Polycrystalline Shell Research Articles

Regioselective encapsulation of Pd clusters in hollow polycrystalline shell

Zeolite catalysts have been widely applied in petroleum and chemical industries. Nano/hierarchical structure could improve the utilization efficiency of active sites, especially for metal species encapsulated in zeolites. However, the uniformity of zeolite surface and crystallinity would decrease, leading to low stabilization effect. Herein, we developed a seed-directed method to prepare Pd clusters (∼1.3 nm) regioselectively encapsulated in hollow polycrystalline shell of Silicalite-1 zeolite. In nitrobenzene hydrogenation reaction, the optimized Pd@S-1-hp showed a high conversion of 92.2 % at 110 °C, which is 40 % higher than that of Pd clusters in bulk zeolite. Sub-nano Pd species in polycrystalline shell could significantly shorten the diffusion length on the basis of strong microporous confinement effect. Catalytic hydrogenation activity remained stable after six cycles. This structure could be extended to heterogeneous reactions suffering from diffusion limitation.

Formation of misoriented blocks during single-crystal CVD diamond growth

This paper presents the results of a CVD diamond crystal growth experiment on an HPHT type IIa substrate (9 × 9 mm). Data acquired through cathodoluminescence and EBSD methods shed light on the aspects of the formation of a polycrystalline shell around a single crystal of CVD diamond. CVD diamond exhibits orientation features of the polycrystalline shell, microblocks, low-angle boundaries, and linear hexagonal macrostructures of dislocation origin. Crystal splitting occurs when the dislocation density increases from the geometric center to the periphery. Such CVD diamonds exhibit a continuous transition from single crystal to polycrystal through a block-type crystal.

Core/shell bimagnetic microwires with asymmetric shell: MOKE and FMR behavior

A new type of multilayered microwires with a partially covering shell and bimagnetic character is here introduced. A precursor Pyrex-coated Fe-based microwire was fabricated by quenching and drawing technique, and subsequently an outer magnetic shell was deposited by magnetron sputtering covering only partly the precursor microwire (contrary to previously reported fully covered wires). Particularly, an amorphous magnetically soft core (Fe77.5B15Si7.5) was selected in this study and a polycrystalline shell (FeNi soft or Co hard) covered only half of microwire along the axis. After performing a geometry and surface roughness characterization, their surface azimuthal static magnetic profile was determined by magneto-optic Kerr-magnetometer, MOKE. In addition, the ferromagnetic resonance, FMR, response was experimentally studied by Network Analyzer up to 16 GHz frequency where the contributions of internal core and partly covering shell are analyzed and compared with fully covered microwires. The magnetoelastic behavior of such asymmetric bimagnetic microwires open up new perspectives to employ such materials for sensing and biomedical tweezer applications.

Epitaxially Driven Phase Selectivity of Sn in Hybrid Quantum Nanowires.

Hybrid semiconductor-superconductor nanowires constitute a pervasive platform for studying gate-tunable superconductivity and the emergence of topological behavior. Their low dimensionality and crystal structure flexibility facilitate unique heterostructure growth and efficient material optimization, crucial prerequisites for accurately constructing complex multicomponent quantum materials. Here, we present an extensive study of Sn growth on InSb, InAsSb, and InAs nanowires and demonstrate how the crystal structure of the nanowires drives the formation of either semimetallic α-Sn or superconducting β-Sn. For InAs nanowires, we observe phase-pure superconducting β-Sn shells. However, for InSb and InAsSb nanowires, an initial epitaxial α-Sn phase evolves into a polycrystalline shell of coexisting α and β phases, where the β/α volume ratio increases with Sn shell thickness. Whether these nanowires exhibit superconductivity or not critically relies on the β-Sn content. Therefore, this work provides key insights into Sn phases on a variety of semiconductors with consequences for the yield of superconducting hybrids suitable for generating topological systems.

Developing First Row Transition Metal Antimonate Oxynitride and Oxysulfide Nanoparticles As Oxygen Reduction Electrocatalysts

Hydrogen fuel cells (FCs) are a promising avenue for replacing global dependence on carbon-intensive fuels especially in the transportation sector. The cathode of a FC carries out the oxygen reduction reaction (ORR), and the kinetics of the ORR is one of the primary sources of inefficiency in commercial FC devices. Commercial FC cathodes are based on expensive platinum-based catalyst architectures owing to their high activity, and therefore identifying highly active, non-precious ORR catalysts could accelerate the deployment of hydrogen fuel cells into the energy landscape. Theory calculations and experiments indicate first-row transition metal antimonates (MSbOx, M = Mn, Fe, Cr, Ni) have highly desirable ORR activity and stability characteristics. Furthermore, the material systems in this family have been extended to ternary and quaternary compositions for the ORR through the inclusion of additional transition metals to identify further opportunities for activity enhancement. In this work, we develop (oxy)sulfides and (oxy)nitrides of first-row transition metals such as manganese (MnSbOxSy and MnSbOxNz, respectively) to extend this class of materials towards enhanced activity when compared to previously reported antimonate materials. These nanoscale electrocatalysts are synthesized via colloidal means followed by high temperature surface modification under various reactive atmospheres (NH3, H2S, O2). Precise control of temperature ramps and holds enables modulation of the degree of oxidation, sulfidation, or nitridation, resulting in fully oxidized manganese antimonate nanocrystal cores with thin polycrystalline shells of (oxy)sulfide and (oxy)nitride. We tune the synthetic parameters of these materials and evaluate their enhanced electrochemical ORR activity in both alkaline and acidic conditions that are relevant to the operating conditions of anion-exchange membrane and proton-exchange membrane FCs, respectively. We perform extensive bulk and surface characterization with techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and gas adsorption BET surface area analysis. We find that converting the pure oxide into oxysulfide or oxynitride causes significant alterations to crystallinity and is accompanied by the appearance of additional crystalline phases that were not present previously as seen in XRD diffractograms. Transmission electron micrographs reveal polycrystalline nanocrystals of roughly 50 nm diameter for all compositions and show varying internal contrast patterns. The pure oxide nanocrystals form a thin shell of ~4 nm thickness after sulfidation or nitridation, and this shell may be converting into an active phase exposed to electrolyte during ORR testing and is expected to be significant for understanding the observed activity enhancement. XPS spectra indicate shifts in Sb and Mn peaks that are suggestive of changing surface-level oxidation state. We also utilize nanometer-resolution elemental mapping with TEM coupled to energy dispersive X-ray spectroscopy (EDS) to visualize the distribution of elements both before and after catalysis to understand how degradation affected observed activity trends. Lastly, on-line degradation measurements during potential cycling on an electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) reveal potential and current density dependence of corrosion to provide insight into degradation mechanisms. Utilization of a diverse set of physical and electrochemical characterization techniques provides a thorough description of how manganese antimonate (oxy)sulfides and (oxy)nitrides carry out the ORR and could inform the future development of advanced non-precious FC cathode catalysts to drive down device cost.

Room-temperature gas sensor based on in situ grown, etched and W-doped ZnO nanotubes functionalized with Pt nanoparticles for the detection of low-concentration H2S

The direct in-situ growth of sensitive materials on the gas-sensing electrode is a subject of great interest for the gas-sensing field. However, combining the device design with the modification of materials is still of great challenge. Herein, a high-performance room-temperature gas sensor was constructed by functionalizing the in situ grown, etched and W-doped ZnO nanotubes (EW-ZnO NTs) with Pt nanoparticles (NPs) on new-type FTO gas-sensing electrode. The effect of etching treatment on the surface characteristics was adjusted by controlling the hydrothermal reaction time to obtain ZnO NTs with uniformly W-doped polycrystalline shells. The thus-produced EW-ZnO NTs sensor was found to be improved by 223 % in response to 10 ppm H2S at room temperature compared to the ZnO nanorods (NRs) sensor. Additionally, the effect of different Pt loading amounts on the gas-sensing performance of sensitive materials was investigated. Benefiting from the good contact of sensitive materials and gas-sensing electrodes, the formation of defects and the functionalization of noble metal Pt, the Pt@EW-ZnO NTs gas sensors exhibit numerous superior advantages, including high response, low detection limit (100 ppb) and excellent selectivity. This study shows that the gas detection capability of ZnO sensors can be significantly boosted with the in-situ growth, surface modification and noble metal functionalization, which brings insights into gas sensors.

Early Stages of Aluminum-Doped Zinc Oxide Growth on Silicon Nanowires.

Aluminum-doped zinc oxide (AZO) is an electrically conductive and optically transparent material with many applications in optoelectronics and photovoltaics as well as in the new field of plasmonic metamaterials. Most of its applications contemplate the use of complex and nanosized materials as substrates onto which the AZO forms the coating layer. Its morphological characteristics, especially the conformality and crystallographic structure, are crucial because they affect its opto-electrical response. Nevertheless, it was difficult to find literature data on AZO layers deposited on non-planar structures. We studied the AZO growth on silicon-nanowires (SiNWs) to understand its morphological evolution when it is formed on quasi one-dimensional nanostructures. We deposited by sputtering different AZO thicknesses, leading from nanoclusters until complete incorporation of the SiNWs array was achieved. At the early stages, AZO formed crystalline nano-islands. These small clusters unexpectedly contained detectable Al, even in these preliminary phases, and showed a wurtzite crystallographic structure. At higher thickness, they coalesced by forming a conformal polycrystalline shell over the nanostructured substrate. As the deposition time increased, the AZO conformal deposition led to a polycrystalline matrix growing between the SiNWs, until the complete array incorporation and planarization. After the early stages, an interesting phenomenon took place leading to the formation of hook-curved SiNWs covered by AZO. These nanostructures are potentially very promising for optical, electro-optical and plasmonic applications.

Plasma engineering of microstructured piezo – Triboelectric hybrid nanogenerators for wide bandwidth vibration energy harvesting

We introduce herein the advanced application of low-pressure plasma procedures for the development of piezo and triboelectric mode I hybrid nanogenerators. Thus, plasma assisted deposition and functionalization methods are presented as key enabling technologies for the nanoscale design of ZnO polycrystalline shells, the formation of conducting metallic cores in core@shell nanowires, and for the solventless surface modification of polymeric coatings and matrixes. We show how the perfluorinated chains grafting of polydimethylsiloxane (PDMS) provides a reliable approach to increase the hydrophobicity and surface charges at the same time that keeping the PDMS mechanical properties. In this way, we produce efficient Ag/ZnO convoluted piezoelectric nanogenerators supported on flexible substrates and embedded in PDMS compatible with a contact–separation triboelectric architecture. Factors like crystalline texture, ZnO thickness, nanowires aspect ratio, and surface chemical modification of the PDMS are explored to optimize the power output of the nanogenerators aimed for harvesting from low-frequency vibrations. Just by manual triggering, the hybrid device can charge a capacitor to switch on an array of color LEDs. Outstandingly, this simple three-layer architecture allows for harvesting vibration energy in a wide bandwidth, thus, we show the performance characteristics for frequencies between 1 Hz and 50 Hz and demonstrate the successful activation of the system up to ca. 800 Hz.

When More Is Less: Plastic Weakening of Single Crystalline Ag Nanoparticles by the Polycrystalline Au Shell.

It is well-known that in the case of bulk polycrystalline metals, a reduction in the grain size leads to material hardening, since the grain boundaries represent efficient barriers for slip transfer between the adjacent crystalline grains. Here, we show that coating single crystalline Ag nanoparticles with a thin polycrystalline Au layer leads to a weakening of the particles. Moreover, while the single crystalline Ag nanoparticles yield in a single large displacement burst when loaded in compression, their Ag-Au core-shell counterparts demonstrate a more homogeneous deformation with signs of strain hardening. Our molecular dynamics simulations demonstrate that particle weakening at low strains is attributed to the plasticity confinement in the polycrystalline shell, in which the grain boundaries play a dual role of dislocations sources and sinks. At higher strains, the plasticity within the Ag core is initiated by the dislocations nucleating at the Ag-Au interphase boundary. The widespread of energy barriers for dislocations nucleation at the interphase boundaries and their lower value as compared to the barriers for surface nucleation ensure particle weakening and more homogeneous deformation. The results of this study show that adding imperfect material to superstrong single crystalline metal nanoparticles makes them weaker. At the same time, thin nanocrystalline coatings can be employed to improve the formability of metals at the nanoscale.

Crystal growth of clathrate hydrate formed with H2\u2009+\u2009CO2 mixed gas and tetrahydropyran

Hydrate-based gas separation technology is applicable to the CO2 capture and storage from synthesis gas mixture generated through gasification of fuel sources including biomass. This paper reports visual observations of crystal growth dynamics and crystal morphology of hydrate formed in the H2 + CO2 + tetrahydropyran (THP) + water system with a target for developing the hydrate-based CO2 separation process design. Experiments were conducted at a temperature range of 279.5–284.9 K under the pressure of 4.9–5.3 MPa. To simulate the synthesis gas, gas composition in the gas phase was maintained around H2:CO2 = 0.6:0.4 in mole fraction. Hydrate crystals were formed and extended along the THP/water interface. After the complete coverage of the interface to shape a polycrystalline shell, hydrate crystals continued to grow further into the bulk of liquid water. The individual crystals were identified as hexagonal, tetragonal and other polygonal-shaped formations. The crystal growth rate and the crystal size varied depending on thermodynamic conditions. Implications from the obtained results for the arrangement of operating conditions at the hydrate formation-, transportation-, and dissociation processes are discussed.

Crystallization of PECVD grown Si nanowire and its characterization using confocal Raman and EELS studies

In the present report, systematic studies on nanoclustered Au catalyzed vapour-liquid–solid growth of silicon nanowires (SiNWs) by plasma enhanced chemical vapour deposition (PECVD) technique are described. The growth of SiNWs is performed on not so conventional sapphire substrate as well as commonly used Si substrate. SiNWs grown on different substrates are characterized for their morphologies and structures. Field emission scanning electron microscopy analysis of grown SiNWs, having an average diameter of ~ 800 nm and a typical length of ~ 10 µm, shows a slight increase in density of nanowires grown on Si substrate when compared to that for the samples grown on sapphire without any morphological changes. Structural studies using high resolution transmission electron microscopy confirm the crystalline nature of SiNWs where the crystalline core with polycrystalline shell is encapsulated in the amorphous phase. Confocal Raman analysis, unambiguously, confirms the presence of amorphous silicon oxide and amorphous Si (a-Si) in the SiNWs. Electron energy loss spectroscopy for the Si L2,3 shows the presence of a-Si and SiO2 at the surface of nanowire affirming the encapsulation of SiNWs in amorphous phase. Annealing of the samples conducted at 900 °C for 15 min restores the crystalline nature within the SiNWs which is confirmed further from Raman analysis.

Controlling Pt Crystal Defects on the Surface of Ni–Pt Core–Shell Nanoparticles for Active and Stable Electrocatalysts for Oxygen Reduction

A strategy of direct growth of Pt on Ni was used to create and control Pt crystal defects on the surface of Ni–Pt core–shell nanoparticles. The control over the types of defects was easily achieved by changing the surfactant system. In this work, two types of crystal defects have been introduced into Ni–Pt core–shell nanoparticles: polycrystalline shells with multiple grain boundaries and step-edge shells with undercoordinated atoms at corners and steps. We show that the step-edge shell has a higher specific activity for the oxygen reduction reaction (ORR), while the thinner polycrystalline shell results in a higher activity per mass and stability. Our results suggest that Ni–Pt core–shell nanoparticles with a thin Pt shell that have high density of crystal defect should be targeted for high performance ORR catalysts.

Understanding the Stability of Hollow Nanoparticles with Polycrystalline Shells

The existence of polycrystalline shells has been widely reported in the synthesis of hollow nanoparticles; however, the exact role displayed by the grain boundaries on the stability has been scarce...

Nanoindentation of polycrystalline Pd hollow nanoparticles: Grain size role

Polycrystalline hollow nanoparticles present a unique combination of strength and flexibility. However, the exact role displayed by their grain structure in mechanical properties has not been yet fully understood. Here, by means of molecular dynamics simulations, the role of grain boundary structure during the nanoindentation of metallic hollow nanoparticles with a polycrystalline shell was investigated. Our simulations were performed for a range of grain sizes and shell thicknesses, including the large strain regime. Our results show that hNP mechanical properties can be controlled by tuning the grain size of the polycrystalline shell, following an inverse Hall-Petch type dependence with the grain size. Deformation involves dislocation activity, twin hardening, grain boundary sliding, coalescence, and rotation. For single crystal shells at large strain there is hardenning following the closure of the internal cavity. For nanocrystalline shells at large strains a constant flow stress regime is observed even for deformations as high as 80%, thanks to grain boundary activity. Surprisingly, some particular grain size not only leads to an improvement in strength, but also a flow stress higher than the observed in their single-crystalline counterparts. Our work, suggest that grain boundary structure can be employed to improve and tailor desired mechanical properties in hollow nanostructures.

Polyol-Made Luminescent and Superparamagnetic β-NaY0.8Eu0.2F4@γ-Fe2O3 Core-Satellites Nanoparticles for Dual Magnetic Resonance and Optical Imaging.

Red luminescent and superparamagnetic β-NaY0.8Eu0.2F4@γ-Fe2O3 nanoparticles, made of a 70 nm-sized β-NaY0.8Eu0.2F4 single crystal core decorated by a 10 nm-thick polycrystalline and discontinuous γ-Fe2O3 shell, have been synthesized by the polyol process. Functionalized with citrate ligands they show a good colloidal stability in water making them valuable for dual magnetic resonance and optical imaging or image-guided therapy. They exhibit a relatively high transverse relaxivity r2 = 42.3 mM−1·s−1 in water at 37 °C, for an applied static magnetic field of 1.41 T, close to the field of 1.5 T applied in clinics, as they exhibit a red emission by two-photon excited fluorescence microscopy. Finally, when brought into contact with healthy human foreskin fibroblast cells (BJH), for doses as high as 50 µg·mL−1 and incubation time as long as 72 h, they do not show evidence of any accurate cytotoxicity, highlighting their biomedical applicative potential.

A core–shell Y zeolite with a mono-crystalline core and a loosely aggregated polycrystalline shell: a hierarchical cracking catalyst for large reactants

Increased external surface areas promote the pre-cracking of reactants, and the core–shell structure guarantees that reactants undergo a hierarchical cracking process.

General strategy for doping rare earth metals into Au–ZnO core–shell nanospheres

Abstract

Enhanced gas sensing performance based on the fabrication of polycrystalline Ag@TiO2 core-shell nanowires

This study demonstrates a novel one-dimensional core-shell structure based on the coating of silver nanowires (Ag NWs) with a layer of titanium oxide (TiO2) nanoparticles. This approach for generating core-shell structures is facile and straightforward, utilizing a sol-gel method followed by the crystallization of TiO2 using a simple open-air hydrothermal method. The Ag nanowires are ˜10 μm in length and have a diameter of 100 nm, and their TiO2 polycrystalline shell is 10–15 nm in thickness. These novel structures offer large surface area and high stability, which are qualities that favor gas sensing performance. Gas sensing tests have demonstrated that the generated Ag NWs@TiO2 core-shell nanocomposites exhibit better sensing properties (response, selectivity, optimized working temperature, minimum concentration, and response and recovery time) when compared to sensors containing pure TiO2 nanoparticles. The mechanism of sensing enhancement can be attributed to the Schottky barrier that exists at the interface between the Ag NWs and the TiO2. The Ag core has an excellent conductive property for electronic transfer and further accelerates the oxygen ionization and surface redox reactions. These results may shed light on the design and construction of TiO2-based nanocomposites for gas sensor applications.

3D core-multishell piezoelectric nanogenerators

The thin film configuration presents obvious practical advantages over the 1D implementation in energy harvesting systems such as easily manufacturing and processing, and long-lasting and stable devices. However, ZnO-based piezoelectric nanogenerators (PENGs) generally rely on the exploitation of single-crystalline nanowires because of their self-orientation in the c-axis direction and ability to accommodate long deformations resulting in high piezoelectric performance. Herein, we show an innovative approach to produce PENGs by combining polycrystalline ZnO layers fabricated at room temperature by plasma-assisted deposition with supported small-molecule organic nanowires (ONWs) acting as 1D scaffolds. Such hybrid nanostructures present convoluted core-shell morphology, formed by a single-crystalline organic nanowire conformally surrounded by a poly-crystalline ZnO shell and combine the organic core mechanical properties with the ZnO layer piezoelectric response. In a step forward towards the integration of multiple functions within a single wire, we have also developed ONW-Au-ZnO nanoarchitectures including a gold shell acting as inner electrode achieving output piezo-voltages up to 170 mV. The synergistic combination of functionalities in the ONW-Au-ZnO devices promotes an enhanced performance generating piezo-currents one order of magnitude larger than the ONW-ZnO nanowires and superior to the thin film nanogenerators for equivalent and higher thicknesses.

Effective shell wall thickness of vertically aligned ZnO-ZnS core-shell nanorod arrays on visible photocatalytic and photo sensing properties

Development of hierarchical core-shell semiconductor heterostructures ensue significant advancement in catalytic functional structures with improvised optical functionalities. Shell wall controlled vertically aligned ZnO-ZnS core-shell nanorod (NR) heterostructures were grown on transparent conductive substrates along the c-axis by sulfidation of aligned ZnO nanorod arrays for visible photocatalytic properties. The effects of the sulfidation time on the morphology, crystalline properties, optical property, photocurrent response, and photocatalytic activity of the catalyst arrays were studied under UV and visible light irradiation. The shell wall thickness of these heterostructures influenced in great extent the effective photo responsive charge separation and improved carrier mobility. ZnO-ZnS core-shell heterostructure having the shell wall thickness of 20 nm has exhibited more efficient visible photocatalytic behavior due to effective separation of carriers and improved visible absorption. On further increasing the wall thickness the catalytic efficiency was reduced due to the poor carrier (hole) mobility in the polycrystalline shell grains which induced the higher recombination rate. Stability and reusability of ZnO-ZnS core-shell nanostructures reveals that the ZnS acted as a protective layer over the ZnO NR arrays. In appraisal with ZnO NR arrays, the control over the shell wall thickness of ZnO-ZnS core-shell NR array attributed to the excellent visible photocatalytic activity and improvised absorption of light in visible region at ZnO-ZnS interface and effective separation of photogenerated electron-hole pairs at ZnO-ZnS heterojunctions.