Jing Zhang

A library of the dielectric functions of the ten binary alloys formed by combinations of gold, silver, copper, palladium, and platinum is presented. Accuracy is confirmed by comparison of calculated and measured optical extinction spectra. The results reveal non‐trivial trends that pave the way for optimization of a wide range of nanophotonic applications.

Abstract

Accurate complex dielectric functions are critical to accelerate the development of rationally designed metal alloy systems for nanophotonic applications, and to thereby unlock the potential of alloying for tailoring nanostructure optical properties. To date, however, accurate alloy dielectric functions are widely lacking. Here, a time‐dependent density‐functional theory computational framework is employed to compute a comprehensive binary alloy dielectric function library for the late transition metals most commonly employed in plasmonics (Ag, Au, Cu, Pd, Pt). Excellent agreement is found between electrodynamic simulations based on these dielectric functions and selected alloy systems experimentally scrutinized in 10 at% composition intervals. Furthermore, it is demonstrated that the dielectric functions can vary in very non‐linear fashion with composition, which paves the way for non‐trivial optical response optimization by tailoring material composition. The presented dielectric function library is thus a key resource for the development of alloy nanomaterials for applications in nanophotonics, optical sensors, and photocatalysis.

This review summarizes recent advances in the rapidly developing fields related to the Moiré‐pattern‐tuned electronic structures of van der Waals heterostructures, including Moiré bands, Dirac cones, surface reconstruction, Moiré excitons, Moiré valleytronics, stoner magnetism, superconductivity, and superfluidity. Some potential applications based on the fascinating characteristics of the Moiré pattern are also presented.

Abstract

Moiré patterns are quasi‐periodic geometric patterns generated by the incommensurate stacking between two monolayers; they have rapidly attracted enormous attention due to their profound ability to modulate the electronic properties of 2D materials. For instance, the Bloch band of the Moiré superlattice, which is known as the Moiré band, can become flat at a specific series of discrete angles, and these flat bands are capable of exhibiting strong correlation behaviors such as the high‐temperature superconductivity reported recently. Moiré patterns can alter electronic properties, while surface reconstruction can modify Moiré patterns. In this review, the fundamental geometry is discussed and the basic electronic structure modification is summarized. Surface reconstruction is a method of tuning the electronic properties of a Moiré superlattice. Strong correlation phenomena, such as superconductivity, superfluidity, and magnetism induced by the flat bands, have been confirmed experimentally in recent years, which will be discussed in detail. Some possible application opportunities based on the fascinating characteristics of the Moiré pattern will also be presented. Because of the growing interest in Moiré patterns and related physical phenomena, it is anticipated that a deeper understanding of the fundamental physics of Moiré systems and further progress in the investigation of strong correlation phenomena are forthcoming.

The structural superlubricity (SSL), a state of near-zero friction between two contacted solid surfaces, has been attracting rapidly increasing research interest since it was realized in microscale graphite in 2012. An obvious question concerns the implications of SSL for micro- and nanoscale devices such as actuators. The simplest actuators are...

In two-dimensional (2D) solids, point defects, i.e., vacancies and interstitials, are bound states of topological defects of edge dislocations and disclinations. They are expected to play an important role in the thermodynamics of the system. Yet very little is known about the detailed dynamical processes of these defects. Two-dimensional colloidal...

The design rules for a graphene ferroelectric transistor with multi‐level resistance states through patterning the ferroelectric gate are outlined and validated. The multiplicity is demonstrated in transistors with polymeric ferroelectric gate insulator. The methodology to create deterministic multiple resistance states in transistors is generic and applicable to other material systems that are envisioned for nonvolatile memory and neuromorphic applications.

Abstract

Conventional memory elements code information in the Boolean “0” and “1” form. Devices that exceed bistability in their resistance are useful as memory for future data storage due to their enhanced memory capacity, and are also a necessity for contemporary applications such as neuromorphic computing. Here, with the aid of an experimentally validated device model, design rules are outlined and more than two stable resistance states in a graphene ferroelectric field‐effect transistor are experimentally demonstrated. The design methodology can be extrapolated for on‐demand introduction of multiple resistance states in ferroelectric transistors for applications both in data storage and neuromorphic computing.

Nature Nanotechnology, Published online: 25 May 2020; doi:10.1038/s41565-020-0682-9

Lattice reconstruction in twisted transition metal dichalcogenides manifest in intrinsic asymmetry of electronic wavefunctions for 3R homo-bilayers and strong piezoelectric textures in 2H homo-bilayers.

Upon photon excitation of 2D materials, the temperatures of optical and acoustic phonons are distinguished by constructing different interphonon energy transport states. A breakthrough is made on measuring the intrinsic in‐plane thermal conductivity of 2D materials by excluding interphonon branch thermal nonequilibrium. The phonon branch energy cooping factor is determined and agrees well with theoretical prediction.

Abstract

Under photon excitation, 2D materials experience cascading energy transfer from electrons to optical phonons (OPs) and acoustic phonons (APs). Despite few modeling works, it remains a long‐history open problem to distinguish the OP and AP temperatures, not to mention characterizing their energy coupling factor (G ). Here, the temperatures of longitudinal/transverse optical (LO/TO) phonons, flexural optical (ZO) phonons, and APs are distinguished by constructing steady and nanosecond (ns) interphonon branch energy transport states and simultaneously probing them using nanosecond energy transport state‐resolved Raman spectroscopy. ΔT _{OP −AP} is measured to take more than 30% of the Raman‐probed temperature rise. A breakthrough is made on measuring the intrinsic in‐plane thermal conductivity of suspended nm MoS₂ and MoSe₂ by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman‐based thermal conductivity measurement of 2D materials. G _OP↔AP for MoS₂, MoSe₂, and graphene paper (GP) are characterized. For MoS₂ and MoSe₂, G _OP↔AP is in the order of 10¹⁵ and 10¹⁴ W m⁻³ K⁻¹ and G _ZO↔AP is much smaller than G _LO/TO↔AP. Under ns laser excitation, G _OP↔AP is significantly increased, probably due to the reduced phonon scattering time by the significantly increased hot carrier population. For GP, G _LO/TO↔AP is 0.549 × 10¹⁶ W m⁻³ K⁻¹, agreeing well with the value of 0.41 × 10¹⁶ W m⁻³ K⁻¹ by first‐principles modeling.

Photochemical activity of black phosphorus (BP) for controlled in situ biomineralization is investigated. Near infrared (NIR) light can promote the degradation of BP and enhance their chemical activity to accelerate the mineralization process. BP@Hydrogel with NIR irradiation exhibit greatly improved biomineralization and can be controlled by modulating the irradiation time and location, thus promising high potential in bioengineering.

Abstract

The photochemical activity of black phosphorus (BP) in near‐infrared (NIR) light controlled in situ biomineralization is investigated. Owing to the excellent NIR absorption, irradiation with NIR light not only promotes degradation of BP into PO₄ ³⁻, but also enhances the chemical activity to accelerate the reaction between PO₄ ³⁻ and Ca²⁺ and promote in situ biomineralization. Mineralization of hydrogels is demonstrated by the preparation of BP incorporated hydrogel (BP@Hydrogel) which delivers greatly improved biomineralization performance under NIR illumination. The biomineralization process which can be controlled by modulating the light irradiation time and location has a high potential in controlling the mechanical properties and osteoinductive ability in tissue engineering. This study also provides insights into the degradation, photochemical activity, and new biological/biomedical applications of BP.

Nature Nanotechnology, Published online: 25 May 2020; doi:10.1038/s41565-020-0695-4

Super-resolution microscopy of defects in a two-dimensional material unveils the transport of single proton charges at solid/water interfaces.

In article number https://doi.org/10.1002/adfm.2020005742000574, Achim Harzheim, Pascal Gehring, and co‐workers report U‐shaped graphene strips comprising one wide and one narrow leg that form a single material thermocouple due to the increased influence of electron edge scattering in a narrow channel. This leads to different Seebeck coefficients in the two legs, which for the devices tested, yield a maximum sensitivity of ΔS ≈ 39 μV K⁻¹.

Correlated 4d² SrMoO₃ thin films appeal for oxygen‐vacancy‐endurable transparent conductors. Strong correlation effects between 4d electrons cause persistent metallicity even in 10 nm thin films with enhanced transmittance. These films have a higher ultraviolet–visible transmittance and lower sheet resistance than those of 3d¹ SrVO₃ due to the reduced binding energy of the 4d orbitals and higher carrier density, respectively.

Abstract

Degenerately doped wide‐bandgap semiconductors, e.g., Sn‐doped In₂O₃, are the most conventional transparent conductors (TCs), but degradation of the TC performance by a doping bottleneck or instability due to oxygen vacancies is encountered. Recently, nondoped correlated metals have attracted great attention as a new strategy for developing next‐generation TCs. To date, most studies of this brand‐new type of TC have been biased toward 3d¹ vanadates. Here, compared with 3d¹ SrVO₃, it is found that the 4d² SrMoO₃ thin films show promising TC properties: higher ultraviolet–visible transmittance of 80% and extremely low resistivity of 100 µΩ cm at room temperature. This enhancement in the SrMoO₃ is ascribed to a p‐4d transition occurring at higher photon energy and a higher number of electrons in the outermost 4d orbitals, respectively. In addition, the TC properties of the correlated SrMoO₃ are resistive to oxygen vacancies. Using spectroscopic ellipsometry, it is found that this robustness is attributed to the lack of formation of defect states near the Fermi level, which is different from the observation in conventional TCs. Taken together, the correlated 4d² SrMoO₃ is appealing for next‐generation oxygen‐vacancy‐endurable conductors with enhanced transparency.

Solution‐phase exfoliated graphene has the potential to be a flexible, non‐toxic, and elementally abundant thermoelectric material, but is typically limited by its poor power factor and lack of effective doping strategies. Here, non‐oxidized graphene flakes with adsorbed dopants are shown to produce extremely high power factors for both n‐type and p‐type sides of a thermoelectric device.

Abstract

Solution‐phase exfoliated graphene has always been an attractive material for flexible thermoelectric applications, but traditional oxidative routes suffer from poor flake quality and a lack of quality doping techniques to make complementary n‐type and p‐type films. Here, it is demonstrated that by changing the adsorbed surfactant during the intercalation‐exfoliation process (polyvinylpyrrolidone for n‐type, pyrenebutyric acid for p‐type), both extremely high electrical conductivity (3010 and 2330 S cm⁻¹) and high Seebeck coefficients (53.1 and −45.5 µV K⁻¹) can be achieved. The result is that both of these films show remarkable power factors, over 600 µW m⁻¹ K⁻² at room temperature, which is over an order of magnitude better than that in previous works demonstrating complementary n‐type and p‐type graphene thermoelectric films. Based on these films, a full all‐graphene thermoelectric device is constructed as a proof of concept, where a peak power of 5.0 nW is recorded at a temperature difference of 50 K.

A high‐quality single crystalline freestanding Fe₃O₄ thin film with strong magnetism has been synthesized by pulsed laser deposition using water‐dissolvable Sr₃Al₂O₆ sacrificial layer, and the resulting freestanding film is highly flexible. When transferred to the polydimethylsiloxane support layer, the Fe₃O₄ film can be bent with large deformation without affecting its magnetization, demonstrating its robust magnetism.

Abstract

Magnetic materials and devices that can be folded and twisted without sacrificing their functional properties are highly desirable for flexible electronic applications in wearable products and implantable systems. In this work, a high‐quality single crystalline freestanding Fe₃O₄ thin film with strong magnetism has been synthesized by pulsed laser deposition using a water‐dissolvable Sr₃Al₂O₆ sacrificial layer, and the resulting freestanding film, with magnetism confirmed at multiple length scales, is highly flexible with a bending radius as small as 7.18 µm and twist angle as large as 122°, in sharp contrast with bulk magnetite that is quite brittle. When transferred to a polydimethylsiloxane support layer, the Fe₃O₄ film can be bent with large deformation without affecting its magnetization, demonstrating its robust magnetism. The work thus offers a viable solution for flexible magnetic materials that can be utilized in a range of applications.

All‐optical synapses based on a 2D Bi₂O₂Se/graphene hybrid structure can yield positive photoresponses under visible light and negative photoresponses under 365 nm illumination without the extra electrical control. Contributing to this unique optoelectronic property, the single two‐terminal device with fully optical operations is demonstrated for the photodetector, optoelectronic synapses, and optical logic functions.

Abstract

Neuromorphic computing has been extensively studied to mimic the brain functions of perception, learning, and memory because it may overcome the von Neumann bottleneck. Here, with the light‐induced bidirectional photoresponse of the proposed Bi₂O₂Se/graphene hybrid structure, its potential use in next‐generation neuromorphic hardware is examined with three distinct optoelectronic applications. First, a photodetector based on a Bi₂O₂Se/graphene hybrid structure presents positive and negative photoresponsibility of 88 and −110 A W⁻¹ achieved by the excitation of visible wavelength and ultraviolet wavelength light at intensities of 1.2 and 0.3 mW cm⁻², respectively. Second, this unique photoresponse contributes to the realization of all optically stimulated long‐term potentiation or long‐term depression to mimic synaptic short‐term plasticity and long‐term plasticity, which are attributed to the combined effect of photoconductivity, bolometric, and photoinduced desorption. Third, the devices are applied to perform digital logic functions, such as “AND” and “OR,” using full light modulation. The proposed Bi₂O₂Se/graphene‐based optoelectronic device represents an innovative and efficient building block for the development of future multifunctional artificial neuromorphic systems.

The dielectric and magnetic polarizations of quantum paraelectrics and paramagnetic materials have in many cases been found to initially increase with increasing thermal disorder and hence, exhibit peaks as a function of temperature. A quantitative description of these examples of “order-by-disorder” phenomena has remained elusive in nearly ferromagnetic metals and...

Nature Nanotechnology, Published online: 18 May 2020; doi:10.1038/s41565-020-0684-7

Experimental realizations of magnetic skyrmions, particle-like spin swirls with topological protection, so far have required inversion symmetry breaking or a geometrically frustrated lattice. In centrosymmetric GdRu2Si2, in which a geometrically frustrated lattice is absent, a skyrmion lattice phase emerges, which is probably stabilized by four-spin interactions mediated by itinerant electrons in the presence of easy-axis anisotropy.

In article number https://doi.org/10.1002/adfm.2020005612000561, Tiejun Wang and co‐workers show that cobalt‐doping and Mo vacancies on the molybdenum carbide (β‐Mo₂C) surface synergistically tune the electron density and d‐band center, which effectively enhance the hydrogen evolution reaction (HER) performance over the sphere‐type porous β‐Mo₂C‐based catalysts. With an optimal degree of etching treatment, the Co₅₀‐Mo₂C‐12 catalyst exhibits high activity and excellent durability for HER.

A monolayer of CrI 3 is a two-dimensional crystal in its equilibrium configuration is a ferromagnetic semiconductor. In contrast, two coupled layers can be ferromagnetic, or antiferromagnetic depending on the stacking. We study the magnetic phase diagram upon the strain of the antiferromagnetically coupled bilayer with C2/m symmetry. We found that strain can be an efficient tool to tune the magnetic phase of the structure. A tensile strain stabilizes the antiferromagnetic phase, while a compressive strain turns the system ferromagnetic. We associate that behavior to the relative displacement between layers induced by the strain. We also study the evolution of the magnetic anisotropy, the magnetic exchange coupling, and how the Curie temperature is affected by the strain.

α -RuCl 3 , a layered 2D material, was recently identified as a promising candidate for realizing a Kitaev quantum spin liquid. However one fundamental property, the crystal structure, has not been well resolved yet due to difficulty of bulk diffraction techniques caused by layer stacking faults. Furthermore, the surface relaxation of monolayer-level thin films is completely unknown yet. In this report, surface sensitive low energy electron diffraction technique with µ m selectivity ( µ -LEED) combined with dynamical LEED analysis were used to reveal the detailed crystal structure of the surface monolayer of α -RuCl 3 . A surface structural distortion that breaks the inversion symmetry of the ideal bulk structure was revealed. To be specific, we found the surface Cl sub-lattice is buckled with one Cl atom approximately 0.16 Å below the other two Cl atoms, in the unit cell. The Ru atomic layer shows an even larger buckling of approximatel...

The number of two-dimensional (2D) materials has grown steadily since the discovery of graphene. Each new 2D material demonstrated unusual physical properties offering a large flexibility in their tailoring for high-tech applications. Here, we report on the formation and characterization of an uncharted 2D material: ‘Cu 2 Te alloy monolayer on Cu(111) surface’. We have successfully grown a 2D binary Te-Cu alloy using a straightforward approach based on chemical deposition method. Low electron energy diffraction (LEED) and scanning tunneling microscopy (STM) results reveal the existence of a well-ordered alloy monolayer characterized by (√3 × √3)R30° superstructure, while the x-ray photoemission spectroscopy (XPS) measurements indicate the presence of single chemical environment of the Te atoms associated with the Te-Cu bonding. Analysis of the valence band properties by angle resolved photoemission spectroscopy (ARPES); in particular the electronic states close to the F...

Ion irradiation is a versatile tool to introduce controlled defects into two-dimensional (2D) MoS 2 on account of its unique spatial resolution and plethora of ion types and energies available. In order to fully realise the potential of this technique, a holistic understanding of ion-induced defect production in 2D MoS 2 crystals of different thicknesses is mandatory. X-ray photoelectron spectroscopy, electron diffraction and Raman spectroscopy show that thinner MoS 2 crystals are more susceptible to radiation damage caused by 225 keV Xe + ions. However, the rate of defect production in quadrilayer and bulk crystals is not significantly different under our experimental conditions. The rate at which S atoms are sputtered as a function of radiation exposure is considerably higher for monolayer MoS 2 , compared to bulk crystals, leading to MoO 3 formation. P-doping of MoS 2 is observed and attributed to the accept...

The study of topological materials possessing nontrivial band structures enables exploitation of relativistic physics and development of a spectrum of intriguing physical phenomena. However, previous studies of Weyl physics have been limited exclusively to semimetals. Here, via systematic magnetotransport measurements, two representative topological transport signatures of Weyl physics, the negative...

Developing field effect transistors with channels made of 2D semiconducting materials can produce substantial advances in several key technologies. However, their production at the wafer level for industrial application remains a big challenge. This work provides critical advice on materials synthesis, the effect of the electrode/channel contact resistance, dielectric environment, channel length, and channel thickness.

Abstract

The continuous miniaturization of field effect transistors (FETs) dictated by Moore's law has enabled continuous enhancement of their performance during the last four decades, allowing the fabrication of more powerful electronic products (e.g., computers and phones). However, as the size of FETs currently approaches interatomic distances, a general performance stagnation is expected, and new strategies to continue the performance enhancement trend are being thoroughly investigated. Among them, the use of 2D semiconducting materials as channels in FETs has raised a lot of interest in both academia and industry. However, after 15 years of intense research on 2D materials, there remain important limitations preventing their integration in solid‐state microelectronic devices. In this work, the main methods developed to fabricate FETs with 2D semiconducting channels are presented, and their scalability and compatibility with the requirements imposed by the semiconductor industry are discussed. The key factors that determine the performance of FETs with 2D semiconducting channels are carefully analyzed, and some recommendations to engineer them are proposed. This report presents a pathway for the integration of 2D semiconducting materials in FETs, and therefore, it may become a useful guide for materials scientists and engineers working in this field.

This review discusses the progress of key experimental and theoretical components of scanning thermal microscopy including thermal probes, experimental methods, heat transfer mechanisms, and calibration strategies and highlights the recent applications to novel materials and devices, with emphasis on thermoelectric, biological, phase change, and 2D materials.

Abstract

As the size of materials, particles, and devices shrinks to nanometer, atomic, or even quantum scale, it is more challenging to characterize their thermal properties reliably. Scanning thermal microscopy (SThM) is an emerging method to obtain local thermal information by controlling and monitoring probe–sample thermal exchange processes. In this review, key experimental and theoretical components of the SThM system are discussed, including thermal probes and experimental methods, heat transfer mechanisms, calibration strategies, thermal exchange resistance, and effective heat transfer coefficients. Additionally, recent applications of SThM to novel materials and devices are reviewed, with emphasis on thermoelectric, biological, phase change, and 2D materials.

This work introduces a useful protocol for discovering promising 2D insulators for all‐2D transistors based on MoS₂, using density functional theory and quantum dynamics simulations. Promising 2D materials are considered, including 2D SiC, hBN, and BeO. It is demonstrated how this protocol can consider external effects that are present in devices, such as bias potentials and finite temperatures.

Abstract

Two‐dimensional dielectric materials that can inhibit electronic leakage are vital for developing next‐generation all‐2D electronic devices. However, few comprehensive studies of the atomistic nature of 2D insulating dielectrics currently exist. In this work, computational design strategies based on density functional theory and quantum dynamics simulations are used to assess the charge permeability through dielectric materials. Promising 2D dielectrics are considered, including monolayer SiC, hBN, and BeO, which possess promising properties and a honeycomb structure compatible with that of MoS₂, currently the most commonly used channel material in all‐2D transistors. A useful protocol for discovering promising dielectrics is described. The atomic structures of the interfaces are determined and their stabilities are evaluated by studying the interface formation energies and the presence of stress/strain at the interfaces. The interface electronic structures are characterized by studying the band structures, band offsets, and charge transfer at the interface. These important quantities reveal that all three materials chosen are good dielectric materials, but SiC is the poorest among them, BeO has the best insulting properties as a monolayer and hBN prevents the most charge leakage at the interface. It is shown how this protocol can also consider the effects of external potentials and temperatures.

A data‐driven materials science approach utilizing high‐throughput methodologies, including inkjet printing and scanning droplet electrochemical cell measurements, is used to create and evaluate functional passivation and electron‐selective contact materials for p‐type CuBi₂O₄ to solve the common issues of charge carrier recombination and material degradation.

Abstract

Metal oxide semiconductors are promising for solar photochemistry if the issues of excessive charge carrier recombination and material degradation can be resolved, which are both influenced by surface quality and interface chemistry. Coating the semiconductor with an overlayer to passivate surface states is a common remedial strategy but is less desirable than application of a functional coating that can improve carrier extraction and reduce recombination while mitigating corrosion. In this work, a data‐driven materials science approach utilizing high‐throughput methodologies, including inkjet printing and scanning droplet electrochemical cell measurements, is used to create and evaluate multi‐element coating libraries to discover new classes of candidate passivation and electron‐selective contact materials for p‐type CuBi₂O₄. The optimized overlayer (Cu_1.5TiO_z) improves the onset potential by 110 mV, the photocurrent by 2.8×, and the absorbed photon‐to‐current efficiency by 15.5% compared to non‐coated photoelectrodes. It is shown that these enhancements are related to reduced surface recombination through passivation of surface defect states as well as improved carrier extraction efficiency through Fermi level engineering. This work presents a generalizable, high‐throughput method to design and optimize passivation materials for a variety of semiconductors, providing a powerful platform for development of high‐performance photoelectrodes for incorporation into solar‐fuel generation systems.

Ferroelectric BaTiO₃ crystallites are covered by a native cubic TiO _x surface layer of 1–2 nm as is found by chemical and crystallographic cross‐characterization at the atomic scale. Computer modeling supports the observations and suggests that further growth of the TiO _x layer is possible. Such patterning of the metal‐oxide surface is achievable with sub‐nanometer resolution with electron‐beam irradiation.

Abstract

Surfaces and interfaces of ferroelectric oxides exhibit enhanced functionality, and therefore serve as a platform for novel nano and quantum technologies. Experimental and theoretical challenges associated with examining the subtle electro‐chemo‐mechanical balance at metal‐oxide surfaces have hindered the understanding and control of their structure and behavior. Here, combined are advanced electron‐microscopy and first‐principles thermodynamics methods to reveal the atomic‐scale chemical and crystallographic structure of the surface of the seminal ferroelectric BaTiO₃. It is shown that the surface is composed of a native <2 nm thick TiO _x rock‐salt layer in epitaxial registry with the BaTiO₃. Using electron‐beam irradiation, artificial TiO _x sites with sub‐nanometer resolution are successfully patterned, by inducing Ba escape. Therefore, this work offers electro‐chemo‐mechanical insights into ferroelectric surface behavior in addition to a method for scalable high‐resolution beam‐induced chemical lithography for selectively driving surface phase transitions, and thereby functionalizing metal‐oxide surfaces.

Hierarchical Pt–MXene–carbon nanotube (CNT) heterostructures in which Pt is 0D, MXene is 2D, and CNTs are 1D, are constructed by a simple, efficient, yet scalable solution method. The heterostructures enable high stability and low overpotential for hydrogen evolution reaction. This general strategy is expected to construct other hierarchical 0D/2D/1D heterostructures for high‐performance catalysts, sensors and so on.

Abstract

Developing nano‐ or atom‐scale Pt‐based electrocatalysts for hydrogen evolution reaction (HER) is of considerable importance to mitigate the issues associated with low abundance of Pt. Here, a protocol for constructing a hierarchical Pt––MXene–single‐walled carbon nanotubes' (SWCNTs) heterostructure for HER catalysts is presented. In the heterostructure, highly active nano/atom‐scale metallic Pt is immobilized on Ti₃C₂ T_x MXene flakes (MXene@Pt) that are connected with conductive SWCNTs' network. The hierarchical heterostructure is constructed by filtrating a mixed colloidal suspension containing MXene@Pt and SWCNTs. Taking the advantages of the hydrophilicity and reducibility of MXene, the MXene@Pt colloidal suspension is prepared by spontaneously reducing Pt cations into metallic Pt without additional reductants or post‐treatments. The so‐fabricated hierarchical HER catalysts, in the form of membrane, show high stability during 800 h operation, a high volume current density of up to 230 mA cm⁻³ at −50 mV versus reversible hydrogen electrode (RHE) and a low overpotential of −62 mV versus RHE at the current density of −10 mA cm⁻². This solution‐processed strategy offers a simple, efficient, yet scalable approach to construct stable and efficient HER catalysts. Given the properties and the structure–activity relationships of the hierarchical Pt–MXene–SWCNTs' heterostructure, other MXenes probably show greater promise in HER electrocatalysis.

A novel family of in‐plane, chemically‐ordered quaternary MAX phases (i‐MAX), possessing ordered Sc within a Mo‐dominated M layer, renders a novel MXene family (i.e., i‐MXenes), with unique surface structure and ordered divacancies. Herein, fundamental investigations of i‐MAX and i‐MXene phases are summarized to demonstrate the potential of i‐MXenes in energy storage and catalytic applications.

Abstract

In 2017, a new family of in‐plane, chemically‐ordered quaternary MAX phases, coined i‐MAX, has been reported since 2017. The first i‐MAX phase, (Mo_2/3Sc_1/3)₂AlC, garnered significant research attention due to the presence of chemically ordered Sc within the Mo‐dominated M layer, and the facilitated removal of both Al and Sc upon etching, resulting in 2D i‐MXene, Mo_1.33C, with ordered divacancies. The i‐MXene renders an exceptionally low resistivity of 33.2 µΩ m⁻¹ and a high volumetric capacitance of ≈1150 F cm⁻³. This discovery has been followed by the synthesis of, to date, 32 i‐MAX phases and 5 i‐MXenes, where the latter have shown potential for applications including, but not limited to, energy storage and catalysis. Herein, fundamental investigations of i‐MAX phases and i‐MXenes, along with their applicability in supercapacitive and catalytic applications, are reviewed. Moreover, recent results on ion intercalation and post‐etching treatment of Mo_1.33C are presented. The charge storage performance can also be tuned by forming MXene hydrogel and through inert atmosphere annealing, where the latter renders a superior volumetric capacitance of ≈1635 F cm⁻³. This report demonstrates the potential of the i‐MXene family for catalytic and energy storage applications, and highlights novel research directions for further development and successful employment in practical applications.

First‐principles calculations combined with the reference‐interaction‐site model reveal that the hydration shell prevents orbital coupling between MXene and the intercalated cations, which maintains charge separation and leads to the capacitive behavior. Once the cations are partially dehydrated and adsorbed onto the MXene surface, charge transfer occurs because of the orbital coupling, resulting in a pseudocapacitive behavior.

Abstract

MXene electrodes in electrochemical capacitors have a distinctive behavior that is both capacitive and pseudocapacitive depending on the electrolyte. In this work, to better understand their electrochemical mechanism, first‐principles calculations based on the density functional theory combined with the implicit solvation model are used (termed as 3D reference‐interaction‐site model). From the viewpoint of their electronic states, the hydration shell prevents orbital coupling between MXene and the intercalated ions, which leads to the formation of an electric‐double layer and capacitive behavior. However, once the cations are partially dehydrated and adsorbed onto the MXene surface, because of orbital coupling of the cation states with the MXene states, particularly for surface‐termination groups, charge transfer occurs and results in a pseudocapacitive behavior.