Inverted Core Shell Research Articles

Inverted core–shell structured hollow particles for improved volatile organic compounds removal and photocatalytic regeneration

As the amount of time people spend indoors increases, the impact of indoor air quality on health has become more important, and numerous studies have been conducted to eliminate indoor air pollutants such as volatile organic compounds (VOCs). Among the several methods for removing VOCs, the adsorption is the most widely used in that it requires a simple mechanism and low energy, but has a limitation in that it is difficult to regenerate after saturation. In order to solve this problem, composite materials combining adsorbents and photocatalysts have been developed, but there is still an issue that the photocatalytic activity can be lowered by covering the surface of the photocatalyst with the adsorbent. Here, mesoporous carbon/silica/titania double shell hollow nanoparticles (MC/SiO2/TiO2 HNPs) are prepared through etching, calcination and carbonization of polystyrene/polyacrylonitrile/SiO2/TiO2 multi-layered core/shell nanoparticles. Attributed to the high specific surface area of the MC/SiO2/TiO2 HNPs, high VOCs adsorption efficiency can be achieved. Moreover, the photocatalytic activity can be maximized since the TiO2 shell is exposed to the surface without any covering materials.

On the synthesis of bi-magnetic manganese ferrite-based core-shell nanoparticles.

Multifunctional nano-heterostructures (NHSs) with controlled morphology are cardinal in many applications, but the understanding of the nanoscale colloidal chemistry is yet to be fulfilled. The stability of the involved crystalline phases in different solvents at mid- and high-temperatures and reaction kinetics considerably affect the nucleation and growth of the materials and their final architecture. The formation mechanism of manganese ferrite-based core–shell NHSs is herein investigated. The effects of the core size (8, 10, and 11 nm), the shell nature (cobalt ferrite and spinel iron oxide) and the polarity of the solvent (toluene and octanol) on the dissolution phenomena of manganese ferrite are also studied. Noteworthily, the combined use of bulk (powder X-ray diffraction, 57Fe Mössbauer spectroscopy, and DC magnetometry) and nanoscale techniques (HRTEM and STEM-EDX) provides new insights into the manganese ferrite dissolution phenomena, the colloidal stability in an organic environment, and the critical size below which dissolution is complete. Moreover, the dissolved manganese and iron ions react further, leading to an inverted core–shell in the mother liquor solution, paving the way to novel synthetic pathways in nanocrystal design. The MnFe2O4@CoFe2O4 core–shell heterostructures were also employed as heat mediators, exploiting the magnetic coupling between a hard (CoFe2O4) and a soft phase (MnFe2O4).

Inverted core–shell potential energy landscape of icosahedral clusters in deeply undercooled metallic liquids and glasses and its effect on the glass forming ability of bcc and fcc metals

Current understanding of the origin of icosahedral clusters or icosahedral short-range ordering in undercooled metallic liquids or glasses is based on Frank’s consideration of an isolated icosahedron whose core has lower potential energy than the shell. Using large scale atomistic simulations and statistical analysis of several bcc (body-centered-cubic) and fcc (face-centered-cubic) metals, here we show that the shells of icosahedrons spontaneously formed inside deeply undercooled metallic liquids or glasses in fact have lower (averaged) potential energy than the cores. The shell potential energy deficiency occurs only to the icosahedral clusters but not to the equilibrium-crystal clusters, and, for icosahedral clusters, this deficiency grows with decreasing temperature. Compared with fcc metals, bcc metals exhibit greater potential energy deficiency on the icosahedral shells and produce significantly more icosahedral clusters upon liquid quenching, which explains the higher tendency of bcc metals to be vitrified observed in ultrafast cooling experiments. Inspecting the potential energy deficiency on the icosahedral shells through computation provides a new avenue to the search for amorphous metals (i.e. metallic glasses) with high glass forming ability and processability.

Formulation of polyphthalaldehyde microcapsules for immediate UV-light triggered release

Triggered release from responsive drug reservoirs activated by remote stimuli is desired in a range of fields. Critical bottlenecks are cost-efficient formulation avenues applicable for industrial scale-up, viable triggers and immediate release rather than continuous release upon activation. UV-sensitive microcapsules based on self-immolating polymers in combination with thin shells and morphological weak spots should allow for immediate triggered release. Polyphthalaldehyde-based microcapsules were prepared using several variations of the internal phase separation route. In addition, a fluorescence microscopy method was developed to study both the microcapsule morphology and the triggered release in-situ. The microcapsule formation was driven by the surface activity of the stabilizer, effectively lowering the high polymer-water interfacial tension, which is in sharp contrast to conventional encapsulation systems. Contrary to previous findings, a core–shell morphology was obtained via slow emulsion-to-suspension transformation. Rapid transformation captured intermediate inverted core–shell structures. The capsules were highly sensitive to both acid- and UV-mediated triggers, leading to an unzipping and rupturing of the shell that released the core content. Poly(methacrylic acid)-stabilized microcapsules displayed immediate UV-triggered release provided by their stimuli-sensitive blueberry morphology. Both capsules in aqueous and dry environment started to lose their core content after less than one minute of UV light exposure.

Design and fabrication of PVAc-based inverted core/shell (ICS) structured adhesives for improved water-resistant wood bonding performance: II. Influence of copolymerizing-grafting sequential reaction

PVAc-based inverted core/shell (ICS) structured latex adhesive with improved water- and heat-resistance was prepared by a copolymerizing-grafting sequential reaction approach. The sequential reaction process combines the function of a copolymer layer and an active grafting site to controllably achieve the thermodynamic non-equilibrium ICS morphology with hydrophilic poly(vinyl acetate) (PVAc) cores and hydrophobic polystyrene (PS) shells. The difference between the sequential reaction approach and the chemical grafting method was explored. Structured latex particle morphology, transferring from strawberry-like particles, over particles with large and irregular PS protuberances, to nearly spherical particles with a smooth surface, was microscopically visualized by changing the amount of butyl acrylate (BA) in the PVAc copolymer cores. The heat- and water-resistant bonding of PVAc-based adhesives on wood blocks were evaluated by shear strength at both dry and wet conditions, as well as the boiling water failure time. The results showed that a significant improvement of water- and heat-resistance, especially for boiling water resistance, was successfully obtained. This novel approach paves a way for controlled design and manufacture of high-performance emulsion adhesives through tailoring particle morphology.

Subband energy of an inverted core shell quantum dot

The subband energy of an Inverted core shell structure is computed for a GaAs/AlxGa1−xAs Quantum dot in the effective mass approximation. The effect of shell thickness and size of the dot on the subband energy is studied. It is seen that the subband energy is affected by core as well as shell size. These studies aid in the construction of electronic levels of the system and hence can act as a probe to study the fine structure splitting of excitonic levels. The change in subband energy is larger for change in shell thickness value than for core radius. Subband energy decreases as aluminum concentration decreases. For lower shell thickness there is substantial change in the subband energy between aluminum concentration of 0.4 and 0.1.

Design and fabrication of PVAc-based inverted core/shell (ICS) structured adhesives for improved water-resistant wood bonding performance: I. Influence of chemical grafting

Strategy for fabricating inverted core/shell (ICS) structured latex particles to improve the water- and heat-resistance of PVAc-based adhesives was investigated. A grafting layer was fabricated with acrylonitrile (AN) to achieve a thermodynamic non-equilibrium ICS morphology with a hydrophilic poly(vinyl acetate) (PVAc) core and a hydrophobic polystyrene (PS) shell. The function of the grafting layer was explored by tuning the AN loading levels and core-to-shell ratios. Special latex particle morphologies, transferring from strawberry-like with secondary PS particles, over neat strawberry-like, to final ball-like morphology, were observed and verified by electron microcopies. The wood adhesive performance of PVAc-based latex adhesives, particularly for heat- and water-resistant bonding, were evaluated by shear strength at both dry and wet conditions, as well as the boiling water failure time, showing a significant enhancement of boiling water resistance. This general approach opens an avenue for controlled design and manufacture of high-performance emulsion-based adhesives containing structured particles.

Effect of Shell Growth on the Morphology of Polyvinyl Acetate/Polystyrene Inverted Core-Shell Latex Fabricated by Acrylonitrile Grafting.

A novel strategy for fabricating inverted core-shell structured latex particles was implemented to investigate the morphology and properties of polyvinyl acetate (PVAc)-based latex. In this study, active grafting points were synthesized onto the surface of PVAc latex cores via grafting acrylonitrile (AN) to obtain a controllable coating growth of the shell monomer, styrene (St). The effect of shell growth on the morphological evolvement was explored by tuning the time of shell monomer polymerization. Unique particle morphologies, transferring from “hawthorn” type, over “peeled pomegranate” type, to final “strawberry-like” type, were observed and verified by electron microscopy. The morphological structure of latex particles exerted a significant effect on the particle size, phase structure, and mechanical properties of the obtained emulsions. The water-resistance of PVAc-based latex was also evaluated by the water absorption of latex films. More importantly, the experimental results provided a reasonable support for the controlled growth of St monomer, that is, the self-nucleation of dispersive St monomer can be transformed to in-situ coating growth on the PVAc core surface depending on the AN-active grafting points. This fabricating approach provides a reference for dynamical design and control of the latex particle morphology.

Spin–orbit interaction effects in ZnO/ZnS core–shell and ZnS/ZnO inverted core–shell quantum dots

Rashba and Dresselhaus spin–orbit interactions (SOIs), external fields and dielectric environment effects on hydrogenic impurity states in core–shell (ZnO/ZnS) and inverted core–shell (ZnS/ZnO) quantum dots were studied. We employed the finite difference method to achieve energy eigenvalues of the system. Our results demonstrate that SOIs, external fields and dielectric environment change drastically the impurity states. Besides, the spin degeneracy is removed when both the SOIs and the Zeeman effect are simultaneously considered.

Self-polarization effects in spherical inverted core–shell quantum dot

Single electron in spherical shell in infinite potential barrier is investigated. Schrodinger equation in effective mass approximation for this spherical heterosystem is solved to get eigenenergies and corresponding wave functions. In case of infinite potential barrier the ground state energy depends only on the shell width. Self-polarization potential is obtained by solving Poisson equation in three concentric spherical regions: core, shell and surrounding medium. Shell self-polarization energy depends on geometry (core and shell radius) and the dielectric mismatch at the quantum dot boundaries. Self-polarization energy is used as perturbation. One electron ground state energy results are presented in this paper. CdSe is placed in the shell. Surrounding medium is dielectric of smaller permittivity (vacuum or water), that is the most realistic case. The core dielectric permittivity influence on ground state energy is analysed. The self-polarization corrections to the ground state energy for different compositions, i.e. core and shell radius, are presented. For smaller core radius and the same shell thickness (which imply the same unperturbed ground state energy) bigger is the energy correction, i.e. perturbed energy state, for fixed value of the core dielectric permittivity. Increase of core dielectric permittivity produces decrease of energy correction.

A comparative study of conventional type II and inverted core–shell nanostructures based on CdSe and ZnS

The objective of this work is to investigate structural, morphological and optical properties of conventional CdSe/ZnS core–shell and inverted ZnS/CdSe core–shell nanostructures for opto-electronic device applications. For this purpose both nanostructures were synthesized using chemical bath deposition technique in thin film form. The structural properties were studied using X-ray diffraction technique with Rietveld refinement and transmission electron microscopy (TEM). The surface morphology of synthesized thin film was illustrated in the form 2D and 3D images using atomic force microscopy (AFM). The optical properties were explained using UV–Vis absorption spectroscopy and photo luminescence (PL) spectroscopy in in situ monitoring process. A comparison of estimated particle size from XRD, high resolution AFM and TEM images was resulted in good agreement as 2.1, 2.4 and 2.1 nm respectively for conventional CdSe/ZnS core–shell and as 2.5, 2.5 and 2.2 nm respectively for inverted ZnS/CdSe core–shell nanostructures.

Novel highly ordered core–shell nanoparticles

Core–shell nanoparticles have potential for a wide range of applications due to the tunability of their magnetic, catalytic, electronic, optical, and other physicochemical properties. A frequent drawback in the design of core–shell nanoparticles and nanocrystals is the lack of control over an extensive, disordered, and compositionally distinct interface that occurs due to the dissimilarity of structural and compositional phases of the core and shell. In this work, we demonstrate a new hydrothermal nanophase epitaxy (HNE) technique to synthesize highly structurally ordered α-Cr2O3@α-Co0.38Cr1.62O2.92 inverted core–shell nanoparticles (CSNs) with evidence for the nanoscale growth of corundum structure beginning from the core and extending completely into the shell of the CSNs with minimal defects at the interface. The high-resolution TEM results show a sharp interface exhibiting epitaxial atomic registry of shell atoms over highly ordered core atoms. The XPS and Co K-edge XANES analyses indicate the +2 oxidation state of cobalt is incorporated in the shell of the CSNs. Our XPS and EXAFS results are consistent with oxygen vacancy formation in order to maintain charge neutrality upon substitution of the Co2+ ion for the Cr3+ ion in the α-Co0.38Cr1.62O2.92 shell. Furthermore, the CSNs exhibit the magnetic exchange bias effect, which is attributed to the exchange anisotropy at the interface made possible by the nanophase epitaxial growth of the α-Co0.38Cr1.62O2.92 shell on the α-Cr2O3 core of the nanoparticles. The combination of a well-structured, sharp interface and novel nanophase characteristics is highly desirable for nanostructures having enhanced magnetic properties.

Structural and Magnetic Properties of Well-Ordered Inverted Core-Shell α-Cr2O3/ α-MxCr2-xO3 (M=Co, Ni, Mn, Fe) Nanoparticles

ABSTRACTMagnetic core shell nanoparticles (NPs) have potential for applications in magnetic random access memory, spintronic devices, and drug delivery systems. Our investigations are focused on the synthesis of inverted core shell nanoparticles and characterization of their structural and magnetic properties. By using our hydrothermal nanophase epitaxy technique, we are able to synthesize well-ordered α-Cr2O3@α-MxCr2-xO3 (M = Co, Ni, Mn, Fe) inverted core-shell nanoparticles. This typically results in the formation of novel phases of MxCr2-xO3 shells having ferromagnetic/ferrimagnetic (FM/FiM) spin ordering and an antiferromagnetic (AFM) Cr2O3 core structure. The combined results from XRD and high-resolution TEM (HRTEM) provide evidence of the presence of corundum phase both in the shell and in the core regions. HRTEM results also show a sharp interface exhibiting epitaxial atomic registry of shell atoms over highly ordered core atoms whereas TEM-EDX analyses show that the M atoms reside predominantly in the shell regions. The XPS analyses of the NPs indicate the M transition metals incorporated in the shell are in the +2 oxidation state. Magnetic measurements show well developed hysteresis loops: The field cooled hysteresis loops reveal horizontal shifts in the applied field axis and vertical shifts in the magnetization axis, relative to the zero-field cooled hysteresis loops. This provides direct evidence for the exchange bias effect between the AFM α-Cr2O3 core and the FM/FiM α-MxCr2-xO3 shell. The XPS data are consistent with oxygen vacancy formation in order to maintain charge neutrality upon substitution of the M2+ ion for the Cr3+ ion in the α-MxCr2-xO3 shell. The FM/FiM ordering in the shell may at least partially result from the F-center exchange coupling between the oxygen-vacancy induced bound magnetic polaron and nearby cations.

Cyclodextrin encapsulated monometallic and inverted core–shell bimetallic nanoparticles as efficient free radical scavengers

A facile and green route for monometallic and core–shell bimetallic Ag and Au nanoparticles using cyclodextrin as reducing and stabilizing agent for radical scavenging.

Ultrafast Charge Carrier Delocalization in CdSe/CdS Quasi-Type II and CdS/CdSe Inverted Type I Core–Shell: A Structural Analysis through Carrier-Quenching Study

We have employed femtosecond transient absorption spectrocopy to monitor charge carrier delocalization in CdSe/CdS quasi-type II and CdS/CdSe inverted type I core–shell nanocrystals (NCs). Interestingly, CdSe and CdS QD pairs can make both type I and quasi-type II core–shell structures, depending on their band alignment and charge carrier localization. Steady-state optical absorption and luminescence studies show a gradual red-shift in both optical absorption and emission spectra in CdSe/CdS core–shell with increasing CdS shell thickness. The luminescence quantum yield in CdSe/CdS core–shell drastically increases with shell thickness. Notably, CdS/CdSe inverted core–shell shows a huge red-shift both in absorption and luminescence which closely matches with the band edge photoluminescence (PL) of pure CdSe QDs (shell). However, the luminescence quantum yield does not change much with shell thickness. Depending on their band energy level alignment, the charge carrier (electron and hole) delocalization in bo...

Structural, optical and photocatalytic properties of TiO2/SnO2 and SnO2/TiO2 core–shell nanocomposites: An experimental and DFT investigation

We employed an efficient and cost effective method to synthesize core–shell TiO2/SnO2 and inverted core–shell SnO2/TiO2 nanocomposites and investigated their visible light photo catalytic activity for degradation of dye methyl orange. We carried out techniques such as XRD and TEM for the structural verification while UV–Visible and photoluminescence spectra for the optical characterization. BET is done to reveal pore diameter and surface area of prepared nanosystems. We have also performed DFT based calculation using VASP 5.2 to calculate density of states. The analyses of density of states indicate a higher photocatalytic efficiency of core–shell TiO2/SnO2 nanostructures and which we indeed observe through experiment as well.

Surface Enhanced Raman Scattering Studies of Silver-gold Normal and Inverted Core-shell Nanostructures on their Efficiency of Detecting Molecules

The intense significance of biological molecules in our everyday lives has prompted the research into the analysis, identification and quantification of these molecules. Recently, surface-enhanced Raman scattering (SERS) study is identified as one of the effective techniques to detect and quantify such bio-molecules, as these processes are indispensable in order to use them in biological applications. Such detection of molecules by SERS depends predominantly upon SERS active metal substrates such as the typical colloidal silver-gold nanostructures. However, identifying and fabricating suitable metal nanostructures for detecting such biomolecules is an essential need to implement them for practical applications particularly in extensive physiological environments. Herein, we report the synthesis of Ag-Au normal and inverted core-shell nanostructures and investigation of their plasmonic properties to detect drug molecules such as methylene blue and methyl orange using the surface enhanced Raman scattering technique. Our investigation promises for an easy reproduction of biocompatible silver-gold bimetallic core-shell nanostructures that possess excellent chemical stability which can be ideal candidates for a wide range of biomedical applications.

Chiral monolayer-protected Au–Pd bimetallic nanoclusters: Effect of palladium doping on their chiroptical responses

We present an effect of palladium doping on the chiroptical responses of optically active gold nanoclusters. Monolayer-protected Au–Pd bimetallic nanoclusters of 1.5–2.0nm in diameter are synthesized by the reduction in mixtures of Au and Pd salts in the presence of chiral thiol, glutathione (GSH), and isolated by electrophoretic separation. For synthetic feed mole ratios of Au/Pd=3/1 and 1/1, the isolated nanocluster compounds have much higher Au/Pd ratios of ∼8.4 (7.9–8.8) and ∼5.1 (4.7–5.4), respectively. We find that circular dichroism (CD) offers a greater advantage in detecting a foreign atom doping in the nanocluster as compared to absorption spectroscopy; namely, there is a significant difference in Cotton effects between the Au–Pd bimetallic nanoclusters and the monometallic counterparts (Au or Pd nanoclusters), whereas almost no difference can be seen between them in the UV–vis absorption spectra. In the bimetallic nanoclusters, interestingly, their CD profiles are almost independent on the Pd dopant content. The X-ray photoelectron spectroscopy (XPS) reveals that Au atoms are predominated in the surface (including staples) layer of these Au–Pd nanoclusters, indicating a Pdcore–Aushell (inverted core–shell) structure, so the similarity in the CD responses observed would be due both to the predominance of Au in the outermost chiral surface structure and possession of their similar metal core configuration.

Electric field effect on the third-order nonlinear optical susceptibility in inverted core–shell nanodots with dielectric confinement

Third-order nonlinear optical processes associated with the interlevel transitions in ZnS/CdSe core–shell quantum dots under electric fields are theoretically investigated. Taking into account the dielectric mismatch with the surrounding matrix, the electronic structure of the dots is obtained within the effective mass and parabolic band approximations. It is shown that large applied electric fields break the symmetry of the confinement potential and lead to a significant blue-shift of the peak positions in the nonlinear optical spectrum. The size effect is also discussed and it is proved that large nonlinear susceptibility can be obtained by increasing the thickness of the nanocrystal shell. Our results suggest that external factors such as the applied electric field and orientation of the incident light polarization can be used – in addition to spatial confinement – to improve the performances of the optical devices.

Computation of intersubband transition energy in normal and inverted core–shell quantum dots using finite difference technique

In this paper, intersubband transition energy is computed for core–shell (normal and inverted) quantum dots (CSQD) of cubic and spherical geometries by solving time-independent Schrödinger equation using finite-difference technique. Sparse, structured Hamiltonian matrices of order N3×N3 for cubic and N×N for spherical dots are produced considering N discrete points in spatial direction. The matrices are diagonalized to obtain eigenstates for electrons. Computed results for the lowest three eigenstates and intersubband transitions are shown for different structural parameters taking GaAs/AlxGa1−xAs based CSQD as example. Transition energy decreases with increase in core thickness. When compared, spherical CSQDs show higher transition energy between two subbands than cubic CSQDs of similar size and same material composition. Also, in inverted configuration, transition energy decreases for a cubic dot while increases for a spherical dot as core size is increased. Wide tuning range for intersubband transition by tailoring dot dimensions indicates important applications for optical emitters/detectors.