Spectroelectrochemical and DFT Study of Thiourea Adsorption on

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Spectroelectrochemical and DFT Study of Thiourea Adsorption on Gold Electrodes in Acid Media William Cheuquepán, Juan Manuel Pérez, Jose Manuel Orts, and Antonio Rodes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp503694m • Publication Date (Web): 03 Jul 2014 Downloaded from http://pubs.acs.org on July 8, 2014

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The Journal of Physical Chemistry

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

William Cheuquepán, Juan Manuel Pérez, José Manuel Orts*, Antonio Rodes Departamento de Química Física e Instituto Universitario de Electroquímica Universidad de Alicante, Apartado 99, E-03080 Alicante (Spain) *Corresponding autor: [email protected]

Tel: +34 965909814 Fax: +34 965909814

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 reflectionabsorption 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 DFT calculations for adsorbed TU corresponds to a unidentate bonding through the sulfur atom, with the Au-S bond slightly tilted (13º) from the surface normal, 1 ACS Paragon Plus Environment


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whereas the C-S bond appears to be tilted by 45 º. 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 V RHE can be interpreted on the basis of DFT results as due to the deprotonation of adsorbed TU to form adsorbed thioureate. KEYWORDS: thiourea; thioureate; gold electrodes; infrared spectroscopy; ATR-SEIRAS; SERS; DFT.

1. Introduction Thiourea (SC(NH2)2), TU) is a small sulfur-containing molecule that, due to its strong adsorption on metal surfaces, has interesting properties both from technological and fundamental points of view. TU has been used as an additive for metal deposition

1-4

and corrosion protection.

5;6

TU complexing

capability makes this molecule also useful for extracting some metals such as gold from their ores. 7;8 Adsorption and oxidation of TU have been studied, among others, at gold,8-20 platinum,21-26 silver,27-35 copper 19;27;36;37 and bismuth 38 electrodes. Electrochemical and spectroscopic evidences indicate that, in the case of platinum electrodes, and similarly to the chemical oxidation in the presence of mild oxidants, electrooxidation of dissolved TU in acidic solutions at moderate potentials gives rise to the formation of formamidine disulfide (FDS, (NH2)(NH)CS-SC(NH)(NH2))

21;22;24;26

2 NH2CSNH2 (NH2)(NH)CS-SC(NH)(NH2) + 2H+ + 2e

(1)

At higher potentials the formation of cyanamide (NH2CN) and adsorbed thiocyanate has been detected, these species being further oxidized to yield carbon dioxide, nitrogen and (bi)sulphate anions. Similar processes have been described in the case of gold electrodes in diluted TU solutions.

22;24;26

16;17

At

higher TU concentrations, oxidative gold dissolution takes place via the formation of gold-TU complexes in parallel to the formation of FDS 16-18 Au + 2 NH2CSNH2 [Au(I)(NH2CSNH2)2 ]+ + e

(2)

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Even if all these electrode reactions involve mainly dissolved TU, the presence of adsorbed TU at metal electrodes has also been described. The irreversible adsorption of TU can be expected due to the strong sulfur-metal interaction and some indirect evidences on this process have been derived for gold 15

electrodes from electrochemical,

scanning tunneling microscopy (STM),

13

differential

electrochemical mass spectrometry (DEMS) 17 and electrochemical quartz crystal nanobalance (ECQN) 14

experiments. The nature of the adsorbates formed on gold electrodes in TU-containing solution has

been investigated spectroscopically.

8;10;17

Adsorbed TU has been detected on gold electrodes from

surface enhanced Raman Spectroscopy (SERS) experiments, which proved the bonding of TU through the sulfur atom. 17

at gold

8-10

Moreover, in situ infrared experiments have allowed the detection of adsorbed TU

but not at platinum

26

electrodes. In the former case, the observed potential-dependent

behaviour includes the existence of bands for the consumption of dissolved TU together with additional bands for the related adsorbed TU species, both ascribed to the corresponding symmetric and asymmetric NCN stretching modes. 17 In the case of the adsorbed species, the presence of a band for the asymmetric NCN stretching mode in the infrared spectra has been linked to the tilting of the S-C axis with respect to the surface normal.

17

In the case of platinum electrodes, the absence of bands in the

corresponding infrared spectra for the adsorbates detected voltammetrically 23;26 has been interpreted as due to the adsorption of TU with its molecular plane parallel to the electrode surface.

26

These results

are in contrast with SERS data reported for rough platinum electrodes that showed bonding though the sulfur atom in a tilted configuration, with the tilting angle being dependent on the electrode potential. 25 The formation of a surface sulfide adlayer was detected when TU adsorption was carried out under open circuit conditions. 25 Finally, it can also be mentioned that, whereas associative adsorption of TU seems to be assumed in most of the previously published papers, some authors proposed the existence of a deprotonation process leading to the formation of adsorbed thioureate on platinum, gold

39

23

copper

33;34

and

electrodes. This process could involve the tautomeric form of the TU molecule NH2CSNH2 ↔ NHCSHNH2 (NHCSNH2)ads + H+ + e

(3)

The experimental results on the adsorption of TU at metal electrodes can be compared with those derived from Density Functional Theory (DFT) calculations,

35;40-44

In this respect, some reports exist

regarding the theoretical study of the microsolvation of TU and thioureate anions (CSNH2NH-) 43 with up to 6 and 7 water molecules respectively, and the effects on structure and energetics. The interaction of TU and urea with different anions was studied by B3LYP/6-311+G** calculations, as these molecules or their derivatives have potentiality for their use in the identification of anions in biological systems.

42

Other authors studied the vibrational behaviour of TU and TU-d4 by comparing the 3 ACS Paragon Plus Environment


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experimental spectra in argon and nitrogen matrices with their respective theoretical vibrational frequencies obtained at the B3LYP/6-31+G(2d,p) level.40 A further detailed study including anharmonicity was carried out using MP2 and QCISD methods.44 We are aware of only one theoretical work dealing with the adsorption of thiourea and thioureate (on a Ag(111) surface), authored by Patrito et al. , 35 where electric field and solvent effects were considered. Spectroelectrochemical studies similar to that reported in this paper on the adsorption behavior of simple molecules have been published by our group recently. Namely, the adsorption and reactivity on Au thin-film and single crystal electrodes of some simple amino acids

45-47

and a number of acetic acid

48-51

have been studied by combining electrochemical measurements and in situ vibrational

spectroscopies.

In-situ SERS and infrared spectroscopy (either using surface-enhanced infrared

derivatives

reflection-absorption spectroscopy under attenuated total reflection conditions (ATR-SEIRAS) or external reflection configurations) provided molecular level information on the bonding of these adsorbed molecules and on their interactions with coadsorbed water and anions. 45-51 This experimental spectroelectrochemical approach has been complemented with the use of DFT to calculate theoretical vibrational spectra from which the observed experimental bands have been assigned. On the other hand, the reported experiments combine the use of single-crystal electrode surfaces (external reflection infrared experiments) and gold thin film electrodes (ATR-SEIRAS and SERS). The use of metal thin films allows the enhanced surface sensitivity of the ATR-SEIRAS experiments for which the combination of a low deposition rate (either by thermal sputtering

54

52

or electron beam

53

evaporation or by argon

) with subsequent electrochemical annealing in solutions containing specifically adsorbed

anions such as (bi)sulphate 52;53 or acetate preferential (111) orientation.

52-54

54

allowed the preparation of ordered gold thin-films with a

The gold thin-film electrodes prepared under these experimental

conditions preserve the granular nanostructure being at the origin of the so-called SEIRA effect providing also a noticeable SER effect

54

55;56

that allows the comparison of complementary infrared and

Raman spectra for the same adsorbate under the same kind of electrode surface. This paper reports a spectroelectrochemical study of TU adsorption on gold electrodes. The nature of the adsorbed species formed from diluted TU solutions (namely, with TU concentrations below 0.1 mM) is investigated in a low potential range in order to prevent the oxidation of the TU in solution and/or the dissolution of the gold electrode via the formation of TU complexes. Voltammetric and spectroscopic data obtained in acidic media with Au(111) and Au(100) single crystal are compared to those for gold thin-film electrodes and with the previously published results. In order to better support the assignment of experimental vibrational frequencies, DFT calculations will be used for obtaining 4 ACS Paragon Plus Environment

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optimized adsorbate geometries and theoretical harmonic vibrational frequencies of adsorbed TU and thioureate anions.

2. Model and Computational Details The theoretical study of the adsorbed TU and thioureate species was carried out using a cluster for modelling the unreconstructed Au(111) surface. This choice is justified both by the preferential (111) orientation of the gold thin film electrodes used in this work and by the results of previous studies with metal surfaces of low Miller indices that show that experimental and harmonic calculated frequencies of adsorbed carboxylate anions do not depend significantly on the crystallographic orientation of the metal surface. 48;49;57 The model consists of a metal cluster of 31 gold atoms arranged in two layers of 19 and 12 atoms, plus either a TU or a thioureate species. The size of the metal cluster is big enough to prevent, during a full optimization of the adsorbate geometry, the bonding of the adsorbate species to the metal atoms in the cluster border. The geometry of the metal cluster was kept fixed, with the gold nuclei located at their positions in the truncated crystal, and the same distances between neighbouring gold atoms as in the bulk metal (0.28837 nm).

58

We studied the effect of adding a water molecule and of applying an

external electric field in the normal direction to the metal surface, of 0.01 atomic units (equivalent to 5.14·109 V m-1), plus a water molecule. These two ingredients in the model account (at least partially) for the importance of specific interactions with water (formation of hydrogen bonds, and screening of charges and dipoles in adsorbates) and applied potential (that affects to the charge density on the metal, influencing the electronic structure of the adsorbates). The field is applied as to generate a positive charge density at the metal surface in contact with the adsorbate. This situation corresponds with experimental electrode potentials positive to the potential of zero charge (PZC). The inclusion of a water molecule and the application of an external electric field were also needed in some cases in order to reproduce the experimental frequencies. The added water molecule is important, as the formation of hydrogen bonds affects the frequency values, especially in the range used for the identification of the adsorbates (1700-1200 cm-1). A systematic study of the effect of adding more water molecules was not carried out. A full optimization of the geometry of the adsorbate system was carried out, using the B3LYP functional as implemented in the Gaussian 03 code. parameter hybrid exchange functional of Becke

60

59

The B3LYP functional combines the three-

with the Lee-Yang-Parr correlation functional.

61



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This functional, in combination with the 6-31+G(d)

62-64

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basis set for the C, O and H atoms, and the

LANL2DZ 65 effective core potential and associated double zeta basis set for describing the gold metal atoms, has been used to estimate theoretical values of harmonic frequencies that agree remarkably well with the experimental values measured for adsorbed acetate on Cu,

66;67

Ag

57

and Au

48

surfaces. A

good agreement between theoretical and experimental frequency values was also found for some acetate derivatives, such as fluoroacetates,

49

glyoxylate

51

and glycolate

50

anions adsorbed on gold surfaces.

All frequency values are given without scaling. Assignments of the calculated frequencies are based on the visualization of the vibrational normal modes using Molden. 68

3. Experimental Section Perchloric acid working solutions were prepared from the concentrated acid (Merck Suprapur®) and ultrapure water (18.2 MΩ·cm, Elga Vivendi). TU (> 99%, Sigma-Aldrich) was added to the perchloric acid solution to obtain the desired concentrations. Solutions were deaerated with Ar (N50, Air Liquide). Solutions in deuterated water were prepared with deuterium oxide (99.9 atom %D, Aldrich), which was used as received. The working electrodes used in the electrochemical and in situ external reflection infrared spectroscopy experiments were gold single crystals with diameters around 2.0 and 4.5 mm, respectively. They were prepared from a high purity gold wire (99,9998% Alfa-Aesar) following Clavilier’s method.

69;70

Prior

to each experiment, the electrodes were heated in a gas-oxygen flame, cooled down in air and protected with a droplet of ultrapure water. 70-72 In the internal reflection infrared spectroscopy experiments, a 25 nm-thick gold thin film thermally evaporated on a silicon prism was used as the working electrode. Film deposition was carried out in the vacuum chamber of a PVD75 coating system (Kurt J. Lesker Ltd.) equipped with a turbomolecular pump. Before deposition, pressure was lowered to ca. 10-6 Torr. The deposition rate, which was fixed at 0.006 nm s-1, and the thin-film thickness were monitored with a quartz crystal microbalance. Once assembled the spectroelectrochemical cell, the gold film was electrochemically annealed by cycling the electrode potential between 0.10 and 1.20 V RHE in a 0.1 M HClO4 + 10 mM CH3COONa solution at 20 mV s-1 for 1-2 hours. 54 Then the solution was replaced by an acetate-free perchloric acid solution. All the voltammetric and in situ infrared experiments were performed in glass cells using a gold wire as the counter electrode and a reversible hydrogen electrode (RHE) as the reference electrode. The cells 6 ACS Paragon Plus Environment

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used in spectroelectrochemical experiments

73;74

are equipped with a prismatic window beveled at 60°.

This prism was made from Si or CaF2, for the internal and external reflection experiments, respectively. All the spectra were obtained with a resolution of 8 cm-1 with a Nexus 8700 (Thermo Scientific) spectrometer equipped with a MCT-A detector. They were calculated as – log (Ro/R) (where R and Ro stand, respectively, for the reflectance measured at sample and reference conditions) and are presented in absorbance units (a.u.). Thus, positive- and negative-going bands correspond, respectively, to species being formed/consumed when collecting the sample single beam spectrum. In some of the external reflection experiments each spectrum was obtained from twenty sets of 100 interferograms, which were accumulated alternately at the reference and sample potentials using the so called SNIFTIR (subtractively normalized interfacial Fourier transform infrared) technique.75 In the rest of in situ infrared experiments, sets of 100 or 200 interferograms were collected at increasing sample potentials above 0.10 V and referred to a reference single beam spectrum obtained from the same number of interferograms collected at this latter potential value. In the case of the ATR-SEIRA spectra this single beam spectrum was obtained before dosing TU in the 0.1 M HClO4 solution. Either p- or s-polarized radiation was used in the external reflection experiments. P-polarization was chosen to collect all the ATR-SEIRA spectra. For SERS experiments the nanostructured electrode was prepared by evaporating a gold thin film onto a polycrystalline polished gold disk (2mm in diameter) sheathed in a threaded poly(tetrafluoroethylene) (PTFE) piece. A 35 nm-thick gold film was deposited by thermal evaporation at 0.080 nm s-1 on the flat surface of a polished polycrystalline gold cylinder.

54

The use of the gold substrate is necessary in our

SERS experimental set-up in order to have electric contact with the gold thin-film electrode. Moreover, electrochemical annealing of the sample was avoided since it causes a significant decrease of the SERS enhancement.

54

The substrate was then mounted on an electrochemical PTFE flow cell specifically

designed for the in situ Raman measurements.

76

A saturated Ag/AgCl electrode was used as reference

electrode and a Pt wire was used as counter electrode. Raman spectra were obtained with a NRS-5000 Laser Raman Spectrometer (Jasco). The excitation line was provided by a 17mW He–Ne laser at 632.8 nm. The laser beam was focused through a 50× long-working distance objective (0.5NA) into a 2 µm spot at the electrode surface. The spectrometer resolution was better than 5 cm-1 and the detector was a Peltier-cooled charge coupled device (CCD) (1064×256 pixels).



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4. Experimental results.

4.1. Thiourea adsorption at gold single crystal electrodes.

Figures 1 and 2 show typical voltammetric profiles obtained in the double layer potential region for Au(100) and Au(111) electrodes in TU-containing perchloric acid solutions for various concentrations ranging from 0.05 up to 10 mM. These curves were obtained in the first potential cycle after immersing the flame-treated electrode in the working solution at 0.10 V. The upper potential limit in Figure 1 was chosen at 0.60 V to avoid the irreversible oxidation processes that, as shown in Figure 2 for the Au(111) electrode, take place at more positive potentials for high enough TU concentrations. In this sense, curve b in Figure 2A shows that no significant further irreversible oxidation takes place for potentials between 0.60 and 1.0 V for the Au(111) electrode in the 0.05 mM TU solution whereas irreversible TU oxidation clearly takes place above 0.60 V in a 10 mM TU solution (curve b in Figure 2B). On the other hand, the unbalanced oxidation and reduction voltammetric charges in Figure 1B show that irreversible oxidation processes also appear for the Au(111) electrode at potentials below 0.60 V for TU concentrations equal or higher than 1 mM. The corresponding peak current density increases with TU concentration (although not linearly), and presents the tail typical of currents limited by diffusive transport. Noticeably, no irreversible oxidation of TU seems to take place below 0.60 V for the Au(100) electrode irrespective of the TU concentration (Figure 1A).

Figure 1. Cyclic voltammograms (first cycle after immersion at 0.10 V) for A) Au(100) and B) Au(111) single crystal electrodes in 0.1 M HClO4 + x mM thiourea solutions. a) x = 0 ; b) x = 0.05 ; c) x = 0.2 ; d) x = 1 ; e) x = 5; f) 10 . Scan rate = 50 mV s-1. 8 ACS Paragon Plus Environment

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Figure 2. Cyclic voltammograms recorded for a Au(111) single crystal electrode in a 0.1 M HClO4 + x mM thiourea solution up to a) 0.60 and b) 1.0 V. A) x = 0.05 ; B) x = 10. Scan rate = 50 mV s-1.

Besides the irreversible TU oxidation features described above for the Au(111) electrode, Figure 1 shows structure-sensitive voltammetric profiles below 0.60 V for the Au(100) and Au(111) electrodes in the presence of low TU concentrations. As a common characteristic for these two electrode orientations, for TU concentrations below 1 mM, and similarly to the behavior previously reported in previous works for polycrystalline and Au(111) electrodes,15;17 an increase of the voltammetric charge is observed in the TU-containing solutions with respect to that measured in the TU-free perchloric acid solution for potentials below 0.60 V. This voltammetric behaviour can be related to either a potential-dependent reversible TU adsorption-desorption process or to a surface redox process at constant TU coverage. The voltammetric curves reported in Figure 1B show that the excess of charge density recorded up to 0.60 V for the Au(111) electrode increases with the TU concentration up to 1mM. In the case of the Au(100) electrode, it can be noted that the voltammetric curve recorded in the 0.05 mM TU solution (curve b in Figure 1A) shows a pair of sharp well-defined peaks at ca. 0.42 and 0.37 V in the positive- and negative-going sweeps, respectively, that shifts to less positive potentials as the TU concentration increases. The voltammetric peak appearing between 0.39 and 0.42 V in the positive-going sweep in Figure 1A could be related to the potential-dependent lift of the surface reconstruction of the flame9 ACS Paragon Plus Environment


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annealed Au(100) electrode. As observed in curve a in Figure 1A, this peak is observed at 0.85 V for the TU-free perchloric acid solution 71 and is known to shift towards less positive potentials in the presence of adsorbates that stabilize the unreconstructed surface.11;71;72 Hamm and Kolb

11

reported a shift of

the peak for the lifting in the positive-going sweep of the surface reconstruction of the Au(100) electrode from 0.35 to 0.30 V SCE (i.e. from 0.59 to 0.54 V RHE in our experiments) for TU concentrations ranging from 10-8 to 10-5 M. The existence of a potential-dependent lifting of the reconstructed surface obtained after flame annealing for the Au(111) electrode has also been described. 71;72

However, no clearcut peak for this process can be appreciated in Figure 1B for the solutions

containing TU at concentrations higher than 10-5M. In the experiments reported in Figure1A for Au(100), the cyclic voltammograms recorded for TU concentrations above 1 mM show a decrease of the height of the peak for the lifting of the surface reconstruction together with a lowering of the integrated voltammetric charge up to 0.60 V. Besides, this charge decreases steadily with cycling in the latter potential range for a given TU concentration (not shown). This latter behavior can also be observed in the case of the Au(111) electrode, thus suggesting the existence of some process leading to the irreversible blocking of the electrode surface in the TUcontaining solutions. Thus, it could be expected that both the Au(100) and Au(111) electrodes are probably covered by TU and thus not reconstructed in the whole potential region above the hydrogen evolution reaction in the presence of high enough TU concentrations. In order to obtain molecular level information on the electrode processes taking place at the gold electrodes in the TU-containing solution, in situ infrared spectroscopy has been used to characterize the species present on the electrode surface under these conditions. Some external reflection infrared spectra obtained with a Au(100) electrode at different sample potentials in TU-containing D2O solutions are shown in Figures 3 and 4. In these experiments, deuterium oxide was chosen as the solvent in order to avoid interferences from uncompensated O-H bending bands which are more important in the external than in the internal reflection experiments (see below). In fact, no TU bands in the spectral region around 1600 cm-1 have been observed in the in situ spectra obtained in external reflection experiments carried out in water solutions (not shown). Figure 3 shows a set of potential-dependent spectra collected with the Au(100) electrode in (A) 0.05 and (B) 10 mM TU solutions. In both cases, the corresponding reference spectrum was obtained at 0.10 V after immersing the flame-annealed electrode and fitting it against the infrared window. Then, the electrode potential was changed in 0.10 V intervals up to 1.0 V and a sample spectrum collected at each potential. As previously reported by García et al

17

for the

Au(111) electrode in similar experiments, the resulting spectra in the 10 mM solution (Figure 3B) show 10 ACS Paragon Plus Environment

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negative-going bands at ca. 1380 and 1520 cm-1. These bands were assigned, respectively, to the asymmetric and symmetric NCN stretching modes of dissolved deuterated TU.

17

The sign of these

features indicates that they correspond to the consumption of species existing at/near the electrode surface at the reference potential. Positive-going bands appear at ca. 1390 and 1585 cm-1 for sample potentials up to 0.50 V. Based on their absence in the spectra collected with s-polarised light, García et al. ascribed similar bands for the Au(111) electrode to the symmetric and asymmetric NCN stretching modes of adsorbed TU.

17

Similar but less intense bands as those described above for the 10 mM TU

solution are observed in the spectra collected in the 0.05 mM solution for potentials below 0.60 V (Figure 3A). At higher potentials new positive-going features are observed in Figure 3B at ca. 1560 and 1631 cm-1 for the 10 mM solution. Similar bands have been ascribed, respectively, to the asymmetric NCN stretching mode of dissolved [Au(I)-Tu2]+ and FDS molecules formed upon gold and TU oxidation.

17

The assignment of these features to dissolved species was based on the observation of

absorption bands at the same frequencies in the spectra collected with s-polarised light. 17

Figure 3. In situ external reflection infrared spectra collected with p-polarized radiation for a Au(100) single crystal electrode at different sample potentials in A) 0.05 and B) 10 mM thiourea + 0.1M HClO4 solutions prepared in D2O. Reference potential 0.10 V. 200 Interferograms were collected at each potential.



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Figure 4. SNIFTIR spectra collected with p- and s-polarized radiation for a Au(100) single crystal electrode at 0.3 and 0.6 V in A) 0.05 and B) 10 mM thiourea + 0.1M HClO4 solutions prepared in D2O. Reference potential 0.10 V. 2000 Interferograms were collected at each potential.

In an attempt to improve the signal to noise ratio of the in situ infrared spectra obtained under external reflection conditions, additional experiments were carried out with both Au(100) and Au(111) electrodes by averaging a higher number of interferograms with the SNIFTIR

75

approach, i.e. by

alternating the electrode potential from the reference to the sample values while collecting the corresponding spectra. The spectra collected under these conditions for sample potentials of 0.30 and 0.60 V with s- and p-polarised light in 0.05 and 10 mM TU solutions are shown in Figure 4 for the Au(100) surface. Spectra collected under similar conditions for a Au(111) electrode are reported as supporting information (Figure S1). Observed features in these spectra appear as bipolar bands centered at ca. 1380 and 1540-1580 cm-1. The intensities of these features increase when increasing the sample electrode potential from 0.30 and 0.60 V, being somewhat higher in the 10 mM solution. Except for the 12 ACS Paragon Plus Environment

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spectra collected at 0.60 V in the 10 mM solution, the disappearance of the positive- and negative-going features when recording the spectra with s-polarized radiation indicates that these features are related to the presence of adsorbed species at both the sample and the reference potentials. This statement is based on the surface selection rule for external reflection infrared spectroscopy, which precludes the observation of bands for adsorbed species with s-polarised light.

77

At 0.60 V, similar spectral features

are observed irrespective of the polarization of the infrared beam, except for a small feature at 1584 cm-1 in the spectrum obtained with p-polarised light for the Au(100) electrode. The observation of absorption bands in the spectra collected with s-polarised light suggests that they have some contribution from dissolved species under these conditions. This behaviour is consistent with the observation of irreversible oxidation processes in the voltammetric experiments involving a net consumption of dissolved TU molecules and, probably, the formation of the gold-TU complex as witnessed by the absorption band at ca. 1560 cm-1. 17 Regarding the comparison of the spectroelectrochemical behavior of the Au(100) and Au(111) electrode in the TU-containing solutions, it can be pointed out that the observed bands for both electrodes are alike,

with similar effect of electrode potential and TU

concentration on the bands recorded with p- and s-polarised light. As a significant difference between the spectra collected for the Au(100) and Au(111) electrodes in the TU-containing solutions, some of the characteristic features for adsorbates (those disappearing when changing beam polarization from p to s) in the spectral region above 1500 cm-1 have frequency values somewhat higher in the case of the Au(100) electrode (1580-1590 cm-1 instead of 1570-1580 for the Au(111) electrode). It has to be noted that the collecting mode of the spectra reported in Figures 4 and S1 favors the accumulation either at the electrode surface or in the thin solution layer of species being formed upon TU adsorption/oxidation at the more positive potentials. In this way, negative-going adsorbate bands in these spectra originate from adsorbed species being accumulated when stepping the electrode potential from the reference to the sample values and viceversa. On the other hand, it can not be concluded from the external reflection experiment whether adsorbed TU could already be formed at the reference potential when fitting the electrode surface against the infrared window. However, this verification can be done, as described below, in the internal reflection experiments for which no limitations exist for the transport of molecules being dosed at a controlled potential and a proper reference spectrum can be collected before dosing TU. Besides, adsorbed species formed from TU can be more easily detected from the ATR-SEIRA spectra obtained with gold thin-film electrodes taking advantage of the enhanced sensitivity associated to the SEIRA effect. 53;55;56



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4.2.

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Adsorption of thiourea at gold thin-film electrodes.

The spectroelectrochemical study reported above for TU adsorption on gold single crystal electrodes has been extended to gold thin film electrodes deposited on silicon substrates. The stationary cyclic voltammograms recorded in perchloric acid solutions in the presence of 0.05 mM TU for these (111)preferentially oriented gold thin film electrodes (in the following noted as Au(111)-25nm) are shown in Figure 5 for various upper potential limits ranging from 0.68 to 1.0 V. The increase of the voltammetric charge density related to the presence of TU takes place in the same potential region as in the case of the Au(111) single crystal electrode surface and reaches its maximum when limiting the potential excursion to 0.68 V. Irreversible oxidation processes take place at potentials above 0.70 V giving rise in successive cycles to a decrease of the voltammetric charge at lower potentials.

Figure 5. Cyclic voltammograms recorded for a Au(111)-25 nm thin film electrode in a 0.1 M HClO4 + 0.05 mM thiourea solution with increasing upper potential limits. Scan rate = 50 mV s-1.


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Figure 6. Series of time-dependent ATR-SEIRA spectra obtained at 0.10 V with a Au(111)25 nm thin film electrode in a 0.1M HClO4 solution after the dosing of thiourea to reach a final concentration equal to 0.05 mM. The reference spectrum was collected at 0.10 V before the addition of thiourea. 100 interferograms collected with p-polarized radiation were co-added to obtain each spectrum. The inset shows the plot as a function of time of the integrated intensities of absorbance bands observed at 1654 and 1411 cm-1.

Figure 6 shows a set of time-dependent ATR-SEIRA spectra collected at 0.10 V for the Au(111)-25 nm electrode in the perchloric acid test solution after the addition of 0.05 mM TU. These spectra are referred to the single beam spectrum collected at the same electrode potential before the addition of TU. Several positive-going bands appear in the resulting absorbance spectrum under these conditions with their intensities increasing with time after dosing. The surface specificity of the SEIRA effect suggests that these features correspond to adsorbed species. As it will be discussed below, some of these features can be clearly related to adsorbed TU, namely the band at 1423 cm-1 and the small feature at 1500 cm-1. 15 ACS Paragon Plus Environment


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Also a strong band is observed at 1650 cm-1 that could be tentatively related in a first approximation to the O-H bending of interfacial water molecules.53;55;78 This preliminary assignment would be consistent with the observation of a strong broad band above 3000 cm-1, with shoulders at 3587, 3351 and 3228 cm-1, which is typical of the O-H stretching bands related to interfacial water molecules.

53;55;78

However, DFT calculations and experiments performed in deuterium oxide solutions suggest that the band at 1650 cm-1 has a strong contribution related to adsorbed TU (see below). Another piece of information that can be useful for the adscription of the band at 1650 cm-1 in the spectra reported in Figure 6 is its time-dependent behavior after TU dosage. Plots of the integrated intensity of the bands at 1650 and 1416 cm-1 (reported in the inset in Figure 6) show a similar time dependence that suggests that both features could be associated to the same species. On the other hand, the time plot in Figure 6 indicates that after ca. 10 min the band at 1416 cm-1 reaches it maximum intensity whereas that for the band at 1654 cm-1 still increases for longer times in parallel to the growth of the bands for the O-H stretching mode of interfacial water. Finally, it has to be pointed out the observation in Figure 6 of another absorption band at ca. 1114 cm-1 that can be related to interfacial perchlorate anions

78

that,

together with water molecules, would be co-adsorbed with TU at the gold electrode surface. It is to be remarked that the accumulation of adsorbed TU does not produce a significant decrease of the perchlorate band. A similar behavior was reported in the case of amino acids adsorbed on gold in acidic media.

45;47

It those cases the perchlorate anions were retained in the close vicinity of the chemisorbed

layer in order to compensate the positive charge of the ammonium group of the specifically adsorbed zwitterionic aminoacid. In the case of adsorbed TU, no positively charged group is expected to be present, but it must be taken into account that TU can bind to a number of anionic species.

42

In the

case of perchlorate, the TU-anion interaction would probably involve the formation of hydrogen bonds between the oxygens of the anion and the hydrogen atoms of amino groups of TU (imino groups in the case of thioureate). Some contribution coming from the TU-related adsorbates can also be expected in the region around 1100 cm-1, as theoretical calculations also yielded frequency values in this range, due to vibrational modes involving NH2 rocking in combination with symmetric NCN stretch (see below). Figure 7A shows a selected group of the time-dependent ATR-SEIRA spectra collected in the experiment reported in Figure 6. These spectra can be compared to those shown in figure 7B, which correspond to a similar experiment performed in a deuterium oxide solution. Under these latter conditions, bands for adsorbed species are expected to shift to lower wavenumbers if some of their hydrogen atoms are replaced by deuterium atoms.

17

In this way, the band at ca. 1654 cm-1 in water is

observed as a splitted band with peaks at ca. 1546 and 1565 cm-1 in deuterium oxide solutions. The relative intensities of these features change with time after dosing, with that at 1546 cm-1 prevailing for 16 ACS Paragon Plus Environment

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long times (see bottom spectrum in Figure 7B). On the other hand, the bands at 1496 and 1411 cm-1 in Figure 7A seem to shift, respectively, to 1380 and 1212 cm-1 in deuterium oxide solutions. Note that, in the latter solvent, bands around 1200 cm-1 in the ATR-SEIRA spectra could have some contribution from the O-D bending mode of interfacial deuterium oxide as well as from the uncompensated absorption from the silicon window. Regarding the features at 1546-1565 and 1380 cm-1, it can be said that they appear in the same spectral regions as the bipolar bands observed for Au(111) and Au(100) also in deuterium oxide solutions (Figures 4 and S1 and reference

17

). As mentioned above, these

features were assigned to the asymmetric and symmetric N-C-N stretching modes of adsorbed thiourea. 17

The band around 1114 cm-1 is not significantly affected by the change of the solvent, in agreement

with its preliminary assignment to perchlorate anions.

Figure 7. Selected time-dependent ATR-SEIRA spectra obtained at 0.10 V with a Au(111)25 nm thin film electrode in a 0.1M HClO4 solution prepared in A) water and B) deuterium oxide after dosing thiourea to reach a final concentration equal to 0.05 mM. The reference spectrum was collected at 0.10 V before the addition of thiourea. 100 interferograms collected with p-polarized radiation were co-added to obtain each spectrum.



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Figure 8. Potential-dependent ATR-SEIRA spectra obtained with an Au(111)-25 nm thin film electrode in a 0.05 mM thiourea + 0.1M HClO4 solution prepared A) in water B) in deuterium oxide. The reference spectrum was collected at 0.10 V before the addition of thiourea. 100 interferograms collected with p-polarized radiation were co-added to obtain each spectrum.

The potential-dependent behavior of the ATR-SEIRA spectra obtained for adsorbed TU in water solution after dosing at 0.10 V is reported in Figure 8A. A very small shift of the bands appearing at 1650 and 1420 cm-1 can be appreciated for potentials between 0.10 and 0.70 V. Noticeably, the small feature at ca. 1500 cm-1 is replaced at this latter electrode potential value by a new small band at around 1546 cm-1. It is worth mentioning that these potential-dependent changes (namely, the observation of the new band at ca. 1546 cm-1) can also be appreciated in the spectra obtained in experiments in which dissolved TU was removed from the working solution at 0.10 V (by flushing a generous amount of pure test electrolyte) after the adsorption step and before stepping the electrode potential to more positive values up to 1.0 V (see Figure S2 in the supporting information). Observed bands are basically similar to those reported in Figure 8A proving that TU remains irreversibly adsorbed at the gold electrode surface and that the surface process responsible for the appearance of the band at ca. 1546 cm-1 takes place irrespective of the presence of dissolved TU. 18 ACS Paragon Plus Environment

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Experiments similar to that reported in Figure 8A were carried out in deuterium oxide solutions. The corresponding spectra, shown in Figure 8B, show also significant potential-dependent changes for adsorbed TU bands. Namely, the intensity of the band observed at 1380 cm-1 at 0.10 V decreases when the electrode potential is increased. This change is paralleled by the development of a new feature at ca. 1322 cm-1 for potentials above 0.50 V. Note that the spectrum collected at 1.0 V in the deuterium oxide solution still shows a significant feature at 1380 cm-1. When comparing the spectra collected at 1.0 V in water or in deuterium oxide, it could be concluded that the characteristic feature appearing at 1546 cm-1 in the water solution is redshifted to 1322 cm-1 when the solvent is deuterium oxide. Again, the observed shifts can be related to the effect of replacing hydrogen by deuterium in the corresponding adsorbed species. However, direct relationships between the observed bands in both solvents can only be established after their assignment on the basis of the results of DFT calculations (see below). Another relevant effect of the electrode potential that can be appreciated in the spectra reported in Figure 8B is related to the splitting of the main band appearing at 1546 cm-1 in the spectrum collected at 0.10 V. For potentials between 0.30 and 0.70 V, the intensity of the shoulder appearing at higher wavenumbers increases and appears as a well-defined peak at 1581 cm-1. Finally, the intensity of this feature decreases again at a potential of 1.0 V for which a single peak is observed at 1558 cm-1.

Figure 9. Potential-dependent SER spectra collected for a gold thin film electrode in a 0.1M HClO4 solution prepared in water after dosing thiourea at 0.10 V from a 0.1 mM thiourea + 0.1M HClO4 solution and flushing the spectroelectrochemical cell with thiourea-free perchloric acid. The vibrational information obtained from the ATR-SEIRA spectra presented above can be complemented with some Raman scattering data. Raman spectroscopy allows the observation of 19 ACS Paragon Plus Environment


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specific vibrational features in a wide spectral region that includes the low wavenumber region below 1000 cm-1 (not accessible in IR experiments because of the higher cutoff of the detector and window) where metal-adsorbate vibrations appear. Even if not so high as for other substrates, the gold thin film electrodes used in the ATR-SEIRAS experiments present a high enough enhancement of the surface scattering from adsorbates that allows the observation of the corresponding bands in the SER spectra. To achieve this goal, and as a difference with the samples used in the ATR-SEIRAS experiments, the thermally evaporated gold thin films used in the SERS experiments were prepared at a somewhat higher deposition rate and were not electrochemically annealed. A set of potential-dependent in situ SER spectra collected in a 0.10 M HClO4 solution prepared in water is presented in Figure 9. As in the case of the experiment reported in Figure S2, spectra were obtained after dosing TU at 0.10 V and, subsequently, replacing the working solution at constant electrode potential by TU-free perchloric acid solution. Then SER spectra were collected first at 0.10 V and, subsequently, at increasing electrode potentials up to 1.0 V. Bands observed at 938 and 637 cm-1 in Figure 9 can be assigned to the symmetric (Cl-O) stretching of perchlorate anions

79

(either co-adsorbed or in solution) whereas the

features at 1107, 714 and 239 cm-1 could be related to adsorbed TU. Note that similar bands have been reported by Parker and Hope 8 and Holze and Schomaker.

10

The assignment of these bands to specific

vibrational modes will be discussed below. Now, we can anticipate that the band at 239 cm-1 appears in the typical region for the Au-S vibration thus suggesting the adsorption of TU to the surface gold atoms via the sulfur atom.

8;10

On the other hand, it has to be noted that no significant changes from the

spectrum reported in Figure 9 are observed in the spectra collected at increasing electrode potentials up to 0.60 V. On the other hand, the desorption of TU adsorbates seems to take place at 1.0 V from the decrease of the corresponding scattering bands in the spectrum collected at this electrode potential.

5. Discussion. Results reported above deal with the potential-dependent spectroelectrochemical behavior of adsorbates formed from TU-containing solutions on gold single crystal and thin film electrodes. The results obtained for the Au(111) and Au(100) electrodes can be compared to those previously published in the literature. García et al discussed the in situ potential-dependent infrared spectra obtained for polycrystalline and Au(111) electrodes in TU-containing D2O solutions.

17

From the effect of the

polarization of the infrared beam on the spectra collected in a 10 mM TU solution, the authors concluded the existence of a potential-dependent adsorption-desorption process for potentials below 0.4 V. Namely, they reported bands corresponding to the reference potential of 0.10 V (which are also 20 ACS Paragon Plus Environment

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observed with s-polarized light) that were ascribed to dissolved TU molecules disappearing from the thin solution layer upon adsorption. These bands, appearing at ca. 1515 and 1380 cm-1, were assigned respectively to the symmetric and asymmetric N-C-N stretching modes. These bands were blue-shifted for adsorbed TU molecules, disappearing when the polarization of the infrared beam was changed from p to s. The authors explained the observation of a band for the asymmetric NCN stretching of adsorbed TU assuming the existence of some tilting of the S-C axis with respect to the surface normal. The presence of irreversibly adsorbed species at 0.10 V was derived by the authors from the shift towards more negative potentials of the signal from hydrogen evolution reaction in DEMS experiments performed in the TU-containing solution. García et al. also reported the observation of bands for dissolved species formed upon TU oxidation at potentials above 0.50 V.

17

These bands were assigned to the formation of [Au(I)(TU)2]+ complexes (a

band at ca. 1560 cm-1 for potentials above 0.50 V) and FDS (a feature around 1635 cm-1 at slightly more positive potentials). Note that the former frequency value is similar to that of one of the bands observed in Figures 4 and S1 at 0.60 V in the 10 mM TU solution, suggesting that the gold-TU complex is also formed under these conditions. The absorption band at 1631 cm-1 reported by García et al

17

for the

formation of FDS has also been observed in this work for the Au(100) electrode at 0.90 V in the 10 mM TU solution (Figure 3B). Bands reported by García et al. 17 for cyanamide (H2NCN), carbon dioxide or bisulfate anions appear at potentials above the region explored in this paper. Whereas the use of single crystal electrodes allows the study of the structure sensitive behavior of the adsorption and oxidation processes, the improved sensitivity of the ATR-SEIRA spectra obtained for the gold thin-film electrodes facilitates the detection of absorption bands for adsorbed TU. This much higher signal-to-noise ratio achieved for the gold thin film electrodes is related to the surface specificity of the SEIRA effect and the absence of interferences from features associated to the consumption of TU in solution. This favorable situation allows the observation of the absorption bands coming from adsorbed TU or related species at lower TU concentration (i.e. lower adsorbate coverages) than in the case of the external reflection experiments. Moreover, and in contrast with external reflection experiments, spectral features associated to adsorbed TU molecules can also be clearly observed in water solutions due to lower interference from water bands. Another observation allowed by the use of the Kretschmann’s configuration in the ATR-SEIRA spectra is the formation of adsorbed species at the dosing potential of 0.10 V. Besides, small features not being detected in the external reflection experiments have been observed in the ATR-SEIRA spectra and related to the processes undergone by adsorbed TU at potentials just below the beginning of its irreversible oxidation. Namely, these new 21 ACS Paragon Plus Environment


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features appear, at potentials equal or higher than 0.60 V, at 1322 cm-1 in the spectra collected in D2O solution and at 1546 cm-1 in water solutions. Other than the irreversible oxidation of the gold surface atoms to form the corresponding gold-TU complex and that of TU itself to give rise to FDS, some authors have proposed the partial deprotonation (involving charge transfer) of one of the –NH2 groups of the adsorbed TU molecule leading to the formation of a thioureate anion.

33;34;39

The formation of this species from adsorbed TU can be

spectroscopically confirmed from the assignment of the features at 1546 and 1322 cm-1 to adsorbed (NHCSNH2)ads and (NDCSND2)ads species, respectively. This is supported by the comparison of the experimental spectra discussed above with the calculated DFT frequencies for these species.

Figure 10. A) Side and B) top views of the optimized geometry (at the B3LYP/LANL2DZ,6-31+G(d) level) corresponding to thiourea adsorbed on a Au31(111) model cluster.

Similarly to previous studies for other adsorbates,

45;47-51

the system composed of either a TU or a

thioureate species adsorbed on a Au31(111) cluster was fully optimized at the B3LYP/631+G*,LANL2DZ level. This model and theory level have been previously shown to provide a good description of adsorbate geometries and vibrational frequencies. Irrespective of the application of an external electric field of 0.01 a.u., the optimized geometry for adsorbed TU shows that the molecular (NCSN) plane is perpendicular to the cluster surface and misaligned by 30º from the dense rows of the outermost metal layer, with unidentate bonding to the metal surface through the sulfur atom. The Au-S bond is slightly tilted (around 13º, with water and applied field) from the surface normal (Figure 10). In 22 ACS Paragon Plus Environment

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turn, the C-S bond is tilted by 45º from the normal, making one of the hydrogen atoms of the –NH2 group closer to the metal to be near a tetrahedral (fcc) surface site. The absence or presence in the calculations of the external fields slightly modifies the bonding distances and angles in the adsorbate.

Figure 11. A) Side and B) top views of the optimized geometry (at the B3LYP/LANL2DZ,631+G(d) level) corresponding to thioureate adsorbed on a Au31(111) model cluster under an external electric field of 0.01 a.u.

Partial deprotonation (involving charge transfer) of one of the –NH2 groups of the TU molecule would lead to the formation of an adsorbed thioureate species, which presents significant changes in its calculated geometry and orientation when adsorbed at the gold surface with respect to adsorbed TU. The optimized geometry of adsorbed thioureate (in the presence of a water molecule and external field) corresponds to bridge bonding to the gold through the sulfur and the nitrogen atom of the NH group with a bidentate configuration (Figure 11). Both the bonded S and N atoms lie close to top adsorption position. On the other hand, and similarly to the adsorbed TU, the molecular plane is perpendicular to metal surface. However, and as a consequence of bidentate bonding, it is aligned with the dense surface metal rows. Finally, the C-S bond is more strongly tilted (around 60-70º from the normal) than in the case of TU. As in the case of TU, the application of external field and addition or one water modify slightly the bond distances and angles of the adsorbate, without affecting neither the orientation of the molecular plane nor the bonding mode. The calculated B3LYP/6-31+G*,LANL2DZ harmonic vibrational frequencies corresponding to the equilibrium geometries for adsorbed TU and thioureate species described above are summarized in 23 ACS Paragon Plus Environment


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Tables 1 to 4, for the spectral range between 1100 and 1800 cm-1 and both for the non-deuterated and deuterated species. The difference between calculated and experimental frequencies for the TU and thioureate adsorbates are in general comparable to, or

somewhat higher than the experimental

-1

uncertainty, 8 cm , and amount to relative errors that are below 3% for the calculated frequencies. It is well known that the harmonic frequencies calculated using the B3LYP functional in combination with Pople basis sets similar to the one used by us, differ from the experimental values for gas phase molecules, typically by 2-3%. This is usually corrected by applying an average scaling factor that has been obtained from the comparison of calculated and experimental frequencies for a set of test molecules. We have not applied any scaling to the calculated frequencies in this paper, as there is no reported scaling factor for the cases involving adsorption on metals described using LANL2DZ pseudopotentials. We do not expect, however, the scaling factor to differ significantly from those reported for gas-phase calculations. Taking into account this remark, the agreement between experimental and calculated frequencies can be considered as rather satisfactory. In the case of thioureate, we report the values corresponding to the calculations with added water and field, as these provide the best agreement with the experimental values. The calculated infrared frequencies in the latter spectral range correspond mostly to combination modes that include contributions from the NH and NH2 bendings and from the stretchings (both asymmetric and symmetric) of the NCN group. All these modes are affected by the isotopic substitution of the hydrogen atoms in the –NHx groups and, in some cases, have distinct characteristic frequencies for adsorbed TU and thioureate species. The calculated B3LYP/LANL2DZ,6-31+G(d) harmonic frequencies for the combination modes of adsorbed TU lie in the same spectral region than the frequencies of the bands observed in the external (Figures 3, 4 and S1) and internal (Figures 6-10 and S2) reflection infrared spectra obtained in the presence of TU at low electrode potentials. According to our DFT calculations, the main band at ca. 1650 cm-1 in the ATR-SEIRA spectra collected at low potentials in water has, in addition to some eventual contribution from the OH bending mode of interfacial water, a contribution from the bending of the NH2 group. Calculations for the adsorbed deuterated TU molecule show that the corresponding ND2 bending mode should appear at ca. 1523 cm-1. This redshift is consistent with the experimental observation of an adsorbate band around 1560 cm-1 in deuterium oxide solutions. In the experimental spectrum two maxima are observed. This band splitting is not obtained in a single DFT calculation. We propose, as a tentative explanation, that it could originate from different hydrogen bonding environments, as it has been seen that different orientations of the water species hydrogen bonded to TU and thioureate slightly affect to the calculated vibrational frequencies in this spectral range. Note that the NH2 bending band can be distinguished in the SNIFTIR spectra collected for gold single crystal 24 ACS Paragon Plus Environment

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electrodes in D2O but can not be observed if the experiment was carried out in water due to the interference of uncompensated water bands (spectra not shown). Regarding the spectra collected at potentials higher than 0.60 V, no significant changes are observed for the bands that appear at ca. 1650 cm-1 in water and 1530 cm-1 in D2O with respect to their wavenumber at low potentials. The DFT calculations predict a slight blueshift of the NH bending band of thioureate compared to that for adsorbed TU.

The modes with main contributions of the asymmetric and symmetric NCN stretchings have calculated frequency values of 1497 and 1400 cm-1, respectively, for adsorbed TU in the absence of external field. These frequency values match well with the experimental frequencies at ca. 1496 and 1411 cm-1 in the spectra reported in Figure 7A. The calculated frequencies obtained for these modes of TU under electric field were 1420 and 1329 cm-1, very different from the experiment. The application of the field does not modify the orientation of the molecular plane, although it changes the electronic distribution in the molecule, which results in small changes in geometry, and significant changes in force constants. This indicates that the new frequencies appearing in the positive potential region are more likely due to an adspecies different from TU. For deuterated adsorbed TU, calculated frequencies for the same vibrational modes of TU appear (without field) at 1377 and 1209 cm-1, respectively whereas absorption bands are observed in Figure 7B at 1380 and 1214 cm-1, respectively. Note that the latter feature may have some contribution from uncompensated OD bending bands from interfacial deuterium oxide molecules as well as from Si-O features from the silicon window. Regarding the thioureate species, the main calculated bands appearing between 1540 and 1300 cm-1 can be assigned to the asymmetric (1530 cm-1) and to the symmetric (1396 cm-1) NCN stretchings respectively. Thus, the observed shift in the ATR-SEIRA spectra of the band at ca. 1500 cm-1 to ca. 1546 cm-1 when stepping the electrode potential from 0.10 V to potentials above 0.60 V can be tentatively related to the potential-driven deprotonation of adsorbed TU to form adsorbed thioureate species. According to our DFT calculations, this process takes place with a change from unidentate (with S-Au bond) to bidentate bonding through S and N(H) atoms. Taking into account the surface selection rule for adsorbed species both in the ATR-SEIRA and in the external reflection

77

80

spectra, which is related to the existence of a null component of the

electric field parallel to a metal surface interacting with electromagnetic radiation, the experimental observation of a band for the asymmetric NCN stretching implies the tilting of the S-C bond both for adsorbed TU and thioureate species with respect to the surface normal so that the variation of the dynamic dipole associated to the asymmetric NCN vibrational mode has a non-zero contribution normal to the electrode surface. On the contrary, an S-C bond normal to the electrode surface would imply a 25 ACS Paragon Plus Environment


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variation of the dynamic dipole of the asymmetric NCN stretching parallel to the metal surface, which would not be observable in the infrared spectra. Tilting of the S-C bond has been confirmed from the optimized geometries of both adsorbed TU and thioureate species.

The formation of adsorbed

thioureate species at potentials higher than 0.60 V is also consistent with the observation of the band at 1322 cm-1 in the spectra collected in deuterium oxide solution since the calculated frequency of the symmetric NCN stretch for adsorbed deuterated thioureate is found to be at 1329 cm-1. The spectra of deuterated TU obtained upon changing the electrode potential from an initial value of 0 V to different potentials up to 0.90 V show that with increasing potential the band around 1322 cm-1, characteristic of the mode of deuterated thioureate with main contribution of the symmetric NCN stretch, increases steadily. Conversely, the band with experimental frequency of 1380 cm-1, assigned to the asymmetric NCN stretch of deuterated TU, decreases with increasing potential. Both bands are the only ones that allow to identify the adsorbates, because, as it results from the DFT calculated frequencies, both adspecies have vibrations with frequencies around 1100, 1210 and 1550 cm-1. On this ground, it can be said that above 0.6 V the formation of thioureate is detected, its amount increasing with electrode potential. However, at potentials as high as 0.9 V, significant amounts of TU remain unreacted. This suggests the existence of a potential-driven equilibrium between both surface species. Even if both non-deuterated TU and thioureate adsorbates have theoretical frequencies around 1110 cm1

(see tables 1 and 3), the observation in Figure 6 and 7-10 of clearcut bands at ca. 1100 cm-1 suggests

the existence in this spectral region of significant contributions due to coadsorbed perchlorate anions. 78 The calculated bands of TU and thioureate in this region involve mainly contributions from NH2 rocking, and would be significantly redshifted upon deuteration. In this respect, it has to be recalled that the in situ ATR-SEIRA spectra presented in this work have been referred to the single beam spectrum collected at 0.10 V in the TU-free perchloric acid solution, thus corresponding to the electrode surface free of adsorbed TU but also free of adsorbed perchlorate, since the potential-dependent adsorption of this species does not take place at this potential.78 In the spectra reported in Figures 6 and 7, collected also at 0.10 V in the TU-containing solution, the observation of the perchlorate band implies that the coadsorption of this species is induced by the adsorption of TU. On the other hand, the potentialdependent spectra reported in Figures 8 and S2, for which the intensity of the perchlorate band is nearly constant, suggest that no additional adsorption of perchlorate takes place when increasing the electrode potential up to 1.0 V. This behavior seems to be linked to the existence of a nearly constant coverage of adsorbed TU, specially in the experiment reported in Figure S2, which was carried out in a TU-free


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solution. Finally, it seems that the formation of adsorbed thioureate upon deprotonation of adsorbed TU does not affect in a significant way to the co-adsorption of perchlorate anions. Regarding the band assignment of the SER spectra, table 5 shows the calculated Raman shifts for adsorbed TU and thioureate, respectively, on a (111) gold cluster, together with the experimental values corresponding to the SER spectrum reported in Figure 9 for an electrode potential of 0V. Again, a good agreement is found between most of calculated and experimental band frequencies. Namely, bands at 1107 and 714 cm-1 in the experimental spectra can be assigned, respectively, to the NH2 rocking and CS stretching modes of adsorbed TU which appear at 1069 and 727 cm-1 in the calculated spectra. Regarding the band at 239 cm-1, its frequency is significantly higher than the calculated frequency value for the Au-S stretching band which appears at 147 cm-1. Despite of this difference, which is typical for calculated frequency values for vibrations involving surface atoms, which in the cluster models are maintained fixed, it can be concluded from the comparison of the calculated and experimental Raman spectra that TU adsorbs to the gold surface via the sulfur atom. Potential-dependent SER spectra reported in Figure 9 do not show significant changes in the band frequencies in the potential region above 0.60 V for which the ATR-SEIRA spectra suggest the formation of adsorbed thioureate. From the calculated frequency values in table 5, the only expected change in the observed Raman frequencies when forming thioureate from adsorbed TU would be a shift of the Au-S stretching band towards higher wavenumbers. As mentioned above, the limited accuracy of the calculated metal-adsorbate bands in the cluster model does not allow extracting any conclusion from the SER spectra about the eventual formation of thioureate. On the other hand, the absence of changes in the SER spectra reported for potentials above 0.60 V can be connected to results reported in a previous work

8

regarding the absence

of SER bands for adsorbed FDS that could be formed upon TU oxidation at gold electrodes

6. Conclusions. The spectroelectrochemical behavior of TU has been studied on Au(111) and Au(100) single crystal electrodes and on preferentially (111)-oriented gold thin-films deposited on Si (Au(111)-25nm). Experimental conditions for the spectroscopic experiments reported in this paper (namely, TU concentrations below 0.1 mM and electrode potentials below 1.0 V), were chosen in order to avoid the electrodissolution of gold associated to the formation of gold(I)-TU complexes. Even if cyclic voltammetry profiles in the double layer potential region and in-situ infrared spectra in the same potential region could suggest that, in acidic solutions, the TU molecule adsorbs reversibly, in situ ATR27 ACS Paragon Plus Environment


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SEIRA experiments have provided evidence that irreversible adsorption takes place yielding adsorbed TU even at very low TU concentrations in a wide potential range above 0 V RHE. In situ SERS experiments confirm the observation of a low frequency band that can be associated to the Au-S vibration, indicating the bonding of the TU molecule via the sulfur atom. On the other hand, the main experimental infrared bands for non-deuterated adsorbed TU appear between 1100 and 1700 cm-1 and are due to normal modes with significant contributions from the NH2 bending and from the symmetric and asymmetric NCN stretchings. From the observation in the experimental spectra of a band due to the asymmetric NCN stretch, the surface selection rule allows to conclude that the S-C axis of adsorbed TU is tilted with respect to the surface normal. If the S-C axis were perpendicular to the electrode surface, the dynamic dipole corresponding to the asymmetric NCN stretch would be parallel to the surface, and, consequently, indetectable in these spectroscopic experiments because of having a null component in the direction of the surface normal. This is in agreement with the DFT-calculated optimized geometry of the adsorbed TU on cluster models of the Au(111) surface, that indicates an unidentate adsorption configuration, with the molecular plane perpendicular to the metal surface and misaligned 30º from dense surface metal rows. The molecule is bonded via the sulfur atom in on-top position slightly tilted (around 13º) and with the C-S bond tilted by 45º from the normal. The observation of a surface redox process at potentials between 0.60 and 0.90 V can be related to the deprotonation of adsorbed TU to form adsorbed thioureate species. This conclusion is based on the observation of changes in the characteristic frequencies of the adsorbate in this potential region, namely, the appearance of a new band around 1550 cm-1 in water solutions that, according to DFT calculations, can be assigned to the assymetric NCN stretching of adsorbed thioureate anions. None of the theoretical calculations with TU yielded vibrational frequencies near this value. The same holds for the observed band at ca. 1322 cm-1 in the ATR-SEIRA spectra collected in D2O solutions that can only be ascribed to the symmetric NCN stretching of adsorbed deuterated thioureate. The optimized adsorption geometry derived from the DFT calculations for adsorbed thioureate implies bidentate bridge bonding via the S atom and the N atom of the NH group, with the molecular plane perpendicular to the surface and the CS bond tilted around 60-70º from the normal. The presence in the spectra collected in D2O solutions of a band at 1380 cm-1 characteristic of deuterated TU, at potentials as high as 0.90 V indicates the existence of significant amounts of TU coexisting with thioureate coadsorbed at this potential. It must be stressed the point that most of the experimental bands found in the ATR-SEIRA spectra collected in the TU-containing solutions are characterized by very similar frequency values irrespective of the electrode potential. These values are in agreement with the corresponding theoretical B3LYP 28 ACS Paragon Plus Environment

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frequencies for adsorbed thiourea and thioureate. In addition, some vibrations with similar wavenumbers correspond to different normal modes of thiourea and thioureate as concluded from the calculated frequency values. This is due to changes in the values of the force constants arising from the modifications in electronic structure influenced by the different bonding, but also in some extent by the external electric field and the formation of hydrogen bonds. This system can be taken as an example of the subtleties that can arise in the analysis of experimental spectra, and of the value of the DFT calculations in providing geometry and frequency data which can greatly help in disentangling and correctly interpreting the experimental spectra.

Acknowledgements The authors thank the financial support from Ministerio de Economía y Competitividad (project CTQ2009-13142, Fondos FEDER), Generalitat Valenciana (ACOMP/2011/200 and ACOMP/2012/137, Fondos FEDER) and Universidad de Alicante. Prof. Juan Feliu is gratefully acknowledged for kindly providing the gold single crystal electrodes used in this work. William Cheuquepán is endebted to the Ministerio de Economía y Competitividad for the award of a FPI grant.

Supporting information available: SNIFTIR spectra collected with p- and s-polarized radiation for a Au(111) single crystal electrode thiourea-containing perchloric acid solutions prepared in D2O and potential-dependent ATR-SEIRA spectra obtained with an Au(111)-25 nm thin film electrode in a perchloric acid solution after dosing thiourea at 0.10 V and flushing thoroughly the spectroelectrochemical cell with thiourea-free perchloric acid solution. This material is available free of charge via the Internet at http://pubs.acs.org.



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Table 1. Comparison of the experimental and calculated (B3LYP/6-31+G(d),LANL2DZ) harmonic vibrational frequencies of thiourea adsorbed on a Au31(111) model cluster (without external field).

νexp / cm-1

νcalc / cm-1

1650

1685

δNH2 + υs NCN ….

1663

δNH2 + υas NCN ….

1496

1497

υas NCN + δNH2

1411

1400

υs NCN+ δNH2

1100

1089, 1069

Assignment

ρ NH2 + υs NCN

Table 2. Comparison of the experimental and calculated (B3LYP/6-31+G(d),LANL2DZ) harmonic vibrational frequencies of deuterated thiourea adsorbed on a Au31(111) model cluster.

νexp / cm-1

νcalc / cm-1

1565 / 1546

1523

δND2 + ….

1380

1377

υas NCN + δND2

1214

1209

υs NCN+ δND2

Assignment


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Table 3. Comparison of the experimental and calculated (B3LYP/6-31+G(d),LANL2DZ) harmonic vibrational frequencies of thiourea and thioureate adsorbed on a Au31(111) model cluster.

Thiourea

Thioureate + water

νcalc / cm-1 νexp / cm-1

(without external

νcalc / cm-1 νexp / cm-1

field)

(External field

Assignment

0.01 a.u.)

1654

1685 / 1663

1650

1718/1699

δNH2 + δOH

1502

1497

1546

1530

υas NCN + δNH + δNH2

1407

1400

1421

1396

υsym NCN + δNH2 + δNH

1100

1089, 1069

1100

1115

ρ NH2 + υs NCN

Table 4. Comparison of the experimental and calculated (B3LYP/6-31+G(d),LANL2DZ) harmonic vibrational frequencies of deuterated thioureate adsorbed on a Au31(111) model cluster.

νexp / cm-1

νcalc / cm-1

Assignment

1558

1527

υas NCN + δND2 + δND

1322

1329

υsym NCN + δND2 + δND + υ CS

1214

1248

δND2 + δND + υas NCN

1217

δND2 + δND + υsym NCN



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Table 5. Calculated (B3LYP/6-31+G(d),LANL2DZ) harmonic Raman shifts of thiourea and thioureate adsorbed on a Au31(111) model cluster.

νcalc / cm-1

νexp / cm-1

Assignment Thiourea

Thioureate

1107

1069

1114

ρ(NH2)

714

727

743

ν (C-S)

239

147

256

ν (Au-S)


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Reference List

(1) Plieth, W. Additives in the Electrocrystallization Process. Electrochim.Acta 1992, 37, 21152121. (2) Suarez, D. F.; Olson, F. A. Nodulation of Electrodeposited Copper in the Presence of Thiourea. J.Appl.Electrochem. 1992, 22, 1002-1010. (3) Lacconi, G. I.; Macagno, V. A. Electrochemical Study of Thiourea in Relation to the Silver Electrodeposition Process. Electrochim.Acta 1994, 39, 2605-2610. (4) Fanigliulo, A.; Mele, C.; Bozzini, B. An Electrochemical and in Situ SERS Study of Au Electrodeposition From a Thiourea Solution. Trans.Inst.Met.Finish. 2003, 81, 75-78. (5) Ateya, B. G.; El-Anadouli, B. E.; El-Nizamy, F. M. The Effect of Thiourea on the Corrosion Kinetics of Mild Steel in Sulfuric Acid. Corros.Sci. 1984, 24, 497-507. (6) Stankovic, Z. D.; Vukovic, M. The Influence of Thiourea on Kinetic Parameters on the Cathodic and Anodic Reaction at Different Metals in H2SO4 Solution. Electrochim.Acta 1996, 41, 25292535. (7) Deschenes, G.; Ghali, E. Leaching of Gold From a Chalcopyrite Concentrate by Thiourea. Hydrometallurgy 1988, 20, 179-202. (8) Parker, G. K.; HOPE, G. A. Spectroelectrochemical Investigations of Gold Leaching in Thiourea Media. Miner.Eng. 2008, 21, 489-500. (9) Gao, P.; Patterson, M. L.; Tadayyoni, M. A.; Weaver, M. J. Gold As a Ubiquitous Substrate for Intense Surface-Enhanced Raman Scattering. Langmuir 1985, 1, 173-176. (10) Holze, R.; Schomaker, S. New Results on the Electrosorption of Urea and Thiourea on Gold Electrodes. Electrochim.Acta 1990, 35, 613-620. (11) Hamm, U. W.; Kolb, D. M. On the Stability of Reconstructed Gold(100) Surfaces in the Presence of Organic Molecules. J.Electroanal.Chem. 1992, 332, 339-347. (12) Vandeberg, P. J.; Johnson, D. C. A Study of the Voltammetric Response of Thiourea and Ethylene Thiourea at Gold Electrodes in Alkaline Media. J.Electroanal.Chem. 1993, 362, 129139. (13) Azzaroni, O.; Andreasen, G.; Blum, B.; Salvarezza, R. C.; Arvia, A. J. Scanning Tunneling Microscopy Studies of the Electrochemical Reactivity of Thiourea on Au(111) Electrodes. J.Phys.Chem.B 2000, 104, 1395-1398. (14) Zhang, H.; Ritchie, I. M.; La Brooy, S. R. Electrochemical Oxidation of Gold and Thiourea in Acidic Thiourea Solutions. J.Electrochem.Soc. 2001, 148, D146-D153. (15) Garcia, G.; Macagno, V. A.; Lacconi, G. I. Study of Thiourea Adsorption Onto Polycrystalline Gold Electrodes. Electrochim.Acta 2003, 48, 1273-1277. (16) Bolzan, A. E.; Iwasita, T.; Arvia, A. J. In Situ FTIRRAS Study of the Electro-Oxidation Reactions of Thiourea and Gold in Aqueous Acid Solutions. J.Electroanal.Chem. 2003, 554555, 49-60. (17) Garcia, G.; Rodriguez, J. L.; Lacconi, G. I.; Pastor, E. Spectroscopic Investigation of the Adsorption and Oxidation of Thiourea on Polycrystalline Au and Au(111) in Acidic Media. Langmuir 2004, 20, 8773-8780. (18) Tian, M.; Conway, B. E. Effects of Thiourea on Anodic Dissolution of Au and Surface Oxidation Behaviour in Aqueous HClO4 Studied by Means of an EQCN. J.Appl.Electrochem. 2004, 34, 533-543. (19) Gomez, H.; Lizama, H.; Suarez, C.; Valenzuela, A. Effect of Thiourea Concentration on the Electrochemical Behavior of Gold and Copper Electrodes in Presence and Absence of Cu(II) Ions. J.Chil.Chem.Soc. 2009, 54, 439-444. 33 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

(20) Yang, X.; Moats, M. S.; Miller, J. D. The Interaction of Thiourea and Formamidine Disulfide in the Dissolution of Gold in Sulfuric Acid Solutions. Miner.Eng. 2010, 23, 698-704. (21) Yan, M.; Liu, K.; Jiang, Z. Electrochemical Oxidation of Thiourea Studied by Use of in Situ FTIR Spectroscopy. J.Electroanal.Chem. 1996, 408, 225-229. (22) Bolzan, A. E.; Wakenge, I. B.; Salvarezza, R. C.; Arvia, A. J. Electrochemical Response of Thiourea and Formamidine Disulphide on Polycrystalline Platinum in Aqueous 0.5 M Sulphuric Acid. J.Electroanal.Chem. 1999, 475, 181-189. (23) Bolzan, A. E.; Piatti, R. C. V.; Salvarezza, R. C.; Arvia, A. J. Electrochemical Study of Thiourea and Substituted Thiourea Adsorbates on Polycrystalline Platinum Electrodes in Aqueous Sulfuric Acid. J.Appl.Electrochem. 2002, 32, 611-620. (24) Bolzan, A. E.; Schilardi, P. L.; Piatti, R. C. V.; Iwasita, T.; Cuesta, A.; Gutierrez, C.; Arvia, A. J. Comparative Voltammetric and FTIRRAS Study on the Electro-Oxidation of Thiourea and Methyl-Thioureas on Platinum in Aqueous Acid Solutions. J.Electroanal.Chem. 2004, 571, 5972. (25) Zheng, J. Z.; Ren, B.; Wu, D. Y.; Tian, Z. Q. Thiourea Adsorption on a Pt Surface As Detected by Electrochemical Methods and Surface-Enhanced Raman Spectroscopy. J.Electroanal.Chem. 2005, 574, 285-289. (26) Garcia, G.; Rodriguez, J. L.; Lacconi, G. I.; Pastor, E. Adsorption and Oxidation Pathways of Thiourea at Polycrystalline Platinum Electrodes. J.Electroanal.Chem. 2006, 588, 169-178. (27) Fleischmann, M.; Hill, I. R.; Sundholm, G. A Raman Spectroscopic Study of Thiourea Adsorbed on Silver and Copper Electrodes. J.Electroanal.Chem.Interfacial Electrochem. 1983, 157, 359368. (28) Tian, Z. Q.; Lian, Y. Z.; Fleischmann, M. In-Situ Raman Spectroscopic Studies on Coadsorption of Thiourea With Anions at Silver Electrodes. Electrochim.Acta 1990, 35, 879-883. (29) Milkowska, M. Adsorption of Thiourea at the Polycrystalline Silver Electrode From Aqueous Potassium Fluoride Solutions - I. Electrochim.Acta 1987, 32, 159-162. (30) Bukowska, J.; Jackowska, K. Influence of Thiourea on Hydrogen Evolution at a Silver Electrode As Studied by Electrochemical and SERS Methods. J.Electroanal.Chem. 1994, 367, 41-48. (31) Szklarczyk, M.; Hoa, N. N.; Zelenay, P. Interaction of Thiourea With Silver Electrodes. J.Electroanal.Chem. 1996, 405, 111-116. (32) Reents, B.; Plieth, W.; Macagno, V. A.; Lacconi, G. I. Influence of Thiourea on Silver Deposition: Spectroscopic Investigation. J.Electroanal.Chem. 1998, 453, 121-127. (33) Brunetti, V.; Blum, B.; Salvarezza, R. C.; Arvia, A. J.; Schilardi, P. L.; Cuesta, A.; Gayone, J. E.; Zampieri, G. Scanning Tunneling Microscopy, Fourier Transform Infrared ReflectionAbsorption Spectroscopy, and X-Ray Photoelectron Spectroscopy of Thiourea Adsorption From Aqueous Solutions on Silver (111). J.Phys.Chem.B 2002, 106, 9831-9838. (34) Brunetti, V.; Blum, B.; Salvarezza, R. C.; Arvia, A. J. Comparative Molecular Resolution STM Imaging of Thiourea, Ethylthiourea, and Sulfur Self-Assembled Adlayer Structures on Silver (111). Langmuir 2003, 19, 5336-5343. (35) Patrito, E. M.; Cometto, F. P.; Paredes-Olivera, P. Quantum Mechanical Investigation of Thiourea Adsorption on Ag(111) Considering Electric Field and Solvent Effects. J.Phys.Chem.B 2004, 108, 15755-15769. (36) Bolzan, A. E.; Wakenge, I. B.; Piatti, R. C. V.; Salvarezza, R. C.; Arvia, A. J. The Behaviour of Copper Anodes in Aqueous Thiourea-Containing Sulphuric Acid Solutions. Open Circuit Potentials and Electrochemical Kinetics. J.Electroanal.Chem. 2001, 501, 241-252. (37) Lukomska, A.; Smolinski, S.; Sobkowski, J. Adsorption of Thiourea on Monocrystalline Copper Electrodes. Electrochim.Acta 2001, 46, 3111-3117. (38) Grozovski, V.; Kasuk, H.; Kallip, S.; Lust, E. Adsorption of Thiourea on Bi(111) Electrode Surface. J.Electroanal.Chem. 2014, 712, 103-112. 34 ACS Paragon Plus Environment

Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Bolzan, A. E.; Arvia, A. J. Adsorption and Electro-Oxidation of N-Alkyl and N,N'-Dialkyl Thioureas on Gold Electrodes in Acid Solutions. A Combined FTIRRAS and Voltammetry Study. J.Solid State Electrochem. 2008, 12, 529-543. (40) Bencivenni, L.; Cesaro, S. N.; Pieretti, A. Matrix and Ab Initio Infrared Spectra of Thiourea and Thiourea-D4. Vib.Spectrosc. 1998, 18, 91-102. (41) Zhou, W.; Wen, Y.; Qiu, L. Structure and Stability of Thiourea With Water, DFT and MP2 Calculations. J.Mol.Struct.: THEOCHEM 2005, 730, 133-141. (42) Jose, D. A.; Singh, A.; Das, A.; Ganguly, B. A Density Functional Study Towards the Preferential Binding of Anions to Urea and Thiourea. Tetrahedron Lett. 2007, 48, 3695-3698. (43) Yang, W.; Zhou, W.; Peng, K. Structure and Stability of Thioureate Anions With Water, DFT Calculations. J.Mol.Struct.: THEOCHEM 2008, 858, 101-106. (44) Kowal, A. T. Ab Initio Molecular Geometry and Anharmonic Vibrational Spectra of Thiourea and Thiourea-D4. J.Comput.Chem. 2011, 32, 718-729. (45) Sandoval, A. P.; Orts, J. M.; Rodes, A.; Feliu, J. M. Adsorption of Glycine on Au(Hkl) and Gold Thin Film Electrodes: an in Situ Spectroelectrochemical Study. J.Phys.Chem.C 2011, 115, 16439-16450. (46) Sandoval, A. P.; Orts, J. M.; Rodes, A.; Feliu, J. M. In Vibrational Spectroscopy at Electrified Interfaces; Wieckowski, A., Korzeniewski, C., Braunschweig, B., Eds.; 2013; p 241. (47) Sandoval, A. P.; Orts, J. M.; Rodes, A.; Feliu, J. M. A Comparative Study of the Adsorption and Oxidation of L-Alanine and L-Serine on Au(100), Au(111) and Gold Thin Film Electrodes in Acid Media. Electrochim.Acta 2013, 89, 72-83. (48) Berna, A.; Delgado, J. M.; Orts, J. M.; Rodes, A.; Feliu, J. M. Spectroelectrochemical Study of the Adsorption of Acetate Anions at Gold Single Crystal and Thin-Film Electrodes. Electrochim.Acta 2008, 53, 2309-2321. (49) Delgado, J. M.; Blanco, R.; Orts, J. M.; Pérez, J. M.; Rodes, A. DFT and in-Situ Spectroelectrochemical Study of the Adsorption of Fluoroacetate Anions at Gold Electrodes. J.Phys.Chem.C 2009, 113, 989-1000. (50) Delgado, J. M.; Blanco, R.; Orts, J. M.; Pérez, J. M.; Rodes, A. Glycolate Adsorption at Gold and Platinum Electrodes: A Theoretical and in Situ Spectroelectrochemical Study. Electrochim.Acta 2010, 55, 2055-2064. (51) Delgado, J. M.; Blanco, R.; Perez, J. M.; Orts, J. M.; Rodes, A. Theoretical and Spectroelectrochemical Studies on the Adsorption and Oxidation of Glyoxylate and Hydrated Glyoxylate Anions at Gold Electrodes. J.Phys.Chem.C 2010, 114, 12554-12564. (52) Ataka, K.; Osawa, M. In Situ Infrared Study of Water-Sulfate Coadsorption on Gold(111) in Sulfuric Acid Solutions. Langmuir 1998, 14, 951-959. (53) Wandlowski, T.; Ataka, K.; Pronkin, S.; Diesing, D. Surface Enhanced Infrared SpectroscopyAu(1 1 1-20 Nm)/Sulphuric Acid - New Aspects and Challenges. Electrochim.Acta 2004, 49, 1233-1247. (54) Delgado, J. M.; Orts, J. M.; Pérez, J. M.; Rodes, A. Sputtered Thin-Film Gold Electrodes for in Situ ATR-SEIRAS and SERS Studies. J.Electroanal.Chem. 2008, 617, 130-140. (55) Osawa, M. Dynamic Processes in Electrochemical Reactions Studied by Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS). Bull.Chem.Soc.Jpn. 1997, 70, 2861-2880. (56) Aroca, R.Surface Enhanced Vibrational Spectroscopy; Wiley: Chichester, 2006. (57) Delgado, J. M.; Rodes, A.; Orts, J. M. B3LYP and in Situ ATR-SEIRAS Study of the Infrared Behavior and Bonding Mode of Adsorbed Acetate Anions on Silver Thin-Film Electrodes. J.Phys.Chem.C 2007, 111, 14476-14483. (58) Lide, D. R., Ed.;CRC Handbook of Chemistry and Physics, 86th Edition; CRC Press: Boca Raton, FL, 2005.



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(59) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J.; Vreven, T.; Kudin, K. N.; Burant, J.C. et al. Gaussian 03, Revision D.02; Gaussian, Inc.: Wallingford CT, 2004. (60) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J.Chem.Phys. 1993, 98, 5648-5652. (61) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys.Rev.B 1988, 37, 785-789. (62) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules. J.Chem.Phys. 1972, 56, 2257-2261. (63) Hariharan, P. C.; Pople, J. A. Influence of Polarization Functions on MO Hydrogenation Energies. Theor.Chim.Acta 1973, 28, 213-222. (64) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V. R. Efficient Diffuse FunctionAugmented Basis Sets for Anion Calculations. III. The 3-21+G Basis Set for First-Row Elements, Li-F. J.Comput.Chem. 1983, 4, 294-301. (65) Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for K to Au Including the Outermost Core Orbitals. J.Chem.Phys. 1985, 82, 299-310. (66) York, S. M.; Haq, S.; Kilway, K. V.; Phillips, J. M.; Leibsle, F. M. STM, FTIR and Quantum Chemical Calculation Studies of Acetate Structures on Cu(1 1 0) Surfaces. Surf.Sci. 2003, 522, 34-46. (67) Phillips, J. M.; Leibsle, F. M.; Holder, A. J.; Keith, T. A Comparative Study of Chemisorption by Density Functional Theory, Ab Initio, and Semiempirical Methods: Carbon Monoxide, Formate, and Acetate on Cu(110). Surf.Sci. 2003, 545, 1-7. (68) Schaftenaar, G.; Noordik, J. H. Molden: a Pre- and Post-Processing Program for Molecular and Electronic Structures. J.Comput.-Aided Mol.Des. 2000, 14, 123-134. (69) Clavilier, J.; Armand, D.; Sun, S.-G.; Petit, M. Electrochemical Adsorption Behaviour of Platinum Stepped Surfaces in Sulphuric Acid Solutions. J.Electroanal.Chem. 1986, 205, 267277. (70) Rodes, A.; Herrero, E.; Feliu, J. M.; Aldaz, A. Structure Sensitivity of Irreversibly Adsorbed Tin on Gold Single-Crys Tal Electrodes in Acid Media. J.Chem.Soc.Faraday.Trans. 1996, 92, 37693776. (71) Kolb, D. M. Reconstruction Phenomena at Metal-Electrolyte Interfaces. Prog.Surf.Sci. 1996, 51, 109-173. (72) Hamelin, A. Cyclic Voltammetry at Gold Single-Crystal Surfaces .1. Behaviour at Low Index Faces. J.Electroanal.Chem. 1996, 407, 1-11. (73) Rodes, A.; Pérez, J. M.; Aldaz, A. In Handbook of Fuel Cells. Fundamentals, Technology and Applications.; Vielstich, W., Gasteiger, H. A., Lamm, A., Eds.; John Wiley & Sons Ltd.: Chichester, 2003; Vol. 2, p 191. (74) Delgado, J. M.; Orts, J. M.; Rodes, A. ATR-SEIRAS Study of the Adsorption of Acetate Anions at Chemically Deposited Silver Thin Film Electrodes. Langmuir 2005, 21, 8809-8816. (75) Zamlynny, V.; Lipkowski, J. In Diffraction and Spectroscopic Methods in Electrochemistry; Alkire, R. C., Kolb, D. M., Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: Weinheim, 2006; Vol. 9, p 315. (76) Gómez, R.; Pérez, J. M.; Solla-Gullón, J.; Montiel, V.; Aldaz, A. In Situ Surface Enhanced Raman Spectroscopy on Electrodes With Platinum and Palladium Nanoparticle Ensembles. J.Phys.Chem.B 2004, 108, 9943-9949. (77) Greenler, R. G. Infrared Study of Adsorbed Molecules on Metal Surfaces by Reflection Techniques. J.Chem.Phys. 1966, 44, 310-315.


Page 37 of 37

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(78) Ataka, K.; Yotsuyanagi, T.; Osawa, M. Potential-Dependent Reorientation of Water Molecules at an Electrode/e Lectrolyte Interface Studied by Surface-Enhanced Infrared Absorption Spectroscopy. J.Phys.Chem. 1996, 100, 10664-10672. (79) Herzberg, G.Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Co.Inc.: New York, 1945. (80) Osawa, M.; Ataka, K.; Yoshii, K.; Nishikawa, Y. Surface-Enhanced Infrared Spectroscopy : the Origin of the Absorption Enchancement and Band Selection Rule in the Infrared Spectra of Molecules Adsorbed on Fine Metal Particles. Appl.Spectrosc. 1993, 47, 1497-1502.

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