All-Solid-State Plasmon-Assisted Interfacial Engineered Ag-Seeded ZnO/ZnS Core–Shell Nanorods for Z‑Scheme Photocatalysis

Abstract

The persistent contamination of water resources by organic dyes and inorganic pollutants presents serious environmental and public health concerns. However, conventional photocatalysts are hindered by rapid charge recombination, limited redox capability, and inefficient interfacial charge transfer, while traditional heterojunctions struggle to simultaneously deliver strong oxidation and reduction potentials. Herein, a nature-inspired all-solid-state, plasmon-assisted interface-engineered Z-scheme photocatalyst comprising ZnO/ZnS core–shell nanorods (NRs) on an Ag seed layer is reported to address the limitations of conventional photocatalyst. The metallic Ag seed layer acts as a localized surface plasmon resonance (LSPR) platform, facilitating electron injection into the ZnO NRs core and suppressing photogenerated charge recombination. A controlled ZnS shell thickness (∼20 nm, obtained by 1.5 h sulfidation) optimizes interfacial charge transfer, promotes selective recombination of low-energy carriers, and maximizes solid-state Z-scheme efficiency. Trapping experiments indicate that ^•O₂ ^– and ^•OH are the dominant reactive species, enabling efficient degradation of methylene blue and tetracycline and reduction of Cr (VI) under UV irradiation. This work provides a rational interfacial engineering strategy for developing high-performance all-solid-state Z-scheme photocatalysts for environmental remediation.

1. Introduction

The increasing release of organic and inorganic contaminants into aquatic environments has become a critical global concern driven by industrial activities, pharmaceutical consumption, and rapid population growth. Organic dyes, pharmaceutical residues, and toxic heavy metal ions are among the most prevalent pollutants in aquatic environments, exhibiting high chemical stability, persistence, and resistance to conventional wastewater treatment processes. − Persistent organic dyes such as methylene blue (MB), antibiotic residues including tetracycline hydrochloride (TC), and toxic heavy metal ions such as hexavalent chromium [Cr (VI)] are frequently detected in wastewater and pose significant risks to human health and ecosystems. Their chemical stability, bioaccumulative nature, and resistance to conventional treatment processes make their effective removal a critical environmental challenge. − Developing efficient and sustainable remediation strategies capable of addressing these chemically diverse pollutants remains an urgent challenge.

Semiconductor photocatalysis has emerged as a promising strategy for environmental remediation because it enables the conversion of light energy into chemical redox reactions under mild and sustainable conditions. , Upon photoexcitation, photogenerated electrons and holes participate in reduction and oxidation reactions, enabling the degradation of organic pollutants and the detoxification of hazardous species. However, the performance of many photocatalytic systems remains limited by rapid charge-carrier recombination, insufficient redox capability, and inefficient light utilization. − These limitations become particularly significant in systems involving simultaneous oxidation and reduction reactions. In such cases, efficient charge separation and the retention of highly energetic electrons and holes are crucial for achieving high photocatalytic efficiency.

To address these limitations, heterostructure engineering has been widely explored to enhance charge separation and interfacial charge transfer. Although conventional type-II heterojunction promotes spatial separation of photogenerated electron–hole pairs by driving electrons toward the lower-energy conduction band (CB) and holes toward the higher-energy valence band (VB) of the counterpart semiconductor. However, this charge migration pathway often weakens the redox capability because the accumulated electrons and holes possess reduced reduction and oxidation potentials. , In type-II systems, the weakened oxidation and reduction potentials restrict their effectiveness toward persistent organic pollutants and the reduction of highly toxic Cr (VI) ions. , In contrast, a Z-scheme photocatalytic system, inspired by the natural photosynthetic process, enables efficient charge separation through the selective recombination of low-energy electrons and holes at the interface between two semiconductors. This charge-transfer pathway preserves the highly energetic electrons in the conduction band (CB) and holes in the valence band (VB), thereby maintaining strong reduction and oxidation potentials for enhanced photocatalytic reactions. ,,,− This configuration effectively suppresses bulk charge recombination and preserves the intrinsic oxidation–reduction capability of the constituent semiconductors, thereby enhancing overall photocatalytic efficiency. Moreover, the Z-scheme charge-transfer pathway promotes efficient charge separation, rapid electron transport, and enhanced surface reaction kinetics while preserving strong redox capability. Consequently, Z-scheme architectures are highly attractive for photocatalytic applications requiring efficient charge utilization and robust redox activity.

ZnO and ZnS are attractive semiconductor components for Z-scheme photocatalysts due to their complementary electronic structures, chemical stability, and environmental compatibility. − ZnO exhibits high electron mobility and strong oxidative capability, whereas ZnS possesses a more negative conduction band position favorable for reduction reactions. In ZnO/ZnS core–shell architecture, photoexcited electrons in the ZnO conduction band recombine with photogenerated holes in the ZnS valence band at the core–shell interface, while the remaining high-energy holes in ZnO and electrons in ZnS participate directly in oxidation and reduction reactions, respectively. This Z-scheme charge transfer pathway effectively suppresses bulk recombination while maintaining superior redox capability compared to type-II systems. Feifei et al. successfully synthesized a hollow spherical ZnO/ZnS heterojunction using hydrothermal process photocatalytic reduction of CO₂. In a separate study, Fang et al. fabricate a core–shell like ZnO/ZnS heterojunction for photocatalytic degradation applications. However, optimizing light absorption and interfacial charge transfer in ZnO/ZnS heterostructures remains challenging. In particular, non-optimal ZnS shell thickness can increase carrier recombination or weaken interfacial coupling, thereby reducing photocatalytic performance and stability. Moreover, most ZnO/ZnS photocatalysts are powder-based, suffering from agglomeration, difficult recovery, and poor reusability, which limit their practical environmental applications.

To address these challenges, a substrate-supported plasmonic Ag seed layer provides an effective strategy to enhance light–matter interaction and charge-carrier dynamics. It enables the growth of vertically aligned ZnO nanorods (NRs) arrays while introducing localized surface plasmon resonance (LSPR), a phenomenon arising from the collective oscillation of conduction-band electrons in metallic nanostructures under light excitation. Under UV illumination, the LSPR effect can enhance light–matter interactions and facilitate interfacial charge transfer, thereby establishing an all-solid-state photocatalytic platform. ,, Although several previous studies have investigated Ag-doped or Ag-modified ZnO/ZnS heterostructures, − the incorporation of Ag as a dopant or surface-deposited component may introduce defect-related states or poorly controlled interfaces, which can limit charge transport and increase carrier recombination. In contrast, the present work introduces a distinct all-solid-state plasmon-assisted architecture in which a continuous Ag seed layer is deliberately positioned beneath vertically aligned ZnO/ZnS core–shell NRs, rather than being incorporated into or randomly deposited on the semiconductor. This rational architecture enables the Ag layer to function simultaneously as a conductive interfacial platform and a plasmonic component, while maintaining intimate contact with the ZnO NRs core. Under UV illumination, the Ag layer may further contribute plasmon-mediated hot-electron transfer to ZnO, potentially improving photogenerated-carrier utilization. Moreover, the Ag seed layer promotes the formation of vertically aligned ZnO NRs arrays, providing direct one-dimensional pathways for charge transport and facilitating intimate coupling with the conformal ZnS shell. Meanwhile, the controlled sulfidation further enables regulation of the ZnS shell thickness, which is crucial for optimizing interfacial charge transfer while minimizing carrier-transport resistance. Therefore, the novelty of this all-solid-state heterostructure lies not simply in combining Ag, ZnO, and ZnS, but in the rational integration of a continuous plasmonic Ag seed layer with an optimized ZnO/ZnS core–shell NRs architecture to establish a spatially defined charge-transfer platform. This synergistic interfacial design facilitates efficient charge separation and transport while preserving strong redox capability, thereby enhancing the photocatalytic performance of the heterostructure.

In this work, we demonstrate multifunctional pollutant remediation using an all-solid-state, substrate-supported plasmon-assisted Z-scheme photocatalyst consisting of interface-engineered ZnO/ZnS core–shell NRs arrays directly grown on a plasmonic Ag seed layer. Vertically aligned ZnO/ZnS core–shell NRs were fabricated on the Ag seed layer, and the ZnS shell thickness was systematically tuned by controlling the sulfidation time (0.5–2.0 h) to optimize interfacial charge transfer and carrier diffusion pathways. An optimal ZnS shell thickness of ∼20 nm was achieved at a sulfidation time of 1.5 h, which preserved the structural integrity of the ZnO NRs and the metallic nature of the Ag seed layer. The ZnO/ZnS core–shell architecture on the Ag seed layer exhibits a Z-scheme band alignment at the heterointerface, as evidenced by binding energy shifts in the Zn 2p and O 1s core levels from X-ray photoelectron spectroscopy (XPS) and corresponding changes in photoluminescence (PL) behavior following the sulfidation process. In addition, the metallic Ag seed layer functions as a plasmonic platform, LSPR assisted electron injection and transport into the ZnO NRs core, thereby suppressing bulk charge recombination and enhancing carrier utilization.

To the best of our knowledge, this study presents the first report of solid-state, plasmon-assisted ZnO/ZnS core–shell NRs array on an Ag seed layer for organic and inorganic pollutants remediation. Unlike conventional powder-based photocatalysts that require post-reaction separation and recovery, the immobilized architecture enables straightforward handling, excellent reusability, and practical applicability. − Moreover, compared with conventional type-II heterojunctions, Ag-induced LSPR, interfacial engineering, and optimized ZnS shell thickness enable plasmon-enhanced Z-scheme charge transfer with superior charge separation and redox capability, leading to enhanced photocatalytic activity. As a proof of concept, the photocatalytic performance was evaluated using representative organic dyes (methylene blue (MB)), pharmaceutical contaminants (tetracycline hydrochloride, (TC)), and toxic inorganic ions Cr (VI) under UV irradiation with an intensity of 3.5 mW/cm² and a wavelength of 365 nm. The optimized Z-scheme photocatalyst with an optimal shell thickness exhibited superior activity, achieving nearly complete degradation of MB within 100 min, TC within 60 min, and effective reduction of Cr (VI) within 60 min. The significantly enhanced activity compared with conventional heterostructures, ,, and physically mixed Ag, ZnO, and ZnS systems, , highlights the effectiveness of the proposed architecture. The improved photocatalytic performance and stability are attributed to synergistic effects of the Ag seed layer and the ZnO/ZnS core–shell heterostructure, and efficient Z-scheme charge transfer. These findings provide valuable insight into plasmon-enhanced Z-scheme photocatalysis and offer a rational strategy for the design of advanced photocatalytic systems.

2. Results and Discussion

2.1. Structure and Morphology Evolution

Figure (a–c) schematically illustrates the spin-coated metallic Ag seed layer, ZnO NRs grown on the Ag seed layer, and ZnO/ZnS core–shell NRs on the Ag seed layer. The fabrication process begins with the deposition of the Ag seed layer, followed by a two-step hydrothermal method involving the growth of ZnO NRs on the Ag seed layer and a subsequent sulfidation treatment to form the ZnS shell, as depicted in Figure S1. The FESEM micrograph of the spin-coated Ag seed layer (Figure (d)) exhibits a uniform surface with minor voids, which are attributed to solvent evaporation during annealing at 140 ^°C. The sintered Ag nanoparticles function as effective nucleation centers for NRs growth, with their size and spatial distribution critically determining the NRs density, orientation, and morphology during the hydrothermal process. Furthermore, the interconnected Ag network promotes vertical alignment of the ZnO NRs and enhances photophysical and photocatalytic performance by increasing the active surface area, facilitating charge transfer, and improving light absorption.

1.

(a–c) Schematic diagram of metallic Ag seed layer, ZnO NRs on Ag seed layer and ZnO/ZnS core–shell NRs on Ag seed layer. (d–i) FESEM micrographs of (d) sintered Ag seed layer, (e) ZnO NRs on Ag seed layer and ZnO/ZnS core–shell NRs on Ag seed layer with different sulfidation times (f) 0.5 h, (g) 1.0 h, (h) 1.5 and (i) 2.0 h. (j) Sulfidation process for ZnS shell formation. Different structural analysis (k,l) XRD patterns, (m) Raman spectra and (n) FTIR spectra of the Ag seed layer, ZnO NRs on Ag seed layer, and ZnO/ZnS core–shell NRs on Ag seed layer along with the sulfidation times.

Figure (e) shows low temperature hydrothermally grown ZnO NRs on the Ag seed layer with an average length of ∼2 μm and a diameter of ∼200 nm. The sintered Ag seed layer promotes dense, vertically aligned, and homogeneous NRs growth by reducing the nucleation barrier and suppressing disordered nucleation, thereby providing preferential sites for ZnO NRs formation. Following the successful growth of ZnO NRs, a ZnS shell was uniformly deposited onto the ZnO cores through an ion-exchange sulfidation process. Figure (f–i) present the ZnO/ZnS core–shell NRs on the Ag seed layer synthesized with sulfidation times ranging from 0.5 to 2.0 h. A uniform and conformal ZnS shell is obtained after 1.5 h of sulfidation (Figure (h)) while preserving the hexagonal morphology of the ZnO NRs.

The formation of the ZnS shell on the ZnO NRs arrays proceeds via a surface-driven ion-exchange sulfidation mechanism, as schematically illustrated in Figure (j). The process involves three sequential steps. Initially, thioacetamide (TAA) undergoes hydrolysis in aqueous solution to form intermediate species, followed by further decomposition to release H₂S. Subsequently, the generated H₂S reacts with ZnO at the NRs surface, leading to the nucleation of ZnS through an anion–exchange reaction between O^2– and S^2– ions. Finally, continued sulfidation promotes the outward growth of a conformal ZnS shell along the ZnO NRs surface, while preserving the underlying hexagonal ZnO framework. The overall mechanism can be expressed by the following chemical reaction equations such as ,

$${\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{S}\mathrm{N}\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{O}\mathrm{N}\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{S}$$ 1

$${\mathrm{H}}_2\mathrm{S}+\mathrm{Z}\mathrm{n}\mathrm{O}\to \mathrm{Z}\mathrm{n}\mathrm{S}+{\mathrm{H}}_2\mathrm{O}$$ 2

However, excessive sulfidation can reduce the effective diameter of the ZnO cores and compromise their structural integrity. As the sulfidation time increases, progressive surface conversion of ZnO promotes core thinning through ion-exchange and interfacial diffusion processes, which can induce structural strain, reduce crystallinity, and compromise the mechanical stability of the NRs. Such reduction in core dimensions may also hinder charge transport pathways and enhance carrier recombination, ultimately leading to diminished photophysical and photochemical performance. ,

Therefore, precise control of the sulfidation duration is essential to balance shell formation and core preservation. An optimized ZnS shell thickness ensures strong interfacial contact and efficient charge separation while maintaining the structural integrity, electron transport pathways, and long-term stability of the ZnO NRs. In this study, the ZnS shell thickness was tuned by varying the sulfidation time from 0.5 to 2.0 h in 0.5 h intervals, with 1.5 h identified as the optimal condition with effective ZnS shell thickness approximately ∼20 nm as shown in Figure (h). FESEM images in Figure (g–i) reveal a progressive increase in shell thickness with sulfidation time, while prolonged sulfidation (2.0 h) leads to excessive and nonuniform ZnS shell growth (Figure (i)), resulting in partial reduction of the ZnO core diameter due to ion-exchange reactions. , Moreover, overly thick ZnS shells can impede charge diffusion and enhance recombination losses, thereby diminishing the efficiency of the Z-scheme charge transfer process. ,,

The XRD patterns of Ag seed layer, corresponding to ZnO NRs and ZnO/ZnS core–shell NRs on Ag seed layer depicts in Figure (k). The XRD pattern of the Ag seed layer exhibits prominent diffraction peaks at 2θ = 38.14°, 44.38°, 64.46°, and 77.46°, corresponding to the (111), (200), (220), and (311) planes of the face-centered cubic (FCC) Ag crystal structure. These peak positions are in good agreement with the standard XRD data for face-centered cubic (FCC) crystal structure of Ag. ,, XRD analysis of ZnO NRs grown on Ag seed layer confirms the successful integration of both materials, as the diffraction peaks correspond to the characteristic planes of hexagonal wurtzite-structured ZnO and the FCC crystal structure of Ag, indicating that the ZnO NRs were effectively grown without altering the crystalline integrity of the underlying Ag seed layer.

Following the sulfidation process, characteristic XRD peaks appear at 2θ = 28.5°, 47.5°, and 56.3°, corresponding to the (111), (220), and (311) planes of the cubic zinc blende (ZB) ZnS phase, in addition to the prominent diffraction peaks of the ZnO NRs core (2θ ≈ 34.46°) and the Ag seed layer (2θ = 38.14°), consistent with the JCPDS database (card no. 01-073-8765). The comparative XRD patterns of ZnO/ZnS core–shell NRs prepared with different sulfidation durations (Figure (l)) reveal a progressive sharpening and intensification of the ZnS diffraction peaks with increasing sulfidation time. In contrast, the diffraction intensities of ZnO and Ag decrease over 1.5 h sulfidation, which is attributed to partial reduction of the ZnO core diameter and a concomitant decrease in the crystallinity of both ZnO and the Ag seed layer. Accordingly, a sulfidation duration of 1.5 h is identified as optimal for forming a conformal ZnS shell while preserving the crystalline integrity of the ZnO core and the metallic Ag seed layer. Structural properties such as lattice constants, crystallite size, full width at half maximum (FWHM), macrostrain, and dislocation density were calculated from the XRD data using the Scherrer equation and related models, summarizing in Table S1 (Supporting Information).

Figure (m) shows the room temperature Raman spectra of ZnO NRs on Ag seed layer as a function of sulfidation time, confirming the progressive formation of the ZnO/ZnS core–shell heterostructure. Pristine ZnO NRs exhibit dominant E ₂ (high) (∼438 cm^–1) and A ₁ (LO) (∼584 cm^–1) modes, characteristic of highly crystalline wurtzite ZnO. , Progressive sulfidation (0.5–1.5 h) induces the emergence and systematic intensification of ZnS transverse and longitudinal optical phonon modes (∼248 and ∼324 cm^–1), while preserving the ZnO E ₂ (high) mode, indicating coherent shell growth without structural degradation of the ZnO NRs core. − Notably, at the optimized sulfidation time of 1.5 h, the strong enhancement of Ag-related Raman features (∼1405 and ∼1590 cm^–1) indicates maximized plasmon-semiconductor coupling at the well-preserved Ag/ZnO/ZnS interfaces. ,, This coupling promotes selective recombination between ZnO conduction-band electrons and ZnS valence-band holes, preserving high-energy electrons and holes for redox reaction. Such characteristic phenomena are indicative of a Z-scheme charge-transfer pathway, in contrast to conventional type-II heterojunctions that primarily promote spatial carrier separation without interfacial recombination and often result in diminished redox strength and limited plasmonic contribution. ,,− In contrast, insufficient sulfidation limits interfacial coupling, whereas excessive sulfidation (2.0 h) induces ZnO core thinning, enhanced plasmon-phonon coupling, and increased disorder, ultimately hindering charge transport. These results identify 1.5 h as the optimal sulfidation condition for achieving efficient Z-scheme charge transfer.

The FTIR spectra of the Ag seed layer, ZnO NRs on Ag, and ZnO/ZnS core–shell NRs on Ag seed layer in Figure (n), confirm successful heterostructure formation and interfacial coupling. Weak bands at ∼2928 and ∼2853 cm^–1 originate from residual organic species, while features in the 2205–1980 cm^–1 and 1604–1401 cm^–1 regions are attributed to adsorbed CO₂, carbonate species, and surface hydroxyl groups. , The characteristic Zn–O stretching vibration at ∼570 cm^–1 confirms ZnO NR formation. After ZnS shell deposition, an additional Zn–S stretching mode appears at ∼763 cm^–1, while the Zn–O band remains, evidencing the formation of a ZnO/ZnS core–shell heterojunction. ,,, The slight modulation of metal–oxygen and metal–sulfur vibrational modes indicates strong interfacial interaction between ZnO and ZnS. Collectively, the intimate interfacial coupling, together with surface hydroxylation, is favorable for selective interfacial and efficient charge separation, supporting a Z-scheme charge transfer mechanism in the Ag/ZnO/ZnS system.

2.2. Atomic Structure and Optical Properties Analysis

Figure (a,b) display bright-field TEM images of ZnO NRs on Ag seed layer and ZnO/ZnS core–shell NRs on Ag seed layer, respectively. The hydrothermally grown ZnO NRs exhibit an average length of approximately 2 μm and a diameter of about 200 nm, as shown in Figure (a). After sulfidation of the ZnO NRs for 1.5 h, a uniform ZnS shell with a thickness of approximately ∼20 nm is clearly observed (Figure (b)), resulting the successful formation of ZnO/ZnS core–shell NRs on the Ag seed layer. The HRTEM lattice-fringe image in Figure (c) shows a lattice spacing of ∼0.235 nm, which corresponds to the (002) crystallographic plane of ZnO, indicating its high crystallinity and preferred orientation. Furthermore, the HRTEM lattice-fringe micrograph in Figure (d) depicts lattice spacings of ∼0.235 nm and ∼0.316 nm, attributed to the (002) plane of ZnO and the (111) plane of ZnS, respectively. Additionally, the presence of all elements was confirmed through STEM–EDX mapping of randomly selected ZnO/ZnS core–shell NRs from the same TEM specimen, as shown in Figure (e–h). The elemental mapping revealed that ZnS shell uniformly covered ZnO NRs across the Ag seed layer, further validating the formation of ZnO/ZnS core–shell NRs on Ag seed layer.

2.

(a,b) Bright-field TEM image of (a) ZnO NRs on Ag seed layer, (b) ZnO/ZnS core–shell NRs on Ag seed layer (1.5 h sulfidation). (c,d) High-resolution TEM (HRTEM) lattice fringe images taken at the tip of the (c) ZnO NRs structure and (d) ZnO/ZnS core–shell NRs structure. (e–h) Elemental mapping of ZnO/ZnS core–shell NRs on Ag seed layer recorded in STEM mode. (i) Full-range and high-resolution XPS spectra of (j) Ag 3d, (k) Zn 2p, (l) O 1s and (m) S 2p for ZnO/ZnS core–shell NRs on Ag seed layer. (n) EDS analysis of ZnO/ZnS core–shell NRs on Ag seed layer.

The full range XPS spectrum of the interface engineered ZnO/ZnS core–shell NRs on Ag seed layer shown in Figure (i). The survey spectrum confirms the presence of Zn, O, S, and Ag without detectable impurities. However, the presence of C is due to hydrocarbon contaminants in the XPS analysis instrument. High-resolution Ag 3d spectra in Figure (j) shows characteristic peaks at ∼368.2 eV (Ag 3d_5/2) and ∼374.2 eV (Ag 3d_3/2), indicating the metallic nature of the Ag seed layer. , The Zn 2p spectrum in Figure (k) exhibits peaks at ∼1021.5 eV (Zn 2p_3/2) and ∼1044.6 eV (Zn 2p_1/2), corresponding to Zn²⁺ species. These values are characteristic of Zn²⁺, confirming the presence of Zn in an oxidized state within both ZnO and ZnS lattices. , The O 1s spectrum (Figure (l)) is dominated by the lattice oxygen component of ZnO, accompanied by a weak high-binding-energy shoulder attributed to oxygen vacancies and/or surface hydroxyl groups. The S 2p spectrum (Figure (m)) exhibits a well-defined doublet centered at ∼161.6 and ∼162.8 eV, corresponding to S^2– species in ZnS, confirming the successful formation of ZnO/ZnS core–shell heterostructures on the Ag seed layer. The absence of peaks in the higher binding energy region (∼168–170 eV) indicates that sulfur predominantly exists in the sulfide state rather than as sulfate species. ,,

The oxygen vacancies in the O 1s spectrum can serve as electron trapping and transfer centers, thereby enhancing interfacial charge migration. Under UV illumination, electrons accumulated in the conduction band of ZnS, and the valence band holes selectively recombine with the ZnO conduction band electrons at the interface, while the remaining high-energy electrons in ZnO and holes in ZnS participate in surface redox reactions. This charge transfer behavior is consistent with a Z-scheme mechanism, in which the Ag seed layer further facilitates electron injection and transport into the ZnO NR core. The EDS spectrum in Figure (n) further confirms the presence of Zn, O, S, and Ag, consistent with the XPS and XRD results, while the homogeneous elemental distribution observed in the inset SEM image indicates uniform growth of ZnO/ZnS core–shell NRS on Ag seed layer.

Figure (a) presents the normalized transmission spectra of the Ag seed layer, pristine ZnO NRs, and ZnO/ZnS core–shell NRs on Ag seed layer. The spin-coated Ag seed layer exhibits significantly lower optical transmittance than pristine ZnO NRs, accompanied by a sharp and intense absorption feature in the ultraviolet (UV) region. This behavior is attributed to the localized surface plasmon resonance (LSPR) of Ag nanoparticles. After sulfidation, the ZnO/ZnS core–shell NRs deposited on the Ag seed layer show an intermediate transmittance higher than that of the bare Ag seed layer but lower than that of pristine ZnO NRs, indicating enhanced light absorption induced by the ZnS shell and Ag/ZnO interfacial interactions. ,,

3.

(a) Normalized transmission spectra of Ag seed layer, ZnO NRs, and ZnO/ZnS core–shell NRs on Ag seed layer. (b,c) Calculated optical band gap and comparison band gap with sulfidation time of ZnO/ZnS core–shell NRs on Ag seed layer using the transmission spectrum Data are expressed as mean ± SD (n = 3 independent replicates). (d) PL spectra of ZnO NRs on Ag seed layer and different sulfidation time (0.5–2.5 h) dependent ZnO/ZnS core–shell NRs on Ag seed layer. (e) Experimental setup for photocurrent measurement. (f–h) The comparative photocurrent of pristine ZnO NRs, ZnO NRs and ZnO/ZnS core–shell NRs on Ag seed layer with varying sulfidation times (UV light; wavelength 365 nm, intensity ∼5 μW/cm².). (i–k) The wavelength-dependent (365 to 580 nm) photocurrent measurements were performed for all representative samples.

The calculated optical band gap of ZnO/ZnS core–shell NRs on Ag seed layer from transmission spectra using Tauc plots as shown in Figure (b). Interestingly, the estimated band gap revealing a triple band gap region with 3.13 eV for Ag seed layer, 3.20 eV for ZnO NRs core and 3.44 eV for ZnS shell, collectively define the energetic framework required for a direct Z-scheme heterostructure. Additionally, the presence of three distinct band regions enhances light absorption, while the Z-scheme band structure facilitates the generation of energetic charge carriers for redox reactions. Although direct band-structure measurements such as UPS and Mott–Schottky analyses were not conducted in this study, the proposed mechanism is supported by the experimental observations and previous reports in the literature. − More detailed band alignment analyses will be considered in future work. While bulk Ag is metallic and lacks a true band gap, the Ag seed layer, composed of sintered nano particles, exhibits an apparent optical energy gap, an intriguing and noteworthy phenomenon. ,, This optical band gap arises due to several factors, including the size of the nanomaterials, LSPR effects, and the interaction of light through absorption and scattering. ,− Through comprehensive analyses, including XRD and UV–vis–NIR spectroscopy, we successfully demonstrated that the band gap of the Ag seed layer is 3.13 eV, with no evidence of silver oxide (AgO) or other Ag composites forming.

The progressive blue shift in the band gaps of both the ZnO core and ZnS shell with increasing sulfidation time as shown in Figure (c), reflects effective defect passivation and interfacial strain-driven band edge modulation during shell growth. The gradual widening of the ZnO band gap arises from the suppression of oxygen vacancy-induced sub-band states and strain-mediated electronic reconstruction at the heterointerface. ,, At 1.5 h sulfidation, the ZnS shell exhibits a band gap of 3.44 eV, slightly lower than bulk ZnS (3.6–3.7 eV), indicative of interfacial ZnO_1–x S _x alloy formation and lattice mismatch-induced band renormalization. Additionally, the excessive sulfidation (2.0 h), further shell thickening reduces alloying effects and enhances crystallinity, driving the ZnS band gap toward bulk-like values. ,,, Importantly, the underlying Ag seed layer establishes a Schottky junction and facilitates Fermi level equilibration, generating pronounced band bending and an intensified internal electric field. Under UV excitation, plasmon-induced hot electron injection from Ag into the semiconductor, coupled with selective interfacial recombination, supports a Z-scheme like charge transfer pathway. ,,, This synergistic band structure engineering and plasmonic modulation collectively enable directional carrier separation and superior photodegradation performance.

Figure (d) presents the room-temperature PL spectra of interface-engineered ZnO/ZnS core–shell NRs on Ag seed layer, with ZnO NRs on Ag used as the reference. The PL spectrum of pristine ZnO NRs exhibits two characteristic emission bands: a strong near-band-edge violet emission centered at ∼384 nm and a weak green emission around ∼570 nm. The green emission is commonly attributed to deep-level defect-related transitions associated with intrinsic point defects, such as zinc interstitials (Zn_i) and oxygen vacancies (V₀), involving recombination between defect states and the valence band. ,, After sulfidation, the ZnO/ZnS core–shell structures exhibit a broad violet-to-orange emission with complete suppression of the green band as shown in Figure (d), indicating effective defect passivation and interfacial recombination modulation induced by the wide band gap (3.44 eV) ZnS shell. Notably, as the increase of sulfidation time leads to a pronounced decrease of the near-band-edge emission accompanied by a systematic blue shift, reflecting enhanced interfacial charge transfer and band-structure modulation.

The maximum blue shift, from 384 to 374 nm as shown in Figure (d), is observed for a sulfidation time of 1.5 h, confirming the formation of ZnS shell and the presence of interfacial strain and defect passivation effects. ,, These features are consistent with efficient Z-scheme charge separation in the ZnO/ZnS core/shell heterostructure. In contrast, prolonged sulfidation (2.0 h) results in a red shift of the PL emission, which is attributed to over-sulfidation-induced interfacial disorder and the generation of sulfur-related defect states that promote trap-assisted recombination and partially weaken the Z-scheme charge-transfer pathway. ,, Furthermore, the emission band at ∼756 nm arising after sulfidation is attributed to interfacial radiative recombination of spatially separated charge carriers across the ZnO/ZnS heterointerface, enhanced by localized surface plasmon resonance induced hot-electron injection from the Ag seed layer. ,,

Figure (e–h) shows the photoresponse characteristics of pristine ZnO NRs, ZnO NRs on an Ag seed layer, and ZnO/ZnS core–shell NRs on an Ag seed layer, respectively. The measurement procedure and subsequent data analysis are fully automated through in-house software developed in LabVIEW, as shown in Figure (e). The photoresponse of the ZnO/ZnS core–shell NRs was evaluated under UV illumination using a UV light source (L8388-01 Spot Light Source, Hamamatsu Photonics, Japan) under ambient conditions. The effective active area of all samples was maintained same (1 × 1 mm²) to ensure consistent and reliable comparison of photoresponse performance. The light intensity at the sample surface was calibrated to ∼5 μW/cm² (365 nm). Photocurrent measurements were carried out using a Keithley 2400 Source Meter (Tektronix, Inc., USA) operated through a LabVIEW-controlled data acquisition system. The measurements were performed in a two-probe configuration under a fixed bias voltage of 2.0 V, and the light was periodically switched on and off to assess the transient photoresponse and carrier stability.

Under UV illumination, the ZnO/ZnS core–shell NRs on the Ag seed layer exhibit a markedly higher photocurrent than pristine ZnO NRs and Ag-assisted ZnO NRs, as evidenced by the stable consecutive on–off switching cycles in Figure (f) and the enlarged single-cycle response in Figure (g). This enhancement is attributed to the Ag/ZnO Schottky junction, which facilitates electron extraction and suppresses charge recombination through the electron-sink effect of metallic Ag. The photocurrent progressively increases with sulfidation time and reaches a maximum at 1.5 h, indicating optimized ZnS shell thickness and interfacial charge separation. Notably, the pristine ZnO NRs exhibit a relatively slower photocurrent decay after switching off the UV light is attributed to defect-mediated carrier trapping and inefficient charge separation, resulting in persistent photoconductivity. In contrast, the Ag seeded ZnO/ZnS heterointerface promotes spatial charge separation and interfacial charge redistribution, thereby accelerating carrier recombination and relaxation after removal of the excitation. Excessive sulfidation (2.0 h), reduces the photocurrent as shown in Figure (h), likely because the thick ZnS shell hinders efficient carrier transport. Thus, the non-monotonic photocurrent behavior highlights the critical role of ZnS shell thickness in optimizing UV absorption and interfacial charge separation. ,,,

To further investigate the spectral dependence of the photoresponse, wavelength-dependent photocurrent measurements were performed for all representative samples, as shown in Figure (i–k). Under monochromatic illumination at 365, 430, 520, and 580 nm with a constant incident optical power of approximately 5 μW cm^–2, the Ag/ZnO/ZnS heterostructure exhibited the highest photocurrent response at all investigated wavelengths (Figure (j)). For the Ag/ZnO/ZnS sample, the photocurrent decreased progressively from 14.2 μA at 365 nm to 4.8, 2.3, and 1.1 μA at 430, 520, and 580 nm, respectively (Figure (k)). The strong response under 365 nm illumination is primarily attributed to the intrinsic photoexcitation of ZnO core and ZnS shell, while the measurable photoresponse in the visible region can be associated with the localized surface plasmon resonance (LSPR) of Ag seed layer, together with plasmon-mediated interfacial charge-transfer processes. The enhanced response near the UV region is also consistent with the pronounced optical absorption of the Ag seed layer in the 360–380 nm range, suggesting a possible contribution of Ag LSPR to the broadened spectral photoresponse of the Ag/ZnO/ZnS heterostructure. , Notably, the Ag seed layer exhibits a distinct optical absorption feature in the 360–380 nm region, which is consistent with the spectral region of the strongest photoresponse. These results provide experimental support for the contribution of Ag LSPR to the enhanced and broadened photoresponse of the Ag/ZnO/ZnS heterostructure. Collectively, the transient photoresponse behavior, XPS core-level binding energy shifts, and room-temperature PL quenching strongly support a plasmon-enhanced direct Z-scheme charge-transfer mechanism.

2.3. Photocatalytic Activity Evaluation

2.3.1. Photocatalytic Mechanism and Dye Degradation

The proposed photocatalytic degradation mechanisms of ZnO NRs on an Ag seed layer and ZnO/ZnS core–shell NRs on an Ag seed layer under UV irradiation, based on the experimental results, are illustrated in Figure (a,b) respectively. Figure (a) shows the degradation mechanism of ZnO NRs on metallic Ag seed layer with localized surface plasmon resonance (LSPR) effect of the Ag nanoparticles (Figure (c)). Upon UV illumination, LSPR excitation induces energetic (hot) electrons in the Ag seed layer, which are subsequently injected into the conduction band (CB) of ZnO. The increased electron density at the ZnO surface promotes the formation of superoxide radicals (^•O₂ ^–) through the reduction of dissolved oxygen, while the photogenerated holes in the valence band (VB) of ZnO oxidize surface-adsorbed water or hydroxyl ions to generate highly reactive hydroxyl radicals (^•OH). ,, These reactive oxygen species (ROS) actively participate in the degradation of organic pollutants and the reduction of toxic heavy metal ions, ultimately leading to mineralization and environmentally benign products. However, in ZnO NRs, the relatively weak separation efficiency of photogenerated charge carriers results in significant electron–hole recombination, which limits the overall photocatalytic efficiency.

4.

(a,b) Proposed mechanism of plasmon induced Z-scheme photocatalyst. (c) Schematic representation of the localized surface plasmon resonance phenomenon of Ag NPs (seed layer). (d) Comparative full range XPS spectra and (e,f) high-resolution XPS spectra of the Zn 2p and O 1s pristine ZnO NRs, ZnO NRs on Ag seed layer and ZnO/ZnS core–shell NRs on Ag seed layer. (g–j) Schematic illustration of the comparative photocatalytic dye degradation mechanisms. (k–m) Photocatalytic degradation properties of different catalyst under UV irradiation, (k) methylene blue (MB) degradation profile (C _t /C ₀), (l) first order kinetics rates of the MB dye degradation and (m) degradation efficiency under UV irradiation for 100 min. Data are represented as mean ± SD (n = 3 independent replicates).

To overcome this intrinsic limitation, an efficient plasmon induced Z-scheme photocatalytic system is proposed by introducing a ZnS shell onto the ZnO NR core via a facile low-temperature sulfidation process as shown in Figure (b). Under UV irradiation both ZnO (E _g ≈ 3.20 eV) and ZnS (E _g ≈ 3.44 eV) are photoexcited, and Fermi-level equilibration at the ZnO/ZnS interface establishes an internal electric field that selectively drives interfacial charge recombination. Specifically, photogenerated electrons in the conduction band of ZnO recombine with holes in the valence band of ZnS at the core–shell interface, while high-energy electrons remain in the ZnS conduction band and strongly oxidizing holes are preserved in the ZnO valence band. This recombination pathway, characteristic of a Z-scheme heterostructure, avoids the loss of redox potential inherent to type-II systems. Additionally, the metallic Ag seed layer functions as a plasmonic platform, LSPR assisted electron injection and transport into the ZnO NRs core, thereby suppressing bulk charge recombination and enhancing carrier utilization.

The proposed Z-scheme charge transfer mechanism was evaluated through comparative full-range and high-resolution XPS analyses of pristine ZnO NRs, ZnO NRs on an Ag seed layer, and ZnO/ZnS core–shell NRs on an Ag seed layer, as shown in Figure (d–f). The full-range XPS spectrum (Figure (d)) confirms the successful formation of the ZnO/ZnS core–shell heterostructure on the Ag seed layer by the presence of Ag, Zn, O, and S without detectable impurities. The high-resolution Zn 2p spectra (Figure (e)) and corresponding O 1s spectra (Figure (f)) reveal notable binding-energy shifts upon heterostructure formation. Compared with pristine ZnO and Ag-assisted ZnO NRs, the Zn 2p states of the ZnO/ZnS core–shell NRs shift toward lower binding energies, whereas the O 1s states exhibit a clear shift toward higher binding energies. In addition, the Ag 3d peaks of the ZnO/ZnS core–shell NRs on the Ag seed layer show a lower binding-energy shift relative to Ag/ZnO and bare Ag seed layers as shown in Figure S2 (Supporting Information).

These systematic binding-energy shifts indicate electron migration from Ag to ZnO and subsequently from ZnO to ZnS (Ag → ZnO → ZnS), confirming strong interfacial electronic coupling within the heterostructure. This electron redistribution originates from Fermi-level equilibration between ZnO and ZnS, leading to electron transfer from ZnS to ZnO and the accumulation of positive charge on the ZnS shell, thereby establishing a weak internal electric field at the ZnO/ZnS interface as shown in Figure (b). During XPS measurements, photoexcitation of the Ag, ZnO, and ZnS components further promotes electron migration under the influence of the internal electric field, with energetic hot electrons injected from the Ag seed layer into the ZnO core. These lower-energy electrons are preferentially recombined with holes in the valence band of the ZnS shell at the ZnO/ZnS interface (Figure (b)). This interfacial charge migration pathway is consistent with a metal-assisted direct Z-scheme mechanism, in which the Ag seed layer promotes selective charge recombination at the interface. As a result, high-energy electrons and strongly oxidizing holes are retained, preserving strong redox capability and enabling efficient charge separation. Although supported by the observed performance, we acknowledge that direct validation using electron paramagnetic resonance (EPR) spectroscopy would further clarify reactive species and charge-transfer pathways. , However, such measurements were beyond the scope of the present study and are identified as an important direction for future work.

Figure (g–j) presents a comparative schematic illustration of the methylene blue (MB) degradation mechanisms under UV irradiation (wavelength 365 nm, intensity 3.5 mW/cm²) using pristine ZnO NRs, ZnO NRs on an Ag seed layer, and ZnO/ZnS core–shell NRs with different ZnS shell thicknesses on an Ag seed layer. As shown in Figure (i), an optimized ZnS shell thickness (∼20 nm, corresponding to 1.5 h sulfidation) promotes efficient Z-scheme charge transfer by enhancing charge separation while selectively recombining low-energy carriers, resulting in superior MB degradation performance. In contrast, an excessively thick ZnS shell (Figure (j)) increases carrier transport resistance and bulk recombination, screens the interfacial electric field, and partially blocks light absorption by the ZnO core, thereby suppressing interfacial Z-scheme charge transfer and reducing photocatalytic efficiency.

Figure (k–m) present the time-dependent photocatalytic degradation profiles of pristine ZnO NRs, Ag seed-assisted ZnO NRs, and ZnO/ZnS core–shell NRs on Ag seed layer with different sulfidation times under UV irradiation. The absorption spectrum in Figure S3­(a–g) shows the decrease in the absorbance of MB at 664 nm with increasing the UV irradiation times, indicating the progressive degradation of MB molecules. Additionally, the gradual decolorization of MB with increasing degradation time in Figure S3­(g) indicates the progressive decomposition of the MB dye. The photocatalytic degradation kinetics of MB were analyzed using a pseudo-first-order Langmuir–Hinshelwood model, and the corresponding rate constants (k) were extracted from the kinetic plots shown in Figure (k,l). Compared with pristine ZnO NRs and ZnO NRs on an Ag seed layer, the ZnO/ZnS core–shell NRs on Ag seed layer sulfidated for 1.5 h exhibit significantly enhanced photocatalytic activity. Notably, the degradation efficiency initially increases and then decreases with increasing ZnS shell thickness, indicating the existence of an optimal shell thickness as shown in Figure S3­(h,i). The ZnO/ZnS core–shell NRs with a ZnS shell thickness of approximately 20 nm (1.5 h sulfidation) achieve nearly 100% MB degradation within 100 min under UV irradiation, demonstrating the critical role of optimized shell thickness in maximizing photocatalytic performance. This trend is further supported by the linear ln (C ₀/C) versus irradiation time plots (Figure (l)), which show the highest rate constant for the 1.5 h sulfidated sample. In addition, the maximum MB degradation within 100 min under UV illumination is observed for the 1.5 h sulfidated ZnO/ZnS core–shell NRs on an Ag seed layer, as summarized in Figure (m) and Figure S5­(g). Table summarizes the degradation efficiency of ZnO/ZnS core–shell NRs on Ag seed layer-based Z-scheme photocatalysts toward industrial pollutants, pharmaceutical contaminants, and toxic heavy metals.

1. Comparison of the Degradation Efficiency of ZnO/ZnS Core–shell NRs Based Photocatalysts Toward Industrial Pollutants, Pharmaceutical Contaminants, and Toxic Heavy Metals.

photocatalyst	dye	experiment condition	efficiency	reference
ZnO/ZnS nanocomposite	MB	1 g/L, xenon lamp (150 W)	95%, 120 min
ZnO/ZnS core–shell NRs	MB	(1 cm × 1 cm), visible light (75 W)	89.4%, 135 min
ZnO/ZnS core–shell NRs	MB	∼, xenon lamp (200 W)	65%, 120 min
ZnO/CdS core–shell NRs	MB	20 mg, xenon lamp (500 W)	83.1%, 60 min
ZnS/ZnO nanosheets	Cr (VI)	10 mg, Xe lamp (300 W)	62%, 120 min
ZnO/ZnS nanoparticles	Cr (VI)	1 mg/mL, laser diode (2 W/cm2)	70%, 180 min
ZnO/ZnS nanocomposite	Cr (VI)	5 mg, UV light (25 W)	92%, 120 min
rGO-Sm2MoO6–TiO2	Cr (VI)	50 mg, visible light (∼)	96%, 70 min
ZnO/ZnS core–shell NRs	TC	(3 cm × 3 cm), xenon lamp (500 W)	78.8%, 140 min
ZnO/CuS composite	TC	10 mg, xenon lamp (300 W)	85.28%, 60 min
CdS/g-C3N4 composite	TC	5 mg, xenon lamp (300 W)	95.32%, 90 min
Ag@ZnO nanorods	TC	0.2 g/L, visible light (75 W)	92%, 90 min
ZnO/ZnS core–shell NRs on Ag seed layer	MB	160 mm2, UV lamp (3.5 mW/cm2)	∼100%, 100 min	this work
ZnO/ZnS core–shell NRs on Ag seed layer	Cr (VI)	160 mm2, UV lamp (3.5 mW/cm2)	∼99.60%, 60 min	this work
ZnO/ZnS core–shell NRs on Ag seed layer	TC	160 mm2, UV lamp (3.5 mW/cm2)	∼99.31%, 60 min	this work

Radical trapping experiments were conducted (Figure S4 (f)) to identify the active species responsible for the photocatalytic degradation of methylene blue (MB) over the optimized ZnO/ZnS core–shell NRs on the Ag seed layer under UV irradiation. In the absence of scavengers, nearly complete degradation (∼100%) was achieved while the addition of benzoquinone (BQ), a superoxide radical (^•O₂ ^–) scavenger, drastically reduced the degradation efficiency to 22.26%, indicating that ^•O₂ ^– is the dominant reactive species as shown in Figure S4­(f). Additionally, isopropanol (IPA), a hydroxyl radical (^•OH) scavenger, was introduced, the degradation decreased to 54.60%, confirming significant involvement of ^•OH radicals. Meanwhile, EDTA-2Na, a hole (h⁺) scavenger, lowered the degradation efficiency to 68.45%, suggesting moderate participation of photogenerated holes. Based on these results, the reactive species contribution order can be summarized as ^•O₂ ^–≫ ^•OH > h⁺. The simultaneous preservation of strong reductive and oxidative pathways along with the XPS binding energy shifting, PL quenching and enhance photocurrent supports a plasmon-assisted direct Z-scheme charge-transfer mechanism in the ZnO/ZnS/Ag heterostructure rather than a conventional type-II heterojunction.

Furthermore, the optimized catalyst maintains high degradation efficiency over six consecutive photocatalytic cycles (Figure S4­(a–e)) with negligible (2.8%) photocorrosion (Figure S5­(h)), indicating robust operational stability. The optimized photocatalyst exhibits excellent structural stability and recyclability. As shown in the XRD patterns (Figure S4­(i), Supporting Information), no noticeable changes are observed before and after the degradation tests, confirming the preservation of the crystal structure and phase integrity. Collectively, these results confirm that the interface-engineered ZnO/ZnS core–shell NRs with an optimized ZnS shell thickness on Ag seed layer construct an efficient direct Z-scheme heterostructure, promoting enhanced charge separation, accelerated reaction kinetics, and superior photocatalytic degradation performance toward organic dyes.

2.3.2. Tetracycline Hydrochloride Degradation and Reduction of Cr (VI) Ion

Figure (a) illustrates the degradation behavior of tetracycline hydrochloride (TC) under ultraviolet (UV) irradiation in the presence and absence of a catalyst. The degradation of TC under UV light without catalyst is slow and incomplete due to the lack of sufficient catalytic redox radicals, whereas the Z-scheme-based ZnO/ZnS core–shell NRs on an Ag seed layer enable rapid and complete degradation of TC, ultimately leading to mineralized products. The time dependent absorption spectra shown in Figure S5­(a–g) exhibits a gradual decrease in the characteristic absorbance peak of tetracycline (TC) at ∼260 nm with increasing UV irradiation time in the presence of different catalysts, indicating the progressive degradation of TC molecules.

5.

(a) Schematic illustration of the mechanism and impact of proposed Z-scheme catalyst for TC degradation. (b–d) Time dependent photocatalytic degradation properties of different catalyst under UV irradiation (b) TC degradation profile, (c) the pseudo-first-order kinetics rates of the TC degradation and (d) degradation efficiency under UV irradiation for 60 min. (e–h) Time dependent photocatalytic reduction of Cr (VI) ion, (e) photocatalytic reduction rate, (f) the pseudo-first-order kinetics rates of the Cr (VI) reduction, (g) reduction efficiency under UV illumination for 60 min and (h) recyclability of different catalysts over five consecutive photocatalytic cycles (each cycle for 60 min) for Cr (VI) reduction. Time dependent photocatalytic degradation properties of Ag–ZnO/ZnS composite catalyst under UV irradiation (i) degradation profile, (j) the pseudo-first-order kinetics rates and (k) degradation efficiency under UV irradiation for 60 min. Photocatalytic data are presented as mean ± SD from three independent experiments (n = 3).

Figure (b) presents the time-dependent photocatalytic decomposition TC under UV illumination using pristine ZnO NRs, ZnO NRs on an Ag seed layer, and ZnO/ZnS core–shell NRs on an Ag seed layer with varying sulfidation times. The corresponding degradation kinetics, expressed as linear ln (C ₀/C) versus irradiation time plots, are shown in Figure (c). Compared with pristine ZnO NRs and Ag-assisted ZnO NRs, the ZnO/ZnS core–shell NRs sulfidated for 1.5 h exhibit significantly enhanced photocatalytic degradation of TC within 60 min of UV irradiation as shown in Figure (d) and Figure S5­(g,h). Notably, the degradation efficiency initially increases and then decreases with increasing ZnS shell thickness, indicating the presence of an optimal shell thickness. The ZnO/ZnS core–shell NRs with an approximately 20 nm ZnS shell (1.5 h sulfidation) achieve nearly complete (∼100%) TC degradation within 60 min, highlighting the critical role of optimized shell thickness in maximizing photocatalytic performance.

Furthermore, the highest TC degradation under UV is observed for the 1.5 h sulfidated ZnO/ZnS core–shell NRs on an Ag seed layer, as summarized in Figure (d). This optimized photocatalyst exhibits structural and operational stability, maintaining high degradation efficiency (Figure S5­(i)) over six consecutive photocatalytic cycles. Time-dependent absorption spectra in Figure S6­(a–e) confirm sustained performance with negligible (∼2.5%) photocorrosion. Collectively, these results confirm that interface-engineered ZnO/ZnS core–shell NRs with an optimized ZnS shell thickness on an Ag seed layer form an efficient Z-scheme heterostructure, promoting effective charge separation, improved reaction kinetics, and superior photocatalytic degradation of pharmaceutical contaminants.

The developed interface-engineered Z-scheme ZnO/ZnS core–shell photocatalyst on an Ag seed layer was further evaluated for its photocatalytic reduction capability toward aqueous Cr (VI) as shown in Figure (e–h). Figure S7­(a–g) shows the absorption spectra of Cr (VI) display a gradual decrease in the characteristic absorbance peak at ∼376 nm with increasing UV irradiation time in the presence of different catalysts, indicating the progressive reduction of toxic Cr (VI) to Cr (III) ions. The Z-scheme ZnO/ZnS core–shell photocatalyst facilitates the transformation of aqueous Cr (VI) into Cr (III) through the following reaction steps ,,

$${\mathrm{C}\mathrm{r}}_2{\mathrm{O}}_7^{2-}+{14\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\to 2{\mathrm{C}\mathrm{r}}^{3+}+{7\mathrm{H}}_2\mathrm{O}$$ 3

$$2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{h}}^{+}\to {\mathrm{H}}_2+{4\mathrm{H}}^{+}$$ 4

The photocatalytic reduction performance of pristine ZnO NRs, ZnO NRs on an Ag seed layer, and sulfidation-time-dependent ZnO/ZnS core–shell NRs on an Ag seed layer is presented in Figure (e). The highest reduction of Cr (VI) to Cr (III) is observed for the ZnO/ZnS core–shell NRs sulfidated for 1.5 h, attributed to the optimized ZnS shell thickness that enhances Z-scheme charge transfer during the reaction. The corresponding reduction kinetics, shown in Figure (f) as ln­(C ₀/C) versus irradiation time plots, reveal significantly higher reduction rates for the 1.5 h sulfidated sample. Notably, the reduction efficiency initially increases and then decreases with increasing ZnS shell thickness as shown in Figure S7­(g,h), indicating the critical role of shell thickness in governing Z-scheme operation and overall photocatalytic performance.

The overall Cr (VI) reduction efficiencies of all catalysts are summarized in Figure (g) and Figure S7­(i), where the 1.5 h sulfidated ZnO/ZnS core–shell NRs on an Ag seed layer achieve nearly 100% reduction within 60 min under UV irradiation. The optimized photocatalyst demonstrates high reduction efficiency over five consecutive photocatalytic cycles (Figure (h)) with negligible (∼3.8%) photocorrosion, as confirmed by the corresponding absorption spectra in Figure S8­(a–e). In addition, the FESEM micrographs of ZnO/ZnS core–shell NRs on the Ag seed layer shown in Figure S10­(a–d) illustrate the morphological characteristics after the photocatalytic degradation experiments of multiple pollutants. The ZnO/ZnS core–shell NRs exhibit only slight surface deterioration, indicating minimal photocorrosion and good structural stability during the photodegradation process.

To verify whether the enhanced photocatalytic performance arises from the engineered Ag-seed-layer-supported ZnO/ZnS core–shell architecture rather than simply from the coexistence of Ag, ZnO, and ZnS, an Ag–ZnO/ZnS composite was prepared as a control sample. Its photocatalytic activity toward MB, TC, and Cr (VI) was evaluated under identical UV irradiation conditions for 60 min. As shown in Figure S9­(a–c), the characteristic absorption bands progressively decreased with irradiation time. The corresponding normalized concentration profiles (C/C ₀) in Figure (i) reveal a distinct, pollutant-dependent degradation behavior. Additionally, the pseudo-first-order kinetic analysis (Figure (j)) further confirms the differences in degradation rates, with MB exhibiting the highest apparent reaction rate among the three pollutants. After 60 min, the composite achieved approximately 68%, 61% and 57.52% removal of MB, TC, and Cr (VI), respectively, with MB exhibiting the highest apparent degradation rate (Figure (k)). These results demonstrate that the physically mixed Ag–ZnO/ZnS composite exhibits lower photocatalytic activity than the Ag-seed-layer-supported ZnO/ZnS core–shell NRs. The enhanced performance of the core–shell architecture can be attributed primarily to its well-defined Ag/ZnO and ZnO/ZnS heterointerfaces, which facilitate interfacial charge transfer and suppress charge-carrier recombination, rather than merely to the coexistence of Ag, ZnO, and ZnS. Collectively, these findings confirm that interface-engineered ZnO/ZnS core–shell NRs with an optimized ZnS shell thickness on an Ag seed layer form an efficient Z-scheme heterostructure, enabling effective charge separation, enhanced reaction kinetics, and superior photocatalytic removal of toxic heavy metal ions from the environment.

3. Conclusions

In conclusion, an all-solid-state, interface-engineered Z-scheme photocatalyst based on ZnO/ZnS core–shell NRs integrated on a plasmonic Ag seed layer was successfully developed to address the intrinsic limitations of conventional semiconductor systems. The ZnS shell thickness was systematically tuned via sulfidation time (0.5–2.0 h), achieving an optimal ∼20 nm at 1.5 h that preserved ZnO NRs integrity and the metallic nature of the Ag seed layer while optimizing interfacial charge transfer. The ZnO/ZnS core–shell NRs on Ag seed layer exhibit Z-scheme band alignment at the heterointerface, evidenced by binding-energy shifts in the Zn 2p and O 1s XPS spectra and corresponding changes in PL behavior after sulfidation. Moreover, the metallic Ag seed layer acts as a plasmonic platform, enabling LSPR-assisted electron injection into the ZnO NRs core, thereby suppressing charge recombination and enhancing carrier utilization.

The synergistic coupling between the plasmonic Ag seed layer and the ZnO/ZnS core–shell heterostructure, together with controlled ZnS shell thickness, effectively strengthens the Z-scheme charge transfer pathway while preserving strong redox capability. To evaluate its practical applicability, photocatalytic performance was assessed using representative organic dyes (methylene blue, MB), pharmaceutical contaminants (tetracycline hydrochloride, TC), and toxic inorganic ions Cr (VI) under UV irradiation. The optimized Z-scheme photocatalyst exhibited superior activity, achieving near-complete degradation of MB within 100 min, TC within 60 min, and efficient reduction of Cr (VI) within 60 min with enhanced stability and rapid kinetics relative to pristine ZnO and conventional heterostructures. The enhanced performance and stability are attributed to LSPR-assisted electron transport from the Ag seed layer into the ZnO NRs, an optimized core–shell interface, and an efficient Z-scheme charge transfer mechanism. This work provides mechanistic insight into plasmon-enhanced Z-scheme photocatalysis and presents a rational design strategy for advanced photocatalysts capable of treating chemically diverse pollutants.

4. Experimental Method

4.1. ZnO/ZnS Core–Shell NRs Arrays on Ag Seed Layer

ZnO/ZnS core–shell nanorods (NRs) arrays were synthesized on spin-coated Ag seed layers via a two-step low-temperature hydrothermal process, as illustrated in Figure S1. Initially, an Ag seed layer was deposited onto precleaned glass substrates (microscope slides, Marienfeld Superior, Germany) using a spin-coating technique. A commercial Ag nanopaste ink (NPS-JL, Harima Silver Nano Paste-Ink Jet, Japan) was spin-coated at 2500 rpm for 30 s under ambient conditions (25 °C, 35% relative humidity). In this study, we employed the thinnest possible Ag layer using a single spin-coating process, considering the high material cost of Ag, particularly for potential large-area applications. The Ag layer thickness (∼120 nm) was kept constant to ensure a systematic study of plasmon-assisted Z-scheme ZnO/ZnS nanorods with controlled ZnS shell thickness, enabling direct comparison and isolating shell-thickness-dependent structural, optical, and charge-transfer effects. The coated substrates were subsequently sintered in air at 140 °C for 20 min on a hot plate (SP131320-33, Thermo Scientific, China).

Following seed layer preparation, vertically aligned NRs were grown on Ag-coated substrates using a low-temperature hydrothermal method, as described in our previous work. ,, Subsequently, ZnO/ZnS core–shell NRs arrays were formed via a controlled sulfidation process. The ZnO NRs arrays were immersed in a 0.2 M thioacetamide (TAA) aqueous solution at 70 °C in an oil bath for varying sulfidation times (0.5–2 h) to achieve different ZnS shell thicknesses. After sulfidation, the samples were thoroughly rinsed with distilled water, dried at room temperature, and finally annealed at 240 °C for 1 h under a nitrogen atmosphere.

4.2. Characterizations Techniques

The surface morphology of the samples was examined using field-emission scanning electron microscopy (FESEM; MIRA LMH, TESCAN, Czech Republic) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector (X-FLASH 4010). The microstructure, core–shell architecture, and crystallite size were analyzed by transmission electron microscopy (TEM; JEM-F200, JEOL, Japan), coupled with EDS for elemental mapping and compositional analysis. The surface chemical states and elemental composition were investigated using X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific, USA) with monochromatic Cu Kα radiation.

The crystal structure and phase composition were determined by X-ray diffraction (XRD; MiniFlex 600, Rigaku, Japan) using Cu Kα radiation (λ = 1.5406 Å). Raman and room-temperature photoluminescence (PL) spectra were recorded using a LabRAM HR (Horiba Jobin Yvon, France) system with excitation wavelengths of 325 and 514 nm, respectively, to evaluate vibrational modes and charge-carrier recombination behavior. The optical properties were measured using a UV–vis–NIR spectrophotometer (SolidSpec-3700, Shimadzu, Japan) over the relevant wavelength range.

4.3. Photoresponse Test

The photocurrent measurement was conducted using a UV light source (wavelength 365 nm, L8388-01 Spotlight Source from Hamamatsu Photonics, Japan) under ambient conditions. The UV light intensity was calibrated by a UV light meter (LS125, Linshang, China). Additionally, the wavelength-dependent photocurrent measurements were conducted using different wavelength light sources (5 W power, Light Emitting Diode, (430 nm, HBL0216), (520 nm, HBL0324) and (580 nm, HBL0212)). Photocurrent response measurements were completed using a Keithly 2400 (Tektronix, Inc., USA). The measurement procedure and subsequent data analysis are fully automated through in-house software developed in LabVIEW.

4.4. Photocatalytic Activity Test

Photocatalytic activity of the samples was estimated by monitoring the degradation of methylene blue (MB) dye, tetracycline hydrochloride (TC) and the reduction of chromium Cr (VI) ion under UV irradiation at an intensity of 3.5 mW/cm² and a wavelength of 365 nm. To ensure meaningful comparison under identical conditions, the effective UV-exposed surface area of all samples was kept constant at 20 × 8 mm². Initially, MB dye solutions were prepared in DI water at concentration of 10 ppm. Prior to the start of the photocatalysis experiment, the glass beaker containing the dye solution and catalyst was kept in the dark for 20 min to facilitate adsorption–desorption equilibrium. A reference spectrum was recorded to estimate the baseline degradation efficiency of the dye in the absence of any catalyst. To evaluate the rate of degradation facilitated by the catalyst, UV–visible absorption spectra of the dye solution were recorded at 20 min intervals over a duration of 100 min for each sample.

The photocatalytic degradation of TC was carried out under same conditions, using a TC concentration of 0.1 g/L with absorption spectra recorded every 10 min for 60 min. To avoid the influence of pH on the photocatalytic reaction, all experiments were conducted under neutral pH conditions (pH = 7). Additionally, the photocatalytic reduction of Cr (VI) ions (0.03 g/L) was evaluated under the same experimental conditions, with absorption spectra recorded at 10 min intervals for 60 min. The recyclability and photochemical stability of the catalyst were assessed through multiple successive photocatalytic cycles to examine its resistance to photocorrosion. To evaluate the effectiveness of the ZnO/ZnS core–shell NRs structure supported on the Ag seed layer, an Ag–ZnO/ZnS composite was prepared as a control sample using an estimated mass ratio of approximately 2:98 wt %. The components were thoroughly mixed, and the resulting composite was subjected to the same photocatalytic testing conditions as the Ag-seeded ZnO/ZnS core–shell NRs arrays for direct comparison.

In addition, scavenger test was performed to find out the reactive species accountable for MB dye degradation. Isopropyl alcohol (IPA), ethylenediaminetetraacetic acid (EDTA), and benzoquinone (BQ) were employed as scavengers for hydroxyl (^•OH), hole (h⁺), and superoxide (^•O₂ ^–) radicals, respectively. For the trapping experiments, 100 μL of each scavenger solution (10 mM) was added to the MB solution prior to UV irradiation. The photocatalytic degradation process was then carried out under identical experimental conditions to evaluate the contribution of each reactive species. Note that, all photocatalytic analysis were performed at least three times, and the reported values are presented as mean ± standard deviation.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c08853.

• The Supporting Information includes the synthesis schematic, structural parameters, Ag 3d XPS spectra, detailed photocatalytic degradation and recyclability results for MB, TC, and Cr (VI), radical scavenger experiments, comparative XRD analysis before and after photocatalysis, control experiments using a physically mixed Ag–ZnO/ZnS composite, and post-reaction FESEM characterization (Figures S1–S10 and Table S1) (PDF)

Elius Hossain: data curation, conceptualization, writing-original draft, methodology, investigation etc. Kwon Kye-Si: writing-review & editing, supervision, project administration, investigation, funding acquisition.

The authors declare no competing financial interest.