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Langmuir 1991, 7, 2004-2006
Silver-Coated Faujasitic Zeolite Crystals as Surface-EnhancedRaman Spectroscopic Substrates Prabir K. Dutta' and Daniel Robins Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 Received April 5, 1991. In Final Form: May 23, 1991 This study focuses on generation of A clusters on the surface of zeolite particles by chemical reduction of &+-exchanged zeolites. The growt of silver appears to occur by a diffusion-limited aggregation, resulting in 'snowflake"1ike patterns. Surface-enhancedRaman spectra (SERS) of molecules absorbed on the silver surface can be observed at nanogram detection levels. The advantage of a zeolite substrate stems from the stability of the silver clusters toward aggregation upon heating and their ability to exhibit SERS even after extended heating to temperatures as high as 450 O C .
Introduction The phenomenon of significant enhancement of the Raman signal of molecules adsorbed on specially prepared metal surfaces, referred to as surface enhanced Raman spectroscopy (SERS) has been extensively studied from theoretical and experimental viewpointa.l8 More recently, there has been considerable research aimed at exploiting this effect in analytical chemistry and In order to make general analytical use of this phenomenon, it is important to stabilize the metal particles on substrates. Research has focused on generation of silver particles on substrates such as cellulose, filter paper, silica, frosted glass, alumina, and The crucial issues in generating these materials involve the control of metal deposition levels, aggregation, and long-term stability. In this paper, we examine for the first time the use of zeolites as substrates. Zeolites are crystalline aluminosilicates with unique pore structures, which have been the site for catalytic transformations of hydrocarbons in a variety of important industrial processes.B The cations that are present inside the zeolite cages can be readily ion-exchanged at controllable levels by other cations, including Ag+ ions. The chemical reduction of these Ag+ ions by hydrazine to generate Ag clusters suitable for SERS has been examined. The advantages of a zeolite support stem from its unique ion-exchanging capability and thereby a novel way to generate metal clusters, ita enhanced thermal stability, and the possibility of using SERS to study the mechanism of the large number of chemical reactions that can be carried out on zeolites. To the best of our knowledge, there have been no previous studies on using Ag-zeolites as SERS substrates, though the generation of very small clusters of Ag atoms and their spectroscopic and photochemical properties have been examined.lOJ1 (1)Chang, R. K.; Furtak, E., MS. Surface-Enhanced Raman Scattering; Plenum Press: New York, 1982. (2)Moekovita, M. Rev. Mod. Phys. 1986,57,783. (3)Vo-Dinh, T.; Alak,A.; Moody, R. L. Spectrochim. Acta 1988,43B, 605. (4)Sequaris, J.; Koglin, E. Anal. Chem. 1987,59,527. (5)Tran, C. D. A d . Chem. 1984,56,824. (6)Fan Ni;Cotton, T. M.Anal. Chem. 1986,58,3159. (7)Torres, E.L.; Winefordner, J. 0. Anal. Chem. 1987,59,1626. (8) Soper, S. A.; Kuwana, T. Appl. Spectrosc. 1989,43,1180. (9)Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974. (10)Baker, M. 0.; Godber, J.; Ozin, G. A. J. Phys. Chem. 1985,89, 2299. (11)Baumann, J.;Berr, R.; Calznferri, G.; Waldeck, B. J.Phys. Chem. 1989,93,2292.
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Experimental Section Zeolite Y samples were obtained from Union Carbide (LZY52). The large crystals of NaX were synthesized following Charnell's procedure.12 Ag+-exchangedzeolites were prepared by ion exchangingwith0.005M AgNOssolutionein the dark. Reduction of 200 mg of Ag+-zeolitedispersed in 15mL of water was done by adding 25 pL of hydrazine in an anaerobic atmosphere. The samples were extensively washed with water to remove any unreacted hydrazine. Heating of Agzeolitewas done under vacuum. A 10sM solution of riboflavin mononucleotide (FMN)was used for impregnation on the Ag-zeolite. The loading of FMN was estimated from the electronic spectra of the solution (444 nm, molar absorptivity 1.0 X 1@L cm-' mol-') before and after interaction with the zeolite. Exchange of tetramethylammonium (TMA+)into the zeolite was carried out from a 0.5 M solution. Diffraction patterns were collected with a Rigaku Geigerflex D/Max2B diffractometer using Ni-filtered Cu Ka radiation. Micrographs of zeolite sampleswere taken with the JEOL JMS820scanning electron microscope. Raman spectrawere collected with a Spectra Physics Model 171 Argon ion laser using 514.5nm radiation. The power at the sample was of the order of 15 mW. The scattered light was filtered with a Spex 1403 double monochromater and detected with a RCA C31036 photomultiplier. Typical scanningtimes of 1s cm-1 at a slit width of -6 em-' were used.
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Results Generation of silver clusters was carried out by reduction of silver-exchanged NaY by hydrazine. Reaction of a completely exchanged AgY (-55 Ag+/unit cell) with hydrazine led to an almost instantaneous destruction of the crystalline framework of the zeolite, as shown in Figure la. No damage to the zeolite framework was evident upon exposure of NaY to hydrazine for as long as 2 weeks, clearly suggesting that it is the reduction of Ag+ in a fully exchanged zeolite that brings about the destruction of the zeolite framework. Loweringthe Ag+loading to 14Ag+/ unit cell led to retention of the framework upon reduction (Figure lb), with the appearance of two new reflections at 28 values of 38.18' and 44.31O. The presence of new reflections is related to the formation of Ag clusters that are at least 200 A in size and therefore must be forming on the zeolite surface. This also indicates that the Ag+ upon reduction is migrating from the interior of the zeolite to the surface. In order to examine this reduction process, scanning electron micrographs @EM) were obtained from the reduced samples. However, instead of working with 1-
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(12)Charnell, J. F. J. Cryst. Growth 1971.8,291.
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Figure 1. XRD pattern of a hydrazine-reduced sample of (a) completelyexchangedAg-Y zeoliteand (b) a partially exchanged Ag NaY zeolite (Ag+ loading 14 ions/unit cell).
pm crystals of Nay, larger, well-defined single crystals of NaX (-20 pm) were used. Zeolites NaY and NaX have the same framework architecture but differ in their Si/A1 ratios, being 1.2 for NaX and 2.6 for Nay. The chemistry on both these supports is not expected to be different, consideringtheir similar pore structure. Figure 2a,b shows the SEM of a surface of AgNaX (14 Ag ions/unit cell) crystal upon reduction and upon heating at 450 "C for 45 min, respectively. The Ag appears to form as a twodimensional "snowflakenlike pattern on the zeolite faces. There is a nucleus for each snowflake, with veins growing out from it. The typical dimensions are -2 pm. These films were unstable to thermal treatment and collapse to form microspheresof Ag atoms on the zeolite (Figure 2b). These clusters range in size from tens to thousands of nanometers. There is no change in morphology of the clusters or any further agglomeration upon heating from up to 4 h, between 300 and 450 "C. The surface-enhancedRaman spectra of two molecules, flavin monoculeotide (FMN) and tetramethylammonium ion (TMA+)were examined. The choice of these molecules was dictated by their size, FMN with dimensions greater than 7 81 can only be present on the zeolite outer surface, whereas TMA+ (-6 A) can ion-exchange into the inner pores of the zeolite via the 7-81 ring opening of the supercages. Figure 3 compares the SERS spectra obtained from 2.1 X104 moles of FMN/g of zeolite for a reduced AgNaX sample and FMN impregnated onto the Ag-zeolite after heating at 300 "C for 4 h. In both cases, SERS can be observed, though the unheated sample produces a better spectrum. These spectra are comparable to SERS of FMN obtained from Ag s01s.l~ It is clear that SERS of FMN adsorbed on the Ag on the zeolite surface can be (13) Lee, N.;Sheng, R.;Moms, M. D.;Schopper,L.M. J. Am. Chem. Soc. 1986,108,6179.
Figure 2. SEM patterns of Ag metal formed on the surface of a large crystal of zeolite X (a) under ambient conditions and (b) heat to 400 "C for 45 min. I-
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Figure 3. SERS of FMN (2.1 X 1 Pmol g of zeolite) absorbed on the surface of reduced AgNaX (a) un er ambient conditions and (b) after heating Ag-zeolite to 300 "C for 4 h.
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readily observed. Similar spectroscopic data were also obtained for Ag-zeolite Y samples. The sensitivity of detection by SERS from the Ag-loaded zeolites must be of the order of nanograms or less since -1 nmol of FMN is adsorbed on a gram of the zeolite, and the laser samples only -10-50 pm of the zeolite surface.
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Figure 4. Raman spectra of TMA+ exchanged into supercages of (a) NaX and (b) reduced AgNaX. Figure 4a shows the SERS of TMA+-exchanged Agcoated zeolite X. Only a weak signal due to the symmetric C-N stretching at 754 cm-l is observed. The spectrum obtained for similar loading of TMA+ in a regular NaX zeolite was considerably better (Figure 4b), indicating that the presence of Ag crystallites on the surface of the zeolite leads to a deenhancement of the Raman spectrum.
Discussion A variety of reducing agents are available to reduce Ag+ to Ag, including hydrazine, sodium borohydride, and radiation. Our choice of hydrazine was based on the simple products that are formed upon reduction, Nz and hydronium ions. These H30+ions essentially serve to neutralize the negative aluminosilicate framework, replacing the Ag+ ions. The destruction of the zeolite for the completely Ag+ exchanged zeolite (Figure la) could arise from the strong propensity of the Ag atoms to form clusters that are too large to be accommodated in the -13-A zeolite supercages. Another destructive route could be via the generation of significant quanitites of hydronium ions. Lowering the Ag+loading allowsthe silver atoms to migrate to the surface without growing too large. The snowflakelike pattern of Ag formed on the zeolite surface (Figure 2a) is indicative of diffusion-limited aggregation.14J5
The exact mechanism of migration of the silver from the zeolite interior onto the surface is unclear. However the fractallike pattern would indicate that growth occurs by aggregation of atomic clusters. There are two possibilities. The Ag+ may be reduced to Ag atoms in the interior of the zeolite and then migrate to the surface. The problem with this mechanism is that Ag atoms are not expected to be diffusive in a hydrated zeolite under ambient conditions. The more likely possibility is that the Ag+ ions migrate to the surface and are subsequently reduced. However this process needs a driving force and may occur as follows. The NzHd begins reduction of the Ag+ on the surface layers of the zeolite, which promotes further migration from the Ag+ in the interior on to the depleted surface layers. Thus the Ag cluster essentially grows by atomic aggregationon the surface. The migration of Ag+ ions is considerably aided by its mobility in water. The flakes of Ag atome on the zeolite surface are unstable to heat treatment and collapse to form Ag clusters that are submicron (-0.1 pm) in size. Both the unheated and heated samples exhibit SERS from FMN adsorbed onto them. The analytical advantage of the zeolite as support appears to be the stability of these Ag particles. These samples could be heated a t 300-400 "C for hours and stored for weeks under ambient conditions without any precautions and would still produce a FMN SERS spectrum. However, the TMA+ exchanged into the Ag-containing zeolite cages does not produce a SERS effect. As a matter of fact, its Raman intensity is diminished as compared to a Ag-free Na+ zeolite X, which is representative of the spontaneous nonenhanced Raman spectrum. The loadings on these two zeolites were comparable, as determined by the intensity of the C-H stretch in the infrared spectrum. The enhancement could be expected if there are Ag clusters embedded within the pore structure of zeolite. This supports the fact that the Ag clusters are forming primarily on the zeolite surface. The Raman signal from the Agzeolite could also be reduced as compared to NaX because the coating of Ag particles on the zeolite may act as a reflective surface and prevent penetration of the incident radiation into the zeolite. Experiments are continuing to examine if it is possible to prevent migration of Ag+ ions to the surface of the zeolite and generate long enough Ag strands within the zeolitesuch that moleculeswithin the cages can exhibit SERS effects. Registry NO. FMN, 146-17-8; TMA, 51-92-3. (14) Wittan, T. A., Jr.; Scinder, L. M. Phya.Reu. Lett. 1981,47,1400. (15) Sciman, 0.; Feilchenfeld, H. J. Phys.Chem. 1988, 92,463.
