【作者】 孟卫；

【导师】 陆路德；

【作者基本信息】 南京理工大学 ， 材料科学与工程， 2014， 博士

【摘要】 自Fleischmann等人首次在粗糙的银电极表面观察到吡啶的表面增强拉曼光谱(SERS)以来,银一直是最为广泛使用的SERS活性基底材料。目前,单分子的SERS信号已经可以在银溶胶纳米粒子表面获得。本论文制备了一系列纳米银溶胶,并以结晶紫(CV)为探针分子研究了这些银溶胶的SERS活性,探讨了它们的SERS增强机理。论文主要内容包括：(1)制备了六种具有不同ζ电位的银溶胶。测试了CV在银溶胶纳米粒子上的吸附等温线,其规律不满足Langmuir和Freundlich方程。ζ电位测定表明纳米银粒子带负电,说明与带正电荷的CV分子除发生单分子层吸附外,还可以发生多分子层的吸附。计算了CV在银溶胶粒子表面的吸附量和SERS增强因子,并研究了银溶胶的SERS活性与其粒子的大小、ζ电位之间的关系。由此确定了制备高SERS活性银溶胶较理想的条件。(2)用DFT方法在B3LYP水平上,对CV的拉曼光谱进行了计算,通过GaussView和VEDA程序模拟了CV分子的振动模式。计算结果表明,CV的二维振动模式E在SERS光谱中被选择性的增强,这与CV在半导体材料ZnO表面的SERS增强特性类似,说明银溶胶纳米粒子和CV分子之间发生了明显的电荷转移。因此,SERS增强中存在化学增强的成分。(3)用UV-Vis光谱研究了柠檬酸钠还原硝酸银制备银溶胶的成核动力学过程。考察了反应温度和过饱和度(S)对纳米银成核诱导期的影响。结果表明,其成核诱导期随着反应温度的升高和溶液过饱和度的增加而下降。同时结合均相成核理论,得出了当溶液的过饱度大于或等于4×108时,银溶胶的形成为均相成核过程。这一结论为制备形貌均一的,粒径窄分布的纳米银溶胶提供了理论依据。(4)根据经典的成核理论,结合反应体系的实际情况,摸索出银溶胶的两步合成法,即高温(100℃)成核和低温(92℃)生长。与一步法相比,所得银溶胶纳米粒子形貌单一、粒径分布窄,并具有更高的SERS活性。(5)用硼氢化钠、柠檬酸钠还原硝酸银溶液,得到球形纳米银粒子。这些球形粒子在光照条件下可得到三棱柱形纳米银粒子。实验中探讨了反应物浓度、反应过程中加样速率对三棱柱形纳米银粒子形成的影响,并对光照条件下,球形粒子转变成三棱柱形纳米银的机理进行了探讨。SERS研究结果表明三棱柱形纳米银溶胶比球形纳米银溶胶具有更高的SERS活性。(6)制备了金银合金纳米溶胶,其稳定性和分散性均较好。在UV-Vis光谱中,该合金体系只出现一个最大吸收峰,并且位于纯银和纯金的最大吸收峰之间,表明该溶胶体系是金银合金纳米粒子,而不是金粒子与银粒子的混合物。该体系是局域表面等离子体共振(LSPR)可调的合金材料,在特定激发波长条件下,具有比单一金溶胶和银溶胶更高的SERS活性。更多还原

【Abstract】 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.更多还原