Antibacterial Activity and the Mechanism of the Z-Scheme Bi2MoO6/Bi5O7I Heterojunction under Visible Light

Z-scheme Bi2MoO6/Bi5O7I heterojunction was constructed by an in situ solvothermal method, which was composed of Bi2MoO6 nanosheets growing on the surface of Bi5O7I microrods. The antibacterial activities under illumination towards Escherichia coli (E. coli) were investigated. The Bi2MoO6/Bi5O7I composites exhibited more outstanding antibacterial performance than pure Bi2MoO6 and Bi5O7I, and the E. coli (108 cfu/mL) was completely inactivated by BM/BI-3 under 90 min irradiation. Additionally, the experiment of adding scavengers revealed that h+, •O2− and •OH played an important role in the E. coli inactivation process. The E. coli cell membrane was damaged by the oxidation of h+, •O2− and •OH, and the intracellular components (K+, DNA) subsequently released, which ultimately triggered the apoptosis of the E. coli cell. The enhanced antibacterial performance of Bi2MoO6/Bi5O7I heterojunction is due to the formation of Z-scheme heterojunction with the effective charge transfer via the well-contacted interface of Bi2MoO6 and Bi5O7I. This study provides useful guidance on how to construct Bi5O7I-based heterojunction for water disinfection with abundant solar energy.


Introduction
As the world economy continues to develop, water pollution is becoming more and more serious, and the safety of drinking water is becoming increasingly prominent.Water is one of the most abundant resources on earth, and the health risks brought about by the spread of pathogenic microorganisms in drinking water have become a focal point for researchers around the world [1,2].Many efforts including ultraviolet irradiation, chlorination and ozonation are applied to disinfect most pathogens in drinking water, but the high energy consumption and toxic byproducts restrict their development [3].It is imperative to develop an environmentally friendly method for the efficient removal of pathogenic microorganisms.Photocatalytic technology, as an emerging advanced oxidation process, has the potential to become a rising star in the field of water disinfection.With the utilized light, the electrons (e − ) in valence band (VB) of semiconductor are excited to the conduction band (CB) resulting in the production of electron/hole (e − /h + ) pairs.Moreover, the e − can reduce O 2 to •O 2 − and the h + can oxidate H 2 O or OH − to •OH, respectively.Ultimately, the active species including •O 2 − , •OH and h + damage the bacterial cell membranes and contribute to the leakage of intracellular components, accompanied by bacterial inactivation [4,5].On account of the solar spectrum, the development of stable, efficient and visible-light-responsive photocatalysts is a prerequisite for the application of photocatalytic disinfection techniques.
Bismuth-based semiconductors are considered to be promising photocatalysts by reason of their great variety, favorable stability and low toxicity.Bismuth oxyhalide (BiOX) has a stable structure with an internal electric field (IEF) formed by a (Bi 2 O 2 ) 2+ layer alternately arranged with an I − layer.IEF can be conducive to the migration of photoinduced charges, and BiOX has been widely studied in the fields of pollutant removal from water or air [6][7][8][9], water splitting [10,11], CO 2 reduction [12,13], nitrogen fixation [14,15], selective oxidation [16,17] and so on.Among them, BiOI is an excellent visible-light-responsive photocatalyst based on the band gap of about 1.8 eV [18][19][20].On the other hand, the narrow band gap of BiOI results in rapid recombination of photoinduced electrons and holes, in addition to poor redox ability, so the photocatalytic activity is unsatisfactory.Similar to other photocatalytic materials, various strategies such as crystal plane regulation [21,22], element doping [23,24], surface oxygen vacancies [25,26] and construction of heterojunctions [27,28] can be applied to boost the photocatalytic performance of BiOI.Furthermore, the construction of Bi x O y I z through the bismuth-rich strategy has been shown to be useful for modulating the band structure and enhancing redox capacity [29][30][31].Furthermore, the bismuth-rich strategy is easy to implement and cannot introduce other elements.Bi 5 O 7 I, as a member of Bi x O y I z , possesses suitable band structure for photocatalytic application, but the limited separation efficiency of photogenerated carriers hinders its performance.Therefore, it is urgent to seek suitable modification methods to enhance the photogenerated carrier separation efficiency of Bi 5 O 7 I.
The formation of heterojunctions is a proven effective way to enhance the separation efficiency of photogenerated carriers.Heterojunctions are usually composed of two semiconductors with suitable band structures, which can effectively encourage the charge transfer and separation.It is acknowledged that charge transfer pathways such as Type I, Type II, and Z-scheme have been researched widely for different heterojunctions [32,33].For Z-scheme heterojunction, the e − in the CB of one photocatalyst with more positive potential recombine with the h + in the VB of another photocatalyst with more negative potential, which not only promote the separation of the photoinduced e − and h + , but also reserve the higher redox capacity.Based on the band structure of Bi 5 O 7 I, another bismuthbased semiconductor material Bi 2 MoO 6 comes into view.Bi 2 MoO 6 is an Aurivilius oxide photocatalyst with layer structure consisting of a [Bi 2 O 2 ] 2+ layer and a [MoO 4 ] 2− layer.Due to its high photooxidation potential, appropriate band structure and environmental friendliness, Bi 2 MoO 6 shows promise for building a heterojunction with Bi 5 O 7 I, thereby improving photocatalytic performance.So far, the application of the Bi 2 MoO 6 /Bi 5 O 7 I heterojunction as a photocatalytic antibacterial has not been reported.
This work focuses on the synthesis, characterization and photocatalytic antibacterial activity of the Bi 2 MoO 6 /Bi 5 O 7 I heterojunction.The photocatalytic antibacterial activity was evaluated through the inactivation of Escherichia coli (E.coli) under illumination.The inactivation mechanism for E. coli with the Bi 2 MoO 6 /Bi 5 O 7 I heterojunction was also illustrated.

Material Characterization
The XRD patterns of Bi 5 O 7 I, Bi 2 MoO 6 and BM/BI composites are presented in Figure 1.The diffraction peaks of Bi 5 O 7 I in Figure 1a [34].As for pure Bi 2 MoO 6 , the diffraction peaks can be identified at 28.   1b, the corresponding diffraction peaks of Bi 2 MoO 6 cannot be recognized for BM/BI-1, BM/BI-2 and BM/BI-3 XRD patterns, but the peak intensities become weaker with the increasing amount of Bi 2 MoO 6 .As for BM/BI-4, the weak peak at 32.4 • can be discovered, which corresponds to the (002) plane of Bi 2 MoO 6 , and the peak at 28.1 • is broadened, which may be because it is composed of the characteristic peaks of Bi 5 O 7 I (28.The optical absorption property is an important factor affecting the photocatalytic performance of semiconductor materials.The UV-vis diffuse reflectance spectra (UV-vis DRS) of the Bi5O7I, Bi2MoO6 and BM/BI composites are exhibited in Figure 2a.Pure Bi5O7I showed the absorption edge at 450 nm.With the incorporation of Bi2MoO6, the red shift could be discovered for the absorption edges of the BM/BI composites, which could be conducive to improving the production of photoinduced carriers.As displayed in Figure 2b,c, the band gap energies (Eg) of Bi5O7I and Bi2MoO6 are 2.77 and 2.67 eV, respectively.To determine the band structure of Bi5O7I and Bi2MoO6, Mott-Schottky (M-S) plots were measured (Figure 2d,e).Bi5O7I and Bi2MoO6 are both n-type semiconductors in accordance with the positive slope of M-S plots.As a result, the flat band potentials (Efb) of Bi5O7I and Bi2MoO6 are at −0.85 and −0.32 eV (vs.Ag/AgCl), which can be converted to −0.65 and −0.12 eV (vs.NHE) according to the formula ENHE = EAg/AgCl + 0.197 [36].The Efb is usually positive by 0.1 eV over the conduction band potential (ECB) [37], so the ECB of Bi5O7I and Bi2MoO6 are estimated to be −0.75 and −0.22 eV (vs.NHE).Consequently, the valence band potentials (EVB) of Bi5O7I and Bi2MoO6 were calculated to be 2.02 and 2.45 eV on account of the formula ECB = EVB − Eg.SEM images of the Bi5O7I, Bi2MoO6 and BM/BI composites were measured to determine their micromorphology.Bi5O7I is composed of uniform microrods with width of 200-800 nm and length of 2-8 µm (Figure 3a).Bi2MoO6 exhibits the micromorphology of The optical absorption property is an important factor affecting the photocatalytic performance of semiconductor materials.The UV-vis diffuse reflectance spectra (UV-vis DRS) of the Bi 5 O 7 I, Bi 2 MoO 6 and BM/BI composites are exhibited in Figure 2a.Pure Bi 5 O 7 I showed the absorption edge at 450 nm.With the incorporation of Bi 2 MoO 6 , the red shift could be discovered for the absorption edges of the BM/BI composites, which could be conducive to improving the production of photoinduced carriers.As displayed in Figure 2b,c, the band gap energies (E g ) of Bi 5 O 7 I and Bi 2 MoO 6 are 2.77 and 2.67 eV, respectively.To determine the band structure of Bi 5 O 7 I and Bi 2 MoO 6 , Mott-Schottky (M-S) plots were measured (Figure 2d,e).Bi 5 O 7 I and Bi 2 MoO 6 are both n-type semiconductors in accordance with the positive slope of M-S plots.As a result, the flat band potentials (E fb ) of Bi 5 O 7 I and Bi 2 MoO 6 are at −0.85 and −0.32 eV (vs.Ag/AgCl), which can be converted to −0.65 and −0.12 eV (vs.NHE) according to the formula E NHE = E Ag/AgCl + 0.197 [36].The E fb is usually positive by 0.1 eV over the conduction band potential (E CB ) [37], so the E CB of Bi The optical absorption property is an important factor affecting the photocatalytic performance of semiconductor materials.The UV-vis diffuse reflectance spectra (UV-vis DRS) of the Bi5O7I, Bi2MoO6 and BM/BI composites are exhibited in Figure 2a.Pure Bi5O7I showed the absorption edge at 450 nm.With the incorporation of Bi2MoO6, the red shift could be discovered for the absorption edges of the BM/BI composites, which could be conducive to improving the production of photoinduced carriers.As displayed in Figure 2b,c, the band gap energies (Eg) of Bi5O7I and Bi2MoO6 are 2.77 and 2.67 eV, respectively.To determine the band structure of Bi5O7I and Bi2MoO6, Mott-Schottky (M-S) plots were measured (Figure 2d,e).Bi5O7I and Bi2MoO6 are both n-type semiconductors in accordance with the positive slope of M-S plots.As a result, the flat band potentials (Efb) of Bi5O7I and Bi2MoO6 are at −0.85 and −0.32 eV (vs.Ag/AgCl), which can be converted to −0.65 and −0.12 eV (vs.NHE) according to the formula ENHE = EAg/AgCl + 0.197 [36].The Efb is usually positive by 0.1 eV over the conduction band potential (ECB) [37], so the ECB of Bi5O7I and Bi2MoO6 are estimated to be −0.75 and −0.22 eV (vs.NHE).Consequently, the valence band potentials (EVB) of Bi5O7I and Bi2MoO6 were calculated to be 2.02 and 2.45 eV on account of the formula ECB = EVB − Eg.SEM images of the Bi5O7I, Bi2MoO6 and BM/BI composites were measured to determine their micromorphology.Bi5O7I is composed of uniform microrods with width of 200-800 nm and length of 2-8 µm (Figure 3a).Bi2MoO6 exhibits the micromorphology of SEM images of the Bi 5 O 7 I, Bi 2 MoO 6 and BM/BI composites were measured to determine their micromorphology.Bi 5 O 7 I is composed of uniform microrods with width of 200-800 nm and length of 2-8 µm (Figure 3a).Bi 2 MoO 6 exhibits the micromorphology of nanosheets with an average size of 300 nm (Figure 3b).For BM/BI composites (Figure 3c-f), Bi 2 MoO 6 nanosheets growing on the surface of Bi 5 O 7 I microrods can be discovered.As the loading of Bi 2 MoO 6 increases, the nanosheets covered on the nanorods gradually grow.The TEM image of BM/BI-3 was also displayed in Figure 3g.The nanosheets binding with microrods could be observed in accordance with the results of SEM.In addition, the elemental distribution of BM/BI-3 was obtained by energy disperse spectroscopy (EDS).In Figure 3h, Bi, O, I and Mo elements can be detected and they are evenly distributed, further confirming that Bi 5 O 7 I and Bi 2 MoO 6 were successfully combined together.
Molecules 2023, 27, x FOR PEER REVIEW 4 of 13 nanosheets with an average size of 300 nm (Figure 3b).For BM/BI composites (Figure 3cf), Bi2MoO6 nanosheets growing on the surface of Bi5O7I microrods can be discovered.As the loading of Bi2MoO6 increases, the nanosheets covered on the nanorods gradually grow.The TEM image of BM/BI-3 was also displayed in Figure 3g.The nanosheets binding with microrods could be observed in accordance with the results of SEM.In addition, the elemental distribution of BM/BI-3 was obtained by energy disperse spectroscopy (EDS).
In Figure 3h, Bi, O, I and Mo elements can be detected and they are evenly distributed, further confirming that Bi5O7I and Bi2MoO6 were successfully combined together.The element compositions and chemical states of Bi5O7I, Bi2MoO6 and BM/BI-3 were analyzed through the XPS measurement.In Figure S1, the survey spectrum confirmed the presence of Bi, I, O and Mo elements, which is in agreement with the elemental mappings of EDS.As exhibited in Figure 4a, the Bi 4f spectrum of BM/BI-3 is composed of two peaks at 159.4 and 164.9 eV, which are correlated with the Bi 4f7/2 and Bi 4f5/2 of Bi 3+ [38,39].In the high resolution XPS spectrum of I 3d for BM/BI-3 (Figure 4b), two peaks at 619.4 and 630.8 eV can be obviously detected, which are assigned to the I 3d5/2 and I 3d3/2, respectively [34].Two peaks at 232.6 eV and 235.7 eV can be detected in the Mo 3d spectrum of BM/BI-3 (Figure 4c), corresponding to Mo 3d5/2 and Mo 3d3/2 of Mo 6+ from the Bi2MoO6 [40].In addition, the O 1 s spectrum of BM/BI-3 (Figure 4d) can be deconvoluted into three peaks at 529.5, 531.0 and 532.5 eV, which are attributed to the Bi-O, Mo-O and the adsorbed H2O on the surface, respectively [41].Compared with pure Bi2MoO6, the Bi 4f and Mo 3d peaks of BM/BI-3 shifted to a higher-energy region, indicating a decrease in the electron density in Bi2MoO6.Meanwhile, the Bi 4f and I 3d peaks of BM/BI-3 shifted to lower-energy region in contrast with pure Bi5O7I, suggesting an increase in the electron density in the Bi5O7I.The element compositions and chemical states of Bi 5 O 7 I, Bi 2 MoO 6 and BM/BI-3 were analyzed through the XPS measurement.In Figure S1, the survey spectrum confirmed the presence of Bi, I, O and Mo elements, which is in agreement with the elemental mappings of EDS.As exhibited in Figure 4a, the Bi 4f spectrum of BM/BI-3 is composed of two peaks at 159.4 and 164.9 eV, which are correlated with the Bi 4f 7/2 and Bi 4f 5/2 of Bi 3+ [38,39].In the high resolution XPS spectrum of I 3d for BM/BI-3 (Figure 4b), two peaks at 619.4 and 630.8 eV can be obviously detected, which are assigned to the I 3d 5/2 and I 3d 3/2 , respectively [34].Two peaks at 232.6 eV and 235.7 eV can be detected in the Mo 3d spectrum of BM/BI-3 (Figure 4c), corresponding to Mo 3d 5/2 and Mo 3d 3/2 of Mo 6+ from the Bi 2 MoO 6 [40].In addition, the O 1 s spectrum of BM/BI-3 (Figure 4d) can be deconvoluted into three peaks at 529. of electrons from Bi2MoO6 to Bi5O7I.
To illustrate the separation efficiency of photoinduced carriers, transient photocurrent response and electrochemical impedance spectra (EIS) could be performed.In Figure 5a, the photocurrent can be detected during each light on for Bi5O7I, Bi2MoO6 and BM/BI-3, but the photocurrent intensity of BM/BI-3 was higher than that of Bi5O7I and Bi2MoO6, indicating the optimal photoinduced carrier separation efficiency of BM/BI-3.Compared with Bi5O7I and Bi2MoO6, BM/BI-3 exhibits a lower arc radius in EIS plots (Figure 5b), suggesting more effective separation of photoinduced carriers.Due to the construction of the Bi5O7I/Bi2MoO6 heterojunction, the contact interface would facilitate the migration of photoinduced electrons and holes between Bi5O7I and Bi2MoO6, and the recombination of photoinduced carriers is successfully suppressed.

Photocatalytic Antibacterial Activity
To evaluate the photocatalytic performance of fabricated materials, inactivation of E. coli under illumination was accomplished and the results were displayed in Figure 6a.Under visible light without synthesized materials, only a slight decrease occurred for the To illustrate the separation efficiency of photoinduced carriers, transient photocurrent response and electrochemical impedance spectra (EIS) could be performed.In Figure 5a, the photocurrent can be detected during each light on for Bi 5 O 7 I, Bi 2 MoO 6 and BM/BI-3, but the photocurrent intensity of BM/BI-3 was higher than that of Bi The results demonstrated the formation of the heterojunction and suggested the migration of electrons from Bi2MoO6 to Bi5O7I.
To illustrate the separation efficiency of photoinduced carriers, transient photocurrent response and electrochemical impedance spectra (EIS) could be performed.In Figure 5a, the photocurrent can be detected during each light on for Bi5O7I, Bi2MoO6 and BM/BI-3, but the photocurrent intensity of BM/BI-3 was higher than that of Bi5O7I and Bi2MoO6, indicating the optimal photoinduced carrier separation efficiency of BM/BI-3.Compared with Bi5O7I and Bi2MoO6, BM/BI-3 exhibits a lower arc radius in EIS plots (Figure 5b), suggesting more effective separation of photoinduced carriers.Due to the construction of the Bi5O7I/Bi2MoO6 heterojunction, the contact interface would facilitate the migration of photoinduced electrons and holes between Bi5O7I and Bi2MoO6, and the recombination of photoinduced carriers is successfully suppressed.

Photocatalytic Antibacterial Activity
To evaluate the photocatalytic performance of fabricated materials, inactivation of E. coli under illumination was accomplished and the results were displayed in Figure 6a.Under visible light without synthesized materials, only a slight decrease occurred for the

Photocatalytic Antibacterial Activity
To evaluate the photocatalytic performance of fabricated materials, inactivation of E. coli under illumination was accomplished and the results were displayed in Figure 6a.
Under visible light without synthesized materials, only a slight decrease occurred for the survival rate of E. coli, signifying the effect of visible light on E. coli is limited.As for Bi 5 O 7 I and Bi 2 MoO 6 under 90 min illumination, the survival rates of E. coli were 48.7% and 58.7%, respectively.The enhanced destructive abilities to E. coli can be detected for the Bi 2 MoO 6 /Bi 5 O 7 I composites.Furthermore, BM/BI-3 displayed the optimal photocatalytic performance to inactivate E. coli and all E. coli were inactivated after 90 min illumination.The antibacterial activities of synthesized samples under dark were also determined and the results were exhibited in Figure S2

Mechanism of Improved Photocatalytic Antibacterial Activity for Bi2MoO6/Bi5O7I Heterojunction
The charge transfer mechanism of the Bi2MoO6/Bi5O7I heterojunction was summarized based on the aforementioned analysis.As exhibited in Figure 8, if Bi2MoO6 and Bi5O7I formed the traditional type-II heterojunction, the e − in the CB of Bi5O7I would transfer to that of Bi2MoO6 because the ECB of Bi5O7I (−0.75 eV) is more negative than Bi2MoO6 (−0.22 eV).Simultaneously, the h + would migrate from the VB of Bi2MoO6 to that of Bi5O7I based on the more positive EVB of Bi2MoO6 (2.45 eV).But the accumulated e -in the CB of Bi2MoO6 are unable to convert O2 to •O2 − based on the facts that the ECB of Bi2MoO6 (−0.22 eV) is more positive than the potential of O2/•O2 − (−0.33 eV vs. NHE) [49,50].According to the results of adding scavengers, •O2 − participated in the E. coli inactivation with BM/BI-3, so it is unreasonable for the type-II charge transfer mechanism.Consequently, a Zscheme charge transfer mechanism of the Bi2MoO6/Bi5O7I heterojunction was put forward The dead/live E. coli cells can be identified by LSCM.Stained by propidium iodide (PI) and SYTO9, live E. coli cells glow green fluorescent, while dead display red fluorescent [42,43].The E. coli cells treated by BM/BI-3 under visible light were stained and observed as shown in Figure 6b.With the extension of the illumination time, the number of green dots progressively reduced while red dots increased, indicating that E. coli cells were progressively experiencing apoptosis.For the inactivation of E. coli by BM/BI-3, the growth of colonies corresponding to the plate count are exhibited in Figure S3 in the supporting information.The number of colonies was gradually reduced as irradiation time increased, which was in agreement with the results of the fluorescence staining measurements.SEM was carried out to investigate the morphological changes in BM/BI-3-treated E. coli cells.As exhibited in Figure 6c the untreated E. coli cells were bluntly rounded and rod-shaped at both ends and the cell surface was intact.With illumination, wounds appeared on the surface of some cells and gradually became more severe with prolonged irradiation.Furthermore, wounds caused the intracellular components to release and the cell to collapse, and the deactivated cells tended to clump together.
Photoinduced electrons (e − ) and holes (h + ) can be originated from semiconductor materials under photoexcitation, and subsequently active species including •O 2 − and •OH are also produced.To inactivate the bacteria, h + , •O 2 − and •OH can destroy the membrane permeability of E. coli cell by means of oxidation, thereby leading to the cell apoptosis [44,45].In order to analyze the involvement of different active species for E. coli inactivation with BM/BI-3, three scavengers including ammonium oxalate (AO, 5 mM), p-benzoquinone (BQ, 5 mM) and isopropanol (IPA, 5 mM) were added for detecting h + , •O 2 − and •OH, respectively.In Figure 7a, the antibacterial activity of BM/BI-3 decreased with adding BQ, AO or IPA, indicating that h + , •O 2 − and •OH contributed for the E. coli inactivation, and the sequence of the effect was •O 2 − > •OH > h + .

Mechanism of Improved Photocatalytic Antibacterial Activity for Bi2MoO6/Bi5O7I Heterojunction
The charge transfer mechanism of the Bi2MoO6/Bi5O7I heterojunction was summarized based on the aforementioned analysis.As exhibited in Figure 8, if Bi2MoO6 and Bi5O7I formed the traditional type-II heterojunction, the e − in the CB of Bi5O7I would transfer to that of Bi2MoO6 because the ECB of Bi5O7I (−0.75 eV) is more negative than Bi2MoO6 (−0.22 eV).Simultaneously, the h + would migrate from the VB of Bi2MoO6 to that of Bi5O7I based on the more positive EVB of Bi2MoO6 (2.45 eV).But the accumulated e -in the CB of Bi2MoO6 are unable to convert O2 to •O2 − based on the facts that the ECB of Bi2MoO6 (−0.22 eV) is more positive than the potential of O2/•O2 − (−0.33 eV vs. NHE) [49,50].According to the results of adding scavengers, •O2 − participated in the E. coli inactivation with BM/BI-3, so it is unreasonable for the type-II charge transfer mechanism.Consequently, a Zscheme charge transfer mechanism of the Bi2MoO6/Bi5O7I heterojunction was put forward in Figure 8.The e -in the CB of Bi2MoO6 migrated to the VB of Bi5O7I and recombined with the h + , resulting in the accumulation of surplus e -in the CB of Bi5O7I and h + in the VB of Bi2MoO6, respectively.The e − in the CB of Bi5O7I could interact with O2 to produce •O2 − , The active species containing h + , •O 2 − and •OH destroy the membrane of the E. coli cell, and the wounds on the cell membrane may bring about the release of intracellular components.K + is one of the important intracellular components of E. coli, and the extracellular K + concentration of E. coli solution was measured at different illumination time (Figure 7b).As treated by Bi 5 O 7 I, Bi 2 MoO 6 or BM/BI-3, the extracellular K + concentration increased gradually, suggesting that the leakage of K + enhanced as the irradiation time was prolonged.Additionally, the increase of the extracellular K + concentration induced by BM/BI-3 was the most significant, indicating the strong destructive effect of BM/BI-3.The loss of intracellular nucleic acid is lethal for E. coli [46], so it is more meaningful to determine the extracellular DNA content of E. coli solution (Figure 7c).The extracellular DNA induced by BM/BI-3 was more than that by Bi 5 O 7 I and Bi 2 MoO 6 under the same illumination time.With the destruction of active species, the wounds on the E. coli cell membrane appeared and the intracellular components released, which triggered the E. coli cells to ultimately experience apoptosis.Additionally, the destructive ability of active species produced by BM/BI-3 was greater than those produced by Bi 5 O 7 I or Bi 2 MoO 6 .
To further verify whether E. coli was completely inactivated after the photocatalytic treatment, the bacterial regrowth experiment was performed.After the photocatalytic antibacterial experiment, BM/BI-3 was removed from the E. coli solution.The E. coli solution was stored under dark for 4 h and then was directly coated on the LB plate and incubated at 37 • C for 24 h.It was discovered that there was no E. coli colony on the plate, demonstrating that photocatalytic disinfection by BM/BI-3 caused irreversible destruction of E. coli.The result is consistent with other photocatalytic disinfection reports [47,48].

Synthesis of Materials
Add 5 mmol Bi(NO3)3•5H2O into 30 mL distilled water and stir magnetically until uniform.Dissolve 5 mmol KI into 30 mL distilled water and stir magnetically until dissolved.Then, add KI solution dropwise to Bi(NO3)3 solution under continue stirring for 1 h, and the pH of mixed liquor was disposed to 12.5 with dropping NaOH solution (2 mol L −1 ).After stirring for 30 min, the suspension was poured into a hydrothermal autoclave and heated at 160 °C for 10 h.When the autoclave cooled to room temperature, centrifugal washing with distilled water and ethanol was carried out and the precipitate was dried and collected as Bi5O7I.
The suspension composed of 500 mg Bi5O7I and 50 mL deionized water was sonicated for 1 h.A certain amount of NaMoO4•2H2O and Bi(NO3)3•5H2O (molar ratio 1:2) were dissolved into•20 mL ethylene glycol.The above solution was dropwise added into Bi5O7I dispersion under ultrasonication for 30 min and stirred for another 30 min, which was poured into a hydrothermal autoclave with heat at 160 °C for 4 h.After cooling, centrifugal washing and drying were followed up and the collected powder was the Bi2MoO6/Bi5O7I composite.According to the above process, the amount of Bi(NO3)3•5H2O added was 0.01 and 0.02, 0.03, 0.04 mmol, and the products were named BM/BI-1, BM/BI-2, BM/BI-3 and BM/BI-4, respectively.The schematic diagram for synthesis of Bi2MoO6/Bi5O7I composite is shown in Figure 9.
The electrochemical experiments were performed on an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) using the three-electrode system.The synthesized powder (10 mg) was mixed with 1 mL ethanol and 20 µL Nafion solution (5%) under ultrasonic and then coated on the FTO glass as the working electrode.The Ag/AgCl electrode was selected as the reference electrode and the Pt wire was chosen as the counter electrode.The Na2SO4 solution (0.1 M) was taken as the electrolyte for the photoelectrochemical experiments.Additionally, transient photocurrent response tests were performed under 300W Xe lamp irradiation with 420 nm cut-off filter Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) plots were measured in the dark.Moreover, EIS were investigated in a frequency range from 1 Hz to 10 kHz and M-S plots were studied at the frequency of 1000 Hz.

Photocatalytic Inactivation of E. coli
The antibacterial performance with visible light was investigated by the inactivation of E. coli (ATCC 8739, Shanghai Beinuo Biotechnology Co., Ltd.Shanghai, China).The operational vessel must be autoclaved at 121 °C for half an hour and the following antimicrobial procedures need to be fulfilled in the sterile environment.A 300 W Xe lamp with a 420 nm cut-off filter was adopted as visible light.The antibacterial photocatalytic experiment was performed by 50 mL E. coli solution (10 8 cfu/mL) with 20 mg photocatalyst.At 15 min intervals, 3 mL suspension was taken out and centrifuged.The supernatants were employed to determine intracellular components (K + and DNA) release.The precipitate was rinsed with a PBS buffer solution three times and suspended in a PBS buffer solution
The electrochemical experiments were performed on an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) using the three-electrode system.The synthesized powder (10 mg) was mixed with 1 mL ethanol and 20 µL Nafion solution (5%) under ultrasonic and then coated on the FTO glass as the working electrode.The Ag/AgCl electrode was selected as the reference electrode and the Pt wire was chosen as the counter electrode.The Na 2 SO 4 solution (0.1 M) was taken as the electrolyte for the photoelectrochemical experiments.Additionally, transient photocurrent response tests were performed under 300W Xe lamp irradiation with 420 nm cut-off filter.Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) plots were measured in the dark.Moreover, EIS were investigated in a frequency range from 1 Hz to 10 kHz and M-S plots were studied at the frequency of 1000 Hz.

Photocatalytic Inactivation of E. coli
The antibacterial performance with visible light was investigated by the inactivation of E. coli (ATCC 8739, Shanghai Beinuo Biotechnology Co., Ltd.Shanghai, China).The operational vessel must be autoclaved at 121 • C for half an hour and the following antimicrobial procedures need to be fulfilled in the sterile environment.A 300 W Xe lamp with a 420 nm cut-off filter was adopted as visible light.The antibacterial photocatalytic experiment was performed by 50 mL E. coli solution (10 8 cfu/mL) with 20 mg photocatalyst.At 15 min intervals, 3 mL suspension was taken out and centrifuged.The supernatants were employed to determine intracellular components (K + and DNA) release.The precipitate was rinsed with a PBS buffer solution three times and suspended in a PBS buffer solution.To determine the cell density of E. coli, the plate count method was applied.An amount of 1 mL of the above solution was diluted with a gradient of 10 −1 and coated on a Luria-Bertani plate with incubation at 37 • C for 24 h.The number of colonies on the plate was counted to evaluate the antibacterial performance of the synthesized samples under different irradiation times.

Fluorescence Microscopy Assays and Microstructure of E. coli
To further identify the survival state, laser scanning confocal microscopy (LSCM) and scanning electron microscope (SEM) were operated.To identify the dead/live E. coli cells, the bacteria were stained by propidium iodide (PI) and SYTO9.The PI solution (5 µg/mL) and SYTO9 solution (5 µg/mL) were mixed with a volume ratio of 1:1.The PBS buffer solution with E. coli and the PI/SYTO9 solution were uniformly mixed and reacted in the dark for 10 min.The stained E. coli was then centrifuged and washed with PBS three times and observed by LSCM.
To study the morphological change in E. coli cells during the photocatalytic antibacterial process, 2.5% (v/v) glutaraldehyde solution was used to fix the E. coli cells at 4 • C for 6 h.Next, after washing with a PBS buffer solution, the E. coli cells were gradually dehydrated with ethanol solution (30%, 50%, 70%, 90% and 100%) for 10 min each time and tert-butanol for 20 min.Eventually, SEM was operated to observe the microstructure of the E. coli cells.

Measurement of Intracellular Components Leakage
The leakage of intracellular components was determined using the supernatant from the E. coli suspension at different irradiation times.The released K + from E. coli cells was detected by inductively coupled plasma optical emission spectroscopy (ICP-OES).Extracellular DNA content was determined by NanoDrop One at 260 nm.

Conclusions
In summary, a Bi 2 MoO 6 /Bi 5 O 7 I heterojunction was constructed via an in situ solvothermal process for antibacterial application.The Z-scheme charge transfer through the interface of Bi 2 MoO 6 and Bi 5 O 7 I constrained the recombination of photoinduced carriers and enhanced their antibacterial performance under visible light.On the basis of the experiment of adding scavengers, h + , •O 2 − and •OH played an important role in E. coli inactivation.The membrane permeability of the E. coli cell was damaged by the oxidation of h + , •O 2 − and •OH, and the intracellular components (K + , DNA) subsequently released, which ultimately triggered the apoptosis of the E. coli cell.This study offers an opportunity to construct a Z-scheme Bi 5 O 7 I-based heterojunction for water disinfection with abundant solar energy.

Data Availability Statement:
The data of the study can be provided by corresponding author upon reasonable request.
5, 531.0 and 532.5 eV, which are attributed to the Bi-O, Mo-O and the adsorbed H 2 O on the surface, respectively [41].Compared with pure Bi 2 MoO 6 , the Bi 4f and Mo 3d peaks of BM/BI-3 shifted to a higher energy region, indicating a decrease in the electron density in Bi 2 MoO 6.Meanwhile, the Bi 4f and I 3d peaks of BM/BI-3 shifted to lower energy region in contrast with pure Bi 5 O 7 I, suggesting an increase in the electron density in the Bi 5 O 7 I.The results demonstrated the formation of the heterojunction and suggested the migration of electrons from Bi 2 MoO 6 to Bi 5 O 7 I.
5 O 7 I and Bi 2 MoO 6 , indicating the optimal photoinduced carrier separation efficiency of BM/BI-3.Compared with Bi 5 O 7 I and Bi 2 MoO 6 , BM/BI-3 exhibits a lower arc radius in EIS plots (Figure 5b), suggesting more effective separation of photoinduced carriers.Due to the construction of the Bi 5 O 7 I/Bi 2 MoO 6 heterojunction, the contact interface would facilitate the migration of photoinduced electrons and holes between Bi 5 O 7 I and Bi 2 MoO 6 , and the recombination of photoinduced carriers is successfully suppressed.Molecules 2023, 27, x FOR PEER REVIEW 5 of 13
. Without irradiation, the antibacterial performances of the Bi 5 O 7 I, Bi 2 MoO 6 and Bi 2 MoO 6 /Bi 5 O 7 I composites were inadequate in 90 min.The synergistic effects of the Bi 2 MoO 6 /Bi 5 O 7 I composite and visible light are favorable for the inactivation of bacteria.Molecules 2023, 27, x FOR PEER REVIEW 7 of 13 at 37 °C for 24 h.It was discovered that there was no E. coli colony on the plate, demonstrating that photocatalytic disinfection by BM/BI-3 caused irreversible destruction of E. coli.The result is consistent with other photocatalytic disinfection reports [47,48].

Figure 6 .
Figure 6.Photocatalytic antibacterial activities toward E. coli of synthesized samples (a), LSCM images of stained E. coli (b) and SEM images of E. coli cells (c) treated by BM/BI-3 under different irradiation time.

Figure 6 .
Figure 6.Photocatalytic antibacterial activities toward E. coli of synthesized samples (a), LSCM images of stained E. coli (b) and SEM images of E. coli (c) treated by BM/BI-3 under different irradiation time.

Figure 6 .
Figure 6.Photocatalytic antibacterial activities toward E. coli of synthesized samples (a), LSCM images of stained E. coli (b) and SEM images of E. coli cells (c) treated by BM/BI-3 under different irradiation time.
Activity for Bi 2 MoO 6 /Bi 5 O 7 I Heterojunction The charge transfer mechanism of the Bi 2 MoO 6 /Bi 5 O 7 I heterojunction was summarized based on the aforementioned analysis.As exhibited in Figure 8, if Bi 2 MoO 6 and Bi 5 O 7 I formed the traditional type-II heterojunction, the e − in the CB of Bi 5 O 7 I would transfer to that of Bi 2 MoO 6 because the E CB of Bi 5 O 7 I (−0.75 eV) is more negative than Bi 2 MoO 6 (−0.22 eV).Simultaneously, the h + would migrate from the VB of Bi 2 MoO 6 to that of Bi 5 O 7 I based on the more positive E VB of Bi 2 MoO 6 (2.45 eV).But the accumulated e − in the CB of Bi 2 MoO 6 are unable to convert O 2 to •O 2 − based on the facts that the E CB of Bi 2 MoO 6 (−0.22 eV) is more positive than the potential of O 2 /•O 2 − (−0.33 eV vs. NHE) [49,50].According to the results of adding scavengers, •O 2 − participated in the E. coli inactivation with BM/BI-3, so it is unreasonable for the type-II charge transfer mechanism.Consequently, a Z-scheme charge transfer mechanism of the Bi 2 MoO 6 /Bi 5 O 7 I was put forward in Figure 8.The e − in the CB of Bi 2 MoO 6 migrated to the VB of Bi 5 O 7 I and recombined with the h + , resulting in the accumulation of surplus e − in the CB of Bi 5 O 7 I and h + in the VB of Bi 2 MoO 6 , respectively.The e − in the CB of Bi 5 O 7 I could interact with O 2 to produce •O 2 − , since the E CB of Bi 5 O 7 I (−0.75 eV) is more negative than the potential of 2 /•O 2 − (−0.33 eV vs. NHE).Meanwhile, the h + in the VB of Bi 2 MoO 6 could interact with H 2 O or OH − to generate •OH, based on the fact that the E VB of Bi 2 MoO 6 (2.45 eV) is more positive than the potentials of •OH/OH − (1.99 eV vs. NHE) [51] and •OH/H 2 O (2.34 eV vs. NHE) [52].Under the oxidation of h + , •O 2 − and •OH, the membrane permeability of the E. coli cell was damaged and the release of intracellular components subsequently happened, which ultimately triggered the apoptosis of the E. coli cell.Molecules 2023, 27, x FOR PEER REVIEW 8 of 13 since the ECB of Bi5O7I (−0.75 eV) is more negative than the potential of O2/•O2 − (−0.33 eV vs. NHE).Meanwhile, the h + in the VB of Bi2MoO6 could interact with H2O or OH − to generate •OH, based on the fact that the EVB of Bi2MoO6 (2.45 eV) is more positive than the potentials of •OH/OH -(1.99 eV vs. NHE) [51] and •OH/H2O (2.34 eV vs. NHE) [52].Under the oxidation of h + , •O2 − and •OH, the membrane permeability of the E. coli cell was damaged and the release of intracellular components subsequently happened, which ultimately triggered the apoptosis of the E. coli cell.

Figure 8 .
Figure 8. Schematic diagrams for Type Ⅱ and Z-scheme charge transfer mechanism of the Bi2MoO6/Bi5O7I heterojunction.

Figure 8 .
Figure 8. Schematic diagrams for Type II and Z-scheme charge transfer mechanism of the Bi 2 MoO 6 /Bi 5 O 7 I heterojunction.

3. Experiment Section 3 . 1 .
Synthesis of MaterialsAdd 5 mmol Bi(NO 3 ) 3 •5H 2 O into 30 mL distilled water and stir magnetically until uniform.Dissolve 5 mmol KI into 30 mL distilled water and stir magnetically until dissolved.Then, add KI solution dropwise to Bi(NO 3 ) 3 solution under continue stirring for 1 h, and the pH of mixed liquor was disposed to 12.5 with dropping NaOH solution (2 mol L −1 ).After stirring for 30 min, the suspension was poured into a hydrothermal autoclave and heated at 160 • C for 10 h.When the autoclave cooled to room temperature, centrifugal washing with distilled water and ethanol was carried out and the precipitate was dried and collected as Bi 5 O 7 I.The suspension composed of 500 mg Bi 5 O 7 I and 50 mL deionized water was sonicated for 1 h.A certain amount of NaMoO 4 •2H 2 O and Bi(NO 3 ) 3 •5H 2 O (molar ratio 1:2) were dissolved into•20 mL ethylene glycol.The above solution was dropwise added into Bi 5 O 7 I dispersion under ultrasonication for 30 min and stirred for another 30 min, which was poured into a hydrothermal autoclave with heat at 160 • C for 4 h.After cooling, centrifugal washing and drying were followed up and the collected powder was the Bi 2 MoO 6 /Bi 5 O 7 I composite.According to the above process, the amount of Bi(NO 3 ) 3 •5H 2 O added was 0.01 and 0.02, 0.03, 0.04 mmol, and the products were named BM/BI-1, BM/BI-2, BM/BI-3 and BM/BI-4, respectively.The schematic diagram for synthesis of Bi 2 MoO 6 /Bi 5 O 7 I composite is shown in Figure9.

Figure 9 .
Figure 9. Schematic diagram for construction of the Bi 2 MoO 6 /Bi 5 O 7 I composite.

Funding:
This research was funded by the Henan Provincial Science and Technology Research Project (No. 222102320224), the Key Scientific Research Project of Colleges and Universities in Henan Province (No. 21A180005 and 21A610008).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.