School of Chemistry - Theses
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ItemSynthesis and Modification of ZnSe Nanoplatelets( 2021)Nanoplatelets (NPLs) are a class of nanoparticles which have garnered significant interest in the research community. Unfortunately, much of the focus has been on the usual workhorse material: cadmium selenide. Zinc selenide is a close relative of cadmium selenide, both belonging to the II-VI family of semiconductors, but little research exists on ZnSe NPLs beyond its initial synthesis. In this thesis, ZnSe NPLs are addressed from the bottom up. In Chapter 2, the formation mechanism of ZnSe NPLs and MSCs is investigated. The evolution of nanoparticles in the reaction is monitored while exploring the reaction space. It is demonstrated that the concept of surface reversibility can be used to predict the formation of NPLs and MSCs. Additionally, it is found that MSCs and NPLs compete in the reaction, and selective formation can be induced by varying selenium precursor and the ripening behaviour of the ligand. Along the way, five unreported ZnSe magic-sized clusters (MSCs) are found. Chapter 3 of the thesis is a demonstration of Mn 2+ doping into the ZnSe and ZnS NPLs. Mn 2+dopant incorporation can be confirmed via its photoluminescence and photoluminescence excitation spectra and the photostability is measured. Additionally, the unique Mn 2+ emission is used a probe to investigate the evolution of various ZnSe species. Finally, Chapter 4 is concerned with the post-synthetic shelling of ZnSe nanoplatelets. ZnSe NPLs as synthesized from literature are colloidally and photo-unstable. A common solution to this is to coat the surface of the nanocrystal with a suitable semiconductor material. By modifying the process introduced for CdSe NPLs, the synthesized ZnSe NPLs are shelled successfully via colloidal atomic layer deposition (C-ALD). This allows us to improve its photoluminescence properties and observe unique features associated with Type-II ZnSe/CdS heterostructures.
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ItemPorous titania-based composite materials and their high-throughput photocatalytic evaluation for environmental remediation( 2016)Semiconductor-mediated photocatalysis is a promising technology for environmental remediation. Among various materials, titania is a well-known photocatalyst, yet much improvement is still required to further improve its activity. This thesis presents some approaches used to optimize the photocatalytic activity of porous titania-based materials that are physically viable for practical operations. Specifically, the effect of the addition of nitrogen during synthesis and silver nanoparticles combined with various templating methods were examined. A high-throughput testing system based on parallelization and miniaturization of methylene blue photodegradation reactions was also developed to facilitate the photocatalytic evaluation in an efficient manner. Hierarchically porous, anatase titania thin films of varied thickness were fabricated by a one-pot, soft-templating technique combined with a phase separation route. The pore structure was readily tuned by adjusting the concentration of the polymeric components added during the sol-gel synthesis. Poly(vinylpyrrolidone) (PVP) altered the three dimensional pore structure, generating macroporous networks within the films. The highest photocatalytic activity under UV irradiation normalized by the accessible surface area was obtained by porous titania thin films prepared using 1:1:0 poly(ethylene glycol):PVP:F127. The addition of F127 did increase the overall photocatalytic activity, but lowered the activity per unit area because of obstructed light penetration. In order to effectively utilize the visible light, mesoporous anatase titania with nitrogen doping was prepared by a template-free, sol-gel synthesis route. The effect of calcination conditions and the type of titania precursor were investigated, highlighting their profound influence upon the adsorption and visible light activity. The titania crystallization in the presence of nitrogen was also studied using in situ synchrotron powder diffraction. The nitrogen modified titania prepared from titanium (IV) butoxide and diethanolamine calcined at 350 °C for 10 h exhibited a high methylene blue adsorption capacity (85 mg g-1) and high photocatalytic activity under visible light. The prominent photocatalytic performance was attributed to the synergetic effect from the abundant nitrogen content (10.91 at. %), relatively high specific surface area (154.8 m2 g-1), and enhanced surface acidity (isoelectric point ≈ 2.7). To further enhance the practicality of the titania composites with nitrogen modification, the synthesis method was then extended to obtain porous monolithic structures. The goal of this study was to investigate the relationship between the photocatalytic activity and the diverse porous morphologies produced using the phase separation route and agarose gel templating. The amount of polymer used in the phase separation induced monoliths and the infiltration method in the preparation of agarose templated monoliths were shown to affect both the physicochemical and optical properties. This comparative study showed that the highest UV and visible light activity for methylene blue removal was achieved by the agarose-templated monoliths that were infiltrated at 60 °C. This was accredited to their higher surface area and higher nitrogen content compared to those of the monoliths obtained from phase separation. The addition of nitrogen and silver nanoparticles was carried out simultaneously with a hard templating technique using silica spheres packed into a three dimensional “opal” structure to further optimize the performance of titania under visible light. All of the opal templated samples in this work performed better than the commercial titania, P25. The highest photocatalytic enhancement, showing more than eight times higher activity than the non-modified titania, was achieved by the opal templated sample prepared with 1.0 mol % of silver. Although both the nitrogen and silver addition and templating enhanced the visible light activity, the most significant improvement was afforded by the utilization of the silica opal template that gave rise to high surface area (>100 m2 g-1) and promoted the surface charge interaction.