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Butterfly
wings may rank among the most delicate structures in nature, but they
have given researchers powerful inspiration for new technology that
doubles production of hydrogen gas—a green fuel of the future—from water
and sunlight. The researchers presented their findings here today 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.
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.
The
scientists acknowledged funding from National Natural Science
Foundation of China (No.51172141 and 50972090), Shanghai Rising-star
Program (No.10QH1401300).
Article Abstract
We
probe into the art of blackness in butterfly wings by measuring and
modeling the antireflection behavior of the black butterfly wing scales.
The results demonstrate Mother Nature’s excellent talent in devising
creations with elaborate architectures and great light harvesting
performance. Then as a typical prototype, artificial butterfly wing
architecture TiO2 (ABWA-TiO2) has been produced using the original
butterfly wings as templates and enhanced photocatalytic efficiency has
been achieved. The hierarchical architecture borrowed from butterfly
wing template works on several levels to enhance catalytic activities of
Pt loaded TiO2 and enhance the hydrogen evolution rate by 2.3
times.This is due to advantages brought about by the hierarchical
antireflection architecture.
Source: American Chemical Society
