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1. Spin- and angle-resolved photoemission studies of the electronic structure of Si(110)'16x2' surfaces

Author

Lewis, N. K., Lassailly, Y., Martinelli, L., Vobornik, I., Fujii, J., Bigi, C., Brunkow, E., Clayburn, N. B., Gay, T. J., Flavell, W. R., and Seddon, E. A.

Subjects
Condensed Matter - Materials Science, Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

The electronic structure of Si(110)"16 x 2" double-domain, single-domain and 1 x 1 surfaces have been investigated using spin- and angle-resolved photoemission at sample temperatures of 77 K and 300 K. Angle-resolved photoemission was conducted using horizontally- and vertically-polarised 60 eV and 80 eV photons. Band-dispersion maps revealed four surface states ($S_1$ to $S_4$) which were assigned to silicon dangling bonds on the basis of measured binding energies and photoemission intensity changes between horizontal and vertical light polarisations. Three surface states ($S_1$, $S_2$ and $S_4$), observed in the Si(110)"16 x 2" reconstruction, were assigned to Si adatoms and Si atoms present at the edges of the corrugated terrace structure. Only one of the four surface states, $S_3$, was observed in both the Si(110)"16 x 2" and 1 x 1 band maps and consequently attributed to the pervasive Si zigzag chains that are components of both the Si(110)"16 x 2" and 1 x 1 surfaces. A state in the bulk-band region was attributed to an in-plane bond. All data were consistent with the adatom-buckling model of the Si(110)"16 x 2" surface. Whilst room temperature measurements of $P_y$ and $P_z$ were statistically compatible with zero, $P_x$ measurements of the enantiomorphic A-type and B-type Si(110)"16 x 2" surfaces gave small average polarisations of around 1.5\% that were opposite in sign. Further measurements at 77 K on A-type Si(110)"16 x 2" surface gave a smaller value of +0.3\%. An upper limit of $\sim1\%$ may thus be taken for the longitudinal polarisation., Comment: Main paper: 12 pages and 11 figures. Supplemental information: 5 pages and 2 figures

Published
2019
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2. Domain formation mechanism of the Si(110)'16 x 2' reconstruction

Author

Lewis, N. K., Clayburn, N. B., Brunkow, E., Gay, T. J., Lassailly, Y., Fujii, J., Vobornik, I., Flavell, W. R., and Seddon, E. A.

Subjects
Condensed Matter - Materials Science
Abstract

The main factor that determines which of the two domains form upon reconstruction of the Si(110)"16 x 2" surface has been investigated. LEED and STM images showed that the domain orientation was independent of the heating current direction used to induce the Si(110)"16 x 2" reconstruction. Reciprocal-space lattice models of the reconstruction allowed for the correct identification of the domain orientations in the LEED images and confirm that the reconstruction is 2D-chiral. It is proposed that the domain orientation upon surface reconstruction is determined by the direction of monoatomic steps present on the Si(110) plane. This is in turn determined by the direction at which the surface is polished off-axis from the (110) plane., Comment: 7 pages, 10 figures

Published
2018
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3. Spin-orbit effects at chiral surfaces

Author

Lewis, N. K., Durham, P. J., Flavell, W. R., and Seddon, E. A.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Two-dimensional hexagonal and oblique lattices were investigated theoretically with the aim of observing differences in the spin expectation values between chiral and achiral systems. The spinresolved band structures were derived from the energy eigenvalues and eigenfunctions of a Hamiltonian that includes the lattice potential and the spin-orbit interaction. The spin texture of the achiral hexagonal system was shown to have two non-zero components of the spin polarisation, whereas all three components were calculated to be non-zero for the chiral system. The longitudinal component, found to be zero in the achiral lattice, was observed to invert between the enantiomorphs of the chiral lattice. A heuristic model was introduced to discern the origin of this inverting spin polarisation by considering the dynamics of an electron in chiral and achiral lattices. This model was further used to demonstrate the change in magnitude of the spin polarisation as a function of the lattice parameters and an electric field perpendicular to the lattice., Comment: 8 pages, 6 figures

Published
2018
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6. Characterization of buried interfaces using Ga Kα hard X-ray photoelectron spectroscopy (HAXPES)

Author

Spencer, B. F., primary, Church, S. A., additional, Thompson, P., additional, Cant, D. J. H., additional, Maniyarasu, S., additional, Theodosiou, A., additional, Jones, A. N., additional, Kappers, M. J., additional, Binks, D. J., additional, Oliver, R. A., additional, Higgins, J., additional, Thomas, A. G., additional, Thomson, T., additional, Shard, A. G., additional, and Flavell, W. R., additional

Published
2022
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8. Electron spectroscopy of metal oxides

Author

Flavell, W. R.

Subjects
546, Inorganic chemistry
Abstract

The validity of the classical dielectric theory of HREELS is investigated. Group theory is employed to obtain a tabulation of SO phonon modes expected to appear strongly in the spectra of given faces of many common crystal structures. The effects of crystalline anisotropy and surface defects are considered in detail. The theoretical investigation is used in conjunction with experiment to obtain a more detailed understanding of the HREELS of rutile (110), (100) and (001) surfaces than has been obtained previously. XPS, UPS and HREELS are used to investigate the surface composition and electronic structure of Sn-doped In2O3 ceramics (containing 1-6 at.% Sn) and thin films. XPS of the well-equilibrated ceramics reveals substantial tin enrichment in the surface atomic layer, with a heat of segregation of ~-20kJmol-1, and provides evidence for a sub-surface region partly depleted in tin. UPS and HREELS results are consistent with a free-carrier concentration close to the surface considerably below the bulk nominal value. XPS of the thin films reveals considerably less surface tin segregation, suggesting that thermal equilibrium is not attained during film production. Vacuum annealing dramatically increases the free carrier concentration, as shown by the shift in the surface plasmon frequency in HREELS. There is a substantial discrepancy between the bulk plasmon frequency predicted from HREELS, and that measured directly from optical transmission. The shift and attenuation of the HREELS plasmon is compared with a model where the surface layer is completely depleted of free carriers. Surface depletion layers have been created on rutile (001), and Sb-doped SnO2 ceramics containing 0.1 and 1 at.% Sb, by adsorption of Cl2 and NO2. The surface coverage of adsorbate is monitored by XPS. Shifts in work function and valence band edge are measured by UPS. HREELS of Cl2-dosed Sb-doped SnO2 show changes consistent with a depletion layer model.

Published
1986

11. Spin- and angle-resolved photoemission studies of the electronic structure of Si(110)“16×2” surfaces

Author

Lewis, N. K, Lassailly, Y., Martinelli, L., Vobornik, I., Fujii, J., Bigi, C., Brunkow, Evan M., Clayburn, Nathan B., Gay, Timothy J., Flavell, W. R., Seddon, E. A., Lewis, N. K, Lassailly, Y., Martinelli, L., Vobornik, I., Fujii, J., Bigi, C., Brunkow, Evan M., Clayburn, Nathan B., Gay, Timothy J., Flavell, W. R., and Seddon, E. A.

Abstract

The electronic structure of Si(110)“16×2” double-domain, single-domain, and 1×1 surfaces have been investigated using spin- and angle-resolved photoemission at sample temperatures of 77K and 300K. Angleresolved photoemission was conducted using horizontally and vertically polarized 60 eV and 80 eV photons. Band-dispersion maps revealed four surface states (S1 to S4) which were assigned to silicon dangling bonds on the basis of measured binding energies and photoemission intensity changes between horizontal and vertical light polarizations. Three surface states (S1, S2, and S4), observed in the Si(110)“16×2” reconstruction, were assigned to Si adatoms and Si atoms present at the edges of the corrugated terrace structure. Only one of the four surface states, S3, was observed in both the Si(110)“16×2” and 1×1 band maps and consequently attributed to the pervasive Si zigzag chains that are components of both the Si(110)“16×2” and 1×1 surfaces. A state in the bulk-band region was attributed to an in-plane bond. All data were consistent with the adatom-buckling model of the Si(110)“16×2” surface. Whilst room temperature measurements of Py and Pz were statistically compatible with zero, Px measurements of the enantiomorphic A-type and B-type Si(110)“16×2” surfaces gave small average polarizations of around 1.5% that were opposite in sign. Further measurements at 77K on A-type Si(110)“16×2” surfaces gave a smaller value of +0.3%. An upper limit of ∼1% may thus be taken for the longitudinal polarization.

Published
2019

13. Electron spectroscopy of metal oxides

Author

Flavell, W

Abstract

The validity of the classical dielectric theory of HREELS is investigated. Group theory is employed to obtain a tabulation of SO phonon modes expected to appear strongly in the spectra of given faces of many common crystal structures. The effects of crystalline anisotropy and surface defects are considered in detail. The theoretical investigation is used in conjunction with experiment to obtain a more detailed understanding of the HREELS of rutile (110), (100) and (001) surfaces than has been obtained previously. XPS, UPS and HREELS are used to investigate the surface composition and electronic structure of Sn-doped In2O3 ceramics (containing 1-6 at.% Sn) and thin films. XPS of the well-equilibrated ceramics reveals substantial tin enrichment in the surface atomic layer, with a heat of segregation of ~-20kJmol-1, and provides evidence for a sub-surface region partly depleted in tin. UPS and HREELS results are consistent with a free-carrier concentration close to the surface considerably below the bulk nominal value. XPS of the thin films reveals considerably less surface tin segregation, suggesting that thermal equilibrium is not attained during film production. Vacuum annealing dramatically increases the free carrier concentration, as shown by the shift in the surface plasmon frequency in HREELS. There is a substantial discrepancy between the bulk plasmon frequency predicted from HREELS, and that measured directly from optical transmission. The shift and attenuation of the HREELS plasmon is compared with a model where the surface layer is completely depleted of free carriers. Surface depletion layers have been created on rutile (001), and Sb-doped SnO2 ceramics containing 0.1 and 1 at.% Sb, by adsorption of Cl2 and NO2. The surface coverage of adsorbate is monitored by XPS. Shifts in work function and valence band edge are measured by UPS. HREELS of Cl2-dosed Sb-doped SnO2 show changes consistent with a depletion layer model.

Published
2017

16. Electronic properties of the interface between p-CuI and anatase-phase n-TiO2 single crystal and nanoparticulate surfaces: A photoemission study.

Author

Kumarasinghe, A. R., Flavell, W. R., Thomas, A. G., Mallick, A. K., Tsoutsou, D., Chatwin, C., Rayner, S., Kirkham, P., Warren, S., Patel, S., Christian, P., O'Brien, P., Grätzel, M., and Hengerer, R.

Subjects
*PHOTOEMISSION, *HETEROJUNCTIONS, *BINDING energy, *SPECTRUM analysis, *PHYSICAL & theoretical chemistry
Abstract

We present a study of the growth of the p-type inorganic semiconductor CuI on n-type TiO2 anatase single crystal (101) surfaces and on nanoparticulate anatase surfaces using synchrotron radiation photoemission spectroscopy. Core level photoemission data obtained using synchrotron radiation reveal that both the substrate (TiO2) and the overlayer (CuI) core levels shift to a lower binding energy to different degrees following the growth of CuI on TiO2. Valence band photoemission data show that the valence band maximum of the clean substrate differs from that of the dosed surface which may be interpreted qualitatively as due to the introduction of a new density of states within the band gap of TiO2 as a result of the growth of CuI. The valence band offset for the heterojunction n-TiO2/p-CuI has been measured using photoemission for both nanoparticulate and single crystal TiO2 surfaces, and the band energy alignment for these heterojunction interfaces is presented. With the information obtained here, it is suggested that the interface between p-CuI and single crystal anatase-phase n-TiO2 is a type-II heterojunction interface, with significant band bending. The measured total band bending matches the work function change at the interface, i.e., there is no interface dipole. In the case of the nanoparticulate interface, an interface dipole is found, but band bending within the anatase nanoparticles remains quite significant. We show that the corresponding depletion layer may be accommodated within the dimension of the nanoparticles. The results are discussed in the context of the functional properties of dye-sensitized solid state solar cells. [ABSTRACT FROM AUTHOR]

Published
2007
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17. Investigations of chemical and electronic inhomogeneities in BaPb[sub 1-x]Bi[sub x]O[sub 3] via highly spatially resolved electron energy loss spectroscopy.

Author

Gutiérrez-Sosa, A., Bangert, U., and Flavell, W. R.

Subjects
TRANSMISSION electron microscopes, ENERGY dissipation, CRYSTALLOGRAPHY, ELECTRONIC structure, SEMICONDUCTORS
Abstract

BaPb[sub 1-x]Bi[sub x]O[sub 3] crystals, doped to various nominal x, were investigated in a scanning transmission electron microscope equipped with energy dispersive x-ray and electron energy loss spectroscopy facilities. Large compositional variations on the nm scale were found in single crystalline regions of any nominal x value. These were accompanied by changes in the core-loss and low-loss energy loss spectra. The low-loss energy loss spectra, furthermore, demonstrate the evolution of electronic structure with doping (i.e., transitions from semiconductor to semimetal) and fluctuations with variations in the local composition. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Published
2003
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19. Tuning the Charge Carriers Migration in Epitaxial BaTiO3Thin-Film Photoanodes

Author

Magnan, H., Deleuze, P. M., Brehin, J., Plays, T., Stanescu, D., Flavell, W. R., Silly, M. G., Domenichini, B., and Barbier, A.

Abstract

We studied the growth of epitaxial ferroelectric layers of BaTiO3/Pt(001) in the framework of solar water splitting. The stoichiometry, that is, the Ti/Ba ratio, appears as a crucial parameter not only for the crystallographic structure but also for the photocurrent generation when the sample is used as a photoanode. We established an electrochemical poling method using a modulated potential, without altering the sample to control the electric polarization. Importantly, we show that the photoanode efficiency is improved when the polarization of the film is oriented toward the substrate. We demonstrate that this improvement is only due to the electrical field created by screening charges, which allows the separation of photogenerated charges.

Published
2020
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20. Domain formation mechanism of the Si(110) “16 × 2” reconstruction

Author

Lewis, N. K., Clayburn, Nathan B., Brunkow, Evan M., Gay, Timothy J., Lassailly, Y., Fujii, J., Vobornik, I., Flavell, W. R., Seddon, E. A., Lewis, N. K., Clayburn, Nathan B., Brunkow, Evan M., Gay, Timothy J., Lassailly, Y., Fujii, J., Vobornik, I., Flavell, W. R., and Seddon, E. A.

Abstract

The main factor that determines which of the two domains forms upon reconstruction of the Si(110) “16 × 2” surface has been investigated. Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) images showed that the domain orientation was independent of the heating current direction used to induce the Si(110) “16 × 2” reconstruction. Reciprocal-space lattice models of the reconstruction allowed for the correct identification of domain orientations in the LEED images, and they confirmed that the reconstruction is two dimensionally chiral. It is proposed that the domain orientation upon surface reconstruction is determined by the direction of monatomic steps present on the Si(110) plane. This is determined in turn by the direction at which the surface is polished off-axis from the (110) plane.

Published
2017

21. Dynamics in next-generation solar cells: time-resolved surface photovoltage measurements of quantum dots chemically linked to ZnO (101[combining macron]0)

Author

Spencer, B, Cliffe, M, Graham, D, Hardman, S, Seddon, E, Syres, K, Thomas, A, Sirotti, F, Silly, MG, Akhtar, J, O'Brien, P, Fairclough, S, Smith, J, Chattopadhyay, S, and Flavell, W

Abstract

The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (101[combining macron]0) using a laser pump-synchrotron X-ray probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS-ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells.

Published
2014

23. Charge dynamics at heterojunctions for PbS/ZnO colloidal quantum dot solar cells probed with time-resolved surface photovoltage spectroscopy

Author

Spencer, B. F., primary, Leontiadou, M. A., additional, Clark, P. C. J., additional, Williamson, A. I., additional, Silly, M. G., additional, Sirotti, F., additional, Fairclough, S. M., additional, Tsang, S. C. E., additional, Neo, D. C. J., additional, Assender, H. E., additional, Watt, A. A. R., additional, and Flavell, W. R., additional

Published
2016
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26. Reflection anisotropy spectroscopy of biological molecules with the 4GLS source

Author

Weightman, P., primary, Bowler, M., additional, Clarke, J. A., additional, Farrell, T., additional, Flavell, W., additional, Harrison, P., additional, Martin, D. S., additional, Papiz, M. Z., additional, Poole, M. A., additional, Quinn, F., additional, Seddon, E. A., additional, Smith, C. I., additional, Smith, S., additional, and Surman, M., additional

Published
2008
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27. Electronic properties of the interface between p-CuI and anatase-phase n-TiO2 single crystal and nanoparticulate surfaces: A photoemission study

Author

Kumarasinghe, A. R., primary, Flavell, W. R., additional, Thomas, A. G., additional, Mallick, A. K., additional, Tsoutsou, D., additional, Chatwin, C., additional, Rayner, S., additional, Kirkham, P., additional, Warren, S., additional, Patel, S., additional, Christian, P., additional, O’Brien, P., additional, Grätzel, M., additional, and Hengerer, R., additional

Published
2007
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28. Comparison of the electronic structure of anatase and rutileTiO2single-crystal surfaces using resonant photoemission and x-ray absorption spectroscopy

Author

Thomas, A. G., primary, Flavell, W. R., additional, Mallick, A. K., additional, Kumarasinghe, A. R., additional, Tsoutsou, D., additional, Khan, N., additional, Chatwin, C., additional, Rayner, S., additional, Smith, G. C., additional, Stockbauer, R. L., additional, Warren, S., additional, Johal, T. K., additional, Patel, S., additional, Holland, D., additional, Taleb, A., additional, and Wiame, F., additional

Published
2007
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29. A PHOTOEMISSION-STUDY TO CONFIRM THE 2ND-ORDER NATURE OF ANOMALOUS O 2S RESONANT ENHANCEMENT OF BI2SR2CACU2O8(001) FERMI LEVEL STATES

Author

Wincott, P, Thomas, A, Lindsay, R, Muryn, C, Flipse, C, Raiker, G, Thornton, G, Cummins, T, Cohen, O, Egdell, R, Laverty, J, Mitchell, P, Ross, J, Law, D, Holland, D, Leonyuk, L, Morris, B, and Flavell, W

Abstract

We have performed a synchrotron-excited angle resolved photoemission study of the valence band region of cleaved Bi2Sr2CaCu2O8(001) samples. By recording spectra between 16 and 30eV on two separate beamlines with differing second order light contributions, we have investigated the origin of previously reported "anomalous" O 2s resonant enhancement of Fermi level states, in which the features showing resonant enhancement apparently disperse with photon energy. Our results indicate that anomalous O 2s resonant enhancement may be explained by second order effects. © 1991.

Published
1991

30. Resonant photoemission of single-crystal RBaCo2O5+δ (R=Gd, Dy)

Author

Flavell, W., primary, Thomas, A., additional, Tsoutsou, D., additional, Mallick, A., additional, North, M., additional, Seddon, E., additional, Cacho, C., additional, Malins, A., additional, Patel, S., additional, Stockbauer, R., additional, Kurtz, R., additional, Sprunger, P., additional, Barilo, S., additional, Shiryaev, S., additional, and Bychkov, G., additional

Published
2004
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36. The 4GLS Project: update and technological challenges.

Author

Quinn, F. M., Seddon, E. A., Flavell, W. F., Weightman, P., Poole, M. W., Todd, B., Hirst, G., and Warwick, T.

Subjects
LIGHT sources, ELECTRON beams, LINEAR accelerators, PHOTON beams, FREE electron lasers, FAR ultraviolet radiation, MAGNETS
Abstract

The fourth generation light source, 4GLS, exploits high brightness electron beam and Energy Recovery Linac (ERL) technology to provide ultra-bright photon beams from infra-red to XUV with pulse lengths down to the femtosecond regime. The suite of light sources covers three free-electron lasers (FELs) and conventional spontaneous radiation from undulators and bending magnets. Two cavity-based lasers will provide infra-red and vacuum-ultraviolet radiation with a high gain, single pass FEL for XUV radiation from 10 –100 eV. The science case for 4GLS has passed peer review in 2002. This paper will provide an update on the status of the project as it moves towards the detailed design phase. It will also review the technological challenges for the accelerator and experiment systems. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Published
2004
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38. Resonance photoemission of LaCoO3(111) and La0.9Sr0.1CoO3(111)

Author

Thomas, A G, primary, Flavell, W R, additional, Dunwoody, P M, additional, Mitchell, C E J, additional, Warren, S, additional, Grice, S C, additional, Marr, P G D, additional, Jewitt, D E, additional, Khan, N, additional, Dhanak, V R, additional, Teehan, D, additional, Seddon, E A, additional, Asai, Kichizo, additional, Koboyashi, Yoshihiko, additional, and Yamada, Nobuyoshi, additional

Published
2000
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40. Resonant Photoemission from Complex Cuprates and Nickelates

Author

Flavell, W. R., primary, Hollingworth, J., additional, Howlett, J. F., additional, Thomas, A. G., additional, Sarker, Md. M., additional, Squire, S., additional, Hashim, Z., additional, Mian, M., additional, Wincott, P. L., additional, Teehan, D., additional, Downes, S., additional, and Hancock, F. E., additional

Published
1995
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42. Book reviews

Author

Barron, A. R., primary, Flavell, W., additional, Gavezzotti, A., additional, and Plater, M. J., additional

Published
1995
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45. Book reviews

Author

Dunmur, D., primary, Whyman, R., additional, Flavell, W., additional, Moseley, P. T., additional, and Cantwell, W. J., additional

Published
1994
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48. H2O adsorption onBi2Sr2CaCu2O8(001)

Author

Flavell, W. R., primary, Laverty, J. H., additional, Law, D. S-L., additional, Lindsay, R., additional, Muryn, C. A., additional, Flipse, C. F. J., additional, Raiker, G. N., additional, Wincott, P. L., additional, and Thornton, G., additional

Published
1990
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50. Electronic Structure and Reactivity of TM-Doped La[sub 1-x]Sr[sub x]CoO[sub 3] (TM = Ni, Fe) Catalysts.

Author

Grice, S. C., Flavell, W. R., Thomas, A. G., Warren, S., Marr, P. G., Jewitt, D. E., Khan, N., Dunwoody, P. M., and Jones, S. A.

Subjects
*CATALYSTS, *SEMICONDUCTOR doping, *ELECTRONIC structure
Abstract

The catalytic properties of LaCoO[sub 3] in the oxidation of organic molecules in aqueous solution are explored as a function of doping with both Sr substitution for La and Fe and Ni substitution for Co. VUV photoemission is used to explore the surface reactivity of the ceramic catalysts in aqueous solution, using H[sub 2]O as a probe molecule. These measurements are complemented by EXAFS and XANES measurements designed to probe the local defect structure and by GC measurements of catalytic activity in the aqueous epoxidation of crotyl alcohol. We relate the observed catalytic activity to the defect structure of the doped materials. In Ni-doped materials, oxygen vacancies appear to be the predominant defect, whereas in Fe-doped samples, electron holes are stabilised on Fe, leading to very different behaviour in oxidation. The surface reactivity to water is also influenced by the TM d electron count, with water binding more strongly to Fe-doped materials than to those containing Ni. The influence of these factors on the rate of the unwanted hydrogen peroxide decomposition reaction and hence on activity in epoxidation is discussed. [ABSTRACT FROM AUTHOR]

Published
2002
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