FT-IR Spectra of Nanoparticles: Surface and Adsorbate Modes

Abstract

Nanocrystals and amorphous nanoparticles (similar to 20 nm diameter) have been prepared in the vapor phase by the rapid expansion of a dilute mixture of volatile substances (H2O, CO2, mesitylene, etc.) in N-2/He into a precooled (30-130 K) static infrared cell. The nanoparticles can be studied spectroscopically while suspended in the inert carrier gas, or as deposits on an infrared window, formed by use of multiple load-pump cycles. Since the particles are of a size for which similar to 10% of the molecules are at the surface, it is possible to obtain high quality spectra of the surface-localized vibrational modes and/or of monolayer quantities of adsorbate molecules, the latter quite generally, whereas direct observation of the surface-localized modes has been limited to H-bonded samples for which the frequencies differ significantly from interior mode frequencies. For example, from the difference between spectra for ''large'' vs ''small'' nanocrystals, the absorbance from bulk ice modes can be largely eliminated so that it is possible to observe several O-H(D) stretch and bend modes of surface molecules. With guidance From simulated spectra of an ice cluster and a slab of cubic ice, it has been possible to assign bands to vibrations of three classes of surface water molecules. The simulations show that a relaxed crystalline ice surface has three types of surface water molecules: 3-coordinated with a dangling H(D), 3-coordinated with a dangling O, and surface 4-coordinated with a distorted tetrahedral structure. With mode assignments for the surface water molecules, it becomes possible to interpret the interactions between small adsorbate molecules and the ice surface through the magnitude of the shifts and intensity variations of specific modes. Of course, interpretations based on these effects can be complemented by simultaneous observations on the modes of the adsorbate molecules. In the case of monolayer CF4 on surfaces of a variety of nanoparticles, the antisymmetric stretch mode appears as an intense TO-LO doubler with a large (similar to 80 cm(-1)) splitting that is a sensitive probe of the surface structure and tecture.