Controlled Fabrication of Hexagonally Close-Packed Langmuir–Blodgett Silica Particulate Monolayers from Binary Surfactant and Solvent Systems

We describe a controllable method to fabricate hexagonally close-packed Langmuir–Blodgett (LB) monolayers with stearic acid (SA) as co-surfactant and methanol as co-solvent. The optimal SA concentrations and volume ratios of chloroform to methanol are 0.8 mg/mL and 3:1 for particles of 140 nm, 0.50 mg/mL and 4:1 for particles of 300 nm, and 0.05 mg/mL and 5:1 for particles of 550 nm, respectively. Additionally, SEM detections of the monolayers transferred at different surface pressures indicate that the monolayers deposited from the binary systems are more compressible. The experimental results indicate that the interparticle repulsions and particle–water interactions can be enhanced without decreasing the particle hydrophobicity by adding SA and methanol; thus, particulate monolayers with large hexagonally close-packed domains composed of small silica particles can be successfully fabricated using LB technique. We propose that the enhanced interparticle repulsion is attributed to the Columbic repulsion resulting from the attachment of SA molecules to the CTAB modified particles around the three phase contact line

Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production on CdS/Cu₇S₄/g‑C₃N₄ Ternary Heterostructures

Hydrogen production through photocatalytic water splitting has attracted much attention because of its potential to solve the issues of environmental pollution and energy shortage. In this work, CdS/Cu₇S₄/g-C₃N₄ ternary heterostructures are fabricated by ion exchange between CdS and Cu⁺ and subsequent ultrasonication-assisted self-assembly of CdS/Cu₇S₄ and g-C₃N₄, which provide excellent visible-light photocatalytic activity for hydrogen evolution without any noble metal cocatalyst. With the presence of p–n junction, tuned band gap alignments, and higher charge carrier density in the CdS/Cu₇S₄/g-C₃N₄ ternary heterostructures that can effectively promote the spatial separation and prolong the lifetime of photogenerated electrons, a high hydrogen evolution rate of 3570 μmol g^–1 h^–1, an apparent quantum yield of 4.4% at 420 nm, and remarkable recycling stability are achieved. We believe that the as-synthesized CdS/Cu₇S₄/g-C₃N₄ ternary heterostructures can be promising noble metal-free catalysts for enhanced hydrogen production from photocatalytic water splitting

Bifunctional Nitrogen-Doped Microporous Carbon Microspheres Derived from Poly(<i>o</i>‑methylaniline) for Oxygen Reduction and Supercapacitors

Heteroatom-doped carbon materials have attracted significant attention because of their applications in oxygen reduction reaction (ORR) and supercapacitors. Here we demonstrate a facile poly­(<i>o</i>-methylaniline)-derived fabrication of bifunctional microporous nitrogen-doped carbon microspheres (NCMSs) with high electrocatalytic activity and stability for ORR and energy storage in supercapacitors. At a pyrolysis temperature of 900 °C, the highly dispersed NCMSs present a high surface area (727.1 m² g^–1), proper total content of doping N, and high concentration of quaternary N, which exhibit superior electrocatalytic activities for ORR to the commercial Pt/C catalysts, high specific capacitance (414 F g^–1), and excellent durability, making them very promising for advanced energy conversion and storage. The presented conducting polymer-derived strategy may provide a new way for the fabrication of heteroatom-doped carbon materials for energy device applications

Shell Thickness-Dependent Microwave Absorption of Core–Shell Fe₃O₄@C Composites

Core–shell composites, Fe₃O₄@C, with 500 nm Fe₃O₄ microspheres as cores have been successfully prepared through in situ polymerization of phenolic resin on the Fe₃O₄ surface and subsequent high-temperature carbonization. The thickness of carbon shell, from 20 to 70 nm, can be well controlled by modulating the weight ratio of resorcinol and Fe₃O₄ microspheres. Carbothermic reduction has not been triggered at present conditions, thus the crystalline phase and magnetic property of Fe₃O₄ micropsheres can be well preserved during the carbonization process. Although carbon shells display amorphous nature, Raman spectra reveal that the presence of Fe₃O₄ micropsheres can promote their graphitization degree to a certain extent. Coating Fe₃O₄ microspheres with carbon shells will not only increase the complex permittivity but also improve characteristic impedance, leading to multiple relaxation processes in these composites, thus the microwave absorption properties of these composites are greatly enhanced. Very interestingly, a critical thickness of carbon shells leads to an unusual dielectric behavior of the core–shell structure, which endows these composites with strong reflection loss, especially in the high frequency range. By considering good chemical homogeneity and microwave absorption, we believe the as-fabricated Fe₃O₄@C composites can be promising candidates as highly effective microwave absorbers

Synthesis of Electromagnetic Functionalized Fe₃O₄ Microspheres/Polyaniline Composites by Two-Step Oxidative Polymerization

Composites consisting of Fe₃O₄ microspheres (FMS) and polyaniline (PANI), FMS/PANI, have been successfully prepared through a two-step oxidative polymerization of aniline monomers in the presence of Fe₃O₄ microspheres. In our two-step polymerization technique, Fe³⁺ and ammonium persulfate (APS) are used as the oxidants in each step. It is discovered that the two-step oxidative process plays a dominant role in the morphology of these composites: aniline oligomers oxidized by Fe³⁺ are mainly produced in the first stage, and “egg-like” PANI aggregates are obtained in the second stage. It can be found that embedding Fe₃O₄ microspheres in the polymer matrixes will not only modulate the complex permittivity but also produce magnetic resonance and loss in the composites. Therefore, the characteristic impedance and reflection loss of these composites are greatly improved. Especially, the composite with equal amount of FMS and PANI, FMS/PANI₅₀, displays very strong reflection loss over a wide frequency range that can be manipulated by the absorber thickness. More importantly, the composites prepared from the two-step chemical oxidative polymerization using hierarchical magnetic materials have better microwave absorption and environmental stability as compared with those composites from Fe₃O₄ nanoparticles, one-step oxidative polymerization, and physical mixture. We believe the two-step oxidative polymerization technique can be a novel route for the design and preparation of lightweight and highly effective microwave absorbers in the future

Ultrasmall MnO Nanoparticles Supported on Nitrogen-Doped Carbon Nanotubes as Efficient Anode Materials for Sodium Ion Batteries

Sodium ion batteries (SIBs) have attracted increasing attentions as promising alternatives to lithium ion batteries (LIBs). Herein, we design and synthesize ultrasmall MnO nanoparticles (∼4 nm) supported on nitrogen-doped carbon nanotubes (NDCT@MnO) as promising anode materials of SIBs. It is revealed that the carbonization temperature can greatly influence the structural features and thus the Na-storage behavior of the NDCT@MnO nanocomposites. The synergetic interaction between MnO and NDCT in the NDCT@MnO nanocomposites provides high rate capability and long-term cycling life due to high surface area, electrical conductivity, enhanced diffusion rate of Na⁺ ions, and prevented agglomeration and high stability of MnO nanoparticles. The resulting SIBs provide a high reversible specific capacity of 709 mAh g^–1 at a current density of 0.1 A g^–1 and a high capacity of 536 mAh g^–1 almost without loss after 250 cycles at 0.2 A g^–1. Even at a high current density of 5 A g^–1, a capacity of 273 mAh g^–1 can be maintained after 3000 cycles

MOFs-Derived Hollow Co/C Microspheres with Enhanced Microwave Absorption Performance

Rational construction of a profitable microstructure in carbon-based electromagnetic composites is becoming a promising strategy to reinforce their microwave absorption performance. Herein, the microstructure design is innovatively coupled with a metal–organic frameworks (MOFs)-derived method to produce hollow Co/C microspheres (Co/C-HS). The resultant composites combine the advantages of hollow microstructures and good chemical homogeneity. It is found that the pyrolysis temperature plays an important role in determining the electromagnetic properties of these hollow Co/C microspheres, where high pyrolysis temperature will increase relative complex permittivity and decrease relative complex permeability. When the pyrolysis temperature is 600 °C, the sample (Co/C-HS-600) will show improved impedance matching and good attenuation ability, and thus an excellent microwave absorption performance with strong reflection loss (−66.5 dB at 17.6 GHz) and wide response bandwidth (over −10 dB, 3.7–18.0 GHz) can be achieved. By comparing with Co/C composites derived from conventional ZIF-67, it can be validated that a hollow microstructure is greatly helpful to upgrade the performance by boosting dielectric loss ability and suppressing a negative interaction between the carbon matrix and incident electromagnetic waves, as well as providing multiple reflection behaviors. We believe that this study may open a new avenue to promote the electromagnetic applications of MOFs-derived carbon-based composites

Amino Acid-Assisted Synthesis of Hierarchical Silver Microspheres for Single Particle Surface-Enhanced Raman Spectroscopy

We demonstrate the use of amino acids as directing agents to synthesize hierarchical silver microspheres assembled by nanosheets with well-defined morphologies, in the absence of any other surfactants or capping agents. This fabrication method avoids the absorption of macromolecules and enables clean surface on the Ag microspheres. The chemical nature of the amino acids plays a vital role in the hierarchical structure of the Ag microspheres. As found, amino acids with simple structures and 2–3 carbon atoms like alanine and glycine lead to more loosely packed Ag microspheres, and those with more complicated structures and more carbon atoms, e.g. glycine, glutamine, and asparagine, result in close-packed Ag particles assembled by thinner nanosheets. By adjusting the concentration of AgNO₃ solution, size as well as the surface roughness of the Ag microspheres can be well controlled. Individual particles of the constructed hierarchical Ag microspheres with highly roughened surface can act as sensitive SERS platforms. Detection of chemical molecules and monitoring of the plasmon-driven chemical reactions have been carried out through a single particle SERS technique

Fabrication of Thorny Au Nanostructures on Polyaniline Surfaces for Sensitive Surface-Enhanced Raman Spectroscopy

Here we demonstrate, for the first time, the fabrication of Au nanostructures on polyaniline (PANI) membrane surfaces for surface enhanced Raman spectroscopy (SERS) applications, through a direct chemical reduction by PANI. Introduction of acids into the HAuCl₄ solution leads to homogeneous Au structures on the PANI surfaces, which show only sub-ppm detection levels toward the target analyte, 4-mercaptobenzoic acid (4-MBA), because of limited surface area and lack of surface roughness. Thorny Au nanostructures can be obtained through controlled reaction conditions and the addition of a capping agent poly (vinyl pyrrolidone) (PVP) in the HAuCl₄ solution and the temperature kept at 80 °C in an oven. Those thorny Au nanostructures, with higher surface areas and unique geometric feature, show a SERS detection sensitivity of 1 × 10^–9 M (sub-ppb level) toward two different analyte molecules, 4-MBA and Rhodamine B, demonstrating their generality for SERS applications. These highly sensitive SERS-active substrates offer novel robust structures for trace detection of chemical and biological analytes

Constructing Uniform Core–Shell PPy@PANI Composites with Tunable Shell Thickness toward Enhancement in Microwave Absorption

Highly uniform core–shell composites, polypyrrole@polyaniline (PPy@PANI), have been successfully constructed by directing the polymerization of aniline on the surface of PPy microspheres. The thickness of PANI shells, from 30 to 120 nm, can be well controlled by modulating the weight ratio of aniline and PPy microspheres. PPy microspheres with abundant carbonyl groups have very strong affinity to the conjugated chains of PANI, which is responsible for the spontaneous formation of uniform core–shell microstructures. However, the strong affinity between PPy microspheres and PANI shells does not promote the diffusion or reassembly of two kinds of conjugated chains. Coating PPy microspheres with PANI shells increases the complex permittivity and creates the mechanism of interfacial polarization, where the latter plays an important role in increasing the dielectric loss of PPy@PANI composites. With a proper thickness of PANI shells, the moderate dielectric loss will produce well matched characteristic impedance, so that the microwave absorption properties of these composites can be greatly enhanced. Although PPy@PANI composites herein consume the incident electromagnetic wave by absolute dielectric loss, their performances are still superior or comparable to most PANI-based composites ever reported, indicating that they can be taken as a new kind of promising lightweight microwave absorbers. More importantly, microwave absorption of PPy@PANI composites can be simply modulated not only by the thickness of the absorbers, but also the shell thickness to satisfy the applications in different frequency bands