SIZE AND SHAPE EFFECTS IN MAGNETIC NANOPARTICLES

Abstract

Nanoscale materials are of immense interest because their physicochemical properties (e.g. optical, magnetic, electrical, reactivity, etc) are significantly different from that of bulk materials. Among various nanoparticles, magnetic nanoparticles have gained a special importance because of their unique characteristics. The properties of magnetic nanoparticles strongly depend on their size and shape and they are useful in applications such as data storage, environmental remediation, catalysis and biomedicine. Different types of magnetic nanoparticles such as metals, metal oxides, alloys, multi-functional materials and self-assembled nanoparticles have been well studied in the literature. Various synthetic methods have been reported for the preparation of the magnetic nanoparticles. Simple and economical routes for the synthesis of magnetic nanoparticles with good control over size and shape are highly desirable since their magnetic properties are highly dependent on them. In the present study, three different types of magnetic nanoparticles have been investigated. The systems that were investigated include: (i) self-assembled prismatic iron oxide nanoparticles, (ii) iron oxide microspheres formed by the self-assembly of iron oxide nanoparticles and multi-functional iron oxide@Ag core−shell nanoparticles, and (iii) MgO supported iron oxide and cobalt oxide nanoparticles. The nanoparticles were synthesized using thermal decomposition approach at low temperatures without the need for any special conditions. A thorough characterization of the synthesized magnetic nanoparticles was carried out using various analytical techniques. After the characterization, the magnetic properties of the nanoparticles were investigated using a superconducting quantum interference device. The effect of size and shape on the magnetic properties of the different types of nanoparticles has been investigated. Further, some of the interesting applications of the magnetic nanoparticles, prepared in the present study, have also been explored. The thesis consists of seven chapters and a brief summary on each chapter is given below. Chapter 1 deals with a brief historical background on nanotechnology, an introduction to magnetic nanoparticles, their types and synthetic methods. A short description on superparamagnetism has been given. Various examples of size and shape dependent magnetic properties of nanoparticles have been discussed. At the end, some of the important applications of the magnetic nanoparticles in different areas have been described. ii Chapter 2 deals with the various analytical techniques that were employed for the characterization of the synthesized magnetic nanoparticles and their methods of sample preparation. The techniques that were used include powder X-ray diffraction, Fourier transform infrared spectroscopy, thermal gravimetric analysis, differential thermal analysis, atomic absorption spectroscopy, carbon, hydrogen and nitrogen elemental analysis, field emission-scanning electron microscopy coupled with energy dispersive X-ray analysis, transmission electron microscopy, selected area electron diffraction, UV-Visible spectroscopy (diffuse reflectance spectroscopy) and BET surface area measurements. The magnetic properties of the synthesized magnetic nanoparticles were investigated using the superconducting quantum interference device as a function of magnetic field and temperature. Chapter 3 deals with the synthesis of hexagonal prismatic iron oxide nanostructures and studies on their magnetic properties. Thermal decomposition of an iron-urea complex, [Fe(CON2H4)6](NO3)3, in diphenyl ether at 200 oC, leads to the formation of prismatic iron oxide particles. They were characterized using an array of analytical techniques. SEM results indicated that the prismatic particles are made up of self-assembled iron oxide nanoparticles. The iron oxide nanoparticles were found to be superparamagnetic at room temperature with a characteristic blocking temperature. Zero field cooled and field cooled measurements indicated strong interparticle interactions among the iron oxide nanoparticles. The mechanism of formation of prismatic iron oxide nanoparticles has also been proposed. Chapter 4 deals with the synthesis of iron oxide microspheres formed by the self-assembly of iron oxide nanoparticles and iron oxide@Ag core−shell nanoparticles by a novel thermal decomposition approach. These systems have been discussed separately in two different sections. In Section 1, the preparation of iron oxide microspheres by the thermal decomposition of the iron-urea complex in a mixture of solvents (dimethylformamide and diphenyl ether, DMF:DPE = 7:3 v/v) at 200 oC has been discussed. SEM and TEM results indicated that the iron oxide microspheres are uniform in size with an average diameter of 1.2 ± 0.3 μm. The iron oxide microspheres are in turn made up of small iron oxide nanoparticles. Magnetic measurement results indicated superparamagnetic behavior for the iron oxide microspheres which confirm that the microspheres consist of iron oxide nanoparticles. The mechanism of the formation of iron oxide microspheres has also been investigated. iii In Section 2, the synthesis of multi-functional iron oxide@Ag core–shell nanoparticles in which iron oxide microspheres serve as the core and silver nanoparticles form the shell has been studied. Thermal decomposition of silver acetate, in the presence of iron oxide microspheres in diphenyl ether, leads to the formation of iron oxide@Ag core–shell nanoparticles. The core–shell nanoparticles were characterized using a variety of analytical techniques and their magnetic behavior as a function of field and temperature was investigated. SEM and TEM images indicated uniform coating of silver nanoparticles on the surface of iron oxide microspheres. Magnetic measurements revealed superparamagnetic behaviour with a characteristic blocking temperature for the iron oxide@Ag core−shell nanoparticles. It was found that the blocking temperature decreases on coating the surface of iron oxide microspheres with silver. The mechanism of formation of iron oxide@Ag core–shell nanoparticles has also been discussed. Chapter 5 deals with the synthesis of iron oxide and cobalt oxide nanoparticles supported on macro-crystalline and nanocrystalline MgO. These systems have been discussed in two separate sections. In the first section, g-Fe2O3 nanoparticles supported on MgO (macro-crystalline and nanocrystalline) were prepared by the thermal decomposition of iron (III) acetylacetonate in diphenyl ether, in the presence of the MgO supports followed by calcination at 350 oC. The X-ray diffraction results indicated the stability of g-Fe2O3 phase on MgO (macro-crystalline and nanocrystalline) up to 1150 oC. TEM images indicated the presence of smaller g-Fe2O3 nanoparticles on nanocrystalline MgO compared to that on macro-crystalline MgO. The magnetic properties of the supported magnetic nanoparticles at various calcination temperatures (350 oC−1150 oC) were investigated which indicated superparamagnetic behavior at all the calcination temperatures. The zero field cooled and field cooled measurements indicated uniform distribution of iron oxide nanoparticles on nanocrystalline MgO compared to that on macro-crystalline MgO. In the second section, an easy single step synthesis of Co3O4 nanoparticles supported on two different MgO supports (macro-crystalline and nanocrystalline) has been reported. The preparation involves the thermal decomposition of cobalt (II) acetylacetonate, in the presence of the supports in diphenyl ether followed by calcination. The supported cobalt oxide nanoparticles were characterized by a variety of analytical techniques. The XRD and TEM iv results indicated that the cobalt oxide nanoparticles are dispersed uniformly on nanocrystalline MgO compared to macro-crystalline MgO. For the supported cobalt oxide nanoparticles, the M-H curves indicated paramagnetic behavior at 300 K and a weak superparamagnetic behavior at 5 K. In the zero field cooled and field cooled magnetic measurements, paramagnetic behavior at low temperatures in the supported Co3O4 nanoparticles due to crystal defects was also observed. Chapter 6 deals with various applications that were explored using the magnetic nanoparticles synthesized in the present study. Iron oxide prisms and iron oxide microspheres were used as photocatalysts for the degradation of rhodamine B in an aqueous solution using H2O2 under UV illumination. Iron oxide prisms were found to exhibit better catalytic performance compared to iron oxide microspheres. Iron oxide@Ag core−shell nanoparticles were explored as catalysts for the reduction of 4-nitrophenol and methylene blue in the presence of NaBH4 in aqueous solution. The iron oxide@Ag core−shell nanoparticles were found to be better catalyst compared to pure silver nanoparticles and iron oxide microspheres. MgO supported iron oxide nanoparticles were explored as adsorbent for the removal of Cr(VI) from an aqueous solution. It was found that maximum percentage of Cr(VI) removal is achieved using the iron oxide nanoparticles supported on nanocrystalline MgO. Chapter 7 deals with the overall summary of the work done in the thesis and also discusses the future prospects.