Excellent NiO–Ni Nanoplate Microwave Absorber via Pinning Effect of Antiferromagnetic–Ferromagnetic Interface

Materials with strong magnetic property that can provide excellent microwave absorption performance are highly desirable, especially if their dielectric and magnetic properties can be easily modulated, which make minimal thickness and ultrawide bandwidth become achievable. The magnetic properties of ferromagnetic (FM) and antiferromagnetic (AFM) composite materials are closely related to their ratio of composition, size, morphology, and structure. AFM–FM composites have become a popular alternative for microwave absorption; however, the controllable design and preparation need to be urgently optimized. Here, we have successfully prepared a series of platelike NiO–Ni composites and demonstrated the potential of such composites for microwave absorption. Strong magnetic coupling was found from NiO–Ni nanoparticles by electron holography, which makes NiO–Ni composites a highly efficient microwave absorber (strong reflection loss: −61.5 dB and broad bandwidth: 11.2 GHz, reflection loss < −10 dB). Our findings are helpful to develop a strong microwave absorber based on magnetic coupling

Copper- and Cobalt-Codoped CeO₂ Nanospheres with Abundant Oxygen Vacancies as Highly Efficient Electrocatalysts for Dual-Mode Electrochemical Sensing of MicroRNA

Oxide materials with redox properties have aroused growing interest in many applications. Introducing dopants into crystal lattices provides an effective way to optimize the catalytic activities of the oxides as well as their redox properties. Herein, CeO2 nanospheres codoped with Cu and Co (CuCo–CeO2 NSs) were first synthesized and exploited as efficient electrocatalysts for dual-mode electrochemical sensing of microRNA (miRNA). With the doping of Cu and Co into the CeO2 lattice, large amounts of extra oxygen vacancies were generated, remarkably enhancing the redox and electrocatalytic properties of the CeO2 material. The abundant oxygen vacancies of the CuCo–CeO2 NSs were further identified by X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), and electron-energy-loss spectroscopy (EELS). Moreover, Mg2+-induced DNAzyme-assisted target recycling was introduced for ultrasensitive determination. The dual-mode sensing with generality was conducted as follows: First, the CuCo–CeO2 NSs acted as a direct redox mediator to generate a differential-pulse-voltammetry (DPV) signal, which was then greatly amplified by the efficient electrocatalysis of CuCo–CeO2 NSs toward H2O2 decomposition. Second, under the electrocatalysis of CuCo–CeO2 NSs, 3,3-diaminobenzidine (DAB) was oxidized to form nonconductive insoluble precipitates (IPs), leading to great amplification of the electrochemical-impedimetric-spectroscopy (EIS) signal. The dual-mode electrochemical sensor showed a wide linear range (0.1 fM to 10 nM) with a low detection limit (33 aM), paving a new way for constructing ultrasensitive electrochemical sensors

Electron Holography of Yolk–Shell Fe₃O₄@mSiO₂ Microspheres for Use in Microwave Absorption

Yolk–shell nanostructures are very promising alternatives as next-generation microwave absorption materials. However, it still remains a great challenge to understand structure–property relationships and develop highly efficient yolk–shell structured microwave absorbers. Herein, a series of dielectric-magnetic yolk–shell Fe3O4@mesoporous SiO2 microspheres (denoted as Fe3O4@mSiO2) were synthesized by combining a modified Stöber method and hydrothermal etching process. The microwave absorption performance of the Fe3O4@mSiO2 yolk–shell microspheres could be optimized by tuning the void space and shell number easily. By increasing the average diameter of the void space from 380 to 490 nm, the maximum reflection loss (RL) value of −38.7 dB at 8 GHz was achieved with a composite thickness of 2 mm, and the effective bandwidth (RL < −10 dB) was beyond 4 GHz. Compared with the single-shelled counterparts, the double-shelled Fe3O4@mSiO2 microspheres exhibited significantly enhanced absorption properties. Importantly, electron holography analysis confirmed that the enhanced microwave absorption could be ascribed to the unique architecture. Our research can give insights into understanding the structure–property relationships and provide an avenue for fabrication of high-performance and lightweight microwave absorbers based on yolk–shell structures

Hierarchical Fe₂O₃@C@MnO₂@C Multishell Nanocomposites for High Performance Lithium Ion Batteries and Catalysts

The Fe2O3@C@MnO2@C (FCMC) nanocomposites containing spindle-like Fe2O3 as a core and MnO2 nanoflakes as a sandwiched shell and double carbon layers have been successfully prepared by a facile method. As anode materials of lithium ion batteries (LIBs), the cycling stability, rate performance, and conductivity of the prepared FCMC nanocomposites are far beyond those of the carbon-free Fe2O3@MnO2 (FM) nanocomposites. The hierarchical structure with double layers of carbon effectively enhances the ion conductivity and electrochemical performance of transitional metal oxides, indicating that carbon in FCMC played an important role during lithium ion storage. The initial discharge/charge capacity of the FCMC electrode reaches as high as 1240.2/1215.9 mAh g–1, and the discharge capacity is over 1000 mAh g–1 at 500 mA g–1 after 50 cycles. Additionally, the unique hierarchical structural characteristic with double layers of green carbon with a high degree of graphitization makes FCMC an excellent catalyst in removing methylene blue (MB) dye from solution with H2O2 under a slight heating with the degradation time as short as 10 min. Our work presents a new perspective on carbon modified multilayer core–shell oxide structure, which can be applied to many fields such as energy storage and catalyst

“Matryoshka Doll”-Like CeO₂ Microspheres with Hierarchical Structure To Achieve Significantly Enhanced Microwave Absorption Performance

Recently, it is still a great challenge to develop a new type of absorber that possesses special advantages of low cost, ultrawide bandwidth, and strong absorption intensity. Herein, the unique “Matryoshka doll”-like CeO₂ microspheres with tunable interspaces were successfully synthesized by a facile and template-free method. The as-synthesized hierarchical yolk–shell CeO₂ microspheres were constructed by a layer of outer shell and multiple inner cores. The interspace gap of the microspheres can be simply adjusted only by altering the solvothermal reaction time. Simultaneously, Ostwald ripening, Kirkendall effect, and self-etching process contribute a synergetic growth mechanism responsible for this amazing hierarchical architecture. Importantly, the “Matryoshka doll”-like CeO₂ microspheres exhibited significantly strong microwave absorption in the frequency range of 2–18 GHz, with a reflection loss of −71.3 dB at 14.5 GHz and an effective absorption bandwidth of 5.4 GHz (<−10 dB), which is superior to the multicomponent absorbers. Such an outstanding microwave absorption performance stems from the unique hierarchical yolk–shell structure and the designable interspaces, leading to the multiple scattering, interfacial polarization, and plasma dielectric oscillation from the abundant interfaces and curved surfaces, which can be illustrated by the related results from electron holography and electron energy loss spectroscopy. To the best of our knowledge, the “Matryoshka doll”-like CeO₂ microspheres with a facile synthesis process, low cost, and excellent microwave absorption performance are believed to be an optimal candidate of single-component absorbers and helpful in the study of absorption mechanism

Self-Assembly-Magnetized MXene Avoid Dual-Agglomeration with Enhanced Interfaces for Strong Microwave Absorption through a Tunable Electromagnetic Property

Multilayered microwave absorbers which can provide massive interfaces are highly needed for electromagnetic-wave absorption property enhancement. Meanwhile, how to effectively avoid agglomeration and further widen the absorption band is still a challenge. Herein, accordion-like magnetized MXene/Ni composites were fabricated by the electrostatic self-assembly interaction between multilayer MXene and Ni­(OH)2 nanoplates and subsequent in situ reduction in the H2/Ar atmosphere. Ni nanoparticles were uniformly distributed without magnetic agglomeration between the multilayered gaps of the adjacent 2D (2 dimension) MXene (Ti3C2Tx) of MXene/Ni nanocomposites (magnetized MXene), which hold the distinct absorption performance that the reflection loss maximum measures up to −50.5 dB at 5.5 GHz. Moreover, dynamic magnetic response of the magnetized MXene absorber was first researched by the electron holography analysis. The related key mechanism includes the enhanced magnetic loss, less dual-agglomeration (multilayer MXene itself and magnetic agglomeration), and more interfaces and intrinsic defects for related polarization. A broadened absorption bandwidth can further be obtained by changing the mass ratio of MXene to Ni that possesses the widest absorption bandwidth of 5.28 GHz. This work provides a new route for the balance among strong absorption intensity, tunable electromagnetic properties, and wide absorption bandwidth of the MXene-based nanocomposites

Self-Assembly-Magnetized MXene Avoid Dual-Agglomeration with Enhanced Interfaces for Strong Microwave Absorption through a Tunable Electromagnetic Property

Multilayered microwave absorbers which can provide massive interfaces are highly needed for electromagnetic-wave absorption property enhancement. Meanwhile, how to effectively avoid agglomeration and further widen the absorption band is still a challenge. Herein, accordion-like magnetized MXene/Ni composites were fabricated by the electrostatic self-assembly interaction between multilayer MXene and Ni­(OH)2 nanoplates and subsequent in situ reduction in the H2/Ar atmosphere. Ni nanoparticles were uniformly distributed without magnetic agglomeration between the multilayered gaps of the adjacent 2D (2 dimension) MXene (Ti3C2Tx) of MXene/Ni nanocomposites (magnetized MXene), which hold the distinct absorption performance that the reflection loss maximum measures up to −50.5 dB at 5.5 GHz. Moreover, dynamic magnetic response of the magnetized MXene absorber was first researched by the electron holography analysis. The related key mechanism includes the enhanced magnetic loss, less dual-agglomeration (multilayer MXene itself and magnetic agglomeration), and more interfaces and intrinsic defects for related polarization. A broadened absorption bandwidth can further be obtained by changing the mass ratio of MXene to Ni that possesses the widest absorption bandwidth of 5.28 GHz. This work provides a new route for the balance among strong absorption intensity, tunable electromagnetic properties, and wide absorption bandwidth of the MXene-based nanocomposites

High-Density Anisotropy Magnetism Enhanced Microwave Absorption Performance in Ti₃C₂T_<i>x</i> MXene@Ni Microspheres

Two-dimensional materials, especially the newly emerging MXene, have attracted numerous interests in the fields of energy conversion/storage and electromagnetic shielding/absorption. However, the inherently inevitable aggregation and absence of magnetic loss of MXene considerably limit its electromagnetic absorption application. The introduction of magnetic component and favorable structural engineering are the alternatives to improve the microwave absorption (MA) performance. Herein, we report a spheroidization strategy to assemble double-shell MXene@Ni microspheres, where the commonly lamellar MXene are reshaped into three-dimensional microspheres that provide the substrate for oriented growth of Ni nanospikes. Whereas this structural feature offers massive accessible active surfaces that effectively promote the dielectric loss ability, the introduction of magnetic Ni nanospikes enables the additional magnetic loss capacity. Benefiting from these merits, the synthesized 3D MXene@Ni microspheres exhibit superior MA performance with the minimum reflection loss value of −59.6 dB at an ultrathin thickness (∼1.5 mm) and effective absorption bandwidth of 4.48 GHz. Moreover, the electron holography results reveal that the high-density anisotropy magnetism plays an important role in the improvement of MA performance, which provides an insight for the design of MXene-based materials as high-efficient microwave absorbers