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Campus Currents May 2007: Volume 4, Number 2

The Nanowire Forest

No, it’s not a new series on the Sci-Fi channel. Electrical engineers are growing forests of nanowire trees to capture solar energy, and are using it to generate hydrogen fuel from water. 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 a large scale.

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

The vertical structure of trees grabs and absorbs light more efficiently than flat surfaces. Within this vertical nanotree structure, photons bounce between single nanowires, which eventually absorb them. The nanowires’ vertical branches also maximize hydrogen gas output, says 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 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 as fuel.

—Catherine Hockmuth