Article
pubs.acs.org/JPCC

Integration of Active Nickel Oxide Clusters by Amino Acids for Water
Oxidation
Masaaki Yoshida,*,†,‡,§ Sho Onishi,†,§ Yosuke Mitsutomi,† Futaba Yamamoto,† Masanari Nagasaka,#
Hayato Yuzawa,# Nobuhiro Kosugi,# and Hiroshi Kondoh††

Department of Chemistry, Keio University, Yokohama 223-8522, Japan
Cooperative Research Fellow, Institute for Catalysis, Hokkaido University, Hokkaido 060-0808, Japan#
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan‡S Supporting Information*

ABSTRACT: The move toward sustainable hydrogen production from water using renewable energy, a highly eﬃcient
oxygen evolution electrocatalyst, is crucial because watersplitting eﬃciency is restricted to the oxygen evolution
capability, which is insuﬃcient compared to the hydrogen
evolution reaction. Herein, we report a new method that
improves the oxygen evolution activity by integration of active
nickel oxide clusters using amino acids, meaning that theamount of electrodeposited nickel oxides is increasing withmaintaining the catalytic activity. This method enhances thecatalytic activity because the reaction sites drastically increasein three dimensions. The detailed reaction mechanism wasinvestigated using operando UV/vis absorption and Ni K-edge X-ray absorption spectroscopic techniques, which suggested thatamino acids such as glycine, alanine, and glutamine promoted the electrodeposition of NiO6 octahedral structure clusters.
Meanwhile, the analysis of N and O K-edge X-ray absorption spectra showed that the amino acid (glycine) in the nickel
electrocatalyst was present in the molecular state. Therefore, it was spectroscopically demonstrated that amino acids are bound tonickel oxide clusters accompanied by oxygen evolution activity.

1. INTRODUCTION
Electrochemical water splitting using renewable non-fossil fuelenergy sources such as sunlight, wind, and tides is a fascinating
candidate for hydrogen production toward a sustainable
society.1−6 This reaction consists of two half reactions ofhydrogen (2H+ + 2e− → H2) and oxygen (2H2O → O2 + 4H++ 4e−) evolution. However, the eﬃciency of the oxygen
evolution reaction (OER) on general electrode materials is
insuﬃcient to develop a commercial water-splitting system dueto the high overpotentials required to extract oxygen atoms
from water molecules.5−10 Therefore, the development ofeﬃcient OER electrocatalysts has received the most attention
owing to the potential toward highly eﬃcient hydrogenproduction.5−28
Generally, transition-metal oxides, such as MnOx,7−10
CoOx,11−17 and NiOx,18−21 are known to function as eﬃcient
OER electrocatalysts. Thus, substantial eﬀorts have been madeto develop OER electrocatalysts for improvement of the wateroxidation eﬃciency. For example, Nocera and colleagues
reported that cobalt−phosphate electrodeposited from a dilute
Co2+ solution in a phosphate-buﬀered electrolyte (Co−Pi)
functions as an eﬃcient OER electrocatalyst,11−14 as do similarmaterials of cobalt−borate (Co−Bi)11,13 and nickel−borate(Ni−Bi)18−21 complexes. Meanwhile, it was recently reported© 2016 American Chemical Societythat the addition of certain organic molecules to metal oxide
catalysts enhanced the OER activity.22−28 Singh et al. andHoang et al. proposed that the addition of diaminoethane, 3,5diamino-1,2,4-triazole, or its derivatives to the Ni2+ solution in aborate-buﬀered electrolyte enhanced the catalytic activity forwater oxidation because of the diﬀerence in ﬁlm morphology.22−24 Moreover, Allen et al. suggested the enhancement of
OER activity for NiOx electrocatalysts by amino acids (Ni−AA), and concluded that the presence of primary amines andphosphate is the key to the formation of the catalytically active
nickel hydroxide species.24 These OER catalysts are composedof inexpensive and earth-abundant materials and operate safely
with high activity. However, the exploration of more eﬃcient
catalysts is still imperative to attain sustainable hydrogenproduction.
The reaction rate of transition-metal oxides is not suﬃciently
high, although the water oxidation reaction proceeds at a small
overpotential.6 Thus, increasing the reaction sites is the key todrastically enhancing the water oxidation eﬃciency. To increase
the surface area of catalysts, various strategies have been applied
Received: August 31, 2016
Revised: November 12, 2016
Published: December 12, 2016
255

DOI: 10.1021/acs.jpcc.6b08796
J. Phys. Chem. C 2017, 121, 255−260

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Figure 1. (a) OER current densities at various electrode potentials and (b) time courses of the OER current densities for blank Ni (black), Ni−Gly(red), Ni−Ala (orange), Ni−Gln (green), Ni−AcGly (purple), Ni−EA (blue), and Ni−AcOH (brown) catalysts on the ITO substrates in a K−Pi
buﬀer of pH 11 containing Ni(NO3)2 and molecular compounds. (c) Schematic models for molecular compounds.

for development of the nanoparticle assemblies.29−35 For
example, the Nishihara group reported that Pd nanoparticle
assemblies can be formed using an organic linker with a rigid
tetrahedral core.34 Meanwhile, Martis et al. reported that Pd
nanoparticles can be immobilized within the pores of metal−organic frameworks (MOF).35 In such a situation, it wasrecently proposed that some OER electrocatalysts such as Co−
Pi, Co−Bi, and Ni−Bi are formed by assembly of nanosizedclusters of transition-metal oxides.11,17,19 The unique characteristics of these assemblies enable utilization of the thin ﬁlm
inside for water oxidation sites because the proton-coupledelectron hopping proceeds within the bulk hydrated oxideﬁlms.14,15,20 Therefore, an assembly technique to eﬃciently
utilize the catalyst inside is of critical importance to develop
three-dimensional (3D) functional materials with many
reaction sites.
Here, we successfully demonstrated that amino acids
integrate nickel oxide clusters in the Ni−AA electrocatalysts
to drastically increase the number of reaction sites for OER
activity. Operando observation under catalysis using diﬀerential
UV/vis absorption and Ni K-edge X-ray absorption ﬁne
structure (XAFS) measurements supported the proposal
concerning integration of nickel oxide clusters because theelectrodeposition of nickel oxide species with the NiO6
structure was promoted by amino acids. N and O K-edge
XAFS spectra also implied that the nitrogen and oxygen speciesof the amino acid (glycine) in the nickel electrocatalyst were
present in a similar chemical state as the isolated amino acidmolecule. In this report, we discuss the detailed mechanism forthe water oxidation reaction.

containing 0.4 mM Ni(NO3)2 and 1.6 mM amino acids (Ni−AA) such as glycine (Ni−Gly), alanine (Ni−Ala), andglutamine (Ni−Gln) (none: blank Ni). To compare withother molecular compounds, ethylamine (Ni−EA), acetylglycine (Ni−AcGly), and acetic acid (Ni−AcOH) were also usedinstead of amino acids. The prepared Ni−Gly sample wasanalyzed using scanning electron microscopy (SEM), X-ray
photoelectron spectroscopy (XPS), and a surface proﬁlometer,
conﬁrming that the substrate was totally covered with Ni−Gly
at a ﬁlm thickness of ca. 100 nm with elemental composition of
Ni, N, K, and P (Figures S1−S3 of the Supporting
Information).
Operando diﬀerential UV/vis absorption spectra wereobtained in diﬀuse transmission mode using a UV/vis
spectrometer, according to previous work.36 The operando Ni
K-edge XAFS spectra were obtained in ﬂuorescence mode at
BL-9A in the Photon Factory (PF) and at BL01B1 in the
SPring-8 facility, in accordance with previous work.37 Soft X-ray
XAFS measurements were performed in the total electron yieldmode at the BL3U of the UVSOR Synchrotron and in the
ﬂuorescence yield mode at the BL-7A in the PF. Additional
experimental details are included in the Supporting Information.

3. RESULTS AND DISCUSSION
Time courses of current densities for Ni−Gly in a K−Pi
aqueous electrolyte containing Ni(NO3)2 and glycine areshown in Figure 1a. The OER current of Ni−Gly was higher
than that of the blank Ni at all electrode potentials, whichindicates that the glycine molecule enhances the catalytic OER
activity, as previously reported.25 Moreover, the OER currentsof Ni−Ala, Ni−Gln, and Ni−Gly also increased (Figure 1b),
indicating that amino acids in the Ni−AA improve the OER
activity. Considering that the currents of Ni−EA, Ni−AcGly,
and Ni−AcOH were almost the same as that of blank Ni, thepresence of NH2 and COOH groups in the amino acids is the
key for the enhancement of OER activity (Figure 1c). The
OER current was maintained for at least 3 h, indicating that the

2. EXPERIMENTAL SECTIONA Teﬂon electrochemical cell was equipped with a Pt wirecounter electrode and a Ag/AgCl (saturated KCl) referenceelectrode for all electrochemical experiments. Ni-AA thin ﬁlms
were electrodeposited on an indium tin oxide (ITO) or a Au
thin ﬁlm by controlled potential electrolysis at 1.0 V in a 0.1 M
potassium phosphate (K−Pi) aqueous electrolyte of pH 11
256

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Figure 2. Operando (a) diﬀerential UV/vis absorption against 0.0 V of reference electrode voltage (inset: photographic images (see Figure S6)) and(b) Ni K-edge XAFS spectra (inset: estimation of nickel valence state using half edge energy (NiPPI: nickel potassium paraperiodate)) after 1 h
maintained at 1.0 V in a K−Pi buﬀer of pH 11 containing Ni(NO3)2 and various molecular compounds. (c) k3-weighted and (d) FT EXAFS spectraof γ-NiOOH powder (black) and Ni−Gly (red) catalysts maintained at 1.0 V.

electrode surface but also in the thin ﬁlm inside as 3D
assembled electrocatalysts (Figure S7), as described previously.14,15,20 The average nickel valences were roughly
estimated to be ca. +3.1 to +3.3 for Ni−AA from the halfedge energies of nickel reference samples, similar to those of βNiOOH (Ni3.0+) and γ-NiOOH (Ni3.6+) materials (inset of
Figure 2b).
To investigate the local structure of Ni species in the Ni−
Gly, extended X-ray absorption ﬁne structure (EXAFS) spectra
were taken for the Ni−Gly sample as shown in Figure 2c,
together with the γ-NiOOH powder reference. The ﬁrst (Ni−O) and second (Ni−Ni) peaks for Ni−Gly were observed at
the same peak position and intensity as those of the γ-NiOOH
reference (Figure 2d), which showed that the local structure of
Ni species in the Ni−Gly was composed of an edge-sharing
NiO6 octahedral (γ-NiOOH) structure. In similar work, thenickel species in the Ni−Bi electrocatalyst at higher potential
has been reported to be a nanosized NiO6 cluster18,19 because
the second peak (Ni−Ni) intensity for Ni−Bi was weaker thanthat for γ-NiOOH and no peak was observed above 3 Å in the
EXAFS spectra. However, in this study, considering that all
peak intensities for Ni−O and Ni−Ni bonds of Ni−Gly were
the same as those for γ-NiOOH, the NiO6 cluster is likely to beover a few nanometers in size. The SEM image of Ni−Glysuggests that the NiO6 clusters are a few hundred nanometersin size (Figure S1).
The XAFS technique using soft X-rays is a powerful tool toinvestigate electronic states composed of light elements such as
C, N, and O species.41 Recently, we reported that NiO6octahedral domains in Ni−Bi can be detected by O K-edge
XAFS.42 Thus, to learn about the glycine molecule, we
observed N and O K-edge XAFS spectra for Ni−Gly underultrahigh vacuum using ﬂuorescence and electron yield modes,
together with those for glycine and NiOOH (Figure 3). In the
N K-edge XAFS spectrum using the electron yield mode, twopeaks associated with nitrogen species were observed at ca.

Ni−AA catalysts can function as stable OER catalysts (FigureS4). The diﬀerential molar amount of the oxidation reaction
estimated from the diﬀerential currents between Ni−Gly andblank Ni catalysts for 3 h (ca. 74 μmol) was markedly higher
than those of nickel ion (ca. 8 μmol) and glycine molecule (ca.
32 μmol) in the initial electrolyte solution, conﬁrming that
OER on the Ni−AA catalysts proceeded catalytically without
the decomposition of glycine by the electrode potential. The
Faradaic eﬃciency for the Ni−Gly catalyst during steady state
was estimated to be ca. 96%, as shown in Figure S5, meaningthat the observed oxidation currents for the Ni−AA catalysts
are almost all used for the OER process.
Figure 2a shows the operando diﬀerential UV/vis absorption
spectra at 1.0 V for 1 h in a 0.1 M K−Pi electrolyte containing
Ni(NO3)2 and various molecular compounds. A broad
absorption peak was observed for all samples, indicating thatnickel oxide species (probably NiOOH, see below) were
electrodeposited on the ITO substrates. The peak intensities of
UV/vis absorption spectra for Ni−Gly, Ni−Ala, and Ni−Glnwere considerably higher than those of Ni−AcGly, Ni−EA,
Ni−AcOH, and blank Ni samples. These results suggest thatthe electrodeposition of the samples proceeded for the Ni−AA
samples but did not proceed for the other samples. This can be
conﬁrmed by the photographic image of Ni−AA electrocatalysts, as shown in Figure S6. Similar results by electrochemical quartz crystal microbalance (EQCM) are obtained fornickel and copper oxide ﬁlms using glycine molecule as asemiconductor and electrochromic material by Ogura etal.38−40 The same behavior in UV/vis absorption spectra wasobserved for the operando Ni K-edge XAFS measurements, as
shown in Figure 2b. Considering that XAFS intensity is
generally related to the abundance of the target element, it isconcluded that amino acids integrate the NiO6 clusters in the
Ni−AA using nickel ion in the electrolyte. The OER activity
increased little by little upon increasing the ﬁlm thickness,
indicating that the OER reaction proceeds not only at the257

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investigation, it is likely to be related to complex formation
between Ni ion and amino acids in the electrolyte solution(Figure S9). From some previous reports, the domain edge ofNiO6 octahedra is known to function as an active wateroxidation site.20,41 Therefore, we believe that the integration of
NiO6 octahedral clusters increases and extends the number ofreaction sites for OER to three dimensions. The Ni−AA is
likely to function not only at the electrode surface but also in
the thin ﬁlm inside as a 3D assembled electrocatalyst (Figure4b), as described in similar work.14,15,20 Further investigation isin progress.
Transition-metal oxides such as MnOx, CoOx, and NiOx are
generally considered to exhibit high OER activity.5−28
However, there is a disadvantage that OER current density is
low in spite of the small overvoltage, compared with Pt
electrodes. Our present technique shows that 3D assemblies oftransition-metal oxides using amino acids are fascinating
candidates to drastically increase the amount of reaction sites.
This method is applied not only to amino acids but also tosome other organic molecules. (Further investigation is inprocess.). The assembly technique of OER materials discoveredby this study has provided the ﬁrst successful proof-of-concept
and there is the potential for it to be applied to many other
OER electrocatalysts composed of transition-metal oxides suchas MnOx, CoOx, and NiOx to increase the reaction sites for the
OER process. The knowledge obtained here will be useful todevelop more eﬃcient OER catalysts toward sustainable watersplitting systems.
In conclusion, the function of amino acids was investigatedby several spectroscopic techniques. Operando diﬀerential UV/
vis absorption spectroscopy showed that the amino acids
function to increase the amount of electrodeposited nickel
species on the electrode surface. The result of operando Ni Kedge XAFS measurements showed that the local structure of
the nickel species was composed of NiO 6 (NiOOH).
Moreover, the observation of N and O K-edge XAFS suggested
that the amino acid (glycine) in the nickel electrocatalyst
maintained a similar chemical state with the amino acidmolecule. On the basis of these studies, it was demonstrated
that each nickel oxide cluster is combined with amino acids andthe integration of active nickel oxide clusters is attained with alarge amount of interface area between nickel oxide clusters andwater molecules, accompanied by highly eﬃcient OER activity.

Figure 3. (a) N K-edge and (b) O K-edge XAFS spectra for the Ni−
Gly electrocatalyst. It should be noted that the N K-edge XAFS
spectrum for glycine molecule with electron yield mode was slightly
distorted by the charge-up eﬀect.

401.0 and 405.2 eV for Ni−Gly, which were similar to those in
the glycine molecule reference and previous work,43 indicatingthat Ni−Gly includes the glycine molecule. In the O K-edge
XAFS measurements using the electron yield mode, two peaks
were observed at ca. 528.6 eV for NiOOH and 532.4 eV for
glycine. The presence of glycine molecule is likely to contributeto the combination with each NiO6 cluster by binding with
NH2 and COOH groups. However, although the glycine
species was also observed by X-ray photoelectron spectroscopy(XPS) measurements (Figure S3), it was not observed by XAFS
using ﬂuorescence yield (Figure 3) mode and X-ray
ﬂuorescence analysis (Figure S8), indicating that the glycine(amino acid) molecule is present only near the electrodesurface (typically within 10 nm) of the OER thin ﬁlm. On thebasis of the XPS results of nickel and nitrogen species, theatomic ratio of Ni:N was ca. 10:3, indicating that several glycine
molecules are likely to cover the NiO6 clusters near theelectrode surface.
Figure 4 shows a schematic model for the electrodeposition
and water oxidation of the Ni−AA electrocatalyst. The positive
potential supply to the Au electrode results in generation of
Ni−AA with a NiO6 cluster of octahedral structure by theelectrodeposition of nickel ion in the electrolyte (Figure 4a).
The amino acid combines the NiO6 clusters by binding with
NH2 and COOH groups accompanied by OER activity. In
contrast, molecular compounds without NH2 and COOH
groups did not exhibit the binding function between nanoclusters. Although the mechanism of catalyst assembly is under

Figure 4. Proposed schematic model of (a) the formation of the Ni−AA electrocatalyst and (b) the water oxidation reaction processes. Nickel ion
was integrated as active nickel oxide clusters by amino acids for water oxidation catalysis.
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The Journal of Physical Chemistry C■(9) Huynh, M.; Bediako, D. K.; Nocera, D. G. Functionally Stable
Manganese Oxide Oxygen Evolution Catalyst in Acid. J. Am. Chem.
Soc. 2014, 136, 6002−6010.
(10) Huynh, M.; Shi, C.; Billinge, S. J. L.; Nocera, D. G. Nature of
Activated Manganese Oxide for Oxygen Evolution. J. Am. Chem. Soc.
2015, 137, 14887−14904.
(11) Bediako, D. K.; Ullman, A. M.; Nocera, D. G. Catalytic Oxygen
Evolution by Cobalt Oxido Thin Films. Top. Curr. Chem. 2015, 371,
173−213.
(12) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+.
Science 2008, 321, 1072−1075.
(13) Surendranath, Y.; Dincǎ, M.; Nocera, D. G. ElectrolyteDependent Electrosynthesis and Activity of Cobalt-Based Water
Oxidation Catalysts. J. Am. Chem. Soc. 2009, 131, 2615−2620.
(14) Bediako, D. K.; Costentin, C.; Jones, E. C.; Nocera, D. G.;
Savéant, J.-M. Proton−Electron Transport and Transfer in Electrocatalytic Films. Application to a Cobalt-Based O2-Evolution Catalyst. J.
Am. Chem. Soc. 2013, 135, 10492−10502.
(15) Klingan, K.; Ringleb, F.; Zaharieva, I.; Heidkamp, J.; Chernev,
P.; Gonzalez-Flores, D.; Risch, M.; Fischer, A.; Dau, H. Water
Oxidation by Amorphous Cobalt-Based Oxides: Volume Activity andProton Transfer to Electrolyte Bases. ChemSusChem 2014, 7, 1301−
1310.
(16) González-Flores, D.; Sánchez, I.; Zaharieva, I.; Klingan, K.;
Heidkamp, J.; Chernev, P.; Menezes, P. W.; Driess, M.; Dau, H.;
Montero, M. L. Heterogeneous Water Oxidation: Surface Activity
versus Amorphization Activation in Cobalt Phosphate Catalysts.
Angew. Chem. 2015, 127, 2502−2506.
(17) Risch, M.; Ringleb, F.; Kohlhoff, M.; Bogdanoff, P.; Chernev, P.;
Zaharieva, I.; Dau, H. Water Oxidation by Amorphous Cobalt-Based
Oxides: In Situ Tracking of Redox Transitions and Mode of Catalysis.
Energy Environ. Sci. 2015, 8, 661−674.
(18) Risch, M.; Klingan, K.; Heidkamp, J.; Ehrenberg, D.; Chernev,
P.; Zaharieva, I.; Dau, H. Nickel-oxido Structure of a Water-oxidizing
Catalyst Film. Chem. Commun. 2011, 47, 11912−11914.
(19) Bediako, D. K.; Lassalle-Kaiser, B.; Surendranath, Y.; Yano, J.;
Yachandra, V. K.; Nocera, D. G. Structure-activity Correlations in a
Nickel-borate Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2012, 134,
6801−6809.
(20) Bediako, D. K.; Surendranath, Y.; Nocera, D. G. Mechanistic
Studies of the Oxygen Evolution Reaction Mediated by a Nickel−
Borate Thin Film Electrocatalyst. J. Am. Chem. Soc. 2013, 135, 3662−
3674.
(21) Dincă, M.; Surendranath, Y.; Nocera, D. G. Nickel-Borate
Oxygen-Evolving Catalyst That Functions under Benign Conditions.
Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 10337−10341.
(22) Singh, A.; Chang, S. L. Y.; Hocking, R. K.; Bach, U.; Spiccia, L.
Anodic Deposition of NiOx Water Oxidation Catalysts fromMacrocyclic Nickel(ii) Complexes. Catal. Sci. Technol. 2013, 3,
1725−1732.
(23) Singh, A.; Chang, S. L. Y.; Hocking, R. K.; Bach, U.; Spiccia, L.
Highly Active Nickel Oxide Water Oxidation Catalysts Deposited from
Molecular Complexes. Energy Environ. Sci. 2013, 6, 579−586.
(24) Wang, D.; Ghirlanda, G.; Allen, J. P. Water Oxidation by a
Nickel-Glycine Catalyst. J. Am. Chem. Soc. 2014, 136, 10198−10201.
(25) Hoang, T. T. H.; Gewirth, A. High Activity Oxygen Evolution
Reaction Catalysts from Additive-Controlled Electrodeposited Ni and
NiFe Films. ACS Catal. 2016, 6, 1159−1164.
(26) Lu, C.; Wang, J.; Chen, Z. Water Oxidation by Copper−Amino
Acid Catalysts at Low Overpotentials. ChemCatChem 2016, 8, 2165−
2170.
(27) Yu, F.; Li, F.; Zhang, B.; Li, H.; Sun, L. Efficient Electrocatalytic
Water Oxidation by a Copper Oxide Thin Film in Borate Buffer. ACS
Catal. 2015, 5, 627−630.
(28) Li, T.-T.; Cao, S.; Yang, C.; Chen, Y.; Lv, X.-J.; Fu, W.-F.
Electrochemical Water Oxidation by In Situ-Generated Copper Oxide
Film from [Cu(TEOA)(H2O)2][SO4] Complex. Inorg. Chem. 2015,
54, 3061−3067.

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.6b08796.
Detailed experimental procedures, SEM images, ﬁlm
thickness, XPS spectra, stability of catalyst, detection of
evolved gas, photographic image, relationship between
ﬁlm thickness and catalysis, X-ray ﬂuorescence, andnickel complex with glycine molecule (PDF)■

AUTHOR INFORMATION

Corresponding Author*Tel.: +81-45-566-1592. Fax: +81-45-566-1697. E-mail:yoshida@chem.keio.ac.jp.
ORCID

Masaaki Yoshida: 0000-0002-3656-4444
Author Contributions§

These authors contributed equally.

Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was performed at the UVSOR Synchrotron (28-205,
27-218), the PF (2014G539, 2015G629), and SPring-8(2015B1082). This work was supported by Keio Kogakukai
Fund, Keio Gijuku Koizumi Memorial Fund, The Sumitomo
Foundation (150050), Tonen General Sekiyu Foundation,
Iketani Science and Technology Foundation (0281023-A), andCooperative Research Program of Hokkaido University(14A1004, 15B1002, 16B1009). The authors would like tothank M. Kawamura, M. Sasai, D. Kim, T. Hiue, H. Kurosu, and
the Workshop in Keio University for experimental support, andProf. Asakura at Hokkaido University, Prof. Amemiya at KEK,
and Dr. Ina at JASRI for valuable discussions.

■

REFERENCES(1) Christopher, K.; Dimitrios, R. A Review on Energy Comparisonof Hydrogen Production Methods from Renewable Energy Sources.
Energy Environ. Sci. 2012, 5, 6640−6651.
(2) Dincer, I.; Acar, C. Review and Evaluation of Hydrogen
Production Methods for Better Sustainability. Int. J. Hydrogen Energy
2015, 40, 11094−11111.
(3) Tachibana, Y.; Vayssieres, L.; Durrant, J. R. Artificial Photosynthesis for Solar Water-splitting. Nat. Photonics 2012, 6, 511−518.
(4) Hisatomi, T.; Kubota, J.; Domen, K. Recent Advances in
Semiconductors for Photocatalytic and Photoelectrochemical Water
Splitting. Chem. Soc. Rev. 2014, 43, 7520−7535.
(5) Nocera, D. G. The Artificial Leaf. Acc. Chem. Res. 2012, 45, 767−
776.
(6) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi,
Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev.
2010, 110, 6446−6473.
(7) Najafpour, M. M.; Renger, G.; Hołyńska, M.; Moghaddam, A. N.;
Aro, E.-M.; Carpentier, R.; Nishihara, H.; Eaton-Rye, J. J.; Shen, J.-R.;
Allakhverdiev, S. I. Manganese Compounds as Water-Oxidizing
Catalysts: From the Natural Water-Oxidizing Complex to Nanosized
Manganese Oxide Structures. Chem. Rev. 2016, 116, 2886−2936.
(8) Robinson, D. M.; Go, Y. B.; Mui, M.; Gardner, G.; Zhang, Z.;
Mastrogiovanni, D.; Garfunkel, E.; Li, J.; Greenblatt, M.; Dismukes, G.
C. Photochemical Water Oxidation by Crystalline Polymorphs ofManganese Oxides: Structural Requirements for Catalysis. J. Am.
Chem. Soc. 2013, 135, 3494−3501.
259

DOI: 10.1021/acs.jpcc.6b08796
J. Phys. Chem. C 2017, 121, 255−260

Article

The Journal of Physical Chemistry C(29) Wang, T.; LaMontagne, D.; Lynch, J.; Zhuang, J.; Cao, Y. C.
Colloidal Superparticles from Nanoparticle Assembly. Chem. Soc. Rev.
2013, 42, 2804−2823.
(30) Min, Y.; Akbulut, M.; Kristiansen, K.; Golan, Y.; Israelachvili, J.
The Role of Interparticle and External Forces in Nanoparticle
Assembly. Nat. Mater. 2008, 7, 527−538.
(31) Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzán, L. M.
Directed Self-Assembly of Nanoparticles. ACS Nano 2010, 4, 3591−
3605.
(32) Wang, L.; Xu, L.; Kuang, H.; Xu, C.; Kotov, N. A. Dynamic
Nanoparticle Assemblies. Acc. Chem. Res. 2012, 45, 1916−1926.
(33) Shenhar, R.; Norsten, T. B.; Rotello, V. M. Polymer-MediatedNanoparticle Assembly: Structural Control and Applications. Adv.
Mater. 2005, 17, 657−669.
(34) Yamanoi, Y.; Yamamoto, Y.; Miyachi, M.; Shimada, M.; Minoda,A.; Oshima, S.; Kobori, Y.; Nishihara, H. Nanoparticle Assemblies via
Coordination with a Tetrakis(terpyridine) Linker Bearing a Rigid
Tetrahedral Core. Langmuir 2013, 29, 8768−8772.
(35) Martis, M.; Mori, K.; Fujiwara, K.; Ahn, W.-S.; Yamashita, H.
Amine-Functionalized MIL-125 with Imbedded Palladium Nanoparticles as an Efficient Catalyst for Dehydrogenation of Formic Acid
at Ambient Temperature. J. Phys. Chem. C 2013, 117, 22805−22810.
(36) Takashima, T.; Hashimoto, K.; Nakamura, R. Mechanisms of
pH-dependent Activity for Water Oxidation to Molecular Oxygen by
MnO2 Electrocatalysts. J. Am. Chem. Soc. 2012, 134, 1519−1527.
(37) Yoshida, M.; Mineo, T.; Mitsutomi, Y.; Yamamoto, F.; Kurosu,
H.; Takakusagi, S.; Asakura, K.; Kondoh, H. Structural Relationshipbetween CoO6 Cluster and Phosphate Species in a Cobalt-Phosphate
Water Oxidation Catalyst Investigated by Co and P K-edge XAFS.
Chem. Lett. 2016, 45, 277−279.
(38) Nakaoka, K.; Ueyama, J.; Ogura, K. Semiconductor andElectrochromic Properties of Electrochemically Deposited Nickel
Oxide Films. J. Electroanal. Chem. 2004, 571, 93−99.
(39) Nakaoka, K.; Ogura, K. Electrochemical Preparation of p-type
Cupric and Cuprous Oxides on Platinum and Gold Substrates fromCopper (II) Solutions with Various Amino Acids. J. Electrochem. Soc.
2002, 149, C579−C585.
(40) Ogura, K.; Nakaoka, K.; Nakayama, M.; Tanaka, S. A New Typeof Electrochemical Formation of Copper Oxide during Redox
Processes of the Copper(II)−Glycine Complex at High pH. J.
Electroanal. Chem. 2001, 511, 122−127.
(41) Yoshida, M.; Mitsutomi, Y.; Mineo, T.; Nagasaka, M.; Yuzawa,
H.; Kosugi, N.; Kondoh, H. Direct Observation of Active Nickel Oxide
Cluster in Nickel−Borate Electrocatalyst for Water Oxidation by In
Situ O K-Edge X-ray Absorption Spectroscopy. J. Phys. Chem. C 2015,
119, 19279−19286.
(42) Chen, J. G. NEXAFS Investigations of Transition Metal Oxides,
Nitrides, Carbides, Sulfides and Other Interstitial Compounds. Surf.
Sci. Rep. 1997, 30, 1−152.
(43) Zubavichus, Y.; Zharnikov, M.; Schaporenko, A.; Grunze, M.
NEXAFS Study of Glycine and Glycine-based Oligopeptides. J.
Electron Spectrosc. Relat. Phenom. 2004, 134, 25−33.

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DOI: 10.1021/acs.jpcc.6b08796
J. Phys. Chem. C 2017, 121, 255−260