Micro-Structure of Functional Particles and Particle Systems

Deng, Mao

Nanoparticles and particle systems exhibit exceptional properties in various applications. In the present dissertation the results of the transmission electron microscopy (TEM) analysis in microstructures of nanoparticles and of hybrid nanocomposites with different length scales are presented. The functional particles and particle systems under study include samples from: 1. MoS2 based catalysts and their composites. A photocatalyst MoS2/carbon/phosphorus composite is synthesized, and high resolution TEM (HRTEM) visualizes the expanded (002) lattice planes. Electron energy loss spectroscopy (EELS) results suggest an ordered and sp2 bonding dominated carbon species. In situ electron beam irradiation shows instability of the composite sample. The intercalation of graphitic carbon inside the 250 °C sample and the nanoslabs with more active sites in the 350 °C sample are responsible for their higher photocatalytic activities as compared to bulk MoS2. With regard to an amorphous MoS2 (PX) sample, in situ electron beam irradiation and in situ thermal annealing inside TEM both lead to an increased crystallinity. Yet statistic studies comparing the size distribution of the MoS2 slabs reveal different crystallization mechanisms of the two pathways. The inherent coordinatively unsaturated sites inside the PX sample account for the high photocatalytic activity. In hydrodesulferizaton (HDS) catalysts where PX is promoted with cobalt, HRTEM reveals microstructural difference between samples prepared under different conditions, and their HDS catalytic activity is related accordingly. Quantitative analyses of electron diffraction show a good agreement with size distributions of MoS2 nanoparticles. Kinematic and dynamic simulations of a diffraction pattern reveal the influence of HDS reaction on crystallinity and microstructure of MoS2 phase. Computer simulation of the interface between MoS2 and Co is performed for understanding promoter effect. For MoS2/carbon composite as a potential candidate for catalyst and electrode, EELS analyses suggest a mixture of graphitic and carbide-like carbons. Synthesis conditions play an important role on the structure of the sample. 2. Quasi 1D MoO3, SnO2 nanorods and core-shell composite. For the MoO3 nanorods, TEM structural and compositional studies reveal the growth direction and the highly defective structure of the orthorhombic MoO3. The performance of the MoO3 nanorods sensor toward ethanol vapor is related to the surface catalytic reaction on the lattice oxygen of MoO3. The reaction is further facilitated by the defect rearrangement of the crystallographic structure. The fast gas response is due to the dimension of the MoO3 nanorods. For the SnO2/Ga2O3/GaN:Ox composite, a comprehensive approach of energy filtered TEM, diffraction, HRTEM and in situ heating is applied to reveal the complex structure of the sample. The influences of thermal annealing to the structure are demonstrated via ex situ and in situ heating experiments. 3. Carbon microtube (Aerographite, AG) and its variants. Beam sensitivity of AG is investigated with 80 kV and 300 kV EELS. The quantitative EELS result shows an increased π*/ σ* peak ratio of the carbon K-edge when lowering the acceleration voltage of TEM, which indicates the 300 kV electron beam modifies the graphitic carbon into the amorphous carbon. A high temperature variant of the AG-like amorphous carbon is presented. The bonding state of carbon atoms is studied by examining both the high loss and the low loss regions of the carbon K-edge. A mixture of graphite- and diamond-like carbon bonding is signified. EFTEM elemental mapping reveals small traces of light elements like oxygen, nitrogen and carbon on AG based composites down to micro-/nano-scale. Furthermore, a novel hollow tubular structure of Aero-GaN is shown. Diffraction patterns and corresponding computer simulations suggest a possible intergrowth of GaN and ZnO phases. HRTEM simulation proves the structure of GaN and near-atomic resolution imaging discloses defective structure. 4. A 0-3 nanocomposite of CdSe (crystalline nanoparticles) and Cr2Se3 (amorphous matrix) is studied. The transformation of the 0-3 composite is studied under TEM in situ electron beam irradiation and thermal annealing. In situ irradiation removes the CdSe nanoparticles and a porous Cr2Se3 matrix with a slightly increased crystallinity is produced. In situ thermal annealing eliminates CdSe particles and generates crystalline nano- and micro Cr2Se3 particles, with an additional formation of chromium enriched crystallites. The results are compared with those of conventional ex situ methods.

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Deng, Mao: Micro-Structure of Functional Particles and Particle Systems. 2015.

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