Lina Wu

SiC–Fe3O4 dielectric–magnetic hybrid nanowires: controllable fabrication, characterization and electromagnetic wave absorption

Controllable dielectric–magnetic coaxial hybrid nanowires, having a core of SiC nanowires and a shell of Fe3O4 nanoparticles, have been synthesized using a straightforward polyol approach. The morphology, microstructure and magnetic properties of the SiC–Fe3O4 hybrid nanowires have been characterized by transmission electron microscope, powder X-ray diffractometer and vibrating sample magnetometer. The characterization confirms that monodisperse Fe3O4 nanoparticles of core size 10 nm have been successfully coated on the surface of SiC nanowires. The coverage density of the nanoparticles may be adjusted simply by changing the weight ratio of the precursors. Measurement of the electromagnetic (EM) parameters indicates that the Fe3O4 nanoparticles increase the magnetic loss and improve the impedance matching conditions compared to untreated SiC nanowires. When the coverage density of Fe3O4 is optimal, the reflection loss of an EM wave can be as low as −51 dB. By changing the loading density of Fe3O4, the best microwave absorption state was obtained in the 2–18 GHz band. These results suggest that SiC–Fe3O4 hybrid nanowires will be valuable in EM absorption applications.

Enhanced microwave absorption of Fe3O4 nanocrystals after heterogeneously growing with ZnO nanoshell

To enhance the microwave absorption of Fe3O4 nanocrystals, ZnO nanoshells with a thickness of 2 nm were grown on Fe3O4 nanocrystals by heterogeneous nucleation. After being coated with ZnO nanoshells, the material possesses a far more improved ability for microwave absorption. A minimum reflection loss of −3.31 dB for Fe3O4 nanocrystals alone was improved to a minimum reflection loss of −22.69 dB and with an effective absorption band (RL < −10 dB) covering a frequency range of 10.08–15.97 GHz. The reasons for the enhanced microwave absorption were studied by the use of X-ray absorption near-edge structures at O K-edge, electron spin resonance analysis and microwave electromagnetic parameters mapping. The results indicate that the decoration of the dielectric ZnO shell had varied the dielectric property as well as the oxidization environment and distribution of Fe ions on the surface of the Fe3O4. This well balances the permeability and the permittivity of the nanomaterials and decreases the difficulty of impedance matching the microwave absorber within the free space. This leads to the Fe3O4@ZnO nanohybrids possessing vastly improved microwave absorption as compared to Fe3O4 nanocrystals alone.

Covalent interaction enhanced electromagnetic wave absorption in SiC/Co hybrid nanowires

The interaction between components in hybrids is an indispensable factor in designing and fabricating composites with distinguished electromagnetic (EM) absorption performances. Herein, covalently bonded SiC/Co hybrid nanowires (NWs) have been fabricated, which present significantly enhanced EM absorption compared to a simple physical mixture of SiC and Co. The hybrids are characterized by transmission electron microscopy, X-ray diffraction, Raman spectroscopy, vector network analysis, and X-ray absorption near-edge spectroscopy at the Si K-edge, C K-edge, Co L3,2-edge and O K-edge. Microstructure analysis indicates the formation of Si–O–Co bonds between SiC NWs and magnetic Co nanocrystals. Charge transfer takes place in the covalently bonded SiC/Co hybrid NWs. The induced synergistic coupling interaction in SiC/Co leads to an effective EM absorption band (RL < −10 dB) covering the frequency range of 10–16.6 GHz when the Co content is 25.1 wt% in the hybrid.

Fabrication of core–multishell MWCNT/Fe3O4/PANI/Au hybrid nanotubes with high-performance electromagnetic absorption

Core–multishell MWCNT/Fe3O4/polyaniline (PANI)/Au hybrid nanotubes are synthetized by a facile layer-by-layer technique which well integrates the dielectric components (MWCNT and PANI) and the magnetic absorber (Fe3O4) as well as the electromagnetic (EM) wave reflector (Au) into one unit. The hybrid nanotubes are characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy and vector network analysis. Microstructure analysis indicates that the coverage of the gold nanoparticles could be adjusted simply by varying the weight ratio of the precursors. When gold is introduced into the hybrid nanotubes, the EM absorption is sharply enhanced. The lowest reflection loss value reduces from −22 dB to −60 dB. This may be ascribed to the fact that the introduction of gold nanoparticles is beneficial for the multi-reflection of EM waves within the absorbers, balances the impedance matching between the absorbers and free space, and induces charge redistribution within the hybrid nanotubes generating significant interfacial polarization.

Large-scale gold nanoparticle superlattice and its SERS properties for the quantitative detection of toxic carbaryl

Large scale and well-ordered gold nanoparticle superlattices were fabricated by self-assembly as an active substrate for surface-enhanced Raman scattering (SERS) that can quantitatively detect carbaryl with a detection limit of 1 ppm. These fabricated superlattices with a dimension of several hundred micrometers exhibited high, reproducible SERS activity.

Ultrahigh Mass Activity for Carbon Dioxide Reduction Enabled by Gold–Iron Core–Shell Nanoparticles

Wide application of carbon dioxide (CO2) electrochemical energy storage requires catalysts with high mass activity. Alloy catalysts can achieve superior performance to single metals while reducing the cost by finely tuning the composition and morphology. We used in silico quantum mechanics rapid screening to identify Au-Fe as a candidate improving CO2 reduction and then synthesized and tested it experimentally. The synthesized Au-Fe alloy catalyst evolves quickly into a stable Au-Fe core-shell nanoparticle (AuFe-CSNP) after leaching out surface Fe. This AuFe-CSNP exhibits exclusive CO selectivity, long-term stability, nearly a 100-fold increase in mass activity toward CO2 reduction compared with Au NP, and 0.2 V lower in overpotential. Calculations show that surface defects due to Fe leaching contribute significantly to decrease the overpotential.

Luminescent Au11 nanocluster superlattices with high thermal stability

The poor thermal stability of nanoparticle superlattices heavily inhibits their practical applications. In present research, using stable thiolate-capped Au11(SCH2CH2COO−)7([CH3(CH2)7]4N+)7 nanoclusters as the building blocks, novel luminescent Au11 nanocluster superlattices with high thermal stability have been fabricated by self-assembly. The nanocluster superlattices have a blade-like morphology and extend on a micrometre length scale with the largest over 50 μm. Under an excitation at 400 nm, the fabricated Au11 nanocluster superlattices emit a blue luminescence with the emission peak of 473 nm. The native stability of thiolate-capped gold nanoclusters and the steric repulsion induced by the high-density ligands (SCH2CH2COO−)7([CH3(CH2)7]4N+)7 endows the fabricated superlattices with high thermal stability. The differential scanning calorimetry and thermogravimetric analysis indicates that the superlattices undergo irreversible endothermic transitions in the range of room temperature to 200 °C, which starts at 124 °C and reaches a peak at 160 °C. When processed with heat treatment below the transition temperature or stored for six months at room temperature, there is no obvious difference detected in the emission intensity of the fabricated Au11 nanocluster superlattices. Such thermostability gives the fabricated nanocluster superlattices great potential for many applications, especially for optical devices.

Durian-like multi-functional Fe3O4–Au nanoparticles: synthesis, characterization and selective detection of benzidine

A novel durian-like multi-functional water soluble Fe3O4–Au nanocomposite is fabricated via a facile layer-by-layer technology in which a mercapto-silica shell is utilized as a functional coating on the central Fe3O4 nanoparticle cluster. Then gold nanoparticles are loaded onto the surface of Fe3O4, stabilized by Au–S chemical bonding. The fabricated nanocomposites inherit excellent physical and chemical properties from their building blocks, simultaneously exhibiting superparamagnetic, surface plasmon resonance and surface enhanced Raman scattering (SERS) active properties. Based on the magnetic separability of the inner Fe3O4 nanoparticle clusters and the SERS active properties of the suface gold, a selective detection method for benzidine has been developed for rapid detection and ease of operation with a low detection limit of 0.18 ppm.

Light and Strong Hierarchical Porous SiC Foam for Efficient Electromagnetic Interference Shielding and Thermal Insulation at Elevated Temperatures

A novel light but strong SiC foam with hierarchical porous architecture was fabricated by using dough as raw material via carbonization followed by carbothermal reduction with silicon source. A significant synergistic effect is achieved by embedding meso- and nanopores in a microsized porous skeleton, which endows the SiC foam with high-performance electromagnetic interference (EMI) shielding, thermal insulation, and mechanical properties. The microsized skeleton withstands high stress. The meso- and nanosized pores enhance multiple reflection of the incident electromagnetic waves and elongate the path of heat transfer. For the hierarchical porous SiC foam with 72.8% porosity, EMI shielding can be higher than 20 dB, and specific EMI effectiveness exceeds 24.8 dB·cm3·g-1 at a frequency of 11 GHz at 25-600 °C, which is 3 times higher than that of dense SiC ceramic. The thermal conductivity reaches as low as 0.02 W·m-1·K-1, which is comparable to that of aerogel. The compressive strength is as high as 9.8 MPa. Given the chemical and high-temperature stability of SiC, the fabricated SiC foam is a promising candidate for modern aircraft and automobile applications.

Enhanced electrochemical reduction of CO2 to CO on Ag electrocatalysts with increased unoccupied density of states

CO2 conversion through catalytic processes in a selective and efficient manner is an essential technology for a sustainable carbon economy at present and in the future. Here, we fabricated five nanostructured Ag electrocatalysts and studied their CO2 electro-reduction properties compared to commercial Ag. Ag L-edge X-ray absorption near-edge spectroscopy was employed to characterize the electronic structure variation among the catalysts. It is found that an increased unoccupied density of states (DOS) of d-character of Ag endows the electrocatalyst with a higher selectivity and efficiency for CO2 reduction. The introduction of Ni into the Ag matrix reduces the unoccupied DOS of d-character as there is charge redistribution between Ag and Ni, worsening the active efficiency of CO production. Density functional theory calculations show that an increased unoccupied DOS optimizes the affinity of the catalyst surface to the intermediate COOH and CO, closer to the activity volcano, which facilitates CO formation. The nanoporous Ag electrocatalyst, made through anodization–reduction, possesses the highest unoccupied DOS of d-character among the samples, showing the best catalytic performance in the reduction of CO2 to CO. Meanwhile, its onset overpotential is 0.19 V and the highest faradaic efficiency reaches 90%. The electrocatalyst stays at this level for 10 h without any noticeable activity change, possessing great potential for applications in industry. This research provides a useful insight on the structure–property relationships of CO2 reduction catalysts, and a guideline for designing high-performance electrocatalysts.