Enhanced visible-light photocatalytic activity of BiVO4 microstructures via annealing process

Accepted Manuscript Enhanced visible-light photocatalytic activity of BiVO4 microstructures via annealing process Yanjie Lu, Huishan Shang, Huijuan Guan, Yafei Zhao, Hongsong Zhang, Bing Zhang PII:

S0749-6036(15)30235-4

DOI:

10.1016/j.spmi.2015.10.016

Reference:

YSPMI 4020

To appear in:

Superlattices and Microstructures

Received Date: 18 September 2015 Revised Date:

9 October 2015

Accepted Date: 12 October 2015

Please cite this article as: Y. Lu, H. Shang, H. Guan, Y. Zhao, H. Zhang, B. Zhang, Enhanced visible-light photocatalytic activity of BiVO4 microstructures via annealing process, Superlattices and Microstructures (2015), doi: 10.1016/j.spmi.2015.10.016. 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.

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Graphical abstract

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Enhanced visible-light photocatalytic activity of BiVO4 microstructures via annealing process Yanjie Lu a, Huishan Shang a, Huijuan Guan a, Yafei Zhao *a, Hongsong Zhang *b and Bing Zhang a

School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China

b

Henan Institute of Engineering, Zhengzhou 451191, P. R. China

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a

Corresponding author: Tel: 86-371-67781724; fax: 86-371-67781724.

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E-mail: [email protected] (Y Zhao), [email protected] (H Zhang)

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ABSTRACT

The peanut-shaped monoclinic scheelite BiVO4 crystals were synthesized via a facile one step hydrothermal method without using any templates, followed by annealing at different temperatures. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron

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microscope (SEM), Brunauer-Emmet-Teller (BET) and UV-vis diffuse reflectance spectra (DRS), showing that the annealing procedure could not only change the surface morphology and crystallinity

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of BiVO4 but also increase the specific surface area without damaging the peanut shape. The band gap values of the samples annealed at different temperatures could be reduced to 2.44~2.46 eV as

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compare with that of 2.47 eV of the original sample. The photocatalytic activities of BiVO4 crystals were evaluated by degradation of methylene blue in aqueous solution under artificial solar-light. Results demonstrated that the sample annealed at 450 °C exhibited the highest activity, and the photocatalytic conversion of methylene blue could reach above 94% after 150 min of irradiation.

Keywords: BiVO4; hydrothermal method; annealing; photodegradation; methylene blue

ACCEPTED MANUSCRIPT 1. Introduction Since Bard reported for the first time that semiconductor photocatalysts could remove cyanide in industrial wastewater [1], semiconductor photocatalysts have been regarded as a promising

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alternative to degrade organic pollutant in wastewater under solar light irradiation. To accomplish photo-electrochemical water purification, the development of applicable semiconductors is of key importance because it must satisfy several strict requirements, such as efficient light absorption,

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chemical stability, environmental friendliness and low cost [2-4]. A number of photocatalysts have

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been prepared and studied in recent years, including TiO2, NiO, ZnS, and Cu2O, etc. [5-8]. However, many of them either have wide band gaps or display photochemical activity only under UV light which accounts for only a small fraction (4%) of the sun’s energy compared to visible light (43%). Therefore, it is necessary to construct visible-light-driven photocatalysts to efficiently utilize the

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natural sunlight in the visible light region. Many efforts have been devoted to design and synthesize visible-light responsive photocatalysts, such as BiVO4, WO3, and CdS, etc [9-11]. Among them, BiVO4 is commonly used due to low cost, non-toxicity and high stability against photocorrosion,

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[12-14].

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high photocatalytic activity in water splitting and organics degradation under visible-light irradiation

According to the previous reports, bismuth vanadate exits in three crystalline phase forms: monoclinic scheelite (s-m), tetragonal scheelite (s-t) and tetragonal zircon (z-t) structures [15]. Tetragonal BiVO4 with a band gap of 3.1 eV mainly possesses a UV absorption, while monoclinic scheelite BiVO4 (2.4 eV) has both a visible light absorption and a UV absorption band [16]. Thus, monoclinic scheelite bismuth vanadate (m-BiVO4) presents strong photocatalytic activity in chemical reaction induced with visible light irradiation. It is well known that the photocatalytic activity of

ACCEPTED MANUSCRIPT m-BiVO4 is strongly associated with its morphology. A number of approaches including sonochemical method [17], solution combustion synthesis (SCS) [18], co-precipitation [19], solid phase method [20], microwave-assisted route [21] and hydrothermal progress [22] have been

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developed to realize the tailored fabrication of m-BiVO4 with different morphologies. Among them, hydrothermal route is one of the most efficient methods for the synthesis of m-BiVO4 with different morphologies [23, 24], such as microspheric [25], decahedral [26], rode-like [27] and star-like.

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Adding a variety of organic templates (surfactants or chelating agents), such as CTAB, sodium

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dodecyl sulfonate (SDS), triblock copolymer-P123 and ethylenediamine tetraacetic acid (EDTA), is commonly used to obtain crystals with specific morphologies and perfect structures. But this process involves adding impurities and increasing the overall cost of production. Recently, some researches have revealed that the structure transformation through annealing-treatment could enhance the

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photocatalytic activity of photocatalysts [28, 29]. For example, the annealing-treatment of TiO2 not only produces the intermedia or transition state between anatase and rutile phases through phase transformation, but also broadens its absorption in the near-UV range, which results in the

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improvement of photocatalytic activity [30-32]. However, there are few studies about the influence

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of the heat-treatment on the crystal structure and photocatalytic activity of m-BiVO4 photocatalysts. In this work, we synthesized a well-defined m-BiVO4 with peanut-like shape by a facile and straightforward hydrothermal route without using any template, followed by annealing procedure at different temperatures. The as-prepared BiVO4 exhibited high photocatalytic activity and endowed this material with a bright perspective in degradation of organic pollutant.

2. Materials and methods

ACCEPTED MANUSCRIPT 2.1 Materials and characterizations

Bi(NO3)3·5H2O, NH4VO3, ethylene glycol and methylene blue were purchased from Tianjin

purification, and all solutions were prepared with distilled water.

2.2 Preparation of BiVO4

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chemical reagent factory. All these chemicals were analytical grade and were used without further

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In a typical hydrothermal synthesis process, 4.851 g (10 mmol) Bi(NO3)·5H2O was dissolved in

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40 ml ethylene glycol and the mixture was kept stirring for 0.5 h to obtain a cleared solution (named as solution A); 1.17 g (10 mmol) NH4VO3 was added into 40 ml deionized water with stirring for 1 h to obtain a milk-like suspension (named as suspension B). The solution A was added dropwise into the suspension B with constant stirring to form a yellow suspension and kept stirring for 30 min.

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Then the final solution was transferred into a 100 ml Teflon-lined autoclave and maintained at 160 °C for 24 h. After being cooled to room temperature, the precipitates were centrifuged and

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washed with deionized water and absolute ethanol alternately for three times and then dried at 50 °C for 12 h. Afterwards, the harvested samples were subjected to further annealing at temperatures of

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400, 450 and 550 °C respectively for 3 h with a constant heating rate of 3 °C min-1. The final BiVO4 products were named as BV-400, BV-450, BV-550 according to the annealing temperature. To make a comparison, the BiVO4 sample without further annealing was also prepared, and was named as BV-0.

2.3 Characterization

The crystal structure of synthetic BiVO4 samples were characterized by X-ray diffraction (XRD)

ACCEPTED MANUSCRIPT on D/max-2000 diffractometer in the range of 10-80°. The morphologies and microstructures of the products were observed by a JEOL JSM-6701F field emission scanning electron microscopy (FE-SEM) operating at 10 KV in high vacuum. The Brunauer-Emmet-Teller (BET) surface area of

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the BiVO4 powders was measured by a specific surface area analyzer (Quantachrome Corporation, USA). UV-vis diffuse reflectance spectra (DRS) were conducted by a UV-vis spectrophotometer

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(UV-3600) and BaSO4 was used as the reflectance standard.

2.4 Photocatalytic activity

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The photocatalytic activities of the as-prepared BiVO4 samples were evaluated by degradation of methylene blue in aqueous solution under artificial solar-light irradiation. The visible light source was provided by a 300 W Xe-illuminator with a 420 nm optical filter. In each test, 0.1 g of BiVO4

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sample was added into 100 ml methylene blue solution (10 mg/l). Before irradiation, the suspension was stirred for 30 min in the dark to reach an adsorption-desorption equilibrium of methylene blue on the surface of the material. In this case, the BiVO4 has already reached adsorption saturation, so

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the physical or chemical adsorption effect is negligible, which can be ignored during the following

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irradiation. Then, the mixture was exposed to solar-light with constant stirring. At given time intervals, about 4 ml of the mixture was extracted and centrifuged to remove the catalysts; the supernatant fluid was analyzed using a UV-vis spectrophotometer to get the concentration of methylene blue. The ration of Ct / C0 was used to evaluate the efficiency of photocatalytic degradation, where C0 is the initial concentration of methylene blue before illumination and Ct is the concentration of methylene blue at illumination time t. 3. Results and discussion

ACCEPTED MANUSCRIPT 3.1 XRD analysis XRD study was carried out to investigate the change of the BiVO4 phase after annealing. Fig. 1 shows the X-ray diffraction (XRD) patterns of the as-prepared BiVO4 annealed at different

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temperatures. All the diffraction peaks of BV-0 could be indexed to monoclinic scheelite BiVO4 with lattice constants of a=5.195Å, b=11.701Å, c=5.092 Å, which agrees well with the standard data for monoclinic BiVO4 (JCPDS No. 14-0688). No other peaks were found, indicating that the sample was

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single-phase and pure. After being annealed at different temperatures (400, 450 and 550 °C), the

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BiVO4 crystals were still monoclinic scheelite. However, the intensity of the diffraction peaks was slightly different, which could be attributed to the discrepancy of the crystalline degree. For BV-450, all diffraction peaks were higher and narrower than the other samples, indicating that BV-450 had a higher crystallinity.

When

the

annealing

temperature

was

increased

up

to

550

°C,

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the crystallinity of BiVO4 was reduced, indicating that excessively high temperature did not help to improve the crystallinity of BiVO4. XRD analysis revealed that the annealing temperature played

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an important role in determining the crystallinity of the as-prepared BiVO4 samples, and the best

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suitable annealing temperature was 450 °C.

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Fig. 1. XRD patterns of as-synthesized BiVO4 annealed at different temperatures

3.2 Morphologies

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FE-SEM was used to observe the morphological structures and the sizes of the BiVO4 samples annealed at different temperatures. As shown in Fig. 2a, BiVO4 microparticle without annealing has the shape of peanut, and the overall length is about 1 µm and diameter is about 0.6-0.7 µm. An

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enlarged SEM image of BV-0 is shown in Fig. 2b, it is clear that many nano-sized particles aggregate

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on the surface of the peanut-like shaped BiVO4. After being annealed at 400 °C and 450 °C, the original morphology of peanut-like shaped BiVO4 can be basically maintained, while the nano-particles on the surface of the peanut-like shaped BiVO4 become larger with a rise of annealing temperature (Fig. 2c~f), which may be caused by the growth of BiVO4 crystal during the annealing process. When the annealing temperature increases up to 550 °C, the symmetry of the peanut-like morphology is slightly damaged (Fig. 2g and h), indicating that excessively high annealing temperature (550 °C) can lead to structural damage of the peanut-like shaped BiVO4.

ACCEPTED MANUSCRIPT Obviously, the annealing treatment has an important effect on the morphological structures and the sizes of the BiVO4 samples, and the peanut-like morphology of BiVO4 can be well kept at the

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annealing temperature of 450 ºC.

ACCEPTED MANUSCRIPT Fig. 2. SEM images of BiVO4 samples annealed at different temperatures: (a and b) without annealing; (c and d) 400 °C; (e and f) 450 °C; (g and h) 550 °C.

3.3 UV-vis diffuse reflectance spectrum The UV-vis diffuse reflectance spectra of BiVO4 samples before and after annealing are shown in

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Fig. 3. All the original and annealed BiVO4 samples show a broad absorbance in the visible light region as well as in the UV region. This provides the peanut-like shaped BiVO4 ability to respond to

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a wide range of solar spectrum including both visible and UV light for photocatalytic degradation of dyes in wastewater. The steep absorption in the visible-light region can be ascribed to the band gap

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transition [33]. The density functional theory (DFT) calculations by Walsh et al. show that the valence band of monoclinic scheelite BiVO4 is composed of hybrid orbitals of Bi 6s and O 2p

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orbitals, whereas the conduction band is composed of V 3d orbitals [22].

Fig. 3. UV-vis diffuse reflectance spectrum of BiVO4 samples annealed at different temperatures

The band gaps (Eg) of m-BiVO4 samples annealed at different temperatures can be obtained according to the formula: (αhν)=C (hν-Eg)n/2, where α, ν, h and C are the absorption coefficient, the

ACCEPTED MANUSCRIPT incident light frequency, Planck's constant and a constant, respectively. The variable n depends on the characteristics of the optical transition of the semiconductor. n equals to 1 for direct transition semiconductor and n equals to 4 for indirect transition semiconductor [34]. Since m-BiVO4 is

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typically a direct transition semiconductor, so the value of n should be 1 [35]. Thus, the band gap energy of the as-synthesized BiVO4 powders can be estimated from a plot of (αhν)2 versus photon energy (hν). The results are shown in Fig.4. The intercept of the tangent to the X-axis is

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approximately equal to the Eg values of the BiVO4 samples. The estimated Eg values of BiVO4

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samples are 2.47 eV (BV-0), 2.46 eV (BV-400), 2.44 eV (BV-450), and 2.46 eV (BV-550), respectively, which are in accordance with those of monoclinic BiVO4 in the previous reports [36, 37]. It shows that the band gap energy of BV-450 sample is lower than that of other samples. Therefore, the BV-450 sample can be excited to produce more electron-hole pairs under the same

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visible-light illumination, which can result in higher photocatalytic activity.

Fig. 4. The estimated band gaps of BiVO4 samples annealed at different temperatures with plots of (αhν)2 versus (hν).

ACCEPTED MANUSCRIPT 3.4 Photocatalytic activity The photocatalytic activities of the as-prepared BiVO4 samples were examined by the degradation of methylene blue in aqueous solution under simulated solar irradiation (λ > 420 nm). As shown in

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Fig. 5, it can be clearly seen that the annealed BiVO4 samples show better photocatalytic performance than the BiVO4 sample without annealing. After 150 min of irradiation, the degradation rate of methylene blue is only 72.6% for the original BiVO4 without annealing; however, it can reach

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up to 91.5% 94.3%, and 89.2% after being annealed at 400, 450, and 550 °C, respectively. The result

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indicates that the annealing process can enhance photocatalytic activity of m-BiVO4 samples, and the sample annealed at 450 °C exhibits the best photocatalytic activity. The possible reasons for that are

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better crystallinity and lower band gap.

Fig. 5. The photocatalytic degradation of methylene blue under simulated solar irradiation over BiVO4 samples annealed at different temperatures.

In order to investigate the photoreaction model of these m-BiVO4 series, the relation between ln(Cd/Ct) (Cd is the concentration of methylene blue after adsorption-desorption equilibrium) and irradiation time (t) is plotted in Fig. 6. All the plots of ln(Cd/C0) νs. t give straight lines, confirming

ACCEPTED MANUSCRIPT that the degradation of methylene blue follows the first-order kinetics, and the corresponding kinetics formula is ln(Cd/C0)=kt, where k is the reaction rate constant. The regression coefficient R2 of BV-0, BV-400, BV-450 and BV-550 are 0.987, 0.995, 0.998 and 0.999, respectively. The excellent fitness

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also indicates that the photoreaction follows the first-order kinetics model. The rate constants k are 0.008 (BV-0), 0.017 (VB-400), 0.018 (BV-450) and 0.015 (BV-550), respectively. The result reveals that BV-450 give a higher rate constant for methylene blue dye degradation and it is 2.3 times higher

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than that of the BV-0 photocatalyst under visible light irradiation.

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Fig. 6. The reaction rate constants of BiVO4 samples annealed at different temperatures.

According to previous studies, the photocatalytic performance of BiVO4 is strongly related to the degree of crystallinity, specific surface area, the band gap (optical absorbance) and crystal structure morphology [38-40]. It is well known that photon-exited electrons and holes have fewer recombination centers for the crystals with higher perfection [41]. Therefore, the highest crystallinity of BV-450 may cause the lowest probability of photon-exited electron-hole recombination. The BET surface areas, band gap energy, adsorption in the dark and photocatalytic conversions of these

ACCEPTED MANUSCRIPT as-prepared BiVO4 samples are listed in Table. 1. From the results, we can see that BV-450 has a lower Eg value than that of the other samples, suggesting that more light energy can be harvested to excite electrons and holes to participate in the photocatalytic reaction. Additionally, compared with

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the surface areas of the other samples, BV-450 has a higher specific surface area of 24.78 m2/g, which provides more active sites to adsorb and degrade the reactants. Therefore, the superior photocatalystic activity of BV-450 may be attributed to the good crystallization, the lowest band gap

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energy, the highest specific surface area and its unique peanut like morphology.

Catalyst

Surface area

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Table. 1. Specific surface areas, band gap energy, adsorption in the dark and photocatalytic conversions of as prepared BiVO4 samples.

Band gap energy

(m2/g)

(eV)

15.62

2.47

BV-400

19.05

2.46

BV-450

24.78

BV-550

23.67

Photocatalytic conversion

(%)

(%)

5.7

72.6

3.4

91.5

2.44

4.2

94.3

2.46

2.0

89.2

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BV-0

Adsorption in the dark

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Fig. 7 shows the UV-vis spectral changes of methylene blue as a function of irradiation time in the

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presence of BV-450. With the increase of irradiation time, an obvious decrease of methylene blue absorption at 664 nm can be observed. It is also clearly seen that the deep blue color of the starting methylene blue solution fades almost completely with exposure time increasing to 150 min. In addition, the main absorption peak of methylene blue weakens from 664 nm to 608 nm during irradiation. The phenomenon of hypsochromic shift is caused by photocatalytic degradation of methylene blue. After 150 min of simulated solar irradiation, the degradation rate of methylene blue solution could reach above 94%, indicating that most of the methylene blue has been degraded. The

ACCEPTED MANUSCRIPT high photocatalytic activity of the m-BiVO4 annealed at 450 °C indicates that it can be widely used

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in the treatment of dye-containing wastewater.

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Fig. 7. UV-vis spectral changes of methylene blue as a function of irradiation time in the presence of BiVO4

times.

4. Conclusions

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annealed at 450 °C. The inset shows the photo images of corresponding dye solution after irradiation for different

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Monoclinic scheelite BiVO4 crystals with peanut-like morphology have been successfully prepared by a simple hydrothermal method without using any templates. The as-prepared BiVO4 samples exhibited photocatalytic activities in the degradation of methylene blue under irradiation of simulated solar-light. BiVO4 samples annealed at different temperatures showed better photocatalytic performance than the samples without annealing. The highest photocatalytic activity of BiVO4 annealed at 450 °C might be attributed to the good crystallization, the lowest band gap energy, the highest specific surface area and its unique peanut shape.

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Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 20871105 and

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21576247 ).

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characterization and photocatalytic properties, Mater. Chem. Phys. 115 (2009) 9-13. [36] X. Wang, H. Liu, J. Wang, L. Chang, N. Song, Z. Yan, X. Wan, Additive-free solvothermal preparation, characterization, and photocatalytic activity of 3D butterfly-like BiVO4, Res. Chem.

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ACCEPTED MANUSCRIPT mater. 172 (2009) 338-344. [40] G. Tan, L. Zhang, H. Ren, S. Wei, J. Huang, A. Xia, Effects of pH on the hierarchical structures and photocatalytic performance of BiVO4 powders prepared via the microwave hydrothermal

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method, ACS Appl. mater. interfaces 5 (2013) 5186-5193. [41] X. Xing, Y. Ma, J. Li, G. Fan, H. Ding, X. Ma, L. Yang, G. Xi, Facile one-pot synthesis and photocatalytic properties of hierarchically structural BiVO4 with different morphologies,

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CrystEngComm 16 (2014) 10218-10226.

Figure captions

Figure 1. XRD patterns of as-synthesized BiVO4 annealed at different temperatures. Figure 2. SEM images of BiVO4 samples annealed at different temperatures: (a and b) without

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annealing; (c and d) 400 °C; (e and f) 450 °C; (g and h) 550 °C.

Figure 3. UV-vis diffuse reflectance spectrum of BiVO4 samples annealed at different temperatures.

of (αhν)2 versus (hν).

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Figure 4. The estimated band gaps of BiVO4 samples annealed at different temperatures with plots

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Figure 5. The photocatalytic degradation of methylene blue under simulated solar irradiation over BiVO4 samples annealed at different temperatures. Figure 6. The reaction rate constants of BiVO4 samples annealed at different temperatures. Figure 7. UV-vis spectral changes of methylene blue as a function of irradiation time in the presence of BiVO4 annealed at 450 °C. The inset shows the photo images of corresponding dye solution after irradiation for different times.

ACCEPTED MANUSCRIPT Highlights
Peanut-like m-BiVO4 was prepared by hydrothermal method without using any template.
The effect of annealing procedure on photocatalytic activity was examined.

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The annealed BiVO4 showed advanced activity for dye sunlight-driven degradation.

had the most excellent catalytic activity.

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The m-BiVO4 annealed at 450