The upconversion and enhanced visible light photocatalytic activity of Er3+-doped tetragonal BiVO4

Article abstract of DOI:10.1039/c4ra12322k

Er³⁺-doped BiVO₄ with tetragonal structure is prepared by the microwave hydrothermal method. X-ray diffraction and Rietveld refinement demonstrate that the structure is transformed from the monoclinic (C2/c:c3) phase to the tetragonal (I41/amd:2) phase by doping with Er³⁺ ions. Er³⁺ doping also influences the morphology change of BiVO₄ from irregular flake-like crystal to rod-like crystal, which leads to the increase of the surface areas from 3.25 to 11.96 m² g^-1. Compared with the monoclinic BiVO₄, the upconversion of the Er³⁺-doped tetragonal BiVO₄ occurs through the transitions from the ⁴I_15/2 ground state to ⁴F_7/2, ²H_11/2, and ⁴F_9/2 states, respectively. The photocatalytic experiment indicates that the tetragonal BiVO₄ (8 at.%) with a larger specific surface area (9.88 m² g^-1) shows the best photocatalytic activity under visible light irradiation, which can efficiently improve the degradation rate of RhB up to 97.2% at 150 min. This journal is

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The upconversion and enhanced visible light photocatalytic activity of Er3+-doped
tetragonal BiVO4
Wei Yang, Guoqiang Tan*, Huijun Ren, Lili Zhang, Chengcheng Zhao, Ao Xia
Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of
Education, Shaanxi University of Science and Technology, Xi’an, 710021, Shaanxi,
China
Abstract: Er3+ꢀdoped BiVO4 with tetragonal structure is prepared by the microwave
hydrothermal method. Xꢀray diffraction and Rietveld refinement demonstrate that the
structure is transformed from the monoclinic (C2/c:c3) phase to the tetragonal (I41/amd:2)
phase by doping Er3+ ions. Er3+ doping also influences the morphology change of BiVO4
from irregular flakeꢀlike crystal to rodꢀlike crystal, which leads to the increase of the
surface areas from 3.25 to 11.96 m2/g. Compared with the monoclinic BiVO4, the
upconversion of the Er3+ꢀdoped tetragonal BiVO4 occurs through the transitions from the
4
2
4
4I15/2 ground state to F7/2, H11/2, and F9/2 states, respectively. The photocatalytic
experiment indicates that the tetragonal BiVO4 (8at.%) with a larger specific surface area
(9.88 m2/g) shows the best photocatalytic activity under visible light irradiation, which
can efficiently improve the degradation rate of RhB up to 97.2% at 150min.
Keywords: Semiconductors; BiVO4; Er3+; Upconversion; Photocatalytic activity
1. Introduction
Semiconductor photocatalysts are widely used in the degradation of organic
contaminants in wastewater [1ꢀ3]. According to the early reports, the monoclinic BiVO4
*
Corresponding author. Tel.: +8613759878391.
Eꢀmail address: tan3114@163.com (G. Tan).
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(mꢀBiVO4) is the best visibleꢀlightꢀdriven photocatalyst, while the photocatalytic activity
under visible light irradiation of tetragonal BiVO4 (tꢀBiVO4) is negligible [4, 5].
Therefore the monoclinic BiVO4 has been extensively studied as a promising
photocatalyst for organic photocatalytic degradation with its narrow band gap (2.4eV) [6,
7]. However, the rapid recombination rate of electronꢀhole pairs is always the main
drawback for monoclinic BiVO4 to be applied. To solve this problem, different methods
have been used to improve the photocatalytic activity of the monoclinic BiVO4, such as
coꢀcatalysts loading, impurity doping, and the formation of heterojunction [8, 9].
Recently, S. Usai and Sergio Obregón reported that Y3+ or Er3+ doping could induce the
monoclinicꢀtetragonal heterostructure and improve the photocatalytic performance under
sunꢀlike excitation [10, 11]. On this basis, we obtained the tetragonal BiVO4 after doping
Er3+ ions. This phenomenon is quite different from the previous reports. In this work,the
Er3+–doped tetragonal BiVO4 system exhibits higher photocatalytic activity than that of
monoclinic BiVO4 for the degradation of RhB under visible light irradiation.
2. Experimental
All the reagents were of analytical grade and were used without any further
purification. In a typical preparation process, 0.01mol Bi(NO3)3·5H2O was dissolved in
25mL deionized water and stirred for 20min at room temperature. A quantity of 0.01mol
NH4VO3 was dissolved in 25mL 100℃ deionized water and stirred for 20min until the
dissolution was complete. Then the NH4VO3 solution was added into the Bi(NO3)3·5H2O
solution with magnetically stirring for 20 min, thus forming the jacinth slurry A. The pH
of the slurry A was adjusted to 8.5 by adding concentrated NaOH (5mol/L). After stirred
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for several minites, the appropriate contents of Er(NO3)3·6H2O were added to slurry A
until the molar ratio of nEr to nBi was 0at.%ꢀ12at.%, generating precursors B. Then each
precursor was transferred into the polytetrafluoroethylene (PTFE) autoclave (the filling
density was 60%) and heated under microwave irradiation using a microwave
hydrothermal reactor (Model MDSꢀ8, Sineo, ShangHai). The specific program was as
follow. Firstly, the microwave hydrothermal reaction power was set to 300W and the
temperature was from room temperature up to 100℃ for 6min.Secondly, the microwave
hydrothermal reaction temperature was from 100℃to 150℃for 6min and from 150℃to
200℃for 40min, and then it was allowed to cool to room temperature naturally. Finally,
the obtained precipitates were washed three times with deionized water and absolute
ethyl alcohol, and then dried at 80℃for 12h in air.
The structural properties of the asꢀprepared samples were determined by an Xꢀray
diffractometer (XRD, D/Maxꢀ2200, Rigaku, Japan) with Cu Kα radiation (λ=0.15406nm).
The morphology was analyzed by using a scanning electron microscope (SEM, Sꢀ4800,
Hitachi, Japan). The microstructures were investigated by a transmission electron
microscopy (TEM) and a high resolution transmission electron microscopy (HRTEM, J
EMꢀ2100, Japan). The specific surface area was measured by a specific surface area
analyzer (3Hꢀ2000BETꢀA, BJ, China). The adsorbate was N2 while the carrier gas was
He. The UVꢀvis diffuse reflection spectra were recorded on a UVꢀvis spectrophotometer
(Hitachi Uꢀ3900H, Japan), using BaSO4 as a reference. The upconversion fluorescence
spectra were measured using YAG:Nd3+ (Quanta Ray Labꢀ170) pulse laser and Ti
sapphire sapphire femtosecond laser (Miraꢀ900) as excitation sources, and the output
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wavelength was 980 nm. The collection and detection of luminescence were carried out
on a spectrometer (SP2750i) with a spectral resolution of 0.008nm. All of the
measurements were performed at room temperature. The Zeta potential was identified by
Nano Particle and Zeta potential analyzer (Malvern NanoꢀZS, UK).
In a process, a 500W xenon lamp was used as a visibleꢀlight source. 0.05g
photocatalyst was dispersed in 50mL RhB solution (5mg/L) under ultrasonic treatment.
Prior to the irradiation, the solution was magnetically stirred in the dark for 30min to
achieve the adsorptionꢀdesorption equilibrium. The concentration of RhB was determined
by recording the absorbance at 553 nm using a UVꢀVis spectrophotometer (Model
SPꢀ756p).
3. Results and discussion
Fig. 1
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Fig. 1(a) shows the XRD patterns of pure BiVO4 and Er3+ꢀdoped BiVO4
photocatalysts. It is worth noting that the monoclinic BiVO4 (JCPDS NO.14ꢀ0688) was
changed into the tetragonal BiVO4 (JCPDS NO.14ꢀ0133) when Er3+ is incorporated. The
observed evolution of the crystalline structure of Er3+ꢀdoped BiVO4 is significantly
different from that previously reported [11], that is merely the tetragonal phase is present
when Er3+ content is above 4at.%. In other words, Er3+ꢀdoped BiVO4 is considered to
stabilize the tetragonal structure. In addition, the intensities of the (101) and (200) peaks
of the tetragonal BiVO4 are reduced gradually so that the crystallinity of the composite is
decreased.
The Zeta potential of the precursor slurry is 41.20 with the Er3+ꢀdoped BiVO4 (0at.%)
in Fig. 1(c), suggesting the larger electrostatic repulsion, which is responsible for the
–
shortage of BiVO4 crystal nuclei. The Bi3+ and VO3 will grow up into more stable
thermodynamic monoclinic BiVO4 crystal grains according to certain spatial
configurations and then irregular monoclinic BiVO4 in the microwave hydrothermal
process [12]. The Zeta potentials of Er3+ꢀdoped BiVO4 (4ꢀ12at.%) are 35.71, 32.76, 32,
10.09, 1.94, respectively, suggesting the decreasing electrostatic repulsion, which can
form a large number of BiVO4 crystal nuclei. The crystal nucleus will generate
amorphous BiVO4 precipitate, which can be crystallized into tetragonal BiVO4 and
Er8V2O17 in the drying oven at 80℃(Fig. 1(d)). The tetragonal BiVO4 and Er8V2O17 are
dissolved and crystallized into the tetragonal Er3+ꢀdoped BiVO4. The increase of Er3+
doping content give rise to the decrease of Er3+ꢀdoped BiVO4 crystal grains with
recrystallization process (Tab. 1), resulting in the widened diffraction peak with the
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Er3+ꢀdoped BiVO4 (12at.%).
To further analyze the crystal structure of the Er3+ꢀdoped BiVO4, the Rietveld
refinement on the XRD patterns of pure BiVO4 and the Er3+ꢀdoped BiVO4 (8at.%) is
performed using the Maud program [13, 14]. In the light of the refinement results, the
schematics of the BiVO4 crystal structure are shown in Fig. 1(b). The structural transition
of Er3+ꢀdoped BiVO4 can be found, where the monoclinic BiVO4 and the Er3+ꢀdoped
BiVO4 (8at.%) having C2/c:c3 and I41/amd:2 space groups, respectively, can be
observed.
Tab. 1
Samples
Crystal
Space
group
Lattice parameters
tetragonal
crystallite size
(nm)
structure
Undoped
Monoclinic
C2/c:c3
a=5.1881Å b=5.0788Å
c=11.6757Å
4at.% Er3+
6at.% Er3+
8at.% Er3+
10at.% Er3+
12at.% Er3+
Tetragonal
Tetragonal
Tetragonal
Tetragonal
Tetragonal
a=b=7.2769Å c=6.4413 Å
a=b=7.2854Å c=6.4424 Å
a=b=7.2894Å c=6.4464 Å
a=b=7.2876Å c=6.4468 Å
a=b=7.2852Å c=6.4465 Å
67.9
69.4
69.1
59.3
49.4
I41/amd:2
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Fig. 2
The morphologies of the samples synthesized by microwaveꢀhydrothermal process
were characterized by a scanning electron microscopy (SEM), as shown in Fig. 2. When
the content of Er3+ is 0 at%, the crystal possesses irregular flakeꢀlike morphology with
100 nm thickness (Fig. 2 (a)). The distribution of crystal grains is nonuniform and the
gain surface is smooth, resulting in small specific surface area (3.25m2/g). With the
increase of doping Er3+ content (4ꢀ12at.%), the flakeꢀlike crystal grains are dissolved
gradually and recrystallized to the wellꢀdefined rodꢀlike crystals of the tetragonal BiVO4
(Fig. 2(b)ꢀ(f)). This implies that the formation of crystals in nucleationꢀdissolutionꢀ
recrystallization may be predominated by Er3+ ions. The increase of the specific surface
area (from 7.42 m2/g to 11.96 m2/g) can therefore be correlated with these changes of the
morphology and smaller gain size. The changes of morphologies and the specific surface
area attribute to the different Zeta potentials, resulting in the different growth of BiVO4
crystal. The tetragonal BiVO4 and Er8V2O17 crystals are dissolved gradually and
recrystallized to form the wellꢀdefined rodꢀlike crystals. These results are in accordance
with the XRD patterns.
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Fig. 3
It can also be seen in the TEM images that the irregular flakeꢀlike crystal is the
monoclinic BiVO4 (0at.%) with 100nm thickness (Fig. 3(a)) and the rodꢀlike crystal is
Er3+ꢀdoped BiVO4 (8at.%) with the length of 300nm (Fig. 3(c)). From the insets in Fig.
3(b) and (d), the SEAD diffraction patterns are the bright spots of electron diffraction,
which indicates that both monoclinic and tetragonal BiVO4 are crystalline. The HRTEM
images obtained from the (b) and (d) areas marked with rectangles are shown in Fig. 3(b)
and (d). An interval of 0.2910 nm between the uniform fringes is in good agreement with
the (040) lattice plane of monoclinic BiVO4. Meanwhile, the spaces of 0.4810 nm and
0.3626 nm of the clear lattice fringes with corresponding to (101) and (200) planes of
tetragonal BiVO4 imply the high crystallinity. These results are consistent with the initial
XRD analyses.
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Fig. 4
The optical properties of monoclinic BiVO4 and Er3+ꢀdoped tetragonal BiVO4 are
measured by using UVꢀvis diffuse reflection spectrum in the wavelength range of
200ꢀ800nm. From the diffuse reflectance spectra (Fig. 4(a)), two clear absorption edges
can be noticed both in the UV and visible light regions, which corresponds to the band
gap energy of 2.4 eV and 2.9 eV of the monoclinic and tetragonal phases, respectively
[15]. Compared with the monoclinic BiVO4, three new absorption bands at ca. 490, 524
and 655 nm of Er3+ꢀdoped BiVO4 can be observed, which can be assigned to the Er3+
transitions from the 4I15/2 ground state to 4F7/2, 2H11/2, and 4F9/2 states, respectively [16].
In order to understand the luminescent property of Er3+ doping in the catalysts, we
have performed the upconversion fluorescence spectra of Er3+ꢀdoped tetragonal BiVO4
(8at.%) upon 980nm laser excitation (Fig. 4(b)). In the spectrum, there are two emission
parts: the green emission from 490 nm to 570 nm is corresponding to the transitions from
4
2H11/2/4F7/2 to I15/2 when Er3+ ions are doped and the red one in a range 640ꢀ690 nm is
assigned to the transition of 4F9/2ꢀ4I15/2 when Er3+ ions are doped [17]. The result indicates
the existence of the upconversion process in the Er3+ꢀdoped tetragonal BiVO4
photocatalyst. Generally, the Er3+ꢀdoped upconversion process can increase the number
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of incoming photons with adequate energy. Then the photons can be absorbed by the
photocatalyst, which will generate more photogenerated electronꢀhole pairs [18].
Therefore, the photocatalytic activities of Er3+ꢀdoped tetragonal BiVO4 with
upꢀconversion effect are expected to be efficiently enhanced.
Fig. 5
The photocatalytic activities of the Er3+ꢀdoped BiVO4 are evaluated by measuring
the degradation of RhB aqueous solution under visible light irradiation shown in Fig. 5(a).
For the monoclinic BiVO4, the photodegradation of RhB is 63.13% after 150min
irradiation. By introducing the upꢀconversion Er3+ and optimizing the contents (4ꢀ12at.%),
the photodegradation rates of RhB have promptly reached to 96.80%, 96.98%, 97.2%,
96.38% and 88.35% respectively, which exhibits much higher photocatalytic activity than
that of the monoclinic BiVO4. When Er3+ is increased to 10at.%, the photocatalytic
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activity begins to be decreased because the excessive Er3+ ions may cover the active sites
on BiVO4 surface and also may act as the recombination centers of photogenerated
electrons and holes, resulting in the decrease of photocatalytic activity [19]. Therefore,
the optimum content ofdoping Er3+ is 8at.%.
The temporal evolution of spectral changes during the photodegradation process of
RhB for Er3+ꢀdoped BiVO4 (8at.%) is displayed in Fig. 5(b). From the spectra, it can be
seen that the absorbance of RhB at the maximum absorption wavelength is gradually
decreased, which realizes the deꢀethylation. Under visible illumination, the intense bright
cherryꢀred color of the starting solution gradually disappears with increasing exposure
time. The deꢀethylation of the N, N, N, Nꢀtetraethylated rhodamine molecules appears in
the wavelength position of its major absorption band moving toward the blue region, and
it is as follows: λmax, RhB, 554 nm; N, N, Nꢀtriethylated rhodamine, 539nm; N,
Nꢀdiethylated rhodamine, 522nm; and Nꢀethylated rhodamine, 510nm; and rhodamine,
498nm. RhB has completely been changed into rhodamine [20]. Then through the
destruction of their conjugate structure, the further degradation is achieved. After a series
of complicated oxidation reaction, the RhB is decomposed to smaller organics and
minerals, which can be proved by the results of the TOC measurement [21]. The inset in
Fig. 5(b) displays that the ability of Er3+ꢀdoped tetragonal BiVO4 (8at.%) to mineralize
RhB is evaluated by monitoring changes of organic carbon in TOC. As can be seen, TOC
diminishes gradually and ca. 36% TOC removal is obtained after 150 min irradiation,
implying that RhB degradation is accompanied by partial mineralization.
The recycle experiments are performed to evaluate the photocatalytic repeatability
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and stability of Er3+ꢀdoped tetragonal BiVO4 (8at.%) photocatalyst under visible light
irradiation [22]. Fig. 5(c) shows the results from the four successive runs of the
photodegradation of RhB under the same experimental conditions. It is clear to see that
no significant photoactivity loss is observed after four times of successive recycles. The
results indicate that Er3+ꢀdoped tetragonal BiVO4 (8at.%) photocatalyst is stable during
the photodegradation of RhB.
Fig. 5(d) shows the mechanism of the upconversion of Er3+ꢀdoped tetragonal BiVO4.
For monoclinic BiVO4, the absorbable light is limited to 519 nm (Fig. 4(a)), while the
proper doping content of Er3+ in the Er3+doped BiVO4 with can effectively be converted
from the long wavelength light to the short wavelength light so as to generate the excited
electrons with higher energy and sequentially to capture low energy photons as shown in
Fig. 5(d). When these excited electrons are on the ground state by means of irradiation or
energy transformation, the emitted light or the transformed energy will make the
tetragonal BiVO4 yield more photoinduced electronꢀhole pairs so as to enhance the
photocatalytic activities. Then the highly oxidative holes accumulated in the BiVO4 can
directly decompose the organic pollutants or react with H2O molecules to produce •OH
[23, 24], which indicates that the upconversion process can enhance the utilization of
efficient photons of Er3+ꢀdoped tetragonal BiVO4 photocatalysts. On the basis, the
upconversion plays a significant role in the improvement of the photocatalytic activities
in the Er3+ꢀdoped tetragonal BiVO4 system.
4. Conclusions
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The tetragonal BiVO4 with different Er3+ꢀdoped contents has been successfully
synthesized by microwave hydrothermal method. The morphology of Er3+ꢀdoped BiVO4
has been changed from irregular flakeꢀlike to rodꢀlike crystals with the structural
transition from monoclinic to tetragonal BiVO4. Compared with the monoclinic BiVO4,
the upconversion of the Er3+ꢀdoped tetragonal BiVO4 occurs through the transitions from
the 4I15/2 ground state to 4F7/2, 2H11/2, and 4F9/2 states, which makes the tetragonal BiVO4
generate more photoinduced electronꢀhole pairs so as to enhance the photocatalytic
activities. Furthermore, the Er3+ꢀdoped tetragonal BiVO4 exhibits significantly higher
photocatalytic activity than that of the monoclinic BiVO4 for the degradation of RhB
under visible light irradiation.
Acknowledgments
This work is supported by the Project of the National Natural Science Foundation of
China (Grant No.51172135); the Academic Leaders Funding Scheme of Shaanxi
University of Science & Technology (2013XSD06); Doctorate Scientific Research Initial
Fund Program of Shaanxi University of Science & Technology(BJ4ꢀ13); the Graduate
Innovation Fund of Shaanxi University of Science & Technology (SUSTꢀA04).
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Figure Captions
Fig. 1 (a) XRD patterns of different Er3+ꢀdoped contents. (b) Rietveld refined XRD
patterns and the insets show schematics of the monoclinic BiVO4 and Er3+ꢀdoped BiVO4
(8at.%). (c) Zeta potential of different Er3+ꢀdoped contents. (d) XRD patterns of
monoclinic BiVO4 and Er3+ꢀdoped BiVO4 (8at.%) in the drying oven at 80℃.
Fig. 2 SEM images of Er3+ꢀdoped BiVO4 (a) 0at.% (b) 4at.% (c) 6at.% (d) 8at.% (e)
10at.% (f) 12at.%. (g) Specific surface areas of the monoclinic BiVO4 and Er3+ꢀdoped
BiVO4.
Fig. 3 (a, b) TEM images and HRTEM of monoclinic BiVO4 (0at.%); (c, d) TEM images
and HRTEM of Er3+ꢀdoped BiVO4 (8at.%) (the insets in (b) and (d) show SEAD
patterns).
Fig. 4 (a) UVꢀvis diffuse reflection absorption spectra of Er3+ꢀdoped BiVO4. (b) The
upconversion fluorescence spectra of Er3+ꢀdoped tetragonal BiVO4 (8at.%) upon 980nm
laser excitation.
Fig. 5 (a) Photocatalytic degradation to RhB with different Er3+ doping contents; (b)
absorption spectra changes during the photocatalytic degradation of the Er3+ꢀdoped
tetragonal BiVO4 (8at.%) in RhB solution under visible light irradiation (inset show the
TOC analysis of the degradation of RhB solution by the Er3+ꢀdoped tetragonal BiVO4
(8at.%) photocatalyst) (c) Cycling runs in the photocatalytic degradation of RhB in the
presence of Er3+ꢀdoped tetragonal BiVO4 (8at.%) photocatalyst. (d) A schematic
illustration of the energy conversion in the Er3+ꢀdoped BiVO4 photocatalyst.
Tab. 1 Rietveld refined structural parameters and crystallite size of the catalysts.
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