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Publications (10 of 12) Show all publications
Wang, B., Århammar, C., Jiang, X., Arauji, C. M. & Ahuja, R. (2014). A Comparison Between Hybrid Functional, GW Approach and the Bethe Salpether Equation: Optical Properties of High Pressure Phases of TiO2. Science of Advanced Materials, 6(6), 1170-1178
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, p. 1170-1178Article 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.

Keywords
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
Wang, B. (2014). Electronic Structure and Optical Properties of Solar Energy Materials. (Doctoral dissertation). Stockholm: KTH Royal Institute of Technology
Open this publication in new window or tab >>Electronic Structure and Optical Properties of Solar Energy Materials
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. p. viii, 82
Keywords
GW, Bethe-Salpeter equation, Kinetic Monte Carlo, Density Functional Theory
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-145625 (URN)978-91-7595-190-4 (ISBN)
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
Wang, B. & Skorodumova, N. (2014). Structure and optical properties of (CdSxSe1-x) 42 nanoclusters. Physical Chemistry, Chemical Physics - PCCP, 16(27), 13956-13963
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, p. 13956-13963Article 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.

Keywords
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
Li, X., Wang, B., Zhang, T.-Y. & Su, Y. (2014). Water Adsorption and Dissociation on BaTiO3 Single-Crystal Surfaces. The Journal of Physical Chemistry C, 118(29), 15910-15918
Open this publication in new window or tab >>Water Adsorption and Dissociation on BaTiO3 Single-Crystal Surfaces
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 29, p. 15910-15918Article in journal (Refereed) Published
Abstract [en]

Experimental and theoretical studies of water-molecule adsorption on BaTiO3 single-crystal surfaces are presented in this paper. The Fourier transform infrared spectrum shows that there are three types of energy-nonequivalent active modes for water-molecule adsorption on the in-plane-polarized BaTiO3(100) surface. The X-ray photoelectron spectroscopic results illustrate hydroxyl group on the surface, thereby indicating that the adsorbed water molecules are dissociated. The first-principles calculations of the 1/4-, 1/2-, and 1-monolayer water coverage demonstrate that H bonds are formed between the hydrogen of water and the surface oxygen of BaTiO3 and between the hydrogen of hydroxyl and the surface oxygen of BaTiO3, and the difference in the water adsorption behavior on the BaO- and TiO2-terminated surfaces. The calculation results are in good agreement with the experimental observations.

Keywords
Barium-Titanate, Humidity, XPS
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-149503 (URN)10.1021/jp5051386 (DOI)000339540700039 ()2-s2.0-84904994747 (Scopus ID)
Note

QC 20140822

Available from: 2014-08-22 Created: 2014-08-22 Last updated: 2017-12-05Bibliographically approved
Wang, B., Kanhere, P. D., Chen, Z., Nisar, J., Pathak, B. & Ahuja, R. (2013). Anion-Doped NaTaO3 for Visible Light Photocatalysis. The Journal of Physical Chemistry C, 117(44), 22518-22524
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, p. 22518-22524Article 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.

Keywords
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
Nisar, J., Wang, B., Araújo, C. M., da Silva, A. F., Kang, T. W. & Ahuja, R. (2012). Band gap engineering by anion doping in the photocatalyst BiTaO4: First principle calculations. International journal of hydrogen energy, 37(4), 3014-3018
Open this publication in new window or tab >>Band gap engineering by anion doping in the photocatalyst BiTaO4: First principle calculations
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2012 (English)In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 37, no 4, p. 3014-3018Article in journal (Refereed) Published
Abstract [en]

We have shown the effect of mono and co-doping of non-metallic anion atoms on the electronic structure in BiTaO4 using the first-principles method. It can improve the photocatalytic efficiency for hydrogen production in the presence of visible sunlight. It is found that the band gap of BiTaO4 has been reduced significantly up to 54% with different nonmetallic doping. Electronic structure analysis shows that the doping of nitrogen is able to reduce the band gap of BiTaO4 due to the impurity N 2p state at the upper edge of the valence band. In case of C or C-S doped BiTaO4, double occupied (filled) states have been observed deep inside the band gap of BiTaO4. The large reduction of band gap has been achieved, which increases the visible light absorption. These results indicate that the doping of non-metallic element in BiTaO4 is a promising candidate for the photocatalyst due to its reasonable band gap.

Keywords
Band gap engineering, Photocatalysis, Anionic doping in BiTaO4
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-93949 (URN)10.1016/j.ijhydene.2011.11.068 (DOI)000301615100004 ()2-s2.0-84856587728 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20120503Available from: 2012-05-03 Created: 2012-05-03 Last updated: 2017-12-07Bibliographically approved
Wang, B., Nisar, J., Pathak, B., Kang, T. W. & Ahuja, R. (2012). Band gap engineering in BiNbO4 for visible-light photocatalysis. Applied Physics Letters, 100(18), 182102
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, p. 182102-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.

Keywords
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
Nisar, J., Pathak, B., Wang, B., Kang, T. W. & Ahuja, R. (2012). Hole mediated coupling in Sr2Nb2O7 for visible light photocatalysis. Physical Chemistry, Chemical Physics - PCCP, 14(14), 4891-4897
Open this publication in new window or tab >>Hole mediated coupling in Sr2Nb2O7 for visible light photocatalysis
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2012 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, no 14, p. 4891-4897Article in journal (Refereed) Published
Abstract [en]

The band gap reduction and effective utilization of visible solar light are possible by introducing the anionic hole-hole mediated coupling in Sr2Nb2O7. By using the first principles calculations, we have investigated the mono-and co-anionic doping (S, N and C) in layered perovskite Sr2Nb2O7 for the visible-light photocatalysis. Our electronic structure and optical absorption study shows that the mono- (N and S) and co-anionic doped (N-N and C-S) Sr2Nb2O7 systems are promising materials for the visible light photocatalysis. The calculated binding energies show that if the hole-hole mediated coupling could be introduced, the co-doped systems would be more stable than their respective mono-doped systems. Optical absorption curves indicate that doping S, (N-N) and (C-S) in Sr2Nb2O7 can harvest a longer wavelength of the visible light spectrum as compared to the pure Sr2Nb2O7 for efficient photocatalysis.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-93862 (URN)10.1039/c2cp23912d (DOI)000301494400026 ()2-s2.0-84858690654 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20120502Available from: 2012-05-02 Created: 2012-05-02 Last updated: 2017-12-07Bibliographically approved
Wang, B., Nisar, J. & Ahuja, R. (2012). Molecular Simulation for Gas Adsorption at NiO (100) Surface. ACS Applied Materials and Interfaces, 4(10), 5691-5697
Open this publication in new window or tab >>Molecular Simulation for Gas Adsorption at NiO (100) Surface
2012 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 4, no 10, p. 5691-5697Article in journal (Refereed) Published
Abstract [en]

Density functional theory (DFT) calculations have been employed to explore the gas-sensing mechanisms of NiO (100) surface on the basis of energetic and electronic properties. We have calculated the adsorption energies of NO2, H2S, and NH3 molecules on NiO (100) surface using GGA+U method. The calculated results suggest that the interaction of NO2 molecule with NiO surface becomes stronger and contributes more extra peaks within the band gap as the coverage increases. The band gap of H2S-adsorbed systems decrease with the increase in coverage up to 0.5 ML and the band gap does not change at 1 ML because H2S molecules are repelled from the surface. In case of NH3 molecular adsorption, the adsorption energy has been increased with the increase in coverage and the band gap is directly related to the adsorption energy. Charge transfer mechanism between the gas molecule and the NiO surface has been illustrated by the Bader analysis and plotting isosurface charge distribution. It is also found that that work function of the surfaces shows different behavior with different adsorbed gases and their coverage. The work function of NO2 gas adsorption has a hill-shaped behavior, whereas H2S adsorption has a valley-shaped behavior. The work function of NH3 adsorption decreases with the increase in coverage. On the basis of our calculations, we can have a better understanding of the gas-sensing mechanism of NiO (100) surface toward NO2, H2S, and NH3 gases.

Keywords
gas sensing, NiO (100) surface, density functional theory (DFT), conductivity
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-106139 (URN)10.1021/am3016894 (DOI)000310109000084 ()2-s2.0-84867755515 (Scopus ID)
Funder
Swedish Research Council
Available from: 2012-11-30 Created: 2012-11-29 Last updated: 2017-12-07Bibliographically approved
Nisar, J., Almeida Silva, L., Gomes Almeida, C., Santos Mascarenhas, J. A., Wang, B., Moysés Araújo, C., . . . Ferreira da Silva, A. (2012). Study of electronic and optical properties of BiTaO 4 for photocatalysis. Paper presented at 16th International Semiconducting and Insulating Materials Conference (SIMC-XVI) Location: Royal Inst Technol (KTH), Stockholm, Sweden, Date: JUN 19-23, 2011. Physica Status Solidi. C, Current topics in solid state physics, 9(7), 1593-1596
Open this publication in new window or tab >>Study of electronic and optical properties of BiTaO 4 for photocatalysis
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2012 (English)In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 9, no 7, p. 1593-1596Article in journal (Refereed) Published
Abstract [en]

We present the optical absorption spectrum of BiTaO 4 using the photo acoustic spectroscopy (PAS) technique and first principles approach. Band gap have been estimated 2.65 and 2.45 eV using PAS method and DFT calculations, respectively. Position of reduction and oxidation level with respect to vacuum level are identified, which shows that BiTaO 4 can be used as photocatalyst for hydrogen production. Electronic structure is explained by plotting total density of states (TDOS).

Keywords
Hydrogen production, Photocatalysts, Water splitting
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-100492 (URN)10.1002/pssc.201100654 (DOI)000306479300019 ()2-s2.0-84864024948 (Scopus ID)
Conference
16th International Semiconducting and Insulating Materials Conference (SIMC-XVI) Location: Royal Inst Technol (KTH), Stockholm, Sweden, Date: JUN 19-23, 2011
Note

QC 20120810

Available from: 2012-08-10 Created: 2012-08-09 Last updated: 2017-12-07Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-7321-8594

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