Two-Dimensional Janus Transition Metal Oxides and Chalcogenides: Multifunctional Properties for Photocatalysts, Electronics, and Energy Conversion

Metallic CoSb and Janus Co2AsSb monolayers as promising anode materials for metal-ion batteries

Structural symmetry breaking plays a pivotal role in fine-tuning the properties of nano-layered materials. Here, based on the first-principles approaches we propose a Janus monolayer of metallic CoSb by breaking the out-of-plane structural symmetry. Specifically, within the CoSb monolayer by replacing the top-layer 'Sb' with 'As' atoms entirely, the Janus Co2AsSb monolayer can be formed, whose structure is confirmed via structural optimization and ab initio molecular dynamics simulations. Notably, the Janus Co2AsSb monolayer demonstrates stability at an elevated temperature of 1200 K, surpassing the stability of the CoSb monolayer, which remains stable only up to 900 K. We propose that both the CoSb and Janus Co2AsSb monolayers could serve as capable anode materials for power-driven metal-ion batteries, owing to their substantial theoretical capacity and robust binding strength. The theoretical specific capacities for Li/Na reach up to 1038.28/1186.60 mA h g-1 for CoSb, while Janus Co2AsSb demonstrates a marked improvement in electrochemical storage capacity of 3578.69/2215.38 mA h g-1 for Li/Na, representing a significant leap forward in this domain. The symmetry-breaking effect upgrades the CoSb monolayer, as a more viable contender for power-driven metal-ion batteries. Furthermore, electronic structure calculations indicate a notable charge transfer that augments the metallic nature, which would boost electrical conductivity. These simulations demonstrate that the CoSb and Janus Co2AsSb monolayers have immense potential for application in the design of metal-ion battery technologies.

Effect of replacing titanium with zirconium on the solubility of lithium in the LixZryTi1-yS2 system

Solid solutions of ZryTi1-yS2 (y = 0.1, 0.2) compositions have been synthesized. The electrochemical introduction of lithium is carried out by the galvanostatic intermittent titration technique. An isothermal section of the phase diagram of the LixZryTi1-yS2 system at room temperature has been constructed. The limiting solubility of lithium in solid solutions decreases with an increasing amount of titanium substituted for zirconium. It was noted that at low lithium concentrations (x < 1), there is no change in the E(x) dependence when titanium is replaced by zirconium. This may indicate the formation of separate TiS2 and ZrS2 layers.

Rational design of 2D Janus P3m1 M2N3 (M = Cu, Zr, and Hf) and their surface-functionalized derivatives: ferromagnetic, piezoelectric, and photocatalytic properties

In this study, first-principles calculations were employed to rationally design two-dimensional (2D) Janus transition metal nitrides of P3m1 M2N3 phases, where M is a d-block element (Sc-Zn, Y-Cd, Hf-Hg). Among the 29 examined 2D M2N3, three 2D phases, namely P3m1 Cu2N3, Zr2N3, and Hf2N3, exhibit excellent thermodynamic, dynamic, mechanical, and thermal stabilities. These novel Janus 2D materials exhibit ferromagnetic metallic and half-metallic behavior. The related 2D Janus surface-functionalized derivatives, Cu2N3H, Cu2N3F, Cu2N3Cl, Zr2N3H, Hf2N3H, and Hf2N3F, are all dynamically stable. The 2D Janus P3m1 phases of Zr2N3H, Hf2N3H, and Hf2N3F, all with M in the +IV oxidation state, act as semiconductors in the visible region, with energy band gaps of 2.26-2.70 eV at the HSE06 level of theory. On the other hand, the 2D Janus P3m1 Cu2N3X phases (where X = H, F, and Cl) are ferromagnetic half-metals. Additionally, it has been unveiled that there are high hole mobilities (∼6 × 103 cm2 V-1 s-1) derived from the moderate deformation potential and effective mass in the 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F phases. Uniaxial strain engineering has demonstrated the outstanding in-plane piezoelectric properties of 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F with high d11 values (∼99.91 pm V-1). Furthermore, the desirable band-edge alignments and high anisotropic carrier mobilities of 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F phases indicate their potential as visible light-driven photocatalysts for water splitting reactions on different facets. These properties render 2D Janus P3m1 Zr2N3H, Hf2N3H, and Hf2N3F phases promising for use in optoelectronics, piezoelectric sensing, and photocatalysis applications.

Strain-induced topological phase transition in ferromagnetic Janus monolayer MnSbBiS2Te2

We investigate a strain-induced topological phase transition in the ferromagnetic Janus monolayer MnSbBiS2Te2 using first-principles calculations. The electronic, magnetic, and topological properties are studied under biaxial strain within the range of -8 to +8%. The ground state of monolayer MnSbBiS2Te2 is metallic with an out-of-plane magnetic easy axis. A band gap is opened when a compressive strain between -4% and -7% is applied. We observe a topological phase transition at a biaxial strain of -5%, where the material becomes a Chern insulator exhibiting a quantum anomalous hall (QAH) effect. We find that biaxial strain and spin-orbit coupling (SOC) are responsible for the topological phase transition in MnSbBiS2Te2. In addition, we find that biaxial strain can alter the direction of the magnetic easy axis of MnSbBiS2Te2. The Curie temperature is calculated using the Heisenberg model and is found to be 24 K. This study could pave the way to the design of topological materials with potential applications in spintronics, quantum computing, and dissipationless electronics.

Janus monolayer PXC (X = As/Sb) for photocatalytic water splitting with a negative Poisson's ratio

Two-dimensional (2D) Janus materials have attracted considerable attention in photocatalysis owing to their robust redox capability and efficient segregation. In this study, we propose a novel Janus monolayer structure, denoted as PXC (X = As/Sb), exhibiting favorable stability in terms of dynamics, thermal properties, and mechanical characteristics. The PXC monolayers demonstrate a relatively smaller Young's modulus (132.5/119.5 N m-1 for PAsC/PSbC) and large negative Poisson's ratios (-0.15/-0.101 for PAsC/PSbC). Moreover, the HSE06 + SOC functional results show that PAsC/PSbC are indirect semiconductors with a 2.33/1.43 eV band gap, exhibiting a suitable band alignment for photocatalytic water splitting. The calculated high carrier mobility (104 cm2 V-1 s-1), along with a significant discrepancy, determined by the deformation potential theory and the built-up field induced by the large intrinsic dipole, effectively suppresses the recombination of photogenerated carriers. Furthermore, PXC monolayers possess a strong absorption capacity in the visible and ultraviolet light region (105 cm-1). Therefore, our results indicate that PXC monolayers hold great potential for application in the field of photocatalytic water splitting.

Janus layers and electronic structure of 1T-(TiSeS)2

TiS2-TiSe2 is one of the most studied titanium based solid solution systems. However, so far, all research on it has only focused on its disordered phase. Here, we systematically investigate its ordered phases. Using a structure search method based on the particle swarm optimization (CALYPSO) algorithm, we identify TiSeS-156 and discover a new structure (1T-(TiSeS)2). Based on first principles theory, their phonon spectra, formation energy, mechanical, electronic, thermal, and optical properties, as well as chemical bond analysis and synthetic pathways, have been investigated. The primitive cell of TiSeS-156 has three atoms and has a space group of P3m1 (no. 156). 1T-(TiSeS)2 has six atoms and has P3̄m1 symmetry (no. 164). TiSeS-156 and 1T-(TiSeS)2 are constructed by stacking the S-Ti-Se Janus layer materials. TiSeS-156 and 1T-(TiSeS)2 are narrow-gap semiconductors. The localized nature of the Ti(3d) states of TiSeS-156 and 1T-(TiSeS)2 leads to their semiconductor properties. 1T-(TiSeS)2 and TiSeS-156 have very similar mechanical, electronic, thermal, and optical properties of 1T-TiS2 and 1T-TiSe2, and are members of the 2D hexagonal lattice transition metal dichalcogenide layered material family. However, compared with 1T-TiS2 and 1T-TiSe2, TiSeS-156 and 1T-(TiSeS)2 have a wider range of potential applications, such as photovoltaic devices and photocatalysis, due to their S-Ti-Se Janus layer structure. They also provide a pathway for the preparation of Janus TiSeS monolayer and multi-layer materials. Moreover, our findings provide crucial insights for understanding the rich and complex crystal structures of the TiS2-TiSe2 system, which have broad implications for further exploration of this class of promising materials.

Substitutional doping of MoTe2/ZrS2 heterostructures for sustainable energy related applications

Stacking and/or substitutional doping are effective strategies to tune two-dimensional materials with desired properties, greatly extending the applications of the pristine materials. Here, by employing first-principles calculations, we propose that a pristine MoTe2/ZrS2 heterostructure is a distinguished lithium-ion battery anode material with a low Li diffusion barrier (∼0.26 eV), and it has a high maximum Li storage capacity (476.36 mA h g-1) and a relatively low open-circuit voltage (0.16 V) at Li4/MoTe2/Li/ZrS2/Li4. The other heterostructures with different types can be achieved by substitutional doping and their potential applications in sustainable energy related areas are further unraveled. For instance, a type-II TeMoSe/ZrS2 heterostructure could be a potential direct Z-scheme photocatalyst for water splitting with a high solar-to-hydrogen conversion efficiency of 17.62%. The TeMoSe/SZrO heterostructure is predicted to be a potential candidate for application in highly efficient solar cells. Its maximum power conversion efficiency can be as high as 19.21%, which is quite promising for commercial applications. The present results will shed light on the sustainable energy applications of pristine or doped MoTe2/ZrS2 heterostructures in the future.

Dimensionality reduction induced synergetic optimization of the thermoelectric properties in Bi2Si2X6 (X = Se, Te) monolayers

Different from three-dimensional bulk compounds, two-dimensional monolayer compounds exhibit much better thermoelectric performance on account of the quantum confinement and interface effect. Here, we present a systematic study on the electronic and thermal transport properties of bulk and monolayer Bi2Si2X6 (X = Se, Te) through theoretical calculations using density functional theory based on first-principles and Boltzmann transport theory. Monolayer Bi2Si2X6 are chemically, mechanically and thermodynamically stable semiconductors with suitable band gaps, and they have lower lattice thermal conductivity (κL) in the a/b direction than their bulk counterparts. The calculated κL of monolayer Bi2Si2Se6 (Bi2Si2Te6) is as low as 0.72 (0.95) W m-1 K-1 at 700 K. Moreover, monolayer Bi2Si2X6 exhibit a higher Seebeck coefficient compared with bulk Bi2Si2X6 owing to the sharper peaks in the electronic density of states (DOS). This results in a significant increase in power factor by dimensionality reduction. Combined with the synergetically suppressed thermal conductivity, the maximum ZT values for monolayer Bi2Si2Se6 and Bi2Si2Te6 are significantly enhanced up to 5.03 and 2.87 with p-type doping at 700 K, which are more than 2 times that of the corresponding bulk compounds. These results demonstrate the superb thermoelectric performance of monolayer Bi2Si2X6 for promising thermoelectric conversion applications.

Novel Piezoelectricity in Two-Dimensional Metallic/Semimetallic Materials with Out-of-Plane Polarization

Normally, the good conductivity of metals and semimetals is incompatible with the piezoelectricity since the internal electric current will dismiss any polarization. However, here, we reveal that the out-of-plane piezoelectric effect can exist in two-dimensional (2D) metallic/semimetallic materials due to their giant anisotropy. A method is developed to calculate the out-of-plane polarization in 2D systems, where the modern theory of polarization based on a Berry-phase approach is not applicable. Detailed calculation and analysis on a Dirac material, the FeB2 monolayer, show that it has an out-of-plane polarization of 8.3 pC/m and the piezoelectric coefficient of e31 = -59.3 pC/m and d31 = -0.25 pm/V. This work provides a formalism to discover more piezoelectric materials within the vast 2D metallic/semimetallic materials.

Moderate direct band-gap energies and high carrier mobilities of Janus XWSiP2 (X = S, Se, Te) monolayers via first-principles investigation

Two-dimensional (2D) Janus materials with extraordinary properties are promising candidates for utilization in advanced technologies. In this study, new 2D Janus XWSiP2 (X = S, Se, Te) monolayers were constructed and their properties were systematically analyzed by using first-principles calculations. All three structures of SWSiP2, SeWSiP2, and TeWSiP2 exhibit high energetic stability for the experimental fabrication with negative and high Ecoh values, the elastic constants obey the criteria of Born-Huang, and no imaginary frequency exists in the phonon dispersion spectra. The calculated results from the PBE and HSE06 approaches reveal that the XWSiP2 are semiconductors with moderate direct band-gaps varying from 1.01 eV to 1.06 eV using the PBE method, and 1.39 eV to 1.44 eV using the HSE06 method. In addition, the electronic band structures of the three monolayers are significantly affected by the applied strains. Interestingly, the transitions from a direct to indirect semiconductor are observed for different biaxial strains εb. The transport parameters including the carrier mobility values along the x direction μx and y direction μy were also calculated to study the transport properties of the XWSiP2. The results indicate that the XWSiP2 monolayers not only have high carrier mobilities but also anisotropy in the transport directions for both holes and electrons. Together with the moderate and tunable energy gaps, the XWSiP2 materials are found to be potential candidates for application in the photonic, photovoltaic, optoelectronic, and electronic fields.

Excellent thermoelectric properties of the Tl2S3 monolayer for medium-temperature applications

Exploring materials with high thermoelectric (TE) performance can alleviate energy pressure and protect the environment, and thus, TE materials have attracted extensive attention in the new energy field. In this paper, we systematically study the TE properties of Tl₂S₃ using first-principles combined with Boltzmann transport theory (BTE). The calculation results show an excellent power factor (1.12 × 10^-2 W m^-1 K^-2) and ultra-low lattice thermal conductivity (k_l = 0.88 W m^-1 K^-1) at room temperature. Through analysis, we attribute the ultra-low k_l of Tl₂S₃ to the lower phonon group velocity (v_g) and larger phonon anharmonicity. Meanwhile, discussion of chemical bonding showed that the filling of the anti-bonding state leads to the weakening of the Tl-S chemical bond, resulting in low v_g. Furthermore, this research also investigates the scattering processes (the out-of-plane acoustic mode (ZA) + optical mode (O) → O (ZA + O → O), the in-plane transverse acoustic mode (TA) + O → O (TA + O → O), and the in-plane longitudinal acoustic mode (LA) + O → O (LA + O → O)), from which we find that 2D Tl₂S₃ possesses strong acoustic-optical scattering. Based on the analysis of electron transport properties under electron-phonon coupling, 2D Tl₂S₃, as a novel TE material, exhibits a ZT value as high as 2.8 at 400 K. Our calculations suggest that Tl₂S₃ is a potential TE material at medium temperature.

Toward a Rigorous Theoretical Description of Photocatalysis Using Realistic Models

This Perspective aims at providing a road map to computational heterogeneous photocatalysis highlighting the knowledge needed to boost the design of efficient photocatalysts. A plausible computational framework is suggested focusing on static and dynamic properties of the relevant excited states as well of the involved chemistry for the reactions of interest. This road map calls for explicitly exploring the nature of the charge carriers, the excited-state potential energy surface, and its time evolution. Excited-state descriptors are introduced to locate and characterize the electrons and holes generated upon excitation. Nonadiabatic molecular dynamics simulations are proposed as a convenient tool to describe the time evolution of the photogenerated species and their propagation through the crystalline structure of photoactive material, ultimately providing information about the charge carrier lifetime. Finally, it is claimed that a detailed understanding of the mechanisms of heterogeneously photocatalyzed reactions demands the analysis of the excited-state potential energy surface.

Structural, electronic and thermoelectric properties of GeC and MXO (M = Ti, Zr and X = S, Se) monolayers and their van der Waals heterostructures

Vertical stacking of two-dimensional materials into layered van der Waals heterostructures is considered favourable for nanoelectronics and thermoelectric applications. In this work, we investigate the structural, electronic and thermoelectric properties of GeC and Janus monolayers MXO (M = Ti, Zr; X = S, Se) and their van der Waals (vdW) heterostructures using first-principles calculations. The values of binding energies, interlayer distances and thermal stability confirm the stability of these vdW heterostructures. The calculated band structure shows that GeC monolayer have a direct band gap while MXO (M = Ti, Zr; X = S, Se) and their van der Waals heterostructures show indirect band nature. Partial density of states confirms the type-II band alignment of GeC-MXY vdW heterostructures. Our results shows that ZrSeO (GeC) monolayers and GeC-ZrSO vdW heterostructures have higher power factor, making them promising for thermoelectric device applications.

First-principles study of the structural and electronic properties of tetragonal ZrOX (X = S, Se, and Te) monolayers and their vdW heterostructures for applications in optoelectronics and photocatalysis

Using first-principles calculations, we have studied the structural and electronic properties of ZrOX (X = S, Se, and Te) monolayers and their van der Waals heterostructures in the tetragonal structure. Our results show that these monolayers are dynamically stable and are semiconductors with electronic bandgaps ranging from 1.98 to 3.16 eV as obtained with the GW approximation. By computing their band edges, we show that ZrOS and ZrOSe are of interest for water splitting applications. In addition, the van der Waals heterostructures formed by these monolayers show a type I band alignment for ZrOTe/ZrOSe and a type II alignment for the other two heterostructures, making them potential candidates for certain optoelectronic applications involving electron/hole separation.

Two-dimensional magnetic Janus monolayers and their van der Waals heterostructures: a review on recent progress

A magnetic Janus monolayer, a special type of material which has asymmetric arrangements of its surface at the nanoscale, has been shown to present rather exotic properties for applications in spintronics and its intersections. This review aims to offer a comprehensive review of the emergent physical properties of magnetic Janus monolayers and their van der Waals heterostructures from a theoretical point of view. The review starts by introducing the theoretical methodologies composed of the state-of-the-art methods and the challenges and limitations in validations for the descriptions of the magnetic ground states and thermodynamic properties in magnetic materials. The built-in polarization field induced physical phenomena of magnetic Janus monolayers are then presented. The tunable electronic and magnetic properties of magnetic Janus monolayer-based van der Waals heterostructures are discussed. Finally, the conclusions and future challenges in this field are prospected. This review serves as a complete summary of the two-dimensional magnetic Janus library and emergent electronic and magnetic properties in magnetic Janus monolayers and their heterostructures, and provides guidelines for the design of electronic and spintronic devices based on Janus materials.

A first-principles study on the electronic, piezoelectric, and optical properties and strain-dependent carrier mobility of Janus TiXY (X ≠ Y, X/Y = Cl, Br, I) monolayers

Janus transition metal dichalcogenide monolayers (TMDs) have attracted wide attention due to their unique physical and chemical properties since the successful synthesis of the MoSSe monolayer. However, the related studies of Janus monolayers of transition metal halides (TMHs) with similar structures have rarely been reported. In this article, we systematically investigate the electronic properties, piezoelectric properties, optical properties, and carrier mobility of new Janus TiXY (X ≠ Y, X/Y = Cl, Br, I) monolayers using first principles calculations for the first time. These Janus TiXY monolayers are thermally, dynamically, and mechanically stable, and their energy bands near the Fermi level (E_F) are almost entirely contributed by the central Ti atom. Besides, the Janus TiXY monolayers exhibit excellent in-plane and out-of-plane piezoelectric effects, especially with an in-plane piezoelectric coefficient of ∼4.58 pm V^-1 for the TiBrI monolayer and an out-of-plane piezoelectric coefficient of ∼1.63 pm V^-1 for the TiClI monolayer, suggesting their promising applications in piezoelectric sensors and energy storage applications. The absorption spectra of Janus TiXY monolayers are mainly distributed in the visible and infrared regions, implying that they are fantastic candidates for photoelectric and photovoltaic applications. The obtained carrier mobilities revealed that TiXY monolayers are hole-type semiconductors. Under uniaxial compressive strain, the hole mobilities of these monolayers are gradually improved, indicating that TiXY monolayers have potential applications in the field of flexible electronic devices.

Active Asymmetric Electron-Transfer Effect on the Enhanced Piezoelectricity in MoTO (T = S, Se, or Te) Monolayers and Bilayers

Development of piezoelectric materials is limited partly due to the incompleteness of internal mechanism and the lack of vertical piezoelectricity. Herein, we theoretically identify the stable MoTO (T = S, Se, or Te) monolayers and bilayers. When two elements are given but another element can be changed, the larger the electronegativity difference ratio R_ratio is, the stronger the piezoelectricity will be. Vertical piezoelectric coefficient d₃₃ of the MoTeO bilayer reaches 38.907 pm/V, which is 12 times larger than that of the bulk GaN. The "active asymmetric electron-transfer" strategy mainly contributes to the spontaneous remarkable piezoelectricity of MoTO. Importantly, we proposed the new method for calculating the piezoelectric coefficients of two-dimensional (2D) materials, which corresponds to the fact that 2D materials have a certain thickness. This study not only provides novel extraordinary candidates for energy conversion and touch-sensor nanodevices but also promotes a deeper understanding of piezoelectricity of 2D materials.

Cu Nanoparticles Modified Step-Scheme Cu2O/WO3 Heterojunction Nanoflakes for Visible-Light-Driven Conversion of CO2 to CH4

In this study, Cu and Cu2O hybrid nanoparticles were synthesized onto the WO3 nanoflake film using a one-step electrodeposition method. The critical advance is the use of a heterojunction consisting of WO3 flakes and Cu2O as an innovative stack design, thereby achieving excellent performance for CO2 photoreduction with water vapor under visible light irradiation. Notably, with the modified Cu nanoparticles, the selectivity of CH4 increased from nearly 0% to 96.7%, while that of CO fell down from 94.5% to 0%. The yields of CH4, H2 and O2 reached 2.43, 0.32 and 3.45 mmol/gcat after 24 h of visible light irradiation, respectively. The boosted photocatalytic performance primarily originated from effective charge-transfer in the heterojunction and acceleration of electron-proton transfer in the presence of Cu nanoparticles. The S-scheme charge transfer mode was further proposed by the in situ-XPS measurement. In this regard, the heterojunction construction showed great significance in the design of efficient catalysts for CO2 photoreduction application.

Two-dimensional cobalt telluride as a piezo-tribogenerator

Two-dimensional (2D) materials have been shown to be efficient in energy harvesting. Here, we report the use of waste heat to generate electricity via the combined piezoelectric and triboelectric properties of 2D cobalt telluride (CoTe₂). The piezo-triboelectric nanogenerator (PTNG) produced an open-circuit voltage of ∼5 V under 1 N force and the effect of temperature in the range of 305-363 K shows a four-fold energy conversion efficiency improvement. The 2D piezo-tribogenerator shows excellent characteristics with a maximum voltage of ∼10 V, fast response time, and high responsivity. Density functional theory was used to gain further insights and validation of the experimental results. Our results could lead to energy harvesting approaches using 2D materials from various thermal sources and dissipating waste heat from electronic devices.

Rational Design of Heteroanionic Two-Dimensional Materials with Emerging Topological, Magnetic, and Dielectric Properties

Designing and tuning the physical properties of two-dimensional (2D) materials at the atomic level are crucial to the development of 2D technologies. Here, we introduce heteroanions into metal-centered octahedral structural units of a 2D crystal breaking the O_h symmetry, together with the synergistic effect of anions' electrons and electronegativity, to realize ternary 2D materials with emerging topological, magnetic, and dielectric properties. Using an intrinsic heteroanionic van der Waals layered material, VOCl, as a prototype, 20 2D monolayers VXY (X = B, C, N, O, or F; Y = F, Cl, Br, or I) are obtained and investigated by means of first-principles calculations. The anion engineering in this family significantly reshapes the electronic properties of VOCl, leading to nonmagnetic topological insulators with nontrivial edge states in VCY, ferromagnetic half-semimetals with a nodal ring around the Fermi energy in VNY, and insulators with dielectric constants in VOY higher than that of h-BN. This work demonstrates the rationality and validity of the design strategy of multiple-anion engineering to achieve superior properties in the 2D monolayers with potential application in electronics and spintronics.

Engineering 2D Materials for Photocatalytic Water-Splitting from a Theoretical Perspective

Splitting of water with the help of photocatalysts has gained a strong interest in the scientific community for producing clean energy, thus requiring novel semiconductor materials to achieve high-yield hydrogen production. The emergence of 2D nanoscale materials with remarkable electronic and optical properties has received much attention in this field. Owing to the recent developments in high-end computation and advanced electronic structure theories, first principles studies offer powerful tools to screen photocatalytic systems reliably and efficiently. This review is organized to highlight the essential properties of 2D photocatalysts and the recent advances in the theoretical engineering of 2D materials for the improvement in photocatalytic overall water-splitting. The advancement in the strategies including (i) single-atom catalysts, (ii) defect engineering, (iii) strain engineering, (iv) Janus structures, (v) type-II heterostructures (vi) Z-scheme heterostructures (vii) multilayer configurations (viii) edge-modification in nanoribbons and (ix) the effect of pH in overall water-splitting are summarized to improve the existing problems for a photocatalytic catalytic reaction such as overcoming large overpotential to trigger the water-splitting reactions without using cocatalysts. This review could serve as a bridge between theoretical and experimental research on next-generation 2D photocatalysts.

Janus monolayer HfSO with improved optical properties as a novel material for photovoltaic and photocatalyst applications

First principles calculations were performed to investigate the photocatalytic behavior of 2D Janus monolayer HfSO at equilibrium and under the influence of strains and external electric fields.

Rational design of metal oxide catalysts for electrocatalytic water splitting

Electrocatalytic energy conversion between electricity and chemical bonding energy is realized through redox reactions with multiple charge transfer steps at the electrode-electrolyte interface. The surface atomic structure of the electrode materials, if appropriately designed, will provide an energetically affordable pathway with individual reaction intermediates that not only reduce the thermodynamic energy barrier but also allow an acceptably fast kinetic rate of the overall redox reaction. As one of the most abundant and stable forms, oxides of transitional metals demonstrated promising electrocatalytic activities towards multiple important chemical reactions. In this topical review, we attempt to discuss the possible avenues to construct the electrocatalytic active surface for this important class of materials for two essential chemical reactions for water splitting. A general introduction of the electrochemical water splitting process on the electrocatalyst surface with applied potential will be provided, followed by a discussion on the fundamental charge transfers and the mechanism. As the generally perceived active sites are chemical reaction dependent, we offer a general overview of the possible approaches to construct or create electrocatalytically active sites in the context of surface atomic structure engineering. The review concludes with perspectives that summarize challenges and opportunities in electrocatalysis and how these can be addressed to unlock the electrocatalytic potentials of the metal oxide materials.

Theoretical Prediction of a Bi-Doped β-Antimonene Monolayer as a Highly Efficient Photocatalyst for Oxygen Reduction and Overall Water Splitting

The photo-/electrocatalysts with high activities for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) are of significance for the advancement of photo-/electrochemical energy systems such as solar energy to resolve the global energy crisis, reversible water electrolyzers, metal-air batteries, and fuel cells. In the present work, we have systematically investigated the photochemical performance of the 2D β-antimonene (β-Sb) monolayer. From density functional theory investigations, β-Sb with single-atom doping possesses a trifunctional photocatalyst with high energetics and thermal stabilities. In particular, it is predicted that the performance of the HER activity of β-Sb will be superior to most of the 2D materials. Specifically, β-Sb with single atom replacement has even superior that the reference catalysts IrO2(110) and Pt(111) with relatively low overpotential values for ORR and OER mechanisms. The superior catalytic performance of β-Sb has been described by its electronic structures, charge transfer mechanism, and suitable valence and conduction band edge positions versus normal hydrogen electrode. Meanwhile, the low overpotential of multifunctional photocatalysts of the Bi@β-Sb monolayer makes them show a remarkable performance in overall water splitting (0.06 V for HER, 0.25 V for OER, and 0.31 V for ORR). In general, the Bi@β-Sb monolayer may be an excellent trifunctional catalyst that exhibits high activity toward all electrode reactions of hydrogen and oxygen.

Two-Dimensional Janus Antimony Selenium Telluride with Large Rashba Spin Splitting and High Electron Mobility

Janus two-dimensional materials with large Rashba spin splitting and high electron mobility are rarely reported but highly desired for nanoscale spintronics. Herein, using density functional theory calculations, we predicated Janus Sb2Se x Te3-x (x = 1 or 2) monolayers simultaneously harboring these fascinating properties. The predicated monolayers are indirect semiconductors with great dynamical, thermal, and mechanical stability. The spin-orbital coupling (SOC) and the out-of-plane asymmetry lead to Rashba spin splitting at the conduction band minimum (CBM), which can be effectively tuned by the small uniaxial strain. The strong band dispersion at the CBM leads to small electron effective mass, consequently enabling a high electron mobility that reaches up to 6816.63 cm2 V-1 s-1. Moreover, Janus Sb2Se x Te3-x monolayers possess great light absorption capability within the visible and infrared regions of solar light. Our findings highlight promising candidates for high-speed spintronic devices and may motivate more research efforts on carrier transport and SOC effects in Janus group V and VI monolayers.

Role of Ce concentration on the structural and magnetic properties of functional magnetic oxide particles

Functional magnetic oxide particles offer exceptional GHz frequency capabilities, which can significantly enhance the utility of communication and signal processing devices. In the present work, we have investigated the structural and magnetic properties of rational multifunctional oxide Y2.9-x Ce x Bi0.1Fe5O12 particles - a full series with x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1 via a conventional solid-state route. The X-ray diffraction pattern validated the Ia3̄d cubic garnet phase in all samples. From Rietveld refinement, it is observed that the ceric oxide (CeO2) impurity increases with an increase of Ce concentration, evincing a partial substitution of cerium (Ce) element into the garnet structure. The magnetic oxide particles with Ce concentration x = 0.4 showed a better crystallite size, dodecahedral site occupancy and solubility of cerium in the garnet phase. The morphological visualization of random shaped grains in the micrometer range was performed using the scanning electron microscopy (SEM) technique. The static magnetic properties showed that the saturation magnetization (M s) decreases up to 43% and coercivity increases up to 59% with the increase of Ce concentration. The dynamic investigation on these oxide particles exhibits various intriguing and novel properties. Various intrinsic material parameters such as saturation magnetization (M s), gyromagnetic ratio (γ), Gilbert damping constant (α) and extrinsic contribution (ΔH o) to linewidth were determined from the fitting of resonance field (H r) and field linewidth (ΔH r) data. We ascertained that the damping constant increases with the increase of Ce concentration, which can be explained in terms of two magnon scattering and local defects caused by CeO2 inhomogeneity. The proposed doped garnets can be a potential candidate for high frequency microwave applications and spin-transfer-torque devices.

Induced half-metallic characteristics and enhanced magnetic anisotropy in the two-dimensional Janus V2I3Br3 monolayer by graphyne adsorption

The recent emergence of two-dimensional (2D) Janus materials has opened a new avenue for spintronic and optoelectronic applications. However, 2D magnetic Janus materials and Janus monolayer-based magnetic heterostructures are yet to be fully studied. Herein, the stability and electronic structure of 2D Janus V₂I₃Br₃ and V₂I₃Cl₃ monolayers, and the electronic and magnetic properties of 2D graphyne/Janus V₂I₃Br₃ (γ-GY/V₂I₃Br₃) heterostructures are calculated based on the density functional theory. Janus V₂I₃Br₃ and V₂I₃Cl₃ monolayers are ferromagnetic semiconductors with good stability and direct band gap. By combing the graphyne layer, the Janus V₂I₃Br₃ monolayer shows half-metallic characteristics. The electrical conductivity of the Janus V₂I₃Br₃ monolayer in γ-GY/V₂I₃Br₃ heterostructures is further improved, which is very favorable for the applications of the γ-GY/V₂I₃Br₃ heterostructure in battery anodes. Moreover, the Janus V₂I₃Br₃ monolayer possesses a smaller perpendicular magnetic anisotropy (PMA), and the PMA can be effectively enhanced by combing γ-GY. Herein, the enhanced PMA was discovered to depend on the stacking patterns of γ-GY and V₂I₃Br₃ monolayers. Biaxial strains can further affect the PMA of the γ-GY/V₂I₃Br₃ heterostructure. Meanwhile, at a compressive strain, the Janus V₂I₃Br₃ monolayer in the γ-GY/V₂I₃Br₃ heterostructure realizes the transition from the magnetic half-metallic to the magnetic metal state. These results can enrich the applications and designs of γ-GY/V₂I₃Br₃ magnetic heterostructures in spintronic devices and energy fields.

Electronic and photochemical properties of hybrid binary silicon and germanium derived Janus monolayers

The electronic structures and optical properties of a novel class of hybrid binary Janus materials derived from IV-V groups were investigated using first principles calculations. The computational results demonstrated that, except for Ge₂NAs, all the other five structures of M₂XY monolayers (M = Si, Ge; X, Y = N, P, As; X ≠ Y) have excellent thermal and dynamical stabilities. Janus Si₂NP, Si₂NAs, Si₂PAs and Ge₂NP are semiconductors with direct band gaps spanning the range between 0.82 and 2.49 eV. Notably, the hybrid M₂XY materials exhibit highly efficient absorption within the visible light region, which are greatly higher than their pristine MX structures. Janus Si₂PAs and Ge₂PAs possess appropriate band edge alignments that straddle the water redox potentials in the pH range from 0 to 14, making them promising photocatalysts for water splitting under visible light. Our calculations further demonstrate that the catalytic selectivity for the water splitting reaction could be achieved through the hybrid Janus M₂XY, where, for instance, Ge₂NP appears to facilitate only the oxidation, but not the reduction of water under certain conditions. This outcome provides a new route for the design of novel photocatalysts with improved efficiency and selectivity.

Two-dimension on two-dimension growth: hierarchical Ni0.2Mo0.8N/Fe-doped Ni3N nanosheet array for overall water splitting

Developing advanced electrocatalysts with low cost for electrocatalytic water splitting are highly desirable. Herein, we report the design of two-dimension on two-dimension growth of hierarchical Ni0.2Mo0.8N nanosheets on Fe-doped Ni3N nanosheets supported on Ni foam (Ni0.2Mo0.8N/Fe-Ni3N/NF) via hydrothermal reaction and nitridation treatment. In the hierarchical structures, small Ni0.2Mo0.8N nanosheets were uniformly anchored on Fe-Ni3N nanosheets. Due to enhanced electron transfer between Ni0.2Mo0.8N and Fe-Ni3N, Ni0.2Mo0.8N/Fe-Ni3N/NF exhibits superior electrocatalytic activity for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). After stability tests for 50 h, Ni0.2Mo0.8N/Fe-Ni3N/NF exhibits negligible degradation of the current density for the OER (91% remain) and HER (95% remain), suggesting excellent stability. Owing to the outstanding performance, Ni0.2Mo0.8N/Fe-Ni3N/NF display a cell voltage of 1.54 V (10 mA cm-2) for electrocatalytic overall water splitting.

Design of 2D materials - MSi2CxN4-x (M = Cr, Mo, and W; x = 1 and 2) - with tunable electronic and magnetic properties

Two-dimensional (2D) materials have attracted increasing interest in the past decades due to their unique physical and chemical properties for diverse applications. In this work, we present a first-principles design on a novel 2D family, MSi₂C_xN_4-x (M = Cr, Mo, and W; x = 1 and 2), based on density-functional theory (DFT). We find that all MSi₂C_xN_4-x monolayers are stable by investigating their mechanic, dynamic, and thermodynamic properties. Interestingly, we see that the alignment of magnetic moments can be tuned to achieve non-magnetism (NM), ferromagnetism (FM), anti-ferromagnetism (AFM) or paramagnetism (PM) by arranging the positions of carbon atoms in the 2D systems. Accordingly, their electronic properties can be controlled to obtain semiconductor, half-metal, or metal. The FM states in half-metallic 2D systems are contributed to the hole-mediated double exchange, while the AFM states are induced by super-exchange. Our findings show that the physical properties of 2D systems can be tuned by compositional and structural engineering, especially the layer of C atoms, which may provide guidance on the design and fabrication of novel 2D materials with projected properties for multi-functional applications.

Stacking effects in van der Waals heterostructures of blueP and Janus XYO (X = Ti, Zr, Hf: Y = S, Se) monolayers

Using first-principles calculations, the geometry, electronic structure, optical and photocatalytic performance of blueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers and their corresponding van der Waal heterostructures in three possible stacking patterns, are investigated. BlueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers are indirect bandgap semiconductors. A tensile strain of 8(10)% leads to TiSeO(ZrSeO) monolayers transitioning to a direct bandgap of 1.30(1.61) eV. The calculated binding energy and AIMD simulation show that unstrained(strained) blueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers and their heterostructures are thermodynamically stable. Similar to the corresponding monolayers, blueP-XYO (X = Ti, Zr, Hf: Y = S, Se) vdW heterostructures in three possible stacking patterns are indirect bandgap semiconductors with staggered band alignment, except blueP-TiSeO vdW heterostructure, which signifies straddling band alignment. Absorption spectra show that optical transitions are dominated by excitons for blueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers and the corresponding vdW heterostructures. Both E VB and E CB in TiSO, ZrSO, ZrSeO and HfSO monolayers achieve energetically favorable positions, and therefore, are suitable for water splitting at pH = 0, while TiSeO and HfSeO monolayers showed good response for reduction and fail to oxidise water. All studied vdW heterostructures also show good response to any produced O2, while specific stacking reduces H+ to H2.

A first-principles study of Janus monolayer TiSSe and VSSe as anode materials in alkali metal ion batteries

Anode materials play an important role in the performance of rechargeable batteries and have been attracting much research interest. In this work, we have investigated the electrochemical properties of two-dimensional (2D) Janus MSSe (M = Ti or V) for potential applications as anode materials in alkali metal ion batteries from density functional theory (DFT), following the recent successful synthesis of 2D Janus MoSSe. Our DFT calculations suggest that 2D Janus TiSSe and VSSe are stable in the 1T phase and 1H phase, respectively. It is found that alkali metal atoms X (X = Li, Na or K) can be stably adsorbed on the surfaces of Janus MSSe, and have low diffusion energy barriers. Additionally, small volume changes are observed in Janus MSSe after the adsorption of alkali metal atoms. It is predicted that the MSSe-2X systems have low open circuit voltages and high capacities. Our results suggest that 2D Janus TiSSe and VSSe are potential anode materials for alkali metal ion batteries.

Enhanced out-of-plane electromechanical response of Janus ZrSeO

Two-dimensional piezoelectric materials have attracted great attention as they could play a vital role in nano-electromagnetic systems. Herein, we investigate the compelling piezoelectric properties of Janus ZrSeO in monolayer and bulk structures using density functional theory calculations with a van der Waals correction. One of the two independent out-of-plane piezoelectric coefficients (e31) of the bulk ZrSeO is as high as 287.60 pC m-1, which is over five times larger than that of monolayer ZrSeO due to charge changes in the internal structure within each Zr, Se, and O layer. Interestingly, another large negative out-of-plane piezoelectric stress coefficient (e33) of bulk ZrSeO (-467.40 pC m-1) results from the displacement difference between the electronic and ionic center positions, which is at least three times larger than those previously reported for Janus Mo/W/Hf-based transition metal dichalcogenides. The charge transformation between atoms under strain induces negative piezoelectric stress, a process that is clarified using maximally localized Wannier functions (MLWF) and Bader charge analysis. This research also reveals the dependence of piezoelectricity in Janus MXY on the metal (M = Zr, Hf, W, Mo) and chalcogenide (X,Y = S, Se, O) components, which are directly proportional to the electronegativity and the atomic size difference.

Exploring the electronic band gap of Janus MoSeO and WSeO monolayers and their heterostructures

Electronic band structure of TMSeO monolayers.

Strain-Controllable High Curie Temperature, Large Valley Polarization, and Magnetic Crystal Anisotropy in a 2D Ferromagnetic Janus VSeTe Monolayer

Two-dimensional (2D) intrinsic ferromagnetic semiconductors are important for spintronics. A highly stable ML (monolayer) Janus 2H-VSeTe with intrinsic ferromagnetism is investigated by density functional theory. The biaxial strain could effectively tune the magnetic and electronic properties of Janus VSeTe. Specifically, the magnetic moment, band gap, Curie temperature (T_c), and valley splitting (Δ) could be modulated, as the states near the Fermi level are mainly contributed by the in-plane atomic orbitals. The VSeTe could be switched from ferromagnetic (FM) order to antiferromagnetic (AFM) ground state, under biaxial strains. And the corresponding T_c is tuned from 360 K (4%) to 0 K (-10.7%). However, VSeTe can be modulated from bipolar magnetic semiconductor (BMS) to half-semiconductor (HSC), spin gapless semiconductor (SGS), half-metal (HM), and even normal metal as the biaxial strain varies from -13 to 10%. Moreover, the easy and hard axes could be switched from each other, and the magnetocrystalline anisotropy (MCA) energy is also controlled by the strains. The Δ is also increased from 158 to 169 meV as the strain varies from 3.3 to -3.0%. The magnetic and electronic phase transitions in the strained VSeTe are observed, which could help researchers to investigate the controllable electronic and magnetic properties in electronics, spintronics, and valleytronics.

Predicted 2D ferromagnetic Janus VSeTe monolayer with high Curie temperature, large valley polarization and magnetic crystal anisotropy

The development of two-dimensional (2D) intrinsic ferromagnetic semiconductors is urgent in the spintronic field. Motivated by the recent experiments on the successful synthetization of monolayer (ML) Janus transition-metal dichalcogenides (MoSSe) and ferromagnetic (FM) VSe2, a highly stable ML Janus 2H-VSeTe is fabricated by density functional theory and confirmed by a global minimum search. The Janus VSeTe shows a large valley polarization of 158 meV as the space- and time-reversal symmetry is broken. The VSeTe shows FM order with Curie temperature (Tc) of 350 K and a sizable magnetocrystalline anisotropy (MCA) of -8.54 erg cm-2. The high Tc and large valley polarization suggest the 2D Janus VSeTe is a promising magnetic material for potential applications in electronics, spintronics, and valleytronics.

Asymmetric Schottky Contacts in van der Waals Metal-Semiconductor-Metal Structures Based on Two-Dimensional Janus Materials

Physical and electronic asymmetry plays a crucial role in rectifiers and other devices with a directionally variant current-voltage (I-V) ratio. Several strategies for practically creating asymmetry in nanoscale components have been demonstrated, but complex fabrication procedures, high cost, and incomplete mechanistic understanding have significantly limited large-scale applications of these components. In this work, we present density functional theory calculations which demonstrate asymmetric electronic properties in a metal-semiconductor-metal (MSM) interface composed of stacked van der Waals (vdW) heterostructures. Janus MoSSe has an intrinsic dipole due to its asymmetric structure and, consequently, can act as either an n-type or p-type diode depending on the face at the interior of the stacked structure (SeMoS-SMoS vs. SMoSe-SMoS). In each configuration, vdW forces dominate the interfacial interactions, and thus, Fermi level pinning is largely suppressed. Our transport calculations show that not only does the intrinsic dipole cause asymmetric I-V characteristics in the MSM structure but also that different transmission mechanisms are involved across the S-S (direct tunneling) and S-Se interface (thermionic excitation). This work illustrates a simple and practical method to introduce asymmetric Schottky barriers into an MSM structure and provides a conceptual framework which can be extended to other 2D Janus semiconductors.

Deep eutectic solvent-based manganese molybdate nanosheets for sensitive and simultaneous detection of human lethal compounds: comparing the electrochemical performances of M-molybdate (M = Mg, Fe, and Mn) electrocatalysts

Potentially hazardous chemical contaminants endanger the environment and human well-being, challenging scientists and policy makers to develop holistic alternative approaches for remediation. The addition or accumulation of these chemicals can have a series of far-reaching consequences and have direct and indirect effects at multiple levels of ecological organization. Therefore, the development of a sensitive tool for the comprehensive evaluation of chemical concentrations is highly relevant. Herein, we thus report the simultaneous electrochemical detection of highly toxic hydroquinone (HQ), Hg²⁺, and nitrite (NO₂^-) compounds using nanostructured metal molybdate (M = Mg, Fe and Mn) catalysts. These functional nanomaterials are synthesized using a deep eutectic solvent (DES) modified hydrothermal method that provides sustainable aspects and energy efficient synthesis strategies. Choline chloride (ChCl)-urea DES used in this study exhibits an all-in-one behaviour by simultaneously acting as a template, reducing agent, and homogeneous means for stabilizing metal ions. This stimulates the fabrication of hierarchical structures of metal molybdates with high surface activities that cause their remarkable properties with minimal waste generation. The structural, morphological, catalytic, and electrochemical capacities of the as-synthesized MgMoO₄, Fe₂(MoO₄)₃, and MnMoO₄ materials are explored through various techniques and comparatively, MnMoO₄ presents superior characterization features such as a reduced particle size, increased surface area and hierarchical architectures. Owing to the exceptional physicochemical attributes, the MnMoO₄ modified glassy carbon electrode (GCE) demonstrates superior electrochemical activities towards the individual and simultaneous detection of HQ, Hg²⁺, and NO₂^-. Well-defined and separate peaks are observed for the simultaneous detection of HQ, Hg²⁺, and NO₂^- which is influenced by the binding energies of these pollutants. Furthermore, the modified electrode exhibits a high sensitivity of 23.8, 17.7 and 10.2 μA μM^-1 cm^-2 with a limit of detection (LOD) of 0.026, 0.05, and 0.01 μM for HQ, Hg²⁺, and NO₂^- respectively under ideal conditions. Also, the reproducibility and anti-interference ability reinforce the application potential of the MnMoO₄ modified electrode for the simultaneous electrochemical detection of HQ, Hg²⁺, and NO₂^- in real samples with better recoveries, thus assessing the effect of these hazardous chemicals on humanity.

Recent Advancements and Future Prospects in Ultrathin 2D Semiconductor-Based Photocatalysts for Water Splitting

Ultrathin two-dimensional (2D) semiconductor-mediated photocatalysts have shown their compelling potential and have arguably received tremendous attention in photocatalysis because of their superior thickness-dependent physical, chemical, mechanical and optical properties. Although numerous comprehensions about 2D semiconductor photocatalysts have been amassed up to now, low cost efficiency, degradation, kinetics of charge transfer along with recycling are still the big challenges to realize a wide application of 2D semiconductor-based photocatalysis. At present, most photocatalysts still need rare or expensive noble metals to improve the photocatalytic activity, which inhibits their commercial-scale application extremely. Thus, developing less costly, earth-abundant semiconductor-based photocatalysts with efficient conversion of sunlight energy remains the primary challenge. In this review, it begins with a brief description of the general mechanism of overall photocatalytic water splitting. Then a concise overview of different types of 2D semiconductor-mediated photocatalysts is given to figure out the advantages and disadvantages for mentioned semiconductor-based photocatalysis, including the structural property and stability, synthesize method, electrochemical property and optical properties for H2/O2 production half reaction along with overall water splitting. Finally, we conclude this review with a perspective, marked on some remaining challenges and new directions of 2D semiconductor-mediated photocatalysts.

Boron-dopant enhanced stability of diamane with tunable band gap

The structural, electronic, and superconducting properties of B-doped cubic and hexagonal diamane (single layer diamond) were investigated based on the first-principles methods. B atom tends to stay in the substitutional site, and the most stable configuration is the structure with vertical B-B dimer. The formation energy of B-doped diamane is lower than the counterpart of pristine diamane indicating that B dopant can facilitate the synthesis of diamane. The configurations with vertical B-B dimers are semiconductors with tunable band gaps, which decrease with the B concentration increasing due to the interaction between B-B dimers. For example, the band gap of 3.125 mol% and 6.25 mol% B-doped cubic diamane is 1.82 eV and 1.44 eV, respectively. Moreover, configurations with meta-stable B distributions are metals, which have comparable superconducting transition temperatures with B-doped diamond (~4 K).

Structures, stabilities and piezoelectric properties of Janus gallium oxides and chalcogenides monolayers

Due to losing out-of-surface symmetry and the induced built-in dipole, two-dimensional (2D) Janus structures were believed to have various potential applications in the fields of piezoelectric devices, photocatalysis and electrochemical catalysis. In this work, 2D gallium oxides and chalcogenides (GaX) derived Janus monolayers Ga₂XY (X/Y  =  O, S, Se, and Te) were designed. Our first-principles calculations showed that apart from Ga₂OTe, all the monolayers are thermodynamically, dynamically and mechanically stable. These monolayers are demonstrated to be semiconductors with the band gaps in a wide range of 1.00-3.24 eV. The calculated in-plane piezoelectric coefficients of Janus monolayers (d ₁₁  =  3.09-5.67 pm V^-1) are demonstrably enhanced with respect to the pristine monolayers (d ₁₁  =  0.41-3.04 pm V^-1). Meanwhile, the yielding additional out-of-plane piezoelectric coefficients range from 0.11 to 0.34 pm V^-1. Interestingly, O-participated Janus monolayers display distinctive properties: dipole direction flipping from O to S/Se, high stabilities and moderate direct-band-gaps. Particularly, Ga₂OSe monolayer was found to have the largest piezoelectric coefficient of 5.67 pm V^-1, which can be ascribed to the highly imbalanced charge distribution on O and Se as well as the largest bond length differences between Ga-O and Ga-Se. Our study revealed that the Janus Ga₂XY monolayers, especially O-related systems, could be pretty promising as candidates in optoelectronic, piezoelectric sensors and energy conversion devices.

First-principles investigation of ScX2 (X = Cl, Br, or I) monolayers for flexible spintronic and electronic applications

The promising two-dimensional ScX₂ monolayer candidates for flexible spintronics applications.

High stability and visible-light photocatalysis in novel two-dimensional monolayer silicon and germanium mononitride semiconductors: first-principles study

The two-dimensional SiN and GeN semiconductors are expected to become novel photocatalysts for water-splitting.

Superhigh out-of-plane piezoelectricity, low thermal conductivity and photocatalytic abilities in ultrathin 2D van der Waals heterostructures of boron monophosphide and gallium nitride

A stable 2D van der Waals (vdW) heterobilayer, constituted by boron monophosphide (BP) and Gallium Nitride (GaN) monolayers, has been explored for different kinds of energy conversion and nanoelectronics. The nearly matched lattice constants of GaN and BP are commensurate with each other in their lattice structures. The out-of-plane inversion asymmetry coupled with the large difference in atomic charges between the GaN and BP monolayers induces in the heterobilayer a giant out-of-plane piezoelectric coefficient (|d33|max ≈ 40 pm V-1), which is the highest ever reported in 2D materials of a finite thickness. It is much higher than the out-of-plane piezoelectric coefficient reported earlier in multilayered Janus transition metal dichalcogenide MXY (M = Mo, W; X, Y = S, Se, Te) (|d33|max = 10.57 pm V-1). Such a high out-of-plane piezoelectricity found in a BP/GaN heterobilayer can bring about gigantic strain-tunable top gating effects in nanopiezotronic devices based on the same. Moreover, electron mobility (∼104 cm2 V-1 s-1) is much higher than that of transition metal dichalcogenides and conventional semiconductors. The origin of low lattice thermal conductivity (κL ∼ 25.25 W m-1 K-1) in BP/GaN at room temperature, which is lower than that of black phosphorene (78 W m-1 K-1), buckled arsenene (61 W m-1 K-1), BCN (90 W m-1 K-1), MoS2 (34.5 W m-1 K-1) and WS2 (32 W m-1 K-1) monolayers, has been systematically investigated via phonon dispersion, lattice thermal conductivity, phonon lifetime and mode Grüneisen parameters. The valence band maximum (VBM) and conduction band minimum (CBM) arising from GaN and BP monolayers respectively result in a type II vdW heterobilayer, which is found to be thermodynamically favorable for photocatalytic water splitting in both acidic and neutral media. The exciton binding energies are comparable to those of MoS2 and C3N4 single layers, while the absorbance reaches as high as ∼105 cm-1 in the visible wavelength region. The emergence of high piezoelectricity, high carrier mobility, low lattice thermal conductivity and photocatalytic water splitting abilities in the proposed vdW heterobilayer signifies enormous potential for its versatile applications in nanoscale energy harvesting, e.g., nano-sensors in medical devices, future nanopiezotronics, 2D thermoelectrics and solar energy conversion.