Defect state of indium-doped bismuth molybdate nanosheets for enhanced photoreduction of chromium(vi) under visible light illumination

Article abstract of DOI:10.1039/c8dt01807c

The construction of defect states is an effective method for regulating the energy band structure of photocatalytic semiconductor materials. Establishing defect states effectively is a particularly significant strategy to improve the photoelectric conversion efficiency. Herein, we report a peculiar method to manufacture defect states in indium-doped Bi₂MoO₆ by changing the valence state of the doped indium element. By increasing the amount of doped indium, different doping forms and valence states are produced, which causes distortion of crystal structure, generation of oxygen vacancies and variation in the elemental valence state. Specifically, when Bi is substituted with In^(3-x)+, the acceptor energy level becomes higher than the valence band due to the p-type-like doping, which widens the width of the valence band and drives the up-shift of the valence band edges, resulting in narrowed bandgap and improved photoresponse ability. In addition, the photoelectrochemical performance tests using the In-doped Bi₂MoO₆ show that low-valent indium doping effectively improves the carrier separation efficiency, reduces the transmission impedance, and greatly improves the photocatalytic reduction performance of Cr(vi). This study provides a new insight to the design of efficient photocatalysts for the green and sustainable treatment of Cr(vi) pollution.

Full text of DOI:10.1039/c8dt01807c

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Defect-sate of Indium Doped Bismuth Molybdate Nanosheets for
Enhanced Photoreduction of Chromium (VI) under Visible Light
Illumination
Zhongfu Li a, b, c †, Zhaohui Wu b, †, Shumin Zhang a, b, Jie Shen b,Wenhui Feng b, Yi Du c, Long Wan a, *, Shiying
Zhang b, *
a College of Materials Science and Engineering, Hunan University, Changsha, 410082, People’s Republic of China
b Hunan Key Laboratory of Applied Environmental Photocatalysis, Changsha University, Changsha 410005,
Hunan Province, People’s Republic of China
c Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province,
Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, People’s Republic of China
† These authors contributed equally to this work.
Corresponding author
*Shiying Zhang Tel: +86ꢀ731ꢀ84261297. Eꢀmail: cdzhangshiying@163.com.
* Long Wan Tel: +86ꢀ13055177892. Eꢀmail: wanlong1799@126.com.
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Abstract
The construction of defectꢀstate is an effective method for regulating the energy band structure of
photocatalytic semiconductor materials. Establishing defectꢀstate effectively is particularly significant strategy to
improve the photoelectric conversion efficiency. Herein, we report a peculiar method to manufacture defect state in
indium doped Bi2MoO6 by changing the valence state of the doping indium element. Increasing the amount of
doped indium, different doping forms and valence states are produced, causing crystal structure distortion, oxygen
vacancies generation and elemental valence variation. That is, when In(3ꢀx)+ substituted for Bi site, the acceptor
energy level generates above the valence band due to the pꢀtypeꢀlike doping, which widens the width of valence
band and drives the upꢀshift of valence band edges, resulting in narrowed bandgap and improved photoresponse
ability. In addition, photoelectrochemical performance tests over Inꢀdoped Bi2MoO6 show that lowꢀvalence indium
doping can effectively improve the carrier separation efficiency, reduce the transmission impedance, and greatly
improve the photocatalytic reduction performance of Cr (VI). This work provides a new insight to design efficient
photocatalysts for the green and sustainable treatment of Cr (VI) pollution.
Keyword: Indium, Bi2MoO6 nanosheet, Defect sate, Hexavalent chromium, Visible photocatalyst.
Introduction
Chromium (Cr) pollution, stemming from the waste gas, waste water and waste residue during the process of
metallurgy, metal processing, electroplating, leather and other sectors, is the primary and serious threat to public
health and ecosystem.1ꢀ3 Because Cr ions are easily oxidized to soluble Cr (VI) compounds and transferred into
underground water by leaching, which are easily absorbed by human body and accumulated in liver, kidney and
other organs, resulting in rhinitis, pharyngitis, liver damage, renal function failure or even cancer.4,5 Therefore,
removal of highꢀrisk Cr (VI) from waste water or residue is emergency.6ꢀ8 So far, various studies and multifarious
methods have been conducted to remove soluble Cr (VI), including chemical reduction,2,9 membrane filtration,10,11
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adsorption,12,13 microbial reduction14 and photocatalytic reduction.15ꢀ18 Among them, the semiconductor
photocatalytic technology with low cost, environmentꢀfriendly and recyclable characteristics is considered as an
appealing route to remove Cr (VI) pollution completely.19ꢀ21 Herein, various of semiconductors with diverse shapes
(such as including TiO2, ZnO, SrTiO3 and ZrO2), have been employed to enhance the photocatalytic efficiency of
soluble Cr (VI) removal.16,19,22 For example, about 96% Cr (VI) was reduced by TiO2 hollow spheres within 2 h
under ultraviolet light irradiation at pH = 2.8.23 However, such pristine semiconductors are still significantly
limited in practical application under solar light irradiation attributing to their wide bandgap energy, weak solar
light response and low photocatalytic efficiency.24ꢀ26 Therefore, developing visible lightꢀdriven semiconductor
photocatalyst with stable, cheap and nonꢀtoxic characteristics to improve the treatment efficiency of Cr (VI) has
attracted numerous attentions.27
Bismuth based semiconductors, as potential and efficient visible light catalytic materials with various unique
crystal structures, have been widely studied and applied in various fields.28ꢀ30 Particularly, nontoxic bismuth
molybdate (Bi2MoO6), a typical Aurivillius oxide, is composed of fluorite [Bi2O2]2+ layer and cornerꢀshared
octahedral [MoO4]2ꢀ layer, where [Bi2O2]2+ layers are interval embedded in [MoO6] octahedrite layers, presenting
typical sandwich structures.31ꢀ33 Such unique structure of Bi2MoO6 is available to drive the photoelectron transfer
from valence band (VB) to conduction band (CB). Furthermore, the transference rate of electrons is able to altered
feasibly by adjusting the element ratio of Bi, O and Mo, because VB consists of the O 2p orbitals, while the CB
composes of Mo 4d orbitals derived from MoO6 octahedral and Bi 6p orbitals from the (Bi2O2)2+ layers.34,35
Furthermore, Bi2MoO6 with an appropriate bandgap energy (Eg = 2.6 eV) and visible light responsibility, brings
out stronger photocatalytic efficiency and light resistance than other bismuth materials, such as Bi2WO6, BiOCl
and Bi2S3.33 But for pristine Bi2MoO6 photocatalyst, rapid recombination of charge carriers, slow carrier migration
and relatively weak visible light response are still the main issues of low photoreduction performance.36,37 To solve
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those problems, some effective strategies have been facilitated, such as morphology design,34,38 composite
heterostructure,31,39,40 element doping,41,42 surface sensitization43 and crystal defect.44,45 Particularly, element doped
Bi2MoO6 is a common and effective route to improve its photoreduction performances, on basis of the crystal and
band structure of Bi2MoO6.31,34 For instance, when fluorine replaced O in Bi2MoO6, anion oxygen vacancies were
obtained to trap electrons and largely boost the separation of photogenerated electrons and holes. Moreover, such
fluorine doped Bi2MoO6 also changed the energy band structure and contributed to the valence band position
moving toward the positive direction. Thus, more active oxidizing species (h+, ·OH, ·O22ꢀ) were produced over
F
0.20ꢀBi2MoO6 photocatalyst.46 Additionally, crystal defects in Bi2MoO6 lattice generated by cerium doping caused
the upꢀshift of the CB position and optimum electron dynamics, resulting in a significant increased amount of
reactive oxygen species for enhanced photocatalytic activity. Besides, the oneꢀelectron and twoꢀelectron reactions
also effectively promoted by introducing redox couples of Ce3+/Ce4+ and Mo4+/Mo6+.36
Recently, indium (In) element with novel characteristics, such as multiple oxidation states (In0, In1+, In3+),
higher electron production, trapping and mobility and lower toxicity, is regarded as appealing doping element to
improve the photoreduction ability and widely used in semiconductor catalysis, gas sensitive components and
other fields.47,48 Doping indium into TiO2 samples was used to improve CO2 reduction efficiency under visible
light irradiation. Such promoted photoreduction efficiency of Inꢀdoped TiO2 were explained that the doped indium
increases the active surface area and suppresses the recombination of photogenerated electronꢀhole (eꢀ/h+) pairs
during the photoreduction process.47 Additionally, Yaan Cao prepared the Inꢀdoped TiO2 samples with indium
chloride as precursors and found out that the surficial OꢀInꢀClx (x = 1 or 2) configuration could induce a new
surface defective state energy level under the CB of TiO2, improving the photodegradation performance toward
organic pollutant under visible light illumination.48 Based on previous studies, it can be concluded that the indium
doping can regulate the catalytic activity of semiconductors through different routes. However, in terms of
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Bi2MoO6, the effects of indium doping on its crystal structure and corresponding photocatalytic performance have
been scarcely reported until now.
Therefore, we prepared Inꢀdoped Bi2MoO6 nanosheets successfully via a facile hydrothermal method. And
then the effects of indium doping amount on the crystal and band energy structure of Bi2MoO6 nanosheets are
studied through the crystal structures, morphologies, band structures and photoelectrochemical characteristics of
Bi2MoO6 sheets, systematically. When indium is doped into Bi2MoO6 crystals, the novel characteristics of indium
may present different doping forms and chemical valence states, resulting in distortion of the crystal structure,
charge compensation and elemental valence variation. Thus, these variation generated by indium doping may
produce a defect state in energy band structure or crystal structure to improved the photoelectrochemical properties
under visible light illumination.
Experimental section.
Materials
Bismuth nitrate (Bi(NO3)3ꢁ5H2O), sodium molybdate (Na2MoO4·2H2O) and indium nitrate pentahydrate
(In(NO3)3ꢁ5H2O) were purchased from Aladdin (Shanghai, China) without any purification before used.
Ultrapure water was used in the experiment.
Synthesis of indium doped Bi2MoO6 nanosheets
The samples were prepared through a hydrothermalꢀcalcination process and the detail processes were carried
out as follows. 2 mmol of Bi(NO3)3·5H2O, 1 mmol of Na2MoO4·2H2O were dissolved into dilute nitric acid
solution (10 ml, 2 M) to form transparent homogenous solution. And then, 50 ml deionized water and different
amount of In(NO3)3ꢁ5H2O were added into the above solution under vigorously stirring. Subsequently, the pH
value of the resulted transparent mixtures was adjusted to ca. 8 by adding ammonium hydroxide (25%), then the
transparent mixture was turned to white suspension slowly, which were stirred for 30 min at ambient temperature.
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Finally, the suspensions were transferred into 100 mL Para polyphenylene autoclave and kept at 160 °C for 16 h.
The resulted products were collected after centrifugation and washing, and then dried overnight at 80 °C. The
obtained InꢀBi2MoO6 was denoted as Ini ꢀBi2MoO6 (i is the molar ratio of indium to bismuth molybdate, i=0, 0.05,
0.1, 0.15 and 0.2, respectively).
Characterization
The crystal phase of the samples was characterized by Xꢀray diffractometer (XRD, Brooker D8 Advance).
The morphology and crystalline state of prepared samples were performed with a fieldꢀemission scanning electron
microscope (SEM, JEOL, JSMꢀ6700F) and transmission electron microscope (TEM, JEOL, JEMꢀ3010). The
surface element states of the doped samples were characterized by Xꢀray photoꢀelectron spectrometer (XPS)
(
ESCALAB 250Xi, USA). Diffuse reflectance spectrum (DRS) of the samples was recorded on a UVꢀvis
spectrophotometer (UVꢀ2600, Shimadzu). The photoluminescence emission spectra (PL) of prepared samples are
recorded on a fluorescence spectrometer (Hitachi Fꢀ4500, Japan). Raman spectra of these tested samples were
recorded by a laser Raman spectrometer (Labramꢀ010, France) using 320 nm lasers. The defect states in crystals is
characterized by electron spin resonance spectra (EPR) (JESꢀFA200, Japan). Photoelectrochemical measurements
were accomplished with an electrochemical workstation equipped with threeꢀelectrode system (CHIꢀ660E, China).
The working electrode prepared by traditional knife coating method on FTO glass (1cm × 2 cm) and maintained at
80 °C for 5 h. During the measurement process, the reference electrode and counter electrode was saturated
Ag/AgCl and platinum wires, respectively, and the electrolyte solution was 0.5 M Na2SO4 solution. The excitation
light source was LED lamp (3 W).
Photocatalytic Cr (VI) reduction
The photocatalytic activity of the prepared samples was characterized by photoreduction of Cr (VI) under
visible light irradiation. The photoreduction test was conducted in a 50 mL sealed quartz reactor with 20 mg of
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photocatalysts, 5 mg of oxalic acid and 40 mL of Cr (VI) (20 mg/L) aqueous solution. Diluted H2SO4 or NaOH
was used to adjust the pH value of the solution. After advocating nitrogen and stirring for 60 minutes to achieve
the adsorptionꢀdesorption equilibrium of photocatalysts, the sample was then transferred to a lightꢀfree
environment of a 300 W Xe lamp (PLSꢀSXE 300C, Beijing Perfect light Co. Ltd.) with 420 nm filter. During
photocatalysis experiment, about 2 mL of suspension was taken out and centrifuged at each 5 min interval. All the
concentration of centrifuged Cr (VI) solution were measured with a mini UVꢀvis Spectrophotometer (UVꢀ1280,
Shimadzu) at 540 nm, according to the diphenyl carbazide method.49
Results and Discussion
Crystal structure, Morphology and Composition
Fig. 1 (a) XRD patterns of the synthesized samples, (b) the magnified spectra of (131) plane of Bi2MoO6 and
IniꢀBi2MoO6 nanosheets.
XRD patterns of the pure Bi2MoO6 and Ini ꢀBi2MoO6 samples are shown in Fig. 1. Apparently, all the peaks of
pure Bi2MoO6 and Ini ꢀBi2MoO6 (i=0.05, 0.1 and 0.15) are indexed to the orthorhombic bismuth molybdate phase
(JCPDS No. 21ꢀ0102). Moreover, no characteristic peaks of other impurity phases could be observed in all these
samples (Fig.1 (a)). However, the diffraction intensity of all peaks is decreased gradually in all samples when
increasing indium doping amount, suggesting that indium ions may decrease the crystalline of resulted Ini
ꢀBi2MoO6. Meanwhile, the (131) facet is shifted toward higher 2θ value when increasing doped indium, as
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presented in the high magnification of XRD patterns (Fig. 1(b)), indicating the faintly lattice shrinkage of the
detected crystals based on Bragg’s law.46,50 Such shrinkage of crystal lattice probably is caused by the crystal
defects of Ini ꢀBi2MoO6.
Fig. 2 SEM images of (a) pure Bi2MoO6 and (b) In0.15ꢀdoped Bi2MoO6 nanosheets. TEM images of (c) pure
Bi2MoO6 and (d) In0.15ꢀdoped Bi2MoO6 nanosheets. HRTEM and inserted SAED patterns of (e) pure
Bi2MoO6 and (f) In0.15ꢀdoped Bi2MoO6 nanosheets.
The morphology and crystal structure of pure and Inꢀdoped Bi2MoO6 were investigated by SEM and TEM. As
shown in Fig. 2(a), pure Bi2MoO6 presents morphology of twoꢀdimensional (2D) bandꢀlike nanosheets with
irregular edges, average size in microꢀscale and broad size distribution. Comparatively, although the morphology
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of Inꢀdoped Bi2MoO6 sample is still maintained 2D sheets, the size of asꢀprepared Inꢀdoped Bi2MoO6 sheets (Fig.
2(b)) decreases to nanoꢀscale and the corresponding thickness is of 60ꢀ80 nm. Therefore, it can be seen that the
doped indium retards the growth of Bi2MoO6 in a [100] direction and changes its stripꢀlike morphology.
Furthermore, the corresponding TEM images of pure Bi2MoO6 and indiumꢀdoped Bi2MoO6 samples (Fig. 2(c) and
2(d)) also further confirm the sheetꢀlike morphology and altered size of Bi2MoO6 and Inꢀdoped Bi2MoO6 product.
In addition, selectedꢀarea electron diffraction (SAED) patterns (Fig. 2(e) and 2(f)) suggest that the crystal structure
of pure Bi2MoO6 strip with monocrystalline is modulated to polycrystalline bismuth molybdate structure after
indium doping (Fig. 2(f)).
Fig. 3 (a) Nitrogen adsorptionꢀdesorption isothermal curve and (b) pore size distribution of pure Bi2MoO6 and
In0.15ꢀBi2MoO6.
Table 1. Specific surface area and pore structure of pure Bi2MoO6 and In0.15ꢀBi2MoO6.
Sample
Pore Volume (cm3/g)
Pore Diameter (nm)
4.740
Specific Surface Area (m3/g)
Pure Bi2MoO6
In0.15ꢀBi2MoO6
0.083
0.131
14.34
17.62
3.625
The specific surface areas of pure Bi2MoO6 and In0.15ꢀBi2MoO6 nanosheets were investigated by the nitrogen
adsorptionꢀdesorption isotherms. The specific surface area of the prepared pure sheetꢀlike Bi2MoO6 sample is
14.34 m3/g (Fig. 3 and Table 1). While, the specific surface area of In0.15ꢀBi2MoO6 nanosheets is increased to
17.62 m3/g and the corresponding pore volume is also improved to 0.131 cm3/g when indium was introduced. In
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general, large specific surface area of photocatalyst provides sufficient active sites for catalytic reactions and
promotes the separation of electronsꢀholes effectively. However, the surface area and pore volume of Bi2MoO6 and
In0.15ꢀBi2MoO6 nanosheets is altered slightly, indicating the effect of indium doping on special surface area and
pore structure is negligible. Herein, the influence of the variation of pore structure and surface area of Inꢀdoped
Bi2MoO6 nanosheets on photocatalytic activity could be ignored.
Fig. 4 XPS spectra of as synthesised Bi2MoO6 photocatalyst (a) Bi 4f, (b) Mo 3d, (c) In 3d and (d) O 1s,
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respectively. (e) Raman spectrum of pure Bi2MoO6 and In0.15ꢀBi2MoO6 sample, (f) the magnified Raman
spectra, (g) the EPR spectrum of the prepared Bi2MoO6 samples
XPS was applied to analyze the surface chemical composition and element status of pure Bi2MoO6 and
In0.15ꢀBi2MoO6 sample. According to the survey spectra of Bi2MoO6 and In0.15ꢀBi2MoO6 sample (Fig. S1), C, Bi,
Mo and O element are presented in both spectra, while additional peak of indium is presented in the survey
spectrum of In0.15ꢀBi2MoO6 sample. In Fig. 4(a), the peaks centered at 159.1 and 164.4 eV are assigned to Bi 4f7/2
and Bi 4f5/2 of Bi3+.51 Obviously, the banding energy of 4f7/2 and Bi 4f5/2 shifts of ca. 0.5 eV to lower banding
energy in In0.15ꢀBi2MoO6, probably due to the substitution of indium for bismuth position.52 Two divergent peaks
located at 232.2 and 235.4 eV are ascribed to Mo 3d5/2 and Mo 3d3/2 of Mo6+ in samples (Fig.4 (b)). Notably, two
additional new peaks located at 234.4 eV and 230.9 eV are presented when indium element was introduced,
attributing to the lowꢀvalence Mo(6ꢀx)+ 53
. In the highꢀresolution of In 3d spectra (Fig. 4(c)), two peaks centered at
452.5 eV and 445.1 eV are assigned to In 3d3/2 and In 3d5/2 of trivalent indium.54,55 The peak at low binding energy
(442.2 eV) can be ascribed to the low valence state of indium (In(3ꢀx)+).56 More obviously, the peak area of indium
increases gradually with the raising amount of doped indium, indicating that the proportion of low valence indium
increases in samples (Fig. S2). Additionally, the O1s XPS spectrum (Fig. 4(d)) can be separated into three peaks at
530.4 eV, 531.6 eV and 532.1 eV, which are assigned to lattice oxygen in Bi2MoO6, native defects of oxygen
vacancies resulted from the introduction of indium and surface adsorbed hydrocarbons, respectively.53,57,58
According to the peaks shifting in Bi 4f and In 3d spectra, it can be concluded that bismuth may be substituted by
indium. As well known, the ionic radius of In3+ (0.080 nm) is between Bi3+ (0.103 nm) and Mo6+ (0.059 nm).
When In (III) with small radius replaces the site of Bi (III), indium and molybdenum present lower valence to
achieve the ideal lattice matching because the cation with low valence possesses a large ion radius. Meanwhile, the
positive oxygen vacancy will give excess electrons to molybdenum and indium to complete the valence state
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variation and keep the charge balance of the system.59,60 In addition, the Bader charges of oxygen atoms around the
substituted Bi atom in Inꢀdoped Bi2MoO6 also change correspondingly in comparison with that in the pristine
Bi2MoO6, suggesting the charge redistribution around the dopant of Bi2MoO6, which would change the internal
electric field, favoring better separation of photoexcited electrons and holes and thus improving the photocatalytic
activity.61
In order to illustrate the structural model reasonably, the XPS spectra of samples with different indium doping
amount are carefully examined. When increasing amount of indium, the peak area corresponding to In3+ (445.1 eV)
and In(3ꢀx)+ (442.2 eV) are respective gradually decreased and increased (Fig. 4(c)). Meanwhile, the peak areas of
oxygen vacancy also increase with the increase of the lowꢀvalence indium content (Fig. S3), implying the possible
coupling effect between lowꢀvalence indium and oxygen vacancies. Although the molybdenum element also
presents changeable valence states, its evolution is not as obvious as that of indium (Fig. S4). Therefore, to figure
out the change of molybdenum valence, the MoꢀO bond changes before and after doping were illustrated by
Raman spectroscopy (Fig. 4(e)). In0.15ꢀBi2MoO6 exhibits typical vibration mode of Bi2MoO6 without other
miscellaneous peaks, comparing with pure Bi2MoO6. In the magnified Raman spectra (Fig. 4(f)), the peaks at
794.13 cmꢀ1 and 713.77cmꢀ1 shift toward low wavenumber but the peak at 849.22cmꢀ1 shifts in the opposite
direction, suggesting the disordered distribution of MoꢀO bond length in MoO6 octahedron. In addition, a
significant broadened peak at 794.13cmꢀ1 indicates that the doped indium destroys the ordered structure of the
Bi2MoO6 crystal, and the MoO6 octahedron exhibits multiple MoꢀO bonds along the central axis of the
octahedron28. Besides, to investigate the degree of crystal distortion, the MoꢀO bond length was calculate with
empirical equation of RMoꢀO=0.48239 ln(32895/v), where v is the Raman stretching frequency and R is the
metalꢀoxygen bond length.62 The calculated results of MoꢀO bond length are listed in Table S1. The increased
chemical bond length suggests the appearance of a lowꢀvalence molybdenum atom. Therefore, combining XPS and
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Raman spectra, it can be concluded that indium doping can lead to low valence of molybdenum and may result the
defectꢀstate of In3+/In(3ꢀx)+. To further confirm the defect state in the crystals, ESR spectra were performed (Fig.
4(g)). Two signals (g1=2.00 and g2=1.93) corresponds to oxygen vacancy and low valence state Mo ions (Mo4+) in
the solid, respectively.36,45 It is obvious that peak (g3=1.97) corresponding to low valence indium appears after
indium doping and the oxygen vacancy peak is enhanced, which suggests the presence of In(3ꢀx)+ sites in the solid.63
All the results indicate that indium ions are indeed doped into the Bi2MoO6 lattice successfully.
Photoreduction of Cr (VI) under visible light illumination
Fig. 5 (a) Photocatalytic activities of prepared samples for the photoreduction of Cr (VI) at pH=3, (b)
photoreduction kinetics of Cr (VI) over pure Bi2MoO6 and In0.15ꢀBi2MoO6 sample, (c) photoreduction of Cr (VI) at
different pH value and (d) cycling photoreduction performance of prepared In0.15ꢀBi2MoO6 sample at pH=3.
The photocatalytic performances of indiumꢀdoped Bi2MoO6 samples were characterized by the
photoreduction of Cr (VI) under simulate visible light source (Fig. 5). Obviously, Cr (VI) is scarcely reduced
without visible light (Fig. 5 (a)). About 29% of Cr (VI) is reduced by pure Bi2MoO6 sheets after 50 minutes light
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irradiation. Interestingly, the photoreduction performance of IniꢀBi2MoO6 is gradually improved with the
increasing indium doping when the doping molar ratio of indium is limited within 0.15. Especially, the
In0.15ꢀBi2MoO6 sample presents the superior reduction performance that about 98% of Cr (VI) is reduced after 30
minutes visible light irradiation. Furthermore, the photoreduction of Cr (VI) approximately followed
pseudoꢀfirstꢀorder kinetics within 30 min according to the reduction kinetics plot of ln (C0/C) (Fig. 5(b)). Besides,
the reduction rate constant of In0.15ꢀBi2MoO6 (0.0626 minꢀ1) is much higher than that of pure Bi2MoO6 (0.00449
minꢀ1), suggesting indium doping can significantly enhance the photoreduction performance of Cr (VI) under
visible light. Additionally, the influences of pH value on the photoreduction performance of the prepared samples
are also investigated. The photoreduction performance of pure and Inꢀdoped Bi2MoO6 sheets under acidic
conditions is superior to that under neutral or alkaline conditions. Particularly, optimal reduction activity of all
samples is observed at pH=3 (Fig. 5(c)). The stability of the In0.15ꢀBi2MoO6 sample has also been studied by
cycling photoreduction experiments (Fig. 5(d)). Apparently, the photocatalytic efficiency of In0.15ꢀBi2MoO6 is still
maintained over ~90% even after 4 recycles, indicating that the Inꢀdoped Bi2MoO6 sheets possess good stability
and reusability under visible light irradiation.
Photoelectrochemical property
Fig. 6 (a) UVꢀvisible diffuse reflectance spectra, (b) the band gap energies of pure Bi2MoO6 and IniꢀBi2MoO6.
The DRS was employed to detect the photoꢀresponsive performance of the samples and the results were
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shown in Fig.6 (a). Pure Bi2MoO6 possesses strong photoresponse from ultraviolet region to visible light region
and the absorption edge is around 500 nm. The absorption edges of IniꢀBi2MoO6 show an obvious redꢀshift in
visible light region when increasing the doping amount of indium, indicating the decreased bandgap energy. The
bandgap values of the prepared samples are estimated by the formula as follow:
n
α
hv =C hv − E
(1)
(
)
(
)
g
where Eg is band gap energy, h is Planck constant, α is the absorption coefficient, v and C are light frequency and
constant, respectively. Here, n is 2 for Bi2MoO6 and Inꢀdoped Bi2MoO6. As a result, the approximate band gap
energies of the samples are ∼2.86 eV for pure Bi2MoO6 and ~2.71 eV for In0.15ꢀBi2MoO6, respectively (Fig. 6 (b)).
Fig. 7 PL spectra of the pure Bi2MoO6 and IniꢀBi2MoO6 samples.
PL spectra of different products were used to study the separation of photoinduced electrons and holes with
excitation wavelength of 320 nm. As shown in Fig. 7, emission peaks centered at ~470 nm is observed and the
intensity of the emission peaks decreases significantly with the increasing amount of indium dopant. Specially,
In0.15ꢀBi2MoO6 presents the lowest intensity in the PL spectra, suggesting higher separation efficiency of charge
carriers. Hence, the photogenerated charge carriers in In0.15ꢀBi2MoO6 sample presents the longest lifetime,
contributing that more excited electrons are able to participate in photocatalytic reactions rather than recombining
with holes. However, the intensity of In0.20ꢀBi2MoO6 in PL spectrum increases, probably due to the excessive
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defects of excessive doping as recombination center of charge carriers.
Fig. 8 (a) Photocurrent measurements (iꢀt), (b) Nyquist plots of pure Bi2MoO6 and IniꢀBi2MoO6 samples.
To further understand the improvement of photocatalytic activity of indium doped Bi2MoO6,
photoelectrochemistry performances of pure Bi2MoO6 nanosheets and Inꢀdoped Bi2MoO6 nanosheets were
investigated. According to the photocurrent current response (iꢀt), the photocurrent intensity is enhanced with the
increasing amount of doping indium under visible light irradiation (Fig. 8(a)). Moreover, the photocurrent intensity
of In0.15ꢀBi2MoO6 is about 3 times higher than that of pure Bi2MoO6, indicating an enhancement of photogenerated
carriers over indium doping Bi2MoO6. Furthermore, such evolution of photocurrent intensity is also consistent
with the variation of emission peak intensity in PL spectra (Fig. 7). Electrochemical impedance spectroscopy (EIS)
measurements were carried out to elucidate the interface transfer of charge carriers and the separation efficiency
between the photogenerated electrons and holes. The Nyquist plots (Fig.8 (b)) of pure Bi2MoO6 and indiumꢀdoped
Bi2MoO6 are fitted with the resistance and capacitance circuit model (RC) in the inserted Fig. 8 (b). Here, Rs is
regarded as solution resistance of 0.5 M Na2SO4 aqueous solution, and RC fitted circuit (Rct, CPE) is considered as
the interface capacitance and resistance between electrodes and electrolyte, respectively.64,65 As shown in Fig. 8(b),
the arc radius of prepared samples decreases gradually when increasing the amount of doped indium in Bi2MoO6.
Especially, the In0.15ꢀBi2MoO6 presents the smallest arc radius, suggesting its more effective transference
efficiency of electrons between interfaces and higher separation efficiency of photoelectronꢀholes. Furthermore,
comparatively, the equivalent circuit fitting result of In0.15ꢀBi2MoO6 is also the smallest Rst value (Table 2).
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Therefore, indium doping could remarkably enhance the separation and interface transfer of charge carriers
generated by Bi2MoO6 sheets.
Table 2 Fitting data of the Nyquist plots equivalent circuit.
Samples
Rs
Pure Bi2MoO6
24.13
In0.05ꢀBi2MoO6
23.46
In0.1ꢀBi2MoO6
18.12
In0.15ꢀBi2MoO6
14.53
In0.2ꢀBi2MoO6
20.06
CPE
Rst
7.89×10ꢀ6
1.31×105
9.19×10ꢀ5
8.10×104
2.37×10ꢀ5
2.24×104
3.20×10ꢀ5
9212
2.63×10ꢀ5
3.52×104
Possible photocatalytic mechanism
Fig. 9 (a) MottꢀSchottky (MS) plots and (b) valence band XPS spectra of pure Bi2MoO6 and In0.15ꢀBi2MoO6.
To investigate the influence of indium doping on the band structures of the prepared samples, MottꢀSchottky
and valence band XPS spectrum were introduced (Fig. 9). In MottꢀSchottky plots, the positive slope of the
In0.15ꢀBi2MoO6 (Fig. 9 (a)) indicates that the sample of In0.15ꢀBi2MoO6 is remained characteristics of n type
semiconductor after doping with indium. In terms of n type semiconductor, the flat band position is usually located
near the CB.66 Therefore, the variation of flatꢀband position is available to reflect the change of the CB. The
intercept of MottꢀSchottky plots indicated that the flatꢀband potential of pure Bi2MoO6 and In0.15ꢀBi2MoO6
nanosheets are about −0.53 V (vs. Ag/AgCl, pH = 6.8).67 The slope of the tangent decreases obviously in
In0.15ꢀBi2MoO6, suggesting the increase of charge carrier concentration according to the formula (2), which is in
accordance with the results of iꢀt test (Fig. 8(a)).
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1
C2
2
kT



=
× −E + Efb −
(2)



e
εε0 Na
e
where C, Na, E, T, ε, ε0 e and k are the capacitance of the electrode, carrier density, applied potential, temperature,
dielectric constant, permittivity of vacuum (8.85×10ꢀ14 F•cmꢀ1), elementary charge (1.6×10ꢀ19 C) and Boltzmann
constant (1.38×10ꢀ23 J•Kꢀ1), respectively.68 In addition, the VB of pure Bi2MoO6 is estimated to 2.05 eV, according
to the valence band XPS spectrum (Fig. 9(b)). Comparatively, the VB of In0.15ꢀBi2MoO6 sample is shifted to 1.89
eV due to the indium doping. On basis of DRS, MottꢀSchottky and valence band XPS spectrum, the VB position
of In0.15ꢀBi2MoO6 (1.89 eV) is raise up by 0.16 eV, which is mainly attributed to the impurity levels formed by the
coexistence of multivalent indium (In3+/In(3ꢀx)+) after indium doping. When indium occupies the Bi position,
indium will capture electrons from the surrounding atoms to form lowꢀvalence indium atoms, as confirmed by
XPS results (Fig. 4(c)). In order to achieve local charge balance, the electronic energy structure will be broken and
positive charged oxygen vacancies will be generated (Fig. 4(d)). Therefore, due to the presented lowꢀvalue indium,
pꢀtypeꢀlike doping will be achieved and introduces an acceptor level above the valence band.69,70
Based on the above analysis, it can be concluded that the doping of indium mainly improves the
photocatalytic efficiency of Bi2MoO6 through the following aspects, as showed in Fig. 10. (1) The doped indium
drives the formation of defectꢀstate on VB, narrowing the bandgap energy of Bi2MoO6, increasing visible light
response and producing more photoelectrons (e−). (2) Indium doping can reduce the interface transfer resistance of
charge carriers and enhance the separation efficiency of photoinduced charge carriers in Bi2MoO6, resulting in
more participation of photoelectrons in the light reduction reaction.
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Fig. 10 Mechanism of photocatalytic reduction of Cr (VI) over pure and Inꢀdoped Bi2MoO6.
Conclusions
In summary, Inꢀdoped Bi2MoO6 nanosheets was successfully prepared via a facile hydrothermal method and
corresponding photocatalytic ability was evaluated by the photoreduction of Cr (VI) under simulated light (λ > 420
nm) illumination. In the Inꢀdoped Bi2MoO6 samples, trace Bi3+sites are substituted by In(3ꢀx)+ in the crystal lattice
to achieve pꢀtypeꢀlike doping, resulting in a defect state on VB, widening the upꢀshift of VB, narrowing bandgap
for increased visible light response, and producing more photoelectrons. The doped Indium significantly improves
the photocatalytic performance of Bi2MoO6 not only due to the enhanced visible light response, but also because
of the enhancement in the interface transportation and separation of charge carriers. Especially, In0.15ꢀBi2MoO6
presents the optimal photocatalytic reduction performance for Cr (VI) at pH value of 3 under visible light
illumination. Furthermore, the photoreduction efficiency of In0.15ꢀBi2MoO6 is still maintained up to ~90% even
after 4 recycles. This work provides a solution of developing efficient visible light photocatalysts to promote the
practical application of Bi2MoO6 photocatalyst.
Author Information
Corresponding author
* Long Wan Tel: +86ꢀ13055177892. Eꢀmail: wanlong1799@126.com.
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*Shiying Zhang Tel: +86ꢀ731ꢀ84261297. Eꢀmail: cdzhangshiying@163.com.
Notes
The authors declare no competing financial interest.
Acknowledgement
This work is supported by the National Natural Science Foundation of China (51272032), the Hunan
Provincial Natural Science Foundation of China (14JJ5010) and the Open Project of Hunan Key Laboratory of
Applied Environmental Photocatalysis, China (no.ccsuꢀKFꢀ1501).
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