Fabrication of a Novel AgBr/Ag2MoO4@InVO4 Composite with Excellent Visible Light Photocatalytic Property for Antibacterial Use

A novel AgBr/Ag2MoO4@InVO4 composite photocatalyst with different heterojunction structures was successfully constructed by compounding InVO4 with Ag2MoO4 and AgBr. According to the degradation, antibacterial and free radical trapping data, the photocatalytic antibacterial and antifouling activities of AgBr/Ag2MoO4@InVO4 composite were evaluated, and the corresponding photocatalytic reaction mechanism was proposed. Adding AgBr/Ag2MoO4@InVO4 composite, the degradation rate of ciprofloxacin (CIP) achieved 95.5% within 120 min. At the same time, the antibacterial rates of Escherichia coli (E. coli), Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa) achieved 99.99%. The AgBr/Ag2MoO4@InVO4 composite photocatalyst showed promising usage in photocatalytic antibacterial and purification areas.


Introduction
In recent years, new green photocatalysis technology based on semiconductors has developed rapidly. It has been widely used in pollutant degradation, water separation and sterilization through using solar energy as an energy source [1,2]. Semiconductor photocatalysis technology has attracted more and more attention due to its energy efficiency, simple operation, low expense and high stability, and because it is also green and non-toxic, it has no secondary pollution, among other advantages [3][4][5]. In order to use sunlight effectively, it is necessary to design new photocatalytic heterojunction materials with a visible light band response. To date, a number of excellent semiconductor photocatalysts have been developed rapidly, such as metal oxides [6][7][8], metal sulfides [7,9,10], metal oxynitrides [11] and polymer materials [12]. InVO 4 is a metal vanadate photocatalyst with a band gap width of 2.0 eV, and it has excellent photocatalytic performance under visible light [13,14]. InVO 4 has gained widespread attention in many fields, such as degradation, air purification, water decomposition, organic pollutants, etc. However, the competence of the InVO 4 photocatalyst is largely affected by its size and micro-morphology, resulting in low efficiency of photogenerated carriers [15]. Use of a supporting cocatalyst on the surface of the photocatalyst is considered as one of the Nanomaterials 2020, 10, 1541 2 of 12 most effective methods to drive the separation of photogenerated electrons and holes and enhance photocatalytic activity [15,16]. Ag 2 MoO 4 has many advantages such as excellent antibacterial activity, high conductivity, good electrochemical energy storage, suitable valence band position, generation of a large number of hydroxyl radicals, controllable morphology, photoluminescence, etc. [17,18]. AgBr has been extensively studied as a promoter of Ag-based semiconductors. Similar studies, such as with Ag/AgBr/AgVO 3 [19], Ag/AgBr-Bi 2 MoO 6 [20], Ag@AgBr/CaTiO 3 [21] and Ag/AgBr@InVO 4 [22], have also confirmed that AgBr is promising to prepare efficient and stable photocatalysts.
As InVO 4 , Ag 2 MoO 4 and AgBr have their own advantages and disadvantages. Many researchers combined them with other substances to learn their strengths and make up for their shortcomings, and then they prepared photocatalysts with better performance for various studies. Chen et al. [23] fabricated a new β-Ag 2 MoO 4 /BiVO 4 heterojunction photocatalyst by a simple precipitation method at room temperature. The degradation rates for rhodamine B (RhB) of pure β-Ag 2 MoO 4 and BiVO 4 were not good, but the as-created β-Ag 2 MoO 4 /BiVO 4 photocatalyst had about 92.6% RhB degradation rate. The combination of Ag 2 MoO 4 and BiVO 4 increased the absorption of visible light, improved the transfer speed of photogenerated electrons and reduced the recombination of holes and electrons, and it had excellent photocatalytic performance. Yang et al. [24] constructed an InVO 4 /β-AgVO 3 nanocomposite photocatalyst by a facile hydrothermal method and subsequent in situ growth process. The as-prepared InVO 4 /β-AgVO 3 composite photocatalyst had an enhanced photocatalytic performance in reducing CO 2 to CO under visible light. Li et al. [25] reported a g-C 3 N 4 /graphene oxide-Ag/AgBr composite photocatalyst used to prepare hydrogen. Due to the synergistic effect of silver bromide, with good photosensitivity, and silver plasma, the photocatalyst improved the hydrogen evolution performance and provided a feasible method for developing hydrogen energy.
To date, there are no reports on the composite photocatalyst of AgBr/Ag 2 MoO 4 @InVO 4 . In this work, a hydrothermal method and in situ growth method were used to produce AgBr/Ag 2 MoO 4 @InVO 4 photocatalytic composites with different molar ratios. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), energy-dispersive X-ray spectroscopy (EDX) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the microstructure and composition of the prepared composite photocatalyst. Using visible light as a light source, the photocatalytic degradation of the organic pollutant ciprofloxacin (CIP) was tested. The degradation rates of different molar ratios of AgBr/Ag 2 MoO 4 @InVO 4 photocatalyst to CIP solution were calculated under the same test conditions. At the same time, E. coli, S. aureus and P. aeruginosa were selected as model bacteria to carry out antibacterial experiments on the prepared AgBr/Ag 2 MoO 4 @InVO 4 photocatalytic composite materials in order to study the bactericidal performance of the photocatalyst. In addition, the photocatalytic reaction mechanism of AgBr/Ag 2 MoO 4 @InVO 4 heterojunction was proposed based on free radical trapping experiments, degradation and sterilization data.

Synthesis of AgBr/Ag 2 MoO 4 @InVO 4 Photocatalysts
All chemical reagents in our experiments were analytical reagent grade. In a classic order for the preparation of InVO 4 materials, 0.117 g NH 4 VO 3 (Shanghai, China) was dissolved in 50 mL water firstly. After that, it was sonicated and stirred continuously for 20 min at normal room temperature to get a homogeneous solution. After dissolving and stirring 0.382 g of In(NO 3 ) 3 (Shanghai, China) in 10 mL water, this liquid was added dropwise slowly to the former solution. The pH value was controlled to 4.0 using 0.25 wt. % NH 3 ·H 2 O (Shanghai, China) and 2 mol/L HNO 3 (Shanghai, China). The mix solution was stirred in succession for 30 min until a yellow colloidal solution was acquired. The mix solution was moved to a 100mL Teflon-lined stainless-steel autoclave, which had been heated to 200 • C for 24 h. Having been cleaned many times using ultrapure water and absolute ethyl alcohol, the obtained solid composite materials were centrifuged to obtain yellow solid powders. Finally, the composite materials were dried at 60 • C for 6 h.
The 0.2068 g InVO 4 semiconductor material prepared above was put into 30 mL distilled water and treated ultrasonically for 30 min. After that, 0.238 g AgNO 3 (Shanghai, China) was added into the above solution and mixed for 30 min to make them disperse evenly. Subsequently, 0.164 g of cetyltrimethylammonium bromide (CTAB, Beijing, China) and 0.109 g of Na 2 MoO 4 .2H 2 O (Shanghai, China) were dissolved in 20 mL distilled water. Next, this solution was slowly added into the above solution drop by drop. The reaction mixture was stirred continuously for 2 h in a dark environment. The composite material gained was cleaned three times using water and ethanol respectively. Then, the precipitates were dried for 12 h in an oven at 60 • C. The prepared sample was marked as "1.0AgBr/Ag 2 MoO 4 @InVO 4 ". In this way, composite materials with different molar ratios were prepared and labeled as X AgBr/Ag 2 MoO 4 @ InVO 4 (x = 0.2, 0.6, 1.0 and 1.4).

Characterization
XRD (Rigaku D/max-3C, Tokyo, Japan) was used to characterize the crystalline structure of the samples. The microstructure of the prepared photocatalyst was examined by SEM (Hitachi S-4800, Tokyo, Japan), TEM (Tecnai G2F20, Oregon, USA) and HRTEM (FEI Company, Hillsboro, OR, USA). A UV-vis spectrophotometer (U-2900, Tokyo, Japan) was used to characterize the absorption spectra, and ultra-pure water was used as the reference.

Photocatalytic Performance
In this experiment, ciprofloxacin (CIP), an organic substance, was chosen as a model molecule to judge the photocatalytic degradation property of the material. Before the reaction, condensed water was acquired, and an 800W Xe lamp (XPA-7, Xujiang Electromechanical Plant, Nanjing, China) deployed with a 420 nm cut-off filter was turned on. Then, 40 mg photocatalyst was put into 50 mL CIP solutions, and quartz tubes were inserted into the photochemical reactor filled with condensed water. In the photocatalytic reaction process, the solution was blended magnetically in the dark for 30 min so CIP and the synthetic materials were well-distributed. After the light shield had been pulled up, the sample solution extracted in the same time interval was filtered through a membrane to eliminate solid particles. The residual amount of CIP in the extracted solution was measured by an ultraviolet-visible spectrometer (Hitachi U-2900, Hitachi, Tokyo, Japan). The amount of CIP was determined by comparing the peak-to-peak value of the sample between the standard sample. In the measurement experiment, ultra-pure water was used as a reference, and the scanning range was 200-700 nm.
In this experiment, S. aureus, E. coli and P. aeruginosa were selected to evaluate the antibacterial performance of the photocatalyst. An 800W Xe lamp using a 420 nm cut-off filter was adopted as the light source. Typically, 45 mL phosphate-buffered saline (PBS), 30 mg photocatalyst and 5 mL bacterial suspension were added into 50mL quartz tubes. The mingled liquids were stirred for 30 min using a magnetic stirrer in total darkness to balance the adsorption/desorption. During 800W Xe lamp irradiation, 2 mL of mixed solution was taken out every 20 min and diluted with PBS in different gradients. Next, LB agar plates were used for cultivating the diluted bacterial suspension at the temperature of 37 • C for 24 h. Then, the number of bacteria was calculated via plate counts. In each group, the survival rate and antibacterial rate were calculated through triplicate parallel experiments.
The survival rate was calculated by the formula [26]: Nanomaterials 2020, 10, 1541 4 of 12 Among them, N 0 and N t are the number of bacteria in the blank group and the number of bacteria in the antibacterial experiment, respectively. The formula of antibacterial rate [5] is Antimicrobial rate (%) = 100-survival rate (2) At present, many researchers have confirmed that free radical active substances (·OH, ·O 2 − , h + , etc.) play a major role in photocatalytic reactions. In this experiment, free radical trapping experiments were used to study the types of free radicals. In this experiment, isopropyl alcohol (IPA), p-benzoquinone (BQ) and sodium oxalate (MSDS) were used as the ·OH capture agent, ·O 2 − capture agent and h + capture agent, respectively. The photocatalytic reaction mechanism was studied in combination with the above experiments. The operation steps were consistent with the photocatalytic degradation experiment. Then, the degradation rate was calculated. The active species were also analyzed.

Characterization of AgBr/Ag 2 MoO 4 @InVO 4
The XRD patterns of InVO 4 crystals and AgBr/Ag 2 MoO 4 @InVO 4 photocatalytic composites are shown in Figure  experiments were used to study the types of free radicals. In this experiment, isopropyl alcohol (IPA), p-benzoquinone (BQ) and sodium oxalate (MSDS) were used as the ·OH capture agent, ·O2 -capture agent and h + capture agent, respectively. The photocatalytic reaction mechanism was studied in combination with the above experiments. The operation steps were consistent with the photocatalytic degradation experiment. Then, the degradation rate was calculated. The active species were also analyzed.
In AgBr/Ag2MoO4@InVO4 samples, no obvious change in the diffraction peak position of InVO4 was observed, which means that the introduction of AgBr/Ag2MoO4 did not destroy the crystal structure of InVO4. In addition, AgBr/Ag2MoO4@InVO4 composite material had no other miscellaneous peaks. Therefore, it could be proved that the AgBr/Ag2MoO4@InVO4 composite was correctly prepared.  In AgBr/Ag 2 MoO 4 @InVO 4 samples, no obvious change in the diffraction peak position of InVO 4 was observed, which means that the introduction of AgBr/Ag 2 MoO 4 did not destroy the crystal structure of InVO 4 . In addition, AgBr/Ag 2 MoO 4 @InVO 4 composite material had no other miscellaneous peaks. Therefore, it could be proved that the AgBr/Ag 2 MoO 4 @InVO 4 composite was correctly prepared.
The morphology of the composite photocatalyst was observed by SEM. SEM images of 1.0Ag 2 MoO 4 @InVO 4 are shown in Figure 2a  The morphology of the composite photocatalyst was observed by SEM. SEM images of 1.0Ag2MoO4@InVO4 are shown in Figure 2a-c, showing a clear waxberry-like structure with an average diameter of 8 μm. Some polygonal grains could also be clearly found on the surface. The resultant SEM images (Figure 2d-f) of 1.0AgBr/Ag2MoO4@InVO4 composite indicated that the spherical morphology of InVO4 was saved. Furthermore, AgBr and Ag2MoO4 particles were uniformly distributed on the InVO4 surface forming AgBr/Ag2MoO4@InVO4 heterostructures. To some extent, the AgBr/Ag2MoO4@InVO4 heterostructures could enhance the specific surface area of the material and offer more active sites for photocatalytic reaction. The morphologies of 1.0AgBr/Ag2MoO4@InVO4 composite were further illustrated by TEM and HRTEM. The amplified TEM images in Figure 3a-b distinctly display the anomalistic particles around the microsphere circumambience, proving that the AgBr/Ag2MoO4@InVO4 particles were tightly bound together. Figure 3c exhibits the HRTEM image of 1.0AgBr/Ag2MoO4@InVO4, in which three sets of different crystal streaks were observed expressly. Consequently, these results further demonstrate that a well-defined heterojunction structure had taken shape between AgBr, Ag2MoO4 and InVO4. Moreover, EDS measurements revealed the elemental composition of AgBr/Ag2MoO4@InVO4 ternary composites, which provided direct evidence for the coexistence of AgBr, Ag2MoO4 and InVO4. As shown in Figure 3d, the above AgBr/Ag2MoO4@InVO4 composite was composed of O, Br, Mo, Ag, In and V, illustrating the formation of AgBr/Ag2MoO4@InVO4 composite photocatalyst including Ag2MoO4, AgBr and InVO4. In addition, SEM elemental mapping described the composition and distribution of the different elements. As shown in the elemental mapping images (EMIs), the five Ag, Br, In, Mo and V elements (Figure 3e) could be observed existing homogeneously within the selected area on the AgBr/Ag2MoO4@InVO4 photocatalyst. The morphologies of 1.0AgBr/Ag 2 MoO 4 @InVO 4 composite were further illustrated by TEM and HRTEM. The amplified TEM images in Figure 3a-b distinctly display the anomalistic particles around the microsphere circumambience, proving that the AgBr/Ag 2 MoO 4 @InVO 4 particles were tightly bound together. Figure 3c exhibits the HRTEM image of 1.0AgBr/Ag 2 MoO 4 @InVO 4 , in which three sets of different crystal streaks were observed expressly. Consequently, these results further demonstrate that a well-defined heterojunction structure had taken shape between AgBr, Ag 2 MoO 4 and InVO 4 . Moreover, EDS measurements revealed the elemental composition of AgBr/Ag 2 MoO 4 @InVO 4 ternary composites, which provided direct evidence for the coexistence of AgBr, Ag 2 MoO 4 and InVO 4 . As shown in Figure 3d

Photocatalytic Property Study
Ciprofloxacin (CIP), as an antibiotic, protects people's health, but it also causes some environmental pollution. Using a photocatalyst to degrade CIP has low cost and no secondary pollution. Photocatalysts can generate photo-generated carriers under certain light illumination, and the photo-generated carriers can react with water to generate active hydroxyl groups (·OH) and superoxide radicals (·O2 -), which can decompose CIP into small molecular inorganic substances. In addition, the generated holes (h + ) can also oxidize CIP directly or indirectly [30]. The photocatalytic decontamination performances of the obtained AgBr/Ag2MoO4@InVO4 samples were appraised according to the CIP degradation rate in visible light [5]. As demonstrated in Figure 4a, under the condition of no photocatalyst, no degradation of CIP was observed, indicating that CIP was stabilized in visible light. In visible light irradiation, the degradation rate of InVO4 for CIP was lower than that of composite AgBr/Ag2MoO4@InVO4. For composite materials, the degradation efficiency of AgBr/Ag2MoO4@InVO4 for CIP was above 95.5%, and the degradation efficiency of 1.0 AgBr/Ag2MoO4@InVO4 for CIP was the highest. In Figure 4, a linear relationship between -ln(C/C0) and reaction time (T) is displayed, which indicates that the reaction process conformed to the pseudofirst-order reaction kinetic process [31].

Photocatalytic Property Study
Ciprofloxacin (CIP), as an antibiotic, protects people's health, but it also causes some environmental pollution. Using a photocatalyst to degrade CIP has low cost and no secondary pollution. Photocatalysts can generate photo-generated carriers under certain light illumination, and the photo-generated carriers can react with water to generate active hydroxyl groups (·OH) and superoxide radicals (·O 2 − ), which can decompose CIP into small molecular inorganic substances.
In addition, the generated holes (h + ) can also oxidize CIP directly or indirectly [30]. The photocatalytic decontamination performances of the obtained AgBr/Ag 2 MoO 4 @InVO 4 samples were appraised according to the CIP degradation rate in visible light [5]. As demonstrated in Figure 4a, under the condition of no photocatalyst, no degradation of CIP was observed, indicating that CIP was stabilized in visible light. In visible light irradiation, the degradation rate of InVO 4 for CIP was lower than that of composite AgBr/Ag 2 MoO 4 @InVO 4 . For composite materials, the degradation efficiency of AgBr/Ag 2 MoO 4 @InVO 4 for CIP was above 95.5%, and the degradation efficiency of 1.0 AgBr/Ag 2 MoO 4 @InVO 4 for CIP was the highest. In Figure 4, a linear relationship between -ln(C/C 0 ) and reaction time (T) is displayed, which indicates that the reaction process conformed to the pseudo-first-order reaction kinetic process [31].
−ln (C/C 0 ) = K app T C is CIP concentration with reaction time T, C 0 is initial CIP concentration, and K app is the apparent rate constant. According to the above formula, the K app of InVO 4 , 0.  A photocatalyst can not only degrade organic pollutants, but it can also kill bacteria in sewage. Many researchers believe that photocatalysts can produce holes (h + ) and other active substances under the action of light, which can interact with bacterial cell membranes and affect the permeability of cell membranes, leading to disorder of bacterial physiological processes and eventually leading to bacterial death [33]. In this paper, E. coli (9.8 × 106 cfu/mL), S. aureus (2.1 × 106 cfu/mL) and P. aeruginosa (2.7 × 106 cfu/mL) were selected to study the photocatalytic antibacterial activity of photocatalysts in visible light [13]. As shown in Figure 5a, it can be seen from the survival curve of P. aeruginosa that the number of P. aeruginosa did not noticeably change in blank control experiments. This revealed that the influence of visible light and the toxicity of the photocatalyst itself on bacterial activity could be ignored. In addition, as can be seen from Figure 5a, for P. aeruginosa, the 1.0AgBr/Ag2MoO4@InVO4 composite had better antibacterial activity than pure InVO4 and other molar ratios of AgBr/Ag2MoO4@InVO4 composites. In addition, as shown in Figure 5b, under the photocatalytic condition of 1.0AgBr/Ag2MoO4@InVO4 composite after 60 min, the sterilization rates of E. coli, S. aureus and P. aeruginosa were 99.9999%, 99.9998% and 99.9997%, respectively, indicating that the catalyst had higher antibacterial and antifouling activity. In addition, compared with other reported antifouling photocatalysts such as g-C3N4@Ag/AgVO3 [34], AgBr/TiO2/graphene aerogel [35], AgI/BiVO4 [36] and InVO4/AgVO3 [27], the 1.0 AgBr/Ag2MoO4@InVO4 composite photocatalyst in this experiment showed quite outstanding photocatalytic antibacterial performance, revealing potential application value in sterilization and marine antifouling.  A photocatalyst can not only degrade organic pollutants, but it can also kill bacteria in sewage. Many researchers believe that photocatalysts can produce holes (h + ) and other active substances under the action of light, which can interact with bacterial cell membranes and affect the permeability of cell membranes, leading to disorder of bacterial physiological processes and eventually leading to bacterial death [33]. In this paper, E. coli (9.8 × 10 6 cfu/mL), S. aureus (2.1 × 10 6 cfu/mL) and P. aeruginosa (2.7 × 10 6 cfu/mL) were selected to study the photocatalytic antibacterial activity of photocatalysts in visible light [13]. As shown in Figure 5a, it can be seen from the survival curve of P. aeruginosa that the number of P. aeruginosa did not noticeably change in blank control experiments. This revealed that the influence of visible light and the toxicity of the photocatalyst itself on bacterial activity could be ignored. In addition, as can be seen from Figure 5a, for P. aeruginosa, the 1.0AgBr/Ag 2 MoO 4 @InVO 4 composite had better antibacterial activity than pure InVO 4 and other molar ratios of AgBr/Ag 2 MoO 4 @InVO 4 composites. In addition, as shown in Figure 5b, under the photocatalytic condition of 1.0AgBr/Ag 2 MoO 4 @InVO 4 composite after 60 min, the sterilization rates of E. coli, S. aureus and P. aeruginosa were 99.9999%, 99.9998% and 99.9997%, respectively, indicating that the catalyst had higher antibacterial and antifouling activity. In addition, compared with other reported antifouling photocatalysts such as g-C 3 N 4 @Ag/AgVO 3 [34], AgBr/TiO 2 /graphene aerogel [35], AgI/BiVO 4 [36] and InVO 4 /AgVO 3 [27], the 1.0 AgBr/Ag 2 MoO 4 @InVO 4 composite photocatalyst in this experiment showed quite outstanding photocatalytic antibacterial performance, revealing potential application value in sterilization and marine antifouling.  A photocatalyst can not only degrade organic pollutants, but it can also kill bacteria in sewage. Many researchers believe that photocatalysts can produce holes (h + ) and other active substances under the action of light, which can interact with bacterial cell membranes and affect the permeability of cell membranes, leading to disorder of bacterial physiological processes and eventually leading to bacterial death [33]. In this paper, E. coli (9.8 × 106 cfu/mL), S. aureus (2.1 × 106 cfu/mL) and P. aeruginosa (2.7 × 106 cfu/mL) were selected to study the photocatalytic antibacterial activity of photocatalysts in visible light [13]. As shown in Figure 5a, it can be seen from the survival curve of P. aeruginosa that the number of P. aeruginosa did not noticeably change in blank control experiments. This revealed that the influence of visible light and the toxicity of the photocatalyst itself on bacterial activity could be ignored. In addition, as can be seen from Figure 5a, for P. aeruginosa, the 1.0AgBr/Ag2MoO4@InVO4 composite had better antibacterial activity than pure InVO4 and other molar ratios of AgBr/Ag2MoO4@InVO4 composites. In addition, as shown in Figure 5b, under the photocatalytic condition of 1.0AgBr/Ag2MoO4@InVO4 composite after 60 min, the sterilization rates of E. coli, S. aureus and P. aeruginosa were 99.9999%, 99.9998% and 99.9997%, respectively, indicating that the catalyst had higher antibacterial and antifouling activity. In addition, compared with other reported antifouling photocatalysts such as g-C3N4@Ag/AgVO3 [34], AgBr/TiO2/graphene aerogel [35], AgI/BiVO4 [36] and InVO4/AgVO3 [27], the 1.0 AgBr/Ag2MoO4@InVO4 composite photocatalyst in this experiment showed quite outstanding photocatalytic antibacterial performance, revealing potential application value in sterilization and marine antifouling.  Based on the research of many scholars, it could be concluded that free radical active species play a vital role in photocatalytic reactions. To further prove the influence of free radical active substance on the photocatalytic reaction, we conducted a free radical capturing experiment. Isopropyl alcohol (IPA), p-benzoquinone (BQ) and sodium oxalate (MSDS) served as the ·OH capture agent, ·O 2 − capture agent and h + capture agent, respectively [16,37]. As shown in Figure 6, 94.9% of ciprofloxacin (CIP) was degraded without the capture agent after 120 min illumination. After adding 1 mmol IPA, the degradation rate of CIP decreased to 93.7%. Moreover, after adding 1 mmol BQ and 1 mmol MSDS, the antibacterial rates decreased to 30.0% and 31.3%, indicating that the photocatalytic performance of 1.0AgBr/Ag 2 MoO 4 @InVO 4 was significantly inhibited. Therefore, these experiment results proved that ·O 2 − and h + played a crucial role in photocatalytic degradation of CIP. To sum up, we can conclude that the main active substances of the AgBr/Ag 2 MoO 4 @InVO 4 photocatalyst for CIP degradation were ·O 2 − and h + . CIP was oxidized by ·O 2 − and h + generated by the photocatalyst into a small molecular product. The possible reaction process can be shown as follows [38]: CIP + h + + ·O 2 − → CO 2 + small molecules (6) Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 12 Based on the research of many scholars, it could be concluded that free radical active species play a vital role in photocatalytic reactions. To further prove the influence of free radical active substance on the photocatalytic reaction, we conducted a free radical capturing experiment. Isopropyl alcohol (IPA), p-benzoquinone (BQ) and sodium oxalate (MSDS) served as the ·OH capture agent, ·O2 -capture agent and h + capture agent, respectively [16,37]. As shown in Figure 6, 94.9% of ciprofloxacin (CIP) was degraded without the capture agent after 120 min illumination. After adding 1 mmol IPA, the degradation rate of CIP decreased to 93.7%. Moreover, after adding 1 mmol BQ and 1 mmol MSDS, the antibacterial rates decreased to 30.0% and 31.3%, indicating that the photocatalytic performance of 1.0AgBr/Ag2MoO4@InVO4 was significantly inhibited. Therefore, these experiment results proved that ·O2 -and h + played a crucial role in photocatalytic degradation of CIP. To sum up, we can conclude that the main active substances of the AgBr/Ag2MoO4@InVO4 photocatalyst for CIP degradation were ·O2 -and h + . CIP was oxidized by ·O2 -and h + generated by the photocatalyst into a small molecular product. The possible reaction process can be shown as follows [38]： photocatalyst + hv → e -+ h + (4) O2 + e -→·O2 - CIP + h + + ·O2 -→ CO2 + small molecules (6) A Z-type photocatalytic mechanism of composite AgBr/Ag2MoO4@InVO4 was proposed, based on the analysis of experimental results. The energy band structures of AgBr, InVO4 and Ag2MoO4 as well as the degradation of CIP and sterilization mechanism of AgBr/Ag2MoO4@InVO4 photocatalyst are shown in Figure 7. When the photon energy was greater than the bandgap, electrons (e -) in the valence band of AgBr (with narrow band gap) were excited to the guide band easily, and photoelectrons and holes (h + ) appeared in visible light [39]. At the same time, electrons (e -) on the AgBr conduction band (-0.30 eV) could transfer to Ag2MoO4 (-0.18 eV) readily. At this time, the electrons (e -) on the Ag2MoO4 conduction band quickly moved to the valence band of AgBr through the heterojunction and newly combined with the holes (h + ) on the valence band of AgBr [4,5]. In addition, holes (h + ) in the valence band of InVO4 [13] could also be transferred to the conduction band of AgBr, which characterizes the band potential difference. A Z-type mechanism was set up. Electrons (e -) in the conduction band (-0.57 eV) of InVO4 had very strong reduction performance [27], while holes (h + ) in the valence band (3.02 eV) of Ag2MoO4 had good oxidation capability. The Z-type structure effectively separated electrons (e -) and holes (h + ) and enhanced the photocatalytic capability of AgBr/Ag2MoO4@InVO4 composite. InVO4 has a more negative charge conducting potential (-0.57 A Z-type photocatalytic mechanism of composite AgBr/Ag 2 MoO 4 @InVO 4 was proposed, based on the analysis of experimental results. The energy band structures of AgBr, InVO 4 and Ag 2 MoO 4 as well as the degradation of CIP and sterilization mechanism of AgBr/Ag 2 MoO 4 @InVO 4 photocatalyst are shown in Figure 7. When the photon energy was greater than the bandgap, electrons (e − ) in the valence band of AgBr (with narrow band gap) were excited to the guide band easily, and photoelectrons and holes (h + ) appeared in visible light [39]. At the same time, electrons (e − ) on the AgBr conduction band (−0.30 eV) could transfer to Ag 2 MoO 4 (−0.18 eV) readily. At this time, the electrons (e − ) on the Ag 2 MoO 4 conduction band quickly moved to the valence band of AgBr through the heterojunction and newly combined with the holes (h + ) on the valence band of AgBr [4,5]. In addition, holes (h + ) in the valence band of InVO 4 [13] could also be transferred to the conduction band of AgBr, which characterizes the band potential difference. A Z-type mechanism was set up. Electrons (e − ) in the conduction band (−0.57 eV) of InVO 4 had very strong reduction performance [27], while holes (h + ) in the valence band (3.02 eV) of Ag 2 MoO 4 had good oxidation capability. The Z-type structure effectively separated electrons (e − ) and holes (h + ) and enhanced the photocatalytic capability of AgBr/Ag 2 MoO 4 @InVO 4 composite. InVO 4 has a more negative charge conducting potential (−0.57 eV) Nanomaterials 2020, 10, 1541 9 of 12 than E 0 (O 2 /·O 2 − ) (−0.046 eV vs. Normal Hydrogen Electrode (NHE)), which can produce the active substance ·O 2 − [40]. These free radicals can not only oxidize and degrade CIP, but they also have bactericidal effects. The mechanism showed that photocatalytic antibacterial antifouling technology has the advantages of high efficiency, environmental protection and no secondary pollution.
Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 12 eV) than E 0 (O2/·O2 -) (-0.046 eV vs. Normal Hydrogen Electrode (NHE)), which can produce the active substance ·O2 - [40]. These free radicals can not only oxidize and degrade CIP, but they also have bactericidal effects. The mechanism showed that photocatalytic antibacterial antifouling technology has the advantages of high efficiency, environmental protection and no secondary pollution.

Conclusions
In this paper, a new AgBr/Ag2MoO4@InVO4 photocatalytic composite with a microsphere-like morphology was produced successfully by hydrothermal and in situ growth methods, and the photocatalytic antibacterial activity was determined. The chemical composition and morphology of the AgBr/Ag2MoO4@InVO4 photocatalytic composites were proved by XRD, SEM, EDS, EDX and HRTEM. Under visible light, the photocatalytic experiments showed that the 1.0 AgBr/Ag2MoO4@InVO4 photocatalytic composite had a much higher photocatalytic performance compared to pure InVO4 and other AgBr/Ag2MoO4@InVO4 composites. Furthermore, the antibacterial activity of this photocatalyst was excellent. Almost all E. coli, S. aureus and P. aeruginosa could be eliminated, and the antimicrobial performance reached 99.999%. The experiment of active free radical trapping showed that ·O2 -and h + were the main active substances in the AgBr/Ag2MoO4@InVO4 photocatalyst. InVO4 was compounded with Ag2MoO4 and AgBr to construct a composite photocatalyst with different heterojunction structures, which facilitated the separation of photogenerated holes and electrons, enhanced the light capture capability, prolonged the light absorption region, restrained recombination of holes and electrons and further improved the photocatalytic performance. Due to its outstanding photocatalytic performance, AgBr/Ag2MoO4@InVO4 shows good prospect in photocatalytic sterilization and environmental pollution control areas.

Conclusions
In this paper, a new AgBr/Ag 2 MoO 4 @InVO 4 photocatalytic composite with a microsphere-like morphology was produced successfully by hydrothermal and in situ growth methods, and the photocatalytic antibacterial activity was determined. The chemical composition and morphology of the AgBr/Ag 2 MoO 4 @InVO 4 photocatalytic composites were proved by XRD, SEM, EDS, EDX and HRTEM. Under visible light, the photocatalytic experiments showed that the 1.0 AgBr/Ag 2 MoO 4 @InVO 4 photocatalytic composite had a much higher photocatalytic performance compared to pure InVO 4 and other AgBr/Ag 2 MoO 4 @InVO 4 composites. Furthermore, the antibacterial activity of this photocatalyst was excellent. Almost all E. coli, S. aureus and P. aeruginosa could be eliminated, and the antimicrobial performance reached 99.999%. The experiment of active free radical trapping showed that ·O 2 − and h + were the main active substances in the AgBr/Ag 2 MoO 4 @InVO 4 photocatalyst. InVO 4 was compounded with Ag 2 MoO 4 and AgBr to construct a composite photocatalyst with different heterojunction structures, which facilitated the separation of photogenerated holes and electrons, enhanced the light capture capability, prolonged the light absorption region, restrained recombination of holes and electrons and further improved the photocatalytic performance. Due to its outstanding photocatalytic performance, AgBr/Ag 2 MoO 4 @InVO 4 shows good prospect in photocatalytic sterilization and environmental pollution control areas.