Investigating the optical properties of gold decorated CdS nanoparticles via physical techniques
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Date
2019-04
Authors
Nqayi, Sibusiso
Journal Title
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Publisher
University of the Free State
Abstract
Quantum dots are very important in modern technology, which is driven by decreasing machinery size, while enhancing performance. Quantum dots form an integral part of nanoparticles (NPs) with a particle size that varies between 2-10 nm. The preparation of these materials is very important, with structural, optical and morphological studies showing dependence on synthesis conditions. In this work, cadmium sulphide (CdS) NPs were prepared using the chemical precipitation method. To control the particle size, thioglycerol (TG) was used as a capping agent with particle size dependent on the TG concentration. Scanning electron microscope and transmission electron microscope investigations showed that agglomerated particles were formed due to the high surface energies that are associated with very small particles. In order to study the role of the reaction conditions on the stabilizing of the particle surface, the particles were prepared using two different solvents: water and ethanol. X-ray diffraction (XRD) data showed that the use of water solvent resulted in particles consisting of a mixed phases of wurtzite and cubic structures. The quantum confinement effect was first observed in the colour change of the prepared samples. In the absence of TG, the sample had an orange colour, however, by introducing 0.1 mL of TG, the colour of the particles turned yellow while at 0.8 mL of TG the particles had a whitish-lemon colour. This effect was confirmed by the blue shift to lower wavelengths of the absorbance spectra obtained with the ultraviolet-visible (UV-Vis) technique. The blue shift is associated with a decreasing particle size. The increased sulphur content with an increasing TG concentration also increased the photoluminescence (PL) recombination rate. Thus preparation of CdS NPs with the water solvent resulted in luminescence from green, yellow, and red emission. The annealing of the 0.3 mL sample resulted in sintering of the small particles making up bulk particles with a hexagonal structure upon a phase transformation temperature. This change in phase introduced infrared emission in the PL spectra. This emission was obtained in the as-prepared samples in the ethanol solvent. Its source was unreacted cadmium chloride (CdCl2) species. Ethanol solvent could not completely dissolve the starting material in the reaction. This resulted in the appearance of strong Cd(OH)2 peaks in the XRD pattern. Also, the XRD pattern showed a cubic zinc blende structure to be dominant in the formed particles with a size of 6.3 nm, 3.0 nm, 3.0 nm, 2.9 nm, and 2.6 nm for S0 (no TG), S1 (0.1 mL TG), S2 (0.2 mL TG), S3 (0.3 mL TG), and S5 (0.8 mL TG), respectively. These were all in the domain of quantum dots and the introduction of TG in this batch took the NP size to sizes below the exciton Bohr radius of CdS. Thereafter the deposition of gold (Au) co-catalyst on the CdS surface was done using physical methods in the top-down approach. It was observed that Au nanoclusters deposited with pulsed laser deposition (PLD) and sputter coating (SPC) enhanced the absorbance of CdS in the UVVis spectra. Nanocomposites prepared with the SPC technique showed the highest absorbance enhancement due to the larger cluster formation which was observed with time-of-flight secondary ion mass spectroscopy as agglomerated clusters. PL spectra showed a decreased luminescence, which showed a decrease in the electron-hole recombination. This is of high importance in the field of photocatalysis for water splitting. This occurred due to the transfer of electrons from the highest occupied states in the CdS semiconductor to the lowest unoccupied states in the Au metal.
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Keywords
Dissertation (M.Sc. (Physics))--University of the Fee State, 2019, Quantum confinement effect, Cadmium sulphide, Schottky junction, Pulsed laser deposition, Nanoparticles, Sputter coating, Gold nanoclusters