by Earth is a mixed bag of wavelengths ranging from ultraviolet to visible to infrared. Each wavelength carries inherent energy that, if effectively harnessed, holds great potential to facilitate solar hydrogen production and diminish reliance on non-renewable energy sources. Nonetheless, existing solar hydrogen production technologies face limitations in absorbing light across this broad spectrum, particularly failing to harness the potential of near infrared (NIR) light energy that reaches Earth. Recent research has identified that both Au and Cu7S4 nanostructures exhibit a distinctive optical characteristic known as localized surface plasmon resonance (LSPR).