Abnormal infrared effects of nanostructured rhodium thin films for CO adsorption at solid/gas interfaces

Nanometer-scale thin films of rhodium supported on glassy carbon (nm-Rh/GC) were prepared by electrochemical deposition using cyclic voltammetry. STM studies demonstrated that the Rh film is made of layered crystallites of an average size 230 nm (1) x 90 nm (w) x 10-30 nm (h). In situ FTIR spectroscopic investigations revealed, for the first time, that the nanostructured film exhibits abnormal infrared effects (AIREs) for CO adsorption at solid/gas interfaces. The AIREs are characterized by the inverting of the direction of IR bands of adsorbed CO (COad), an enhancement of IR absorption intensity, and an increase in the full width at half-maximum (FWHM) of the bands. The inversion of the direction of the absorption bands and the increase in the FWHM are observed in spectra of CO on nm-Rh/GC of different Rh film thicknesses; the enhancement of IR absorption depends strongly on the thickness of the Rh film. The maximum enhancement factor has been determined to be 10.38 for an Rh film thickness of 79 nm. The present study demonstrated that the AIREs are general phenomena exhibited by nanostructured thin-film materials at both solid/liquid and solid/gas interfaces; they are of importance in revealing the intrinsic properties of 2D nanomaterials.