Synthesis, Characterization And The SERS Activity Of Au And Au@Pd Nanocubes

The unique properties and potential applications of metal nanoparticles are mainly determined by their size and shape. So far there have been many methods to prepare metal nanostructures and most of the products are spherical nanoparticles. It is therefore a great challenge to develop some simple and effective methods of synthesizing nanoparticles whose size and shape can be controlled at will.Surface-enhanced Raman scattering (SERS) is a phenomenon of nanoscience, its extremely high surface sensitivity is critically dependent on metal nanostructure. The strategy of extending the SERS substrate from the rough and ill-defined surface to the atomically flat and well-defined surface is essentially important to develop SERS into a powerful tool in surface science. The surface plasmon resonance as the major contribution to the SERS effect can be excited efficiently on certain nanostructures but not atomic flat surfaces of massive electrodes. The SERS obtained from atomically flat single crystal surfaces, if possible, will be greatly helpful for studying the orientation of adsorbates unambiguously. Therefore, single crystal nanoparticles may be a promising material to bridge the gap between highly rough and single crystal surfaces, and hence allows us to obtain a deep insight into the SERS mechanism and make contributions to nanoscience. In this work, we synthesized Au nanocubes and Pd film coated Au (Au@Pd) nanocubes with controllable size by a simple and effective method. Their surface structure, size, shape, optical and electronic properties, and SERS activity were investigated using SEM, TEM, HRTEM, SAED, UV-Visible and Confocal Raman microscopy. The major results of the thesis are outlined as follows:1. Au nanocubes with controllable size were synthesized by using the seed method. SEM, TEM and HRTEM were used to characterize the nanocubes and confirm the particles are monodispersed and with the well defined (100) surface. UV-visible absorption spectra indicate that When the size of cube Au increased, the plasmon absorption maximum red shift obviously. Moreover, the electrochemical SERS study showed that the potential dependent SERS intensity of pyridine (Py) as probe molecule depends greatly on the cube size. The 83 nm nanocube has the highest Raman intensity and then the Raman intensity decreased in the sequence of 95,105, 70 and 53 nm.2. Au@Pd nanocubes with controllable Pd shell thickness (1-15 layers of Pd atoms) were synthesized by using a wet chemical method. The electrochemical cyclic voltametry and SERS results confirm that the surface of the Au@Pd nanocubes is uniform and the pinhole is not exist. UV-visible absorption spectra indicate that as the Pd shell becomes thicker, the plasmon absorption maximum of cube Au core is gradually damped. Furthermore, the electrochemical SERS result reveals that the SERS intensity of CO and Py decays exponentially with the increase of the shell thickness. The thinner the shell is, the stronger the SERS intensity will be. Here the highest SERS intensity of CO and Py can reach 1330 cps and 10,000 cps, which is about two orders higher than that of bare Pd nanoparticles, roughened Pd electrode and Pd nanowire array systems. Moreover, the theoretical approach using Finite Difference Time Domain (FDTD) method was adopted to simulate the inter-particle coupling of the Au@Pd nanocube dimmers. The calculation result is consistent well with the experiment data, i.e., The SERS activity of Au@Pd nanocubes is decreased exponentially with the increase of Pd shell thickness.3. Au@Pd bimetellic nanocubes were synthesized by the combination of the seed method and the wet chemical method. UV-visible absorption spectra indicate that when the small polyhedral Au seed (3-4 nm) is used, the plasmon absorption maximum of Au can not be seen in the Au@Pd nanocubes. The Au@Pd nanocubes can be takes as the pure Pd nanocubes and the SERS intensities of CO and Py as probe molecules are almost the same as the pure Pd nanocubes. When we used the big polyhedral Au seed (40 nm), the SERS activity of Au@Pd nonocubes is about one order higher than that of Pd nanoparticles, roughened Pd electrodes and Pd nanowire array systems. Here, the plasmon absorption maximum of Au can be observed in the Au@Pd nanocubes. As a consequence, SERS could be in turn a powerful technique in studying the optical property of metal nanoparticles.