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Oxidative Coupling or Reductive Coupling? Effect of Surroundings on the Reaction Route of the Plasmonic Photocatalysis of Nitroaniline Liu-Bin Zhao,*,† Xiao-Xiang Liu,† and De-Yin Wu*,‡ †

Department of Chemistry, School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China State Key Laboratory of Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

‡

S Supporting Information *

ABSTRACT: Recent studies demonstrated that aromatic amines and aromatic nitro compounds could be converted to the corresponding azo species during surface-enhanced Raman experiments. It is very interesting to study the reaction mechanism for molecules that contain both an amino group and a nitro group, nitroaniline isomers. DFT calculations are applied to study the surface-enhanced Raman scattering and plasmonic photocatalysis of nitroaniline isomers on silver surfaces. The normal Raman and surface Raman spectra of nitroaniline isomers are first simulated and compared with experimental results. The calculated Raman spectra of onitroaniline (ONA), m-nitroaniline (MNA), and p-nitroaniline (PNA) correspond to their solid-state Raman spectra. However, the simulated surface Raman spectra of nitroaniline−silver complexes are significantly different from the experimental SERS spectra. According to the theoretical simulation, the appearance of new peaks in the SERS experiments of nitroaniline is attributed to the formation of new surface species. Two possible reaction routes, an oxidative coupling route and a reductive coupling route, are suggested to be involved in surface plasmon-mediated photocatalysis of nitroaniline on silver. It is found that the reaction route of nitroaniline on silver is strongly affected by the surroundings. The potential energy curves for the photocatalysis of PNA in the air and in the solution are presented. In the case that PNA is exposed in the air in the presence of oxygen, PNAs are oxidized to dinitroazobenzene (DNAB) by the surface activated oxygen species. In the case that PNA is immersed in the solution in the absence of oxygen, PNAs are reduced to diaminoazobenzene (DAAB) by the excited hot electrons. Finally, the Raman spectra of oxidative coupling product DNAB and reductive product DAAB are simulated. They are in good agreement with the abnormal Raman signals in SERS experiments of nitroaniline on silver.

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INTRODUCTION Surface-enhanced Raman spectroscopy (SERS) and tipenhanced Raman spectroscopy (TERS) are powerful spectroscopic techniques for in situ probing of adsorbed species and monitoring of surface reaction process.1−5 The ultrahigh sensitivity is mainly due to the huge enhancement of the local electromagnetic field as a result of surface plasmon resonance (SPR).6 SPR can be viewed as the collective oscillation of conductive electrons upon light irradiation with a specific frequency that matches the nature frequency of surface electrons of noble metals. Following the optical excitation, surface plasmons can decay by either a radiative scattering of photons or a nonradiative relaxation.7,8 As for the radiative decay, the SPR effect gives rise to the significant focus of light and the giant enhancement of local electromagnetic field, enabling observation of SERS and TERS with ultrahigh detection sensitivity.1,9 For the nonradiative process, the SPR effect can generate energetic electron−hole pairs. These hot © 2016 American Chemical Society

charge carriers are capable of inducing chemical reactions for molecule in the vicinity of the excited nanostructures.10−12 The photophysical effect of surface plasmon for electromagnetic field enhancement was the mainstream of research direction in SERS study. Different nanostructures were designed to obtain high enhancement factor.13 However, the photochemical effect of surface plasmon drew less attention. The possibility that probing molecules transform to other species through surface plasmon-mediated photocatalytic chemical reactions was often neglected.14−16 Taking the unique advantage of SPR to enhance local electric field, generate high energy electron−hole pairs, and convert the light energy into heat, surface plasmon-enhanced photocatalysis has recently come into focus as a promising technique for high Received: October 19, 2015 Revised: January 6, 2016 Published: January 7, 2016 1570

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The Journal of Physical Chemistry C performance solar energy conversion.17 Plasmonic silver and gold nanostructures show excellent ability for both catalytic oxidation reaction and catalytic reduction reaction. It has been demonstrated that both aromatic amines18−27 and aromatic nitro compounds28−35 can be selectively converted to the corresponding aromatic azo compounds during SERS or TERS measurement. In our previous work, the photo-oxidation of paminothiophenol (PATP) and photoreduction of p-nitrothiophenol (PNTP) were chosen as model system to study the reaction mechanism of the plasmonic photocatalysis.21,27 It is found that the oxidative coupling of PATP and reductive coupling of PNTP are influenced by the irradiation wavelength, the irradiation power, the solution pH, and the nature of metal substrate. More importantly, the reaction mechanisms for the reaction occurring at the metal−gas interface and at the metal− liquid interface are quite different. At the metal−gas interface in the presence of redox agents O2 or H2, O2 or H2 is first activated by the surface plasmon excitons and then reacts with adsorbed PATP or PNTP.24,26,36 At the metal−liquid interface in the absence of O2 and H2, PATP is directly oxidized by the excited hot holes while PNTP is directly reduced by the excited hot electrons.21,27 It is very interesting to study whether nitroaniline, a molecule that contains both an oxidizable amino group and a reducible nitro group, can be converted to the azo compound in the SERS experiments and whether the conversion is an oxidative coupling or a reductive coupling. Ortho-, meta-, and paranitroaniline (ONA, MNA, and PNA, respectively) have been studied extensively using conventional Raman spectroscopy,37,38 preresonant and resonant Raman spectroscopy,39−42 and SERS.43−51 Holtz studied the adsorption of PNA on silver and gold electrodes by using SERS spectroscopy.43 PNA was proposed to adsorb on metal electrodes with a perpendicular orientation via the nitro group based on SER spectra recorded at various electrode potentials. Muniz-Miranda reported on the adsorption of PNA in silver hydrosol and interpreted the SERS bands in terms of aromatic-to-quinonoidic transition44 and charge-transfer enhancement mechanism.46 Jbarah and Holtz studied the oxidative and reductive electrochemistry of three isomeric nitroanilines on gold electrode by cyclic voltammetry and SERS.47 It was found that the position of substituent groups and the pH of the electrolyte had a substantial effect on oxidation and reduction processes of the nitroanilines at surfaces. Tanaka et al. investigated the Raman spectra of PNA in acetonitrile solution and their SERS spectra in gold and silver colloidal solutions.45 Ma and Fang reported recently a joint experimental (Raman and SERS) and theoretical study (DFT) on the adsorption of the three isomers of nitroaniline on gold nanoparticles.48,49 Also, a combination of SERS and DFT study on the adsorption of nitroaniline on silver was performed by Chis et al.51 Surface-enhanced IR adsorption (SEIRA) and SERS were used by Posey et al. to study the adsorption behavior of nitroaniline isomers on silver surfaces.50 The differences in SEIRA versus SERS enhancement of the individual isomers were explained as resonance effects, adsorbate geometries, and hydrogen-bonding variations combined with changes in the strength of adsorption. In prior SERS studies on nitroaniline isomers, the reported SERS spectral shapes were quite different from their ordinary Raman spectra and some abnormal intense SERS peaks were observed in the frequency region around 1200 and 1400 cm−1.47,50,52,53 The abnormal SERS signals were not well explained previously and the potential photochemical reaction

of detection molecules, e.g., nitroaniline isomers, did not draw much attention in the SERS field. Until very recently, Ding et al. proposed that a similar molecule, 2-amino-5-nitrobenzenethiol, was selectively reduced to an azobenzene-like species from the time-dependent SERS experiment.54 In this work, we perform a systematic theoretical study on surface-enhanced Raman spectra and plasmonic photocatalysis of nitroaniline isomers on silver surface. First, the normal Raman and surface Raman spectra of nitroaniline isomers are simulated. By comparing the theoretical and experimental Raman spectra of nitroaniline on silver surfaces, we propose that the appearance of abnormal SERS signals is attributed to the formation of new surface species. Then, the reaction mechanisms for the photocatalysis of nitroaniline in the air and in the solution are discussed. The potential energy curves for the oxidative coupling route and reductive coupling route are performed. Finally, the Raman spectra of oxidative coupling product dinitroazobenzene (DNAB) and reductive product diaminoazobenzene (DAAB) are simulated.

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COMPUTATIONAL DETAILS Cluster model was used to mimic the adsorption of nitroaniline isomers on the surfaces of silver nanoparticles, where the active sites of metal surfaces were modeled as Ag10 clusters. It was found that the perpendicular orientation via nitro group was the most stable adsorption configuration of nitroaniline isomers on Ag10 cluster. Density functional theory (DFT) calculations were carried out with the hybrid exchange−correlation functional B3LYP55,56 and generalized gradient approximation functional PW91PW91.57 The basis sets for C, H, N, and O atoms of investigated molecules were 6-311+G(d,p), including a polarization function to all the atoms and a diffuse function to C, N, and O atoms.58,59 For all silver atoms, the valence electrons and the inner shells were described by the basis set, LANL2DZ, and the corresponding relativistic effective core potentials, respectively.60,61 All calculations including structure optimization and thermodynamic energy computation were carried out by using the Gaussian 09 package.62 Raman intensities in on- and off-resonance Raman scattering processes were estimated in terms of the derivative of the polarizability with respect to a given normal coordinate on the basis of the optimized geometries. We employed the general formula in the harmonic approximation, where the differential Raman scattering cross section (DRSC) was given by63,64 (ω0 − ωi)4 ⎛ dσ ⎞ (2π )4 h ⎜ ⎟ = Si ⎝ dΩ ⎠i 45 8π 2cωi 1 − exp[−hcωi /(kBT )]

(1)

⎛ dα ⎞ 2 ⎛ dγ ⎞ 2 ⎟⎟ + 7⎜⎜ ⎟⎟ Si = 45⎜⎜ ⎝ dQ i ⎠ ⎝ dQ i ⎠

(2)

where h, c, kB, and T are the Planck constant, light speed, Boltzmann constant, and Kelvin temperature, respectively. ω0 and ωi are the frequencies (in cm−1) of incident light and the ith vibrational mode. Si is the Raman scattering factor (RSF, in Å4/amu), which can be directly obtained from the Gaussian program. dα/dQi and dγ/dQi are derivatives of isotropic and anisotropic polarizabilities with respect to the ith normal coordinate, respectively. In order to make direct comparison with the SERS experiments, the simulated Raman spectra were presented in terms of a Lorentzian expansion of the DRSC with a line width of 10 cm−1.65 For preresonance Raman spectral 1571

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The Journal of Physical Chemistry C simulation, the frequency-dependent RSF magnitudes were calculated by using the coupled perturbation methods.66 The assignments of vibrational modes are based on the potential energy distributions (PED) derived from the SQM-DFT force field by Wilson’s GF matrix method.67,68 The frequency calculations were performed on all the intermediates and transition states. For stable intermediate geometries, we verified that there was no imaginary frequency. In the case of the transition states we verified that there was only one imaginary frequency along the reaction coordinate. These frequency calculations also provided us with the thermochemical analysis at a pressure of 1 atm and temperature of 298.15 K. The solvation model of density (SMD) approach was used to consider the solvation effect, which included both nonelectrostatic and electrostatic terms and was recommended to predict well the solvation Gibbs free energies of ions and molecules.69 Electronic excitation was studied within the time-dependent DFT (TD-DFT) approaches, which was an efficient method and offered an orbital picture for a physical understanding of the electronic excitation process.70 The charge transfer (CT) direction of nitroaniline metal complexes was determined by analyzing the orbital components involved in the optical transitions. This method was successfully used in previous studies on the CT issues of pyridine and benzenethiol derivatives adsorbed on metal surfaces.71−73 The simulated absorption spectra were presented in terms of the Lorentzian function with a peak half-width at half height of 0.333 eV.

bending and NO2 rocking, the ring in-plane deformation, and the mixed vibration of ring breathing and NO2 scissoring. The calculated frequencies match well with experimental data at 419, 560, and 817 cm−1.47 The doublet bands at 1052 and 1085 cm−1 correspond with experimental values of 1067 and 1100 cm−1.47 They are assigned to the mixed vibration of C−C stretching and NH2 rocking and mixed vibration of C−NO2 stretching and ring in-plane deformation. The nearby nitro group and amino group form an intramolecular hydrogen bond in ONA. As a result, the bond length of the N−O bond (1.241 Å) in proximity to the NH2 group is longer than that of the N− O bond (1.226 Å) away from NH2 group and the symmetric NO2 stretching splits into several bands around 1300 cm−1 due to the vibrational coupling with other vibrations. The most intense Raman bands at 1262 and 1347 cm−1 are assigned to the mixed vibrations from the C−NH2 stretching and the NO2 stretching. The former peak observed experimentally at 1247 cm−1 was assigned to the C−H bending in a previous study.47 According to potential energy distribution (PED) analysis, the 1262 cm−1 peak is mainly composed of the C−NH2 stretching (17.9%) and the NO2 stretching (32.8%), and the Raman intensity of the C−NH2 stretching is enhanced by coupling with the NO2 stretching. The detailed theoretical and experimental vibrational assignments of ONA are listed in Table S1, Supporting Information. The Raman spectrum of MNA is dominated by the intense peak at 1345 cm−1 (1350 cm−1 in experiment47), which is assigned to the NO2 symmetric stretching. Since there is no intramolecular hydrogen bond formed in MNA, the NO2 stretching is not coupled with other vibrations. As compared to ONA, the Raman intensity of the C−NH2 stretching at 1268 cm−1 is very weak. The peak appearing at 991 cm−1 is assigned to the ring triangular deformation. The calculated frequencies of MNA match well with Raman47 and IR74 experimental results. The detailed theoretical and experimental vibrational assignments of MNA are listed in Table S2, Supporting Information. The Raman spectrum of PNA is sensitive to the phase state. The Raman spectrum of solid PNA is characterized by the intense peak at 1282 cm−1, and the Raman spectrum in ethanol solution has a strong peak at 1316 cm−1.51 Because the NO2 group and NH2 group in two nearby PNA molecules can form an intermolecular hydrogen bond, the frequency of NO2 stretching is sensitive to the surroundings. The simulated Raman spectrum of PNA dimer match well with the solid-state Raman spectrum of PNA.47,51 The most intense Raman peaks calculated at 1294 and 1327 cm−1 well reproduce the doublet bands at 1282 and 1315 cm−1 observed in experiments.49 According to the PED analysis, the C−NH2 stretching and the symmetric NO2 stretching are strongly coupled. The 1302 cm−1 peak is a mixed vibration of the C−NH2 stretching (27.6%) and the NO2 stretching (32.8%) and the 1327 cm−1 peak is a mixed vibration of the C−NH2 stretching (22.5%) and the NO2 stretching (40.0%). The medium intense peaks at 550, 858, 1104, and 1610 cm−1 are assigned to the NH2 wagging, the ring breathing, the C−NO2 stretching, and the C−C stretching, which are consistent with experimental values at 531, 861, 1107, and 1591 cm−1.51 The hydrogen bond is not expected to be formed in solution phase due to large intermolecular distances.51 The simulated Raman spectrum of PNA monomer agrees well the Raman spectrum in ethanol solution (see Figure S1, Supporting Information). The calculated doublet peaks at 1301 and 1332

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RESULTS AND DISCUSSION Raman Spectra of Nitroaniline. Figure 1 shows the simulated normal Raman spectra of ONA, MNA, and PNA. Despite the same functional group, the relative position of nitro group and amino group on the benzene ring strongly influences its spectral shape. For ONA, the Raman peaks at 401, 559, and 818 cm−1 are assigned to the mixed vibration of C−NH2

Figure 1. Simulated normal Raman spectra of ONA, MNA, and PNA by B3LYP/6-311+G(d,p) with an excitation wavelength of 632.8 nm and a line width of 10 cm−1. 1572

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The Journal of Physical Chemistry C cm−1 are consistent with experimental values at 1316 and 1333 cm−1.51,75 They are assigned to the C−NH2 stretching and the NO2 stretching. Both of these two peaks undergo a blue-shift with respect to PNA dimer due to the lack of intermolecular hydrogen bond. The detailed theoretical and experimental vibrational assignments of PNA are listed in Table S3, Supporting Information. Figure 2 shows the simulated off-resonance Raman spectra of ONA, MNA, and PNA adsorbed on silver surfaces. The

the NO2 stretching mode of MNA undergoes a 12 cm−1 redshift in the adsorption state. The simulated surface Raman spectrum of PNA is consistent with SERS from PNA adsorbed on silver electrode.44 The calculated frequencies of the C−NH2 stretching (1302 cm−1) and the NO2 symmetric stretching (1323 cm−1) of the Ag10− PNA complex are lower than those in free PNA (1301 and 1332 cm−1). In the SERS study from PNA adsorbed on silver surfaces, Chis et al. observed that the Raman frequencies of these two peaks move from 1316 and 1333 cm−1 to 1307 and 1331 cm−1.51 Also, the red-shifts of the 1332 cm−1 peak to 1327 cm−1 and of the 1312 cm−1 peak to 1294 cm−1 were demonstrated by Muniz-Miranda.44 As shown in Figures 1 and 2, in the simulated normal Raman and surface Raman spectra of PNA, there are no strong peaks near 1200 and 1400 cm−1. However, new peaks were observed in the SERS spectra of PNA.50,51 Intense SERS peaks of PNA appeared at 1186 and 1406 cm−1,50 which were not found in its solid-state and solution spectra. These abnormal peaks were also detected in the SERS of ONA and MNA.47 However, the appearance of abnormally strong peaks had not attracted great attention in prior publications. The change of the Raman line shape provides important information on the physical and chemical processes occurring at metal−molecule interface. The appearance of new peaks in SERS spectra can be attributed to either the enhancement of specific modes via photon-driven charge transfer mechanism76 or the formation of new surface species by photocatalytic surface reactions.27 In order to study the effect of photon-driven charge transfer mechanism on the surface Raman spectra of nitroanilines, the preresonance Raman spectra of Ag10−ONA, Ag10−MNA, and Ag10−PNA complexes are simulated by using frequencydependent polarizability (see Figures S2−S4, Supporting Information). As the excitation wavelength changes from 785 to 488 nm, the absolute Raman intensities of three nitroanilines first increase and then decrease, which is in agreement with the theoretical illustration of the photon-driven charge transfer enhancement in SERS.9 Our calculation demonstrates that the Raman intensity of nitroaniline adsorbed on silver surface can be enhanced in case that the incident light energy approaches the charge transfer excitation energy. However, the relative Raman intensities of nitroanilines are not strongly influenced; especially, no intense Raman peaks appear near 1200 and 1400 cm−1. Therefore, the appearance of abnormal SERS signals of

Figure 2. Simulated surface Raman spectra of ONA, MNA, and PNA complexes by B3LYP/6-311+G(d,p)/Lanl2DZ with an excitation wavelength of 632.8 nm and a line width of 10 cm−1.

optimized structures of three nitroanilines bonding to an Ag10 cluster are shown in the inset of Figure 2. According to previous experimental study, nitroaniline is adsorbed via the nitro group in a perpendicular orientation with respect to the silver surface, as evidenced by the downshift of symmetric stretching of the nitro group.44 For Ag10−ONA, the NO2 stretching frequency red-shifts from 1347 to 1344 cm−1 with respect to the free ONA molecule, in agreement with the 1353−1347 cm−1 frequency shift from experiment.47 Similarly

Figure 3. Reaction mechanism for the plasmonic photocatalysis of nitroaniline in the air and in the solution. 1573

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molecule.26,27 Following the oxygen activation process, the four hydrogen atoms of two nearby nitroaniline molecules can be consecutively abstracted by the activated surface oxygen species and nitroanilines are transformed to DNAB through an oxidative coupling reaction. The photothermal effect of SPR allows for very rapid and local heating on the metal surface, which can provide the necessary energy input that is required for the cleavage of N−H bonds. As seen in Figure 4, after oxygen molecule is dissociated on metal surface, the oxygen atoms are inserted into the bridge site of two silver atoms. Ag2O clusters are extracted from Figure 4 and are employed to model the activated oxygen species that react with the amino group of PNA. Figure 5 compares the

nitroanilines should be attributed to the formation of new surface species during SERS measurement. According to our previous studies, both aromatic amines and aromatic nitro compounds could be converted to the corresponding aromatic azo compounds via the surface plasmon-mediated photocatalysis.18−21,26,27 Nitroaniline, containing an amino group and nitro group, was therefore possibly transformed to dinitroazobenzene (DNAB) by an oxidative coupling reaction or to diaminoazobenzene (DAAB) by a reductive coupling reaction.54,77 The reaction mechanism of plasmonic catalytic oxidative coupling to p-aminothiophenol (PATP) and reductive coupling of p-nitrothiophenol (PNTP) has been investigated intensively in our recent studies.21,27 It was found the plasmonic photocatalytic redox reactions can be driven directly by the excitation of hot electrons and hot holes or induced by the surface plasmon-mediated formation of surface active redox agents.27 For nitroaniline, the oxidation of amino group and reduction of nitro group are competitive processes. The branching ratio between these two routes depends on the surroundings. The reaction mechanism for the plasmonic photocatalysis of nitroaniline is proposed in Figure 3. DNAB is the major product via the oxidative coupling route in the air, and DAAB is the major product via the reductive coupling route in the solution. Plasmonic Catalytic Oxidative Coupling. Figure 4 shows a schematic illustration of plasmonic photocatalysis of oxidative

Figure 5. Potential energy surfaces of the oxidative coupling of PNA to DNAB.

potential energy curves of the oxidation coupling of PNA to DNAB by surface activated oxygen species Ag2O and free O2. The oxidative coupling of PNA to DNAB adopts an OCO (oxidation−coupling−oxidation) mechanism, which is similar to the oxidation mechanism of p-aminothiophenol.26,27 The transition states for the consecutive four hydrogen abstraction reactions are labeled as TS1−TS4. PNA molecules are first oxidized to their neutral radical intermediates (O2N−C6H4− NH) via TS1 and TS2. Then, the spontaneous coupling of two nearby radicals produces a hydrazo compound DNHAB. Finally, DNHAB is oxidized to DNAB by cleavage of two N−H bonds via TS3 and TS4. As shown in Figure 5, the calculated standard molar Gibbs free energy of PNA oxidation by Ag2O is 40 kcal/mol more negative than that by O2 molecules. The activation energies for PNA oxidized by free oxygen are much higher than those by surface activated oxygen species. The rate-determining step for the oxidation of PNA by Ag2O is the second dehydrogenation step with an activation energy of 4.2 kcal/mol, while the ratedetermining step for oxidation of PNA by oxygen is the first dehydrogenation step with an activation energy of 40.9 kcal/ mol. The formation of surface activated species, as a result of surface plasmon-mediated oxygen dissociation, strongly enhances the oxidation ability of oxygen and reduces the reaction barrier of oxidative dehydrogenation reactions.26,27 Additionally, the energy required for the cleavage of N−H bond can be provided by the photothermal effect of SPR.79,80 Due to the bifunctional group, PNA molecules may be trapped into the gap between silver nanoparticle aggregates. The photothermal heating effect can be amplified in the presence of several nanoparticles.81,82 Figure 6 shows the simulated Raman spectra of the oxidative coupling products of ONA, MNA, and PNA by PW91PW91

Figure 4. Schematic illustration of plasmonic photocatalysis of oxidative coupling of nitroaniline to dinitroazobenzene.

coupling of nitroaniline in the air. The overall reaction can be viewed as the initial oxygen activation reaction and the subsequent dehydrogenation of amines. The nonradiative decay of surface plasmon gives rise to hot electron−hole pairs. The excited electrons and holes can transfer to the adsorbates if a suitable acceptor level is available. In the case that nitroaniline and oxygen are coadsorbed on silver surfaces, the hot electrons are more likely to transfer to oxygen.26,27 Because the LUMO level of oxygen (−3.36 eV) is lower than that of PNA (−2.80 eV) and the oxygen reduction potential (0.60 V vs NHE in 0.1 M HClO4 solution78) is more positive than the PNA reduction potential (0.12 V vs NHE in 0.1 M HClO4 solution47). Oxygen molecules approaching the silver surface are then activated and dissociated through the surface plasmon-mediated hot electron injection to the antibonding orbital of oxygen. The activated surface oxygen species shows a very strong oxidation ability with respect to the free oxygen 1574

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Figure 7. Schematic illustration of plasmonic photocatalysis of reductive coupling of nitroaniline to diaminoazobenzene.

Figure 6. Simulated Raman spectra of o,o′-DNAB, m,m′-DNAB, and p,p′-DNAB by PW91PW91/6-311+G(d,p) with an excitation wavelength of 632.8 nm and a line width of 10 cm−1.

nitro group may be reduced by the excited hot electrons, and the amino group may be oxidized by the excited hot holes. The HOMO and LUMO levels of PNA adsorbed on silver is −6.76 and −2.80 eV, respectively. Apparently, the LUMO level of PNA is closer to the Fermi level of silver (−4.52 to −4.74 eV).83 The calculated lowest metal-to-molecule (electron transfer) and molecule-to-metal (hole transfer) transition energies of Ag10−PNA complexes are 2.42 and 3.95 eV, respectively. So the most likely CT transition occurring at the PNA−silver interface should be a metal-to-molecule CT process upon visible light irradiation. The energy levels of frontier orbitals and the optical excitation properties are listed in Table S4, Supporting Information. It was demonstrated that aromatic nitro compounds could be reduced to the corresponding azo compounds by the excited hot electrons.21,27,28,31,84 Thus, DAAB should be the major product for the plasmonic photocatalysis of PNA in the solution. Figure 7 shows two possible charge transfer channels for the attachment of the hot electron from silver to adsorbed nitroaniline. For the direct CT channel, the optical excitation of metal−molecule complex was damped by the chemical interface, resulting in a small probability for a direct transition of electron from silver cluster to the adsorbate.7 Alternatively, electrons in silver are first excited to the unoccupied states above the Fermi level by surface plasmon decay and then tunnel to an adsorbed nitroaniline.27 The possibilities of the above two competitive processes can be estimated from the oscillator strengths of the corresponding transitions. Figure S5 shows the simulated absorption spectra of surface complexes of nitroaniline. As seen, the oscillator strength of surface plasmon excitation round 380 nm is much stronger than that of the direct metal-to-molecule transition round 520 nm. Thus, the plasmon-induced hot electron injection mechanism should be the major charge transfer channel. Nitroanilines can be reduced to DAAB via multiple electron transfer and proton transfer processes, in which the energy of electrons and the chemical potential of protons are determined by the excitation wavelength and the solution pH, respectively.27 Figure 8 shows the wavelength-dependent potential energy curves of reduction of PNA to DAAB in a neutral solution (pH = 7). The reaction mechanism of reduction of nitrobenzene to azobenzene has been proposed in our previous study.21,27,64 As the irradiation wavelength decreases from 1064 to 514.5 nm, PNA is first reduced to a nitroso compound PNSA and then to

functional, which is found to be the best functional for the simulation of aromatic azo compounds.18,21 The optimized strucutres of the oxidative coupling product, o,o′-DNAB, m,m′DNAB, and p,p′-DNAB, are shown in the inset of Figure 6. Posey et al. investigated the SERS of ONA, MNA, and PNA adsorbed on silver film.50 The measured SERS spectrum of PNA is obviously different from its normal Raman spectrum.51 Excpet for the strong band of NO2 stretching at 1337 cm−1, a set of abnormal intense peaks were observed at 1127, 1186, 1402, and 1450 cm−1, which cannot be found from the normal Raman spectrum of PNA. These abnormal Raman signals were also observed in other SERS studies on nitroaniline.46,47,51−53 In the experiment by Posey et al., nitroaniline films for SERS studies were formed on vacuum-evaporated silver films via the drop method by pipeting nitroaniline solution and allowing the solvent to evaporate.50 So nitroanilines were exposed in the air in their experiment. As proposed above, nitroanilines adsorbed on silver surface can catalyze oxidative coupling reaction to produce DNAB in the apperance of oxygen in the air. The simulated Raman spectrum of p,p′-DNAB is in good agreement with the experimental SERS of PNA. As seen in Figure 6, the symmetric NO2 stretching modes are marked with green star and their frequencies are very close to those in the three nitroanilines. The peaks that are labeled with dashed red frames are assigned to the mixed vibration of the C−N stretching and the C−H bending, and the peaks that are labeled with dashed blue frames are assigned to the mixed vibration of the N−N stretching, the C−C stretching, and the C−H bending. For p,p′-DNAB, the calculated frequencies at 1124 1172, 1394, and 1436 cm−1 match well with experimental values at 1127, 1186, 1402, and 1450 cm−1 from the SERS of PNA on silver surface.50 In addition, the simulated Raman spectrum of p,p′-DNAB is also consistent with experimental SERS of DNAB.77 Thus, we can conclude that nitroaniline are transformed to DNAB through a catalyzed oxidative coupling reaction during SERS experiment proceeded in the presence of oxygen in the air. Plasmonic Catalytic Reductive Coupling. Figure 7 shows a schematic illustration of plasmonic photocatalysis of reductive coupling of nitroaniline in the solution. In the absence of oxygen, nitroanilines can interact directly with the excited charge carriers generated by surface plasmon decay. The 1575

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Figure 8. Potential energy curves of the reductive coupling of PNA to DAAB as a function of irradiation wavelength.

duced to DAAB.54 New SERS peaks from 2A-5NBT in the aqueous environment were observed at 1141, 1185, 1390, and 1436 cm−1, which were consistent with the calculation results at 1129, 1193, 1399, and 1451 cm−1. Xu et al. studied the catalytic reduction of PNA on silver nanobowl arrays by SERS.53 Although they clamined that PNA was reduced to pphenylenediamine, the characteristic SERS peaks (1137, 1192, 1392, and 1449 cm−1) of the reduction product indeed correspond to the simulated Raman spectrum of DAAB. The agreement of the simulated Raman spectra of DAAB with the experimental SERS spectrum of PNA makes us believe that nitroanilines are transformed to DAAB through the catalyzed reductive coupling reaction during SERS experiment proceeding in the solution. Both of the oxidative coupling product DNAB and the reductive coupling product DAAB exhibit strong Raman signals in the frequency region of C−N stretching and N−N stretching as showns in the red and blue frames in Figure 6 and Figure 9. However, the spectral shapes for DNAB and DAAB in these two reigions are quite different. It is found that the frequency of the N−N stretching peak in DAAB is lower than that in DNAB. The most intense N−N stretching peak in o,o′-DNAB, m,m′DNAB, and p,p′-DNAB is located at 1499, 1429, and 1436 cm−1. The most intense N−N stretching peak in o,o′-DAAB, m,m′-DAAB, and p,p′-DAAB is located at 1380, 1413, and 1399 cm−1. Additionaly, there are more peaks appearing in the frequency region from 1100 to 1200 cm−1 in DNAB than in DAAB due to the coupling of the C−NO2 stretching and the C−NH2 stretching. These spectral differences can be used to distinguish the two reaction products of nitroaniline. The detailed theoretical vibrational assignments of DNAB and DAAB are listed in Tables S5 and S6, Supporting Information. As discussed above, the reaction products and reaction mechanisms of the photocatalysis of nitroaniline are significantly affected by the surroundings. At solid−gas interface in the presence of oxygen, PNAs are oxidized to DNAB by the activated surface oxygen species. The simulated Raman spectrum of DNAB is in agreement with the SERS spectrum of PNA measured in the air.50 At solid−solution interface in the absence of oxygen, PNAs are reduced to DAAB by the excited hot electrons. The simulated Raman spectrum of DAAB is consistent with the SERS spectrum of PNA measured in the solution.53

a hydroxylamine compound PHOA by the excited hot electrons. PHOA cannot be further reduced by the hot electron because of its high LUMO level. However, it can be reoxidized to PNSA by the hot holes.27 The PNSA/PHOA redox pair accumulated on surfaces undergoes a condensation reaction to produce an azoxy compound DAAOB, which can be further reduced to DAAB.85,86 As seen from Figure 8, the one electron−one proton reduction of PNA to its hydrogen adduct PNA−H is the most difficult step during the overall reduction process of PNA. The electrons excited by the 1064 and 785 nm irradiation are not able to drive the reduction of PNA, which can be otherwise driven by the 514.5 and 632.8 nm irradiation. This is in agreement that PNA can be reduced to DAAB during SERS measurement with an incident wavelength of 632.8 nm.54 Figure 9 shows the simulated Raman spectra of the reductive coupling products, o,o′-DAAB, m,m′-DAAB, and p,p′-DAAB.

Figure 9. Simulated Raman spectra of o,o′-DAAB, m,m′-DAAB, and p,p′-DAAB by PW91PW91/6-311+G(d,p) with an excitation wavelength of 632.8 nm and a line width of 10 cm−1.

As the nitro group is reduced, the symmetric NO2 stretching cannot be found while new peaks appear in the regions from 1100 to 1200 cm−1 and from 1350 to 1450 cm−1. The peaks that are labeled with dashed red frames are assigned to the mixed vibration of the C−N stretching and the C−H bending, and the peaks that are labeled with dashed blue frames are assigned to the mixed vibration of the N−N stretching, the C− C stretching, and the C−H bending. Ding et al. proposed that 2-amino-5-nitrobenzenethiol (2A-5NBT) was selectively re1576

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CONCLUSIONS We have theoretically investigated the surface-enhanced Raman scattering and plasmonic photocatalysis of nitroaniline isomers on silver surfaces. The normal Raman and surface Raman spectra of ONA, MNA, and PNA are simulated and compared with experimental results. The calculated Raman spectra of three nitroaniline isomers are consistent with their solid-state Raman spectra. The most intense Raman peaks from the NO2 stretching for the three nitroaniline molecules decrease according to the trend of MNA (1345 cm−1) > PNA (1294 cm−1) > ONA (1262 cm−1) because an intramolecular hydrogen bond is formed in ONA and intermolecular hydrogen bonds are formed in PNA aggregates. However, the simulated surface Raman spectra of nitroaniline silver complexes are significantly different from the experimental SERS spectra. Abnormal intense SERS peaks appear in the frequency region near 1200 and 1400 cm−1, which are not found in their normal Raman spectra. The possibility of spectral change as a result of charge transfer enhancement of specific vibrational modes is excluded from the simulated preresonance Raman spectra by using the frequency-dependent polarizability. Thus, the appearance of new peaks in the SERS of nitroaniline is attributed to the formation of new surface species. We propose that nitroanilines undergo two possible reaction routes by the surface plasmon-mediated photocatalysis on silver, the oxidative coupling of amino group to DNAB, and the reductive coupling of nitro group to DAAB. It is found that the reaction route of nitroaniline on silver is strongly affected by the surroundings. The potential energy curves for the photocatalysis of PNA in the air and in the solution are performed. In the case that nitroaniline is exposed in the air in the presence of oxygen, oxygen is first activated on silver surface by the excited hot electrons due to its low electron acceptor level. The amino hydrogen atoms of PNA are successively removed by the generated active oxygen species, producing the oxidative coupling product DNAB. In the case that nitroaniline is in solution in the absence of oxygen, the nitro group can be reduced by the excited hot electrons instead of be oxidized by the excited hot holes due to its high moleculeto-metal transition energy. PNAs are reduced to DAAB via multiple electron-transfer and proton-transfer steps that are mediated by the irradiation wavelength and solution pH. Finally, the Raman spectra of the oxidative coupling product DNAB and the reductive coupling product DAAB are simulated. They are in good agreement with the abnormal Raman signals in SERS experiments of nitroaniline on silver.

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Article

AUTHOR INFORMATION

Corresponding Authors

*L.-B.Z: e-mail, [email protected]. *D.-Y.W.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grants 21373712, 21321062, and 21533006), National Key Basic Research Program of China (Grant 2015CB932303), Fundamental Research Funds for the Central Universities (Grants SWU114076 and XDJK2015C100), and Open Funds of State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University No. 201416). L.-B.Z. thanks Li-Wei Chen for improving English writing.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10207. Experimental and theoretical vibrational assignments of ONA, MNA, and PNA (Table S1−S3); frontier orbitals energy levels and low lying charge transfer excitation energies of nitroaniline silver complexes (Table S4); theoretical vibrational assignments of DNAB and DAAB (Table S5 and S6); simulated normal Raman spectrum of PNA monomer (Figure S1); simulated preresonance Raman spectra of nitroaniline silver complexes with different excitation wavelength; simulated absorption spectra of ONA, MNA, and PNA complexes (Figure S2, S3, and S4) (PDF) 1577

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