SYNTHESIS OF METAL OXIDE NANOPARTICLES AND THEIR NANOCOMPOSITES FOR NOVEL APPLICATIONS

Abstract

Nanotechnology deals with research related to materials falling in the nano size range (1-100 nm). Nanotechnology involves manipulation of matter on an atomic and molecular scale and holds immense promise across various domains. Metal oxide nanoparticles (MO NPs) are nanoscale materials composed of metal atoms bonded with oxygen atoms to form oxides. These NPs exhibit unique and enhanced properties compared to their bulk counterparts. The most studied MO NPs include those derived from metals such as Ti, Zn, Fe, Cu, and others. Metal oxide nanocomposites are advanced materials consisting of metal oxide nanoparticles dispersed within a matrix material. Nanocomposites offer unique properties that are distinct from the individual components and bulk materials. Metal oxide nanocomposites exhibit remarkable enhancement in mechanical, electrical, magnetic, optical, and catalytic properties, making them valuable in diverse industries such as electronics, energy, healthcare, and environmental remediation. The synthesis of metal oxide nanocomposites involves control over factors such as particle size, shape, and distribution within the matrix, allowing for tailored properties suited to specific applications. The integration of metal oxides into nanocomposites opens avenues for advanced functionalities, increased surface area, enhanced reactivity and improved stability. In the present work, metal oxide nanoparticles and their nanocomposites have been synthesized using thermal decomposition and homogeneous precipitation methods. The systems studied are: (i) CdFe2O4 nanoparticles, (ii) SnO2-Ag nanocomposites, (iii) Fe2O3@SnO2 core-shell nanocomposites, and (iv) TiO2@NiCo2O4 core-shell nanoparticles. The synthesized metal oxide NPs and their nanocomposites were characterized using XRD, TGA, FT-IR, FE-SEM, TEM, EDXA, SAED, Raman, EIS, and XPS. BET surface area analyzer and zeta potential analyzer were used to measure surface area and zeta potential of the metal oxide NPs and their nanocomposites. UV-Vis DRS and PL spectroscopy were used to study their optical properties. LC-MS analysis was used to understand the mechanistic pathway involved in the photodegradation of toxic dyes. Magnetic investigations of metal oxide NPs and their nanocomposites were carried out using PPMS and SQUID magnetometer. The synthesized metal oxide nanoparticles and their nanocomposites were used for various applications such as catalyst for peroxidase-like activity, photocatalytic degradation of congo red, catalytic reduction of 4-nitrophenol, adsorption of congo red, and photocatalytic degradation of tetracycline.The present thesis consists of six chapters and a brief discussion of each chapter is as follows. Chapter 1 starts with a brief introduction to nanotechnology followed by an introduction to metal oxide nanoparticles. Then, a brief introduction to mixed metal oxide nanoparticles, metal oxide-based nanocomposites and core-shell nanocomposites is given. Next, different physical and chemical routes for the synthesis of metal oxide NPs and their nanocomposites have been discussed. Further, this chapter discusses various interesting physicochemical properties of metal oxide nanoparticles and their nanocomposites. In the end, different important applications of metal oxide nanoparticles and their nanocomposites have been discussed. Chapter 2 describes various analytical techniques that were used in the present study to characterize the synthesized metal oxide NPs and their nanocomposites and also sample preparation methods for the measurements. The different analytical techniques used for the characterization include powder XRD, FT-IR spectroscopy, TGA, FESEM, EDXA, TEM, SAED, XPS, EIS, Raman spectroscopy, LC-MS analysis, zeta potential and BET surface area analyses. Optical properties of the metal oxide nanoparticles and their nanocomposites were studied using diffuse reflectance spectroscopy and PL spectroscopy. Magnetic properties of the metal oxide nanoparticles and their nanocomposites were studied using a physical property measurement system and a superconducting quantum interference device. Chapter 3 deals with the effect of different synthetic conditions on the morphology, magnetic properties, and peroxidase-like activity of CdFe2O4 nanoparticles synthesized via thermal decomposition of metal glycolates. XRD analysis confirm the formation of Cd-Fe glycolates at different synthetic temperatures (130 ℃, to 220 ℃). CdFe2O4 nanoparticles are obtained after the calcination of Cd-Fe glycolates at 500 °C. Rietveld refinement of XRD data confirms the presence of more Fe3+ ions in octahedral sites in CdFe2O4 nanoparticles synthesized from Cd-Fe glycolate prepared at 190 ℃ and 220 ℃ as compared to the CdFe2O4 nanoparticles derived from Cd-Fe glycolates synthesized at 160 ℃ and at 130 ℃. Scanning and transmission electron microscopy studies indicate that flake-like, raspberry, and hexagonal particles are formed, respectively, in Cd-Fe glycolates synthesized at 130 ℃, 160 ℃, 190 ℃, and 220 ℃. EDX analysis results confirm the presence of Cd, Fe and O in Cd-Fe glycolates and CdFe2O4 nanoparticles with uniform elemental distribution. TEM studies demonstrate that the flake-like, raspberry, and hexagonal particles of CdFe2O4 are made up of small nanoparticles. XPS studies indicate the presence of divalent Cd and trivalent Fe in octahedral and tetrahedral sites of CdFe2O4. Magnetic measurements (M-H and M-T) were carried out to understand the effect of morphology, crystallite size and cationic distribution on the magnetic properties of CdFe2O4 nanoparticles at 5 K and 300 K. The M-H studies indicate superparamagnetic behavior at room temperature (300 K) and weak ferromagnetic behavior at low temperature (5 K) for the CdFe2O4 nanoparticles. The M-T measurements indicate that blocking temperature of CdFe2O4 nanoparticles varies from 38 K to 51 K. The CdFe2O4 NPs exhibit peroxidase-like activity in the presence of H2O2, and the activity is better than that of horseradish peroxidase (natural enzyme). The catalytic activity of CdFe2O4 NPs is affected by their particle size, surface area, differences in exposed facets and concentration of Fe3+ ions. Chapter 4 deals with the synthesis of SnO2 based nanocomposites via novel thermal decomposition approach that includes (i) SnO2-Ag nanocomposites and (ii) Fe2O3@SnO2 core-shell nanocomposites. This chapter consists of two sections and each of them has been discussed separately. The first section describes a thermal decomposition method for the synthesis of SnO2-Ag nanocomposites and their application for photocatalytic degradation of congo red and catalytic reduction of 4-nitrophenol. SnO2-Ag nanocomposites with different particle size of Ag nanoparticles were synthesized by thermal decomposition approach in two steps. In the first step, SnO2 nanoparticles were synthesized via thermal decomposition of tin chloride pentahydrate along with hydrazine hydrate. Silver salicylate complex (Ag(Hsal) precursor for Ag) was then synthesized using a precipitation method. In the second step, SnO2-Ag nanocomposites were synthesized by refluxing silver salicylate complex with SnO2 in diphenyl ether at about 200 ℃ in air. The as-prepared samples were calcined at 500 ℃ to obtain SnO2-Ag nanocomposites. The particle size of silver nanoparticles in the SnO2-Ag nanocomposites could be controlled by changing the amount of Ag(HSal) used during the synthesis. The presence of SnO2 and Ag nanoparticles in the SnO2-Ag nanocomposites was confirmed using powder X-ray diffraction. The purity of SnO2-Ag nanocomposites was proved by FT-IR spectroscopy. The presence of Sn4+, Ago, surface hydroxyls and lattice oxygen were confirmed by XPS studies. SEM and TEM studies confirm that pure silver nanoparticles show close to spherical morphology, while as prepared and calcined SnO2 nanoparticles and their nanocomposites (SnO2-Ag) show particles with irregular morphology. UV-visible DRS spectroscopy studies show that the SnO2-Ag nanocomposites absorb both in UV and visible regions. Raman spectral analysis results confirm the presence of cubic Ag and tetragonal rutile SnO2 in the SnO2-Ag nanocomposites. The results obtained from photoluminescence spectroscopy and EIS studies suggest that electron-hole recombination is suppressed, and interfacial charge transfer is more favorable in the SnO2-Ag nanocomposites. The SnO2-Ag nanocomposites perform as better catalyst for the photocatalytic degradation of congo red compared to the constituents. LC-MS analysis was used to understand the mechanistic pathway involved in the photodegradation of congo red. The nanocomposites show faster kinetics (within 30 minutes) as compared to the previously reported SnO2 based systems in the literature. The SnO2-Ag nanocomposites were also explored for the catalytic degradation of 4-nitrophenol in an aqueous solution. The second section describes a thermal decomposition method for the synthesis of Fe2O3@SnO2 core-shell nanocomposites and their application as adsorbent for removal of congo red. The Fe2O3@SnO2 core-shell nanocomposites with different shell thickness were synthesized in two steps. In the first step, the Fe2O3 microspheres were synthesized via thermal decomposition of [Fe(CoN2H4)6](NO3)3 (synthesized using precipitation method) in a DMF-diphenyl ether mixture. In the second step, Fe2O3@SnO2 core-shell nanocomposites in which Fe2O3 microspheres is the core and SnO2 is the shell were synthesized by refluxing tin chloride pentahydrate (precursor for SnO2) with Fe2O3 microspheres in DMF at 100 ℃, followed by the addition of hydrazine hydrate. The as-prepared samples were calcined at 500 ℃ to obtain Fe2O3@SnO2 core-shell nanocomposites. The thickness of shell (SnO2) was altered by varying the amount of tin chloride pentahydrate. Various characterization techniques were used to prove successful formation of Fe2O3@SnO2 core-shell nanocomposites. XRD results prove the presence of SnO2, α-Fe2O3, and Fe3O4 in the calcined Fe2O3@SnO2 core-shell nanocomposites. FE-SEM and TEM studies reveal uniform deposition of SnO2 nanoparticles over the iron oxide microspheres. XPS analysis demonstrates the presence of Sn4+, Fe2+, Fe3+ and O2− in the Fe2O3@SnO2 core-shell nanocomposites. Optical spectral studies show that the Fe2O3@SnO2 core-shell nanocomposites absorb UV and visible regions. M-H studies on Fe2O3@SnO2 core-shell nanocomposites indicate weak ferromagnetic and superparamagnetic behavior of the nanocomposites at 5 K and 300 K, respectively. In M-T measurement results, pure α-Fe2O3 shows characteristic Morin transition, but the Fe2O3@SnO2 core-shell nanocomposites do not show such transition. The Fe2O3@SnO2 core-shell nanocomposites were tested as adsorbent for the removal of congo red (from an aqueous solution). The Fe2O3@SnO2 core-shell nanocomposites adsorb congo red completely in 5 minutes with good adsorption capacity (88.7 mg g-1) as compared to their pure counterparts and other adsorbents reported in the literature.Chapter 5 deals with the synthesis of TiO2@NiCo2O4 core-shell nanoparticles via calcination of TiO2@NiCo-LDH precursors and their application in photocatalytic degradation of tetracycline. First, TiO2 spheres were prepared using a known method and they were surface modified with NaOH (TiO2-NaOH). The TiO2@NiCo-LDH precursors were prepared by homogeneous precipitation method and, on calcination of the precursors, TiO2@NiCo2O4 core-shell nanoparticles were obtained. The LDH precursors and the TiO2@NiCo2O4 core-shell nanoparticles were characterized using XRD, FT-IR, FESEM, TEM, DRS, BET, and XPS analyses. XRD results indicate the presence of NiCo2O4 and TiO2 phase in the TiO2@NiCo2O4 samples. FE-SEM analysis confirms uniform deposition of NiCo2O4 NPs on TiO2 spheres. TEM images reveal core-shell morphology of the TiO2@NiCo2O4 samples. Optical spectral studies indicate d-d and charge transfer transitions in the TiO2@NiCo2O4 core-shell nanoparticles due to the presence of NiCo2O4 as the shell. BET results indicate higher surface area (134.5 m2 g–1 − 118.8 m2 g–1) of TiO2@NiCo2O4 core-shell nanoparticles compared to the constituents (NiCo2O4 (115.5 m2 g–1) and TiO2-NaOH (8.8 m2 g–1)). The TiO2@NiCo2O4 core-shell nanoparticles show better catalytic activity towards photodegradation of tetracycline compared to pure NiCo2O4 NPs and TiO2. The catalytic properties of TiO2@NiCo2O4 core-shell NPs result from synergic effect due to the presence of both NiCo2O4 and TiO2. Coupling of NiCo2O4 shell with TiO2 core NPs results in higher surface area, smaller band gap, and more active sites, allowing faster photodegradation of tetracycline under natural sunlight. TiO2@NiCo2O4 core-shell NPs showed 98 % removal efficiency for tetracycline under visible light. The tetracycline degradation pathways were investigated using LC-MS. The TiO2@NiCo2O4 core-shell NPs exhibit good stability up to four catalytic cycles without major loss of catalytic performance, indicating their practicability as photocatalyst for industrial application in wastewater treatment. Chapter 6 deals with an overall summary of work done in the present thesis and also discusses future prospects.