Oxygen Vacancy-Engineered Cd_1–x Ag _x O_1–y Nanostructures for Visible-Light-Driven Hazardous Dye Remediation

Abstract

Addressing the intertwined challenges of sustainable wastewater remediation and the development of efficient heterogeneous photocatalysts for visible-light applications remains a pivotal objective in environmental nanotechnology. This study presents the rational design and development of oxygen-vacancy-engineered Cd_1–x Ag _x O_1–y nanostructures as next-generation visible-light-active photocatalysts for environmental remediation. The incorporation of Ag dopants in a CdO matrix not only modulates the lattice architecture but also promotes the formation of electronically active oxygen vacancies, thereby enhancing the material’s optoelectronic properties. Detailed structural and spectroscopic investigations confirm that 3 wt % Ag doping induces lattice strain, narrows the bandgap, and introduces midgap states conducive to visible-light absorption and prolonged charge carrier lifetimes. These tailored features culminate in superior photocatalytic degradation of Amido Black 10B dye, achieving 91% removal within 24 min under 450 nm LED illumination, adhering to zero-order kinetics and yielding a high apparent quantum yield of 27.5%. Mechanistic insights derived from scavenger assays and ESR spectroscopy reveal that superoxide radicals (O₂ ^•–) are the principal reactive species driving oxidative degradation. Electrochemical analyses, including Mott–Schottky profiling and impedance spectroscopy, further substantiate the enhanced charge separation, elevated carrier density, and favorable energy band alignment induced by Ag doping and oxygen vacancy generation. The photocatalyst exhibits robust operational durability over five cycles without structural compromise or significant metal leaching. Phytotoxicity evaluation via chickpea (Cicer arietinum L.) germination assays confirms the nontoxic nature of the treated effluent, underscoring its environmental compatibility. Collectively, this work advances Cd_1–x Ag _x O_1–y as a high-efficiency, defect-engineered photocatalyst with significant promise for sustainable, real-world wastewater remediation under ambient visible-light conditions.

1. Introduction

The pervasive discharge of dye-laden industrial effluents, particularly from textile manufacturing and biochemical applications, constitutes a significant global challenge to the ecosystem integrity and public health. Synthetic azo dyes, characterized by their resilient azo (−NN−) chromophores, dominate approximately 65% of the commercial dye market and exhibit pronounced environmental persistence due to their extensive π-conjugation and molecular stability. , A prominent representative, Amido Black 10B (AB10B), is widely employed in textile dyeing and histochemical staining. Its recalcitrance stems from a synergistic combination of strong intermolecular π–π stacking, inherent aromatic rigidity, and the presence of sulfonated functional groups, which collectively impede natural attenuation and confer mutagenic and carcinogenic potential upon environmental dissemination.

Conventional water treatment strategies, such as coagulation–flocculation, adsorption, ozonation, membrane filtration, and electrochemical oxidation, have demonstrated only partial efficacy in the mineralization of AB10B. − These methodologies are frequently hampered by significant drawbacks, including excessive energy or chemical reagent consumption, the generation of secondary solid waste (sludge), incomplete degradation leading to dye fragments, and the potential formation of toxic transformation byproducts. Consequently, Advanced Oxidation Processes (AOPs) have emerged as promising alternatives, leveraging the in situ generation of highly reactive oxygen species (ROS), including hydroxyl radicals (^•OH), superoxide anions (O₂ ^•–), and photogenerated holes (h⁺) to mineralize refractory organic pollutants. − Notwithstanding their potent oxidative mechanisms, conventional AOPs are constrained by several operational challenges. These encompass a narrow operational pH window (often acidic), the precipitation of ferric sludge (in Fenton-based systems), inefficient radical utilization yields, and a reliance on stoichiometric chemical oxidants like hydrogen peroxide (H₂O₂), peroxymonosulfate (PMS), or peroxydisulfate (PDS), which collectively impede their scalable implementation. −

To overcome these limitations, extensive efforts have focused on designing photocatalysts capable of harnessing solar or visible-light irradiation for sustainable dye degradation. A range of nanostructured semiconductors including Co/TiO₂, ZnO@Ag, polyoxometalates, CeO₂, Eu-TiO₂, and Mn₃O₄ has been evaluated for AB10B degradation. − While these systems exhibit noteworthy performance under UV illumination, their wide bandgaps necessitate high-energy photons and their photocatalytic activity typically depends on external electron scavengers to suppress ultrafast charge recombination. Moreover, the requirement for acidic media, oxidant addition, and sluggish charge-transfer kinetics continues to restrict their applicability in natural sunlight.

In response, research attention has gradually shifted toward narrow-band gap semiconductors capable of visible-light activation. Cadmium oxide (CdO) is particularly attractive due to its n-type conductivity, direct bandgap (2.2–2.7 eV), and favorable band-edge positions for redox reactions. However, pristine CdO suffers from rapid electron–hole recombination, limited catalytic surface sites, and photocorrosion, all of which reduce the quantum efficiency. To address this, two complementary approaches have been widely explored: (i) dopant engineering to tune electronic structure and enhance charge separation and (ii) defect engineering, especially oxygen vacancies to increase active sites and retard recombination. ,

Specifically, silver (Ag) doping has shown potential in modulating charge carrier density and improving charge extraction efficiency in metal oxides. Nevertheless, nonequilibrium doping can inadvertently introduce deep-level trap states that paradoxically serve as recombination centers. A systematic investigation into the synergistic interplay between Ag doping and engineered oxygen vacancies for the photocatalytic degradation of AB10B remains an underexplored area.

Motivated by these knowledge gaps and building upon our earlier contributions in nanomaterial design, − we report the facile synthesis of oxygen-vacancy-enriched Ag-doped CdO nanostructures (Cd_1–x Ag _x O_1–y ) via a controlled chemical precipitation method, followed by an annealing step. The resulting nanomaterials demonstrated superior photocatalytic activity in the degradation of the AB10B dye under visible-light irradiation. To our knowledge, this is the first systematic study evaluating the photocatalytic efficacy of Cd_1–x Ag _x O_1–y toward AB10B degradation, filling a critical gap in the literature. Detailed morphological, structural, and optical characterizations were performed to elucidate the role of Ag dopants and oxygen vacancies in the charge transfer dynamics. In this system, Ag⁺ plays a pivotal role by acting as an aliovalent dopant that facilitates the formation and stabilization of oxygen vacancies through charge compensation. Its ionic radius, which closely matches that of Cd²⁺, enables substitutional incorporation into the CdO lattice without significant structural disruption. Additionally, the closed-shell 4d¹⁰ configuration of Ag⁺ suppresses deep-level trap formation, promotes efficient charge separation, and enhances visible-light absorption. Together, these features make Ag⁺ particularly effective in modulating the electronic structure and defect landscape of CdO, thereby improving the photocatalytic performance. Furthermore, electron paramagnetic resonance (EPR) spectroscopy and radical scavenging experiments were employed to identify the nature of ROS and to unravel the degradation pathway. Finally, the experimental insights led to the formulation of a mechanistic model that explains the enhanced photocatalytic response, wherein the thermodynamic feasibility of ROS generation is rationalized through the modified redox potential landscape imparted by Ag incorporation.

2. Experimental Section

2.1. Materials

Cadmium acetate (Cd­(CH₃COO)₂·6H₂O), sodium hydroxide (NaOH), silver nitrate (AgNO₃) (purity, 99.98%), and Amido Black 10B (Figure S1) were obtained from Sigma-Aldrich. No further purification was done for the reagents and they were used as obtained. All of the solutions were prepared in double-distilled water.

2.2. Preparation of the Photocatalyst

CdO nanomaterials were synthesized via chemical precipitation using 0.2 M cadmium acetate hexahydrate (Cd­(CH₃COO)₂·6H₂O) as the precursor and 1 M sodium hydroxide (NaOH) as the precipitating agent. The reaction was conducted under continuous stirring at 60 °C for 4 h, resulting in a white precipitate. The precipitate was filtered using Whatman filter paper, thoroughly washed with deionized water and ethanol, and dried in a hot air oven at 110 °C. Subsequently, calcination was performed at 400 °C for 2 h (heating rate: 10 °C/min) in a muffle furnace to produce bare CdO powder. Further, Ag-doped CdO nanomaterials were prepared using the same method, with AgNO₃ added as a dopant to achieve 1, 3, and 5 wt % Ag doping. The final dried powders were ground in an agate mortar to minimize agglomeration, ensuring the formation of crystalline CdO- and Ag-doped CdO nanomaterials.

2.3. Characterization Techniques

The Supporting Information offers comprehensive characterization data, including detailed optical, electrochemical, and photoelectrochemical analyses. It further elaborates on the experimental methodology, provides specifications of the instrumentation employed, and includes assessments of the water quality parameters. Additionally, it presents the outcomes of catalyst recyclability and stability investigations.

2.4. Photocatalytic Experiment

A solution of AB10B (20 mL) was prepared, to which 6 mg of the photocatalyst was added. The mixture was initially kept in the dark for 5 min to establish adsorption–desorption equilibrium before being transferred to a photoreactor. The reaction was conducted at ambient temperature under continuous stirring using a 50 W 450 nm LED as the light source. Samples were collected at 3 min intervals throughout the reaction. After each cycle, the photocatalyst was recovered via centrifugation (10,000 rpm for 10 min) and reused in subsequent degradation experiments. The photocatalytic efficiency of the prepared samples was analyzed using a UV–visible spectrophotometer, while the progression of degradation efficiency was quantified using eq

$$\mathrm{E}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\%=(A_0-A/A_0)\times 100$$ 1

where A ₀ represents the initial absorbance of the dye and A is the absorbance of the dye after photoirradiation.

2.5. Seed Germination Study

Seeds of chickpea (variety CSJ 515) were obtained from the Rajasthan Agricultural Research Institute (RARI). Seeds were washed using distilled water to remove dust particles, and then sterilization treatment was done using 70% ethanol for 1 min followed by 0.1% of Mercuric Chloride for 5 min and subsequently washed by distilled water. The sterilized seeds were soaked in wastewater for 24 h at room temperature. The bed of cotton was placed on petriplates, and then the cotton bed was covered by a blotting paper and moistened by wastewater. For the experiment, soaked seeds of each species were placed in their respective petriplates at room temperature in the dark. The petriplates were examined throughout the experiment. After 24 h, the seed germination percentage was calculated by using eq

$$\mathrm{G}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\%=\frac{\mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{g}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{d}}{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{e}\mathrm{d}\mathrm{s}}\times 100$$ 2

3. Results and Discussion

Cd_1–x Ag _x O_1–y nanostructures were synthesized via a precisely tuned chemical coprecipitation approach, followed by postsynthetic thermal annealing. This strategy enabled uniform Ag incorporation into the CdO lattice, improved crystallinity, and induced surface oxygen vacancies, all of which are pivotal in modulating charge carrier dynamics for enhanced photocatalytic functionality. The structural, morphological, optical, and electronic properties of the resulting nanostructures were comprehensively characterized using advanced analytical techniques, providing deep insight into the defect chemistry and dopant–vacancy interactions.

X-ray diffraction analysis (Figure a,b,d,e) confirms the phase purity and structural evolution of oxygen vacancy-engineered Cd_1–x Ag _x O_1–y nanomaterials. All diffraction patterns exhibit precise alignment with the cubic CdO reference (JCPDS 075-0592), with characteristic Bragg reflections (Figure c,f) indexed to (111), (200), (220), (311), and (222) planes at 2θ = 33.20°, 38.37°, 55.62°, 65.01°, and 69.66°, respectively. The absence of extraneous peaks associated with metallic Ag or secondary oxide phases indicates that Ag incorporation does not disrupt the host lattice integrity or induce phase separation. Notably, peak broadening and uniformity support the nanocrystalline nature of the materials.

1.

(a) XRD pattern, (b) crystal structure, and (c) crystallographic planes of bare CdO and (d) XRD pattern, (e) crystal structure, and (f) crystallographic planes of 3% Ag-doped CdO.

A gradual reduction in unit cell volume with increasing Ag content (Table S1) reflects lattice strain due to the substitution of Cd²⁺ ions (0.95 Å) by larger Ag⁺ ions (1.26 Å). This substitution is energetically favorable and likely accompanied by the generation of charge-compensating oxygen vacancies to maintain electroneutrality. These intrinsic vacancies serve as key defect centers that perturb the local lattice environment. Rietveld refinement using PDXL software confirmed the cubic symmetry and rocks salt crystal structure (space group Fm3̅m), with refined lattice constants (a = b = c = 4.6964 Å) and a calculated unit cell volume of 103.58 ų. The Cd–O bond length was estimated to be 2.55 Å. The refinement convergence parameters R _p = 4.95, R _wp = 6.80, R _f = 0.624, and χ² = 2.91 underscore the robustness of the structural model. The average crystallite size, calculated via the Scherrer equation, was ∼60 ± 2 nm.

High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses provide critical nanoscale insights that corroborate and extend the crystallographic findings obtained by X-ray diffraction. As illustrated the bare CdO nanocrystals exhibit irregular, quasi-spherical morphologies with an average particle diameter of approximately 70 nm (Figures a and S2a), indicative of predominantly isotropic growth mechanisms. Upon the introduction of 3% Ag dopant, a marked morphological transformation is observed, wherein the nanostructures evolve into faceted, hexagonal architectures with an increased mean size of ∼80 nm (Figures d and S2b). This anisotropic growth behavior can be ascribed to Ag⁺ substitution at Cd²⁺ lattice positions, which alters the surface energy distribution and promotes directional crystal growth. ,

2.

(a) HRTEM image, (b) fringes image, (c) SAED pattern of bare CdO and (d) HRTEM image, (e) fringes image, (f) SAED pattern of 3% Ag-doped CdO.

The HRTEM micrographs offer further resolution of the local structural distortions induced by Ag incorporation. In the bare CdO sample (Figure b), well-defined lattice fringes with an interplanar spacing (d ₁₁₁) of 0.28 nm are observed, corresponding to the (111) planes of the cubic CdO phase. In contrast, the Ag-doped CdO sample (Figure e) exhibits a reduced fringe spacing of 0.27 nm. This contraction, despite the substitution of smaller Cd²⁺ ions (0.95 Å) with larger Ag⁺ ions (1.26 Å), is attributed to the generation of oxygen vacancies, a defect type known to exert compressive strain on the lattice. Such contraction is consistent with the decrease in the unit cell volume revealed by Rietveld refinement of the XRD data, suggesting that vacancy-induced lattice distortion supersedes the expansive strain typically associated with cationic size mismatch. SAED patterns (Figure c,f) acquired from Cd_1–x Ag _x O_1–y samples reveal sharp, well-defined diffraction spots and concentric rings, corresponding to the (111), (200), and (220) planes of the cubic CdO phase. These patterns confirm the single-crystalline nature and high phase purity of the synthesized materials. This multiscale structural coherence-spanning atomic (HRTEM), nanodomain (SAED), and long-range (XRD) order validates the successful synthesis of Cd_1–x Ag _x O_1–y with engineered oxygen vacancy concentrations.

SEM micrographs (Figure S3) of Cd_1–x Ag _x O_1–y nanomaterials corroborate the morphological evolution observed in HRTEM, where bare CdO exhibits predominantly spherical nanostructures, while Ag incorporation induces a transition to well-defined hexagonal geometries. Complementary energy-dispersive X-ray (EDX) elemental mapping confirms the homogeneous spatial distribution of Cd and O in pristine samples (Figure S4), and the uniform colocalization of Cd, O, and Ag in doped counterparts (Figure S5). The compositional integrity and stoichiometric consistency of the Cd_1–x Ag _x O_1–y nanostructures were further validated through inductively coupled plasma-optical emission spectrometry (ICP-OES) analyses, with the results in excellent agreement with the EDX findings.

Building upon the elemental validation obtained from EDX and ICP-OES analyses, X-ray photoelectron spectroscopy (XPS) was employed to probe the surface chemical states and electronic environments of the Cd_1–x Ag _x O_1–y nanostructures (Tables S2 and S3). The wide-scan survey spectrum of bare CdO (Figure a) confirms the exclusive presence of cadmium (Cd) and oxygen (O), underscoring its compositional purity. High-resolution deconvolution of the Cd 3d core-level (Figure b) reveals characteristic spin–orbit doublets at 404.9 eV (Cd 3d_5/2) and 411.5 eV (Cd 3d_3/2), consistent with Cd²⁺ species in the CdO lattice. The O 1s spectrum of bare CdO (Figure d) exhibits two discernible peaks: a dominant signal at ∼528.0 eV attributed to lattice oxide ions (O^2–) and a higher binding energy component at ∼531.1 eV, which is ascribed to surface-adsorbed oxygen and oxygen vacancy states. In the Cd_1–x Ag _x O_1–y samples, the survey spectrum (Figure a) reveals the presence of Ag in addition to Cd and O, validating successful incorporation of the dopant. The Ag 3d region exhibits two distinct sets of spin–orbit doublets: the first, at 368.65 and 374.82 eV, corresponds to metallic silver (Ag⁰), while the second set, located at 368.17 and 374.18 eV (Figure c), is assigned to Ag⁺ species. Moreover, the Cd 3d peaks in the Ag-doped system (Figure b) show a slight shift toward higher binding energies relative to those of undoped CdO, indicating an altered electronic environment due to Ag incorporation and the formation of oxygen-deficient sites. A parallel upshift in the O 1s core-level (Figure d) further supports an increased density of oxygen vacancies. These vacancies are crucial for photocatalysis, as they serve as active sites for charge trapping, thereby suppressing recombination of photogenerated charge carriers and enhancing photocatalytic efficiency.

3.

(a) XPS survey of bare CdO and 3% Ag-doped CdO, (b) Cd 3d of bare CdO and 3% Ag-doped CdO, (c) Ag 3d, (d) O 1s of bare CdO 3% Ag-doped CdO.

The findings from XPS analysis are further corroborated by Fourier-transform infrared (FTIR) spectroscopy (Figure S6), which provides additional insights into the vibrational dynamics and the chemical environment of the synthesized nanostructures. For bare CdO, the FTIR spectrum exhibits a prominent and broad absorption band in the range of 500–700 cm^–1, corresponding to the fundamental Cd–O stretching vibrations characteristic of the cubic rocksalt phase. , Upon doping with Ag, this absorption band not only intensifies but also exhibits notable broadening, indicative of lattice distortion and altered phonon modes. This spectral evolution strongly suggests the successful substitutional incorporation of Ag into the CdO lattice and the concomitant formation of oxygen vacancies, which locally disrupt the bonding environment. A distinct vibrational feature emerging at 1003 cm^–1 in the doped sample serves as a spectral fingerprint for integration of Ag within the CdO matrix. Furthermore, absorption bands in the 1400–1600 cm^–1 region can be attributed to the symmetric and asymmetric bending modes of surface-adsorbed carbonate species (CO₃ ^2–) as well as the δ­(O–H) bending vibrations associated with physisorbed water and hydroxyl groups. The relative intensification of these bands in Ag-doped CdO implies an increase in surface-active sites, likely due to dopant-induced roughness and vacancy-mediated adsorption. Additionally, the broad feature observed within the 3000–3600 cm^–1 region is assigned to O–H stretching vibrations, stemming from hydrogen-bonded surface hydroxyls or chemisorbed water molecules. The increased breadth and intensity of this band in the doped sample are consistent with a higher concentration of surface-bound hydroxyls, arising from enhanced oxygen deficiency and the creation of hydrophilic sites on the nanoparticle surface. BET analysis revealed that bare CdO possessed a surface area of 25.53 m² g^–1, whereas the 3% Ag-doped CdO sample showed an increased surface area of 32.00 m² g^–1. The observed enhancement in the surface area upon Ag incorporation may contribute to improved photocatalytic performance by providing a greater number of accessible active sites (Figure ).

4.

BET spectra of (a) bare CdO and (b) 3% Ag-doped CdO.

Figure a presents the ultraviolet–visible (UV–vis) absorption spectra of the synthesized Cd_1–x Ag _x O_1–y nanostructures with the corresponding Tauc plots displayed in the inset for optical band gap estimation. The bare CdO sample exhibits a pronounced absorption edge within the 350–450 nm range, consistent with its intrinsic direct band gap of approximately 2.22 eV, in agreement with previous reports. , Upon progressive incorporation of Ag dopants, a notable redshift in the absorption onset is observed, indicative of a band gap narrowing effect induced by the dopant. The estimated optical band gaps for the Ag-doped variants were found to be 2.13, 2.10, and 2.11 eV for 1, 3, and 5 wt % Ag content, respectively (Figure b). This systematic reduction in band gap energy is attributed to the perturbation of the electronic structure caused by Ag incorporation. The substitutional doping introduces impurity states within the band gap, primarily through s–d and p–d orbital hybridization between the host Cd²⁺ ions and the Ag dopant atoms. Such electronic interactions result in the formation of localized states that effectively lower the energy barrier for electronic transitions, thereby extending the absorption into the visible region. Additionally, the incorporation of Ag promotes the formation of oxygen vacancies, which act as shallow donor levels or midgap states. These vacancy-induced states facilitate easier photoexcitation of electrons from the valence band (VB) to the conduction band (CB) by enabling sub-band gap optical transitions. The combination of impurity-induced band tailing and defect states leads to enhanced visible light harvesting. The observed redshift and increased absorption intensity across the visible spectrum not only confirm successful band gap modulation but also suggest a substantial improvement in the generation rate of photoinduced charge carriers. This is expected to significantly augment the photocatalytic activity of the Cd_1–x Ag _x O_1–y nanostructures by promoting efficient light utilization and charge separation under visible light irradiation.

5.

(a) UV spectra, (b) Tauc’s plots, and (c) PL analysis of Cd_1–x Ag _x O_1–y samples, (d) transient photocurrent response and (e) electrochemical impedance spectroscopy (EIS) analysis of Cd_1–x Ag _x O_1–y samples.

Figure c shows the photoluminescence (PL) spectra of Cd_1–x Ag _x O_1–y nanomaterials, recorded at room temperature with an excitation wavelength of 380 nm, aimed at probing the electronic structure and defect-associated recombination phenomena. The undoped CdO displays intense emission peaks at 410 and 427 nm, which are attributed to near-band-edge (NBE) recombination processes, likely involving transitions of electrons from the CB to the VB mediated by cadmium interstitial sites. In addition, broad visible emission bands located in the 480–550 nm range are observed, which are characteristic of deep-level emissions arising from oxygen vacancy-related defect states. Specifically, the 484 nm emission is linked to the recombination of photogenerated holes with electrons trapped at neutral oxygen vacancies, whereas the 520 nm emission corresponds to deeper donor-to-valence band transitions associated with ionized oxygen vacancies. Upon Ag doping (3 wt %), a notable suppression in both UV and visible PL intensities is observed. This quenching effect aligns with the UV–vis absorption results, which revealed a redshift and bandgap narrowing in the Cd_1–x Ag _x O_1–y samples. The diminished UV emission intensity suggests a lower probability of radiative band-edge recombination, consistent with enhanced charge separation efficiency due to the introduction of localized impurity states by Ag incorporation. Moreover, the modified visible emission in Cd_1–x Ag _x O_1–y supports the hypothesis of an increased oxygen vacancy concentration, corroborating the observed enhancement in visible light absorption from UV–vis studies. These vacancies serve as electron trapping sites, effectively inhibiting rapid recombination and thereby facilitating prolonged charge carrier lifetimes. This synergistic behavior between bandgap narrowing and defect engineering is instrumental in improving the photocatalytic performance of the material under visible-light irradiation.

To further probe the efficiency of photogenerated charge carrier separation in the Cd_1–x Ag _x O_1–y series, transient photocurrent measurements were conducted under intermittent visible light irradiation in chopping mode , (Figure d). Among the tested compositions, the 3 and 5 wt % Cd_1–x Ag _x O_1–y samples exhibited the highest and most stable photocurrent densities, indicative of markedly improved separation efficiency and prolonged lifetimes of photoexcited charge carriers compared to bare CdO and lower-doped counterparts. These results are in excellent agreement with the PL quenching behavior, where reduced radiative recombination in the doped samples pointed to suppressed electron–hole recombination dynamics. The enhanced photocurrent response in Ag-doped CdO can be attributed to dual engineering, namely, silver ion incorporation and the concurrent formation of oxygen vacancies, which together facilitate efficient charge extraction and minimize recombination losses. To gain further insights into the interfacial charge transport kinetics, EIS , was performed and analyzed through Nyquist plots (Figure e). The radius of the high-frequency semicircular arc, which reflects the charge transfer resistance at the electrode–electrolyte interface, was significantly smaller for the 3% and 5% Cd_1–x Ag _x O_1–y . This reduced impedance implies more efficient interfacial electron transfer and lower recombination losses, consistent with the enhanced photocurrent and PL results. Overall, these electrochemical and optoelectronic investigations collectively demonstrate that optimal Ag doping (3–5 wt %), along with oxygen vacancy modulation, substantially boosts charge separation efficiency, interfacial conductivity, and light harvesting ability, key attributes for improved photocatalytic performance under visible light irradiation.

Initial control experiments were conducted to evaluate the photocatalytic system using AB10B dye, employed as a model contaminant at a concentration of 70 mg/L. Each trial utilized a 20 mL aliquot of the dye solution combined with 6 mg of the catalyst material. The photocatalytic degradation efficiency was quantified by tracking the temporal evolution of the dye’s characteristic absorbance spectrum using UV–visible spectrophotometry. All assays were performed under ambient, neutral pH conditions (Table ). Prior to visible light exposure, the reaction mixture was magnetically stirred in complete darkness for 15 min to establish adsorption–desorption equilibrium between the dye molecules and the catalyst surface. Subsequently, photocatalytic degradation was initiated by irradiating the mixture with visible light (a 50 W blue LED).

1. Optimization of the Parameters Governing the Degradation Process .

catalyst	time	degradation percentage (%)
-	2 h	3
CdO NPs (6 mg)	2 h	45
1% Ag doped CdO NP (6 mg)	2 h	65
3% Ag doped CdNPs (6 mg)	24 min	91
5% Ag doped CdO NP (6 mg)	90 min	90
3% Ag doped CdO NP (3 mg)	24 min	78
3% Ag doped CdO NP (8 mg)	24 min	91
3% Ag doped CdNPs (6 mg)	4 h	5

^aThe experiments were carried out using a 70 mg/L aqueous dye solution at its inherent pH, under blue LED light illumination.

^bUnder dark condition.

Figure S7a depicts the temporal evolution of UV–vis absorption spectra for an AB10B dye solution undergoing photocatalytic treatment using 3% Ag-doped CdO, recorded at discrete time intervals. The characteristic absorption bands exhibited a progressive attenuation in intensity as a function of the irradiation duration. Complete disappearance of the primary absorption features was observed after 24 min, signifying near-total degradation of the AB10B dye (Figure S7b).

The degradation kinetics were evaluated using a zero-order kinetics model (eq ).

$$C_0-C_t=kt$$ 3

The reaction rate constant (k), expressed in mol^–1 min^–1, reflects the efficiency of the photodegradation process, where C ₀ and C _t denote the initial and time-dependent absorptions of the AB10B dye, respectively. Although various kinetic models (first-order, second-order) were considered, the observed degradation behavior aligns best with zero-order kinetics, , as evidenced by the linearity of the C ₀ – C _t vs time plot in (Figures S7c and S8). This suggests that the oxidation process is primarily dictated by the availability and generation rate of ROS, rather than by the dye concentration itself, indicating a surface-saturated reaction regime. Among all synthesized photocatalysts, 3 wt % Ag-doped CdO exhibited the highest degradation efficiency. The calculated rate constants (k) for bare CdO, 1%, 3%, and 5% Ag-doped CdO were found to be 0.00415, 0.00705, 0.02618, and 0.02497 mol^–1 min^–1, respectively (Figure a). Notably, the rate constant for the 3% Ag-doped sample was over 6-fold higher than 1% Ag–CdO and more than 20 times greater than that of bare CdO, underscoring the synergistic effect of moderate Ag doping and induced oxygen vacancies in promoting charge separation and surface redox activity. However, the 5% Ag-doped CdO sample shows a slight decrease in photocatalytic activity relative to the optimized 3% Ag composition. This minor decline arises from a shift in the dopant–defect equilibrium at higher Ag content, where excess Ag introduces additional defect states that function as incipient recombination centers rather than solely enhancing charge separation (Figure S9). Consequently, a modest increase in defect-assisted recombination marginally shortens charge-carrier lifetimes and results in a slight reduction in overall photocatalytic efficiency. Furthermore, catalyst loading studies revealed that 6 mg of photocatalyst per 20 mL of dye solution represented the optimal dosage for maximum degradation efficiency.

6.

(a) Effect of catalyst concentration, (b) AB10B dye concentrations degradation dependent with time, (c) zero-order kinetics with respect to AB10B concentration, (d) effect of various pH with time, (e) apparent quantum yield (AQY) of photocatalytic degradation of AB10B with various catalysts.

The photocatalytic activity of the synthesized materials was systematically investigated over a broad concentration range of AB10B dye (10–100 mg/L) to understand the influence of substrate loading on the degradation kinetics (Figure b). As anticipated, higher initial dye concentrations required longer irradiation times to reach near-complete removal. However, the calculated reaction rate constants remained nearly constant across the entire concentration range, substantiating the dominance of zero-order kinetics with respect to AB10B concentration (Figures c and S10). This observation implies that the rate-determining step is independent of the bulk dye concentration and is primarily governed by surface or photon-limited processes, such as the availability of photogenerated reactive species or active catalytic sites. The graph distinctly illustrates that the degradation rate does not vary with changes in the initial AB10B concentration, reinforcing the zero-order kinetic behavior under the tested conditions. Interestingly, this result diverges from commonly reported trends, where zero-order kinetics in photocatalytic systems are typically observed at higher substrate concentrations (often exceeding 100 mg/L) due to active site saturation or photon-limited regimes. The manifestation of zero-order behavior even at comparatively lower dye concentrations in this study could be attributed to a rapid adsorption–desorption equilibrium and efficient generation of ROS facilitated by optimal surface characteristics and the unique electronic structure of the Cd_1–x Ag _x O_1–y photocatalyst. Effluents discharged from textile industries often exhibit highly variable pH levels, which profoundly affect photocatalytic degradation efficiency by altering catalyst surface properties and charge transfer dynamics. To elucidate the influence of pH on the photocatalytic degradation of AB10B, a series of controlled experiments were performed over a pH range of 2 to 11 (Figure d), wherein the pH was finely tuned using dilute aqueous solutions of NaOH and HCl. Experimental findings demonstrated a pronounced enhancement of photocatalytic activity under acidic conditions. Notably, at pH 3, 3% Ag-doped CdO nanomaterials facilitated the degradation of AB10B with an efficiency reaching approximately 96%. This exceptional activity in acidic media is attributed to multiple synergistic factors. First, the proton-rich environment promotes the efficient separation of photogenerated charge carriers by stabilizing holes and suppressing recombination events. Second, enhanced generation of ROS, particularly superoxide anions (O₂ ^•–), under low-pH conditions accelerates oxidative degradation pathways. Additionally, the positive surface charge of the catalyst in acidic environments fosters stronger electrostatic interactions with the anionic dye molecules, promoting efficient adsorption and subsequent degradation. In contrast, under strongly alkaline conditions (pH 11), a marked reduction in the photocatalytic efficiency was observed. This decline can be ascribed to several interrelated phenomena: (i) the catalyst surface becomes increasingly negatively charged, which electrostatically repels anionic dye species, reducing adsorption efficiency and (ii) elevated pH may enhance charge carrier recombination due to diminished interfacial electric fields, thereby attenuating photocatalytic activity.

The photocatalytic potential of the engineered Cd_1–x Ag _x O_1–y nanomaterials was rigorously evaluated through AQY measurements under monochromatic blue LED irradiation (λ = 450 nm) for 24 min (Figure e). AQY, defined as the ratio of photochemically transformed molecules to incident photons, offers a quantitative benchmark for gauging photon-to-chemical energy conversion efficiency and elucidates charge carrier utilization in semiconductor-mediated redox processes. Bare CdO exhibited a relatively low AQY of 2.5%, reflecting its inherent limitations, namely, rapid radiative and nonradiative recombination of photogenerated electron–hole pairs, poor surface reactivity due to limited active sites, and suboptimal absorption in the visible regime. These intrinsic bottlenecks severely restrict photonic utilization and constrain catalytic turnover. Upon Ag incorporation, a significant modulation of the photoresponse was observed. The 1 wt % Ag-doped CdO sample displayed a 4-fold enhancement in AQY (10.1%), suggesting initial improvements in charge carrier dynamics and lattice polarization. This is attributed to partial substitution of Cd²⁺ with Ag⁺ ions, which introduce mild lattice strain and electronic perturbations, fostering shallow trap states conducive to suppressed charge recombination. Strikingly, the AQY peaked at 27.5% for the 3 wt % Ag-doped CdO nanomaterial, coinciding with maximal AB10B dye degradation. This marked improvement results from a synergistic interplay between Ag doping and oxygen vacancy engineering. The substitution of Cd²⁺ by Ag⁺ not only narrows the bandgap by inducing hybridized Ag 4d–Cd 5s and Ag 4d–O 2p orbitals but also triggers the formation of compensatory oxygen vacancies due to charge imbalance. These vacancies function as dual-purpose catalytic centers, enhancing optical absorption through the creation of midgap states and promoting exciton dissociation by acting as transient electron traps. The net result is a more efficient separation of charge carriers and an accelerated interfacial electron transfer, thereby enhancing quantum efficiency. However, at 5 wt % Ag doping, a slight decline in AQY (24.6%) was observed. This attenuation can be rationalized by dopant oversaturation, where excess Ag may lead to the formation of recombination centers or disrupt the long-range order of the lattice. Moreover, agglomeration of Ag at higher concentrations can reduce surface area accessibility, impede light penetration due to scattering, and potentially shift redox potentials away from optimal windows for ROS generation. The AQY results delineate a pronounced structure activity correlation, wherein 3 wt % Ag incorporation optimally balances band structure engineering, defect generation, and charge dynamics in Cd_1–x Ag _x O_1–y nanomaterials, establishing it as the most efficient composition for visible-light-driven photocatalytic applications.

To delineate the active species responsible for the photocatalytic degradation of AB10B, a series of radical quenching experiments , were conducted employing specific scavengers (Figure S11): potassium iodide (KI) for photogenerated holes (h⁺), isopropanol (IPA) for hydroxyl radicals (^•OH), and benzoquinone (BQ) for superoxide radicals (O₂ ^•–). Among these, BQ exhibited the most pronounced inhibitory effect on the degradation efficiency, implying that O₂ ^•– is the dominant reactive species in the photocatalytic process. These findings were corroborated by EPR spectroscopy using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin-trapping agent (Figure ). Under visible-light irradiation, the EPR spectrum of the 3 wt % Ag-doped CdO nanomaterial displayed a characteristic quartet signal with a 1:1:1:1 intensity ratio, confirming the formation of DMPO–O₂ ^•– adducts. Additionally, a minor quartet signal with a 1:2:2:1 pattern was also detected, corresponding to the DMPO–^•OH adduct. The relatively weaker ^•OH signal intensity suggests that hydroxyl radicals are generated in smaller quantities, likely via secondary reactions involving O₂ ^•– and protons (H⁺), consistent with the indirect formation pathway: O₂ ^•– + H⁺ → HO₂ ^• → ^•OH. Importantly, the EPR signal intensities for both DMPO–O₂ ^•– and DMPO–^•OH adducts were observed to increase with higher Ag doping levels, with the 3 and 5 wt % Ag-doped CdO samples exhibiting significantly stronger signals compared to undoped CdO. The pronounced signal associated with the 3 wt % Ag-doped sample highlights its superior ROS generation capacity, which is attributed to enhanced charge separation, accelerated interfacial electron transfer, and oxygen vacancy-mediated activation of molecular oxygen. These results substantiate the hypothesis that the photocatalytic degradation pathway is predominantly governed by superoxide radical-driven oxidation.

7.

EPR spectra of radical (a) DMPO–O₂ ^•– and (b) DMPO–^•OH.

To gain mechanistic insights into the electronic properties and charge carrier dynamics of the Cd_1–x Ag _x O_1–y nanostructures, Mott–Schottky (MS) measurements were conducted (Figures a and S12). The resulting plots exhibit a positive slope for all samples, confirming their intrinsic n-type semiconducting behavior, where electrons are the dominant charge carriers. From the linear region of the MS plots, , the flat-band potentials (Fb) and donor carrier densities (N _d) were extracted using eq S5, and the detailed values are summarized in Table . For n-type semiconductors, the CB typically lies approximately 0.1 V more negative than the Fb potential. Accordingly, the CB and corresponding VB positions were calculated versus those of the normal hydrogen electrode (NHE). The 3 wt % Ag-doped CdO sample exhibited a more negative Fb compared to bare CdO, signifying a shift in the Fermi level toward the CB, indicative of enhanced electron accumulation and elevated reductive capacity. Ag incorporation significantly modulates the band energetics and increases the free carrier density, as evidenced by a 2.5-fold enhancement in N _d for the 3% Ag-doped sample relative to pristine CdO. This enhancement is attributed to two synergistic factors: (i) substitutional Ag⁺ doping at Cd²⁺ lattice sites and (ii) the concurrent generation of oxygen vacancies, which serve as shallow donor states, thereby boosting carrier concentration and extending charge carrier lifetimes. Additionally, the work function (Φ) of the Cd_1–x Ag _x O_1–y samples was calculated from the Fermi level positions (Table S4). A pronounced decrease in Φ with increasing Ag content was observed, implying enhanced band bending at the surface and improved interfacial charge transfer efficiency. Lower work function values correlate with reduced electron binding energy, which facilitates more efficient photogenerated charge separation and transport across the semiconductor–electrolyte interface. Band structure diagrams demonstrate that Ag doping induces an upward shift in both the conduction and valence bands, resulting in an effective narrowing of the bandgap. This shift aligns well with the optical bandgap trends observed from UV–vis absorption data and supports enhanced visible-light-driven photocatalytic performance. From a redox perspective (Figure b), the CB of 3% Ag-doped CdO lies at a more negative potential than the redox couple of O₂/O₂ ^•– (−0.33 V vs NHE), enabling thermodynamically favorable one-electron reduction of O₂ to O₂ ^•–. These superoxide radicals can undergo protonation and subsequent reactions to form hydrogen peroxide (H₂O₂), which may further decompose or react with additional O₂ ^•– to generate hydroxyl radicals (^•OH)-potent oxidative species responsible for dye degradation. Simultaneously, photogenerated holes oxidize surface-adsorbed dye molecules, resulting in the formation of cationic dye radicals (Dye^•+). These oxidized species subsequently undergo oxidative degradation primarily mediated by superoxide radicals (O₂ ^•–) and hydroxyl radicals (^•OH) generated in the system.

8.

(a) Mott–Schottky plots of Ag doped CdO materials (b) probable mechanism.

2. Conduction (CB) and VB Potential, Flat Band Potential, Band Gap Values of Ag-Doped CdO Materials.

S.N.	materials type	band gap (eV)	FB (vs Ag/AgCl)	CB vs NHE	VB vs NHE	carrier density (N d) (×1017)
1	bare CdO	2.22	–0.68	–0.58	1.64	5.3
2	1% Ag-doped CdO	2.13	–0.73	–0.63	1.50	8.54
3	3% Ag-doped CdO	2.10	–0.77	–0.67	1.43	13.8
4	5% Ag-doped CdO	2.11	–0.80	–0.70	1.41	11.5

The long-term operational stability and reusability of the 3 wt % Ag-doped CdO nanophotocatalyst were systematically evaluated through consecutive photocatalytic degradation cycles, as illustrated in Figure S13. The material demonstrated excellent stability, maintaining high degradation efficiencies of 91%, 91%, 89%, 89%, and 87% across five successive cycles without significant performance deterioration. Following each cycle, the catalyst was recovered via centrifugation at 10,000 rpm for 15 min, washed, and reused under identical reaction conditions. Postreaction characterizations (Figures S14 and S15) confirmed the structural integrity, chemical stability, and morphological preservation of the photocatalyst after reuse. These findings underscore the robust durability of the Ag-doped CdO system, positioning it as a promising candidate for practical applications in large-scale photocatalytic wastewater remediation, particularly in the textile sector where persistent dye pollutants such as AB10B are prevalent.

To verify the heterogeneous nature of the photocatalytic process, a catalyst removal experiment was performed under the optimized conditions. After 10 min of visible light irradiation, the catalyst was separated from the reaction mixture, and the residual solution was stirred continuously for an additional 3 h. No further degradation of the dye was observed during this period, strongly indicating that the photocatalytic reaction was exclusively driven by the solid-state Cd_1–x Ag _x O_1–y material, and not by any soluble catalytic species. Complementary ICP-OES analysis confirmed the negligible leaching of Ag or Cd ions into the solution, further substantiating the heterogeneous and chemically stable nature of the catalyst. A comparative analysis of the current system with previously reported AB10B photodegradation methodologies reveals significant advantages (Table S5). Unlike conventional approaches that often require UV irradiation, high catalyst dosages, or auxiliary oxidants, the Ag-doped CdO nanocatalyst operates efficiently under visible-light illumination with a low catalyst loading and without external oxidizing agents. This not only improves energy efficiency and operational simplicity but also enhances environmental compatibility and cost-effectiveness. In conclusion, the excellent recyclability, structural robustness, negligible metal leaching, and high visible-light-driven photocatalytic efficiency of the 3% Ag-doped CdO nanomaterials underscore their practical utility for sustainable and scalable dye remediation technologies.

Dissolved oxygen (DO), which denotes the concentration of molecular oxygen freely available in aqueous systems, serves as a vital indicator of water quality and aerobic environmental health. In parallel, the mineralization and degradation efficiency of organic contaminants can be quantitatively assessed using Total Dissolved Solids (TDS), Biological Oxygen Demand (BOD), and Chemical Oxygen Demand (COD) analyses. As depicted in Figure S16, the raw textile wastewater exhibited elevated TDS, BOD, and COD levels, alongside markedly low DO content, indicative of a high organic pollutant load and poor water quality. Upon treatment with 3 wt % Ag-doped CdO photocatalyst under visible light, substantial improvements in all water quality indicators were observed. Specifically, TDS, BOD, and COD values were reduced by 65%, 83%, and 80.06%, respectively, signifying efficient breakdown and mineralization of the dye molecules. Concurrently, the DO level increased significantly by approximately 72%, reflecting reduced oxygen consumption by organic pollutants and enhanced water reoxygenation. The pronounced changes across these parameters validate the photocatalyst’s ability not only to decolorize the dye but also to mineralize complex organics into simpler, less harmful end-products. This highlights the 3% Ag-doped CdO nanostructures as a potent photocatalytic system for real-world wastewater remediation, with demonstrable efficacy in improving ecological water quality.

To evaluate the post-treatment ecotoxicological safety of photocatalytically remediated effluent, the wastewater resulting from AB10B dye degradation using 3% Ag-doped CdO nanocatalysts was employed in a seed germination assay using chickpea seeds (Cicer arietinum L., variety CSJ 515). The study aimed to assess potential phytotoxic effects and gauge the suitability of the treated water for agricultural reuse (Figure S17). The results revealed that the seeds irrigated with treated wastewater exhibited a faster germination rate (16.00–16.50 h) compared to those irrigated with distilled water (control, 17.00 h), indicating enhanced metabolic activation and low residual toxicity in the treated effluent. Furthermore, the germination efficiency in treated samples reached 87–89%, surpassing the control group (85%), while no germination was observed in seeds exposed to untreated dye-laden wastewater, underscoring the severe phytotoxicity of the raw effluent. Postgermination growth assessments conducted after 3 days showed notable improvements in morphological parameters. These findings suggest that the photocatalytically treated wastewater not only mitigates the toxic effects of AB10B dye but may also positively influence early plant development, likely due to residual micronutrients or enhanced water quality. Overall, the data confirm that the 3% Ag-doped CdO-treated wastewater is nontoxic and agriculturally viable, offering a sustainable route for safe effluent reuse in crop irrigation systems.

4. Conclusion

In this work, oxygen-vacancy-engineered Ag-doped CdO (Cd_1–x Ag _x O_1–y ) nanostructures were synthesized via a controlled chemical coprecipitation and annealing method. Ag⁺ incorporation induced lattice distortions and oxygen vacancies, which together modulated the electronic structure, enhanced visible-light absorption, and improved charge separation. The 3 wt % Ag-doped sample exhibited optimal optoelectronic properties, including narrowed bandgap, quenched photoluminescence, enhanced photocurrent, and reduced charge-transfer resistance. ESR and radical quenching experiments confirmed superoxide radicals (O₂ ^•–) as the main reactive species. Under visible-light irradiation, near-complete AB10B degradation occurred within 24 min, with a peak AQY of 27.5%. The material demonstrated excellent stability over five cycles and negligible metal leaching, while phytotoxicity tests confirmed environmental safety.

Beyond validating the performance of Ag-modified CdO, this work emphasizes the broader relevance of dopant-vacancy coupling as a rational framework for designing efficient visible-light photocatalysts. While Ag⁺ proves particularly effective owing to its compatible ionic radius, closed-shell configuration, and strong tendency to stabilize oxygen vacancies, the underlying strategy is not inherently limited to silver. Dopants with comparable valence states or vacancy-modulating characteristics may similarly alter vacancy-formation energies, adjust band-edge positions, or generate shallow trap states that promote charge mobility. Accordingly, future studies should explore systematic dopant selection, codoping schemes, and complementary defect-engineering approaches (e.g., reductive annealing, plasma surface activation), supported by computational evaluations to quantify dopant–vacancy interactions and to predict their impact on ROS generation and carrier dynamics.

Overall, this work establishes Cd_1–x Ag _x O_1–y as an effective visible-light-responsive photocatalyst for degrading recalcitrant azo dyes and provides a mechanistic basis that can guide the rational development of next-generation oxide materials through the synergistic tuning of dopant chemistry and intrinsic defect populations.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c10075.

• Detailed synthetic methodologies, characterization protocols, analytical results, instrumentation specifications, and additional supporting data (PDF)

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S.M., S.M., and A.G. contributed equally.

The authors declare no competing financial interest.