Tuning the Photocatalytic Activity of Layered Double Hydroxides with Loading of Certain Transition Metals-metal Oxides Nanocomposites

Abstract

Chapter-1 The structural and chemical composition, adsorption, and photocatalytic properties of 2-D nanostructured layered double hydroxides (LDH) are covered in this chapter. LDH behaviour and alterations have been explored in relation to adsorption and photocatalysis. This chapter also includes descriptions of the literature evaluation, research gaps, objectives, experimental procedures, and characterization approaches. Chapter-2 Recently, the efficient removal of antibiotics like Tetracycline (TC) from wastewater is emerging as a crucial problem. However, low adsorption capacity and high cost of photocatalysts limit their utility. In this context, monoclinic tungsten trioxide (WO3) loaded ZnCr layered double hydroxide (LDH) composites (ZnCr-xWO3 x= 10 and 20%) were prepared by co-precipitation method and characterized by several techniques viz. X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Raman, Photoluminescence (PL), Field-Emission Scanning Electron Microscopy (FE-SEM), Brunauer Emmett Teller (BET), Diffuse Reflectance Spectroscopy (DRS) and Dynamic Light Scattering (DLS). The XRD pattern confirms the formation of ZnCr LDH and its composites with monoclinic-WO3. Bond vibrations and phase transitions were explored with the help of FTIR and Raman spectra. The BET analysis for ZnCr LDH and ZnCr-10WO3 composite shows the surface area as 27.439 m²/g and 47.691m²/g. The higher photocatalytic performance of the composites formed is due to enhanced specific surface area and better separation efficiency of photo-generated electrons and holes through conventional class II hetero junction between ZnCr LDH and monoclinic-WO3. The kinetics studies performed show a better fit for pseudo-first-order reaction with maximum degradation efficiency and rate constant for ZnCr-10WO3 (i.e., 86.7% and 1.5 × 10-2 min-1). Chapter-3 An environmental friendly approach was utilized to remove pharmaceutical drugs from wastewater, employing adsorption and photocatalysis. The emphasis centered on synthesizing ternary composites comprising silver-deposited tungsten trioxide (Ag@WO3; 2, 4 wt.%), and MgAl layered double hydroxide (LDH) for the remediation of wastewater from the pharmaceutical industry. Combining silver-loaded WO3 nanoparticles with MgAl LDH resulted in the enhancement of the adsorption and photocatalytic properties of MgAl LDH. The studies demonstrated that by incorporating Ag@WO3 into MgAl LDH, the formed ternary composites exhibited superior performance, achieving 88% tetracycline (TC) adsorption in 90 minutes, providing the best fit for the Langmuir model, suggesting monolayer adsorption following second-order kinetics and 77.9% of ciprofloxacin (CIP) degradation under visible light irradiation in 160 minutes with rate constant of 0.96 × 10-2 min-1 for pseudo-first-order kinetics. The enhancement was attributed to the increased surface area and electron-accepting ability of silver NPs (surface plasmonic effect). Additionally, the Ag2@WO3-LDH heterostructure exhibited high stability and recyclability for up to 5 cycles with little loss in performance. High-resolution mass spectrometry (HRMS) studies and free radical trapping experiments were conducted to reveal the intermediates generated during the degradation process, leading to the proposed mechanism via two degradation pathways. Chapter-4 The study aimed to enhance the photocatalytic efficiency of NiCo layered double hydroxide by forming composites with Fe2O3 and further improving its responsiveness to visible light by Ag loading. In comparison to bare NiCo LDH and its binary composites, the optimized ternary composite: NCF10A3 LDH (Ag3@10Fe2O3/NiCo LDH), showed significantly improved photoactivity in degrading malachite green, achieving degradation rates and rate constant of 81.2% (k = 1.25 × 10-2 min-1) and 98.6% (k = 3.33 × 10-2 min-1) under visible light and sunlight, respectively. This is caused by the active visible light response from Fe2O3 NPs and Ag NPs' SPR effect. The band gap of material decreased from 2.40 eV to 1.74 eV. The noteworthy enhancement in visible-light response and faster transfer of photoinduced carriers are attributed to the collaborative effect of NiCo LDH, Fe2O3, and Ag NPs in the ternary composite. Furthermore, malachite green was partially mineralized, as indicated by TOC analysis and corroborated by HR-MS chromatograms, with mineralization efficiencies of 41.3% and 49.4% under visible light and sunlight, respectively. Through examination of the fragments found at different m/z ratios, the degradation routes were proposed. In comparison to samples exposed to visible light, the HRMS spectra of the degraded product under sunlight exhibit fewer fragments, which could be due to the accelerated rate of a reaction under high-intensity and high-temperature reaction conditions. The catalyst’s durability was assessed by reusability and recyclability tests, which showed that the catalyst stayed stable and reusable for up to five cycles.