Hyouk Soo Han

Adsorption and stability of phthalic acid on a colloidal silver surface: surface-enhanced Raman scattering study

This work was supported by the Korea Science and Engineering Foundation(KOSEF) through the Center for Molecular Catalysis at Seoul National University (SNU) and by the Korea Research Foundation through the Research Institute for Basic Sciences at SNU. S.W.J. thanks the KOSEF for an Intern Research Fellowship.

Infrared and Raman spectra of 4-cyanobenzoic acid on powdered silver

Abstract We have attempted to record the infrared and Raman spectra of 4-cyanobenzoic acid (4CBA) adsorbed on 2-μm-sized silver particles. The diffuse reflectance infrared Fourier transform (DRIFT) spectrum taken for the sample was little different from the reflection–absorption infrared (RAIR) spectrum taken for the same molecules on vacuum-evaporated thick silver films, suggesting that the usual surface selection rule should be applicable even to the surface of fine metal particles. The Raman spectrum of 4CBA on powdered silver was a surface-enhanced Raman (SER) spectrum, exhibiting little difference from that taken on vacuum-evaporated thin, rough silver films. Thus, the commercially available powdered silver seemed to be an efficient substrate for the infrared and Raman spectroscopic characterization of molecular adsorbates prepared in a similar way on silver surfaces.

Diffuse Reflectance Infrared Spectra of 4-Nitrobenzoic Acid and 4-Cyanobenzoic Acid Self-Assembled on Fine Silver Particles

Adsorption of 4-nitrobenzoic acid and 4-cyanobenzoic acid on fine silver powders was investigated by diffuse reflectance infrared Fourier transform spectroscopy. It was concluded that both molecules were adsorbed on the surfaces of silver powders as carboxylate, after deprotonation, assuming a perpendicular orientation with respect to the silver surface. In a comparison with the reflection-absorption infrared spectra taken for the same molecules on vacuum-evaporated thick silver films, the usual surface selection rule that applied to flat metal surfaces also seemed applicable to the surface of fine metal particles, as long as the diameter of the particles was near 2 μm. Metal powders with diameters greater than 5μm appeared inappropriate as an adsorbent, however, probably because of particle sizes quite close to the wavelength of the infrared light used.

Phototransformation of alkanethiol-derivatized noble metal nanoparticles

Photon-initiated shape transformation of n-alkanethiol-derivatized noble metal nanoparticles has been studied with variations of metal, alkanethiol, and solvent. Silver nanoparticles undergo fragmentation upon irradiation while gold ones barely do. Silver/gold composite particles follow the case of silver with a reduced efficiency. The efficiency decreases as alkanethiol length or solvent dipole moment increases. Following the conduction of thermalized photon energy, alkanethiol can dissociate in a period of heat dissipation, and some of dethiolated particles fragment within the recombination time. Prior to the thermal conduction, shape transformation via melt and vaporization also occurs for both metals but this effect is less apparent for silver because of more notable fragmentation followed. The difference in the transformation of two metals is ascribed to the differences in work function, oxidation potential, atomization enthalpy, and particle size. Smaller fragmentation efficiency with more polar solvent or longer alkanethiol is attributed mainly to relatively smaller dissociation rate compared with heat dissipation rate.

OPTIMUM GEOMETRY OF CO DIMER AND FT-IR SPECTRA OF CO IN SOLID ARGON

Abstract To gain insight into the optimum geometry of the CO dimer, conventional ab initio and density functional calculations have been carried out. The potential energy surface of the CO dimer appeared to be very flat. Nonetheless, the planar distorted T-shaped structure was computed to be the most stable structure. The slipped antiparallel structure was also computed to be a local minimum. Both the linear and the nonplanar crossed structures seemed unstable. The slipped parallel structure was calculated to correspond to a saddle point. In addition to computing the optimum geometry of (CO) 2 , the matrix isolated infrared spectra were taken for CO dispersed in solid argon. Three peaks were identified at 2136.4, 2138.4, and 2139.9 cm −1 . The most distinct peak at 2138.4 cm −1 could be assigned to monomeric CO. The other two peaks were attributed to distorted T-shaped dimeric CO species, the 2136.4 cm −1 peak due to (CO) 2 bound through the CO…OC interaction and the 2139.9 cm −1 peak due to (CO) 2 bound through the OC…CO interaction.

New Strategy for Ready Application of Surface-Enhanced Resonance Raman Scattering/Surface-Enhanced Raman Scattering to Chemical Analysis of Organic Films on Dielectric Substrates

Dropping of appropriately concentrated AgNO3 and NaBH4 solutions, as well as laser-ablated Ag sols, onto organic molecules results in the formation of aggregated Ag nanoparticles that can induce surface-enhanced Raman scattering (SERS) for the molecules. The addition of flocculating agents such as alkali halides can further increase the Raman signals. We demonstrate in this work that Raman spectra can be obtained even for 0.01 monolayers of R6G on Si simply by spreading silver nanoparticles and/or fabricating Ag nanoparticles and nanoaggregates at the gaps and vacant sites of R6G molecules. The application prospect of the present methodology is extremely high, not only because of its simplicity but also because of the fact that the observation of vibrational spectra is one of the most incisive methods for understanding the chemical and physical phenomena on a variety of surfaces.

Preparation, Characterization, and Photolysis of Dodecanethiol-Derivatized Noble Metal Nanoparticles

Abstract Dodecanethiol-derivatized nanoparticles of Ag, Ag/Au, and Au with an average size of 5.5 nm have been prepared via a two-phase synthetic route in water/toluene and characterized with various methods. In particular, their photochemical dynamics and mechanisms have been studied with laser flash photolysis. The surface plasmon absorption peak varies linearly from 532 to 429 nm as the Ag fraction of Ag/Au alloy clusters, in which Ag atoms are more abundant at the outer part, increases. Excited silver nanoparticles dispersed in cyclohexane undergo dethiolation in 8 ns and then fragmentation in 40 ns. Thereafter, the fragments gradually aggregate owing to the presence of less soluble bare metal sides.