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Comparative study of rutile and anatase SnO2 and TiO2: Band-edge structures, dielectric functions, and polaron effects
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.ORCID iD: 0000-0002-9050-5445
2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 8, 083703- p.Article in journal (Refereed) Published
Abstract [en]

SnO2 and TiO2 polymorphs (rutile and anatase) are oxides with similar crystal structures, comparable bond lengths, and electronic band-gap energies, but different optical and electronic properties. In this work, we have studied the origin of these differences from the band-edge structures and electron-phonon coupling. The band-edge structures, dielectric functions, and effective masses were calculated by means of a first-principles approach with the exchange-correlation described by a hybrid functional. The phonon frequencies were calculated using a finite displacement method with non-analytic correction, and the phonon contribution to the dielectric functions was modeled using a multi-phonon Lorentz model. The calculated band-edge structures show that the bottommost conduction bands are highly dispersive for SnO2 polymorphs but flat dispersive for TiO2 polymorphs because of the strongly localized Ti-3d states. Consequently, SnO2 polymorphs present small effective electron masses and a weak optical absorption, whereas the TiO2 polymorphs present a strong optical absorption and larger effective electron masses. Due to the strong ionic bonds, TiO2 have larger Born effective charges than that of SnO2, result in stronger polaron effect and larger average static dielectric constant epsilon(0). For example, epsilon(0) = 115 for rutile TiO2 whereas epsilon(0) = 9.5 for rutile SnO2. Moreover, it is interesting to note that the epsilon(0) in rutile TiO2 is much larger than in anatase TiO2 (epsilon(0) = 28) although they have the same chemical compositions, which related to the local structure distortion of the phases.

Place, publisher, year, edition, pages
2013. Vol. 113, no 8, 083703- p.
Keyword [en]
Born effective charge, Chemical compositions, Electron phonon couplings, Electronic band gaps, Exchange-correlations, First-principles approaches, Optical and electronic properties, Static dielectric constants
National Category
Other Physics Topics
URN: urn:nbn:se:kth:diva-121488DOI: 10.1063/1.4793273ISI: 000315667500038ScopusID: 2-s2.0-84874863000OAI: diva2:619898
Swedish Research Council

QC 20130507

Available from: 2013-05-07 Created: 2013-04-29 Last updated: 2015-04-23Bibliographically approved
In thesis
1. First principles study of oxide semiconductors for solar energy applications
Open this publication in new window or tab >>First principles study of oxide semiconductors for solar energy applications
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The objectives of this thesis are to understand the electronic structures of oxides and oxynitrides for photocatalytic water splitting, examine the Casimir interaction between oxides, and explore possible approach to bridge the Casimir force and material properties for advanced material research. The studies were performed in the framework of the density functional theory, many-body perturbation theory, i.e, the GW approximation and Bethe-Salpeter equation, as well as the Casimir-Lifshitz approach.

The thesis consists of two sets of results. In the first part (papers I-VI), the electronic structures of oxynitrides, i.e., ZnO-GaN and ZnO-InN, with different compositions and local structures have been studied. The oxynitrides reduce the band-gap energies significantly compared to the binary counterparts, enabling the oxynitrides to act as visible light active photocatalysts. Formation of cluster--like structures further reduces the band-gap and delocalizes the valence bands, benefiting higher optical absorption. Furthermore, the energy levels between oxynitride and water were aligned using a surface model adapted from semiconductor heterostructure.

In the second part (papers V-IX), the electronic structures of oxides as well as the Casimir interactions have been examined. In particular, we investigated the differences of optical and electronic properties between SnO2 and TiO2 polymorphs in terms of band-edge characters and electron-phonon coupling. In addition, we synthesized a mesoporous material possessing two types of pore structures (one is hexagonal ordered with pore diameter of 2.60 nm and the other is disordered with pore diameter of 3.85 nm). The pore framework contains four-coordinated titanium and oxygen vacancies, verified by both experimental measurements and density-functional theory calculations. Utilizing the predicted properties of the materials, we studied the Casimir interactions. A stable equilibrium of Casimir force is achieved in planar geometry containing a thin film and porous substrates. Both the force and equilibrium distance are tuned through modification of the material properties, for instance, optical properties and porosity. Furthermore, we adapted this concept to study the interactions between gas bubbles and porous SiO2 in water. A transition from repulsion to attraction is predicted, which highlights that the bubbles may interact differently at different surface regions.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. viii, 96 p.
photocatalysis; water splitting; oxynitrides; dielectric function; first-principles calculation; density functional theory; electronic structures; Casimir interaction
National Category
Materials Engineering
Research subject
Materials Science and Engineering
urn:nbn:se:kth:diva-165070 (URN)978-91-7595-451-6 (ISBN)
Public defence
2015-05-22, D3, Lindstedtsvägen 5, KTH, Stockholm, 13:00 (English)

QC 20150423

Available from: 2015-04-23 Created: 2015-04-22 Last updated: 2015-04-23Bibliographically approved

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