Drawing Inspiration from Nature: Trinitarian Strategies for Designing Polyoxometalates and Metal-Organic Framework-Based Biomimetic Microhoneycomb Electromagnetic Wave-Absorbing Materials.

Trinitarian designs in the morphology, components, and microstructure remain challenging for advanced electromagnetic wave absorption (EMWA) materials with light weight, strong absorption, and well-defined structure-function relationships. Herein, a series of X-doped MoS2/Cu9S5 with multilevel honeycomb structures (X-MoS2/Cu9S5 MHs, X = P, Si, Ge) were designed by space-confined growth and in situ sulfidation of a polyoxometalate-based metal-organic framework. X-MoS2/Cu9S5 MHs possess low density, high surface area, and abundant cation-cuprum and anion-sulfur double vacancies (VCu and VS) simultaneously that are unmatched by conventional EMWA materials. Also, the systematic investigation of the doping effect of various polyoxometalate heteroatoms on VCu and VS in the microhoneycomb has been conducted. Experimental results and density functional theory calculations reveal that the excellent EMWA performance (-56.21 dB) results from the synergistic effect of morphology design, component optimization, and vacancy regulation. This study not only provides an important opportunity to understand a morphology-component-microstructure strategy in electromagnetic wave absorption but also builds a noteworthy bridge between bioinspired engineering and microscale absorbers.

Highly sensitive detection of glucose at a novel non-enzyme electrochemical sensing based on Mo-doped CoO Nanosheets.

In this work, a Mo doped CoO nanosheet grown on nickel foam (labeled as: Mo-CoO) with defect-rich and improved electron transfer capacity was designed to be used as a novel non-enzyme electrode material. Physical characterizations demonstrated the Mo elements were doped inside of the samples and they were mutually stabilized with each other, resulting in a high structural stability electrochemical catalytical activity even if the content of Mo was low. For non-enzymatic glucose electrochemical sensing, the prepared Mo-CoO-1 showed a remarkable sensitivity of 89.3 mA cm-2 mM-1 , and a low detection limit of 0.43 µM. Density functional theory (DFT) studies revealed that the doped Mo atom exhibited a higher d-band center compared to the Co atom. A stronger p-d orbital hybridization between the glucose and the Mo atoms indicated the enhancement of glucose adsorption and activation. Importantly, Mo-CoO-1 provided a good selectivity and long-term stability, which can be expected to be used in future practical applications.

Crystal Structure, Chemical Bond, and Microwave Dielectric Properties of Ba1-xSrx(Zn1/3Nb2/3)O3 Solid Solution Ceramics.

Ba1-xSrx(Zn1/3Nb2/3)O3 (BSZN) perovskite ceramics are prepared using the traditional solid-state reaction method. X-ray diffraction (XRD), Scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used to analyze the phase composition, crystal structure, and chemical states of BSZN ceramics, respectively. In addition, the dielectric polarizability, octahedral distortion, complex chemical bond theory, and PVL theory were investigated in detail. Systematic research showed that Sr2+ addition could considerably optimize the microwave dielectric properties of BSZN ceramics. The change in τf value in the negative direction was attributed to oxygen octahedral distortion and bond energy (Eb), and the optimal value of 1.26 ppm/°C was obtained at x = 0.2. The ionic polarizability and density played a decisive role in the dielectric constant, achieving a maximum of 45.25 for the sample with x = 0.2. The full width at half-maximum (FWHM) and lattice energy (Ub) jointly contributed to improving the Q × f value, and a higher Q × f value corresponded to a smaller FWHM value and a larger Ub value. Finally, excellent microwave dielectric properties (εr = 45.25, Q × f = 72,704 GHz, and τf = 1.26 ppm/°C) were obtained for Ba0.8Sr0.2(Zn1/3Nb2/3)O3 ceramics sintered at 1500 °C for 4 h.

Influences of Metal Core and Carbon Shell on the Microwave Absorption Performance of Cu-C Core-Shell Nanoparticles.

Metal-C core-shell nanoparticles have been recently demonstrated to be promising candidates for microwave absorption applications. However, the underlying absorption mechanism, such as the contributions of the metal cores and C shells on their absorption performance, remains far from clear due to the complicated interfaces and synergetic effects between metal cores and C shells, as well as the significant challenges in the preparation of samples with well-defined comparability. In this study, Cu-C core-shell nanoparticles and their derivatives, i.e., bare Cu and hollow C nanoparticles, were synthesized for a comparative study on their microwave absorption properties. Electric energy loss models of the three samples were established, and based on these models, the comparative study suggested that the polarization loss could be significantly improved by C shells, and Cu cores had negligible influences on the conduction loss of Cu-C core-shell nanoparticles. The interface between C shells and Cu cores tuned the conduction loss and polarization loss to establish improved impedance matching and achieve optimal microwave absorption performances. A wide effective bandwidth of 5.4 GHz and a low reflection loss of -42.6 dB were achieved for Cu-C core-shell nanoparticles. This work provides new insights into how metal nanocores and C nanoshells affect the microwave absorption of core-shell nanostructures from experimental and theoretical points of view, which has reference values for the construction of highly efficient metal-C-based absorbers.

Crystal Structures and Microwave Dielectric Properties of Novel MgCu2Nb2O8 Ceramics Prepared by Two-Step Sintering Technique.

In this work, novel MgCu2Nb2O8 (MCN) ceramics were synthesized by the two-step sintering (TSS) technique, and the phase composition, crystal structures, and microwave dielectric properties were comprehensively studied. X-ray diffraction (XRD) and Raman analysis demonstrated that MCN ceramics are multi-phase ceramics consisting of MgNb2O6 and CuO phases. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the chemical composition and element valence of MgCu2Nb2O8 ceramics. Scanning electron microscopy (SEM) analysis demonstrated dense microstructures in the MCN ceramics prepared at a sintering temperature of 925 °C. The microwave dielectric properties were largely affected by the lattice vibrational modes and densification level of the ceramics. The outstanding microwave dielectric properties of εr = 17.15, Q × f = 34.355 GHz, and τf = -22.5 ppm/°C were obtained for the MCN ceramics sintered at 925 °C, which are results that hold promise for low temperature co-fired ceramic (LTCC) applications.

Enhancement of Electrochromic Properties of Polyaniline Induced by Copper Ions.

Driven by the urgent need for adaptive infrared (IR) electrochromic devices, the improvement in electrochromic performance based on polyaniline (PANI) conducting polymers has become an outstanding challenge. In recent years, the acid doping strategy has been proven to increase the IR modulation ability of PANI, in particular for the Bronsted acid doping. Herein, the effects of copper ions, a Lewis acid, on the structure and electrochromic properties of polyaniline were investigated. Compared to pure polyaniline, the Cu-doped PANI porous films show better IR modulation ability. With the increasing concentration of copper ions, the Cu-doped PANI porous films exhibit a trend in volcanic patterns for the emittance variation (∆ε), depending on the number of polarons and bipolarons. The optimal IR emissivity (ε) modulation obtained on Cu-doped PANI films shows the ∆ε modulation of 0.35 and 0.3 in the wavelength range of 8-14 µm and 2.5-25 µm, superior to previously reported pure sulfuric acid-doped PANI. Furthermore, a flexible IR electrochromic device was fabricated with the present Cu-doped PANI porous films. The modulation of the emittance variation varied between 0.513 and 0.834 (∆ε = 0.32 in ranges of wavelength 8-12 µm), suggesting the great potential for applications in military camouflage and intelligent IR thermal management. We believe that the results in this work will provide a novel perspective and avenue for improving the IR modulation ability of electrochromic devices based on polyaniline conducting polymers.

Hexagonal Single-Crystal CoS Nanosheets: Controllable Synthesis and Tunable Oxygen Evolution Performance.

Cobalt-based sulfides with variable valence states and unique physical and chemical properties have shown great potential as oxygen evolution reaction (OER) catalysts for electrochemical water-splitting reactions. However, poor morphological characteristics and a small specific surface area limit its further application. Here, hexagonal single-crystal two-dimensional (2D) CoS nanosheets with different thicknesses are successfully prepared by an atmospheric-pressure chemical vapor deposition method. Because of the advantages of the 2D structure, more exposed catalytic active sites, better reactant adsorption ability, accelerated electron transfer, and enhanced electrical conductivities can be achieved from the thinnest 5 nm CoS nanosheets (CoS-5), significantly improving OER performance. The electrochemical tests manifest that CoS-5 show an overpotential of 290 mV at 10 mA cm-2 and a Tafel slope of 65.6 mV dec-1 in the OER in an alkaline solution, superior to those for other thicknesses of CoS, bulk CoS, and RuO2. For the mechanistic investigation, the lowest charge transfer resistance (Rct) and the highest double-layer capacitance (Cdl) were obtained for CoS-5, demonstrating the faster OER kinetics and the larger active area. Density functional theory calculations further reveal the enhanced density of states around the Fermi level and higher H2O molecule adsorption energy for thinner CoS nanosheets, promoting its intrinsic catalytic activity. Moreover, the two-electrode system with CoS-5 as the anode and Pt/C as the cathode requires only 1.56 V to attain 10 mA cm-2 in the overall water-splitting reaction. We believe that this study will provide a fresh view for thickness-dependent catalytic performance and offers a new material for the study of electronic and energy devices.

N,N-Dimethyl Formamide Regulating Fluorescence of MXene Quantum Dots for the Sensitive Determination of Fe3.

Due to the wide use of iron in all kinds of areas, the design and construction of direct, fast, and highly sensitive sensor for Fe3+ are highly desirable and important. In the present work, a kind of fluorescent MXene quantum dots (MQDs) was synthesized via an intermittent ultrasound process using N,N-dimethyl formamide as solvent. The prepared MQDs were characterized via a combination of UV-Vis absorption, fluorescence spectra, X-ray photoelectron energy spectra, and Fourier-transform infrared spectroscopy. Based on the electrostatic-induced aggregation quenching mechanism, the fluorescent MQDs probes exhibited excellent sensing performance for the detection of Fe3+, with a sensitivity of 0.6377 mM-1 and the detection limit of 1.4 µM, superior to those reported in studies. The present MQDs-based probes demonstrate the potential promising applications as the sensing device of Fe3+.

Porous Carbon Substrate Improving the Sensing Performance of Copper Nanoparticles Toward Glucose.

An accurate sensor to rapidly determine the glucose concentration is of significant importance for the human body health, as diabetes has become a very high incidence around the world. In this work, copper nanoparticles accommodated in porous carbon substrates (Cu NP@PC), synthesized by calcinating the filter papers impregnated with copper ions at high temperature, were designed as the electrode active materials for electrochemical sensing of glucose. During the formation of porous carbon, the copper nanoparticles spontaneously accommodated into the formed voids and constituted the half-covered composites. For the electrochemical glucose oxidation, the prepared Cu NP@PC composites exhibit much superior catalytic activity with the current density of 0.31 mA/cm2 at the potential of 0.55 V in the presence of 0.2 mM glucose. Based on the high electrochemical oxidation activity, the present Cu NP@PC composites also exhibit a superior glucose sensing performance. The sensitivity is determined to be 84.5 µA /(mmol.L) with a linear range of 0.01 ~ 1.1 mM and a low detection limit (LOD) of 2.1 µmol/L. Compared to that of non-porous carbon supported copper nanoparticles (Cu NP/C), this can be reasonable by the improved mass transfer and strengthened synergistic effect between copper nanoparticles and porous carbon substrates.

A mV-level real-time peak-voltage detection circuit based on differential structure.

A peak-voltage detection circuit based on a differential comparison structure is proposed to synchronize the control and the input signal. The detection circuit is hence free of reset signals for the sampling capacitors. Furthermore, a two-channel parallel sample and hold structure (i.e., S/H circuit) is used, and a correlated double sampling technique is used, in combination with the ping-pong technique, to sample the signal value and offset voltage within one sample cycle. Consequently, the parallel connected S/H structure can not only extract the offset voltage of the op-amp but also effectively reduce the detection error, which is caused by circuit noise and leakage current. Measurements of the implemented peak detector show that in the case that the detection signal frequency is 20 kHz and the amplitude is 10 mV, the detection error is decreased to 30 µV with the equivalent output noise of 71 nV/Hz.

A comparative study on the dielectric response and microwave absorption performance of FeNi-capped carbon nanotubes and FeNi-cored carbon nanoparticles.

The mechanisms responsible for the dielectric response of C-based microwave absorbers remain a long-standing theoretical question. Uncovering these mechanisms is critical to enhance their microwave absorption performance. To determine how different C forms alter the dielectric response of C-based absorbers, FeNi-capped carbon nanotubes (FeNi-CNTs) and FeNi-cored carbon nanoparticles (FeNi-CNPs) are synthesized, and a comparative study of their dielectric responses is then carried out in this study. The as-synthesized FeNi-CNTs and FeNi-CNPs have similar magnetic properties and complex permeabilities, but differ in complex permittivities. It is shown that FeNi-CNTs have a much stronger dielectric loss than FeNi-CNPs. At a thickness of 2.8 mm, a low optimal reflection loss of -32.2 dB and a broad effective absorption bandwidth of 8.0 GHz are achieved for FeNi-CNTs. Meanwhile, equivalent circuit models reveal that the CNT network of the FeNi-CNTs could introduce an electrical inductance that can effectively improve its dielectric loss capability. This study demonstrates that designing a composite with a tailored C form and composition is a successful strategy for tuning its microwave absorption performance. Furthermore, the equivalent circuit modeling is an effective tool for analyzing the dielectric response of the microwave absorbers, as is expected to be applicable for other metal-C composites.

Multipurpose chemical liquid sensing applications by microwave approach.

In this work, a novel sensor based on printed circuit board (PCB) microstrip rectangular patch antenna is proposed to detect different ratios of ethanol alcohol in wines and isopropyl alcohol in disinfectants. The proposed sensor was designed by finite integration technique (FIT) based high-frequency electromagnetic solver (CST) and was fabricated by Proto Mat E33 machine. To implement the numerical investigations, dielectric properties of the samples were first measured by a dielectric probe kit then uploaded into the simulation program. Results showed a linear shifting in the resonant frequency of the sensor when the dielectric constant of the samples were changed due to different concentrations of ethanol alcohol and isopropyl alcohol. A good agreement was observed between the calculated and measured results, emphasizing the usability of dielectric behavior as an input sensing agent. It was concluded that the proposed sensor is viable for multipurpose chemical sensing applications.

C0.3N0.7Ti-SiC Toughed Silicon Nitride Hybrids with Non-Oxide Additives Ti3SiC2.

In situ grown C0.3N0.7Ti and SiC, which derived from non-oxide additives Ti3SiC2, are proposed to densify silicon nitride (Si3N4) ceramics with enhanced mechanical performance via hot-press sintering. Remarkable increase of density from 79.20% to 95.48% could be achieved for Si3N4 ceramics with 5 vol.% Ti3SiC2 when sintered at 1600 °C. As expected, higher sintering temperature 1700 °C could further promote densification of Si3N4 ceramics filled with Ti3SiC2. The capillarity of decomposed Si from Ti3SiC2, and in situ reaction between nonstoichiometric TiCx and Si3N4 were believed to be responsible for densification of Si3N4 ceramics. An obvious enhancement of flexural strength and fracture toughness for Si3N4 with x vol.% Ti3SiC2 (x = 1~20) ceramics was observed. The maximum flexural strength of 795 MPa for Si3N4 composites with 5 vol.% Ti3SiC2 and maximum fracture toughness of 6.97 MPa·m1/2 for Si3N4 composites with 20 vol.% Ti3SiC2 are achieved via hot-press sintering at 1700 °C. Pull out of elongated Si3N4 grains, crack bridging, crack branching and crack deflection were demonstrated to dominate enhance fracture toughness of Si3N4 composites.

A sensitivity-enhanced capacitance readout circuit with symmetric cross-coupling structure.

This paper presents a proposed capacitance readout circuit that enables a quadrupled (x4) output strength. A symmetric cross-coupling structure is proposed to amplify the voltage difference between two adjacent channels; hence, the detected signal can be integrated twice every clock cycle. Compared with conventional schematics, the proposed readout circuit shows an increased output strength for integration times within dozens of µs. In addition, the measurements show that the integrator resistors should be less than 1 kΩ to suppress the resistance-capacitance delay effects. Although the proposed capacitance readout circuit is implemented using discrete transistors, it has a good signal integrity at an operating clock cycle of 100 µs. Therefore, the proposed readout circuit is a promising way to detect small capacitance variations with short integration times.

Novel Metamaterials-Based Hypersensitized Liquid Sensor Integrating Omega-Shaped Resonator with Microstrip Transmission Line.

In this paper, a new metamaterials-based hypersensitized liquid sensor integrating omega-shaped resonator with microstrip transmission line is proposed. Microwave transmission responses to industrial energy-based liquids are investigated intensively from both numerical and experimental point of view. Simulation results concerning three-dimensional electromagnetic fields have shown that the transmission coefficient of the resonator could be monitored by the magnetic coupling between the transmission line and omega resonator. This sensor structure has been examined by methanol-water and ethanol-water mixtures. Moreover, the designed sensor is demonstrated to be very sensitive for identifying clean and waste transformer oils. A linear response characteristic of shifting the resonance frequency upon the increment of chemical contents/concentrations or changing the oil condition is observed. In addition to the high agreement of transmission coefficients (S21) between simulations and experiments, obvious resonant-frequency shift of transmission spectrum is recognized for typical pure chemical liquids (i.e., PEG 300, isopropyl alcohol, PEG1500, ammonia, and water), giving rise to identify the type and concentration of the chemical liquids. The novelty of the work is to utilize Q factor and minimum value of S21 as sensing agent in the proposed structure, which are seen to be well compatible at different frequencies ranging from 1-20 GHz. This metamaterial integrated transmission line-based sensor is considered to be promising candidate for precise detection of fluidics and for applications in the field of medicine and chemistry.

Molybdenum Disulfide Quantum Dots Prepared by Bipolar-Electrode Electrochemical Scissoring.

A convenient bipolar-electrode (BPE) electrochemical method was engineered to produce molybdenum disulfide (MoS2) quantum dots (QDs) using pure phosphate buffer (PBS) as the electrolyte and the MoS2 powder as the precursor. Meanwhile, the corresponding by-product precipitate was studied, in which MoS2 nanosheets were observed. The BPE design would not be restricted by the shape and size of the MoS2 precursor. It could lead to the defect generation and 2H → 1T phase variation of the MoS2, resulting in the formation of nanosheets and finally the QDs. The as-prepared MoS2 QDs exhibited high photoluminescence (PL) quantum yield of 13.9% and average lateral size of 4.4 ± 0.2 nm, respectively. Their excellent PL property, low cytotoxicity, and good aqueous dispersion offer promising applicability in PL staining and cell imaging. Meanwhile, the as-obtained byproduct containing the nanosheets could be used as an effective electromagnetic wave (EMW) absorber. The minimum reflection loss (RL) value was -54.13 dB at the thickness of 3.3 mm. The corresponding bandwidth with efficient attenuation (<-10 dB) was up to 7.04 GHz (8.8-15.84 GHz). The as-obtained EMW performance was far superior over most previously reported MoS2-based nanomaterials.

Mxenes Derived Laminated and Magnetic Composites with Excellent Microwave Absorbing Performance.

Two dimensional materials have been widely identified as promising microwave absorbers, owing to their large surface area and abundant interfaces. Here, a novel laminated and magnetic composite derived from Mxene was designed and successfully synthesized via facile hydrothermal oxidization of nickel ion intercalated Ti3C2. Highly disordered carbon sheets were obtained by low temperature hydrothermal oxidization, and the in-situ produced TiO2 and NiO nanoparticles embedded closely between them. This layered hybrid exhibits excellent microwave absorbing performance with an effective absorbing bandwidth as high as 11.1 GHz (6.9-18 GHz) and 9 GHz (9-18 GHz) when the thickness is 3 and 2 mm, respectively. Besides the high dielectric loss, magnetic loss and ohmic loss of the composite, the amorphous nature of obtained carbon sheets and multi-reflections between them are believed to play a decisive role in achieving such superior microwave absorbing performance.

Design of a Broadband Coplanar Waveguide-Fed Antenna Incorporating Organic Solar Cells with 100% Insolation for Ku Band Satellite Communication.

A broadband coplanar waveguide (CPW)-fed monopole antenna based on conventional CPW-fed integration with an organic solar cell (OSC) of 100% insolation is suggested for Ku band satellite communication. The proposed configuration was designed to allow for 100% insolation of the OSC, thereby improving the performance of the antenna. The device structure was fabricated using a Leiterplatten-Kopierfrasen (LPKF) prototyping Printed circuit board (PCB) machine, while a vector network analyzer was utilized to measure the return loss. The simulated results demonstrated that the proposed antenna was able to cover an interesting operating frequency band from 11.7 to 12.22 GHz, which is in compliance with the International Telecommunication Union (ITU). Consequently, a 3 GHz broadband in the Ku band was achieved, along with an enhancement in the realized gain of about 6.30 dB. The simulation and experimental results showed good agreement, whereby the proposed structure could be specifically useful for fixed-satellite-services (FSS) operating over the frequency range from the 11.7 to 12.22 GHz (downlink) band.

Facile approach to fabricate BCN/Fe x (B/C/N) y nano-architectures with enhanced electromagnetic wave absorption.

Carbon-based materials have excited extensive interest for their remarkable electrical properties and low density for application in electromagnetic (EM) wave absorbents. However, the processing of heteroatoms doping in carbon nanostructures is an insuperable challenge for attaining effective reflection loss and EM matching. Herein, a facile method for large-scale synthesis of boron and nitrogen doped carbon nanotubes decorated by ferrites particles is proposed. The BCN nanotubes (50-100 nm in diameter) imbedded with nanosized Fe x (B/C/N) y (10-20 nm) are successfully constructed by two steps of polymerization and carbonthermic reduction. The product exhibits an outstanding reflection loss (RL) performance, in that the minimum RL is -47.97 dB at 11.44 GHz with a broad bandwidth 11.2 GHz (from 3.76 to 14.9 GHz) below -10 dB indicating a competitive absorbent in stealth materials. Crystalline and theoretical studies of the absorption mechanism indicate a unique dielectric dispersion effect in the absorbing bandwidth.

Facile synthesis and excellent microwave absorption properties of FeCo-C core-shell nanoparticles.

FeCo-C core-shell nanoparticles (NPs) with diameters of 10-50 nm have been fabricated on a large scale by one-step metal-organic chemical vapor deposition using the mixture of cobalt acetylacetonate and iron acetylacetonate as the precursor. The Fe/Co molar ratio of the alloy nanocores and graphitization degree of C shells, and thus the magnetic and electric properties of the core-shell NPs, can be tuned by the deposition temperature ranging from 700 °C to 900 °C. Comparative tests reveal that a relatively high Fe/Co molar ratio and low graphitization degree benefit the microwave absorption (MA) performance of the core-shell NPs. The composite with 20 wt% core-shell NP obtained at 800 °C and 80 wt% paraffin exhibits an optimal reflection loss [Formula: see text] of -60.4 dB at 7.5 GHz with a thickness of 3.3 mm, and an effective absorption bandwidth (frequency range for RL ≤10 dB) of 9.2 GHz (8.8-18.0 GHz) under an absorber thickness of 2.5 mm. Our study provides a facile route for the fabrication of alloy-C core-shell nanostructures with high MA performance.