SERS Chemical Enhancement of Water Molecules from Halide Ion

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SERS Chemical Enhancement of Water Molecules from Halide Ion Coadsorption and Photoinduced Charge Transfer on Silver Electrodes Ran Pang, Xia-Guang Zhang, Jian-Zhang Zhou, De-Yin Wu, and Zhong-Qun Tian J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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The Journal of Physical Chemistry

SERS Chemical Enhancement of Water Molecules from Halide Ion Coadsorption and Photo-induced Charge Transfer on Silver Electrodes Ran Pang*, Xia-Guang Zhang, Jian-Zhang Zhou, De-Yin Wu*, Zhong-Qun Tian State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China

ABSTRACT: The surface-enhanced Raman scattering (SERS) signal of water is hard to be measured due to its very small Raman scattering cross section and weak adsorption on coinage metals, only electrochemical SERS spectra of water have been observed in electrode/electrolyte interfaces so far. Our present work focuses on the chemical enhancement from halide ions on SERS signals of water adsorbed on silver electrodes, by combining the metallic cluster model and hybrid density functional theory (DFT) calculations. The interfacial structures, binding interactions and the anion effect from different halides including chloride, bromide, and iodide ions have been analyzed and compared with experimental measurements in literatures. Then the excited states of halide ions modified active sites on roughened silver electrodes have been discussed. Especially, our time-dependent DFT (TD-DFT) calculations predicted that halide ions

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can form low-lying excited states of surface complexes, like the photon-induced charge transfer (PICT) states, and finally contribute to the chemical enhancement of SERS signals of water. Furthermore, we proposed that the halide effect on the relative SERS intensities of water is a good example for understanding the chemical enhancement of SERS active sites modified by halide ions in electrochemical systems. This is different from the chemical enhancement of SERS of water at metal cathodes.

Introduction Water is one of the commonest solvents and plays vital roles in life, energy and surface sciences.1-3 Clarifying the interfacial water at the microscopic level can greatly improve our fundamental understanding of the electrode/electrolyte interface, which is an eternal issue in electrochemistry and corrosion sciences.4-8 Surface-enhanced Raman spectroscopy (SERS) as a highly sensitive tool has been used to characterize and analyze the interfacial water on several metal electrodes.9-13 However, water has not only a very small scattering cross section, but also a weak adsorption interaction on noble metal electrodes. Thus, the interactions between water molecules and electrolyte ions,14-18 as well as those between water and electrode surfaces9-13,19 exist either as competitive adsorption or co-adsorption, resulting in a very complex phenomenon in different potentials. A detailed insight on interfacial structures and the interaction of water, electrolyte ions, and electrode surfaces is necessary at a molecular level. In halide-containing systems, the enhanced SERS signals of water molecules have been firstly observed at an electrochemically roughened silver electrode.6,18,20-24 The enhanced Raman scattering processes was observed at applied potentials close to the potential of zero charge


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(PZC). It was proposed that localized surface coadsorbed complexes involving water and halide ions are responsible for the observed SERS of water. Such signals could not be obtained when the halide solution was replaced by non-halides, such as sulphate and propanoate.18 Surprisingly, the SERS signals of water molecules and silver-halide vibrations disappeared when the electrode was polarized sufficiently negative.20 The enhancement mechanism was proposed due to disrupting the hydrogen bonds of adsorbed water molecules, after adding high concentration halide ions.21 Besides, halide ions also play a considerable role in other coadsorption or competitive adsorption systems. It is the fact that the vibrational band intensities of adsorbates are considerably enhanced by one to three orders of magnitude in the presence of chloride or bromide in the electrolyte.25,26 In single-molecule SERS, the halide activated SERS sites are always essential.27,28 The SERS enhancements accompanied by the halide activation have been explained by aggregation of metal particles and the formation of stable surface complexes of halide and probe molecules.29 The electromagnetic enhancement of the halide effect can be reflected easily from the ultraviolet-visible absorption spectroscopy, but the chemical enhancement related to the electronic structural of probe species adsorbed at active sites still is a question that cannot be directly answered by the experiments at present. Combining density functional theory (DFT) calculations and Raman scattering theory, it could give better assignments for Raman spectra of adsorbed molecules, and analyze the change of SERS spectra. Wu et al. investigated the influence of the hydrogen bonding interaction and suggested that the strong polarization of the proton acceptor causes a significant enhancement on the Raman intensity of the HOH bending mode in water molecules as proton donors.30 At very negative potentials, Li et al. proposed that interfacial water molecules were in an adsorption


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configuration with hydrogen bonding interactions, i.e., the HO-H…H–Pt dihydrogen bond for platinum and the HO-H…Ag(Au) for silver and gold electrodes.13 Then, the charge transfer from the interfacial electron to a hydrated proton were proposed to contribute to the SERS signals of interfacial water.31 Recently, Sanchéz-Lozano et al. investigated the SERS spectra of water interacting with a 20-atom silver cluster, and proposed that the symmetry of the dipole moment and the symmetry of the vibrational modes influence the Raman intensities of the SERS spectrum.32 All these above studies focused on the Raman signals of water molecule or hydrated species interacting directly with metal cathodes only. Furthermore, Zhang et al. calculated electronic ground and excited states of water molecules on silver clusters, and discussed the influence of the photo-induced electron transfer (PICT) excitation on dissociation of water.33,34 But the transition energy is quite higher than general excitation energies used in SERS measurements. Further study is, however, much necessary to gain a better understanding of the halide effect on the SERS enhancement of water, as well as complex interfacial structures and interaction of water molecules, halide ions and metallic surface active sites. The goal of the present work is to explore the chemical enhancement of the halide effect on SERS of water on silver electrodes from the viewpoint of adsorption and electronic structures. We first calculated the interfacial structures of water molecules and halide ions coadsorbed at silver nanostructures by using DFT methods. To simulate the solvation effect in electrochemical interfaces, the implicit solvation model of SMD approaches have been considered. Then different structures of halide modified silver clusters as SERS active sites have been discussed in detail. Time dependent DFT (TD-DFT) calculations predicted that there exist several low-lying excited states in the visible light region due to adding the halide ions. On the basis of the TD-DFT calculations, we further investigated that the PICT from halide ions to silver clusters and the


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intraband transition of silver contribute to the chemical enhancement of SERS signals of water molecules. Finally, we estimated the enhancement factor of the charge transfer mechanism on the SERS of water.

Computational Details To gain deep insight on the halide effect for SERS of water on silver electrodes, the cluster models we selected are H2O-AgnXδ (n = 2 – 7; δ = 0 and -1), where water molecules adsorbed on halide modified active sites of silver surfaces. Meanwhile, the structures of water directly adsorbed on silver clusters (H2O-Agn, n = 2 - 7) have also been calculated for a comparison. To mimic the electrode surface site with the polarization of applied potentials, we selected neutral clusters of Agn, and positively charged clusters by adding a halide ion into AgnX- and AgnX complexes. Here, X is the chloride, bromide or iodide ions, while the fluoride ion with special hydrogen bonds and weak adsorption35,36 is not considered in the present paper. All

DFT

calculations,

including

geometry

optimization,

vibrational

analysis

and

thermodynamic energies, were carried out by using Gaussian 09 program.37 The Becke’s threeparameters exchange and Lee-Yang-Parr correlation hybrid functional (B3LYP) approach38,39 was used for geometry optimization and electronic energy calculations. There is not any constraint for the optimization calculations. For O and H atoms, we used the augmented tripletzeta splitting Dunning’s correlation consistent basis set aug-cc-pVTZ.40 As for metal atoms of silver, the electrons in the valence and internal shells were described by the basis set LANL2DZ and the corresponding relativistic effective core potentials, respectively.41 This is a good combination and has been used for the simulation SERS of water in our previous studies.13,31,42 Considering the halide ionic effect, the basis set used for Cl and Br atoms is 6-311+G**. While


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for the I atom, a triple-ζ valence basis set with the pseudopotential (SBK CEP-121) was employed.43-45 All the calculated structures are without any imaginary frequencies, indicating that they are true local minimums. Besides, binding energies (BE) are calculated according to BE = -[E(Mol-Mn) - E(Mol) - E(Mn)], and the counterpoise method was used to remove the basis set superposition error (BSSE) in calculating the interaction energies.46,47 To consider the solvation effect in electrochemical interfaces, the implicit solvation model of density (SMD)48 has been used for comparisons. In this paper, we choose water with a static dielectric constant (ε = 78.3) as the solvent at 298.15 K and 1atm. Furthermore, the charge population on these hydrogen-bonded networks is estimated on the basis of natural bonding orbital (NBO) analysis.49 The validity of the theoretical method and all the basis sets on the prediction of Raman spectra has been demonstrated in our previous studies.31,42,50 In static polarizability calculations, the Raman scattering factor (RSF) of a given vibrational mode is, Si = 45α i′2 + 7γ i′2

where

(1)

αi′ and γ i′2 are the mean polarizability tensor derivative, and the anisotropic polarizability

derivative of the ith vibrational mode. To obtain information of absolute Raman intensity, we further calculate the differential Raman scattering cross section (DRSCS) at a given excitation wavelength.51,52

( v%0 − v%i ) h I = 2 ⋅ S 8π cv%i 45 1 − exp ( − hcv%i k BT )  i 4

R i

where

v%0

and

v%i

(2)

denote the frequency of the incident light and the vibrational frequency of the ith

mode, respectively. In this paper, the static Raman intensity presented in simulated Raman spectra is in the DRSCS with the Lorentzian expansion in a line width of 10 cm-1 at laser line


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514.5 nm. The unit of cm2.sr-1.mole-1 means the effective scattering cross section cm2 per steradian per molecule. In dynamic polarizability calculations, the TD-DFT method was used to calculate the property of electronic transitions of low-lying excited states. The coupled perturbation Hartree-Fock (CPHF) method was used to predict the preresonance Raman intensity related to the charge transfer states at a given excitation frequency. The frequency-dependent polarizabilities were calculated as occ vac

α (ω ) = −∑∑  Aki( − ) + Aki( + )  H ki(1) i

(3)

k

where A(±) is a matrix of mixing coefficients and is determined by solving the following randomphase-approximation equation53  M 11  21 M

M 12   A( + )   J ( + )   =  M 22   A( − )   J ( − ) 

(4)

where J ki( ± ) = −2 H ki(1) / (ε k + iΓ k − ε i ± hω )

M kiab,k 'i ' = δ abδ kk 'δ ii ' + [2 / (ε k + iΓk − ε i ± hω )]R ab ki ,k ' i '

(5) (6)

In which the indices i and k, etc. are the molecular orbital labels, H(1) the dipole matrix in the molecular orbital basis, ε the orbital energy obtained by the unperturbed HF calculation, Γ the damping constant, and ω the wavenumber of the external electric field, which corresponds to that of the incident light in the Raman scattering. Rab is the difference between the two-electron integrals in the two molecular orbital bases. In our cases the preresonance Raman scattering processes was considered, the energy difference between the excited-state energy and the incident photonic energy is larger than 0.1 eV, and the damping constant Γ can be neglected.54


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Then the frequency-dependent polarizability derivatives are used to calculate the DRSCS for the dynamic Raman intensities of preresonance Raman scattering at a given excitation wavelength.

Result and Discussion SERS Intensities of Water Directly Adsorbed on Silver Electrodes Firstly, the intensities of water directly adsorbed on silver clusters have been calculated as a reference. It was reported that the SERS signal of water is hard to be measured due to its very small Raman scattering cross section and weak adsorption ability.6,12,19 Figure S1 (see supporting information) shows the structures of water molecules adsorbed on silver clusters. In accordance with previous theoretical prediction,30,34,55-57 and spectroscopic measurements58-61, the adsorption orientation of water molecules are all with their O-end approaching the silver surfaces in neutral structures. The calculated BE values are about 3.47 ~ 7.86 kcal/mol after the corrections of the BSSE effect46,47, and the BE values change to 2.31~6.34 kcal/mol after correcting with the zeropoint energy (Table S1, see supporting information). This is in agreement with the calculated adsorption energies of 0.14 eV for water adsorbed on the Ag(111) on a atop site.34 Our results also agree with 5.68 - 8.06 kcal/mol in AgnH2O (n = 2 - 4) calculated using the PBE1PBE functional.56 All these BE values and NBO analysis show that water is weakly physical adsorbed on a Ag surface. Therefore, we can conclude that there exists only weak adsorption for a water molecule through its O-end on silver electrodes at the potential close to the PZC. Table 1 summarizes the calculated Raman intensities of intramolecular vibrations of water and adsorbed water molecules. In a free water molecule, the calculated RSF values are 1.0, 101.3, and 26.5 Å4/amu, comparable to the experimental data of 0.9 ± 2, 108 ± 14, and 19.2 ± 2.1


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Å4/amu, respectively.30,62,63 Accordingly, we estimated the ratio of differential Raman crosssections between the bending and symmetric stretching vibrations to be about 0.03 at an excitation line of 514.5 nm. As shown in Table 1, the ratios do not change remarkably, although the absolute Raman intensities display a significant increase in H2O-Ag5 and H2O-Ag6. For example, the RSF values of symmetric stretching vibration in H2O-Ag5 is 2435.4 Å4/amu, and the absolute Raman intensity is as high as to 2.44 × 10-29 cm2.sr-1.mole-1, which is even stronger than the ring breathing vibration of pyridine of 1.1 × 10-29 cm2.sr-1.mole-1 at the same excitation wavelength64. In consequence, without the halide effect, specific surface active sites may have an enhancement factor about only 20-fold on Raman signals of water molecules. Chloride Modified Active Sites on Silver Electrodes To consider the halide effect, the interfacial structures of water adsorbed on the chloride modified active sites of silver were calculated. Figure 1 presents the optimized structures of H2O-AgnClδ (n = 2 – 7; δ = 0 and -1). In neutral complexes, the bridge site on silver clusters are selected to accommodate a water molecule and a halide ion. This agrees with previous calculations that bridge sites are the most stable ones for halide ions on silver.65,66 As shown in Figure 1, a typical optimized configuration is adopted in planar structures. Moreover, the formation of a five-member-ring of Ag…Cl…O-H…Ag presented in a rectangle configuration due to the water–halide hydrogen bonding interaction. Here, chloride ions interact with water molecules through the strong hydrogen bonds with the Cl…H bond length of 2.024 ~ 2. 156 Å, which are close to 2.155 Å in isolated water-chloride anion complexes30. This shows that highconcentration halide ions could disrupt the hydrogen networks of the adsorbed water molecules and form surface complexes in electrochemical interfaces.18,21 In the case, these water molecules


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prefer to an O-end configuration interacting with silver clusters. NBO analysis shows that the positive charge is distributed on silver clusters about 0.70 au, while chloride ions are negative. This indicates that these small silver clusters are positive to the PZC, which is similar to the experimental condition in early studies at the potential of -0.2 V.18 For the modeling complexes of H2O-AgnCl-, there is no remarkable structural deformation for the metal clusters, except H2O-Ag4Cl- displaying a large deformation. NBO analysis shows that the surface charge of silver decreases to about -0.20 au in these negatively charged complexes. Figure 1 presents two types of geometries for negatively charged clusters. In larger clusters of H2O-AgnCl- (n = 5 - 7), water molecules still adopt a configuration through its O-end interacting with silver clusters. This is similar to those in the neutral clusters. In small clusters of n= 2 - 4, however, the water molecules adopt an H-end configuration interacting with silver clusters. Meanwhile, the Cl…H distances significantly increase to 2.290 ~ 2.406 Å, indicating that the hydrogen bonding interaction becomes weak upon cathodic potential excursion. This situation is associated with halide desorption at negative potential.20,67 Therefore, these modeling complexes of H2O-AgnCl- (n = 2 - 4) should be reliable to model the water directly adsorbed on silver electrodes at potentials negative to the PZC. Next we will further check the influence of the structural change on Raman spectra of adsorbed water. Raman Spectra of Halide-Water-Silver Structures The calculated frequencies and Raman intensities of H2O-AgnClδ (n = 2 – 7; δ = 0 and -1) are shown in Table 2 and S2. After adding a chloride ion, the static Raman intensities of bending and stretching bands of interfacial water are slightly larger than that of free water molecule. In neutral complexes, the ratios of Raman intensities for bending and stretching vibrations of water


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are about 0.02 ~ 0.05 as shown in Table 2. This agrees with the observed SERS that in halidecontaining systems, the Raman intensity of the bending mode of water is much weaker than that of the OH stretching mode (about only 4% of the peak height).18 Besides, our results predicted the scaled frequencies of Ag-Cl vibrations at around 239.9 ~245.6 cm-1 as shown in Table 2, in accordance with the observations that Fleischmann et al assigned a Raman peak at 240 cm-1 to the Ag-Cl bond in the SERS spectra.18 Therefore, we conclude that our calculated neutral complexes correspond to the interfacial structures observed in the chloride-containing SERS of water. We also calculated the Raman intensities of H2O-AgnCl-. As shown in Table S2 (see supporting information), the bands of HOH bending vibrations present abnormal intensities in H2O-AgnCl- (n = 2 - 4). The ratios of bending and stretching vibrations reach to 0.76 and 0.35 in H2O-Ag3Cl- and H2O-Ag4Cl-, respectively. It is the fact that electrode potential plays an important role in adsorption orientation and relative Raman intensities of interfacial water molecules.9-13 The H-end configurations results in such abnormal intense enhancement of the HOH bending vibration observed at very negative potentials.6,13,31,68 In this case, the chloride ions are expected to desorb from the silver surface. While the Raman signal of the Ag-Cl vibration decreases in both intensity and frequency and finally disappears.20,64 Correspondingly, our calculated result showed that the Raman intensities of Ag-Cl vibration are very low. Besides, the frequencies of the Ag-Cl vibration decrease to 204.0 ~ 208.5 cm-1 in H2O-AgnCl- (n = 5 - 7), which are lower than the observed peak at 213 cm-1 with cathodic polarization to -0.7 V versus SCE.69 This is due to the electrochemical Stark effect weakening the Ag-Cl bonds.70 Thus, our calculated spectral data showed that the neutral complexes H2O-AgnCl are better describing interfacial structures at applied potentials positive to the PZC, while the negatively charged


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clusters present situations negative to the PZC, in which the Ag-Cl bond is weakened due to the electrostatic repulsion.

Charge-Transfer Enhancement Since the above DFT calculations only take account for the electronic ground state of the modeling complexes, we further insight on the influence of their low-lying excited states, especially focusing on PICT state on the SERS. Figure 2 presents the energy levels of occupied and unoccupied orbitals of H2O-Ag5 and H2O-Ag5X (X= Cl, Br, I). In H2O-Ag5, the frontier orbitals are mainly from the silver cluster. After adding a halide ion, their lone-paired orbitals locate at higher energy positions. For example, in H2O-Ag5Cl and H2O-Ag5Br, the energy levels of the lone paired orbitals from Cl- and Br- are approaching to that of HOMO-1, the lowest 5s bonding orbitals of silver. They have energies about -7.02 eV for Cl- and -6.63 eV for Br- in the corresponding complexes. While in H2O-Ag5I, the orbital of iodide with higher energy of -6.22 eV is in between the 5s bonding orbitals of silver. Take chloride-containing clusters as example, we first inspect the low-lying excited states in H2O-AgnCl. Table 3 lists the transition energies and oscillator strengths of these excited states. These states of H2O-AgnCl complexes are closely associated with the orbitals of chloride, corresponding to the charge transfer transition. The excitation energies higher than 3.2 eV are not considered here, because they involved in the d orbitals and the interband transition will decrease SPR enhancement efficiency in silver substrates.71 For a comparison, the charge transfer excited states of H2O-Agn are also listed in Table S2 (see supporting information). Their transition energies are dependent on the size of silver clusters, and low-lying excited states are from the 5s-


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orbital intraband transition of silver clusters. Here, the structures of open-shelled systems with a single electron are not considered here. Figure 3 shows the transition orbitals in H2O-Ag5Cl and H2O-Ag7Cl. The natural transition orbitals (NTO) on the basis of the S0 structure were used to analyze the S0 → S1 transition properties. The NTO eigenvalues of all molecules are close to 1, indicating that the NTO analysis is reliable. In H2O-Ag5Cl, NTO analysis shows that the S0 → S1 electronic transition is from the 5s bonding orbital of silver clusters mixed with the lone paired orbital of chloride (Figure 3a) to the antibonding orbital of Ag5 (Figure 3b). In H2O-Ag7Cl, Figure 3c and 3d show that the hole and particle wavefunctions were localized on the chloride ion and silver cluster, respectively. This clearly shows that the electron transfer arises from the lone paired orbital of chloride to the antibonding orbital of Ag7. Moreover, it has a lower transition energy than that in H2O-Ag5Cl. Figure 4 presents simulated Raman spectra with the enhancement from the charge transfer mechanism. For example, the first singlet low-lying excited state S1 belongs to a charge transfer excitation with an excited energy of 2.69 eV and an oscillator strength f = 0.0604. Figure 4a and b presents the simulated Raman spectra of off-resonance and preresonance Raman spectra of H2O-Ag5Cl complex. Correspondingly, the excitation wavelength used in the preresonance Raman spectrum is 478.9 nm, corresponding to the photonic energy about 2.59 eV. There is an energy separation about 0.1 eV between the S1 excitation energy and photonic energy. The intramolecular vibrations of water have been significantly enhanced by the PICT. The RSF values of HOH bending and hydrogen bonded O-H stretching vibrations of water significantly increase to 690.0 and 39373.9 Å4/amu, respectively. Their absolute Raman intensities are about 3.47 × 10-29 and 7.59 × 10-28 cm2·sr-1·mol-1, respectively. Chemical enhancement factors increase


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to 400 fold for SERS signals of interfacial water molecules from Figure 4a to 4b. Meanwhile, the Ag-Cl vibration is also enhanced about 640 fold, with the RSF value of 2949.8 Å4/amu and DRSCS value of 1.83 × 10-29 cm2·sr-1·mol-1. Similar enhancement was also found in H2O-Ag7Cl. The charge transfer leads to the enhancement of characteristic bands of water molecules. Moreover, the enhancement for the stretching vibrations of water is more significant than the bending one, like their RSF values about 10150.3 and 67.5 Å4/amu, respectively, as shown in Figure 4d. The low-frequency peaks are also enhanced. The RSF value for the Ag-Cl vibration at 240.5 cm-1 increased to 272.0 Å4/amu estimated at the excitation wavelength of 598.9 nm, with the energy separation of 0.15 eV to the excited states. Another Ag-O vibration at 257.2 cm-1 is enhanced to 634.3 Å4/amu. Therefore, the PICT mechanism gives a good explanation for the SERS of water observed in the solution containing chloride ions. The interfacial charge transfer from chloride ions to silver and the intraband transition of silver clusters contributed to the chemical enhancement of intramolecular vibration of water.

The Halide Effect on SERS Enhancement Figure S2 (see supporting information) presents the optimized structures with different halides of Cl-, Br- and I- in aqueous solution. Here, the structures have been further optimized under the solvation model of SMD to consider the situations in electrochemical interfaces. In the aqueous structures, the hydrogen bonding interaction are weakened, increasing the Ag…X-, X-…H-O and Ag…O distances compared with that in the gas phase. Table 4 presents the calculated frequencies and Raman intensities for these complexes in gas phase and with solvation models. Static Raman intensities show about 2 – 5 fold enhancement, due to the solvation effect.72 This is


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consistent with the solvation effect on hydrated protons in different concentrated acid solution.42 However, the calculated frequencies are underestimated at 1566.1 cm-1 in H2O-Ag5Cl with the SMD model, respectively, which agree with the bending frequencies of 1552.5 cm-1 of free water molecules calculated under the same model. Figure 5 shows static Raman spectra of aqueous H2O-Ag5X (X = Cl, Br, I) with B3LYP/SMD methods. The simulated Raman spectra containing different halide ions exhibit a reliable trend on changes in frequency and intensity. In off-resonance Raman spectra, the frequency of HOH bending vibration of water in the complex containing iodide ion is about 10 – 15 cm-1 lower than that in the complexes containing chloride or bromide ions. This agrees with previous theoretical calculations and observations in aqueous solutions and electrochemical interfaces.6,30,73-80 In static polarizability calculation, the Raman intensities of the bending vibration of water in aqueous H2O-Ag5I are about 1.03×10-30 cm2·sr-1·mol-1, which is significantly larger than that of H2O-Ag5Cl and H2O-Ag5Br. Due to the large polarizability of halide ions, the chemical enhancement factors for bending vibrations are about 3, 4 and 26 folds for Cl-, Br- and I-, respectively, compared with that of aqueous water molecule42. As for the stretching vibrations, the hydrogen bonded O-H stretching ones exhibit more intense peaks than that of free O-H ones. Also their Raman intensities are increasing with the order of Cl- < Br- < I-, of 6.05, 6.98 and 16.05 ×10-30 cm2·sr-1·mol-1,respectively.The chemical enhancement factors for stretching ones are 3, 4 and 9 folds for Cl-, Br-, I-, respectively. Moreover, as shown in Figure 5c, the largest radio of 0.06 between Raman intensities for bending and stretching bands is also present in the Icomplex, which agrees with previous observation.6,73-80 Figure 6 presents the simulated Raman spectra of H2O-Ag5X (X = Cl, Br, I) from the charge transfer enhancements. For different halide ions, the excited energies of their S1 states are at 2.55,


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2.59, and 2.59 eV in H2O-Ag5Cl, H2O-Ag5Br, and H2O-Ag5I, respectively. When the energy difference was adopted about 0.1 and 0.2 eV, the excitation wavelengths used here are 527.6, 518.7, and 518.7 nm for Cl-, Br- and I-, respectively. In these Raman spectra, the charge transfer mechanism is responsible to their different enhancements. For H2O-Ag5Cl, the RSF values of bending and stretching vibrations of water can be enhanced to 3870.3 and 466084.0 Å4/amu, with the DRSCS values of 1.32×10-28 and 4.03×10-27 cm2·sr-1·mol-1, respectively, as shown in Figure 6a. In the case, these Raman signals are enhanced about 1200 and 1900 fold for bending and stretching modes, respectively, compared with those in the off-resonance Raman spectrum (Figure 5a). For H2O-Ag5Br, as shown in Figure 6b the charge transfer enhanced the Raman signal of the OH stretching vibration. The DRSCS values of bending and stretching vibrations are 3.14×10-28 and 8.53×10-27 cm2·sr-1·mol-1, where the enhancement factors are estimated to 1800 and 3400 fold, respectively. Surprisingly, the charge transfer enhancement is not obvious in H2O-Ag5I. The DRSCS value of the HOH bending vibration is 3.16 ×10-29 cm2·sr-1·mol-1, with an enhancement factor only 30 fold. This is much smaller than 220 fold estimated from the OH stretching vibration with the DRSCS value of 3.56 ×10-27 cm2·sr-1·mol-1. This can be explained that the O-H stretching mode is more sensitive to the charge transfer state than the HOH bending one in H2O-Ag5I. Thus, our results show that the enhancement factors for the O-H stretching vibrations of water are about 2200, 4600 and 1900 folds in chloride, bromide and iodine solutions, respectively. After comparing the different chemical enhancement factors in these clusters containing halide ions, we can conclude that in the static polarizability calculation, their Raman intensities increase with the order of Cl- < Br- < I-. The iodide ion with a larger static polarizability plays an important role in the enhancement of static Raman intensities. This results in a significant


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enhancement of the HOH bending vibrations due to the large static polarizability of halide ions. While in the dynamic polarizability calculations, the PICT are the main contribution to the chemical enhancement of SERS signals of water molecules. The selective enhancement strongly depends on the displacement of the equilibrium positions in electronic excited and ground states. The chemical enhancement factors can reach to about 1900 - 3400 fold compared with their offresonance Raman intensities.

Conclusion The halide effect on SERS of water on silver electrodes has been investigated by using hybrid DFT methods combining the metallic cluster model. On the top of optimized structures of water molecules and halide ions coadsorbed on silver nanostructures, our calculated results show that among the three halide ions, the iodide ion has a significant influence on the relative Raman intensity due to the larger static polarizability. The influence of the halide anionic effect on the Raman intensities increases with the order of Cl- < Br- < I- in the static polarizability calculation. Besides, the surface negative charge plays an important role on the orientation of adsorbed water molecules and the relative Raman intensities of interfacial water molecules. It is worth noting that the Raman signals of the HOH bending vibration are selectively enhanced from the large static polarizability in off-resonant Raman scattering processes. When the dynamic polarizability calculations were considered, the charge transfer mechanism plays a more significant role in the simulated Raman spectra of water molecules. This gives a good explanation for the SERS of water observed in the electrolyte solution containing halide ions. Halide ions can tune the frontier orbitals to lower energies, so that the PICT can be excited from a halide ion to silver in visible light. The charge transfer state and the intraband transition


17


Page 18 of 38

states of silver clusters themselves are the main contribution for the chemical enhancement of intramolecular vibration of water. Halide activation plays a significant role in electrochemical SERS and single molecule SERS. Although the enhancement of the halide ions on SERS intensities of probe molecules is complex from aggregation of nanoparticles, and/or the production of new surface states from the formation of surface complexes, our calculated results of static and dynamic polarizabilities clearly present the contributions from the halide modified surface active sites and the PICT to the SERS chemical enhancement.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Binding energies and NBO analysis of H2O-Agn (n = 2 - 7) complexes, calculated vibrational frequencies and Raman intensities in negative H2O-AgnCl- complexes, low-lying excited states in structures of water adsorbed on silver clusters, optimized structures of H2O-Agn complexes, and optimized structures of coadsorbed water molecules and halide ions on silver clusters in aqueous solution.

AUTHOR INFORMATION Corresponding Author Ran Pang


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Address: State Key Laboratory of Physical Chemistry of Solid Surfaces, and Department of Chemistry, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. Phone Number: +86-592-2189023 Fax number: +86-592-2186979 E-mail address: [email protected]

De-Yin Wu Address: State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and Department of Chemistry, and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. Phone Number: +86-592-2189023 Fax number: +86-592-2186979 E-mail address: [email protected]

Acknowledgements National Natural Science Foundation of China (21321062, 21533006, 21273182 and 21373172), National Key Basic Research Program of China (No. 2015CB932303), and Funds of State Key Laboratory of Physical Chemistry of Solid Surfaces.


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Page 20 of 38

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(72) Weber, A. Raman Spectroscopy of Gases and Liquids; Springer: Berlin, 1979. (73) Busing, W. R.; Hornig, D. F. The effect of dissolved KBr, KOH or HCl on the Raman spectrum of water J. Phys. Chem. 1961, 65, 284-292. (74) Schultz, J. W.; Hornig, D. F. The effect of dissolved alkali halides on the Raman spectrum of water. J. Phys. Chem. 1961, 65, 2131-2138. (75) Walrafen, G. E. Raman spectral studies of the effects of electrolytes on water. J. Chem. Phys. 1962, 36, 1035-1042. (76) Rull, F.; De Saja, J. Effect of electrolyte concentration on the Raman spectra of water in aqueous solutions. J. Raman Spectrosc. 1986, 17, 167-172. (77) Weston, R. Raman spectra of electrolyte solutions in light and heavy water. Spectrochim. Acta 1962, 18, 1257-1277. (78) Wall, T. T.; Hornig, D. F. Raman intensity in binary solutions. J. Chem. Phys. 1966, 45, 3424-3430. (79) Liu, D.; Ma, G.; Levering, L. M.; Allen, H. C. Vibrational spectroscopy of aqueous sodium halide solutions and air-liquid interfaces: Observation of increased interfacial depth. J. Phys. Chem. B 2004, 108, 2252-2260. (80) Burikov, S.; Dolenko, T.; Velikotnyi, P.; Sugonyaev, A.; Fadeev, V. The effect of hydration of ions of inorganic salts on the shape of the Raman stretching band of water. Opt. Spectrosc. 2005, 98, 235-239.


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Table 1. Calculated vibrational frequencies (cm-1), Raman scattering factors (SR, Å4/amu), differential Raman scattering cross-sections (IR, 10-30 cm2.sr-1.mole-1), and Raman intensity ratios of bending and stretching bands (Ib/Is) of intramolecular vibrations in free water and water interacting with neutral silver clusters. Species

Frequencies

Scaled frequencies a

SR

IRb

H2O

1627.4

1596.5

1.0

0.03

3796.4

3655.9

101.3

1.00

3899.0

3754.7

26.5

0.25

1622.8

1592.4

10.1

0.38

3771.6

3647.2

949.7

9.45

3873.3

3745.5

74.0

0.70

1624.7

1594.3

4.9

0.18

3775.4

3650.8

403.8

4.01

3875.5

3747.6

60.7

0.57

1620.9

1590.6

3.9

0.14

3781.5

3656.7

238.2

2.36

3882.1

3754.0

34.6

0.33

1619.9

1589.6

55.3

2.06

3758.2

3643.2

2435.4

24.40

3861.7

3734.3

176.8

1.68

1622.1

1591.8

10.2

0.38

3772.6

3648.1

1163.0

11.57

3874.3

3746.4

91.5

0.86

1619.6

1589.3

8.2

0.31

3778.7

3654.0

806.6

8.00

H2O-Ag2

H2O-Ag3

H2O-Ag4

H2O-Ag5

H2O-Ag6

H2O-Ag7

Ib/IS

0.03

0.04

0.04

0.06

0.08

0.03

0.04


25


3881.1

3753.0

Page 26 of 38

85.2

0.80

a

The factors of 0.981 and 0.963 extracted from the previous work (ref.30) at the same theoretical level were used to scale the theoretical vibrational frequencies of the bending and stretching modes of free water and adsorbed water. b Differential Raman scattering cross sections were calculated at the excitation wavelength of 514.5 nm.


26

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Table 2. Calculated vibrational frequencies (cm-1) and Raman scattering factors (SR, Å4/amu), differential Raman scattering cross-sections (IR, 10-30 cm2.sr-1.mole-1) and Raman intensity ratios of bending and stretching bands (Ib/Is) of intramolecular vibrations of water and Ag-Cl vibration in neutral H2O-AgnCl (n = 2 – 7) complexes. Species

Frequencies

Scaled frequenciesa

SR

IRb

H2O-Ag2Cl

245.1

240.5

16.6

7.99

1637.9

1607.2

1.5

0.05

3352.4

3241.7

77.4

0.96

3833.2

3706.7

184.9

1.78

249.1

244.4

6.35

2.97

1639.9

1609.3

1.45

0.05

3262.6

3155.0

65.8

0.86

3839.3

3712.6

189.2

1.82

249.0

244.4

19.5

9.13

1636.1

1605.5

0.9

0.03

3205.7

3099.9

97.7

1.31

3842.8

3716.0

154.4

1.48

250.3

245.6

6.1

2.84

1637.4

1606.7

2.2

0.08

3166.4

3061.9

95.9

1.31

3843.6

3716.7

171.4

1.64

244.5

239.9

27.0

13.02

1632.8

1602.3

2.27

0.08

3229.3

3122.7

149.2

1.98

3842.1

3715.3

260.1

2.50

245.1

240.5

17.5

8.41

H2O-Ag3Cl

H2O-Ag4Cl

H2O-Ag5Cl

H2O-Ag6Cl

H2O-Ag7Cl

Ib/IS

0.03

0.03

0.02

0.05

0.03


27


Page 28 of 38

1634.3

1603.7

1.68

0.06

3244.9

3137.9

102.2

1.34

3841.4

3714.7

325.0

3.12

0.02

a

The factors of 0.981 and 0.963 are used to scale the theoretical vibrational frequencies of the bending and stretching modes of water. b Differential Raman scattering cross sections were calculated at the excitation wavelength of 514.5 nm.


28

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Table 3. Electronic transition energies (E/eV) and oscillator strengths (f) of low-lying singlet excited states in H2O-AgnCl (n = 3, 5, 7) complexes calculated at the B3LYP/aug-cc-pVTZ(O, H)/6-311+G**(Cl)/LANL2DZ(Ag) level. Here, σAg and σ*Ag denote the 5s bonding and antibonding orbitals of silver, respectively; n denotes the lone paired orbitals of chloride ion. Species

H2O-Ag3Cl E/eV

f

H2O-Ag5Cl

Property

E/eV

f

Property

H2O-Ag7Cl E/eV

f

Property

S1

3.36

0.1450 n → σ*

2.69

0.0604 (nσ)→ σ*

2.22

0.0167 n → σ*

S2

3.79

0.2353 n → σ*

3.25

0.6778 (nσ) → σ*

3.19

0.3668 σ → σ*

S3

3.91

0.1573 n → σ*

3.48

0.0637 (nσ) → σ*

3.21

0.0140 n → π*

a

Here only the strong electronic transitions with the oscillator strengths larger than 0.01 were listed.


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Table 4. Experimental and theoretical vibrational frequencies (cm-1) and Raman intensity (IR, 1030

cm2.sr-1.mole-1) of intramolecular vibrations of water in H2O-Ag5X (X = Cl, Br, I) complexes Frequencies a Modes

B3LYP/ B3LYP

Intensities Expt.b

B3LYP/ B3LYP

SMD

SMD

H2O-Ag5Cl

vb

1606.7

1566.1

1610

0.08

0.11

vs,b

3061.9

3295.9

3498

1.31

6.05

vs,f

3716.7

3676.9

3570

1.64

2.16

vb

1601.8

1570.2

1610

0.09

0.17

vs,b

3159.2

3312.4

1.82

6.98

vs,f

3712.4

3676.2

1.62

2.50

1596.8

1555.8

0.36

1.03

H2O-Ag5Br

3523

H2O-Ag5I 1595 vb

1610


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vs,b

3231.4

3467.4

3493

2.50

16.1

vs,f

3707.4

3678.6

3553

1.60

1.73

a

Vibrational frequencies are scaled using SQMF procedure. b the observed Raman frequencies are extracted from Ref18.


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Figure 1. Optimized structures of H2O-AgnClδ (n = 3 – 7; δ = 0 and -1) complexes calculated at the B3LYP/aug-cc-pVTZ(O, H)/6-311+G**(Cl)/LANL2DZ(Ag) level.


32

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Figure 2. Energy levels of occupied and unoccupied molecular orbitals in H2O-Ag5 and H2OAg5X (X = Cl, Br, I). σAg and σ*Ag denote the 5s bonding and antibonding orbitals of silver, respectively. nX denotes the lone paired orbitals of halide ions.


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Figure 3. The natural transition orbitals (NTO) of H2O-Ag5Cl and H2O-Ag7Cl calculated at the B3LYP/aug-cc-pVTZ(O, H)/6-311+G**(Cl)/LANL2DZ(Ag) level. (a) and (b) particle and hole of S0 → S1 transition in H2O-Ag5Cl; (c) and (d) particle and hole of S0 → S1 transition in H2OAg7Cl, respectively;


34

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Figure 4. Off-resonance Raman spectra and pre-resonance charge-transfer Raman spectra of water-chloride-silver complexes. (a) and (c) are the off-resonance Raman spectra of H2O-Ag5Cl and H2O-Ag7Cl, respectively. The excitation wavelength of 514.5 nm was used; (b) and (d) are the pre-resonance Raman spectra of H2O-Ag5Cl and H2O-Ag7Cl with excitation wavelengths of 478.7 and 598.9 nm, respectively. The Lorentzian line width used in simulated Raman spectra is 10 cm-1.


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Figure 5. Off-resonance Raman spectra of H2O-Ag5X (X = Cl, Br, I) in the aqueous solution containing halide ions. (a) (b) and (c) Off-resonance Raman spectrum of H2O-Ag5Cl, H2O-Ag5Br, and H2O-Ag5I, respectively. The excitation wavelength of 514.5 nm was used; The Lorentzian line width used in simulated Raman spectra is 10 cm-1.


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Figure 6. Pre-resonance Raman spectra of H2O-Ag5X (X = Cl, Br, I) in the aqueous solution containing halide ions. (a) (b) and (c) Pre-resonance charge-transfer Raman spectrum of H2OAg5Cl, H2O-Ag5Br, and H2O-Ag5I with excitation wavelengths of 527.6, 518.7, and 518.7 nm, respectively; The Lorentzian line width used in simulated Raman spectra is 10 cm-1.


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TOC Graphic:


38

SERS Chemical Enhancement of Water Molecules from Halide Ion

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