Functionalized MBenes as promising anode materials for high-performance alkali-ion batteries: a first-principles study.

The discovery of novel electrode materials based on two-dimensional (2D) structures is critical for alkali metal-ion batteries. Herein, we performed first-principles computations to investigate functionalized MXenes, Mo2BT2(T = O, S), which are also regarded as B-based MXenes, or named as MBenes, as potential anode materials for Li-ion batteries and beyond. The pristine and T-terminated Mo2BT2(T = O, S) monolayers reveal metallic character with higher electronic conductivity and are thermodynamically stable with an intrinsic dipole moment. Both Mo2BO2and Mo2BS2monolayers exhibit high theoretical Li/Na/K storage capacity and low ion diffusion barriers. These findings suggest that functionalized Mo2BT2(T = O, S) monolayers are promising for designing viable anode materials for high-performance alkali-ion batteries.

Electronic properties and enhanced photocatalytic performance of van der Waals heterostructures of ZnO and Janus transition metal dichalcogenides.

Vertical stacking of two-dimensional materials into layered van der Waals heterostructures has recently been considered as a promising candidate for photocatalytic and optoelectronic devices because it can combine the advantages of the individual 2D materials. Janus transition metal dichalcogenides (JTMDCs) have emerged as an appealing photocatalytic material due to the desirable electronic properties. Hence, in this work, we systematically investigate the geometric features, electronic properties, charge density difference, work function, band alignment and photocatalytic properties of ZnO-JTMDC heterostructures using first-principles calculations. Due to the different kinds of chalcogen atoms on both sides of JTMDC monolayers, two different possible stacking patterns of ZnO-JTMDC heterostructures have been constructed and considered. We find that all these stacking patterns of ZnO-JTMDC heterostructures are dynamically and energetically feasible. Moreover, both ZnO-MoSSe and ZnO-WSSe heterostructures are indirect band gap semiconductors and present type-I and type-II band alignments for model-I and model-II, respectively. The Rashba spin polarization of the ZnO-WSSe heterostructure for model-I is greater than that in the others. Furthermore, valence (conduction) band edge potentials are calculated to understand the photocatalytic behavior of these systems. Energetically favorable band edge positions in ZnO-Janus heterostructures make them suitable for water splitting at zero pH. We found that the ZnO-Janus heterostructures are promising candidates for water splitting with conduction and valence band edges positioned just outside of the redox interval.

Optoelectronic and solar cell applications of Janus monolayers and their van der Waals heterostructures.

Janus monolayers and their van der Waals heterostuctures are investigated by hybrid density functional theory calculations. MoSSe, WSSe, MoSeTe and WSeTe are found to be direct band gap semiconductors. External electric fields are used to transform indirect MoSTe and WSTe to direct band gap semiconductors. MoSSe-WSSe, MoSeTe-WSeTe and MoSTe-WSTe vdW heterostructures are also indirect band gap semiconductors with type-II band alignment. Similar to the corresponding monolayers, in some of the above mentioned vdW heterostructures an external electric field and tensile strain can transform indirect to direct band gaps, while sustaining type-II band alignment. Janus monolayers have lower values of the work function (φ) than their vdW heterostructure counterparts. Furthermore, absorption spectra, absorption efficiency, and valence(conduction) band edge potentials are calculated to understand the optical and photocatalytic behavior of these systems. Red and blue shifts are observed in the position of excitonic peaks due to the induced strain in Janus monolayers. Strong device absorption efficiencies (80-90%) are observed for the WSeTe, MoSTe and WSTe monolayers in the visible, infra-red and ultraviolet regions. Energetically favourable band edge positions in Janus monolayers make them suitable for water splitting at zero pH. We find that the MoSSe-WSSe heterostructure and the MoSTe monolayer are promising candidates for water splitting with conduction and valence band edges positioned just outside of the redox interval.

Electronic structure, optical and photocatalytic performance of SiC-MX2 (M = Mo, W and X = S, Se) van der Waals heterostructures.

The stacking of monolayers in the form of van der Waals heterostructures is a useful strategy for band gap engineering and the control of dynamics of excitons for potential nano-electronic devices. We performed first-principles calculations to investigate the structural, electronic, optical and photocatalytic properties of the SiC-MX2 (M = Mo, W and X = S, Se) van der Waals heterostructures. The stability of most favorable stacking is confirmed by calculating the binding energy and phonon spectrum. SiC-MoS2 is found to be a direct band gap type-II semiconducting heterostructure. Moderate in-plane tensile strain is used to achieve a direct band gap with type-II alignment in the SiC-WS2, SiC-MoSe2 and SiC-WSe2 heterostructures. A difference in the ionization potential of the corresponding monolayers and interlayer charge transfer further confirmed the type-II band alignment in these heterostructures. Furthermore, the optical behaviour is investigated by calculation of the absorption spectra in terms of ε2(ω) of the heterostructures and the corresponding monolayers. The photocatalytic response shows that the SiC-Mo(W)S2 heterostructures can oxidize H2O to O2. An enhanced photocatalytic performance with respect to the parent monolayers makes the SiC-Mo(W)Se2 heterostructures promising candidates for water splitting.

Alkali to alkaline earth metals: a DFT study of monolayer TiSi2N4 for metal ion batteries.

A significant problem in the area of rechargeable alkali ion battery technologies is the exploration of anode materials with overall high specific capacities and superior physical properties. By using first-principles calculations, we have determined that monolayer TiSi2N4 is precisely such a potential anode candidate. Its demonstrated dynamic, thermal, mechanical, and energetic stabilities make it feasible for experimental realization. An important benefit of the electrode conductivity is that the electronic structure reveals that the pristine system experiences a change from a semiconductor to a metal throughout the entire alkali adsorption process. What's more interesting is that monolayer TiSi2N4 can support up to double-sided 3-layer ad-atoms, resulting in extremely high theoretical capacities for Li, Na, Mg, and K of 1004, 854, 492 and 531 mA h g-1 and low average open-circuit voltages of 0.55, 0.25, 0.55, and -1.3 V, respectively. Alkali diffusion on the surface has been demonstrated to occur extremely quickly, with migration energy barriers for Li, Na, Mg, and K as low as 0.25, 0.14, 0.10, and 0.07 eV, respectively. The results reveal that the migration barrier energy is the lowest for Li and Mg from path-2 and Na and K from path-1. Overall, these findings suggest that monolayer TiSi2N4 is a suitable anode candidate for use in high-performance and low-cost metal-ion batteries.

First-principles study of BX-SiS (X = As, P) van der Waals heterostructures for enhanced photocatalytic performance.

The vertical integration of two-dimensional (2D) materials through weak van der Waals (vdW) interactions is gaining tremendous attention for application in nanotechnology and photovoltaics. Here, we performed first-principles study of the electronic band structure, optical and photocatalytic properties of vertically stacked heterostructures based on boron pnictides BX (X = As, P) and SiS monolayers. Both heterobilayers possess a stable geometry and reveal type I band alignment with a direct band gap, indicating substantial transfer of charge across the junction of the same layer. Interestingly, a redshift is found in the visible light region of the optical absorption spectra of BX-SiS heterobilayers. The comparatively larger hole mobility (14 000 cm2 V-1 s-1) of BP-SiS preferably allows hole conduction in the zigzag-direction. More importantly, the excellent band edge values of the standard redox potential and smaller Gibbs free energy for the adsorption of hydrogen (ΔGH*) make them ideal for performing the hydrogen evolution reaction (HER) mechanism under solar irradiation. These findings offer exciting opportunities for developing next-generation devices based on BX-SiS heterobilayers for promising applications in nanoelectronics, optoelectronic devices and photocatalysts for water dissociation into hydrogen to produce renewable clean energy.

Correction: First principles study of electronic and optical properties and photocatalytic performance of GaN-SiS van der Waals heterostructure.

[This corrects the article DOI: 10.1039/D1RA06011B.].

Correction: Theoretical prediction of the electronic structure, optical properties and photocatalytic performance of type-I SiS/GeC and type-II SiS/ZnO heterostructures.

[This corrects the article DOI: 10.1039/D3RA01061A.].

Theoretical prediction of the electronic structure, optical properties and photocatalytic performance of type-I SiS/GeC and type-II SiS/ZnO heterostructures.

Nowadays, it would be ideal to develop high-performance photovoltaic devices as well as highly efficient photocatalysts for the production of hydrogen via photocatalytic water splitting, which is a feasible and sustainable energy source for addressing the challenges related to environmental pollution and a shortage of energy. In this work, we employ first-principles calculations to investigate the electronic structure, optical properties and photocatalytic performance of novel SiS/GeC and SiS/ZnO heterostructures. Our results indicate that both the SiS/GeC and SiS/ZnO heterostructures are structurally and thermodynamically stable at room temperature, suggesting that they are promising materials for experimental implementation. The formation of SiS/GeC and SiS/ZnO heterostructures gives rise to reduction of the band gaps as compared to the constituent monolayers, enhancing the optical absorption. Furthermore, the SiS/GeC heterostructure possesses a type-I straddling gap with a direct band gap, while the SiS/ZnO heterostructure forms a type-II band alignment with indirect band gap. Moreover, a red-shift (blue-shift) has been observed in SiS/GeC (SiS/ZnO) heterostructures as compared with the constituent monolayers, enhancing the efficient separation of photogenerated electron-hole pairs, thereby making them promising candidates for optoelectronic applications and solar energy conversion. More interestingly, significant charge transfers at the interfaces of SiS-ZnO heterostructures, have improved the adsorption of H, and the Gibbs free energy ΔH* becomes close to zero, which is optimal for the hydrogen evolution reaction to produce hydrogen. The findings pave the path for the practical realization of these heterostructures for potential applications in photovoltaics and photocatalysis of water splitting.

Intriguing interfacial characteristics of the CS contact with MX2 (M = Mo, W; X = S, Se, Te) and MXY ((X ≠ Y) = S, Se, Te) monolayers.

Using (hybrid) first principles calculations, the electronic band structure, type of Schottky contact and Schottky barrier height established at the interface of the most stable stacking patterns of the CS-MX2 (M = Mo, W; X = S, Se, Te) and CS-MXY ((X ≠ Y) = S, Se, Te) MS vdWH are investigated. The electronic band structures of CS-MX2 and CS-MXY MS vdWH seem to be simple sum of CS, MX2 and MXY monolayers. The projected electronic properties of the CS, MX2 and MXY layers are well preserved in CS-MX2 and CS-MXY MS vdWH. Their smaller effective mass (higher carrier mobility) render promising prospects of CS-WS2 and CS-MoSeTe as compared to other MS vdWH in nanoelectronic and optoelectronic devices, such as a high efficiency solar cell. In addition, we found that the effective mass of holes is higher than that of electrons, suggesting that these heterostructures can be utilized for hole/electron separation. Interestingly, the MS contact led to the formation of a Schottky contact or ohmic contact, therefore we have used the Schottky Mott rule to calculate the Schottky barrier height (SBH) of CS-MX2 (M = Mo, W; X = S, Se, Te) and CS-MXY ((X ≠ Y) = S, Se, Te) MS vdWH. It was found that CS-MX2 (M = Mo, W; X = S, Se, Te) and CS-MXY ((X ≠ Y) = S, Se, Te) (in both model-I and -II) MS vdWH form p-type Schottky contacts. These p-type Schottky contacts can be considered a promising building block for high-performance photoresponsive optoelectronic devices, p-type electronics, CS-based contacts, and for high-performance electronic devices.

First principles study of optoelectronic and photocatalytic performance of novel transition metal dipnictide XP2 (X = Ti, Zr, Hf) monolayers.

Low cost and highly efficient two dimensional materials as photocatalysts are gaining much attention to utilize solar energy for water splitting and produce hydrogen fuel as an alternative to deal with the energy crisis and reduce environmental hazards. First principles calculations are performed to investigate the electronic, optical and photocatalytic properties of novel two dimensional transition metal dipnictide XP2 (X = Ti, Zr, Hf) monolayers. The studied single layer XP2 is found to be dynamically and thermally stable. TiP2, ZrP2 and HfP2 systems exhibit semiconducting nature with moderate indirect band gap values of 1.72 eV, 1.43 eV and 2.02 eV, respectively. The solar light absorption is found to be in energy range of 1.65-3.3 eV. All three XP2 systems (at pH = 7) and the HfP2 monolayer (at pH = 0) that straddle the redox potentials, are promising candidates for the water splitting reaction. These findings enrich the two dimensional family and provide a platform to design novel devices for emerging optoelectronic and photovoltaic applications.

A first principles study of a van der Waals heterostructure based on MS2 (M = Mo, W) and Janus CrSSe monolayers.

The strategy of stacking two-dimensional materials for designing van der Waals heterostructures has gained tremendous attention in realizing innovative device applications in optoelectronics and renewable energy sources. Here, we performed the first principles calculations of the geometry, optoelectronic and photocatalytic performance of MS2-CrSSe (M = Mo, W) vdW heterostructures. The mirror asymmetry in the Janus CrSSe system allows the designing of two models of the MS2-CrSSe system by replacing S/Se atoms at opposite surfaces in CrSSe. The feasible configurations of both models of the MS2-CrSSe system are found energetically, dynamically and thermally stable. The studied heterobilayers possess an indirect type-I band alignment, indicating that the recombination of photogenerated electrons and holes in the CrSSe monolayer is hence crucial for photodetectors and laser applications. Remarkably, a red-shift in the optical absorption spectra of MS2-CrSSe makes them potential candidates for light harvesting applications. More interestingly, all heterobilayers (except W(Mo)S2-CrSSe of model-I(II)) reveal appropriate band edge positions of the oxidation and reduction potentials of the photocatalysis of water dissociation into H+/H2 and O2/H2O at pH = 0. These results shed light on the practical design of the MS2-CrSSe system for efficient optoelectronic and photocatalytic water splitting applications.

Intriguing electronic, optical and photocatalytic performance of BSe, M2CO2 monolayers and BSe-M2CO2 (M = Ti, Zr, Hf) van der Waals heterostructures.

Using density functional (DFT) theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M2CO2 (M = Ti, Zr, Hf) monolayers and their corresponding BSe-M2CO2 (M = Ti, Zr, Hf) van der Waals (vdW) heterostructures. Optimized lattice constant, bond length, band structure and bandgap values, effective mass of electrons and holes, work function and conduction and valence band edge potentials of BSe and M2CO2 (M = Ti, Zr, Hf) monolayers are in agreement with previously available data. Binding energies, interlayer distance and Ab initio molecular dynamic simulations (AIMD) calculations show that BSe-M2CO2 (M = Ti, Zr, Hf) vdW heterostructures are stable with specific stacking and demonstrate that these heterostructures might be synthesized in the laboratory. The electronic band structure shows that all the studied vdW heterostructures have indirect bandgap nature - with the CBM and VBM at the Γ-K and Γ-point of BZ for BSe-Ti2CO2, respectively; while for BSe-Zr2CO2 and BSe-Hf2CO2 vdW heterostructures the CBM and VBM lie at the K-point and Γ-point of BZ, respectively. Type-II band alignment in BSe-M2CO2 (M = Ti, Zr, Hf) vdW heterostructures prevent the recombination of electron-hole pairs, and hence are crucial for light harvesting and detection. Absorption spectra are investigated to understand the optical behavior of BSe-M2CO2 (M = Ti, Zr, Hf) vdW heterostructures, where the lowest energy transitions are dominated by excitons. Furthermore, BSe-M2CO2 (M = Ti, Zr, Hf) vdW heterostructures are found to be potential photocatalysts for water splitting at pH = 0, and exhibit enhanced optical properties in the visible light zones.

First principles study of electronic and optical properties and photocatalytic performance of GaN-SiS van der Waals heterostructure.

The vertical stacking of two-dimensional materials via van der Waals (vdW) interaction is a promising technique for tailoring the physical properties and fabricating potential devices to be applied in the emerging fields of materials science and nanotechnology. The structural, electronic and optical properties and photocatalytic performance of a GaN-SiS vdW heterostructure were explored using first principles calculations. The most stable stacking configuration found energetically stable, possesses a direct staggered band gap, which is crucial for separating photogenerated charged carriers in different constituents and is efficacious for solar cells. Further, the charge transfer occurred from the SiS to GaN layer, indicating that SiS exhibits p-type doping in the GaN-SiS heterobilayer. Interestingly, a systematic red-shift was observed in the optical absorption spectra of the understudy heterobilayer system. Moreover, the conduction band edge and valence band edge of the monolayers and corresponding heterostructure were located above and below the standard redox potentials for photocatalytic water splitting, making these systems promising for water dissociation for hydrogen fuel production. The results provide a route to design the GaN-SiS vdW heterostructure for the practical realization of next-generation light detection and energy harvesting devices.

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.