XAFS Studies On ZnCdSe And NiP Nano Systems

The main content in this dissertation is as follows:1. In-situ XAFS studies on the growth of Zn1-xCdxSe quantum dotsWe have developed a time resolved XAFS method aiming at the study of growth progress of nano materials in solution by utilizing a flow reactor and synchrotron radiation. By utilizing this method and QXAFS technology, the growth mechanism of L-glutathione capped Zn1-xCdxSe quantum dots (QDs) obtained from the aqueous synthesis method is in situ studied as a model system. Detailed analysis of the Se K-edge EXAFS data reveals that the growth of ZnSe QDs from aqueous synthesis undergoes a fast nucleation process and then a slow growth stage, which follows Ostwald ripening mechanism. The ZnSe QDs are compressively strained to decrease the surface energy then to minimize the total free energy, and the contraction of Se-Zn bond relaxes with time. After doping with Cd, the bond length of the first shell in QDs increases compared to ZnSe QDs, and the structural disorder of Zn1-xCdxSe QDs increases with the reaction time. This shows the promising capability of in situ XAFS to investigate the growth process of nano-materials synthesize by chemical solution method.2. X-ray absorption spectrum studies on structural evolution of Ni100-xPx alloysX-ray absorption fine structure (XAFS) and x-ray diffraction (XRD) were utilized to investigate the atomic and electronic structures of Ni100-xPx alloys with different P contents prepared by chemical reduction method. Their catalytic properties are characterized by the reaction of benzene hydrogenation. Relationship between the P content and properties has been established. The results show that as x = 10, the doped P element leads to expansion and distortion of the Ni fcc lattice in NiP sample, and the RNi-Ni increases by about 0.03 ?. With the P content increasing, the expansion and distortion are strengthened. It is found when x increases to a critical value of 14, the lattice of fcc-Ni is completely destroyed, and thus the amorphous NiP alloy is formed. The X-ray absorption near edge structure (XANES) results indicate that at a low concentration (x < 10), no significant change of the electronic structure was observed. However, as P content increases, the distribution of the Ni 4p states becomes diffuser and broader. When x = 26, the white line feature of Ni K-edge implies that a large number of charges are transferred from Ni atoms to P atoms.