Tadaoki Mitani

Roles of Surface Steps on Pt Nanoparticles in Electro-oxidation of Carbon Monoxide and Methanol

Design of highly active nanoscale catalysts for electro-oxidation of small organic molecules is of great importance to the development of efficient fuel cells. Increasing steps on single-crystal Pt surfaces is shown to enhance the activity of CO and methanol electro-oxidation up to several orders of magnitude. However, little is known about the surface atomic structure of nanoparticles with sizes of practical relevance, which limits the application of fundamental understanding in the reaction mechanisms established on single-crystal surfaces to the development of active, nanoscale catalysts. In this study, we reveal the surface atomic structure of Pt nanoparticles supported on multiwall carbon nanotubes, from which the amount of high-index surface facets on Pt nanoparticles is quantified. Correlating the surface steps on Pt nanoparticles with the electrochemical activity and stability clearly shows the significant role of surface steps in enhancing intrinsic activity for CO and methanol electro-oxidation. Here, we show that increasing surface steps on Pt nanoparticles of approximately 2 nm can lead to enhanced intrinsic activity up to approximately 200% (current normalized to Pt surface area) for electro-oxidation of methanol.

Charge Transfer Exciton in Halogen-Bridged Mixed-Valent Pt and Pd Complexes: Analysis Based on the Peierls-Hubbard Model

Polarized reflection and luminescence have been measured for the single crystals of [MA 2 ][MX 2 A 2 ](ClO 4 ) 4 (M=Pt, Pd, X=Cl, Br, I and A=ethylenediamine, cyclohexanediamine). The strong absorption bands due to the charge-transfer (CT) exciton transitions between the mixed-valent metal ions have been investigated in detail in the visible or infrared energy regions. The dependence of the CT excitation energies on the species M and X is shown to be consistent with the prediction by the Peierls-Hubbard model which incorporates the effect of the electron-electron correlation on inter-metal sites. The oscillator strength of the CT excitons are observed to be enhanced by substituting heavier halogen ions. This enhancement is interpreted by a halogen-linked super-transfer mechanism. The unusually large values of the oscillator strength can be qualitatively explained in terms of the trimer CT model.

Highly dispersed ruthenium oxide nanoparticles on carboxylated carbon nanotubes for supercapacitor electrode materials

To enhance its pseudocapacitance, ruthenium oxide must be formed with a hydrated amorphous and porous structure and a small size, because this structure provides a large surface area and forms conduction paths for protons to easily access even the inner part of the RuO2. In this study, we report that highly dispersed RuO2 nanoparticles could be obtained on carboxylated carbon nanotubes. This could be achieved by preventing agglomeration among RuO2 nanoparticles by bond formation between the RuO2 and the surface carboxyl groups of the carbon nanotubes. Highly dispersed RuO2 nanoparticles on carbon nanotubes showed an increased capacitance, which can be explained by the fact that with the decrease in size protons were able to access the inner part of RuO2, so that its utilization was increased. The high dispersion of RuO2 is therefore a key factor to increase the capacitance of nanocomposite electrode materials for supercapacitors.

Drastic change of electric double layer capacitance by surface functionalization of carbon nanotubes

In this letter we show that it is possible to drastically change electric double layer capacitance, from a large increase to complete disappearance, by introducing various surface functional groups on carbon nanotubes. The introduction of surface carboxyl groups created a 3.2 times larger capacitance due to the increased hydrophilicity of the multiwalled carbon nanotubes in an aqueous electrolyte. In contrast, the introduction of alkyl groups resulted in a marked decrease of capacitance. Notably, we unexpectedly observed the complete disappearance of capacitance for samples functionalized with longer alkyl groups than octyl, indicating the perfect block of proton access to the carbon nanotubes’ surfaces by extreme hydrophobicity.

Neutral-to-ionic transition in tetrathiafulvalene-p-chloranil as investigated by optical reflection spectra

Abstract Polarized reflection spectra and their temperature-dependence as well are investigated on tetrathiafulvalene-p-chloranil (TTFCA) single crystals over the temperature range from 2 K to room temperature. Abrupt changes are observed at Tc = 84.4 ± 0.5 K in the charge transfer and intramolecular excitation bands. These experimental results strongly indicate that the dominant part of the neutral-to-ionic transition in this crystal takes place at a sharply defined critical temperature, in contrary to the recent observation by Torrance et al. [Phys. Rev. Lett. 47, 1747 (1981)] that the transition occurs in a temperature range of about 30 K in width. A simple model is presented to account for the observed shift of the TTF+ intramolecular transition bands at Tc.

Control of CDW state in halogen-bridged metal complexes

Abstract Chemical modifications of one-dimensional (1D) halogen-bridged mixed-valence metal complexes have been made in order to control the amplitude of the charge density wave (CDW) state in -M-X-chains (M = Pt, Pd or Ni, and X = Cl, Br or I). By changing chemical parameters, such as strength of hydrogen-bonds between the ligands and the counter anions, and the ionic radii of M and X, the characteristic diagram of the amplitude of CDW has been obtained as a function of the M-X-M distances. This provides a new basic idea for understanding the 1D electronic state of the M-X compounds, especially as related to dynamical phenomena originated in the charge transfer instability in the ground state and the photo-excited state.

Ambipolar operation of fullerene field-effect transistors by semiconductor/ metal interface modification

We report an ambipolar operation in field-effect transistors of C60 and metallofullerene Dy@C82 by modification of semiconductor/metal electrode interface with perfluoroalkylsilane (FAS) molecules. Kelvin probe experiments revealed that the work function of the gold surface modified with FAS molecules increased by 0.55eV as compared to the untreated gold. Hole injection into fullerenes is qualitatively understood in terms of this work-function change induced by the FAS molecules. The present results indicate that the charge injection from electrodes to organic semiconductors can be controlled simply by modification of semiconductor/metal interface without changing materials themselves.

Effect of Substitutional Impurities on the Neutral-to-Ionic Phase Transition in TTF-p-Chloranil Crystal

In order to investigate the effect of doping on the neutral-to-ionic phase transition, reflection spectra due to the intramolecular excitations have been measured on TTF- p -chloranil single crystals doped with the substitutional weak acceptor, trichloro- p -benzoquinone, at various temperatures and dopant concentrations. The thermal shifts of the spectra are considerably blurred by doping in contrast with a sharp spectral change at T c =84 K in pure crystal. It is concluded that the phase transition in doped crystals occurs inhomogeneously due to a binary phase separation into quasi-ionic and quasi-neutral regions even below T c . On the basis of these results, a microscopic model is presented for the effect of impurities on the neutral-to-ionic phase transition in TTF-chloranil crystal.

IR Study of the H-Bond Coupled with the Mixed-Valence State of Halogen-Bridged Metal Complexes

IR spectroscopic measurements were made on the halogen bridged metal complexes of {M(chxn) 2 Br}Br 2 (M=Pt, Pd and Ni) which have interchain N-H···Br bonds strongly coupled with the electronic state of chains. The splitting of the N-H stretching modes for Pt and Pd complexes show a good correlation with the amplitude of the CDW in the chain, i.e. with the displacement of bridging halogen ion. No splitting of the N-H band was observed for the Ni complex, which is an evidence of a transformation from a mixed valence (CDW) state to a mono-valence (the Mott-Hubbard) state by a replacement of M=Pt (or Pd) by Ni. Such IR studies were applied for the estimation of the CDW amplitude under hydrostatic pressure. The results indicated that more detailed information can be derived by the IR study than the X-ray analysis.

Mott-Hubbard State in One-Dimensional Iodo-Bridged Binuclear Metal Dithioacetato Complexes, M2(dta)41 (M=Pt and Ni)

Abstract Polarized reflection, electrical conductivity, magnetic susceptibility measurements and X-ray crystal structure predetermination have been carried out on one- dimensional iodo-bridged binuclear metal dithioacetato complexes, M2(dta)4I (M=Pt and Ni; dta=CH3CS2 −) in order to elucidate their electronic property. The chain structure consists of -M2-5+-M2,5+-I-M2-5+-M2-5+- with no Peierls distortion, and two metal atoms in binuclear moieties are bridged by four dithioacetato ligands to give a cage structure. The magnetic behavior of these compounds revealed very strong anti ferromagnetic coupling between electronic spins (S=1/2) localized on the M-M dimers. They show semi conducting behavior with high conductivities and small activation energies compared with those of the MX compounds. From these results, it may be concluded that the compounds are one-dimensional Mott-Hubbard insulators, in contrast to Peierls insulators of R4[Pt2(pop)4X].nH20 complexes (…M2+-M2+…X-M3+ -M3+-X‥).