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Langmuir 1994,10,3940-3942
Osmium Carbonyl Cluster Growth on Self-Assembled (3-Mercaptopropy1)trimethoxysilaneon a Gold Surface A. Morneau, A. Manivannan, and C. R. Cabrera* Department of Chemistry and Materials Research Center, P.O. Box 23346, University of Puerto Rico, Rio Piedras Campus, Sun Juan, Puerto Rico 00931 -3346 Received June 30, 1994. In Final Form: September 2, 1994@ Surface modification of metal surfaceswith inorganic clusters is very important for electrocatalysis. We present here the growth of an osmium carbonyl cluster on a self-assembled alkylsiloxanethiol monolayer (1)with a self-assembled on a Au surface. This has been done by the surface reaction of OS~(CO)~~(NCCH~) monolayer of (3-mercaptopropyl)trimethoxysilane(MPS) on Au( 111). The determination of the surface morphology and chemical composition has been done by using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS).The reaction of the osmium cluster with the alkylsiloxanethiol monolayer destroys the well-orderedthiol surface by forming several cluster aggregates which range from 10 to 22 A in diameter. In recent years, metal-supported complexes on inorganic oxides have received considerable interest as important catalysts in the petroleum Several different heterogeneous catalysts, such as metal-silica1ayb*e*2 (alumina1c-f92d-for magnesium oxidele>g)and metalsubstrate3,have been used. Recently, metal clusters have been covalently bound to fuse quartz and on the native oxide of the (100) p-doped Si wafer.4 Our interest is on the surface modificationof surfaces with inorganic clusters that can be used in electrocatalytic applications. We present here the growth of a triosmium carbonyl cluster on a self-assembled alkylsiloxanethiol monolayer on a Au surface. This was done by the surface reaction of Os3(CO)11(NCCH3)5(1)with a self-assembled monolayer of (3-mercaptopropy1)trimethoxysilane (MPS) on Au(111). The determination of the structural surface modification of the Os cluster on Au surface has been done by using scanning tunneling micro~copy69~ and X-ray photoelectron spectroscopy.8 The MPS self-assembled monolayer was prepared as follows. A gold bead or a gold on mica surface, both having (111)facet^,^ was placed in a 1mM solution of MPS, in ethanol, for ca. 12 h. This MPS modified gold surface, which we will call MPS/Au, was thoroughly washed with ethanol and air-dried before being analyzed by STM and XPS. Parts A and B of Figure 1shows the STM pictures Abstract published inAdvame ACS Abstracts, October 15,1994. (1) (a)Roberto, D.; Psaro, R.; Ugo, R. Organometallics 1993,12,2292. (b) Roberto, D.; Psaro, R.; Ugo, R. J. Organomet. Chem. 1993,451,123. (c)Kou, Y.; Wang, H.-L.; Te, M.; Tanaka, T.; Nomura, M. J.Catal. 1993, 141,660. (d) Kawi, S.; Chang, J.-R.; Gates, B. C. J. Phys. Chem. 1993, 97,5375. (e) Chang, T.; Cheng, C. P.; Yeh, C. J.Phys. Chem. 1992,96, 4151. (0 Deutsch, S. E.; Chang, J.-R.; Gates, B. C. Langmuir 1993,9, 1284. (g)Van Zon, F. B. M.; Maloney, S. D.; Gates, B. C.; Koningsberger, D. C. J. Am. Chem. SOC.1993,115, 10317. (2) (a) Choplin, A.; Besson, B.; D'Ornelas, L.; Sanchez-Delgado, R.; Basset, J. M. J. Am. Chem. SOC.1988, 110, 2783. (b) Walter, T. H.; Frauenhoff, G. R.; Shapley, J. R.; Oldfield, E. Inorg. Chem. 1991,30, 4732. (c) Walter, T. H.; Frauenhoff, G. R.; Shapley, J. R.; Oldfield, E. Inorg. Chem. 1988,27,2563. (d) Psaro, R.; Ugo, R.; Zanderighi, G. M.; Besson, B.; Smith, A. It;Basset, J. M. J. Organomet. Chem. 1981,213, 215. (e) Smith, A. K; Besson, B.; Basset, J. M; Psaro, R.; Fusi, A.; Ugo, R. J. Organomet. Chem. 1980,192, C31. (D Frauenhoff, G. R. Coord. Chem. Rev. 1992,121,131. (g) Lopez, T.; Bosch, P.; Moran, M.; Gomez, R. J. Phys. Chem. 1993,97, 1671. (h) Prignano, A. L.; Trogler, W. C. J.Am. Chem. SOC.1987,109,3586. (i)Adams, R. D.; Cortopassi, J. E.; Pompeo, M. P. Inorg. Chem. 1991,30,2960. (3) (a) Zubimendi, J. L.; VBzquez, L.; O c h , P.; Vara, J. M.; Triaca, W. E.; Salvarezza, R. C.; Arvia, A. J. J.Phys. Chem. 1993,97,5095. (b) Clark, G. W.; Kesmodel, L. L. J. Vac. Sei. Technol. B 1993, 11, 131. (4) Li, DeQuan; Moore, Louise W.; Swanson, Basil 1.Langmuir 1994, 10,1177. . (5) Johnson, B. F. G.; Lewis, J.;Pippard, D. A. J. Chem. SOC.,Dalton Trans. 1981,407. @
igure 1. (A) STM image (5 nm x 5 nm) of a Au(111)surface (i = 19.33nA, Vbias = 7.6 mV). (B) STM image (35nm x 35 nm) of the MPS/Au surface (i = 11.3 nA, Vbias = -147.5 mV). Both
images were taken in a constant height mode.
of the Au(ll1) and MPS/Au surfaces, respectively. The STM image of the MPS/Au surface showed a n ordered (6) Bonnell, D. A., Ed. Scanning Tunneling Microscopy and Spectroscopy: Theory, Techniques,and Applications; VCH Publishers, Inc.: New York, 1993. (7) Experimental STM images were obtained with a commercial scanning probe microscope, NanoScope I11 instrument (Digital Instruments). Pt/Ir nanotips were used. Both constant current or constant height images were taken. (8)XPS spectra were recorded on Perkin-Elmer instrumentoperating at 15.0 kV and 300 mA with a Al K a monochromated beam. A pass energy of 25 eV was used for high-resolution spectra. (9)Snyder, S. R. J. Electrochem. SOC.1992,139, 5C.
0743-7463/94/2410-3940$04.50/0 0 1994 American Chemical Society
Langmuir, Vol.10,NO.11, 1994 3941
Letters
Table 1. XPS Binding Energies (eWof the Modified Surfaces and Cluster@ surface or compound
Au(4f7/2) Si(2p) S(2p) C(ls) os(4f7n)
( 1)MPS/Au
83.7
(2) OalMPSIAu
83.8
(3)oSdCo)i2 (4) 0~3(CO)lo@-H)-
102.3 163.0 284.5 102.2 162.5 287.0 287.9 102.0
287.6
51.7 51.9 51.7
b-OSi(CH(CHdd31 (5) Os(literature)
Figure 2. Schematic representation of the MPS on a Au surface. columnlike structure over a surface area of 1225nm2with a line periodicity varying between 23 and 33A. This STM observation is quite different from the previously reported n-alkanethiollAu results.1° In the n-alkanethiol case, a hexagonal structure was reported with a spacin of 5 A between the nearest S atoms, representing a ( 3 x d 3 ) R30" adlayer structure onAu(ll1). The MPS/Au surface structure may be due to a A u ( l l 1 ) surface reconstruction or annealing induced by the alkylsiloxanethiol. Au(ll1) has surface reconstructions ofthe typep x d3 facet where p may be 22-30."-13 The corrugation amplitude of the line pairs of this type of Au surface reconstruction is 0.2 A and the periodicity is between 63 and 66 A. In our case, the height corrugation of the MPSIAu surface is around 2 A, which is 1order of magnitude larger than the expected value for t h e p x 4 3 facet. However, this may be due to the alkyl chain length of the MPS. On the other hand, the periodicity of the lines varied from 23 to 33 A, which is half of thep x 43 surface reconstruction. This periodicity, however, may be the intra air distance. The pair to pair distance would be 46-66 , which is close to the values reported for p x 4 3 surface reconstruction. This type of surface reconstruction is observed when [Ru(bpy)z(bpy(CH&bpy)l is adsorbed on Au(lll).14 Thus, we believe that MPS adsorption is causing a surface reconstruction or annealing of the Au(ll1) surface. STM images ofn-alkanethiol self-assembled monolayers always show h01es.l~In our case, the STM images of the MPS/Au surfaces did not show any pit holes. This absence of pit holes in our STM images may be due to the formation of a polymeric siloxane-type network on the surface, such as in silica material. Trace amounts of water may cause the trimethoxysilane tail groups of the MPS to react with each other and form a polymeric siloxane-type network. Figure 2 shows a schematic representation of a polymeric siloxane surface. The lacking of molecular resolution for the MPS/Au surface may be due, as well, to the siloxane network a t the surface of the monolayer. This surface was analyzed by XPS. The Si (2p)binding energy, a t 102.3 eV, was present on the XPS analysis16(see Table 1)of the MPS/Au surface. This is similar to the Si binding energy in ~i1icates.l~ In addition, a weak and broad S(2p) binding energy peak, a t 163.0 eV, was observed. This is within
J
B
(10) (a) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. SOC. 1991, 113, 2805. (b) Kim,Y.-T.;McCarley, R. L.; Bard, A. J. J . Phys. Chem. 1992,96, 7416. (11) Barth, J. V.; Brune, H.; Ertl, G. Phys. Rev. B 1990,42, 9307. (12) Schott, J. H.; White, H. S. Langmuir 1992, 8, 1955. (13) Oden, P. I.; Tao, N. J.; Lindsay, S. M. J . Vac. Sci. Technol. B 1993. 11 (2). ~... ,-.. 137.. (14) Schoot, J. H.;Arana,C.R.;Abmfia,H.D.; Petach,H. H.;Elliott, C. M.; White, H. S. J . Phys. Chem. 1992, 96, 5222. (15) SchGnenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G . J. Lungmuir 1994, 10, 611. (16) The XPS results were normalized with respect to the C(1s) binding energy peak at 284.57 eV. (17)Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook ofx-rayPhotoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1992, and reference therein. I
~~
50.7 The XPS (ESCA) results were normalized with respect to the C(ls) binding energy peak at 284.5 eV.
the value expected for the RS species on Au, where R is a n alkabe group.lS As expected for this surface, the Au(4f7~) binding energy peak was found at 83.7 eV.18The COS) binding energy was found a t 284.5 eV, which is representative of the CH2 carbon of the propyl chain. No indication of OC& moieties was found. This means that the hydrolysis of the MPS monolayer on gold occurs. These XPS results confirm the presence of MPS on Au and the presence of Si as a silicate-type structure. The clusters O S ~ ( C O(2) ) ~and ~ O S ~ ( C O ) ~ O ( N C C(3) H~)~ are known to react with silica and silanol derivatives to form Oss(CO)&-H)@-OSi-3 m ~ i e t i e s . ~ * - Thus, t ~ J ~ these clusters are expected to react with the siloxane network present on the MPS/Au surface. In order to do the surface reaction, the MPS/Au surface was placed, for ca. 9 h, in a 0.6 mM solution of 1 in dichloromethane. The resulting modified surface was thoroughly washed with dichloromethane and air-dried before being analyzed by STM andXPS. An STM image of this surface is shown in Figure 3A. The resulting surface is quite different from the STM image of the MPS/Au surface shown in Figure 1B. The ordered column-like structure of the MPS surface was modified by the reaction with 1. The formation of randomly oriented clusters with sizes ranging from 10 to 22 in diameter and with a corrugation amplitude of up to 7 A were found. These different island sizes indicate that several cluster moleculesmight react together in order to form a larger aggregate. The size of an osmium carbonyl cluster, such as Os~(CO)lo~-H)~-OSiR~),lga~c is around 10 A in diameter. The sizes of the islands, seen with the STM, seem to increase with a longer contact time of the MPS/Au surface with the cluster solution. For example, when the MPS/Au surface was placed in the cluster solution for over 12 h, the island size ranged from 26 to 55 A (see Figure 3B). These results show that the cluster island size depends on the time of exposure of the MPS/ Au surface to the cluster solution. The surface obtained after the reaction of the osmium cluster with the MPS/Au surface was analyzed by XPS (see Table 1). This Os cluster modified surface will be called Os/MPS/Au. The surface analysis showed the presence of Au, S, Si, 0, C, and Os. The Au(4f7,2), S(2p), and Si(2p)bindingenergypeakswere found at 83.8,162.5, and 102.2 eV, respectively. These values are similar to those found in the MPS/Au surface. This suggests that we have the same type of Au, S, and Si on both surfaces.
a
(18)(a) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.;Nuzzo, R. G. J . Am. Chem. SOC.1991,113,7152. (b) Ihs, A.; Uvdal, K.; Liedberg, B. Langmuir 1993, 9, 733. ( c ) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323. (d) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D. Langmuir 1992,8, 2707. (e) Huang, J.; Hemminger, J. C. J . Am. Chem. SOC.1993, 115, 3342. (0 Nuzzo, R. G.;Zegarski, B. R.; Dubois, L. H. J . Am. Chem. SOC. 1987,109, 733. (g) Zak, J.;Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772. (19) (a) D'Ornelas, L.; Choplin, A.; Basset, J. M.; Hsu, L.-Y.;Shore, S. Nouu. J . Chim. 1985, 9, 155. (b) Besson, B.; Moraweck, B.; Smith, A. K.; Basset, J. M.; Psaro, R.; Fusi, A.; Ugo, R. J. Chem. SOC.,Chem. Commun. 1980,569. (c) Liu, J.;Wilson, S. R.; Shapley, J. R.; Feher, F. J. Inorg. Chem. 1990,29, 5138.
3942 Langmuir, Vol. 10,No. 11, 1994
Figure 3. (A)STM image ofthe osmium cluster modified MPS/ Au surface after (A) 9 (40 nm x 40 nm) (z = 11.5 nA, Vbias = -862.5 mV) and (B) 12 (130 nm x 130 nm) (i = 480 PA, Vbias = 100 mV) h of reaction time. Both images were taken at a constant current mode.
For the carbon (1s)binding energy, we had some differences in peak shape. For the Os/MPS/Au surface we found a broad shoulder, at 287.0 eV, for the C(1s)binding energy. Based on the literature results this C(1s)binding energy
Letters is characteristic of an organic carbonyl g r ~ ~ We p . ~ believe that this shoulder is due to the CO ligands of the Os3(CO)10moieties. The Os(4f7&) binding energy peak was at 51.7 eV. The osmium present on the surafce is not metallic osmium since the metallic Os has a binding energy of 50.7 eV.16 Due to the limited XPS data on Os clusters in the literature, it was difficult to assign the oxidation state for the observed Os binding energy. To overcome this problem, films of triosmium cluster 2 and O S ~ ( C O ) ~ ~ ( M - H ) ~ - O S ~ ( C H ( C H(4) ~ ) on ~ ) ~gold ] ~ ~were analyzed by XPS. The films were prepared by placing a drop of a cluster saturated solution, in CH2C12, on a gold surface. The surface was air-dried before being analyzed by XPS. The C(1s)binding energypeak for the CO ligands of the clusters 2 and 4 were found a t 287.9 and 287.6 eV, respectively. These values do correspond to the energy range observed for the organic carbonyls. The OS(4f712) binding energy for 2 and 4 was found a t 51.9 and 51.7 eV, respectively, and the Si (2p) binding energy for 4 was observed a t 102.0 eV. These two Os(4f712) binding energies and the Si(2p) binding energy are very similar to the values observed for the Os/MPS/Au surface. These XPS data suggest that the Os/MPS/Au surface may have osmium clusters with similar chemical nature as that of compound 2 and 4. In summary, we have shown for the first time that cluster 1 reacts with the self-assembled MPS/Au surface to form random island growth of osmium clusters. The most important fact is the molecular level reaction and time-dependent modification of a self-assembled MPS on Au surface by the reaction with cluster 1. We have presented a method to modi@metallic surfaces,with metal carbonyl clusters, which can be used for electrocatalytic processes.
Acknowledgment. We acknowledge the financial support of NSF-EPSCoR, Grant Number EHR-9108775, NIH-MBRS, Grant Number SO6-GMO8102-21,and NSFRIMI, Grant Number HRD-9353197. (20) (a)Bain, C. D.; Troughton, E. B.; Tao,Y.-T.; Evall, J.;Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC.1989,111, 321. (b) Song, S.; Clark, R. A.; Bowden, E. F.; Tarlov, M. J. J.Phys. Chem. 1993,97,6564. (c) NUZZO, R. G.; Dubois, L. H.; Allara, D. L. J.Am. Chem. SOC.1990, 112, 558. (21) Morneau, A, Cabrera, C. R. Unpublished results.
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