Fabrication And Research Of Silver Nanobowls, Nanocages, And Hollow Porous Nanosheets

It is well known that the properties of gold and silver nanostructures are strongly dependent on their size, morphology, and composition. Very recently, the synthesis of cavity-containing metal nanoparticles with reduced symmetry, such as nanobowls, nanocups and hollow nanospheres has attracted great attention, as it could have specific applications in optics, magnetics, catalysts, biosensors, microchip reactors. Therefore it's important to do research on the special appearance, specific size of gold and silver nanoparticles.This paper reviews the progress of research on metallic silver nanostructure with hollow cavity at home and abroad. The nanobowl and nanocage obtained by template recently are often polycrystalline or amorphous, and the method is limited. This paper presents a simple chemical deposition method without any templates. In our work, we have developed a simple chemical deposition method for the preparation of silver nanobowl, nanocage and hollow porousnanoplate through controlling the reaction conditions and the morphology of precursor. Their morphology and structure were characterized. The mechanism and influence factors of the formation of these structures were discussed. This may provides a new ideas for the new preparation and application of reduced symmetric structure. In this paper, the content and conclusions are as follows:Silver nanobowls with the diameter of 80-150 nm have been successfully synthesized in this paper at 0℃through chemical deposition method using silver nitrate as precursor, formaldehyde as reductant, and PVP as dispersing agent. The changes of morphology at different stages were characterized, and the effect of the sodium hydroxide concentration, reaction temperature, the reductant concentration, PVP content on the the final morphology were studied. Results show that, the deposition and growth of Ag reduced by formaldehyde on the surface of silver oxide and the formation of bubbles during the reactions are the key for the formation of bowllike structure. Moreover, the size of bowl particles can tuned by the concentration of sodium hydroxide. Bowllike structures were also obtained without PVP, which indicates that PVP only plays the main role of dispersant in this reaction. The UV/vis absorption spectra showed the maximum absorption peaks at 478 nm. Compared to nanoparticles and nanowires, it has red shift which show these structures has larger absorbtion region.Based on the bowlike structure, we have synthesis 100nm cagelike structure with internal hollow and some holes on the surface, using sodium borohydride as reducing agent. The ring-shaped, frame, and irregular particles are present following the change of reaction conditions, such as temperature and the concentration of sodium hydroxide, etc. Through the mechanism analysis, these structures obtained are attributed to the collapse of the framework,the enlarge and merging of the holes on the surface during the formation of cagelike structure.On the basis of the above results, the processes of bowlike nanoparticles can be possibly summarized:(i) formation of Ag2O nanocrystallites; (ii) spherical aggregation of primary Ag2O crystallites; (iii) reductive conversion of Ag2O to Ag; (iv) deposition of Ag atoms on Ag2O and formation of hollow structures with pinholes on the surface; (v) growth of the main pore on the surface caused by the releasing of bubbles and formation of the bowllike morphology. The smooth surface is due to the Ostwald ripening process as the reaction time proceeded. Compared to the bowllike structures, cagelike structures are different in the fifth step. The difference in the generating and releasing rate of bubbles lead to the diffence on the morphology of the particles.Based on the formation mechanism of bowlike and cagelike structures, we have obtained hollow and porous silver nanosheets with a side length of 1 micrometer and thickness less than 100 nm by turning the composition of the precursor. Through the morphology and structure change at each step in the formation, we concluded the formation mechanism of these stuctures.