Designing a Dipole‐Scheme Heterostructure Based on Janus TMDCs for Highly Efficient Photocatalytic Overall Water Splitting

Direct Z‐scheme heterostructures, with strong redox abilities, are promising candidates for solar‐driven water splitting. However, carriers' recombination at the interface results in the loss of at least half of the photogenerated carriers, leading to low efficiency of carrier utilization. To solve this issue, a novel dipole‐scheme for photocatalytic water splitting is proposed, by utilizing the synergistic effects of the intrinsic dipole in Janus materials and the interfacial electric field. This approach can separate the photogenerated carriers with strong redox ability and suppress the recombination of photogenerated carriers at interface. Density functional theory calculations indicate that, for the heterostructures including Janus materials, the total electric dipole points to the oxygen evolution photocatalyst from the hydrogen evolution photocatalyst decreases the redox potential for water splitting. Additionally, the opposite intrinsic dipole of two Janus layers can offset the total dipole of Janus heterostructure to obtain a moderate vacuum level difference ΔV for reducing the redox potential. Based on these findings, a highly efficient dipole‐scheme photocatalyst CrSSeCrSeTe (SeTe interface), with opposite intrinsic Janus dipole is predicted, which achieves a high solar‐to‐hydrogen efficiency of 44%. This study provides a promising avenue for the design of highly efficient photocatalysts for overall water splitting with potential commercial applications.


Introduction
Photocatalytic water splitting can convert solar energy to hydrogen fuels and has been considered as a promising and sustainable technology to solve the increasing energy and environmental issues. [1,2] For single-component photocatalysts, it is hard to achieve the effective separation of photogenerated carriers, due to the recombination of the electron-hole pairs induced by the strong Coulombic force. [3] Type-II photocatalysts (Scheme 1a) consisting of two semiconductors can promote the spatial separation of charge carriers, but they sacrifice the materials' strong redox ability. [4] Moreover, highly efficient photocatalyst for water splitting usually requires a wide light absorption range and strong redox ability simultaneously, which is hard to achieve in conventional single semiconductors or type-II heterostructures. [1,4] This is because the strong redox ability requires a large bandgap, while wide light-harvesting range needs a narrow bandgap. [5,6] Direct Z-scheme photocatalytic systems have been proposed to solve these problems, inspired by natural photosynthesis. These systems can broaden the light absorption range while retaining strong redox ability and separating the photogenerated carriers. [5,[7][8][9] In direct Z-scheme photocatalytic systems (Scheme 1b), the photogenerated carriers with lower redox ability would recombine fast due to the interfacial electric field, and the photogenerated carriers with stronger redox ability remained to participate in the redox reactions. [8,10] However, it also means that at least half of the photogenerated carriers are recombined during the catalytic processes, limiting the efficiency of carrier utilization cu . Furthermore, the redox potentials for water splitting (1.23 eV) and the required overpotentials to drive hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) also limit the further improvement of the catalytic efficiency for photocatalytic water splitting. [11,12] In conventional direct Z-scheme or type-II photocatalysts, interfacial electric field determines the motion and recombination of photogenerated carriers. [13,14] The polarized materials, such as ferroelectric materials, [15,16] III 2 -VI 3 materials, [17,18] Scheme 1. a) The Schematic of the different photocatalytic modes of type-II mode, b) Z-scheme mode, and c) dipole-scheme mode. In dipole-scheme mode, driven by the intrinsic dipole in individual Janus monolayers, the photogenerated electrons would transfer to the interface of the heterostructures to participate into the HER, while the photogenerated holes transfer to the surface to participate into the OER.
Li-III-VI compounds [19] and Janus transition metal dichalcogenides (TMDCs), [20,21] possess an intrinsic electric field due to the asymmetric structure, which can drive the photogenerated holes and electrons to separate effectively. Therefore, when introducing the polarized materials, the motion and recombination of photogenerated carriers can be further affected by the intrinsic dipole. [22,23] Our previous work has achieved a high solarto-hydrogen STH efficiency of 19.26% in the direct Z-scheme In 2 Se 3 /SnP 3 heterostructure, [13] by utilizing the synergistic effects of the intrinsic dipole in polarized In 2 Se 3 layer and the interfacial electric field in heterostructure. However, the recombination of photogenerated carriers at the interface limited the efficiency of carrier utilization cu (only 35.35%). Considering the direction of intrinsic dipole in polarized layers, the different stacking order of Janus monolayers might result in different motion paths of photogenerated carriers. Therefore, utilizing the intrinsic dipole of polarized materials with different stacking orders might reduce the recombination of photogenerated carriers at interface in direct Z-scheme heterostructures, thus further improving the cu for higher STH efficiency.
In this contribution, we propose a novel dipole-scheme mode for photocatalytic water splitting, utilizing the synergistic effects of the intrinsic Janus dipole, and the interfacial electric field. This approach can maintain the redox ability of materials while preventing the recombination of photogenerated carriers at the interface of the heterostructures (Scheme 1c). Using density functional theory (DFT), we systematically investigate a series of heterostructures based on Janus TMDCs monolayers with different contacting interfaces. Our findings reveal that for the redox abilities for water splitting to be enhanced, the total electric dipole pointing from the hydrogen evolution photocatalyst (HEP) to the oxygen evolution photocatalyst (OEP) is required to reduce the redox potentials. Moreover, tuning the redox potentials for water splitting requires a moderate vacuum level difference (ΔV), which is determined by the intrinsic Janus dipole and the interfacial electric field. We show that stacking two Janus monolayers with opposite intrinsic dipoles can obtain a moderate ΔV, resulting in strong redox abilities. Finally, we screen out the CrSSe-CrSeTe (Se-Te interface) dipole-scheme heterostructure for overall water splitting, which exhibits high STH ef-ficiency of 44%, exceeding conventional type-II or Z-scheme photocatalysts.

Results and Discussion
Compared to conventional 2D TMDCs (MX 2 ) (Figure 1a), Janus TMDCs (MXY) monolayers break the centrosymmetry of the 2D plane, with one metal atom M sandwiched between two different chalcogenide atoms (X and Y atoms; Figure 1b). The difference in electronegativity of atoms at two surfaces results in uneven charge distribution in Janus TMDCs, which induces an intrinsic dipole along the z direction. The intrinsic dipole leads to a vacuum level difference (ΔV) between the two surfaces of Janus TMDCs (Figure 1c), which can tune the redox potentials for water splitting to break the bandgap limitation (1.23 eV) for photocatalytic water splitting. For MXY (M = Cr, Mo, W; X/Y = S, Se, Te) monolayers, ΔV of the MSTe monolayers is nearly twice as that of monolayers with MSSe and MSeTe, due to the larger difference in electronegativity; while HfXY, ZrXY, and TiXY monolayers exhibit quite small ΔV (as shown in Table 1) due to the H-phase structure. Our band structures calculations from the hybrid HSE06 functional ( Figure S1, Supporting Information) show that MoSSe, MoSeTe, WSSe, WSeTe, CrSSe, and CrSeTe are direct bandgap semiconductors with the bandgaps ranging from 1.04 to 2.04 eV, which are expected to have higher efficiency for light harvesting compared with indirect bandgap semiconductors. These results are in good agreement with other theoretical calculations. [24,25] Compared with the single photocatalyst, heterostructures assembled by two different materials can separate the photogenerated carriers more effectively. Based on the above discussions, we further use MoSSe, MoSeTe, WSSe, WSeTe, CrSSe, and CrSeTe monolayers to form heterostructures in following investigations. For conventional heterostructures without polarized materials, the direction of interfacial electric field would lead to different photogenerated carriers transfer modes (type-II or Z-scheme mode) under the light irradiation, which is determined by the work function of two layers. [26,27] For the heterostructures consisting of Janus materials, the photogenerated carriers transfer mode is determined by the intrinsic dipole of Janus layers and the  interfacial electric field in heterostructures. As shown in Figure 1d, Janus TMDCs exhibit two different work functions at two different surfaces, which means that different atomic stacking orders might result in different photogenerated carriers transfer modes with distinct directions of interfacial electric field and intrinsic dipole.
To investigate the effects of the atomic stacking order for Janus materials on the interface properties in heterostructures, we further constructed a series of 2D TMDCs-Janus TMDCs heterostructures: MoSe 2 -MoSeTe, WSe 2 -MoSeTe, MoSe 2 -WSeTe, and WSe 2 -WSeTe systems, with two stacking modes (Se-Se and Se-Te interface) for each system. Owing to their close lattice constants, the heterostructures are built by their unit cells containing six atoms, with two stacking types (AA and AB stacking types) considered as shown in Figure S2 Table S1 (Supporting Information). The similar interlayer bandgap E g and vacuum level difference (ΔV) of AA and AB stacking heterostructures shown in Table S1 (Supporting Information) indicate that the stacking mode has minimal impact on the intrinsic dipole and interfacial properties of heterostructures. Taking into account the stability of structures, we have opted to focus on the AB stacking structures for further investigation in this study.
Based on the calculated projected band structures of these heterostructures (Figure 2b), we found that different contacting interfaces lead to different band alignments (Figure 2c). Heterostructures with Se-Se interface formed type-I band alignment, in which the photogenerated carriers are limited in the same layer and cannot separate effectively as the valence band maximum VBM and the conduction band minimum CBM are contributed by TMDCs monolayer. Heterostructures with Se-Te interface formed the staggered band structure configuration that allowed for the spatial separation of photogenerated carriers, with larger band offset (ΔE C ≈ 0.52-1.00 eV and ΔE V ≈ 0.44-0.72 eV) and narrower interlayer bandgap (0.58-0.91 eV) compared with those with Se-Se interface, due to the larger difference of electronegativity and work function between Se and Te atoms at the interface (Table 1).
In the practical photocatalytic process, the required overpotentials for HER and OER are assumed to be 0.2 and 0.6 eV to drive the redox reactions, respectively, considering the energy loss during carrier migration between different materials. [18] For single Janus materials, ΔV induced by the intrinsic diploe can effectively tune the redox potentials for water splitting. For the heterostructures including Janus materials, ΔV is determined by both the intrinsic dipole and interfacial electric field. We further calculated the electrostatic potential of the MoSe 2 -WSeTe (Se-Te) heterostructure and found a total electric dipole E total pointing to MoSe 2 from WSeTe, resulting in a ΔV (0.64 eV) of MoSe 2 -WSeTe (Se-Te) heterostructure smaller than that of Janus WSeTe monolayer (0.81 eV) (Figure 3a), indicating that there is an interfacial electric field E in opposite to the intrinsic dipole of Janus monolayer; the similar results can be seen in other heterostructures with Se-Te interface ( Figure S3, Supporting Information). The ΔV of heterostructures at two surfaces induces the shift of potentials for H + /H 2 and O 2 /H 2 O in Figure 3b, which gives rise to the redox potentials for water splitting (>1.23 eV), due to the total electric dipole pointing to the hydrogen evolution photocatalyst (HEP) from the oxygen evolution photocatalyst (OEP). This enlarged redox potentials (1.23 + 0.64 eV) significantly reduce the overpotentials (H 2 ) and (O 2 ) for HER (0.8 eV) and OER (0.2 eV), thus weakening the driving force for catalytic reactions. We found that all the investigated TMDCs-Janus TMDCs heterostructures fail to achieve spontaneous solar-driven overall water splitting.
Based on the above discussions, we found that only when the total electric dipole points from HEP to OEP, the redox potentials for water splitting can be reduced and redox abilities for HER and OER can be enhanced. To further confirm the conclusion, we investigated a series of heterostructures by stacking two different Janus monolayers together to explore the structures that can fulfill the above requirements.  According to the band alignments in Figure 1d, we constructed MoSSe-CrSeTe(S-Te), MoSeTe-CrSeTe (S-Te), WSSe-CrSeTe (Se-Te), and WSeTe-CrSeTe (Se-Te) heterostructures (Figure 4a) with large difference of work function at the interface. We found that although the intrinsic dipoles for the two Janus monolayers are opposite to the interfacial electric field of these heterostructures, there is a large vacuum level difference ΔV at the two surfaces of heterostructures due to the strong interfacial electric field, which significantly reduces the redox potential and thus enhances the overpotentials (H 2 ) and (O 2 ) as the total dipole of the heterostructure points to the OEP from the HEP, as shown in Figure 4b. Table 2, and among these heterostructures only MoSSe-CrSeTe (S-Te) and WSSe-CrSeTe (Se-Te) heterostructures possess large (H 2 ) over 0.2 eV and large (O 2 ) over 0.6 eV simultaneously, which can supply sufficient driving force for spontaneous solar-driven overall water splitting. Notably, the large ΔV gives a significant increase of (H 2 ) and (O 2 ) to enhance the redox abilities, but it also leads to the redox potentials of H + /H 2 and H 2 O/O 2 shift a lot and even shift over the VBM of HEP (Figure 4b). In this condition, the photogenerated electrons at the CBM of HEP would prefer to recombine with the photogenerated holes rather than participating in the HER, and thus the STH efficiency  to enter the valence band of HEP or the conduction band of OEP, resulting in the recombination of photogenerated electrons and holes upon the Coulomb interaction. On the contrary, for the heterostructures stacked by two Janus layers with opposite intrinsic dipoles, the opposite dipole can offset the total dipole of the heterostructure to obtain a suitable ΔV for tuning the redox potential. Since light absorption efficiency is strongly dependent on the bandgap, we further use CrSSe (1.24 eV) and CrSeTe (1.04 eV) monolayers with narrower bandgaps to construct two different contacting interfaces: Se-Te and S-Se interface with opposite intrinsic Janus dipole.

The detailed (H 2 ) and (O 2 ) are listed in
As shown in Figure 5a,b, the redox potential for water splitting of the heterostructures with Se-Te and S-Se interface are significantly reduced for the total electric dipole E total points from HEP to OEP. The opposite intrinsic dipole of CrSSe (0.93 eV) and CrSeTe (0.98 eV) layers results in an offset, thus the magnitude and direction of ΔV are largely dependent on the interfacial electric field E in . For Se-Te stacking heterostructure, although the direction of intrinsic dipoles of CrSSe and CrSeTe is opposite, there is a strong interfacial electric field with the same direction of the intrinsic dipole for CrSeTe monolayer, resulting in a suitable vacuum level difference ΔV (0.49 eV), which is sufficient to reduce the redox potential to drive HER and OER. However, for S-Se stacking heterostructure, the opposite intrinsic dipole of Janus layers results in a small ΔV (0.26 eV) that is insufficient to tune the redox potential. We found that Se-Te stacking heterostructure can enhance the redox abilities with increased www.advancedsciencenews.com www.advmatinterfaces.de respectively, unlike the heterostructures in Figure 4b. Therefore, CrSSe-CrSeTe heterostructure with Se-Te interface is a promising photocatalyst for overall water splitting. Notably, distinguished from the conventional Z-scheme charge transfer mode, the photogenerated carriers transfer mode of CrSSe-CrSeTe heterostructure with opposite Janus dipole is shown in Figure 5c, and we define this photocatalytic mode as dipole-scheme mode. In this dipole-scheme mode, the photogenerated holes would accumulate at the surface of the heterostructure and the photogenerated electrons would accumulate at the interface of the heterostructure, driven by the intrinsic Janus dipole under the light irradiation. The interfacial electric field would further drive the photogenerated electrons at the OEP transfer to the HEP to participate in the HER. Eventually, the total electric dipole of the heterostructures enables the photogenerated holes at the HEP layer to transfer to the surface of OEP, to participate in OER. We think this dipole-scheme mode can effectively suppress the recombination of photogenerated carriers at interface and improves the efficiency of carrier utilization, as the photogenerated holes and carriers are spatially separated at the surface and interface, respectively.
The adsorption of H 2 O on the surface of photocatalysts is an important parameter that characterizes their activity in photocatalytic water-splitting applications. Three different adsorption sites of H 2 O on both CrSSe and CrSeTe sides of the heterostructure are considered as shown in Figure S4 (Supporting Information), and the most stable configuration for both sides is the hollow site with the O atom located above the hexagonal center of the lattice and one of the two H atoms pointing to a surface Se or S atom. The calculated adsorption energies of these configurations are −0.23 and −0.21 eV, respectively, which are lower than those on the CrSSe (−0.17 eV) and CrSeTe (−0.16 eV) monolayers.
Relative to the redox reactions of the water splitting, there are two half-reactions: OER in anode: H 2 O → 1/2O 2 + 2H + + 2e − ; HER in cathode: 2H + + 2e − → H 2 . The CrSSe-CrSeTe (Se-Te) heterostructure exhibits the most stable adsorptions of intermediates (OH * , O * , OOH * , and H * ) during the redox reaction, as depicted in Figure 6a,b. The OER and HER processes occur on the CrSSe and CrSeTe layers, respectively. To further elucidate the thermodynamic energy barrier for HER and OER on the heterostructure, free energy diagrams of the HER and OER pathway were calculated and shown in Figure 6c,d. For HER, the free energy change for step 1 (ΔG H ) is found to be 0.89 eV at pH 0 without light irradiation. Upon switching on the light, the photogenerated electrons can supply a large potential U e of 0.35 V, resulting in a reduced ΔG H of 0.59 V. For OER, it is evident that under the external potential provided by the photogenerated holes at pH 0 or 7, all the free energy profiles become downhill, suggesting that the OER can proceed spontaneously without any barriers under these conditions. The results of OER and HER indicate the enhanced photocatalytic activity and performance of CrSSe-CrSeTe (Se-Te) heterostructure, making it favorable for achieving high overall photocatalytic efficiency.
We then explore the energy conversion efficiency of CrSSe-CrSeTe (Se-Te) heterostructure for photocatalytic water splitting. Upon the strong interfacial effects of CrSSe-CrSeTe (Se-Te) heterostructure, the bandgaps of individual Janus CrSSe and CrSeTe monolayers are reduced to 1.13 and 0.98 eV, respectively. The hybridization of electronic state between two layers facilitates the electron excitation between them, resulting in an obvious "red shift" of CrSSe-CrSeTe (Se-Te) heterostructure, with a wider light absorption range and higher absorbance compared to the CrSSe and CrSeTe monolayers, as shown in Figure S5 (Supporting Information). The quite small bandgaps and good light-harvesting capacity enable the large efficiency of light absorption abs (80.3%), and the efficiency of carrier utilization cu (66.44%) is significantly improved in the special dipole-scheme heterostructure. Finally, a high STH efficiency of 53.36% was obtained in CrSSe-CrSeTe (Se-Te) heterostructure and the corrected STH efficiency is 44%.

Conclusion
In conclusion, we proposed a novel dipole-scheme photocatalytic mode for highly efficient water-splitting, utilizing heterostructures with opposite Janus dipole. This photocatalytic mode can not only separate the photogenerated carriers with strong redox abilities but also suppress the recombination of photogenerated carriers to enhance carrier utilization. By investigating a series of heterostructures based on Janus TMDCs based on density functional theory calculations, we found that the different contacting interfaces result in different band alignments for the different work functions at the interface of heterostructures due to the intrinsic diploe of Janus layers. We found that the total electric dipole E total points from HEP to OEP enhanced the redox abilities of heterostructures, and the offset of the opposite intrinsic dipole in two individual Janus layers produced a suitable ΔV to reduce the redox potential effectively. Based on these findings, a highly efficient CrSSe-CrSeTe dipole-scheme photocatalyst was obtained with a high STH efficiency of 44%. Our theoretical computations offer not only a promising photocatalyst for highefficiency overall water splitting but also a novel avenue for the design of highly efficient photocatalysts.

Experimental Section
Structural optimizations based on DFT were performed using the plane-wave basis set Vienna Ab initio simulation package (VASP) code, [28,29] along with the generalized gradient approximation (GGA) of Perdew-Burke-Ernzernhof (PBE) for exchange and correlation functional. [30,31] The vdW correction model [32] within the PBE functional was involved to describe the long-range interlayer interaction. A kinetic energy cutoff of 450 eV was employed in the simulations. The Brillouin Zone sampling was set as 9 × 9 × 1 in the Monkhorst-Pack grid. [33] The vacuum thickness was 20 Å. Because the GGA-PBE exchange-correlation function tends to underestimate the bandgap of semiconductors, the screened hybrid HSE06 functional, [34] implemented in the Fritz Haber Institute ab initio molecular simulations (FHI-aims) code, [35] was utilized to determine the electronic band structures for all the systems. The dipole correction was applied to all the systems involved in Janus materials. The lattice of the heterostructures studied was optimized, and the maximum lattice mismatch was <5%.
Both U e and U h were defined relative to the normal hydrogen electrode (NHE) and thus dependent on the pH value, according to U e = U e (pH 0) −0.059 × pH and U h = U h (pH 0) + 0.059 × pH, respectively. Notably, U e and U h could be correlated with (H 2 ) and (O 2 ) with U e = (H 2 ) and U h = 1.23 + (O 2 ). The efficiency of light absorption abs and the efficiency of carrier utilization cu for the dipole-scheme mechanism systems could be defined as follows: [36] where P(ℏ ) is the AM1.5G solar energy flux at the photon energy ℏ , and E g is the band gap of the layer with a larger bandgap in heterostructures.
The term ∞ ∫ 0 P(ℏ )d(ℏ ) represents incident solar radiation (AM1.5G), and ∞ ∫ Eg P(ℏ )d(ℏ ) represents power density that can be absorbed by the materials. ΔG is the potential difference of 1.23 eV for water splitting, and ∞ ∫ E P(ℏ ) ℏ d(ℏ ) is the effective photocurrent density. E is the energy of a photon that can be used for water splitting, which can be determined using Equation 3: The solar-to-hydrogen efficiency (STH) is the product of abs and cu , which is defined as: As the intrinsic dipole in Janus monolayers did positive work for the separation of photogenerated carriers during the process of photocatalytic water splitting. Then the corrected STH efficiency of photocatalytic water splitting for Janus materials could be calculated as follows [18] : where ΔV is the vacuum level difference between the two surfaces of 2D Janus materials, and the second term in the denominator represents the work done by the intrinsic dipole of Janus materials.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.