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Contribution of ZnO to Charge-Transfer Induced Surface-Enhanced Raman Scattering in Au/ZnO/PATP Assembly Libin Yang,†,‡ Weidong Ruan,† Xin Jiang,‡ Bing Zhao,*,† Weiqing Xu,† and John R. Lombardi*,§ State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China, College of Chemistry and Pharmacology, Jiamusi UniVersity, Jiamusi 154007, People’s Republic of China, and Department of Chemistry, The City College of New York, New York 10031 ReceiVed: August 18, 2008; ReVised Manuscript ReceiVed: NoVember 5, 2008
The assembly of metal/semiconductor/molecule was fabricated with gold nanoparticles, ZnO, and 4-aminothiophenol (PATP) molecules through the layer-by-layer self-assembly method. Raman spectra of PATP adsorbed on Au/ZnO were considerably enhanced relative to those observed on Au particles alone. By examining the relative enhancement of the nontotally symmetric (b2) modes, this effect is shown to arise from a charge transfer (CT) contribution from the metal to molecule. Introduction 1
Fleischmann first discovered in 1974 that high-intensity Raman scattering of small molecules could be obtained on an electrochemically roughened silver electrode surface, which was attributed to the large number of the molecules adsorbed on the roughened surface. In 1977, Jeanmaire et al.2 and Creighton et al.3 independently discovered that the enhancement of the Raman scattering is related to an intrinsic surface enhancement, marking the beginning of surface enhanced Raman scattering (SERS). Since then, considerable interest has been focused on the mechanism of the extraordinarily large enhancement of Raman signals for molecules adsorbed on roughened metal surfaces. It is now generally accepted that there are two interacting mechanisms which explain the overall SERS effect: the electromagnetic (EM) mechanism and the charge-transfer (CT) mechanism.4-8 The former is based on the interaction of the transition moment of an adsorbed molecule with the electric field of a surface plasmon resonance induced by the incoming light at the metal surface, whereas the latter is related to the interaction of the adsorbates adsorbed on metal and the metal, mostly from the charge-transfer between adsorbates and the metal. At present, SERS active substrates have been restricted to some noble metals (Ag, Au, and Cu) and transitional metals (Pt, Pd, Ru, Rh, Fe, Co, and Ni).9-13 Very few studies of SERS on nonmetallic surfaces have been reported. Quagliano14 has detected Raman signals from pyridine molecules adsorbed on InAs/GaAs surfaces. Yamamoto15 has reported some evidence for SERS on small GaP particles. SERS from molecules adsorbed on surfaces of semiconductor oxides such as NiO,16 TiO2,17 and R-Fe2O318 was also reported. A lack of SERS substrate generally limits the breadth of practical applications for SERS in various materials, especially in semiconductor materials, widely used in electrochemistry, catalysis, and other industries. Furthermore, the nature of the enhancement mechanism of semiconductors-based SERS substrate is still ambigu* Corresponding authors. Fax: +86-431-85193421 (B.Z.). E-mail:
[email protected] (B.Z.),
[email protected] (J.R.L.). † Jilin University. ‡ Jiamusi University. § The City College of New York.
ous. Therefore, SERS studies on semiconductors are still necessary for the application of SERS on nonmetallic surfaces. In this paper, the Au/ZnO/PATP assembly was successfully prepared by layer-by-layer self-assembly method. SERS behavior of PATP molecule adsorbed on the Au/ZnO was investigated, which was verified to arise from CT from the metal to the molecule. We suggest that the ZnO introduced in the Au/ZnO/PATP assembly augments CT from the metal to the molecule, resulting in a stronger SERS signal than in the case of the absence of ZnO. Experimental Section Chemicals. 4-Aminothiophenol (PATP) purchased from Acros Organics Chemical Co. and poly(diallyldimethylammonium) chloride (PDDA) purchased from Aldrich Chemical Co. were used without further purification. The other chemicals were all analytical grade without further purification. Purified water produced by Hangzhou Wahaha Group Co.,Ltd. was used in all experiments. Sample Preparation. Gold colloid was prepared by the reduction of gold chloride (AuCl3 · HCl · 4H2O) aqueous solution with sodium citrate according to the literature.19,20 100 mL of 0.01% gold chloride aqueous solution was brought to boiling under magnetic stirring, and subsequently a 4 mL of 1% sodium citrate aqueous solution was added quickly, which resulted in a change in solution color from pale yellow to amaranth. After the color change, the boiling was continued for an additional 15 min under reflux, and finally, gold colloid was cooled to room temperature. The Au/ZnO/PATP and Au/PATP assemblies were carried out by the following procedure: glass slides were first derivatized with PDDA by immersing the slides into a 0.5% PDDA aqueous solution for 1 h. After thoroughly rinsing with water, the derivatized slides were exposed to the gold colloid suspension for 3 h. A layer of the gold nanoparticles was assembled on the surface of the slides. Then, the glass slide assembled with the gold nanoparticles was laid flat in a 100 mL of 0.05 M zinc nitrate and 0.05 M hexamethylenetetramine (HMT) mixed aqueous solution, which was heated at 90 °C for 3 h. The asprepared Au/ZnO assembly was then rinsed with water and blown dry with a stream of nitrogen. The adsorption of the
10.1021/jp8074095 CCC: $40.75 2009 American Chemical Society Published on Web 12/09/2008
118 J. Phys. Chem. C, Vol. 113, No. 1, 2009
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Figure 1. SEM images of (A) the gold nanoparticles and (B) the Au/ZnO assemblies.
PATP molecules on the Au/ZnO assembly and Au nanoparticles assembled on the slides was carried out by immersion of the Au/ZnO and Au nanoparticle substrates in a 1 mM PATP ethanol solution for 24 h. After that, the slides were washed with ethanol and water, respectively. Sample Characterization. The surface morphologies of the samples were measured on a JEOL JSM-6700F field emission scanning electron microscope (FE-SEM) operated at 3.0 kV. The electronic absorption spectra were recorded on a Shimadzu UV-3600 UV-vis spectrophotometer, whose slit width was set at 2 nm. Raman spectra were obtained with a Renishaw Raman system model 1000 spectrometer. The 514.5 nm radiation from a 20 mw air-cooled argon ion laser was used as exciting source. Data acquisition was the results of two 60s accumulations for PATP molecules adsorbed on Au/ZnO and Au nanoparticles thin films. Replicate measurements on different areas of each sample were made at least three times to verify the spectra were reproduceable. The resolution of the Raman instrument was approximately 4 cm-1. Moreover, 632.8 nm laser excitation experiments were also conducted with a Horiba Jobin-Yvon Labram HR800 spectrometer.
Figure 2. Absorption spectra of the gold nanoparticles monolayer (a), the gold colloid (b), the Au/PATP (c), the Au/ZnO/PATP (d), and the Au/ZnO (e).
Results and Discussion Structures of Assemblies. The gold nanoparticles were assembled on the PDDA derivatized glass slide by the electrostatic attractive interaction between the positively charged PDDA molecules and the negatively charged gold nanoparticles. Because of the electrostatic repulsion between the negatively charged gold nanoparticles, the assembled gold nanoparticles remained isolated from each other. The SEM image of the gold nanoparticles assembled on the glass slide is shown in Figure 1A. It can be clearly seen that gold nanoparticles form a submonolayer structure on the derivatized glass slide surface. Most of the particles are isolated and uniformly distributed on the surface. ZnO crystals were deposited on the surface of gold nanoparticles monolayer by the hydrolysis reaction between the HMT and the zinc nitrate. It can be seen from Figure 1B that ZnO particles dispersively distributed on the gold nanoparticles monolayer surface in the form of single particle or multiparticle aggregation, which do not change the apparent distribution of gold nanoparticles. Furthermore, no remarkable change in the assembly structure was observed after the adsorption of the PATP molecules on the Au/ZnO assembly. The adsorption of the PATP molecules on the Au/ZnO substrate is likely through the thiol group of the molecules because of the formation of partial Au-S chemical bonds21 and Zn-S bonds,22 respectively. Electronic Absorption Spectra. Figure 2 shows UV-vis absorption spectra of the gold colloid and the different assemblies. The absorption spectra of all samples exhibit an
Figure 3. Raman spectrum of PATP solid powders.
absorption band with a maximum absorption wavelength (λmax) of about 518 ( 6 nm, which are caused by the localized surface plasmon resonance (LSPR) of gold nanoparticles. It can be seen clearly from Figure 2 that, compared with the gold colloid, the LSPR λmax of the gold nanoparticles monolayer assembled on the glass slide take place slightly blue-shifted from 523 to 517 nm, due to the dipole-dipole interaction between the metal particles assembled in a two-dimensional manner.23,24 Whereas, after the adsorption of the PATP molecules on the gold nanoparticles monolayer, the LSPR λmax shifted to 522 nm (Figure 2 c), which most likely is the result of charge transfer from the gold nanoparticle to the PATP molecule.25 Compared with the gold nanoparticles monolayer (Figure 2 a), the characteristic gold plasmon peak in the Au/ZnO (Figure 2 e) was observed to shift from 517 to 512 nm. The LSPR λmax of metal is represented by the following equation: λmax ) [4π2c2meffε0/ Ne2]1/2 where meff is the effective mass of the free electron of the metal and N is the electron density of the metal. For Au, the λmax position is related to the electron density of the metal. When increasing the electron density of the metal, the λmax of
Contribution of ZnO in Au/ZnO/PATP Assembly
Figure 4. Surface-enhanced Raman spectra of PATP molecules adsorbed on the gold nanoparticles monolayer (a), the Au/ZnO assembly (b), and ZnO substrate (c).
Figure 5. Surface-enhanced Raman spectra of PATP molecules adsorbed on the gold nanoparticles monolayer (a) and the Au/ZnO assembly (b) with 632.8 nm laser excitation.
metal is decreased, which results in the blue shift of the plasmon absorption band. In this experiment, the blue shift of the plasmon peak of gold in the Au/ZnO suggests that the electron density of Au was increased. The increase of electron density of Au nanoparticles may be due to transfer of the electrons from the ZnO to Au. Besides the LSPR absorption band of gold nanoparticles, the absorption spectra of both the Au/ZnO and the Au/ZnO/PATP exhibit a new absorption band at around 369 nm, which results from the band-band transition of the semiconductor ZnO according to its band energy 3.37 eV.26,27 It can also be seen that, after the adsorption of the PATP molecules on the Au/ZnO, the absorption band at 369 nm increases in the intensity and becomes broader (Figure 2 d). These changes are attributed to the interaction of PATP molecules and ZnO nanocrystals. Similar results have been observed from dye molecules adsorbed on TiO2 surfaces28 and 4-Mpy surface modified CuO nanocrystals.22 SERS Spectra of PATP on Assemblies and Enhanced Mechanism Analysis. Figures 3 and 4 show the Raman spectra of PATP solid powders and PATP molecules adsorbed on the different assemblies respectively.
Figure 6. Sketch map of charge transfer process.
J. Phys. Chem. C, Vol. 113, No. 1, 2009 119 It can be seen in Figures 3 and 4 that the two spectra are significantly different. The predominant bands in the spectrum of PATP solid powders (Figure 3) are located at 1595, 1171, and 1089 cm-1, which have been assigned to the a1 modes of the PATP molecules,8,29 whereas the strong bands are observed at 1574, 1436, 1391, and 1142 cm-1 in the Au/PATP assembly (Figure 4 a), which are assigned to the nontotally symmetric b2 vibrational modes of the PATP molecule.8,29 Except for 1077 cm-1, the other a1 modes Raman bands are not observed. That means that the b2 vibrational modes of the PATP molecule on the gold nanoparticles monolayer were selectively enhanced. The selective enhancement of the b2 modes can not be explained by the EM mechanism. According to the EM model proposed by Creighton30 and by Moskovits,31 the totally symmetric (a1) modes should be most strongly enhanced and the SERS enhancement order for nontotally symmetric modes with C2V symmetry should be b1 ) b2 > a2 in the case of the molecules with a standing-up orientation. If the molecule were lying flat, the enhancement order instead should be a2 ) bl > b2. The selective enhancement of only the b2 mode among the three modes can not be explained by the EM models for any other reasonable orientation. Osawa et al.8 have experimentally demonstrated that the CT mechanism dominantly contributes to the SERS of PATP adsorbed on silver. The CT is from the Fermi level of the metal to the LUMO of the molecule. This is responsible for the selective enhancement of the b2 modes of PATP molecules adsorbed on the gold nanoparticles monolayer in our case. Furthermore, by comparing curves a and b in Figure 4, it can be seen that, except for 1434 and 1389 cm-1, the frequencies of the other Raman bands do not display any changes as PATP absorbed on the Au/ZnO. This illustrates that the SERS of PATP adsorbed on the Au/ZnO also resulted from the CT from gold to PATP molecule. It is more interesting in our case, however, that the intensity of Raman signals of PATP on the Au/ZnO is obviously higher than that on the gold nanoparticles monolayer. Moreover, when 632.8 nm excitation line was used as exciting source, the same result can be observed (as shown in Figure 5). This is despite the fact that the Raman signal of PATP was not observed on the single ZnO substrate without Au particles (as shown in Figure 4c). This obvious Raman enhancement of PATP on the Au/ZnO must be related to the introduced ZnO and the interaction between ZnO and gold nanoparticles. We suggest that we observe more enhancement on the Au/ZnO than on the gold nanoparticles monolayer for two possible reasons: (1) Since ZnO is a typical n-type semiconductor, its surface congregates more negative charges (electrons) than its bulk phase. When it is in contact with the gold nanoparticles, the electrons congregated at the surface of ZnO will be injected into the Fermi level of the gold particles to balance charges at the junction of gold and ZnO. As mentioned above, the UV-vis absorption spectrum displays a
120 J. Phys. Chem. C, Vol. 113, No. 1, 2009 blue shift in the LSPR of Au. The electrons injected from ZnO to gold result in the elevation of the gold Fermi level, which augments the CT from the gold nanoparticles to the PATP molecules in the Au/ZnO/PATP system (Figure 6b). The elevation of the gold Fermi level has previously been reported in the ZnO/Au system by Kamat32 and Wood.33 (2) In the Au/ ZnO/PATP assembly, ZnO junction between gold and PATP also serves the function of transmitting the electron by its conduction band. These additional electrons are obtained from the Fermi level of the metal via the LUMO of molecule (Figure 6c), forming the similar “donor-bridge-acceptor” system. This kind of the electronic transmission role of one-dimensional ZnO nanostructure has been extensively applied in the field of dyesensitized solar cells.34,35 It should be pointed out that, in the Au/ZnO/PATP assembly, the SERS of PATP molecules also resulted from the direct CT between gold and PATP (Figure 6a) besides the contribution of the synergetic effect of gold and ZnO (Figure 6b,c). Conclusions In this work, a Au/ZnO/PATP system has been fabricated by the layer-by-layer self-assembly. The Au/ZnO assembly has distinct Raman scattering enhancement to the adsorbed PATP molecule, which was attributed to the dominant contribution of the CT from the metal to molecule. The CT mechanism was assumed to involve the direct CT from gold to PATP as well as the indirect CT process assisted by ZnO from gold to PATP, which was responsible for a larger enhancement of PATP on the Au/ZnO than on the single gold nanoparticle substrate. It is expected that this work will be helpful to design SERS substrates based on semiconductors and to understand chemisorption and reaction mechanisms of molecules on semiconductor materials by SERS. Acknowledgment. The research was supported by National Natural Science Foundation (Grants 20473029 and 20573041) of People’s Republic of China, Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), Program for New Century Excellent Talents in University (NCET), Innovative Scholars of Jilin University (2004CX035), Fund for the Doctoral Program of Higher Education (20040183048), Scientific Research Foundation for the Returned Overseas Chinese Scholars initiated by State Education Ministry, Program of Introducing Talents of Discipline to Universities (B06009), and Scientific and Technical Project of Jiamusi University (S2008-045). References and Notes (1) Fleischmann, M.; Hendra, P. J.; Mcquillan, A. J. Chem. Phys. Lett. 1974, 26, 163–166.
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