Size Control Of Spherical Silver Nanoparticles In Aqueous Solutions

Noble metal nanoparticles are intensively studied in the past twenty years because they exhibit unique optical, electrical, catalytic and magnetic properties that are consequence of nanometer dimensions. Silver nanoparticles is an especially attractive metal due to its extremely high electrical conductivity in the bulk and the ability to efficiently interact with light. Spherical silver particles play an important role in many applications, such as SERS, single-molecule labeling and recognition, and many others. The widely applied preparations of spherical silver nanoparticles in aqueous solutions are the reduction of silver nitrate with borohydride or sodium citrate. Although the particles through the reduction of borohydride were small (less than 10 nm) and the one through citrate were large (larger than 100 nm), the distribution were poor and the size was not conthollable by either. The use of borohydride, a strong reductant, the reduction ability was not controlled. As citrate is a weaker reductant, the reduction rate is slower and larger particles can be prepared, but the size distribution was far from narrow probably due to the time of nucleation too long and the size was not changed much even the reduction potential was adjusted by change of pH values. However, the size of spherical silver nanoparticles between 10 nm to 100 nm could be got by the seed growth method. Although the size could be adjusted by changing the ratio between silver precursor and seed, the process was fussy and the secondary nucleation was difficult to avoid as the ratio was large. To synthesize the particles between 10 nm to 100 nm, we selected ascorbic acid as reductant, the reduction ability between borohydride and citrate. As the number and the size of the nuclei generated in the solution depend on the reductant, the number of the nuclei in the system of ascorbic acid reduction were between the system of borohydride and citrate, the size could be got according to LaMer model.In this work, we found that temperature of reaction was important and the size distribution of particles of reduction by ascorbic acid at 30℃was narrow. So in this work, the temperature of reduction by ascorbic acid was 30℃and the concentration of AgNO3 was 1 mM and the citrate was 3 mM. Fist,we synthesized the silver particles by changing the ratio of L-AA/AgNO3 and the temporal evolution of Ag+ ion concentration for the reaction varied little when the ratio lager than 0.5, the stoichiometric ratio. In our previous work, we demonstrated a new synthetic route for the synthesis of nearly monodisperse gold nanocrystals with size-control by simly varying the solution pH with fixed concentrations of HAuCl4 and citrate. This inspired us to carry out the reduction of silver precursor with ascorbic acid under different pH values. In our studies, a H3Ct or NaOH solution was added to adjust the reaction solution to pH values from 6.0, 7.0, 8.0, 9.0, 10.0, 10.5 and the temporal evolution of monomer concentration of above pH values showed the the reaction rate accelerated as pH values elevated that was correspond to the reduction potential for C6H6O6/C6O8H6 systems raising from the Nernst equation. The average sizes and standard deviations of the particles were 72 nm (±22%), 63 nm (±15%), 56 nm (±20%), 50nm (±19%), 40nm (±17%), 30nm (±19%) as pH gradually increased from 6.0 to 10.5 which were consistent to the blue shift of surface plasmon peak. The TEM images also indicated the shape of primary particles were not spherical and irregular. To further improve the particles, the factors of temperature and Na3Ct and L-AA and hydrogen ion were studied and the result showed the higer temperate is necessary to make the primary particles improve further. However, the placement of the solution for 24 h would cause the size distribution broader,as the intraparticle ripening was dominant in the first 2 h and then Ostwald ripening was regnant which caused the size distribution broadens and should be avoided for the formation of relatively monodisperse nanoparticles. Above all, the optimum condition to improve the particles was the primary particles aging for 2 h at 100℃and the TEM images showed the final particles are spherical and mostly monodisperse.As the ratio of L-AA/AgNO3 less than 0.5, the stoichiometric ratio, the rate of reduction of different ratio of L-AA/AgNO3 changed a lot. So in this wok, the concentration of AgNO3 was 1 mM and the citrate was 3 mM, the ratio of L-AA/AgNO3 was adjust to 0:1,0.05:1,0.1:1,0.2:1,0.3:1,0.5:1, respectively, the reaction were proceeded in the water bath of 30℃for 10 min and then placed to the water bath of 70℃. The pH value of above solution was 10.0. The temporal evolution of Ag+ ion concentration for the reaction at 30℃showed the rates of reduction increase as the elevation of ratio of L-AA/AgNO3, so the nuclei would increase and the size of final particles would decrease. The TEM images of the final particles showed the size decrease as the increase of ratio of L-AA/AgNO3 except the ratio of 0:1 as the size distribution of this particles was very broad probably due to poor balance between nucleation and growth in the pure reduction by citrate. When the solution were placed to the water bath of 70℃, the extinction spectra showed the intraparticle ripening exist in all above except the ratio of 0:1 and the final result was almost particles of all the solution above were spherical except the one of the ratio of 0:1 was spherical and club-shaped.