Core@shell structured Co-CoO@NC nanoparticles supported on nitrogen doped carbon with high catalytic activity for oxygen reduction reaction

Article abstract of DOI:10.1039/c8ra01680a

A composite with a hierarchical structure consisting of nitrogen doped carbon nanosheets with the deposition of nitrogen doped carbon coated Co-CoO nanoparticles (Co-CoO@NC/NC) has been synthesized by a simple procedure involving the drying of the reaction mixture containing Co(NO₃)₂, glucose, and urea and its subsequent calcination. The drying step is found to be necessary to obtain a sample with small and uniformly sized Co-CoO nanoparticles. The calcination temperature has a great effect on the catalytic activity of the final product. Specifically, the sample prepared at the calcination temperature of 800 °C shows better catalytic activity of the oxygen reduction reaction (ORR). Urea in the reaction mixture is crucial to obtain the sample with the uniformly sized Co-CoO nanoparticles and also plays an important role in improving the catalytic activity of the Co-CoO@NC/NC. Additionally, there exists a strong electronic interaction between the Co-CoO nanoparticles and the NC. Most interestingly, the Co-CoO@NC/NC is highly efficient for the ORR and can deliver an ORR onset potential of 0.961 V vs. RHE and a half-wave potential of 0.868 V vs. RHE. Both the onset and half-wave potentials are higher than those of most catalysts reported previously and even close to those of the commercial Pt/C (the ORR onset and half-wave potential of the Pt/C are 0.962 and 0.861 V vs. RHE, respectively). This, together with its high stability, strongly suggests that the Co-CoO@NC/NC could be used as an efficient catalyst for the ORR.

Full text of DOI:10.1039/c8ra01680a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Core@shell structured Co–CoO@NC nanoparticles
supported on nitrogen doped carbon with high
catalytic activity for oxygen reduction reaction†
bc
Cite this: RSC Adv., 2018, 8, 14462
Zihao Zhen,a Zhongqing Jiang,
Xiaoning Tian,c Lingshan Zhou,a Binglu Deng,a
Bohong Chena and Zhong-Jie Jiang
a
*
A composite with a hierarchical structure consisting of nitrogen doped carbon nanosheets with the
deposition of nitrogen doped carbon coated Co–CoO nanoparticles (Co–CoO@NC/NC) has been
synthesized by a simple procedure involving the drying of the reaction mixture containing Co(NO3)2,
glucose, and urea and its subsequent calcination. The drying step is found to be necessary to obtain
a sample with small and uniformly sized Co–CoO nanoparticles. The calcination temperature has a great
effect on the catalytic activity of the final product. Specifically, the sample prepared at the calcination
temperature of 800 ꢀC shows better catalytic activity of the oxygen reduction reaction (ORR). Urea in
the reaction mixture is crucial to obtain the sample with the uniformly sized Co–CoO nanoparticles and
also plays an important role in improving the catalytic activity of the Co–CoO@NC/NC. Additionally,
there exists a strong electronic interaction between the Co–CoO nanoparticles and the NC. Most
interestingly, the Co–CoO@NC/NC is highly efficient for the ORR and can deliver an ORR onset
potential of 0.961 V vs. RHE and a half-wave potential of 0.868 V vs. RHE. Both the onset and half-wave
potentials are higher than those of most catalysts reported previously and even close to those of the
commercial Pt/C (the ORR onset and half-wave potential of the Pt/C are 0.962 and 0.861 V vs. RHE,
respectively). This, together with its high stability, strongly suggests that the Co–CoO@NC/NC could be
used as an efficient catalyst for the ORR.
Received 26th February 2018
Accepted 12th April 2018
DOI: 10.1039/c8ra01680a
rsc.li/rsc-advances
stability, poor tolerance to poisoning, scarcity in earth, and high
cost, which have largely hindered their applications as the
Introduction
electrocatalysts for the ORR in fuel cells and metal air
batteries.9,10 Developing alternative catalysts that can boost the
ORR kinetics and simultaneously meet the requirements of low
cost and high stability is therefore highly desirable.
The ever-increasing global demands in sustainable energy have
prompted worldwide efforts in developing alternative energy
conversion and storage devices, such as fuel cells, and metal air
batteries.1,2 Exploiting efficient electrocatalysts that can
enhance the kinetics of the oxygen reduction reaction (ORR) has
therefore attracted a great deal of attention, since the slug-
gishness of the ORR has greatly limited practical applications of
these devices.3,4 Currently, the best catalysts reported for the
ORR are Pt and its alloys, such as Pt/Ni, Pt/Co, and Pt/Cu, etc.5–8
These catalysts, however, suffer from severe problems of low
In response to reduce the cost and improve the catalytic
activities for the ORR, efforts on developing ORR catalysts have
been mainly focused on noble metal-free or even metal-free
materials.11,12 Transition metal based catalysts are of particular
interest due to their advantages of superior catalytic activity and
earth abundance.13,14 As reported previously, transition metals
(TMs) and their oxides (TMOs) can exhibit excellent catalytic
activities for the ORR when they are synthesized properly.15–18 The
main problem associated with the practical applications of the
TMs and TMOs, however, is their poor stability in the compli-
cated electrochemical environments.19,20 Strategies to address
this issue are therefore needed to promote the applications of the
TMs and TMOs as the ORR electrocatalysts. Encapsulating with
the carbon shell is a promising way to improve the stability of the
TMs and TMOs.16,21–23 As demonstrated recently, the encapsula-
tion cannot only protect the encapsulated TMs and TMO cores
from degradation through avoiding their direct contact with the
electrolyte solutions, but also improve the catalytic activities of
aGuangzhou Key Laboratory for Surface Chemistry of Energy Materials, New Energy
Research Institute, College of Environment and Energy, South China University of
Technology, Guangzhou 510006, Guangdong, P. R. China. E-mail:
Zhongjiejiang1978@hotmail.com; eszjiang@scut.edu.cn
bDepartment of Physics, Key Laboratory of ATMMT Ministry of Education, Zhejiang Sci-
Tech University, Hangzhou 310018, P. R. China
cSchool of Materials and Chemical Engineering, Ningbo University of Technology,
Ningbo 315211, Zhejiang, P. R. China
† Electronic supplementary information (ESI) available: Element mapping; TGA
curve; TEM images; XPS survey spectra; XRD patterns; Raman spectra; LSVs and
K-L plots; elemental composition; performance comparison. See DOI:
10.1039/c8ra01680a
14462 | RSC Adv., 2018, 8, 14462–14472
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
the TMs and TMOs since the ultrathin carbon shells can greatly activities higher than or even close to the commercial Pt/C has
promote penetration of electrons from the TM and TMO cores to less been reported. The work presented here is therefore of great
the carbon surface, participating in the catalytic reactions.24,25 In interest since it develops a new and high efficient catalyst using
particular, when the carbon shell is doped with heteroatoms, the a simple synthetic method and would be also helpful to design
synergistic interaction between heteroatom doped carbon shell other catalysts.
and the TM/TMO core can increase the electron density on the
carbon shell, further improving their capability for the catalytic
applications.24,26,27
Metallic cobalt (Co) and its oxide (CoO) are typical TM based
materials, both of which have been demonstrated to be usable
Experimental
Chemicals and reagents
as the catalysts for the ORR.28,29 The catalytic activities of the
single component Co or CoO material, however, remain lower
in comparison to the Pt-based catalysts, although they might be
enhanced through the integration of Co or CoO with carbona-
ceous materials.20,30 Recently, the composite materials consist-
ing of two or more components that are individually active for
the ORR are receiving growing attention, since the synergistic
between multiple components might generate more fascinating
properties for the catalytic applications.2,12,31–36 For example, the
Fe–Fe2O3 particles deposited N-doped graphene shows
enhanced catalytic activity for the ORR.12 The Ni–NiO nano-
particles on PDDA-modied graphene are highly active for
oxygen reduction reaction.37 In this sense, the combination of
Co with CoO is therefore expectable to produce a composite
particle with higher catalytic activity for the ORR.
In this work, we investigate the ORR catalytic activity of
nitrogen doped carbon coated Co–CoO nanoparticles (Co–
CoO@NC). Specically, a composite material with a hierar-
chical structure consisting of the nitrogen doped carbon
nanosheets with the deposition of the Co–CoO@NC (Co–
CoO@NC/NC) has been synthesized through a simple calcina-
tion of a mixture containing Co(NO3)2, glucose, and urea. The
effects of the drying step and the calcination temperature on the
catalytic activity of the nal product has been studied. It shows
that the drying step is indispensable in obtaining the sample
with the small and uniformly sized Co–CoO nanoparticles. The
sample prepared at the calcination temperature of 800 ꢀC shows
better catalytic activity for the ORR, since at this temperature
glucose and urea are well carbonized and the obtained product
consists of small and uniform sizes of the Co–CoO nano-
particles (NPs) and a high content of the doping nitrogen in the
Urea (99%) and glucose (99%) were obtained from Guangzhou
Chemical Reagent Co. Ltd. Cobalt(II) nitrate hexahydrate
(Co(NO3)2$6H2O, 99%) was obtained from Tianjin Kermel
Chemical Reagent Co. Ltd. Potassium hydroxide (KOH, 85%),
methanol and isopropanol were obtained from Tianjin
chemical reagent Co. Ltd. Naon (5.0 wt%) was purchased
from DuPont Company. All the chemicals were used as
received without further purication. Deionized (DI) water
with a resistance of ꢁ18 MU cmꢂ1 was used in all the
reactions.
Material synthesis
In a typical synthesis, 50.0 mg of glucose and 1.0 g of urea were
dissolved with 40 mL of DI water. The obtain mixture was stirred
vigorously for 30 min, followed by addition of 0.6 mmol of
Co(NO3)2. The obtained pink homogenous solution was heated at
80 ꢀC under stirring to remove water. Aer vaporization, the
obtained solid was loaded on a crucible, then thoroughly dried at
200 ꢀC for 2 h, and nally calcined at 700, 800 or 900 ꢀC for 3 h in
a tube furnace under the N2 ow. The products obtained from the
ꢀ
calcination temperature of 700, 800 and 900 C were named as
Co–CoO@NC/NC-700, Co–CoO@NC/NC-800, and Co–CoO@NC/
NC-900, respectively. The same synthetic procedure was per-
formed for the synthesis of the Co–CoO@C/C-800 and the NC in
the absence of urea or Co(NO3)2, respectively, while keeping other
parameters constant.
Physical characterizations
carbon. Urea in the reaction mixture is found to play an X-ray diffraction patterns (XRD) of the samples were recorded on
important role in the formation of the Co–CoO@NC/NC with a Bruker D8 X-ray diffractometer with the Cu Ka radiation and
higher catalytic activity. It does not only provide a nitrogen were analyzed using Bruker EVA and Bruker TopAs 4.2 soware.
source for the NC, but also facilitate the formation of the Co– Scanning electron microscopy (SEM) was performed on Merlin
CoO nanoparticles with small and uniform sizes. Most notably, with an operation voltage of 20.0 kV. Transmission electron
there exists a strong interaction between the Co–CoO nano- microscopy (TEM) and high-resolution transmission electron
particles and the NC. When used as the catalyst, the Co– microscopy (HRTEM) measurements were performed on a JEM-
CoO@NC/NC at the calcination temperature of 800 ꢀC (Co– 2100F with an accelerating voltage of 200 kV. Raman spectra
CoO@NC/NC-800) could exhibit an ORR onset potential of were collected on a LabRAM HR Evolution system using an Ar
0.961 V vs. RHE and a half-wave potential of 0.868 V vs. RHE. laser with excitation wavelength of 514.5 nm. The chemical
These values are higher than the onset and half-wave potentials compositions of the samples were determined by X-ray photo-
of the single component Co and CoO based catalysts and even electron spectroscopy (XPS) performed on a PHI X-tool (Japan)
close to those of the commercial Pt/C, strongly suggesting that with an Al Ka X-ray radiation (1486 eV). The Brunauer–Emmett–
the Co–CoO@NC/NC is an efficient catalyst for the ORR. Up to Teller (BET) surface areas and pore size distributions of the
now, although a signicant amount of the catalysts has been samples were determined by the N2 adsorption/desorption
reported to be catalytically active for the ORR, those with isotherms recorded at 77 K using Micromeritics ASAP 2020.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 14462–14472 | 14463

View Article Online
RSC Advances
Paper
Electrochemical measurement
Results and discussion
A standard three-electrode cell was used for the electrochemical
measurements using an electrochemical workstation (CHI
760D, China). Platinum wire and a saturated calomel electrode
served as the counter and the reference electrodes, respectively.
The working electrode was prepared by dropping drops of the
catalyst ink onto the glassy carbon electrode and then naturally
dried at room temperature. The catalyst ink with a catalysts
concentration of 2 mg mLꢂ1 was prepared by dispersing the
catalyst and 10 mL of Naon (5 wt%) into a mixture of water and
isopropanol with a volume ratio of 1.0 : 3.0, followed by ultra-
sonication to obtain a homogeneous solution. The loading of
the catalyst on the working electrode was well controlled to
ꢁ0.1 mg cmꢂ2. The working electrode with the Pt/C (20 wt% Pt
on the carbon, Johnson Matthey) used for comparison was
prepared in a similar manner. The cyclic voltammograms (CVs)
were recorded at ambient temperature in an 0.1 M KOH solu-
tion. Before the measurements, the solution was aerated with
N2 or O2 for at least 30 min. The N2 or O2 ow was maintained
above the solution to keep the N2 or O2 saturation during the
electrochemical measurements. The linear sweep voltammo-
grams (LSVs) obtained from the rotating disk electrode (RDE) at
the different rotating rates were collected in the O2 saturated
0.1 M KOH at a rate of 5 mV sꢂ1 from 0.3 V to ꢂ0.8 V (vs. SCE).
The kinetic analysis was performed using the Koutecky–Levich
(K–L) equations shown below:
Fig. 1a shows a typical SEM image of the Co–CoO@NC/NC-800
synthesized by a procedure involving the drying of the reaction
mixture of Co(NO3)2, glucose and urea at 200 ꢀC and its
subsequent calcination at the high temperature of 800 ꢀC. It
reveals that the Co–CoO@NC/NC-800 has
a morphology
resembling to the crumpled papers with the deposition of the
small sized particles. This is also veriable by its TEM image in
Fig. 1b, which shows a structure consisting of the sheet-like
carbon material and uniformly sized nanoparticles. The
average size of the nanoparticles is ꢁ14.8 nm, as estimated by
the particle size distribution histogram in Fig. 1c. The high-
resolution TEM image of the Co–CoO@NC/NC-800 in Fig. 1d
indicates that the nanoparticles are composed of mixed phases
of Co and CoO. As shown in Fig. 1d, the lattice fringes corre-
sponding to the (111) and (200) planes of the face-centered
cubic (fcc) structure cobalt and the (220) plane of the rock salt
cubic structure CoO could be identied. The presence of the
mixed phases of Co and CoO suggests that a fraction of Co2+ has
been reduced by glucose and urea or their calcination products
during the calcination process, while the residual has changed
into CoO through the decomposition of Co(NO3)2 or through its
reaction with oxygen from the decomposed glucose and urea.
Most interestingly, each Co/CoO nanoparticle is surrounded by
a crystallized shell of 1.2 nm (Fig. 1d). The d-spacing measured
from the lattice fringes of the shell is 0.335, well consistent with
the interlayer spacing of the (002) plane of the graphitic carbon.
This suggests that the Co/CoO nanoparticles are coated with the
carbon shells. These carbon shells could be attributed to the
1
1
1
1
1
¼
þ
¼ þ
jk Bu1=2
(1)
j
jk jL
B ¼ 0.62nFCOD2/3 ꢂ1/6
v
(2) carbonized product of glucose and urea formed during the
high-temperature calcination processes.
where j, jk, and jL represent the measured current, the kinetic
current, and the limiting current densities, respectively. CO is the
concentration of molecular oxygen in O2-saturated 0.1 M KOH
(CO ¼ 1.2 ꢃ 10ꢂ6 mol cmꢂ3), and u the electrode rotating rate,
The X-ray diffraction (XRD) pattern of the Co–CoO@NC/NC-
800 is given in Fig. 2a. It shows three distinct peaks at 2q ¼
44.2ꢀ, 51.6ꢀ and 75.8ꢀ, corresponding to the (111), (200), and
(220) reections of the fcc structure metallic Co (PDF no. 15-
0806), and two weak peaks 2q ¼ 36.6 and 42.3ꢀ, corresponding
to the (111) and (200) reections of the rock salt cubic structure
CoO (PDF no. 48-1719). This further demonstrates the co-
existence of Co and CoO in the Co–CoO@NC/NC-800. The
reason that the Co–CoO@NC/NC-800 exhibits low diffraction
peaks corresponding to CoO could be attributed to its relatively
low content in comparison to that of the metallic cobalt (as
demonstrated by the XPS results shown below) and the low X-
ray scattering factor of cobalt oxide. Besides these peaks, the
XRD pattern of the Co–CoO@NC/NC-800 also exhibits a broad
peak at 2q ¼ 25.9ꢀ, which could be assigned to the (002)
reection of the layer-by-layer aligned graphitic nanosheets,
suggesting the presence of the graphitic carbon in the Co–
CoO@NC/NC-800. This could be further corroborated by the
Raman spectrum of the Co–CoO@NC/NC-800 in Fig. 2b, which
B
the Levich constant,
F
the Faraday constant
(F ¼ 96 500C molꢂ1), n transferred electron number, D the O2
diffusion coefficient of the electrolyte (D ¼ 1.9 ꢃ 10ꢂ5 cmꢂ2 sꢂ1),
v the kinetic viscosity (v ¼ 0.01 cm2 sꢂ1).
The rotating ring-disk electrode (RRDE) technique was also
employed to analyze the ORR properties of the catalysts. The
ꢂ
electron transfer number and the amount of HO2 generated
during ORR from RRDE test were determined through the
equations below:
200IR
HO2ð%Þ ¼
(3)
jIDjN þ IR
4jIDj
n ¼
(4)
jIDj þ IR=N
where IR is the ring current, ID is the disk current, N is the
collection efficiency with a value of 0.4. The potential reported
in this work was all referenced to RHE. In the 0.1 M KOH
solution, the potentials versus the SCE reference electrode were
shows pronounced peaks at 1350 cmꢂ1 and 1580 cmꢂ1
,
assignable to the D band (arising from the disorder carbon) and
G band (associated with tangential stretching mode of highly
order graphitic carbon) of the graphitic structure. Worthnoting
is that the Raman spectrum of the Co–CoO@NC/NC-800 also
converted to the RHE through the calibration equation: ERHE
ESCE + 0.2438 + 0.0591 ꢃ 13.
¼
14464 | RSC Adv., 2018, 8, 14462–14472
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Fig. 1 . (a) SEM and (b) TEM images of the Co–CoO@NC/NC-800. (c) Particle size distribution histogram of the Co–CoO NPs in the Co–
CoO@NC/NC-800. (d) High resolution TEM image of the Co–CoO@NC/NC-800.
exhibits peaks at 473, 517, and 679 cmꢂ1, assignable to active corresponding to Co 2p3/2 and 2p1/2 of Co(0) is observable at
vibrational modes Eg, F2g, and A1g of the cubic structure CoO, 778.8 eV and 794.1 eV, respectively. Based on the deconvoluted
respectively.38,39 This further supports the presence of CoO in spectrum shown in Fig. 3b, the relative atomic ratio of
the Co–CoO@NC/NC-800.
Co(0) : Co(II) is estimated to ꢁ3 : 2. It implies that Co in the Co–
The XPS measurements were carried out to gain information CoO@NC/NC-800 mainly exists in the metallic form. Fig. 3c
on the elemental composition of the Co–CoO@NC/NC-800. displays the deconvoluted C 1s spectrum of the Co–CoO@NC/
Fig. 3a shows the XPS survey spectrum of the Co–CoO@NC/ NC-800, which shows four components at binding energies of
NC-800, which indicates that the Co–CoO@NC/NC-800 is 284.5, 285.3, 286.3, and 288.2 eV, corresponding to sp2 C, C]N/
composed of Co, O, N, and C. The deconvolution of the Co 2p C–O, C–O–C/C–N, and O–C]O, respectively. The domination of
spectrum in Fig. 3b clearly demonstrates the presence of both the peak corresponding to sp2 C suggests the carbon in the Co–
Co(0) and Co(II) in the Co–CoO@NC/NC-800. As shown in Fig. 3b, CoO@NC/NC-800 is well graphitized. This is in line with the
the peaks corresponding to Co 2p3/2 and 2p1/2 of Co(II) could be result shown by the Raman spectrum (Fig. 2b), which exhibits
observed at 780.5 eV and 796.5 eV, respectively, while the peak pronounced peaks corresponding to the D and G bands of the
Fig. 2 (a) XRD patterns, (b) Raman spectra of the Co–CoO@NC/NC-800. (c) N2 adsorption/desorption isotherms of the Co–CoO@NC/NC-800,
the NC, the Co–CoO@C/C-800, the Co–CoO@NC/NC-700, the Co–CoO@NC/NC-900.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 14462–14472 | 14465

View Article Online
RSC Advances
Paper
graphitic structure. The existence of the peaks corresponding to size distribution (the TEM image, XPS spectra, XRD diffraction
C]N and C–N in the deconvoluted C 1s spectrum indicates that pattern, and Raman spectrum of the Co–CoO@C/C-800 is given
carbon in the Co–CoO@NC/NC-800 is doped with nitrogen. This in Fig. S3a, S4, S5, and S6,† respectively). This suggests that urea
could be also veried by the deconvoluted N 1s spectrum in in the reaction mixture does not simply serve as the nitrogen
Fig. 3d, which shows four peaks at binding energies of 397.9, source for the doping of carbon, but plays an important role in
399.6, 400.7, and 402.3 eV, assignable to pyridinic, pyrrolic, promoting the formation the Co–CoO@NC/NC-800 with
graphitic, and oxidized nitrogen, respectively. This is similar to uniformly nanosized Co–CoO particles. It is imaginable that
the NC reported previously.40,41 Since the Co–CoO@NC/NC-800 during the high temperature calcinations the explosive pyrol-
was synthesized from a mixture, in which the reactants were ysis of urea would facilitate the doping of nitrogen into
well dispersed, we believe that both the carbon shell of the Co– graphitic structure of carbon with different forms, such as
CoO nanoparticles and the carbon sheet observed in the SEM pyridinic, pyrrolic, graphitic, and oxidized nitrogen. Due to the
and TEM images of the Co–CoO@NC/NC-800 (Fig. 1a and b) are high electronegativity of nitrogen, the doping sites in the forms
doped with nitrogen. The elemental composition of the Co– of pyridinic, pyrrolic, and graphitic nitrogen are negatively
CoO@NC/NC-800 could be further demonstrated by its charged,42,43 making them capable of adsorbing the Co2+ ions
elemental mapping image in Fig. S1,† which clearly shows the through the coordination and electrostatic interactions. This
uniform distribution of Co, O, N, and C. Based on the XPS would prevent the aggregation of the Co2+ ions during its
results and the TGA curve of the Co–CoO@NC/NC-800 in transformation into Co–CoO nanoparticles, facilitating the
Fig. S2,† the weight percentage of Co–CoO in the Co–CoO@NC/ formation of the Co–CoO nanoparticles with a uniform and
NC-800 is estimated to be 28.7% (the detailed calculation is small size.
given in the ESI†).
The calcination temperature in the synthetic procedure is
The results shown above make it clear that the Co–CoO@NC/ found to have a profound effect on the nal product. As
NC-800 has a structure consisting of the sheet-like NC with the demonstrated by the TEM image of the Co–CoO@NC/NC-700 in
deposition of the NC coated Co–CoO nanoparticles. To under- Fig. S3b,† the low temperature calcination would lead to the
stand the role of urea in the formation of the Co–CoO@NC/NC- formation of smaller Co–CoO nanoparticles with a more
800, the same synthetic procedure was performed in the uniform size. The XPS spectrum in Fig. S4 and Table S1† show
absence of urea while keeping other parameters constant. It that the Co–CoO@NC/NC-700 contains relatively higher atomic
shows the formation of the Co–CoO@C/C-800 consisting of percentages of oxygen and nitrogen, suggesting that the urea
largely sized Co–CoO particles with a big inhomogeneity in the and glucose are not well carbonized at the low calcination
Fig. 3 (a) XPS survey spectra of the Co–CoO@NC/NC-800 and the NC. (b) High resolution Co 2p spectra of the Co–CoO@NC/NC-800 and the
Co–CoO@C/C-800. (c) High resolution C 1s spectra of the Co–CoO@NC/NC-800 and the NC. (d) High resolution N 1s spectra of the Co–
CoO@NC/NC-800 and the NC.
14466 | RSC Adv., 2018, 8, 14462–14472
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
temperature. On the other hand, when the reaction mixture was lower than that of the Co–CoO@NC/NC-800. This suggests that
calcined at the high temperature of 900 ꢀC, the formation of the the calcination temperature also has a great effect on the BET
Co–CoO nanoparticles with the relatively larger size and big size specic surface area of the sample. As demonstrated above, the
inhomogeneity is observed, as indicated by the TEM image of calcination temperature inuences the average size of Co–CoO
the Co–CoO@NC/NC-900 in Fig. S3c.† This indicates that the nanoparticles and the doping content of nitrogen in the NC,
mild reaction condition at the lower temperature is favorable and it also affects the pyrolysis rate of Co(NO3)2, glucose, and
for the formation of the Co–CoO nanoparticles with small and urea. These might be the reasons leading to the variations of the
uniform sizes, while at the higher calcination temperature, the BET specic surface areas of the samples with the calcination
improved uncontrollability of the decomposition of Co(NO3)2, temperature. The high specic BET surface area is helpful for
glucose, and urea due to the increase of the pyrolysis rates the application of the Co–CoO@NC/NC-800 as the electro-
would promote the formation of large Co–CoO nanoparticles catalyst for the ORR, since it allows for its good accessibility to
with a big inhomogeneity in the size distribution (the XPS the electrolyte.
spectrum, XRD diffraction pattern, and Raman spectra of the
The catalytic activity of the Co–CoO@NC/NC-800 for the ORR
Co–CoO@NC/NC-900 is given in Fig. S4, S5, and S6,† respec- is rst assessed by its CV curves in the N2 or O2 saturated 0.1 M
tively). Table S1† shows that the Co–CoO@NC/NC-900 contains KOH solution. As shown in Fig. 4a, the CV curve of the Co–
relatively lower atomic percentages of oxygen and nitrogen, CoO@NC/NC-800 in the N2 saturated 0.1 M KOH solution
suggesting that the high calcination temperature would shows featureless voltammetric currents, while that in the O2
decrease the contents of oxygen and nitrogen containing groups saturated 0.1 M KOH solution shows a well-dened cathodic
in the nal product. This is well consistent with the results re- peak corresponding to the reduction of oxygen. This clearly
ported previously,44,45 which showed that the high calcination demonstrates the catalytic activity of the Co–CoO@NC/NC-800
temperature would lead to the signicant decomposition of toward the ORR. Fig. 4b displays the LSV of the Co–CoO@NC/
nitrogen and oxygen containing groups in the NC. Additionally, NC-800 for the ORR in the O2 saturated 0.1 M KOH solution.
we also found that the drying step in the synthetic procedure is It clearly shows that the Co–CoO@NC/NC-800 can exhibit an
necessary to obtain the Co–CoO@NC/NC-800 with uniformly ORR onset potential of 0.961 V vs. RHE and half-wave potential
sized Co–CoO nanoparticles. As shown in Fig. S3d,† when the of 0.868 V vs. RHE. Most interestingly, the ORR onset and half-
reaction mixture was directly calcined at 800 ꢀC, the signicant wave potentials of the Co–CoO@NC/NC-800 are very close to
aggregation of the Co–CoO nanoparticles is observed in the those of the Pt/C (the ORR onset and half-wave potential of the
nal product. This might be attributed to the highly corrosive Pt/C are 0.962 and 0.861 V vs. RHE, respectively, which are close
nature of water vapor at the high temperature. Its presence in to the reported values48,49). This clearly suggests that the Co–
the reaction mixture would greatly accelerate the pyrolysis of CoO@NC/NC-800 is an efficient electrocatalyst for the ORR.
Co(NO3)3, glucose, and urea, leading to uncontrollable aggre- Comparison in Fig. 4d shows that the Co–CoO@NC/NC-800 can
gation and coalescence of the Co–CoO nanoparticles during even exhibit the ORR half-wave potential higher than those of
their nucleation and growth.
most catalysts reported previously (Table S2† shows a more
The textural property of the Co–CoO@NC/NC-800 was detailed comparison of the ORR onset and half-wave poten-
investigated by the N2 adsorption/desorption isotherms. As tials), strongly suggesting that the Co–CoO@NC/NC-800 is one
shown in Fig. 2c, the N2 adsorption/desorption isotherms of the of the best ORR catalysts among those reported (Table 1).
Co–CoO@NC/NC-800 exhibit a rapid uptake at p/p0 < 0.04 fol-
To understand the origin of the high ORR activity of the Co–
lowed by a H3-type hysteresis in the mesopore region. This CoO@NC/NC-800, the LSVs of the NC (synthesized from a same
indicates the co-existence of both the micropores and synthetic procedure while in the absence of Co(NO3)2), the Co–
macropores in the sample. Additionally, the big uptake of N2 at CoO@C/C-800, the Co–CoO@NC/NC-700, and the Co–
p/p0 > 0.9 in the adsorption/desorption isotherms also demon- CoO@NC/NC-900 in the O2 saturated 0.1 M KOH solutions were
strates the existence of macropores in the Co–CoO@NC/NC- also recorded. Fig. 4a and b shows that although the NC is
800. Base on the N2 adsorption/desorption isotherms in electrochemically active for the ORR, its activity is much lower
Fig. 2c, the specic surface area of the Co–CoO@NC/NC-800 than that of the Co–CoO@NC/NC-800. This clearly indicates
estimated using the BET method is 475 m2 gꢂ1. The presence a great importance of the Co–CoO nanoparticles in the high
of urea in the reaction precursor is found to have a signicant catalytic activity of the Co–CoO@NC/NC-800. The nitrogen
effect on the BET specic surface area of the sample. As shown doped structure of the carbon is also found to be important in
in Fig. 2c, the Co–CoO@C/C exhibits signicantly lower BET improving its catalytic activity of the Co–CoO@NC/NC-800 for
specic surface area than that of the Co–CoO@NC/NC-800. This the ORR. As shown in Fig. 4a and b, the Co–CoO@C/C-800
might be due to the reason that the thermal decomposition of exhibits signicantly lower catalytic activity for the ORR than
urea during the calcination would produce a large amount of that of the Co–CoO@NC/NC-800. Previous work has demon-
gases, such as CO2, NH3, and other gaseous products,46,47 which strated that the NC could be also highly active for the ORR,
are helpful to the formation of the porous structure in the Co– when its composition is well controlled.50–53 We therefore
CoO@NC/NC-800. Most interestingly, although the Co– assume that the high catalytic activity of the Co–CoO@NC/NC-
CoO@NC/NC-700 and the Co–CoO@NC/NC-900 exhibit the N2 800 is a result of a synergistic effect between two electroactive
adsorption/desorption isotherms comparable to those of the catalysts of the Co–CoO nanoparticles and the NC. Worthnoting
Co–CoO@NC/NC-800, their BET specic surface areas are much is that the catalytic activity of the sample is also effected by the
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 14462–14472 | 14467

View Article Online
RSC Advances
Paper
Fig. 4 (a) CV curves of the Co–CoO@NC/NC-800, the NC, the Co–CoO@C/C-800, the Co–CoO@NC/NC-700, and the Co–CoO@NC/NC-
900 in O2 or N2 saturated 0.1 M KOH solution at a scan rate of 5 mV sꢂ1 (b) LSVs for the ORR by the Co–CoO@NC/NC-800, the NC, the Co–
CoO@C/C-800, the Co–CoO@NC/NC-700, the Co–CoO@NC/NC-900, and the Pt/C in an O2-saturated 0.1 M KOH solution at a scan rate of
5 mV sꢂ1 (c) EIS spectra of the Co–CoO@NC/NC-800, the NC, the Co–CoO@C/C-800, the Co–CoO@NC/NC-700, and the Co–CoO@NC/NC-
900. (d) Comparison of the ORR half-wave potential of the Co–CoO@NC/NC-800 with those of the catalysts reported. (e) LSVs for the ORR by
the Co–CoO@NC/NC-800 in an O2-saturated 0.1 M KOH solution at various rotation rates at a scan rate of 5 mV sꢂ1 (f) K–L plots of the ORR by
the Co–CoO@NC/NC-800. (g) Electron transfer number and (h) kinetic current density of the ORR by the Co–CoO@NC/NC-800, the NC, the
Co–CoO@C/C-800, the Co–CoO@NC/NC-700, the Co–CoO@NC/NC-900 and the Pt/C.
calcination temperature. As shown in Fig. 4a and b, the catalytic composite materials.48 The nitrogen doping could promote the
activity of the Co–CoO@NC/NC-800 is higher than those of the electronic coupling between transition metal based catalysts
Co–CoO@NC/NC-700 and Co–CoO@NC/NC-900. This might be and carbon.39,48 It is therefore expectable that the strong elec-
due to the fact that the Co–CoO@NC/NC-800 synthesized at the tronic coupling between the Co–CoO nanoparticles and the NC
calcination temperature of 800 ꢀC has small and uniformly is an additional reason leading to the high catalytic activity of
sized Co–CoO nanoparticles and exhibits a higher BET specic the Co–CoO@NC/NC-800 for the ORR. To demonstrate the
surface area, which expose more active sites and increase the presence of the electronic coupling between the Co–CoO
accessibility of the Co–CoO@NC/NC-800 to the ORR. Addi- nanoparticles and the NC, the XPS spectra of the NC were
tionally, the Co–CoO@NC/NC-800 has a well carbonized struc- carefully analyzed. The XPS survey spectra in Fig. 3a shows the
ture with the doping of an appropriate amount of nitrogen, absence of the peak corresponding to Co in the NC. Most
which increases the electrical conductivity and hydrophilicity of notably, although the high-resolution N 1s spectrum of the NC
the Co–CoO@NC/NC-800. Fig. 4c shows the electrochemical exhibits the components similar to that of the Co–CoO@NC/
impedance spectra (EISs) of the Co–CoO@NC/NC-800, the NC, NC-800, their peak positions are slightly shied to the lower
the Co–CoO@C/C-800, the Co–CoO@NC/NC-700, and the Co– binding energies, as shown in Fig. 3d. This clearly corroborates
CoO@NC/NC-900, which clearly demonstrate that the Co– the presence of the strong electronic coupling between the Co–
CoO@NC/NC-800 has higher electrical conductivity.
CoO nanoparticles and the NC with a possible electron transfer
The work reported previously has suggest that the deposition from the Co–CoO nanoparticles to the NC. Indeed, the strong
of transition metal based catalysts onto the surface of carbon electronic coupling between the Co–CoO nanoparticles and the
would impose a strong electronic coupling between two mate- NC could also be demonstrated by the C 1s spectra in Fig. 3c,
rials, which could also improve the catalytic activities of the which shows the shis of the peak positions of C]N/C–O, and
14468 | RSC Adv., 2018, 8, 14462–14472
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
Table 1 Comparison of the ORR catalytic activities of the Co–
CoO@NC/NC-800, the NC, the Co–CoO@C/C-800, the Co–
CoO@NC/NC-700, the Co–CoO@NC/NC-900, and the Pt/C
potentials (Fig. 4f). This indicates that the ORR by the Co–
CoO@NC/NC-800 follows a rst order reaction kinetics with
respect to the concentration of dissolved oxygen. According to
the slopes of the K–L plots, the electron transfer number (n) of
the ORR can be extracted. Fig. 4g shows that the electron
transfer number of the ORR by the Co–CoO@NC/NC-800 is
always higher than 3.8 and close to that of the Pt/C over the
potential range covered in this work, demonstrating that the
ORR by the Co–CoO@NC/NC-800 proceeds mainly via a favor-
able four-electron pathway. This is different from the ORR by
the NC, the Co–CoO@C/C-800, the Co–CoO@NC/NC-700, and
Co–CoO@NC/NC-900 (the K-L plots of the NC, the Co–CoO@C/
C-800, the Co–CoO@NC/NC-700, and Co–CoO@NC/NC-900 are
given in Fig. S7†), in which a two-electron pathway plays an
increasing role in the reduction of oxygen. As shown in Fig. 4g,
the electron transfer numbers of the ORR by the NC, the Co–
CoO@C/C-800, the Co–CoO@NC/NC-700, and Co–CoO@NC/
NC-900 are much lower than 4.0. Fig. 4h presents the ORR
kinetic current density (jk) of the catalysts. It shows that the Co–
CoO@NC/NC-800 exhibits higher ORR kinetic current density
than the NC, the Co–CoO@C/C-800, the Co–CoO@NC/NC-700,
and Co–CoO@NC/NC-900, further demonstrating that the Co–
CoO@NC/NC-800 is a more efficient catalyst for the ORR than
the NC, the Co–CoO@C/C-800, the Co–CoO@NC/NC-700, and
Co–CoO@NC/NC-900.
The rotating ring-disk electrode (RRDE) measurement was
perform to further investigate the ORR by the Co–CoO@NC/NC-
800. Fig. 5a shows the LSV of the ORR by the Co–CoO@NC/NC-
800 obtained from the disk electrode of the RRDE shows
a prole similar to that directly obtained from the rotating disk
electrode (Fig. 4b). Most interestingly, the ORR by the Co–
CoO@NC/NC-800 shows a comparable ring current corre-
sponding to hydrogen peroxide ions (H2Oꢂ) to that by the Pt/C.
Fig. 5b reveals that the H2Oꢂ yield of the ORR by the Co–
CoO@NC/NC-800 is below 5% with an electron transfer number
higher than 3.9 over the potential range from 0.2 to 0.8 V. The
H2Oꢂ yield and electron transfer number of the ORR by the Co–
CoO@NC/NC-800 are very close to those by the Pt/C, as shown
Fig. 5b. Most interestingly, the H2Oꢂ yield of the ORR by the Co–
CoO@NC/NC-800 is lower than those of most catalysts reported
previously, while its electron transfer number is higher than
those of the ORR by most catalysts reported previously.36,59,60
This further demonstrates that the Co–CoO@NC/NC-800 has
a catalytic activity close to that of the Pt/C.
Onset potential/V
Half-wave potential/V
vs. RHE
Catalyst
vs. RHEa
Co–CoO@NC/NC-800
NC
Co–CoO@C/C-800
Co–CoO@NC/NC-700
Co–CoO@NC/NC-900
Pt/C
0.961
0.889
0.899
0.921
0.920
0.961
0.868
0.777
0.817
0.841
0.842
0.862
a
The ORR onset potential is dened at which the current density
reaches to 0.1 mA cmꢂ2
.
C–O–C/C–N in the NC to lower binding energies in comparison
to those in the Co–CoO@NC/NC-800, and by Co 2p spectra,
which shows the shis of the peaks of Co(0) and Co(II) in the Co–
CoO@NC/NC-800 to higher binding energies in comparison to
those in the Co–CoO@C/C-800. Additionally, Fig. 3d also shows
a difference between the N 1s spectra proles of the NC and the
Co–CoO@NC/NC-800. This might be due to the fact that the
growth of the Co–CoO nanoparticles on the nitrogen doping
sites makes some of the nitrogen atoms undetectable by the XPS
spectroscopy due to the low penetrability of X-ray photoelec-
trons, or the presence of the Co precursor in the reaction
mixture changes the relative percentages of the different
nitrogen components in the Co–CoO@NC/NC-800.
Previous work have demonstrated that the presence of metal–
N bond in the metal and nitrogen coexisting system is respon-
sible for the high catalytic activity for the ORR.54,55 In the work
presented here, however, the metal–N bonds are not detected in
both the XPS spectra of Co 2p and N 1s, which clearly rules out
the presence of the metal–N bond in the Co–CoO@NC/NC-800
and its contribution to the higher catalytic activity of the Co–
CoO@NC/NC-800. On the basis of this and the results shown
above, we therefore attribute the following aspects to the main
reasons responsible for the high catalytic activity of the Co–
CoO@NC/NC-800: (1) strong electronic interaction between the
Co–CoO nanoparticles and the NC, (2) high specic surface area
of the Co–CoO@NC/NC-800, (3) small and uniformly sized Co–
CoO nanoparticles, (4) good electric conductivity of the Co–
CoO@NC/NC-800 due to its well carbonized structure, (5) good
hydrophilicity of the Co–CoO@NC/NC-800 due to the presence of
the NC.30,56 Additionally, the interaction between Co and CoO
could be an additional reason leading to the high catalytic activity
of the Co–CoO@NC/NC-800. As reported previously, the
composites consisting of TMs and their oxides exhibit greatly
enhanced catalytic activities in comparison to their single
component counterparts.36,57,58
In view that the Co–CoO@NC/NC-800 is highly active for the
ORR with great potential for the replacement of the commercial
Pt/C, its stability and durability towards the methanol crossover
were tested. The current–time (i–t) chronoamperometric curve
in Fig. 5c shows that the current density of the ORR by the Pt/C
decreases rapidly with increase of the operating time. This
could be attributed to the low stability of the Pt/C in the alkaline
environment, which undergoes a fast degradation of the Pt/C
over the time due to the dissolution and aggregation of the Pt
nanoparticles and their surface oxidation.61,62 The introduction
of methanol causes a sharp decrease in its ORR current density
due to the poisoning of the ORR active sites by the oxidized
products of methanol (Fig. 5d).62,63 The stability of the Co–
To gain insight into the reaction kinetics and mechanism of
the ORR by the Co–CoO@NC/NC-800, its LSVs at the different
rotating rates were recorded. Fig. 4e shows an improvement of
the measured current density with increase of the rotating rate.
Analysis of the LSVs using the K–L equation shows a linear
relationship between the inverse of the current density (jꢂ1) and
the inverse of the root of the rotating rate (uꢂ1/2) at the different
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 14462–14472 | 14469

View Article Online
RSC Advances
Paper
Fig. 5 (a) LSV curves of the Co–CoO@NꢂC/NC-800 and the Pt/C obtained from the RRDE measurements at a rotation rate of 1600 rpm in the O2
saturated 0.1 M KOH solution. (b) HO2 yields (top) and electron transfer number (n, bottom) of the Co–CoO@NC/NC-800 and the Pt/C
obtained from the RRDE measurements. (c) Chronoamperometric curves of the Co–CoO@NC/NC-800 and the Pt/C at 0.67 V vs. RHE in the O2-
saturated 0.1 mol Lꢂ1 KOH solution. (d) Chronoamperometric curves of the Co–CoO@NC/NC-800 and the Pt/C electrodes with addition of
0.3 M methanol. The arrow indicates the addition of methanol.
CoO@NC/NC-800 and its durability toward the methanol catalyst for practical uses. The work presented here does not
crossover are much higher than those of the Pt/C. As shown in simply introduce a new and simple method for the synthesis of
Fig. 5c and d, the current density of the ORR by the Co– the ORR catalysts of high efficiency, and the same method
CoO@NC/NC-800 decreases much slowly in comparison to that might be also applicable to design and synthesize other cata-
by the Pt/C, and the introduction of methanol has a negligible lysts for various applications.
effect on its ORR current density. These results strongly suggest
that the specic coating structure of the Co–CoO nanoparticles
by the NC shell in the Co–CoO@NC/NC-800 could effectively
Conflicts of interest
protect the Co–CoO nanoparticles from the degradation and
poisoning of the ORR active sites by the oxidized products of
methanol.
There are no conicts to declare.
Acknowledgements
This work is nancially supported from the Chinese National
Natural Science Foundation (No. 11474101 and U1532139), the
“Outstanding Talent and Team Plans Program” of South China
University of Technology, the Guangdong Provincial Natural
Science Foundation (No. 2017A030313092), the Guangdong
Innovative and Entrepreneurial Research Team Program (No.
2014ZT05N200), and the Ningbo Natural Science Foundation
(No. 2017A610059).
Conclusions
In summary, the Co–CoO@NC/NC-800, which consists of the
NC nanosheets with deposition of the NC coated Co–CoO
nanoparticles, has been synthesized through a simple calcina-
tion of the dried reaction mixture of Co(NO3)2, glucose, and
urea. The Co–CoO@NC/NC-800 is highly active for the ORR and
can deliver an onset potential of 0.961 V vs. RHE and a half-wave
potential of 0.868 V vs. RHE. The onset and the half-wave
potentials of the Co–CoO@NC/NC-800 are very close to those
of the Pt/C. This, in conjunction with its higher stability and
better durability towards the methanol crossover, strongly
suggests that the Co–CoO@NC/NC-800 is a promising ORR
References
1 S. K. Singh, V. M. Dhavale and S. Kurungot, ACS Appl. Mater.
Interfaces, 2015, 7, 21138–21149.
14470 | RSC Adv., 2018, 8, 14462–14472
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
RSC Advances
2 L. Li, C. Liu, G. He, D. Fan and A. Manthiram, Energy Environ. 29 K. Kreek, A. Sarapuu, L. Samolberg, U. Joost, V. Mikli,
Sci., 2015, 8, 3274–3282.
3 K. Mohanraju and L. Cindrella, RSC Adv., 2014, 4, 11939–
11947.
4 Y. Jiao, Y. Zheng, M. Jaroniec and S. Z. Qiao, Chem. Soc. Rev.,
2015, 44, 2060–2086.
5 L. Bu, S. Guo, X. Zhang, X. Shen, D. Su, G. Lu, X. Zhu, J. Yao,
J. Guo and X. Huang, Nat. Commun., 2016, 7, 11850–11859.
M. Koel and K. Tammeveski, ChemElectroChem, 2015, 2,
2079–2088.
30 C. Han, X. Bo, Y. Zhang, M. Li, A. Nsabimana and L. Guo,
Nanoscale, 2015, 7, 5607–5611.
31 X. Zhang, R. Liu, Y. Zang, G. Liu, G. Wang, Y. Zhang,
H. Zhang and H. Zhao, Chem. Commun., 2016, 52, 5946–
5949.
¨
6 G. Hu, E. Gracia-Espino, R. Sandstrom, T. Shari, S. Cheng, 32 H. Zhong, Y. Luo, S. He, P. Tang, D. Li, N. Alonso-Vante and
˚
H. Shen, C. Wang, S. Guo, G. Yang and T. Wagberg, Catal.
Sci. Technol., 2016, 6, 1393–1401.
Y. Feng, ACS Appl. Mater. Interfaces, 2017, 9, 2541–2549.
33 K. Liu, X. Huang, H. Wang, F. Li, Y. Tang, J. Li and M. Shao,
ACS Appl. Mater. Interfaces, 2016, 8, 34422–34430.
34 A. B. Yousaf, M. Imran, N. Uwitonze, A. Zeb, S. J. Zaidi,
T. M. Ansari, G. Yasmeen and S. Manzoor, J. Phys. Chem.
C, 2017, 121, 2069–2079.
7 J. Ding, X. Zhu, L. Bu, J. Yao, J. Guo, S. Guo and X. Huang,
Chem. Commun., 2015, 51, 9722–9725.
8 Y. Zhao, J. Liu, Y. Zhao and F. Wang, Phys. Chem. Chem.
Phys., 2014, 16, 19298–19306.
9 D. U. Lee, M. G. Park, H. W. Park, M. H. Seo, X. Wang and 35 Y. Cheng, S. Dou, M. Saunders, J. Zhang, J. Pan, S. Wang and
Z. Chen, ChemSusChem, 2015, 8, 3129–3138. S. P. Jiang, J. Mater. Chem. A, 2016, 4, 13881–13889.
10 P. Sennu, M. Christy, V. Aravindan, Y.-G. Lee, K. S. Nahm and 36 Z. Wu, J. Wang, L. Han, R. Lin, H. Liu, H. L. Xin and D. Wang,
Y.-S. Lee, Chem. Mater., 2015, 27, 5726–5735. Nanoscale, 2016, 8, 4681–4687.
11 C. Zhu, H. Li, S. Fu, D. Du and Y. Lin, Chem. Soc. Rev., 2016, 37 T.-Y. Yung, L.-Y. Huang, T.-Y. Chan, K.-S. Wang, T.-Y. Liu,
45, 517–531.
P.-T. Chen, C.-Y. Chao and L.-K. Liu, Nanoscale Res. Lett.,
12 H. J. Choi, N. A. Kumar and J. B. Baek, Nanoscale, 2015, 7,
6991–6998.
13 J. E. Kim, J. Lim, G. Y. Lee, S. H. Choi, U. N. Maiti, W. J. Lee,
2014, 9, 444.
38 X. Dong, H. Xu, X. Wang and Y. Huang, ACS Nano, 2012, 6,
3206–3213.
H. J. Lee and S. O. Kim, ACS Appl. Mater. Interfaces, 2016, 8, 39 Z. J. Jiang and Z. Jiang, Sci. Rep., 2016, 6, 27081.
1571–1577.
40 S. Wang, J. Zhang, P. Shang, Y. Li, Z. Chen and Q. Xu, Chem.
Commun., 2014, 50, 12091–12094.
41 Z. Lin, G. Waller, Y. Liu, M. Liu and C.-P. Wong, Adv. Energy
Mater., 2012, 2, 884–888.
42 C. Ma, X. Shao and D. Cao, J. Mater. Chem. A, 2012, 22, 8911–
8915.
14 M. Shen, C. Ruan, Y. Chen, C. Jiang, K. Ai and L. Lu, ACS
Appl. Mater. Interfaces, 2015, 7, 1207–1218.
15 Y. Zhan, M. Lu, S. Yang, C. Xu, Z. Liu and J. Y. Lee,
ChemCatChem, 2016, 8, 372–379.
16 J. Masa, W. Xia, I. Sinev, A. Zhao, Z. Sun, S. Grutzke,
P. Weide, M. Muhler and W. Schuhmann, Angew. Chem., 43 Y.-X. Yu, Phys. Chem. Chem. Phys., 2013, 16819–16827.
Int. Ed., 2014, 53, 8508–8512.
17 M. Sun, H. Liu, Y. Liu, J. Qu and J. Li, Nanoscale, 2015, 7,
1250–1269.
44 C. V. Rao, C. R. Cabrera and Y. Ishikawa, J. Phys. Chem. Lett.,
2010, 1, 2622–2627.
¨
45 S. Yang, X. Feng, X. Wang and K. Mullen, Angew. Chem., Int.
18 J. Xu, Q. Yu, C. Wu and L. Guan, J. Mater. Chem. A, 2015, 3,
21647–21654.
19 S. Guo, S. Zhang, L. Wu and S. Sun, Angew. Chem., Int. Ed.,
2012, 51, 11770–11773.
20 H. Yu, Y. Li, X. Li, L. Fan and S. Yang, Chem.–Eur. J., 2014, 20,
3457–3462.
21 M. Li, X. Bo, Y. Zhang, C. Han, A. Nsabimana and L. Guo, J.
Mater. Chem. A, 2014, 2, 11672–11682.
Ed., 2011, 50, 5339–5343.
46 X. Zhang, D. Yu, Y. Zhang, W. Guo, X. Ma and X. He, RSC
Adv., 2016, 6, 104183–104192.
47 S. Cao, H. Chen, F. Jiang and X. Wang, Appl. Catal., B, 2018,
224, 222–229.
48 Y. Liang, H. Wang, J. Zhou, Y. Li, J. Wang, T. Regier and
H. Dai, J. Am. Chem. Soc., 2012, 134, 3517–3523.
49 Y. Liang, H. Wang, P. Diao, W. Chang, G. Hong, Y. Li,
M. Gong, L. Xie, J. Zhou, J. Wang, T. Z. Regier, F. Wei and
H. Dai, J. Am. Chem. Soc., 2012, 134, 15849–15857.
22 Z. Wang, S. Xiao, Z. Zhu, X. Long, X. Zheng, X. Lu and
S. Yang, ACS Appl. Mater. Interfaces, 2015, 7, 4048–4055.
23 Y. Li, F. Cheng, J. Zhang, Z. Chen, Q. Xu and S. Guo, Small, 50 D. Gao, R. Shibuya, C. Akiba, S. Saji, T. Kondo and
2016, 12, 2839–2845. J. Nakamura, Science, 2016, 351, 361–365.
24 J. Deng, P. Ren, D. Deng and X. Bao, Angew. Chem., 2015, 127, 51 L. Lai, J. R. Potts, D. Zhan, L. Wang, C. K. Poh, C. Tang,
2128–2132.
H. Gong, Z. Shen, J. Lin and R. S. Ruoff, Energy Environ.
Sci., 2012, 5, 7936–7942.
25 D. Deng, L. Yu, X. Chen, G. Wang, L. Jin, X. Pan, J. Deng,
G. Sun and X. Bao, Angew. Chem., Int. Ed., 2013, 52, 371–375. 52 Z. Jiang, Z.-j. Jiang, X. Tian and W. Chen, J. Mater. Chem. A,
26 Y. Wang, X. Ding, F. Wang, J. Li, S. Song and H. Zhang,
Chem. Sci., 2016, 7, 4284–4290.
2014, 2, 441–450.
53 Z.-J. Jiang and Z. Jiang, J. Mater. Chem. A, 2014, 2, 14071–
27 M. Zhuang, X. Ou, Y. Dou, L. Zhang, Q. Zhang, R. Wu,
14081.
Y. Ding, M. Shao and Z. Luo, Nano Lett., 2016, 16, 4691–4698. 54 C. Li, Z. Han, Y. Yu, Y. Zhang, B. Dong, A. Kong and Y. Shan,
28 T. Kottakkat and M. Bron, ChemElectroChem, 2014, 1, 2163–
RSC Adv., 2016, 6, 15167–15174.
2171.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 14462–14472 | 14471

View Article Online
RSC Advances
Paper
55 Y. Jiang, Y. Lu, X. Wang, Y. Bao, W. Chen and L. Niu, 59 C. Han, X. Bo, Y. Zhang, M. Li, A. Wang and L. Guo, Chem.
Nanoscale, 2014, 6, 15066–15072. Commun., 2015, 51, 15015–15018.
56 D.-W. Wang and D. Su, Energy Environ. Sci., 2014, 7, 576–591. 60 G. L. Tian, M. Q. Zhao, D. Yu, X. Y. Kong, J. Q. Huang,
57 V. M. Dhavale, S. K. Singh, A. Nadeem, S. S. Gaikwad and
Q. Zhang and F. Wei, Small, 2014, 10, 2251–2259.
S. Kurungot, Nanoscale, 2015, 7, 20117–20125.
61 Y. Nie, L. Li and Z. Wei, Chem. Soc. Rev., 2015, 44, 2168–2201.
58 X. X. Liu, J. B. Zang, L. Chen, L. B. Chen, X. Chen, P. Wu, 62 H. Uchida, K. Izumi and M. Watanabe, J. Phys. Chem. B,
S. Y. Zhou and Y. H. Wang, J. Mater. Chem. A, 2017, 5,
5865–5872.
2006, 110, 21924–21930.
63 F. Tian and A. B. Anderson, J. Phys. Chem. C, 2008, 112,
18566–18571.
14472 | RSC Adv., 2018, 8, 14462–14472
This journal is © The Royal Society of Chemistry 2018