On the electrochemical and in-situ Fourier transform infrared spectroscopy characterization of urea adlayers at Pt(100) electrodes

The urea adlayers formed at the surface of Pt(100) electrodes immersed in urea-containing solutions have been characterized in-situ with a combination of cyclic voltammetry, charge displacement, and Fourier transform IR spectroscopy experiments. The infrared data indicate that the urea molecule is bonded through the two nitrogen atoms with the C-O group normal to the electrode surface. At the same time, the cyclic voltammetry and charge displacement data agree with previously reported coverage data if urea adsorption takes place with a net transfer of two electrons per adsorbed urea molecule. The existence of lateral interactions between neighbouring adsorbed molecules has been determined from the analysis of the band frequency and bandwidth together with their dependence on both the urea coverage and the electrode potential. At constant urea coverage the adsorbate band is shifted towards higher wavenumbers as the electrode potential increases, with a measured tuning rate of 50 cm(-1) V-1, which does not depend significantly on the urea coverage. The effect of the surface charge density on the electronic distribution in the urea molecule is suggested to play a key role on the observed potential dependent band shift and narrowing. The coexistence of an electrochemical Stark effect can not be excluded from the present data.