Sonochemical Synthesis of Copper-doped BiVO4/g-C3N4 Nanocomposite Materials for Photocatalytic Degradation of Bisphenol A under Simulated Sunlight Irradiation

Copper-doped bismuth vanadate/graphitic carbon nitride (BiVO4/g-C3N4) nanocomposite materials were successfully fabricated using a sonochemical approach. Cu-doped BiVO4/g-C3N4 nanocomposite photocatalysts could improve electron/hole (e−/h+) pair separation, stability, and light-harvesting efficiency compared to pristine BiVO4 or g-C3N4, resulting in the enhancement of photocatalytic activity. The optimal parameters, such as pH value at 10, photocatalyst dosage of 0.4 g L−1, and 10 mol% Cu-doped BiVO4/g-C3N4 photocatalyst, were determined to degrade initial concentration of 20 ppm Bisphenol A, which could be completely removed after 90 min. Furthermore, the excessive doping of copper (> 10 mol%) could not synthesize the pure monoclinic scheelite phase, which substantially resulted in the reduction of the photocatalytic activity.


Introduction
Bismuth vanadate (BiVO 4 ) has three crystal phases, such as tetragonal-scheelite, monoclinic scheelite, and zircon. Among those, the monoclinic phase, in particular, exhibits outstanding visible-light photocatalytic properties because monoclinic scheelite BiVO 4 possesses the small bandgap 2.4 eV [1][2][3][4][5]. Dong et al. (2016) [6] prepared monoclinic scheelite BiVO 4 semiconductors with porous structures using a two-step method. The porous BiVO 4 material prepared at hydrothermal temperatures of 200 • C displays enhanced photocatalytic efficiency by degrading methylene blue (MB). Sun et al. (2019) revealed that the two-dimensional BiVO 4 nanosheets/reduced graphene oxide (RGO) exhibited higher photocatalytic activity [7]. In addition, the heterojunction of semiconductor composite has been reported to be an effective strategy to fabricate efficient photocatalysts [6]. Xie et al. (2018) exhibited that the BiVO 4 /Mn-Zn ferrite (Mn 1−x Zn x Fe 2 O 4 )/RGO photocatalyst could degrade rhodamine B (RhB) about 96% [8]. Thus, in this article, we chose graphitic carbon nitride (g-C 3 N 4 ) to modify BiVO 4 to enhance photocatalytic efficiency due to graphene-like structure, good thermal and chemical stability, good visible light absorption, and photocatalytic properties of g-C 3 N 4 [6]. In addition, g-C 3 N 4 nanosheets could enhance an easier electron transfer during the reaction [9]. Smykalova et al. (2019) indicated that g-C 3 N 4 nanosheets possessed a large specific surface area, which resulted in the higher degradation ratio of medicines in water [10]. Zhang et al. (2017) indicated that the composite with 7 wt% g-C 3 N 4 had better photocatalytic efficiency than bare BiVO 4 [11]. The stable core/shell structure of the BiVO 4 /g-C 3 N 4 composite material not only enlarges the surface area but also enhances the photo-generated charge separation. Therefore, a new composite photocatalyst is expected to improve the stability, solar light utilization, charge separation, and transfer by combining with the two semiconductors.
BPA (bisphenol A) is not only a synthetic organic chemical with various applications in the polymer industry used as intermediate for the production of epoxy resins and plastics but also an endocrine disruptor, which would lead to serious ecology exacerbation [12,13]. Some researchers have shown that exposure to very low BPA levels may result in reduced fertility and increased incidence of breast, ovarian, and testicular cancers [13]. Therefore, BPA should be completely removed from the water for safety consideration to human beings. In this research, we prepared the BiVO 4 nanocomposite photocatalysts at 60 • C, 1 atm, and 1.5 h via a sonochemical synthesis way. The advantage of sonochemistry in fabricating nanostructured samples occurs mostly from acoustic cavitation [14]. In addition, we also introduced the synthesis of copper and g-C 3 N 4 -modified BiVO 4 and removal of BPA using photocatalytic reaction under simulated sunlight irradiation. Some parameters that affect the efficiency of BPA removal via the photocatalytic degradation process, including the pH values in solution, the dosages of BiVO 4 nanocomposite material, and the various BiVO 4 nanocomposite materials, were studied.

Characterization of BiVO 4 Nanocomposite Photocatalysts
The morphologies were examined by the JEOL JSM-7800F model and the JEOL JEM-2010 model. The XRD patterns were examined by the Rigaku Ultima III diffractometer (Rigaku Japan Sales Division, Tokyo, Japan). Particle size distribution, bandgap, and photoluminescence (PL) properties were measured using the Shimadzu SALD-2300 model, Shimadzu UV-2600 and Shimadzu RF-3501 spectrometer (Shimadzu Corporation, Tokyo, Japan), respectively. The flat-band potential of the sample was recorded using potentiostat/galvanostat PGSTAT302N, Metrohm Autolab (Metrohm AG, Herisau, Switzerland). The surface area, pore size, and pore volume were measured by using a Micrometrics ASAP-2020 nitrogen adsorption instrument (Micrometrics Headquarters, Norcross, GA, USA).

Photocatalytic Reaction
Typically, an appropriate amount of BiVO 4 nanocomposite material was put into 100 mL of BPA solution (4,4'-dihydroxy-2,2-diphenylpropane, Bisphenol A, C 15 H 16 O 2 , ECHO) with a concentration of 20 ppm. The reactor was irradiated with a 350 W Xenon light (KIT-XENON-ADJ350W) for 6 h. During the experiment, about 1.5 mL sample was withdrawn at predetermined time intervals, and the sample was immediately filtered through the 0.22 µm polyvinylidene difluoride (PVDF) syringe filter to remove the powders, and the clarified solution was analyzed by High-performance liquid chromatography (HPLC, LC-20A, Shimadzu Corporation, Tokyo, Japan).

Characterization of BiVO 4 Nanocomposite Photocatalysts
Reaction conditions could effectively affect the morphology of photocatalysts. Figure 1a-d show when irradiation time was extended, the particle size of BiVO 4 nanoparticles could gradually enlarge. As time was irradiated for 1.5 h, BiVO 4 samples had the uniform particle size and the rough surface. No other impurities could be detected. This also indicated that the ultrasonic time did not influence the crystal structure. According to our previous study, surfactant would affect the formation of various morphological structures [15]. Therefore, we used three types of surfactants to control the particle size of BiVO 4 , such as CIT (chelating agent), CTAB (cationic surfactant), and PEG (non-ionic surfactant). Figure 1f-h show FE-SEM images and particle size distribution of BiVO 4 with various surfactants. The particle size distribution (D 50 ) of BiVO 4 with surfactants significantly decreased from 11.971 µm to 0.330 µm, which could be attributed to better dispersibility by adding the surfactants with long molecular chain and hydrophilic end during the synthetic process [16][17][18]. Therefore, the smaller particle size could be formed than that of pristine BiVO 4 .
The BiVO 4 photocatalyst was further modified by g-C 3 N 4 and copper in order to promote the photocatalytic activity. The results were confirmed by HR-TEM measurement on the 10Cu/BiVO 4 /g-C 3 N 4 powders ( Figure 2). The corresponding selected area electron diffraction (SAED) pattern indicated rings; the as-synthesized BiVO 4 nanocomposite photocatalysts was the polycrystalline structure ( Figure 2b). Furthermore, the element of Bi, V, O, C, N, and Cu substantially existed in the sample from the EDX image and TEM-EDX element mapping (Figure 2c-i). The bandgap of g-C 3 N 4 and copper-modified BiVO 4 slightly reduced from 2.47 eV to 2.28 eV (Figure 3a). The PL spectra of the BiVO 4 samples are exhibited in Figure 3b. BiVO 4 photocatalyst displayed two peaks at 530.7 nm and 357.7 nm [19]. The g-C 3 N 4 had its characteristic peak around at 450.4 nm [20]. In addition, the pristine BiVO 4 sample had higher PL intensity than that of g-C 3 N 4 and copper-modified BiVO 4 samples. In other words, the e − /h + recombination rate could be efficiently restrained in the g-C 3 N 4 and Cu-modified BiVO 4 samples. In the high concentration of copper (> 10 mol%), the PL peak of BiVO 4 nanocomposite photocatalysts shifted from 530.7 nm to 492.7-469.4 nm due to the fabrication of the tetragonal-scheelite structure BiVO 4 , as shown in Figure 4. Therefore, the excessive doping of copper (> 10 mol%) could not synthesize the pure monoclinic scheelite phase, which might result in the reduction of the photocatalytic activity.

Photocatalytic Degradation Activity
The photocatalytic degradation activities of the BiVO 4 nanocomposite photocatalysts were performed for the degradation of wastewater pollutants containing BPA under simulated sunlight irradiation. Photocatalytic parameters of degrading BPA were investigated as below, including the pH values of BPA solution, the dosages of BiVO 4 nanocomposite photocatalyst, and the content of copper.   4,7,9,10, and 12, respectively. With the increase of the pH value to 10, the BPA could be completely removed to 100% for 120 min. This result could be illustrated by the net signs of the surface charge of the BPA species and BiVO 4 nanocomposite photocatalysts at different pH values. Because the dissociation constants (pK a1 and pK a2 ) of BPA are 9.6 and 10.2 [21][22][23], the surface charge of the BPA species is negatively charged under alkaline conditions. In addition, the pH changes also could affect the surface properties of 10Cu/BiVO 4 /g-C 3 N 4 photocatalyst. Meanwhile, we measured the point of zero charge of 10Cu/BiVO 4 /g-C 3 N 4 photocatalyst (pH pzc = 11.3). Therefore, BPA molecules were adsorbed onto the surface of 10Cu/BiVO 4 /g-C 3 N 4 photocatalyst via electrostatic interaction (pH smaller than 11.3), which is an important step in the process of photocatalytic degradation. Conversely, BPA was adsorbed weakly onto the negatively charged 10Cu/BiVO 4 /g-C 3 N 4 surfaces due to the coulombic repulsion (pH greater than 11.3). Based on the above results, the best pH value of the BPA solution for the photocatalytic degradation of BPA was at 10.

Photocatalyst Dosage
The photocatalytic efficiency by using metal oxides can be determined by their physical and chemical properties. Physical properties, such as pore size, surface area, and surface charge, could affect the photocatalytic activity [24]. Therefore, the function of the photocatalysts is able to provide effective surface area and play the role of the active center. The effect of photocatalyst dosage on the degradation efficiency was evaluated by various amounts of 10Cu/BiVO 4 /g-C 3 N 4 from 0.2 g to 0.6 g in a 1000 mL BPA solution. The result of the photo-degradation performance has been depicted in Figure 5c, which exhibits that the rate of degradation increased linearly with an increase in the amount of photocatalyst up to 0.4 g and then decreased. The presence of 10Cu/BiVO 4 /g-C 3 N 4 photocatalyst could provide more active centers, which led to increased photocatalytic activity. As the amount of photocatalyst increased, the number of BPA molecules adsorbed were increased, leading to an increase in BPA degradation. At higher concentrations of photocatalyst, more surface areas were available for constant BPA molecules. However, more photocatalyst would also induce greater aggregation of the photocatalyst, and the specific surface area decreased, leading to a reduction in the reaction rate. In addition, the inactivation of activated molecules by collision with ground-state molecules might also hinder the photocatalytic efficiency. Hence, above a certain level, additional catalyst amounts were not involved in catalysis reaction, and thus the rate might level off. Hence, the appropriate photocatalyst dosage showed an outstanding performance of photocatalytic degradation.

Cu Doping Content
In view of the above results, we conducted the experiments of the photocatalytic degradation of BPA by BiVO 4 nanocomposite photocatalyst equal to 0.4 g L −1 at pH 10. Figure 6a exhibits the photocatalytic activity of BiVO 4 nanocomposite photocatalysts with various Cu contents. It could be seen that the Cu content in the BiVO 4 presented an excellent influence on the photocatalytic activity of BiVO 4 nanocomposite materials. The conduction band (CB) edge position and valence band (VB) edge position of bare BiVO 4 were about 0.75 eV and 3.22 eV, respectively, leading to a weak reduction ability, as shown in Figure 6b. The 10 mol% Cu-doped BiVO 4 photocatalyst could not only shift the CB edge position more negative than the others but also decrease the intensity of PL emission (Figure 3b). Therefore, 10Cu/BiVO 4 /g-C 3 N 4 photocatalyst had the highest activity (k = 0.0426 min −1 ) for photocatalytic degradation. Moreover, the addition of excess Cu contents (> 10 mol%) to synthesize BiVO 4 nanocomposite photocatalysts resulted in a reduction in the photocatalytic activity because BiVO 4 formed the tetragonal-scheelite phase, as shown in Figure 4. According to the references, we know that the crystal structure of monoclinic type BiVO 4 exhibits excellent photocatalytic properties [2][3][4]. In addition, copper-doped photocatalyst can fabricate the trap energy levels (TEL), and the activated electrons could transfer to TEL Cu , which leads to the useful e − /h + separation [15,16,[25][26][27]. Therefore, the appropriate addition of Cu contents displayed an excellent photocatalytic degradation performance. Furthermore, the 10Cu/BiVO 4 /g-C 3 N 4 was assigned to the type IV isotherm, displaying with a type H3 hysteresis loop, which indicated the presence of mesopores (2-50 nm), as shown in Figure 7. The BiVO 4 /g-C 3 N 4 nanocomposite materials with various amounts of copper addition are compared in Table 1, revealing a slight decrease in the surface areas with copper doping. However, the slight loss of specific surface area did not compete with the enhanced activity of doping copper as reaction centers for photocatalytic degradation of BPA.

Conclusions
BiVO 4 nanocomposite photocatalysts were prepared using the sonochemical approach. Cu-doped BiVO 4 /g-C 3 N 4 nanocomposite photocatalysts exhibited a remarkable improvement in degradation performance. The pH values of BPA solution, the dosages of 10Cu/BiVO 4 /g-C 3 N 4 , and the various types of BiVO 4 nanocomposite material had a great influence on the removal efficiency of BPA. The optimal parameters, such as pH value at 10, photocatalyst dosage of 0.4 g L −1 , and 10 mol% Cu-doped BiVO 4 /g-C 3 N 4 photocatalyst, were determined to degrade the initial concentration of 20 ppm bisphenol A, which could be completely removed after 90 min.

Conflicts of Interest:
The authors declare no conflict of interest.