Two-dimensional Layered Materials Research Articles

Structure, Synthesis, and Catalytic Performance of Emerging MXene-Based Catalysts.

As traditional fossil fuel energy development faces significant challenges, two-dimensional layered materials have become increasingly popular in various fields and have generated widespread research interest. MXene is an exceptional catalytic material that is typically integrated into functional composite materials with other substances to enhance its catalytic-reaction performance. Improving the thermal stability, electrical conductivity, and electrochemical activity, as well as enhancing the specific surface structure, can make the material an excellent catalyst for photoelectrocatalysis and energy-regeneration reactions. The article mainly outlines the structural characteristics, preparation methods, and applications of MXene in the field of catalysis. This text highlights the latest progress and performance comparison of MXene-based catalytic functional materials in various fields such as electrochemical conversion, photocatalysis, renewable energy, energy storage, and carbon capture and conversion. It also proposes future prospects and discusses the current bottlenecks and challenges in the development of MXene-based catalytic materials.

Germanene: demonstration of the conversion from mode-locked to Q-switched mode-locked in Er-doped fiber laser

Some two-dimensional layered mono-elemental materials have been reported as saturable absorbers (SAs) for the generation of various soliton phenomena, and many excellent results have been achieved. In our experiment, we made thin films of germanene-polyvinyl alcohol (Ge-PVA) and applied them as SA in Er-doped fiber lasers, investigated Ge’s characteristics, and demonstrated the conversion from mode-locked to Q-switched mode-locked (QML). The conventional mode-locked operation with a repetition rate of 9.63 MHz and a central wavelength of 1559.7 nm was stably triggered when the pump power exceeded the threshold of 40 mW. QML pulse with a central wavelength of 1530.26 nm and the maximum pulse energy of 86 nJ can be obtained by changing the pump power and polarization state of the light in the cavity. This work reveals the excellent optical properties of Ge SA in ultrafast fiber lasers and provide a new approach for the generation of QML pulses.

One-step synthesizing Ti3C2Tx doped with slight amount of Ni2+ to enhance the electrochemical stability

Two-dimensional layered material of Ti3C2Tx has attracted widespread attention due to its high conductivity, rich surface functional groups and excellent hydrophilicity. However, its practical application is hindered by the problems of re-stacking and the instability rooted from easy oxidation of low valent Ti. Here, a simple synthesis strategy that the surface of Ti3C2Tx is decorated by Ni2+ has been developed: a small amount of Ni2+ is added into the Ti3C2Tx dispersion to bind to the surface vacancies to obtain Ti3C2Tx-Ni during the ultrasonic stripping process. Therefore, the interlayer spacing of Ti3C2Tx-Ni-2 is expanded from 1.12 nm (for Ti3C2Tx) to 1.22 nm, meaning that the stacking and aggregation effect has been alleviated. Compared with the short storage time of Ti3C2Tx, the suspension of Ti3C2Tx-Ni-2 can keep stability for more than 270 days. The self-supporting Ti3C2Tx-Ni-2 film exhibits a specific capacitance of 208.0 F g−1 in 1.0 M H2SO4 and 82.8 F g−1 in 1.0 M Na2SO4 electrolyte at a scan rate of 2 mV s−1. Furthermore, it can still maintain 100 % of the initial capacitance after 10,000 cycles at a current density of 5 A g−1 in neutral electrolyte, while the Ti3C2Tx film can only retain 74.8 % of its initial capacitance.

Flower-like carbon spheres formed by self-assembly of nitrogen-doped porous nanoribbons to boost SnS2 nanosheets for advanced lithium storage

SnS2 is a two-dimensional layered material with high theoretical capacity, but its further application in lithium-ion batteries is limited by problems such as low intrinsic conductivity and dramatic volume expansion during lithiation process. Therefore, SnS2 nanosheets were designed to anchor in the interlayer nanospaces of flower-like carbon spheres self-assembled from nitrogen-doped porous carbon nanoribbons (SnS2/N-FC). Nitrogen-doped porous nanoribbons facilitated the uniform distribution of SnS2 nanosheets in the interlayer nanospaces of the flower-like carbon spheres, establishing a face-to-face interaction with the nanoribbons and the SnS2 nanosheets. This composite structure enhances the transfer of electrons between the components. In addition, the integration of spherical flower-like electrodes with the reserved interlayer nanospaces provided a buffering space for volume expansion which ensured an even distribution of mechanical stress. As a result, SnS2/N-FC exhibited a high reversible specific capacity of 1322.3 mAh g−1 after 200 cycles at 0.1 A g−1, with the capacity kept growing to 1329.4 mAh g−1 after 1000 cycles at 1 A g−1. This study introduces a novel approach for the design of anode materials which demonstrate excellent lithium storage.

MXenes vs MBenes: Demystifying the materials of tomorrow’s carbon capture revolution

Two emerging materials, MXenes and MBenes, have garnered significant attention as promising candidates for CCS applications. Both materials possess unique properties that make them well-suited for CO2 adsorption, such as high surface area, porosity, and tunable chemical functionality. This perspective article presents a comparative evaluation of MXenes and MBenes for CO2 capture, leveraging advanced computational simulations and experimental data to elucidate their respective adsorption capacities, kinetic performance, and stability. The simulations reveal that both materials exhibit superior CO2 adsorption performance compared to conventional CCS materials, with MXenes demonstrating a slight edge in adsorption capacity and selectivity. Furthermore, the potential of MXenes and MBenes for CCS applications is discussed, including their layer thickness, selective affinity to CO2, advantages over conventional sorbents, regeneration, stability, and durability. The findings provide valuable insights into the structure–property relationships of MXenes and MBenes in the context of CO2 capture and shed light on the technology readiness of these materials for specific CCS applications. Finally, this perspective article aims to advance the fundamental understanding of these novel 2D materials for CCS, paving the way for future developments in sustainable CO2 capture technologies.Graphical abstractHighlightsMXenes and MBenes are two-dimensional layered materials with the potential to revolutionize carbon capture and storage (CCS). MXenes have several advantages over other CCS materials, such as greater porosity, higher CO2 adsorption capacity, and easier and less expensive production. MBenes are more stable in humid environments and have higher oxidation resistance and thermal conductivity than MXenes, making them a better choice for CCS applications where the CO2 stream is humid, hot, and/or corrosive. MXenes and MBenes have the potential to make CCS more efficient, cost-effective, and versatile.DiscussionWhy are MXenes and MBenes ideal for carbon capture applications?In terms of carbon capture efficiency, how do MXenes and MBenes stack up against other materials such as MOFs, zeolites, and activated carbons?Which are better, MXenes or MBenes, for carbon capture?Why do MXenes and MBenes have a selective affinity to CO2 compared to other gases such as N2 and O2?What is the optimal number of layers for MXenes/MBenes for carbon capture, and does interlayer spacing affect performance?What is the best surface termination for CO2 capture?What happens to the CO2 after it is absorbed onto MXene and MBene surfaces, and how can one remove CO2 that has been adsorbed?What are the major challenges, besides scalability, that need to be overcome for these materials to be practical?How durable and stable are MXenes and MBenes?

Dipolar interactions enhanced by two-dimensional dielectric screening in few-layer van der Waals structures

We theoretically examined how the dielectric screening of two-dimensional (2D) layered materials affects the dipolar interaction between interlayer excitons (IXs) in few-layer van der Waals structures. Our analysis indicates that the dipolar interaction is largely enhanced by 2D dielectric screening at an inter-exciton separation of several nanometers or larger. The underlying mechanism can be attributed to the induced-charge densities in layered materials, which give rise to induced-dipole densities at large distances with directions parallel to that of the IX. The interaction between quadrupolar excitons in trilayer structures are found to be enhanced even larger, with a magnitude one to two orders stronger than that without 2D dielectric screening. The strengths of these dipolar and quadrupolar interactions can be further tuned by engineering the dielectric environment.

Spectroscopy of monolayer and multilayer tungsten disulfide under high pressure

Recently exfoliated monolayer and multilayered transition metal dichalcogenides have gathered significant interest based on their tunable bandgap and extremely high carrier mobility. We have investigated the Raman and photoluminescence spectra of monolayer and multilayer WS2 as a function of pressure. The Raman-inactive mode B1u, which is activated by structural disorder, was revealed at 6.7 GPa in monolayers, at 8.0 GPa in bilayers, and at 13.7 GPa in multilayers, respectively. With the enhancement of pressure-induced interlayer interaction, the crystal phase transition due to layer sliding like 2Hc to 2Ha occurs at 14.8 and 18.7 GPa in bilayers and multilayers, as evidenced by the split of E12g and B1u. The electronic phase transition of the monolayer is supposed to be a direct K-K bandgap changing to an indirect Λ-K bandgap at 2.6 GPa. These observations contribute to a better understanding of the impact of interlayer interactions on the modulation of WS2 energy bands and structure, as well as fundamental studies of two-dimensional layered materials, which can inform the development of device applications.

Brain-inspired computing: can 2D materials bridge the gap between biological and artificial neural networks?

The development of excellent non-volatile storage and computing devices based on two-dimensional layered materials is necessary for overcoming the memory bottleneck of the traditional von-Neumann structure-based devices.

High-purity C3N quantum dots for enhancing fluorescence detection of metal ions.

A new type of two-dimensional layered material, namely C3N, has been fabricated by polymerization and recommended to have great potential in various applications such as the development of electronic devices or photo-detectors, due to its enhanced conductivity, electronegativity, and unique optical properties. Actually, most of the present research on C3N is limited in the scope of theoretical calculation, while experimental research is blocked by inefficient synthesis with low purity and homogeneity. Here, we report an optimized efficient synthesis method of high-purity C3N QDs in aqueous solution by polymerization of DAP with combined centrifugation and filtration of products, with the synthesis yield up to 33.1 ± 3.1%. Subsequently, the C3N QDs have been used as novel metal ion probes exhibiting a sensitive fluorescent response to various metal ions including monovalent alkaline metals (Li+, Na+, and K+), divalent alkaline-earth metals (Mg2+, Ca2+, and Sr2+), and multivalent transition metals (Cu2+, Co2+, Ni2+, and Au3+, Fe3+, Cr3+) due to the high electronegativity of the C3N surface. Particularly, the fluorescent quenching response of Al3+, Ga3+, In3+, and Sc3+ ions is significantly different from the fluorescent enhanced response of most other carbon-based QDs, which is promising for enriching the detection methods of these metal ions and beneficial to improve the accuracy of ion recognition.

Tunable structured AlGaN-based nanoporous distributed Bragg reflectors for light-coupling enhancement in monolayer MoS2

In this work, we demonstrated tunable structured AlGaN-based nanoporous (NP) distributed Bragg reflectors (DBRs) with high-reflectivity through one-step vertical-lateral electrochemical etching (ECE). The aperture of nanopores in the doping layer and the size and density of V-pits on the surface of NP-DBRs can be tuned by controlling the etching bias. Furthermore, by further tuning the thickness of alternating layers, we achieved high reflectivity of NP-DBRs at multiple center wavelengths within the wavelength range from 280 to 800 nm. To explore its potential applications, MoS2 with a photoluminescence (PL) emission in the red light region was grown for the first time on the NP-DBRs. Due to the light localized effect of surface V-pits and the high reflectivity of the bottom DBRs, the PL intensity of MoS2 was significantly enhanced by a factor of 37.3 compared to the region without NP-DBRs. The PL enhancement of MoS2 on porous AlGaN and different periodic NP-DBRs further validated the PL enhancement mechanism. The corresponding theoretical simulations also revealed that the PL enhancement should be attributed to the combined effect of enhanced light absorption and NP-DBRs induced emission interference. The fabricated AlGaN-based NP-DBRs provide a new approach for improving the quantum efficiency of two-dimensional layered materials, which holds significant importance for advancing the development of miniaturized and integrated 2D/3D heterogeneous structure systems in the field of optoelectronics.

The enhanced adsorption mechanism of ethanol on (Fe-Zn) bimetallic-doped Ti2CO2 monolayers: a DFT study

Ti2C as a new two-dimensional layered MXene material has a certain potential in the sensor field. The adsorption properties of gases (NH3, NO, ethanol, CO) on intrinsic Ti2CO2 and transition metal atoms (Fe, Zn) doped Ti2CO2 were studied by density functional theory systematically. The geometric structure, adsorption energy, molecular dynamics, energy band structure, density of states, charge transfer, and differential charge density of these molecules at five sites of intrinsic Ti2CO2, Fe-doped Ti2CO2, Zn-doped Ti2CO2, FeZn-adjacent co-doped Ti2CO2 and FeZn-pair co-doped Ti2CO2 were analyzed. The results show that the activity of the substrate can be improved by the introduction of Fe-Zn transition metals. The high adsorption energy of ethanol gas at (A) FeZn site is −1.98eV with the short adsorption distance. The high charge transfer between the metal co-doped Ti2CO2 substrate and ethanol gas was also found. The results suggest that the Fe-Zn bimetallic-doped Ti2CO2 monolayer may be a potential sensing material for the detection of ethanol gas.

Room-Temperature, Current-Induced Magnetization Self-Switching in A Van Der Waals Ferromagnet.

2D layered materials with broken inversion symmetry are being extensively pursued as spinsource layers to realize high-efficiency magnetic switching. Such low-symmetry layered systems are, however, scarce. In addition, most layered magnets with perpendicular magnetic anisotropy show a low Curie temperature. Here, the experimental observation of spin-orbit torque magnetization self-switching at room temperature in a layered polar ferromagnetic metal, Fe2.5 Co2.5 GeTe2 is reported. The spin-orbit torque is generated from the broken inversion symmetry along the c-axis of the crystal. These results provide a direct pathway toward applicable 2D spintronic devices.

Highly selective and ultra-sensitive gas sensor based on Fe2O3/Ti3C2Tx MXene heterostructure for ppb-level n-butanol detection

The sensitive and rapid detection of toxic gases, particularly at concentrations as low as parts per billion (ppb), is crucial in preventing explosion and gas leakage accidents. Herein, we have developed a ppb-level n-butanol sensor by constructing a heterojunction structure through in situ growth of n-type oxide semiconductor Fe2O3 nanoparticles on two-dimensional layered carbide materials Ti3C2Tx MXene. The obtained results from that the Fe2O3/MXene-1% sensors display ultra-low detection limit, exhibiting a 1.31 response towards 70 ppb n-butanol. In addition, the Fe2O3/MXene-1% sensors show a remarkable response of 83.7 towards 100 ppm n-butanol at 150 °C, which represents a 22-fold improvement when comparison with the pure Fe2O3 sensors. The sensors also exhibit brilliant selectivity, reproducibility and excellent long-term stability. The remarkable enhancement in gas sensitive performance depends on Schottky heterojunctions formed between metal oxides and Ti3C2Tx MXene and the synergistic effect. The incorporation of MXene decreases materials’ resistance and enhances the carrier transfer rate. The design of this study provides a novel approach for achieving ppb-level n-butanol detection.

Exploring charge transfer mechanisms and optical properties in vdW heterostructures of MoS2 and Bi2Se3 at nanoscale regime

We report here the two-dimensional layered material of MoS2/Bi2Se3 vdW heterostructures formed by ultrasonication of different concentrations of exfoliated MoS2 and Bi2Se3 nanosheets. The vdW heterostructures show unique optical characteristics compared to individual exfoliated nanosheets. The enhanced lifetime of heterostructures explained the charge transfer with increasing efficacy for producing optoelectronic devices. Raman spectroscopy confirms the presence of intrinsic Raman active modes of Bi2Se3, MoS2, and respective modes of vdW heterostructures. TEM images show characteristic lattice planes in both materials and clear overlapping regimes, confirming successful heterojunction formation. Further, Zeta potential confirms the enhanced stability of the heterostructures formed while the significant PL quenching, strongly suggests the presence of charge transfer processes within the heterostructures. Ultrafast pump-probe spectroscopy further confirms the non-linear charge transfer and relaxation mechanism of the heterostructures. We employed Surface Xplorer and Glotaran software for model fitting and Glotaran's global fitting revealed unresolved charge carrier dynamics.

Type-II CoMoO4/Graphdiyne heterojunction promotes visible-light-driven photocatalytic hydrogen production activity

Photocatalytic hydrogen production technology has great challenges in designing and synthesizing stable photocatalyst and inhibiting high recombination rate of carriers. In this paper, CoMoO4/GDY type-II heterojunctions were successfully constructed by coupling a new two-dimensional layered carbon material graphdiyne (GDY) with molybdate (CoMoO4), and the positive effect of GDY on photocatalytic hydrogen production system was studied. The introduction of GDY provides a large number of reaction sites for the reaction. Moreover, due to the strong light absorption and excellent electrical conductivity of GDY, the photoelectrochemical properties of molybdate (CoMoO4) are enhanced. The construction of type-II heterojunctions effectively promotes the spatial separation of photogenerated electrons and holes. The electron transfer mechanism of the photocatalyst was verified based on DFT and in-situ irradiation X-ray photoelectron spectroscopy. The experimental results show that the photocatalytic performance of CoMoO4/GDY photocatalyst is the best (121 μmol), which is 2 times and 45 times that of CoMoO4 and GDY, respectively. This study provides a unique idea for the application of GDY in the field of photocatalysis.

Studying the Adsorption of Gas Molecules and Defects on Modulating the Electronic Transport Characteristics of Monolayer Penta-BN2-Based Devices.

Two-dimensional atomic layer materials, as an important part of the post-Moore era, have recently become an ideal choice for the preparation of high-efficiency, low-power, and miniaturized gas sensors. In this work, our study utilized density functional theory and the nonequilibrium Green's function method to investigate the electronic properties of the pentagonal BN2 (P-BN2) monolayer, as well as its gas-sensing properties for organic and inorganic gases. We also investigated how defects affect the quantum transport properties of the P-BN2-based device. Our findings demonstrate that the CO, H2S, NH3, SO2, C2H5OH, C3H6OH, CH3OH, and CH4 undergo physisorption on the P-BN2 monolayer, while NO, NO2, C2H2, C2H4, and HCHO undergo chemisorption. Then, we analyzed the impact of gas molecules chemisorbed on the P-BN2 monolayer on the electronic transport properties of the P-BN2-based gas sensor. When these five gas molecules are adsorbed, the current of the P-BN2-based gas sensor is greatly reduced. In addition, the effect of defects on the quantum transport properties of the P-BN2-based device is investigated. The results indicate that defects of N, B, and BN atoms lead to a decrease in the current of P-BN2-based nanodevices. Moreover, both the adsorption of gas molecules and the formation of vacancy defects leading to a decrease in device current can be revealed by the local device density of states near the zero-bias Fermi level, elucidating their microscopic mechanisms. Finally, gas molecules can also cause a decrease in the current of defect systems. These theoretical studies are of great significance for exploring two-dimensional atomic layer materials as high-efficiency gas sensors.

Two-dimensional layered materials for modifying solid-state electrolytes in lithium batteries via interface engineering

Lithium-ion batteries have been widely used in mobile electronic devices and electric vehicles, but the safety issues of lithium-ion batteries have become increasingly prominent owing to the risk of leak out and explosion with liquid electrolytes. Solid-state electrolytes (SSEs) with high ionic conductivity and low cost are considered as one of the most attractive alternatives to replacing liquid electrolytes. However, the poor interfacial compatibilities of SSE in lithium batteries lead to the failure, which severely hinders their development. Recent studies have shown that two-dimensional (2D) layered materials can improve the interface stability of SSE to lithium metal. In this Review, we will mainly introduce the fundamentals of SSEs and interface issues, and then summarize 2D materials-based modification strategy for SSEs. Recent advances have been highlighted in the aspects of using 2D materials for improving the interface of SSEs, which are very promising for energy applications. The availability of the 2D materials offers a rich playground, which not only improves the electrochemical performance of solid-state lithium batteries, but also conceives deep understanding the mechanism of interface modulation. Lastly, the perspective is provided to address the current challenges and issues of SSEs in lithium batteries.

Macroscale ultradurable superlubricity on passivated transition-metal diborides

Superlubricity is a sliding regime with almost vanishing friction sustained under stringent material conditions. This remarkable phenomenon was first found between surfaces with incommensurate lattice structures, and later extended to materials with weak shear, as in many two-dimensional layered materials. Such materials, however, face great challenges that limit their applications, including the difficulty to scale up to meet the needs of many practical implementations and low durability stemming from the intrinsic structural weakness. Overcoming these limitations hinges on new design and fabrication of advanced superlubricating materials. Here, we report on the realization of macroscale ultradurable superlubricity on passivated transition-metal diborides (TMB2) following an unconventional design principle that first construct materials with strong anti-wear ability then create superlubricity via effective surface passivation. TMB2 possess superior mechanical strength and durability, along with the ability to activate in-situ load-induced passivation layers via tribochemical actions at the surface. The resultant passivated TMB2 exhibits characteristic superlubricating and ultradurable friction coefficient (0.002) and wear rate (10−19 m3/N·m). These findings introduce a fresh approach toward rational design and implementation of superlubricating materials, opening a new avenue for versatile applications at large (up to industrial) scales.

Nitrogen-doped carbon quantum dots assembled on layered Ti3C2Tx exhibit excellent tribological properties

Nitrogen-doped carbon quantum dots (NCQDs) as a lubricant additive, is limited by its single tribological mechanisms and difficult to surface functionalization. Therefore, it will be anticipated to combine NCQDs with two-dimensional layered materials, so as to improve lubrication and interface bonding performance. Due to the active groups existing on the surface of Ti3C2Tx, the NCQDs is assembled on the two-dimensional material Ti3C2Tx, thus the NCQDs@Ti3C2Tx composite can be used as a favorable lubricant additive. The NCQDs@Ti3C2Tx composite can exert synergistic lubrication and chemical reactions during the friction process in comparison to NCQDs as lubricant additives, resulting in forming a friction transfer film between the surface. Therefore, this study confirms that NCQDs@Ti3C2Tx composite exhibits enormous potential as a lubricant additive.

Rotation angle dependent Li intercalation and the induced phase transition in bilayer MoTe2

In this work, taking twisted bilayer MoTe2 (tb-MoTe2) as an example, density functional theory (DFT) calculations are done to investigate the interlayer rotation angle dependent lithium intercalation as well as the induced 2H → 1T phase transition in two-dimensional layered materials. Both static equipotential energy diagrams and dynamic ab initio molecular dynamics simulations (AIMD) are utilized to obtain the reliable lithium diffusion pathways and barriers. The results reveal that the interlayer rotation leads to increased interlayer separation in tb-MoTe2, and provides a stable environment for Li insertion. Moreover, the diffusion barrier is substantially reduced with increasing twist angle, promoting the fast Li diffusion. At room temperature, the mobility of Li in tb-MoTe2 with the twist angle of θ = 21.79˚ and θ = 60˚ is increased by a factor of 10 and 105, respectively. The energy barrier for the 2H → 1T phase transition is lowered from 1.23 eV in the non-twisted MoTe2 to 0.88 eV in the tb-MoTe2 with a twist angle θ = 21.79˚ for a given concentration of inserted Li. Above all, the enhanced diffusion and lower critical electron concentration facilitate the 2H-1T phase transition in tb-MoTe2. The results provide valuable insights into the non-uniform intercalation and present an opportunity to modulate the phase transition of twisted 2D systems.