Investigation of oxygen vacancy variation via chemical reduction methods and its effect on the photocatalytic and electrochemical behavior of coatings formed by plasma electrolytic oxidation on commercial pure titanium

نوع: Type: thesis

مقطع: Segment: PHD

عنوان: Title: Investigation of oxygen vacancy variation via chemical reduction methods and its effect on the photocatalytic and electrochemical behavior of coatings formed by plasma electrolytic oxidation on commercial pure titanium

ارائه دهنده: Provider: Reza hosseini

اساتید راهنما: Supervisors: Dr. Arash Fattah–alhosseini

اساتید مشاور: Advisory Professors: Dr. Minoo Karbasi

اساتید ممتحن یا داور: Examining professors or referees: Dr. saeed azizian, Dr. Behzad koozehgar kaleji, Dr. Ahmad keyvani

زمان و تاریخ ارائه: Time and date of presentation: 27/09/2023

مکان ارائه: Place of presentation: Amphitheater of Boali Sina University Faculty of Engineering

چکیده: Abstract: Creating optimal amounts of defects in semiconductors like oxygen vacancies and Ti3+ species, can improve the photocatalytic activity of semiconductors. The aim of this research is to develop TiO2 coatings containing different amounts of active oxygen vacancies under the visible light range in order to remove the tetracycline pharmaceutical pollutant. The coating containing different amounts of oxygen vacancies is obtained from the plasma electrolytic oxidation (PEO) process and subsequently chemical reduction as the post-treatment at room temperature and hydrothermal conditions using different concentrations of the reducing agent NaOH. Surface oxygen vacancies act as charge carrier traps after chemical reduction post treatment on PEO coatings, helping increment of photocatalytic activity through separating electron-hole pairs and preventing their rapid recombination. Also, the resulted narrow band gap of post-treated PEO coatings can be attributed to the overlap of intermediate energy levels stemmed from oxygen vacancies with the conduction band, leading to a downward shift of the conduction band. The chemical reduction at room temperature showed that the microstructure and the amount of crystalline phases in the coating (anatase and rutile) are not affected and via changing the concentration of the reducing agent, the amounts of surface defects would change. In addition to change of the microstructure (nano-sheet like), increasing the amount of rutile phase and creating a new Na2Ti3O7 photocatalytic phase, the hydrothermal chemical reduction treatment causes an increase in the amount of oxygen vacancies. The photocatalytic degradation of tetracycline experiments under visible light radiation using optimally produced coatings have shown that by increasing the concentration of NaOH to the optimal amount (the optimal level of oxygen vacancy defect) in the chemical reduction operation at room temperature, the photocatalytic activity can be increased to a large extent (3.41 times). In addition, the photocatalytic degradation rate constant of tetracycline by PEO coating after hydrothermal chemical reduction was 10.15 times higher than that of PEO coating. Also, in order to improve and fasten the photocatalytic reaction, the effect of peroxymonosulfate (PMS) oxidant activation in the presence of PEO coating and post treated one on the photocatalytic degradation rate constant of tetracycline under visible light was compared. The optimal coating with a significant amount of oxygen vacancies acted as sites for PMS activation, leading to the rapid degradation of tetracycline. In addition, appropriate scavengers and electron spin resonance spectroscopy (ESR) were used to investigate the role of different species involved in the photocatalytic process and subsequently describe the photocatalytic mechanism. Also, possible intermediate products of photocatalytic degradation of tetracycline were identified by RT-2M-16h/PMS/Vis system by LC-MS method. Finally, the photocatalytic behavior of TiO2 PEO coated containing oxygen vacancy surface defects and Ti3+ (TiO2-x coatings) was also investigated using different reducing agents.

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