Loading sulfur and nitrogen co-doped carbon dots onto g-C3N4 nanosheets for an efficient photocatalytic reduction of 4-nitrophenol

Article abstract of DOI:10.1039/c8dt00735g

A favorable interface for hybrid photocatalysts makes an important contribution in enhancing photocatalytic reactions. Herein we located sulfur and nitrogen co-doped carbon dots (SN-CDs) onto g-C₃N₄ nanosheets and achieved a novel SN

Full text of DOI:10.1039/c8dt00735g

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Yang, L.Q. Li, C.R. Xue, Y. Li, Y.Z Wang, S. Hu, J. Yang and F. Zhang, Dalton Trans., 2018, DOI:
1
0.1039/C8DT00735G.
Volume 45 Number
1
7
January 2016 Pages 1–398
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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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3 4
Loading sulfur and nitrogen co-doped carbon dots onto g-C N nanosheets for
efficient photocatalytic reduction of 4-nitrophenol
a∗
a
a
a
a
a
a*
b
Q. Chang , S.S. Yang , L.Q. Li , C.R. Xue , Y. Li , Y.Z. Wang , S.L. Hu , J.L. Yang
c
and F. Zhang
a
North University of China, School of Energy and Power Engineering, School of
Materials Science and Engineering, Xueyuan Road 3, Taiyuan 030051, China
b
Tsinghua University, State Key Laboratory of New Ceramics and Fine Processing,
Beijing 100084, China
c
Jinzhong Institute of Quality and Technical Supervision Inspection & Measurement,
Yuci 030600, China
Abstract
A favorable interface for the hybrid photocatalysts makes an important contribution in
enhancing photocatalytic reactions. Herein we located sulfur and nitrogen coꢀdoped
carbon dots (SNꢀCDs) onto gꢀC N nanosheets and achieved a novel SNꢀCDs/gꢀC N
4
3
4
3
composite with high visibleꢀlight photocatalytic reduction of 4ꢀnitrophenol (4ꢀNP).
Compared to gꢀC nanosheets, SNꢀCDs/gꢀC composite possessed wider light
absorption. More importantly, the interaction of surface groups led to a favorable
interfacial bonding between SNꢀCDs and gꢀC nanosheets which facilitated the
3
N
4
3 4
N
3
N
4
transfer of photoexcited electrons and holes, giving rise to an improved electron
transport ability of the composite. In addition, the upward shift of the energy band of
3 4
gꢀC N nanosheets due to the location of SNꢀCDs led to a stronger reducibility of
3 4
charge carriers. As a consequence, the photocatalytic performance of SNꢀCDs/gꢀC N
∗
Corresponding authors. Tel.: +86 351 3559638; Fax: +86 351 3559638; E-mail address: changneu@gmail.com (Q. Chang),
hsliang@yeah.net (S.L. Hu).
1

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has been remarkably improved in photocatalytic reduction of 4ꢀnitrophenol.
2

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1. Introduction
Hybrid photocatalysts, with enhanced photocatalytic performance due to the
synergetic effects of each component, have drawn intensive interest. Carbon dots
(CDs), a new star of carbon nanomaterials, have been proven to be a potential
photocatalyst due to their unique properties including UVꢀvisible light absorption,
low cost and toxicity, whilst being easily functionalized with various species and
1
ꢀ6
capable of acting as both electron acceptors and donors. Among all the merits of
CDs, one of the most important ones is the surfaces of CDs can be easily tailored or
modified with inorganic, organic, metallic ingredients, or polymeric molecules. Not
7
ꢀ12
only can the introduction of many kinds of heteroatoms tune the behaviors of CDs,
13ꢀ15
and also facilitates their combination with other materials.
To date, CDsꢀbased
photocatalytic composites have been widely reported and they showed much superior
16ꢀ23
photocatalytic activity to single CDs.
3 4
Graphitic carbon nitride (gꢀC N ), as a new class of photocatalyst, has been
2
4
extensively studied in the recent years since its firs report in 2009 because of its
visibleꢀlight driven band structure and high stability, plus ease of synthesis via
2
5,26
costꢀeffective routes.
3 4
However, the photocatalytic performance of gꢀC N is still
limited. In order to improve the photoactivity of gꢀC
3
4
N , many efforts have been
27ꢀ31
32ꢀ35
36ꢀ39
made, such as structural changes,
doping,
coupling with other materials,
4
0ꢀ42
and fabricating gꢀC N based Zꢀscheme systems.
Considering the remarkable
3
4
advantages of CDs, researchers have developed metalꢀfree CDs/gꢀC
3
3 4
N

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heterostructures and demonstrated that the photocatalytic activity of the composites
43ꢀ48
have been significantly improved due to the synergetic effect of CDs and gꢀC
3 4
N .
It is known that the transport in the interfaces plays a key role in promoting the
11
photocatalytic activity. However, the transfer of the photoexcited charge carriers for
the hybrids is more complex because the transfer not only occurs on the surfaces but
49
also at the newꢀformed interfaces. For CDs, there are rich/different functional
groups on their surfaces that could unavoidably lead to the interaction between CDs
50ꢀ53
and gꢀC
3
N
4
so as to alter the surface features. Suggested by the references,
the
alteration of surface features would definitely affect the properties of the materials.
However, the understanding about the mechanism of light absorption and conversion
for the carbon based composites is still limited.
By far, many researchers have focused on the nitrogen and sulfur coꢀdoped CDs
because coꢀdoping with heteroatoms might introduce more active sites to CDs and
11,54
diversify the electron transition pathways.
CDs and the superiority of gꢀC nanosheets to bulk gꢀC
type of CDs/gꢀC composite by loading sulfur and nitrogen coꢀdoped carbon dots
SNꢀCDs) onto gꢀC nanosheets in this study. By introducing SNꢀCDs to gꢀC
nanosheets, two remarkable features could be created. One is the reaction between
surface groups can lead to a good interfacial bonding between SNꢀCDs and gꢀC
nanosheets which could be the potential chargeꢀcarriers transport channel. The other
Considering the advantage of coꢀdoped
29
3
N
4
3 4
N , we developed a new
3
N
4
(
3
N
4
3 4
N
3
N
4
is the change of the surface states can alter the band structure of gꢀC
4
3 4
N nanosheets.

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The obtained SNꢀCDs/gꢀC
structure, photophysical behavior, photoelectrochemical performance and
photocatalytic activity. It was found that the SNꢀCDs/gꢀC composite showed
excellent photocatalytic performance in the reduction of 4ꢀnitrophenol.
3 4
N composite was evaluated in terms of morphology and
3
N
4
2. Experimental
2.1 Synthesis of SNꢀCDs
Sulfur and nitrogen coꢀdoped carbon dots were synthesized through a hydrothermal
method using pꢀaminobenzenesulfonicacid as carbon source. In detail, 0.35 g of
pꢀaminobenzenesulfonicacid was dissolved into 35 mL water. After vigorously
stirring for 30 min, the solution was transferred into a Teflonꢀlined autoclave and
heated at 200 °C for 5 h. After cooling to room temperature, SNꢀCDs were obtained.
3 4
2.2 Synthesis of SNꢀCDs/gꢀC N composite
The bulk gꢀC
50 °C for 4 h in static air with a ramp rate of 2.3 °C/min. Then the obtained bulk
gꢀC powders were placed in an open ceramic container and were heated at 500 °C
for 2 h with a ramp rate of 5 °C/min. The naturally cooled product ꢀ fluffy and light
yellow powders was gꢀC nanosheets.
3 4
N was first prepared by the thermal polymerization of melamine at
5
3
N
4
3 4
N
5

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Preparation of SNꢀCDs/gꢀC
3
N
4
3 4
composite: 0.1 g gꢀC N nanosheets were dispersed in
10 ml water, and 3ml of SNꢀCDs aqueous solution was added. The mixed solution
was vigorously stirred for 30 min and transferred to the Teflonꢀlined autoclave and
heated at 180 °C for 4 h. After the autoclave cooled to room temperature, the
precipitates were separated by centrifugation and washed with distilled water, then
dried at 50 °C for 12 h in vacuum oven. Finally, the SNꢀCDs/gꢀC
obtained. The formation of SNꢀCDs/gꢀC composite is shown schematically in Fig.
. In addition, to study the concentration effect of SNꢀCDs on the properties of the
3 4
N composite was
3
N
4
1
composite, a series of samples with different volume of SNꢀCDs (1ml, 5ml, and 7ml)
were also prepared via the identical procedure, and the corresponding final products
3 4 3 4 3 4
were named as SNꢀCDs/gꢀC N ꢀa, SNꢀCDs/gꢀC N ꢀb and SNꢀCDs/gꢀC N ꢀc.
Fig. 1 Schematic illustration of the formation of SNꢀCDs/gꢀC
.3 Characterization
3 4
N composite.
2
6

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The microstructure was examined using a scanning electron microscope (SEM) and
transmission electron microscopy (TEM). The phase and crystallinity were analyzed
α
via Xꢀray diffraction (XRD) using CuK radiation at 40kV and 30mA. Fourier
transform infrared spectra (FTIR) were recorded with a Thermo Nicolet 360 FTIR
spectrometer. The compositions and chemical states of the samples were analyzed
using Xꢀray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250) equipped
with a multichannel detector. The surface area was calculated using the
BrunauerꢀEmmettꢀTeller (BET) method. Photoluminescence (PL) spectra and UVꢀvis
absorption spectra were collected on a Hitachi F4500 fluorescence spectrophotometer
and a Shimadzu UVꢀ2550 spectrometer, respectively. Timeꢀresolved fluorescence
emission spectra were recorded at room temperature with a Hamamatsu C11367
fluorescence spectrophotometer.
2.4 Photoelectrochemical measurements
The photoelectrochemical measurements were performed in a standard threeꢀelectrode
system using prepared samples as working electrodes, a Pt foil as the counter
electrode, and Ag/AgCl as the reference electrode, respectively. 0.5 M Na
solution was used as the electrolyte. A 300W Xe arc lamp with an UVꢀcutoff filter
λ<420nm) was served as a light source. The working electrodes were prepared by
2 4
SO
(
spreading aqueous slurries of samples in ethanol over ITO glass substrate. The
electrodes were dried in a vacuum oven at 200°C for 1h to improve adhesion. The
7

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electrochemical impedance spectroscopy (EIS) were measured by applying an AC
6
ꢀ1
voltage of 10 mV amplitude in the frequency range of 10 Hz to 10 Hz with the
initial potential (0 V). The transient photocurrent tests with onꢀoff light of 50 s were
carried out under an applied voltage of +0.5 V versus Ag/AgCl. In addition, the linear
potential scans (10 mV/s) were also tested.
2.5 Photocatalytic test
The photocatalytic performance of the prepared samples was evaluated by the
photocatalytic reduction of 4ꢀnitrophenol (4ꢀNP) under visibleꢀlight irradiation at
ambient temperature. 0.02g photocatalyst was suspended in 10 mL of 4ꢀNP solution
4
(0.1mM). Then, a freshly prepared aqueous solution of 10 mL of NaBH (10mM) was
added. The reaction solution was subsequently positioned 20 cm away from the
reactor and irradiated under continuous stirring. A 300W Xe arc lamp with an
UVꢀcutoff filter (λ<420 nm) was used as a visible light source. After irradiation for
some time, about 2mL of the reaction solution was withdrawn and centrifuged. The
absorbance spectrum of supernate was measured using a UVꢀvis absorption
spectrophotometer.
3. Results and discussion
3.1 Morphology and structure
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The TEM image and size distribution of synthesized SNꢀCDs were shown in Fig. S1.
It can be seen that SNꢀCDs have spherical morphology and the diameter ranged from
3
~6 nm. A Gaussian fitting curve revealed that the average size of SNꢀCDs was
approximately 4.5 nm. The morphology of obtained gꢀC nanosheets is shown in
Fig. S2. It is clearly observed that gꢀC appeared as loose and nanosheetsꢀliked
agglomerates. The characterization of the SNꢀCDs/gꢀC composite by TEM is
3
N
4
3
N
4
3
N
4
shown in Fig. 2. The porous grains of the composite were observed (Fig. 2A). The
highꢀmagnification TEM image (Fig. 2B) shows the SNꢀCDs (3~6nm) were
distributed in the gꢀC
interplanar spacing of 0.208 nm and 0.32 nm, which corresponded to (101) lattice
planes of CDs and (002) diffraction planes of gꢀC , respectively.
3 4
N nanosheets. The HRTEM image (Fig. 2C) shows the
3 4
N
A
C
B
Fig. 2 (A) TEM image of SNꢀCDs/gꢀC
3 4
N composite, (B) Highꢀmagnification TEM
image of (A) marked in red square, and (C) HRTEM image.
3 4 3 4
The XRD patterns of prepared gꢀC N nanosheets and SNꢀCDs/gꢀC N composite are
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3 4
given in Fig. S3. It can be seen that the gꢀC N nanosheets exhibited a strong
o
diffraction peak centered at 27.8 associated with the interlayer stacking and a weak
o
diffraction peak centered at 13.1 associated with the intralayer hydrogen bonds. This
28,29
is consistent with the reports in literatures.
With respect to the gꢀC N nanosheets,
3 4
3 4
the SNꢀCDs/gꢀC N composite gave a stronger peak originated from the interlayer
o
stacking at 27.8 , implying an increase of the periodic stacking of the layers due to the
interaction between SNꢀCDs and gꢀC
prepared gꢀC nanosheets and SNꢀCDs/gꢀC
adsorptionꢀdesorption (Fig. S4). The BET surface area of SNꢀCDs/gꢀC N composite
3
N
4
nanosheets. We tested the surface area of
3
N
4
3
N
4
composite via N
a
2
3
4
2
is 72.77 m /g, which is smaller than that of the obtained gꢀC
3
N
4
nanosheets (122.0
2
m /g) in this study. The reduction of the surface area further demonstrated that the
disrupted stacking of the layers in the gꢀC
3
N
4
nanosheets became more ordered after
o
the location of SNꢀCDs, giving rise to the increase of peak intensity at 27.8 as
revealed by the XRD patterns.
FTIR spectra were used to investigate the structure of SNꢀCDs/gꢀC
3 4
N composite. As
shown in Fig. S5, the spectrum of the composite represented the overlap of the spectra
ꢀ1
ꢀ1
of both SNꢀCDs and gꢀC
3
N
4
nanosheets. The peaks at 1030 cm , 1087 cm and 1211
ꢀ1
54
cm were related to the vibration of S=O, CꢀO and CꢀS from SNꢀCDs, respectively.
ꢀ1
ꢀ1
The peak at around 810 cm and the peaks in the region from 1250 to 1650 cm
could be ascribed to the vibration of heptazine ring and trigonal CꢀN(ꢀC)ꢀC/bridging
28,29
CꢀNHꢀC units of gꢀC
3
N
4
.
3 4
In addition, compared to those pristine gꢀC N , it is
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ꢀ1
clearly observed the bands in the 1250 to 1650 cm region for the SNꢀCDs/gꢀC
composite represented slight redꢀshifts, which might be associated with the interaction
between SNꢀCDs and gꢀC that led to a weak bond strength of CꢀN. The interaction
between SNꢀCDs and gꢀC N could give rise to a good contact interface between the
3 4
N
3
N
4
3
4
two components.
3 4
The compositions and chemical states of SNꢀCDs/gꢀC N composite were further
investigated by XPS. Fig. S6 shows the XPS spectra of SNꢀCDs. It can be seen that
the obtained CDs were composed of carbon, nitrogen, oxygen and sulfur elements,
3 4
suggesting that S and N coꢀdoped CDs were achieved. For SNꢀCDs/gꢀC N composite,
the highꢀresolution XPS spectra of C, N, O and S elements are displayed in Fig. 3. In
detail, the C1s spectrum (Fig. 3A) shows three deconvoluted peaks. The peak at 284.8
4
3,54
eV could be attributed to the carbonꢀcontaining groups (eg. CꢀC or C=C bonds).
54
The peak at 285.8 eV could be assigned to the CꢀS/CꢀN bonds, implying the
presence of SNꢀCDs in the composite. The strong peak at 288.3 eV corresponded to
28
the threeꢀcoordinated carbon of C(N)
B) also shows three deconvoluted peaks centered at 398.8, 400.0 and 401.0 eV,
which could be ascribed to the nitrogen species in the N(C) , N(C) and N(CH)
3 3 4
groups in the gꢀC N . The N1s spectrum (Fig.
3
2
3
28
groups, respectively. As shown in the O1s spectrum (Fig. 3C), two peaks at 531.9
eV and 532.6 eV were observed, which could be contributed to presence of C=O/S=O
and CꢀO groups of SNꢀCDs. Moreover, for the S2p spectrum (Fig. 3D), the two peaks
centered at 163.8 eV and 168.5 eV were observed, which can be ascribed to the CꢀS
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and CꢀSO
XRD and FIIR, we can conclude that SNꢀCDs were successfully loaded on the surface
of gꢀC nanosheets due to the interface reaction between sulfonic groups of
3
species in SNꢀCDs. Based on these results together with the above TEM,
3
N
4
SNꢀCDs and remained amine groups of gꢀC N nanosheets.
3
4
C1s
N1s
N-C=N
A
N-(C)2
B
C-C/C=C
N-(C)3
N-(CH)
C-S/C-N
286
282
284
288
290
292 394
396
398
400
402
404
Binding energy / eV
Binding energy / eV
O1s
S2p
C
D
C-SO3
S=O
C-S
C-O
528
530
532
534
536
538
540
160 162 164 166 168 170 172 174
Binding energy / eV
Binding energy / eV
Fig. 3 XPS spectra of SNꢀCDs/gꢀC N composite: (A) C1s, (B) N1s, (C) O1s and (D)
3
4
S2p.
3.2 Optical properties
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1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
A
B
SN-CDs/g-C N
3
4
SN-CDs/g-C N4
3
g-C N4 nanosheets
3
g-C N nanosheets
3
4
Eg=2.92 eV
2.5
SN-CDs
Eg=2.98 eV
3
00
400
500
600
700
800
1.5
2.0
3.0
ν / eV
3.5
4.0
Wavelength / nm
h
C
g-C N nanosheets
D
3
4
SN-CDs/g-C N4
3
g-C N nanosheets
3
4
SN-CDs/g-C N
3
4
4
00
450
500
550
600
650
700
30
40
50
60
70
80
Wavelength / nm
Time / ns
2
Fig. 4 (A) UVꢀvisible absorption spectra, (B) (ahυ) versus hυ curves, (C)
Photoluminescence emission spectra under the excitation of 350 nm, and (D)
Timeꢀresolved photoluminescence decay spectra of gꢀC
3
N
4
nanosheets and
SNꢀCDs/gꢀC composite measured by monitoring the emission at 465 nm when
3
N
4
excited at 335 nm.
Fig. 4A shows the UVꢀvis absorption spectra of prepared samples. It is seen that
gꢀC
to gꢀC
wavelength (470ꢀ600 nm). For SNꢀCDs/gꢀC N composite, the intrinsic absorption
3
N
4
nanosheets exhibited an intrinsic absorption edge at around 463nm. Compared
3
4
N sheets, SNꢀCDs exhibited better light absorption performance at a longer
3
4
remarkably increased and an obvious light absorption in the region of 500ꢀ700 nm
was observed. The strengthened light absorption can be well explained as a result of
3 4
the successful incorporation of SNꢀCDs with gꢀC N nanosheets. This indicated that
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3 4
the SNꢀCDs/gꢀC N composite could be excited to produce more electronꢀhole pairs
under the visible light irradiation, which is beneficial for improving photocatalytic
2
performance. The optical bandgaps were estimated from the Tauc plots of (ahυ)
versus (n=2 for the direct bandgap) hυ from the UVꢀvis spectra, as shown in Fig. 4B.
The bandgap of gꢀC
3
N
4
nanosheets was determined to 2.98 eV, which is consistent
with the reports. The bandgap of SNꢀCDs/gꢀC composite was 2.92 eV, a little
lower than that of gꢀC nanosheets within experimental error, which is also helpful
29
3
N
4
3
N
4
for improving the light absorbance.
The introduction of SNꢀCDs also led to substantial suppression of radiative
electronꢀhole recombination, as indicated by the greatly weakened PL peak in Fig. 4C.
3 4
gꢀC N nanosheets exhibited a strong emission peak at 465 nm. But the intensity of
this PL peak decayed significantly upon the couple of SNꢀCDs. The significant
reduction of PL peak was related to two potential reasons: first, the interaction
3 4
between SNꢀCDs and gꢀC N nanosheets provided efficient transport channels for
electrons. Second, there are abundant shallow charge trapping states on the surface of
7
CDs. These effectively suppressed the electronꢀhole recombination. Timeꢀresolved
3 4 3 4
fluorescence decay spectra of gꢀC N nanosheets and SNꢀCDs/gꢀC N composite
were also recorded, as shown in Fig. 4D. The derived three components of the
lifetime and their relative amplitudes are given in Table S1. The average PL lifetime
of the composite had a little decrease (5.353 ns) comparing with gꢀC
3 4
N nanosheets
(7.317 ns). The slight decrease of PL lifetime could also be attributed to the
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introduction of SNꢀCDs that led to an increased absorption density of bandꢀtail (as
shown in Fig. 4A) of gꢀC which shortened the gap between the valence band and
3 4
N
conduction band, therefore resulting in a little faster electronꢀhole recombination
processes in the SNꢀCDs/gꢀC N composite so as to reduce PL lifetime. The result
3
4
2
8
was consistent with the report.
3.3 Photoelectrochemical performance
To further understand the photophysical behaviors of photoexcited charge carriers, the
photoelectrochemical activities were evaluated, as shown in Fig. 5. It is seen from the
EIS Nyquist plots (Fig. 5A) that the semicircular radius of the SNꢀCDs/gꢀC
3 4
N
composite was smaller than that of gꢀC nanosheets, indicating a lower
3
N
4
chargeꢀtransfer resistance that could lead to a faster chargeꢀtransfer rate. The same
result was obtained using a transient photocurrent responses experiment. As shown in
Fig. 5B, comparison to gꢀC
3
N
4
nanosheets, about doubly enhanced photocurrent
composite, further indicating that the
response was observed in the SNꢀCDs/gꢀC
3
N
4
photoexcited electrons and holes were effectively separated. It is believed that the
improved electron transport ability of the composite could be associated with the
favorable interfacial bonding.
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1
0
8
6
4
2
0
0.8
g-C N nanosheets
SN-CDs/g-C
g-C nanosheets
off
3 4
N
A
3
4
B
SN-CDs/g-C N
N
3 4
3
4
on
0.6
0.4
0.2
0.0
0
2
4
6
8
10
12
14
45
95 145 195 245 295 345 395 445
Z' / ohm
Time / s
Fig. 5 (A) Electrochemical impedance spectroscopy (EIS) Nyquist plots and (B) The
photocurrentꢀtime (I-t) curves of gꢀC nanosheets and SNꢀCDs/gꢀC composite.
3
N
4
3 4
N
3.4 Photocatalytic activities
The photocatalytic performance was evaluated by the photocatalytic reduction of
55,56
4ꢀnitrophenol (4ꢀNP) to 4ꢀaminophenol (4ꢀAP) in the presence of NaBH
4
.
The
absorption peak at about 315 nm of 4ꢀNP could red shift to 400 nm owing to the
56
formation of 4ꢀnitrophenolate ion in alkali medium. And the peak intensity of
ꢀ
nitrophenolate at 400 nm remains unaltered since BH
4
is incapable of reducing 4ꢀNP
in the absence of the photocatalysts. Fig. 6 shows the UVꢀvis absorption spectra of
ꢀNP in the presence of SNꢀCDs, gꢀC nanosheets and SNꢀCDs/gꢀC composite.
It was clearly observed that SNꢀCDs/gꢀC composite showed a superior
photocatalytic activity comparing with SNꢀCDs and gꢀC nanosheets. The peak at
00 nm almost disappeared after 5min irradiation in the presence of the composite
Fig. 6C), whereas the reduction efficiency reached 10% and 50% in the presence of
SNꢀCDs and gꢀC nanosheets (Fig. 6A and B). The control experiments without
light irradiation or no photocatalysts were evaluated, as shown in Fig. S7. Almost no
4
3
N
4
3 4
N
3
N
4
3
N
4
4
(
3
N
4
16

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reduction of 4ꢀNP was observed under dark conditions with SNꢀCDs, gꢀC
3 4
N
nanosheets or SNꢀCDs/gꢀC . Similarly, illumination in the absence of
3
N
4
photocatalysts did not lead to the photocatalytic reduction of 4ꢀNP. In addition, as
shown in Fig. S7, it can be seen that increasing the content of SNꢀCDs led to
increased photocatalytic reduction efficiency of 4ꢀNP, but excess SNꢀCDs resulted in
a decrease of photocatalytic activity. This was related to two potential reasons: first,
the SNꢀCDs tended to form aggregates that make the recombination of electronꢀhole
pairs inevitably happen. Second, too much SNꢀCDs could mask the active sites of
gꢀC N .
3
4
00min
01min
03min
05min
07min
00min
01min
03min
05min
07min
A
B
300
350
400
450
500
550
300
350
400
450
500
550
Wavelength / nm
Wavelength / nm
1
00
0
0
0
0
0
0min
C
1min
3min
5min
7min
D
80
60
40
20
0
3
00
350
400
450
500
550
1
2
3
Wavelength / nm
Cycle
Fig. 6 UVꢀvisible absorption spectra of the 4ꢀnitrophenol (4ꢀNP) reduction in the
presence of (A) SNꢀCDs, (B) gꢀC N nanosheets and (C) SNꢀCDs/gꢀC N . (D)
3
4
3
4
Recyclability of SNꢀCDs/gꢀC N as photocatalyst in reduction of 4ꢀNP under visible
3
4
light irradiation.
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3 4
To investigate the stability of SNꢀCDs/gꢀC N , we repeated the photocatalytic
reduction of 4ꢀNP three times and the corresponding results are shown in Fig. 6D. No
obvious decay was observed, indicating the high stability of SNꢀCDs/gꢀC N . These
3
4
3 4
above results strongly suggested that SNꢀCDs/gꢀC N could be an effective
visibleꢀlightꢀresponsive photocatalyst. The remarkably improved photocatalytic
activity of the composite can be explained as follows.
For comparison, the photocatalytic activities of CDs/gꢀC N , NꢀCDs/gꢀC N and
3
4
3
4
3 4
SNꢀCDs/gꢀC N were also tested by measuring the amounts of •OH radicals using the
57
method reported previously. It is known that the reaction between •OH and
terephthalic acid (TA) in basic solution can generate 2ꢀhydroxyterephthalic acid
(TAOH) which emits a fluorescent signal at 426 nm. Fig. S8 shows the fluorescent
spectra associated with TAOH generated from CDs/gꢀC N , NꢀCDs/gꢀC N and
3
4
3
4
SNꢀCDs/gꢀC
3 4
N suspended in TA solution after visibleꢀlight irradiation for 5 min. It
could be seen that the strongest fluorescent intensity from the suspension solution of
SNꢀCDs/gꢀC
NꢀCDs/gꢀC
more active sites and diversify the electron transition pathways.
3
N
4
containing TA was achieved in comparison to CDs/gꢀC
3
N
4
and
3
N
4
. This strongly suggested that combining SNꢀCDs with gꢀC
3
N
4
create
3.5 Photocatalytic mechanism
18

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The photocatalytic performance of a photocatalyst is dependent on the synergistic
effects of light absorption, separation and transfer of charge carriers as well as
3 4
suitable band structure. In the case of SNꢀCDs/gꢀC N composite, the introduction of
SNꢀCDs contributed to not only extend visible light absorption as confirmed by
UVꢀvis absorption spectra (Fig. 4A), but also improve exciton separation efficiency as
verified by PL (Fig. 4C) since CDs can act as both electron acceptors and donors that
3,21
could make the electrons transfer easily to the surface/interface.
These definitely
favored the photocatalytic reactions. In addition, the interaction between sulfonic
groups of SNꢀCDs and remained amine groups of gꢀC N nanosheets led to a good
3
4
interfacial bonding between SNꢀCDs and gꢀC
potential transfer channels of the charge carriers. Once the charge carriers reach the
interface, they can be easily transferred to the surface of SNꢀCDs and gꢀC N
4
3 4
N nanosheets, which could be the
3
nanosheets, giving rise to efficient suppression of the radiative electronꢀhole
recombination and a faster chargeꢀtransfer rate in the composite as indicated by the
greatly weakened PL peak in Fig. 4 and smaller radius of semicircle in the Nyquist
plots as well as enhanced photocurrent response in Fig. 5. The efficient separation and
fast transfer of charge carriers definitely enhanced the photocatalytic activity.
Fig. 7 illustrates the separation and transfer behaviors of photogenerated electrons and
holes of the composite. Electronꢀhole pairs were created once excited by visible light.
Because SNꢀCDs can act as both the electron acceptors and donors, plus have good
3 4
interfacial bonding with gꢀC N nanosheets, the electrons could be easily transferred
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3 4 3 4
to the surface of gꢀC N nanosheets. Meanwhile, the redundant electrons on gꢀC N
nanosheets can also transfer to SNꢀCDs. Hence the electronꢀhole recombination was
effectively suppressed. During the photocatalytic process, the electrons could reduce
4ꢀNP to 4ꢀAP.
4ꢀNP 4ꢀAP
4
ꢀNP
e-
e- e-
-
e
SN-CDs 4ꢀAP
3
g-C N4
+
h
h+ h+
3 4
Fig. 7 Photocatalytic mechanism for reduction of 4ꢀNP by SNꢀCDs/gꢀC N .
Moreover, we analyzed the band structure by a combined analysis of optical
58
absorption spectra and linear potential scans. Fig. S9 shows the cathodic and anodic
scan results. The conduction band (CB) values and valence band (VB) values of
gꢀC
V, ꢀ0.99 V and 1.935 V (vs. Ag/AgCl), which were equivalent to ꢀ0.67 V, 2.30V,
0.79 V and 2.135V versus NHE, respectively. The bandꢀgap energy obtained from
this potential scan was 2.97 eV for gꢀC nanosheets and 2.925 eV for
SNꢀCDs/gꢀC , which were similar with that determined from the absorption spectra
Fig. 4B). The bandꢀgap structures of gꢀC nanosheets and SNꢀCDs/gꢀC are
shown in Fig. 8. Clearly, the energy bands of gꢀC N nanosheets shifted upward to the
3 4 3 4
N nanosheets and SNꢀCDs/gꢀC N composite were approximately ꢀ0.87 V, 2.10
ꢀ
3
N
4
3
N
4
(
3
N
4
3 4
N
3
4
negative potential after the introduction of SNꢀCDs. The changed electronic structure
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3 4
that was attributed to the interaction between SNꢀCDs and gꢀC N nanosheets could
lead to a stronger reducibility of photoexcited electrons. Thus, it is reasonable to
believe that the composite, with wider visibleꢀlight absorption range, higher efficient
separation and transfer of charge carriers, suitable band structure, exhibited
significantly improved photocatalytic activity in reduction of 4ꢀNP. However, further
studies are needed so as to achieve insightful understanding on the structure and
photocatalytic mechanism of the SNꢀCDs/gꢀC
3
N
4
composite.
-
-
-
1.5
1.0
0.5
SN-CDs/g-C N
3
4
g-C N nanosheets
3
4
0.0
0.5
1.0
1.5
2.0
2.5
Fig. 8 Schematic energy level diagrams of gꢀC
composite.
3 4 3 4
N nanosheets and SNꢀCDs/gꢀC N
4. Conclusions
3 4
We loaded SNꢀCDs onto gꢀC N nanosheets to obtain an innovative metalꢀfree
photocatalyst for reduction of 4ꢀNP using a simple hydrothermal method. The
introduction of SNꢀCDs extended visible light absorption by allowing low energy
photons induced excitations. The good interfacial bonding between SNꢀCDs and
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gꢀC
3 4
N nanosheets created the transfer channel of electrons, giving rise to efficient
charge separation and fast chargeꢀtransfer rate. In addition, the energy band of gꢀC
3
N
4
nanosheets after the location of SNꢀCDs shifted upward to the negative potential,
leading to a stronger reducibility of photoexcited electrons. As a consequence of these
favorable changes, the composite showed excellent photocatalytic activity in
reduction of 4ꢀNP. The results achieved in this study may provide valuable
information for further development of carbon dots based composites with improved
abilities in energy transformation.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank the financial support by the National Natural Science Foundation of
China (No. 51502270, U1510125). The authors also appreciate the financial support
by Shanxi Province Science Foundation (201601D021059), Program for the
Outstanding Innovative Teams of Higher Learning Institutions of Shanxi (OIT),
Program for the Young Academic Leaders of North University of China and the North
University of China Fund for Scientific Innovation Team.
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