First-Principles Investigation of Optoelectronic and Redox Properties

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First-Principles Investigation of Optoelectronic and Redox Properties of (Ta1-xNbx)ON Compounds for Photocatalysis Moussab Harb J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511878g • Publication Date (Web): 13 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

First-Principles Investigation of Optoelectronic and Redox Properties of (Ta1-xNbx)ON Compounds for Photocatalysis Moussab Harb* Division of Physical Sciences and Engineering, KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), 4700 KAUST, Thuwal 23955-6900, Kingdom of Saudi Arabia

1 Environment ACS Paragon Plus


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ABSTRACT We investigate essential fundamental properties of monoclinic (Ta1-xNbx)ON (x = 0.0625, 0.125, 0.25, and 0.5) solid solution semiconductor materials for water splitting using first-principles computations on the basis of density functional theory (DFT) and density functional perturbation theory (DFPT) using the PBE and HSE06 functionals. The formation energies, band gaps, UVvisible optical absorption coefficients, dielectric constants, charge carrier effective masses and band edge energy positions of these compounds are evaluated. Similarly to TaON, our calculations reveal strongly 3D delocalized characters of the band edge electronic states through the crystal lattices, high dielectric constants, small hole effective masses along [001] direction and small electron effective masses along [100] direction. This leads to good exciton dissociation ability into free charge carriers, good hole mobility along [001] direction and good electron mobility along [100] direction. The main difference, however, is related to their band edge positions with respect to water redox potentials. TaON with a calculated bandgap energy of 3.0 eV is predicted as a good candidate for water oxidation and O2 evolution while the (Ta1-xNbx)ON materials (for 0.25 ≤ x ≤ 0.5) with calculated bandgap energies between 2.8 and 2.9 eV reveal suitable band edge positions for water oxidation and H+ reduction. These results offer a grand opportunity for these compounds to be properly synthesized and tested for solar-driven overall water-splitting reactions.

Keywords Nb-modified TaON, dielectric constant, charge carrier transport, band edge energy positions.


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1. Introduction The challenging solar-driven water splitting using powder semiconducting compounds is one of the most economical processes to produce hydrogen in large scale.1-4 In addition to the adequate band gap (< 3.0 eV) needed for the material to absorb photons in the visible region, the design of a suitable photocatalyst for overall water-splitting reactions requires three other fundamental parameters to be simultaneously satisfied: (1) high dielectric constant to obtain a good exciton dissociation in the bulk into free charge carriers; (2) strongly delocalized photogenerated charge carrier throughout the crystal structure along with small effective masses to obtain a good mobility from bulk to photocatalyst surface; (3) suitable valence band (VB) and conduction band (CB) edge energy levels with respect to water spitting limits to drive the photogenerated hole for oxidizing water and similarly the photoexcited electron for reducing H+. Transition metal oxynitrides have been invoked in the literature as active photocatalysts for H2 or O2 evolution in the presence of appropriate sacrificial reagents.2,4 In particular, TaON has received particular attention for its suitable bandgap energy estimated at 2.5 eV.5-11 TaON powders were obtained from high-temperature ammonolysis of Ta2O5 under various heating and nitridation regimes.5-13 The obtained samples were analyzed by neutron powder diffraction technique which revealed a monoclinic crystal structure (known as β-phase).13 Nevertheless, overall water splitting using TaON has not yet been achieved. It was reported that the photocatalytic properties of the prepared TaON samples varied with the preparation method (temperature, flow rate and nitridation time). This was related to the variation of the O/N ratio indicating that the prepared samples at high temperatures are not fully stoichiometric. Theoretically, a few studies11,14,15 have been reported only the density of states of TaON calculated by standard density functional theory (DFT)-GGA, -PBE and -LDA methods and



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bandgap energies of 1.8, 2.1 and 1.92 eV were obtained, respectively. Although these three methods are quite good for the structural parameters of semiconductors, they are well-known to strongly underestimate their band gaps.16-18 Advanced hybrid functionals such as HSE06 provide a much more accurate description of these bandgap energies.17-25 In the present work, we report an extended systematic theoretical study on the structural, energetic, dielectric, optoelectronic and redox properties of mixed tantalum-niobium oxynitride (Ta1-xNbx)ON (x = 0.0625, 0.125, 0.25, and 0.5) solid solution semiconducting materials as new photocatalysts for solar-driven overall water-splitting reactions. Our present study brings rational insights at the atomic level into essential fundamental properties of these materials obtained from first-principles computations based on DFT and DFPT using the PBE and HSE06 formalisms. In addition to the structural parameters and energetics of the (Ta1-xNbx)ON solids, we systematically investigated their electronic structure and optical absorption response with various Nb concentrations. Then, we calculated their high-frequency (electronic contribution) and static (ionic and electronic contributions) dielectric tensors of these compounds. Besides, we analyzed the hybridization (localized or delocalized) character of the VB and CB edge electronic states generated upon Nb incorporation and we calculated the effective mass tensors of photogenerated holes and electrons at the band edges. Moreover, we calculated the band edge energy positions of (Ta1-xNbx)ON solids with respect to the vacuum level. We stress here that the good dielectric and charge carrier transport properties as well as the suitable band positions predicted for (Ta1xNbx)ON

materials (with 0.25 ≤ x ≤ 0.5), offer a grand opportunity for these compounds to be

properly synthesized and tested for solar-driven overall water-splitting reactions.


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2. Computational Methods 2.1. Structural optimization Calculation For (Ta1-xNbx)ON structures simulation, we built the 2 × 2 × 2 monoclinic TaON supercell model containing 32 TaON functional units (96 atoms). We generated several geometrical configurations with various Nb concentrations by a direct substitution of neutral Ta atoms by neutral Nb atoms (Ta in the Wyckoff position 4e). We explored key structures showing gathered or well-separated Nb to identify the most stable systems. We simulated the structures of (Ta1xNbx)ON

with 6.25, 12.5, 25, and 50 at. % Nb occupying Ta sites or x = 0.0625, 0.125, 0.25 and

0.5, respectively (see the Supporting Information S1). The various structures were optimized using DFT as implemented in VASP program26-29 with PBE functional30 and PAW approach.31 The Brillouin zone was sampled with 3 × 3 × 3 Monkhorst–Pack k-point grid for the various studied materials.32 Our test performed with 5 × 5 × 5 k-point mesh revealed that the energy does not vary more than 0.02 meV per TaON unit. 2.2. Formation Energy Consideration To identify the thermodynamic feasibility of assembling these compounds from individual elements, we considered the following reaction:

(1 − x )Ta + xNb + 12 O 2 +

1 2

N 2 → (Ta 1− x Nb x ) ON

(1)

Then, we computed the formation energy (or reaction energy) using the following expression: K E form (1) = E 0form (1) − ∆µO − ∆µ N

(2)

0K where E form (1) is the electronic energy at 0 K and expressed by:

K E 0form (1) = Etot ((Ta1− x Nbx )ON ) − (1 − x) Etot (Ta) − xEtot ( Nb) − 12 EO2 − 12 EN2


(3)


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Equation (3) includes the total energies at 0 K of (Ta1-xNbx)ON, Ta, Nb solids in their groundstate structures and of gas phase O2 and N2 molecules. ∆ µ O and ∆µ N in equation (2) are the chemical potentials of oxygen and nitrogen that depend on the temperature (T) and pressure (p) via the enthalpy (h) and entropy (s) corrections of each gas phase molecule as follows:

∆µO , N = hO2 , N2 (T ) − TsO2 , N2 (T ) + RTLn(

p(O2 , N 2 ) ) p0

(4)

The enthalpy and entropy corrections as a function of T were obtained from DMol33 using PBE functional and DNP basis set.34 All electronic energies were calculated using VASP program. In what follows, we fixed ∆ µ O and ∆µ N at -0.22 and -0.18 eV, respectively, for T = 298 K and p(O2) = p(N2) = 1 atm (standard thermodynamic conditions). Negative or positive formation energy corresponds to stable or unstable material.

2.3. Electronic Structure, UV-Vis Absorption and Dielectric Constant Calculations Density-of-state calculations of (Ta1-xNbx)ON were investigated using the screened non-local hybrid (HSE06) exchange-correlation formalism35 implemented in VASP.26-29 The tetrahedron method with Blöchl corrections was adopted for the Brillouin zone integration (see Supporting Information S1 for more details). UV-visible absorption calculations were carried out within the density functional perturbation theory (DFPT) implemented in VASP,26-29 using the HSE06 functional.35 The optical absorption coefficient α (ω ) of each compound was calculated using the equation α (ω ) = (4π / λ )k (ω ) . ω and λ represent the frequency and wavelength of the light. k (ω ) is the extinction coefficient given by k (ω ) = {[(ε 12 + ε 22 )1 / 2 − ε 1 ] / 2}1 / 2 .36-38 ε1 (ω ) and ε 2 (ω) represent the real and imaginary parts of the frequency-dependent complex dielectric function, respectively


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(Supporting Information S1). Electronic and ionic contributions to the dielectric constants of these materials were obtained from DFPT using the linear response method implemented in VASP,26-29 with the PBE functional.

2.4. Band Edge Energy Position Calculations To evaluate the band positions of the semiconductor, we built a semiconductor-vacuum interface to provide a reference to the electrostatic potential.39,40 We first reproduced the bandgap energy of the bulk material. Then, we aligned the electrostatic potential in the solid with respect to vacuum level. The band edge energy levels of the semiconductor with respect to vacuum level were hence obtained by subtracting the vacuum energy from the band edge energies obtained using the slab calculation. For TaON–vacuum interface modelling, we selected the (001) surface by generating the (2ܽറ × 1ܾሬറ) slab. A crystal thickness of 15 Å (6 atomic layers) and a vacuum thickness of 15 Å provided a good reproduction of the bulk properties of this material (see Supporting Information S1 for more details). To determine the energy of vacuum level, we computed the averaged electrostatic potential of the (001) TaON surface using DFT and HSE06 functional as defined in VASP.26-29 Note that the dipole corrections to the local potential were also added to avoid such possible error induced by the periodic boundaries.41

3. Results and Discussion 3.1. Structural Parameters and Energetics To elucidate the Nb preferred structural location in TaON monoclinic lattice, the energetics of the various explored (Ta1-xNbx)ON structures with respect to TaON is discussed. The lowestenergy structures of (Ta1-xNbx)ON materials with x = 0, 0.0625, 0.125 and 0.5 are given in



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Supporting Information (Figure S2). Their formation energy together with the lattice parameters are reported in Table 1. For TaON, our calculated lattice parameters are found to be in excellent agreement with the available experimental data.11 The crystal structure of this material is composed of edge-sharing irregular TaO3N4 polyhedra (Figure S2a). Each Ta is coordinated to three O (that are three-fold coordinated) and four N (that are four-fold coordinated) with Ta-O bond lengths ranging from 2.03 to 2.15 Å and Ta-N bond lengths ranging from 2.06 to 2.13 Å. The calculated formation energy is found to be -5.542 eV, and so, this crystalline phase is stable. For (Ta1-xNbx)ON materials (with x = 0.0625, 0.125, 0.25, and 0.5) modeled by replacing 2, 4, 8, and 16 neutral Ta atoms by 2, 4, 8, and 16 neutral Nb atoms, respectively, the lowest-energy structures reveal well-separated Nb species by nearest neighbor Nb-Nb distances ranging from 5.0 to 7.23 Å. Interestingly, the various relaxed Nb positions upon geometry optimization are found to be very slightly moved with respect to the initial Ta positions in TaON (Figure S2b-e), giving very close Nb-O and Nb-N bond lengths to those obtained for Ta-O and Ta-N in TaON. This leads to very similar lattice parameters to those obtained for the monoclinic TaON phase (Table 1). The calculated formation energies of the lowest-energy (Ta1-xNbx)ON structures with x = 0.0625, 0.125, 0.25, and 0.5 are found to be -5.508, -5.463, -5.410, and -5.301 eV, and so, these compounds within the monoclinic phase are also likely to be stable (Table1). Although these materials are found 0.03, 0.07, 0.13, and 0.24 eV less stable than TaON, they should not be neglected because such slightly metastable compounds can be obtained using high-temperature conditions as adopted in the case of TaON. Other (Ta1-xNbx)ON structural configurations (with x = 0.0625, 0.125, 0.25, and 0.5) displaying Nb species separated by shorter nearest neighbor NbNb distances ranging from 3.22 to 3.95 Å, were found 0.03, 0.04, 0.05, and 0.07 eV less stable


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than the previous cases, respectively. Consequently, our calculated lattice parameters and formation energies show remarkable similarities between the pure and the Nb-modified TaON materials. These results highlight the (Ta1-xNbx)ON compounds with the monoclinic crystalline lattice as being competitive structures with that of TaON. Table 1. Formation Energies (at 298 K, in eV) and Lattice Parameters of the Various Optimized (Ta1-xNbx)ON Structures Obtained using the DFT/PBE Method. stoichiometry

structure

absolute (relative) formation energy a (Å) (eV)

lattice parameters b (Å)

c (Å)

α (º)

β (º)

γ (º)

TaON

Expt11

-

4.95

5.01

5.16

90

99.6

90

TaON

(S2a)

-5.542 (0.00)

4.97

5.03

5.18

90

99.7

90

Ta0.94Nb0.06ON

(S2b)

-5.508 (0.03)

4.96

5.03

5.18

90

99.7

90

Ta0.88Nb0.12ON

(S2c)

-5.463 (0.07)

4.96

5.03

5.18

90

99.7

90

Ta0.75Nb0.25ON

(S2d)

-5.410 (0.13)

4.96

5.03

5.18

90

99.7

90

Ta0.5Nb0.5ON

(S2e)

-5.301 (0.24)

4.96

5.03

5.18

90.1

99.7

89.8

3.2. Electronic Structure and UV-Visible Absorption Modifications with Increasing Nb Content Our calculated DOS for TaON predicts bandgap energy of 3.0 eV (Figure 1a). The upper part of VB is governed by filled N 2p orbitals with weak contributions of O 2p and Ta 5d orbitals while the deeper part is dominated by occupied O 2p orbitals, in clear relation with the lower electronegativity of O versus N. The CB of this material is mainly made by empty Ta 5d states. Its calculated absorption response shows an absorption edge at 414 nm (Figure 2a). Note that our DOS calculated with PBE gave much smaller band gap energy of 1.9 eV (Figure S4 in Supporting Information) in agreement with the previous theoretical works.11,14,15 Our predicted band gaps with HSE06 for TaO0.90N1.06 (2.7 eV) and TaO0.81N1.12 (2.5 eV) matches the



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experimental data reported on TaON powders.5-11 This result indicates that the experimentally prepared tantalum oxynitride materials might not be fully stoichiometric but slightly enriched in N, closer to TaO0.81N1.12 rather than TaON (see Supporting Information S5 and S6). In contrast, our formation energy calculations of TaO0.90N1.06 and TaO0.81N1.12 revealed metastable compounds with respect to TaON with higher formation energies of 0.30 and 0.58 eV than that obtained for TaON. Discussing now the (Ta1-xNbx)ON materials (with x = 0.0625, 0.125, 0.25, and 0.5) in their lowest-energy configurations, the band gaps predicted by HSE06 are found to be very close to that of TaON (Figure 1b-e). The DOS analysis for the compound with x = 0.0625 or 6.25% Nb also gives a bandgap energy of 3.0 eV (same as TaON) as shown in Figure 1b. Increasing the Nb content to 12.5% (x = 0.125) or 25% (x = 0.25) gives slightly narrower band gap of 2.9 eV (Figure 1c and d). A further increase in the Nb content to 50% (x = 0.5) slightly decreases the band gap to 2.8 eV (Figure 1e). Their calculated absorption spectra reveal a slight red-shift giving new absorption edges at 428 and 443 nm, respectively (Figure 2b-e). An increase in the Nb content substituting Ta sites leads to higher density of Nb 4d states and lower density of Ta 5d states located in the bottom of CB as a result of the stacking of Nb 4d states (Figure 1b-e). The contribution of Nb 4d orbitals to the bottom of CB of the compound with 25% Nb becomes slightly lower than that of Ta 5d orbitals (Figure 1d), while the bottom of CB of the compound with 50% Nb reveals the major contribution from empty Nb 4d orbitals (Figure 1e). In all cases, as expected, the upper part of VB is made by occupied N 2p orbitals similar to TaON. The lowest-energy band gaps in the compounds with 25% and 50% Nb involve also new transitions between N 2p6 orbitals and Nb 4d0 orbitals. Note that we checked the spin-orbit coupling effect on the electronic structures of (Ta1-xNbx)ON materials at the DFT/PBE level and very small


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decreases of 0.01 eV in the band gaps were obtained in all cases. Although the electronic structures of (Ta1-xNbx)ON materials (with x = 0.0625, 0.125, 0.25, and 0.5) are similar to TaON, their photocatalytic capabilities for water splitting might be different depending on their exciton dissociation ability, their charge carrier transport properties and also on their band positions relative to water redox potentials.

Figure 1. Electronic density of states (DOS) of (Ta1-xNbx)ON materials obtained at the DFT/HSE06 level: (a) 0% Nb, (b) 6.25% Nb, (c) 12.5% Nb, (d) 25% Nb, and (e) 50% Nb. Color legend: total DOS in black, partial contribution from Ta 5d orbitals in blue, Nb 4d orbitals in pink, O 2p orbitals in green, and N 2p orbitals in red. Fermi levels are defined by the black dashed lines.

Figure 2. Optical absorption coefficient spectra of (Ta1-xNbx)ON obtained at the DFPT/HSE06 level: (a) 0% Nb, (b) 6.25% Nb, (c) 12.5% Nb, (d) 25% Nb, and (e) 50% Nb.



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3.3. Dielectric Constant and Charge Carrier Effective Masses After electronic excitation, the photogenerated holes and electrons must be easily separated in the bulk and quickly transferred through two different crystallographic directions to the semiconductor surface to enable the water oxidation and reduction reactions on two different facets by avoiding the electron-hole recombination at the surface. Previous experimental works on common semiconductors used in photovoltaic devices showed that a value of 10 or more for the static dielectric constant is quite enough to obtain a good exciton dissociation into free charge carriers.42-47 They also showed that the charge carrier effective masses should be smaller than 0.5m0 (m0 is the free electron mass), at least in one crystallographic direction, to obtain a good mobility.42-47 These two values can be considered as a requirement for the exciton dissociation and the charge carrier transport properties of the semiconductor. For TaON, we calculated the three nonzero components of the static dielectric tensor along the three principal directions [001], [010] and [001] and we found 20.82, 33.71 and 22.51, respectively. These values are quite higher than 10 indicating that TaON has excellent dielectric properties. Our calculated average value of 25.68 matches well the experimental one.48 Incorporating 25% Nb at Ta sites, as in (Ta1-xNbx)ON (x = 0.25), gives slightly higher dielectric constants (22.27, 35.85 and 23.26) along the three principal directions with an average value of 27.12. A further increase in the Nb content to 50% (x = 0.5) leads again to slightly higher dielectric constants (23.65, 37.99, 24.03) along the three principal directions with an average value of 28.55. Consequently, substituting Ta by Nb in the structure of TaON, as in (Ta1xNbx)ON,

tends to slightly increase the dielectric constant, and so, their ability for exciton

dissociation into free charge carriers is expected to be slightly improved.


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As the photogenerated charge carriers utilized for water splitting are positioned on the band edges, it is hence necessary to analyze their hybridization character in order to understand their mobility tendency through the crystal structure of the semiconductor. Figure 3a-c shows the electron density maps (in the bc plane) associated with the VB edge states of (Ta1-xNbx)ON materials with 0%, 25%, and 50% Nb. In all cases, the analysis gives very similar results revealing important contributions from p-orbitals on all N species in the lattice accompanied by weak contributions from p-orbitals on all O species. Similar pictures were also obtained from the electron density analysis in the ab plane (see the Supporting Information S7 for more details). This behavior is in line with the DOS results exhibiting very similar VB edge shapes in the three materials (Figure 1). Hence, the strongly delocalized character of the VB edges of (Ta1-xNbx)ON compounds is expected to give a good ability of the photogenerated hole to migrate through their crystal structure. The electron density maps (in the ab plane) associated with the CB edge states with 0%, 25%, and 50% Nb, are shown in Figure 3d-f. For TaON (0% Nb), the analysis reveals strong contributions from d-orbitals on all Ta species in the lattice. Similarly, the analyses of the compounds with 25% and 50% Nb show important contributions from d-orbitals on all Nb and Ta species in the lattices. Note that similar pictures were also obtained from the electron density analysis in the bc plane (see the Supporting Information S7 for more details). This trend is also in line with the DOS results revealing very similar CB edge shapes in the three compounds (Figure 1). As a consequence, substituting Ta by Nb in the structure of TaON, as in (Ta1-xNbx)ON, tends to keep the strong 3D delocalization character of the photoexcited electron, and so, its ability to migrate through their crystal structure is expected to be similar.



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Figure 3. Electron density maps for the VB (on the top) and CB (on the bottom) edge states of (Ta1-xNbx)ON materials obtained at the DFT/HSE06 level: (a) 0% Nb, (b) 25% Nb, and (c) 50% Nb. Isovalue is 0.003 au.

To obtain rational insights into the charge carrier transport properties of (Ta1-xNbx)ON materials with 0%, 25%, and 50% Nb, we computed the effective masses of photogenerated holes and electrons at their band edges using finite difference method and by considering the band structure obtained at DFT/PBE level. In all cases, the smallest hole effective mass is found along [001] direction with 0.3m0 for 0% Nb, 0.43m0 for 25% Nb and 0.48m0 for 50% Nb. In contrast, the smallest electron effective mass is found along [100] direction in all cases with 0.33m0 for 0% Nb, 0.35m0 for 25% Nb and 0.42m0 for 50% Nb. This means that the highest hole mobility is expected to be along [001] direction while the highest electron mobility is expected to be along [100] direction. As the various effective masses are relatively small (lower than 0.5m0), good charge carrier transport properties are expected along these two specific directions. Interestingly, the holes and electrons tend to migrate easily through two different crystallographic directions, and so, efficient electron-hole separation is expected at the surfaces


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of these compounds. Note that the hole effective masses along [100] and [010] directions as well as the electron effective masses along [010] and [001] directions were found to be larger than 0.5 m0, and so, lower charge carrier mobilities and poor transport properties are expected along these directions.

3.4. Band Edge Energy Positions Modification with Increasing Nb Content The photogenerated holes at semiconductor surface must oxidize water while the photogenerated electrons must reduce H+. Thermodynamically, the VB edge energy level of the semiconductor is required to be below the O2/H2O level whereas its CB edge energy level must be above the H+/H2 level. Our calculated DOS of the (001) TaON slab reveals a good bandgap reproduction of the bulk material (3.0 eV) with respective VB and CB edge energies of -1.74 and +1.26 eV (Figure 4a). Besides, our calculated electrostatic potential profile gives a vacuum energy of 6.08 eV (Figure 4b). The band edge energy positions of TaON material with respect to the vacuum level were then obtained by subtracting the vacuum energy from VB and CB edge energies obtained using the slab model. For (Ta1-xNbx)ON, we deduced the band positions from DOS results (Figure 1) based on their relative positions with respect to TaON. Discussing first TaON, our calculated CB edge energy level is found to be 0.3 eV lower than H+/H2 level as shown in Figure 5a. Its calculated VB edge energy level is found 2.07 eV lower than O2/H2O level. Therefore, TaON is predicted to be a good candidate for oxygen evolution reaction because of its unsuitable CB edge position with respect to H+/H2 level. Even if we take into account such error bars in the orbital energy positions (which is close to 0.2 eV on molecular systems), the CB edge energy position, in any case however, will not be above the H+/H2 level,



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and so, the photoexcited electrons to CB of TaON will suffer from very limited capacity for H+ reduction. In contrast, as its VB edge level is deeply located below the O2/H2O level, the holes created upon photon absorption will hence have a strong ability for water oxidation.

Figure 4. (a) Density of states (DOS) of (001) TaON slab obtained at the DFT/HSE06 level. The band gap of the bulk material is well reproduced. Color legend: total DOS in black, partial contributions from subsurface Ta in blue, from subsurface O in green, and from subsurface N in red. (b) Averaged electrostatic potential profile over plans parallel to the (001) TaON surface obtained at the DFT/HSE06 level. Considering now the (Ta1-xNbx)ON materials (with x = 0.0625, 0.125, 0.25, and 0.5), the predicted band edge positions are found to be different from the previous case (Figure 5b-e), and consequently, different photocatalytic behaviors are expected. Figure 5 reveals a particular trend showing a linear upward shift of both VB and CB edge energy levels with increasing Nb content. Indeed, an increase in the Nb content to 6.25% or 12.5% upward shifts the CB edge level by 0.2 or 0.3 eV over TaON, respectively. This leads to new CB edge energy positions located near or at the same H+/H2 level. Hence, the evolution here is in the right direction although the ability of the new modified compounds to reduce H+ still remains very limited. Also, their respective VB


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edge energy level is also shifted upward by 0.2 and 0.4 eV over TaON (becoming 3.1 and 2.9 eV lower in energy than O2/H2O level), and therefore, their power to oxidize water is maintained. One important result here is that a further increase in the Nb content to 25% or 50% leads again to an upward shift in the CB edge energy position by 0.7 or 1.4 eV over TaON, respectively. Interestingly, the new CB edge energy levels here become 0.4 and 1.1 eV higher than H+/H2 potential, and consequently, their capability for H+ reduction becomes possible. Moreover, their respective VB edge energy level is also found to be moved upward by 0.8 and 1.6 eV (e.g., they become 2.5 and 1.7 eV lower in energy than O2/H2O level), and therefore, their ability to oxidize water remains preserved although it is expected to be slightly lower than TaON. As a consequence, our calculated band edge energy levels relative to water redox potentials clearly show that the substitution of Ta by Nb in the TaON monoclinic structure significantly improves its capability for H+ reduction and towards H2 evolution by preserving its ability for water oxidation and towards O2 evolution. To the best of our knowledge, these significant effects of substituting Nb for Ta sites in TaON on its photocatalytic features for overall water-splitting reactions have never been reported in previous theoretical works.



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Figure 5. VB and CB edge energy levels of (Ta1-xNbx)ON materials with various Nb contents obtained at the DFT/HSE06 level: (a) 0% Nb, (b) 6.25% Nb, (c) 12.5% Nb, (d) 25% Nb, and (e) 50% Nb. The values are given versus vacuum level (in eV) as well as versus NHE potential (in V).

4. Conclusion Essential fundamental properties of monoclinic (Ta1-xNbx)ON (x = 0.0625, 0.125, 0.25, and 0.5) solid solution semiconductor materials for water splitting were investigated using first-principles computations on the basis of DFT and DFPT within the PBE and HSE06 formalisms. The formation energies, the band gaps, the UV-visible optical absorption coefficients, the dielectric constants, the charge carrier effective masses and the band edge energy positions of these compounds were evaluated. Our calculations revealed strongly 3D delocalized characters of the band edge electronic states through the crystal lattices, high dielectric constants (> 10), small hole effective masses (< 0.5m0) along [001] direction and small electron effective masses (< 0.5m0) along [100] direction. This leads to good exciton dissociation ability, good hole mobility along [001] direction and good electron mobility along [100] direction similarly as obtained for TaON. Despite the remarkable similarities of optoelectronic, dielectric, and charge carrier transport properties between the pure and the Nb-modified materials, their photocatalytic abilities for water splitting showed different trends because of the main difference on the band edge energy levels with respect to water redox potentials. TaON with calculated bandgap energy of 3.0 eV showed a good potential for water oxidation and O2 evolution because the CB edge energy level was found 0.3 eV below the H+/H2 level. The VB edge energy level of this material was 2.07 eV below the O2/H2O level. Substituting Ta by


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Nb into the TaON lattice led to a linear upward shift of both VB and CB edge energy positions with increasing Nb content, thereby improving its ability for H+ reduction and towards H2 evolution by preserving its power for water oxidation and O2 evolution. This resulted in suitable band edge energy positions of (Ta1-xNbx)ON materials (with 0.25 ≤ x ≤ 0.5) for both water oxidation and H+ reduction. Consequently, the good dielectric and charge carrier transport properties as well as the suitable photoelectrochemical properties predicted for (Ta1-xNbx)ON materials (0.25 ≤ x ≤ 0.5) will certainly offer a grand opportunity for these compounds to be properly synthesized and tested for solar-driven overall water-splitting reactions. In conclusion, we have shown positive effects of substituting Ta by Nb on relevant properties to water splitting using mixed tanlalum-niobium oxynitride (Ta1-xNbx)ON compounds. The computational methodology adopted in this work will be applied to other semiconductors in order to identify novel good candidate photocatalysts for visible-light-driven overall watersplitting reactions.

Associated Content Supporting Information. S1: Computational details. S2: Lowest-energy structures of (Ta1xNbx)ON

(with x = 0, 0.0625, 0.125 and 0.25) obtained using the DFT/PBE method. S3:

Fractional coordinates of the lowest-energy (Ta1-xNbx)ON structures. S4: Electronic density of states of TaON obtained using the DFT/PBE method. S5: Lowest-energy structures along with the formation energies and lattice parameters of TaO0.90N1.06 and TaO0.81N1.12 obtained using the DFT/PBE method. S6: Electronic density of states of TaO0.90N1.06 and TaO0.81N1.12 obtained using the DFT/HSE06 method. S7: Electron density contour plots (over the ab and bc planes) for



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the band edge states of TaON, Ta0.75Nb0.25ON, and Ta0.5Nb0.5ON obtained using the DFT/HSE06 method. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author *Moussab Harb. E-mail: [email protected]. Phone: +966.2.808.07.88. Fax: +966.2.802.12.72.

Acknowledgments The author gratefully thanks the KAUST Supercomputing Laboratory (KSL) at King Abdullah University of Science and Technology (KAUST) for the computational time granted to this work.

References

(1)

Esswein, A. J.; Nocera, D. G. Hydrogen Production by Molecular Photocatalysts. Chem.

Rev. 2007, 107, 4022–4047. (2)

Maeda, K.; Domen, K. New Non-Oxide Photocatalysts Designed for Overall Water

Splitting Under Visible Light. J. Phys. Chem. C 2007, 111, 7851–7861. (3)

Kudo, A.; Miseki, Y. Heterogenous Photocatalyst Materials for Water Splitting. Chem.

Soc. Rev. 2009, 38, 253–278.


Page 20 of 27

Page 21 of 27

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)

Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future

Challenges. J. Phys. Chem. Lett. 2010, 1, 2655–2661. (5)

Ritala, M.; Kalsi, P.; Riihelä, D.; Kukli, K.; Leskelä, M.; Jokinen, J. Controlled Growth

of TaN, Ta3N5, and TaOxNy Thin Films by Atomic Layer Deposition. Chem. Mater. 1999, 11, 1712–1718. (6)

Orhan, E.; Tessier, F.; Marchand, R. Synthesis and Energetics of Yellow TaON. Solid

State Sci. 2002, 4, 1071–1076. (7)

Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. An Oxynitride,

TaON, as an Efficient Water Oxidation Photocatalyst Under Visible Light Irradiation (λ ≤ 500 nm). Chem. Commun. 2002, 1698–1699. (8)

Hara, M.; Chiba, E.; Ishikawa, A.; Takata, T.; Kondo, J. N.; Domen, K. Ta3N5 and

TaON Thin Films on Ta Foil: Surface Composition and Stability. J. Phys. Chem. B 2003, 107, 13441–13445. (9)

Hara, M.; Hitoki, G.; Takata, T.; Kondo, J. N.; Kobayashi, H.; Domen, K. TaON and

Ta3N5 as New Visible Light Driven Photocatalysts. Catal. Today 2003, 78, 555–560. (10) Chun, W.-J.; Ishikawa, A.; Fujisawa, H.; Takata, T.; Kondo, J. N.; Hara, M.; Kawai, M.; Matsumoto, Y.; Domen, K. Conduction and Valence Band Positions of Ta2O5, TaON, and Ta3N5 by UPS and Electrochemical Methods. J. Phys. Chem. B 2003, 107, 1798–1803. (11) Yashima, M.; Lee, Y.; Domen, K. Crystal Structure and Electronic Density of Tantalum Oxynitride, a Visible Light Responsive Photocatalyst. Chem. Mater. 2007, 19, 588–593.



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

Page 22 of 27

(12) Brauer, G.; Weidlein, J.; Strähle, J. Über das Tantalnitrid Ta3N5 und das Tantaloxynitrid TaON. Z. Anorg. Allg. Chem. 1966, 348, 298–308. (13) Armytage, D.; Fender, B. E. F. Anion Ordering in TaON: A Powder Neutron-Diffraction Investigation. Acta Crystallorg. B 1974, 30, 809–812. (14) Fang, C. M.; Orhan, E.; de Wijs, G. A.; Hintzen, H. T.; de Groot, R. A.; Marchand, R.; Saillard, J. Y.; de With, G. The Electronic Structure of Tantalum (Oxy)nitrides TaON and Ta3N5. J. Mater. Chem. 2001, 11, 1248–1252. (15) Li, P.; Fan, W.; Li, Y.; Sun, H.; Cheng, X.; Zhao, X.; Jiang, M. First-Principles Study of the Electronic, Optical Properties and Lattice Dynamics of Tantalum Oxynitride. Inorg. Chem. 2010, 49, 6917–6924. (16) Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Ángyán, J. G. Screened Hybrid Denity Functionals Applied to Solids. J. Chem. Phys. 2006, 124, 154709/1–154709/13. (17) Harb, M.; Sautet, P.; Raybaud, P. Origin of the Enhanced Visible-Light Absorption in NDoped Bulk Anatase TiO2 from First-Principles Calculations. J. Phys. Chem. C 2011, 115, 19394–19404. (18) Harb, M.; Sautet, P.; Nurlaela, E.; Raybaud, P.; Cavallo, L.; Domen, K.; Basset, J.-M.; Takanabe, K. Tuning the Properties of Visible-Light-Responsive Tantalum (Oxy)nitride Photocatalysts by Non-Stoichiometric Compositions: A First-Principles Viewpoint. Phys. Chem. Chem. Phys. 2014, 16, 20548–20560.


Page 23 of 27

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


(19) Harb, M.; Sautet, P.; Raybaud, P. Anionic or Cationic S-Doping in Bulk Anatase TiO2: Insights on Optical Absorption from First Principles Calculations. J. Phys. Chem. C 2013, 117, 8892–8902. (20) Harb, M. Screened Coulomb Hybrid DFT Study on Electronic Structure and Optical Properties of Anionic and Cationic Te-Doped Anatase TiO2. J. Phys. Chem. C 2013, 117, 12942–12948. (21) Harb, M. New Insights into the Origin of Visible-Light Photocatalytic Activity in SeModified Anatase TiO2 from Screened Coulomb Hybrid DFT Calculations. J. Phys. Chem. C 2013, 117, 25229–25235. (22) Harb, M.; Masih, D.; Ould-Chikh, S.; Sautet, P.; Basset, J.-M.; Takanabe, K. Determination of the Electronic Structure and UV-Vis Absorption Properties of (Na2xCux)Ta4O11

from First-Principle Calculations. J. Phys. Chem. C 2013, 117, 17477–17484.

(23) Harb, M.; Masih, D.; Takanabe, K. Screened Coulomb Hybrid DFT Investigation of Band Gap and Optical Absorption Predictions of CuVO3, CuNbO3 and Cu5Ta11O30 Materials. Phys. Chem. Chem. Phys. 2014, 16, 18198–18204. (24) Harb, M.; Cavallo, L.; Basset, J.-M. Major Difference in Visible-Light Photocatalytic Features Between Perfect and Self-Defective Ta3N5 Materials: A Screened Coulomb Hybrid DFT Investigation. J. Phys. Chem. C 2014, 118, 20784–20790. (25) Nurlaela, E.; Ould-Chikh, S.; Harb, M.; del Gobbo, S.; Aouine, M.; Puzenat, E.; Sautet, P.; Domen, K.; Basset, J.-M.; Takanabe, K. Critical Role of the Semiconductor–Electrolyte



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

Page 24 of 27

Interface in Photocatalytic Performance for Water-Splitting Reactions Using Ta3N5 Particles. Chem. Mater. 2014, 26, 4812–4825. (26) Kresse, G.; Hafner, J. Ab-Initio Molecular Dynamics Simulation of the Liquid-MetalAmorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251–14269. (27) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab-Initio Total Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186. (28) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15–50. (29) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector AugmentedWave Method. Phys. Rev. B 1999, 59, 1758–1775. (30) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approxmation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (31) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953–17979. (32) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188–5192. (33) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69–96. (34) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polymeric Molecules. J. Chem. Phys. 1990, 92, 508–517.


Page 25 of 27

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


(35) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207–8215. (36) Saha, S.; Sinha, T. P.; Mookerjee, A. Electronic Structure, Chemical Bonding and Optical Properties of Paraelectric BaTiO3. Phys. Rev. B 2000, 62, 8828–8834. (37) Launay, M.; Boucher, F.; Moreau, P. Evidence of a Rutile-Phase Characteristic Peak in Low-Energy Loss Spectra. Phys. Rev. B 2004, 69, 035101/1–035101/9. (38) Gajdoš, M.; Hummer, K.; Kresse, G.; Furthmüller, J.; Bechstedt, F. Linear Optical Properties in the Projector-Augmented Wave Methodology. Phys. Rev. B 2006, 73, 045112/1– 045112/9. (39) Van de Walle, C. G.; Martin, R. M. Theoretical Study of Ban Offsets at Semiconductor Interfaces. Phys. Rev. B 1987, 35, 8154–8165. (40) Franciosi, A.; Van de Walle, C. G. Heterojunction Band Offset Engineering. Surf. Sci. Reports 1996, 25, 1–140. (41) Makov, G.; Payne, M. C. Periodic Boundary Conditions in Ab Initio Calculations. Physical Review B 1995, 51, 4014–4022. (42) Taguchi, T.; Shirafuji, J.; Inuishi, Y. Excitonic Emission in Cadmium Telluride. phys. status solidi b 1975, 68, 727–738. (43) Vurgaftman, I.; Meyer, J. R.; Ram-Mohan, L. R. Band Parameters for III–V Compounds Semiconductors and Their Alloys. J. Appl. Phys. 2001, 89, 5815–5875.



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

(44) Young, K. F.; Frederikse, H. P. R. Compilation of Static Dielectric Constant of Inorganics Solids. J. Phys. Chem. Ref. Data 1973, 2, 313–409. (45) Gilleo, M. A.; Bailey, P. T.; Hill, D. E. Free-Carrier and Exciton Recombination Radiation in GaAs. Phys. Rev. 1968, 174, 898–905. (46) Madelung, O. Semiconductors: Data Handbook, 3rd ed.; Springer: New York, 2004. (47) Adashi, S. GaAs and Related Materials; World Scientific Publishing Co. Pte. Ltd.: Singapore, 1994. (48) Kato, K.; Toyota, H.; Jin, Y.; Ono, T. Characterization of Tantalum Oxy-nitride by ECR Sputtering. Vacuum 2008, 83, 592–595.


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Graphical Abstract


First-Principles Investigation of Optoelectronic and Redox Properties

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