Surface Structure Dependent Electrocatalytic Activity of Co3O4 Anchored on Graphene Sheets toward Oxygen Reduction Reaction

Abstract

Catalytic activity is primarily a surface phenomenon, however, little is known about Co3O4 nanocrystals in terms of the relationship between the oxygen reduction reaction (ORR) catalytic activity and surface structure, especially when dispersed on a highly conducting support to improve the electrical conductivity and so to enhance the catalytic activity. Herein, we report a controllable synthesis of Co3O4 nanorods (NR), nanocubes (NC) and nano-octahedrons (OC) with the different exposed nanocrystalline surfaces ({110}, {100}, and {111}), uniformly anchored on graphene sheets, which has allowed us to investigate the effects of the surface structure on the ORR activity. Results show that the catalytically active sites for ORR should be the surface Co(2+) ions, whereas the surface Co(3+) ions catalyze CO oxidation, and the catalytic ability is closely related to the density of the catalytically active sites. These results underscore the importance of morphological control in the design of highly efficient ORR catalysts.

Full text

Surface Structure Dependent
Electrocatalytic Activity of Co
3
O
4
Anchored on Graphene Sheets toward
Oxygen Reduction Reaction
Junwu Xiao
1,2
, Qin Kuang
1
, Shihe Yang
1
, Fei Xiao
2
, Shuai Wang
2
& Lin Guo
3
1
Department of Chemistry, William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong,
2
Department of Chemistry and Chemical Engineering, Hubei Key Laboratory of
Matieral Chemistry and Service Failure, Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, Huazhong
University of Science & Technology, Wuhan, PR China,
3
School of Chemistry & Environment, Beihang University, Beijing, PR China.
Catalytic activity is primarily a surface phenomenon, however, little is known about Co
3
O
4
nanocrystals in
terms of the relationship between the oxygen reduction reaction (ORR) catalytic activity and surface
structure, especially when dispersed on a highly conducting support to improve the electrical conductivity
and so to enhance the catalytic activity. Herein, we report a controllable synthesis of Co
3
O
4
nanorods (NR),
nanocubes (NC) and nano-octahedrons (OC) with the different exposed nanocrystalline surfaces ({110},
{100}, and {111}), uniformly anchored on graphene sheets, which has allowed us to investigate the effects of
the surface structure on the ORR activity. Results show that the catalytically active sites for ORR should be
the surface Co
21
ions, whereas the surface Co
31
ions catalyze CO oxidation, and the catalytic ability is closely
related to the density of the catalytically active sites. These results underscore the importance of
morphological control in the design of highly efficient ORR catalysts.
T
he advent of nanotechnology and the ability to synthesize a marvelous panoply of nanocrystals have
breathed a new life to the catalysis science
1–5
. The notion that catalysts are necessarily nanomaterials is
rooted in the importance of surface in activating chemical bonds. Although there have been numerous
reports on the catalytic activities of nanomaterials, detailed understanding of how surface structure affects
catalytic performance is still lacking. There is therefore a need to systematically study the catalytic activity as a
function of nanocrystalline morphology other than the size since the surface structure is tunable by varying the
morphology. The prerequisite is the selective synthesis of differently shaped nanocrystal catalysts with uniform
crystal surfaces, preferably dispersed on a supporting substrate.
The spinel type Co
3
O
4
, in which the Co
21
and Co
31
ions occupy the tetrahedral and octahedral sites, respect-
ively
6
, is known to be a promising catalytic material
7–12
. It has been reported that different morphologies of Co
3
O
4
nanocrystals have a direct bearing on their catalytic activities for CO oxidation. For example, the {110} faces of
Co
3
O
4
nanocrystals have a higher catalytic activity for CO oxidation than {100} and {111}, because of the more
abundant catalytically active Co
31
sites on the former
12
. For the CH
4
combustion, however, the catalytic activity of
the nanocrystalline surfaces was found to be in the order of {112} . {011} ? {001}, depending instead on the
surface energy
13
. In the main, the catalytic activity of a given catalyst is therefore determined by the nature of
adsorption/activation/desorption of the reactants and products on the catalytically active sites
12–15
.
The spinel-type Co
3
O
4
nanocrystals are also a potential alternate for the high cost Pt and its alloys to catalyze
the oxygen reduction reaction (ORR), a critical reaction which underlies a battery of renewable-energy tech-
nologies such as fuel cell. To our knowledge, however, no study has been reported on the correlation between the
shape and the ORR catalytic activities of Co
3
O
4
nanocrystals. Such a study requires anchoring the Co
3
O
4
nanocrystals onto a substrate, which is preferably conductive and thus can enhance the ORR activity and stabilize
the catalyst system. As a relatively new class of carbon-based nanomaterials, graphene and carbon nanotube
(CNT) have high electrical conductivity, large surface area, high mechanical strength, and structural flexibility,
making them ideal substrates for supporting such nanocrystal catalysts. Indeed, graphene and CNT supported
Co-based electro-catalysts have already been used for ORR with improved catalytic activity and stability
16–19
.
OPEN
SUBJECT AREAS:
SYNTHESIS AND
PROCESSING
ENERGY
ORGANIC-INORGANIC
NANOSTRUCTURES
ELECTROCATALYSIS
Received
2 May 2013
Accepted
9 July 2013
Published
29 July 2013
Correspondence and
requests for materials
should be addressed to
J.W.X. (chjwxiao@
hust.edu.cn) or S.H.Y.
(chsyang@ust.hk)
SCIENTIFIC REPORTS | 3 : 2300 | DOI: 10.1038/srep02300 1

However, shape-controllable synthesis of Co
3
O
4
nanocrystals on
graphene and CNT as composites is still an unmet challenge.
In this paper, we report the controllable synthesis of Co
3
O
4
nanor-
ods, nanocubes and nano-octahedrons with difference exposed sur-
faces uniformly immobilized in situ on graphene sheets. This series
of nanocrystals showed much enhanced ORR catalytic activity when
dispersed on graphene. More significantly, the quantitative catalytic
activity depends on the detailed nanocrystalline morphology and
thus the surface structure of the nanocrystals, namely, {111} .
{100} . {110}, pointing to the Co
21
ions as the ORR active sites.
Results
Shape-selective synthesis of Co
3
O
4
nanocrystals. Detailed proce-
dures for the synthesis of Co
3
O
4
nanoparticles (NP), nanorods (NR),
nanocubes (NC) and nano-octahedrons (OC) on the surface of
reduced graphene oxides (RGO) have been given in the experimen-
tal section, and are here illustrated in Scheme 1. The crystalline
phases of these nanocomposites were ascertained by XRD patterns
(Figure SI-1), with the help of the standard crystal structure of Co
3
O
4
(JCPDS 65-3103). Co
3
O
4
NP around 10 nm across were formed by
thermally decomposing the precursors nucleated from the super-
saturated metal bicarbonate solution accompanied by the slow
release of CO
2
(Figure SI-2), as we reported previously
20–22
.
Presumably, through the Ostwald ripening process, the initially
nucleated precursors were transformed into cobalt carbonate
hydroxide (Co(CO
3
)
0.5
OH) nanorods with a length of several hun-
dred nanometers and a diameter of ,10 nm (Figure SI-3)
21
. The
subsequent calcination process caused a spontaneous transforma-
tion into the Co
3
O
4
NR with the overall morphology conserved.
Figure 1A shows a typical low-magnification transmission electron
microscopy (TEM) image of the synthesized Co
3
O
4
NR, densely
distributed on the surface of RGO sheets. We further examined the
crystallographic nature of the individual Co
3
O
4
NR through high
resolution TEM (HRTEM) observations. Shown in Figure 1B is a
section of the Co
3
O
4
NR observed in the [001] orientation, which
extends along the [110] direction. The side walls are parallel to the
(2–20) plane (Figure 1B, C). When the nanorod was titled to the [1–
10] zone axis, the (004) side planes and the (222) crystal planes could
also be clearly observed (Figure 1D, E). On the basis of these results,
the nanorod morphology can be approximately laid out as shown in
Figure 1F: the nanorod assumes its axial direction along [110] and is
bounded by the side planes of {001} and {1–10}. Similar Co
3
O
4
NR
were prepared previously by the calcination of cobalt carbonate
hydroxide nanorod precursors obtained by the precipitation of
cobalt acetate and sodium carbonate in ethylene glycol, and used
for low temperature catalytic CO oxidation
12
.
When H
2
O
2
was added, during the Ostwald ripening process, the
cobalt carbonate hydroxide (Co(CO
3
)
0.5
OH
.
0.11H
2
O) nanorods
were gradually transformed into Co
3
O
4
nanocrystals, as seen from
SEM images of the intermediates (Figure SI-4). After complete trans-
formation, well-defined Co
3
O
4
OC were uniformly dispersed on the
surface of graphene sheets (Figure 2A). To reveal the exposed sur-
faces, we refer to the TEM images in Figure 2. According to the
HRTEM image of a single Co
3
O
4
OC particle taken along the
[001] direction (Figure 2B), the two-dimensional (2D) projection
appears to be square (see the model octahedron in the bottom left
inset in Figure 2B). By tilting the Co
3
O
4
OC to the [011] zone axis, the
2D projection of the corresponding TEM image (Figure 2C) becomes
diamond-like. These results are consistent with the octahedron mor-
phology of the Co
3
O
4
nanocrystals anchored on the surface of RGO
sheets with eight exposed {111} surfaces (see the model structure in
Figure 2D).
Figure 1
|
Co
3
O
4
nanorods anchored on the RGO sheets. Low magnification TEM image (A); HRTEM images viewed along the (B, C) [001] and (D, E)
[1–10] axes; (F) Schematic illustration of the nanorod morphology highlighting the exposed surfaces. Note: [uvw] is a crystal axis index, (hkl) is a
crystal plane index.
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SCIENTIFIC REPORTS | 3 : 2300 | DOI: 10.1038/srep02300 2

Finally, Co
3
O
4
NC were formed on the surface of RGO sheets by
co-precipitation of Co
21
and ammonia followed by hydrothermal
treatment in the presence of H
2
O
2
. TEM observations revealed that
the as-formed Co
3
O
4
nanocrystals have a perfect cubic morphology
and a uniform crystallite size of about 10 nm (Figure 3A). Excellent
crystallinity of the Co
3
O
4
NC was confirmed by the HRTEM image
in Figure 3B. The lattice fringes of d
200
(0.418 nm) and d
220
(0.285 nm) of Co
3
O
4
are clearly observed from the [001] direction,
indicating the nanocubes bounded by the (001) facets (see the con-
structed nanocube model in Figure 3C).
The composition of the prepared cobalt oxides was investigated by
XPS spectra, which are shown in Figure SI-5A. The Co 2p spectra all
show a doublet consisting of a low energy band (Co 2p
3/2
at 780.
6 eV) and a high energy band (Co 2p
1/2
at 796.0 eV) for the Co
3
O
4
NP, NR, NC and OC, in agreement with the standard spectra of
Co
3
O
4
23,24
. The energy difference between the peak of Co 2p
3/2
and
2p
1/2
splitting is approximately 15 eV, indicating the presence of
both Co
21
/Co
31
species in the cobalt oxides samples
23–25
. The RGO
contents in the composites were obtained from the TGA curves
(Figure SI-5B), and calculated based on the weight loss below
400uC. According to that analysis, the mass percentages of RGO
are around 16.1 wt% for Co
3
O
4
NP/RGO, 16.0 wt% for Co
3
O
4
NR/RGO, 17.4 wt% for Co
3
O
4
NC/RGO, and 14.0 wt% for Co
3
O
4
OC/RGO.
Shape-dependent ORR catalytic activity of the Co
3
O
4
nanocry-
stals. To assess their ORR catalytic activity, the nanocrystal mate-
rials were loaded (with equal mass loading) onto glassy carbon
Figure 2
|
Co
3
O
4
nano-octahedrons anchored on the RGO sheets. (A) Low magnification TEM image; (B) HRTEM image along the [001] direction;
(C) HRTEM image along the [01-1] direction; and (D) Schematic illustration of a nano-octahedron bounded by the eight {111} surfaces.
Figure 3
|
Co
3
O
4
nanocubes grown on the RGO sheets. (A) Low magnification TEM image; (B) HRTEM image; and (C) Schematic illustration of the
nanocube morphology with the six exposed {100} surfaces.
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SCIENTIFIC REPORTS | 3 : 2300 | DOI: 10.1038/srep02300 3

electrodes. The electrodes were interrogated by cyclic voltammetry
(CV) in O
2
- and for reference, N
2
-, saturated 0.1 M KOH solutions,
and the data are shown in Fig. 4A. The Co
3
O
4
NP (size ,10 nm) on
the surface of RGO sheets exhibited very poor ORR activity: with an
onset potential of ,20.25 V vs. Hg/HgO. The Co
3
O
4
NR/RGO
hybrids with exposed surfaces dominated by {110} showed a much
more positive ORR onset potential (,20.1 V), suggesting higher
ORR catalytic activity than Co
3
O
4
NP/RGO. Remarkably, the
Co
3
O
4
NC/RGO with the six exposed {100} surfaces and the
Co
3
O
4
OC/RGO nanocomposites with the eight exposed {111}
surfaces achieved even more positive onset potentials, e.g.,
,20.06 V for Co
3
O
4
NC/RGO, and ,20.04 V for Co
3
O
4
OC/
RGO, closely approaching that of Pt/C, the gold standard for ORR
catalysts.
The ORR kinetics of the Co
3
O
4
/RGO composites was investigated
using the rotating-disk electrode (RDE) technique in O
2
-saturated
0.1 M KOH electrolyte. As can be seen from the LSV curves in
Figure 4B, the ORR process is diffusion controlled when the potential
is negative to 20.20 V, mixed diffusion kinetic controlled in the
potential region from 20.20 to 20.10 V, and kinetic controlled in
the potential range from 20.01 to 0 V. Unsupported Co
3
O
4
NP
prepared by thermal decomposition of Co(CO
3
)
1/2
OH precursors,
as we reported previously
26
, exhibited much lower onset potential
and diffusion-limited current density than those of the present four
Co
3
O
4
/RGO composite electrodes, suggesting the positive effect of
RGO on the ORR catalytic activity of Co
3
O
4
17
. Among the four
Co
3
O
4
/RGO composite electrodes, the Co
3
O
4
NP/RGO composite
catalyst shows the lowest onset potential, whereas the Co
3
O
4
OC/
RGO composite shows the highest one. The half-wave potentials of
the composite catalysts are in the sequence of Co
3
O
4
OC/RGO
(20.14 V) . Co
3
O
4
NC/RGO (20.16 V) . Co
3
O
4
NR/RGO
(20.20 V) . Co
3
O
4
NP/RGO (20.33 V). In the diffusion controlled
region, the diffusion-limited current densities follow the trend of the
half-wave potentials for the composite catalyst series. Together, these
results suggest that the Co
3
O
4
OC/RGO composite catalyst exhibits
the highest ORR catalytic activity among the four samples, and the
different catalytic activities can be attributed to the different
morphologies of the Co
3
O
4
nanocrystals in the composites.
A series of rotating disk voltammograms of oxygen reduction are
shown in Figure SI-(6–9)A with the commercial Pt/C and the Co
3
O
4
/
RGO composite catalysts at different rotation rates in O
2
-saturated
0.1 M KOH electrolyte. The RDE data were analyzed using the
Koutecky-Levich equation (Eq. 1 in the experimental section),
according to which a plot of the inverse current density J
21
versus
V
21/2
, shown in Figure SI-(6–9)B, should yield a straight line with the
intercept corresponding to J
k
and the slope reflecting the so-called B
factor. The electron transfer number for the O
2
reduction process can
be calculated from the B factor according to Eq. 2 (see the experi-
mental section). The linearity of the Koutecky-Levich plots and the
near parallelism of the fitting lines are consistent with the first-order
reaction kinetics with respect to the concentration of the dissolved
oxygen and implicate similar electron transfer numbers for the ORR
at different potentials in the region of 20.30 V to 20.50 V. The
calculated electron transfer numbers (n) for the commercial Pt/C
and the Co
3
O
4
/RGO composite catalysts in the potential region of
20.30 to 20.50 V are shown in Figure 4C. We can see that the Co
3
O
4
OC/RGO composite electrode can catalyze the ORR via a 4 e process,
in much the same way as a high-quality commercial Pt/C catalyst
does, which is impressive for a non-Pt catalyst. However, the ORR
electron transfer number for the Co
3
O
4
NR/RGO and Co
3
O
4
NC/
RGO composite catalysts were calculated to be ,3.5, suggesting
incomplete reduction of oxygen, but still domination by the 4e
process.
The ORR catalytic activity of the Co
3
O
4
/RGO hybrid catalysts can
also be gleaned from the Tafel slopes at low and high overpotentials
in O
2
-saturated 0.1 M KOH aqueous solution. The Tafel data are
Figure 4
|
(A) CV curves of Co
3
O
4
nanocrystals/RGO composites on glassy carbon electrodes in N
2
-saturated (solid line) or O
2
-saturated 0.1 M KOH
(dash line); (B) Rotating-disk voltammograms, (C) The electron transfer number (n) profiles obtained from (B), and (D) Tafel plots for the Co
3
O
4
/RGO
composite electrodes and the commercial Pt/C electrode; (E) J-T chronoamperometric responses at 20.40 V versus Hg/HgO reference electrode at a
rotating rate of 2400 rpm. The 0.1 M KOH solution electrolyte is firstly bubbled by N
2
for 30 min, and then is introduced by O
2
gas for around
3000 s, and is finally added by 20 vol% of methanol; (F) Chronoamperometric responses (percentage of current retained versus operation time) of the
kept at 20.40 V versus Hg/HgO reference electrode in O
2
-saturated 0.1 M KOH electrolyte at a rotating rate of 2400 rpm.
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SCIENTIFIC REPORTS | 3 : 2300 | DOI: 10.1038/srep02300 4

shown in Figure 4D. The E versus log (-J ) curves of the samples
similarly show two Tafel slopes at low and high overpotentials,
respectively, indicating a similar change in reaction mechanisms
with the potential. The two slopes can be explained in term of the
isotherms at two different O
2
coverages; i) the Temkin isotherm
(high O
2
coverage) associated with an intermediate oxide coverage
arising from ORR at low overpotential, whereby the first electron
transfer step involving an adsorbed product such as OH
2
is the rate-
determining step; and ii) the Langmuir isotherm (low O
2
coverage) at
high overpotential wherein significant oxide coverage ceases to exist,
which is commonly the case when a two-electron transfer reaction is
the rate-determining step. This is a characteristic feature of ORR on
mixed valence spinel oxide, e.g., Co-based ORR catalysts
25,27
. As can
be observed in Figure 4D, the Co
3
O
4
NC (83.7 mV/decade) and
Co
3
O
4
OC (101.4 mV/decade) on RGO sheets exhibit smaller
Tafel slopes at the over-potentials from 20.05 V to 20.10 V than
the Co
3
O
4
NR/RGO hybrid (115.6 mV/decade) in 0.1 M KOH elec-
trolyte, demonstrating high ORR catalytic activities close to that of
the commercial Pt/C catalyst (93.9 mV/decade).
Other important performance metric for an ORR catalysts include
the tolerance of the commonly used fuel molecules and cycle
stability, which are especially relevant for fuel cells such as direct
methanol fuel cell. To examine the possible crossover effect for the
catalytic performance, we measured the electrocatalytic selectivity of
the Co
3
O
4
/RGO composite electrode against electro-oxidation of
methanol molecules. The current density-time (J-T) chronoampero-
metric response profiles are shown in Figure 4E. For the commercial
Pt/C electrode, a sharp decrease (,85%) in current density was
observed upon methanol addition (20 vol%) into the O
2
-saturated
0.1 M KOH electrolyte. However, the amperometric responses of the
Co
3
O
4
/RGO composite electrodes are strong and stable, showing a
retention ratio of at least 80% after the addition of methanol. Such
high selectivity of the Co
3
O
4
/RGO composite electrodes toward
the ORR and the remarkably good tolerance to crossover effect can
be attributed to the much lower ORR potential than required for
oxidation of the fuel molecules
28
. Moreover, the Co
3
O
4
/RGO hybrid
electrodes also exhibited excellent stability as measured by chron-
oamperometric measurements (Figure 4F). At a constant voltage of
20.40 V vs Hg/HgO, the ORR current density produced in the
hybrid catalysts almost had no decay over 10000 s of continuous
operation, whereas the commercial Pt/C catalyst exhibited ,22%
decrease in current density. Thus, in comparison with the commer-
cial Pt/C catalyst, our Co
3
O
4
/RGO composite electrodes are more
insensitive to methanol molecules, thus more resistive to poisoning
by the possible methanol crossover from the anode of a fuel cell, and
are more stable under operating conditions.
Discussion
Co
3
O
4
has the normal-spinel structure Co
21
Co
2
31
O
4
, in which the
Co
21
ion in the formula unit occupies the tetrahedral site, while the
two Co
31
ions occupy the octahedral sites
6
, as shown in Figure 5A.
Figure 5B–D depict the close-packed planes of {001}, {111} and {110},
and their surface atomic configurations of the spinel-type Co
3
O
4
crystals. Experimental and theoretical measurements have demon-
strated that the three low Miller index planes ({100}, {110} and {111})
of such metallic oxide particles with a fcc structure differ not only in
the surface atomic density but also in the electronic structure, geo-
metric bonding and chemical reactivity
29
. As a result, those planes
have different surface energies, following the order of c{111} ,
c{100} , c{110}, which is closely parallel to the catalytic activities
for CO and CH
4
oxidation
12,13,30,31
.
For catalyzing CO oxidation, the CO molecule interacts preferably
with the surface Co
31
cations, which is the only favorable site for CO
adsorption, as confirmed both theoretically
32
and experimentally
33,34
.
The oxidation of the adsorbed CO then occurs by abstracting the
surface oxygen that had been coordinated with the Co
31
cations. The
partially reduced cobalt site, i.e., Co
21
cation with a neighboring
oxygen vacancy, is re-oxidized by a gas-phase oxygen molecule back
to the active Co
31
form. Consequently, the surface Co
31
cations are
regarded as the active sites for CO oxidation, whereas the surface
Co
21
cations are almost inactive
12,31,35–37
. It is known that in the
Co
3
O
4
crystal structure, the {001} and {111} planes contain only
Co
21
cations, while the {110} plane is composed mainly of Co
31
cations (Figure 5B–D). This scenario has been proved by surface
differential diffraction studies concluding that the Co
31
cations are
present solely on the {110} plane
38,39
. Similarly, in our own experi-
ment with the Co
3
O
4
NR/RGO composite catalyst, the initial trans-
formation temperatures for CO oxidation is 60uC, considerably
lower than that with Co
3
O
4
NC/RGO (100uC) and Co
3
O
4
OC/
RGO (120uC) (Figure SI-10). Although the catalytic activities of
the Co
3
O
4
/RGO composites for CO oxidation are by no means opti-
mized, our study suffices to conclude that the Co
3
O
4
NRs with the
predominantly {110} exposed surfaces have higher catalytic activity
for CO oxidation than the Co
3
O
4
NCs with the sole six {100} exposed
surfaces and the OCs with the only eight {111} exposed surfaces, in
excellent agreement with the literature reports
12,30,31
.
In sharp contrast, for ORR catalysis, the Co
3
O
4
OC enclosed by the
eight {111} facets on the RGO sheets was found to exhibit the highest
catalytic activity among the four Co
3
O
4
/RGO nanocomposite cata-
lysts we have studied in the present work, followed by Co
3
O
4
NC/
RGO, and then Co
3
O
4
NR/RGO, with Co
3
O
4
NP/RGO being the
least active (Figure 4B). Surprisingly, this ORR catalytic activity
order correlates very well with the surface Co
21
density order of
Figure 5
|
Structure models of the spinel Co
3
O
4
nanocrystals. (A) Three-dimensional atomic arrangement and (B–D) Surface atomic configurations in
the {100}, {111} and {110} planes.
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SCIENTIFIC REPORTS | 3 : 2300 | DOI: 10.1038/srep02300 5

the corresponding nanocrystals on RGO excepting the unsupported
nanoparticles, namely, {111} . {100} . {110}. This strongly suggests
that the surface Co
21
ions are the catalytically active sites for ORR.
Note that the measured specific surface areas of the composites are
139.4 m
2
g
21
for Co
3
O
4
NP/RGO, 110.0 m
2
g
21
for Co
3
O
4
NR/RGO,
116.6 m
2
g
21
for Co
3
O
4
NC/RGO, and 98.9 m
2
g
21
for Co
3
O
4
OC/
RGO composites (see the N
2
adsorption isotherms in Figure SI-11).
And the contents of Co
3
O
4
in the composites are around 83.9 wt%
for Co
3
O
4
NP/RGO, 84.0 wt% for Co
3
O
4
NR/RGO, 82.6 wt% for
Co
3
O
4
NC/RGO, and 86.0 wt% for Co
3
O
4
OC/RGO (Figure SI-5B).
The similar specific surface areas together with the similar amounts
of catalysts used for the ORR testing exclude the possibility of the
specific surface area being an important factor that determines the
ORR catalytic activity. Thus, it is the exposed crystal planes of
the Co
3
O
4
nanocrystals that play a vital role in determining the
ORR catalytic activity.
Assuming that the adsorption/desorption process of O
2
on the
catalytic active sites is involved in the rate-determined step of the
ORR, the surface bonding of O
2
to the composite catalysts should be
critical. In general, O
2
molecules with the bond length of 0.12 nm
adsorbs on the catalytic active sites mainly via three modes (Griffths,
Bridge, and Pauling)
40
. For the mode of Griffths, the angle of the
adsorbed O
ads
…Co…O
ads
is around 36u. In comparison with the
angle of O-Co-O in the bulk of Co
3
O
4
(95.5u for O-Co
31
-O, and
109.5u for O-Co
21
-O), such a huge angle mismatch would induce a
large intra-molecular stress, resulting in the weak adsorption of O
2
on the catalytic active sites. In the Bridge mode (Co…(O
ads
5
O
ads
)…Co), the bond length of O
2
molecules (,0.12 nm) fails to
match the distance between the adjacent catalytically active sites
(0.1956 nm for Co
31
-O, and 0.1902 for Co
21
-O). Thus we are left
with the possibility that O
2
molecules be preferably adsorbed on the
catalytically active sites via the Pauling mode (Co…(O
ads
5 O)).
Conceivably, when O
2
molecules are absorbed on the catalytically
active sites via the Pauling mode, it is the surface Co
21
(3d
5
4s
2
)
cations rather than the surface Co
31
(3d
5
4s
1
) cations which prefer
to transfer electrons to the absorbed O
2
molecules to weaken and to
assist breaking the O-O bond, meanwhile leaving themselves oxi-
dized to Co
31
. This suggests that the surface Co
21
sites should be the
catalytically active sites instead of Co
31
sites for ORR, and can nat-
urally explain why the Co
3
O
4
nanocrystals with the predominant
{111} and {110} exposed surfaces exhibited higher catalytic activity
for ORR than that with the {110} exposed surfaces. Such explanation
also applies to the observation that CoO exhibits better catalytic ORR
performance than Co
3
O
4
16,41
. In addition, the density of Co
21
cations
in {111} planes (4=
ffiffiffi
3
p
a
2
) is higher than that in {100} planes (2=a
2
),
resulting in catalytic activity of the Co
3
O
4
OC/RGO composite cat-
alysts enclosed by the eight {111} facets than that of the Co
3
O
4
NC/
RGO composite catalyst surrounded by the six {100} exposed
surfaces.
It is widely known that the RGO can serve as both a supporter for
the catalyst dispersion and a conduction path for shuttling electrons
involved in redox reactions. Here it is clearly the case as well, and this
would indiscriminately enhance the catalytic activity of the different
nanocomposite catalysts in our study. It is also possible, however,
that due to the specific interaction of the different crystal faces with
the RGO, the catalytic activity enhancement may be different for
different nanocomposites. In particular, the interaction of the
Co
21
-rich surface with the RGO may be more beneficial to the
ORR catalysis. Indeed, some studies along thins line have appeared
in recent years
16,17
. Nevertheless, this aspect still underlines the role of
the nanocrystal surfaces in ORR catalysis and adds to source of
inspiration for tuning nanocrystal morphologies for optimizing cata-
lytic efficiency.
In sum, we have demonstrated the morphological control of Co
3
O
4
nanocrystals uniformly immobilized in situ on RGO sheets by judi-
ciously choosing the oxidant and tuning the reaction conditions such
as pH value. The resulting nanorods predominantly exposing the
{110} surfaces, nanocubes surrounded by the six {100} facets, and
nano-octahedrons enclosed by the eight {111} facets have allowed
us to further investigate the crystal face effects on the ORR catalytic
activity. We found that, while the surface Co
31
ions are the catalyti-
cally active sites for CO oxidation, and the surface Co
21
ions act as the
catalytically active sites for ORR. Additionally, the density of the
catalytically active sites on the surface is closely related to the catalytic
activity. Accordingly, we have established that the catalytic activity for
ORR of these crystalline facets decreases in the sequence of {111} .
{100} ? {110}. This fundamental understanding shows that morpho-
logical control of metal oxide catalysts is a promising surface engin-
eering strategy for the development of nanostructured catalysts in
general and non-precious metal free nano-catalysts in particular for
ORR in alkaline media.
Methods
Materials and reagents. Graphite flake (natural, ,325 mesh, Alfa Aesar), potassium
permanganate (KMnO
4
, Riedel-de Hae¨n), hydrogen peroxide solution (30 wt%,
H
2
O
2
, BDH), nickel chloride hexahydrate (NiCl
2
?6H
2
O, Fisher), cobalt chloride
hexahydrate (CoCl
2
?6H
2
O), sodium hydrogen carbonate (NaHCO
3
, BDH),
ammonia water (28 , 29 wt%) and hydrazine monohydrate (min 98.0 wt%,
N
2
H
4
?H
2
O, Wako) were used without further purification.
Synthesis of graphene oxide (GO) sheets. Graphene oxide sheets were synthesized
from natural graphite by a modified Hummers method
42
. Briefly, 0.5 g of graphite
(,325 mesh, Alfa Aesar) and NaNO
3
(0.5 g; Aldrich, .99%) were dispersed into
concentrated H
2
SO
4
(20 mL; Fisher Scientific, 98%) with an ice bath. Under vigorous
stirring, KMnO
4
(2.0 g; Riedel-de Hae¨n, .99%) was then add ed gradually. After
removing the ice bath, the mixture was stirred at room temperature for 24 h. As the
reaction progressed, the mixture became pasty with a brownish color. Successively,
20 mL of H
2
O was slowly added to the pasty mixture while keeping the mixture in an
ice bath, since the addition of water into the concentrated H
2
SO
4
medium will release
a large amount of heat. After dilution with 40 mL of H
2
O, 5 mL of 30% H
2
O
2
(VMR)
was added to the mixture, accompanied by bubbling and changing to brilliant yellow
color. After continuously stirring for 2 h, the mixture was filtered and w ashed with DI
water. Then, the products were dispersed in 10 wt% HCl aqueous solution, and
washed with DI water again three times to remove impurity ions. Finally, the products
were dispersed in DI water via ultrasonication, and then centrifuged at 7000 rpm for
1 h. The supernatant was collected as a GO aqueous solution with a concentration of
,1mgmL
21
.
Controllable synthesis of differently shaped Co
3
O
4
nanocrystals on reduced
graphene oxide (RGO) sheets. To start with, 20 mM of CoCl
2
?6H
2
O and 40 mM of
NaHCO
3
were dissolved into 90 mL of DI water and mixed with 10 mL of GO
solution, and the mixture was flushed with gaseous CO
2
for 2 h forming Solution A.
Solution A was stirred at room temperature for 12 h, and subsequently refluxed at
100uC for 10.0 h after adding in 0.1 mL N
2
H
4
, forming Solution B. The precipitates in
Solution B were centrifuged and washed with DI water three times, and then freeze
dried. Finally, the products were thermally treated at 400uC for 1 h in a N
2
atmosphere with a heating rate of 5uC/min, to form the Co
3
O
4
nanoparticles/RGO
(Co
3
O
4
NP/RGO) composites. In parallel experiments, when Solution B was poured
into a 70 mL capacity autoclave with Teflon liner and then hydrothermally treated at
100uC for 12 h before the thermal treatment process, the as-formed products were
labeled as Co
3
O
4
nanorods/RGO (Co
3
O
4
NR/RGO) composites. When Solution B
and 5.0 mL of 30 wt% hydrogen dioxide (H
2
O
2
) were poured into a 70 mL capacity
autoclave with Teflon liner and then hydrothermally treated at 100uC for 12 h before
the thermal treatment process, the final products were labeled as Co
3
O
4
octahedrons/
RGO (Co
3
O
4
OC/RGO) composites.
As for Co
3
O
4
NC on RGO sheets, a typical synthesis is as follows. First, 1 mmol
CoCl
2
?6H
2
O was dissolved in a mixture of 1 mL of 30 wt% hydrogen peroxide
(H
2
O
2
) and 40 mL of distilled water. When the solution was clarified, the solution
was maintained at pH 9.0 by adding ammonia solution (25 , 28 wt%). Then 5 mL of
GO solution was added into the above solution, followed by stirring for 1 h. The
reaction mixture was then charged into a 70 mL capacity autoclave with Teflon liner,
which was then kept at 180uC for 12 h. After the reaction was completed, the auto-
clave was allowed to cool down to room temperature naturally and opened for
product collection. The precipitates were washed with DI water three times and freeze
dried. Finally, the products were thermally treated at 400uC for 1 h in a N
2
atmo-
sphere with a heating rate of 5uCmin
21
, to form the Co
3
O
4
nanocubes/RGO (Co
3
O
4
NC/RGO) composites.
General Materials Characterization. The product morphologies were directly
examined by scanning electron microscopy (SEM) using JEOL JSM-6700F at an
accelerating voltage of 5 kV. Transmission electron microscopy (TEM) observations
were carried out on a JEOL 2010 microscope operating both at 200 kV. X-ray
diffraction (XRD) was performed on a Philips PW-1830 X-ray diffractometer with Cu
ka irradiation (l 5 1.5406 A
˚
). The step size and scan rate are set as 0.05u and 0.025u/s,
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SCIENTIFIC REPORTS | 3 : 2300 | DOI: 10.1038/srep02300 6

respectively. X-ray photoelectron spectroscopy (XPS) was measured on a Perkin-
Elmer model PHI 5600 XPS system with a resolution of 0.3–0.5 eV from a
monochromated aluminum anode X-ray source with Ka radiation (1486.6 eV). The
thermogravimetric analysis (TGA) was performed from 30 to 700uC on a TGA Q5000
(TA Instruments Ltd) at a heating rate of 5uCmin
21
under an air flow of 25 mL
min
21
. Brunauer-Emmett-Teller (BET) surface areas were measured on a Coulter SA
3100 surface area analyzer.
Catalytic measurements for oxygen reduction reaction (ORR). Electrochemical
measurements were carried out by cyclic voltammetry (CV) on a CHI 660D
electrochemical workstation. A conventional, three-electrode cell consisting of glassy
carbon electrode (GCE) with an area of 0.125 cm
2
was used as the working electrode,
Pt foil was employed as the counter electrode and Hg/HgO (1.0 M KOH) (MMO,
0.098 V vs. SH E) was used as the reference electrode. The working electrode was
modified with a catalyst layer by dropping a suitable amount of catalyst ink on the
GCE. The catalyst ink was prepared by ultrasonically dispersing 10 mg of the carbon
supported catalysts in a 2.0 mL solution (1.9 mL of ethanol and 0.1 mL of 5 wt%
Nafion solution) for 30 min to obtain a homogeneous solution. 10 mLofthe
dispersion was pipetted out and dropped onto a glassy carbon rotating disk electrode
of 3 mm in diameter, which was then dried in air. CV experiments were conducted at
room temperature in 0.1 M KOH solution saturated with nitrogen. For all of the
experiments, stable voltammogram curves were recorded after scanning for 20 cycles
in the potential region from 0 to 0.6 V in 0.1 M KOH solution. Polarization curves for
the oxygen reduction reaction (ORR) were obtained in 0.1 M KOH solution using the
rotating ring disk electrode (RRDE-3A). Before the RRDE study, the electrodes were
cycled at 50 mV s
21
between 0 and 0.6 V until reproducible cyclic voltammograms
were obtained. Normalized currents are given in terms of geometric weight (mA
cm
22
). The working electrode was scanned cathodically at a rate of 5 mV s
21
with
varying rotating speed from 400 rpm to 2400 rpm. Koutecky-Levich plots (J
21
vs.
v
21/2
) were analyzed at various electrode potentials. The slopes of their best linear fit
lines were used to calculate the number of electrons transferred (n) on the basis of the
Koutecky-Levich equation:
1
J
~
1
J
k
z
1
J
L
~
1
J
k
z
1
Bv
1=2
ð1Þ
B~0:62nFC
o
D
o
2=3
n
{1=6
ð2Þ
J
k
~nFkC
o
ð3Þ
Where J is the measured current density, J
k
and J
L
are the kinet ic- and diffusion-
limiting current densities, v is the angular velocity, n is transferred electron number,
F is the Faraday constant (96485 C mol
21
), C
o
is the bulk concentration of O
2
(1.2 3
10
26
mol cm
23
), n is the kinematic viscosity of the electrolyte (0.01 cm
2
s
21
), D
o
is the
O
2
diffusion coefficient (1.9 3 10
25
cm
2
s
21
), and k is the electron-transfer rate
constant.
Catalytic measurements for CO oxidation. The catalytic activity toward CO
oxidation was eva luated in a continuous flow reactor. In brief, the reaction gas, 5% CO
in nitrogen (99.999%) (10 mL min
21
) and air (99.999%) (40 mL min
21
) was fed to a
catalyst (22.5 mg) containing fixed-bed flow reactor made of glass with an inner
diameter of 2.4 mm. Steady-state catalytic activity was measured at each chosen
temperature, from room temperature to 200uC in a step of 20u C. The effluent gas was
analyzed on-line by an on-stream gas chromatograph (Ramiin GC 2060) equipped
with a TDX-01 column.
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Acknowledgements
This work was supported by the HK-RGC General Research Funds (GRF no. HKUST
605710 and 604809), and the Fundamental Research Funds for the Central Universities
(Project No. 2013QN158).
Author contributions
S.Y. designed the experiments. J.X. carried out the experiments. Q.K. carried out the CO
oxidation experiment. J.X. and S.Y. analyzed the data and wrote the manuscript. Q.K., F.X.,
S.W. and L.G. contributed to the data analysis. All the authors discussed the research.
Additional information
Supplementary information accompanies this paper at http://www.nature.com/
scientificreports
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Xiao, J.W. et al. Surface Structure Dependent Electrocatalytic
Activity of Co
3
O
4
Anchored on Graphene Sheets toward Oxygen Reduction Reaction. Sci.
Rep.
3
, 2300; DOI:10.1038/srep02300 (2013).
This work is licensed under a Creative Commons Attribution 3.0 Unported license.
To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0
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SCIENTIFIC REPORTS | 3 : 2300 | DOI: 10.1038/srep02300 8