A nanoscale look at a photocatalyst that is both durable and very efficient. This high-resolution transmission electron microscope image of a titanium dioxide nanocrystal after hydrogenation reveals engineered disorder on the crystal’s surface, a change that enables the photocatalyst to absorb infrared light.
A little disorder goes a long way, especially when it comes
to harnessing the sun’s energy. Scientists from the U.S. Department of Energy’s
Lawrence Berkeley National Laboratory (Berkeley Lab) jumbled the atomic
structure of the surface layer of titanium dioxide nanocrystals, creating a
catalyst that is both long lasting and more efficient than most materials in
using the sun’s energy to extract hydrogen from water.
Their photocatalyst, which accelerates light-driven chemical
reactions, is the first to combine durability and record-breaking efficiency,
making it a contender for use in several clean-energy technologies.
It could offer a pollution-free way to produce hydrogen for
use as an energy carrier in fuel cells. Fuel cells have been eyed as an
alternative to combustion engines in vehicles. Molecular hydrogen, however,
exists naturally on Earth only in very low concentrations. It must be extracted
from feedstocks such as natural gas or water, an energy-intensive process that
is one of the barriers to the widespread implementation of the technology.
“We are trying to find better ways to generate hydrogen from
water using sunshine,” says Samuel Mao, a scientist in Berkeley Lab’s
Environmental Energy Technologies Division who led the research. “In this work,
we introduced disorder in titanium dioxide nanocrystals, which greatly improves
its light absorption ability and efficiency in producing hydrogen from water.”
Mao is the corresponding author of a paper on this research
that was published online in Science Express with the title
“Increasing Solar Absorption for Photocatalysis with Black, Hydrogenated Titanium
Dioxide Nanocrystals.” Co-authoring the paper with Mao are fellow Berkeley Lab
researchers Xiaobo Chen, Lei Liu, and Peter Yu.
Mao and his research group started with nanocrystals of
titanium dioxide, which is a semiconductor material that is used as a
photocatalyst to accelerate chemical reactions, such as harnessing energy from
the sun to supply electrons that split water into oxygen and hydrogen. Although
durable, titanium dioxide isn’t a very efficient photocatlayst. Scientists have
worked to increase its efficiency by adding impurities and making other
The Berkeley Lab scientists tried a new approach. In
addition to adding impurities, they engineered disorder into the ordinarily
perfect atom-by-atom lattice structure of the surface layer of titanium dioxide
nanocrystals. This disorder was introduced via hydrogenation.
Berkeley Lab scientist Samuel Mao leads a research team that is searching for sustainable ways to generate hydrogen for use in clean-energy technologies. In a first-of-its-kind development, they jumbled the surface layer of titanium dioxide nanocrystals, a feat that turned the material from white to black. It also created a photocatalyst whose efficiency outpaces others in using the sun’s energy to extract hydrogen from water. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
The result is the first disorder-engineered nanocrystal. One
transformation was obvious: the usually white titanium dioxide nanocrystals
turned black, a sign that engineered disorder yielded infrared absorption.
The scientists also surmised disorder boosted the
photocatalyst’s performance. To find out if their hunch was correct, they
immersed disorder-engineered nanocrystals in water and exposed them to
simulated sunlight. They found that 24% of the sunlight absorbed by the
photocatalyst was converted into hydrogen when using a sacrificial reagent, a
production rate that is about 100 times greater than the yields of most
semiconductor photocatalysts under the same conditions. More work needs to be
done in order to reach comparable efficiency without the use of a sacrificial
reagent, according to Mao.
In addition, their photocatalyst did not show any signs of
degradation during a 22-day testing period, meaning it is potentially durable
enough for real-world use.
Its landmark efficiency stems largely from the
photocatalyst’s ability to absorb infrared light, making it the first titanium
dioxide photocatalyst to absorb light in this wavelength. It also absorbs
visible and ultraviolet light. In contrast, most titanium dioxide
photocatalysts only absorbs ultraviolet light, and those containing defects may
absorb visible light. Ultraviolet light accounts for less than ten percent of
“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, who is also
an adjunct engineering professor in the Univ.
of California at Berkeley.
The team’s intriguing experimental findings were further
elucidated by theoretical physicists Peter Yu and Lei Liu, who explored how
jumbling the latticework of atoms on the nanocrystal’s surface via
hydrogenation changes its electronic properties. Their calculations revealed
that disorder, in the form of lattice defects and hydrogen, makes it possible
for incoming photons to excite electrons, which then jump across a gap where no
electron states can exist. Once across this gap, the electrons are free to
energize the chemical reaction that splits water into hydrogen and oxygen.
“By introducing a specific kind of disorder, mid-gap
electronic states are created accompanied by a reduced band gap,” says Yu, who
is also a professor in the University
of California at Berkeley’s Physics Department. “This makes it
possible for the infrared part of the solar spectrum to be absorbed and
contribute to the photocatalysis.”