Theoretical Study on Thermodynamic and Spectroscopic Properties of

Article pubs.acs.org/JPCC

Theoretical Study on Thermodynamic and Spectroscopic Properties of Electro-Oxidation of p‑Aminothiophenol on Gold Electrode Surfaces Liu-Bin Zhao, Meng Zhang, Bin Ren, Zhong-Qun Tian, and De-Yin Wu* State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China S Supporting Information *

ABSTRACT: The electro-oxidation of p-aminothiophenol (PATP) on gold electrodes has been investigated by means of density functional theory. A combination of thermodynamic calculations and surface Raman and infrared (IR) spectral simulations has allowed us to reveal the electro-oxidation mechanism and reaction products of PATP on gold electrodes in acidic, neutral, and basic solutions. PATP can be first oxidized to the radical cation PATP(NH2•+) or the neutral radical PATP(NH•) depending on the pH of aqueous solutions, and this is the rate-determining step. The radical cation or neutral radical can then transform to the dimerized products through a radical coupling reaction. In the acidic medium, the radical cation reacts with its resonance hybrid through a N−C4 coupling to form 4′-mercapto-N-phenyl-1,4-quinone diimine (D1), which can further undergo hydrolysis to yield 4′-mercapto-N-phenyl-1,4-quinone monoimine (D2). In the neutral medium, the neutral radical reacts with its resonance hybrid through the N−C2(6) coupling to form 4,4′-dimercapto-N-phenyl-1,2-quinone diimine (D3). In the basic medium, the neutral radical reacts with its resonance structure through the N−N coupling to form 4,4′-dimercaptoazobenzene (D4). The adsorbed dimer products exhibit reversible redox properties. The calculated standard electrode potentials of the above four species decrease in the order D3, D1, D2, and D4. Finally, the characteristic bands for the surface Raman and IR spectra of D1 to D4 redox pairs are clearly assigned. This study provides mechanistic insight into the electrochemical reaction properties of PATP on metal electrodes.

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INTRODUCTION Surface-enhanced vibrational spectroscopies, including surfaceenhanced Raman spectroscopy (SERS) and surface-enhanced infrared spectroscopy (SEIRS), can provide in situ molecular fingerprint information for electrochemical interface processes.1−3 Because of the enhanced local field as a result of the surface plasmon resonance effect, the spectral signals from surface species can be greatly amplified.4,5At least two different mechanisms, the electromagnetic (EM) and chemical enhancement (CE), are proposed to contribute to the total enhancement in SERS and SEIRS. The EM mechanism originates from the collective excitation of delocalized electrons on metal surfaces and gives rise to great enhancement of local electromagnetic field due to surface plasmon resonance.4−6 The CE effect assumes a modification of molecular electronic structures (dipole moments and polarizability derivatives) as a results of chemical interactions between probing molecules and metal surfaces. The ability to amplify spectral signals of molecules on the surfaces of nanostructures makes SERS and SEIRS powerful tools for monitoring not only the adsorption− desorption behavior but also the redox process of the surface species by providing direct information on the adsorbed © XXXX American Chemical Society

intermediates and/or products formed during complex electrochemical processes.7−9 When combined with conventional electrochemical measurements, SERS and SEIRS can be used to identify the intermediates and evaluate the reaction pathways of the electrochemical reaction. p-Aminothiophenol (PATP) can be chemisorbed on metal electrodes or nanoparticle surfaces to form well-organized selfassembled monolayers (SAMs). PATP is one of the most important probe molecules in surface science and nanoscience.10,11 The use of SERS and SEIRS to study the adsorption structure of PATP can be dated to 1990s.10−14 However, there are fewer attempts to use surface vibrational spectroscopy to investigate its electrochemical reactions.15−17 Combined cyclic voltammetry and Fourier transform infrared (FTIR) spectroscopy has been used to study the electrooxidation of PATP adsorbed on gold electrodes in acidic and neutral solutions to reveal the C−N coupling dimer product.15,17 Ab initio MO calculations were performed to Received: August 7, 2014 Revised: October 20, 2014

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dx.doi.org/10.1021/jp507987x | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Reaction framework of PATP adsorbed on a gold electrode. PATP is first oxidized to the radical cation PATP(NH2•+) or the neutral radical PATP(NH•) as shown in the top portion of the figure. The formed radical cation or neutral radical can transform to a dimer product through the radical coupling reaction. In route 1, the N−C4 coupling gives the D1 redox pair, which undergoes hydrolysis to yield the D2 redox pair. In route 2, the N−C2(6) coupling gives the D3 redox pair. In route 3, the N−N coupling gives the D4 redox pair.

abnormal SERS spectra of PATP.19,21 These studies show that the electro-oxidation mechanism of PATP is extremely complex because of the multiple reaction pathways and various reaction intermediates and products. Although surface vibrational spectroscopies have been used to understand the electrochemical interfacial processes, the intrinsic complexity of the electrochemical system makes the assignment of spectra very difficult. In this case, theoretical simulation and analysis can play an important role in identifying the surface species. A comprehensive analysis of the spectral features from experimental and theoretical aspects is essential for obtaining the crucial information at the electrode surfaces.

simulate the infrared (IR) spectra of PATP and its oxidation product; however, the molecule−metal interaction and the solvent effect were not considered.17 In situ SERS study of the electro-oxidation of aniline on a gold electrode has been reported and identified three redox pairs, including tail-to-tail, tail-to-head, and head-to-head dimers. The proportion of these products were found to strongly depend on solution pH values.18 Recently, our group has demonstrated that PATP undergoes the photo-oxidation reaction through the N−N coupling to form azo-like species on noble nanostructures in neutral or basic solutions.19−24 The simulated Raman spectrum of the predicted N−N coupling product well reproduces the B



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the ECE mechanism (sequential heterogeneous electron transfer, chemical reaction, and heterogeneous electron transfer).37 The reaction framework of PATP adsorbed on a gold electrode is illustrated in Figure 1. The nitrogen lone pair orbital of amino group in PATP is protonated to an ammonium ion. In the acidic solution, the protonation of PATP gives the PATP cation.12,38−40 PATP losses one electron to produce the PATP radical cation, which can further convert to the PATP neutral radical. The formed radical cation or neutral radical can transform to a dimer product through different radical coupling pathways. In the first route, the radical cation reacts with its resonance hybrid through the N−C4 coupling to form 4′mercapto-N-phenyl-1,4-quinone diimine (D1), which can further undergo hydrolysis to yield 4′-mercapto-N-phenyl-1,4quinone monoimine (D2).16,17 In the second route, the neutral radical reacts with its resonance structure through the N− C2(6) coupling to form 4,4′-dimercapto-N-phenyl-1,2-quinone diimine (D3).15,41−43 In the last route, the neutral radical reacts with its resonance structure through the N−N coupling to form 4,4′-dimercaptoazobenzene (D4, or DMAB).18,43−45 The dimer products D1 to D4 adsorbed on the surface exhibit reversible redox properties. Their adsorption structures are shown in Figure 1, and the oxidized forms are marked as Dn-ox and the reduced forms are labeled as Dn-red (n = 1−4). The electro-oxidation of PATP involves multiple electrontransfer and proton-transfer steps. Figure 2 shows the possible

In the present paper, a systematic study of the thermodynamic and spectroscopic properties on the electro-oxidation of PATP on gold electrodes is performed by means of density functional theory (DFT) calculations and compared with reported experimental results in previous studies. With this study, we successfully build a bridge between the electrochemical reaction properties and spectral features for the surface species to understand the electrochemical reaction mechanism of PATP on the gold electrode in different solutions. The reaction products and the reaction mechanisms of electro-oxidation of PATP in acidic, neutral, and basic solutions are revealed according to the calculated standard electrode potentials and simulated Raman and IR spectra of reaction species.

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COMPUTATIONAL DETAILS The metallic cluster model was employed to investigate the electro-oxidation of PATP on gold surfaces. The probing molecules adsorbed on the metal electrode surfaces were modeled as the metal−molecule complexes. DFT calculations were carried out with the hybrid exchange-correlation functional B3LYP25,26 and generalized gradient approximation functional PW91PW91.27 The basis sets for C, H, N, O, and S atoms of investigated molecules were 6-311+G**, including the polarization function to all the atoms and the diffuse function to C, N, O, and S atoms.28,29 For all metal atoms, the valence electrons, and the inner shells were described by the basis set, LANL2DZ, and the corresponding relativistic effective core potentials, respectively.30,31 The solvent effect was considered by integral equation formalism polarization continuum model (PCM).32 Water with dielectric constant (ε = 78.3) was chosen as the solvent. The frequency calculations were performed to obtain the thermodynamic data. The standard electrode potential and acid dissociation constant were calculated from the Gibbs free-energy changes at 298.15 K and 1 atm. All calculations including structure optimization and frequency calculations were carried out by using the Gaussian 09 package.33 Spectral simulations were performed on the basis of the optimized structures. To make sure that all the structures reported here were the minima on potential energy surfaces, we verified that there is no imaginary frequency. A scaling factor of 0.981 was used in the B3LYP/6-311+G** level calculation.19,34 IR intensities and Raman activities of the optimized structures were obtained from the integral absorption coefficient and the differential Raman scattering cross sections within the doubleharmonic approximation.35 The scaled quantum mechanics force field procedure was used to assign all the fundamentals.36 To allow a direct comparison with experimental spectra, the simulated IR and Raman spectra were presented in terms of a Lorentzian expansion with a line width of 10 cm−1 and an excitation wavelength of 632.8 nm was used in the calculation of Raman spectra. By comparing the simulated spectra of PATP and its dimer products with the experimental results, we found that the B3LYP functional is better for PATP monomers while the PW91PW91 is better for PATP dimer products. As for the thermodynamic calculations, the calculated standard electrode potentials by the two methods give very similar results and agree well with the experimental redox potentials.

Figure 2. Electron transfer and proton transfer involved in the initial oxidation of PATP.

existing elementary reaction steps during the initial oxidation of PATP. The optimized structures of the involved surface species can be found in Figure S1 in Supporting Information. The oxidation potential E1 for PATP to its radical cation can be calculated directly from the adiabatic ionization energy − PATP(NH 2) → PATP(NH•+ 2 ) + e

(1)

It is noted that the oxidation potential of reaction 1 is independent of the solution pH. However, cyclic voltammograms show that the oxidation potential of PATP decreases with the increasing pH.17 Therefore, the reaction is an electrontransfer and proton-transfer coupled process.46 Two possible concerted electron and proton transfers can be written as

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+ − PATP(NH+3 ) → PATP(NH•+ 2 ) + H + e

(2)

PATP(NH 2) → PATP(NH•) + H+ + e−

(3)

The Gibbs free-energy changes in reaction 2 and 3 depend on the electrode potential U and solution pH through the chemical potentials of e− and H+. Using the standard hydrogen electrode (SHE) as the reference47,48

RESULTS AND DISCUSSION Reaction Framework of PATP Adsorbed on a Gold Electrode. Generally, the electro-oxidation of PATP adopts C



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H+ + e− → 1/2H 2(g)

the range from 1.06 to 1.12 V. These values are very close to the calculated standard electrode potential of reaction 3. Experimentally, the initial oxidation peak is assigned to the oxidation of PATP cation to its radical cation (reaction 2).15,16 However, according to our calculations, this voltammetric peak should be attributed to the oxidation of PATP to its neutral radical in reaction 3. It was noted that the electro-oxidation of PATP will experience interference due to the oxidation of silver because the oxidation potential of PATP exceeds that of silver itself. Nevertheless, PATP can undergo photolysis on silver surfaces with laser irradiation.20 The calculated acidic dissociation constant for the conjugated acid of PATP is 4.99, which is close to the value of aniline (4.63).49 PATP shows weak basicity because of the protonation of amino group. Figure 3 shows the simulated surface Raman and IR spectra of neutral PATP and its ammonium ion adsorbed on a gold electrode. Here, one Au5 cluster was used to model the active site on the gold electrode surface. Raman and IR Spectra of PATP Adsorbed on a Gold Surface. The surface Raman spectrum of neutral PATP is dominated by the two major peaks at 1072 and 1598 cm−1, which are assigned to the C−S stretching mode and the C−C stretching mode, respectively. The weak peaks at 1178 and 1489 cm−1 are assigned to the C−H in-plane bending mode. The 1272 and 1623 cm−1 bands are assigned to the C−N stretching mode and the NH2 scissoring mode, respectively.20 After protonation, the transformation of NH2 to NH3 from sp2 to sp3 hybridization breaks the conjugation between nitrogen lone pair orbital and benzene ring π orbitals. Thus, the C−N bond length increases from 1.393 Å in PATP(NH2) to 1.482 Å in PATP(NH3+). Moreover, the peak of the C−N stretching mode in PATP(NH3+) red-shifts to 1171 cm−1. Meanwhile, the NH2 scissoring mode disappears and a new peak appears at 1522 cm−1 corresponding to the umbrella mode of NH3+ group. The umbrella mode of NH3+ group was detected at 1520 cm−1 from the 2D-SERS experiment in the acidic solution.39 In general, the surface Raman spectra of neutral PATP and PATP cation are quite similar. The spectral differences in the C−N stretching and NH2 related modes are hard to distinguish because of their weak Raman intensities. However, these spectral evolutions before and after protonation can be identified easily from the surface IR spectroscopy, as shown in Figure 3b. The most intense IR peak at 1272 cm−1

(4)

it is possible to relate the chemical potential of proton and electron to that of 1/2H2. This allows us to calculate the Gibbs free-energy changes of reaction 2 and 3 at pH = 0 and U = 0 V from the following dehydrogenation reactions PATP(NH+3 ) → PATP(NH•+ 2 ) + 1/2H 2(g)

(5)

•

PATP(NH 2) → PATP(NH ) + 1/2H 2(g)

(6)

The standard electrode potentials of reactions 1, 2, and 3 can be calculated from the change in Gibbs free energy (7) ΔG = −nFE 0 The Gibbs free-energy changes of acidic dissociation reactions are obtained from the energy differences between reactions 2 or 3 and 1.

ΔG = −RT ln Ka = 2.303RT pKa

(8)

Table 1 lists the calculated thermodynamic data involved in Figure 1. The calculated standard electrode potentials of Table 1. Calculated Standard Electrode Potential, E0, Acidic Dissociation Constant, pKa, and Gibbs Free-Energy Changea reaction Au5−PATP(NH2) → Au5− PATP(NH2•+) Au5−PATP(NH3+) → Au5− PATP(NH2•+) Au5−PATP(NH2) → Au5− PATP(NH•) Au5−PATP(NH3+) → Au5− PATP(NH2) Au5−PATP(NH2•+) → Au5− PATP(NH•) a

E0 (V)

ΔG (kcal/mol)

pKa

0.75

17.34

0.46

10.54

1.25 (1.12)16

28.92 4.99 (4.63) −8.49

49

6.80 −11.58

Experimental values are given in parentheses.

reactions 1, 2, and 3 are 0.752, 0.457, and 1.254 V, respectively. The oxidation potential of PATP adsorbed on a gold electrode was measured by cyclic voltammetry. The experimental results are 0.77 V vs statured calomel electrode (SCE) in 0.5 M HClO4 by Lukkari et al.16 and 0.5 V vs Ag/AgCl in pH 7.2 buffer solution by Raj et al.17 When converted to standard hydrogen electrode in pH = 0 solution, the experimental values fall into

Figure 3. Simulated surface Raman (a) and IR (b) spectra of Au5−PATP(NH2) and Au5−PATP(NH3+) complexes at the B3LYP/6-311+G**/ LANL2DZ level of theory. The simulated IR and Raman spectra were presented in terms of a Lorentzian expansion with a line width of 10 cm−1, and an excitation wavelength of 632.8 nm was used in the calculation of the Raman spectra. D



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disappears. A new peak appearing at 1522 cm−1 is assigned to NH3 umbrella mode. The NH2 scissoring mode at 1623 cm−1 splits into two peaks at 1627 and 1647 cm−1 after protonation. Experimentally, the C−N stretching band at 1265 cm−1 and NH2 scissoring band at 1620 cm−1 were not detected for PATP on gold electrode in an acidic solution.14 Raj et al. observed broad bands around 1546 and 1628 cm−1 in the SEIRA spectrum of PATP adsorbed on a gold electrode in the acidic solution, but they did not observe those peaks in the neutral solution.17 The appearance of vibrational modes in surface IR spectra is subject to the surface dipole selection rule, which can be used to infer the adsorption configurations of adsorbates. This states that only those vibrational modes which give rise to an oscillating dipole perpendicular to the surface are IR active and give rise to an observable absorption band. According to the surface selection rule, PATP adopts an inclined configuration because the relative IR intensities of the C−N stretching and the NH2 scissoring modes in the SEIRA of PATP are weaker than in normal IR spectrum.10,11 Radical Coupling Reactions of PATP in Acidic, Neutral, and Basic Solutions. As illustrated in Figure 2, the oneelectron-one-proton transfers of PATP cation and neutral PATP give PATP radical cation and PATP neutral radical, respectively. The formed PATP radicals can react with their resonance structures to produce corresponding dimer products through radical coupling reactions. Figure 4 presents the

first positive scan, the large irreversible anodic peak was assigned to the initial single-electron oxidation of PATP.15−17 On the first reverse and subsequent scans, reversible redox waves were observed at the potentials more negative than the initial oxidation potential.16,43 According to our calculation, the four possible redox pairs (D1 to D4 in Figure 1) may appear during the electro-oxidation of PATP. The calculated standard electrode potentials of D1 to D4 as well as PATP are listed in Table 2 along with experimental results.15−17 Note that the Table 2. Comparison of Calculated Standard Electrode Potentials of PATP and Its Dimer Products and Experimental Results reaction Au5−PATP(NH2) → Au5− PATP(NH•) D1-ox D2-ox D3-ox D4-ox

→ → → →

D1-red D2-red D3-red D4-red

Ecalc (V)

Eexp (V)

Eexp (V)*c

1.25

ref

0.50a

1.12

17

0.72 0.57 0.90 0.22

0.77b 0.53b 0.30b 0.23a −0.17a

1.06 0.82 0.59 0.85 0.45

16 16 16 17 17

a

pH 7.2 buffer solution, reference electrode Ag/AgCl electrode. b0.1 M HClO4 solution, statured calomel electrode (SCE) as reference electrode. cRescaled to standard hydrogen electrode in pH 0 solution.

values in column 2 are the original experimental results from references and the data in column 3 are the rescaled experimental values at the standard condition (pH 0) with a reference electrode of the standard hydrogen electrode which can be directly compared with the calculated standard electrode potentials in column 1. Cyclic voltammetry measures the electrochemical process in transient states, and the theoretical calculations predict the electron transfer in a steady state. This may lead to delicate differences between the theoretical and experimental values in Table 2. The calculated oxidation potentials of PATP are higher than the redox potentials of its dimer products, which is in a good agreement with the ECE mechanism derived from cyclic voltammograms.15−17 The redox potentials of D1 to D4 decrease as D3 > D1 > D2 > D4. Lukkari et al. studied the electro-oxidation of PATP in acidic solution (0.5 M HClO4).16 Two reversible waves were observed at 0.53 and 0.30 V vs SCE. Normalized to the standard hydrogen electrode at pH = 0, the experimental values are consistent with our calculation results. According to our calculation, the reversible voltammetric peak at 0.53 V is assigned to D1-ox/D1-red redox and the peak at 0.30 V is assigned to the D2-ox/D2-red pair. D2-ox is the hydrolysis product of D1-ox, and the hydrolysis of D1-ox to D2-ox is a thermodynamically spontaneous reaction. The Gibbs freeenergy change for the hydrolysis reaction is calculated to be −3.50 kcal/mol. Raj et al. studied the electro-oxidation of PATP in the neutral solution (pH 7.2 phosphate buffer solution).17 A symmetrical reversible redox peak at 0.23 V vs Ag/AgCl and an irreversible wave at −0.17 V vs Ag/AgCl were observed. The authors assigned the redox peak at 0.23 V to the D1-ox/D1-red pair. Nevertheless, our calculation indicates that the radical coupling product of PATP in a neutral solution should be the D3-ox/ D3-red pair. The calculated standard electrode potential of D3 redox pair is 0.90 V, which is in agreement with the experimental value of 0.85 V (rescaling to pH 0 and referring to standard hydrogen electrode). Therefore, the voltammetric

Figure 4. Calculated spin densities of the PATP radical cation (a) and PATP neutral radical (b). In the PATP radical cation, the spin is localized on N and C4 atoms. In the PATP neutral radical, the spin is populated on N, C2, C4, and C6 atoms.

calculated spin densities of Au5−PATP(NH2•+) and Au5− PATP(NH•) complexes. The former is the major reaction intermediate in the acidic solution, and the latter is dominant in neutral and basic solutions. As seen in Figure 4a, the PATP radical cation is adsorbed on the gold surface in a flat configuration. The single electron is mainly distributed on the N atom (0.236) and C4 atom (0.241). In this case, the N−C4 coupling reaction gives the D1 dimer (see Figure 1). The PATP neutral radical is adsorbed on the gold surface in a perpendicular configuration. The single electron is delocalized on the N atom (0.554), C2 atom (0.298), C4 atom (0.323), and C6 (0.243) atom. The coupling between N atom and C2 or C6 atom gives the D3 dimer and the coupling between two N atoms gives the D4 dimer. Cyclic voltammetry was applied to reveal the electrooxidation mechanism of PATP on a gold electrode. In the E



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Table 3. Assignments of Vibrational Fundamentals of Reduced Forms of Dimer Productsa D1-red

a

D2-red

D3-red

D4-red

frequency

assignment

frequency

assignment

frequency

assignment

frequency

assignment

1067 1166 1217 1244 1269 1331 1464 1497 1586 1615

νCS + νCC βCH νCN + βCH βNH + βCH νCN + βCH νCC + βCH βNH + βCH νCN + βCH νCC + βCH νCC + δNH

1068 1154 1158 1167 1216 1236 1331 1355 1465 1498 1585 1590 1610

νCS + νCC βOH + βCH βOH + βCH βCH νCN + βCH νCO + βCH νCC + βCH νCC + βCH βNH + βCH νCN + βCH νCC + βCH νCC + βOH νCC

1067 1071 1163 1206 1260 1292 1319 1472 1585 1601

νCS + νCC νCS + νCC βCH νCN + βCH νCN + βCH νCN + βCH νCC + βCH βNH + βCH νCC + δNH νCC + δNH

1064 1066 1156 1164 1237 1289 1323 1458 1468 1565 1586 1593

νCS + νCC νCS + νCC βCH βCH νCN + βCH νNN + νCC νCC βNH + βCH νCN + βCH νCC + βNH νCC + βCH νCC + βCH

ν, stretching; β, bending; δ, scissoring.

Table 4. Assignments of Vibrational Fundamentals of Oxidized Forms of Dimer Productsa D1-ox

a

D2-ox

D3-ox

D4-ox

frequency

assignment

frequency

assignment

frequency

assignment

frequency

assignment

1062 1151 1155 1220 1343 1452 1497 1554 1564 1577 1628

νC−S + νCC βN−H + βC−H βC−H νC−N βN−H + βC−H νC−N + βC−H νCN νCN + νCC νCC νCN + νCC νCC + βC−H

1061 1137 1157 1226 1358 1451 1487 1553 1569 1582 1620

νC−S + νCC βC−H βC−H νC−N βC−H νC−N + βC−H νCN + νCO νCC νCC + νCO νCN + νCO νCC + βC−H

1061 1155 1222 1292 1341 1401 1453 1483 1571 1609

νC−S + νCC βC−H νC−N + βN−H βC−H βN−H + βC−H βN−H + βC−H βC−H νCN + βC−H νCN + νCC νCC + νCN + βC−H

1057 1121 1133 1181 1235 1382 1418 1456 1457 1542 1573 1579

νC−S + νCC νC−N + βC−H βC−H νC−N + βC−H νC−N + βC−H νNN + νCC νNN + βC−H βC−H νNN + βC−H νCC + βC−H νCC + βC−H νCC + βC−H

ν, stretching; β, bending.

Figure 5. Simulated surface Raman spectra of dimer products D1 to D4 (from left to right) adsorbed on gold surfaces at the PW91PW91/6311+G**/LANL2DZ level of theory. The upper panels are spectra of dimer products in their reduced forms, and the lower panels are spectra of dimer products in their oxidized forms.

wave at 0.23 V should be assigned to the D3-ox/D3-red redox pair. As for the irreversible wave observed at −0.17 V, as seen in Table 2, this cathodic wave should be assigned to the reduction of D4-ox to D4-red, an azo-to-hydrazo reduction. D4-ox, also

called DMAB, is the major product of photolysis of PATP and p-nitrothiophenol (PNTP) on noble nanostructures.23,50−54 In our previous work, DMAB was synthesized and its electrochemical property in 0.1 M NaClO4 solution was tested.20 A F



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Figure 6. Simulated surface IR spectra of dimer products D1 to D4 (from left to right) adsorbed on gold surfaces at PW91PW91/6-311+G**/ LANL2DZ level of theory. The upper panels are apectra of dimer products in their reduced forms, and the lower panels are spectra of dimer products in their oxidized forms.

peaks at 1497, 1487, and 1486 cm−1, respectively. They are assigned to the CN double bond stretching, the mixed stretching mode of CN and CO bonds, and the mixed stretching mode of CN and CC bonds, respectively. As seen in Figure 5, the Raman spectrum of D4-ox is quite different from the other three dimer products. The Raman peaks at 1121 and 1161 cm−1 are related to the C−N stretching and the peaks at 1383 and 1418 cm−1 are related to the NN stretching. In situ SERS study of the electro-oxidation of aniline on a gold electrode in 0.1 M HClO4 revealed a strong peak at 1490 cm−1 during the positive-going potential scan. This band was assigned to the characteristic bands of the C−N coupling product N-phenyl-1,4-phenylenediimine.16 In 0.1 M KOH solution, intense peaks at 1150 and 1435 cm−1 were observed during the positive-going potential scan, indicating the formation of the N−N coupling product azobenzene.18 Because the characteristic Raman bands of D1-ox to D4-ox are attributed directly to their functional group, it is very convenient to distinguish these surface species from their Raman spectra. Figure 6 shows the simulated surface IR spectra of dimer products D1 to D4 on gold surfaces. The upper and lower spectra are the reduced and oxidized forms of D1 to D4, respectively. The spectral shapes of D1-red, D2-red, and D3-red are very similar. The peaks at 1166 and 1497 cm−1 are assigned to the C−H in-plane bending modes, while those at 1331 and 1585 cm−1 are assigned to the C−C stretching modes. The most obvious differences among the three species are IR bands around 1270 cm−1. The 1269 cm−1 peak in D1-red is assigned to the C−NH2 stretching vibration. The 1236 cm−1 peak in D2red is assigned to the C−OH stretching vibration. The double peaks at 1260 and 1292 cm−1 are the C−N stretching vibrations of primary and secondary amine, respectively. The IR spectrum of D4-red shows three major peaks at 1237, 1458, and 1586 cm−1. They are assigned to the C−N stretching, C−H bending, and C−C stretching, respectively. The lower part of Figure 6 presents the simulated IR spectra of D1 to D4 in their oxidized forms. The characteristic functional groups in D1-ox and D2-ox are the CN double bond and the CO double bond. In D1-ox, the stretchings of two CN bonds are strongly coupled so that the peaks at 1497 and 1577 cm−1 can be attributed to the symmetric and

cathodic process clearly occurred at the electrode potential more negative than −0.4 V. The reduction potential of DMAB agrees with our calculated result. Therefore, the irreversible voltammetric wave at −0.17 V observed by Raj et al. should be assigned to the reduction of D4-ox to D4-red. During the reduction of D4-ox to D4-red, the adsorption configurations of surface species changes significantly (see Figure S2 in Supporting Information). After the electro-reduction, D4-red may desorb from the electrode surface and then diffuse to the solution. As a result, no corresponding anodic wave was found and the cathodic wave gradually disappeared upon repeating CV cycling.15,17 Raman and IR Spectra of PATP Electro-oxidation Dimer Products. Conventional electrochemical measurements such as cyclic voltammetry can provide only the macroscopic information about the electrochemical reactions. By contrast, surface vibrational spectroscopies are able to detect the fingerprint information on surface species and thus can help to investigate the electrochemical interfacial processes at the molecular level (Tables 3 and 4). Figure 5 shows the simulated surface Raman spectra of dimer products D1 to D4 on gold surfaces. The upper and lower spectra are the reduced and oxidized forms of D1 to D4, respectively. As illustrated in Figure 5, the Raman spectra for the reduced forms of D1 to D4 are quite similar. Prominent peaks around 1060 and 1600 cm−1 are assigned to the C−S stretching mode and the benzene ring C−C stretching mode. Note that the C−C stretching modes correspond to doublet peaks in D1 and D2, while they are single peaks in D3 and D4. This is due to the fact that D1 and D2 are adsorbed on gold surfaces in a single-end adsorption configuration, resulting in significant differences in the chemical environment of the two benzene rings. By contrast, D3 and D4 are adsorbed on gold surfaces in a double-end configuration. The two benzene rings in D3 and D4 have a similar chemical environment, and the C− C stretchings of two benzene rings are strongly coupled. The weak peaks around 1170 and 1470 cm−1 are assigned to the C− H in-plane bending mode. The weak peaks around 1220 and 1270 cm−1 are assigned to the C−N stretching mode. The Raman intensities of the oxidized forms of D1 to D4 are 10 times stronger than their corresponding reduced forms. For D1-ox, D2-ox, and D3-ox, their Raman spectra show prominent G



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asymmetric CN stretching modes. In D2-ox, the CN stretching red-shifts to 1487 cm−1 while the CO bond appears at 1582 cm−1. In D3-ox, the most intense IR peak at 1486 cm−1 is assigned to the CN stretching and the IR intensity of another CN related mode at 1563 cm−1 is very weak. The other strong peaks at 1292, 1389, and 1601 cm−1 are assigned to the C−H bending, N−H bending, and C−C stretching. Note that the relative IR intensities of 1516 and 1486 cm−1 (CN stretchings) are different in acidic solutions.15 The 1516 cm−1 peak is stronger in acidic solution, and it is assigned to the CN stretching of D1-ox. The 1486 cm−1 peak is stronger in neutral solution, and it is assigned to the CN stretching of D3-ox. The simulated IR spectrum of D4-ox matches well with the IR spectrum of the synthesized DMAB.55 The peaks at 1057, 1235, 1456, and 1573 cm−1 are assigned to the C−S stretching, C−N stretching, C−H bending, and C−C stretching vibrations, respectively. Although in situ FTIR was applied to study the electro-oxidation of PATP on gold electrodes in acidic and neutral solutions, the observed spectra are not well-defined. Present calculations provide clear assignments for each surface species involved in electro-oxidation of PATP. When compared with the simulated Raman spectra shown in Figure 5, the IR spectra in Figure 6 are more complicated. It is hard to distinguish each species according to their IR spectra. Nevertheless, the present calculations of standard IR spectra can provide more fingerprints of reaction species.

Article

ASSOCIATED CONTENT

S Supporting Information *

Optimized structure of PATP, PATP cation, PATP radical cation, and PATP neutral radical adsorbed on gold surfaces and optimized structure of dimer products D1 to D4 adsorbed on gold surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 592-2186979. Tel: +86 592-2189023. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of this work by the NSF of China (21373172 and 21321062).

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REFERENCES

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CONCLUSION The electro-oxidation of p-aminothiophenol (PATP) on a gold electrode has been investigated by means of density functional theory. Integration of thermodynamic calculations and surface Raman and IR spectral simulations has allowed us to reveal the electro-oxidation mechanism and reaction products. The electro-oxidation of PATP adopts a typical ECE mechanism in which multiple electron transfers and proton transfers are involved. PATP is first oxidized to PATP radicals, which is the rate-determining step, followed by chemical coupling reactions to form different dimer products. In the first route, the N−C4 coupling reaction gives D1, which can further hydrolyze to D2 in the acidic solution. In the second route, the N−C2(6) coupling reaction gives D3 in the neutral solution. In the third route, the N−N coupling reaction gives D4 in the basic solution. The calculated standard electrode potentials of the above four species decrease in the order D3, D1, D2, and D4. The characteristic bands for the surface Raman and IR spectra related to surface species are clearly assigned. Theoretical simulation indicates that PATP and its protonation form can be identified according to their IR spectroscopy. The neutral PATP molecule shows characteristic IR bands of the C−N stretching at 1272 cm−1 and the NH2 scissoring at 1623 cm−1. After protonation, the frequency of the C−N stretching shows red-shift and its IR intensity dramatically decreases while the NH2 scissoring mode splits into two peaks. As for the oxidation dimer products, Raman spectroscopy can be used to identify the reaction products of PATP. The characteristic functional groups of the electro-oxidation products are easily distinguished according to their Raman spectra. D1-ox and D3ox are characterized with their CN stretchings at 1497 and 1483 cm−1. D2-ox is characterized with its CO stretching at 1497 cm−1. The NN stretchings at 1383 and 1418 cm−1 can be viewed as the fingerprints of D4-ox. H



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