Surface Termination Research Articles

Assembly of Multi-Dimensional Microstructures of MXene towards Wearable Electromagnetic Attenuating Devices

MXene is a competitive and attractive 2D material used in wearable electromagnetic devices due to its laminated nanostructure, abundant surface terminations and high conductivity. Assembling MXene nanosheets into multi-dimensional microstructures is considered to be an effective method for improving the overall performance of MXene-based composites, especially their electromagnetic attenuation and wearability performance. This article focuses on the strategies for assembling multi-dimensional MXene microstructures, from 0D spheres and 1D fibers to 2D films and 3D architectures. The origin of the electromagnetic response of MXene microstructures is revealed, and the corresponding electromagnetic absorption and interference shielding performance are shown. Furthermore, additional extended functions that serve wearable electromagnetic attenuation devices are highlighted. Finally, the application prospects and challenges of wearable electromagnetic devices, as well as the function integration of multi-dimensional MXene composites, are summarized.

Tunable electronic properties and Schottky barrier in Janus Ti3C2FO and TMD heterostructures by interface atomic species and disorder

In two-dimensional (2D) Janus MXenes materials, asymmetric disordered surface termination can greatly enhance material functionality. In this work, considering surface termination composition and disorder in 2D Janus MXenes, we designed five models of Janus Ti3C2FO, and then systematically investigated the electronic and contact properties of the corresponding Janus Ti3C2FO and TMD (TMD=WS2, MoS2 and WSe2) heterostructures using the density functional theory method. When the top and bottom surfaces of Janus Ti3C2FO are purely occupied by F and O atoms, respectively, Janus Ti3C2FO shows a pseudogap, which disappears in the presence of the mixed F and O atoms. It was found that the surface termination atomic species and disorder of Janus Ti3C2FO can effectively control the contact nature and Schottky barrier height (SBH) of these Ti3C2FO/TMD heterostructures and lead to a different response of their contact properties to the interlayer distance and external electric field. The presence of pure F atoms at the interface induces in the SBHs a strong dependence on the decrease in interlayer distance, while the pure O atoms causes both ΦB,n and ΦB,p to be almost pinned. Our study provides important new insight into the tuning of contact properties of MXenes/TMD heterostructures.

Atomic Scale Design of MXenes and Their Parent Materials─From Theoretical and Experimental Perspectives.

More than a decade after the discovery of MXene, there has been a remarkable increase in research on synthesis, characterization, and applications of this growing family of two-dimensional (2D) carbides and nitrides. Today, these materials include one, two, or more transition metals arranged in chemically ordered or disordered structures of three, five, seven, or nine atomic layers, with a surface chemistry characterized by surface terminations. By combining M, X, and various surface terminations, it appears that a virtually endless number of MXenes is possible. However, for the design and discovery of structures and compositions beyond current MXenes, one needs suitable (stable) precursors, an assessment of viable pathways for 3D to 2D conversion, and utilization or development of corresponding synthesis techniques. Here, we present a critical and forward-looking review of the field of atomic scale design and synthesis of MXenes and their parent materials. We discuss theoretical methods for predicting MXene precursors and for assessing whether they are chemically exfoliable. We also summarize current experimental methods for realizing the predicted materials, listing all verified MXenes to date, and outline research directions that will improve the fundamental understanding of MXene processing, enabling atomic scale design of future 2D materials, for emerging technologies.

Stacking and spin order in a van der Waals Mott insulator 1T-TaS2

Quasi-two-dimensional charge density wave system of 1T-TaS2 has attracted recent interest due to topological excitations, emergent superconductivity, ultrafast synaptic functionality, and the possibility of a quantum spin liquid state. While electron correlation has been known to be essential in this system, the nature of its insulating phase is currently under debate. Here, we reinvestigate the origin of the insulating band structures of the 1T-TaS2 surface using density-functional theory calculations to consider the recently-raised issues such as interlayer coupling, surface effect, and interlayer spin ordering. We identify four distinct electronic states of the surface layer such as a 2D Mott phase, a strongly-coupled antiferromagnetic insulator, a weakly-coupled ferromagnetic insulator, and a small-gap semiconductor, depending on types of the surface termination and the interlayer spin configuration. These distinct surface electronic states explain the different sizes of spectroscopic band gaps observed in scanning tunneling microscopy, revealing the complexity of the interlayer charge and spin couplings in layered correlated materials.

Stability and superconductivity of freestanding two-dimensional transition metal boridene: M4/3B2

The small atomic mass of boron indicates strong electron–phonon coupling (EPC), so it may have a brilliant performance in superconductivity. Recently, a new 2D boride sheet with ordered metal vacancies and surface terminals (Mo4/3B2−x ) was realized in experiments (Zhou et al 2021 Science 373 801). Here, the 2D monolayer freestanding Mo4/3B2 is evidenced to be thermodynamically stable. Through electronic structure, phonon spectrum and EPC, monolayer Mo4/3B2 is found to be an intrinsic phonon-mediated superconductor. The superconducting transition temperature (Tc ) is determined to be 4.06 K by the McMillian–Allen–Dynes formula. Remarkably, the Tc of monolayer Mo4/3B2 can be increased to 6.78 K with an appropriate biaxial tensile strain (+5%). Moreover, we predict that other transition metal replacing Mo atoms is also stable and retaining the superconductivity. Such as monolayer W4/3B2 is also a superconductor with the Tc of 2.37 K. Our research results enrich the database of 2D monolayer superconductors and boron-related formed materials science.

Tuning the electronic and optical properties of Ti2X (X = C, N)-based MXenes through surface termination with oxygen and fluorine functional groups

Tuning the electronic and optical properties of Ti2X (X = C, N)-based MXenes through surface termination with oxygen and fluorine functional groups

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio‐Inspired Bimetallic MXene Electrocatalyst

AbstractMo‐ and Fe‐containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO3RR) catalyzed by an Fe‐substituted two‐dimensional molybdenum carbide of the MXene family, viz., Mo2CTx : Fe (Tx are oxo, hydroxy and fluoro surface termination groups). Mo2CTx : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo2CTx) and features a Faradaic efficiency (FE) and an NH3 yield rate of 41 % and 3.2 μmol h−1 mg−1, respectively, for the reduction of NO3− to NH4+ in acidic media and 70 % and 12.9 μmol h−1 mg−1 in neutral media. Regardless of the media, Mo2CTx : Fe outperforms monometallic Mo2CTx owing to a more facile reductive defunctionalization of Tx groups, as evidenced by in situ X‐ray absorption spectroscopy (Mo K‐edge). After surface reduction, a Tx vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO3RR in close analogy to the prevailing mechanism of the natural Mo‐based nitrate reductase enzymes.

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio-Inspired Bimetallic MXene Electrocatalyst.

Mo- and Fe-containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO3 RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo2 CTx : Fe (Tx are oxo, hydroxy and fluoro surface termination groups). Mo2 CTx : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo2 CTx ) and features a Faradaic efficiency (FE) and an NH3 yield rate of 41 % and 3.2 μmol h-1 mg-1 , respectively, for the reduction of NO3 - to NH4 + in acidic media and 70 % and 12.9 μmol h-1 mg-1 in neutral media. Regardless of the media, Mo2 CTx : Fe outperforms monometallic Mo2 CTx owing to a more facile reductive defunctionalization of Tx groups, as evidenced by in situ X-ray absorption spectroscopy (Mo K-edge). After surface reduction, a Tx vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO3 RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.

Surface Termination on Unstable Methylammonium-based Perovskite Using a Steric Barrier for Improved Perovskite Solar Cells.

Compared to widely adopted low-dimensional/three-dimensional (LD/3D) heterostructure, functional organic cation based surface termination on perovskite can not only realize advantage of defect passivation but also prevent potential disadvantage of the heterostructure induced intercalation into 3D perovskite. However, it is still very challenging to controllably construct surface termination on organic-inorganic hybrid perovskite because the functional organic cations' substitution reaction is easy to form LD/3D heterostructure. Here, we report using a novel benzyltrimethylammonium (BTA) functional cation with rational designed steric hindrance to effectively surface terminate onto methylammonium lead triiodide (MAPbI3) perovskite, which is composed of the most unstable MA cations. The BTA cation is difficult to form a specific 1.5-dimensional perovskite of BTA4Pb3I10 by cation substitution with MA cation, which then provides a wide processing window (around 10 minutes) for surface terminating on MAPbI3 films. Moreover, the BTAI surface terminated BTAI-MAPbI3 shows better passivation effect than BTA4Pb3I10-MAPbI3 heterojunction. Finally, BTAI surface terminated solar cell (0.085 cm2) and mini-module (11.52 cm2) obtained the efficiencies of 22.03% and 18.57%, which are among the highest efficiencies for MAPbI3 based ones.

Surface Termination on Unstable Methylammonium‐based Perovskite Using a Steric Barrier for Improved Perovskite Solar Cells

AbstractCompared to widely adopted low‐dimensional/three‐dimensional (LD/3D) heterostructure, functional organic cation based surface termination on perovskite can not only realize advantage of defect passivation but also prevent potential disadvantage of the heterostructure induced intercalation into 3D perovskite. However, it is still very challenging to controllably construct surface termination on organic–inorganic hybrid perovskite because the functional organic cations’ substitution reaction is easy to form LD/3D heterostructure. Here, we report using a novel benzyltrimethylammonium (BTA) functional cation with rational designed steric hindrance to effectively surface terminate onto methylammonium lead triiodide (MAPbI3) perovskite, which is composed of the most unstable MA cations. The BTA cation is difficult to form a specific 1.5‐dimensional perovskite of BTA4Pb3I10 by cation substitution with MA cation, which then provides a wide processing window (around 10 minutes) for surface terminating on MAPbI3 films. Moreover, the BTAI surface terminated BTAI‐MAPbI3 shows better passivation effect than BTA4Pb3I10‐MAPbI3 heterojunction. Finally, BTAI surface terminated solar cell (0.085 cm2) and mini‐module (11.52 cm2) obtained the efficiencies of 22.03 % and 18.57 %, which are among the highest efficiencies for MAPbI3 based ones.

MXene-Integrated Perylene Anode with Ultra-Stable and Fast Ammonium-Ion Storage for Aqueous Micro Batteries.

The aqueous micro batteries (AMBs) are expected to be one of the most promising micro energy storage devices for its safe operation and cost-effectiveness. However, the performance of the AMBs is not satisfactory, which is attributed to strong interaction between metal ions and the electrode materials. Here, the first AMBs are developed with NH4 + as charge carrier. More importantly, to solve the low conductivity and the dissolution during the NH4 + intercalation/extraction problem of perylene material represented by perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), the Ti3 C2 Tx MXene with high conductivity and polar surface terminals is introduced as a conductive skeleton (PTCDA/Ti3 C2 Tx MXene). Benefitting from this, the PTCDA/Ti3 C2 Tx MXene electrodes exhibit ultra-high cycle life and rate capability (74.31% after 10000 galvanostatic chargedischarge (GCD) cycles, and 91.67 mAh g-1 at 15.0 A g-1 , i.e., capacity retention of 45.2% for a 30-fold increase in current density). More significantly, the AMBs with NH4 + as charge carrier and PTCDA/Ti3 C2 Tx MXene anode provide excellent energy density and power density, cycle life, and flexibility. This work will provide strategy for the development of NH4 + storage materials and the design of AMBs.

Laminated 2D-Ti3C2-MXenes as high-performance counter electrode for triiodide reduction in dye-sensitized solar cells

The different surface terminals, sizes, and sheet quality of laminated 2D-Ti3C2-MXenes were prepared by selective etching intermediate Al atoms layer form the strongly bonded compound Ti3AlC2 as precursor at low temperature using different etchants of HF, HF + HCl, LiF + HCl, and NH4HF2 for 24 h, respectively. Then, 2D-Ti3C2-MXenes were further instead of traditional Pt as counter electrodes (CEs) in triiodide reduction for dye-sensitized solar cells (DSSCs). The selection of different etchant directly affects the properties and performance of the laminated 2D-Ti3C2-MXenes catalyst. Herein, the DSSCs with 2D-Ti3C2-MXenes CEs yielded power conversion efficiencies (PCE) of 7.15 % (HF-), 6.06 % (HF + HCl-), 6.38 % (LiF + HCl-), and 6.05 % (NH4HF2-) for regenerate traditional I3−/I− shuttles via the photocurrent-voltage (J-V) test, respectively. The outstanding electrocatalytic performances of laminated 2D-Ti3C2-MXenes can be attributed to the formation of regular inter-stratification structures by etching to expose larger specific surface areas and further generate more catalytic reaction sites, as well as the acceleration of I3−/I− shuttle diffusion channels by adjusting the interlayer d-spacing, implying a potentially candidate of Pt catalyst for applications in the encapsulated DSSCs as Pt-free CEs.

Combined experimental and DFT study on the adsorption of congo red dye using self-assembled hierarchical microspheres of lanthanum hydroxide

Self-assembled hierarchical microspheres of lanthanum hydroxide (La(OH)3) were used to investigate the adsorption of the azo-dye Congo Red (C.R.) in an aqueous medium. The versatile coordination chemistry of lanthanum ions makes them very efficient in adsorption processes involving organic ligands. The microspheres were synthesized using the reaction-diffusion framework (RDF) at ambient temperature, which consists of diffusing a concentrated ammonia solution onto an agar hydrogel matrix containing the dissolved lanthanum salt. The microspheres, which emerged with distinct Liesegang bands ranging in size from ∼300 nm to ∼70 µm, demonstrated rapid adsorption, reaching equilibrium within 15 min, with optimal adsorption of 395 mg g–1 at pH 6 and 30 °C. The kinetic and thermodynamic data were analyzed and fitted to a pseudo-second-order equation and the Sips model. The adsorption mechanism, driven by the dye's structure, the adsorbent's surface chemistry, and the medium, involves complex interplays of electrostatic attractions, physisorption, and chemisorption, with water playing a pivotal role. The positive enthalpy of adsorption extracted from a Sips isotherm, diverging from Langmuir behavior, and FTIR spectral comparisons pre- and post-adsorption provide empirical insights. Density functional theory (DFT) calculations, complemented by implicit solvation models, reveal significant solvation effects on the adsorption enthalpy of the sulfonate group of C.R. on the La(OH)3 surface, highlighting the solvent's vital role. The adsorption enthalpy of +62.63 kJ mol–1 closely aligns with the primary termination of the lanthanum hydroxide surface by water molecules. Remarkably, the bond length between the surface lanthanum and water's oxygen undergoes a significant shift when bonded to the sulfonate group. This study offers a deeper understanding of C.R. dye adsorption dynamics, emphasizing both experimental findings and theoretical insights, underscoring the complex interplay of forces and the central role of water in the process.

Hydrogen-mediated polarity compensation on the (110) surface terminations of ABO3 perovskites.

Polar surfaces undergo polarity compensation, which can lead to significantly different surface chemistry from their nonpolar counterparts. This process in turn can substantially alter the binding of adsorbates on the surface. Here, we find that hydrogen binds much more strongly to the polar (110) surface than the nonpolar (100) surface for a wide range of ABO3 perovskites, forming a hydroxyl layer on the O24- termination and a hydride layer on the ABO4+ termination of the (110) surface. The stronger adsorption on the polar surfaces can be explained by polarity compensation: hydrogen atoms can act as electron donors or acceptors to compensate for the polarity of perovskite surfaces. The relative stability of the surface terminations is further compared under different gas environments and several perovskites have been found to form stable surface hydride layers under oxygen-poor conditions. These results demonstrate the feasibility of creating stable surface hydrides on perovskites by polarity compensation which might lead to new hydrogenation catalysts based on ABO3 perovskites.

Theoretical prediction of hafnium-terminated diamond (100) surface with promising negative electron affinity for electron emission applications

Controlling the chemical termination of diamond surface shows great promise in achieving negative electron affinity (NEA). In this work, we explore the feasibility of obtaining stable NEA properties by hafnium (Hf) termination on bare and oxidized diamond (100) surfaces based on swarm-intelligence structural search and first-principles calculations. The negative adsorption energies demonstrate that Hf-terminated diamond surfaces are energetically favorable. Remarkably, the lowest-energy structure at 0.25 monolayer Hf coverage on an oxidized surface has an extremely large adsorption energy of −11.15 eV, which is consistent with the largest bonding orbital population of 7.28 eV. The most negative NEA values of bare and oxidized surfaces are −3.50 and −3.19 eV at 0.25 monolayer Hf coverage, respectively. Both structures have dynamic stability, and the oxidized surface exhibits excellent thermal stability even up to 1200 K. Owing to their super stability and outstanding NEA properties, Hf-terminated diamond surfaces are suggested as promising candidate materials for electron emission applications.

Effect of mixed surface terminations on the work function and quantum capacitance of Sc2CT2 monolayer

MXenes are two-dimensional transition metal carbides and nitrides, which are promising for supercapacitor applications. The surface of MXenes is usually covered by a mixture of O, OH, and F groups during etching. The effect of mixed termination on the quantum capacitance and work function (WF) of MXenes is investigated by first principles calculation. The WFs of Sc2C(OH)2-R are smaller than those of Sc2CO2-R and Sc2CF2-R when the OH ratio is larger than about 27 %. The quantum capacitances of Sc2C MXenes with different ratio of F, O, and OH groups (named as Sc2CF2-R, Sc2CO2-R, Sc2C(OH)2-R) are explored to predict the electrode type of materials. Negative binding energies confirm the stability of the studied mixed systems. The increasing ratio of O group can make the system have decreased TDOS near Fermi level, thus decreasing quantum capacitance. The top quantum capacitance of Sc2CO2-R and Sc2C(OH)2-R systems at negative bias increase with the increasing ratio of O/OH group. All systems have larger storage charge at negative potential, and are all potential cathode materials, especially for Sc2CO2-R and Sc2CF2-R MXene with much smaller |Qa|/|Qc| than 1 and larger Cc. The quantum capacitance under wide voltage is also considered.

Hydrogen and oxygen mixed-termination for nitrogen-vacancy quantum sensors in hexagonal diamond

Controlling the chemical termination of hexagonal diamond surface opens up new possibilities for shallow nitrogen-vacancy (NV) centers in quantum sensing applications. In this work, the H/O/OH mixed terminations on hexagonal diamond (100) surfaces to host NV centers are investigated by first-principles calculations. Results indicate that H–O, O–OH, and H–O–OH mixed surfaces produce positive electron affinity with neither spin noise nor surface-related state. It is concluded that the surface-connected ether bonds should be maintained, and unconnected ether bonds should be avoided, preserving the shallow NV centers unaffected. Based on the surface stability analysis, we further confirm that the H–O surface has a higher possibility to be fabricated. Moreover, we make an intensive study by placing an NV center ∼12 Å from the hexagonal (100) surface, and the results elucidate that the H–O surface has no obvious effect on the defect states of the NV center. Therefore, we identify a combination of surface terminators that is a prospective candidate for NV-based quantum sensors.

Bioinspired Design Rules from Highly Mineralized Natural Composites for Two-Dimensional Composite Design.

Discoveries of two-dimensional (2D) materials, exemplified by the recent entry of MXene, have ushered in a new era of multifunctional materials for applications from electronics to biomedical sensors due to their superior combination of mechanical, chemical, and electrical properties. MXene, for example, can be designed for specialized applications using a plethora of element combinations and surface termination layers, making them attractive for highly optimized multifunctional composites. Although multiple critical engineering applications demand that such composites balance specialized functions with mechanical demands, the current knowledge of the mechanical performance and optimized traits necessary for such composite design is severely limited. In response to this pressing need, this paper critically reviews structure-function connections for highly mineralized 2D natural composites, such as nacre and exoskeletal of windowpane oysters, to extract fundamental bioinspired design principles that provide pathways for multifunctional 2D-based engineered systems. This paper highlights key bioinspired design features, including controlling flake geometry, enhancing interface interlocks, and utilizing polymer interphases, to address the limitations of the current design. Challenges in processing, such as flake size control and incorporating interlocking mechanisms of tablet stitching and nanotube forest, are discussed along with alternative potential solutions, such as roughened interfaces and surface waviness. Finally, this paper discusses future perspectives and opportunities, including bridging the gap between theory and practice with multiscale modeling and machine learning design approaches. Overall, this review underscores the potential of bioinspired design for engineered 2D composites while acknowledging the complexities involved and providing valuable insights for researchers and engineers in this rapidly evolving field.

Phase Stability of MXenes: Understanding the Role of Coordination Symmetries, Transition Metals, and Surface Terminations

Phase Stability of MXenes: Understanding the Role of Coordination Symmetries, Transition Metals, and Surface Terminations

Mean inner potential of elemental crystals from density-functional theory calculations: Efficient computation and trends

The mean inner potential (V0) of crystals plays an important role in electron microscopy. In a few cases, it has been measured experimentally, using mostly electron holography; however, it is not uncommon to find reports that disagree by a few volts regarding the mean inner potential of the same material. Different levels of theory have also been used to estimate its value, often by building the crystal as a superposition of isolated atoms or ions—an independent-atom approximation that does not take bonding into account. In a few cases, density-functional theory (DFT) calculations were done to capture such bonding, frequently using computer-intensive all-electron approaches. In this article, we describe in detail a faster implementation based on postprocessing files produced by a DFT code that relies on the projector-augmented wave method. We deployed this approach to compute values of V0 for 44 elemental solids, and we provide the first quantum-mechanical calculation of the mean inner potential beyond the independent-atom approximation for many of them. We also report instances in which different surface terminations for the same material led to differences in V0 of more than 3 V, highlighting the dependence of the mean inner potential on the boundary conditions of the sample. Finally, by comparing our values of V0 with other material properties, we show that it correlates mostly linearly with the mass density, that it can be used to compute a good approximation to the orbital diamagnetic contribution to the magnetic susceptibility, and that it provides a simple route to compute atomic scattering amplitudes for forward scattering of electrons.