Emerging 2D Materials Research Articles

Ti3C2Tx MXene Electrode‐Electrolyte Interface Reactions at Different Stages of Charge/Discharge in Lithium and Sodium Half‐Cells

AbstractEnergy storage technologies necessitates efficient, cost effective, and durable storage systems like Li‐ion batteries (LIBs), with high energy density. Emerging 2D materials like MXenes have become significant for battery applications. Herein, titanium carbide (Ti3C2Tx) synthesized and lattice engineered via ‐OH surface terminations removal by thermal processing is well explained. The synthesized samples were subjected to annealing at 250 and 500 °C. All the samples were characterized using XRD, TEM, XPS, etc. Subsequently, they were tested in the half‐cell configuration for both lithium and sodium ion batteries (NIBs). It is observed that the best performance for lithium‐ion storage capacity was 200 mAh/g at 50 mA/g and 125 mAh/g at the same specific current for sodium‐ion storage for the 500 °C processed sample. However, for both the systems the cycling stability was exceptional maintaining high retention till the end of 1000 cycles. To establish the performance, electrochemical impedance and ex situ XPS results at different voltage of 1st charge/discharge were correlated for the best sample. Thus, providing information that is unavailable in the literature on MXene‐electrolyte interactions, kinetics and the chemical nature of solid‐electrolyte interface layer for both lithium and sodium‐ion batteries.

Flexible all-solid-state asymmetric supercapacitor based on Ti3C2Tx MXene/graphene/carbon nanotubes

As the emerging 2D materials, MXenes have been regarded as promising electrode materials in supercapacitor as power supply for wearable and portable electronic devices due to their metallic conductivity, hydrophilic feature, abundant functional groups, and large theoretical specific surface area etc. The capacity and rate capability of MXene-based electrodes is limited by the sheet restacking, hindering their further development. Herein, Ti3C2Tx MXene/graphene/carbon nanotubes (MXene/graphene/CNTs) flexible film was successfully fabricated by vacuum assisted filtration. Intercalation and confinement of graphene and carbon nanotubes between the Ti3C2Tx MXene nanosheets could increase the spacing and specific surface area, alleviate the stacking of MXene nanosheets, thus enhancing the electrochemical properties. The prepared MXene/graphene/CNTs film possesses increased specific surface area, average pore size, pore volume (112.259 m2g−1, 9.982 nm, 0.276 cm3g−1), compared to MXene (93.088 m2g−1, 3.786 nm, 0.088 cm3g−1). Consequently, MXene/graphene/CNTs delivers high area specific capacitance of 1862.5 mF cm−2 at 1 mA cm−2 and excellent rate capability (1525 mF cm−2, 81.88 % at 20 mA cm−2). Flexible asymmetric supercapacitor constructed with MXene/graphene/CNTs film and active carbon as positive and negative electrodes, and PVA/H2SO4 gel as solid electrolyte, achieves an energy density of 133.75 μWh cm−2 at the power density of 750 μW cm−2. In addition, there is still 94.9 % of the initial performance after cycling 8000 charge/discharge cycles at 10 mA cm−2, corresponding to a single-turn decay rate down to 0.00019 %. Besides, this flexible supercapacitor manifests outstanding electrochemical stability at different bending times. These advantages make the flexible supercapacitor device very promising for wearable electronics applications.

Electrochemical performance of MXene/Fe3O4 composite derived from recycled carbon for vanadium redox flow battery

MXenes are emerging 2D materials that have demonstrated excellent combined metallic electrical conductivity and hydrophilicity. They have been extensively studied in applications like energy storage, catalysis, optoelectronics, and so on. In this work, the carbon sourced from carbon soot extracted from fossil fuel combustion wastes was used to produce Ti3C2Tx MXene for the first time. The recyclable carbon-derived MXene/iron oxide (Fe3O4) composites with different concentrations of Fe3O4 were synthesized via a hydrothermal route. The successful synthesis of the MXene/Fe3O4 composites was confirmed by the SEM, HRTEM, EDX, UV–Vis, XRD, FTIR, and XPS analyses. Electron images morphologically revealed the existence of Fe3O4 within the as-synthesized composites, while UV–Vis spectra detected the surface plasmon resonance peaks of MXene and Fe3O4, respectively. The XRD spectra identified the crystalline structure of MXene at 5.7° (002) and 25.3° (006), as well as the magnetite Fe3O4 at 18.2° (111) and 38.2° (311), which aligned well with the presence of Ti-O and Fe-O bands observed in the FTIR data. The XPS study also confirmed the presence of the Fe element within the composites. The electrochemical performance of composite as electrode material towards the cathode VO2+/VO2+ redox reaction was investigated. The synergistic effect of both MXene and metal oxide improved the electrode kinetic activity towards the redox reaction, as evidenced by the voltage peak separation (ΔE) and the ratio of peak currents (IO/IR) values determined in cyclic voltammetry (CV), and the charge transfer process probed by electrochemical impedance spectroscopy (EIS). The specific capacity of the battery with the composites was increased along with the improvement of retention strength. The MXene/Fe3O4 5 wt% composite displayed the highest electrochemical catalytic activity with the lowest ΔE (124 mV), IO/IR of about 1, producing specific capacities of 10.2 Ah L-1 and energy efficiency of 70.1 %. The nature of MXene and Fe3O4, morphological features, and higher surface area of composites increased the kinetic activity of the electrode, enabling the composites to be potentially applied in the vanadium redox flow battery (VRFB) system.

Mid‐Infrared Optoelectronic Waveguide Devices with 2D Materials

AbstractMid‐infrared (Mid‐IR) integrated optics has tremendous applications in spectroscopic sensing, imaging, and ranging. Compared with visible light and near‐IR wavelengths, the study of mid‐IR photonic integrated devices is limited due to the need for more suitable materials and designs for constructing high‐performance on‐chip optoelectronic devices. Integrating emerging 2D materials with novel waveguide devices opens an avenue to boost the development of high‐performance optoelectronic waveguide devices operating in the mid‐IR wavelength range. This review summarizes the previous progress, current status, and future trends in exploring mid‐IR optoelectronic waveguide devices with 2D materials. Specifically, the authors focus on the research efforts of developing passive photonic devices, modulators, photodetectors, and light sources. Then, the challenges and prospects in this area are discussed. The paper provides a valuable reference for researchers in infrared physics, optoelectronics, integrated optics, material science, sensing, and spectroscopy.

Understanding epitaxial growth of two-dimensional materials and their homostructures.

The exceptional physical properties of two-dimensional (2D) van der Waals (vdW) materials have been extensively researched, driving advances in material synthesis. Epitaxial growth, a prominent synthesis strategy, enables the production of large-area, high-quality 2D films compatible with advanced integrated circuits. Typical 2D single crystals, such as graphene, transition metal dichalcogenides and hexagonal boron nitride, have been epitaxially grown at a wafer scale. A systematic summary is required to offer strategic guidance for the epitaxy of emerging 2D materials. Here we focus on the epitaxy methodologies for 2D vdW materials in two directions: the growth of in-plane single-crystal monolayers and the fabrication of out-of-plane homostructures. We first discuss nucleation control of a single domain and orientation control over multiple domains to achieve large-scale single-crystal monolayers. We analyse the defect levels and measures of crystalline quality of typical 2D vdW materials with various epitaxial growth techniques. We then outline technical routes for the growth of homogeneous multilayers and twisted homostructures. We further summarize the current strategies to guide future efforts in optimizing on-demand fabrication of 2D vdW materials, as well as subsequent device manufacturing for their industrial applications.

Gate‐Modulated and Polarization‐Sensitive Photodetector Based on the MoS2/PdSe2 Out‐Of‐Plane Van Der Waals Heterostructure

AbstractPhotodetectors with good polarization detection ability are promising in many applications, such as remote sensing imaging and environmental monitoring. However, the traditional polarization detection systems fall short in meeting integration demands of the integrated‐circuits field due to additional optical elements. The emerging 2D materials with in‐plane anisotropic structures provide a possible method to fabricate remarkable polarization detectors. Modulating the band structure by gate voltage is an important strategy for developing optoelectronic devices. Herein, a polarized photodetector based on PdSe2/MoS2 out‐of‐plane heterojunction is fabricated. Due to its unique out‐of‐plane heterostructure, the device exhibits excellent photoresponse characteristics and polarization sensitivity, including an excellent responsivity of 10.19A/W, an extremely high external quantum efficiency of 2429%, a fast rise/decay time of 68/192 µs, and a high photocurrent anisotropy ratio of 3.09. Based on the adjustment of the built‐in electric field through gate voltage, the performance of the device can be accordingly modulated. As the gate voltage increases from −30 to 30 V, the responsivity gradually increases from 7.5 to 13A/W and the detectivity increases from 1.53 to 2.63 × 109Jones. Finally, its olarization imaging ability is demonstrated at different polarization angles. The findings indicate that PdSe2/MoS2 devices exhibit significant potential for polarized photoelectric detection.

2D Materials-Based Thermal Interface Materials: Structure, Properties, and Applications.

The challenges associated with heat dissipation in high-power electronic devices used in communication, new energy, and aerospace equipment have spurred an urgent need for high-performance thermal interface materials (TIMs) to establish efficient heat transfer pathways from the heater (chip) to heat sinks. Recently, emerging 2D materials, such as graphene and boron nitride, renowned for their ultrahigh basal-plane thermal conductivity and the capacity to facilitate cross-scale, multi-morphic structural design, have found widespread use as thermal fillers in the production of high-performance TIMs. To deepen the understanding of 2D material-based TIMs, this review focuses primarily on graphene and boron nitride-based TIMs, exploring their structures, properties, and applications. Building on this foundation, the developmental history of these TIMs is emphasized and a detailed analysis of critical challenges and potential solutions is provided. Additionally, the preparation and application of some other novel 2D materials-based TIMs are briefly introduced, aiming to offer constructive guidance for the future development of high-performance TIMs.

Thermal properties of carbon-based materials

Phonons and related processes govern the thermal properties of materials. They can be excited externally using light, heat, mechanical energy, etc. This article aims to elucidate the methodology used in studying thermal properties, specifically employing temperature-dependent Raman spectroscopy. However, unique challenges arise when applying these techniques to low-dimensional materials. We summarize theoretical and experimental studies highlighting the significance of phonon hydrodynamics—a phenomenon typically observed only at low temperatures in bulk materials, but now found at room temperature in 2D materials like graphene. This discovery calls for caution when utilizing temperature-dependent Raman spectroscopy-based techniques, such as the optothermal Raman method, which has been extensively employed to experimentally determine the thermal conductivity of graphene. Moreover, temperature-dependent Raman spectroscopy remains a valuable tool for investigating phonon anharmonicity, a key factor governing phonon transport and decay processes in a wide array of emerging 2D materials and their heterostructures.

Two-Dimensional SnS Mediates NiFe-LDH-Layered Electrocatalyst toward Boosting OER Activity for Water Splitting.

NiFe-layered double hydroxides (NiFe-LDHs), as promising electrocatalysts, have received significant research attention for hydrogen and oxygen generation through water splitting. However, the slow oxidation kinetics of NiFe-LDH, due to the limited number of active sites and the low conductivity, hinders the improvement of the water-splitting efficiency. Therefore, to overcome the obstacles, two-dimensional (2D) SnS was first explored to tailor the prepared NiFe-LDH via the hydrothermal method. A NiFe-LDH/SnS heterojunction is built, which is observed from the microstructural investigations. SnS incorporation could greatly improve the conductivity of the NiFe-LDH sheets, which was reflected by the reduced charge transfer resistance. Moreover, SnS layers modulated the electronic environment around the active sites, favoring the adsorption of intermediates during the oxygen evolution reaction (OER) process, which was verified by density functional theory calculations. A synergistic effect induced by the NiFe-LDH/SnS heterostructure promoted the OER activities in electrical, electronic, and energetic aspects. Consequently, the as-prepared NiFe-LDH/SnS electrocatalyst greatly improved the electrocatalytic performance, exhibiting 20% and 27% reductions in the overpotential and Tafel slope compared with those of pristine NiFe-LDH, respectively. The results provide a strategy for regulating NiFe-based electrocatalysts by using emerging 2D materials to enhance water-splitting efficiency.

Progress, Prospects and Challenges of MXene Integrated Optoelectronics Devices

AbstractRecently, the emerging 2D materials MXene have gained a surge of attention to the production of optoelectronics devices such as solar cells, plasmonic, phototransistors, photodetectors, light‐emitting diodes, photothermal therapy, and so on. Its outstanding optical and electrical characteristics, unique structure, and large specific surface area make it suitable for future use in modern optoelectronics including ultrafast lasers, light emitters, modulators, and plasmonic generators. There is a lack of critical analysis on the prospects, challenges, overview of synthesis methods, mechanisms, and future research directions of MXene despite having some reviews have been published on the applications of MXene. Therefore, this study critically analyzed the existing challenges of MXene, such as poor stability in an oxygen environment, inadequate mechanical properties, ease of stacking, temperature barrier, and so on. In addition, the fundamentals, preparation techniques, properties, and applications of MXene have been summarized. The mechanism, limitations, and benefits of different preparation methods have been mentioned. A comprehensive analysis and guidelines have been provided to improve the existing synthesis methods. The ways to overcome these challenges, prospects, and future markets of the MXene‐based optoelectronic devices have been described.

Environmental and Health Impacts of Graphene and Other Two-Dimensional Materials: A Graphene Flagship Perspective.

Two-dimensional (2D) materials have attracted tremendous interest ever since the isolation of atomically thin sheets of graphene in 2004 due to the specific and versatile properties of these materials. However, the increasing production and use of 2D materials necessitate a thorough evaluation of the potential impact on human health and the environment. Furthermore, harmonized test protocols are needed with which to assess the safety of 2D materials. The Graphene Flagship project (2013-2023), funded by the European Commission, addressed the identification of the possible hazard of graphene-based materials as well as emerging 2D materials including transition metal dichalcogenides, hexagonal boron nitride, and others. Additionally, so-called green chemistry approaches were explored to achieve the goal of a safe and sustainable production and use of this fascinating family of nanomaterials. The present review provides a compact survey of the findings and the lessons learned in the Graphene Flagship.

Boron Nitride Nanosheets Induce Lipid Accumulation and Autophagy in Human Alveolar Lung Epithelial Cells Cultivated at Air-Liquid Interface.

Hexagonal boron nitride (hBN) is an emerging 2D material attracting significant attention due to its superior electrical, chemical, and therapeutic properties. However, inhalation toxicity mechanisms of hBN in human lung cells are poorly understood. Here, cellular interaction and effects of hBN nanosheets is investigated in alveolar epithelial cells cultured on porous inserts and exposed under air-liquid interface conditions for 24h. hBN is taken up by the cells as determined in a label-free manner via RAMAN-confocal microscopy, ICP-MS, TEM, and SEM-EDX. No significant (p > 0.05) effects are observed on cell membrane integrity (LDH release), epithelial barrier integrity (TEER), interleukin-8 cytokine production or reactive oxygen production at tested dose ranges (1, 5, and 10µgcm-2). However, it is observed that an enhanced accumulation of lipid granules in cells indicating the effect of hBN on lipid metabolism. In addition, it is observed that a significant (p < 0.05) and dose-dependent (5 and 10µgcm-2) induction of autophagy in cells after exposure to hBN, potentially associated with the downstream processing and breakdown of excess lipid granules to maintain lipid homeostasis. Indeed, lysosomal co-localization of lipid granules supporting this argument is observed. Overall, the results suggest that the continuous presence of excess intracellular lipids may provoke adverse outcomes in the lungs.

Cytotoxicity assessment of exfoliated MoS2 using primary human mast cells and the progenitor cell-derived mast cell line LAD2.

Molybdenum disulfide is an emerging 2D material with several potential applications in medicine. Therefore, it is crucial to ascertain its biocompatibility. Mast cells are immune cells that are found in many organs and tissues in contact with the extracellular environment, and can be cultured from progenitor cells present in the bone marrow. Given the long period required for differentiation and proliferation of primary mast cells, human mast cell lines have emerged as a tractable model for biological and toxicological studies. Here, we compare two types of industrial MoS2 using CD34+-derived primary human mast cells and the LAD2 cell line. Minimal effects were observed on early-stage activation endpoints such as β-hexosaminidase release and expression of surface markers of mast cell activation. Transmission electron microscopy revealed limited uptake of the tested materials. Overall, MoS2 was found to be biocompatible, and the LAD2 cell line was validated as a useful in vitro model of mast cells.

Electrochemical exfoliation of 2D materials beyond graphene.

After the discovery of graphene in 2004, the field of atomically thin crystals has exploded with the discovery of thousands of 2-dimensional materials (2DMs) with unique electronic and optical properties, by making them very attractive for a broad range of applications, from electronics to energy storage and harvesting, and from sensing to biomedical applications. In order to integrate 2DMs into practical applications, it is crucial to develop mass scalable techniques providing crystals of high quality and in large yield. Electrochemical exfoliation is one of the most promising methods for producing 2DMs, as it enables quick and large-scale production of solution processable nanosheets with a thickness well below 10 layers and lateral size above 1 μm. Originally, this technique was developed for the production of graphene; however, in the last few years, this approach has been successfully extended to other 2DMs, such as transition metal dichalcogenides, black phosphorous, hexagonal boron nitride, MXenes and many other emerging 2D materials. This review first provides an introduction to the fundamentals of electrochemical exfoliation and then it discusses the production of each class of 2DMs, by introducing their properties and giving examples of applications. Finally, a summary and perspective are given to address some of the challenges in this research area.

Electrolytic preparation of two-dimensional layered sulphur nanomaterials and their application in lithium-sulphur batteries

Currently, two-dimensional (2D) layered materials have gained much attention for their outstanding properties. Among these materials, layered sulphur, which is an emerging 2D material, has shown great potential for various applications due to its unique chemical and physical properties. In this study, we present a new electrolytic method for synthesizing layered sulphur using thiourea as the sulphur source and poly-3,4-ethylenedioxy thiophene/polystyrene sulfonate (PEDOT: PSS) as the stabilizer. This method is simple, fast, and has a high yield. Furthermore, a composite material was prepared containing carbon nanotubes (CNTs) and layered sulphur, which was used a cathode material in lithium-sulphur batteries. The prepared cathode exhibited a discharge specific capacity of 450 mAhg−1 and a reversible capacity of 46.6% after 40 cycles of reciprocation at 0.1 C cycling current. Electrode design based on layered sulphur composites provides a new idea for the structural design of lithium-sulphur battery cathodes.

Alkalized MXene/carbon nanotube composite for stable Na metal anodes.

Ti3C2 MXenes are emerging 2D materials and have attracted increasing attention in sodium metal anode fabrication because of their high conductivity, multifunctional groups and excellent mechanical performances. However, the severe self-restacking of Ti3C2 MXenes is not conducive to dispersing Na+ and limits the function of regulating sodium deposition. Herein, an alkalized MXene/carbon nanotube (CNT) composite (named A-M-C) is introduced to regulate Na deposition behavior, which consists of Na3Ti5O12 microspheres, Ti3C2 MXene nanosheets and CNTs. Ti3C2 MXene nanosheets with large interlayer spaces and "sodiophilic" functional groups can provide abundant active sites for uniform nucleation and deposition of Na. Plenty of nanosheets are grown on the surface of the microsphere, thereby reducing the local current density, which can guide initial Na nucleation and promote Na dendrite-free growth. Furthermore, CNTs increase the electrical conductivity of the composite and achieve fast Na+ transport, improving the cycling stability of Na metal batteries. As a result, at a capacity of 1 mA h cm-2, the A-M-C electrode achieves a high average coulombic efficiency (CE) of 99.9% after 300 cycles at 2 mA cm-2. The symmetric cells of A-M-C/Na provide a long cycling life of more than 1400 h at 1 mA cm-2 with a minimal overpotential of 19 mV at an areal capacity of 1 mA h cm-2. The A-M-C/Na//NVP@C full cell presents a high coulombic efficiency of 98% with 100 mA g-1 in the first cycle. The strategy in this work provides new insights into fabricating novel MXene-based anode materials for dendrite-free sodium deposition.

Mechanical Characterization of Emerging 2D Materials for Electronics: Review

The rapid growth in the field of electronics has created the need for studying new materials that offer improved qualities. Among these options, materials which exist in two dimensions (referred to as 2D materials) emerged as the most popular choices because of their unique mechanical and electrical properties. The present study provides a thorough review of the mechanical properties of new two-dimensional (2D) materials, with a particular focus on their potential utility throughout the electronics sector. In this research, It provide a comprehensive examination of the mechanical properties of multiple two-dimensional (2D) materials, such as graphene, which transition-metals dichalcogenides (TMDs), and hexagonal nitride of boron (h-BN), under different conditions that mimic the operational environment of electronic systems. The elastic moduli, strength, and flexibility of the components are evaluated by the utilization of sophisticated characterization methods such as atomic force microscopy (AFM), and nanoindentation. Also, this study examines the impact of environmental factors, specifically temperature and humidity, on the characteristics mentioned earlier. Results illustrate that the 2D materials exhibit remarkable mechanical properties, distinguished by both strength and flexibility, leaving them well-suited for applications in flexible electronic devices and high-performance nanodevices.

Characterization of emerging 2D materials after chemical functionalization

Characterizing functionalized 2D materials is not easy. We present a critical overview of the challenges, the spectroscopic, microscopic and analytical techniques available and practical examples in the literature to illustrate their correct use.

Emerging 2D materials hardware for in-sensor computing

This review covers recent advancements and future directions in 2DM-based devices for in-sensor computing, focusing on unique physical mechanisms for sensory responses, biomimetic synaptic features, and potential applications.

(Invited) Revealing Charge Storage Processes at the Interfaces of Supercapacitor Electrodes

The fast development of electrochemical energy storage devices has revolutionized almost every aspect of modern life by enabling portable electronic devices, electric vehicles, and grid storage of renewable energy. To satisfy the growing industrial and consumer needs for energy storage systems with high output power and short recharge time, the electrode materials should have excellent high-rate performance.1 Electrical double-layer (EDL) capacitors exhibit high-rate capability and long-cycling life as they store charges electrostatically. Using ionic liquid (IL) as the electrolyte leads to a large voltage window but low capacitance. In this first session of the presentation, I will introduce a mathematical model to simulate the IL ion packing structure in nanopores. This model is capable of estimating the EDLC capacitance based on pore size distribution. Then, we further revealed that mixture IL ions can be selectively driven into the pores of different confinement levels, which was confirmed by solid-state NMR, and further explained by DFT simulation2.In the second part of the presentation, I will shift to pseudocapacitors, which are expected to have a much higher charge storage capacity than EDL capacitors and a much higher rate than batteries as they store energy through fast surface redox reactions.3 The emerging 2D material family, 2D transition metal carbides/nitrides MXenes, shows outstanding pseudocapacitive performance due to their ionophilicity, metallic conductivity, and highly reactive surfaces. The proper coupling between electrolyte and electrode is critical to increase energy and power density. In the organic electrolyte, we found that the solvent has a strong impact on the ion/solvent arrangement in 2D MXene material and hence the charge storage capability.4 In the aqueous electrolytes, we successfully introduced surface redox reactions to MXene electrodes by using the water-in-salt electrolytes and by adjusting the initial valence of Ti in Ti3C2 MXenes, which dramatically increases the capacitance of MXene in the neutral aqueous electrolytes5.References Simon, P.; Gogotsi, Y., Nat. Mater. 2020, 19 (11), 1151-1163. Wang, X.; Mehandzhiyski, A. Y.; et al., J. Am. Chem. Soc. 2017, 139 (51), 18681-18687. Fleischmann, S.; Mitchell, J. B.; et al., Chem. Rev. 2020, 120 (14), 6738-6782. Wang, X.; Mathis, T. S.; et al., Nat. Energy 2019, 4 (3), 241-248. Wang, X.; Mathis, T. S.; et al., ACS Nano 2021, 15 (9), 15274-15284.