Preparation of Cadmium Sulfide Nanoparticles in Gel Media PDF Download

Author: Tharwat Mokalled Mokalled
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Languages : en
Pages : 256

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Book Description
We present in the first chapter of this thesis an overview about the properties of cadmium sulfide (CdS) nanoparticles and their quantum size effects which renders them useful in various applications (light emitting diodes, photovoltaic devices, sensors, etc). Then we describe some of the methods for determining the size of these nanoparticles. This is followed by an overview of the synthetic techniques for the preparation of CdS quantum dots (aqueous and non aqueous), description of their nucleation and growth, and their photophysical properties. In chapter two, we discuss the behavior of CdS nanoparticles in gel (agar and gelatin) matrix in the presence of various capping agents in order to explore regimes of concentrations. From these observations, we introduce a new pulse-exhibiting reaction-diffusion system which consists of sulfide ions as an outer diffusing electrolyte into a gel containing mercaptoethanol-capped cadmium ions as an inner electrolyte, producing cadmium sulfide nanoparticles. These nanoparticles form a transparent faint yellow propagating front (connection between the bulk region and the nuclei region) which propagates starting from the junction between the two electrolytes; however, when subject to UV light, a very thin yellow fluorescing region appears leading this front region which is defined as the pulse (spatially localized region that propagates heading the a solitary wave). Ultraviolet/visible (UV/Vis) absorption and X-ray diffraction (XRD) measurements show that the average size of CdS nanoclusters in the front region is around 6-8 nm with a cubic phase, while UV/Vis measurements show that the average size of the CdS nanoparticles in the pulse region is about 4 nm. The distance travelled by the pulse versus time exhibits a diffusion-like profile, and it is found that the speed of the pulse propagation fluctuates as the concentration of the capping agent is varied. We observe that the width of the pulse remains constant with time (approximately 2 mm) and is independent of the outer electrolyte concentration (S0). This yellow fluorescence originates from emission from surface states as seen through the emission spectra of the pulse region. Time-correlated-single-photon counting (TCSPC) measurements are performed on both the pulse and the front regions in order to understand the origin of the weak fluorescence in the front region. In the third chapter of the thesis, the spatiotemporal dynamics of this pulse- propagating system is theoretically studied using a competitive particle growth model (Müller -Polezhaev (MP) model) wherein the evolution equations for the reactants and the product are written and solved, and the results are found to be in good agreement with the experimental findings. The densities of the small particles in the pulse region are found to lie within the density distribution of the larger particles in the front region which confirms the evolution of the pulse while leading the front. Moreover, the pulse has shown a diffusion-like profile of x peak equal to time power half, as was experimentally verified. A power law of the form xpeak equal to the initial concentration (S0) to the power 1/4 relates the distance travelled by the pulse (xpeak) to the concentration of the outer electrolyte (S0). Finally, the width of the pulse obeys a time power law, w equal to time power alpha, with crossover between an early time regime (alpha = 1/3) and an intermediate time regime (alpha = 0). Thus, this system provides a method to study the early dynamics of Liesegang systems.