Insights into the photocatalytic mechanism of the C4N/MoS2 heterostructure: A first-principle study

Constructing heterostructures by combining COFs and TMD is a new strategy to design efficient photocatalysts for CO₂ reduction reaction (CO₂RR) due to their good stability, tunable band gaps and efficient charge separation. Based on the synthesis of completely novel C₄N−COF in our previous reported work, a new C₄N/MoS₂ heterostructure was constructed and then the related structural, electronic and optical properties were also studied using first principle calculations. The interlayer coupling effect and charge transfer between the C₄N and MoS₂ layer are systematically illuminated. The reduced band gap of the C₄N/MoS₂ heterostructure is beneficial to absorb more visible light. For the formation of type-II band alignment, a built-in electric field appears which separates the photogenerated electrons and holes into different layers efficiently and produces redox active sites. The band alignment of the heterostructure ensures its photocatalytic activities of the whole CO₂ reduction reaction. Furthermore, the charge density difference and charge carrier mobility confirm the existence of the built-in electric field at the interface of the C₄N/MoS₂ heterostructure directly. Finally, the high optical absorption indicates it is an efficient visible light harvesting photocatalyst. Therefore, this work could provide strong insights into the internal mechanism and high photocatalytic activity of the C₄N/MoS₂ heterostructure and offer guiding of designing and synthesizing COF/TMD heterostructure photocatalysts.     

g-C3N4/SnS2 Heterostructure: a Promising Water Splitting Photocatalyst

Graphite-like carbon nitride (g-C₃N₄) based heterostrutures has attracted intensive attention due to their prominent photocatalytic performance. Here, we explore the g-C₃N₄/SnS₂ coupling effect on the electronic structures and optical absorption of the proposed g-C₃N₄/SnS₂ heterostructure through performing extensive hybrid functional calculations. The obtained geometric structure, band structures, band edge positions and optical absorptions clearly reveal that the g-C₃N₄ monolayer weakly couples to SnS₂ sheet, and forms a typical van der Waals heterojunction. The g-C₃N₄/SnS₂ heterostructure can effectively harvest visible light, and its valence band maximum and conduction band minimum locate in energetically favorable positions for both water oxidation and reduction reactions. Remarkably, the charge transfer from the g-C₃N₄ monolayer to SnS₂ sheet leads to the built-in interface polarized electric field, which is desirable for the photogenerated carrier separation. The built-in interface polarized electric field as well as the nice band edge alignment implys that the g-C₃N₄/SnS₂ heterostructure is a promising g-C₃N₄ based water splitting photocatalyst with good performance.     

Theoretical Design of an MoxW1-xS2/Graphene (x=0.25/0.75) Heterojunction with Adjustable Band Gap: Potential Candidate Materials for the Next Generation of Optoelectronic Devices

Multicomponent two-dimensional (2D) transition metal dichalcogenides (TMDCs) semiconductors based on adjustable band gap are increasingly used to design optoelectronic devices with specific spectral response. Here, we have designed the Mo_xW_1-xS₂/graphene heterostructure with adjustable band gap by adopting the combination idea of alloying and multiple heterogeneous recombination. The contact type, stability and photoelectric properties of Mo_xW_1-xS₂/graphene heterojunction were investigated theoretically. At the same time, by applying external vertical electric field to Mo_xW_1-xS₂/graphene, the regulate of heterojunction Schottky contact type was realized. The results show that Mo_xW_1-xS₂/graphene heterojunction has broad application prospects in the field of photocatalysis and Schottky devices, and is suitable for being a potential candidate material for next generation of optoelectronic devices. The design of Mo_xW_1-xS₂/graphene heterostructure enables it to obtain the advanced characteristics that are lacking in the one-component intrinsic 2D TMDCs semiconductors or graphene materials, and provides a theoretical basis for the experimental preparation of such heterojunctions.     

Electronic and optical properties of α‐MoO3/TiO2 heterostructures: A DFT study

The coupling of metal oxide semiconductors has become an effective method to improve the separation of photon‐generated carriers and light absorption efficiency. In this study, we explored electronic and optical properties of monolayer and bilayer α‐MoO₃ on TiO₂ (001) surface. It is observed that α‐MoO₃/TiO₂ heterostructures can form a stable Mo‐O‐Ti bonding mode at the interface. Electrons transfer from TiO₂ (001) surface to the α‐MoO₃, leading to the enhancement of the valence band and the optical absorption spectrum in visible light region. In addition, this proper charge transfer generates a built‐in electric field between the interface regions of bilayer α‐MoO₃/TiO₂ heterostructure and forms a favorable type‐II band alignment between the two α‐MoO₃ layers. The α‐MoO₃/TiO₂ heterostructure can prevent the recombination of the electron‐hole pairs; thus, excite electrons can easily move from TiO₂ to the inner layer, and then to the outer layer of α‐MoO₃. These results demonstrate that the bilayer α‐MoO₃/TiO₂ heterostructure, especially the outer layer α‐MoO₃, has efficient photoelectric performance.     

Polyether Adducts of d‐Block Metal Compounds as Starting Materials for New Cluster Compounds

New crystalline cobalt, nickel, zinc, and mercury halide adducts with polyethers as ligands have been isolated, characterized, and identified as [Co(μ‐Cl)₂CoCl₂(glyme)₂] (glyme = 1,2‐dimethoxyethane), cis‐[CoI₂(H₂O)₂(glyme)₂]²⁺[CoI₄]^2–, [NiI₂(glyme)₂], [ZnI₂(glyme)], [HgCl₂(glyme)], [CoI₂(diglyme)] (diglyme = bis(2‐methoxyethyl)ether), [ZnI₂(diglyme)], [HgI₂(diglyme)], [CoCl(μ‐Cl)(diglyme)]₂, [NiI(μ‐I)(diglyme)]₂, [Co(μ‐Cl)(triglyme)]₂²⁺[CoCl₂(μ‐Cl)]₂^2–, and cis‐[(NiI₂)(triglyme)]_n. These compounds were obtained from the metal halide salts in solution of glyme or diglyme, and some of them show unusual coordination numbers and arrangements.     

Two Dimensional Materials Beyond MoS2: Noble‐Transition‐Metal Dichalcogenides

The structure and electronic structure of layered noble‐transition‐metal dichalcogenides MX₂ (M=Pt and Pd, and chalcogenides X=S, Se, and Te) have been investigated by periodic density functional theory (DFT) calculations. The MS₂ monolayers are indirect band‐gap semiconductors whereas the MSe₂ and MTe₂ analogues show significantly smaller band gap and can even become semimetallic or metallic materials. Under mechanical strain these MX₂ materials become quasi‐direct band‐gap semiconductors. The mechanical‐deformation and electron‐transport properties of these materials indicate their potential application in flexible nanoelectronics.     

Band‐Edge Electronic Structure of β‐In2S3: The Role of s or p Orbitals of Atoms at Different Lattice Positions

As a promising solar‐energy material, the electronic structure and optical properties of Beta phase indium sulfide (β‐In₂S₃) are still not thoroughly understood. This paper devotes to solve these issues using density functional theory calculations. β‐In₂S₃ is found to be an indirect band gap semiconductor. The roles of its atoms at different lattice positions are not exactly identical because of the unique crystal structure. Additonally, a significant phenomenon of optical anisotropy was observed near the absorption edge. Owing to the low coordination numbers of the In3 and S2 atoms, the corresponding In3‐5s states and S2‐3p states are crucial for the composition of the band‐edge electronic structure, leading to special optical properties and excellent optoelectronic performances.     

Two Step Chemical Vapor Deposition of In2Se3/MoSe2 van der Waals Heterostructures

Two-dimensional transition metal dichalcogenides heterostructures have stimulated wide interest not only for the fundamental research,but also for the application of next generation electronic and optoelectronic devices.Herein,we report a successful two-step chemical vapor deposition strategy to construct vertically stacked van der Waals epitaxial In₂Se₃/MoSe₂ heterostructures.Transmission electron microscopy characterization reveals clearly that the In₂Se₃ has well-aligned lattice orientation with the substrate of monolayer MoSe₂.Due to the interaction between the In₂Se₃ and MoSe₂ layers,the heterostructure shows the quenching and red-shift of photoluminescence.Moreover,the current rectification behavior and photovoltaic effect can be observed from the heterostructure,which is attributed to the unique band structure alignment of the heterostructure,and is further confirmed by Kevin probe force microscopy measurement.The synthesis approach via van der Waals epitaxy in this work can expand the way to fabricate a variety of two-dimensional heterostructures for potential applications in electronic and optoelectronic devices.     

Origin of the insulating state in NaCuO2

We study the electronic structure of NaCuO₂ by analysing experimental core level photoemission and X-ray absorption spectra using a cluster as well as an Anderson impurity Hamiltonianincluding the band structure of the oxygen sublattice. We show that the X-ray absorption results unambiguously establish a negative value of the charge transfer energy, Δ. Further, mean-field calculations for the edge-shared one-dimensional CuO₂ lattice of NaCuO₂ within the multiband Hubbard Hamiltonian show that the origin of the insulating nature lies in the band structure rather than in the correlation effects. LMTO-ASA band structure calculations suggest that NaCuO₂ is an insulator with a gap of around 1 eV.     

Zur thermischen Zersetzung und Sublimation des NiI2

Thermical Decomposition and Sublimation of NiI₂ In a membran manometer the thermical decomposition and the sublimation of NiI₂ was measured and in ampuls the sublimation of NiI₂ studied. From the total pressure and the sublimation pressure the enthalpy of formation ΔH°(f,NiI₂,f,298) = ?20 ± 2 kcal/mole and ΔH°(f,NiI₂,g,298) = +31.2 ± 5 kcal/mole was derived. The entropy dates are: S°(NiI₂,f,298) = 35 ± 2 cl, S°(NiI₂,g,298) = 80 ± 1 cl and S°(Ni₂I₄,g,298) = 128 ± 3 cl respectively. The Ni formed with NiI₂ an eutectical system.     

Über die Systeme AJ/CoJ2 und die kristallchemischen Beziehungen in der Gruppe der Doppelhalogenide AnCoX(n+2) mit X = Cl,Br, J [1]

The Systems AI/CoI₂ (A = Alkali Metal, Tl, Ag) and the Crystal Chemistry of the Double Halides A_nCoX_(n+2) with X = Cl, Br, I The systems AI/CoI₂ (A = Cs, Rb, K, Tl, Na, Ag) were investigated by differencethermal analysis. The systems of NaI and AgI are found to be eutectical. A compound A₂CoI₄ always exists in the other systems. Cs₂CoI₄ crystallizes in the β-K₂SO₄ type with a coordination number (C.N.) for Cs equal to 9/10. Results obtained with single crystal technique reveal for the first time that among the double halides Rb₂CoI₄ is of the monoclinic Sr₂GeS₄ type (C. N. for Rb = 6(+2)). The compounds K₂CoJ₄, Tl₂CoJ₄, T-K₂CoBr₄, and T-Tl₂CoBr₄ are isotypic. Both structure groups are characterized by isolated CoX₄^2? tetrahedra. Reflectance spectra and magnetic susceptibilities can be explained on the basis of crystal field theory. – Our results close presently existing gaps in the knowledge on systems of CoBr₂ and CoCl₂ too.     

Electronic structure and quadrupole interactions in triple molybdates Li<Subscript>2</Subscript>M<Subscript>3</Subscript>Al(MoO<Subscript>4</Subscript>)<Subscript>4</Subscript>, M = Cs,Rb

Within the density functional theory the electronic structure of triple molybdates Li₂M₃Al(MoO₄)₄, where M = Cs, Rb, is studied for the first time. It is found that all molybdates studied belong to wide band insulators with a band gap of ~4 eV. Quadrupole frequencies and asymmetry parameters of the electric field gradient near magnetic ⁷Li, ²⁷Al, ⁸⁷Rb, and ¹³³Cs nuclei are calculated and experimental NMR spectra are interpreted.     

An indirect-to-direct band gap transition of NaSbS2 via minor Ga doping: A theoretical study

The efficiency of NaSbS₂ is limited by its wide indirect band gap. Alloying is a very effective strategy to tune the band gap over a wide range for the mixed-anion NaSb(S,Se)₂ alloys. However, these compounds are still indirect band gap semiconductors. The influence of Ga doping on the structure, electronic, and optical properties of NaSbS₂ is studied for the first time. Our calculated results show that NaSbS₂ is an indirect band gap semiconductor, and the difference between the indirect and direct band gaps is less than 0.1 eV. Moreover, the forbidden transition is discovered for the fundamental direct bandgap of NaSbS₂. The results indicate that the NaSb_1-xGa_xS₂ alloys are predicted to be synthesized in the proper conditions. An indirect-to-direct band gap transition is observed from NaSbS₂ to NaSb_1-xGa_xS₂. The minor Ga doping (less than10 %) has little effect on the electronic and optical properties of NaSbS₂. Importantly, the weak transition of the fundamental direct bandgap is allowed for NaSb_1-xGa_xS₂. This study can provide a route to explore the high efficiency of novel based-NaSbS₂ materials.     

The Analysis of Os≡C Bond and Electric Field Influence on the Properties in the Osmium Carbyne Complex OsCl3 (≡CCH2CMe3 )(PH3 )2: A Theoretical Insight

In this paper, we study the effect of electric field on the dipole moment, electronic structure, and frontier orbital energy in the osmium carbyne complex OsCl₃ (≡CCH₂CMe₃ )(PH₃ )₂ using MPW1PW91 quantum chemical computations. We demonstrate the nature of the chemical bond between the [OsCl₃ (PH₃ )₂]⁻ and [CCH₂CMe₃ ]⁺ fragments through energy and charge decomposition analyses. We also estimate the percentage composition in terms of the specified groups of frontier orbitals for this complex to investigate the feature in the metal–ligand bonds. Quantum theory of atoms in molecules (QTAIM ) is applied to elucidate the Os≡C bond in the complex. Also, the influence of external electric field on the energy, frontier orbital energies, and HOMO–LUMO gap values is analyzed.     

Engineering Graphdiyne for Solar Photocatalysis

Graphdiyne (GDY) with a direct band gap, excellent carrier mobility and uniform pores, is regarded as a promising photocatalytic material for solar energy conversion, while the research on GDY in photocatalysis is a less developed field. Herein, the distinctive structure, adjustable band gap, and electronic properties of GDY for photocatalysis is firstly summarized. The construction and progress of GDY-based photocatalysts for solar energy conversion, including H₂ evolution reaction (HER), CO₂ reduction reaction (CO₂RR) and N₂ reduction reaction (NRR) are then elaborated. At last, the challenges and perspectives in developing GDY-based photocatalysts for solar fuel production are discussed. It is anticipated that a timely Minireview will be helpful for rapid progress of GDY in solar energy conversion.     

First-principles investigation of structural,electronic, and optical properties of transition metal-doped C40 CrSi2

Although CrSi₂ silicide is an attractive advanced functional material, the improvement of electronic and optical properties is still a challenge for its applications. Here, we apply the first-principles calculations to investigate the influence of transition metals (TMs) on the electronic and optical properties of C40 CrSi₂ silicide. Five possible TMs, Ti, V, Pd, Ag, and Pt, are considered in detail. The calculated results show that the additive metals Ti, V, Pd, and Pt are thermodynamically stable in C40 CrSi₂ because the calculated impurity formation energy of TM-doped C40 CrSi₂ is lower than zero. In particular, the V dopant is more thermodynamically stable than that of the other TMs. The calculated electronic structure shows that the band gap of C40 CrSi₂ is 0.391 eV, which is in good agreement with the other results. In particular, the additive TMs improve the electronic properties of C40 CrSi₂ due to the role of the d-state of TMs. Naturally, the additive TMs result in band migration (Cr-3d state and Si-3p state) from the valence band to the conduction band. Interestingly, the additive TMs lead to a red shift for optical adsorption of C40 CrSi₂ silicide.     

Two-Dimensional GaTe/Bi2Se3 Heterostructure: a Promising Direct Z-scheme Water Splitting Photocatalyst

Among various photocatalytic materials, Z-scheme photocatalysts have drawn tremendous research interest due to high photocatalytic performance in solar water splitting. Here, we perform extensive hybrid density functional theory calculations to explore electronic structures, interfacial charge transfer, electrostatic potential profile, optical absorption properties, and photocatalytic properties of a proposed two-dimensional (2D) small-lattice-mismatched GaTe/Bi\begin{document}$ _2 $\end{document}Se\begin{document}$ _3 $\end{document} heterostructure. Theoretical results clearly reveal that the examined heterostructure with a small direct band gap can effectively harvest the broad spectrum of the incoming sunlight. Due to the relative strong interfacial built-in electric field in the heterostructure and the small band gap between the valence band maximum of GaTe monolayer and the conduction band minimum of Bi\begin{document}$ _2 $\end{document}Se\begin{document}$ _3 $\end{document} nanosheet with slight band edge bending, these photogenerated carriers transfer via Z-scheme pathway, which results in the photogenerated electrons and holes effectively separating into the GaTe monolayer and the Bi\begin{document}$ _2 $\end{document}Se\begin{document}$ _3 $\end{document} nanosheet for the hydrogen and oxygen evolution reactions, respectively. Our results imply that the artificial 2D GaTe/Bi\begin{document}$ _2 $\end{document}Se\begin{document}$ _3 $\end{document} is a promising Z-scheme photocatalyst for overall solar water splitting.     

Structural,elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn2P2

The present study explores the structural, elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn₂P₂ using a pseudopotential plane-wave method in the framework of density functional theory within the generalized gradient approximation. The calculated lattice constants and internal coordinates are in very good agreement with the experimental findings. Independent single-crystal elastic constants as well as numerical estimations of the bulk modulus, the shear modulus, Young's modulus, Poisson's ratio, Pugh's indicator of brittle/ductile behaviour and the Debye temperature for the corresponding polycrystalline phase were obtained. The elastic anisotropy of BaIn₂P₂ was investigated using three different indexes. The calculated electronic band structure and the total and site-projected l-decomposed densities of states reveal that this compound is a direct narrow-band-gap semiconductor. Under the influence of hydrostatic pressure, the direct D–D band gap transforms into an indirect B-D band gap at 4.08 GPa, then into a B–Γ band gap at 10.56 GPa. Optical macroscopic constants, namely, the dielectric function, refractive index, extinction coefficient, reflectivity coefficient, absorption coefficient and energy-loss function, for polarized incident radiation along the [100], [010] and [001] directions were investigated.     

Electronic structure and disorder in NaxCoO2 and SrRh2O4

We discuss the electronic structure of Na_xCoO₂ from the point of view of first principles electronic structure calculations. The band structure contains low spin Co ions, with average charge 5 + x leading to a nearly full Co t_2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the e_g bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from a_g symmetry states and small hole pockets. We find significant effects due to Na disorder on these small sections, with the result that they should be localized. This is discussed in relation to recent photoemission experiments. For comparison, we present a virtual crystal band structure of beta-SrRh₂O₄. Like Na_xCoO₂ it shows a large crystal field gap between narrow t_2g and e_g manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.     

Creating Zipper‐Like van der Waals Gap Discontinuity in Low‐Temperature‐Processed Nanostructured PbBi2nTe1+3n: Enhanced Phonon Scattering and Improved Thermoelectric Performance

Nanoengineered materials can embody distinct atomic structures which deviate from that of the bulk‐grain counterpart and induce significantly modified electronic structures and physical/chemical properties. The phonon structure and thermal properties, which can also be potentially modulated by the modified atomic structure in nanostructured materials, however, are seldom investigated. Employed here is a mild approach to fabricate nanostructured PbBi_2nTe_1+3n using a solution‐synthesized PbTe‐Bi₂Te₃ nano‐heterostructure as a precursor. The as‐obtained monoliths have unprecedented atomic structure, differing from that of the bulk counterpart, especially the zipper‐like van der Waals gap discontinuity and the random arrangement of septuple‐quintuple layers. These structural motifs break the lattice periodicity and coherence of phonon transport, leading to ultralow thermal conductivity and excellent thermoelectric z T.