Akihisa Yanase

IN SITU OBSERVATION OF OXIDATION AND REDUCTION OF SMALL SUPPORTED COPPER PARTICLES USING OPTICAL ABSORPTION AND X-RAY DIFFRACTION

Abstract Oxidation and reduction of small copper particles on supports has been studied at temperatures between 298 and 673 K, using in situ observation of optical absorption and X-ray diffraction. The change in chemical composition with time was traced by observing the change in optical absorption spectra at wavelengths between 300 and 800 nm and in X-ray diffraction peak intensity for Cu(111), Cu 2 O(111) and CuO(111) when the ambient gas was alternated between 5% H 2 /He and 1% O 2 /He. We found that the oxidation of small supported copper particles involves a fast reaction of Cu to CuO x ( x ~ 0.67) with Cu 2 O structure and a slow reaction of CuO x ( x ~ 0.67) to CuO x ( x ~ 1). The activation energy of the CuO x ( x ~ 0.67) to CuO x ( x ~ 1) reaction is found to b

Two different in-plane orientations in the growths of cuprous halides on MgO(001)

Abstract Heteroepitaxial growths of CuCl and CuBr, zinc-blende-structure ionic compounds, on (001) surfaces of MgO with rock-salt structure have been studied. Textured films of CuCl and CuBr with (111) orientations were grown from molecular beams at a typical substrate temperature of 333 K. We found that CuCl and CuBr had in-plane alignments with respect to the substrate lattice: CuCl[110] was parallel to the MgO[110], [110], [110] and [110] axes; CuBr[110] was parallel to the MgO[100], [010], [100] and [010] axes. This difference is explained by the change in the atomic configurations of the initial nuclei with respect to the substrate lattice arising from a 5% difference in lattice parameter between CuCl and CuBr. An atomic force microscopic observation showed no preferential nucleation of CuCl on step edges appearing on MgO(001). These results strongly suggest an occurrence of the initial nucleation on terraces of MgO(001), not at ledge and kink sites.

Optical observation of oxidation and reduction of small supported copper particles

Abstract In situ and real-time optical absorption measurements of supported copper particles (4–10 nm) at wavelengths of 300 to 800 nm are carried out under H 2 , CO, and O 2 respectively as ambient gases in the temperature range of 300 to 673 K. We observe a reversible change in the optical spectra caused by oxidation of copper and reduction of copper oxide. The data strongly indicate that the oxidation of small copper particles is composed of a fast process of Cu to CuO x ( x ≃ 0.67) and a slow process of CuO x ( x ≃ 0.67) to CuO.

Nucleation and morphology evolution in the epitaxial growth of CuCl on MgO(001) and CaF2(111)

Abstract Nucleation and morphology evolution in the growth of CuCl on MgO(001) and CaF 2 (111) investigated by atomic force microscopy is reported. CuCl nucleates on MgO(001) without a discernible relationship to the step edges running parallel to the MgO[100] and [010] axes, forming triangular (111)-oriented islands in the initial stage. Further growth causes the island shape to change to hexagonal with a highly concave top surface. The morphology evolution is explained by a migration of CuCl-related surface species from the substrate surface to (111) top surfaces of the islands through three specific side facets composed of CuCl{111} and/or {110}. Step edges on a lattice-matched substrate of CaF 2 (111) act as dominant nucleation sites for CuCl.

Gas-Sensitive Optical Absorption of Ultrafine Silver Particles Dispersed in a Porous Silica Film

In situ, real-time measurements of the optical absorption of ultrafine silver particles of about 9 nm dispersed in a porous silica film are carried out under defined atmospheres over a temperature range of 300 K to 653 K. A reversible change in the optical absorption spectra resulting from the atmosphere exchange between 1% O2 and 20% CO is observed for the first time, which indicates a shape change of silver particles induced by the adsorption and desorption of oxygen.

Stranski-Krastanov growth of CuCl on MgO(001)

Abstract Initial growth of CuCl, a zinc-blende-structure I–VII ionic compound, on MgO(001) with NaCl-type structure has been studied using reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). An intermediate layer of adsorbed CuCl, which has a surface structure strongly influenced by the host lattice, was formed before islanding. This layer is easily distinguished from (111) top surfaces of the CuCl islands with trigonal symmetry which have been grown subsequently. This implies that CuCl grows in the Stranski-Krastanov growth mode, which also explains why the nucleation of the islands is insensitive to the step edges of MgO(001).

Adsorbate-induced lattice relaxation of small supported silver particles observed by an in situ X-ray diffraction technique

Abstract In situ X-ray diffraction measurements of small supported silver particles are performed under defined atmospheres in the temperature range of 298–723 K. We find a reversible change in lattice parameter of silver particles against the atmosphere exchange between 5%H 2 /He and 1%O 2 /He. The data indicate that the oxygen-induced lattice dilatation observed here is due to the reduction in surface stress which is caused by not only the static effects of oxygen adsorption but also the induced change in the structure of silver particles.

Real-time optical observation of morphological change of small supported copper particles during redox treatments

Abstract In situ optical observation was used as a technique to study the effects of cyclic oxidation and reduction (redox) treatment made at temperatures below 673 K on small copper particles supported on porous glass. The in situ real-time observation revealed decreases both in size of copper particles and volume fraction of metallic copper during the cyclic redox treatment. The decreases correspond to an increase in fraction of nonmetallic copper species on the support surface. This is confirmed by comparing the measured with calculated optical spectra and tracing the change in the rate of oxidation of copper particles. This phenomenon is enhanced by using 500 ppm O 2 rather than 1% O 2 , and at higher temperatures. The role of a lower oxygen concentration in the mechanism of the phenomenon is discussed.

Heteroepitaxial growth of CuCl on MgO(001) substrates

Abstract Textured CuCl films with a (111) orientation were grown on MgO(001), in spite of the large lattice mismatch, at 333 K by molecular beam epitaxy. The CuCl overlayer contains four types of domains with specific crystallographic relationships with respect to the lattice of MgO; the CuCl[1 1 0] axis was parallel to the MgO[110], [ 1 10], [ 1 1 0] and [1 1 0] axes in the different domains. An atomic force microscope topograph showed that the size of the domains was about 1 μm.

Nucleation from ionic molecules in Stranski–Krastanov growth of CuCl on MgO(001)

The nucleation of islands during Stranski-Krastanov growth of CuCl on MgO(001) is studied by molecular beam epitaxy and atomic force microscopy. The dependence of island density (N) on deposition flux (R) is analyzed using the rate equation, for the first time, for a compound-material growth system in which the impinging particles are ionic molecules. The scaling of N ∼ R 0.5± 0.1 observed at growth temperatures of 293, 313 and 333 K indicates a critical nucleus size of 2 to 3, leading to the smallest stable island of (Cu 3 Cl 3 ) n (n = 3-4).