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Title: Homojunction and heterostructure engineering on zinc and copper sulfides for enhanced photocatalytic hydrogen evolution from water
Authors: Liu, Wei
Degree: Ph.D.
Issue Date: 2019
Abstract: The development in semiconductor nanomaterials has brought many opportunities for efficient conversion of solar energy to clean and sustainable fuels for utilization and storage. Solution-based synthesis of earth-abundant metal sulfide nanomaterials is an ideal technique that offers the ease of preparation and the possibility of upscaling to meet industrial needs. This thesis focuses on the interface tuning of nanoscale zinc and copper sulfides as well as Au nanomaterials/copper sulfide, to give a fundamental understanding on the role of nanoscale interface in photocatalytic reaction. Zinc sulfide (ZnS) nanocrystals with multiple parallel interfaces between wurtzite and sphalerite phases were prepared using a simple solvothermal method. The zinc atoms at these parallel interfaces have different electronic environments, creating abundant active sites on the surface. The ZnS nanocrystals were of good yield and high quality and exhibited a dramatic enhancement in H₂ gas evolution rate compared with that of normal ZnS. This rate was further increased by loading Pt nanoparticles on the ZnS surface. In a parallel electrocatalytic investigation, substantially lower overpotential for electrocatalytic hydrogen evolution was observed in the phase-junctions rich ZnS nanocrystals, probably attributed to the synergistic effect of easier proton adsorption and enhanced H₂ desorption on the active sites. Although the ZnS nanocrystals with multiple phase-junctions exhibited drastically improved photocatalytic activity, the intrinsic wide bandgap limits the use of this material in visible-light driven photochemical reaction.
To further understand the impact of multiple phase-junctions, homo-interfaces were introduced to CuxS (where 1 ≤ x ≤ 2) as a p-type semiconductor photocatalyst. The crystal phases of CuxS can be tuned by varying the Cu/S stoichiometry using an ethylenediamine-mediated hydrothermal method, yielding CuS, Cu₁.₇₅S, Cu₁.₈S and Cu₂S nanocrystals. By fine-tuning the ratio of ethylenediamine, multiple phase-junctions were introduced to CuxS nanocrystals to modify the surface states, which promote the H₂ gas evolution from water. The multiple interfaces give an optimal optical and photocatalytic performance when the CuxS nanocrystal is comprising of Cu₁.₇₅S and , with ca. 230% enhancement in H₂ gas evolution rate compared with single phase Cu₁.₈S. An active surface arising from multiple interfaces within phase-junctions is believed to lower down the energy barrier of the CuxS nanocrystals, and improves the photocatalytic performance for H₂ gas evolution. To push up the photocatalytic performance of copper sulfide, plasmonic Au nanoparticles were embedded into hollow copper sulfide. In this study, different types of yolk-shell structures based on hollow copper sulfide encapsulating Au nanoparticles of various shapes were designed, prepared and characterized with the aims to enhance the light absorption, to improve the charge carrier separation and transfer, and subsequently to enhance the photochemical activity. The results of photocatalytic hydrogen generation indicated that copper sulfide nanocages embedded with multi-spike Au nanoparticles performed better than pure copper sulfide nanoparticles or other Au-copper sulfide yolk/shell nanoparticles. This enhancement in catalytic activity highlights the size and shape effects of the Au nanomaterials, which have strong influence on the surface plasmonic resonance property. Multi-spike Au cores exhibit stronger plasmonic effect with increased pathways for exciton transport, which consequently generate an improved photocatalytic activity in the Au-copper sulfide nanoparticles. To maximize the surface plasmon effect on photocatalysis, Au nanorod-copper sulfide nanocages with multiple interfacial contacts were prepared. This Au-copper sulfide hetero-nanostructure enhances the light-harvesting of copper sulfide over the visible region with hot electrons injected from Au. The supramolecular structured materials performed much better in photocatalytic hydrogen evolution than hollow copper sulfide nanocages alone and Au-copper sulfide mixture without interfacial contacts. This superior photocatalytic property could come from the plasmonic effect of the Au nanorod as the hot electron generator and the photoactive copper sulfide as the electron accepter. This study introduced a unique hetero-structure of noble metal with semiconductor nanomaterial, and offers an alternative structural design of plasmonic metal-semiconductor nano-catalysts in photochemical reactions.
Subjects: Hong Kong Polytechnic University -- Dissertations
Nanostructured materials
Renewable energy resources
Pages: xii, 219 pages : color illustrations
Appears in Collections:Thesis

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