Preparation And Catalytic Performances Of Gold Nanoparticles With Different Particle Sizes Or Morphologies

This paper explores the application of hydroxylamine hydrochloride,sodium-borohydride as reducing agents in preparation of spherical GNPs. In asolution system composed of cetyltrimethyl-ammonium bromide (CTAB) and-Cyclodextrin, hydroxylamine hydrochloride was too weak to reduce chloroauricacid completely to GNPs. It was found that, the GNPs prepared by the combinationuse of hydroxylamine hydrochloride followed by sodium borohydride in sequence asreducing agents were of smallest particle size of4nm in mean diameter and of bettersize distribution, and therefore showed the best catalytic performance in the reductionof p-nitrophenol into p-aminophenol.In order to explore the relationship between particle size and catalyticperformances, GNPs with sizes in the range of2~57nm were prepared following amultiple-step seeding-growth approach. Seed was prepared witht sodium borohydrideas the reductant and sodium citrate as stabilizer, while in the growth process; ascorbicacid was applied as a weak reducing agent and CTAB as the surfactant. The effects ofthe surfactant concentration, aging time of seeds, and the ratio of gold salts to seedson particle sizes were explored in the seeding-growth process. GNPs with differentsizes were prepared with CTAB concentration of0.08M, mole ratio of gold salts toseeds kept at3:2in the first growth step, and particle sizes increased with theincreasing of growth steps. Except seeds, an increase in sizes of the GNPs led to adecrease in catalytic activity under this condition.One-step growth method was also applied to prepare GNPs and meanwhilesilver ion or halide ions were introduced to study their effects on morphologies andsizes of GNPs. CTAB concentrations varied from10-3M to4×10-1M. In thepresence of silver ions, many different shapes of GNPs appeared such as spherical,tree-like branched, triangular, quadrilateral, triangular pyramid. Spherical GNPs withaverage diameter of15nm, showed relatively good catalytic activities. In thepresence of iodide ions, at low concentration of0.26μM, hexagonal and triangular GNPs both exposed (111) crystal facet were prepared, and spherical GNPs exposed(110) crystal facet were also observed. The particles sizes increased with theincreasing of CTAB concentration, which varied in a large range of30~650nm. Adecrease in catalytic activity was observed when the mean diameter of GNPs waslonger than90nm. In the presence of chloride ions, spherical, triangular, rod-likeGNPs with small sizes were also prepared with different concentration of CTAB.To prepare composite nano-materials with special new features, A amphiphilicblock copolymer poly (N-isopropylacrylamide)-Maleic anhydride-block-poly (4-vinylpridine)(PNIPAM-MAh-b-P4VP) was synthesized by reversible addition andfragmentation chain transfer. When pH value was higher than6.5and temperatuelower than20℃, the copolymer associated into spherical core-shell micells with thecore formed by the P4VP block and the shell formed by the PNIPAM block. In amixture of ethanol and water in the presence of sodium citrate under alkalineconditions, with the mole ratio of polymers to gold salts at20:1, hexagonal andtriangular GNPs with sizes of10~17nm were prepared. After being reduced for5hours and then aged for12hours, the prepared GNPs showed relatively good catalyticactivity.