Mahmoud S. Kaba

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 ∼11 A, 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 respe...

Investigation of redox potential and negative differential resistance behavior of heteropolyacids by scanning tunneling microscopy

Abstract Nanoscale images and tunneling spectra of cation-exchanged or polyatom-substituted heteropolyacids (HPAs) were probed by scanning tunneling microscopy (STM) at room temperature. All the HPAs formed two-dimensional ordered arrays on graphite surfaces, and their molecular dimensions were in good agreement with values determined by X-ray crystallography. It was observed that the negative differential resistance (NDR) peak voltage measured by STM was closely related to the reduction potential of the corresponding bulk HPA. The higher the reduction potential of the HPA, the lower the applied voltage at which NDR was observed.

Investigation of Wells-Dawson Heteropoly Oxofluorotungstates by Scanning Tunneling Microscopy and Tunneling Spectroscopy

Ammonium salts of Wells-Dawson heteropoly oxofluorotungstates with different framework compositions. (NH 4 ) 9 Mn(II)W 17 O 56 F 6 NaH 4 .9H 2 O, (NH 4 ) 9 Cu(II)W 17 O 56 F 6 NaH 4 .9H 2 O, (NH 4 ) 8 Mn(III)W 17 O 56 F 6 NaH 4 .8H 2 O, and (NH 4 ) 8 Fe(III)W 17 O 56 F 6 NaH 4 .8H 2 O, were deposited on highly oriented pyrolytic graphite surfaces and imaged by scanning tunneling microscopy. All samples formed monolayers of two-dimensional, well-ordered, close-packed arrays. The lattice constants of these arrays in each case were roughly ca. 13 × 16 A; these values were consistent with the molecular dimensions of typical Wells-Dawson heteropolyanions (10.98 × 14.48 A), plus charge-compensating NH + 4 cations (ionic radius of 1.43 A) between the heteropolyanions. The presence and positions of these cations in the surface arrays were subsequently confirmed by tunneling spectroscopy (TS), and the positions were consistent with those found in the bulk crystal structures of these salts. TS measurements also showed that the oxofluorotungstates exhibited negative differential resistance (NDR) behavior in the negative sample bias regime of their current-voltage spectra. Their NDR peak voltages varied depending on the identity of the monosubstituted polyatom, Mn 2+ , Cu 2+ , Mn 3+ , and Fe 3+ . The reduction potentials of heteropoly oxofluorotungstates increased and NDR peak voltages appeared at less negative applied values with the decreasing electronegativity of the monosubstituted polyatom. The correlation between reduction potentials and NDR peak voltages established for these heteropolyacids (HPAs) was consistent with trends observed in the cation-exchanged and heteroatom-substituted HPAs; the more reducible HPAs showed NDR behavior at less negative applied voltages. However, the trends in NDR peak voltage and reduction potential with respect to the electronegativity of the monosubstituted polyatom were opposite of those observed for the cation-exchanged and heteroatom-substituted HPAs.