Bidding - the International Association of the Catalysis Societies

C&EN Volume 77, Number 26, p. 10-11 (June 28, 1999)
Photocatalysts split water in high yield
The prospect of using solar energy to convert water into hydrogen and oxygen has received a boost with
the discovery of a new class of semiconducting photocatalysts that are highly efficient at splitting water
under ultraviolet irradiation.
The new photocatalysts were prepared from materials known as layered perovskites by chemical
engineering professor Jae S. Lee and coworkers at Pohang University of Science & Technology, South
Korea [Chem. Commun.,1999, 1077].
The group showed that by impregnating the layered perovskite powders with nickel, quantum yields of
up to 23% can be achieved. The quantum yield is the percentage of photons used to generate products in a
photochemical reaction. The previous best quantum yield for UV-photocatalytic water splitting, 5%, was
achieved by chemistry professor Kazunari Domen and his group at Tokyo Institute of Technology using
other types of layered perovskites.
"Our work has achieved not only unprecedented high quantum yields but also demonstrates a strong
relationship between structure and performance of the semiconductor photocatalysts," Lee tells C&EN.
The perovskite materials prepared by the Korean group have the generic composition A m B m O 3m+2 , where
A is Ca, Sr, or La; B is Nb or Ti; and m is 4 or 5. The group showed that the highest rates of water
splitting occur when the materials are loaded with 1.0% by weight of nickel.
"The discovery that this class of compounds makes good UV-photocatalysts is important," comments
Thomas E. Mallouk, chemistry professor at Pennsylvania State University, University Park. "To go from
5% to 23% quantum efficiency for overall water splitting is a big step. However, the process requires UV
light. To be useful, a catalyst would need to work with visible light."
In general, photocatalytic water splitting over a semiconductor occurs when the catalyst is illuminated
with light of sufficient energy to generate excited electron-hole pairs that can be separated. It is not yet
clear, Mallouk points out, how these new materials work or what their microstructures are.
"If we can understand the mechanism of photochemical charge separation in this new class of compounds,
then it may help us to improve on existing materials which operate, albeit inefficiently, with visible light,"
Mallouk says.
Lee's group is already looking at ways to modify these materials for use with visible light. "We are
working on new preparation methods to increase their surface areas," he says. "With high surface areas,
such materials might find applications not only in photocatalytic water splitting but also in the
photodestruction of toxic materials and in the reduction of carbon dioxide."
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