Noboru Taguchi

First-Principles Calculations of Pd/Au(100) Interfaces with Adsorbates

Atomic and electronic structures of H-adsorbed Pd overlayers on Au(100) substrates have been studied by first-principles calculations. The geometric strain effects change the electronic structure and local reactivity of the surface. The lattice strained Pd overlayers on Au surfaces have larger adsorption energies for atomic hydrogen than the unstrained Pd slabs. Adsorption energies for several adsorption sites on the models with different numbers of Pd overlayers have been analyzed from the viewpoints of strains and H-Pd and H-substrate interactions.

First-Principles Calculations of the Atomic and Electronic Structures in Au-Pd Slab Interfaces

We performed first-principles calculations using the projector augmented-wave (PAW) method for Au/Pd slab interface models. The calculations of relaxed configurations and energies for the thin Pd layers (3 layers) stacking on Au (111) and Au (100) slabs with an epitaxial relationship represent that Pd overlayers have a lateral expansion in both cases. This trend is in good agreement with experimental results for Pd/Au slabs and Au-Pd core-shell nanoparticles, obtained by electron microscopy, X-ray diffraction, and positron annihilation. In addition, an intermixing configuration near the Au-Pd interface was shown to be more stable than the binary separated one.

Si-Doping Effects in Cu(In,Ga)Se2 Thin Films and Applications for Simplified Structure High-Efficiency Solar Cells

We found that elemental Si-doped Cu(In,Ga)Se2 (CIGS) polycrystalline thin films exhibit a distinctive morphology due to the formation of grain boundary layers several tens of nanometers thick. The use of Si-doped CIGS films as the photoabsorber layer in simplified structure buffer-free solar cell devices is found to be effective in enhancing energy conversion efficiency. The grain boundary layers formed in Si-doped CIGS films are expected to play an important role in passivating CIGS grain interfaces and improving carrier transport. The simplified structure solar cells, which nominally consist of only a CIGS photoabsorber layer and a front transparent and a back metal electrode layer, demonstrate practical application level solar cell efficiencies exceeding 15%. To date, the cell efficiencies demonstrated from this type of device have remained relatively low, with values of about 10%. Also, Si-doped CIGS solar cell devices exhibit similar properties to those of CIGS devices fabricated with post deposition alkali halide treatments such as KF or RbF, techniques known to boost CIGS device performance. The results obtained offer a new approach based on a new concept to control grain boundaries in polycrystalline CIGS and other polycrystalline chalcogenide materials for better device performance.

Synthesis of Au Nanorods by Using Gamma-ray Irradiation

To synthesize Au nanorods without the use of seed crystals, we applied gamma-ray irradiation from 60Co radioactive source to reduce aqueous solution containing Au ions. The total dose was fixed to 6.0 kGy with various dose rates from 0.4 to 13.6 kGy/h. After the irradiation, two surface plasmon bands in the range around 530 nm and above 700 nm were observed in UV/vis spectra. We observed by a transmission electron microscopy (TEM) that rod-shaped Au nanoparticles with the length of 20–25 nm, and the width of about 7–15 nm were produced as well as a small amount of spherical particles. The size and the aspect ratio of the Au nanorods are well correlated with gamma-ray irradiation dose rate; with increasing the dose rate, their size decreases and the aspect ratio increases.

Study of the hydrate-melt/Li4Ti5O12 interphase by scanning electron microscopy based spectroscopy.

To develop safe and low-cost Li-ion batteries (LIBs), recently, an aqueous-based electrolyte so-called hydrate-melt (HDM) electrolyte is proposed. Li4Ti5O12 is a promising negative electrode material for a LIB with such a HDM electrolyte because of its unexpected reversible Li insertion and extraction properties without usually inevitable water reduction. The solid-electrolyte interphase formation is one of the reasons for this stable reaction, although a detailed analysis is not yet performed. Here, a Li4Ti5O12 electrode surface reacted in a HDM electrolyte is investigated by scanning electron microscopy-based analysis. Surface reaction products are clearly observed on the Li4Ti5O12 surface after the Li insertion reaction in a HDM electrolyte. Energy-dispersive X-ray spectroscopy and Auger electron spectroscopy indicated that the products do not contain any components originated from Li salts, whereas anion-derived passivation films seem to cover a bare surface below the products. Further, the surface products are identified as Li2O by the feature of Li-K-edge reflection electron energy-loss spectrum. The Li2O formation would be one of the key issues for stable Li insertion and extraction of a Li4Ti5O12 electrode in a HDM electrolyte.

First-Principles Calculations of C2H4 Adsorption on Pd Surface Stacked on Fcc-Au

We performed first-principles calculations of stable atomic structure and partial density of states (PDOS) analysis of Ethylene (C2H4) adsorption on Pd(100) surface stacked on fcc-Au using the projector augmented-wave (PAW) method. In case of Pd overlayers on the Au(100) substrate, from the analysis of PDOS around surface, highest occupied states become sharpen compared with the case of C2H4 adsorption on Pd monometallic slab surface in both di-uf073 and uf070 adsorption model.

Analytical TEM study of the core-shell structure of Au-Pd nano-particles prepared by sonochemical technique

The structure of Au-Pd nanoparticles prepared by the sonochemical method was observed by an analytical TEM. The core (Au)-shell (Pd) structure was confirmed by ADF-STEM images. The ratio of Au and Pd contents was controlled by adjusting the ratio of precursors, and the Pd-shell thickness was also controlled by the ratio of Au and Pd contents. The electron diffraction patterns showed that the lattice constant of the Pd shell is quite larger than that of bulk Pd and rather similar to that of bulk Au even in the systems with larger ratios of Pd, which supports the pseudomorphic growth of the Pd shell on the Au core.