A Universal Seeding Strategy to Synthesize Single Atom Catalysts on 2D Materials for Electrocatalytic Applications

Single‐atom catalysts (SACs) are attracting significant attention due to their exceptional catalytic performance and stability. However, the controllable, scalable, and efficient synthesis of SACs remains a significant challenge. Herein, a new and versatile seeding approach is reported to synthesize SACs supported on different 2D materials such as graphene, boron nitride (BN), and molybdenum disulfide (MoS₂). This method is demonstrated on the synthesis of Ni, Co, Fe, Cu, Ag, Pd single atoms as well as binary atoms of Ni and Cu codoped on 2D support materials with the mass loading of single atoms in the range of 2.8–7.9 wt%. In particular, the applicability of the new seeding strategy in electrocatalysis is demonstrate on nickel SACs supported on graphene oxide (SANi‐GO), exhibiting excellent catalytic performance for electrochemical CO₂ reduction reaction with a turnover frequency of 325.9 h^?1 at a low overpotential of 0.63 V and high selectivity of 96.5% for CO production. The facile, controllable, and scalable nature of this approach in the synthesis of SACs is expected to open new research avenues for the practical applications of SACs.     

Enhancing the Energy Storage Capabilities of Ti3C2Tx MXene Electrodes by Atomic Surface Reduction

MXenes are a large class of 2D materials that consist of few-atoms-thick layers of transition metal carbides, nitrides, or carbonitrides. The surface functionalization of MXenes has immense implications for their physical, chemical, and electronic properties. However, solution-phase surface functionalization often leads to structural degradation of the MXene electrodes. Here, a non-conventional, single-step atomic surface reduction (ASR) technique is adopted for the surface functionalization of MXene (Ti₃C₂T_x) in an atomic layer deposition reactor using trimethyl aluminum as a volatile reducing precursor. The chemical nature of the modified surface is characterized by X-ray photoelectron spectroscopy and nuclear magnetic resonance techniques. The electrochemical properties of the surface-modified MXene are evaluated in acidic and neutral aqueous electrolyte solutions, as well as in conventional Li-ion and Na-ion organic electrolytes. A considerable improvement in electrochemical performance is obtained for the treated electrodes in all the examined electrolyte solutions, expressed in superior rate capability and cycling stability compared to those of the non-treated MXene films. This improved electrochemical performance is attributed to the increased interlayer spacing and modified surface terminations after the ASR process.     

Opportunities and Challenges in Precise Synthesis of Transition Metal Single-Atom Supported by 2D Materials as Catalysts toward Oxygen Reduction Reaction

Transition metal single-atom catalysts (SACs) are currently a hot area of research in the field of electrocatalytic oxygen reduction reaction (ORR). In this review, the recent advances in transition metal single-atom supported by 2D materials as catalysts for ORR with high performance are reported. Due to their large surface area, uniformly exposed lattice plane, and adjustable electronic state, 2D materials are ideal supporting materials for exploring ORR active sites and surface reactions. The rational design principles and synthetic strategies of transition metal SACs supported by 2D materials are systematically introduced while the identification of active sites, their possible catalytic mechanisms as well as the perspectives on the future of transition metal SACs supported by 2D materials for ORR applications are discussed. Finally, according to the current development trend of ORR catalysts, the future opportunities and challenges of transition metal SACs supported by 2D materials are summarized.     

Simple and Scalable Mechanochemical Synthesis of Noble Metal Catalysts with Single Atoms toward Highly Efficient Hydrogen Evolution

Designing a facile strategy to access active and atomically dispersed metallic catalysts are highly challenging for single atom catalysts (SACs). Herein, a simple and fast approach is demonstrated to construct Pt catalysts with single atoms in large quantity via ball milling Pt precursor and N‐doped carbon support (K₂PtCl₄@NC‐M; M denotes ball‐milling). The as‐prepared K₂PtCl₄@NC‐M only requires a low overpotential of 11 mV and exhibits 17‐fold enhanced mass activity for the electrochemical hydrogen evolution compared to commercial 20 wt% Pt/C. The superior hydrogen evolution reaction (HER) catalytic activity of K₂PtCl₄@NC‐M can be attributed to the generation of Pt single atoms, which improves the utilization efficiency of Pt atoms and the introduction of Pt‐N₂C₂ active sites with near‐zero hydrogen adsorption energy. This viable ball milling method is found to be universally applicable to the fabrication of other single metal atoms, for example, rhodium and ruthenium (such as Mt‐N₂C₂, where Mt denotes single metal atom). This strategy also provides a promising and practical avenue toward large‐scale energy storage and conversion application.     

Recent Progress on 2D Noble‐Transition‐Metal Dichalcogenides

Emerging classes of 2D noble‐transition‐metal dichalcogenides (NTMDs) stand out for their unique structure and novel physical properties in recent years. With the nearly full occupation of the d orbitals, 2D NTMDs are expected to be more attractive due to the unique interlayer vibrational behaviors and largely tunable electronic structures compared to most transition metal dichalcogenide semiconductors. The novel properties of 2D NTMDs have stimulated various applications in electronics, optoelectronics, catalysis, and sensors. Here, the latest development of 2D NTMDs are reviewed from the perspective of structure characterization, preparation, and application. Based on the recent research, the conclusions and outlook for these rising 2D NTMDs are presented.     

Tailoring Ultrahigh Energy Density and Stable Dendrite-Free Flexible Anode with Ti3C2Tx MXene Nanosheets and Hydrated Ammonium Vanadate Nanobelts for Aqueous Rocking-Chair Zinc Ion Batteries

Exploiting Zn metal-free anode materials would be an effective strategy to resolve the problems of Zn metal dendrites that severely hinder the development of Zn ion batteries (ZIBs). However, the study of Zn metal-free anode materials is still in their infancy, and more importantly, the low energy density severely limits their practical implementations. Herein, a novel (NH₄)₂V₁₀O₂₅ · 8H₂O@Ti₃C₂T_x (NHVO@Ti₃C₂T_x) film anode is proposed and investigated for constructing “rocking-chair” ZIBs. The NHVO@Ti₃C₂T_x electrode shows a capacity of 514.7 mAh g⁻¹ and presents low potential which is 0.59 V (vs Zn²⁺/Zn) at 0.1 A g⁻¹. The introduction of Ti₃C₂T_x not only affords an interconnected conductive network, but also stabilizes the NHVO nanobelts structure for a long cycle life (84.2% retention at 5.0 A g⁻¹ over 6000 cycles). As a proof-of-concept, a zinc metal-free full battery is successfully demonstrated, which delivers the highest capacity of 131.7 mAh g⁻¹ (mass containing anodic and cathodic) and energy density of 97.1 Wh kg⁻¹ compared to all reported aqueous “rocking-chair” ZIBs. Furthermore, a long cycling span of 6000 cycles is obtained with capacity retention reaching up to 92.1%, which is impressive. This work is expected to provide new moment toward V-based materials for “rocking-chair” ZIBs.     

Tetrabutylammonium Bromide Functionalized Ti3C2Tx MXene as Versatile Cathode Buffer Layer for Efficient and Stable Inverted Perovskite Solar Cells

2D Ti₃C₂T_x MXene, possessing facile preparation, high electrical conductivity, flexibility, and solution processability, shows good application potential for enhancing device performance of perovskite solar cells (PVSCs). In this study, tetrabutylammonium bromide functionalized Ti₃C₂T_x (TBAB-Ti₃C₂T_x) is developed as cathode buffer layer (CBL) to regulate the PCBM/Ag cathode interfacial property for the first time. By virtue of the charge transfer from TBAB to Ti₃C₂T_x demonstrated by electron paramagnetic resonance and density functional theory, the TBAB-Ti₃C₂T_x CBL with high electrical conductivity exhibits significantly reduced work function of 3.9 eV, which enables optimization of energy level alignment and enhancement of charge extraction. Moreover, the TBAB-Ti₃C₂T_x CBL can effectively inhibit the migration of iodine ions from perovskite layer to Ag cathode, which synergistically suppresses defect states and reduce charge recombination. Consequently, utilizing MAPbI₃ perovskite without post-treatment, the TBAB-Ti₃C₂T_x based device exhibits a dramatically improved power conversion efficiency of 21.65% with significantly improved operational stability, which is one of the best efficiencies reported for the devices based on MAPbI₃/PCBM with different CBLs. These results indicate that TBAB-Ti₃C₂T_x shall be a promising CBL for high-performance inverted PVSCs and inspire the further applications of quaternary ammonium functionalized MXenes in PVSCs.     

Removal of Interlayer Water of two Ti3C2Tx MXenes as a Versatile Tool for Controlling the Fermi-Level Pinning-Free Schottky Diodes with Nb:SrTiO3

Striving for the sixth-generation communication technology discovery, semiconductors beyond Si with wider bandgaps as well as non-conventional metals are actively being sought to achieve high speeds whilst maintaining devices miniaturization. 2D materials may provide the potential for downsizing, but their functional advantage over existing counterparts still longs to be discovered. Along that path, surface-adsorbed or bulk-intercalated water molecules remaining after wet-chemical synthesis of 2D materials are generally seen as obstacles to high-performance achievement. Herein, the control of such water within the interlayers of solution-processed metallic 2D titanium carbide (MXene) by vacuum annealing duration is demonstrated. Moreover, the impact of water removal on work function (WF) and functional terminations is unveiled for the first time. Furthermore, the usefulness of such water for controlling a novel Schottky diode in contact with an n-type oxide semiconductor, niobium-doped strontium titanate (Nb:SrTiO₃) is observed. The advantage of MXene compared to conventional gold as facile processing, WF tunability, and lower turn-on voltage in the Schottky anode application is highlighted. This fundamental study shows the way for a novel Schottky diode preparation in atmospheric conditions and provides implications for further research directions aiming at commercialization.     

2D,Metal-Free Electrocatalysts for the Nitrogen Reduction Reaction

Environmentally friendly ammonia production is important for addressing the carbon emissions and substantial energy consumption that are currently associated with the chemical industry. In recent decades, many achievements are made in this area; however, low production yield, poor selectivity, and unsatisfactory Faradaic efficiency hinder large-scale applications. 2D, metal-free electrocatalysts stand out from other candidates because of their physical, electronic, and chemical properties. In this study, recent developments of 2D-based electrochemical materials for converting dinitrogen into ammonia in ambient conditions are systematically reviewed. First, recent unique progress and challenges on novel 2D electrocatalysts for the nitrogen reduction reaction are summarized. Then, various synthetic strategies for electrochemical materials and the influence of these methods have on the intrinsic material performance are highlighted. Last, by comparing current engineering strategies, electrochemical tests, and computational calculations, the opportunities, critical issues, and scientific challenges for 2D nanomaterials as stable, efficient catalysts, are analyzed. On the basis of this comparison, technology solutions are provided and rational principles for future studies are proposed.     

Pillared MXene with Ultralarge Interlayer Spacing as a Stable Matrix for High Performance Sodium Metal Anodes

Sodium (Na) metal is a promising alternative to lithium metal as an anode material for the next‐generation energy storage systems due to its high theoretical capacity, low cost, and natural abundance. However, dendritic/mossy Na growth caused by uncontrollable plating/stripping results in serious safe concerns and rapid electrode degradation. This study presents Sn²⁺ pillared Ti₃C₂ MXene serving as a stable matrix for high‐performance dendrite‐free Na metal anode. The intercalated Sn²⁺ between Ti₃C₂ layers not only induces Na to nucleate and grow within Ti₃C₂ interlayers, but also endows the Ti₃C₂ with larger interlayer space to accommodate the deposited Na by taking advantage of the “pillar effect,” contributing to uniform Na deposition. As a result, the pillar‐structured MXene‐based Na metal electrode could enable high current density (up to 10 mA cm^?2) along with high areal capacity (up to 5 mAh cm^?2) over long‐term cycling (up to 500 cycles). The full cell using MXene‐based Na metal anode exhibits superior electrochemical performance than that using host‐less commercial Na. It is believed that the well‐controlled MXene‐based Na anode not only extends the application scope of MXene, but also provides guidance in designing high‐performance Na metal batteries.     

2D MXenes as a Promising Candidate for Surface Enhanced Raman Spectroscopy: State of the Art,Recent Trends,and Future Prospects

Surface-enhanced enhanced Raman spectroscopy (SERS) has emerged as a powerful analytical technique for ultrasensitive and label-free detection of chemical species, with numerous applications in various fields. Recently, 2D MXenes, have evoked substantial intrigue as promising substrates for SERS. Hence, a comprehensive understanding of the developments in the Raman effect and the mechanisms involved in SERS is highly crucial. The review reflects the advances, working principle, and dual mechanisms, including SERS's electromagnetic and chemical mechanisms. Noble metal nanostructures are highly prioritized as SERS substrates owing to their excellent sensitivity. However, due to certain disadvantages that they pose, metal-free SERS substrates with exceptional tunable properties are extensively researched in the current days. The combination of 2D MXenes and nanostructures can be effective in producing enhanced SERS signals. SERS performance of different MXene-based materials is emphasized. The performance of this combination is credited to their large surface-to-volume ratio, good electrical conductivity, and surface-terminated functionalities. The recent advancements in MXenes and MXenes-based heterostructures driven SERS sensing concerning the structural design of the material, its performance, and the mechanisms are studied. Finally, a detailed conclusion is provided with the challenges and future perspectives for designing 2D materials for efficient SERS sensors.     

Surface‐Modified Metallic Ti3C2Tx MXene as Electron Transport Layer for Planar Heterojunction Perovskite Solar Cells

MXenes are a large and rapidly expanding family of 2D materials that, owing to their unique optoelectronic properties and tunable surface termination, find a wide range of applications including energy storage and energy conversion. In this work, Ti₃C₂T_x MXene nanosheets are applied as a novel type of electron transport layer (ETL) in low‐temperature processed planar‐structured perovskite solar cells (PSCs). Interestingly, simple UV‐ozone treatment of the metallic Ti₃C₂T_x that increases the surface Ti? O bonds without any change in its bulk properties such as high electron mobility improves its suitability as an ETL. Improved electron transfer and suppressed recombination at the ETL/perovskite interface results in augmentation of the power conversion efficiency (PCE) from 5.00% in the case of Ti₃C₂T_x without UV‐ozone treatment to the champion PCE of 17.17%, achieved using the Ti₃C₂T_x film after 30 min of UV‐ozone treatment. As the first report on the use of pure MXene layer as an ETL in PSCs, this work shows the great potential of MXenes to be used in PSCs and displays their promise for applications in photovoltaic technology in general.     

Robust Self-Catalytic Reactor for CO2 Methanation Fabricated by Metal 3D Printing and Selective Electrochemical Dissolution

The methanation of CO₂ has been actively pursued as a practical approach to mitigating global climate change. However, the complexity of the catalyst development process has hindered the development of new catalysts for CO₂ methanation; as a result, few catalysts are commercially available. Herein, a multifunctional self-catalytic reactor (SCR) is prepared via metal 3D printing and selective electrochemical dissolution as a method to not only simplify the catalyst development process but also fabricate active catalysts for CO₂ methanation. The combination of metal 3D printing and selective electrochemical dissolution is demonstrated as a feasible method to prepare active catalysts for the methanation of CO₂ in a short time. In addition, the use of an electrochemical method enables the formation of galvanic cells on the SCR; these cells continuously generate active sites via self-dissolution during a simple refresh process, resulting in high reusability of the SCR. The proposed method represents a new facile technique to fabricate highly reusable catalysts that exhibit superior performance for CO₂ methanation, and the results provide a guideline for preparing metal 3D-printed catalysts that will satisfy industrial demand.     

Reversible Crumpling of 2D Titanium Carbide (MXene) Nanocoatings for Stretchable Electromagnetic Shielding and Wearable Wireless Communication

In the emerging Internet of Things, stretchable antennas can facilitate wireless communication between wearable and mobile electronic devices around the body. The proliferation of wireless devices transmitting near the human body also raises interference and safety concerns that demand stretchable materials capable of shielding electromagnetic interference (EMI). Here, an ultrastretchable conductor is fabricated by depositing a crumple‐textured coating composed of 2D Ti₃C₂T_x nanosheets (MXene) and single‐walled carbon nanotubes (SWNTs) onto latex, which can be fashioned into high‐performance wearable antennas and EMI shields. The resulting MXene‐SWNT (S‐MXene)/latex devices are able to sustain up to an 800% areal strain and exhibit strain‐insensitive resistance profiles during a 500‐cycle fatigue test. A single layer of stretchable S‐MXene conductors demonstrate a strain‐invariant EMI shielding performance of ≈30 dB up to 800% areal strain, and the shielding performance is further improved to ≈47 and ≈52 dB by stacking 5 and 10 layers of S‐MXene conductors, respectively. Additionally, a stretchable S‐MXene dipole antenna is fabricated, which can be uniaxially stretched to 150% with unaffected reflected power <0.1%. By integrating S‐MXene EMI shields with stretchable S‐MXene antennas, a wearable wireless system is finally demonstrated that provides mechanically stable wireless transmission while attenuating EM absorption by the human body.     

Hybrid Metasurfaces of Plasmonic Lattices and 2D Materials

Plasmonic metasurfaces can significantly enhance the interaction between light and 2D materials. Hybrid structures of plasmonic lattices and 2D materials show great promise for both fundamental and practical studies because of their unprecedented ability for precise manipulation of light at the nanoscale. This review starts with an overview of the basic concepts of plasmonic lattices and optical properties of 2D materials, as well as fabrication strategies for hybrid metasurfaces. Then, the enhanced photoluminescence, quantum emission, optoelectronic detection, nonlinear process, and valleytronics in hybrid metasurfaces are summarized, and their development for nanophotonic functional devices are reviewed. Further, several compelling topics are also outlined that provide outlooks for future directions of hybrid metasurfaces such as novel structural design and high-quality fabrication, all-dielectric metasurfaces, dynamic metasurfaces, and plasmonic mediation of chemical reactions and physical processes. It is believed that hybrid metasurfaces of plasmonic lattices and 2D materials can open prospects for versatile platforms for light-matter interactions and contribute to the revolutions on nanophotonic devices.     

Graphene-Enhanced Metal Transfer Printing for Strong van der Waals Contacts between 3D Metals and 2D Semiconductors

2D semiconductors have shown great potentials for ultra-short channel field-effect transistors (FETs) in next-generation electronics. However, because of intractable surface states and interface barriers, it is challenging to realize high-quality contacts with low contact resistances for both p- and n- 2D FETs. Here, a graphene-enhanced van der Waals (vdWs) integration approach is demonstrated, which is a multi-scale (nanometer to centimeter scale) and reliable (≈100% yield) metal transfer strategy applicable to various metals and 2D semiconductors. Scanning transmission electron microscopy imaging shows that 2D/2D/3D semiconductor/graphene/metal interfaces are atomically flat, ultraclean, and defect-free. First principles calculations indicate that the sandwiched graphene monolayer can eliminate gap states induced by 3D metals in 2D semiconductors. Through this approach, Schottky barrier-free contacts are realized on both p- and n-type 2D FETs, achieving p-type MoTe₂, p-type black phosphorus and n-type MoS₂ FETs with on-state current densities of 404, 1520, and 761 µA µm⁻¹, respectively, which are among the highest values reported in literature.     

Flexible Ti3C2Tx@Al electrodes with Ultrahigh Areal Capacitance: In Situ Regulation of Interlayer Conductivity and Spacing

Although Ti₃C₂ MXene has shown great potential in energy storage field, poor conductivity and restacking between MXene flakes seriously hinders the maximization of its capacitance. Herein, a new strategy to solve the problems is developed. Gallery Al atoms in Ti₃AlC₂ are partially removed by simple hydrothermal etching to get Ti₃C₂T_x reserving appropriate Al interlayers (Ti₃C₂T_x@Al). Ti₃C₂T_x@Al keeps stable layered structure rather than isolated Ti₃C₂T_x flakes, which avoids flake restacking. The removal of partial Al frees up space for easy electrolyte infiltration while the reserved Al as “electron bridges” ensures high interlayer conductivity. As a result, the areal capacitance reaches up to 1087 mF cm^?2 at 1 mA cm^?2 and over 95% capacitance is maintained after 6000 cycles. The all‐solid‐state supercapacitor (ASSS) based on Ti₃C₂T_x@Al delivers a high capacitance of 242.3 mF cm^?2 at 1 mV s^?1 and exhibits stable performance at different bending states. Two ASSSs in tandem can light up a light‐emitting diode under the planar or wrapping around an arm. The established strategy provides a new avenue to improve capacitance performances of MXenes.     

2D Titanium Carbide (MXene) Based Films: Expanding the Frontier of Functional Film Materials

2D titanium carbide (Ti₃C₂T_x) MXene films, with their well-defined microstructures and chemical functionality, provide a macroscale use of nano-sized Ti₃C₂T_x flakes. Ti₃C₂T_x films have attractive physicochemical properties favorable for device design, such as high electrical conductivity (up to 20 000 S cm^–1), impressive volumetric capacitance (1500 F cm^–3), strong in-plane mechanical strength (up to 570 MPa), and a high degree of flexibility. Here, the appealing features of Ti₃C₂T_x-based films enabled by the layer-to-layer arrangement of nanosheets are reviewed. We devote attention to the key strategies for actualizing desirable characteristics in Ti₃C₂T_x-based functional films, such as high and tunable electrical conductivity, outstanding mechanical properties, enhanced oxidation-resistance and shelf life, hydrophilicity/hydrophobicity, adjustable porosity, and convenient processability. This review further discusses fundamental aspects and advances in the applications of Ti₃C₂T_x-based films with a focus on illuminating the relationship between the structural features and the resulting performances for target applications. Finally, the challenges and opportunities in terms of future research, development, and applications of Ti₃C₂T_x-based films are suggested. A comprehensive understanding of these competitive features and challenges shall provide guidelines and inspiration for the further development of Ti₃C₂T_x-based functional films, and contribute to the advances in MXene technology.     

High Linearity,Low Hysteresis Ti3C2Tx MXene/AgNW/Liquid Metal Self-Healing Strain Sensor Modulated by Dynamic Disulfide and Hydrogen Bonds

Flexible wearable strain sensors have received extensive attention in human–computer interaction, soft robotics, and human health monitoring. Despite significant efforts in developing stretchable electronic materials and structures, developing flexible strain sensors with stable interfaces and low hysteresis remains a challenge. Herein, Ti₃C₂T_x MXene/AgNWs/liquid metal strain sensors (MAL strain sensor) with self-healing function are developed by exploiting the strong interactions between Ti₃C₂T_x MXene/AgNWs/LM and the disulfide and hydrogen bonds inside the self-healing poly(dimethylsiloxane) elastomers. AgNWs lap the Ti₃C₂T_x MXene sheets, and the LM acts as a bridge to increase the lap between Ti₃C₂T_x MXene and AgNWs, thereby improving the interface interaction between them and reducing hysteresis. The MAL strain sensor can simultaneously achieve high sensitivity (gauge factor for up to 3.22), high linearity (R² = 0.98157), a wide range of detection (e.g., 1%–300%), a fast response time (145 ms), excellent repeatability, and stability.In addition, the MAL strain sensor before and after self-healing is combined with a small fish and an electrothermally driven soft robot, respectively, allowing real-time monitoring of the swinging tail of the small fish and the crawling of the soft robot by resistance changes.     

Periodical Ripening for MOCVD Growth of Large 2D Transition Metal Dichalcogenide Domains

2D semiconductors, especially 2D transition metal dichalcogenides (TMDCs), have attracted ever-growing attention toward extending Moore's law beyond silicon. Metal–organic chemical vapor deposition (MOCVD) has been widely considered as a scalable technique to achieve wafer-scale TMDC films for applications. However, current MOCVD process usually suffers from small domain size with only hundreds of nanometers, in which dense grain boundary defects degrade the crystalline quality of the films. Here, a periodical varying-temperature ripening (PVTR) process is demonstrated to grow wafer-scale high crystalline TMDC films by MOCVD. It is found that the high-temperature ripening significantly reduces the nucleation density and therefore enables single-crystal domain size over 20 µm. In this process, no additives or etchants are involved, which facilitates low impurity concentration in the grown films. Atom-resolved electron microscopy imaging, variable temperature photoluminescence (PL) spectroscopy, and electrical transport results further confirm comparable crystalline quality to those observed in mechanically exfoliated TMDC films. The research provides a scalable route to produce high-quality 2D semiconducting films for applications in electronics and optoelectronics.