In Situ XAFS Method And Studies Of Kinetics Of Functional Materials

In the 21st century, the materials research center will shift from the study of equilibrium state to the study of dynamic process of how they are formed and, ultimately, how they work. The goal is to achieve the controllable synthesis and to manipulate the novel functional materials with tailored properties, so as to to meet the material requirements in the modern civilization, and to provide a strong material support for energy, safety and environment of our country. With the fast transformation from the research on equilibrium to on inequilibrium state, how to characterize and control the formation of materials and the physical and chemical reaction kinetics in the inequilibrium state at the atomic and electronic level still remains a great challenge in the future scientific research. To effectively control the synthesized paths and the reaction kinetics of materials, it is particularly important to build and develop new experimental methods to improve research condition from the average, equilibrium to local, inequilibrium.Synchrotron Radiation (SR) has many excellent features, such as high brightness, time structure, coherence, wide wavelength and polarization, and is an advanced setup for the multi-disciplinary basic and applied research. In the past 20 years, many significant research advances in the resolution of structure and function of materials in equilibrium state have been achieved by using the merit of high brightness and continue adjuatable of wavelength of SR. For example, the SR X-ray determination of the structure and function of the ion channel of the cell membrane, and the ribosome have received Nobel Prize in Chemistry (2003 and 2009). It is most likely to deep insight into the synthesized and reactional process based on the previous observation, if we combine the conventional SR with the in situ measurement and time-resolved technique to investigate in real time the dynamic process in the inequilibrium state. SR-XAFS has become a powful method for the in situ characterization because it is sensitive to the local atomic and electronic structure, and it can be applied to study on most condensed matters such as solid and solution. Now, the structural kinetics of the phase transition under the high pressure-temperature, and the active atoms in the catalyzed reaction can be obtained bu using in situ and time-resolved XAFS method. Therefore, developing the in situ and time-resolved XAFS method is helpful to obtain the information on the evolutions of atomic and electronic structures in the real conditions, and will realize the break through in solving the key scientific problem of nantechnology, energy and life science.This dissertation develops several in situ (including temperature-dependence and chemical reaction) XAFS spectroscopy methods based on the X-ray stations in National Synchrotron Radiation Laboratory, USTC, and performs the interdisciplinary research on kinetic process of some important and typical functional materials, such as metal-insulator transition (MIT) in strongly correlated systems and initial nucleation of nanomaterials. Also, the developed in situ XAFS method provides a well public research platform for the domestic SR users. This dissertation has made the following research advances:1. The development of the in situ SR XAFS methodThe phase transition and nanocrystal formation are the most important topic in tht field of condensed matter and nanoscience, respectively. To reveal these kinetic mechanisms, the experimental sections of ChapterⅡand III have described the setup of temperature-dependent in situ XAFS and XRD measurement, and an appropriate in situ time—resolved XAFS method for the study on the chemical solution. For the temperature-dependent in situ XAFS and XRD instrument, the temperature variation is in the range of 10-1200 K. This setup can be worked in the vacuum or under the different gas atmosphere, which provide a well environment for the study of temperature induced phase transition for the metastable materials. For the in situ time-resolved XAFS method, a recirculation system where the reactional solution is continuously circulated along the microtubes by peristaltic pump is used in this equipment. Further, combined with quick XAFS method which reduces the data-collection time to s level, improving the time resolution, the in situ probe on the nanocrystal formation in the chemical solution can be achieved.2. Clarification of metal-insulator transition of VO2 by temperature-dependent in situ XAFS technologyVanadium dioxide (VO2) has become an important smart energy-saving material material owing to its special electronic and optical properties behaved during its MIT. The MIT mechanism of VO2, however, is a hot topic in condensed state physics. In order to address this issue, the work of ChapterⅡmainly presents the temperature-dependent in situ XAFS inverstigation on the coorelation of atomic and electronic structures during the VO2 MIT triggerd by temperature. A clear figure of atomic and electronic structural evolutions near the VO2 transition temperature at 341 K is established. The author found that the V-V bond distance and structural distortion play an important role on the evolution of electronic structures, and the metallization process in VO2 is developed from the intermediate state with monoclinic feature. From the atomic level, the author proposed a correlative mechanism of structurally driven transition (Peierls) and the electron correlation (Mott) for the MIT of VO2, clarifying the longstanding controversy for the MIT mechanism of VO2. This research will provide solid theoretic and experimental foundation for both the understanding of the interplay of lattice and electron in strong-coorelated system and the application of novel smart window materials. These results have been published on Physical Review Letters 105,226405 (2010).3. In situ time-resolved XAFS study on the initial kinetic nucleation of gold nanocrystalsTo achieve the controlled synthesis of various new-type nanomaterials with desired properties, an important issue in nano-material field is the understanding of the nucleation and growth process of nanomaterials for a long time. Although a large number of research works have been launched, the information of the initial nucleation process still remains remains obscure. Therefore, the work of ChapterⅢmainly presents the design of the in situ time-resolved XAFS setup, and its studies on the formation kinetics of Au nancrystals in the chemical solution. The author proposed a novel nucleation mechanism that the initial nucleation undergo the formation of intermediate'Cl3-Au-AuCl3-'dimer and the subsequent higher complexes'AunCln+x'. Also, a kinetic three-step mechanism involving the initial nucleation, slow growth, and eventual coalescence for the Au nancrystals formation is presented. This research will be helpful for mediating the nucleation and growth process in the synthesis of nanocrystals. These results have been published on Journal of the American Chemical Society 132,7696 (2010).4. The local structure and magnetic study on the DMS nano-materialsDMS nanowires are the promising candidate in the spintronic devices due to their advantage of well single-crystal structure and isotropy, as well as the high effective in the spin-polarized carrier injection. The work of ChapterⅣmainly present the XAFS combined with the first principle calculations studies on the atomic, electronic structures and magnetism for the Co-doped ZnO based nano-wires, and-rods with diverse morphology. The author found that the coeffect of the homogeneity of Co dopants and the size effect are the main reasons for the enhanced ferromagnetism in the Zn0.98Co0.02O nanowire. Also, in Ag-ZnCoO hybrid nanostructure, interstitial Ag atoms which forms donor impurity band, plays an important role in mediating the high-temperature ferromagnetism. These researches clarify the basic relation of the atomic distribution, electronic structure and the magnetic property in ZnCoO DMS nanomaterials. These results have been published on The Journal of Physical Chemistry C113,3581 (2009); and 113,14114(2009).