Spectroelectrochemical and DFT Study of Thiourea Adsorption on Gold Electrodes in Acid Media

Abstract

The adsorption of thiourea (TU) at Au(111) and Au(100) single crystal and evaporated gold thin-film electrodes with preferential (111) orientation was studied in perchloric acid solutions with TU concentrations below 0.1 mM. For this purpose, cyclic voltammetry with gold single crystals was combined with external reflection infrared spectroscopy and surface-enhanced infrared reflection−absorption spectroscopy under attenuated total reflection conditions (ATR-SEIRAS) with gold thin film electrodes. In situ surface enhanced Raman spectroscopy (SERS) experiments were also carried out with these latter samples. In addition, optimized geometries and theoretical harmonic vibrational frequencies, obtained from B3LYP/LANL2DZ, 6-31+G(d) calculations for TU and thioureate species adsorbed on Au clusters with (111) orientation, were used for the interpretation of the experimental spectra. ATR-SEIRAS experiments show irreversible adsorption of TU at 0.10 V, whereas the SERS experiments have confirmed the bonding of the TU molecule to the metal surface through the S atom. The optimized geometry obtained from density functional theory (DFT) calculations for adsorbed TU corresponds to unidentate bonding through the sulfur atom, with the Au−S bond slightly tilted (13°) from the surface normal, whereas the C−S bond appears to be tilted by 45°. In the case of adsorbed thioureate, under the application of an external electric field of 0.01 a.u., a bidentate asymmetrical bridge adsorption configuration is obtained with one N(H) and the S atoms bonded to Au in positions close to "top" adsorption sites and with the molecular plane perpendicular to the metal surface. The observation of an adsorbate band for the asymmetric NCN stretching in the experimental infrared spectra confirms the tilting of the S−C bond of the adsorbed TU at low potentials. Changes of the adsorbate bands in the ATR-SEIRA spectra at potentials around 0.60 vs reversible hydrogen electrode (RHE) can be interpreted on the basis of DFT results as due to the deprotonation of adsorbed TU to form adsorbed thioureate.

On the electrochemical behavior of formamidine disulfide on gold electrodes in acid media

“…The optimized adsorption geometry for TU on clusters modeling the Au(111) surface was found to correspond to a unidentate configuration for adsorbed TU on top surface sites via the sulfur atom and with the C-S bond tilted by 45º from the normal. This result confirmed the conclusions derived from the observation of the asymmetric NCN band in the in situ infrared spectra [6,17].…”

“…These authors interpreted the observed ladder-like structures as due to adsorbed FDS, which can be reduced to adsorbed TU or oxidized yielding adsorbed sulfur. On the other hand, a surface redox process at potentials between 0.60 and 0.90 V RHE for gold electrodes in acidic TU-containing solutions has been related to the deprotonation of adsorbed TU to form adsorbed thioureate species [17]. Note that this proposal is connected to previous suggestions regarding the existence of a deprotonation process leading to the formation of adsorbed thioureate on platinum [26], copper [27,28] and gold [29] electrodes that would involve the tautomeric form of the TU molecule…”

“…In that study, the effects of external electric field and solvation were also addressed. Regarding the interaction of TU and related species with gold surfaces, we are not aware of any theoretical study other than the DFT study carried out by our group [17]. The optimized adsorption geometry for TU on clusters modeling the Au(111) surface was found to correspond to a unidentate configuration for adsorbed TU on top surface sites via the sulfur atom and with the C-S bond tilted by 45º from the normal.…”

“…As expected from the strong sulfur-metal interaction, a variety of surface techniques has allowed the detection of irreversible adsorbed TU on gold and platinum electrodes in contact with TUcontaining solutions [4,6,[12][13][14][15][16]. Surface-Enhanced Raman Scattering (SERS) spectroscopy experiments proved the bonding of TU to the metal surface through the sulfur atom [12,13,[16][17][18].…”

“…Moreover, both external reflection infrared spectra [6] and surface-enhanced infrared reflectionabsorption spectra obtained under attenuated total reflection conditions (ATR-SEIRA) [17] showed bands assigned to the symmetric and asymmetric NCN stretching modes of adsorbed TU. The observation of the latter band implies a significant tilting of the S-C axis with respect to the surface normal.…”