Visible Light Water Splitting Using Dye-
Sensitized Oxide Semiconductors
W. JUSTIN YOUNGBLOOD,†SEUNG-HYUN ANNA LEE,
KAZUHIKO MAEDA,‡AND THOMAS E. MALLOUK*
Department of Chemistry, The Pennsylvania State University, University Park,
RECEIVED ON SEPTEMBER 15, 2009
ical energy in the form of fuels. Hydrogen is a key solar fuel
because it can be used directly in combustion engines or fuel
cells, or combined catalytically with CO2to make carbon con-
taining fuels. Different approaches to solar water splitting
include semiconductor particles as photocatalysts and pho-
toelectrodes, molecular donor-acceptor systems linked to cat-
alysts for hydrogen and oxygen evolution, and photovoltaic
cells coupled directly or indirectly to electrocatalysts.
Despite several decades of research, solar hydrogen gen-
eration is efficient only in systems that use expensive pho-
tovoltaic cells to power water electrolysis. Direct photo-
catalytic water splitting is a challenging problem because the reaction is thermodynamically uphill. Light absorption results
in the formation of energetic charge-separated states in both molecular donor-acceptor systems and semiconductor par-
ticles. Unfortunately, energetically favorable charge recombination reactions tend to be much faster than the slow multi-
electron processes of water oxidation and reduction. Consequently, visible light water splitting has only recently been achieved
in semiconductor-based photocatalytic systems and remains an inefficient process.
This Account describes our approach to two problems in solar water splitting: the organization of molecules into assem-
blies that promote long-lived charge separation, and catalysis of the electrolysis reactions, in particular the four-electron
oxidation of water. The building blocks of our artificial photosynthetic systems are wide band gap semiconductor particles,
photosensitizer and electron relay molecules, and nanoparticle catalysts. We intercalate layered metal oxide semiconduc-
tors with metal nanoparticles. These intercalation compounds, when sensitized with [Ru(bpy)3]2+derivatives, catalyze the
photoproduction of hydrogen from sacrificial electron donors (EDTA2-) or non-sacrificial donors (I-). Through exfoliation of
layered metal oxide semiconductors, we construct multilayer electron donor-acceptor thin films or sensitized colloids in which
individual nanosheets mediate light-driven electron transfer reactions. When sensitizer molecules are “wired” to IrO2·nH2O
nanoparticles, a dye-sensitized TiO2electrode becomes the photoanode of a water-splitting photoelectrochemical cell.
Although this system is an interesting proof-of-concept, the performance of these cells is still poor (∼1% quantum yield)
and the dye photodegrades rapidly. We can understand the quantum efficiency and degradation in terms of competing kinetic
pathways for water oxidation, back electron transfer, and decomposition of the oxidized dye molecules. Laser flash pho-
tolysis experiments allow us to measure these competing rates and, in principle, to improve the performance of the cell
by changing the architecture of the electron transfer chain.
esearchers are intensively investigating photochemical
water splitting as a means of converting solar to chem-
Visible light water splitting is a long-standing prob-
lem in photochemistry.1,2Efficient photocatalytic
water-splitting systems could have practical value
for solar energy conversion, particularly if they
could be coupled to higher temperature catalytic
reactions for making liquid fuels.3In addition, the
lessons learned from the design and study of
ACCOUNTS OF CHEMICAL RESEARCH
1966-1973 December 2009 Vol. 42, No. 12
Published on the Web 11/11/2009
10.1021/ar9002398 CCC: $71.50
© 2009 American Chemical Society
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Visible Light Water Splitting Youngblood et al.
Vol. 42, No. 12December 2009 1966-1973
ACCOUNTS OF CHEMICAL RESEARCH