Physics

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Browsing Physics by Subject "Visible-light photocatalysis"

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    Ab initio defect engineering and green synthesis of ca-mg Codoped titania and zirconia for photocatalytic Applications
    (Chuka University, 2025-10) Mbae, Jane Kathure
    The growing demand for long-term solutions to environmental pollution and energy concerns has sparked interest in photocatalysts that work efficiently in visible light. In particular, TiO2 and ZrO2 photocatalysts have shown great potential in photocatalytic degradation and disinfection applications. However, traditional TiO2 and ZrO2photocatalysts feature metastable phases that are better photocatalysts, large bandgaps, low solar energy utilization, and rapid charge carrier recombination, limiting their usefulness. This study addresses these limitations by investigating Ca-Mg codoping as a defect engineering strategy for improving the photocatalytic performance of TiO2 and ZrO2. The primary objectives were to determine the phase stability of Ca-Mg codoped TiO2 and ZrO2 polymorphs using Density Functional Theory (DFT), to evaluate the effects of oxygen vacancies on their electronic structures, to establish optimal dopant concentrations for visible light activation, to green synthesize and characterize the predicted visible-active materials, to assess photocatalytic activity via Rhodamine B degradation under visible light and to evaluate antibacterial performance against E. coli and S. aureus. In this study, a combined ab initio and experimental approach was undertaken. Density Functional Theory (DFT) method was used to simulate the codoped polymorphs at different doping levels. Optimal codoped systems found suitable for visible light photocatalysis were green synthesized and characterized using XRD, ICP-MS and UV-VIS spectroscopy. The photocatalytic properties of the synthesized materials were evaluated by degradation of Rhodamine B (RhB) under visible light and the disinfection capability was tested against Escherichia coli and Staphylococcus aureus bacteria strains. The results yielded reasonable agreement between experimental and theoretical results, as well as a better understanding of the system. DFT results showed that Ca-Mg codoping stabilizes the anatase phase in TiO2 by increasing the c/a ratio and induces tetragonalization in monoclinic ZrO2 through lattice distortions and oxygen vacancy formation. Defective oxygen vacancy Localized mid-gap states were discovered in Ca-Mg codoped TiO2 and ZrO2 systems. These states served as trapping sites for the photogenerated charge carriers and significantly decreased the energy band, improving visible light absorption. Experimentally, the green synthesized codoped materials revealed smaller crystallite sizes (5.567 nm for TiO2 and 8.647 nm for ZrO2) and narrower bandgaps (1.92 eV and 2.4 eV, for TiO2 and ZrO2 respectively). Codoped TiO2 degraded 99.3% of Rhodamine B in 120 minutes and completely inactivated S. aureus under visible light in 60 minutes. RhB degradation % for 5 cycles of codoped TiO2 and ZrO2 nanoparticles have remarkable stability and reusability, which is the key reason for their widespread use in photocatalytic degradation of dyes. Based on these findings, Ca-Mg codoping is recommended as an economical, non-toxic and effective strategy for tuning the structural and electronic properties of TiO2 and ZrO2 to enhance visible-light photocatalysis. Future work should focus on advancing the green synthesis technique and testing against a larger spectrum of contaminants and microbial strains to further establish its environmental application.