Strong Core@Shell Dependence in Surface-Enhanced Raman

Article pubs.acs.org/JPCC

Strong Core@Shell Dependence in Surface-Enhanced Raman Scattering of Pyridine on Stable 13-Atom Silver-Caged Bimetallic Clusters Lei Chen,†,‡ Yang Gao,‡ Yingkun Cheng,† Yanbin Su,⊥ Zhigang Wang,*,‡ Zhengqiang Li,*,† and Rui-Qin Zhang*,∥ †

Key Laboratory for Molecular Enzymology and Engineering of Ministry of Education, College of Life Sciences, Jilin University, Changchun, 130012, China ‡ Institute of Atomic and Molecular Physics, Jilin University, Changchun, 130012, China ⊥ College of Chemical & Pharmaceutical Engineering Institution, Jilin University of Chemical Technology, Jilin, 132022, China ∥ Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, Hong Kong SAR, China S Supporting Information *

ABSTRACT: On the basis of calculations using time-dependent density functional theory, we conducted detailed analyses of the surface-enhanced Raman scattering (SERS) of pyridine adsorbed on 13-atom icosahedral M@Au12 and M@Ag12 (M = Mo, W) clusters in this article. Surprisingly, we find that, although the SERS enhancements for all complexes can reach the order of 104, the signals of pyridine on M@Ag12 at charge transfer (CT) transition excitations are twice as much as that of pyridine on M@Au12, and the corresponding energies used for SERS excitations are significantly different in the low-energy region of 1.63−2.10 eV. The interactive modulation between the core and shell can produce varying strong CT transitions from metal clusters to pyridine, which tunes the SERS enhancements with altered optical properties. The complexes of pyridine on silver-caged clusters are more easily influenced by the tunability of the core than that of pyridine on gold-caged clusters. Our analyses are expected to provide a theoretical basis for experimentally synthesizing multicomponent SERS substrates and exploring the dependence of SERS enhancement on the synergies between the different components in core@shell binary metal clusters. determining their chemical reactivity.22 Experimental results show that SERS enhancement based on core@shell bimetallic NPs is closely related to the interactive modulation of the electronic structures between the core atoms and the shell atoms.17,23−26 However, the quantum mechanism behind this synergistic effect is still not clear in terms of theory.18−21 Therefore, a comprehensive theoretical study is essential for further exploration and synthesis of core@shell materials for SERS detection. Coinage metals have special valence shell electrons (3d104s1 for copper, 4d105s1 for silver, 5d106s1 for gold). Among them, gold has been used as a biological material in the life science field because of its good biocompatibility.27,28 To enhance the stability of gold clusters and improve their chemical activity, many theoretical and experimental studies have been done on encapsulating foreign atoms in gold nanocages.29−34 For instance, Pyykko and Runeberg35 predicted that the M@Au12

1. INTRODUCTION Since the discovery of surface-enhanced Raman scattering (SERS) in the 1970s,1,2 Raman spectroscopy has been widely used as a powerful analytical tool,3−6 even for single molecule detection,7−9 because the weak Raman signal can be greatly enhanced by a factor of 1014−1015 when the analyte molecule is absorbed on a coarse coinage metal surface. It is generally believed that enormous SERS enhancement arises mainly from four mechanisms:10−12 (a) enhanced local electromagnetic (EM) fields due to excitations in the metal cluster (the EM mechanism), (b) resonance enhancements caused by charge transfer (CT) between the adsorbate and metal cluster due to excitations (the CT mechanism), (c) a molecular resonance mechanism where the incident beam is resonant with a molecular excitation, and (d) an enhancement due to nonresonant interaction between the surface and the adsorbate. Core@shell bimetallic nanoparticles (NPs)13−17 based on coinage metals have been widely used as the SERS substrate in surface analysis, biological and chemical sensors, biomedical detection, and other uses.18−21 The surface structures, surface compositions, and aggregation characteristics are important in © XXXX American Chemical Society

Received: May 8, 2015 Revised: July 1, 2015

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DOI: 10.1021/acs.jpcc.5b04453 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (M = Mo, W) cluster containing an icosahedral Au12 cage and a central heteroatom would be a highly stable 18-eletron cluster. Li and colleagues34 confirmed this experimentally using photoelectron spectroscopy (PES). The measured HOMO− LUMO gap of the W@Au12 cluster is 1.68 eV and very close to 1.77 eV, the value of the tetrahedral cluster Au20.36 Mo@Ag12 and W@Ag12 clusters also aroused widespread interest because they have the same valence structure as silver atoms and gold atoms.37 Compared with the Au20 cluster, the bimetallic M@ Au12 cluster has attracted extensive attention in the new materials field, catalysis field, and other fields because of its higher symmetry and perfect electronic structure determined by the core atom and the golden cage.38−40 However, its possible application in SERS has not been reported yet. Accordingly, in light of the fact that M@Au12 clusters have the property that they meet the requirements of SERS substrates (such as good stability and tunable optical properties), these bimetallic materials may be promising cluster-assembled nanomaterials for SERS detection. In recent years, more and more studies have focused on theoretical investigation of microscopic mechanistic insights of SERS using electronic structure calculation methods.12,41−46 Lombardi and colleagues elucidated metal-molecule CT and molecular resonances using a Herzberg−Teller expansion of polarizability.12,41 Arenas and colleagues investigated the effect of CT on relative Raman intensity using a two-state resonance Raman model.42 Jensen and colleagues presented a detailed study of the SERS mechanism of pyridine on Ag20 and other small silver clusters using time-dependent density functional theory (TDDFT) based on a short-time approximation for the Raman cross section.10,45,47−49 Numerous studies have indicated that quantum chemical calculations can provide an adequate measure of SERS enhancements originating from analyte adsorption onto a metal cluster or surface. Moreover, silver and gold atoms have the same valence electron arrangement (n − 1)d10 ns1, and they are both important materials for SERS enhancement. Therefore, in this article, we present a systematic study of SERS enhancement of pyridine interacting with M@Au12 and M@ Ag12 (M = Mo, W) clusters34,37 using the TDDFT method. Figure 1 presents the structures of the pyridine molecule on M@Au12 and M@Ag12 clusters. This study can help clarify the tunable SERS mechanism that may be affected by the interactive modulation between the core atoms and the shell atoms.

Figure 1. Optimized geometric structures of (a1) Mo@Au12-Py, (a2) W@Au12-Py, (b1) Mo@Ag12-Py, and (b2) W@Ag12-Py complexes.

For M@Au12 and M@Ag12 (M = Mo, W) clusters and pyridine-metal complexes, only electronic transitions from the ground state to singlet excited states are dipole allowed, so only singlet−singlet excitations to these states were evaluated for optical absorption spectrum. A total of 200 states were examined for metal clusters and pyridine-metal complexes. In order to verify the reliability of the electronic transition energy, we also used the statistical averaging of orbital potential (SAOP)54,55 to provide another test of the validity of our approach. The SAOP that displays the correct asymptotic behavior has been specially designed for calculating optical properties.56,57 In this article, we use a series of 13-atom Ih clusters M@Au12 and M@Ag12 (M = Mo, W) as SERS substrates as shown in Figure 1, and we present a detailed analysis of the enhanced Raman scattering of pyridine interacting with these clusters using the TDDFT method, which is based on a short-time approximation to the Raman scattering cross section. The 13-atom bimetallic clusters contain a W or Mo atom encapsulated in the center of Au12 and Ag12 cages and the properties of 12 gold or silver atoms are equivalent. On the basis of the obtained structures, we calculated static Raman spectra and CT resonance-enhanced Raman spectra.58 Absolute Raman intensities are presented as the differential Raman scattering cross-section (DRSC). For Stokes scattering with an experimental setup of a 90° scattering angle and perpendicular plan-polarized light, the cross-section is59

2. METHOD All calculations in this study were performed with a local version of the Amsterdam Density Functional (ADF) 2012 program.50 The Becke-Perdew (BP86) XC-potential was used because this function usually gives harmonic frequencies close to experimental results without the use of scaling factors.51 We used a triple-polarized slater type (TZP) basis set with a [1s23d10] frozen core for Ag or Mo, a [1s2-4f14] frozen core for Au or W, and with a [1s2] frozen core for the first row elements.52,53 We used the zeroth-order regular approximation (ZORA) in our calculations to account for the scalar relativistic effects. The geometric optimizations and frequency calculations have been performed at the same level without imposing any symmetry constraint for each structure, and all vibrational frequencies obtained for the electronic ground states are real to ensure the reliability of the results. Excited state calculations were performed using TDDFT.

IRaman =

(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 ⎠ B

(2) DOI: 10.1021/acs.jpcc.5b04453 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. Vibrational modes of selected frequencies and the static Raman spectrum of pyridine at the BP86/TZP level. The differential cross section is in units of 10−30 cm2/sr and wavenumbers are in cm−1. Spectra were broadened by a Lorentzian having a width of 20 cm−1.

Figure 3. Static Raman spectra: (a) Mo@Au12-Py, (b) W@Au12-Py, (c) Au2-Py, (d) Mo@Ag12-Py, (e) W@Ag12-Py, (f) Ag2-Py. The differential cross section is in units of 10−30 cm2/sr and wavenumbers are in cm−1. Spectra have been broadened by a Lorentzian having a width of 20 cm−1.

where ω0 and ωi, respectively, denote the frequencies of the incident light and the ith vibrational mode; (dσ/dQi) and (dγ/ dQi) denote the derivatives of the isotropic and anisotropic polarizability of the ith vibrational mode, respectively; and h (Planck constant), c (light speed), kB (Boltzmann constant), and T (Kelvin temperature) denote corresponding physical quantities. The electronic polarizability of both on- and offresonance Raman scattering is calculated by including a finite lifetime (using a damping parameter Γ ≈ 0.004 au) of the electronic excited states in the TDDFT polarization calculations.60,61 The six-membered heterocyclic aromatic compounds containing a nitrogen atom exist widely in DNA bases, amino acids, proteins, and other biological molecules. In this work, the pyridine molecule, which is a six-membered heterocyclic aromatic compound, is used as an adsorbate interacting with metal clusters. This is because it is often used as a probe molecule due to its notable Raman characteristic signals in SERS experiments. Figure 2 presents the six Raman characteristic peaks of pyridine at 598, 978, 1022, 1199, 1467, and 1572 cm−1, which are important for the discussion and can be assigned to asymmetric ring deformation mode υ6a, ring breathing mode υ1, symmetric ring deformation mode υ12, C−H in-plane bending mode υ9a and υ19a, and ring C−C stretching mode υ8a under a C2v point group, respectively.62 We

see that, for pyridine alone, the static differential cross section, which is dominated by two intense peaks at 978 and 1022 cm−1, is of the order of 10−31 cm2/sr and consistent with the results calculated by Zhao and colleagues.10 In addition, the six modes shown in Figure 2 are susceptible to SERS substrates and the corresponding frequencies and intensity are closely related to the adsorption sites, sizes, and compositions of the metal clusters.

3. RESULTS AND DISCUSSION 3.1. Static Raman Scattering of Pyridine-Metal Complexes. In order to explore SERS enhancement based on core@shell bimetallic clusters, we first systematically studied the static Raman properties and CT enhanced Raman scattering of pyridine molecules adsorbed on M@Au12 and M@Ag12 clusters. Calculations of bare metal clusters show that the M@Ag12 clusters with Ih symmetry are similar to the reported M@Au12 clusters34,35 and can exist stably with a large band gap of 1.89 eV for the Mo@Ag12 cluster and 1.60 eV for the W@Ag12 cluster, respectively. The specific analyses are shown in Table S1 (Supporting Information). On the basis of constructed initial structures, we obtained four stable structures of the pyridine molecule adsorbed on M@Au12 and M@Ag12 (M = W/Mo) clusters. These structures are denoted as Mo@Au12-Py, W@Au12-Py, Mo@Ag12-Py, W@ C


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σ−π cooperation strengthens the stability of pyridine on metal clusters.48 Compared with silver-caged clusters, the CT transitions from the HOMO of gold-caged clusters to LUMO of pyridine need to overcome a larger amount of energy because the frontier orbital energy levels of silver-caged clusters are higher than that of gold-caged clusters and closer to the LUMO of pyridine. The core atom in metal clusters plays the role of tuning the energy level of frontier orbitals so as to adjust the CT transition energy from metal to pyridine. After the orbital characteristics analysis, we assessed the absorption spectra of optimized complexes using TDDFT calculations to determine the strong CT transitions energy in the low-energy region. We also calculated the absorption spectra of bare metal clusters as presented in Figure S1 (Supporting Information), which clearly indicated the specific changes in the low-energy region before and after the adsorption of pyridine. The interaction between pyridine and metal clusters can result in new CT excitation transitions from the metal to the molecule, and these strong CT transitions have very obvious signal strength in absorption spectra in Figure 5. As mentioned in the Introduction, the CT resonance mechanism is very important when the incident light is resonant with a molecular or CT excitation of the system. The CT transitions of pyridine on gold-caged clusters are mainly limited in the range of 550− 650 nm, while the CT transitions of pyridine on silver-caged clusters are entirely limited in the range of 700−800 nm. These results fit with the orbital characteristics analysis, which found that the CT transitions of pyridine on gold-caged clusters need to overcome a larger energy barrier than that of pyridine on silver-caged clusters. The variety of CT transition energy in different complexes is due to the interactive modulation of core atoms and cage atoms in metal clusters. However, variation of core atoms can tune the energy of the CT transitions, such as from 721 to 759 nm for complexes containing silver-caged clusters and from 600 to 590 nm for complexes containing gold-caged clusters as the core atom changes from Mo to W. Obviously, the strong CT transitions of pyridine on silver-caged clusters are more easily affected by the alteration of core atoms than that of pyridine on gold-caged clusters. In addition, to further understand the electron transition behavior of the adsorption systems, we listed the CT transitions properties of pyridine adsorbed on M@Au12 and M@Ag12 clusters in Table 1, in comparison with those of pyridine adsorbed on Ag2 and Au2 clusters. From Table 1, we note that the CT excitation energies of Ag2-Py and Au2-Py complexes are higher than 2.8 eV and close to the ultraviolet region, which makes it hard to use as excitation light in SERS experiments. Meanwhile, for pyridine on gold-caged clusters and silver-caged clusters, the CT transitions occur in the low-energy region from 1.63 to 2.10 eV and skirt the near-infrared light region, which can be used to facilitate SERS experiments. In addition, it is apparent that the sources that contribute to the initial orbitals of CT transitions are all from the 6s5d hybrid orbital of gold metal for Au2-Py and the 5s5p hybrid orbital of silver metal for Ag2-Py as presented in Table 1. Meanwhile, for pyridine on caged clusters, the initial orbitals of CT transitions contain an indispensable 4d5d of Mo atoms and 5d6d of W atoms besides 6s5d of gold metal and 5s5p of silver metal. Molecular orbital analysis indicates that the HOMO-1 of every complex is totally distributed on the metal, while the unoccupied orbitals corresponding to CT transitions are all distributed on the π

Ag12-Py. On the basis of geometric optimization, we simulated the static Raman spectra of every stable structure as shown in Figure 3, together with the Raman spectra of pyridine on Au2 and Ag2 clusters, which are models of stable small clusters commonly used in previous studies.11,44,62 Comparing the simulated spectra of pyridine on different metal clusters with that of pyridine alone, we find that the DRCSs of pyridine on M@Au12 (Figure 3a,b) and M@Ag12 (Figure 3d,e) show two-order enhancement compared to pyridine alone (Figure 2). Furthermore, pyridine on M@Au12 and M@Ag12 clusters show larger DRSCs than pyridine on Au2 and Ag2 clusters (Figure 3c,f). For Mo@Au12-Py and W@Au12Py complexes, although the core atoms are different, their Raman spectra show almost the same DRSC, sharing the value of 8 × 10−30 cm2/sr, which is twice as much as that of pyridine on Au2 clusters. For silver-caged clusters, the DRCSs of pyridine on Mo@Ag12 and W@Ag12 clusters are 1.6 × 10−29 cm2/sr and 1.2 × 10−29 cm2/sr, respectively, and there is one order enhancement more than that of pyridine on Ag2 cluster. So, whether for gold-caged clusters or silver-caged clusters, the signal enhancements of pyridine are more obvious than for corresponding diatomic clusters. The results indicate static enhancements mainly depend on the shell material when pyridine is adsorbed on core@shell clusters. In addition, every spectrum presented in Figure 3 shows that the ring breathing mode υ1 and ring C−C stretching mode υ8a, which are expected to be influenced more by the interaction with the metal clusters,10 exhibit larger DRCS than other modes. Their relative intensities are sensitive to metal clusters and adsorption site. However, shifts in vibrational frequencies are observed for all complexes, and the largest shift observed is for the modes υ6a when pyridine is adsorbed on metal clusters as shown in Table S3 (Supporting Information). These observations are in line with the theoretical findings by Jensen and Wu that pyridine would interact with silver clusters containing just a few atoms.49,62 3.2. CT Enhanced Raman Scattering. To obtain CT transition energy of pyridine on different metal clusters, we first analyzed the orbital characteristics of pyridine and bare metal clusters. Figure 4 presents the frontier orbitals of pyridine, M@ Au12, and M@Ag12 including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The adsorption of pyridine on metal clusters is dominated by σ-bonding between the nitrogen atom and metal atom with a small contribution from π-backbonding, and the

Figure 4. HOMO and LUMO energy levels of pyridine and Mo@ Au12, W@Au12, Mo@Ag12, and W@Ag12 bare clusters. The dotted line indicates the orbitals primarily responsible for σ-bonding, and the dashed line indicates the orbitals primarily responsible for πbackbonding. D


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Figure 5. Calculated major CT transition absorption spectra of (a) M@Au12-Py complexes and (b) Mo@Ag12-Py complexes. Red lines denote the results of complexes including clusters with W atom encapsulated in Au12 and Ag12 cages. Blue lines denote the complexes including clusters that have Mo atoms encapsulated in Au12 and Ag12 cages.

Table 1. Calculated Excitation Energies (E in eV), Excitation Wavelengths (λ in nm), Oscillator Strengths (f), Contributions to the Initial Orbitals of CT Transitions from Core Atoms (C1), Contributions to the Initial Orbitals of CT Transitions from Cage Atoms (C2), and Orbital Transitions of CT Excitations for Pyridine Adsorbed on M@Au12, M@Ag12, Ag2, and Au2 Clusters, Respectively geometry

E/λ

f

Ag2-Py Mo@Ag12-Py W@Ag12-Py Au2-Py Mo@Au12-Py W@Au12-Py

2.88/431 1.63/759 1.72/721 2.87/432 2.07/600 2.10/590

0.0001 0.0297 0.0283 0.0691 0.0256 0.0245

transitions HOMO-1 HOMO-1 HOMO-1 HOMO-1 HOMO-1 HOMO-1

→ → → → → →

LUMO LUMO LUMO+1 LUMO LUMO+5 LUMO+5

C1 39%, 4d and 5d 36%, 5d and 6d 24%, 4d and 5d 20%, 5d and 6d

C2 100%, 5s and 5p 61%, 5s and 5p 64%, 5s and 5p 100%, 6s and 5d 76%, 6s and 5d 80%, 6s and 5d

Figure 6. Simulated enhanced-Raman spectra with CT excitation of the (a) Mo@Au12-Py complex, (b) W@Au12-Py complex, (c) Mo@Ag12-Py complex, and (d) W@Ag12-Py complex. Differential cross sections are in units of 10−30 cm2/sr and wavenumbers are in cm−1. Spectra have been broadened by a Lorentzian having a width of 20 cm−1.

modes of the pyridine molecule as listed in Figure 2, which have very obvious signals in SERS experiments. This is due to the interaction between the caged cluster and the pyridine molecule, which results in a strong CT transition emerging in the low-energy region, and the new CT excitation state can be resonant with the excitation light leading to enhanced resonant Raman scattering. Compared to intracluster excitations in the higher energy region of 300−450 nm (see the Supporting Information), the CT excitation transitions appear in the low-

electron orbit of the pyridine ring. However, in order to confirm the accuracy of the CT excited energy at the BP86/ TZP level, we calculated the absorption spectrum of the W@ Au12-Py complex at the SAOP/TZP level and found that the typical CT excitation energy (2.18 eV) increased by only 0.08 eV compared with that (2.10 eV) at the BP86/TZP level. In order to explore how the interactive modulation between the core atoms and the shell atoms affects the intensity of the Raman scattering, we analyzed the well-known six vibrational E


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respectively. All data are based on the BP86/TZP level and the scalar relativistic effects have been taken into account through zeroth order regular approximation (ZORA) in our calculations. Compared with corresponding diatomic clusters, the binding distances of pyridine interacting with M@Au12 and M@Ag12 clusters increase no more than 0.03 Å. Although Table 2 indicates that the binding energy of pyridine on gold caged clusters is about 5 kcal/mol smaller than that of pyridine on Au2, a substantial portion of this difference in binding energy could be attributed to an omission of basis set supposition error (BSSE) correction, as presented in our previous work that the BSSE correction reaches nearly 50% in total binding energy of pyridine adsorbed on Ac@Au7,46 and the BSSE correction in Jensen’s calculation can reach more than 34% in total binding energy of pyridine adsorbed on Ag20.10 However, the calculated binding energy of pyridine on M@ Au12 is about 19 kcal/mol in our manuscript, which is consistent with 17.31 kcal/mol, the result of pyridine on Au20 calculated by Schatz and colleagues.63 The binding between pyridine and caged clusters is dominated by σ-bonding between nitrogen atoms and metal atoms, with a small contribution from π-backbonding. These σ−π synergies can greatly enhance the stability of pyridine adsorbed on metal clusters. In addition, as depicted in Figure 4, σ-bonding between pyridine and goldcaged clusters is stronger than that of pyridine on silver-caged clusters, which fits with the results in Table 2. Whether for pyridine on M@Au12 or M@Ag12, the σ-bonding varies with the core atom and the σ−π synergies between pyridine and different caged clusters can be tuned with the change of interactive modulation between the core atom and cage atoms. Deformation density analysis can reveal visually photoinduced charge transfer, which has been used in SERS studies.10,64 We analyzed the redistribution of the charge density when forming complexes calculating the Voronoi deformation density (VDD) charges. We found that 0.171e is transferred from pyridine to Mo@Ag12 for the Mo@Ag12-Py complex, whereas 0.281e is transferred for the W@Ag12-Py complex. The charge transfer difference is 0.11e between the two complexes due to their different core atoms. However, the charge transfer difference is only 0.005e between Mo@Au12-Py and W@Au12-Py complexes, indicating that the alteration of the core atom has a bigger impact on M@Ag12-Py than on M@ Au12-Py complexes. In order to clarify the specific difference of charge density in metal clusters, Figure 7 presents a graphical representation of the calculated deformation density isosurfaces for each complex. We can clearly see that a majority of the electron exchanges between metal clusters and pyridine take place around the junction of molecule-metal complexes, and the electron exchange occurs in overall systems. Zayak and colleagues have confirmed that the intense electron exchange in the interface is beneficial to chemical enhancement.65 In addition, we are surprised to find that the deformation density is obviously increased in the area between the core atom and the connected metal atom, especially for the W@Ag12-Py complex, although the change is most evident in all complexes. Yet, compared with complexes based on the Mo atom as the core, complexes using the W atom as the core show conspicuous improvement of the deformation density in the area between the core atom and the connected metal atom. So the interactive modulation between the core atoms and the shell atoms can alter the deformation density of every complex of pyridine on metal clusters.

energy region of 550−800 nm and relatively independent from intracluster excitations, which can be more conducive to SERS experiments. On the basis of the calculated CT excitation energy, Figure 6 shows the SERS spectra at CT resonant excitations in the range of 400 to 2000 cm−1. By comparison, we found that the CT resonance-enhanced Raman spectra are mainly dominated by C−H in-plane bending mode υ9a and ring C−C stretching mode υ8a because these two modes both involve motions of the atoms where the unoccupied orbitals associating with CT transitions is localized.10 However, compared with the static DRCS (10−31 cm2/sr) of pyridine, the intensities are enhanced by about a factor of 104 for pyridine on silver- or gold-caged clusters. In addition, we found that the extent to which excitation light of CT enhances SERS spectra varies for different metal clusters. For pyridine on silver-caged clusters, the CT transition excitations are distributed in the range of 700−800 nm and correspond to 721 nm for W@Ag12-Py and 759 nm for Mo@Ag12-Py, while for pyridine on gold-caged clusters, the CT transition excitation arises in the range of 550− 600 nm and corresponds to 591 nm for W@Au12-Py and 600 nm for Mo@Au12-Py. It is clear that the CT transition excitations of pyridine on different caged clusters can be tuned by interactive modulation between the core atoms and the shell atoms. By comparison, we found that the SERS enhancement of pyridine on silver-caged clusters is twice as much as that of pyridine on gold-caged clusters. So the enhancement of pyridine on different core@shell NPs is determined by cage material, and the change of core atoms can alter the CT transition energy for SERS excitation. The specific comparison is shown in Part 3 of the Supporting Information. 3.3. Binding Properties of Pyridine on Gold- And Silver-Caged Clusters. To further understand the SERSenhancement mechanism associated with the interactive modulation between core atoms and shell materials in bimetallic NPs, we exhaustively analyzed the binding properties of pyridine interacting with M@Au12 and M@Ag12 clusters as listed in Table 2, including charge transfer, binding distance, Table 2. Binding Interactions between Pyridine and the M@ Au12, M@Ag12, Ag2, and Au2 Clustersa complexes

q(Py → cluster)

R(N−Ag/ Au)

ΔE

BE for BSSE

Ag2-Py Mo@Ag12-Py W@Ag12-Py Au2-Py Mo@Au12-Py W@Au12-Py

0.136b 0.171 0.281 0.266 0.213 0.208

2.261b 2.289 2.278 2.110 2.208 2.210

−14.4b −12.76 −13.26 −25.39 −19.75 −19.92

−10.14 −9.72 −10.70 −4.92 −9.21 −10.48

a

Bond length between N and Ag/Au atoms, R(N−Ag/Au) is in angstroms; ground state charge transfer between pyridine and clusters q(Py → cluster) is in units of electron charge; and total binding energy ΔE = Ecomplex − EPy − Emetal is in kcal/mol, BE for BSSE represents total binding energy corrected with BSSE. bFrom ref 43.

and binding energy. In order to facilitate comparison, we also present the binding properties of pyridine on coinage diatomic clusters Ag2 and Au2. The total binding energy is calculated with the equation ΔE = Ecomplex − EPy − Emetal, where the Ecomplex, EPy, and Emetal denote the total bond energy of pyridine-metal complexes, the total bond energy of the pyridine molecule, and the total bond energy of the metal cluster, F


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vibrational analysis and CT-enhanced Raman vibrational analysis of pyridine interacting with Mo@Au12/W@Au12/ Mo@Ag12/W@Ag12 clusters, calculated deformation density isosurface for complexes and the VDD charge distribution analysis before and after pyridine adsorption, important orbitals involved in major CT excitation for complexes of pyridine on Mo@Au12/W@Au12/Mo@Ag12/W@Ag12 clusters, Cartesian coordinates (in Å) of pyridine on Mo@Au12/W@Au12/Mo@ Ag12/W@Ag12 clusters obtained using BP86 and TZP. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04453.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

Figure 7. Calculated deformation density isosurface (Δρ = ρComplex − ρMetal cluster − ρPyridine) for complexes: (a) Mo@Au12-Py, (b) W@Au12Py, (c) Mo@Ag12-Py, (d) W@Ag12-Py. The isosurface value of 0.004 au is shown with enhanced density in blue and depletion density in red; A represents the coinage metal atom connected to the pyridine molecule.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work described in this paper is supported by grants from the National Science Foundation of China under Grant Nos. 11374004 and 30870533 and the Research Grants Council of Hong Kong SAR [Project Nos. CityU 103812 and G_HK005/ 12]. Z.W. acknowledges the High Performance Computing Center (HPCC) of Jilin University.

4. CONCLUSION Detailed analyses of the SERS of pyridine adsorbed on icosahedral 13-atom M@Au12 and M@Ag12 (M = Mo, W) clusters reveal the SERS mechanism based on core−shell bimetallic nanoclusters. Although the SERS enhancements for all complexes can reach the order of 104, the signal enhancements of pyridine on M@Ag12 with charge transfer (CT) excitations are twice as much as that of pyridine on M@ Au12. The corresponding wavelengths used for SERS excitation can be tuned with the interactive modulation of the core atoms and the shell atoms. So, the CT enhancements of pyridine on different core−shell bimetallic clusters mainly depend on the shell materials. In addition, there are strong CT transitions from metal clusters to pyridine in the low-energy area as pyridine molecules are adsorbed on M@Au12 and M@Ag12 clusters, and the interactive modulation of the electronic structure between the core atoms and shell atoms can produce varied strong CT transitions in the range of 1.63−2.10 eV, which tune the SERS enhancements with altered optical properties. This finding can guide material synthesis for controllable SERS enhancement in experiments. Our molecular orbital analysis reveals that, in complexes based on M@Au12 and M@Ag12 (M = Mo, W) clusters, the 4d orbital of Mo and 5d orbital of W are the major source contributing to SERS enhancement in initial orbitals of CT transitions for SERS excitation. The interactive modulation of the electronic structure between the core atoms and shell atoms is very significant, and the alteration of the core atom has a greater impact on complexes based on silver-caged clusters than on gold-caged clusters. Our analyses are expected to provide a theoretical basis for experimentally synthesizing multicomposition SERS substrates and exploring the dependence of SERS enhancement on the synergies between the different compositions of binary metal clusters.

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REFERENCES

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S Supporting Information *

Calculations of HOMO−LUMO gap for Mo@Au12/W@Au12/ Mo@Ag12/W@Ag12 clusters, absorption spectra of bare metal clusters and corresponding pyridine complexes, static Raman G


Article

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Strong Core@Shell Dependence in Surface-Enhanced Raman

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