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Ding Z, Goldsmith ZK, Selloni A. Pathways for Electron Transfer at MgO–Water Interfaces from Ab Initio Molecular Dynamics. J Am Chem Soc 2022; 144:2002-2009. [DOI: 10.1021/jacs.1c13250] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Zhutian Ding
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Zachary K. Goldsmith
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Annabella Selloni
- Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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2
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Schwab T, Thomele D, Aicher K, Dunlop JWC, McKenna K, Diwald O. Rubbing Powders: Direct Spectroscopic Observation of Triboinduced Oxygen Radical Formation in MgO Nanocube Ensembles. THE JOURNAL OF PHYSICAL CHEMISTRY. C, NANOMATERIALS AND INTERFACES 2021; 125:22239-22248. [PMID: 34676020 PMCID: PMC8521521 DOI: 10.1021/acs.jpcc.1c05898] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 07/02/2021] [Revised: 09/14/2021] [Indexed: 06/13/2023]
Abstract
Powder compaction-induced surface chemistry in metal oxide nanocrystal ensembles is important for very diverse fields such as triboelectrics, tribocatalysts, surface abrasion, and cold sintering of ceramics. Using a range of spectroscopic techniques, we show that MgO nanocube powder compaction with uniaxial pressures that can be achieved by gentle manual rubbing or pressing (p ≥ 5 MPa) excites energetic electron-hole pairs and generates oxygen radicals at interfacial defect structures. While the identification of paramagnetic O- radicals and their adsorption complexes with O2 point to the emergence of hole centers, triboemitted electrons become scavenged by molecular oxygen to convert into adsorbed superoxide anions O2 - as measured by electron paramagnetic resonance (EPR). By means of complementary UV-photoexcitation experiments, we found that photon energies in the range between 3 and 6 eV produce essentially the same EPR spectroscopic fingerprints and optical absorption features. To provide insights into this effect, we performed density functional theory calculations to explore the energetics of charge separation involving the ionization of low-coordinated anions and surface-adsorbed O2 - radicals at points of contact. For all selected configurations, charge transfer is not spontaneous but requires an additional driving force. We propose that a plausible mechanism for oxygen radical formation is the generation of significant surface potential differences at points of contact under loading as a result of the highly inhomogeneous elastic deformations coupled with the flexoelectric effect.
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Affiliation(s)
- Thomas Schwab
- Department
of Chemistry and Physics of Materials, Paris-Lodron
University Salzburg, Jakob-Haringer-Straße 2a, A-5020 Salzburg, Austria
| | - Daniel Thomele
- Department
of Chemistry and Physics of Materials, Paris-Lodron
University Salzburg, Jakob-Haringer-Straße 2a, A-5020 Salzburg, Austria
| | - Korbinian Aicher
- Department
of Chemistry and Physics of Materials, Paris-Lodron
University Salzburg, Jakob-Haringer-Straße 2a, A-5020 Salzburg, Austria
| | - John W. C. Dunlop
- Department
of Chemistry and Physics of Materials, Paris-Lodron
University Salzburg, Jakob-Haringer-Straße 2a, A-5020 Salzburg, Austria
| | - Keith McKenna
- Department
of Physics, University of York, Heslington, YO10 5DD York, U.K.
| | - Oliver Diwald
- Department
of Chemistry and Physics of Materials, Paris-Lodron
University Salzburg, Jakob-Haringer-Straße 2a, A-5020 Salzburg, Austria
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3
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Kuklja MM, Tsyshevsky R, Zverev AS, Mitrofanov A, Ilyakova N, Nurmukhametov DR, Rashkeev SN. Achieving tunable chemical reactivity through photo-initiation of energetic materials at metal oxide surfaces. Phys Chem Chem Phys 2020; 22:25284-25296. [PMID: 33136098 DOI: 10.1039/d0cp04069j] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Known applications of high energy density materials are impressively vast. Despite this, we argue that energetic materials are still underutilized for common energy purposes due to our inability to control explosive chemical reactions releasing energy from these materials. The situation appears paradoxical as energetic materials (EM) possess massive amounts of energy and, hence, should be most appropriate for applications in many energy-intensive processes. Here, we discover how chemical decomposition reactions can be stimulated with laser excitation and therefore, highly controlled by selectively designing energetic material - metal oxide interfaces with an example of pentaerythritol tetranitrate (PETN)-MgO and trinitrotoluene (TNT)-MgO composite samples. Density functional theory and embedded cluster method calculations were combined with measurements of the optical absorption spectra and laser initiation experiments. We found that the first (1064 nm, 1.17 eV), second (532 nm, 2.33 eV), and third (355 nm, 3.49 eV) laser harmonics, to all of which pure energetic materials are transparent, can be effectively used to trigger explosive reactions in the PETN-MgO samples. We propose a consistent electronic mechanism that explains how specific sub-band optical transitions initiate decomposition chemistry. Also, this selectivity reveals a fundamental difference between materials chemistry at interfaces as we show on examples of PETN and TNT energetic materials.
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Affiliation(s)
- Maija M Kuklja
- Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA.
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4
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Rivera M, Dommett M, Crespo-Otero R. ONIOM(QM:QM′) Electrostatic Embedding Schemes for Photochemistry in Molecular Crystals. J Chem Theory Comput 2019; 15:2504-2516. [DOI: 10.1021/acs.jctc.8b01180] [Citation(s) in RCA: 17] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Affiliation(s)
- Miguel Rivera
- School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, U.K
| | - Michael Dommett
- School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, U.K
| | - Rachel Crespo-Otero
- School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, U.K
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5
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Hung CH, Chen CS, Sheu HS, Chang JR. Deactivation and Rejuvenation of Pellet MgO/SiO2 Catalysts for Transesterification of Soybean Oil with Methanol to Biodiesel: Roles of MgO Morphology Change in Catalysis. Ind Eng Chem Res 2018. [DOI: 10.1021/acs.iecr.7b02859] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Chien-Hsiu Hung
- Department
of Food Science and Biotechnology, National Chung Hsing University, Taichung 402, Taiwan
| | - Chin-Shuh Chen
- Department
of Food Science and Biotechnology, National Chung Hsing University, Taichung 402, Taiwan
| | - Hwo-Shuenn Sheu
- National Synchrotron
Radiation Research Center, Hsinchu 300, Taiwan
| | - Jen-Ray Chang
- Department
of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan
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6
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Fuks D, Komisarchik G, Kaller M, Gelbstein Y. Doping in controlling the type of conductivity in bulk and nanostructured thermoelectric materials. J SOLID STATE CHEM 2016. [DOI: 10.1016/j.jssc.2016.05.029] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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7
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8
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Huang B, Kobayashi H, Kitagawa H. Facile Synthesis of Small MgO Nanoparticle/Metal–Organic Framework Hybrid Material. CHEM LETT 2014. [DOI: 10.1246/cl.140429] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Affiliation(s)
- Bo Huang
- Division of Chemistry, Graduate School of Science, Kyoto University
| | - Hirokazu Kobayashi
- Division of Chemistry, Graduate School of Science, Kyoto University
- JST CREST
| | - Hiroshi Kitagawa
- Division of Chemistry, Graduate School of Science, Kyoto University
- JST CREST
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9
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Siedl N, Koller D, Sternig AK, Thomele D, Diwald O. Photoluminescence quenching in compressed MgO nanoparticle systems. Phys Chem Chem Phys 2014; 16:8339-45. [DOI: 10.1039/c3cp54582b] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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10
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McKenna KP. Electronic and chemical properties of a surface-terminated screw dislocation in MgO. J Am Chem Soc 2013; 135:18859-65. [PMID: 24279391 PMCID: PMC3892727 DOI: 10.1021/ja408342z] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Dislocations represent an important and ubiquitous class of topological defect found at the surfaces of metal oxide materials. They are thought to influence processes as diverse as crystal growth, corrosion, charge trapping, luminescence, molecular adsorption, and catalytic activity; however, their electronic and chemical properties remain poorly understood. Here, through a detailed first-principles investigation into the properties of a surface-terminated screw dislocation in MgO we provide atomistic insight into these issues. We show that surface dislocations can exhibit intriguing electron trapping properties which are important for understanding the chemical and electronic characteristics of oxide surfaces. The results presented in this article taken together with recent experimental reports show that surface dislocations can be equally as important as more commonly considered surface defects, such as steps, kinks, and vacancies, but are now just beginning to be understood.
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Affiliation(s)
- Keith P McKenna
- Department of Physics, University of York , Heslington, York YO10 5DD, United Kingdom
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11
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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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12
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Vingurt D, Fuks D, Landau MV, Vidruk R, Herskowitz M. Grain boundaries at the surface of consolidated MgO nanocrystals and acid–base functionality. Phys Chem Chem Phys 2013; 15:14783-96. [DOI: 10.1039/c3cp51086g] [Citation(s) in RCA: 12] [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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13
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Sousa C, Tosoni S, Illas F. Theoretical Approaches to Excited-State-Related Phenomena in Oxide Surfaces. Chem Rev 2012. [DOI: 10.1021/cr300228z] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- Carmen Sousa
- Departament de Química
Física and Institut de Química Teòrica i Computacional
(IQTCUB), Universitat de Barcelona, C/Martí
i Franquès 1, 08028 Barcelona, Spain
| | - Sergio Tosoni
- Departament de Química
Física and Institut de Química Teòrica i Computacional
(IQTCUB), Universitat de Barcelona, C/Martí
i Franquès 1, 08028 Barcelona, Spain
- Departamento de Química, Universidad de Las Palmas de Gran Canaria, Campus Universitario
de Tafira, 35017 Las Palmas de Gran Canaria, Spain
| | - Francesc Illas
- Departament de Química
Física and Institut de Química Teòrica i Computacional
(IQTCUB), Universitat de Barcelona, C/Martí
i Franquès 1, 08028 Barcelona, Spain
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14
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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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15
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Maoz BM, Tirosh E, Sadan MB, Popov I, Rosenberg Y, Markovich G. Highly defective MgO nanosheets from colloidal self-assembly. ACTA ACUST UNITED AC 2011. [DOI: 10.1039/c1jm10181a] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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16
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Affiliation(s)
- Peter V. Sushko
- Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom, WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan, and Department of Physics, St. Petersburg State University, St. Petersburg 198504, Russia
| | - Igor V. Abarenkov
- Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom, WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan, and Department of Physics, St. Petersburg State University, St. Petersburg 198504, Russia
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17
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Stoneham M. The strange magnetism of oxides and carbons. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2010; 22:074211. [PMID: 21386389 DOI: 10.1088/0953-8984/22/7/074211] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Magnetism is not usually expected in simple sp oxides like MgO or in carbons like graphite. Yet basic intrinsic defects in these systems can be magnetic in ways that seem to be shared by more complex oxides. A second puzzle comes from reports of possible room temperature ferromagnetism in simple oxides, where experiments are not always in agreement. This paper discusses what determines whether point defects like cation vacancies in oxides have magnetic or non-magnetic ground states. It also discusses the possible connections between point defect ground states and oxide ferromagnetism. The connectivity issue raises questions about possible diffuse states in nanocrystalline oxides, several possibilities being outlined. These ideas raise the further possibility that the magnetism might be written in these oxides at the nanoscale, perhaps using atomic force microscopy.
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Affiliation(s)
- Marshall Stoneham
- London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, WC1E 6BT, UK
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18
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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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19
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Alimohammadi M, Fichthorn KA. Molecular dynamics simulation of the aggregation of titanium dioxide nanocrystals: preferential alignment. NANO LETTERS 2009; 9:4198-4203. [PMID: 19719155 DOI: 10.1021/nl9024215] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
We use classical molecular-dynamics simulations to study the aggregation of various titanium dioxide (anatase) nanocrystals in vacuum. In all cases, we observe a strong tendency for the nanocrystals to aggregate with certain preferred orientations in a "hinge" mechanism. Although some of the nanocrystals possess significant dipole moments, dipole-dipole interactions do not direct aggregation, implying that higher-order multipole moments are the driving force for preferential alignment. These high-order multipole moments originate from under-coordinated O and Ti surface atoms on the edges between nanocrystal facets, which create localized regions of positive and negative charge. The observed mechanism for preferential alignment may be a driving force for oriented attachment and the growth of anisotropic structures during crystallization.
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Affiliation(s)
- Mozhgan Alimohammadi
- Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
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20
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Bromley ST, Moreira IDPR, Neyman KM, Illas F. Approaching nanoscale oxides: models and theoretical methods. Chem Soc Rev 2009; 38:2657-70. [DOI: 10.1039/b806400h] [Citation(s) in RCA: 95] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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21
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McKenna KP, Shluger AL. Electron-trapping polycrystalline materials with negative electron affinity. NATURE MATERIALS 2008; 7:859-862. [PMID: 18849977 DOI: 10.1038/nmat2289] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/21/2008] [Accepted: 09/11/2008] [Indexed: 05/26/2023]
Abstract
The trapping of electrons by grain boundaries in semiconducting and insulating materials is important for a wide range of physical problems, for example, relating to: electroceramic materials with applications as sensors, varistors and fuel cells, reliability issues for solar cell and semiconductor technologies and electromagnetic seismic phenomena in the Earth's crust. Surprisingly, considering their relevance for applications and abundance in the environment, there have been few experimental or theoretical studies of the electron trapping properties of grain boundaries in highly ionic materials such as the alkaline earth metal oxides and alkali halides. Here we demonstrate, by first-principles calculations on MgO, LiF and NaCl, a qualitatively new type of electron trapping at grain boundaries. This trapping is associated with the negative electron affinity of these materials and is unusual as the electron is confined in the empty space inside the dislocation cores.
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Affiliation(s)
- Keith P McKenna
- Department of Physics and Astronomy and The London Centre for Nanotechnology, University College London, Gower Street, London, WC1E 6BT, UK.
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22
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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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