Upward Shift in Conduction Band of Ta2o5 Due to Surface Dipoles Induced by N‑Doping † ‡ † † ‡ Ryosuke Jinnouchi,*, Alexey V

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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*, † Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ‡ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

*S Supporting Information

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 nondoped 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 is not high enough to compensate positive charges induced at the surface defects, the remaining 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 photogenerated 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.

1. INTRODUCTION The photocathode catalyst is a key material in the ffi Artificial photosynthesis under visible light to produce organic photosynthesis device. To achieve e cient and selective species is an important energy conversion method to resolve conversion of CO2 to the desired product, formic acid in the 1−9 above reaction R3, the photocatalyst requires efficient electron the fossil fuel shortage and global warming problems. One 10−12 13−19 of the promising methods to realize artificial photosynthesis is injections and selective catalytic conversions. The Z-scheme,2,8 where two semiconductor electrodes are used to former is driven by the suitable energy alignment between activate two half-cell redox reactions. In the photosynthesis semiconductor and MCE; the LUMO level must be higher than 2 − fi the redox level of the CO2 reduction, i.e., 4.4 eV in vacuum device proposed by Sato et al., a semiconductor modi ed with 20 metal-complex electrocatalyst (SC/MCE) used as a photo- scale for the case of reaction R1, and the energy level of the conduction band minimum (CBM) must be further higher than cathode activates the following CO2 reduction, the LUMO level to make the electron injections possible.10,18 It

Downloaded via UNIV OF SOUTHERN CALIFORNIA on November 8, 2019 at 00:12:43 (UTC). +− See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 2CO2 ++→ 4H 4e 2HCOOH (R1) should be also noted that the band gap of the semiconductors must be narrow enough to make the solar light available for the 18,21,22 while a Pt loaded TiO2 semiconductor used as a photoanode electron excitations. To meet these requirements, a wide − activates the following oxygen evolution reaction, variety of semiconductors has been developed,2,18,21 23 but +− little is known on the mechanisms dominating the energy 2HO22→+ O 4H + 4e (R2) alignments. A typical example is shown by N-doped Ta O (N-Ta O ) In the photocathode semiconductor, such as InP, GaP, and N- 2 5 2 5 modiﬁed with Ru complexes, which is the ﬁrst photocathode doped Ta2O5, excited electrons are injected from the 18 conduction band of the semiconductor to the LUMO of utilized for selectively reducing CO2 under visible light. As shown in Figure 1, redox levels of Ru complexes, [Ru- MCE, such as Ru complex, and the injected electrons 2+ ′ (bpy)2(CO)2] (bpy: 2,2 -bipyridine), [Ru(dcbpy)(bpy)- participate in the CO2 reduction reaction R1.Inthe 2+ ′ ′ (CO)2] (dcbpy: 4,4 -dicarboxy-2,2 bipyridine), [Ru- photoanode TiO2, photogenerated holes oxidize water 2+ ′ molecules to evolve oxygen molecules through reaction R2. (dcpby)2(CO)2] , and [Ru(dpbpy)(Cl)2CO)2] (dpbpy: 4,4 - By combining the two semiconductor electrodes, the following net photosynthesis reaction is realized: Received: July 17, 2015 Revised: November 6, 2015 2CO22+→ 2H O 2HCOOH + O 2 (R3) Published: November 9, 2015

diphosphonate-2,2′-bipyridine), are −3.8 to −3.5 eV in vacuum − 24 scale, while the CBM of nondoped Ta2O5 is 4.03 eV. Hence, the conduction band of the nondoped 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 nondoped Ta2O5 is used. When N-Ta2O5 with 8.9 atom % of nitrogen is used, 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 plays a key role in realizing 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 24 by Chun et al. indicated that the CBMs of TaON and Ta3N5 are not significantly different from that for the nondoped 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 Figure 1. Experimentally measured alignments of conduction band semiconductor surfaces are calculated and compared with the minimum (CBM) and valence band maximum of Ta O , N-doped 2 5 theoretically obtained redox levels of Ru complexes as well as Ta2O5 (N-Ta2O5), and redox levels (LUMO levels) of four Ru ff complexes a−d. Small white spheres are H atoms, medium gray experimentally obtained energy alignments, and the e ects by spheres are C atoms, medium blue spheres are N atoms, medium red N-doping are discussed. spheres are O atoms, medium orange spheres are P atoms, medium light green spheres are Cl atoms, and large green spheres are Ru 2. COMPUTATIONAL METHOD atoms. 2.1. Models. Nondoped Ta2O5 and N-doped Ta2O5 bulk and surface models including several types of N-dopants,

− − ﬁ ﬁ Figure 2. Bulk models of nondoped Ta2O5 (a d) and N-Ta2O5 (e) (i). Upper gures show the top views, and lower gures 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.

26926 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925−26936 The Journal of Physical Chemistry C Article

− − Figure 3. Surface models of nondoped Ta2O5 (a d) and N-Ta2O5 (e n). For the clarity of the positions of doped nitrogen atoms and defects of fi oxygenated species, the composition of each plane is shown in the gure, and the locations of OH vacancies (OHv) are shown as dashed circles in panels k and l. defects and impurities were examined in this study. Symbols in Figure 2b−d were determined by examining the stability of denoting the constructed models are tabulated in Tables S1 and all nonequivalent defect and impurity sites by the DFT S2 in section A in the Supporting Information, and their calculations as summarized in Tables S4 and S5. structures are summarized in Figures 2 and 3. The details of the As discussed in section 3.1, the calculations on the bulk λ- constructed models are described in following subsections. Ta2O5 indicated that Ta2O5 without Ov and Hint are stable in 2.1.1. Bulk and Surface Models of Ta2O5. Ta2O5 is known practically important environmental conditions of the sample 19,21 to transform from tetragonal phase to orthorhombic phase at preparations and the CO2 reductions. The Ta2O5 surface ° 25−27 λ 1350 C, and the experimentally used Ta2O5 was model should be, therefore, constructed from the bulk -Ta2O5 confirmed to be the orthorhombic phase by X-ray diffraction 18,21 without the defects and impurities. However, there still measurements. The semiconductor surface model should remained arbitrariness in the surface terminations which can be, therefore, constructed on the basis of the orthorhombic depend sensitively on environments surrounding the surface. phase. The orthorhombic phase, however, has a complex crystal Under slightly humidified conditions as in actual experimental structure, where 55 Ta atoms and 22 O atoms are included in 18 28 conditions, similarly to surfaces of other transition metal its primitive lattice, and locations of several O atoms are not 41,42 fi fi oxides, such as TiO2, the metal oxide surface is likely identi ed as shown in Figure S1. Simpli cations are, therefore, terminated by hydroxyl groups as shown in Figure 3a. The necessary to construct computationally tractable surface expectation is supported by our DFT results summarized in models. Similar problems were also reported in past theoretical fi section 3.2, where OH-terminated Ta2O5 surface is shown to be studies, and several simpli ed bulk Ta2O5 models were 29−39 thermodynamically more stable than Ta2O5 surfaces with other suggested. Among four representative bulk Ta2O5 models, λ 39 terminations, such as H-, O-, and Ta-terminations shown in the -Ta2O5 model shown in Figure 2a was chosen in this study because it reproduces both lattice structures and Figures 3b, 3c, and 3d, respectively. The Ta2O5 electrode electronic structures accurately as shown in section 3.1. surface was, therefore, modeled by the OH-terminated Ta2O5(001) surface slab. The slab thickness was set as 4 Detailed comparisons among the four bulk Ta2O5 models are × described in section B in the Supporting Information. atomic layer, and the 1 1 periodicity was imposed to the constructed surface model. Although the system size is As discussed in past studies on Ta2O5 and other transition metal oxides,34,38,40 oxygen defects and hydrogen impurities can relatively small, the constructed surface models based on the λ exist in the Ta O bulk. Their stabilities and effects on the low-energy high-symmetry -Ta2O5 bulk model correctly 2 5 − electronic structure were also examined by the DFT describe the formal oxidation states of +5 and 2 for Ta and calculations. The defects and impurities examined in this O atoms, respectively, and the experimentally observed study are the oxygen vacancy (Ov) shown in Figure 2b, triangular lattice symmetry of the Ta sublattice. The use of interstitial hydrogen impurity (Hint) shown in Figure 2c, and the high-symmetry surface models combined with the highly combination of them (Hint+Ov) shown in Figure 2d. The accurate but computationally demanding hybrid functional × models were constructed by introducing one Hint into the 1 1 method described in section 2.3 is expected to provide essential × λ × ff 1 -Ta2O5 bulk model and by removing one Ov from the 1 electronic e ects by the N-doping within a modest computa- × λ 1 2 -Ta2O5 bulk model. The locations of Ov and Hint shown tional time.

26927 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925−26936 The Journal of Physical Chemistry C Article − 2.1.2. Bulk and Surface Models of N-Ta2O5. The atoms: a homogeneous distribution (Figures 33e 3g, 3k, and fi − experimentally used N-Ta2O5 was con rmed to have the 3m) and distribution with surface segregation (Figures 3h 3j, orthorhombic structure with lattice constants close to the 3l, and 3n). Although other distributions may exist in actual N- nondoped Ta2O5, and X-ray photoelectron spectroscopy (XPS) Ta2O5, the comparison of the DFT results between these two measurements indicated that an N1s peak at 396 eV evolves distributions highlights the essence of the mechanism of the with the increase in the amount of the doped nitrogen CBM and VBM energy rise, as discussed in section 3.2.2. The atoms.18,21 These experimental results indicate that most of the comparison presents the key factors allowing one to understand nitrogen atoms are introduced substitutionally at the oxygen the properties of the other distributions that are not explicitly sites. The doped N atoms were, therefore, modeled as the demonstrated in this study. The homogeneously doped h h substitutional nitrogen atoms (Nsub). The N-doped Ta2O5 was nitrogen atoms are denoted as N sub and N int, and the nitrogen s s constructed by substituting one oxygen atom in the 1 × 1 × 1 atoms segregated on the surfaces are denoted as N sub and N int λ bulk -Ta2O5 model with one nitrogen atom. The resulting later in this article. concentration of nitrogen atoms corresponds to 7.1 atom %, Similarly to the OH-terminated Ta2O5(001) surface slab which is within the experimental range of 2−9 atom %. The model, the slab thickness was set as 4 atomic layer, and the 1 × location of Nsub was determined as the lowest energy site in the 1 periodicity was imposed. For the slabs with the homogeneous λ fi -Ta2O5 bulk model as summarized in Table S6 and Figure S4. nitrogen distribution, their middle two Ta2O3 planes were xed The obtained lowest energy structure is shown in Figure 2e. at positions determined from the calculations on N-doped Similarly to the case of the nondoped Ta O bulk, energetics of Ta2O5 bulks, while for the slabs with the surface segregations, 2 5 fi Ov,Hint, and Hint+Ov formations in the N-Ta2O5 bulk were also their middle two Ta2O3 planes were xed at positions examined by the DFT calculations. The same numbers of H determined from the calculations on the nondoped Ta2O5 int ff and Ov as those in the nondoped Ta2O5 were introduced into bulk. A test calculation to examine the e ects by the constraints the N-Ta2O5 after examining the stability of their nonequivalent on the middle two Ta2O3 planes was carried out on the N- locations as summarized in Tables S7 and S8 and Figures S5 doped Ta2O5 slab with Ov and Nsub (Ov+Nsub), structure of and S6. The determined structures are shown in Figures 2f−2h. which especially deformed during the relaxation, and the result Although the experimentally measured lattice constants and indicated that the significant features discussed in this article are 18,21 ff XPS indicated that Nsub is the major dopant, the XPS also not a ected by this constraint as shown in section D in the showed a broad N1s peak at 400 eV which is likely attributed to Supporting Information. the small amount of interstitially doped N atoms (Nint). Their 2.1.3. Molecular Models of Ru Complex. Calculations were energetics and effects on the lattice constants and electronic executed on redox levels of four Ru complexes shown in Figure structures were also examined by the DFT calculations. 1 isolated in the bulk solution. Although actual Ru complexes Similarly to the case of Nsub, one Nint was introduced into are bound to the Ta2O5 surfaces through their bpy ligands, the × × λ the 1 1 1 bulk -Ta2O5 model. The resulting concentration redox levels of free Ru complexes are considered to be one of of N is 6.7 atom %, which is also within a range of the the essential physical properties dominating the electron int 18 experimental concentration. Similarly to other defects and injections as discussed in past studies. The redox levels dopants, the most stable site was determined by the DFT were, therefore, examined by the DFT calculations. In the calculations to examine the stability of nonequivalent interstitial calculations, the solution was modeled by a continuum nitrogen sites as summarized in Table S9 and Figure S7. The solvation model described by a polarizable continuum model determined structure is shown in Figure 2i. (PCM), and the Ru complexes were placed in the continuum 43 As discussed in section 3.1, DFT calculations on the N- medium. Ta O bulk models as well as the past experiments18,21 2.2. Physical Properties Calculated by DFT. DFT 2 5 44 indicated that Ov+Nsub and Hint+Nsub are dominant types of methods were used to obtain band gaps and optical N dopants, and therefore, the N-Ta2O5 surface models properties of the bulk systems, and energy alignments of including the Ov+Nsub and Hint+Nsub as well as Nsub were CBMs and VBMs of the surface systems. Besides those physical constructed as shown in Figures 3e−3j and examined by the properties, calculations were also executed on free energies DFT calculations. The detailed analysis on their electronic forming the N-Ta2O5 bulk and surface models from the structures indicated that the substitutional N atoms are nondoped Ta2O5 bulk and surface models, respectively, to examine their stabilities. The redox potentials of the stabilized by introducing Ov and Hint because unstable unpaired electrons formed at the doped N atoms are removed by experimentally used Ru complexes were also calculated to electron donations by their introductions. The same phenom- compare them with the theoretically obtained CBMs and VBMs of the Ta O surfaces. The chemical processes and enon can also occur when OH vacancies (OHv) are created on 2 5 equations used for obtaining the free energies and redox the surface. Hence, N-Ta2O5 surface models including the potentials are explained below. OHv+Nsub as shown in Figures 3k and 3l were also constructed and examined. 2.2.1. Formation Free Energies of N-Ta2O5 Bulks and Although most of the nitrogen atoms were judged to be Surfaces. In experiments, an ammonia treatment or sputtering in a N2/Air atmosphere was adopted to dope nitrogen atoms substitutionally doped on the basis of both experimental and 18,21 theoretical results, a small amount of interstitially introduced into Ta2O5. Under the former conditions, N-Ta2O5 is likely nitrogen atoms can also exist on the surfaces as described formed through the following reaction: previously. Surface models including the N were, therefore, int +→ + constructed as shown in Figures 3m and 3n, and the effects of TaO25xz NH 3 NHTaOxy 2(5)− z HO 2 Nint on the electronic structure were also examined. +−− fi [(3/2)xy ( /2) z ]H2 (R4) For each of ve types of N-dopants, Nsub,Ov+Nsub,Hint+Nsub, OHv+Nsub, and Nint, two types of nitrogen distribution were Hence, DFT calculations were carried out on the reaction free ff considered to examine the e ects by the distribution of N energies of reaction R4 forming the N-Ta2O5 bulk and surface

26928 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925−26936 The Journal of Physical Chemistry C Article − models from the nondoped Ta2O5 bulk and surface models, R5 R8) states, respectively. The Gibbs free energies are respectively, to examine the stability of the constructed N- described by eq 2, and the used Hn and entropy Sn are tabulated Ta2O5 bulk and surface models. The reaction free energy of in Table S10. reaction R4 is described as 2.3. Computational Parameters. Calculations on the Ta2O5 and N-Ta2O5 bulks and surfaces were executed using the ΔGG=+[Nxy H Ta2(5) O− z ] zG [H 2 O] ab initio density functional code Vienna Ab initio Simulation Package (VASP)48,49 using plane wave basis sets50 with an +−−−[(3/2)xyzGG (1/2) ] [H ] [Ta O ] 225 energy cutoff of 400 eV and projector augmented wave (PAW) 51 − xG[NH3 ] (1) method. Band gaps, optical properties, and energy alignments were obtained by self-consistent field (SCF) calculations using − where G indicates the Gibbs free energy of the system denoted the Heyd−Scuseria−Ernzerhof functional (HSE06)52 54 on in the square brackets and was approximately calculated as a optimized structures. The structural optimizations of the bulk sum of the total energy Etot at 0 K and enthalpy and entropy systems were carried out using the HSE06 functional, while for − contributions Hn TSn from the nuclear motions on the basis 45 the surface systems, the optimizations were carried out using of the ideal gas model and harmonic oscillator model as the generalized gradient approximation with Perdue−Burke− follows: Ernzerhof (GGA-PBE) functional55 to decrease the computa- GE≅−tot+ H n TS n (2) tional cost. To avoid unphysical relaxations of the slabs, middle two atomic layers were fixed during the structural optimiza- The used Hn and entropy Sn are tabulated in Table S10. tions. Besides the physical properties described above, the fi On the basis of this simpli ed model and equation, the Gibbs reaction free energies ΔG of reaction R4 forming the (N- free energies of NH ,H, and H O are approximated by the 3 2 2 )Ta2O5 bulk and surface models were executed as described in equation section 2.2. The calculations were executed using the GGA- PBE functional. In the GGA-PBE calculations, Brillouin zone GG[s]≅+Δ=0322 [s] G [s] (s NH , H , or H O) (3) integrations were executed by using Monkhorst−Pack k-point 0 56 × × × ΔGkTpp[s]= ln / meshes of 4 3 6 for the (N-)Ta2O5 bulks with the 1 1 B ss (4) × × × × 1 periodicity, 4 3 3 for the (N-)Ta2O5 bulks with the 1 × × × where ps indicates the partial pressure of the species s, and G0 1 2 periodicity, and 5 4 1 for the (N-)Ta2O5 surfaces 0 indicates the Gibbs free energy at the pressure p s that was set with the 1 × 1 periodicity. The same k-point meshes were used as 0.1 MPa. ΔG, therefore, varies with changes in the partial for the HSE06 calculations on the bulks, while for the surfaces, pressure ps, indicating that the stable phase depends on the the k-point mesh was reduced to 2 × 2 × 1 to decrease the gaseous compositions surrounding the N-Ta2O5. In other computational cost. Vacuum layers with a thickness of 20 Å words, by exploring the bulk and surface phases giving the were placed between the slabs to avoid the interactions among lowest ΔG with changing the Gibbs free energies ΔG[s] (s = the repeated slabs, and dipole corrections57,58 were applied to NH3,H2,orH2O), stable phases can be determined. It should eliminate the long-ranged electrostatic interactions among the Δ Δ be noted that G[H2] and G[H2O] are probably negative in repeated slabs. the fabrication conditions where only the nonpressurized NH3/ DFT calculations on the Ru complexes were executed by Ar gas was introduced into the reactor vessel. Hence, stable using Gaussian09 package.59 The HSE06 functional was used in types of N-dopants can be determined as bulk and surface these calculations, too. 6-31G** basis sets60,61 were used for H, phases appearing in the third quadrant in the phase diagram C, N, O, P, and Cl atoms, and LANL2DZ basis set62 and Δ − Δ mapped on a G[H2] G[H2O] plane. effective core potential were used for Ru atoms. A polarizable 2.2.2. Redox Reactions of Ru Complexes. Redox reactions continuum model using the integral equation formalisms of the four Ru complexes are described as follows: variant (IEFPCM)43 was used to describe the experimentally 18 2+− + used acetonitrile solution. [Ru(bpy)22 (CO) ]+→ e [Ru(bpy)22 (CO) ] (R5) 3. RESULTS [Ru(dcbpy)(bpy)(CO) ]2+−+ e 2 3.1. Ta O and N-Ta O Bulks. 3.1.1. Phase Diagrams of + 2 5 2 5 → [Ru(dcbpy)(bpy)(CO)2 ] (R6) Ta2O5 and N-Ta2O5 Bulks. Before discussing the theoretically obtained surface properties of the Ta O and N-Ta O , their 2+− + 2 5 2 5 [Ru(dcpby)22 (CO) ]+→ e [Ru(dcpby)22 (CO) ] bulk properties, such as phase diagrams, lattice constants, (R7) electronic structures, and optical properties, are shown and − compared with the experimental results. [Ru(dpbpy)(CO)22 (Cl) ]+ e Figure 4 shows the theoretically obtained phase diagrams at − 848 and 298 K. The N-Ta2O5 was synthesized at the former → [Ru(dpbpy)(CO)22 (Cl) ] (R8) 21 temperature, and the synthesized N-Ta2O5 was used as a A conventional statistical thermochemistry method was used photosensitizer at the latter temperature.18 The Gibbs free 46,47 fi to obtain their redox potentials Uredox in the vacuum scale. energy G[NH3]ofNH3 was xed at that at 0.1 MPa when the In this method, Uredox is calculated by using the Gibbs free phase diagrams were calculated. energies of the Ru complexes before and after the electron The calculated phase diagram indicates that the nondoped transfers as follows, Ta2O5 bulk without any defects and impurities corresponding to phase (a) is stable in a wide free energy range including Uredox =−([Red]GG − [Ox])/e (5) ≈− atmospheric pressures of H2 and H2O (e.g., G[H2] 0.3 eV × −6 where Red and Ox indicate reductant (right-hand side in corresponding to 5 10 MPa for H2 gas in air and G[H2O] reactions R5−R8) and oxidant (left-hand side in reactions ≈−0.1 eV corresponding to 3.5 × 10−3 MPa for vapor in

26929 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925−26936 The Journal of Physical Chemistry C Article

Table 1. Experimental and Theoretical Lattice Constants (Å) and Band Gaps (eV) of Nondoped and N-Doped Ta O a 2 5 Bulks

type lattice constants band gap Calculated (a) nondoped a = 6.17, b = 40.07, c = 3.77 3.7 (a = 6.21, b = 40.38, c = 3.80) (2.1)

(b) Ov (a = 6.19, b = 39.95, c = 3.82) (1.3)

(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 2.0 (a = 6.21, b = 40.69, c = 3.80) (0.24)

(f) Ov+Nsub a = 6.23, b = 40.51, c = 3.73 3.2 (a = 6.25, b = 40.77, c = 3.76) (2.1)

(g) Hint+Nsub a = 6.18, b = 40.54, c = 3.80 3.9 (a = 6.22, b = 40.78, c = 3.83) (2.4)

(h) Hint+Ov+Nsub a = 6.23, b = 40.26, c = 3.75 0.3 (a = 6.26, b = 40.57, c = 3.78) (0.0)

Figure 4. Phase diagrams of nondoped and N-doped Ta2O5 bulk (i) Nint a = 6.39, b = 40.84, c = 3.80 2.4 models at 298 K (I) and 848 K (II). The symbols (a)−(i) indicate the (a = 6.39, b = 41.00, c = 3.83) (0.6) models shown in Figure 2 and Table S1. Experimentalb

nondoped Ta2O5 a = 6.16, b = 40.30, c = 3.89 3.8 N-Ta O (8.9 atom % of N) a = 6.20, b = 40.26, c = 3.78 2.4 63 2 5 equilibrium with liquid water at 298 K). In more negative aThe theoretical lattice constant b was calculated by multiplying the regions of G[H2] and/or G[H2O], four types of N dopants and lattice constant of the simplified model with the 1 × 1 × 1 periodicity defects appear in the phase diagrams: substitutional nitrogen by 11/2. Definitions of the theoretical models are listed in Table S1. atoms with interstitial hydrogen impurities (Hint+Nsub) (g), Calculated data in the parentheses indicate the results obtained by the substitutional nitrogen atoms with oxygen defects (Ov+Nsub) GGA functional, and other calculated data indicate the results obtained b (f), interstitial nitrogen atoms (Nint) (i), and substitutional by the HSE functional. Taken from refs 18 and 21. nitrogen atoms and interstitial hydrogen impurities plus oxygen defects (Hint+Ov+Nsub) (h). Among the four phases, the Δ the nondoped Ta O bulk model agree well with the Hint+Ov+Nsub phase (h) appears only when G[H2] is highly 2 5 positive, indicating that this phase is stable only when the experimental ones, and the lattice constants of the N-Ta2O5 models e−h other than the N -doped Ta O model (i) are partial pressure of H2 is raised. This situation does not occur in int 2 5 close to those of the nondoped Ta O similarly to the any experimental conditions, and therefore, the Hint+Ov+Nsub 2 5 phase is omitted from the evaluations. experiments. The lattice constant a of the Nint-doped Ta2O5 model is significantly larger than the experimental result, The Hint+Nsub (g) and Ov+Nsub (f) phases appear in the low Δ indicating that the N phase is not dominant in the N-Ta O . G[H2O] region, while the Nint (i) phase appears in the low int 2 5 Δ Calculated partial densities of states (PDOSs) of the G[H2]region.TheHint+Nsub and Ov+Nsub phases are Δ nondoped Ta O and N-Ta O bulk models are summarized stabilized by decreasing the water activity G[H2O] because 2 5 2 5 in Figure 5, and theoretical and experimental band gaps are H2O is generated by the following reactions forming those phases: summarized in Table 1. The imaginary parts of the frequency- dependent dielectric functions are also shown in Figure 6. The NH3410492+→ Ta O HNTa O + H O (R9) theoretical band gap of the nondoped Ta2O5 agrees well with the experimental one. The band gap is significantly narrowed 2NH3+→ Ta 8 O 20 N 2 Ta 8 O 17 + 3H 2O (R10) when Nsub and Nint are introduced into Ta2O5 as indicated by Similarly, the Nint phase is stabilized by decreasing the H2 (e) and (i), while the band gap of N-Ta O with N is Δ 2 5 sub activity G[H2] because H2 is generated by the following widened again when Hint and Ov are additionally introduced as reaction forming the Nint phase: indicated by (f) and (g). The similar trend also appears in the dielectric function, where the trend in the absorption edge is NH34104102+→ Ta O NTa O + (3/2)H (R11) summarized as Hint+Nsub

26930 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925−26936 The Journal of Physical Chemistry C Article

Figure 5. Partial densities of states (PDOSs) of nondoped and N-toped Ta2O5 models: (a) nondoped, (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.

ε Figure 6. Imaginary parts ( 2) of frequency-dependent dielectric functions of nondoped and N-toped Ta2O5 models: (a) nondoped, (e) Nsub, (f) Ov+Nsub, (g) Hint+Nsub, and (i) Nint. bonding and antibonding molecular-like orbitals of NO formed in the Ta2O5 shown as circles in Figure 2i. By the generations of those bands, 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 Figures 5f and 5g. 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 Figure 7I 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 Figure 7. Schematic of N-doping and formations of O-defects (I) and defect and impurity formations. By this mechanism, the Nsub OH defects (II). Black circles indicate electrons, and white circles are phase disappears from the phase diagram shown in Figure 4. the holes. The figure indicates the partial bulk and surface systems. 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 stable as expected from the reported results on the TiO2 41,42 bulk, while the small amount of the isolated Nint and Nsub can surfaces. fi signi cantly contribute on the band gap narrowing. N-doped Ta2O5 surfaces are stable in more negative regions Δ Δ 3.2. Ta2O5 and N-Ta2O5 Surfaces. 3.2.1. Phase Diagrams of G[H2O] or G[H2] similarly to the bulk systems; h s of Ta2O5 and N-Ta2O5 Surfaces. The theoretically obtained OHv+N sub (k) and OHv+N sub (l) are stable in the negative Δ h phase diagrams of nondoped and N-doped Ta2O5 surfaces with G[H2O] region, and N int (m) is stable in the negative Δ and without the surface segregation of the doped N atoms are G[H2] region. Analyses on the PDOSs shown in Figure 9 h s summarized in Figure 8. Similarly to the phase diagrams of the indicate that the OHv+N sub (k) and OHv+N sub (l) are bulk systems shown in Figure 4, the nondoped Ta2O5 surface is stabilized by a mechanism similar to that described in the stable in the wide free energy range. Among the nondoped previous section. As indicated in PDOSs of the N-Ta2O5 with h s Ta2O5 surface models, the OH-terminated one (a) is especially N sub and N sub shown in Figures 9e and 9h, the introductions

26931 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925−26936 The Journal of Physical Chemistry C Article

Figure 8. Phase diagrams of nondoped and N-doped Ta2O5 surface models at 298 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, Δ ﬁ − G[NH3] was xed at 0 eV. The symbols (a) (m) indicate the models shown in Figure 3 and Table S2.

Figure 9. Partial densities of states (PDOSs) of nondoped Ta2O5 and N-doped Ta2O5 surfaces shown in Figure 3 and Table S2. of the substitutional N atoms generate unstable unoccupied surfaces are summarized in Figure 9.InTable 3, experimental bands (shown as A in the ﬁgures) below the CBM. Those and theoretical redox levels of the Ru complexes are also unoccupied bands are compensated by redistributions of tabulated, and all the calculated energy alignments are electrons located near the OHv defects to the doped nitrogen summarized in Figure 10. atoms as schematically shown in Figure 7II. The calculated CBM and VBM of the nondoped Ta2O5 Accordingly, the surface phase diagrams indicate that OHv terminated with OH adsorbates agree well with the 24 can be created on the N-Ta2O5 surface, and therefore, experimental results obtained by Chun et al., and the h s electronic alignments of OHv+N sub (and OHv+N sub) as well experimentally observed upward shifts in CBM and VBM by h s h s 18,21 as Ov+N sub (and Ov+N sub) and N int (and N int) are examined N-doping are reproduced well only by the N-Ta2O5 surface h h in the following subsection. models (f) and (k) (Ov-N sub and OHv-N sub, respectively), 3.2.2. Electronic Alignments of Ta2O5 and N-Ta2O5 where oxygen and hydroxyl defects, respectively, are introduced Surfaces. The experimentally and theoretically obtained into the surface Ta2O3 planes, and nitrogen atoms are CBMs and VBMs of the Ta2O5 and N-Ta2O5 surfaces are homogeneously doped. The redox levels of Ru complexes are tabulated in Table 2, and theoretically obtained PDOSs of all also reproduced well by the theory. Although the calculated

26932 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925−26936 The Journal of Physical Chemistry C Article

Table 2. Theoretically and Experimentally Obtained to realize the electron injections from the Ta2O5 surface to the ε Conduction Band Minimums VBM and Valence Band Ru complexes by the N-doping. Maximums ε of Ta O and N-Ta O Surfaces Shown in To clarify the mechanisms of the upward shifts of CBM and a CBM 2 5 2 5 Figure 3 VBM in the N-Ta2O5 surface models (f) and (k), charge ε ε Δε Δε distributions in the surface systems were analyzed. The resulted type VBM CBM VBM CBM Bader charges in models (f) and (k) are summarized in Figure b Experimental 11. The Bader charges in other surface models are summarized − − nondoped Ta2O5 7.9 4.0 − − N-Ta2O5 5.7 3.3 +2.2 +0.7 Calculated nondoped (a) −7.99 −4.25 h − − − N sub (e) 7.89 5.75 +0.10 1.50 h − − Ov+N sub (f) 5.51 3.67 +2.48 +0.58 h − − − Hint+N sub (g) 7.95 5.41 +0.04 1.16 s − − − N sub (h) 7.65 5.52 +0.34 1.27 s − − − Ov+N sub (i) 7.71 4.35 +0.28 0.10 Figure 11. Calculated excess Bader charges in nondoped and N-doped h H +Ns (j) −7.33 −4.01 +0.66 +0.24 Ta2O5 surface models: (a) nondoped, (f) Ov+N sub,and(k) int sub h h − − OHv+N sub. Gray squares indicate the surface layers, and the black OHv+N sub (k) 5.74 3.74 +2.25 +0.51 fi s − − − squares indicate the inner layers. De nitions of the surfaces are OHv+N sub (l) 7.36 4.52 +0.63 0.27 h − − − summarized in Table S2. N int (m) 7.17 4.61 +0.82 0.36 s − − − N int (n) 7.49 5.21 +0.82 0.95 aΔε Δε in section F in the Supporting Information. As clearly indicated VBM and CBM indicate the relative shift caused by the N-doping b by those figures, only the two surface models have prominent and defect formations. Experimental data was taken from refs 18 and fi 24. dipole moments normal to the surfaces at the rst and second Ta2O3 planes. The dipole moments are oriented to raise the Table 3. Experimentally and Theoretically Obtained Redox electrostatic potentials inside the surfaces relative to the Levels (eV in Vacuum Scale) of the Ru Complexes Shown in vacuum, and the potential rises of about 1.0 eV, which are a Figure 1 close to the theoretically obtained rises in CBM, are in fact observed in the calculated local potentials across the surfaces as molecule calcd exptlb shown in Figure 12. The upward shifts in the CBM and VBM − − − − [Ru(dpbpy) (CO)2(Cl)2] 3.34 ( 1.26) 3.5 ( 1.1) are, therefore, judged to be caused by the dipole moments 2+ − − − − [Ru(bpy)2(CO)2] 3.46 ( 1.14) 3.6 ( 1.0) formed near the surface. 2+ − − − − [Ru(dcbpy) (bpy) (CO)2] 3.89 ( 0.71) 3.7 ( 0.9) 2+ − − − − [Ru(dcbpy)2(CO)2] 3.96 ( 0.64) 3.8 ( 0.8) aThe values in the parentheses are the redox potentials scaled in SHE obtained by eq 5 described in section 2.2. bExperimental data were taken from ref 18.

Figure 12. Local potentials in the nondoped and N-doped Ta2O5 h h surface models: (a) nondoped, (f) Ov+N sub, and (k) OHv+N sub.

The mechanism of the surface dipole formations can be understood by taking account of the schemes shown in Figures 7I and 7II. As described previously, when oxygen atoms and Figure 10. Theoretically obtained energy alignments of nondoped hydroxyl groups are removed from the surfaces, unstable h − Ta2O5, N-doped Ta2O5 (Ov+N sub (f)), and Ru complexes (a) (d) unpaired electrons remain at the Ta atoms next to the defected shown in Figure 1. LUMO levels indicate the redox potentials sites. These unpaired electrons are redistributed to pair with the calculated by eq 5 described in section 2.2. unpaired electrons located at the nitrogen atoms. Hence, by concurrent formations of the N-doping and defect, positive CBMs of N-Ta2O5 (f) and (k) are still lower than the calculated charges are induced near the defect sites, and negative charges redox levels of [Ru(dpbpy)(CO)2(Cl)2]and[Ru- are induced near the nitrogen atoms. Because the concen- 2+ (bpy)2(CO)2] , unlike the experimental results as shown in trations of the doped nitrogen atoms in the surface layers of the Figure 1, probably because of the errors caused by the N-Ta2O5 models (f) and (k) are not high enough to fully constructions of the surface models, approximations in the accept the unpaired electrons from the Ta atoms next to the exchange-correlation functional, and descriptions of the defect sites, the negative charges compensating the positive solvation medium surrounding the Ru complexes, the theory charges near the defect sites in the surface layers are induced indicates that the energy alignments can be drastically changed near the nitrogen atoms not only in the surface layer but also in

26933 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925−26936 The Journal of Physical Chemistry C Article the inner layer. The dipole moments normal to the surfaces are, ■ AUTHOR INFORMATION therefore, generated in these two models. In contrast, in other Corresponding Authors models, because the defects are not created, or concentrations *E-mail: [email protected] of the doped nitrogen atoms in the surface layers are high *E-mail: [email protected]. Tel: 213-821-3116. enough, dipole moments normal to the surfaces are not generated. Notes The analyses indicate that similar dipole moments can be The authors declare no competing financial interest. generated by other distributions of Nsub,Ov, and OHv, besides those examined in the current study, by the same mechanism ■ ACKNOWLEDGMENTS illustrated in Figure 7. For example, when Nsub is introduced We acknowledge partial financial support from the Advanced only in the inner layers, and OHv and/or Ov are generated at Catalytic Transformation Program for Carbon Utilization the surface layers, similar dipole moments will be generated. (ACT-C), JST, supported by Japan. We also thank T. Hence, more generally, the CBM and VBM of the Ta2O5 Morikawa, S. Sato, and T. Suzuki in Toyota Central R&D surface can be raised by N-doping when the resulting surface Laboratories., Inc., for their fruitful discussions. A.V.A and concentration of the substitutionally doped nitrogen atoms is O.V.P. acknowledge financial support of the U.S. Department ffi not su cient to compensate the positive charges induced by of Energy, DE-SC0014429. the surface defects of the oxygenated species. Similar dipole moments can be generated when Hint is introduced at the ■ REFERENCES surface layer separately from the N in the inner layers, sub (1) Gust, D.; Moore, T. A.; Moore, A. L. 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26936 DOI: 10.1021/acs.jpcc.5b06932 J. Phys. Chem. C 2015, 119, 26925−26936