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Upward Shift in Conduction Band of Ta2O5 due to Surface Dipoles Induced by N-doping Ryosuke Jinnouchi, Alexey V. Akimov, Soichi Shirai, Ryoji Asahi, and Oleg V. Prezhdo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06932 • Publication Date (Web): 09 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

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The Journal of Physical Chemistry

Upward Shift in Conduction Band of Ta2O5 due to Surface Dipoles Induced by N-doping Ryosuke Jinnouchi,1 Alexey V. Akimov,2 Soichi Shirai, 1 Ryoji Asahi1 and Oleg V. Prezhdo2,* 1

2

Toyota Central Research and Development Laboratories, Inc.

Department of Chemistry, University of Southern California, Los Angeles, CA 90089

*Corresponding author. Email: [email protected] Tel: 213-821-3116

1

ACS Paragon Plus Environment


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Abstract Density functional theory calculations were executed to clarify the mechanism of the experimentally-observed upward shift in conduction band minimum (CBM) and valence band maximum (VBM) of N-doped Ta2O5, which is used as a photosensitizer in CO2 reduction. Calculations reproduce well the experimental energy levels (with respect to vacuum) of non-doped Ta2O5 and N-doped Ta2O5. Detailed analyses indicate that N-doping induces formations of defects of oxygenated species, such as oxygen atom and surface hydroxyl group, in the Ta2O5, and the defect formations induce charge redistributions to generate excess negative charges near the doped nitrogen atoms and excess positive charges near the defect sites. When the concentration of the doped nitrogen atoms at the surface are not high enough to compensate positive charges induced at the surface defects, the remained positive charges are compensated by the nitrogen atoms in inner layers. Dipole moments normal to the surface generated in this situation raise the CBM and VBM of Ta2O5, allowing photo-generated electrons to transfer from N-doped Ta2O5 to the catalytic active sites for CO2 reduction as realized with Ru-complex on the surface in experiment.

2


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1. Introduction Artificial photosynthesis under visible light to produce organic species is an important energy conversion method to resolve the fossil fuel shortage and global warming problems.1–9 One of the promising methods to realize artificial photosynthesis is Z-scheme,2,8 where two semiconductor electrodes are used to activate two half-cell redox reactions. In the photosynthesis device proposed by Sato et al.,2 a semiconductor modified with metal-complex electrocatalyst (SC/MCE) used as a photocathode activates the following CO2 reduction, 2CO2 + 4H+ + 4e− → 2HCOOH,

(R1)

while Pt loaded TiO2 semiconductor used as a photoanode activates the following oxygen evolution reaction, 2H2O → O2 + 4H+ + 4e−.

(R2)

In the photocathode semiconductor, such as InP, GaP and N-doped Ta2O5, excited electrons are injected from the conduction band of the semiconductor to the LUMO of MCE, such as Ru-complex, and the injected electrons participate in the CO2 reduction reaction (R1). In the photoanode TiO2, photogenerated holes oxidize water molecules to evolve oxygen molecules through the reaction (R2). By combining the two semiconductor electrodes, the following net photosynthesis reaction is realized, 2CO2 + 2H2O → 2HCOOH + O2.

(R3)

The photocathode catalyst is a key material in the photosynthesis device. To achieve efficient and 3



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selective conversion of CO2 to the desired product, formic acid in the above reaction (R3), the photocatalyst requires efficient electron injections10–12 and selective catalytic conversions.13–19 The former is driven by the suitable energy alignment between semiconductor and MCE; the LUMO level must be higher than the redox level of the CO2 reduction, i.e., −4.4 eV in vacuum scale for the case of the reaction (R1),20 and the energy level of the conduction band minimum (CBM) must be further higher than the LUMO level to make the electron injections possible.10,18 It should be also noted that the band gap of the semiconductors must be narrow enough to make the solar light available for the electron excitations.18,21,22 To meet these requirements, a wide variety of semiconductors has been developed,2,18,21–23 but little is known on the mechanisms dominating the energy alignments. A typical example is shown by N-doped Ta2O5 (N-Ta2O5) modified with Ru-complexes, which is the first photocathode utilized for selectively reducing CO2 under visible light.18 As shown in Fig. 1, redox levels of Ru-complexes, [Ru(bpy)2(CO)2]2+ (bpy: 2,2’-bipyridine), [Ru(dcbpy)(bpy)(CO)2]2+ (dcbpy: 4,4’-dicarboxy-2,2’bipyridine)，[Ru(dcpby)2(CO)2]2+ and [Ru(dpbpy)(Cl)2CO)2] (dpbpy: 4,4’-diphosphonate-2,2’bipyridine), are −3.8 to −3.5 eV in vacuum scale, while the CBM of non-doped Ta2O5 is −4.03 eV.24 Hence, the conduction band of the non-doped Ta2O5 is lower than the LUMO levels of Ru-complexes, and electrons cannot be injected from the semiconductor to the Ru-complexes. The experiments,2 in fact, indicated that no photocurrents are generated by the CO2 reduction when the non-doped Ta2O5 is used. When N-Ta2O5 with 8.9 atomic % of nitrogen is used, 4


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the photocurrent is generated. Detailed analysis using electrochemical measurements and photoelectron spectroscopy in air (PESA) indicated that the CBM of Ta2O5 is raised to −3.3 eV by N-doping.18,21 These experimental results indicate that the upward shift by the N-doping makes a key role to realize the electron injections from the Ta2O5 to the Ru-complexes. The mechanism of the upward shift of the CBM by N-doping is, however, not clear at all. Electrochemical measurements and ultraviolet photoelectron spectroscopy (UPS) done by Chun et al.24 indicated that the CBMs of TaON and Ta3N5 are not significantly different from that for the non-doped Ta2O5. Inconsistency of CBM in the above experiments suggests that desired energy alignments could be achieved by carefully controlling the concentrations and distributions of dopants, motivating us to clarify the mechanism underlying the observed phenomena. In this study, density functional theory (DFT) calculations are executed on modeled Ta2O5 surfaces with and without N-doping. The CBM and valence band maximum (VBM) of the semiconductor surfaces are calculated and compared with the theoretically-obtained redox levels of Ru-complexes as well as experimentally-obtained energy alignments, and the effects by N-doping are discussed. 2. Computational method 2.1 Models Non-doped Ta2O5 and N-doped Ta2O5 bulk and surface models including several types of N-dopants, defects and impurities were examined in this study. Symbols denoting the constructed 5



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models were tabulated in Tables S1 and S2 in Section A in Supporting Information, and their structures are summarized in Figs. 2 and 3. The details of the constructed models are described in following subsections. 2.1.1 Bulk and surface models of Ta2O5 Ta2O5 is known to transform from tetragonal phase to orthorhombic phase at 1350 °C,25–27 and the experimentally used Ta2O5 was confirmed to be the orthorhombic phase by X-ray diffraction measurements.18,21 The semiconductor surface model should be, therefore, constructed on the basis of the orthorhombic phase. The orthorhombic phase, however, has a complex crystal structure, where 55 Ta atoms and 22 O atoms are included in its primitive lattice,28 and locations of several O atoms are not identified as shown in Fig. S1 in Supporting Information. Simplifications are, therefore, necessary to construct computationally tractable surface models. Similar problems were also reported in past theoretical studies, and several simplified bulk Ta2O5 models were suggested.29–39 Among representative four bulk Ta2O5 models, λ-Ta2O5 model39 shown in Fig. 2 (a) was chosen in this study because it reproduces both lattice structures and electronic structures accurately as shown in Section 3.1. Detailed comparisons among the four bulk Ta2O5 models are described in Section B in Supporting Information. As discussed in past studies on Ta2O5 and other transition metal oxides,34,38,40 oxygen defects and hydrogen impurities can exist in the Ta2O5 bulk. Their stabilities and effects on the electronic structure were also examined by the DFT calculations. The defects and impurities examined in this 6


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study are the oxygen vacancy (Ov) shown in Fig. 2 (b), interstitial hydrogen impurity (Hint) shown in Fig. 2 (c) and combination of them (Hint+Ov) shown in Fig. 2 (d). The models were constructed by introducing one Hint into the 1×1×1 λ-Ta2O5 bulk model and by removing one Ov from the 1×1×2 λ-Ta2O5 bulk model. The locations of Ov and Hint shown in Fig. 2 (b)−(d) were determined by

examining the stability of all non-equivalent defect and impurity sites by the DFT calculations as summarized in Tables S4 and S5 in Supporting Information. As discussed in Section 3.1, the calculations on the bulk λ-Ta2O5 indicated that Ta2O5 without Ov and Hint are stable in practically important environmental conditions of the sample preparations and the CO2 reductions.19,21 The Ta2O5 surface model should be, therefore, constructed from the bulk λ-Ta2O5 without the defects and impurities. However, there still remained arbitrariness in the surface terminations which can depend sensitively on environments surrounding the surface. Under slightly humidified conditions as in actual experimental conditions,18 similarly to surfaces of other transition metal oxides, such as TiO2,41,42 the metal oxide surface is likely terminated by hydroxyl groups as shown in Fig. 3 (a). The expectation is supported by our DFT results summarized in Section 3.2, where OH-terminated Ta2O5 surface is shown to be thermodynamically more stable than Ta2O5 surfaces with other terminations, such as H-, O- and Ta-terminations shown in Figs. 3 (b), (c) and (d), respectively. The Ta2O5 electrode surface was, therefore, modeled by the OH-terminated Ta2O5(001) surface slab. The slab thickness was set as 4 atomic layer, and the 1×1 periodicity was imposed to the constructed surface model. Although the system size is relatively small, the constructed surface 7



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models on the basis of the low-energy high-symmetry λ-Ta2O5 bulk model correctly describe the formal oxidation states of +5 and −2 for Ta and O atoms, respectively, and the experimentally observed triangular lattice symmetry of the Ta sublattice. The use of the high-symmetry surface models combined with the highly accurate but computationally-demanding hybrid functional method described in Section 2.3 is expected to provide essential electronic effects by the N-doping within a modest computational time. 2.1.2 Bulk and surface models of N-Ta2O5 The experimentally used N-Ta2O5 was confirmed to have the orthorhombic structure with lattice constants close to the non-doped Ta2O5, and X-ray photoelectron spectroscopy (XPS) measurements indicated that an N1s peak at 396 eV evolves with the increase in the amount of the doped nitrogen atoms.18,21 These experimental results indicate that most of nitrogen atoms are introduced substitutionally at the oxygen sites. The doped N atoms were, therefore, modeled as the substitutional nitrogen atoms (Nsub). The N-doped Ta2O5 was constructed by substituting one oxygen atom in the 1×1×1 bulk λ-Ta2O5 model with one nitrogen atom. The resulted concentration of nitrogen atoms corresponds to 7.1 atomic % that is within the experimental range of 2−9 atomic %. The location of Nsub was determined as the lowest energy site in the λ-Ta2O5 bulk model as summarized

in

Table

S6

and

Fig.

S4

in

Supporting

Information.

The

obtained

lowest-energy-structure is shown in Fig. 2 (e). Similarly to the case of the non-doped Ta2O5 bulk, energetics of Ov, Hint and Hint+Ov formations in the N-Ta2O5 bulk were also examined by the DFT 8


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calculations. The same numbers of Hint and Ov as those in the non-doped Ta2O5 were introduced into the N-Ta2O5 after examining the stability of non-equivalent locations of them as summarized in Tables S7 and S8 and Figs. S5 and S6 in Supporting Information. The determined structures are shown in Figs. 2 (f)−(h). Although the experimentally measured lattice constants and XPS indicated that Nsub is the major dopant,18,21 the XPS also showed a broad N1s peak at 400 eV which is likely attributed to the small amount of interstitially doped N atoms (Nint). Their energetics and effects on the lattice constants and electronic structures were also examined by the DFT calculations. Similarly to the case of Nsub, one Nint was introduced into the 1×1×1 bulk λ-Ta2O5 model. The resulted concentration of Nint is 6.7 atomic %, which is also within a range of the experimental concentration. Similarly to other defects and dopants, the most stable site was determined by the DFT calculations to examine the stability of non-equivalent interstitial nitrogen sites as summarized in Table S9 and Fig. S7 in Supporting Information. The determined structure is shown in Fig. 2 (i). As discussed in Section 3.1, DFT calculations on the N-Ta2O5 bulk models as well as the past experiments18,21 indicated that Ov+Nsub and Hint+Nsub are dominant types of N dopants, and therefore, the N-Ta2O5 surface models including the Ov+Nsub and Hint+Nsub as well as Nsub were constructed as shown in Figs. 3 (e)-(j) and examined by the DFT calculations. The detailed analysis on their electronic structures indicated that the substitutional N atoms are stabilized by introducing Ov and Hint because unstable unpaired electrons formed at the doped N atoms are removed by electron 9



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donations by their introductions. The same phenomenon can also occur when OH vacancies (OHv) are created on the surface. Hence, N-Ta2O5 surface models including the OHv+Nsub as shown in Figs. 3 (k) and (l) were also constructed and examined. Although most of nitrogen atoms were judged to be substitutionally doped on the basis of both experimental and theoretical results, a small amount of interstitially introduced nitrogen atoms can also exist on the surfaces as described previously. Surface models including the Nint were, therefore, constructed as shown in Figs. 3 (m) and (n), and the effects of Nint on the electronic structure were also examined. For each of five types of N-dopants, Nsub, Ov+Nsub, Hint+Nsub, OHv+Nsub and Nint, two types of nitrogen distribution were considered to examine the effects by the distribution of N atoms: a homogeneous distribution (Figs. 3 (e)-(g), (k) and (m)) and distribution with surface segregation (Figs. 3 (h)-(j), (l) and (n)). Although other distributions may exist in actual N-Ta2O5, the comparison of the DFT results between these two distributions highlights the essence of the mechanism of the CBM and VBM energy rise, as discussed in Section 3.2.2. The comparison presents the key factors allowing one understand the properties of the other distributions that are not explicitly demonstrated in this study. The homogeneously doped nitrogen atoms are denoted as Nhsub and Nhint, and the nitrogen atoms segregated on the surfaces are denoted as Nssub and Nsint later in this article. Similarly to the OH-terminated Ta2O5(001) surface slab model, the slab thickness was set as 4 10


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atomic layer, and the 1×1 periodicity was imposed. For the slabs with the homogenous nitrogen distribution, their middle two Ta2O3 planes were fixed at positions determined from the calculations on N-doped Ta2O5 bulks, while for the slabs with the surface segregations, their middle two Ta2O3 planes were fixed at positions determined from the calculations on the non-doped Ta2O5 bulk. A test calculation to examine the effects by the constraints on the middle two Ta2O3 planes was carried out on the N-doped Ta2O5 slab with Ov and Nsub (Ov+Nsub), structure of which especially deformed during the relaxation, and the result indicated that the significant features discussed in this article are not affected by this constraint as shown in Section D in Supporting Information. 2.1.3 Molecular models of Ru-complex Calculations were executed on redox levels of four Ru-complexes shown in Fig. 1 isolated in the bulk solution. Although actual Ru-complexes are bounded to the Ta2O5 surfaces through their bpy ligands, the redox levels of free Ru-complexes are considered to be one of essential physical properties dominating the electron injections as discussed in past studies.18 The redox levels were, therefore, examined by the DFT calculations. In the calculations, the solution was modeled by a continuum solvation model described by a polarizable continuum model (PCM), and the Ru-complexes were placed in the continuum medium.43 2.2 Physical properties calculated by DFT DFT methods44 were used to obtain band gaps and optical properties of the bulk systems, and energy alignments of CBMs and VBMs of the surface systems. Besides those physical properties, 11



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calculations were also executed on free energies forming the N-Ta2O5 bulk and surface models from the non-doped Ta2O5 bulk and surface models, respectively, to examine their stabilities. The redox potentials of the experimentally used Ru-complexes were also calculated to compare them with the theoretically-obtained CBMs and VBMs of the Ta2O5 surfaces. The chemical processes and equations used for obtaining the free energies and redox potentials are explained below. Formations free energies of N-Ta2O5 bulks and surfaces In experiments, an ammonia treatment or sputtering in a N2/Air atmosphere was adopted to dope nitrogen atoms into Ta2O5.18,21 Under the former condition, N-Ta2O5 is likely formed through the following reaction: Ta2O5 + xNH3 → NxHyTa2O(5−z) + zH2O + (3/2x−y/2−z)H2.

(R4)

Hence, DFT calculations were carried out on the reaction free energies of the reaction (R4) forming the N-Ta2O5 bulk and surface models from the non-doped Ta2O5 bulk and surface models, respectively, to examine the stability of the constructed N-Ta2O5 bulk and surface models. The reaction free energy of the reaction (R4) was described as,

[

]

∆G = G N x H y Ta 2 O (5− z ) + zG [H 2 O ] + (3 2 x − 1 2 y − z )G [H 2 ] − G [Ta 2 O 5 ] − xG [NH 3 ],

(1)

where G indicates the Gibbs free energy of the system denoted in the square bracket and was approximately calculated as a sum of the total energy Etot at 0 K and enthalpy and entropy contributions Hn − TSn from the nuclear motions on the basis of the ideal gas model and harmonic oscillator model45 as follows, 12


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G ≅ Etot + H n − TS n .

(2)

The used Hn and entropy Sn are tabulated in Table S10 in Supporting Information. On the basis of these simplified model and equation, the Gibbs free energies of NH3, H2 and H2O are approximated by a following equation,

G[s ] ≅ G0 [s ] + ∆G[s ] (s = NH3 , H 2 or H 2 O) ,

(3)

∆G [s ] = k B T ln p s p s0 ,

(4)

where ps indicates the partial pressure of the species s, and G0 indicates the Gibbs free energy at the pressure p0s that was set as 0.1 MPa. ∆G, therefore, varies with changes in the partial pressure ps, indicating that the stable phase depends on the gaseous compositions surrounding the N-Ta2O5. In other words, by exploring the bulk and surface phases giving the lowest ∆G with changing the Gibbs free energies G[s] (s = NH3, H2 or H2O), stable phases can be determined. It should be noted that ∆G[H2] and ∆G[H2O] are probably negative in the fabrication conditions where only the non-pressurized NH3/Ar gas was introduced into the reactor vessel. Hence, stable types of N-dopants can be determined as bulk and surface phases appearing in the third quadrant in the phase diagram mapped on a ∆G[H2]−∆G[H2O] plane. Redox reactions of Ru-complexes Redox reactions of the four Ru-complexes are described as follows, [Ru(bpy)2(CO)2]2+ + e− → [Ru(bpy)2(CO)2]+,

(R5)

[Ru(dcbpy)(bpy)(CO)2]2+ + e− → [Ru(dcbpy)(bpy)(CO)2]+,

(R6)

13



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[Ru(dcpby)2(CO)2]2+ + e− → [Ru(dcpby)2(CO)2]+,

(R7)

[Ru(dpbpy)(CO)2(Cl)2] + e− →[Ru(dpbpy)(CO)2(Cl)2]−.

(R8)

A conventional statistical thermochemistry method was used to obtain their redox potentials Uredox in the vacuum scale.46,47 In this method, Uredox is calculated by using the Gibbs free energies of the Ru-complexes before and after the electron transfers as follows,

U redox = − (G[Red] − G[Ox]) e ,

(5)

where Red and Ox indicate reductant (right hand side in (R5)-(R8)) and oxidant (left hand side in (R5)-(R8)) states, respectively. The Gibbs free energies are described by Eq. (2), and the used Hn and entropy Sn are tabulated in Table S10 in Supporting Information. 2.3 Computational parameters Calculations on the Ta2O5 and N-Ta2O5 bulks and surfaces were executed using the ab initio density functional code Vienna Ab initio Simulation Package (VASP)48,49 using plane wave basis sets50 with an energy cutoff of 400 eV and projector augmented wave (PAW) method.51 Band gaps, optical properties and energy alignments were obtained by self-consistent field (SCF) calculations using the Heyd-Scuseria-Ernzerhof functional (HSE06)52–54 on optimized structures. The structural optimizations of the bulk systems were carried out using the HSE06 functional, while for the surface systems, the optimizations were carried out using the generalized gradient approximation with Perdue-Burke-Ernzerhof (GGA-PBE) functional55 to decrease the computational cost. To avoid unphysical relaxations of the slabs, middle two atomic layers were fixed during the structural 14


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optimizations. Besides the physical properties described above, the reaction free energies ∆G of the reaction (R4) forming the (N-)Ta2O5 bulk and surface models were executed as described in Section 2.2. The calculations were executed using the GGA-PBE functional. In the GGA-PBE calculations, Brillouin zone integrations were executed by using Monkhorst-Pack k-point meshes56 of 4×3×6 for the (N-)Ta2O5 bulks with the 1×1×1 periodicity, 4×3×3 for the (N-)Ta2O5 bulks with the 1×1×2 periodicity, and 5×4×1 for the (N-)Ta2O5 surfaces with the 1×1 periodicity. The same k-point meshes were used for the HSE06 calculations on the bulks, while for the surfaces, the k-point mesh was reduced to 2×2×1 to decrease the computational cost. Vacuum layers with a thickness of 20 Å were placed between the slabs to avoid the interactions among the repeated slabs, and dipole corrections57,58 were applied to eliminate the long-ranged electrostatic interactions among the repeated slabs. DFT calculations on the Ru-complexes were executed by using Gaussian09 package.59 The HSE06 functional was used in these calculations, too. 6-31G** basis sets60,61 were used for H, C, N, O, P and Cl atoms, and LANL2DZ basis set62 and effective core potential were used for Ru atoms. A polarizable continuum model using the integral equation formalisms variant (IEFPCM)43 was used to describe the experimentally-used acetonitrile solution.18 3

Results

3.1 Ta2O5 and N-Ta2O5 bulks 3.1.1 Phase diagrams of Ta2O5 and N-Ta2O5 bulks 15



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Before discussing the theoretically-obtained surface properties of the Ta2O5 and N-Ta2O5, their bulk properties, such as phase diagrams, lattice constants, electronic structures and optical properties, are shown and compared with the experimental results. Figure 4 shows the theoretically-obtained phase diagrams at 848 K and 298 K. The N-Ta2O5 was synthesized at the former temperature,21 and the synthesized N-Ta2O5 was used as a photosensitizer at the latter temperature.18 The Gibbs free energy G[NH3] of NH3 was fixed at that at 0.1 MPa when the phase diagrams were calculated. The calculated phase diagram indicates that the non-doped Ta2O5 bulk without any defects and impurities corresponding to the phase (a) is stable in a wide free energy range including atmospheric pressures of H2 and H2O (e.g., G[H2] ≈ −0.3 eV corresponding to 5×10−6 MPa for H2 gas in air and G[H2O] ≈ −0.1 eV corresponding to 3.5×10−3 MPa for vapor in equilibrium with liquid water at 298 K).63 In more negative regions of G[H2] and/or G[H2O], four types of N dopants and defects appear in the phase diagrams: substitutional nitrogen atoms with interstitial hydrogen impurities (Hint+Nsub) (g), substitutional nitrogen atoms with oxygen defects (Ov+Nsub) (f), interstitial nitrogen atoms (Nint) (i), and substitutional nitrogen atoms and interstitial hydrogen impurities plus oxygen defects (Hint+Ov+Nsub) (h). Among the four phases, the Hint+Ov+Nsub phase (h) appears only when ∆G[H2] is highly positive, indicating that this phase is stable only when the partial pressure of H2 is raised. This situation does not occur in any experimental conditions, and therefore, the Hint+Ov+Nsub phase is omitted from the evaluations. 16


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The Hint+Nsub (g) and Ov+Nsub (f) phases appear in the low ∆G[H2O] region, while the Nint (i) phase appears in the low ∆G[H2] region. The Hint+Nsub and Ov+Nsub phases are stabilized by decreasing the water activity ∆G[H2O] because H2O is generated by the following reactions forming those phases: NH3 + Ta4O10 → HNTa4O9 + H2O,

(R9)

2NH3 + Ta8O20 → N2Ta8O17 + 3H2O.

(R10)

Similarly, the Nint phase is stabilized by decreasing the H2 activity ∆G[H2] because H2 is generated by the following reaction forming the Nint phase: NH3 + Ta4O10 → NTa4O10 + 3/2H2.

(R11)

A more negative value of ∆G[H2] is necessary for stabilizing the Nint phase than ∆G[H2O] for stabilizing the Hint+Nsub and Ov+Nsub phases. The result likely indicates that the Nint phase is more difficult to synthesize. Quantitative discussion is, however, difficult because the detailed experimental gaseous compositions are not clear. Hence, the lattice structures and electronic structures of all three bulk models are examined later in this article. 3.1.2 Lattice constants and electronic structures of Ta2O5 and N-Ta2O5 bulks Table 1 summarizes the experimental and theoretical lattice constants of the non-doped Ta2O5 and N-doped Ta2O5 bulk models. The calculated lattice constants of the non-doped Ta2O5 bulk model agree well with the experimental ones, and the lattice constants of the N-Ta2O5 models (e)−(h) other than the Nint-doped Ta2O5 model (i) are close to those of the non-doped Ta2O5 similarly to the 17



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experiments. The lattice constant a of the Nint-doped Ta2O5 model is significantly larger than the experimental result, indicating that the Nint phase is not dominant in the N-Ta2O5. Calculated partial densities of states (PDOSs) of the non-doped Ta2O5 and N-Ta2O5 bulk models are summarized in Fig. 5, and theoretical and experimental band gaps are summarized in Table 1. The imaginary parts of the frequency-dependent dielectric functions are also shown in Fig. 6. The theoretical band gap of the non-doped Ta2O5 agrees well with the experimental one. The band gap is significantly narrowed when Nsub and Nint are introduced into Ta2O5 as indicated by (e) and (i), while the band gap of N-Ta2O5 with Nsub is widened again when Hint and Ov are additionally introduced as indicated by (f) and (g). The similar trend also appears in the dielectric function, where the trend in the absorption edge is summarized as Hint+Nsub < Ov+Nsub < Nint < Nsub. The PDOSs shown in Figs. 5 (a) and (e) for the non-doped and Nsub-doped Ta2O5, respectively, indicate that unoccupied bands (shown as A in Fig. 5 (e)), which are partially attributed to the doped Nsub, are additionally generated below the CBM of the non-doped Ta2O5 by the N-doping, and the PDOSs shown in Figs. 5 (a) and (i) for the non-doped and Nint-doped Ta2O5, respectively, indicate that the occupied and unoccupied bands, which are partially attributed to the doped Nint, are generated above the VBM and below the CBM, respectively. The unoccupied bands in the Nsub-doped Ta2O5 correspond to the holes generated at the doped N atom, and the occupied and unoccupied bands in the Nint-doped Ta2O5 correspond to the bonding and anti-bonding molecular-like orbitals of NO formed in the Ta2O5 shown as circles in Fig. 2 (i). By the generations of those bands, 18


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band gaps are narrowed. When Ov and Hint are introduced into the Nsub-doped Ta2O5, the unoccupied bands corresponding to the holes located at the doped N atoms disappear as shown in Figs. 5 (f) and (g). This is because the holes are compensated by redistributions of electrons located near the Ov-defects and Hint-impurities to the doped nitrogen atoms as schematically shown in Fig. 7 (I) for the case of Ov. Those electron redistributions also affect the energetics of the Nsub, Ov+Nsub and Hint+Nsub phases. Because the unstable unpaired electrons located near the Nsub atoms are removed by the electron donations, the bulk system is stabilized by the defect and impurity formations. By this mechanism, the Nsub phase disappears from the phase diagram shown in Fig. 4. In summary, both the theoretically-obtained phase diagrams and lattice structures indicate that Nsub plus Ov-defect or Hint-impurity is the dominant type of N-dopants in the N-Ta2O5 bulk, while the small amount of the isolated Nint and Nsub can significantly contribute on the band-gap narrowing. 3.2 Ta2O5 and N-Ta2O5 surfaces 3.2.1 Phase diagrams of Ta2O5 and N-Ta2O5 surfaces The theoretically-obtained phase diagrams of non-doped and N-doped Ta2O5 surfaces with and without the surface segregation of the doped N atoms are summarized in Fig. 8. Similarly to the phase diagrams of the bulk systems shown in Fig. 4, the non-doped Ta2O5 surface is stable in the wide free energy range. Among the non-doped Ta2O5 surface models, the OH-terminated one (a) is especially stable as expected from the reported results on the TiO2 surfaces.41,42 19



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N-doped Ta2O5 surfaces are stable in more negative regions of ∆G[H2O] or ∆G[H2] similarly to the bulk systems; OHv+Nhsub (k) and OHv+Nssub (l) are stable in the negative ∆G[H2O] region, and Nhint (m) is stable in the negative ∆G[H2] region. Analyses on the PDOSs shown in Fig. 9 indicate that the OHv+Nhsub (k) and OHv+Nssub (l) are stabilized by a mechanism similar to that described in the previous section. As indicated in PDOSs of the N-Ta2O5 with Nhsub and Nssub shown in Figs. 9 (e) and (h), the introductions of the substitutional N atoms generate unstable unoccupied bands (shown as A in the figures) below the CBM. Those unoccupied bands are compensated by redistributions of electrons located near the OHv defects to the doped nitrogen atoms as schematically shown in Fig. 7 (II). Accordingly, the surface phase diagrams indicate that OHv can be created on the N-Ta2O5 surface, and therefore, electronic alignments of OHv+Nhsub (and OHv+Nssub) as well as Ov+Nhsub (and Ov+Nssub) and Nhint (and Nsint) are examined in the following subsection. 3.2.2 Electronic alignments of Ta2O5 and N-Ta2O5 surfaces The experimentally- and theoretically-obtained CBMs and VBMs of the Ta2O5 and N-Ta2O5 surfaces are tabulated in Table 2, and theoretically-obtained PDOSs of all surfaces are summarized in Fig. 9. In Table 3, experimental and theoretical redox levels of the Ru-complexes are also tabulated, and all the calculated energy alignments are summarized in Fig. 10. The calculated CBM and VBM of the non-doped Ta2O5 terminated with OH adsorbates agree well with the experimental results obtained by Chun et al.,24 and the experimentally-observed 20


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upward shifts in CBM and VBM by N-doping18,21 are reproduced well only by the N-Ta2O5 surface models (f) and (k) (Ov-Nhsub and OHv-Nhsub, respectively), where oxygen and hydroxyl defects, respectively, are introduced into the surface Ta2O3 planes, and nitrogen atoms are homogeneously doped. The redox levels of Ru-complexes are also reproduced well by the theory. Although the calculated CBMs of N-Ta2O5 (f) and (k) are still lower than the calculated redox levels of [Ru(dpbpy)(CO)2(Cl)2] and [Ru(bpy)2(CO)2]2+ unlikely the experimental results as shown in Fig. 1 probably because of the errors caused by the constructions of the surface models, approximations in the exchange-correlation functional and descriptions of the solvation medium surrounding the Ru-complexes, the theory indicates that the energy alignments can be drastically changed to realize the electron injections from the Ta2O5 surface to the Ru-complexes by the N-doping. To clarify the mechanisms of the upward shifts of CBM and VBM in the N-Ta2O5 surface models (f) and (k), charge distributions in the surface systems were analyzed. The resulted Bader charges in the models (f) and (k) are summarized in Fig. 11. The Bader charges in other surface models are summarized in Section F in Supporting Information. As clearly indicated by those figures, only the two surface models have prominent dipole moments normal to the surfaces at the first and second Ta2O3 planes. The dipole moments are oriented to raise the electrostatic potentials inside the surfaces relative to the vacuum, and the potential rises of about 1.0 eV, which are close to the theoretically-obtained rises in CBM, are in fact observed in the calculated local potentials across the surfaces as shown in Fig. 12. The upward shifts in the CBM and VBM are, therefore, judged to be 21



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caused by the dipole moments formed near the surface. The mechanism of the surface dipole formations can be understood by taking account of the schemes shown in Figs. 7 (I) and (II). As described previously, when oxygen atoms and hydroxyl groups are removed from the surfaces, unstable unpaired electrons are remained at the Ta atoms next to the defected sites. These unpaired electrons are redistributed to pair with the unpaired electrons located at the nitrogen atoms. Hence, by concurrent formations of the N-doping and defect, positive charges are induced near the defect sites, and negative charges are induced near the nitrogen atoms. Because the concentrations of the doped nitrogen atoms in the surface layers of the N-Ta2O5 models (f) and (k) are not high enough to fully accept the unpaired electrons from the Ta atoms next to the defect sites, the negative charges compensating the positive charges near the defect sites in the surface layers are induced near the nitrogen atoms not only in the surface layer but also in the inner layer. The dipole moments normal to the surfaces are, therefore, generated in these two models. In contrast, in other models, because the defects are not created, or concentrations of the doped nitrogen atoms in the surface layers are high enough, dipole moments normal to the surfaces are not generated. The analyses indicate that similar dipole moments can be generated by other distributions of Nsub, Ov and OHv, besides those examined in the current study, by the same mechanism illustrated in Fig. 7. For example, when Nsub is introduced only in the inner layers, and OHv and/or Ov are generated at the surface layers, similar dipole moments will be generated. Hence, more generally, the CBM and 22


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VBM of the Ta2O5 surface can be raised by N-doping when the resulted surface concentration of the substitutionally doped nitrogen atoms are not sufficient to compensate the positive charges induced by the surface defects of the oxygenated species. Similar dipole moments can be generated when Hint is introduced at the surface layer separately from the Nsub in the inner layers, because a positive charge is induced at Hint, and a negative compensating charge is induced at Nint. However, this situation is unlikely, since the DFT calculations summarized in Section C in Supporting Information indicate that Hint unbound to Nsub is energetically unstable. Accordingly, Nsub, Ov and OHv, and their configurations are concluded to be the cause of the CBM and VBM energy rise in N-Ta2O5. 4

Conclusion DFT calculations were executed to clarify the mechanism of the experimentally-observed upward

shifts in CBM and VBM of Ta2O5 by N-doping. The experimental energy alignments of the non-doped Ta2O5 surface, N-doped Ta2O5 surface and Ru-complexes were semi-quantitatively reproduced by the calculations. Calculations indicated that substitutional N-doping of the Ta2O5 surface generates unstable unpaired electrons at the doped nitrogen atoms, and these unpaired electrons are removed by electron redistributions to the nitrogen atoms through the formations of defects of oxygenated species. Hence, by the N-doping, negative excess charges are induced near the nitrogen sites, while positive excess charges are induced near the defect sites. When the defects of the oxygenated species are segregated near the surface, and the concentration of the doped nitrogen atoms is not high enough to compensate the positive excess charges induced near the surface defect 23



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sites, dipole moments normal to the surface are generated. The generated dipole moments create electrostatic potential gaps to raise the electrostatic potential inside the N-doped Ta2O5 surface relative to the vacuum, and then, the CBM and VBM are shifted positively. In other words, the energy alignments are highly sensitive to the distributions of the doped nitrogen atoms and defects of oxygenated species near the semiconductor surface, and therefore, their controls are essential to realize desired energy alignments accelerating the CO2 reductions. Supporting Information Supporting Information Available: DFT results on four models describing the non-doped Ta2O5 bulk, definitions of the symbols denoting the bulk and surface models, results of DFT calculations exploring stable N-dopant and defect sites, DFT results on the effects caused by the middle layer relaxation, enthapies and entropies of nuclear motions, and Bader charge distributions in the N-doped Ta2O5 surface models. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgement We acknowledge partial financial support from the Advanced Catalytic Transformation Program for Carbon Utilization (ACT-C), JST supported by the Japan. We also thank T. Morikawa, S. Sato, and T. Suzuki in Toyota Central R&D Labs., Inc. for their fruitful discussions. A.V.A and O.V.P. acknowledge financial support of the U.S. Department of Energy, DE-SC0014429. The authors declare no competing financial interest 24


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Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270–283.

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CRC Handbook of Chemistry and Physics; Haynes, W. M., Ed.; 95th ed.; CRC Press: Boca Raton, 2015.

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Page 32 of 50

Table 1 Experimental and theoretical lattice constants (Å) and band gaps (eV) of non-doped and N-doped Ta2O5 bulks. The theoretical lattice constant b was calculated by multiplying the lattice constant of the simplified model with the 1×1×1 periodicity by 11/2. Definitions of the theoretical models are listed in Table S1. Calculated data in the parenthesis indicate the results obtained by the GGA functional, and other calculated data indicate the results obtained by the HSE functional.

Cal.

Exp.a

Type

Lattice constants

Band gap

(a) non-doped

a=6.17, b=40.07, c=3.77 (a=6.21, b=40.38, c=3.80)

3.7 (2.1)

(b) Ov

− (a=6.19, b=39.95, c=3.82)

− (1.3)

(c) Hint

− (a=6.23, b=41.04, c=3.81)

− (0.0)

(d) Hint+Ov

− (a=6.17, b=40.37, c=3.79)

− (0.1)

(e) Nsub

a=6.17, b=40.46, c=3.77 (a=6.21, b=40.69, c=3.80)

2.0 (0.24)

(f) Ov+Nsub

a=6.23, b=40.51, c=3.73 (a=6.25, b=40.77, c=3.76)

3.2 (2.1)

(g) Hint+Nsub

a=6.18, b=40.54, c=3.80 (a=6.22, b=40.78, c=3.83)

3.9 (2.4)

(h) Hint+Ov+Nsub

a=6.23, b=40.26, c=3.75 (a=6.26, b=40.57, c=3.78)

0.3 (0.0)

(i) Nint

a=6.39, b=40.84, c=3.80 (a=6.39, b=41.00, c=3.83)

2.4 (0.6)

non-doped Ta2O5

a=6.16, b=40.30, c=3.89

3.8

N-Ta2O5 (8.9 atomic % of N)

a=6.20, b=40.26, c=3.78

2.4

a. Taken from Refs. (18) and (21).

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Table 2 Theoretically- and experimentally-obtained conduction band minimums εVBM and valence band maximums εCBM of Ta2O5 and N-Ta2O5 surfaces shown in Fig. 3. ∆εVBM and ∆εCBM indicate the relative shift caused by the N-doping and defect formations.

Exp.a

Cal.

Type

εVBM

εCBM

non-doped Ta2O5

−7.9

−4.0

N-Ta2O5

−5.7

−3.3

∆εVBM ∆εCBM − +2.2

− +0.7 − −1.50

non-doped (a)

−7.99 −4.25

Nhsub (e) Ov+Nhsub (f) Hint+Nhsub (g) Nssub (h) Ov+Nssub (i) Hint+Nssub (j) OHv+Nhsub (k) OHv+Nssub (l) Nhint (m) Nsint (n)

−7.89 −5.75

− +0.10

−5.51 −3.67

+2.48

+0.58

−7.95 −5.41

+0.04

−1.16

−7.65 −5.52

+0.34

−1.27

−7.71 −4.35

+0.28

−0.10

−7.33 −4.01

+0.66

+0.24

−5.74 −3.74 −7.36 −4.52

+2.25

+0.51

+0.63

−0.27

−7.17 −4.61

+0.82

−0.36

−7.49 −5.21

+0.82

−0.95

a. Experimental data was taken from Refs. 18 and 24.

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Page 34 of 50

Table 3 Experimentally- and theoretically-obtained redox levels (eV in vacuum scale) of the Ru-complexes shown in Fig. 1. The values in the parenthesis are the redox potentials scaled in SHE obtained by Eq. (5) described in Section 2.2. Molecule [Ru(dpbpy)(CO)2(Cl)2] 2+

[Ru(bpy)2(CO)2] [Ru(dcbpy)(bpy)(CO)2]2+ [Ru(dcbpy)2(CO)2]2+

Cal.

Exp.a

−3.34 (−1.26)

−3.5 (−1.1)

−3.46 (−1.14) −3.89 (−0.71) −3.96 (−0.64)

−3.6 (−1.0) −3.7 (−0.9) −3.8 (−0.8)

a. Experimental data were taken from Ref. (18).

34


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Figure captions Figure 1

Experimentally measured alignments of conduction band minimum (CBM) and valence

band maximum of Ta2O5, N-doped Ta2O5 (N-Ta2O5), and redox levels (LUMO levels) of four Ru-complexes (a)-(d). Small white spheres are H atoms, medium gray spheres are C atoms, medium blue spheres are N atoms, medium red spheres are O atoms, medium orange spheres are P atoms, medium light green spheres are Cl atoms, and large green spheres are Ru atoms (color online). Figure 2

Bulk models of non-doped Ta2O5 (a)−(d) and N-Ta2O5 (e)−(i). Upper figures show the top

views, and lower figures show the side views. Small white spheres are H atoms, medium blue spheres are N atoms, medium red spheres are O atoms, and large light blue spheres are Ta atoms. Squares indicate the unit cell. Dotted circles indicate the locations of oxygen vacancies (Ov), and solid circles indicate the locations of NO formed by introducing interstitial N atoms (Nint). Details of the models are described in Sections 3.1 and 3.2 and Table S1 in Supporting Information. Figure 3

Surface models of non-doped Ta2O5 (a)-(d) and N-Ta2O5 (e)-(n). For the clarity of the

positions of doped nitrogen atoms and defects of oxygenated species, the composition of each plane is shown in the figure, and the locations of OH vacancies (OHv) are shown as dashed circles in (k) and (l). Figure 4

Phase diagrams of non-doped and N-doped Ta2O5 bulk models at 298K (I) and 848 K (II).

The symbols (a)−(i) indicate the models shown in Fig. 2 and Table S1 in Supporting Information. Figure 5

Partial densities of states (PDOSs) of non-doped and N-toped Ta2O5 models: (a) 35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

non-doped, (e) Nsub, (f) Ov+Nsub, (g) Hint+Nsub and (i) Nint. Meanings of these symbols are described in Sections 2.1 and 2.2 and are tabulated in Table S1 in Supporting Information. Figure 6

Imaginary parts (ε2) of frequency-dependent dielectric functions of non-doped and

N-toped Ta2O5 models: (a) non-doped, (e) Nsub, (f) Ov+Nsub, (g) Hint+Nsub and (i) Nint. Figure 7

Schematic of N-doping and formations of O-defects (I) and OH defects (II). Black circles

indicate electrons, and white circles are the holes. The figure indicate the partial bulk and surface systems. Figure 8

Phase diagrams of non-doped and N-doped Ta2O5 surface models at 298K and 848 K.

Diagrams (I) and (II) are the results on the surfaces with homogeneously doped N atoms, and diagrams (III) and (IV) are the results on the surfaces with surface segregations of N atoms. In all results, ∆G[NH3] was fixed at 0 eV. The symbols (a)−(m) indicate the models shown in Fig. 3 and Table S2 in Supporting Information. Figure 9

Partial densities of states (PDOSs) of non-doped Ta2O5 and N-doped Ta2O5 surfaces

shown in Fig. 3 and Table S2 in Supporting Information. Figure 10

Theoretically-obtained energy alignments of non-doped Ta2O5, N-doped Ta2O5

(Ov+Nhsub(f)) and Ru-complexes (a)-(d) shown in Fig. 1. LUMO levels indicate the redox potentials calculated by Eq. (5) described in Section 2.2. Figure 11

Calculated excess Bader charges in non-doped and N-doped Ta2O5 surface models: (a)

non-doped, (f) Ov+Nhsub and (k) OHv+Nhsub. Gray squares indicate the surface layers, and the black 36


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squares indicate the inner layers. Definitions of the surfaces are summarized in Table S2 in Supporting Information. Figure 12

Local potentials in the non-doped and N-doped Ta2O5 surface models: (a) non-doped, (f)

Ov+Nhsub and (k) OHv+Nhsub.

37



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Figure 1

38


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Figure 2

39



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Figure 3

40


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Figure 4

41


(a) NonNon-doped

1.0 0.5 0.0

1.5

0 ε - εF / eV

(g ) Hint+Nsub

1.0 0.5 0.0 -10

-5

0

ε - εF / eV

5

(e) Nsub

1.5 1.0 0.5

A

0.0

5

-10

PDOS / eV-1 atom-1

2.0

-5

2.0

2.0

-5

0 ε - εF / eV

5

1.5

(f) O v+Nsub

1.0 0.5 0.0 -10 Total N (2p) O (2p) Ta (5d)

1.0 0.5

B

C

0.0 -10

2.0

-5

0

-5

0

ε - εF / eV

(i ) N int

1.5

Page 42 of 50

PDOS / eV-1 atom-1

1.5

PDOS / eV-1 atom-1

2.0

-10

PDOS / eV-1 atom-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PDOS / eV-1 atom-1


5

ε - εF / eV

Figure 5

42


5

Page 43 of 50

10

(a) nonnon-doped (f) Ov+Nsub (i) Nint

8

(e) Nsub (g) Hint+Nsub

6

ε2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 2 0 100

200

300

400

λ / nm

Figure 6

43


500


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(I)

O 2-

Ta2O5

Ta5+

N-Ta2O5 without defects

Page 44 of 50

O 2-

O2 -

Ta5+

Ta5+

O2 -

O 2-

O 2-

O2 -

O2 -

O2-

O 2-

O2 -

Ta5+

Ta5+

O2 -

O2-

O2-

Ta5+

N2 -

O 2-

O 2O2 -

N 2-

Ta5+

Ta5+ O 2-

Defect formation O 2-

O 2-

Ta5+

Ta5+

O 2-

O2 -

O 2-

O 2-

H+

H+

H+

H+

O 2-

O2 -

O 2-

Ta5+

Ta5+

O2 -

O2-

O 2-

H+

H+

H+

H+

O 2-

O2-

O 2-

O2 -

Ta5+

Ta5+

O 2-

O2 -

O2-

H+

H+

O 2-

O2-

O 2-

Ta5+

Ta5+

O 2-

O2 -

O2 N-Ta2O5 with OO-defects Ta5+

(II) OHOH-terminate Ta2O5

OHOH-terminated N-Ta2O5 without defects

OHOH-terminated N-Ta2O5 with OHOH-defects

N2 -

O2Ta5+

Ta5+

Ta5+

O2-

N 2-

N 2-

O 2-

O2N 2-

O2 -

Ta5+

Ta5+ O2 -

N2-

Ta5+ O 2-

Defect formation H+ H+ O2 N2-

Figure 7

44


Ta5+ O 2-

Page 45 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8

45


-10 -8

-6

-4

A

-4

1.5

(i) Ov+Nssub

0.5

(l) OHv+Nssub

1.0

1.5 1.0

0.5

-10 -8 -6 -4 -2 ε / eV (vacuum scale)

1.5

(j) Hint+Nssub

1.0 0.5 0.0

-10 -8 -6 -4 -2 ε / eV (vacuum scale) 1.5 (n) Nsint 1.0

0.5

0.0

-10 -8 -6 -4 -2 ε / eV (vacuum scale)

-10 -8 -6 -4 -2 ε / eV (vacuum scale) (m) Nhint

0.0

0.0

PDOS / eV-1⋅atom-1

-10 -8 -6 -4 -2 ε / eV (vacuum scale)

0.5

0.5

-2

0.0

0.0

1.5

-6

0.5

0.0 -10 -8 -6 -4 -2 ε / eV (vacuum scale)

(g) Hint+Nhsub

1.0

1.0

ε / eV (vacuum scale)

1.0

1.0 0.5

-10 -8

-2

(h) Nssub

A

0.0



1.5

0.5

1.5

(f) Ov+Nhsub


0.0

1.0

1.5

Page 46 of 50


0.5

(e) Nhsub


1.0

1.5



(a) nonnon-doped


1.5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



0.0 -10 -8

-6

-4

-2


Figure 9

46


-10 -8

-6

-4

-2

ε / eV (Vacuum scale) 1.5

(k) OHv+Nhsub

1.0 0.5 0.0 -10 -8 -6 -4 -2 ε / eV (vacuum scale) Total N (2p) O (2p) Ta (5d)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electron energy ε / eV (vacuum scale)

Page 47 of 50

RuRuTa2O5 N-Ta2O5 complexes -3.3 -3.7 -4.0 -4.3

CBM

-5.5

-8.0

CBM

LUMOs

VBM

VBM

Figure 10

47


(a) (b) (c) (d)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11

48


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Figure 12

49



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Graphical abstract

50


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Upward Shift in Conduction Band of Ta2O5 Due ... - ACS Publications

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