Browsing by Author "Sakata, O."

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    The 3D atomic scale and electronic structure characterization of novel fcc ruthenium nanoparticles using synchrotron light source
    (Faculty of Science, University of Kelaniya, Sri Lanka, 2016) Kumara, L.S.R.; Sakata, O.; Kohara, S.; Song, C.; Yang, A.; Kusada, K.; Kobayashi, H.; Kitagawa, H.
    Ruthenium (Ru) is a 4d transition metal that in the bulk adopts hexagonal closepacked (hcp) structure at all temperature ranges, and novel face-centered cubic (fcc) Ru nanoparticles (NPs) have been observed to be more efficient than conventional hcp Ru NPs larger than 3 nm. It has recently attracted much attention as a potential application in removal of car exhausts due to high catalytic activity for CO oxidation and preventing CO poisoning in fuel-cell system. We here report the 3-dimentational atomic-scale structures of fcc and hcp Ru NPs using high-energy X-ray diffraction (HEXRD), Rietveld analysis, pair distribution function (PDF), and reverse Monte Carlo (RMC) modelling. Hard X-ray photoelectron spectroscopy (HAXPES) can provide important information on the influence of NP size on electronic properties. The HEXRD and HAXPES of Ru NPs were performed at BL04B2 and BL15XU at SPring-8, the world largest third-generation (8 GeV) synchrotron radiation facility located in Hyōgo prefecture, Japan. We observed higher stability of the lattice distortion of fcc Ru NPs with increasing particle size. The PDF analysis results show that the structural disordered Ru NPs at short- to intermediate-range atomic distances. The order parameter for fcc Ru NPs decreased with increasing particle size due to the loosely packing atomic arrangement and may explain an origin of higher catalytic activity of fcc Ru NPs. In this study, the observed trend of increasing catalytic activity of fcc Ru NPs was also discussed using their core-levels and valence band electronic structures. This work was partly supported by ACCEL, Japan Science and Technology Agency (JST) and also partly supported by Ministry of Education, Culture, Sports, Science and Technology of Japan (OS: 15K04616).
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    Cu2O Homojunction Solar Cells: Efficiency Enhancement with a High Short Circuit Current
    (2024) Thejasiri, S. A. A. B.; Jayathilaka, K. M. D. C.; Kafi, F. S. B.; Kumara, L. S. R.; Seo, O.; Yasuno, S.; Sakata, O.; Siripala, W.; Wijesundera, R. P.
    Cu2O homojunction solar cells were fabricated using potentiostatic electrodeposition technique. n-Cu2O thin films were grown in an acetate bath while p-Cu2O thin films were grown in a lactate bath. In the growth of n-Cu2O films, cupric acetate concentration, pH and temperature of the bath, deposition potential and duration (film thickness) and annealing temperature were investigated. In the growth of p-Cu2O on n-Cu2O, concentration of copper sulphate and lactic acid solutions, pH and temperature of the bath, deposition potential and duration were investigated. In addition, the procedure of sulfidation of p-Cu2O film surface using (NH4)2S vapor, before depositing Au front contact, was also optimized to enhance the photoactive performance. The structural, morphological and optoelectronic properties of the Cu2O films were investigated using scanning electron microscopy (SEMs), high energy X-ray diffraction (HEXRD), hard X-ray photoelectron spectroscopy (HAXPES), spectral response and current–voltage (J-V) measurements. The best Cu2O homojunction solar cell exhibited Voc = 460 mV, Jsc = 12.99 mA·cm−2, FF = 42% and η = 2.51%, under AM 1.5 illumination. Efficiency enhancement with the record high Jsc value for the Cu2O homojunction solar cell has mainly been due to the optimization of pH of the n-Cu2O deposition bath and lactic acid concentration of the p-Cu2O deposition bath.