Hydrothermal synthesis of Sm-doped Bi2MoO6 and its high photocatalytic performance for the degradation of Rhodamine B

Article abstract of DOI:10.1016/j.jallcom.2017.09.066

Serials Sm doped Bi₂MoO₆ photocatalyts with different Sm contents (xSm-Bi₂MoO₆, x% = 0.0%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%) were prepared via hydrothermal method to improve the photocatalytic performance of pure Bi

Full text of DOI:10.1016/j.jallcom.2017.09.066

Accepted Manuscript
Hydrothermal synthesis of Sm-doped Bi MoO and its high photocatalytic
2
6
performance for the degradation of Rhodamine B
Min Wang, Meiyan You, Pengyao Guo, Hongyang Tang, Chunmei Lv, Yu Zhang,
Tong Zhu, Jin Han
PII:
S0925-8388(17)33104-3
10.1016/j.jallcom.2017.09.066
JALCOM 43122
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 29 June 2017
Revised Date: 22 August 2017
Accepted Date: 6 September 2017
Please cite this article as: M. Wang, M. You, P. Guo, H. Tang, C. Lv, Y. Zhang, T. Zhu, J.
Han, Hydrothermal synthesis of Sm-doped Bi MoO and its high photocatalytic performance
2
6
for the degradation of Rhodamine B, Journal of Alloys and Compounds (2017), doi: 10.1016/
j.jallcom.2017.09.066.
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ACCEPTED MANUSCRIPT
Hydrothermal synthesis of Sm-doped Bi2MoO6 and its high
photocatalytic performance for the degradation of Rhodamine B
1
Min Wang1*, Meiyan You1, Pengyao Guo3, Hongyang Tang2, Chunmei LV , Yu
Zhang1, Tong Zhu1, Jin Han1
1.School of Mechanical Engineering and Automation, Northeastern University,
Shenyang 110011, China
2.College of Environmental and Chemical Engineering, Shenyang Ligong University,
Shenyang 110165, China
3. School of Resources and Civil Engineering, Northeastern University, Shenyang
11004, China
* Corresponding author:
Tel.: +86 -24-83679926
Fax: +86 -24-83679926
E-mail: wangmin@me.neu.edu.cn
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ABSTRACT: Serials Sm doped Bi2MoO6 photocatalyts with different Sm contents
(xSm-Bi2MoO6, x%=0.0%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%) were prepared via
hydrothermal method to improve the photocatalytic performance of pure Bi2MoO6.
The samples are characterized by XRD, XPS, SEM, TEM, HRTEM, and PL
techniques. It indicates that Sm3+ ions were successfully doped into Bi2MoO6, and the
band gap value was gradually decreased with Sm dopant concentration increasing.
The photocatalytic activity of Sm-doped Bi2MoO6 photocatalyst was significantly
affected by the Sm doping content. Compared to pure Bi2MoO6, the Sm-doped
Bi2MoO6 samples exhibited much higher photocatalytic activity and 0.8% Sm-doped
Bi2MoO6 shows the highest RhB photodegradation rate, being 89% in 50 min under
visible light irradiation. The improved photocatalytic activity of Sm-doped Bi2MoO6
could be attributed to the enhanced visible light absorption, narrower band gaps and
higher separation rate of electron-hole pairs derived from Sm trapping photogenerated
electrons. But the phase structure, special surface areas and morphology have little
effects on the higher photocatalytic activity of Sm-doped Bi2MoO6.
Key words: Bi2MoO6ꢀSm-dopedꢀhydrothermal methodꢀRhodamine Bꢀvisible-light
photoactivity
1. Introduction
In recent years, the application of semiconductor in hydrogen production from
water splitting, environmental pollutants degradation, sensor, antibacterial, etc. have
attracted great attention [1-9].The bismuth-based compounds, such as, BiVO4,
Bi2WO6, Bi2MoO6 and BiOX (X=Cl, Br or I) are typical visible-light-induced
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photocatalyst [10-13]. Among them, Bi2MoO6 has been extensively investigated and
applied due to its distinguished intrinsic properties, such as good chemical stability,
narrow energy band gap (2.5~2.8 eV) [14, 15]. Bi2MoO6 is characterized with a
layered Aurivillius-type phase constructing of alternating (Bi2O2)2+ layers and
perovskite (MoO4)2+ slaps [16]. However, the photocatalytic efficiency of pure
Bi2MoO6 is still low due to its rapid recombination of photoinduced electrons and
holes [17,18]. To solve this problem, some efforts have been made, such as metal
and(or) nonmetal elements doping, heterojunction composites, morphology control
and surface exposure [19-22].
Recently, earth metals that have incompletely occupied 4f and empty 5d orbitals
are often chosen to doping photoctatlysts to improve their photocatalytic performance
because doping earth metals have the ability to promoted the efficient separation of
photoinduced electron-hole pairs. For example, Hanifehpour et al. [23] prepared
samarium-doped ZnS nanoparticles via hydrothermal process. Among the different
amounts of dopant agent used, 4% Sm-doped ZnS nanoparticles indicated the highest
decolorization for Reactive Red 43 solutionunder visible light irradiation. Hu et al.
[24] prepared Sm-doped BiFeO3 via a simple sol-gel route. When the Sm doping
concentration was 3%, the catalyst presented the highest degradation efficiency,
which was about 1.5 times that of pure BiFeO3. Badiei et al. [25] synthesized
samarium doped ZnO nanoparticles by a sonochemical method. The results indicated
that the Sm-doped ZnO with 0.4 mol% of Sm concentration shows high sonocatalytic
activity (59%) in comparison with pure ZnO (51%) and other Sm-doped ZnO
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nanoparticles.
Kashfi-Sadabad et al. [26] prepared hollow spherical structures of ternary
bismuth molybdenum oxide doped with samariumvia development of a P123-assisted
solvothermal technique. And the density functional theory (DFT) calculations have
been performed to further understand the effects of Sm doping on the electronic band
structure, density of states, and band gap of the material. But to the best of our
knowledg, there is no report about the effect of Sm doping on the photocatalytic
acitivity of Bi2MoO6. Furthermore, different synthesis methods should lead to
different results, even when doping the same materials with the same element [27]. so,
we prepared Sm-doped Bi2MoO6 by a hydrothermal method and reveal the effect of
Sm ions doing on the photocatalytic activities of Bi2MoO6 photocatalyst in this work.
Sm doped Bi2MoO6 with different Sm doping contents were synthesized by a
hydrothermal method. The as prepared serials xSm-Bi2MoO6 samples were
characterized by XPS, XRD, SEM, TEM, BET, UV-vis, and PL. The effect of doping
content on photocatalytic activity of Bi2MoO6 was investigated through RhB
photocatalytic degradation under visible light irradiation. The possible catalytic
mechanism is also discussed based on the results of characterization and activity
testing.
2 Experimental
2.1. Synthesis of photocatalysts
In a typical synthesis process, 0.01 mol of Bi(NO3)3·5H2O was dissolved in 50
mL of HNO3 (3mol/L) to form tansparent solution, named as solution A. 5/7 mmol of
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(NH4)6Mo7O24·4H2O was dissolved in 50ml of NaOH (2mol/L) to generate solution B.
Then solution A was slowly dropped into solution B With constant magnetic stirring.
The pH value of the mixtures was adjusted to 7 using 2mol/L of NaOH solution. And
a given amount of Sm2O3 was added into the mixtures. After 30 mintunes, the catalyst
precursors were transferred into a Teflon-lined stainless steel autoclave and heated at
160
for 16 h. Finally, the reactor was naturally cooled to room temperature. The
yellow precipitates were collected by vacuum filtration and washed with absolute
ethanol and distilled water for several times, and finally dried in at 80 for 12 h. For
reference, the pure Bi2MoO6 was also prepared by the similar method. For
convenience, the sample was identified as xSm-Bi2MoO6, where x % symbolized the
initial molar ratio of Sm to Bi (x=0.4, 0.6, 0.8, 1.0, 1.2, respectively) .
2.2. Characterization of the prepared catalysts
The surface composition and binding energy of elements were characterized by
X-ray photoelectron spectroscopy (XPS) equipped with Mg Ka excitation
(MULTILAB2000, VG, Britain). The C 1s peak at Eb=284.6 eV was used for a
correction of the absolute binding energy. The crystal structures of catalysts were
analyzed by X-ray diffraction (XRD) with Cu Kα radiation (D/max-RB, Rigaku,
Japan). The accelerating voltage, the applied current and scanning range were 40 kV,
40 mA and 10ꢁ~ 80ꢁ, respectively. The morphology of all samples were observed by
scanning electron microscope (SEM) with an accelerating voltage of 20 kV (S-3400N,
Hitachi, Japan). The BET surface areas of photocatalysts were acquired from low
temperature nitrogen adsorption-desorption on a specific surface area analyzer
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(F-Sorb 3400, Gold App, China). The photoluminescence (PL) spectra were
investigated on an F-2700 fluorescence spectrophotometer (Hitachi, Japan) at room
temperature.
2.4. Evaluation of photocatalytic activity and photostability of the catalysts
The photocatalytic activity of the catalysts was evaluated by photocatalytic
degradation of RhB under visible light irradiation. A 250 W halogen lamp equipped
with a 420 nm cutoff filter was used as the visible light source, which was placed at
14 cm from the reactor. The details are as follows: 0.02g of catalyst was added into
100 mL of RhB (5 mg/L). Then the suspension was stirred in dark for 30 min to
achieve an adsorption/desorption equilibrium. At every 10 minutes, 5ml of suspension
centrifuged and filtered through a millipore filter to separate catalyst particles. The
concentration of RhB was determined by the absorbance at 554 nm measured by 721
UV-vis spectrophotometer. To study the stability and reusability of the photocatalyst,
cyclic degradation tests were carried out. After each cycle of degradation, the catalyst
was separated by filtration and washed thoroughly with distilled water, dried at 60◦C
for 10 h and reused to the next cycle.
3. Results and discussion
3.1. XRD analysis
The detailed information on the crystal phase and structure of the as prepared
pure Bi2MoO6 and serials xSm-Bi2MoO6 (x %= 0.4, 0.6, 0.8, 1.0, 1.2,) with different
Sm dopant contents are determined by X-ray diffraction patterns. Fig. 1 shows XRD
patterns of pure Bi2MoO6 and xSm-Bi2MoO6 samples. The pure Bi2MoO6 and
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xSm-Bi2MoO6 samples show strong diffraction peaks at 2θ = 10oꢂ70o which were
perfectly indexed as the orthorhombic crystal (JCPDS No. 77-1246) [10]. But no
characteristic peaks from other impurities from Sm dopant are detected. But
interestingly, the intensity of (060) plane for Sm doped Bi2MoO6 samples is higher
than that of the pure Bi2MoO6 and also increase with the Sm doping contents
increasing, which means that Sm doping maybe benefit to preferential orientation
growth along the [060] direction [12, 16]. Furthermore, upon doping with Sm, the
XRD peaks of Sm-doped Bi2MoO6 remained at the same angles as those of pure
orthorhombic Bi2MoO6. The (131) diffraction peek was designated to calculate the
average crystallite sizes of all samples based on Scherrer equation. And the the
average crystallite sizes are 52.4 nm for pure Bi2MoO6 and 50.7~51.2 nm for serials
Sm doped Bi2MoO6 samples.
3.2. XPS analysis
XPS was performed to determine the elemental composition and chemical states
of pure Bi2MoO6 and 0.8Sm-Bi2MoO6. As shown in Fig.2 (a), the binding energies of
Bi3+ located at at Eb=159.1 and 164.4 eV for pure Bi2MoO6, Eb=159.2 and 164.4 eV
for 0.8Sm-Bi2MoO6, are assigned to Bi 4f5/2 and Bi 4f7/2. The results indicate that Sm
doping had no effect on the chemical state of bismuth. From Fig.2 (b), the Mo 3d XPS
spectra of Bi2MoO6, consists of two symmetrical peaks at Eb=232.4 and 235.5 eV,
which correspond to Mo 3d5/2 and Mo 3d3/2, respectively. The Mo 3d XPS spectra of
0.8Sm-Bi2MoO6, also consists of two symmetrical peaks at Eb=232.8 and 235.9 eV,
corresponding to Mo 3d5/2 and Mo 3d3/2, respectively. All peaks are characteristic of
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the Mo6+ species [28]. The binding energy of Mo 3d increased after Sm doping.The O
1s XPS spectra of the pure Bi2MoO6 and 0.8Sm-Bi2MoO6 are showed in Fig.2 (c). As
for the O 1s XPS spectra, the asymmetric peak can be de-convolved into two peaks at
Eb=529.8 and 530.5 eV for pure Bi2MoO6 and Eb=529.7 and 530.4 eV for
0.8Sm-Bi2MoO6, indicating the presence of two forms of oxygen on these samples
surface. The binding energies located at 529.7~8 eV of O1s are attributed to the lattice
oxygen (Olatt) and at 530.4~5 eV are assigned to the adsorbed oxygen (Oads) species,
respectively[10]. According to the peak integrated areas of Olatt and Oads speaks , the
molar ratios of Olatt/Oads can be calculated to be 0.39 and 0.65 for Bi2MoO6 and
0.8Sm-Bi2MoO6 respectively.The Oads/Olatt molar ratios increase after Sm doping,
which would benefit improve the photocatalytic activity. Furthermore, the Bi/Mo
molar ratios for Bi2MoO6 and 0.8Sm-Bi2MoO6 from XPS are 2.03 and 2.08, closed
to 2.0, indicating that the pure Bi2MoO6 and 0.8Sm-Bi2MoO6 are well-proportioned.
In the Sm 3d core level spectrum (Fig. 2(d)), the doublet peaks for Sm3d 5/2 and
Sm 3d 3/2 were located at 1083.5 and 1109.8 eV, respectively, indicating that the
dopant Sm ion was in the trivalent oxidation state and no other valence state of Sm
present in the 0.8Sm-Bi2MoO6 sample[1, 24, 25]. The XPS results further verify the
Sm was successfully doped into Bi2MoO6. But, the real percent of Sm content doped
in Bi2MoO6 from XPS results is about 0.3 % which is less than the theoretical amount
of dopant in experiment (0.8%).
3.3. SEM, TEM and BET analysis
The morphology and micro-structures of pure Bi2MoO6 and 0.8Sm-Bi2MoO6
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were examined by SEM,TEM and HRTEM as shown in Fig. 3. From Fig.3 (a), a sheet
structure can be observed for all samples, and the nanosheets are not uniform in size.
As shown in Fig.3 b, there is a little difference in the morphologies and sheet shapes
of the samples after Sm doping.
To further confirm the morphology of pure Bi2MoO6 and 0.8Sm-Bi2MoO6, TEM
images were also used. TEM images shown in Fig.3c and d further proved that they
are sheet structure. The pure Bi2MoO6 and 0.8Sm-Bi2MoO6 were irregular in shape.
The SEM and TEM images imply that the doping of Sm would not change the
morphological feature of Bi2MoO6. In addition, the HRTEM images of the pure
Bi2MoO6 and 0.8Sm-Bi2MoO6 (Fig. 3e and f ) clearly display the lattice fringes with
d-spacing of 0.32 nm, which is in good agreement with the (111) lattice plane of the
orthorhombic Bi2MoO6.
The BET specific surface area is one of the important affecting factors of
photocatalyic activity. The as prepared samples were evaluated by low temperature
nitrogen adsorption-desorption method and the results are 4.21, 3.89, 4.01, 4.14, 4.56,
3.95 m2/g, indicating that Sm doping has little impact on increasing the specific
surface area.
3.4 UV-Vis diffuse reflectance spectra (DRS)
The optical absorption properties of pure Bi2MoO6 and xSm-Bi2MoO6 were
studied using UV-Vis diffuse reflectance spectroscopy (UV-vis DRS) and the
absorption spectra are shown in Fig. 4. Apparently, all samples exhibit strong
absorption in the UV and visible light regions. In comparison to pure Bi2MoO6, the
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absorption of serials Sm doped Bi2MoO6 samples presents a slightly red shift at 528
nm, indicating the absorption ability was greatly enhanced by the introduction of
doping Sm.The band gap energy (Eg) values of all samples were calculated with the
formula of (ahv)2=A(hv-Eg)n, where a is the absorption coefficient, hv is the incident
photon energy, A is a constant (A=1) and Eg is the band gap energy[27, 29]. In the
above formula, n is a constant and it is dependent on the type of semiconductor
material. In case of Bi2MoO6, the value of n is 1 for direct band gap. The band gap
energies of pure Bi2MoO6, 0.4Sm-Bi2MoO6, 0.6Sm-Bi2MoO6, 0.8Sm-Bi2MoO6 ,
1.0Sm-Bi2MoO6 and 1.2Sm-Bi2MoO6, are 2.65, 2.58, 2.58, 2.55, 2.55 and 2.60 eV,
respectively. It shows that incorporating Sm species in the Bi2MoO6 introduces an
impurity level below the conduction band, thus reducing the band gap energy from
2.65 to 2.55eV. The yield of the photogenerated electron-hole pair depends primarily
on the intensity of the incident photons with energies exceeding or equaling the
photocatalyst band gap energy. Therefore, the photocatalytic performance of Sm
doped Bi2MoO6 photocatalysts would be enhanced, as will be demonstrated below.
3.5 Influence of the Sm doping concentration on thephotocatalytic activity of
Bi2MoO6 and mechanism
RhB is one of the most important and widely used cationic xanthene dyes, which
has high toxicity and stability under natural.The direct drainage not only pollutes
environment, but also causes great harm to aquatic life and human beings [30].
Therefore, the efficient removal of RhB has drawn great concerns. So, in this work,
RhB was chosen as a representative photodegradation target to measure the
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photocatalytic activities of as-prepared catalysts under visible light irradiation and the
results are shown in Fig. 5(A). In the present work, the self-photodegradation of RhB
under visible light irradiation is about 4%ꢃwhich is almost negligible, and the
efficiency of RhB degradation over pure Bi2MoO6 is as low as 55.6 % within 50 min.
However, the photocatalytic activity of Bi2MoO6 can be significantly enhanced after
Sm doping, and the RhB degradation over 0.8Sm- Bi2MoO6 reaches the highest, about
89 %. The photocatalytic activity increases with Sm doping content increasing when
the initial molar ratio of Sm to Bi is less than 0.8%, whereas the photocatalytic
activity decreases with further increasing the initial molar ratios of Sm to Bi.
From Fig. 5(B), the photocatalytic degradation of RhB follows pseudo-first-order
reaction kinetics, which is confirmed by the linear transforms of -ln (C/C0) ~ t (C and
C0 are RhB concentration after degradation and initial concentration, respectively).
The reaction rate constant k values acquired from various catalysts with different Sm
doping content are 0.01564, 0.02157, 0.02237, 0.04336, 0.02999, 0.01990 min-1 for
Bi2MoO6, 0.4Sm-Bi2MoO6 , 0.6Sm-Bi2MoO6, 0.8Sm-Bi2MoO6, 1.0Sm-Bi2MoO6, and
1.2Sm-Bi2MoO6, respectively. The larger reaction rate constant k values, the faster the
photocatalytic reaction rate will be. It can be determined that a maximum rate
constant of RhB degradation over 0.8Sm- Bi2MoO6 is nearly 2.8 times that over pure
Bi2MoO6 under visible light irradiation.
Generally, the photoluminescence (PL) spectra of the semiconductor materials is
used to reflect the transfer, migration, and recombination processes the recombination
efficiency of photogenerated electron-hole pairs stimulated under light illumination.
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The stronger PL intensity usually means that the photo-induced electron-hole pairs are
prone to recombine and the lifetime of the photogenerated electrons are short, thus
photocatalytic performance is enhanced [31-33]. Fig.6(a) demonstrated the PL spectra
of as-prepared samples excited by 325 nm. All the samples possess prominent PL
peaks centering at approx. 513 nm at room temperature. It shows that the relative
intensity of the emission spectra of pure Bi2MoO6 is the greatest, which means that
electrons and holes of Bi2MoO6 are easy to recombine. But there is reduction in the
intensities of PL spectra with increasing in Sm doping concentration, which means
that the Sm doping into Bi2MoO6 lattice could suppress the recombination probalitiy
of electron-holes pairs, because of the separately charge transfer between the Sm3+
and Bi2MoO6. The intensity of the peaks of 0.8Sm-Bi2MoO6 sample is the lowest
compared the other Sm doping Bi2MoO6 samples. The PL intensity increased in the
sequence
of
Bi2MoO6>1.2Sm-Bi2MoO6>0.4Sm-Bi2MoO6>0.6Sm-Bi2MoO6,
1.0Sm-Bi2MoO6 > 0.8Sm-Bi2MoO6, which was coincide with the photocatalytic
performance of the as prepared photocatalysts.
The photocatalytic degradation rate of organic pollutants is generally affected by
●
three major reactive species including holes (h+), hydroxyl radicals ( OH) and
−
●
superoxide radicals ( O2 ) [35]. To elucidate the dominant active species in the
photocatalytic process, the radical trapping experiments are carried out and the results
are shown in Fig. 6(b). Benzoquinone (BZQ), Ammonium oxalate (AO) and
−
+
●
●
isopropanol (IPA) are employed to act as O2 , h , and OH scavenger, respectively.
The addition of AO (an
hole scavenger, 2 mM) could decreased the
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photodegradation efficiency of RhB to 52.8%, suggesting that the consumption of
●
excited holes could apparently affect the photodegradation efficiency. When IPA ( OH
scavenger, 2 mM) was added, the photodegradation efficiency of RhB was slightly
suppressed, indicating the minor role of hydroxyl radicals in the RhB
−
●
photodegradation process. Additionally, the addition of BQ ( O2 scavenger, 0.5 mM)
dramatically suppressed the RhB photodegradation efficiency, demonstrating that the
superoxide radicals were also of great importance in RhB photocatalytic process.
+
−
●
Consequently, these results clearly evidenced that photogenerated h and O2 are the
dominant reactive species for the degradation of RhB.
On the basis of the above characterization and photocatalytic performance results,
the photocatalytic mechanism for RhB degradation by the Sm-doped Bi2MoO6
photocatalysts under visible light irradiation was proposed. Fig. 7 shows a possible
mechanism for the highest photocatalytic degradation of RhB by the 0.8Sm-Bi2MoO6.
Firstly, with respect to the Sm-doped Bi2MoO6 photocatalyst, its band gap structure
could be modulated by the Sm dopant ions. In Sm-doped Bi2MoO6, the Sm 4f level
was located below the conduction band of Bi2MoO6 [10, 26], which would play as an
acceptor lever for photo-excited electrons. Therefore, compared to the pure Bi2MoO6,
the band gap of Sm-doped Bi2MoO6 would be narrower and the visible-light
absorption would be enhanced, which can be confirmed by UV-vis diffuse reflectance
spectra (Fig. 4). Secondly, Sm3+ ions have a unique 4f 5 electron configuration and can
accept photogenerated electrons and generate Sm2+ ions which are very unstable, then
the electrons can easily transfer to the surface-adsorbed O2 and finally form a reactive
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−
●
superoxide radical ion ( O2 ); consequently, the recombination of the photogenerated
electrons and holes is inhibited [35]. Thirdly, Sm doping can increase the amount of
oxygen vacancies compared with pure Bi2MoO6. The oxygen vacancies in the
catalysts also can act as electron trapping centers, hence significantly boosting the
separation of photogenerated electrons and holes. Thus, more active species would be
produced over 0.8Sm-Bi2MoO6 photocatalyst. Simultaneously, the holes migrating to
●
the photocatalyst surface could react with H2O to form OH for the degradation of
−
+
●
RhB or directly oxidize RhB. On the basis of the trapping experiments, O2 and h
were the predominant reactive species for RhB degradation, while hydroxyl radicals
●
OH might also contribute a little to the RhB degradation process as shown in Fig.6c.
In addition, it is generally accepted that the phase, morphology, specific surface
areas affect the photocatalytic activity of photocatalyst [36]. In this work, the samples
possessed similar orthorhombic phase structure, so the phase structure is not the main
reason for higher photocatalytic acitivity of 0.6Sm- Bi2MoO6. Also, the results of
SEM and BET specific surface areas changes little, so, the morphology and specific
surface areas would not promoted remarkably the photocatalytic activity.
However, when the molar ratio of Sm to Bi is more than 0.8%, the photocatalytic
activity decrease significantly, which means there was an optimal concentration of Sm
for the effective separation of electron-hole pairs. The reason may be due to that an
excessively high concentration of Sm would become recombination centers for
electron–hole pairs and also shield the surface of Bi2MoO6 from light absorption that
resulting in a lower photo activity [10].
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3.7 Evaluation of photostability
The stability and recyclability are key issue for practical application. The recycle
experiments for RhB degradation were carried out for four cycles in the presence of
0.8Sm-Bi2MoO6 photocatalyst under the same experimental conditions. After each
run, the photocatalyst was collected by simple infiltration and dried in the oven 60◦C
for 8 h before reused. The degradation rates for RhB of each run are shown in Fig.8
(a). It shows that the degradation efficiency of 0.8Sm-Bi2MoO6 effectively maintained
about 83% after four cycling reactions. Furthermore, to verify the stability of the
sample, XRD patterns of 0.8Sm- Bi2MoO6 after four cycled photodegradation of RhB
were measured. As shown in Fig. 8(b), the XRD pattern of 0.8Sm- Bi2MoO6 after four
successive cycles exhibits the similar diffraction peaks as-synthesized 0.8Sm-
Bi2MoO6. These recycling results indicate that the photocatalyst is stable and
recyclable, which favor a long time application in the photocatalytic removal rate of
environmental pollutants.
4. Conclusions
The Sm-doped Bi2MoO6 photocatalysts were successfully synthesized by a
hydrothermal method and the effect of Sm doping content on the photocatalytic
perfromance was studied. Based on XRD and XPS analyses, Sm is successfully doped
into the Bi2MoO6 .From UV-vis DRS results, the band gap of Sm-doped Bi2MoO6
photocatalysts decreased and the absorption ability enhanced compared with pure
Bi2MoO6. It was found that the photocatalytic activity of asprepared samples is
strongly dependent on the Sm: Bi molar ratios. The highest photocatalytic activity is
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obtained when the initial molar ratio of Sm to Bi is 0.8% and the degradation rate of 5
mg/LRhB within 50 min is 89 %. And the 0.8Sm-Bi2MoO6 also has good stability for
practical application.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China for Youth (21207093), by Liaoning Excellent Talents in University
ꢄLJQ2014023).
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Figure Captions
Fig.1 XRD patterns of as-prepard samples
Fig. 2 XPS spectra of Bi2MoO6 and 0.8Sm-Bi2MoO6 for Bi 4f (a), Mo 3d (b), O 1s (c),
Sm 3d (d)
Fig. 3 SEM, TEM and HRTEM images of Bi2MoO6(a, b, c), 0.8Sm-Bi2MoO6 (d, e, f)
Fig. 4 UV-Vis DRS spectra of (a) Bi2MoO6 , (b) 0.4Sm-Bi2MoO6, (c) 0.6Sm-Bi2MoO6,
(d) 0.8Sm-Bi2MoO6 , (e)1.0Sm-Bi2MoO6 and (f) 1.2Sm-Bi2MoO6
Fig.5 (A) RhB degradation under visible light irradiation in the presence of the
samples; (B) -ln(C/C0) as a function of irradiation time on the catalysts for (a) blank,
(b) Bi2MoO6, (c)0.4Sm-Bi2MoO6, (d)0.6Sm-Bi2MoO6, (e) 0.8Sm-Bi2MoO6,
(f)1.0Sm-Bi2MoO6, and (g) 1.2Sm-Bi2MoO6
Fig.6 (a) Photoluminescence spectra of the samples. The excitation wavelength is
325 nm; (b) Photocatalytic experiment in presence of Benzoquinone (BZQ),
Ammonium oxalate (AO) and isopropanol (IPA) respectively under visible light
irradiation over 0.8Sm-Bi2MoO6 photocatalyst;
Fig.7 Schematic illustration of the mechanism for the photocatalytic degradation of
RhB on the Sm-doped Bi2MoO6 photocatalyst.
Fig. 8 (a) Repeated photocatalytic degradation of RhB over 0.8Sm-Bi2MoO6 sample
under visible light irradiation, (b) XRD patterns of cycling 0.8Sm-Bi2MoO6
22

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1.2Sm-Bi2MoO
6
1.0Sm-Bi2MoO
6
0.8Sm-Bi2MoO
6
0.6Sm-Bi2MoO
6
0.4Sm-Bi2MoO
6
Bi2MoO
6
10
20
30
40
50
60
70
2θ(°)
Fig.1

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159.1
(b) Mo 3d 232.4
(a) Bi 4f
159.2
164.4
235.5
235.9
232.8
164.5
Bi MoO
2
6
Bi2MoO6
0.8Sm-Bi2MoO6
162
0.8Sm-Bi MoO
2
6
230
231
232
233
234
235
236
237
238
156
158
160
164
166
Binding Energy/eV
Binding Energy/eV
(d) Sm 3d
Sm 3d5/2
(c) O 1s
Sm 3d3/2
529.7
Sm 3d5/2
530.4
Bi2MoO6
529.9
530.6
0.8Sm-Bi2MoO6
527
528
529
530
531
532
533
1060
1080
1100
1120
1140
Binding Energy/ev
Binding Energy/eV
Fig.2

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I
D=0.32 nm
J
Fig.3

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1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
(f)
(a)
200
300
400
500
600
700
800
Wavelength/nm
(b)
(f)
(a)
v
h
a
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
hv(eV)
Fig.4

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1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(a)
blank
Bi MoO
2
6
0.4Sm-Bi MoO
2
6
0.6Sm-Bi MoO
2
6
0.8Sm-Bi MoO
2
6
1.0Sm-Bi MoO
2
6
1.2Sm-Bi MoO
2
6
0
10
20
30
40
50
Irradiation time/min
2.4
(b)
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
a: k=0.00085, R=0.98981
b: k=0.01564, R=0.97758
c: k=0.02157, R=0.99468
d: k=0.02237, R=0.99632
e: k=0.04336, R=0.99808
f: k=0.02999, R=0.98652
g: k=0.01990, R=0.98564
0
10
20
30
40
50
Irradiation time/min
Fig.5

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(a)
Bi2MoO6
0.4Sm-Bi2MoO
0.6Sm-Bi2MoO6
0.8Sm-Bi2MoO6
1.0Sm-Bi2MoO6
1.2Sm-Bi2MoO6
400 425 450 475 500 525 550 575 600 625
Wavelength/nm
(b)
80
60
40
20
0
No quencher
BQ
AO
IPA
Fig.6

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Fig.7