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Sterrer M, Berger T, Diwald O, Knözinger E, Sushko PV, Shluger AL. Chemistry at corners and edges: Generation and adsorption of H atoms on the surface of MgO nanocubes. J Chem Phys 2005; 123:64714. [PMID: 16122342 DOI: 10.1063/1.1997108] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We used UV light to generate site-selective O- hole centers at three-coordinated corner oxygen sites on MgO nanocubes. These highly reactive O- radicals split H2 homolytically and, in the course of this reaction, become hydroxylated and produce hydrogen atoms. The hydrogen atoms adsorb predominantly at cube edges and dissociate into surface-trapped electrons and protons. We propose that the experimentally observed (H+)(e-) centers are formed adjacent to the hydroxyl groups generated in the homolytic splitting process and can be defined as (H+)3C...(e-)(H+)NC centers where 3C and NC refer to the coordination numbers of the corresponding hydroxylated oxygen sites. Our ab initio embedded cluster calculations reveal that the electronic properties of (H+)3C...(e-)(H+)4C centers situated along MgO nanocube edges are consistent with both the electron-paramagnetic-resonance signal parameters and the reported optical-absorption properties. The transformation of corner O- centers into the (H+)3C...(e-)(H+)NC-type centers prevents their recombination with electronic surface centers and, hence, significantly alters the electronic structure of MgO nanocubes by introducing shallow electron traps.
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Affiliation(s)
- Martin Sterrer
- Department of Chemical Physics, Fritz-Haber-Institute of the Max-Planck-Society, Faradayweg 4-6, D-14195 Berlin, Germany
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MgO-Nanowürfel: über die Abhängigkeit der optischen Eigenschaften von der Teilchengröße. Angew Chem Int Ed Engl 2005. [DOI: 10.1002/ange.200500663] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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53
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Chiesa M, Paganini MC, Spoto G, Giamello E, Di Valentin C, Del Vitto A, Pacchioni G. Single Electron Traps at the Surface of Polycrystalline MgO: Assignment of the Main Trapping Sites. J Phys Chem B 2005; 109:7314-22. [PMID: 16851837 DOI: 10.1021/jp044783c] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Paramagnetic centers at the surface of ionic oxides in the form of trapped electrons can be generated by exposure of the material to alkali metal or hydrogen atoms or of molecular hydrogen under UV irradiation. For many years, it has been assumed that the resulting paramagnetic centers consist of oxygen vacancies filled by one electron. High-resolution electron spin resonance spectra and ab initio quantum chemical calculations show that the paramagnetic centers consist of (H(+))(e(-)) electron pairs formed at morphological irregularities of the surface. At least three different kinds of (H(+))(e(-)) centers, [A], [B], and [C], have been identified with abundances of 80%, 10%, and 8%, respectively. In this work, we compare a wide set of measured and computed g-factors and hyperfine coupling constants of the unpaired electron with the surrounding (25)Mg, (17)O, and (1)H nuclei and we propose a general assignment of the centers. (H(+))(e(-)) pairs formed at Mg(4c) ions at steps and edges account for species [A], centers formed at Mg(4c) ions at reverse corners correspond to species [B], and species [C] originates from (H(+))(e(-)) pairs formed at Mg(3c) ions at corners and kinks.
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Affiliation(s)
- Mario Chiesa
- Dipartimento di Chimica IFM, Università di Torino, via P. Giuria 7, 10125 Torino, Italy
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Chiesa M, Paganini MC, Giamelloal E. Bidimensional Solvation and Delocalisation of Electrons at the Surface of an Insulating Oxide: The Role of Surface Hydroxyl Groups on MgO. Chemphyschem 2004; 5:1897-900. [PMID: 15648139 DOI: 10.1002/cphc.200400466] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Affiliation(s)
- Mario Chiesa
- Dipartimento di Chimica IFM, Università di Torino N.I.S., Nanostructured Interfaces and Surfaces Center of Excellence, Torino, Italy.
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Sterrer M, Berger T, Stankic S, Diwald O, Knözinger E. Spectroscopic Properties of Trapped Electrons on the Surface of MgO Nanoparticles. Chemphyschem 2004; 5:1695-703. [PMID: 15580929 DOI: 10.1002/cphc.200400266] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
To characterise electron-trapping sites on the surface of MgO nanoparticles, surface colour centres were generated using UV light in conjunction with selected hydrogen-based electron sources. Four different colour-centre species, including the characteristic (e-)(H+) or F(S)+(H) centre, were identified due to the distinct shape of the respective electron paramagnetic resonance (EPR) signals. The analysis of the EPR saturation behaviour down to microwave powers of 5 x 10(-3) mW reveals an enhanced spin-relaxation probability of the (e-)(H+) centre compared to all other F(S)+ centres that do not exhibit significant magnetic interactions with hydroxylic protons. Beside the dipolar magnetic interaction in the (e-)(H+) centre observed by EPR, the electronic interaction between the unpaired electron and the proton of a closely spaced OH group produces a redshift of the OH stretching band by about 70 to 170 cm(-1), as observed by infrared spectroscopy. EPR and IR spectroscopic data obtained after the selective address of individual reaction channels for surface colour-centre formation point to the fact that (e-)(H+) centres are formed by trapping electrons from H atoms. Consequently, the underlying surface defect does not belong to the sites of the MgO surface, which chemisorb hydrogen via a heterolytic splitting process.
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Affiliation(s)
- Martin Sterrer
- Institut für Materialchemie, Technische Universität Wien, Veterinärplatz 1/GA, 1210 Wien, Austria
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56
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Xu YJ, Li JQ, Zhang YF, Chen WK. The adsorption and dissociation of Cl2 on the MgO (001) surface with vacancies: Embedded cluster model study. J Chem Phys 2004; 120:8753-60. [PMID: 15267807 DOI: 10.1063/1.1697375] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
The adsorption of Cl(2) at a low-coordinated oxygen site (edge or corner site) and vacancy site (terrace, edge, corner F, F(+), or F(2+) center) has been studied by the density functional method, in conjunction with the embedded cluster models. First, we have studied the adsorption of Cl(2) at the edge and corner oxygen sites and the results show that Cl(2), energetically, is inclined to adsorb at the corner oxygen site. Moreover, similar to the most advantageous adsorption mode for Cl(2) on the MgO (001) perfect surface, the most favorable adsorption occurs when Cl(2) approaches the corner oxygen site along the normal direction. A small amount of electrons are transferred from the substrate to the antibonding orbital of the adsorbate, leading to the Cl-Cl bond strength weakened a little. Regarding Cl(2) adsorption at the oxygen vacancy site (F, F(+), or F(2+) center), both large adsorption energies and rather much elongation of the Cl-Cl bond length have been obtained, in particular at the corner oxygen vacancy site, with concurrently large amounts of electrons transferred from the substrate to the antibonding orbital of Cl(2). It suggests, at the oxygen vacancy site, that Cl(2) prefers to dissociate into Cl subspecies. And the potential energy surface indicates that the dissociation process of molecular Cl(2) to atomic Cl is virtually barrierless.
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Affiliation(s)
- Yi-Jun Xu
- Department of Chemistry, Fuzhou University, Fuzhou, 350002,
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Berger T, Sterrer M, Diwald O, Knözinger E. The Color of the MgO SurfaceA UV/Vis Diffuse Reflectance Investigation of Electron Traps. J Phys Chem B 2004. [DOI: 10.1021/jp036336n] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Thomas Berger
- Institut für Materialchemie, Technische Universität Wien, Veterinärplatz 1/GA, A-1210 Wien, Austria, and Department Chemische Physik, Fritz-Haber-Institut der Max Planck Gesellschaft, D-14195 Berlin, Germany
| | - Martin Sterrer
- Institut für Materialchemie, Technische Universität Wien, Veterinärplatz 1/GA, A-1210 Wien, Austria, and Department Chemische Physik, Fritz-Haber-Institut der Max Planck Gesellschaft, D-14195 Berlin, Germany
| | - Oliver Diwald
- Institut für Materialchemie, Technische Universität Wien, Veterinärplatz 1/GA, A-1210 Wien, Austria, and Department Chemische Physik, Fritz-Haber-Institut der Max Planck Gesellschaft, D-14195 Berlin, Germany
| | - Erich Knözinger
- Institut für Materialchemie, Technische Universität Wien, Veterinärplatz 1/GA, A-1210 Wien, Austria, and Department Chemische Physik, Fritz-Haber-Institut der Max Planck Gesellschaft, D-14195 Berlin, Germany
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Affiliation(s)
- Gianfranco Pacchioni
- Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Istituto Nazionale per la Fisica della Materia via R. Cozzi, 53, 20125 Milano, Italy.
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Brazzelli S, Di Valentin C, Pacchioni G, Giamello E, Chiesa M. Alkali Metal Doping of MgO: Mechanisms of Formation of Paramagnetic Surface Centers. J Phys Chem B 2003. [DOI: 10.1021/jp0341321] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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Ricci D, Di Valentin C, Pacchioni G, Sushko PV, Shluger AL, Giamello E. Paramagnetic defect centers at the MgO surface. An alternative model to oxygen vacancies. J Am Chem Soc 2003; 125:738-47. [PMID: 12526673 DOI: 10.1021/ja0282240] [Citation(s) in RCA: 125] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
On the basis of embedded cluster calculations, we propose a new model for the structure of paramagnetic color centers at the MgO surface usually denoted as F(S)(H)(+) (an electron trapped near an adsorbed proton). These centers are produced by exposing the surface of polycrystalline MgO to H(2) followed by UV irradiation. We demonstrate that properties of H atom absorbed at surface sites such as step edges (MgO(step)) and reverse corner sites (MgO(RC)), formed at the intersection of two step edges, are compatible with a number of features observed for F(S)(H)(+). Our calculations suggest that (i) H(2) dissociates at the reverse corner site heterolytically and that there is no barrier for this exothermic reaction; (ii) the calculated vibrations of the resulting MgO(RC)(H(+))(H(-)) complex are fully consistent with the measured ones; (iii) desorption of a neutral H atom from the diamagnetic precursor requires UV light and leads to the formation of stable neutral paramagnetic centers at the surface, MgO(step)(H(+))(e(-))(trapped) and MgO(RC)(H(+))(e(-))(trapped). The computed isotropic hyperfine coupling constants and optical transitions of these centers are in broad agreement with the existing experimental data. We argue that these centers, which do not belong to the class of "oxygen vacancies", are two of the many possible forms of the F(S)(H)(+) defect center.
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Affiliation(s)
- Davide Ricci
- Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Istituto Nazionale per la Fisica della Materia, Via R. Cozzi, Italy
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Sterrer M, Berger T, Diwald O, Knözinger E. Energy transfer on the MgO surface, monitored by UV-induced H2 chemisorption. J Am Chem Soc 2003; 125:195-9. [PMID: 12515522 DOI: 10.1021/ja028059o] [Citation(s) in RCA: 99] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Surface anions on edges (4-coordinated = 4C) and on corners (3-coordinated = 3C) of cubic MgO nanoparticles exhibit UV resonance absorptions around 5.5 and 4.6 eV, respectively. After monochromatic excitation of either site the electron paramagnetic resonance (EPR) spectrum exhibits exclusively signal components related to 3-coordinated O- radicals (O-(3C), electron hole centers), which are perfectly bleached by H(2) addition. The disappearance of the O-(3C) EPR signal components is paralleled by a depletion of the UV resonance absorption of the 3-coordinated O(2-) only and the appearance of one single band in the OH stretching region of the IR spectrum. Obviously the sites of UV excitation and subsequent UV induced surface reaction with H(2) are not the same. This may coherently be explained in terms of mobility of the exciton (O(2-)(4C)* or--after ionization--of the corresponding electron hole O-(4C) along the edge where it was created. Finally the mobile state is trapped at a corner site where the O(3C)H group is formed.
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Affiliation(s)
- Martin Sterrer
- Institut für Materialchemie, Technische Universität Wien, c/o Veterinärplatz 1/GA, A-1210 Vienna, Austria
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Sterrer M, Diwald O, Knözinger E, Sushko PV, Shluger AL. Energies and Dynamics of Photoinduced Electron and Hole Processes on MgO Powders. J Phys Chem B 2002. [DOI: 10.1021/jp026733i] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Martin Sterrer
- Institut für Materialchemie, Technische Universität Wien, c/o Veteriärplatz 1/GA, A-1210 Wien, Austria, Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K
| | - Oliver Diwald
- Institut für Materialchemie, Technische Universität Wien, c/o Veteriärplatz 1/GA, A-1210 Wien, Austria, Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K
| | - Erich Knözinger
- Institut für Materialchemie, Technische Universität Wien, c/o Veteriärplatz 1/GA, A-1210 Wien, Austria, Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K
| | - Peter V. Sushko
- Institut für Materialchemie, Technische Universität Wien, c/o Veteriärplatz 1/GA, A-1210 Wien, Austria, Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K
| | - Alexander L. Shluger
- Institut für Materialchemie, Technische Universität Wien, c/o Veteriärplatz 1/GA, A-1210 Wien, Austria, Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K
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63
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Ricci D, Pacchioni G, Sushko PV, Shluger AL. Electron trapping at neutral divacancy sites on the MgO surface. J Chem Phys 2002. [DOI: 10.1063/1.1491405] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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