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Financial support by the European Union project CRONOS (grant
number 280879-2), the Deutsche Forschungsgemeinschaft
(SPP1391), the Korea Foundation for International Cooperation of
Science and Technology (Global Research Laboratory project,
K20815000003), and the Italian Fondo per gli Investimenti della
Ricerca di Base (Flashit project) is gratefully acknowledged.
C.A.R. and E.M. acknowledge the Partnership for Advanced
Computing in Europe (project LAIT) for awarding us access to
supercomputing resources at CINECA, Italy, and useful discussions
with C. Cocchi and Y. Kanai. S.M.F. is grateful for a Ph.D.
fellowship from Stiftung der Metallindustrie im Nord-Westen.
C.L. and G.C. acknowledge support from the European Community
(Seventh Framework Programme INFRASTRUCTURES-2008-1,
Laserlab Europe II contract no. 228334); A.R. acknowledges
financial support from the European Research Council
(ERC-2010-AdG-267374), Spanish grant (FIS2010-21282-C02-01),
Grupos Consolidados (IT578-13), Ikerbasque. G.C. acknowledges
financial support by the European Research Council (ERC-2011-AdG
no. 291198). C.L., S.M.F., G.C., C.A.R., and E.M. initiated this
work. S.M.F. prepared the samples. S.M.F., D.B., M.M., E.S.,
and A.D.S. performed the ultrafast spectroscopy experiments.
C.A.R. and M.A. performed DFT and TDDFT simulations. S.M.F.,
A.D.S., D.B., G.C., and C.L. analyzed and discussed the experimental
data. C.A.R., M.A., E.M., and A.R. analyzed and discussed the
theoretical data. C.A.R., E.M., G.C., A.D.S., and C.L. designed the
paper. All authors discussed the implications and contributed
to the writing of the paper. The authors declare no competing
Materials and Methods
Figs. S1 to S10
Movies S1 and S2
16 December 2013; accepted 7 May 2014
Amorphous TiO2 coatings stabilize
Si, GaAs, and GaP photoanodes for
efficient water oxidation
Shu Hu,1,2 Matthew R. Shaner,1,2 Joseph A. Beardslee,1 Michael Lichterman,1,2
Bruce S. Brunschwig,3 Nathan S. Lewis1,2,3,4*
Although semiconductors such as silicon (Si), gallium arsenide (GaAs), and gallium
phosphide (GaP) have band gaps that make them efficient photoanodes for solar fuel
production, these materials are unstable in aqueous media. We show that TiO2 coatings
(4 to 143 nanometers thick) grown by atomic layer deposition prevent corrosion, have
electronic defects that promote hole conduction, and are sufficiently transparent to
reach the light-limited performance of protected semiconductors. In conjunction with a
thin layer or islands of Ni oxide electrocatalysts, Si photoanodes exhibited continuous
oxidation of 1.0 molar aqueous KOH to O2 for more than 100 hours at photocurrent
densities of >30 milliamperes per square centimeter and ~100% Faradaic efficiency.
TiO2-coated GaAs and GaP photoelectrodes exhibited photovoltages of 0.81 and 0.59 V
and light-limiting photocurrent densities of 14.3 and 3.4 milliamperes per square
centimeter, respectively, for water oxidation.
The oxidation of water to O2 is a key process in the direct photoelectrochemical (PEC) production of fuels from sunlight (1, 2). A fuel-forming reductive half-reaction in- volving the reduction of CO2 to lower
hydrocarbons or the reduction of H2O to H2
requires an oxidative half-reaction, such as the
oxidation of water to O2. Metal oxide photo-
anodes can oxidize water to O2 in alkaline or
acidic media, but thus far have been inefficient
because their band gaps are too large and be-
cause the potential of their valence band edge is
much more positive than the formal potential
for water oxidation, Eo′(O2/H2O) (3). Although
many semiconductors, including silicon (Si), gal-
lium arsenide (GaAs), and gallium phosphide
(GaP), have valence-band edges at more negative
potentials than metal oxides and also typical-
ly have optimal band gaps for efficient solar-
driven water splitting, these semiconductors
are unstable when operated under photoanodic
conditions in aqueous electrolytes. Specifically,
in competition with oxidizing water to O2, these
materials either anodically photocorrode or pho-
topassivate (3, 4). Furthermore, passive and in-
trinsically safe solar-driven water-splitting systems
can only be constructed (5) in either alkaline or
acidic media, and the development of general
strategies to stabilize existing photoelectrode
materials under water-oxidation conditions is
an important goal.
Various coating strategies have been explored
to stabilize semiconductors with optimal band
gaps (1.1 to 1.7 eV) for direct water splitting (6).
Deposition of thin films of Pd (7), Pt (8), Ni (9), or
metal-doped SiOx (10) onto n-type Si or n-GaAs
photoanodes yields improved stability under
water-oxidation conditions, primarily near neutral pH, as well as in strongly alkaline (9) or
acidic (10) media for up to 12 hours. However,
these stabilized photoanodes generally exhibit low photovoltages; additionally, the protective metal coating is either too thick to be highly
optically transmissive or too thin to afford extended stability during water oxidation, particularly in alkaline or acidic media. Transparent
conductive oxide (TCO) coatings on Si and GaAs
are not stable in strongly alkaline or acidic media, and also produce low voltages because of
defective semiconductor/TCO interfaces. (11, 12)
Coatings of Ni islands (9), as well as MnOx (13)
and NiOx (14) films on Si, have been used to catalyze the oxidation of water and thus provide
some degree of stability enhancement. However,
such coatings do not enable prolonged operation
in alkaline media and/or yield very low photovoltages because of a large density of interface
states at the Si/Ni interface.
Conformal layers of 2-nm thin TiO2 formed
by atomic layer deposition (ALD) have been
used to stabilize Si, and in conjunction with an
IrOx catalyst, to effect water oxidation (15). Deposition of such electrically insulating films at
larger thickness, to reliably prevent pinholes
and thus suppress active corrosion over macroscopic areas, creates a tunneling barrier for
photogenerated holes. As this barrier becomes
thicker, it no longer conducts holes via a tunneling mechanism and also introduces a large
series resistance to a PEC device, degrading its
efficiency to low values. Specifically, as the TiO2
thickness was increased, the overpotential for
the oxygen-evolution reaction (OER) increased
linearly at a rate of ~21 mV nm−1 (16), resulting
in an additional voltage loss of ~200 mV for a
12-nm thick TiO2 overlayer, even at a current
density of 1 mA cm−2.
We describe a general approach to significantly improve the stability of Si, GaAs, and
GaP photoanodes against both photocorrosion
and photopassivation for water oxidation in
alkaline media [all results reported below are
1Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, CA 91125, USA. 2Joint
Center for Artificial Photosynthesis, California Institute of
Technology, Pasadena, CA 91125, USA. 3Beckman Institute
and Molecular Materials Research Center, California Institute
of Technology, Pasadena, CA 91125, USA. 4Kavli Nanoscience
Institute, California Institute of Technology, Pasadena, CA
*Corresponding author. E-mail: email@example.com