Realizing the regulated carrier separation and exciton generation of Bi24O31Cl10: Via a carbon doping strategy

Article abstract of DOI:10.1039/c8ta08598f

The insufficient efficiency of carrier separation and faint exciton generation are the major limitations of photocatalytic performance. We herein report a dual-purpose strategy for enabling the carrier separation and exciton production of Bi₂₄O₃₁Cl₁₀via carbon doping. An impurity state appears after carbon doping in Bi₂₄O₃₁Cl₁₀, which can furnish the charge transport channels and promote carrier separation. DFT calculations confirm that a more localized distribution of conduction and valence band charges strengthens the electron-hole interaction, significantly boosting exciton generation. Besides, the hierarchical structure endows carbon-doped Bi₂₄O₃₁Cl₁₀ with a higher specific surface area. As a consequence, the photoreactivity towards photocatalytic CO₂ reduction and singlet oxygen (¹O₂) generation is dramatically enhanced in carbon-doped Bi₂₄O₃₁Cl₁₀. This study develops an effective pathway to manipulate the behavior of photoexcited species, which allows for the optimum design of excellent photocatalysts with emergent properties in energy conversion and environmental remediation.

Full text of DOI:10.1039/c8ta08598f

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Materials Chemistry A
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Realizing the regulated carrier separation and
exciton generation of Bi24O31Cl10 via a carbon
doping strategy†
Cite this: J. Mater. Chem. A, 2018, 6,
24350
*
Xiaoli Jin, Chade Lv, Xin Zhou, Congmin Zhang, Biao Zhang, Huan Su
*
and Gang Chen
The insufficient efficiency of carrier separation and faint exciton generation are the major limitations of
photocatalytic performance. We herein report a dual-purpose strategy for enabling the carrier separation
and exciton production of Bi24O31Cl10 via carbon doping. An impurity state appears after carbon doping
in Bi24O31Cl10, which can furnish the charge transport channels and promote carrier separation. DFT
calculations confirm that a more localized distribution of conduction and valence band charges
strengthens the electron–hole interaction, significantly boosting exciton generation. Besides, the
hierarchical structure endows carbon-doped Bi24O31Cl10 with
a higher specific surface area. As
Received 4th September 2018
Accepted 15th October 2018
a consequence, the photoreactivity towards photocatalytic CO2 reduction and singlet oxygen (1O2)
generation is dramatically enhanced in carbon-doped Bi24O31Cl10. This study develops an effective
pathway to manipulate the behavior of photoexcited species, which allows for the optimum design of
excellent photocatalysts with emergent properties in energy conversion and environmental remediation.
DOI: 10.1039/c8ta08598f
rsc.li/materials-a
internal electric eld (IEF) induced by the unique layered
structure provides the driving force for spatially separating
photogenerated electron–hole pairs.15–17 Besides, the simulta-
neous localization of the conduction and valence states near the
band edges on Bi atomic sites formed in conned [Bi2O2] slabs
is benecial for electron–hole interaction, which greatly inu-
ences photocatalytic exciton behavior.18 The coulombic inter-
action of electrons and holes, as well as the stability, is strongly
dependent on the exciton binding energies (Eb) of materials.19
However, how to tune Eb of BiOX remains elusive.
1 Introduction
Semiconductor-based photocatalysis, showing great promise
for CO2 conversion, hydrogen generation, molecular oxygen
activation and mineralization of organic pollutants, has been
regarded as the most promising technology to address the
energy crisis and environmental pollution.1–4 The photocatalytic
efficiency strongly depends on the photoinduced species (elec-
trons, holes and excitons) behavior of the photocatalyst. The
separated electrons or holes contribute to the redox reactions
on the surface, and therefore carrier separation is widely
accepted as a critical determinant that affects the photocatalytic
performance.5,6 In addition to carrier separation, the interaction
between electrons and holes (that is exciton effects), which has
long been neglected during the photocatalytic process, is now
arousing considerable and interdisciplinary attention due to its
intriguing applications.7–10 A comprehensive understanding of
the process of charge transfer and energy transfer is indis-
pensable, which enables us to pursue advanced photocatalysts.
Bismuth oxyhalides (BiOX; X ¼ Cl, Br and I), with [Bi2O2]2+
sandwiched between halogen atom slabs, are potentially in
favor of bulk carrier separation and exciton formation.11–14 The
Ion doping is considered to be a promising strategy to alter
the intrinsic structures and charge distribution of materials.20
A
pioneering study of tuning the IEF by incorporating carbon into
the Bi3O4Cl lattice revealed that carbon doping causes
maximum polarization of the nonuniform charge distribution
of BiOX.15 It should be noted that this study only focused on the
IEF tuning of C-doped Bi3O4Cl; the electron–hole interaction
was ignored. The defects caused by doping would tend to
localize the excited electrons, which would hinder the electron
transfer process and induce signicant electron–hole interac-
tion.21 Therefore, carbon doping may play an important role in
the design and fabrication of BiOX with substantial carrier
separation and exciton generation. Bearing this in mind, we
focus on the carbon doping of BiOX to explore its potential in
carrier separation and exciton generation.
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and
Storage, School of Chemistry and Chemical Engineering, Harbin Institute of
Technology, Harbin, P. R. China. E-mail: gchen@hit.edu.cn; zhoux@hit.edu.cn; Fax:
+86-451-86413753
Herein, a novel carbon doping strategy of a bismuth-rich
Bi24O31Cl10 photocatalyst was developed via a hydrothermal
synthesis process with subsequent calcination, taking the
organic salt oxytetracycline hydrochloride (OTC) as the chlorine
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta08598f
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Scheme 1 Schematic illustration of the proposed carbon doping strategy consisting of hydrothermal processing and subsequent thermal
treatment with oxytetracycline hydrochloride as the halide and carbon source.
and carbon source (Scheme 1). The existence of impurity levels over the 2q range of 10–80ꢀ at a scan rate of 2ꢀ minꢁ1 with a scan
and the larger local charge density induced efficient carrier width of 0.02ꢀ. The morphologies of the samples were observed
separation and plenty of exciton production in C-doped using a HELIOS NanoLab 600i eld emission scanning electron
Bi24O31Cl10, which possessed exceptional photocatalytic prop- microscope (FE-SEM). Transmission electron microscopy (TEM)
1
erties for CO2 reduction and O2 generation. Furthermore, the and high-resolution TEM (HRTEM) images were obtained on
universality of this carbon doping strategy was also investigated a JEOL JEM-2100F (UHR) eld emission electron microscope.
in detail.
X-ray photoelectron spectroscopy (XPS) was performed on
a Thermo Scientic ESCALAB 250Xi X-ray photoelectron spec-
trometer with a pass energy of 20.00 eV using Al Ka (1486.6 eV)
monochromatic X-ray radiation. Raman spectra were measured
with a Renishaw Invia micro-Raman microscope (Witec, Alpha
300) using a 514.5 nm argon ion laser and a 458 nm argon ion
laser. Thermogravimetric-differential scanning calorimetry (TG-
DSC) analysis was performed on a SDT Q600 TG-DSC instru-
ment. Fourier transform infrared (FT-IR) spectra were recorded
on a Nicolet IS10 spectrometer using KBr pellets. The electron
paramagnetic resonance (EPR) spectra were obtained at room
temperature with a Bruker A200 EPR spectrometer operating
with a micro frequency at 9.86 GHz. The ultraviolet–visible
diffuse reectance spectra (DRS) were recorded on a spectro-
photometer (PG, TU-1901) using BaSO4 as a reference. Photo-
2 Experimental
2.1 Materials
All of the reagents were purchased from Aladdin Chemical Co.,
Ltd., and used without any further purication.
2.2 Synthesis
Synthesis of bismuth oxychloride hierarchical precursors. In
a typical solvothermal route, 2 mmol Bi(NO3)3$5H2O and 1
mmol oxytetracycline hydrochloride were dissolved in 20 mL of
deionized (DI) water, respectively. Aerwards, the oxytetracy-
cline hydrochloride solution was added into the above
Bi(NO3)3$5H2O solution drop by drop to form a homogeneous
solution under vigorous magnetic stirring for 30 min. Subse-
quently, the solution was transferred to a 50 mL Teon-lined
autoclave to carry out the hydrothermal process at 160 ꢀC for 16
h. Aer the reaction was completed, the black-colored precipi-
tate was collected by centrifugation, then washed with DI water
luminescence (PL) spectra were recorded on
a HORIBA
FluoroMax-4 operating at room temperature. The BET surface
area was determined using the Barrett–Joyner–Halenda method
to analyze the adsorption data in the relative pressure (P/P0)
range from 0 to 0.3.
ꢀ
and ethanol several times, and nally dried at 80 C for 12 h.
2.4 Photocatalytic properties for molecular oxygen activation
Synthesis of C-doped Bi24O31Cl10. A series of products were
The photocatalytic activity of molecular oxygen activation and
photodegradation were studied under a 300 W high pressure
xenon lamp (Trusttech PLS-SXE 300, Beijing) covered with
a cutoff lter (l > 400 nm).
3,30,5,50-Tetramethylbenzidine (TMB, exhibiting an absorp-
tion maximum at 380 nm), nitrotetrazolium blue chloride (NBT,
exhibiting an absorption maximum at 259 nm) and terephthalic
acid (TA, exhibiting an absorption maximum at 422 nm when
excited at 315 nm) were used as molecular probes to detect the
generation of singlet oxygen (1O2), superoxide anion radicals
prepared by calcining the precursor in a tube furnace at varioꢀus
ꢀ
temperatures (250–600 C) for 3 h with a heating rate of 5 C
minꢁ1 in air. The C-doped Bi24O31Cl10 samples with different
amounts of carbon were obtained with the calcination
ꢀ
ꢀ
ꢀ
ꢀ
temperatures of 350 C, 400 C, 500 C and 600 C, and were
denoted as BOC-350, BOC-400, BOC-500 and BOC-600,
respectively.
2.3 Characterization
The crystalline structure of the as-prepared samples was (O2cꢁ) and hydroxyl radicals ($OH), respectively. In a typical test,
conrmed by powder X-ray diffraction (XRD) on a Rigaku 20 mg catalysts were dispersed into TMB (0.02 g Lꢁ1, HAc/NaAc
D/max-2000 diffractometer equipped with a Cu Ka radiation buffer solution), NBT (1.25 ꢂ 10ꢁ5 M) or TA (5 ꢂ 10ꢁ4 M, 2 ꢂ
source (l ¼ 0.15406 nm). The diffraction patterns were taken 10ꢁ3 M NaOH) solution. The suspensions were kept under
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ꢁ1
˚
constant magnetic stirring. At the given time intervals, 4 mL the force and total energy on all atoms converged to 0.01 eV A
mixture was collected and centrifuged to remove the photo- and 10ꢁ5 eV Aꢁ1, respectively. A 4 ꢂ 10 ꢂ 2 k-point mesh was
catalyst. The upper clear liquid of TMB and NBT was analyzed used for the structure relaxation and 6 ꢂ 15 ꢂ 3 was used for
using a UV-vis spectrophotometer (HITACHI UH-5300). The the subsequent electronic structure calculations.
TA–NaOH solution was detected on the PL spectrouorometer
˚
(HORIBA FluoroMax-4) with lexc ¼ 315 nm.
3 Results and discussion
2.5 Photocatalytic activity measurements
The bismuth–oxygen–chlorine–carbon precursor with the
morphology of a hierarchical microsphere is rst prepared
through the hydrothermal method with an organic salt as the
chlorine source (Fig. S1†), and it exhibits a step-like weight loss
below 450 ꢀC and two phase transition points (Fig. S2†). During
the subsequent calcination process, the morphology of the
precursor is retained below 500 ꢀC (Fig. 1a and S3†). The
Photocatalytic CO2 reduction. Photocatalytic CO2 reduction
was carried out on an off-line system and the results were
observed with a Labsolar-III AG closed 350 mL gas system
(Beijing Perfectlight Technology Co., Ltd, China) with a 300 W
high pressure xenon lamp as the light source. 0.05 g of photo-
catalysts were evenly dispersed on a 28.26 cm2 glass sheet and
then 1.7 g NaHCO3 was put into the reaction cell (Pyrex glass).
Prior to irradiation, the above system was thoroughly vacuum-
treated to remove the air inside the reactor completely. Subse-
quently, 4 M H2SO4 (5 mL) was injected into the reaction cell to
achieve 1 atm CO2 gas with NaHCO3. The photoreaction
temperature was kept at 20 ꢀC with a DC-0506 low-temperature
thermostat bath system. At given time intervals, about 1 mL of
gas was taken out using a syringe injector for qualitative anal-
ysis using a GC9790II gas chromatograph equipped with
a GDX-502 ame ionization detector and a TDX-01 thermal
conductivity detector. A standard curve is obtained to estimate
the production yield. The outlet gases were determined to be
CO, CH4 and CO2.
Photodegradation of contaminants. Bisphenol-A (BPA),
oxytetracycline hydrochloride (OTC), Rhodamine B (RhB) and
Cr(VI) are used to evaluate the degradation performance of the
materials. 20 mg of photocatalysts were added to the above
solution (100 mL, 10 mg Lꢁ1) with stirring (about 30 min) to
attain adsorption–desorption equilibrium. The suspensions (4
mL) were sampled at each time interval and centrifuged to
remove the powders. The concentration was determined at
ꢀ
precursor and the as-calcined samples obtained below 300 C
are mainly BiOCl (Fig. S4†). By calcining the precursor at 350 ꢀC
(denoted as BOC-350), Bi24O31Cl10 (JCPDF Card no. 75-0887) can
be obtained, and a peak shi towards lower angles between 31
and 33ꢀ is discovered in the X-ray photoelectron spectroscopy
(XRD) patterns (Fig. 1b and c), indicating that some foreign
atom might be incorporated into the Bi24O31Cl10 lattices to give
rise to the lattice deformations.25,26 According to the survey X-ray
photoelectron spectroscopy (XPS) spectra (Fig. 1d), the Bi, O, Cl,
and C elements are present in BOC-350 and the atomic ratio of
Bi/Cl is 2.4 : 1 which is within the experimental error, further
suggesting that the photocatalyst is pure Bi24O31Cl10. Fig. 1e
and f are the TEM images of BOC-350, and it can be clearly seen
that the microsphere is composed of nanosheets.
The lattice fringes are measured to be 0.33 nm in the high-
resolution transmission electron microscope (HRTEM) images,
their maximum absorption wavelength with
spectrophotometer.
a
UV-vis
2.6 Photoelectrochemical measurement
An AUTOLAB-PGSTAT302N electrochemical work station with
a standard three-electrode electrochemical cell under visible
light was employed to measure the photoelectrochemical
characteristics of BOC-350 and BOC-600 using Na2SO4 (0.5 M)
as the electrolyte solution. FTO glass with the coated photo-
catalysts, an Ag/AgCl electrode and a piece of a Pt sheet were
used as the working electrode, the reference electrode and the
counter electrode, respectively. Mott–Schottky plots were
measured under direct current potential polarization at
different frequencies (1500 and 2000 Hz). The potential ranged
from ꢁ1.0 to 1.0 V (vs. Ag/AgCl).
2.7 DFT theoretical calculations
Fig.
1 Morphologic and structural characterization of C-doped
The Vienna Ab initio Simulation Package (VASP) with a gener-
alized gradient approximation (GGA) was used.22–24 Carbon
doping was modeled by inserting one carbon atom into the
Bi24O31Cl10. (a) SEM image, (b and c) XRD pattern and the magnified
XRD pattern, (d) the survey XPS spectrum, (e and f) TEM images and (g)
the HRTEM image of BOC-350. (h–k) The XEDS energy mapping of an
middle space of two Cl atoms. In the optimization procedure, individual BOC-350 microsphere.
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matching well with the (015) plane of Bi24O31Cl10 (Fig. 1g). In the narrowest Eg of 1.64 eV according the plots of (ahv)1/2 versus
order to affirm the doping element, the corresponding hv (Fig. S7b†).31 The accurate content of carbon doping in
elemental mapping of BOC-350 is conducted (Fig. 1h–k). The carbon-doped Bi24O31Cl10 can be obtained by the XPS
well-distributed carbon element in the microsphere demon- measurement. The content of Bi, O, Cl and C based on the XPS
strates that carbon has been successfully introduced into data is listed in Table S1.† As shown in Fig. 2c, two types of
Bi24O31Cl10. No carbon species can be observed in the HRTEM carbon can be distinguished from the C 1s XPS spectra of BOC-
images of BOC-350, demonstrating that carbon does not exist in 350, CI and CII. CI and CII represent the carbonaceous impu-
the surface.
rities and doping carbon, respectively.27,32 By taking the area
When the calcination temperature is increased to 600 ꢀC, ratio of the CI and CII peaks, the content of doping carbon in
Bi24O31Cl10 with sharp peaks is formed and the regular shi to BOC-350 is estimated to be 0.86%. The Cl 2p peaks of BOC-350
a lower angle is also observed between 39.5 and 45ꢀ (Fig. S5†). show a slight shi to lower binding energy, indicating that the
Besides, the D and G bands (at 1340 cmꢁ1 and 1580 cmꢁ1
,
doping carbon may be connected with chlorine atoms and thus
respectively) of disordered and graphitic carbon are absent in changes its chemical environment (Fig. S8†). To further conrm
the Raman spectra (Fig. S6†), providing essential evidence of the doped position of carbon, Fourier transform infrared
carbon doping.27 This lattice defect concentration can be pro- spectroscopy (FT-IR) of BOC-350 and BOC-600 is performed
bed qualitatively by the typical electron paramagnetic reso- (Fig. 2d). Apart from the strong peak of the Bi–O stretching
nance (EPR) technology, and it is gradually diminished while vibration at 510 cmꢁ1, an obvious new peak emerges at 847
increasing the calcination temperature (Fig. 2a).28 To quantify cmꢁ1 in BOC-350 which is assignable to the C–Cl stretching
the concentration of carbon dopants in the Bi24O31Cl10 samples, modes, which might have originated from the close connection
thermogravimetric analysis (TGA) in the temperature range of Cl and C.33,34 Based on the above analysis, it can be concluded
ꢀ
100–700 C is performed on BOC-350, BOC-400, BOC-500 and that the carbon atoms are successfully inserted into the chlorine
BOC-600 (Fig. 2b). BOC-350 and BOC-400 show approxꢀimately atom layer of Bi24O31Cl10 with a maximum content of 0.86%.
1.26% and 0.54% weight loss in the range of 350–600 C, cor-
Theoretical calculations are performed to predict the dual-
responding to the decomposition of carbon dopants, while purpose tuning of carrier separation and exciton generation by
nearly no noticeable weight loss is found in BOC-500 and BOC- this carbon doping strategy. As shown in Fig. 3a and b, the
600 due to their very low concentration of dopants, in confor- carbon doping does not change the intrinsic band position of
mity with the EPR results. The low doping concentration (below Bi24O31Cl10, but leads to an impurity state localizing near the
2%) suggests that the carbon atom prefers to be doped at VBM. To corroborate the band structure experimentally, the
interstitial sites in the Bi24O31Cl10 crystal.29 Generally, doping in relative positions of the CB and VB edges are further obtained
the lattice of the host semiconductor photocatalyst could from at-band potential measurement and XPS valence band
generate a doping energy level in the band gap and thus spectra, respectively. BOC-350 and BOC-600 exhibit an intrinsic
enhance the intrinsic optical absorption.30 As expected, a strong n-type characteristic with the positive slope presented in the
tail absorption of BOC-350 and BOC-400 across the entire linear region (Fig. 3c and d). The at-band potential is esti-
visible light region accompanied by a slight absorption edge red mated to be approximately ꢁ0.35 V for the two samples, relative
shiing occurs in the UV-vis diffuse reectance spectra (DRS) to the Ag/AgCl electrodes. It is found from Fig. 3e that the VB
(Fig. S7a†). BOC-350 with the highest carbon content exhibits potential of BOC-350 has a slightly negative shi compared with
that of BOC-600, which is almost in agreement with the calcu-
lation. Based on the above data, combined with the band gap
energy, the corresponding band structure schematic of
Bi24O31Cl10 and C-doped Bi24O31Cl10 is illustrated in Fig. 3f. The
doping state existing in C-doped Bi24O31Cl10 would act as the
springboard to ensure that excited electrons migrate from the
VB to the doping state, and then to the CB, thus promoting
carrier separation and migration.35 The density of states (DOS)
is plotted in Fig. S9,† which shows that the CB is mainly
composed of the Bi 6p and O 2p states, while the VB is primarily
contributed to by the Cl 3d and O 2p states. The C 2p states are
located near the Fermi level. The enhanced photocatalytic
performance of Cr(VI) reduction of BOC-350 under 475 nm and
500 nm monochromatic light further veries the energy band
changes via carbon doping (Fig. S10†).
The excitonic effect of photocatalysts can be evaluated by the
exciton binding energies (Eb), which is related to the number of
localized valence and conduction states, with the linear rela-
tionship of Eb f (NcNv)1/2 (Nc and Nv represent the number of
localized conduction and valence electrons, respectively).17,36
Fig. 2 EPR spectra (a) and TG curves (b) of BOC-350, BOC-400, BOC-
500 and BOC-600. High-resolution C 1s XPS spectra (c) and the FT-IR
spectra (d) of BOC-350 and BOC-600.
Fig. 4a–c present the isosurface charge density near the
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Fig. 3 The band structure of carbon-doped and undoped Bi24O31Cl10. (a and b) Theoretical calculation of electronic band structures, (c and d)
Mott–Schottky plots, (e) valence-band XPS spectra and (f) the schematic illustration of the electronic band structures of BOC-350 and BOC-600.
conduction band, impurity state and valence band edges of intensity, which is nearly three times higher than that of BOC-
C-doped Bi24O31Cl10. Compared to those delocalized electrons 600. This indicates the rapid separation and transfer of photo-
of undoped Bi24O31Cl10 (Fig. 4b and d), the charge density near generated charge carriers in C-doped Bi24O31Cl10.37 Meanwhile,
the CB and VB edges tends to be more localized aer intro- electrochemical impedance spectroscopy (EIS) is another valid
ducing a carbon atom, implying the larger Nv and Nc of C-doped electrochemical strategy to monitor electron generation and
Bi24O31Cl10. In addition, an emerging Ni caused by the localized charge transport characteristics. From the observation in Fig. 5b,
charge density appears in the impurity state, which indicates BOC-350 features a remarkably smaller radius with respect to
the strong excitonic effects of Bi24O31Cl10 by carbon doping.
BOC-600, reecting the lower electron transfer resistance and
To further corroborate the separation of photoexcited carriers efficaciously elevated separation of the electron–hole pairs of
and excitonic effects experimentally, photoelectrochemical Bi24O31Cl10 (Fig. 5b).38 Singlet oxygen (1O2), generated during
measurements and molecular oxygen activation are recorded. As exciton photocatalytic processes via energy transfer between
presented in Fig. 5a, BOC-350 exhibits a superior photocurrent photocatalysts and molecular oxygen, possesses powerful reac-
tivity towards photocatalytic oxidizability. The exciton effect is
evaluated by detecting 1O2 using 3,30,5,50-tetramethylbenzidine
(TMB), nitroblue tetrazolium (NBT) and terephthalic acid (TA) as
probe molecules.39–44 The persistent increase of the absorbance
peak evolution at 380 nm manifests the rapid oxidation of TMB
with BOC-350, indicating that an increased amount of ROS is
generated as compared to BOC-600 (Fig. 6a).45 Furthermore, no
discernible O2cꢁ or $OH is detected during the molecular oxygen
activation of BOC-350, as observed from the time-dependent
absorption spectra of NBT and TA oxidation (Fig. S11†).46 Obvi-
ously, it is 1O2 that mainly accounts for the photogenerated
reactive oxygen species (ROS) during the excitonic photocatalytic
process by BOC-350. The above results show that efficient carrier
Fig.
4 Theoretical calculation of Eb for undoped and C-doped
Bi24O31Cl10. (a–c) isosurface conduction, impurity level and valence
charge density of C-doped Bi24O31Cl10, respectively. (d and e) Iso-
surface conduction and valence charge density of un-doped Fig. 5 The behavior of carrier separation. (a) Photocurrent responses
Bi24O31Cl10, respectively.
and (b) electrochemical impedance plots of BOC-350 and BOC-600.
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conversion in the rst cycle might be due to the initial
adsorption of CO2 on the catalyst.47 In addition to the effective
CO2 reduction of BOC-350 in gas, the same result is also ob-
tained with Cr(VI) reduction in the liquid phase (Fig. S12†). The
results show that increasing the amount of carbon tends to
favor the photocatalytic reduction activity involving electrons.
Moreover, the time-dependent experiments conrm that CO2 is
the carbon source for CO. To further evidence the deduction
that CO comes from the CO2 reduction process rather than from
the doping carbon in BOC-350, an atmosphere control experi-
ment is carried out (Fig. S13†). When fed with Ar in the reactor,
nearly no CO is detected, indicating that CO2 is the origin of CO
over BOC-350.48
1
Fig. 6 The behavior of O2 generation. (a) Time-dependent absorp-
tion spectra of TMB oxidation in air over BOC-350 and BOC-600. (b)
The absorbance of TMB oxidation with BOC-350 monitored under air
and N2 gas conditions.
1
In view of its extremely high oxidizability of O2 and carrier
separation and 1O2 generation are successfully achieved through
the carbon doping strategy. Additionally, the controlled experi-
ment monitored with N2 gas blowing and without catalyst in air
under visible light irradiation demonstrates that the generation
of 1O2 is associated with molecular oxygen and the catalyst
(Fig. 6b).
separation, BOC-350 is also considered to be a promising pho-
tocatalyst for degrading pollutants. As predicted, BOC-350
presents much more splendid photocatalytic activities towards
colorless bisphenol-A (BPA), oxytetracycline hydrochloride
(OTC) and the colored dye Rhodamine B (RhB) degradation
than BOC-600, proving its good universality for decomposing
various pollutants (Fig. S14†). The trapping experiment
conrms that holes and 1O2 are the main active species for RhB
degradation (Fig. S14d†).
The highly improved carrier and energy transfer are realized
by the carbon doping strategy on Bi24O31Cl10. Besides, the
particle size of BOC-350 is obviously smaller than that of BOC-
600, which may lead to the larger surface areas of BOC-350. As
an important parameter for the improvement of photocatalytic
activity, Brunauer–Emmet–Teller (BET) specic surface areas of
BOC-350 and BOC-600 are obtained by N2 adsorption–desorp-
tion experiments (Fig. S15†). Aer calculation, the BET specic
surface area of BOC-350 is determined to be 17.67 m2 gꢁ1, which
was higher than that of BOC-600 (6.61 m2 gꢁ1). The higher
specic surface area can create more active sites for adsorption
and surface reaction, which is benecial for photocatalytic CO2
reduction. On the basis of the above results, we proposed the
photocatalytic mechanism of C-doped Bi24O31Cl10 (Fig. 8). The
carbon doping induces an impurity state near the valence band,
thus promoting the separation and transport of electrons and
holes. As a result, improved photocatalytic activity for CO2
reduction is obtained via this carbon doping strategy. Mean-
while, the large Eb of C-doped Bi24O31Cl10 provides abundant
1O2 generated during the exciton photocatalytic process. The
Beneting from the dual-purpose regulation of Bi24O31Cl10
by carbon doping, BOC-350 could be a promising candidate for
reduction and oxidation reactions. As elucidated in Fig. 7a,
abundant CO and less CH4 gas can be produced on BOC-350,
while a small amount of CO and nearly no detectable CH4 gas is
generated over BOC-600, revealing the high photocatalytic effi-
ciency and selectivity of C-doped Bi24O31Cl10 for converting CO2
to CO. The yield of CO gas generated by the as-prepared
samples increased with the irradiation time and accumulated
linearly as a function of time (Fig. 7b). It is noteworthy that the
yield decreased with the decreasing doping content and
BOC-350 possesses the highest rate of CO generation with 2.54
mmol gꢁ1 hꢁ1, which is nearly more than 3 times higher than
that of BOC-600 (0.73 mmol gꢁ1 hꢁ1) (Fig. 7c). And the cycling
experiment revealed that the reduction of the CO evolution rate
was about 28% aer four cycles (Fig. 7d). The somewhat higher
Fig. 7 Photocatalytic activities for CO2 reduction. (a) CO2 conversion,
(b) CO generation, (c) CO generation rate and (d) recycling of BOC- Fig. 8 Schematic illustration of the carrier and energy transfer in the
350 for CO rate.
C-doped Bi24O31Cl10.
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In this study, the dual purpose of enhancing the carrier sepa-
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rst time. The fabricated C-doped Bi24O31Cl10 enables standout
CO2 reduction and molecular oxygen activation for producing
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good universality of the designed synthetic method provide
opportunities to develop advanced photocatalysts for solar fuel
generation.
¨
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Conflicts of interest
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24 G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter Mater.
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Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (21471040, 21303030 and
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