DFT+U Study of the Surface Structure and Stability of Co<sub>3</sub>O<sub>4</sub>(110): Dependence on U

Selçuk, Sencer; Selloni, Annabella

doi:10.1021/acs.jpcc.5b02298

Cited by 126 publications

(97 citation statements)

References 39 publications

(66 reference statements)

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“…The proper symmetry of the top and bottom slab terminations was assured to minimize the dipole moment development in the supercells. We considered three differently terminated models of the (100) surface of various composition: an oxidized (oxygen rich) termination (Co 46 Table S1. A comprehensive description of these terminations is discussed in the main manuscript, whereas detailed definition of the slab models used for DFT modeling can be found in Figure S1.…”

Section: Methodsmentioning

confidence: 99%

Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into the Structure and Surface Thermodynamics of the (100) Facet

2015

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Periodic spin unrestricted DFT-PW91+U calculations together with ab initio thermodynamic modeling were used to study the structure, defects, and stability of different terminations of the (100) surface of cobalt spinel under various redox conditions imposed by different oxygen partial pressure and temperature. Three terminations containing under-stoichiometric (100)-O, stoichiometric (100)-S, and overstoichiometric (100)-R amount of cobalt ions were analyzed, and their atomic and defect structure, reconstruction, and stability were elucidated. For the most stable (100)-S and (100)-O facets, formation of cationic and anionic vacancies was examined, and a surface redox state diagram of possible spinel (100) terminations in the stoichiometry range from Co 2.75 O 4 to Co 3 O 3.75 was constructed and discussed in detail. The results revealed that the bare (100)-S surface is the most stable at temperatures and pressures of typical catalytic processes (T ∼ 200°C to ∼500°C, p O2 /p°∼ 0.001 to ∼1). In more reducing conditions (T > 600°C and p O2 /p°< 0.0001), the (100)-S facet is readily reduced by formation of oxygen vacancies, whereas in the oxidizing conditions (T < 200°C and p O2 /p°> 10), coexistence of (100)-S and (100)-O terminations was revealed. Formation of the oxygen vacancies involves reduction of the octahedral trivalent cobalt and is accompanied by migration of the divalent tetrahedral cobalt into empty, interstitial octahedral positions. It was also found that the constituent octahedral Co cation proximal to the interstitial cobalt adopts a low spin configuration in contrast to the distal one that preserves its surface high spin state. In the case of the Co depleted surfaces, the octahedral vacancies are thermodynamically disfavored with respect to the tetrahedral ones in the whole range of the examined T and p O2 values. The obtained theoretical results, supported by TPD-O 2 and TG experiments, show that the octahedral cobalt ions are directly involved in the redox processes of Co 3 O 4 .

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Section: Methodsmentioning

confidence: 99%

Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into the Structure and Surface Thermodynamics of the (100) Facet

2015

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“…The{100}a nd {111}f acets present highlyu nsaturated surface Co 2 + ions;t he {110} plane is characterizedb yt he coexistence of di-and trivalent cobalt cations,a nd it has two possible terminations, usually denoted the Aa nd Bp atterns. [79,80] The oxygen-poor( cationic)A termination exposes two tetrahedral cobalt atoms, two octahedralc obalt atoms, and four threefold-coordinated oxygen atoms;t he oxygen-rich (anionic) Btermination exposes two octahedral cobalt atoms, two threefold-coordinated oxygen atoms, and two twofold-coordinated oxygen atoms. Hence, the Atermination is stable in ac obalt-rich/oxygen-poor environment, whereas the Btruncation is stable in ac obalt poor/oxygen-rich one.…”

Section: Co 3 Omentioning

confidence: 99%

Shape Engineering of Oxide Nanoparticles for Heterogeneous Catalysis

Zhou

Shen

2016

Chemistry An Asian Journal

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The fabrication of oxide particles with tunable sizes and shapes at the nanoscale is one of the most crucial issues for the design and development of highly efficient heterogeneous catalysts. The shape of oxide nanoparticles has been demonstrated to affect their catalytic properties remarkably. Tuning the shape of oxide particles allows preferential exposure of specific reactive facets; this can maximize the number of active sites available to the reactants, which can improve the activity and also mediate the reaction route to a specific channel to achieve higher selectivity for a particular chemical reaction. In addition, the shape of the oxide particles affects their interaction with metal particles or clusters, and this involves interfacial strain and charge transfer. Metal particles or clusters dispersed on the reactive or polar facets of the oxide support often provide superior catalytic performance, primarily because of strong metal-support interactions. However, the geometric and electronic features of the metal-oxide interface may change during the course of the reaction, induced by chemisorption of reactive molecules at elevated temperatures, which should be taken into account in proposing a structure-reactivity relationship.

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“…Although the Hubbard parameters chosen are commonly accepted in the literature, they still are a subject of discussion. 38 Therefore, DFT+U calculations of Co 3 O 4 should be carried out with care: the systems under consideration might have several solutions and there is no guarantee whether the lowest energy solution corresponds to the global minimum. For such a reason, we have checked that our conclusions do not depend in a sensitive way on these Coulomb parameters.…”

Section: Methodsmentioning

confidence: 99%

Interface magnetism and electronic structure: ZnO(0001)/ Co3O4 (111)

et al. 2018

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We have studied the structural, electronic and magnetic properties of spinel Co 3 O 4 (111) surfaces and their interfaces with ZnO (0001) using density functional theory (DFT) within the Generalized Gradient Approximation with on-site Coulomb repulsion term (GGA+U). Two possible forms of spinel surface, containing Co 2+ or Co 3+ ions and terminated with either cobalt or oxygen ions were considered, as well as their interface with zinc oxide. Our calculations demonstrate that Co 3+ ions attain non-zero magnetic moments at the surface and interface, in contrast to the bulk, where they are not magnetic, leading to the ferromagnetic ordering. Since heavily Co-doped ZnO samples can contain Co 3 O 4 secondary phase, such a magnetic ordering at the interface might explain the origin of the magnetism in such diluted magnetic semiconductors (DMS). PACS numbers: 73.20.-r, 75.70.-i Acknowledgments 16References 17 I. INTRODUCTIONMagnetic semiconductors (MS) and diluted magnetic semiconductors (DMS) exhibit both ferromagnetic and semiconducting properties. Therefore, they are promising materials for spintronics, which utilizes for information processing not only the electron charge but also its spin. Historically, the first DMS with a high Curie temperature up to about 200 K was GaAs doped with Mn ions. 1,2 In that compound, the ferromagnetism is promoted by hole carriers, which align along the local Mn magnetic moments and called carrier-induced ferromagnetism or Zener p − d exchange. It is crucial for this mechanism that Mn at the Ga site becomes Mn 2+ instead of Ga 3+ , thus providing at the same time a local spin and a hole charge carrier. Extension of the mechanism, proposed in a very influential paper 3 of Dietl and co-workers, allows a prediction that the above room-temperature ferromagnetism in ZnO:Co and GaN:Mn is due to the same carrier-induced mechanism. This would be responsible for the ferromagnetism with a sufficiently high number of hole charge carriers.First experiments after that prediction 4 seemed to confirm the mechanism proposed and has also been supported by ab-initio calculations. 5 However, it soon turned out that the Co impurity is in fact isovalent to the Zn ion 6 and provides no charge carriers at all, while the situation in GaN:Mn is similar. 7We are going to concentrate here on ZnO:Co, where the experimental reports demon-2 strate that the above room-temperature ferromagnetism in ZnO:Co persist. Even though its origin is still not clarified, there are clear indications in more recent experiments that the magnetism in the ZnO:Co system is attributed to the formation of the Co 3 O 4 phase in ZnO. 8-12 Therefore, we will focus here on the role of the Co 3 O 4 phase, although several attempts to explain the mechanism of the ferromagnetism in realistic ZnO:Co systems exist including, for instance, spinodal decomposition 13 or Lieb-Mattis ferrimagnetism, 14 to cite just two ideas. The typical doping level of Co in ZnO can be relatively high (in the range between 10% and 30%). This leads to the secondar...

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DFT+U Study of the Surface Structure and Stability of Co₃O₄(110): Dependence on U

Cited by 126 publications

References 39 publications

Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into the Structure and Surface Thermodynamics of the (100) Facet

Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into the Structure and Surface Thermodynamics of the (100) Facet

Shape Engineering of Oxide Nanoparticles for Heterogeneous Catalysis

Interface magnetism and electronic structure: ZnO(0001)/ Co3O4 (111)

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DFT+U Study of the Surface Structure and Stability of Co3O4(110): Dependence on U

Cited by 126 publications

References 39 publications

Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into the Structure and Surface Thermodynamics of the (100) Facet

Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into the Structure and Surface Thermodynamics of the (100) Facet

Shape Engineering of Oxide Nanoparticles for Heterogeneous Catalysis

Interface magnetism and electronic structure: ZnO(0001)/ Co3O4 (111)

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Product

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DFT+U Study of the Surface Structure and Stability of Co₃O₄(110): Dependence on U