Synthesis and Characterization of Ag Doped TiO2, CdS, ZnS Nanoparticles for Photocatalytic, Toxic Ions Detection, and Antimicrobial Applications

Abstract

The progresses of nanoparticles (NPs) research have been passed through several advancements, such as simple spherical NPs to different shapes (anisotropic), hollow, core/shell, doped, movable core/shell or yolk shell, etc. These NPs have more advanced properties in several applications, such as catalysis, biomedical, electronics, solar cells, sensors, and so on because of high surface area to volume ratio, the presence of more loosely bound surface atoms, etc. When the particles are made of multimaterials it’s not only show improved property of the main material but also developed multifunctionality. Because of these reasons the multimaterials NPs are continuously drawing significant research attentions in the recent years. Under the multi-materials nanoparticles category, doped nanoparticles are also considered as an important class. This thesis is focused on synthesis, characterization, properties, and applications of Ag doped semiconductor nanoparticles. More specifically, TiO2, CdS, and ZnS were considered as the host materials and Ag as the dopant to form single, core/shell, hollow, and hollow bi-layer NPs for the applications in visible light induced photocatalytic degradation of organic compounds (nitrobenzene, metronidazole, methylene blue dye), antifungal agent (against Fusarium solani and Venturia inaquaelis), and sensor for the detection of arsenic and fluoride ions in aqueous media. The abstracts of the studied works are organized sequentially in the following paragraphs. Continuous increasing consumption of antibiotics in health care results to increase concentration of these compounds in surface water through wastewater treatment systems, which in turn, cause adverse effects on the aquatic ecosystems of the receiving water bodies, because of the intrinsic biological activity of these compounds. However, there are limited efforts on remediation of water pollution because of antibiotics using an effective and clean technology. In this study, photocatalytic activity of TiO2, CdS, and ZnS semiconductor nanoparticles were employed to degrade the metronidazole antibiotic in visible light irradiation. The particle size of pure TiO2, CdS, and ZnS was 33.39 ± 1.67, 4.06 ± 0.63, and 5.85 ± 0.5 nm, respectively. The particle size of Ag doped TiO2, CdS, and ZnS was 27.6 ± 2.08, 3.44 ± 0.76, and 4.91 ± 0.45 nm, respectively. The maximum degradation efficiencies of the pure TiO2, CdS and ZnS nanoparticles were 80.78, 82.46, and 81.66%, respectively. These particles were also modified by silver doping to improve its degradation efficiency. Doping of silver greatly enhanced the degradation efficiency of these nanoparticles. The particular concentrations of silver dopant were 1.00, 1.5, and 1.25% for TiO2, CdS, and ZnS nanoparticles for achieving the maximum degradation efficiency and the corresponding maximum degradation efficiencies were 94.39, 94.9%, and 95.11%. The basic mechanism of doping and the photocatalytic processes was explored in detail. A kinetic study of the degradation reaction shows first order kinetics fits well for all three cases. The reusability and stability of these photocatalyst were confirmed by the cyclic degradation test. In addition to the antibiotics, contamination of water because of other organic pollutants, especially synthetic dyes, causes severe environmental problems because of its toxic nature to microorganisms, aquatic life, and human beings. In this regard, an effective and clean remediation process for the remediation of dye contaminated effluent waters becomes more demanding to reduce the environmental impact. This section reports the photocatalytic behaviour of methylene blue using pure and silver doped semiconductor heterogeneous nanocatalysts (TiO2, CdS, and ZnS) under visible light. The photodegradation studies show there is a significant enhancement in degradation efficiency of all three nanoparticles after silver doping. For all nanoparticles, there is an optimum doping concentration to get the maximum degradation efficiency, which again depends on the material. The maximum degradation efficiencies for the three Ag doped TiO2, ZnS, and CdS nanoparticles were 95.9, 95.33, and 94.99% for 1.00, 1.25, and 1.50% Ag, respectively. The first order rate constant value of 1.00% Ag doped TiO2, 1.5% Ag doped CdS, and 1.25% Ag doped ZnS is 5.21, 5.72, and 7.71 times higher compared to their respective pure nanoparticles. The maximum degradation efficiency with minimum doping concentration among all three materials studied here was again found for TiO2. Further, silver doped hollow TiO2 (Ag-h-TiO2) nanoparticles were also synthesized by a sacrificial core (AgBr) method to enhance the surface area for higher photocatalytic activity. The Ag doping and the core removal was done simultaneously during the dissolution of the core in (NH4)OH solution. The mean particle size of synthesized Ag-h-TiO2 nanoparticles was 17.76 ± 2.85 nm with the wall thickness ~2.5 nm. The hollow structured nanoparticles have the specific surface area of 198.3 m2/g, where as solid TiO2 nanoparticles have the specific surface area of 95.1 m2/g. The suitability of this synthesized hollow nanoparticles as photocatalyst were tested for the photocatalytic degradation of three important different classes of organic compounds such as nitrobenzene (NB), metronidazole (MTZ) antibiotic, and methylene blue dye (MBD) in aqueous solution under irradiation of visible light. The maximum NB degradation was obtained 95.5%, and the metronidazole degradation efficiency was found to be 96.55 and 94.77% under the irradiation of visible light for the initial MTZ concentration of 15 and 30 mg/L with catalyst dose of 0.5 g/L. Photodegradation studies show there is a significant enhancement of the degradation efficiency of the TiO2 after the hollow structure formation and silver doping. The recycling tests of the catalysts show only ~ 10% decrease in efficiency for NB and MTZ degradation after sixth cycle of reuse. The light emission capacity in terms of quantum yield (QY) is enhanced by 18.7% for Ag-h-TiO2 than that of pure TiO2 nanoparticles.
The above mentioned hollow TiO2 NPs were also used as photoinduced antifungal agent. The chemical based pesticides are widely used in agricultural farming to protect crops from insect infestation and diseases. However, the excessive use of highly toxic pesticides causes several human health (neurological, tumour, cancer) and environmental problems. So, nanoparticles based green pesticides are of special importance in recent years. Antifungal activities of the pure and Ag doped (solid and hollow) TiO2 nanoparticles were studied against two potent phytopathogens, Fusarium solani (causing Fusarium wilt disease to potato, tomato etc.) and Venturia inaquaelis (causing apple scab disease) and found hollow nanoparticles are more effective than other two. The antifungal activities of the nanoparticles enhanced further under visible light exposure against these two phytopathogens. Fungicidal effect of the nanoparticles depends on different parameters, , such as particle concentration, and intensity of visible light. The minimum inhibitory dose of the nanoparticles for V.inaquaelis and F.solani are 0.75 and 0.43 mg/plate. Presence of Ag as a dopant helps to the formation of stable Ag-S and di-sulfide bond (R-S-S-R) in cellular protein, which leads to the cell damage. During photocatalysis generated OH radicals loosen the cell wall structure and finally lead to the cell death. The mechanisms of fungicidal effect of nanoparticles against these two phytopathogens are supported by biuret and triphenyl tetrazolium chloride analyses, and field emission electron microscopy. Apart from the fungicidal effect, at very low dose (0.015 mg/plate) the nanoparticles are successfully arrest production of toxic napthoquinone pigment for F.solani which is related to the fungal pathogenecity. The nanoparticles are found to be effective to protect spoiling of potato affected by F.solani or other fungus. The doped nanoparticles can also be used effectively for the easy detection of toxic ions in water. In this regard, fluoride ion detection has taken a considerable research interest in recent years because of its typical nature. It is an essential anion for biological and medical systems, as well as for some industrial applications. But, the fluoride ions above its permissible level can cause different diseases, such as fluorosis, urolithiasis, kidney failure, cancer, and even leading to death. Because of this reason a simple and low cost method is highly desirable for the detection of fluoride ion. In this study a fluorometric method based
on Ag-CdS/Ag-ZnS nanoparticle is developed for the fluoride ion detection. The developed nanoparticles were of size range 5.92 ± 0.76 nm with shell layer of 0.75 nm and it showed the quantum yield of 77.57%. The method was tested in aqueous solution at different pH. The selectivity and sensitivity of the fluorescence probe was checked in the presence of other anions (Cl-, Br-, I-, OH-, NO3- SO42-, HCO3-, HPO42-, CH3COO-, H2PO4-). The fluoride ion concentration was varied in the rage 190 – 22,800 μg/L and the lower detection limit was obtained as 99.7 μg/L.
Arsenic poisoning from drinking water is also an important global issue in recent years. Because of high level toxicity of arsenic to human health, an easy, inexpensive, and low level and highly selective detection technique is of great importance to take any early precautions. This study reports the synthesis of Ag doped hollow CdS/ZnS bi-layer (Ag-h-CdS/ZnS) nanoparticles for easy fluorometric determination of As(III) ions in aqueous phase. The hollow bi-layer structures are synthesized by a sacrificial core method using AgBr as the sacrificial core and the core is removed by dissolution in ammonium hydroxide solution. The synthesized nanoparticles were characterized by using different instrumental techniques. The particle size of Ag-h-CdS/ZnS nanoparticles is ~ 76.02 ± 2.47 nm with the shell thickness of CdS layer is 1.5 nm and ZnS layer is 1.8 nm. The QY of the Ag-h-CdS/ZnS nanoparticles is 88.14%. A good linear relationship is obtained between fluorescence quenching intensity and the As(III) concentration in the range of 750 – 22500 ng/L at neutral pH with a limit of detection as low as 226 ng/L.