Investigation of framework and cation substitutions in Keggin-type heteropoly acids probed by scanning tunneling microscopy and tunneling spectroscopy

Scanning tunneling microscopy (STM) and spatially resolved tunneling spectroscopy were used to characterize surface arrays of heteropoly acids (HPAs) of the Keggin structural class. STM images revealed that these discrete oxide anions formed single, ordered monolayers on inert graphite surfaces, with lattice spacings of similar to 11 Angstrom, which is the characteristic dimension of the Keggin anion. However, increases in the lattice constants were observed in the STM images upon the substitution of the protons in H3PMo12O40 With larger counter cations such as Cs and K. This finding is consistent with those observed for bulk crystals of these salts. Substitution of metal ions in the Keggin ion framework (such as the replacement of Mo with W, or vice versa), led to insignificant variations in the lattice constants of the surface arrays. Tunneling spectroscopy measurements on discrete polyanions in the STM images revealed that the HPAs (both cation-exchanged and framework-substituted) exhibited current peaks in their respective current-voltage spectra, but at different applied potentials. The location of the current peak, or negative differential resistance (NDR) feature, was characteristic of the reducibility of the Keggin-type HPA in each case-the more reducible the HPA, the smaller the magnitude of the NDR peak voltage. (C) 1997 American Vacuum Society.