Browsing by Author "Xu, Q"

Now showing 1 - 3 of 3
  • No Thumbnail Available
    Item
    90° Rotation of orbital stripes in bilayer manganite PrCa2Mn2O7 studied by in situ transmission electron microscopy
    (Academic Press Inc Elsevier Science, 2013-04-01) He, ZB; Deng, G; Tian, H; Xu, Q; Van Tendeloo, G
    We present an in situ transmission electron microscopy study on the half-doped bilayer manganite PrCa2Mn2O7 to reveal the rotation process of the orbital stripes. Between the reported initial and final ordering phases, we identified an intermediate state with two sets of satellite spots to bridge the 90° rotation of the orbital stripes. Furthermore, we determined that the rotation of the orbital stripes does not always occur. Some restricted conditions for the orbital rotation to occur were found and reasons are discussed. © 2013, Elsevier Inc.
  • No Thumbnail Available
    Item
    Microstructure of bilayer manganite PrCa2Mn2O7 showing charge/orbital ordering
    (American Institute of Physics, 2013-05-27) He, ZB; Tian, H; Deng, G; Xu, Q; Van Tendeloo, G
    The microstructure of the charge/orbital ordering Ruddleden-Popper phase PrCa2Mn2O7 was studied by transmission electron microscopy along both the [001] and the [110] orientation. Three coexisting charge/orbital ordering phases CO1, CO2, and CO3 were observed along the [001] orientation at room temperature. Different from the one-dimensional modulation in the CO1 and CO2 phase, the CO3 phase is characterized by two sets of mutually perpendicular structural modulations. From [110] high angle annular dark field-scanning transmission electron microscopy, we found that the Pr atoms locate in-between the bilayer MnO6 octahedra, which is different from the previous reports. © 2013, American Institute of Physics.
  • No Thumbnail Available
    Item
    Spectroscopic identification of hydrogen spillover species in ruthenium-modified high surface area carbons by diffuse reflectance infrared fourier transform spectroscopy
    (American Chemical Society., 2012-12-27) Blackburn, JL; Engtrakul, C; Bult, JB; Hurst, K; Zhao, Y; Xu, Q; Parilla, PA; Simpson, LJ; Rocha, JDR; Hudson, MR; Brown, CM; Gennett, T
    In recent years, carbon-based sorbents have been recognized for their potential application within vehicular hydrogen storage applications. One method by which sorbents have been reported to store appreciable hydrogen at room temperature is via a spillover process: where molecular hydrogen is first dissociated by metal nanoparticle catalysts and atomic hydrogen subsequently migrates onto the carbon substrate. Many reports have invoked the spillover mechanism to explain enhancements in reversible room temperature hydrogen uptake for metal-decorated sorbents. However, there is a lack of experimental evidence for the proposed chemical species formed as well as several differing theoretical explanations describing the process. In this report, we utilize diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to identify the various chemical species formed upon room temperature H-2 charging of ruthenium-decorated high surface area carbons. Room temperature H-2 loading of a control sample with no ruthenium nanoparticles (Ru NPs) leads to broad reversible peaks in the DRIFTS spectrum that correspond to the vibration-rotation transitions of weakly bound physisorbed hydrogen molecules. In contrast, the sample modified with Ru NPs shows a variety of reversible and irreversible peaks in addition to the physisorbed H-2 peaks. Rigorous experimental and theoretical analysis enables the assignment of the peaks to ruthenium-mediated formation of water, surface hydroxyl groups (R-OH, where R = carbon or ruthenium), and C-H bonds. The low-energy DRIFTS peaks assigned to spillover C-H bonds were additionally confirmed using inelastic neutron spectroscopy. Reversible vibrational peaks consistent with ruthenium-mediated formation of C-H bonds provide much-needed spectroscopic evidence for the spillover process. The results demonstrated here should facilitate future mechanistic investigations of hydrogen sorption on transition metal nanoparticles and high surface area activated carbons. © 2012, American Chemical Society.