A novel enhanced visible-light-driven photocatalyst via hybridization of nanosized BiOCl and graphitic C3N4

Article abstract of DOI:10.1039/c4dt02082k

Nanosized C₃N₄/BiOCl composites were synthesized via a facile method in the presence of arabic gum. Arabic gum acts as the structure-directing agent and helps the formation of BiOCl nanoparticles. A significant enhanced photodegradation is observed and a possible mechanism is proposed by combining photosensitization and photocatalysis. This journal is

Full text of DOI:10.1039/c4dt02082k

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A novel enhanced visible-light-driven
photocatalyst via hybridization of nanosized
BiOCl and graphitic C3N4†
Cite this: DOI: 10.1039/c4dt02082k
Liwen Lei,a,b Huihui Jin,a Qi Zhang,a Jun Xu,a Da Gaoa and Zhengyi Fu*b
Received 9th July 2014,
Accepted 4th November 2014
Nanosized C3N4/BiOCl composites were synthesized via a facile method in the presence of arabic gum.
Arabic gum acts as the structure-directing agent and helps the formation of BiOCl nanoparticles. A signifi-
cant enhanced photodegradation is observed and a possible mechanism is proposed by combining
photosensitization and photocatalysis.
DOI: 10.1039/c4dt02082k
www.rsc.org/dalton
internal static electric fields that can help the photoinduced
hole–electron separation and the charge transport process,
1. Introduction
Semiconductor photocatalysis has attracted a great deal of which promote the visible light photocatalytic activity.3,4 Up to
attention as an effective way for organic pollutant remediation now, much effort has been made to improve the activity of the
and the conversion of solar energy into chemical energy. pristine catalyst by controlling the morphology,5,6 the selected
Although it is very effective under near-UV light irradiation orientation of crystal growth,7 self-doping and surface modifi-
with a wavelength less than 387.5 nm, conventional TiO2 cation.8 However, it is still necessary to further improve the
photocatalyst is not ideal for such applications because it per- photocatalytic efficiency for practical applications.
forms rather poorly in the processes associated with natural
One of the techniques for increasing the separation
sunlight photocatalyst, which is induced by its wide band gap efficiency of photogenerated hole–electron pairs is to form a
(3–3.2 eV). Considering that most solar light sources include hetero-structure photocatalyst using two kinds of semiconduc-
plenty of visible light, the narrow optical absorption range of tors. Construction of a hetero-structure by coupling two
the photocatalyst and the low solar energy conversion different semiconductors can help achieve the better separ-
efficiency are the major problems limiting the energy ation of photoinduced electrons and holes and thus improve
efficiency. Therefore, much effort has been devoted to develop- the photocatalytic efficiency dramatically. Recently, formation
ing more efficient and stable photocatalysts with good photo- of a hetero-structure by coupling with conjugative π structure
generated charge separation as well as a wide response materials has been extensively reported as an effective way to
wavelength range.1,2
In recent years, bismuth-based photocatalytic materials charge recombination.9–12
improve the photoinduced separation and a relatively slow
such as BiOX (X = Cl, Br, I), BiVO4, BiWO6, Bi5Nb3O15 etc. have
Graphitic carbon nitride (g-C3N4) with natural lamellar
aroused great interest in the scientific community due to their structure can be readily synthesized by pyrolysis with several
intriguing electronic structures. Of these photocatalysts, BiOCl cheap nitrogen rich precursors.13,14 This metal free semicon-
exhibits excellent photocatalytic activity to decompose organic ductor can form a π-conjugated plane and possess high
compounds into inorganic substances for purifying textile dye thermal and chemical stability since the structure of g-C3N4
polluted waste water. Although BiOCl has a similar band gap has the s-triazine ring with high condensation.15 More impor-
to TiO2, its layered structure endows it with the self-built tantly, g-C3N4 with a band gap of 2.7 eV is chemically reactive
in itself and was found to exhibit good photocatalytic activity
for hydrogen production and organic pollution degradation
under visible light irradiation. However, the photocatalytic per-
formance of g-C3N4 is still limited to the high recombination
rate of the photo-induced electron–hole pair. The literature
contains numerous reports relating to g-C3N4 heterostructure
aSchool of Materials Science and Engineering, Wuhan University of Technology,
188 Luoshi Road, Hubei, P. R. China. E-mail: leiliwen@whut.edu.cn;
Fax: +86 27 87214521; Tel: +86 27 87866762
bState Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, 188 Luoshi Road, Hubei, 430070,
P. R. China. E-mail: shswhut@gmail.com
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4dt02082k
catalysts,16 such as ZnO/g-C3N4,17 Bi2WO6/g-C3N4,18 WO3/
20
g-C3N4 19, BiOP4/g-C3N4
etc., which show the enhanced
photocatalytic activities. Accordingly, g-C3N4 is an excellent
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component with a π-conjugated structure to fabricate the het- another 30 min. Next, 0.97 g Bi(NO3)3·5H2O was dissolved in it
erojunction catalysts to increase the photocatalytic activity of a and stirred for an additional 60 min to obtain a stable, homo-
semiconductor.
geneous precursor. Subsequently, the mixture was put into a
As a result, the combination of g-C3N4 with the π-conju- water bath and heated at 80 °C for 12 h. After completion of
gated structure and BiOCl with an open structure may be an the reaction, the obtained precipitate was collected and rinsed
ideal system to achieve an enhanced charge separation. To the several times with deionized water and absolute ethanol, alter-
best of our knowledge, very few reports have focused on the nately. Finally, the precipitate was dried at 60 °C. A series of
synthesis of BiOCl coupling with g-C3N4 to highlight its photo- C3N4/BiOCl photocatalysts with various g-C3N4 mass contents
catalytic performance. Very recently, Wang et al. have reported of 0%, 5%, 10%, 20%, 30% and 100% were synthesized by this
BiOCl photocatalysts hybridized with g-C3N4 prepared using method, which can be denoted as BiOCl, 5C3N4/BiOCl,
ionic liquid [HMIm]Cl as templates and Cl− as sources to 10C3N4/BiOCl, 20C3N4/BiOCl, 30C3N4/BiOCl and g-C3N4 for
enhance the photocatalytic activities.21 However, [HMIm]Cl is convenience. In order to understand the role of AG in the syn-
slightly expensive and is also harmful to the environment. thesis of BiOCl and the composite, prisitine BiOCl and C3N4/
Herein, we first report the synthesis of BiOCl and g-C3N4 het- BiOCl photocatalysts with 20% g-C3N4 mass content were pre-
erostructured photocatalysts under mild conditions. A nano- pared in the absence of AG under the same conditions for
flower structured BiOCl was synthesized using arabic gum (AG) comparison, which is assigned as BIOCL and 20C3N4–BiOCl,
as a template. AG is a biopolymer gelatin used as a surface pro- respectively.
tecting agent and is known to contain many polar amino
acids. The g-C3N4/BiOCl inorganic–organic hybrid photocata-
2.2 Characterization
lysts showed superior photosensitized degradation activity of Powder X-ray diffraction (XRD) patterns of the samples were
rhodamine B (RhB) to those of pure phases. The results show obtained with a D/MAX-RB system with the Cu Kα radiation
that the addition of AG can improve evidently the specific from 10° to 80° with a step size of 0.02°. Morphological investi-
surface area of these hetero-structure catalysts and the inter- gations were carried out with scanning electron microscopy
face conditions, which are vital for the photocatalytic perform- (SEM, Hitachi-S3400N), field emission scanning electron
ance of the hetero-structure photocatalyst. Furthermore, microscopy (FESEM, Hitachi-S4800), transmission electron
among the heterostructure photocatalysts relating to BiOCl, microscopy (TEM, Philips-CM12) and high-resolution trans-
the mechanism is rarely related to the photosensitiza- mission electron microscopy (HRTEM, JEOL-2010). The charge
tion.12,21,22 However, visible light with λ > 420 nm could not effect was calibrated using the binding energy of C 1s. UV-vis
excite BiOCl because of its large band gap. Hence, the mechan- diffuse reflectance spectra (DRS) of the samples were recorded
ism of combining photosensitization with photocatalyst for on a UV-2550 (Shimadzu) using BaSO4 as the background. The
the degradation of RhB over the g-C3N4/BiOCl photocatalysts Brunauer–Emmett–Teller (BET) surface area and pore size
was proposed.
measurements were measured using nitrogen adsorption via a
Micromeritics ASAP 2020 (USA) surface area analyzer. Fourier
transform infrared (FTIR) spectra were acquired with a DTGS
KBr detector using a Nicolet Nexus 670 spectrometer.
2. Experimental
2.1 Catalyst preparation
2.3 Photocurrent experiments
All starting materials were guaranteed reagents and used The electrochemical measurements were carried out on an
without further purification. The g-C3N4 powder was prepared electrochemical system (CHI600C, China). A standard three elec-
by direct polymerization of urea according to the literature.23 trode system using the as-prepared samples as the working elec-
In a typical synthesis run, 25 g of urea was placed into a semi- trodes, a Pt wire as a counter electrode, and Ag/AgCl (saturated
closed alumina crucible to prevent the sublimation of the pre- KCl) as a reference electrode were used in photoelectric studies.
cursors in a muffle furnace. The crucible was heated to 550 °C A 500 W Xe arc lamp with a UV-cut off filter (λ > 420 nm) served
for 2 h with a ramp rate of 2 °C min−1. Next, 100 mg of the as a light source. 0.5 M Na2SO4 aqueous solution was used as
obtained yellow g-C3N4 powder was dispersed in 100 mL of the electrolyte. The working electrodes were FTO glass (0.5 ×
water, and then ultrasonicated for about 16 h. The resulting 1 cm2) coated with the as-prepared samples (5 mg).
suspension was centrifuged at low speed to remove the
2.4 Photocatalytic experiments
residual unexfoliated g-C3N4, followed by centrifugation at
10 000 rpm to obtain g-C3N4 nanosheets.
Photocatalytic activities of the samples were evaluated by the
The typical preparation of nanosized g-C3N4/BiOCl photo- decolorization of rhodamine B(RhB) under visible light
catalysts was as follows: first, 0.1 g AG was added to 100 mL irradiation at ambient temperature. A 500 W Xe-illuminator
glycerol and stirred for 30 min to dissolve AG completely. was used as a light source and set about 10 cm apart from the
Then, the appropriate amount of g-C3N4 was completely dis- reactor. The 420 nm (or 532 nm) cut-off filter was placed
persed into the saturated solution assisted by ultrasonication between the Xe-illuminator and the reactor to completely
for 30 min. After that, 0.45 g KCl and 0.02 mol carbamide were remove all incoming wavelengths shorter than 420 nm (or
added to the above solution and stirred in a fume hood for 532 nm) to provide visible light. Experiments of RhB decolori-
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zation were carried out as follows: 10 mg of photocatalyst relatively lower, which could be attributed to the smaller
powder was added to 100 mL of 20 mg L−1 RhB solution with lateral size oriented along the [110] direction.25,26
constant stirring. Before illumination, the solutions were
The diffraction peaks of C3N4/BiOCl composite photocata-
stirred for 1 h in the dark to obtain adsorption–desorption lysts show that all the samples are well crystallized and the
equilibrium and eliminate the error due to any initial adsorp- crystal phase of BiOCl does not change after the introduction
tion effect. At given time intervals, about 5 mL suspensions of g-C3N4. No crystalline impurity phase emerges in the C3N4/
were collected, and centrifuged to separate the photocatalyst BiOCl photocatalysts. However, no characteristic g-C3N4 dif-
particles. The concentrations of the remnant RhB were then fraction peak is observed with low g-C3N4 addition. With
monitored using a UV-vis spectrometer (Shimadzu UV-2550) at increasing the g-C3N4 content of the composite, the peaks
a wavelength of 553 nm.
assigned to g-C3N4 become stronger. Moreover, when the mass
The detection of active species in the photocatalytic reacti- ratio of g-C3N4 reaches 30%, the intensity of the diffraction
vity is similar to the photodegradation experiment. tert-Butyl peaks of C3N4/BiOCl composite photocatalysts decreases evi-
alcohol (t-BuOH), 1,4-benzoquinone (BQ) and ethylenediamine- dently compared to those with a lower g-C3N4 mass content.
tetraacetic acid (EDTA-2 Na) were selected to detect the During the synthetic process, the nucleation and growth of
hydroxyl radicals (•OH), superoxide radical (O2•−) and hole (h+), BiOCl grains may preferentially occur on the surface of porous
respectively. 1 mM of the scavenger was introduced into the g-C3N4 sheets for the relatively lower nucleation energy of het-
RhB solution prior to addition of the photocatalyst. The erogeneous nucleation. The relative high content of g-C3N4
detailed experimental process and the dosage of these scaven- could wrap the BiOCl grains and decrease the crystallinity of
gers were referred to in the previous studies.24
the g-C3N4/BiOCl photocatalyst. Furthermore, it is apparent
that no impurity phase is detected, confirming the high purity
of g-C3N4/BiOCl photocatalysts.
3.2 Morphology and surface area
3. Results and discussion
The particle sizes and morphology of the representative photo-
catalysts were investigated by SEM and FESEM. Fig. 2a shows a
typical SEM image of pristine BiOCl without g-C3N4. It is clear
that the product exhibits a homogeneous flower-like nano-
structure sphere with an average diameter of approximately
200 nm. A TEM image of an individual pristine BiOCl particle,
as shown in Fig. 2g, clearly reveals that the flowerlike 3D hier-
archical structure is constructed by many nanoflakes with a
thickness of around 5–8 nm and a lateral size of about
100 nm. The nanoflakes conglutinate at the centre and are
composed of self-assembled nanoparticles. The ultrafine par-
ticles of BiOCl could be conducive to the coupling with g-C3N4
for the formation of the hetero-structure photocatalyst due to
the high surface energy. The detailed structures for BiOCl are
further characterized by the selected-area electron diffraction
(SAED) pattern (Fig. 2h) and show the continuous diffraction
rings, revealing the highly crystalline nature of the nano-
flowers. It is worth noting that BiOCl microstructures are
highly sensitive to intensive electron beam irradiation, which
was observed and reported in some groups.27,28 The SEM
picture of BIOCL in the absence of AG is shown in Fig. 2f. The
shape of the products is still a flower-like sphere. However, the
size of the products is far larger than the BiOCl synthesized in
the presence of AG. The above results are consistent with those
of XRD and further demonstrate that the presence of AG plays
a key role in the precipitation process and is responsible for
the formation of BiOCl particles with controlled size. It is
known that the isoelectric point of AG in aqueous solution is
around 2.4. Hence, the AG molecules are expected to possess
negative charges in this experimental reaction system at the
pH value of 6.8. For the flower-like BiOCl, the single nanosheet
grows along the [110] direction and the side surfaces are {110}
facets. Then, the negative carboxylate charge groupings in AG
3.1 Phase structures
Fig. 1 shows the XRD profiles of g-C3N4, BiOCl, BIOCL and
C3N4/BiOCl photocatalysts with different g-C3N4 mass con-
tents. Fig. 1a shows that two pronounced peaks are found in
the pristine g-C3N4 at 27.4° and 13.1°, which corresponds to
the characteristic interplanar stacking of the conjugated aro-
matic system and the interlayer structural packing peak and
can be indexed to (002) and (100) diffraction planes, respect-
ively. Meanwhile, the diffraction peaks of BiOCl could be
readily indexed to the tetragonal phase of BiOCl (JCPDS card
no. 06-0249). The XRD patterns of BIOCL are similar to those
of BiOCl. It is worth noting that compared with BIOCL, the
diffraction intensity ratio of (110)/(001) planes for BiOCl is
Fig. 1 XRD patterns of as-prepared photocatalysts in different
compositions.
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Table 1 Measured parameters for the specimens
Kinetic rate
Samples
name
Surface area
constant (k)
Adsorption
capability (%)
(m2 g−1
)
(min−1
)
g-C3N4
BiOCl
5C3N4/BiOCl
10C3N4/BiOCl
20C3N4/BiOCl
30C3N4/BiOCl
BIOCL
46.85
49.51
48.82
50.47
49.37
51.03
16.87
32.16
0.0078
0.1143
0.1875
0.1967
0.3532
0.1003
0.0871
0.1535
9.2
21.9
21.7
20.8
20.2
14.1
17.7
17.8
20C3N4–BiOCl
to the XRD patterns are marked in Fig. S1(C),† confirming the
existence of C3N4 and BiOCl. The interplanar spacing value of
0.663 nm in Fig. S1(C)† corresponds to the (001) planes of
BiOCl, which means that the lattice structure of BiOCl shows
no obvious changes after the addition of g-C3N4. Smooth inter-
faces are found between BiOCl and C3N4. The specific surface
area pore size distribution of the as-synthesized specimens
was investigated by nitrogen adsorption–desorption analysis
(as shown in Table 1). It can be found that the specific surface
area of BiOCl is 49.51 m2 g−1, which is roughly equal to that of
g-C3N4. Hence, the specific surface areas for the C3N4/BiOCl
composites are in the similar range from 45 to 51 m2 g−1
.
However, BIOCL exhibits a relatively low specific surface area
of 16.87 m2 g−1. Therefore, AG acts as the structure-directing
agent and help control the size of BiOCl particles. The nano-
sized BiOCl with higher specific surface area may favour the
formation of intense interaction during the hybridization for
the higher surface energy, producing more interfaces between
g-C3N4 and BiOCl.
3.3 FT-IR spectroscopy analysis
Fig. 3 displays a comparison of FT-IR spectra of g-C3N4, BiOCl,
and C3N4/BiOCl composites prepared in the presence of AG
and 20C3N4–BiOCl prepared in the absence of AG. In the FT-IR
spectra of g-C3N4, the adsorption peak at 811 cm−1 corres-
ponds to the characteristic breathing mode of an s-triazine
ring system. The peak at 1638 cm−1 is attributable to CvN
stretching vibration modes, while the 1208–1571 cm−1 are
related to aromatic C–N stretching. The broad absorption
band at 3100–3300 cm−1 can be assigned to the stretching
modes of the secondary and primary amines and their inter-
molecular hydrogen-bonding interactions. Both of the charac-
teristic peaks of BiOCl (527 cm−1) and those of g-C3N4 were
observed on C3N4/BiOCl photocatalysts, which means the
maintenance of the C3N4 and BiOCl structures during the
hybridization. The characteristic peaks of g-C3N4 for 20C3N4–
BiOCl do not move after combination with BiOCl nanoparti-
cles. Nevertheless, compared to the 20C3N4–BiOCl photo-
catalyst, the red shift of all the main characteristic peaks
belonging to g-C3N4 implies that the bond strengths of CvN
and C–N would be attenuated. This phenomenon also
appeared in the g-C3N4/ZnO and g-C3N4/ZnWO4 systems,
which means that the conjugated system of C3N4 was stretched
Fig. 2 SEMand images of (a) BiOCl, (b) 5C3N4/BiOCl, (c) 10C3N4/BiOCl,
(d) 20C3N4/BiOCl, (e) 30C3N4/BiOCl, (f) BIOCL and TEM images of (g)
and (h) BiOCl.
are supposed to interact with {110} planes which enrich in Bi
cations and suppress its growth. Therefore, the nanosheets of
BiOCl in the presence of AG are shorter than those of BIOCL.
As seen from Fig. 2b to d, BiOCl particles are well wrapped
by the g-C3N4 sheet system and their shapes show no signifi-
cant changes with a relatively low g-C3N4 amount. However,
the regular flower-like shape may be disrupted with the
increasing content of g-C3N4, which means that the excessive
g-C3N4 in the reaction system can destroy the self-assembly
process of BiOCl. To provide better insights into the mor-
phology and the microstructure of the as-prepared samples,
TEM and HRTEM images of C3N4, and 20C3N4/BiOCl are
shown in Fig. S1, ESI.† It is clearly seen that g-C3N4 (Fig. S1a†)
has a two-dimensional slice-like structure with many pores
and irregular shapes, which is consistent with the reference.29
The BiOCl nanoflowers were wrapped by the g-C3N4 sheets.
The lattice fringes with an interplanar distance corresponding
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estimated to be 3.21 and 2.75 eV, respectively. The UV-vis
spectra of the C3N4/BiOCl composites can be seen as the
overlap of the two components. Compared with that of BiOCl,
the absorption edge of the C3N4/BiOCl composites shows an
apparent red shift due to the presence of g-C3N4 on the BiOCl
surface, which means that the light response range of BiOCl is
extended further in the visible region by the addition of
g-C3N4. Additionally, the spectral range covered increases with
the increase of the g-C3N4 content, indicating more production
of the electron–hole pairs and the higher photocatalytic
performance.
In order to further determine the energy band structure of
C3N4/BiOCl composites, the valence band (VB) and the con-
duction band (CB) edge positions of BiOCl and g-C3N4 are esti-
mated by EVB = X − Ee + 0.5Eg, where EVB is the VB edge
potential, X is the absolute electronegativity of the semicon-
ductor, which is the geometric mean of the electronegativity of
the constituent atoms, Ee is the energy of free electrons on the
hydrogen scale (about 4.5 eV). The CB edge potential can be
obtained by ECB = EVB − Eg. Hence, the EVB of g-C3N4 and
BiOCl is 1.595 and 3.465 eV, respectively. The ECB of g-C3N4
and BiOCl is −1.155 and 0.255 eV, respectively.
Fig. 3 FT-IR spectra of g-C3N4, BiOCl and C3N4/BiOCl photocatalysts.
and a more widely conjugated system was composed of g-C3N4
and BiOCl.30,31 Furthermore, the intensity of the strong
absorption peaks at 527 cm−1 due to the Bi–O stretching mode
is correlated with their concentration.32
3.5 Photocatalytic activity
The photocatalytic performances of all the as-prepared C3N4/
BiOCl composites with different g-C3N4 concentrations are
evaluated by degradation of RhB which is chosen as the target
pollutant in aqueous solution. As can be seen from Fig. 5a,
RhB can be rarely degraded under visible light illumination in
the absence of the photocatalyst, indicating that RhB is stable
under experimental conditions. The degradation rate of RhB
in the presence of BiOCl reaches 99.5% in 30 min while g-
C3N4 only reaches 27% in 40 min. Although BiOCl cannot be
activated by visible-light for its wide band gap, it shows
superior degradation of RhB due to the self-sensitized degra-
dation activity. To evaluate the photocatalytic efficiency quanti-
tatively, the apparent photodegradation rate constants were
obtained by fitting the lines of ln(C0/C) vs. time. The photo-
catalytic degradation of organic pollutants follows the pseudo-
first-order kinetic, and the values of the apparent rate constant
k were calculated and are shown in Table 1. The introduction
of an appropriate amount of g-C3N4 could enhance the photo-
catalytic activity of BiOCl and the photocatalytic activity of
C3N4/BiOCl composites increases gradually with an increasing
proportion of g-C3N4. When the content of g-C3N4 reached
20 wt% it exhibits the maximum photocatalytic activity. Then
3.4 Optical and photocatalytic properties
The optical properties of the specimens were studied using a
UV-vis spectrometer. As reviewed from Fig. 4, the absorption
edge of BiOCl is located at about 370 nm while that of g-C3N4
is approximately 460 nm in the visible-light region. Based on
the equation (αhν)2/n = A(hν − Eg), where α, hν, A and Eg are,
respectively, the absorption coefficient, the photo energy, a
constant and the optical band gap. In this equation, n is deter-
mined by the type of optical transition of a semiconductor (n =
1 for direct transition and n = 4 for indirect transition). Accord-
ing to the previous reports, the n value of BiOCl is 4 and that
of g-C3N4 is 1. Thus, Eg of the as-prepared BiOCl and g-C3N4 is
apparent rate constant k for 20C3N4/BiOCl is 0.3532 min−1
,
which is about 3 times as high as that of BiOCl and 44 times
that of g-C3N4. Further increase in the g-C3N4 concentration
results in a significant decrease in the photocatalytic activity.
When the content of g-C3N4 reaches 30 wt%, the degradation
rate is lower than that of BiOCl, but is still higher than that of
g-C3N4. Therefore, the ratio of the C3N4/BiOCl composites is
pivotal for the demands of both the charge separation and
light harvesting to achieve enhanced photocatalytic activity.
The existence of the optimal ratio for the photocatalyst compo-
Fig. 4 UV-Vis spectra of g-C3N4, BiOCl and C3N4/BiOCl in different
compositions.
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capability was evaluated by the remaining concentration of the
dye stirring in the dark in the presence of BiOCl until it
reached the dye adsorption/desorption equilibrium. For BiOCl,
the adsorption capability contributes more than the conduc-
tion band condition to the high photosensitized degradation
rate.26 The adsorption amounts of 20C3N4/BiOCl and 20C3N4–
BiOCl toward different kinds of dyes are shown in Fig. S2,
ESI.† As expected, the adsorption of RhB over 20C3N4/BiOCl
with a larger specific surface area reaches 21.9%, which is
higher than that of 20C3N4–BiOCl. Another possible reason
related to the difference of adsorption capability may be due to
the surface characteristics. The surface anionic sites will
increase in the presence of AG, relying on the negative charge
of AG. Generally, RhB dyes exist in two forms: cationic form
and zwitterionic form. However, RhB dyes in this system may
be regarded as cationic due to the acidic environment. There-
fore, they tend to be adsorbed on the negatively charged sur-
faces. As a result, the adsorption capability of RhB of 20C3N4/
BiOCl is larger than that of 20C3N4–BiOCl. To validate the
above assumption, the anionic dye MO is chosen as a model
target dye and the two composite photocatalysts toward MO
are also shown in Fig. S2.† Evidently, compared with RhB, the
adsorption ability toward MO of 20C3N4/BiOCl is relatively
poor. In addition to the adsorption capability of dyes, the
contact between the two components is also an important
factor affecting the photocatalytic activity because the presence
of the interface is needed to allow the highly efficient interpar-
ticle charge transfer.33 The poor contact between the two semi-
conductors would retard the charge transfer and decrease the
separation efficiency. Therefore, the prominent performance
of 20C3N4/BiOCl could also be ascribed to the better
cooperation of BiOCl and g-C3N4. From the results discussed
above, the interface contact of 20C3N4/BiOCl is stronger than
that of 20C3N4–BiOCl. Although the C3N4/BiOCl composites
exhibit a higher photocatalytic activity, considering the practi-
cal applications, the stability of the photocatalytic performance
is also very important. The recycling experiments on 20C3N4/
BiOCl were carried out five times under the same reaction con-
ditions to evaluate the stability of the composites (as shown in
Fig. 5b). It can be seen that the photocatalytic activity of
20C3N4/BiOCl is retained at about 90% of its original activity
after five cycles under visible light irradiation, indicating its
superior stability during photocatalysis. Therefore, the C3N4/
BiOCl composites have a promising practical application in
environmental protection originating from its high activity and
stability.
Fig. 5 (a)Photodegradation of RhB over the as-prepared photocatalyst
and (b) cyclic photocatalytic degradation experiments of RhB by
20C3N4/BiOCl composite photocatalyst.
site is in agreement with the reports in the literature,27,28
which may be related to the synergetic effect between C3N4
and BiOCl. The suitable g-C3N4 content may favour it to bond
with the BiOCl to produce more interfaces needed to allow the
highly efficient interparticle charge transfer. In contrast, at
much higher g-C3N4 content, the soft sheet-shaped g-C3N4
would cover a larger surface of the BiOCl and shield visible
light off of BiOCl and RhB due to the relative g-C3N4 higher
content, inducing the decrease of photocatalytic activity. In
addition, the adsorption of RhB is the crucial factor for the
photocatalyst. Accordingly, the poor activity of 30C3N4/BiOCl
should also be attributed to the reduced adsorption of RhB (as
shown in Table 1).
3.6 Photocurrent–time measurements
The influences of AG on the photocatalytic activities of
C3N4/BiOCl composites were studied, and are also shown in
Table 1. As seen in Table 1, the apparent rate constant of
20C3N4/BiOCl is nearly two times higher than that of 20C3N4–
BiOCl. The prominent photocatalytic performance of 20C3N4/
BiOCl composites could be ascribed to two key points: the
high adsorption capability toward the contaminant molecules
and the cooperation of the components.30 The adsorption
Photocurrent–time measurements were performed with C3N4/
BiOCl and BiOCl electrodes, elucidating the interfacial charge
transfer dynamics between the interfacial surface of the BiOCl
and g-C3N4. Generally, the photocurrent is widely regarded as
the best evidence demonstrating the charge separation for the
photocatalysts. Fig.
6 shows the transient photocurrent
response of 20C3N4/BiOCl, 20C3N4–BiOCl, BiOCl and g-C3N4
measured by two on-and-off cycles of visible-light irradiation.
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Fig. 6 Transient photocurrent density versus time for g-C3N4, BiOCl,
20C3N4/BiOCl and 20C3N4–BiOCl photocatalysts.
It is obvious that the photocurrent response of both specimens
switch reversibly and reproducibly when the visible light is
turned on and off, indicating the high photophysical stability
of the specimens. Furthermore, as can be seen from Fig. 6, the
photocurrent intensities of 20C3N4/BiOCl and 20C3N4–BiOCl
are both higher than that of BiOCl and g-C3N4, indicating the
longer living photoinduced carriers and the suppression of the
charge recombination after the hybridization.31 It is note-
worthy that 20C3N4/BiOCl photocatalyst presents the highest
photocurrent intensity, which is higher than that of 20C3N4–
BiOCl, demonstrating a lower recombination and a more
efficient separation of photoinduced electron–hole pairs for
hybrid at the 20C3N4/BiOCl interfaces. The results are consist-
ent with photocatalytic testing results.
Fig. 7 (a) Photodegradation of RhB over 20C3N4/BiOCl under different
visible light irradiation (λ ≥ 420 nm and λ ≥ 532 nm) and (b) trapping
experiment of active species for RhB degradation over 20C3N4/BiOCl
under visible light irradiation.
3.7 Photocatalytic mechanism
decrease is observed with the addition of the hydroxyl radical
quencher (t-BuOH), indicating that the hydroxyl radicals are
not the main oxidative species of this photocatalyst. Therefore,
the superoxide radicals and holes would play a dominant role
in the visible-light-induced photocatalyst for the C3N4/BiOCl
composite system.
The photocatalytic decomposition of RhB using BiOCl g-C3N4,
and 20C3N4/BiOCl systems was also investigated under
different light irradiation. As depicted in Fig. 7a, the photo-
catalytic degradation for 20C3N4/BiOCl systems under λ ≥
532 nm irradiation is lower than that by λ ≥ 420 nm. As men-
tioned above, g-C3N4 can only be excited by the light with λ <
455 nm irradiation (as shown in Fig. 4). Hence, the photo-
catalytic activity of the g-C3N4/BiOCl composite system mainly
resulted from the BiOCl under λ ≥ 532 nm irradiation. Consid-
ering that BiOCl cannot be excited by visible light because of
its large energy band gap (3.21 eV), photosensitization would
play an important role in the photodegradation of the C3N4/
BiOCl composite system.
To investigate the photocatalytic behaviours of C3N4/BiOCl
photocatalysts, photoinduced holes, superoxide radical and
the hydroxyl radicals are selected for the oxidization of organic
pollutants in aqueous media to reveal the photocatalytic
mechanism. As can be seen from Fig. 7b, the photodegrada-
tion of RhB for 20C3N4/BiOCl photocatalyst is significantly
suppressed after the injection of a scavenger for superoxide
radicals (BQ) and holes (EDTA-2 Na). In contrast, only a slight
On the basis of experimental and theoretical results, a pro-
posed mechanism for the enhanced C3N4/BiOCl photocatalyst
is shown as below (as shown in Scheme 1): when the compo-
site photocatalyst is illuminated by visible light, BiOCl cannot
be excited by visible light due to its large energy band gap.
Nevertheless, both RhB and g-C3N4 would be excited and gene-
rate the photoinduced electron–hole pairs. Because the CB
edge potential of g-C3N4 is more negative than that of BiOCl,
the photoinduced electrons on the g-C3N4 surfaces get easily
transferred to the CB of BiOCl through the closely contacted
interfaces, leaving the holes on the VB of g-C3N4. On the other
hand, RhB adsorbed on the surface of BiOCl is photosensi-
tized. Photoexcited electrons from the LUMO of RhB would
also be injected into the CB of BiOCl, since the LUMO levels
of RhB is more negative than the CB edge potential of BiOCl.
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activity of the as-prepared samples under visible light. The
C3N4/BiOCl composite with proper proportion showed excel-
lent performance and 20C3N4/BiOCl possessed the highest
photocatalytic activity. The synergistic effect between C3N4 and
BiOCl is elucidated, as proven by photocurrent measurement.
Based on the experimental and theoretical results, a possible
photodegradation mechanism relating to the combination of
photosensitization and photocatalyst is proposed. It is expected
that our present work could provide useful information for
the fabrication of other heterostructured nanocomposites.
Acknowledgements
We acknowledge the financial support from the National
Science Foundation of China (A3 Foresight Project no.
51161140399), and the Fundamental Research Funds for the
Central Universities (WUT: 143201002).
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