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Applying mechanical principles from the visible world to the nanorealm

​Mechanical engineer Xiaolin Zheng uses nanomaterials to split water molecules in a way that turns H20 into a battery, and employs fire to fashion nanomaterials into tiny wires.

drawing of molecules

If nanomaterials are going to impact society at a large scale, we have to find a way to make them affordably. | Illustration by Stefani Billings

Ever since our ancestors sharpened sticks and shaved flints around the campfire, humans have been tool makers.

Today, engineers apply this maker impulse to the realm of nanotechnology. In her Stanford lab, Xiaolin Zheng, associate professor of mechanical engineering, has one initiative underway to turn water into a sort of nanotech battery to store solar energy. A second research thread aims to turn flames into nanomaterial factories.

“By making the materials really, really small, we can tune their properties to perform better,” says Zheng, who offers one concrete example from her team.

Solar panels absorb sunlight and convert it into electricity. But unless you use the power immediately or have a bank of expensive batteries, the electricity is wasted.

Zheng and her team are working on a solution. They create nanomaterials that utilize the sun’s power to produce energy that is both clean and plentiful. When submerged in water, these nanomaterials would absorb sunlight. That absorbed energy would trigger an electrochemical reaction to split water molecules into oxygen and clean-burning hydrogen. Both elements can be stored for later use, getting around the need for massive batteries.

“Water splitting is promising for utilizing and storing solar energy,” Zheng says.

Or it would be, if it were easy, but solar water splitting is challenging in terms of finding the right materials and the right methods to create those materials. To solve the first problem, Zheng is seeking to fine-tune molybdenum disulfide, a well-known engine lubricant.

Smaller is stronger and stretchy

Nano-engineers understand that bulk materials have defects and imperfections that weaken their overall structure. As we shrink objects to the nanoscale, the presence of defects is greatly reduced, making nanomaterials stronger than their large-scale counterparts. A brittle plastic that snaps in your hand, for example, becomes strong and stretchy at the nanoscale.

Nanomaterials are also chemically more reactive than their macroscale equivalents. That’s because catalytic chemical reactions occur when molecules meet surfaces in an energetic state—more surface area translates to more reactions. So breaking a bulk catalyst down into billions of nanoparticles provides a vastly greater active surface area.

This is not merely a theoretical concept. Car companies have saved millions of dollars in materials costs by fragmenting large particles of precious metals down to the nanoscale to greatly improve the pollution-cleansing properties of catalytic converters.

Zheng is following that tack with molybdenum disulfide. In its bulk state, it is oily and a great engine lubricant. In its nano state, molybdenum disulfide is a catalyst for splitting water molecules due to the active edges of the nanoscale material. Until recently, it was assumed that only the edges, not the face, of molybdenum disulfide had catalytic activity. But Zheng’s team was able to tweak the molecular structure of its monolayer to make the face reactive as well. They did this by removing certain sulfur atoms from the faces to create active sites critical to catalytic activity. To fine-tune it, they stretched the monolayer, seeing when it reached its most effective state.

When submerged in water, Zheng’s modified molybdenum disulfide can successfully split water to generate hydrogen to be stored for later energy use. Zheng envisions that such a system could be integrated with solar panels or it could be used as a standalone energy generation system. But first engineers must solve the problem of custom manufacturing billions of nanomaterials.

Quest for fire

“If nanomaterials are going to impact society at a large scale, we have to find a way to make them cheaply,” says Zheng, who believes that one of humankind’s oldest technologies, flame, could hold the key to nanoscale mass production.

Reaching back into her academic past, when she studied the ancient art of combustion, Zheng and her team are developing a flame-based way to grow nanomaterials in quantities.

This involves exposing metal to a flame to create a vapor, and then creating the conditions to condense that vapor onto a prepared surface. The result is solid nanowires. The advance is the rapid growth of high-quality and long nanowires.

Though Zheng’s main focus at the moment is further developing the flame-based method and using the materials for water-splitting, she is already collaborating with many other departments to use nanomaterials to address other issues.

“People have been branching out to all kinds of applications, not only energy but electronics and medicine—anything you can imagine,” Zheng says. “It’s exciting to work on so many different things.”

Continue reading articles from
Mechanical Engineering: Big and Small
Studying macro and nano scale forces to improve transportation, materials, health care and more.

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