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A Nanowire Forest for Clean Fuel

Electron microscope image of a nanoforest, or “3D branched nanowire array.” Nanoforests suggest a way to capture solar energy and use it to generate clean hydrogen fuel.

Electrical engineers are growing forests of nanowire trees that capture solar energy and then use that clean energy to generate hydrogen fuel from water without using fossil fuels. The nanowires, which are made from abundant natural materials like silicon and zinc oxide, suggest a cheap way to generate eco-friendly hydrogen fuel at large scale.

“This is a clean way to generate clean fuel,” said electrical engineering professor Deli Wang.

The vertical structure of trees grabs and adsorbs light more efficiently than flat surfaces. Within the vertical nanotree structure, photons bounce between single nanowires, which eventually absorb them. The nanowires' vertical branches also maximize hydrogen gas output, said Ph.D. candidate Ke Sun. For example, on the flat wide surface of a pot of boiling water, bubbles must become large to come to the surface. In the nanotree structure, nano-scale bubbles of hydrogen gas can be extracted much faster. This “3D branched nanowire array” structure has enhanced the surface area for chemical reactions by 400,000 times.

The vertical nanotree structure allows photons to bounce between single nanowires, which absorb them.

The array uses a process called photoelectrochemical water-splitting to produce hydrogen gas. Water splitting refers to the process of separating water into oxygen and hydrogen in order to extract hydrogen gas to be used as fuel.

Long-term, the team's research is focused on a larger energy goal: artificial photosynthesis. In photosynthesis, as plants absorb sunlight they also collect carbon dioxide (CO2) and water from the atmosphere to create carbohydrates to fuel their own growth. The team wants to mimic this process to also capture CO2 from the atmosphere, reducing carbon emissions, and convert it into hydrocarbon fuel.

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