3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Synthesis and characterization of Cu-based telluride semiconductor materials for application in photovoltaic cells(2017) Ntholeng, NthabisengThe colloidal method has extensively been used to synthesize ternary and quaternary copper sulfides and selenides. Although tellurides form part of the chalcogenides, little has been reported on them particularly the synthesis of these nanostructures. Achieving high-quality nanocrystals through colloidal synthesis requires thorough monitoring of parameters such as time, solvent, precursor as they affect nucleation and growth of the nanocrystals. Herein, we report on the colloidal synthesis of ternary CuInTe2 and quaternary CuIn1-xGaxTe2 nanostructured semiconductor materials. A typical synthesis of CuInTe2 entailed varying reaction temperature. At temperatures below 250 °C, no formation of CuInTe2 was seen. At 250 °C formation of CuInTe2 could be observed with the formation of binary impurities. A change in the sequence in which precursors were added at 250 °C yielded pure CuInTe2. Applying different surfactants aided in achieving differently structured morphologies of CuInTe2 nanocrystals. Morphology varied from rods, cubes, nanosheets etc. Different morphologies resulted in different optical properties with the high optical band gap of 1.22 eV measured for 1D rods. Different precursors were employed in the synthesis of quaternary CuIn1-xGaxTe2. Precursor 2 (entailed the use of Cu (acac)2, In (acac)3 and Ga(acac)3) yielded pure CuIn1-xGaxTe2 phase with no formation of impurities. Variation in reaction time influenced the optical properties of the quaternary CuIn1-xGaxTe2 with high band gap obtained at low reaction time (30 min). A change in Ga and In concentration resulted in reduced lattice parameters a and c with lowest values obtained with the highest Ga concentration. However, achieving the intended concentration proved challenging due to the loss of the material during synthesis. Increasing the Ga concentration resulted in a high optical band gap. Conducting the reaction with Hexadecylamine (HDA) resulted in a relatively high optical band though the formation of impurities was evident. The obtained band gap can be attributed to small sized particles as evident from TEM results. Heterojunction ZnO/CIT and ZnO/CIGT solar cell devices were fabricated through a simple solution approach. The performance of ZnO/CIGT device was superior to that of ZnO/CIT in which efficiency increased from 0.26-0.78%. In the ZnO/CIT device, high Voc of 880 mV was recorded while 573.66 mV was measured for ZnO/CIGT device. Chemical and thermal treatments were performed on the ZnO/CIGT devices. The efficiency increased from 0.78 1.25% when the device was chemically treated with a short-chain EDT ligand. A high conversion efficiency of 2.14% was recorded for devices annealed at 300 °C. High annealing temperatures resulted in poor device performance with the lowest efficiency of 0.089% obtained at annealing temperatures of 500 °C attributed to the leaching out of In and Ga into the ZnO layer.Item Investigating the effect of gold-palladium bimetallic nanoparticles on TiO2 and the catalytic activity in CO oxidation(2013-04-29) Ntholeng, NthabisengIn recent years, studies have shown that supported Au catalysts have high activity for CO oxidation at low or ambient temperatures. However, the activity of these catalysts is dependent on a lot of synthesis conditions as reproducibility of small sized gold particles is hard. In this study supported Au catalysts were prepared via deposition precipitation-method (DP). The small sized Au particles were supported on TiO2 (P25). The suitable synthesis conditions such as pH, aging, metal loading and catalyst pre-treatment were investigated in order to obtain optimum synthesis conditions. The catalysts were characterized with TEM, XRD, and HRTEM. It was found that 3.7 nm Au particles were best synthesized when Au metal loading is 3% at pH 8 and aged for 72 h. The suitable calcination temperature was 200 °C. It was found that the Au particle size was 4.5 nm when Au was supported on SiO2 thus making TiO2 a suitable support. Bimetallic catalyst was synthesized via DP where Pd metal was incorporated as the second metal. It was found that the type of bimetallic formed was heterostructed where both metals where separately attached on the support. The interatomic distance measured from HRTEM results confirmed that both metal were individually attached on the support. XRD results showed that there was no Au-Pd alloy phase or PdO confirming that the Pd metal on the support was indeed in metallic form. Carbon monoxide (CO) oxidation reactions were undertaken in a tubular glass flow reactor. The monometallic Au catalyst showed superior activity at 200 °C with almost 100% CO conversion. It was also observed that the activity of these catalyst decreased as temperature increased. The CO-TPD studies showed that as temperature increased there was a low CO adsorption due to a decrease in adsorption sites. Varying Pd composition in the bimetallic catalyst did not enhance catalytic activity. However, 25Au75Pd catalyst showed a better conversion as compared to other Au-Pd catalysts. Temperatures studies on bimetallic catalysts showed that as temperature increased there was a decrease in activity. The observed decrease could be attributed to catalyst formation of large particle aggregates. It was also assumed that the low activity was due to how these catalysts were prepared as there was no surfactant utilized during preparation.