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Charge-Transfer Effect on Surface-Enhanced Raman Spectroscopy in Ag/PTCA: Herzberg-Teller Selection Rules Hao Ma, Yufeng Chen, He Wang, Xiaolei Wang, Xiaolei Zhang, Xiao Xia Han, Chengyan He, and Bing Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07281 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017
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The Journal of Physical Chemistry
Charge-Transfer Effect on Surface-Enhanced Raman Spectroscopy in Ag/PTCA: Herzberg-Teller Selection Rules Hao Ma,† Yufeng Chen,§ He Wang,† Xiaolei Wang,† Xiaolei Zhang,† Xiaoxia Han,† Chengyan He,‡ Bing Zhao*† † State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China §
College of Chemistry and Chemical Engineering, Mudanjiang Normal University, Mudanjiang 157012, China
‡
China-Japan Union Hospital of Jilin University, Changchun 130033, P. R. China
Abstract The adsorption of 3,4,9,10-perylene tetracarboxylic acid (PTCA) on Ag and Au self-assembled films has been investigated through surface-enhanced Raman spectroscopy (SERS). The spectrum of PTCA reveals a strong excitation wavelength dependence. On the basis of Herzberg-Teller selection rules, it is deduced that appearance of a new b1 mode of PTCA is selectively enhanced by charge-transfer resonance. Moreover, density functional theory (DFT) calculations were also performed to identify the expected vibrational modes and electronic transitions. The information obtained from the adsorption of PTCA on Ag/Au by SERS is helpful to understand the Herzberg-Teller selection rule. Meanwhile, it provides evidence to explain Raman vibronic modes observed when charge transfer resonances appear in metal-dye systems.
Introduction The charge-transfer (CT) process plays a crucial role in a variety of applications, such as chemical sensing,1,2 photocatalysis,3,4 dye-sensitized solar cells (DSSC),5,6,7 et al.8-12 The current demand for performance improvement in these devices entails a better understanding of CT process. Surface-enhanced Raman scattering (SERS), as one of the most powerful tools for probing CT process, has widely been applied to many areas due to its ultrasensitive chemical analysis, and nondestructive trace detection.13 It also provides information as to the orientation of the molecule on a substrate.14,15 Both the electromagnetic mechanism (EM) and chemical enhancement mechanism (CM) contribute to the high SERS intensity.16 The EM contribution requires the coupling of metallic nanoparticles and incident radiation, whereas CM includes a CT process between substrate and absorbate. Usually it is observed that the EM contributes more to the SERS intensity. Hence, many researchers concentrate on new substrates which can provide strong electromagnetic field and so-called hot spots.17-20 However, CT also plays a crucial role, where experimental SERS enhancement, induced by CT, has been widely
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reported.21-24 As for SERS intensities in the molecule-metal system, both molecule-to-metal and metal-to-molecule transfer can contribute to the CT intensity. In order to explain the contributions of charge-transfer, Lombardi and his coworkers derived Herzberg-Teller surface selection rules.25,26 They showed that this effect could increase the non-totally symmetric intensities, which reveals strong wavelength or voltage dependence. The simplest expression for Herzberg-Teller surface selection rules is Γ(Qk)=∑ ΓCT⊥xΓK The Γ(Qk) is the irreducible representation to which the allowed SERS vibration ⊥ belongs, Γ CT and ΓK are the irreducible representations to the component of the charge-transfer dipole perpendicular to the surface and the optical transition allowed excited-state of the molecule. In addition, for SERS, occurrence of CT should be accompanied with a molecule chemically bonded or physically adsorbed on the substrate in a definite orientation. When molecules are adsorbed on the substrate surface, a loss of inversion symmetry (i) occurs to molecules with high molecular symmetry such as pyrazine and benzene. Their symmetry was shown to be lowered to a subgroup (for example, D2h is lowered to C2v for pyrazine). These considerations are of great importance when invoking Herzberg-Teller selection rules. 3,4,9,10-perylene tetracarboxylic acid (PTCA) is a perylene derivative, which contains four carboxylate groups, with inversion symmetry. The utilization of PTCA and its diimide derivatives in various applications has been widely reported, such as dye-sensitized solar cells,27-30 biological and chemical sensing,31 photocatalysis,32 electrochemical sensor,33-35 et al.36-38 The Raman and SERS spectra of 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) and its derivatives have already been reported by Aroca et al.39-41 They proposed an empirical assignment of peaks based on perylene, and also observed abnormal peaks which were ascribed to the Herzberg-Teller mechanism in SERRS. To investigate charge-transfer resonance of PTCA on SERS substrates and explore the corresponding mechanism, herein we examine wavelength-dependent SERS spectra of PTCA on Ag and Au self-assembled films. DFT calculation is applied to obtain basic information of PTCA and PTCDA including normal Raman and electronic states. Based on Herzberg-Teller selection rules, the mechanism of the CT process in Ag/PTCA is proposed.
Experimental Section Materials 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), Sodium citrate, AgNO3, Amidazole, p-aminobenzoic acid (PABA), Chlorauric acid, KOH and poly(diallyldimethylammonium chloride) (Mw = 200,000−350,000, 20 wt% aqueous solution) were obtained from Sigma-Aldrich. All chemicals above were analytical-grade reagents and used without further purification. Milli-Q water was used in the study.
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SERS and normal Raman spectra were measured on Horiba Jobin Yvon T64000 with 514nm, a Renishaw Raman system model 1000 spectrometer with the 532 nm an air-cooled frequency doubled Nd:Yag laser, and a Jobin Yvon/HORIBA LabRam ARAMIS Raman spectrometer equipped with 632.8 nm and 785nm laser. The lasers were focused onto a spot with approximately 1 µm using an objective microscope of 50×. The typical exposure time for SERS measurement applied in this work was 1 s with one-time accumulation, and for normal Raman was 5 s with two-time accumulation. The ultraviolet-visible (UV-vis) spectra of the samples were obtained with a SHIMADTU ultraviolet spectrophotometer (UV-3600). The X-ray photoelectron spectra (XPS) were obtained with PREVAC R3000/VUV5K/MX-650. The infrared reflection-absorption spectroscopy (IRRAS) were acquired with Bruker VERTEX-80V FT-IR spectrometer (Accessory: A513), equipped with a MCT detector at a resolution of 4 cm-1.
Theoretical calculations The geometries of PTCA were optimized by DTF using the basis set B3LYP, which is a hybrid of Becke’s three-parameter exchange functional with Lee-Yang-Parr correlation function. We adapted the triple split valence basis set of 6-311++G**. At this geometry, the excited states calculation and frequency calculation were obtained at the same level. The calculations of PTCDA and PBI were obtained in the same way. All calculations were carried out by the Gaussian 09 program.42
Preparation of Ag/PTCA chips PTCA aqueous was obtained by hydrolyzation of PTCDA in aqueous KOH. Ag NPs modified chips were immersed in PTCA solution (10-5 M) for 12 h at room temperature. The samples were removed from the solution and rinsed with distilled water five times. For convenience, we provide the detailed information of silver assembled glass chips in the supporting information. The preparation of PBI is also reviewed in the supporting information. Ag/PBI was obtained in the same way.
Results and discussion Raman and SERS spectra of PTCA PTCA is an approximate planar molecule consists of 44 atoms, which undergoes 126 normal modes of vibrations. The restricted PTCA belongs to the D2h symmetry ignoring four hydrogen atoms, and it is convenient for spectrum discussion and vibrational assignments. Thus the 114 normal modes of vibrations are distributed as: Tvib=20Ag+19B1g+10B2g+8B3g+9Au+10B1u+19B2u+19B3u There are 57 Raman active vibrations and 48 infrared active vibrations. Since PTCA is a large planar molecule, the change of the polarizability ellipsoid with out-of-plane should be small, and therefore B2g and B3g are expected to be observed with very low intensity in normal Raman spectra.39 Note that B2u and B3u are not allowed in the Raman spectrum. Hence, Ag and B1g modes should be assigned to be the most intense Raman band. The observed and calculated Raman vibration modes are shown in
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Figure 1. The resonance Raman spectrum in Figure 1 (red line) shows several bands at 623, 1048, 1303, 1378, 1450, 1569, 1588, 1612 and 1780 cm-1, which correspond to Ag and B1g vibrations.43 Comparing with the resonance Raman obtained at 477 nm, only four major bands at 1288, 1377, 1568 and 1587 cm-1 which can be ascribed as Ag vibrations appeared at 785 nm in Figure 1 (blue line). As can be seen from Figure 1, most Raman vibrations in resonance agreed with the theoretically predicted frequencies well. In the case of SERS, we observed strongly wavelength-dependent spectra (shown in Figure 2). Compared with resonance Raman spectra, no obvious new peaks are observed under excitation of 514nm. Both RR and SERS spectra in 514nm display a weak peak located at 1780cm-1, which is ascribed to the vibration of carboxyl.43 This result indicates that PTCA is attached to the silver surface through weak Ag-O bonds, which can be confirmed by infrared reflection-absorption spectroscopy (Figure S1) and XPS (Figure S2) in the supporting information. On the other hand, there is only small red shift of Ag (a1) symmetry (ring stretch) at 1571 cm-1 and disappearance of some characteristic peak of Ag and B1g types with no appearance of other vibration types. These facts indicate that PTCA is attached on the silver in which PTCA plane is tilted an angle (θ) from the metal surface.44 Meanwhile, there are no Raman-forbidden bands observed. However, comparison of SERS spectra under different excitation wavelengths of PTCA on Ag show that the intensity of characteristic peaks observed were quite different. It is worth noting that 514nm, 532nm, 633nm reveal the similar spectra with some weak peaks emerging. When the laser energy become lower to 785nm, a number of bands appeared at 548, 784, 891cm-1, which were assigned to non-totally-symmetric vibrations of B3u (b1) symmetry. A number of peaks are located at 1192, 1238, 1605 cm-1. These peaks were ascribed to non-totally-symmetric vibrations of B3g, B2u (b2) symmetry. This enhancement is expected for a metal to molecule resonance involving a specific vibronic state of PTCA. UV-vis Absorption of PTCA The experimental absorption spectra of PTCA in solution shows an absorption band (400-500nm) consisting of three peaks (Figure 3). These three peaks are associated with π-π* transitions of aromatic group, presenting a well-defined vibronic structure.30 As shown in Figure 3A, absorption maxima observed at 466, 438, 412 nm corresponding to (ν0-0, ν0-1, and ν0-2) transitions from the electronic ground state to different vibronic levels in the first electronic excited state, respectively. From the DFT calculations, the HOMO-LUMO gap is estimated to be 2.50 eV at B3LYP/6-311++G** level. Converting to wavelength, the absorption of the last maxima would be 496 nm, which agrees well with experimental result. According to previous work,45 using the calculated absorption spectra of PTCDA, we can also assign the transition symmetries to the D2h point group (ignoring four hydrogen atoms), and calculate absorption spectra of PTCA. As a result, the calculated UV-vis of PTCA is dominated by the transition to B3u at 2.5 eV, (ƒ=0.6451). Several weak transitions to B1g at 3.38 eV (ƒ=0.001), B3g at 3.54 eV (ƒ=0.0003), and Au at 3.72 eV
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(ƒ=0.065) should be negligible. We can obviously see that there are no allowed transitions in the range of 400-500 nm except π-π* transitions, which indicates one electronic transition and its vibronic progressions. Finally, we conclude that the three peaks correspond to π-π* with transition symmetry of B3u. The calculated UV-vis of PTCDA is also dominated by transition to B3u at 1.98 eV, (ƒ=0.5741), several weak intense transitions Au at 2.86 eV (ƒ=0.0023), B1u at 2.87 eV (ƒ=0.0083), and Au at 3.21 eV (ƒ=0.048) should be negligible. Both PTCA and PTCDA, when attached on the surface, should display orbital symmetry. B3u in D2h should be lowered to B1 in C2v for PTCA and for PTCDA, it should be lowered to B2 in C2v.46 UV-Vis spectra of Ag, Ag/PTCA were also obtained in Figure 3B. The surface plasmon resonance (SPR) absorption peak of silver film was located at 381nm. After the adsorption of PTCA molecules on silver chips, the SPR absorption reveals an obvious red shift from 381 to 393 (Ag/PTCA, Figure 3B), which is caused by the chemical adsorption of PTCA on the silver chip. Thus, it can be inferred that the wavelength-dependent phenomenon results from this new surface energy state in a possible charge transfer process of Ag/PTCA system. Mechanism of Charge Transfer To explain this phenomenon, discussing with PTCA and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) together should make sense. From a structural point of view, the difference in PTCA and PTCDA exists only in substituent group of perylene. Ignoring the four hydrogen atoms of PTCA, both belong to the same point group (D2h). However, when they are adsorbed on metal, they have totally different adsorption orientation. For PTCA, we have proved it is attached to the surface through weak Ag-O bonds, and its plane is tilted an angle (θ) from surface by IRRAS. Unlike PTCA, PTCDA lies flat on the surface, which Aroca and his coworkers had proved through polarized spectra. This is to say, the z-axis of PTCDA becomes the y-axis in C2v, and both of x-axis and y-axis are parallel to the surface. The scheme of adsorption orientation of PTCA and PTCDA are shown in Scheme 1. These facts provide the basic information of dipole moment operator for Herzberg-Teller surface selection rules. Let us turn to the task of explaining the observed SERS spectra. The SERS spectrum of PTCA (see Figure 2) reveals a wavelength-dependent phenomenon. We have proved that the molecule is attached to the surface through a weak Ag-O bond, which lead to the conclusion that D2h point group should be lowered to C2v with no inversion symmetry element. Note that several symmetry species are active in SERS, lower to a1, b2, a2, b1 corresponding to C2v. We have calculated transitions in the UV spectrum of PTCA. Unlike pyrazine, PTCA only has one allowed transition of π-π* B3u (B1, ƒ=0.6451) at 2.50eV. In addition of B3u, there is no intense transition. Now we invoke the Herzberg-Teller-surface selection rules. The selection rules require two irreducible representations: charge-transfer dipole moment operator which is perpendicular to surface (A1 in C2v, θ=90°) and allowed excited-state (B1). Thus, Γ(Qk)=∑ ΓµCT⊥xΓK =A1xB1=b1 The charge-transfer state must be of symmetry B1 which in turn allow normal mode of
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b1 symmetry as observed. Also, an enhancement (b2 mode in Figure 2) would occur when the PTCA plane is tilted an angle from the surface (θ
