Co-reporter:Wei Zhou
RSC Advances (2011-Present) 2017 vol. 7(Issue 33) pp:20542-20547
Publication Date(Web):2017/04/05
DOI:10.1039/C6RA28878B
Realization of efficient p-type conductivity in wide gap oxides is a challenging task partly due to the localized nature of non-bonding oxygen 2p states of which the valence band maximum consists. In this study, effects of anisotropic strain on an accepter level of Al-doped anatase TiO2 are investigated using LDA+U calculations. Strain engineering effectively increases the cation states in the valence band maximum of TiO2. It is demonstrated that a deep acceptor level induced by substitutional Al for Ti is turned into a delocalized shallow level under a tensile strain of as much as 8%. This effect is confirmed by the analysis of thermodynamic transition level which is largely shifted from 0.8 eV above to below the valence band maximum, being a shallow acceptor, as the tensile strain is increased.
Co-reporter:Junjie Wang;Hideo Hosono
Advanced Energy Materials 2016 Volume 6( Issue 1) pp:
Publication Date(Web):
DOI:10.1002/aenm.201501190
Van der Waals (vdW) heterostructures, which can be assembled by combining 2D atomic crystals in a precisely chosen sequence, enable a wide range of potential applications in optoelectronics, photovoltaics, and photocatalysis. However, the difficulty of peeling isolated atomic planes and the lattice mismatch between different materials is the main obstacle to hinder vdW materials from more practical applications. In this work, the mixed valence tin oxides, SnxOy (0.5 < x/y < 1), are proposed as a new member of vdW materials and these mixed valence tin oxides show promise to overcome the above-mentioned obstacle. Density-functional theory calculations are combined with an evolutionary algorithm to predict the crystal structures of a series of previously reported tin oxides (Sn2O3, Sn3O4, Sn4O5, and Sn5O6), unreported compositions (Sn7O8, Sn9O10, and Sn11O12), and a new β-SnO phase. These structures consist of β-SnO, Sn2O3, and Sn3O4 monolayers. Their band gaps can be engineered in the 1.56–3.25 eV range by stacking the monolayers appropriately. The band gap depends linearly on the interlayer distance, as understood from interlayer Sn2+–Sn2+ and intralayer Sn2+–O interactions. SnxOy structures exhibit high photoabsorption coefficients and suitable band-edge positions for photoexcited H2 evolution; this indicates potential for environmentally benign solar energy conversion in photovoltaic and photocatalytic applications.
Co-reporter:Mu Li, Junjie Wang, Peng Li, Kun Chang, Cuiling Li, Tao Wang, Bo Jiang, Huabin Zhang, Huimin Liu, Yusuke Yamauchi, Naoto Umezawa and Jinhua Ye
Journal of Materials Chemistry A 2016 vol. 4(Issue 13) pp:4776-4782
Publication Date(Web):26 Feb 2016
DOI:10.1039/C6TA00487C
Finding a highly efficient, selective and economic approach for electrochemical reduction of aqueous carbon dioxide is a great challenge in realizing an artificial system for a sustainable carbon cycle. Novel mesoporous palladium–copper bimetallic electrocatalysts with superior activity and high faradaic efficiencies (FEs) are reported for the first time. The mesoporous nanostructure provides a roughened surface which is abundant in active sites and promotes selective conversion of CO2 to CO. First-principles calculations exhibit that Pd atoms on the catalyst surface serve as reactive centers and highly selective CO formation is attributed to the geometric and electronic effects within the palladium–copper bimetallic alloys. The CO2 and COOH* intermediate adsorption ability and the CO desorption ability on Pd atoms are effectively enhanced in the presence of Cu. Our results provide wide ranging implications for further improving the design and preparation of CO2 reduction electrocatalysts.
Co-reporter:Y. Shiga, N. Umezawa, N. Srinivasan, S. Koyasu, E. Sakai and M. Miyauchi
Chemical Communications 2016 vol. 52(Issue 47) pp:7470-7473
Publication Date(Web):12 May 2016
DOI:10.1039/C6CC03199D
A visible-light-sensitive tin sulfide photocatalyst was designed based on a ubiquitous element strategy and density functional theory (DFT) calculations. Computational analysis suggested that tin monosulfide (SnS) would be more efficient than SnS2 as a photocathode for hydrogen production because of the low ionization potential and weak ionic character of SnS. To test this experimentally, nanoparticles of SnS were loaded onto a mesoporous electrode using a wet chemical method, and the bandgap of the synthesized SnS quantum dots was found to be tunable by adjusting the number of successive ionic layer adsorption and reaction (SILAR) cycles, which controls the magnitude of the quantum confinement effect. Efficient hydrogen production was achieved when the bandgap of SnS was wider than that of the bulk form.
Co-reporter:Wei Zhou and Naoto Umezawa
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 11) pp:7860-7865
Publication Date(Web):05 Feb 2016
DOI:10.1039/C6CP00039H
The effects of in-plane biaxial strain on the electronic structure of a photofunctional material, single-layer SnS2, were systematically investigated using hybrid density functional calculations. The bonding diagram for the band gap was firstly proposed based on the crystal orbital overlap population analysis. The conduction band-edge of single-layer SnS2 is determined by the anti-bonding interaction between Sn-5s and S-3p orbitals, while the valence band-edge comes from the anti-bonding between the neighboring S atoms. It is found that the compressive strain not only decreases the indirect band gap of single-layer SnS2, but also effectively promotes the band-edges of the conduction band to realize the overall water splitting. Besides, the dispersion of the valence band of single-layer SnS2 becomes weaker with increasing tensile strain which is beneficial for the photo-excitation through direct transitions.
Co-reporter:Dr. Naoto Umezawa; Anderson Janotti
ChemSusChem 2016 Volume 9( Issue 9) pp:1027-1031
Publication Date(Web):
DOI:10.1002/cssc.201600040
Abstract
Band-gap engineering of oxide materials is of great interest for optoelectronics, photovoltaics, and photocatalysis applications. In this study, electronic structures of perovskite oxynitrides, LaTiO2N and SrNbO2N, and solid solutions, (SrTiO3)1−x(LaTiO2N)x and (SrTiO3)1−x(SrNbO2N)x, are investigated using hybrid density functional calculations. Band gaps of LaTiO2N and SrNbO2N are much smaller than that of SrTiO3 owing to the formation of a N 2p band, which is higher in energy than the O 2p band. The valence- and conduction-band offsets of SrTiO3/LaTiO2N and SrTiO3/SrNbO2N are computed, and the adequacy for H2 evolution is analyzed by comparing the positions of the band edges with respect to the standard hydrogen electrode (SHE). The band gap of (SrTiO3)1−x(LaTiO2N)x and (SrTiO3)1−x(SrNbO2N)x solid solutions are also discussed.
Co-reporter:Hungru Chen
The Journal of Physical Chemistry C 2016 Volume 120(Issue 10) pp:5549-5556
Publication Date(Web):February 16, 2016
DOI:10.1021/acs.jpcc.5b12681
Metal/TiO2 interfaces have been extensively studied because of their importance in electronic devices, electrochemical cells, and photocatalysis. In this article, we present our studies on electronic structures for anatase TiO2(001)/fcc-metal(001) (metal = Pt, Pd, or Au) interfaces using first-principles calculations. It is demonstrated that the Schottky barrier height depends on the metal work function and significantly decreases at an interface with strong adhesion between the metal and TiO2. The sizable reduction of the barrier height is a consequence of dipole formation at the interface due to electron transfer from TiO2 to the metal. The formation of dipoles at the Pt/TiO2 interface is supported by our experimental results for a core-level binding-energy shift in Pt clusters loaded on the surface of TiO2. Differences in the bonding and antibonding characters of metal–O bonds for the three metals are discussed based on the projected densities of states given by our density-functional theory calculations.
Co-reporter:Naoto Umezawa
The Journal of Physical Chemistry C 2016 Volume 120(Issue 17) pp:9160-9164
Publication Date(Web):April 18, 2016
DOI:10.1021/acs.jpcc.5b11625
Photoreduction of CO2 for fuel production is considered to be an ultimate solution to today’s energy crisis. Platinum (Pt) particles are known to promote photocatalysis reactions when loaded on the surface of titanium dioxide (TiO2). In this study, we investigate the initial step of the reduction process of CO2 with water, i.e., the formation of formate, HCOO–, from surface bound CO2 and H2O on rutile TiO2(110) in terms of energetics of initial and final states using density functional theory calculations. To understand the role of a Pt cocatalyst, chemisorption energies of HCOO and OH on TiO2(110) are investigated with and without a Pt cluster. It is revealed that free electrons provided by the Pt cluster dramatically decrease the chemisorption energy thanks to the electron transfer from high-lying Pt states to unoccupied valence states induced by the adsorbates, which facilitates ionization of HCOO– and OH– on the TiO2 surface near the Pt cluster. Direct adsorption of HCOO and OH on the surface of the Pt cluster is also energetically favored.
Co-reporter:Peng Li, Naoto Umezawa, Hideki Abe and Jinhua Ye
Journal of Materials Chemistry A 2015 vol. 3(Issue 20) pp:10720-10723
Publication Date(Web):16 Apr 2015
DOI:10.1039/C5TA01416F
Two vanadates, Ag2Sr(VO3)4 and Sr(VO3)2, have been studied as visible-light-driven water oxidation photocatalysts with the help of density-functional theory calculations. Our computational results for the density of states and partial charge densities implied that Ag2Sr(VO3)4 and Sr(VO3)2 possess desirable electronic structures for the water oxidation reaction, i.e., the valence band (VB) maximum of Ag2Sr(VO3)4 consists of multiple orbitals of Ag d and O p, while Sr(VO3)2 has a broad VB associated with oxygen non-bonding states. We have experimentally demonstrated that these vanadates efficiently oxidize water to O2 under irradiation of visible light in the presence of the sacrificial agent.
Co-reporter:Wenqiang Dang, Hungru Chen, Naoto Umezawa and Junying Zhang
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 27) pp:17980-17988
Publication Date(Web):04 Jun 2015
DOI:10.1039/C5CP02110C
Sensitizing wide band gap photo-functional materials under visible-light irradiation is an important task for efficient solar energy conversion. Although nitrogen doping into anatase TiO2 has been extensively studied for this purpose, it is hard to increase the nitrogen content in anatase TiO2 because of the aliovalent nitrogen substituted for oxygen, leading to the formation of secondary phases or defects that hamper the migration of photoexcited charge carriers. In this paper, electronic structures of (TiO2)1−x(TaON)x (0 ≤ x ≤ 1) solid solutions, in which the stoichiometry is satisfied with the co-substitution of Ti for Ta along with O for N, are investigated within the anatase crystal structure using first-principles calculations. Our computational results show that the solid solutions have substantially narrower band gaps than TiO2, without introducing any localized energy states in the forbidden gap. In addition, in comparison with the pristine TiO2, the solid solution has a direct band gap when the content of TaON exceeds 0.25, which is advantageous to light absorption. The valence band maximum (VBM) of the solid solutions, which is mainly composed of N 2p states hybridized with O 2p, Ti 3d or Ta 5d orbitals, is higher in energy than that of pristine anatase TiO2 consisting of non-bonding O 2p states. On the other hand, incorporating TaON into TiO2 causes the formation of d–d bonding states through π interactions and substantially lowers the conduction band minimum (CBM) because of the shortened distance between some metal atoms. As a result, the anatase (TiO2)1−x(TaON)x is expected to become a promising visible-light absorber. In addition, some atomic configurations are found to possess exceptionally narrow band gaps.
Co-reporter:Wei Zhou and Naoto Umezawa
Physical Chemistry Chemical Physics 2015 vol. 17(Issue 27) pp:17816-17820
Publication Date(Web):04 Jun 2015
DOI:10.1039/C5CP02255J
The effects of interlayer lone-pair interactions on the electronic structure of SnO are explored using density-functional theory. Our comprehensive study reveals that the band gap of SnO opens with the increase in the interlayer Sn–Sn distance. The effect is rationalized by the character of band edges which consist of bonding and anti-bonding states from interlayer lone pair interactions. The band edges for several nanosheets and strained double-layer SnO are estimated. We conclude that the double-layer SnO is a promising material for visible-light driven photocatalysts for hydrogen evolution.
Co-reporter:Maidhily Manikandan, Toyokazu Tanabe, Peng Li, Shigenori Ueda, Gubbala V. Ramesh, Rajesh Kodiyath, Junjie Wang, Toru Hara, Arivuoli Dakshanamoorthy, Shinsuke Ishihara, Katsuhiko Ariga, Jinhua Ye, Naoto Umezawa, and Hideki Abe
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 6) pp:3790
Publication Date(Web):March 10, 2014
DOI:10.1021/am500157u
A mixed-valence tin oxide, (Sn2+)2(Sn4+)O4, was synthesized via a hydrothermal route. The Sn3O4 material consisted of highly crystalline {110} flexes. The Sn3O4 material, when pure platinum (Pt) was used as a co-catalyst, significantly catalyzed water-splitting in aqueous solution under illumination of visible light (λ > 400 nm), whereas neither Sn2+O nor Sn4+O2 was active toward the reaction. Theoretical calculations have demonstrated that the co-existence of Sn2+ and Sn4+ in Sn3O4 leads to a desirable band structure for photocatalytic hydrogen evolution from water solution. Sn3O4 has great potential as an abundant, cheap, and environmentally benign solar-energy conversion catalyst.Keywords: mixed valence; photocatalyst; tin oxide; visible light; water splitting;
Co-reporter:Adisak Boonchun, Naoto Umezawa, Takahisa Ohno, Shuxin Ouyang and Jinhua Ye
Journal of Materials Chemistry A 2013 vol. 1(Issue 22) pp:6664-6669
Publication Date(Web):01 May 2013
DOI:10.1039/C3TA10249A
The mechanism of photocatalytic reactions that occur at platinum co-catalysts loaded on TiO2 is not well understood at the atomic scale. The photoexcited electrons that are generated in TiO2 should play an important role in the evolution of hydrogen by water splitting on a platinum surface. Our density-functional calculations reveal that electrons introduced in the conduction band of TiO2 contribute to an upward shift of the Fermi level of platinum via accumulation at the Pt/TiO2 interface, which results in weakened adsorption of H2 molecules on the Pt surface. We study how the electronic structure of the Pt/TiO2 interface is modified by photoexcited electrons which are represented by extra electrons introduced into the conduction band by substituting fluorine for oxygen. One important consequence of doping is that bonding–antibonding splitting associated with the hybridization of H2-sigma and Pt-d states is narrowed, which weakens the H2–Pt binding and enhances the evolution of H2.
Co-reporter:Pakpoom Reunchan, Shuxin Ouyang, Naoto Umezawa, Hua Xu, Yuanjian Zhang and Jinhua Ye
Journal of Materials Chemistry A 2013 vol. 1(Issue 13) pp:4221-4227
Publication Date(Web):21 Dec 2012
DOI:10.1039/C2TA00450J
SrTiO3 is a promising photocatalyst for the production of hydrogen from water splitting under solar light. Cr doping is an effective treatment for adjusting its absorption edge to the visible-light range, although the performance of Cr-doped SrTiO3 is strongly affected by the oxidation number of the Cr ions. In this study, we theoretically predict that elevating the Fermi level, i.e., n-type carrier doping in SrTiO3, can stabilize the desirable oxidation number of chromium (Cr3+), contributing to a higher activity for H2 evolution. Our computational results, based on hybrid density-functional calculations, reveal that such an n-type condition is realized by substituting group-V metals (Ta, Sb, and Nb), group-III metals (La and Y), and fluorine atoms for the Ti, Sr, and O sites in SrTiO3, respectively. From our systematic study of the capability of each dopant, we conclude that La is the most effective donor for stabilizing Cr3+. This prediction is successfully evidenced by experiments showing that the La and Cr codoped SrTiO3 dramatically increases the amount of H2 gas evolved from water under visible-light irradiation, which demonstrates that our guiding principle based on Fermi level tuning by the codoping scheme is valid for the design of advanced photocatalysts.
Co-reporter:Naoto Umezawa and Jinhua Ye
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 17) pp:5924-5934
Publication Date(Web):18 Jan 2012
DOI:10.1039/C2CP24010F
Deep impurity states associated with a substitutional nitrogen at an oxygen site (NO) are believed to be the source of the visible-light absorption of nitrogen-doped titanium dioxide (TiO2). Our comprehensive study using density functional theory (DFT) plus onsite Coulomb interaction (U) reveals that a titanium atom at an interstitial site (Tii) is highly mobile and strongly binds with NO. Hybridizations of N p with Ti d states of Tii give rise to a new band at the valence band edge, eliminating the hole-trapping centers originated from the deep NO states. The suggested mechanism explains the photocatalytic oxidation reactions as well as the visible-light absorption observed on N-doped anatase TiO2.
Co-reporter:Pakpoom Reunchan, Naoto Umezawa, Shuxin Ouyang and Jinhua Ye
Physical Chemistry Chemical Physics 2012 vol. 14(Issue 6) pp:1876-1880
Publication Date(Web):01 Dec 2011
DOI:10.1039/C2CP23348G
We used hybrid density-functional calculations to clarify the effect of substituting chromium for titanium (CrTi) on photocatalytic activities of Cr-doped SrTiO3. A singly negative Cr−Ti, which is relevant to a lower oxidation state of Cr, is advantageous for the visible light absorption without forming electron trapping centers, while other charge states are inactive for the photocatalytic reaction. Stabilizing the desirable charge state (Cr−Ti) is feasible by shifting the Fermi level towards the conduction band. Our theory sheds light on the photocatalytic properties of metal-doped semiconductors.
Co-reporter:Mu Li, Junjie Wang, Peng Li, Kun Chang, Cuiling Li, Tao Wang, Bo Jiang, Huabin Zhang, Huimin Liu, Yusuke Yamauchi, Naoto Umezawa and Jinhua Ye
Journal of Materials Chemistry A 2016 - vol. 4(Issue 13) pp:NaN4782-4782
Publication Date(Web):2016/02/26
DOI:10.1039/C6TA00487C
Finding a highly efficient, selective and economic approach for electrochemical reduction of aqueous carbon dioxide is a great challenge in realizing an artificial system for a sustainable carbon cycle. Novel mesoporous palladium–copper bimetallic electrocatalysts with superior activity and high faradaic efficiencies (FEs) are reported for the first time. The mesoporous nanostructure provides a roughened surface which is abundant in active sites and promotes selective conversion of CO2 to CO. First-principles calculations exhibit that Pd atoms on the catalyst surface serve as reactive centers and highly selective CO formation is attributed to the geometric and electronic effects within the palladium–copper bimetallic alloys. The CO2 and COOH* intermediate adsorption ability and the CO desorption ability on Pd atoms are effectively enhanced in the presence of Cu. Our results provide wide ranging implications for further improving the design and preparation of CO2 reduction electrocatalysts.
Co-reporter:Adisak Boonchun, Naoto Umezawa, Takahisa Ohno, Shuxin Ouyang and Jinhua Ye
Journal of Materials Chemistry A 2013 - vol. 1(Issue 22) pp:NaN6669-6669
Publication Date(Web):2013/05/01
DOI:10.1039/C3TA10249A
The mechanism of photocatalytic reactions that occur at platinum co-catalysts loaded on TiO2 is not well understood at the atomic scale. The photoexcited electrons that are generated in TiO2 should play an important role in the evolution of hydrogen by water splitting on a platinum surface. Our density-functional calculations reveal that electrons introduced in the conduction band of TiO2 contribute to an upward shift of the Fermi level of platinum via accumulation at the Pt/TiO2 interface, which results in weakened adsorption of H2 molecules on the Pt surface. We study how the electronic structure of the Pt/TiO2 interface is modified by photoexcited electrons which are represented by extra electrons introduced into the conduction band by substituting fluorine for oxygen. One important consequence of doping is that bonding–antibonding splitting associated with the hybridization of H2-sigma and Pt-d states is narrowed, which weakens the H2–Pt binding and enhances the evolution of H2.
Co-reporter:Naoto Umezawa and Jinhua Ye
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 17) pp:NaN5934-5934
Publication Date(Web):2012/01/18
DOI:10.1039/C2CP24010F
Deep impurity states associated with a substitutional nitrogen at an oxygen site (NO) are believed to be the source of the visible-light absorption of nitrogen-doped titanium dioxide (TiO2). Our comprehensive study using density functional theory (DFT) plus onsite Coulomb interaction (U) reveals that a titanium atom at an interstitial site (Tii) is highly mobile and strongly binds with NO. Hybridizations of N p with Ti d states of Tii give rise to a new band at the valence band edge, eliminating the hole-trapping centers originated from the deep NO states. The suggested mechanism explains the photocatalytic oxidation reactions as well as the visible-light absorption observed on N-doped anatase TiO2.
Co-reporter:Pakpoom Reunchan, Naoto Umezawa, Shuxin Ouyang and Jinhua Ye
Physical Chemistry Chemical Physics 2012 - vol. 14(Issue 6) pp:NaN1880-1880
Publication Date(Web):2011/12/01
DOI:10.1039/C2CP23348G
We used hybrid density-functional calculations to clarify the effect of substituting chromium for titanium (CrTi) on photocatalytic activities of Cr-doped SrTiO3. A singly negative Cr−Ti, which is relevant to a lower oxidation state of Cr, is advantageous for the visible light absorption without forming electron trapping centers, while other charge states are inactive for the photocatalytic reaction. Stabilizing the desirable charge state (Cr−Ti) is feasible by shifting the Fermi level towards the conduction band. Our theory sheds light on the photocatalytic properties of metal-doped semiconductors.
Co-reporter:Wenqiang Dang, Hungru Chen, Naoto Umezawa and Junying Zhang
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 27) pp:NaN17988-17988
Publication Date(Web):2015/06/04
DOI:10.1039/C5CP02110C
Sensitizing wide band gap photo-functional materials under visible-light irradiation is an important task for efficient solar energy conversion. Although nitrogen doping into anatase TiO2 has been extensively studied for this purpose, it is hard to increase the nitrogen content in anatase TiO2 because of the aliovalent nitrogen substituted for oxygen, leading to the formation of secondary phases or defects that hamper the migration of photoexcited charge carriers. In this paper, electronic structures of (TiO2)1−x(TaON)x (0 ≤ x ≤ 1) solid solutions, in which the stoichiometry is satisfied with the co-substitution of Ti for Ta along with O for N, are investigated within the anatase crystal structure using first-principles calculations. Our computational results show that the solid solutions have substantially narrower band gaps than TiO2, without introducing any localized energy states in the forbidden gap. In addition, in comparison with the pristine TiO2, the solid solution has a direct band gap when the content of TaON exceeds 0.25, which is advantageous to light absorption. The valence band maximum (VBM) of the solid solutions, which is mainly composed of N 2p states hybridized with O 2p, Ti 3d or Ta 5d orbitals, is higher in energy than that of pristine anatase TiO2 consisting of non-bonding O 2p states. On the other hand, incorporating TaON into TiO2 causes the formation of d–d bonding states through π interactions and substantially lowers the conduction band minimum (CBM) because of the shortened distance between some metal atoms. As a result, the anatase (TiO2)1−x(TaON)x is expected to become a promising visible-light absorber. In addition, some atomic configurations are found to possess exceptionally narrow band gaps.
Co-reporter:Wei Zhou and Naoto Umezawa
Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 27) pp:NaN17820-17820
Publication Date(Web):2015/06/04
DOI:10.1039/C5CP02255J
The effects of interlayer lone-pair interactions on the electronic structure of SnO are explored using density-functional theory. Our comprehensive study reveals that the band gap of SnO opens with the increase in the interlayer Sn–Sn distance. The effect is rationalized by the character of band edges which consist of bonding and anti-bonding states from interlayer lone pair interactions. The band edges for several nanosheets and strained double-layer SnO are estimated. We conclude that the double-layer SnO is a promising material for visible-light driven photocatalysts for hydrogen evolution.
Co-reporter:Wei Zhou and Naoto Umezawa
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 11) pp:NaN7865-7865
Publication Date(Web):2016/02/05
DOI:10.1039/C6CP00039H
The effects of in-plane biaxial strain on the electronic structure of a photofunctional material, single-layer SnS2, were systematically investigated using hybrid density functional calculations. The bonding diagram for the band gap was firstly proposed based on the crystal orbital overlap population analysis. The conduction band-edge of single-layer SnS2 is determined by the anti-bonding interaction between Sn-5s and S-3p orbitals, while the valence band-edge comes from the anti-bonding between the neighboring S atoms. It is found that the compressive strain not only decreases the indirect band gap of single-layer SnS2, but also effectively promotes the band-edges of the conduction band to realize the overall water splitting. Besides, the dispersion of the valence band of single-layer SnS2 becomes weaker with increasing tensile strain which is beneficial for the photo-excitation through direct transitions.
Co-reporter:Pakpoom Reunchan, Shuxin Ouyang, Naoto Umezawa, Hua Xu, Yuanjian Zhang and Jinhua Ye
Journal of Materials Chemistry A 2013 - vol. 1(Issue 13) pp:NaN4227-4227
Publication Date(Web):2012/12/21
DOI:10.1039/C2TA00450J
SrTiO3 is a promising photocatalyst for the production of hydrogen from water splitting under solar light. Cr doping is an effective treatment for adjusting its absorption edge to the visible-light range, although the performance of Cr-doped SrTiO3 is strongly affected by the oxidation number of the Cr ions. In this study, we theoretically predict that elevating the Fermi level, i.e., n-type carrier doping in SrTiO3, can stabilize the desirable oxidation number of chromium (Cr3+), contributing to a higher activity for H2 evolution. Our computational results, based on hybrid density-functional calculations, reveal that such an n-type condition is realized by substituting group-V metals (Ta, Sb, and Nb), group-III metals (La and Y), and fluorine atoms for the Ti, Sr, and O sites in SrTiO3, respectively. From our systematic study of the capability of each dopant, we conclude that La is the most effective donor for stabilizing Cr3+. This prediction is successfully evidenced by experiments showing that the La and Cr codoped SrTiO3 dramatically increases the amount of H2 gas evolved from water under visible-light irradiation, which demonstrates that our guiding principle based on Fermi level tuning by the codoping scheme is valid for the design of advanced photocatalysts.
Co-reporter:Peng Li, Naoto Umezawa, Hideki Abe and Jinhua Ye
Journal of Materials Chemistry A 2015 - vol. 3(Issue 20) pp:NaN10723-10723
Publication Date(Web):2015/04/16
DOI:10.1039/C5TA01416F
Two vanadates, Ag2Sr(VO3)4 and Sr(VO3)2, have been studied as visible-light-driven water oxidation photocatalysts with the help of density-functional theory calculations. Our computational results for the density of states and partial charge densities implied that Ag2Sr(VO3)4 and Sr(VO3)2 possess desirable electronic structures for the water oxidation reaction, i.e., the valence band (VB) maximum of Ag2Sr(VO3)4 consists of multiple orbitals of Ag d and O p, while Sr(VO3)2 has a broad VB associated with oxygen non-bonding states. We have experimentally demonstrated that these vanadates efficiently oxidize water to O2 under irradiation of visible light in the presence of the sacrificial agent.
Co-reporter:Y. Shiga, N. Umezawa, N. Srinivasan, S. Koyasu, E. Sakai and M. Miyauchi
Chemical Communications 2016 - vol. 52(Issue 47) pp:NaN7473-7473
Publication Date(Web):2016/05/12
DOI:10.1039/C6CC03199D
A visible-light-sensitive tin sulfide photocatalyst was designed based on a ubiquitous element strategy and density functional theory (DFT) calculations. Computational analysis suggested that tin monosulfide (SnS) would be more efficient than SnS2 as a photocathode for hydrogen production because of the low ionization potential and weak ionic character of SnS. To test this experimentally, nanoparticles of SnS were loaded onto a mesoporous electrode using a wet chemical method, and the bandgap of the synthesized SnS quantum dots was found to be tunable by adjusting the number of successive ionic layer adsorption and reaction (SILAR) cycles, which controls the magnitude of the quantum confinement effect. Efficient hydrogen production was achieved when the bandgap of SnS was wider than that of the bulk form.
Co-reporter:Adisak Boonchun, Pakpoom Reunchan and Naoto Umezawa
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 43) pp:NaN30046-30046
Publication Date(Web):2016/10/10
DOI:10.1039/C6CP05798E
The energetics and electronic structures of native defects in anatase TiO2 are comprehensively studied using hybrid density functional calculations. We demonstrate that oxygen vacancies (VO) and titanium interstitials (Tii) act as shallow donors, and can form at substantial concentrations, giving rise to free electrons with carrier densities from 1011 to 1019 cm−3 under oxygen-rich and oxygen-poor conditions, respectively. The titanium vacancies (VTi), identified as deep acceptors and induced hole carriers, are incapable of fully compensating for the free electrons originating from the donor-type defects at any oxygen chemical potential. Even under extreme oxygen-rich conditions, the Fermi level, which is determined from the charge neutrality condition among charge defects, electron and hole carriers, is located 2.34 eV above the valence band maximum, indicating that p-type conductivity can never be realized under any growth conditions without external doping. This is consistent with common observations of intrinsic n-type conductivity of TiO2. At a typical annealing temperature and under a typical oxygen partial pressure, the carrier concentration is found to be approximately 5 × 1013 cm−3.
