Time-resolved in situ ATR-SEIRAS study of adsorption and 2D phase formation of uracil on gold electrodes

Abstract

The adsorption and phase formation of uracil on massive Au[ n(111)–(110)] single crystal and Au(111-20 nm) film electrodes in 0.1 M H 2SO 4 was studied by electrochemical measurements and ATR surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS). At E<0.15 V uracil molecules are disordered and planar oriented, co-adsorbed with weakly hydrogen-bonded interfacial water (region I). Around the potential of zero charge a 2D condensed, physisorbed film of planar oriented molecules, interconnected by directional hydrogen bonds is formed (region II). At more positive potentials the carbonyl combination bands, υ C2O and υ C4O increase significantly in intensity and exhibit a large negative shift, characteristic to uracil co-ordinated to metal ions (region III). Band intensities and peak positions reach rather constant values at E>0.80 V (region IV). Uracil undergoes an orientational change from planar to perpendicular accompanied by the formation of a chemisorbed adlayer composed of molecular islands. The organic molecule is co-ordinated to the positively charged electrode surface via O2N3O4. The chemisorbed uracil adlayer (first layer) facilitates also the co-adsorption of hydrogen bonded water and sulphate species in the second layer. The kinetics of dissolution of uracil chemisorbed on Au[ n(111)–(110)] or Au(111-20 nm) electrodes (region IV) was studied by chronoamperometry and time-resolved ATR-SEIRAS employing either the rapid-scan or the step-scan regime. The macrokinetics of the i– t transients could be described by two processes: (i) hole nucleation according to an exponential law coupled with detachment-controlled growth (final potentials in region III) or surface diffusion controlled growth (final potentials in region II) and (ii) Langmuir-type desorption of disordered species from defect sites. Time-resolved ATR-SEIRAS experiments demonstrate that the transformation of chemisorbed uracil into lower coverage adlayers proceeds according to the following scenario: (i) Perpendicularly oriented uracil molecules change their orientation toward a tilted or planar arrangement depending on the final potential. (ii) Desorption of strongly hydrogen-bonded second-layer water and sulphate species. (iii) Adsorption of weakly hydrogen-bonded water. The spectroscopic transient responses of these three processes can be approximated by first order rate equations. Macroscopic signals, such as i, q M or C, do not reflect the entire complexity of the phase formation kinetics of the system investigated. Simultaneously recorded structure-sensitive transient data are of ultimate importance to develop a ‘real system understanding’.