Engineering the dimension and crystal structure of bismuth molybdate photocatalysts via a molten salt-assisted assembly approach

Article abstract of DOI:10.1016/j.jallcom.2020.156231

In this work, a facile molten salt-assisted assembly strategy was studied to achieve controlled synthesis of bismuth molybdate photocatalysts including Bi₂MoO₆, Bi₆Mo₂O₁₅, Bi₄MoO₉ and Bi₁₄MoO₂₄. The transition of bismuth molybdates from Aurivillius-type to Fluorite-type could be selectively and efficiently achieved by adjusting the molten salt synthesis time. It was observed that the bismuth molybdate samples exhibited an obvious redshift in the absorption band with increasing of Bi/Mo molar ratio (from 3 to 14) in the materials, indicating a reduced band-gap. As a result, the photocatalytic activity of the Fluorite-type Bi₁₄MoO₂₄ sample for TC removal was higher than that of Bi₂MoO₆, Bi₆Mo₂O₁₅ and Bi₄MoO₉ samples under a visible light irradiation. Furthermore, the mechanisms of phase change between Aurivillius-type and Fluorite-type as well as their band structures of the bismuth molybdates were systematically investigated in this work. This study not only provides a new strategy for the controlled synthesis of bismuth molybdates with different dimensionalities and structures, but also opens a new pathway to the design and optimization of advanced bismuth molybdates-based photocatalysts for applications in energy harvesting and environmental remediation.

Full text of DOI:10.1016/j.jallcom.2020.156231

Journal Pre-proof
Engineering the dimension and crystal structure of bismuth molybdate photocatalysts
via a molten salt-assisted assembly approach
Gui Yang, Yujun Liang, Kai Li, Jian Yang, Kun Wang, Rui Xu, Xianjun Xie
PII:
S0925-8388(20)32595-0
DOI:
https://doi.org/10.1016/j.jallcom.2020.156231
JALCOM 156231
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 9 September 2019
Revised Date: 27 June 2020
Accepted Date: 27 June 2020
Please cite this article as: G. Yang, Y. Liang, K. Li, J. Yang, K. Wang, R. Xu, X. Xie, Engineering
the dimension and crystal structure of bismuth molybdate photocatalysts via a molten salt-assisted
assembly approach, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/
j.jallcom.2020.156231.
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Author Contributions Section
Gui Yang, Kai Li and Jian Yang designed the system, Gui Yang conducted
the experiment and wrote the manuscript, Rui Xu, Yujun Liang and
Xianjun Xie checked and revised this manuscript, Gui Yang and Kun
Wang did measurement. Gui Yang and Yujun Liang analyzed the data. All
authors took part in this work.

Graphical Abstract

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Engineering the dimension and crystal structure of bismuth
molybdate photocatalysts via a molten salt-assisted assembly
approach
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Gui Yang a, Yujun Liang a, *, Kai Li a, Jian Yang a, Kun Wang a, Rui Xu b, Xianjun Xie
b,*
a
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Engineering Research Center of Nano-Geomaterials of Ministry of Education,
Faculty of Materials Science and Chemistry, China University of Geosciences，Wuhan
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430074, China
b
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State Key Laboratory of Biogeology and Environmental Geology & School of
Environmental Studies, China University of Geosciences, Wuhan 430074, China.
*Corresponding author: Tel.: +86 27 67884814; fax: +86 27 67883733.
E-mail address: yujunliang@sohu.com (Yujun Liang); xjxie@cug.edu.cn (Xianjun
Xie)
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Abstract
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In this work, a facile molten salt-assisted assembly strategy was studied to achieve
controlled synthesis of bismuth molybdate photocatalysts including Bi2MoO6,
Bi6Mo2O15, Bi4MoO9 and Bi14MoO24. The transition of bismuth molybdates from
Aurivillius-type to Fluorite-type could be selectively and efficiently achieved by
adjusting the molten salt synthesis time. It was observed that the bismuth molybdate
samples exhibited an obvious redshift in the absorption band with increasing of Bi/Mo
molar ratio (from 3 to 14) in the materials, indicating a reduced band-gap. As a result,
the photocatalytic activity of the Fluorite-type Bi14MoO24 sample for TC removal was
higher than that of Bi2MoO6, Bi6Mo2O15 and Bi4MoO9 samples under a visible light
irradiation. Furthermore, the mechanisms of phase change between Aurivillius-type
and Fluorite-type as well as their band structures of the bismuth molybdates were
systematically investigated in this work. This study not only provides a new strategy
for the controlled synthesis of bismuth molybdates with different dimensionalities and
structures, but also opens a new pathway to the design and optimization of advanced
bismuth molybdates-based photocatalysts for applications in energy harvesting and
environmental remediation.
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18 Keywords: photocatalysts, bismuth molybdates, molten salt, Aurivillius-type,
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Fluorite type
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1. Introduction
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Bismuth-based oxides have attracted great research interest due to their promising
physical and chemical properties such as oxygen ion conductivity, ferro-electricity,
and catalytic capability [1-6], and thus have potential application in photochemical
catalysts. Most of the bismuth-based oxides in the Bi-M-O (M = Nb, Ta, Cr, W, or Mo)
system have Fluorite-type crystal structure (Fig. 1a) which shows a high ionic
conductivity due to the high polarizability caused by Bi-6s single-molecule pairs in
the structure. This high ionic conductivity enables a rapid migration of electrons and
holes during the photochemical catalysis process and thus benefits the catalytical
efficiency [9-11].
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Fortunately, the Type II Bi2O3-MoO3 phases have been proven to crystallize in
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the superspace group Fm3m(α,α,α)q00(α,α,α)q00(α,α,α)000 (no. 225.3.215.7; P: Fm3
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m: Fm3m) with incommensurate modulation parameters in a relatively short range of
observations (0.292 < ε < 0.299), i.e., these crystals do not expose the d-glide plane
for the modulated structure (Fig. 1a). The highly symmetrical internal structures may
lead to the tetrahedral coordination of the surrounding dopant atoms (Mo). Therefore,
the Mo6+-doped phases often exhibit the Fluorite type sublattice for the heavy atoms.
Recently, several phases in the Bi2O3-MoO3 binary system (Bi/Mo ≥ 2) have been
extensively studied as photochemical catalysts and ionic conductors, including
Bi14MoO24 [12], Bi3.64Mo0.36O6.55 [13], Bi38Mo7O78 [14], Bi4MoO9 [15], Bi6Mo2O15
[16], Bi26Mo10O69 [17] and Bi2MoO6 (γ -phase) [18, 19]. As displayed in Fig. 1b,
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these Bi-Mo-O materials can be considered as the binary combination of Bi2O3 and
MoO3, and the material crystalline structures and related physical properties depend
on the concentration ratio of Bi2O3 and MoO3 in the materials. A lower Mo metal
oxide concentration (5% - 30%) will lead to a Fluorite-type crystal structure, while a
higher concentration will lead to the formation of Aurivillius phase Bi2MoO6. The
catalytical behavior difference between Fluorite and Aurivillius phase bismuth
molybdates is caused by their different crystalline structures. Therefore, it is essential
to have a deep understanding of the transition between the Aurivillius phase and
Fluorite phase in these bismuth molybdates which will benefit the design and
fabrication of highly efficient catalysts based on bismuth molybdates.
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Furthermore, because the photocatalytic performance of a catalyst strongly
depends on its particle size dimensions and crystal structure [25], it is crucial to
synthesize catalysts with controlled dimensions and structure. Various physical and
chemical methods including solvothermal (hydrothermal) reaction [26-30], solid-state
reactions [31], co-precipitation [12] and sol-gel methods [14], have been used to
synthesize bismuth molybdates with different dimensions and crystal structures. For
example, Long et al. [18] synthesized 1-dimensional (1D) Bi2MoO6 nanobelts with
(010) facets by using an oleylamine-mediated hydrothermal approach, and the
Bi2MoO6 nanobelts exhibited much better photocatalytic activity than those of
nanorod-based microspheres with hierarchical structures. Wang et al. [12, 14]
synthesized 0D Bi14MoO24 and Bi38Mo7O78 nanoparticles with the Fluorite-type
crystal structure via co-precipitation approach and sol-gel method, respectively. The
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synthesized nanoparticles showed highly open structure due to the rich oxygen
vacancies, resulting in a high photocatalytic activity. However, it should be noted that
conventional methods for synthesizing bismuth molybdate such as hydrothermal and
solid-state reaction, showed their severe limitations such as poor safety, complex
operation, low product yield, etc [28, 31]. Thus, it is important to explore a new
approach with features of high yield, low cost, increase safety and environment
friendly for synthesizing bismuth molybdate photocatalysts.
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In this work, a facile molten salt-assisted assembly strategy was developed to
achieve controlled synthesis of a series of bismuth molybdate photocatalysts including
Bi2MoO6, Bi6Mo2O15, Bi4MoO9 and Bi14MoO24. By adjusting the molten-salt
synthesis time, we could achieve the phase-structural and dimensional transition of
bismuth molybdates from Aurivillius-type to Fluorite-type selectively and efficiently.
The phases, crystal structures, morphologies and optical properties of bismuth
molybdate photocatalysts were investigated. Moreover, the photocatalytic degradation
efficiency of these bismuth molybdate photocatalysts was also evaluated by the
oxidation of tetracycline (TC) using a 5 W white LED light illumination. It was
observed that the Fluorite-type Bi14MoO24 material showed a better catalytical
performance than that of Bi2MoO6, Bi6Mo2O15 and Bi4MoO9. Besides, the formation
mechanism and band structure change of bismuth molybdates were also studied in
detail.
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2. Experimental section
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2.1. Synthesis of the photocatalysts
Samples of the compounds including Bi2MoO6, Bi6Mo2O15, Bi4MoO9 and Bi14MoO24
were prepared in a special salt system [19]. Typically, bismuth nitrate
(Bi(NO3)3·5H2O) of 1.2127 g and sodium molybdate (Na2MoO4·2H2O) of different
weights were used as the precursors and grinded in an agate mortar for about 15 min.
Subsequently, KNO3 and NaNO3 salt mixtures with a weight ratio of KNO3: NaNO3 =
1:1 were taken as the co-solvents, and the weight ratio of raw materials to molten salts
was 1:8. After being grinded for over 30 mins, the mixtures including salts and raw
materials were placed in an alumina crucible and then calcined at a certain
temperature for 10 h. Different Bi/Mo molar ratios and calcination temperatures were
chosen for the molten salt reaction in order to synthesize different materials: (2:1,
400 °C) for Bi2MoO6; (2:3, 500 °C) for Bi6Mo2O15; (2:1, 500 °C) for Bi4MoO9; and
(4:1, 500 °C) for Bi14MoO24. After the molten salt reaction, the as-obtained catalysts
were washed repeatedly with deionized water and absolute alcohol to remove the
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molten salts, respectively, and the resultant products were dried at 80 C for 24 h for
further characterization and testing. For comparative study, Bi2MoO6 was also
prepared by using solid-phase method (400 °C, 10 h) and hydrothermal method (pH =
5, 150 °C, 10 h).
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2.2. Characterization
Phases in photocatalytic materials were studied by X-ray powder diffractometer
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(XRD) Analysis (Bruker D8 Focus diffractometer) using the Cu-Kα radiation with a
wavelength of 1.5406 Å in the 2θ range of 5°-70°. Fourier transform infrared (FT-IR)
spectra were undertaken using a Nicolet 6700 spectrometer with KBr pellet (Thermo
scientific, USA). The morphologies and microstructures of materials were examined
by the field-emission scanning electron microscope (FE-SEM; Japan SU8010) and
high-resolution transmission electron microscopy (HRTEM; America G2 T20).
Energy Dispersive Spectrometer (EDS) was also performed to detect the component
elements in materials. UV-vis diffuse reflectance spectra (DRS) were recorded on an
UV-2550PC spectrophotometer (Shimadzu Corporation, Japan) equipped with a 150
W xenon lamp as the excitation source, using BaSO4 as a reference. The
photoluminescence (PL) spectra were undertaken with a spectrofluorometer (Horiba
Jobin Yvon FLUOROMAX-4P) under the excitation of a 340 nm line laser. The PL
spectra ranged from 400 to 700 nm, and the emission slit was 2 nm. The
electrochemical impedance spectroscopy (EIS) and transient photocurrent of
photocatalysts were undertaken on an electrochemical workstation (Chenhua
Instruments Co. Shanghai china) in the electrolyte of 0.2 M Na2SO4.
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2.3. Evaluation of photocatalytic performance
Photocatalytic performances of the as-prepared catalysts were investigated by the
photocatalytic decomposition of tetracycline (TC) under visible light irradiation. The
visible light irradiation was applied using a 5 W white LED. In each photocatalytic
experiment, 10 mg of as-obtained catalyst was added into 50 mL of 10 mg·L-1
pollutant aqueous solution. Prior to the visible light illumination, the TC solution was
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magnetically stirred for about 0.5 h to reach the adsorption/desorption equilibrium in
darkness. During the reaction process, about 5 mL reaction solution was extracted and
centrifuged (10000 rpm, 2 min) to separate the catalysts. The peak absorbency of TC
at the wavelength of 357 nm was used to detect its concentrations with an UV-vis
spectrophotometer, where the TC absorbance can be indicated by the corresponding
ratio of residual TC concentration to initial concentration (C/C0).
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3. Results and discussion
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3.1. Crystal phase composition
Fig. 2 showed the typical XRD patterns of the as-prepared catalysts. It is observed
that the peaks shown in the XRD pattern of Bi2MoO6 could be well indexed to the
crystal planes of orthorhombic Bi2MoO6 (JCPDS No. 21-0102). For the Bi6Mo2O15
sample, the XRD peaks were also consistent with the characteristic crystal planes of
Bi6Mo2O15 phase (JCPDS No. 26-0216). Similarly, the XRD peaks of the Bi4MoO9
and Bi14MoO24 samples were also consistent with the characteristic diffraction peaks
listed in the standard card (JCPDS No. 36-0105) and (JCPDS No. 89-6667),
respectively.
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Fig. 3a-d show the crystal structures of Bi2MoO6, Bi6Mo2O15, Bi4MoO9 and
Bi14MoO24 displayed down along the [001] direction and emphasizing the [MoO6]
octahedra and [MoO4] tetrahedra. It could be seen that Bi2MoO6 had a typical
Aurivillius-type structure consisting of alternate stacking of [MoO6] octahedral layers
and [Bi2O2] atomic layers. In contrast, Bi6Mo2O15, Bi4MoO9 and Bi14MoO24 exhibited
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a Fluorite-type super structure which was made up of [MoO4] tetrahedra interspersed
with Bi-O polyhedra. Furthermore, some Fluorite-type bismuth molybdates could
contain Mo-O polyhedra rather than the [MoO4] tetrahedra [32]. The difference
between the Fluorite-type structure and Aurivillius-type structure for bismuth
molybdates was mainly in the cations (Bi3+, Mo6+) distribution. The Fluorite-type
bismuth molybdates were mainly composed by a Fluorite-type super cell with an
infinite channel of Bi-O polyhedra surrounded by [MoO4] tetrahedra. Fig. 3e shows
the initial structural unit based on the above super space model, which considers only
the O atoms surrounded by Bi atoms. It was interesting to note that the
Aurivillius-phase Bi2MoO6 (Bi/Mo = 2) exhibited the alternating [Bi2O2] sheets,
while the Bi6Mo2O15 (Bi/Mo = 3) contained the so-called [Bi12O14] columnar structure.
As the molar ratio of Bi/Mo increased, the favorable topology of Mo-O lattice
imparted a high degree of rotational freedom to the Bi-O bond. Therefore, the
structure of Bi4MoO9 (Bi/Mo = 4) and Bi14MoO24 (Bi/Mo = 14) contained the typical
Bi-O ([Bi-Ox], [Bi-Oy]) polyhedra. The Bi-O polyhedra in the bismuth molybdate
phases could promote the migration of photogenerated electrons and holes, thus
enhancing the photocatalytic activities of the photocatalysts [33].
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To further understand the structural difference between the Aurivillius-type and
Fluorite-type bismuth molybdates, the corresponding FT-IR spectra were measured to
examine the present functional groups and the molecular structures of the as-obtained
photocatalysts, as illustrated in Fig. 4. For the spectrum of Bi2MoO6, two absorption
peaks at 843 and 796 cm-1 could be attributed to the asymmetric and symmetric
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stretching modes of the [MoO6] octahedral structure, respectively, which were
correlated to the vibration of the oxygen atoms distributed at the top plane of the
crystal lattice. The peak at 731 cm-1 was assigned to the asymmetric stretching mode
of [MoO6] octahedral structure caused by the vibration of the oxygen atoms at the
equator in its crystal structure. In addition, the peak at 575 cm-1 could be attributed to
the bending vibration of the [MoO6] octahedra [34]. However, for the FTIR spectra of
Bi6Mo2O15, Bi4MoO9 and Bi14MoO24, all the characteristic peaks of the [MoO6]
octahedra disappeared, while a typical absorption peak related to the [MoO4]
tetrahedra at 805 cm-1 was observed [35]. This indicated that the [MoO6] octahedral
structure of the Aurivillius-type phase has completely vanished and some new
Fluorite-type structures of bismuth molybdate phases were formed.
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To further verify the chemical composition and elemental chemical status of the
bismuth molybdate samples, XPS spectra were measured, which are shown in Fig.5. It
was observed that Bi, Mo and O elements were all detected with strong characteristic
peaks, and no impurities were detected. For the Aurivillius-type Bi2MoO6, two
predominant XPS peaks were found at the binding energies of 158.9 and 164.2 eV
(Fig. 5b), which could be assigned to the Bi 4f7/2 and Bi 4f5/2. The peaks located at
235.2 and 232.1ꢀeV in the Mo 3d spectrum (Fig. 5c) were assigned to the Mo 3d5/2
and Mo 3d3/2 of Mo6+, respectively [19]. In addition, three peaks at the binding energy
values of 529.2, 529.9 and 530.9ꢀeV were observed in the O 1s core level spectrum
(Fig. 5d), which were attributed to the existence of Bi-O, Mo-O and adsorbed oxygen,
respectively [19, 23]. Because of the chemical environment change of the
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Fluorite-type bismuth molybdates (Bi6Mo2O15, Bi4MoO9 and Bi14MoO24), the peak
position and intensity of Bi, Mo and O were found to slightly shift in comparison to
that of Aurivillius-type Bi2MoO6. This indicated the interrelation between the type of
crystal structure of materials and their chemical bonding environment.
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3.2. Morphology and formation mechanism of the photocatalysts
The morphologies of the as-prepared bismuth molybdate catalysts were investigated
by SEM and HRTEM. As displayed in Fig. 6a, the as-obtained Bi2MoO6 sample
possessed 2D sheet-like microstructure with a thickness ranging from 20 nm to 50 nm.
According to Fig. 6b, the Bi6Mo2O15 material exhibited a well-defined morphology of
1D nanowires with a diameter of 30~150 nm. For Bi4MoO9 and Bi14MoO24, irregular
3D bulk-shaped nanostructures could be observed, as shown in Fig. 6c and d. It was
obvious that all the bismuth molybdates displayed clean and smooth surfaces. The
TEM images (Fig. 6e-g) further confirmed the crystal structures of the bismuth
molybdates. To further study the detailed crystal structures of the samples, HRTEM
measurements were undertaken. Fig. 6i shows the clear lattice fringes with spacings
of 0.275 and 0.274 ꢀnm, which were close to the spacings expected for the (002) and
(200) planes of Bi2MoO6. This suggested that Bi2MoO6 sheets preferentially grew
along the [010] direction [36]. For the Fluorite-related bismuth molybdates, the lattice
fringes shown in Fig. 6g-6j with the interplanar spacings of 0.324, 0.327 and
0.325ꢀnm could be clearly identified, which could be assigned to the Bi6Mo2O15 (-116),
Bi4MoO9 (111) and Bi14MoO24 (213) planes, respectively [37-39]. Furthermore, the
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EDS results shown in Fig. 7 confirmed that the Bi, Mo and O elements were
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uniformly distributed in the bismuth molybdates, further indicating that the bismuth
molybdates with various crystal structures have been successfully fabricated via the
molten salt method.
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Based on the above discussion, it was proposed that the phase-structural and
dimensional transition from the Aurivillius-type to the Fluorite-type in the bismuth
molybdates could be controlled by simply adjusting the reaction time in the
salt-assisted assembly system. Fig. 8 showed the sketched synthesis pathway. It was
obvious that the salt-assisted assembly strategy was more conducive to the formation
of sheet-like Bi2MoO6 with a well-defined morphology than the high temperature
solid-state method (Fig. 8a). The molten salts (eutectic mixture of NaNO3 and KNO3)
would accelerate migration and enhance the electrostatic interaction between the ionic
components and material surfaces. Actually, the difference in the phases and
morphologies of bismuth molybdate photocatalysts was mainly determined by the
transformation involving Bi and Mo components. During the molten salt synthesis
process, Bi(NO3)3·5H2O, a strongly acidic and weakly alkaline salt, ionized into Bi3+
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and NO3 , which would further react with MoO4 to form the Aurivillius-type
Bi2MoO6. At this stage, the Mo stoichiometric concentration in the Fluorite-type
phases was lower than that of the Aurivillius-type Bi2MoO6, indicating the higher Bi
concentration. With increasing the reaction time, Bi gradually penetrated into the
Aurivillius-type phase, leading to the formation of the known Fluorite-related material
systems. Multitudinous Fluorite-type phases (Bi6Mo2O15, Bi4MoO9 and Bi14MoO24)
with different morphologies could be formed (Fig. 8b) at different molar ratios of
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Bi/Mo. This molten salt-assisted strategy for synthesizing the bismuth molybdates
could be considered to be an ionization nucleation - crystallization growth -
recrystallization pathway, and this is supported by the fact that the bismuth
molybdates with various morphologies could be rapidly and easily obtained in a mild
liquid environment.
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3.3. Photoabsorption performance and band structure
The optical absorption behaviors of the as-obtained catalysts including Bi2MoO6,
Bi6Mo2O15, Bi4MoO9 and Bi14MoO24 were measured by using UV-vis diffuse
reflectance spectroscopy (DRS), and the results are shown in Fig. 9. Except for the
Bi6Mo2O15 sample, all the bismuth molybdate photocatalysts exhibited the absorption
in the visible region. Most notably, with increasing the molar ratio of Bi/Mo (from 3
to 14), the Fluorite-related bismuth molybdates exhibited an obvious redshift of the
visible absorption band, indicating the reduced band-gap energy. Moreover, the
optical images of the as-synthesized catalysts are also shown in the right side of Fig. 9.
It was noted that the color of Bi2MoO6 sample was deep yellow, and the color of the
Fluorite-type bismuth molybdate samples changed from white to faint yellow with
increasing molar ratio of Bi/Mo. Additionally, the band gap of Bi2MoO6, Bi6Mo2O15,
Bi4MoO9 and Bi14MoO24 could be estimated by the Kubelka-Munk equation [40]:
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αhv = A(hv-Eg)n/2
(1)
, in which α, h, v, A , and Eg are the absorption coefficient, the Planck constant, the
light frequency, a constant and band gap energy, respectively. The n values usually
decide the type of optical transition that occurs in the photocatalytic materials (n = 1
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for direct band optical transition, and n = 4 for indirect band optical transition). Here,
n = 1 for Bi6Mo2O15, Bi4MoO9 and Bi14MoO24, and n = 4 for Bi2MoO6 [37-39, 41].
Therefore, the band gaps of Bi2MoO6, Bi6Mo2O15, Bi4MoO9 and Bi14MoO24 were
estimated to be 2.81, 3.18, 2.77 and 2.57 eV, respectively. To thoroughly investigate
the energy band structures of the bismuth molybdate catalysts, a Mott-Schottky (M-S)
analysis was performed and applied to measure both the donor density and flat band
potential (Efb) of the semiconductors, the results of which are elucidated in Fig. 10.
The positive slopes observed in the M-S plots indicated that all of the as-obtained
bismuth molybdates were typical n-type semiconductors. Furthermore, the values of
Efb were determined based on the M-S equation [42]:
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ꢀ
ꢃ
ꢌꢍ
ꢄ
=
(ꢉ − ꢉꢊꢋ
ꢄꢅꢅ ꢇ
ꢆ
ꢈ
−
)
(2)
ꢂ
ꢁ
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, in which C is the capacitance per unit surface area, e is the elementary charge, ε is
the dielectric constant of the materials, ε0 is the permittivity of free space, Nd is the
number of donor density, E is the applied voltage, Efb is the flat band potential, K is
the Boltzmann constant, and T is the absolute temperature. The calculated Efb values
of the Bi2MoO6, Bi6Mo2O15, Bi4MoO9 and Bi14MoO24 were -0.34, -0.51, -0.37 and
-0.30ꢀV (vs Ag/AgCl), respectively. Therefore, the potentials of conduction band (CB)
edges of Bi2MoO6, Bi6Mo2O15, Bi4MoO9 and Bi14MoO24 was estimated to be -0.14,
-0.31, -0.17 and -0.10ꢀV (vs NHE). Based on the bandgaps of the bismuth molybdate
catalysts, the potential of the valence band (VB) edge were located at 2.67, 2.87ꢀ, 2.60
and 2.47 eV (vs NHE), respectively [42]. With the increase of the molar ratio of
Bi/Mo, the CB and VB edges of the Fluorite-related bismuth molybdates shifted down
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and up, respectively. So, compared with Bi6Mo2O15, the band gaps of Bi4MoO9 and
Bi14MoO24 were narrower. Such change of the bandgaps was attributed to the
incorporation bond of Mo ionic components in the bismuth molybdates. For the
Fluorite-related Bi2O3-MoO3 binary compounds, the Mo components often exhibited
tetrahedral coordination consisting of the regular [MoO4] tetrahedra (Bi6Mo2O15).
Meanwhile, the Mo components exhibited distorted [MoOx] polyhedra in Bi4MoO9
and Bi14MoO24. A previous study [43] has reported that the coordination value x of
[MoOx] polyhedra could affect the charge transfer and the light absorption capacity in
molybdates. In general, a higher x value could lead to a lower energy of the charge
transfer occurring in the molybdates, which inevitably shifted the absorption edge to a
longer wavelength. This phenomenon could also be understood by the interactions
bewteen Mo and O orbitals. For photocatalysts in Bi2O3-MoO3 compounds, the CB
comprised the Mo-4d orbital, while the VB mainly comprised the O-2p orbital. The
interaction between the Mo6+ and O2- ions was stronger for the bismuth molybdate
photocatalysts with smaller coordination numbers, which increased the bandgap
energy. These results indicated that Bi14MoO24 could absorb more visible light, thus
benefiting the the photocatalytic efficiency.
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3.4. Photocatalytic activities
The photocatalytic performances of the as-obtained bismuth molybdate catalysts were
systematically investigated by the degradation of TC under the illumination of a 5 W
white LED light. As depicted in Fig. 11a, the change of the TC concentration is
negligible within 5 h of visible light irradiation without photocatalyst. In contrast,
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with the presence of Aurivillius-type Bi2MoO6 photocatalyst, ca. 90.54% of TC was
removed with the same light irradiation time. For the Fluorite-type bismuth molybdate
photocatalysts, the photocatalytic activitiy significantly increased with increasing the
mole ratio of Bi/Mo. Specifically, the photocatalytic efficiencies of Bi6Mo2O15,
Bi4MoO9 and Bi14MoO24 for TC were measured to be 7.49%, 78.79% and 97.98%,
respectively, for the same visible light exposure time. The Bi6Mo2O15 exhibited
relatively low visible light photocatalytic efficiency due to its poor visible light
absorption capability.
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As shown in Fig. 11b, to quantitatively analyze the catalytic efficiency, the
pseudo-first-order rate constant (k) was further evaluated. The k values were
calculated using the following function [32]:
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-ln(C/C0) = k t
, where C is the remaining concentration of the TC solution at an irradiation time of t,
C0 is the initial concentration, and is the apparent reaction rate constant. The k
(3)
k
values of Bi2MoO6, Bi6Mo2O15, Bi4MoO9 and Bi14MoO24 were calcalated to be
0.4130, 0.0113, 0.2686 and 0.5710 h-1, respectively.
To study the influence of material synthesis method on the photocatalytic
reaction, Bi2MoO6 catalysts were synthesized with molten salt method (labelled as
sample MS-Bi2MoO6), solid-state method (labelled as sample SS-Bi2MoO6) and
hydrothermal method (labelled as sample H-Bi2MoO6), respectively. The XRD
patterns and SEM images are displayed in Fig. S1. It could be observed that the
photocatalytic activity of MS-Bi2MoO6 was much higher than those of SS-Bi2MoO6
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and H-Bi2MoO6 (Fig. S2). Compared to the solid-phase and hydrothermal methods,
molten salt method could produce Bi2MoO6 nanosheets with a better crystallinity. As
the solvent and stabilizer, molten salts could also be used as an inhibitor to inhibit
grain growth and produce smaller particles. All these features led to the excellent
photocatalytic ability for MS-Bi2MoO6.
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The Photoluminescence (PL) spectra can be used to study the transition behavior
of the photogenerated electrons and holes in the catalysts. It is generally known that a
weaker PL intensity usually indicates a lower recombination efficiency of the
photoexcited charge carriers [44, 45]. Fig. 11c presents the PL spectra of the Bi2MoO6,
Bi6Mo2O15, Bi4MoO9 and Bi14MoO24 samples obtained under the excitation of 350
nm laser. It was found that the PL peaks of Bi2MoO6 and Bi14MoO24 was obviously
weaker compared with those of Bi6Mo2O15 and Bi4MoO9. In addition, the PL
emission intensity of the Fluorite-type bismuth molybdate system (Bi6Mo2O15,
Bi4MoO9 and Bi14MoO24) decreased obviously with increasing the molar ratio of
Bi/Mo, suggesting that the recombination of the photogenerated charge carriers could
be effectively reduced, resulting in the increased photocatalytic performance. The
excitation and transfer of the charge carriers in the bismuth molybdate catalysts were
further investigated by photoelectrochemical measurements. As presented in Fig. S3,
the photocurrent intensities of the Bi2MoO6 and Bi14MoO24 samples were higher than
those of Bi4MoO9 and Bi6Mo2O15. Meanwhile, the photocurrent intensities of the
Fluorite-type bismuth molybdates (Bi6Mo2O15, Bi4MoO9 and Bi14MoO24) decreased
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dramatically with increasing the molar ratio of Bi/Mo, which were consistent with the
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PL analyses. Moreover, the electrochemical impedance spectroscopy (EIS) data were
also recorded, and these results are presented in Fig. 11d. As expected, a relatively
small arc was noticed for the Bi14MoO24 in comparison to the Bi6Mo2O15 and
Bi4MoO9, which indicated a rapid migration of electron and hole pairs in the
Bi14MoO24. However, the arc of Aurivillius-type Bi2MoO6 was larger than that of
Fluorite-type Bi4MoO9 but smaller than that of Bi14MoO24. Therefore, the above
analyses showed that the Fluorite-type Bi14MoO24 photocatalyst exhibited a better
visible-light-driven photocatalytic activity than Aurivillius-type Bi2MoO6 and could
be a promising candidate material for applications in the wastewater treatment field.
The stability and reusability of catalysts are essential for practical applications.
Therefore, cycling experiments were carried out to investigate the photocatalytic
stability of the catalysts. In each cycling experiment, the TC aqueous solution was
illuminated for approximately 5 h. After each cycling experiment, the bismuth
molybdate powders were collected and washed. Finally, the as-obtained powders were
dried at 80 °C for 10 h and used for the next cycling test. As displayed in Fig. S4, it
could be observed that no obvious decrease in the degradation efficiencies of TC was
observed after 5 cycling tests. Furthermore, the structural stability of the recycled
samples was also studied by XPS spectroscopy (Fig. S5). No significant change was
observed for the position and the ratio of XPS peaks after 5 successive cycles,
indicating that the structure of the bismuth molybdates were relatively stable during
the pollutant degradation.
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4. Conclusions
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In this work, molten salt-assisted assembly route was studied to achieve controlled
synthesis of the bismuth molybdates in terms of phase composition and morphology.
Both Aurivillius-type and Fluorite-type bismuth molybdates could be obtained by
changing the synthesis conditions. It was found that the as-obtained Bi14MoO24
sample exhibited much higher photocatalytic activity than that of Bi2MoO6,
Bi6Mo2O15 and Bi4MoO9. The relationship between the change of crystal
structure/band structure and the photocatalytic activities were systematically
investigated to understand the mechanism of the photocatalytic activity change for
bismuth molybdates with different phases and compositions. Compared with other
synthesis methods, the molten salt-assisted assembly technique applied in this work
could be developed into an economical and feasible method to synthesize bismuth
molybdates with controlled dimensions and types of crystal structures. The molten
salt could facilitate the migration of precursors and enhance the electrostatic
interaction effect. Thus, the molten salt-assisted synthesis strategy could provide a
new insight into the design and optimization of the advanced bismuth
molybdate-based photocatalysts for applications in environmental protection.
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19 Acknowledgements
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This research was supported by the Major Science and Technology Program for
Water Pollution Control and Treatment “Research and demonstration of key
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technologies for water ecological environment remediation and water safety guarantee
in Baiyangdian Lake and Daqing River Basin (Xiong'an New Area)”
(2018ZX07110)-“Research and Engineering Demonstration of Groundwater Pollution
Control and Remediation Technology for Typical Sewage Reclamation Ponds in
Tanghe Sewage Reservoir and Xiong’an New Area” (2018ZX07110005), and the
National Natural Science Foundation of China (Grant Nos. 21571162).
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