SYNTHESIS OF METAL OXIDE NANOPARTICLES USING LAYERED DOUBLE HYDROXIDES AS PRECURSORS AND THEIR APPLICATIONS

Abstract

Nanotechnology deals with understanding and utilization of materials in the nanoscale size range (1-100 nm). Nanomaterials exhibit unique properties which depend on their size, shape, composition, etc compared to their bulk counterparts. Metal oxide nanoparticles play a major role in the field of materials science and technology. They possess different physical and chemical properties due to the presence of large number of surface sites. The interesting properties of metal oxide nanoparticles make them useful for different applications, e.g. optics, electronics, energy storage, catalysis, catalyst support, adsorption, sensors, anode material for Li-ion batteries, dye-sensitized solar cells, supercapacitors, etc. Metal oxide nanoparticles (NPs) derived from layered double hydroxide (LDH) precursors are of immense interest. Thermal decomposition of layered metal hydroxides and LDHs at suitable temperatures yields metal oxide NPs and mixed metal oxide NPs. In layered metal hydroxides, intercalated anions can be easily exchanged with various organic or inorganic anions. By substituting the anions present in between the hydroxide layers with other ions, various properties of the metal oxide NPs can be enhanced. In the present work, layered metal hydroxides and LDH are used as precursors for the synthesis of metal oxide NPs and their core-shell nanoparticles. Various metal oxide NPs synthesized in the present work are: (i) NiO nanoparticles using SDS intercalated Ni(OH)2 precursors, (ii) Zn2TiO4@NiO core-shell nanoparticles using Zn2TiO4@Ni(OH)2 precursors, (iii) Zn2TiO4@NiCo2O4 core-shell nanoparticles using NiCo-LDH and (iv) SiO2@MnCo2O4 core-shell nanoparticles using MnCo-LDH precursors. The synthesized metal oxide NPs and also the precursors have been characterized using various analytical techniques such as XRD, FT-IR, TGA, FESEM, TEM, EDXA, SAED and XPS. BET surface area analyzer and zeta potential analyzer were used to measure the surface area and zeta potential of the metal oxide NPs. UV-Vis DRS and PL spectroscopy were used to study optical properties of the metal oxide nanoparticles. Magnetic investigations of the metal oxide NPs were carried out using VSM, PPMS and SQUID magnetometer. The synthesized metal oxide nanoparticles and core-shell nanoparticles were used for various applications such as adsorption of congo red, adsorption of ciprofloxacin and as catalyst for peroxidase-like activity. 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. Next, different physical and chemical routes for the synthesis of metal oxide NPs have been discussed. Then, a brief introduction to layered metal double hydroxide precursors is given. Further, this chapter discusses various interesting physicochemical properties of metal oxide nanoparticles. In the end, different important applications of metal oxide nanoparticles in different fields have been discussed. Chapter 2 describes various analytical techniques that were used in the present study to characterize the synthesized LDH precursors and metal oxide NPs and also their 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, zeta potential and BET surface area analyses. Optical properties of the metal oxide nanoparticles were studied using diffuse reflectance spectroscopy (DRS) and PL spectroscopy. Magnetic properties of the metal oxide nanoparticles were studied using a vibrating sample magnetometer (VSM), physical property measurement system (PPMS) and a superconducting quantum interference device (SQUID). Chapter 3 deals with the synthesis of NiO nanoparticles using sodium dodecyl sulfate (SDS) intercalated Ni(OH)2 precursors and their use as an adsorbent. A simple synthetic route has been explored to synthesize NiO nanoparticles using SDS intercalated layered nickel hydroxides as precursors followed by calcination at 400 C. Homogeneous precipitation method was used to synthesize the layered metal hydroxide precursors. The precursors and NiO nanoparticles were characterized using XRD, FT-IR, FESEM, TEM, BET, and VSM. XRD results confirm the intercalation of SDS in between the nickel hydroxide layers and the formation of nickel oxide nanoparticles after calcination. FTIR analysis reveals the presence of dodecyl sulfate anion (DS-) in between the Ni(OH)2 gallery. FESEM and TEM analyses show formation of hierarchical micro-flower-like structures made up of nanosheets and they can be converted into NiO nanoparticles via calcination. XPS analysis was used to confirm the presence of Ni2+, Ni3+ and O2- in NiO NPs. The NiO NPs obtained from the thermal treatment of SDS intercalated Ni(OH)2 have high surface area (110 m2/g to 123 m2/g) and smaller crystallite size (2.2 nm to 2.5 nm). Zeta potential measurement shows positive surface charge of NiO nanoparticles. Magnetic studies of NiO NPs indicate their superparamagnetic behavior at 300 K. The synthesized NiO NPs acts as good adsorbent for the removal of congo red a toxic dye from an aqueous solution. Chapter 4 deals with the synthesis of Zn2TiO4-based core-shell nanoparticles (CSNPs) using layered double hydroxide precursors. Zn2TiO4@NiO and Zn2TiO4@NiCo2O4 CSNPs were synthesized using Zn2TiO4@Ni(OH)2 and Zn2TiO4@NiCo-LDH precursors, respectively. This chapter consists of two sections. The first section describes, a simple soft chemical approach for the synthesis of Zn2TiO4@NiO CSNPs via calcination of Zn2TiO4@Ni(OH)2 precursors and their application as adsorbent. The precursors were prepared by homogeneous precipitation method and, on calcination of the precursors at 350 C, Zn2TiO4@NiO CSNPs were obtained. The precursors and the Zn2TiO4@NiO CSNPs were characterized using XRD, FT-IR, FESEM, TEM, BET, and SQUID. XRD analysis indicates the presence of NiO NPs and Zn2TiO4 in the CSNPs. FE-SEM analysis confirms uniform deposition of NiO NPs on the surface of Zn2TiO4 spheres. TEM images show formation of core-shell morphology in the Zn2TiO4@NiO samples. BET results indicate higher surface area of Zn2TiO4@NiO CSNPs compared to the constituents. XPS measurements prove the presence of Zn2+, Ti4+, Ni2+ and O2- in the core-shell NPs. Magnetic investigations indicate soft ferromagnetic behaviour of the Zn2TiO4@NiO core-shell nanoparticles at 5 K. Zeta potential measurements show positive surface charge of Zn2TiO4@NiO core-shell NPs. The Zn2TiO4@NiO CSNPs were tested as adsorbent for the removal of ciprofloxacin (CIP), a common antibiotic pollutant from an aqueous solution. The Zn2TiO4@NiO CSNPs have higher capacity (60.6 mg/g) compared to the constituents. The adsorption of CIP follows Langmuir isotherm model and pseudo 2nd order kinetics. The second section deals with a facile synthetic approach for the synthesis of Zn2TiO4@NiCo2O4 core-shell nanoparticles (NPs) using Ni-Co layered double hydroxide (LDH) as precursors and the core-shell nanoparticles are explored as catalyst for peroxidase-like activity using tetramethylbenzidine as the substrate. The LDH precursors were prepared by homogeneous precipitation method and, on calcination of the precursors, Zn2TiO4@NiCo2O4 core-shell nanoparticles were obtained. The precursors and the Zn2TiO4@NiCo2O4 CSNPs were characterized using XRD, FT-IR, FESEM, TEM, BET, and SQUID. XRD results confirm the presence of both Zn2TiO4 and NiCo2O4 phases in the Zn2TiO4@NiCo2O4 core-shell NPs. FESEM and TEM analyses indicate formation of hierarchical porous NiCo2O4 shell with varying thickness on the Zn2TiO4 spheres. Optical studies indicate bandgap of ⁓2.2 eV corresponding to NiCo2O4 NPs in the Zn2TiO4@NiCo2O4 core-shell NPs. The Zn2TiO4@NiCo2O4 core-shell nanoparticles possess higher surface area compared to the constituents as measured from BET analysis. Magnetic studies of Zn2TiO4@NiCo2O4 core-shell nanoparticles indicate soft ferromagnetic and hard ferromagnetic behavior at 300 K and 5 K, respectively. The Zn2TiO4@NiCo2O4 core-shell nanoparticles act as nanozyme and they exhibit better peroxidase-like activity as compared to pure NiCo2O4 NPs and natural horseradish peroxidase. The Zn2TiO4@NiCo2O4 CSNPs can be used in different applications, e.g. detection of glucose, hydroquinone, protein and dopamine. Chapter 5 deals with the synthesis of SiO2@MnCo2O4 core-shell nanorattles via calcination of SiO2@MnCo-LDH precursors and their peroxidase-like activity. The LDH precursors were prepared by homogeneous precipitation method and, on calcination of the precursors, SiO2@MnCo2O4 core-shell nanorattles were obtained. The LDH precursors and the SiO2@MnCo2O4 core-shell nanorattles were characterized using XRD, FT-IR, FESEM, TEM, DRS, BET, and XPS analyses. XRD results indicate the presence of MnCo2O4 phase in the SiO2@MnCo2O4 samples. FE-SEM analysis confirms uniform deposition of MnCo2O4 NPs on the surface of SiO2 spheres. TEM images reveal core-shell nanorattle morphology of the SiO2@MnCo2O4 samples. Optical studies indicate the presence of d-d and charge transfer transitions in the SiO2@MnCo2O4 nanorattles due to the presence of MnCo2O4 as the shell. BET results indicate higher surface area of SiO2@MnCo2O4 core-shell nanorattles compared to the constituents. The SiO2@MnCo2O4 core-shell nanorattles were explored as catalyst for peroxidase-like activity. The SiO2@MnCo2O4 core-shell nanorattles show better peroxidase-like activity compared to pure MnCo2O4 and horseradish peroxidase. The SiO2@MnCo2O4 core-shell nanorattles can be used as an artificial nanozyme in different biomedical applications. Chapter 6 deals with an overall summary of work done in the present thesis and also discusses future prospects.