Article pubs.acs.org/JPCC
Design of High-Efficiency Visible-Light Photocatalysts for Water Splitting: MoS2/AlN(GaN) Heterostructures Jiamin Liao,† Baisheng Sa,† Jian Zhou,‡ Rajeev Ahuja,§ and Zhimei Sun*,‡ †
College of Materials, Xiamen University, 361005 Xiamen, P. R. China School of Materials Science and Engineering, and Center for Integrated Computational Materials Engineering, International Research Institute for Multidisciplinary Science, Beihang University, 100191 Beijing, P. R. China § Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden ‡
S Supporting Information *
ABSTRACT: Hydrogen fuel produced from water splitting using solar energy and a catalyst is a clean and renewable future energy source. Great efforts in searching for photocatalysts that are highly efficient, inexpensive, and capable of harvesting sunlight have been made for the last decade, which, however, have not yet been achieved in a single material system so far. Here, we predict that MoS2/AlN(GaN) van der Waals (vdW) heterostructures are sufficiently efficient photocatalysts for water splitting under visible-light irradiation based on ab initio calculations. Contrary to other investigated photocatalysts, MoS2/AlN(GaN) vdW heterostructures can separately produce hydrogen and oxygen at the opposite surfaces, where the photoexcited electrons transfer from AlN(GaN) to MoS2 during the photocatalysis process. Meanwhile, these vdW heterostructures exhibit significantly improved photocatalytic properties under visible-light irradiation by the calculated optical absorption spectra. Our findings pave a new way to facilitate the design of photocatalysts for water splitting.
1. INTRODUCTION Photocatalytic water splitting using a semiconductor as a catalyst involves photogenerated electrons and holes migrating to the semiconductor surface and serving as redox sources which react with the adsorbed water to produce H2 and O2.1,2 For direct photocatalytic water splitting, the width of the band gap and levels of the conduction and valence bands are important points in semiconductor photocatalysts. The redox potential for water splitting requires that the conduction band minimum (CBM) should be more negative than the redox potential of H+/H2 (0 V vs NHE), and the valence band maximum (VBM) should be more positive than the redox potential of O2/H2O (1.23 V).1,2 Therefore, the theoretical minimum band gap for water splitting is 1.23 eV which corresponds to the infrared wavelength. Usually, a band gap of larger than 1.23 eV is needed for effective water splitting due to additional overpotential associated with each electron transfer and gas evolution steps.1,2 As visible light consisting of half of the solar radiation, searching for active semiconductor photocatalysts that directly split water under visible-light irradiation is an ambitious mission for solar-energy utilization, which, however, remains one of the most challenging tasks.3−7 On the other hand, the inefficient separation and transportation of the photogenerated charges as well as the material instability are other big problems for practical solar fuel production by photocatalytic water splitting.2 Despite great interest and the significant efforts that have been made so far, researchers have © 2014 American Chemical Society
not yet developed a single material system that can satisfy all the requirements and overcome the problems.1 Herein, we design and predict MoS2/AlN(GaN) van der Waals (vdW) heterostructures for highly efficient visible-light photocatalysts that can separately produce hydrogen and oxygen at the opposite surfaces of the heterostructures. This type of layered vdW heterostructures has a direct band gap, where AlN(GaN) and MoS2 behave as the electron donor and electron acceptor, respectively. The present work opens a new door to design desirable semiconductor photocatalysts for water splitting under solar energy radiation. Monolayer MoS2 exhibits a direct band gap of ∼1.9 eV, the specific electronic structure of which is of great interest for photocatalytic water splitting under visible-light irradiation.8−10 However, due to the less water splitting activity of pristine MoS2, the catalytic efficiency of overall water splitting is not sufficient, and thus a cocatalyst is needed.1 The formation of heterostructures is an effective way for cocatalysis, which will enhance the photoinduced charge separation and improve the photocatalytic efficiency.2 Many efforts have been made to obtain MoS2-based heterostructures for efficient photocatalytic water splitting; however, the desired materials have not yet been produced due to the problems of band gaps or lattice Received: April 17, 2014 Revised: July 15, 2014 Published: July 15, 2014 17594
dx.doi.org/10.1021/jp5038014 | J. Phys. Chem. C 2014, 118, 17594−17599
The Journal of Physical Chemistry C
Article
Figure 1. Schematic views of the MoS2/AlN(GaN) heterostructures with various stackings. If the rotation angle of configuration (a) were set to be 0°, then the rotations of AlN(GaN) with respect to MoS2 for the rest of the configurations are (b) 60°, (c) 120°, (d) 180°, (e) 240°, and (f) 300°. Herein, the small green balls are N atoms; the middle blue balls are Al or Ga atoms; and the light magenta and large yellow balls are Mo and S atoms, respectively.
mismatch between the constituent structures.11−17 The group III nitrides (hereafter referred to as XNs, X = Al and Ga) exhibit good chemical and thermal stability as well as high thermal conductivity.18−20 Recently, theoretical predictions have deomonstrated the possible application of monolayer XNs as photocatalysts.21,22 Meanwhile, the band gap of monolayer XNs lies within the range of visible light.22 Our analysis on the structures of AlN(GaN) and MoS2 monolayers shows that they share the same hexagonal crystal structure with only around 2% lattice mismatch, which makes the high-quality and intimate heterostructures possible.14,23 In this work, we design new vdW heterostructures based on AlN(GaN) and MoS2 monolayers and predict that these materials will work as highly efficient visible-light photocatalysts for water splitting.
considered. All configurations were under structure optimization with convergence criteria in terms of both energy and force. After structure optimization, the strain energy due to the small lattice mismatch between the two layers is accommodated by small atomic adjustment. Table 1 lists the calculated energy Table 1. Energy Difference ΔE (eV) between Various Configurations, the Lowest Energy Configuration, the Layer Distance d (Å) for MoS2/AlN(GaN) van der Waals Heterostructures, as Well as the Mo−S and Al(Ga)−N Bond Length for Monolayers and Heterostructures Calculated by DFT-D2
■
system
configuration
MoS2 AlN GaN MoS2/AlN
METHODS Our calculations are based on the density functional theory (DFT) in conjunction with the projector-augmented-wave (PAW) potential which is implemented in the Vienna ab initio Simulation Package. 24,25 We used the Perdew−Burke− Ernzerhof (PBE) exchange-correlation functionals with the approach of Grimme (DFT-D2)26−29 considering the weak van der Waals like interaction in the MoS2-based heterostructures.14 The calculation details have been fully described in our previous work.30 For accurate band gap calculations, the hybridDFT with 10% Hartree−Fock exchange energy is used, which can reproduce the experimental band gap of monolayer MoS2, for which further details can be found in the Supporting Information.
layer distance (Å)
LMo−S (Å)
LAl(Ga)−N (Å)
2.412
a b c d e f a b c d e f
MoS2/GaN
3. RESULTS AND DISCUSSION 3.1. Crystal and Electronic Structures of MoS2/ AlN(GaN) vdW Heterostructures. By the Perdew−Burke− Ernzerhof (PBE) exchange-correction functionals with the approach of Grimme (DFT-D2),26,28,29 our calculated lattice parameters for MoS2, AlN, and GaN monolayers are 3.187, 3.126, and 3.258 Å, respectively, which agree well with previous works.22,31 The lattice mismatch between MoS2 and AlN(GaN) monolayers is only −1.9% (+2.3%) which is good for constructing MoS2/AlN and MoS2/GaN heterostructures. By the hybrid functional with the mixing of the Hartree−Fock and DFT exchange terms,32,33 the calculated rather accurate band gaps for MoS2, AlN, and GaN monolayers are 1.92, 3.39, and 2.48 eV, respectively, which are in good agreement with previous experimental34,35 or theoretical works.22 For MoS2/ AlN(GaN) heterostructures, there are six most possible configurations as illustrated in Figure 1, where six special rotation angles between the adjacent sheets have been
ΔE (eV)
0.006 0 0.055 0.135 0.136 0.056 0.018 0.019 0 0.087 0.092 0.008
2.776 2.722 2.990 3.457 3.454 3.050 2.989 2.975 2.972 3.411 3.422 3.054
2.406 2.408 2.406 2.405 2.405 2.407 2.422 2.423 2.422 2.421 2.421 2.421
1.805 1.880 1.825 1.827 1.823 1.821 1.820 1.823 1.865 1.867 1.864 1.863 1.863 1.864
difference between the total energy of various stacking configurations and the most stable one, the interlayer distance between MoS2 and AlN(GaN) layers, as well as the Mo−S and Al(Ga)−N bond lengths for the MoS2/AlN(GaN) heterostructures. The energy difference ΔE is defined as ΔE = E − E0
(1)
where E0 is the total energy of the most stable configuration and E is the total energy of each configuration. The most stable structures for MoS2/AlN and MoS2/GaN (with ΔE = 0 eV) are configurations b and c, respectively. As seen in Table 1, the most stable stackings have the smallest interlayer distance for the two heterostructures. The interlayer distances in different configurations vary from 2.722 to 3.457 Å, which covary with the total energies. For all the configurations, in MoS2/AlN the Mo−S bond length is smaller than that in MoS2 while the Al− N bond length is larger than that in AlN; in MoS2/GaN the 17595
dx.doi.org/10.1021/jp5038014 | J. Phys. Chem. C 2014, 118, 17594−17599
The Journal of Physical Chemistry C
Article
Mo−S bond length is larger than that in MoS2, while the Ga−N bond length is smaller than that in GaN. These bond-length changes in the heterostructures compared to the monolayers are due to the small atomic adjustments to accommodate the strain energy induced by the few percent (
