Plasmonic Ag/AgCl/Bi₂MoO₆ photocatalyst: Dual-functional bacteriostatic and sulfamerazine degradation via synergistic LSPR and Z-scheme charge transfer

2.1. Materials

The materials details are provided in the Supporting Information.

2.2. Preparation of BMO

In the experimental procedure, Bi(NO₃)₃·5H₂O (5 mmol) was introduced into beakers with 30 mL of glycol solution and agitated until complete dissolution was achieved. Subsequently, Na₂MoO₄·2H₂O (0.36 mmol) was added to the solution, and the mixture was subjected to vigorous stirring for 30 min to ensure the homogeneous dispersion. The pH of the resulting solution was meticulously adjusted to 6 using HCl (0.1 M) or ammonia solution, and the mixture was allowed to equilibrate for 24 h at ambient temperature. The solution was then subjected to thermal treatment at 140°C for 24 h. Following centrifugal washing with water and ethanol (8000 rpm for 10 min), the product was filtered and isolated. The sample was then drying at 60°C to obtain the Bi₂MoO₆ sample, henceforth referred to as BMO.

2.3. Preparation of silver chloride/bismuth molybdate (AgCl/BMO)

During synthesis process, 60 mL of ultra-pure water was measured into a beaker, and 0.40 g of the synthesized BMO was introduced, followed by ultrasonication for 30 min to obtain a uniform suspension. Subsequently, 0.18 g of NaCl was added, and the mixture keep stirring for 30 min. Concurrently, AgNO₃ (0.0714 g) was dissolved in a separate beaker containing 20 mL of ultra-pure water, and this solution was slowly introduced to the mixture under dark conditions, with continuous stirring for 60 min. The impurities were removed by centrifugal washing (8000 rpm for 10 min) and the resulting sediment was collected, followed by overnight drying at 60°C. After grinding, the final sample was obtained and designated as AgCl/BMO.

2.4. Preparation of silver/silver chloride/bismuth molybdate (Ag/AgCl/BMO)

60 mL ultra-pure water together with 0.30 g of prepared AgCl/BMO were added into a beaker. After stirring for 30 min, the sample was irradiated with a 300 W xenon lamp (CEL-HXF300, Beijing) for 60 min. The sediment was separated by centrifugal washing (8000 rpm for 10 min), and then dried overnight at 60°C. Finally, the prepared sample denoted as Ag/AgCl/BMO.

2.5. Characterization

The characterization details are provided in the Supporting Information.

2.6. Photocatalytic sterilization experiment

The photocatalytic bactericidal activity of Ag/AgCl/BMO was assessed using E. coli (10⁵ cfu/mL in phosphate-buffered saline (PBS)). In sterile quartz tubes, 10 mg catalyst was mixed with 50 mL bacterial suspension and irradiated under 300 W Xe lamp with a light intensity of 160 mW/cm². The distance between the lamp and the surface of the reaction solution was fixed at 10 cm. At 15 min intervals, 1 mL of the solution was extracted, diluted with PBS, and evenly spread onto agar plate. The plates were inverted and incubated at overnight 37°C to promote bacterial growth. Controls included: (1) dark control (light-shielded), and (2) blank control (no catalyst). All experiments were performed in triplicate, with survival concentrations and bactericidal rates calculated from mean colony counts.

The OD600 test, preparation of luria-bertani broth​ (LB) solid medium and tablet, determination of bacterial ROS production, and determination of the permeability of bacterial cell membrane details are provided in the Supporting Information.

2.7. Photocatalytic degradation of SM

SM was selected as the model pollutant to assess the photocatalytic activity. A 50 mL solution of 10 mg/L SM was combined with 20 mg of photocatalyst in a sterile quartz tube. The mixture was stirred in the dark for 30 min to establish adsorption-desorption equilibrium. Photocatalytic degradation was initiated using a 300 W xenon lamp with 420 nm cut-off filter (130 mW/cm²). The distance between the lamp and the surface of the reaction solution was fixed at 10 cm. During the 2.5 h reaction under continuous stirring, 4 mL aliquots were collected every 30 min for high-performance liquid chromatography (HPLC) analysis of SM concentration. The total organic carbon (TOC) content of the samples was determined using a TOC analyzer. The experiment on the effect of cations and anions on degradation efficiency involved adding 5 mM solutions of NaCl, KCl, CaCl₂, NaNO₃, Na₂CO₃, Na₂SO₄ respectively into the photocatalytic system, while keeping all other procedures unchanged.

2.8. Active free radical capture and cyclic stability experiment

The experiment details are provided in the Supporting Information.

3.1. Crystal structure and morphology of Ag/AgCl/BMO

The crystal phase structures of BMO, AgCl/BMO and Ag/AgCl/BMO were shown in Figure 1. The peak position at 2θ=28.3^o, 32.5^o, 32.6^o, 33.1^o, 46.7^o, 55.5^o, and 75.9^o corresponds to (131), (200), (002), (060), (202), (331) and (391) crystal faces of the pristine BMO, respectively, which is consistent with the Bi₂MoO₆ standard card (JCPDS: 72-1524) [27]. The AgCl/BMO photocatalyst exhibited diffraction peaks for both BMO and AgCl. The 2θ values of 27.8°, 32.2°, 46.2°, and 55.4° aligning with the (131), (200), (220), and (222) planes are consistent with the standard peaks of AgCl (JCPDS: 85-1355) [28]. The Ag/AgCl/BMO photocatalyst further displayed a principal Ag peak at 38.3°, corresponding to the (111) plane (JCPDS: 87-0719) [28], confirming the simultaneous presence of BMO, AgCl, and Ag. These findings indicate that Ag/AgCl has been effectively deposited on BMO without altering its crystalline phase or structure.

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Figure 1.

The XRD pattens of BMO, AgCl/BMO and Ag/AgCl/BMO.

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The morphology and crystal structure of the synthesized BMO and Ag/AgCl/BMO were further characterized by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The catalysts modified with AgCl and Ag/AgCl retained the nanoflake-like morphological characteristics of pristine BMO (Figures 2a-c). Upon Ag/AgCl loading, numerous irregular Ag/AgCl nanoparticles are observed on the BMO surface. HRTEM analysis reveals distinct lattice fringes with spacings of 0.236, 0.167, and 0.276 nm, corresponding to the (111) plane of Ag, the (311) plane of AgCl, and the (200) plane of BMO, respectively (Figure 2d) [29]. These results confirm the successful deposition of Ag/AgCl on the BMO surface, and the Ag NPs are spherical with a narrow size distribution uniformly dispersed on the heterostructure substrate.

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Figure 2.

TEM images of (a) BMO, (b) AgCl/BMO and (c) Ag/AgCl/BMO, and (d) HRTEM image of Ag/AgCl/BMO.

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The surface chemical composition and chemical states of BMO, AgCl/BMO and Ag/AgCl/BMO were investigated using x-ray photoelectron spectroscopy (XPS). The survey spectrum confirmed the presence of Bi, Mo, O, Ag, and Cl elements in the Ag/AgCl/BMO composite (Figure 3a). XPS analysis revealed that in pristine BMO, the Bi 4f spectrum exhibits two characteristic peaks at binding energies of 158.9 eV and 164.2 eV, corresponding to Bi³⁺ 4f_7/2 and Bi³⁺ 4f_5/2 states, respectively [30]. Notably, upon formation of the Ag/AgCl/BMO heterostructure, the Bi 4f peaks exhibit a positive shift toward higher binding energies compared to pure BMO (Figure 3b), suggesting altered electronic interactions in the composite system.

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Figure 3.

High resolution XPS (a) the survey spectra, (b) Bi 4f, (c) Mo 3d, (d) Cl 2p, (e) Ag 3d spectra of BMO, AgCl/BMO and Ag/AgCl/BMO.

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The high-resolution Mo 3d XPS spectrum of BMO exhibits two characteristic peaks at 232.2 eV and 235.3 eV, corresponding to Mo⁶⁺ 3d_5/2 and Mo⁶⁺ 3d_3/2 states, respectively (Figure 3c) [30]. Notably, in Ag/AgCl/BMO, the Mo 3d peaks undergo a positive shift in binding energy, consistent with the behavior observed for Bi 4f. This shift suggests electron depletion resulting from interfacial charge transfer within the heterojunction structure. The Cl 2p spectrum of Ag/AgCl/BMO displays two distinct peaks at 198.0 eV (Cl 2p_3/2) and 199.8 eV (Cl 2p_1/2), consistent with the presence of chloride species (Figure 3d) [31]. Furthermore, in Ag/AgCl/BMO, the Ag 3d peaks appear at 367.0 eV (Ag 3d_5/2) and 372.9 eV (Ag 3d_3/2) (Figure 3e). Deconvolution of these peaks reveals contributions from both Ag⁰ and Ag⁺ species, with fitted peaks at 367.0 eV (Ag⁺), 368.3 eV (Ag⁰), 372.9 eV (Ag⁺), and 373.6 eV (Ag⁰), confirming the coexistence of metallic Ag and AgCl in the composite, which aligns with the XRD findings. Based on the synthesis procedure, Ag/AgCl accounts for approximately 20% of the substrate. According to XPS analysis, Ag⁰ constitutes about 32% of all Ag species. Therefore, it can be inferred that Ag nanoparticles represent roughly 6.7% of the BMO substrate. Notably, compared to AgCl/BMO, the Ag 3d peaks in Ag/AgCl/BMO exhibit a negative binding energy shift. This shift likely arises from the interfacial electronic interactions among Ag, AgCl, and BMO, which modulate the local electronic environment of Ag species.

3.2. Photochemical properties of Ag/AgCl/BMO

The light absorption properties of BMO, AgCl/BMO and Ag/AgCl/BMO were investigated by UV-vis spectroscopy. As shown in Figure 4(a), the pristine BMO exhibited an absorption edge at 498 nm, corresponding to its intrinsic bandgap. Upon incorporation of Ag/AgCl, the absorption edge of Ag/AgCl/BMO red-shifted to 539 nm, indicating extended light harvesting capability. Notably, the Ag/AgCl/BMO composite demonstrated significantly enhanced absorption intensity across the visible spectrum compared to both BMO and AgCl/BMO, suggesting improved photon utilization efficiency.

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Figure 4.

(a) UV-vis absorption, (b) PL spectra, (c) EIS and (d) time-photocurrent curve diagram of BMO, AgCl/BMO and Ag/AgCl/BMO.

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To evaluate the charge carrier dynamics, the photogenerated carrier separation efficiency of BMO, AgCl/BMO, and Ag/AgCl/BMO were systematically investigated using photoluminescence spectroscopy (PL) spectroscopy, electrochemical impedance spectroscopy (EIS), and photocurrent response measurements. The PL spectra revealed a sequential quenching of emission intensity upon modification with AgCl and Ag/AgCl, with Ag/AgCl/BMO exhibiting the most pronounced suppression (Figure 4b) [32]. The marked decrease in fluorescence intensity indicates that the integration of AgCl and Ag nanoparticles significantly enhances charge separation, thereby boosting the photocatalytic performance of the composite system. EIS analysis showed that Ag/AgCl/BMO possessed the smallest arc radius in Nyquist plots compared to pristine BMO, indicating superior charge transport properties and more efficient interfacial charge transfer (Figure 4c). Furthermore, transient photocurrent measurements demonstrate that Ag/AgCl/BMO generated the highest current density under visible light illumination, providing direct evidence for enhanced separation of photogenerated electron-hole pairs (Figure 4d). The enhanced charge transport and separation efficiency can be attributed to the synergistic effect between Ag/AgCl and the LSPR of Ag NPs, which collectively facilitate charge carrier separation and migration.

3.3. Photocatalytic performance of Ag/AgCl/BMO

The visible-light photocatalytic performance of BMO, AgCl/BMO, and Ag/AgCl/BMO was assessed using E. coli as a model pollutant. The E. coli cell density remained stable, confirming the non-toxic nature of all three materials under dark conditions (Figure 5a). As shown in Figure 5(b), under visible-light irradiation, the blank control (Light control) exhibited negligible E. coli inactivation, demonstrating that visible light alone had no bactericidal effect. After 60 min of irradiation, the Ag/AgCl/BMO composite exhibited significantly enhanced photocatalytic activity compared to BMO and AgCl/BMO, achieving complete sterilization (∼5 log₁₀ cfu/mL reduction).

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Figure 5.

Temporal course of E. coli survival curve in aqueous dispersions containing photocatalysts (a) in dark and (b) under visible light irradiation, (c) Photocatalytic degradation efficiency of SM and (d) curve fitting of SM degradation over BMO, AgCl/BMO and Ag/AgCl/BMO.

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SM was chosen as a model antibiotic contaminant to assess the visible-light photocatalytic degradation activity of Ag/AgCl/BMO. Negligible degradation of SM occurred in the presence of BMO, AgCl/BMO, or Ag/AgCl/BMO under dark conditions, confirming that degradation was photo-driven. Under visible-light irradiation, pristine BMO exhibited minimal photocatalytic activity, with almost no degradation after 60 min (Figure 5c). In contrast, AgCl/BMO achieved 65% degradation within 60 min, while Ag/AgCl/BMO completely degraded SM (100%) in 30 min, exhibiting exceptional photocatalytic performance. The photocatalytic degradation kinetics of SM were analyzed using a first-order reaction model. Linear fitting of the degradation profiles (Figure 5d) yielded the rate constants presented in Table S1. The calculated rate constants for BMO, AgCl/BMO, and Ag/AgCl/BMO were 0.00096, 0.01682, and 0.08718 min⁻¹, respectively. Notably, the Ag/AgCl/BMO heterojunction exhibited a remarkable 90-fold enhancement in apparent rate constant compared to pure BMO. The enhanced degradation performance of Ag/AgCl/BMO and AgCl/BMO compared to pristine BMO can be attributed to: (1) The formation of a heterostructure, facilitating charge separation, (2) the LSPR effect of Ag NPs, which enhances light absorption and promotes reactive species generation [33]. The comparison on photocatalytic activity of Ag/AgCl/BMO with other reported dual-functional photocatalyst has been shown in Table S2. The effect of common anions (Cl^-, NO₃^-, CO₃^2-, SO₄^2-) and cations (Na⁺, K⁺, Ca²⁺) on the photocatalytic performance of Ag/AgCl/BMO have been evaluated (Figure S1). The results indicate that all ions caused only a slight suppression of activity, which we attribute to competitive adsorption at the active sites. This finding underscores the potential of Ag/AgCl/BMO for application in complex aqueous environments.

The comparative analysis of TOC values for SM degradation under light only, dark/Ag/AgCl/BMO, and light/Ag/AgCl/BMO were shown in Figure S2. After 30 min of reaction, the light/Ag/AgCl/BMO system achieved the highest mineralization rate of 29%. These results demonstrate the superior degradation efficiency of the Ag/AgCl/BMO photocatalyst system, which can be attributed to its ability to generate reactive species that promote deep SM degradation.

The stability evaluation of Ag/AgCl/BMO through repeated photocatalytic sterilization cycles (60 min each) (Figure S3). The results demonstrate that Ag/AgCl/BMO exhibits not only high photocatalytic activity but also excellent chemical stability. After four consecutive cycles, the antibacterial efficiency against E. coli decreased by only ∼0.8 log₁₀ CFU/mL, indicating minimal loss of catalytic performance. This confirms the robust and sustainable photocatalytic capability of Ag/AgCl/BMO.

3.4. Discussion of photocatalytic mechanism of Ag/AgCl/BMO

3.4.1. Active free radical detection

To elucidate the bactericidal mechanism of the Ag/AgCl/BMO composite catalyst, the active radicals involved in the photocatalytic process were investigated. Radical scavengers including tert-butyl alcohol​ (TBA) (for ·OH), 4-Hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) (for ·O₂^-), KI (for h⁺), and Cr(VI) (for e^-) were employed to identify the key reactive species. Under dark conditions, the E. coli cell density remained stable (Figure 6a), confirming that the scavengers themselves exhibited no bactericidal effect. As shown in Figure 6(b), the addition of Cr(VI) did not significantly hinder bacterial inactivation, indicating that e^- played a minor role. Similarly, KI only slightly reduced the sterilization efficiency, suggesting a limited contribution of h⁺. In contrast, both TBA and TEMPOL dramatically suppressed the bactericidal activity, demonstrating that ·OH and ·O₂^- were the dominant reactive species responsible for E. coli inactivation. Thus, our findings confirm that ·OH and ·O₂^- are the primary active radicals in the photocatalytic sterilization process mediated by Ag/AgCl/BMO. Moreover, the results of radical trapping experiments in SM degradation system over Ag/AgCl/BMO indicating that the ·O₂⁻ is the dominant reactive spicy (Figure S4).

Figure S4

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Figure 6.

Effect of scavengers on survival curve for Ag/AgCl/BMO (a) under visible light irradiation and (b) in dark, the yield of (c)·OH and (d)·O₂^- at different time intervals. (e) Proposed degradation pathway of SM during Ag/AgCl/BMO system.

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To further verify the generation of ·O₂^- and ·OH in the Ag/AgCl/BMO system, quantitative radical detection experiments were performed. The Ag/AgCl/BMO composite exhibited significantly enhanced ·O₂^- and ·OH production under 60 min of illumination, reaching 2.8 µmol/L and 0.78 µmol/L, respectively, compared with pristine BMO (Figures 6c-d). These results provide direct evidence for the efficient generation of these ROS in the Ag/AgCl/BMO system.

Based on the product distribution in the current catalytic system, as analyzed by HPLC-MS (Figure S5), and in consideration of relevant literature, we propose the reaction pathways for the degradation of SM by the catalyst (Figure 6e). The active species (holes, ·OH, and ·O₂^-) can selectively degrade the target pollutant into various intermediates or directly into end products (CO₂ and H₂O). The cleavage of the N–S bond in SM is proposed as the primary degradation pathway, leading to the formation of intermediates C2 and C3 (pathway A). Additionally, intermediate C4 is generated through the cleavage of S–C bonds (pathway B). Throughout the process, hydrogen abstraction and oxidation by ·OH/·O₂^- radicals are involved.

Figure S5

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To elucidate the photocatalytic bactericidal mechanism, flow cytometry was employed to analyze ROS generation and cell membrane permeability in E. coli (Figures 7a and b) [34]. The BMO treatment group exhibited a 2.1-fold increase in ROS production compared to the control after 60 min of visible light irradiation. The AgCl/BMO composite showed further enhancement, with ROS levels reaching 4.2 times that of the control. Notably, the Ag/AgCl/BMO system demonstrated the highest ROS generation, exhibiting an 11.2-fold increase relative to the untreated group, confirming its superior photocatalytic activity. These findings collectively demonstrate that the synergistic effect in Ag/AgCl/BMO significantly promotes ROS production, which plays a crucial role in bacterial inactivation. The effect of Ag/AgCl/BMO on E. coli cell membrane permeability under photoexcitation was investigated. After 60 min of visible light irradiation, the cell membrane permeability increased by 1.8-fold in pure BMO treatment groups compared to the control. The AgCl/BMO composite showed enhanced activity, with membrane permeability reaching 2.4 times that of the control. Notably, the Ag/AgCl/BMO treatment demonstrated superior performance, exhibiting a 4.8-fold increase in membrane permeability compared to the control, significantly surpassing both BMO and AgCl/BMO treatments. Under solar irradiation, Ag/AgCl/BMO generates various ROS, which oxidize E. coli cells, damaging their membrane integrity. This increases membrane permeability, ultimately leading to membrane rupture, leakage of intracellular components, and cell death [35]. Moreover, the morphological changes of E. coli cells before and after the photocatalytic disinfection were examined. Prior to treatment, E. coli exhibited a well-defined rod-shaped structure with intact intracellular components, including faint nucleoid regions in the cytoplasm (Figure 7c). After 0.5 h of treatment, the central cytoplasmic area became translucent and indistinct, indicating leakage of cellular contents and initial disruption of the cells (Figure 7d). Upon extending the reaction time to 1 h, the rod-like morphology was markedly distorted, suggesting irreversible damage to the cell wall and cytoplasmic membrane (Figure 7e) [36,37].

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Figure 7.

Fold changes of (a) ROS production and (b) cell membrane permeability of BMO, AgCl/BMO and Ag/AgCl/BMO. The individual morphology changes of the E. coli (c) before and after disinfection for (d) 0.5 h, (e) 1h over Ag/AgCl/BMO under light irradiation.

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3.4.2. Photocatalytic degradation mechanism

The optical bandgap was determined by applying the Tauc plot method, assuming a direct bandgap semiconductor model [38]. The bandgap narrowed from 2.74 eV to 2.59 eV after Ag/AgCl modification (Figure 8a). This reduction demonstrates enhanced visible-light absorption, primarily attributed to the synergistic effects of Ag’s LSPR and interfacial interactions between Ag/AgCl and BMO. Notably, Ag NPs in the composite originates from the partial photoreduction of AgCl during synthesis. Mott-Schottky analysis confirms the n-type semiconductor nature of all samples, as indicated by the positive slopes of their respective curves (Figure 8b) [39]. Consequently, the flat-band potentials of BMO, AgCl/BMO and Ag/AgCl/BMO were determined to be -1.02 eV, -0.53 eV and -0.43 eV vs. Ag/AgCl, corresponding to -0.41 eV, 0.08 eV, 0.18 eV vs. NHE, respectively. Furthermore, the energy differences between the Fermi level and valence band (VB) potential of BMO, AgCl/BMO and Ag/AgCl/BMO were determined to be 1.69 eV, 1.99 eV and 1.97 eV, respectively, as measured by VB XPS (Figure 8c) [40]. Thus, the valence band (VB) potentials of BMO, AgCl/BMO, and Ag/AgCl/BMO were determined to be 1.28 eV, 2.07 eV, and 2.15 eV, respectively. It is noteworthy that the downward shift of the overall valence band of the catalyst implies a higher oxidation potential of photogenerated holes (h⁺), thereby significantly enhancing the catalyst’s oxidative capacity towards pollutants.

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Figure 8.

(a) The bandgap, (b) Mott-Schottky plots, and (c) Valence-band XPS spectra of BMO, AgCl/BMO and Ag/AgCl/BMO.

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Combining band structure analysis with radical trapping experiments, a photocatalytic mechanism for the Ag/AgCl/BMO system was proposed. The band alignment shows distinct potential differences: AgCl (E_CB=-0.06 eV, E_VB=3.2 eV) [41] and BMO (E_CB=-1.46 eV, E_VB=1.28 eV). This favorable band structure facilitates efficient charge separation through Z-scheme electron transfer between the components, accounting for both the enhanced bactericidal activity and organic pollutant degradation performance.

As illustrated in Figure 9, under visible light irradiation, the LSPR effect of Ag NPs facilitates the generation of electron-hole pairs. According to the theoretical calculations, the energy level alignment reveals that the Fermi level of metallic silver (Ag) is higher than that of silver chloride (AgCl) (Figure S6 and Table S3). This disparity drives electron transfer from Ag to AgCl upon contact, resulting in the formation of a built-in electric field and an upward band bending of AgCl at the interface. The LSPR-excited hot electrons (e^-) possess energies ranging from 1.0 to 4.0 eV relative to the metal’s Fermi level, endowing these excited e^- with sufficient energy to transfer from Ag NPs to the conduction bands (CB) of both AgCl and BMO [42]. The holes (h⁺) remaining on the Ag NPs surface recombine with the photogenerated e^- from AgCl, forming a Z-scheme-like structure. This configuration effectively preserves the photogenerated e^- with strong reduction capability and the h⁺ with potent oxidation capability, thereby significantly enhancing the spatial separation of photogenerated electron-hole pairs.

Figure S6

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Table S3

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Figure 9.

Photocatalytic mechanism of Ag/AgCl/BMO.

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The CB edge potential of BMO (-1.86 eV vs. NHE) is more negative than the standard redox potential of O₂/·O₂^- (-0.33 eV vs. NHE), enabling the reduction of molecular oxygen to superoxide radicals (·O₂^-) by BMO’s CB electrons. Meanwhile, the valence band (VB) potential of AgCl exceeds the standard redox potential of ·OH/H₂O (+1.99 eV vs. NHE), allowing the oxidation of surface-adsorbed water molecules to hydroxyl radicals (·OH) by AgCl’s VB holes.

In the Ag/AgCl/BMO ternary system, ·O₂^- and ·OH serve as the primary ROS responsible for both bacterial inactivation and pollutant degradation. The antibacterial mechanism involves: (1) ROS attacking and disrupting E. coli cell walls and membranes, (2) subsequent oxidation of intracellular proteins and nucleic acids, and (3) eventual cell lysis through membrane rupture and cytoplasmic leakage. Simultaneously, ·O₂^- oxidizes SM, ultimately mineralizing it into harmless inorganic compounds.