CHEMICAL-REACTIONS ON THE SURFACE OF METAL NANOPARTICLES STUDIED BY OPTICAL SPECTROSCOPY

The influence of molecular adsorbate layers on surface plasmon excitat ion in small supported metal particles has been investigated and explo ited to study adsorption reactions on their surfaces. For this purpose sodium nanoclusters on quartz and LiF substrates served as model syst ems. Their optical transmission spectra are dominated by two maxims wh ich are due to the excitation of surface plasmon resonances in the dir ection of the long and short axes of the oblate particles. By recordin g the spectra under ultrahigh-vacuum conditions and, subsequently, aft er exposure to gases such as O-2, N2O, CO2, H-2 and N-2, changes in th e optical spectra can be identified if the clusters are covered by as little as half a molecular monolayer, Depending on the adsorbed molecu les, different modifications of the maximum position, the width and th e amplitude of the surface plasmon resonances are observed. The result s of a series of measurements together with calculations using the qua sistatic approximation indicate that the variations in the spectra all ow one to distinguish between physisorption and chemisorption, i.e. to characterize the strength of the chemical bond. In addition, diffusio n of the molecules into the bulk of the particles can be detected. Par ticularly interesting is the observation that the clusters can experie nce a change in their shape if gases such as O-2 or CO2 react with the ir surface. (C) 1998 John Wiley & Sons, Ltd.