3.1. Structure and morphology characterizations
Figure 1 shows the XRD patterns of BiSnSbO6-ZnO composite photocatalytic materials with different molar ratios. The crystal structures of BiSnSbO6, ZnO and BiSnSbO6-ZnO were characterized by XRD. In the ZnO spectrum, the characteristic peaks at 2θ = 31.1°, 34.0°, 35.6°, 47.0°, 56.0°, 62.3° and 67.5° correspond to (100), (002), (101), (102), (110), (103) and (112) crystal planes, the same as the ZnO standard card (PDF#01-0562), are wurtzite hexagonal phases (V. Kononenko et al. 2017; A. Sundararaj et al. 2017). The characteristic diffraction peaks located at 2θ = 29.4°, 34.0°, 48.9°, 58.0°, 60.8° and 71.8° correspond to (222), (400), (440), (622), (444) and (800) of BiSnSbO6 diffraction crystal plane, which is consistent with the literature report (J. F. Luan et al. 2018). Compared with the characteristic peaks at the molar ratios of 1:2 and 1:3, the characteristic peaks of the composites with the molar ratios of 1:4 and 1:5 are clearer and sharper, indicating that the composites with the molar ratios of 1:4 and 1:5 have higher crystallinity. In the spectrum of BiSnSbO6-ZnO composite photocatalytic material, both the characteristic peaks of BiSnSbO6 and ZnO appeared, which indicated that the BiSnSbO6-ZnO composite photocatalytic material was successfully synthesized.
The morphology, structure and elemental composition of BiSnSbO6-ZnO composite photocatalytic materials were analyzed by SEM, TEM and EDS. Figure 2(a) is an SEM image of BiSnSbO6, showing particles with random three-dimensional shapes. Figures 2(b) and (c) are the SEM and TEM images of the composite with a molar ratio of 1:4, respectively, while Figs. 2(d) and (e) are the SEM and TEM images of the composite with a molar ratio of 1:5. Figure 2(b) and (d) are the SEM pictures of BiSnSbO6-ZnO composite photocatalytic material. It can be seen from the figure that the irregularly shaped BiSnSbO6 particles are attached to ZnO nanoparticles with smaller particle size, indicating that BiSnSbO6 and ZnO are successfully compounded together to form a BiSnSbO6-ZnO composite photocatalytic material. From Fig. 2 (c), (e) BiSnSbO6-ZnO composite photocatalytic material TEM pictures can be seen from the larger BiSnSbO6 particles and smaller diameter of ZnO. The results further proved that the BiSnSbO6-ZnO composite photocatalytic materials in 1:4 and 1:5 molar ratios were successfully composited. Among them, when scanning and observing the BiSnSbO6-ZnO composite photocatalytic materials with a molar ratio of 1:2 and 1:3, it was found that the composite photocatalytic materials of these two ratios appeared material agglomeration. This phenomenon may be because BiSnSbO6 accounts for more. According to the literature, it can be known that BiSnSbO6 is prone to agglomeration when sintered at high temperature for a long time [30], so the 1:2 and 1:3 molar ratio composite materials were not used in the subsequent antibacterial experiments.
The BiSnSbO6-ZnO composite photocatalytic material was scanned by EDS to further verify whether the effective components in the composite material were evenly distributed during the material composite process. Scanning the marked area of the SEM image, all elements (Bi, Sn, Sb, Zn and O) are uniformly distributed in the BiSnSbO6-ZnO composite photocatalytic material as shown in Fig. 3. It shows that BiSnSbO6 and ZnO are successfully coupled to form BiSnSbO6-ZnO composites.
3.2. Antibacterial activity and in vitro cytotoxicity evaluation
Using 35 W LED lamp as excitation light source, the photocatalytic antibacterial properties of pure BiSnSbO6 material, pure ZnO material and BiSnSbO6-ZnO composite photocatalytic material were determined by colony counting method. Compared with pure BiSnSbO6 and ZnO materials, the antibacterial properties of BiSnSbO6, ZnO and BiSnSbO6-ZnO composite photocatalytic materials against E. coli, S. aureus, P. aeruginosa and Candida albicans were compared. Firstly, the photocatalytic efficiency of BiSnSbO6-ZnO composite photocatalytic materials with different molar ratios of 1:4 and 1:5 was compared with S. aureus as model bacteria, so as to determine the optimal composite material ratio for subsequent experiments.
Using a control group without catalytic materials in a light environment, the effect of LED light on the inhibition or killing of pathogenic microorganisms was removed. Set up a dark group with catalytic materials in a dark environment, and test whether the materials themselves have the ability to inhibit or kill microorganisms. Provided 35 W LED lighting simultaneously under the same ambient conditions. Using S. aureus as model bacteria, the antibacterial ability of BiSnSbO6-ZnO composite photocatalytic materials at different ratios and concentrations against S. aureus was compared. It can be clearly seen from Fig. 4 that the BiSnSbO6-ZnO composite photocatalytic material with a molar ratio of 1:4 achieved 100% antibacterial efficiency against S. aureus at 4 h. The antibacterial efficiency against S. aureus is higher than that of the BiSnSbO6-ZnO composite photocatalytic material with a molar ratio of 1:5. Therefore, the BiSnSbO6-ZnO composite photocatalytic material with the best molar ratio of 1:4 was selected for the subsequent experiments.
Figure 5(a) shows the photocatalytic antibacterial efficiency of the BiSnSbO6-ZnO composite photocatalytic material with different concentrations of 1:4 molar ratio on E. coli. In the antibacterial process, the best antibacterial efficiencies of single BiSnSbO6 and ZnO against E. coli within 6 h were 67.46% and 28.57%, respectively, indicating that both BiSnSbO6 and ZnO have a certain inactivation effect on E. coli under LED light, but the effect is not ideal, and different concentrations of BiSnSbO6-ZnO composite photocatalytic materials have better antibacterial properties than single BiSnSbO6 and ZnO. After 6 h of light irradiation, 500 mg/L BiSnSbO6-ZnO composite photocatalytic material had the highest antibacterial efficiency against E. coli, reaching 99.63%. Figure 5(c) is a graph showing the antibacterial efficiency of BiSnSbO6-ZnO composite photocatalytic material against S. aureus. It can be seen that the highest antibacterial efficiencies of the monomers BiSnSbO6 and ZnO are 21.60% and 84.79% respectively within 6 h, and the BiSnSbO6-ZnO composite photocatalytic material exhibits better antibacterial properties. The antibacterial efficiency of the catalytic material reached 100% at 4 h. Nano-ZnO has been shown to be selectively toxic to S. aureus, and the experimental results are consistent with the reported literature (L. R. Zheng et al. 2012). Figure 5(e) is a graph showing the antibacterial efficiency of BiSnSbO6-ZnO composite photocatalytic material against P. aeruginosa. Compared with the monomers BiSnSbO6 and ZnO, the BiSnSbO6-ZnO composite photocatalytic material exhibited stronger antibacterial properties against P. aeruginosa, It can be clearly seen that the antibacterial efficiency of the 500 mg/L BiSnSbO6-ZnO composite photocatalytic material against P. aeruginosa reached 100% when the light was irradiated for 4 h. Compared with the antibacterial efficiency of other different concentrations, the antibacterial efficiency of the BiSnSbO6-ZnO composite photocatalytic material at a concentration of 500 mg/L is better, indicating that the optimal antibacterial concentration of the BiSnSbO6-ZnO composite photocatalytic material for bacteria is 500 mg/L. Too much concentration or too little concentration will affect the photocatalytic activity of BiSnSbO6-ZnO composite photocatalytic material. Figure 5(b), (d), (f) are the antibacterial plate images of monomer BiSnSbO6, ZnO and BiSnSbO6-ZnO composite photocatalytic materials with different concentrations against E. coli, S. aureus and P. aeruginosa.
In order to explore the antibacterial efficiency of materials with different concentrations, the antibacterial time of light exposure was extended to continue to observe. From Fig. 6, it can be found that at 12 h, the BiSnSbO6-ZnO composite photocatalytic materials with four different antibacterial concentrations of 125 mg/L, 250 mg/L, 500 mg/L and 1000 mg/L had better effects on E. coli, S. aureus and P. aeruginosa three bacteria were 100% antibacterial efficiency. It shows that under the condition of maintaining light conditions, the BiSnSbO6-ZnO composite photocatalytic material with different concentrations can also continue to exert antibacterial effect to kill bacteria.
According to the above research results, it can be found that the optimal antibacterial concentration of BiSnSbO6-ZnO composite photocatalytic material for bacteria is 500 mg/L, but there are species differences in the antibacterial effect on different bacteria.
In order to understand the antibacterial properties of BiSnSbO6-ZnO composite photocatalytic materials on fungi, this paper also used Candida albicans as model bacteria to conduct antibacterial experiments on fungi. Figure 7 showed the antibacterial efficiency diagram and antibacterial plate diagram of single BiSnSbO6, ZnO and BiSnSbO6-ZnO composite photocatalytic materials with different concentrations against Candida albicans. The antibacterial efficiency against Candida albicans reached the highest at h, which was 63.80%.
The cytotoxicity of antibacterial materials is of great significance for further practical applications of the materials. In this study, normal mouse skin fibroblasts (L929) were selected to detect the cytotoxicity of BiSnSbO6-ZnO composite photocatalytic materials. According to the results of the previous antibacterial experiments, three different concentrations were set, which were 250 mg/L, 500 mg/L, and 1000 mg/L, respectively. Figure 8 showed the results of MTT assay. There was no significant difference in the survival rate of L929 cells between the three different concentrations of BiSnSbO6-ZnO composite photocatalytic material and the control group (p > 0.05). This can explain the non-cytotoxicity of BiSnSbO6-ZnO composite photocatalytic material. However, the survival rate of L929 cells incubated with monomeric ZnO at a concentration of 500 mg/L was significantly reduced, which was significantly different from that of the blank control group (p < 0.05).
3.3. Electrochemical Characterization
Figure 9 showed the UV-vis DRS spectra of BiSnSbO6, ZnO and BiSnSbO6-ZnO. It can be seen from Fig. 9(a) that ZnO absorbs light with a wavelength below 400 nm, mainly absorbing ultraviolet light, and the absorption band edges of BiSnSbO6 and BiSnSbO6-ZnO are above 400 nm, indicating that they are responsive to visible light. With the addition of ZnO, the light absorption intensity of BiSnSbO6-ZnO was enhanced. The band gap energy (Eg) can be estimated according to the formula: ɑhv = A(hv-Eg)n/2 (L. Yosefi et al. 2018; S. Y. Li et al. 2014). Since ZnO is a direct transition semiconductor material, n = 2 in the calculation process (R. Kumar et al. 2014). From Fig. 9(b), (c), (d), it can be concluded that the BiSnSbO6-ZnO composite photocatalytic material, the forbidden band widths of BiSnSbO6 and ZnO materials are 2.81 eV, 2.66 eV and 3.14 eV, respectively. This is similar to the data reported in the previous literature (J. F. Luan et al. 2018). The EVB and ECB edge potentials of BiSnSbO6 and ZnO calculated according to the formula are shown in Table 1. According to the edge potentials of EVB and ECB, BiSnSbO6-ZnO composites belong to type II heterojunction structure composites.
Table 1
Eg, EVB and ECB values of BiSnSbO6and ZnO photocatalytic materials
Photocatalysts
|
Eg (eV)
|
EVB (eV)
|
ECB (eV)
|
BiSnSbO6
|
2.80
|
3.23
|
0.57
|
ZnO
|
3.14
|
2.83
|
-0.31
|
Semiconductor photocatalytic materials can understand the interfacial mobility of photogenerated electron-hole pairs in semiconductor materials through electrochemical impedance, where the smaller the radius of the semicircle arc, the higher the charge transport efficiency of semiconductor photocatalytic materials. As shown in Fig. 10, compared with pure BiSnSbO6 and pure ZnO materials, BiSnSbO6-ZnO composite photocatalytic material has the smallest arc radius, indicating that BiSnSbO6-ZnO composite photocatalytic material has the smallest charge transfer resistance. Therefore, the BiSnSbO6-ZnO composite photocatalytic material has the strongest photocatalytic antibacterial performance.
Figure 11 is the PT map of different photocatalytic materials under intermittent illumination conditions. The photocurrent of the BiSnSbO6-ZnO composite photocatalytic material is stronger than that of the two single materials. The higher the photocurrent intensity, the faster the separation rate of electron-hole pairs. It shows that the BiSnSbO6-ZnO composite photocatalytic material has better photocatalytic antibacterial efficiency than the single BiSnSbO6 and ZnO materials.
PL analysis can study the process of photoinduced electron-hole pair migration, transfer and recombination in semiconductor photocatalytic materials. The PL spectra of BiSnSbO6-ZnO composite photocatalytic material and ZnO are shown in Fig. 12. After exciting the material with 469 nm light, it can be found that the material shows a strong emission peak in the range of 800–900 nm, and the emission peak intensity of the BiSnSbO6-ZnO composite photocatalytic material is lower than that of the pure ZnO material, indicating that the recombination rate of BiSnSbO6 and ZnO inhibited the charge recombination rate. It is further demonstrated that the BiSnSbO6-ZnO composite photocatalytic material can reduce the charge recombination rate, thereby improving the photocatalytic antibacterial activity.
3.4. Understanding of the mechanism for antimicrobial activity
In order to understand the role of different active substances in the antibacterial process of BiSnSbO6-ZnO composite photocatalytic materials, the photocatalytic antibacterial mechanism of BiSnSbO6-ZnO composite photocatalytic materials was explored through radical scavenging experiments. BQ, IPA, Na2C2O4 and K2Cr2O7 were used as scavengers of ·O2−, ·OH, h+ and e−, respectively. It can be seen from Fig. 13 that after adding BQ, the antibacterial efficiency against the model bacteria S. aureus was enhanced, reaching 100% in 1 h. This should be because the conduction band position of ZnO is -0.31 eV, which is more correct than EθO2/·O2− (-0.33 eV), so the e- on the conduction band of ZnO cannot combine with O2 to form ·O2− (K. N. Abbas et al. 2017). After adding BQ, BQ cannot combine with ·O2−, and because BQ is a broad-spectrum antibacterial agent, it can interact with DNA, protein, mitochondria, etc. in cells, resulting in bacterial death (Y. Liu et al. 2009; H. L. Wong et al. 2017). This may be the reason why the antibacterial efficiency against Staphylococcus aureus was improved after adding BQ, indicating that ·O2− was not generated during the photocatalytic antibacterial process. The valence band position of BiSnSbO6 is 2.81 eV, which is more correct than Eθ·OH/H2O (2.38 eV) (D. L. Jiang et al. 2017), so the h+ at the valence band of BiSnSbO6 can react with H2O to generate OH, and then react with bacteria. After adding IPA, the antibacterial efficiency decreased slightly, indicating that the role of ·OH in the photocatalytic antibacterial process was almost negligible. When the Na2C2O4 solution was added, the antibacterial efficiency decreased, indicating that the effect of h+ was weak in the photocatalytic antibacterial process. After adding K2Cr2O7 solution, the antibacterial efficiency decreased to a large extent, which indicated that the active substance e− played a major role. The free radical clearing experiment shows that the BiSnSbO6-ZnO composite photocatalytic material can generate ROS under light conditions, and ·OH, h+ and e− all have certain antibacterial effects, of which e− plays the main antibacterial effect.
In order to more clearly understand the effect of BiSnSbO6-ZnO composite photocatalytic material on the morphological changes of bacteria and fungi. SEM was used to observe the morphology of experimental bacteria treated with added material light and the control bacteria only treated with light. From Fig. 14, it can be found that compared with the control group, the structures and shapes of bacteria and fungi treated with BiSnSbO6-ZnO composite photocatalytic materials are significantly different. E. coli, S. aureus, P. aeruginosa and Candida albicans all exhibited wrinkled atrophy, rupture and irregular morphology to a certain extent, unlike bacteria that had not been treated with the material whose surface was intact. The results showed that the BiSnSbO6-ZnO composite photocatalytic material contacted the surface of bacteria and fungi, causing the surface of bacteria and fungi to atrophy or even rupture, preventing bacteria and fungi from maintaining normal physiological activities until death.
In summary, it is found that the BiSnSbO6-ZnO composite photocatalytic material produces active substances ·OH, h+ and e− in the photocatalytic antibacterial process, ·OH, h+ and e− all have a certain antibacterial effect, and the role of e− occupies the leading position. After the active substance is in contact with the bacteria, the structural integrity of the bacteria is destroyed, and the internal substances of the bacteria leak, thereby achieving an antibacterial effect. According to published studies, BiSnSbO6 and ZnO are p-type and n-type semiconductors, respectively (P. Huang et al. 2019; Z. H. Jaffari et al. 2019). At the same time, combining the UV-vis valence and conduction band positions of the two, it is concluded that the BiSnSbO6-ZnO composite photocatalytic material is a type II heterojunction structure, as shown in Fig. 15.
3.5. Application of composite photocatalytic materials in livestock and poultry wastewater
A variety of bacteria have been found in the collected livestock and poultry wastewater, and these bacteria are extremely harmful to humans and the environment. This serious problem needs to be solved urgently to protect the environment. The practical application of antibacterial properties of BiSnSbO6-ZnO composite photocatalytic materials was tested with livestock and poultry wastewater. As shown in Fig. 16, after the livestock and poultry wastewater was treated by light with the BiSnSbO6-ZnO composite photocatalytic material with the optimal antibacterial concentration of 500 mg/L, with the increase of time, the colonies in the livestock and poultry wastewater diluted 20 times were obviously reduced or even disappeared. After 3 h of LED light illumination, the antibacterial rate of BiSnSbO6-ZnO composite photocatalytic material reached 99.41%. When the time was extended to 6 h, it was found that the bacteria had been completely killed. It is proved that the BiSnSbO6-ZnO composite photocatalytic material has a good antibacterial effect on a variety of coexisting bacteria, which also shows that the BiSnSbO6-ZnO composite photocatalytic material is a broad-spectrum antibacterial material, which can be applied in practical applications.