Insight to the enhanced microwave absorption of porous N-doped carbon driven by ZIF-8:Competition between graphitization and porosity

Porous carbon-based materials are promised to be lightweight dielectric microwave absorbents.Deeply understanding the influence of graphitization grade and porous structure on the dielectric parameters is urgently required.Herein,utilizing the low boiling point of Zn,porous N-doped carbon was fabricated by carbonization of ZIF-8(Zn)at different temperature,and the microwave absorption performance was investigated.The porous N-doped carbon inherits the high porosity of ZIF-8 precursor.By increasing the carbonization temperature,the contents of Zn and N elements are decreased;the graphitization degree is improved;however,the specific surface area and porosity are increased first and then decreased.When the carbonization temperature is 1000°C,the porous N-doped carbon behaves enhanced microwave absorption.With an ultrathin thickness of 1.29 mm,the ideal RL reaches-50.57 dB at 16.95 GHz and the effective absorption bandwidth is 4.17 GHz.The mechanism of boosted microwave absorption is ascribed to the competition of graphitization and porosity as well as N dopants,resulting in high dielectric loss capacity and good impedance matching.The porous structure can prolong the pathways and raise the contact opportunity between microwaves and porous carbon,causing multiple scattering,interface polarization,and improved impedance matching.Besides,the N dopants can induce electron polarization and defect polarization.These results give a new insight to construct lightweight carbon-based microwave absorbents by regulating the graphitization and porosity.

Ultrathin MoS2 with expanded interlayers supported on hierarchical polypyrrole-derived amorphous N-doped carbon tubular structures for high-performance Li/Na-ion

Layered molybdenum disulfide （MoS2） has received much attention as one of the most promising energy-storage and conversion materials for Li/Na ion batteries. Here, a simple and effective approach is proposed for the rational design and preparation of hierarchical three-d imensional （3D） amorphous N-doped carbon nanotube@MoS2 nanosheets （3D-ANCNT@MoS2） via a simple hydrothermal method, followed by an annealing process. With such a unique nanoarchitecture, ultrathin MoS2 nanosheets grown on the external surfaces of polypyrrole-derived ANCNTs are assembled to form a hierarchical 3D nanoarchitecture, where the adopted ANCNTs serve not only as the template and continuous conductive matrix, but can also prevent MoS2 from aggregating and restacking, and help to buffer the volumetric expansion of MoS2 during cycling. More importantly, when evaluated as an anode material for lithium-ion batteries, the 3D-ANCNT@MoS2 composite exhibits excellent cycling stability, superior rate performance, and reversible specific capacity as high as 893.4 mAh·g^-1 at 0.2 A·g^-1 after 200 cycles in a half battery, and 669.4 mAh·g^-1 at 0.2 A·g^-1 after 100 cycles in the 3D-ANCNT@Mo2//LiCoO2 full battery. With respect to sodium-ion batteries, the outstanding reversible capacity, excellent rate behavior, and good cycling performance of 3D-ANCNT@MoS2 composites are also achieved.

Dynamically observing the formation of MOFs-driven Co/N-doped carbon nanocomposites by in-situ transmission electron microscope and their application as high-efficient microwave absorbent

Metal-organic frameworks(MOFs)derived magnetic carbon-based nanocomposites have drawn widespread attentions due to the well distributed nanocrystals in carbon matrix.Dynamically observing the formation process is urgently needed.Herein,taking zeolitic imidazolate framework(ZIF)-67 as an example,the pyrolysis process is investigated by in-situ transmission electron microscopy(TEM)assisted with ex-situ characterizations.Co nanocrystals are evenly distributed in carbon at the initial stage of carbonization.By increasing pyrolysis temperature,the nanocrystals grow bigger and migrate to carbon surface.The carbon texture transfers from amorphous to crystalline at 600°C,and thoroughly converts at 800°C.In-situ heating TEM shows that more tiny Co nanocrystals move out from the carbon texture by increasing temperature from 700 to 800°C.At 1,000°C,some escaped tiny Co nanocrystals are volatilized and disappeared.The residual escaped Co nanocrystals catalyze the formation of carbon nanotubes(CNTs).Due to the synergistic effect between Co and carbon as well as porous structure,the nanocomposites show high-efficient microwave absorption performance,which can be tuned by pyrolysis temperature,heating rate,and mass fraction.When the mass fraction is 30 wt.%,the nanocomposites obtained at 600 or 700°C display remarkable microwave absorption with optimal reflection loss(RL)smaller than−70 dB and effective absorption band larger than 4.9 GHz.Combining the in-situ and ex-situ techniques,some key findings were observed:(1)graphitization of carbon;(2)volatilization of Co nanocrystals;(3)formation process of CNTs by Co catalyst.These findings are helpful to understand the formation of MOFs derived carbon-based composites and expand their practical applications,especially for microwave absorption.

Cation ratio and oxygen defects for engineering the magnetic transition of monodisperse nonstoichiometric zinc ferrite nanoparticles

Monodisperse nonstoichiometric zinc ferrite nanoparticles with a tunable size of 4.1–32.2 nm are fabricated via thermal decomposition. An extrinsic impurity phase of the ZnO component is present in the zinc ferrite nanoparticles with a size of <10 nm, but this phase can be eliminated after the air annealing treatment. The atom ratio of Zn/Fe and concentration of oxygen vacancies decrease as the particle size of zinc ferrite increases, causing magnetic transition from superparamagnetism to ferromagnetism. The X-ray magnetic circular dichroism spectra reveal that the spin magnetic moments of Fe^(3+)are reduced, and the orbital magnetic moments are frozen with the increasing atom ratio of Zn/Fe. Therefore,saturation magnetization decreases. The saturation magnetizations of all the zinc ferrite nanoparticles decrease after the air annealing treatment, suggesting that oxygen vacancies considerably influence the magnetic properties. The air annealing treatment can minimize the number of oxygen defects,which trigger some of the Fe^(3+)–OV–Fe^(3+)ferrimagnetic couplings to transfer into the Fe^(3+)–O^(2-)–Fe^(3+)antiferromagnetic couplings. This work provides new insights regarding the magnetic performance of spinel ferrites by tuning the stoichiometric ratio and oxygen defects.