Synthesis And Characterization Of One-Dimensional Oxide Nanostructures
Vanithakumari, S C
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Nanostructured materials especially, one-dimensional (1D) nanostructures have unique physical, chemical, mechanical properties and are the building blocks for a range of nanoscale devices. The procedure employed for the synthesis of nanostructures involves the use of sophisticated instruments or rigorous chemical reactions. The motivation of our work is to develop a strategy that is simple, cost effective and applicable to a host of oxide materials. Nanostructures of various oxides have been grown from the metal as the source material. 1D ZnO nanostructures have been obtained by simply heating Zn metal in ambient air at temperatures below 600 °C. The nanostructures grow on the surface of the source material and the morphology is controlled by monitoring the curvature of the source material. This technique has an added advantage that neither any catalyst nor any gas flow is required. Tetrapods of ZnO are obtained when Zn is heated above 700 °C in ambient air. It has been shown that the morphology and the aspect ratio (length-to-diameter ratio) of the tetrapods depend on the temperature and the temperature gradient. Photoluminescence studies reveal good optical quality ZnO nanostructures. The technique employed to synthesize 1D ZnO nanostructures has been checked for other oxides. The temperature required for the synthesis of Ga2O3 nanostructures is 1200 °C. Many researchers have shown that Ga2O3 emits in the blue-green region. A red emission is required to get the impression of white light which has been seen for nitrogen doped Ga2O3. As the temperature is very high and Ga is heated in ambient air, unintentional nitrogen doping of 1D Ga2O3 nanostructures is obtained which is the reason for white light emission. The morphology of Ga2O3 nanostructures has been controlled by monitoring the curvature of the starting material as is the case of ZnO. Similar technique has also been employed for the synthesis of CuO nanostructures. The morphology is temperature dependent and 1D CuO nanostructures are obtained when the synthesis temperature is between 400 and 600 C. Possible growth mechanisms have been proposed for all these oxide materials. The entire thesis is based on the results discussed above. It has been organized as follows: Chapter 1 deals with the introduction to nanostructures, importance of 1D nanostructures, the specific applications of different morphologies, materials that are widely explored in the synthesis of nanostructures and different approaches to the synthesis of nanostructures. Growth mechanisms like VLS, VS and SLS are briefly discussed. A brief review on the basic physical properties, applications and different morphologies of ZnO, Ga2O3 and CuO is outlined with emphasis to the various synthesis techniques. Finally the aim and scope of the present work is discussed. Chapter 2 describes the experimental setup used for the synthesis and the basic principles of characterization techniques like x-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), energy dispersive spectrum (EDS), electron energy loss spectroscopy (EELS), photoluminescence (PL), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), UV-Visible spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and thermogravimetric analysis (TGA). Chapter 3 deals with the synthesis of 1D ZnO nanostructures with different morphologies such as nanoneedles, nanorods, nanobelts from Zn powder/granule. The growth process is found to be different from the conventional VS mechanism. The advantage and the versatility of the method is emphasized. In this method, neither a catalyst nor any gas flow is required for the synthesis of oxide nanostructures. Depending upon the Zn powder or Zn granules as the starting material different nanostructures of ZnO have been synthesized. The as-synthesized materials are characterized by XRD, SEM, HRTEM, EDS, TGA and Raman spectroscopy and the results are discussed. Chapter 4 describes the controlled growth of ZnO tetrapods and the influence of temperature and temperature gradient on the growth process. Though there are several methods to synthesize ZnO tetrapods and it has been established that ZnO tetrapods can be synthesized by heating Zn in air, it is advantageous to grow tetrapods of different morphologies with different lengths. The large scale synthesis of ZnO tetrapods by heating Zn in air ambient is discussed in this chapter. The key parameters that control the diameter, length, and morphology of tetrapods are identified. It is shown that the morphology and dimensions of the tetrapods depend not only on the vaporization temperature but also on the temperature gradient of the furnace. The influence of vaporization temperature and growth temperature on the morphology of the tetrapods is discussed elaborately. Chapter 5 explains the one-step synthesis of nitrogen doped Ga2O3 nanostructures of different morphologies and the different growth mechanisms. The experimental method employed for the synthesis of nanostructures is simple and is different from the other reported methods. Neither any catalyst/substrate preparation nor any gas flow is required for the synthesis of Ga2O3 nanostructures. The synthesis involves the heating of molten Ga at high temperatures. Single crystalline monoclinic phase of nitrogen-doped Ga2O3 nanorods, nanobelts and nanoneedles are obtained by this method. The morphology is controlled by monitoring the curvature of the Ga droplet which is achieved by using different substrates. Possible growth processes of different morphology have been proposed. Chapter 6 includes some surprising results on the white light emission of Ga2O3 nanorods. High synthesis temperature generates a high vapor pressure suitable for the growth of Ga2O3 nanorods, creates oxygen vacancy and incorporates nitrogen from the ambient. The oxygen vacancy is responsible for the bluish-green emission, while nitrogen is responsible for the red emission. As a consequence, white light emission is observed from Ga2O3 nanorods when irradiated with UV light. The interesting point is that neither post-treatment of the nanorods nor size control is required for white light emission. Chapter 7 describes the synthesis of CuO nanostructures by heating Cu foil in air ambient. This is an attempt to check whether the synthesis technique employed for ZnO and Ga2O3 is applicable to other oxides. The as-synthesized CuO nanostructures are characterized by XRD, SEM, HRTEM, EDS, TGA, UV-visible, FTIR and the results are discussed. Chapter 8 gives the conclusions and the overall summary of the thesis.
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