Abstract:Surfaceenhanced Raman scattering (SERS) of thiourea [SC(NH,),] was studied in acidic solutions of 0.1 M H,SO, and 0.1 M HCI. Very strong SERS bands of thiourea were observed in the acidic solutions, in contrast to very weak SERS spectra of thiourea in standard SERS solutions employing K,SO, or KCI. Thiourea seems to have a strong and stable surface adsorption at a silver electrode in the acidic solutions where protonation and anionic co-adsorption may enhance this surface stability of thiourea.
“…On the Mechanism of Thiourea − Thioureate Conversion. Several conclusions may be drawn from the results of previous sections concerning the nature of the adsorbed species: (1) of all of the species considered, the strongest bonding to the surface occurs for the canonical TU radical suggesting that this should be most stable adsorbate; (2) the surface bonding of this radical has no major differences with that of the methanethiol radical supporting the XPS experimental evidence in this direction; and (3) the enlargement of the SC bond upon adsorption for molecular TU is negligible, whereas for the canonical TU radical, this bond considerably increases, indicating a weakening of the bond strength compatible with the SC frequency downshift observed experimentally. − Therefore, the general conclusion is that TU interconverts upon adsorption into a canonical TU radical that bears a negative charge, giving rise to a thioureate anion, which has the same surface bonding as that of alkanethiols. 10 Oxidative adsorption of thiourea.…”
Section: Resultsmentioning
confidence: 67%
“…Therefore, under low-coverage conditions, adsorption with the molecular plane parallel to the surface is the most stable configuration in vacuum, whereas adsorption via the sulfur atom has the lowest binding energy. These results seem to contradict the spectroscopic data of TU adsorption in solution, − which indicate that TU adsorbs via the sulfur atom. Clearly, the solvent and the electric field at the interphase must play an important role in determining the adsorption geometry of thiourea.…”
Section: Resultsmentioning
confidence: 79%
“…Surface-enhanced Raman spectroscopy has been extensively used to study the adsorption of thiourea and several isostructural compounds such as thioamide, thiohydrazide, and thiocarbohydrazide on silver and copper surfaces. − In most cases, it was found that these molecules are bonded via the sulfur atom to Ag and Cu surfaces. The substantial downshift in the frequencies of vibrational modes having a predominant contribution from CS stretching suggested considerable electronic interactions between these molecules and metal surfaces.…”
The adsorption of different thiourea species was investigated on the 111 surface of silver considering electric
field and solvent effects with the objective of (a) elucidating the nature of the adsorbed species, mainly in an
electrochemical environment, and (b) understanding the energetics and mechanisms of the surface reactions,
particularly the oxidative adsorption of thiourea. We first considered the adsorption in vacuum of molecular
species such as thiourea (TU), canonical thiourea, and formamidine disulfide as well as the adsorption of the
canonical thiourea radical resulting from the cleavage of the S−H bond. The molecular adsorption of TU in
vacuum has the highest binding energy (−33.2 kcal/mol) when the molecular plane is parallel to the surface.
However, the strongest surface bond was observed for the canonical thiourea radical (−42.2 kcal/mol) whose
nature is the same as that of the methanethiol radical. Externally applied electric fields perpendicular to the
surface have an important effect on the orientation and surface bonding of thiourea, leading to an upright
configuration of the molecule and to a strengthening of the sulfur−surface bond. Solvent effects were first
considered to investigate the equilibria of thiourea in solution with the purpose of elucidating the nature of
the predominant species which, in turn, constitutes the initial adsorbate on the surface. In neutral and basic
pHs, the thiourea molecule is the predominant species, whereas in acidic electrolytes, the thiouronium ion
readily forms. The microsolvation of thiourea with up to 16 water molecules was investigated to determine
the structure of the hydration shell and the interaction energy between TU and water molecules. Water molecules
form a compact cage around thiourea characterized by linked tetramer and pentamer structures. The NH2
groups form hydrogen bonds with water molecules, whereas the sulfur atom is poorly hydrated. The implications
of the structure of water around thiourea on the adsorption geometry of the molecule are discussed. The
oxidative adsorption mechanism of thiourea resulting in chemisorbed canonical thiourea was investigated
considering charge-transfer processes, solvent effects, and the presence of a counterion.
“…On the Mechanism of Thiourea − Thioureate Conversion. Several conclusions may be drawn from the results of previous sections concerning the nature of the adsorbed species: (1) of all of the species considered, the strongest bonding to the surface occurs for the canonical TU radical suggesting that this should be most stable adsorbate; (2) the surface bonding of this radical has no major differences with that of the methanethiol radical supporting the XPS experimental evidence in this direction; and (3) the enlargement of the SC bond upon adsorption for molecular TU is negligible, whereas for the canonical TU radical, this bond considerably increases, indicating a weakening of the bond strength compatible with the SC frequency downshift observed experimentally. − Therefore, the general conclusion is that TU interconverts upon adsorption into a canonical TU radical that bears a negative charge, giving rise to a thioureate anion, which has the same surface bonding as that of alkanethiols. 10 Oxidative adsorption of thiourea.…”
Section: Resultsmentioning
confidence: 67%
“…Therefore, under low-coverage conditions, adsorption with the molecular plane parallel to the surface is the most stable configuration in vacuum, whereas adsorption via the sulfur atom has the lowest binding energy. These results seem to contradict the spectroscopic data of TU adsorption in solution, − which indicate that TU adsorbs via the sulfur atom. Clearly, the solvent and the electric field at the interphase must play an important role in determining the adsorption geometry of thiourea.…”
Section: Resultsmentioning
confidence: 79%
“…Surface-enhanced Raman spectroscopy has been extensively used to study the adsorption of thiourea and several isostructural compounds such as thioamide, thiohydrazide, and thiocarbohydrazide on silver and copper surfaces. − In most cases, it was found that these molecules are bonded via the sulfur atom to Ag and Cu surfaces. The substantial downshift in the frequencies of vibrational modes having a predominant contribution from CS stretching suggested considerable electronic interactions between these molecules and metal surfaces.…”
The adsorption of different thiourea species was investigated on the 111 surface of silver considering electric
field and solvent effects with the objective of (a) elucidating the nature of the adsorbed species, mainly in an
electrochemical environment, and (b) understanding the energetics and mechanisms of the surface reactions,
particularly the oxidative adsorption of thiourea. We first considered the adsorption in vacuum of molecular
species such as thiourea (TU), canonical thiourea, and formamidine disulfide as well as the adsorption of the
canonical thiourea radical resulting from the cleavage of the S−H bond. The molecular adsorption of TU in
vacuum has the highest binding energy (−33.2 kcal/mol) when the molecular plane is parallel to the surface.
However, the strongest surface bond was observed for the canonical thiourea radical (−42.2 kcal/mol) whose
nature is the same as that of the methanethiol radical. Externally applied electric fields perpendicular to the
surface have an important effect on the orientation and surface bonding of thiourea, leading to an upright
configuration of the molecule and to a strengthening of the sulfur−surface bond. Solvent effects were first
considered to investigate the equilibria of thiourea in solution with the purpose of elucidating the nature of
the predominant species which, in turn, constitutes the initial adsorbate on the surface. In neutral and basic
pHs, the thiourea molecule is the predominant species, whereas in acidic electrolytes, the thiouronium ion
readily forms. The microsolvation of thiourea with up to 16 water molecules was investigated to determine
the structure of the hydration shell and the interaction energy between TU and water molecules. Water molecules
form a compact cage around thiourea characterized by linked tetramer and pentamer structures. The NH2
groups form hydrogen bonds with water molecules, whereas the sulfur atom is poorly hydrated. The implications
of the structure of water around thiourea on the adsorption geometry of the molecule are discussed. The
oxidative adsorption mechanism of thiourea resulting in chemisorbed canonical thiourea was investigated
considering charge-transfer processes, solvent effects, and the presence of a counterion.
“…Several electrochemical methods as well as in situ and ex situ spectroscopic techniques have been employed to investigate the adsorption and oxidation reactions of TU on different metals [7,13,[18][19][20][21][22][23][24][25][26][27][28][29]. Voltamperometric studies have shown the formation of residual sulphur on the surface after TU molecule oxidation [20,21,30,31].…”
“…In situ vibrational spectroscopies were also used to investigate these reactions. ,,,,− Surface-enhanced Raman spectroscopy has shown that TU adsorbs strongly on the metal surface via the sulfur atom in a wide potential range and desorbs at sufficiently negative potentials. ,,,− On the other hand, the combination of Fourier transform infrared spectroscopy (FTIRS) with electrochemical techniques enables the acquisition of molecular information concerning the direct interaction of the adsorbate at the metal/electrolyte interface. , Consequently, in situ FTIRS appears also to be a useful technique to study the interaction of organic plating additives.…”
In the present paper, a systematic electrochemical investigation on thiourea (TU) electrooxidation was developed on polycrystalline and (111) single-crystal gold electrodes in 0.1 M perchloric acid. The combination of cyclic voltammetry with in situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry techniques have allowed the nature of the species formed during the electroadsorption and electrooxidation of TU to be established. FTIRS experiments were performed in D2O to clean up the region of the H2O bending around 1600 cm(-1). It was concluded that TU adsorbs tilted on the surface in the 0.05-0.40 VRHE potential range. A dual-path reaction mechanism was evidenced in the oxidation process. The first pathway takes place from adsorbed TU at E > 0.40 VRHE and implies the formation of [Au(I)-(TU)2]+, which is oxidized to NH2CN and S0 at E > 0.80 VRHE. In a following oxidation step at E > 1.20 V, N2, CO2, and HSO4-/SO4(2-) were produced. The second parallel reaction occurs from TU in solution at E > 0.50 VRHE to form (TU)2(2+). All these species were characterized from the spectroscopic experiments. Similar results were obtained for both surfaces.
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