Scanning Tunneling Microscopy of Ordered Arrays of Heteropolyacids Deposited on a Graphite Surface

Song, In Kyu; Kaba, Mahmoud S.; Coulston, George W.; Kourtakis, K.; Barteau, Mark A.

doi:10.1021/cm960177l

Cited by 52 publications

(82 citation statements)

References 30 publications

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“…The absence of such variations among different regions of the same sample probed by STM and TS suggests that the composition of the surface HPA arrays is spatially uniform. In agreement with other STM reports, 10,18 the TS measurements also indicated that the HPAs formed monolayers on the graphite surfaces, as the tunneling spectrum of graphite was obtained at high symmetry interstitial sites in these arrays.…”

Section: Introductionsupporting

confidence: 92%

“…10,[15][16][17][18][19][20] In tunneling spectroscopy experiments, we have consistently observed that HPAs deposited on graphite exhibit distinctive current-voltage (I-V) behavior, referred to as negative differential resistance (NDR). 10,[18][19][20] In this work, several water-soluble and water-insoluble cationexchanged 12-molybdophosphates were prepared and deposited on a highly oriented pyrolytic graphite (HOPG) surface to obtain images and tunneling spectra by STM. These data serve to establish a correlation between NDR behavior in the tunneling spectrum of the HPA monolayer and its reduction potential, i.e., the oxidizing power of the heteropoly acid catalyst.…”

Section: Introductionmentioning

confidence: 99%

“…HPAs are known to exhibit NDR behavior in their tunneling spectra, and the potential at which this behavior is observed can be used as a fingerprint of the HPA. 10 Both the array spacing 18 and NDR peak voltage were found to vary for samples in which the proton content was deliberately varied by ion exchange, e.g., for the series H 3-x Cs x PMo 12 O 40 . The absence of such variations among different regions of the same sample probed by STM and TS suggests that the composition of the surface HPA arrays is spatially uniform.…”

Section: Introductionmentioning

confidence: 99%

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Ordered Array Formation and Negative Differential Resistance Behavior of Cation-Exchanged Heteropoly Acids Probed by Scanning Tunneling Microscopy

1996

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Cation-exchanged 12-molybdophosphates deposited on a graphite surface were imaged by scanning tunneling microscopy (STM), and their current-voltage behavior in tunneling spectroscopy (TS) measurements was examined. The periodicity of the polyanion arrays in the STM images was ca. 11-14 Å, and the spacing was dependent on the identity of the countercation. It was also observed that the characteristic negative differential resistance (NDR) behavior of 12-molybdophosphates, observed in TS measurements, is closely related to the reduction potential of the polyoxometalate and to the electronegativity of the countercation. The applied potential (sample to tip) at which NDR behavior was observed decreased with increases in the reduction potential of the HPA as well as with increases in electronegativity of the countercation. More electronegative countercations facilitated electron transfer between the cation and the heteropoly anion, apparently by acting as electron reservoirs, leading to NDR behavior at lower applied voltages and generating higher reduction potentials. This work demonstrates the first correlation between tunneling spectra measured by STM and the chemical properties of polyoxometalates.

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Section: Introductionsupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Ordered Array Formation and Negative Differential Resistance Behavior of Cation-Exchanged Heteropoly Acids Probed by Scanning Tunneling Microscopy

1996

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“…3. Literature survey shows that the particles of Cs-salts obtained by the precipitation from aqueous solutions are in the form of irregular aggregates of size within wide range [33,34]. This shape is also observed for our Cs2-CO 3 and Cs2-Cl samples.…”

Section: Cs-salts Of H 3 Pw 12 O 40supporting

confidence: 82%

Methanolysis of Castor Oil Catalysed by Solid Potassium and Cesium Salts of 12-Tungstophosphoric Acid

et al. 2008

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Methanolysis of castor oil to methyl esters, a key step in Biodiesel production, was studied with the use of KOH, H 2 SO 4 and 12-tungstophosphoric acid (H 3 PW 12 O 40 , HPW) as the homogeneous catalysts. Reaction was also performed in the presence of solid salts of HPW, namely M x H 3-x PW 12 O 40, where M = K or Cs and x = 2, 2.5 and 3 (abbreviated as K2, K2.5, K3 and Cs2, Cs2.5, Cs3, respectively). The HPW salts were precipitated by K 2 CO 3 and Cs 2 CO 3 or CsCl. Their properties were characterized by BET, electron microscopy (SEM, EDS) and colloidal particles size distribution (laser diffraction technique). The potassium doped HPW samples, K2 and K2.5, prove to be much more active catalysts (ca. 3 times) than their Cs-containing analogues. Among the K, Cs salts, K2 salt was the most active catalyst. The activity of catalysts was found to depend on preparation stages such as the temperature of drying or annealing and aging of the samples. Microscopic studies evidenced colloidal form of Cs and K salts particles under the catalytic reaction. The size of colloidal particles was found to depend upon the type of cation, Cs ? or K ? , as well as the ''history'' of catalysts preparation (temperature of drying). Based on the results obtained in this work, we concluded that activity was determined by the accessibility of the reactants to acid sites which is facilitated by the high surface area and open structure of the colloidal form. This may lead to better utilization of acid sites and higher activity of samples with lower content of K ? or Cs ? cations in the HPW.

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“…Calibration procedures for the STM measurements are described elsewhere. 35,36 No STM image presented in this report has been filtered, and all lattice dimensions reported were determined from two-dimensional fast fourier transforms (2-D FFT).…”

Section: Methodsmentioning

confidence: 99%

STM Investigation of Pyridine Interaction with Heteropoly Acid Monolayers

1996

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Images and tunneling spectra of H 3 PMo 12 O 40 and Cs 3 PMo 12 O 40 deposited on graphite were obtained in air using scanning tunneling microscopy before and after pyridine adsorption, and the effects of pyridine adsorption on the HPA arrays and on their tunneling spectra were examined. Fresh H 3 PMo 12 O 40 formed well-ordered two-dimensional arrays with a nearly square symmetry on the graphite surface, with a characteristic lattice spacing of about 10.8 Å, as determined from STM images. Upon interaction with pyridine, the STM images showed not only a change in array symmetry from square to hexagonal but also an increase in the lattice constant to ca. 16.5 Å, as well. This indicated that even in the two-dimensional array of H 3 PMo 12 O 40 , the ∼6 Å pyridine molecules were mainly adsorbed on the acid sites in the interstices between the polyanions, causing an increase in the interstitial spacing. In contrast, STM images of both fresh Cs 3 PMo 12 O 40 and pyridineadsorbed Cs 3 PMo 12 O 40 showed similar two-dimensional ordered array formation with hexagonal symmetry and a lattice constant of about 13.7 Å (reflecting the larger size of Cs + than H + ). This similarity of the surface arrays of Cs 3 PMo 12 O 40 with and without pyridine exposure is due to the absence of acid adsorption sites for pyridine in the two-dimensional structure of Cs 3 PMo 12 O 40 . Confirmation of pyridine adsorption by H 3 PMo 12 O 40 and not by Cs 3 PMo 12 O 40 was also obtained by infrared spectroscopy and by spatially resolved tunneling spectroscopy; the latter technique distinguished the anion, cation, and void positions in these arrays.

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Scanning Tunneling Microscopy of Ordered Arrays of Heteropolyacids Deposited on a Graphite Surface

Cited by 52 publications

References 30 publications

Ordered Array Formation and Negative Differential Resistance Behavior of Cation-Exchanged Heteropoly Acids Probed by Scanning Tunneling Microscopy

Ordered Array Formation and Negative Differential Resistance Behavior of Cation-Exchanged Heteropoly Acids Probed by Scanning Tunneling Microscopy

Methanolysis of Castor Oil Catalysed by Solid Potassium and Cesium Salts of 12-Tungstophosphoric Acid

STM Investigation of Pyridine Interaction with Heteropoly Acid Monolayers

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