Steering photoinduced charge kinetics: Via anionic group doping in Bi2MoO6 for efficient photocatalytic removal of water organic pollutants

Article abstract of DOI:10.1039/c7ra04615d

In this work, we report a novel anionic group doped Bi₂MoO₆ with noble metal loading as an excellent photocatalytic material for the efficient photodegradation of organic pollutants, including rhodamine B (RhB) and colorless o-phenylphenol (OPP). Several characterization techniques were conducted to investigate the effect of anionic group doping and noble metal loading on the lattice structure, electronic structure, defect chemistry as well as photocatalytic performance. It's found that CO₃^2- doping in a Bi₂MoO₆ host matrix led to lattice expansion, local symmetry distortion, oxygen vacancy generation and band gap narrowing from ～2.60 to ～2.31 eV. VB-XPS, Mott-Schottky plots and DFT results indicated that the variation of the band gap energy for Bi₂MoO₆ mainly originated from the upward shift of the valence band edge, which is attributed to the presence of the midgap states of the C 1s orbital. Noble metals (Au, Ag and Pd) as cocatalysts forcefully improved the photogenerated charge separation. With contributions from the doping effects and cocatalysts, the photocatalytic activity of 0.5% Pd-3C/BMO was robustly enhanced about 5-fold for RhB degradation within 40 min under UV + visible light irradiation and 29-fold for OPP degradation within 120 min under visible light irradiation in comparison with pristine Bi₂MoO₆, respectively.

Full text of DOI:10.1039/c7ra04615d

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Steering photoinduced charge kinetics via anionic
group doping in Bi2MoO6 for efficient
photocatalytic removal of water organic
pollutants†
Cite this: RSC Adv., 2017, 7, 35883
*
Yongxing Xing, Jing Zhang, Zhiliang Liu
and Chunfang Du
In this work, we report a novel anionic group doped Bi2MoO6 with noble metal loading as an excellent
photocatalytic material for the efficient photodegradation of organic pollutants, including rhodamine B
(RhB) and colorless o-phenylphenol (OPP). Several characterization techniques were conducted to
investigate the effect of anionic group doping and noble metal loading on the lattice structure,
2ꢀ
electronic structure, defect chemistry as well as photocatalytic performance. It's found that CO3
doping in a Bi2MoO6 host matrix led to lattice expansion, local symmetry distortion, oxygen vacancy
generation and band gap narrowing from ꢁ2.60 to ꢁ2.31 eV. VB-XPS, Mott–Schottky plots and DFT
results indicated that the variation of the band gap energy for Bi2MoO6 mainly originated from the
upward shift of the valence band edge, which is attributed to the presence of the midgap states of the C
1s orbital. Noble metals (Au, Ag and Pd) as cocatalysts forcefully improved the photogenerated charge
separation. With contributions from the doping effects and cocatalysts, the photocatalytic activity of
0.5% Pd–3C/BMO was robustly enhanced about 5-fold for RhB degradation within 40 min under UV +
visible light irradiation and 29-fold for OPP degradation within 120 min under visible light irradiation in
comparison with pristine Bi2MoO6, respectively.
Received 25th April 2017
Accepted 13th July 2017
DOI: 10.1039/c7ra04615d
rsc.li/rsc-advances
transport or even regulation of the band gap and work function,
both of which are benecial for improving the photocatalytic
activity.7–10 Most of the element-doping researches are con-
1. Introduction
Photocatalytic decomposition or mineralization of organic
pollutants such as phenol, p-chlorephenol and o-phenylphenol
with high degradation rates and low toxicity has been recog-
nized as a promising strategy in environmental remediation
since these organic pollutants are difficult to eliminate using
conventional water treatment process.1–4 Photocatalysis with
naturally abundant solar light is a perfect development direc-
tion that need all mankind to make efforts to try. In such a way,
photocatalysts are required to respond in a broad wavelength
range. Element doping and noble metal loading are two effi-
cient approaches to enhance the visible-light harvest and thus
improve the photocatalytic performance of photocatalysts
under visible-light irradiation.5,6
cerned about metal or non-metal ions, such as Er3+/Fe3+ co-
doped porous Bi5O7I,11 Mo doped Bi2WO6,7 B doped Bi2WO6,12
N–F co-doped BiVO4,13 etc. In comparison with cationic or
anionic ions, anionic groups constituted by various elements
and compositions possibly make more contributions to the
regulation and management on the crystal lattice, optical
harvest and photocatalytic performance.14–17 Self-doping of the
CO32ꢀ anionic group into Bi2O2CO3 could induce a larger lattice
spacing, extend the photoresponsive range from UV to visible
light and the band gap could be also continuously tuned.14 PO4-
doped Bi2WO6 synthesized by a urea-precipitation way showed
the higher photocatalytic activity than that of pristine Bi2WO6
for the degradation of Cr(VI), RhB dye, phenol and antibiotics,
etc.17 These excellent results imply that anionic group doping,
a new concept, is a potential way to dramatically enhance the
photocatalytic performance and makes the photocatalysts more
practical for environmental requirement.15,16
Element doping is a fundamental strategy to introduce
a small amount of atoms into the host crystal lattice, resulting
in the modication of electronic dynamics and phonon
College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot,
Inner Mongolia, 010021, P. R. China. E-mail: cedchf@imu.edu.cn; Fax: +86-471-
4994375; Tel: +86-471-4994375
Bismuth molybdate (Bi2MoO6), a member of the Aurivillius
oxide family, has attracted considerable attention due to its
unique physicochemical properties relating to its layered
structure consisting of perovskite layers and bismuth oxide
layers.18,19 However, the high recombination of photogenerated
† Electronic supplementary information (ESI) available: Raman spectra, TG
curves and FTIR spectra of carbonated doped samples; XRD patterns, XPS
spectra, SEM and TEM images, BET analysis and photocatalytic properties of
noble metals loaded samples. See DOI: 10.1039/c7ra04615d
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carriers and low utilization of solar energy inhibit its practical 3C/BMO of 0.25, 0.5, 0.75 and 1.0 wt% were denoted as 0.25%
application.10,20 Until now, various strategies have been explored Pd–3C/BMO, 0.5% Pd–3C/BMO, 0.75% Pd–3C/BMO and 1.0%
to solve these problems, including element doping even self- Pd–3C/BMO, respectively. Under the same condition, Au loaded
doping,21–25 heterojunction,26–30 noble metal loading,31–34 etc. on 3C/BMO (Au–3C/BMO) and Ag loaded on 3C/BMO (Ag–3C/
However, anionic group doping has not been tried. Thus, in this BMO) with various Au or Ag weight ratios were respectively
paper, the anionic group CO32ꢀ was doped into the MoO6 sites synthesized.
in the host lattice of Bi2MoO6 to modulate the crystal and band-
gap structures and extend the photoresponsive range for the
rst time. Then noble metals (Au, Ag and Pd) were loaded on
2.2 Characterization
Bi2MoO6 surfaces as the cocatalysts to further improve the The phase structure of the synthesized samples was obtained on
photocatalytic activity. DFT calculation and series of charac- an X-ray diffractometer (EmpyreanPanalytical) with Cu Ka
terization techniques were conducted to study the crystal radiation (l ¼ 0.15406 nm). The detailed morphology and
structure, electronic band structure and optical properties. The structure were recorded by transmission electron microscopy
photocatalytic performance was evaluated by the degradation of (TEM) and high resolution TEM (HRTEM) on a JEM-2010
rhodamine B (RhB) and colorless o-phenylphenol (OPP) under apparatus with an acceleration voltage of 200 kV. Scanning
UV + visible light or visible light irradiation. The mechanism of electron microscopy (SEM) images were recorded on the Hitachi
enhanced photocatalytic activity was also investigated. This S-4800 apparatus with an accelerating voltage of 15 kV. The
work could possibly provide a new strategy for regulation and thermogravimetric analysis (TG) was conducted on a thermal
management of the crystal and band gap structures and thus analyzer (NETZSCH STA 449F3) when the sample was heated
enhance the photocatalytic performance of photocatalysts.
from 30 to 900 ꢂC with a raising ramp rate of 10 ꢂC minꢀ1 under
nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS)
was performed on a Thermo Escalab 250Xi with a mono-
chromatic Al Ka (hn ¼ 1486.6 eV) to determine the surface state
and chemical composition. Raman spectra were recorded on
2. Experimental
2.1 Sample synthesis
Synthesis of carbonate doped Bi2MO6 (C/BMO). All the a Horiba JobinYvonLabRAM HR800 instrument with the laser
chemical reagents used in this work were purchased from excitation of 320 nm. Fourier transform infrared spectroscopy
commercial sources and used without further purication. (FT-IR) was carried out using a Bruker Tensor 27 spectropho-
Carbonate doped Bi2MO6 (C/BMO) and pristine Bi2MO6 (BMO) tometer using KBr powder-pressed pellets. The UV-vis absorp-
were synthesized via a one-pot hydrothermal method. In tion spectra were measured using a UV-vis spectrophotometer
a typical synthesis procedure of C/BMO, 2.425 g Bi(NO3)3$5H2O (Lambda 750s) in the range of 200–800 nm. The specic surface
was dissolved into 25 mL of nitric acid (1 mol Lꢀ1) to obtain area (SBET) of the samples was obtained from N2 adsorption–
a transparent solution (solution A) and 0.445 g (NH4)6- desorption isotherms at 77
K (ASAP 2020). The photo-
Mo7O24$4H2O was dissolved into 25 mL of deionized water to luminescence (PL) spectra were measured on an Edinburgh
obtain solution B. Then, solution A was added into solution B Instruments FLS920 spectrouorimeter equipped with both
drop by drop to obtain a homogeneous suspension. Aer stir- continuous (450 W) and pulsed xenon lamps, with the excita-
ring for 1 h, a certain amount of sodium citrate was added into tion wavelength of 416 nm.
this suspension and the pH value of this mixture was adjusted
The photocurrent transient response measurement, elec-
to be 8.2 using NH3$H2O solution (25%). Flowingly, this clear trochemical impedance spectroscopy (EIS) and Mott–Schottky
solution was transferred into a 100 mL Teon-lined stainless measurements were performed on a PGSTAT 302 N electro-
ꢂ
steel autoclave for a hydrothermal process (170 C, 12 h). The chemistry workstation. The measurement system contains
obtained products were ltrated, washed with deionized water a sample chamber, a lock-in amplier (SR 830, Stanford
and ethanol, and dried at 60 ꢂC for 12 h. The samples with Research Systems, Inc.) with a light chopper (SR540, Stanford
molar ratios of sodium citrate to Bi(NO3)3$5H2O of 1, 2, 3 and 4 Research Systems, Inc.) and a source of monochromatic light
were denoted as 1C/BMO, 2C/BMO, 3C/BMO and 4C/BMO, (500 W xenon lamp, CHF-XM 500, Trusttech) as well as
respectively. For comparison, pristine Bi2MoO6 (BMO) was a monochromator (Omni-l 300, Zolix). The analyzed product is
also synthesized under identical condition in the absence of assembled as a sandwich-like structure of ITO–product–ITO,
sodium citrate.
which ITO indicates an indium tin oxide electrode. All the
Synthesis of noble metal (Au, Ag or Pd) loaded on C/BMO. measurements were performed in air atmosphere and at room
The syntheses process of noble metal loaded on C/BMO, taking temperature.
Pd for an example (Pd–3C/BMO), is as follows: 100 mg of 3C/
Oxygen vacancy and active species of hydroxyl (cOH) and
BMO was dispersed in 15 mL of deionized water via sonica- superoxide radicals (cO2ꢀ) were detected by electron spin reso-
tion. Then 2.0 mL of K2PdCl6 (1 mg mLꢀ1) solution was dropped nance (ESR) measurements on JEOL JES FA200 spectrometer.
under magnetic stirring for 0.5 h. Aerwards, 1.0 mL NaBH4 Oxygen vacancy measurement was conducted at 77 K. ESR
with a concentration of 0.1 mol Lꢀ1 was added into the solution. spectra for hydroxyl radicals and superoxide radicals were
Aer stirring for 0.5 h, the product was separated by ltering, conducted in methylbenzene solution (2.0 mL) and methyl-
washed with water and ethanol for several times, and dried at benzene solution containing methyl alcohol (2 mL, the volume
60 ꢂC for 12 h. The samples with weight ratios of noble metals to ratio of methyl alcohol being 20%), respectively. The
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experiments were performed in dark and under visible light
irradiation with adding 4 mg sample and 0.05 mol Lꢀ1 DMPO
(5,50-dimethyl-1-pyrroline-N-oxide).
To investigate the crystal structure and electronic band
structure of carbonate doped Bi2MoO6, density functional
theory (DFT) calculation was performed using the CASTEP
program package. The kinetic energy cutoff is 420 eV, using the
generalized gradient approximation (GGA) with the Perdew–
Burke–Ernzerhof (PBE) to treat the models. Geometry optimi-
zation is carried out until the residual forces were smaller than
0.01 eV Aꢀ1, and the convergence threshold for self-consistent
˚
iteration was set at 5 ꢃ 10ꢀ7 eV.
2.3 Photocatalytic activity measurements
The photocatalytic activity was evaluated by degradation of
water organic pollutants rhodamine B (RhB, 0.02 mol Lꢀ1) and
colorless o-phenylphenol (OPP, 20 mg Lꢀ1) under various light
irradiation. The photodegradation process was conducted in
a
photochemical reactor with different luminous power
including 300 W and 500 W Xe lamp. Moreover, the photo-
catalytic performance was also investigated under UV + visible
light irradiation (500 Xe lamp without cut-off lter) or visible
light irradiation (500 W Xe lamp with a 420 nm cut-off lter),
respectively.
Fig. 1 XRD pattern (a and b), Raman spectra (d) of pristine Bi2MoO6
and carbonate doped Bi2MoO6 and XRD patterns of various noble
metals loaded on 3C/BMO (c). Vertical bars in (a) indicate the standard
data for Bi2MoO6 (JCPDS card no. 77-1246, red line) and Bi2O3 (JCPDS
card no. 16-0654, black line).
In every experiment, 30 mg of the prepared sample was
suspended in 30 mL of RhB or OPP solution. Prior to irradia-
tion, the suspension was kept in the dark with stirring for
60 min to reach adsorption–desorption equilibrium. During
irradiation, 3 mL of suspension was sampled and centrifuged at
certain time intervals. The absorbance of the as-obtained
ltrates was measured at 552 nm and 282 nm using a UV-vis
spectrophotometer (U-3900, Hitachi, Japan) to obtain the RhB
and OPP concentrations, respectively. The normalized concen-
tration change (C/C0) of the pollutant concentration was adop-
ted to evaluate the photocatalytic performance, where C is the
pollutant concentration at a certain time and C0 is the initiative
concentration before illumination.
that carbonate groups were incorporated in Bi2MoO6 host
matrix. On the other hand, it is noted that the diffraction peaks
in Fig. 1a greatly broadened with an increase of carbonate
doping content. Using Scherrer formula, the average size of
pristine Bi2MoO6 was calculated to be 52.9 nm from peaking
broadening. For 3C/BMO sample, the average size was greatly
reduced to 4.1 nm (Fig. S1†). Low grain size of 3C/BMO predicts
large surface to volume ratios and more active sites that is
benet for enhancement of photocatalytic performance. When
the molar ratio of sodium citrate to Bi(NO3)3$5H2O increases to
4, the crystal phase transforms to Bi2O3, thus the product 4C/
BMO will not be referred in the following discussion.
As known that Raman scattering is regarded as an effective
technique to check the lattice strain of semiconductor.14,35 Thus,
to further understand the crystal defects and crystal structure
3. Results and discussion
3.1 Sample characterization
The XRD patterns of pristine Bi2MoO6 (BMO) and carbonate distortion in carbonate doped Bi2MoO6 structures, Raman
doped Bi2MoO6 (C/BMO) with various molar ratios of sodium spectra were measured and displayed in Fig. 1d and S2.† As
citrate to Bi(NO3)3$5H2O are depicted in Fig. 1. All the diffrac- shown in Fig. S2† and enlarged spectra in Fig. 1d, all of the C/
tion peaks can be indexed to the orthorhombic Bi2MoO6 (JCPDS BMO samples exhibit the typical vibration mode of Bi2MoO6.
card no. 77-1246) when the molar ratio of sodium citrate to The vibration modes near 292 cmꢀ1 are assigned to the Eg
Bi(NO3)3$5H2O is below 4 (Fig. 1a). With an increase of the bending vibrations, while the peaks located at 334 and 366
carbonate doping amount, the amplifying XRD pattern (Fig. 1b) cmꢀ1 are attributed to Eu symmetric bending.20,21 The Raman
displays that the peak (131) located at about 28.2ꢂ shows a slight vibrations, while the peaks located at 334 and 366 cmꢀ1 are
shi toward smaller angles compared with pristine Bi2MoO6. attributed to Eu symmetric bending.20,21 As illustrated in Fig. 1d,
According to Bragg's law (2d sin q ¼ nl), a smaller 2q indicates the Raman signal in the range of 550–950 cmꢀ1 can be well
a larger lattice spacing.7,10,14 By a least-squares method, the reproduced by three vibration peaks. The Raman vibrations at
lattice parameters for carbonate doped Bi2MoO6 were given in 732 cmꢀ1 (Eu mode), 811 cmꢀ1 (A1g mode), 857 cmꢀ1 (A2u mode)
Fig. S1.† As depicted in Fig. S1,† a monotonous increase in the are assigned to the asymmetric, symmetric and asymmetric
lattice volume was observed with an increase of initial molar stretching vibrations of the MoO6 octahedrons, respectively.36,37
ratios of sodium citrate to Bi(NO3)3$5H2O, which may suggest Compared with those of pristine Bi2MoO6, the Raman peaks of
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carbonate dope Bi2MoO6, taking the intense peak at 811 cmꢀ1
in Fig. 1d and 334 cmꢀ1 in Fig. S2† as examples, shi slightly
toward lower wavenumbers with increasing the carbonate
doping content. In addition, the Mo–O bond length calculated
by the empirical equation RMo–O ¼ 0.48239 ln(32 895/n) is
usually adopted to evaluate the crystal distortion, where R is the
metal–oxygen bond length in angstroms and n is the Raman
stretching frequency in wavenumbers.10,36 According to this
equation, the RMo–O calculated based on the Raman mode at
811 cmꢀ1 changed from 1.7862 A for pristine Bi2MoO6 to 1.7886
˚
˚
A for 3C/BMO. This result forcefully conrms the result that the
carbonate has been doped into the lattice crystal structure of
Bi2MoO6, interpreting the distinct distortion and symmetry
breaking of the MoO6 octahedron, which result is in consistent
with that of Ce doped Bi2MoO6.10
Sodium citrate was adopted as carbonate source to dope into
the crystal lattice of Bi2MoO6, which generally decomposes
when accelerating the temperature. Thus, thermogravimetric
(TG) curves are measured to prove the existence of carbonate
(Fig. S3†). The TG results reveal that the total weight loss of
Bi2MoO6 is 4.7%, which can be attributed to the removal of
adsorbed water.21 In comparison with pristine Bi2MoO6, the
total weight loss of carbonate doped Bi2MoO6 increases with
raising the carbonate doping content. This result is in accor-
dance with that of carbonate doped Bi2O2CO3,14 which conrms
the existence of carbonate in Bi2MoO6.
Fig. S4† presents the FTIR spectra of various samples. For
pristine Bi2MoO6, the bands located at 841 and 800 cmꢀ1 are
assigned to the asymmetric and symmetric stretching of MoO6
referring vibration of the apical oxygen atoms, respectively.38
The band at 728 cmꢀ1 is specied as the asymmetric stretching
vibration of equatorial oxygen atoms of MoO6 octahedrons, and
the band at 570 cmꢀ1 is attributed to the bending vibration of
Fig. 2 High-resolution XPS spectra of Bi (a), Mo (b), O (c), C (d), Au (e),
MoO6.5 The band located at 445 cmꢀ1 corresponds to the Bi–O Ag (f), Pd (g) and ESR spectra (h) at room temperature.
stretching vibration. The bands at 3438 and 1388 cmꢀ1 are
assigned to the stretching vibration and deformation vibration
of O–H.39 It should be noted that the FTIR spectra of carbonate found that the Bi 4f and Mo 3d peaks shi to the lower bonding
doped Bi2MoO6 resemble that of pristine Bi2MoO6. However, energy region aer carbonate doping, which result is consistent
a new broad band at 1569 is observed aer carbonate doping, with those of Ce doped Bi2MoO6 and La,
which is assigned to the C]C bonds of sodium citrate.39,40 This Bi2MoO6.10,23 This phenomenon conrms the incorporation of
phenomenon further conrms the presence of carbonate. carbonate into the crystal lattice structure of Bi2MoO6, which
F co-doped
The XRD patterns of noble metals (Au, Ag, Pd) loaded on 3C/ results in the change of chemical environment for Mo center in
BMO samples are shown in Fig. 1c. As displayed in Fig. 1c, the [MoO6] octahedron.23 Fig. 2c shows the high-resolution XPS
loading of noble metals on the surface of 3C/BMO has a negli- spectra of O 1s. For pristine Bi2MoO6, the spectra can be
gible inuence on the XRD patterns of 3C/BMO. No peaks deconvoluted into two peaks at 530.0 and 531.3 eV, which are
assigned to metal or metal oxide are observed even the Pd assigned to the crystal lattice oxygen and surface hydroxyl
loading content reaching 1.0 wt% (Fig. S5†), which could be groups, respectively.46,47 Aer carbonate doping, a new peak
attributed to its low content or high dispersion of metal located at 532.0 eV could be observed, which is possibly attri-
nanoparticles.41–43
bute to the oxygen vacancies.48,49 To conrm the existence of
XPS technique is usually employed to investigate the chem- oxygen vacancies, the ESR spectra were measured. As can be
ical state and prove the effect of doping.44,45 The survey XPS seen in Fig. 2h, the 3C/BMO sample presents a strong signal
spectra and high-resolution XPS spectra of various elements for peak at g ¼ 2.002, which is regarded as the typical characteristic
pristine BMO, 3C/BMO, 0.5% Pd–3C/BMO are displayed in of surface oxygen vacancies.49,50 It is generally accepted that
Fig. S6† and 2. Two strong peaks located at 159.6 eV and doping could induce the crystal defects. Thus, the existence of
164.6 eV in Fig. 2a are assigned to Bi 4f7/2 and Bi 4f5/2, respec- oxygen vacancies could further conrm the successful doping of
tively. Another two strong peaks at 232.5 eV and 235.6 eV in carbonate into the crystal lattice of Bi2MoO6. The high-
Fig. 2b are attributed to Mo 3d5/2 and Mo 3d3/2, respectively. It is resolution XPS spectra of C 1s are displayed in Fig. 2d. The
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spectra of 3C/BMO can be deconvoluted into three peaks at result reveals that Bi, Mo, O, C and Pd are found and the cor-
284.8, 286.4 and 288.2 eV. The former peak belongs to the responding element mapping images presented in Fig. 3e
adventitious carbon from the instrument.14 The latter two peaks display the distribution of individual elements Bi, C and Pd in
are assigned to C–OH and C–OOH bond,51,52 respectively, which the 0.5% Pd–3C/BMO sample, conrming that Pd nanoparticles
further conrms the presence of carbonate aer doping. The are uniformly dispersed in the 0.5% Pd–3C/BMO. From SEM
absence of band at 283.1 eV excludes a Bi–C bond formation or analysis, it is seen an apparent particle size reduction, which is
carbon doping.53 The binding energies at 83.8 and 87.4 eV in further conrmed by zeta potential measurement (Fig. S8†).
Fig. 2e are attributed to Au 4f7/2 and Au 4f5/2, respectively, which From Fig. S8,† it is seen that the particle size distribution of
are characteristic of metallic Au.18 Fig. 2f shows two obvious BMO appeared at 50 nm, which is larger than that of 33 nm for
peaks located at 368.2 and 374.2 eV, assigning to Ag 3d5/2 and Ag 3C/BMO. Particle size reduction oen accomplishes with
3d3/2, respectively. The span between the two peaks is 6.0 eV, increase of surface to volume ratios, which is of great impor-
indicating the presence of Ag0.33 For the high-resolution Pd 3d tance to photocatalytic activity. Fig. S9† shows the N2 adsorp-
XPS spectra in Fig. 2g, the peaks located at 335.2 and 340.5 eV tion–desorption isotherms of pristine BMO, 3C/BMO and
belong to Pd 3d5/2 and Pd 3d3/2, respectively, which is an various noble metal loaded samples and the resultant specic
evidence of metallic Pd.34 The quantitative elemental analysis surface areas and pore volumes are listed in Table S2.† Aer
was conducted from the XPS data, as shown in Table S1.† From carbonate doping, the reduced thickness and width remarkably
XPS spectra, the surface Bi/Mo molar ratio was determined to be enhanced the specic surface area, which increases from 1.80
2.15 for pristine BMO, which is smaller than that of 2.69 for 3C/ m2 gꢀ1 to 40.8 m2 gꢀ1 (nearly 22 times enhanced). The loading
BMO. The variation of Bi/Mo molar ratio suggested the incor- of Au, Ag and Pd also contributes to the specic surface area,
poration of carbonate groups into Bi2MoO6 host matrix. For 3C/ which would be benecial for the photocatalytic activity.
BMO, the Bi/C molar ratio was determined to be 2.34. Moreover,
the loading content of Ag, Au, and Pd was also determined by 3C/BMO and 0.5% Pd–3C/BMO. The TEM image of pristine
XPS peaks. BMO in Fig. 4a reveals the nanosheet-like morphology, which is
Fig. 4 displays the TEM and HRTEM images of pristine BMO,
The SEM images of pristine BMO, 3C/BMO and 0.5% Pd–3C/ consistent with the SEM result. The lattice fringes with the
BMO are displayed in Fig. 3. The pristine BMO presents a sheet- d spacing of 0.275 nm shown in Fig. 4b match well with the
like morphology with no more than 500 nm in width. In (002) crystalline plane of Bi2MoO6. In contrast to pristine
contrast to the morphology of pristine BMO, the morphology of Bi2MoO6, carbonate doping makes no obvious changes on the
3C/BMO has no obvious changes except the reduced thickness
and width, which result resembles that of Ce-doped Bi2MoO6.10
Moreover, the loading of noble metals on the surface of 3C/BMO
also makes no effect on the morphology of 3C/BMO (Fig. 3c and
S7†). To prove the presence of noble metals (taking Pd loaded
sample as an example), the energy dispersive X-ray (EDX)
analysis was employed and the results is shown in Fig. 3d. The
Fig. 3 (a, b) SEM images of BMO and 3C/BMO, (c) SEM image, (d) EDX
spectra and (e) STEM image and the corresponding element mapping Fig. 4 TEM and HRTEM images of BMO (a, b), 3C/BMO (c, d) and 0.5%
of 0.5% Pd–3C/BMO.
Pd–3C/BMO (e, f).
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morphology of BMO but with reduced thickness and width for incident light frequency, constant, and band gap energy,
nanosheets (Fig. 4c). Fig. 4d shows that the lattice fringes with respectively. As a result, the band gap energies are determined
the d spacing of 0.232 nm correspond with the (221) crystalline to be ꢁ2.60 eV and ꢁ2.31 eV for pristine BMO and 3C/BMO,
plane of Bi2MoO6. Fig. 4e displays the TEM image of 0.5% Pd– respectively. Apparently, a red shi of ꢁ0.29 eV for the band
2ꢀ
3C/BMO. It can be clearly seen that Pd nanoparticles with an gap energy occurred aer CO3 doping. Fig. 5b shows the VB-
average diameter of 5–10 nm are uniformly dispersed on the XPS spectra of BMO and 3C/BMO samples. The pristine BMO
surface of nanosheets. In HRTEM image of Fig. 4f, the lattice sample showed the edge of the maximum energy at about
fringes with the d spacing of 0.328 nm are ascribed to the (140) 2.28 eV, whereas the valence band edge of 3C/BMO shied to
crystalline plane of Bi2MoO6 and the lattice fringes with the 1.98 eV. This red shi value of the valence band edge is in
d spacing of 0.225 nm match well with the (111) crystalline accordance with that of the band gap energy narrowing, which
plane of Pd, which conrms the successful loading of Pd on the gives an evidence that the variation of the band gap energy aer
2ꢀ
surface of Bi2MoO6. The TEM and HRTEM images as well as CO3 doping mainly originates from the upward shi of the
EDS data of 1.2% Au–3C/BMO and 0.9% Ag–3C/BMO are pre- valence band edge. To further verify this presumption, Mott–
sented in Fig. S10.† The lattice fringes with the d spacing of Schottky measurement was used to investigate the conduction
0.235 and 0.238 nm respectively belonged to the (111) crystal- band positions of the as-prepared samples. Positive plots reveal
line plane of Au and (111) crystalline plane of Ag and EDS data an n-type character of both pristine BMO and 3C/BMO (Fig. 5c).
also prove the existence of Au and Ag nanoparticles on the The at band potential of both pristine BMO and 3C/BMO is
surface of Bi2MoO6. Moreover, the particle sizes of Ag and Au is nearly identical, which is determined to be ꢀ0.85 V versus Ag/
close to that of Pd nanoparticle, which showed diameter of 5– AgCl electrode, being equivalent to ꢀ0.63 V versus NHE. As
10 nm.
well described in previous literatures,55 the conduction band of
Associated with the photocatalytic activity is the electronic semiconductor is close to the at band potential. The conduc-
structure and band edge potentials of semiconductors that are tion band edge of both pristine BMO and 3C/BMO is estimated
highly dependent on the perturbation of lattice structure. As to be ꢀ0.63 V versus NHE, suggesting high redox potential for
2ꢀ
shown in Fig. 5a, either pristine BMO or CO3 doped BMO photocatalytic reactions. Moreover, the valence band edge of
samples exhibited intense absorption band in the range of 200– both pristine BMO and 3C/BMO can also be estimated to be
500 nm, assigning to the typical electronic transition from O 2p/ 1.97 V and 1.68 V versus NHE, respectively. The Mott–Schottky
Bi 6s to Mo 4d orbitals, which is veried by the following DFT result is greatly consistent with the VB-XPS observation. As for
calculations. Interestingly, it is noted that a relatively weakened the enhanced absorption spectra among 500–800 nm for
broad absorption ranging from 500 to about 800 nm is observed carbonate doped samples is possibly related to oxygen vacan-
for all CO32ꢀ doped BMO samples. Inset of Fig. 5a displays the cies,48,56 which can be further conrmed by the following DFT
color changes of the as-prepared samples that varies from light- calculations. Fig. 5d shows the UV-vis reectance spectra of
2ꢀ
yellow to brown, providing indirect evidence of CO3 anionic noble metals loaded on 3C/BMO samples. As shown in Fig. 5d,
group doping rather than carbon doping.14 Since Bi2MoO6 gives cocatalysts modication has minor impact on the absorption
an indirect electronic transition feature,54 the band gap energy edge of 3C/BMO. For instance, the absorption feature of 0.5%
Eg can be estimated by the following equation: ahn ¼ A(hn ꢀ Pd loaded 3C/BMO is close to that of 3C/BMO. However, for Ag
Eg)1/2, where a, n, A, and Eg are the absorption coefficient, and Au loaded samples, a weakened absorption band in the
range of 500–700 nm appeared, which can be ascribed to the
surface plasmon resonance of Ag and Au, respectively.
Having the above results in mind, it is therefore necessary to
2ꢀ
understand the doping effect of CO3 anionic group on the
modication of the electronic structure and properties. The
2ꢀ
detailed knowledge of CO3
anionic group doping on the
electronic structure modulation of Bi2MoO6 is obtained by
calculating the band structure and partial density of states on
the basis of DFT. On the grounds of the Bi2MoO6 unit cell,
a supercell containing 72 atoms is employed for pure Bi2MoO6,
as illustrated in Fig. 6a. Two different doping models based on
this supercell are considered: one model is obtained by
replacing one Mo with a C atom (denoted as C/Bi2MoO6). One
defective model is obtained by deleting an oxygen near Mo site
(denoted as VO/Bi2MoO6) because oxygen vacancy was detected
in 3C/BMO sample (Fig. 2h) and meanwhile the doping of
2ꢀ
CO3
anionic group in Bi2MoO6 host matrix can produce
Fig. 5 UV-vis reflectance spectra of various samples (a). VB-XPS
spectra of pristine BMO and 3C/BMO (b). Mott–Schottky plots of
pristine BMO and 3C/BMO (c). UV-vis reflectance spectra of noble
metals loaded on 3C/BMO (d). Inset figure reflects the colour of the as-
prepared samples.
oxygen vacancies due to the charge compensation effect. As
shown in Fig. 6b, Bi2MoO6 shows an indirect optical transition
character, which is in accordance with previous literature.54 The
band gap energy for pure Bi2MoO6 is calculated to be 1.899 eV.
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Fig. 6 Supercell structural models of Bi2MoO6 used for calculation (a), band structures and the corresponding density of states of pure Bi2MoO6
(b), CO32ꢀ doped Bi2MoO6 (c) and Bi2MoO6 with oxygen vacancy (d).
The optical band edge is originated from the electronic transi- electronic band was observed at ꢀ0.02 eV, which mainly
tion between O 2p/Bi 6s and Mo 4d orbitals. From Fig. 6b, it is consists of Bi 5s, Bi 5p orbitals and a little hybridization of Mo
seen that the minimum conduction band of Bi2MoO6 mainly 4d orbital, being close to the conduction band minimum. The
consists of the Mo 4d orbital and a minor hybridization of Bi 5p electronic gap was calculated to be about 1.73 eV between the
orbital in the energy region from 1.79 to 2.97 eV, as is illustrated electronic transition from O 2p, Bi 6s and Bi 5p orbitals to Bi 5s,
in Fig. S11.† The maximum valence band consists of the Bi 5p and Mo 4d orbitals. Since both C/Bi2MoO6 and VO/
hybridization of O 2p and Bi 6s orbitals in the energy region Bi2MoO6 models predict new electronic bands, the band gap
from 0 eV to ꢀ6.21 eV (Fig. S11†). With the model of C/Bi2MoO6, narrowing and weakened visible light absorption is thought to
2ꢀ
a new band contributed from the hybridization of O 2p and Bi be related to CO3 doping and defect chemistry. Moreover,
6s orbitals is observed, which is centered at about 0.52 eV from Fig. 6d, it is clear that the construction of oxygen vacancy
(Fig. S12†). The appearance of a new electronic band can in Bi2MoO6 host matrix can narrow the band gap energy and
evidently result in the band gap narrowing of Bi2MoO6. The gap meanwhile lead to the downward shi of the conduction band.
energy between the new band and Mo 4d orbital is determined However, on the basis of VB-XPS and Mott–Schottky results, an
to be 1.22 eV, which contracts about 35.8% in comparison to upward shi of the valence band is observed, which is contrary
that of pure Bi2MoO6. However, the contraction of band gap to the oxygen vacancy model. Therefore, the valence band edge
2ꢀ
energy of 3C/BMO with pristine BMO was calculated to be shi may be mainly attributed to CO3 doping effects.
11.2%, which is much smaller than the theoretical predication.
On the other hand, the original electronic gap between O 2p/Bi
6s and Mo 4d orbitals is about 1.72 eV (Fig. 6c), which shows the
3.2 Photocatalytic test
contraction of about 9.43% in comparison with pure Bi2MoO6, Organic dye RhB and colorless OPP are selected as the typical
being close to the experimental data. Therefore, the new elec- pollutants to evaluate the photocatalytic activity of as-prepared
tronic band probably contributes to the weakened absorption in samples. Xe lamp with two kinds of luminous power including
the range of 500–800 nm. As for VO/Bi2MoO6 model (Fig. 6d), the 300 W and 500 W are employed as the light source. Before
minimum conduction band consists of the hybridization Mo irradiation, the suspension was placed in dark with continu-
4d, O 2p and Bi 5p orbitals in the energy region from 0.20 eV to ously stirring to obtain adsorption–desorption equilibrium
1.18 eV (Fig. S13†). The maximum valence band consists of the (Fig. S14†). The photocatalytic activities of various samples
hybridization of O 2p, Bi 6s and Bi 5p orbitals in the energy toward RhB with different luminous power are shown in Fig. 7.
region from ꢀ1.75 eV to ꢀ7.89 eV (Fig. S13†). Moreover, a new As shown in Fig. 7a, the pristine Bi2MoO6 showed poor
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Fig. 7 Photocatalytic activities of pristine BMO with different carbonate doping contents (a), 3C/BMO with different Pd loading contents (b) with
luminous power of 300 W and pristine BMO, 3C/BMO as well as various noble metals loaded samples with luminous power of 300 W (c) and
500 W (d) under UV + visible light irradiation toward RhB.
photocatalytic activity toward RhB under UV + visible light in Fig. S15a.† As shown in Fig. S15a,† the noble metals loading
irradiation with a luminous power of 300 W. With an increase of on the surface of pristine Bi2MoO6 is also favourable for
carbonate doping content, the photodegradation efficiency was enhancing the photocatalytic activity of Bi2MoO6. The sample
heavily enhanced, which could reach nearly 100% within 0.5% Pd–BMO showed the best photodegradation efficiency,
180 min. Fig. 7b shows the photodegradation efficiency of 3C/ which could degrade nearly 90% of RhB within 4 h. This pho-
BMO with various Pd loading content. The photocatalytic tocatalytic performance is signicantly poorer than sample
activity was further enhanced aer Pd loading and could be 0.5% Pd–3C/BMO, which indicates that carbonate doping plays
adjusted via changing the Pd loading content. The sample 0.5% a more essential role in enhancing the photocatalytic activities.
Pd–3C/BMO showed the best activity, which could completely Incontrast to the photodegradation efficiencies of various
degrade RhB within 90 min. It is obvious that the photo- samples in Fig. 7c, the photodegradation efficiencies were
degradation time could be shorten in half aer Pd loading. The further enhanced by using the 500 Xe lamp instead of 300 W Xe
photodegradation efficiencies of samples Ag–3C/BMO and Au– lamp (Fig. 7d). The sample 0.5% Pd–3C/BMO showed the best
3C/BMO with various loading contents are shown in Fig. S15b photodegradation efficiency, which could reach nearly 100%
and c.† The results presented that samples 0.9% Ag–3C/BMO within 40 min. The photodegradation time is shorten in half
and 1.2% Au–3C/BMO showed the best photodegradation effi- again, which reveals that the luminous power plays an impor-
ciencies. Fig. 7c displays the photodegradation efficiencies of tant role in the photocatalytic process. Based on the above
pristine Bi2MoO6, carbonate doped BMO (3C/BMO) and 3C/ results, it could be concluded that the photocatalytic activity of
BMO loaded various noble metals. It could be found that all pristine Bi2MoO6 could be remarkably enhanced by carbonate
the noble metals, including Au, Ag as well as Pd loading doping and noble metals loading as well as increasing the
contributed to the photocatalytic activity. As shown in Fig. 7c, luminous power. The photodegradation time could be shorten
the photodegradation efficiency order was as follows: 0.5% Pd– from 180 min for pristine Bi2MoO6 to 40 min for 0.5% Pd–3C/
3C/BMO > 1.2% Au–3C/BMO > 0.9% Ag–3C/BMO > 3C/BMO > BMO under UV + visible light irradiation. Moreover, the pho-
BMO. For comparison, pristine Bi2MoO6 without carbonate tocatalytic performance of commercial TiO2 was also given for
doping but with the same Au, Ag as well as Pd loading contents comparison (Fig. 7d). It is seen that the photocatalytic activity of
(0.5% Pd–BMO, 1.2% Au–BMO and 0.9% Ag–BMO) were also 0.5% Pd–3C/BMO is higher than that of P25 TiO2. In addition,
synthesized and the photocatalytic performances are displayed stability of a photocatalyst is indispensable for its practical
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application. Thus, cycling experiments were conducted to check a 550 nm bandpass lter (Fig. S16†), the RhB degradation was
the stability of sample 0.5% Pd–3C/BMO under UV + visible selected as the model reaction to test the surface plasmon
light irradiation with the luminous power of 500 W and the resonance effect of Ag and Au on the photocatalytic perfor-
results are shown in Fig. S15d.† There is about 28% reduction mance. As shown in Fig. 9, under 550 nm light irradiation,
aer ve successive cycles. This nding indicates that 0.5% Pd– either 3C/BMO or noble metal modied 3C/BMO displayed
3C/BMO possesses certain stability during the photocatalytic superior photocatalytic activities than that of pristine BMO.
process, which implies the potential application of this Meanwhile, the photocatalytic activity of 0.5% Pd–3C/BMO and
photocatalyst.
0.9% Ag–3C/BMO is close to that of 3C/BMO. This result may
The photocatalytic performances of as-prepared samples are imply that the surface plasmon resonance of Ag may have little
further checked by degrading OPP under UV + visible light or impact on the photodegradation of RhB. On the other hand, the
visible light irradiation with the same luminous power of 500 W photodegradation efficiency of 1.2% Au–3C/BMO is about 44%,
and the results are shown in Fig. 8. As shown in Fig. 8, pristine which is higher than that of 3C/BMO and 0.5% Pd–3C/BMO,
Bi2MoO6 displays no photocatalytic activity toward OPP under suggesting that the surface plasmon resonance effect of Au
either UV + visible light or visible light. Aer carbonate doping, plays a critical role in the RhB photocatalytic degradation
the photodegradation efficiency of Bi2MoO6 was dramatically process. According to scavenger trapping experiment, photo-
enhanced, which reached 40% within 60 min under UV + visible generated holes act as dominant active species toward RhB
light irradiation and 50% within 120 min under visible light degradation (Fig. 11). The injection of photogenerated electrons
irradiation, respectively. Various noble metals loading make from Au nanoparticles to the conduction band of 3C/BMO,
considerable contribution to enhancing the photodegradation resulting positively charged Au nanoparticles. The oxidation
efficiency. The photodegradation efficiencies of 0.9% Ag–3C/ capability of the positively charged Au nanoparticles may be
BMO, 1.2% Au–3C/BMO and 0.5% Pd–3C/BMO are 67.2%, weaker than the photogenerated holes in 0.5% Pd–3C/BMO.
76.3%, 96.8% under UV + visible light irradiation within 60 min
and 72.7%, 86.1%, 100.0% under visible light irradiation under
within 120 min, respectively. Similarly, the sample 0.5% Pd–3C/
BMO also showed the best photodegradation efficiency. Similar
to the result of RhB degradation, the photocatalytic activity of
0.5% Pd–3C/BMO toward OPP degradation is much higher than
that of commercial TiO2.
Basically, the addition of noble metal nanoparticles to
semiconductors can accelerate the charge separation efficiency
under light irradiation. Particularly, certain nanoparticles, such
as Ag, and Au, were also found to be effective in enhancing
visible light absorption and photocatalytic activity, which is
ascribed to plasmon enhanced electric elds and electron
injection from the plasmonic nanoparticles to the conduction
band of semiconductors. To specify the surface plasmon reso-
nance effect of Ag, and Au on the photocatalytic performance of
Bi2MoO6 photocatalyst, we conducted control experiment by
using a bandpass lter (550 nm) with luminous power of 500 W.
Since the as-prepared Bi2MoO6 based photocatalysts exhibited
very weak photocatalytic activity toward OPP degradation using
Fig. 9 The photocatalytic activities of pristine BMO, 3C/BMO as well
as various noble metals loaded samples toward RhB degradation by
using a bandpass filter (550 nm) with luminous power of 500 W.
Fig. 8 The photocatalytic activities of pristine BMO, 3C/BMO as well as various noble metals loaded samples under UV + visible light (a) and
visible light (b) irradiation with luminous power of 500 W toward OPP.
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Therefore, it is expected that 0.5% Pd–3C/BMO possesses the
optimal photocatalytic performance.
The photocatalytic processes toward RhB and OPP could be
further quantitatively assessed through employing the pseudo-
rst-order kinetics, which is expressed as ln(C0/C) ¼ kt, where
C0 and C is the concentration of RhB or OPP at t ¼ 0 and at time
t, respectively. k is the pseudo-rst-order rate constant, which
can be obtained from the reduction of the peak intensity at
552 nm for RhB and 282 nm for OPP with increasing time. The
photodegradation rates of RhB and OPP over different samples
are shown in Fig. S17† and the calculated constant k are dis-
played in Fig. 10. As shown in Fig. 10a, the k values with lumi-
nous power of 500 W are considerably higher than the
counterparts with luminous power of 300 W. For sample 0.5%
Pd–3C/BMO, the k value (500 W) is nearly 5 times as that of value
Fig. 11 Effects of various scavengers on the photodegradation of RhB
with luminous power of 300 W. Fig. 10b displays the degrada- and OPP over 0.5% Pd–3C/BMO under UV + visible light and visible
light irradiation, respectively.
tion constants toward OPP under different light irradiation.
Whether under visible light or UV + visible light irradiation, the
k values of pristine Bi2MoO6 are nearly close to zero. Aer
ꢀ
role in the photodegradation of RhB as well as OPP and cO2
properly contributes to the photocatalytic efficiency.
carbonate doping and noble metals loading, the maximum k
value could reach 0.0234 and 0.0286 minꢀ1 under visible light
or UV + visible light irradiation, respectively.
To further specify the active species involved in the photo-
catalytic process, ESR technique was carried and the results are
shown in Fiꢀg. 12. In brief, DMPO was used as trapping agent to
capture cO2 and cOH active species. For comparison, the ESR
3.3 Photocatalytic mechanism
To comprehensively understand the photocatalytic process, spectra of DMPO–cO2ꢀ and DMPO–cOH without photocatalysts
scavenger trapping experiments were conducted. Fig. 11 were also measured, which rꢀesults are shown in Fig. S19.† No
displays the results aer adding various scavengers in the signal of either DMPO–cO2 or DMPO–cOH was observed,
ꢀ
photodegradation process of RhB and OPP under visible light indicating that active species of cO2 and cOH could not be
irradiation. Benzoquinone (BQ) employed as an cO2ꢀ scavenger, produced without photocatalysts. As illustrated in Fig. 12,
isopropanol (IPA) as an cOH scavenger, AgNO3 as a scavenger of pristine BMO exhibited no apparent ESR signal of DMPO–cOH
eꢀ and ammonium oxalate (AO) as the scavenger of h+, were even prolonging the irradiation time. Whereas the character-
ꢀ
added to the photocatalytic reaction system, respectively. istic sextet peaks of DMPO–cO2 ESR signal was observed with
Details of UV-visible absorption spectra recorded during pho- an increase of visible light irradiation time for Bi2MoO6
tocatalytic degradation of OPP and the residual fraction of OPP (Fig. 12b). As for 3C/BMO and 0.5% Pd–3C/BMO, the ESR signal
in solution as a function of reaction time over 0.5% Pd–3C/BMO of DMPO–cOH appeared under visible light irradiation (Fig. 12c
2ꢀ
by adding various scavengers were given in Fig. S18.†
and e), predicting that CO3
doping and Pd loading can
As shown in Fig. 11, it is clearly seen that the photo- improve the capability of cOH generation for Bi2MoO6. More-
degradation of RhB and OPP are heavily depressed by adding over, it is seen that the intensity of DMPO–cO2ꢀ ESR signal also
2ꢀ
AO and slightly inhibited aer the addition of BQ. The partici- greatly increased aer CO3 doping and noble metal loading
2ꢀ
pation of IPA and AgNO3 has no inuence on the photocatalytic (Fig. 12d and f). This observation hinted that CO3 doping in
efficiency. This phenomenon reveals that h+ plays the dominant Bi2MoO6 can enhance the charge separation efficiency and the
Fig. 10 The photocatalytic constant k toward RhB (a) and OPP (b) over various samples.
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Fig. 12 ESR spectra of DMPO–cOH (a, c and e) and DMPO–cO2ꢀ (b, d and f) adducts of pristine BMO, 3C/BMO and 0.5% Pd–3C/BMO in darkness
and under visible light irradiation with different irradiation times.
subsequent photocatalytic activity, which is veried by the factors should be responsible for the boosting photocatalytic
following photocurrent measurement. In addition, noble activity of Bi2MoO6 by CO32ꢀ doping and noble metals loading.
metals loading can further improve the intensity of DMPO–cOH Since photocatalytic reaction occurs on semiconductor
ꢀ
and DMPO–cO2 ESR signal, which can further increase the surfaces, the surface to volume ratio may impose great impact
charge separation of photogenerated carriers as well the pho- on the photocatalytic performance of Bi2MoO6 samples. For the
tocatalytic performance.
present Bi2MoO6 samples, the BET surface areas are deter-
It is now necessary to specify the origin of the highly mined to be 52–57 m2 gꢀ1 for noble metals loaded 3C/BMO
2ꢀ
enhanced photocatalytic activity of Bi2MoO6 by CO3 doping samples, which are much larger than that of 1.8 m2 gꢀ1 for
and noble metals loading. Basically, several factors should be pristine Bi2MoO6 sample. Principally, an increase of surface
taken into consideration, including electronic structure, area leads to more active sites and enhances the photocatalytic
surface area, doping effects, cocatalysts and so forth. In view of activity of semiconductors. Hence, the increased surface areas
solid state physics, any perturbation of lattice structure can of noble metals loaded 3C/BMO samples possibly contributed
2ꢀ
have consequences on the electronic structure. By CO3
to the improvement of the photocatalytic activity, nearly being
doping, upward valence band edge of Bi2MoO6 was achieved, in accordance with the trend of the photocatalytic performance
which promises to weakened driving force of photogenerated toward RhB and OPP degradation. As for doping effects, it is
holes. The higher valence band edge of Bi2MoO6 induces poor well accepted that the incorporation of foreign doping ions into
photocatalytic activity toward oxidation reaction, which is in semiconductors can have consequences on the microstructure,
contrast to the observed results in Fig. 10. Therefore, other electronic structure as well as charge transfer process.
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Fig. 13 Transient photocurrent responses (a), electrochemical impedance spectroscopy (b) and photoluminescence spectra (c) for pristine
BMO, 3C/BMO and 0.5% Pd–3C/BMO. The mechanism illustration of 0.5% Pd–3C/BMO (d).
Moreover, the impurity ions can not only decrease the photo- efficiency, charge transfer rate greatly enhance the photo-
threshold energy but also serve as trap centers for charge catalytic activity. On the basis of the above results, a plausible
separation and improvement of electrical conductivity.57 The photocatalytic mechanism was proposed, as illustrated in
separation and transfer efficiency of the photogenerated charge Fig. 13d. Under visible light or UV + visible light irradiation, the
carriers were explored using transient photocurrent responses electrons are excited from valence band or defect states to
with light on/off cycles. As illustrated in Fig. 13a, the photo- conduction band of Bi2MoO6, which subsequently migrate to
current density of pure Bi2MoO6 reaches at about 6.25 mA, the surface of noble metals and are captured by O2 to produce
2ꢀ
which was greatly improved to 10.48 mA aer CO3 doping. cO2ꢀ. Subsequently, theꢀholes remained in valence band and
Moreover, the photocurrent density was further increased to the photogenerated cO2 in conduction band are involved in
13.54 mA via noble metal loading. Stronger photocurrent the photocatalytic reaction.
density oen promises higher separation efficiencies of pho-
togenerated electrons and holes.58 The improved charge sepa-
ration efficiency of Bi2MoO6 aer CO3 doping and noble
4. Conclusions
2ꢀ
metal loading was further investigated by photoluminescence Anionic group doping and noble metal loading as two prom-
spectra (Fig. 13b). As a self-activating phosphor, Bi2MoO6 ising strategies to construct a novel photocatalytic material for
exhibits a broad, intrinsic blue-green emission band centered efficient photodegrading water organic pollutants. Carbonate
at about 472 nm under UV light excitation. The incorporation doping induced the generation of oxygen vacancies and band
2ꢀ
of CO3 groups and noble metal loading on Bi2MoO6 caused gap narrowing, which are benecial for visible light harvest and
efficient depression of the blue-green luminescence, which photogenerated charges separation. Nobel metals (Au, Ag and
suggests promoted separation of photogenerated charge Pd) as cocatalysts further improved the photogenerated charges
2ꢀ
carriers and promises enhanced photocatalytic activity. The separation efficiency. By CO3
doping and noble metal
Nyquist plot data of the as-prepared samples were measured in loading, the photocatalytic activities of Bi2MoO6 toward RhB
order to investigate the process of electron transfer. As illus- and OPP were enormously enhanced. The synthesized 0.5% Pd–
trated in Fig. 13c, the arc radius of the Nyquist plot of 3C/BMO 3C/BMO photocatalyst is of potential importance for water
sample is smaller than that of pure Bi2MoO6, implying a faster organic pollutants.
transfer rate of charge carriers in the former, leading to the
effective separation of electron–hole pairs.59 Moreover, noble
metal loading further improves the charge separation effi-
Acknowledgements
ciency and the subsequent photocatalytic reactivity. As This work was supported by the National Natural Science
a
consequence, the highly improved charge separation Foundation of China (NSFC, 51462025).
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27 J. Di, J. Xia, M. Ji, H. Li, H. Xu, H. Li and R. Chen, Nanoscale,
2015, 7, 11433–11443.
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