Electronic microscopic image of a nanoforest, or 3D branched nanowire array. Green tint added for contrast. Image: Wang Research Group, UC San Diego Jacobs School of Engineering
University of California, San
Diego electrical engineers are building a forest of
tiny nanowire trees in order to cleanly capture solar energy without using
fossil fuels and harvest it for hydrogen fuel generation. Reporting in Nanoscale, the team said nanowires,
which are made from abundant natural materials like silicon and zinc oxide,
also offer a cheap way to deliver hydrogen fuel on a mass scale.
“This is a clean way to generate clean fuel,” said Deli
Wang, professor in the Department of Electrical and Computer Engineering at the
UC San Diego Jacobs School of Engineering.
The trees’ vertical structure and branches are keys to
capturing the maximum amount of solar energy, according to Wang. That’s because
the vertical structure of trees grabs and adsorbs light while flat surfaces
simply reflect it, Wang said, adding that it is also similar to retinal
photoreceptor cells in the human eye. In images of Earth from space, light
reflects off of flat surfaces such as the ocean or deserts, while forests
Wang’s team has mimicked this structure in their 3D branched
nanowire array which 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. This process uses clean energy with no greenhouse gas byproduct. By
comparison, the current conventional way of producing hydrogen relies on
electricity from fossil fuels.
“Hydrogen is considered to be clean fuel compared to fossil
fuel because there is no carbon emission, but the hydrogen currently used is
not generated cleanly,” said Ke Sun, a PhD student in electrical engineering
who led the project.
By harvesting more sun light using the vertical nanotree
structure, Wang’s team has developed a way to produce more hydrogen fuel
efficiently compared to planar counterparts. Wang is also affiliated with the
California Institute of Telecommunications and Information Technology and the
Material Science and Engineering Program at UC San Diego.
The vertical branch structure also maximizes hydrogen gas
output, said 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, very small gas bubbles of hydrogen can be extracted much faster. “Moreover, with this structure, we have enhanced, by at least 400,000 times,
the surface area for chemical reactions,” said Sun.
In the long run, what Wang’s team is aiming for is even
bigger: artificial photosynthesis. In photosynthesis, as plants absorb sunlight
they also collect carbon dioxide and water from the atmosphere to create
carbohydrates to fuel their own growth. Wang’s team hopes to mimic this process
to also capture carbon dioxide from the atmosphere, reducing carbon emissions,
and convert it into hydrocarbon fuel.
“We are trying to mimic what the plant does to convert
sunlight to energy,” said Sun. “We are hoping in the near future our ‘nanotree’
structure can eventually be part of an efficient device that functions like a
real tree for photosynthesis.”
The team is also studying alternatives to zinc oxide, which
absorbs the sun’s ultraviolet light, but has stability issues that affect the
lifetime usage of the nanotree structure.