SYNTHESIS OF p-Fe203 BASED NANOSTRUCTURES STUDY OF THEIR OPTICAL AND MAGNETIC PROPERTIES

Abstract

In recent years, nanoscience and nanotechnology have made a global impact on the society because of their vast scientific and technological applications. Many physical and chemical methods have been developed for synthesizing and enhancing the properties of nanoparticles for applications in optics, magnetic and catalytic to biomedicines. In this context, the processing of colloids using wet chemistry offer an advantage of synthesizing size- and shape-dependent nanostructures in solution enabling to fine-tune their optical, electronic, magnetic and photophysical properties with ease. Metal-semiconductor assemblies have become interesting because of the weak van der Waals interaction between them and are, therefore, expected to have several beneficial effects in terms of their organization, increased interface, improved charge carrier dynamics, reactivity and selectivity. Among various semiconductors, Fe203 and its composites have been investigated widely in recent years because of their extensive applications in ferrofluids, refrigeration, targeted drug delivery, MRI, cancer diagnosis, and protein seperation. A better understanding of these nanosystems, however, requires characterizing these systems in terms of their size, shape, structure, and the interaction between metal and semiconductor components. It also requires to analyze their optical, magnetic and electronic properties, and to study the dynamics of charge carriers in the irradiated system(s). The present thesis has been divided into six chapters. The chapter wise details have been furnished below: Thefirst chapter presents a brief overview of the progress of scientific research carried out on various nanosystems for about last one and a half decade. The synthesis and photophysical aspects of certain nanoceramics; nanosized metals and their hybrids; semiconductors and their hybrids, and metal-semiconductor nanocomposite/nanohybrids focusing on iron oxide systems have been presented. The binary metal-semiconductor nanocomposites/nanohybrids have been reported to construct generally the core-shell structure having either of metal(s) or semiconductor(s) as a shell and vice-versa. Some reports, available on ternanry metal-semiconductor systems have also been included. This chapter also lists the objectives of the present work. The second chapter deals with experimental details of the used materials, equipment and techniques. A brief account of the methodology used for carrying out various measurements on different techniques has been included. It also gives a brief description of methods employed for the synthesis of metal and semiconductror nanocolloids used in the present work. The third chapter presents the synthesis of P-Fe203 nanostructures of different shapes by the hydrolysis of FeCl3 in the absence and presence of Co2+ ion. These materials have been characterized by UV-vis spectroscopy, X-ray diffraction, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), superconducting quantum interference device magnetometry (SQUID) and C-V analyzer. The addition of Co2+ (0.12% to 0.59%) slowly transforms the shape of nanosized p-Fe203 from rods to become spherical. These nanostructures demonstrate superparamagnetic behavior. The extent of saturation magnetization is enhanced bya factor of 3, i.e. from 0.49 to 1.56 emu/g upon addition of 0.12% Co2+ The fourth chapter has been divided in two sections. Section A gives an account of synthesis of silver iron oxide nanoparticles in the absence and presence of Co2+. It produces nanoparticles with a narrow size distribution by the interaction of colloidal PFe203 and silver nanoparticles. Surface morphology and size of these particles have been analyzed by usingAFM, FESEMand TEM. Their structural analysis has been carried out by employing XRD, SAED, optical and infrared spectroscopic techniques. Aging of these particles exhibit the formation of self-assembly, possibly involving weak supramolecular interactions between Ag'04 and Fem04 species. These particles display the onset of absorption in the near infrared region and have higher absorption coefficient in the visible range compared to that of its precursors. Magnetic measurements reveal an interesting transition in their magnetic behavior from diamagnetic at room temperature to superparamagnetic at 7 T and 5 K. The magnetic moment of these particles attains a limiting value of about 0.19 emu/cm2, which is more than two times higher to that of colloidal P-Fe203 measured under identical conditions. Analysis of electrical properties of this system exhibits non-ohmic behavior. Section B describes synthesis of silver encapsulated P-Fe203 core-shell hollow nanotubes. These nanostructures are produced by the hydrolysis of FeCl3 in the presence of colloidal Ag nanoparticles and are characterized by AFM, FESEM, TEM, XRD, IR, UV and magnetic measurements. A variation in the amount of silver (0.23x10"* mol dm"3 - 0.76x10"4 mol dm"3) regularly blue shifts the excitonic band due to P-Fe203, and reduces the thickness of P-Fe203 in the shell besides changing the morphology of the nanostructures. Atypical amount ofsilver (0.58x10'4 mol dm"3) leads to the development of core-shell hollow tubes in which the core consists of Ag nanoparticles with average in diameter of 3.5 nm and the shell is made of hollow nanotubes consisting of CI" and N03" ions with an average thickness and the inner diameter of shell of 3 nm and 9 nm, respectively. Unlike pure P-Fe203 nanorods, the core-shell structure at 7 T exhibits superparamagnetic behavior at a relatively higher temperature (100 K), whereas P-Fe203 under these conditions depicted paramagnetic behavior. However, at 7 T and 5 K both of the samples exhibited superparamagnetic behavior, but the magnetic moment for the core-shell structure was about 5.6 times higher compared to that of pure P-Fe203. The fifth chapter presents the synthesis, optical and photophysical behavior of GMP-templated binary (p-Fe203/CdS) and ternary (p-Fe203/Ag/CdS) nanohybrids. In these systems the amount of different phases (GMP, P-Fe203 and Ag) and pH have been optimized so as to achieve the maximum emission. Analysis of the morphology of these nanohybrids by AFM, FE-SEM and TEM measurements shows that the fresh sample of binary and ternary nanohybrids contains spherical nanoparticles, which transformed into nanorods and nanowires, respectively upon aging. Both SAED and XRD analysis of the nanohybrid depicts them to contain CdS and Ag in the hexagonal phase and P-Fe203 in orthorhombic phase. In these nanohybrids interaction among different phases has been analyzed using IR spectroscopy. The excitation of binary nanohybrids by 340 nm light results in the quenching of emission of CdS phase without bringing any shift in the emission maxima. In a control experiment the quenching rate constant has been estimated to be 2.1xl010 dm3 mol"1 s"1 using both steady state and time resolved techniques. The presence of CdS in the binary hybrid results in the reduction ofsaturation magentization value by an order ofmagnitude 10 as compared to P-Fe203 and a change in the magnetic behavior from IV superparamagnetic to ferromagnetic having coercivity and remanance values to be 1200 Oe and 7.9 x 10"5 emu/cm2. On the other hand excitation of CdS by 340 nm in the ternary nanohybrid complex results in enhancement of fluorescence intensity by a factor of ~ 3 compared to the binary nanohybrid, associated with a red shift in the emission maximum. It also results in an increase in the values of saturation magentization, coercivity and remanance to 0.0026 emu/cm2, 1273 Oe and 1.78 x 10"4 emu/cm2, respectively. An analysis of the fluorescence lifetime data reveals an increase in average lifetime value of ternary complex (52 ns) compared to that of binary (30 ns). The relaxation of charge carriers in both binary and ternary nanohybrids is observed to take place over a period ranging from sub-nanasecond to hundred(s) of nanosecond time domain. An examination of fluorescence anisotropy in both the cases depict an increase in the value of anisotropy and rotational correlation time (02) upon aging. The sixth chapter presents a summary and discussion of the results presented in the third, fourth and fifth chapters. The structures of P-Fe203, P-AgFe02 and different nanocomposites have been analyzed based on data obtained by using XRD, electron microscopy and different spectroscopic techniques. For binary (P-Fe203/CdS) and ternary (P-Fe203/Ag/CdS) nanohybrids the photophysics is understood by monitoring the fluorescence in these irradiated system under various experimental conditions. The observed physicochemical characteristics viz. optical, electronic and magnetic behavior of different nanostructures suggest them to be the promising materials for optoelectronic, photonics, fluorescence imaging, molecular recognition, MRI, targeted drug delivery and sensing applications.