Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2

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Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2 Xin Yong, Jianqi Zhang, Xiangchao Ma* School of Physics and Optoelectronic Engineering, Xidian University, Xi’an, 710071, China

highlights

graphical abstract


Proper intrinsic defects cannot destroy the water redox ability of PtSe2.
The carrier recombination rate in PtSe2 can be reduced by proper intrinsic defects.
The optical absorption of PtSe2 can be improved by proper intrinsic defects.
The Pt@Se anti-site defect in PtSe2 notably enhances its interaction with H2O.

article info

abstract

Article history:

PtSe2 monolayer is previously predicted to be a two-dimensional water-splitting photo-

Received 6 August 2019

catalyst. However, the weak van der Waals (vdW) interaction between H2O and the basal

Received in revised form

surface of PtSe2 significantly undermines its photocatalytic water-splitting activities. In

18 December 2019

this work, we explore the possibility of various intrinsic defects of PtSe2 in remedying this

Accepted 10 January 2020

deficiency on the basis of first-principles calculations. It is interesting to find that the

Available online xxx

introduction of Pt@Se, Se@Pt, and Se interstitial defect not only fully retain the water redox

Keywords:

but also can extend optical absorption range and absorption coefficients. Moreover,

Two-dimensional photocatalyst

introduction of the three kinds of defects increase the initial weak vdW interactions be-

abilities of pure PtSe2 and realize spatial separation of photogenerated electrons and holes,

Water-splitting

tween H2O and the PtSe2 surface to different extent. In particular, Pt@Se anti-site defect

Electronic structures

transform the initial weak vdW to strong chemical interaction between H2O and PtSe2

First-principles calculations

surface, and function as active reaction site. These insights demonstrate that introduction

Intrinsic defect

of intrinsic defects, especially the Pt@Se anti-site defect, are effective means for improving the photocatalytic water-splitting activities of PtSe2 monolayer. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. E-mail address: [email protected] (X. Ma). https://doi.org/10.1016/j.ijhydene.2020.01.066 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Yong X et al., Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.066

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Introduction Finding photocatalyst which is more suitable for watersplitting is a key to solve the problem of solar to hydrogen energy conversion. Since the first work on photocatalytic water splitting on TiO2 in 1972 [1], the study on photocatalytic materials with high efficiency and stability has stimulated intensive research in recent decades. The requirement for a material to be a water-splitting photocatalyst is that the energy levels of photogenerated electrons and holes must respectively match the reduction and oxidation potentials of water. If this is satisfied, the optical absorption range and efficiency, recombination rate of photogenerated carriers, and surface reactive sites for a material determine its solar to hydrogen conversion efficiency. Early studies on water-splitting photocatalysts mainly focused on three-dimensional (3D) materials, such as metal oxides [2,3], nitrogen oxides [4,5], and sulfides [6e8]. However, these materials often suffer several drawbacks: the optical absorption is mainly in the ultraviolet range, the photostability is poor, and the recombination rate of photogenerated carriers is high [9]. In addition, the low volume-tosurface ratio of three-dimensional structure results in fewer active reaction sites. Despite the large efforts to solve these problems, such as band gap engineering [10,11], cocatalysts loading [12], nano/microstructure engineering [13,14], etc., the overall photocatalytic water-splitting efficiency of the modified photocatalysts can still not satisfy the demands for practical applications. Recently, the successful preparation of graphene has tremendously inspired the development of two-dimensional (2D) monolayer materials. Therefore, they are also explored for photocatalytic water-splitting application. In general, the carrier mobilities of 2D materials are much higher than those of traditional 3D ones, thus ensuring a lower carrier recombination rate [15]. Moreover, 2D materials have theoretically infinite surface-to-volume ratio, which can provide as many surface reactive sites as possible. On the other hand, the band gaps and edges of 2D materials can be effectively modulated by methods, such as mechanical strain and electric field [15e17]. Therefore, 2D materials can play crucial roles in designing the ideal photocatalysts for water-splitting application. Indeed, many 2D monolayer materials, such as g-C3N4, MoS2, MoSSe, ZrS2, and PtSe2 had recently been researched as water-splitting photocatalyst [18e24]. In 2013, Houlong Zhuang et al. theoretically predicted that PtSe2 can be used as water-splitting photocatalyst for its proper band edge positions [20]. In 2015, 2D monolayer PtSe2 was epitaxially grown successfully on Pt surface [25]. In 2016, Zegao Wang et al. reported the synthesis of high-quality single crystal PtSe2 nanosheets with large carrier mobility [26]. Then, according to the DFT calculation and experiments [27], Yuda Zhao showed that 2D PtSe2 has high air-stability, and H2O and O2 prefer physisorption on PtSe2. However, for photocatalytic water-splitting application, the weak vdW interaction between H2O and PtSe2 is unfavorable. In this work, we explore the possibility of various intrinsic defects of PtSe2 in remedying this deficiency on the basis of first-principles calculations. As is known, the intrinsic defects are unavoidable and

can significantly affect the electronic, optical and catalytic properties of a semiconductor. For 2D monolayer PtSe2, Junfeng Gao et al. studied the structure and stability of intrinsic vacancy defects [28]; Combining experiments and firstprinciples calculations, Husong Zheng et al. studied the intrinsic point defects in ultrathin layered PtSe2, determining the existence of Pt vacancy (hereafter denoted as Pt-v), Se vacancy (hereafter denoted as Se-v), and Se@Pt anti-site defects [29]; In 2019, Ahmet Avsar et al. reported that the intrinsic Pt vacancy in PtSe2 are responsible for the observed layer-dependent magnetism [30]. All these results indicate the ability of intrinsic defects to introduce new properties that are absent in pristine PtSe2. In this work we explore the ability of intrinsic defects to improve the photocatalytic water-splitting properties of PtSe2 by investigating the electronic structures, optical absorption, and interactions with water molecules of PtSe2 with six kinds of intrinsic defects, including Pt vacancy, Se vacancy, Pt@Se anti-site, Se@Pt anti-site, Pt interstitial and Se interstitial defects. It is found that the PtSe2 with Pt@Se, Se@Pt, and Se interstitial defects have proper band edges for photocatalytic water-splitting reactions. Moreover, the three defects improve the optical absorption and interaction strength with water of PtSe2. In particular, Pt@Se anti-site defect transform the initial weak vdW to chemical interaction between PtSe2 and H2O. These results indicate that the three defects are beneficial for improving the photocatalytic water-splitting activities of PtSe2 monolayer.

Computational method The first-principles calculations in this paper are conducted using the Vienna Ab initio Simulation Package (VASP) [31]. Pure and defected PtSe2 are modeled with 4  4 supercells, and a vacuum thickness larger than 30
A is used for separating it from its periodic images. The plane wave cutoff energy is set to 400 eV and the accuracy for self-consistent iteration is set to 105 eV. For geometric optimization, the generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof [32] is used. In order to accurately calculate the electronic structure of pure and defected PtSe2, HSE06 functional [33] is used. The Brillouin zones for the unit cell of pure and defected PtSe2 monolayers are sampled with 3  3  1 Gcentered k-points [34], and their structures are fully relaxed until the residual forces on each atom are smaller than 0.02 eV/
A. Because of the existence of dipole moments in some structures with defects, dipole corrections have been made to these structures [35]. The adsorption of H2O molecules on pure and defected PtSe2 monolayers is determined as follows: first, several configurations of placing one H2O molecule on the 4  4 supercell of PtSe2 are constructed; the composite structures are then fully relaxed until the residual force on each atom is smaller than 0.02 eV/
A; the most stable structures are regarded as the adsorption structures of H2O on the monolayers. The adsorption energy of H2O on the monolayer is defined as the difference between the sum of the energies of an isolated H2O molecule and the monolayer and the total energy of the H2O adsorbed system.

Please cite this article as: Yong X et al., Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.066

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Results and discussion Geometric and electronic structures of PtSe2 with different defects For the defected PtSe2 monolayer, three group of defects are considered: (1) Pt-v [Fig. 1 (b)] and Se-v [Fig. 1 (c)] defects; (2) Pt@Se [Fig. 1 (d)] and Se@Pt [Fig. 1 (e)] anti-site defects; (3) Pt [Fig. 1 (f)] and Se [Fig. 1 (g)] interstitial defects. Note that the most stable Pt and Se interstitial sites are determined as follows: the interstitial atoms are placed in several different positions of the 4  4 monolayer structure and then fully optimized, the most stable structures are regarded as the corresponding interstitial sites. Fig. 1 shows the pure and defected structures after structural relaxation. The formation processes and energies of the various defects are previously studied by Zhen et al. and Absor et al., which indicate that the defects are unavoidable and can be purposely introduced by optimizing experimental conditions. Although the introduction of defect breaks the mirror symmetry of the PtSe2, it is found that the introduced dipole potentials along the surface normal are less than 0.05eV, for all the defected structures, which should not considerably contribute to their photocatalytic activities. The fundamental prerequisite for a material to photocatalyze water-splitting is that the positions of its CBM and VBM of PtSe2 must be lower and higher than the reduction level of hydrogen (VHþ =H2 ¼ 4:44eV with respect to the vacuum level) and the oxidation level of oxygen (VO2 =H2 O ¼  5:67eV with respect to the vacuum level) [20], respectively. Therefore, on the basis of HSE06 functional, we calculated the density of states of pure and defected PtSe2, and locate the redox levels of H2O within them. With respect to the vacuum level, the positions of redox levels for H2O in Fig. 2 are determined. That is, we first determine the position of Fermi level of monolayer with respect to vacuum level, and then determine the redox levels of H2O with respect to the Fermi level. Fig. 2 shows the calculated results, in which the reduction and oxidation potentials of water are respectively marked with red dotted and solid lines. As shown in Fig. 2(a), the pure PtSe2 has a band gap of about 1.8 eV, the O2/H2O oxidation level is higher than the VBM by about 0.5 eV, the Hþ/H2 reduction level is lower than the CBM by about 0.2 eV. These results indicate that pure PtSe2 is able to fully photocatalyze water-splitting, which is consistent with previous reports[20]. For the defected PtSe2 with Pt-v defect [Fig. 2(b)], the defect introduced unoccupied gap states are lower than the Hþ/H2 reduction level, indicating that the defect may act as trapping center; while both the O2/H2O oxidation and Hþ/H2 reduction levels are located in the defect introduced gap states of PtSe2 with Se-v defect [Fig. 2(c)], rendering it useless in photocatalyze water-splitting. For PtSe2 with Pt@Se anti-site defect [Fig. 2(d)], the band gap and position of the redox level of water is almost the same as that of pure PtSe2. For PtSe2 with Se@Pt anti-site defect [Fig. 2(e)], although some gap states are introduced, the reduction (oxidation) level of water is still lower (higher) than its CBM (VBM). Therefore, PtSe2 with the two kinds of anti-site defects can still fully photocatalyze water-splitting. For PtSe2 with Pt interstitial defect [Fig. 2(f)], the O2/H2O

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oxidation level is located lower than the defected introduced occupied gap states, and the Hþ/H2 reduction level is within the conduction band; For PtSe2 with Se interstitial defect [Fig. 2(g)], the band gap and position of the redox level of water is also almost the same as that of pure PtSe2. Therefore, only PtSe2 with Se interstitial defect can fully photocatalyze watersplitting. From the above results, only PtSe2 with Pt@Se, Se@Pt, and Se interstitial defects can fully photocatalytic water-splitting. To further investigate the effect of these defects on the separation rate of photo-generated electrons and holes, the spatial distributions of the electronic states near the CBM and VBM are calculated and shown in Fig. 3. For comparison, the spatial distributions of charge densities for the electronic states near the CBM and VBM of pure PtSe2 are shown in Fig. 3(a), as can be seen, the electronic states near the CBM and VBM are evenly distributed on the Pt and Se atoms, respectively. These indicate that the photo-generated electrons and holes are spatially adjacent to each other, which is beneficial for carrier recombination. On the contrary, the electronic states near the CBM and VBM of PtSe2 with Pt@Se, Se@Pt, Se interstitial defects are located in separate parts of the monolayer, realizing spatial separation of the photo-generated electrons and holes. This can reduce the recombination rate of photo-generated carriers. In general, it is reasonable to infer that the recombination rate of carriers is reduced if the electrons and holes become more separable in space, which is often used in previous works, such as Ref. 22, 24 and 26. Specifically, in PtSe2 with Pt@Se anti-site defect [Fig. 3(b)], the electronic states near the CBM is located on and near the anti-site defect, indicating that the Pt@Se anti-site defect should act as reduction center for photocatalytic water-splitting; Whereas the electronic states near the VBM is distributed far away from the anti-site defect and on the Se atoms. In PtSe2 with Se@Pt defect [Fig. 3(c)], the electronic states near the CBM is mainly located on the Pt atoms far away from the Se@Pt anti-site defect; Whereas the electronic states near the VBM are distributed on the anti-site defect and Se atoms adjacent to the anti-site defect, indicating that the they should act as oxidation centers for photocatalytic water-splitting. In PtSe2 with Se interstitial defect [Fig. 3(d)], the electronic states near the CBM is almost evenly located on the Pt atoms; Whereas the electronic states near the VBM are mostly located on the Se interstitial site, indicating that the Se interstitial defect should act as oxidation center for photocatalytic water-splitting.

Optical absorption of PtSe2 with different kinds of defects For optical properties, the imaginary part of the frequencydependent dielectric function ε2 ðuÞis first calculated in the independent-particle picture without including the local field effects, and the real part of the dielectric function ε1 ðuÞis obtained by Kramers-Kronig transformation. The optical absorption coefficients aðuÞare evaluated according to the following equation (where c is the speed of light in vacuum): 1=2 pffiffiffi uqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ε21 ðuÞ þ ε22 ðuÞ  ε1 ðuÞ aðuÞ ¼ 2 c

Please cite this article as: Yong X et al., Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.066

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Fig. 1 e Top and side views of pure and defected PtSe2 after structural relaxation. (a) Pure PtSe2, (b) PtSe2 with Pt-v defect, (c) PtSe2 with Se-v defect, (d) PtSe2 with Pt@Se anti-site defect, (e) PtSe2 with Se@Pt anti-site defect, (f) PtSe2 with Pt interstitial defect, (g) PtSe2 with Se interstitial defect. Please cite this article as: Yong X et al., Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.066

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Fig. 2 e Density of states (DOS) and location of redox levels of water for pure and defected PtSe2. (a) pure PtSe2, (b) PtSe2 with Pt-v defect, (c) PtSe2 with Se-v defect, (d) PtSe2 with Pt@Se defect, (e) PtSe2 with Se@Pt defect, (f) PtSe2 with Pt interstitial defect, (g) PtSe2 with Se interstitial defect. The Fermi levels are set to 0 eV. The reduction level of hydrogen (Hþ/H2) is indicated by red dashed lines, and the oxidation level of oxygen (O2/H2O) is indicated by red solid lines. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3 e Top and side views of the charge density distribution for the states near the CBM (in the energy range of 0e0.3 eV relative to the CBM) and VBM (the energy range of ¡0.3 to 0 eV relative to the VBM) of pure PtSe2 (a) and PtSe2 with Pt@Se anti-site (b), Se@Pt anti-site (c), and Se interstitial (d) defects. The values of isosurfaces for all the four structures are set to 0.005 e/Å3. Efficient and wide-range visible light absorption are important prerequisites for high-efficiency solar to hydrogen conversion. To examine how the defects affect the optical

absorption properties of PtSe2, we calculated the optical absorption coefficients of pure and defected PtSe2. The detailed calculation procedure is the same as our previous work[36].

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The optical calculations are performed with the polarization vector of E field along the x, y and z directions, respectively. Because of the symmetry of pure PtSe2 monolayer structure[36], the optical absorption intensity is the same for polarization of E field along the in-plane x and y directions.

Moreover, the introduction of Pt@Se, Se@Pt, Se interstitial defects does not notably break the in-plane structural symmetry, thus the optical absorption intensities for polarization of E field along the in-plane x and y directions are almost the same. In Fig. 4(a), for pure and defected PtSe2 only the optical

Fig. 4 e Optical absorption coefficients a(u) for pure PtSe2 and PtSe2 with Pt@Se, Se@Pt, Se interstitial defects. (a) For polarization of E field along the in-plane x direction. (b) For the out-of-plane z direction.

Fig. 5 e Top and side views of the most stable adsorption structures of H2O on the pure PtSe2 (a), and PtSe2 with Pt@Se (b), Se@Pt (c), and Se interstitial (d) defects. Please cite this article as: Yong X et al., Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.066

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absorption coefficients for polarization of E field along the inplane x direction are shown. As can be seen, all the absorption coefficients are very strong with values more than 104e105 cm1, which are superior to that of the famous 2D gC3N4 water-splitting photocatalyst by 101e102 [37]. In addition, the optical absorption edges are slightly extended to lower energy values in the defected PtSe2, and the extension for PtSe2 with Pt@Se anti-site defect is the largest with a value of about 0.2 eV; This is consistent with the slight decrease in the band gap of the defected PtSe2, as shown in Fig. 3(d), (e), and (g). Fig. 4 only shows the results for energy range with large optical absorption coefficients. To present full information about optical absorption, Fig. S1 shows the results for energy range between 1 and 3 eV. As shown in Fig. 4(b), in comparison with the optical absorption edge for polarization of E field along the in-plane directions, the optical absorption edge for polarization of E field along the out-of-plane z direction is blue-shifted by about 0.5 eV in both pure and defected PtSe2. Moreover, the introduction of defects has only tiny effects on the optical absorption edge and intensity. Based on the microscopic theory ! of dielectric function[38], the dot product between the E field of incident light and moment operator p^ of electron plays central role in the absorption coefficient. For 2D monolayer material, the momentum vectors of most electrons are within the layer, thus the optical absorption coefficients should be very small when the polarization of E field is out of the plane.

Adsorption of water molecules on pure and defected PtSe2 Adsorption of H2O on the PtSe2 monolayer is another important parameter characterizing its activity in the photocatalytic water-splitting application. To determine the most stable adsorption structures of H2O on the monolayer, many different configurations of H2O on the surfaces of PtSe2 are firstly constructed, which are then geometrically optimized. Fig. 5 shows the most stable adsorption structures of H2O on pure PtSe2 and PtSe2 with Pt@Se, Se@Pt and Se interstitial defects. From Fig. 5(a), it is observed that H2O interacts with the pure PtSe2 mainly by forming HeSe bonds, and the bond lengths are about 2.8
A, which is within the vdW interaction length between Se and H atoms, but larger than the lengths of any chemical bonds that can form between Se and H atoms. This indicates that H2O is bonded to the surface mainly by vdW force. For PtSe2 with Pt@Se defect, H2O interacts directly with the Pt@Se defect by forming OePt bonds, and the bond lengths are about 2.35
A, which is very similar to the sum of van Bohr radii of O and Pt (2.25
A), indicating certain chemical interaction between them. For PtSe2 with both Se@Pt and Se interstitial defects, H2O does not interact directly with the defects, but interacts with the surface Se atoms near the defects, and the adsorption structures are very similar to that on the pure PtSe2, but the SeeH interaction lengths are decreased to some extent in the defected PtSe2, indicating stronger interaction between them. The increased interactions between H2O and defected PtSe2 are also manifested by the larger adsorption energy of H2O on the PtSe2 surface. As listed in Table 1, the adsorption energy of H2O on pure PtSe2 is 47.8 meV. After introducing defects, the adsorption energy of H2O on PtSe2 with Pt@Se

Table 1 e Bond length (in Å) between H2O and the surface atom of pure PtSe2 and PtSe2 with Pt@Se, Se@Pt, Se interstitial defects. Adsorption energy (in meV) of H2O on pure PtSe2 and PtSe2 with Pt@Se, Se@Pt, Se interstitial defects. Structure O-surface bond length H-surface bond length Adsorption energy

Pure-PtSe2

Pt@Se

Se@Pt 3.51

Se interstitial

3.56

2.35

2.81/2.84

2.56/2.60 2.80/2.83

2.79/2.80

47.8

336.6

61.6

49.9

3.49

defect is significantly increased (becoming 336 meV), and the adsorption energies of H2O on PtSe2 with the other two kinds of defects are also increased to different extent. These changes in the adsorption energy are consistent with the changes in bond length between H2O and the defected PtSe2. The increased interactions between H2O and defected PtSe2 are beneficial for photocatalyzing water-splitting reactions. In particular, the significantly increased interaction between H2O and PtSe2 with Pt@Se defect can notably improve the photocatalytic water-splitting activity of PtSe2 monolayer, and the Pt@Se defect site will function as a highly active reaction site.

Conclusion In conclusion, to study the effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2, we have investigated the geometric, electronic, optical, and H2O adsorption properties of pure PtSe2 and PtSe2 with vacancy, anti-site, and interstitial defects. It is found that PtSe2 with Pt@Se, Se@Pt and Se interstitial defects can still retain the water redox abilities of PtSe2 and realize spatial separation of photogenerated electrons and holes, whereas the introduction of Pt-v, Se-v and Pt interstitial defects destroy the water oxidation and/or reduction abilities of PtSe2, and they may act as carrier recombination centers. Moreover, the introduction of Pt@Se, Se@Pt and Se interstitial defects not only slightly extends the optical absorption range, but also increase the optical absorption coefficients in 1.6e2.1 eV. In addition, the introduction of Se@Pt and Se interstitial defects increase the vdW interaction between H2O and PtSe2 surface to some extent, and the introduction of Pt@Se significantly increases the interaction between H2O and PtSe2 surface. The above results indicate that the introduction of Pt@Se, Se@Pt and Se interstitial defects are beneficial for the photocatalytic watersplitting activities of PtSe2, and Pt@Se anti-site defect may most notably improve the activities of PtSe2.

Acknowledgements This work is supported by the National Natural Science Foundation of China (11704298), the 2018 Postdoctoral Innovation Talent Support Program (BX20180233), the Key Industry Innovation Chain of Shaanxi (2018JQ1054), the China

Please cite this article as: Yong X et al., Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.066

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Postdoctoral Science Foundation (2019M653549), and the 111 Project (B17035). [14]

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.066.

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Please cite this article as: Yong X et al., Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.066