American Ceramic Society Bulletin

ceramics in energy
Disordered TiO 2 nanocrystal surface yields durable, more efficient photocatalyst
By tinkering with the outer layer of
titanium oxide nanocrystals, researchers
at Lawrence Berkeley National
Lab have figured out a way to turn the
material into a tough and more effective
photocatalyst for environmental
and energy applications. They claim
this is the first time durability and efficiency
have been combined in a photocatalyst.
Samuel Mao, an investigator with
the Advanced Energy Technologies
Department of the Lab’s Environmental
Energy Technologies Division, says
they were trying to improve hydrogen
production from organic materials in
water when they had the idea to introduce
disorder in nanophase TiO 2 and
hopefully expand its light-absorption
ability.
The group’s work, “Increasing solar
absorption for photocatalysis with
black, hydrogenated titanium dioxide
nanocrystals,” is published in Science
Express (doi:10.1126/science.1200448),
and may offer a path for generating
hydrogen from organic compounds
found in natural and polluted water
sources.
Mao leads a research team that is
searching for sustainable ways to generate
hydrogen for use in clean-energy
TEM image of a Ti0 2 nanocrystal after
hydrogenation reveals engineered disorder
on the crystal’s surface, a change
that enables the photocatalyst to absorb
infrared light.
(Credit S. Mao, et al; Science Express.)
Mao shows how the disorder-engineered titanium dioxide nanocrystals turned the
material from white to black.
technologies. In a first-of-its-kind
development, the team jumbled the
surface layer of TiO 2 nanocrystals,
a feat that turned the material from
white to black.
Mao’s group used hydrogenation
to engineer disorder in the TiO 2 . The
researchers had a hint the nanocrystals
might be effective over a wider
spectrum of light when they saw that
the material had turned from white to
black after hydrogenation.
After 22 days of lab test using a
full-spectrum solar light simulator
with methanol serving as a sacrificial
reagent, they report that, “We found
that one hour of solar irradiation generated
0.2 Formula 0.02 mmol of H 2
using 0.02 g of disorder-engineered
black TiO 2 nanocrystals (10 mmol·hour
–1 ·g –1 of photocatalysts). This H2 production
rate is about two orders of
magnitude greater than the yields of
most semiconductor photocatalysts.
The energy conversion efficiency for
solar hydrogen production, defined as
the ratio between the energy of solarproduced
hydrogen and the energy of
the incident sunlight, reached 24% for
disorder-engineered black TiO 2 nanocrystals,”
which they attribute to the
nanocrystals new ability to absorb light
from the infrared part of the spectrum.
The group also demonstrated similar
effects when they substituted phenol
and methylene blue for the methanol.
According to an LBL news release,
the group says this is the first time a
TiO 2 -based photocatalyst is able to
convert infrared, visible and ultraviolet
light. “The more energy from the sun
that can be absorbed by a photocatalyst,
the more electrons can be supplied
to a chemical reaction, which makes
black titanium dioxide a very attractive
material,” says Mao in the release.
Theoretical physicist Peter Yu
explains in the release that, “by introducing
a specific kind of disorder, midgap
electronic states are created accompanied
by a reduced band gap.” Yu,
who also is a professor in the University
of California at Berkeley’s Physics
Department, furher states, “This makes
it possible for the infrared part of the
solar spectrum to be absorbed and contribute
to the photocatalysis.”
Mao and his group say they are now
tackling how to reach similar energy
conversion levels in water containing
more commonplace organic compounds.
Visit: http://eetd.lbl.gov/aet n
18 American Ceramic Society Bulletin, Vol. 90, No. 4
(Credit S. Mao, et al; Science Express.)