Hexagonal nanoplates of NiO/CoO/Fe2O3 composite acting as an efficient photocatalytic and electrocatalytic water oxidation catalyst

Article abstract of DOI:10.1039/c5dt02046h

A unique hexagonal sheet-shaped NiO/CoO/Fe₂O₃ composite with irregularly shaped nanoparticles was fabricated for the first time through a simple co-precipitation and hydrothermal method. The NiO/CoO/Fe₂O₃ composite was characterized by numerous techniques (TEM, HRTEM, PXRD, EDX, ICP-AES, BET, and XPS) to confirm its structure and composition. This structure of the NiO/CoO/Fe₂O₃ composite may enhance the photocatalytic and electrocatalytic performance for water oxidation. Compared with NiO, CoO and Fe₂O₃, the NiO/CoO/Fe₂O₃ composite exhibits a lower overpotential and a much smaller Tafel slope of 49 mV dec^-1 for water oxidation. At the same time, the composite possesses beneficial ferromagnetic properties and superior stability; thus, it can be used repeatedly without any loss in activity.

Full text of DOI:10.1039/c5dt02046h

View Article Online
View Journal
Dalton
Transactions
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Y. Zhao, Y. Zhang,
Y. Ding and M. Chen, Dalton Trans., 2015, DOI: 10.1039/C5DT02046H.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/dalton

Please do not adjust margins
Dalton Transactions
Page 1 of 9
View Article Online
DOI: 10.1039/C5DT02046H
Journal Name
ARTICLE
Hexagonal nanoplates of NiO/CoO/Fe2O3 composite acting as
efficient photocatalytic and electrocatalytic water oxidation
catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Yukun Zhaoa, Yan Zhanga, Yong Dinga,b*and Mindong Chen b
DOI: 10.1039/x0xx00000x
A unique hexagonal sheet-shaped NiO/CoO/Fe2O3 composite with disordered shaped nanoparticles was fabricated for the
first time through a simple co-precipitation and hydrothermal method. The NiO/CoO/Fe2O3 composite was characterized
by multiple experiments (TEM, HRTEM, PXRD, EDX, ICP-AES, BET, XPS) to confirm the structure and the component. This
structure of NiO/CoO/Fe2O3 composite may strengthen photocatalytic and electrocatalytic performance of water oxidation.
Compared with NiO, CoO and Fe2O3, NiO/CoO/Fe2O3 composite exhibits lower overpotential and a much smaller Tafel
slope of 49 mV dec-1 in water oxidation. At the same time, the composite possesses beneficial ferromagnetic properties
and superior stability so that it was used repeatedly without the loss in activity.
www.rsc.org/
finite supplies make them not suitable for large-scale
application.19 Especially in recent years, copious amounts of
non-noble metal oxides have been reported as efficient WOCs
to replace expensive compounds based on iridium and
Introduction
Artificial photosynthesis that directly converts solar energy
into chemical energy is one of the most promising systems for
satisfying humans’ thirsty of renewable clean energy.1-4 One
convenient way of energy conversion is to split water which
can produce hydrogen and oxygen. In the process of water
20-22
ruthenium. Among which, WOCs containing nickel10,
,
cobalt23-27 and iron28-31 attract significant attentions on
account of their earth-abundance and robust activity.
However, NiO, CoO and Fe2O3 without any further
modification exhibit poor water oxidation activity. To improve
their catalytic activity and further simplify synthesis of those
WOCs, a mixed-metal composite which is composed of nickel,
cobalt and iron should be manufactured to reinforce their
photocatalytic and electrocatalytic activity.
splitting, the oxidation of water to O2 (2H2O
→
4H+ + 4e- + O2)
is a more complicated reaction as four electrons are needed
for the formation of an oxygen molecule.5, 6 Compared to the
reduction of water to hydrogen, the water oxidation process
usually holds sluggish kinetics and requires large
overpotentials.7 In order to improve the reaction rate and/or
lower the overpotentials, tremendous water oxidation
catalysts (WOCs) should be exploited.8-11 Therefore, extensive
efforts have so far been devoted to developing efficient WOCs
which can convert solar energy into chemical energy.
Herein, we firstly report the synthesis of mixed-metal oxides
NiO/CoO/Fe2O3 composite and the application as a highly
active and robust catalyst for visible light-driven water
2+
2-
oxidation with Ru(bpy)3 as a photosensitizer and S2O8 as a
sacrificial oxidant, respectively. The NiO/CoO/Fe2O3 composite
exhibits a unique hexagonal sheet-shaped morphology with
some disordered shaped nanoparticles. The special structure
generates surface charge effect which can strengthen the
photocatalytic activity through accelerating electron transfer.15,
Since oxido bridged calcium and manganese cubical
subcluster was reported as the active site of water oxidation in
the photosystem
Ⅱ (PS Ⅱ)
12, 13, multifarious homogeneous
and heterogeneous catalysts based on precious metals such as
Ir and Ru were reported by many groups14-18. Although these
16, 32-34
Compared with that of simple oxides NiO, CoO and
Fe2O3, the photocatalytic activity of NiO/CoO/Fe2O3 composite
is the highest. At the same time, NiO/CoO/Fe2O3 composite
shows a strong electrochemical activity with an extraordinary
Tafel slope of 49 mV dec-1 in water oxidation. Additionally, the
ferromagnetic properties of NiO/CoO/Fe2O3 composite are so
beneficial that the catalyst can be recovered from the reaction
solution without any obvious loss. In the future, there will be a
widespread prospect in industrial application.
catalysts behaved vigorous catalytic activity in PS
Ⅱ, their
Results and discussion
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 2 of 9
View Article Online
DOI: 10.1039/C5DT02046H
ARTICLE
Journal Name
Scheme 2. Synthesis process for NiO/CoO/Fe2O3 composite.
irradiation without catalyst of NiO/CoO/Fe2O3 composite, the
degradation of photosensitizer reaches to about 30%.
However, the photosensitizer only has less than 10%
degradation in the presence of catalyst, indicating the catalyst
(NiO/CoO/Fe2O3 composite) can stabilize the photosensitizer
to make it recycle efficiently (Scheme 1).
Scheme 1. Catalytic cycle of visible light-driven water oxidation with Na2S2O8 and
Ru(bpy)32+ using NiO/CoO/Fe2O3 composite as catalyst.
The photocatalytic activity of the mixed-metal oxides was
performed under visible light-driven water oxidation with
Ru(bpy)3Cl2 as a photosensitizer and Na2S2O8 as two-electron
oxidant, respectively. The overall catalytic cycle of the visible
light-driven water oxidation is depicted in Scheme 1. At first,
The mixed-metal oxides composite (NiO/CoO/Fe2O3) was
synthesized for the first time according to a simple co-
precipitation and hydrothermal method (Scheme 2). Fig. 2
elucidates the transmission electron microscope (TEM) image
of the sample, from which unique hexagonal sheet-shaped
morphology with some disordered shaped nanoparticles can
be observed. Moreover, the sizes of hexagonal nanoplates are
approximately 100–200 nm, on which surface there are many
nanoparticles about 10-30 nm. The different sizes and
morphologies nanoparticles represent different metal oxides.
The metal oxides adhere to each other and generate surface
charge effect which makes great contributions to electron
transfer.32, 37 Fig. 3(a) manifests a high-resolution TEM (HRTEM)
image of a piece of a cracked NiO/CoO/Fe2O3 nanoplate, which
further reveals the lattice planes with fringe spacing of 0.21,
0.28 and 0.30 nm corresponding to the (200) plane of NiO, the
(100) plane of CoO and the (220) plane of Fe2O3, respectively.
two equiv. of photosensitizers, 2 Ru(bpy)32+, which response to
2+
the visible light, absorb two photons and form 2 Ru*(bpy)3
(
where the * denotes the excited states). The excited states of
photosensitizers can be oxidized by the oxidant of Na2S2O8,
2-
•-
forming Ru(bpy)33+, SO4 and SO4•-. The produced SO4 which
is known to be a very strong oxidant [E0(SO4•-/SO42-) = 2.6 V]35
2+
can oxidize Ru(bpy)3 to generate two equiv. of Ru(bpy)3
3+
.
3+
Eventually, Ru(bpy)3 can oxidize water to evolve O2 in the
3+
presence of the WOCs. The Ru(bpy)3 cannot steadily exist so
it will rapidly and irreversibly decompose to inactive products
in the absence of catalysts(Fig. 1). The competitive
degradation of Ru(bpy)32+ appears to be dominant especially at
the absence of catalyst where the bimolecular scavenging of
the reactive Ru(bpy)33+ state is slowed down.36 After the
Fig. 1 UV-Vis absorption of a freshly prepared reaction solution before irradiation (black
trace), after irradiation without catalyst (red trace) and after irradiation with catalyst
(blue trace).
Fig. 2 Transmission electron microscope (TEM) image of NiO/CoO/Fe2O3
composite.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 3 of 9
View Article Online
DOI: 10.1039/C5DT02046H
Journal Name
ARTICLE
Table 1. Apparent turnover frequency (TOF) normalized by a catalyst surface area
and oxygen yield (O2 yield) in photocatalytic water oxidation with different catalysts.a
a
c
b
Catalysts
Apparent TOFb
(μmol s-1 m-2)
O2 yield (%) c
Ref.
NiO
(200)
d = 0.21 nm
NiO/CoO/Fe2O3
NiO
0.20
0.06
-
64
25
45
34
47
52
34
This work
This work
This work
This work
This work
38
CoO
Fe2O3
0.19
-
NiO+CoO+Fe2O3
γ-Fe2O3@NiO
γ-Fe2O3@NiO
0.19
0.29
38
d
a Conditions: LED lamp (≥420 nm), 15.8 mW; 1.0 mM Ru(bpy)3Cl2, 5.0 mM
Na2S2O8, 0.33 g L-1 NiO/CoO/Fe2O3 composite, 80 mM sodium borate buffer
(initial pH 8.5); total reaction volume 15 mL and head space volume 13.5 mL;
vigorous agitation using a magnetic stirrer. b Turnover frequency normalized by
a catalyst surface area for O2 evolution. c Yield is defined as twice the number
of moles of O2 per mole of Na2S2O8.
d = 0.30 nm
(220)
CoO
(100)
Fe2O3
d = 0.28 nm
Fig. 3 High resolution TEM images of NiO/CoO/Fe2O3 composite.
The more distinct fringe spacing of NiO lattice plane and Fe2O3
lattice plane can be observed in Fig. 3(b) and Fig. 3(d).
Although the fringe spacing of CoO lattice plane is some vague,
the fringe spacing can be still obtained from the HRTEM image
(Fig. 3c). The vague fringe spacing of CoO (100) plane may be
assigned to the altitude of TEM detection probe.
The catalyst was characterized by the powder X-ray
diffraction (PXRD) measurement as shown in Fig. 4. The
dominated peaks of the catalyst are in accordance with the
characteristic patterns of NiO, CoO and Fe2O3. The (200) plane
of NiO, the (100) plane of CoO and the (220) plane of Fe2O3
can agree well with the planes of HRTEM observed. Fig. S1
(ESI†) demonstrates the energy dispersive X-Ray (EDX)
spectrum which indicates the catalyst is composed of the
elements Ni, Co, Fe and O. Element mapping characterizations
Fig. 5 Time courses of O2 evolution under photoirradiation (LED lamp, λ ≥420 nm,
15.8 W) of a borate buffer solution (pH 8.5, 15.0 mL) containing Na2S2O8 (5.0
mM), Ru(bpy)3Cl2 (1.0 mM).
(Fig. S2, ESI†) were carried out to further investigate the main
constituent of the samples. The data of EDX spectrum
manifests that the ratio of these atoms (Ni: Co: Fe) is
approximately 1.1: 0.9: 1. The radio of these atoms is also
authenticated through X-ray photoelectron spectra (XPS) and
inductively coupled plasma atomic emission spectroscopy (ICP-
AES) analyses. These results match well with the atomic
percentages of Ni, Co and Fe in the composite. The Brunauer-
Emmett-Teller (BET) surface area determined by N2 absorption
measurements at 77 K is 76 m2 g-1 for NiO/CoO/Fe2O3
composite.
The photocatalytic water oxidation activity of
NiO/CoO/Fe2O3 composite was investigated in the borate
buffer solution (initial pH 8.5) with Ru(bpy)3Cl2 as
photosensitizer and Na2S2O8 as sacrificial electron acceptor.
The time courses of evolved O2 with different catalysts are
shown in Fig. 5. Table 1 tabulates that the apparent turnover
frequency (TOF) and O2 yield were obtained for all of the metal
oxides studied. Compared with NiO (26%), Fe2O3 (35%) and
Fig. 4 Powder XRD pattern of NiO/CoO/Fe2O3 composite.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 4 of 9
View Article Online
DOI: 10.1039/C5DT02046H
ARTICLE
Journal Name
Fig. 7 Cyclic voltammograms (CVs) of 80 mM sodium borate buffer solution at pH 8.5
with 1.0 mM Ru(bpy)3Cl2 (red line) and NiO/CoO/Fe2O3 composite (blue line). The black
line displays the CV of 80 mM sodium borate buffer solution (pH 8.5).
Fig. 6 Time courses of O2 evolution under photoirradiation (LED lamp, λ ≥ 420 nm,
15.8W) of a borate buffer solution (pH 8.5, 15.0 mL) containing Na2S2O8 (5.0 mM),
Ru(bpy)3Cl2 (1.0 mM) and NiO/CoO/Fe2O3 composite (0.33 g L-1) at room temperature
in five repetitive experiments.
Conditions: CV was recorded on a CHI660D electrochemical analyzer with a glassy
carbon pasting 0.01 mg catalyst, Ag/AgCl, and Pt wire electrodes as the working,
reference, and auxiliary electrode, respectively, at room temperature with a scanning
CoO (45%), oxygen yield of NiO/CoO/Fe2O3 composite can
reach to 64% and it has a considerable apparent TOF (0.20
μmol s-1 m-2). However, the physical mixture of NiO, CoO and
Fe2O3 exhibits lower oxygen yield (47%) than the composite,
which may be attributed to the surface charge effect of
NiO/CoO/Fe2O3 composite.38 The high catalytic activity of
NiO/CoO/Fe2O3 composite was maintained perfectly even
after the fifth run (Fig. 6). As illustrated in Fig. S3 (ESI†), the
catalyst demonstrated extremely strong ferromagnetic
properties, which can explain the same high catalytic activity in
repetitive experiments. Isotope-labelling water oxidation
experiments using 18O enriched water instead of H216O were
carried out with NiO/CoO/Fe2O3 composite to confirm that the
water is the source of the evolved oxygen. The ratio of 16O16O :
16O18O : 18O18O is determined to be 83 : 16: 1, which is in great
agreement with the simulated ratio of 81 : 18 : 1 for oxygen
coming exclusively from water (Fig. S4, ESI†). The above data
clearly reveal that the evolved O2 originates exclusively from
the water. Thus, NiO/CoO/Fe2O3 composite is a robust and
efficient water oxidation catalyst and can be recycled many
times.
rate of 100 mV s-1
.
Fig. 8 The powder XRD patterns of NiO/CoO/Fe2O3 composite before the reaction
(black); after the reaction (red).
3+
1.12
V
(Fig. S5, ESI†). Therefore, Ru(bpy)3
is
The catalysis behavior of NiO/CoO/Fe2O3 composite for
visible light-driven water oxidation was also scrutinized under
electrocatalysis conditions. The cyclic voltammograms (CVs)
using a working electrode modified with NiO/CoO/Fe2O3
composite in a pH 8.5 borate buffer solution are demonstrated
as Fig. 7. The anodic currents with NiO/CoO/Fe2O3 composite
started growing at a lower potential (0.70 V) than that of
Ru(bpy)3Cl2 (1.05 V) and it could reach approximately 120 μA
at 1.50 V, which is a strong catalytic current. The result reveals
that the composite is active for the water oxidation. The onset
of the catalytic wave due to water oxidation is observed at ca.
0.70 V, and the redox potential of Ru(bpy)32+/3+ in our system is
thermodynamically capable of facilitating water oxidation.
In contrast to the homogeneous catalysts, the
heterogeneous ones possess an enviable preponderance that
they are easy recycling for many times. Due to the strong
ferromagnetic properties of the catalyst, the high catalytic
activity of NiO/CoO/Fe2O3 composite maintained well even
after the fifth run (Fig. 6). Compared to the powder XRD
patterns before and after the reaction (Fig. 8), all the peaks of
two samples were consistent completely, which indicates that
it is a stable catalyst in the process of water oxidation. As
depicted in Fig. 9 and Fig. S6 (ESI†), the changes of the surface
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 5 of 9
View Article Online
DOI: 10.1039/C5DT02046H
Journal Name
ARTICLE
Fig. 10 Linear sweep voltammograms (LSVs) of the catalysts in 1 M potassium
hydroxide with a scan rate of 1 mV s-1
.
results from the TEM images before and after the reaction (Fig.
S7, ESI†). Therefore, NiO/CoO/Fe2O3 composite is a highly
stable catalyst in the visible light-driven water oxidation
process.
Fig. 9 X-ray photoelectron spectra (XPS) of NiO/CoO/Fe2O3 composite before (black)
and after (red) the reaction in the energy regions of (a) Ni 2p, (b) Co 2p, (c) Fe 2p and (d)
O 1s.
conditions and the valence changes of each component before
and after photocatalytic reaction were observed by X-ray
photoelectron spectra. The XPS was performed in the Ni 2p,
Co2p, Fe 2p, O 1s and survey binding energy regions, which
were corrected using the C 1s peak from residual carbon
(284.8 eV). Fig. 9(a) displays the XPS spectrum for the Ni 2p3/2
and Ni 2p1/2 peaks at 854.3 and 872.1 eV with strong satellite
peaks before the reaction, at 854.5 and 872.3 eV with strong
satellite peaks after the reaction. The binding energies of these
peaks indicate that the Ni species in the samples are Ni2+ by
comparison with the Ni 2p peak positions of NiO.39 The Co
2p3/2 and Co 2p1/2 peaks were located at 779.3 and 795.1 eV
with satellite peaks before the photocatalytic reaction (Fig. 9b),
and at 779.6 and 795.6 eV with satellite peaks after the
reaction. These satellite peaks are characteristic of Co2+
species.40 As illustrated in Fig. 9(c), the XPS spectrum of Fe
2p3/2 and Fe 2p1/2 exhibited characteristic peaks at 709.9 and
723.1 eV for NiO/CoO/Fe2O3 composite before the reaction
and that at 710.1 and 723.8 eV after the reaction. These peaks
are assigned to Fe3+ species by comparison with the Fe 2p
peaks of Fe2O3.41 Fig. 9(d) elucidates the XPS spectrum of O 1s
peak for NiO/CoO/Fe2O3 composite which appeared at 530.0
eV and 530.2 eV before and after the reaction. At the same
time, they both have a weak shoulder peak on the main peak,
which may be ascribed to the common influence between
these three different metal oxides. These data insinuates that
the surface conditions of two samples were the same even
after the photocatalytic reaction.
The electrochemical activity of NiO/CoO/Fe2O3 composite
and reference samples in the water oxidation was evaluated in
1 M potassium hydroxide solution using a standard three-
electrode system. Catalysts (0.01 mg) were uniformly drop-
casted on a glassy carbon electrode. The electrocatalytic
activity of water oxidation was investigated by linear sweep
voltammograms (LSVs) with a scan rate of 1 mV s-1. As shown
in Fig. 10, the electrocatalytic water oxidation started at an
electrode potential of ~+1.52 V vs RHE for NiO/CoO/Fe2O3
composite which is the minimum one among the samples
measured. At the same overpotential, the highest anodic
current density was obtained for NiO/CoO/Fe2O3 composite in
comparison to other samples. To reach a current density of 10
mA cm-2 (Fig. S9, ESI†), the as-prepared NiO/CoO/Fe2O3
composite requires 0.41 V whereas 0.43 V and 0.45 V are
required for NiO and CoO, respectively. Moreover, Fe2O3
requires much more than 0.50 V to reach the same current
density. Thus, the good activity of NiO/CoO/Fe2O3 composite
obviously outperformed those of the rest materials over the
entire potential range (1.5 V to 1.7 V vs RHE) of water
oxidation.
The Tafel slopes curves were calculated by plotting
overpotential (η) against log(J), which are significant kinetic
parameters of water oxidation. Fig. 11 shows that the Tafel
slope of NiO/CoO/Fe2O3 composite is 49 mV dec-1, which is
lower than that of NiO (60 mV dec-1) and CoO (66 mV dec-1).
The Tafel slope of the Fe2O3 sample is 183 mV dec-1 (Fig. S10,
ESI†), which is much higher than the value of NiO/CoO/Fe2O3
composite. The excellent Tafel slopes of 49 mV dec-1 for
NiO/CoO/Fe2O3 composite is comparable to 49 mV dec-1 for
IrO2 and superior to other mixed metal oxide composites such
as CoFeOn (89 mV dec-1), Co3O4-carbon (70 mV dec-1) and
MnCo2O4+δ (84 mV dec-1 ). 42, 43 The Tafel slopes are frequently
influenced by electron and mass transport while they might be
used to examine the mechanism of WOCs.44 Recently, Hu et al.
Although the peaks of Ni 2p, Co 2p, Fe 2p and O 1s from the
samples before and after the reaction shifted slightly, the
identical separation between different peaks in both samples
and the consistency of peak shapes obviously verify that there
were no changes in the valence state of Ni2+ ,Co2+, Fe3+ and O2-.
The surface conditions of the composite before and after the
photocatalytic water oxidation were the same which can also
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 6 of 9
View Article Online
DOI: 10.1039/C5DT02046H
ARTICLE
Journal Name
Materials
All chemicals used for synthesizing catalysts were obtained
from chemical company and used without further
a
purification. Purified water (18.2MΩ cm) for the preparation of
solutions was provided by a Milli-Q water purification system
(Millipore, Direct-Q 3 UV).
Synthesis of NiO52
Porous NiO nanostructure powders were obtained by
annealing as-prepared Ni(OH)2 samples at 300 oC for 2 h. After
calcination, a color change from light green to gray was
visualized, confirming the transformation of Ni(OH)2 into NiO.
Synthesis of CoO53
Fig. 11 Tafel plot of NiO/CoO/Fe2O3 composite.
72.8 mg Co(NO3)2·6H2O was dissolved in 10 mL deionized
water, and then 10 mL 0.1M NH3·H2O was added to the
solution slowly. After stirring for about 15 min, the
precipitation was washed with deionized water by
centrifugation for 5-7 times, then transferred into a 20 mL
Teflon-lined autoclave, which was filled with solvents
(water/methanol = 1/1, V/V) up to 80% of the total volume.
reported that the slow electron transport gave larger Tafel
slopes for hydrogen evolution.45 Therefore, the lower Tafel
slope of NiO/CoO/Fe2O3 composite probably results from
faster electron transportation on the sample nanoplates. The
stability of the catalyst was studied at η = 300 mV in 1 M
potassium hydroxide solution (Fig. S11, ESI†). The anodic
current density sostenuto increased from 0.5 to 0.8 mA cm-2 in
2 hours. Some speculations were reported previously that the
determined step for water oxidation of CoOx was the proton-
o
And then these systems were sealed and heated at 250 C for
24 h.
6
Synthesis of Fe2O3
coupled one-electron oxidation of Co -OH to Co -O.46-51 The
Ⅲ
Ⅳ
To an aqueous solution (30 mL) containing FeSO4 (1.0 M) was
added NaOH (6.0 M) drop wisely to pH 11 with magnetic
stirring in room temperature. The mixtures were stirred
vigorously for 1.5 h with bubbling air. The products were
collected by filtration and washed with water several times,
dried at 65 oC.
increase of current density may be attributed to the
accumulation of Co(Ⅳ) species or other element high state
species to enhance the water oxidation activity.42 These high
state species come from the continuous anodic conditioning.
When the species accumulated to some certain level, the
anodic current density will remain stationary.
Synthesis of NiO/CoO/Fe2O3 composite
NiCl2·6H2O (2.0 mmol), Co(NO3)2·6H2O (2.0 mmol) and
Fe(NO3)3·9H2O (2.0 mmol) were dissolved in 24 mL deionized
water with strong stirrings. Subsequently, 24 mL KOH solution
(2 M) was added to this aqueous solution with magnetic
stirring in room temperature. After continuous stirring for 10
min, the mixture was transferred into a Teflon-lined autoclave
and maintained at 160 oC overnight. After being cooled to
ambient temperature naturally, the brown powders in the
autoclaves were collected by centrifugation, washed with
deionized water, followed by ethanol (99.7%) three times, and
Conclusions
In summary, we synthesized mixed-metal oxides
NiO/CoO/Fe2O3 composite to develop a highly efficient visible
light-driven water oxidation catalyst for the first time. The
unique hexagonal nanoplate morphology of the catalyst, to
which some nanoparticles adhere, produces surface charge
effect which may strengthen photocatalytic and
electrocatalytic performance. The mixed-metal oxides
composite exhibited lower overpotential and a much smaller
Tafel slope than that of CoO, NiO and Fe2O3. Moreover, this
catalyst possesses superior durability and stability in the
recycling experiments. The beneficial durability and stability
are certified by multiple experiments and are assigned to the
beneficial ferromagnetic properties of the catalyst. There will
be important applications for developing novel and robust
water oxidation catalysts in the future.
o
dried at 60 C for 8 h. The synthesis process for mixed-metal
oxides composite is shown in scheme 2.
Catalysts characterization
Transmission electron microscope (TEM) images were
obtained with a TecnaiG2F30. Crystalline structures were
determined by X-ray diffraction (XRD) using a Rigaku D/MAX
2400 diffractometer (Japan) with Cu Kα radiation (λ = 1.5418
Å) operating at 40 kV and 60 mA. X-ray photoelectron spectra
(XPS) were measured by ESCALAB250Xi with X-Ray
monochromatisation. The binding energy of each element was
corrected by the C 1s peak (284.8 eV) from residual carbon.
Nitrogen adsorption-desorption at 77 K was performed within
Experimental section
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 7 of 9
View Article Online
DOI: 10.1039/C5DT02046H
Journal Name
ARTICLE
a relative pressure range from 0.01 to 101.3 kPa. A sample detector was tuned for maximum sensitivity (quadruple
o
o
mass of about 200 mg was used for adsorption analysis after temperature, 150 C; ion source temperature, 230 C). The
pretreatment at 437 K for 30 min under vacuum conditions single ion mode was used to scan for the ions m/z = 28, 32, 34,
and kept in
a
N2 atmosphere until N2-adsorption 36 with a dwell time of 100 ms, resulting in 8.3 cycles per
measurements. Cyclic voltammetry (CV) was recorded on a second. Ions in the m/z range 30 to 50 were also scanned in
CHI660D electrochemical analyser with a glassy carbon, order to observe the abundance change of 16O18O and 18O18O,
Ag/AgCl and Pt wire electrode as the working, reference and which evolved from H216O and H218O, respectively. The total
auxiliary electrodes, respectively, using sodium borate buffer flow rate into the spectrometer was limited to 0.6 mL min-1.
solutions (pH 8.5, 80 mM) as a supporting electrolyte at room The GC was equipped with a molecular sieve column (30 m
temperature with a scanning rate of 100 mV s-1.
×
0.32 mm × 15 μm), and the vaporizing chamber temperature
and column temperature was set for 100 oC and 35 oC,
respectively.
Photocatalytic and electrocatalytic water oxidation
Photocatalytic water oxidation was performed as follows: the
desired concentration of catalyst (0.33 g L-1) was prepared by Definition of half-wave potential (E1/2
)
dissolving the appropriate amount of catalyst in a borate E1/2 = (Epa + Epc) / 2
2+/3+
2+/3+
buffer solution (80 mM, pH 8.5) containing Ru(bpy)3Cl2 (1.0 Epa : the oxidation potential of Ru(bpy)3
mM) and Na2S2O8 (5.0 mM). The above solution was deaerated Epc : the reduction potential of Ru(bpy)3
by purging with Ar gas for 10 min in a flask (28.5 mL) sealed
with a rubber plug (the total volume of the reaction solution
Acknowledgements
was 15 mL). The reaction was then started by irradiating the
solution with a LED light source54 (light intensity 15.8 mW,
beam diameter 2 cm) through a transmitting glass filter (λ ≥
420 nm) at room temperature. After each sampling time, 100
μL of Ar was injected into the flask and then the same volume
of gas sample in the headspace of the flask was withdrawn by
a SGE gas-tight syringe and analyzed by gas chromatography
(GC). The O2 in the sampled gas was separated by passing
through a 2 m × 3 mm packed molecular sieve 5 Å column with
an Ar carrier gas and quantified by a Thermal Conductivity
Detector (TCD). The total amount of evolved O2 was calculated
based on the concentration of O2 in the headspace gas.
Contamination of the head-space with air was corrected by
measuring the N2 concentration present in the head-space.
The solution pH was monitored after the reaction by a
METTLER TOLEDO FEP20 pH meter.
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 21173105) and the
open fund by Jiangsu Key Laboratory of Atmospheric
Environment Monitoring and Pollution Control (KHK1402), A
Project Funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
Notes and references
1
2
M. Yagi and M. Kaneko, Chem. Rev., 2001, 101, 21-35.
M. W. Kanan and D. G. Nocera, Science, 2008, 321, 1072-
1075.
3
4
5
B. Li, F. Li, S. Bai, Z. Wang, L. Sun, Q. Yang and C. Li, Energy
Environ. Sci., 2012,
F. Song, Y. Ding, B. Ma, C. Wang, Q. Wang, X. Du, S. Fu and J.
Song, Energy Environ. Sci., 2013, , 1170-1184.
5, 8229-8233.
Linear scan voltammograms (LSVs) were recorded on a
CHI660D electrochemical analyser with a glassy carbon,
Ag/AgCl and Pt wire electrode as the working, reference and
auxiliary electrodes, respectively. LSVs were conducted in
6
H. Lv, Y. V. Geletii, C. Zhao, J. W. Vickers, G. Zhu, Z. Luo, J.
Song, T. Lian, D. G. Musaev and C. L. Hill, Chem. Soc. Rev.,
2012, 41, 7572-7589.
6
7
8
D. Hong, Y. Yamada, T. Nagatomi, Y. Takai and S. Fukuzumi, J.
Am. Chem. Soc., 2012, 134, 19572-19575.
J. Rosen, G. S. Hutchings and F. Jiao, J. Am. Chem. Soc., 2013,
135, 4516-4521.
Q. Yin, J. M. Tan, C. Besson, Y. V. Geletii, D. G. Musaev, A. E.
Kuznetsov, Z. Luo, K. I. Hardcastle and C. L. Hill, Science,
2010, 328, 342-345.
potassium hydroxide solutions (1 M) as
a supporting
electrolyte at room temperature with a scanning rate of 1 mV
s-1. The experiments were measured from 0.4 to 0.7 V vs.
Ag/AgCl with a glassy carbon pasting 0.01 mg catalyst as a
working electrode. Tafel slopes were calculated from the LSVs
curves by plotting overpotential η against log (J).
9
J. W. Ko, W. H. Ryu, I. D. Kim and C. B. Park, Chem. Commun.,
2013, 49, 9725-9727.
10 B. M. Hunter, J. D. Blakemore, M. Deimund, H. B. Gray, J. R.
Isotope-labelled experiment
The 10.8 atom% H218O of borate buffer solution (pH 8.5, 80
mM) containing NiO/CoO/Fe2O3 composite (0.33 g L-1),
Ru(bpy)32+ (1.0 mM) and Na2S2O8 (5.0 mM) was deaerated with
helium gas before irradiation by LED light ( ≥ 420 nm) in a
quartz flask that was sealed with a rubber plug. After 10 min,
50 μL of gas sample was withdrawn using a gas-tight syringe
for gas analysis. An Agilent Series 7890A model chromatograph
interfaced with an Agilent Series 5975C model mass
spectrometer operating in electron impact ionization mode
was used to collect the mass spectrometry data. The MS
Winkler and A. M. Muller, J. Am. Chem. Soc., 2014, 136
13118-13121.
11 Y. Cui, L. Shi, Y. Yang, W. You, L. Zhang, Z. Zhu, M. Liu and L.
Sun, Dalton Trans., 2014, 43, 17406-17415.
12 A. W. Rutherford and A. Boussac, Science, 2004, 303, 1782-
1784.
13 Y. Umena, K. Kawakami, J. R. Shen and N. Kamiya, Nature,
2011, 473, 55-60.
14 M. Hara, C. C. Waraksa, J. T. Lean, B. A. Lewis and T. E.
Mallouk, J. Phys. Chem. A, 2000, 104, 5275-5280.
,
15 N. D. Morris and T. E. Mallouk, J. Am. Chem. Soc., 2002, 124
11114-11121.
,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 8 of 9
View Article Online
DOI: 10.1039/C5DT02046H
ARTICLE
Journal Name
16 J. D. Blakemore, N. D. Schley, D. Balcells, J. F. Hull, G. W. 47 J. G. McAlpin, Y. Surendranath, M. Dinca, T. A. Stich, S. A.
Olack, C. D. Incarvito, O. Eisenstein, G. W. Brudvig and R. H. Stoian, W. H. Casey, D. G. Nocera and R. D. Britt, J. Am.
Crabtree, J. Am. Chem. Soc., 2010, 132, 16017-16029. Chem. Soc., 2010, 132, 6882-6883.
17 Y. V. Geletii, C. Besson, Y. Hou, Q. Yin, D. G. Musaev, D. 48 M. W. Kanan, J. Yano, Y. Surendranath, M. Dinca, V. K.
Quin˜onero, R. Cao, K. I. Hardcastle, A. Proust, P. K. gerler
and C. L. Hill, J. Am. Chem. Soc., 2009, 131, 17360-17370.
Yachandra and D. G. Nocera, J. Am. Chem. Soc., 2010, 132,
13692-13701.
18 S. Neudeck, S. Maji, I. Lopez, S. Meyer, F. Meyer and A. 49 Y. Surendranath, M. W. Kanan and D. G. Nocera, J. Am.
Llobet, J. Am. Chem. Soc., 2014, 136, 24-27. Chem. Soc., 2010, 132, 16501-16509.
19 X. Zhou, T. Zhang, C. W. Abney, Z. Li and W. Lin, ACS Appl. 50 D. J. Wasylenko, C. Ganesamoorthy, J. Borau-Garcia and C. P.
Mater. Interfaces, 2014, , 18475-18479. Berlinguette, Chem. Commun., 2011, 47, 4249-4251.
20 D. Hong, Y. Yamada, A. Nomura and S. Fukuzumi, Phys. 51 R. L. Doyle, I. J. Godwin, M. P. Brandon and M. E. Lyons, Phys.
Chem. Chem. Phys., 2013, 15, 19125-19128. Chem. Chem. Phys., 2013, 15, 13737-13783.
21 M. Gong, Y. Li, H. Wang, Y. Liang, J. Z. Wu, J. Zhou, J. Wang, T. 52 H. Y. Wang, E. Mijangos, S. Ott and A. Thapper, Angew.
6
Regier, F. Wei and H. Dai, J. Am. Chem. Soc., 2013, 135, 8452-
8455.
Chem. Int. Ed., 2014, 53, 14499-14502.
53 G. Chen, L. Chen, S. M. Ng and T. C. Lau, ChemSusChem,
22 X. Du, Y. Ding and C. Li, ChemCatChem, 2015, DOI:
10.1002/cctc.201500308.
2014, 7, 127-134.
54 T. S. Natarajan, K. Natarajan, H. C. Bajaj and R. J. Tayade, Ind.
Eng. Chem. Res., 2011, 50, 7753-7762.
23 F. Jiao and H. Frei, Energy Environ. Sci., 2010,
24 Y. Zhao, Y. Liu, X. Du, R. Han and Y. Ding, J. Mater. Chem. A,
2014, , 19308-19314.
3, 1018-1027.
2
25 H. Lv, J. Song, Y. V. Geletii, J. W. Vickers, J. M. Sumliner, D. G.
Musaev, P. Kogerler, P. F. Zhuk, J. Bacsa, G. Zhu and C. L. Hill,
J. Am. Chem. Soc., 2014, 136, 9268-9271.
26 J. Zhao, Y. Zou, X. Zou, T. Bai, Y. Liu, R. Gao, D. Wang and G.
D. Li, Nanoscale, 2014, 6, 7255-7262.
27 T. Nakazono, A. R. Parent and K. Sakai, Chem. Eur. J., 2015,
21, 6723-6726.
28 Y. Hou, F. Zuo, A. Dagg and P. Feng, Angew. Chem. Int. Ed.,
2013, 52, 1248-1252.
29 D. K. Zhong and D. R. Gamelin, J. Am. Chem. Soc., 2010, 132
,
4202-4207.
30 Y. Zhang, Z. Zhou, C. Chen, Y. Che, H. Ji, W. Ma, J. Zhang, D.
Song and J. Zhao, ACS Appl. Mater. Interfaces, 2014,
12844-12851.
6,
31 Y. Liu, R. Xiang, X. Du, Y. Ding and B. Ma, Chem. Commun.,
2014, 50, 12779--12782.
32 N. S. McCool, D. M. Robinson, J. E. Sheats and G. C.
Dismukes, J. Am. Chem. Soc., 2011, 133, 11446-11449.
33 F. Evangelisti, R. Guttinger, R. More, S. Luber and G. R.
Patzke, J. Am. Chem. Soc., 2013, 135, 18734-18737.
34 L. Tong, Y. Wang, L. Duan, Y. Xu, X. Cheng, A. Fischer, M. S.
Ahlquist and L. Sun, Inorg. Chem., 2012, 51, 3388-3398.
35 Y. Yamada, K. Yano, D. Hong and S. Fukuzumi, Phys. Chem.
Chem. Phys., 2012, 14, 5753-5760.
36 R. Al-Oweini, A. Sartorel, B. S. Bassil, M. Natali, S. Berardi, F.
Scandola, U. Kortz and M. Bonchio, Angew. Chem. Int. Ed.,
2014, 53, 11182-11185.
37 G. Chen, L. Chen, S.-M. Ng, W.-L. Man and T.-C. Lau, Angew.
Chem. Int. Ed. , 2013, 52, 1789 -1791.
38 C. Miao, S. Ji, G. Xu, G. Liu, L. Zhang and C. Ye, ACS Appl.
Mater. Interfaces, 2012, 4, 4428-4433.
39 M. A. Peck and M. A. Langell, Chem. Mater., 2012, 24, 4483-
4490.
40 X. C. Song, E. Yang, R. Ma, H. F. Chen and Y. Zhao, J.
Nanopart. Res., 2007, 10, 709-713.
41 X. Du, J. Wei, J. Zhao, R. Han and Y. Ding, Chem. Asian J.,
2014, 9, 2745-2750.
42 F. Song and X. Hu, J. Am. Chem. Soc., 2014, 136, 16481-
16484.
43 Y. Gao, X. Ding, J. Liu, L. Wang, Z. Lu, L. Li and L. Sun, J Am
Chem Soc, 2013, 135, 4219-4222.
44 H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan and P.
Strasser, ChemCatChem, 2010, 2, 724-761.
45 H. Vrubel, T. Moehl, M. Gratzel and X. Hu, Chem. Commun.,
2013, 49, 8985-8987.
46 B. S. Brunschwig, M. H. Chou, Q. Creutz, P. Ghosh and N.
Sutin, J. Am. Chem. Soc., 1983, 105, 4833-4835.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 9
Dalton Transactions
View Article Online
DOI: 10.1039/C5DT02046H
Graphic abstract
Hexagonal nanoplates of NiO/CoO/Fe2O3 composite acting as an
efficient photocatalytic and electrocatalytic water oxidation catalyst
Yukun Zhaoa, Yan Zhanga, Yong Dinga,b* and Mindong Chen b
The unique hexagonal sheet-shaped NiO/CoO/Fe2O3 composite fabricated through a
co-precipitation and hydrothermal method demonstrates robust photocatalytic and
electrocatalytic performance of water oxidation.