Adsorption of thiourea on Bi(1Â 1Â 1) electrode surface

The impedance spectroscopy and in situ STM methods have been used for investigation of the thiourea (TU) adsorption at the electrochemically polished Bi(1 1 1) electrode from weakly acidified (pH ≈ 5) Na2SO4 supporting electrolyte solution. Influence of the Bi(1 1 1) electrode potential and TU concentration on the adsorption kinetics of thiourea has been demonstrated. In the region of maximum adsorption, E > −0.85 V (Ag|AgCl in saturated KCl), the heterogeneous adsorption and diffusion steps are the rate determining stages for TU adsorption. It was found that for more concentrated TU solutions | Bi(1 1 1) interface (cTU ⩾ 1.0 M), the stable TU adlayer is detectable by the in situ STM method only at the weakly negatively and positively charged Bi(1 1 1) electrode surface. At lower TU concentrations the adsorbed TU molecules are mobile and cannot be visualized using in situ STM method. At moderately negatively charged Bi(1 1 1) electrode surface, there are no chemisorbed TU molecules detectable by in situ STM method. The in situ STM data are in a good agreement with the impedance data indicating that the weak slow chemisorption with the partial charge transfer step takes place at E > −0.85 V.