METAL OXIDES BASED NANOCOMPOSITES AND THEIR APPLICATIONS

Abstract

Anthropogenic population increase, uncontrolled urbanization, rapid industrialization, and reckless exploitation of natural water resources are the key factors contributing to the degradation of water quality over time. As a consequence, access to clean drinking water is becoming difficult and a pressing problem in most parts of the world. The pollution of water by industrial wastes is a significant issue in the twenty-first century. Today, waste from industries and urbanized areas is dumped onto agricultural land and nearer water bodies, contaminating the water, severely influencing human health, and significantly damaging aquatic life. Industries include textile, paper, rubber, petrochemical, leather, pharmaceutical, and insecticides are the main producers of organic pollutants. Two important pollutants produced by the industries mentioned above that are highly hazardous in nature are nitro substituted phenols and colouring organic dyes. They are known to be poisonous, carcinogenic and mutagenic, and their biodegradability in ecosystems is extremely low. Many simple and advanced chemical, physical, and biological techniques, have been developed to treat organic contaminants from wastewater. It is essential to eliminate these pollutants using efficient and safe methods for the environment. Metal oxides are an important class of catalysts. Due to their distinctive qualities, including high surface area, surface reactivity, morphological versatility, and chemical and thermal stability, they have attracted considerable attention. In many areas of science and technology, metal oxides have shown excellent potential. In recent years, nanocomposite materials have received a great deal of attention due to their superior optical, electrical, mechanical, and structural properties compared to their components. Nanocomposites are unique in that they have multifunctional characteristics resulting from mixing various nanomaterials with distinct properties. Among nanocomposites, metal oxide based nanocomposites such as metal/metal oxide and mixed metal oxides nanocomposites have been reported to improve their efficiency, and use them in new application fields in comparison to precursor metal oxides. Some examples of reported noble metal/metal oxide composites are ZnO/Ag nanoarrays, Ag-TiO2 nanotube, Au-ZnO arrays, Ag-CuO nanosheets, Pt-ZnO microsphere and Pd/TiO2 nanosheets which are used as efficient catalysts. Metal oxides serve as a useful support in all these examples because they provide a large surface area for small noble metal nanoparticles, and prevent them from aggregating during catalysis. Further, mixed metal oxides nanocomposites containing two or more metal oxides are gaining attention and applications in different scientific and engineering fields. Each metal oxide can serve as a standalone entity, bringing its own set of properties to the composite, or it can have properties resulting from new metal-metal or metal-oxygen-metal interactions at interfaces. These metal oxides based nanocomposites are being utilized in various applications, including sensing, photovoltaic applications, energy generation, environmental remediation, biomedical applications, electronics and catalysis. The present study aims to synthesize metal oxide based nanocomposites such as noble metal/metal oxide nanocomposites (Ag/NiO) and mixed metal oxides nanocomposites (binary NiO/ZnO and ternary Ag2O/NiO/ZnO). The synthesized nanocomposites were thoroughly characterized by various techniques. Nanocomposites were used as a catalyst for wastewater remediation using adsorption and reduction methods. Further synthesized silver based nanocomposites e.g. Ag/NiO and Ag2O/NiO/ZnO were tested as antiviral agents against Chikungunya virus. Chapter 1: This chapter deals with general introduction of water contamination, organic contaminants, and wastewater treatment methods such as photocatalysis, adsorption and reduction. Metal oxides and metal oxides based composites: metal/metal oxide and mixed metal oxides composites and their applications as catalyst for wastewater remediation and as antimicrobial agents have also been compiled in tabular form and discussed in detail. Chapter 2: This chapter includes the detailed specifications of the materials and chemicals utilized including their formula, purity and manufacturers for their procurement. The specification of various instruments and methods of experimental techniques have also been described. The structural, morphological, compositional, and electronic characteristics of metal oxides and metal oxide-based nanocomposites were characterized using several experimental techniques, such as powder X-ray diffraction (PXRD), Fourier-transform infrared spectroscopy (FTIR), UV-Visible diffuse reflectance spectroscopy (UV-DRS), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area analysis. The catalytic performance of composites was studied with the help of UV-Vis spectrophotometer and gas chromatography-mass spectrometry (GC-MS). Chapter 3: This chapter describes the synthesis of NiO/ZnO nanocomposites using the sol-gel method in various Ni:Zn molar ratios of 1:4, 2:3, 1:1, 3:2, and 4:1, denoted as N1Z4, N2Z3 N1Z1, N3Z2 and N4Z1, respectively. Various techniques, such as XRD, FTIR, FE-SEM, TEM, BET, and XPS, were used to characterize these nanocomposites. The nanocomposites have mixed morphologies with particle sizes in the range of 15−45 nm, according to FESEM and TEM studies. The NiO, ZnO, and NiO/ZnO nanocomposites were employed for adsorption studies of CONG-R (congo red) dye. N3Z2 outperformed all other nanocomposites as the best adsorption catalyst for CONG-R, having a maximum adsorption capacity of 169.77 mg/g. The pseudo-second-order and Langmuir isotherm model best describe the adsorption kinetics of CONG-R. Moreover, all NiO/ZnO nanocomposites were tested for catalytic reduction of para-np (4-nitrophenol), N1Z1 completed catalytic reduction of para-np to para-aminophenol in the shortest duration of 7 min with a rate constant of 0.512 min-1. Reusability of N3Z2 for removal of CONG-R by adsorption was checked up to five cycles, and reduction of para-np for three cycles using N1Z1. Chapter 4: This chapter illustrates the synthesis of three Ag/NiO composites by anchoring silver nanoparticles on NiO octahedrons with varying concentrations of silver nanoparticles (abbreviated as AN-5% (5% Ag), AN-10% (10% Ag), and AN-15% (15% Ag). SEM analysis confirms the octahedral morphology of pure NiO with a particle size of 0.63 ± 0.13 μm. Also, TEM images of AN-5% composite were used to evaluate the anchoring of silver nanoparticles with an average particle size of 16.54 ± 1.88 nm. According to the BET surface area analysis, the average surface area of composites is between 49 and 52 m2g-1. All synthesized composites (AN-5%, AN-10% and AN-15%) show outstanding catalytic activity towards the reduction of para-np in the presence of NaBH4. As AN-5% has the least amount of anchoring silver (5%), it was chosen to optimize various parameters (catalyst’s amount and para-np’s amount) that affect the reduction rate of para-np. AN-5% (50 μL) demonstrated outstanding catalytic activity for the reduction of nitrophenols (0.1587 mM) viz. ortho-np (2-nitrophenol), meta-np (3-nitrophenol), para-np (4-nitrophenol) and tri-np (2,4,6-trinitrophenol). The order of catalytic reduction of nitrophenols is as follows: meta-np > para-np = ortho-np > tri-np. In addition, AN-5% (29.41 μg mL-1) reduced >95% of the colouring dyes (10 ppm) such as CONG-R (congo red: 95% in 6 min), METH-O (methyl orange: 97.5% in 7 min), METH-B (methylene blue: 98.3% in 10 min) and RHOD-B (rhodamine B: 99.2% in 5 min). AN-5% also showed excellent activity for reduction of the mixtures of nitrophenols/dyes and for treatment of simulated industrial effluent samples (EFF1 and EFF2) and a real sample (dye-bath effluent: DBE). AN-5% can also be reused up to several cycles with almost same efficiency.