In-situ environmental transmission electron microscopy on nickel-based nanostructures

This thesis explores the synthesis, phase transformations, and structural development of nickel-based nanoparticles (Ni-based NPs) using ex-situ and in-situ environmental transmission electron microscopy (ETEM).
The study starts with exploring the transformation of carbamide and ammonia into NiO nanoparticles (NPs). We focus on the mechanism of this transformation via nickel hydroxide intermediates, β-Ni(OH)2 and Ni3(OH)4(NO3)2, which are achieved through thermal annealing. The ex-situ thermal decomposition of these intermediates resulted in NiO NPs with distinct shapes, sizes, and surface properties. NiO obtained by the carbamide route forms nanorods with polar {111}c facets with a bandgap of 3.7 eV. NiO obtained by the ammonia route forms flower-like 3D nanostructures with neutral {002} facets and a lower bandgap of 3.2 eV, associated with an increased number of surface states due to their three times smaller size compared to NiO obtained by the carbamide route. Both sets of NPs are Ni-deficient.
Next, we study these phase transformation mechanisms by utilising the electron beam as a driving force. In-situ (E)TEM was employed to observe the real-time transformation of the intermediates, β-Ni(OH)2 and Ni3(OH)4(NO3)2, under electron beam irradiation, as well as in an annealing and O₂ environment. In both cases, the transformation proceeded via β-NiOOH intermediates to NiO, with 3D flower-like morphology preserved in both cases. In-situ annealing at 350–400 °C formed <3 nm NiO particles retained the original structure, in comparison to ex-situ treatments that led to distinct NiO NPs morphologies.
Next, we focused on NiO NPs obtained by physical ultra-high vacuum methods. MBE-grown Ni ultrathin films were transformed to Ni NPs under annealing in H2 atmosphere and vacuum, resulting in Ni NPs with varied morphologies. Ni NPs oxidation was performed in a 1 Pa background of O2, H2O, and H2O + CO2. O2 background resulted in the formation of spherical NiO NPs; H2O background yielded single-crystal NiO NPs, faceted predominantly by {111} surfaces, while CO2 + H2O mixtures produced polycrystalline NiO NPs, with irregular shape.
Finally, we studied the reduction and oxidation of core-shell Ni/NiO NPs. We follow this process dynamically using in-situ ETEM, which includes shell grains coalescence and changes in crystallinity. The oxidation of pristine Ni NPs results in core-shell formation of Ni/NiO, which follows the Cabrera-Mott and logistic models of self-limited oxide formation.
The findings in this work support the rational design of Ni-based nanostuctures with potential use in catalysis.