Butterfly wings may rank among the most delicate structures in nature, but
they have given researchers powerful inspiration for developing better solar
technology. The researchers presented their findings at the American Chemical
Society's (ACS') 243rd National Meeting & Exposition.
Tongxiang Fan, Ph.D., who reported on the use of two swallowtail
butterflies — Troides aeacus (Heng-chun birdwing butterfly) and Papilio helenus
Linnaeus (Red Helen) — as models, explained that finding renewable sources of
energy is one of the great global challenges of the 21st century. One promising
technology involves producing clean-burning hydrogen fuel from sunlight and
water. It can be done in devices that use sunlight to kick up the activity of
catalysts that split water into its components, hydrogen and oxygen. Better
solar collectors are the key to making the technology practical, and Fan's team
turned to butterfly wings in their search for making solar collectors that
gather more useful light.
"We realized that the solution to this problem may have been in
existence for millions of years, fluttering right in front of our eyes," Fan
said. "And that was correct. Black butterfly wings turned out to be a natural
solar collector worth studying and mimicking," Fan said.
Source: American Chemical
Society
Scientists long have known that butterfly wings contain tiny scales that
serve as natural solar collectors to enable butterflies, which cannot generate
enough heat from their own metabolism, to remain active in the cold. When
butterflies spread their wings and bask in the sun, those solar collectors are
soaking up sunlight and warming the butterfly's body.
Fan's team at Shanghai Jiao Tong University in China used an electron
microscope to reveal the most-minute details of the scale architecture on the
wings of black butterflies — black being the color that absorbs the maximum
amount of sunlight. "We were searching the 'art of blackness' for the secret
of how those black wings absorb so much sunlight and reflect so little," Fan
explained.
Scientists initially thought it was simply a matter of the deep inky black
color, due to the pigment called melanin, which also occurs in human skin. More
recently, however, evidence began to emerge indicating that the structure of the
scales on the wings should not be ignored. Fan's team observed elongated
rectangular scales arranged like overlapping shingles on the roof of a house.
The butterflies they examined had slightly different scales, but both had ridges
running the length of the scale with very small holes on either side that opened
up onto an underlying layer.
The steep walls of the ridges help funnel light into the holes, Fan
explained. The walls absorb longer wavelengths of light while allowing shorter
wavelengths to reach a membrane below the scales. Using the images of the
scales, the researchers created computer models to confirm this filtering
effect. The nano-hole arrays change from wave guides for short wavelengths to
barriers and absorbers for longer wavelengths, which act just like a high-pass
filtering layer.
The group used actual butterfly-wing structures to collect sunlight,
employing them as templates to synthesize solar-collecting materials. They chose
the black wings of the Asian butterfly Papilio helenus Linnaeus, or Red Helen,
and transformed them to titanium dioxide by a process known as dip-calcining.
Titanium dioxide is used as a catalyst to split water molecules into hydrogen
and oxygen. Fan's group paired this butterfly-wing patterned titanium dioxide
with platinum nanoparticles to increase its water-splitting power. The
butterfly-wing compound catalyst produced hydrogen gas from water at more than
twice the rate of the unstructured compound catalyst on its own.
"These results demonstrate a new strategy for mimicking Mother Nature's
elaborate creations in making materials for renewable energy. The concept of
learning from nature could be extended broadly, and thus give a broad scope of
building technologically unrealized hierarchical architecture and design
blueprints to exploit solar energy for sustainable energy resources," he
concluded.
Source: American Chemical Society
Top image: Butterfly Club
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