Novel Two-Dimensional Janus MoSiGeN4 and WSiGeN4 as Highly Efficient Photocatalysts for Spontaneous Overall Water Splitting

Enhanced electron transport and optical properties of experimentally synthesized monolayer Si9C15: a comprehensive DFT study for nanoelectronics and photocatalytic applications

Two-dimensional silicon-carbide (SixCy) materials stand out for their compatibility with current silicon-based technologies, offering unique advantages in nanoelectronics and photocatalysis. In this study, we employ density functional theory and nonequilibrium Green's function methods to investigate the electronic properties, electron transport characteristics, and optoelectronic qualities of experimentally synthesized monolayer Si9C15. Utilizing the modified deformation potential theory formula, we unveil Si9C15's significant directional anisotropy in electron mobility (706.42 cm2 V-1 s-1) compared to holes (432.84 cm2 V-1 s-1) in the a direction. The electrical transport calculations reveal that configurations with a 3 nm channel length demonstrate an ON state when biased, reaching a peak current of 150 nA. Moreover, this maximum current value escalates to 200 nA under tensile strain, marking an increase of approximately 100 times compared to the 5 nm channel, which remains in an OFF state. Si9C15 exhibits high light absorption coefficients (∼105 cm-1) and suitable band edge positions for water splitting at pH 0-7. Applying 1-5% tensile strain can tune the conduction band minimum and valence band maximum closer to the standard redox potentials, enhancing photocatalytic water splitting efficiency. Remarkably, under illumination at pH 0 and 7, Si9C15 can spontaneously catalyze water splitting, demonstrating its potential as a highly efficient photocatalyst. Our findings emphasize the importance of strain control and device length optimization for performance enhancement in nanoelectronics and renewable energy applications, positioning Si9C15 as a promising material for high-performance field-effect transistors and photocatalytic water splitting.

Two-dimensional Janus monolayers SPtAZ2 (A = Si and Ge; Z = N, P, and As): insight into their photocatalytic properties via first-principles calculations

As a derivative of the two-dimensional material family, two-dimensional Janus materials have garnered widespread attention in recent years. Consequently, in this work, we systematically investigated the stability, electronic properties, photocatalytic properties, optical properties, and carrier mobility of SPtAZ2 (A = Si and Ge; Z = N, P, and As) monolayers using first-principles calculations. In the equilibrium state, we identified four stable structures that exhibited the properties of indirect band gap semiconductors using the HSE06 hybrid functional. Through the exploration of the photocatalytic and optical properties of these four stable structures, we observed that SPtSiN2, SPtSiP2, and SPtGeAs2 monolayers possess favorable band edge positions, high solar-to-hydrogen efficiency (up to 30.74%), and light absorption efficiency, thus endowing these three structures with commendable photocatalytic and light absorption performance. We additionally calculated the carrier mobility of these three structures and identified significant differences in electron and hole mobilities in the same direction, facilitating the effective separation of electrons and holes. Finally, we explored the effects of biaxial strain on the electronic properties, photocatalysis, and light absorption of stable SPtAZ2 monolayers. Our research results not only expand the 2D Janus material family, but also successfully predict a type of photocatalyst capable of utilizing visible light for overall water splitting.

Ferroelectric polarization promotes a CdS/In2Se3 heterostructure for photocatalytic water splitting

The rapid recombination of photogenerated electrons and holes greatly limits the efficiency of photocatalyst based on semiconductor. In order to address this issue, we predicted a novel ferroelectric polarized heterojunction photocatalyst, CdS/In2Se3, which enables the spontaneous overall water splitting reaction. The CdS/In2Se3 heterojunction exhibits a band-edge staggered alignment and it is easy to form a direct Z-scheme charge transfer pathway. Besides, the built-in electric field (Eint) in the CdS/In2Se3 heterojunction promoted the charge transfer of CdS/In2Se3, leading to an improved separating efficiency of photo-generated carriers. Moreover, the vertical intrinsic polarized electric field (Ep) not only alters the position of the band edge but also reduces the bandgap limitations commonly associated with photocatalytic materials. Furthermore, the CdS/In2Se3 heterojunctions demonstrate separate catalytic activity for the hydrogen evolution reaction (HER) on the surface of the CdS monolayer and oxygen evolution reaction (OER) on the surface of In2Se3, respectively. Notably, the CdS/In2Se3-down configuration enables spontaneous photocatalytic water splitting in pH = 7, while the CdS/In2Se3-up configuration efficiently facilitates the HER process. This study highlights the significant advantages of CdS/In2Se3 heterojunctions as photocatalytic materials, offering unique insights into the development and research of this promising heterojunction architecture.

A novel two-dimensional Janus TiSiGeN4 monolayer with N vacancies for efficient photocatalytic nitrogen reduction

The photocatalytic nitrogen reduction reaction (pNRR) is a clean technology that converts H2O and N2 into NH3 under environmental conditions using inexhaustible sunlight. Herein, we designed a novel two-dimensional (2D) Janus TiSiGeN4 structure and evaluated the pNRR performance of the structure with the presence of nitrogen vacancies at different positions using density functional theory (DFT) calculations. The intrinsic dipoles in the Janus TiSiGeN4 structure generate a built-in electric field, which promotes the migration of photogenerated electrons and holes towards the (001) and (00-1) surfaces, respectively, to achieve efficient charge separation. For the pNRR, the Si atoms exposed after the formation of top N vacancies can realize the efficient activation of N2 through the "acceptance-donation" mechanism, while the presence of middle N vacancies not only suppresses the hydrogen evolution reaction, a competition reaction, but also lowers the reaction barrier for the protonation of N atoms. The limiting potential of TiSiGeN4 with the coexistence of both top and middle N vacancies (TiSiGeN4-VN-mt) is as low as -0.44 V. In addition, the introduction of N vacancies generates defect levels, narrowing the band gap and improving the light response. This work provides theoretical guidance for the design of efficient pNRR photocatalysts under mild conditions.

Biaxial strain modulated electronic structures of layered two-dimensional MoSiGeN4 Rashba systems

The two-dimensional (2D) MA2Z4 family has received extensive attention in manipulating its electronic structure and achieving intriguing physical properties. However, engineering the electronic properties remains a challenge. Herein, based on first-principles calculations, we systematically investigate the effect of biaxial strains on the electronic structure of 2D Rashba MoSiGeN4 (MSGN), and further explore how the interlayer interactions affect the Rashba spin splitting (RSS) in such strained layered MSGN systems. After applying biaxial strains, the band gap decreases monotonically with increasing tensile strains but increases when the compressive strains are applied. An indirect-direct-indirect band gap transition is induced by applying a moderate compressive strain (<5%) in the MSGN systems. Due to the symmetry breaking and moderate spin-orbit coupling (SOC), the monolayer MSGN possesses an isolated RSS near the Fermi level, which could be effectively regulated to the Lifshitz-type spin splitting (LSS) by biaxial strain. For instance, the LSS ← RSS → LSS transformation of the Fermi surface is presented in the monolayer and a more complex and changeable LSS ← RSS → LSS → RSS evolution is observed in bilayer and trilayer MSGN systems as the biaxial strain varies from -8% to 12%, which actually depends on the appearance, variation, and vanish of the Mexican hat band in the absence of SOC under different strains. The contribution of the Mo-dz2 orbital hybridized with the N-pz orbital in the highest valence band plays a dominant role in band evolution under biaxial strains, where the RSS → LSS evolution corresponds to the decreased Mo-dz2 orbital contribution. Our study highlights the biaxial strain controllable RSS, in particular the introduction and even the evolution of LSS near the Fermi surface, which makes the strained MSGN systems promising candidates for future applications in spintronic devices.

Tunable polarization properties of charge, spin, and valley in Janus VSiGeZ4 (Z = N, P, As) monolayers

Polarization, as an important characterization of the symmetry breaking systems, has attracted tremendous attention in two-dimensional (2D) materials. Due to their significant symmetry breaking, Janus 2D ferrovalley materials provide a desirable platform to investigate the charge, spin, and valley polarization, as well as their coupling effects. Herein, using first-principles calculations, the polarization properties of charge, spin, and valley in Janus VSiGeZ4 (Z = N, P, and As) monolayers are systematically studied. The mirror symmetry breaking leads to a non-zero dipole moment and surface work function difference, indicating the presence of out-of-plane charge polarization. Magnetic properties calculations demonstrate that VSiGeN4 is a 2D-XY magnet with a Berezinskii-Kosterlitz-Thouless temperature of 342 K, while VSiGeP4 and VSiGeAs4 have an out-of-plane magnetization with a Curie temperature below room temperature. The magnetization can be rotated by applying biaxial strain, allowing manipulation of the spin polarization via nonmagnetic means. The spontaneous valley polarization is predicted to be 46, 49, and 70 meV for VSiGeN4, VSiGeP4, and VSiGeAs4, respectively, whose physical origin can be elucidated by employing the model analysis. In particular, the biaxial strain can induce the valley polarization switching from the valence (conduction) band to conduction (valence) band, but it hardly changes the valley polarization strength. Meanwhile, the valley extremum is transformed from the K' (K) to K (K') points. The present work not only provides an underlying insight into the polarization properties of Janus VSiGeZ4 but also offers a class of promising materials for spintronic and valleytronic devices.

Two-dimensional SPdAZ2 (A = Si, Ge; Z = N, P, As) monolayers with an intrinsic electric field for high-performance photocatalysis

Two-dimensional materials exhibiting exceptional photocatalytic properties and a low carrier recombination rate have garnered significant attention. However, such attributes are relatively scarce among conventional two-dimensional materials. Two-dimensional Janus materials, owing to their intrinsic electric field, hold substantial promise in the realm of photocatalysis. In this study, we conducted a comprehensive investigation of the electronic, optical and photocatalytic properties, as well as the carrier mobility of SPdAZ2 (A = Si, Ge; Z = N, P, As) monolayers employing first-principles calculations. Employing the HSE06 hybrid density functional, we discovered that all six structures exhibit semiconductor characteristics with indirect band gaps under equilibrium conditions. Notably, SPdSiP2, SPdSiAs2, and SPdGeP2 monolayers displayed advantageous band edge positions, facilitating effective photocatalytic water decomposition. Furthermore, we computed the carrier mobility of SPdAZ2 monolayers, revealing significant variations in the electron and hole mobility along the same direction, which enhances the effective separation of electrons and holes. Finally, we explored the impact of biaxial strain and an applied electric field on the electronic properties, photocatalysis, and light absorption of SPdAZ2 monolayers. These compelling features underscore the broad potential applications of SPdAZ2 (A = Si, Ge; Z = N, P, As) monolayers in the realm of photocatalytic water decomposition.

Tunable covalent benzo-heterocyclic rings constructed using two-dimensional conjugated polymers for visible-light-driven water splitting

Developing highly efficient, stable, and cost-effective two-dimensional (2D) conjugated polymers (CPs) for overall water splitting (OWS) is critical for producing clean and renewable hydrogen energy, yet it remains a great challenge. Here, we designed eight 2D CPs through the topological assembly of diacetylene and benzene-derived molecular linkers that can offer active sites for hydrogen and oxygen evolution reactions, and explored their structural, electronic, optical, and photocatalytic OWS properties by performing first-principles computations. It is shown that incorporating benzo-heterocyclic rings into CPs can significantly modulate the electronic structures of CPs and broaden the spectral absorption, suitable for visible-light-driven OWS. Remarkably, through a range of screening criteria, including stability, electronic band structures, band edge alignments, and photocatalytic activity, we found that CP-4 based on diacetylene and benzotrifuran can spontaneously trigger the OWS in a neutral environment under its own light-induced bias, eliminating the need for sacrificial agents or cocatalysts. Specifically, the HER active site is primarily located at diacetylene moieties, while the OER active site is mainly concentrated on the benzo-heterocyclic rings. Moreover, the ideal STH efficiency for OWS on CP-4 was estimated to be 13.87%, highlighting its potential as a prospective photocatalyst for large-scale industrial OWS. Our findings open a door to the rational design of novel polymer photocatalysts for OWS.

Modulating the electronic properties and band alignments of the arsenene/MoSi2N4 van der Waals heterostructure via applying strain and electric field

The two-dimensional (2D) MoSi2N4 monolayer fabricated recently has attracted extensive attention due to its exotic electronic properties and excellent stability for future applications. Using first-principles calculations, we have shown that the electronic properties of the arsenene/MoSi2N4 van der Waals (vdW) heterostructure can be effectively modulated by applying in-plane/vertical strain and vertical electric field. The arsenene/MoSi2N4 vdW heterostructure has type-II band alignment, facilitating the separation of photogenerated electron-hole pairs. The heterostructure is predicted to have an indirect bandgap of about 0.52 eV by using the PBE functional (0.87 eV by using the hybrid functional). Furthermore, under εz = 0.5 Å vertical tensile strain or -0.05 V Å-1 vertical electric field, the arsenene/MoSi2N4 heterostructure can not only experience transition from an indirect to a direct bandgap semiconductor, but also exhibit type-II to type-I band alignment transition. The calculated optical absorption properties reveal that the formation of the vdW heterostructure can effectively enhance the light absorption, and the absorption coefficient in visible and ultraviolet regions is much higher than those of the arsenene and the MoSi2N4 monolayer. Most importantly, based on charge transfer analysis, we proposed the modulation mechanism of the electronic properties of the vdW heterostructure influenced by vertical strain and electric field. Our study provides physical insights into manipulating the electronic and optoelectronic properties of MoSi2N4 based vdW heterostructures, which may be helpful for their practical applications.

Phonon dynamics in MoSi2N4: insights from DFT calculations

We have reported the density functional theory investigations on the monolayered, 2 layered and bulk MoSi2N4 in three structural modifications called α1 [Y.-L. Hong, et al., Chemical Vapor Deposition of Layered Two-Dimensional MoSi2N4 Materials, Science, 2020, 369(6504), 670-674, DOI: 10.1126/science.abb7023], α2 and α3 [Y. Yin, Q. Gong, M. Yi and W. Guo, Emerging Versatile Two-Dimensional MoSi2N4 Family, Adv. Funct. Mater., 2023, 2214050, DOI: 10.1002/adfm.202214050]. We showed that in the case of monolayers the difference in total energies is less than 0.1 eV between α1 and α3 phases, and less than 0.2 eV between α1 and α2 geometries. The most energetically favorable layer stacking for the bulk structures of each phase was investigated. All considered modifications are dynamically stable from a single layer to a bulk structure in energetically favorable stacking. Raman spectra for the monolayered, 2 layered and bulk structures were simulated and the vibrational analysis was performed. The main difference in the obtained spectra is associated with the position of the strongest band which depends on the Mo-N bond length. According to the obtained data, we can conclude that the Raman line at 348 cm-1 in the experimental spectra of MoSi2N4 can have more complex explanation than just Γ-point Raman-active vibration as was discussed before in [Y.-L. Hong, et al., Chemical Vapor Deposition of Layered Two-Dimensional MoSi2N4 Materials, Science, 2020, 369(6504), 670-674, DOI: 10.1126/science.abb7023].

Janus MoAZ3H (A = Ge, Si; Z = N, P, As) monolayers: a new class of semiconductors exhibiting excellent photovoltaic and catalytic performances

Due to the asymmetrical structure in the vertical direction, Janus two-dimensional (2D) monolayer (ML) materials possess some unique physical properties, holding great promise for nanoscale devices. In this paper, based on the newly discovered MoA2Z4 (A = Si, Ge; Z = N, P, As) ML, we propose a class of 2D Janus MoAZ3H ML materials with good stability and excellent mechanical properties using first-principles calculations. We demonstrate that the novel Janus MoAZ3H ML materials are all semiconductors with bandgaps ranging from 0.69 to 2.44 eV, giving rise to good absorption in the visible light region. Especially, both MoSiN3H and MoGeN3H MLs can be used as catalysts for producing hydrogen through water splitting. This catalytic property is much more efficient than that of the MoA2Z4 ML, attributed to the intrinsic electric field induced by the vertical asymmetry effectively separating electrons and holes. More importantly, the carrier mobility of the MoAZ3H ML is up to 103-104 cm2 V-1 s-1 due to the large elastic modulus or small effective mass. Additionally, the electronic properties of the MoAZ3H ML can be easily tuned by strain. Our results suggest a new strategy for designing novel 2D Janus materials, which not only expands the members in the 2D MA2Z4-based Janus family, but also provide candidates with excellent performances in photovoltaic and catalytic fields.

Strain and electric field induced electronic property modifications in two-dimensional Janus SZrAZ2 (A = Si, Ge; Z = P, As) monolayers

Recently, significant attention has been directed towards two-dimensional Janus materials owing to their unique structure and novel properties. In this work, we have introduced novel two-dimensional Janus monolayers, SZrAZ2 (A = Si, Ge; Z = P, As), through first principles. Our primary focus was the investigation of the controllable electronic properties exhibited by the Janus SZrAZ2 structures under the influence of strain and an external electric field. Our research findings indicate the dynamic and thermodynamic stability of Janus SZrAZ2 (A = Si, Ge; Z = P, As) monolayers. In the equilibrium state, these monolayers exhibit properties of an indirect band gap semiconductor. When subjected to biaxial strain and an external electric field, we observed that the dependency of SZrSiAs2 and SZrGeAs2 monolayers on an external electric field is very weak. Their electronic properties can only be modulated by applying biaxial strain. For SZrSiP2 and SZrGeP2 monolayers, their electronic properties can be modulated under biaxial strain and an external electric field, resulting in a transition from semiconducting to metallic behavior. Finally, we calculated the carrier mobility of these four structures and observed that the SZrGeAs2 monolayer exhibits a hole mobility of up to 597.52 cm2 s-1 V-1 in the x-direction, whereas the SZrSiP2 monolayer demonstrates an electron mobility of up to 479.30 cm2 s-1 V-1 in the y-direction. In the x-direction, the electron mobility of SZrSiAs2 and SZrGeP2 monolayers was measured to be 189.88 and 528.44 cm2 s-1 V-1, respectively. These values are greater than or equivalent to that of experimentally synthesized MoS2 (∼200 cm2 s-1 V-1). Our research lays the foundation for utilizing two-dimensional Janus materials in electronic devices.

Recyclable and flexible Bi(Ho3+-Yb3+)OBr/g-C3N4 composite porous fiber for efficient water purification and real-time temperature sensing

Herein, an electrospinning porous nanofiber with large specific surface area, excellent flexibility, remarkable tensile strength, and high stability of thermal degradation has been developed by loading Ho3+/Yb3+ co-doped BiOBr/g-C3N4 (BHY/CN) heterojunction photocatalysts on polyacrylonitrile (PAN) nanofibers. The optimized BHY/CN-2 nanofiber demonstrates outstanding photocatalytic activity for the degradation of 98.83% tetracycline (TC, 60 min) and 99.06% rhodamine B (RhB, 90 min) under simulated sunlight irradiation, and maintains a high level of reusability and recycling stability in three cycles. In addition, temperature monitoring of the catalytic degradation process can be feedback by (5F4, 5S2) → 5I8 and 5F5 → 5I8 radiation transitions of Ho3+ with excellent sensitivity. More importantly, the nanofiber luminescence performance is enhanced by the doping of g-C3N4, which maintain the effective upconversion luminescence properties even in water, providing a reliable reference for real-time monitoring and feedback of the operating temperature, and further expanding the application fields of photocatalysts.

Kagome-like BiP3 Monolayer: An Emerging Quasi-Direct Auxetic Semiconductor Coupled with High Anisotropic Mobility toward Visible-Light-Driven Photoelectrocatalytic pH-Robust Overall Water-Splitting

Two-dimensional (2D) Janus materials exhibit an outstanding potential that can meet the rigorous requirements of photocatalytic water splitting resulting from their unique atomic arrangement. However, these materials are quite scarce. Through ab initio density functional theory calculations, we introduce a kagome topology into the honeycomb lattice of blue phosphorene using phosphorus and bismuth atoms to build a hybrid honeycomb-like kagome lattice, realized by a hitherto unknown kagome-like Janus-like BiP3 monolayer with robust stability. Excitingly, the out-of-plane asymmetry benefiting from kagome and honeycomb topologies gives rise to a significantly negative out-of-plane Poisson's ratio and an obvious built-in electric field pointing from the sublayer of the P atom to the sublayer of the Bi atom. In conjunction with the investigations that encompass semiconducting properties, such as a quasi-direct gap, suitable band-edge positions, effective visible-light absorption, and high carrier mobility, the BiP3 monolayer achieves overall water splitting at pH 0-14 regardless of strain. Moreover, this intrinsic electric field provides a sufficient photogenerated carrier driving force for water splitting. The bare BiP3 comprises P and Bi atoms that function as catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) active sites, respectively. Upon exposure to light, the reaction of water into H2 and O2 can be observed across a pH range of 0-14. Meanwhile, by designing a transition-metal single-atom catalyst (TM@BiP3), our investigations have shown that embedding a single TM on BiP3 is a feasible route to improving the HER/OER activity by reducing the overpotentials to -0.039 and 0.58 eV for Mo and Os atoms, respectively. In this case, the positive value of the external potential acts as a sufficient OER driving force, i.e., in the light environment, the Os@BiP3 system can promote water molecules spontaneously oxidized into O2 at pH 0-14.

A First-Principles Study of Regulating Spin States of MoSi2N4 Supported Single-Atom Catalysts Via Doping Strategy for Enhancing Electrochemical Nitrogen Fixation Activity

Regulating the spin states of catalysts to enhance activity is fascinating but challenging. Herein, by using first-principles calculations, single transition-metal (TM) atoms Mo, Re, and Os embedded in nitrogen vacancy of the MoSi2N4 monolayer (TM1/VN-MoSi2N4) were screened out as potential catalysts for electrochemical nitrogen reduction reaction to ammonia. Our findings suggest that the spin states of these active centers can be precisely and gradually tuned through a simple doping strategy. Additionally, doping one O atom into the Mo1/VN-MoSi2N4 system as an example significantly improves catalytic activity. The spin state of Mo1 transitions from high to intermediate while simultaneously breaking the C3v symmetry of the supported atom. These factors synergistically lead to better orbital overlap between the catalyst and intermediates, facilitating subsequent protonation processes and overall catalytic activity. This work provides novel insight into designing, precisely controlling, and revisiting the spin-related catalytic performance in heterogeneous catalysis.

Tuning of the electronic and photocatalytic properties of Janus WSiGeZ4 (Z = N, P, and As) monolayers via strain engineering

Recently, MA₂Z₄ materials have received tremendous attention due to their amazing electronic, spintronic, and optoelectronic properties. In this work, we propose a class of 2D Janus materials WSiGeZ₄ (Z = N, P, and As). It was found that their electronic and photocatalytic properties are sensitive to the change of the Z element. Biaxial strain results in an indirect-direct band gap transition in WSiGeN₄ and a semiconductor-metal transition in WSiGeP₄ and WSiGeAs₄. Comprehensive studies demonstrate that these transitions as well as valley-contrasting physics are closely related to the crystal field induced orbital distribution. By taking into account several features of the excellent photocatalysts reported for water splitting, we predict three promising photocatalytic materials WSi₂N₄, WGe₂N₄, and WSiGeN₄. Their optical and photocatalytic properties can be well modulated by applying biaxial strain. Our work not only provides a class of potential electronic and optoelectronic materials but also enriches the study of Janus MA₂Z₄ materials.

Novel Janus MoSiGeN4 nanosheet: adsorption behaviour and sensing performance for NO and NO2 gas molecules

A novel Janus MoSiGeN4 nanosheet is proposed for detecting poisonous gas molecules. Herein, the adsorption behaviour and sensing performance of both sides of the MoSiGeN4 monolayer to NO and NO2 gas molecules were investigated by first-principles calculations. Firstly, it is found that the MoSiGeN4 monolayer exhibits structural stability and indirect gap semiconductor characteristics. The largest adsorption energy of NO2 molecules on the MoSiGeN4 monolayer is -0.24 eV, which is higher than the -0.13 eV for NO molecules. Of course, the physisorption between gas molecules and the MoSiGeN4 monolayer appears with slight charge transfer. It is confirmed that NO molecules and NO2 molecules act as electron donors and electron acceptors, respectively. Meanwhile, the generation of small band gaps and impurity levels in the electronic structures after gas adsorption is in favour of the enhancement of electronic conductivity. Furthermore, the longest recovery times of NO and NO2 molecules are predicted to be 0.15 and 10.67 ns at room temperature, and the lateral diffusion at the surface requires crossing a large energy barrier. These findings provide indisputable evidence for further design and fabrication of highly sensitive gas sensors based on the MoSiGeN4 monolayer.

The First-Principle Study on Tuning Optical Properties of MA2Z4 by Cr Replacement of Mo Atoms in MoSi2N4

Recently, with the successful preparation of MoSi2N4, an emerging family of two-dimensional (2D) layered materials has been predicted with a general formula of MA2Z4 (M: an early transition metal, A: Si or Ge and Z: N, P, or As). In terms of this new type of 2D material, how to effectively tune its light absorption properties is unclear. We systematically discuss the effects of replacing Mo with Cr atoms on the lattice structure, energy bands, and light absorption properties of 2D monolayer MoSi2N4 using density functional theory (DFT) and the Vienna Ab initio Simulation Package (VASP). Additionally, the results show that the single replacement of the atom Cr has no significant effect on the lattice structure of the outermost and sub-outer layers but plays a major role in the accumulation of electrons. In addition, the 2D MoSi2N4, Mo0.5Cr0.5Si2N4, and CrSi2N4 all have effective electron-hole separation properties. In the visible region, as the excited state increases, the required excitation energy is higher and the corresponding wavelength of light is shorter. It was found that the ultraviolet (UV)-visible spectra are red-shifted when Cr atoms replace Mo atoms in MoSi2N4; when Cr atoms and Mo atoms coexist, the coupling between Cr atoms and Mo atoms achieves modulation of the ultraviolet (UV)-visible spectra. Finally, we reveal that doping M-site atoms can effectively tune the light absorption properties of MA2Z4 materials. These results provide a strategy for the design of new 2D materials with high absorption properties.

2D Nb3SBr7 and Ta3SBr7: Experimentally Achievable Janus Photocatalysts with Robust Coexistence of Strong Optical Absorption, Intrinsic Charge Separation, and Ultrahigh Solar-to-Hydrogen Efficiency

Recently, two-dimensional (2D) Janus semiconductors have attracted great attention in photocatalytic applications owing to their extraordinary properties, especially the intrinsic polarization-induced spontaneous carrier separation, strong optical absorption, and ultrahigh solar-to-hydrogen (STH) efficiency. However, experimental achievable candidates for 2D intrinsic Janus semiconductors are rarely reported. Herein, based on density functional theory (DFT) calculations, we uncovered two new 2D photocatalysts, namely, Janus Nb₃SBr₇ and Ta₃SBr₇ bilayers. We revealed that both structures are highly feasible to be obtained from their bulk counterparts. Excitingly, intrinsic charge separations emerge in both structures, which are beneficial to the repression of recombinations of their photoexcited carriers. Optical absorptions of both structures can be activated in the visible and even infrared regions. Most interestingly, Nb₃SBr₇ and Ta₃SBr₇ bilayers can exhibit ultrahigh STH efficiencies of 35% and 31%, respectively, which are larger than those of most 2D Janus structures. In addition, we further found that these distinguished photocatalytic properties are rather robust and are independent of their stacking modes. Experimental feasibilities and robust coexistences of intrinsic charge separations, ultrahigh STH efficiencies, and strong absorptions endow Nb₃SBr₇ and Ta₃SBr₇ bilayers as hopeful photocatalysts for water splitting.

Direct Z-scheme WTe2/InSe van der Waals heterostructure for overall water splitting

WTe2/InSe is a direct Z-scheme vdW heterostructure for water splitting. The Te-vacancy can effectively lower the energy of the HER, and the overall water splitting can proceed spontaneously on the surface of the WTe2/InSe heterostructure when pH > 7.

Boost the large driving photovoltages for overall water splitting in direct Z-scheme heterojunctions by interfacial polarization

Using direct Z-scheme heterojunction photocatalysts has been regarded as one of the most effective methods to separate photogenerated carriers for photocatalytic redox reactions of water splitting.

First principles prediction of two-dimensional Janus XMoGeN2 (X = S, Se and Te) materials

Motivated by the successful synthesis of two-dimensional MoSi$_2$N$_4$ [Science 369 (2020) 670-674] and Janus MoSSe [Nature Nanotechnol. 12 (2017) 744-749], in this work, we propose a novel 2D Janus XMoGeN$_2$...

First principles prediction of electronic, mechanical, transport and optical properties of the silicane/Ga2SSe heterostructure

In this work, we investigated the electronic structure, and mechanical, transport and optical properties of the van der Waals heterostructure formed from silicane (SiH) and Janus Ga₂SSe monolayers using first-principles prediction. The out-of-plane symmetry in the Janus Ga₂SSe monolayer leads to the formation of two different types of Ga₂SSe/SiH heterostructure, namely SGa₂Se/SiH and SeGa₂S/SiH stacking patterns. All stacking patterns of the SiH/Ga₂SSe heterostructure are thermodynamically, mechanically and energetically stable at room temperature. Furthermore, the generation of the SiH/Ga₂SSe heterostructure gives rise to a reduction in the band gap, demonstrating that the electrons move faster from the valence bands to the conduction bands. The SiH/Ga₂SSe heterostructure is a semiconductor with a direct band gap of about 0.68 or 0.95 eV, depending on the stacking pattern. The SiH/Ga₂SSe heterostructure forms type-II band alignment for all stacking patterns, indicating that the photogenerated carriers are separated effectively, thus enhancing the photocatalytic performance. Moreover, the carrier mobilities for electrons and holes of the Ga₂SSe/SiH heterostructure are higher than those of the constituent SiH and Ga₂SSe monolayers in both the x and y directions, suggesting that the performances of electronic devices based on the Ga₂SSe/SiH heterostructure would be excellent and reliable. The formation of the Ga₂SSe/SiH heterostructure also gives rise to an enhancement of the absorption coefficient in both the visible and ultraviolet regions. Our findings could give valuable guidance for the design of high-efficiency devices based on the SiH/Ga₂SSe heterostructure.

In this work, we investigated the electronic structure, and mechanical, transport and optical properties of the van der Waals heterostructure formed from silicane (SiH) and Janus Ga₂SSe monolayers using first-principles prediction.

Stacking Engineering: A Boosting Strategy for 2D Photocatalysts

Two-dimensional (2D) photocatalytic material is a vital project for modern solar energy conversion and storage. Despite a vast family of potential 2D photocatalysts that is demonstrated, their commercial applications are severely limited because of fast photogenerated electron-hole recombination. Here, based on first-principles, we propose a general paradigm to boost the separation of photoexcited charge carriers in 2D photocatalysts by stacking engineering. Taking the emerging water splitting photocatalyst MoSi₂N₄ as an example, we show that specific interlayer stacking-induced electric polarization plays a significant role in altering the electronic properties and thus the suppressed recombination rate of photoexcited carriers. Moreover, we find that the catalytic performance can be further controlled by vertical strain. These generalized findings not only highlight the importance of stacking-induced electric polarization but also offer new prospects for the design and application of 2D photocatalysts.