Efficient visible-light-driven photocatalytic removal of Acid Blue 92, E. coli, and S. aureus over Ag-AgCl nanoparticles-decorated bismuth sulfide microparticles

Bismuth sulfide particles were modified with Ag-AgCl nanoparticles to make a visible light active plasmonic photocatalyst. The powder x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive x-ray (EDX), elemental mapping, nitrogen adsorption–desorption isotherms (BET-BJH), photoluminescence (PL), and diffuse reflectance spectroscopy (DRS) techniques were served to analyze the morphological and structural properties. To evaluate the photocatalytic performance, Acid Blue 92 (AB92) azo dye was degraded in the aqueous solution under visible light irradiation. According to the results, 0.025 g of the photocatalyst powder was able to remove more than 98% of AB92 at 15 ppm concentration under neutral acidity, following pseudo first-order kinetics. Superoxide anion radicals (O2 •−) were also recognized as the most key species promoting the photodegradation pathway. Also, the antibacterial activity of the materials was explored against E. coli and S. aureus pathogenic bacteria under irradiation and dark conditions. Using transmission electron microscopy (TEM) images of the treated cells, it was found that the plasmonic photocatalyst damaged the cell wall structure of both gram-positive and negative bacteria within 2 h significantly, which could be attributed to the efficient production of destructive superoxide anion radicals on the surface of Ag-AgCl/Bi2S3 particles under illumination.


Introduction
In recent years, human communities have been involving in a large variety of critical environmental and energy issues.The continuous release of pollutants from the agricultural and industrial sectors has induced serious adverse effects on the ecosystem life quality and human health [1].Currently, large amounts of pesticides and insecticides are utilized in agricultural processes in developing countries which contain dangerous carcinogens, causing a series of bad diseases depending upon the contaminant type and exposure time [2].Besides, pesticides show great potential to impose harmful effects on non-target organisms such as animals, birds, fungi, algae, and bacteria [3].Improper disposal of pharmaceutical sewages leads to the entry of dangerous persistent pollutants into the surface and underground water, which have severe implications for human health and the life quality of organisms [4].As another important source of threatening the earth planet ecosystems, textile industry effluents release large amounts of carcinogenic dye compounds such as anthraquinone, triphenylmethane, reactive, and azo dyes to the rivers and lakes atmosphere [5].
The aforementioned contaminants change the appearance of water, prevent light entrance to the deeper regions, harm the aquatic animals living in that environment, disrupt photosynthesis and as-related phenomena, and finally interrupt growth and vital life factors [6].Therefore, the development of efficient methods for the treatment of toxic organic pollutants in wastewater such as pesticides [7], pharmaceuticals [3], dyes [6], and other industrial organic macromolecules [7] has become a global challenge.Accordingly, many researchers have focused on sunlight as a clean and eco-friendly energy source and attempted to develop photoactive materials towards the effective harvesting of solar light photons [8].During the past decades, photocatalysis has become very popular due to the efficient degradation and mineralization of organic pollutants into H 2 O and CO 2 by under suitable light sources [9,10].During photocatalysis, semiconductors are excited by absorbing photons with suitable energies to create photogenerated charge carriers and reactive oxygen species such as active radicals to trigger the degradation reactions [11,12].
Bismuth sulfide (Bi 2 S 3 ) has gained serendipitous popularity as a semiconducting material for photocatalysis due to its exceptional electrical and optical properties, such as a relatively narrow band gap (1.3-1.9 eV), high absorption coefficient, good visible light harvesting, and low toxicity [13,14].However, its photocatalytic application has been limited mainly due to the significant charge carrier recombination rate and photo corrosion.Several anti-photo corrosion strategies include ion imprinted polymer [15], element doped [16], cocatalyst coupling [17], and tandem heterojunction structures [18] have been reported to be efficient for the abovementioned goal.Recently, boosting the photocatalytic performance of Bi 2 S 3 through combining it with other semiconductors such as BiOCl/Cu [19], MoS 2 /BiVO 4 [20], AgIO 4 [21], Ag 6 Si 2 O 7 [22], and so on have been reported.Doping copper element has been also shown to be incredibly helpful for improving light harvesting properties of Bi 2 S 3 [23].Other metals such as Mn [16] and dual Er/Yb [24] as impurities have resulted in the same observation in Bi 2 S 3 lattices which are assigned to the formation of new energy levels and easier charge transfer.Non-metal defects such as sulfur and halogens have shown great potential to boost photoactivity through increasing surface active sites, leading to improve photocatalytic degradation activity [25].
Silver species as co-catalyst are shown to eminently increase the capacity of visible light absorption and improve the photocatalytic performance of several systems [26][27][28].AgCl has suitable potential for use in various purposes, including photography, pharmaceuticals, and electronic sectors, due to its favorable antibacterial, catalytic, and optical characteristics [29].AgCl is a light-sensitive material with a wide indirect band gap (about 3.25 eV) that is convertible to the visible-light-responsive Ag/AgCl composite upon light absorption [30].Subsequently, the photogenerated electron combines with a silver ion (Ag + ) on the surface to form metallic silver (Ag°), which benefits the local surface plasmon resonance (LSPR) effect and exhibits remarkable photocatalytic efficiency [31].Ag/AgCl as a co-catalyst in many photocatalysts led to excellent synergistic outcomes, among which Ag/AgCl/α-Fe 2 O 3 [27], Ag/AgCl@T-C 3 N 4 [32], Ag/AgCl/FeOCl [33], Ag/AgCl/ZnTiO 3 [34], and Ag/AgCl/Bi 2 O 3 /BiFeO 3 [35] are comprehensively explored.Inspired by the above considerations, we synthesized Ag/AgCl/Bi 2 S 3 photocatalyst and studied its synergistic effect on photocatalytic activity.
In this work, we successfully fabricated the Ag/AgCl/Bi 2 S 3 ternary composite and examined its photocatalytic activity under visible light toward the AB92 organic dye compound removal.The materials were well analyzed by powder x-ray Diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive x-ray Spectroscopy (EDX), Elemental Mapping, Differential Reflectance Spectroscopy (DRS), nitrogen adsorption-desorption isotherms, and photoluminescence (PL) spectroscopy.The photocatalytic activity and degradation process were mechanistically discussed too, supported by well-designed experiments and appropriate techniques.

Materials synthesis
Bismuth sulfide powder, 1.5 g, was poured into 45 ml distilled water and sonicated for 20 min at room temperature.3 ml ionic liquid, 1-hexyl-3-methylimidazolium chloride, was added to the suspension and magnetically stirred for 1 h.0.015 g of AgNO 3 was dissolved in 25 ml of distilled water and then gradually poured into the previous suspension.The medium was further stirred at 50 °C for 2 h.Finally, the precipitate was isolated by centrifugation, washed with water and ethanol repeatedly, and dried at 70 °C overnight to give Ag/AgCl/Bi 2 S 3 .

Photocatalytic experiments
AB92 azo dye was chosen as typical wastewater from the textile industries.The aqueous solution of this organic wastewater was poured into a double-jacketed Pyrex reactor surrounded by circulating water at 25 °C which keeps the temperature of the photodegradation reactions constant (scheme 1).The reactor is placed on a magnetic stirrer and 10 cm below the illumination source, OSRAM lamp 125 W (the emission spectrum of the lamp is provided at: [36]).The whole collection is located in a well aluminum foil-covered wood box to concentrate the photons on the reaction medium.After 15 min of treating the wastewater solution with the photocatalyst powder at dark, the lamp was turned on. 1 ml of the solution was taken in various time intervals and analyzed by UV-vis spectrophotometer to evaluate the progress of the reaction.

Characterization method
The morphological features of the materials were evaluated on an XL30 SEM instrument coupled with EDX analysis.TEM images of the bacteria were provided by an FEI Tecnai G2 f20 s-twin TEM with a 200-kV acceleration voltage.The XRD analysis was conducted to find the crystalline phases of the materials using a Bruker D8 Advance diffractometer with Cu-Kα radiation, λ = 0.15406 nm.FTIR analysis was performed using a Bruker Vertex 70 spectrophotometer.Examining the optoelectronic properties based on the DRS analysis was conducted using a UV-vis spectrophotometer Shimadzu Solidspec3300 DUV.To demonstrate the porosity, N 2 adsorption-desorption isotherms were provided at 77 K using a Belsorp apparatus.Cryo Eclipse fluorescence spectrometer was used to record the PL analysis of the samples.

Functional groups analysis
The photocatalyst surface functional groups and chemical bonds nature were examined using FTIR analysis in the 4000-400 cm −1 range.According to the spectrum (figure 2), the bands at 3442 and 1629 cm −1 are attributed to the stretching and bending vibration modes of water molecules adsorbed on the nanocomposite surface.The obvious bands at 1383 and 1095 cm −1 belong to the stretching vibration modes of Bi-S bonds [37].The peaks located at 2856-2926 cm −1 correspond to the symmetric and asymmetric stretching modes of CH 2 and CH 3 moieties of imidazolium ionic liquid alkyl chains, respectively [38].The characteristic bands between 400 and 600 cm −1 represent the Bi-S bond vibrations, which confirms the formation of the Bi 2 S 3 structure [39].However, the weak IR signal overlapping with other bands may prevent the appearance of the characteristic Ag/ AgCl peaks in the FTIR spectrum.

Morphological properties
The morphology and microstructure of Ag/AgCl/Bi 2 S 3 photocatalyst were studied using scanning electron microscopy (SEM).As presented in figures 3(a)-(b), Ag/AgCl/Bi 2 S 3 particles exhibit an irregular shape.Also, Scheme 1. Schematic representation of the photocatalytic reactor.
the particle size distribution is between 100-700 nm, as shown in figure 3(c).The distribution of key elements on the surface of nanomaterials was determined using the EDX-mapping technique (figure 4).While S and Bi exhibit a less uniform distribution, Ag and Cl show higher population.Mapping analysis indicates that the Ag/ AgCl particles are distributed well on the Bi 2 S 3 surface.Also, the sulfur element represents a higher percentage of distribution than bismuth which was expectable in terms of the Bi 2 S 3 intrinsic stoichiometry.EDX-elemental mapping simply provides clear evidence of how Ag/AgCl nanocrystals tend to grow on the surface of bismuth sulfide particles previously covered with ionic liquid as chlorine ions nodes.The EDX quantitative results, representing the atomic and weight abundances of various elements found in the main pattern, are illustrated in table 1.

Optical properties
The optical properties and electronic structure of the pure Bi 2 S 3 and Ag/AgCl/Bi 2 S 3 photocatalysts were studied using the UV-vis DRS analysis and Tauc plot method (figure 5).Compared to pure Bi 2 S 3 , the small red-shift in the absorption edge and increased absorption intensity in the Ag/AgCl/Bi 2 S 3 composite may be due to the presence of silver materials.Due to the SPR effect of the Ag NPs, the Ag/AgCl/Bi 2 S 3 sample has a narrower bandgap width (1.75 eV) and superior light-harvesting performance than pure Bi 2 S 3 (1.90 eV).Accordingly, it was expected that Ag/AgCl/Bi 2 S 3 composite shows improved photocatalytic activity toward the removal of dye compounds under visible-light irradiation.

Porosity analysis
The surface area and porosity of the Ag/AgCl/Bi 2 S 3 photocatalyst were determined using N 2 adsorptiondesorption isotherms, as shown in figure 6.The hysteresis loop in the relative pressure range of 0.3 to 1 corresponds to a type IV isotherm, which indicates a well-developed porous structure [40].According to the sharp pore distribution with an average diameter of 1.91 nm obtained by the BJH method, the microstructure shows microporous properties.The BET surface area of the Ag/AgCl/Bi 2 S 3 compound is determined to be  9.23 m 2 g −1 , which is pretty acceptable compared to the similar bismuth sulfide-based materials reported in the literature [41].As it has been repeatedly reported earlier, the addition of Ag/AgCl on the surface can ameliorate the BET specific surface area of the support materials due to its nano-dimension sizes and behavior which favorably affects both adsorption capacity and catalytic activity through providing more accessible active sites on the surface [42].

PL analysis
To further study the charge separation efficiency, the PL spectra of both pristine Bi 2 S 3 and Ag/AgCl/Bi 2 S 3 photocatalysts were investigated at room temperature.From figure 7, the intensity of the Ag/AgCl/Bi 2 S 3 emission band at around 450 nm is pretty lower than pure Bi 2 S 3 which implies the higher efficiency of   electron-hole separation over the former case.Accordingly, it was expected that the plasmonic photocatalyst promotes the AB92 degradation reaction more efficiently under irradiation due to the higher capacity of producing oxidative radicals on the surface.

Photodegradation of AB92 wastewater
AB92 solutions were treated with the photocatalyst at various conditions to evaluate the photoactivity of the asprepared material.The absorbance is dramatically decreased in all wavelengths for the AB92 spectra with the treatment time, implying that the photocatalyst can efficiently degrade the chromophore structure under illumination (figure 8(a)).Also, fitting the concentration values with the kinetic plots well illustrates that the photoreaction obeys pseudo first order kinetic (Ln (C 0 /C) is linear versus time) with the rate constant of 0.0171 min −1 (figure 8(b)).To clarify the effect of photoactivity on removal percentage, some primary tests including, dark test (to estimate the sorption capacity of the photocatalyst powder), photolysis (light alone contribution towards AB92 removal), and the pristine Bi 2 S 3 utilization as pure support were conducted at room temperature.As shown in figure 8(c), the photocatalyst powder alone (in the absence of light, dark test) and photolysis conditions indicated negligible contributions (less than 3 percent) in the removal yield which reveals that the removal is predominantly due to the photocatalytic activity of Ag/AgCl/Bi 2 S 3 .Furthermore, the pristine Bi 2 S 3 showed much less activity than the modified photocatalyst, demonstrating the effect of loaded Ag/ AgCl components on boosting the photocatalytic performance.In fact, Ag/AgCl makes the surface more capable of harvesting visible light photons to generate higher concentrations of charge carriers in the bulk structure.Due to migrating the photogenerated negative electrons and positive holes between the interfaces of support and silver species on the surface, charge carriers can survive for longer periods, resulting in a lower e-h recombination rate [43].The net effect is the production of more radical species in the aqueous medium, which is the main reason for degrading the chemical structure of AB92 molecules and better photocatalytic performance.
In order to optimize the operational conditions toward reaching the highest photocatalytic yield, some supplemental experiments were performed such as investigating the effect of catalyst dosage, AB92 concentration, and pH.From the obtained results, the photocatalytic efficiency after two hours of the reaction time increased with the enhancement of catalyst dosage from 0.015 to 0.025 g and then decreased (figure 9(a)).The blackish color of the photocatalyst powder makes the dye solution turned quickly opaque so that photons cannot easily penetrate the medium and promote the photodegradation pathway.Accordingly, the photocatalytic yield decreased dramatically when the catalyst amount increased from 0.025 to 0.045 g.To find the concentration of AB92 in which photodegradation is performed with a higher reaction rate, the removal experiments were conducted with dye solutions ranging from 5 to 35 ppm. Figure 9(b) indicated that the best performance was obtained at 15 ppm where 94.3% of photocatalytic removal was achieved and then reduced when the AB92 concentration enhanced to 25 and 35 ppm.This was also attributed to the effect of increasing solution turbidity by raising the dye molecules population, which retards the penetration of visible light photons into the photoreaction medium.The most suitable acidity for the better photodegradation performance of the Ag/AgCl/Bi 2 S 3 photocatalyst was recognized to be pH = 7.5 where the photocatalytic efficiency of 98.4% was obtained after 120 min of reaction time (figure 9(c)).At higher pH values, anionic AB92 dye molecules in the aqueous environment compete with OH − species to be adsorbed on the photocatalyst surface, which adversely affects the photocatalytic pathway and removal efficiency.At smooth basicity, pH of 7.5, the population of hydroxyl anions is still sufficient to generate hydroxyl radicals from the reactions with photogenerated positive holes, not such high to compete with the dye molecules for being adsorbed on the photocatalyst surface.
From a mechanistic point of view, the photodegradation of AB92 over the prepared photocatalyst can be illustrated as follow: Bi 2 S 3 and Ag nanoparticles on the surface are excited by photon absorption to generate electron-hole carriers.Electrons can transfer from Ag nanoparticles to the conduction band of AgCl and accumulate there.Oxygen molecules on the surface of the particles trapped these electrons to be converted to superoxide anion-radicals (O 2

•−
), which have great potential to degrade the AB92 chromophore.Making use of the Schottky barrier, the negative electrons of the Bi 2 S 3 conduction band migrate to the Ag nanoparticles to be neutralized by positive holes [42].On the other hand, photogenerated holes of the Bi 2 S 3 valence band transfer to the AgCl nanoparticles on the surface and react with chloride ions to create Cl • radicals which enable them to oxidize the AB92 chemical structure strongly and degrade it.As the previous researchers repeatedly highlighted this in their reports, the enhancement of the electron-hole recombination efficiency toward reaching an improved photocatalaytic performance owes the prominent bridge-like role of Ag nanoparticles between the Bi 2 S 3 and AgCl counterparts on the matrix surface of the plasmonic photocatalysts [36].The presence of Ag crystalline phase in our work was already confirmed by XRD analysis results.All the abovementioned statements are schematically explained in Scheme 2, demonstrating the charge-carrier separation through a logical S-scheme mechanism [44].
As depicted in figure 10, the results of quenching experiments revealed what we explained about the photoreaction mechanism.Through that, three compounds, isopropyl alcohol (IPA), triethanolamine (TEOA), and p-benzoquinone (p-BQ), were used to trap hydroxyl radicals, positive holes, and superoxide anion radicals, respectively.The highest amount of decrease in photocatalytic yield after 2 h of the reaction time with 0.025 g of catalyst powder and 15 ppm of dye concentration in pH 7 were related to TEOA and p-BQ, while isopropyl alcohol didn't affect it remarkably.This well demonstrates that hydroxyl radicals didn't play an eminent role in AB92 photodegradation over Ag/AgCl/Bi 2 S 3 in the aqueous phase and the key species were superoxide anionradical and positive photogenerated holes.To highlight the enhanced photoactivity of the Ag/AgCl/Bi 2 S 3 composite, its performance is compared with that of recently-reported photocatalysts based on the same support.From table 2, within 2 h of reaction time, our plasmonic photocatalyst showed a higher efficiency than most of the other Bi 2 S 3 -based photocatalysts towards removing organic contaminants under visible illumination.Although pollutant structural persistency against light-driven destructive processes is a key parameter in achieving higher removal efficiencies, the improved photon harvesting properties and suppressed electron-hole recombination on its surface play a prominent role in reaching a faster reaction rate.Considering its facile preparation method along with boosted antibacterial capacity, Ag/AgCl/Bi 2 S 3 can be introduced as a suitable candidate to remediate organic and biological contaminants from the environment.

Photoinactivation of E. coli and S. aureus pathogenic microorganisms
Silver-contained materials have been shown to be promising candidates for disinfecting bacterial wastewater [55,56].Accordingly, the antibacterial performance of Ag/AgCl/Bi 2 S 3 photocatalyst was explored against two bacterial strains, including gram-negative E. coli and gram-positive S. aureus.The initial concentration of the bacterial cell samples was adjusted to 10 7 CFU (colony-forming unit).At various time intervals, the sample suspension was analyzed based on the counting protocols explained in our previous work in detail [57].
From the results (figure 11), the number of colonies found in both bacterial strains became zero when the samples were treated under light, which illustrates the photoactivity of the powder toward the successful removal of pathogenic bacteria.On the other hand, the antibacterial performance decreased dramatically at dark, and according to the previous reports, this low antibacterial activity can be assigned to the chemical content of the materials (Ag species mainly) [58].To verify the destructive performance of the as-prepared photoactive structure against bacterial cells under light, TEM images of the bacteria were provided after 2 h of the treatment time with Ag/AgCl/Bi 2 S 3 .As shown in figure 12, the cell walls of both gram-negative and grampositive bacteria were seriously damaged with the photocatalytic treatment.During the reaction time, reactive oxygen species, predominantly superoxide anion radicals, are generated in the medium, which efficiently attacks  the outer shell of the bacterial cell wall and degrades it [59].Due to the thicker and more tenacious cell wall structure, gram-positive bacteria indicate more persistency against such attacks, and the rate of photoinactivation is lower for these organisms, as revealed in figure 11.Briefly, the observations implied the great potential of the prepared photocatalyst for being introduced as an antibacterial material.

Conclusions
Plasmonic Ag/AgCl/Bi 2 S 3 photocatalyst was prepared by a facile single-step co-precipitation method.In addition to the characteristic crystalline phases of Bi 2 S 3 , the growth of silver and silver chloride nanocrystals on the surface was well revealed from XRD.Based on DRS, the bandgap decreased from 1.90 eV for the pristine  Bi 2 S 3 to 1.75 eV for the plasmonic materials, manifesting the key role of surface modification in reducing the bandgap and improving the visible light harvesting properties.AB92 catalytic photodegradation was efficiently conducted over the photocatalysts under visible irradiation, and the highest efficiency of 98.4% was obtained for the plasmonic structure under the optimized conditions, including 0.025 g of photocatalyst dosage, 15 ppm of dye concentration, and pH 7.5.Quenching experiments revealed significant photo-generation of superoxide anion radicals in the reaction medium, which properly served to propose a logical mechanism of charge carriers' migration in the interfaces.Ag/AgCl/Bi 2 S 3 structure was also recognized to be a strong antibacterial material against E. coli and S. aureus microorganisms under the light.The sharp slope of Log(N) decay to zero within 2 h of photocatalytic treatment supports the abovementioned statement, which was further confirmed by TEM analysis of the bacterial cell walls.

Figure 3 .
Figure 3. (a), (b) The SEM images and (c) histograms of SEM length data for the Ag/AgCl/Bi 2 S 3 composite.

Figure 4 .
Figure 4. EDX spectrum and elemental mappings images of Bi, S, Ag, and Cl (inset) for the ternary Ag/AgCl/Bi 2 S 3 composite.

Figure
Figure The results of AB92 photocatalytic degradation over the prepared materials (a) UV-vis spectra of the solution during the treatment time, (b) Pseudo first-order kinetic of the reaction, and (c) Removal efficiency under various conditions.

Figure 9 .
Figure 9. Optimizing the operational conditions to achieve the highest AB92 photodegradation yield over the Ag/AgCl/Bi 2 S 3 photocatalyst: effect of (a) catalyst dosage, (b) AB92 concentration, and (c) pH.

Figure 11 .
Figure 11.The results of bacterial inactivation in the presence of Ag/AgCl/Bi 2 S 3 powder under visible light and dark conditions.

Figure 12 .
Figure 12.TEM images of S. aureus (a, b) and E. coli (c, d) bacterial cells after being treated with the final photocatalyst powder under visible light.

Table 1 .
The results of EDX analysis for the Ag/AgCl/Bi 2 S 3 composite.