Page 14 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin
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Chapter 1
1.1 Conventional Nanomaterials Synthesis
Due to the low dimensionality and large surface areas, nanomaterials exhibit unique properties compared to bulk metals, making them versatile and attractive for applications including catalysis,1 sensing,2 drug delivery,3 imaging,4 data/energy storage5,6 and bio- medicine.7 It is well-known that product properties are closely related to their composition, size, crystalline phase and microstructures. One typical example is the significant difference of the properties between the diamond and graphite, which can be attributed to the difference of their atomic structures. The fluorescence intensity and light absorption of Au nanoparticles are size dependent, with the surface plasmon resonance being red shifted from 517 nm to 532 nm when particle size increases from 10 nm to 50 nm.8 Another example is Ni nanoparticles utilized as catalysts for the nucleation and growth of carbon nanotubes (CNTs). It is reported that the formation rate of CNTs depends inversely on the Ni nanoparticles size, and no CNTs are formed by particles with diameters larger than 7 nm.9,10 In addition to particle size, crystalline facets also proved to affect the photoexcitation performance of Ag nanoparticles, in which oxygen is shown to exist in molecular form on the (111) crystalline surface but is dissociated into atomic form on both (100) and (110) surfaces.11
In the past few decades, many efforts have been devoted to fabricate nanomaterials with controllable size, shape and structures, aiming to understand how those parameters affect the properties of nanoparticles, and ultimately, to tailor them for specific applications. On the other hand, such extensive studies also resulted in a variety of novel or already well- established methods for nanomaterial synthesis. Generally they can be subdivided into three categories: The first one consists of physical approaches such as evaporation/condensation,12 sputtering,13 milling14 and laser ablation.15 The second one includes chemical methods such as colloidal route,16 sol-gel,17 microemulsion18 and solvothermal route.19 The last one is biological-assisted nanomaterials synthesis, like microorganism assisted,20 bio-templated assisted21 and plant extracts assisted methods.22
However, physical approaches always implement time/energy consuming procedures and take place in in an inert atmosphere, requiring substantial equipment and operation costs. By contrast, chemical methods are relatively simple, low-temperature operation and easily to be controlled. Fine quality products can be obtained when the starting materials are properly chosen and processes are well controlled. One inevitable problem is the introduction or generation of byproducts (e.g. surfactants, reductants and stabilizers) which requires subsequent purification steps after the synthesis. In some cases post treatments like calcination or annealing are required to improve products’ crystallinity. Moreover, the majority of present chemical techniques are lab scale batch processes, while the upscale towards industrial manufacturing with retaining product quality represents very challenging task 23. As to the biological-assisted nanomaterials synthesis, due to the complex process and long reactions time, it is difficult to control and adjust the products properties. Meanwhile, for scaled-up behavior, the obtained products have low uniformity, since the source of the plant/microorganism is independent and has considerable influence on the characteristics of the nanoparticles.24 Therefore, the controllable synthesis of nanomaterials with desired properties in a simple, environmental friendly and efficient way is still a challenge.
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