Metal doped TiO2 nanoparticles for photodegradation of pollutants under visible light

Abstract

Dyes are one of the major pollutants that concern around the world, which reduction of light penetration, photosynthetic activity of aquatic plants and dissolved oxygen in the water. The dye molecule itself is toxic to humans and fish and cause for water pollution. Recently, pharmaceutical and hormonal contaminants including paracetamal and antibiotics in the water are continuously increasing which can damage the living organisms via bioaccumulation and food chain. Not only these two pollutants, pathogenic microorganisms are other common pollutants which are found in the environement. The efficient treatment technology is important to improve the water quality and avoid the negative impacts to human and the environment. Since there are many pollutans contaminated in the water, the development of applicable treatment technology for variuos pollutants removal, simplicity, low cost and energy consumption is concermed in this study.

Among treatment technologies, photocatalysis of titanium dioxide (TiO2) has a potential process for organic compound removal because of its nontoxicity and inexpensiveness. TiO2 photocatalysts exhibit the decomposition of organic pollutants and the inhibition of microorganisms in the environment. However, TiO2 has low utilization of the sunlight spectrum as a wide band gap (anatase: 3.2 eV, rutile: 3.0 eV). This limit needs to improve to enhance photocatalytic performance. Therefore, the aim of this study was to synthesize metal-doped TiO2 to improve photocatalytic and antibacterial activities. TiO2 was doped with copper (Cu) and manganese (Mn) in various content of 0.1 to 1.0% and 0.1 to 0.3%, respectively. The Cu-doped TiO2 nanoparticles were synthesized by hydrothermal method. At the same time, the Mn-doped TiO2 nanoparticles were synthesized by the sol-gel method. The physico-chemical properties of the synthesized nanoparticles were characterized by X-ray diffractometer (XRD), Brunauer–Emmett–Teller (BET), transmission electron microscopy (TEM), scanning electron microscopic (SEM) and energy dispersive X-ray (EDX). The photocatalysis performance of synthesized nanoparticles on methylene blue (MB) and salbutamol (SAL) degradation were examined. In addition, the antimicrobial activity of the nanoparticles also was observed.

The results revealed that the anatase phase of TiO2 was well maintained in all undoped and Cu-doped TiO2 nanoparticles. The crystal sizes were approximately 10 nm and the specific surface areas of the nanoparticles were approximately 180-182 m2/g. The band gap energy slightly decreased from 3.20 eV for undoped TiO2 to 3.10 for Cu-doped TiO2. However, the Cu-doped TiO2 nanoparticles were agglomerated by increasing Cu doping. For MB degradation, the 0.10% Cu-doped TiO2 obtained the best MB degradation efficiency (approximately 100% under 30 min UV-A irradiation) as similar as P25. The undoped TiO2 obtained the lowest MB degradation efficiency of 70%. In addition, the best E. coli photocatalytic activity was obtained in the 0.1% Cu-doped TiO2 nanoparticles which completely killed in 180 minutes under UV-A irradiation, whereas the undoped TiO2 showed the lowest bacterial photokilling activity which many bacterial colony was still observed in 180 minutes under UV-A irradiation. The significant drawback of Cu-doped TiO2 was that it required UV-A light, was higher energy than visible light, to activate the photocalytic activity.

On the other hand, the another metal doped TiO2 was Mn-doped TiO2. Due to the physico-chemical properties, the anatase and rutile phases were observed on the undoped and Mn-doped TiO2 nanoparticles. However, the major phase is the anatase phase. The specific surface was enhanced by Mn doping; the 0.1%Mn-doped TiO2 obtained the highest specific surface area of 120.16 m2/g. The band gap energy of 0.1%Mn-doped TiO2, 0.2%Mn-doped TiO2 and 0.3%Mn-doped TiO2 was 2.91, 2.80 and 2.88 eV which fell in the visible light region. The photocatalytic degradation of MB and SAL was tested under visible irradiation. The best MB removal efficiency of 97% was obtained in the 0.2%Mn-doped TiO2 after 60 minutes irradation, followed by 0.3%Mn-doped TiO2, 0.1%Mn-doped TiO2, and undoped TiO2 nanoparticles. In addition, the highest SAL removal efficiency of 95% was found in the 0.2%Mn-doped TiO2 after 180 minutes. Moreover, the 0.2%Mn-doped TiO2 nanoparticles also achieved the best E.coli photokilling activity; the complete killing was observed under visible light for 240 minutes whereas some E.coli colony was found in undoped TiO2, 0.1%Mn-doped TiO2 and 0.3%Mn-doped TiO2.

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