| Anisotropic gold nanoparticles have been widely used in optics,catalysis,sensing,biomedicine and other fields because of their unique physical and chemical properties.Their properties and applications are closely related to their size,morphology,composition and structure.Meanwhile,the final morphology of Au NPs is affected by many factors,such as capping agents,reducing agents,stabilizers/surfactants,reaction time and solution temperature,etc.The shape-oriented substances（organic capping agents and iodide）play a key role in the morphology and structure of Au NPs.Based on the above,the morphology and structure of Au NPs synthesized with different capping agents（hexadecyltrimethylammonium chloride（CTAC）,hexadecyltrimethylammonium bromide（CTAB）,polyvinyl pyrrolidone（PVP））and with or without potassium iodide（KI）were characterized by transmission electron microscopy（TEM）,and in combination with UV-Vis spectrum and the time change in the color of reaction solution,the effect of capping agents and halogen on the morphology and structure of anisotropic Au NPs synthesized by one-pot seedless approach was studied.In addition,other experimental parameters（reaction temperature,reaction time,and ascorbic acid（AA）concentration）were studied to reveal their influences on morphology and size of the synthetic anisotropic Au NPs.The main research results are summarized as follows.Firstly,the morphology and structure of gold nanoparticles synthesized with KI and without KI in the reaction system using different cationic surfactants（CTAC and CTAB）as capping agents were compared by TEM and UV-Vis spectra.In CTAC-contained solution growths,the introduction of KI led to the morphological change from thermodynamically-favorable truncated octahedra（TO）to kinetically-controlled triangular nanoprisms.The size of Au nanoprisms decreased with the extension of reaction time to 1 h,and when the reaction time was extended to 12 h,Au nanoprisms with high surface energy tended to transform into spherical Au NPs,resulting in a decrease in the yield of Au nanoprisms.Meanwhile,the size of Au nanoprisms could be adjusted by changing the concentration of AA.For CTAB-contained solution growths,the decrease of both population density and aspect ratios of Au nanorods were evidently seen after the addition of KI,accompanied with the emergence of gold nanoparticles with other morphologies.Different from the reaction system with CTAC as capping agents,due to the nature of CTAB,the prolongation of reaction time was beneficial to the synthesis of anisotropic Au NPs and the high temperature could promote the reaction and contribute to the synthesis of regular Au NPs.Secondly,in polymer PVP-contained solution growths without iodide ions（I-）,PVP served a dual purpose.On the one hand,PVP prefers to adsorb on the Au{111}faces,thus acting as a stabilizer and promoting the synthesis of a small amount of Au nanoprisms;on the other hand,polymer PVP acted as the reducing agent owing to the presence of the hydroxyl end groups,which promoted rapid reduction and crystallization during the reaction.As a result,the thermodynamically less favorable nanostructures were formed because of the lack of structural relaxation during the rapid growth process.However,with the addition of I-,we found that the thermodynamically less stable irregular Au NPs with bumpy surface were largely replaced with the stable（quasi-）spherical structures,which was attributed to the occurrence of oxidative etching on low-coordination sites by the tri-iodide ions（I3-）formed by the combination of I-and oxygen.Meanwhile,the study showed that changing the experimental parameter（increasing the reaction time）was conducive to the synthesis of large-size Au NPs.The study thus manifests a synergistic effect of shape-direction agents on the growth pathways and shape formation of Au NPs.Combined role of surfactants/polymers and iodide additives promotes or restrains the formation of the anisotropic gold nanostructures.Meanwhile,the importance of experimental conditions for the controlled synthesis of anisotropic Au NPs is demonstrated. |
