Page 45 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Multiphase Operational Microplasma Setup and the Involved Instrumentation Techniques
3) Crystalline structure
For specific nanoparticles under a given condition, a particular structure (amorphous, body- centered cubic (bcc), face centered cubic (fcc) or hexagonal close packed (hcp)) is determined by the chemical composition, the number of atoms and the chemical interaction between atoms, which correspond to the optimum stable configurations.32 It is well-known that the majority of metals adopt a crystalline arrangement, since greatest stabilization is achieved in this way by anchoring interactions between neighboring atoms to minimize the total internal energy.33
The determination or confirmation of a structure can be performed by a series of analytical techniques, among them X-ray diffractometer (XRD) is the most popular instrumentation for metallic nanoparticles analysis. In XRD electrons are generated from a heated filament in a cathode ray tube, which are accelerated toward a target material (Cu, Fe, Mo, Cr) with the external high voltages. When electrons have sufficient energy to dislodge inner shell electrons of the target material, characteristic X-ray spectra are generated. During the measurement the X-rays are directed to the sample. Meanwhile, the sample and the detector are rotated at a certain rate, with the intensity of the reflected X-rays being recorded. Once the geometry of the incident X-rays impinging the sample satisfies the Bragg equation, constructive interference occurs and generates an intensive peak.
The crystalline structure of the metallic nanoparticles can be also studied by the selected area electron diffraction (SAED) technique, which is mostly performed inside a transmission electron microscope. During the measurement the crystalline specimen is subjected to a high- energy electron beam (100-400 keV). Electrons in this case are treated as wave-like, rather than particle-like. Due to the short wavelength of the high-energy electrons (~10-3 nm) as well as the relatively larger atom spacing, the atoms act as a diffraction grating to the electrons. When the electrons pass through atoms, some will be scattered to particular angles to form a spot on the screen, determined by the crystal structure of the sample, while others continue to pass through without deflection. As a result, a diffraction pattern is formed by a series of spots, and each spot corresponds to a satisfied diffraction condition of the sample's crystal structure.34 Compared to XRD, SAED has higher intensity of diffracted X-rays, making them particularly suitable for low atomic number materials. It also has the ability to examine areas as small as several hundred nanometers in size instead of several centimeters. However, the operation of SAED is more complicated, expensive and time-consuming.
Sometimes
In order to apply nanoparticles for specific applications, it is essential to have a comprehensive understanding of their physicochemical properties. The rapid development of 33
both the transmitted and the scattered beams are used in the transmission electron
microscopy to create a high-resolution image (HRTEM). In this case the electrons are
modulated to wave at very low angles and interfere with itself during propagation through
the objective lens. All electrons are combined at a point in the image plane to form an
interference image, which can be used for the identification of the crystal structures and
lattice imperfections of nanoparticles. In this thesis HRTEM, SEAD and XRD are used
commonly used as commentary techniques to confirm the crystalline structures of the
obtained products.