Enhanced Crystal Violet dye degradation using Artemisia plant-based synthesized Indium- doped ZnO nanocomposites under UV- Irradiation

Technical Summary: Enhanced Crystal Violet Dye Degradation Using Artemisia Plant-Based Synthesized Indium-Doped ZnO Nanocomposites

Problem Statement
The discharge of synthetic dyes, specifically Crystal Violet (CV), into aquatic ecosystems poses significant environmental and health risks, including water deoxygenation, reduced light penetration, and the introduction of endocrine disruptors. While semiconductor photocatalysts like Zinc Oxide (ZnO) offer a promising solution for degrading organic pollutants, their performance is often limited by a wide bandgap (~3.37 eV) and rapid electron-hole recombination. Furthermore, conventional synthesis methods often involve toxic chemicals. This study addresses the need for an eco-friendly synthesis route and a method to enhance the photocatalytic efficiency of ZnO through doping.

Methodology
The researchers employed a green synthesis approach using Artemisia vulgaris (mugwort) extract as a reducing and capping agent to synthesize pure ZnO (P-ZnO) and Indium-doped ZnO (5In-ZnO) nanocomposites.

• Synthesis: Zinc nitrate hexahydrate served as the precursor for ZnO, while Indium (III) nitrate was used to introduce a 5 atomic percent (at.%) Indium dopant. The process involved mixing the plant extract with the metal salts, adjusting the pH to 10, stirring at 70°C, and subsequent drying and annealing.
• Characterization: The physical properties were analyzed using X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM) with EDX, Transmission Electron Microscopy (TEM), UV-Visible spectroscopy, Photoluminescence (PL), and Vibrating Sample Magnetometry (VSM).
• Photocatalytic Testing: The degradation of CV dye was evaluated under UV irradiation (365 nm) and natural sunlight. Key parameters investigated included pH, initial dye concentration, and the role of reactive species using tert-butanol as a scavenger. Mineralization was assessed via Chemical Oxygen Demand (COD) measurements.

Key Contributions and Results

• Structural and Morphological Properties:

  • Both P-ZnO and 5In-ZnO exhibited a hexagonal wurtzite crystal structure. XRD analysis revealed that Indium doping caused a slight shift in diffraction peaks toward lower angles and a reduction in crystallite size from 57.3 nm (P-ZnO) to 53.1 nm (5In-ZnO).
  • No secondary phases of Indium oxide were detected, suggesting successful substitution of Zn²⁺ ions by In³⁺ ions within the lattice.
  • SEM and TEM analyses showed heterogeneous grain shapes and agglomeration, with particle sizes ranging from 20 to 50 nm.

• Optical and Electronic Properties:

  • UV-Vis absorption spectra indicated a blue shift and a reduction in the optical bandgap from 3.24 eV (P-ZnO) to 3.22 eV (5In-ZnO). This reduction is attributed to the formation of defect states (such as oxygen vacancies and zinc interstitials) and the introduction of shallow donor levels by Indium.
  • Photoluminescence (PL) analysis showed two emission peaks: a UV peak (band-to-band transition) and a visible peak (defect-related). The visible emission at 568 nm for 5In-ZnO was stronger than that of P-ZnO (566 nm), indicating an increased concentration of point defects.
  • The increased defect density was linked to the generation of weak ferromagnetism at room temperature, confirmed by M-H hysteresis loops. The saturation magnetization ($M_s$) increased from 0.0343 emu/g (P-ZnO) to 0.0429 emu/g (5In-ZnO).

• Photocatalytic Performance:

  • Degradation Efficiency: Under UV irradiation, the 5In-ZnO nanocomposite achieved 99.9% decolorization of CV dye within 25 minutes, significantly outperforming pure ZnO, which reached only 72% degradation after 60 minutes under the same conditions.
  • Kinetics: The degradation followed pseudo-first-order kinetics (Langmuir-Hinshelwood model). The reaction rate constant ($k_{app}$) was highest at lower initial dye concentrations (10 ppm).
  • Mechanism: Scavenger studies confirmed that hydroxyl radicals (•OH) play a dominant role in the degradation process. The incorporation of Indium is proposed to enhance the separation of photo-generated electron-hole pairs, reducing recombination and promoting the formation of reactive radicals ($O_2^{\bullet-}$ and $OH^{\bullet}$).
  • Solar vs. UV: The catalyst performed more efficiently under natural sunlight, achieving nearly 100% removal in 20 minutes, compared to 90% under artificial UV light over the same duration.
  • Mineralization: Complete mineralization (conversion to inorganic products) was slower than decolorization, reaching 95% COD removal after 300 minutes of irradiation.

Significance and Claims
The paper claims that the green synthesis method using Artemisia vulgaris is effective for producing high-quality, doped nanocomposites. The primary significance lies in demonstrating that Indium doping, combined with a plant-mediated synthesis, significantly enhances the photocatalytic activity of ZnO for Crystal Violet degradation. The authors attribute this enhancement to:

1. Bandgap Modulation: A slight reduction in bandgap energy facilitating better light absorption.
2. Defect Engineering: An increase in point defects (oxygen vacancies) which act as traps for charge carriers, thereby extending their lifetime and reducing recombination.
3. Magnetic Properties: The induction of weak ferromagnetism, which may facilitate catalyst recovery, though the paper focuses primarily on the photocatalytic mechanism.

The study concludes that 5In-ZnO is a highly efficient, eco-friendly photocatalyst for the remediation of textile wastewater, capable of achieving near-complete decolorization in short irradiation times under both UV and solar light.