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Sternig A, Diwald O. Surface Decoration of MgO Nanocubes with Sulfur Oxides: Experiment and Theory. THE JOURNAL OF PHYSICAL CHEMISTRY. C, NANOMATERIALS AND INTERFACES 2013; 117:7727-7735. [PMID: 23616910 PMCID: PMC3632092 DOI: 10.1021/jp401432j] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 02/08/2013] [Revised: 03/15/2013] [Indexed: 06/02/2023]
Abstract
We investigated the effect of surface sulfate formation on the structure and spectroscopic properties of MgO nanocubes using X-ray diffraction, electron microscopy, several spectroscopic techniques, and ab initio calculations. After CS2 adsorption and oxidative treatment at elevated temperatures the MgO particles remain cubic and retain their average size of ∼6 nm. Their low coordinated surface elements (corners and edges) were found to bind sulfite and sulfate groups even after annealing up to 1173 K. The absence of MgO corner specific photoluminescence emission bands at 3.4 and 3.2 eV substantiates that sulfur modifies the electronic properties of characteristic surface structures, which we attribute to the formation of (SO3)2- and (SO4)2- groups at corners and edges. Ab initio calculations support these conclusions and provide insight into the local atomic structures and spectroscopic properties of these groups.
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52
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Cadigan CA, Corpuz AR, Lin F, Caskey CM, Finch KBH, Wang X, Richards RM. Nanoscale (111) faceted rock-salt metal oxides in catalysis. Catal Sci Technol 2013. [DOI: 10.1039/c2cy20373a] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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53
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Conte M, Liu X, Murphy DM, Whiston K, Hutchings GJ. Cyclohexane oxidation using Au/MgO: an investigation of the reaction mechanism. Phys Chem Chem Phys 2012; 14:16279-85. [PMID: 23132082 DOI: 10.1039/c2cp43363j] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The liquid phase oxidation of cyclohexane was undertaken using Au/MgO and the reaction mechanism was investigated by means of continuous wave (CW) EPR spectroscopy employing the spin trapping technique. Activity tests aimed to determine the conversion and selectivity of Au/MgO catalyst showed that Au was capable of selectivity control to cyclohexanol formation up to 70%, but this was accompanied by a limited enhancement in conversion when compared with the reaction in the absence of catalyst. In contrast, when radical initiators were used, in combination with Au/MgO, an activity comparable to that observed in industrial processes at ca. 5% conversion was found, with retained high selectivity. By studying the free radical autoxidation of cyclohexane and the cyclohexyl hydroperoxide decomposition in the presence of spin traps, we show that Au nanoparticles are capable of an enhanced generation of cyclohexyl alkoxy radicals, and the role of Au is identified as a promoter of the catalytic autoxidation processes, therefore demonstrating that the reaction proceeds via a radical chain mechanism.
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Affiliation(s)
- Marco Conte
- Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff, CF10 3AT, UK.
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54
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Surnev S, Fortunelli A, Netzer FP. Structure-property relationship and chemical aspects of oxide-metal hybrid nanostructures. Chem Rev 2012; 113:4314-72. [PMID: 23237602 DOI: 10.1021/cr300307n] [Citation(s) in RCA: 141] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Affiliation(s)
- Svetlozar Surnev
- Surface and Interface Physics, Institute of Physics, Karl-Franzens University, Graz A-8010 Graz, Austria
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55
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Sternig A, Stankic S, Müller M, Siedl N, Diwald O. Surface exciton separation in photoexcited MgO nanocube powders. NANOSCALE 2012; 4:7494-7500. [PMID: 23100068 DOI: 10.1039/c2nr31844j] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
In MgO nanocube powders surface excitons can separate and the resulting charge carriers provide reactive adsorption sites at well-defined surface elements. We employed photoluminescence (PL) emission bands originating from the photoexcitation of nanocube corners and edges as quantitative probes to explore their chemical reactivity towards molecular hydrogen. Surface excitons which form at corners and edges exhibit similar cross-sections for separation in vacuum. The separation of edge excitons, however, is significantly enhanced in hydrogen atmosphere when hydrogen adsorption occurs as a simultaneous surface process. The electronic structure of MgO nanocube edges which split hydrogen heterolytically upon generation of surface hydroxyls and hydrides is unaffected by the photoexcitation of corners. Respective edges, however, are efficient absorption sites for UV photons. Transfer of exciton energy to oxygen ions in corners is followed by exciton separation which transforms corner ions into surface radicals leading to a well-defined starting point for the site selective functionalization of metal oxide nanostructures.
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Affiliation(s)
- Andreas Sternig
- Cluster of Excellence - Engineering of Advanced Materials (EAM), Friedrich-Alexander University Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany
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56
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Pacchioni G, Freund H. Electron Transfer at Oxide Surfaces. The MgO Paradigm: from Defects to Ultrathin Films. Chem Rev 2012; 113:4035-72. [DOI: 10.1021/cr3002017] [Citation(s) in RCA: 241] [Impact Index Per Article: 20.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Affiliation(s)
- Gianfranco Pacchioni
- Dipartimento di Scienza dei
Materiali, Università di Milano-Bicocca, Via R. Cozzi, 53−20125,
Milano, Italy
| | - Hajo Freund
- Fritz-Haber-Insitut
der MPG,
Department of Chemical Physics, Faradayweg 4-6, 14195 Berlin, Germany
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57
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Zhang H, Gheisi AR, Sternig A, Müller K, Schowalter M, Rosenauer A, Diwald O, Mädler L. Bulk and surface excitons in alloyed and phase-separated ZnO-MgO particulate systems. ACS APPLIED MATERIALS & INTERFACES 2012; 4:2490-2497. [PMID: 22530613 DOI: 10.1021/am300184b] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
The rational design of composite nanoparticles with desired optical and electronic properties requires the detailed analysis of surface and bulk contributions to the respective overall function. We use flame spray pyrolysis (FSP) to generate nanoparticles of the ternary Zn-Mg-O system the compositions of which range from solid solutions of Zn(2+) ions in periclase MgO to phase separated particle mixtures which consist of periclase (cubic) MgO and wurtzite (hexagonal) ZnO phases. The structure and composition of the composite Zn(x)Mg(1-x)O (0 ≤ x ≤ 0.3) particles are investigated using X-ray diffraction and high-resolution transmission electron microscopy, whereas UV diffuse reflectance and photoluminescence (PL) spectroscopy are used for the investigation of their optical properties. Vacuum annealing has been carried out to track the effects of stepwise elimination of surface adsorbates on the photoexcitation and PL emission properties. We demonstrate that for Zn(0.1)Mg(0.9)O particles, the admixed ZnO suppresses the MgO specific surface excitons and produces a PL emission band at 470 nm. Although gaseous oxygen partially reduces the emission intensity of hydroxylated particles, it leads to entire quenching in completely dehydroxylated samples after vacuum annealing at 1173 K. Consequently, surface hydroxyls at the solid-gas interface play a significant role as protecting groups against the PL-quenching effects of O(2). The obtained results are relevant for the characterization of ZnO-based devices as well as for other metal oxide materials where the impact of the surface composition on the photoelectronic properties is usually neglected.
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Affiliation(s)
- Huanjun Zhang
- Foundation Institute of Materials Science (IWT), Department of Production Engineering, University of Bremen, Badgasteinerstrasse 3, 28359 Bremen, Germany
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58
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Hua M, Zhang S, Pan B, Zhang W, Lv L, Zhang Q. Heavy metal removal from water/wastewater by nanosized metal oxides: a review. JOURNAL OF HAZARDOUS MATERIALS 2012; 211-212:317-31. [PMID: 22018872 DOI: 10.1016/j.jhazmat.2011.10.016] [Citation(s) in RCA: 901] [Impact Index Per Article: 75.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/08/2011] [Revised: 09/29/2011] [Accepted: 10/04/2011] [Indexed: 05/17/2023]
Abstract
Nanosized metal oxides (NMOs), including nanosized ferric oxides, manganese oxides, aluminum oxides, titanium oxides, magnesium oxides and cerium oxides, provide high surface area and specific affinity for heavy metal adsorption from aqueous systems. To date, it has become a hot topic to develop new technologies to synthesize NMOs, to evaluate their removal of heavy metals under varying experimental conditions, to reveal the underlying mechanism responsible for metal removal based on modern analytical techniques (XAS, ATR-FT-IR, NMR, etc.) or mathematical models, and to develop metal oxide-based materials of better applicability for practical use (such as granular oxides or composite materials). The present review mainly focuses on NMOs' preparation, their physicochemical properties, adsorption characteristics and mechanism, as well as their application in heavy metal removal. In addition, porous host supported NMOs are particularly concerned because of their great advantages for practical application as compared to the original NMOs. Also, some magnetic NMOs were included due to their unique separation performance.
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Affiliation(s)
- Ming Hua
- State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR China
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59
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Krishnamoorthy K, Moon JY, Hyun HB, Cho SK, Kim SJ. Mechanistic investigation on the toxicity of MgO nanoparticles toward cancer cells. ACTA ACUST UNITED AC 2012. [DOI: 10.1039/c2jm35087d] [Citation(s) in RCA: 178] [Impact Index Per Article: 14.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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60
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Shao X, Jerratsch JF, Nilius N, Freund HJ. Probing the 4f states of ceria by tunneling spectroscopy. Phys Chem Chem Phys 2011; 13:12646-51. [PMID: 21674086 DOI: 10.1039/c1cp21113g] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Low-temperature scanning tunneling microscopy and spectroscopy have been employed to analyze the local electronic structure of the (111) surface of a ceria thin film grown on Ru(0001). On pristine, defect-free oxide terraces, the empty 4f states of Ce(4+) ions appear as the only spectral feature inside the 6 eV oxide band gap. In contrast, occupied states are detected between -1.0 and -1.5 eV below E(Fermi) in conductance spectra of different point and line defects, such as surface oxygen vacancies, grain boundaries and step edges. They are assigned to partially filled 4f states localized at the Ce(3+) ions. The presence of excess electrons indicates the oxygen-deficient nature of the direct oxide environment. The f state spectroscopy with the STM allows us to probe the spatial distribution of Ce(3+) ions in the ceria surface, providing unique insight into the local reduction state of this chemically important material system.
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Affiliation(s)
- Xiang Shao
- Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany
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61
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McKenna KP, Koller D, Sternig A, Siedl N, Govind N, Sushko PV, Diwald O. Optical properties of nanocrystal interfaces in compressed MgO nanopowders. ACS NANO 2011; 5:3003-9. [PMID: 21443262 PMCID: PMC3082970 DOI: 10.1021/nn200062d] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/06/2023]
Abstract
The optical properties and charge trapping phenomena observed on oxide nanocrystal ensembles can be strongly influenced by the presence of nanocrystal interfaces. MgO powders represent a convenient system to study these effects due to the well-defined shape and controllable size distributions of MgO nanocrystals. The spectroscopic properties of nanocrystal interfaces are investigated by monitoring the dependence of absorption characteristics on the concentration of the interfaces in the nanopowders. The presence of interfaces is found to affect the absorption spectra of nanopowders more significantly than changing the size of the constituent nanocrystals and, thus, leading to the variation of the relative abundance of light-absorbing surface structures. We find a strong absorption band in the 4.0-5.5 eV energy range, which was previously attributed to surface features of individual nanocrystals, such as corners and edges. These findings are supported by complementary first-principles calculations. The possibility to directly address such interfaces by tuning the energy of excitation may provide new means for functionalization and chemical activation of nanostructures and can help improve performance and reliability for many nanopowder applications.
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Affiliation(s)
- Keith P. McKenna
- WPI-AIMR, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan
- Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom
- Address correspondence to ;
| | - David Koller
- Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Wien, Austria
| | - Andreas Sternig
- Friedrich-Alexander Universitat, Erlangen-Nurnberg, Cauerstrasse 4, Erlangen D-91058, Germany
| | - Nicolas Siedl
- Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Wien, Austria
- Friedrich-Alexander Universitat, Erlangen-Nurnberg, Cauerstrasse 4, Erlangen D-91058, Germany
| | - Niranjan Govind
- Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States
| | - Peter V. Sushko
- Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom
| | - Oliver Diwald
- Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Wien, Austria
- Friedrich-Alexander Universitat, Erlangen-Nurnberg, Cauerstrasse 4, Erlangen D-91058, Germany
- Address correspondence to ;
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62
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63
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Xie S, Han X, Kuang Q, Zhao Y, Xie Z, Zheng L. Intense and wavelength-tunable photoluminescence from surface functionalized MgO nanocrystal clusters. ACTA ACUST UNITED AC 2011. [DOI: 10.1039/c1jm10745c] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
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64
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Jean ST, Her YC. Growth mechanism and cathodoluminescence properties of indium hydroxide nanocubes synthesized from a simple aqueous solution. CrystEngComm 2011. [DOI: 10.1039/c0ce00480d] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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65
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Freund HJ, Nilius N, Risse T, Schauermann S, Schmidt T. Innovative Measurement Techniques in Surface Science. Chemphyschem 2010; 12:79-87. [DOI: 10.1002/cphc.201000812] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2010] [Indexed: 11/08/2022]
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66
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Stankic S, Sternig A, Finocchi F, Bernardi J, Diwald O. Zinc oxide scaffolds on MgO nanocubes. NANOTECHNOLOGY 2010; 21:355603. [PMID: 20693618 DOI: 10.1088/0957-4484/21/35/355603] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
Powders of isolated and well-dispersed oxide nanocubes are promising components for photoelectronic applications that benefit from tunable optical properties, surface reactivity and the ease of realization of their controlled assembly. Here, we demonstrate that combustion of zinc and magnesium metal vapors at reduced pressures followed by subsequent vacuum annealing of the resulting nanoparticle powders yields single-crystalline Zn(x)Mg(1-x)O nanocubes of exceptional regular cubic shape and edge lengths below 25 nm. In line with ab initio calculations, which predict preferential Zn(2+) segregation into low coordinated surface elements of the MgO nanocubes, we track the occupation of edge sites by chains of Zn(2+)-O(2-) units through their spectroscopic signatures. As a method to generate composite nanostructures with controlled spatial distribution of the chemical components, the annealing induced ion segregation can be extended to other well-dispersed metastable nanoparticles. We expect that the energy of segregation mainly depends on the site coordination number, which can promote controlled demixing within the nanoparticles.
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Affiliation(s)
- Slavica Stankic
- Institut des Nanosciences de Paris, UMR 7588 CNRS and Université Paris 6, Paris, France
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67
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Niu KY, Yang J, Sun J, Du XW. One-step synthesis of MgO hollow nanospheres with blue emission. NANOTECHNOLOGY 2010; 21:295604. [PMID: 20601755 DOI: 10.1088/0957-4484/21/29/295604] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
MgO hollow nanospheres were produced via one-step laser synthesis in both gas and liquid media. The formation mechanism of MgO hollow nanospheres was investigated by adopting high-speed photography and performing control experiments under different oxidization conditions. The results indicated that the in situ Kirkendall effect is responsible for the formation of the hollow nanospheres. Blue emission was observed from the MgO hollow nanospheres produced in a liquid medium, and is ascribed to the surface state arising from the organic modification.
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Affiliation(s)
- Kai-Yang Niu
- School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People's Republic of China
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68
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Zhao G, Shi J, Liu G, Liu Y, Wang Z, Zhang W, Jia M. Efficient porous carbon-supported MgO catalysts for the transesterification of dimethyl carbonate with diethyl carbonate. ACTA ACUST UNITED AC 2010. [DOI: 10.1016/j.molcata.2010.05.007] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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69
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Khassin AA, Filonenko GA, Minyukova TP, Molina IY, Plyasova LM, Larina TV, Anufrienko VF. Effect of anionic admixtures on the copper–magnesium mixed oxide reduction. REACTION KINETICS MECHANISMS AND CATALYSIS 2010. [DOI: 10.1007/s11144-010-0211-1] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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70
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Myrach P, Nilius N, Levchenko S, Gonchar A, Risse T, Dinse KP, Boatner L, Frandsen W, Horn R, Freund HJ, Schlögl R, Scheffler M. Temperature-Dependent Morphology, Magnetic and Optical Properties of Li-Doped MgO. ChemCatChem 2010. [DOI: 10.1002/cctc.201000083] [Citation(s) in RCA: 91] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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71
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Panchakarla LS, Sundarayya Y, Manjunatha S, Sundaresan A, Rao CNR. On the Defect Origin of the Room-Temperature Magnetism Universally Exhibited by Metal-Oxide Nanoparticles. Chemphyschem 2010; 11:1673-9. [DOI: 10.1002/cphc.201000011] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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72
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Sternig A, Müller M, McCallum M, Bernardi J, Diwald O. BaO clusters on MgO nanocubes: a quantitative analysis of optical-powder properties. SMALL (WEINHEIM AN DER BERGSTRASSE, GERMANY) 2010; 6:582-8. [PMID: 20029851 DOI: 10.1002/smll.200901662] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/12/2023]
Abstract
Uniformly sized and shaped nanoparticles are well suited for the quantitative characterization of optical-powder properties. For the first time, quantum yields related to photoluminescence emissions that originate from the photoexcitation of MgO nanocube corners and edges are measured. In addition, the surfaces of these nanoparticles are doped with submonolayer barium, which oxidizes during adsorption onto the MgO nanocrystal surfaces and transforms in O(2) atmosphere into BaO. UV-Vis diffuse reflectance and photoluminescence spectroscopy is employed to explore whether 10(-3) monolayer equivalents of these dopants affect the MgO specific optical properties. Surface-admixed BaO produces additional absorption and photoluminescence emission features but does not significantly affect those specific to MgO nanocubes. On this basis the number of optically active sites that can be sampled inside a powder of alkaline earth oxide nanoparticles using a standard spectrometer system is estimated.
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Affiliation(s)
- Andreas Sternig
- Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, Vienna 1210, Austria
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73
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Petitjean H, Chizallet C, Krafft JM, Che M, Lauron-Pernot H, Costentin G. Basic reactivity of CaO: investigating active sites under operating conditions. Phys Chem Chem Phys 2010; 12:14740-8. [DOI: 10.1039/c0cp00855a] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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74
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Pashchanka M, Hoffmann RC, Schneider JJ. Controlled synthesis and characterisation of MgOnanoparticles, thin films and polycrystalline nanorods derived from a Mg(ii) single source precursor. ACTA ACUST UNITED AC 2010. [DOI: 10.1039/b917715a] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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75
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Defect induced ferromagnetism in MgO nanoparticles studied by optical and positron annihilation spectroscopy. Chem Phys Lett 2009. [DOI: 10.1016/j.cplett.2009.07.037] [Citation(s) in RCA: 99] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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76
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Mi Y, Huang Z, Zhou Z, Hu F, Meng Q. Room-temperature Preparation of BaMoO4Nano-octahedra by Microemulsion Method. CHEM LETT 2009. [DOI: 10.1246/cl.2009.404] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
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77
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Riss A, Elser MJ, Bernardi J, Diwald O. Stability and Photoelectronic Properties of Layered Titanate Nanostructures. J Am Chem Soc 2009; 131:6198-206. [DOI: 10.1021/ja810109g] [Citation(s) in RCA: 96] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Alexander Riss
- Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria, University Service Centre for Transmission Electron Microscopy, Vienna University of Technology, Wiedner Hauptstrasse 8-10/052, A-1040 Vienna, Austria, and Institute of Particle Technology, Friedrich-Alexander-University Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany
| | - Michael J. Elser
- Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria, University Service Centre for Transmission Electron Microscopy, Vienna University of Technology, Wiedner Hauptstrasse 8-10/052, A-1040 Vienna, Austria, and Institute of Particle Technology, Friedrich-Alexander-University Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany
| | - Johannes Bernardi
- Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria, University Service Centre for Transmission Electron Microscopy, Vienna University of Technology, Wiedner Hauptstrasse 8-10/052, A-1040 Vienna, Austria, and Institute of Particle Technology, Friedrich-Alexander-University Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany
| | - Oliver Diwald
- Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria, University Service Centre for Transmission Electron Microscopy, Vienna University of Technology, Wiedner Hauptstrasse 8-10/052, A-1040 Vienna, Austria, and Institute of Particle Technology, Friedrich-Alexander-University Erlangen-Nuremberg, Cauerstrasse 4, 91058 Erlangen, Germany
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78
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Riss A, Diwald O. Partikel-Morphologien und Festkörper-Eigenschaften. Gestalten in der Nanowelt. CHEM UNSERER ZEIT 2009. [DOI: 10.1002/ciuz.200900474] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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79
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Lu HB, Liao L, Li H, Wang DF, Tian Y, Li JC, Fu Q, Zhu BP, Wu Y. Hollow MgO Nanotube Arrays by Using ZnO Nanorods as Templates. Eur J Inorg Chem 2008. [DOI: 10.1002/ejic.200800095] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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80
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Riss A, Berger T, Stankic S, Bernardi J, Knözinger E, Diwald O. Charge Separation in Layered Titanate Nanostructures: Effect of Ion Exchange Induced Morphology Transformation. Angew Chem Int Ed Engl 2008; 47:1496-9. [DOI: 10.1002/anie.200703817] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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81
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Riss A, Berger T, Stankic S, Bernardi J, Knözinger E, Diwald O. Ladungstrennung in nanoskaligen Titanat-Schichten: Einfluss von Ionenaustausch und Morphologieumwandlung auf die photoelektronischen Eigenschaften. Angew Chem Int Ed Engl 2008. [DOI: 10.1002/ange.200703817] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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82
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Desikusumastuti A, Laurin M, Happel M, Qin Z, Shaikhutdinov S, Libuda J. Strong Size Effects in Supported Ionic Nanoparticles: Tailoring the Stability of NO
x
Storage Catalysts. Catal Letters 2007. [DOI: 10.1007/s10562-007-9340-1] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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83
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Stankic S, Bernardi J, Diwald O, Knözinger E. Optical surface properties and morphology of MgO and CaO nanocrystals. J Phys Chem B 2007; 110:13866-71. [PMID: 16836335 DOI: 10.1021/jp061741a] [Citation(s) in RCA: 73] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Optical absorption and photoluminescence emission properties of dehydroxylated MgO and CaO nanocrystals are discussed with respect to particle morphology and size. On MgO nanocubes with pronounced corner and edge features two emission bands at 3.4 and 3.3 eV result from the excitation of 4-coordinated surface O(4C)(2-) anions in edges at 5.4 eV and of regular oxygen-terminated corners at 4.6 eV, respectively. Morphologically ill-defined CaO particles are a factor of 5 larger, do not display regular corner features, and show only one photoluminescence emission band at 3.0 eV. The associated excitation spectrum indicates electronic excitations above the energy required to excite regular oxygen-terminated CaO corners. It is concluded that in the case of morphologically well-defined MgO nanocubes variations in the next coordination of oxygen-terminated corners can effectively be probed by photoluminescence spectroscopy and thus allows for discrimination between 3-coordinated surface O(2-) in regular corner sites and kinks.
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Affiliation(s)
- Slavica Stankic
- Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria
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84
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Müller M, Stankic S, Diwald O, Knözinger E, Sushko PV, Trevisanutto PE, Shluger AL. Effect of Protons on the Optical Properties of Oxide Nanostructures. J Am Chem Soc 2007; 129:12491-6. [PMID: 17892290 DOI: 10.1021/ja0736055] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Site-specific functionalization of oxide nanostructures gives rise to novel optical and chemical surface properties. In addition, it can provide deeper insights into the electronic surface structure of the associated materials. We applied chemisorption of molecular hydrogen, induced by ultraviolet (UV) light, followed by vacuum annealing to MgO nanocubes to selectively decorate three-coordinated oxygen ions (oxygen corner sites, for simplicity) with protons. Fully dehydroxylated nanocubes exhibit 3.2 +/- 0.1 eV photoluminescence induced by 4.6 eV light, where both emission and absorption are associated with three-coordinated oxygen sites. We find that partially hydroxylated nanocubes show an additional photoluminescence feature at 2.9 +/- 0.1 eV. Interestingly, the excitation spectra of the 2.9 and 3.2 eV emission bands, associated with protonated and nonprotonated oxygen corner sites, respectively, nearly coincide and show well-pronounced maxima at 4.6 eV in spite of a significant difference in their local atomic and electronic structures. These observations are explained with the help of ab initio calculations, which reveal that (i) the absorption band at 4.6 eV involves four-coordinated O and Mg ions in the immediate vicinity of the corner sites and (ii) protonation of the three-coordinated oxygen ions eliminates the optical transitions associated with them and strongly red-shifts other optical transitions associated with neighboring atoms. These results demonstrate that the optical absorption bands assigned to topological surface defects are not simply determined by the ions of lowest coordination number but involve contributions due to the neighboring atoms of higher coordination. Thus, we suggest that the absorption band at 4.6 eV should not be regarded as merely a signature of the three-coordinated O2- ions but ought to be assigned to corners as multiatomic topological features. Our results also suggest that optical absorption signatures of protonated and nonprotonated sites of oxide surfaces can be remarkably similar.
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Affiliation(s)
- Markus Müller
- Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria
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85
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Sterrer M, Berger T, Diwald O, Knözinger E, Allouche A. Ozonide ions on the surface of MgO nanocrystals. Top Catal 2007. [DOI: 10.1007/s11244-007-0321-9] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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86
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McKenna KP, Sushko PV, Shluger AL. Inside Powders: A Theoretical Model of Interfaces between MgO Nanocrystallites. J Am Chem Soc 2007; 129:8600-8. [PMID: 17569535 DOI: 10.1021/ja071602m] [Citation(s) in RCA: 52] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The electron- and hole-trapping and optical properties of a wide variety of interfaces between MgO nanocrystallites are investigated for the first time using a quantum-mechanical embedded-cluster method and time-dependent density functional theory. We conclude that delocalized holes can be transiently trapped at a large number of places within a powder. However, it is more energetically favorable for holes to trap on low-coordinated anions on the nanocrystallite surface, forming O- species. Electrons are trapped at few interfaces but are readily trapped by surface kink and corner sites. Contrary to common perception, our calculations of optical absorption spectra indicate that a variety of features buried within a powder can be exited with photon energies less than 5 eV, usually used to selectively excite low-coordinated surface sites.
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Affiliation(s)
- Keith P McKenna
- Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.
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87
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Srnec M, Zahradník R. Small Group IIa–VIa Clusters and Related Systems: A Theoretical Study of Physical Properties, Reactivity, and Electronic Spectra. Eur J Inorg Chem 2007. [DOI: 10.1002/ejic.200600816] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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88
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Liu Q, Yan Z, Sun G, Zheng W. Solvothermal Preparation and Characterization of Barium Titanate Nanocubes. CHEM LETT 2007. [DOI: 10.1246/cl.2007.458] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
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89
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Zhu K, Hu J, Kübel C, Richards R. Efficient Preparation and Catalytic Activity of MgO(111) Nanosheets. Angew Chem Int Ed Engl 2006. [DOI: 10.1002/ange.200602393] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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90
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Zhu K, Hu J, Kübel C, Richards R. Efficient Preparation and Catalytic Activity of MgO(111) Nanosheets. Angew Chem Int Ed Engl 2006; 45:7277-81. [PMID: 17024713 DOI: 10.1002/anie.200602393] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Affiliation(s)
- Kake Zhu
- International University Bremen, 28759 Bremen, Germany
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91
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Chiesa M, Paganini MC, Giamello E, Murphy DM, Di Valentin C, Pacchioni G. Excess electrons stabilized on ionic oxide surfaces. Acc Chem Res 2006; 39:861-7. [PMID: 17115726 DOI: 10.1021/ar068144r] [Citation(s) in RCA: 135] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Surface excess electrons are remarkable chemical entities that provide great opportunities for the design of new materials with precisely tuned electronic and magnetic properties. In this Account, we describe the structure and electronic properties of excess electron centers generated at the surface of insulating oxides. We also outline the elementary mechanisms that are at the basis of the generation of excess electrons at solid surfaces, setting a comparison to the general problem of excess electron localization in condensed media. Emphasis is given to morphological aspects relative to the surface-trapping sites as deduced from combined electron paramagnetic resonance and accurate quantum chemical calculations. The remarkable reactivity featured by the so formed "electron-rich" surfaces is illustrated, describing the reduction of simple diatomic molecules that form adsorbed radical anions via direct surface to adsorbate electron transfer.
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Affiliation(s)
- Mario Chiesa
- Dipartimento di Chimica IFM, Università di Torino and NIS, Nanostructured Interfaces and Surfaces Center of Excellence, Via P. Giuria 7, I-10125 Torino, Italy
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92
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Glaspell G, Hassan HMA, Elzatahry A, Fuoco L, Radwan NRE, El-Shall MS. Nanocatalysis on Tailored Shape Supports: Au and Pd Nanoparticles Supported on MgO Nanocubes and ZnO Nanobelts. J Phys Chem B 2006; 110:21387-93. [PMID: 17064082 DOI: 10.1021/jp0651034] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Active gold and palladium nanoparticles supported on MgO nanocubes and ZnO nanobelts and transition-metal-containing MgO nanobelts were synthesized by combining evaporation and deposition-precipitation techniques. The high activity and stability of the Au/CeO2 and Pd/CeO2 nanoparticle catalysts deposited on the MgO cubes are remarkable and imply that a variety of efficient catalysts can be designed and tested using this approach. The significant increase in the concentration of corner and edge sites in MgO nanocubes make them well-defined supports to study the detailed mechanism of the catalytic activity enhancement.
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93
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Benedetti S, Benia H, Nilius N, Valeri S, Freund H. Morphology and optical properties of MgO thin films on Mo(001). Chem Phys Lett 2006. [DOI: 10.1016/j.cplett.2006.08.130] [Citation(s) in RCA: 78] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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94
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Teo JJ, Chang Y, Zeng HC. Fabrications of hollow nanocubes of Cu(2)O and Cu via reductive self-assembly of CuO nanocrystals. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2006; 22:7369-77. [PMID: 16893240 DOI: 10.1021/la060439q] [Citation(s) in RCA: 144] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/02/2023]
Abstract
In this work, a template-free synthetic approach for generating single-crystalline hollow nanostructures has been described. Using the small optical band-gap cuprous oxide Cu(2)O as a model case, we demonstrate that, instead of normally known spherical aggregates, primary nanocrystalline particles can first self-aggregate into porous organized solids with a well-defined polyhedral shape according to the oriented attachment mechanism, during which chemical conversion can also be introduced. In contrast to the spherical aggregates, where the nanocrystallites are randomly joined together, the Cu(2)O nanocrystallites in the present case are well organized, maintaining a definite geometric shape and a global crystal symmetry. Due to the presence of intercrystallite space, hollowing and chemical conversion can also be carried out in order to create central space and change the chemical phase of nanostructured polyhedrons. It has been revealed that Ostwald ripening plays a key role in the solid evacuation process. Using this synthetic strategy, we have successfully prepared single-crystal-like Cu(2)O nanocubes and polycrystalline Cu nanocubes with hollow interiors. For the first time, we demonstrate that nanostructured polyhedrons of functional materials with desired interiors can be synthesized in solution via a combination of oriented attachment and Ostwald ripening processes.
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Affiliation(s)
- Joong Jiat Teo
- Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
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95
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Kang ES, Takahashi M, Tokuda Y, Yoko T. Template-free magnesium oxide hollow sphere inclusion in organic-inorganic hybrid films via sol-gel reaction. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2006; 22:5220-3. [PMID: 16732641 DOI: 10.1021/la060307x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
Magnesium oxide hollow spheres without a template core were conveniently prepared by stabilized bubble formation in a hybrid solution containing a magnesium acetate precursor, thus avoiding the complicated preparation process using a template. The hollow sphere could be aligned along the radial striation by spin coating, and its diameter from a micrometer to submicrometer dimension could be easily modified by the solution composition. It was also possible to control the open or closed hollow sphere by changing the solvent. Thus, the produced magnesium oxide hollow sphere is envisioned to have applications in many areas such as medicine, analysis, optics, and so on.
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Affiliation(s)
- Eun-Seok Kang
- Laboratory of Inorganic Photonics Materials, Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
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96
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Chizallet C, Costentin G, Krafft JM, Lauron-Pernot H, Che M. Kinetic Model of Energy Transfer Processes Between Low-Coordinated Ions on MgO by Photoluminescence Decay Measurements. Chemphyschem 2006; 7:904-11. [PMID: 16596614 DOI: 10.1002/cphc.200500580] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Photoluminescence decay studies of emitting species on MgO nanocubes at room temperature provide evidence of three surface species characterized by an excitation and emission wavelength couple {lambda(exc);lambda(em)}. Species A corresponds to {lambda(exc)=240 nm; lambda(em)=380 nm}, whereas the couple {lambda(exc)=280 nm; lambda(em)=470 nm} is assigned to two species: B and B', the former is involved in energy transfer from excited state A* and the latter in direct emission from excited state B'*. A simple model for energy transfer from species A* to B is proposed. The numerical resolution of equations corresponding to this model is in good agreement with experimental data. This method quantifies the kinetics of intrinsic emission and energy transfer processes. Lifetime values indicate that phosphorescence is taking place, and species A, B and B' are identified as edge O(2-) (4 C), corner O(2-) (3 C) and kink O(2-) (3 C) oxide ions respectively.
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Affiliation(s)
- Céline Chizallet
- Laboratoire de Réactivité de Surface, UMR 7609 CNRS, Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France.
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97
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Elser MJ, Berger T, Brandhuber D, Bernardi J, Diwald O, Knözinger E. Particles Coming Together: Electron Centers in Adjoined TiO2 Nanocrystals. J Phys Chem B 2006; 110:7605-8. [PMID: 16610847 DOI: 10.1021/jp0607465] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
To include particle attachment and porosity of nanostructured materials in the discussion of their electronic properties is critical to our understanding of charge transfer across grain boundaries. We report the condensation of isolated TiO(2) nanocrystals via the application of a simple hydration-dehydration cycle. After contact with water and subsequent removal of adsorbed water, these nanocrystals form a mesoporous structure with altered properties as compared with the original material: first, the energy needed for defect formation is substantially reduced, and second, electron paramagnetic resonance measurements reveal the presence of polarizable conduction band electrons not detectable in samples which have not been in contact with water.
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Affiliation(s)
- Michael J Elser
- Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria
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98
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Berger T, Sterrer M, Diwald O, Knözinger E. Charge Trapping and Photoadsorption of O2 on Dehydroxylated TiO2 Nanocrystals—An Electron Paramagnetic Resonance Study. Chemphyschem 2005; 6:2104-12. [PMID: 16208752 DOI: 10.1002/cphc.200500161] [Citation(s) in RCA: 114] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
The interaction of photogenerated charges with molecular oxygen was investigated on TiO2 nanocrystals by means of paramagnetic resonance (EPR) spectroscopy. Compared to photoactivation experiments in vacuum at P < 10(-6) mbar and T = 140 K, the presence of O2 enhances the concentration of persistently trapped electron and hole centres--by a factor of ten--due to the formation of adsorbed O2- species. The photoadsorption of oxygen was also tracked quantitatively by pressure measurements, and the number of trapped charges, hole centres and O2- was found to correspond to ten electron-hole pairs per TiO2 nanocrystal. Conversely, in experiments at P < 10(-6) mbar with one trapped electron-hole pair per particle, charge separation is not persistent and completely reversible with respect to temperature. Heating to 298 K causes the total annihilation of photogenerated and trapped charges.
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Affiliation(s)
- Thomas Berger
- Institut für Materialchemie, Technische Universität Wien, Veterinärplatz 1/GA, 1210 Vienna, Austria
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99
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Stankic S, Sterrer M, Hofmann P, Bernardi J, Diwald O, Knözinger E. Novel optical surface properties of Ca2+ -doped MgO nanocrystals. NANO LETTERS 2005; 5:1889-93. [PMID: 16218704 DOI: 10.1021/nl0511418] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/04/2023]
Abstract
Surface doping of oxide nanoparticles is important in fields ranging from heterogeneous catalysis to optoelectronics. Here, we report the solvent-free synthesis of mixed calcium-magnesium oxide nanocrystals in the size range between 5 and 40 nm. Although CaMgO mixtures are thermodynamically forbidden on a macroscopic scale, Calcium ions can be distributed homogeneously in MgO using chemical vapor deposition (CVD) as a nonequilibrium technique. Subsequent thermal activation leads to calcium ion segregation into the nanocrystal surface which, in addition to the synthesis parameters, provides efficient means for manipulating the optical surface properties of insulating oxide nanocrystals. A novel material with unexpected photonic behavior, such as enhanced photoluminescence emission which is also red-shifted with respect to those of CaO and MgO, was found.
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Affiliation(s)
- Slavica Stankic
- Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria
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100
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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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