Photocatalytic degradation of Methyl Orange under ultraviolet-visible light irradiation using WO3-ZnO nanocomposites synthesized via Saffron petal extract

Technical Summary: Photocatalytic Degradation of Methyl Orange Using Green-Synthesized WO₃-ZnO Nanocomposites

Problem Statement
The paper addresses the critical issue of water pollution caused by industrial organic dyes, specifically Methyl Orange (MO), which is characterized by complex structures, high molecular weight, and carcinogenic properties. Conventional water treatment methods (chemical, physical, and biological) are often limited by high energy consumption, low efficiency, and the generation of toxic byproducts. While semiconductor photocatalysis offers an environmentally friendly alternative, single-component semiconductors like ZnO suffer from a wide band gap (limiting activity to the UV region, <5% of the solar spectrum) and rapid electron-hole recombination. Conversely, WO₃ has a narrower band gap but limitations in generating superoxide radicals. The study aims to overcome these individual limitations by developing WO₃-ZnO nanocomposites to enhance charge separation and extend light absorption into the visible region, utilizing a sustainable "green" synthesis approach to avoid the environmental drawbacks of traditional chemical methods.

Methodology
The researchers synthesized a series of WO₃-ZnO nanocomposites with varying weight ratios of ZnO to WO₃ (100:0, 80:20, 60:40, 40:60, 20:80, and 0:100), labeled 100Z through 100W.

• Green Synthesis: The synthesis employed saffron petal extract (SPE) as a reducing and stabilizing agent. The process involved dissolving WO₃ in SPE, adding zinc nitrate hexahydrate, adjusting the pH to 10 with ammonia, stirring, drying at 100°C, and calcining at 450°C for 2 hours.
• Characterization: The resulting materials were analyzed using X-ray diffraction (XRD) for crystal structure, Brunauer-Emmett-Teller (BET) analysis for surface area and porosity, Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive Spectroscopy (EDS) for morphology and elemental composition, Dynamic Light Scattering (DLS) for particle size distribution, Fourier Transform Infrared Spectroscopy (FTIR) for functional groups, and Diffuse Reflectance Spectroscopy (DRS) for optical band gap determination.
• Photocatalytic Testing: The photocatalytic activity was evaluated by degrading MO (100 ppm initial concentration) in an aqueous solution under UV-visible light irradiation (500 W xenon lamp). The experiments utilized 0.1 g of catalyst in 50 mL of solution, with hydrogen peroxide (H₂O₂) added to enhance the process. Degradation efficiency was calculated based on absorbance changes at 487 nm over time intervals of 5, 15, and 30 minutes.

Key Results

• Structural and Morphological Properties: XRD confirmed the formation of hexagonal ZnO and monoclinic WO₃ phases in the composites without impurities. The mean crystallite size increased with WO₃ addition, ranging from 22 nm (pure ZnO) to 43 nm (pure WO₃). FESEM revealed that pure ZnO exhibited a cauliflower-like morphology, while pure WO₃ formed layered nanosheets. The composites, particularly the 40WZ sample, showed a mixed morphology with WO₃ nanosheets homogeneously deposited on ZnO particles, enhancing interfacial contact.
• Textural Properties: BET analysis indicated that the 40WZ nanocomposite possessed the optimal textural properties, with the highest specific surface area (20.413 m²/g) and total pore volume (0.99023 cm³/g), compared to pure ZnO (10.898 m²/g) and pure WO₃ (3.968 m²/g).
• Optical Properties: DRS analysis showed a red shift in the absorption edge for the composites compared to pure ZnO, extending light absorption into the visible region. The direct optical band gaps were calculated as 3.29 eV for 100Z, 2.99 eV for 100W, and intermediate values for the composites, with 40WZ showing a band gap of 3.09 eV.
• Photocatalytic Performance: The 40WZ nanocomposite (40% WO₃) demonstrated the highest photocatalytic efficiency, achieving 100% degradation of Methyl Orange within 15 minutes of UV-visible light irradiation. Pure ZnO and pure WO₃ showed significantly lower efficiency. The degradation followed pseudo-first-order kinetics, with the 40WZ sample exhibiting the highest rate constant.
• Mechanism: The enhanced performance is attributed to the formation of a heterojunction between WO₃ and ZnO. Upon irradiation, photoexcited electrons transfer from the conduction band of ZnO to WO₃, while holes move in the reverse direction. This spatial separation suppresses electron-hole recombination. The separated charges react with adsorbed oxygen and water to generate reactive oxygen species (•O₂⁻ and •OH), which drive the degradation of the dye.

Significance and Claims
The authors claim that this study represents the first report on the green synthesis of WO₃-ZnO nanocomposites using saffron petal extract for the photocatalytic degradation of Methyl Orange under UV-visible light. The significance of the work lies in demonstrating that:

1. Green Synthesis: Saffron petal extract is an effective, eco-friendly alternative to chemical reagents for synthesizing high-performance nanocomposites.
2. Synergistic Effect: The specific 40:60 ratio of WO₃ to ZnO creates an optimal heterojunction that maximizes charge separation and surface area, leading to superior photocatalytic activity compared to single-component semiconductors or other ratios.
3. Efficiency: The 40WZ nanocomposite achieves complete dye removal in a remarkably short timeframe (15 minutes), outperforming many previously reported WO₃-ZnO systems synthesized via conventional methods (e.g., hydrothermal, precipitation, or ball-milling) which often require longer durations or show lower degradation percentages.

The paper concludes that the WO₃-ZnO nanocomposite, specifically the 40WZ variant synthesized via this green method, is a versatile and efficient photocatalyst for wastewater treatment, offering a sustainable solution to the limitations of traditional semiconductor photocatalysts.