Insight Into Local Structure And Formation Mechanism Of Ni-P Nanoparticles By In Situ Synchrotron Radiation Techniques

Noncrystalline Ni-P nanoparticles have attracted wide interest due to their excellent properties including catalytic activity, magnetocaloric effect, magneto-resistance, and Li intercalation behavior. To further explore the application of noncrystalline Ni-P nanoparticles, it is important to study their local structure, formation process, activation of the formation process and thermal stabilities of the structure.Liquid-phase pulse discharge method has great superiorities in the preparation of nanomaterials due to its facility, economic and convenience. While, techniques based on synchrotron radiation facilities have been widely used in the analysis of materials structures for its unique X-rays. Therefore, in this thesis, the synchrotron radiation techniques will be used together with common laboratory methods to study the formation process, local structures, activation of the formation process, and thermal stabilities of noncrystalline Ni-P nanoparticles.Firstly, the local structure of noncrystalline Ni-P nanoparticles were characterized through XRD, HRTEM, ICP-MS, EDS and XAFS. As the noncrystalline Ni-P nanoparticles are amorphous, having only short-range ordered structures and no long-range ordered structures. Therefore, the near neighbor structure of Ni and P atoms were studied respectively through XAFS spectra. The atoms distributions around Ni centers is similar with of crystalline Ni, while the atom distributions around P centers are much like crystalline Ni3P. Accordingly, a local structure model containing 23 Ni atoms and 3 P atoms was configurated to describe the short-range ordered structure of noncrystalline Ni-P nanoparticles.Secondly, formation process of noncrystalline Ni-P nanoparticles were studied. HRTEM was used to catch the morphology changes, in situ QXAFS was used to follow the near neighbor structure changes of Ni atoms, while the ex situ XAFS was used to follow the near neighbor structure changes of P atoms, the in situ SAXS was used to follow the particles size distribution changes during the formation process of Ni-P nanoparticles. The results showed that the initially formed nuclei were Ni nanocrystals. As the reaction proceed, the reduced P atoms attached to the Ni nanocrystals as they grew bigger. And then these attached P atoms diffused into the lattice structure of Ni nanocrystals, leading to the structure distortion of the Ni crystal. As a result, transformation from crystalline phase to noncrystalline phase occurred, resulting in the formation of noncrystalline Ni-P nanoparticles. Then, the formed noncrystalline Ni-P nanoparticles grew steadly through auto-catalysis of Ni. Therefore, the formation process of noncrystalline Ni-P nanoparticles were divided into four stages:formation of Ni crystal, formation of Ni-P nanoparticles, transformation from crystalline phase to noncrystalline phase, and growth of noncrystalline Ni-P nanoparticles.Thirdly, the beginning of the formation process of Ni-P nanoparticles was studied through emission spectra. The self-established emission equipment were used to characterize the discharge pulse and active radicals. UV-Vis spectra analysis was used to characterize Ni2+concentration changes in chemical reactions. The results showed that the formation process of noncrystalline Ni-P nanoparticles were activated by active radicals of H, P+, OH released during the pulse discharge. Ni2+ were reduced directly into Ni atoms, while P atoms were generated indirectly by the disproportionation of H2PO2-. Therefore, the formation of Ni atoms is earlier than that of P atoms. The total chemical reaction formula was given, and the reaction rate of the given formula is 3.77×10-6 (mol/L)/s.Fourthly, local structure thermal stabilities of the as-prepared noncrystalline Ni-P nanoparticles were studied through XRD, HRTEM and XAFS from both Ni and P K-edges. The structure turned out to be unstable at high temperature. During heat treatment, Ni, Ni3P and metastable pahses occurred simultaneously, and the growth of crystalline Ni3P and Ni are completing. The main phases within the particles are crystalline Ni3P and Ni, while the metastable phases presented as coating shell of the particles. As P atoms precipitated from the amorphous matrix, crystalline Ni was formed. The outwards diffused P atoms formed crystalline Ni3P. Metastable phase was formed in the P-rich zones between crystalline Ni and Ni3P zones. The structure evolution of noncrystalline Ni-P nanoparticles could be separated into three phases:formation of crystalline Ni and Ni3P, competing growth of Ni and Ni3P, and the decomposition of the metastable phase. According to the XAFS fitting results, the standard deviation of P-Ni bondlength could be used as a quantitative criterion for the structure evolution of the as-prepared noncrystalline Ni-P nanoparticles.