In-Situ TEM Observation of Au Nanoparticles Growth in Liquid
|關鍵字:||金奈米粒子;臨場穿透式電子顯微鏡;液態試片;四氯金酸;動態成長;成核;Au nanoparticle;in-situ TEM;liquid cell;HAuCl4;dynamic growth;nucleation|
Gold nanoparticles(NPs) have been widely used in catalysis, sensor, electronics and biological application due to their unique optical, electrical, and biocompatible properties. These properties mainly related to the shape and size of Au nanoparticles. If we can control the properties during Au NPs synthesis, we can design and apply to many functional nanodevices. However, the lack of information that how NPs growing and reaction process are remain unclear. Therefore, the technology of liquid cells was used for in-situ TEM observation which can provide direct evident and extend the study of reaction kinetics for modifying the morphology of NPs. In this work, we used salt reduction method that preparing Au NPs by reducing HAuCl4 with citrate acid owing to its low cost. The solution was sealed in liquid cell and then the dynamic-growth process of Au NPs can be observed via in-situ TEM. The drive force for synthesis process resulted from the heating effect of electron beam. The synthesis process of Au NPs can be classified into nucleation and growth. The nucleation of Au NPs mainly followed LaMer model, the monomers of Au3+would reduce to Au0. Moreover, Au NPs grew as a result of either single nanoparticle growth which tended to form twin structures or aggregation of nanoparticles to minimize its free energy. In addition to the orientation of twin planes, the thickness of liquid space was also one of the parameters to affect the NPs growth. The thicker solution layer tended to form multi-twinned nanostructures; on the contrary, nanoplates were easy to form in the thinner solution layer. For aggregation, single crystalline NP generated when the lattice arrangement in two NPs almost in the same direction; on the other hand, if the direction is different, it would form polycrystalline NP. Furthermore, Au NPs would like to aggregate at the vertex sites where the surface atoms have low coordination number for stabilizing structures. However, the solution irradiated by electron beam would cause the formation of hydroxyl radical which resulted in the production of hydrogen bubble to hinder the in-situ observation. Although the bubble limited the observation time of liquid cell, we still successfully observed the nucleation and growth process. We revealed the synthesis process of Au NPs in thermodynamic and kinetic viewpoints and provided direct evident of the synthesis process associated with theoretical mechanism. These experimental results sheds light on the salt reduction method.