Tailored nanocrystal catalysts
<p>The thesis explores the synthesis, characterization, and simulation of the heteroepitaxial monometallic (Ni, Ag, Pd) and bimetallic (Ni@Ag) systems on SrTiO3 substrates.</p> <p>In monometallic systems, the morphology of nanocrystals is shaped by the combined influence of thermo...
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| Format: | Thesis |
| Language: | English |
| Published: |
2023
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| _version_ | 1826311947616780288 |
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| author | Zhao, Y |
| author2 | Castell, M |
| author_facet | Castell, M Zhao, Y |
| author_sort | Zhao, Y |
| collection | OXFORD |
| description | <p>The thesis explores the synthesis, characterization, and simulation of the heteroepitaxial monometallic (Ni, Ag, Pd) and bimetallic (Ni@Ag) systems on SrTiO3 substrates.</p>
<p>In monometallic systems, the morphology of nanocrystals is shaped by the combined influence of thermodynamics and kinetics. Factors like crystal volume, metal species, substrate terminations, deposition temperature, post-annealing temperature, and duration collectively influence the nanocrystal’s shape. Extended post-annealing shows a strain relaxation characterized by increased crystal height.</p>
<p>In some monometallic systems, a transition is observed from multi-twinned nanoparticles to single crystals. Comparisons across these systems reveal that factors such as surface/interface energy, elastic constants, surface stress effect, and twin boundary energy collectively influence the critical size at which this transition happens.</p>
<p>The synthesis of Ni@Ag core-shell nanoparticles on SrTiO3(001) is achieved via sequential deposition and annealing at 500◦C. Scanning tunneling microscopy (STM) images highlight a transition from mixed shapes to uniform truncated pyramidal shapes with increased annealing temperature. The h/l ratio analysis, based on STM images, further suggests a core-shell structure. X-ray photoelectron spectroscopy (XPS) results display a pure Ni peak only in the annealed sample, and the selective Ag removal by sputtering rules out the possibility of Ni-Ag alloy formation.</p>
<p>The strain energy in the Ni and Ni@Ag systems was simulated using the finite-element method. The strain energy displays a layered distribution. A simulation for the observed heightening effect for strain energy relaxation is provided. In the Ni@Ag-SrTiO3(001) system, the Ag shell consistently exhibits compressive strain, while the Ni core shows tensile strain. The simulations further reveal that the smaller height-to-length ratio in Ni@Ag, in comparison to the pure Ni core, results from the compressive strain in the shell, corroborating previous experimental findings.</p>
<p>These findings broaden our understanding of heteroepitaxial systems on SrTiO3 substrates and the influence of epitaxial strain on morphologies. This knowledge serves as a stepping-stone towards enhanced synthesis of tailored nanomaterials for a broad range of applications.</p> |
| first_indexed | 2024-03-07T08:18:49Z |
| format | Thesis |
| id | oxford-uuid:7ef1d5bc-7025-43c3-b995-2e6321f32e30 |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T08:18:49Z |
| publishDate | 2023 |
| record_format | dspace |
| spelling | oxford-uuid:7ef1d5bc-7025-43c3-b995-2e6321f32e302024-01-19T07:30:23ZTailored nanocrystal catalystsThesishttp://purl.org/coar/resource_type/c_db06uuid:7ef1d5bc-7025-43c3-b995-2e6321f32e30Scanning tunneling microscopyMetal nanoparticlesSurface scienceEnglishHyrax Deposit2023Zhao, YCastell, M<p>The thesis explores the synthesis, characterization, and simulation of the heteroepitaxial monometallic (Ni, Ag, Pd) and bimetallic (Ni@Ag) systems on SrTiO3 substrates.</p> <p>In monometallic systems, the morphology of nanocrystals is shaped by the combined influence of thermodynamics and kinetics. Factors like crystal volume, metal species, substrate terminations, deposition temperature, post-annealing temperature, and duration collectively influence the nanocrystal’s shape. Extended post-annealing shows a strain relaxation characterized by increased crystal height.</p> <p>In some monometallic systems, a transition is observed from multi-twinned nanoparticles to single crystals. Comparisons across these systems reveal that factors such as surface/interface energy, elastic constants, surface stress effect, and twin boundary energy collectively influence the critical size at which this transition happens.</p> <p>The synthesis of Ni@Ag core-shell nanoparticles on SrTiO3(001) is achieved via sequential deposition and annealing at 500◦C. Scanning tunneling microscopy (STM) images highlight a transition from mixed shapes to uniform truncated pyramidal shapes with increased annealing temperature. The h/l ratio analysis, based on STM images, further suggests a core-shell structure. X-ray photoelectron spectroscopy (XPS) results display a pure Ni peak only in the annealed sample, and the selective Ag removal by sputtering rules out the possibility of Ni-Ag alloy formation.</p> <p>The strain energy in the Ni and Ni@Ag systems was simulated using the finite-element method. The strain energy displays a layered distribution. A simulation for the observed heightening effect for strain energy relaxation is provided. In the Ni@Ag-SrTiO3(001) system, the Ag shell consistently exhibits compressive strain, while the Ni core shows tensile strain. The simulations further reveal that the smaller height-to-length ratio in Ni@Ag, in comparison to the pure Ni core, results from the compressive strain in the shell, corroborating previous experimental findings.</p> <p>These findings broaden our understanding of heteroepitaxial systems on SrTiO3 substrates and the influence of epitaxial strain on morphologies. This knowledge serves as a stepping-stone towards enhanced synthesis of tailored nanomaterials for a broad range of applications.</p> |
| spellingShingle | Scanning tunneling microscopy Metal nanoparticles Surface science Zhao, Y Tailored nanocrystal catalysts |
| title | Tailored nanocrystal catalysts |
| title_full | Tailored nanocrystal catalysts |
| title_fullStr | Tailored nanocrystal catalysts |
| title_full_unstemmed | Tailored nanocrystal catalysts |
| title_short | Tailored nanocrystal catalysts |
| title_sort | tailored nanocrystal catalysts |
| topic | Scanning tunneling microscopy Metal nanoparticles Surface science |
| work_keys_str_mv | AT zhaoy tailorednanocrystalcatalysts |