Nanoscale Investigation of Mixed Arrays of Keggin-Type and Wells

Feb 21, 2002 - The nanoscale investigation of mixed arrays of Keggin-type (H3PW12O40) and Wells−Dawson-type (H6P2W18O62) heteropolyacids (HPAs) depo...
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Nanoscale Investigation of Mixed Arrays of Keggin-Type and Wells-Dawson-Type Heteropolyacids (HPAs) by Scanning Tunneling Microscopy (STM) In K. Song,† Mahmoud S. Kaba, and Mark A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received July 26, 2001. In Final Form: December 17, 2001 The nanoscale investigation of mixed arrays of Keggin-type (H3PW12O40) and Wells-Dawson-type (H6P2W18O62) heteropolyacids (HPAs) deposited on a graphite surface was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS). Pure H3PW12O40 and H6P2W18O62 formed self-assembled and well-ordered two-dimensional arrays, and their molecular dimensions were in good agreement with values determined by XRD. In tunneling spectroscopy, H3PW12O40 and H6P2W18O62 exhibited negative differential resistance (NDR) behavior at applied voltages of -1.14 and -0.80V, respectively. Mixtures of H3PW12O40 and H6P2W18O62 deposited from aqueous solution formed less ordered two-dimensional arrays than did the pure components. The mixed arrays exhibited a bimodal distribution of NDR peak voltages which was represented as a combination of the monomodal distributions of NDR peak voltages of pure H3PW12O40 and H6P2W18O62. Individual molecules in the mixed array were successfully identified by differences in molecular size (STM image) and electronic properties (NDR peaks). The ability to probe sites on a heterogeneous surface on a site-by-site basis is of potential importance in the design, characterization, and utilization of surfaces molecularly tailored with HPAs or other reactive complexes.

Introduction An important step toward real space imaging of chemical reactions on surfaces, and thus to understand site requirements for different reactions, is the ability to image mixtures of different adsorbed species and to distinguish individual species by scanning tunneling microscopy (STM).1-3 The ability to distinguish between closely related molecules is of importance in applications where molecular recognition or sequencing is desired. An STM image contains both geometric and electronic information about the sample, but the tunneling spectrum contains information only about the electronic states. In general, it is difficult to resolve the molecular structures of adsorbates on surfaces. These difficulties may arise from rapid surface diffusion, electric field effects,4 or the absence of molecular states near the Fermi level (EF).5 When one cannot easily distinguish between chemically inequivalent sites or adsorbates with nearly identical geometric structures and sizes, or when the STM image is of low resolution, one may utilize tunneling spectroscopy (TS).6 Together with STM images, current-voltage (I-V) behavior measured by TS has been used to distinguish inequivalent chemical sites.7,8 In a STM study on the initial stages of Li adsorption * To whom all correspondence should be addressed. Tel: 302831-8905. Fax: 302-831-8201. E-mail: [email protected]. † Present address: Department of Industrial Chemistry, Kangnung National University, Kangnung, Kangwondo 210-702, Korea. (1) Boland, J. J.; Villarrubia, J. S. Science 1990, 248, 838. (2) Cernota, P. D.; Yoon, H. A.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1998, 415, 351. (3) Silva, S. L.; Patal, A. A.; Pham, T. M.; Leibsle, F. M. Surf. Sci. 1999, 441, 351. (4) Gimzewski, J. K.; Stoll, E.; Schlittler, R. R. Surf. Sci. 1987, 181, 267. (5) Chiang, S.; Wilson, R. J.; Mate, C. M.; Ohtani, R. J. Microscopy 1988, 152, 567. (6) Bode, M.; Pascal, R.; Wiesendanger, R. J. J. Vac. Sci. Technol., A 1997, 15, 1285. (7) Piancastelli, M. N.; Motta, N,; Sgarlata, A.; Balzarotti, A.; Crescenzi, M. D. Phys. Rev. B 1993, 48, 17892. (8) Bode, M.; Pascal, R.; Wiesendanger, R. J. Vac. Sci. Technol., A 1997, 15, 1285.

on Si(001) surface, for example, Johansson et al.9 showed that two types of Li clusters formed on the Si(001) surface could be distinguished by their I-V characteristics. Heteropolyacids (HPAs) are strong inorganic acids that possess well-defined molecular structures.10-12 STM images of HPAs deposited on conductive substrates such as graphite and silver have been reported by several researchers.13-15 Recent papers reporting STM images of HPAs showed that two-dimensional ordered arrays of these molecules on a graphite surface exhibited a distinctive I-V behavior referred to as negative differential resistance (NDR) in their tunneling spectra.16-23 Simple one-dimensional theory predicts that the transmission probability between two electrically equivalent electrodes should increase monotonically with increasing applied potential.24 NDR behavior is manifested as local maxima and minima in the I-V spectrum. Such peaks in the I-V (9) Johansson, M. K.-J.; Gray, S. M.; Johansson, L. S. O. J. Vac. Sci. Technol., B 1996, 14, 1015. (10) Kozhevnikov, I. V. Catal. Rev.sSci. Eng. 1995, 37, 311. (11) Okuhara, T.; Mizuno, N.; Misono, M. Adv. Catal. 1996, 41, 113. (12) Mizuno, N.; Misono, M. Chem. Rev. 1998, 98, 199. (13) Ke¨ita, B.; Chauveau, F.; The´obald, F.; Be´langer, D.; Nadjo, L. Surf. Sci. 1992, 264, 271. (14) Watson, B. A.; Barteau, M. A.; Haggerty, L.; Lenhoff, A. M.; Weber, R. S. Langmuir 1992, 8, 1145. (15) Ge, M.; Zhong, B.; Klemperer, W. G.; Gewirth, A. A. J. Am. Chem. Soc. 1996, 118, 5812. (16) Song, I. K.; Kaba, M. S.; Coulston, G.; Kourtakis, K.; Barteau, M. A. Chem. Mater. 1996, 8, 2352. (17) Song, I. K.; Kaba, M. S.; Barteau, M. A. J. Phys. Chem. 1996, 100, 17528. (18) Kaba, M. S.; Song, I. K.; Barteau, M. A. J. Phys. Chem. 1996, 100, 19577. (19) Kaba, M. S.; Song, I. K.; Barteau, M. A. J. Vac. Sci. Technol., A 1997, 15, 1299. (20) Kaba, M. S.; Song, I. K.; Duncan, D. C.; Hill, C. L.; Barteau, M. A. Inorg. Chem. 1998, 37, 398. (21) Song, I. K.; Kaba, M. S.; Barteau, M. A.; Lee, W. Y. Catal. Today 1998, 44, 285. (22) Kaba, M. S.; Barteau, M. A.; Lee, W. Y.; Song, I. K. Appl. Catal., A 2000, 194, 129. (23) Kinne, M.; Barteau, M. A. Surf. Sci. 2000, 447, 105. (24) Hamers, R. J. In Scanning Tunneling Microscopy and Spectroscopy; Bonnel, D. A., Eds.; VCH Publishers: New York, 1993; p 90.

10.1021/la0111811 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002

Keggin-Type and Wells-Dawson-Type Heteropolyacids

spectrum result in negative values of dI/dV, and thus the phenomenon is referred to as negative differential resistance. The NDR phenomenon has been explained in terms of resonant tunneling through a double barrier quantum well25-27 and has been observed consistently for arrays of pure HPAs.17-23 It has been observed that NDR peak position correlates well with the reduction potential of HPAs18,19,21,23 and, thus, can serve to distinguish chemically different HPA monolayers. It is anticipated that the NDR features in tunneling spectra measured at spatially wellresolved positions in two-dimensional arrays of HPA mixtures might be utilized to distinguish inequivalent sites, i.e., to distinguish different molecules in a mixed array. In this work, nanoscale investigation of mixed arrays of H3PW12O40 (a typical Keggin-type28 HPA) and H6P2W18O62 (a typical Wells-Dawson-type29 HPA) was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) to distinguish individual molecules in the mixed arrays. A mixture having both different molecular size and different anionic charge was investigated as a model mixed array for this purpose. The identities of individual HPAs in the mixed array were determined from differences in molecular size and electronic properties. This step may be important for surface functionalization with reactive species and for the design of mixed HPA catalysts. Experimental Section Sample Preparation and Deposition. H3PW12O40 was purchased from Aldrich Chemical Co. (Milwaukee, WI), and H6P2W18O62 was synthesized according to published procedures.30-32 Approximately 0.01 M aqueous solutions of each sample were then prepared to make mixed solutions for the STM measurements. A drop of single or mixed solution was deposited on a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface and allowed to dry in air for 1 h. STM Imaging and TS Measurements. STM images were acquired using a Topometrix TMX-2010 STM (Discoverer) in air. Mechanically formed Pt/Ir (90/10) tips were used. Images were obtained in the constant current mode at a tunneling current of 1-2 nA and a positive sample bias of 100 mV. The mechanically formed tips were first calibrated by imaging bare HOPG to confirm the standard periodicity of HOPG (2.46 Å). Several tunneling spectra were then taken on the bare graphite section of the surface to ensure the stability of the tip and the reproducibility of the tunneling spectrum of HOPG. Once these had been established, the “good” tip would be moved to the HPAcovered section on graphite to image and obtain tunneling spectra of the HPA sample. Detailed imaging procedures have been enumerated elsewhere.16-23 All STM images presented in this work were unfiltered, and the reported periodicities (lattice constants) represent average values determined by performing two-dimensional fast Fourier transform (2D-FFT) analyses on at least three images for each sample which were obtained with different tips. Tunneling spectroscopy (TS) measurements were taken using both Topometrix and LK Technologies LK-1000 instruments to ensure consistency and reproducibility. The STM tip was positioned above a corrugation (heteropolyacid molecule) (25) Nakashima, H.; Uozumi, K. J. Vac. Sci. Technol., B 1997, 15, 1411. (26) Nosho, B. Z.; Weinberg, W. H.; Barvosa-Carter, W.; Bracker, A. S.; Mango, R.; Bennett, B. R.; Culbertson, J. C.; Shanabrook, B. V.; Whitman, L. J. J. Vac. Sci. Technol., B 1999, 17, 1786. (27) Petukhov, A. G.; Demchenko, D. O.; Chantis, A. N. J. Vac. Sci. Technol., B 2000, 18, 2109. (28) Keggin, J. F. Nature 1933, 131, 908. (29) Dawson, B. Acta Crystallogr. 1953, 6, 113. (30) Strandberg, R. Acta Chem. Scand. A 1975, 29, 350. (31) Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: New York, 1983. (32) Hu, C.; Hashimoto, M.; Okuhara, T.; Misono, M. J. Catal. 1993, 143, 437.

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Figure 1. Polyhedral representations of the molecular structures of (a) Keggin-type [PW12O40]3- and (b) Wells-Dawsontype [P2W18O62]6- heteropolyanions. of interest for TS measurements, and the tunneling current was monitored while the bias voltage was ramped from -2 to +2 V. The voltage axis in the tunneling spectrum represents the potential applied to the sample relative to that of the tip.

Results and Discussion Self-Assembled Arrays of H3PW12O40 and H6P2W18O62. Figure 1a shows the polyhedral representation of the molecular structure of a Keggin-type28 [PW12O40]3- heteropolyanion. The molecular structure of [PW12O40]3- consists of a heteroatom, P, at the center of the anion cluster, tetrahedrally coordinated to four oxygen atoms. This tetrahedron is surrounded by 12 WO6 octahedra. The Keggin-type HPA has a soccer ball-like shape, and its molecular size is ca. 10-12 Å as determined by X-ray crystallography28,33,34 and STM.13-23 Figure 1b shows the polyhedral representation of the molecular structure of a Wells-Dawson-type29 [P2W18O62]6- heteropolyanion. The molecular structure of [P2W18O62]6consists of two defect Keggin-type fragments, [PW9O34]9-. Each fragment is chiral and consists of a central PO4 tetrahedron sharing corners with nine WO6 octahedras the octahedra are somewhat distorted from an ideal octahedron. Three WO6 octahedra form a compact group by sharing edges, while the remaining six octahedra in each of the [PW9O34]9- fragments form a zigzag ring by alternately sharing edges and corners. The two fragments are linked by six nearly linear W-O-W bonds. The WellsDawson-type29 HPA has a rugby ball-like shape, and its molecular size is ca. 11 Å × 14.5 Å as determined by X-ray crystallography29,30 and STM.20 Figure 2 shows STM images and unit cells of pure H3PW12O40 and H6P2W18O62 monolayers on graphite. STM images clearly indicate that these molecules formed wellordered two-dimensional arrays on the graphite surface. As shown in Figure 2a, the periodicity of the H3PW12O40 array, i.e., the center-to-center spacing between bright features in the image, is 11.7 Å × 11.7 Å, consistent with the value determined by XRD.28,33,34 The unit cell constructed on the basis of lattice constants determined from 2D-FFT shows that the arrays of H3PW12O40 have nearly square symmetry (included angle ) 79.5°). The area/H3PW12O40 molecule in these arrays is 1.35 nm2/molecule for these unit cell dimensions. In the case of H6P2W18O62, the unit cell constructed on the basis of lattice constants determined from 2D-FFT shows that the arrays have both the primitive unit cell (rhombus) and the conventional (33) Brown, G. M.; Noe-Spirlet, M.-R.; Busing, W. R.; Levy, H. A. Acta Crystallogr. B 1977, 33, 1038. (34) Hayashi, H.; Moffat, J. B. J. Catal. 1982, 77, 473.

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Figure 2. STM images and unit cells of (a) H3PW12O40 and (b) H6P2W18O62 deposited on a graphite surface.

unit cell (centered oblique rectangle) as presented in Figure 2b. The primitive cell has sides of 14.4 Å with an included angle of 40.5°. The conventional unit cell has a minor axis of 11.2 Å with an included angle of 72°. The area/ H6P2W18O62 molecule in this array is 1.53 nm2/molecule. The unit cell size of 11.2 Å × 14.4 Å is close to the molecular size of H6P2W18O62 determined by XRD.29,30 The array structure and orientation of H6P2W18O62 presented in Figure 2b are different from those of H7P2Mo17V1O62 described previously.20 Graphite itself is chemically inert, and there is no special reason to expect any interactions between HPA molecules and graphite substrate. Therefore, it is believed that the differences in molecular array structure and orientation between these Wells-Dawsontype HPAs, H6P2W18O62, and H7P2Mo17V1O62, may arise from their different intermolecular interactions rather than from molecule-substrate interactions. In fact, we have observed that the structures of two-dimensional arrays of Keggin-type HPAs strongly depended on molecular identities and vary with changes in HPA composition.16-23 Tunneling Spectroscopy of H3PW12O40 and H6P2W18O62. Typical tunneling spectra taken at the bright features in the images of H3PW12O40 and H6P2W18O62 samples in Figure 2 are shown in Figure 3. The spectra show a distinctive I-V behavior, referred to as negative differential resistance (NDR), where dI/dV is negative.14,17-23 The NDR peak voltage was defined as the voltage at which the maximum current was observed in this region. The NDR peak voltages in the individual spectra for H3PW12O40 and H6P2W18O62 shown in Figure 3 were -1.20 and -0.80 V, respectively. Tunneling spectra taken at the dark interstitial spaces between bright corrugations in each image of Figure 2 showed the typical I-V response of graphite, indicating that the two-dimensional arrays of H3PW12O40 and H6P2W18O62 in these images are monolayers, as previously demonstrated.18-23 Although the detailed mechanism of NDR for HPA samples has not yet been elucidated, we have shown that the characteristic NDR peak position of HPAs can serve as a probe of molecular identity.20 In fact, there is some variation in both NDR peak position and intensity between spectra even on single component arrays, most likely due to variations in positioning the tip precisely for each measurement. The I-V curves shown in Figure 3, therefore, represent typical examples of tunneling spectra exhibiting NDR for the corresponding HPA arrays. To

Figure 3. Typical I-V curves of (a) H3PW12O40 and (b) H6P2W18O62 taken at bright features in Figure 2a,b, respectively.

Figure 4. Distribution of NDR peak voltages of pure HPA components: (a) H3PW12O40 taken at bright features in Figure 2a with a statistical mean of -1.14 ( 0.09 V; (b) H6P2W18O62 taken at bright features in Figure 2b with a statistical mean of -0.80 ( 0.08 V.

obtain more accurate and reproducible results, and to provide a basis for statistical analyses, tunneling spectra were measured several times with at least three different tips for each sample. Figure 4 shows the distribution of NDR peak voltages of pure H3PW12O40 and H6P2W18O62 arrays plotted as a function of the applied potential at which NDR was observed. The NDR peak voltages of pure HPAs exhibited a typical Gaussian distribution. The statistical means of NDR peak voltages determined by fitting the data to normal (Gaussian) curves were -1.14 ( 0.09 V for H3PW12O40 and -0.80 ( 0.08 V for H6P2W18O62. The NDR peak voltages of H3PW12O40 were observed in the range from -1.30 to -0.85 V, and those of H6P2W18O62 were in the range from -0.95 to -0.65 V. On the basis of the statistical analyses, the representative NDR peak voltages of H3PW12O40 and H6P2W18O62 can be taken as -1.14 and

Keggin-Type and Wells-Dawson-Type Heteropolyacids

-0.80 V, respectively. Considering the resolution of I-V data points, an important criterion in spectroscopic studies of mixtures is that the difference between the NDR voltages of the pure HPA components be significantly greater than the resolution of the TS measurements (0.05 V or less), which depends on the instrument used. This is important for the discrimination of different HPA molecules in the mixed arrays on the basis of electronic properties measured by tunneling spectroscopy. Identification of H3PW12O40 and H6P2W18O62 in Mixed Arrays by STM. Several mixed arrays of HPAs have been examined in our laboratories,35,36 including mixtures of H4PMo11V1O40-K12.5Na1.5[NaP5W30O110], H3PW12O40-H3PMo12O40, and H3PW12O40-H3PMo9W3O40. In the mixed arrays of H4PMo11V1O40 (a Keggin-type28 HPA) and K12.5Na1.5[NaP5W30O110] (a Pope-Jeannin-Preysslertype37 HPA), which have different molecular sizes, anionic charges, countercations, and polyatom identities, most attempts to produce and image mixed arrays containing both HPA species were unsuccessful.35 Most regions of the sample surface showed only one kind of HPA, indicating that each HPA species in the mixed array segregated into separate domains on graphite. Regions containing both HPA species were found on a small fraction of the sample surface, near boundaries between pure component domains, with less order than observed for the pure components. In the mixed arrays of H3PW12O40-H3PMo12O4036 and H3PW12O40-H3PMo9W3O40,35 which have essentially identical molecular sizes, anionic charges, and countercations, it was also observed that mixtures of HPA species were found on a small fraction of the sample surface, as observed for the mixed array of H4PMo11V1O40-K12.5Na1.5[NaP5W30O110].35 However, regions where both species were found formed well-ordered arrays similar to those formed by their pure components. In these mixed arrays of HPAs having identical molecular sizes, individual molecules could be identified by their NDR peak positions.36 From TS measurements, distributions of each HPA species in the mixed arrays were found to be fairly random, even though equal amounts of each HPA were used to form the mixed samples. To obtain insights about the structure of mixed HPA arrays for the rational design of mixed-HPA catalysts, STM experiments in this work were designed to investigate mixed arrays of H3PW12O40 and H6P2W18O62. These HPAs have identical chemical elements but different molecular sizes and anionic charges. With mixed samples of different proportions, many attempts were made to locate and image regions containing both HPA species on the graphite surfaces, but most were unsuccessful as in our previous works.35,36 With mixed arrays of H3PW12O40 and H6P2W18O62 deposited from an equimolar solution on graphite, however, we could locate and image regions containing both HPA species occasionally. An STM image showing a mixed array is presented in Figure 5. This image shows features of two characteristic sizes and, not surprisingly, exhibits less order than the pure component arrays in Figure 2. Some other regions on the sample surface from which Figure 5 was acquired showed only one kind of HPA, as previously observed for HPA arrays deposited from mixed solutions.35,36 Regions such as that imaged in Figure 5 may represent a small fraction of the total surface area in which both species may be found, (35) Kaba, M. S. Ph.D. Dissertation, University of Delaware, Newark, DE, 1998. (36) Kaba, M. S.; Song, I. K.; Barteau, M. A. J. Phys. Chem. B 2002, in press. (37) Alizadeh, M. H.; Harmalker, S. P.; Jeannin, Y.; Martin-Fre`re, J.; Pope, M. T. J. Am. Chem. Soc. 1985, 107, 2662.

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Figure 5. STM image of the mixed array of H3PW12O40 and H6P2W18O62 deposited from equimolar solution on graphite. “A” and “B” represent the Keggin-type H3PW12O40 and WellsDawson-type H6P2W18O62 molecules, respectively.

Figure 6. Another STM image of the mixed array of H3PW12O40 and H6P2W18O62 deposited from equimolar solution on graphite. “A” and “B” represent the Keggin-type H3PW12O40 and WellsDawson-type H6P2W18O62 molecules, respectively.

perhaps near boundaries between pure HPA domains. The paucity of such mixed arrays suggests that interactions between like HPA species are more favorable in the two-dimensional arrays than those between unlike species. Figure 5 apparently shows larger features marked “B” and features of smaller diameter marked “A”. Solely on the basis of their molecular sizes, it can be concluded that the corrugations labeled “B” and “A” are H6P2W18O62 and H3PW12O40 molecules, respectively. Importantly, the average area/molecule determined within the outlined area in Figure 5 is 1.45 nm2/molecule. This value is between that of the two pure component arrays, as might be expected for a mixed array of H3PW12O40 and H6P2W18O62. Figure 6 shows another STM image of a mixed array of H3PW12O40 and H6P2W18O62 obtained with a different tip and at a different sample position. It is again apparent that the two-dimensional arrays of the mixed HPA sample are less ordered than those of pure HPA components presented in Figure 2. The less ordered arrays undoubtedly result from the size difference between the two HPA components in the surface layer. Although the image resolution is not high enough to identify all the molecules, the average area per HPA molecule in this image was found to be approximately 1.38 nm2/molecule, again between those of the pure components. Despite the low resolution, Figure 6 also shows both large and small features in some regions, demonstrating that the image indeed corresponds to a mixed array. The larger corrugations marked “B” are attributed to H6P2W18O62, and smaller corrugations marked “A” represent H3PW12O40 molecules. In some lower contrast regions in Figure 6, however, it is very difficult to identify the HPA molecules by size and shape alone. To probe the identity of individual molecules in the mixed array, TS measurements were taken on the individual molecules to determine the position of their NDR features.

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Figure 7. Bimodal distribution of NDR peak voltages of the mixed HPA array taken at random positions of Figure 6 showing statistical means of -1.14 ( 0.09 and -0.81 ( 0.07 V.

Identification of H3PW12O40 and H6P2W18O62 in the Mixed Array by NDR. Evidence that the electronic structures of the features in Figure 6 differ was obtained from spatially resolved tunneling spectroscopy measurements. The characteristic NDR peak voltages of the mixed HPA arrays were measured to identify the individual molecules by their electronic properties. TS measurements taken with the STM tip positioned atop the bright features marked “B” and “A” in Figure 6 showed the NDR behavior of H6P2W18O62 and H3PW12O40, respectively. This means that the identities of the bright features in Figure 6 can be distinguished by their size difference (in the STM image) as well as by their NDR behavior (in tunneling spectroscopy). Figure 7 shows the distribution of NDR peak voltages of HPA arrays probed atop random positions of Figure 6, regardless of the contrast. The distribution of NDR peak voltages shown in Figure 7 compares favorably with the superposition of the pure component distributions in Figure 4. As shown in Figure 7, the HPA molecules in the mixed array show a bimodal distribution of NDR peak voltages with statistical means of -1.14 ( 0.09 and -0.81 ( 0.07 V. These values are well matched with the statistical means determined for pure HPA components (-1.14 ( 0.09 V for H3PW12O40 and -0.80 ( 0.08 V for H6P2W18O62). One lobe appears in the range from -1.30 to -0.95 V, while the other appears in the range from -0.95 to -0.65 V. The two lobes of the distribution can be assigned to the two components. Those molecules with NDR voltages between -1.30 and -0.95 V can be assigned to H3PW12O40, and those between -0.95 and -0.65 V can be assigned to H6P2W18O62 molecules. The number of molecules showing NDR behavior at -0.95 V is only about 3% of the total, and these molecules may be either H3PW12O40 or H6P2W18O62. The HPA species in lower contrast regions

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of Figure 6, where the molecular shapes are not clear, can also be successfully identified by their NDR behavior. Thus, TS provides a valuable technique to distinguish individual species in the mixed arrays. One should remember that the tip structure may change electronically during imaging and that the tunneling area (about 5 Å in diameter) may vary as the tip is moved from corrugation to corrugation. Thus, the ability to distinguish individual molecules on the basis of their tunneling spectra represents an important complement to imaging in the analysis of heterogeneous surfaces. It is concluded that the mixed arrays of H3PW12O40 and H6P2W18O62 deposited on graphite surfaces were successfully imaged in air by STM. The individual sites of the mixed arrays were identified by their molecular size and NDR behavior. It is expected that the site-by-site approach to surface of the mixed HPA arrays may play an important role in the design of novel mixed HPA catalysts where the molecular sequence is crucial. This technique might also be utilized for the development of chemical sensors using HPAs as reference materials. Conclusions Nanoscale STM investigation of pure and mixed arrays of Keggin-type H3PW12O40 and Wells-Dawson-type H6P2W18O62 HPAs, with both different molecular size and different anionic charge, was carried out for the identification of the individual molecules. Pure H3PW12O40 and H6P2W18O62 formed well-ordered arrays with molecular dimensions of ca. 11.7 Å × 11.7 Å and 11.2 Å × 14.4 Å, respectively, in good agreement with the values determined by XRD. The mixed arrays of H3PW12O40 and H6P2W18O62 formed less ordered two-dimensional arrays than did the pure components. In tunneling spectroscopy studies, H3PW12O40 and H6P2W18O62 exhibited a distribution of NDR peak voltages with statistical means of -1.14 ( 0.09 and -0.80 ( 0.08 V, respectively. The mixed arrays exhibited a bimodal distribution of NDR peak voltages which was represented as a combination of the monomodal distributions of NDR peak voltages of pure H3PW12O40 and H6P2W18O62. The individual molecules in the mixed array were thus identified by both their molecular size and NDR behavior, demonstrating the site-by-site identification of active sites for model HPA catalysts. Acknowledgment. The Topometrix TMX-2010 was acquired via an equipment grant from the U.S. Department of Energy. I.K.S. acknowledges support from Korea Science and Engineering Foundation. LA0111811