Interactions between the Keggin-Type Lacunary Polyoxometalate, α

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Langmuir 2003, 19, 8934-8942

Interactions between the Keggin-Type Lacunary Polyoxometalate, r-SiW11O398-, and Electrode Surfaces Jongwon Kim and Andrew A. Gewirth* Department of Chemistry and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received April 25, 2003. In Final Form: July 25, 2003 Cyclic voltammetry, scanning tunneling microscopy (STM), and surface-enhanced Raman spectroscopy (SERS) are used to investigate the interactions between lacunary polyoxometalate, R-SiW11O398-, and glassy carbon, Au(111), and Ag(111) electrode surfaces. The cyclic voltammetry and STM measurements show that R-SiW11O398- anions adsorb strongly only on Ag(111). R-SiW11O398- species adsorbed on Ag(111) retains its redox activity on the Ag surface. STM images reveal the formation of monolayers on Ag(111), which have hexagonal adlattice structures with spacings between 1.22 and 1.37 nm, somewhat larger than the 1.0-nm spacing observed for the complete Keggin ion. SERS measurements show that R-SiW11O398is present on the Ag surface at potentials between 0.0 and -0.9 V. A model rationalizing the larger spacing is proposed.

1. Introduction Polyoxometalates (POMs), constituting a large class of metal oxide molecules, are known to have a variety of sizes, structures, electrochemical properties, and chemical reactivities.1 The modification of electrode surfaces with POMs has been extensively studied because POMs have many practical applications such as catalysis,2,3 molecular materials,4 and corrosion inhibition.5 POMs have been shown to adsorb on highly oriented pyrolytic graphite (HOPG),6,7 Au,7,8 and Hg9 electrode surfaces from their acidic solutions. It is also known that POMs can be immobilized on electrode surfaces by either achieving multilayer structures via electrostatic interactions between POMs and cationic species10,11 or by electrodeposition onto electrode surfaces.12,13 In these studies, it should be noted that the electrochemical response of the POM-modified electrode shows the same features as that of the solution species. On Ag surfaces, the behavior of POMs is also quite interesting. Our group showed that R-dodecatungstosilicate anions, R-SiW12O404-, passivate the Ag(111) electrode toward subsequent solution redox chemistry.14 We showed that R-SiW12O404- forms ordered monolayer arrays on Ag(111) surfaces from an acidic aqueous solution.15 In situ * Author to whom correspondence should be addressed. Telephone: 217-333-8329. Fax: 217-333-2685. E-mail: [email protected]. (1) Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: Berlin, 1983. (2) Mizuno, N.; Misono, M. Chem. Rev. 1998, 98, 199-217. (3) Sadakane, M.; Steckhan, E. Chem. Rev. 1998, 98, 219-237. (4) Coronado, E.; Gomez-Garcia, C. J. Chem. Rev. 1998, 98, 273296. (5) Katsoulis, D. E. Chem. Rev. 1998, 98, 359-387. (6) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1993, 354, 295-304. (7) Rong, C. Y.; Anson, F. C. Inorg. Chim. Acta 1996, 242, 11-16. (8) Keita, B.; Nadjo, L.; Belanger, D.; Wilde, C. P.; Hilaire, M. J. Electroanal. Chem. 1995, 384, 155-169. (9) Rong, C. Y.; Anson, F. C. Anal. Chem. 1994, 66, 3124-3130. (10) Ingersoll, D.; Kulesza, P. J.; Faulkner, L. R. J. Electrochem. Soc. 1994, 141, 140-147. (11) Kuhn, A.; Anson, F. C. Langmuir 1996, 12, 5481-5488. (12) Wang, B.; Dong, S. J. Electroanal. Chem. 1992, 328, 245-257. (13) Tang, Z. Y.; Liu, S. Q.; Wang, E. K.; Dong, S. J.; Wang, E. B. Langmuir 2000, 16, 5806-5813. (14) Lee, L.; Gewirth, A. A. J. Electroanal. Chem. 2002, 522, 11-20. (15) Ge, M.; Zhong, B.; Klemperer, W. G.; Gewirth, A. A. J. Am. Chem. Soc. 1996, 118, 5812-5813.

scanning tunneling microscopy (STM) images of the adsorbed monolayers on Ag(111) revealed that R-SiW12O404forms a four-fold square adlattice, which was ascribed to the fact that the Td-symmetric R-SiW12O404- anion was arranged with its S4 axis perpendicular to the Ag(111) surface.16,17 A similar monolayer arrangement of AsMo11VO404- anions was observed on Au(111) surfaces, which also showed square adlattices.18 Recently, we showed that R-PVW11O404- forms monolayers on Ag(111) by exposure of the surface to R-PVW11O404- in an acidic aqueous solution.19 In this system, the STM images evinced hexagonal ordered layers, which was ascribed to R-PVW11O404- anions being oriented such that a pseudo C3 axis is perpendicular to the Ag(111) surface. The different structures observed for different POMs on Ag raise the question concerning the origin of this structural behavior. In this paper, we examine factors determining the structure of monolayers of POMs on electrode surfaces by constructing monolayers from the lacunary derivative of the Keggin structure R-SiW12O404anion. Although there have been many studies devoted to the modification of electrode surfaces with POMs, most of them have focused on two structural types of POMs, the Keggin- and Dawson-type structures. In particular, the STM studies on the monolayer structures of POMs have been reported only for complete Keggin-type POMs. The purpose of this work is to investigate the interactions between electrode surfaces and POMs with different structures than those studied previously. For this purpose, Keggin-type lacunary POMs derived from the Keggin structure are utilized, and their interactions with various electrode surfaces are investigated in this work. As shown in Figure 1, lacunary POMs are deficient in a single metaloxygen unit from their complete Keggin structures. They have received much attention as precursors of metal(16) Ge, M.; Gewirth, A. A.; Klemperer, W. G.; Wall, C. G. Pure Appl. Chem. 1997, 69, 2175-2179. (17) Ge, M.; Niece, B. K.; Wall, C. G.; Klemperer, W. G.; Gewirth, A. A. Mater. Res. Soc. Symp. Proc. 1997, 451, 99-108. (18) Tang, Z. Y.; Liu, S. Q.; Wang, E. K.; Dong, S. J. Langmuir 2000, 16, 4946-4952. (19) Powell, J. D.; Gewirth, A. A.; Klemperer, W. G. In Polyoxometalate Chemistry From Topology via Self-Assembly to Applications; Pope, M. T., Muller, A., Eds.; Kluwer Academic Publishers: Dordrecht, 2001; pp 329-334.

10.1021/la034708d CCC: $25.00 © 2003 American Chemical Society Published on Web 09/03/2003

R-SiW11O398- and Electrode Surface Interactions

Figure 1. Ball-and-stick models of (A) R-SiW12O404- and (B) R-SiW11O398- anions. W atoms are represented by gray circles, O atoms by green circles, and Si atoms by blue circles.

substituted POMs, which have electrocatalytic activities on electrode surfaces.20-22 It is also known that lacunary POMs can be modified with organic groups23-25 and can be used as building blocks for the controlled assembly of POMs.26 In addition to these applications, lacunary POMs have several unique properties distinct from their parent structures, such as reduced symmetry, higher charge density, and increased reactivity.1 In this report, the interactions between one of the lacunary POMs, R-SiW11O398-, and electrode surfaces have been investigated. The electrochemical responses of R-SiW11O398- on three different electrode surfaces are examined and compared with each other. For the Ag(111) electrode surface, on which R-SiW11O398- forms ordered monolayers, the detailed interaction mechanism was investigated by STM and surface-enhanced Raman spectroscopy (SERS). On the basis of these results, a possible model for the monolayer structure is proposed. 2. Experimental Section All solutions were prepared using purified water (Milli-Q UV plus, 18.2 MΩ cm). The supporting electrolyte was acetate buffer, prepared by partial neutralization of 0.1 M CH3COOH (99.9985%, (20) Toth, J. E.; Anson, F. C. J. Am. Chem. Soc. 1989, 111, 24442451. (21) Sun, W. L.; Liu, H. Z.; Kong, J. L.; Xie, G. Y.; Deng, J. Q. J. Electroanal. Chem. 1997, 437, 67-76. (22) Sun, W. L.; Zhang, S.; Liu, H. Z.; Jin, L. T.; Kong, J. Anal. Chim. Acta 1999, 388, 103-110. (23) Knoth, W. H. J. Am. Chem. Soc. 1979, 101, 759-760. (24) Judeinstein, P.; Deprun, C.; Nadjo, L. J. Chem. Soc., Dalton Trans. 1991, 1991-1997. (25) Katsoulis, D. E.; Keryk, J. R. U.S. Patent 5,548,052, 1996. (26) Sadakane, M.; Dickman, M. H.; Pope, M. T. Angew. Chem., Int. Ed. 2000, 39, 2914-2916.

Langmuir, Vol. 19, No. 21, 2003 8935 Alfa Aesar, Ward Hill, MA) to pH 4.7. The pH of the Na2SO4 electrolyte solution was adjusted to 4.7. R-K8SiW11O39 was prepared following published procedures27 and recrystallized from hot water. The typical R-K8SiW11O39 concentration was 0.5 mM. A commercially available glassy carbon electrode (Bioanalytical Systems, West Lafayette, IN) was employed. The surface was mechanically polished with Al2O3 powder and rinsed with Milli-Q water following sonication. HOPG was obtained from Advanced Ceramics (ZYH grade, Cleveland, OH), and clean surfaces were prepared by cleaving a fresh surface using Scotch tape to give a flat, shiny surface immediately before use. Ag electrodes for electrochemical measurements were prepared from a Ag(111) single crystal (Monocrystals, Richmond Heights, OH) etched according to published procedures using a CrO3/HCl solution.28 Ag(111) films for STM experiments were obtained in a bell-jar system by evaporating Ag (99.999%, Alfa Aesar Premium shot) onto freshly cleaved V2 grade 4 mica (Asheville Mica, Newport News, VA) at 10-7 Torr. After obtaining a film of 200-nm thickness, the surface was annealed at about 145 °C for 1 h. Atomic resolution in situ STM images revealed the expected (111) texture.29 Ag electrodes for SERS measurement were prepared from a Ag(111) crystal electrochemically roughened using oxidation-reduction cycles in 0.1 M KCl according to published procedures.30 Au(111) surfaces were prepared by annealing Au-coated glass samples (Dirk Schro¨er, Inc., Germany) with a high-purity scientific grade H2 flame prior to use.31 After this treatment, Au(111) terraces with domain sizes larger than 100 nm were readily achieved. STM images were obtained with a NanoScope III E electrochemical STM microscope (ECSTM) equipped with a fluid cell (Digital Instruments, Santa Barbara, CA). The tip was formed from a Pt/Ir wire coated with polyethylene except at the very end of the tip to minimize the faradaic background current. SERS spectra were obtained by using a modification of an in situ cell described previously.32 An Ar ion laser (Coherent) provided the Raman excitation at 514.5 nm, and the scattered radiation was collected with a camera lens (85 mm, Canon) and focused at the entrance slit of a three-stage monochromator (1877 Triplemate, Spex). The system was allowed to equilibrate for 2 min at each potential before acquiring spectra. The spectral resolution was estimated to be 5 cm-1. The baseline for each SERS spectrum was corrected prior to presentation. Cyclic voltammograms were obtained using a Pine AFRDE-5 potentiostat (Pine Instrument Co., Grove City, PA) and a twocompartment cell that was purged with Ar prior to use. A freshly flame-annealed Pt or Au wire was used as the counter electrode. A gold oxide wire was used as the reference electrode for STM measurements and a Ag/AgCl reference was used for the voltammetry and SERS measurements. All potentials are reported relative to the Ag/AgCl reference electrode (+0.20 vs the normal hydrogen electrode). X-ray photoelectron spectroscopy (XPS) was performed on a Physical Electronics PHI 5400 spectrometer (Eden Prairie, MN) or a Kratos Axis Ultra spectrometer (Kratos Analytical, Inc., Chestnut Ridge, NY) equipped with a sample rotation stage.

3. Results 3.1. Electrochemical Response. Figure 2 shows cyclic voltammagrams of three different electrode surfaces in a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer solution. Figure 2A was obtained on a glassy carbon electrode and shows two redox waves with midpoint potentials Emid ) (27) Teze, A.; Herve, G. Inorg. Synth. 1990, 27, 85-96. (28) Smolinski, S.; Zelenay, P.; Sobkowski, J. J. Electroanal. Chem. 1998, 442, 41-47. (29) Obretenov, W.; Schmidt, U.; Lorenze, W. J.; Staikov, G.; Budevski, E.; Carnal, D.; Muller, U.; Siegenthaler, H.; Schmidt, E. J. Electrochem. Soc. 1993, 140, 692-703. (30) Leung, L. W. H.; Gosztola, D.; Weaver, M. J. Langmuir 1987, 3, 45-52. (31) Will, T.; Dietterle, M.; Kolb, D. M. In Nanoscale Probes of the Solid/Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; Kluwer Academic Publisher: Dordrecht, 1995; pp 137-162. (32) Biggin, M. E. Ph.D. Thesis, University of Illinois at UrbanaChampaign, Urbana, Illinois, 2001.

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Figure 2. Cyclic voltammograms obtained from solutions containing a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer on (A) glassy carbon, (B) Au(111), and (C) Ag(111) electrodes. The scan rate was 100 mV s-1.

-0.68 and -0.88 V. Here, Emid ) (Ea + Ec)/2 and Ea and Ec are the anodic and cathodic peak potentials, respectively. The peak locations of the two redox waves on the glassy carbon electrode correspond well to the values reported in previous studies,27,33 where these two waves were assigned to two two-electron redox couples. On a Au(111) electrode, the voltammogram shows one wave with Emid ) -0.69 V, which corresponds to the first wave on a glassy carbon electrode (Figure 2B). The second redox wave recorded on the glassy carbon electrode could not be obtained on the Au electrode due to the onset of hydrogen evolution. For glassy carbon and Au electrodes, peak currents for all of the redox couples shown in parts A and B of Figure 2 follow a linear relationship with the square root of the scan rate, which indicates that all of these redox events are diffusion-controlled.34 Figure 2C shows the cyclic voltammogram of R-K8SiW11O39 on a Ag(111) electrode obtained from a solution containing a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer. The wave corresponding to this redox couple has Emid ) -0.70 V and a peak separation of 40 mV at a scan rate of 100 mV s-1. The total charge density under the cathodic peak in Figure 2C is 5.3 µC/cm2. The redox position on a Ag electrode is close to that of the first redox wave on glassy carbon and Au electrodes. A plot of the peak current versus the scan rate (not shown here) follows a linear relationship up to a scan rate of 500 mV s-1. The scan-rate dependence of the peak current as well as the small peak separation value indicates that the wave at -0.70 V on the Ag electrode corresponds to a surface-confined species.34 In a previous study, we showed that R-SiW12O404passivates the Ag surface and that one redox wave was present, corresponding to a surface-confined species with a peak position the same as that of the second redox couple of R-SiW12O404- in the solution phase.14 On the other hand, the lacunary species, R-SiW11O398-, shows a redox wave on the Ag surface with the same peak position as that of the first redox wave on glassy carbon and Au surfaces, (33) Teze, A.; Herve, G. J. Inorg. Nucl. Chem. 1977, 39, 999-1002. (34) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; John Wiley & Sons: New York, 2001.

Kim and Gewirth

Figure 3. (A) Cyclic voltammograms of a Ag(111) electrode emersed from a solution containing a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer and then reimmersed in a solution containing only a 0.1 M acetate buffer obtained at scan rates of 100, 200, 300, 400, and 500 mV s-1 (solid lines) and a bare Ag(111) electrode at 100 mV (dashed line) in a 0.1 M acetate buffer solution. (B) Scan rate dependence of the anodic (squares) and cathodic (circles) peak currents.

which indicates that R-SiW11O398- is fully redox-active on the Ag surface. To further investigate the electrochemical behavior of the surface-confined species shown in Figure 2C, the Ag electrode was emersed from the R-K8SiW11O39-containing solution following potential cycling, rinsed with water, and placed in a pure electrolyte solution. Figure 3A shows the cyclic voltammograms of the electrochemically treated Ag(111) electrode in a 0.1 M acetate buffer at various scan rates. This figure shows a redox wave with Emid ) -0.70 V, which is absent on the bare Ag(111) electrode (dashed line in Figure 3A). The electrochemical response of a R-K8SiW11O39-modified Ag(111) electrode is almost same as that observed in R-K8SiW11O39-containing solution. The total charge density under the cathodic peak at the scan rate of 100 mV s-1 was calculated to be 5.2 µC/cm2, which is nearly the same as that found in R-K8SiW11O39containing solution shown in Figure 2C. This is smaller than the expected value, 21.2 µC/cm2, for a full monolayer of a two-electron redox species, which occupies approximately 1.75 nm2 per molecule (vide infra). The peak separation between the anodic and cathodic peaks was measured as 37 mV at 100 mV s-1. At higher scan rates, the peak separation became larger and reached 65 mV at the scan rate of 500 mV s-1, which generally indicates slow electron-transfer rates or high film resistance.35 Figure 3B shows a linear relationship between the scan rates and the anodic and cathodic peak currents. Therefore, the redox couple shown in Figure 3A can be assigned to the redox wave of the surface-adsorbed lacunary species. 3.2. STM. Figure 4A shows a 70 × 70 nm STM image of a Ag(111) surface obtained in a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer solution following application of a potential of -0.20 V. This image shows partially wellordered monolayers of R-SiW11O398- anions with a domain (35) Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 114, 89115.

R-SiW11O398- and Electrode Surface Interactions

Figure 4. (A) 70 × 70 nm in situ STM image of a Ag(111) surface obtained in a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer solution at -0.20 V. Parts B-D are magnified images from part A (10 ×10 nm)

size of around 100 nm2 on top of several different Ag terraces. The size of the ordered domain is similar to that of previously reported systems such as R-SiW12O404- on Ag(111)15-17 and R-SiW12O404- on Ag(100)36 but is smaller than that found for R-PVW11O404- on Ag(111).19 The images shown here can be obtained only at negative potentials ranging from -0.05 to -0.35 V. No ordered layers can be imaged at a potential range more positive than 0.0 V. Utilizing potentials more negative than -0.35 V was precluded due to H2 evolution at the end of the Pt/It tip. In a pure 0.1 M acetate buffer solution, we could image bare Ag surfaces consisting of flat terraces and step edges as shown in Figure 5A. In addition to these ordered layers, bright spots, a few of which are marked with arrows in Figure 4A, were always seen at negative potentials. In the absence of R-SiW11O398species, these spots were not imaged in a pure acetate buffer solution (Figure 5A). As shown in Figure 5B, upon the addition of the R-K8SiW11O39-containing solution, step edges on the Ag surfaces were first decorated with R-SiW11O398- anions and the ordered layers, as well as the bright spots, started to form on the flat terrace areas. Figure 5C is a magnified image from Figure 5B, which shows well-ordered monolayers and bright spots (marked with arrows). These bright spots were not imaged at potentials more positive than -0.05 V. Although there is not enough experimental evidence to determine the identity of these spots, we note that these are found only in the presence of R-SiW11O398- anions at negative (36) Lee, L.; Wang, J. X.; Adzic, R. R.; Robinson, I. K.; Gewirth, A. A. J. Am. Chem. Soc. 2001, 123, 8838-8843.

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potentials. We speculate that the spots result from aggregation mixtures consisting of R-SiW11O398- and cations present in the system. In other work, we found that R-SiW12O404- forms multilayers on Ag surfaces and in negative potential regions.37 Therefore, these bright spots might be due to the formation of multilayers in these potential regions. To determine the structure of the ordered layers, three different domains on three different terraces in Figure 4A were magnified and are shown in Figure 4B-D. Although there are several defects apparent in Figure 4A, the zoomed-in images clearly show a hexagonal adlattice structure. The spacing between two adjacent R-SiW11O398- anions was measured to be between 1.22 and 1.37 nm. We showed that the R-SiW12O404- species, which corresponds to a complete Keggin structure, from which R-SiW11O398- is derived, forms monolayers on Ag(111) and Au(111) surfaces with square adlattice structures.15-17 The spacing of the square adlattice was 1.02 ( 0.05 nm, which matches the diameter of R-SiW12O404-.38 In the case of the R-SiW11O398- species, the monolayers on Ag(111) show spacings ranging from 1.22 to 1.37 nm, which is much larger than that of the R-SiW12O404- monolayers on Ag surfaces. Although exact crystallographic data are not available for R-K8SiW11O39, the structures of similar compounds exhibit sizes similar to that found for R-SiW12O404-.39-41 The difference in spacing and structure between R-SiW12O404- and R-SiW11O398- must reflect different ways in which the two molecules associate with the Ag surface. To compare the adsorption phenomena of R-K8SiW11O39 on Ag(111) with other electrode surfaces, similar STM experiments as described before were performed on HOPG and Au(111) surfaces. On HOPG surfaces, no ordered layers were imaged in the 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer solution. Throughout the potential ranges between -0.35 and +0.9 V, the bare HOPG surfaces were always imaged. On the Au(111) surfaces, no adsorbed layers were observed at potentials between -0.35 and +0.5 V. However, at potentials more positive than +0.5 V, irregular textures were found. Keggin-type POM anions such as R-SiW12O404- and R-PVW11O404- have been shown to form well-ordered monolayers on both Au(111) and Ag(111) surfaces.17,42 By contrast, R-SiW11O398- showed ordered monolayer structures on only Ag(111) surfaces. Finally, we examined the effect of the cation on the observed structures. In the R-H4SiW12O40/Ag system, only H+ ions exist as countercations. However, in the R-K8SiW11O39/Ag system, there are three kinds of countercations, H+, K+ and Na+ ions, where the Na+ comes from the NaOH used to partially neutralize acetic acid. In an attempt to investigate the effect of the countercation on the observed structure, acetate buffer neutralized with CsOH instead of NaOH was used as a supporting electrolyte. Because R-SiW11O398- is less soluble in a solution containing a large amount of Cs+, the supernatant of a 0.5 mM R-K8SiW11O39 + 0.1 M cesium acetate buffer solution was taken for the STM experiments. Figure 6 (37) Kim, J.; Lee, L.; Wang, J. X.; Gewirth, A. A. Manuscript in preparation. (38) Fuchs, J.; Thiele, A.; Palm, R. Z. Naturforsch., B: Anorg. Chem., Org. Chem. 1981, 36B, 161-171. (39) Fuchs, J.; Thiele, A.; Palm, R. Z. Naturforsch., B: Anorg. Chem., Org. Chem. 1981, 36B, 544-550. (40) Zhang, B. L.; Wang, E. K. J. Electroanal. Chem. 1995, 388, 207213. (41) Kim, G. S.; Hagen, K. S.; Hill, C. L. Inorg. Chem. 1992, 31, 5316-5324. (42) Powell, J. D. Ph.D. Thesis, University of Illinois at UrbanaChampaign, Urbana, Illinois, 2001.

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Figure 5. 100 × 100 nm in situ STM images of a Ag(111) surface obtained in the absence (A) and presence (B) of 0.5 mM R-K8SiW11O39 in a 0.1 M acetate buffer solution at -0.20 V. Part C is a 50 × 50 nm magnified image from part B.

Figure 7. Normal Raman spectra obtained from (A) a pressed pallet of R-K8SiW11O39 powder and (B) a 0.1 M acetate buffer solution saturated with R-K8SiW11O39 species and (C) a SERS spectrum obtained on a roughened Ag surface in a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer solution at 0 V. Figure 6. 50 × 50 nm in situ STM image of a Ag(111) surface obtained in a 0.1 M cesium acetate buffer solution containing R-K8SiW11O39 at -0.20 V.

shows a 50 × 50 nm in situ STM image of a Ag(111) surface obtained in a 0.1 M cesium acetate buffer solution containing R-K8SiW11O39 at -0.20 V. Although this image shows less-ordered structures than that obtained in a sodium acetate buffer, several domains exhibit hexagonal structures. The spacings in these hexagonal structures were measured as between 1.2 and 1.3 nm, which are almost the same as those measured in a sodium acetate buffer solution. It was also reported that changing countercations does not affect the structure of the monolayers of R-PVW11O404- on Ag(111).19 3.3. SERS. To obtain further information about the mode of association of the R-K8SiW11O39 molecule with the Ag surface, we obtained SERS spectra of this species on a roughened Ag surface. SERS has the advantage of providing information in the low-wavenumber region, where R-K8SiW11O39-specific bands are known to occur.43 To our knowledge, this is the first time SERS has been reported for any POM species. The normal Raman spectra obtained from a pressed pallet of R-K8SiW11O39 powder and a 0.1 M acetate buffer solution containing R-K8SiW11O39 species are shown in spectra A and B of Figure 7, respectively. RocchiccioliDeltcheff et al. reported the normal Raman spectra obtained from an aqueous solution containing R-SiW11O39 (43) Bridgeman, A. J. Chem. Phys. 2003, 287, 55-69.

species and suggested tentative peak assignments for the bands shown in the spectra.44,45 Recently, density functional study of the vibrational frequencies of R-Keggintype POMs has been reported.43 There are some 147 normal modes expected for the lacunary species, all of which apparently will occur in a range of about 1000 cm-1 extending to lower energies.43 The lacunary species has Cs symmetry, which means that all bands are both IR- and Raman-allowed. Because only about 15-20 bands are seen in each spectrum in Figure 7, each band likely consists of several overlapping modes. By reference to the calculation for the Keggin ion,43 group frequencies are assigned. The bands between 900 and 1000 cm-1 are associated with the stretching modes of W and terminal oxygen Ot. Two broad bands between 800 and 900 cm-1 are due to the stretches of W-O2c1-W and W-O2c2-W. Here, O2c1 and O2c2 correspond the oxygen atoms inside a W3O13 unit and between W3O13 units, respectively. The lower energy region from 300 to 600 cm-1 is populated with several modes associated with the bending mode of W-O2c1-W, W-O2c2-W, and O-Si-O. As the symmetry of the POM is lowered from the Td presented by the Keggin ion to the Cs of the lacunary species, peak splittings in the W-Ot region might be expected. Parts A and B of Figure 7 do not show this expected splitting, a result reported in previous measurements.45 Peak assignments are listed in Table 1. (44) Rocchiccioli-Deltcheff, C.; Thouvenot, R. C. R. Acad. Sci., Ser. C 1974, 278, 857-860. (45) Rocchiccioli-Deltcheff, C.; Thouvenot, R. J. Chem. Res., Synop. 1977, 46-47.

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Table 1. Normal Raman and SERS Band Assignments peak position (cm-1) powder Ramana 996 973 950 923 ∼865 805 524 ∼380

solution Ramanb

SERSc at 0 V

995 963

995 971 936

∼885 812 516 ∼375

∼887 795 512 381

tentative assignment νs W-Ot νs W-Ot νs W-Ot νs W-Ot νas W-O2c1-W νas W-O2c2-W δ W-O2c1-W; δ W-O2c2-W δ W-O2c1-W; δ W-O2c2-W; δ O-Si-O

a Obtained from a pressed pallet of R-K SiW O 8 11 39 powder. Obtained from a 0.1 M acetate buffer solution saturated with R-K8SiW11O39 species. c Obtained on a roughened Ag surface in a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer solution.

b

Figure 7C shows the SERS spectra of the roughened Ag surface obtained in a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer solution at 0 V. Table 1 gives the frequencies of the bands. The spectrum shows considerable similarity with the solution and powder spectra. The bands between 300 and 900 cm-1 in the SERS spectra correspond well to those obtained in the normal Raman spectrum of the R-SiW11O398- containing solution. However, in the W-Ot stretching region, two strong bands are observed at 936 and 971 cm-1. In their study of WO42- adsorbed on Ag particles, Feilchenfeld and Siiman46 showed that WO42- on Ag exhibited bands that were both broader and red-shifted relative to the solution tungstate. They attributed the broadening to heterogeneity on the roughened Ag surface. However, the bands SERS obtained from R-K8SiW11O39 adsorbed on Ag are neither red-shifted nor broader than those present in powder or solution. The lack of broadening and the correspondence in the peak position suggest strongly that the lacunary species has not decomposed on the Ag surface. Figure 8A shows the potential dependence of SERS spectra between 0.0 and -0.9 V obtained in a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer solution. The positions of the SERS peaks obtained at various potentials between 0 and -0.9 V correspond to those obtained in the normal Raman spectrum of the R-SiW11O398- containing solution. Although there are changes in the peak positions and intensities, the existence of SERS peaks due to R-SiW11O398- species indicates that the R-SiW11O398species still remains on the Ag surface throughout this potential range. Interestingly, the two dominant peaks at 936 and 971 cm-1 appear at the electrode potential of 0 V, labeled as 1 and 2, respectively. As the applied potential moves to more negative values, peak 1 exhibits a blue shift, whereas peak 2 shows a red shift ending up at 944 and 964 cm-1, respectively. The relative intensity of peak 1 increases up to -0.4 V, after which potential the intensity slightly decreases. Intensity changes with the other peaks are not nearly as pronounced. We note that potentialdependent intensity changes in SERS can be associated with a combination of both electronic47 and number density factors.48 The lack of distinct, separated bands makes the evaluation of these factors difficult in this case. One origin of the 936 cm-1 band is from the acetate electrolyte. SERS obtained from a Ag surface immersed in a 0.1 M acetate buffer solution without added R-K8(46) Feilchenfeld, H.; Siiman, O. J. Phys. Chem. 1988, 92, 453-464. (47) Furtak, T. E.; Roy, D. Phys. Rev. Lett. 1983, 50, 1301-1304. (48) Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon, J. G.; Philpott, M. R. J. Electroanal. Chem. 1984, 160, 321-333.

Figure 8. SERS spectra between 0.0 and -0.9 V obtained on a roughened Ag surface in a 0.5 mM R-K8SiW11O39 + 0.1 M (A) acetate buffer and (B) Na2SO4 solutions.

SiW11O39 reveals a peak at 942 cm-1 at 0 V. This peak is associated with the C-O stretching mode of the acetate ion for adsorption on a Ag electrode.49 Upon the negative potential excursion, this peak shows a blue shift that finally locates at 936 cm-1 at -0.9 V. The intensity of the peak sharply decreases as the potential approaches -0.4 V and then stays almost constant up to -0.9 V. In an attempt to eliminate interference from vibrational modes in the supporting electrolyte, 0.1 M Na2SO4 was used instead of the 0.1 M acetate buffer solution. The S-O stretch of SO42- is found at 979 cm-1, which is somewhat higher than that for acetate. Figure 8B shows SERS obtained from a Ag surface immersed in a solution containing 0.5 mM R-K8SiW11O39 + 0.1 M Na2SO4 at pH ) 4.7. While the detailed behavior of the bands in the W-Ot region shows some differences from that obtained with acetate, the spectrum still features a split band with peak maxima at 930 and 956 cm-1 at 0.0 V that coalesces to a single feature at 927 cm-1 at -0.9 V. In this case, the low-energy band cannot be associated with acetate but rather likely reflects the lacunary species on the surface. 3.4. XPS. To assist in the identity of the surface-confined species, XPS spectra were taken for a Ag electrode immersed in a 0.5 mM R-K8SiW11O39 + 0.1 M acetate buffer solution for 10 min followed by emersion and rinsing with Milli-Q water. A survey scan spectrum of the R-K8SiW11O39-modified Ag electrode showed contributions from Ag, W, Si, O, and C. The presence of W and Si on the Ag surface indicates that R-K8SiW11O39 anions still stay on the Ag surface after the electrode is emersed from the solution and rinsed with water. A multiplex scan spectrum for W was also collected and showed well-resolved peaks corresponding to W(4f5/2) and W(4f7/2). (49) Fleischmann, M.; Tian, Z. Q. J. Electroanal. Chem. 1987, 217, 385-395.

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4. Discussion The results presented above show that the lacunary species examined in this report exhibits structures on the Ag(111) surface different from those reported previously for the corresponding Keggin ion species. SERS shows that the species on the Ag surface is likely the lacunary ion; there are subtle differences in peak splitting that change with the applied potential. Voltametric measurements show that the lacunary ion is still electrochemically active following emersion and rinsing. 4.1. Monolayer Structures of r-SiW11O398- on Ag(111). There are several reasons that monolayers on Ag(111) obtained from R-SiW12O404- and R-SiW11O398- anions could differ in both spacing and structure. These reasons include effect of the countercation, size, charge, and symmetry (shape). Figure 6 shows that changing the countercation for the lacunary species does not affect the spacing or structure of the monolayer on the surface. Similar results were reported for monolayers of R-PVW11O404- on Ag(111).19 We, thus, do not expect that the countercation participates in determining the structure of these monolayers. A somewhat different conclusion was reached by Barteau et al. in his study of films of phosphomolybdates fabricated by solvent evaporation on HOPG, where different structures were observed with different cations.50,51 In the present case, it is clear that only monolayer films are constructed, while this may not be true in the case of the films on HOPG. Size is another possibility that could control the structure on the electrode surface. The HPW11O396- species,39 analogous to the lacunary molecule considered here, exhibits dimensions that are very similar to those reported for R-SiW12O404-.38 Certainly the 30% (1.22-1.37 vs 1.02 nm) spacing difference observed in STM cannot be recovered in the size differences between the two molecules. Another factor to be considered is the charge difference between two adsorbates. It is known that the R-H4SiW12O40 is a very strong acid;1 thus, R-SiW12O404anions exist without being protonated in acidic aqueous solution at pH 1. R-K8SiW11O39 dissociates in aqueous solution to produce R-SiW11O398- anions. Because the pKa1 and pKa2 values of R-H2SiW11O396- are known to be 4.0 and 3.4, respectively,33 most of the R-SiW11O398- anions would exist without being protonated in an acetate buffer solution at pH 4.7. Therefore, there is a difference in charge densities between R-SiW12O404- and R-SiW11O398- in their aqueous solutions. The higher overall charge density of the lacunary species may cause a stronger electrostatic repulsion, which in turn could result in an enlarged spacing between two adjacent molecules. A final possibility is that the lowered symmetry afforded by the lacunary species does not allow for the close-packed approach of anions found with R-SiW12O404-. The larger spacings may be indicative of a different packing geometry. Indeed, the observation of a hexagonal structure as opposed to the square adlattice seen previously strongly supports a different type of association for the lacunary species as opposed to the Keggin ion. The different mode of association could derive from the presence of reactive O groups formed during the removal of the W-Ot moiety during the formation of the lacunary species. If these groups were to associate with the surface, then the molecular footprint of the lacunary species would be quite (50) Song, I. K.; Kaba, M. S.; Coulston, G.; Kourtakis, K.; Barteau, M. A. Chem. Mater. 1996, 8, 2352-2358. (51) Song, I. K.; Kaba, M. S.; Barteau, M. A. J. Phys. Chem. 1996, 100, 17528-17534.

Kim and Gewirth

different from that found for the Keggin ion. This point is taken up further in the next section. 4.2. Possible Models for the Association of r-SiW11O398- with the Ag(111) Surface. We next examine possible models to account for our structural observations. We explained the structures formed by the Keggin ion on Ag(111) and Ag(100) by reference to specific interactions between the terminal oxo groups of the molecule and the Ag surface. We showed that the structures could be explained if the molecule associated with the substrate has its S4 axis perpendicular to the surface. This orientation is more favored thermodynamically than the other structure with the C3 axis perpendicular to the surface because the former has four terminal oxygen atoms that can interact with Ag surfaces, whereas the latter has only three.15-17,36 In addition, X-ray reflectivity studies on Ag(100) showed that the terminal oxo groups were likely associated with fourfold hollow sites on this surface.36 The lacunary species, R-SiW11O398-, exhibits a lower symmetry relative to the Keggin ion (Cs versus Td). The lacunary species is likely not associated with the Ag surface using the same terminal oxo groups as the Keggin ion because this would produce structures of the same symmetry and spacing as found previously. Our observation of a hexagonal structure and a larger moleculemolecule spacing suggests that a different molecular footprint relative to the surface is operative. The removal of a W-Ot moiety from the Keggin structure to make the lacunary species leaves four new terminal oxygen atoms, which can contact the surface. These four and an additional pair of terminal oxo groups can contact the surface when the molecule is oriented as shown in Figure 9A. This would lead to a total of six Ag-O interactions and, thus, be more thermodynamically stable than the other orientations featuring three or four contacts. We next consider how these six contacts could be arranged on the surface. The structure proposed in Figure 9B features all of the terminal oxygen atoms at or near threefold hollow sites on the Ag(111) surface. These sites are assumed to be the locus of the Ag-O interaction, by reference to the Ag(100) case described previously. Figure 9B shows that the distance of closest approach for molecules arranged in this manner is 1.32 nm, consistent with the observations made by STM. To push the molecules closer together would require either heterogeneity in the Ag-O interaction among the six terminal groups or reorientation of the molecule. The model shown does provide an explanation for the larger spacings observed with the lacunary species relative to our previous observations using the Keggin ion. Finally, we note that there are many other possible ways to orient the lacunary species on the surface to obtain the spacing and structure found with STM. However, all of these other models, which feature Ag association by three or four terminal oxygens, do not provide an explanation for the increased molecular separation observed with the lacunary species. 4.3. Potential Dependence of SERS. There are two noteworthy effects found in the SERS spectra of R-SiW11O398- on Ag. First, there is a generally good correspondence between the spectroscopy of the solution, solid-state, and Ag-supported lacunary species. This suggests that the lacunary species is not decomposed on the Ag surface. In addition, the lacunary species is clearly present on the surface throughout the potential range studied here. This behavior, also inferred for the Keggin ion, means that the polyoxotungstate-Ag interaction is much more complicated than that of a simple anion.

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Figure 9. (A) Side and top views of the proposed R-SiW11O398- orientation relative to the Ag surface and (B) proposed model for the R-SiW11O398- monolayer on Ag(111). W atoms are represented by gray circles, O atoms by green circles, and O atoms interacting with Ag surfaces by black circles. The van der Waals radii for terminal O atoms are included.

Second, potential-dependent spectroscopy shows subtle changes in the W-Ot spectral region as the potential is varied. The spectroscopy shows that at positive potentials, the SERS spectra obtained in both acetate and sulfate exhibit a split peak in the W-Ot region, in contrast to the solution or solid-state spectra. Association of both peaks with the lacunary species is of course complicated by the potential overlap of the W-Ot region with the C-O stretch of acetate. However, the same behavior is observed in sulfate, where the S-O stretch occurs at somewhat higher energy. As mentioned, a split of this sort was anticipated in the solution spectroscopy of the molecule but was not observed.45 We suggest that the split peak observed at at least some potentials results from the lowered symmetry afforded by interaction of the molecule with the Ag surface. While the details of this lowered symmetry are difficult to understand given the number of bands expected in this region and the large bandwidths, we do expect that the W-Ot moieties in close proximity to the Ag surface will exhibit red shifts relative to those that are not close. In the model shown in Figure 9B, 6 of the 15 terminal oxygen atoms are interacting with the Ag surface, leaving 9 which are not. Interestingly, little splitting or broadening is observed in the W-O-W region between the solution and Ag-supported spectra, suggesting that these moieties are not strongly affected by Ag-surface association. The partial convergence of features as the potential moves to

more negative values seen in both sulfate and acetate could reflect the weaker association of the molecule with the surface at these more cathodic potentials. Conclusions The lacunary POM, R-SiW11O398-, is shown to adsorb on a Ag(111) electrode surface to produce ordered monolayers. Electrochemical analysis shows that the R-SiW11O398- species adsorbed on the Ag surface exhibits a redox wave with the same peak position as that observed on glassy carbon and Au surfaces in the solution phase. Association of R-SiW11O398- with a Ag(111) surface gives rise to different monolayer structures from those found with the parent R-SiW12O404- species. The origin of this behavior likely resides in the lower symmetry of the former compound, which enables a different footprint with the Ag surface. Vibrational spectroscopic measurements show that R-SiW11O398- is present on the surface even at very negative potentials. Acknowledgment. J.K. thanks the Department of Chemistry for financial support in the form of a University Block Grant. The authors thank Mr. Craig Teague for providing R-K8SiW11O39 used in this work and J. O. White of the Laser Laboratory in the Frederick Seitz Materials

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Research Laboratory at the University of Illinois for his assistance in Raman data acquisition. The Laser Laboratory is funded by Department of Energy Grant DE-FG0296ER45439 through the Materials Research Laboratory

Kim and Gewirth

at the University of Illinois. This work was funded by the NSF (CHE-0237683), which is gratefully acknowledged. LA034708D