Preparation of a Novel Bi₂MoO₆/Ag/Ag₂CrO₄ Catalyst with Promoted Visible Light Photodegradation of RhB Dye

Rui Wang ^1,†, Dong Lin ^1,†, Minghuan Gao ¹, Linna Guo ^1,*, Tiesheng Li ^1,*, Minghua Liu ^2,3,*

1. College of Chemistry, Zhengzhou University, 75 North University Road, Zhengzhou, P. R. China

2. Henan Institute of Advanced Technology, Zhengzhou University, Zhengzhou, Henan Province, P.R. China

3. Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun North First Street 2, Beijing, P. R. China

† These authors contributed equally to this work.

* Correspondences: Linna Guo, Tiesheng Li and Minghua Liu

Academic Editor: Md Ariful Ahsan

Special Issue: Applications of Environmental Catalysis

Received: January 29, 2022 | Accepted: March 28, 2022 | Published: April 07, 2022

Catalysis Research 2022, Volume 2, Issue 2, doi:10.21926/cr.2202008

Recommended citation: Wang R, Lin D, Gao MH, Guo LN, Li TS, Liu MH. Preparation of a Novel Bi₂MoO₆/Ag/Ag₂CrO₄ Catalyst with Promoted Visible Light Photodegradation of RhB Dye. Catalysis Research 2022;2(2):22; doi:10.21926/cr.2202008.

© 2022 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

With the development of several industries, environmental pollution has attracted considerable attention. Untreated industrial wastewater contains numerous organic pollutants that are discharged into the river, thus posing a severe threat to the ecological environment [1,2,3,4]. So far, these organic pollutants are degraded by adsorption degradation, photodegradation, and biological method [5,6,7,8,9,10]. Photodegradation has the unique advantage of high efficiency, and visible light photocatalyst has emerged as the mainstream of research due to its effectiveness in using visible light [11,12,13,14,15].

Bi₂MoO₆, having a low bandgap (2.4-2.8 eV), has gradually attracted the attention of researchers because of its effective utilization of visible light [16,17]. Zhang et al. obtained a series of Bi₂MoO₆ with different morphologies by changing the pH in a hydrothermal manner [18]. Photocatalytic property of degradation of dyes by Bi₂MoO₆ with other pH values of the solution was investigated，that revealed different degradation efficiencies. Nevertheless, Bi₂MoO₆ is still confronted with low quantum yield and low photocatalytic activity under visible light irradiation [19,20,21,22,23,24].

Researchers have tried to explore several methods to enhance photocatalytic activity, such as by supporting precious metal nanoparticles and constructing heterojunctions [25,26,27,28,29]. The heterojunction can effectively inhibit the recombination of electrons and holes and enhance photocatalytic activity [30,31,32,33,34]. For example, Li et al. successfully designed and prepared heterojunctions, which exhibited a high photocatalytic ability to degrade RhB under solar light irradiation [35,36,37,38,39,40].

Silver-based photocatalysts have been extensively investigated [41,42,43,44,45,46,47]. Ag₂CrO₄ is an ideal semiconductor with an appropriate bandgap (1.75 eV) and better visible light photocatalytic activity [48]. Ouyang et al. [49] prepared AgAlO₂, AgCrO₂, and Ag₂CrO₄ using the cation exchange method. The photo-oxidation of MO and gaseous benzene catalyzed by AgAlO₂, AgCrO₂, and Ag₂CrO₄ was investigated. The activity order of the two reactions is Ag₂CrO₄ > AgAlO₂ > AgCrO₂. Moreover, heterojunctions composed of Bi₂MoO₆/silver-based photocatalysts used for photodegrading dyes were reported [50,51,52]. However, Bi₂MoO₆/Ag/Ag₂CrO₄ having a heterostructure as a photocatalyst has not been studied.

Herein, a series of new Bi₂MoO₆/Ag/Ag₂CrO₄ photocatalysts were prepared, and their photodegradation properties for degrading RhB under visible light irradiation were systematically investigated.

2. Materials and Methods

2.1 Materials

Ethylene glycol (EG) was commercially purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Silver nitrate (AgNO₃), bismuth nitrate pentahydrate (Bi₂(NO₃)₃∙5H₂O), sodium molybdate dihydrate (Na₂MoO₄∙2H₂O), potassium chromate (K₂CrO₄), polyethylene glycol (PEG), and RhB were commercially purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). All chemicals (analytical grade) were purchased from a commercial market and used as received without any further purification.

2.2 Synthesis of Bi₂MoO₆

In total, 0.97 g of Bi₂(NO₃)₃∙5H₂O and 0.24 g of Na₂MoO₄∙2H₂O were dissolved in 5 mL of EG, separately and mixed. Next, 20 mL of ethanol was added to the solutions under magnetic stirring until they were dissolved completely. Ammonia was added drop-by-drop to the solution, and the pH was adjusted to 3, 5, 7, and 9. Afterward, the solutions were placed into a 50 mL Teflon-lined stainless autoclave. The autoclave was sealed and maintained at 160 °C for 20 h. After cooling to room temperature, the products were washed with DI and ethanol thrice. Finally, faint yellow Bi₂MoO₆ powders obtained at different pH values of 3, 5, 7, and 9 were dried in a 60 °C vacuum oven for 12 h [18,32,53,54,55,56].

2.3 Preparation of Bi₂MoO₆/Ag/Ag₂CrO₄

In total, 0.10 g (0.5 mmol) of K₂CrO₄ was dissolved in 20 mL of DI until it was dissolved completely. Next, 2 g of PEG 4000 was added to the solution under magnetic stirring until it was dissolved completely. A certain mass proportion of Bi₂MoO₆ 1%, 3%, 5%, 10% (1.66 mg, 5.0 mg, 8.3 mg, 16.6 mg, respectively) was added and subjected to ultrasound for 30 min to evenly disperse the mixture. Afterward, 0.17 g (1 mmol) of AgNO₃ was added to the dispersion of Bi₂MoO₆ and stirred in the dark for 4 h. Finally, Bi₂MoO₆/Ag/Ag₂CrO₄ composite materials were obtained by washing with deionized water and ethanol thrice and dried in a 60 °C vacuum oven for 12 h [57]. Bi₂MoO₆/Ag/Ag₂CrO₄ with different dosages of Bi₂MoO₆ were labeled as BA-1, BA-3, BA-5, and BA-10. Calcination of BA-5 and BA-10 was performed at 550 °C for 2 h in a muffle furnace [58].

2.4 Characterizations

The microstructure and elemental composition of Bi₂MoO₆, Ag₂CrO₄, and the BA-X series (ratio: X = 1, 3, 5, 10) were investigated by scanning electron microscopy (SEM ; Hitachi S-4700) and energy-dispersive X-ray spectroscopy (EDX Bruker). Diffuse reflectance spectroscopy (DRS) in the region of 300-800 nm was performed using a PerkinElmer Lambda 950 spectrometer, whereas BaSO₄ was selected for background measurements. X-ray diffraction (XRD) was performed on a Bruker D8 VENTURE diffractometer (Bruker, Germany) using Cu-Ka radiation. Photoluminescence (PL) spectra were obtained by an F-4600 fluorescence spectrophotometer with an excitation wavelength of 460 nm. The photoelectron chemical measurements were performed with an electrochemical system (CHI-660D) at room temperature. In these systems, a 1 cm² Pt sheet and a saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. Indium-Tin Oxide(ITO) glass electrodes were seared by as-prepared samples (Bi₂MoO₆, Ag₂CrO_4, and BA-5) as the working electrode.

2.5 Photocatalytic Test

The photocatalytic activity of the prepared catalyst was assessed by degrading RhB (10 ppm) under the irradiation of a 350 W Xenon lamp. First, 5 mg of RhB was weighed and dispersed in 500 mL of deionized water to obtain the RhB aqueous solution. Next, 30 mg of the photocatalyst was weighed and evenly dispersed in 30 mL of the RhB solution and stirred in darkness for 1 h to achieve the dynamic equilibrium of adsorption and desorption. Afterward, it was stirred under the Xenon lamp irradiation for 1 h, and 3 mL of the solution was absorbed every 10 min. The extracted solution was centrifuged in a high-speed centrifuge, and the supernatant was absorbed as a sample for subsequent testing. Finally, the concentration of RhB in the supernatant was determined by ultravioletvisible spectrophotometry. The experimental process of o-chlorophenol and p-chlorophenol solution photodegradation was similar to that of RhB degradation, such that the pre-configured 10 ppm o-chlorophenol, p-chlorophenol, MO, MB, and tetracycline solution were degraded under a Xenon lamp. Finally, the concentration of pollutants in the supernatant was determined by high-performance liquid chromatography.

2.6 Recycle Experiment

The photocatalyst was collected from the remaining solution after the reaction was over by centrifuging and washed with water several times. The photocatalyst treated above was added to 30 mL of RhB (10 ppm solution) again. Recycling experiments were repeated until the photocatalyst lost its degradation ability.

3. Results and Discussion

3.1 Characterization of Photocatalysts

3.1.1 Characterization of Bi₂MoO₆ Prepared with Different pH Values

The XRD patterns of Bi₂MoO₆ synthesized at different pH values were measured, as shown in Figure 1. The peaks at 28.2°, 32.5°, 47.1°, 56.2°, 58.4°, 68.1°, 76.0,° and 77.6° belonged to (131), (200), (062), (191), (262), (400), (391), and (402) facets, respectively, of Bi₂MoO₆ (JCPDS No. 76-2388) [59]. No new peaks were detected, indicating that Bi₂MoO₆ had a higher purity. Moreover, the position of diffraction peaks did not change as compared with Bi₂MoO₆ prepared at different pH values, indicating that the change in pH values did not affect the crystal structure of Bi₂MoO₆.

Figure 1 XRD patterns of Bi₂MoO₆ prepared at different pH values (3, 5, 7, and 9).

Morphologies of Bi₂MoO₆ synthesized at various pH (3, 5, 7, and 9) values were measured by SEM, as depicted in Figure 2. Spherical images of Bi₂MoO₆ with an average diameter of about 2 µm were observed (Figure 2a, pH = 3). Several thin sheets on the spherical surface were slowly dispersed, and the spherical morphology was slowly destroyed with an increase in the pH value (Figure 2b, c; pH = 5,7). Finally, the spherical structure disappeared completely, and the flakes with a width of 300 nm and a thickness of 80 nm appeared (Figure 2d; pH = 9). The results obtained above indicated that the morphology of Bi₂MoO₆ could be controlled by changing the pH.

Figure 2 SEM images of Bi₂MoO₆ prepared with different pH values. (a) 3, (b) 5, (c) 7, and (d) 9.

3.2 Characterization of Bi₂MoO₆/Ag/Ag₂CrO₄ (BA-X, X = 1, 3, 5 and 10)

The XRD patterns of Bi₂MoO₆, Ag₂CrO_4, and Bi₂MoO₆/Ag/Ag₂CrO₄ were measured, as shown in Figure 3. Peaks at 31.0°, 31.1°, 31.4°, 32.3°, 33.7°, 35.1°, 39.3°, 44.3°, 45.4°, 48.0°, 52.1°, 56.4°, 57.1°, 62.6°, 65.0,° and 67.6° were determined as (220), (031), (211), (002), (131), (221), (122), (240), (222), (051), (400), (160), (213), (402), (062), and (004) facets, respectively, for Ag₂CrO₄ (PDF #26-0952) [57]. The XRD patterns of Bi₂MoO₆/Ag/Ag₂CrO₄ composites were the same as that of Ag₂CrO₄ because of the majority of Ag₂CrO₄ in these composites. However, the intensity of diffraction peaks of the (131) facets at 2θ of 28.2° belonging to Bi₂MoO₆ increased with an increase in the amount of Bi₂MoO₆ doped, suggesting the formation of heterojunction photocatalysts.

Figure 3 XRD patterns of as-prepared Bi₂MoO₆, Ag₂CrO₄, BA-1, BA-3, BA-5, and BA-10.

The SEM images of Ag₂CrO₄ and BA-X were measured (Figure 4). The sheets of Bi₂MoO₆ and blocks having a plane length of 200 to 400 nm with a thickness of about 200 nm for Ag₂CrO₄ were observed (Figure 4a, b). The SEM images of BA-1, BA-3, BA-5, and BA-10 are shown in Figure 4c-f. The images of BA-X revealed the bulk morphology of Ag₂CrO₄ and the sheets of Bi₂MoO₆, indicating the formation of heterojunction materials. In addition, an increase in Bi₂MoO₆ resulted in the attachment of more number of Bi₂MoO₆ sheets to the surface of Ag₂CrO₄, which was consistent with our design.

Figure 4 SEM images. (a) Bi₂MoO₆ (pH = 9), (b) Ag₂CrO₄, (c) BA-1, (d) BA-3, (e) BA-5, and (f) BA-10.

The TEM and high-magnification HR-TEM characterization images of Bi₂MoO₆ (pH = 9), Ag₂CrO_4, and BA-5 are shown in Figure 5. It could be seen that Bi₂MoO₆ has a sheet-like structure of about 100 nm (Figure 5a). Its crystal spacing was about d = 0.65 nm in the HR-TEM image, as seen in the upper right corner of the figure. The spherical structure Ag₂CrO₄ has numerous black silver particles fixed on its surface (Figure 5b), and the crystal spacing of Ag₂CrO₄ was about d = 0.350 nm, its crystal plane belonging to (200). The lattice spacing of metal Ag particles was about d = 0.231 nm, and it belonged to the (111) crystal plane. Figure 5c shows the microscopic structure of the composite sample; the microstructure of Bi₂MoO₆ and Ag₂CrO₄, and the HR-TEM image in the upper right corner show that the crystal spacing d = 0.314 nm, d = 0.320 nm, and d = 0.304 nm belonged to the Bi₂MoO₆ (131) crystal plane, d = 0.268 nm and d = 0.249 nm belonged to the Ag₂CrO₄ (002), (200) crystal plane, and d = 0.198 nm belonged to the (111) crystal plane of metal Ag particles. The above results demonstrate the formation of a heterojunction between Bi₂MoO₆ and Ag/Ag₂CrO₄, and the formation of a homojunction between metal Ag and Ag₂CrO₄, indicating that the composite material was successfully prepared.

Figure 5 TEM images. (a) Bi₂MoO₆ (pH = 9), (b) Ag₂CrO₄, and (c) BA-5.

Nitrogen adsorption--desorption isotherms of Bi₂MoO₆, Ag₂CrO₄, and BA-5 were measured (Figure 6a), and the relative pore size distributions (RPSD) were obtained (Figure 6b). The pristine Bi₂MoO₆ yielded a type IV isotherm having an H2 hysteresis loop, indicating the mesoporous nature of the material. Ag₂CrO₄ exhibited a type III isotherm having an approximate H3 hysteresis loop, indicating a narrow gap. BA-5 showed a type III shape and H3 hysteresis loop [35,60]. The results showed that the addition of even a small amount of Bi₂MoO₆ maintained the primary structure of BA-5. The specific surface areas for Bi₂MoO₆, Ag₂CrO₄, and BA-5 were 22.74 m² g^-1, 3.60 m² g,^-1, and 4.88 m² g^-1, respectively (Figure 6b). BA-5 had a larger specific surface area compared with Ag₂CrO₄, indicating that a small amount of Bi₂MoO₆ doped led to more active centers in its heterostructure, which could further enhance its photocatalytic activity. The RPSD of Bi₂MoO₆, Ag₂CrO_4, and BA-5 were 21.52 nm, 9.65 nm, and 11.66 nm for Bi₂MoO₆, Ag₂CrO_4, and BA-5, indicating the mesoporous nature of materials (Figure 6b).

Figure 6 (a) N₂ adsorption--desorption isotherms and (b) the relative pore size distribution of Bi₂MoO₆, Ag₂CrO_4, and BA-5.

X-ray photoelectron spectroscopy (XPS) could be used to investigate the element composition and valence [35]. The XPS survey spectra of BA-5, Bi₂MoO₆, and Ag₂CrO₄ were measured, as shown in Figure 7. High-resolution XPS spectra of Ag 3d, Cr 2p, Bi 4f, and Mo 3d are shown in Figure 8. Ag, Cr, Bi, Mo, and O elements in BA-5 could be observed in the survey (Figure 7). The BE peaks at 367.62 eV and 373.60 eV were denoted to Ag 3d_5/2 and Ag 3d_3/2, indicating the existence of metallic Ag and Ag⁺ in high-resolution XPS (Figure 8a). The BE peaks at 578.76 eV and 588.19 eV were denoted to Cr 2P_3/2 and Cr 2p_1/2 in Figure 8b, indicating that the valence state was +6. The BE peaks at 158.98 eV and 164.29 eV were classified as Bi 4F_7/2 and Bi 4F_5/2, and their valence states were Bi³⁺ (Figure 8c). Figure 8d showed that the characteristic BE peaks at 232.26 eV and 235.51 eV were 3d_3/2 and 3d_5/2 of Mo having +6 valence.

Figure 7 XPS survey spectra of the BA-5, Bi₂MoO₆, and Ag₂CrO₄.

Figure 8 High-resolution XPS spectra of (a) Ag 3d, (b) Cr 2p, (c) Bi 4f, and (d) Mo 3d.

The optical absorption is related to the electronic structure of the semiconductor [28,32]. UV spectra of BA-X (X = 1, 3, 5, 10), Bi₂MoO₆, and Ag₂CrO₄. were assessed to determine the light absorption range. As shown in Figure 9a, a weak absorption in the visible region for Bi₂MoO₆ was observed. Ag₂CrO₄ had a stronger absorption ranging from 500 to 680 nm. However, the light absorption range of BA-5 was similar to that of Ag₂CrO₄, which was caused by doping Bi₂MoO₆. The bandgap widths for Bi₂MoO₆ and Ag₂CrO₄ were calculated from Figure 9b, in which the Eg values were 2.78 eV and 1.80 eV for Bi₂MoO₆ and Ag₂CrO₄, respectively.

Figure 9 (a) UV diffuse reflectance spectra of pristine Bi₂MoO₆, Ag₂CrO₄, BA-1, BA-3, BA-5, and BA-10. (b) Eg of pristine Bi₂MoO₆ and Ag₂CrO₄.

To study the electron--hole conversion process of Bi₂MoO₆ and Ag₂CrO₄, the CB of the material was calculated using the equation, E_CB = X−E^θ−0.5Eg (Eg: the forbidden bandwidth, E^θ: the energy of free electrons on the hydrogen scale [4.5 eV], and X: the electronegativity). The X values were 5.55 and 5.86 for Bi₂MoO₆ and Ag₂CrO₄, as reported previously [61,62,63]. The edge of the VB (E_VB) could be obtained from the E_VB = E_CB + Eg equation. The calculated E_CB values of Bi₂MoO₆ and Ag₂CrO₄ were −0.34 eV and 0.46 V, and E_VB values were 2.44 eV and 2.26 eV.

3.3 Investigation of Photocatalytic Properties

To obtain Bi₂MoO₆/Ag/Ag₂CrO₄ with excellent photocatalytic performance, Bi₂MoO₆ prepared at different pH values was selected to degrade RhB (10 PPM solution) under Xenon lamp irradiation. First, considering the effect of adsorption, the solution of RhB was mixed with Bi₂MoO₆ and stirred for 1 h in the dark to reach absorption-desorption equivalence. In contrast, when the prepared Bi₂MoO₆ pH was 9, the degradation capacity of RhB was better (Figure 10, green line), and the degradation rate reached 22% after irradiation for 1 h. According to the SEM image in Figure 2, the pH value could affect photocatalytic performance by influencing the morphology of Bi₂MoO₆ [18]. When pH = 9, the particle size of Bi₂MoO₆ was small and had more sufficient contact with pollutants. Therefore, Bi₂MoO₆ prepared (at pH = 9) was selected in the following experiment.

Figure 10 Photocatalytic activity of Bi₂MoO₆ prepared at pH = 3 (blue), 5 (red), 7 (black) and 9 (green) used to degrade RhB (10 ppm solution) under 350 W Xenon lamp irradiation.

Similarly, the photocatalytic properties of BA-5 (black, blue) and BA-10 (red, green) with and without calcination were assessed by desorption of RhB (10 ppm solution). Considering the adsorption of pollutants by the catalyst, RhB and BA-5 or BA-10 solutions were stirred for 1 h in the dark to reach the adsorption--desorption equilibrium. As shown in Figure 11, the degradation rates of BA-5 and BA-10 before and after calcination were almost the same, which could be because calcination did not change the crystal structure of as-prepared BA-5 and BA-10 (Figure 12).

Figure 11 Photocatalytic activity of BA-5 and BA-10 prepared before and after calcination at 550 °C for 2 h to degrade RhB (10 ppm solution) under Xenon lamp irradiation.

Figure 12 XRD spectra of Bi₂MoO₆/Ag/Ag₂CrO₄ samples prepared before and after calcination at 550 °C for 2 h.

Photocatalytic properties of BA-1, BA-3, BA-5, and BA-10 were investigated by degrading RhB (Figure 13a). The prepared BA-X (X: 1, 3, 5, 10) had more vital photocatalytic degradation ability than Bi₂MoO₆ (pH = 9) and Ag₂CrO₄. BA-5 and BA-10 had a high photocatalytic property, and their degradation rate of RhB was up to 99.9% in 60 min, whereas the degradation rates of pure Bi₂MoO₆ (pH = 9) and Ag₂CrO₄ were only 22.0%, and 53.8%, respectively. Figure 13b shows the linear relationship between −ln(C/C0) and t, the degradation curve of RhB was consistent with the pseudo-first-order kinetic equation [64,65]. The degradation rate constant k_ap of BA-5 was 0.07857 min⁻¹, which was 24.63 times of pure Bi₂MoO₆ and 6.44 times of pure Ag₂CrO₄. It indicated that the photocatalytic performance of Bi₂MoO₆/Ag/Ag₂CrO₄ was considerably higher than that of the pure Bi₂MoO₆ or Ag₂CrO₄ due to the formation of a heterojunction structure.

Figure 13 (a) Photocatalytic activity of Bi₂MoO₆ prepared at pH = 9, Ag₂CrO_4, and BA-X (X = 1, 3, 5, 10) to degrade RhB (10 ppm solution) under Xenon lamp irradiation. (b) Pseudo first-order kinetic constants of RhB photodegradation.

The photocatalytic performance of BA-5 for decomposing phenol was also investigated under Xe lamp irradiation. The peak intensities of o-chlorophenol and p-chlorophenol decreased rapidly when irradiated for 1 h, indicating that BA-5 also had a better degrading efficiency for phenol (Figure 14). In addition to the photodegradation of RHB, o-chlorophenol, and p-chlorophenol, we tried to degrade MB and MO, and the degradation efficiencies were 82.34% and 61.02% in 60 min, respectively. Unfortunately, BA-5 showed a low degradation activity for tetracycline under the same condition.

Figure 14 Photocatalytic activities of BA-5 were characterized under Xenon lamp irradiation at different times by high-performance liquid chromatography (HPLC): (a) o-chlorophenol (10 ppm solution) and (b) p-chlorophenol (10 ppm solution).

3.4 Photocatalysis Stability

Photostability is one of the important indexes for a catalyst. The recycling capacity and photostability of BA-5 were systematically studied. The degradation rate of BA-5 for RhB was up to 99.9% in the first recycle, 87.6% in the second cycle, 74.5% in the third cycle, and 60.2% in the fourth cycle. The degradation rate of BA-5 for RhB was still up to 51.3% in the fifth recycling (Figure 15), indicating that BA-5 exhibited a better recycling performance in the photocatalytic process.

Figure 15 Recycling stability of BA-5 for degrading RhB irradiated by Xenon lamp.

The photocatalytic activities of Bi₂MoO₆/Ag/Ag₂CrO₄ compared with other similar photocatalysts reported are listed in Table 1.

Table 1 Comparison with similar photocatalysts for degrading RhB.

To study the deactivation reason for the degradation of performance, XRD characterization of BA-5 was measured before and after recycling. The position and intensity of diffraction peaks for 0 min, 10 min, 20 min, 30 min, and 60 min did not change significantly, indicating that the crystal structure of BA-5 did not change during the degradation process (Figure 16).

Figure 16 XRD patterns of BA-5 during the first catalytic process.

XRD tests were performed for five catalytic cycles of BA-5. The position and intensity of diffraction peaks of the first and second times did not change significantly. The intensity of diffraction peaks of 31.0°, 31.1°, and 31.4° gradually weakened after the third cycle, indicating that the crystal structure of BA-5 changed with the increase in the number of cycles (Figure 17).

Figure 17 XRD patterns of BA-5 in five catalytic processes.

To observe the morphological changes of samples during the degradation process, SEM images of BA-5 during the process were characterized (Figure 18). Morphologies of BA-5 did not change significantly during degradation, indicating that BA-5 had excellent stability. Compared with Figure 18a and Figure 18i, the morphology of BA-5 changed considerably in the fifth cycle. Therefore, the more significant change in morphology was one of the reasons for the decreased activity.

Figure 18 SEM image of BA-5 during catalytic process under Xenon lamp irradiation. (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 60 min, (f) SEM image of BA-5 after 2 cycles, (g) after 3 cycles, (h) after 4 cycles, and (i) after 5 cycles.

3.5 Investigation of the Photocatalytic Mechanism

3.5.1 Catalytic Active Species in Bi₂MoO₆/Ag/Ag₂CrO₄

Various scavengers, such as isopropyl alcohol (IPA), benzoquinone (BQ), and triethanolamine (TEOA), were applied to the photodegradation of RhB to elucidate what reactive species were the key influencing factors. As shown in Figure 19, the degradation was inhibited completely by adding TEOA, meaning that hole (h⁺) was the main influencing factor. In the case of BQ (•O₂^-) and IPA (•OH), less effect was observed in the photocatalytic progress, and the degradation rates were 66.8% and 77.0%. The results indicated that the hole (h⁺) played the most crucial role in the whole degradation process [66].

Figure 19 Capture experiments for BA-5 to photodegrade RhB under Xenon lamp irradiation.

3.5.2 Photoluminescence Spectra

The recombination rate of electron--hole pairs during the photodegradation process was explored with the PL spectra. The fluorescence spectra of Bi₂MoO₆, Ag₂CrO₄, and BA-5 were recorded (Figure 20), and the wide emission peak of pure Bi₂MoO₆was observed at around 465 nm, which was attributed to its rapid electron--hole recombination (Figure 20, black line). Pure Ag₂CrO₄ had a low recombination efficiency (Figure 20, red line) [66]. The emission peak of BA-5 was similar to that of pure Ag₂CrO₄ after the combination; however, the luminous position of BA-5 was weaker than that of Bi₂MoO₆ and Ag₂CrO₄, indicating that the combination of BA-5 had a low photogenerated carrier recombination efficiency (Figure 20, blue line) [66]. The result showed that BA-5 had lower recombination of the photoexcited electron-holes due to the formation of a heterojunction.

Figure 20 Photoluminescence emission spectra of Bi₂MoO₆ (black line), Ag₂CrO₄ (red line), and BA-5 (blue line).

The separation of electron--hole in the photocatalyst was studied by the photocurrent method. Generally, a higher photocatalytic degradation ability was associated with higher photocurrent property of the photocatalyst. The transient photocurrent response diagrams of Ag₂CrO₄, Bi₂MoO₆, and BA-5 under optical switching conditions were measured (Figure 21). BA-5 had a higher separation and transfer efficiency of photogenerated electron--hole pairs under visible light irradiation; the photocurrent response of the BA-5 composites increased significantly in comparison with that of pure Ag₂CrO₄ and Bi₂MoO₆, and hence the higher photocatalytic activity (Figure 21a, blue line). Electrochemical impedance experiments wereperformed on Ag₂CrO₄, Bi₂MoO₆, and BA-5 (Figure 21b), which showed the semicircular curves in the intermediate frequency region, in which BA-5 had the smallest arc radius. BA-5 had a high charge carrying rate and slow charge separation, showing that the heterojunction structure could effectively enhance the separation of charge carriers.

Figure 21 (a) Typical transient photocurrent responses diagrams of Ag₂CrO₄ (red), Bi₂MoO₆ (black), and BA-5 (blue) deposited on the ITO electrodes under intermittent UV-visible irradiation. (b) Typical electrochemical impedance diagrams of Ag₂CrO₄, Bi₂MoO₆, and BA-5 deposited on ITO electrodes.

3.5.3 Proposal Photodegradation Mechanism

The process of charge carrier transfer was proposed according to the valence states and CB positions of Ag₂CrO₄ and Bi₂MoO₆ (Figure 22). Bi₂MoO₆/Ag/Ag₂CrO₄ could be judged as a Type I in the coupling process. Under light irradiation, the photogenerated electrons in the conduction band (CB) of Bi₂MoO₆ were transferred to the conduction band (CB) of Ag₂CrO₄ due to the higher stabilization with the lower potential energy of Bi₂MoO₆. From the capture experiment (Figure 19), it was inferred that a small amount of •OH and •O₂^- were involved, whereas the VB edge potential (2.26 eV) and the CB edge potential (0.46 eV) of Ag₂CrO₄ were lower than •OH/H₂O (2.4 eV) and O₂/•O₂^- (-0.046 eV), respectively. Thus, the holes in the valence band (VB) of Ag₂CrO₄ could not oxidize H₂O to •OH, and the photogenerated electrons in CB could not reduce O₂ to •O₂^-. Although the potential at the edge of VB (2.44 eV) and CB (−0.34 eV) of Bi₂MoO₆ was higher than at •OH/H₂O (2.4 eV) and O₂/•O₂^- (−0.046 eV), the holes in the VB of Bi₂MoO₆ oxidized H₂O to •OH, and the photogenerated electrons in CB reduced O₂ to •O₂^-. Most importantly, the holes in the VB of Ag₂CrO₄ participated in the degradation of organic pollutants. Simultaneously, the surface plasmon resonance (SPR) effect of the metallic Ag facilitated electron transfer in the homojunction structure.

Figure 22 Plausible photocatalytic mechanism.

4. Conclusions

Several morphologies of Bi₂MoO₆ were prepared at different pH values using a hydrothermal method. Bi₂MoO₆ was prepared at pH 9 and had the best degradation effect on RhB, showing that the morphology could affect their photocatalytic performance. A series of Bi₂MoO₆/Ag/Ag₂CrO₄ photocatalysts were synthesized and characterized. The photocatalytic performance of Bi₂MoO₆/Ag/Ag₂CrO₄ used to degrade RhB under irradiation was investigated. Trapping agent experiments showed that holes played an essential role in the degradation of RhB, and the degradation mechanism was proposed as Type I. Furthermore, BA-5 had better stability and could be reused at least five times. With the increase in the number of cycles, the intensity of the diffraction peak corresponding to the XRD of the catalyst weakened, indicating a change in the crystal structure of the catalyst. The morphology of the catalyst had changed significantly through the SEM. Therefore, BA-5 had the best catalytic performance due to its excellent light absorption and high carrier separation efficiency. The enhanced performance could be ascribed to the Bi₂MoO₆/Ag/Ag₂CrO₄ structure, improved charge transfer efficiency, and suppressed photoelectron−hole recombination.

Author Contributions

Rui Wang was responsible for data analysis and manuscript writing; Dong Lin was in charge of instrument operation; Minghuan Gao was responsible for the experimental design; Linna Guo and Tiesheng Li were responsible for providing the overall experimental idea.

Funding

This work was supported by the National Natural Science Foundation of China (21861132002) and the Henan Natural Science Foundation of China (192102210046).

Competing Interests

The authors have declared that no competing interests exist.