Multifunctional Ag-doped CuS surface nanostructures for simultaneous dye degradation and bacterial inactivation via visible-light activation

Abstract

This work reports a new dual-strategy approach for the preparation of nanostructured Ag-doped CuS (Cu_1−xAg_xS, x = 0.03) photocatalysts designed by the synergy of morphology control and band gap engineering for enhanced environmental cleanup. The hexagonal covellite phase, the successful lattice substitution of Ag ions, and the change from strongly agglomerated 65 nm particles to well-dispersed 55 nm nanoparticles were confirmed by systematic characterization using XRD, FTIR, SEM, and EDX analyses. In addition, I–V analysis demonstrated a remarkable increase in conductivity. Together with UV-vis DRS, PL, electrochemical impedance spectroscopy (EIS), and transient photocurrent response analyses, the results demonstrated that Ag doping decreased the bandwidth from 1.69 eV to 1.57 eV, strongly inhibited charge recombination, and improved interfacial charge transfer. Thus, 3% Ag-doped CuS with the best performance shows excellent photocatalytic activity, with almost 98.2% MB degraded in 80 min (k = 0.029 min⁻¹), which is 3.6 times higher than the activity of pristine CuS. In addition, an anti-bacterial assay revealed a 2.12-fold increase in the zone of inhibition against Escherichia coli (23.3 mm). Mechanistic information based on radical scavenging experiments and electron paramagnetic resonance (EPR) spectra indicated that ˙OH and ˙O₂⁻ were the major reactive species for dye mineralization and bacterial inactivation. Cyclic degradation experiments and after-cycling XRD analysis demonstrated that the material could be reused several times without any structural degradation.

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1 Introduction

The development of industries and the medical sector guarantees human development and human health security. Unfortunately, most industries, including textile, leather, paint, food, cosmetic, hospital and pharmaceutical units, directly discharge their effluents into natural water bodies without treatment.¹ These industrial discharges, which mainly consist of several organic and inorganic pollutants, i.e., synthetic dyes, mineral acids, pesticides, antibiotics, hexavalent chromium, other heavy metals, and pathogenic microorganisms, severely endanger aquatic life and human beings when they enter the food chain.^2,3

Pollution of water sources also affects the agricultural sector, leading to a loss of crop quality and quantity due to irrigation with contaminated water. In addition, the consumption of contaminated water without treatment promotes the dissemination of a number of fatal water-borne diseases such as cholera and typhoid.⁴ The environmental and human health risks are further exacerbated by non-biodegradable, carcinogenic organic dyes and antibiotic residues, which can promote the growth of multiple-drug-resistant bacteria.⁵ As a result, before the disposal of polluted water into natural waterways, it is important to treat the water, particularly for the inactivation of pathogenic microbes (bacteria) and the degradation of synthetic dyes.⁶ Thus, there is an urgent need for innovative approaches to simultaneously combat bacterial proliferation and remove recalcitrant dyes. Many different processes have been implemented to remove integrated pollutants (dyes and pathogenic microbes) from ecosystems. These include physical processes (e.g., sedimentation, adsorption, flocculation, and filtration), chemical methods (e.g., ozone and ion exchange), and biological processes (e.g., bioremediation and activated sludge).^7–9 Unfortunately, each of these conventional techniques has several limitations, including high operational costs, the generation of hazardous secondary residues, low efficiency, and the need for extreme operating conditions (e.g., very high pressures or temperatures).¹⁰

Photocatalysis is increasingly considered a green alternative to conventional methods for wastewater treatment, as structurally modified semiconductor photocatalysts can efficiently produce electron–hole pairs upon irradiation with photons of energy greater than their band gap.^11,12 These charge carriers then interact with oxygen and water, forming reactive oxygen species (ROS).¹³ The ROS chemically interact with toxic dyes later on, annihilate them into non-polluting final products, and enhance bacterial inactivation by damaging intracellular components.¹⁴ Transition metal sulfides can serve as efficient photocatalysts on account of their adjustable band gaps and natural availability on Earth,¹⁵ but their serious application is restricted by fast charge carrier recombination and inferior charge transport kinetics.^16,17

There are many ways of enhancing catalytic performance, including doping, composites, and heterojunctions.^18–20 Copper sulfide is an n-type semiconductor photocatalyst. In its pristine form, its photocatalytic activity is limited because of its limited light-harvesting and fast charge recombination.²¹ The doping strategy has attracted attention for improving the bandgap for better light harvesting and charge separation/conductivity. Report after report shows the value of doping. Many reported works disclose the importance of doping in photocatalysis. Vinotha et al. successfully achieved 98% degradation of rhodamine B (RhB) with indium-doped CuS,²² while Cao et al. synthesized nanotube-like Zn-doped CuS with improved light absorption and thermal stability and successfully achieved 96% degradation of MB.²³ Ahmad et al. synthesized hexagonal In-doped CuS nanoparticles that showed superior antimicrobial activity against Mycobacterium smegmatis.²⁴

Herein, we present a hydrothermal approach for the preparation of novel Ag-doped CuS (Cu_1−xAg_xS) nanostructures that can be utilized as a unique multifunctional platform. After thorough physicochemical analysis, the bifunctional remediation potential of the material was assessed by means of the dye methylene blue (MB), an obstinate textile dye, and Escherichia coli, a common pathogenic bacterium. This new single-matrix concept could lead to dual-therapy treatment for the simultaneous removal of organic contaminants and microbial agents from challenging wastewater solutions. This new single-matrix concept could lead to dual-therapy treatment for the simultaneous removal of organic contaminants and microbial agents from challenging wastewater solutions.

2 Experimental section

2.1 Chemicals

Copper(II) nitrate trihydrate [Cu(NO₃)₂·3H₂O], thiourea (CH₄N₂S), and silver nitrate (AgNO₃) with high purity (>99%) were sourced from Sigma-Aldrich. The synthesis was conducted using deionized water.

2.2 Synthesis of Ag-doped CuS

In order to prepare 3 mol% silver-doped CuS (Cu_1−xAg_xS, where x = 0.03) NPs, a solution containing 2.345 g (10 mmol) of copper(II) nitrate trihydrate and 0.051 g (0.3 mmol) of silver nitrate was prepared and stirred. Then, 40 mL of deionized water was added to thiourea (0.761 g, 0.01 mol), and the mixture was stirred for 25 minutes. The thiourea solution was slowly added to the metal precursor solution while stirring until a homogeneous mixture formed, after which all the material was transferred to the autoclave and heated for 18 h at 180 °C. The precipitate was centrifuged after cooling to room temperature, then carefully washed with deionized water and ethanol until the liquid reached a neutral pH. Cu_1–xAg_xS nanopowder was dried overnight in an oven at 80 degrees Celsius. Undoped CuS nanoparticles were synthesized using the same technique, with the difference that the silver nitrate was removed from the precursor solution. The 1 mol%, 3 mol%, and 5 mol% Ag-doped CuS samples were synthesized and characterized to determine the best photocatalyst. Of all of them, the 3 mol% Ag-doped sample showed the strongest visible-light absorption and charge separation efficiency; thus, further studies concentrate on this optimized composition.

2.3 Photocatalytic experiment

The photocatalytic efficacies of pure CuS and doped Cu_1−xAg_xS were examined using MB as a model contaminant. Each catalyst was individually dispersed at a dosage of 10 mg in 50 mL of a methylene blue solution (5–25 ppm). The suspensions were stirred in the dark to reach a balance between desorption and adsorption after 30 minutes. When equilibrium was achieved, a sample of the suspension was removed to obtain the initial absorbance using a UV-vis spectrophotometer. A tungsten filament lamp was used to expose the catalyst-dye solution to visible light. The time exposure for Cu_1−xAg_xS was 80 minutes and that for pure CuS was 90 minutes. At 10 min intervals, aliquots (5 mL) were taken off and centrifuged, and the supernatants were analyzed for MB at its maximum absorption wavelength by UV-visible spectrophotometry. The equation given below was used to calculate the degradation efficiency:²⁵

% Photodegradation = [(C₀ − C_t)/C₀] × 100.

In this equation, C_t represents the absorbance at time = t min, while C₀ represents the starting absorbance at t = 0 min.

3 Results and discussion

3.1 XRD and FTIR

The crystalline structure of the synthesized nanomaterials was confirmed by X-ray diffraction studies. The XRD patterns of the freshly synthesized CuS and Cu_1−xAg_xS samples are shown in Fig. 1a. Seven sharp and well-defined diffraction peaks were observed in the XRD pattern of the unmodified CuS. These peaks, recorded at 2θ values of 27.9°, 29.5°, 32.0°, 33.25°, 48.4°, 53.2°, and 59.9° respectively correspond to the (101), (102), (103), (006), (110), (108), and (116) planes. These planes are in excellent agreement with JCPDS no. 01-078-0878,²⁶ confirming the synthesized hexagonal covellite-phased CuS. The observed X-ray diffraction pattern of the doped sample shows all the characteristic diffraction peaks of the hexagonal covellite structure of CuS. This confirms that the incorporation of Ag ions into the CuS lattice substitutes Cu⁺ ions within the lattice without forming any secondary phases.²⁷ In contrast, the XRD peaks of the doped sample are less intense, relatively broadened, and shifted towards lower diffraction angles compared to the CuS diffraction peaks. The decline in the peaks' intensity is ascribed to a decrease in crystallinity, possibly due to structural defect creation,^28,29 the peak broadening clearly suggests a decrease in the crystallite size, possibly due to limited grain boundary growth and controlled nucleation, while the peak shifting suggests lattice expansion, possibly due to the substitution of smaller Cu⁺ ions (0.77 Å) by bigger Ag⁺ ions (1.26 Å), as predicted by Bragg's law.

Fig. 1 (a) XRD patterns and (b) FTIR spectra of CuS and Cu_1−xAg_xS.

The findings from the FTIR analysis (Fig. 1b) support key insights regarding the chemical bonding and also provide key information about the surface chemistry. The FTIR spectrum for pristine CuS (i.e., CuS without any dopants) shows an intense Cu–S vibrational band at 650 cm⁻¹, which confirms that copper sulfide was successfully formed during synthesis.³⁰ In addition to the Cu–S vibrational band, the FTIR spectrum of pristine CuS shows secondary transmission bands of relatively low intensity, including C–H vibrations (1146 cm⁻¹, 2981 cm⁻¹),^31–33 nitrate remnants (1368 cm⁻¹),³⁴ O–H vibrations that correspond to adsorbed water (1585 cm⁻¹, 3491 cm⁻¹),^35,36 and atmospheric CO₂ (2336 cm⁻¹).³⁷ The spectrum of the Cu_1−xAg_xS retains all of its characteristic peaks (confirming its structural integrity); however, the main Cu–S band (peaking at approximately 627 cm⁻¹) shifts to lower wavenumbers and is less intense as a result of bond elongation and weakening due to the larger Ag⁺ ions effectively replacing Cu⁺ cations (which are smaller).³⁸ In addition, peaks corresponding to adsorbed water and CO₂ become more pronounced, indicating that the Ag dopant enhances surface porosity and active sites.

3.2 SEM and EDX

We conducted a SEM analysis to study the surface topography, surface defects, particle size, and particle distribution of the synthesized CuS and Cu_1−xAg_xS samples. The SEM micrograph of the CuS sample (Mag. 40 500×), shown in Fig. 2a, indicates that the sample has quasi-spherical agglomerated nanoparticles of uneven distribution. The inset histogram in Fig. 2a indicates that the nanoparticles of CuS are between 62 and 69 nm, with an average size of 65 nm. Fig. 2b shows the Cu_1−xAg_xS sample's SEM micrograph taken at a magnification of 42 000×. The Ag-doped CuS exhibits spherical nanoparticles in a well-dispersed state, reducing aggregation. The inset histogram in Fig. 2b shows a smaller particle size range of 53–57 nm, with a reduced average particle size of 55 nm. The doped sample demonstrated greater morphological consistency, smaller particle size, and better dispersion, resulting in a larger surface area. This enables more effective pollutant adsorption and increased photocatalytic effectiveness.³⁹

Fig. 2 (a) SEM image of CuS and (b) SEM image of Cu_1−xAg_xS. (c) EDX results of CuS and (d) EDX results of Cu_1−xAg_xS.

The elemental compositions of pure and Ag-doped CuS were determined using EDX. The EDX spectra for both synthesized samples are shown in Fig. 2c. The EDX spectrum of virgin CuS displayed four unique peaks: three associated with copper (Cu) at 0.93 keV (Lα), 8.04 keV (Kα), and 8.90 keV (Kβ) and a significant peak at 2.31 keV (Kα) attributable to sulfur (S).⁴⁰ In contrast, the EDX spectrum of Cu_1−xAg_xS (Fig. 2d) not only retains the original peaks of copper and sulfur but also has additional peaks at ∼3.0 keV (Lα), confirming the presence of silver (Ag) as a dopant.⁴¹ Statistical analysis of the Cu_1−xAg_xS sample's EDX spectrum showed 49.10 at% (63.95 wt%) for copper and 49.23 at% (32.35 wt%) for sulfur, while silver accounted for only 1.67 at% (3.70 wt%).

3.3 Optical and electronic characterization

The synthesized nanomaterials' optical characteristics were analyzed by UV-vis DRS to establish the effect of Ag substitution on the electronic structure and light-harvesting ability. As shown in Fig. 3a, the Cu_1−xAg_xS displays a distinct bathochromic shift of its absorption edge with respect to pristine CuS.⁴² This qualitative change indicates the high ability of the material to harvest photons from the visible range of the electromagnetic spectrum, which is important for visible-light-driven applications, i.e., photocatalysis. Tauc plots were extracted from the Kubelka–Munk function to quantify this electronic modulation.⁴³ The inception of Ag ion doping in the CuS is analyzed and clearly reduces the optical band gap energy considerably, with the band gap energy for undoped CuS found to be 1.69 eV and that for Ag-doped CuS being 1.57 eV (Fig. 3b).

Fig. 3 (a) UV-vis reflectance spectra, (b) Tauc plots, (c) PL spectra, (d) I–V profiles, (e) EIS plots, and (f) transient photocurrent responses of CuS and Cu_1−xAg_xS.

The finding of this 0.12 eV narrowing may be an important observation and can be attributed to the Ag-induced intermediate energy states in the forbidden band gap. These states lower the energy needed for electron transitions, thus creating charged particles through the reaction with photons of lesser energy. The improved photocatalytic performance is mainly due to this strategic band gap engineering. The narrow band gap directly relates to the higher generation rate of photogenerated electron–hole pairs (e⁻–h⁺) under visible-light irradiation. Also, the doped Ag⁺ ions can function as helpful electron-trapping sites, which spatially separate charge carriers and reduce their recombination. The improved photocatalytic degradation and antibacterial inactivation by the Ag-doped CuS nanocatalyst can be attributed to the enhanced light absorption and better charge separation dynamics, resulting in its superior efficiency. PL spectroscopy was employed to investigate charge-carrier dynamics. The intensity of the emission from Cu_1−xAg_xS was much lower than that from CuS, as can be seen in Fig. 3c. This means that the recombination rate of e⁻–h⁺ photogenerated in Cu_1−xAg_xS is much lower compared to that in CuS.⁴⁴ The strong emission band of the CuS sample is centered above 600 nm, which may be ascribed to the band-edge emission, while for the Cu_1−xAg_xS, this band shows a red shift and is observed above 650 nm, indicating that Ag doping has successfully tuned the band structure. Notably, the PL spectrum of the doped sample shows a pronounced hump at higher wavelengths, which may be attributed to defect-state-derived emissions.⁴⁵ Ag doping not only reduced the band gap to maximize quantum efficiency but also acted as an electron-trap center, prolonging charge-carrier lifetimes and making them available for surface redox reactions.⁴⁶

I–V analysis was used to assess the effect of silver doping on charge transport. As shown in Fig. 3d, the charge transport in Cu_1−xAg_xS is greatly improved, as indicated by the increased slope of the I–V curve. Based on the modified Ohm's law, the increased slope indicates greatly improved electrical conductance.^47,48 The greater improvement in conductance for Cu_1−xAg_xS can be ascribed to the changes in the local electronic environment caused by the partial replacement of Cu⁺ ions with Ag⁺ ions. The partial replacement of Cu⁺ ions with Ag⁺ ions makes it easier to transport charge carriers to the catalytic surface.

In addition, electrochemical impedance spectroscopy (EIS) and transient photocurrent response were performed to further understand the charge transfer kinetics. As shown in Fig. 3e, the arc radius of the Nyquist plot for the Cu_1−xAg_xS sample is much smaller than that of the pristine CuS. The decrease in the diameter of the semicircle represents the decrease in the charge transfer resistance at the interface of the electrode/electrolyte and faster interfacial charge transport.⁴⁹ In line with this, the transient photocurrent response in Fig. 3f demonstrates that the Ag-doped sample has significantly larger photocurrent density under visible light irradiation. The increased photocurrent further validates the efficient inhibition of e–h recombination and the rapid charge migration, also confirming the highest photocatalytic activity of the doped nanocatalyst.^50,51

3.4 Antimicrobial activity

As shown in Fig. 4a and b, the antibacterial activities of the CuS and Cu_1−xAg_xS nanocatalysts on the growth of the bacterium Escherichia coli were determined through the agar-well diffusion method. After incubation at 37 °C for 24 hours, both samples showed concentration-dependent antibacterial behavior, and Cu_1−xAg_xS was significantly more bactericidal at all concentrations tested (4–16 µg). For the optimal concentration of 16 µg, Cu_1−xAg_xS registered a markedly larger inhibition zone of 23.3 mm, which is 2.12 times the inhibition zone of 11 mm for the undoped CuS (Fig. 4c). The antibacterial mechanism for both materials is based on the photocatalytic generation of reactive oxygen species (ROS) upon visible-light irradiation (Fig. 4d). However, the antibacterial effect of CuS is inferior to that of Cu_1−xAg_xS owing to the lack of defect states or Ag+ states, leading to a high rate of charge recombination that reduces the total ROS population and limits its activity. In comparison, the enhanced and synergistic antibacterial performance of Cu_1−xAg_xS can be attributed to the introduction of Ag⁺ ions into the CuS lattice that intensifies photocatalysis from several aspects:⁵² (1) narrowing the bandgap down to 1.57 eV makes better use of visible light for higher efficiency, (2) Ag⁺ ions serve as an electron trapping site that reduces interfacial charge recombination, thus prolonging exciton lifetimes, and (3) extra antimicrobial behavior is associated with controlled release of Ag⁺ ion.

Fig. 4 Antibacterial activities of (a) CuS and (b) Ag-doped CuS. (c) Zone of inhibition comparison between CuS and Ag-doped CuS. (d) Photocatalytic destruction of E. coli.

3.5 Dye degradation activity

The photocatalytic activities of the pure CuS and Cu_1−xAg_xS nanocatalysts were systematically assessed using methylene blue (MB) as a model organic pollutant. The degradation process can be defined as two sequential steps: an initial stage of physical adsorption followed by photoinduced catalytic degradation. To achieve adsorption–desorption equilibrium, the catalyst-dye suspensions were swirled in the dark for 30 minutes before irradiation. UV-vis spectra at the characteristic absorbance of MB (665 nm) clearly revealed that the Ag-doped sample had a better adsorption capacity (11.5%) than the undoped CuS (6.4%) (Fig. 5a and b). This enhancement is attributed to the higher surface-to-volume ratio and the suitable surface morphology of the Cu_1−xAg_xS nanocatalyst.⁵³

Fig. 5 Photocatalytic degradation of MB dye over (a) CuS and (b) Cu_1−xAg_xS. (c) Comparative bar chart of the total removal efficiencies. (d) Pseudo-first-order kinetic plots illustrating the significantly enhanced degradation rate of Cu_1−xAg_xS.

Later, dye-degradation tests under light irradiation with a photocatalyst showed that the Cu_1−xAg_xS photocatalyst exhibited superior photocatalytic activity compared to bare CuS. Time-resolved UV-vis spectra showed that the degradation rate of Cu_1−xAg_xS was much faster, with 98.2% of total methylene blue (MB) degraded in 80 minutes (86.7% via photodegradation). CuS, in contrast, took 90 minutes to reach 53.1% (46.7% by photodegradation) removal (Fig. 5c). The observed significant improvement in the degradation efficiency of the Cu_1−xAg_xS, almost 45.1% higher than that of bare CuS, can be directly attributed to the synergistic effect of incorporating Ag⁺: a reduced band gap (1.57 eV versus 1.69 eV) enables easier access to visible light, and an increase in charge-separation efficiency reduces electron–hole recombination.

Kinetic analysis showed that the degradation of MB over both catalysts follows the pseudo-first-order kinetic model, and the correlation coefficients for the degradation process are high (Adj. R² > 0.97 in Cu_1−xAg_xS and 0.99 in CuS) as follows:⁵⁴

−ln (C_t/C₀) = kt.

The rate constant of Cu_1−xAg_xS (0.029 min⁻¹) is also 3.6 times greater than that of pure CuS (0.008 min⁻¹) (Fig. 5d), which definitely proves the catalytic superiority of the Ag-modified system.³⁸ These results support Cu_1−xAg_xS as a highly efficient visible-light-active photocatalyst in which the dual enhancement strategy, involving morphological optimization and band gap engineering, yields a synergistic effect that is much more efficient than the undoped version. The outstanding photocatalytic activity together with its proven antibacterial properties make Cu_1−xAg_xS a promising, versatile nanomaterial for a wide range of wastewater remediation processes.

3.6 Photocatalytic process optimization

The key operating conditions (MB dye concentration, Cu_1−xAg_xS dosage, pH, and temperature) for the Cu_1−xAg_xS-driven MB dye degradation reaction were optimized by studying their effects on the rate constant, as shown in Fig. 6a–d. The initial MB concentration had a significant impact on degradation productivity, and the optimal concentration (10 ppm) removed 98.2% (Fig. 6a). This concentration effect can be explained by the equilibrium between active sites and the photon penetration depth: below 10 ppm, the concentration is too low to allow full utilization of the catalyst; above that level, the light is screened, and the active sites become saturated.^55,56 The loading of catalysts showed a volcano-shaped dependence of the degradation efficiency, with a peak at 10 mg (98.2% removal) (Fig. 6b). However, catalyst overload (>10 mg) causes agglomeration (particle light scattering) and reduces the efficiency of photon utilization, thereby reducing the degradation rate.⁵⁷

Fig. 6 Effects of (a) MB dye concentration, (b) catalyst dosage, (c) pH value, and (d) temperature on the photocatalytic rate constant for MB dye degradation using Cu_1−xAg_xS.

The pH of the solution is one of the important characteristics that will affect the physical interaction between the catalytic surface and the MB molecules. The photocatalytic activity is at its maximum at a pH of 9 with k = 0.029 min⁻¹ (Fig. 6c). In alkaline conditions, the Cu_1−xAg_xS surface will have a hydroxyl negative charge, leading to an efficient electrostatic interaction with cationic MB dye, enabling efficient electron transfer. Meanwhile, the present resources of OH⁻ serve as the precursor for the generation of ˙OH, which is the primary oxidizing agent in the degradation process.⁵⁸ The temperature dependence showed an optimal operating temperature of 45 °C (k = 0.029 min⁻¹), and rate constants decreased at both lower and higher temperatures (Fig. 6d). The process is highly correlated with the Arrhenius law as the temperature rises from 25 °C to 45 °C to increase the rate of charge movement and enhance surface reaction rates. High temperatures (above 45 °C) may induce surface reconstruction or alteration, thereby promoting electron–hole recombination rates and ultimately reducing photocatalytic efficiency. These optimization studies determined the optimum operation parameters (10 ppm MB, 10 mg catalyst, pH 9, 45 °C) to achieve optimum photocatalytic efficiency of Cu_1−xAg_xS, which is useful in practical application in experimental wastewater treatment systems.

3.7 Analysis of active species and reaction mechanism

Radical-trapping experiments and electron paramagnetic resonance (EPR) spectroscopy were performed to specifically identify the role of reactive species in the photocatalytic pathway. The photocatalytic degradation of the substrate with no radical-trapping agent (as represented in Fig. 7a) had a rate constant of 0.029 min⁻¹, while the inclusion of a radical-trapping agent (P-BQ or IPA) decreased the rate constants to 0.009 and 0.013 min⁻¹, respectively, indicating that superoxide (˙O₂⁻) and hydroxyl (˙OH) free-radical species result from the photocatalytic process.

Fig. 7 Scavenger tests and EPR analysis. (a) Effects of scavengers on the rate constant k. (b) EPR signals of DMPO-˙O₂⁻ and DMPO-˙OH radicals in visible light.

The additional observation of significant inhibition of photocatalytic activity due to the presence of AgNO₃ and EDTA further suggests that both the electrons and holes generated by the photocatalytic process are necessary precursors for the formation of the radical species. The free-radical generation was confirmed by EPR spectra of both DMPO-˙O₂⁻ and DMPO-˙OH only under visible light, with no appearance of significant signals under dark conditions (Fig. 7b), providing direct evidence that the combined use of both radical species is responsible for the degradation of the photocatalytic substrate.⁵⁹ The photocatalytic and antibacterial properties of the Cu_1−xAg_xS nanocatalyst depend on the modification of its electronic structure and the control of its charge carrier density. To understand this, we first calculated the conduction band (CB) and valence band (VB) edge potentials using the absolute electronegativity of the semiconductor.

The band edge potentials of Cu_1−xAg_xS were calculated using the following two relations:⁶⁰

E_CB = χ − E_e − 0.5 × E_g,

E_VB = E_g + E_CB,

where χ is absolute electronegativity (6.0 for Cu_1−xAg_xS), E_e is the free electron energy on the hydrogen scale (4.5 eV), and E_g is the band gap. The conduction band potential of Cu_1−xAg_xS was estimated to be about +0.71 V (vs. NHE), which does not allow the single electron reduction of molecular oxygen to superoxide (O₂/˙O₂⁻, −0.33 eV), but the formation of ˙O₂⁻ radical was supported by EPR and scavenger tests. This apparent contradiction can be understood by the surface-controlled reduction mechanism with Ag dopants. The Ag positions act as binary centres at which molecular oxygen can be adsorbed. Substantial chemisorption of O₂ on the Ag surface decreases the activation energy and overpotential of the reduction reaction. This surface interaction stabilizes the transition state and alters the local reduction potential so that the otherwise prohibited reaction can proceed at a high rate. Plus, the current–voltage (I–V) curve of the doped material shows that charge carriers can be quickly transported to the catalyst surface once separated. This increased charge transport results in a higher reaction rate, as the photocatalytic rate constant of the doped sample (k = 0.029 min⁻¹) is significantly higher than that of the undoped sample (k = 0.008 min⁻¹).

Hence, under visible-light irradiation, the high rate of electron excitation, the optimized charge separation, and the high charge-transport rate, all facilitated by Ag doping, result in the effective formation of ROS. The Ag-mediated red shift in the band gap allows the utilization of a wide range of the visible spectrum as the initial stimulus for the photocatalytic process. The excited electrons are then effectively trapped by the Ag dopant, where they reduce adsorbed molecular oxygen (O₂) to form superoxide anion radicals (˙O₂⁻).⁶¹ At the same time, the photogenerated holes (h⁺) in the valence band (+2.28 eV) have a high positive potential and effectively oxidize the adsorbed hydroxyl (⁻OH) or water molecules (H₂O) to form hydroxyl radicals (˙OH), as OH⁻/˙OH is 1.99 eV vs. NHE.^62,63 Fig. 8 illustrates the proposed dual-functionality mechanism for both dye degradation and bacterial inactivation. As confirmed by our scavenger experiments, the ˙OH and ˙O₂⁻ radicals play the main role in the multifunctionality of the system. These non-selective species attack the chromophore group in the MB dye and mineralize it to harmless CO₂ and H₂O. Additionally, the reactive oxygen species (ROS) create an environment of lethal oxidative stress for E. coli bacteria, thus degrading the integrity of the cell membrane and causing the bacteria to die. The stepwise photocatalytic reactions that occur here are as follows:

Ag-doped-CuS + hν → e⁻ (trap) + h⁺ (VB),

e⁻ (trap) + O₂ → ˙O₂⁻,

H₂O/OH⁻ + h⁺ (VB) → ˙OH,

˙O₂⁻/˙OH + MB dye → Degraded products,

˙O₂⁻/˙OH → Cell Inactivation.

Fig. 8 Schematic of the proposed mechanism of antibacterial activity and methylene blue (MB) dye degradation facilitated by the Cu_1−xAg_xS nanocatalyst.

3.8 Stability and reusability assessment

To explore the reusability of the doped photocatalyst, cyclic degradation experiments were conducted over five runs. Fig. 9a clearly shows that the Ag-doped CuS maintained stable performance during the photocatalytic reactions, with only a slight decline in initial activity at the completion of five successive runs, which may be due to surface passivation or to inevitable catalyst loss during the recovery process.⁶⁴ Fig. 9b shows that the diffraction patterns of the sample before (red) and after (black) the photocatalytic experiments were almost identical, revealing that catalyst's crystal structure remains chemically and structurally stable during the photocatalytic process.

Fig. 9 (a) Reusability assessment of the Ag-doped CuS photocatalyst over five cycles. (b) XRD patterns of the fresh and used catalyst.

4 Limitations and future outlook

Herein, we demonstrate nanostructured Ag-doped CuS as a multifunctional material with excellent visible-light-driven photocatalytic activity for methylene blue degradation and E. coli inactivation which provides great potential for the simultaneous treatment of these two pollutants. However, the practical use of the as-prepared photocatalyst in realistic wastewater treatment is challenging due to the complexity of multiple pollutants and possible ion interferences. In addition, the present study is limited by the lack of ICP-AES, LSPR, and XPS analyses to obtain quantitative elemental composition, plasmonic properties, and confirmation of surface chemical states. In the future, these gaps will be filled to the full extent to understand the degradation mechanisms and surface electronic properties.

5 Conclusions

This study was motivated by the issue of slow charge transport and fast recombination in CuS. We were interested in narrowing the band gap, modifying the structure, and enhancing conductivity through controlled Ag doping. I–V analyses showed that electrical conductivity was enhanced by Ag, enabling greater charge transfer and thus better results. At pH 9 and 45 °C, this silver doping in CuS significantly enhanced the degradation rate and antibacterial activity by 3.6-fold. The results indicate that lattice-selective doping converts CuS into a multifunctional photocatalyst with integrated features that simultaneously achieve efficient pollutant degradation and rapid antimicrobial aptitude.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data will be made available upon request.

Acknowledgements

The authors express their gratitude to the Princess Nourah bint Abdulrahman University Researchers Supporting Project (Grant No. PNURSP2026R12) and the Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. The authors extend their appreciation to the Deanship of Scientific Research, Islamic University of Madinah, Saudi Arabia, for funding this research work.

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