NanoPhotoBioSciences
Volume 1 (2013), Article ID xxxxxx, 23 pages
doi:xxxxxxxxxxx
Review Article
Toward Solar Fuel Production using
Manganese/Semiconductor Systems to Mimic Photosynthesis
Wanshu He1, Kai-Hong Zhao2, and Harvey J.M. Hou3,*
Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth, North
Dartmouth, Massachusetts, 02747, USA; 2State Key Laboratory of Agricultural Microbiology,
Huazhong Agricultural University, Wuhan 430070, P.R. China; 3Department of Physical
Sciences, Alabama State University, Montgomery, Alabama 36104, USA, *Corresponding
author, hhou@alasu.edu
1
Received xx May xxxx; Accepted xx June xxxx
Academic Editor: XXXXXXXXXXX
Copyright © 2013 Harvey J.M. Hou. This is an open access article distributed under the
Creative Commons Attribution License, which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.
Abstract
To address the current energy crisis and global warming issues, novel renewable carbonfree or carbon-neutral energy sources must be identified and generated. Natural
photosynthesis is a unique and an excellent example for design and mimic for solar energy
storage and renewable fuel production on the large scale via water splitting chemistry. The
oxygen-evolving complex of photosystem II is an oxomanganese complex that is able to
catalyze water-splitting reaction to achieve energy storage on the large scale at room
temperature and neutral pH in green plants, algae, and cyanobacteria. Great progress and
breakthroughs in illustrating the structure and mechanism of water oxidation in
photosystem II have been made using the combination of modern molecular genetics and
sophiscated biophysical techniques in the past decade. In particular, the three-dimensional
structure of photosystem II with oxygen-evolving activity has been determined at an atomic
level, which provides a complete picture with the specific position of each atom in the
Mn4CaO5 cluster and interaction between the each of atoms with its own amino acid ligand.
These progresses have significantly enhanced our understanding of the mechanisms of
water splitting in natural photosynthesis and offered a unique opportunity for transforming
solar energy into our energy system to solve the global energy crisis. Toward solar fuel
production by mimic the water oxidation of photosystem II oxygen evolving complex,
appealing Mn-containing catalytic materials were discovered. In this article, the synthesis,
structural characterization, electrochemistry, mechanism, and photo water splitting models
of oxomanganese complexes mimicking photosynthesis are presented and discussed. Mnoxo complex/Nafion and Mn-oxo oligomer/semiconductor systems show a compelling
working principle by combing the active catalysts in water splitting with Nafion or
semiconductor hetero-nanostructures for effective solar energy harnessing.
1. Introduction
Energy is increasingly becoming the top priority of national and international
issues. This is because that global energy need is expected to double by midcentury
and triple by the end of the century (Cook et al., 2010; Lewis & Nocera, 2006),
largely due to the growing world population. As documented in the literature,
current the energy sources are insufficient to keep pace with the global energy
demand. The main energy source, fossil fuels, is nonrenewable and produces
enormous amount of net greenhouse gases, which have substantial negative impact
on the environment, as well as has limited source supply on earth. To address
these issues, novel renewable carbon-free or carbon-neutral energy sources must
be identified and generated in next 10 to 50 years. Nuclear energy is problematic to
build fast and has been a concern in public safety. The wind energy is too low in
producing enough energy density. Compared with all other energy options, solar
energy is the most promising and the only source of truly renewable, plentiful, and
secure energy (Cook et al., 2010; Lewis & Nocera, 2006).
The sun, one member of the solar system, provides all of the energy and
generates oxygen molecules by water splitting reaction to support the life on our
planet over several billion years. Natural photosynthesis is a unique and an
excellent example for design and mimic for solar energy storage and renewable fuel
production on the large scale via water splitting chemistry. Through a variety of
pigment and their protein complexes, sunlight energy is harvested and storied via
photosynthesis on the large scale. The process can be performed at room
temperature and neutral pH by green plants, cyanobacteria, and algae. Sunlight is
far exceeds what is necessary to support the society. The ability of solar to meet
the global energy need of the future is well documented. The major challenge for
the development of solar energy on a large scale is its storage. The solar energy
storage has been successfully accomplished by the water splitting reaction via
rearranging the chemical bonds in photosynthesis. In the chemical reaction, the
breaking of four O-H covalent bonds and forming of two H-H bond and one O-O
bond result in the conversion of energy-deficient water molecule into the energyrich hydrogen and oxygen molecules. The water splitting reaction stores significant
may be released in the form of electric energy via a fuel cell through the reverse
reaction.
The water splitting reaction can be achieved by electrolysis. A key determinant
of energy storage in artificial photosynthesis is the efficiency of the water splitting
catalysts. These catalysts must operate close to the Nernstian potential for the halfcell reaction. In general, an extra potential in addition to E, designated
overpotential, limits the efficiency of the conversion of light to catalytic current. The
water oxidation reaction is more complex as it requires a four-electron oxidation of
two water molecules coupled to the removal of four protons. In addition, a catalyst
must to tolerate prolonged exposure to highly oxidized conditions, which is able to
cause most chemical functional groups to degrade. In artificial photosynthesis,
water oxidation is considered a substantial challenging task. A deeper
understanding of solar energy conversion, such as photosynthesis, is the key. The
fundamental investigations of water splitting chemistry will provide a firm
foundation for facilitating this transformation. Solar PV panel, solar energy cell, and
fuel cell working together will promise to transform solar energy into affordable
mainstream energy.
2. Photosynthetic Water Splitting
Nature uses photosynthetic organisms to collect sunlight efficiently from the sun
and to convert the solar energy into organic molecules. At the heart of the
photosynthetic process, is the splitting of water by sunlight into oxygen and
‘hydrogen’ (NADPH). The oxygen is released into the atmosphere for us to breathe
and for burning fuels to drive our technologies. The ‘hydrogen’ is combined with
greenhouse gas, carbon dioxide, to make sugars and other organic molecules. To
extract one electron from water and transfer into carbon dioxide, two photons of
light are required by two separated photosystems (PS I and PS II). One photon is
absorbed by PS II to generate a strong oxidizing species (P680+), which is able to
drive the water splitting reaction. The other photon is used by PS I to produce a
strong reducing species, NADPH, and a weak oxidant P700+.
Water splitting chemistry driven by sunlight for solar energy conversion occurs
in the reaction center of PS II, which is located in the thylakoid membranes of
green plants, cyanobacteria, and algae (Barber, 2009; Diner & Rappaport, 2002;
Nanba & Satoh, 1987). PS II is the water-plastoquinone photooxidoreductase or
oxygen-evolving enzyme. It performs a series of light-induced electron transfer
reactions leading to the splitting of water into protons and molecular oxygen. The
products of PS II, namely chemical energy and oxygen, are vital for sustaining life
on earth. The three-dimensional structures of PS II with oxygen-evolving activity
were determined in the past six years (Ferreira, Iverson, Maghlaoui, Barber, &
Iwata, 2004; Loll, Kern, Saenger, Zouni, & Biesiadka, 2005; Yano et al., 2006) and
have laid solid foundation for mechanistic study of solar energy conversion at the
molecular level. When the primary donor P680 is excited by light, charge separation
(2-20 ps) takes place mainly between the chlorophyll, ChlD1, and the pheophytin,
PhD1 (Step 1). The cation is stabilized mainly in chlorophyll PD1, designated P680+.
PhD1- species transfers an electron (~400 ps) to the quinone, QA (Step 2). P680+ is
able to oxidize (~20 ns) the tyrosine-161 of D1 protein, TyrZ, which loses a proton
to the neighboring histidine (Step 3). TyrZ• oxidizes (~30 μs) the Mn cluster (S1 to
S2) (Step 4). QA- transfers an electron (~100 μs) to the second quinone, QB (Step
5). Subsequent turnovers give similar reactions but with kinetic differences at steps
affected by charge accumulation on the Mn cluster and on QB. The second electron
on QB triggers the uptake of two protons and replaces a plastoquinone (PQ) from
the pool in the membrane. The enzyme accumulates four positive chargeequivalents and releasing O2. The valence of the Mn ions increases on the S0 to S1
to S2 steps. However, it is unknown for the S3 and S4 states.
Due to the photosensitivity of PS II to X-ray radiation and the resolution of Xray crystallographic data, the model of Mn4Ca cluster in the PS II oxygen evolving
complex (OEC) is proposed but remains to be confirmed (Sproviero, Gascon,
McEvoy, Brudvig, & Batista, 2008; Yano et al., 2006). Recently, to suppress the
possible radiation damage to a minimum level, using a slide-oscillation method, a
full data set of oxygen-evolving photosystem II was collected and process to a
resolution of 1.9 Å (Umena, Kawakami, Shen, & Kamiya, 2011). The 1.9 Å crystal
structure reveals the geometric arrangement of the Mn4CaO5 cluster including its
oxo bridges and ligands (Figure 1). Three manganese, one calcium and four oxygen
atoms form a cubane-like structure, but the Mn3CaO4 is not an ideal, symmetric
one. The fourth manganese (Mn5) is located outside the cubane and is liked to two
manganeses (Mn1 and Mn3). The calcium is linked to all four manganeses by oxo
bridges. In addition to the five oxygen, four water molecules (W1 to W4) were
found to be associated with the Mn4CaO5 cluster. Two waters are coordinated to the
Mn4 and two to the calcium. The direct ligands of the Mn4CaO5 cluster are
identified: D1-Glu 189, D1-Asp 342, D1-Glu 333, D1-Asp 342, D1-Ala 344, CA43Glu 354, and D1-His 332. The second coordination sphere includes D1-Asp 61, D1His 337, and CP43-Arg 357. The O5 is likely a hydroxide ion in the S1 state. The OO bond formation may occurs in two of the three species O5, W2 and W3. The highresolution structure of PS II at 1.9 Å resolution provides a basis for unraveling the
mechanism of water splitting and O-O bond formation, one of the most fascinating
and important reactions in nature.
In photosynthesis, PS II water splitting chemistry involves four-oxidation steps
with five intermediates known as S-states (Kok, Forbush, & McGloin, 1970).
However, limited information at the molecular level due the complexity of PS II and
its sensitivity to environment. There were several mechanistic proposals in probing
PS II water splitting chemistry (Hoganson & Babcock, 1997; McEvoy, Gascon,
Batista, & Brudvig, 2005; W. Ruettinger, Yagi, Wolf, Bernasek, & Dismukes, 2000).
A body of evidence provides strong support for binding of the substrate water
molecules as terminal ligands to manganese and calcium and for a direct role of
calcium in the water-oxidation chemistry as a Lewis acid to activate a substrate
water molecule as a nucleophile. Mn model chemistry also supports the possibility
that water is activated for O-O bond formation in the OEC by binding to a highvalent manganese ion. It is generally established that the active catalytic species is
Mn(V)=O or Mn(V)-oxo radical, which is capable of releasing oxygen and closes the
S-state cycle (Brudvig, 2008).
The artificial water splitting chemistry is pioneered by the discovery of Ru-based
catalyst, blue dimer, which was reported with a moderate number of turnover
(Gersten, Samuels, & Meyer, 1982; F. Liu et al., 2008). The mechanism involves
Ru(V)-oxo active intermediate. Recently one new Iridium-based family of catalysts
was reported (McDaniel, Coughlin, Tinker, & Bernhard, 2008; Meyer, 2008). In
addition, two all-inorganic catalysts by two independent groups were synthesized
(Geletii et al., 2008; Sartorel et al., 2008). However, the low abundance and high
expense of Ru- and Ir-based catalysts are problematic for large scale solar energy
conversion. It is urgent to develop earth abundant metal catalysts, such as Mn, Fe,
Co, Ni, and Cu-based catalysts. The reason is obvious for practical purpose. The
invention of earth-abundant metal-oxo catalysts is extremely important for
transforming solar energy to affordable energy source in the next ten to fifty years.
Figure 1. Structure of the Mn4CaO5 cluster in the oxygen-evolving photosystem II at a
resolution of 1.9 Å (Umena et al., 2011). (Reproduced with permission from Macmillan
Publisher)
2. Manganese-Based Catalytic Materials
In nature, the oxygen-evolving complex of photosystem II is an oxomanganese
complex that is able to catalyze water-splitting reaction, which inspires the
synthesis and development of manganese-based material in energy research (Hou,
2011; Rivalta, Brudvig Gary, & Batista, 2012; Young et al., 2012). In the field of
artificial photosynthesis, the first functional mimic of Mn4Ca center in PS II is a Mnoxo tetramer complex (W. Ruettinger et al., 2000; W. F. Ruettinger, Campana, &
Dismukes, 1997). The compound is synthesized and contains a cubical [Mn4O4]n+
core with six bidentate ligands chelating to the manganese ions, (dpp)6Mn4O4 (dpp=diphenylphosphinate anion). UV light absorption by the Mn ion produces a Mn-O
charge-transfer excited state, which efficiently release one dioxygen molecule. The
development of the Mn-oxo tetrameric model offer novel insights into the possible
nature of PS II oxygen evolving complex in water splitting and play a vital role in
illustrating photosynthetic oxygen evolving mechanism. However, the oxygen
evolution is not continuous due to the light-induced decomposition of Mn-oxo
tetramer cubane core.
Brudvig and co-workers synthesize a dimeric Mn-oxo complex to probe the
active site of Mn4Ca center in PS II. The experimental procedures for the synthesis
are described in the literature (Limburg et al., 1999). Mn(II) acetate and terpy were
dissolved in water. Oxone in solution was added dropwise and caused yellow
solution to dark green. The green solid precipitation, Mn-oxo dimer, was produced
when the solution was cooled to 0C. The formula of the Mn-oxo dimer is
C30H40Mn2N9O19.
The
ORTEP
diagram
of
Mn-oxo
dimer,
III
IV
[H2O(terpy)Mn (O)2Mn H2O(terpy)](NO3)3 (terpy is 2,2’:6’,2’’-terpyridine), is
shown in Figure 2 (right panel). The Mn-oxo dimer is able to continuous evolve
oxygen gas in the presence of chemical oxidant such as oxone as shown in Figure 2
(left panel). Oxygen-18 labeling shows that water is the source of the oxygen
atoms in the molecular oxygen evolved. The Mn-oxo is the first functional model for
photosynthetic water oxidation with continuous catalytic activity (Limburg et al.,
1999).
Figure 2. Structure of a binuclear di-
-oxo Mn(III,IV) water-oxidation catalyst,
[H2O(terpy)Mn (O)2Mn H2O(terpy)](NO3)3,, which is a functional model for oxygenIII
IV
evolving complex of PS II (Limburg et al., 1999) (Reproduced with permission from
Elsevier)
It is important to know the redox chemistry of the oxomanganese dimer
complex, [H2O(terpy)MnIII(O)2MnIVH2O(terpy)](NO3)3, to understand the mechanism
of as a catalyst for water splitting. It is reported that the oxomanganese dimer is
soluble and stable for several hours in aqueous solution at pH 4. The cyclic
voltammogram showed a well-defined irreversible reduction peak at 0.64 V vs
Ag/AgCl and a poorly defined oxidation at around 1.10 V (M.-N. Collomb, Deronzier,
Richardot, & Pecaut, 1999). The shape and potentials of the electrochemical signal
strongly depends on the nature of the supporting electrolyte. The reason is likely
the some adsorption phenomena at the working electrode.
It is proposed that the reduction peak at Epc1’=0.64 V is assigned to the
formation of the mononuclear complex, MnII(Terpy) complex as the product with
the possible electron transfer number n=3. The low intensity irreversible oxidation
peak at Epa3=0.87 V is the 3-electron oxidation of MnII(Terpy) to
[H2O(terpy)MnIII(O)2MnIVH2O(terpy)](NO3)3. The partially reversible one-electron
redox signals were observed at Epa1=1.10 V and Epc1=1.10 V, which have been
nonambiguously assigned to a one-electron exchange process of Mn(III/IV) to
Mn(IV/IV) (M. N. Collomb, Deronzier, & Piron, 1999; M. N. Collomb, Deronzier,
Piron, Pradon, & Menage, 1998; M. N. Collomb, Deronzier, Pradon, Menage, &
Philouze, 1997). A reduction peak at less positive potential, Epc2=0.89 V, and its
intensity decreases as the scan rate increases. This reduction peak is due to the
reduction of a Mn(IV/IV) tetramer, which is chemically transformed from the
Mn(III/IV) dimer and reduced at a slightly less positive potential.
To identify the nature of the species involvement in the electrochemistry of Mnoxo dimer, [H2O(terpy)MnIII(O)2MnIVH2O(terpy)](NO3)3, exhaustive electrolysis was
performed (Baffert et al., 2005). Controlled potential electrolysis at 1.20 V in
aqueous solutions revealed the consumption of about 2-5 electrons per molecule
depending on the supporting electrolytes and a pronounced color change of the
solution from green to red. The red oxidized species has been identified as the
tetranulear Mn(IV/IV) complex, [H2O(terpy)Mn2IV(O)5Mn2IVH2O(terpy)](NO3)6. The
Mn-oxo dimer in green color shows the four bands located at 275, 330, 553, and
654 nm. The 553 nm absorption is ascribed to the contribution of d-d transition of
the Mn ion. The 654 nm band can be ascribed to the metal to oxo ligand charge
transfer. During the electrolysis at 1.20 V, a new intense visible band at 477 nm,
which is assigned to the d-d transition of Mn(IV) species. The final product shows
the five absorption bands at 275, 324, 477, 650, and 780 nm, which is identical to
those
of
a
synthetic
Mn(IV/IV)
tetramer,
[H2O(terpy)Mn2IV(O)5Mn2IVH2O(terpy)](NO3)6 (M. N. Collomb et al., 1999; M. N.
Collomb et al., 1998; M. N. Collomb et al., 1997).
Figure 3 summarizes the electrochemical mechanism of the Mn-oxo dimer. The
electrochemical oxidation of MnII(Terpy) monomer complex allows the quantitative
formation of Mn(III/IV)-oxo dimer, [H2O(terpy)MnIII(O)2MnIVH2O(terpy)](NO3)3,
involving three electron transfer process. The Mn(III/IV)-oxo dimer can
electrochemically
oxidized
to
Mn(IV/IV)-oxo
dimer,
IV
IV
[H2O(terpy)Mn (O)2Mn H2O(terpy)](NO3)3,
by a one-electron step, which is
moderately stable and quantitatively yields the stable Mn(IV/IV) tetranulear
complex, [H2O(terpy)Mn2IV(O)5Mn2IVH2O(terpy)](NO3)6. The Mn-oxo tetramer is a
linear structure, the “dimer-of-dimer.”
Figure 3. Electrochemical interconversion of Mn-oxo complexes (Baffert et al., 2005)
(Reproduced with permission from American Chemical Society)
The
role
of
the
terminal
water
in
the
Mn-oxo
dimer,
[H2O(terpy)MnIII(O)2MnIVH2O(terpy)](NO3)3, is probed by the comparison of the pHdependent oxidation of oxomanganese complexes was examined (Cady & Brudvig,
2008; Cady, Shinopoulos, Crabtree, & Brudvig). The electrochemical oxidation of
Mn-oxo dimer containing terminal water ligands showed pH dependence with a
slope of 59 mV per pH. In contrast, the oxidation of the Mn-oxo dimer without
water ligands is pH independent. This observation indicated that the terminal water
ligand is important for the proton-coupled electron transfer reaction, which is vital
for the water oxidation. The presence of terminal water ligands in the oxygenevolving complex may play a role in the redox “leveling effect” in the four-electron
transfer cycle.
The calculations predict that the redox potentials of Mn-oxo dimer in aqueous
solutions are linear dependence with pH at a rate of 59 mV per pH. The prediction is
in agreement with the experimental data. In the presence of acetate ion, the redox
potentials and pKa values are shifted. The pKa values of terminal water in the Mnoxo dimer are 1.2 and 13.3, respectively. The pKa values of terminal water ligands
depend strongly on the oxidation states of the Mn centers, changing by ~13 pH
units during the Mn(III/IV) to Mn(IV/IV) transition (Wang et al., 2010).
According to Raman spectroscopic, EPR, MS, and enzymatic kinetic data in the
presence of variety of oxidants, such as, oxone, Ce4+, and hyperchrite (Brudvig,
2008; Cady, Crabtree, & Brudvig, 2008; Limburg et al., 2001), the key feature of
the Mn-oxo dimer is Mn(III)/Mn(IV) mix-valence and the presence of one terminal
water molecule on each Mn ion. The catalytic mechanism of Mn-oxo dimer involves
the valence change of Mn(III/IV) to Mn(IV/V) by the oxidant and followed by the
molecular oxygen release from water splitting step as shown in Figure 4. The
chemical oxidant, XO, including oxone and hypochlorite, is the oxygen atom
transfer reagent. The rate-limiting step is formation of 4 by oxidation of 1. The
oxygen release step are proposed in two different pathways: (1) The solvent H2O
attacks the terminal oxo ligand of 4, which form O-O band and is associated with
the reduction of Mn(IV/V) to Mn(II/III); (2) The chemical oxidant, XO, attacks the
terminal oxo ligand and produces O2.
Figure 4. Mechanism of the function model, Mn-oxo dimer,
[H2O(terpy)MnIII(O)2MnIVH2O(terpy)](NO3)3, (terpy is 2,2’:6’,2’’-terpyridine), in the
presence of chemical oxidant (Limburg et al., 2001) (Reproduced with permission from
American Chemical Society)
It is suggested that PS II photoinhibition is triggered by a direct absorption of
UV light in the Mn4Ca cluster (Hakala, Tuominen, Keranen, Tyystjarvi, & Tyystjarvi,
2005). The Mn-oxo dimer was unstable to UV light, as judged by the measurement
of increasing absorption at 400 nm, which is assigned to the Mn(IV)/Mn(IV) species
(Limburg et al., 2001). It seemed that the photodamage of the Mn-oxo dimer may
be associated with a valence change from Mn(III) to Mn(IV) (Wei, Cady, Brudvig, &
Hou, 2011). The oxygen-evolution activity of the Mn-oxo dimer was decreased upon
UV treatment, supporting the occurrence of photodamage. The action spectrum of
Mn(III/IV)-oxo dimer under strong light at six wavelengths (254, 312, 365, 452,
555, and 655 nm) revealed the presence of a stable species peaking at 440 nm.
The absorbance at 440 nm increases with a lifetime of about 30 min when the
illumination time at 312 nm increases. The continuing illumination causes a
decrease of the absorbance at 440 nm and increase of the absorbance at 400 nm,
suggesting the photodamage of Mn-oxo dimer is a two-step reaction accompanying
with the formation of two new species, denoted as Mn(IV/IV)440 and Mn(IV/IV)400.
The photodamage induced by UV radiation showed strong pH dependence,
indicating that protons play a role in the photodamage reaction.
The UV-induced product has an intense fluorescence peak at 513 nm, confirming
the formation of a novel stable species. The two-step kinetics of the photodamage
shows that the lifetime of the first step in forming the fluorescence species is 30
min. After 60 min, the fluorescence emission is decreased in the second step and is
associated with the formation of Mn(IV/IV)440. There is no report of fluorescent high
valent Mn-oxo complex in the literature. The observed fluorescence peak at 513 nm
is likely emitted by the Terpy ligand and not by the central Mn ion. The bipyridine
and phenanthroline compounds are fluorescence active due to the hydration (Henry
& Hoffman, 1979). The fluorescent Mn(IV/IV)440 species is likely involved in
hydration or protonation of the Terpy ligand (Wei et al., 2011).
The native Mn-oxo dimer sample shows a 16-line signal in the range of 28004100 G in the acetate buffer, which is characteristic for the Mn(III/IV) mixedvalence species. The 16-line signal is decreased by a factor of 90% with 10 min,
indicating the Mn-oxo dimer is converted into an EPR silent Mn(IV/IV) species. This
observation shows that the Mn-oxo dimer is unstable and is decomposed at the
temperature of 60C (Zhang, Cady, Brudvig Gary, & Hou, 2011).
The thermal decomposition reaction kinetics of Mn-oxo dimer revealed that the
lifetime of 3.5 min in the first fast step and of 19 min in the following slow step.
Using the kinetic data of decomposition reaction at the different temperatures and
Arrhenius plots, the activation energies for step 1 and step 2 are determined to be
68 and 82 kJ/mol, respectively. The decomposition reaction of Mn-oxo dimer is
accompanied with formation of new products, judged by the formation of brown
precipitates in solution and the observation of the colored Mn-oxo dimer from green
to colorless (Zhang et al., 2011). Unexpectedly, the oxygen evolution
measurements showed an activity increase after the decomposition reaction was
completed. We concluded that one solid water-splitting material with higher
activity, thereafter designated Mn-oxo oligomer, is formed in the solution (Zhang et
al., 2011). The two-step mechanism for decomposition of the Mn-oxo dimer under
elevated temperature involves the a fast stem with a valence change in Mn. In the
first step, the Mn(III/IV)-oxo dimer may have disproportionated into Mn(II)-Terpy,
Mn(IV/IV)-oxo dimer, and an unknown water-soluble species with high catalytic
activity. The following slow step leads to a highly active Mn-oxo oligomer
precipitate.
FTIR data shows that the solid product has a different IR spectrum than MnO2,
suggesting the Mn-oxo oligomer is not MnO2. The EPR signal confirmed that the Mnoxo oligomer is different from Mn(III/IV)-oxo dimer. The elemental analysis showed
that the Mn-oxo oligomer contains terpyridine ligand. The TEM data indicated Mnoxo oligomer is amorphous on the nanometer scale. The XANES and EXAFS data
suggested that the rising edge energy of the Mn oligomer is slightly shifted to
higher energy compared to the Mn Terpy sample, likely indicating an increased
fraction of Mn(IV). However, this should still be a mixture of Mn(III) and Mn(IV)
oxidation state. The EXAFS data indicated that the Mn-Mn distances are increased
from ~2.7Å to 2.9Å (average) after the oligomerization. These lines of evidence
suggested that the Mn-oxo oligomer has unique new structural feature with
bounded terpy ligands. This material is thermal stable in nanoscale size and highly
active in photosynthetic water splitting, which may be unique for fabricating novel
catalysts in solar fuel production.
4. Manganese/Semiconductor Catalytic Systems
It has been demonstrated that these charges can be readily utilized for water
splitting (Fujishima & Honda, 1972). With an energy gap between the conduction
and valence bands, semiconductor represents an appealing candidate to effectively
absorb photons and transform the optical energy into free charges (electrons and
holes). Theoretical calculations have shown that the power conversion efficiency of
using semiconductor for water photo-splitting can be as high as that of solid-state
solar cells (Bolton, 1996). More recently, significant efforts have been attracted to
fabricate nanoscale semiconductor materials as photoelectrodes to further improve
the performance of water splitting by adding advantages of high surface area and
improved conductivity (Lin, Zhou, Liu, Sheehan, & Wang, 2009; Mor, Shankar,
Paulose, Varghese, & Grimes, 2005; Yang et al., 2009). Combining semiconductor
nanomaterials with the Mn-oxo catalyst overcomes a key challenge in using
semiconductor directly – the low catalytic activity of semiconductors. The low
reactivity often leads to a high overpotential and results in significant reduction in
the overall energy conversion efficiency (Kudo & Miseki, 2009). Using a research
scale commercial ALD reactor, various oxides that can be used for photo water
splitting were successfully grown, including TiO2, WO3, Cu2O and Fe2O3 (Lin, Zhou,
Liu, Sheehan, & Wang, 2009). By interfacing these semiconductor materials with a
highly conductive nanonet structure, the performance of splitting water is greatly
boosted.
The Mn-oxo compounds including Mn(II)-terpy compound and Mn(III/IV)-oxo
dimer can be attached to the surface of TiO2 nanomaterial via direct adsorption or
in situ synthesis and showed an efficient electron transfer (Abuabara et al 2007,
McNamara et al 2008, Li et al., 2009; McNamara et al., 2009). As shown in Figure
5, the resulting Mn-oxo dimer/TiO2 is able to reversibly change mixed valent
Mn(III/IV) to Mn(IV/IV) state by photoexcitation and interfacial electron injection
into the conducting band of TiO2. This Mn-based TiO2 material appears to be
promising for developing an inexpensive water splitting catalyst in the
photocatalytic solar cells. Recently, Mn-oxo dimer is immobilized in Nafion
membranes to achieve photocatalytic water oxidation (Young, Gao, & Brudvig Gary,
2011).
Figure 5. Catalytic water oxidation system by attaching the Mn-oxo dimers to the TiO2
nanoparticles (McNamara et al., 2009) (Reproduced with permission from RCS
Publishing)
Our hypothesis is to use n-type semiconductor to generate holes. When
irradiated by light, n-type semiconductor will cooperate with Mn-oxo complex to
efficiently split water using solar energy (Figure 6). The advantage is the
combination of highly active water splitting catalytic ability of Mn-oxo oligomer and
highly efficient photoconversion of semiconductor. The Mn layer is expected to be
within a few nanometer in thickness to ensure the high electric conductivity for
photocatalytic water splitting. The Mn-oxo oligomer with high catalytic activity,
which is the decomposition product of Mn(III/IV)-oxo dimer, may be an ideal
material for fabricating robust water-splitting catalysts.
Figure 6. Working model of a Mn-oxo oligomer/tungsten oxide photo water oxidation
catalyst.
The solar light radiations are absorbed by tungsten oxide semiconductor and
cause the charge separation to produce electrons and holes. The electrons are transferred
to the cathode by an electric wire to produce hydrogen gas. The holes receive electrons
from Mn-oxo oligomer, which is the precipitate of Brudvig catalyst (Mn-oxo dimer) under
thermal conditions (R. Liu et al., 2011). (Reproduced with permission from Wiley-VCH)
As shown in Figure 7, various evidence supports that the detected oxygen by
capillary GC analysis is the direct product of water splitting (R. Liu et al., 2011).
The Mn-oxo oligomer/tungsten oxide material is able to directly generate oxygen
and hydrogen for solar energy harness (R. Liu et al., 2011). The amount of
hydrogen is approximately twice that of oxygen, consistent with complete
decomposition of water. Control experiments with H218O confirmed that O is the gas
phase comes from water. The experimental results also demonstrated that the
water splitting reaction requires the cooperation of Mn-oxo catalytic material and
tungsten oxide semiconductor.
Figure 7. Water splitting reaction by Mn-oxo oligomer/tungsten oxide catalytic system.
(a) the rate of hydrogen production is approximately twice that of oxygen. (b) isotopic
labeling experiments verify that oxygen atoms in oxygen come from water (R. Liu et al.,
2011). (Reproduced with permission from Wiley-VCH)
It is vital to have the synergistic design of WO3 and Mn-oxo oligomer to achieve
robust and efficient photo water splitting. Without the Mn catalyst, the amount of
O2 measured is only approximately 50% of that with the Mn catalyst after 3h at pH
4. This phenomena is more obvious when the pH in the solution increase. At pH 7,
WO3 without the Mn catalyst decayed more quickly (60% loss in 1h) than at pH 4.
In contrast, the Mn/WO3 system shows approximately 4 % performance impairment
for up to 2 h. It took more than 19 h in the Mn/WO3 case for the efficiency to drop
to 50 % of the initial value.
Light intensity affects the photocurrents of photo water splitting of the WO3 and
Mn/WO3 systems, roughly following the linear relationship. At the high light
intensity of (>70 mW/cm2), the dependence for WO3 system is curved. This may
be due the electron transfer rate between the H2O and WO3 is limited and likely
reach its saturation. In contrast, the highly active Mn-oxo oligomer complex is
efficiently transfer electron transfer from H2O mimicking photosynthetic water
oxidation. The stability test of the water splitting reaction catalyzed by the
Mn/tungsten oxide is carried out when the reaction vial is purged with inert gas
nitrogen every 7 h. The oxygen evolution driven by light is increased steady to
2
2 /cm in each of 7 h cycle, demonstrating that the photo
water splitting reaction is robust and efficient over the experimental period of 35 h.
To evaluate the incident photon to electron conversion efficiency (IPCE) of a
photoelectrochemical cell, the energy conversion efficiency of Mn/tungsten oxide
system is varied to the applied bias voltages (Varghese & Grimes, 2008). The
efficiency is increased when the applied voltage is 0 to 0.2 V (vs. RHE) and follows
by a slightly decrease at 0.2 to 0.3 V. When the voltages increase rapidly at 0.3 to
0.8 V, the efficiency reaches its maximum, which is 1.10 %. After the voltage at 0.8
V, the efficiency decreases dramatically to 0.3 % at 1.4 V. At the voltage of 1.23 V
(vs. RHE), the efficiency is 0.59 %.
The experimental data show that the Mn/tungsten oxide is a robust efficient
catalytic system in photo water splitting. To explore the role of Mn in the catalytic
cycle, the Mn is monitored before and after photo reaction for 19 h. As shown in
Figure 8, The W content is almost unchanged during the 19 h period. It agrees with
the observation that Mn/tungsten oxide system is much more stable and robustness
in photo water splitting than the tungsten oxide material without Mn-oxo oligomer.
However, Mn on the surface of tungsten oxide is undetectable by XPS after 19 h.
This suggests that Mn is likely diffused into the aqueous solution in the form of
Mn(II) ion. Similar observation is reported in the tetramanganease/Nafion catalytic
system (Hocking et al., 2011). In the case of Mn(II) ion presented in the aqueous
solution, Mn signal is detected after 19 h of photoreaction. This confirmed that
Mn(II) is involved in the photo catalytic cycle of water splitting. It is likely that
Mn(II) ions in aqueous solution is oxidized and form active high valent species on
the surface of tungsten oxide and close the catalytic cycle.
Figure 8 XPS of Mn/tungsten oxide system (Chou et al., 2012). (Reproduced
with permission from Elsevier)
As shown in Figure 9, a possible mechanism of Mn/tungsten oxide system in photo
water splitting mimicking multi electron and proton transfer reaction in
photosynthetic water oxidation is proposed. Four light photos are required to
oxidize the Mn-terpy species, which is accompanied by four proton-coupled
electron transfer steps mimicking the Kok s-state cycle. At each step, the photo
causes charge separation in tungsten oxide. The hole generated in tungsten oxide
receives an electron from Mn-terpy complex via Mn valence changes. The Mn(V)
intermediate species is formed by the fourth photo-driven reaction and splits water
to dioxygen and regenerates the active Mn-terpy catalyst.
Figure 9. Possible mechanism of Mn/tungsten oxide system in photo water splitting (Hou,
2011). (Reproduced with permission from MDPI).
To avoid the production of corrosive byproducts, the ALD growth of tungsten
oxide without production is successfully established. The synthetic technique makes
it easy to form heteronanostructures. The Mn catalyst derived from the oxo-bridged
Mn dimer is easy to prepare and exhibits good stability and catalytic properties.
When interfaced with tungsten oxide, it acts as a protecting layer without adverse
effect on the water-splitting properties. To the best of our knowledge, this is the
first time that tungsten oxide photoelectrodes stable in neutral solution have been
prepared. The heteronanostructures design combines multiple components, each
with unique complementary and critical functions, and offers combinations of
properties that are not available in single-component materials. The versatility of
this method will find applications in numerous areas where the availability of
materials is the limiting factor.
5. Conclusions
Nature uses the water-splitting reaction via photosynthesis driven by sunlight in
plants, algae, and cyanobacteria to store the vast solar energy and to provide vital
oxygen to life on earth. In the recent five years, revolutionary developments in
photoelectrochemical water splitting using Mn-oxo complexes and Co-based
molecular catalysts (Cady et al., 2008; Dismukes et al., 2009; Hou, 2010, 2011) as
well as Ru- and Ir-based compounds (Concepcion et al., 2009; Sala, Romero,
Rodriguez, Escriche, & Llobet, 2009) associated with dye-sensitized semiconductors
(Woodhouse & Parkinson, 2008; Youngblood, Lee, Maeda, & Mallouk, 2009) have
been made. In particular, the developed Mn/Nafion, Mn/TiO2, Mn/WO3, Co/Fe2O3,
Co/ZnO systems may be extended to heterostructures of a variety of
semiconductors (Hou, 2010). The protocols are suit for preparing earth-abundant
metal/semiconductor catalysts.
One of the most challenges in renewable energy production is the fabrication of
efficient catalysts for splitting water into hydrogen and oxygen. The Mn-oxo
tetramer cubane-like compound developed by Dismukes and co-workers (Wolfgang
Ruettinger & Dismukes, 1997; W. Ruettinger et al., 2000; W. F. Ruettinger et al.,
1997) was doped into the Nafion membrane (3-8 m) to make a Mn cubium/Nafion
photoanode, which is able to oxidize water upon activation with visible light
(Brimblecombe, Dismukes, Swiegers, & Spiccia, 2009; Brimblecombe, Koo,
Dismukes, Swiegers, & Spiccia, 2010; Brimblecombe, Swiegers, Dismukes, &
Spiccia, 2008; Dismukes et al., 2009). The key feature of the design is two aspects
(1) a photoinduced charge separation system, which is Ru(II)-bipy complex and
TiO2-coated film, and (2) a molecular catalyst, which is Mn-oxo cubic species in a
Nafion membrane.
In additional to manganese catalyst, a Co-based catalytic material that forms
electrochemically on an ITO electrode in phosphate buffered water containing cobalt
(II) ions was reported to operate in neutral water under room temperature (Kanan
et al 2009, Kanan et al 2008). This type of Co-base catalyst was able to oxidize
water in aqueous solutions containing 0.5 M NaCl (Surendranath et al 2009). The
active species is proposed to be the Co-oxo cubane-like structure, which oxidizes
water to produce O2 by forming a Co(IV) intermediate via a proton-coupled electron
transfer step. Phosphate ion may be the key player for the proton transfer reaction
(Kanan, Surendranath, & Nocera, 2009; Lutterman, Surendranath, & Nocera,
2009). Further analysis revealed that the Co-Pi material is a robust heterogeneous
water splitting catalyst and able to self-repair by self-assembly.
XAS and EPR studies showed that the active Co-Pi film functions as a molecular
cobaltate cluster model (Kanan et al., 2010; McAlpin et al., 2010). The high
catalytic activity of Co-Pi suggests molecular cobaltate cluster structure promote
water oxidation and that the Co valency is greater that 3. The “edge” of cobaltate
may have terminal waters. By truncating the extended cobaltate lattice, the
number of edges is maximized and maximum activity is realized. The extended
cobaltate lattices have few terminal oxygens and hence are unable to splitting
water.
It has been reported that the Co-based water splitting catalyst can be
electrochemically and photochemically deposited on the surface of semiconductor
Fe2O3 and ZnO, respectively (Zhong & Gamelin, 2010). The resulting Co/Fe2O3 and
Co/ZnO photoanodes showed a dramatic improvement in solar water splitting.
These results demonstrate that integration of promising water splitting catalysts
with a photo-absorbing substrate can provide a substantial reduction in the external
power needed to drive the catalytic water splitting chemistry and can be used as a
general route to deposit the molecular catalysts on any semiconductor electrode. In
addition to Co-Pi, a homogeneous catalyst, B-type [Co4(H2O)2(-PW9O34)2]10-, which
is free of carbon-based ligands, was synthesized and demonstrated high catalytic
turnover frequencies for O2 production at pH 8 (Yin et al., 2010). The key element
of the complex is a Co4O4 core stabilized by oxidatively resistant polytunstate
ligands. Although the mechanism of the complex is unclear, the catalytic material
provides a basis for further understanding of Co-based water splitting catalysis in
general. Artificial leaf using a ternary alloy (NiMoZi) and cobalt phosphate (Co-OEC)
system associated with silicon photovoltaic is developed and provides a means for
an inexpensive and viable system for solar energy supply (Nocera, 2012).
It is envisioned that the progresses in the filed of nanomaterial and
photosynthesis will offer novel technology for transforming the solar energy into our
future energy systems. In nature, the production of oxygen by oxidation of water is
catalyzed by an Mn4Ca inorganic center in the oxygen-evolving complex of
photosystem II. Using synthetic biology and fundamental knowledge of
photosynthesis, one might be able to enhance natural and artificial photosynthesis
for improved solar energy conversion efficiency (Blankenship et al., 2011). Hence
the use of a light harvester, a water splitting catalyst, and an electron acceptor is a
promising way for solar energy conversion (Hou, 2011). Grand challenges remains,
including the discovery of inexpensive, robust, and efficient water oxidation
catalysts. In particular, the future endeavors will be placed on improvement in
efficiency and durability of the catalytic system for its practical application as well
as on usage of visible and infrared light. It is highly likely open a new area of
fabricating next generation of highly efficient water splitting catalysts in solar fuel
production.
Acknowledgments
The work was supported by the Alabama State University and in part by USDA
CSREES program. We would like to thank Gary Brudvig and Clyde Cady at Yale
University for continuous support and Dunwei Wang, Rui Liu, Yongjing Lin, and
Yang Xu at Boston College for fruitful collaborative work. We also thank Lien-Yang
Chou, Xuejing Hou, Ndi Geh, Robert Mulkern, Aaron Raposo, Joy Patel for technical
assistance and insightful discussions.
References
Baffert, C., Romain, S., Richardot, A., Lepretre, J. C., Lefebvre, B., Deronzier, A., et
al. (2005). Electrochemical and chemical formation of
[Mn4(IV)O5(terpy)4(H2O)2]6+, in relation with the photosystem II oxygenevolving center model [Mn2(III,IV)O2(terpy)2(H2O)2]3+. J Am Chem Soc,
127(39), 13694-13704.
Barber, J. (2009). Photosynthetic energy conversion: natural and artificial. Chem
Soc Rev, 38(1), 185-196.
Blankenship, R. E., Tiede, D. M., Barber, J., Brudvig, G. W., Fleming, G., Ghirardi,
M., et al. (2011). Comparing Photosynthetic and Photovoltaic Efficiencies and
Recognizing the Potential for Improvement. Science, 332(6031), 805-809.
Bolton, J. R. (1996). Solar photoproduction of hydrogen: A review. Solar Energy,
57(1), 37-50.
Brimblecombe, R., Dismukes, G. C., Swiegers, G. F., & Spiccia, L. (2009). Molecular
water-oxidation catalysts for photoelectrochemical cells. Dalton Trans (43),
9374-9384.
Brimblecombe, R., Koo, A., Dismukes, G. C., Swiegers, G. F., & Spiccia, L. (2010).
Solar-driven Water Oxidation by a Bio-inspired Manganese Molecular Catalyst. J
Am Chem Soc, 132(9), 2892-2894.
Brimblecombe, R., Swiegers, G. F., Dismukes, G. C., & Spiccia, L. (2008).
Sustained water oxidation photocatalysis by a bioinspired manganese cluster.
Angew Chem Int Ed, 47(38), 7335-7338.
Brudvig, G. W. (2008). Water oxidation chemistry of photosystem II. Philos Trans R
Soc B, 363(1494), 1211-1219.
Cady, C. W., & Brudvig, G. W. (2008). Functional manganese model chemistry
relevant to the oxygen-evolving complex of photosystem II: oxidation of a
Mn(III,IV) complex coupled to deprotonation of a terminal water ligand. In:
Allen JP, Osmond B, Golbeck GH, and Gantt E Eds., Photosynthesis: Energy from
the Sun, Springer, pp. 377-382.
Cady, C. W., Crabtree, R. H., & Brudvig, G. W. (2008). Functional models for the
oxygen-evolving complex of photosystem II. Coord Chem Rev, 252(3+4), 444455.
Cady, C. W., Shinopoulos, K. E., Crabtree, R. H., & Brudvig, G. W.
[(H2O)(terpy)Mn(micro -O)2Mn(terpy)(OH2)](NO3)3 (terpy = 2,2':6,2''terpyridine) and its relevance to the oxygen-evolving complex of photosystem II
examined through pH dependent cyclic voltammetry. Dalton Trans, 39(16),
3985-3989.
Chou, L.-Y., Liu, R., He, W., Geh, N., Lin, Y., Hou, E. Y. F., et al. (2012). Direct
Oxygen and Hydrogen Production by Water Splitting Using a Robust Bioinspired
Manganese-oxo Oligomer Complex/Tungsten Oxide Catalytic System. Int J
Hydrogen Energy, 37, 8889-8896.
Collomb, M.-N., Deronzier, A., Richardot, A., & Pecaut, J. (1999). Synthesis and
characterization of a new kind of Mn2III,IV -oxo complex:
[Mn2O2(terpy)2(H2O)2](NO3)3.6 H2O, terpy= 2,2':6',2''-terpyridine. New J Chem,
23(4), 351-354.
Collomb, M. N., Deronzier, A., & Piron, A. (1999). Electrochemical behaviour of
[Mn2III,IVO2(phen)4]3+ complex in aqueous phen-phenH+ buffer; phen=1,10phenanthroline. J Electranal Chem, 463, 119-122.
Collomb, M. N., Deronzier, A., Piron, A., Pradon, X., & Menage, S. (1998). New
Chemical and Electrochemical Synthesis of the [Mn4IVO6(bpy)6]4+ Cluster.
Electrochemical Interconversion with Corresponding Bi- and Mononuclear
Complexes. J Am Chem Soc, 120, 5373-5380.
Collomb, M. N., Deronzier, A., Pradon, X., Menage, S., & Philouze, C. (1997).
Electrochemical Interconversion of Mono-, Bi-, and Tetranuclear (Bipyridyl)
Manganese Complexes in Buffered Aqueous Solution. J Am Chem Soc, 119,
3173-3174.
Concepcion, J. J., Jurss, J. W., Brennaman, M. K., Hoertz, P. G., Patrocinio, A. O.
T., Murakami Iha, N. Y., et al. (2009). Making Oxygen with Ruthenium
Complexes. Acc Chem Res, 42(12), 1954-1965.
Cook, T. R., Dogutan, D. K., Reece, S. Y., Surendranath, Y., Teets, T. S., & Nocera
Daniel, G. (2010). Solar energy supply and storage for the legacy and nonlegacy
worlds. Chem Rev, 110, 6474-6502.
Diner, B. A., & Rappaport, F. (2002). Structure, dynamics, and energetics of the
primary photochemistry of photosystem II of oxygenic photosynthesis. Annu Rev
Plant Biol, 53, 551-580.
Dismukes, G. C., Brimblecombe, R., Felton, G. A. N., Pryadun, R. S., Sheats, J. E.,
Spiccia, L., et al. (2009). Development of bioinspired Mn4O4-cubane water
oxidation catalysts: lessons from photosynthesis. Acc Chem Res, 42(12), 19351943.
Ferreira, K. N., Iverson, T. M., Maghlaoui, K., Barber, J., & Iwata, S. (2004).
Architecture of the photosynthetic oxygen-evolving center. Science, 303(5665),
1831-1838.
Fujishima, A., & Honda, K. (1972). Electrochemical Photolysis of Water at a
Semiconductor Electrode. Nature, 238(5358), 37-38.
Geletii, Y. V., Botar, B., Kogerler, P., Hillesheim, D. A., Musaev, D. G., & Hill, C. L.
(2008). An all-inorganic, stable, and highly active tetraruthenium homogeneous
catalyst for water oxidation. Angew Chem Int Ed, 47(21), 3896-3899.
Gersten, S. W., Samuels, G. J., & Meyer, T. J. (1982). Catalytic oxidation of water
by an oxo-bridged ruthenium dimer. J Am Chem Soc, 104(14), 4029-4030.
Hakala, M., Tuominen, I., Keranen, M., Tyystjarvi, T., & Tyystjarvi, E. (2005).
Evidence for the role of the oxygen-evolving manganese complex in
photoinhibition of Photosystem II. Biochim Biophys Acta, 1706(1-2), 68-80.
Henry, M., & Hoffman, M. (1979). Photophysics and photochemistry of aromatic
nitrogen heterocycles. Fluorescence from 2,2'-bipyridine and 1,10phenanthroline. J Phys Chem, 83, 618-625.
Hocking, R. K., Brimblecombe, R., Chang, L.-Y., Singh, A., Cheah, M. H., Clover, C.,
et al. (2011). Water-oxidation catalysis by manganese in a geochemical-like
cycle. Nature Chemistry, 3, 461-466.
Hoganson, C. W., & Babcock, G. T. (1997). A metalloradical mechanism for the
generation of oxygen from water in photosynthesis. Science 277(5334), 19531956.
Hou, H. J. M. (2010). Structural and mechanistic aspects of Mn-oxo and Co-based
compounds in water oxidation catalysis and potential application in solar fuel
production. J. Integr Plant Biol, 52, 704-711.
Hou, H. J. M. (2011). Manganese-based materials inspired by photosynthesis for
water-splitting. Materials, 4, 1693-1704.
Kanan, M. W., Surendranath, Y., & Nocera, D. G. (2009). Cobalt-phosphate
oxygen-evolving compound. Chem Soc Rev, 38(1), 109-114.
Kanan, M. W., Yano, J., Surendranath, Y., Dinca, M., Yachandra, V. K., & Nocera, D.
G. (2010). Structure and Valency of a Cobalt-Phosphate Water Oxidation
Catalyst Determined by in Situ X-ray Spectroscopy. J Am Chem Soc., 132(39),
13692-13701.
Kok, B., Forbush, B., & McGloin, M. (1970). Cooperation of charges in
photosynthetic O2 evolution-I. A linear four step mechanism. Photochem
Photobiol, 11(6), 457-475.
Kudo, A., & Miseki, Y. (2009). Heterogeneous photocatalyst materials for water
splitting. Chem Soc Rev 38(1), 1.
Lewis, N. S., & Nocera, D. G. (2006). Powering the planet: chemical challenges in
solar energy utilization. Proc Natl Acad Sci U S A, 103(43), 15729-15735.
Li, G., Sproviero, E. M., Snoeberger, R. C., III, Iguchi, N., Blakemore, J. D.,
Crabtree, R. H., et al. (2009). Deposition of an oxomanganese water oxidation
catalyst on TiO2 nanoparticles: computational modeling, assembly and
characterization. Energy Environ Sci, 2(2), 230-238.
Limburg, J., Vrettos, J. S., Chen, H., de Paula, J. C., Crabtree, R. H., & Brudvig, G.
W. (2001). Characterization of the O2-evolving reaction catalyzed by
[(terpy)(H2O)Mn(III)(O)2Mn(IV)(OH2)(terpy)](NO3)3 (terpy = 2,2':6,2"terpyridine). J Am Chem Soc, 123(3), 423-430.
Limburg, J., Vrettos, J. S., Liable-Sands, L. M., Rheingold, A. L., Crabtree, R. H., &
Brudvig, G. W. (1999). A functional model for O-O bond formation by the O2evolving complex in photosystem II. Science, 283(5407), 1524-1527.
Lin, Y., Zhou, S., Liu, X., Sheehan, S., & Wang, D. (2009). TiO2/TiSi2
Heterostructures for High-Efficiency Photoelectrochemical H2O Splitting. J Am
Chem Soc, 131(8), 2772-2773.
Liu, F., Concepcion Javier, J., Jurss Jonah, W., Cardolaccia, T., Templeton Joseph,
L., & Meyer Thomas, J. (2008). Mechanisms of water oxidation from the blue
dimer to photosystem II. Inorg Chem, 47(6), 1727-1752.
Liu, R., Lin, Y., Chou, L.-Y., Sheehan, S. W., He, W., Zhang, F., et al. (2011). Water
splitting by tungsten oxide prepared by atomic layer deposition and decoraed
with an oxygen-evolving catalyst. Angew Chem Int Ed, 50, 499-502.
Loll, B., Kern, J., Saenger, W., Zouni, A., & Biesiadka, J. (2005). Towards complete
cofactor arrangement in the 3.0 A resolution structure of photosystem II.
Nature, 438(7070), 1040-1044.
Lutterman, D. A., Surendranath, Y., & Nocera, D. G. (2009). A Self-Healing
Oxygen-Evolving Catalyst. J Am Chem Soc, 131(11), 3838-3839.
McAlpin, J. G., Surendranath, Y., Dinca, M., Stich, T. A., Stoian, S. A., Casey, W.
H., et al. (2010). EPR Evidence for Co(IV) Species Produced During Water
Oxidation at Neutral pH. J Am Chem Soc, 132(20), 6882-6883.
McDaniel, N. D., Coughlin, F. J., Tinker, L. L., & Bernhard, S. (2008).
Cyclometalated Iridium(III) Aquo Complexes: Efficient and Tunable Catalysts for
the Homogeneous Oxidation of Water. J Am Chem Soc, 130(1), 210-217.
McEvoy, J. P., Gascon, J. A., Batista, V. S., & Brudvig, G. W. (2005). The
mechanism of photosynthetic water splitting. Photochem Photobiol Sci, 4(12),
940-949.
McNamara, W. R., Snoeberger, R. C., III, Li, G., Richter, C., Allen, L. J., Milot, R. L.,
et al. (2009). Hydroxamate anchors for water-stable attachment to TiO2
nanoparticles. Energy Environ Sci, 2(11), 1173-1175.
Meyer, T. J. (2008). The art of splitting water. Nature, 451(7180), 778-779.
Mor, G. K., Shankar, K., Paulose, M., Varghese, O. K., & Grimes, C. A. (2005).
Enhanced Photocleavage of Water Using Titania Nanotube Arrays. Nano Lett,
5(1), 191-195.
Nanba, O., & Satoh, K. (1987). Isolation of a photosystem II reaction center
consisting of D-1 and D-2 polypeptides and cytochrome b-559. Proc Natl Acad
Sci U S A, 84(1), 109-112.
Nocera, D. G. (2012). The Artificial Leaf. Acc Chem Res, 45(5), 767-776.
Rivalta, I., Brudvig Gary, W., & Batista, V. S. (2012). Oxomanganese complexes for
natural and artificial photosynthesis. Current Opinion Chem Biol, 16, 11-18.
Ruettinger, W., & Dismukes, G. C. (1997). Synthetic Water-Oxidation Catalysts for
Artificial Photosynthetic Water Oxidation. Chem Rev, 97(1), 1-24.
Ruettinger, W., Yagi, M., Wolf, K., Bernasek, S., & Dismukes, G. C. (2000). O2
Evolution from the Manganese-Oxo Cubane Core Mn4O46+: A Molecular Mimic of
the Photosynthetic Water Oxidation Enzyme. J Am Chem Soc, 122(42), 1035310357.
Ruettinger, W. F., Campana, C., & Dismukes, G. C. (1997). Synthesis and
Characterization of Mn4O4L6 Complexes with Cubane-like Core Structure: A
New Class of Models of the Active Site of the Photosynthetic Water Oxidase. J
Am Chem Soc, 119(28), 6670-6671.
Sala, X., Romero, I., Rodriguez, M., Escriche, L., & Llobet, A. (2009). Molecular
catalysts that oxidize water to dioxygen. Angew Chem Int Ed, 48(16), 28422852.
Sartorel, A., Carraro, M., Scorrano, G., De Zorzi, R., Geremia, S., McDaniel, N. D.,
et al. (2008). Polyoxometalate Embedding of a Tetraruthenium(IV)-oxo-core by
Template-Directed Metalation of [-SiW10O36]8-: A Totally Inorganic OxygenEvolving Catalyst. J Am Chem Soc, 130(15), 5006-5007.
Sproviero, E. M., Gascon, J. A., McEvoy, J. P., Brudvig, G. W., & Batista, V. S.
(2008). Quantum Mechanics/Molecular Mechanics Study of the Catalytic Cycle of
Water Splitting in Photosystem II. J Am Chem Soc, 130(11), 3428-3442.
Umena, Y., Kawakami, K., Shen, J. R., & Kamiya, N. (2011). Crystal structure of
oxygen-evolving photosystem II at a resolution of 1.9 A. Nature, 473, 55-61.
Varghese, O. K., & Grimes, C. A. (2008). Appropriate strategies for determining the
photoconversion efficiency of water photoelectrolysis cells: A review with
examples using titania nanotube array photoanodes. Sol Energy Mater Sol Cells,
92(4), 374-384.
Wang, T., Brudvig, G. W., & Batista, V. S. (2010). Study of Proton Coupled Electron
Transfer in a Biomimetic Dimanganese Water Oxidation Catalyst with Terminal
Water Ligands. J Chem Theory Comput, 6(8), 2395-2401.
Wei, Z., Cady, C., Brudvig, G. W., & Hou, H. J. M. (2011). Photodamage of a
Mn(III/IV)-oxo mix valence compound and photosystem II complexes: Evidence
that high-valent manganese species is responsible for UV-induced photodamage
of oxygen evolving compelx in photosystem II. J Photochem Photobiol B, 104,
118-125.
Woodhouse, M., & Parkinson, B. A. (2008). Combinatorial approaches for the
identification and optimization of oxide semiconductors for efficient solar
photoelectrolysis. Chem Soc Rev, 38(1), 197-210.
Yang, X., Wolcott, A., Wang, G., Sobo, A., Fitzmorris, R. C., Qian, F., et al. (2009).
Nitrogen-Doped ZnO Nanowire Arrays for Photoelectrochemical Water Splitting.
Nano Lett, 9(6), 2331-2336.
Yano, J., Kern, J., Sauer, K., Latimer, M. J., Pushkar, Y., Biesiadka, J., et al. (2006).
Where water is oxidized to dioxygen: structure of the photosynthetic Mn4Ca
cluster. Science, 314(5800), 821-825.
Yin, Q., Tan, J. M., Besson, C., Geletii, Y. V., Musaev, D. G., Kuznetsov, A. E., et al.
(2010). A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based
on Abundant Metals. Science, 328(5976), 342-345.
Young, K. J., Gao, Y., & Brudvig Gary, W. (2011). Photocatalytic Water Oxidation
Using Manganese Compounds Immobilized in Nafion Polymer Membranes.
Australian J Chem, 64, 1219-1226.
Young, K. J., Martini, L. A., Milot, R. L., Snoeberger, R., Batisa, V. S.,
Schmuttenmaer, C., et al. (2012). Light-driven Water Oxidation for Solar Fuels.
Coord Chem Rev, 256, 2503-2520.
Youngblood, W. J., Lee, S.-H. A., Maeda, K., & Mallouk, T. E. (2009). Visible Light
Water Splitting using Dye-Sensitized Oxide Semiconductors. Acc Chem Res,
42(12), 1966-1973.
Zhang, F., Cady, C. W., Brudvig Gary, W., & Hou, H. J. M. (2011). Thermal Stability
of [Mn(III)(O)2Mn(IV)(H2O)2(Terpy)2](NO3)3 (Terpy = 2,2':6',2"-terpyridine) in
aqueous solution. Inorg Chim Acta, 366, 128-133.
Zhong, D. K., & Gamelin, D. R. (2010). Photoelectrochemical Water Oxidation by
Cobalt Catalyst "Co-Pi"/-Fe2O3 Composite Photoanodes: Oxygen Evolution and
Resolution of a Kinetic Bottleneck. J Am Chem Soc, 132(12), 4202-4207.