•Multi-shelled Co3O4 hollow spheres were synthesized by an easy hydrothermal method.•Interconnected triple-shelled Co3O4 hollow spheres were synthesized.•The Co3O4 samples show unexpected high specific discharge capacity.We report a facile, one-pot hydrothermal synthesis of Co3O4 solid spheres and multi-shelled Co3O4 hollow spheres with a controlled number of movable internal Co3O4 shells. Moreover, the magnetic properties of the multi-shelled Co3O4 hollow spheres were first investigated by the SQUID magnetometer. Interestingly, the Co3O4 solid spheres calcined at 430 °C deliver an unexpected high specific discharge capacity of 1976 and 1129 mAh g−1 for the 17th and 100th cycle at 100 mA g−1, respectively. In addition, the Co3O4 solid spheres calcined at 430 °C also show good capacity retention (i.e., 1129 mAh g−1 after 100 cycles). The significant performance improvement offers the potential to open up an avenue for next-generation LIBs.Download high-res image (105KB)Download full-size image

•The ZnFe2O4 with controlled morphologies and structures have been investigated.•We summarized the synthetic methods as well as the modifications of the ZnFe2O4.•The review also dealt with applications of the ZnFe2O4.ZnFe2O4 is an attractive material due to its unique properties and various applications. A large volume of works on the synthesis of ZnFe2O4 have been reported such as mechanochemical synthesis, co-precipitation method, sol-gel auto-combustion method, electrospinning method, hydrothermal/solvothermal method, and spray drying. The synthetic methodologies have significant influence on the properties, morphologies and structures of the ZnFe2O4. Moreover, with the Fe3+ and Zn2+ substituted by different metal ions, the properties of ZnFe2O4 can be altered. Besides, the zinc ferrite composites with controlled morphologies and structures have been investigated. Owing to the enhanced magnetic, electrical and catalytic properties of the zinc ferrite composites, extensive applications of zinc ferrite composites have been achieved. In this review, we summarized the synthetic methods as well as the modifications of the ZnFe2O4. The review also dealt with applications of the ZnFe2O4 and its composites in the fields of sensors, photocatalysis and lithium ion batteries, etc.Download high-res image (42KB)Download full-size image

•Multi-shelled NiO hollow spheres were synthesized by an easy hydrothermal method.•Quintuple-shelled NiO hollow spheres were synthesized for the first time.•The samples show promising electrochemical performances for lithium-ion battery.Complex metal oxide hollow structures are mostly prepared by hard template-based multistep procedures. It is still desirable yet challenging to develop new efficient strategies to fabricate high-quality complex metal oxide hollow structures. Herein, uniform multi-shelled NiO hollow spheres were synthesized; especially quintuple-shelled NiO hollow spheres were synthesized for the first time by a simple shell-by-shell self-assembly capable of controlling the shell numbers, which are achieved by controlling the heat treatment. The research makes a significant contribution to the synthetic methodology of multi-shelled hollow structures and opens up new opportunities for deeper understanding formation mechanism of the shell-by-shell self-assembly. The obtained quadruple/quintuple-shelled NiO hollow spheres show promising electrochemical performance in anodic lithium storage for Li-ion battery.The lithium ion battery performance of the quadruple/quintuple-shelled NiO hollow spheres was clearly superior to that of the other types of multi-shelled NiO hollow spheres synthesized.

Nanostructured 3D CeO2 have been synthesized by using PEG 2000 assisted simple hydrothermal technique. The impact of this novel approach on the structure, morphology, spectroscopy, complex permittivity and permeability was discussed. CeO2 was studied by employing X-ray diffraction (XRD), Raman spectra, field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TG) and vector network analyzer. It can be seen that the microspheres of 1–3 μm in diameter were prepared at 600 °C with a mole ratio of 2:1, while the decreasing concentration of PEG-2000 resulted in the appearance of dim side-shape geometry. Obviously, the morphology of the synthesized CeO2 gradually changed from sphere with relatively smooth to irregular side-shape annealed at 600 °C with the increase of mole ratio for reactants. The electromagnetic (EM) wave absorption properties of the synthesized CeO2 were investigated over the range from 2 to 16 GHz, and minimum reflection loss (RL) with −19.3 dB was observed at 15.8 GHz with the thickness of 2.0 mm. This phenomenon may be attributed to both intrinsic characteristic of CeO2 and preparation with the function of PEG-2000.

Amorphous hierarchical NiCo2O4–CoNiO2 hybrids have been successfully fabricated via a facile one-pot hydrothermal route, followed by morphological conversion into urchin-like structured NiCo2O4–CoNiO2 nanorods and irregular-shaped hierarchical NiCo2O4–CoNiO2 polyhedral nanocrystals through air-annealing treatment at 450 °C and 650 °C, respectively. The phase structure, morphology and chemical composition have been characterized in detail. Calcined hierarchical NiCo2O4–CoNiO2 hybrids show improved microwave absorption properties, which are ascribed to the synergistic effect of dielectric CoNiO2 and NiCo2O4 phases. In particular, the calcined hierarchical NiCo2O4–CoNiO2 hybrids at 450 °C exhibit significant enhancement in complex permittivity with respect to others due to their remarkable dipole polarization and interfacial polarization. The maximum reflection loss (RL) of the calcined hierarchical NiCo2O4–CoNiO2 hybrids at 450 °C reaches −42.13 dB at 11.84 GHz with a matching thickness of 1.55 mm, and a relatively broad absorption bandwidth (RL ≤ −10 dB) in the 13.12–17.04 GHz range. Very interestingly, the electromagnetic (EM) wave absorption performance of the hierarchical NiCo2O4–CoNiO2 hybrids shows dependence on the Co2+/Co3+ ratio. The calcined NiCo2O4–CoNiO2 hybrids at 450 °C of the most defect concentration possess the best EM wave absorption ability among all the samples. The results suggest that appropriate interactions between the building blocks in hybrids can guide us to design and fabricate highly efficient EM wave absorption materials.

•γ-Fe2O3@C nanorod-carbon sphere was synthesized via a facile hydrothermal method.•The measured EM parameters show that its maximum RL can reach −8.11 dB at 3.92 GHz.•This is mainly attributed to the multiple-interfacial polarization.Core-shell structured γ-Fe2O3@C nanorod-carbon sphere composites have been synthesized via a facile hydrothermal method employing FeCl3·6H2O as the iron source and d-glucose as the carbon source. The core–shell structured γ-Fe2O3@C nanorods possess a uniform size with ∼500–800 nm in length, the carbon shell's thickness is ∼3–10 nm, and the γ-Fe2O3 core is homogeneously coated by amorphous carbon layer. The measured electromagnetic (EM) parameters show that its maximum reflection loss (RL) can reach −8.11 dB at 3.92 GHz. This is mainly attributed to the multiple-interfacial polarization among the core–shell structured γ-Fe2O3@C nanorod-carbon sphere composites. The core–shell structured γ-Fe2O3@C nanorod-carbon sphere composites can be a promising candidate for lightweight microwave materials.

•Peculiar porous iron oxide nanospheres were synthesized by a facile method.•Microwave absorption properties of porous iron oxide nanospheres were investigated.•Porous α-Fe2O3 nanosphere/paraffin composites exhibited a higher permittivity level.•The composites exhibited better absorption properties than porous γ-Fe2O3 and Fe3O4.We reported a facile approach to prepare peculiar porous α-Fe2O3, γ-Fe2O3 and Fe3O4 nanospheres by combining a facile hydrothermal route with a calcination process in Ar or H2 atmosphere. The synthesized monodisperse porous α-Fe2O3 nanospheres with uniform average diameters of ~ 60 nm in fact contained randomly distributed pores. A close view further revealed that there are two types of pores, one is large mesopores (ca. 15–20 nm) in the center, and the other is small mesopores (ca. < 10 nm) in the outside. After calcining in Ar or H2, the obtained α-Fe2O3, γ-Fe2O3 and Fe3O4 nanospheres preserved the similar morphology and particle size as the uncalcined α-Fe2O3 nanospheres, indicating the as-prepared α-Fe2O3 nanospheres are stable under Ar and H2-annealing heat treatment. Comparing with all the paraffin composites, it was found that the porous α-Fe2O3 nanosphere/paraffin composites exhibit a higher permittivity level. A minimum reflection loss (RL) of − 25 dB was observed at ~ 13 GHz for the porous α-Fe2O3 nanosphere/paraffin composites with a thickness of 3.5 mm, and the effective absorption frequency (RL < − 10 dB) ranged from 9.9 to 15.1 GHz. The composites exhibited better absorption properties than the magnetic porous γ-Fe2O3 and Fe3O4 nanosphere/paraffin composites.

•Urchin-like ZnO hollow spheres were synthesized by a simple hydrothermal method.•Microwave absorption properties of urchin-like ZnO hollow spheres were investigated.•The ZnO calcined at 500 °C exhibited dual peaks with increasing thickness.•The ZnO calcined at 500 °C exhibited better absorption properties than others.Urchin-like ZnO hollow spheres were synthesized by a simple hydrothermal method and their electromagnetic and microwave absorbing properties were investigated. The enlarged SEM image shows that as-prepared ZnO precursor is composed of randomly close-packed porous nanosheets. On the contrary, well-crystallized ZnO spheres are obtained after calcination. The TEM images provide further insight into the detailed structure, showing the typically urchin-like and hollow morphology of ZnO calcined at 500 °C. The SAED pattern clearly demonstrates the single-crystalline feature of urchin-like ZnO hollow spheres. It can be found that the urchin-like ZnO calcined at 500 °C exhibit relatively higher EM wave absorption properties than others. The maximum RL is up to −20 dB at 14.3 GHz and the EM wave absorption shows dual peaks with increasing thickness.

•Porous and hollow ZnO spheres were synthesized by a facile method.•Microwave absorption properties of porous and hollow ZnO spheres were investigated.•It was found that the ZnO exhibited a better performance of microwave absorption.•The relationship of the ZnO and the microwave absorption properties was revealed.Three-dimensional (3D) porous ZnO nanostructures were synthesized via one-pot solvothermal treatment. The structural, morphological and spectral properties were investigated using X-ray diffraction (XRD), N2 sorption measurement, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman spectroscopy, and photoluminescence (PL) spectroscopy. It was found that the minimum reflection loss value of ZnO calcined at 500 °C reached − 5.62 dB at 16.24 GHz with the thickness of 2.5 mm, which had a superior performance of microwave absorption than those uncalcined and calcined at 600 °C. The possible mechanism for the formation of porous ZnO hollow spheres was proposed. The relationship of the ZnO microstructure and the microwave absorption properties was revealed via studying the dielectric loss and interference multi-reflection absorption in this paper as well. In addition, the photoluminescence results show that the uncalcined ZnO and porous hollow ZnO calcined at 500 °C and 600 °C show a narrow and sharp UV emission at 355.8 nm and a relatively broad visible spectra emission at around 423 nm.Download full-size image

Bamboo-like and cubic single-crystalline silicon carbide nanowhiskers (SiCNWs) were synthesized using multiwalled carbon nanotube via a process of calcination in the molten-salt circumstance. The system was heated to 1,250 °C and maintained for 6 h in argon atmosphere, and obtained the sample. The as-prepared sample was characterized by a series of techniques. Especially, the microwave absorption properties of SiCNWs/paraffin composites (30 wt%) were investigated over 2–14 GHz. The result shows the optimal reflection loss can reach −48.1 dB at 13.52 GHz when the thickness of the match is only 1.9 mm. The excellent microwave absorption properties of the SiCNWs/paraffin composites due to the dielectric loss would make it as a promising candidate for the application of absorbing materials. In addition, a possible growth mechanism of SiCNWs was also discussed.

Electromagnetic (EM) and microwave absorbing properties of highly ordered mesoporous carbon (OMC) supported by ultra-fine gold nanoparticle absorber was investigated for the first time. A minimum reflection loss (RL) value of −32 dB at 9.7 GHz and a broad absorption band with the RL values under −10 dB was obtained in the whole Ku-band and X-band when the thickness was 2.0 mm and 2.8 mm, respectively. Au-doped OMC/paraffin composite showed far more excellent microwave absorbing property with respect to undoped absorber. This could be attributed to the promotion effects of Au nanoparticles on better impedance match between absorber and air.Graphical abstractIt was interesting to find that Au/OMC/paraffin shows much higher microwave absorption than OMC/paraffin without doping Au nanoparticles due to the promotion effects of Au nanocrystals on the impedance matching between composite and air.Highlights► Au/OMC/paraffin was successfully prepared by the colloidal deposition method. ► A minimum reflection loss value of −32 dB at 9.7 GHz was obtained with the thickness of 2.8 mm. ► A broad absorption band with the RL values under −10 dB was obtained in the whole Ku-band and X-band. ► Au-doped composite showed excellent microwave absorption with respect to undoped absorber. ► This could be attributed to the promotion effects of Au nanoparticles on impedance match.

We propose and demonstrate a new scheme to improve microwave absorption property through polyaniline (PANI)-functionalized Ni-doped ordered mesoporous carbon (OMC) by in situ polymerization method. The polymer-functionalized nanocomposites, embedding polyaniline within ordered mesoporous carbon, exhibit strong and broadband microwave absorption due to its better dielectric loss characteristic. OMC-Ni0.15/PANI exhibits an effective absorption bandwidth (i.e., reflection loss (RL) ≤ −10 dB) of 4.7 GHz and an absorption peak of −51 dB at 9.0 GHz. The absorption peak intensity and position can be tuned by controlling the thickness of the coating.Highlights► OMC-Ni/PANI nanocomposites were prepared by in situ polymerization method. ► The effective absorption bandwidth was 4.7 GHz for OMC-Ni0.15/PANI. ► OMC-Ni/PANI showed excellent microwave absorption with respect to OMC-Ni. ► This effect could be mainly attributed to the improvement of impendence matching.

Microwave absorbing materials carbonyl iron (CI)-doped Ag/ordered mesoporous carbon (OMC) paraffin wax composites were prepared by colloidal deposition and impregnation methods, and their electromagnetic and microwave absorbing properties were investigated in the frequency ranging from 2 to 18 GHz. The microstructures and chemical compositions of the Ag/OMC and Ag/OMC-CI paraffin wax composites were characterized by TEM, XRD, XPS, SEM and EDS, respectively. The complex permittivity of the paraffin wax composites show dual resonance behavior, resulting from the multi-interfaces among Ag nanoparticles, OMC nanorods, CI and paraffin wax. The magnetic loss was mainly caused by natural resonance and eddy current loss, respectively. The minimum reflection loss (RL) value of Ag/OMC-CI was below −10 dB at 12 GHz, which were superior to those of OMC-CI and Ag/OMC. This phenomenon is attributed to the enhancement of dielectric polarization and magnetic loss.Highlights► Ag/OMC-CI paraffin wax composites were successfully prepared. ► Reflection loss value below −10 dB at 12 GHz was obtained. ► Ag/OMC-CI showed excellent microwave absorption with respect to OMC-CI and Ag/OMC. ► This could be attributed to the enhancement of interfacial polarization.

Gold supported on cobalt oxide has been successfully synthesized through a colloidal precipitation method and tested in toluene and p-xylene total oxidation. It has been demonstrated that the catalytic activity of Au/Co3O4 for toluene and p-xylene oxidation is much higher than that of Au/Al2O3 and Au/MgO in spite of its lower BET surface area and larger gold crystalline size. The enhanced catalytic activity in toluene and p-xylene oxidation has been linked to a high concentration of superficial electrophilic oxygen species and oxygen vacancies, which may be originated from a strong interaction in the colloidal Au–Co3O4 system.Graphical abstractHighlights► Au/Co3O4 has been successfully prepared by means of a colloidal precipitation method. ► Au/Co3O4 exhibits higher activity than Au/Al2O3 or Au/MgO in toluene and p-xylene total oxidation. ► The enhanced activity may be due to the strong metal-oxide interaction between Au and Co3O4. ► This effect causes abundant electrophilic oxygen species and oxygen vacancies on the surface.

Amorphous hierarchical NiCo2O4–CoNiO2 hybrids have been successfully fabricated via a facile one-pot hydrothermal route, followed by morphological conversion into urchin-like structured NiCo2O4–CoNiO2 nanorods and irregular-shaped hierarchical NiCo2O4–CoNiO2 polyhedral nanocrystals through air-annealing treatment at 450 °C and 650 °C, respectively. The phase structure, morphology and chemical composition have been characterized in detail. Calcined hierarchical NiCo2O4–CoNiO2 hybrids show improved microwave absorption properties, which are ascribed to the synergistic effect of dielectric CoNiO2 and NiCo2O4 phases. In particular, the calcined hierarchical NiCo2O4–CoNiO2 hybrids at 450 °C exhibit significant enhancement in complex permittivity with respect to others due to their remarkable dipole polarization and interfacial polarization. The maximum reflection loss (RL) of the calcined hierarchical NiCo2O4–CoNiO2 hybrids at 450 °C reaches −42.13 dB at 11.84 GHz with a matching thickness of 1.55 mm, and a relatively broad absorption bandwidth (RL ≤ −10 dB) in the 13.12–17.04 GHz range. Very interestingly, the electromagnetic (EM) wave absorption performance of the hierarchical NiCo2O4–CoNiO2 hybrids shows dependence on the Co2+/Co3+ ratio. The calcined NiCo2O4–CoNiO2 hybrids at 450 °C of the most defect concentration possess the best EM wave absorption ability among all the samples. The results suggest that appropriate interactions between the building blocks in hybrids can guide us to design and fabricate highly efficient EM wave absorption materials.