A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

Based on the extensive investigation on graphene and graphene derivatives asymmetrically functionalized with a variety of molecular groups and the concept of Janus materials with asymmetric facial properties, Janus two-dimensional (2D) materials as derivatives of the 2D material family, including Janus graphene, Janus transition metal chalcogenides, etc., have attracted much attention in recent years. Janus 2D materials have been demonstrated experimentally and theoretically to possess unique properties such as an out-of-plane piezoelectric polarization and strong Rashba effect due to their out-of-plane asymmetry. Here, the recent progress of emerging Janus 2D materials, including Janus graphene, prediction and fabrication of various different Janus 2D materials and Janus 2D van der Waals heterostructures, is presented. The investigations on the unique properties and potential device applications of Janus 2D materials are summarized. Finally, the conclusion and prospects for the future explorations on Janus 2D materials are also provided.

Recent advances in emerging Janus two-dimensional materials: from fundamental physics to device applications

Stable photocatalysts with excellent optical adsorption and low reaction barrier are the key for the water splitting. Here, we find that a two-dimensional Janus WSSe monolayer possesses the compelling photocatalytic properties from density functional theory simulations, which can be well modulated with strain deformation. Comprehensive investigations indicate that the Janus material not only exhibits strong optical absorbance in the visible spectrum, suitable band edge potentials, high carrier separation, and transfer efficiency but also has adequate driving forces of photoexcited carrier for water redox reaction and good resistance against photoinduced corrosion. Janus WSSe is therefore predicted to be a promising photocatalyst for water splitting. Moreover, we also find that tensile strains could further improve the photocatalytic performance for water splitting by effectively increasing the energy conversion efficiency and reducing the exciton binding energy. Our results not only predict a photocatalyst, which can utilize the visible light for overall water splitting, but also propose an effective path to extend the absorption spectra and raise the photocatalytic efficiency.

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Janus WSSe Monolayer: An Excellent Photocatalyst for Overall Water Splitting.

Efficient and selective CO2 electroreduction into chemical fuels promises to alleviate environmental pollution and energy crisis, but it relies on catalysts with controllable product selectivity and reaction path. Here, by means of first-principles calculations, we identify six ferroelectric catalysts comprising transition-metal atoms anchored on In2Se3 monolayer, whose catalytic performance can be controlled by ferroelectric switching based on adjusted d-band center and occupation of supported metal atoms. The polarization dependent activation allows effective control of the limiting potential of CO2 reduction on TM@In2Se3 (TM = Ni, Pd, Rh, Nb, and Re) as well as the reaction paths and final products on Nb@In2Se3 and Re@In2Se3. Interestingly, the ferroelectric switching can even reactivate the stuck catalytic CO2 reduction on Zr@In2Se3. The fairly low limiting potential and the unique ferroelectric controllable CO2 catalytic performance on atomically dispersed transition-metals on In2Se3 clearly distinguish them from traditional single atom catalysts, and open an avenue toward improving catalytic activity and selectivity for efficient and controllable electrochemical CO2 reduction reaction. Electroreduction of CO2 into chemical fuels holds promise for mitigating environmental pollution and energy crisis. This work presents a distinct design of ferroelectric catalysts with high catalytic activity and selectivity for efficient and controllable electrochemical CO2 reduction reaction.

Controllable CO2 electrocatalytic reduction via ferroelectric switching on single atom anchored In2Se3 monolayer.

Janus two-dimensional (2D) materials, referring to the layers with different surfaces, have attracted intensive research interest due to the unique properties induced by symmetry breaking, and promising applications in energy conversion. Based on the successful experimental synthesis of Janus transition metal dichalcogenides (TMDC), here we present a review on their potential application in photocatalytic overall water splitting, from the perspectives of the latest theoretical and experimental progress. Four aspects which are related to photocatalytic reaction, including the adsorption of water molecules, utilization of sunlight, charge separation and transport, and surface chemical reactions have been discussed, and it is concluded that the Janus structures have better performances than symmetric TMDCs. At the end of this review, we raise further challenges and possible future research directions for Janus 2D materials as water-splitting photocatalysts.

https://iopscience.iop.org/article/10.1088/2515-7639/ab7c57/pdf

Janus transition metal dichalcogenides: a superior platform for photocatalytic water splitting

The recently synthesized two-dimensional (2D) MoSi2N4 material with a proper band gap and excellent environmental stability promises potential optoelectronic applications. On the basis of first-principles calculations and electron–phonon interaction theory, we systematically analyze the structural, optical, carrier distribution and transport, and photocatalytic water-splitting properties of pristine and strained MoSi2N4 monolayers. They are all found to be dynamically stable. Their optical absorption efficiencies are very high for photon energy larger than the respective band gap. Surprisingly, the optical absorption coefficient of tensile MoSi2N4 is as large as 106 cm–1 across 300–880 nm. To the best of our knowledge, this is the best among optoelectronic materials in the visible region. In addition, compressive strain results in spatial separation of photogenerated electrons and holes, which is beneficial for increasing their separation rates. Transport properties indicate that the lifetime and mean free path (MFP) of photogenerated electrons and holes are, respectively, on the scale of several femtoseconds and within 1.5 nm. Tensile strain notably increases them, becoming comparable to conventional semiconductor Si. Finally, pristine and strained MoSi2N4 monolayers are predicated to be able to photooxidize and photoreduce H2O with proper band edges and H2O adsorption capacities. These systematic characterizations not only deepen understanding of the physical and optoelectronic properties of MoSi2N4 but also indicate promising applications of MoSi2N4 in electronics, optoelectronics, and photocatalytic water splitting.

Strained MoSi2N4 Monolayers with Excellent Solar Energy Absorption and Carrier Transport Properties

Janus MoSSe monolayers are recently proposed as a potential water-splitting photocatalyst. However, the weak van der Waals (vdW) interaction between water molecules and the surfaces of MoSSe signif...

Transition Metal-Functionalized Janus MoSSe Monolayer:A Magnetic and Efficient Single-Atom Photocatalyst for Water-SplittingApplications

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

Developing efficient N2 and O2 gas sensors is of great importance to our daily life and industrial technology In this work, first-principles calculations are performed to study the N2 and O2 gas-sensing properties of pure and defected PtSe2 It is found that both N2 and O2 adsorb weakly on pure PtSe2, and adsorption of the molecules induces negligible changes in the electrical and optical properties Whereas the Pt@Se anti-site defect significantly improves the N2 adsorption capacity of PtSe2 and induces notable changes in the electrical property Similar results are also observed for the Pt and Se vacancies and Pt@Se anti-site defects when examining O2 adsorption In addition, notable changes in the optical absorption spectra of the PtSe2 with Pt@Se defect are induced upon N2 adsorption, which also occurs for PtSe2 with Pt and Se vacancies and Pt@Se anti-site defects upon O2 adsorption These results demonstrate that PtSe2 with the corresponding defects can be both excellent electrical and optical sensors for detecting N2 and O2 gases Our work offers a new avenue for preparing efficient gas sensors

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Xin Yong

Papers

Strained MoSi2N4 Monolayers with Excellent Solar Energy Absorption and Carrier Transport Properties

Transition Metal-Functionalized Janus MoSSe Monolayer:A Magnetic and Efficient Single-Atom Photocatalyst for Water-SplittingApplications

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

Efficient N2- and O2-Sensing Properties of PtSe2 With Proper Intrinsic Defects.