A template induced method to synthesize nanoporous graphitic carbon nitride with enhanced photocatalytic activity under visible light

Article abstract of DOI:10.1039/c4ra12033g

A template induced method was developed to synthesize nanoporous graphitic carbon nitride (npg-CN) by directly heating Triton X-100-modified-melamine sulfate. The effects of the added amount of Triton X-100 and heating temperature on the formation of npg-CN were investigated. The npg-CN samples were characterized by XRD, FT-IR, and elemental analysis; nitrogen adsorption isotherms, SEM and TEM images; and UV-vis DRS and photoluminescence spectra. The results showed that the npg-CN samples possessed a laver-like layered structure with a high surface area (50-135 m² g^-1). Meanwhile, compared to the bulk g-CN (synthesized by directly heating melamine), the npg-CN samples possessed stronger optical absorptions, narrower band gaps, and significantly reduced fluorescence intensity. The photocatalytic activity was investigated by degradation of Rhodamine B under visible light irradiation. As a result, the photodegradation rate of Rhodamine B dye on the npg-CN samples was greatly improved over that on bulk g-CN.

Full text of DOI:10.1039/c4ra12033g

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A template induced method to synthesize
nanoporous graphitic carbon nitride with enhanced
Cite this: RSC Adv., 2014, 4, 61877
photocatalytic activity under visible light†
Qianjing Fan, Jianjun Liu,* Yingchun Yu and Shengli Zuo
A template induced method was developed to synthesize nanoporous graphitic carbon nitride (npg-CN) by
directly heating Triton X-100-modified-melamine sulfate. The effects of the added amount of Triton X-100
and heating temperature on the formation of npg-CN were investigated. The npg-CN samples were
characterized by XRD, FT-IR, and elemental analysis; nitrogen adsorption isotherms, SEM and TEM
images; and UV-vis DRS and photoluminescence spectra. The results showed that the npg-CN samples
2
ꢀ1
possessed a laver-like layered structure with a high surface area (50–135 m
g
). Meanwhile, compared
to the bulk g-CN (synthesized by directly heating melamine), the npg-CN samples possessed stronger
optical absorptions, narrower band gaps, and significantly reduced fluorescence intensity. The
photocatalytic activity was investigated by degradation of Rhodamine B under visible light irradiation. As
a result, the photodegradation rate of Rhodamine B dye on the npg-CN samples was greatly improved
over that on bulk g-CN.
Received 9th October 2014
Accepted 7th November 2014
DOI: 10.1039/c4ra12033g
www.rsc.org/advances
But hard templating methods are step tedious and environ-
mentally harmful because the synthesis procedure involves
1
. Introduction
Recently, graphitic carbon nitride (g-CN) has attracted consid- using silica or alumina as hard templates, which need to be
erable attention because of its catalytic and photocatalytic subsequently removed by aqueous ammonium biuoride
1
–3
activity as a new metal-free catalyst. g-CN materials have been (NH
HF ), hydrogen uoride (HF) or NaOH solutions. There-
4
2
found useful for many applications: as metal-free photo- fore, a novel procedure for the synthesis of npg-CN without hard
4,5
6–8
9,10
21
catalyst, in water splitting, in pollutant degradation, and templates is of particular interest. Wang and co-workers rst
more. Its unique optical, chemical, and catalytic properties, tried to explore a so-template method to synthesize npg-CN by
along with its low price and remarkably high stability against using a variety of surfactants as structure-directing agents and
oxidation, make it a very attractive material for photocatalytic dicyandiamide as precursor. Most experiments yielded disor-
applications. However, the photocatalytic activities of pristine g- dered pore systems and high molar ratio of C/N (0.8–2.0)
CN are seriously limited by its intrinsic drawbacks, including because of the carbon residues generated from the decompo-
22
relatively low surface area, fast charge recombination, and poor sition of so templates. Yan used melamine (MA) and P123 to
mass diffusion/transfer. Many modications of g-CN have been synthesize npg-CN, and obtained the npg-CN samples with low
1
1,12
developed, including texture modication,
nonmetal molar ratios of C/N (0.65–0.69) as well as low BET surface areas
2
ꢀ1
13–15
5,16
doping,
and noble metal deposition;
and constructing (about 30 m g ) without further calcination in air. In general,
17–19
heterojunctions with other materials.
Among these the npg-CN oen possesses low surface areas and high molar
methods, the texture modication have been demonstrated a ratios of C/N because of the surfactant self-decomposition and
successful and efficient pathway to promote charge migration the side-reaction of the surfactant with the precursor during the
11
and separation, as well as the mass diffusion/transfer during thermal treatment. To the best of our knowledge, there are few
12
photoredox reactions, greatly enhancing the photocatalytic reports trying to improve so-template methods of npg-CN
performance. Hard templating strategy has been widely repor- synthesis by solving the problems mentioned above.
ted and successfully used in the synthesis of nanoporous g-CN
Herein, a template induced method derived from the so-
template method was proposed to synthesize npg-CN, by
which the side-reactions of surfactants can be avoided
completely. Because the nonionic surfactant was just used to
modify the structure of precursor and then totally removed
before the thermal treatment. Melamine sulfate was chosen as
the precursor because the sublimation of melamine can be
signicantly suppressed by treating melamine with sulfuric
12,20
(npg-CN) with desired surface properties and morphologies.
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical
Technology, Beijing 100029, China. E-mail: jjliu717@aliyun.com; Fax: +86 10
6
†
1
4434898; Tel: +86 10 64431056
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra12033g
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23
acid and the insolubility of melamine sulfate make it can be specic surface area was calculated by applying the Brunauer–
easily separated from the nonionic surfactant. Triton X-100 was Emmett–Teller (BET) model to the adsorption data. Elemental
used to modify the structure of melamine sulfate as it can be analyses (C, H, N, S) were performed on a EuroVector S.P.A.
easily removed by simple washing. As a result, the as-prepared Euro EA3000 elemental analyzer. UV-vis diffuse reectance
npg-CN samples possessed high BET surface areas and low spectra (UV-vis DRS) were measured using a Shimadzu UV3600
molar ratios of C/N. The nal samples were characterized in spectrophotometer. The photoluminescence (PL) spectra were
terms of chemical structure, morphology, and optical proper- performed on a Hitachi F-7000 uorescence spectrophotometer
ties. The photocatalytic activity of the obtained samples was with photomultiplier tube voltage of 400 V and scanning speed
ꢀ
1
tested by evaluating the degradation of Rhodamine B (RhB) dye of 1200 nm min . The TG-DTA curves were recorded on DTG-
ꢀ
1
under visible light illumination.
60A (Shimadzu) at a heating rate of 10 K min under the
ow of N gas.

2
2
. Experimental section
2.4 Photocatalytic evaluation
2.1 Synthesis of Triton X-100-modied-melamine sulfate
The photocatalytic activity of the samples was evaluated by
degradation of RhB under visible light irradiation. A 500 W
Xenon lamp (Institute of Electric Light Source, Beijing) with 400
nm cutoff lter was chosen as a visible light source. The
All reagents were of analytical grade without further treatment.
In a typical synthetic procedure, 5.0 g melamine and a certain
amount of Triton X-100 (with mass ratios to melamine of 0.3,
0
.5, 0.7 and 1.0) was added into 100 mL distilled water, and then
ꢁ
temperature of the reaction solution was kept at approximately
the mixture was heated at 100 C in an oil bath with stir for 1
hour under reuxing. Then 2 mL concentrated sulfuric acid (98
wt%) was added to the solution dropwise along with white
ꢁ
2
0 C by equipped with a cooling water sleeve surrounding the
lamp. The distance between the light source and the surface of
the reaction solution was 15 cm. In a typical experiment, 0.1 g of
photocatalyst was suspended in RhB aqueous solution (100 mL,
precipitates gradually formed, and the mixture was stirred at
ꢁ
1
00 C for another hour. Aer naturally cooling down to room
ꢀ1
1
0 mg L ), and then stirred in the dark for 1 hour to reach the
temperature, the precipitate was ltrated and washed three
ꢁ
adsorption–desorption equilibrium. During the visible light
irradiation, an aliquot of 5 mL containing the sample was taken
from the reaction suspensions at given time intervals, and then
centrifuged to remove the photocatalyst particles. Subse-
quently, the solutions were measured with the UV-vis spectro-
photometer at wavelength of 554 nm. The concentration
times with distilled water, and then dried in an oven at 80 C
overnight. The obtained samples were the Triton X-100-
modied-melamine sulfate, denoted as MST-0.3, 0.5, 0.7 and
1.0, respectively. For comparison, melamine sulfate (MAS) was
also prepared by the same method without adding Triton X-100.
0 0
changes were described by C/C , where C is the initial
2
.2 Synthesis of nanoporous graphitic carbon nitride
concentration of RhB (aer subtraction of the concentration of
RhB which was adsorbed to the catalysts) and C is the remained
concentration of RhB. The RhB degradation ratio (DR) was
5.0 g MST was put into an alumina crucible with a cover and
ꢁ
then heated to 500 C in a muffle furnace for 2 hours in air with
a heating rate of 2 C min . Further heat treatment was set at
5
ꢁ
ꢀ1
determined by DR ¼ [1 ꢀ (C/C
0
)] ꢂ 100%. The degradation rate
ꢁ
20, 550, 580 and 600 C under the same condition for another 2
was monitored by the rate constant (k), which can be calculated
hours, respectively. The nal products were denoted as npg-CN-
xy, in which x represents the mass ratios (0.3, 0.5, 0.7, and 1.0)
of Triton X-100 to melamine, and y ¼ a, b, c and d, represents
from the tted curve of the rst-order equation: ln(C
0
/C) ¼ kt.
ꢁ
3. Results and discussion
the nal heating temperature of 520, 550, 580 and 600 C,
respectively. For comparison, the melamine and melamine
sulfate were treated with the same heating program, while the
further heat treatment was just kept at 550 C for 2 hours. The
obtained samples were denoted as bulk g-CN and CN–MAS,
respectively.
3
.1 Chemical structure and texture of npg-CN samples
The chemical structure of the npg-CN samples was character-
ized by means of XRD, FT-IR, and elemental analysis. Fig. 1
shows the XRD patterns and FT-IR spectra of npg-CN samples
prepared with different mass ratios of Triton X-100 to melamine
at 550 C. The XRD patterns (Fig. 1a) of all the npg-CN samples
are similar to that of bulk g-CN. The strongest peak around 2q ¼
ꢁ
ꢁ
2.3 Characterization
ꢁ
X-ray diffraction (XRD) patterns were collected in a Bruker 27.4 is attributed to the typical graphitic interlayer (002) peak
ꢁ
D8FOCUS powder diffractometer with Cu Ka irradiation (l ¼ with d ¼ 0.326 nm, and another peak at about 2q ¼ 13 , cor-
0.15406 nm). The Fourier transform infrared spectra (FTIR) of responding to a distance of 0.67 nm and indexed as (100)
the samples were performed using a Nicolet iS10 spectrometer. spacing of g-CN, can be attributed to the in-plane structural
24
The scanning electron microscopy (SEM) and transmission packing motif of g-CN. Compared with that of bulk g-CN, the
electron microscopy (TEM) images were taken with Hitachi (002) peaks of npg-CN samples shi to slightly lower angle and
S4700 and Tecnai G220 microscope, respectively. Nitrogen becomes broader with reducing intensity, indicating the
sorption measurements were accomplished with N at 77 K aer increased interlayer distance and the less crystallinity of npg-CN
2
ꢁ
degassing the samples at 300 C under vacuum for 3 hours samples. But the higher heating temperature results in higher
using a Quantachrome Quadrasorb SI-MP porosimeter. The crystallinity of npg-CN (Fig. S1†), corresponding to previous
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Fig. 1 XRD patterns (a) and FT-IR spectra (b) of bulk g-CN, CN–MAS and npg-CN-(0.3b–1.0b).
23,25
reports.
The FT-IR (Fig. 1b) spectra show that all samples
exhibit several major bands centered at about 3180, 2185, 1200–
ꢀ1
ꢀ1
1
700 and 803 cm . The broad 3180 cm band can be attrib-
uted to the stretching vibration of N–H groups. All samples
ꢀ
1
show stretching modes in the 1200–1700 cm range, which are
typical for aromatic C–N heterocycles and originate from the
extended CN network. In addition, the characteristic breathing
ꢀ
1
26
mode of the triazine unit is observed at 803 cm
. The FT-IR
spectra of the samples prepared at different temperatures also
show the same IR bands (Fig. S2†). Elemental analysis results
(Table 1) show the atomic C/N ratio of npg-CN samples are all of
same as 0.66, very close to the bulk g-CN (0.65). The lower
atomic C/N ratios than the theoretical value (0.75) indicates that
the g-CN samples obtained in our experimental conditions are
nitrogen-enriched. Besides, all the samples contain H elements
indicates that the uncondensed amino groups exist in the
samples, corresponding to the FT-IR results. This result
demonstrates that there is no carbon residues of Triton X-100
le in the npg-CN samples. The very little S content of npg-
CN samples demonstrates that all the sulfate radicals had
decomposed.
The morphology and textural structure of npg-CN were
investigated by using SEM and TEM. Fig. 2a and b show typical
SEM images of npg-CN-0.5c viewed from different areas. The
network layers with irregular pores can be observed in Fig. 2a,
and the stacking motif of the network layers can be observed in
Fig. 2 SEM images (a and b) viewed from different areas and TEM
images (c and d) with different magnifications of npg-CN-0.5c.
Fig. 2b. TEM images of npg-CN-0.5c with different magnica-
tions are shown in Fig. 2c and d. Curved layers of g-CN with
rolled, buckled edges interlaced with each other and formed a
laver-like structure. It can conclude that irregular pore system is
formed by the stacking of the porous network layers. The
porosity of the npg-CN samples was further quantied by
nitrogen sorption measurements. The isotherms (Fig. 3a) of all
Table 1 Preparation conditions, physicochemical properties, and photocatalytic activity of bulk g-CN, CN–MAS and npg-CN samples
a
b
2
ꢀ1
c
d
e
ꢀ1
Samples
Product mass (g)
C/N (atomic)
H (wt%)
S (wt%)
S
BET (m g
)
Band gap (eV)
PR (%)
DR (%)
k (min )
Bulk g-CN
CN–MAS
0.46
0.41
0.39
0.36
0.35
0.34
0.38
0.33
0.12
0.65
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.67
1.94
2.05
2.14
1.78
1.88
2.21
1.93
1.71
2.13
—
15.1
44.5
77.8
87.1
66.7
64.8
49.2
70.8
2.58
2.61
2.55
2.53
2.53
2.51
2.56
2.47
2.57
6.0
18
31
40
26
20
19
40
60
9.0
31
47
70
56
45
45
80
62
0.00083
0.00386
0.00672
0.01310
0.00879
0.00625
0.00638
0.01710
0.01042
0.019
0.010
0.008
0.007
0.010
0.010
0.005
0.003
npg-CN-0.3b
npg-CN-0.5b
npg-CN-0.7b
npg-CN-1.0b
npg-CN-0.5a
npg-CN-0.5c
npg-CN-0.5d
135.1
a
b
c
d
e
1
g of starting melamine. The BET surface area. The physical adsorption ratio. The RhB degradation ratio under visible light for 90 min. The
photodegradation rate constant.
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2
Fig. 3 (a) N adsorption–desorption isotherms and (b) corresponding pore size distributions of npg-CN-(0.3b, 0.5b, 1.0b, 0.5c and 0.5d).
npg-CN samples exhibit a hysteresis loop range at p/p0
¼
and CN–MAS results in apparent upward trailing. The corre-
0
.8–1.0, indicating a macroporous system presents in the sponding band gaps (estimated from the absorption edge using
27
material. The BET surface areas of npg-CN samples increases Tauc plot) of bulk g-CN and npg-CN samples were summarized
with increasing r value at 550 C and to a maximum value of in Table 1, from which narrower band gaps of npg-CN samples
ꢁ
2
ꢀ1
8
7.1 m g at r ¼ 0.5, as shown in Table 1. Besides, with the are found than that of bulk g-CN. The npg-CN-0.5c shows the
increase of the heating temperature, the BET areas and pore narrowest band gap, which is 0.11 eV narrower than that of bulk
11,12
volumes increases (Table S1†). Fig. 3b show the corresponding g-CN, as shown in Fig. 4 (inset). According to previous study,
BJH pore size distribution curves of npg-CN samples deter- these results can be attributed to the multiple reections of
mined from the adsorption branch of the isotherms. The incident light within the porous structures, which increases the
ꢁ
samples of npg-CN-0.3b, 0.5b and 1.0b calcined at 550 C show effective path length for light absorption, or an additional
broad pore size distributions centered around 45 nm, while the absorption of increasing number of defects derived from the
28
samples npg-CN-0.5c and npg-CN-0.5d show broad pore size nanoporous structure. The photoluminescence (PL) spectra
distributions centered around 60 nm. This result indicates that excited by 340 nm of npg-CN-0.5c were compared to the spectra
the pore structure will be enlarged when the heating tempera- of bulk g-CN (Fig. S4†). The much lower uorescence intensity
ꢁ
ture over 550 C because the samples begin to suffer from a of npg-CN-0.5c than that of bulk g-CN indicates that the higher
25
decomposition process, which can be demonstrated by the TG separation efficiency of photogenerated electrons and holes of
11
thermograms of MST-0.5 (Fig. S3†) and the relative low yield of npg-CN samples.
npg-CN-0.5d (Table 1).
The optical absorption and energy bandgap of the npg-CN
3.2 Evaluation of photocatalytic activity of npg-CN samples
sample was examined by using UV-vis DRS measurements. In
Fig. 4, the UV-vis DRS spectra of npg-CN samples were
compared with that of bulk g-CN and CN–MAS. The enhanced
absorption at 450–650 nm of npg-CN samples over bulk g-CN
The photocatalytic activities of all the samples were evaluated
for Rhodamine B (RhB) photodegradation under visible light (l
>
400 nm) irradiation. The physical adsorption properties of all
samples were evaluated by stirring in the dark for 0.5 h, as
shown in Fig. S5.† All samples have reached adsorption equi-
librium during 10 min, and the physical adsorption ratio
increased with the increase of BET area. The photodegradation
curves of all the samples are shown in Fig. 5a. The bulk g-CN
and the CN–MAS show poor photocatalytic degradation ratios
as only 9.0% and 31% during 90 min, respectively, while all the
samples of npg-CN show better activity (45–80%), as listed in
Table 1. Besides, the photocatalytic activities of npg-CN samples
are mostly in line with the BET areas. These results can be
attributed to that the increased surface area of npg-CN can
29
provide more catalytic active sites for photocatalytic reaction.
Meanwhile, the kinetics of photodegradation on the catalyst
surface can be well described by the rst-order equation:
9
ꢀ1
ln(C /C) ¼ kt, where k is the reaction rate constant (min ), as
0
shown in Fig. 5b. The obtained reaction rate constants of all
samples were listed in Table 1. The sample npg-CN-0.5c, with
the narrowest band gap (2.47 eV) and relatively large BET area,
Fig. 4 UV-vis DRS spectra and corresponding Tauc plot spectra (inset)
of bulk g-CN, CN–MAS, and npg-CN samples.
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Fig. 5 (a) Photocatalytic degradation of RhB and (b) the corresponding reaction rate curves for all the samples.
ꢀ
1
exhibits the best degradation rate (0.017 min ), which is 20 These absorption peaks completely disappeared aer illumi-
times of the bulk g-CN and 4.4 times of CN–MAS. While the nation for 5 hours, suggesting the chromophoric structure of
sample npg-CN-0.5d, with the largest BET area, exhibits a rela- the RhB dye can be decomposed.
ꢀ
1
tively low degradation rate (0.010 min ) because of its broad
The stability of photocatalytic activity was investigated by
band gap (2.57 eV). These results indicate that the band gap using npg-CN-0.5c as the photocatalyst example (as shown in
plays an important role to the photocatalytic activity of npg-CN Fig. 7a). It can be seen that the photocatalytic activity of the
as well as the BET area, because narrower band gap means more catalyst does not show any decrease aer four cycle evaluations.
effective light harvesting of the visible light under same irradi- In addition, the XRD patterns (as shown in Fig. 7b) of the used
ation intensity.
photocatalyst aer 4th cycling experiment are almost the same
Generally, decolorization does not mean that the dyes have as that of the fresh photocatalyst, and there is no other phases
been completely oxidized into harmless nal products such as in the XRD patterns. These results demonstrate that the pre-
9
H
2
O and CO
2
. In order to determine the photodegradation of sented npg-CN photocatalysts are stable under present reaction
RhB is
a
decolorization or mineralization process, the conditions.
variation of UV-vis absorption spectra of RhB degradation using
npg-CN-0.5c is shown in Fig. 6. The characteristic absorption
band of RhB at 554 nm decreased signicantly with increasing
irradiation time, accompanied by a remarkable blue-shi of the
maximum absorption peak. The stepwise blue-shi of the main
peak from 554 to 496 nm can be attributed to the step-by-step
3
.3 Discussion about the formation mechanism of npg-CN
The FT-IR spectra of MA, Triton X-100 and the mixture of MA
and Triton X-100 (obtained by drying the mixed solution of MA
and Triton X-100 (r ¼ 0.5) directly) were compared with that of
MAS and MST to demonstrate the template of Triton X-100 was
totally removed. From Fig. 8a, the characteristic bands of Triton
X-100 at 2800–3000 cm and 1111 cm can be easily found in
the spectra of the mixture of MA and Triton X-100, while it
cannot be found in the spectra of MST-0.5 and MAS. Meanwhile,
MST-0.5 show same peaks with that of MAS, and the broad 3364
14
de-ethylation of RhB molecule, while the decrease in absor-
bance indicates the destruction of the conjugated structure.
ꢀ
1
ꢀ1
ꢀ
1
ꢀ1
cm
band and 1090 cm
band can be attributed to the
30
combined water and sulfate radical, respectively. These results
indicate the Triton X-100 had been fully pre-removed in our
method. The XRD patterns of MSTs were compared with that of
MAS, as shown in Fig. 8b. The same peak positions indicate that
MSTs have a same crystal form as MAS, corresponding to the
FT-IR result. But the measured XRD peak intensity distributions
for MSTs are different from the intrinsic one for MAS,
indicating there are obvious crystal preferred orientations in
MST samples that inferred that the polycrystalline samples of
31
MST have nonrandom distribution of orientations because of
the template induced effect from Triton X-100. Besides, the
different concentrations of Triton X-100 can also result in
different crystal preferred orientations of MST, because the
shape of Triton X-100 micelles changes with the concentra-
Fig. 6 UV-vis spectral changes of RhB aqueous solution in the pres-
ence of npg-CN-0.5c under visible light irradiation.
32
tions. As a conclusion, the mechanism of npg-CN formation
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Fig. 7 (a) Cycling runs for the photocatalytic degradation of RhB in the presence of npg-CN-0.5c and (b) XRD patterns of npg-CN-0.5c before
and after 4th run of photodegradation of RhB.
Fig. 8 (a) FT-IR spectra of MA, Triton X-100, MAS, MST-0.5, and the mixture of MA and Triton X-100 (MA+TX-100); and (b) XRD patterns of MA,
MST-0.3, MST-0.5, and MST-1.0.
was supposed as that Triton X-100 modied the process of the
formation of MAS and result in a different crystal preferred
Acknowledgements
orientation of melamine sulfate, which determined the nal This work was nancially supported by the Natural Science
porous structures of npg-CN.
Foundation of China under Grant no. 10972025.
4
. Conclusions
References
In conclusion, the npg-CN with large surface area and low C/N
ratios (0.66) was successfully prepared by directly heating Triton
X-100-modied-melamine sulfate. The obtained npg-CN
samples possessed high surface areas, which could be medi-
ated by the heating temperature and the adding amount of
Triton X-100. Compared with the bulk g-CN, the npg-CN
samples exhibited stronger absorbance of visible light, nar-
rower band gaps and reduced uorescence intensity. As a result,
the photocatalytic activity of npg-CN samples was greatly
improved in the photodegradation of RhB with 20 times of the
bulk g-CN and 4.4 times of CN–MAS at the optimum condition;
meanwhile, the stability of photocatalytic activity of npg-CN
samples shows better recycling life even aer four cycles.
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