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Electronic Structure and Optical Properties of Solar Energy Materials
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Multiscale Materials Modelling.ORCID iD: 0000-0001-7321-8594
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, we have studied the electronic and optical properties of solar energy m-terials. The studies are performed in the framework of density functional theory (DFT), GW, Bethe-Salpeter equation (BSE) approaches and Kinetic Monte Carlo (KMC). We present four sets of results. In the first part, we report our results on the band gap engineering issues for BiNbO4and NaTaO3, both of which are good photocatalysts. The band gap tuning is required for these materials in order to achieve the maximum solar to hydrogen conversion efficiency. The most common method for the band gap reduction is an introduction of foreign elements. The mono-doping in the system generates electrons or holes states near band edges, which reduce the efficiency of photocatalytic process. Co-doping with anion and cation or anion and anion can provide a clean band gap. We have shown that further band gap reduction can be achieved by double-hole mediated coupling between two anionic dopants. In the second part, the structure and optical properties of (CdSxSe1x)42nanoclusters have been studied. Within this study, the structures of the (CdS)42, (CdSe)42, Cd42Se32S10, Cd42Se22S20, and Cd42Se10S32 clusters have been determined using the simulated annealing method. Factors influencing the band gap value have been analyzed. We show that the gap is most significantly reduced when strongly under coordinated atoms are present on the surface of the nanoclusters. In addition, the band gap depends on the S concentration as well as on the distribution of the S and Se atoms in the clusters. We present the optical absorption spectra calculated with BSE and random phase approximation (RPA) methods based on the GW corrected quasiparticle energies. In the third part, we have employed the state-of-art computational methods to investigate the electronic structure and optical properties of TiO2high pressure polymorphs. GW and BSE methods have been used in these calculations. Our calculations suggest that the band gap of fluorite and pyrite phases have optimal values for the photocatalytic process of decomposing water in the visible light range. In the fourth part we have built a kinetic model of the first water monolayer growth on TiO2(110) using the kinetic Monte Carlo (KMC) method based on parameters describing water diffusion and dissociation obtained from first principle calculations. Our simulations reproduce the experimental trends and rationalize these observations in terms of a competition between different elementary processes. At high temperatures our simulation shows that the structure is well equilibrated, while at lower temperatures adsorbed water molecules are trapped in hydrogen-bonded chains around pairs of hydroxyl groups, causing the observed higher number of molecularly adsorbed species at lower temperature.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , viii, 82 p.
Keyword [en]
GW, Bethe-Salpeter equation, Kinetic Monte Carlo, Density Functional Theory
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-145625ISBN: 978-91-7595-190-4 (print)OAI: oai:DiVA.org:kth-145625DiVA: diva2:719212
Public defence
2014-06-13, FB53, AlbaNova, Roslagstullsbacken, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140603

Available from: 2014-06-03 Created: 2014-05-23 Last updated: 2014-06-03Bibliographically approved
List of papers
1. Band gap engineering in BiNbO4 for visible-light photocatalysis
Open this publication in new window or tab >>Band gap engineering in BiNbO4 for visible-light photocatalysis
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2012 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 100, no 18, 182102- p.Article in journal (Refereed) Published
Abstract [en]

We have investigated the electronic structure of anionic mono- (S, N, and C) and co-doping (N-N, C-N, S-C, and S-N) on BiNbO4 for the visible-light photocatalysis. The maximum band gap reduction of pure BiNbO4 is possible with the (C-S) co-doping and minimum with N mono-doping. The calculated binding energies show that the co-doped systems are more stable than their mono-doped counterparts. Our optical absorption curves indicate that the mono- (C) and co-anionic doped (N-N and C-S) BiNbO4 systems are promising materials for visible light photocatalysis.

Keyword
Band gap engineering, Band gap reduction, Co-doped, Co-doping, Visible-light photocatalysis
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-100947 (URN)10.1063/1.4709488 (DOI)000303598600026 ()2-s2.0-84862524607 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20120822Available from: 2012-08-22 Created: 2012-08-22 Last updated: 2017-12-07Bibliographically approved
2. Mo- and N-doped BiNbO(4) for photocatalysis applications
Open this publication in new window or tab >>Mo- and N-doped BiNbO(4) for photocatalysis applications
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2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 99, no 5, 051909- p.Article in journal (Refereed) Published
Abstract [en]

The electronic structure of pure BiNbO(4) has been calculated and their electronic band positions have been aligned with respect to the water oxidation/reduction potential. The effect of cationic (Mo), anionic (N), and co-doping (Mo-N) on BiNbO(4) has been studied and discussed with respect to the standard redox potential levels. Our results show that co-doping of Mo and N in BiNbO(4) reduces the band gap up to 31.8%, thus making it a potential candidate for the photocatalysis of water for hydrogen production. The relative stability between the mono-and co-doped BiNbO(4) materials show that co-doped material is more stable and feasible in comparison to the mono-doped materials.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-40668 (URN)10.1063/1.3622659 (DOI)000293617300023 ()2-s2.0-80051584758 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20110926Available from: 2011-09-26 Created: 2011-09-20 Last updated: 2017-12-08Bibliographically approved
3. Anion-Doped NaTaO3 for Visible Light Photocatalysis
Open this publication in new window or tab >>Anion-Doped NaTaO3 for Visible Light Photocatalysis
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2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 44, 22518-22524 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, we have employed DFT and HSE06 methods to study the doping effects on the NaTaO3 photocatalyst. N, S, C, and P monodoping and N-N, C-S, P-P, and N-P codoping have been studied. The redopants' formation energies have been calculated, and we find S monodoping is energetically more favorable than any other elemental doping. The mechanism of anion doping on the electronic properties of NaTaO3 is discussed. We find the band gap reduces significantly if we dope with anionic elements whose p orbital energy is higher than the O 2p orbitals. N and S can shift the valence band edge upward without losing the ability to split water into H-2 and O-2. Double-hole-mediated codoping can decrease the band gap significantly. On the basis of our calculations, codoping with N-N, C-S, and P-P could absorb visible light. However, they can only decompose water into H-2 when the valence band edge is above the water oxidation level.

Keyword
Initio Molecular-Dynamics, Augmented-Wave Method, Hydrogen Evolution, Water, H-2, Lanthanum, Exchange, Solids, O-2
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-137471 (URN)10.1021/jp407025r (DOI)000326845400007 ()2-s2.0-84889841428 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20131217

Available from: 2013-12-17 Created: 2013-12-13 Last updated: 2017-12-06Bibliographically approved
4. Structure and optical properties of (CdSxSe1-x) 42 nanoclusters
Open this publication in new window or tab >>Structure and optical properties of (CdSxSe1-x) 42 nanoclusters
2014 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 27, 13956-13963 p.Article in journal (Refereed) Published
Abstract [en]

The structures of the (CdS)(42), (CdSe)(42), Cd42Se32S10, Cd42Se22S20, and Cd42Se10S32 clusters have been determined using the simulated annealing method. Factors influencing the band gap value have been analysed. We show that the gap is most significantly reduced when strongly under coordinated atoms are present on the surface of the nanoclusters. In addition, the band gap depends on the S concentration as well as on the distribution of the S and Se atoms in the clusters. We present the optical absorption spectra calculated with BSE and RPA methods based on the GW corrected quasiparticle energies. Strong electron-hole coupling is observed for all the clusters, shifting the calculated RPA onset of optical absorption to lower energies. The absorption edge is shifted to higher photon energies as S concentration increases. The calculated energy separation of the first bright exciton and first dark exciton increases with S concentration.

Keyword
Light-Emitting-Diodes, Cdse Quantum Dots, Initio Molecular-Dynamics, Augmented-Wave Method, Binding-Energy, Solar-Cells, Band-Gaps, In-Vivo, Semiconductor, Nanocrystals
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-145887 (URN)10.1039/c4cp01008f (DOI)000338116700047 ()2-s2.0-84902668581 (Scopus ID)
Note

QC 20140808. Updated from manuscript to article in journal.

Available from: 2014-06-03 Created: 2014-06-03 Last updated: 2017-12-05Bibliographically approved
5. A Comparison Between Hybrid Functional, GW Approach and the Bethe Salpether Equation: Optical Properties of High Pressure Phases of TiO2
Open this publication in new window or tab >>A Comparison Between Hybrid Functional, GW Approach and the Bethe Salpether Equation: Optical Properties of High Pressure Phases of TiO2
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2014 (English)In: Science of Advanced Materials, ISSN 1947-2935, E-ISSN 1947-2943, Vol. 6, no 6, 1170-1178 p.Article in journal (Refereed) Published
Abstract [en]

Titanium dioxide has good corrosion resistance in aqueous solutions and is a good candidate for photoelectrodes. The limitation of the anatase phase of TiO2 is its large band gap. High pressure phases of TiO2 like fluorite, pyrite and cotunnite may possess a more suitable band gap than the well known atmospheric phases. In this paper, the electronic properties of high pressure phases of TuO(2), fluorite, pyrite and cotunnite, have been investigated by hybrid functional and GW methods. Our calculations suggest that the band gap of fluorite and pyrite phases have optimal band gaps to absorb visible light for photocatalysis to decompose water. The imaginary part of the dielectric function has also been calculated for fluorite, pyrite, cotunnite and anatase phases using the Bethe-Salpether (BSE) method. The dielectric function calculated by BSE for the anatase phase agrees well with experiment and with previous studies, using the same level of theory. Therefore we expect that we are also able to predict the optical properties of the high pressure phases of TiO2 by the BSE method. The spatial properties and the localization character of excitons in these high pressure phases were investigated and discussed in terms of photoconversion efficiency.

Keyword
High Pressure, Titanium Dioxide, Hybrid Functional, GW, BSE
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-145886 (URN)10.1166/sam.2014.1883 (DOI)000337268100012 ()2-s2.0-84904627960 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20140603

Available from: 2014-06-03 Created: 2014-06-03 Last updated: 2017-12-05Bibliographically approved
6. Kinetics of water adsorption on TiO2(110)
Open this publication in new window or tab >>Kinetics of water adsorption on TiO2(110)
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(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-145889 (URN)
Note

QS 2014

Available from: 2014-06-03 Created: 2014-06-03 Last updated: 2014-06-03Bibliographically approved

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