Hexagonal Nanocrystals Research Articles

Low Temperature Synthesis of Nanoscale Magnesium Hydroxide under Normal Pressure

In the present paper, nano-scale magnesium hydroxide has been prepared at low temperature via a wet chemical process using MgCl2· 6H2O (or Mg(NO3)2) and NaOH as the starting reagents under normal pressure. The fresh synthesized magnesium hydroxide nanocrystals were characterized by X-ray diffraction (XRD), selected area electronic diffraction (SAED) and transmission electron microscopy (TEM) measurements. Patterns of XRD and SAED exhibited that the products were high-quality hexagonal crystal system nanocrystals. Though the TEM pictures, large quantities of lamellar-like and needle-like magnesium hydroxide nanocrystals could be observed without obvious aggregation. Using the present method, high yield magnesium hydroxide nanoparticles would be produced without expensive equipments like the high temperature and high-pressure reactor.

Structure dependent photoluminescence and magnetic properties of Co:ZnS nanostructures

Co:ZnS nanostructures have been synthesized via the solvothermal method by modifying the stoichiometry of Zn and S sources in the solvothermal process. The structure and morphology analysis demonstrates that the increase of the Zn/S molar ratio leads to changes in the morphology and structure, from Co:ZnS nanorods and hexagonal nanocrystals to core–shell Co:ZnS/ZnO nanocrystals. The photoluminescence spectrum for Co:ZnS/ZnO exhibits strong visible light emissions due to the presence of a large amount of structural defects, whereas the emissions for Co:ZnS nanorods almost disappear because of the high crystallinity. All the samples display obvious room-temperature ferromagnetism. The saturated magnetic moment of the samples strongly depends on the concentration of structural defects in the nanostructures and can be effectively enhanced by the introduction of abundant defects.

Synthesis of Uniform Disk-Shaped Copper Telluride Nanocrystals and Cation Exchange to Cadmium Telluride Quantum Disks with Stable Red Emission

We present the synthesis of novel disk-shaped hexagonal Cu2Te nanocrystals with a well-defined stoichiometric composition and tunable diameter and thickness. Subsequent cation exchange of Cu to Cd at high temperature (180 °C) results in highly fluorescent CdTe nanocrystals, with less than 1 mol % of residual Cu remaining in the lattice. The procedure preserves the overall disk shape, but is accompanied by a substantial reconstruction of the anion sublattice, resulting in a reorientation of the c-axis from the surface normal in Cu2Te into the disk plane in CdTe nanodisks. The synthesized CdTe nanodisks show a continuously tunable photoluminescence (PL) peak position, scaling with the thickness of the disks. The PL lifetime further confirms that the CdTe PL arises from band-edge exciton recombination; that is, no Cu-related emission is observed. On average, the recombination rate is about 25-45% faster with respect to their spherical quantum dots counterparts, opening up the possibility to enhance the emission rate at a given wavelength by controlling the nanocrystal shape. Finally, with a PL quantum efficiency of up to 36% and an enhanced PL stability under ambient conditions due to a monolayer of CdS formed on the nanocrystal surface during cation exchange, these flat quantum disks form an interesting enrichment to the current family of highly fluorescent, shape-controlled nanocrystals.

Preparation and gas-sensing property of parallel-aligned ZnO nanofibrous films

Parallel-aligned zinc oxide (ZnO) nanofibrous films fabricated by using electrospinning technique were used in gas sensors for the detection of ethanol and formaldehyde. The morphologies and crystal structures of the films were characterized by field-emission scanning electron microscopy (FE–SEM) and X-ray diffraction (XRD), respectively. FE–SEM results showed that ZnO nanofibres had an approximate diameter of 100–300 nm and consisted of hexagonal wurtzite structure ZnO nanocrystals with a primary particle diameter of 20–50 nm. The results of resistance–temperature characteristics and responses to ethanol and formaldehyde indicated that the parallel-aligned ZnO nanofibrous film had a low activation energy (0· 246 eV), a low optimum operating temperature and a high response. The response and recovery had a high rate in the initial stage and a low rate in the later stage. The parallel-aligned ZnO nanofibrous film had excellent potential application for formaldehyde sensor.

Single-crystal-like NiO colloidal nanocrystal-aggregated microspheres with mesoporous structure: Synthesis and enhanced electrochemistry, photocatalysis and water treatment properties

Abstract A new microwave-assisted hydrothermal synthetic route based on the self-assembly and subsequently controlled thermal decomposition process is proposed to fabricate nickel oxide colloidal nanocrystal aggregated microspheres (CNAMs) with mesoporous structure. XRD, EDS, SEM, TEM. FTIR, and N2 adsorption and desorption isotherm techniques are employed for morphology and structure characterizations. The as-prepared nickel oxide CNAMs, which has a high surface area (234 m2/g) with narrow pore distribution at around 3.25 nm, are composed of numerous hexagonal mesoporous nanocrystals of approximately 50–60 nm in size, and present a single-crystal-like characteristic. The experimental results also demonstrated that the CNAMs showed outstanding performance in electrochemistry, photocatalysis and waste water treatment due to their special hierarchical and mesoporous structure, presenting the promising candidate for catalysis and catalysis support materials.

Cubic and Hexagonal NaGdF4 Crystals Precipitated from an Aluminosilicate Glass: Preparation and Luminescence Properties

Depending on the composition of the glass and the annealing procedure supplied, thermal annealing of silicate glasses in the system SiO2/Al2O3/AlF3/Na2O/Gd2O3/SmF3 led to the precipitation of cubic or hexagonal NaGdF4 nanocrystals. The glasses were already phase separated after casting and formed a droplet phase supposedly enriched in fluorides and rare earths. The droplets had sizes mainly in the range from 80 to 120 nm. During annealing at temperatures ≥600 °C, multicore particles of cubic or hexagonal NaGdF4 were precipitated inside the amorphous droplet phase. Completely transparent hexagonal NaGdF4 containing nano-glass-ceramics could be derived from this system. Fluorescence spectra and fluorescence decay curves of Sm3+ show crystal phase dependent effects. Sm3+ doped hexagonal NaGdF4 exhibits notably different fluorescence emission spectra and longer fluorescence lifetimes than cubic NaGdF4.

A hydroquinone sensor based on a new nanocrystals modified electrode

AbstractBACKGROUNDCuS nanocrystals with excellent properties are widely applied as semiconductor materials, but little work has been done on their application to electrochemical analysis. This paper reports on a very straightforward method to synthesize pure hexagonal phase CuS nanocrystals with an average diameter of 20 nm by a solvothermal method. A glassy carbon electrode was modified by drop casting the suspension of CuS nanocrystals and chitosan onto its surface. The new electrochemical sensor was developed for the determination of hydroquinone.RESULTSThe crystallization, morphology and composition of as‐prepared CuS nanocrystals were characterized by XRD, SEM, Raman and EDS, respectively. The electrochemical oxidation behavior of hydroquinone at a CuS nanocrystals/chitosan/glassy carbon electrode was investigated by cyclic voltammetry. The electrode exhibited a wide linear dynamic range between 4.5 µmol L−1 and 4.5 mmol L−1 hydroquinone, and a detection limit of 1.5 µmol L−1.CONCLUSIONSThe electrode was successfully applied to the determination of hydroquinone in real water samples with satisfactory accuracy and precision. © 2013 Society of Chemical Industry

Preparation and luminescence properties of glass–ceramics containing Sm3+-doped hexagonal NaGdF4 crystals

Rare earth-doped hexagonal NaGdF4 nanocrystals were precipitated from a glass. The glass with the mol% composition 70.1SiO2·4.3Al2O3·1.8AlF3·2.3Na2O·18.5NaF·3.0Gd2O3 doped with 8 × 1019 Sm3+ ions per cm3 was already phase separated after casting. The formed droplets enriched in rare earths and fluoride had sizes in the range mainly from 50 to 90 nm. During annealing at temperatures ≥600 °C, multi core particles of hexagonal NaGdF4 were formed inside the amorphous droplet phase. The fluorescence spectra show that Sm3+ is incorporated into the NaGdF4 lattice. This is confirmed by electron dispersive X-ray analyzes performed in a transmission electron microscope.

Fabrication and Characterization of ZnO Thin Films by Sol-Gel Spin Coating Method for the Determination of Phosphate Buffer Saline Concentration

The fabrication, characterization and application of nanostructured zinc oxide (ZnO) thin films on interdigitated silver electrodes were described for the determination of phosphate buffer saline (PBS) concentration. The ZnO thin films were synthesized on a silicon dioxide wafer using a sol-gel spin coating technique. Two different seed solutions were prepared by dissolving Zn-acetate dihydrate in methanol and isopropanol in presence of a stabilizer, monoethanolamine. The field emission scanning electron microscope, atomic force microscope, X-ray diffractometer and Fourier transform infrared characterization revealed the presence of hexagonal ZnO nano-crystals in all thin films. However, the smaller sized and homogeneous ZnO nano-crystals were observed in isopropanol derived thin films. These thin films were used to discriminate the concentrations of different PBS solutions and the discriminatory signals were captured using a low-cost dielectric analyzer and a source meter. The frequency-capacitance curve reflected 2.85 fold increase in capacitance values when the sensor was exposed to 1000-fold diluted PBS in deionized water. A change in PBS concentration from 1000 fold to 10 fold increased the current flow from 6uF to 122uF. Thus the capacitance and current flow demonstrated a proportional relationship with the concentration of PBS, suggesting the application of the fabricated sensor in the determination and discrimination of chemicalspecies concentration in various solutions.

Rare earth doped downshifting glass ceramics for photovoltaic applications

The ability of fluorochlorozirconate (FCZ) glass ceramics containing hexagonal barium chloride nanocrystals doped with the rare earth elements, holmium and europium, to downshift ultraviolet light to wavelengths more usable by polycrystalline silicon photovoltaic cells was investigated. Six rare-earth-doped and one undoped FCZ glass samples were synthesized and subsequently heat treated to produce glass ceramics containing barium chloride nanocrystals in the hexagonal phase. The glasses were characterized by differential scanning calorimetry to determine crystallization temperatures for the heat treatment process. The resulting glass ceramics were characterized by X-ray diffraction, phosphorimetry, and spectrophotometry. All samples produce light centered at 470nm when excited by ultraviolet radiation. The excitation spectra of FCZ glass ceramics containing hexagonal barium chloride nanocrystals doped or co-doped with divalent europium more closely matches the solar spectrum at the earth's surface than the excitation spectrum of an undoped sample. This feature of europium doped glass ceramics makes them suitable for the process of downshifting to improve the efficiency of polycrystalline silicon photovoltaic cells. The addition of holmium to the glass ceramics gives rise to additional emission at higher wavelengths and nearer to the band gap energy of polycrystalline silicon photovoltaic cells, reducing heating of the cells and thereby increasing cell efficiency. Additionally, the strong absorption by europium in the ultraviolet region of the spectrum may allow these glass ceramics to serve as a protective layer for ultraviolet sensitive materials.

Synthesis, characterization, shape evolution, and optical properties of copper sulfide hexagonal bifrustum nanocrystals

The hexagonal bifrustum-shaped copper sulfide (CuS) nanocrystals were selectively and facilely synthesized by a hydrothermal method for the first time at 120 °C. The products were characterized by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, ultraviolet–visible (UV–Vis) spectroscopy, and photoluminescence spectroscopy. The results showed that the CuS hexagonal bifrustum nanocrystal was bounded by two top hexagons with edge length of about 50–70 nm and twelve lateral trapezoids with a base of about 100 nm and that the length of each hexagonal bifrustum was about 250 nm. Tetradecylamine (TDA), as an effective capping agent, was found to be critical for this special shape. Using different amounts of TDA, two kinds of CuS hexagonal bifrustum nanocrystals were obtained: “lender hexagonal bifrustum” and “pancake hexagonal bifrustum.” Furthermore, we studied the formation mechanism of hexagonal bifrustum, which is related to the intrinsic crystalline structure of CuS and Ostwald ripening. And, the results revealed that the CuS nanocrystal evolved from hexagonal plate to hexagonal bifrustum and finally to hexagonal bipyramid as the heating time increased. The UV–Vis absorption spectrum showed that these CuS hexagonal bifrustum nanocrystals exhibited strong absorption in the near-infrared region and had a potential application for photothermal therapy and photocatalysis.

Nanocrystallinity and the Ordering of Nanoparticles in Two-Dimensional Superlattices: Controlled Formation of Either Core/Shell (Co/CoO) or Hollow CoO Nanocrystals

Here it is demonstrated that the diffusion process of oxygen in Co nanoparticles is controlled by their 2D ordering and crystallinity. The crystallinity of isolated Co nanoparticles deposited on a substrate does not play any role in the oxide formation. When they are self-assembled in 2D superlattices, the oxidation process is slowed and produces either core/shell (Co/CoO) nanoparticles or hollow CoO nanocrystals. This is attributed to the decrease in the oxygen diffusion rate when the nanoparticles are interdigitated. Initially, polycrystalline nanoparticles form core/shell (Co/CoO) structures, while for single-domain hexagonal close-packed Co nanocrystals, the outward diffusion of Co ions is favored over the inward diffusion of oxygen, producing hollow CoO single-domain nanocrystals.

A synthesis and up-conversional photoluminescence study of hexagonal phase NaYF4:Yb,Er nanoparticles

In this article, hexagonal phase NaYF4:Yb,Er (β-NaYF4:Yb,Er) nanoparticles with a controlled size and morphology were synthesized via a simple and environmentally friendly method under relatively mild conditions. The different F−/Y3+ molar ratios were used in the synthesis of hexagonal Yb3+,Er3+ codoped NaYF4 nanocrystals as a means of controlling the size and morphology of the nanocrystals. Subsequently, the morphology and structure of the products were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The effect of the F−/Y3+ molar ratio on the size and morphology of the products was examined. When the F−/Y3+ molar ratio was gradually increased, the products transformed from spherical hexagonal nanocrystals to regular hexagonal nanocrystals and the size of the products changed from 8 to 400 nm. That is to say, excessive F− ions were capable of accelerating for the formation of larger β-NaYF4:Yb,Er nanoparticles in our synthesis procedure. Meanwhile the upconversion photoluminescence of the nanocrystals was investigated in detail by fluorescent spectroscopy to reveal the relationship between the optical properties and the morphology and size of the products. As a consequence, the upconversion photoluminescence of the nanocrystals demonstrated a morphology and size dependence. Thus the fabrication of β-NaYF4:Yb,Er nanoparticles with different sizes and morphologies would satisfy the diverse requirements in various fields.

Great enhancements in the thermoelectric power factor of BiSbTe nanostructured films with well-ordered interfaces

An innovative concept of twin-enhanced thermoelectricity was proposed to fundamentally resolve the high electrical resistance while not degrading the phonon scattering of the thermoelectric nanoassemblies. Under this frame, a variety of highly oriented and twinned bismuth antimony telluride (BixSb2-xTe3) nanocrystals were successfully fabricated by a large-area pulsed-laser deposition (PLD) technique on insulated silicon substrates at various deposition temperatures. The significant presence of the nonbasal- and basal-plane twins across the hexagonal BiSbTe nanocrystals, which were experimentally and systematically observed for the first time, evidently contributes to the unusually high electrical conductivity of ~2700 S cm(-1) and the power factor of ~25 μW cm(-1) K(-2) as well as the relatively low thermal conductivity of ~1.1 W m(-1) K(-1) found in these nanostructured films.

Size- and shape-controlled synthesis of ZnIn2S4 nanocrystals with high photocatalytic performance

A facile route for synthesizing size- and shape-controlled ternary hexagonal ZnIn2S4 nanocrystals with narrow size distributions is developed using oleylamine as the ligand and noncoordinating octadecene as the solvent. Tunable sizes from 2.1 nm to 9.2 nm of the ZnIn2S4 nanocrystals are achieved through manipulation of reaction temperatures. Furthermore, the obtained ZnIn2S4 presents a nanoplate structure by replacing the sulfur powder with thiourea as the sulfur source. Optical measurements of the ZnIn2S4 nanocrystals demonstrate that their optical properties are related to the sizes of the products. The band gap energy varies from 3.28 to 2.35 eV, corresponding to the size from 2.1 nm to 9.2 nm. Compared with the bulk material, the blue-shift of the absorption spectra is mainly due to the size-dependent quantum confined effect. Photodegradation investigation demonstrates that the annealed ZnIn2S4 nanocrystals reveal higher photocatalytic activity for degradation of methylene orange (MO) solution in the visible region than the annealed ZnIn2S4 nanoplates and unannealed ZnIn2S4 nanocrystals.

稀土掺杂氟化物纳米晶体的合成机理和光谱特性

Cubic phase, hexagonal phase NaYF 4 : Eu 3+ and orthogonal phase YF 3 : Eu 3+ nanocrystals were prepared using a mild hydrothermal method. Transition from cubic phase to hexagonal phase NaYF 4 : Eu 3+ nanocrystals was realized only by simply changing the concentrations of starting materials, and orthogonal-phase YF 3 : Eu 3+ nanocrystals were also synthesized by varying the ratio of rare-earth ions to sodium fluoride. X-ray diffraction (XRD), transmission electron microscopy (TEM) and fluorescence spectroscopy were used to investigate the grown mechanism of nanocrystals controlled by precursor concentrations in detail and the crystallinity was evaluated by spectroscopic approach as well. To further improve crystallinity and eliminate the hydroxyl at the surface of nanocrystals, we have treated the cubic phase, hexagonal phase NaYF 4 : Eu 3+ and orthogonal phase YF 3 : Eu 3+ nanocrystals by annealing. Results showed that hexagonal phase NaYF 4 : Eu 3+ nanocrystals are stabilized in thermodynamics and the phase transitions from cubic phase NaYF 4 : Eu 3+ and orthogonal phase YF 3 : Eu 3+ to hexagonal phase YOF nanocrystals are easy to happen. The samples after sintering treatment are all agglomerate to some extent. Photoluminescence properties of Tb 3+ ions doped these three fluoride matrices showed that the crystal-field splitting was obviously observed in YF 3 : Tb 3+ nanocrystals, even at room temperature, which revealed that we can effectively control the growing speed by reducing the content of sodium fluoride, leading to crystalline grow well.

An extreme biomimetic approach: hydrothermal synthesis of β-chitin/ZnO nanostructured composites

β-Chitinous scaffolds isolated from the skeleton of marine cephalopod Sepia officinalis were used as a template for the in vitro formation of ZnO under conditions (70 °C) which are extreme for biological materials. Novel β-chitin/ZnO film-like composites were prepared for the first time by hydrothermal synthesis, and were thoroughly characterized using numerous analytical methods including Raman spectroscopy, HR-TEM and XRD. We demonstrate the growth of hexagonal ZnO nanocrystals on the β-chitin substrate. Our chitin/ZnO composites presented in this work show antibacterial properties against Gram positive bacteria and can be employed for development of inorganic-organic wound dressing materials.

In situ-grown hexagonal silicon nanocrystals in silicon carbide-based films

Silicon nanocrystals (Si-NCs) were grown in situ in carbide-based film using a plasma-enhanced chemical vapor deposition method. High-resolution transmission electron microscopy indicates that these nanocrystallites were embedded in an amorphous silicon carbide-based matrix. Electron diffraction pattern analyses revealed that the crystallites have a hexagonal-wurtzite silicon phase structure. The peak position of the photoluminescence can be controlled within a wavelength of 500 to 650 nm by adjusting the flow rate of the silane gas. We suggest that this phenomenon is attributed to the quantum confinement effect of hexagonal Si-NCs in silicon carbide-based film with a change in the sizes and emission states of the NCs.

Development and study of the structural and optical properties of hexagonal ZnO nanocrystals

AbstractZnO is a promising member of the semiconducting materials of II-VI group. ZnO nanocrystals have shown potential applications in various novel technologies. In the present investigation of ZnO nanocrystals, a novel chemical route using Zinc acetate as organic precursor is being reported. ZnO nanocrystals were characterized using X-ray diffraction, scanning electron microscopy UV-visible (UV–vis) spectroscopy, and photoluminescence measurements. The X-ray diffraction studies reveal the typical hexagonal structure of ZnO nanocrystals along the preferred orientation of (101) and (100) planes. The optical bandgap of ZnO nanocrystals was found to be 3.50 eV from the absorbance spectrum, which is higher than that of the bulk ZnO material. A blueshift of 21 nm is observed in the excitonic transitions, which clearly indicates the formation of ZnO nanocrystals. Photoluminescence spectroscopy of the ZnO nanocrystals showed a strong emission peak at 365 nm near the band edge along with a weak green-yellow emission peak spanning the range of 450 to 600 nm. The blueshifting was also observed in the photoluminescence spectrum, in accordance with the UV–vis spectroscopy in contrast to the bulk ZnO material.

Influence of iodide ions on morphology of silver growth on gold hexagonal nanoplates

Different morphologies of Au@Ag nanocrystals have been successfully synthesized using iodide ions in growth solutions consisting of ascorbic acid, NaOH, and CTAB. Without I− ions in growth solution, there were unnoticeable changes in morphology. On the other hand, with addition of I− ions, remarkable changes were observed including formation of hexagonal bipyramidal nanocrystals. Ag+ ions were reduced and deposited on {111} facets of gold hexagonal nanoplates (GHNs), and hexagonal bipyramidal nanoparticles were formed more quickly in the presence of I− ions than without I− ions. Our studies reveal that I− ion affects the selective growth of Ag onto GHNs. This can be explained by the decrease in the surface redox potential of nanocrystals that occurs by combining with the AgI complex on the metallic surface. These results were confirmed by FESEM images, UV–vis–NIR spectra, and TEM images.