Page 41 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Multiphase Operational Microplasma Setup and the Involved Instrumentation Techniques
In addition to direct imaging and OES, another important technique for in-situ characterization of the precursor dissociation process is the aerosol measurement for determining the as-grown particle size and size distributions on line. It usually consists of a differential mobility analyzer (DMA) and an ultrafine condensation particle counter (CPC). The aerosol measurement is based on particle electrical mobility in the carrying gas and corresponds to the projected area. Briefly, particles firstly flow into a bipolar charger and expose to ionizing radiation from a sealed 85Kr β-source, where they are positively or negatively charged. The particle electrical mobility is a function of its projected area. Once they are directed into the DMA, particles of different sizes have distinct trajectories. Therefore, by adjusting the applied voltage, nanoparticles of a specific narrow electrical mobility are transmitted and introduced into a CPC, where the concentration of classified nanoparticles is measured and counted.13–15 The combined operation of the DMA and CPC allows monitoring the particle size and size distributions on line with speed and sensitivity, which enables us to get the direct information during the microplasma dissociation process. Due to the lack of DMA-CPC instrument, the investigation of the as-grown particle size and size distributions is out of scope in this thesis. Detailed information can be referred to the research of Mohan et al.16
2.2.2 Ex-situ Characterization of the Generated Nanoparticles
As motioned above, the product properties depend critically on the microstructure, i.e. the arrangement of the atoms (the atomic structure), the chemical composition, and the size of a solid in one, two or three dimensions.17 Here the ex-situ characterization of the generated nanoparticles are discussed in three general aspects which have essential influences on their properties, including particle size, size distributions and morphology; composition; crystalline structure. It is also important to know that for a specific instrument, it usually provides a great deal of information on different aspects. For example, transmission electron microscope (TEM) equipped with certain detectors can be utilized to characterize the particle size, morphology, crystalline structures and even chemical compositions.
1) Particle size, size distributions and morphology
In addition to the aerosol measurement for on line monitoring the as-grown nanoparticle size and size distributions, TEM is the mostly used instrument for determining the particle size and size distributions (Figure 2.2). For TEM characterization an electron beam is transmitted through an ultrathin specimen and interacts with it as it passes through it. An image is formed from the electrons transmitted through the specimen. By magnifying and focusing the image using electron lenses and transmitting to imaging devices (e.g. fluorescent screen) or a CCD camera, the particle size and shapes are directly observed, and the particle size distribution can be derived by randomly counting enough particle numbers. Additionally, owing to the short electron wavelengths, it enables the instrument to capture ultra-fine details at high magnifications, even as small as a single column of atoms. In this manner the lattice fringes and lattice space of metallic nanoparticle can be observed and calculated, which in turn, helps to confirm their crystalline structure. Therefore, TEM characterization can be regarded as one of the most important techniques for characterizing nanoparticles, which is involved in each chapter of this thesis.
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