A Rapid Microwave-Assisted Thermolysis Route to Highly Crystalline Carbon Nitrides for Efficient Hydrogen Generation

Article abstract of DOI:10.1002/anie.201608453

Highly crystalline graphitic carbon nitride (g-C₃N₄) with decreased structural imperfections benefits from the suppression of electron–hole recombination, which enhances its hydrogen generation activity. However, producing such g-C

Full text of DOI:10.1002/anie.201608453

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International Edition: DOI: 10.1002/anie.201608453
German Edition: DOI: 10.1002/ange.201608453
Carbon Materials Very Important Paper
A Rapid Microwave-Assisted Thermolysis Route to Highly Crystalline
Carbon Nitrides for Efficient Hydrogen Generation
Yufei Guo, Jing Li, Yupeng Yuan,* Lu Li, Mingyi Zhang, Chenyan Zhou, and Zhiqun Lin*
[9,10]
Abstract: Highly crystalline graphitic carbon nitride (g-C N )
the polycondensation of g-C N .
The characterization of
3
4
3
4
with decreased structural imperfections benefits from the
suppression of electron–hole recombination, which enhances
its hydrogen generation activity. However, producing such
g-C N materials by conventional heating in an electric furnace
g-C N reveals the presence of structural defects that are due
3
4
to incompletely reacted intermediates, including amino and/
[
12,13]
or cyano groups.
The photocatalytic activity is sensitively
affected by these structural defects because they can serve as
charge trap sites in photocatalytic reactions. To address this
issue, one strategy is to avoid the complicated reaction
pathways, thus reducing the formation of intermediate
products during the polycondensation into g-C N . The
3
4
has proven challenging. Herein, we report on the synthesis of
high-quality g-C N with reduced structural defects by judi-
3
4
ciously combining the implementation of melamine–cyanuric
acid (MCA) supramolecular aggregates and microwave-
3
4
assisted thermolysis. The g-C N4 material produced after
second strategy involves the preparation of highly crystalline
3
optimizing the microwave reaction time can effectively gen-
erate H2 under visible-light irradiation. The highest H2
evolution rate achieved was 40.5 mmolh , which is two times
g-C N4 to reduce the number of structural defects. It is
3
noteworthy that the conventional synthetic approach, which
involves heating in an electric furnace, cannot yield high-
quality g-C N as noted above. Clearly, an effective synthetic
À1
higher than that of a g-C N4 sample prepared by thermal
3
3
4
polycondensation of the same supramolecular aggregates in an
electric furnace. The microwave-assisted thermolysis strategy is
simple, rapid, and robust, thereby providing a promising route
for the synthesis of high-efficiency g-C N photocatalysts.
strategy for the synthesis of highly crystalline g-C N is
3 4
desirable yet challenging.
Compared with single-component N-rich precursors, the
advantages of using supramolecular aggregates as starting
materials for creating g-C N are twofold. First, the supra-
3
4
3
4
A
mong various renewable energy sources, solar production
molecular aggregates possess a preorganized network that is
structurally similar to the local arrangement in g-C N
4
of hydrogen (H ) fuel through photocatalytic water splitting is
2
3
widely recognized as an ideal route to utilizing solar energy as
both solar energy and water are highly abundant on Earth. Of
building blocks at the molecular level. Second, because of
the hydrogen-bonded supramolecular structure, the supra-
molecular aggregates reduce the sublimation of the N-rich
molecules (e.g., melamine) during polycondensation at ele-
the various materials that have been reported for active H
2
[1–8]
generation under UV and visible-light irradiation,
graph-
[
14,15]
itic carbon nitride (g-C N ) has garnered much attention as it
vated temperatures.
Consequently, g-C N4 samples
3
3
4
is a highly active and stable photocatalyst for H generation
obtained from supramolecular aggregates as starting precur-
sors benefit from reduced structural defects and enhanced
photocatalytic H generation.
2
2
[
9–11]
upon exposure to visible light.
To date, several nitrogen-
[14,15]
rich molecules, such as dicyandiamide, melamine, and urea,
have been routinely used as precursors to produce g-C N at
Herein, we report on a simple route for the synthesis of
highly crystalline g-C N with a significantly reduced number
3
4
[
9,10]
high temperatures (400–6008C) in air or inert atmosphere.
3
4
Notably, these N-rich molecules can follow several reaction
pathways with the formation of various intermediates during
of defects for high-efficiency H generation by judiciously
2
combining the use of melamine–cyanuric acid supramolecular
aggregates with rapid, microwave-assisted thermolysis. 1,3,5-
Triazine-2,4,6-triol, a planar molecule with carbonyl groups at
each vertex (compound 2 in Figure 1) was complexed with
melamine (1) to produce hydrogen-bonded melamine–cya-
nuric acid (MCA) supramolecular aggregates (3). Intrigu-
ingly, the MCA supramolecular aggregates possess structural
similarity to g-C N (4). They can thus be directly polymerized
[
*] Y. Guo, J. Li, Dr. Y. Yuan, C. Zhou
School of Chemistry and Chemical Engineering
Anhui University
Hefei 230601 (P. R. China)
E-mail: yupengyuan@ahu.edu.cn
3
4
L. Li, Dr. M. Zhang
School of Physics and Electronic Engineering
Harbin Normal University
to yield g-C N , thereby eliminating the complicated reaction
3 4
pathways involved in the formation of intermediates and
producing g-C N with greatly reduced defects. Moreover, in
3
4
Harbin 150025 (P. R. China)
sharp contrast to the conventional approach based on heating
in an electric furnace, the microwave-assisted thermolysis
approach enables the production of g-C N within minutes
Dr. Y. Yuan, Prof. Z. Lin
School of Materials Science and Engineering
Georgia Institute of Technology
Atlanta, GA 30332 (USA)
3
4
with greatly improved crystallinity owing to the strong
rotation, friction, and collision of N-rich molecules under
E-mail: zhiqun.lin@mse.gatech.edu
[16–18]
microwave irradiation.
The photocatalytic H generation
2
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201608453.
rate of a g-C N sample thus obtained was two times higher
3
4
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Figure 2. a) XRD patterns and b) FTIR spectra of CN samples pro-
t
Figure 1. g-C N formation. Molecular structures of melamine (1),
duced by microwave-assisted thermolysis of MCA precursors for 10,
12, and 16 min, respectively. The XRD pattern and FTIR spectrum of
a reference sample (CN540) prepared by heating MCA precursors at
3
4
cyanuric acid (2), MCA supramolecular aggregates (3), and g-C N (4).
3
4
5408C for 4 h are also provided for comparison. The right panel in (a)
than that of a g-C N sample obtained by polymerizing MCA
supramolecular aggregate in an electric furnace.
shows close-up XRD patterns of g-C samples from 2q=25–308.
N
3 4
3
4
1
3
c) C MAS NMR spectra of MCA supramolecular aggregates and
CN . d) XPS spectra showing the binding energies of C 1s and N 1s
1
6
Specifically, MCA supramolecular aggregates (3) were
precipitated from an equimolar mixture of melamine and
cyanuric acid in dimethyl sulfoxide (DMSO). The MCA
aggregates possess a spherical morphology with an average
size of about 3 mm, as shown by scanning electron microscopy
for the CN16 sample.
pattern of CN540 is essentially the same as that reported in
[20]
(
SEM; see the Supporting Information, Figure S1a). Further
the literature. The two characteristic diffraction peaks at
analysis of the MCA spheres revealed that they are composed
of closely assembled nanoparticles with an average size of
2q = 138 and 27.58 (!) correspond to the periodic in-plane tri-
s-triazine stacking and the interlayer structural aromatic
[
16,21]
2
00 nm or smaller (Figure S1b). The intense peaks at 2q = 108
packing, respectively (Figure 2a).
The wide diffraction
and 288 in the XRD pattern were attributed to the periodic
arrays of intraplanar stacking and interlayer aromatic stack-
ing of MCA, respectively (Figure S2). Compared to g-C N ,
peak of the CN540 sample suggested that low-crystalline
g-C N was produced by conventional heating. It is interesting
3
4
to note that in spite of the different heating process with
microwave-assisted heating of MCA for only 8 min, the CN8
sample showed essentially the same XRD pattern as MCA
with the emergence of a new diffraction peak at 2q = 128,
indicating the polymerization of melamine and cyanuric acid
(Figure S2). Importantly, the CN10 sample obtained after
microwave-assisted thermolysis for 10 min displayed two
distinct diffraction peaks at 2q = 138 and 27.58, reflecting
that g-C N had been formed after microwave-assisted heat-
3
4
with peaks located at 2q = 138 and 27.58 as described below
Figure 2a), the different peak positions of MCA (2q = 108
(
and 288) may be due largely to the hydrogen-bonded network
in MCA (3). Nonetheless, this XRD result further substan-
tiated that the structure of MCA (3) bears a strong resem-
blance to g-C N (4).
3
4
Subsequently, the MCA supramolecular aggregates were
thermalized for various periods of time using the microwave-
assisted heating strategy (see the Experimental Section in the
Supporting Information). CuO was used as the microwave
absorber as it can intensely and effectively absorb microwaves
3
4
ing for 10 min. Moreover, in comparison to the XRD pattern
of the CN540 sample with a broad and weak peak at 2q =
27.58, a relatively intense and narrower diffraction peak was
observed in the CN10 sample, indicating improved crystal-
linity. Prolonged reaction times of 12 and 16 min yielded
g-C N (CN and CN ) with progressively increasing dif-
[19]
to increase the temperature up to 1285 K in less than 7 min.
The resulting products were labeled CN , where t is the
t
microwave heating time in minutes. A control sample was also
prepared by polymerizing the MCA supramolecular aggre-
gates at 5408C for 4 h in Ar atmosphere by conventional
heating in an electric furnace (CN540). After 10 min poly-
condensation by microwave irradiation (yielding CN ), the
3
4
12
16
fraction peak intensities (Figure 2a). We note that the 27.58
peak was shifted to a higher angle with increased reaction
time (Figure 2a, right), suggesting a decreased interlayer
distance in the resulting CN12 and CN16 samples. Clearly, the
increased XRD peak intensity and the decreased interlayer
1
0
products showed the yellow color typical of g-C N . The XRD
3
4
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distance upon increasing the reaction time revealed that the
polycondensation of the MCA precursors is enhanced by
microwave-assisted thermolysis, resulting in highly crystalline
g-C N .
3
4
FTIR measurements were performed to analyze the MCA
polycondensation at different microwave reaction times
(
Figure 2b). The white CN8 sample gave rise to several
bands at 3396, 3232, 1748, 1652, 1535, 1450, 1074, 1025, 910,
À1
8
10, and 765 cm , corresponding to the MCA supramolec-
ular aggregates (Figure S3). The increased intensity of these
peaks in CN is indicative of the polycondensation of MCA;
8
À1
this is particularly true for the enhanced peak at 810 cm ,
[
14]
a signature of the formation of tri-s-triazine (Figure S3). In
the CN10 sample, the characteristic peaks of the MCA
precursors have disappeared, and new peaks have emerged
at 810, 1240, 1318, 1411, 1460, 1568, and 1640 cm , which
belong to the stretching vibrations of the CN heterocycles,
signifying the formation of tri-s-triazine. The intensity of
these peaks increased as the polycondensation reaction
progressed (from CN10 to CN12 to CN ). The NH stretching
vibration mode was observed at approximately 3200 cm .
The structure of the CN sample was further investigated
À1
[16]
[
16]
16
2
À1
1
6
1
3
Figure 3. SEM images of a) CN540 and b) CN16. TEM images of
c) CN540 and d) CN . Whereas CN540 has a sphere-like morphology,
CN16 has an irregular morphology resulting from the collapse of the
MCA sphere precursors. Moreover, the CN16 nanosheets are thinner
than those of CN540.
by C MAS NMR spectroscopy (Figure 2c). The NMR
1
6
spectrum showed two resolved resonances at d = 156.2 and
1
d2 = 164.6 ppm, corresponding to the C(i) atoms in the CN
3
groups and the C(e) atoms in CN (ÀNH )C, respectively
2
2
1
3
(
Figure S4). The C MAS NMR spectrum clearly corrobo-
[16]
rated the g-C N frameworks in CN . Notably, the MCA
3
4
16
supramolecular aggregates also displayed two resonances at
Figure 3 shows representative SEM and TEM images of
CN16 and CN540 samples. The CN540 sample displayed
a loose and porous sphere-like morphology with an average
sphere size of about 4 mm (Figure 3a). The overall appearance
of CN540 resembles that of the MCA precursors. Interest-
ingly, the CN540 spheres are composed of velvet-like nano-
sheets with a thickness of less than 30 nm (Figure 3c). In
contrast, the CN16 sample exhibited an irregular morphology.
Obviously, the MCA spheres shown in Figure S1 had com-
pletely collapsed after microwave-assisted heating for 16 min
(Figure 3b), which is due to the large amount of gases, such as
d = 153.6 and d = 165.3 ppm, corroborating the structural
1
2
similarity to CN16 (Figure 2c).
To further investigate the structural details of CN ,
1
6
samples were analyzed by X-ray photoelectron spectroscopy
XPS; Figure 2d). The XPS analysis confirmed the presence
(
of carbon and nitrogen in CN16 (Figure S5). The O 1s peak is
due to doped cyanuric acid (Figure S5). The carbon species
2
with a binding energy of 286.05 eV arises from the C(sp )=N
bonds in the s-triazine rings. The peak at 283.07 eV is a result
of the graphitic carbon while the peak at 287.03 eV is due to
CÀO bonds. The N 1 s binding energy can be deconvoluted
NH , spontaneously generated during the polycondensation
3
into three peaks of 396.72, 398.40, and 402.32 eV (Fig-
of the MCA supramolecular aggregates. The quick escape of
gases led to the formation of highly fluffy and porous
structures. The g-C N nanosheets produced by microwave-
[
21,22]
ure 2d).
The strongest N 1s peak at 396.72 eV corre-
2
sponds to sp -hybridized aromatic N atoms (C=NÀC) in the
3
4
s-triazine rings. The N 1s peak at 398.4 eV is indicative of
amino groups (CÀNÀH), which is consistent with the FTIR
assisted heating were also further visualized by TEM (Fig-
ure 3d).
measurements (Figure 2b). The weakest N 1s peak at
UV/Vis absorption spectra of g-C N samples produced by
3
4
[
14,23]
4
02.32 eV is due to the charging effect.
Elemental analysis revealed the C/N molar ratios of
CN540, CN , CN , and CN to be 0.674, 0.671, 0.679, and
microwave-assisted heating for different periods of time
(CN , CN , and CN ) and the control example (CN540) are
1
0
12
16
shown in Figure S6. The band gap of the CN16 sample was
estimated to be 2.7 eV based on the onset of the absorption
1
0
12
16
0
.684, respectively. The nitrogen richness of the g-C N4
3
[
16]
samples arises from the dangling NH groups in g-C N , as
edge. It is noticeable that the intensity of the absorption in
the visible region increased with the microwave reaction time.
The effective transfer of photogenerated charge carriers
depends sensitively on the trap sites in the semiconductor,
which are closely related to the structural defects of the
material. In this context, the steady-state photoluminescence
(PL) upon excitation at 350 nm was measured in order to
study the charge recombination in g-C N . As shown in
2
3
4
revealed by FTIR spectroscopy. Although the measured C/N
molar ratios are lower than the theoretical ratio of 0.75 for g-
C N , the progressively increasing C/N ratios clearly reflect
3
4
the improved structural integrity of the CN samples. We note
t
that the suite of characterizations discussed above substan-
tiate the rapid and effective formation of g-C N by micro-
3
4
wave-assisted thermolysis of MCA supramolecular aggre-
gates.
3
4
Figure 4a, the CN540 sample gave rise to a strong emission
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Figure 4. a) Steady-state PL spectra and b) time-resolved PL spectra of
CN16 and CN540 samples. The quenching of the PL intensity in the
CN16 sample signifies the reduced structural defects within the frame-
work. In addition, the shorter lifetime in CN16 indicates that charge
transfer is more rapid than in the CN540 sample.
Figure 5. a) Photocatalytic H generation rates of CN and CN540
2
t
peak centered at 470 nm. In contrast, a significantly smaller
samples. b) H generation with CN , CN540, and M540 samples as
2
16
emission peak was observed for CN , suggesting decreased
1
6
a function of time. The M540 sample was prepared by polycondensa-
tion of melamine alone at 5408C for 4 h in Ar. c) Photoluminescence
spectra of TAOH formed by the reaction of TA with COH radicals
generated from CN16 and CN540 samples under visible-light irradiation
for 10 min. d) Transient photocurrent response of ITO/CN16 and ITO/
CN540 at À1.0 V vs. Ag/AgCl in 0.5m Na SO exposed to visible light
recombination of photogenerated electron–hole pairs. This
observation further supported the reduced structural defects
in the CN16 sample as defects commonly serve as electron–
hole recombination centers. More importantly, the improved
2
4
crystallinity of the CN samples (Figure 2a) resulted in rapid
t
(lꢀ420 nm, 300 W Xe lamp).
charge transfer in the CN16 sample compared to CN540, as
confirmed by time-resolved PL spectroscopy measurements
(
Figure 4b). The average PL lifetimes of CN540 and CN16
CN540 sample and more than six times larger than that of the
M540 sample. The H2 evolution rate of 40.5 mmolh is
À1
were 3.96 and 2.79 ns, respectively. The shorter lifetime of the
charge carriers in CN16 signifies the rapid transfer of photo-
generated electron–hole pairs involved in the redox reaction
for CN , thereby potentially promoting the photocatalytic H
relatively high as the specific surface area of CN16
2
À1
(45.3 m g ) is 1.5 times smaller than that of CN540
2
À1
(68.3 m g ), which further substantiates the effectiveness of
the present microwave-assisted thermolysis strategy for high-
quality g-C N production. Interestingly, the initial H gen-
1
6
2
[24]
production.
Visible-light-induced photocatalytic H2 generation was
3
4
2
then attempted by capitalizing on the produced g-C N4
samples (20 mg) in the presence of triethanolamine
eration rates of CN16 and CN540 were essentially the same
during the first hour (Figure 5b). As the microwave reaction
3
(
TEOA) as the sacrificial electron donor. Pt (0.5 wt%) was
time was increased, the H generation rate was comparatively
2
photoloaded in situ onto all g-C N samples as a co-catalyst to
reduced for the CN540 sample, revealing the enhanced
3
4
boost H generation. Control experiments showed that no H2
stability of the CN16 sample for photocatalytic H generation,
2
2
generation occurred in the absence of either photocatalyst or
which is a direct consequence of the improved crystallinity of
light irradiation, confirming that H generation requires both
the CN samples as determined by XRD (Figure 2a).
2
t
light irradiation and a g-C N photocatalyst. The photocata-
As noted above, the enhanced photocatalytic H produc-
3
4
2
lytic H2 generation rate of CN was determined to be
tion can be ascribed to the reduced structural defects and the
10
À1
1
9.8 mmolh , which is comparable to that of a CN540 sample
improved crystallinity of the CN samples and thus the
t
À1
(
22.4 mmolh ; Figure 5a). Notably, the g-C N sample pre-
decreased number of trap sites in g-C N4 for efficient
3
4
3
pared by polycondensation of melamine alone at 5408C for
separation of photogenerated electron–hole pairs. To confirm
the effectiveness of the charge separation, we carried out COH
radical concentration measurements. As the COH radicals can
oxidize terephthalic acid (TA) to 2-hydroxyterephthalic acid
4
h in Ar in an electric furnace (M540) only showed a H
2
À1
generation rate of 6 mmolh under the same experimental
conditions. This result clearly demonstrates the advantage of
using the MCA supramolecular aggregates as precursors
instead of melamine alone to create g-C N . The photo-
[
25,26]
(TAOH), which has a strong emission peak at 426 nm,
the
fluorescence intensity of TAOH can be used to monitor the
charge separation efficiency. Compared to the CN540 sample,
the higher fluorescence intensity of TAOH (Figure 5c) in the
CN16 sample revealed that more holes in the excited CN16
sample are involved in the oxidation reaction of TA than in
3
4
catalytic H generation activity was even higher with the CNt
2
samples with increased microwave reaction times. The highest
À1
H evolution rate of 40.5 mmolh was achieved with the CN16
2
sample; this value is nearly two times larger than that of the
4
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the CN540 sample, indicating the more effective charge
separation in CN16 compared to CN540.
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dark current subtracted). The photocurrent responses were
prompt and reproducible during the on/off cycles of visible-
light excitation. Despite the relatively small value, the
photocurrent generated in the CN16 sample was still compa-
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (51572003), Anhui Provincial
Natural
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(1508085ME105,
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Chinese Scholar, Ministry of Personnel of China.
2
2
Keywords: graphitic carbon nitride ·
microwave-assisted thermolysis · photocatalysis ·
supramolecular aggregates · water splitting
8
9
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2] A. Kudo, Y. Miseki, Chem. Soc. Rev. 2009, 38, 253 – 278.
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Carbon Materials
Highly crystalline graphitic carbon nitride
g-C N ) was synthesized within 16 min
(
3
4
Y. Guo, J. Li, Y. Yuan,* L. Li, M. Zhang,
by microwave-assisted thermolysis of
melamine–cyanuric acid supramolecular
C. Zhou, Z. Lin*
&&&& — &&&&
aggregates. The H generation rate ach-
2
A Rapid Microwave-Assisted Thermolysis
Route to Highly Crystalline Carbon
Nitrides for Efficient Hydrogen
Generation
ieved with the as-obtained g-C N mate-
3
4
rial is two times higher than that of
a sample prepared by heating the same
supramolecular aggregates in a furnace
at 5408C for 2 h.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!