Scanning tunneling microscopy of ordered arrays of heteropolyacids deposited on a graphite surface

Keggin-type heteropolyacids (water-soluble H3PMo12O40, H4PMo11VO40, and H8PMo10VCuO40 and water-insoluble K3PMo12O40 and H3-xCsxPMo12O40, x = 1, 2, 2.5, 3) deposited on highly oriented pyrolytic graphite (HOPG) surfaces were successfully imaged by scanning tunneling microscopy (STM). All of these heteropoly acids (HPAs) formed clear two-dimensional ordered arrays on graphite, and their periodicities were in good agreement with values determined by X-ray crystallography. Spatially resolved tunneling spectroscopy was used to demonstrate that the arrays imaged represented HPA monolayers, as the characteristic tunneling spectrum of graphite was obtained at high-symmetry interstitial sites in these arrays. The periodicity of the ordered arrays of the water-soluble Keggin-type HPAs was ca. 10-11 Angstrom. However, ordered arrays of the water-insoluble K3PMo12O40 and H3-xCsxPMo12O40 showed periodicities of ca. 12-14 Angstrom, values somewhat larger than that of the pure acid form. The array spacing increased monotonically with increasing substitution of larger (Cs) cations, as expected. However, the nearly invariant periodicity of 2-D arrays of the water-soluble HPAs with changes in the number of protons suggests that there is little or no water of crystallization in these arrays, in contrast to the bulk compounds, STM permitted observation of the ordered structures of HPAs when deposited on a chemically inert surface, as well as the determination of the effects of changing framework and charge-compensating cations on these structures. The ubiquity of ordered array formation of these HPAs suggests that they can be utilized to create well-defined surfaces with more complex chemical functions than one typically encounters in studies of metal or oxide single-crystal surfaces.