Rational design direct Z-scheme BiOBr/g-C3N4 heterojunction with enhanced visible photocatalytic activity for organic pollutants elimination

Article abstract of DOI:10.1039/c9ra10146b

A rapid recombination of photo-generated electrons and holes, as well as a narrow visible light adsorption range are two intrinsic defects in graphitic carbon nitride (g-C₃N₄)-based photocatalysts. Inspired by natural photosynthesis, an artificially synthesized Z-scheme photocatalyst can efficaciously restrain the recombination of photogenerated electron-hole pairs and enhance the photoabsorption ability. Hence, to figure out the above problems, BiOBr/g-C₃N₄ composite photocatalysts with different mass ratios of BiOBr were successfully synthesized via a facile template-assisted hydrothermal method which enabled the BiOBr microspheres to in situ grow on the surface of g-C₃N₄ flakes. Furthermore, to explore the origin of the enhanced photocatalytic activity of BiOBr/g-C₃N₄ composites, the microstructure, photoabsorption ability and electrochemical property of BiOBr/g-C₃N₄ composites were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), electrochemical impedance spectroscopy (EIS) and photocurrent (PC) response measurements. As a result, the introduction of BiOBr on g-C₃N₄ to constitute a direct Z-scheme heterojunction system can effectively broaden the light absorption range and promote the separation of photo-generated electron-hole pairs. Hence, compared with pure g-C₃N₄ and BiOBr, the resultant BiOBr/g-C₃N₄ composites exhibit the remarkable activity of photodegradated rhodamine B (RhB) and tetracycline hydrochloride (TC-HCl) under visible light irradiation. Simultaneously, the optimal BiOBr content of the BiOBr/g-C₃N₄ composites was obtained. The BiOBr/g-C₃N₄ composites exhibit an excellent photostability and reusability after four recycling runs for degradation RhB. Moreover, the active-group-trapping experiment confirmed that ·OH, ·O₂^- and h⁺ were the primary active groups in the degradation process. Based on the above research results, a rational direct Z-scheme heterojunction system is contrastively analyzed and proposed to account for the photocatalytic degradation process of BiOBr/g-C₃N₄ composites.

Full text of DOI:10.1039/c9ra10146b

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Rational design direct Z-scheme BiOBr/g-C3N4
heterojunction with enhanced visible
photocatalytic activity for organic pollutants
elimination
Cite this: RSC Adv., 2020, 10, 4681
*
Hongfei Li, Aiqiong Ma,
Dian Zhang, Yunqin Gao and Yonghao Dong
A rapid recombination of photo-generated electrons and holes, as well as a narrow visible light adsorption
range are two intrinsic defects in graphitic carbon nitride (g-C3N4)-based photocatalysts. Inspired by natural
photosynthesis, an artificially synthesized Z-scheme photocatalyst can efficaciously restrain the
recombination of photogenerated electron–hole pairs and enhance the photoabsorption ability. Hence,
to figure out the above problems, BiOBr/g-C3N4 composite photocatalysts with different mass ratios of
BiOBr were successfully synthesized via a facile template-assisted hydrothermal method which enabled
the BiOBr microspheres to in situ grow on the surface of g-C3N4 flakes. Furthermore, to explore the
origin of the enhanced photocatalytic activity of BiOBr/g-C3N4 composites, the microstructure,
photoabsorption ability and electrochemical property of BiOBr/g-C3N4 composites were investigated by
X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron
microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), electrochemical impedance
spectroscopy (EIS) and photocurrent (PC) response measurements. As a result, the introduction of BiOBr
on g-C3N4 to constitute a direct Z-scheme heterojunction system can effectively broaden the light
absorption range and promote the separation of photo-generated electron–hole pairs. Hence,
compared with pure g-C3N4 and BiOBr, the resultant BiOBr/g-C3N4 composites exhibit the remarkable
activity of photodegradated rhodamine B (RhB) and tetracycline hydrochloride (TC-HCl) under visible
light irradiation. Simultaneously, the optimal BiOBr content of the BiOBr/g-C3N4 composites was
obtained. The BiOBr/g-C3N4 composites exhibit an excellent photostability and reusability after four
recycling runs for degradation RhB. Moreover, the active-group-trapping experiment confirmed that
Received 4th December 2019
Accepted 3rd January 2020
$OH, $O2 and h+ were the primary active groups in the degradation process. Based on the above
ꢀ
DOI: 10.1039/c9ra10146b
research results, a rational direct Z-scheme heterojunction system is contrastively analyzed and
proposed to account for the photocatalytic degradation process of BiOBr/g-C3N4 composites.
rsc.li/rsc-advances
utilization visible light to excite semiconductor materials to
produce highly efficient active groups.6,7 Nevertheless, most
1. Introduction
semiconductor photocatalysts with a wide band gap can merely
utilize the ultraviolet light approximately, which accounts for
only 4% of the solar energy.8,9 Although CdS,10 BiVO4,11
Ag3PO4,12 etc. with relatively narrow band gaps possess a visible
light photocatalytic activity, they usually suffer from a rapid
recombination between excited electrons and holes during light
irradiation.13 As a consequence, it is attractive and signicant to
design and develop photocatalytic materials with the applicable
band gaps, wide visible-light adsorption ranges and highly
photocatalytic activity.14
Graphitic carbon nitride (g-C3N4), a familiar metal-free
semiconductor photocatalyst, has been frequently investigated
because of its numerous excellent physicochemical traits, such
as outstanding thermology, electricity and photology proper-
ties.15 Such performances result mainly from its stratiform
With the development of dye and pharmaceuticals industry,
organic pollutants of industrial emission have given rise to
severe water pollution which could constitute a bad threat to the
living environment of humanity. Thereinto, aromatic
compounds with diffusely conjugated chromophores, such as
RhB and TC-HCl, possess a physicochemical stability in water
environment such that the traditional treatment measures
hardly prove effective. Therefore, plenty of new technologies
which can effectively decompose organic pollutants through
redox reactions have emerged continuously.1–3 Semiconductor
photocatalytic degradation technology has attracted a wide
research interest,4,5 due to its unique advantages of the
College of Materials Science and Engineering, Xi'an University of Architecture and
Technology, Xi'an, Shaanxi 710055, China. E-mail: maaiqiong@xauat.edu.cn
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structure of strong covalent C–N bonds in each layer and weak
van der Waals force between layers.16,17 The g-C3N4 with
a narrow band gap (2.7 eV) possesses an appropriate conduction
band (CB) position (ECB ¼ ꢀ1.07 eV), which is more negative
than the reduction potential of E0(O2/$O2 ) (about ꢀ0.33 eV).18
ꢀ
Therefore, the photo-generated electrons on the CB of g-C3N4
are capable of reacting with O2 to generate superoxide radicals
($O2ꢀ) which can subsequently participate in the process of
degradation of organic pollutants. Nevertheless, on account of
the weak van der Waals interactions between adjacent CN layers
in the g-C3N4 crystal structure, g-C3N4 has high recombination
Fig. 1 Abridged summarization of the synthesis process of the BiOBr/
odds of photo-generated electron–hole pairs which lessens the g-C3N4 composites.
quantum efficiency of the photocatalytic degradation proce-
dures.19,20 Therefore, for further improving the inherent short-
comings of g-C3N4 and enhancing its photocatalytic efficiency,
several research works have been carried out and showed that
combination with metal-oxide semiconductors to construct
composite photocatalyst can remarkably enhance the absorp-
tion ability of sunlight and prolong the lifetime of the photo-
generated carrier.21
Recently, BiOBr, as one of the highly active bismuth oxy-
halide (BiOX, X ¼ Cl, Br, I) photocatalytic materials, has
received intensive attention in the photocatalytic eld. It
exhibits a high photocatalytic activity on account of its suitable
band gap (2.81 eV) and wide visible light adsorption range (>410
nm).22 It is noteworthy that its unique lamellar crystal structure,
just as the tetragonal matlockite structure, can afford adequate
space to polarize the relevant atoms and orbitals, resulting in
the generation of an internal electric eld between the [Bi2O2]2+
layer and the Brꢀ layer.23 On the basis of previous reports, BiOBr
was used to combine with g-C3N4 to form an indirect Z-scheme
heterojunction and showed excellent photocatalytic activity. In
2019, Zhang Mingming et al. prepared an indirect Z-scheme
L-lysine with the polar functional groups of amino and
hydroxyl, as a special surfactant, was used for regulating the
microstructure of BiOBr microspheres so as to increase the
efficacious active sites on the BiOBr surface.25–27 To the best of
our knowledge, only a few researchers have focused on direct Z-
scheme BiOBr/g-C3N4 composites synthesized by a template-
assisted hydrothermal method and the number of efficient
direct Z-scheme photocatalysts is still nite.
In this study, a direct Z-scheme BiOBr microsphere/g-C3N4
ake heterojunction photocatalysts was successfully synthe-
sized via a facile template-assisted hydrothermal method which
enabled BiOBr microspheres to in situ grow on the surface of g-
C3N4 akes (Fig. 1). Through exploring the microstructure,
optical and electrochemical properties of the composites, the
BiOBr/g-C3N4 composites exhibit
a higher photocatalytic
activity proved by the degradation of RhB and TC-HCl, which
could be attributed chiey to the formation of a direct Z-scheme
heterojunction system. Ultimately, a reasonable photocatalytic
mechanism of the BiOBr/g-C3N4 composites for the degradation
of RhB is contrastively analyzed and proposed by free radical
trapping experiments.
BiOBr/CDs/g-C3N4 photocatalyst by
a facile hydrothermal
method and the hybrids can degrade 17.5% of tetracycline (TC)
within 180 min for which the reaction rates were 31.92 and 9.76
times those of pristine BiOBr and g-C3N4, respectively.24 Hence,
this indirect Z-scheme-type composite shows remarkable
2. Experimental
potential as an efficient photocatalyst for the utilization of solar 2.1. Chemicals
energy to decompose contaminants. To further improve the
All the chemicals were used as received without further purica-
photocatalytic performance of such a Z-scheme BiOBr/g-C3N4
system, a high-efficiency direct Z-scheme type photocatalyst is
proposed in which the photo-induced electrons can reduce
unnecessary loss during transfer process by direct contact
between BiOBr and g-C3N4. Furthermore, for the past few years,
BiOBr has been synthesized via different methods, such as
solvothermal, chemical precipitation, hydrothermal and so on.
Among the different synthesis methods, a BiOBr microsphere
tion. Bismuth nitrate pentahydrate (Bi(NO3)3$5H2O) was acquired
from Tianjin Kemiou Chemical Reagent Co. (Tianjing, China).
Sodium bromide (NaBr) was acquired from Wuhan Tiantai
Chemical Co. (Wuhan, China). Ethylene glycol ((CH2OH)2),
sodium hydroxide (NaOH), ethyl alcohol absolute (C2H6O) and
ammonium chloride (NH4Cl) were purchased from Tianjin Tianli
Chemical Reagent Co. (Tianjing, China). Tetracycline hydrochlo-
ride (TC-HCl, C22H24N2O8$HCl) was purchased from Shanghai
Macklin Biochemical Technology Co. (Shanghai, China).
structure obtained by
a hydrothermal method reveals
a remarkably photocatalytic activity thanks to its abundant
porous structures providing BiOBr with sufficient active sites for
the elimination of organic pollutants. To improve the micro-
2.2. Synthesis of g-C3N4 akes
structure on the basis of BiOBr microspheres, a facile template- Fig. 1 illustrates the synthesis procedure of the BiOBr/g-C3N4
assisted hydrothermal method was selected for the synthesis of composites and the particular processes were as follows: 10 g
BiOBr microspheres. Among the precursor solutions, ethylene melamine was sintered at 550 ꢁC with a heating rate of
glycol was used for prompting BiOBr nanosheets to self-
assemble into microspheres. Simultaneously, the bio-template obtained aer cooling down to room temperature. Aer 4.5 g of
3
ꢁC minꢀ1 in a muffle furnace for 4 h. The bulk-g-C3N4 was
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NH4Cl particles were dissolved in 50 mL of deionized water, 6 g under a 500 W Xe-lamp with a cut off lter (l $ 420 nm). Briey,
of the obtained bulk-g-C3N4 was added in solution under the process was performed through immersing 50 mg of the as-
vigorous stirring and sonicated for 2 h. Thereaer, the prepared samples into 50 mL of RhB (TC-HCl) at an initial
suspension was dried at 80 ꢁC in a drying oven overnight. concentration of 15 mg Lꢀ1 (20 mg Lꢀ1). Before lighting up the
Aerwards, the obtained dry g-C3N4 was sintered at 550 ꢁC once illuminant, the adsorption–desorption equilibrium was ach-
again with a heating rate of 3 ꢁC minꢀ1 in a muffle furnace for ieved by continuously stirring the suspensions in the dark for
4 h. The akes of g-C3N4 were obtained aer cooling down to 40 min. At a certain time interval, 4 mL of the suspension was
room temperature.
fetched and centrifuged to obtain a puried liquid. Finally, the
RhB and TC-HCl concentrations were detected via the UV-vis
spectrophotometer (UV-1200, Macy) at an absorption wave-
length of 554 nm for RhB and 356 nm for TC-HCl. The stability
of the B-20/CN composites was evaluated by four successive
cycling experiments.
2.3. Synthesis of BiOBr/g-C3N4 nanocomposites
In a peculiar template-assisted hydrothermal process (Fig. 1),
0.16 mmol Bi(NO3)3$5H2O and 0.16 mmol NaBr were dissolved
into 50 mL of ethylene glycol solution and magnetically stirred
for 30 min to form a clear solution. Then, 0.16 mmol L-lysine
was dispersed into the above mixture solution and stirred for
30 min, followed by adding 0.95 g of as-prepared g-C3N4 akes
under vigorous stirring and sonicated for 2 h. Subsequently, the
pH value of the mixture was adjusted to 5 by adding NaOH
solution under constant stirring. Aer the suspension was
transferred to a Teon-lined stainless-steel autoclave and
maintained at 120 ꢁC for 12 h, the resultant suspension was
respectively washed with deionized water and ethyl alcohol
three times. The wet products were dried at 80 ꢁC to obtain the
BiOBr/g-C3N4 composite with a BiOBr mass ratio of 5%. BiOBr/
g-C3N4 nanocomposites with different mass ratios of BiOBr
(5%, 10%, 15%, 20% and 25%) were synthesized similarly by
tuning the dosage of BiOBr. The corresponding BiOBr/g-C3N4
samples are abbreviated as B-5/CN, B-10/CN, B-15/CN, B-20/CN
and B-25/CN. For comparison, pure BiOBr and g-C3N4 photo-
catalysts were prepared under the same conditions.
3. Results and discussion
3.1. Crystal structures of the samples
Fig. 2 exhibits XRD patterns of all the synthesized samples. Pure
g-C3N4 shows distinct diffraction peaks at approximately 27.5ꢁ,
corresponding to the (002) planes of g-C3N4 (JCPDS no. 87-
1526). The diffraction peaks at 2q ¼ 10.9ꢁ, 25.2ꢁ, 32.2ꢁ, 40.6ꢁ,
46.2ꢁ, and 57.2ꢁ correspond to the (001), (101), (110), (112),
(200), and (212) crystal planes of BiOBr (JCPDS No. 09-0393).
Moreover, no other diffraction peaks were detected, revealing
the high purity and single phase of the as-obtained samples.
However, no other characteristic peaks of BiOBr were detected
in B-5/CN except diffraction peaks at 2q ¼ 25.2ꢁ and 32.2ꢁ,
owing to the low BiOBr content in the B-5/CN composite. With
the increase of BiOBr content, the characteristic peaks intensity
of BiOBr also have increases of varying degree. Meanwhile, the
characteristic peak intensities of g-C3N4 gradually weaken
thanks to the decrease of g-C3N4 content. The strong diffraction
peaks correspond with g-C3N4 and BiOBr, which indicates that
the BiOBr/g-C3N4 composites were successfully synthesized
through the facile template-assisted hydrothermal method.
2.4. Characterization
The samples were characterized by X-ray diffraction (XRD) (D/
max-2500, Rigaku), eld emission scanning electron micros-
copy (FESEM) (Gemini SEM 300, Zeiss) and transmission elec-
tron microscope (TEM) (H-7650, Hitachi). Optical diffuse
reectance spectra (DRS) were measured by using UV-vis DRS
(CARY 5000, Agilent).
3.2. Morphology characteristics of the samples
The microstructures and morphology of the as-prepared
samples were observed by FESEM and TEM, and the results
are displayed in Fig. 3. As can be seen from the FESEM image of
pure g-C3N4 in Fig. 3a, the pure g-C3N4 synthesized by simple
polymerization and gas phase methods consists of thin akes
The electrochemical impedance spectroscopy (EIS) and
photocurrent (PC) responses measurements were performed by
using an electrochemical workstation (CHI660E, CH Instru-
ments Ins.) with a standard three-electrode conguration at
room temperature. The as-prepared sample, a Pt wire and Ag/
AgCl were used as the working electrode, counter electrode
and reference electrode, respectively. The working electrode was
prepared by a uorinated tin oxide (FTO) sheet glass with an
active area of 1 ꢂ 1 cm2 containing 10 mg of photocatalyst and
ꢁ
ultimately calcined at 105 C for 2 h. The EIS experiment was
executed in an aqueous 0.5 M Na2SO4 solution under dark
conditions. For the PC measurement, a 500 W Xe lamp and
Na2SO4 (0.5 M) aqueous solution served as the light source and
electrolyte, respectively.
2.5. Photocatalytic activity
The photocatalytic activities of the as-prepared samples were
Fig. 2 XRD patterns of (a) pure BiOBr, (b) pure g-C3N4, (c) B-5/CN, (d)
evaluated by means of the degradation of the RhB solutions B-10/CN, (e) B-15/CN, (f) B-20/CN and (g) B-25/CN.
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with an approximate thickness of 200 nm. The small thickness physical junction between the BiOBr and g-C3N4 semi-
of g-C3N4 akes is conducive to the separation of photo- conductors that unquestionably will accelerate the generation
generated carriers and the increase of built-in electric eld and separation of photo-generated carriers due to the direct Z-
intensity perpendicular to the lamellae.28 From Fig. 3b, BiOBr scheme heterojunction structure of the composites.29,30
microspheres are hardly detected on the surface of g-C3N4 akes Furthermore, TEM images of template-free BiOBr and BiOBr
on account of the lower BiOBr content of the B-5/CN sample of prepared by using a bio-template are shown in Fig. 3h and i,
than others. As you can see from Fig. 3c–f, BiOBr microspheres reecting the effect of a bio-template on the microsphere
are randomly dispersed on the surface of the g-C3N4 akes and structures within the BiOBr photocatalyst. Obviously, BiOBr
between the g-C3N4 akes. With the increase of BiOBr content, microspheres consist of vast BiOBr nanosheets via a self-
the number of BiOBr microspheres on the surface of the g-C3N4 assembly process. Meanwhile, the bio-template L-lysine25–27
akes gradually multiplies. Furthermore, from the FESEM with polar functional groups of amino and hydroxyl, as a special
image of B-20/CN in Fig. 3e, a mass of BiOBr microspheres surfactant, can selectively adsorb on specic crystal faces and
existed between the g-C3N4 akes which provided multiple control the crystal growth and self-assembly process of BiOBr.
cellular structures for the BiOBr/g-C3N4 composites. Such Compared with template-free BiOBr (Fig. 3h), the internal
structures are convincing evidence that the B-20/CN sample structure of BiOBr microspheres (diameter: about 400 nm)
possesses a better photoabsorption performance than the other prepared by using the bio-template (Fig. 3i) shows more loose
samples in below UV–vis DRS spectra.
structures and a further exposure degree of the BiOBr nano-
A TEM image of B-20/CN is shown in Fig. 3g, providing sheets in order to enhance the amount of active sites for the
a more distinct observation about the two components. The degradation of organic pollutants.
darker area with a spherical shape ought to be BiOBr and the
lighter area should be g-C3N4, which further testies the good
3.3. UV-vis DRS and band gap analysis
dispersion of BiOBr microspheres on the g-C3N4 akes and the
direct contact between the two components. Homogeneous
dispersion of the BiOBr microspheres constructs an effective
The optical properties of the as-prepared samples with different
BiOBr content were evaluated by UV-vis DRS. From Fig. 4a, the
Fig. 3 FESEM images of (a) pure g-C3N4, (b) B-5/CN, (c) B-10/CN, (d) B-15/CN, (e) B-20/CN, (f) B-25/CN, TEM images of (g) B-20/CN, (h)
template-free BiOBr and (i) BiOBr prepared by using bio-templates.
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light absorption edges of g-C3N4 and BiOBr are approximately electron–hole pairs. The EIS spectra of pure BiOBr, pure g-C3N4
465 and 428 nm, respectively. Aer g-C3N4 combines with and the B-20/CN composite are shown in Fig. 4c. The diameter
BiOBr, the light absorption edge of the B-5/CN composite has of the EIS Nyquist plot of B-20/CN is smaller than those of pure
red-shied to 470 nm, which indicates that the BiOBr micro- BiOBr and g-C3N4. Given that the diameters on the EIS spectra
spheres are likely to settle onto the surface of g-C3N4 akes. It's reveal the reaction rate at the surface of the electrode, the plots
worth noting that the photoabsorption performance of the in Fig. 4c display that B-20/CN possesses a lower resistance than
blended samples seems stronger than those of pure g-C3N4 and pure BiOBr and g-C3N4. It's worth noting that lower resistance is
BiOBr in the range of 470–495 nm. This phenomenon can be conducive to accelerating the interfacial charge transfer and
ascribed to the irregular cellular structure of BiOBr micro- separating photo-generated electron–hole pairs.35 Fig. 4d
spheres scattering and reecting light, as well as the strong reveals the photoelectric conversion ability for pure g-C3N4,
interfacial interaction between BiOBr and g-C3N4 in the BiOBr/ pure BiOBr and the B-20/CN electrodes. As is well-known,
g-C3N4 hetero-phase junction.31
during the electron transfer from the CB of samples to the
Fine approximations of the band gaps of BiOBr and g-C3N4 electrodes, the photocurrent is simultaneously generated.
semiconductors are provided in Fig. 4b. From the spectra, the Generally speaking, the sample with a higher photocurrent
band gaps (Eg) of g-C3N4 and BiOBr are 2.50 and 3.05 eV, signies the presence of a longer lifetime of the photo-
respectively. Furthermore, the conduction band (CB) and generated electrons and holes which enhances the photo-
valence band (VB) edge redox potentials of g-C3N4 and BiOBr catalytic activity.36 As is shown in Fig. 4d, high-frequency and
can be estimated on the basis of the extensively accepted consistent photocurrent responses are visibly observed for
equations:
each switch-on and switch-off event in the electrodes. The
photocurrent of the B-20/CN electrode is higher than those of
the pure g-C3N4 and BiOBr electrodes. This conclusion is in
accordance with the EIS analyses, and explicitly indicates that
the introduction of BiOBr into g-C3N4 can effectively promote
the separation of photogenerated electron–hole pairs.
ECB ¼ X ꢀ Ee ꢀ Eg/2
EVB ¼ ECB + Eg
(1)
(2)
The X values for BiOBr and g-C3N4 are 6.18 eV and 4.73 eV,
respectively.
32,33
E
and ECB stand for the VB and CB potentials,
VB
respectively, and Ee is the energy of free electrons on the 3.5. Photocatalytic activity
hydrogen scale (about 4.5 eV).34 According to eqn (1) and (2), the
ECB and EVB of g-C3N4 are ꢀ1.02 and +1.48 eV, respectively.
Furthermore, the ECB and EVB of BiOBr are +0.16 and +3.21 eV,
respectively.
The photocatalytic activity of all the semiconductor photo-
catalysts was measured by degrading RhB under visible light
irradiation. Before irradiation, the adsorption–desorption
equilibrium between the RhB solution and photocatalyst was
attained by magnetic stirring for 40 min under dark conditions.
From Fig. 5a, it can be observed that the direct photolysis of
RhB is negligible without the addition of a photocatalyst. Pure
g-C3N4 possesses a relatively lower photocatalyst activity than
those of the other BiOBr/g-C3N4 composites, and only 53.2% of
RhB was degraded within 120 min. In addition, the pure BiOBr
has a relatively higher adsorptive property and photocatalyst
activity than those of pure g-C3N4 due to the special structure of
the microspheres, and 59.9% of RhB was degraded within
120 min. With an incremental BiOBr content, the photocatalytic
efficiency of the composites was gradually improved due to the
formation of direct Z-scheme heterostructures and the resulting
promotion of light absorption properties in the visible light
range.37 The B-20/CN composite with an optimal photocatalytic
efficiency can decompose 97.1% of RhB. Nonetheless, while the
mass ratio of BiOBr was higher than 20%, the photocatalytic
efficiency of B-25/CN decreased by 11.5% for the degradation of
RhB. This phenomenon can be attributed to the excessive BiOBr
covering the active sites of g-C3N4, leading to a low photo-
catalytic performance.
3.4. EIS analysis and photocurrent response measurements
EIS analysis and photocurrent response measurements were
applied to testify the high efficiency of direct Z-scheme BiOBr/g-
C3N4 hybrids in facilitating the segregation of photogenerated
Moreover, aiming to intensively research the kinetics curves
of the photodegradation of RhB, the experimental data were
applied to a pseudo-rst-order model:
ꢀ
ꢁ
Fig. 4 (a) DRS spectra; (b) plot of (ahn)2 vs. hn; (c) EIS Nyquist plot and
(d) transient photocurrent response of the as-prepared samples with
light on/off cycles.
C0
Ct
ln
¼ kt
(3)
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Fig. 5 (a) Photocatalytic activity for the degradation of RhB; (b) the relative pseudo-first-order kinetic plots; (c) UV-vis spectral curves of RhB
degradation over B-20/CN; (d) photocatalytic activity for the degradation of TC-HCl; (e) the relative pseudo-first-order kinetic plots of the as-
prepared samples; (f) four recycling runs of B-20/CN for the degradation of RhB; (g) the XRD patterns of B-20/CN samples before and after the
photocatalytic degradation experiments for RhB and (h) the effects of various scavengers on the degradation efficiency of B-20/CN toward RhB.
where C0 and Ct are the concentrations of the RhB solution at solution over B-20/CN every 30 min. As can be seen from the
times 0 and t, respectively, and k is the pseudo-rst-order rate spectra, the absorption peak intensity of the RhB solution
constant (minꢀ1). From Fig. 5b and Table 1, the degradation of gradually reduces with the extension of irradiation time and
B-20/CN is 0.02427 minꢀ1, which is 3.97 and 3.14 times higher tends to be gentle aer 120 min. The corresponding solution
than those for pure g-C3N4 and BiOBr, respectively. The result color gradually fades away.
further illustrates that BiOBr coupled with g-C3N4 can enhance
To shield against specic degradation of the photocatalysts,
the transfer and separation efficiency of photo-generated TC-HCl, regarded as an antibiotic residue, was applied to eval-
carriers, and thus increase the photocatalytic activity.38 Fig. 5c uate the photocatalytic efficiency of the as-prepared samples
displays the evolution of the absorption spectra of the RhB under visible light irradiation.39 From Fig. 5d, it can be observed
that the direct photolysis of TC-HCl is negligible without the
photocatalysts. Not surprisingly, the B-20/CN sample exhibits
the highest photodegradation efficiency for TC-HCl, which can
Table 1 The kinetics of the degradation reaction under visible light
reach 81.1% within 150 min. With the increasing BiOBr
irradiation
content, the photocatalytic efficiency of the composites was
gradually promoted compared with pure g-C3N4 and BiOBr.
However, for the BiOBr content higher than 20%, a further
augmentation of the BiOBr content leads to a decrease in the
photocatalytic efficiency of TC-HCl degradation. The photo-
degradation process of TC-HCl accords with a pseudo-rst order
model (Fig. 5e). The degradation rate of B-20/CN is 1.87, 1.57,
1.53, 1.39, 1.08 and 1.11 times higher than those for pure g-
C3N4, pure BiOBr, B-5/CN, B-10/CN, B-15/CN and B-25/CN,
respectively (Table 1). The above conclusion testies that the
Degradation rate (Vis)
Samples
RhB (minꢀ1
)
TC-HCl (minꢀ1
)
Pure g-C3N4
Pure BiOBr
B-5/CN
B-10/CN
B-15/CN
B-20/CN
B-25/CN
0.0061
0.0077
0.0018
0.0044
0.0128
0.0242
0.0139
0.0063
0.0075
0.0077
0.0088
0.0109
0.0118
0.0106
4686 | RSC Adv., 2020, 10, 4681–4689
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B-20/CN composite possesses a great potential to decompose C3N4 can transmit to the CB of BiOBr and the h+ from VB of
antibiotics from the sewage of the pharmaceuticals industry. BiOBr can simultaneously transfer to the VB of g-C3N4. As
As shown in Fig. 5f, in order to evaluate the stability of the as- a result, the eꢀ and h+ may respectively concentrate on the CB of
prepared photocatalyst, B-20/CN was selected for a recycling test BiOBr and the VB of g-C3N4, which effectually restrains the
to degrade RhB under visible light irradiation. Aer four recy- recombination of eꢀ and h+. Nevertheless, during the migration
cling runs, the degradation efficiency diminished from 97.1% to of eꢀ/h+ to more positive/negative energy levels, the reducing
95.3%, indicating that the B-20/CN composite exhibits an capacity of the photo-generated electrons and oxidizing capacity
excellent photocatalysis stability. Furthermore, to further testify of the photo-generated holes will be subject to inevitable loss.44
the stability of the B-20/CN photocatalyst, the XRD pattern aer
Obviously, in the direct Z-scheme BiOBr/g-C3N4 hetero-
the photocatalytic degradation experiments for RhB was junction structure (Fig. 6b), the contact surface between BiOBr
measured.35 The result (Fig. 5g) exhibits that the crystal struc- and g-C3N4 serves as a charge transmission bridge to facilitate
ture did not change during the photocatalytic degradation of annihilation of electron–hole pairs with a weaker oxidation–
RhB. Hence, the B-20/CN composite has a remarkable stability reduction ability, which efficiently gures out the above
and recyclability.
deciency.45,46 According to energy band theory, the photo-
To illustrate the photocatalytic mechanism of the B-20/CN generated eꢀ in the CB of BiOBr can quickly transfer to the
photocatalyst, the primary active groups generated in the pho- contact surface through the Schottky barrier47,48 and recombine
todegradation process were detected via an active-groups- with h+ on the VB of g-C3N4, leaving the eꢀ/h+ with a stronger
trapping experiment. In this experiment, tertiary butanol oxidation-reduction ability in the CB/VB of g-C3N4/BiOBr.
(TBA), benzoquinone (p-BQ) and ammonium oxalate (AO) Moreover, the CB potential of g-C3N4 is more negative than
ꢀ
served as the scavengers for $OH, $O2 and h+, respectively.40 E0(O2/$O2ꢀ), which means the eꢀ on the CB of g-C3N4 can more
ꢀ
49,50
Fig. 5h demonstrates the effects of the three scavengers on the easily reduce O2 to $O2
.
Meantime, the VB potential of
photodegradation process of RhB. As TBA, p-BQ and AO were BiOBr is more positive than E0(H2O/$OH), so the h+ in the VB of
added into the photocatalytic process, the photocatalytic BiOBr is capable of oxidizing H2O to $OH. The active groups,
ꢀ
degradation efficiency of B-20/CN was reduced from 97.1% to such as $O2 and $OH, are able to directly react with RhB to
53.4%, 75.4% and 83.3%, respectively. The results indicate that produce CO2, H2O or intermediate products.51,52 The procedure
the $OH, $O2ꢀ and h+ play major roles in the degradation course can be depicted as follows:
of RhB.
visible light
ꢀ
BiOBr
h
þ þ eCB
þ þ eCB
ƒƒƒƒ!
VB
visible light
ꢀ
g-C N
3
h
VB
ƒƒƒƒ!
4
ꢀ
eCBꢀ þ O2/$O2
3.6. Photocatalytic mechanism
þ
hVB þ H2O/$OH þ Hþ
According to the free radical trapping experiments, as well as
optical and electrochemical analysis, a possible photocatalytic
mechanism of BiOBr/g-C3N4 photocatalyst is elaborated in
Fig. 6. Followed by BiOBr and g-C3N4 being stimulated under
visible light irradiation, photoexcited electrons (eꢀ) and holes
(h+) are located in their VB and CB, respectively. For BiOBr/g-
C3N4, the transfer process of the photo-generated of eꢀ and h+
can be classied as a type II heterojunction structure (Fig. 6a).41
Chiey, the VB potential of g-C3N4 (+1.48 eV) is more negative
than that of BiOBr (+3.21 eV) and the CB potential of g-C3N4
(ꢀ1.02 eV) is more negative than that of BiOBr (+0.16 eV).42
Secondly, since the band gap of g-C3N4 (Eg ¼ 2.50 eV) is nar-
rower than that of BiOBr (Eg ¼ 3.05 eV), the VB of g-C3N4 is likely
to generate more carriers than BiOBr under visible light irra-
diation.43 For the above two reasons, the eꢀ from the CB of g-
$OH or hVBþ þ RhB/H2O þ CO2 or intermediate products
The deduction is well in accord with the above conclusion of
the radical trapping experiments that the $O2ꢀ, $OH and h+ are
the primary oxidation active groups for the degradation of RhB.
4. Conclusions
In summary, a direct Z-scheme BiOBr/g-C3N4 heterojunction
system has been successfully constructed via a facile template-
assisted hydrothermal method. Compared with pure g-C3N4
and BiOBr, the BiOBr/g-C3N4 composite with a BiOBr mass ratio
of 20% (B-20/CN) exhibits a more remarkable photocatalytic
performance for the degradation of RhB and TC-HCl. In such
a direct Z-scheme system, the intimate synergistic interactions
between BiOBr and g-C3N4, the signicant extension of the light
absorption edge, as well as the segregation of the strong
oxidation–reduction capacity of eꢀ and h+ lead to the remark-
able photocatalytic activity of BiOBr/g-C3N4 comꢀposites. The
trapping experiments manifest that the $OH, $O2 and h+ are
the main active groups for the degradation of RhB. Moreover,
the as-prepared B-20/CN composite exhibits excellent photo-
stability and reusability aer 4 runs under the same conditions.
This work is supposed to open up new insights into the struc-
Fig. 6 Schematic of charge carriers transfer in (a) BiOBr/g-C3N4
heterojunction and (b) direct Z-scheme BiOBr/g-C3N4 heterojunction. tural design of novel direct Z-scheme photocatalysts with a high
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Conflicts of interest
23 L. D. Cao, D. K. Ma, Z. L. Zhou, C. L. Xu, C. Cao, P. Y. Zhao
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The authors declare that there are no conicts of interest.
Acknowledgements
¨
¨
¨
This work was supported by the Natural Science Foundation of 25 P. Alam, P. Fagerlund, P. Hagerstrand, J. Toyryla, S. Amini,
Shaanxi Province, China [grant no. 2020318029].
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