Improvement of Visible-Light Photocatalytic Efficiency in a Novel InSe

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Improvement of Visible-Light Photocatalytic Efficiency in a Novel InSe/ZrCO Heterostructure for Overall Water Splitting 2

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Yong He, Min Zhang, Jun-jie Shi, Yu-lang Cen, and Meng Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01175 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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The Journal of Physical Chemistry

Improvement of Visible-Light Photocatalytic Efficiency in a Novel InSe/Zr2CO2 Heterostructure for Overall Water Splitting Yong He,† Min Zhang,∗,† Jun-jie Shi,∗,‡ Yu-lang Cen,‡ and Meng Wu‡ †College of Physics and Electronic Information, Inner Mongolia Normal University, Hohhot 010022, People’s Republic of China ‡State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, People’s Republic of China E-mail: [email protected]; [email protected]

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Abstract The unexpected visible-light absorption, low recombination of electron-hole pairs and high carrier mobility are found in a novel two-dimensional (2D) InSe/Zr2 CO2 van der Waals (vdW) heterostructure overall water splitting photocatalyst. The photocatalytic mechanism has been systematically investigated using first-principles calculations for the first time. We prove that 2D InSe/Zr2 CO2 heterostructure is a robust and promising visible-light photocatalyst with the following several distinctive advantages. It has a direct bandgap of 1.81 eV, a more favorable bandgap for visible-light photocatalysis. Its type-II band alignment directly leads to a significant electron-hole separation with electrons (holes) localized in InSe (Zr2 CO2 ) monolayer. The indirect bandgap of InSe (Zr2 CO2 ) monolayer further suppresses the electron-hole recombination in it. Naturally, the recombination of the photo-generated electrons and holes is greatly suppressed in InSe/Zr2 CO2 heterostructure, which improves the solar energy utilization effectively. Moreover, a large optical absorption coefficient (105 cm−1 ) has been confirmed in 2D InSe/Zr2 CO2 heterostructure with the electron (hole) mobility reaching up to 104 (103 ) cm2 V−1 s−1 , which is highly beneficial and desirable for enhancing its photocatalytic efficiency.

INTRODUCTION Nowadays, in response to depletion of fossil fuels and serious environmental problems, developing clean fuels by solar energy conversion has become a necessary and urgent effort. Hydrogen (H2 ) prepared by photocatalytic water splitting is supposed to be one of the most attractive way to utilize clean energy. 1–3 Since 1970s, overall water splitting has attracted extensive attention because of its clean, environmentally friendly and low-cost properties when photocatalytic water splitting generates H2 through solar energy. Firstly, Fujishima et al. found that titanium oxide (TiO2 ) can achieve overall water splitting. 4 Subsequently, various photocatalysts, such as metal oxides, nitrides with d0 or d10 transition metal cations, sulfides,

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MXenes (Zr2 CO2 and Hf2 CO2 ), some new types of 2D semiconductors (TiO2 , MoS2 , SnS2 , g-C3 N4 , BiVO4 ) and their heterostructures, 5–11 are successively proposed, among which TiO2 is one of the most hopeful photocatalysts. Unfortunately, some characteristics of these catalysts, such as large energy bandgap, poor stability, high electron-hole recombination and low carrier mobility, seriously inhibit the photocatalytic water splitting efficiency. For instance, photocorrosion arises in CdS. 6 Photocatalysts with highly effective solar-to-hydrogen conversion and broad spectrum of solar light, namely, from UV to near-infrared-region (NIR), 12 have not been found yet. Therefore, it is urgently needed to search for new and valid visiblelight photocatalysts both theoretically and experimentally. As a novel material, 2D InSe has been successfully exfoliated from bulk InSe (γ-phase) by mechanical exfoliation method 13 and chemical-vapor-transport (CVT), 14 which has been effectively utilized in photodetectors characterized by a broadband response ranging from visible-light to near-infrared area. Bandurin et al. 15 demonstrated that the electron mobility of atomically thin InSe reaches 103 and 104 cm2 V−1 s−1 in room and liquid-helium temperatures, severally. Simultaneously, their optical bandgaps of bilayer and bulk InSe are 1.90 and 1.25 eV, respectively, observed by photoluminescence experiments, which is caused by the strong quantum confinement of 2D InSe. The theoretical results of Zhuang et al. 16 manifested that as a typical III-VI semiconductor characterized by a high optical absorption and an indirect electronic energy bandgap of 2.83 eV, 2D InSe is a powerful visible-light photocatalyst. Furthermore, 2D group-III monochalcogenide nanosheets have also been examined by using first-principles calculations, including its exciton binding energy, carrier mobility and optical absorption. 17 The results indicate that InSe monolayer can be recognized as a promising photocatalyst because of its low recombination of electron-hole pairs, brilliant optical response and high carrier mobility. Wei et al. 18 synthesized a cubic InSe nanosheet photocatalyst, which was used for H2 production in visible-light irradiation. Above facts make it clear that in electronic, optoelectronic and photocatalytic device applications, 2D InSe is a potential semiconducting material.

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Apart from the aformentioned 2D InSe photocatalyst, some other catalysts have also been investigated. MXenes, including a series of 2D transition metal carbides/nitrides, have also attracted widespread attention due to their outstanding optoelectronic properties and potential device applications. Experimentally, these compounds can be prepared by chemical etching method, template method, plasma enhanced pulsed laser deposition (PEPLD) and chemical vapor deposition (CVD). 19 Here, M, A and X are transition metal, IIIA and IVA elements and C or N atoms, respectively. 20 In 2016, the photocatalytic activities and carrier mobility of MXenes, including Ti2 CO2 , Zr2 CO2 , Hf2 CO2 , Sc2 CF2 and Sc2 CO2 , were investigated by using density functional theory (DFT). 11 The results show that only Zr2 CO2 and Hf2 CO2 monolayer can be considered as suitable photocatalysts for overall water splitting among these materials because of their low-cost, excellent optoelectronic property with good thermal stability in liquid water and high hole mobility. The 2D M2 CO2 (M=Sc, Zr and Hf) multilayers are not to be viewed as suitable photocatalysts. This is because their bandgap decreases with increase of the layer number, which directly leads to the inappropriate band-edge position. 21 In 2017, Khan et al. investigated the strain affect of MXenes on electronic structures and photocatalytic performance. 22 The results showed that a biaxial tensile strain of 8% applied in Zr2 CO2 multilayer can achieve a transition from the indirect to direct bandgap, which can be used as a photocatalyst for overall water splitting. As stated in the above, we know that Zr2 CO2 monolayer has a great potential for overall water splitting. Similar to the other photocatalysts, however, the efficiency of H2 production for 2D Zr2 CO2 remains low. This disadvantage can be interpreted using two physical insights. The first, Zr2 CO2 is only able to absorb photons with energy from 2.5 to 4.1 eV, which brings about a low efficiency of solar energy conversion. The second, the electronic mobility is still low in Zr2 CO2 , although it exhibits a high hole mobility. Consequently, the photocatalytic activity of Zr2 CO2 needs to be further enhanced in virtue of surface co-catalysts. Actually, oxidation or reduction active sites can be provided by catalyst accelerator. Besides, this catalyst can also catalyze surface reaction through lowering the activation energy, suppress

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the recombination of photo-generated electrons and holes, and trap the charge carriers. 23 Thus, suitable co-catalysts should be searched for improving the catalytic performance of 2D Zr2 CO2 monolayer. It is worth mentioning that construction of heterostructure photocatalysts via co-catalysts has also been widely studied, which indicates an enormous potential to improve photocatalytic performance. Compared with the ordinary photocatalysts, 2D semiconductor heterostructure photocatalysts show a better photocatalytic performance, such as high carrier mobility, efficient separation of electron-hole pairs, low recombination of photo-generated carriers and strong visible-light absorption ability. For instance, g-C3 N4 -based heterostructures, g-C3 N4 /TiO2 , 24–26 g-C3 N4 /MoS2 , 27 g-C3 N4 /CdS, 28 g-C3 N4 /BiVO4 29 and g-C3 N4 /Zn2 GeO4 , 30 have showed an enhanced photocatalytic performance. Moreover, the other complex photocatalysts, such as TiO2 - 31,32 and CdS-based semiconductor heterostructures, 33,34 also exhibit much excellent photocatalytic performance than the individual catalysts. We thus know that constructing nanocomposite catalyst is an effective way to improve photocatalytic efficiency. To the best of our knowledge, 2D Zr2 CO2 -based semiconductor nanocomposite has rarely been investigated up to now. Hence, it is necessary and timely to design a novel Zr2 CO2 based heterostructure in order to increase its visible-light photocatalytic performance. Considering the water redox potentials and the suitable band edge position of photocatalyst, we choose 2D InSe monolayer as a co-catalyst to improve the photocatalytic performance of Zr2 CO2 . The novel InSe/Zr2 CO2 hybrid heterostructure can get rid of some weaknesses of the other photocatalysts mentioned above, such as low carrier mobility, poor visible-light absorption, instability of photocatalyst, high electron-hole recombination rate and so on. According to refs., 24–34 we believe that InSe/Zr2 CO2 heterostructures can expect to be synthesized by using one of the following approaches, such as ball milling method, 24 mechanical transfer method, 35 facile hydrothermal synthesis, 31 electrophoretic deposition method, 36 and immersion-precipitation technique. 37 In order to understand the photocatalytic mechanism, the atomic details at the interface and charge transfer across the hetero-interface between

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the light harvesting Zr2 CO2 nanosheet and the monolayer InSe should be explored in depth. The present work aims to provide a new insight into hetero-interface of 2D InSe/Zr2 CO2 heterostructure by virtue of the powerful DFT calculations, and then to get an atomic-level explaining of its improved visible-light catalytic mechanism. Our calculations prove that 2D InSe/Zr2 CO2 hybrid heterostructure displays a typical type-II band alignment with a direct bandgap of 1.81 eV and a significant electron transfer from Zr2 CO2 to InSe and vice versa for hole. These phenomena evidently improve visible-light absorption and photocatalytic properties by reducing electron-hole recombination. The high electron mobility of InSe (103 cm2 V−1 s−1 ) and hole mobility of Zr2 CO2 (103 cm2 V−1 s−1 ) are also extremely beneficial for enhancing the hydrogen-evolution reaction (HER) activity. The strong visible-light absorption (105 cm−1 ) has also been confirmed. Therefore, 2D InSe/Zr2 CO2 hybrid heterostructure is a energetic visible-light photocatalyst with higher photocatalytic hydrogen evolution (PHE) rate than individual Zr2 CO2 and InSe, which opens up a new route to design novel next-generation high performance visible-light photocatalysts.

COMPUTATIONAL DETAILS All calculations were carried out on the basis of framework of DFT and Vienna ab initio simulation package (VASP). 38,39 The projector augmented wave (PAW) method and the Perdew-Burke-Ernzerhof (PBE) functional under the generalized gradient approximation (GGA) were adopted. 40,41 The empirical van der Waals (vdW) correction method (DFT-D2) recommended by Grimme 42 was chosen to fully optimize the geometric structure, because it can more felicitously describe vdW interaction between Zr2 CO2 nanosheet and InSe monolayer. The valence electron configurations of C (2s2 2p2 ), O (2s2 2p4 ), Se (4s2 4p4 ), Zr (4d2 5s2 ) and In (4d10 5s2 5p1 ) have been taken into consideration. The kinetic energy was cut by 500 eV in following studies. In the 2D Brillouin zone, 4×4×1 and 8×8×1 k-point grids were utilized for geometry optimization and electronic structure calculation, separately. For all

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calculations, to break interaction of repeated two slabs, a vacuum space of 20 Å along the z -axis was adopted. For each atom, the criterion on total energy difference and remainder force were less than 10−4 eV and 10−2 eV/Å, separately, employing to fully optimize structures and atom positions. In order to verify dynamic stability of the InSe and Zr2 CO2 monolayers, the phonon spectra have been studied using PHONOPY code. 43,44 A 4×4×1 supercell containing 64 (80) atoms for InSe (Zr2 CO2 ) is constructed to calculate the phonon dispersion curves, in which a mesh of k-point 3×3×1 is employed. For InSe, Zr2 CO2 layers and InSe/Zr2 CO2 heterostructure, the correctness of electronic properties is the key of investigating their catalytic mechanism. It is well known that semiconductor bandgap is usually underestimated by using the general PBE functional, 45 so we adopt GW methods to accurately describe the electronic properties of 2D InSe and Zr2 CO2 . 46 In self-consistent GW 0 calculations, the energy cutoff and the frequency grid point number are set to 450 eV and 64, respectively, and the K-point grid is set to 11×11×1 for 2D InSe and Zr2 CO2 . The quasi-particle band structure has been fitted by Wannier functions implanted in the Wannier 90 package. 47 As is known to all, GW methods can give accurate electronic band structures of semiconductors, but it requires tremendously expensive computational cost, which is a formidable task for some complicated structures. Therefore, the GGA-1/2 scheme, 48,49 an accurate bandgap calculation and time-consuming approach, is used to revise the underestimated PBE bandgap for 2D InSe/Zr2 CO2 heterostructure. According to previous researches, 48,49 the difference between the all-electron potentials of the atom and those of the half-ion can be expressed using atomic self-energy potential,

Vs ≈ V (0, r) − V (−1/2, r).

(1)

For Se and O atoms, the p-orbitals have been revised using half ionization. The long-range

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Coulomb tail of potential Vs can be expressed using a function,    [1 − ( r )n ]3 , (r 6 CU T ), CU T Θ(r) =   0, (r > CU T ).

(2)

The optimum n and CU T values can be chosen in accordance with the energy bandgap of the semiconductor reaching the experimental or accurate GW band gap. In our calculations, the power n=100 (90) and the trimming parameter CU T =3.05 (1.94) a.u. for Se (O) atom were selected. The reason is that the bandgaps calculated by these parameters are 2.71 and 2.21 eV for InSe and Zr2 CO2 , individually (see Table 1), which is in excellent agreement with the corresponding GW values. Consequently, the electronic structures of InSe/Zr2 CO2 heterostructure are calculated using PBE functional together with the GGA-1/2 bandgapcorrection in the present work. In order to further investigate the interband optical absorption, the frequency-dependent dielectric function has firstly been calculated. We calculate the imaginary part of the dielectric function, according to the following formula, 50 (2)

αβ (ω) =

∗ 1 X 4π 2 e2 lim 2 2ωk δ(ck − vk − ω) × huck+eα q |uvk i uck+eβ q |uvk . Ω q→0 q c,v,k

(3)

Here, q represents the wave-number of the incident electromagnetic wave, eα is the unit vector for the α direction, Ω denotes the unit cell volume and v and c are the valance and conduction band, respectively. From the imaginary part of dielectric function, using Kramers-Kronig transformation, 51 the real part can be derived as follows,

(1) αβ (ω)

2 =1+ P π

∞

Z 0

(2)

αβ (ω 0 )ω 0 ω 0 2 − ω 2 + iη

dω 0 ,

(4)

where the P represents the principle value. The significant optical absorption coefficient

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α(ω) can thus be calculated based on the widely-used formula, 52–59

α(ω) =

√

2ω[

q

(1) (ω)2 + (2) (ω)2 − (1) (ω)]1/2 .

(5)

Some previous calculations have clearly shown that, for several important semiconductors including BaTiO3 and InN, the optical absorptions derived from eq 5 are in excellent agreement with the experiments only if the underestimated DFT bandgaps are revised to their experimental bandgaps. 52,60,61 Moreover, a detailed comparison for the dielectric property of Si, calculated by using the standard DFT, random phase approximation (RPA) quasiparticales with single-shot GW and Bethe-Salpeter equation (BSE) methods, has been given and a good agreement can be found in the Ref. 62 In our following optical property calculations, the underestimated DFT bandgaps of monolayer InSe and Zr2 CO2 together with InSe/Zr2 CO2 heterostructure have been revised according to the aforementioned GGA-1/2 method. We adopt PBE functional to calculate the imaginary part of the dielectric function and optical absorption of monolayer InSe and Zr2 CO2 (1×1×1 primitive cell) with different K-point meshes of 150×150×1 and 30×30×1 and find that a negligible difference exists in their optical absorption spectra. Therefore, we choose the K-point mesh of 10×10×1 in our √ √ InSe/Zr2 CO2 (3×3×1 InSe and 13× 13×1 Zr2 CO2 supercell) optical property calculations. The Gaussian smearing width of 0.05 eV has been carefully tested. Generally, the carrier mobility µ2D of 2D semiconductor, with a view to studying the properties of the migration of electrons and holes, can be obtained as follows, 63

µ2D =

where m∗ and md =

e~3 C2D , KB T m∗ md E12

(6)

p ∗ ∗ mx my are carrier effective mass along transport direction and average

effective mass, respectively. The E1 = ∆V /(∆l/l0 ) is deformation potential constant, defined as the shift of band edges induced by strain. Here, ∆V represents the energy difference about CBM or VBM with the lattice applied by proper dilatation or compression. l0 and ∆l are 9



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the lattice constant along the x or y direction and their deformation, correspondingly. In 2D semiconductor, in-plane stiffness C2D can be calculated using 2[∂ 2 E/∂(∆l/l0 )2 ]/S0 , where E and S0 stand for the total energy and the area of xy plane for the supercell , individually. The T is temperature, which was set to 300 K in our calculation.

RESULTS AND DISCUSSION Table 1: The Lattice Parameter a, b (Å) and the Bandgap E g (eV) Calculated Using GGA, GGA-1/2 and GW Methods for InSe Monolayer and Zr2 CO2 Nanosheet. By Way of Comparison, Some Preceding Theoretical Results Are Given as Well InSe Zr2 CO2

a b E g (GGA) 4.08 7.07 1.38 4.09a 7.08a 1.49b 3.31 5.73 1.05 3.31d 5.73d 0.97e a ref. 53 b ref. 16 c ref. 64

E g (GGA-1/2) E g (GW ) 2.71 2.75 2.74c 2.21 2.26 d

ref. 11

e

ref. 65

Figure 1: Top view of (a) InSe and (b) Zr2 CO2 monolayers. The blue and red dashed lines shows hexagonal primitive cells and orthogonal supercells. (c) Side view of designed InSe/Zr2 CO2 heterostructure. Specifically, C, O, Se, Zr and In atoms are plotted by the black, blue, green, yellow and purple balls, separately. (d) In first Brillouin zone (FBZ), the hexagonal primitive cell is plotted using blue line with labels of K, M and Γ points, and the orthogonal supercell has been presented by red line with points of Y, Γ, X and M. Before studying InSe/Zr2 CO2 heterostructure (see Figure 1), we firstly investigate the 10


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freestanding InSe and Zr2 CO2 monolayers aiming to test the correctness of our calculations, and results are listed in Table 1. From Table 1 and Figure 1, we can find InSe monolayer accesses a honeycomb lattice with a=4.08 and b=7.07 Å, In-Se bond length 2.68 Å and vertical In-In distance 2.82 Å. These are in line with experimental and theoretical results. 13,53 As shown in Figure 1(b) and (c), the Zr2 CO2 nanosheet has a hexagonal-like lattice (top view) and a sandwich configuration (side view). Here, the transition metal (Zr atom) layers are terminated by O atoms. It can be seen that the C atom is completely saturated by the surrounding four Zr atoms, the Zr atom is connected with two C atoms and two O atoms, and all O atoms only have two nearest Zr atoms. The gained lattice parameter a (b) is 3.31 (5.73) Å, also supported by earlier theoretical calculations. 11

Figure 2: Calculated phonon dispersion curves of (a) InSe and (b) Zr2 CO2 monolayers, respectively. Phonon spectra, as shown in Figure 2, are used to examine lattice stability of monolayer InSe and Zr2 CO2 . All positive frequency modes in phonon dispersion curves demonstrate that both InSe and Zr2 CO2 monolayers are dynamically stable. Moreover, the thermal stability of 2D few-layer InSe has been confirmed in experiments at room temperature. 15 The calculations of the enthalpy of solvation, performed by Zhuang et al., 16 show that 2D InSe is stable in aqueous solution. Based on ab initio molecular dynamics (AIMD) simulations, 2D Zr2 CO2 is also proved to be stable in liquid water at room temperature. 11 Hence, both 2D InSe and Zr2 CO2 nanosheets are thermal stable in water at room temperature. 11



Figure 3: Calculated band structures and corresponding PDOSs for InSe (a) and Zr2 CO2 (c) monolayers with GW method. For the purpose of comparison, the PBE results with the GGA-1/2 bandgap correction are also plotted for InSe (b) and Zr2 CO2 (d) nanosheets. It is well-known that the bandgaps of semiconductors are seriously underestimated in GGA-PBE calculations. Hence, the Heyd-Scuseria-Ernzerhof (HSE06) and GW schemes are usually adopted to correct band structures, 46,66 in which the GW method is recognized as the most accurate strategy to describe band structures for semiconductors. 67,68 The modification band structures of GW and corresponding projected density of states (PDOS) for InSe and Zr2 CO2 nanosheets are presented in Figure 3(a) and (c). Meanwhile, with the purpose of comparison, the PBE combined with the GGA-1/2 scheme results are also given in Figure 3(b) and (d). The InSe monolayer is an indirect bandgap semiconductor with a gap of 2.75 eV (see Figure 3(a) and (b)), where the CBM lies on the Γ point while the VBM on a point between Γ and K. The VBM state of InSe monolayer is primarily determined by Se-4p and In-5p orbits, while the CBM is dominated by Se-4p and In-5s orbits. These results are also confirmed by previous calculations. 16,64 12


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Figure 3(c) and (d) shows that Zr2 CO2 monolayer is an indirect bandgap semiconductor because the VBM and CBM locate high symmetry points of Γ and M, respectively. The calculated bandgap of Zr2 CO2 is 1.05 eV by GGA-PBE scheme (see Table 1), which has been confirmed by other theoretical calculations (0.97 eV). 11 We further note that Zr2 CO2 monolayer has a gap value of 1.62-1.76 eV, certified by all the previous HSE06 band structure calculations. 11,21,22,69 In order to get more accurate gap value, the band structure of Zr2 CO2 was calculated by the GW methods. The result shows that the gap value of Zr2 CO2 is 2.26 eV. According to the PDOS, the 2p orbits of C and O atoms primarily offer VBM of Zr2 CO2 , while the CMB is primarily derived from the 4d orbits of Zr atoms, which is in good agreement with previous works. 11,65 Based on the above discussion, a conclusion can be arrived that the band structures of InSe and Zr2 CO2 derived from the PBE+GGA-1/2 scheme are well in line with GW calculated results. Therefore, the PBE+GGA-1/2 method will be applied to study InSe/Zr2 CO2 heterostructure. In order to combine the photocatalytic advantages of InSe monolayer and Zr2 CO2 nanosheet, we construct the innovative InSe/Zr2 CO2 heterostructure to apply in overall water splitting under sunlight irradiation. The side view of InSe/Zr2 CO2 heterostructure is plotted in Fig√ √ ure 1 (c), in which a 13× 13×1 hexagonal supercell of Zr2 CO2 nanosheet is stacked on a hexagonal supercell of 3×3×1 InSe monolayer. The lattice mismatch of our designed nanocomposite is no more than 2.6%. After geometry structure was fully optimized, the measured vertical distance between InSe and Zr2 CO2 is about 3.02 Å, which can be regarded as a moderate vdW equilibrium space. For the sake of assessing thermodynamic stability of the constructed 2D InSe/Zr2 CO2 heterostructure, the formation energy is calculated according to the following formula,

Ef = EInSe/Zr2 CO2 − EInSe − EZr2 CO2 ,

(7)

where EInSe/Zr2 CO2 , EInSe and EZr2 CO2 are the total energies of fully relaxed InSe/Zr2 CO2

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heterostructure, isolated InSe monolayer and Zr2 CO2 nanosheet, respectively. As expressed by the above equation, a negative Ef value denotes a stable heterostructure. The calculated Ef value is -33.44 meV/per-atom-pair for the fully optimized 2D InSe/Zr2 CO2 heterostructure, similar to the previous theoretical value -39.96 meV/per-atom-pair of C2 N/MoS2 vdW heterostructure. 70 The negative Ef indicates that the Zr2 CO2 nanosheet has an attractive interaction with the InSe monolayer via vdW force. Moreover, the formation energy of 33.44 meV/per-atom-pair is larger than the thermal energy at room temperature, which indicates the thermal stability of InSe/Zr2 CO2 heterostructure. Hence, the stability of the designed 2D InSe/Zr2 CO2 nanocomposite can thus be expected. (a)

(b)

(c)

CBM

(d)

VBM

3

2

InSe/Zr 2CO2 Energy (eV)

Energy (eV)

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InSe

1

1.81 eV

0

1.6

1.2

0.8

DOS (arb. units)

Zr2CO2

InSe -1

Zr 2CO2

-2

M

K

M

DOS (arb. units)

Figure 4: (a) Projected band structure of InSe/Zr2 CO2 heterostructure and (b) the corresponding DOS. Here, the red and blue lines denote the contribution of InSe and Zr2 CO2 layers, separately. Fermi level is set to 0 eV. The band-decomposed charge density distributions of InSe/Zr2 CO2 have been plotted using (c) for CBM and (d) for VBM (ρ=1.6 × 10−5 eÅ−3 ). To understand well the interaction mechanism between InSe and Zr2 CO2 , we further present the projected band structure and corresponding DOSs of 2D InSe/Zr2 CO2 vdW heterostructure, as shown in Figure 4 (a) and (b). The result shows that InSe/Zr2 CO2 heterostructure possesses a direct bandgap 1.81 eV at the Γ point. Compared with gap value

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of InSe (2.75 eV) and Zr2 CO2 (2.26 eV), we note that the gap value of InSe/Zr2 CO2 decreases by 0.94 and 0.45 eV, respectively. The finding can be attributed to the band offset of InSe and Zr2 CO2 nanosheets. From the projected band structure and the calculated DOSs, we can find the VBM of heterostructure is completely contributed by Zr2 CO2 layer, and that the CBM is entirely dominated by InSe layer. Interestingly, in contrast with the band structure of freestanding InSe and Zr2 CO2 , the distinctive properties of the calculated band structure projected on two different layers in the constructed InSe/Zr2 CO2 hybrid nanoheterostructure are similar to that of InSe for the CBM and Zr2 CO2 for the VBM. This can be easily proved because of the large separation (∼3.02 Å) between InSe and Zr2 CO2 layers, which apparently indicates that the InSe-Zr2 CO2 interaction is of weakness because the hybrid interface fails to form a covalent bond. The discussions mentioned above manifest the transition of electrons from VBM to CBM in InSe/Zr2 CO2 heterostructure gets easier and easier because of its direct and favourable band gap, beneficial to enhancing the efficiency of photoabsorption. To obtain more physical properties of InSe/Zr2 CO2 heterostructure, the band-decomposed charge densities of the CBM and VBM have been calculated, as plotted in Figure 4 (c) and (d). We find the lowest-energy hole and electron states are lain in the Zr2 CO2 and InSe layer, respectively. InSe/Zr2 CO2 heterostructure forms a type-II band alignment and the spatial separation of holes and electrons can thus be predictable. In addition, we can predict the water reduction and oxidation process mainly occur in InSe monolayer and Zr2 CO2 nanosheet under sunlight irradiation, respectively (see the following Figure 8). The separation of photo-generated holes and electrons in 2D InSe/Zr2 CO2 nanocomposite further endorses its photocatalytic efficiency. As an effective way to explore the charge transfer mechanism across the heterointerface of 2D InSe/Zr2 CO2 nanoheterostructure in-depth, the charge density difference of InSe/Zr2 CO2 has been calculated by the following equation, 71,72

∆ρ = ρInSe/Zr2 CO2 − ρInSe − ρZr2 CO2 ,

15


(8)


Figure 5: Charge density difference of InSe/Zr2 CO2 is plotted by top panel. The accumulation and depletion of electrons are presented using cyan and green color isosurfaces, respectively (ρ=1.5 × 10−4 eÅ−3 ). The bottom panel shows plane-averaged plot. where ρInSe/Zr2 CO2 , ρInSe and ρZr2 CO2 are the charge densities of InSe/Zr2 CO2 heterostructure, InSe and Zr2 CO2 layers, respectively. From Figure 5, we find that the charge density is reconstructed near the heterointerface area, accumulated close to InSe layer because of the positive ∆ρ and depleted near Zr2 CO2 layer owing to negative ∆ρ. In addition, the effective electron accretion close to the InSe layer reaches 3.02×10−4 eÅ−1 . It demonstrates that the electrons are transferred from Zr2 CO2 to InSe layers through the vdW gap, which is chiefly on account of its band offset. A built-in electric field has formed in heterostructure because of charge movement, eventually conducing to the Fermi level alignment of Zr2 CO2 and InSe layers. To sum up, the electrons and holes are accumulated in InSe and Zr2 CO2 , respectively. Undoubtedly, Figures 4 and 5 indicate that there is an amazing e− -h+ separation in InSe/Zr2 CO2 heterostructure. Meanwhile, the holes are situated in the Zr2 CO2 layer with high hole mobility about 103 cm2 V−1 s−1 and the electrons are lain in the InSe layer with high electron mobility about 103 -104 cm2 V−1 s−1 (see Table 2). These facts are beneficial to enhancing the photocatalytic activity.

16


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Figure 6: The electrostatic potentials for (a) InSe, (b) Zr2 CO2 monolayers, and (c) InSe/Zr2 CO2 heterostructure. The Fermi level and vacuum energy level are labelled by blue and red dashed lines, separately. Here the vacuum energy level has been set to 0 eV as a reference. The work function of a material is a critical parameter commonly used as an intrinsic reference for band alignment. 73 In a general way, the work function can be expressed as follows, Φ = Evac − EF .

(9)

Here, the Evac and EF are the energy of a fixed electron in the vacuum and the Fermi level of the material, respectively. The electrostatic potentials for InSe monolayer, Zr2 CO2 nanosheet and 2D InSe/Zr2 CO2 vdW heterostructure are showed in Figure 6 (a)-(c). The results indicate that the work function of InSe, Zr2 CO2 and InSe/Zr2 CO2 are 5.51, 5.26 and 5.41 eV, respectively. It is the higher Fermi level of Zr2 CO2 nanosheet that impels the photo-generated electrons to migrate from its valence band into the conduction band of InSe monolayer across the vdW gap because of its lower Fermi level. These facts are also strongly supported by the above results of Figures 4 and 5. As we all know, the optical absorption plays a critical role in the photocatalytic overall water splitting. Thus, we further investigate the optical properties of 2D InSe/Zr2 CO2 nanocomposite. Figure 7 illustrates the light absorption capability of 2D InSe/Zr2 CO2 vdW heterostructure, freestanding InSe and Zr2 CO2 layers. It is shown that the 2D InSe exists

17



Figure 7: The optical absorption of InSe and Zr2 CO2 monolayers together with 2D InSe/Zr2 CO2 vdW heterostructure, in which the underestimated DFT bandgaps have been revised according to GGA-1/2 method. For the sake of comparison, the results of gC3 N4 /MoS2 heterostructure 27 and the previous calculated InSe monolayer 53 are also given. The inset is our calculated imaginary part of the dielectric function of Zr2 CO2 monolayer together with the other calculations. 21 We can find that our optical absorption spectra for InSe monolayer and the imaginary part of the dielectric function for Zr2 CO2 monolayer are supported by previous results, 21,53 just except for a minor variance caused by the trivial difference in the band dispersions of our PBE and HSE06 calculations. a reasonable energy bandgap (2.75 eV), which can be viewed as a promising photocatalyst. However, the major light absorption of InSe monolayer locates a high energy region, that is to say, it has a low photocatalytic efficiency for water splitting under sunlight irradiation. In addition, photo-generated holes and electrons show an objectionable spatial separation because of their ultrathin thickness. These clearly indicate that InSe monolayer should not be looked upon as a highly efficient visible-light photocatalyst for overall water splitting although it has a sensible gap value (2.75 eV). The Zr2 CO2 nanosheet possesses a favorable bandgap (2.26 eV), which is a promising visible-light photocatalyst. Compared with InSe, obviously, Zr2 CO2 has an enhanced absorption in visible-light region and a large sunlight absorption coefficient. But, for Zr2 CO2 nanosheet, the sunlight absorption edge does not reach extreme requirement of water redox reaction (1.23 eV). Additionally, the photo-generated electrons and holes in the spatial separation are also not good enough due to the same reason with InSe monolayer. Hence, 18


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we construct a novel 2D InSe/Zr2 CO2 vdW heterostructure (see Figure 1) to overcome the disadvantages mentioned above. Figure 7 clearly shows that the original InSe/Zr2 CO2 heterostructure improves optical response in visible-light region with absorption edge of 1.8 eV, and reveals more effective UV absorption. In comparison with the isolated InSe and Zr2 CO2 layers, the absorption edge of InSe/Zr2 CO2 is about 0.95 and 0.46 eV, smaller than that of InSe and Zr2 CO2 nanosheet. Moreover, the absorption coefficient of InSe/Zr2 CO2 nanoheterostructure increases up to about 4 (30) times compared with Zr2 CO2 (InSe) monolayer in the visible-light region at 2.75 eV, which combines the advantage of InSe and Zr2 CO2 layers. Meanwhile, in the 2D InSe/Zr2 CO2 heterostructure, the photo-generated electrons and holes are located in the InSe and Zr2 CO2 layers, respectively, which reaches spatial separation (∼3 Å) of e− -h+ pairs. And it decreases recombination of holes and electrons. In contrast with the optical absorption of the well-known g-C3 N4 /MoS2 heterostructure photocatalyst, 27 the optical absorption coefficient of InSe/Zr2 CO2 is about eight times as large as that of g-C3 N4 /MoS2 under visible-light irradiation. Based on the above discussion, the excellent sunlight absorption and low recombination of electron-hole pairs are conducive to improve the photocatalytic efficiency of InSe/Zr2 CO2 photocatalyst. It is well-known that carrier mobility is another significant indicator for a good photocatalyst. Hence, the carrier effective masses and mobilities along x and y directions were calculated, in which the orthorhombic lattices are used instead of the hexagonal cells for 2D InSe, Zr2 CO2 and InSe/Zr2 CO2 vdW heterostructure. The selected supercells and corresponding first Brillouin zone are plotted in Figure 1. And the obtained effective masses (m∗ ), E1 , C2D and the carrier mobilities (µ) are listed in Table 2. First, for the 2D InSe, the mobility of electron in the x (y) direction is up to 1619.51 (1779.16) cm2 V−1 s−1 . However, the hole mobilities only have 134.90 and 65.70 cm2 V−1 s−1 along Zigzag and Armchair directions, separately. Specifically, the reason behind the above results is that the hole has a heavier m∗ than the electron. These calculations have also been supported by preceding theoretical 17 and experimental results. 15,75 Second, for 2D Zr2 CO2 , the electron mobilities are 57.55 cm2 V−1 s−1

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Table 2: Calculated Carrier Mobility µ (cm2 V−1 s−1 ) in InSe, Zr2 CO2 Layers and InSe/Zr2 CO2 vdW Heterostructure along x (Zigzag-Chain) and y (ArmchairChain) Directions at 300 K. Effective Mass is labelled by m∗ (m0 ). Deformation Potential Constant has been given using E1 (eV). In-Plane Stiffness is presented using C2D (Nm−1 ). We Also List Some Former Theoretical and Experimental Results to Verify the Accuracy of Our Calculations material electrons InSe

m∗x

m∗y

E1x

E1y

C2D−x

C2D−y

0.18 0.19 4.27 3.97 45.99

46.10

µx

µy

InSe/Zr2 CO2

1619.51 1779.16 1404.89a 1873.79a 2.57 0.35 6.02 7.57 264.31 264.71 57.55 612.12 82.59c 1096.46c 814d 144d 0.16 0.21 4.35 4.13 315.73 317.24 10942.98 9293.66

holes InSe

2.14 2.02 1.28 1.89 45.99

Zr2 CO2

Zr2 CO2

InSe/Zr2 CO2

46.10

134.90 100.04a 0.40 0.43 2.52 3.16 264.31 264.71 4828.88 2299.98c 81500d 0.39 0.41 2.61 3.21 315.73 317.24 5716.60 a ref. 17 b ref. 15 c ref. 11 d ref. 74 e ref. 75

65.70 159.94a 2859.25 1695.65c 4290d 3797.36

µexp

103b

40e

for x direction and 612.12 cm2 V−1 s−1 for y direction. Interestingly, the hole mobilities are 4828.88 and 2859.25 cm2 V−1 s−1 for x and y directions, respectively. Our calculated results coincide with previous calculations. 11,74 Nevertheless, the rate of carrier migration is limited by low hole mobility in InSe and low electron mobility in Zr2 CO2 , tremendously increasing the recombination of the photo-generated electron-hole pairs. This drawback is harmful to improving the photocatalytic activity for freestanding InSe and Zr2 CO2 layers. Surprisingly, for designed InSe/Zr2 CO2 nanocomposite, the mobility of electron in the Zigzag (Armchair) direction increases by 5-6 times, the value of InSe monolayer from 1619.51 to 10942.98 cm2 V−1 s−1 for x direction and from 1779.16 to 9293.66 cm2 V−1 s−1 for y direction. However, the hole mobility has a favorable enhancement compared with Zr2 CO2 monolayer, but is still much less than the electron mobility. The low hole mobility in InSe/Zr2 CO2 can be expected to be improved by means of the strain engineering 64,76 or exerting an external

20


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electric field. 77 From Table 2, we can note the effective mass m∗ and the deformation potential E1 of the VBM and CBM in InSe/Zr2 CO2 are almost unchanged, similar to InSe and Zr2 CO2 monolayers, respectively. Nevertheless, compare the InSe/Zr2 CO2 heterostructure with freestanding 2D InSe. The carrier mobilities along both the x- and y-direction, especially the electron mobilities, have an extraordinary improvement. The physical explanation is a consequence of the increasing on in-plane stiffness C2D (see Table 2), greatly reducing the electron-phonon scattering probability as analyzed by ref. 78

Figure 8: In relation to the standard hydrogen electrode (SHE) potential, bandgaps and band edge positions of InSe and Zr2 CO2 layers, and InSe/Zr2 CO2 nanocomposite are presented by (a) and (b), separately. The standard water redox potentials are drawn using black dashed lines. High photocatalytic performance would be predicted in InSe/Zr2 CO2 photocatalyst. It is essential that accurate band edge position of InSe/Zr2 CO2 relative to the standard water redox potentials has been investigated deeply. As we all know, the gap value is always underestimated by GGA-PBE scheme. In other words, it can not accurately calculate band edge positions. Fortunately, Perdew et al. proved that the DFT scheme is formally exact to calculate the band gap center (BGC). 79 Toroker et al. showed that the BGC is insensitive to the type of exchange-correlation functionals. 80 Furthermore, the calculated different BGC energies are merely 0.01-0.14 eV by HSE06 and PBE functionals. 16 In the present work, the BGC energies and gap values are calculated by GGA-PBE functional and GW methods

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(GGA-1/2 scheme was adopted for InSe/Zr2 CO2 heterostructure). Herein, with respect to the standard hydrogen electrode (SHE), we can accurately evaluate the band edge positions of InSe and Zr2 CO2 layers as follows, 16,80–82 1 ECBM = EBGC + Eg , 2 1 EVBM = EBGC − Eg . 2

(10)

Here, the EBGC and Eg are BGC energies and gap values, respectively. According to the eq 10, the band edge of InSe is 2.48 (-0.27) eV for VBM (CBM), which is consistent with the results of ref. 16 Similarly, the CBM and VBM of Zr2 CO2 can be estimated to be -0.73 and 1.53 eV, which has band edges located at energetically favorable positions for photocatalysis, indicating that Zr2 CO2 monolayer is suitable for water splitting (see Figure 8 (a)). Figure 8 (b) shows that the band edges of InSe/Zr2 CO2 nanocomposite exactly straddle the water redox potentials. The calculated CBM of InSe/Zr2 CO2 is 0.24 eV (known-as reducing capacity) lower than reduction potential and the VBM is 0.34 eV (named as oxidizing ability) higher than oxidation potential. The overall water splitting can thus be achieved in the novel 2D InSe/Zr2 CO2 vdW heterostructure photocatalyst under visible-light irradiation. From the band edges of InSe and Zr2 CO2 nanosheet, we can find that the significant valence (conduction) band offset, defined as the comparative band edge difference of VBM (CBM) between the InSe and Zr2 CO2 layers, is 0.95 (0.46) eV, tremendously enhancing the photocatalytic performance. To be specific, the photo-genearted electrons in Zr2 CO2 tend to migrate to conduction band of InSe, while the holes in InSe are inclined to move to valence band of Zr2 CO2 . The highly desirable electron-hole separation can thus be realized in InSe/Zr2 CO2 nanoheterostructure (see Figures 4 and 5). As a result, our designed InSe/Zr2 CO2 nanocomposite, as an efficient photocatalyst for overall water splitting, has broad prospect.

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CONCLUSIONS In summary, on the basis of DFT calculations, we systematically assessed the structural, electronic properties and photocatalytic mechanism toward overall water splitting catalyzed by 2D InSe/Zr2 CO2 nanocomposite. The calculated lattice mismatch is no more than 2.6% and formation energy is 33.44 meV/per-atom-pair, which verifies that InSe monolayer can combine with Zr2 CO2 nanosheet to form a stable nanocomposite. The novel 2D InSe/Zr2 CO2 vdW heterostructure is considered as a prospective photocatalyst with exceptional visiblelight absorption for overall water splitting. The above viewpoints can be supported by the following profound physical reasons. (1) It possesses an unexpected direct band gap (1.81 eV) and a significant amount of optical absorption in the visible-light region (∼3×105 cm−1 ). (2) Higher efficiency of electron-hole separation and transfer can be realized in InSe/Zr2 CO2 heterostructure, with the photo-generated electrons transferring from Zr2 CO2 to InSe and vice versa for holes, because of its critical type-II band alignment with valence (conduction) band offset of 0.95 (0.46) eV. (3) The spatial separation (∼3.02 Å) of photo-generated electron-hole pairs, intrinsic built-in electric field and the indirect band gap of InSe and Zr2 CO2 monolayers prohibit the energy-wasted recombination of electrons and holes in heterostructure and thus extremely improve performance of photocatalyst. (4) The electron and hole mobilities in InSe/Zr2 CO2 heterostructure are high up to 104 and 103 cm2 V−1 s−1 , respectively, which can significantly enhance the catalytic efficiency of InSe/Zr2 CO2 photocatalyst. We thus recommend the innovative 2D semiconductor nanoheterostructure photocatalyst with improved visible-light catalytic activity and give a thorough understanding of its photocatalytic mechanism. The current work concludes that 2D InSe/Zr2 CO2 heterostructure with broad visible-light response is a favorable candidate photocatalyst in further overall water splitting, propounding a new approach to construct highly active and visible-light harvesting photocatalysts in the near future.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported jointly by the National Key Research and Development Program of China (Grant No. 2017YFA0206303, MOST of China) and the National Natural Science Foundation of China (11474012 and 11364030). We used computational resource of the "Explorer 100" cluster system of Tsinghua National Laboratory for Information Science and Technology and part of the analysis was performed on the Computing Platform of the Center for Life Science of Peking University.

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Improvement of Visible-Light Photocatalytic Efficiency in a Novel InSe

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