Preparation and structural characterization of coinage metal loaded ZnO and TiO2 nanostructures for photodegradation of organic pollutants

Abstract

Chapter-1 This chapter briefly introduces semiconductor photocatalysis, focusing on two semiconductor oxides, ZnO and TiO2, as photocatalysts for the decomposition of organic pollutants. It highlights the benefits and drawbacks of these materials and presents various strategies to boost their photocatalytic efficiency. This chapter also discusses the role of various morphologies of nanostructures in the improvement of photodegradation abilities and how discusses the unique characteristics of the different shapes. A thorough review of relevant literature and a summary of characterization techniques used to evaluate the synthesized metal@ZnO or metal@TiO2 nanostructures are included. The chapter also emphasizes the importance of combining ZnO and TiO2 as a binary nanohybrid to enhance performance beyond that of the pristine forms. An outline of the photocatalytic performance assessments, including model pollutants, is provided. Finally, the research gaps identified are discussed in relation to the aims of the current study. Chapter-2 This work assesses the role of the noble metal silver in the visible-light sensitization of UV-active ZnO by introducing a localized plasmonic resonance effect and enhancing the spatial charge carrier separation. We herein synthesize ZnO nanorods via a solvothermal approach, yielding nanorods of length and diameter 1.356 ± 0.619 μm and 120.56 ± 25.09 nm, respectively. Silver nanoparticles are reduced on the surface of ZnO nanorods via the photodeposition route. XRD spectra reveal a high crystallinity and wurtzite-type structure for ZnO with new peaks for cubic phase Ag upon Ag-loading. The diffraction peaks shift upon Ag modification, indicating partial incorporation of Ag into the ZnO lattice. Further, upon Ag-decoration, a slight increase in dislocation density and micro strain is obtained, which could suppress the recombination rate of charge carriers. The DRS spectra reveal a band gap contraction of materials from 3.19 to 2.98 eV with increasing Ag density. The PL spectra confirmed that the optimum Ag concentration of 3 wt% is proficient in harvesting visible light towards high photocatalytic degradation efficiencies of tetracycline (92.1%) and amoxicillin (76.4%) in 90 min as compared to 49.4% and 38% over pristine ZnO nanorods. TOC studies reveal only partial mineralization of ~42.7% (tetracycline) and ~31.3% (amoxicillin) due vii to the formation of reaction intermediates identified by HR-MS chromatograms. In addition, degradation pathways are proposed through the fragments at different m/z ratios. Chapter-3 The removal of pollutants via photocatalysis involves two steps: adsorption and photodegradation. This study aimed to construct a plasmonic metal-loaded semiconductor and examine its synergic adsorption–photocatalysis effect for the removal of sucrose. Plasmonic metals augment photocatalytic activity by improving visible light absorption and charge transfer mechanisms. ZnO and TiO2 nanoparticles were synthesized via wet chemical processes followed by photodeposition of two noble metals, Cu and Ag, at each photocatalyst. The detailed photocatalytic information was investigated to evaluate the performance of various fabricated materials toward sucrose. With qmax = 156.1 mg g-1 (TiO2) > qmax = 126.1 mg g-1 (Cu@TiO2) > qmax = 109.1 mg g-1 (ZnO) > qmax = 96.8 mg g-1 (Ag@TiO2) > qmax = 95.7 mg g-1 (Cu@ZnO) > qmax = 66.9 mg g-1 (Ag@ZnO), TiO2 displayed superior adsorption capabilities towards sucrose. However, the overall removal efficiencies were Ag@ZnO(100%) > Cu@ZnO(89%) > Ag@TiO2(86%) > Cu@TiO2(72%) > ZnO(62%) > TiO2(58%). Thus, Ag@ZnO NPs emerged as a promising photocatalyst for the synergic adsorptive-photocatalytic removal of sucrose contamination due to the combined LSPR and interfacial charge transfer effects of Ag NPs. These NPs were then employed for the solar-irradiated removal of sucrose from sucrose-contaminated wastewater, showing a high efficiency of 91% in 60 min. Chapter-4 The current study investigated 3-D ZnO nanoflowers augmented with elongated 1-D TiO2 nanostructures and gold (Au) nanoparticles. ZnO flowers were effectively fabricated via a facile hydrothermal synthesis. Thereafter, the hydrothermal technique was employed to load various concentrations of TiO2 onto the ZnO surface, resulting in the TiO2/ZnO heterostructure. Additionally, Au nanoparticles were photo-deposited superficially at TiO2/ZnO to construct Au@TiO2(x)/ZnO hybrid structures. Samples were characterized using XRD, FE-SEM, EDX, HR-TEM, and DRS analyses. The photocatalytic abilities of ZnO nanoflowers and their consequent enhancements were studied. The study found that the indicated surface-loading materials can improve the photocatalytic capabilities of pristine ZnO nanoflowers. After 50 minutes of exposure to LED light, the synthesized photocatalysts demonstrated the following viii removal efficiency: TiO2(32%) < ZnO(45%) < TiO2(1.0)/ZnO(54%) <TiO2(10.0)/ZnO(57%) < TiO2(3.0)/ZnO(59%) < TiO2(5.0)/ZnO(65%) < Au@TiO2(5.0)/ZnO(98%). Further, the Au@TiO2(5.0)/ZnO nanostructures could mineralize the toxic PCM (76.7%) ~4.2 times higher than pristine ZnO (18.5%) and ~6.2 times higher than bare TiO2 (12.4%). The Au@TiO2(5.0)/ZnO samples outperformed the others.