Quantitative three-dimensional density mapping of Ni and PtNi nanoparticles during morphological transformation using coherent X-ray diffraction imaging

Abstract

In this dissertation, the study of metal nanoparticles was performed using coherent X-ray diffraction imaging (CXDI). CXDI has the advantage of non-destructive imaging of the specimen due to the high penetration of X-rays due to weak interactions with materials or small scattering cross-sections. Quantitative three-dimensional (3D) density mapping of Ni and PtNi nanoparticles during morphological transformation was obtained using two-dimensional (2D) projection images reconstructed by the phase retrieval algorithm and computed tomography techniques, and the details of the study are as follows.

In Chapter 1, we introduces imaging techniques mainly used in material research and CXDI techniques used in this dissertation. Principles, the theories and features related to CXDI and reconstruction method of 3D image are introduced in Chapter 2. Chapter 3 and 4 contains the contents of experiments using CXDI and related research. In Chapter 3, 2D and 3D images of Ni nanoparticles produced as single crystals were reconstructed before and after oxidation by CXDI technique, and the total number of electrons, volume change, porosity, and distribution due to oxidation were analyzed before and after oxidation. It was analyzed from image comparison. It has been suggested that 3D analysis of oxides with complex structures is useful. In Chapter 4, the morphology and crystal structure of PtNi alloy particles formed by nanosecond laser were confirmed by 2D image analysis such as SEM, EDS and TEM, and it was confirmed that Pt- and Ni-rich were divided into core-shell forms. The 3D electron density image obtained by CXDI clearly visualized the Pt- and Ni-rich regions, and that more clear information such as the spatial distribution according to the shape and chemical composition of the nano-cavity, which is difficult figure out from the 2D image. In Chapter 5, analyzed the morphological changes of oxidized PtNi particles as an extension of Chapter 4. It was confirmed from the pores formed between the NiO formed on the outside and the Pt boundary inside the Pt boundary that there was shrinkage due to thermal annealing of Pt during the oxidation process. NiO clusters were formed between Pt and pores due to internal diffusion of oxygen, and Pt was aggregated into one cluster, and the structure by faceting of pores according to surface energy was confirmed.