Janus Chromium Dichalcogenides Monolayers with Low Carrier

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Janus Chromium Dichalcogenides Monolayers with Low Carrier Recombination for Photocatalytic Overall Water-Splitting under Infrared Light Pei Zhao, Yan Liang, Yandong Ma, Baibiao Huang, and Ying Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11240 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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The Journal of Physical Chemistry

Janus Chromium Dichalcogenides Monolayers with Low Carrier Recombination for Photocatalytic Overall Water-Splitting under Infrared Light Pei Zhao, Yan Liang, Yandong Ma*, Baibiao Huang and Ying Dai* School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Shandanan Str. 27, Jinan 250100, People's Republic of China *Corresponding author: [email protected] (Y. M); [email protected] (Y. D)

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ABSTRACT Photocatalytic overall water-splitting is known as one of most promising methods to alleviate energy crisis. Searching for stable and efficient photocatalysts plays a critical role in this process. Here, we propose a novel class of Janus chromium dichalcogenides (CrXY, X/Y = S, Se, Te) monolayers serving as efficient photocatalysts for overall water-splitting under infrared light irradiation. We reveal that these Janus monolayers harbor an intrinsic dipole, which promotes the spatial separation of photo-generated carriers. More significantly, these systems exhibit suitable band gaps as well as band edge positions, enabling preeminent infrared optical absorption and high carrier mobility. Furthermore, the nonradiative recombination of photoinduced charge carriers in CrXY monolayers are evaluated based on time-domain density functional theory. The obtained long-lived excited carriers (~ 2 ns) are even comparable with that in transition-metal dichalcogenides heterostructures, which benefits for the photocatalytic reaction with high efficiency. Our results provide a new guidance for designing brand new photocatalytic systems with broad optical absorption and low carrier recombination.

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1. INTRODUCTION Searching for clean and renewable energy is an effective way to solve environmental pollution and energy shortage. Hydrogen production utilizing solar energy based on photocatalytic water-splitting technology has attracted widespread attention during the past decades.1,2 In photocatalytic field, exploring robust photocatalysts with high conversion efficiency from solar energy to hydrogen energy is one of the most urgent issue. To this end, excellent optical absorption, spatial separation of photogenerated carriers and energy level are the prerequisites for achieving high photocatalytic efficiency. Nevertheless, conventional three dimensional (3D) photocatlysts, such as TiO2, SrTiO3, CdS and WO3 etc.,3-9 exhibit a wide band gap, which restricts their absorption in the ultraviolet light region, accounting for only about 5% of solar energy. And most of the reactive sites of these 3D photocatalysts cannot be exposed on the surface. Although the carriers can migrate from the interior to the surface of bulk, such migration would increase the possibility of their recombination.10,11 As a result, most of the 3D photocatalysts display a low photocatalytic efficiency. Despite extensive efforts have been devoted to enhancing their performances in photocatalytic reaction (i.e., band gap engineering, surface modification and developing new reactive mechanism, etc.12-14), the photocatalytic water-splitting efficiency still call for further improvement. Since the birth of graphene,15-19 a wide variety of two-dimensional (2D) materials

have been

applied in photocatalytic field, such as, 2D oxides, 2D chalcogenides and 2D g-C3N4 etc..20-25 Compared with 3D photocatalytic materials, 2D photocatalysts hold high carrier mobility, tunable band gap and light absorption, large specific surface area and short migration distance for the generated electrons and holes.21 Recently, Janus MoSSe monolayer was successfully synthesized via chemical vapour deposition (CVD) method.26 Due to the broken structural symmetry, the Janus structure has an intrinsic dipole moment in the out-of-plane direction, thus inducing a built-in electric field. More recently, Janus MoSSe monolayer has been demonstrated to be a promising 3



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photocatalyst.27-29 However, its large band gap makes it missing the energy of infrared spectrum accounting for even 43% of solar energy. In this case, it is imperative to hunt for novel photocatalysts to fully utilize solar energy and improve the energy conversion efficiency. Inspired by the unique properties of Janus monolayers,30-32 we design a series of novel Janus CrXY monolayers and explore their potential applications in photocatalytic overall water-splitting based on first principles calculations. Their stability is examined by ab initio molecular dynamics (AIMD) and phonon calculations. Interestingly, all these Janus monolayers exhibit suitable band edge positions relative to redox levels of water, and the intrinsic electrostatic potential difference between the top and bottom surfaces could effectively promote spatial separation of photo-generated carriers. The most remarkable point is that Janus monolayers hold excellent optical absorption under the whole solar spectrum including the near-infrared light. Meanwhile, the detailed time-domain ab initio atomistic simulations prove that the electron-hole recombination rate in CrXY is slow, making these materials bear a high potential for applications in photocatalysis.

2. COMPUTATIONAL METHODS First principles calculations are carried out by using projector-augmented wave (PAW) scheme implemented in Vienna ab-initio simulation package (VASP).33,34 For

geometrical optimizations,

we use the generalized gradient approximation (GGA) as formulated by Predew-Burke-Ernzerhof (PBE) to describe the interaction between exchange and correlation interaction between electrons.35,36 Cut-off energy is set as 500 eV for the plane-wave expansion of wave functions, and a vacuum space of about 15 Å perpendicular to 2D plane is included to avoid the interaction between adjacent layers. A 15×15×1 grid for k-point sampling is adopt. The structures are fully relaxed until the residual force and energy difference are, respectively, less than 0.01 eV/Å and 10-5 eV. It is worth mentioning that HSE functional may give wrong electronic structures of CrXY compounds, and for the sake of safety, the electronic properties of CrXY monolayers are checked not only based on PBE and HSE functional but also based on G0W0 method.37-39 In one shot G0W0 calculation, we set 10-8 eV and 150 eV as the 4


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energy convergence and cutoff energy. And the number of empty band is set as 144 to obtain accurate results. Besides, thermal stability is demonstrated with a 5×5×1 supercell at 300 K based on AIMD, and dynamical stability is examined by 6×6×1 supercell based on PHONOPY code.40,41 The nonadiabatic molecular dynamic simulations (NAMD) for electron-hole recombination with decoherence correction in CrXY are carried out using the PYXAID code.42,43 The methodology and justification are presented elsewhere.44,45 A 4 ps MD simulation at 300 K in the microcanonical ensemble is performed with a time step of 1 fs, then the generated trajectories are used to obtain the nonadiabatic (NA) Hamiltonian. 500 initial configurations are randomly sampled from the first 1ps MD trajectory for NAMD simulations.

3. RESULTS AND DISCUSSION 3.1 Structure and stability of the CrXY monolayer The crystal structure of CrXY monolayer is formed by one Cr layer sandwiched by one X (S, Se, Te) layer and one Y layer (X≠Y; S, Se, Te). As shown in Figure 1(a), CrXY monolayer share similar honeycomb structures with that of pristine CrX2 monolayer. To provide more information on the surface of crystal for experiments, the simulated scanning tunneling microscope (STM) is plotted in Figure 1(b). Due to the atomic difference of top and bottom surfaces, the mirror symmetry of the structure is broken, thus leading to a dipole moment along the out-of-plane direction. The optimized lattice constants of pristine CrX2 and Janus CrXY monolayers are listed in Table I. We can see that the lattice constant of CrSSe monolayer is found to be 3.13 Å, approaching to the average value of those of CrS2 and CrSe2. The similar conclusion is also obtained in CrSeTe and CrSTe monolayers. To confirm their stability, AIMD and phonon calculations are performed. Figure 1(d) and S1 display the top and side views of proposed Janus monolayers after heating for 5ps at temperature of 300K. Clearly, there is no bond broken or structural reconstruction in all these monolayers. And the 5



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vibrations of total energy per unit cell for these monolayers are within the range of 0.2 eV. The phonon dispersions of Janus CrXY monolayers are shown in Figure 1(c) and S2, no imaginary frequencies appear for all proposed monolayers. In addition, we also calculate their formation energies (Ef) based on the following equation: Ef = (Etotal-nECr-nE1-nE2)/3n, where Etotal is the total energy of monolayer structures, ECr, E1 and E2 are the energies of Cr, X and

Figure 1. (a) Top and side views and (b) Simulated STM image for CrSSe monolayer. (c) Phonon dispersion curves of CrSSe monolayer. (d) Side views and total energy per unit cell of CrSSe snapshotted from MD simulations at 300 K during the time of 5ps. (e) The calculated formation energy of CrX2 and CrXY monolayers. The orange arrow indicates the intrinsic electric-field direction. Yellow, orange and light blue balls represent S, Se and Cr atoms, respectively. Y atoms in their stable bulk phases, n is the number of atoms. From Figure 1(e), we find that all the formation energy of these structures are less than the value of the Sb monolayer (~ 0.9 eV/atom), which has been synthesized in experiment.16 These results could serve as convincing evidences to demonstrate the stability of these proposed Janus structures at room temperature. And they are expected to be synthesized in experiment by CVD method. 6


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3.2 Electronic properties and band alignment As shown in Figure S3, all 2H-CrX2 monolayers are non-magnetic semiconductors with a direct band gap of 1.34, 1.14 and 0.96 eV for CrS2, CrSe2 and CrTe2 monolayers, which is consistent with previous studies.46-48 Due to the mirror symmetry, there is no electrostatic potential difference between top and bottom surfaces. For both CrSSe and CrSeTe monolayers, they are also nonmagnetic semiconductors with a direct band gap of 1.24 and 1.04 eV, respectively, with valence band maximum (VBM) and conduction band minimum (CBM) locating at the K point. While for CrSTe monolayer, it holds an indirect band gap of 0.69 eV, with VBM and CBM locating at  and K points, respectively. In order to obtain the accurate band gaps of Janus CrXY monolayers, the band structures are examined on G0W0 level. From Figure 3, we can see that the band gaps of CrSSe, CrSeTe and CrSTe monolayers increase to 1.57, 1.32 and 1.10 eV, respectively, while the main features of the bands are similar to those based on HSE06 functional.

Figure 2. Band structures of (a) CrSSe, (b) CrSeTe and (c) CrSTe based on HSE06 (blue line) and G0W0 (red line) functional. EQP is the quasiparticle bandgap and insert picture is the 2D Brillouin zone of the hexagonal structure. The Fermi levels are set as 0 eV. As expected, the intrinsic dipole moment exists for all Janus CrXY monolayers arising from the mirror symmetry broken, which generates a built-in electric field along the out-of-plane direction. As

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displayed in Table I and Figure S4, the dipole moment of CrSSe monolayer is 0.2 D, and a built-in electric field along vertical direction pointing from Se atom to S atom forms, thus inducing electrostatic potential difference of 0.88 eV. Similarly, for CrSeTe and CrSTe, the electrostatic potential difference between the top and bottom surface are 0.98 and 1.82 eV, respectively, which results in a dipole moment of 0.25 and 0.45D. Table I. Optimized lattice constants (a), formation energy Ef, band gaps based on HSE06, band gaps based on G0W0, the quasiparticle and optical band gaps EQP and Eopt, exciton binding energies (Eb), dipole moments (μ) and difference of electrostatic potential (ΔV) between top and bottom surfaces for CrX2 and Janus CrXY monolayers. Materials

a (Å)

Ef(eV/atom)

CrS2 CrSe2 CrTe2 CrSSe CrSeTe CrSTe

3.04 3.21 3.48 3.13 3.35 3.27

-2.13 -1.48 -0.79 -1.79 -1.11 -1.38

EHSE(eV) EGW(eV) EQP(eV) EOpt(eV)

1.34 1.14 0.96 1.24 1.04 0.69

1.57 1.32 1.10

1.57 1.32 1.29

1.15 0.88 1.06

Eb (eV)

μ(debye)

V (eV)

0.42 0.44 0.23

0 0 0 0.20 0.25 0.45

0 0 0 0.88 0.98 1.82

Figure 3. (a) Schematic diagram of absolute positions of band edges with respect to vacuum level according to the band gap center (BGC) theory. Band alignments of (b) CrSSe, (c) CrSeTe and (d) CrSTe monolayers based on G0W0 level. As we know, it is indispensable for fully photocatalytic water-splitting to hold suitable band edge 8


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positions, where CBM is higher than the reduction level of hydrogen and VBM is lower than the oxidation level of oxygen. We initially investigate the band edges of pristine CrX2 monolayers based on HSE06 level. Unfortunately, their band edge positions could not meet the redox potential requirement, reflecting that CrX2 monolayers are not good candidates as being photocatalyts. Then, we examine the band edge positions of Janus CrXY monolayers. According to reaction mechanism proposed by Yang et al.,49 the existence of electrostatic potential difference makes band gap (> 1.23 eV) of photocatalysts no limitation for fully water-splitting. As shown in Figure S5, the proposed Janus monolayers well satisfy the band edge requirements for fully photocatalytic water-splitting. In order to accurately obtain the band edges, GW is a preferred method. However, its well-converged process of band edge is considerately costly. In such situation, approximations that determine the absolute band edge positions but avoid a fully converged GW calculation have been proposed, in which of their validity has yet been verified in TMDs.50-53 As presented in Figure 3(a), the energy of band gap center (EBGC) is obtained by HSE06, Eg is the calculated band gap based on G0W0. Generally, the band edge positions of VBM and CBM can be determined by EVBM = EBGC – 1/2 Eg, ECBM = EBGC +1/2 Eg, which has been verified by previous reports.50,52 As we mentioned above, the CBM and VBM would move to outside under the internal electric field. And the BGC theory tells us that the BGC of one certain material is insensitive to the calculated method. In this case, we redefine the band edge positions (CBM and VBM) as EVBM = EBGC – 1/2 (Eg+ V), ECBM = EBGC +1/2 (Eg + V) for polar systems. As shown in Figure 3(b)-(d), all Janus CrXY monolayers hold suitable band alignments for overall water-splitting, and the energy differences between band edges and redox levels are about 0.5 eV. These results demonstrate that these proposed Janus structures have a good redox ability, indicating that they can be potential photocatalysts for water-splitting. To further explain the redox mechanism for splitting water, we investigate the partial charge density near the VBM and CBM of pristine CrX2 and Janus CrXY monolayers. From Figure S6, we can clearly see that the partial charge density near CBM and VBM of pristine CrX2 monolayers are localized around Cr atoms. While for 9



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Janus CrSSe monolayer, the partial charge density near CBM mainly distributes around Se and Cr atoms, and that near VBM mainly resides at S and Cr atoms. This can be attributed to the electrostatic potential difference between two surfaces of CrSSe monolayer. In this case, the photo-generated electrons and holes tend to move towards opposite directions, which leads to the spatial separation of carriers. According to photocatalytic reaction: 4H2O+4e-→2H2+4OH-, 2H2O +4h+→O2+4H+, the hydrogen production mainly occurs in the Se surface, while the oxygen production mainly occurs in S surface, thus being able to improve the photocatalytic efficiency. And similar reactive mechanism is also obtained in CrSeTe and CrSTe monolayers. 3.3 Optical absorption and Photoexcitation Dynamics In view of their photocatalytic applications, it is of great significant to investigate the optical absorption behaviors of CrXY monolayers. It is well known that 2D materials exhibit relatively weak screening, we thus study the optical properties of Janus CrXY monolayers using G0W0-BSE method, which takes the electron-hole Coulomb interactions into consideration. The imaginary part of the dependent complex dielectric function is calculated according to:54

α (ω) = 2ω



ε1 (ω) 2 + ε2 (ω) 2 - ε1 (ω)



1/2

As shown in Figure 4(a), all Janus CrXY monolayers exhibit good optical absorption, which are superior to that of the well-known 2D g-C3N4 and MoSSe photocatalysts.28,55 More remarkably, the initial peaks of the optical absorption for CrSSe, CrSeTe and CrSTe monolayers are 1.15, 0.88 and 1.06 eV, respectively, illustrating that these proposed photocatalysts can be active under near-infrared light region. In addition, we study the exciton binding energy of CrXY monolayers on the basis of G0W0-BSE. The exciton binding energy is the difference between quasiparticle band gaps and optical band gaps, the calculated quasiparticle band gaps and exciton energy are displayed in Table I, which 10


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basically follows the Eb ~ 1/4EQP law proposed in previous study. 56,57 And these values are smaller than those of 2D g-C3N4, BP, and MoS2 etc., which is favorable for the separation of photo-generated electrons and holes. Besides, the carrier mobility is another vital factor determining the photocatalytic efficiency. Therefore, the carrier mobility of Janus CrXY monolayers is calculated using the deformation potential method, which is expressed by the following equation:58

2 D 

2eh3C 2

3K BT m* Ed 2

. Here, e and ℏ are electron charge and reduced Planck constant. Kb, T and Ed represent Boltzmann constant, room temperature (300 K) and deformation potential constant. m  h2 (

d 2 Ek 1 ) is the dk 2

carries effective mass, where Ek and k are total energies and momentum, respectively. C  [ 2 E /  2 ] / S0 is the elastic constant of the 2D system, wherein E and S0 are total energy and

system’s area, respectively. The calculated hole mobility for CrSSe monolayer is about 0.37×103 and 0.31×103 𝑐𝑚2𝑉 ―1𝑠 ―1along zigzag (x) and armchair (y) directions, respectively. While for electrons, the mobilities are 0.53×102 and 0.43×102 𝑐𝑚2𝑉 ―1𝑠 ―1 along x and y directions, respectively. The huge disparity of mobility between electron and hole are commonly beneficial for their separation, further enhancing the photocatalytic efficiency. More detailed descriptions are listed in Table SI.

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Figure 4. (a) The optical absorption coefficients of Janus CrXY monolayers based on G0W0-BSE. (b) Decay of the excited-state population at 300 K. (c) Spectral density obtained from Fourier transforms of the autocorrelation functions for the band gap fluctuations of Janus CrXY monolayers. (d) Averaged absolute nonadiabatic (NA) coupling. The photo-generated carrier lifetime plays a crucial role in affecting photocatalytic efficiency.59 To quantitatively describe the lifetime of photo-generated carriers, the evolutions of the population of the first excited state in these Janus CrXY monolayers are calculated using time-domain density functional theory combined with NAMD, as displayed in Figure 4(b). The electron-hole recombination in CrSSe monolayer needs 71 ps, in the same time scale of TMD monolayers (several hundreds of picoseconds).60,61 Emotionally, the lifetimes of photo-generated carriers in CrSeTe and CrSTe monolayers can reach up to 2.4 and 1.9 ns, respectively, even comparable with those of TMDs heterostructures.62,63 This indicates CrXY prone to possess long-lived photo-generated electrons and holes to be involved in the redox reaction, thus leading to high efficiency in photocatalytic overall water-splitting. 12


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The different recombination rate of these systems can be rationalized by the following two factors. First, spectral density by Fourier transforms of the fluctuations for band gaps characterizes the phonon modes of vibrational motions inducing electron-hole recombination. As depicted in Figure 4(c), the dominant vibrational mode peak of CrXY locate at frequency around 200 cm-1, and the mode intensity vanishes when the frequency is larger than 500 cm-1. Particularly, the vibrational modes for CrSSe monolayer is widespread with highest intensities, while are relatively weak for CrSeTe and CrSTe monolayers. Thus, least intense phonon modes in CrSeTe monolayer coupling to the electronic subsystem participate in the relaxation of excited electrons and create longest recombination time.64,65 Second, the recombination rate is proportional to the NA coupling squared between CBM and VBM,66 smaller NA coupling favors slower nonradiative decay as it determines the hopping probability of hot electrons from one state to another. The NA coupling between two electronic states j and k can be expressed as:67

d jk   j |

 j |  R H | k   | k  R, k   j t

so NA coupling is mainly determined by energy difference term  k   j , electron-phonon (e-ph) coupling term

 j |  R H | k



and the nuclear velocity term R . As shown in Figure 4(d), the

strength of NA coupling is in the order of CrSSe > CrSTe > CrSeTe, mainly because the atomic mass is in the order of Te> Se> S, and the e-ph coupling is in the sequence of CrSSe > CrSTe > CrSeTe. In comparison, less active vibrations and small NA coupling cause longest lifetime of photogenerated carriers in CrSeTe.

4. CONCLUSIONS In summary, we propose a series of novel Janus CrXY structures, and demonstrate that all these CrXY monolayers are promising photocatalysts under near-infrared light. The thermal and dynamic stabilities are confirmed. These Janus structurs are all semiconductors with a band gap of 1.57, 1.32 and 1.10 eV for CrSSe, CrSeTe and CrSTe monolayers, respectively. Due to the broken symmetry, 13



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Janus CrXY monolayers display an intrinsic electric field. This, makes the reactions of hydrogen and oxygen production happening on different surfaces, thus improving the photocatalytic efficiency. Moreover, all these Janus monolayers hold suitable band edges, which ensures their redox abilities in fully water-splitting. The optical absorption spectrum demonstrates that these Janus monolayers can be active even under near-infrared light, which is of great significance for increasing the utilization of solar energy. Besides, the electron-hole recombination of Janus CrXY monolayers is evaluated as well. All these systems hold long–lived photogenerated carriers, especially for CrSeTe and CrSTe monolayers. Given all this, the proposed Janus monolayers can be promising candidates with high efficiency applied in photocatalytic overall water-splitting.

CONFLICTS OF INTEREST There are no conflicts to declare.

SUPPORTING INFORMATION The carrier mobility of Janus CrXY monolayers, MD simulation and phonon dispersion for CrSeTe and CrSTe monolayers, the band structures of CrX2 monolayer based on HSE level, the electrostatic potential and band alignments of CrX2 and CrXY monolayers based on HSE level, partial charge density of CrX2 and CrXY monolayers.

ACKNOWLEDGEMENT This work is supported by the National Natural Science foundation of China (No. 11804190 and 21333006), Qilu Young Scholar Program of Shandong University, Taishan Scholar Program of Shandong Province, and 111 Project (No. B13029).

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Janus Chromium Dichalcogenides Monolayers with Low Carrier

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