Structure determination of submicron-sized porous materials by EM and XRD

Structure determination of submicron-sized porous materials is always a great challenge for widening their industrial application. There are a few reasons for this challenge, such as 1) their big unit cell dimensions cause serious peak overlapping in powder X-ray diffraction (PXRD); 2) typical disordered guest molecules in the big portion of pores lower the resolution; 3) their relatively low stability prevents an easy application of high resolution transmission electron microscope image (HRTEM) and STEM techniques. PXRD and electron diffraction (ED) techniques are complimentary for the structure determination of such materials. The PXRD technique gives very accurate intensities and the major difficulty for the structure determination from PXRD is the peak overlapping while there is almost no peak overlapping in ED since the single-crystal-like ED patterns can be obtained from nano-sized crystals. For the ED techniques, the intensities suffer severely from dynamical effects which make it less reliable for the structure determination in many cases. The combination of them can overcome both shortcomings and solve most difficulties in the structure determination of submicron-sized porous materials. Here we will use a few examples to demonstrate how this combination facilitates the structure determination, such as ITQ-37, PKU-16, PKU-3, ITQ51.

CdSe/ZnO Core/Shell Semiconductor Nanocrystals: Synthesis and Characterization

CdSe/ZnO core/shell semiconductor nanocrystals which show high luminescence quantum yield have been synthesized through a simple routine without the use of any pyrophoric organometallic precursors. Transmission electron microscope image demonstrates the shape, monodispersity, average size, size distribution and core-shell structure of CdSe/ZnO nanocrystals. We use a combination of X-ray diffraction, UV-Vis absorption spectroscopy and photoluminescence to analyze the core/shell nanocrystals and determine their chemical composition, optical character and internal structure.

Improvement of the Electrochemical Properties in Nano-Sized Al2O3 and AlF3-Coated LiFePO4 Cathode Materials

The surface conditions of LiFePO4 powder were modified by adding AlF3 and Al2O3 by using the sol-gel process to improve its electrochemical properties. The surface of LiFePO4 powders was partially covered with nano-sized AlF3 and Al2O3, which is confirmed by using a transmission electron microscope image. The states of coated Al materials were examined by using X-ray photoelectron spectrometer results. The nano-sized AlF3- and Al2O3-coated LiFePO4 powders showed no difference in the bulk structure compared with the pristine one. However, the AlF3- and Al2O3-coating on LiFePO4 powders improved the overall electrochemical properties such as the discharge capacity, the cyclability, and the rate capability compared with those of a pure LiFePO4. Such enhancements were attributed to the presence of a stable AlF3 and Al2O3 layer which acts as an interfacial stabilizer on the surface of LiFePO4 powders.