Controllable Synthesis And Properties Research Of Small Size Au Nanocages Supported On Reduced Graphene Oxide

Gold nanoparticles fascinate tremendous interests due to its unique photophysical properties and good biocompatibility for various biomedical applications. Among them, the unique morphology, such as hollow interiors and porous walls, providing gold nanocages with excellent properties. Recent years, researchers have been focused on finding supporters for multivariate composite to enrich the application realm of gold nanoparticles. Graphene, a two-dimensional monolayer of carbon atoms packed into a dense, honeycomb crystal structure, has also shown fascinating properties and holds the promise for future carbon-based device architectures. Hence the integration of the above two types of nanomaterials as new generation hybrid materials for obtaining composite materials with better performance and synergetic enhanced properties has attracted widespread concern in recent years. In our investigation, there is still a certain improvement space for the synthesis of Au nanocages/Graphene hybrids. Firstly, typical methods for the Au nanocages preparation usually employed Ag nanocubes or quasispherical Ag nanoparticles as sacrificial templates. Yet the side lengths or the diameters of the Ag nanoparticles prepared via typical methods mostly distributed in dozens of nanometers, which were quite difficult to regulate effectively. That is to say, the sacrificing templates were relatively large. Consequently, the as-prepared Au nanocages were relatively large, the interior diameters of which also distributed in tens of nanometers, corresponding to the Ag nanoparticles. Thus, there is still a certain improvement space for the preparation and property studies of small size Au nanocages/Graphene hybrids. Secondly, the existing synthetic technology for Au nanocages/Graphene hybrids preferred the assembly methods, the reaction condition of which were complicated and harsh. Besides, the processes involved the templates fabrication, surfaces modification and coupling assemblies. To obtain the final products, additional centrifugations and re-dispersions were inevitable and reduced the products yield. Moreover, the surfaces of the as-prepared Au nanocages modificated by surfactants were not ‘clean’ enough, which was not conducive to improving the activities of the composite materials in catalysis, SERS or other fields. Therefore, the Au nanocages/Graphene hybrids with ‘clean’ surfaces have certain advantages and important scientific value in practice. Based on the above analysis, the main contents of this paper are as follows:Firstly, an in-situ growth method, through which silver ions(Ag+) and graphene oxide(GO) were simultaneously reduced into Ag nanoparticles(Ag NPS) and reduced graphene oxide(RGO), has been described. Meanwhile, the deposition of Ag nanoparticles onto RGO took place. Then, the galvanic replacement process for fabricating small size Au nanocages/RGO hybrids using the as-prepared Ag NPs/RGO hybrids as sacrificial templates has been studied. All experiments were simple and rapid and the process could be accomplished within 1 minute. UV-vis, TEM, HRTEM, FTIR, XRD, XPS and other characterization techniques were carried out to confirm the formation of Au nanocages and RGO structure. The morphological evolution presented during the reaction demonstrated the transformation process. Thus, our work undoubtedly provided direct and unambiguous experimental evidence toward understanding the galvanic replacement mechanism. The replacement reaction on Ag nanoparticles generated a complete shell at the very beginning of the reaction. Then the Ag nanoshells evolved into hollow and porous octahedral Au-Ag nanocages. By varying the molar ratio of HAu Cl4 to Ag, we were able to adjust the pore size and the density of pores. In addition, the effects of several experimental conditions during the galvanic replacement reaction were discussed. We found the optimal conditions for the synthesis of Au nanocages/RGO was when the p H value was 2.16 and the molar ratio of L-aa /Na3 Cit was 4/1. Explanation for the phenomenon was that high p H value and low molar ration of L-aa /Na3 Cit would decrease the activity of HAu Cl4 and the reducing performance of the reductant respectively.Secondly, we have demonstrated the use of the as-prepared Au nanocages/RGO hybrids as catalysts for the model reduction of p-nitroaniline(4-NA). We have also obtained the relationship between the catalytic activity and the particle morphology: the more open the particle morphology was and the thinner the wall was, the higher the catalytic activity would be. When the molar ratio of HAu Cl4/Ag was 0.35, the inner diameter of the as-prepared Au nanocage was about 11.3 nm. The highest catalytic activity could be achieved then and the reaction rate constant(k) of which was-1.00737 min-1. Moreover, controlled experiments, synthesising the relatively large Au nanocages(d=40 nm) supported on RGO with the same concentration through assembly methods in accordance with literature were also conducted for contrast. We found that the Au nanocages/RGO hybrids prepared in our work had higher catalytic activity and better stabilty. The ‘clean’ surface and the small size of a Au nanocage supported on RGO made it a much better catalyst than the assembled one.