Hongjing Wu

Co2+/Co3+ ratio dependence of electromagnetic wave absorption in hierarchical NiCo2O4–CoNiO2 hybrids

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.

Co3O4 nanocrystals and Co3O4–MOx binary oxides for CO, CH4 and VOC oxidation at low temperatures: a review

Among the possible substitutes for noble metals, cobalt-based catalysts represent promising alternative systems. In recent years, many articles have been devoted to the synthesis, characterization and reactivity of cobalt oxides. This article provides a comprehensive review of the state-of-the-art activities that concentrate on the synthesis, structural properties and catalytic applications of Co3O4 nanocrystals and Co3O4–MOx binary oxides in CO, CH4 and VOC oxidation at low temperatures. It begins with the major synthetic approaches and basic properties of Co3O4 nanocrystals and Co3O4–MOx binary oxides and subsequently highlights the relationship between the peculiar structure of Co3O4 nanocrystals and their catalytic activity (or between the redox properties of Co3O4–MOx binary oxides and their catalytic activity). Finally, the active sites and key factors determining the catalytic oxidation over Co3O4 and Co3O4–MOx are discussed. The perspective with respect to future research on Co3O4 nanocrystals and Co3O4–MOx binary oxides is considered.

Facile synthesis and application of multi-shelled SnO2 hollow spheres in lithium ion battery

A simple template-free hydrothermal method has been developed to prepare multi-shelled SnO2 hollow spheres with controlled interior texture. The interior texture and size of the multi-shelled SnO2 hollow spheres were found to be strongly dependent on the carbon source, Sn salt precursor and molar ratio of G : M. The multi-shelled SnO2 hollow spheres exhibit good electrochemical performance as the anode material in lithium ion batteries.

Multishelled Metal Oxide Hollow Spheres: Easy Synthesis and Formation Mechanism

Uniform multishelled NiO, Co3 O4 , ZnO, and Au@NiO hollow spheres were synthesized (NiO and Co3 O4 hollow spheres for the first time) by a simple shell-by-shell self-assembly allowing for tuning of the the size, thickness and shell numbers by controlling the heat treatment, glucose/metal salt molar ratio, and hydrothermal reaction time. These findings further the development of synthetic methodologies for multishelled hollow structures and could open up new opportunities for deeper understanding of the mechanisms of shell-by-shell self-assembly. Moreover, the double-shelled NiO hollow sphere exhibits a higher photocatalytic activity for degradation of methyl orange than its morphological counterparts.

Co3O4 particles grown over nanocrystalline CeO2: influence of precipitation agents and calcination temperature on the catalytic activity for methane oxidation

Crystalline cobalt oxides were prepared by a precipitation method using three different precipitation agents, (NH4)2CO3, Na2CO3 and CO(NH2)2. Cobalt oxide nanoparticles corresponding to a Co3O4 loading of 30 wt% were also deposited over high-surface area nanocrystalline ceria by the same precipitation agents. The effect of calcination temperature, 350 or 650 °C, on the morphological and structural properties was evaluated. Characterization by BET, XRD, SEM, TEM, Raman spectroscopy, H2-TPR, XPS and NH3-TPD was performed and the catalytic properties were explored in the methane oxidation reaction. The nature of the precipitation agent strongly influenced the textural properties of Co3O4 and the Co3O4–CeO2 interface. The best control of the particle size was achieved by using CO(NH2)2 that produced small and regular crystallites of Co3O4 homogeneously deposited over the CeO2 surface. Such a Co3O4–CeO2 system precipitated by urea showed enhanced low-temperature reducibility and high surface Co3+ concentration, which were identified as the key factors for promoting methane oxidation at low temperature. Moreover, the synergic effect of cobalt oxide and nanocrystalline ceria produced stable full conversion of methane in the entire range of investigated temperature, up to 700–800 °C, at which Co3O4 deactivation usually occurs.

Enhanced Microwave Absorption Properties of α-Fe2O3-Filled Ordered Mesoporous Carbon Nanorods

A novel kind of α-Fe2O3-filled ordered mesoporous carbon nanorods has been synthesized by a facial hydrothermal method. Compared with dendritic α-Fe2O3 micropines, both a broader effective absorption range—from 10.5 GHz to 16.5 GHz with reflection loss (RL) less than −10 dB—and a thinner matching thickness of 2.0 mm have been achieved in the frequency range 2–18 GHz. The enhanced microwave absorption properties evaluated by the RL are attributed to the enhanced dielectric loss resulting from the intrinsic physical properties and special structures.

Fast Synthesis of Pt Nanocrystals and Pt/Microporous La2O3 Materials Using Acoustic Levitation

Usually, we must use an appropriate support material to keep the metal species stable and finely dispersed as supported metal nanoparticles for industry application. Therefore, the choice of support material is a key factor in determining the dispersion and particle size of the noble metal species. Here, we report the synthesis of a single-atom Pt material in the solution and supported Pt nanoclusters on microporous La2O3 by a one-step acoustic levitation method without any pretreatment/modification of raw oxide. We have strongly contributed to the synthetic methodology of the surface/interfacial heterogeneous catalysts in this study, and this finding could open another door for synthesis of supported metal nanoparticles on porous materials for environmental catalysis.

Acoustic levitation of liquid drops: Dynamics, manipulation and phase transitions

The technique of acoustic levitation normally produces a standing wave and the potential well of the sound field can be used to trap small objects. Since no solid surface is involved it has been widely applied for the study of fluid physics, nucleation, bio/chemical processes, and various forms of soft matter. In this article, we survey the works on drop dynamics in acoustic levitation, focus on how the dynamic behavior is related to the rheological properties and discuss the possibility to develop a novel rheometer based on this technique. We review the methods and applications of acoustic levitation for the manipulation of both liquid and solid samples and emphasize the important progress made in the study of phase transitions and bio-chemical analysis. We also highlight the possible open areas for future research.

Degenerate seaweed to tilted dendrite transition and their growth dynamics in directional solidification of non-axially oriented crystals: a phase-field study.

We report the results of a phase-field study of degenerate seaweed to tilted dendrite transition and their growth dynamics during directional solidification of a binary alloy. Morphological selection maps in the planes of (G, Vp) and (ε4, Vp) show that lower pulling velocity, weaker anisotropic strength and higher thermal gradient can enhance the formation of the degenerate seaweed. The tip undercooling shows oscillations in seaweed growth, but it keeps at a constant value in dendritic growth. The M-S instability on the tips and the surface tension anisotropy of the solid-liquid interface are responsible for the formation of the degenerate seaweed. It is evidenced that the place where the interfacial instability occurs determines the morphological transition. The transient transition from degenerate seaweed to tilted dendrite shows that dendrites are dynamically preferred over seaweed. For the tilted dendritic arrays with a large tilted angle, primary spacing is investigated by comparing predicted results with the classical scaling power law, and the growth direction is found to be less sensitive to the pulling velocity and the primary spacing. Furthermore, the effect of the initial interface wavelength on the morphological transition is investigated to perform the history dependence of morphological selection.

EFFECT OF CARBONIZATION TEMPERATURE ON DIELECTRIC AND MICROWAVE ABSORBING PROPERTIES OF COBALT DOPED MESOPOROUS CARBON COMPOSITES

Co doped mesoporous carbon composites (MC–Co) have been synthesized at different carbonization temperatures via evaporation-induced multicomponent co-assembly strategy. The nanostructures and chemical compositions of MC–Co composites were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM) and X-ray diffraction analysis (XRD), etc. Their electromagnetic and microwave absorption properties were investigated in the frequency ranging from 2 GHz to 18 GHz. By the measurement of electromagnetic parameters and theoretical simulation of reflection loss (RL), the results showed that the minimum RL value of the optimum composite MC–Co-800 reached up to -41.9 dB at 13.5 GHz with a thickness of only 1.80 mm, and the effective absorption bandwidth (< -10 dB) is 4.4 GHz (from 11.4 GHz to 15.8 GHz). This work demonstrated that the optimum carbonization temperature of the obtained composites is 800°C, which is critical to its electromagnetic properties. The excellent microwave absorption properties can be attributed to dielectric loss, Ohmic loss, and characteristic impedance matching. Therefore, the as-synthesized composites could be acted as a candidate of microwave absorber, especially for the light weight demand.