Negative Differential Resistance Peak Voltage Research Articles

Scanning tunneling microscopy investigation of nano-structured α-K5PW11(M x OH2)O39(M = Mn(II), Co(II), Ni(II), and Zn(II)) Keggin heteropolyacid catalyst monolayers.

Nano-structured α-K5PW11(M x OH2)O39 (M = Mn(II), Co(II), Ni(II), and Zn(II)) Keggin heteropolyacids (HPAs) were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) measurements in order to elucidate their redox property and oxidation catalysis. HPA molecules formed two-dimensional self-assembled monolayer arrays on highly oriented pyrolytic graphite (HOPG) surface. Furthermore, HPAs exhibited a distinctive current-voltage behavior referred to as negative differential resistance (NDR) phenomenon. The measured NDR peak voltage of HPAs was correlated with the reduction potential and the absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. NDR peak voltage of HPAs appeared at less negative voltage with increasing reduction potential and with decreasing UV-visible absorption edge energy. The correlations strongly suggested that NDR phenomenon was closely related to the redox property of HPAs. Vapor-phase oxidation of benzyl alcohol to benzaldehyde was carried out as a model reaction to track the oxidation catalysis of HPAs. NDR peak voltage appeared at less negative voltage with increasing yield for benzaldehyde.

Redox properties of α2-K8P2W17(M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson heteropolyacids probed by scanning tunneling microscopy and tunneling spectroscopy

Abstract Scanning tunneling microscopy (STM) and tunneling spectroscopy investigations of α 2 -K 8 P 2 W 17 (M·OH 2 )O 61 (M = Mn II , Zn II , Fe II , Co II , and Ni II ) Wells–Dawson heteropolyacids (HPAs) were conducted. HPAs formed self-assembled and well-ordered arrays on graphite surface. Negative differential resistance (NDR) phenomena were observed in the tunneling spectra of the HPAs. NDR peak voltage of the HPAs appeared at less negative voltage with increasing reduction potential and with decreasing UV–visible absorption edge energy. HPAs were then applied to the electro-oxidation of methanol as a redox mediator. The oxidation activity for residual intermediates in the reaction increased as NDR peak voltage of HPA appeared at less negative voltage.

Scanning Tunneling Microscopy Study of Nano-Structured Polyatom-Substituted H4PW11M1O40 Keggin and H7P2W17M1O62 (M = Nb, Ta) Wells-Dawson Heteropolyacid Catalysts

Polyatom-substituted H4PW11M1O40 Keggin and H7P2W17M1O62 (M = Nb, Ta) Wells-Dawson heteropolyacid (HPA) catalysts were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy to elucidate their redox property and oxidation catalysis. STM images clearly showed that HPAs formed nano-structured monolayer arrays on graphite surface. In tunneling spectroscopy, HPAs exhibited a distinctive current-voltage behavior called negative differential resistance (NDR). NDR peak voltage of the HPAs was then correlated with reduction potential determined by electrochemical method in solution. NDR peak voltage of the HPAs appeared at less negative voltage with increasing reduction potential. Vapor-phase oxidative dehydrogenation of isobutyraldehyde to methacrolein was also carried out as a model reaction to probe oxidation catalysis of the HPAs. NDR peak voltage of the HPAs appeared at less negative voltage with increasing yield for methacrolein. NDR peak voltage could be utilized as a correlating parameter for the reduction potential and as a probe of oxidation catalysis in the oxidative dehydrogenation of isobutyraldehyde.

Scanning tunneling microscopy and tunneling spectroscopy studies of niobium-containing H6+x P2W18−x Nb x O62 (x=0, 1, 2, 3)Wells-Dawson heteropolyacid catalysts to probe their redox property and oxidation catalysis

Niobium-containing H6+xP2W18−xNbxO62 (x=0, 1, 2, 3) Wells-Dawson heteropolyacids (HPAs) were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in order to elucidate their redox properties. The HPAs formed two-dimensional well-ordered monolayer arrays on graphite surface and exhibited a distinctive current-voltage behavior called negative differential resistance (NDR) in their tunneling spectra. NDR peak voltage measured on HPA molecule was correlated with reduction potential and absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. NDR peak voltage of H6+xP2W18−xNbxO62 Wells-Dawson HPAs appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. Oxidative dehydrogenation of isobutyraldehyde was also carried out as a model reaction to probe oxidation catalysis of the HPAs. The trend of NDR peak voltage of H6+xP2W18−xNbxO62 Wells-Dawson HPAs was well consistent with the trend of yield for methacrolein.

Nanostructured H<SUB>3+<I>x</I></SUB>PW<SUB>12−<I>x</I></SUB>Nb<SUB><I>x</I></SUB>O<SUB>40</SUB> (<I>x</I> = 0–3) Keggin Heteropolyacid Catalysts

Nanostructured H(3+x)PW(12-x)NbxO40 (x = 0, 1, 2, 3) Keggin heteropolyacid (HPA) catalysts were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy to probe their redox property and oxidation catalysis. STM image showed that the HPAs formed two-dimensional well-ordered monolayer arrays on graphite surface. In tunneling spectra of the HPAs deposited on graphite, they exhibited a distinctive current-voltage behavior referred to as negative differential resistance (NDR). NDR peak voltage measured atop HPA molecule was then correlated with reduction potential and absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. It was revealed that NDR peak voltage of the HPAs appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. In order to correlate NDR peak voltage of H(3+x)PW(12-x)NbxO40 Keggin HPAs with oxidation catalysis, oxidative dehydrogenation of isobutyraldehyde to methacrolein was carried out as a model reaction. NDR peak voltage of the HPAs appeared at less negative voltage with increasing yield for methacrolein.

Scanning Tunneling Microscopy and Tunneling Spectroscopy of Nano-structured H<SUB>6</SUB>P<SUB>2</SUB>Mo<SUB><I>x</I></SUB>W<SUB>18−<I>x</I></SUB>O<SUB>62</SUB> (<I>x</I> = 0, 3, 9, 15, 18) Wells-Dawson Heteropolyacids

Scanning tunneling microscopy (STM) and tunneling spectroscopy studies of nano-structured H6P2MoxW(18-x)O62 (x = 0, 3, 9, 15, 18) Wells-Dawson heteropolyacids (HPAs) were carried out to examine redox properties of the HPAs. STM images of H6P2MoxW(18-x)O62 HPAs clearly showed self-assembled and well-ordered 2-dimensional arrays on graphite surface. Tunneling spectroscopy measurements revealed that all H6P2MoxW(18-x)O62 HPAs exhibited a negative differential resistance (NDR) behavior in their tunneling spectra. NDR peak voltage of H6P2MoxW(18-x)O62 HPAs appeared at less negative applied voltage with increasing molybdenum substitution. Reduction potential of H6P2MoxW(18-x)O62 HPAs measured by an electrochemical method increased and absorption edge energy determined by UV-visible spectroscopy shifted to lower value with increasing molybdenum substitution. In other words, NDR peak voltage of H6P2MoxW(18-x)O62 HPAs appeared at less negative applied voltage with increasing reduction potential and with decreasing absorption edge energy of the HPAs; more reducible H6P2MoxW(18-x)O62 HPAs showed NDR behavior at less negative applied voltage. These results indicate that NDR peak voltage of nano-structured HPAs measured by STM could be utilized as a correlating parameter for the redox properties of bulk HPAs.

Scanning Tunneling Microscopy Study of H6+x P2Mo18−x V x O62 (x = 0–3) Wells–Dawson Heteropolyacid Catalysts: Correlation of NDR Peak Voltage with Reduction Potential and Oxidation Catalysis

Scanning tunneling microscopy (STM) and tunneling spectroscopy studies were carried out to examine the redox properties of vanadium-containing H6+x P2Mo18−x V x O62 (x = 0, 1, 2, 3) Wells–Dawson heteropolyacid (HPA) catalysts. The HPAs formed two-dimensional well-ordered monolayer arrays on a graphite surface and exhibited a distinctive current–voltage behavior called negative differential resistance (NDR). The NDR peak voltages of H6+x P2Mo18−x V x O62 HPAs were correlated with reduction potentials determined by temperature-programmed reduction and with catalytic activity for oxidative dehydrogenation of isobutyraldehyde to methacrolein. The NDR peak voltage of H6+x P2Mo18−x V x O62 appeared at less negative voltage with increasing reduction potential and oxidation catalysis. Negative differential resistance (NDR) peak voltage of H6+x P2Mo18−x V x O62 determined by scanning tunneling microscopy appeared at less negative voltage with increasing reduction potential and with increasing oxidation catalysis.

Room temperature negative differential resistance of CdF2∕CaF2 double-barrier resonant tunneling diode structures grown on Si(100) substrates

The authors have demonstrated the crystal growth of CaF2∕CdF2∕CaF2 multilayered heterostructures on Si(100) substrates as double-barrier resonant tunneling diode structures by a low-temperature growth technique. Current-voltage characteristics were investigated and the authors observed negative differential resistance (NDR) characteristics at room temperature. The peak-to-valley current ratio was 2–8, and 13 at maximum, and peak current density was 80–90A∕cm2. The quantum-well layer thickness dependence of NDR peak voltages is also discussed on the basis of qualitative analytical model using the Esaki-Tsu formula.

Reduction potentials of heteropolyacid catalysts probed by scanning tunneling microscopy and UV-visible spectroscopy

Reduction potentials of heteropolyacid (HPA) catalysts were probed by scanning tunneling microscopy (STM) and UV-visible spectroscopy. Correlations between reduction potentials and NDR (negative differential resistance) peak voltages, and between reduction potentials and absorption edge energies of HPA catalysts were established. The reduction potentials of HPA catalysts have been shown to follow similar trends to the NDR peak voltages of selfassembled HPA monolayers and the absorption edge energies of bulk HPAs. In the UV-visible spectra of HPA catalysts, lower absorption edge energies corresponded to higher reduction potentials of the HPAs.

UV–visible spectroscopy as a probe of heteropolyacid redox properties: application to liquid phase oxidations

UV–visible spectra of heteropolyacids (HPAs) in solution were measured as a simple method of correlating and predicting the redox properties of HPAs. The range of Keggin-type HPAs included H3PMo12O40, H3PW12O40, and variations of cations, heteroatoms, and/or the framework metals. The absorption edges of 0.01 molar aqueous HPA solutions were determined and compared with absorption edges of solid HPAs and with results from other methods that probe redox properties. The absorption edges of the solutions followed similar trends to the absorption edges of bulk solid HPAs, NDR (negative differential resistance) peak voltages from STM measurements on HPA monolayers, and reduction potentials measured electrochemically. HPAs that are stronger oxidizing agents are characterized by absorption edges at longer wavelength (lower energy), and NDR peaks and reduction potentials at more positive voltages. Comparison of edge energies measured in aqueous solution with the performance of HPA catalysts for liquid phase oxidations demonstrates threshold behavior for oxidation activity in each case examined. These results illustrate the potential use of UV–visible spectroscopy in solution as a simple diagnostic indicator in the selection of HPAs as oxidation catalysts.

Scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) studies of polyoxometalates (POMs) of the Wells–Dawson structural class

Presented in this work is an extensive scanning tunneling microscopy (STM) study of self-assembled monolayers of polyoxometalates (POMs) belonging to the Wells–Dawson structural class. The effects of cation and framework atom substitutions in these POMs and their salts have been examined. The Wells–Dawson POMs and their salts formed self-assembled and well-ordered arrays on graphite surfaces. Tunneling spectroscopy (TS) measurements revealed that these POMs exhibited negative differential resistance (NDR) behavior in their tunneling spectra. The arrays of vanadium-substituted POMs, H 6+ x [P 2Mo 18− x V x O 62] ( x = 1, 2, 3), showed periodicities of ca. 11 Å × 14 Å. The shapes and periodicities of the corrugations in the STM images are consistent with the structure and the characteristic dimensions of the Wells–Dawson polyanion, [P 2Mo 18O 62] 6−, as determined from X-ray crystallography studies. Moreover, the STM results indicated that framework substitutions and increases in the number of charge-compensating protons had negligible effects on the array periodicities. Various salts of trisubstituted Wells–Dawson POMs, Q[P 2W 15Nb 3O 62] (Q = Na 9, Cs 9, (Bu 4N) 9, (Bu 4N) 5Na 3(Re(CO) 3)), also formed two-dimensional monolayer arrays with various periodicities. For this class of POM salts, the periodicities were greater than the anion dimensions, reflecting the size of the counter cation (Q). TS measurements revealed the positions of the counter cations in the POM salt arrays, and the positions were consistent with the cation positions found in the bulk crystal structures. For each class of Wells–Dawson anions, it was shown that the packing of the polyanions in the monolayers was not identical to, but resembled that found in bulk cleavage planes of the corresponding POM. A correlation between NDR peak voltage and reduction potential established for both POM series revealed that more reducible POMs showed NDR peaks at less negative applied voltage. In other words, a less negative NDR peak voltage corresponded to a higher reduction potential. This trend is consistent with those previously elucidated for POM catalysts with the Keggin structure.

On the reduction potential of cation-exchanged heteropolyacids (HPAs)

UV-Visible spectroscopy and scanning tunneling microscopy (STM) studies were performed to explore reduction potentials of cation-exchanged Keggin-type heteropolyacid (HPA) catalysts. Absorption band edge and negative differential resistance (NDR) peak voltage of cation-exchanged HPA samples determined by UV-Visible spectroscopy and STM, respectively, were colsely related to their reduction potentials. It was observed that HPAs with higher reduction potentials showed absorption band edges at longer wave lengths and exhibited NDR peak voltages at less negative applied values. The reduction potentials of cation-exchanged HPA catalysts could also be correlated with the electronegativities of counter-cations. Substitution of more electronegative counter-cations increased reduction potentials of the HPAs. The NDR peak voltage and the absorption band edge of HPAs could be utilized as a correlating parameter for their reduction potentials.

Correlation of alkane oxidation performance with STM and tunneling spectroscopy measurements of heteropolyacid catalysts

A comprehensive study of the surface electronic properties of nanostructured heteropolyacid (HPA) monolayers was carried out using scanning tunneling microscopy (STM) with the aim of developing tools for rational selection of HPAs as oxidation catalysts. A wide set of Keggin-type HPAs with different counter-cation, framework polyatom, and central heteroatom substitutions were examined. HPA samples deposited on graphite surfaces formed two-dimensional well-ordered monolayer arrays, and exhibited negative differential resistance (NDR) behavior in their tunneling spectra. Substitution of more electronegative atoms for counter-cations or for the central heteroatom shifted the NDR peaks to less negative voltages, corresponding to increased reduction potentials of the HPAs. However, substitution of more electronegative framework polyatoms shifted the NDR peaks to more negative voltages, corresponding to decreased reduction potentials. Irrespective of the exchanged/substituted positions, it was consistently observed that NDR peaks appeared at less negative potentials for higher reduction potentials of the HPAs, across all families of HPAs examined in this work. The NDR peak voltage could be utilized as a correlating parameter (as an alternative parameter) for the reduction potential; a less negative NDR peak voltage corresponds to a higher reduction potential of the HPA. A “volcano” plot constructed for the first time by relating NDR peak voltage to catalytic performance of HPAs in propane oxidation (a target reaction) showed that there was an optimum range of NDR peak voltages for the best performance in this reaction. The ability to tune NDR peak voltages of HPAs by exchange/substitution can be a strong advantage in designing HPAs in a systematic way as selective oxidation catalysts. This work has successfully demonstrated that NDR peak voltages of HPAs can serve as selection, prediction, and design tools in identifying efficient HPAs for propane oxidation.

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

Ammonium salts of Wells-Dawson heteropoly oxofluorotungstates with different framework compositions, and 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. these values were consistent with the molecular dimensions of typical Wells-Dawson heteropolyanions plus charge-compensating cations (ionic radius of 1.43 Å) 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, and 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. © 2002 The Electrochemical Society. All rights reserved.

Nanoscale Investigation of Mixed Arrays of Keggin-Type and Wells−Dawson-Type Heteropolyacids (HPAs) by Scanning Tunneling Microscopy (STM)

The nanoscale investigation of mixed arrays of Keggin-type (H3PW12O40) and Wells−Dawson-type (H6P2W18O62) heteropolyacids (HPAs) deposited on a graphite surface was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS). Pure H3PW12O40 and H6P2W18O62 formed self-assembled and well-ordered two-dimensional arrays, and their molecular dimensions were in good agreement with values determined by XRD. In tunneling spectroscopy, H3PW12O40 and H6P2W18O62 exhibited negative differential resistance (NDR) behavior at applied voltages of −1.14 and −0.80V, respectively. Mixtures of H3PW12O40 and H6P2W18O62 deposited from aqueous solution formed less ordered two-dimensional arrays than did the pure components. The mixed arrays exhibited a bimodal distribution of NDR peak voltages which was represented as a combination of the monomodal distributions of NDR peak voltages of pure H3PW12O40 and H6P2W18O62. Individual molecules in the mixed array were successfully identified by differences in mol...

Nanoscale characterization of redox and acid properties of keggin-type heteropolyacids by scanning tunneling microscopy and tunneling spectroscopy: effect of heteroatom substitution.

Nanoscale characterization of acid and redox properties of Keggin-type heteropolyacids (HPAs) with different heteroatoms, H(n)MW(12)O(40) (M = P, Si, B, Co), was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in this study. HPA samples were deposited on highly oriented pyrolytic graphite surfaces to obtain images and tunneling spectra by STM before and after pyridine adsorption. All HPA samples formed well-ordered 2-dimensional arrays on graphite before and after pyridine exposure. NDR (negative differential resistance) peaks were observed in the tunneling spectra. Those measured for fresh HPA samples appeared at less negative voltages with increasing reduction potential of the HPAs and with increases in the electronegativity of the heteroatom, but with decreases in the overall negative charge of the heteropolyanions. These results support the conclusion that more reducible HPA samples show NDR behavior at less negative applied voltages in their tunneling spectra. Introduction of pyridine into the HPA arrays increased the lattice constants of the 2-dimensional HPA arrays by ca. 6 A. Exposure to pyridine also shifted NDR peak voltages of H(n)MW(12)O(40) (M = P, Si, B, Co) samples to less negative values in the TS measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with the acid strengths of the HPAs, suggesting that tunneling spectra measured by STM could serve to probe acid properties of HPAs. These results show how one can relate the bulk acid and redox properties of HPAs to surface properties of nanostructured HPA monolayers determined by STM.

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 Å, 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.