Visible-light photocatalytic degradation of glyphosate over BiVO4 prepared by different co-precipitation methods

Accepted Manuscript Title: Visible-light photocatalytic degradation of glyphosate over BiVO4 prepared by different co-precipitation methods Author: Rui Huo Xue-Ling Yang You-Qin Liu Yue-Hua Xu PII: DOI: Reference:

S0025-5408(16)31117-5 http://dx.doi.org/doi:10.1016/j.materresbull.2016.12.012 MRB 9054

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Received date: Revised date: Accepted date:

23-9-2016 27-11-2016 5-12-2016

Please cite this article as: Rui Huo, Xue-Ling Yang, You-Qin Liu, YueHua Xu, Visible-light photocatalytic degradation of glyphosate over BiVO4 prepared by different co-precipitation methods, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.12.012 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.

Visible-light photocatalytic degradation of glyphosate over BiVO4 prepared by different co-precipitation methods

Rui Huoa, Xue-Ling Yangb, You-Qin Liua, Yue-Hua Xua, *

a

Institute of Biomaterial, College of Materials and Energy, South China Agricultural

University, Guangzhou 510642, China b

Guangzhou CAS Test Technical Services Co., Ltd., Guangzhou 510650

*Corresponding author E-mail address: [email protected]

Intensity(a.u.)

Graphical abstract

11 d 4d Dried at 70 oC BiVO4 JCPDS No.14-0688

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Highlights   

BiVO4(s-m) was formed after placing at room temperature in air for several days. The crystallization of BiVO4 at the room temperature depends on moisture. 1-400BiVO4 shows the highest photoactivity for glyphosate degradation. 1

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The high photogenerated carrier separation was proven by EIS and photocurrent.

Abstract: Visible-light-driven bismuth vanadate (BiVO4) photocatalysts were prepared by the co-precipitation method. The BiVO4 samples were characterized using X-ray diffraction, UV−visible diffuse reflectance, electrochemical impedance spectroscopy (EIS), photocurrent, and as well as electron microscopy (SEM, TEM). The photocatalytic activity of the as-prepared BiVO4 samples was tested through the photocatalytic oxidation of glyphosate under visible light irradiation. The dark yellow amorphous BiVO4 powder was prepared by the HAc method after drying at 70 oC, but the bright yellow monoclinic BiVO4 was obtained after keeping at room temperature in the air for several days. The dependence of the room temperature crystallization on moisture can explain the low crystallization temperature found in this work. The BiVO4 sample calcined at 400

o

C by the HAc method showed the highest

photocatalytic activity for glyphosate degradation under visible light irradiation because of its high charge separation efficiency proven by EIS and photocurrent.

Keywords: BiVO4; Visible-light-driven; Photocatalyst; Co-precipitation synthesis method; Glyphosate degradation

1. Introduction 2. Photocatalysis is a promising technology for water splitting, photocatalytic reduction of CO2, water detoxification and gaseous pollutant removal [1-3]. Titanium dioxide (TiO2) is extensively used as photocatalyst because of its 2

nontoxicity, chemical stability and relatively high reactivity under ultraviolet light (λ) < 387nm, which energy exceeds the anatase band gap of 3.2 eV [4-5].The development of photocatalysts, which can yield high reactivity under visible light irradiation, has become particularly attractive in photocatalysis research due to our society’s increasing energy demand. Therefore, many efficient visible-light-driven photocatalysts (WO3 [6], Zn2V2O7 [7], Ag3PO4 [8], ZnCo2O4 [9], LiNiVO4 [10], etc.) have recently been reported. The bismuth vanadate (BiVO4) is of interest because of its high potential for visible-light-driven photocatalysis. It is well known that BiVO4 has three main modifications, namely, monoclinic scheelite (s-m), tetragonal scheelite (s-t) and tetragonal zircon (z-t), and the photocatalytic properties of BiVO4 are strongly dependent on its crystal structure. A reversible phase transition occurs between BiVO4 (s-m) and BiVO4 (s-t) at about 255 oC, while BiVO4 (z-t) can be transformed into BiVO4 (s-m) after heating at 400-600 oC [11]. Among these three crystal phases, monoclinic BiVO4 shows the highest photocatalytic activity [11-13], and the photocatalytic activity of BiVO4 (z-t) is negligible [13]. Therefore, it is very important to control the crystal form of BiVO4 in order to synthesize an efficient visible-light driven photocatalyst. The chemical methods have recently been reported for BiVO4 preparation including: hydrothermal process [11], molten salt method [12], flame spray synthesis [13], sol–gel method [14], solid-state reaction method [15], microemulsion synthesis [16], co-precipitation process [17-23], etc. Ke et al [11] reported that a crystalline mixture consisting of tetragonal and monoclinic phases was prepared through a hydrothermal process, and the pure phase BiVO4 (s-m) was fabricated at a higher hydrothermal temperature (200 oC). Cruz et al 3

reported that the BiVO4 (s-m) was formed even at 70 oC by a co-precipitation method after the slow evaporation of the solvent [17]. Yu et al reported that an amorphous BiVO4 was first synthesized by ammonia co-precipitation method. When the synthesis was followed by heating treatment at about 250 oC, the nanocrystalline BiVO4 was formed, and the pure monoclinic BiVO4 phase was obtained after the heat treatment below 500 oC [18]. Huang et al reported that the pure phase BiVO4 (s-m) photocatalysts were synthesized using a homogeneous co-precipitation process at 90 °C for various durations followed by calcination at 500 °C, and the sample prepared at 90 °C for 24 h showed the best photocatalytic activity [19]. Wan et al [22] reported that BiVO4 (s-t), BiVO4 (z-t), and the mixtures of BiVO4 (s-m) and BiVO4 (s-t) can be successfully synthesized after drying at 25, 80, or 110°C, respectively. Xu et al [23] reported that the BiVO4 (z-t) microspheres were synthesized after drying at 60 °C for 5 h in air. These results show that the BiVO4 samples prepared by co-precipitation method at different conditions were amorphous or contain different crystalline phases before calcination. Bi(NO3)3·5H2O was usually dissolved in HNO3 [11, 16-23] or distilled water [12 ]. In this study, we report a facile co-precipitation synthesis of BiVO4 using Bi(NO3)35H2O and NH4VO3 as precursors, while bismuth nitrate pentahydrate was dissolved in glacial acetic acid (HAc) as compared to nitric acid (HNO3). The synthesis and physiochemical characterization of nanometer-size BiVO4 were undertaken, and it was found that the crystal structures of BiVO4 products can be different. Interestingly, amorphous BiVO4 was prepared after drying at 70 oC by the HAc method, but monoclinic BiVO4 was obtained after placing at room temperature in air for several days. The as-prepared BiVO4 samples show high photocatalytic activity under visible light irradiation toward glyphosate degradation in aqueous 4

solutions. Also, the mechanism of the high photocatalytic activity was proposed on the basis of the photocurrent and electrochemical impedance spectra.

2. Experimental 2.1. Preparation of BiVO4 photocatalysts 2.1.1. Materials Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, ≥99.0%) was obtained from Tianjin Kermel Chemical Reagent Co., China. Ammonium metavanadate (NH4VO3, 99.0%) was obtained from Hunan Nanhua Chemical Reagent Co., China. Acetic acid (CH3COOH, ≥99.5%), nitric acid (HNO3, 65%-68%), sodium hydroxide (NaOH, ≥96.0%), ethanol absolute (C2H5OH, ≥99.7%) and sodium nitrite (NaNO2, ≥99.0%) were obtained from Guangdong Guanghua Chemical Co., Ltd. Glyphosate (99.5%) was obtained by Aladdin Chemistry Co., Ltd. All chemicals used were analytical grade reagents without further purification. 2.1.2. HAc method 8.9850 g of Bi(NO3)3·5H2O was dissolved in 150 mL of 1.34 mol L−1 glacial acetic acid, and 2.1660 g of NH4VO3 was dissolved in 150 mL of 0.5 mol L−1 NaOH at room temperature. Then, these two solutions were mixed quickly under ultrasound to yield a stable and yellow mixture, which pH is 3.6. This suspension was filtered and washed with deionized water and pure ethanol. Then, the resulting yellow product was dried at 70°C. Finally, BiVO4 samples were calcined at different temperatures (300, 400, and 500°C) in air for 2 h at a heating rate of 5 oC/min. They are named 1-300BiVO4, 1-400BiVO4, and 1-500BiVO4, respectively. 2.1.3. HNO3 method 8.9850 g of Bi(NO3)3·5H2O was dissolved in 19 mL of 4.8 mol L−1 nitric acid to 5

yield solution A, and 2.1660 g of NH4VO3 was dissolved in 19 mL of 6 mol L−1 NaOH to produce solution B. Then, the solution B was added dropwise to the solution A under ultrasound to yield a stable and yellow mixture, and the resulting suspension was adjusted to pH=6 using NaOH solution. At the next step, the yellow precipitate was filtered and washed with distilled water and pure ethanol, and dried at 70°C. Lastly, BiVO4 samples were calcined at different temperatures (300, 400, and 500°C) in air for 2 h at a heating rate of 5 oC/min. They are named 2-300BiVO4, 2-400BiVO4, and 2-500BiVO4, respectively. 2.2. Photocatalyst characterization The X–ray diffraction (XRD) data of all resultant BiVO4 samples were collected on a MSAL XD–2 diffractometer using CuK α1 (λ= 1.5406 Å) radiation. The scanning range was from 10o to 80o at a scan rate of 0.02o (2θ) s-1. The microstructure and morphology of the as-prepared BiVO4 particles were studied using a FEI–Tecnai 12 transmission electron microscopy (TEM) operated at 200 keV, where the samples were dispersed in ethanol solution by sonication, and then mounted on carbon-coated holey Cu grids, and a HITACHI S3400–N scanning electron microscope (SEM) at an accelerating voltage of 5 keV, in which the samples were coated with a 15 nm layer of gold before observation. The UV−vis diffuse reflectance spectra (DRS) were measured for all samples on a HITACHI U–2550 UV–vis spectrophotometer equipped with an integrating sphere over a wavelength region of 200−800 nm, and BaSO4 was used as a reflectance standard. Electrochemical measurements were used to provide information about the charge transfer characteristics of the BiVO4 samples. The experiments were carried out with electrochemical impedance spectroscopy (EIS) and photocurrent measurements under visible light irradiation, where a Xe lamp (300 W) with a UV cut-off filter (λ≥420nm) was used as a light source. The photocurrent 6

and EIS were measured on an electrochemical workstation (BAS100 Instruments) in a standard three-electrode system using the as-prepared BiVO4 electrodes as the working electrodes, a Pt plate as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode. 0.1 M Na2SO4 aqueous solution was used as the electrolyte. The working electrodes were prepared as follows: 5 mg of photocatalyst powder was added into 2 mL of ethanol to make a slurry mixture, which was coated onto a 20 mm×35 mm fluorine-doped tin-oxide (FTO) glass substrate. The electrochemical impedance spectra (EIS) were recorded via a computer controlled IM6e impedance measurement unit (Zahner Elektrik, Germany) over a frequency range of 0.01–105 Hz at the ac amplitude of 2 mV in the visible light. 2.3. Photocatalytic reaction test The photocatalytic activity of the as-prepared BiVO4 photocatalysts was studied using glyphosate degradation. For this test, 0.5 g of photocatalyst powder was placed in 600 ml of glyphosate (10–4 mol/L) to form aqueous slurry. Before irradiation, the aqueous slurry was magnetically stirred and bubbled with oxygen in darkness for 30 min to reach the adsorption–desorption equilibrium between the photocatalyst and glyphosate. The flow of oxygen during the stirring without irradiation and during the irradiation of photocatalyst suspension was 2.0 L·min-1. Photocatalytic reactions were carried out by visible light irradiation (λ > 400 nm) with a 125 W high-pressure mercury lamp, which the UV light portion was filtered by a 2.0 mol/L NaNO2 solution. As the photocatalytic reaction proceeded, the suspension was sampled at different time intervals and centrifuged to separate the photocatalyst particles. Finally, the absorbance was recorded to determine the final oxidation product PO43– concentration at characteristic band 710 nm using the molybdenum blue method using a spectrophotometer [24, 25]. 7

The glyphosate degradation rate η was calculated using the following equation 1:  

Ct

 100%

(1)

C0

where C0 is the initial glyphosate concentration, and C t is the PO43− concentration at time t.

3. Results and discussion 3.1. Structure The XRD patterns of the BiVO4 samples prepared by the HAc and HNO3 methods are shown in Fig. 1. For the HAc method, the dark yellow amorphous BiVO4 powder was first prepared after drying at 70 oC (see Supporting Information, Fig. S1). However, after keeping at room temperature for several days in the air, sharp BiVO4 diffraction patterns was observed due to the transition from non-crystalline to crystalline form, and the color of the sample changed to bright yellow (see Supporting Information, Fig. S2). This formation of monoclinic BiVO4 at room temperature may be ascribed to the strong interaction of the BiO+ cation with VO3 anion via electrostatic force [26]. This is a probable reason for the crystallization. However, if the dark yellow amorphous BiVO4 powder was put in a dryer, it was always amorphous. The fact that the crystallization at the room temperature depends on moisture can explain the low crystallization temperature found in this work. The crystallization of amorphous BiVO4 was determined by the existence of water penetration into the primary cores and the effect of this on the electrostatic interactions, which the crystallization process may be attributed to the VO3 anions diffusion to BiO+ ions that resulted in oriented crystallization of amorphous BiVO4 into the monoclinic scheelite structure [27]. For the BiVO4 samples prepared by the HAc method, the diffraction peaks match well to the monoclinic scheelite BiVO4 (Fig. 8

1a), according to the JCPDS file no. 14-0688, while the diffraction peaks at two-theta angle of about 24o and 32o are ascribed to the characteristic peaks of tetragonal scheelite BiVO4 (JCPDS 83-1812), suggesting the existence of little tetragonal scheelite BiVO4. After calcination at 500 oC, only monoclinic scheelite BiVO4 was present. For the BiVO4 samples prepared by the HNO3 method, the XRD pattern analysis (Fig. 1b) shows that the monoclinic (JCPDS No. 14-0688) and tetragonal (JCPDS No. 83-1812) scheelite BiVO4 were obtained after calcination at 300 oC. However, the phase transition from the tetragonal to monoclinic form of BiVO4 occurred below 400 o

C, and the pure monoclinic scheelite BiVO4 was present after calcination at 400 oC.

Thus, monoclinic scheelite BiVO4 is the thermodynamically stable phase at T>400 oC. The results show that the type of acid, used to dissolve bismuth nitrate pentahydrate, can influence the phase composition of the BiVO4 powder products. The products usually vary with the reaction conditions (e.g., concentrations of reactants, temperature, pH, and solvents). 3.2. Morphology The SEM and TEM images of the BiVO4 nanoparticles prepared by different methods are shown in Fig. 2 and Fig. S3 of the Supporting Information, respectively. As it can be seen, the preparation method influences the morphology of the BiVO4 samples, and the 1-BiVO4 nanoparticles are of sphere-like shape with a smooth surface. Figure 2 d and f indicate that the 2-BiVO4 particles are more aggregated, but their shape is comparable to 1-BiVO4. In addition, the particle size in Fig. 2d is obviously larger than in Fig. 2a. This observation may be attributed to the different crystallization pathways under thermodynamic and kinetic control, which appeared for the HNO3 and HAc methods, respectively. It indicates that the one-step route to 9

the crystal phase under thermodynamic control results in the large particles. For both preparation methods, the similar particle size is observed for the uncalcined BiVO4 nanoparticles and 400BiVO4 nanoparticles (Fig. 2a, b; d, e), suggesting that their overall particle sizes and morphologies could be conserved after calcination below 400°C. However, raising the calcination temperature to 500 oC causes an evident increase in the particle size, and we can see the growth in the size of the 500BiVO4 nanoparticles in Fig. 2c and Fig. 2f, suggesting that sintering of crystallized particles occurred as the calcination temperature up to 500 oC. The nanoparticles deposited on the copper grid exhibit a wide dispersion in size. It is noted that the particle diameters of the 1-400BiVO4 particles and 1-500BiVO4 particles are in the range of 45-115 (Fig. S3a) and 125-235 nm (Fig. S3b), respectively, and the particle size of the 2-400BiVO4 particles and 2-500BiVO4 particles are in the range of 30-100 (Fig. S3c) and 300-500 nm (Fig. S3d), respectively. This suggests that the particles of the monoclinic BiVO4 were sintered and grown as the calcination temperature up to 500 oC. 3.3. Optical properties The optical properties of the BiVO4 samples were investigated by UV−vis diffuse reflectance measurements (Fig. 3). The UV−vis diffuse reflectance spectrum is useful to understand light absorption of a sample, resulting in estimating the band gap. The band gap values of the BiVO4 samples were calculated based on the plots of (αhv)2 versus photon energy (hv) [28-30], where α is the absorption coefficient, h is Planck's constant, and ν is the frequency of the light. The intercept of the tangent to the plot gives an approximation of the band gap energy for direct band gap materials such as BiVO4. For the HAc method, the corresponding band gap energy values are calculated to be 2.39, 2.32, 2.41 and 2.39 eV for the uncalcined BiVO4, 1-300BiVO4, 1-400BiVO4 and 1-500BiVO4, respectively (see Supporting Information, Fig. S4a-d). 10

For the HNO3 method, the corresponding band gap energy values are estimated to be 2.31, 2.27, 2.26 and 2.31 eV for the uncalcined BiVO4, 2-300BiVO4, 2-400BiVO4 and 2-500BiVO4, respectively (see Supporting Information, Fig. S4e-h). The band-gaps of BiVO4 samples prepared by the HNO3 method are slightly smaller than those of BiVO4 samples prepared by the HAc method. These band gap energy values are consistent with previous reports [21, 29, 30]. 3.4. Photoelectrochemical properties of 1-400BiVO4 and 2-400BiVO4 Generally, the generation and separation of the photogenerated carriers in the photocatalytic process can be elucidated indirectly by the photocurrent generation, which reveals the interfacial charge transfer. The higher photocurrent suggests the higher electron−hole pair separation efficiency. Fig. 4a shows the photocurrent−time curves for 1-400BiVO4 and 2-400BiVO4 during repeating ON-OFF illumination cycles. Two samples exhibit prompt and reproducible photocurrent. The photocurrent generation of the 1-400BiVO4 electrode surpasses that of 2-400BiVO4. It suggests that 1-400BiVO4 has a lower electron−hole recombination that increases the photocurrent and, subsequently, shows high photocatalytic activity under visible light irradiation, which is in agreement with those observed in other works [31, 32]. The electrochemical impedance spectrum (EIS) can be ascribed to the charge transfer resistance at the contact interface between electrode and electrolyte solution, and EIS is generally used to investigate the charge transfer process. The EIS spectra consist of semicircles and the interface layer resistance occurred on the surface of electrode can be indicated by the radius of the semicircle in the EIS spectra. The smaller semicircle radius relates to higher charge transfer efficiency [31-33]. Fig. 4b shows EIS Nyquist plots of 1-400BiVO4 and 2-400BiVO4. It is observed that the semicircle of 1-400BiVO4 is smaller than that of 2-400BiVO4, indicating that the 11

charge transfer resistance of 1-400BiVO4 is smaller. As compared to 2-400BiVO4, 1-400BiVO4 has faster interfacial electron transfer and smaller recombination rate, which is in accordance with its higher photocatalytic activity. 3.5. Photocatalytic performance of BiVO4 samples The photocatalytic activity of the BiVO4 samples was tested for glyphosate degradation under irradiation by visible light (>400 nm) and the results are shown in Fig. 5. For both preparation methods, it is showed that the photocatalytic activities of the BiVO4 samples increase with increasing calcination temperature, and the photocatalytic activities of all calcined BiVO4 samples are much higher than that of the uncalcined BiVO4. The photocatalytic activity of the BiVO4 sample synthesized using HAc and kept in air for several days is lower than those 1-BiVO4 samples calcined at different temperatures. However, it showed higher photocatalytic activity than the 1-BiVO4 sample dried at 70°C. Under visible light irradiation, the glyphosate degradation rates over BiVO4 prepared by the HAc method follow the order: the BiVO4 dried at 70°C < uncalcined BiVO4< 1-300BiVO4 < 1-500BiVO4 < 1-400BiVO4. However, the activity order of the BiVO4 samples prepared by the HNO3 method was other: the uncalcined BiVO4< 2-500BiVO4 < 2-300BiVO4 < 2-400BiVO4. Among all BiVO4 samples, the 1-400BiVO4 sample shows the largest glyphosate degradation rate under visible light irradiation because of its high charge separation efficiency, which is proven by EIS and photocurrent. Under the same calcination temperature, the photocatalytic activities of the 1-BiVO4 samples are higher than those of the 2-BiVO4 samples, because the 1-BiVO4 samples are pure monoclinic phase, and the 2-BiVO4 samples dried at 70 oC and calcined below 400°C are mainly composed of monoclinic BiVO4 coexisting with limited tetragonal BiVO4. The 2-400BiVO4 sample exhibits the best glyphosate 12

degradation efficiency, which can be attributed to its pure monoclinic phase, high crystallinity and small particle size. This is in agreement with previous reports that monoclinic BiVO4 exhibits higher photocatalytic activity than other modifications of BiVO4 under visible light irradiation [11, 34]. The co-precipitation synthesis is known to result in the photocatalyst particles with lower crystallinity and, thus, less favorable pathway form for the photogenerated carrier migration to surfaces. Furthermore, the high surface defect density may inhibit high photocatalytic rate. Fairly, heat treatment is required to enhance the crystallinity of the BiVO4 particles, which results in the high photocatalytic activity. These results show that controlling the crystallinity of the BiVO4 particle is a key to obtain the high glyphosate photocatalytic degradation rate. The high photocatalytic activity of our 400BiVO4 samples relative to the glyphosate degradation may be due in part to the increased accessible surface produced by the small particle sizes, and the decrease in glyphosate degradation rate with increasing the particle sizes of 500BiVO4 samples is evident. This suggests that the glyphosate degradation rate over the BiVO4 samples depends on the particle size, crystal structure, and particle crystallinity, although they possess similar light absorptions and morphologies.

4. Conclusions The BiVO4 nanoparticles were prepared by the co-precipitation methods, and Bi(NO3)3·5H2O was dissolved in glacial acetic acid or nitric acid, respectively. The dark yellow amorphous BiVO4 powder was prepared after drying at 70 oC through the HAc method, while bright yellow monoclinic BiVO4 formed after placing at room temperature in air for several days. However, the BiVO4 sample synthesized by the HNO3 method was mainly composed of monoclinic BiVO4 coexisting with limited 13

tetragonal BiVO4 after calcination at 300 oC. The1-400BiVO4 sample shows the highest photocatalytic degradation rate of glyphosate under visible light irradiation. We propose that its high photocatalytic activity can be ascribed to the high photogenerated electron−hole pair separation efficiency. It is found that the photocatalytic activity of the BiVO4 samples depends on a balance of high crystallinity, pure monoclinic phase and small particle size, although all BiVO4 samples possess similar light absorptions.

Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21475047).

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nanoparticles in reverse microemulsions, Adv. Functional Mater. 12 (2002) 773–779. [28] H.M. Luo, A.H. Mueller, T. M. McCleskey, A.K. Burrell, E. Bauer, Q. X. Jia, Structural and photoelectrochemical properties of BiVO4 thin films, J. Phys. Chem. C, 112 (2008) 6099–6102. [29] S. Chala, K. Wetchakun, S. Phanichphant, B. Inceesungvorn, N. Wetchakun, Enhanced visible-light-response photocatalytic degradation of methylene blue on Fe-loaded BiVO4 photocatalyst, J. Alloy. Compd. 597 (2014) 129–135. [30] S.P. Berglund, D.W. Flaherty, N.T. Hahn, A.J. Bard, C. B. Mullins, Photoelectrochemical oxidation of water using nanostructured BiVO4 films, J. Phys. Chem. C, 115 (2011) 3794–3802. [31] H.Y. Li, Y.J. Sun, B. Cai, S.Y. Gan, D.X. Han, L. Niu, T.S. Wu, Hierarchically Z-scheme photocatalyst of Ag@AgCl decorated on BiVO4: (040) with enhancing photoelectrochemical and photocatalytic performance, Appl. Catal. B: Environ. 170-171 (2015) 206–214. 18

[32] H.W. Huang, K. Liu, K. Chen, Y.L. Zhang, Y.H. Zhang, S.C. Wang, Ce and F comodification on the crystal structure and enhanced photocatalytic activity of Bi2WO6 photocatalyst under visible light irradiation, J. Phys. Chem. C 118 (2014) 14379−14387. [33] H.M. Yuan, J.L. Liu, J. Li, Y.P. Li, X.P. Wang, Y.Q. Zhang, J.B. Jiang, S.Y. Chen, C. Zhao, D. Qian, Designed synthesis of a novel BiVO4–Cu2O–TiO2 as an efficient visible-light-responding photocatalyst, J. Colloid. Inter. Sci. 444 (2015) 58–66. [34] Y.Y. Liu, B.B. Huang, Y. Dai, X.Y. Zhang, X.Y. Qin, M.H. Jiang, M.-H. Whangbo, Selective ethanol formation from photocatalytic reduction of carbon dioxide in water with BiVO4 photocatalyst, Catal. Commun. 11 (2009) 210–213.

19

HAc method

HNO3 method

(b) Intensity(a.u.)

Intensity(a.u.)

(a)

500oC 400oC 300oC

500oC 400oC 300oC

BiVO4 JCPDS No.14-0688

BiVO4 JCPDS No.14-0688

BiVO4 JCPDS No.83-1812

0

10

20

30

40

50

60

70

80

BiVO4 JCPDS No.83-1812 0

90

10

20

30

40

50

60

70

o

o

2 ( )

2 ( )

Fig. 1. XRD patterns of the BiVO4 samples prepared by different methods.

20

80

90

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 2. SEM images of BiVO4 samples prepared by the HAc method (a-uncalcined, b-400°C and c-500°C) and HNO3 method (d-uncalcined, e-400°C and f-500°C).

21

1.6 1.4

(a) HAc method

1.6

1.2

HNO3 method

1.2

1.0

Absorbance

Absorbance

(b)

1.4

uncalcined

0.8

O

300 C

0.6

O

400 C

0.4

O

1.0 0.8

uncalcined 300OC O 400 C O 500 C

0.6 0.4

500 C

0.2

0.2

0.0

0.0 200

300

400

500

600

700

800

200

300

400

500

600

700

800

Wavelength (nm)

Wavelength (nm)

Fig. 3. UV–vis diffuse reflectance spectra of BiVO4 samples prepared by different methods.

22

1-400BiVO4

(a)

1.4

2-400BiVO4

60000

(b)

1-400BiVO4 2-400BiVO4

1.2 1.0

-Z''(ohm)

-2

Photocurrent Dencity (A cm )

1.6

0.8 0.6

40000

20000

0.4 0.2

0

0.0 0

20

40

60

80

100

120

140

0

Time(s)

50000

100000

Z'(ohm)

Fig. 4. Photocurrent responses (a) and EIS results (b) of 1-400BiVO4 and 2-400BiVO4 under visible light irradiation.

23

60

HAc method

o

300 C

Percentage of degradation(%)

Percentage of degradation(%)

Uncalcined

(a)

50

o

400 C o

40

500 C P25

30

o

Dried at 70 C

20 10 0

0

30

60

90

120

150

(b)

HNO3 method

uncalcined o

300 C

50

o

400 C o

40

500 C

30 20 10 0

180

0

30

60

90

120

150

180

Time (min)

Time(min)

Fig. 5. Photocatalytic degradation rate of glyphosate over BiVO4 prepared by different methods under visible light irradiation.

24