Sp2 C-dominant N-doped carbon sub-micrometer spheres with a tunable size: A versatile platform for highly efficient oxygen-reduction catalysts

Article abstract of DOI:10.1002/adma.201203923

A simple, yet versatile strategy to prepare size-controlled and monodisperse carbon sub-micrometer spheres is developed based on the biomolecule dopamine. Unlike traditional carbon materials, the resulting carbon sub-micrometer spheres contain much less sp³ carbon with high-level electroactive nitrogen. Moreover, metal-carbon hybrid sub-micrometer spheres can be easily obtained, and show highly promising catalytic properties in the oxygen-reduction reaction. Copyright

Full text of DOI:10.1002/adma.201203923

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Sp2 C-Dominant N-Doped Carbon Sub-micrometer Spheres
with a Tunable Size: A Versatile Platform for Highly Efficient
Oxygen-Reduction Catalysts
Kelong Ai, Yanlan Liu, Changping Ruan, Lehui Lu,* and Gaoqing (Max) Lu*
Carbon sub-micrometer spheres (carbon SMSs) are of great
practical interest, not only because they are extraordinary
building blocks for multifunctional hybrid materials, but also
because these carbon SMSs can be used in a broad range of
applications, for instance, for drug delivery or as catalysts or
nanodevices.[1–10] The success of these applications strongly
depends on the availability of high-quality carbon SMSs with
controllable sizes. In particular, controlling the monodispersity
and maintaining a size smaller than 200 nm are essential in
biomedicine.[11]
Until now, the carbonization of phenol/formaldehyde resin
(PFR) SMSs has been accepted as the most effective method
for preparing carbon SMSs, due to several advantages of PFR
including facile preparation, high thermal stability and easy
conversion to carbon materials.[1–10] Despite recent progress,
we believe there is still considerable room for improvement in
the synthesis and properties of the carbon precursor as well
as the as-prepared carbon SMSs. Firstly, it would be desir-
able to use nontoxic carbon resources instead of the strongly
carcinogenic phenol/formaldehyde; this would reduce harm
both to humans and environment. Secondly, maximizing the
reduction in sp3 C occurring in traditional carbon materials,
such as PFR-based carbon SMSs, would be highly beneficial
for enhancing the material’s electroconductivity. Thirdly, the
presence of nitrogen heteroatoms in carbon nanomaterials can
strongly enhance the materials’ electrochemical performance.
However, a lack of nitrogen heteroatoms in PFR results in an
absence of electroactive nitrogen in the final carbon SMSs.
Thus, it is challenging but desirable to explore new polymer
analogues that feature low toxicity, excellent thermal stability,
high monodispersity, and the presence of heteroatom within a
framework to prepare carbon SMSs for practical applications.
With this point of view, we present for the first time a
simple yet versatile strategy for the large-scale manufacture
of monodisperse and size-controlled carbon SMSs, using bio-
molecule dopamine as the carbon resource. In contrast with
phenol/formaldehyde, dopamine is nontoxic, widespread,
and a sustainable resource. It contains carbon and nitrogen
atoms and is well-known for its chelating capability with many
types of metal ions. It can self-polymerize and spontaneously
deposit polydopamine films on virtually any surface in a Tris
buffer.[12–18] Based on this property, hollow carbon spheres have
been prepared by immersing the silica template in a dopamine
Tris-buffer solution, followed by carbonization and HF etching
of the template.[16] However, this approach requires a multistep
synthesis procedure and also uses highly toxic HF. Herein,
dopamine can be directly polymerized into monodisperse sub-
micrometer spheres without using any template, in a mixture
containing water, ethanol, and ammonia at room temperature.
The resulting polydopamine SMSs exhibit excellent thermal sta-
bility and can successfully convert to carbon SMSs without dete-
rioration of their structure and morphology. Their size can be
easily controlled over a wide range, even below 200 nm, which is
a primary challenge in preparing uniform carbon spheres. Spe-
cifically, the as-prepared carbon SMSs contain much less sp3 C
than RFR-based carbon SMSs; thus, they exhibit enhanced elec-
troconductivity. Lastly, this strategy enables the in situ introduc-
tion of high-level electroactive nitrogen into the carbon matrix,
due to the presence of nitrogen in dopamine. This makes the
carbon SMSs highly suitable as a nonprecious metal catalyst for
oxygen reduction reaction.
Indeed, this facile strategy can also provide an important plat-
form for many practical applications. First, polydopamine SMSs
inherit catechol and N–H groups from their starting material,
which facilitates the loading of many types of transition metal
ions. This enables yields of metal-carbon hybrid materials with
specific properties. To demonstrate their versatility, we fab-
ricate nanocatalysts composed of Fe@Fe3C in carbon SMSs,
and this hybrid materials show comparable oxygen reduction
reaction catalytic activity to the commercial 20 wt% Pt/C, but
with higher stability, better catalytic selectivity and potential
applications in energy conversion and storage technologies that
typically proceed in acidic media. Furthermore, porous carbon
SMSs with an extremely large surface area (2006 m2 g−1 ) can
also be obtained based on the as-prepared carbon SMSs. These
features make this strategy highly promising for many practical
applications, such as in catalyst, drug delivery, water treatment,
gas absorption, and supercapacitors.
K. L. Ai, Y. L. Liu, C. P. Ruan, Prof. L. H. Lu
State Key Laboratory of Electroanalytical Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun 130022, P. R. China
E-mail: lehuilu@ciac.jl.cn
Prof. G. Q. (M.) Lu
ARC Centre of Excellence for Functional Nanomaterials Australian
Institute for Bioengineering and Nanotechnology
University of Queensland
QLD 4072, Australia
E-mail: maxlu@uq.edu.au
Our strategy for the preparation of carbon SMSs is schemati-
cally illustrated in Figure 1a. Polydopamine (PDA) SMSs as a
carbon precursor were prepared through polymerization of
dopamine in a mixture containing water, ethanol, and ammonia
DOI: 10.1002/adma.201203923
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carbon precursor before and after carboniza-
tion (Supporting Information, Figure S5).
The peak centered at approximately 150 ppm
was assigned to diphenolic phenoxy C.[19,20]
After carbonization, this peak disappeared,
which indicated the removal of most of the
oxygen. Complementary to the 13C-NMR
data, electron energy-loss spectroscopy
(EELS) mapping and energy-dispersive X-ray
analysis (EDAX) provided further evidences
for this result (Figure 2a,b and Supporting
Information, Figure S6). Interestingly, the
peak at 30–45 ppm corresponding to the
aliphatic signals also nearly disappeared,
suggesting there is little sp3 C in the final
carbon matrix. In contrast, traditional PFR
SMSs retained a much higher amount of sp3
C after carbonization. To better understand
the mechanism underlying the formation of
the carbon SMSs, we next employed X-ray
photoelectron spectroscopy (XPS) analysis to
investigate changes in the C component
of carbon SMSs obtained at different tem-
peratures, and we observed a remarkable
reduction of sp3 C with increasing tempera-
ture (Supporting Information, Figure S7,
Figure 1. a) Schematic illustration of the synthesis of PDA SMSs. b) Typical TEM image of
PDA SMSs with an average diameter of 380 nm. c–h) SEM images of PDA SMSs with different
diameters prepared at different ratios of ammonia to dopamine.
Table S2). This significant reduction of sp3 C
was mainly attributed to the PDA, which
possessed C that was entirely located within
at room temperature. Typical transmission electron micro-
scopy (TEM) and scanning electron microscopy (SEM) images
showed that the resultant PDA SMSs were uniformly spherical
in shape. Their size could be easily controlled in the range from
120 to 780 nm by tuning the ratio of ammonia to dopamine
(Figure 1b–h, Supporting Information, Figure S1). This control
over the size of PDA SMSs is of great importance in practical
applications, particularly in the field of biomedicine. Use in
drug delivery requires the nanoparticles smaller than 200 nm in
order to avoid rapid recognition and clearance by the phagocyte.
The Fourier transform infrared (FTIR) spectrum of PDA SMSs
showed the characteristic spectral peaks of several functional
groups, such as N–H, –OH, C–O, and C–N (Supporting Infor-
mation, Figure S2a). The presence of these functional groups
endowed PDA SMSs with an active surface for absorbing metal
ions, and could also introduce nitrogen heteroatoms in the final
carbon SMSs.
The thermal stability of carbon precursors determines their
successful conversion to carbon materials. Thermogravimetric
analysis (TGA) was performed to examine the thermal stability
of PDA SMSs (Supporting Information, Figure S3). Encourag-
ingly, PDA SMSs exhibited excellent thermal stability; even at
1000 °C, 54.19% of the residues still remained. Such a high
thermal stability suggests that PDA SMSs are excellent carbon
precursors that yield carbon SMSs. As expected, after carboni-
zation, the resultant carbon materials appeared to preserve
the structural integrity, monodispersity, and spherical mor-
phology with a slightly decreased size (Supporting Information,
Figure S4). To verify the successful conversion of the carbon
precursor to carbon, we compared the 13C NMR spectra of the
its rings; this was unlike PRF which had part of the C outside
the rings (Supporting Information, Figure S8a). The C within
the rings could convert into sp2 C more easily than the C out-
side the rings during carbonization. As a result of the signifi-
cant reduction of sp3 C, combined with high-level N-doping, a
substantial improvement in their electroconductivity relative to
PFR-based carbon SMSs was achieved (Supporting Informa-
tion, Figure S9). Using electrochemical impedance spectros-
copy analysis, PDA-based carbon SMSs showed a much smaller
semicircle, as well as a lower intercept of the semicircle with the
real axis (5.84 Ω) in the high frequency region of the Nyquist
plot compared to PFR-based carbon SMSs (6.16 Ω). This
implied that PDA-based carbon SMSs had superior electron
transfer ability. The higher electroconductivity of PDA-based
carbon SMSs was believed to play a crucial role for enhancing
their electrochemical performance.
The oxygen reduction reaction (ORR) is important in many
energy conversion and storage technologies such as fuel cells.
The ORR is sluggish in nature and typically requires the use of
expensive Pt-based catalysts. Metal-free N-doped carbon mate-
rials have been accepted as a potential substitute for Pt as they
reduce the cost of ORR catalysts thereby promoting the commer-
cialization of fuel-cell technology.[21–26] For instance, N-doped
graphitic carbon,[21b] N-CNT,[22,26] and ammonia-treated mesopo-
rous carbon[23,24] have been investigated as catalysts or catalyst
supports for the ORR. Despite their high catalytic activity, these
materials generally require chemical pretreatment with HNO3,
NH3, or HCN to introduce the nitrogen; however, this pretreat-
ment may result in poor control over the chemical homogeneity
and reproducibility, long preparation times, and sometimes the
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limiting current density, and pyridinic N
could improve the onset potential, while other
N species such as pyrrolic N or N−O had little
effect on the electrochemical performance
of carbon materials.[27] Thus, we proceeded
to examine the N component in PDA-based
carbon SMSs using XPS analysis, and only
electroactive pyridinic N and graphitic N
were detected in PDA-based carbon SMSs
(Figure 2c). We also investigated the effect
of carbonization temperature on the catalytic
activity, and we observed that carbon SMSs
obtained at 800 °C showed the highest activity
(Supporting Information, Figure S10a). This
result further confirmed the above conclu-
sion, since carbon SMSs obtained at 800 °C
had the optimal N composition. The eleva-
tion in temperature resulted in a reduction
of electroactive N, while high-level pyrrolic N
was presented in carbon SMSs with carboni-
zation temperature below 800 °C (Supporting
Information, Figure S7, Table S2).
c
Graphitic nitrogen
Pyridinic nitrogen
69.72%
30.28%
Another substantial benefit to this strategy
is that PDA SMSs inherit functional groups
from their starting material, such as catechol
and N–H groups. Thus, they can effectively
absorb transitional metal ions, which ena-
bles the further introduction of transitional
metal-based electroactive substances into
the N-doped carbon matrix. As a proof of
concept, we synthesized monodisperse Fe@
Fe3C-loaded N-doped carbon (Fe@Fe3C/C–
N) spheres and evaluated their use as ORR
catalysts. TEM image demonstrated that
Fe@Fe3C/C–N preserved the spherical mor-
phology with both micropores and mes-
opores (Figure 3a,d). The generation of the
pores was attributed to the formation of Fe3C
and the graphitization of amorphous carbon
catalyzed by Fe metal. This was confirmed by
high-resolution TEM imaging and X-ray dif-
fraction (XRD) analysis (Figure 3b,c). Their
394
396
398
400
402
404
Binding energy (eV)
0
-1
-2
-3
-4
-5
-6
PFR-based carbon SMSs in N2
PFR-based carbon SMSs in O2
PDA-based carbon SMSs in N2
PDA-based carbon SMSs in O2
e
f
Bare glass carbon
PFR-based carbon SMSs
PDA-based carbon SMSs
2
1
0
-1
-2
-0.343 V
-0.218 V
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Potential vs. Ag/AgCl (v)
Potential vs. Ag/AgCl (V)
Figure 2. a,b) EELS mapping analysis of PDA SMSs and PDA-based carbon SMSs, respec-
tively. c) N 1s XPS spectrum of PDA-based carbon SMSs. d) Schematic illustration of chemical
structure of PDA-based carbon SMSs. e) CV plots of PDA- and PFR-based carbon SMSs in N2-
and O2-saturated 0.1 M KOH. f) Polarization curves of PDA- and PFR-based carbon SMSs at
1600 rpm in O2-saturated 0.1 M KOH.
deterioration of the materials’ structure. In contrast, carboni-
zation of PDA SMSs by our proposed method could produce
in situ nitrogen-rich carbon SMSs without the need for addi-
tional treatments. To assess the ORR catalytic activity of PDA-
based carbon SMSs, both this material and traditional PFR-
based carbon SMSs were tested using cyclic voltammetry and
a rotating disk electrode (RDE). As shown in Figure 2e,f, PDA-
based carbon SMSs exhibited a cathodic ORR peak at −0.218 V
and an onset potential at −0.1 V; these were more positive
than those of PFR-based carbon SMSs, suggesting enhanced
ORR catalytic activity. Remarkably, the enhanced catalytic
activity also made it one of the most active metal-free N-doped
carbon ORR catalysts (Supporting Information, Table S1).
We reasoned that N doping in PDA-based carbon SMSs
mainly contributed to an enhancement in ORR catalytic
activity compared to N-free PFR-based carbon SMSs. It has
been demonstrated that graphitic N could greatly increase the
ORR catalytic activity was then investigated in O2-saturated
0.1 M KOH. Relative to metal-free PDA-based carbon SMSs, the
ORR catalytic activity of Fe@Fe3C/C–N was greatly enhanced.
A more positive cathodic ORR peak (−0.138 V vs. −0.218 V)
and onset potential (0 V vs. −0.1 V) were observed (Figure 3e,f).
The half-wave potential was about −0.1 V, similar to that of the
commercial 20 wt% Pt/C, and the catalytic current was even
higher relative to the commercial 20 wt% Pt/C at a voltage
below −0.4 V. We hypothesized that the introduction of iron in
carbon SMSs formed new active sites, which led to the further
enhancement of ORR catalytic activity. To confirm this hypoth-
esis, we performed RDE measurements to check the ORR
activity of Fe@Fe3C/C–N in 0.1 M KOH containing 0.01 M KCN
(Supporting Information, Figure S10b). CN− ions are capable
of coordinating strongly to iron and poisoning the iron-cen-
tered catalytic sites.[28] In the presence of CN−, Fe@Fe3C/C–N
exhibited a significant loss of ORR catalytic activity. The onset
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Calculated from the slope of the K–L plots
(see Supporting Information for the detailed
calculation), the n values for the two elec-
trodes all approached 4. This value indicated
ideal four-electron reaction pathways for
the direct reduction of O2 to OH− instead
−
of HO2 on the two electrodes, similar to
ORR catalyzed by the commercial Pt/C. This
result was visually verified by rotating ring-
disk electrode (RRDE) measurements (Sup-
porting Information, Figure S13), in which
both Fe@Fe3C/C–N and PDA-based carbon
SMSs yielded little H2O2 during the ORR
process. In contrast, 30–60% of H2O2 was
detected for PFR-based carbon SMSs, sug-
gesting a dominant two-electron reaction
pathway.
For applications as effective ORR catalysts
for fuel cells, there should be a high cata-
lytic selectivity for ORR against fuel oxida-
tion, particularly for use in methanol fuel
cells. This is because methanol can cross
over the membrane from anode to cathode,
seriously compromising the whole cell
performance.[25a] We therefore evaluated the
tolerance of both PDA-based carbon SMSs
and Fe@Fe3C/C–N to methanol in compar-
ison with the commercial Pt/C (Supporting
Information, Figure S14a–c). Upon addition
of methanol, we observed a drastic decrease
in the ORR catalytic activity of Pt/C. Con-
versely, neither PDA-based carbon SMSs nor
Fe@Fe3C/C–N exhibited an obvious fading
of activity, which revealed their extraordinary
selectivity for oxygen to evade the crossover
effects of methanol.
800
700
600
500
400
300
200
100
0
c
d
& Graphite
Fe3C
Fe@Fe3C/C-N
*
(031)
Fe
*
(110)
(210)
*
*
(221)
*
*
0
5
10 15 20 25 30 35
Pore size (nm)
*
*
Graphite
&
(002)
*
*
*
(200)
*
BET surface area=422.4 m2 g-1
*
*
*
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
20
30
40
50
60
70
2θ (degree)
f
e
0
-1
-2
-3
-4
-5
-6
-7
Fe@Fe3C/C-N
20 wt% Pt/C
20 wt% Pt/C in N2
20 wt% Pt/C in O2
-0.1V
Fe@Fe3C/C-N in N2
Fe@Fe3C/C-N in O2
-0.138 V
-0.8
-0.6
-0.4
-0.2
0.0
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Potential vs. Ag/AgCl (v)
Potential vs. Ag/AgCl (v)
Figure 3. a,b) TEM (a) and high-resolution TEM (b) images of Fe@Fe3C/C–N. c,d) XRD pat-
tern (c) and nitrogen adsorption and desorption isotherms (d) of Fe@Fe3C/C–N. The inset
in (d) shows the pore-size distribution. e) CV plots of Fe@Fe3C/C–N and the commercial
20 wt% Pt/C in N2- and O2-saturated 0.1 M KOH. f) Polarization curves of Fe@Fe3C/C–N and
the commercial 20 wt% Pt/C at 1600 rpm in O2-saturated 0.1 M KOH.
Another major concern with fuel cell appli-
cations is the durability of the catalysts. PDA-
based carbon SMSs and Fe@Fe3C/C–N were
analyzed for durability using chronoampero-
potential decreased by more than 0.1 V, and was coupled with
a decrease in the diffusion-limiting current. This indicated the
crucial role of iron in improving the catalytic activity of Fe@
Fe3C/C–N. Interestingly however, when the Fe@Fe3C was
loaded on N-free PFR-based carbon SMSs, the resulting Fe@
Fe3C/C was found to be much less active than Fe@Fe3C/C–N
(Supporting Information, Figure S11), clearly clarifying that
nitrogen species were also important to the high ORR catalytic
activity observed in the Fe@Fe3C/C–N.
To shed light on the kinetics of electrochemical catalytic
ORR on both Fe@Fe3C/C–N and PDA-based carbon SMSs,
Koutecky–Levich (K–L) plots derived from RDE measurements
were obtained at different potentials (Supporting Information,
Figure S12). Good linear relationships between the inverse cur-
rent density and the inverse square root of the rotational speed
were observed, regardless of the potential setting. This clearly
suggested first-order reaction kinetics with respect to the
oxygen concentration and a similar electron transfer number
(n) for ORR at different potentials on the two electrodes.
metric measurements (Supporting Information, Figure S14d).
Both displayed excellent catalytic stability with less than 10%
activity loss over 10000 s of continuous operation, whereas Pt/C
suffered more than 30% activity decay over this time period.
In addition, Fe@Fe3C/C–N was found to be an effective ORR
catalyst in acidic media as well. The catalytic activity was much
higher compared with that of Fe@Fe3C/C, PDA-, PFR-based
carbon SMSs, and some carbon materials reported elsewhere
(Figure 4).[29] Moreover, they also possessed low H2O2 yield,
superior catalytic selectivity and stability to the commercial
20 wt% Pt/C (Supporting Information, Figure S15). These fea-
tures make Fe@Fe3C/C–N very attractive as an ORR catalyst for
many important energy conversion and storage technologies
that typically proceed in acidic media, such as proton-exchange
membrane fuel cells.
Finally, the presented strategy can provide an important
platform for other critical applications besides ORR. Firstly,
PDA SMSs could act as a general building block for fabricating
other metal-carbon hybrid materials besides Fe@Fe3C/C–N,
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monodisperse N-rich carbon SMSs without any change in mor-
phology. In contrast with traditional RFR-based carbon SMSs,
the present carbon SMSs contained high-level electroactive
N and much less sp3 C, leading to higher electroconductivity
and thus higher ORR catalytic activity. Moreover, metal-carbon
hybrid materials and porous carbon could also be prepared.
This feasible strategy presents new opportunities for tailoring
the structure, composition, and size of carbon SMSs for appli-
cations in supercapacitors, catalysts, energy conversion, storage,
and as adsorbents for drug delivery and water-treatment
processes.
a
20 wt% Pt/C in N2
20 wt% Pt/C in O2
Fe@Fe3C/C-N in N2
Fe@Fe3C/C-N in O2
Fe@Fe3C/C in N2
Fe@Fe3C/C in O2
PDA-based carbon SMSs in N2
PDA-based carbon SMSs in O2
PFR-based carbon SMSs in N2
PFR-based carbon SMSs in O2
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential vs. Ag/AgCl (V)
b
0
-1
-2
-3
-4
-5
-6
-7
Acknowledgements
K.L.A. and Y.L.L. contributed equally to this work. Financial support by
NSFC (No. 21125521, 21075117), the National Basic Research Program
of China (973 Program, No. 2010CB933600) and the “Hundred Talents
Project” of the Chinese Academy of Science is gratefully acknowledged.
Received: September 19, 2012
Revised: November 19, 2012
Published online: December 13, 2012
Fe@Fe3C/C
PFR-based carbon SMSs
PDA-based carbon SMSs
Fe@Fe3C/C-N
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such as Ni- and Co-C spheres (Supporting Information,
Figure S17,S18). Fe@Fe3C/C–N is highly promising for drug
delivery and water treatment due to its strong ferromag-
netism (Supporting Information, Figure S16), large surface
area, and porous structure, while Ni- and Co-C spheres may
be used for protein separation and catalysis. Secondly, after
activation by KOH, PDA-based carbon SMSs become highly
porous, with a majority of micropores derived from etching
by KOH. Their Brunauer–Emmett–Teller (BET) surface area
reached 2006 m2 g−1 (Supporting Information, Figure S19).
Along with high-level oxygen and nitrogen heteroatoms (Sup-
porting Information, Table S3), these properties may allow the
carbon SMSs to be used as potential electrode materials for
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In summary, we have developed a simple and versatile
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SMSs. In this approach, dopamine was selected as the carbon
resource due to its biocompatibility and self-polymerization
capability at alkaline conditions. The as-prepared polymer SMSs
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