Facile Phase Control and Photocatalytic Performance of BiVO4 Crystals for Methylene Blue Degradation

Emerging contaminants, which mainly exist as organic pollutants and pose adverse biological effects, could be removed using photocatalytic degradation, resulting in a low-cost and environmentally friendly solution. Herein, BiVO4 nanoparticles with different morphologies and photocatalytic performances were synthesized by hydrothermal treatment at different residence times. The XRD and SEM results indicate that the crystal phase of BiVO4 gradually transformed from a single tetragonal phase to a single monoclinic crystal phase as the hydrothermal time increased, and with the extension of the hydrothermal time, the morphology of BiVO4 nanoparticles gradually differentiated from a smooth spherical shape to flower-like shapes composed of polyhedrons; the size of the crystals also increased accordingly. Methylene blue (MB), used as a probe of organic pollutants, was degraded under visible light irradiation by all BiVO4 samples to investigate its photocatalytic activities. The experimental results show that the longer the hydrothermal time, the better the photocatalytic performance. The optimum hydrothermal time was 24 h, at which the sample showed the highest photocatalytic activity for MB degradation. This work shows a convenient strategy for control of the crystal phase of BiVO4-based photocatalysts based on the understanding of the crystal morphology evolution mechanism, which will benefit the researchers in designing new BiVO4-based photocatalysts with high efficiency for emerging contaminants’ degradation.


Introduction
The awareness of green eco-environmental protection has been enhanced due to the recent developments in social science and technology [1]. Photocatalytic technology has become increasingly important as a green and sustainable technology [2][3][4]. Emerging contaminants, such as microplastics, persistent organic pollutants, antibiotics, pesticides, and endocrine disruptor chemicals, could damage the reproduction of human beings and animals [5][6][7]. Most of the emerging contaminants exist as organic pollutants and could be removed by the adsorptive and photocatalytic approach, which has recently gained broad attention [8][9][10].
BiVO 4 is a new type of visible light catalyst that has become popular due to its nontoxicity, high photostability, low cost of production, narrow band gap, response bands in visible light regions, and other advantages [3,11]. The crystallinity phase of BiVO 4 can be easily controlled by adjusting preparation conditions, playing a vital role in its photocatalytic efficiency, which is a low-cost strategy for enhancing the photocatalytic performance for water treatment [12]. It has been found that there are three main crystal structures in the BiVO 4 : a tetrahedral scheelite structure; monoclinic scheelite structure; sonification for 10 min. The NaOH and HNO 3 solution was used to adjust the pH value to 6. Then, the obtained solution was placed in a static condition overnight and transferred to a reaction kettle with a Teflon-lined stainless-steel autoclave, which was followed by a reaction at 180 • C for 0, 3, 6, 12, 14, and 24 h periods. When they had been cooled to room temperature, the as-prepared samples were washed three times with deionized water and ethanol. Then, centrifugal filtering was performed, and a six-hour drying process at 80 • C was completed in order to obtain the purified products [27]. The preparetion illustration is presented as Figure 1. The hydrothermal times were 0, 3, 6, 12, 14, and 24 h, and the prepared products were, respectively, referred to as t = 0 BiVO 4 ; t = 3 BiVO 4 ; t = 6 BiVO 4 ; t = 12 BiVO 4 ; t = 14 BiVO 4 ; and t = 24 BiVO 4 .
Int. J. Environ. Res. Public Health 2023, 20, x FOR PEER REVIEW 3 of 12 In this research study, under the conditions of no template and no surfactant, 2.45 g of Bi(NO3)3·5H2O and 0.6 g of NH4VO3 were dissolved in 40 mL of 4 mol/L HNO3 solution and NaOH solution, respectively. Then, after Bi(NO3)3·5H2O and NH4VO3 were fully dissolved, the two solutions were combined, followed by magnetic stirring for 20 min and sonification for 10 min. The NaOH and HNO3 solution was used to adjust the pH value to 6. Then, the obtained solution was placed in a static condition overnight and transferred to a reaction kettle with a Teflon-lined stainless-steel autoclave, which was followed by a reaction at 180 °C for 0, 3, 6, 12, 14, and 24 h periods. When they had been cooled to room temperature, the as-prepared samples were washed three times with deionized water and ethanol. Then, centrifugal filtering was performed, and a six-hour drying process at 80 °C was completed in order to obtain the purified products [27]. The preparetion illustration is presented as Figure 1. The hydrothermal times were 0, 3, 6, 12, 14, and 24 h, and the prepared products were, respectively, referred to as t = 0 BiVO4; t = 3 BiVO4; t = 6 BiVO4; t = 12 BiVO4; t = 14 BiVO4; and t = 24 BiVO4.

Characterizations
The BiVO4 structure was measured using X-ray powder diffraction (XRD, Smart Lab (3 KW)) between 10° and 75°, which used Cu K radiation (=0.15406 nm) produced at 40 kV and 40 mA. The JCPDS (Joint Committee on Powder Diffraction Standards) files were compared to identify the crystalline phases. The morphological examination used a field emission scanning electron microscope (SEM, FEI Quanta 400 FEG) with an energy dispersive X-ray analyzer system (EX-250). The specific surface area of the samples was characterized using a TriStar II 3020 surface area analyzer at 77 K. UV-Vis diffuse reflection spectra (DRS) were collected with a UV-Vis spectrophotometer (Lambda, 650).

Photocatalytic Activity Tests
30 mg of the BiVO4 prepared in the above-mentioned process was weighed and placed into 20 mg/L of methylene blue solution (30 mL), and then stirred under dark conditions for thirty minutes. The solution was then irradiated by visible light with a wavelength λ > 420 nm (550 W xenon lamp illumination, BL-GHX-V, Shanghai Bilang Instrument Manufacturing Co. LTD). The distance from the light source to the photocatalytic reactor was set as 10 cm. A shading system was equipped to ensure that the photocatalytic degradation was not affected by the light from outside. Sampling was conducted every 30 min, and the supernatant was collected in order to measure the absorbance after centrifugation.

Characterizations
The BiVO 4 structure was measured using X-ray powder diffraction (XRD, Smart Lab (3 KW)) between 10 • and 75 • , which used Cu K radiation (=0.15406 nm) produced at 40 kV and 40 mA. The JCPDS (Joint Committee on Powder Diffraction Standards) files were compared to identify the crystalline phases. The morphological examination used a field emission scanning electron microscope (SEM, FEI Quanta 400 FEG) with an energy dispersive X-ray analyzer system (EX-250). The specific surface area of the samples was characterized using a TriStar II 3020 surface area analyzer at 77 K. UV-Vis diffuse reflection spectra (DRS) were collected with a UV-Vis spectrophotometer (Lambda, 650).

Photocatalytic Activity Tests
30 mg of the BiVO 4 prepared in the above-mentioned process was weighed and placed into 20 mg/L of methylene blue solution (30 mL), and then stirred under dark conditions for thirty minutes. The solution was then irradiated by visible light with a wavelength λ > 420 nm (550 W xenon lamp illumination, BL-GHX-V, Shanghai Bilang Instrument Manufacturing Co., Ltd.). The distance from the light source to the photocatalytic reactor was set as 10 cm. A shading system was equipped to ensure that the photocatalytic degradation was not affected by the light from outside. Sampling was conducted every 30 min, and the supernatant was collected in order to measure the absorbance after centrifugation.

Results and Discussion
3.1. X-ray Diffraction (XRD) Figure 2 details the X-ray diffraction patterns when the hydrothermal times were 0, 3, and 6 h, and 2 θ = 18.6 • , 18 with the diffraction peaks. When the hydrothermal times were zero and three hours, the obtained BiVO 4 was found to display tetragonal diffraction peaks. When the hydrothermal time was six hours, a weak diffraction peak (121) in the monoclinic BiVO 4 appeared. These findings indicated that when the hydrothermal times were zero and three hours, the tetragonal BiVO 4 was obtained. However, when the hydrothermal time was six hours, the mixed crystals of the tetragonal and monoclinic phases were obtained. It was found that when the hydrothermal times were 12, 14, and 24 h, the characteristic peaks at 2 θ values of 18  . Therefore, the resulting product was determined to be a single monoclinic BiVO 4 crystal [28]. It was also indicated that the times of the hydrothermal reactions were directly related to the crystal shape of the BiVO 4 . Furthermore, the longer hydrothermal times were found to be more conducive to the generation of the monoclinic BiVO 4 . The results of this study indicated that the generation process of the BiVO 4 crystals was as follows: The tetragonal phase to mixed crystals of the tetragonal and monoclinic phases, and then to a single monoclinic phase.
3, and 6 h, and 2 θ = 18.6°, 18.8°, 24.9°, 28.8°, 30.4°, 33.1°, 35.1°, 39.9°, 42.4°, 46.0°, 46.6 47.2°, 50.2°, 53.2°, 58.2°, and 59.4° with the diffraction peaks. When the hydrotherma times were zero and three hours, the obtained BiVO4 was found to display tetragonal dif fraction peaks. When the hydrothermal time was six hours, a weak diffraction peak (121 in the monoclinic BiVO4 appeared. These findings indicated that when the hydrotherma times were zero and three hours, the tetragonal BiVO4 was obtained. However, when th hydrothermal time was six hours, the mixed crystals of the tetragonal and monoclini phases were obtained. It was found that when the hydrothermal times were 12, 14, and 2 h, the characteristic peaks at 2 θ values of 18  . Therefore, the resulting product was determined to be a single monoclini BiVO4 crystal [28]. It was also indicated that the times of the hydrothermal reactions wer directly related to the crystal shape of the BiVO4. Furthermore, the longer hydrotherma times were found to be more conducive to the generation of the monoclinic BiVO4. Th results of this study indicated that the generation process of the BiVO4 crystals was a follows: The tetragonal phase to mixed crystals of the tetragonal and monoclinic phases and then to a single monoclinic phase. In order to better clarify the influence degrees of the hydrothermal times on eac crystal plan growth of the BiVO4, (011) was taken as the internal standard of the diffractio peak to normalize the XRD diffraction peak intensity. Table 1 lists the (121)/(011) an (040)/(011) intensity of each diffraction peak. It was observed that when t = 6 h, the (121 and (040) diffraction peaks were relatively weak, and the (121)/(011) and (040)/(011) value were 0.215 and 0.408. However, with the increases in the hydrothermal times, the (121 and (040) diffraction peaks were gradually strengthened, which indicated that certain hy drothermal times were conducive to the formations of the (121) and (040) diffractio peaks. In the m-BiVO4 standard PDF card (JCPDS No. 140688), the (040)/(121) intensit ratio is 0.25, which has been taken as the standard. When t = 6 h, the (040)/(121) had In order to better clarify the influence degrees of the hydrothermal times on each crystal plan growth of the BiVO 4 , (011) was taken as the internal standard of the diffraction peak to normalize the XRD diffraction peak intensity. Table 1 lists the (121)/(011) and (040)/(011) intensity of each diffraction peak. It was observed that when t = 6 h, the (121) and (040) diffraction peaks were relatively weak, and the (121)/(011) and (040)/(011) values were 0.215 and 0.408. However, with the increases in the hydrothermal times, the (121) and (040) diffraction peaks were gradually strengthened, which indicated that certain hydrothermal times were conducive to the formations of the (121) and (040) diffraction peaks. In the m-BiVO 4 standard PDF card (JCPDS No. 140688), the (040)/(121) intensity ratio is 0.25, which has been taken as the standard. When t = 6 h, the (040)/(121) had a value of 1.897, which indicated a weak (121) diffraction peak intensity. Then, as the hydrothermal times increased, the (040)/(121) values were determined to be 0.261, 0.296, and 0.307, which were close to the (040)/(121) values of 0.25 in the m-BiVO 4 standard PDF card (JCPDS No. 140688). In this study, when t = 12 h, the value of 0.261 was almost approached, which confirmed that that particular hydrothermal time was favorable for the formation of m-BiVO 4 diffraction peaks. However, when t was 14 and 24 h, it was observed that the (040)/(121) values displayed slightly increasing trends, indicating that when the hydrothermal time was 12 h, the value of the (040)/(121) was the most similar to the ratio in the m-BiVO 4 standard PDF card (JCPDS No. 140688).  Figure 3 details the UV-Vis spectra of BiVO 4 under different hydrothermal time conditions. It can be seen that the samples in the range of 520 to 580 nm displayed strong absorption abilities. Additionally, the sharp drop in the regional absorbing boundary was due to the transition of the electrons in the semiconductor material during the absorption of certain energies [12]. It can be seen from the figure that when the hydrothermal time was 12 h, 14 h, and 24 h, the absorption band was more red-shifted than at 0 h, 3 h, and 6 h. This was found to be consistent with the conclusions reached regarding the BiVO 4 XRD spectrum. Therefore, it can be seen that during the period ranging from 0 to 12 h, with the increases in the hydrothermal times, the absorption band of the BiVO 4 generated the redshift, and its band gap also decreased. However, when the hydrothermal times were 14 and 24 h, its redshift was not obvious, which may be related to the agglomeration of the BiVO 4 crystals, along with the diameter sizes of the crystals.

UV-Vis Test Analysis
served that the (040)/(121) values displayed slightly increasing trends, indicat when the hydrothermal time was 12 h, the value of the (040)/(121) was the most si the ratio in the m-BiVO4 standard PDF card (JCPDS No. 140688).  Figure 3 details the UV-Vis spectra of BiVO4 under different hydrothermal ti ditions. It can be seen that the samples in the range of 520 to 580 nm displayed absorption abilities. Additionally, the sharp drop in the regional absorbing bound due to the transition of the electrons in the semiconductor material during the abs of certain energies [12]. It can be seen from the figure that when the hydrotherm was 12 h, 14 h, and 24 h, the absorption band was more red-shifted than at 0 h, 3 h. This was found to be consistent with the conclusions reached regarding the BiV spectrum. Therefore, it can be seen that during the period ranging from 0 to 12 h, w increases in the hydrothermal times, the absorption band of the BiVO4 generated shift, and its band gap also decreased. However, when the hydrothermal times and 24 h, its redshift was not obvious, which may be related to the agglomeratio BiVO4 crystals, along with the diameter sizes of the crystals.

SEM Test Analysis
As can be seen from the SEM image displayed in Figure 4, as the hydrothermal time increased, the shape generally became spheroid. Additionally, when the hydrothermal times were 0 and 3 h, its spherical surfaces were relatively smooth. When the hydrothermal times were 6, 12, 14, and 24 h, the smooth spherical surfaces became gradually differentiated into many small polyhedron crystal pieces. Then, as the lengths of the hydrothermal times increased, the volumes of polyhedron crystals on the spherical surfaces were observed to gradually increase. Moreover, when the hydrothermal times were 14 and 24 h, the spheroids gradually showed hollow disintegration trends, and some differentiated polyhedron crystals appeared to be separated from the spheres. It was concluded that, according to the aforementioned process, the BiVO 4 generation process could be speculated as follows: First, a smooth spheroid material was formed due to precipitation; then, the material was gradually differentiated and tiny particles were generated on the surfaces. These tiny particles gradually formed into polyhedron crystals. As the polyhedron crystals gradually increased, the agglomeration and clustering processes appeared to form chrysanthemum-like shapes ( Table 2). Moreover, it was found that when the hydrothermal time was 24 h, the crystal particle sizes were significantly larger than those observed at the 14 h and 12 h stages, and serious agglomeration was evident. These findings indicated that the high-temperature and high-pressure conditions during the process of hydrothermal synthesis were favorable to the generation of monoclinic BiVO 4 crystals. However, as the hydrothermal time increased, the agglomeration phenomenon of the BiVO 4 crystals seemed to become more serious.

SEM Test Analysis
As can be seen from the SEM image displayed in Figure 4, as the hydrothermal time increased, the shape generally became spheroid. Additionally, when the hydrothermal times were 0 and 3 h, its spherical surfaces were relatively smooth. When the hydrothermal times were 6, 12, 14, and 24 h, the smooth spherical surfaces became gradually differentiated into many small polyhedron crystal pieces. Then, as the lengths of the hydrothermal times increased, the volumes of polyhedron crystals on the spherical surfaces were observed to gradually increase. Moreover, when the hydrothermal times were 14 and 24 h, the spheroids gradually showed hollow disintegration trends, and some differentiated polyhedron crystals appeared to be separated from the spheres. It was concluded that, according to the aforementioned process, the BiVO4 generation process could be speculated as follows: First, a smooth spheroid material was formed due to precipitation; then, the material was gradually differentiated and tiny particles were generated on the surfaces. These tiny particles gradually formed into polyhedron crystals. As the polyhedron crystals gradually increased, the agglomeration and clustering processes appeared to form chrysanthemum-like shapes ( Table 2). Moreover, it was found that when the hydrothermal time was 24 h, the crystal particle sizes were significantly larger than those observed at the 14 h and 12 h stages, and serious agglomeration was evident. These findings indicated that the high-temperature and high-pressure conditions during the process of hydrothermal synthesis were favorable to the generation of monoclinic BiVO4 crystals. However, as the hydrothermal time increased, the agglomeration phenomenon of the BiVO4 crystals seemed to become more serious.  It can be seen in the figure that when the hydrothermal times were 0, 3, and 6 h, circular spherical particles with uniform distributions and no agglomeration phenomena were evident. However, when the duration of the hydrothermal time was more than 12 h, it was observed that many small particles were present. These polyhedron crystals were the crystal particles that had been detached from the matrix. When the hydrothermal time was fourteen hours, many large concave parts were presented on the spherical surfaces (within the red circle highlighted in Figure 5). These parts were determined to be the traces of the BiVO4 crystals that had been detached from the matrix, which confirmed the speculation that the BiVO4 crystals were detached from the matrix.   Figure 5 details this study's SEM map with an observational diameter of 10 µm. It can be seen in the figure that when the hydrothermal times were 0, 3, and 6 h, circular spherical particles with uniform distributions and no agglomeration phenomena were evident. However, when the duration of the hydrothermal time was more than 12 h, it was observed that many small particles were present. These polyhedron crystals were the crystal particles that had been detached from the matrix. When the hydrothermal time was fourteen hours, many large concave parts were presented on the spherical surfaces (within the red circle highlighted in Figure 5). These parts were determined to be the traces of the BiVO 4 crystals that had been detached from the matrix, which confirmed the speculation that the BiVO 4 crystals were detached from the matrix.  The results of this study's nitrogen adsorption experiment were carried out using a Tristar 3020 specific surface area analyzer which showed that when the hydrothermal times were 0, 3, 6, 12, 14, and 24 h, the respective specific surface areas were 2.5952 m 2 /g, 2.6115 m 2 /g, 2.7727 m 2 /g, 2.9271 m 2 /g, 3.0971 m 2 /g, and 2.0352 m 2 /g ( Table 2). These results confirmed that with the increases in the durations of the hydrothermal times, the specific surface areas of the BiVO4 crystals displayed increasing trends. However, when the hydrothermal time was 24 h, it was determined that the causes of the sharp declines in specific surface areas were largely related to the agglomeration of the BiVO4 crystals [29]. The results of this study's nitrogen adsorption experiment were carried out using a Tristar 3020 specific surface area analyzer which showed that when the hydrothermal times were 0, 3, 6, 12, 14, and 24 h, the respective specific surface areas were 2.5952 m 2 /g, 2.6115 m 2 /g, 2.7727 m 2 /g, 2.9271 m 2 /g, 3.0971 m 2 /g, and 2.0352 m 2 /g ( Table 2). These results confirmed that with the increases in the durations of the hydrothermal times, the specific surface areas of the BiVO 4 crystals displayed increasing trends. However, when the hydrothermal time was 24 h, it was determined that the causes of the sharp declines in specific surface areas were largely related to the agglomeration of the BiVO 4 crystals [29].

FT-IR and Raman Analyses
As can be seen from the infrared spectrum detailed in Figure 6, the absorption bands in the areas of 3446 cm −1 , 1628 cm −1 , and 1384 cm −1 were different forms of vibrations generated by the H-O-H bond in the water molecules. Meanwhile, in the area of 735 cm −1 , a visible vibration peak had occurred, which had been generated by the VO 4 3− tetrahedron in the BiVO 4 crystals. As can be seen in the figure, when t = 0, 3, and 6 h, the Raman spectra displayed the same type of Raman peak. However, when t = 12, 14, and 24 h, another similar type of Raman peak was presented. Among these, the 838 cm −1 and 850 cm −1 peaks represented the Raman peaks of the V-O bonds in the monoclinic and tetragonal BiVO 4 , respectively, while the peaks at 360 cm −1 and 325 cm −1 were the V-O symmetric bending mode (Ag) and antisymmetric bending mode (Bg) of the VO 4 3− group, respectively. The peak at 221 cm −1 was the outer mode generated by the spin and frequency shift of the V-O bond in the VO 4 3− group. These results were found to be consistent with those of the XRD analysis. The internal structures of the BiVO 4 with different crystal types were observed to be diversified, which resulted in different Raman peaks [30].
Int. J. Environ. Res. Public Health 2023, 20, x FOR PEER REVIEW 8 o mode (Ag) and antisymmetric bending mode (Bg) of the VO4 3-group, respectively. peak at 221 cm −1 was the outer mode generated by the spin and frequency shift of the O bond in the VO4 3-group. These results were found to be consistent with those of XRD analysis. The internal structures of the BiVO4 with different crystal types were served to be diversified, which resulted in different Raman peaks [30].

BiVO4 Generation Mechanism
As displayed in Figure 7, with the extension of the hydrothermal time, the morp logical change of BiVO4 had four steps [12]: Step 1 Oswald ripening process: During the process of the precursor solution pre ration and overnight static placement, tiny particles were observed to have been depos on the surfaces of the larger particles, which gradually formed into spherical particles w smooth surfaces [31].
Step 2 Particle differentiation process: It was observed that with the increases in temperature and hydrothermal times, many fine crystal particles were gradually dif entiated onto the smooth surfaces; Step 3 Crystal detachment: It was found that with the further extension of the du tions of the hydrothermal times, the spheroid matrix structure became damaged, and surface differentiated crystals had gradually fallen off; Step 4 Crystal agglomeration: It was further observed that with the continued ex sions of hydrothermal times, the polyhedron crystals had gradually separated from matrix and agglomeration occurred. Moreover, with the increases in the durations of hydrothermal times, the BiVO4 crystals gradually transformed from single tetragonal tetragonal-monoclinic mixed crystals to monoclinic crystals. These findings indicated the tetragonal phase had converted to a monoclinic phase as the hydrothermal time creased. Meanwhile, the results of this study's degradation experiment using methyl blue solution showed that the photocatalytic performances of the monoclinic BiVO4 c tals were superior to those of the tetragonal BiVO4 crystals. However, the agglomera of the BiVO4 crystals tended to weaken their photocatalytic performances [32].

BiVO 4 Generation Mechanism
As displayed in Figure 7, with the extension of the hydrothermal time, the morphological change of BiVO 4 had four steps [12]:

Photocatalytic Performance Analysis
It can be seen in Figure 8 that, when the hydrothermal time was 24 h, the BiVO4 displayed the best activity. Meanwhile, when hydrothermal times were zero and three hours, the degradation performances of the BiVO4 were observed to be somewhat less effective than when the hydrothermal times were 6, 12, 14, and 24 h. These results were found to be consistent with the absorption boundary values reflected by the solid ultraviolet. It was found that as the durations of the hydrothermal times increased, the photocatalytic performances of the prepared products, as well as the adsorption capacity of the methylene blue solution, were gradually and successively increased. However, when t = 24 h, the adsorption ability of the BiVO4 was found to be slightly weaker than when t = 14 h. This was determined to be related to the BiVO4 surface area. The degradation curve of the MB further confirmed that the photocatalytic performances of the monoclinic BiVO4 were superior to those of the tetragonal BiVO4 [25], which is consistent with the references that monoclinic BiVO4 possessed superior separation of the photo-induced charge carriers to that of tetragonal BiVO4 [12]. Additionally, the photocatalytic activity of BiVO4 in our study was compared with other references and is presented in Table 3. Furthermore, it was reported that the heterojunction structure between tetragonal BiVO4 and monoclinic BiVO4 could exhibit much higher photocatalytic activity [27], which is worth further exploration. Step 1 Oswald ripening process: During the process of the precursor solution preparation and overnight static placement, tiny particles were observed to have been deposited on the surfaces of the larger particles, which gradually formed into spherical particles with smooth surfaces [31].
Step 2 Particle differentiation process: It was observed that with the increases in the temperature and hydrothermal times, many fine crystal particles were gradually differentiated onto the smooth surfaces; Step 3 Crystal detachment: It was found that with the further extension of the durations of the hydrothermal times, the spheroid matrix structure became damaged, and the surface differentiated crystals had gradually fallen off; Step 4 Crystal agglomeration: It was further observed that with the continued extensions of hydrothermal times, the polyhedron crystals had gradually separated from the matrix and agglomeration occurred. Moreover, with the increases in the durations of the hydrothermal times, the BiVO 4 crystals gradually transformed from single tetragonal and tetragonal-monoclinic mixed crystals to monoclinic crystals. These findings indicated that the tetragonal phase had converted to a monoclinic phase as the hydrothermal time increased. Meanwhile, the results of this study's degradation experiment using methylene blue solution showed that the photocatalytic performances of the monoclinic BiVO 4 crystals were superior to those of the tetragonal BiVO 4 crystals. However, the agglomeration of the BiVO 4 crystals tended to weaken their photocatalytic performances [32].

Photocatalytic Performance Analysis
It can be seen in Figure 8 that, when the hydrothermal time was 24 h, the BiVO 4 displayed the best activity. Meanwhile, when hydrothermal times were zero and three hours, the degradation performances of the BiVO 4 were observed to be somewhat less effective than when the hydrothermal times were 6, 12, 14, and 24 h. These results were found to be consistent with the absorption boundary values reflected by the solid ultraviolet. It was found that as the durations of the hydrothermal times increased, the photocatalytic performances of the prepared products, as well as the adsorption capacity of the methylene blue solution, were gradually and successively increased. However, when t = 24 h, the adsorption ability of the BiVO 4 was found to be slightly weaker than when t = 14 h. This was determined to be related to the BiVO 4 surface area. The degradation curve of the MB further confirmed that the photocatalytic performances of the monoclinic BiVO 4 were superior to those of the tetragonal BiVO 4 [25], which is consistent with the references that monoclinic BiVO 4 possessed superior separation of the photo-induced charge carriers to that of tetragonal BiVO 4 [12]. Additionally, the photocatalytic activity of BiVO 4 in our study was compared with other references and is presented in Table 3. Furthermore, it was reported that the heterojunction structure between tetragonal BiVO 4 and monoclinic BiVO 4 could exhibit much higher photocatalytic activity [27], which is worth further exploration.

Photocatalytic Performance Analysis
It can be seen in Figure 8 that, when the hydrothermal time was 24 h, the BiVO4 displayed the best activity. Meanwhile, when hydrothermal times were zero and three hours, the degradation performances of the BiVO4 were observed to be somewhat less effective than when the hydrothermal times were 6, 12, 14, and 24 h. These results were found to be consistent with the absorption boundary values reflected by the solid ultraviolet. It was found that as the durations of the hydrothermal times increased, the photocatalytic performances of the prepared products, as well as the adsorption capacity of the methylene blue solution, were gradually and successively increased. However, when t = 24 h, the adsorption ability of the BiVO4 was found to be slightly weaker than when t = 14 h. This was determined to be related to the BiVO4 surface area. The degradation curve of the MB further confirmed that the photocatalytic performances of the monoclinic BiVO4 were superior to those of the tetragonal BiVO4 [25], which is consistent with the references that monoclinic BiVO4 possessed superior separation of the photo-induced charge carriers to that of tetragonal BiVO4 [12]. Additionally, the photocatalytic activity of BiVO4 in our study was compared with other references and is presented in Table 3. Furthermore, it was reported that the heterojunction structure between tetragonal BiVO4 and monoclinic BiVO4 could exhibit much higher photocatalytic activity [27], which is worth further exploration.

Conclusions
In this work, BiVO 4 was successfully constructed through the hydrothermal method under different hydrothermal times. XRD results showed that the longer hydrothermal times were found to be more conducive to the generation of the monoclinic BiVO 4 . The UV-Vis absorption was more red-shifted when hydrothermal time increased from 3 h to 24 h. SEM results showed that the high temperature and high pressure conditions in the hydrothermal synthesis process are conducive to the formation of monoclinic BiVO 4 crystals, but the agglomeration phenomenon of BiVO 4 becomes more serious with the increase in hydrothermal time. The specific surface area of all the samples was in the range of 2.0~3.1 m 2 /g. Moreover, BiVO 4 had excellent photocatalytic performance toward MB.
Additionally, the photocatalytic activity shows that the crystal phase of BiVO 4 plays a vital role in the photocatalytic performances: the removal efficiency of monoclinic BiVO 4 (45% in 150 min), including adsorption and photocatalysis, was more than twice that of the tetragonal BiVO 4 (20% in 150 min), which provides a facile guideline for the photocatalytic degradation of emerging organic contaminants using BiVO 4 .