Langmuir 1994,10, 1177-1185
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Formation of Metal Clusters on the Surfaces of Covalently Bound Self -Assembled Ligand Monolayers DeQuan Li,* Louie W. Moore, and Basil I. Swanson Isotope and Nuclear Chemistry Division (INC-14),Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545 Received December 1, 199P
This paper describes interactions between metal complexes and self-assemblies of covalently bound and monolayerson surfaces. Covalentlybound molecularself-assembliesof (3-cyanopropy1)trichlorosilane ( PTS) were anchored on the surfaces of fused quartz and the native oxide of the (100)crystallographic orientation of p-doped Si wafers. The formations of a number of ruthenium and osmium metal carbonyl complexes on both the substrates and substrates coated with ligand monolayers have been examined with polarized variable-angle internal attenuated total reflection infrared spectroscopy (PVAI-ATR-IR). Both mononuclear species M(CO),L (n = 2,3), where L = -(CH2)&N, and multinuclear species &(CO)l~(c(COIL were observed on the surface of the covalently bound, self-assembled ligand monolayers when the coated substrates were treated with Ms(C0)12, where M = Ru and Os. Ruthenium porphyrin macrocycle RuTPP(CO).THF which has a “disklike”structure was bound to a CPTS axial ligand and then formed a covalently bound, self-assembled monolayer of “porphyrin-disk”stacking on the quartz surface. This monolayer was analyzed by UV-vis spectroscopy and secondary ion mass spectrometry (SIMS).
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Introduction The behavior of monolayer metal clusters at the surfaces of solid supports has been a topic of great interest to catalysis chemists for decade5.l Fundamental interest and great potential technological applications in these systems are attributed to the possibilities that they allow rational studies of structure-property relationships at the condensed matter surfaces. Moreover, covalently bound, selfassembled monolayers represent an elegant approach to the design of robust microstructures of molecular dimensions exhibiting the desired physical properties, molecular number density, and molecular net orientation. The ability to control local chemical structure and topology a t the surface makes it possible to determine how geometric and structural factors, such as intermolecular packing, chirality, net alignment of the molecular array, etc., determine surface functionalities like catalytic properties or optical nonlinearities. These potentialities have prompted a number of recent studies concerning the formation, structure, and properties of such self-assembled systems. For example, applications of covalently bound, multilayer polar chromophores for nonlinear optical materials have been reported.2 Much of the recent interest in organic/inorganic monolayers is attributable to their resemblance to biological membranes and their ability to mimic enzymes’ catalytic properties.3 Artificially structured self-assemblies of Abstract published in Advance ACS Abstracts, March 1,1994. (1) (a) Evans, J. In Surface Organometallic Chemiutry: Molecular Approaches to Surface Catalysis; Baseet, J. M., et al., Eds.; Kluwer Academic Publinhers: Boaton, 1988; pp 47-73. (b) Malik, I. J.; Hrbek, J . J . Vac. Sci. Technol. 1991,A9 (3),1737-1741. (c) Malik, I. J.; Hrbek, J. J. Chem.Phys. 19W,93(3),2156-7. (d) Psaro, R.; Ugo,R.; Znnderichi, G. M.;Beason,B.; Smith, A. K.;Basset, J. M. J. Organomet. Chem. 1981, 213,216-47. (e)Zacchina, A.; Guglielminotti, E.; Bosei, A.; Camia, M. J. Catal. 1982,74,226-39. (2)(a) Li, D.; Ratner, M. A.; Marks, T. J.; Zhaug, C.; Yang, J. and (b) Dai,D.;Hubbard, Wong,G.K. J.Am.Chem.Soc. 1990,112,738~7390. M.;Li, D.; Park,J.;Ratner, M. A.; Marks,T. J.; Yang, J.; Wong, G. K. New Materials for Nonlinear Optics. ACSSymp. Ser. 1991,455,226-249 (Boston, MA,April 22-27, 1990). (c) Li, D.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, G. K. Nonlinear Optical Properties of Organic Materials III. SPZE BOC. 1990,1337,341-346.
organometallic complexes on surfaces represent a new and intriguing approach to supported catalysts. Particularly interesting properties include highly organized and disperse low-valencemetals on the surface of the substrates,’ more durable catalysts that are covalently bound to the supports, and the relatively high reactivity and selectivity of the surface cluster-derived catalysts compared to those of the conventionally prepared catalysts.s The conversion of COn to chemical carbon or fuel has great potential use. Methanation of C02 is possible with a number of catalysts supported on surfaces of silica? alumina: and molecular sieves.* The higher reactivity and selectivity are attributable to the relatively low concentration of “active surface carbon species”, which favors methane formation rather than carbon accumulation on the surface with subsequent deactivation of the catalyst. Among the transition-metal complexes which catalyze COZinto methane, surface-supported ruthenium carbonyl moieties are probably the best. However, the formation of these surface catalytic species on oxide substrates including ruthenium metal carbonyl clusters is poorly understood. In this paper, we discuss our studies of the molecular interactions of ruthenium and osmium metal carbonyl clusters with a self-assembledligand monolayer on an oxide surface. Our investigations focus on monitoring the formation of surface species by using polarized variableangle internal attenuated total reflection infrared spectroscopy (PVAI-ATR-IR). In addition, we will discuss the formation of ruthenium porphyrin self-assemblies on quartz surfaces. It is important to point out that silica(3)(a)Rubinstein, L;Steinberg,S.;Tor, Y.;Shanzer,A.;Sagiv,J.Nature 1988, 332, 426-9. (b) Steinberg, S.;Rubinstein, I. Langmuir 1992,8, 1183-1187. (4)Ichikawa, M. CHEMTECH 1982,12,674. (5)(a) Simpson, A. F.; Whyman, R. J. Organomet. Chem. 1981,213, 157. (b) Ferkull, H.E.; Stanton, D. J.; McCowan, J . D.; Baird, M. C. J. Chem. Soc.. Chem. Commun.1982,955. (c) Ferkull. H.E.: Stanton.. D. J.; McCowk, J . D.; Baird, M. C. Can. J. .%em. 1983,61,.1306. (6)Zagli, E.; Falconer, J. L. J. Catal. 1981,69,1. (7)Solymosi, F.; Erchohelzi, A.; Kocais, M. J. Chem. SOC.,Farday Trans. 1 1981, 77,1003. (8)Gupta, N. M.; Kambl, V. S.;Annaji Rao, K.; Iyer, R. M. J. Catal. 1979,60,57.
Q743-7463/94/241Q-1177$Q4.5Q/O 0 1994 American Chemical Society
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or alumina-supported catalysts have been examined previously.9 Covalent bonding of metal clusters to the surfaces of supports offers a new approach that generates interesting catalytic species on the surfaces. Furthermore, the chemical structure of these surface catalysts can be tailored, tuned, and controlled by the built-in selfassembled ligand monolayers.
Experimental Section All synthetic procedures described below were carried out under an argon atmosphere. A. Reagents. (5,10,15,20-Tetraphenylporphinato)ruthenium(11) carbonyl, 1 to 1 complex with tetrahydrofuran, RuTPP(CO).THF, was either purchased from Aldrich or synthesized as described in the literature.10 Triruthenium dodecacarbonyl, Rus(CO)lz, triosmium dodecacarbonyl, OSS(CO)IZ,and (2,3,7,8,12,13,17,18-octaethylporphinato)r~thenium(~) carbonyl, 1 to 1 complex with tetrahydrofuran, RuOEP(CO).THF, were purchased from Aldrich. The dry solventa of C&, CHCb, and hexanes were prepared by stirring them with P205, followed by a vacuum distillation, and stored under Ar for future use. B. Substrate Treatment. Fused quartz plates were purchased from General Electric, and Si wafers were obtained from Siltec Corp. The typical thicknesses for the quartz and the p-doped Si wafers are 1mm and 500 pm, respectively. Both the 15 X 15 cm2 fused quartz plates and 10-cm Si wafer disks were cut into approximately 1 x 3 cm2 pieces before use. The fused quartz substrates were ultrasonically cleaned in 10% detergent solution for 10 min and then refluxed in 1% tetrasodium ethylenediaminetetraacetate (EDTA) solution for 10min;this was followed by another 10-minsonication at ambient temperature. Finally, the substrates were thoroughly rinsed with deionized water and acetone and then were sputter-cleaned using an argon plasma at a few millitorr for more than 30min. Polished p-doped Si wafers were cleaned by sonication in 10% detergent solution for 10 min and a thorough rinsing with deionized water and acetone and then sputtered by an argon plasma for 30 min. Conductive I T 0 coated glass substrates were cleaned by sonication in 10% detergent solution for 10 min, followed by rinsing with deionized water and acetone, and finally were sputtered with an argon plasma for 30 min. All substrates were used immediately after cleaning. C. PVAI-ATR-IR Measurement. The infrared spectra of thin films containing surface-bound ruthenium carbonyl species were collected using a Bio-Rad FTS-40 with a Harrick Seagull variable-anglereflection attachment. Both surface-bound metal clusters and self-assembledligand monolayers were studied using the internal attenuated reflection (ATR) mode, in which the p-doped Si wafer-functionalized with an organic/inorganic monolayer on the surface of the (100) crystallographic orientation-was pressed against a ZnSe hemisphere crystal with a miniature pressure device to assure optical contact. Background spectra were also collected in this manner using a clean Si wafer with the same geometry (incident angle, polarization, etc.) as the sample. We collected a single attenuated total reflection from the interface of the ZnSe hemisphere crystal and the Si wafer with 1024 scans at 8-cm-l resolution. This method allows the angle of incidence to be varied from 5O to 85O;therefore, angulardependent results can be probed readily with the current technique. In the vicinity of the critical incident angle, Bc = 39", the maximum signal was observed because a strong interaction of the infrared radiation with the interface of the ZnSe hemisphere crystal and the Si wafer occurs at this geometry. Monolayer Formation of (3-Cyanopropy1)trichlorosilane (CPTS) on Surfaces. A 5-pL samples of CPTS was syringed into a Schlenk tube containing 10 mL of dry CHCla under an inert atmosphere. Both cleaned quartz and Si wafer substrates (9) (a) Kuznetaov, V. L.; Bell, A. T.; Yennakov, Y. I. J. Catal. 1980, 65,374. (b) Darensbourg, D. J.; Ovalles, C. Znorg. Chem. 1986,25,16031609. (10) (a) Tsutaui, M.; Ostfeld, D.; Frances, J. N. J. Coord. Chem. 1971, 1, 115. (b) Tsutaui, M.; Ostfeld, D.; Hoffman, L. M. J. Am. Chem. SOC. 1971, 93, 1820. (c) Rillema, D. P.; Nagle, J. K.; Barringer, Jr., L. F.; Meyer, T. J. J. Am. Chem. SOC.1981,103,66-62.
Li et al. were coated by immersing them in the chloroform solution for more than 12 h a t room temperature. The substrates were then withdrawn, transferred into a neat CHCb solution, and cleaned by 2-min sonication. The cleaning procedure was repeated three times. The coated substrates were then rinsed with acetone and dried in air. Monolayer Formation of Octyltrichlorosilane(OTS) on Surfaces. A 5-pL sample of octyltrichlorosilane was injected into a Schlenk tube containing 10 mL of dry hexanes by syringe. A cleaned Si wafer was dipped in the hexane solution for about 20 min at room temperature. The substrates were then withdrawn, transferred into a hexane solution, and cleaned by a 2-min sonication. This cleaning procedure was repeated three times, and the coated substrates were rinsed with acetone and dried in air. The dense monolayer coverage of the OTS coating can be easily monitored by the water contact angle B(Hz0) 2 100'. Monolayer Formation of RuTPP(C0)L (L = CPTS) on the Surfaces of Quartz Substrates. To a Schlenk tube containing 25.4 mg of RuTPP(CO).THF was added 10 mL of dry CHCb. After all the complexdiseolved,5 pL of CPTS was injected with a syringe under an inert atmosphere. The red solution was mixed by shaking the Schlenk tube occasionally for 0.5 h. Two cleaned quartz substrates were carefully immersed in the red solution for 3 days at room temperature, and the surfaces of the substrates were kept from touching each other by a spacer inserted between them. The substrates were then withdrawn and transferred into a neat CHCb solution and cleaned by a 2-min sonication. The cleaning procedure was repeated three times, and the coated substrates were rinsed with acetone and dried in air. Submonolayer Formation of RuTPP(C0)L (L = CPTS) on the Surface of Conductive I T 0 Thin Films S~pported by Glass. To a Schlenk tube containing 25.4 mg of RuTPP(CO).THF was added 10mL of dry CHCls. After all the complex was dissolved, 5 pL of CPTS was injected with a syringe under an inert atmosphere. The red solution was mixed wellby shaking the Schlenk tube occasionally for 0.5 h, and the clear solution was allowed to stand for 24 h at room temperature. Then, an IT0 glass substratewas carefully immersedin the red chloroform solution for 8 h and 45 min at room temperature. The substrate was then withdrawn, transferred into a neat CHCb solution, and cleaned by a 2-min Sonication. This cleaning procedure was repeated three times. Then the coated substrate was rinsed with acetone and dried in air. Submonolayer Formation of RuOEP(C0)L (L = CPTS) on the Surfaces of Quartz Substrates. A 10-mL portion of dry CHCls was added to a Schlenk tube containing 18.2 mg of RuOEP(CO).THF. Gentle sonication was applied periodically to dissolvethe RuOEP(CO).THF complex. After all the substrate was dissolved, 4 p L of CPTS was added by syringe under an inert atmosphere. The red solution was mixed by shaking the Schlenk tube occasionally for 0.5 h. Two cleaned quartz substrates were immersed in the red solution for 45 h at room temperature, and the substrates were separated from each other by a spacer to ensure good surface-solution contact. The substrates were then withdrawn, transferred into a neat CHCla solution, and cleaned by a 2-min sonication. This cleaning procedure was repeated three times, and then the coated substrates were rinsed with acetone and dried in air. Submonolayer Formation of a Ruthenium Dicarbonyl Cluster on the Surface of Si Wafers Prom RuOEP(CO).THF in the Presence of CPTS. To a Schlenk tube containing 18.2 mg of RuOEP(CO).THF was added 10 mL of dry CHCb. Gentle sonication was applied in order to dissolve all the RuOEP(CO).THF complex promptly. After all the complex was dissolved, 5 pL of CPTS was added by syringe in an Ar atmosphere. The red solution was mixed well by shakiig the Schlenk tube occasionally for 0.5 h. Two cleaned Si wafer substrates were immersed in the red solution for 3 days at room temperature, and the substrates were again separated by a spacer. The substrates were then withdrawn and transferred into aneat CHCls solution and cleaned by a 2-min Sonication. This cleaning procedure was repeated three times. Finally, the coated substrates were rinsed with acetone and dried in air.
Formation of Metal Clusters on Ligand Monolayers
Submonolayer Formation of a Ruthenium Dicarbonyl Cluster on the Surfaceof Si Wafers fromRuOEP(C0)-THF. The same procedure was repeated but without the addition of 5 pL of CPTS to the RuOEP(CO).THF solution. Formation of a Ru(C0)rL (L = CPTS) Cluster on Top of Self-Assembled CPTS Ligand Monolayers. Triruthenium dodecacarbonyl, 3.2 mg, was dissolved in 10 mL of benzene as received from Aldrich. A Si wafer coated with a monolayer of CPTS on either side was immersed in this solution. After 12 h, the coated substrateswere transferred into a CHCb solutionand cleaned by repeated sonication. After cleaning, the substrates were thoroughly rinsed with acetone and dried in air. Monolayer Formation of a Ru(CO)~(HOSI~urface) Cluster on the Surface of Si Wafers. The procedure was repeated with a clean Si wafer of a monolayer-coated wafer. Formation of a Ru,(CO)lo(pCO)L (L = CPTS) Cluster on Top of the Self-Assembled CPTS Ligand Monolayer. Triruthenium dodecacarbonyl, 3.2 mg, was dissolved in 10 mL of dry benzene with a few days of stirring. Si wafers coated with a monolayer of CPTS on either side were immersed in this solution. After 67 h, the coated substrates were transferred into a CHClS solution and cleaned by repeated sonication. After cleaning,the substrateswere thoroughly rinsed with acetone and dried in air. The same procedures were carried out for both clean Si wafers and OTS-coatedwafers, and no measurable metal cluster was observed on the surfaces. Formation of a Osa(CO)&-CO)L(L = CPTS) Cluster on Top of the Self-AssembledCPTS Ligand Monolayer. Triosmium dodecacarbonyl, 4.2 mg, was dissolved in 10 mL of dry CHCIa with a few hours of stirring. A Si wafer coated with a monolayer of CPTS on either side was then immersed in this solution. After 10 days, the coated substrates were transferred into a CHCls solutionand cleaned by repeated sonication. After cleaning,the substrateswere thoroughly rinsed with acetone and dried in air. The same procedures were carried out for both clean Si wafers and OTS-coatedwafers, and no measurable metal cluster was observed on the surfaces.
Results and Discussion A. Surface Characterization. Polarized variableangle internal attenuated total reflection infrared spectroscopy is a powerful tool for analyzing surface-bound monolayers. Traditional techniques for analyzing monolayer ultrathin films involve either reflection at fixed grazing incident angles typically from metallic surfaces or waveguidingmultireflection using expensiveoptical prisms whose surfaces are modified by chemical reactions. The former technique is limited to infrared reflector surfaces such as Au, Ag, and Cu, which prevent collecting a spectrum in the s-polarized geometry because in this geometry the electrons in those metals actively conduct until they destroy the externally applied electric field of the IR light wave. Therefore, molecular orientation cannot be determined experimentally by external reflection spectroscopy on conductive surfaces. Furthermore, the incident radiation is limited to grazing angle only. This technique exaggeratesthe IR response from the extreme surface outer layer rather than from the bulk film because of the grazing incident angle. Although the latter tech 'que gains more sensitivity by waveguiding the multire ection through optical prisms, it suffers from expensive optical quality prisms, fixed incident angle and optical aperture, and handling difficulties in vigorous chemical reactions. These issues have greatly limited the practical usage of such optical prisms. We describe here the application of a versatile technique-polarized variable-angle internal attenuated total reflection-which is used to detect ultrathin surface monolayer assemblies. In this method, self-assembled monolayerthin f i i grown on the ( 100)surface of p-doped Si wafers were pressed against a ZnSe hemispherical crystal by a miniature pressure device to ensure optical contact.
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Wavenumber (cm ) Figure 1. Polarized variable-angle internal attenuated total reflection infrared spectra of (3-cyanopropy1)trichlorosilane covalently bound to the native oxide on the (100) surface of a Si wafer as a function of the incident angle and polarization (1024 scans, 8-cm-1resolution). The baseline was corrected to zero absorbance and offset for display purposes. Optical contact can be confirmed by the observation of a black spot surroundedwith interference rings in the center of the ZnSe hemisphere crystal. Repeatedly pressing the coated Si wafer to the ZnSe hemisphere crystal results in identical infrared spectra for both p- and s-polarized geometries; therefore, the monolayer is apparently not damaged or aligned by pressing it against the ZnSe crystal. Because the substrate is not required to be a conductor, bothp- and s-polarized spectra can be collected, and hence, molecular orientation information can be determined experimentally. Figure 1showsthe PVAI-ATR-IR spectra of a monolayer of (3-cyanopropy1)trichlorosilane(CPTS) covalently bound to the native oxide on the surface of a Si wafer. In addition to the strong cyano, C=N, mode at 2245 cm-l, two CH2 vibrational bands were observed at 2886 and 2940 cm-1 corresponding to the respective symmetric, v,(CH2), and asymmetric, v,(CH2), stretching vibrations. The positions of these surface IR bands, which are slightly higher in energy than generally observed for CH2 modes, are identical to those of vibrational frequencies found in the liquid state for CPTS. First, the strongest infrared absorptions were observed in the vicinity of 30-45O for both CH2 and C=N vibrations because the total internal reflection typically occurs in this region. The absorbance A of thin-film monolayers is governed by integration of the electric field of the evanescent wave.ll By assuming the organic thin film has a typical refractive index of n f b (11) (a) Harrick, N. J.Znternul TotulReflectionZnfruredSpectroecopy; Interscience Publishers: New York, 1967. (b) Harrick, N. J.; Mirabella, F. M. InternalReflectionSpectroscopy Review und Supplement,Harrick Scientific Corp.: New York, 1985.
Li et al.
1180 Langmuir, Vol. 10, No. 4, 1994 = 1.5,a critical incident angle of 8, = 39’ is obtained. Below the critical incident angle, the intensity of the reflection light ray is attenuated due to refraction into the Sisubstrate; thus, weak IR bands are observed. At critical incident angle geometry, there is a strong interaction between the infrared radiation and the interface of the ZnSe hemisphere crystal and the Si wafer, and this yields large infrared absorptions. Above the critical incident angle, the electric field intensity of the evanescent wave diminishes, yielding low IR absorptions. Second, at a geometry close to normal incidence, we found that innerlayer CH2 absorptions are even stronger than C=N vibrations at 2245cm-l. In contrast, as the grazing incident angle was approached, only the outer surface layer, Le., the C=N layer, was observed in the present PVAI-ATRIR geometry and the CH2 bands of the bulk film at 2940 and 2886 cm-l disappeared completely. At 0 = 60°,the CH2 bands at 2940 and 2886 cm-l appear as derivatives rather than as normal peaks. These occurrencesare usually observed around extreme incident angles (normal or grazing incidence) and have been explained by Harrick.“ Orientation of a CPTS overlayer on the surface of a Si wafer can be qualitatively deduced from PVAI-ATR-IR techniques. By using the theoretical model described by Milosevic,12the oscillator strength of a particular mode parallel
