Tailored nanocrystal catalysts

Thesis

The thesis explores the synthesis, characterization, and simulation of the heteroepitaxial monometallic (Ni, Ag, Pd) and bimetallic (Ni@Ag) systems on SrTiO3 substrates.

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.

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.

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.

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.

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.

Authors: Zhao, Y

• DOI: 10.5287/ora-x5mrm46dz
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: epitaxial strain relaxation; scanning tunneling microscopy; strontium titanate; heteroepitaxy; finite-element method; x-ray photoelectron spectroscopy
• Subjects: Scanning tunneling microscopy; Metal nanoparticles; Surface science