TiO2nanorod arrays/Ti3C2TxMXene nanosheet composites with efficient photocatalytic activity.

Semiconductor photocatalysis holds significant promise in addressing both environmental and energy challenges. However, a major hurdle in photocatalytic processes remains the efficient separation of photoinduced charge carriers. In this study, TiO2nanorod arrays were employed by glancing angle deposition technique, onto which Ti3C2TxMXene was deposited through a spin-coating process. This hybrid approach aims to amplify the photocatalytic efficacy of TiO2nanorod arrays. Through photocurrent efficiency characterization testing, an optimal loading of TiO2/Ti3C2Txcomposites is identified. Remarkably, this composite exhibits a 40% increase in photocurrent density in comparison to pristine TiO2. This enhancement is attributed to the exceptional electrical conductivity and expansive specific surface area inherent to Ti3C2TxMXene. These attributes facilitate swift transport of photoinduced electrons, consequently refining the separation and migration of electron-hole pairs. The synergistic TiO2/Ti3C2Txcomposite showcases its potential across various domains including photoelectrochemical water splitting and diverse photocatalytic devices. As such, this composite material stands as a novel and promising entity for advancing photocatalytic applications. This study can offer an innovative approach for designing simple and efficient photocatalytic materials composed of MXene co-catalysts and TiO2for efficient water electrolysis on semiconductors.

Self-formed asymmetric Schottky contacts between graphene and WSiGeN4.

In this paper, the electronic properties and transport characteristics of WSiGeN4/graphene heterostructures were explored by combining the quantum transport method with first-principle calculations. The band structures indicate that the heterostructures can form either p-type or n-type Schottky contacts, depending on the stacking mode. Due to the self-formed asymmetric Schottky contacts, we design an asymmetric van der Waals (vdW) metal-semiconductor-metal (MSM) structure, which exhibits a pronounced asymmetric current-voltage (I-V) curve. The corresponding physical mechanisms are attributed to carrier transport mechanisms, which are primarily governed by thermionic excitation at positive bias voltages and tunneling effects at negative bias voltages. Our study offers a viable strategy for integrating asymmetric Schottky barriers into MSM configurations, laying the groundwork for a wider range of applications in a range of Janus two-dimensional semiconductors.

BTEX sensing potential of elemental-doped graphene: a DFT study.

Elementally-doped graphene demonstrates remarkable gas sensing capabilities as a novel 2D sensor material. In this study, we employed density functional theory calculations, we investigated the impact of various dopants on the BTEX (benzene, toluene, ethylbenzene, and xylene) sensing performance of graphene. Through the systematic analysis of electronic structures and sensitivity, we observed that both the doping method and dopant type significantly influence the interactions between graphene and BTEX molecules. Out of the 22 different elemental doped graphenes studied, N-, O-, and Pd-doped graphenes emerged as promising candidates for BTEX sensor materials. Graphene with N-doping exhibited relatively higher sensitivity towards toluene, ethylbenzene, and xylene compared to O- and Pd-doped graphenes. However, it demonstrated low sensitivity towards benzene. On the other hand, O-doped graphene displayed excellent selectivity for ethylbenzene over the other three gas molecules (benzene, toluene, and xylene). Similarly, Pd-doped graphene also exhibited significant selectivity for ethylbenzene and possessed higher sensitivity than the O-doped graphene. Their distinct characteristics and sensitivities make them potential candidates for future applications in gas sensing technology.

Trapping and diffusion of hydrogen in iron-doped Y2O3.

Y2O3 is a promising material for use as a tritium permeation barrier (TPB) coating and as dispersed particles in oxide dispersion strengthened steels for experimental fusion reactors. By using first-principles approaches, we found that substituting Fe for Y in Y2O3 is the most energetically favourable under O-deficient and H-rich conditions, leading to easier formation of the nearby O vacancies. These O vacancies serve as effective trapping sites for H atoms with a formation energy of -2.36 eV. The presence of Fe defects also makes it more difficult for H atoms to migrate in Y2O3 from three possible H-related defects. These findings suggest that incorporating Fe into Y2O3 could yield a better TPB and provide insight into the improved H trapping ability of Y2O3 with Fe dopants.

Penta-MP5 (M = B, Al, Ga, In) monolayers as high-performance photocatalysts for overall water splitting.

Two-dimensional (2D) phosphorus-rich phosphides generally preserve the excellent electronic properties of phosphorene, making them promising photocatalysts for water splitting. Despite tremendous efforts in the search for potential photocatalysts in 2D phosphides, few known 2D phosphides fully meet the requirements for photocatalytic water splitting. Herein, we systemically investigate a set of penta-MP5 (M = B, Al, Ga, and In) monolayers by first-principles calculations and identify them as potential photocatalysts for water splitting. These penta-MP5 monolayers are found to feature favorable bandgaps of about 2.70 eV with appropriate band edge positions, a high carrier mobility of 1 × 104 cm-2 V-1 s-1, an excellent optical absorption coefficient (OAC) of 1 × 105 cm-1, and a good solar-to-hydrogen (STH) efficiency of 8%. Meanwhile, free energy calculations indicate that these penta-MP5 monolayers present both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) photocatalytic activities under light conditions. All these excellent properties demonstrate that penta-MP5 monolayers are suitable candidates as photocatalysts for promising applications in overall water splitting.

First-principles study of square chalcogen bond interactions and its adsorption behavior on silver surface.

Research on 2Ch⋯2N (Ch = S, Se, Te) square chalcogen interaction has attracted extensive attention in recent years. By search of the Crystal Structure Database (CSD), square chalcogen structures with 2Ch⋯2N interactions were widely found. Herein, dimers of 2,1,3-benzothiadiazole (C6N2H4S), 2,1,3-benzoselenadiazole (C6N2H4Se) and 2,1,3-benzotelluradiazole (C6N2H4Te) from CSD were chosen to construct a square chalcogen bond model. The square chalcogen bond and their adsorption behavior on Ag(110) surfaces have been systematically studied using first principles. Furthermore, partially fluoro-substituted C6N2H3FCh (Ch = S, Se, Te) complexes were also considered for comparison. The results show that in the C6N2H4Ch (Ch = S, Se, Te) dimer, the strength of the 2Ch⋯2N square chalcogen bond is in the order of S < Se < Te. In addition, the strength of the 2Ch⋯2N square chalcogen bond is also enhanced by F atom replacement in partially fluoro-substituted C6N2H3FCh (Ch = S, Se, Te) complexes. The self-assembly behavior of dimer complexes on silver surfaces is guided by van der Waals interactions. This work provides theoretical guidance for the application of 2Ch⋯2N square chalcogen bonds in supramolecular construction and materials science.

A semiconductor Sc2S3 monolayer with ultrahigh carrier mobility for UV blocking filter application.

For humans, ultraviolet (UV) light from sun is harmful to our eyes and eye-related cells. This detrimental fact requires scientists to search for a material that can efficiently absorb UV light while allowing lossless transmission of visible light. Using an unbiased first-principles swarm intelligence structure search, we explored two-dimensional (2D) Sc-S crystals and identified a novel Sc2S3 monolayer with good thermal and dynamical stability. The optoelectronic property simulations revealed that the Sc2S3 monolayer has a wide indirect bandgap (3.05 eV) and possesses an ultrahigh carrier mobility (2.8 × 103 cm2 V-1 s-1). Remarkably, it has almost transparent visible light absorption, while it exhibits an ultrahigh absorption coefficient up to × 105 cm-1 in the ultraviolet region. Via the application of biaxial strain and thickness modulation, the UV light absorption coefficients of Sc2S3 can be further improved. These findings manifest an attractive UV blocking optoelectronic characteristic of the Sc2S3 configuration as a prototypical nanomaterial for the potential application in UV blocking filters.

A new phosphorene allotrope: the assembly of phosphorene nanoribbons and chains.

Phosphorene allotrope monolayers such as blue and red phosphorus are being designed and synthesized to be used in the optoelectronics field due to their tunable bandgap and high mobility. Using the organic molecule self-assembly method similar to the synthesis of graphene allotropes, a novel phosphorene allotrope, P567 monolayer, with five-, six-, and seven-membered rings is designed through the assembly of black phosphorus chains and blue phosphorene nanoribbons. Ab initio molecular dynamics, phonon dispersion, and elastic constants demonstrate the dynamic, thermal, and mechanical stability of the P567 monolayer. Additionally, the first-principles calculations show that the P567 monolayer is an indirect bandgap semiconductor with moderate bandgap and high anisotropic mobility (4.47 × 103 cm2 V-1 s-1). Compared with black phosphorene, the suitable band edge position and higher optical absorption coefficient (105 cm-1) make the P567 monolayer more likely to be used as a photocatalytic hydrolysis material. The P567 monolayer is a viable candidate for use in innovative optoelectronic devices and the assembly method provides a rational approach to designing phosphorus allotropes with high photocatalytic efficiency.

The thermoelectric properties of α-XP (X = Sb and Bi) monolayers from first-principles calculations.

Thermoelectric (TE) materials as one of the effective solutions to the energy crisis are gaining more and more interest owing to their capability to generate electricity from waste heat without generating air pollution. In this work, the TE properties of α-XP monolayers such as the stability, electronic structure, electrical and phonon transport were thoroughly studied in combination with the first-principles calculations and Boltzmann transport equations. We found that α-SbP and α-BiP have indirect bandgaps of 0.85 eV and 0.73 eV, respectively, which are suitable for thermoelectric materials. Furthermore, due to the multiple valleys at the energy band edges and the high carrier mobility, α-XP possesses both large Seebeck coefficients and high electrical conductivities. It is also found that the lattice thermal conductivity of α-BiP is smaller than that of α-SbP due to lower phonon frequencies, smaller phonon group velocities, larger Grüneisen parameters and higher phonon relaxation times. High TE performance was achieved with the ZT values reaching 4.59 (for α-BiP at 500 K) and 1.34 (for α-SbP at 700 K). Our results quantify α-XP monolayers as promising candidates for building outstanding thermoelectric devices.

Promising thermoelectric candidate based on a CaAs3 monolayer: A first principles study.

The CaAs3 monolayer is a newly predicted two-dimensional material with attractive properties, such as a moderate direct bandgap, high carrier mobility, prominent visible-light absorption, etc. To evaluate its potential applications in thermoelectric (TE) fields, herein, the thermoelectric properties of CaAs3 monolayers were comprehensively investigated by a first-principles method in combination with Boltzmann transport theory. Our calculated results indicate that the CaAs3 monolayer has an exceptionally low lattice thermal conductivity of 0.44 W m-1 K-1 at 300 K, mainly because of the small group velocity and strong phonon-phonon scattering. The CaAs3 monolayer also exhibits a high power factor due to the large Seebeck coefficient and electrical conductivity. Therefore, large ZT values of 1.72/1.58 were achieved for the n-type/p-type CaAs3 monolayer at 800 K. Compared with conventional 2D TE materials, the CaAs3 monolayer does not contain expensive heavy elements, which is beneficial for its practical applications as a TE material. Our results qualify the CaAs3 monolayer as a promising candidate for building excellent 2D TE devices.

Exploring the structures and properties of nickel silicides at the pressures of the Earth's core.

Given the highly possible existence of nickel and silicon in the Earth's core, the study of the reaction between Ni and Si and the resulting structures at the pressure corresponding to that of the Earth's core is highly required. Therefore, we have investigated the crystal structures of Ni-Si compounds at pressures of 0-350 GPa by adopting a crystal structure search algorithm in conjunction with first-principles calculations. We uncover two high Ni-content Ni5Si and Ni6Si compounds with 12-coordination Si bonded with Ni, with both showing strong chemical stability in the Earth's core. Bonding analysis reveals that the Ni atoms in these Ni-Si compounds present oxidant features and act as electron acceptors. This distinctive anomaly is the natural result of the energy shifts of the Ni 3d and Si 3p bands, resulting in charge transfer from Si to Ni. By examining the key properties (e.g., density and sound velocities) of the Ni5Si and Ni6Si compounds, the obtained density lies within the range of the Earth's inner core, and the estimated sound velocities are found to be consistent with seismic data. These results indicate that these two compounds could be considered as possible core constituents. Our findings provide valuable insights into the enigmatic Earth's core as well as geophysical and geochemical processes.

2D auxetic material with intrinsic ferromagnetism: a copper halide (CuCl2) monolayer.

The discovery of ferromagnetism in monolayer transition metal halides exemplified by CrI3 has opened a new avenue in the field of two-dimensional (2D) magnetic materials, and more such 2D materials are waiting to be explored. Herein, using an unbiased structure search combined with first-principles calculations, we have identified a novel CuCl2 monolayer, which exhibits not only intrinsic ferromagnetism but also auxetic mechanical properties originating from the interplay of lattice and Cu-Cl tetrahedron symmetries. The predicted Curie temperature of CuCl2 reaches ∼47 K, and its ferromagnetism is associated with the strong hybridization between the Cu 3d and Cl 3p states in the configuration. Moreover, upon biaxial tensile strain or carrier doping, the CuCl2 monolayer can be converted from ferromagnetic to non-magnetic and from half-metal to metal. These properties endow this CuCl2 monolayer with great potential for applications in auxetic/spintronic nanodevices.

Novel two-dimensional beta-XTe (X = Ge, Sn, Pb) as promising room-temperature thermoelectrics.

In this paper, we designed novel low-symmetry two-dimensional (2D) structures based on conventional XTe (X = Ge, Sn, Pb) thermoelectrics with large average atomic mass. The first-principles calculations combined with Boltzmann transport theory show that the beta-XTe exhibit good stability, high electron carrier mobility, and ultralow ΚL. The subsequent analyses show that the ultralow ΚL stems from the coexistence of resonant bonding, weak bonding, and lone-pair electrons in beta-XTe, which leads to large anharmonicities. On the other hand, the lowest energy conduction band of beta-GeTe and beta-SnTe show the convergence of the low-lying Æ© band, which is the source of the high-power factor in the two systems. The calculated maximum ZT of beta-XTe (X = Ge, Sn, Pb) are 3.08, 1.60, and 0.57 at 300 K, respectively, which is significantly greater than that of the previously reported high-symmetry 2D alpha-XTe and the commercial thermoelectrics. We hope that this work can provide important guidance for the development of thermoelectric materials.

Thermal transport in amorphous small organic materials: a mechanistic study.

Understanding the thermal transport mechanisms in amorphous organic materials is of great importance to solve hot-spot issues in organic-electronics nanodevices. Here we studied thermal transport in two popular molecular electronic materials, N,N-dicarbazolyl-3,5-benzene (mCP) and N,N'-diphenyl-N,N'-di(3-methylphenyl)-(1,1'-biphenyl)-4,4'diamine (TPD), in the amorphous state by molecular dynamics simulations. We found that due to the softness of organic materials, the low thermal conductivity of both systems can be greatly enhanced under pressure. Notably, in such systems, the convective term of heat flux provides an important contribution to thermal transport as it cross-correlates with the Virial term in the Green-Kubo formula. Mode diffusivity calculations reveal that low-frequency modes can contribute significantly to thermal transport in both mCP and TPD. By increasing the pressure, the sound velocity and relaxation time of such low-frequency modes can be enhanced, and a part of these modes converts from diffusons to propagons. The cooperation of these three effects is responsible for the strong pressure dependence of thermal transport in amorphous organic systems. Molecular pair heat flux calculations demonstrate that heat transfer mainly happens between pairs of molecules with distances below 1.4 nm. This work paves the way for the optimization of thermal transport in amorphous organic materials widely used in opto-electronics, e.g. as OLED and OPV.

Nanowires with dislocations for ultralow lattice thermal conductivity.

Nanostructures grown by screw dislocations have been successfully synthesized in a range of materials, including thermoelectric materials, but the impact of these extended crystallographic defects on thermal properties of these nanostructures is not known. We investigate thermal transport in PbSe and SiGe nanowires storing screw dislocations via equilibrium molecular dynamics simulations. The inherent one dimensionality and the combined presence of a reconstructed surface and dislocation yield ultralow thermal conductivity values. Our simulations suggest that the large dislocation strain field in nanowires may play a key role in suppressing the thermal conductivity of thermoelectric nanomaterials to increase their thermoelectric figure of merit.

Thermal transport along the dislocation line in silicon carbide.

We elucidate thermal conductivity along the screw dislocation line, which represents a transport direction inaccessible to classical theories. By using equilibrium molecular dynamics simulations, we uncover a Burgers vector dependent thermal conductivity reduction in silicon carbide. The effect is uncorrelated with the classical modeling and originates in the highly deformed core region, which represents a significant source of anharmonic phonon-phonon scattering. High strain reduces the phonon relaxation time, especially in the longitudinal acoustic branches, and creates an effective internal thermal resistance around the dislocation axis. Our results have implications for designing materials useful for high-temperature electronics and thermoelectric applications.

From Porphyrin-Like Rings to High-Density Single-Atom Catalytic Sites: Unveiling the Superiority of p-C2N for Bifunctional Oxygen Electrocatalysis.

With effective utilization of the catalytic site, single-atom catalysts (SACs) supported by nitrogen atoms surrounding built-in pores of two-dimensional (2D) materials, such as porphyrin/phthalocyanine-based covalent organic frameworks, have been highly promising electrocatalysts in the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) processes for the air electrode of the metal-air battery. However, the number of stable single-atom anchoring sites, i.e., accessible single-atom metal sites, has been concerning as a result of the appearance of heterogeneous or large and even supersized pores in substrate materials. 2D porous graphitic carbon nitride (PGCN) with a stronger stability and smaller component is regarded as a more potential alternative owing to similar controllability and designability. In this work, inspired by the robust coordinated TM-N4 environment of porphyrin/phthalocyanine molecules, novel p-C2N with a high density of porphyrin-like organic units is rationally designed. In well-designed p-C2N, a higher homogeneity and uniformity of coordination sites can enhance the electrocatalytic activity in the whole catalytic material and better prevent SACs from sintering and agglomerating into thermodynamically stable nanoclusters. Utilizing density functional theory (DFT), the stability of the p-C2N monolayer, TM@p-C2N, and OER/ORR catalytic activities of TM@p-C2N (TM including Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) are systematically evaluated. Among them, Ir@p-C2N (0.31 V of the OER and 0.36 V of the ORR), Co@p-C2N (0.47 and 0.22 V), and Rh@p-C2N (0.55 and 0.27 V) are screened as promising SACs for the bifunctional ORR and OER. The proposal of p-C2N guides a new direction for the development of TM-N-C-based SAC bifunctional electrocatalysts.

TaF4: A Novel Two-Dimensional Antiferromagnetic Material with a High Néel Temperature Investigated Using First-Principles Calculations.

The structural, electronic, and magnetic properties of a novel two-dimensional monolayer material, TaF4, are investigated using first-principles calculations. The dynamical and thermal stabilities of two-dimensional monolayer TaF4 were confirmed using its phonon dispersion spectrum and molecular dynamics calculations. The band structure obtained via the high-accuracy HSE06 (Heyd-Scuseria-Ernzerhof 2006) functional theory revealed that monolayer two-dimensional TaF4 is an indirect bandgap semiconductor with a bandgap width of 2.58 eV. By extracting the exchange interaction intensities and magnetocrystalline anisotropy energy in a J1-J2-J3-K Heisenberg model, it was found that two-dimensional monolayer TaF4 possesses a Néel-type antiferromagnetic ground state and has a relatively high Néel temperature (208 K) and strong magnetocrystalline anisotropy energy (2.06 meV). These results are verified via the magnon spectrum.

Regulating the thermal conductivity of monolayer MnPS3 by a magnetic phase transition.

In this study, based on ab initio calculations and the phonon Boltzmann transport equation, we found that magnetic phase transitions can lead to a significant change in the thermal conductivity of monolayer MnPS3. Around the Néel temperature (78 K) with the antiferromagnetic-paramagnetic (AFM-PM) phase transition, its thermal conductivity increases from 14.89 W mK-1 (AFM phase) to 103.21 W mK-1 (PM phase). Below 78 K, the thermal conductivity of monolayer MnPS3 can be doubled by applying a magnetic field of 4 T, this value has been reported in a previous experiment for the antiferromagnetic-ferromagnetic (AFM-FM) phase transition. Above 78 K, the thermal conductivity of PM phase can be greatly reduced through the PM-AFM magnetic phase transition. In addition to the value of thermal conductivity, the relative contribution ratio between acoustic and optical modes changes with different magnetic phases. The subsequent analyses demonstrate that this regulation originates from the change in lattice parameter, bonding interaction and phonon anharmonicity. In addition, the different effect on the thermal conductivity between the FM and AFM phases was identified by comparing the corresponding phonon scattering characteristics. This study should shed light on the understanding of phonon thermal conductivity in 2D magnets, and provide a practical method for the realization of 2D thermal switching devices, which would enable a broad range of novel applications including energy conversion and thermal management.

Preparation of Tough, Binder-Free, and Self-Supporting LiFePO4 Cathode by Using Mono-Dispersed Ultra-Long Single-Walled Carbon Nanotubes for High-Rate Performance Li-Ion Battery.

Low-contents/absence of non-electrochemical activity binders, conductive additives, and current collectors are a concern for improving lithium-ion batteries' fast charging/discharging performance and developing free-standing electrodes in the aspects of flexible/wearable electronic devices. Herein, a simple yet powerful fabricating method for the massive production of mono-dispersed ultra-long single-walled carbon nanotubes (SWCNTs) in N-methyl-2-pyrrolidone solution, benefiting from the electrostatic dipole interaction and steric hindrance of dispersant molecules, is reported. These SWCNTs form a highly efficient conductive network to firmly fix LiFePO4  (LFP) particles in the electrode at low contents of 0.5 wt% as conductive additives. The binder-free LFP/SWCNT cathode delivers a superior rate capacity of 161.5 mAh g-1 at 0.5 C and 130.2 mAh g-1 at 5 C, with a high-rate capacity retention of 87.4% after 200 cycles at 2 C. The self-supporting LFP/SWCNT cathode shows excellent mechanical properties, which can withstand at least 7.2 MPa stress and 5% strain, allowing the fabrication of high mass loading electrodes with thicknesses up to 39.1 mg cm-2 . Such self-supporting electrodes display conductivities up to 1197 S m-1 and low charge-transfer resistance of 40.53 Ω, allowing fast charge delivery and enabling near-theoretical specific capacities.