The synergistic effect of copper tungstate produced by solvothermal method on the photocatalytic behavior of titania coatings

نوع: Type: Thesis

مقطع: Segment: PHD

عنوان: Title: The synergistic effect of copper tungstate produced by solvothermal method on the photocatalytic behavior of titania coatings

ارائه دهنده: Provider: hamed bahramian

اساتید راهنما: Supervisors: Dr. Arash Fatah Alhosseini

اساتید مشاور: Advisory Professors: Dr. Minoo Karbasi – Dr. Razieh Charmahali

اساتید ممتحن یا داور: Examining professors or referees: Dr. Saeed Azizian-Dr. Masoud Atapour-Dr. Behzad Kozegar kallege

زمان و تاریخ ارائه: Time and date of presentation: 2025

مکان ارائه: Place of presentation: اتاق کنفرانس

چکیده: Abstract: The presence of hexavalent chromium (Cr⁶⁺) in wastewater, due to its toxicity and carcinogenicity, poses a serious risk to both environmental health and human safety. This research aims to develop an efficient heterogeneous photocatalyst under visible light for the reduction of this pollutant and the mitigation of its hazards. To this end, the advantages of Plasma Electrolytic Oxidation (PEO) were leveraged to create titanium-based coatings, offering benefits such as phase control, suitable porosity and adhesion, and superiority over powdered photocatalysts by overcoming issues like separation difficulties and loss during repeated use. Subsequently, copper tungstate was synthesized via a hydrothermal post-treatment to form a TiO₂/CuWO₄ nanocomposite on these coatings. The hydrothermal method was chosen for its ability to enable controlled and uniform particle synthesis, while copper tungstate was selected for its narrow band gap, low toxicity, and cost-effectiveness. This combined approach ultimately yielded a stable and functional coating that inhibits the rapid recombination of charge carriers and overcomes the limitation of ultraviolet-only absorption in titanium-based photocatalysts, thereby delivering significantly enhanced photocatalytic performance under visible light. FESEM microstructural images confirmed the formation of a hierarchical coating containing uniformly distributed CuWO₄ particles on the microporous TiO₂ substrate. Photocatalytic tests under visible light revealed that the rate constant for the hydrothermally synthesized nanocomposite was 3.77 times higher than that of the pure TiO₂ coating. Based on analyses including AFM, wettability measurements, UV-Vis DRS, XPS, PL and EIS, the performance enhancement mechanism was attributed to multiple factors: higher surface roughness, superhydrophilicity, a narrower band gap, the formation of a type II heterojunction, improved charge carrier separation, reduced charge transfer resistance, and increased photocurrent density. Furthermore, the effects of various scavengers (e.g., oxalic acid, potassium persulfate, sodium azide) and oxidants (H₂O₂, peroxydisulfate/PDS, peroxymonosulfate/PMS) were investigated, clarifying the crucial role of both radical and non-radical species generated. Among the scavengers, oxalic acid and potassium persulfate predominantly indicated the roles of holes and electrons, respectively, in the photocatalytic reduction behavior. Regarding oxidants, PDS demonstrated a higher synergistic effect in generating sulfate radicals compared to PMS, while H₂O₂ was ineffective. A mechanism involving combined radical and non-radical pathways was subsequently proposed for the oxidants. In the subsequent phase of the research, an alkaline post-treatment (chemical reduction) using NaOH as a reducing agent was applied to the TiO₂/CuWO₄ composite coatings synthesized in the first phase, aiming to further enhance the photocatalytic reduction performance. This post-treatment not only altered the surface morphology of the coating to a needle-flower-like nanostructure but also introduced oxygen vacancy defects into the composite. The photocatalytic reduction efficiency of Cr⁶⁺ under visible light reached 100% within 5 hours for this treated sample, a significant improvement compared to the 43% reduction achieved by the first-phase composite after 4 hours. Additionally, the reduction rate constant became 2.92 times faster. The primary reasons for this enhancement were the creation of oxygen vacancies, the formation of a new photocatalytic phase (TiP₂O₇), and the needle-flower-like morphology, which provided more active surface sites.

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