Hollow Polyhedral Arrays Composed of a Co3O4 Nanocrystal Ensemble on a Honeycomb-like Carbon Hybrid for Boosting Highly Active and Stable Evolution Oxygen

Article abstract of DOI:10.1021/acs.inorgchem.8b03236

We synthesize hollow polyhedral arrays composed of honeycomb-like nanosheets of Co₃O₄ nanocrystals imbedded on carbon quantum dots (CQDs)- and nitrogen-codoped carbon matrix via a facile in situ air oxidation pyrolysis for CQDs-incorporated metal-organic framework polyhedral arrays. The function of CQDs hollowing and forming porous nanosheet shells was found. The resulting hierarchical architecture displays excellent oxygen evolution reaction activity with a low overpotential of 301 mV to drive 100 mA cm^-2 in 1.0 M KOH and long-playing durability in oxygen evolution. The high performance can be ascribed to its highly dispersed Co₃O₄ nanocrystals, CQDs and nitrogen codoping, internal cavities, and hierarchical pore system.

Full text of DOI:10.1021/acs.inorgchem.8b03236

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Hollow Polyhedral Arrays Composed of a Co O Nanocrystal
3
4
Ensemble on a Honeycomb-like Carbon Hybrid for Boosting Highly
Active and Stable Evolution Oxygen
,
†
†
,‡
†
†
†
†
Xuan Kuang,* Rui Kuang,* Yanfang Dong, Zhiling Wang, Xu Sun, Yong Zhang,
,
and Qin Wei*
†Key Laboratory of Interfacial Reaction & Sensing Analysis in Universities of Shandong, Shandong Provincial Key Laboratory of
Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022,
China
‡
College of Traffic Civil Engineering, Shandong Jiaotong University, Jinan 250023, China
*
S Supporting Information
ABSTRACT: We synthesize hollow polyhedral arrays composed of honeycomb-like nanosheets of Co O nanocrystals
3
4
imbedded on carbon quantum dots (CQDs)- and nitrogen-codoped carbon matrix via a facile in situ air oxidation pyrolysis for
CQDs-incorporated metal−organic framework polyhedral arrays. The function of CQDs hollowing and forming porous
nanosheet shells was found. The resulting hierarchical architecture displays excellent oxygen evolution reaction activity with a
−
2
low overpotential of 301 mV to drive 100 mA cm in 1.0 M KOH and long-playing durability in oxygen evolution. The high
performance can be ascribed to its highly dispersed Co O nanocrystals, CQDs and nitrogen codoping, internal cavities, and
3
4
hierarchical pore system.
6
,7
1
. INTRODUCTION
mass transport and catalytic kinetics over solid counterparts.
However, the compact shells lead to the inner part being
inactive because of the greatly hampered inward diffusion.
Therefore, hollow Co O micro/nanostructures with perme-
Driven by the ever-growing demand for a clean and highly
efficient hydrogen energy source, many more research efforts
focus on the rational design and preparation of highly efficient,
stable, and cost-efficient oxygen evolution reaction (OER)
electrocatalysts because of their sluggish kinetic barrier, thus
3
4
8
able porous thin shells are highly desired for the OER process.
Further, Co O nanocrystals imbedded on hollow porous
3
4
1
−4
supports, which can disperse and protect the catalysts from
leach-out and/or aggregation, are the optimized alternatives.
Carbon materials have been demonstrated to be ideal
candidates for supporting various nanocatalysts because of
their high surface areas, controllable morphologies, and
leading to a high overpotential as the energy-intensive step.
Spinel Co O is a promising candidate to replace scarce noble-
3
4
metal catalysts like IrO or RuO because of its natural
2
2
5
abundance and proper redox capability in alkaline media. To
overcome the inherent deficiency of a single material, it is very
meaningful for engineering complex Co O micro/nanostruc-
9
outstanding chemical and thermal stability. Doping of
3
4
functional heteroatoms (such as nitrogen) into carbon
networks can effectively tune their intrinsic electronic
structures because of their higher electronegativity (3.04)
relative to carbon (2.55) and thus improve their catalysis
tures to (i) expose intrinsic catalytic sites, (ii) enhance the
electroconductivity, (iii) chance the mass-transfer properties,
4
and (iv) increase the limited electrode surface area.
As a unique architecture, hollow micro/nanostructures with
well-defined internal cavities, functional shell components are
more attractive electrocatalysts in the OER process because of
their more electrochemical active sites and shortened pathways
for gas and electrolyte diffusion and thus facilitated charge/
1
0,11
activity.
Despite this, various contemporary carbon
Received: November 24, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b03236
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Schematic illustration to form Co O @CQDs-CN HPAs/CoF.
3
4
materials such as multiwalled carbon nanotubes and graphenes
have been applied to obtain the required morphologies and
activity and long-term durability. The low overpotential of 301
−
2
mV to attain 100 mA cm in 1.0 M KOH precedes a series of
benchmarking Co O -based OER electrocatalysts in alkaline
1
2
compositions. For the fabrication of hollow polyhedral
structures, the materials do not apply because of the restriction
of their morphologies. A traditional hard-template approach is
known as the most efficient route but suffers from complicated
conditions such as time- and energy-consuming problems for
3
4
media (Table S1). To the best of our knowledge, this is the
first hybrid by CQDs hollowing, which offers a new strategy for
enhancing the electrocatalytic performance.
13
template removal. The soft-template approach, like surfac-
tants and organic polymers, is also limited because this
generates the nanoparticles (NPs) of nonuniform distribution
2. EXPERIMENTAL SECTION
2.1. Synthesis of CQDs. CQDs were easily and quickly prepared
using microwave irradiation. Briefly, an aqueous solution of the
14
in hollow structures. For these, novel and facile strategies,
especially the engineering of self-supported hollow porous
polyhedral nanoarrays as binder-free electrodes that (i) allow
for straightforward template removal and (ii) create a
functional matrix of dispersing Co O nanocrystals to max-
carbon precursor D-glucose (1.67 mmol, 0.3 g) and NaH PO (0.72
2 4
mmol, 0.1 g) acted as the foaming agent to accelerate the reaction,
and ultrapure water (2 mL) in a 350 W domestic microwave oven was
heated for 1.5 min. Thus, a brown-yellow colloidal solid was obtained.
The colloidal solid was dispersed in deionized water, centrifuged at
3
4
1
2000 rpm for 30 min to remove the large particles, and then dried by
imize their surface areas, catalytic active sites, and catalytic
stability, represent alternative ways to accelerate the
microwave at a low power of 250 W. Thus, 56 mg of CQDs was
obtained.
1
,10,15
OER,
which, however, have been not reported.
2.2. Preparation of Co O @CQDs-CN HPAs/CoF and Co O @
3
4
3
4
Herein, we report the novel hollow polyhedral arrays grown
on Co foil (CoF), where polyhedra are composed of porous
CN/CoF. Typically, Co O @CQDs-CN HPAs/CoF was synthesized
3 4
via a facile yet simple two-step process. Typically, CoF was first
treated with diluted hydrochloric acid (1.0 M HCl) and then washed
with ethanol and water several times. DABCO (1.0 mmoL) and
honeycomb-like nanosheet arrays of Co O4 nanocrystals
3
imbedded on a carbon quantum dots (CQDs)- and nitro-
gen-codoped carbon matrix (denoted as Co O @CQDs-CN
H BDC (1.0 mmoL) were added to a mixed solvent of ethanol (1
2
3
4
mL) and DMF (4 mL). Then, a LiOH aqueous solution (1 mL, 3.7%)
was slowly dropped into the mixture to form a transparent ligand
solution. The CQDs (56 mg) were also dispersed in the ligand
HPAs/CoF). The hierarchical architecture is derived from a
facile in situ air oxidation pyrolysis for CQDs-incorporated
−
1
cobalt-based metal−organic framework polyhedral arrays
solution, where the CQDs concentration was 11.2 mg mL . CQDs@
II
II
(
denoted as CQDs@Co -MOF/CoF) at 300 °C. The present
Co -MOF were directly grown on conductive CoF in the ligand
II
solution. A standard three-electrode system was used with a CoF (1
Co -MOF, Co (1,4-BDC) (DABCO)·4DMF·H O (DMF =
2
2
2
cm × 1 cm) as the working electrode and a Pt wire and Hg/HgO as
N,N′-dimethylformamide), was employed because of its
II
the counter and reference electrodes, respectively. CQDs@Co -
porous well-defined 3D crystalline framework constructed
II
MOF/CoF was obtained by applying a constant oxidation potential
from Co cations and organic linkers nitrogen-rich 1,4-
(
+1.5 V) for the solution for 10 min at 15 °C.
diazabicyclo[2.2.2]octane (DABCO) and aromatic 1,4-benze-
II
For comparison, Co -MOF/CoF was also prepared using the same
nedicarboxylic acid (H BDC) at the molecular level. By
II
2
preparation process as that of CQDs@Co -MOF/CoF, except that a
recipe without CQDs was involved.
oxidation pyrolysis, the structure-controlling template is
expected to transform into a porous polyhedron where cobalt
oxide NPs are embedded in a nitrogen-doped carbon matrix.
Further, it will be more promising that the MOF precursor is
loaded into noninnocent guest molecules in framework pores
by host−guest chemistries, which may generate the hierarch-
ical structure with novel functionalities under pyrolysis,
although thus far few researches are reported in this
II
II
The as-prepared CQDs@Co -MOF/CoF and Co -MOF/CoF
−
1
were heated to 300 °C with a heating rate of 5 °C min and kept for
2
h in a tube furnace under air. After the tube furnace was cooled to
−
1
room temperature at a rate of 2 °C min , the products Co O @
CQDs-CN HPAs/CoF and Co O @CN/CoF were harvested.
3
4
3
4
2
.3. Preparation of Ru O/CoF. Powdered RuO was prepared
2
2
16
according to a previous report. Next, 3.4 mg of RuO powder was
2
dispersed in the solvents composed of 720 μL of water, 250 μL of
ethanol, and 30 μL of a Nafion solution (5 wt %), followed by
1
6−18
strategy.
The unique organic−inorganic constitutes
enable MOFs to be especially suitable as precursors for
widespread carbon nanomaterials. CQDs were utilized for
their small and narrow size distribution (<10 nm) and
ultrasonication for 15 min. Then, 100 μL of a homogeneous RuO
suspension was coated onto the 1 cm CoF, followed by drying at 50
2
19
2
°C. The procedure was repeated five times (RuO loading: 1.7 mg
2
−
2
cm ).
abundant surface functional groups (e.g., −NH , −COOH,
2
and −OH) that enable strong interaction (e.g., hydrogen
20,21
3. RESULTS AND DISCUSSION
bonding) with MOFs
and would endow novel function-
ality to the resulting hybrid. Surprisingly, CQDs have a
hollowing function and involved oxidation, accounting for the
shell formation of porous nanosheet arrays. Also, Co O @
Co O @CQDs-CN HPAs/CoF, as shown in Figure 1, was
3 4
synthesized via a facile yet simple two-step process: electro-
II
deposition for the synthesis of CQDs@Co -MOF/CoF and
3
4
CQDs-CN HPAs/CoF display remarkable improved OER
oxidation pyrolysis for the final product. First, the CQDs@
B
DOI: 10.1021/acs.inorgchem.8b03236
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
II
II
Figure 2. (a) FE-SEM images of CQDs@Co -MOF (inset: its high-magnifcation FE-SEM image). (b) XRD patterns of CQDs, CQDs@Co -
MOF, and Co O @CQDs-CN HPAs (c−e) FE-SEM images. (g) TEM and (h) high-magnification TEM images. (i) SEM−EDS elemental
3
4
mapping of Co O @CQDs-CN HPAs.
3
4
II
Co -MOF was directly grown on conductive CoF by a
potentiostatic oxidation in a solution of CQDs and the mixed
uniform honeycomb-like nanosheet arrays with thin walls of
∼60 nm thickness. The images of high-magnification FE-SEM
(Figure 2f) and TEM (Figure 2g) show that numerous small
NPs with ∼20 nm diameter are uniformly imbedded on the
matrix. The lattice spacing of 0.24 nm corresponds to the
ligands DABCO and H BDC. A CoF used as the working
electrode was oxidized, and released Co ions coordinated to
the ligands to fabricate CQDs@Co -MOF/CoF. The field-
emission scanning electron microscopy (FE-SEM) images
clearly reveal that the CQDs@Co -MOF as a well-defined
tetrahedron covers the CoF (Figure 2a). A close-up inspection
inset of Figure 2a) finds that the tetrahedron consists of
2
II
II
26
(311) plane of the spinel Co O , while the crystal lattice
3 4
II
fringes of 0.32 nm match well with the (002) crystallographic
27
planes of graphitic carbon for CQDs (Figures 2h and S3).
The broad PL emission for Co O @CQDs-CN HPAs powders
(
3
4
multiple, closely packed lamellae with smooth surfaces, the
at 350−400 nm (λ = 295 nm) confirms the presence of
ex
edge dimensions of which range from ∼2 to 3 μm. The X-ray
CQDs in the product (Figure S4). Elemental mapping (Figure
2i) from SEM−energy-dispersive X-ray spectroscopy (EDS)
demonstrates uniform distribution of carbon, nitrogen, oxygen,
cobalt, and phosphorus in the material. Moreover, powder
XRD peaks located at 19.2°, 31.5°, 36.8°, and 44.8°
correspond to the crystallographic planes (111), (220),
(311), and (400) of the cubic spinel phase Co O , respectively
II
diffraction (XRD) pattern of CQDs@Co -MOF matches with
the simulated pattern of Co (1,4-BDC) (DABCO)·4DMF·
2
2
22
H O (Figure 2b), indicating the same crystalline structures.
The photoluminescence (PL) spectrum of CQDs@Co -MOF
2
II
powders shows an obvious emission peak of CQDs at ∼365
nm excited at 295 nm (Figures S1 and S2). An evident blue
3
4
II
shift from that of CQDs suggests that Co -MOF can fix CQDs
(Figure 2b). The broad peaks centered at 24.2° and 42.8°
agree well with those of graphitized carbon. The characteristic
peak for CQDs at 26.2° is not found, which may explain the
low content of CQDs.
23
and block the aggregation of CQDs. However, the diffraction
2
4
pattern of CQDs with the lattice planes of graphitic carbon
II
does not appear in the XRD pattern of CQDs@Co -MOF
Figure 2b). From these, we can conclude that CQDs are
incorporated into the Co -MOF pores and not loaded onto
(
To gain further insight into the effect of CQDs on Co O @
3
4
II
CQDs-CN HPAs formation, we first examined the thermo-
stability of CQDs. As shown in Figure S5, the first-stage weight
loss (4.7%) occurs at 60−120 °C, which is assigned to the
escape of adsorbed water molecules. A subsequent significant
weight loss of 47.3%, which starts at 130 °C and ends at 315
°C, is assigned to the oxidation and decomposition of active
moieties in CQDs during the process. From 316 to 400 °C,
about 33.4% weight loss is attributed to the high carbonization
II
25
the outer surfaces of Co -MOF particles.
II
In the second step, CQDs@Co -MOF, regarded as the self-
sacrificing template, was oxidized and pyrolyzed at 300 °C for
2
h in air. Excitedly, the solid tetrahedral arrays covering CoF
have been changed to the hollow polyhedral arrays Co O @
3
4
CQDs-CN HPAs (Figure 2c,d). Also, oxidation pyrolysis
obviously expands the size of the building blocks (Figure 2a,c).
More specifically, a closer observation (Figure 2e,f) clearly
finds that the hollow polyhedra are composed of porous and
28
of the CQD core. This fact reflects that there are partial
CQDs, resulting in the product after oxidation pyrolysis of
C
DOI: 10.1021/acs.inorgchem.8b03236
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
II
II
CQDs@Co -MOF at 300 °C. CQDs-free Co -MOF/CoF and
on a CQDs-, pyrrolic nitrogen-, and pyridinic nitrogen-
codoped sp -hybridized carbon matrix.
2
its pyrolysis product Co O @CN/CoF were also monitored.
FE-SEM images in Figure S6a clearly show that Co -MOFs on
CoF are porous rectangular arrays instead of solid tetrahedral
arrays of CQDs@Co -MOFs with smooth surfaces. The
compacting role of the CQDs for Co -MOF may be the
strong electrostatic interaction between CQDs with carboxyl
−COOH) and hydroxyl (−OH) groups and the Co -MOF
framework. Compared with Co O @CQDs-CN HPAs with
3
4
II
The OER performances of self-supporting integrated
materials directly as electrodes were investigated in 1.0 M
KOH by linear sweep voltammetry (LSV) curves. The loading
II
II
−2
mass of each material on CoF was 1.7 ± 0.1 mg cm . The
LSV curves of the Co O @CQDs-CN/CoF materials with
3
4
II
(
different CQD contents were initially explored. As shown as in
Figure S8, their OER activities are obviously improved with
increasing CQD concentrations in the precursor solution of
3
4
II
hollow and porous nanosheet arrays, Co -MOF-derived
Co O @CN preserves the analogous solid rectangular back-
bone as its precursor (Figure S6b). Thus, we can conclude that
II
−1
CQDs@Co -MOF/CoF up to 11.2 mg mL , above which the
activities decrease. When CQD concentrations exceeds 16.8
3
4
II
−1
II
CQDs in CQDs@Co -MOF play a crucial role in inflating and
mg mL , the electrosynthesis of Co -MOF on NiF becomes
very difficult because of growth of the CQDs-Co complex.
II
hollowing the compacted building blocks into hollow
polyhedra because of their partial pyrolysis. Moreover, CQDs
with excellent electron donors should be involved with air
oxidation of the Co ion into Co O nanocrystals and carbon
hybrid nanosheet arrays.
−
1
Thus, the 11.2 mg mL CQD concentration in the precursor
solution was used in follow-up experiments. Figure S9 shows
the LSV curves of the materials from oxidation pyrolysis of
II
3
4
II
CQDs@Co -MOF/CoF at different temperatures. Clearly, the
X-ray photoelectron spectroscopy (XPS) measurements
reveal that atomic fractions of C, N, O, Co, and P in
Co O @CQDs-CN HPAs (Figure S7) are 32.24, 3.19, 34.26,
OER activities of the materials are highly dependent on the
factor. The above-discussed CQDs-Co O @CN OHPA/CoF
3
4
3
4
corresponding to 300 °C shows the lowest overpotential of
−2
3
0.17, and 0.14%, respectively. Deconvolution C 1s peaks
located at 284.8, 286.1, and 288.3 eV (Figure 3a) show the
301 mV to drive the electric current density of 100 mA cm ,
much smaller than those at 200 °C (392 mV), 400 °C (452
mV), and 500 °C (463 mV). We also compare the OER
properties of the two materials, namely, the above-discussed
CQDs-Co O @CN OHPA/CoF and another CQDs-Co O @
3
4
3
4
CN/CoF, where CQDs-Co O @CN (Figure S10a) existing in
3
4
the form of nanosheets is derived from oxidation pyrolysis of
II
the rod-shaped CQDs@Co -MOF precursor (Figure S10b) at
II
3
00 °C for 2 h under air. The CQDs@Co -MOF precursor
was prepared by a regular hydrothermal method. To achieve
the electric current density of 100 mA cm , the overpotential
of CQDs-Co O @CN/CoF is 1.5 times that of CQDs-
−
2
3
4
Co O @CN OHPA/CoF (446 mV compared to 301 mV;
3
4
Figure S11). The result exhibits the important contribution of
hollow polyhedral arrays composed of porous honeycomb-like
nanosheets for the electrocatalytic performance.
Figure 4a clearly reveals that bare CoF shows negligible
II
catalytic activity. In sharp contrast, the precursor CQDs@Co -
MOF/CoF shows significantly enhanced OER activity over
II
Co -MOF/CoF, implying a crucial role of CQDs for the OER
Figure 3. High-resolution XPS spectra of (a) C 1s, (b) O 1s, (c) Co
performance. Particularly, Co O @CQDs-CN HPAs/CoF
3
4
2
p, and (d) N 1s for Co O @CQDs-CN HPAs.
only needs an overpotential (η) of 301 mV to reach 100 mA
3
4
−
2
cm , achieving the highest OER activity among the materials,
demonstrating that a hollow nanostructure with CQDs
2
existence of sp -hybridized C in CC, C−OH, and COOH,
respectively. Another two at 284.2 and 285.3 eV represent C−
facilitates the OER performance. Also, this compares favorably
2
to the catalytic behavior of RuO
/CoF (η100 mA cm− = 316
2
2
9
C and CN. High-resolution O 1s peaks at 532.0 and 533.2
mV). In addition, the presence of a phosphorus element in the
present Co @CQDs-CN HPAs/CoF has a negligible effect
eV display CO/O−C−OH and C−OH (Figure 3b). The O
O
3 4
•
1
s spectrum at 531.2 eV is assigned to hydroxyl groups ( OH)
on its OER activity, which should be due to the low content of
P in the material (0.14% from XPS analysis).
16
resulting from surface defects. In the Co 2p region (Figure
c), four deconvoluted peaks for 2p3/2/2p1/2 of Co and Co
3
+
2+
3
The Tafel slope of Co
3
O
4
@CQDs-CN HPAs/CoF (115 mV
−1
appear at 781.1/783.2 and 797.1/798.5 eV, while the
corresponding shakeup satellite peaks are at 786.5/802.7 eV.
dec ) is greatly lower than those of CQDs-free Co O @CN/
3
4
−
1
II
CoF (184 mV dec ) and the precursors CQDs@Co -MOF/
3+
2+
−1
II
−1
The atomic ratio of Co /Co calculated by the fitted curve
areas is ∼2:1, which matches well with the formula Co O with
CoF (170 mV dec ) and Co -MOF/CoF (187 mV dec )
−
1
but only slightly lower than that of RuO /CoF (127 mV dec ;
3
4
2
3
0
the spinel crystal. For the N 1s region (Figure 3d),
graphitized nitrogen at 399.1 eV and pyrrolic nitrogen at
Figure 4b). The result suggests that Co O @CQDs-CN
3
4
HPAs/CoF possesses more rapid catalytic kinetics for OER
32
4
00.1 eV are the main nitrogen species, while the peak at 400.8
than others do.
Figure 4c displays the catalytic activity of Co O @CQDs-
31
eV exhibits less pyridinic N. These facts have manifested that
the hollow polyhedral arrays of Co O @CQDs-CN HPAs are
3
4
−
2
CN HPAs/CoF at 40 mA cm being retrained for at least 25
h. Moreover, the LSV curves before and after continuous 1000
cyclic voltammetry (CV) scanning show negligible changes in
3
4
actually composed of uniform porous honeycomb-like nano-
sheets, in which small Co O nanocrystals are highly dispersed
3
4
D
DOI: 10.1021/acs.inorgchem.8b03236
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Inorganic Chemistry
Article
II
Figure 4. (a) LSV curves and (b) Tafel plots for Co O @CQDs-CN HPAs/CoF and its precursor CQDs@Co -MOF/CoF, Co O @CN/CoF
3
4
3
4
II
and its precursor Co -MOF/CoF, RuO /CoF, and bare CoF. (c) Time-dependent current density for Co O @CQDs-CN HPAs/CoF at a 301 mV
2
3
4
overpotential (inset: its multicurrent process). CVs of (d) Co O @CQDs-CN HPAs/CoF and (e) Co O @CN/CoF at different scan rates. (f)
3
4
3
4
Linear relationship for the oxidation peak values and scan rates. (g) EIS comparison. (h and i) CVs for the nonfaradaic capacitance current interval
(
inset: linear dependence of the capacitive currents) at 0.30 V versus RHE on scan rates for (h) Co O @CQDs-CN HPAs/CoF and (i) Co O @
3
4
3
4
CN/CoF. All were tested in 1.0 M KOH.
the current density (Figure S12). The inset in Figure 4c
displays a multicurrent chronopotentiometric curve of
4. CONCLUSIONS
In summary, we successfully fabricate hollow polyhedral arrays
Co O @CQDs-CN HPAs/CoF, composed of a Co O
4
Co O @CQDs-CN HPAs/CoF, with the current being
3
4
3
4
3
−
2
increased from 50 to 275 mA cm at an increment of 25
mA cm per 500 s. Similar results remain in all steps, proving
excellent mechanical properties, mass transportation, and
nanocrystal ensemble on honeycomb-like carbon hybrid
nanosheet arrays. The crucial roles of CQDs that hollow the
polyhedra and form nanosheet arrays are proven. Significantly,
the resulting hierarchical structure (i) can maximize utilization
−
2
33
conductivity for the Co O @CN HPAs/CoF electrode.
3
4
of the Co O catalyst due to the high dispersion of Co O
The SEM image of Co O @CN HPAs/CoF after continuous
3
4
3
4
3
4
nanocrystals, (ii) ensure sufficient highways for charge and
mass storage and delivery because of the hollow hierarchical
porous nanostructure and nanosheet arrays, (iii) offer
enhanced catalyst activity and stability born of CQDs, nitrogen
dopants, and a carbon matrix. This work opens a new route to
preparing carbon-based metal oxide hollow nanoarrays with
high catalytic activity for various energy conversions.
1
000 CV scanning shows negligible changes (Figure S13),
further showing its robustness.
Parts d and e of Figure 4 reveal that the oxidation peak
currents in the CV curves of Co O @CQDs-CN HPAs/CoF
and Co O @CN/CoF are linearly dependent on the scan
rates, suggesting a surface-controlled electrochemical proc-
ess.
3
4
3
4
2
9,30
From slopes of 0.26 for Co O @CQDs-CN HPAs/
3 4
CoF and 0.0025 for Co O @CN/CoF (Figure 4f), the number
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03236.
3
4
■
of active species (m) calculated for Co O @CQDs-CN HPAs/
3
4
*
S
−
7
−9
CoF is 2.8 × 10 mol, significantly higher than 2.7 × 10 mol
for Co O @CN/CoF. The Nyquist plot in Figure 4g for
3
4
Co O @CQDs-CN HPAs/CoF shows smaller charge-transfer
3
4
resistance (7.5 Ω) at the overpotential of 300 mV than that of
Co O @CN/CoF (25 Ω), indicating that the hierarchical
Materials used, electrochemical and structural character-
izations, a table related to the present catalyst in
comparison with MOF-derived and Co O NPs-based
3
4
13
structure can significantly enhance the charge-transfer rate.
3
4
Double-layer capacitance (C ) measured by the CV technique
OER electrocatalysts, and figures related to UV−vis and
PL spectra, TEM and FE-SEM images, a XPS survey
spectrum, and LSV and TG curves (PDF)
dl
is closely related to the electrochemical active surface area
32
−2
(
ECSA) of an electrode. The C value of 637 μF cm for
dl
Co O @CQDs-CN HPAs/CoF is about 20 times larger than
3
4
−
2
that for Co O @CN/CoF (31 μF cm ; Figure 4h,i),
AUTHOR INFORMATION
Corresponding Authors
*E-mail: chm_kuangx@ujn.edu.cn (X.K.).
3
4
■
suggesting that Co O @CQDs-CN HPAs in hierarchical
3
4
architecture owns significantly increased ECSA.
E
DOI: 10.1021/acs.inorgchem.8b03236
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
*
E-mail: kuangr1985@foxmail.com (R.K.).
E-mail: sdjndxwq@163.com (Q.W.).
(13) Wang, X.; Tian, W.; Zhai, T. Y.; Zhi, C. Y.; Bando, Y.; Golberg,
D. Cobalt (II,III) Oxide Hollow Structures: Fabrication, Properties
and Applications. J. Mater. Chem. 2012, 22, 23310−23326.
*
ORCID
(
14) Wang, G. H.; Hilgert, J.; Richter, F. H.; Wang, F.; Bongard, H.
Yong Zhang: 0000-0002-5831-637X
Qin Wei: 0000-0002-3034-8046
Notes
J.; Spliethoff, B.; Weidenthaler, C.; Schu
Bimetallic Nanoparticles in Hollow Carbon Nanospheres for
̈
th, F. Platinum-Cobalt
Hydrogenolysis of 5-Hydroxymethylfurfural. Nat. Mater. 2014, 13,
2
(
93−300.
The authors declare no competing financial interest.
15) Ying, J.; Jiang, G.; Paul Cano, Z.; Han, L.; Yang, X. Y.; Chen, Z.
Nitrogen-Doped Hollow Porous Carbon Polyhedrons Embedded
with Highly Dispersed Pt Nanoparticles as A Highly Efficient and
Stable Hydrogen Evolution Electrocatalyst. Nano Energy 2017, 40,
88−94.
ACKNOWLEDGMENTS
This work was supported by National Key Scientific Instru-
ment, Equipment Development Project of China (Grant
1627809), and National Natural Science Foundation of
■
2
(16) Kuang, X.; Luo, Y. C.; Kuang, R.; Wang, Z. L.; Sun, X.; Zhang,
China (Grants 21605058, 21575050, and 21375047).
Y.; Wei, Q. Metal Organic Framework Nanofibers Derived Co O -
3 4
Doped Carbon-Nitrogen Nanosheet Arrays for High Efficiency
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