Photo-induced Synthesis Of Silver Nanodecahedrons And Studies Of Their SERS Activities

Preparation of nanoparticles, especially the noble metal nanoparticles, with controllable size and morphology is one of the most attractive subjects in the nanoscience and materal science. Plenty of methods have been developed to obtain nanoparticles with special morphology and uniform size in the past few decades. Wet chemistry is widely accepted as the efficient route to produce nanoparticles with various morphologies, such as triangular plate, cube, discal plate, rod, wire, etc. It is well know that noble metal particles in nanoscale possess many unique properties in the field of optics, electronics, magnetics, and catalysis. Among them, silver and gold nanoparticles have attracted more attentions due to their surface plasmon resonance bands lie in the visible and infrared range. Taking silver nanoparticles for example, when the nanoparticle is irradiated by an incident electromagnetic field (an incident light), the free electrons on the surface of silver nanoparticle would be forced into a collective oscilation; when the frequence of the collective oscilation equals to that of incident electromagnetic field, the surface plasmon resonance (SPR) occurs. Under the SPR conditions, the local electromagnetic field (LEF) around the silver nanoparticles would be excited and enhanced. Thus, the light signal in the range of LEF, as well as the chemical reactions can be greatly promoted. As a result, the reaserches on the SPR of nanoparticles give birth to a new subject, the plasmonics. So far as we know, the SPR properties of the nanoparticles are strongly dependent upon their size, shape, component, and some other factors. The size and shape of nanoparticles are thought to be the key factors to determine their SPR properties. How to control the size and shape of nanoparticles efficiently has become a crucial problem. Photochemical method is a newly developed route to controllablly synthesize nanoparticles in recent years. Nanoparticles with different shape and size can be produced by adjusting the excitation wavelength, the components and their molar ratio in the growth system, thus the SPR properties can be manipulated. In this thesis, we focus on the silver nanoparticles. By using a blue LED as the incident light source, silver nanoparticles of decahedron in shape with uniform size have been prepared. The optical properties, SERS activies have been investigated. The growth mechanism of silver nanodecahedrons is discussed as well.Silver nanodecahedrons were prepared by using the 461-nm LED as the light source to irradiate the growth colloid, which contained silver seeds, PVP, and cirate ions, silver ions. The citrate ion in the growth colloid acted as both reducing agent and stabilization, and the PVP acted as the strong stabilization. The TEM technique was used to study the structure of the nanodecahedrons, and the UV-vis spectroscopy was used to investigate their optical properties. The TEM images showed that the nanodecahedrons were 35nm in edge length with narrow size distribution. The UV-vis spectrum showed the LSPR absorption band of the nanodecahedrons lie aound 500 nm, which is very close to the excitation wavelength (514.5 nm)for the Raman detection. Then, the prepared silver nanodecahedrons were self-assembled on the glass slide to use as the SERS substrate. The 4-ATP and 4-MPY were used as the probing molecules to detect the SERS activities of the nanodecahedrons. The results displayed that the nanodecahrons gave a good enhancement of 4-ATP and showed good SERS activities. Two possible factors were concerned to explain the SERS activities of nanodecahedrons. In one hand, the LSPR band of the nanodecahedron was centered at 497 nm, which was very close to the excitation wavelength. The incident laser line might excit the strong LEF around the surface of nanodecahedrons, and thus gave great enhancement of the Raman singals of the probing molecule. On the other hand, the nanodecahedron has seven convexes in the structure, which might be able to enlarge the excited Raman signals via the lighting rod effect.The UV-vis spctra were detected to monitor the growth process of the nanodecahedrons, and the medium samples were characterized by the TEM. The results indicated the growth mechanism of the nanodecahedrons. We divided the whole process into three parts. In part I, the present of PVP might help to form more multi-twinned crystals in the growth colloid during the early irradiation period. In part II, the formed multi-twinned crystals might grow into decahedron in shape, and the single crystals might grow into tetrahedron in shape with the plamon driven of 461-nm light. In part III, decahedrons in the system grew bigger, and the tetrahedrons might aggregate together to form decahedron in structure. Finally, the produced nanodecahedrons were uniform in size. During this growth process, the present of PVP in the system might be the key factor to produce silver nanodecahedron. Moreover, we also irradiated the growth colloid with a 521-nm light. The result showed that the prepared nanoparticles were triangular plate in shape. In this way, the nanodecahedrons might be derived from the growth of multi-twinned crystals, and might be also formed via the aggregation of tetrahedrons derived from single crystals. The growth of decahedron was fast under the irradiation of 461-nm light, and thus became the main produces. When irradiatied by the 521-nm light, the growth of decahedrons might be prevented due to the mismatching of plasmonic environment. So, the formation of decahedron was driven by the wavelength self-limiting effect. Moreover, we found that the light density might influence the thichness and the morphology of the nanodecahedrons. The higher of the light intensity the thicker the nanodecahedrons, and the better of the morphology they formed.