SYNTHESIS OF METAL FERRITE NANOPARTICLES AND STUDIES ON THEIR MAGNETIC PROPERTIES

Abstract

Nanotechnology is a branch of science dealing with research related to nanomaterials with size in the range of 1 to 100 nm. The physicochemical properties of nanomaterials can be easily tailored according to their desired requirements. Metal ferrite (MFe2O4) nanoparticles exhibit fascinating magnetic, electrical and optical properties which can be easily tailored by varying particle size, morphology and composition. Metal ferrite nanoparticles are promising materials in various fields such as electronic devices, magnetic devices, drug delivery, catalysis, photocatalysis, sensing, adsorption and magnetic hyperthermia. Doping is an important method that leads to changes in structure, magnetic, optical and electrical properties of the host materials. The guest component can enter into crystal structure of the host material and change magnetic properties of the parent materials. Doped metal ferrite nanoparticles are very useful in the areas of biomedicine, photocatalysis, catalysis, sensing, optoelectronics, solar cells, electrical and magnetic devices. The current study is focused on the synthesis of pure and doped metal ferrite nanoparticles and studies on their magnetic properties. The systems that have been investigated are: (i) CoFe2O4 nanoparticles, (ii) Cu2+ doped CoFe2O4 nanoparticles, and (iii) ZnFe2O4 nanoparticles. The metal ferrite nanoparticles (pure as well as doped) were synthesized starting from metal glycolate precursors followed by calcination. In the first step, metal glycolate precursors were synthesized using an ethylene glycol mediated route. In the second step, the metal glycolate precursors were calcined at 500 ℃ to obtain pure and doped metal ferrite nanoparticles. The synthesized pure and doped metal ferrite nanoparticles were characterized using various analytical techniques. After characterization, a thorough investigation of their magnetic properties was done using VSM, PPMS and SQUID. The thesis consists of six chapters and a brief description of each chapter is given below. Chapter 1 deals with a brief introduction to nanotechnology and nanomaterials followed a brief introduction to pure and doped metal ferrite nanoparticles. Then, important synthetic methods for the metal ferrite nanoparticles have been briefly discussed. This is followed by a discussion on important physicochemical properties of metal ferrite nanoparticles. At the end, some important applications of metal ferrite nanoparticles have been discussed. Chapter 2 deals with different characterization techniques that have been used for the characterization of pure and doped metal ferrite nanoparticles in the present study. The characterization techniques include powder X-ray diffraction (P-XRD), Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), microwave plasma atomic emission spectroscopy (MP-AES), UV-Visible diffuse reflectance spectroscopy (UV-DRS), X-ray photoelectron spectroscopy (XPS) and BET surface area analysis. Rietveld refinement of XRD data was carried out to understand the distribution of cations in tetrahedral and octahedral sites of metal ferrite nanoparticles. The investigation of magnetic properties of synthesized nanoparticles was done using VSM, PPMS and SQUID. Field dependent (M-H) and temperature dependent (M-T) measurements were carried out. Chapter 3 consists of two sections and each one of them has been discussed separately below. In the first section, Co-Fe glycolates with different morphologies were synthesized at different temperatures (160 °C to 220 °C) and volume of ethylene glycol (20 mL and 30 mL) using an ethylene glycol mediated route. After calcination at 500 ℃, the Co-Fe glycolates were converted into pure cobalt ferrite nanoparticles. The effect of synthetic temperature and volume of ethylene glycol on the morphology and magnetic properties of cobalt ferrite nanoparticles was investigated. XRD analysis confirmed that after calcination, cubic CoFe2O4 is obtained from the Co-Fe glycolates. Electron microscopy studies (FE-SEM and TEM) showed that the Co-Fe glycolate synthesized at 160 °C exhibits rod-like morphology which convert into agglomerated nanoparticles after calcination. The Co-Fe glycolate synthesized at 220 °C and the corresponding cobalt ferrite nanoparticles exhibit hexagonal morphology. The Co-Fe glycolate synthesized at 160 °C using 30 mL ethylene glycol and the corresponding cobalt ferrite nanoparticles exhibit octahedral morphology. From the TEM studies, the hexagon and octahedron were found to be in turn consisting of small CoFe2O4 nanoparticles. The magnetic properties of CoFe2O4 nanoparticles were studied using the VSM. The magnetic studies showed that saturation magnetization and remanent magnetization of the cobalt ferrite nanoparticles depend on crystallite size and distribution of Co2+ and Fe3+ ions in Td and Oh sites of CoFe2O4 at 5 K and 300 K. At 5 K, coercivity depends on morphology and crystallite size of CoFe2O4 nanoparticles. At 300 K, the coercivity of CoFe2O4 nanoparticles depend on crystallite size alone. From ZFC-FC graphs, it was observed that blocking temperature is higher than room temperature (300 K) and it depends on crystallite size of the CoFe2O4 nanoparticles. The second section reports the effect of using different cobalt precursors during the synthesis of metal glycolates on morphology and magnetic properties of CoFe2O4 nanoparticles. First, Co-Fe glycolates with different morphologies (nanosheet, hexagon, flower and sphere) were synthesized using different cobalt salts (cobalt acetate, cobalt acetylacetonate, cobalt chloride and cobalt nitrate). CoFe2O4 nanoparticles were then obtained after calcination of corresponding Co-Fe glycolates at 500 ℃. Before calcination, XRD studies reveal the formation of Co-Fe glycolates and after calcination at 500 ℃, cubic CoFe2O4 nanoparticles are formed. For the understanding of cationic distribution in CoFe2O4 nanoparticles, Rietveld refinement of XRD data was used. Scanning electron microscopy study revealed that the Co-Fe glycolates synthesized using cobalt acetylacetonate, cobalt acetate and cobalt chloride and their corresponding CoFe2O4 nanoparticles exhibit nanosheets, hexagon and flower-like morphology, respectively. The Co-Fe glycolate synthesized using cobalt nitrate consists of spherical particles; after calcination at 500 ℃, the spherical particles are converted into agglomerated nanoparticles. Elemental mapping and EDX analysis confirmed the presence and uniform distribution of Co, Fe and O in the Co-Fe glycolates and their corresponding CoFe2O4 nanoparticles. TEM analysis showed that the nanosheet, hexagon and flower-like CoFe2O4 particles are in turn made up of small nanoparticles. On the other hand, the CoFe2O4 nanoparticles synthesized using cobalt acetate consists of single size population of nanoparticles. SAED patterns confirmed polycrystalline nature of the CoFe2O4 nanoparticles. XPS studies confirmed the presence of divalent Co and trivalent Fe in octahedral and tetrahedral sites of CoFe2O4. VSM and SQUID were used to study the magnetic properties (M-H and M-T) of cobalt ferrite nanoparticles. The M-H graphs reveal wasp-waisted behaviour in CoFe2O4 nanoparticles synthesized from cobalt acetylacetonate, cobalt chloride and cobalt nitrate at 5 K. The degree of wasp-waisted behaviour was found to depend on the cobalt source used during the synthesis of CoFe2O4 nanoparticles. At 5 K and 300 K, saturation magnetization and remanent magnetization depend on cationic distribution in Oh and Td sites of cobalt ferrite nanoparticles. At 5 K, coercivity depends on morphology and crystallite size of CoFe2O4 nanoparticles. However, at 300 K, the coercivity depends on only the crystallite size of CoFe2O4 nanoparticles. M-T studies indicate that the blocking temperature of CoFe2O4 nanoparticles is higher than room temperature (300 K). Chapter 4 deals with Cu2+ substituted cobalt ferrite nanoparticles synthesized using different Cu2+ concentrations (0.10 mmol, 0.20 mmol and 0.30 mmol). First, Cu2+ substituted Co-Fe glycolates were prepared via an ethylene glycol mediated route followed by calcination at 500 ℃. The effect of concentration of Cu2+ ions on the magnetic properties of CoFe2O4 nanoparticles was investigated. XRD studies indicate incorporation of Cu2+ ions in Co-Fe glycolate precursors and Cu2+ substituted CoFe2O4 nanoparticles are obtained after calcination of the Cu2+ substituted Co-Fe glycolates. Rietveld refinement of XRD data indicates that when Cu2+ concentration is 0.10 mmol and 0.20 mmol, Co2+ ions in Oh sites of CoFe2O4 are substituted by Cu2+ ions. When Cu2+ concentration is 0.30 mmol, substitution of Co2+ by Cu2+ occurs in tetrahedral sites in addition to the octahedral sites of CoFe2O4. Morphological studies demonstrate that the morphology of pure Co-Fe glycolate is hexagonal which converts to near micro-spherical particles in Cu2+ substituted Co-Fe glycolates on incorporation of Cu2+ (0.1 mmol to 0.30 mmol). The morphology of the Cu2+ substituted CoFe2O4 nanoparticles remain the same after calcination at 500 °C. TEM study demonstrated that the hexagon (CoFe2O4) and near micro-spherical particles of Cu2+ substituted CoFe2O4 nanoparticles are made up of small nanoparticles. DRS study indicated that the bandgap is increased from 2.70 eV to 3.58 eV as the concentration of Cu2+ increases (x = 0.0 to 0.3) in the Co1-xCuxFe2O4 nanoparticles. M-H studies on Cu2+ substituted CoFe2O4 nanoparticles indicated soft ferromagnetic behaviour at 300 K and hard ferromagnetic magnetic behaviour at 15 K. At 300 K and 15 K, the observed Ms and Mr values could be explained on the basis of concentration of Cu2+ ions and their distribution in tetrahedral and octahedral sites of CoFe2O4. At 300 K, the coercivity decreases on increasing the Cu2+ concentration (0.10 mmol, 0.20 mmol and 0.30 mmol). At 15 K, the coercivity of Cu2+ substituted CoFe2O4 nanoparticles depends on morphology and also Cu2+ concentration. The blocking temperature of Cu2+ substituted CoFe2O4 nanoparticles is greater than 305 K and it depends on crystallite size of CoFe2O4 nanoparticles. Chapter 5 deals with the effect of synthetic conditions (temperature and volume of ethylene glycol) on the morphology and magnetic properties of ZnFe2O4 nanoparticles synthesized via thermal decomposition of metal glycolates. XRD analysis confirm the formation of Zn-Fe glycolates at different synthetic temperatures (140 ℃, 160 ℃ and 180 ℃) and different volumes of ethylene glycol (20 mL and 30 mL). ZnFe2O4 nanoparticles are obtained after the calcination of Zn-Fe glycolates at 500 °C. Rietveld refinement of XRD data confirms the presence of more Fe3+ ions in octahedral sites in ZnFe2O4 nanoparticles synthesized from Zn-Fe glycolate prepared at 140 ℃ as compared to ZnFe2O4 nanoparticles derived from Zn-Fe glycolates synthesized at 160 ℃ and at 180 ℃. Scanning and transmission electron microscopy studies indicated that agglomerated nanoparticles are formed in Zn-Fe glycolates at 140 ℃ and 160 ℃ (20 mL volume of ethylene glycol) and in the Zn-Fe glycolates synthesized at 180 ℃, octahedral particles are observed. On increasing the volume of ethylene glycol from 20 mL to 30 mL during the synthesis of Zn-Fe glycolates at 160 ℃, agglomerated nanoparticles are converted to nanoflakes. EDX and elemental mapping studies confirm the presence of Zn, Fe and O in Zn-Fe glycolates and ZnFe2O4 nanoparticles with uniform distribution. TEM studies demonstrate that the octahedral particles and nanoflakes of ZnFe2O4 are made up of small nanoparticles.XPS studies confirm the presence of divalent Zn and trivalent Fe in octahedral and tetrahedral sites of ZnFe2O4. Magnetic measurements (M-H and M-T) were carried out to undersatnd effect of shape, crystallite size and cationic distribution on the magnetic properties of ZnFe2O4 nanoparticles at 15 K and 300 K. M-H studies indicate superparamagnetic behaviour at 300 K and weak ferromagnetic behaviour at 15 K for the ZnFe2O4 nanoparticles. The saturation magnetization of ZnFe2O4 nanoparticles could be tuned by varying the synthetic temperature and volume of ethylene glycol used during the synthesis of Zn-Fe glycolates. M-T measurements confirm that the blocking temperature of ZnFe2O4 nanoparticles is lower (24.5 K to 28.5 K) than room temperature. Chapter 6 deals with an overall summary of the work done in the present study and future prospects.