Synthetic BiOBr/Bi₂S₃/CdS Crystalline Material and Its Degradation of Dye under Visible Light

Abstract

Constructing heterojunction has attracted widespread concerns in photocatalysis research. BiOBr/Bi₂S₃/CdS composite material with a sea urchin shape was directly obtained by first synthesizing BiOBr microspheres. The morphology, structure and composition of the composite material were characterized by XRD, EDX, SEM and XPS. Dye degradation experiments showed that 83.3% of methylene blue removal was achieved after 2 h of visible light irradiation. The reaction rate under optimal conditions was 0.014 min⁻¹ and the photocatalytic degradation process follows a pseudo-first-order kinetic model. Based on the EPR test results, the main active species involved in the reaction were •O₂⁻ and h⁺. The conduction band and valence band edge potential calculations confirmed the key role of CdS in the production of •O₂⁻.

1. Introduction

As an important part of colorants, dyes are widely used in cosmetics, food, paper, pharmaceuticals, plastics, textile and leather industries and consume a lot of water [1,2,3]. The discharge of organic pollutants such as dyes into freshwater resources often causes a series of serious health risks. The carcinogenicity, teratogenicity and reproductive toxicity of dye wastewater directly threaten aquatic organisms and cause serious water pollution [4,5]. Therefore, the dye concentration in wastewater must be reduced below the safety limit before being released into the environment. However, it is not easy to treat dye-containing wastewater. Due to their high light and thermal stability, dyes are difficult to be biodegraded in natural waters [6,7]. The stability of dyes against soaps, oxidants, detergents, biological and chemical reagents makes them difficult to mineralize [8]. Therefore, the treatment of dye wastewater has become a major problem in industrial wastewater treatment, and it is essential to develop more effective, green and economic dye wastewater treatment processes.

Photocatalytic semiconductor materials have received extensive attention due to their great potential in degrading organic pollutants [9,10,11]. As one of the best types of photocatalysts, Bi-based photocatalysts have shown wide application prospects in various fields including wastewater remediation. Among Bi-based photocatalysts, bismuth oxybromide (BiOBr) has become one of the most popular semiconductor materials for photocatalysis due to its good chemical durability and excellent photocatalytic activity under visible light irradiation [12,13]. BiOBr is a novel two-dimensional material with a layered crystal structure. [Bi₂O₂]²⁺ is sandwiched between halogen ions, and the static internal electric field (IEF) is generated by the electrostatic potential difference between these ions. The static IEF in BiOBr can effectively split and transfer electrons and holes produced by light [14,15]. Pure BiOBr limits its photocatalytic efficiency due to its wide band gap and rapid recombination of photo-generated carriers. Recently, some strategies to improve the photocatalytic activity of BiOBr have been implemented, especially by manufacturing BiOBr-based heterostructure composite materials [16,17,18]. These photocatalysts integrate the synergy between the components, which can enhance the visible light capture ability and photocatalytic activity of the material.

Bismuth sulfide (Bi₂S₃) is another Bi-based semiconductor. Because of its unique narrow band gap (about 1.4 eV) and photocarrier trapping effect, it was used in the photolysis of water to produce hydrogen, and solar energy [19]. At the same time, since the conduction band potential of Bi₂S₃ is more negative than that of BiOBr, the valence band potential of Bi₂S₃ is more positive than that of BiOBr. This suitable energy band structure between them is conducive to charge separation and transfer efficiency to enhance photocatalytic performance. However, because the conduction band edge potential of BiOBr/Bi₂S₃ is often insufficient to reduce O₂ to superoxide radicals, the efficiency of photocatalytic degradation of organics is reduced. Therefore, it is feasible to introduce CdS having a more negative conduction band edge potential (E_CB ≈ −0.52 eV) to form a ternary compound semiconductor material [20].

A sea urchin-shaped hollow BiOBr/Bi₂S₃/CdS composite material was prepared by directly synthesizing BiOBr microspheres in one pot. The research on the photocatalytic performance of the composite material confirmed the view that the ternary composite material has a better catalytic effect. Those results are of great significance to discuss the photodegradation mechanism of dyes in the presence of BiOBr/Bi₂S₃/CdS composites.

2. Experimental Section

2.1. Materials and Preparation of BiOBr/Bi₂S₃/CdS Composite

The description of the synthesis methods of BiOBr/Bi₂S₃/CdS composite material, the required raw materials, as well as the analytical techniques used for characterization of the materials can be found in the Supplementary Materials.

2.2. Photocatalytic Degradation Studies

In order to study the degradation performance of the synthesized catalyst on methylene blue (MeB), experiments proceeded under a 150 W visible light source halogen lamp, and the light source was 15 cm away from the reaction solution. In a reaction vessel, the prepared photocatalysts (50 mg) were dispersed in MeB solution (10 mg·L⁻¹, 50 mL) and maintained in darkness for 1 h to achieve the adsorption–desorption equilibrium. Then the light source was turned on to initiate the photocatalytic reaction and 3 mL samples were collected from different beakers at the same time intervals. Immediately after filtration, a UV/Vis spectrophotometer was used to record the change in absorbance. Each experiment was performed three times to ensure accuracy. The relationship between dye removal rate and absorbance is calculated by the following formula (Equation (1)):

$$Degradation\left(\%\right)=\frac{A_0-A}{A_0}\times 100\%=\frac{C_0-C}{C_0}\times 100\%$$

(1)

where A and A₀ are the absorbance of the dye solution at different times and the initial absorbance of the dye solution (mg·L⁻¹), respectively. C and C₀ are the concentration of the dye solution at different times and the initial concentration of the dye solution, respectively.

3. Results and Discussion

3.1. Characterization of BiOBr/Bi₂S₃/CdS Composite Material

XRD was used to explore the crystal phase and composition of the BiOBr/Bi₂S₃/CdS photocatalyst. Figure 1a shows the XRD patterns of pure Bi₂S₃, BiOBr, CdS and composite material BiOBr/Bi₂S₃/CdS. Among them, the XRD pattern shape of the Bi₂S₃ is consistent with that of its orthogonal phase [21]. The characteristic diffraction peaks at 2θ = 25.0°, 28.7°, 31.9° and 42.0° correspond to crystal planes (130), (230), (221) and (002). For pure CdS, the characteristic diffraction peaks are located at 24.8°, 26.5°, 28.6°, 43.7° and 52.1°, corresponding to the (100), (002), (101), (110) and (112) crystal planes of the hexagonal structure of CdS. The diffraction peaks of the synthesized pure BiOBr at 2θ values of 10.9°, 25.1°, 31.7°, 32.2°, 46.2°, 50.6°, 53.4° and 56.2° are consistent with the structure of tetragonal BiOBr. The characteristic peaks correspond to (001), (101), (102), (110), (200), (104), (211) and (114), respectively [22]. For the composite BiOBr/Bi₂S₃/CdS, its XRD pattern shows the key characteristic peaks of Bi₂S₃ and CdS, indicating that BiOBr is successfully partly vulcanized, while the exposed (001) and (110) crystal planes of the BiOBr microspheres themselves are not covered. The BiOBr/Bi₂S₃/CdS composite prepared using the TAA vulcanization method will not completely change the tetragonal crystal phase of BiOBr [23].

As depicted in Figure 1b, the absorption peak at 606 cm⁻¹ is assigned to the characteristic peak of CdS and Bi₂S₃. BiOBr, Bi₂S₃ and CdS samples displayed a similar spectrum and possessed the same peak at about 1400 cm⁻¹, 1630 cm⁻¹ and 3400 cm⁻¹. All the characteristic peaks above appeared in BiOBr/Bi₂S₃/CdS, indicating that part of BiOBr was successfully vulcanized into Bi₂S₃ and CdS was successfully loaded.

The SEM images of BiOBr, Bi₂S₃ and BiOBr/Bi₂S₃/CdS to determine the morphology of the composite are shown in Figure 2. From Figure 2a, the prepared pure BiOBr has a microsphere structure with an average diameter of 3.5 μm. The partially enlarged view shows that BiOBr microspheres are formed by stacking many BiOBr nanosheet structures. Figure 2b shows the pure Bi₂S₃ with a typical needle-like structure. Figure 2c,d clearly illustrate that the composite BiOBr/Bi₂S₃/CdS has a sea urchin-like structure. The red indicated area is mainly the microsphere structure retained by BiOBr, and some of the microspheres show a cavity structure due to more complete vulcanization. The needle-like structure grown on the microsphere structure is mainly composed of Bi₂S₃ formed by vulcanization and a small amount of CdS. In addition, the irregular surface shape of the microsphere structure is mainly due to the adhesion of CdS nanoparticles on parts of the BiOBr crystal surface [24]. Figure 3 shows the selected area EDS results of the composite BiOBr/Bi₂S₃/CdS, where the selected area is the one shown in Figure 2d. The content of the five elements Bi, Cd, Br, S and O in the composite material basically meets the expectations of the composite material, which further confirms the successful preparation of the material.

XPS was applied to further analyze the chemical state of the sea urchin-like BiOBr/Bi₂S₃/CdS material, and the results are presented in Figure 4. The full measurement spectrum in Figure 4a shows the strong characteristic peaks of Bi, Cd, O and Br and the weak characteristic peak of S, thus confirming the existence of Bi, Cd, O, Br and S in BiOBr/Bi₂S₃/CdS. The peak near the binding energy of 227.6 eV is attributed to S 2s, which indicates the presence of S²⁻ in the complex. The two strong peaks at 164.6 eV and 159.3 eV shown in Figure 4b can be attributed to Bi 4f_5/2 and Bi 4f_7/2 of Bi³⁺ in BiOBr/Bi₂S₃/CdS and the two peaks at 159.2 eV and 161.3 eV can be attributed to S 2p [25,26]. It was confirmed that the precursor BiOBr was sulfided to form Bi₂S₃, a trivalent sulfide of Bi. The strong peaks at the binding energy of 68.3 eV and 69.3 eV shown in Figure 4c belong to the Br 3d_5/2 and Br 3d_3/2 orbital, which is consistent with the XPS results of Br in the BiOBr report in the literature [27]. The Cd 3d orbital in Figure 4d shows the electronic binding energy of Cd 3d_5/2 at 405.05 eV and 411.7 eV. The characteristic peaks at these two positions have been proved to belong to CdS [20]. It shows that there is Cd²⁺ sulfide CdS in the composite material. The above XPS results verify that the composition and valence of each component of sea urchin-like BiOBr/Bi₂S₃/CdS are in line with expectations.

3.2. Photocatalytic Performance

The photocatalytic activity of the BiOBr/Bi₂S₃/CdS composite material was evaluated by degrading MeB solution under an indoor metal halide lamp. Figure 5 shows the experimental results of visible light catalytic removal of MeB in the presence of pure BiOBr, Bi₂S₃, CdS and BiOBr/Bi₂S₃/CdS complexes. MeB is usually relatively stable under the irradiation of metal halide lamps, so the self-photolysis effect is negligible. Dark treatment showed that pure BiOBr, Bi₂S₃, CdS and BiOBr/Bi₂S₃/CdS composite materials had a certain adsorption capacity, especially BiOBr spheres with a large specific surface area have a strong adsorption effect on MeB dye and almost reach equilibrium within 60 min. The efficient adsorption of BiOBr for dyes was similar to other researches [28]. After 120 min of subsequent exposure to the metal halide lamp, pure Bi₂S₃, CdS and BiOBr/Bi₂S₃/CdS all produced a certain degree of removal performance for MeB. Among them, the BiOBr/Bi₂S₃/CdS composite photocatalyst can achieve 83.3% degradation efficiency of MeB dye. This showed that BiOBr/Bi₂S₃/CdS composite material can efficiently remove MeB dye contaminants under visible light.

Subsequently, in order to evaluate the photocatalytic activity of the composite catalyst, the Langmuir–Hinshelwood (L-H) equation was used to fit the reaction kinetics. The results show that the photocatalytic reaction follows the following pseudo-first-order kinetic model (Equation (2)) [29]:

$$ln\frac{C_0}{C}=kt$$

(2)

where C₀ and C are the initial concentration of the dye solution and the concentration of the dye solution at different times, k and t are the rate constants of the system and the time. The result is shown in Figure 6. The degradation of MeB by each material follows a pseudo-first-order kinetic model, and BiOBr/Bi₂S₃/CdS has the largest reaction rate.

Table 1. Rate constants and correlation coefficients of MeB photocatalytic degradation by BiOBr, Bi₂S₃, CdS and BiOBr/Bi₂S₃/CdS.

Catalyst	k (min−1)	(R2)
BiOBr	0.00475	0.9916
Bi2S3	0.00526	0.9941
CdS	0.00656	0.9872
BiOBr/Bi2S3/CdS	0.01404	0.9987

contains the rate constants and correlation coefficients of the pseudo-first-order kinetics of each photocatalyst. It can be seen that compared to the original BiOBr, Bi₂S₃ and CdS photocatalysts, the reaction rate of the BiOBr/Bi₂S₃/CdS composite for MeB degradation has increased by 1.96, 1.67 and 1.14 times, respectively.

3.3. Cycling Experiments

Five continuous degradation cycles in presence of BiOBr/Bi₂S₃/CdS composite photocatalyst were studied. As shown in Figure 7, after the fifth cycle the degradation efficiency of MeB remained at about 78.4% and the catalytic efficiency of the catalyst decreased by only 4.9%. This indicates that the sea urchin-like BiOBr/Bi₂S₃/CdS composite has excellent stability under the MeB degradation.

3.4. Exploration of Photocatalytic Reaction Mechanism

The optical properties of pure BiOBr, Bi₂S₃, CdS and BiOBr/Bi₂S₃/CdS composite materials were verified by UV-Vis, and the results are shown in Figure 8. It can be seen that pure BiOBr has a small amount of absorption at the edge of the visible light region, while CdS can absorb light in a larger range than BiOBr. Bi₂S₃ is different from them, its absorption range covers the entire ultraviolet and visible light regions. Studies have shown that as the vulcanization duration increases, the absorption edge of the composite material formed by BiOBr gradually shifts to a longer wavelength, which indicates that the introduced Bi₂S₃ has a strong visible light response [30]. Therefore, the absorption edge of the BiOBr/Bi₂S₃/CdS composite shifts to higher wavelengths, and it also exhibits a certain intensity of absorption of visible light at higher wavelengths. This fully shows that the introduction of Bi₂S₃ and CdS increases the visible light utilization rate of the original BiOBr, thereby improving its photocatalytic efficiency. Subsequently, in order to more accurately understand the changes in the light absorption characteristics after recombination, the band gap energy E_g of each substance was calculated using the intercept method. The calculation formula of E_g is as follows (Equation (3)) [31]:

$$Eg=\frac{1240}{{\lambda }_g}$$

(3)

The band edge wavelength (λ_g) is determined by extrapolating the sharply rising part of the curve to the abscissa at zero absorption (dashed lines in Figure 8). The band gaps of pure BiOBr, Bi₂S₃, CdS and BiOBr/Bi₂S₃/CdS composites were estimated. The results showed that the band gap widths of BiOBr, Bi₂S₃, CdS and BiOBr/Bi₂S₃/CdS were 2.81 eV, 1.46 eV, 2.36 eV and 2.23 eV, respectively. This shows that the band gap energy of the composite decreases with the sulfidation of BiOBr and the addition of sulfide, which will help to enhance the photocatalytic performance of the composite.

Some reactive species, including holes (h⁺), photo-generated electrons (e⁻), superoxide radical anions (•O₂⁻) and hydroxyl radicals (•OH), have successfully separated the degradation of organic pollutants after generating photogenerated charge carriers on the surface of light and photocatalyst [32]. In order to reveal the reactive species in the process of dye degradation, trap-trap experiments were carried out with 1 mM tert-butyl alcohol (TBA), p-benzoquinone (p-BQ), ammonium oxalate (AO) and potassium bromate (PB). TBA, P-BQ, AO and PB were used to capture •OH, •O₂⁻, h⁺ and e⁻, respectively [33,34]. Figure 9 shows the changes in MeB degradation in the presence of various scavengers. After the introduction of p-BQ and AO, the two-hour removal rate dropped visibly. However, the effect of introducing TBA and PB on the removal of MeB by the catalyst is negligible (close to blank). These results indicate that the superoxide radical •O₂⁻ and h⁺ are the main substances produced by BiOBr/Bi₂S₃/CdS under visible light, and they subsequently degrade MeB. To verify the experimental results of free radical generation, EPR spin trapping was used [35]. The result is shown in Figure 10. Under visible light, the use of BiOBr/Bi₂S₃/CdS catalyst to degrade MeB was recorded with a strong DMPO-•O₂⁻ signal. This proves that in addition to the known h⁺, the composite catalyst can also produce •O₂⁻ as a reactive substance under visible light.

Based on the results of the active species studies discussed above, a possible photocatalytic degradation mechanism is proposed by determining the edge potential of the valence band (VB) and conduction band (CB) of BiOBr, Bi₂S₃ and CdS, and it is illustrated schematically in Figure 11. The VB and CB potentials of BiOBr, Bi₂S₃ and CdS can be calculated using the following formula (Equations (4) and (5)) [36]:

$$E_{VB}=X-E^e+0.5E_g$$

(4)

$$E_{CB}=E_{VB}-E_g$$

(5)

where E_VB and E_CB are the edge potentials of the valence band and conduction band, E^e is the energy of free electrons on the hydrogen scale (about 4.5 eV), and X is the semiconductor electronegativity of BiOBr, Bi₂S₃ and CdS, which are 6.18 eV, 5.35 eV and 5.16 eV, respectively. Using the band gap energies of BiOBr, Bi₂S₃ and CdS being 2.81 eV, 1.46 eV, and 2.36 eV, respectively, the VB and CB potentials of BiOBr were calculated to be 3.09 eV and 0.28 eV, the VB and CB potentials of Bi₂S₃ are 1.58 eV and 0.12 eV, and the VB and CB potentials of CdS were calculated to be 1.84 eV and −0.52 eV.

Since BiOBr is a p-type semiconductor, its Fermi level is close to its valence band, while CdS and Bi₂S₃ are n-type semiconductors, and its Fermi level is close to its conduction band. Therefore, the contact between the semiconductors and the balance of the Fermi level lead to the formation of p-n heterojunctions. This can be achieved by the upward shift of the Fermi level of BiOBr and the downward shift of the Fermi level of CdS and Bi₂S₃. The balance of the Fermi energy level causes the VB and CB of CdS and Bi₂S₃ to move positively, while the VB and CB of BiOBr move negatively. BiOBr, Bi₂S₃ and CdS are all visible active photocatalysts. Therefore, continuous visible light irradiation of the BiOBr/Bi₂S₃/CdS composite causes BiOBr, Bi₂S₃ and CdS to form charge carriers. However, because the VB of BiOBr is in a more positive position than the VB of Bi₂S₃, the holes in the VB of Bi₂S₃ degrade MeB, while the electrons on the CB of Bi₂S₃ transfer to the CB of BiOBr, hindering its reorganization. On the other hand, the width of the CB of CdS makes the electron light excited to a higher CB, and the annihilation of the h⁺on the VB of CdS and the electron (e⁻) on the CB of BiOBr on the conductive medium also reduces the recombination, and the CB of CdS. The e⁻ at the place reacts with O₂ to generate •O₂⁻ for further degradation of MeB [37]. In addition, because the CB of BiOBr and Bi₂S₃ cannot reach the formation potential E₀ of •O₂⁻ (O₂/•O₂⁻= −0.046 eV), the recombination of CdS with a more negative CB plays a key role in the generation of superoxide radicals. In summary, the photo-induced carrier transfer process and MeB removal process at each stage of the photocatalytic process are shown in formula (Equations (6)–(12)) [38]:

$$BiOBr/B{i}_2{S}_3/CdS+h\nu \to BiOBr/B{i}_2{S}_3/CdS(h^{+}+e^{-})$$

(6)

$$BiOBr\left(e^{-}\right)+CdS\left(h^{+}\right) \to h\nu$$

(7)

$$BiOBr\left(h^{+}\right) \to B{i}_2{S}_3\left(h^{+}\right)$$

(8)

$$B{i}_2{S}_3\left(e^{-}\right) \to BiOBr\left(e^{-}\right)$$

(9)

$$O_2+CdS\left(e^{-}\right) \to •O_2^{-}$$

(10)

$$B{i}_2{S}_3\left(h^{+}\right)+MeB \to Degradation products$$

(11)

$$•O_2^{-}+MeB \to Degradation products$$

(12)

4. Conclusions

BiOBr microspheres were synthesized first in this work, and a BiOBr/Bi₂S₃/CdS composite with a sea urchin-like shape was directly obtained by the one-pot method. The morphology, structure and composition of the composite material were fully characterized by XRD, FT-IR, SEM, EDS and XPS. When the initial concentration of MeB aqueous solution is 10 mg·L⁻¹, pH = 7, catalyst dosage 50 mg and the reaction temperature at 30 °C, the best visible light catalytic efficiency can be achieved, and the two-hour removal rate of MeB reaches 83.3%. The MeB photocatalytic degradation process followed a pseudo-first-order kinetic model, and the reaction rate under optimal conditions is 0.014 min⁻¹, which is 1.1, 1.7 and two times higher than that of pure CdS, Bi₂S₃ and BiOBr, respectively. The results of the cycling experiment showed that the degradation rate of MeB under the optimal conditions only decreased by 4.9%for the BiOBr/Bi₂S₃/CdS composite after five cycles, which confirmed the material has good stability. The band gap energy of BiOBr/Bi₂S₃/CdS was calculated to be 2.23 eV from the ultraviolet/visible diffuse reflectance spectrum. •O₂⁻ and h⁺ were the main active species involved in the photocatalytic degradation of MeB through quenching experiment and EPR test results, which explained the reaction mechanism and confirmed the key role of CdS in the production of •O₂⁻. The analysis of the above results proved that the sea urchin-like BiOBr/Bi₂S₃/CdS, as a semiconductor composite material with a moderate band gap, is expected to show more applications in the field of photocatalysis.