398 Inorg. Chem. 1998, 37, 398-406 Molecular Shapes, Orientation, and Packing of Polyoxometalate Arrays Imaged by Scanning Tunneling Microscopy Mahmoud S. Kaba,† In K. Song,† Dean C. Duncan,‡ Craig L. Hill,‡ and Mark A. Barteau*,† Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, and Department of Chemistry, Emory University, Atlanta, Georgia 30322 ReceiVed May 14, 1997 Reported here are both STM images and spatially resolved tunneling spectra of four different polyoxometalate (POM) structural class members: Keggin structure, H3[PW12O40] (spherical); Finke-Droege (FD) structure, Na16- [Cu4(H2O)2(P2W15O56)2] (prolate spheroidal); Wells-Dawson (WD) structure, H7[P2Mo17VO62] (prolate spheroidal); and Pope-Jeannin-Preyssler (PJP) structure, K12.5Na1.5[NaP5W30O110] and (NH4)14 [NaP5W30O110] (oblate spheroidal). In all four cases, the results demonstrate the formation of well-ordered 2-D inorganic POM anion arrays (composed of catalytically active molecular constituents) on graphite. Importantly, the image shapes and lattice spacings accurately reflect the POM anisotropies, permitting the determination of anion orientation with respect to the surface plane. Introduction The distinguishing characteristics of this work are the self- Although the scanning tunneling microscope is capable of assembly and molecular imaging of well-ordered inorganic anion atomic resolution imaging of solid surfaces, accurate imaging monolayers. We have chosen a set of polyoxometalate anions of adsorbates with even molecular resolution is far from routine. (POMs) with distinctive shapes in order to test the ability of STM to resolve molecular anisotropies of inorganic complexes Successful molecular resolution imaging (i.e., images which 0 yield accurate molecular shapes and sizes) has been demon- under ambient conditions. POMs are d early transition metal strated for various small organic molecules adsorbed and imaged oxygen anion clusters which exhibit a wide diversity of in an ultrahigh-vacuum environment. Examples include mixed molecular architectures, surface charge densities, and both C H and CO adlayers on Rh(111),1 aromatics on Pt(111),2 photo- and thermoredox behaviors. These molecules have found 6 6 applications as acid and oxidation catalysts,14,15 electrode perylenetetracarboxylic acid on highly oriented pyrolytic graph- 16 17 3 4,5 functionalization agents, and antiretroviral agents. One of ite (HOPG), and C60 on metal and semiconductor substrates. In cases where the adsorbed layer is not close packed, the the great advantages of POM monolayer assemblies as compared with the more widely studied organic monolayers is the greater apparent molecular dimensions from such images may be 18 distorted owing to adsorbate mobility, tip-dragging, and other thermal stability of POMs. This stability makes monolayers effects.6 Close-packed monolayers of a wide range of organics of these discrete nanoscale clusters good candidates for catalytic can be formed on surfaces by Langmuir-Blodgett and self- and sensor applications which may require harsh environments. assembly techniques, for example, and accurate molecular Several groups have reported the formation of ordered arrays resolution images (and, in some cases, images of intramolecular of POMs on surfaces and have imaged these using STM. Keita 19 features) have been produced by STM under ambient conditions. and Nadjo deposited H3PW12O40 from methanol solution by Examples include alkanes, alkanethiols, alkyl aromatics, and drying on HOPG, reporting resolution of individual anions in a liquid crystals,7-11 as well as several organometallic com- periodic arrangement at the surface of the deposited layers. They plexes.12,13 suggested that their procedures produced “thin layers” which could not be “controlled to be uniformly spread as a monolayer” * To whom correspondence should be addressed. Tel: (302) 831-8056. and noted difficulty in producing films giving images which Fax: (302) 831-2085. E-mail: [email protected]. permitted comparison with the known structure of H3PW12O40. † University of Delaware. ‡ Emory University. (1) Chiang, S.; Wilson, R. J.; Mate, C. M.; Ohtani, R. J. Microsc. 1988, (11) Walba, D. M.; Stevens, F.; Parks, D.; Clark, N.; Wand, M. Science 152, 567. 1995, 267, 1144. (2) Hallmark, V. M.; Chiang, S.; Meinhart, K.-P.; Hafner, K. Phys. ReV. (12) Hudson, J. E.; Abruna, H. D. J. Phys. Chem. 1996, 100, 1036. Lett. 1993, 70, 3740. (13) Snyder, S. R.; White, H. S. J. Phys. Chem. 1995, 99, 5626. (3) Ludwig, C.; Gompf, B.; Glatz, W.; Petersen, J.; Eisenmenger, W.; (14) Misono, M. Catal. ReV.sSci. Eng. 1987, 29, 269. Mo¨bus, M.; Zimmerman, V.; Karl, N. Z. Phys. B 1992, 86, 397. (15) Hill, C. L.; Prosser-McCartha, C. M. Coord. Chem. ReV. 1995, 143, (4) Altman, E. I.; Colton, R. J. J. Vac. Sci. Technol. B 1994, 12, 1906. 407. (5) Chen, D. M.; Xu, H.; Creager, W. N.; Burnett, P. J. Vac. Sci. Technol. (16) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1990, 287, 149. B 1994, 12, 1910. (17) Hill, C. L.; G.-S. Kim; C. M. Prosser-McCartha; Judd, D. In (6) Hallmark, V. M.; Chiang, S. Surf. Sci. 1993, 286, 190. Polyoxometalates: From Platonic Solids to Anti-retroViral ActiVity; (7) Smith, D. P. E.; Ho¨rbe, J. K. H.; Binnig, G.; Nejoh, H. Nature 1990, Pope, M. T., Muller, A., Eds.; Kluwer: Dordrecht, The Netherlands, 344, 641. 1993; p 359. (8) Rabe, J. P.; Buchholz, S. Phys. ReV. Lett. 1991, 66, 2096. (18) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag: (9) Breen J. J.; Flynn, G. W. J. Phys. Chem. 1992, 96, 6825. New York, 1983. (10) Patrick, D. L.; Beebe, T. P. Langmuir 1994, 10, 298. (19) Keita, B.; Nadjo, L. Surf. Sci. 1991, 254, L443. S0020-1669(97)00565-X CCC: $15.00 © 1998 American Chemical Society Published on Web 01/16/1998 Polyoxometalate Arrays Inorganic Chemistry, Vol. 37, No. 3, 1998 399 29 Subsequent STM images of deposited Na6H2[CeW10O36]‚30H2O of four POM structural classes: Keggin, H3[PW12O40]; Finke- 20 30 31 from the same laboratory were compared with AFM images Droege (FD), Na16[Cu4(H2O)2(P2W15O56)2]; Wells-Dawson 32 of the bulk compound and were described as exhibiting short- (WD), H7[P2Mo17VO62]; and Pope-Jeannin-Preyssler (PJP), range order owing to the uncontrolled nature of the deposition. K12.5Na1.5[NaP5W30O110] and (NH4)14 [NaP5W30O110]. Figure Electrochemical deposition was suggested to give more uniform 1 shows spatially accurate structural representations of each class and reproducible thin films,21 and it was noted that “thicker” reconstructed from published X-ray crystal structures.32-35 As regions of the POM deposits were difficult to image and shown below, all four POMs form well-ordered 2-D anion arrays exhibited considerable disorder. on HOPG and, most importantly, their STM images accurately Our initial report22 provided the first evidence for the reflect the shapes and sizes characteristic of their respective formation of POM monolayer arrays by depositing these structural classes, permitting the orientation of anisotropic POMs compounds from solution onto HOPG by drying. Ordered with respect to the surface plane to be determined. arrays of the heteropolyanion derivatives H3PMo12O40,Na4- SiW12O40, and H7SiW9V3O40, as well as the isopoly compound Experimental Section (NH4)6V10O28‚6H2O, were imaged. These ordered surface arrays exhibited periodicities consistent with the dimensions of The POMs, H3[PW12O40], K12.5Na1.5[NaP5W30O110], (NH4)14- [NaP5W30O110], and Na16[Cu4(P2W15O56)2], were prepared according to the respective anions. Tunneling spectroscopy in air was applied 30,36-38 to these materials for the first time, demonstrating (i) negative published procedures. The WD complex, H7P2Mo17VO62, was supplied by the DuPont Co. (Wilmington, DE). The composition and differential resistance in the spectra of all polyanions examined structural integrity of the Keggin, PJP, and FD samples were verified and (ii) the characteristic spectrum of graphite at the interstitial by 31P NMR and FTIR spectroscopy prior to use. The following positions in the isopolyanion arrays, suggesting that the arrays chemical shifts (relative to 85% H3PO4 external standard) were 22 23 31 imaged were indeed monolayers. Recent work by Ge et al. measured by P NMR: H3PW12O40, -14.9 ppm (literature value -14.9 39 also demonstrated self-assembly of silicotungstate anion mono- ppm ); K12.5Na1.5[NaP5W30O110], -10.4 ppm (literature values -10.35 layers on a silver surface by electrochemical methods. Although ppm,32 -9.9 ppm40). Infrared spectra obtained by deposition of the the electronic states of the POMs which contribute to the STM POMs on the diamond probe of a total internal reflectance FTIR images of these arrays have not been considered in detail, both spectrometer, described previously,26 are given in the Supporting the occupied and unoccupied frontier orbitals for the Keggin Information. structure involve the 2p orbitals of the oxygen atoms at bridging Samples for STM imaging were prepared by depositing one drop of positions in the framework.24 The delocalization of the frontier aqueous POM solution (0.01 M) onto a freshly cleaved highly oriented orbitals over the Keggin framework may account for the absence pyrolytic graphite (HOPG) surface. The films were allowed to dry for of intramolecular features (i.e., the relatively isotropic appear- 1 h under ambient conditions with the exception of the PJP sample which required vacuum desiccation to minimize bulk crystal formation. ance) for individual anions