Preparation Of Silver Colloids With High SERS Activities And Study On Its Enhancement Mechanism

Since surface-enhanced Raman scattering (SERS) was first observed from pyridine adsorbed onto a roughened silver electrode by Fleischmann et al. Silver nanoparticles have become the most commonly used nanostructures for SERS owing to their large enhancement factors. The detection of a single molecule has been successfully achieved on colloidal nanoparticles. In our work, a series of silver colloids were prepared and their SERS activities were investigated using crystal violet (CV) as a probe molecular, meanwhile, the SERS enhancement mechanism was further explored. The dissertation is mainly focus on:(1) Six kinds of silver colloids were prepared by the reduction of silver nitrate with different amounts of sodium citrate. Adsorption of CV onto colloidal silver was researched. Adsorption of CV does not satisfy the Langmuir or Freundlich isotherm. Silver colloids are negative charged and very easily adsorb the positive CV through electrostatic attraction, which results in multimolecular layer adsorption. The enhancement factor of SERS was calculated by the use of CV adsorption isotherm. The results show that the amount of sodium citrate is an important influencing factor to size, C potential and SERS activities of silver colloids.(2) Based on the method of the density functional theory (DFT) calculation at the level of B3LYP, the Raman spectra and the structure of CV were investigated. CV is D3point group and the Raman vibration modes belong to A1or E irreducible representations. The intensities of the non-totally symmetric vibration modes E are selectively enhanced in SERS of CV, which is similar to SERS of CV on submicrometer rod-like ZnO, indicating a considerable contribution of charge transfer between CV and silver according to the Herzberg-Teller surface selection rule and a chemical enhancement in SERS.(3) Induction time of silver colloids primary nucleation was determined by UV-Vis spectra, Silver colloids were synthesized by a reaction of silver nitrate and sodium citrate. The effects of temperature and supersaturation (S) on induction time were considered. The results show that induction time decreases with increasing supersaturation and temperature. According to the classical theory of homogeneous nucleation, the primary nucleation of reaction is a homogeneous nucleation when S≥4×10. Based on these results the preparation of more uniform silver colloids can be carried out.(4) According to the LaMer model and our study, the shape of the particles can be well controlled through an explosive nucleation process followed by a slow growth. Silver colloids with improved uniformity were prepared using a two-step reduction method. With this method we first generated the initial small silver seeds by rapid reduction of silver nitrate with sodium citrate at100℃and then let them grow slowly at92℃. The results show that the new silver colloids have increased uniformity in shape and size and stable SERS activities.(5) Small spherical silver nanoparticles were first prepared by reduction of silver nitrate with sodium borohydride in the present of sodium citrate and then transferred into silver triangular nanoprisms under irradiation with a sodium lamp. Based on the study of the concentration of the reactant in preparation, silver triangular nanoprisms with high yield were prepared. SERS study indicates these triangular nanoprisms have high SERS activities.(6) Au-Ag alloy particles were prepared by the co-reduction of HAuCl4and AgNNO3with citrate and sodium borohydride, respectively. These Au-Ag alloy particles are stable and uniform. Meanwhile, they all have a single peak in UV-Vis spectra, which is between the LSPR of pure silver and pure gold. This fact suggests that these nanoparticles in our experiments formed by simple co-reduction of HAuCl4and AgNO3are alloy, which is quite different from UV-Vis spectra of the mixture of gold and silver nanoparticles. The LSPR of Au-Ag alloy can be tailored by changing the gold molar fraction in alloy, which is benefit for SERS.