Hydroxy Benzoic Acid Molecules Adsorbed On Silver Nanoparticles On The Adsorption Behavior Of The Experimental And Theoretical Research

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.