对羟基苯甲酸分子吸附在银纳米颗粒上的吸附行为的实验和理论研究

【作者】 吴慧军；

【导师】 张鹏翔；

【作者基本信息】 昆明理工大学 ， 材料学， 2005， 硕士

【摘要】 自从1974年Fleischman发现表面增强拉曼散射(SERS)以来,以金、银等贵金属为基底的SERS技术在高灵敏度、高荧光淬灭等方面表现出了极大优势,并被广泛应用在分子与基底表面相互作用的研究中。然而其拉曼产生机理仍然有很多争论,而进一步扩大其应用范围又需要对机理进行更为深入的研究,尤其是研究分子与基底表面之间的吸附机理以及吸附取向等问题。 当分子吸附在基底上时,如果基底表面的性质发生变化,吸附物的SERS光谱也会随之发生变化,这就使得光谱的分析变得困难。传统的分析方法是根据拉曼峰的频移以及拉曼峰的增强或减弱来猜测吸附物吸附在基底上的吸附取向、几何结构和吸附的本质,等等。然而,由于SERS会受到很多因素的影响,比如不同的基底、表面形态、结构、温度,甚至是pH值。因此,这种猜测的准确性一直困扰着研究人员来做出正确的判断;而且吸附模型的合理性以及相关理论并没有得到有效的检验,这就给SERS的机理研究和SERS技术的应用都带来了很多的困难。在理论上,其困难在于如何建立一个简单的模型,它既要能恰当描述吸附物和基底之间的吸附关系,同时又要能便于计算吸附物的振动光谱。 在本文中,我们利用红外激发得到了对羟基苯甲酸(PHBA)分子吸附在银胶溶液和沉积了银纳米颗粒的薄层色谱铝箔板(TLCAS)上的拉曼光谱。从中发现PHBA分子在银胶溶液和TLCAS上的吸附行为有很大的不同。在银胶溶液中,PHBA分子是通过羧基直立的吸附在银纳米颗粒表面上的;而在TLCAS上,PHBA分子是倾斜甚至是平躺吸附的。而在TLCAS上时,当银纳米颗粒和PHBA分子之间的比例改变时,吸附行为也会改变。因此,对于PHBA分子吸附在银胶溶液和TLCAS上的这两种情况,我们分别建立了两个模型:第一个模型是PHBA通过羧基吸附在一个银原子上面,第二个模型是PHBA通过羧基和羟基吸附在两个银原子上面。对于这两个模型我们在Gaussian2003软件应用了DFT-B3PW91/lanl2dz方法来计算它们的拉曼光谱。 将实验得到的拉曼光谱与计算得到的拉曼光谱相比较可以得知,虽然我们只用一个银原子来代替银的纳米颗粒来作为衬底,但是相应频率的平均相对误差在2%以内,所以这两个模型的建立是比较合理的,可以认为这两个模型能够比较好的描述这两种实验情况。更多还原

【Abstract】 Surface enhanced Raman scattering (SERS) was discovered by Fleischman in 1974, it has shown the great superiority because of its high sensitivity and high fluorescent quenching. It has been widely used in the study of the interaction of the molecules with the surface of the substrate. However, there’re still many debates about the mechanism of SERS. So a further study on the mechanism is needed if the SERS technique is wanted to be used in a more broaden area, especially in the in the study of the interaction mechanism of the molecules with the surface of the substrate and molecular orientation.When the adsorbates adsorbed on the substrate, the characteristics of the surface of the substrates maybe varied, the SERS spectrum of adsorbates may be different. This makes the analyses difficult. The common analytical method is to hypothesize the adsorption orientation, geometry configuration and adsorption essence of adsorbates on the surface of substrates based on the shift of Raman bands, enhancement or weakness of intensity. However, since SERS is found to be affected by many factors such as the different SERS substrate, surface configuration, structure, temperature and even pH value and so on, therefore the veracity of such guess has frequently troubled the researchers to make definite conclusions. And the reliability of adsorption models and relevant theory cannot be examined effectively, which results in many difficulties in the research of SERS mechanism and in applying the SERS technology. Theoretically, the difficulty lies in the establishment of a simple model, which should be able to describe the combination of substrate and the adsorbed molecule properly, and at the same time, can be used to calculate the vibrational spectra easily.Quantum chemically, however, one can treat the problem in two different ways: the first approach is the periodic density functional theory (DFT) and the second is to use a finite cluster model for the surface studied. In the thesis, the first approach is used. The calculations based on density functional theory (DFT) have been used in many areas, and the results are also in great agreement with the experimental results in calculating vibrational frequencies.In the work of the thesis, high quality Raman spectra of PHBA in silver colloidal solution and on thin-layer chromatoplate aluminum sheets (TLCAS) at IR excitation were obtained. It wasshown that the adsorption behavior of PHBA molecules on the TLCAS is different from that in the silver aqueous colloid. In silver aqueous colloids, PHBA molecules stand perpendicularly on the surfaces of silver nanoparticles through carboxyl, while on the TLCAS, they are tilted and there is a certain angle between the benzene ring and the surfaces of silver nanoparticles, or the benzene ring is even paralleled with the surfaces of the silver nanoparticles. On the TLCAS, when the proportion of silver nanoparticles and PHBA molecules changes, the adsorption behavior changes. And DFT-B3PW91/lanl2dz was applied to calculate the Raman frequencies based on these two models.Comparing the theoretical results with the experimental values, it was found although we only just one Ag atom to model the whole particle as the "substrate," that the mean relative deviation is within 2%. Therefore, one can conclude these two modes is properly reasonable to describe the corresponding adsorption configuration.更多还原