Joule-Heating-Driven Synthesis of a Honeycomb-Like Porous Carbon Nanofiber/High Entropy Alloy Composite as an Ultralightweight Electromagnetic Wave Absorber.

High entropy alloys (HEA) have garnered significant attention in electromagnetic wave (EMW) absorption due to their efficient synergism among multiple components and tunable electronic structures. However, their high density and limited chemical stability hinder their progress as lightweight absorbers. Incorporating HEA with carbon offers a promising solution, but synthesizing stable HEA/carbon composite faces challenges due to the propensity for phase separation during conventional heat treatments. Moreover, EMW absorption mechanisms in HEAs may be different from established empirical models due to their high-entropy effect. This underscores the urgent need to synthesize stable and lightweight HEA/carbon absorbers and uncover their intrinsic absorption mechanisms. Herein, we successfully integrated a quinary FeCoNiCuMn HEA into a honeycomb-like porous carbon nanofiber (HCNF) using electrostatic spinning and the Joule-heating method. Leveraging the inherent lattice distortion effects and honeycomb structure, the HCNF/HEA composite demonstrates outstanding EMW absorption properties at an ultralow filler loading of 2 wt %. It achieves a minimum reflection loss of -65.8 dB and boasts a maximum absorption bandwidth of up to 7.68 GHz. This study not only showcases the effectiveness of combining HCNF with HEA, but also underscores the potential of Joule-heating synthesis for developing lightweight HEA-based absorbers.

Entropy engineering enhances the electromagnetic wave absorption of high-entropy transition metal dichalcogenides/N-doped carbon nanofiber composites.

Entropy engineering strategies provide a broader platform for exploring the behavior of electromagnetic wave (EMW) absorption materials and their absorption mechanisms on the microscopic scale. In this work, a novel entropy engineering strategy was developed to improve the EMW absorption properties of MoS2. A hierarchical N-doped carbon nanofiber/MoS2 (NCNF/MS) composite was synthesized using the electrospinning and hydrothermal methods. Then, the conformational entropy of MoS2 was increased by sequentially integrating elements such as W, Se, and Te. Although MoS2 maintains a single 2H-phase structure throughout the entropy increase process, it triggers a series of complex changes at the microscopic level, including lattice distortion, ingenious electronic structure adjustments, and an increase in defect density. These changes provide more possibilities for the EMW interaction with the absorber, which significantly enhances the dielectric behavior of the composites, including conduction and polarization losses. Owing to the unique hierarchical structure and rich defect structure, the obtained entropy-increased NCNF/MWSST exhibits excellent EMW absorption performance. The minimum reflection loss reaches -60.7 dB, and the maximum effective absorption bandwidth is 6.48 GHz, which is improved by almost 584% and 810% compared to NCNF/MS. This study provides a new way to design efficient and high-performance MoS2-based absorbers and provides valuable insights for exploring the entropy-increasing strategies to optimize the EMW absorption properties.

Fe Engineering on Ru Nanosheets for Enhanced Hydrogen Evolution in pH-Universal Media.

Fe-modified Ru nanosheets are achieved via preintercalated Al species serving as the self-sacrificial template. Benefiting from the amphoteric feature of Al and strong corrosion of Fe3+ ions, Fe is effectively incorporated into pristine Ru nanosheets. Correspondingly, the surface oxophilicity is improved, promoting the Volmer step. The charge density redistribution weakens hydrogen combination on Ru and thus accelerates the desorption kinetics (Heyrovsky step). Meanwhile, more defective sites are exposed, leading to an enhanced hydrogen production in pH-universal electrolytes.

Identifying the Active Site on Graphene Oxide Nanosheets for Ambient Electrocatalytic Nitrogen Reduction.

Identifying the active sites on graphene oxide (GO) nanosheets is of great importance. In situ electroreduction at different potentials is applied to control the oxygenated groups on GO surfaces. Both experiments and theoretical calculations suggest the C═O group is critical for N2 adsorption and activation, guaranteeing the ambient electrocatalytic N2 reduction.

Activating Graphyne Nanosheet via sp-Hybridized Boron Modulation for Electrochemical Nitrogen Fixation.

Exploring new metal-free catalysts with high activity for nitrogen reduction reaction (NRR) is highly desirable but remains a big challenge. Graphyne (GY) is a typical two-dimensional carbon material with many excellent properties. However, the NRR has rarely been envisaged on a GY-based metal-free catalyst up to now. Density functional theory calculations reveal that although pristine GY is inactive for N2 reduction, boron modulation can endow it with efficient activity toward NRR. Natural bond orbitals analysis, spin/charge density distributions, and free energy change diagrams are performed and discussed. Three boron doping formats including sp2-substituted, sp-substituted, and adsorbed configuration are considered. The obtained data show sp-substitution will induce local moderate spin and charge densities at the boron site on the GY surface, which is convenient for N2 adsorption and activation, and conductive to N-related intermediates formation and transformation. Moreover, the incorporated sp-hybridized boron can provide one empty p orbital and one occupied p orbital around itself, which plays a key role as an electron reservoir to accept electrons from and donate electrons to the adsorbed N-related species, and thus facilitate N2 reduction and ammonia synthesis. Henceforth, it provides more opportunities for preparing GY and other carbon materials as efficient catalysts toward renewable energy conversion and storage.

Enhanced Phase Transition Properties of VO2 Thin Films on 6H-SiC (0001) Substrate Prepared by Pulsed Laser Deposition.

For growing high quality epitaxial VO2 thin films, the substrate with suitable lattice parameters is very important if considering the lattice matching. In addition, the thermal conductivity between the substrate and epitaxial film should be also considered. Interestingly, the c-plane of hexagonal 6H-SiC with high thermal conductivity has a similar lattice structure to the VO2 (010), which enables epitaxial growth of high quality VO2 films on 6H-SiC substrates. In the current study, we deposited VO2 thin films directly on 6H-SiC (0001) single-crystal substrates by pulsed laser deposition (PLD) and systematically investigated the crystal structures and surface morphologies of the films as the function of growth temperature and film thickness. With optimized conditions, the obtained epitaxial VO2 film showed pure monoclinic phase structure and excellent phase transition properties. Across the phase transition from monoclinic structure (M1) to tetragonal rutile structure (R), the VO2/6H-SiC (0001) film demonstrated a sharp resistance change up to five orders of magnitude and a narrow hysteresis width of only 3.3 °C.

Holey Ruthenium Nanosheets with Moderate Aluminum Modulation toward Hydrogen Evolution.

Theoretical calculations reveal that aluminum (Al) doping can effectively modulate the electronic structures of 2D ruthenium (Ru) catalysts. Moderate Al incorporation can endow Ru nanosheets with more delocalized electrons and optimal hydrogen adsorption Gibbs free energy, providing opportunities to achieve improved hydrogen evolution performance. Thus, Al-doped Ru nanosheets have been synthesized by a solvothermal strategy, in which they exhibit holey nanosheet structures and have more active sites exposed on the basal plane. The characterizations unraveling the Ru structure can be well maintained, and electrochemical measurements confirm the appropriate amount of Al modulation that can extremely enhance its hydrogen evolution activity.

Fabrication of NiO/NiCo2O4 Mixtures as Excellent Microwave Absorbers.

The NiO/NiCo2O4 mixtures with unique yolk-shell structure were synthesized by a simple hydrothermal route and subsequent thermal treatment. The elemental distribution, composition, and microstructure of the samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and scanning electron microscope (SEM), respectively. The microwave absorption property was investigated by using vector network analysis (VNA). The results indicated that the excellent electromagnetic wave absorption property of the NiO/NiCo2O4 mixtures was achieved due to the unique yolk-shell structure. In detail, the maximum reflection loss (RL) value of the sample reached up to - 37.0 dB at 12.2 GHz and the absorption bandwidth with RL below - 10 dB was 4.0 GHz with a 2.0-mm-thick absorber. In addition, the NiO/NiCo2O4 mixtures prepared at high temperature, exhibited excellent thermal stability. Possible mechanisms were investigated for improving the microwave absorption properties of the samples.

Facile Preparation of Amorphous Fe-Co-Ni Hydroxide Arrays: A Highly Efficient Integrated Electrode for Water Oxidation.

Facile and fast synthesis of functional materials with high catalytic activity is highly demanded to meet the industrial production and applications such as electrolysis. In this study, Ni foam is employed as the current collector and Ni source, which is dipped into the mixture of Fe and Co metal ions solution at room temperature for several minutes, to in situ grow Fe-Co-Ni hydroxide arrays and construct the three-dimensional integrated electrode. This short-time preparation at room temperature is beneficial to avoid the rapid growth of the generated primary nanocrystallites and cause intimate interactions between Fe, Co, and Ni atoms. The obtained self-supported and vertically aligned Fe-Co-Ni hydroxides present an amorphous phase, which exhibit high activity with low overpotentials of 212 mV at 10 mA cm-2 and 319 mV at 100 mA cm-2, associated with a small Tafel slope of 52 mV dec-1 toward the oxygen evolution reaction.

Anion Engineering on Free-Standing Two-Dimensional MoS2 Nanosheets toward Hydrogen Evolution.

On the basis of theoretical predictions, nitrogen was designed and incorporated into free-standing two-dimensional MoS2 nanosheets. Both the amount of electrochemical active sites on the surface and its intrinsic conductivity could be significantly increased as a result of anion engineering, which can extremely improve the electrocatalytic kinetics toward hydrogen evolution.

Elemental two-dimensional nanosheets beyond graphene.

The great success of graphene has encouraged the fast development of other two-dimensional (2D) nanosheets, which have attracted extensive attention in different scientific fields encompassing field effect transistors, lithium-ion batteries, and catalysis. With atomic-scale thickness, almost all of the atoms are exposed on the surface, providing an extremely high specific surface area, in conjunction with special physical, chemical, and electronic properties, owing to the quantum confinement effects, which enable their surface phase to be as important as the bulk counterparts. In this review, we have summarized and discussed the recent advancements of 2D nanomaterials beyond graphene, with an emphasis on their basic fundamentals, preparation strategies, and applications. We believe that this review supplies critical insights for exploring and understanding 2D nanomaterials and puts forward the challenges and opportunities for further developments, such as more precise morphology control, foreign atom doping and surface modification technologies, atomic-scale characterization, and finding wide applications in many different fields.