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

https://doi.org/10.1016/j.saa.2018.11.039Get rights and content

Highlights

  • N@Ga-IV is identified as a candidate for water splitting to produce hydrogen.

  • Significant absorption enhancement of N@Ga-IV in the visible light range is found.

  • Stability of the structures is confirmed by MD simulation and mechanic properties.

  • Band energy edge, mobility of electron and hole can be tuned by the doped elements.

Abstract

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.

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 > 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], [2], [3]]. Therefore, the optimal band energy gap should also be larger than 1.23 eV [[2], [3], [4], [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], [9], [10], [11], [12], [13], [14], [15], [16], [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 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.

Section snippets

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 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

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], [27], [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], [30], [31], [32], [33]]. It

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

Conflicts of Interest

There are no conflicts to declare.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. NSFC-11574125 and NSFC-11874192, as well as the Taishan Scholar Project of Shandong Province (ts201511055).

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