AgZnO/Polyoxometalate Nanocomposites: A Bifunctional Photocatalytic‑Adsorbent System for Efficient Removal of Basic Magenta

Abstract

We report a dual‑function nanocomposite, AgZnO/polyoxometalates (AgZnO/POMs), synthesized via a sonochemical route that couples AgZnO hybrid nanoparticles with Cu‑based polyoxometalates. Transmission electron microscopy (TEM) confirms a uniform, narrowly distributed particle size (~19.5 nm) without agglomeration, while X‑ray diffraction (XRD) and X‑ray photoelectron spectroscopy (XPS) validate the composite’s crystalline phases and elemental composition. Ultraviolet–visible (UV–Vis) and photoluminescence (PL) spectroscopy reveal superior optical characteristics. In aqueous media, the composite removes 94.13 % ± 0.61 % of the triphenylmethane dye basic magenta (BM) through simultaneous adsorption and photocatalysis, following pseudo‑second‑order kinetics. Remarkably, the removal efficiency remains essentially unchanged after five reuse cycles, underscoring the material’s stability. These findings demonstrate that AgZnO/POMs nanocomposites are promising candidates for treating refractory organic dye wastewaters.

Introduction

Industrial effluents increasingly contain toxic, recalcitrant organic dyes such as basic magenta (BM), a triphenylmethane derivative used in textiles, leather, and diagnostic staining. BM’s poor biodegradability, high toxicity, and carcinogenicity pose severe risks to aquatic ecosystems and human health. Conventional removal strategies rely largely on adsorption, yet these approaches suffer from low capacity, sluggish kinetics, and limited regenerability. Polyoxometalates (POMs), known for their structural versatility, thermal stability, tunable acidity, and reversible redox behavior, have emerged as potent adsorbents for diverse dyes. When combined with semiconductor nanoparticles, POMs can enhance adsorption efficiency and introduce photocatalytic functionality.

Ag‑doped ZnO nanostructures exhibit enhanced photocatalytic activity due to Ag’s role as an electron sink and its capacity to improve charge separation and extend light absorption. In this study, we integrate AgZnO nanoparticles with Cu‑based POMs to fabricate a bifunctional nanocomposite (AgZnO/POMs) that harnesses both adsorption and photocatalysis for BM removal. Our work demonstrates superior removal efficiency, kinetic behavior, and recyclability, establishing AgZnO/POMs as a robust platform for dye‑laden wastewater treatment.

Materials and Methods

Materials

Silver acetate, zinc(II) acetylacetonate, PEO‑PPO‑PEO, octyl ether, 1,2‑hexadecanediol, copper perchlorate, sodium molybdate dihydrate, pyridinecarboxamide, and NaOH were sourced from Aladdin (Shanghai). All reagents were used without further purification.

Instrumentation

Structural and morphological analyses were performed using XRD (Bruker X’Pert Pro) and TEM (JEOL JEM‑2100). Optical properties were probed by UV–Vis (Hitachi U4100) and PL (Hitachi F7000). FTIR spectra were recorded with a Nicolet Avatar 360. XPS data were acquired on a Thermo Fisher ESCALAB 250XI using Al Kα radiation.

Synthesis of AgZnO/POMs Nanocomposites

AgZnO nanoparticles were prepared via a nano‑microemulsion route: in a three‑necked flask, octyl ether (10 mL), Zn(acac)₂ (0.0989 g), 1,2‑hexadecanediol (0.6468 g), Ag acetate (0.0259 g), and PEO‑PPO‑PEO (0.7874 g) were mixed and heated to 125 °C, then rapidly to 280 °C. The product was isolated by cooling, washing, and drying. Cu‑POMs were synthesized hydrothermally: copper perchlorate (0.093 g), pyridinecarboxamide (0.061 g), and 15 mL deionized water were stirred; Na₂MoO₄·2H₂O (0.24 g) was added at pH 3, yielding a blue precipitate. Finally, 50 mg POMs and 5 mg AgZnO were dispersed in 5 mL water/ethanol, sonicated to form a homogeneous mixture, and dried to yield the AgZnO/POMs nanocomposite.

Dye Removal Experiments

A 15 mg L⁻¹ BM solution (pH 6.3) was prepared. 5 mg of AgZnO/POMs was introduced into 40 mL of the dye solution, stirred at room temperature, and irradiated with either a 36‑W UV lamp (365 nm) or a 500‑W Xenon lamp. Samples were taken at defined intervals, centrifuged, and the remaining BM concentration was determined by UV–Vis at 545 nm.

Statistical Analysis

All experiments were conducted in triplicate. Results are presented as mean ± SD, and significance was assessed via one‑way ANOVA (p < 0.05).

Results and Discussion

Structural and Morphological Characterization

TEM images reveal a uniform, monodisperse particle distribution with an average diameter of 19.5 nm (Figure 1). HRTEM shows lattice spacings of 1.44 Å (Ag 220) and 2.47 Å (ZnO 101), confirming the coexistence of AgZnO and POM phases. Elemental mapping confirms the spatial distribution of P, O, Ag, Cu, Mo, N, C, and Zn, validating the successful composite formation.

XRD patterns display characteristic peaks of Ag (38.2°, 44.4°, 64.6°, 77.4°) and ZnO (31.7°, 34.5°, 36.5°, 47.6°, 56.7°, 62.8°, 67.7°), along with POMs peaks (8.7°–30.7°). The composite spectrum retains all these features, confirming phase integrity.

FTIR spectra show distinct vibrational bands corresponding to P–O (1120–1008 cm⁻¹), Mo–O (905, 662 cm⁻¹), Zn–O (512 cm⁻¹), and ligand moieties (1680–1133 cm⁻¹), further corroborating composite assembly.

XPS analysis confirms the presence of Zn²⁺, Ag⁰ (shifted due to Ag–ZnO interaction), Cu²⁺, Mo⁶⁺, and P⁵⁺ species, indicating the expected chemical states.

Optical Properties

UV–Vis absorption of the composite shows four bands: 209, 260, 365, and 380–420 nm. The 365 nm band corresponds to ZnO, while 380–420 nm indicates Ag–ZnO hybridization and enhanced interfacial charge transfer. PL spectra under 241 nm and 380 nm excitation display emissions at 393 nm (POMs) and 465/489/596 nm (AgZnO), evidencing efficient charge separation and reduced recombination.

BM Removal Performance

Optimization identified 5 mg AgZnO/POMs and 15 mg L⁻¹ BM as optimal conditions. The composite achieved 94.13 % ± 0.61 % BM removal under UV irradiation, outperforming AgZnO (73.77 % ± 1.17 %) and POMs alone (55.27 % ± 0.83 %). Under visible light, the composite maintained superior performance, highlighting its broad‑spectrum activity.

Adsorption–photocatalysis kinetics follow pseudo‑second‑order models (R² = 0.9997 dark, 0.9736 UV), indicating chemisorption‑dominated uptake and effective electron transfer. The composite’s specific surface area (BET = 33.5 m² g⁻¹) exceeds that of AgZnO alone (28.7 m² g⁻¹), explaining the enhanced adsorption capacity.

ROS scavenging experiments with 1,4‑benzoquinone and isopropanol reduced BM removal to 52.17 % and 57.70 %, respectively, confirming the pivotal roles of superoxide and hydroxyl radicals in the degradation pathway.

Recyclability and Stability

Over five consecutive cycles, the removal efficiency declined by only 7.33 % (from 94.13 % to 86.80 %), with an 96.3 % recovery rate. FTIR spectra before and after cycling remained unchanged, indicating excellent structural integrity and resistance to photocorrosion.

Conclusions

AgZnO/POMs nanocomposites exhibit a synergistic combination of adsorption and photocatalysis, achieving >94 % removal of the hazardous dye basic magenta while retaining stability across multiple uses. Their robust optical properties, high surface area, and efficient charge separation make them promising candidates for industrial wastewater treatment of triphenylmethane dyes.