Preparation And Characterization Of Nanosized Silver Composite Materials And Their Related Properties

Nanotechnology is currently witnessing impressive advances in aspects such as synthesis of nanoparticles, understanding their fundamental physical and chemical properties, and organization of nanoscale matter using noncovalent interactions. Nanosized metal particles possess unique electronic, optical, and catalytic properties that are obviously different from bulk macrocrystallites. In particular, fabrication of metal-coated particles is currently an attractive area of investigation because of their applications in the fields of surface-enhanced Raman scattering, catalysis, or potential uses in optoelectronic device, in ultrasensitive chemical and biological sensor etc.Owing to their excellent properties such as high thermal and electric, high surface activity and catalytic capability, nanosized silver composites have been attracted much attention in recent years. Applications of these composite materials in the electric, catalytic, biological and chemical fields have been reported by many researchers. However, the morphological control in the preparation process of such composite materials is very important and still remains a challenge in this field.In this thesis a series of silver composites with different materials, such as vinyl polymers, nature starch and attapulgite, were prepared. The structure-property relations of the silver composites were investigated. The main content of the thesis is as follows:(1) A strategy for preparation of Ag/PMMA composite nanoparticles with core-shell structure were proposed. The approach has two essential features: (a) "one-pot" route for preparation of nanocomposite in situ by chemical reduction and (b) the nanocomposite is successfully synthesized without typical initiation. Compared with previous methods of preparing nanocomposite, the route presented here is simpler and more effective, which may stimulate technological interests. The ESR results show that the polymerization of MMA follows a free radical mechanism. The silver nanoparticle can stabilize the free radical and prolong its life. TEM and AFM observations show that the nanoparticles with core-shell structure. The results of DLS measurements show that the preparation conditions, such as AgNO₃ concentrations, emulsifier types and AgNO₃/CH₃CH₂SH molar ratios, can influence the size of the particles. Furthermore, Ag/PS core-shell composite nanoparticles with more uniform particle size were prepared.(2) Using chemically modified natural starch particles as core material, the starch/silver core-shell composites were prepared by in-situ chemical reduction method. The chemical composition of the composite was characterized by FT-IR and XPS. The crystal structure of the composite was analyzed by XRD and POM. The morphology of the composite was observed by SEM and TEM.(3) A new method for preparation of hollow silver spheres was established. The strategy for fabrication of starch/silver core-shell composite and hollow silver spheres includes (1) the preparation of mercapto modified starch; (2) the deposition of silver nanoparticle on the starch core by chemical reduction; (3) Removal of the starch core withα- amylase. To the best of our knowledge, no previous studies had explored the fabrication of hollow silver spheres using this biochemical method. The morphology of the starch/silver core-shell composite particle was observed by TEM and AFM. The size of the composite particles is about 50-160nm in diameter with the silver shell thickness of ca.10nm. TEM images of sample after treating withα-amylase showed that the hollow spheres possess a completely shell.(4) Using attapulgite, a natural fibrous hydrous magnesium silicate mineral, as rigid support, the silver/attapulgite nanocomposites were prepared. In the preparation of the nanocomposites, attapulgite was first modified with 3-mercaptopropyltrimethoxysilane or mercaptoacetic acid, so that the mercapto groups were introduced onto the surface of attapulgite, which may be favorable to combine with silver ions. The deposition of silver nanoparticles on the surface of attapulgite took place when silver ions were reduced. TEM and AFM observations showed that the silver nanoparticles were well deposited on the surface of the attapulgite. The nanocomposites were found to be active for catalyzing the reduction reaction of 4-nitrophenol to the corresponding 4-aminophenol.(5) The electric conductivity of polymers filled with the prepared silver composites were investigated. The influence of morphological structure on the electric conductivity of filled polymers was discussed. It was found that polymers filled with silver/attapulgite nanocomposite showed much improved electrical conductivity than that of polymers filled with other Ag composites.