Heterostructured grafting of NiFe-layered double hydroxide@TiO2 for boosting photoelectrochemical cathodic protection

Accelerating the oxidation process at photoanode-electrolyte interfaces can prolong the lifetime of photoexcited electrons and improve the efficiency of photoelectrochemical cathodic protection (PECCP) systems without relying on hole scavengers. However, the systematic design of precisely structured heterostructures for efficient photoanodes remains challenging. Here we meticulously engineered a type-II heterostructure featuring precise spatial organization, wherein NiFe-layered double hydroxide nanosheets (NiFe-LDH NSs) were assembled onto annealed TiO2 nanorod arrays (ATNAs), demonstrating their effectiveness in achieving efficient PECCP. The interfacial electronic coupling and appropriate energy alignment between the NiFe-LDH NSs and ATNAs allowed rapid hole extraction from the ATNAs to the NiFe-LDH NSs. Furthermore, the uniform distribution of the NiFe-LDH NSs on top of ATNAs drastically reduced the overpotential of oxygen evolution reactions (OER) from 370 to 200 mV and Tafel slope from 162 to 56 mV dec-1, leading to significantly improved cathodic protection of 304 stainless steel (SS) under extended illumination and interesting post-illumination protection. In addition, with the increase of testing cycles, the as-prepared NiFe-LDH NSs@ATNAs demonstrated a progressively enhanced cathodic protection potential from 0.15 to 0.13 V vs. RHE over 50 cycles. These findings provide important guidelines for the design of future high-efficiency green metal protection through rational photoanode design.

Engineering Covalent Organic Frameworks Toward Advanced Zinc-Based Batteries

Zinc-based batteries (ZBBs) have demonstrated considerable potential among secondary batteries, attributing to their advantages including good safety, environmental friendliness, and high energy density. However, ZBBs still suffer from issues such as the formation of zinc dendrites, occurrence of side reactions, retardation of reaction kinetics, and shuttle effects, posing a great challenge for practical applications. As promising porous materials, covalent organic frameworks (COFs) and their derivatives have rigid skeletons, ordered structures, and permanent porosity, which endow them with great potential for application in ZBBs. This review, therefore, provides a systematic overview detailing on COFs structure pertaining to electrochemical performance of ZBBs, following an in depth discussion of the challenges faced by ZBBs, which includes dendrites and side reactions at the anode, as well as dissolution, structural change, slow kinetics, and shuttle effect at the cathode. Then, the structural advantages of COF-correlated materials and their roles in various ZBBs are highlighted. Finally, the challenges of COF-correlated materials in ZBBs are outlined and an outlook on the future development of COF-correlated materials for ZBBs is provided. The review would serve as a valuable reference for further research into the utilization of COF-correlated materials in ZBBs.

Heterometallic Electrocatalysts Derived from High-Nuclearity Metal Clusters for Efficient Overall Water Splitting

The development of cost-effective electrocatalysts with an optimal surface affinity for intermediates is essential for sustainable hydrogen fuel production, but this remains insufficient. Here we synthesize Ni2P/MoS2-CoMo2S4@C heterometallic electrocatalysts based on the high-nuclearity cluster {Co24(TC4A)6(MoO4)8Cl6}, in which Ni2P nanoparticles were anchored to the surface of the MoS2-CoMo2S4@C nanosheets via strong interfacial interactions. Theoretical calculations revealed that the introduction of Ni2P phases induces significant disturbances in the surface electronic configuration of Ni2P/MoS2-CoMo2S4@C, resulting in more relaxed d-d orbital electron transfers between the metal atoms. Moreover, continuous electron transport was established by the formation of multiple heterojunction interfaces. The optimized Ni2P/MoS2-CoMo2S4@C electrocatalyst exhibited ultralow overpotentials of 198 and 73 mV for oxygen and hydrogen evolution reactions, respectively, in alkaline media, at 10 mA cm-2. The alkali electrolyzer constructed using Ni2P/MoS2-CoMo2S4@C required a cell voltage of only 1.45 V (10 mA cm-2) to drive overall water splitting with excellent long-term stability.

Data-centric artificial olfactory system based on the eigengraph

Recent studies of electronic nose system tend to waste significant amount of important data in odor identification. Until now, the sensitivity-oriented data composition has made it difficult to discover meaningful data to apply artificial intelligence in terms of in-depth analysis for odor attributes specifying the identities of gas molecules, ultimately resulting in hindering the advancement of the artificial olfactory technology. Here, we realize a data-centric approach to implement standardized artificial olfactory systems inspired by human olfactory mechanisms by formally defining and utilizing the concept of Eigengraph in electrochemisty. The implicit odor attributes of the eigengraphs were mathematically substantialized as the Fourier transform-based Mel-Frequency Cepstral Coefficient feature vectors. Their effectiveness and applicability in deep learning processes for gas classification have been clearly demonstrated through experiments on complex mixed gases and automobile exhaust gases. We suggest that our findings can be widely applied as source technologies to develop standardized artificial olfactory systems.

Hierarchical Spatial Confinement Unlocking the Storage Limit of MoS2 for Flexible High-Energy Supercapacitors

Molybdenum sulfide (MoS2) is a promising electrode material for supercapacitors; however, its limited Mo/S edge sites and intrinsic inert basal plane give rise to sluggish active electronic states, thus constraining its electrochemical performance. Here we propose a hierarchical confinement strategy to develop ethylene molecule (EG)-intercalated Co-doped sulfur-deficient MoS2 (Co-EG/SV-MoS2) for efficient and durable K-ion storage. Theoretical analyses suggest that the intercalation-confined EG and lattice-confined Co can enhance the interfacial K-ion storage capacity while reducing the K-ion diffusion barrier. Experimentally, the intercalated EG molecules with mildly reducing properties induced the creation of sulfur vacancies, expanded the interlayer spacing, regulated the 2H-1T phase transition, and strengthened the structural grafting between layers, thereby facilitating ion diffusion and ensuring structural durability. Moreover, the Co dopants occupying the initial Mo sites initiated charge transfer, thus activating the basal plane. Consequently, the optimized Co-EG/SV-MoS2 electrode exhibited a substantially improved electrochemical performance. Flexible supercapacitors assembled with Co-EG/SV-MoS2 delivered a notable areal energy density of 0.51 mW h cm-2 at 0.84 mW cm-2 with good flexibility. Furthermore, supercapacitor devices were integrated with a strain sensor to create a self-powered system capable of real-time detection of human joint motion.

Covalent Organic Frameworks: Their Composites and Derivatives for Rechargeable Metal-Ion Batteries

Covalent organic frameworks (COFs) have attracted considerable interest in the field of rechargeable batteries owing to their three-dimensional (3D) varied pore sizes, inerratic porous structures, abundant redox-active sites, and customizable structure-adjustable frameworks. In the context of metal-ion batteries, these materials play a vital role in electrode materials, effectively addressing critical issues such as low ionic conductivity, limited specific capacity, and unstable structural integrity. However, the electrochemical characteristics of the developed COFs still fall short of practical battery requirements due to inherent issues such as low electronic conductivity, the tradeoff between capacity and redox potential, and unfavorable micromorphology. This review provides a comprehensive overview of the recent advancements in the application of COFs, COF-based composites, and their derivatives in rechargeable metal-ion batteries, including lithium-ion, lithium-sulfur, sodium-ion, sodium-sulfur, potassium-ion, zinc-ion, and other multivalent metal-ion batteries. The operational mechanisms of COFs, COF-based composites, and their derivatives in rechargeable batteries are elucidated, along with the strategies implemented to enhance the electrochemical properties and broaden the range of their applications.

Zinc-Bismuth Binary Alloy Enabling High-Performance Aqueous Zinc Ion Batteries

Reconfiguration of zinc anodes efficiently mitigates dendrite formation and undesirable side reactions, thus favoring the long-term cycling performance of aqueous zinc ion batteries (AZIBs). This study synthesizes a Zn@Bi alloy anode (Zn@Bi) using the fusion method, and find that the anode surfaces synthesized using this method have an extremely high percentage of Zn(002) crystalline surfaces. Experimental results indicate that the addition of bismuth inhibits the hydrogen evolution reaction and corrosion of zinc anodes. The finite-element simulation results indicate that Zn@Bi can effectively achieve a uniform anodic electric field, thereby regulating the homogeneous depositions of zinc ions and reducing the production of Zn dendrite. Theoretical calculations reveal that the incorporation of Bi favors the anode structure stabilization and higher adsorption energy of Zn@Bi corresponds to better Zn deposition kinetics. The Zn@Bi//Zn@Bi symmetric cell demonstrates an extended cycle life of 1000 h. Furthermore, when pairing Zn@Bi with an α-MnO2 cathode to construct a Zn@Bi//MnO2 cell, a specific capacity of 119.3 mAh g-1 is maintained even after 1700 cycles at 1.2 A g-1 . This study sheds light on the development of dendrite-free anodes for advanced AZIBs.

In Situ Construction of the Fe-Cu Hydroxide Interlocking Structure with Solution-Derived Cu/Ag Current Collectors for Flexible Symmetric Supercapacitors

The current collector serves as a crucial element in supercapacitors, acting as a medium between the electrode material and the substrate. Due to its excellent conductivity, a metal collector is typically favored. Enhancing the binding strength between the collector and the substrate as well as between the collector and the electrode material has emerged as a critical factor for enhancing the capacitance performance. In this study, a Ag film with a grass root-like structure was initially grown on a PI substrate through the surface modification and ion exchange (SMIE) process. This Ag interlocking structure contributes to strong binding between the PI substrate and Ag without compromising the mechanical properties of the Ag film. To further enhance the electrochemical properties at low scan rates, electroless-plated Cu was subsequently deposited on the Ag film to form the Cu/Ag current collector. Moreover, the Cu within the Cu/Ag current collector served as a precursor for the growth of FeOOH-Cu(OH)2 via a two-step in situ method. The resulting FeOOH-Cu(OH)2/Cu/Ag structure as a whole is binder-free. Supercapacitors employing symmetric FeOOH-Cu(OH)2/Cu/Ag structures were assembled, and their energy storage properties were investigated. The solution-based low-temperature process used in this study offers the potential for cost-effective and large-scale applications.

Tensile Strain-Mediated Spinel Ferrites Enable Superior Oxygen Evolution Activity

Exploring efficient strategies to overcome the performance constraints of oxygen evolution reaction (OER) electrocatalysts is vital for electrocatalytic applications such as H2O splitting, CO2 reduction, N2 reduction, etc. Herein, tunable, wide-range strain engineering of spinel oxides, such as NiFe2O4, is proposed to enhance the OER activity. The lattice strain is regulated by interfacial thermal mismatch during the bonding process between thermally expanding NiFe2O4 nanoparticles and the nonexpanding carbon fiber substrate. The tensile lattice strain causes energy bands to flatten near the Fermi level, lowering eg orbital occupancy, effectively increasing the number of electronic states near the Fermi level, and reducing the pseudoenergy gap. Consequently, the energy barrier of the rate-determining step for strained NiFe2O4 is reduced, achieving a low overpotential of 180 mV at 10 mA/cm2. A total water decomposition voltage range of 1.52-1.56 V at 10 mA/cm2 (without iR correction) was achieved in an asymmetric alkaline electrolytic cell with strained NiFe2O4 nanoparticles, and its robust stability was verified with a voltage retention of approximately 99.4% after 100 h. Furthermore, the current work demonstrates the universality of tuning OER performance with other spinel ferrite systems, including cobalt, manganese, and zinc ferrites.

Stabilizing Zn Metal Anode Through Regulation of Zn Ion Transfer and Interfacial Behavior with a Fast Ion Conductor Protective Layer

Aqueous Zn-ion batteries (AZIBs) attract intensive attention owing to their environmental friendliness, cost-effectiveness, innate safety, and high specific capacity. However, the practical applications of AZIBs are hindered by several adverse phenomena, including corrosion, Zn dendrites, and hydrogen evolution. Herein, a Zn anode decorated with a 3D porous-structured Na3 V2 (PO4)3 (NVP@Zn) is obtained, where the NVP reconstruct the electrolyte/anode interface. The resulting NVP@Zn anode can provide a large quantity of fast and stable channels, facilitating enhanced Zn ion deposition kinetics and regulating the Zn ions transport process through the ion confinement effect. The NASICON-type NVP protective layer promote the desolvation process due to its nanopore structure, thus effectively avoiding side reactions. Theoretical calculations indicate that the NVP@Zn electrode has a higher Zn ion binding energy and a higher migration barrier, which demonstrates that NVP protective layer can enhance Zn ion deposition kinetics and prevent the unfettered 2D diffusion of Zn ions. Therefore, the results show that NVP@Zn/MnO2 full cell can maintain a high specific discharge capacity of 168 mAh g-1 and a high-capacity retention rate of 74.6% after cycling. The extraordinary results obtained with this strategy have confirmed the promising applications of NVP in high-performance AZIBs.

Electronic Structure Engineered Heteroatom Doped All Transition Metal Sulfide Carbon Confined Heterostructure for Extrinsic Pseudocapacitor

Ultra-high energy density battery-type materials are promising candidates for supercapacitors (SCs); however, slow ion kinetics and significant volume expansion remain major barriers to their practical applications. To address these issues, hierarchical lattice distorted α-/γ-MnS@Cox Sy core-shell heterostructure constrained in the sulphur (S), nitrogen (N) co-doped carbon (C) metal-organic frameworks (MOFs) derived nanosheets (α-/γ-MnS@Cox Sy @N, SC) have been developed. The coordination bonding among Cox Sy , and α-/γ-MnS nanoparticles at the interfaces and the π-π stacking interactions developed across α-/γ-MnS@Cox Sy and N, SC restrict volume expansion during cycling. Furthermore, the porous lattice distorted heteroatom-enriched nanosheets contain a sufficient number of active sites to allow for efficient electron transportation. Density functional theory (DFT) confirms the significant change in electronic states caused by heteroatom doping and the formation of core-shell structures, which provide more accessible species with excellent interlayer and interparticle conductivity, resulting in increased electrical conductivity. . The α-/γ-MnS@Cox Sy @N, SC electrode exhibits an excellent specific capacity of 277 mA hg-1 and cycling stability over 23 600 cycles. A quasi-solid-state flexible extrinsic pseudocapacitor (QFEPs) assembled using layer-by-layer deposited multi-walled carbon nanotube/Ti3 C2 TX nanocomposite negative electrode. QFEPs deliver specific energy of 64.8 Wh kg-1 (1.62 mWh cm-3 ) at a power of 933 W kg-1 and 92% capacitance retention over 5000 cycles.

Emerging Pristine MOF-Based Heterostructured Nanoarchitectures: Advances in Structure Evolution, Controlled Synthesis, and Future Perspectives

Metal-organic frameworks (MOFs) can be customized through modular assembly to achieve a wide range of potential applications, based on their desired functionality. However, most of the initially reported MOFs are limited to microporous systems and are not sufficiently stable, which restricts their popularization. Heterogeneity is introduced into a simple MOF framework to create MOF-based heterostructures with fascinating properties and interesting functions. Heterogeneity can be introduced into the MOFs via postsynthetic/ligand exchange. Although the ligand exchange has shown potential, it is difficult to precisely control the degree of exchange or position. Among the various synthesis strategies, hierarchical assembly is particularly attractive for constructing MOF-based heterostructures, as it can achieve precise regulation of MOF-based heterostructured nanostructures. The hierarchical assembly significantly expands the compositional diversity of MOF-based heterostructures, which has high elasticity for lattice matching during the epitaxial growth of MOFs. This review focuses on the synthetic evolution mechanism of hierarchical assemblies of MOF-based nanoarchitectures. Subsequently, the precise control of pore structure, pore size, and morphology of MOF-based nanoarchitectures by hierarchical assembly is emphasized. Finally, possible solutions to address the challenges associated with heterogeneous interfaces are presented, and potential opportunities for innovative applications are proposed.

Electronic and Structural Engineering of Atomically Dispersed Isolated Single-Atom and Alloy Architectures

Precise configurations of isolated metal atoms in nitrogen-doped carbon materials with 2D single or multilayers and 3D nanoarchitectures are gaining attention owing to their good stability and activity at high current densities. Atomic metal-N_x moieties, which utilize maximum atoms to attain high intrinsic activity and novel electronic architecture of support materials, facilitate strong interaction between the central metal atom and support matrix. However, resource consumption is considerably high due to the inferior atomic utilization of active sites. Therefore, energy-efficient electrochemical processes are needed to develop advanced isolated single-atom architecture, which would provide high atom-utilization and good durability. Herein, the concepts of atomically dispersed metal sites in single-atom and alloy architectures and their electronic features associated with structural evolution are discussed. Opportunities and challenges associated with the use of isolated single-atoms in 2D materials are discussed based on their unique electronic defects, low-valence central metals, mechanical flexibility, and maximum access to metal sites. This insightful revisit into the engineering of single-atom and alloy architectures would provide a profound understanding of electronic modulations and regulation of geometric characteristics, and unravels potential directions for electrochemical energy conversion, charge storage, and sensing processes.

Multiplexed DNA-functionalized graphene sensor with artificial intelligence-based discrimination performance for analyzing chemical vapor compositions

This study presents a new technology that can detect and discriminate individual chemical vapors to determine the chemical vapor composition of mixed chemical composition in situ based on a multiplexed DNA-functionalized graphene (MDFG) nanoelectrode without the need to condense the original vapor or target dilution. To the best of our knowledge, our artificial intelligence (AI)-operated arrayed electrodes were capable of identifying the compositions of mixed chemical gases with a mixed ratio in the early stage. This innovative technology comprised an optimized combination of nanodeposited arrayed electrodes and artificial intelligence techniques with advanced sensing capabilities that could operate within biological limits, resulting in the verification of mixed vapor chemical components. Highly selective sensors that are tolerant to high humidity levels provide a target for "breath chemovapor fingerprinting" for the early diagnosis of diseases. The feature selection analysis achieved recognition rates of 99% and above under low-humidity conditions and 98% and above under humid conditions for mixed chemical compositions. The 1D convolutional neural network analysis performed better, discriminating the compositional state of chemical vapor under low- and high-humidity conditions almost perfectly. This study provides a basis for the use of a multiplexed DNA-functionalized graphene gas sensor array and artificial intelligence-based discrimination of chemical vapor compositions in breath analysis applications.

Reconfiguring the Electronic Structure of Heteroatom Doped Carbon Supported Bimetallic Oxide@Metal Sulfide Core-Shell Heterostructure via In Situ Nb Incorporation toward Extrinsic Pseudocapacitor

High-energy-density battery-type materials have sparked considerable interest as supercapacitors electrode; however, their sluggish charge kinetics limits utilization of redox-active sites, resulting in poor electrochemical performance. Here, the unique core-shell architecture of metal organic framework derived N-S codoped carbon@Co_x S_y micropetals decorated with Nb-incorporated cobalt molybdate nanosheets (Nb-CMO₄ @C_x S_y NC) is demonstrated. Coordination bonding across interfaces and π-π stacking interactions between CMO₄ @C_x S_y and N and, S-C can prevent volume expansion during cycling. Density functional theory analysis reveals that the excellent interlayer and the interparticle conductivity imparted by Nb doping in heteroatoms synergistically alter the electronic states and offer more accessible species, leading to increased electrical conductivity with lower band gaps. Consequently, the optimized electrode has a high specific capacity of 276.3 mAh g^-1 at 1 A g^-1 and retains 98.7% of its capacity after 10 000 charge-discharge cycles. A flexible quasi-solid-state SC with a layer-by-layer deposited reduced graphene oxide /Ti₃ C₂ T_X anode achieves a specific energy of 75.5 Wh kg^-1 (volumetric energy of 1.58 mWh cm^-3 ) at a specific power of 1.875 kWh kg^-1 with 96.2% capacity retention over 10 000 charge-discharge cycles.

Covalently Interlayer-Confined Organic-Inorganic Heterostructures for Aqueous Potassium Ion Supercapacitors

Artificial assembly of organic-inorganic heterostructures for electrochemical energy storage at the molecular level is promising, but remains a great challenge. Here, a covalently interlayer-confined organic (polyaniline [PANI])-inorganic (MoS₂ ) hybrid with a dual charge-storage mechanism is developed for boosting the reaction kinetics of supercapacitors. Systematic characterizations reveal that PANI induces a partial phase transition from the 2H to 1T phases of MoS₂ , expands the interlayer spacing of MoS₂ , and increases the hydrophilicity. More in-depth insights from the synchrotron radiation-based X-ray technique illustrate that the covalent grafting of PANI to MoS₂  induces the formation of MoN bonds and unsaturated Mo sites, leading to increased active sites. Theoretical analysis reveals that the covalent assembly facilitates cross-layer electron transfer and decreases the diffusion barrier of K⁺ ions, which favors reaction kinetics. The resultant hybrid material exhibits high specific capacitance and good rate capability. This design provides an effective strategy to develop organic-inorganic heterostructures for superior K-ion storage. The K-ion storage mechanism concerning the reversible insertion/extraction upon charge/discharge is revealed through ex situ X-ray photoelectron spectroscopy.

Realizing Superior Redox Kinetics of Hollow Bimetallic Sulfide Nanoarchitectures by Defect-Induced Manipulation toward Flexible Solid-State Supercapacitors

As a typical battery-type material, CuCo₂ S₄ is a promising candidate for supercapacitors due to the high theoretical specific capacity. However, its practical application is plagued by inherently sluggish ion diffusion kinetics and inferior electrical transport properties. Herein, sulfur vacancies are incorporated in CuCo₂ S₄ hollow nanoarchitectures (HNs) to accelerate redox reactivity. Experimental analyses and theoretical investigations uncover that the generated sulfur vacancies increase the active electron states, reduce the adsorption barriers of electrolyte ions, and enrich reactive redox species, thus achieving enhanced electrochemical performance. Consequently, the deficient CuCo₂ S₄ with optimized vacancy concentration presents a high specific capacity of 231 mAh g^-1 at 1 A g^-1 , a ≈1.78 times increase compared to that of pristine CuCo₂ S₄ , and exhibits a superior rate capability (73.8% capacity retention at 20 A g^-1 ). Furthermore, flexible solid-state asymmetric supercapacitor devices assembled with the deficient CuCo₂ S₄ HNs and VN nanosheets deliver a high energy density of 61.4 W h kg^-1 at 750 W kg^-1 . Under different bending states, the devices display exceptional mechanical flexibility with no obvious change in CV curves at 50 mV s^-1 . These findings provide insights for regulating electrode reactivity of battery-type materials through intentional nanoarchitectonics and vacancy engineering.

Highly Dispersed Pt Clusters on F-Doped Tin(IV) Oxide Aerogel Matrix: An Ultra-Robust Hybrid Catalyst for Enhanced Hydrogen Evolution

Dispersing the minuscule mass loading without hampering the high catalytic activity and long-term stability of a noble metal catalyst results in its ultimate efficacy for the electrochemical hydrogen evolution reaction (HER). Despite being the most efficient HER catalyst, the use of Pt is curtailed due to its scarcity and tendency to leach out in the harsh electrochemical reaction environment. In this study, we combined F-doped tin(IV) oxide (F-SnO₂) aerogel with Pt catalyst to prevent metallic corrosion and to achieve abundant Pt active sites (approximately 5 nm clusters) with large specific surface area (321 cm²·g^-1). With nanoscopic Pt loading inside the SnO₂ aerogel matrix, the as-synthesized hybrid F-SnO₂@Pt possesses a large specific surface area and high porosity and, thus, exhibits efficient experimental and intrinsic HER activity (a low overpotential of 42 mV at 10 mA·cm^-2 in 0.5 M sulfuric acid), a 22-times larger turnover frequency (11.2 H₂·s^-1) than that of Pt/C at 50 mV, and excellent robustness over 10,000 cyclic voltammetry cycles. The existing metal support interaction and strong intermolecular forces between Pt and F-SnO₂ account for the catalytic superiority and persistence against corrosion of F-SnO₂@Pt compared to commercially used Pt/C. Density functional theory analysis suggests that hybridization between the Pt and F-SnO₂ orbitals enhances intermediate hydrogen atom (H*) adsorption at their interface, which improves the reaction kinetics.

Recent Advances and Perspectives of Battery-Type Anode Materials for Potassium Ion Storage

Potassium ion energy storage devices are competitive candidates for grid-scale energy storage applications owing to the abundancy and cost-effectiveness of potassium (K) resources, the low standard redox potential of K/K⁺, and the high ionic conductivity in K-salt-containing electrolytes. However, the sluggish reaction dynamics and poor structural instability of battery-type anodes caused by the insertion/extraction of large K⁺ ions inhibit the full potential of K ion energy storage systems. Extensive efforts have been devoted to the exploration of promising anode materials. This Review begins with a brief introduction of the operation principles and performance indicators of typical K ion energy storage systems and significant advances in different types of battery-type anode materials, including intercalation-, mixed surface-capacitive-/intercalation-, conversion-, alloy-, mixed conversion-/alloy-, and organic-type materials. Subsequently, host-guest relationships are discussed in correlation with the electrochemical properties, underlying mechanisms, and critical issues faced by each type of anode material concerning their implementation in K ion energy storage systems. Several promising optimization strategies to improve the K⁺ storage performance are highlighted. Finally, perspectives on future trends are provided, which are aimed at accelerating the development of K ion energy storage systems.

Challenges and Strategies toward Cathode Materials for Rechargeable Potassium-Ion Batteries

With increasing demand for grid-scale energy storage, potassium-ion batteries (PIBs) have emerged as promising complements or alternatives to commercial lithium-ion batteries owing to the low cost, natural abundance of potassium resources, the low standard reduction potential of potassium, and fascinating K⁺ transport kinetics in the electrolyte. However, the low energy density and unstable cycle life of cathode materials hamper their practical application. Therefore, cathode materials with high capacities, high redox potentials, and good structural stability are required with the advancement toward next-generation PIBs. To this end, understanding the structure-dependent intercalation electrochemistry and recognizing the existing issues relating to cathode materials are indispensable prerequisites. This review summarizes the recent advances of PIB cathode materials, including metal hexacyanometalates, layered metal oxides, polyanionic frameworks, and organic compounds, with an emphasis on the structural advantages of the K⁺ intercalation reaction. Moreover, major current challenges with corresponding strategies for each category of cathode materials are highlighted. Finally, future research directions and perspectives are presented to accelerate the development of PIBs and facilitate commercial applications. It is believed that this review will provide practical guidance for researchers engaged in developing next-generation advanced PIB cathode materials.

A reversible and stable doping technique to invert the carrier polarity of MoTe2

Two-dimensional (2D) materials can be implemented in several functional devices for future optoelectronics and electronics applications. Remarkably, recent research on p-n diodes by stacking 2D materials in heterostructures or homostructures (out of plane) has been carried out extensively with novel designs that are impossible with conventional bulk semiconductor materials. However, the insight of a lateral p-n diode through a single nanoflake based on 2D material needs attention to facilitate the miniaturization of device architectures with efficient performance. Here, we have established a physical carrier-type inversion technique to invert the polarity of MoTe₂-based field-effect transistors (FETs) with deep ultraviolet (DUV) doping in (oxygen) O₂and (nitrogen) N₂gas environments. A p-type MoTe₂nanoflake transformed its polarity to n-type when irradiated under DUV illumination in an N₂gaseous atmosphere, and it returned to its original state once irradiated in an O₂gaseous environment. Further, Kelvin probe force microscopy (KPFM) measurements were employed to support our findings, where the value of the work function changed from ∼4.8 and ∼4.5 eV when p-type MoTe₂inverted to the n-type, respectively. Also, using this approach, an in-plane homogeneous p-n junction was formed and achieved a diode rectifying ratio (I_f/I_r) up to ∼3.8 × 10⁴. This effective approach for carrier-type inversion may play an important role in the advancement of functional devices.

On-Chip Chemiresistive Sensor Array for On-Road NO x Monitoring with Quantification

The adverse effects of air pollution on respiratory health make air quality monitoring with high spatial and temporal resolutions essential especially in cities. Despite considerable interest and efforts, the application of various types of sensors is considered immature owing to insufficient sensitivity and cross-interference under ambient conditions. Here, a fully integrated chemiresistive sensor array (CSA) with parts-per-trillion sensitivity is demonstrated with its application for on-road NO x monitoring. An analytical model is suggested to describe the kinetics of the sensor responses and quantify molecular binding affinities. Finally, the full characterization of the system is connected to implement on-road measurements on NO x vapor with quantification as its ultimate field application. The obtained results suggest that the CSA shows potential as an essential unit to realize an air-quality monitoring network with high spatial and temporal resolutions.

Two-dimensional electronic devices modulated by the activation of donor-like states in boron nitride

A two-dimensional (2D) layered material-based p-n diode is an essential element in the modern semiconductor industry for facilitating the miniaturization and structural flexibility of devices with high efficiency for future optoelectronic and electronic applications. Planar devices constructed previously required a complicated device structure using a photoresist, as they needed to consider non-abrupt interfaces. Here, we demonstrated a WSe₂ based lateral homojunction diode obtained by applying a photo-induced effect in BN/WSe₂ heterostructures upon illumination via visible and deep UV light, which represents a stable and flexible charge doping technique. We have discovered that with this technique, a field-effect transistor (FET) based on p-type WSe₂ is inverted to n-WSe₂ so that a high electron mobility is maintained in the h-BN/n-WSe₂ heterostructures. To confirm this hypothesis, we deduced the work function values of p-WSe₂ and n-WSe₂ FETs by conducting Kelvin probe force microscopy (KPFM) measurements, which revealed the decline of the Fermi level from 5.07 (p-WSe₂) to 4.21 eV (n-WSe₂). The contact potential difference (CPD) between doped and undoped junctions was found to be 165 meV. We employed ohmic metal contacts for the planar homojunction diode by utilizing an ionic liquid gate to achieve a diode rectification ratio up to ∼10⁵ with n = 1. An exceptional photovoltaic performance is also observed. The presence of a built-in potential in our devices leads to an open-circuit voltage (V_oc) and short-circuit current (I_sc) without an external electric field. This effective doping technique is promising to advance the concept of preparing future functional devices.

Modulation of Magnetoresistance Polarity in BLG/SL-MoSe2 Heterostacks

Two-dimensional (2D) layered materials have an atomically thin and flat nature which makes it an ultimate candidate for spintronic devices. The spin-valve junctions (SVJs), composed of 2D materials, have been recognized as unique features of spin transport polarization. However, the magnetotransport properties of SVJs are highly influenced by the type of intervening layer (spacer) inserted between the ferromagnetic materials (FMs). In this situation, the spin filtering effect at the interfaces plays a critical role in the observation of the magnetoresistance (MR) of such magnetic structures, which can be improved by using promising hybrid structure. Here, we report MR of bilayer graphene (BLG), single-layer MoSe₂ (SL-MoSe₂), and BLG/SL-MoSe₂ heterostack SVJs. However, before annealing, BLG and SL-MoSe₂ SVJs demonstrate positive MR, but after annealing, BLG reverses its polarity while the SL-MoSe₂ maintains its polarity and demonstrated stable positive spin polarizations at both interfaces due to meager doping effect of ferromagnetic (FM) contacts. Further, Co/BLG/SL-MoSe₂/NiFe determines positive MR, i.e., ~ 1.71% and ~ 1.86% at T = 4 K before and after annealing, respectively. On the contrary, NiFe/BLG/SL-MoSe₂/Co SVJs showed positive MR before annealing and subsequently reversed its MR sign after annealing due to the proximity-induced effect of metals doping with graphene. The obtained results can be useful to comprehend the origin of polarity and the selection of non-magnetic material (spacer) for magnetotransport properties. Thus, this study established a new paragon for novel spintronic applications.

Electrically focus-tuneable ultrathin lens for high-resolution square subpixels

Owing to the tremendous demands for high-resolution pixel-scale thin lenses in displays, we developed a graphene-based ultrathin square subpixel lens (USSL) capable of electrically tuneable focusing (ETF) with a performance competitive with that of a typical mechanical refractive lens. The fringe field due to a voltage bias in the graphene proves that our ETF-USSL can focus light onto a single point regardless of the wavelength of the visible light-by controlling the carriers at the Dirac point using radially patterned graphene layers, the focal length of the planar structure can be adjusted without changing the curvature or position of the lens. A high focusing efficiency of over 60% at a visible wavelength of 405 nm was achieved with a lens thickness of <13 nm, and a change of 19.42% in the focal length with a 9% increase in transmission was exhibited under a driving voltage. This design is first presented as an ETF-USSL that can be controlled in pixel units of flat panel displays for visible light. It can be easily applied as an add-on to high resolution, slim displays and provides a new direction for the application of multifunctional autostereoscopic displays.

Review on Recent Progress in the Development of Tungsten Oxide Based Electrodes for Electrochemical Energy Storage

Current progress in the advancement of energy-storage devices is the most important factor that will allow the scientific community to develop resources to meet the global energy demands of the 21st century. Nanostructured materials can be used as effective electrodes for energy-storage devices because they offer various promising features, including high surface-to-volume ratios, exceptional charge-transport features, and good physicochemical properties. Until now, the successful research frontrunners have focused on the preparation of positive electrode materials for energy-storage applications; nevertheless, the electrochemical performance of negative electrodes is less frequently reported. This review mainly focuses on the current progress in the development of tungsten oxide-based electrodes for energy-storage applications, primarily supercapacitors (SCs) and batteries. Tungsten is found in various stoichiometric and nonstoichiometric oxides. Among the different tungsten oxide materials, tungsten trioxide (WO₃ ) has been intensively investigated as an electrode material for different applications because of its excellent charge-transport features, unique physicochemical properties, and good resistance to corrosion. Various WO₃ composites, such as WO₃ /carbon, WO₃ /polymers, WO₃ /metal oxides, and tungsten-based binary metal oxides, have been used for application in SCs and batteries. However, pristine WO₃ suffers from a relatively low specific surface area and low energy density. Therefore, it is crucial to thoroughly summarize recent progress in utilizing WO₃ -based materials from various perspectives to enhance their performance. Herein, the potential- and pH-dependent behavior of tungsten in aqueous media is discussed. Recent progress in the advancement of nanostructured WO₃ and tungsten oxide-based composites, along with related charge-storage mechanisms and their electrochemical performances in SCs and batteries, is systematically summarized. Finally, remarks are made on future research challenges and the prospect of using tungsten oxide-based materials to further upgrade energy-storage devices.

Phosphorus Regulated Cobalt Oxide@Nitrogen-Doped Carbon Nanowires for Flexible Quasi-Solid-State Supercapacitors

Battery-type materials are promising candidates for achieving high specific capacity for supercapacitors. However, their slow reaction kinetics hinders the improvement in electrochemical performance. Herein, a hybrid structure of P-doped Co₃ O₄ (P-Co₃ O₄ ) ultrafine nanoparticles in situ encapsulated into P, N co-doped carbon (P, N-C) nanowires by a pyrolysis-oxidation-phosphorization of 1D metal-organic frameworks derived from Co-layered double hydroxide as self-template and reactant is reported. This hybrid structure prevents active material agglomeration and maintains a 1D oriented arrangement, which exhibits a large accessible surface area and hierarchically porous feature, enabling sufficient permeation and transfer of electrolyte ions. Theoretical calculations demonstrate that the P dopants in P-Co₃ O₄ @P, N-C could reduce the adsorption energy of OH^- and regulate the electrical properties. Accordingly, the P-Co₃ O₄ @P, N-C delivers a high specific capacity of 669 mC cm^-2 at 1 mA cm^-2 and an ultralong cycle life with only 4.8% loss over 5000 cycles at 30 mA cm^-2 . During the fabrication of P-Co₃ O₄ @P, N-C, Co@P, N-C is simultaneously developed, which can be integrated with P-Co₃ O₄ @P, N-C for the assembly of asymmetric supercapacitors. These devices achieve a high energy density of 47.6 W h kg^-1 at 750 W kg^-1 and impressive flexibility, exhibiting a great potential in practical applications.

Nitrogen-doped carbon integrated nickel–cobalt metal phosphide marigold flowers as a high capacity electrode for hybrid supercapacitors

The fabrication of advanced MOF-derived multicomponent NiCoP/NC marigold flowers electrode for high-performance hybrid supercapacitors.

Metal–organic-framework-derived hierarchical Co/CoP-decorated nanoporous carbon polyhedra for robust high-energy storage hybrid supercapacitors

Electrode materials exhibiting nanostructural design, high surface area, tunable pore size, and efficient ion diffusion/transportation are essential for achieving improved electrochemical performance.

Structural, chemical, and electrical parameters of Au/MoS2/n-GaAs metal/2D/3D hybrid heterojunction

In this study, Au/MoS₂/n-GaAs heterojunction is fabricated with single MoS₂ layer and its structural, chemical and electrical parameters are investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and measurement of current-voltage (I-V) characteristics at room temperature. XRD and XPS analysis results confirm the formation of MoS₂ layer on the n-GaAs surface. The electrical properties of the Au/MoS₂/n-GaAs heterojunction are compared with those of the Au/n-GaAs Schottky junction. Interestingly, the heterojunction possesses a higher barrier height, lower leakage current and higher rectification ratio, in comparison with the Schottky junction. The shunt resistance (R_Sh) and series resistance (R_S) are also assessed for both the junctions. Moreover, the ideality factor (n), barrier height (Φ_b) and series resistance (R_S) are evaluated using Norde, Cheung's and surface potential (Ψ_S-V) plots and the results are well-matched. Furthermore, the current transport mechanism is analyzed based on the forward bias I-V data. Lastly, the Poole-Frenkel emission conduction mechanism is employed to control the reverse bias I-V behavior of both Au/n-GaAs Schottky junction and Au/MoS₂/n-GaAs heterojunction. The results demonstrate that the Au/MoS₂/n-GaAs heterojunction fabricated using a simple technique is suitable for high-quality electronic and optoelectronic device applications.

Imaging and Differentiation of Retinal Ganglion Cells in Ex Vivo Experimental Optic Nerve Degeneration by Differential Interference Contrast Microscopy

Purpose: Apoptotic loss of retinal ganglion cells (RGCs) is involved in various optic neuropathies, and its extent is closely related to visual impairment. Direct imaging and counting of RGCs is beneficial to the evaluation of RGC loss, but these processes are challenging with the conventional techniques, due to the transparency and hypo-reflectivity of RGCs as light-transmitting structures of the retina. Differential interference contrast (DIC) microscopy, which can provide real-time images of transparent specimens, is utilized to image neuronal cells including RGCs in the ganglion cell layer (GCL). Methods: Herein, we show that the neuronal cells within each GCL in an explanted rat retina, including the inner nuclear layer and the outer nuclear layer, can be imaged selectively by transmission-type DIC microscopy. RGCs were also differentiated from non-RGCs by the objective method. Results: RGCs were differentiated from non-RGCs in the GCL by their morphological features on DIC images with the aid of retrograde fluorescence labeling. Loss of RGCs was detected in optic-nerve-transection and retinal-ischemia-reperfusion models by DIC imaging. The images obtained from the reflection-type DIC microscopy were comparable to those from the transmission-type DIC microscopy. Conclusions: This method enables direct optical visualization of RGCs in experimental optic-nerve degeneration, thus providing the opportunity for more accurate evaluation of optic neuropathies as well as more effective investigation of diseases.

A MEMS ultrasound stimulation system for modulation of neural circuits with high spatial resolution in vitro

Neuromodulation by ultrasound has recently received attention due to its noninvasive stimulation capability for treating brain diseases. Although there have been several studies related to ultrasonic neuromodulation, these studies have suffered from poor spatial resolution of the ultrasound and low repeatability with a fixed condition caused by conventional and commercialized ultrasound transducers. In addition, the underlying physics and mechanisms of ultrasonic neuromodulation are still unknown. To determine these mechanisms and accurately modulate neural circuits, researchers must have a precisely controllable ultrasound transducer to conduct experiments at the cellular level. Herein, we introduce a new MEMS ultrasound stimulation system for modulating neurons or brain slices with high spatial resolution. The piezoelectric micromachined ultrasonic transducers (pMUTs) with small membranes (sub-mm membranes) generate enough power to stimulate neurons and enable precise modulation of neural circuits. We designed the ultrasound transducer as an array structure to enable localized modulation in the target region. In addition, we integrated a cell culture chamber with the system to make it compatible with conventional cell-based experiments, such as in vitro cell cultures and brain slices. In this work, we successfully demonstrated the functionality of the system by showing that the number of responding cells is proportional to the acoustic intensity of the applied ultrasound. We also demonstrated localized stimulation capability with high spatial resolution by conducting experiments in which cocultured cells responded only around a working transducer.

Phosphorus-Mediated MoS2 Nanowires as a High-Performance Electrode Material for Quasi-Solid-State Sodium-Ion Intercalation Supercapacitors

Molybdenum disulfide (MoS₂ ) is a promising electrode material for electrochemical energy storage owing to its high theoretical specific capacity and fascinating 2D layered structure. However, its sluggish kinetics for ionic diffusion and charge transfer limits its practical applications. Here, a promising strategy is reported for enhancing the Na⁺ -ion charge storage kinetics of MoS₂ for supercapacitors. In this strategy, electrical conductivity is enhanced and the diffusion barrier of Na⁺ ion is lowered by a facile phosphorus-doping treatment. Density functional theory results reveal that the lowest energy barrier of dilute Na-vacancy diffusion on P-doped MoS₂ (0.11 eV) is considerably lower than that on pure MoS₂ (0.19 eV), thereby signifying a prominent rate performance at high Na intercalation stages upon P-doping. Moreover, the Na-vacancy diffusion coefficient of the P-doped MoS₂ at room temperatures can be enhanced substantially by approximately two orders of magnitude (10^-6 -10^-4 cm² s^-1 ) compared with pure MoS₂ . Finally, the quasi-solid-state asymmetrical supercapacitor assembled with P-doped MoS₂ and MnO₂ , as the positive and negative electrode materials, respectively, exhibits an ultrahigh energy density of 67.4 W h kg^-1 at 850 W kg^-1 and excellent cycling stability with 93.4% capacitance retention after 5000 cycles at 8 A g^-1 .

Environmentally benign and cost-effective synthesis of water soluble red light emissive gold nanoclusters: selective and ultra-sensitive detection of mercuric ions

A green approach to synthesize red emissive gold nanoclusters for nano-molar detection of mercuric ions.

High-Performance Flexible Quasi-Solid-State Supercapacitors Realized by Molybdenum Dioxide@Nitrogen-Doped Carbon and Copper Cobalt Sulfide Tubular Nanostructures

Flexible quasi-/all-solid-state supercapacitors have elicited scientific attention to fulfill the explosive demand for portable and wearable electronic devices. However, the use of electrode materials faces several challenges, such as intrinsically slow kinetics and volume change upon cycling, which impede the energy output and electrochemical stability. This study presents well-aligned molybdenum dioxide@nitrogen-doped carbon (MoO2@NC) and copper cobalt sulfide (CuCo2S4) tubular nanostructures grown on flexible carbon fiber for use as electrode materials in supercapacitors. Benefiting from the chemically stable interfaces, affluent active sites, and efficient 1D electron transport, the MoO2@NC and CuCo2S4 nanostructures integrated on conductive substrates deliver excellent electrochemical performance. A flexible quasi-solid-state asymmetric supercapacitor composed of MoO2@NC as the negative electrode and CuCo2S4 as the positive electrode achieves an ultrahigh energy density of 65.1 W h kg-1 at a power density of 800 W kg-1 and retains a favorable energy density of 27.6 W h kg-1 at an ultrahigh power density of 12.8 kW kg-1. Moreover, it demonstrates good cycling performance with 90.6% capacitance retention after 5000 cycles and excellent mechanical flexibility by enabling 92.2% capacitance retention after 2000 bending cycles. This study provides an effective strategy to develop electrode materials with superior electrochemical performance for flexible supercapacitors.

Carrier Transport Properties of MoS2 Asymmetric Gas Sensor Under Charge Transfer-Based Barrier Modulation

Over the past few years, two-dimensional materials have gained immense attention for next-generation electric sensing devices because of their unique properties. Here, we report the carrier transport properties of MoS₂ Schottky diodes under ambient as well as gas exposure conditions. MoS₂ field-effect transistors (FETs) were fabricated using Pt and Al electrodes. The work function of Pt is higher than that of MoS_2, while that of Al is lower than that of MoS₂. The MoS₂ device with Al contacts showed much higher current than that with Pt contacts because of its lower Schottky barrier height (SBH). The electrical characteristics and gas responses of the MoS₂ Schottky diodes with Al and Pt contacts were measured electrically and were simulated by density functional theory calculations. The theoretically calculated SBH of the diode (under gas absorption) showed that NO_x molecules had strong interaction with the diode and induced a negative charge transfer. However, an opposite trend was observed in the case of NH₃ molecules. We also investigated the effect of metal contacts on the gas sensing performance of MoS₂ FETs both experimentally and theoretically.

Artificial Rod and Cone Photoreceptors with Human-Like Spectral Sensitivities

Photosensitive materials contain biologically engineered elements and are constructed using delicate techniques, with special attention devoted to efficiency, stability, and biocompatibility. However, to date, no photosensitive material has been developed to replace damaged visual-systems to detect light and transmit the signal to a neuron in the human body. In the current study, artificial nanovesicle-based photosensitive materials are observed to possess the characteristics of photoreceptors similar to the human eye. The materials exhibit considerably effective spectral characteristics according to each pigment. Four photoreceptors originating from the human eye with color-distinguishability are produced in human embryonic kidney (HEK)-293 cells and partially purified in the form of nanovesicles. Under various wavelengths of visible light, electrochemical measurements are performed to analyze the physiological behavior and kinetics of the photoreceptors, with graphene, performing as an electrode, playing an important role in the lipid bilayer deposition and oxygen reduction processes. Four nanovesicles with different photoreceptors, namely, rhodopsin (Rho), short-, medium-, and longwave sensitive opsin 1 (1SW, 1MW, 1LW), show remarkable color-dependent characteristics, consistent with those of natural human retina. With four different light-emitting diodes for functional verification, the photoreceptors embedded in nanovesicles show remarkably specific color sensitivity. This study demonstrates the potential applications of light-activated platforms in biological optoelectronic industries.

Vertical and In-Plane Current Devices Using NbS2/n-MoS2 van der Waals Schottky Junction and Graphene Contact

A van der Waals (vdW) Schottky junction between two-dimensional (2D) transition metal dichalcogenides (TMDs) is introduced here for both vertical and in-plane current devices: Schottky diodes and metal semiconductor field-effect transistors (MESFETs). The Schottky barrier between conducting NbS₂ and semiconducting n-MoS₂ appeared to be as large as ∼0.5 eV due to their work-function difference. While the Schottky diode shows an ideality factor of 1.8-4.0 with an on-to-off current ratio of 10³-10⁵, Schottky-effect MESFET displays little gate hysteresis and an ideal subthreshold swing of 60-80 mV/dec due to low-density traps at the vdW interface. All MESFETs operate with a low threshold gate voltage of -0.5 ∼ -1 V, exhibiting easy saturation. It was also found that the device mobility is significantly dependent on the condition of source/drain (S/D) contact for n-channel MoS₂. The highest room temperature mobility in MESFET reaches to approximately more than 800 cm²/V s with graphene S/D contact. The NbS₂/n-MoS₂ MESFET with graphene was successfully integrated into an organic piezoelectric touch sensor circuit with green OLED indicator, exploiting its predictable small threshold voltage, while NbS₂/n-MoS₂ Schottky diodes with graphene were applied to extract doping concentrations in MoS₂ channel.

Redox Additive-Improved Electrochemically and Structurally Robust Binder-Free Nickel Pyrophosphate Nanorods as Superior Cathode for Hybrid Supercapacitors

For several decades, one of the great challenges for constructing a high-energy supercapacitor has been designing electrode materials with high performance. Herein, we report for the first time to our knowledge a novel hybrid supercapacitor composed of battery-type nickel pyrophosphate one-dimensional (1D) nanorods and capacitive-type N-doped reduced graphene oxide as the cathode and anode, respectively, in an aqueous redox-added electrolyte. More importantly, ex situ microscopic images of the nickel pyrophosphate 1D nanorods revealed that the presence of the battery-type redox additive enhanced the charge storage capacity and cycling life as a result of the microstructure stability. The nickel pyrophosphate 1D nanorods exhibited their maximum specific capacitance (8120 mF cm^-2 at 5 mV s^-1) and energy density (0.22 mWh cm^-2 at a power density of 1.375 mW cm^-2) in 1 M KOH + 75 mg K₃[Fe(CN)₆] electrolyte. On the other side, the N-doped reduced graphene oxide delivered an excellent electrochemical performance, demonstrating that it was an appropriate anode. A hybrid supercapacitor showed a high specific capacitance (224 F g^-1 at a current density of 1 A g^-1) and high energy density (70 Wh kg^-1 at a power density of 750 W kg^-1), as well as a long cycle life (a Coulombic efficiency of 96% over 5000 cycles), which was a higher performance than most of those in recent reports. Our results suggested that the materials and redox additive in this novel design hold great promise for potential applications in a next-generation hybrid supercapacitor.

Amorphous Phosphorus-Incorporated Cobalt Molybdenum Sulfide on Carbon Cloth: An Efficient and Stable Electrocatalyst for Enhanced Overall Water Splitting over Entire pH Values

The development of economical, proficient, and highly stable catalysts to substitute the expensive noble metal electrodes for electrocatalytic water-splitting applications is exceedingly desirable. In this context, the most fascinating and challenging approach is the rational design of a nanocomposite encompassing multiple components with unique functionalities. Herein, we describe the fabrication of a strongly catalytic and superb durable phosphorus-incorporated cobalt molybdenum sulfide electrocatalyst grown on carbon cloth (P-CoMoS/CC). The hybrid material exhibited excellent activity for hydrogen and oxygen evolution reactions over a wide range of pH (1-14) with extremely high stability (∼90% retention of the initial current density) after 24 h of electrolysis. Importantly, when P-CoMoS/CC was used as both cathode and anode for overall water splitting, a very low cell voltage of 1.54 V is required to attain the 10 mA cm^-2 current density, and the hybrid material exhibited a long-term stability (89.8% activity retention after 100 h). The outstanding overall water-splitting performance compared to an electrolyzer consisting of the noble-metal-based catalysts Pt/C and RuO₂ makes P-CoMoS one of the most efficient earth-abundant water-splitting catalysts. Phosphorus incorporation was proved to be a vital aspect for the improved charge-transfer properties and catalytic durability of the P-CoMoS/CC catalyst.

Carrier scattering in quasi-free standing graphene on hexagonal boron nitride

Graphene, a two-dimensional material with a honeycomb lattice, has been promoted as a next generation material because of its ultrafast charge carriers and superior electrical properties. Hexagonal boron nitride (h-BN) is an insulator explored as an ideal substrate for graphene with lattice-matching. Using raido-frequency (RF) transmission measurement which provides specific characteristics of carrier scattering in a device, we profoundly investigated the electrical properties of quasi-free standing graphene on h-BN. RF devices with graphene supported and encapsulated with h-BN were fabricated to analyze the RF signal at low temperatures from 100 to 300 K. We demonstrated the carrier behavior in graphene with thermally excited carriers and acoustic photon scattering according to heat energy. Both h-BN supported and encapsulated graphene showed a significant enhancement in RF transmission, which is close to a gold interconnector. Our device with graphene on h-BN exhibited concealed nonlinear characteristics at a specific temperature of 180 K due to the internal effects of acoustic phonon scattering, while a usual device with graphene on SiO₂/Si provided a linear variation. To anticipate the potential for electronic applications, the electrical circuit properties such as impedance, resistance, and inductance were extracted from the results of RF measurement.

Contact Effect of ReS2/Metal Interface

Rhenium disulfide (ReS₂) has attracted immense interest as a promising two-dimensional material for optoelectronic devices owing to its outstanding photonic response based on its energy band gap's insensitivity to the layer thickness. Here, we theoretically calculated the electrical band structure of mono-, bi-, and trilayer ReS₂ and experimentally found the work function to be 4.8 eV, which was shown to be independent of the layer thickness. We also evaluated the contact resistance of a ReS₂ field-effect transistor using a Y-function method with various metal electrodes, including graphene. The ReS₂ channel is a strong n-type semiconductor, thus a lower work function than that of metals tends to lead to a lower contact resistance. Moreover, the graphene electrodes, which were not chemically or physically bonded to ReS₂, showed the lowest contact resistance, regardless of the work function, suggesting a significant Fermi-level pinning effect at the ReS₂/metal interface. In addition, an asymmetric Schottky diode device was demonstrated using Ti or graphene for ohmic contacts and Pt or Pd for Schottky contacts. The ReS₂-based transistor used in this study on the work function of ReS₂ achieved the possibility of designing the next-generation nanologic devices.

Honeycomb-Like Interconnected Network of Nickel Phosphide Heteronanoparticles with Superior Electrochemical Performance for Supercapacitors

Transition-metal-based heteronanoparticles are attracting extensive attention in electrode material design for supercapacitors owing to their large surface-to-volume ratios and inherent synergies of individual components; however, they still suffer from limited interior capacity and cycling stability due to simple geometric configurations, low electrochemical activity of the surface, and poor structural integrity. Developing an elaborate architecture that endows a larger surface area, high conductivity, and mechanically robust structure is a pressing need to tackle the existing challenges of electrode materials. This work presents a supercapacitor electrode consisting of honeycomb-like biphasic Ni₅P₄-Ni₂P (Ni_xP_y) nanosheets, which are interleaved by large quantities of nanoparticles. The optimized Ni_xP_y delivers an ultrahigh specific capacity of 1272 C g^-1 at a current density of 2 A g^-1, high rate capability, and stability. An asymmetric supercapacitor employing as-synthesized Ni_xP_y as the positive electrode and activated carbon as the negative electrode exhibits significantly high power and energy densities (67.2 W h kg^-1 at 0.75 kW kg^-1; 20.4 W h kg^-1 at 15 kW kg^-1). These results demonstrate that the novel nanostructured Ni_xP_y can be potentially applied in high-performance supercapacitors.