Constructing atomic layer g-C3N4-CdS nanoheterojunctions with efficiently enhanced visible light photocatalytic activity

Article abstract of DOI:10.1039/c4cp02846e

Ultrathin two dimensional (2D) materials have triggered extensive interest for their exceptional properties and potential applications. Herein, atomic layer graphitic carbon nitride (g-C₃N₄) was obtained by a simple ultrasonic exfoliation approach, and cadmium sulfide (CdS) nanoparticles were successfully grown on these ultrathin g-C₃N₄ nanosheets (UCNNS) via a facile solvothermal method. The as-prepared UCNNS-CdS nanocomposite exhibits significantly enhanced photocatalytic activity for methyl orange (MO) degradation under visible light irradiation. The enhancement of the photocatalytic activity should be attributed to the well-matched band structure and intimate contact interfaces between the UCNNS and CdS, which lead to the effective transfer and separation of the photogenerated charge carriers. The mechanism for the photodegradation of MO by the composite was also investigated in this study. This study highlights the potential applications of atomic layer g-C₃N₄ based photocatalysts, and we hope our work may provide a new insight for the construction of photocatalysts with efficient visible light activity.

Full text of DOI:10.1039/c4cp02846e

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Constructing atomic layer g-C N –CdS
3
4
nanoheterojunctions with efficiently enhanced
Cite this: Phys.Chem.Chem.Phys.,
visible light photocatalytic activity†
2
014, 16, 21280
Meiliang Lu, Zengxia Pei, Sunxian Weng, Wenhui Feng, Zhibin Fang, Zuyang Zheng,
Mianli Huang and Ping Liu*
Ultrathin two dimensional (2D) materials have triggered extensive interest for their exceptional properties and
potential applications. Herein, atomic layer graphitic carbon nitride (g-C
ultrasonic exfoliation approach, and cadmium sulfide (CdS) nanoparticles were successfully grown on these
ultrathin g-C nanosheets (UCNNS) via a facile solvothermal method. The as-prepared UCNNS–CdS
3 4
N ) was obtained by a simple
3 4
N
nanocomposite exhibits significantly enhanced photocatalytic activity for methyl orange (MO) degradation
under visible light irradiation. The enhancement of the photocatalytic activity should be attributed to the
well-matched band structure and intimate contact interfaces between the UCNNS and CdS, which lead to
the effective transfer and separation of the photogenerated charge carriers. The mechanism for the
photodegradation of MO by the composite was also investigated in this study. This study highlights the
Received 28th June 2014,
Accepted 24th July 2014
DOI: 10.1039/c4cp02846e
3 4
potential applications of atomic layer g-C N based photocatalysts, and we hope our work may provide a
www.rsc.org/pccp
new insight for the construction of photocatalysts with efficient visible light activity.
Recent years have witnessed a burst of research into g-C
This novel metal-free photocatalyst possesses a proper band
D nanosheet materials, which consist of a monolayer or a structure that is suitable for water splitting and photocatalytic
3 4
N .
1
. Introduction
2
1
5–19
few atomic layers, have aroused a broad interest in different degradation of organic pollutants under visible light.
research areas owing to their unique optical and electronic Nevertheless, the photocatalytic efficiency of bulk g-C
properties, as well as their large surface area.
3
N
4
is
These merits far from satisfactory due to its high recombination rate of
render 2D nanosheet materials promising candidates for catalysis, photogenerated charge carriers and relatively small surface
1
–5
2
0,21
energy storage, environmental remediation and water splitting area.
Noticeably, as a p-conjugated material, g-C N has not
3 4
4–8
21,22
applications.
exhibits superiority in properties such as its high carrier mobility, Actually, recent works have reported that ultrathin g-C
large specific surface area and quantum hall effect, is the most nanosheets, obtained by peeling off the bulk counterpart, present
Among various 2D materials, graphene, which really been viewed from the perspective of a 2D material.
3 4
N
2,3
famous and widely investigated. In particular, graphene-based fascinating prospects for improving the transfer and separation of
photocatalysts generally show remarkably enhanced photocatalytic photoexcited charge carriers.
12,21–24
3 4
Compared to bulk g-C N ,
performance due to the accelerated transfer and promoted separa- there are three main apparent advantages of ultrathin nanosheets:
9
–11
tion of photo-generated carriers by graphene.
graphene is a zero bandgap material and does not show any provide abundant reactive sites.
photocatalytic activity, serving just as an electron acceptor or structure can remarkably reduce the distance to transfer the
However, pristine (1) the specific surface area is larger than the bulk one, and can
21,22
(2) The ultrathin nanosheet
2,12
12,21–23
transport intermediate in a photocatalytic reaction.
In addition, photoexcited carriers from one place to the interface.
excess graphene blocks incident light and hampers the external Therefore, there is an improvement in the transportation and
1
3,14
quantum efficiency of a photocatalyst.
separation of the photogenerated electron–hole pairs. (3) The
shift-up conduction and shift-down valence bands of the ultrathin
nanosheets can strengthen the reduction and oxidation abilities
of the electrons and holes, respectively. However, before prac-
tical applications can be considered, the light absorption capacity
23
State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis,
Fuzhou University, Fuzhou 350002, P. R. China. E-mail: liuping@fzu.edu.cn;
Fax: +86-591-8377-9239; Tel: +86-591-8377-9239
3 4
of g-C N still needs further improvement as this material can
only absorb light with a wavelength shorter than 460 nm, which
accounts for a small part of the whole visible light region.
†
Electronic supplementary information (ESI) available: TEM; SEM; UV-visible
16,25,26
absorption spectra; BET and so on. See DOI: 10.1039/c4cp02846e
2
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CdS has long been one of the most attractive visible light active (Cd(CH
PCCP
3
COO)
2
Á2H
2
O), thioacetamide (TAA), ethyl alcohol and
semiconductor photocatalysts due to its efficient absorption of scavengers isopropanol (IPA), ammonium formate (AF) and
visible light (up to 520 nm or even longer) and suitable band edge p-benzoquinone (PBQ) were purchased from Sinopharm
11,27,28
position.
In light of this, CdS can act as an excellent photo- Chemical Reagent Co. Ltd. (Shanghai, China). The deionized
sensitizer in many wide bandgap photocatalyst systems like water used in this work was from local sources.
2
9,30
31,32
CdS–TiO
large barriers that prevent the wide application of pure CdS. Firstly,
the recombination rate of the photo-induced electron–holes is Metal-free g-C
2
and CdS–ZnO.
Regrettably, there are still two
2
.2 Synthesis of the photocatalysts
3 4
N
nanosheets were prepared according to a
11,17
2À
20
undesirably high.
Worse still, S in CdS tends to be oxidized reported method. In detail, 5 g of urea was put into an alumina
by photoexcited holes upon strong irradiation, which is known as combustion boat, then heated to 550 1C at a heating rate of 2 1C
27,33
À1
the photocorrosion phenomenon.
Hence, numerous works min and kept at 550 1C for 4 h, followed by cooling to room
focus on addressing the intrinsic disadvantages of CdS. Approaches temperature. The whole process was performed under a helium
À1
such as using a sacrificial agent and the loading of a proper (He) gas flow (100 mL min ). The product was ground to a
1
1,33–35
co-catalyst have been adopted and have proven to be effective.
Promisingly, fabricating heterojunctions between two semiconduc- g-C
tors is another strategy to tackle both of these barriers. Efficaciously added. After ultrasonication for 8 h, the as-prepared g-C
transferring holes from the CdS domain can alleviate the accumu- exfoliated into ultrathin g-C nanosheets (UCNNS). When the
lation of holes, thus inhibiting charge carrier recombination and ethanol had completely evaporated the UCNNS were obtained.
powder and washed with deionized water. The as-prepared
was then collected in a beaker and 100 mL ethanol was
was
3 4
N
3 4
N
3 4
N
30,36
simultaneously achieving photocorrosion.
When reviewing the
Bulk g-C N (BCN) was prepared according to a reported
3 4
2
4
band alignment of CdS and g-C N , it is fortunate to find that procedure with some modifications. In detail, melamine was
their band edges match quite well to facilitate the separation of heated at 550 1C for 4 h in air at a ramp rate of 2 1C min , and
3
4
À1
e–h pairs. Though some very recent works have demonstrated then cooled to room temperature.
the validity of the above-mentioned strategy by synthesizing
UCNNS–CdS composites were synthesized via a one-step
procedure. A measured amount of UCNNS was redispersed in
17,33,37,38
g-C
the unique properties of atomic layer g-C
tion, not taken into account or investigated in-depth in these with 5 mmol Cd(CH
3 4
N –CdS composites via different preparation methods,
3
N
4
were, without excep- 20 mL ethanol and ultrasonicated again for 1 h. The suspension
COO) O was added to the 20 mL
3
2
Á2H
2
works. However, the novel characteristics of these ultrathin ethanol solution, and was then stirred for 5 h to form a
g-C N nanosheets (UCNNS) are probably extremely important for homogeneous suspension. After that, a 20 mL ethanol solution
3
4
boosting the photocatalytic activity considering the great success of with 5 mmol TAA was added to the above mixed suspension
similar graphene coated photocatalysts. Hitherto, there is no report and stirred for another 30 min. The obtained suspension was
for the in situ growth of CdS nanoparticles on UCNNS.
autoclaved in a 100 mL Teflon-lined stainless steel vessel at
In this study, UCNNS were obtained through thermal condensa- 180 1C for 12 h. After cooling to room temperature, the
tion of urea followed by a facile ultrasonic method. CdS nano- as-prepared yellow product was washed with water and ethanol
particles were then grown in situ on the UCNNS to fabricate several times and collected by centrifugation. Finally, the
nano-heterojunctions via a solvothermal method, during which obtained sample was dried in a vacuum oven at 80 1C. The
the UCNNS provided many binding sites for anchoring CdS nano- UCNNS–CdS composites with weight ratios of the UCNNS to
particles. In addition, intimate contact interfaces between the CdS of 3, 5, 10, 15 and 20 wt% were marked as 3CN, 5CN, 10CN,
UCNNS and CdS were also developed. Consequently, the composite 15CN and 20CN, respectively. The 10 wt% BCN/CdS was
exhibits significantly enhanced capability to degrade MO due to the synthesized by the same procedure without an ultrasonication
dramatically promoted charge separation. It was observed that the step, and the obtained product marked as 10BCN.
optimal photocatalytic activity of the composite with 10 wt% UCNNS
2.3 Characterization
loading was about 6 and 50 times higher than pure the CdS and
UCNNS, respectively, for the degradation of MO, and the photo- The crystal structures of the as-prepared samples was identified
stability of CdS was also apparently reinforced. Moreover, the active by X-ray diffraction using a Bruker D8 ADVANCE X-ray diffracto-
species of the composite for the degradation reaction were investi- meter with Cu K radiation (l = 0.15418 nm) which operated at
a
À1
gated systematically. This study highlights the potential applications 40 kV and 40 mA. The scan rate was 0.051 2y s . Scanning
3 4
of atomic layer g-C N based photocatalysts, and it is hoped that the electron microscopy (SEM) images were obtained using a
present work may provide a new insight for the construction of HITACHI SU8000 field-emission scanning electron microscope.
stable photocatalysts with efficient visible light activity.
Atomic force microscopy (AFM, Nanoscope Multimode IIIa
microscope, Veeco Instruments) was used to observe the thick-
ness of the 2D g-C N nanosheets. Transmission electron
3
4
microscopy (TEM, Tecnai G2 F20 S-TWIN, FEI company), using
a field emission gun which operated at 200 kV, was used to
investigate the morphologies of the samples. Infrared spectro-
2
. Experimental section
2.1 Materials
All reagents in this work were AR grade and used without scopy (IR) analysis was performed on a Nicolet Nexus 670 FT-IR
further purification. Urea ((NH CO), cadmium acetate dihydrate spectrometer. X-ray photoelectron spectroscopy (XPS) analysis
2 2
)
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was done on an ESCALAB 250 photoelectron spectrometer
a
(Thermo Fisher Scientific) with an Al K X-ray beam (1486.6 eV).
Total organic carbon (TOC) measurements of the degradation
solution after 16 min of irradiation were conducted on a TOC
analyzer (TOC-VCPH, Shimadzu). A Varian Cary-500 spectrophoto-
4
meter with BaSO as a reference was employed to obtain diffuse
reflectance spectra of the samples. Photoluminescence (PL) spectra
were recorded on a Varian Cary-Eclipse 500 with an excitation
wavelength of 325 nm. Nitrogen adsorption and desorption
isotherms were recorded on a Micrometrics ASAP 2020 analyzer.
During the degassing process, the samples were held at 180 1C
for 5 h. Electrochemical analysis was conducted using a ZEN-
NIUM electrochemical workstation (Zahner, Germany) with a
conventional three-electrode system. The reference and counter
electrodes were Ag/AgCl and Pt plate, respectively, and 0.2 M
Fig. 1 XRD patterns of pure CdS, pure UCNNS, and UCNNS–CdS
composites.
2 4
Na SO (pH = 6.8) aqueous solution served as the electrolyte.
5
mg of the as-prepared sample was dispersed in 0.5 mL (see Fig. S1, ESI†). Compared with BCN, both the (002) and
N,N-dimethyl formamide (DMF) solution by sonication, and (100) peaks of the UCNNS display a markedly reduced intensity
the slurry was then evenly spread onto a indium tin oxide and are broader. This indicates that the interlayer structure of
(
ITO) conductor glass substrate to serve as a working electrode. the UCNNS was destroyed. This is in agreement with the g-C N
3
4
2
0,22,39–41
A 300 W xenon lamp was used to provide a visible light source nanosheets reported in the literature.
In addition, the
equipped with a UV cut off filter (l Z 420 nm). For electro- layer number of the g-C
chemical impedance spectrscopy (EIS) measurements, the the (002) diffraction peak using the Scherrer formula.
amplitude of the sinusoidal wave was 10 mV, and the frequency According to the Scherrer formula, the average thickness of
ranged from 4 MHz to 0.01 Hz. the UCNNS is estimated to be 1.0 nm. Therefore, the layer
3 4
N nanosheets can be evaluated from
4
2,43
2
4,44
number of the UCNNS is about 2–3,
and this can be further
2
.4 Photocatalytic activity test
confirmed by TEM and AFM, which will be discussed later. The
XRD pattern of pure CdS possesses three discernible diffraction
peaks at 26.61, 43.91 and 52.01, which can be attributed to the
(111), (220) and (311) peaks of the face-centered cubic structure
of CdS (JCPDS 89-0440), respectively. The XRD patterns of the
UCNNS–CdS composites show no difference to pure CdS due to
the low X-ray diffraction intensity of the g-C N nanosheets and
The photocatalytic activity of the as-prepared samples was
evaluated by the photodegradation of MO with a concentration
of 5 mg L under visible light. A PLS-SXE 300 lamp (Beijing
Perfectlight Co., Ltd.), with a UV cut-off filter (l Z 420 nm),
served as the light source. 30 mg of the photocatalyst was added
À1
À1
3
4
to a quartz vial with 80 mL MO solution (5 mg L ). Before
have characteristic peaks overlapping with CdS. Nevertheless,
the presence of g-C N nanosheets in the composites can be
irradiation, the suspension was stirred for 30 min in the dark to
establish an adsorption–desorption equilibrium. During the
irradiation period, 3 mL of the solution was quickly extracted
at a definite time interval. After that, the solution was centri-
fuged at 12 000 rpm to remove the catalyst completely and
analyzed on a UV-vis spectrometer (Cary-50, Varian Co.).
The recycling photocatalytic activity test was performed as
follows. After the 1st run of the reaction, the catalyst was cleaned
and collected for the next run. Similarly, 3rd and 4th runs were
carried out. Controlled activity experiments were performed in a
similar way to the above photocatalytic activity experiment with
different radical scavengers added to the reaction system.
3
4
confirmed by SEM, TEM, FTIR and XPS analyses.
The detailed morphologies and crystal structures of the
UCNNS, CdS and as-prepared composites were directly analyzed
3 4
by SEM and TEM/HRTEM. As shown in Fig. 2a, pure g-C N is
composed of nanosheets with a laminar structure. In the TEM
images (Fig. 2b and Fig. S2, ESI†), the UCNNS look like a silk veil,
similar to graphene or graphene oxide that have been
3
,45
reported,
and look transparent due to their thin nature. The
thickness of the g-C N nanosheets can be further determined by
3
4
atomic force microscopy (AFM). Fig. 2c and d reveal that the
average thickness of the obtained g-C nanosheets is approxi-
mately 0.8 nm, which indicates that the g-C nanosheets are
Compared with the
, which consists of an irregular thick
3 4
N
3 4
N
2
4,44
composed of only about two C–N layers.
as-prepared bulk g-C
3
. Results and discussion
3 4
N
Fig. 1 shows the XRD patterns of pure CdS, pure UCNNS and block-like material (Fig. S3, ESI†), it is apparent that the
their composites with different amounts of the UCNNS. For as-prepared UCNNS possess a larger specific surface area (see
pure g-C N , the distinct diffraction peak at 27.41, corre- Table S1, ESI†). The morphological features of the bare CdS
3
4
sponding to the (002) peak in JPCDS 87-1526, represents the nanoparticles were investigated by SEM. As shown in Fig. 2e, the
interlayer stacking of the conjugated aromatic groups. The CdS nanoparticles are significantly aggregated together to form
weak peak, corresponding to the (100) plane at about 131, is clusters. As shown in Fig. 2f, after introducing CdS through
2
0,22,39
attributed to the in-plane structure of tri-s-triazine units
the solvothermal approach, the CdS nanoparticles are dispersed
2
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Scheme 1 Mechanism for the formation of the UCNNS–CdS nano-
composites.
Fig. 3 FTIR spectra of CdS, UCNNS, and UCNNS–CdS nanocomposites.
Fig. 2 (a) SEM image and (b) TEM image of pure UCNNS. (c) AFM image
and (d) corresponding thickness analysis of pure UCNNS. (e) SEM image of
pure CdS, (f) SEM image of the 10CN sample. (g) TEM image and (h) HRTEM
image of the circle in (g) for the 10CN sample.
attributed to the O–H bending vibration of surface-absorbed water
molecules. For pure UCNNS, the characteristic peaks that appear
À1
around 1246, 1316, 1422, 1569 and 1641 cm can be assigned to
12,41
on the UCNNS. The TEM and HRTEM images (Fig. 2g and h, the typical stretching modes of CN heterocycles in g-C
3 4
N .
The
3 4
respectively) intuitively show that the UCNNS are decorated with representative breathing mode of the triazine units in pure g-C N
À1 12,17,41
CdS nanoparticles, and a lattice spacing of 0.337 nm can be nanosheets can be observed at 813 cm
.
After constructing
assigned to the (111) crystal face of CdS (JCPDS 89-0440). These nanojunctions between CdS and the UCNNS, all of the character-
results indicate that the CdS nanoparticles spread onto the istic bands of CdS are present in the composites because of its
UCNNS to form nanoheterojunctions. The 2D g-C N nanosheets high percentage. Additionally, the characteristic peaks for the
3
4
can provide anchor sites to immobilize the CdS, which can g-C
3
N
4
nanosheets can also be observed in the composites. The
partially prevent aggregation of the CdS nanoparticles. After characteristic bands of g-C gradually increase with the increase
formation of the nano-heterojunctions, the specific surface area in the g-C nanosheet ratio in the composites. As shown in
of the composites increases to some extent compared with CdS Fig. S8 (ESI†), the corresponding peaks for the C–N stretching
see Fig. S7 and Table S1, ESI†). Based on the above results, a vibration mode and triazine units for the mixed-10CN sample
mechanism for the fabrication of UCNNS–CdS composites can show no discrepancies with the pure UCNNS. The mixed-10CN
be proposed and is illustrated in Scheme 1. sample is simply formed by mechanical mixing of the UCNNS
1
1
3 4
N
3 4
N
(
À1
À1
To investigate the interaction between CdS and the UCNNS, with CdS. For 10CN, a 10 cm red shift occurs at 813 cm , and
FTIR spectroscopy was carried out. FTIR spectra of pure CdS, the relevant FTIR band for the C–N stretching vibration is also
pure UCNNS, and UCNNS–CdS composites are compared in weaker than the mixed-10CN. This indicates that an interaction
À1
Fig. 3. For CdS, the peaks between 1100 and 1650 cm
may have developed between UCNNS and CdS, which weakens
À1
are ascribed to the Cd–S bond and the band at 1618 cm is the force between the C and N atoms. The FTIR results of the
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Fig. 5 UV-vis spectra of the as-prepared samples.
17
which sensitizes the UCNNS to strengthen the optical response.
All the as-prepared nanocomposites show broader absorption in
Fig. 4 X-ray photoelectron spectra of (a) C 1s, (b) N 1s, (c) Cd 3d and the visible region compared to the UCNNS, and slightly narrower
d) S 2p in the samples.
absorption than CdS. These observations for the composites are
(
37
attributed to the interaction between CdS and the UCNNS. The
interaction probably plays a significant role in improving the
as-prepared samples are well in accordance with the SEM and separation of the photogenerated electron–hole pairs to enhance
TEM/HRTEM images, and all indicate that the UCNNS–CdS the photocatalytic activity. As a result, the composites may have a
composites are successfully obtained.
To further confirm the existence of g-C
higher photocatalytic activity than pure UCNNS and CdS.
The photocatalytic performances of the CdS–UCNNS compo-
3 4
N in the composites
and study their surface chemical state, XPS measurements were sites were evaluated via MO photodegradation tests under visible
conducted. Fig. 4a-d indicate that the 10CN sample contains C, light (l Z 420 nm) illumination. Each of the photodegradation
N, Cd and S elements. For the UCNNS sample, the C 1s spectrum reactions was carried out after 30 min adsorption equilibrium in
can be fitted with three peaks at 284.6 eV, 285.8 eV and 288.2 eV the absence of light. The photocatalytic activity for each photo-
(
Fig. 4a, top). The peak at 284.6 eV is related to surface adven- catalyst is demonstrated in Fig. 6a. As we can see from the figure,
3
9
titious carbon. The peak centered at 288.2 eV can be attributed
2
to sp C bonded to N-containing aromatic rings (N–CQN), which
3
3,38
represents the carbon species in g-C
3 4
N .
The small peak
3
(
285.8 eV) is ascribed to the sp -bonded carbon species from
3
3,38
defects on the g-C N surface.
Three peaks at 399.0 eV,
3
4
400.3 eV and 401.5 eV, can be identified in the N 1s spectrum
for the UCNNS (Fig. 4b, top). The binding energy of 399.0 eV is
2
derived from the sp -bonded N in the triazine units (C–NQC) in
g-C
3
N
4
. The weak peaks at 400.3 and 401.5 eV originate from
units and amino groups (C–N–H),
From Fig. 4a and b, we can see that the
tertiary nitrogen N–(C)
respectively.
3
3
3,37,38,46
peak for C 1s (285.8 eV) shifts to a higher binding energy when
UCNNS are combined with CdS, and the two peaks at 400.3 and
401.5 eV for N 1s shift to lower binding energies. The shift in
binding energy suggests an electronic interaction between the
4
7
UCNNS and CdS, which is consistent with the IR results. In
Fig. 4c and d, the XPS peaks at 405.1 and 411.9 eV, and the peaks
at 161.1 and 162.2 eV, are ascribed to Cd 3d and S 2p in the 10CN
3
3,37,38
and bare CdS samples, respectively.
The optical performances of the as-prepared samples were
investigated by UV-vis diffuse reflectance spectroscopy (DRS).
As shown in Fig. 5, the absorption edge for pure UCNNS is
about 420 nm, which is shorter than the 450 nm of the
as-prepared BCN. This blue-shift is caused by the quantum size
confinement effect when reducing the dimensionality of the
Fig. 6 (a) Photocatalytic degradation of MO over the as-prepared samples
under visible light illumination (l Z 420 nm). (b) The corresponding rate
2
5
material. The absorption band edge of pure CdS is 520 nm, constant k of the different as-prepared samples.
2
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the activity for BCN is the weakest as almost no MO has decom- g-C
3
N
4
nanosheets are superior to the bulk sample to stabilize
posed after 16 min visible light irradiation. The photoactivity for the CdS particles.
the UCNNS is a little better than BCN due to the larger specific
Based on the aforementioned experimental results, we believe
surface area, but its photocatalytic ability to decompose organic that the enhancement of the photocatalytic activity for the
pollution is still weak due to its limited light absorption capacity. UCNNS–CdS composite is accredited to the efficient interfacial
The composite photocatalysts reveal notably enhanced photo- transfer and separation of photogenerated carriers between the
3 4
activity for MO degradation compared to the bare UCNNS and g-C N nanosheets and CdS particles. This assumption can be
CdS. In our research system, the optimal activity was achieved verified by photoelectrochemical measurements. Fig. 8a shows
with the 10CN sample, which almost completely eliminated MO the transient photocurrent response of 10CN, CdS, UCNNS, and
after 16 min irradiation (see Fig. S3 and S6, ESI†). As depicted BCN under visible light irradiation for several on–off cycles. From
in Fig. 6b, the photoactivity of the 10CN sample is about 6 and Fig. 8a we can clearly see that the photocurrent response of BCN is
50 times higher compared to CdS and the UCNNS, respectively. lower than that of the UCNNS and 10CN is significantly higher
The specific surface area is always a crucial factor in photocata- than that of the UCNNS and CdS under the same visible light
lysis. To describe the photocatalytic performance more exactly, illumination. The generation of the photocurrent is mainly the
48
the reaction rate for photocatalytic degradation before and after result of photoinduced electrons diffusing to the ITO. Therefore,
normalization with the surface area is compared (see Fig. S12, the enhanced photocurrent implies that more effective charge
ESI†). After normalization with the surface area, 10CN still transfer and separation are achieved after constructing the hetero-
exhibits a remarkably higher photocatalytic activity compared to junctions. The strength of the photocurrent is in line with the order
10BCN, which convincingly demonstrates the advantage of the 2D of the photocatalytic activity. To further confirm the above results,
g-C N nanosheet hybrid photocatalysts to the bulk one. These electrochemical impedance spectroscopy (EIS), a useful measure-
3
4
results suggest that the synergic effect between the CdS and ment to characterize charge carrier transportation, was also per-
UCNNS probably plays a pivotal role for accelerating the transfer formed. Fig. 8b displays that the impedance radius of the UCNNS is
and separation of the charge carriers to improve the photocata- smaller than BCN, which indicates a smaller charge transfer
lytic activity. This can be certified through further research.
The stability of the as-prepared samples was evaluated via a the bare CdS and g-C
recycling test under visible light illumination. From Fig. 7, the charge transfer resistance across the interface
resistance for the ultrathin g-C
3 4
N . 10CN is much smaller than
3 4
N
nanosheets, which reflects a decreased
49,50
between CdS and
photocatalytic activity of CdS decreases gradually during the the UCNNS. Therefore, the remarkably improved life-time and more
cycling test due to photocorrosion under irradiation. Mean- efficient separation of the photoexcited carriers induce a notable
while, 10BCN also exhibits a slower decrease in photocatalytic enhancement of the photocatalytic activity in MO degradation.
performance. There is no noticeable decrease in the photo-
catalytic activity for 10CN after four successive cycles of the
reaction. In addition, as shown in Fig. S9 and S10 (ESI†),
neither the structure or the morphology of the 10CN sample
changed after the recycling test, which indicates that the 10CN
sample is stable during the photocatalytic reaction. These
results indicate that the UCNNS–CdS composite not only
significantly enhances the photocatalytic activity, but can also
effectively prevent photocorrosion of the CdS. The 10CN sample
is more stable than 10BCN, which also suggests that the 2D
Fig. 8 Photoelectrochemical properties of the as-prepared CdS, UCNNS,
BCN and 10CN samples. (a) Transient photocurrent response under visible
Fig. 7 Cycling photocatalytic degradation of MO over UCNNS–CdS
composites under visible light irradiation (l Z 420 nm).
light irradiation (l Z 420 nm) and (b) EIS Nyquist plots of the samples; the
inset image is a comparison between the BCN and UCNNS.
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Fig. 10 Effects of the addition of different scavengers on the photo-
catalyzed degradation of MO for the 10CN sample.
Fig. 9 Comparison of the PL spectra for the BCN, UCNNS, and 10CN
samples excited at 325 nm.
Both the images of the composites and the above results
suggest that the enhanced photocatalytic activity is attributed
to the interfacial transfer of the photogenerated charges
between the UCNNS and CdS. This transfer is also investigated
by photoluminescence (PL) measurements. Fig. 9 presents the
PL spectra for the BCN, UCNNS, and the 10CN samples.
The BCN sample shows a strong emission peak centered at
about 465 nm, which can be assigned to the recombination of
3 4
electrons and holes in g-C N . In comparison, the emission
intensity of the UCNNS clearly decreases, which reveals that
ultrathin g-C N undergoes more rapid charge transfer than the
3
4
bulk sample. When the CdS nanoparticles are added, the PL
drops remarkably. The reduction of the PL can be interpreted
as the efficient transfer of photoinduced electrons and holes
between the UCNNS and CdS particles.
Fig. 11 Schematic diagram of the photogenerated electrons and holes
transfer in the UCNNS–CdS composite and photocatalytic degradation of
MO under visible light irradiation (l Z 420 nm).
In order to understand the role of the photoexcited active
species in the degradation of the MO dye, a succession of control
experiments were performed. Different scavengers aiming for
the VB in the UCNNS. Thus, the separation efficiency of the
electron–hole pairs is improved to a large extent, and the
photocorrosion of CdS by holes is greatly alleviated, leading
to a remarkable enhancement of the photocatalytic activity.
ꢀ
+
the hydroxyl radicals ( OH), holes (h ) and superoxide radicals
ꢀ
À
(
O ) were added during the degradation process. Herein,
2
5
1,52
ꢀ
was used to scavenge OH, ammonium
isopropanol (IPA)
formate (AF) was for h , and p-benzoquinone (PBQ) was for
+
35
ꢀ
À 53,54
O
2
.
Fig. 10 presents the photocatalytic activity of the 10CN
4. Conclusions
sample with the addition of the different scavengers. It is clear
that when PBQ was added to the reaction system, the degrada- In summary, atomic layer g-C
3 4
N can be obtained through
tion of MO almost terminated and when AF was added, the thermal condensation of urea and a simple ultrasonic method.
degradation was apparently inhibited. Thus, it can be concluded UCNNS–CdS nanocomposites were successfully synthesized by
that the superoxide radicals play the most pivotal role in the a simple solvothermal method. The photoactivity of MO degra-
ꢀ
À
+
degradation of the MO dye and O
2
and h are the main active dation under visible light irradiation is significantly enhanced
species affecting the degradation rate.
after constructing a heterostructure between the UCNNS and
On the basis of the above experiments and results, a CdS. The result also indicates that such 2D UCNNS can provide
tentative mechanism for the degradation of MO on the more binding sites than the bulk sample to anchor CdS nano-
UCNNS–CdS nanocomposite is illustrated in Fig. 11. Both the particles, resulting in an improved photoactivity. In addition,
UCNNS and CdS can generate photoexcited holes and electrons the as-prepared composites are stable during the photocatalytic
under visible light irradiation. It is reported that the conduc- reaction. Both the photostability and enhanced photocatalytic
tion band (CB) and valance band (VB) of CdS are lower than activity of the composites can be explained by the effective
that of g-C N . After constructing the nanoheterostructure, the transfer and separation of photogenerated charge carriers
3 4
photoinduced electrons in the CB of the UCNNS can directly between the CdS and UCNNS. Furthermore, superoxide radicals
migrate to CdS, while the holes in the VB of CdS can transfer to play the most pivotal role in the degradation of the MO dye.
2
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This study highlights the potential application of ultrathin 17 J. Fu, B. Chang, Y. Tian, F. Xi and X. Dong, J. Mater. Chem. A,
g-C nanosheet hybrid photocatalysts, and we hope that it
2013, 1, 3083.
may provide a new insight for the construction of stable 18 G. Liao, S. Chen, X. Quan, H. Yu and H. Zhao, J. Mater.
3 4
N
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Acknowledgements
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