Metalizing carbon nanotubes with Pd–Pt core–shell nanowires enhances electrocatalytic activity and stability in the oxygen reduction reaction

We describe an electrostatically induced self-assembly method to prepare ultrathin Pd nanowires (NWs) surrounding individual multiwalled carbon nanotubes, i.e., Pd_NW/MWNTs, that are noticeable for improving performance in the oxygen reduction reaction (ORR) of their supported Pt_ML electrocatalyst. The carbonaceous by-products in MWNTs, rather than the nanotubes themselves, are modified with the oxygenated terminals to allow the negatively charged and hydrophilic surface while retaining the intrinsic nature of the MWNTs. Encompassing the nanotubes' length are 2-nm-thick Pd NWs that are closely packed and homogeneously dispersed due to the unique processes for preparing Pd_NW/MWNTs and its components. Although the crystal lattice of the Pd NWs expands somewhat, which should cause an unfavorable interaction with supported Pt_ML, this adverse effect is counterweighed by the shape-determined features of Pd NWs, including their high specific surface area, excellent contiguousness, and low-energy atomic configuration. Consequently, these distinct chemical and physical properties substantially expedite the desorption of the intermediates to refresh the active centers during the reduction of oxygen with the Pt_ML electrocatalyst while ensuring a desirable electron transfer rate, so improving the overall ORR kinetics. Indeed, Pt_ML/Pd_NW/MWNTs exhibits the Pt mass and specific activities of 1.45 A/mg_Pt and 0.65 mA/cm² _Pt, respectively, each of which are several times those of the Pt/C and even higher than those of the Pt_ML supported on Pd nanoparticles. These high activities remained over a long-term stability test using the latest US Department of Energy-established protocol.     

Designing high performance Pt monolayer core–shell electrocatalysts for fuel cells

In proton exchange membrane fuel cells, platinum (Pt) has been the dominant choice for both the cathode and the anode catalysts. The high Pt content and high associated costs particularly at the cathode, and sluggish oxygen reduction reaction (ORR) kinetics and poor stability, remain a challenge. Pt monolayer (ML) catalysts offer a distinctively reduced Pt content while providing considerable possibilities for enhancing their catalytic activity and stability for the ORR. In this opinion, we first review the achievement in active and stable Pt ML on palladium (Pd) nanoparticle catalysts for the ORR. We then describe the mechanisms that rationalize their high activity and durability. Recently, we developed several novel nanostructured cores to further improve the ORR activity and stability by optimizing their surface orientation, composition, and morphology. The results from the Pt ML catalysts significantly impact the research of electrocatalysis and fuel-cell technology, as they demonstrate an exceptionally effective way of design and syntheses of catalysts.     

Synthesis of bifunctional core–shell particles with a porous zeolite core and a responsive polymeric shell

The aim of our work is the synthesis and characterization of colloidal core–shell particles with a zeolite core and an environmentally responsive shell. We have synthesized colloidal ZSM-5 zeolite and modified the surface with 3-(trimethoxysilyl)propyl methacrylate in order to introduce double bonds at the surface. The cross-linked polymeric shell was prepared by precipitation polymerization using the functionalized zeolite particles as seeds. We employed thermoresponsive poly(N-isopropylacrylamide) and pH-responsive poly(vinylpyridine) as the polymeric shell, respectively. The temperature- and pH-depending swelling and deswelling of the core–shell particles were characterized with dynamic light scattering techniques. Transmission electron microscopy pictures show the morphology of the synthesized particles. It is proposed that these types of bifunctional core–shell particles could be of use for controlled uptake and release applications and separation of molecules.     

Electrochemical determination of the surface composition of Pd–Pt core–shell nanoparticles

Different amounts of Pt atoms were deposited onto the surface of Pd nanoparticles supported on carbon black by hydroquinone reduction method in anhydrous ethanol. Here, we surveyed electrochemical probing of surface compositions of Pd–Pt surface alloys. They were calculated from hydrogen desorption, carbon monoxide adlayer oxidation, and reduced carbon dioxide oxidation charges. The surface composition of Pt drastically increased up to Pt[0.3]/Pd/C (23.1 at.% of Pt) and then approached that of pure Pt with the moderate rate of increase.     

Thermoresponsive submicron-sized core–shell hydrogel particles with encapsulated olive oil

Thermoresponsive submicron-sized core–shell hydrogel particles with incorporated olive oil were synthesised and studied. The microspheres having poly(N-isopropylacrylamide-co-methyl methacrylate) core and poly(N-isopropylacrylamide) shell were synthesised by emulsifier-free seed polymerisation method. The morphology, particle size and distribution characteristics of the core microspheres were studied with different amount of initiator, monomer–solvent ratio and polymerisation time using scanning electron microscopy and dynamic light scattering particle size analysis. The prepared core and core–shell microspheres were regularly spherical with average size of around 190.0 and 320.0 nm respectively and nearly monodispersed size distribution. Transmission electron microscopy study revealed the core–shell structure of the microspheres. The thermoresponsive transition temperature (T _t) of the core–shell microspheres was determined as 33 °C by optical absorbance measurement, dynamic light scattering particle size analysis and differential scanning calorimetry. The release rate of olive oil from core–shell microspheres was accelerated by squeezing out the entrapped olive oil as the temperature was increased above T _t. Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy study indicated the formation of copolymer.     

Biomimetic synthesis of raspberry-like hybrid polymer–silica core–shell nanoparticles by templating colloidal particles with hairy polyamine shell

The nanoparticles composed of polystyrene core and poly[2-(diethylamino)ethyl methacrylate] (PDEA) hairy shell were used as colloidal templates for in situ silica mineralization, allowing the well-controlled synthesis of hybrid silica core–shell nanoparticles with raspberry-like morphology and hollow silica nanoparticles by subsequent calcination. Silica deposition was performed by simply stirring a mixture of the polymeric core–shell particles in isopropanol, tetramethyl orthosilicate (TMOS) and water at 25 °C for 2.5 h. No experimental evidence was found for nontemplated silica formation, which indicated that silica deposition occurred exclusively in the PDEA shell and formed PDEA–silica hybrid shell. The resulting hybrid silica core–shell particles were characterized by transmission electron microscopy (TEM), thermogravimetry, aqueous electrophoresis, and X-ray photoelectron spectroscopy. TEM studies indicated that the hybrid particles have well-defined core–shell structure with raspberry morphology after silica deposition. We found that the surface nanostructure of hybrid nanoparticles and the composition distribution of PDEA–silica hybrid shell could be well controlled by adjusting the silicification conditions. These new hybrid core–shell nanoparticles and hollow silica nanoparticles would have potential applications for high-performance coatings, encapsulation and delivery of active organic molecules.     

Hydrophobic modification of cellulose fibres by cationic-modified polyacrylate latex with core–shell structure

In cellulose fibre-based green packaging, the poor resistance or barrier against water or water vapour has remained as one of the key challenges. In this work, cationic polymer latex, butyl acrylate-co-styrene/2-ethylhexylacrylate-co-methyl methacrylate (BA-co-St/EHA-co-MMA), with core–shell structure was especially synthesized and used as a wet-end additive to render the fibre or paper hydrophobic. TEM observation confirmed that the latex particles obtained indeed possessed desired characteristic of core–shell structure. The experimental results showed that the cationic polymer was especially suitable for use in papermaking processes due to its high retention with cellulose fibres. The surface modification of the natural fibre by the adsorption of cationic latex on the fibre surfaces potentially created the thin films of polymers on fibre surfaces. The resulting paper is highly hydrophobic with improved barrier property, as demonstrated by the high contact angles and relatively low WVTR value. Moreover, the mechanical properties of paper were maintained or even improved in the presence of an appropriate level of the latex.     

Amphiphilic polymeric particles with core–shell nanostructures: emulsion-based syntheses and potential applications

The design and synthesis of amphiphilic nano- to micro-sized polymeric particles with core–shell nanostructures have attracted more and more attention because of their wide applicability in modern material science and their technological importance in the areas of colloid and interface science. Many synthetic strategies have been developed for the preparation of amphiphilic core–shell particles that consist of hydrophobic polymer cores and hydrophilic polymeric shells. In this review, we focus on emulsion-based approaches and properties of particles produced. These methods are: (1) grafting to functionalized particle that produces a corona-like particle, (2) grafting from reactive seed particle that produces a brush-like particle, (3) copolymerization of reactive macro-monomer with hydrophobic monomer that produces a corona-like particle, (4) emulsion polymerization in the presence of block or comb-like copolymer containing controlled free-radical moiety that produces a multi-layered particle, and (5) redox-initiated graft polymerization of vinyl monomer from a water-soluble polymer containing amino groups that produces a hairy-like particle. Potential applications of some of these particles in drug and gene deliveries, enzyme immobilization, colloidal nanocatalyst, chemical sensing, smart coating, and thermal laser imaging will be discussed.     

Preparation and properties of silica–gold core–shell particles

Silica-metal core–shell particles, as for instance those having siliceous core and nanostructured gold shell, attracted a lot of attention because of their unique properties resulting from combination of mechanical and thermal stability of silica and magnetic, electric, optical and catalytic properties of metal nanocrystals such as gold, silver, platinum and palladium. Often, the shell of the core–shell particles consists of a large number of metal nanoparticles deposited on the surface of relatively large silica particles, which is the case considered in this work. Namely, silica particles having size of about 600 nm were subjected to surface modification with 3-aminopropyltrimethoxysilane. This modification altered the surface properties of silica particles, which was demonstrated by low pressure nitrogen adsorption at ?196 °C. Next, gold nanoparticles were deposited on the surface of aminopropyl-modified silica particles using two strategies: (i) direct deposition of gold nanoparticles having size of about 10 nm, and (ii) formation of gold nanoparticles by adsorption of tetrachloroauric acid on aminopropyl groups followed by its reduction with formaldehyde.The overall morphology of silica–gold particles and the distribution of gold nanoparticles on the surface of modified silica colloids were characterized by scanning electron microscopy. It was shown that direct deposition of colloidal gold on the surface of large silica particles gives more regular distribution of gold nanopartciles than that obtained by reduction of tetrachloroauric acid. In the latter case the gold layer consists of larger nanoparticles (size of about 50 nm) and is less regular. Note that both deposition strategies afforded silica–gold particles having siliceous cores covered with shells consisting of gold nanoparticles of tunable concentration.     

Silica–silver core–shell particles for antibacterial textile application

The silica–silver core–shell particles were synthesized by simple one pot chemical method and were employed on the cotton fabric as an antibacterial agent. Extremely small (1–2 nm) silver nanoparticles were attached on silica core particles of average 270 nm size. The optimum density of the nano silver particles was found which was sufficient to show good antibacterial activity as well as the suppression in their surface plasmon resonance responsible for the colour of the core–shell particle for antibacterial textile application. The change in the density and size of the particles in the shell were monitored and confirmed by direct evidence of their transmission electron micrographs and by studying surface plasmon resonance characteristics. The colony counting method of antibacterial activity testing showed excellent results and even the least silver containing core–shell particles showed 100% activity against bacterial concentration of 10⁴ colony counting units (cfu). The bonding between core–shell particles and cotton fabric was examined by X-ray photoelectron spectroscopy. The antibacterial activity test confirmed the firm attachment of core–shell particles to the cotton fabric as a result 10 times washed sample was as good antibacterial as that of unwashed sample. The bacterial growth was inhibited on and beneath the coated fabric, at the same time no zone of inhibition which occurs due to the migration of silver ions into the medium was observed indicating immobilization of silver nanoparticles on silica and core–shell particles on fabric by strong bonding.     

Luminescent core–shell nanoparticles with crosslinked aggregation-induced emission core structures: Emission both in solution and solid states

Luminescent core–shell nanoparticles (NPs) with crosslinked aggregation-induced emission (AIE) core structures, which exhibited excellent emission independent of the dispersion state of the NPs, have been developed by a facile one-pot method based on the self-assembly of an amphiphilic block copolymer poly(PEGMA)-b-poly(DB3VT). Core–shell micelles with a poly(DB3VT) core were formed from poly(PEGMA)-b-poly(DB3VT) in tetrahydrofuran (THF)/H₂O condition, and the crosslinked AIE-based structure was selectively incorporated into the core by the Suzuki coupling reaction between poly(DB3VT) blocks and tetraphenylethylene (TPE)-based coupling monomers at the same time. This method afforded a uniform NP with a crosslinked TPE-based AIE core structure. The obtained NP exhibited excellent emission both in diluted solution and solid states. This result indicated that the formed TPE-based AIE core structure was always aggregated regardless of NP dispersion owing to the crosslinking as we expected. The crosslinked TPE-based AIE core structure, which was related to the emission property, was readily tuned by the selection and combination of coupling monomers in the Suzuki coupling reaction. By incorporating electron-deficient units into the core, the emission color could be successfully tuned from yellow-green to orange and red while maintaining the emission property independent of the state of the NP dispersion. These results demonstrated that NPs with the crosslinked AIE core structures are a promising luminescent material design motif to realize emission independent on molecular dispersion.     

Fabrication of polymeric micelles with core–shell–corona structure for applications in controlled drug release

Thermo-responsive polymeric micelles of poly (ethylene glycol)-b-poly(2-hydroxyethyl methacrylate-g-lactide)-b-poly(N-isopropylacrylamide) (PEG-P(HEMA-PLA)-PNIPAM) with core–shell–corona structure were fabricated for applications in controlled drug release. The graft copolymer of PEG-P(HEMA-PLA)-PNIPAM was self-assembled into core–shell micelles with a densely PLA core and mixed PEG/PNIPAM shells at 25 °C in aqueous media. By increasing the temperature above the lower critical solution temperature of PNIPAM, these core–shell micelles could be converted into core–shell–corona micelles because of the collapse of PNIPAM block on the PLA core as the inner shell and the soluble PEG block stretching outside as the outer corona. Anticancer drug doxorubicin (DOX) was loaded in the polymeric micelles as a model drug. Compared with polymeric micelles formed by liner PEG-b-PLA-b-PNIPAM triblock copolymer, these polymeric micelles exhibited higher loading capacity, and release of DOX from the polymeric micelles with core–shell–corona structure was well-controlled.     

A colorimetric assay for measuring iodide using Au@Ag core–shell nanoparticles coupled with Cu

Au@Ag core–shell nanoparticles (NPs) were synthesized and coupled with copper ion (Cu²⁺) for the colorimetric sensing of iodide ion (I⁻). This assay relies on the fact that the absorption spectra and the color of metallic core–shell NPs are sensitive to their chemical ingredient and dimensional core-to-shell ratio. When I⁻ was added to the Au@Ag core–shell NPs-Cu²⁺ system/solution, Cu²⁺ can oxidize I⁻ into iodine (I₂), which can further oxidize silver shells to form silver iodide (AgI). The generated Au@AgI core–shell NPs led to color changes from yellow to purple, which was utilized for the colorimetric sensing of I⁻. The assay only took 10 min with a lowest detectable concentration of 0.5 μM, and it exhibited excellent selectivity for I⁻ over other common anions tested. Furthermore, Au@Ag core–shell NPs-Cu²⁺ was embedded into agarose gels as inexpensive and portable “test strips”, which were successfully used for the semi-quantitation of I⁻ in dried kelps.     

Preparation of Mn2O3 catalyst with core–shell structure via spray pyrolysis assisted with glucose

A novel core–shell Mn₂O₃ catalyst is obtained by facile spray pyrolysis assisted with glucose acting as a structure-directing agent. The detail characterizations include X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption/desorption isotherms, and the thermogravimetric method (TG/DTA). In the probe test of the aqueous degradation of methylene blue (MB), the high catalytic peroxidization activity of core–shell Mn₂O₃ catalyst, compared to the commercial Mn₂O₃, is attributed to the high crystallization of α-Mn₂O₃, the large surface area that resulted from the core–shell structure with porous architecture, and the special core–shell structure that acted as a micro-reactor.     

Preparation of titania hollow particles with independently controlled void size and shell thickness by catalytic templating core–shell polymer particles

Organic/inorganic hybrids were prepared by catalytic hydrolysis and subsequent polycondensation of tetra-n-butyl titanate (TnBT) in shell layers grafted on core particles. The core particles were synthesized by emulsifier-free emulsion polymerization of styrene, N-n-butyl-N-2-methacryloyloxyethyl-N,N-dimethylammonium bromide (C₄DMAEMA), and 2-chloropropionyloxyethyl methacrylate using 2,2′-azobis(2-amidinopropane) dihydrochloride as an initiator. The core diameters were controlled in the range of 70–550 nm by adjusting a C₄DMAEMA feed concentration. The core–shell particles were prepared by surface-initiated activator generated electron transfer–atom transfer radical polymerization of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA). The sizes of core–shell particles were found to increase monotonically with an increase in a DMAEMA concentration. The hybrid particles were fabricated by adding TnBT into a water/ethanol dispersion of core–shell particles. The amounts of titania deposited increased in proportion to the grafted amounts of poly[2-(N,N-dimethylamino)ethyl methacrylate] on the core particles. The X-ray diffraction measurement revealed that the hollow titania particles obtained by heat treatment of hybrids have an anatase crystallographic phase.