Theoretical Insight into Charge-Recombination Center in Ta3N5

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Theoretical Insight Into Charge-Recombination Center in Ta3N5 Photocatalyst: Interstitial Hydrogen Xin Wang, Huiting Huang, Guozheng Fan, Zhaosheng Li, and Zhigang Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09738 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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The Journal of Physical Chemistry

Theoretical Insight Into Charge-Recombination Center in Ta3N5 Photocatalyst: Interstitial Hydrogen Xin Wang,† Huiting Huang,†Guozheng Fan,†Zhaosheng Li,*†Zhigang Zou† †Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructures, and College of Engineering and Applied Sciences, Nanjing University, 22 Hankou Road, Nanjing 210093, People’s Republic of China AUTHOR INFORMATION Corresponding Author * Zhaosheng Li E-mail: [email protected]. Tel: +86-25-83686304(ext. 800)

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ABSTRACT: Ideal Ta3N5 is a promising candidate photocatalyst for solar water splitting. In a common synthetic route, both oxygen and hydrogen impurities are inevitable formed during the nitridation of TaOx precursor by ammonia. The introduced hydrogen impurities would bond with oxygen in form of hydroxyl-groups, resulting in additional bands bracketing band edges. This configuration adds on Ta3N5 electron-hole recombination centers, leading to a high onset potential. Hydrogen impurities would also introduce hydrogen bonds which aggravate charge recombination by additional charge transport paths from anions to hydroxyl recombination centers. Besides, hydride ions of hydroxyl-groups would be activated into protons at high bias, and may relay hole transport in Ta3N5, endowing the material high saturated photocurrent. In a word, hydrogen impurity would aggravate the onset potential of Ta3N5 in the way of high electron-hole recombination. More broadly, hydrogen impurity may be common in (oxy)nitrides and other covalent materials; it may add on photocatalysts high onset potential via electron localizations, and might introduce high charge recombination for covalent semiconductors.


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INTRODUCTION Photoelectrochemical water splitting is a promising strategy for energy crisis.1-4 Photocatalyst with suitable band structure is prerequisite for the practical use of solar water splitting.5-6 One of the most promising candidate is Ta3N5, which has a 2.1 eV band gap and straddles the water redox levels by Ta-5d/N-2p orbitals with up to 15% solar-to-hydrogen conversion efficiency.7-14 Ta3N5 exhibits a high photoelectrochemical performance, for example, saturated photocurrent of Ta3N5 would reach up to 12.3 mA•cm-2 under AM 1.5 G irradiation (100 mW•cm-2), approaching its theoretical predication (~12.9 mA•cm-2).10-12 However, although theoretical works indicate the band edges of Ta3N5 are suitable for overall water splitting, the onset potentials for water oxidization in experimental works are much higher than the theoretical predictions (~0.6-0.9 V vs. 0 V).7-15 Several works ascribe these mismatches to residual oxygen impurities, which are introduced by incomplete nitridation of TaOx precursors by ammonia.14-21 The oxygen impurities would induce band edge shifts, and result in high flatband potentials.15-17 Because onset potential is theoretically equal to flatband potential, these oxygen impurity induced flatband rises indicate high onset potentials.22 Defects above would well explain the high flatband potentials in (oxy)nitrides, except for the mismatch between flatband potential and onset potential. Difference between onset and flatband potential implies dynamic potential-consumptions in Ta3N5, which could be attributed to electron localization. The localized electrons are supposed to be on impurity bands around valance band maximum, and they would consume photo-induced holes at low bias and be delocalized at high bias in photocatalysis. Consequently, the electron localization would induce high charge recombination in the low bias, and would be the key for the high onset potential for nitride and oxynitride photocatalysts.


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Although oxygen impurity would result in band edge shifts and high flatband potentials, it would localize electrons in deep valance bands, and might not be the source for the localized electrons.15-17 Imaging that the photo-induced holes are floating around the shallow valance bands, the localized electrons on oxygen impurities might not take part in electron-hole recombination. Specially, holes would stay on the shallow valance bands, but the localized electrons stay in the deep of valance bands. The holes would not overcome Coulomb force, and skip over covalence electrons of nitrogenous ions in the shallow valance bands, to combine with localized electrons in the deep valance bands; similarly, localized electrons would not get over Coulomb force and electronic repulsions to combine with holes in the shallow valance bands (see Figure S1). As a result, the localized electrons for high onset potentials would be shallow impurity bands near band gap, and would be introduced by cationic defects which would add on bonds for bracketing band gap. Based on the consistency of crystal lattices obtained from theoretical predictions and XRD/TEM results, together with the synthetic atmosphere of ammonia, interstitial hydride ions may be the cation for the electron localization and high onset potential. Hydrogen impurity is common in nitrides and other covalent materials. It could be testified by Nuclear Magnetic Resonance (NMR), and has been deeply investigated on GaN in the luminescent field. Hydrogen impurity would add on GaN semiconductor an inefficient p-type doping, which is taken as the key for high-brightness in GaN blue-LED (light emitting diodes, LED). Researchers make great deals of research works on the preparation method of hydrogenfree GaN, and obtain fruitful results. 23-27 Hydrogen impurity attracts few attention in photoelectronical field. In this paper, the interstitial hydrogen defects and their bonds with anions in Ta3N5 are investigated via DFT calculation. It is


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shown that the hydrogen impurities are stable in Ta3N5 and would bond with anions. The bonds of hydrogen-anion would add on bands at both valance band maximum (VBM) and conduction band minimum (CBM), aggravating electron-hole recombination and leading to high onset potential. Hydrogen-anion bonds exhibit properties of hydroxyl-group, which are known as metastable and might be excited into protons at high bias. These activated hydrogen impurities may play a role of hole transport relays instead of recombination centers in Ta3N5, and may promote hole transport for high photocurrents. COMPUTATIONAL METHODS Calculations in this work are performed using plane wave density function theory (DFT), as implemented in the Vienna Ab Initio Simulation Package (VASP).28-29 The Generalized Gradient Approximations functional (GGA) in Perdew-Burke-Ernzerhof (PBE) with Heyd-ScuseriaErnzerhof (HSE) method are employed as the exchange-correlation energy function.30-34 The parameters for the HSE calculation are α = 0.25 and ω = 0.2 Å-1, which refer to the HSE06 functional.27-29 Band projections of the models are applied with GW method via wannier90 interface to VASP package. Both band structures along high symmetry points and CBM/VBM are obtained by wannier projections. Details for calculations are shown in supporting information. Both oxygen and hydrogen impurities bring charge unbalance in Ta3N5, and the charged formation energies are employed for comparing the stabilities of defect models.35-40 For instance, formation energy of interstitial hydrogen bonding with oxygen impurity is defined as: E(fO

NH

)••

=

1

∑

(E n

t ( O N H )••

t − ETa + µ N − µO − µ H + q (ε f + EV + ∆V ) 3N5

i

)

(1)

i = N ,O ,Ta

t

t where E(ON H )•• is energy of the corresponding defect model, E Ta N is energy of ideal Ta3N5, 3

5

µ H , N ,O are chemical potentials for hydrogen, nitrogen and oxygen, respectively, q is charge


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induced by defects, ε f is fermi level of defective model, EV is VBM energy of ideal Ta3N5, ∆ V is potential corrections between defect model and ideal model,

∑

i =N ,O,Ta

ni is the number of atoms in

lattice. According to the equation (1), a negative value means a stable structure. Activated protons emerge the characteristic of near-free states. Activation energies of these protons could be defined as: E Hf ' ≈ EOt • − E tO N

(

N

H

)••

(

+ E Ht 2O − E tOH (

)•

)

(2) t

t where EON• is formation energy of models with oxygen impurities, E (O N H )•• is formation energy of

t t model with both hydrogen and oxygen impurities, E H 2O is formation energy of water and E (OH )

•

is formation energy of hydroxide-ion. RESULTS AND DISCUSSION There are three kinds of anionic sites for nitrogen in Ta3N5, two sites with fourfold coordination and one with threefold coordination. As shown in Figure 1, nitrogen sites with fourfold coordination emerge tetrahedral configurations, which indicates sp3 hybrid orbitals. These hybrid orbitals bond with Ta-5d orbitals, in the configurations of N-Ta tetrahedrons. Orbitals at threefold sites show 2-dimensional isosceles triangles, exhibiting quadrilateral bonds with Ta. These bond configurations indicate nonequivalent hybridization between anions and Ta, corresponding to the hybrid bonds between sp2 and d orbitals. In addition, the tetrahedrons show edge-sharing along Ta-Ta lines with isosceles triangles. Although these configurations of edgesharing undermine the stability of Ta3N5, they would endow nitrogenous ions saturated bonds at these fourfold sites.41 Configurations of fourfold sites above play a role of cornerstone for the stability of Ta3N5, and anion impurities at the fourfold sites would destroy the saturate state and


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undermine the lattice stability. For example in Figure S2, formation energies of models with defects at both threefold and fourfold sites indicate the stable nitrogenous fourfold sites. As a result, defects would locate at threefold sites rather than fourfold sites, and the content would be consequently limited in 20% (see Figure S3).

Figure 1. Lattice of Ta3N5. Nitrogen sites with threefold coordination show isosceles trigonal planes (in green), sites with fourfold coordination show tetrahedron configurations (in purple). The golden spheres are tantalum ions. Details are shown in Figure S4 in supporting information. Based on outer orbitals of nitrogen and oxygen, anions would show lone pairs at threefold sites. These pairs would exhibit attractions to cations (hydride ions) in the form of Lewis bases. Specially, lone pairs at threefold sites exhibit sp1 orbitals in the planes of hybrid-bonding triangles. Hydrogen impurities would then bond with the anions in the isosceles trigonal planes in Ta3N5 crystal. By bonding with nitrogen or oxygen impurities, there would be two kinds of hydrogen impurities in threefold sites. It should be known that the bonds above are required to be mono-type rather than dual-type. Otherwise the nitrogenous ions with dual hydride ions would have five bonds and show metallicity, consequent bonds of metallic nitrogen and tantalum would


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show characteristic of metallicity. Similarly, hydride ions bonding with oxide ions should also exhibit mono-type rather than dual or treble types. Formation energies of models with defects are shown in Figure 2.29-40 It would be seen that, formation energies of defective models are more negative than the ideal Ta3N5. Based on the definition of formation energy, negative values of these defective models indicate their stablity in Ta3N5. Besides, formation energies of hydrogen bonding with oxygen impurities ( ( O N H ) •• ) are more negative than those of both oxygen impurities ( O N• ) and hydrogen bonding with nitrogen •

( NH i ) ), in different impurity contents. That is to say, the defects of hydrogen bonding with

oxygen impurities, which could be regarded as nitrogenous ions being substituted with hydroxylgroups, should be stable in Ta3N5.

Figure 2. Formation energies of hydrogen bonding with nitrogen, nitrogen substituted with oxygen and hydrogen bonding with oxygen impurities, obtained by HSE06 method. Defect contents are referenced with the number of nitrogenous ions. Hydride ions bonding with oxygen impurities could be simplified as nitrogenous ions substituted with hydroxyl-groups (i.e. ( O N H ) •• ).


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Further evidence for the stability of defects above is that the consistency of formation energies corresponding with defect contents, which could be evaluated by Gibbs energy changes of defects (i.e. formation energy changes of these defects). Stability of these defects means that, Gibbs energy of tiny defect model would approach to the ideal Ta3N5, and a defect content rising would induce rapid decrease of Gibbs energy. As shown in Figure 2, with the decrease of defect content, formation energy shows an asymptotic supremum to ideal Ta3N5, indicating compatibility of Ta3N5 with these defects. Conversely, with the rise of defect content, formation energy goes in acceleration down apart from the ideal Ta3N5, agreeing with a rapid Gibbs energy decrease and implying a tendency of certain defect content inducing higher defect content. As a result, the more defects the Ta3N5 crystal has, the more stable the defective crystal is. Tendency above would work until the defect changes the stability of whole crystal lattice, for instance, high content of hydroxyl impurity would destroy the stability of Ta3N5 lattice and would not be achieved for the saturate states of fourfold sites (see Figure S3).

Figure 3. Band structures for Ta3N5 with defect of hydroxyl-groups ( ( O N H ) •• , in green), together with band projections of hydrogen s orbitals (in red) and oxygen sp1 orbitals (in blue), obtained by GW calculations and wannier functions. The VBM and CBM of Ta3N5 with hydroxyl-group impurities are covered with orbital projections.


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Hydrogen impurity would endow Ta3N5 special cation orbitals, which may play an important role in the photocatalysis. As mentioned above, the remained lone pairs of anions are sp1 hybrid orbitals, thus the hydrogen impurity would bond with oxygen impurity in the form of σ bonds. The bonded hydrogen and oxygen impurity, which exhibits characteristic of hydroxyl-group (partly, the negative second derivative for Gibbs energy in Figure 2 imply covalent connections other than Coulomb forces for the impurities, and donating functionals of hydroxyl over hydroxide-ions). Hydroxyl-groups would provide additional path for photo-electron activations. As shown in Figure 3, both s orbitals of hydrogen and sp1 orbitals of oxygen show overlapping at VBM. The bonds of hydroxyl-groups would then form additional band structures bracketing the Fermi level, which are exactly locate at both VBM and CBM of Ta3N5. Energy dispersions for these localized bonds, including indirect characteristic of band gap, would transform into local perturbations around hydroxyl-groups, other than spatial distribution around bonds between Ta5d and anion-2p orbitals. Hydroxyl-groups and consequent hydrogen bonds would play a role of electron-hole recombination centers. As shown in Figure 4, the charge difference of defect model indicates that, there is an electron aggregated region between hydride and oxide ions, meanwhile the bonds between this oxide ion and tantalum ion nearby are impaired. This aggregated region stands for hybrid bonds between oxygen and hydrogen impurity, specially, s orbitals of hydride ion and sp1 orbitals of oxide ion. As shown in Figure 3, bonds of hydroxyl-groups stretch across fermi surface and build up the VBM and CBM. This bracketing configuration supplies photo-induced electron-hole pairs a localized path to get across the band gap. Photo-induced electrons and holes would preferentially gather together around hydroxyl-groups rather than spreading in the lattice. The gathered electrons and holes around hydroxyl-groups would be easy to combine with each


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other, rather than jumping over N2p-Ta5d bonds and spreading around as charge carriers. As a result, the recombination of electron-hole pairs would be strengthened significantly by hydroxylgroups in the process of photo-activations. Secondary bonds would also be introduced in Ta3N5, in form of hydrogen bonds as shown in Figure 4. There is an electron dilute region at the outside of hydroxyl-groups. This region locates in vacuum space and shows characteristic of hydrogen bonds with the nearby nitrogenous ions. These hydrogen bonds would attract electrons in nitrogenous ions, and hold photo-induced electrons from the nitrogenous ions nearby. Hydrogen bonds here supply another path for excited electrons to go up to conduction bands. Specially, photo-induced electrons are excited from nitrogenous ions and then pumped into hydroxyl-groups via hydrogen bonds. These pumped electrons would gather around hydroxyl-groups and jump into hybridized bonds between tantalum and anions, or be attracted by holes around hydroxyl-groups for electron-hole recombination. From another perspective, holes would travel around VBM in the reciprocal space and combine with electrons from CBM, meanwhile the corresponding electrons jump into Ta-induced conduction bands as charge carriers or move into hydroxyl-group induced bands for recombination. In a word, hydrogen bonds would supply additional path for photo-induced charges to gather around hydroxyl-groups, and aggravate electron-hole recombination by disturbing charge transport. Configurations of

(O N H )

••

would be positive charge states while the outer orbitals for the

hydride/oxide ions are saturated as two/eight electron occupations (see Figure 2). As mentioned above, anions at threefold sites would exhibit lone pairs. The saturated orbitals of hydrogen indicate that, hydride ions are bonded to the left lone pairs of oxide ions to form hydroxyl-groups. As a result, electron density around hydroxyl-groups is impaired and the hydroxyl-groups exhibit


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positive charge aggregations. The redundant positive charges would exhibit Coulomb effects for the electrons around hydroxyl-groups. Namely, the excited electrons around hydroxyl-groups would be localized by these coulomb effects during dynamic process, even though the ionic outer orbitals are fully occupied. Positive charge configurations of ( ( O N H ) •• ) donate oxygen sites strengthened electron localizations and hole-electron recombination. The positive charge aggregations of hydroxyl-groups would pump excited electrons from anionic sites nearby to hydroxyl-groups via hydrogen bonds. Thereafter, electrons at anionic sites nearby would preferentially go combining with remained holes of pumped electrons, rather than spread into tantalum as charge carriers. Dynamic cooperation between hydroxyl-groups and hydrogen bonds would result in intensive charge recombination, until the hydroxyl-groups are broken.

Figure 4. Electron density difference of Ta3N5 with defect of hydroxyl-groups ( ( O N H ) •• ) and the induced hydrogen bonds. White ball is hydride ion, the red is oxide ion, the gray is nitrogenous ion, and the golden is tantalum ion. Red region in profile stands for high density of electrons, and the blue region stands for low density of electrons. Because of the characteristics of hydroxyl functional group, hydroxyl recombination centers are easy to be broken by means of activating hydride ions into protons in high bias. Calculations


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show that the activation energies of these hydride ions may reach up to 0.1 eV and decrease smoothly along with hydrogen content reduction, indicating the metastable states of hydrogen impurities (see Table S1). The activated near-free hydrogen impurities (protons) would act as relay points for charge carriers in Ta3N5, and hence facilitate hole transport and result in high photocurrent at high bias. In additional, these protons would be re-localized on anion sites and to rebuild hydrogen-anion recombination centers. Accordingly, there is a cycle for hydrogen impurities, being activated from anionic ligands into near-free protons and being re-localized on anionic sites. As a result, partial energy of absorbed photons would be wasted on hydrogen activation and localization, to be transformed into anergy rather than exergy.

Figure 5. Band edges of Ta3N5 with defects, versus the normal hydrogen electrode (NHE) reference. Conduction bands (in blue) and valance bands (in red) are obtained by GW method. Hydrogen impurity would also affect band edges and flatband potential. For one hand, hydrogen impurity may not change the band gap for its onefold coordination character, and band gaps of the defect models remain almost the same to the ideal Ta3N5. For the other hand, band edges of the defect models move downwards, as shown in Figure 5. Compared with the ideal Ta3N5, VBM of oxygen impurity model ( O N• ) shifts downwards with 0.10 eV, and the hydroxyl


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impurity model ( ( O N H ) •• ) is 0.43 eV. CBM of ideal Ta3N5 for -0.4 eV. CBM of

(O N H )

••

•

ON

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reach up to -0.35 eV vs NHE, similar to that of

locates at -0.04 eV vs NHE. Accordingly, although the

band edges of these defect models are straddling the water redox levels, their CBM and VBM exhibit shifts downwards and would be harmful to water splitting. Specially, comparing with oxygen impurity, hydrogen impurity would result in an even worse band edges and higher flatband potential, and would be over demands for water oxidization, but may be harmful for stability of Ta3N5 photoanodes. Regardless of the high oxidizing ability of hydrogenous Ta3N5, hydrogen impurities might be a potential threat for the stability of Ta3N5 photocatalyst, because of the near-free protons and anion-hydrogen groups under different surroundings. The protons would be induced in some areas and move around in the crystal, and may neutralize with these groups in the crystal. Accordingly, the existence of hydrogen impurities would be harmful to the stability of both anions and Ta3N5 lattices. Besides, hydrogen impurities would be partly eliminated by a heat treatment in hydrogen via neutralizing these hydroxyl-groups into water vapor, and the treatments would reserve Ta3N5 typical photo-performances of nitrogenous vacancy ( VN••• ) with less recombination centers.42 In consideration of that, availability of hydrogen treatment to eliminate hydrogen impurity would be limited by the various configurations of hydrogen impurities, and the nitrogenous vacancies seems to be easily re-occupied in the Ta3N5 photocatalysis, hydrogen treatment would not solve hydrogen impurity completely. CONCLUSIONS In this paper, we propose the hydrogen impurities as electron-hole combination centers in Ta3N5, resulting in high onset potential. By means of first principle calculations, the hydrogen impurities are found to be stable in Ta3N5 photocatalyst, and would bond with oxygen impurities


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to form hydroxyl-groups. The hydroxyl-groups would endow Ta3N5 band edge shift and high flatband potential, together with electron localizations and additional recombination centers, resulting in high onset potential. Hydrogen impurity may undermine the stability of anions and impair the spatial connectivity of crystal lattices, and would be harmful to the Ta3N5. More broadly, hydrogen impurity would be common in nitrides, oxynitrides and other covalent materials, and it would bond with anions via single covalent bonds. It may undermine extended states overlaps and charge transports, and may be important for charge recombination in photoreactions in the aspect of dynamics. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Detailed computational procedures and supplemental works with Figure S1−S4 and Table-1, as referred to in the text. (PDF) AUTHOR INFORMATION Corresponding Author Zhaosheng Li* E-mail: [email protected]. Tel: +86-25-83686304(ext. 800) Present Address Nanjing University, 22 Hankou Road, Nanjing 210093, People’s Republic of China. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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This work was financially supported by the National Natural Science Foundation of China (Grant Nos. No. 21473090 and U1663228), the 973 Program (Grant No. 2013CB632404), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. The numerical calculations in this paper have been done using the computing facilities of the High Performance Computing Center (HPCC) in Nanjing University. The authors are also grateful to the National Supercomputing Center in Shenzhen for the numerical calculations discussed in this paper. REFERENCES (1) Lewis, N. S. Toward Cost-Effective Solar Energy Use. Science 2007, 315, 798–801. (2) Gratzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338–344. (3) Li, Z. S.; Luo, W.; Zhang, M.; Feng, J.; Zou, Z. Photoelectrochemical Cells for Solar Hydrogen Production: Current State of Promising Photoelectrodes, Methods to Improve their Properties, and Outlook. Energy Environ. Sci. 2013, 6, 347–370. (4) Fujishima, A.; Honda, K. Electrochemical Photocatalysis of Water at Semiconductor Electrode. Nature 1972, 238, 37–38. (5) Bard, A. J.; Fox, M. A. Holy Grails in Chemistry. Acc. Chem. Res. 1995, 28, 141–145. (6) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446–6473. (7) Fang, C. M.; Orhan, E.; de Wijs, G. A.; Hintzen, H. T.; Groot, R.A.; Marchand, R.; Saillard, J. Y.; de With, G. The Electronic Structure of Tantalum (Oxy)nitrides TaON and Ta3N5. Mater. Chem. 2001, 11, 1248–1252. (8) Chun, W.; Ishikawa, A.; Fujisawa, H.; Takata, T.; Kondo, K. N.; Hara, M.; Kawai, M.; Matsumoto, Y.; Domen, K. Conduction and Valence Band Positions of Ta2O5, TaON,


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Theoretical Insight into Charge-Recombination Center in Ta3N5

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