English

• 0.   Introduction

  In recent decades, the continuous growth of the global population, rapid industrialization, human activities, and improper management of various hazardous wastes have led to two major problems that require urgent attention: environmental pollution and the energy crisis^[1]. When considering only water pollution, approximately 80% of industrial and municipal wastewater is discharged into the environment without proper treatment, leading to shortages of freshwater and drinking water. The development of sustainable energy sources is currently the most promising and effective method for addressing fossil fuel reserves and environmental pollution issues. Solar energy possesses advantages such as being an endless and renewable resource. In recent years, the use of solar energy in photocatalysis has been considered a simple, effective, and cost- efficient environmental remediation method, especially for the degradation of organic pollutants in water^[2-4]. Since visible light constitutes the largest portion of the solar spectrum, photocatalysis has been widely utilized with semiconductor photocatalysts to decompose water or degrade organic pollutants under visible light irradiation. This approach has great promise and is sustainable in terms of energy^[5-9]. However, conventional photocatalysts, such as titanium dioxide, have band gaps that are too large to respond to visible light, severely limiting their potential applications. Therefore, researchers are dedicated to developing new semiconductor materials, more than 150 semiconductor materials have been developed. Common bismuth-based compounds in photocatalytic materials include BiOX (X=F, Cl, Br, I), Bi₄Ti₃O₁₂, Bi₂O₃, Bi₂O₂CO₃, BiVO₄, etc. BiVO₄ exhibits three crystal structures: monoclinic scheelite, tetragonal zircon, and tetragonal scheelite. All three structures possess excellent photoelectrochemical properties and are non-toxic, making them widely applied in novel high-performance photocatalytic materials. However, BiVO₄ faces challenges such as a narrow absorption range and high rates of photogenerated carrier recombination, significantly impacting its applications. To address this issue, researchers suggest that loading a single metal onto the surface of BiVO₄ can greatly enhance photocatalytic performance by suppressing the recombination of electrons and holes. Kudo et al.^[10] synthesized an Ag-BiVO₄ catalyst using a dispersant, which exhibited high efficiency in the degradation of phenol-containing wastewater. Ge et al.^[11-12] employed an impregnation method to synthesize Pt-BiVO₄ and Pd-BiVO₄, reporting significant photocatalytic efficiency in the degradation of methyl orange for both photocatalysts. Zhao et al.^[13] synthesized Cu-BiVO₄ metal-loaded catalysts using a hydrothermal and wet impregnation approach, and the metal-loaded BiVO₄ catalyst outperformed BiVO₄ in the photocatalytic degradation of ibuprofen. Wei et al.^[14] synthesized Fe/BiVO₄ through piezoelectric catalysis, and the addition of iron increased charge carrier density, thereby enhancing the piezoelectric catalytic activity of BiVO₄ and effectively degrading dichlorophenol.

  There is currently no research available regarding the improvement of photocatalytic performance for the heterojunction photocatalyst AgNi/BiVO₄ (AgNi/BVO), which involves loading both Ag and Ni onto BVO. In this study, the simple hydrothermal and chemical reduction method was employed to load AgNi bimetallic nanoalloys onto the facets of BVO. The catalyst′s absorption of visible light was enhanced by the surface plasmon resonance effect of silver and the lattice interface effect of nickel, in turn, promoted the separation of photogenerated electrons. The morphology and physicochemical properties of AgNi/BVO composite catalysts were thoroughly examined. Degradation experiments were conducted with AgNi-loaded catalysts of varying mass ratios to investigate the synergistic effects between the two metals. The results showed that compared with BVO, Ag/BVO and Ni/BVO, and AgNi/BVO composites exhibited better and more stable photocatalytic performance.

  1.   Experimental

  All chemicals were of reagen-grade quality obtained from TianJin Damao Chemical Reagent Factory and used as supplied unless otherwise stated. Deionized water was prepared in our laboratory.

  1.1   Synthesis

  BVO was synthesized by a typical hydrothermal method. 5 mmol Bi(NO₃)₃·5H₂O was dissolved in 30 mL 0.3 mol·L^-1 HNO₃ with continuous stirring at room temperature. 5 mmol of NH₄VO₃ was dissolved in 30 mL of deionized water, and it was slowly added drop by drop into the Bi(NO₃)₃ solution. The resulting solution was stirred at room temperature for 30 min and then transferred to a 100 mL hydrothermal reactor, where it was hydrothermally treated at 180 ℃ for 12 h. After cooling to room temperature, the product was separated by centrifugation, washed, and dried at 60 ℃ for 10 h. The obtained pure BiVO₄ sample was named BVO.

  0.5 g of BVO was dispersed in 30 mL of deionized water and subjected to ultrasonication. 0.05 g of AgNO₃·6H₂O and 0.016 6 g of Ni(NO₃)₂·6H₂O were separately added to the above solution (in a mass ratio of 3∶1 for AgNO₃·6H₂O and Ni(NO₃)₂·6H₂O) until uniformly dispersed via ultrasonication. After 30 min, a freshly prepared NaBH₄ solution was added to the solution (with a molar ratio of 5∶1 for NaBH₄ to Ag and 10∶1 for NaBH₄ to Ni). After continuous stirring at room temperature for 2 h, the product was centrifuged, washed with deionized water and ethanol, and then dried overnight at 60 ℃ in a vacuum drying oven to obtain the AgNi/BVO catalyst. Using the same method, Ag/BVO and Ni/BVO were prepared under the conditions, where was added 0.05 g of AgNO₃·6H₂O or 0.016 6 g of Ni(NO₃)₂·6H₂O.

  1.2   Characterization

  X-ray diffraction (XRD) analysis was performed using Bruker D8 advanced diffractometer at 40 kV and 40 mA with Cu Kα radiation (λ=0.154 18 nm) and a scanning range was 20° to 70°. The morphology and microstructure of the materials were characterized using a scanning electron microscope (SEM, Hitachi SU8010, operating voltage: 15 kV) and a transmission electron microscope (TEM, JEM-2100F, operating voltage: 200 kV) produced by Hitachi Corporation in Japan. UV-Vis diffuse reflectance spectrum (DRS) was measured using an Agilent Cary 5000 spectrometer in the United States. X-ray photoelectron spectroscopy (XPS, thermo escalab 250 XPS system) was used to analyze the surface state of the catalyst, and Al Kα radiation was used as a source of excitation. Brunauer-Emmett-Teller (BET) surface area and pore size distribution were analyzed by nitrogen adsorption. Photoluminescence (PL) was measured using the FluoroMax-4 photoluminescence spectrometer from the Japanese company HORIBA. The excitation wavelength was set at 280 nm.

  1.3   Adsorption and photocatalytic performance test

  A 500 W Xe lamp was used as the simulated light source. 50 mL solution of MB (methylene blue) (initial concentration ρ₀=10 mg·L^-1) was placed in the reaction vessel, and 50 mg of the prepared catalyst was added. Continuous stirring was maintained throughout the entire reaction process. A dark reaction was conducted for 30 min initially to reach adsorption equilibrium, followed by 90 min photoreaction. At 30-minute intervals, 3 mL solution was taken, and the absorbance was tested at the end through centrifugation.

  2.   Results and discussion

  2.1   Structure and morphology

  The crystal structure and composition of BVO, Ag/BVO, Ni/BVO, and AgNi/BVO were investigated through XRD. As shown in Fig. 1, the diffraction peaks of the BVO sample were in perfect agreement with the monoclinic structure of BiVO₄, as indicated by the standard card (PDF No.97-003-3243). Compared to other crystal phases of BiVO₄, monoclinic BiVO₄ is known to exhibit the highest photocatalytic efficiency^[15-16]. For the binary samples, Ag/BVO and Ni/BVO, as well as the ternary composite material AgNi/BVO, all the BVO diffraction peaks were identifiable in the XRD patterns. The intervention of AgNi nanoparticles did not alter the crystal structure of BVO. The diffraction peaks at 28.8°, 30.5°, 35.2°, and 50.3° correspond to the (112), (004), (020), and (220) crystal planes of AgNi/BVO. It was found that the relative intensity of (112) and (004) plane peaks of BVO and AgNi/BVO changed. The crystallinity of AgNi/BVO was superior to that of BVO.

  Figure 1

  

  Figure 1.  XRD patterns of the samples

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  Fig. 2a and 2b shows that BVO presented smooth polyhedral morphology. When Ag nanoparticles were introduced into BVO, Ag nanospheres were uniformly distributed on the BVO surface, as shown in Fig. 2c and 2d. Metallic Ni nanoparticles exhibited a random distribution on the BVO surface, as shown in Fig. 2e and 2f. In the AgNi/BVO sample, the particle size of metallic silver was smaller than that of metallic nickel, and its dispersion was notably better, as shown in Fig. 2g and 2h. The above explanation illustrates that the in-situ reduction method allows for the deposition of metal Ag and Ni nanoparticles on the surface of BVO.

  Figure 2

  

  Figure 2.  SEM images of BVO (a, b), Ag/BVO (c, d), Ni/BVO (e, f), and AgNi/BVO (g, h)

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  Further detailed morphological information of BVO and AgNi/BVO samples was obtained through TEM (Fig. 3a and 3b). In Fig. 3a, the TEM image of the BVO sample still exhibited a distinct polyhedral morphology. In Fig. 3b, AgNi nanoparticles were observed to be distributed along the edges of the BVO polyhedra. Therefore, the SEM and TEM results both confirm the successful composite formation of the materials. In Fig. 3c-3g, it was the elemental distribution of the AgNi/BVO composite material, revealing the presence of the elements B, O, V, Ag, and Ni in the composite material. The dispersion of each element was good. Further substantiates the successful synthesis of the composite material.

  Figure 3

  

  Figure 3.  TEM image of BVO (a); TEM image (b) and element mappings (c-g) of AgNi/BVO

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  To investigate the surface morphology and chemical composition of AgNi/BVO composite material, XPS testing was performed. As shown in Fig. 4a, the XPS survey spectrum of AgNi/BVO was primarily composed of five elements: Bi, V, O, Ag, and Ni. Individual elemental energy spectrum analyses were conducted for all elements. As shown in Fig. 4b, in the XPS spectrum of Bi4f, two distinct peaks appeared at binding energies of 159.06 and 164.35 eV, corresponding to the spin-orbit split peaks of Bi4f_7/2 and Bi4f_5/2. Fig. 4c displayed a high-resolution spectrum of V2p, showing the spin-orbit split peaks of V2p_3/2 and V2p_1/2, with binding energies of 516.65 and 524.09 eV. The above results indicate that the Bi and V in the AgNi/BVO sample exist in the +3 and +5 oxidation states. Fig. 4d represented a typical high-resolution spectrum of O1s in which a characteristic peak at a binding energy of 529.68 eV appeared. Fig. 4e displayed the XPS spectra of Ag3d. Two prominent peaks located at 367.83 and 374.01 eV, corresponding to Ag3d_5/2 and Ag3d_3/2. The presence of metallic Ag has been substantiated^[17-18]. In Fig. 4f, the peak located at 855.64 eV corresponded to Ni2p_3/2, while Ni2p_1/2 appeared at 872.96 eV. The two faint peaks appearing at 861.38 and 879.26 eV may be attributed to some oxidation of Ni due to surface exposure or testing, which led to the formation of nickel oxides. Similar phenomena have been reported in other literature^[19-20]. These XPS results align with the SEM findings, providing further evidence that both elemental Ag and Ni have been introduced into the BVO system simultaneously.

  Figure 4

  

  Figure 4.  Survey (a), Bi4f (b), V2p (c), O1s (d), Ag3d (e), and Ni2p (f) XPS spectra of AgNi/BVO

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  2.2   Specific surface area and optical properties

  An N₂ adsorption-desorption was employed for BET testing of BVO, Ag/BVO, Ni/BVO, and AgNi/BVO. As shown in Fig. 5a, all four samples exhibited type Ⅳ isotherms, with AgNi/BVO displaying a pronounced H3 hysteresis loop. This is because the metal AgNi nanoparticles generated by the in-situ reduction method are not only partially loaded on the surface of BVO but also adhere to the pore structure of BVO, further confirming the successful loading of AgNi on BVO. From the pore size distribution curve in Fig. 5b, it can be concluded that the pore sizes of the four samples were mainly distributed in the range of 1 to 10 nm. This indicates that all four materials belong to mesoporous materials. The specific surface area, pore volume, and average pore size can be seen in Table 1. From Table 1, it can be observed that the BET-specific surface areas of BVO, Ag/BVO, Ni/BVO, and AgNi/BVO were 0, 1, 2, and 1 m²·g^-1. It can be concluded that the specific surface areas of all four samples were relatively small and had similar values. Considering the degradation data, it is evident that the specific surface area of the catalyst is not the sole influencing factor for catalyst activity. The loading of AgNi enhances the catalyst′s pore volume, thereby facilitating the diffusion of reactant molecules into the pores.

  Figure 5

  

  Figure 5.  Typical N₂ adsorption-desorption isotherm (a) and pore diameter distributions (b) of the samples

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  Table 1

  

  Table 1.  BET specific surface areas, average pore diameters, and pore volumes of the samples

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  Sample	BET surface area/(m2·g-1)	Average pore diameter/nm	Pore volume/(cm3·g-1)
  BVO	0	1.347 0	0.000 7
  Ag/BVO	1	1.168 2	0004 2
  Ni/BVO	2	1.415 3	0.004 2
  AgNi/BVO	1	1.098 7	0.004 7

  The optical absorption performance of the prepared samples was tested using UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), as shown in Fig. 6a. Within the wavelength range of less than 600 nm, BVO exhibited light absorption characteristics in both the ultraviolet and visible light regions. In comparison to BVO, the absorption peaks of Ag/BVO, Ni/BVO, and AgNi/BVO composite material showed a redshift. The AgNi/BVO composite material exhibited a significantly broadened absorption band in the visible light region, with a noticeable enhancement in light absorption capacity. Indicating that the AgNi/BVO composite material possessed visible light-responsive properties^[21-23].

  Figure 6

  

  Figure 6.  Plots of UV-Vis DRS (a) and the (αhν)² versus hν (b) of the samples

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  Walsh et al.^[24] employed density functional theory (DFT) calculation to demonstrate that BiVO₄ is a direct bandgap semiconductor. The valence band formed by the hybridization of Bi6s and O2p orbitals results in a relatively small bandgap energy for BiVO₄, which can be calculated from the plot of (αhν)² versus photon energy (hν)^[25-26]. The intercept of the X-axis on the plot can effectively approximate the bandgap value of the sample. As shown in Fig. 6b, the calculated bandgap widths (E_g) for BVO, Ni/BVO, Ag/BVO, and AgNi/BVO were 2.35, 2.33, 2.25, and 2.18 eV, respectively. Compared to BVO, the inclusion of AgNi narrows the bandgap of the AgNi/BVO material, which is related to the control of the metal′s morphology on BVO^[27].

  Fluorescence analysis is an analytical method employed to investigate and assess the performance of photocatalysts. PL testing was carried out at room temperature to assess the recombination rate of photo-generated charge carriers in BVO, Ni/BVO, Ag/BVO, and AgNi/BVO catalysts, to explore the role of metals in the electron-hole separation within composite materials. As shown in Fig. 7, all four samples exhibited their most intense photoluminescence at 383 nm. Compared to BVO, the loading of metal cocatalysts decreased the emission peak intensity in the following order: BVO > Ni/BVO > Ag/BVO > AgNi/BVO. This indicates that metal cocatalysts can effectively suppress the recombination of photo-generated electron-hole pairs at the interface. The AgNi/BVO alloy exhibited the lowest emission peak intensity, suggesting that in comparison to single-metal loading, dual-metal loading can further optimize the electron migration pathway through secondary electron transfer, thereby enhancing the separation efficiency of photo-generated charge carriers in BVO more effectively.

  Figure 7

  

  Figure 7.  PL spectra of the samples

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  2.3   Photocatalytic properties

  The photocatalytic degradation efficiency of the samples for MB was evaluated, and reaction kinetics constants were calculated based on the first-order kinetic equation. Fig. 8a illustrates the photocatalytic degradation efficiency of the samples for MB. The relatively smooth surface structure of BVO, which resulted in a higher degree of recombination of photogenerated charge carriers and lower utilization of visible light, its photocatalytic performance was the weakest, degrading only 29.63% of MB. Under the same conditions, the photocatalytic performance was slightly enhanced after loading single metals, Ag and Ni, onto BVO catalysts. After 120 min reaction, Ag/BVO and Ni/BVO could degrade 57.49% and 50.76% of MB. After loading AgNi alloy, the degradation performance of the catalyst could be elevated to over 93%. This is the synergistic effect between the AgNi bimetallic catalyst, which effectively promotes the separation of photogenerated electrons and holes, while enhancing the catalyst′s absorption of visible light, thereby improving the photocatalytic performance of the BVO catalyst. The influence of loading various mass ratios of Ag to Ni (3∶1, 2∶1, 1∶1, 1∶2, 1∶3) on the degradation performance of MB by BVO was investigated, as shown in Fig. 8b. When the ratios were 3∶1, 2∶1, 1∶1, 1∶2, and 1∶3, the degradation rates for MB under the same conditions were 93.00%, 69.54%, 66.94%, 22.55%, and 8.38%, respectively. It was observed that the ratio of 3∶1 resulted in the highest degradation rate. Under this ratio, the electron transfer between Ag and Ni bimetals was maximized, enhancing the interaction between reactants and the catalyst, thus facilitating the photocatalytic reaction. As shown in Fig. 8c, according to the Langmuir-Hinshelwood model, the photocatalytic reaction follows pseudo-first-order kinetics. The initial concentration after adsorption equilibrium was ρ₀′, and ρ was the concentration at the time of reaction (t) after illumination. The apparent first-order reaction rate constant can be obtained. The rate constant for AgNi/BVO was the highest, being 5.4 times that of BVO. The UV-Vis DRS of the MB solution on the AgNi/BVO photocatalyst at different illumination times are shown in Fig. 8d. The absorption peak intensity at 664 nm rapidly decreased, indicating the effective decomposition of MB. Throughout the degradation process, there was no shift in the absorption peak position of the MB solution, and no additional peaks were formed in the ultraviolet-visible absorption spectra of the MB solution. Fig. 8e and 8f present the degradation profiles and first-order kinetic linear relationship graphs of rhodamine B (RhB, 10 mg·L^-1) under the influence of BVO, Ni/BVO, Ag/BVO, and AgNi/BVO catalysts. It was observed that the degradation trend of RhB was consistent with that of MB. Furthermore, upon the loading of the AgNi bimetallic catalyst, the degradation rate of RhB was significantly improved. This method demonstrates that the modification of BVO catalysts is suitable for degrading a wide range of organic pollutants. The synthesized AgNi/BVO catalyst was compared with other bismuth vanadate catalysts reported in existing literature for photocatalytic degradation efficiency, as shown in Table 2. Through this comparison, it is found that the photocatalytic performance of AgNi/BVO obtained in this study has a certain competitive advantage.

  Figure 8

  

  Figure 8.  (a) Degradation activity of MB by different catalysts; (b) Degradation activity of MB by different masses of AgNi/BVO catalysts; (c) Kinetic fitting of MB degradation of different catalysts; (d) UV-Vis DRS of MB by AgNi/BVO catalyst with irradiation time; (e) Degradation activity of RhB by different catalysts; (f) Kinetic fitting of RhB degradation of different catalysts

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  Table 2

  

  Table 2.  Comparison of photocatalytic degradation of MB by different composites under visible light irradiation

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  Catalyst	Light source	Amount of catalyst/mg	Pollutant	Time/min	Degradation rate/%	Ref.
  AgNi/BVO	Xe lamp (500 W)	50	MB	90	93.0	This work
  PANI-BiVO4-GO	Xe lamp (500 W)	100	MB	180	73	[28]
  BiVO4-Cement composites	Two Havells brand bulbs (each 15 W)	100	MB	240	58	[29]
  rGO-BiVO4	Two Havells brand bulbs (each 15 W)	50	MB	180	52	[30]
  BiVO4-Al2O3	25 W fluorescent lamp	200	MB	60	86	[31]
  BiVO4-SiO2	Three 18 W halogen lamps	200	MB	120	88	[32]

  Stability tests were conducted on AgNi/BVO, and the post-experimental catalyst underwent centrifugation, filtration, washing, and drying, as shown in Fig. 9. After four cycles of experiments, a slight decrease in the degradation performance of MB by AgNi/BVO catalyst was observed. This could be attributed to the minor loss of the catalyst during the washing and centrifugation processes, or it might be a small amount of residual MB occupying some active sites on the catalyst′s surface. This still demonstrates that AgNi/BVO can serve as a photocatalyst with excellent cycling stability.

  Figure 9

  

  Figure 9.  Catalytic stability of AgNi/BVO

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  To investigate the active species in the photocatalytic reaction, isopropanol (IPA), p-benzoquinone (BQ), and acid disodium salt (EDTA-2Na) were used as scavengers for hydroxyl radicals (·OH), superoxide radicals (·O₂^-), and holes (h⁺). The mass concentrations of the three kinds of traps were 1 g·L^-1. As shown in Fig. 10, the photocatalytic activity of AgNi/BVO significantly decreased when EDTA-2Na was introduced into the reaction system, indicating that h⁺ was the primary active species. The addition of BQ and IPA resulted in a relatively minor decrease in the degradation rate, suggesting that ·O₂^- and ·OH had a relatively smaller impact on the photocatalytic system.

  Figure 10

  

  Figure 10.  Effects of various scavenger agents on MB degradation rate

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  2.4   Mechanism of removal of MB by photocatalytic degradation

  Based on the above-mentioned research, the photocatalytic degradation mechanism of MB by AgNi/BVO material was proposed, as illustrated in Fig. 11. When the AgNi bimetal was loaded onto the BVO material, under visible light irradiation, electrons in BVO were excited, causing the transition of electrons (e^-) from the valence band (VB) to the conduction band (CB). Under the influence of the AgNi bimetal, e^- reacted with the oxygen (O₂) adsorbed on its surface, thereby generating active free radicals ·O₂^-. Simultaneously, a portion of h⁺ in BVO oxidized H₂O into ·OH. The substantial presence of h⁺ remaining in the VB facilitated the effective separation of electrons and holes. In the end, h⁺, ·OH, and ·O₂^- degrade MB into small molecules such as CO₂ and H₂O, which is in basic agreement with the results of free radical capture experiments.

  Figure 11

  

  Figure 11.  Photocatalytic degradation mechanism of MB for AgNi/BVO

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  3.   Conclusions

  In this study, a novel silver-nickel bimetal-loaded polyhedral bismuth vanadate heterojunction was successfully prepared for the first time through a two-step process involving a simple hydrothermal method and chemical reduction. The enhanced photocatalytic degradation performance of AgNi/BVO compared to BVO can be attributed to two factors: Firstly, the AgNi alloy significantly enhances the utilization of visible light by BVO, and secondly, the synergistic interaction between the bimetal components facilitates more efficient separation of photo-generated electron-hole pairs. Compared to BVO, the introduction of AgNi bimetal resulted in a photodegradation rate of approximately 93% for MB after 1.5 h of illumination, which was 5.4 times more efficient than BVO. Additionally, it exhibited excellent degradation performance for RhB as well. Even after four cycles of photocatalytic experiments, AgNi/BVO maintained its stability and photocatalytic activity. The preparation of AgNi/BVO was simple, environmentally friendly, and exhibited a degree of universality in its synthesis process. This study provides valuable guidance for the development of other catalysts with similar structures that possess high-efficiency photocatalytic performance.

  
  
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  Figure 1  XRD patterns of the samples

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  Figure 2  SEM images of BVO (a, b), Ag/BVO (c, d), Ni/BVO (e, f), and AgNi/BVO (g, h)

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  Figure 3  TEM image of BVO (a); TEM image (b) and element mappings (c-g) of AgNi/BVO

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  Figure 4  Survey (a), Bi4f (b), V2p (c), O1s (d), Ag3d (e), and Ni2p (f) XPS spectra of AgNi/BVO

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  Figure 5  Typical N₂ adsorption-desorption isotherm (a) and pore diameter distributions (b) of the samples

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  Figure 6  Plots of UV-Vis DRS (a) and the (αhν)² versus hν (b) of the samples

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  Figure 7  PL spectra of the samples

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  Figure 8  (a) Degradation activity of MB by different catalysts; (b) Degradation activity of MB by different masses of AgNi/BVO catalysts; (c) Kinetic fitting of MB degradation of different catalysts; (d) UV-Vis DRS of MB by AgNi/BVO catalyst with irradiation time; (e) Degradation activity of RhB by different catalysts; (f) Kinetic fitting of RhB degradation of different catalysts

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  Figure 9  Catalytic stability of AgNi/BVO

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  Figure 10  Effects of various scavenger agents on MB degradation rate

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  Figure 11  Photocatalytic degradation mechanism of MB for AgNi/BVO

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  Table 1.  BET specific surface areas, average pore diameters, and pore volumes of the samples

  Sample	BET surface area/(m2·g-1)	Average pore diameter/nm	Pore volume/(cm3·g-1)
  BVO	0	1.347 0	0.000 7
  Ag/BVO	1	1.168 2	0004 2
  Ni/BVO	2	1.415 3	0.004 2
  AgNi/BVO	1	1.098 7	0.004 7

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  Table 2.  Comparison of photocatalytic degradation of MB by different composites under visible light irradiation

  Catalyst	Light source	Amount of catalyst/mg	Pollutant	Time/min	Degradation rate/%	Ref.
  AgNi/BVO	Xe lamp (500 W)	50	MB	90	93.0	This work
  PANI-BiVO4-GO	Xe lamp (500 W)	100	MB	180	73	[28]
  BiVO4-Cement composites	Two Havells brand bulbs (each 15 W)	100	MB	240	58	[29]
  rGO-BiVO4	Two Havells brand bulbs (each 15 W)	50	MB	180	52	[30]
  BiVO4-Al2O3	25 W fluorescent lamp	200	MB	60	86	[31]
  BiVO4-SiO2	Three 18 W halogen lamps	200	MB	120	88	[32]

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