Nitrogen-doped colloidal graphene quantum dots and their size-dependent electrocatalytic activity for the oxygen reduction reaction

Article abstract of DOI:10.1021/ja309270h

Nitrogen doping has been a powerful way to modify the properties of carbon materials ranging from activated carbon to graphene. Here we report on a solution chemistry approach to nitrogen-doped colloidal graphene quantum dots with well-defined structures. N-doping was demonstrated to significantly affect the properties of the quantum dots, including the emergence of size-dependent electrocatalytic activity for the oxygen reduction reaction.

Full text of DOI:10.1021/ja309270h

Communication
pubs.acs.org/JACS
Nitrogen-Doped Colloidal Graphene Quantum Dots and Their Size-
Dependent Electrocatalytic Activity for the Oxygen Reduction
Reaction
Qiqi Li,† Sheng Zhang,‡ Liming Dai,‡ and Liang-shi Li*,†
†Department of Chemistry, Indiana University, Bloomington Indiana 47405, United States
‡Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
S
* Supporting Information
Colloidal graphene QDs are a new addition to zero-
ABSTRACT: Nitrogen doping has been a powerful way
to modify the properties of carbon materials ranging from
activated carbon to graphene. Here we report on a solution
chemistry approach to nitrogen-doped colloidal graphene
quantum dots with well-defined structures. N-doping was
demonstrated to significantly affect the properties of the
quantum dots, including the emergence of size-dependent
electrocatalytic activity for the oxygen reduction reaction.
dimensional quantum-confined systems, and have some unique
properties due to the dimensionality of graphene and the light
constituent elements.4 The graphene QDs are soluble in
common organic solvents because of the peripheral trialkyl
phenyl groups (R- groups in Scheme 1), which make a flexible,
three-dimensional cage around the graphene core to prevent
aggregation. Combining the solubilization method with well-
developed reactions for carbon−carbon bond formation, we
have synthesized various colloidal graphene QDs with uniform
sizes and well-defined structures.4
In this work, we apply the solution chemistry approach to
synthesis of graphene QDs doped with nitrogen atoms. In
addition to two nitrogen atoms, the conjugated cores (marked
blue in Scheme 1) of QDs 1 and 2 contain 176 and 128 carbon
atoms, respectively. With the solution chemistry approach, we
can not only control the numbers of nitrogen atoms in the
QDs, but also define their bonding configurations so that 1 and
2 were made as aryl-annulated phenazines. This is in sharp
contrast to those methods previously explored for doping
graphene1 and can afford graphene QDs with well-defined
properties. To understand the doping effects in the QDs, we
also synthesized QD 3, an undoped analogue of 2, which
contains 130 conjugated carbon atoms in the graphene core
(marked blue in Scheme 1). Because of the presence of the
solubilizing R- groups, all the QDs are soluble in common
solvents, such as toluene, tetrahydrofuran (THF), and
chloroform.
itrogen substitution, or substitutional N-doping, has been
N
a powerful method to modify graphitic carbon materials
for various applications, ranging from catalysis to micro-
electronics.1,2 N-doped activated carbon or carbon nano-
tubes,1,2 for example, possess catalytic activities for some
important chemical reactions such as oxygen reduction and
dehydrochlorination. In graphene, substitutional nitrogen
atoms can significantly modify the electronic structures of the
host,3 providing a promising way to tailor the transport
properties of graphene for electronic applications. However, N-
doping of the carbon materials so far inevitably requires harsh
reaction conditions such as high temperature or fields.1 As a
result, it is very difficult to control the distribution and bonding
configurations of the nitrogen dopants in the π-conjugated
framework, leading to a poor understanding about the doping
effects and ill-controlled material properties. Herein, we report
on a solution chemistry approach to N-doped colloidal
graphene quantum dots (QDs, 1 and 2 in Scheme 1). This
The synthesis of 1−3 is outlined in Scheme 2. Briefly, we
start with small substituted benzene derivatives such as 2-
bromo-5-iodo-3′-(phenylethynyl)-1,1′-biphenyl (4) and phe-
nylboronic acid to yield a nitrogen-containing intermediate 6
(details in the Supporting Information (SI)). Then 6 undergoes
double Suzuki cross-coupling reactions, leading to 8 and 9, the
precursors for 1 and 2, respectively. Upon treatment with a
large excess of iron(III) chloride under a constant stream of
argon in a dichloromethane/nitromethane mixture at room
temperature,4b these precursors are converted to the N-doped
graphene QDs. Similarly, QD 3 is obtained from an α, β-
diketone 5 with the Wittig reaction,5 followed by the Suzuki
coupling and oxidative dehydrogenation. As for previously
reported graphene QDs, the precursors 8−10 for 1−3 are
Scheme 1. Structures of N-Doped Graphene QDs 1, 2 and an
Undoped QD 3 for Comparison Studies
approach has enabled us to synthesize the QDs with well-
defined structures and various sizes. We further show that the
incorporation of the nitrogen atoms significantly varies the
properties of the resultant QDs. In particular, it was found that
the N-doped QDs could electrochemically catalyze the oxygen
reduction reaction with a size-dependent activity.
Received: September 18, 2012
Published: November 5, 2012
© 2012 American Chemical Society
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Communication
a
completely removed by repetitive washing, as confirmed with
X-ray photoelectron spectroscopy (shown in the SI).
Scheme 2. Synthesis of Quantum Dots 1−3
The presence of the nitrogen atoms in QDs 1 and 2
dramatically affects their optical properties. Figure 2 shows
a
Conditions: (i) phenylboronic acid, Pd(PPh3)4, K2CO3, toluene/
EtOH/H2O, 80 °C; (ii) (a) n-BuLi, THF, −78 °C, (b) 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane, −78 °C to rt, (c) 1,3-
dibromo-5-iodobenzene, Pd(PPh3)4, K2CO3, toluene/EtOH/H2O,
85 °C; (iii) 9,11-diphenyl-10H-cyclop-enta[e]pyren-10-one, Ph2O,
240 °C; (iv) RuCl3, NaIO4, CH2Cl2/CH3CN/H2O, rt; (v) benzene-
1,2-diamine, EtOH/CHCl3, 65 °C; (vi) Pd(PPh3)4, K2CO3, toluene/
EtOH/H2O, 80 °C; (vii) FeCl3, CH2Cl2/CH3NO2, rt; (viii) o-
Xylylenebis(triphenylphosphonium bromide), LiOH(aq, 5 M),
CH2Cl2, rt.
Figure 2. UV−vis absorption spectra of QDs 1−3 (left axis) and
fluorescence spectrum of 3 (right axis) in THF.
UV−vis absorption spectra of 1−3 and the fluorescence
spectrum of 3 (excited at 600 nm). As expected from their
sizes, the absorption edge of 1 appears at a significantly longer
wavelength than those of 2 and 3, whereas 2 and 3 have almost
the same absorption spectra with little differences. More
strikingly, 3 is highly fluorescent, whereas 1 or 2 does not have
detectable photoluminescence (Figure 2). This is consistent
with the absence of n → π*, the lowest-energy transitions, in
the absorption spectra of 1 and 2. The n → π* transitions are
expected for the N-substituted aromatic heterocycles and are
indeed observed in their precursors (8 and 9) (see the SI). The
large size of 1 and 2 explains the diminishing oscillator
strengths of the n → π* transitions, because of the decreasing
orbital overlap between the increasingly delocalized π*
electrons and the nonbonding electrons localized on the
nitrogen atoms. In contrast, the lowest-energy transition in 3 is
π → π*, which is responsible for the emission and has a less
size-dependent oscillator strength.
purified with gel chromatography and characterized with
standard organic characterization techniques. The final
products 1−3 are purified with repetitive dissolution in toluene
and precipitation with methanol. Their structures are confirmed
with isotope-resolved MALDI-mass spectroscopy, showing
excellent uniformity in size (Figure 1, details in the SI). It
should be noted that the concentration control of oxidant
iron(III) chloride is crucial for the success in the oxidative
dehydrogenation step to yield 1 and 2. The precursors cannot
be fully oxidized unless the concentration of the oxidant is as
high as 35 mg/mL in the dichloromethane/nitromethane
mixture. A possible explanation is that, in the presence of the
electron-withdrawing nitrogen atoms, a high concentration of
the oxidant is necessary for the formation of cationic
intermediates so that the dehydrogenation can go to
completion.6 The excess of the iron(III) chloride was
The electrocatalytic activity of the N-doped graphene QDs
for the oxygen reduction reaction (ORR) is of particular
interest and thus has been investigated in our studies. There
Figure 1. MALDI-TOF MS spectra of QDs 1−3 ([1+H]+, [2+H]+, and 3+, left to right), respectively. In the insets are the isotope-resolved patterns
experimentally measured (solid curves) with ones calculated (dotted curves) from the molecular structures of the QDs. In the spectra for 1 and 3,
the small peaks marked by asterisks are due to chlorinated byproducts in the last oxidation step.4b
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Journal of the American Chemical Society
Communication
have been tremendous efforts on finding replacements for Pt to
catalyze the ORR in fuel cells, and N-doped graphitic carbon
materials are among the candidates being investigated.2a,7
Figure 3a shows the cyclic voltammetry (CV) curves of 1
negative potential (Figure 3b), illustrating the importance of
the nitrogen doping, even at a level as low as ∼1% (N/C
atomic ratio) in the conjugated framework, on the ORR
activity. However, our results do not support the positive
correlation between the catalytic activity and the nitrogen
content that has been suggested previously.2a,7 Rather, the
largest N-doped QD 1 studied here, with the lowest doping
level, shows the highest activity.
Further studies show that the ORR catalyzed by the N-doped
graphene QDs can occur via a four-electron reduction pathway
to produce water directly rather than the two-electron pathway
with peroxide as the intermediate.8 The LSV curves measured
with a RDE with 1 at various rotating speeds are presented in
Figure 3c, showing increasing catholic current at increasing
rotating speeds due to improved mass transport at the electrode
surface. From the slope of the Koutecky−Levich plots (Figure
3d) obtained from the data in Figure 3c, the number of
electrons transferred per oxygen molecule in the ORR is
calculated to be about 3.9 (see the SI), consistent with a four-
electron process that have been reported for other N-doped
carbon materials.2a,7 Therefore, similar catalytic mechanisms
may also apply to the graphene QDs investigated in the present
study.
The size-dependent catalytic activity of the N-doped
graphene QDs has important implications in understanding
and further improving the catalytic activities of N-doped carbon
materials. Previous work was based on a hypothetical
correlation between the catalytic activity and the bonding
configurations of nitrogen atoms in the carbon framework.2a,7
Consequently, efforts mostly have been focused on increasing
the abundance of nitrogen atoms with a specific bonding
configuration, resulting in a debate on what configuration is
optimal.2a,7 Our studies clearly indicate the conjugation size in
these materials as an important parameter that has largely been
ignored. This size dependence is consistent with recent
theoretical calculations,9 and may be due to the higher
HOMO levels in larger QDs which as a result can be more
easily oxidized. Furthermore, since the solution chemistry
approach presented herein should be applicable to the synthesis
of QDs doped with nitrogen atoms in other bonding
configurations,1,2a the N-doped colloidal graphene QDs could
provide unique opportunities for systematically investigating
the mechanisms of the catalyzed ORR by N-doped carbon
materials.
Figure 3. (a) CV curves (scanning rate of 50 mV s−1) of 1 on a glassy-
carbon RDE in a N2- and an O2-saturated 0.1 M KOH solution. (b)
LSV curves (10 mV s−1) for 1−3 and Pt/C on a RDE (1600 rpm) in
an O2-saturated 0.1 M KOH solution. Also shown is the LSV curve for
11, a much smaller N-substituted heterocycle with structure shown in
the inset. (c) LSVs (10 mV s−1) for 1 on a RDE in an O2-saturated 0.1
M KOH solution with various rotating speeds. (d) Koutecky−Levich
plots for 1 obtained from the LSV curves in (c) at various voltage
values. A global fitting of the plots reveals that the number of electrons
transferred per O2 molecule is 3.9.
deposited on a glassy-carbon rotating disk electrode (RDE) in
an O2- or a N2-saturated 0.1 M KOH aqueous solution
(experimental details in the SI). A well-defined reduction peak
occurs at around −0.3 V (vs SCE) in the O2-saturated solution,
whereas there is no such a peak in the N2-saturated one,
indicating that O2 is reduced on the electrode coated with 1.
Linear sweep voltammetry (LSV) shows that the N-doped
QDs have high, size-dependent electrocatalytic activity for the
ORR. Figure 3b shows the LSV curves obtained for QDs 1−3
and commercial Pt/C (20% wt. Pt on Vulcan XC-72R) of the
same mass deposited on a glassy-carbon RDE in an O2-
saturated 0.1 M KOH aqueous solution (1600 rpm,
experimental details in the SI). For further comparison, we
also made a much smaller N-substituted aromatic compound
11 (Figure 3b), a derivative of hexabenzocoronene, and
measured its LSV curve under the same condition. The N-
containing 1, 2, and 11 show ORR onset potentials at −0.04,
−0.10, and −0.14 V, respectively, relative to that for the
commercial Pt/C catalyst. In addition, the cathodic current
density increases with size of the QDs, indicating the improved
catalytic activity of larger QDs. The more negative ORR onset
potentials (and thus greater overpotentials) observed with the
N-doped QDs than that with Pt/C, as well as the lower
cathodic current densities with the same mass of the catalysts,
indicate that the electrocatalytic activities of these N-doped
QDs are not as good as that of Pt/C. Nevertheless, it is
remarkable that the N-doped QDs lead to currents comparable
with that by Pt/C, especially if we consider that the QDs have
much greater molecular weights and thus we may expect fewer
active sites2 in the catalysts with the same mass. In contrast, the
undoped QD 3 shows appreciable activity only at a much more
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed procedure for synthesis, characterization of inter-
mediates for QDs 1−3 and 11, electrochemical measurements
and data analysis, absorption spectra of 8−10. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
li23@indiana.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by NSF (DMR-1105185). S. Zhang
and L. Dai acknowledge the funds from NSF (CMMI-
1000768) and AFOSR (FA9550-12-1-0037; FA2386-10-1-
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4071). The mass spectroscopy measurements were conducted
in the Mass Spectrometry Facility in the Department of
Chemistry at Indiana University, Bloomington.
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