Abstract

Abstract

Since the mid-1990s good quality surface-enhanced Raman spectra have been obtained from many transition metal (TM) electrodes. It has been observed quite often that SERS band intensities, i.e., the relative intensities of different vibrational modes, of the adsorbate are very sensitive to the nature of the metal. Since transition metals interact with adsorbed molecules much more strongly than the typical SERS substrates, i.e., Au, Ag, and Cu, it is desirable to give a detailed and quantitative explanation of the spectroscopic behavior on TM electrodes. In the present study, a hybrid density functional approach with 6–311+G**/LanL2DZ basis sets and the B3LYP nonlocal exchange-correlation functionals has been used for the Raman intensity analysis on totally symmetric modes of pyridine adsorbed at transition metal electrodes, e.g., iron, cobalt, nickel, palladium, and platinum. Among all studied metal electrodes, iron and cobalt are predicted to be the most effective SERS substrates involving chemical enhancement, a result in good agreement with the experiments. The chemical bonding enhancement plays a role in pyridine interaction with the transition metal electrodes. The charge transfer enhancement as the most common chemical mechanism is also discussed for comparison.

The binding interactions between pyridine and bimetallic silver–gold clusters are investigated using density functional theory (DFT). The binding energies of pyridine–bimetallic cluster complexes indicate that the bonding depends strongly on the binding site (Au or Ag atom) and bonding molecular orbitals in a given configuration. The donation of the lone-pair electrons of the nitrogen of pyridine to an appropriate unoccupied orbital of each metal cluster plays an important role. The low-lying excited states and charge-transfer states of four stable complexes of interest are calculated on the basis of a time-dependent DFT method. In nonresonance Raman scattering processes, the influence of binding interactions on the relative Raman intensity of totally symmetric pyridine vibrational modes is discussed. These calculated relative Raman intensities are compared with observed surface-enhanced Raman spectra of pyridine adsorbed on silver–gold alloy surfaces.

Abstract

Au core Pd shell (Au@Pd) nanoparticles with controllable size from 35 to 100 nm were prepared by chemical deposition of Pd over pre-formed 12 nm Au seeds. Both transmission electron microscopy and UV–visible spectroscopy studies confirmed the core–shell structure of the synthesized nanoparticles. The Au@Pd nanoparticles dispersed on a polished Pt electrode surface exhibit high surface-enhanced Raman scattering (SERS) effect for the adsorbed pyridine and SCN⁻. With the aid of the long-range effect of the electromagnetic (EM) enhancement created by the SERS-active Au core underneath the Pd shell, the good quality SERS spectra of adsorbates on the palladium metal overlayer can be obtained. Such kind of SERS-active substrate can be used as an alternative substrate to massive metals for investigating adsorption and reactions occurring on the Pd metal catalyst.

Einfach, kontrolliert und reproduzierbar können Metallelektroden mit einem einstellbaren Abstand zwischen etwa 10 nm und wenigen Ångström (in SEM-Messungen bestimmt) mit dem hier beschriebenen Verfahren elektrochemisch hergestellt werden (siehe Bild). Das Funktionsprinzip dieses Potentialrückkopplungssystems mit einzigartiger Elektrodenkonfiguration beruht auf der Potentialverteilung in der elektrischen Doppelschicht.

Potential feedback: Metallic electrodes with controlled gap widths ranging from about 10 nm down to several angstroms (as determined by SEM measurements) can be fabricated electrochemically by using a simple potential feedback system with a unique electrode configuration (see picture). The working principle is based on the potential distribution in the electric double-layer. The process is simple, controllable, and reproducible.

A simple and effective approach to the aqueous-phase synthesis of crystalline silver nanorods and nanowires is demonstrated, using which their diameters and aspect ratios can be effectively controlled. The synthesis involves a template-less and non-seed process to high-quality nanoparticles, which is low-cost and proceeds at moderate temperatures. The nanorods and nanowires were synthesized by the reduction of silver nitrate with tri-sodium citrate in the presence of sodium dodecylsulfonate. The concentration of tri-sodium citrate plays a critical role while sodium dodecylsulfonate, as a capping agent, only plays an assistant role in controlling the diameters and aspect ratios of the products. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) investigations show that the silver nanocrystals are generated with a twinned crystalline structure. We also put forward a primary experimental model to shed light on their growth mechanisms.