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

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 H2SO4 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, υC2O and υC4O 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, qM 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’.