Photocatalytic hydrogen production from water splitting with N-doped β-Ga2O3 and visible light

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 211 (2019) 71–78

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Photocatalytic hydrogen production from water splitting with N-doped β-Ga2O3 and visible light Ze-Cheng Zhao a,b, Chuan-Lu Yang a,⁎, Qing-Tian Meng b, Mei-Shan Wang a, Xiao-Guang Ma a a b

School of Physics and Optoelectronics Engineering, Ludong University, Yantai 264025, People's Republic of China College of Physics and Electronics, Shandong Normal University, Jinan 250014, People's Republic of China

a r t i c l e

i n f o

Article history: Received 12 August 2018 Received in revised form 13 November 2018 Accepted 15 November 2018 Available online 16 November 2018 Keywords: Photocatalytic hydrogen production Water splitting Mobility Visible light Dynamical stability

a b s t r a c t Based on the first principles calculations, the feasibility of the photocatalytic hydrogen production from water splitting driven by N-doped β-Ga2O3 in the visible light is investigated. The formation energy and dynamics properties are used to examine the stability of the doped structures. The absolute positions of the band energy edges are obtained and compared to the redox potentials of the hydrogen production reaction. Moreover, we calculate the carrier lifetime and mobility for both electron and hole of all the considered structures. The optical absorption is also calculated for each structure. The results show that the 5.00 at.% N-doped β-Ga2O3 has the satisfactory band energy edges, obvious difference of mobilities between electron and hole, and significant enhancement of absorption in visible light range, indicating it is a promising photocatalytic material to catalyze hydrogen production from water splitting under the irradiation of the visible light. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen production by photocatalytic water splitting is a promising way in the new energy field. So much attention has been paid to find satisfactory photocatalytic materials. Although a large number of photocatalytic materials have been found, most of them just respond to UV light because of their wide band energy gaps (Eg N 3.2 eV). Moreover, a suitable visible light photocatalytic material requires not only strong absorption in the visible light range but also satisfactory conduction band minimum (CBM) and valence band maximum (VBM) with respect to redox potential. The photocatalytic reaction of water splitting to produce hydrogen requires that CBM is higher than the reduction potential of H+/H2 [0 V] and VBM is lower than the oxidation potential of O2/H2O (1.23 V) [1–3]. Therefore, the optimal band energy gap should also be larger than 1.23 eV [2–5]. Owing to thermodynamic losses and kinetic barriers, the most approximate band gaps are from 2.0 to 2.5 eV [6]. Monoclinic β-Ga2O3 is the most stable structure in comparison with the other phases (α, γ, δ and ε) [7]. Most studies focus on its intrinsic properties in the photocatalyst, solar-blind UV detectors, gas sensors et al. Owing to a wide band-gap of β-Ga2O3, some researchers [8–17] devote to tuning band edges and improve absorption in wider response range of light by band-gap engineering, such as transition mental doped β-Ga2O3 or composite material. The optics, magnetism and electronic ⁎ Corresponding author. E-mail address: [email protected] (C.-L. Yang).

https://doi.org/10.1016/j.saa.2018.11.039 1386-1425/© 2018 Elsevier B.V. All rights reserved.

properties of N-doped β-Ga2O3 are calculated with first-principles calculations [18,19]. However, their investigations mainly focus on the case of O substituted by N, in which the irregular changes of band energy gap or spin polarization phenomenon occur because of odd-even outer valence electron substitution. On the other hand, more possible doping ways, for example, the case of Ga substituted by N, are rarely reported in detail. Moreover, the feasibility of the N-doped β-Ga2O3 for the photocatalytic water splitting is not concerned. In this paper, we will focus on the doping way of Ga substituted by N. By use of the Meta-GGA based on density functional theory (DFT), the geometrical structure, formation energy, dynamic stability, electronic and optical properties are investigated. The effects of the doping concentration on the band energy gap and optical absorption are also examined. The feasibility of N-doped β-Ga2O3 for photocatalytic water splitting under the irritation of the visible light is checked by the energy levels of VBM and CBM, the absorption and the carrier mobility. The results demonstrate that the heavy N-doped β-Ga2O3 structures are promising candidates for the photocatalytic hydrogen production from water splitting driven by the visible light. 2. Computational Methods Based on the intrinsic β-Ga2O3 with C2/m symmetry belonging to the monoclinic structure, a large 1 × 2 × 2 supercell of β-Ga2O3 including 80 atoms is used to model the N-doped structures. Due to the symmetry of β-Ga2O3, there are two types of sites for Ga atoms in the supercell. Both sites doped with N (N@Ga-1 and N@Ga-2) are

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Table 1 Calculated values of binding energy.

Ga-1 site Ga-2 site

1 N@Ga

2 N@Ga(1,2)

2 N@Ga(1,3)

2 N@Ga(2,3)

3.38 eV 3.18 eV

6.56 eV 5.97 eV

6.35 eV 5.71 eV

6.50 eV 6.06 eV

1 Ga-1

2

2 Ga-2 3 3

Fig. 1. N-doped β-Ga2O3 structure. Green, red and blue balls represent to Ga, O and N atoms, respectively.

considered. We evaluate the energy stability by comparing the binding energies of Ga-1 and Ga-2 sites substituted with one and two N atoms.

As shown in Table 1, the binding energy of N@Ga-2 is 0.2 eV lower than that of N@Ga-1. The further calculations on two N substitutions of Ga atoms indicate that the binding energies of N@Ga-1 are 0.59 eV, 0.64 eV, and 0.44 eV higher than those of N@Ga-2 in the three different substituted combination of Ga (1,2), Ga (1,3) and Ga (2,3), respectively. All the calculations demonstrate that the N@Ga-2 is more stable. Hereafter, the N@Ga-2 is considered with four N-doped concentrations of 1.25 at.%, 2.50 at.%, 3.75 at.% and 5.00 at.% which correspond to one to four Ga atoms substituted by N. These preferable N-doped structures are labeled by N@Ga-I, N@Ga-II, N@Ga-III and N@Ga-IV, respectively. The lattice of N@Ga-IV, an example of the doped structures, is demonstrated in the Fig. 1. The supercells are fully relaxed by using DFT with the Perdew–Burke–Ernzerh of generalized gradient approximation (GGA-PBE) [20]. After relaxation, the electronic structure and optical absorption are calculated with Meta-GGA method [21]. The projector augmented wave (PAW) [22] potentials with 4s24p1, 2s22p4 and 2s22p3 valence electrons are adopted for Ga, O and N atoms, respectively. All the calculations are carried out with the Vienna Ab initio Simulation Package 5.4.4 (VASP) [23]. Based on a series of test, the plane wave cutoff energy is set to 450 eV by which the excellent convergent results are obtained. The 2 × 4 × 2 k-mesh of Monkhorst-Pack is used in the whole Brillouin zone. Especially, the k-path along high symmetry points of Z-Γ-Y-A-B-D-E-C is used for the band structure calculations. The convergent criteria in the total energy and the ion inter-atomic forces are 1 × 10−5 eV and 1 × 10−4 eV/Å, respectively. Based on the optimized structures, we perform ab-initio molecular dynamics (AIMD) simulations [24] to confirm the dynamic stability of the doped structures. The total time steps are 5000 with 2.0 fs per step. The Nosé-Hoover method [25] is employed in all calculations with the constant temperature of 300 K.

3. Results and Discussion 3.1. Crystal Structure The present lattice parameters of the pristine β-Ga2O3 are a = 12.45 Å, b = 3.09 Å, c = 5.89 Å and β = 103.76°, which are in good agreement with other theoretical values [18,19,26–28] although a little difference from the experimental values (a = 12.23 Å, b = 3.04 Å, c = 5.80 Å and β = 103.7°). The further calculation for super-cell reveals that the Eg calculated with Meta-GGA method is 4.55 eV, which is close to 4.6–4.9 eV of experimental value [29–33]. It implies that Meta-GGA can really give credible electronic properties for the β-Ga2O3 structure. Using the constructed supercells, we carry out geometrical optimizations for N-doped β-Ga2O3. From Table 2 one can find that, as the doping concentration increases, the lattice parameters just have some slight change within an acceptable deformation range. This results from the differences of the doping concentrations and the different radius between N and Ga atoms. Although the radius of N atom is much smaller than that of Ga atom and the substitution can result in a shorter covalent bond with O atom, the maximum 4 doped N atoms are much less than the 80 of the total atoms. Therefore, the change in the lattice parameters of the supercell is not significant although the heavy concentration usually leads to obvious distortion of lattice parameters. 3.2. Formation Energy and Dynamic Stability of N-Doped β-Ga2O3 Structures To evaluate the energy stability and feasibility of N-doped β-Ga2O3 structures, the formation energy Eform is calculated by using the following formula: [34,35]. E f ¼ EN−doped −Eundoped −nμ N þ nμ Ga

ð1Þ

Table 2 Calculated values of lattice parameters (L), formation energy (Ef) and band energy gap (Eg). Ef (eV)

L (Å)

β-Ga2O3 N@Ga-I N@Ga-II N@Ga-III N@Ga-IV

a

b

c

12.44 12.43 12.42 12.48 12.52

6.18 6.20 6.22 6.25 6.21

11.81 11.80 11.83 11.75 11.87

Eg (eV)

O-rich

Ga-rich

3.06 5.42 7.59 9.17

6.77 9.20

4.55 4.10 2.63 2.44 2.34

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Fig. 2. AIMD simulations of N-doped β-Ga2O3 structures for 10 ps with a time step of 2 fs. XY and XZ plane represents the total energy change and N deviation with respect to an equilibrium position in the entire simulation.

Here, EN-doped, Eundoped, μN, μGa and n represent the total energy of N-doped β-Ga2O3, the total energy of pristine β-Ga2O3, the chemical potential of dopant N atom, the chemical potential of Ga atom, and the number of the substituted atoms, respectively. The chemical potential should satisfy the relation of 2μGa + 3μO = μGa2O3 in the pristine β-Ga2O3. In the O-rich condition, μO can be calculated from one oxygen molecule in 15 × 15 × 15 Å3 cell, and μGa can be derived from the relation to μO and μGa2O3. In the Ga -rich condition, the μGa is the energy per atom in the monoclinic phase of elemental gallium. μN is energy per atom of one nitrogen molecules in 15 × 15 × 15 Å3 cell. The results are given in Table 2. In O-rich cases, Ef increases gradually from 3.06 eV for the low concentration of N@Ga-I to 9.17 eV for the high concentration of N@Ga-IV. On the contrary, the Ef is much larger in Ga-rich cases, especially 9.2 eV of the N@Ca-II. Obviously, the doping behavior of the low N-doped concentration is easier for the experimental preparation in energy, but the modification effect on the properties is also small. In order to get some better properties, higher concentration should be considered in N-doped β-Ga2O3 although it needs a little more energy to synthesize. Fig. 2 shows the total energy and N deviation as a function of simulation time. In the XY plane of Fig. 2a, the total energy of the N@Ga-I structure demonstrates a small energy fluctuation within 2.8 eV due to a slight decline of 0.5 eV at about 6 ps (3000 steps). Compared with N@Ga-I, the other three structures in XY plane (Fig. 2b–d) have a relative small energy fluctuation of 2.0–2.5 eV without regard to the 2 ps of initial simulation, indicating that all the structures are stable in energy. To directly demonstrate the changes of the doped atom, we collect the locations of N atoms with , z) for each cycle and present them in the XZ plane of Fig. 2. The deviated distance di and deviation degree respect to the equilibrium position R0(x, y S can be expressed in the follow formula:

di ¼ jRi −R0 j ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Þ2 þ ðzi −zÞÞ2 ðxi −xÞ2 þ ðyi −y

ð2Þ

Table 3 The calculated elastic constants Cij of N-doped β-Ga2O3 structures. Cij

N@Ga-I

N@Ga-II

N@Ga-III

N@Ga-IV

C11 C22 C33 C44 C55 C66 C12 C13 C23 C15 C25 C35 C46

229.09 326.50 315.42 79.09 52.85 64.00 114.65 133.94 98.76 0.24 0.23 0.24 0.03

219.24 309.63 283.42 81.53 50.86 57.71 110.23 119.06 93.12 0.02 0.03 0.02 0.00

205.25 280.55 255.74 72.82 40.16 49.21 108.85 112.79 89.16 0.03 0.03 0.04 0.02

194.17 289.06 260.28 60.07 47.83 45.21 101.05 109.81 100.83 0.06 0.07 0.09 0.02

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sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N 2 ∑i¼1 di S¼ n

ð3Þ

The data in the XZ plane shows that the max deviated distance for the four cases is 0.046 Å, 0.109 Å, 0.093 Å, and 0.070 Å, respectively. Mostly, the di of N atoms changes within 0.1 Å eventually although some little big values appear at the initial steps as shown in Fig. 2b and d. It should be noted that N atoms in N@Ga-III (Fig. 2c) show a dispersive distribution on a small scale of 0.08 Å, this is caused by the interaction between asymmetrically substituted positions of N. Further calculation on deviation degree S shows a gradual increase from 0.017 of N@Ga-I to 0.050 of N@Ga-IV. The small deviation degree implies that N-doped β-Ga2O3 structures are stable at 300 K. Besides the thermodynamic stability calculations of AIMD, the mechanical properties are also calculated to confirm the stability with external stress impacting on N-doped β-Ga2O3 structures. The relation between stress and deformation should satisfy Hooke's law. Furthermore, the elasticity density U is a quadratic function of stress e, which can be expressed as: U¼

6 X 6 1X C ee 2 i¼1 j¼1 ij i j

ð4Þ

Γ

Γ

Γ

Γ

Fig. 3. (a–d) The band structures calculated with high symmetry line k-path and (e) the band edges alignment with respect to the redox potential of the hydrogen production reaction.

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Table 4 The calculated effective mass (m*), relaxation time (τ) and carrier mobility (μ) for electrons and holes. m∗e(me) β-Ga2O3 N@Ga-I N@Ga-II N@Ga-III N@Ga-IV

2.44 1.99 1.91 1.50 1.47

τe (ps)

μe (m2/Vs)

7.04 × 10−3 9.77 × 10−3 9.11 × 10−3 1.54 × 10−3 2.01 × 10−3

5.08 × 10−4 8.62 × 10−4 8.34 × 10−4 1.81 × 10−3 2.41 × 10−3

m∗h(me)

τh (ps)

μh (m2/Vs)

0.20 0.77 0.26 0.18 0.22

1.61 2.60 0.38 0.91 1.15

1.40 6.16 0.30 0.90 0.91

where the Cij is elastic constant, e1 = exx = εxx, e2 = eyy = εyy, e3 = ezz = εzz, e4 = eyz = εyz + εzy, e5 = ezx = εzx + εxz and e6 = exy = εxy + εyx. If the crystal structure is mechanically stable, the elasticity energy density in Eq. (4) must be positive definite quadratic, which means the Cij coefficient matrix is also a positive definite matrix. Hence, for monoclinic crystals, its mechanical stability criteria can be defined in terms of elastic constants as follows [36,37]:   C ii N0 ði ¼ 1−6Þ; ½C 11 þ C 22 þ C 33 þ 2ðC 12 þ C 13 þ C 23 ÞN0; C 33 C 55 −C 35 2 N0;   C 44 C 46 −C 46 2 N0; ðC 22 þ C 33 −2C 23 ÞN0; h   i C 22 C 33 C 55 −C 35 2 þ 2C 23 C 25 C 35 −C 23 2 C 55 −C 25 2 C 33 N0;

ð5Þ h      i 2 2 2 2 2 2 f2C 15 C 25 ðC 33 C 12 −C 13 C 23 Þ þ C 15 C 35 ðC 22 C 13 −C 12 C 23 Þ þ C 25 C 35 ðC 11 C 23 −C 12 C 13 Þg− C 15 C 22 C 33 −C 23 þ C 25 C 11 C 33 −C 13 þ C 35 C 11 C 22 −C 12 þMC 55 N0; M ¼ C 11 C 22 C 33 −C 11 C 23 2 −C 22 C 13 2 −C 33 C 12 2 þ 2C 12 C 13 C 23

Based on DFT calculations, all the 13 independent elastic constants Cij are obtained and listed in Table 3. One can find that all the Cij of the 4 Ndoped β-Ga2O3 structures satisfy the mechanical stability criteria of Eq. (5), implying these structures are mechanical stable. 3.3. Photocatalytic Properties As a photocatalytic material for hydrogen production from water splitting, the energy levels of both valence band maximum (VBM) and conduction band minimum (CBM) must satisfy the requirement of the redox levels of the reaction. The absolute energy levels of VBM and CBM for each structure are calculated by the following formula based on Meta-GGA method [38]. 1 EVBM ¼ −χ− Eg 2

ð6Þ

1 ECBM ¼ −χ þ Eg 2

ð7Þ

where χ is the Mulliken electronegativity, a value which is corresponding to the energy halfway between conduction and valence band edges [38], 1

n i mþn and it can be calculated by χ S ¼ ðχ m−i , where electronegativity χGa, χO and χN determined by ionization energy and electron Affinity of the Ga χ O χ N Þ corresponding element are 3.1485 eV, 7.5352 eV, and 7.3031 eV, respectively. Ultimately, the calculated χS are 5.3149 eV (i = 0), 5.3711 eV (i = 1), 5.4279 eV (i = 2), 5.4853 eV (i = 3) and 5.5433 eV (i = 4). Eg is the band energy gap of the considered structure. The calculated band structures and the absolute potentials of band edges with respect to the redox potentials are presented in Fig. 3. From the band structure in Fig. 3a–d, we can see that the pristine β-Ga2O3 has a direct band energy gap at Γ point. The N-doped performance will result in a local stress in the structure. With the increase of the N-doped concentrations, the local stress becomes obvious and extents to the entire lattice, which induces the distortion of the lattice structure [39]. Finally, the distortion makes N@Ga-II become an indirect energy gap semiconductor because of the position of VBM shifts from Γ point to Y point. As shown in Fig. 3e, the wide Eg 4.55 eV of the pristine β-Ga2O3 results that the CBM potential (3.04 V) is 1.46 V smaller than the reduction potential of H+ and the VBM (7.58 V) is larger the oxidation potential of H2O by 1.91 V, which is in agreement with the reported potentials (2.95 V and 7.75 V) of Ga2O3 [40]. This indicates that the pristine structure satisfies the requirement of potential for

β

2

ε

ε1

β

Fig. 4. The Dielectric Function of (a) real part Ɛ1(ω) and (b) imaginary part Ɛ2(ω).

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the water splitting reaction. For the lower N-doped concentration cases, no significant effect on the electronic properties is observed. The changes in positions of CBM and VBM for N@Ga-I are just 0.19 eV and 0.16 eV with respect to those of the pristine structure, respectively. However, an obvious narrower band energy gap of 2.63 eV (Fig. 3c) is demonstrated for N@Ga-II. The potential of CBM obviously decreases but still 0.39 eV higher than the reduction potential. Meanwhile, the absolute value of VBM potential also decreases. However, it is also 1.07 eV lower than the oxidation potential. Along with the increase of doping concentration, the band energy gap further decreases to 2.34 eV for N@Ga-IV. Hence, the absolute energy level of CBM is tuned from −3.04 eV of pristine structure to −4.37 eV of N@Ga-IV, and the energy level of VBM from −7.58 eV to −6.72 eV. All the band edge levels still satisfy the requirement of the redox reaction of water splitting. The difference of carrier mobilities between the hole and the electron is a necessary factor for an effective photocatalytic material. For a threedimensional structure, the electronic or hole mobility μe/h and relaxation time τe/h are expressed as [41]: pffiffiffiffiffiffi 2 2π C e=h ℏ4 τe ¼  3=2 3 kB Tme=h  Ee=h 2 μ e=h ¼

ð5Þ

eτe=h me=h 

ð6Þ

where the elastic constant of Ce/h is defined by C e=h ¼ V10

2

∂ Etot j , ∂ðΔl=l0 Þ2 l¼l0

in which Etot, l0, Δl and V0 are the total energy, the lattice constant, corre∂E

edge , in which sponding lattice distortion and equilibrium volume, respectively. The deformation potential constant of Ee/h is defined as Ee=h ¼ ∂ðΔlÞ=l 0 the energy of band edges of Eedge is a function of strain. The effective mass of me/h⁎ can be expressed by me/h∗ = (mx∗my∗m∗z)1/3, where mx⁎, my⁎, and m⁎z represent the electronic effective mass in the directions of x, y, and z, respectively. The effective mass along different directions is given by

−1 2 Ek 2 Þ dk

mi ¼ ℏ2 ðd

(i = x, y, z), where Ek is the energy nearby VBM and CBM, and k is the length in k-path of reciprocal space. The room temper-

ature T = 300 K is used in the present calculations. All the results are listed in Table 4. The mobilities of the electron and hole for the pristine β-Ga2O3 are mediate and much different, which implies that it has a strong ability to separate the photo-generated electrons and holes. This is just the characteristic of photocatalytic materials. One also can find from Table 4 that the carrier mobility of electron increases along with the increase of the N-doped concentrations. Nevertheless, the mobility of the hole does not show a monotone increasing case. The mobility of hole for N@Ga-I reaches 6.16 m2/Vs, which is 4 times more than that of the pristine β-Ga2O3. The mobilities of N@Ga-III and N@Ga-IV increase in comparison to that of N@Ga-II, but still smaller than that of the pristine β-Ga2O3. Fortunately, the mobilities of electrons and holes for all the doped structures are different by 2–3 orders of magnitude, which implies that all the N-doped structures are appropriate for the photocatalytic activity. 3.4. The Optical Absorption The optical absorption is another key factor for effective photocatalysis. To evaluate the absorption of N-doped β-Ga2O3, we calculate the dielectric function by using the first principles calculations. The dielectric function can be written as Ɛ(ω) = Ɛ1(ω) + iƐ2(ω), here, Ɛ2(ω) is calculated from momentum matrix elements of unoccupied and occupied states and Ɛ1(ω) is derived from Ɛ2(ω) based on Kramers-Krönig transformation [34]. The results in Fig. 4b demonstrate that the first dielectric peak for the pristine β-Ga2O3 locates at about 5.0 eV, which corresponds to the electronic transition from VBM to the peak of 4.4 eV in the conduction band, and for the four N-doped structures, the first dielectric peaks are 4.27 eV, 2.75 eV, 2.6 eV, and 2.5 eV, respectively. They are related to the inter-band transition from VBM to the peaks in the conduction bands. The absorption coefficient α can be determined from the dielectric function by using the following equation: α ðωÞ ¼

1=2 pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ω ε 1 ðωÞ2 þ ε2 ðωÞ2 −ε1 ðωÞ

ð7Þ

β

Fig. 5. Optical absorption changing with the increase of N concentration, the colorful area is visible light region.

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Fig. 6. Total DOS and partial DOS of (a) pristine and (b–d) N-doped β-Ga2O3.

The calculated absorption coefficient is shown in Fig. 5. For the pristine β-Ga2O3,the first absorption peak occurs at 5.0 eV, implicating that the pristine β-Ga2O3 cannot respond to the visible light. The peak is close to the experimental 4.83 eV by Zhang et al. [9] and corresponds to the calculated dielectric peak. The low doping concentration N@Ga-I shifts the absorption peak to 4.27 eV, while the more shifts of N@Ga-II and N@Ga-III make the main absorption peaks approach to the violet edge of the visible light. Moreover, a strong absorption region from 2.34 to 3.20 eV is observed for N@ Ga-IV, which is a favorable characteristic to response to the visible light. From this point of view, N@Ga-IV is a promising candidate for the photocatalytic materials for hydrogen production from water splitting. Unfortunately, the other 4 considered structures are not appropriate in the view of absorption although they have satisfied the other requirements or the water splitting reaction. The characteristics of the dielectric function and absorption can be understood through the total density of states (TDOS) and partial density of states (PDOS) of the pristine and N-doped β-Ga2O3 in Fig. 6. From the DOS of pristine β-Ga2O3 in Fig. 6(a) one can find that the VBM dominantly occupied by O-2p orbitals locates at −0.25 eV and the CBM mainly consists of Ga-4 s orbitals and locates at about 4.25 eV. This is the reason why the pristine structure just has an absorption in deep ultraviolet. The PDOS of N@Ga-I in Fig. 6(b) shows that the VBM of the N-doped structure is still occupied by O-2p orbitals while the CBM is occupied by the Ga-4 s orbitals and shifts to 4.0 eV, resulting in an absorption peak at 4.27 eV as shown in Fig. 5. With the increase of N-doped concentration, from the TDOS of N@Ga-II in Fig. 6(c) it shows that VBM and CBM have significant changes. Clearly distinguished from the conduction band of N@Ga-I, the CBM of N@Ga-II has moved to 2.4 eV – down sharply from 4.0 eV. As can be seen from the PDOS of N, the CBM is mainly contributed by N-2p orbitals. Then we can see that the first absorption peak of N@Ga-II locates at 2.75 eV, which is corresponding to the electric translation from VBM of O-2p orbitals to the N-2p orbitals at 2.5 eV. As doping concentration increases continuously, the DOS of N@Ga-IV shows that the location of VBM has no significant change in comparison with above N-doped cases and the CBM has a lower location of 2.0 eV. Therefore, the band energy gap is narrowed to 2.34 eV, which significantly improves the absorption of N@Ga-IV in the visible light.

4. Conclusion In summary, the photocatalytic feasibility of N-doped β-Ga2O3 to produce hydrogen from water splitting under the irritation of the visible light is investigated based on the first-principles DFT calculations. The geometrical and dynamical stabilities of all the N-doped β-Ga2O3 structures are confirmed by optimization, mechanical properties, and AIMD calculations. The formation energies of the N-doped β-Ga2O3 structures increase along with the increase of the N-doped concentration, which implies that the doping structure with high concentration needs more energy to synthesize. The band energy gap decreases from 4.55 eV to 2.34 eV, and the absolute energy levels of band edges are tuned to be more suitable for the requirement of the redox reaction in photocatalytic water splitting. The carrier mobilities demonstrate all the structures have moderate mobility and the difference between the electron

and the hole mobilities is obvious, which is helpful for the separation of the photo-induced electron and hole. Moreover, the present results show that only N@Ga-IV demonstrates obvious optical absorption in visible light of 2.34–3.20 eV while the other considered structures do not. Therefore, N@Ga-IV would be a potential material for hydrogen production from water splitting with the irritation of the visible light because of its satisfactory band energy edges, obvious difference of mobility between electron and hole, and strong absorption in visible light. Conflicts of Interest There are no conflicts to declare. 5.00 at% N-doped β-Ga2O3 is identified as a promising photocatalytic material to catalyze hydrogen production from water splitting under the irradiation of the visible light, by aid of the satisfactory band energy

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edges, obvious difference of mobility between electron and hole, and significant enhancement of absorption in visible light range.

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