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A comprehensive investigation of tetragonal Gd-doped BiVO4 with enhanced photocatalytic performance under sun-light
Accepted Manuscript Title: A comprehensive investigation of tetragonal Gd-doped BiVO4 with enhanced photocatalytic performance under sun-light Author: Yangyang Luo Guoqiang Tan Guohua Dong Huijun Ren Ao Xia PII: DOI: Reference:
S0169-4332(15)03105-0 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.100 APSUSC 32083
To appear in:
APSUSC
Received date: Revised date: Accepted date:
20-5-2015 30-10-2015 14-12-2015
Please cite this article as: Y. Luo, G. Tan, G. Dong, H. Ren, A. Xia, A comprehensive investigation of tetragonal Gd-doped BiVO4 with enhanced photocatalytic performance under sun-light, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.12.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A comprehensive investigation of tetragonal Gd-doped BiVO4 with enhanced photocatalytic performance under sun-light *
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Yangyang Luo, Guoqiang Tan , Guohua Dong, Huijun Ren, Ao Xia School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi'an,
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Shaanxi, 710021, China.
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Abstract
Tetragonal Gd-doped BiVO4 having enhanced photocatalytic activity have been
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synthesized by a facile microwave hydrothermal method. The structural analysis
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indicates that Gd doping can induce the phase transition from monoclinic to tetragonal BiVO4. The reaction results in precursor solutions imply that tetragonal GdVO4 seeds
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as crystal nucleus are the original and determined incentives to force the formation of
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tetragonal Gd-BiVO4. The influences of the surface defect, band structure, and BET
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surface area on the improved photocatalytic activities of tetragonal Gd-doped BiVO4 are investigated systematically. The results demonstrate that the more surface oxygen deficiencies as active sites and the excellent mobility and separation of photogenerated electrons and holes are beneficial to the enhancement of the photocatalytic performance of tetragonal Gd-BiVO4. The RhB photodegradation experiments indicate that the contribution of high photocatalytic activities under simulated sun-light is mainly from UV-light region due to the tetragonal structure feature. The best photocatalytic performance is obtained for tetragonal 10 at%
*
Corresponding author. Tel.: +8613759878391. E-mail address: [email protected] (G.Q. Tan). 1
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Gd-BiVO4, of which the RhB degradation rate can reach to 96% after 120 min simulated sun-light irradiation. The stable tetragonal Gd-BiVO4 with efficient
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mineralization will be a promising photocatalytic material applied in water purification.
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Keywords: Gd-doped BiVO4; Phase transition; Formation mechanism; Photocatalytic
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activity; Sun-light
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1. Introduction
Recently, semiconductor photocatalysis technology as an ideal green technology
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has attracted considerable attention for its potential applications in environmental
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cleaning and solar energy utilization [1-4]. Bismuth vanadate (BiVO4) is regarded as
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one of the most promising semiconductors for its potential applications in solar
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energy conversion and storage [5-7]. BiVO4 mainly exists in three crystalline phases: monoclinic scheelite, tetragonal scheelite, and tetragonal zircon structure [7,8]. Among the three crystalline phases, monoclinic scheelite BiVO4 (ms-BiVO4) with a
band gap of 2.4 eV exhibits the best visible-light-responsive property, while the
visible photocatalytic activity of tetragonal zircon BiVO4 (tz-BiVO4) appears almost negligible for its wide band gap (2.9 eV) [9,10]. Therefore, it is very important to extend the light absorption to visible range to improve the photocatalytic activity of bare BiVO4. So far, there are various modification approaches proposed including
metal/nonmetal doping [11-15], heterojunction structure formation [16,17], and cocatalysts loading [18,19]. Meanwhile, rare earth elements with 4f electron 2
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configuration are deemed to be the better dopants in photocatalysts. Xu and co-workers [20] have reported the preparation of rare earth-loaded BiVO4 by the
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impregnation method. It was found that those rare earth ions had detrimental effect on the photodegradation, except for Gd element. Besides, Eu [21], Y [22] and Er [10,23]
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doped BiVO4 have been synthesized, and the photocatalytic activities of the doped
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photocatalysts were remarkably enhanced.
However, the Gd-doped BiVO4 with tetragonal zircon structure as well as high
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photocatalytic performance under simulated sun-light has been rarely reported. In this work, different contents of Gd-doped BiVO4 were synthesized by a microwave
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hydrothermal method and well-characterized by various techniques. The structure of Gd-doped BiVO4 was analyzed by XRD, Raman spectra and TEM. Moreover, we
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have investigated the formation mechanism of tetragonal Gd-doped BiVO4 in detail by the auxiliary experiments. Subsequently, in order to distinguish the contribution of
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each region of the light spectrum, the photocatalytic activities of the samples were evaluated under simulated sun-light, UV and visible light irradiation, respectively. The effects of the surface defect, band structure, and BET surface area on the tetragonal Gd-doped BiVO4 were investigated systematically. Finally, a possible photodegradation mechanism of tetragonal Gd-doped BiVO4 was proposed.
2. Experimental 2.1. Preparation of photocatalysts All reagents were analytical-reagent grade (Sinopharm Chemical Reagent Co., Ltd) and used without further purification. In a typical process, 0.01 mol 3
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Bi(NO3)3·5H2O was dissolved in 25 mL distilled water and stirred for 20 min at room temperature, while 0.01 mol NH4VO3 was dissolved in 25 mL boiled distilled water
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and stirred with being heated for 20 min. After mixing the two solutions and stirring for a period of time, 5 mL of NaOH solution (5 mol/L) was added to adjust the pH of
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the mixture to about 8. After stirring for 25 min, different contents of Gd(NO3)3·6H2O
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(nGd:nGd+Bi=2, 4, 6, 8, 10 at%) were added into these suspensions with continuously magnetic stirring for 20 min, respectively. Afterwards, each precursor was placed into
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100 mL Teflon-lined stainless autoclave, respectively. The microwave hydrothermal reactions were performed at 180 °C for 40 min. Finally, the prepared precipitates were
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washed with distilled water and absolute ethanol three times, and then dried at 80 °C for several hours. Additionally, the preparation of pure BiVO4 was the same as the
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above-mentioned procedure (with adjusting the pH of the mixture to around 8), except for the addition of Gd(NO3)3·6H2O. To compare tetragonal Gd-doped BiVO4 with
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pure tz-BiVO4 without doping Gd, the pure tz-BiVO4 photocatalyst was solely synthesized by adjusting the pH value of the precursor to 2.55. 2.2. Characterization of photocatalysts The X-ray diffraction (XRD) patterns of photocatalysts were recorded on a
D/max-2200PC diffractometer (Rigaku, Japan), using Cu Kα radiation (λ=0.15418 nm)
with the range of 2θ=15°-70°. The Raman spectra were measured using a Renishaw inVia Raman Microscipe with a laser source of 532 nm wavelength in scanning range from 100 to 1000 cm-1. The components were examined by an energy-dispersive X-ray (EDX) analysis system. The microstructures were investigated by a 4
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transmission electron microscope (TEM, TecnaiG2F20 S-TWIN, FEI). The valence states of ions were tested by an X-ray photoelectron spectroscopy (XPS, XSAM800,
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Kratos Ltd., Britain). The UV-vis diffuse reflectance spectra (DRS) were obtained on an UV-vis spectrophotometer (UV-2550, Shimadzu) in the wavelength range of
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200-800 nm, using BaSO4 as reference. The specific surface areas were measured by a
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Brunauer-Emmett-Teller specific surface area instrument (BET, 3H-2000BET-A, Beishide Instrumentation Technologies Ltd., Beijing) with nitrogen adsorption at 77
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K.
2.3. Evaluation of photocatalytic and photoelectric performance
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The photocatalytic activities of photocatalysts were evaluated by the degradation of Rhodamine B (RhB) aqueous solution under different light irradiations. A 500 W
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xenon lamp was used as the simulated sun-light source. In a typical run, 0.05 g of photocatalyst was added into 50 mL RhB solution (5 mg/L). Before illumination, the
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suspensions were treated by ultrasonic bath for 10 min, and then stirred in dark for 30 min to ensure the adsorption-desorption equilibrium between RhB and photocatalysts.
Afterwards, the suspensions were stirred and exposed to simulated sun-light irradiation. At given time intervals, 5 mL of the suspensions were collected and
centrifuged to remove the photocatalyst particles. The concentrations of RhB
solutions were monitored by checking the absorbance at 554 nm using an UV-vis spectrophotometer (SP-756p). The concentrations of total organic carbon (TOC) were tested on an automated total organic carbon analyzer (liqui TOC П, Germany). In order to distinguish the different contributions of the different ranges of the light 5
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spectrum, UV and visible photocatalytic experiments were also performed by using a 300 W mercury lamp as UV-light source and a 500 W xenon lamp equipped with 420
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nm cut-off filters providing visible light condition, respectively. The photoelectrochemical measurements were performed in a three-electrode
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experimental system, using CHI660E Electrochemical Workstation (Shanghai
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Chenhua Instrument Co., Ltd., Shanghai, China). The Ag/AgCl, Pt electrode, and the prepared thin-film photoelectrode acted as the reference, counter, and working
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electrodes, respectively. The electrolyte was 0.1 molL−1 Na2SO4. The variations of the photocurrent with time (i-t curves) and photocurrent-potential curves were measured
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under simulated sun-light illumination.
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3.1. Structure analysis
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3. Results and discussion
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3.1.1. The structures of prepared samples
Fig. 1 XRD patterns of pure BiVO4 and Gd-doped BiVO4.
The XRD patterns of pure BiVO4 and Gd-doped BiVO4 are shown in Fig. 1. It is 6
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clear that all the diffraction peaks of pure BiVO4 can be well-indexed to pure monoclinic scheelite BiVO4 (PDF 75-1866) [24]. All Gd-doped BiVO4 samples
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exhibit the characteristic diffraction peaks of tetragonal zircon BiVO4 (PDF 14-0133) [8,25], and no any impurities can be detected. The results imply that Gd doping can
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change the phase structure of BiVO4 from monoclinic to tetragonal and induce the stabilization of tetragonal BiVO4. The crystallite sizes of all samples can be estimated
Kλ [5] based on XRD data, where Dhkl is β cos θ
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using the Scherrer formula Dhkl =
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crystallite size; λ is wavelength of the X-ray radiation (0.15418 nm); β is full width at
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half maximum; K is taken as 0.89; and θ is Bragg angle. As a consequence, the crystallite size of pure BiVO4 is estimated as 25.2 nm, and that of Gd-doped BiVO4
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samples are 35.1, 37.6, 31.2, 31.3 and 29.4 nm with increasing Gd content,
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diminution.
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respectively. The crystallite size of Gd-doped BiVO4 presents a tendency of
Fig. 2 Raman spectra of pure BiVO4 and Gd-doped BiVO4.
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Table 1 The lattice parameters, cell volumes, bond lengths of V-O/Bi-O, and bond angles of V-O-Bi for tetragonal Gd-doped BiVO4 obtained from Rietveld refinement. Lattice parameters
Cell volumes
/Å 2at%Gd
Bond lengths
Bond lengths
Bond angles
of V-O /Å
of Bi-O /Å
of V-O-Bi /°
344.6480
1.476
2.57
102.556
344.0214
1.796
2.308
344.7797
1.521
2.553
155.344
344.9415
1.235
2.776
55.569
344.6878
1.418
2.625
100.483
/Å
a=b=7.3039
3
c=6.4605 4at%Gd
a=b=7.2985
6at%Gd
a=b=7.3049 a=b=7.3065 c=6.4614
10at%Gd
a=b=7.3050
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c=6.4593
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c=6.4612 8at%Gd
95.634
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c=6.4583
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Samples
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Raman spectrum is regarded as an effective method of studying the local structure of prepared samples. Fig. 2 shows the Raman spectra of pure BiVO4 and
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Gd-doped BiVO4. For pure BiVO4, there appear the characteristic Raman vibration
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bands at 132.62, 216.03, 329.20, 372.45, 715.34 and 832.62 cm-1, which are in
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agreement with the monoclinic scheelite structure [26,27]. The bands at 132.62 and 216.03 cm-1 are assigned to the external vibrations. However, the Raman spectra of
Gd-doped BiVO4 present the vibration bands at 250.13, 370.85, 764.28, and 856.79 cm-1, which are ascribed to the tetragonal zircon structure BiVO4 [28]. For tetragonal
Gd-BiVO4, the two bands at 856.79 and 764.28 cm-1 are attributed to the symmetric
V-O stretching mode and antisymmetric V-O stretching mode, respectively. The band at 370.85 cm-1 is described as the O-V-O bending mode, and the band at 250.13 cm-1 can be assigned to the Bi-O stretching mode. The Raman bands of Gd-BiVO4 are ascribed to the vibrational modes of V-O bond in the VO43- tetrahedron as known. From Fig. 2, the Raman shifts, widths, and relative intensities of tetragonal Gd-BiVO4 8
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are obviously different from each other, which reflect the different local structure distortions. To further analyze the structural changes of tetragonal Gd-BiVO4, the
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detailed lattice parameters and bond lengths/angles informations are obtained from Rietveld refinement, and the results are summarized in Table 1. It is found that the
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structural information of 4 and 10 at% Gd-BiVO4 appears abnormal, which is mainly
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reflected in the changes of Raman intensity. The lower Raman intensities of 4 and 10 at% Gd-BiVO4 may be related with the expanded bond lengths of V-O. The evolution
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of the V-O bond shows a trend of diminution, but extension in Bi-O bond with Gd
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content, leading to the increase of the cell volumes.
Fig. 3 a(1) and a(2) TEM, a(3) HR-TEM, and a(4) SAED pattern of pure BiVO4; b(1) and b(2) TEM, b(3) HR-TEM, and b(4) SAED pattern of 8 at% Gd-BiVO4; c(1) and c(2) TEM, c(3) HR-TEM, and c(4) SAED pattern of 10 at% Gd-BiVO4.
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Moreover, the microstructures of pure BiVO4 and Gd-doped BiVO4 can be understood by TEM, HR-TEM and selected area electron diffraction (SAED) patterns
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as shown in Fig. 3. From Fig. 3a(1) and 3a(2), the pure BiVO4 is composed of irregular block particles, and these blocks have a solid connection with each other.
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The Fig. 3a(3) and 3a(4) are the HR-TEM and SAED pattern taken from the red
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rectangular area in Fig. 3a(2). The d spacing is 0.3016 nm measured from the HR-TEM image, which is well-matched with the lattice spacing of (112) crystal plane
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of ms-BiVO4. The defined spot pattern in Fig. 3a(4) reveals that the pure BiVO4 is single crystalline [29,30]. With Gd doping, the growth mechanism of the appeared
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rod-like particles can be studied by the HR-TEM. Fig. 3b(3) exhibits the HR-TEM image of the red rectangular area in Fig. 3b(2) for 8 at% Gd-BiVO4, and the d is
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measured as 0.3385 nm consistent with the (200) crystal plane of tz-BiVO4. Since the (200) crystal plane is detected from the side facet of the nanorod, and the average size
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of these nanorods is about 60 nm in width and 700 nm in length, which implies that the growth rate of (200) facets slows down and has been suppressed. The HR-TEM image of 10 at% Gd-BiVO4 (Fig. 3c(3)) taken from the top facet of the nanorod in Fig.
3c(2) shows that the fringe spacing of 0.5102 nm corresponds to the (101) crystal plane of tz-BiVO4. It demonstrates that the growth rate of (101) facets has been extremely promoted. Thus, the grain orientation growth has taken place in the nanorods. The SAED patterns in Fig. 3b(4) and 3c(4) can depict that the Gd-BiVO4 is also single crystalline. The above XRD, Raman, and TEM results have definitely provided the detailed 10
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structural distortion information of Gd-BiVO4 with Gd content. However, a key problem arisen in present is why Gd doping can induce the phase transition. To
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explore the detailed reasons, we have supplemented a series of auxiliary experiments, and the results and discussion are shown below.
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3.1.2. The formation of tetragonal Gd-BiVO4
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Fig. 4 (a) XRD patterns of the products dried at 40 °C in different processes;
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(b) FT-IR spectra of the dried products for Bi(NO3)3·5H2O in HNO3 and
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Bi(NO3)3·5H2O in aqueous solution.
To understand the formation process of tetragonal Gd-BiVO4, we have directly
dried the precursor solutions around room temperature (40 °C). The detailed experiments are as follows:
(1) Drying the Bi(NO3)3·5H2O aqueous solution; (2) Drying the mixture of Bi(NO3)3·5H2O and NH4VO3 aqueous solutions; (3) Drying the mixture of Bi(NO3)3·5H2O and NH4VO3 with adding Gd(NO3)3·6H2O; (4) Drying the mixture of Gd(NO3)3·6H2O and NH4VO3 aqueous solutions. 11
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The phase structures of the dried products are identified via XRD and FT-IR spectra, and the results are shown in Fig. 4. Fig. 4a represents the XRD patterns of the
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products dried at 40 °C. It is seen that the ms-BiVO4 forms after mixing Bi(NO3)3·5H2O and NH4VO3 aqueous solutions as (2) process, but the product is
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almost amorphous. However, with Gd(NO3)3·6H2O introduced (4), the formation of
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tetragonal GdVO4 seeds is easier than that of ms-BiVO4, suggesting the GdVO4 with the higher reaction driving force. Although the ErVO4 seeds have been surmised in
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the report of Er3+-doped BiVO4 [10], herein we have fully provided the adequate evidence to prove it. Therefore, it is induced to the stable existence of tetragonal
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Gd-BiVO4 (3) in this system. The relative chemical reactions can be formulated as follows:
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Bi(NO3 )3 + H 2 O → BiONO3 + 2H + + 2NO3−
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BiONO3 + VO3− → ms − BiVO 4 (amorphous) + NO3 −
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Gd(NO3 )3 + VO3− → GdVO 4 (crystalline) + NO3 −
Phase transition GdVO 4 + ms − BiVO 4 (amorphous) → Gd − BiVO 4 (tz)
To further confirm the formation of slightly soluble BiONO3, the FT-IR spectrum of the dried product of Bi(NO3)3·5H2O aqueous solution (1) is exhibited in Fig. 4b. The
peaks at 571, 726, and 1041 cm-1 correspond to the characteristic FT-IR of BiONO3 products, except for the other peaks of Bi(NO3)3·5H2O. The results demonstrate the formation of BiONO3 in Bi(NO3)3·5H2O aqueous solution, which has already been proposed in previous literatures [31,32]. The formation of tetragonal Gd-BiVO4 has been discussed above. However, 12
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another wonder that whether tetragonal Gd-BiVO4 appears with the special pH value (pH=5) should be considered. Therefore, we have carried out the experiments with
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adjusting pH values of precursor solutions. The XRD patterns of Gd-BiVO4 with different pH values are not shown here (in Fig. S1 of Supporting Information). These
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results reveal that the BiVO4 still maintains tetragonal structure at different pH values,
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which is attributed to the Gd doping, instead of the pH of precursor solutions.
The above discussions indicate that Gd doping in aqueous solution system plays a
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crucial role in the formation of tetragonal Gd-BiVO4. In addition, the tetragonal Gd-BiVO4 with relatively good crystallinity could form at low temperature (40 °C), as
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shown in Fig. 4a. In this case, it is deduced that Gd doping can reduce the formation energy of tz-BiVO4.
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3.2. Property analysis 3.2.1. Photocatalytic results
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The photocatalytic performance of pure BiVO4 and Gd-doped BiVO4 is
investigated by photodegradation RhB under different light irradiations, as shown in Fig. 5. For comparison, the pure tz-BiVO4 and direct photolysis of RhB without photocatalysts were performed under the same conditions. From Fig. 5a, the tetragonal Gd-BiVO4 samples exhibit the higher photocatalytic activities than that of
pure BiVO4 (monoclinic) and pure tz-BiVO4 under simulated sun-light irradiation. The degradation rates of RhB for all Gd-doped BiVO4 can reach more than 95% after 120 min simulated sun-light irradiation, while that of pure BiVO4 is only 37%. The best photocatalytic performance is attained for 10 at% Gd-BiVO4, of which the 13
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degradation rate of RhB can reach up to 96%, indicating that most of RhB molecules
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have been degraded.
Fig. 5 The photodegradation of RhB for all photocatalysts under (a) simulated
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sun-light, (b) UV and (c) visible light irradiation; (d) the corresponding reaction rate
constants of RhB photodegradation under different light irradiations with Gd content.
Under UV-light irradiation (Fig. 5b), the tetragonal Gd-BiVO4 samples also
show the higher photocatalytic activities than that of pure BiVO4 due to the structural feature of tetragonal Gd-BiVO4 mainly responding to UV-light region. Moreover, the
superior photocatalytic performance of tetragonal Gd-BiVO4 with respect to pure tz-BiVO4 suggests the promotion effect of Gd doping in BiVO4. The degradation rate of RhB has reached up to 96% for the best 10 at% Gd-BiVO4 after 30min UV-light irradiation. Fig. 5c indicates that the photocatalytic activity of pure BiVO4 is superior 14
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to that of Gd-BiVO4 under visible light irradiation, which is attributed to the monoclinic structure of pure BiVO4. It is well-known that ms-BiVO4 exhibits the best
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visible photocatalytic activity due to the difference of the energy band structure between ms-BiVO4 and tz-BiVO4 [32]. The narrow band gap (2.41 eV) of pure
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BiVO4 is suitable for visible light excitation (low energy).
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Therefore, the crystal structure of BiVO4 has an important influence on the photocatalytic activity upon irradiation with different spectral ranges. The UV-light
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(approx. 4%) plays a prominent role in the photodegradation processes of tetragonal Gd-BiVO4 due to its high energy. To clearly illustrate the effects of the different
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spectral ranges on the photocatalytic activity of BiVO4, Fig. 5d shows the reaction rate constants of RhB photodegradation under different light irradiations for pure
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BiVO4 and Gd-doped BiVO4. There is no doubt that the contribution of the improved photocatalytic activities of Gd-BiVO4 under simulated sun-light is mainly from
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UV-light region.
Fig. 6 (a) UV-vis absorption spectra changes of RhB (the right inset shows the TOC removal efficiency versus irradiation time); (b) cycling runs in the photodegradation of RhB by tetragonal 10 at% Gd-BiVO4 under simulated sun-light. 15
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Fig. 6a shows the UV-vis absorption spectra changes of RhB during the photodegradation by tetragonal 10 at% Gd-doped BiVO4 under simulated sun-light. It
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is seen that the characteristic absorption peak of RhB at 554 nm declines quickly, accompanied by the blue shifts from 554 to 516 nm of the maximum absorption after
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60 min illumination. After 90 min, the absorption spectrum is almost a straight line.
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As we all know, the RhB photodegradation suffers two competitive mechanisms [33]: a cleavage of the whole conjugated chromophore, causing the reduction of the
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absorption band without a wavelength shift; and a successive de-ethylation from the aromatic rings, producing different de-ethylated intermediates of RhB with blue shifts.
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The spectra changes in Fig. 6a imply the simultaneous occurrence of the de-ethylation and decomposition of conjugated chromophore. The top-left inset in Fig. 6a illustrates
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the temporal evolution of the color of RhB during photodegradation. It is noticed that the color of RhB changes from pink to yellow green and eventually becomes
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colorless.
However, the bleaching of RhB solution is not necessarily the mineralization of
the dye. To accurately evaluate the mineralization, the total organic carbon (TOC) analysis was performed on the supernatant with different irradiation times. The right inset in Fig. 6a presents the TOC removal efficiency in RhB solution by tetragonal 10at% Gd-BiVO4 under simulated sun-light. It is seen that the TOC removal efficiency reaches up to 63.1% after 120 min, which indicates that the RhB molecules can be mineralized by the tetragonal 10 at% Gd-BiVO4. Therefore, the tetragonal Gd-BiVO4 photocatalyst with the ability of efficient mineralization of organic 16
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compounds can be applied in water treatment. The stability of the high photocatalytic performance of tetragonal 10 at%
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Gd-BiVO4 photocatalyst under simulated sun-light irradiation was also confirmed as Fig. 6b. After four cycling runs for RhB photodegradation, the catalyst does not
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exhibit any noticeable loss of activity, suggesting that tetragonal 10 at% Gd-BiVO4
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catalyst has high stability and does not photocorrode during the photodegradation of RhB. These results demonstrate that the tetragonal Gd-BiVO4 photocatalyst is a
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promising material in water purification.
On the basis of the above photocatalytic results, the tetragonal Gd-BiVO4
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photocatalysts can be further characterized by many techniques to explore the crucial factors which affect the photocatalytic activities of Gd-BiVO4.
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3.2.2. The effect of the surface defects
Fig. 7a shows the overall XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4. The
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eight obvious peaks corresponding to Bi5d, Bi4f, C1s, Bi4d5/2, Bi4d3/2, V2p, O1s and Bi4p3/2 can be detected in the both samples. Otherwise, a weak peak of Gd3d appears
at about 1220 eV in the 10 at% Gd-BiVO4. To further identify the chemical state of
Gd in Gd-doped BiVO4, the Gd3d high-resolution XPS spectrum of 10 at% Gd-BiVO4 is shown in Fig. 7b. It is clear that the Gd3d characteristic peak can be detected at 1221.5 eV assigned to Gd3d3/2, indicating the presence of Gd3+ cations in Gd-doped BiVO4. The EDX spectrum in the inset of Fig. 7b shows that the obvious signal of Gd can be detected (except for the O, Bi and V signals), which further suggests the introduction of Gd in Gd-doped BiVO4. 17
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Fig. 7 (a) Overall XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4;
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(b) Gd3d high-resolution XPS spectrum of 10 at% Gd-BiVO4 (the inset shows the
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corresponding EDX spectrum); (c) V2p3/2 and (d) O1s XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4.
The V2p3/2 XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4 are shown in Fig.
7c. The asymmetric V2p3/2 signals can be decomposed into two peaks at Eb=516.08 and 517.12 eV for pure BiVO4, and Eb=516.28 and 516.88 eV for the 10 at% Gd-BiVO4. The two peaks are ascribed to the surface V4+ and V5+, respectively
[14,30]. The molar ratio of V4+/V5+ for 10 at% Gd-BiVO4 (0.33) is much higher than that of pure BiVO4 (0.03). According to electroneutrality principle, the pure BiVO4 and Gd-BiVO4 are oxygen-deficient, and the amount of non-stoichiometric oxygen at the surface of the samples depends on the molar ratio of surface V4+/V5+ [13]. Thus, 18
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the 10 at% Gd-BiVO4 has more oxygen deficiencies on the surface. Fig. 7d exhibits the O1s XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4. The asymmetric O1s
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signals are decomposed into two peaks at Eb=530.24 and 532.78 eV for pure BiVO4, and Eb=529.96 and 532.12 eV for the 10 at% Gd-BiVO4, which correspond to the
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surface lattice oxygen (Olatt) and adsorbed oxygen (Oads) species, respectively [34].
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The molar ratio of Oads/Olatt for 10 at% Gd-BiVO4 (0.76) is higher than that of pure BiVO4 (0.23). Therefore, the 10 at% Gd-BiVO4 possesses the larger concentration of
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surface oxygen vacancies. The more oxygen vacancies as the active sites [14] on the surface can facilitate the separation of photoinduced electron-hole pairs, which is in
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favor of the enhancement of the photocatalytic activity of 10 at% Gd-BiVO4.
Fig. 8 Photocurrent-time (a) and photocurrent-potential curves (b) of pure BiVO4 and 10 at% Gd-BiVO4 thin-film photoelectrodes under simulated sun-light.
The effect of Gd doping on the photocatalytic processes and photogenerated
electron-hole
separation
efficiency
can
be
investigated
through
the
photoelectrochemical test methods. It is well-known that the higher separation efficiency of charge carriers and the higher photocurrent probably give rise to the higher photocatalytic activity [35]. Fig. 8a shows the photocurrent responses of pure 19
Page 19 of 33
BiVO4 and 10 at% Gd-BiVO4 thin-film photoelectrodes under simulated sun-light on and off. The photocurrent of 10 at% Gd-BiVO4 is 1.0 µA, which is about twice than
ip t
that of pure BiVO4 (0.5 µA). The results indicate that 10 at% Gd-BiVO4 possesses the excellent mobility and separation of photogenerated electrons and holes which can be
cr
transferred to the surface of the photocatalyst to participate in the redox reactions with
us
RhB. As a result, the higher electron-hole separation efficiency in Gd-doped BiVO4 is beneficial to the improvement of the photocatalytic activity [36].
an
Furthermore, the linear scan voltammetry can be performed under simulated sun-light to qualitatively evaluate the quantum efficiency of pure BiVO4 and 10 at%
M
Gd-BiVO4 thin-film photoelectrodes at positive and negative bias potential. Fig. 8b exhibits the photocurrent-potential curves of pure BiVO4 and 10 at% Gd-BiVO4 under
te
d
simulated sun-light. As can be seen from Fig. 8b, the cathodic photocurrent is produced under negative bias and the anodic photocurrent occurs under positive bias.
Ac ce p
Since BiVO4 is an n-type semiconductor, the photocurrent is due to the minority holes.
Thus, the pure BiVO4 and 10 at% Gd-BiVO4 hardly present the oxidation reaction currents in dark. However, the pure BiVO4 and 10 at% Gd-BiVO4 can be excited to produce lots of photogenerated electrons and holes under simulated sun-light
illumination, showing the obvious oxidation reaction currents. The photocurrents of pure BiVO4 and 10 at% Gd-BiVO4 photoelectrodes increase with a higher potential supplied, which is due to the higher bias potential can lead to the higher separation efficiency of electron-hole pairs [37]. Moreover, compared with pure BiVO4, the big
change of photocurrent for 10 at% Gd-BiVO4 photoelectrode shows the better 20
Page 20 of 33
photoresponse in this system.
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cr
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3.2.3. The effect of the band structures
an
Fig. 9 UV-vis diffuse reflectance spectra of pure BiVO4 and Gd-doped BiVO4.
M
Fig. 9 shows the UV-vis diffuse reflectance spectra (DRS) of pure BiVO4 and Gd-doped BiVO4. It is noticed that the pure BiVO4 (monoclinic) has strong
te
d
absorption from UV to visible light region with the absorption edge at 515 nm, while the Gd-BiVO4 samples (tetragonal) show the absorption edges (426-440 nm) just in
Ac ce p
the limit between UV and visible light range. The steep spectra of pure BiVO4 and
Gd-doped BiVO4 indicate that the visible light adsorption is due to the band gap
transition, rather than the transition from the impurity level [31]. The band gaps of the samples are calculated from the UV-vis DRS by extrapolating the onset of the rising part to the x-axis (λ/nm) of the plots (as shown in Fig. 9 by the black solid lines) [38]. Thus, the band gap energies (Eg) of all photocatalysts are estimated using the equation Eg=1240/λ [14]. The Eg of pure BiVO4 is estimated to be 2.41 eV, while the Eg of Gd-doped BiVO4 are respectively estimated as 2.82, 2.85, 2.87, 2.90 and 2.91 eV with Gd content increasing. The calculated Eg are similar to the previous reports [32,39]. It 21
Page 21 of 33
is pointed out that the electronic structure of BiVO4 is changed with the phase transition. Compared with pure BiVO4, the wide band gap of Gd-doped BiVO4 with
ip t
the stronger absorption in UV region can decrease the recombination rate of photogenerated electron-hole pairs to enhance the photocatalytic activities of
cr
Gd-doped BiVO4.
M
an
us
3.2.4. The effect of the BET surface area
te
d
Fig. 10 (a) Kinetics of RhB photodegradation (ln(C0/C) versus irradiation time) for all photocatalysts under simulated sun-light; (b) the corresponding values of k/BET in
Ac ce p
different Gd contents (the inset shows the relationship between k and BET).
The Langmuir-Hinshelwood model is selected to quantitatively investigate the
reaction kinetics of RhB photodegradation under simulated sun-light, using the equation ln(C0/C)=kt+a, where k is reaction rate constant, C0 is initial concentration of RhB, and C is concentration of RhB at the reaction time t [16]. From Fig. 10a, it is seen that the relationships between ln(C0/C) and irradiation time are in a good linear, which indicates that the RhB photodegradation follows the pseudo-first-order kinetics [41]. The average reaction rate constant of Gd-doped BiVO4 is 0.02764 min-1, which is much higher than that of pure BiVO4 (0.00322 min-1) and pure tz-BiVO4 (0.00209 22
Page 22 of 33
min-1), suggesting the superior photocatalytic degradation reaction of tetragonal Gd-BiVO4 under simulated sun-light irradiation.
ip t
In general, the BET surface area plays a significant role in the photocatalytic activity of photocatalyst [40,41]. In order to make clear the relationship between k
cr
(corresponding to the degradation rate) and BET in our work, the top-right inset of
us
Fig. 10b exhibits the relationship between k under simulated sun-light and BET in different Gd contents. For Gd-doped BiVO4, there are no big changes in the k values,
an
although the BET is increasing with Gd content. In this case, the influence of the BET surface area on the photocatalytic activities of Gd-BiVO4 is relatively weak. However,
M
the corresponding value of k/BET is regarded as an appropriate means of excluding the effect of BET surface area as shown in Fig. 10b. The higher k/BET values of
te
d
Gd-doped BiVO4 with respect to pure BiVO4 confirm the influences of the crystal structure, oxygen deficiencies and electron-hole separation efficiency as the above
Ac ce p
results discussed.
3.2.5. The photodegradation mechanism of tetragonal Gd-doped BiVO4 Another considerable question is that whether monoclinic Gd-doped BiVO4
catalyst has the similarly outstanding photocatalytic performance under simulated sun-light or not. Thus, series of auxiliary experiments were carried out to prove it. The preparation of monoclinic Gd-doped BiVO4 was performed via adjusting the experimental processes. Fig. 11a displays the XRD pattern of prepared monoclinic 10at% Gd-BiVO4 catalyst, which has a good crystallinity. The RhB photodegradation
by monoclinic 10 at% Gd-BiVO4 under simulated sun-light is shown in Fig. 11b. 23
Page 23 of 33
After 120 min illumination, the degradation efficiency is only 30%, which is lower than that of tetragonal 10 at% Gd-BiVO4 (96%). The result indicates that the
ip t
tetragonal Gd-doped BiVO4 samples prepared in our work possess the excellent photocatalytic performance. Moreover, as can be seen from the inserted TOC removal
cr
efficiency, only 15.3% of TOC has been eliminated, suggesting that the dye
us
mineralization degree of monoclinic 10 at% Gd-BiVO4 is very low. The temporal evolution of the UV-vis absorption spectra implies that the destruction of conjugated
an
chromophore of RhB mainly takes place in the presence of monoclinic 10 at%
Ac ce p
te
d
M
Gd-BiVO4.
Fig. 11 (a) The XRD pattern of prepared monoclinic 10 at% Gd-BiVO4 catalyst; (b) photodegradation of RhB by prepared monoclinic 10 at% Gd-BiVO4 under
simulated sun-light (the insets show the TOC removal efficiency versus irradiation time and UV-vis absorption spectra changes of RhB).
On the above discussion, we have proposed a possible photodegradation mechanism of tetragonal Gd-doped BiVO4 as shown in Fig. 12. When sun-light illumination, the Gd-BiVO4 catalyst is activated and the electrons (e-) in the valence 24
Page 24 of 33
band (V.B.) are stimulated to the conduction band (C.B.), with the same amount of holes (h+) left in V.B.. The electrons will react with O2 adsorbed on the surface of Gd-BiVO4 acting as active sites to produce ·O2- superoxide radicals. In addition,
ip t
according to the XPS analysis, the Gd-BiVO4 catalyst possesses more surface V4+
cr
which can also react with the surface adsorbed O2 to form ·O2- radicals, and the ·O2-
us
superoxide radicals are eventually turned into ·OH hydroxyl radicals. Meanwhile, the photogenerated holes of the V.B. can react with H2O and OH- to generate ·OH
an
hydroxyl radicals [42] as strong oxidizing agents. The ·OH hydroxyl radicals or photogenerated holes as active species [16] can directly attack RhB molecules to
M
decolorize the dye, and the small molecules can be eventually mineralized to form CO2 and H2O. Therefore, the more surface oxygen vacancies in Gd-BiVO4 can
te
d
promote the mobility and separation of photogenerated electrons and holes, and the wide band gap of Gd-doped BiVO4 (2.9 eV) is likely to reduce the recombination rate
Ac ce p
of photogenerated electrons and holes, which jointly lead to the enhancement of the photocatalytic activity.
Fig. 12 The proposed photodegradation mechanism of tetragonal Gd-doped BiVO4. 25
Page 25 of 33
4. Conclusions In summary, the tetragonal Gd-doped BiVO4 with high photocatalytic activities
ip t
have been synthesized by the microwave hydrothermal method. The Gd doping gives rise to the phase transition from monoclinic to tetragonal BiVO4. The tetragonal
cr
GdVO4 seeds with the higher reaction driving force definitively dominate the
us
formation of tetragonal Gd-BiVO4, and the tetragonal Gd-BiVO4 with good crystallinity can exist at low temperature close to room temperature. The improved
an
photocatalytic activities of Gd-BiVO4 are mainly attributed to the more surface oxygen deficiencies and the higher mobility and separation of photogenerated
M
electrons and holes. It is stated that the contribution of high photocatalytic activities under simulated sun-light is mainly from UV-light region, which is determined by the
te
d
tetragonal structure of Gd-BiVO4. The best degradation rate can reach up to 96% after 120 min simulated sun-light irradiation for the tetragonal 10 at% Gd-BiVO4. The
Ac ce p
tetragonal Gd-BiVO4 has the efficient mineralization of organic compounds and does not photocorrode during RhB photodegradation, which provides a promising photocatalytic material applied in water treatment.
Acknowledgements
This work is supported by the Project of the National Natural Science
Foundation of China (Grant No. 51172135); the Graduate Innovation Fund of Shaanxi University of Science & Technology (SUST-A04); the Academic Leaders Funding Scheme of Shaanxi University of Science & Technology (2013XSD06).
26
Page 26 of 33
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Highlights 1. Tetragonal Gd-BiVO4 with enhanced photocatalytic activity was synthesized. 32
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2. Gd doping can induce the phase transition from monoclinic to tetragonal BiVO4. 3. GdVO4 seeds as crystal nucleus dominate the formation of tetragonal Gd-BiVO4.
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4. Tetragonal Gd-BiVO4 exhibits the excellent separation of electrons and holes.
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5. The contribution of high photocatalytic activity under sun-light is from UV-light.
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Graphical Abstract
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Applied Surface Science
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S0169-4332(15)03105-0 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.100 APSUSC 32083
To appear in:
APSUSC
Received date: Revised date: Accepted date:
20-5-2015 30-10-2015 14-12-2015
Please cite this article as: Y. Luo, G. Tan, G. Dong, H. Ren, A. Xia, A comprehensive investigation of tetragonal Gd-doped BiVO4 with enhanced photocatalytic performance under sun-light, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.12.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A comprehensive investigation of tetragonal Gd-doped BiVO4 with enhanced photocatalytic performance under sun-light *
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Yangyang Luo, Guoqiang Tan , Guohua Dong, Huijun Ren, Ao Xia School of Materials Science and Engineering, Shaanxi University of Science & Technology, Xi'an,
cr
Shaanxi, 710021, China.
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Abstract
Tetragonal Gd-doped BiVO4 having enhanced photocatalytic activity have been
an
synthesized by a facile microwave hydrothermal method. The structural analysis
M
indicates that Gd doping can induce the phase transition from monoclinic to tetragonal BiVO4. The reaction results in precursor solutions imply that tetragonal GdVO4 seeds
d
as crystal nucleus are the original and determined incentives to force the formation of
te
tetragonal Gd-BiVO4. The influences of the surface defect, band structure, and BET
Ac ce p
surface area on the improved photocatalytic activities of tetragonal Gd-doped BiVO4 are investigated systematically. The results demonstrate that the more surface oxygen deficiencies as active sites and the excellent mobility and separation of photogenerated electrons and holes are beneficial to the enhancement of the photocatalytic performance of tetragonal Gd-BiVO4. The RhB photodegradation experiments indicate that the contribution of high photocatalytic activities under simulated sun-light is mainly from UV-light region due to the tetragonal structure feature. The best photocatalytic performance is obtained for tetragonal 10 at%
*
Corresponding author. Tel.: +8613759878391. E-mail address: [email protected] (G.Q. Tan). 1
Page 1 of 33
Gd-BiVO4, of which the RhB degradation rate can reach to 96% after 120 min simulated sun-light irradiation. The stable tetragonal Gd-BiVO4 with efficient
ip t
mineralization will be a promising photocatalytic material applied in water purification.
cr
Keywords: Gd-doped BiVO4; Phase transition; Formation mechanism; Photocatalytic
us
activity; Sun-light
an
1. Introduction
Recently, semiconductor photocatalysis technology as an ideal green technology
M
has attracted considerable attention for its potential applications in environmental
d
cleaning and solar energy utilization [1-4]. Bismuth vanadate (BiVO4) is regarded as
te
one of the most promising semiconductors for its potential applications in solar
Ac ce p
energy conversion and storage [5-7]. BiVO4 mainly exists in three crystalline phases: monoclinic scheelite, tetragonal scheelite, and tetragonal zircon structure [7,8]. Among the three crystalline phases, monoclinic scheelite BiVO4 (ms-BiVO4) with a
band gap of 2.4 eV exhibits the best visible-light-responsive property, while the
visible photocatalytic activity of tetragonal zircon BiVO4 (tz-BiVO4) appears almost negligible for its wide band gap (2.9 eV) [9,10]. Therefore, it is very important to extend the light absorption to visible range to improve the photocatalytic activity of bare BiVO4. So far, there are various modification approaches proposed including
metal/nonmetal doping [11-15], heterojunction structure formation [16,17], and cocatalysts loading [18,19]. Meanwhile, rare earth elements with 4f electron 2
Page 2 of 33
configuration are deemed to be the better dopants in photocatalysts. Xu and co-workers [20] have reported the preparation of rare earth-loaded BiVO4 by the
ip t
impregnation method. It was found that those rare earth ions had detrimental effect on the photodegradation, except for Gd element. Besides, Eu [21], Y [22] and Er [10,23]
cr
doped BiVO4 have been synthesized, and the photocatalytic activities of the doped
us
photocatalysts were remarkably enhanced.
However, the Gd-doped BiVO4 with tetragonal zircon structure as well as high
an
photocatalytic performance under simulated sun-light has been rarely reported. In this work, different contents of Gd-doped BiVO4 were synthesized by a microwave
M
hydrothermal method and well-characterized by various techniques. The structure of Gd-doped BiVO4 was analyzed by XRD, Raman spectra and TEM. Moreover, we
te
d
have investigated the formation mechanism of tetragonal Gd-doped BiVO4 in detail by the auxiliary experiments. Subsequently, in order to distinguish the contribution of
Ac ce p
each region of the light spectrum, the photocatalytic activities of the samples were evaluated under simulated sun-light, UV and visible light irradiation, respectively. The effects of the surface defect, band structure, and BET surface area on the tetragonal Gd-doped BiVO4 were investigated systematically. Finally, a possible photodegradation mechanism of tetragonal Gd-doped BiVO4 was proposed.
2. Experimental 2.1. Preparation of photocatalysts All reagents were analytical-reagent grade (Sinopharm Chemical Reagent Co., Ltd) and used without further purification. In a typical process, 0.01 mol 3
Page 3 of 33
Bi(NO3)3·5H2O was dissolved in 25 mL distilled water and stirred for 20 min at room temperature, while 0.01 mol NH4VO3 was dissolved in 25 mL boiled distilled water
ip t
and stirred with being heated for 20 min. After mixing the two solutions and stirring for a period of time, 5 mL of NaOH solution (5 mol/L) was added to adjust the pH of
cr
the mixture to about 8. After stirring for 25 min, different contents of Gd(NO3)3·6H2O
us
(nGd:nGd+Bi=2, 4, 6, 8, 10 at%) were added into these suspensions with continuously magnetic stirring for 20 min, respectively. Afterwards, each precursor was placed into
an
100 mL Teflon-lined stainless autoclave, respectively. The microwave hydrothermal reactions were performed at 180 °C for 40 min. Finally, the prepared precipitates were
M
washed with distilled water and absolute ethanol three times, and then dried at 80 °C for several hours. Additionally, the preparation of pure BiVO4 was the same as the
te
d
above-mentioned procedure (with adjusting the pH of the mixture to around 8), except for the addition of Gd(NO3)3·6H2O. To compare tetragonal Gd-doped BiVO4 with
Ac ce p
pure tz-BiVO4 without doping Gd, the pure tz-BiVO4 photocatalyst was solely synthesized by adjusting the pH value of the precursor to 2.55. 2.2. Characterization of photocatalysts The X-ray diffraction (XRD) patterns of photocatalysts were recorded on a
D/max-2200PC diffractometer (Rigaku, Japan), using Cu Kα radiation (λ=0.15418 nm)
with the range of 2θ=15°-70°. The Raman spectra were measured using a Renishaw inVia Raman Microscipe with a laser source of 532 nm wavelength in scanning range from 100 to 1000 cm-1. The components were examined by an energy-dispersive X-ray (EDX) analysis system. The microstructures were investigated by a 4
Page 4 of 33
transmission electron microscope (TEM, TecnaiG2F20 S-TWIN, FEI). The valence states of ions were tested by an X-ray photoelectron spectroscopy (XPS, XSAM800,
ip t
Kratos Ltd., Britain). The UV-vis diffuse reflectance spectra (DRS) were obtained on an UV-vis spectrophotometer (UV-2550, Shimadzu) in the wavelength range of
cr
200-800 nm, using BaSO4 as reference. The specific surface areas were measured by a
us
Brunauer-Emmett-Teller specific surface area instrument (BET, 3H-2000BET-A, Beishide Instrumentation Technologies Ltd., Beijing) with nitrogen adsorption at 77
an
K.
2.3. Evaluation of photocatalytic and photoelectric performance
M
The photocatalytic activities of photocatalysts were evaluated by the degradation of Rhodamine B (RhB) aqueous solution under different light irradiations. A 500 W
te
d
xenon lamp was used as the simulated sun-light source. In a typical run, 0.05 g of photocatalyst was added into 50 mL RhB solution (5 mg/L). Before illumination, the
Ac ce p
suspensions were treated by ultrasonic bath for 10 min, and then stirred in dark for 30 min to ensure the adsorption-desorption equilibrium between RhB and photocatalysts.
Afterwards, the suspensions were stirred and exposed to simulated sun-light irradiation. At given time intervals, 5 mL of the suspensions were collected and
centrifuged to remove the photocatalyst particles. The concentrations of RhB
solutions were monitored by checking the absorbance at 554 nm using an UV-vis spectrophotometer (SP-756p). The concentrations of total organic carbon (TOC) were tested on an automated total organic carbon analyzer (liqui TOC П, Germany). In order to distinguish the different contributions of the different ranges of the light 5
Page 5 of 33
spectrum, UV and visible photocatalytic experiments were also performed by using a 300 W mercury lamp as UV-light source and a 500 W xenon lamp equipped with 420
ip t
nm cut-off filters providing visible light condition, respectively. The photoelectrochemical measurements were performed in a three-electrode
cr
experimental system, using CHI660E Electrochemical Workstation (Shanghai
us
Chenhua Instrument Co., Ltd., Shanghai, China). The Ag/AgCl, Pt electrode, and the prepared thin-film photoelectrode acted as the reference, counter, and working
an
electrodes, respectively. The electrolyte was 0.1 molL−1 Na2SO4. The variations of the photocurrent with time (i-t curves) and photocurrent-potential curves were measured
M
under simulated sun-light illumination.
te
3.1. Structure analysis
d
3. Results and discussion
Ac ce p
3.1.1. The structures of prepared samples
Fig. 1 XRD patterns of pure BiVO4 and Gd-doped BiVO4.
The XRD patterns of pure BiVO4 and Gd-doped BiVO4 are shown in Fig. 1. It is 6
Page 6 of 33
clear that all the diffraction peaks of pure BiVO4 can be well-indexed to pure monoclinic scheelite BiVO4 (PDF 75-1866) [24]. All Gd-doped BiVO4 samples
ip t
exhibit the characteristic diffraction peaks of tetragonal zircon BiVO4 (PDF 14-0133) [8,25], and no any impurities can be detected. The results imply that Gd doping can
cr
change the phase structure of BiVO4 from monoclinic to tetragonal and induce the stabilization of tetragonal BiVO4. The crystallite sizes of all samples can be estimated
Kλ [5] based on XRD data, where Dhkl is β cos θ
us
using the Scherrer formula Dhkl =
an
crystallite size; λ is wavelength of the X-ray radiation (0.15418 nm); β is full width at
M
half maximum; K is taken as 0.89; and θ is Bragg angle. As a consequence, the crystallite size of pure BiVO4 is estimated as 25.2 nm, and that of Gd-doped BiVO4
d
samples are 35.1, 37.6, 31.2, 31.3 and 29.4 nm with increasing Gd content,
Ac ce p
diminution.
te
respectively. The crystallite size of Gd-doped BiVO4 presents a tendency of
Fig. 2 Raman spectra of pure BiVO4 and Gd-doped BiVO4.
7
Page 7 of 33
Table 1 The lattice parameters, cell volumes, bond lengths of V-O/Bi-O, and bond angles of V-O-Bi for tetragonal Gd-doped BiVO4 obtained from Rietveld refinement. Lattice parameters
Cell volumes
/Å 2at%Gd
Bond lengths
Bond lengths
Bond angles
of V-O /Å
of Bi-O /Å
of V-O-Bi /°
344.6480
1.476
2.57
102.556
344.0214
1.796
2.308
344.7797
1.521
2.553
155.344
344.9415
1.235
2.776
55.569
344.6878
1.418
2.625
100.483
/Å
a=b=7.3039
3
c=6.4605 4at%Gd
a=b=7.2985
6at%Gd
a=b=7.3049 a=b=7.3065 c=6.4614
10at%Gd
a=b=7.3050
an
c=6.4593
us
c=6.4612 8at%Gd
95.634
cr
c=6.4583
ip t
Samples
M
Raman spectrum is regarded as an effective method of studying the local structure of prepared samples. Fig. 2 shows the Raman spectra of pure BiVO4 and
d
Gd-doped BiVO4. For pure BiVO4, there appear the characteristic Raman vibration
te
bands at 132.62, 216.03, 329.20, 372.45, 715.34 and 832.62 cm-1, which are in
Ac ce p
agreement with the monoclinic scheelite structure [26,27]. The bands at 132.62 and 216.03 cm-1 are assigned to the external vibrations. However, the Raman spectra of
Gd-doped BiVO4 present the vibration bands at 250.13, 370.85, 764.28, and 856.79 cm-1, which are ascribed to the tetragonal zircon structure BiVO4 [28]. For tetragonal
Gd-BiVO4, the two bands at 856.79 and 764.28 cm-1 are attributed to the symmetric
V-O stretching mode and antisymmetric V-O stretching mode, respectively. The band at 370.85 cm-1 is described as the O-V-O bending mode, and the band at 250.13 cm-1 can be assigned to the Bi-O stretching mode. The Raman bands of Gd-BiVO4 are ascribed to the vibrational modes of V-O bond in the VO43- tetrahedron as known. From Fig. 2, the Raman shifts, widths, and relative intensities of tetragonal Gd-BiVO4 8
Page 8 of 33
are obviously different from each other, which reflect the different local structure distortions. To further analyze the structural changes of tetragonal Gd-BiVO4, the
ip t
detailed lattice parameters and bond lengths/angles informations are obtained from Rietveld refinement, and the results are summarized in Table 1. It is found that the
cr
structural information of 4 and 10 at% Gd-BiVO4 appears abnormal, which is mainly
us
reflected in the changes of Raman intensity. The lower Raman intensities of 4 and 10 at% Gd-BiVO4 may be related with the expanded bond lengths of V-O. The evolution
an
of the V-O bond shows a trend of diminution, but extension in Bi-O bond with Gd
Ac ce p
te
d
M
content, leading to the increase of the cell volumes.
Fig. 3 a(1) and a(2) TEM, a(3) HR-TEM, and a(4) SAED pattern of pure BiVO4; b(1) and b(2) TEM, b(3) HR-TEM, and b(4) SAED pattern of 8 at% Gd-BiVO4; c(1) and c(2) TEM, c(3) HR-TEM, and c(4) SAED pattern of 10 at% Gd-BiVO4.
9
Page 9 of 33
Moreover, the microstructures of pure BiVO4 and Gd-doped BiVO4 can be understood by TEM, HR-TEM and selected area electron diffraction (SAED) patterns
ip t
as shown in Fig. 3. From Fig. 3a(1) and 3a(2), the pure BiVO4 is composed of irregular block particles, and these blocks have a solid connection with each other.
cr
The Fig. 3a(3) and 3a(4) are the HR-TEM and SAED pattern taken from the red
us
rectangular area in Fig. 3a(2). The d spacing is 0.3016 nm measured from the HR-TEM image, which is well-matched with the lattice spacing of (112) crystal plane
an
of ms-BiVO4. The defined spot pattern in Fig. 3a(4) reveals that the pure BiVO4 is single crystalline [29,30]. With Gd doping, the growth mechanism of the appeared
M
rod-like particles can be studied by the HR-TEM. Fig. 3b(3) exhibits the HR-TEM image of the red rectangular area in Fig. 3b(2) for 8 at% Gd-BiVO4, and the d is
te
d
measured as 0.3385 nm consistent with the (200) crystal plane of tz-BiVO4. Since the (200) crystal plane is detected from the side facet of the nanorod, and the average size
Ac ce p
of these nanorods is about 60 nm in width and 700 nm in length, which implies that the growth rate of (200) facets slows down and has been suppressed. The HR-TEM image of 10 at% Gd-BiVO4 (Fig. 3c(3)) taken from the top facet of the nanorod in Fig.
3c(2) shows that the fringe spacing of 0.5102 nm corresponds to the (101) crystal plane of tz-BiVO4. It demonstrates that the growth rate of (101) facets has been extremely promoted. Thus, the grain orientation growth has taken place in the nanorods. The SAED patterns in Fig. 3b(4) and 3c(4) can depict that the Gd-BiVO4 is also single crystalline. The above XRD, Raman, and TEM results have definitely provided the detailed 10
Page 10 of 33
structural distortion information of Gd-BiVO4 with Gd content. However, a key problem arisen in present is why Gd doping can induce the phase transition. To
ip t
explore the detailed reasons, we have supplemented a series of auxiliary experiments, and the results and discussion are shown below.
M
an
us
cr
3.1.2. The formation of tetragonal Gd-BiVO4
d
Fig. 4 (a) XRD patterns of the products dried at 40 °C in different processes;
te
(b) FT-IR spectra of the dried products for Bi(NO3)3·5H2O in HNO3 and
Ac ce p
Bi(NO3)3·5H2O in aqueous solution.
To understand the formation process of tetragonal Gd-BiVO4, we have directly
dried the precursor solutions around room temperature (40 °C). The detailed experiments are as follows:
(1) Drying the Bi(NO3)3·5H2O aqueous solution; (2) Drying the mixture of Bi(NO3)3·5H2O and NH4VO3 aqueous solutions; (3) Drying the mixture of Bi(NO3)3·5H2O and NH4VO3 with adding Gd(NO3)3·6H2O; (4) Drying the mixture of Gd(NO3)3·6H2O and NH4VO3 aqueous solutions. 11
Page 11 of 33
The phase structures of the dried products are identified via XRD and FT-IR spectra, and the results are shown in Fig. 4. Fig. 4a represents the XRD patterns of the
ip t
products dried at 40 °C. It is seen that the ms-BiVO4 forms after mixing Bi(NO3)3·5H2O and NH4VO3 aqueous solutions as (2) process, but the product is
cr
almost amorphous. However, with Gd(NO3)3·6H2O introduced (4), the formation of
us
tetragonal GdVO4 seeds is easier than that of ms-BiVO4, suggesting the GdVO4 with the higher reaction driving force. Although the ErVO4 seeds have been surmised in
an
the report of Er3+-doped BiVO4 [10], herein we have fully provided the adequate evidence to prove it. Therefore, it is induced to the stable existence of tetragonal
M
Gd-BiVO4 (3) in this system. The relative chemical reactions can be formulated as follows:
d
Bi(NO3 )3 + H 2 O → BiONO3 + 2H + + 2NO3−
te
BiONO3 + VO3− → ms − BiVO 4 (amorphous) + NO3 −
Ac ce p
Gd(NO3 )3 + VO3− → GdVO 4 (crystalline) + NO3 −
Phase transition GdVO 4 + ms − BiVO 4 (amorphous) → Gd − BiVO 4 (tz)
To further confirm the formation of slightly soluble BiONO3, the FT-IR spectrum of the dried product of Bi(NO3)3·5H2O aqueous solution (1) is exhibited in Fig. 4b. The
peaks at 571, 726, and 1041 cm-1 correspond to the characteristic FT-IR of BiONO3 products, except for the other peaks of Bi(NO3)3·5H2O. The results demonstrate the formation of BiONO3 in Bi(NO3)3·5H2O aqueous solution, which has already been proposed in previous literatures [31,32]. The formation of tetragonal Gd-BiVO4 has been discussed above. However, 12
Page 12 of 33
another wonder that whether tetragonal Gd-BiVO4 appears with the special pH value (pH=5) should be considered. Therefore, we have carried out the experiments with
ip t
adjusting pH values of precursor solutions. The XRD patterns of Gd-BiVO4 with different pH values are not shown here (in Fig. S1 of Supporting Information). These
cr
results reveal that the BiVO4 still maintains tetragonal structure at different pH values,
us
which is attributed to the Gd doping, instead of the pH of precursor solutions.
The above discussions indicate that Gd doping in aqueous solution system plays a
an
crucial role in the formation of tetragonal Gd-BiVO4. In addition, the tetragonal Gd-BiVO4 with relatively good crystallinity could form at low temperature (40 °C), as
M
shown in Fig. 4a. In this case, it is deduced that Gd doping can reduce the formation energy of tz-BiVO4.
te
d
3.2. Property analysis 3.2.1. Photocatalytic results
Ac ce p
The photocatalytic performance of pure BiVO4 and Gd-doped BiVO4 is
investigated by photodegradation RhB under different light irradiations, as shown in Fig. 5. For comparison, the pure tz-BiVO4 and direct photolysis of RhB without photocatalysts were performed under the same conditions. From Fig. 5a, the tetragonal Gd-BiVO4 samples exhibit the higher photocatalytic activities than that of
pure BiVO4 (monoclinic) and pure tz-BiVO4 under simulated sun-light irradiation. The degradation rates of RhB for all Gd-doped BiVO4 can reach more than 95% after 120 min simulated sun-light irradiation, while that of pure BiVO4 is only 37%. The best photocatalytic performance is attained for 10 at% Gd-BiVO4, of which the 13
Page 13 of 33
degradation rate of RhB can reach up to 96%, indicating that most of RhB molecules
te
d
M
an
us
cr
ip t
have been degraded.
Fig. 5 The photodegradation of RhB for all photocatalysts under (a) simulated
Ac ce p
sun-light, (b) UV and (c) visible light irradiation; (d) the corresponding reaction rate
constants of RhB photodegradation under different light irradiations with Gd content.
Under UV-light irradiation (Fig. 5b), the tetragonal Gd-BiVO4 samples also
show the higher photocatalytic activities than that of pure BiVO4 due to the structural feature of tetragonal Gd-BiVO4 mainly responding to UV-light region. Moreover, the
superior photocatalytic performance of tetragonal Gd-BiVO4 with respect to pure tz-BiVO4 suggests the promotion effect of Gd doping in BiVO4. The degradation rate of RhB has reached up to 96% for the best 10 at% Gd-BiVO4 after 30min UV-light irradiation. Fig. 5c indicates that the photocatalytic activity of pure BiVO4 is superior 14
Page 14 of 33
to that of Gd-BiVO4 under visible light irradiation, which is attributed to the monoclinic structure of pure BiVO4. It is well-known that ms-BiVO4 exhibits the best
ip t
visible photocatalytic activity due to the difference of the energy band structure between ms-BiVO4 and tz-BiVO4 [32]. The narrow band gap (2.41 eV) of pure
cr
BiVO4 is suitable for visible light excitation (low energy).
us
Therefore, the crystal structure of BiVO4 has an important influence on the photocatalytic activity upon irradiation with different spectral ranges. The UV-light
an
(approx. 4%) plays a prominent role in the photodegradation processes of tetragonal Gd-BiVO4 due to its high energy. To clearly illustrate the effects of the different
M
spectral ranges on the photocatalytic activity of BiVO4, Fig. 5d shows the reaction rate constants of RhB photodegradation under different light irradiations for pure
te
d
BiVO4 and Gd-doped BiVO4. There is no doubt that the contribution of the improved photocatalytic activities of Gd-BiVO4 under simulated sun-light is mainly from
Ac ce p
UV-light region.
Fig. 6 (a) UV-vis absorption spectra changes of RhB (the right inset shows the TOC removal efficiency versus irradiation time); (b) cycling runs in the photodegradation of RhB by tetragonal 10 at% Gd-BiVO4 under simulated sun-light. 15
Page 15 of 33
Fig. 6a shows the UV-vis absorption spectra changes of RhB during the photodegradation by tetragonal 10 at% Gd-doped BiVO4 under simulated sun-light. It
ip t
is seen that the characteristic absorption peak of RhB at 554 nm declines quickly, accompanied by the blue shifts from 554 to 516 nm of the maximum absorption after
cr
60 min illumination. After 90 min, the absorption spectrum is almost a straight line.
us
As we all know, the RhB photodegradation suffers two competitive mechanisms [33]: a cleavage of the whole conjugated chromophore, causing the reduction of the
an
absorption band without a wavelength shift; and a successive de-ethylation from the aromatic rings, producing different de-ethylated intermediates of RhB with blue shifts.
M
The spectra changes in Fig. 6a imply the simultaneous occurrence of the de-ethylation and decomposition of conjugated chromophore. The top-left inset in Fig. 6a illustrates
te
d
the temporal evolution of the color of RhB during photodegradation. It is noticed that the color of RhB changes from pink to yellow green and eventually becomes
Ac ce p
colorless.
However, the bleaching of RhB solution is not necessarily the mineralization of
the dye. To accurately evaluate the mineralization, the total organic carbon (TOC) analysis was performed on the supernatant with different irradiation times. The right inset in Fig. 6a presents the TOC removal efficiency in RhB solution by tetragonal 10at% Gd-BiVO4 under simulated sun-light. It is seen that the TOC removal efficiency reaches up to 63.1% after 120 min, which indicates that the RhB molecules can be mineralized by the tetragonal 10 at% Gd-BiVO4. Therefore, the tetragonal Gd-BiVO4 photocatalyst with the ability of efficient mineralization of organic 16
Page 16 of 33
compounds can be applied in water treatment. The stability of the high photocatalytic performance of tetragonal 10 at%
ip t
Gd-BiVO4 photocatalyst under simulated sun-light irradiation was also confirmed as Fig. 6b. After four cycling runs for RhB photodegradation, the catalyst does not
cr
exhibit any noticeable loss of activity, suggesting that tetragonal 10 at% Gd-BiVO4
us
catalyst has high stability and does not photocorrode during the photodegradation of RhB. These results demonstrate that the tetragonal Gd-BiVO4 photocatalyst is a
an
promising material in water purification.
On the basis of the above photocatalytic results, the tetragonal Gd-BiVO4
M
photocatalysts can be further characterized by many techniques to explore the crucial factors which affect the photocatalytic activities of Gd-BiVO4.
te
d
3.2.2. The effect of the surface defects
Fig. 7a shows the overall XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4. The
Ac ce p
eight obvious peaks corresponding to Bi5d, Bi4f, C1s, Bi4d5/2, Bi4d3/2, V2p, O1s and Bi4p3/2 can be detected in the both samples. Otherwise, a weak peak of Gd3d appears
at about 1220 eV in the 10 at% Gd-BiVO4. To further identify the chemical state of
Gd in Gd-doped BiVO4, the Gd3d high-resolution XPS spectrum of 10 at% Gd-BiVO4 is shown in Fig. 7b. It is clear that the Gd3d characteristic peak can be detected at 1221.5 eV assigned to Gd3d3/2, indicating the presence of Gd3+ cations in Gd-doped BiVO4. The EDX spectrum in the inset of Fig. 7b shows that the obvious signal of Gd can be detected (except for the O, Bi and V signals), which further suggests the introduction of Gd in Gd-doped BiVO4. 17
Page 17 of 33
ip t cr us an M
d
Fig. 7 (a) Overall XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4;
te
(b) Gd3d high-resolution XPS spectrum of 10 at% Gd-BiVO4 (the inset shows the
Ac ce p
corresponding EDX spectrum); (c) V2p3/2 and (d) O1s XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4.
The V2p3/2 XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4 are shown in Fig.
7c. The asymmetric V2p3/2 signals can be decomposed into two peaks at Eb=516.08 and 517.12 eV for pure BiVO4, and Eb=516.28 and 516.88 eV for the 10 at% Gd-BiVO4. The two peaks are ascribed to the surface V4+ and V5+, respectively
[14,30]. The molar ratio of V4+/V5+ for 10 at% Gd-BiVO4 (0.33) is much higher than that of pure BiVO4 (0.03). According to electroneutrality principle, the pure BiVO4 and Gd-BiVO4 are oxygen-deficient, and the amount of non-stoichiometric oxygen at the surface of the samples depends on the molar ratio of surface V4+/V5+ [13]. Thus, 18
Page 18 of 33
the 10 at% Gd-BiVO4 has more oxygen deficiencies on the surface. Fig. 7d exhibits the O1s XPS spectra of pure BiVO4 and 10 at% Gd-BiVO4. The asymmetric O1s
ip t
signals are decomposed into two peaks at Eb=530.24 and 532.78 eV for pure BiVO4, and Eb=529.96 and 532.12 eV for the 10 at% Gd-BiVO4, which correspond to the
cr
surface lattice oxygen (Olatt) and adsorbed oxygen (Oads) species, respectively [34].
us
The molar ratio of Oads/Olatt for 10 at% Gd-BiVO4 (0.76) is higher than that of pure BiVO4 (0.23). Therefore, the 10 at% Gd-BiVO4 possesses the larger concentration of
an
surface oxygen vacancies. The more oxygen vacancies as the active sites [14] on the surface can facilitate the separation of photoinduced electron-hole pairs, which is in
Ac ce p
te
d
M
favor of the enhancement of the photocatalytic activity of 10 at% Gd-BiVO4.
Fig. 8 Photocurrent-time (a) and photocurrent-potential curves (b) of pure BiVO4 and 10 at% Gd-BiVO4 thin-film photoelectrodes under simulated sun-light.
The effect of Gd doping on the photocatalytic processes and photogenerated
electron-hole
separation
efficiency
can
be
investigated
through
the
photoelectrochemical test methods. It is well-known that the higher separation efficiency of charge carriers and the higher photocurrent probably give rise to the higher photocatalytic activity [35]. Fig. 8a shows the photocurrent responses of pure 19
Page 19 of 33
BiVO4 and 10 at% Gd-BiVO4 thin-film photoelectrodes under simulated sun-light on and off. The photocurrent of 10 at% Gd-BiVO4 is 1.0 µA, which is about twice than
ip t
that of pure BiVO4 (0.5 µA). The results indicate that 10 at% Gd-BiVO4 possesses the excellent mobility and separation of photogenerated electrons and holes which can be
cr
transferred to the surface of the photocatalyst to participate in the redox reactions with
us
RhB. As a result, the higher electron-hole separation efficiency in Gd-doped BiVO4 is beneficial to the improvement of the photocatalytic activity [36].
an
Furthermore, the linear scan voltammetry can be performed under simulated sun-light to qualitatively evaluate the quantum efficiency of pure BiVO4 and 10 at%
M
Gd-BiVO4 thin-film photoelectrodes at positive and negative bias potential. Fig. 8b exhibits the photocurrent-potential curves of pure BiVO4 and 10 at% Gd-BiVO4 under
te
d
simulated sun-light. As can be seen from Fig. 8b, the cathodic photocurrent is produced under negative bias and the anodic photocurrent occurs under positive bias.
Ac ce p
Since BiVO4 is an n-type semiconductor, the photocurrent is due to the minority holes.
Thus, the pure BiVO4 and 10 at% Gd-BiVO4 hardly present the oxidation reaction currents in dark. However, the pure BiVO4 and 10 at% Gd-BiVO4 can be excited to produce lots of photogenerated electrons and holes under simulated sun-light
illumination, showing the obvious oxidation reaction currents. The photocurrents of pure BiVO4 and 10 at% Gd-BiVO4 photoelectrodes increase with a higher potential supplied, which is due to the higher bias potential can lead to the higher separation efficiency of electron-hole pairs [37]. Moreover, compared with pure BiVO4, the big
change of photocurrent for 10 at% Gd-BiVO4 photoelectrode shows the better 20
Page 20 of 33
photoresponse in this system.
us
cr
ip t
3.2.3. The effect of the band structures
an
Fig. 9 UV-vis diffuse reflectance spectra of pure BiVO4 and Gd-doped BiVO4.
M
Fig. 9 shows the UV-vis diffuse reflectance spectra (DRS) of pure BiVO4 and Gd-doped BiVO4. It is noticed that the pure BiVO4 (monoclinic) has strong
te
d
absorption from UV to visible light region with the absorption edge at 515 nm, while the Gd-BiVO4 samples (tetragonal) show the absorption edges (426-440 nm) just in
Ac ce p
the limit between UV and visible light range. The steep spectra of pure BiVO4 and
Gd-doped BiVO4 indicate that the visible light adsorption is due to the band gap
transition, rather than the transition from the impurity level [31]. The band gaps of the samples are calculated from the UV-vis DRS by extrapolating the onset of the rising part to the x-axis (λ/nm) of the plots (as shown in Fig. 9 by the black solid lines) [38]. Thus, the band gap energies (Eg) of all photocatalysts are estimated using the equation Eg=1240/λ [14]. The Eg of pure BiVO4 is estimated to be 2.41 eV, while the Eg of Gd-doped BiVO4 are respectively estimated as 2.82, 2.85, 2.87, 2.90 and 2.91 eV with Gd content increasing. The calculated Eg are similar to the previous reports [32,39]. It 21
Page 21 of 33
is pointed out that the electronic structure of BiVO4 is changed with the phase transition. Compared with pure BiVO4, the wide band gap of Gd-doped BiVO4 with
ip t
the stronger absorption in UV region can decrease the recombination rate of photogenerated electron-hole pairs to enhance the photocatalytic activities of
cr
Gd-doped BiVO4.
M
an
us
3.2.4. The effect of the BET surface area
te
d
Fig. 10 (a) Kinetics of RhB photodegradation (ln(C0/C) versus irradiation time) for all photocatalysts under simulated sun-light; (b) the corresponding values of k/BET in
Ac ce p
different Gd contents (the inset shows the relationship between k and BET).
The Langmuir-Hinshelwood model is selected to quantitatively investigate the
reaction kinetics of RhB photodegradation under simulated sun-light, using the equation ln(C0/C)=kt+a, where k is reaction rate constant, C0 is initial concentration of RhB, and C is concentration of RhB at the reaction time t [16]. From Fig. 10a, it is seen that the relationships between ln(C0/C) and irradiation time are in a good linear, which indicates that the RhB photodegradation follows the pseudo-first-order kinetics [41]. The average reaction rate constant of Gd-doped BiVO4 is 0.02764 min-1, which is much higher than that of pure BiVO4 (0.00322 min-1) and pure tz-BiVO4 (0.00209 22
Page 22 of 33
min-1), suggesting the superior photocatalytic degradation reaction of tetragonal Gd-BiVO4 under simulated sun-light irradiation.
ip t
In general, the BET surface area plays a significant role in the photocatalytic activity of photocatalyst [40,41]. In order to make clear the relationship between k
cr
(corresponding to the degradation rate) and BET in our work, the top-right inset of
us
Fig. 10b exhibits the relationship between k under simulated sun-light and BET in different Gd contents. For Gd-doped BiVO4, there are no big changes in the k values,
an
although the BET is increasing with Gd content. In this case, the influence of the BET surface area on the photocatalytic activities of Gd-BiVO4 is relatively weak. However,
M
the corresponding value of k/BET is regarded as an appropriate means of excluding the effect of BET surface area as shown in Fig. 10b. The higher k/BET values of
te
d
Gd-doped BiVO4 with respect to pure BiVO4 confirm the influences of the crystal structure, oxygen deficiencies and electron-hole separation efficiency as the above
Ac ce p
results discussed.
3.2.5. The photodegradation mechanism of tetragonal Gd-doped BiVO4 Another considerable question is that whether monoclinic Gd-doped BiVO4
catalyst has the similarly outstanding photocatalytic performance under simulated sun-light or not. Thus, series of auxiliary experiments were carried out to prove it. The preparation of monoclinic Gd-doped BiVO4 was performed via adjusting the experimental processes. Fig. 11a displays the XRD pattern of prepared monoclinic 10at% Gd-BiVO4 catalyst, which has a good crystallinity. The RhB photodegradation
by monoclinic 10 at% Gd-BiVO4 under simulated sun-light is shown in Fig. 11b. 23
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After 120 min illumination, the degradation efficiency is only 30%, which is lower than that of tetragonal 10 at% Gd-BiVO4 (96%). The result indicates that the
ip t
tetragonal Gd-doped BiVO4 samples prepared in our work possess the excellent photocatalytic performance. Moreover, as can be seen from the inserted TOC removal
cr
efficiency, only 15.3% of TOC has been eliminated, suggesting that the dye
us
mineralization degree of monoclinic 10 at% Gd-BiVO4 is very low. The temporal evolution of the UV-vis absorption spectra implies that the destruction of conjugated
an
chromophore of RhB mainly takes place in the presence of monoclinic 10 at%
Ac ce p
te
d
M
Gd-BiVO4.
Fig. 11 (a) The XRD pattern of prepared monoclinic 10 at% Gd-BiVO4 catalyst; (b) photodegradation of RhB by prepared monoclinic 10 at% Gd-BiVO4 under
simulated sun-light (the insets show the TOC removal efficiency versus irradiation time and UV-vis absorption spectra changes of RhB).
On the above discussion, we have proposed a possible photodegradation mechanism of tetragonal Gd-doped BiVO4 as shown in Fig. 12. When sun-light illumination, the Gd-BiVO4 catalyst is activated and the electrons (e-) in the valence 24
Page 24 of 33
band (V.B.) are stimulated to the conduction band (C.B.), with the same amount of holes (h+) left in V.B.. The electrons will react with O2 adsorbed on the surface of Gd-BiVO4 acting as active sites to produce ·O2- superoxide radicals. In addition,
ip t
according to the XPS analysis, the Gd-BiVO4 catalyst possesses more surface V4+
cr
which can also react with the surface adsorbed O2 to form ·O2- radicals, and the ·O2-
us
superoxide radicals are eventually turned into ·OH hydroxyl radicals. Meanwhile, the photogenerated holes of the V.B. can react with H2O and OH- to generate ·OH
an
hydroxyl radicals [42] as strong oxidizing agents. The ·OH hydroxyl radicals or photogenerated holes as active species [16] can directly attack RhB molecules to
M
decolorize the dye, and the small molecules can be eventually mineralized to form CO2 and H2O. Therefore, the more surface oxygen vacancies in Gd-BiVO4 can
te
d
promote the mobility and separation of photogenerated electrons and holes, and the wide band gap of Gd-doped BiVO4 (2.9 eV) is likely to reduce the recombination rate
Ac ce p
of photogenerated electrons and holes, which jointly lead to the enhancement of the photocatalytic activity.
Fig. 12 The proposed photodegradation mechanism of tetragonal Gd-doped BiVO4. 25
Page 25 of 33
4. Conclusions In summary, the tetragonal Gd-doped BiVO4 with high photocatalytic activities
ip t
have been synthesized by the microwave hydrothermal method. The Gd doping gives rise to the phase transition from monoclinic to tetragonal BiVO4. The tetragonal
cr
GdVO4 seeds with the higher reaction driving force definitively dominate the
us
formation of tetragonal Gd-BiVO4, and the tetragonal Gd-BiVO4 with good crystallinity can exist at low temperature close to room temperature. The improved
an
photocatalytic activities of Gd-BiVO4 are mainly attributed to the more surface oxygen deficiencies and the higher mobility and separation of photogenerated
M
electrons and holes. It is stated that the contribution of high photocatalytic activities under simulated sun-light is mainly from UV-light region, which is determined by the
te
d
tetragonal structure of Gd-BiVO4. The best degradation rate can reach up to 96% after 120 min simulated sun-light irradiation for the tetragonal 10 at% Gd-BiVO4. The
Ac ce p
tetragonal Gd-BiVO4 has the efficient mineralization of organic compounds and does not photocorrode during RhB photodegradation, which provides a promising photocatalytic material applied in water treatment.
Acknowledgements
This work is supported by the Project of the National Natural Science
Foundation of China (Grant No. 51172135); the Graduate Innovation Fund of Shaanxi University of Science & Technology (SUST-A04); the Academic Leaders Funding Scheme of Shaanxi University of Science & Technology (2013XSD06).
26
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Highlights 1. Tetragonal Gd-BiVO4 with enhanced photocatalytic activity was synthesized. 32
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2. Gd doping can induce the phase transition from monoclinic to tetragonal BiVO4. 3. GdVO4 seeds as crystal nucleus dominate the formation of tetragonal Gd-BiVO4.
ip t
4. Tetragonal Gd-BiVO4 exhibits the excellent separation of electrons and holes.
cr
5. The contribution of high photocatalytic activity under sun-light is from UV-light.
us
Graphical Abstract
Ac ce p
te
d
M
an
Applied Surface Science
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