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Correlation of Negative Differential Resistance (NDR) Peak Voltages of Nanostructured Heteropolyacid (HPA) Monolayers with One Electron Reduction Potentials of HPA Catalysts In K. Song*,† and Mark A. Barteau‡ Department of Environmental & Applied Chemical Engineering, Kangnung National University, Kangnung, Kangwondo 210-702, South Korea and Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received July 9, 2003 An extensive and quantitative study on the surface electronic properties of nanostructured Keggin-type heteropolyacid (HPA) monolayers was carried out using scanning tunneling microscopy (STM) to relate surface electronic properties to bulk redox properties of HPAs. Cation-exchanged RPMo12O40 (R ) H3, Zn3/2, Co3/2, Cu3/2, Bi1); heteroatom-substituted HnXW12O40 (X ) P, Si, B, Co) and HnXMo12O40 (X ) P, As, Si); and polyatom-substituted HnPW11M1O40 (M ) W, Mo, V), H3PMoxW12-xO40 (x ) 0, 3, 6, 9, 12), H3+xPMo12-xVxO40 ( x) 0-3), and H3+xPW12-xVxO40 (x ) 0-3) HPAs were examined to elucidate the effect of different substitutions. All HPA samples formed two-dimensional well-ordered monolayer arrays on graphite surfaces and exhibited negative differential resistance (NDR) behavior in their tunneling spectra. Substitution of more electronegative atoms for countercations or for the central heteroatom shifted the NDR peaks to less negative voltages, corresponding to increased reduction potentials of the HPAs. On the other hand, substitution of more electronegative framework polyatoms shifted the NDR peaks to more negative voltages, corresponding to decreased reduction potentials. Irrespective of the exchanged/substituted positions, however, a comprehensive correlation between NDR peak voltage and reduction potential of HPAs established for all families of HPAs examined in this work revealed that NDR peak voltage could be utilized as a correlating parameter for the reduction potential of HPAs; a less negative NDR peak voltage corresponds to a higher reduction potential of the HPA. It is concluded that NDR peak voltages of HPAs can provide a selection and design basis for HPA catalysts efficient for selective oxidation reactions.
* To whom correspondence should be addressed. Phone:+8233-640-2404; fax: +82-33-640-2244; e-mail: inksong@ kangnung.ac.kr. † Kangnung National University. ‡ University of Delaware.
HPA catalysts can be tuned by changing the identity of charge-compensating countercations, central heteroatoms, and framework metal atoms (polyatoms).13 An important experimental finding in STM (scanning tunneling microscopy) studies on heteropoly- and isopolyacids reported by several research groups14-19 is that these molecules form self-assembled and well-ordered monolayer arrays on conductive substrates. Atomic resolution STM images of several HPA structural classes have been reported that show their molecular shape, orientation, and packing on graphite surfaces.19 Another important finding reported by this research group is that selfassembled monolayers (SAMs) of HPA samples deposited on graphite surfaces exhibit a distinctive current-voltage (I-V) behavior referred to as negative differential resistance (NDR) in their tunneling spectra.20-26 Simple onedimensional theory predicts that the transmission prob-
(1) Polyoxometalates: From Platonic Solids to Anti-retroviral Activity; Pope, M. T., Mu¨ller, A., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994. (2) Hill, C. L. (Guest Ed.) Chem. Rev. 1998, 98, No. 1. (3) Keggin, J. F. Nature 1933, 131, 908. (4) Neumann, R. Prog. Inorg. Chem. 1998, 47, 317. (5) Misono, M. Catal. Rev.sSci. Eng. 1987, 29, 269. (6) Kozhevnikov, I. V. Catal. Rev.sSci. Eng. 1995, 37, 311. (7) Hill, C. L.; Prosser-McCartha, C. M. Coord. Chem. Rev. 1995, 143, 407. (8) Mizuno, N.; Iwamoto, M.; Tateishi, M. Appl. Catal., A 1995, 128, 1165. (9) Song, I. K.; Lyons, J. E.; Barteau, M. A. Catal. Today 2003, 81, 137. (10) Barteau, M. A.; Lyons, J. E.; Song, I. K. J. Catal. 2003, 216, 236. (11) Mizuno, N.; Tateishi, M.; Iwamoto, M. J. Chem. Soc. Chem. Comm. 1994, 1411. (12) Mizuno, N.; Suh, D.-J.; Han, W.; Kudo, T. J. Mol. Catal., A 1995, 128, 309.
(13) Okuhara, T.; Mizuno, N.; Misono, M. Adv. Catal. 1996, 41, 113. (14) Keita, B.; Nadjo, L. Surf. Sci. 1991, 254, L443. (15) Ge, M.; Zhong, B.; Klemperer, W. G.; Gewirth, A. A. J. Am. Chem. Soc. 1996, 118, 5812. (16) Lee, L.; Wang, J. X.; Adzˇic´, R. R.; Robinson, I. K.; Gewirth, A. A. J. Am. Chem. Soc. 2001, 123, 8838. (17) Kim, J.; Gewirth A. A. Langmuir 2003, 19, 8934. (18) Song, I. K.; Kaba, M. S.; Coulston, G.; Kourtakis, K.; Barteau, M. A. Chem. Mater. 1996, 8, 2352. (19) Kaba, M. S.; Song, I. K.; Duncan, D. C.; Hill, C. L.; Barteau, M. A. Inorg. Chem. 1998, 37, 398. (20) Kaba, M. S.; Song, I. K.; Barteau, M. A. J. Phys. Chem. 1996, 100, 19577. (21) Kaba, M. S.; Song, I. K.; Barteau, M. A. J. Vac. Sci. Technol., A 1997, 15, 1299. (22) Kinne, M.; Barteau, M. A. Surf. Sci. 2000, 447, 105. (23) Song, I. K.; Kaba, M. S.; Barteau, M. A.; Lee, W. Y. Catal. Today 1998, 44, 285.
Introduction Heteropolyacids (HPAs), also known as polyoxometalates, have advantages of multifunctionality and structural variability.1,2 Among various HPA structural classes, the Keggin-type3 HPAs have been employed as catalysts in homogeneous and heterogeneous systems for acid-base and oxidation reactions.4-7 Recently, HPA catalysts have attracted much attention for the direct oxidation of alkanes to make useful chemicals, including oxidation of propane to acrylic acid8-10 and oxidation of isobutane to methacrolein and methacrylic acid.11,12 The redox properties of
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Figure 1. (a) STM image of H3PMo12O40 array on graphite, and (b) I-V spectra taken at two different sites (Site I and Site II) in the STM image of H3PMo12O40 in Figure 1a.
ability between two electronically equivalent electrodes should increase monotonically with increasing applied potential.27 The NDR phenomenon in tunneling spectra of HPAs is recognized as a consequence of a double-barrier resonant tunneling structure or quantum well in which the electron transmission probability decreases with increasing applied potential at a resonance energy level and has been observed consistently for the arrays of pure HPAs.20-26 A similar explanation for NDR behavior observed for an Anderson-type [PtMo6O24]4- HPA was also reported recently.28 NDR at spatially well-resolved positions in the HPA arrays can distinguish HPAs with different framework compositions on a site-by-site basis in mixed HPA arrays.29 We have also shown that NDR peak voltages of HPAs are closely related to the electronic properties of HPAs and, in turn, to the redox properties of HPAs.9,10,20-25 The NDR peak voltages of HPA monolayers were influenced by the identity of the countercations,20-22 framework transition-metal atoms (polyatoms),21-23 heteroatoms,24 and adsorbed organic molecules.24 Correlations between reduction potentials of Keggin-type HPAs and NDR peak voltage of HPA samples were reported recently to show that more reducible HPAs exhibited NDR behavior at less negative applied potentials in tunneling spectra.25 By correlating NDR peak voltage of HPAs with oxidation activity for some catalytic reactions,9,10 it was shown that NDR peak voltage measured on nanostructured HPA monolayers could serve as a correlating parameter for the performance of bulk HPA catalysts. However, relationships between NDR peak voltages and redox properties of HPAs established thus far have been largely qualitative, because reduction potential data for HPAs measured under consistent conditions were not sufficiently available to cover all families of HPA samples with substituted metal atoms at different positions. It is desirable to establish quantitative relationships between NDR peak voltages and reduction potentials of HPAs to better understand the NDR phenomenon and its utility in evaluating HPA redox properties. In this work, we compare the surface properties of nanostructured HPA monolayers measured by tunneling (24) Song, I. K.; Shnitser, R. B.; Cowan, J. J.; Hill, C. L.; Barteau, M. A. Inorg. Chem. 2002, 41, 1292. (25) Song, I. K.; Barteau, M. A. J. Mol. Catal., A 2002, 182-183, 175. (26) Song, I. K.; Kitchin, J. R.; Barteau, M. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6471. (27) Bonnel, D. A. Scaning Tunneling Microscopy and Spectroscopy; VCH: New York, 1993. (28) Dykhne, A. M.; Vasil’ev, S. Y.; Petrii, O. A.; Rudavets, A. G.; Tsirlina, G. A. Dokl. Akad. Nauk 1999, 368, 467. (29) Kaba, M. S.; Song, I. K.; Barteau, M. A. J. Phys. Chem. B 2002, 106, 2337.
spectroscopy with reduction potentials obtained from electrochemical measurements in aqueous solution.30 Keggin-type HPA samples with different countercation, polyatom, and heteroatom substitutions were examined in a systematic fashion. Experimental Section Materials and Sample Preparation. The following series of HPAs were investigated: cation-exchanged RPMo12O40 (R ) H3, Zn3/2, Co3/2, Cu3/2, Bi1); heteroatom-substituted HnXW12O40 (X ) P, Si, B, Co) and HnXMo12O40 (X ) P, As, Si); and polyatomsubstituted HnPW11M1O40 (M ) W, Mo, V), H3PMoxW12-xO40 (x ) 0, 3, 6, 9, 12), H3+xPMo12-xVxO40 (x ) 0-3), and H3+xPW12-xVxO40 (x ) 0-3). Commercially available H3PMoxW12-xO40 (x ) 0, 3, 6, 9, 12), H3+xPMo12-xVxO40 (x ) 0-3), H3+xPW12-xVxO40 (x ) 0-3), H4SiMo12O40, and H4SiW12O40 samples were purchased from Aldrich Chemical Co. and Nippon Inorganic Color & Chemical Co. H5BW12O40 and H6CoW12O40 were kindly provided by Prof. Craig L. Hill at Emory University. H3AsMo12O40 from Sunoco was provided by Dr. James E. Lyons. Cation-exchanged HPAs were prepared by replacing all protons of H3PMo12O40 with metal atoms, according to published methods.31 Approximately 0.01 M aqueous solutions of each HPA sample were prepared. A drop of solution was deposited on a freshly cleaved HOPG surface and allowed to dry in air for ca. 1 h at room temperature for STM imaging and TS (tunneling spectroscopy) measurements. Scanning Tunneling Microscopy and Tunneling Spectroscopy. STM images were obtained in air using a Topometrix TMX 2010 instrument. Mechanically formed Pt/Ir (90/10) tips were used as probes. Scanning was done in the constant current mode at a positive sample bias of 100 mV and tunneling current of 1-2 nA. Tunneling spectra were measured in air. Both Topometrix TMX 2010 and LK Technologies LK-1000 STM instruments were used to confirm consistency and reproducibility of tunneling spectra. To measure a tunneling spectrum, the sample bias was ramped within the range from -2 to +2 V with respect to the tip and the tunneling current was monitored. The voltage axis in the tunneling spectrum represents the potential applied to the sample relative to that of the tip. Tunneling spectroscopy measurements were performed at least 10 times each using at least three different tips for each sample to obtain more reproducible results and to provide a basis for statistical analyses.
Results Observation of Self-Assembled Monolayer and NDR Behavior. Figure 1a shows the STM image of a H3PMo12O40 array on graphite. The STM image clearly shows the formation of a self-assembled and well-ordered array on the graphite surface. The periodicity constructed on the basis of lattice constants determined from twodimensional fast Fourier transform (2-D FFT) is 10.8 Å. (30) Song, I. K; Barteau, M. A. J. Mol. Catal., A in press. (31) Ai, M. Appl. Catal. 1982, 4, 245.
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The measured periodicity is in good agreement with lattice constants of the Keggin-type HPAs obtained by STM14-25 and X-ray crystallography.32-34 All HPA samples examined in this work formed well-ordered arrays on graphite surfaces over scan areas of at least 200 Å by 200 Å. Figure 1b shows the typical tunneling spectra taken at two different sites, denoted as Site I and Site II in the image of the H3PMo12O40 array in Figure 1a. At sample biases outside the range of -1.5 to +1.5 V the tunneling currents saturated, and these portions of spectra measured over a wider voltage range are not shown. The spectrum taken at a position corresponding to the bright corrugation (Site I) exhibits a distinctive current-voltage (I-V) behavior, referred to as negative differential resistance (NDR). NDR behavior is manifested as local maxima and minima in an I-V spectrum. Such peaks in the I-V spectrum result in negative values of dI/dV. The STM images in this work were obtained at positive sample biases with respect to the tip. This means that electrons flow from the tip to the sample in the imaging mode of operation. NDR behavior in the tunneling spectra of HPAs is consistently observed at negative sample biases, that is, when electrons tunnel from the sample to the tip. Previous quantum-chemical studies35,36of Keggin-type HPAs have suggested that the HOMO (highest occupied molecular orbital), consisting primarily of nonbonding p-orbitals on the terminal oxygens, is little perturbed by framework substitutions, while the LUMO (lowest unoccupied molecular orbital), consisting of an antibonding combination of p-orbitals on the bridge oxygens, is more sensitive to the framework composition. Our interpretation of the chemical sensitivity of NDR in these experiments is that the state through which resonant tunneling occurs is the LUMO of the HPA. NDR arises for electron tunneling from the graphite substrate to the metal tip through the double barrier described above; the LUMO of the individual HPA represents the energy level within this well that defines the energy for resonant tunneling. We define the NDR peak voltage as the voltage at which the maximum current was observed in this region. The NDR peak voltage of the bright corrugation (HPA molecule) in Figure 1a was -0.95 V. A tunneling spectrum taken at the interstitial space (Site II) between bright corrugations showed the same I-V response as bare graphite, indicating that the two-dimensional array of H3PMo12O40 on graphite is a monolayer, as previously demonstrated.18-25 In the tunneling spectroscopy measurements, it was observed that the amplitude of the NDR peaks showed noticeable variations. Therefore, NDR measurements atop HPA sites (Site I) were carried out several times with at least three different tips to obtain more accurate and reproducible results and to provide a basis for statistical analysis of the NDR peak positions. The spectra shown in Figure 1 have not been smoothed and illustrate the representative noise levels observed in individual spectra. It was observed that NDR peak voltages measured for H3PMo12O40 showed a monomodal distribution with a statistical average value of -0.95 ( 0.09 V. The most reproducible and representative NDR peak voltage of each HPA sample examined in this work was determined by this statistitical method, as described previously.25 (32) Pope, M. T. Heteropoly and Isopoly Oxometalates; SpringerVerlag: New York, 1983. (33) Brown, G. M.; Noe-Spirlet, M. R.; Busing, W. R.; Levy, H. A. Acta. Crystallogr., Sect. B 1977, 33, 1038. (34) Hayashi, H.; Moffat, J. B. J. Catal. 1982, 77, 473. (35) Eguchi, K.; Seiyama, T.; Yamazoe, N.; Katsuki, S.; Taketa, H. J. Catal. 1988, 111, 336. (36) Weber, R. S. J. Phys. Chem. 1994, 98, 2999.
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Figure 2. Correlation between NDR peak voltage of cationexchanged RPMo12O40 (R ) H3, Zn3/2, Co3/2, Cu3/2, Bi1) HPAs and Tanaka electronegativity of the countercation (filled squares), and between NDR voltage and reduction potential of RPMo12O40 (R ) H3, Zn3/2, Co3/2, Cu3/2, Bi1) samples (open squares).
Effect of Countercation Substitution. Figure 2 shows the correlation between NDR peak voltage of cationexchanged RPMo12O40 (R ) H3, Zn3/2, Co3/2, Cu3/2, Bi1) HPAs and Tanaka electronegativity37 of the countercation (filled squares) and between NDR voltage and reduction potential of RPMo12O40 (R ) H3, Zn3/2, Co3/2, Cu3/2, Bi1) samples (open squares). The reduction potentials of the HPA samples measured electrochemically were recently reported.30 The correlation of reduction potentials with the NDR peak voltages and the Tanaka electonegativities of the countercations showed consistent trends;25 HPA samples with more electronegative countercations had higher reduction potentials and showed NDR behavior at less negative applied voltages. The Tanaka electronegativity37 takes into account the electron-donating and accepting ability of the metal atom. One explanation for these results is that a more electronegative cation acts as a large electron reservoir to facilitate electron transfer to the heteropolyanion in reducing environments, by providing a route for electron delocalization.38 Effect of Central Heteroatom Substitution. Figure 3 shows the extensive and comparative correlation between NDR peak voltage of heteroatom-substituted HPAs and Tanaka electronegativity of the heteroatom (filled squares) and between NDR voltage and reduction potential of heteroatom-substituted HPA samples (open squares), established for HnXW12O40 (X ) P, Si, B, Co) and HnXMo12O40 (X ) P, As, Si) HPAs. For both families of heteroatom-substituted HPA samples, it is apparent that the NDR peak voltages of HPAs appeared at less negative values with increasing reduction potential of the HPAs and with increases in the electronegativity of the heteroatom. These dependencies of the NDR peak voltage and reduction potential on the electronegativity of heteroatoms exhibit the same trends as those observed for cation-exchanged HPAs (Figure 2), supporting the conclusion that more reducible HPAs show NDR behavior at less negative applied voltages in their tunneling spectra. When taking into account different electron-donating and accepting ability of the heteroatoms, the effect of the heteroatom on the HPA reduction potential may be understood in a similar manner as suggested above for cation-exchanged HPAs. The dependence of HPA reduction potential on the identity of the central heteroatoms is also well supported by previous quantum-chemical molecular orbital calculations35 and STM investigations.25 As mentioned earlier, we have suggested that the state through which resonant tunneling occurs is the LUMO of (37) Tanaka, K.; Ozami, A. J. Catal. 1967, 8, 1. (38) Kim, H. C.; Moon, S. H.; Lee, W. Y. Chem. Lett. 1991, 447.
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Figure 3. Correlation between NDR peak voltage of heteroatom-substituted HPAs and Tanaka electronegativity of the heteroatom (filled squares), and between NDR voltage and reduction potential of heteroatom-substituted HPA samples (open squares), established for families of (a) HnXW12O40 (X ) P, Si, B, Co) and (b) HnXMo12O40 (X ) P, As, Si) HPAs.
Figure 4. Correlation between NDR peak voltage of polyatomsubstituted HnPW11M1O40 (M ) W, Mo, V) HPAs and Tanaka electronegativity of the monosubstituted polyatom (filled squares) and between NDR voltage and reduction potential of HnPW11M1O40 (M ) W, Mo, V) HPA samples (open squares).
the HPA and, therefore, the reduction potential of an HPA is closely related to the LUMO of the HPA. In the previous study35 investigating a set of heteroatom-substituted HnXMo12O40 (X ) As, P, Ge, Si) HPAs, reduction potentials of the HPAs determined by polarographic methods increased in the order Si (0.475 V) < Ge (0.492 V) < P (0.518 V) < As (0.526 V). Theoretical calculations for HnXMo12O40 (X ) As, P, Ge, Si) HPAs revealed that the LUMO (antibonding with respect to Mo-O-Mo bonds) is responsible for reduction of HPAs and the reduction takes place at the bridging oxygens.35 The calculated energy values of the LUMO for HnXMo12O40 (X ) As, P, Ge, Si) HPAs followed the order Si > Ge > P > As, suggesting that H3AsMo12O40 is the most reducible and H4SiMo12O40 is the least reducible in this series. Simple calculation of the Tanaka electronegativity of the heteroatoms in these HPAs gives the sequence Si (17.1) < Ge (18.1) < As (24.0) ≈ P (24.1). The trends in reduction potentials and NDR peak voltages are consistent with this order for both the Mo- and W-framework Keggin ions in Figure 3. Effect of Framework Substitution. Figure 4 shows the correlation between NDR peak voltage of frameworksubstituted HnPW11M1O40 (M ) W, Mo, V) HPAs and Tanaka electronegativities of the polyatoms substituted (filled squares) and between NDR voltages and reduction potentials of HnPW11M1O40 (M ) W, Mo, V) HPA samples (open squares). Reduction potentials of HnPW11M1O40 (M ) W, Mo, V) were measured electrochemically under the same conditions as those in Figures 2 and 3.30 The NDR peak voltages of HnPW11M1O40 (M ) W, Mo, V) arrays appeared at less negative values with increasing reduction potential of the HPAs and with decreasing electronegativity of the polyatom. This result is consistent with our finding that more reducible HPAs show NDR behavior at less negative applied voltages in their tunneling spectra,
Figure 5. Reduction potential (open squares) and NDR peak voltage (filled squares) of polyatom-substituted H3PMoxW12-xO40 (x ) 0, 3, 6, 9, 12) HPA samples with respect to the number of Mo substitution.
as observed for cation-exchangd HPAs (Figure 2) and heteroatom-substituted HPA samples (Figure 3). However, the trends of polyatom electronegativity with respect to NDR peak voltage and reduction potential of polyatomsubstituted HPAs are the opposite of those seen for the cation-exchanged HPAs (Figure 2) and for heteroatomsubstituted HPAs (Figure 3). In other words, the NDR peak voltage appears at a less negative value, and reduction potential increases, with decreasing electronegativity of the polyatom. The same dependencies of the reduction potential and NDR peak voltage on polyatom electronegativity were also observed for a series of polyatom-substituted H3PMoxW12-xO40 (x ) 0, 3, 6, 9, 12) HPAs, as shown in Figure 5. The NDR peak voltages (filled squares) of these HPAs shifted to less negative values and their reduction potentials (open squares) increased in a monotonic fashion with increasing Mo content, supporting the conclusion that more reducible HPAs show NDR behavior at less negative applied voltages. When considering that molybdenum is less electronegative than tungsten, the dependence of NDR peak voltage on the polyatom electronegativity observed for the family of H3PMoxW12-xO40 (x ) 0, 3, 6, 9, 12) HPAs (Figure 5) shows the same trend as observed for HnPW11M1O40 (M ) W, Mo, V) HPA samples (Figure 4). That is, NDR peaks of both families of polyatomsubstituted HPAs appeared at less negative applied voltages with decreasing electronegativity of the polyatom. The effect of vanadium substitution as a polyatom on the NDR peak voltage and the reduction potential of HPAs is somewhat more complicated. Figure 6 shows the reduction potential (open squares) and NDR peak voltage (filled squares) of vanadium-substituted HPA samples with respect to the number of vanadiums substituted, for H3+xPMo12-xVxO40 (x ) 0-3) and H3+xPW12-xVxO40 (x ) 0-3) HPAs. NDR peak voltages of both families of
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Figure 6. Reduction potential (open squares) and NDR peak voltage (filled squares) of polyatom-substituted HPA samples with respect to the number of V substitution, established for families of (a) H3+xPMo12-xVxO40 (x ) 0-3) and (b) H3+xPW12-xVxO40 (x ) 0-3) HPAs.
vanadium-substituted HPAs did not vary monotonically with the number of vanadiums substituted. Instead, the NDR peak voltage and the reduction potential exhibited maxima at intermediate levels (x ) 1 or 2) of vanadium substitution. A previous molecular orbital study36 for HnPM12-xVxO40 (M ) Mo, W; x ) 0-3) HPAs revealed that the energy gap between the HOMO and the LUMO was consistent with reduction potential of the HPAs and that more reducible HPAs showed smaller energy gaps. That study36 also indicated that the HOMO for all HPAs consists primarily of nonbonding p-orbitals on the oxygens of the HPAs, while the LUMO consists of an antibonding combination of d-orbitals on the metal centers and p-orbitals on the neighboring bridging oxygens. Thus, substitution of V ions into either the Mo or W framework does not affect the energies of the HOMOs significantly since they are almost entirely centered on the oxygens. The same substitution, however, stabilizes the LUMOs because these orbitals derive substantially from V d-orbitals which have been assumed to be more stable than those of Mo and W. This indicates that electrons added to the V-substituted HPAs should be localized on the vanadium centers. Therefore, it is believed that the less electronegative vanadium in the series HnPW11M1O40 (M ) W, Mo, V) (Figure 4), H3+xPMo12-xVxO40 (x ) 0-3) (Figure 6a), and H3+xPW12-xVxO40 (x ) 0-3) HPAs (Figure 6b), and molybdenum in the series of H3PMoxW12-xO40 (x ) 0, 3, 6, 9, 12) HPAs (Figure 5) are much more efficient in the role of electron localization than are the metal atoms of the framework into which they are substituted. Discussion We have shown that redox properties of HPAs can be determined from surface electronic properties of nanostructured HPA monolayers and have established correlations between NDR peak voltages and reduction potentials of HPAs by investigating a wide set of cationexchanged (Figure 2), heteroatom-substituted (Figure 3), and polyatom-substituted HPAs (Figures 4-6). We observed systematic variations of NDR peak voltage and reduction potential of HPAs depending on the electronegativity of the exchanged/substituted atoms. Substitution of more electronegative atoms for countercations or for the central heteroatom shifted the NDR peaks to less negative voltages, corresponding to increased reduction potentials of the HPAs. However, substitution of more electronegative metals into the Keggin framework shifted the NDR peaks to more negative voltages, corresponding to decreased reduction potentials. The most important and consistent conclusion is that, regardless of exchange/ substitution positions, NDR peaks appeared at less negative potentials for higher reduction potentials of the
Figure 7. Correlation between reduction potential and NDR peak voltage of Keggin-type HPAs established for all families of HPAs examined in this work. Data points from Figures 2-6 were presented cumulatively in Figure 7.
HPAs. This implies that NDR peak voltage can be utilized as a correlating parameter for reduction potential. The correlations between NDR peak voltages and reduction potentials of HPAs reported in our previous work25 were largely qualitative. Direct comparison of reduction potentials of HPAs from literature data was not a simple task because of the lack of reduction potential data determined on a consistent measurement basis. However, the correlations between NDR peak voltages and reduction potentials of HPAs reported in this work were quantitative ones because reduction potentials of HPA samples were measured electrochemically under consistent conditions.30 This makes it possible for us to compare reduction potentials and NDR peak voltages of HPAs belonging to different HPA families directly. For example, Figure 3 reveals that HnXMo12O40 (X ) P, As, Si) HPAs give rise to NDR peaks at less negative voltages and exhibit higher reduction potentials than those of HnXW12O40 (X ) P, Si, B, Co) HPAs. An important question is whether NDR peak voltages of HPAs can serve as a single correlating parameter for the reduction potentials of HPAs spanning different families, at least across all HPA families examined in this work. Figure 7 shows the comprehensive correlation between reduction potential and NDR peak voltage of HPAs, established for all HPA samples examined in this work. Data points from Figures 2-6 are presented cumulatively in Figure 7. Importantly, the correlation demonstrates that NDR peak voltages are directly correlated with reduction potentials of HPAs regardless of substituted structures; NDR peaks appeared at less negative potentials for higher reduction potentials of the HPAs. The reduction potential represents the potential for electron addition to the LUMO of the heteropolyanion,
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relative to the reference electrode.36,39 Although the reference level is less clear in the case of NDR peaks, since this phenomenon involves resonant tunneling through the LUMO, variations in NDR peak position with changes in HPA composition should represent changes in the accessibility of the LUMO. Thus, the direct correlation between these two different measurements, illustrated in Figure 7, is not surprising. The advantage of the tunneling spectroscopy measurements is that they are free of solvent, of solubility limitations, and of diffusion limitations. They are likely more germane to the redox behavior of HPAs in the solid state, for example, as heterogeneous catalysts, and may provide greater insight into the influence of the associated countercations on the redox properties of solid HPAs. Thus, the NDR peak voltage may provide a selection and design basis for HPA catalysts efficient for selective oxidation and related reactions. Conclusions STM investigation of surface properties of nanostructured HPA monolayers was carried out to relate nanoscale properties to bulk redox properties of HPAs. A wide set of Keggin-type HPAs with different countercation, polyatom, and heteroatom substitutions were examined for this purpose. All HPA samples formed well-ordered monolayer arrays and exhibited negative difference resistance (NDR) (39) Lee, L.; Gewirth, A. A. J. Electroanal. Chem. 2002, 522, 11.
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behavior in their tunneling spectra. The observed NDR peak voltages were correlated with the reduction potential of HPAs and with the electronegativity of the substituted metal atom. The NDR peak voltages observed for both cation-exchanged and heteroatom-substituted HPAs appeared at less negative values with increasing reduction potential of the HPAs and with increases in the electronegativity of countercation and heteroatom. On the other hand, the dependencies of polyatom electronegativity on the NDR peak voltage and reduction potential of polyatom-substituted HPAs were the opposite of those observed for cation-exchanged and for heteroatomsubstituted HPAs. However, it was consistently observed that more reducible HPAs showed NDR behavior at less negative applied voltages. A comprehensive correlation between NDR peak voltage and reduction potential of HPAs, established for all families of HPA samples examined in this work, showed that the NDR peak voltage could be utilized as a correlating parameter (as an alternative parameter) for the reduction potential of the HPAs. A less negative NDR peak voltage corresponds to a higher reduction potential of the HPA. Acknowledgment. This work was supported by the Korea Research Foundation Grant (KRF-2002-041D00126). The authors thank Prof. C. L. Hill and Dr. J. E. Lyons for providing several of the HPA samples. LA030281Z
