Showing papers on "Transmission electron microscopy published in 2007"

Highly Uniform and Monodisperse β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, and Yb/Tm) Hexagonal Microprism Crystals: Hydrothermal Synthesis and Luminescent Properties

Abstract: β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, and Yb/Tm) hexagonal microprisms with remarkably uniform morphology and size have been synthesized via a facile hydrothermal route. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra as well as kinetic decays were used to characterize the samples. It is found that sodium citrate as a shape modifier introduced into the reaction system plays a critical role in the shape evolution of the final products. Furthermore, the shape and size of the products can be further manipulated by adjusting the molar ratio of citrate/RE3+ (RE represents the total amount of Y3+ and the doped rare earth elements such as Eu3+, Tb3+, Yb3+/Er3+, or Yb3+/Tm3+). Under the excitation of 397 nm ultraviolet light, NaYF4:xEu3+ (x = 1.5, 5%) shows the emission lines of Eu3+ corresponding to 5D0-3 → 7FJ (J = 0−4) transitions from 400 to 700 nm (whole visible spectr...

Low Temperature Synthesis and Photocatalytic Activity of Rutile TiO2 Nanorod Superstructures

Abstract: Pure rutile nanorods were synthesized by hydrolysis of TiCl4 ethanolic solution in water at 50 °C. The assembly of rutile nanorods could be controlled through simply changing the molar ratios of TiCl4, ethanol, and water, resulting in different superstructures with flower- or urchinlike morphologies. The resulting samples were characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, nitrogen sorption, and UV−vis diffuse reflectance spectrum. A possible mechanism for the growth and assembly of rutile nanorod superstructures was proposed on the basis of characterization results. More importantly, we found that those low temperature synthesized superstructures showed significantly higher photocatalytic activities than commercial photocatalyst P25 on degradation of rhodamine B in water under artificial solar light. This study provides a simple and inexpensive way to prepare high active rutile nanorods superstructure...

Hydrothermal synthesis of Bi2WO6 uniform hierarchical microspheres

Abstract: Bi2WO6 uniform hierarchical microspheres were grown on a large scale at 180 °C by a simple hydrothermal method with the help of the surfactant poly(vinyl pyrrolidone) (PVP). X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were used to characterize the product. The result indicated that three-dimensional (3D) Bi2WO6 microspheres were constructed layer-by-layer from a large number of two-dimensional (2D) sheets, which were composed of numerous interconnected square nanoplates with a mean side length of 65 nm. Pore-size distribution analysis showed that both mesopores and macropores existed in the 3D microstructures. The formation mechanism was discussed on the basis of the results of time-dependent experiments. It was demonstrated that PVP played a key role in the formation of such hierarchical microspheres. By adjusting the amount of PVP, Bi2WO6 with different morphologies can be attained accor...

Different Microstructures of β-NaYF4 Fabricated by Hydrothermal Process: Effects of pH Values and Fluoride Sources

Abstract: β-NaYF4 microcrystals with a variety of morphologies, such as microrod, hexagonal microprism, and octadecahedron, have been synthesized via a facile hydrothermal route. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra were used to characterize the samples. The intrinsic structural feature of β-NaYF4 seeds and two important external factors, namely, the pH values in the initial reaction solution and fluoride sources, are responsible for shape determination of β-NaYF4 microcrystals. It is found that the organic additive trisodium citrate (Cit3-) as a shape modifier has the dynamic effect by adjusting the growth rate of different facets under different experimental conditions, resulting in the formation of the anisotropic geometries of various β-NaYF4 microcrystals. The possible formation mechanisms for products with various architectures have been presented. A syst...

A versatile strategy to fabricate hydrogel–silver nanocomposites and investigation of their antimicrobial activity

Abstract: In this study, hydrogel–silver nanocomposites have been synthesized by a unique methodology, which involves formation of silver nanoparticles within swollen poly (acrylamide-co-acrylic acid) hydrogels. The formation of silver nanoparticles was confirmed by transmission electron microscopy (TEM) and surface plasmon resonance (SPR) which was obtained at 406 nm. The TEM of hydrogel–silver nanocomposites showed almost uniform distribution of nanoparticles throughout the gel networks. Most of the particles, as revealed from the particle-size distribution curve, were 24–30 nm in size. The X-ray diffraction pattern also confirmed the face centered cubic (fcc) structure of silver nanoparticles. The nanocomposites demonstrated excellent antibacterial effects on Escherichia coli (E. coli). The antibacterial activity depended on size of the nanocomposites, amount of silver nanoparticles, and amount of monomer acid present within the hydrogel–silver nanocomposites. It was also found that immersion of plain hydrogel in 20 mg/30 ml AgNO3 solution yielded nanocomparticle–hydrogel composites with optimum bactericidal activity.

Martensitic and Magnetic Transformation Behaviors in Heusler-Type NiMnIn and NiCoMnIn Metamagnetic Shape Memory Alloys

Abstract: Martensitic and magnetic transformation behaviors of Ni50MnIn, Ni45Co5MnIn, and Ni42.5Co7.5MnIn Heusler alloys were investigated by differential scanning calorimetry (DSC), vibrating sample magnetometry (VSM), and transmission electron microscopy (TEM). The martensitic transformation starting temperature (M s ) decreases with increasing In composition, while the Curie temperatures (T c ) of the parent phase are almost independent in each alloy series. On the other hand, the addition of Co resulted in a decrease of the M s and an increase of the T c , and the degree of the decline of M s was accelerated by magnetic transformation of the parent phase. The M s temperature change induced by the magnetic field was also confirmed. It was found that the degree of M s change is strongly related to the entropy change by the martensitic transformation, which shows a correlation with T c -M s . These behaviors can be qualitatively explained on the basis of thermodynamic considerations.

Synthesis of flower-like gold nanoparticles and their electrocatalytic activity towards the oxidation of methanol and the reduction of oxygen.

Abstract: This article describes the synthesis of branched flower-like gold (Au) nanocrystals and their electrocatalytic activity toward the oxidation of methanol and the reduction of oxygen. Gold nanoflowers (GNFs) were obtained by a one-pot synthesis using N-2-hydroxyethylpiperazine-N-2-ethanesulphonic acid (HEPES) as a reducing/stabilizing agent. The GNFs have been characterized by UV−visible spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and electrochemical measurements. The UV−visible spectra show two bands corresponding to the transverse and longitudinal surface plasmon (SP) absorption at 532 and 720 nm, respectively, for the colloidal GNFs. The GNFs were self-assembled on a sol−gel-derived silicate network, which was preassembled on a polycrystalline Au electrode and used for electrocatalytic applications. The GNFs retain their morphology on the silicate network; the UV−visible diffuse reflectance spectra (DRS) of GNFs on the silicate network show longitudinal and transverse b...

Microstructure of LiCoO2 with and without “AlPO4” Nanoparticle Coating: Combined STEM and XPS Studies

Abstract: “AlPO4”-coated LiCoO2 was shown to exhibit markedly improved capacity retention relative to bare LiCoO2 upon cycling to 4.7 V. Scanning and transmission electron microscopy imaging showed that the ...

Dynamical observation of bamboo-like carbon nanotube growth.

Abstract: The growth dynamics of bamboo-like multiwalled carbon nanotubes (BCNTs) via catalytic decomposition of C2H2 on Ni catalyst at 650 °C was observed in real time using an in situ ultrahigh vacuum transmission electron microscope. During BCNT growth, the shape of the catalyst particle changes constantly but remains metallic and crystalline. Graphene sheets (bamboo knots) within the nanotube preferentially nucleate on the multistep Ni−graphite edges at the point where the graphene joins the catalyst particle, where it is stabilized by both the graphene walls and the Ni catalyst surface. The growth of a complete inner graphene layer growth prior to contraction of the Ni catalyst particle due to restoring cohesive forces will result in a complete BCNT knot whereas partial growth of the inner wall will lead to an incomplete BCNT knot.

Structural and morphological characterization of cerium oxide nanocrystals prepared by hydrothermal synthesis.

Abstract: Colloidal cerium oxide (CeO2) nanocrystals prepared by hydrothermal synthesis were characterized by high-resolution transmission electron microscopy (HRTEM) and three-dimensional electron tomography (3D-ET). HRTEM images of individual CeO2 nanocrystals were then simulated by Blochwave and multislice simulations to determine the atomic arrangement and terminating atoms. The edge length distributions were between 5.0 and 8.0 nm with an average edge length of 6.7 nm. The HRTEM images showed that the CeO2 particles were slightly truncated revealing {220} facets. 3D-ET revealed that the CeO2 nanocrystals exposed predominantly {200} cubic facets. The nanocrystals were truncated at the corners exposing {111} octahedral facets and at the edges {220} dodecahedral facets. Furthermore, 3D-ET revealed the presence of some tetragonal-shaped CeO2 nanocrystals.

Synthesis of a α-Fe2O3 nanocrystal in its different morphological attributes : growth mechanism, optical and magnetic properties

Abstract: Nanospindle and nanorhombohedron and nanocube structured α-Fe2O3 was synthesized by the solvothermal method. An intermixing of ethylenediamine (EN) either with ethanol (EtOH) or water in different volume ratios (either 15:85, 50:50 or 85:15 in particular) was used to generate the structural forms of α-Fe2O3. The study showed that, during synthesis, EN functioned as a ligand and facilitated the growth of nanostructured samples. The probable growth mechanism is discussed in this paper. Field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) investigations revealed that the nanostructures were formed through oriented attachment of primary nanocrystals. Fourier transform infrared spectroscopy (FTIR) results showed the presence of Fe–O or Fe–O–Fe vibrational bands whereas UV–vis–NIR optical absorbance spectra showed two prominent absorption bands around 540–560 and 670–680 nm. The room temperature magnetization measurement revealed that the remanence and coercivity depend on the morphological attributes of the nanocrystals. The magnetic hysteresis measurement also revealed that α-Fe2O3 nanostructures displayed weak ferromagnetic behaviour at room temperature.

Synthesis of ZnO nanoparticles by hydrothermal method

Abstract: Stable, OH free zinc oxide (ZnO) nanoparticles were synthesized by hydrothermal method by varying the growth temperature and concentration of the precursors. The formation of ZnO nanoparticles were confirmed by x-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies. The average particle size have been found to be about 7-24 nm and the compositional analysis is done with inductively coupled plasma atomic emission spectroscopy (ICP-AES). Diffuse reflectance spectroscopy (DRS) results shows that the band gap of ZnO nanoparticles is blue shifted with decrease in particle size. Photoluminescence properties of ZnO nanoparticles at room temperature were studied and the green photoluminescent emission from ZnO nanoparticles can originate from the oxygen vacancy or ZnO interstitial related defects.

Intermetallic FexAly-phases in a steel/Al-alloy fusion weld

Abstract: The microstructure of joints between an Al-alloy and a zinc coated ferritic steel sheet manufactured by the so-called CMT joining method is investigated. The joint consists of a weld between the Al-alloy and Al 99.8 filler and a brazing of the filler to the zinc coated steel. The morphology, the structure and the defects of the intermetallic phases that developed at the interface between the steel and the Al 99.8 filler are characterised using scanning and transmission electron microscopy. The intermetallic phase seam is only about 2.3 μm thick and consists of trapezoidal nearly equiaxial Fe2Al5 grains surrounded by finger-like remains of the steel and mostly elliptical FeAl3 grains extending into the Al 99.8 filler material. Both the Fe2Al5 and the FeAl3 grains contain crystal defects.

Hydrothermal Synthesis and Photocatalytic Properties of Pyrochlore La2Sn2O7 Nanocubes

Abstract: Nanocubic La2Sn2O7 photocatalysts with pyrochlore structure have been successfully synthesized by a one-pot hydrothermal method. The effects of alkaline concentration, reaction time, and hydrothermal temperature on the structures and morphologies of the resultant products were investigated. On the basis of characterization results from X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected area electron diffraction (SAED), a possible growth mechanism of the nanocubes under the hydrothermal conditions was proposed. The absorption spectra of as-prepared cubic La2Sn2O7 photocatalyst were determined by UV−vis spectrometer. Photocatalytic experiments showed that the La2Sn2O7 samples not only had a high activity for degradation of methyl orange, but also had the activity for generating H2 with a rate of 39 μmol/h under ultraviolet light irradiation.

Controlling the growth and luminescence properties of well-faceted ZnO nanorods

Abstract: In this work, we synthesized ZnO nanorods by a wet chemical process. By varying different parameters of the synthesis process like precursor concentration, solvent used, surfactant, temperature, and reaction time, we were able to control the aspect ratio (l/d) and sharpness factor (Sf = h/b) of the rods. Rods with an aspect ratio of ∼20 and a sharpness factor of 10 were repeatedly obtained and characterized from scanning electron microscopy. High-resolution transmission electron microscopy and dark field analysis revealed that nanorods grow as a single crystal along the c axis [0001]. UV and visible emission was obtained in both flat- and sharp-terminated rods. According to the XPS analysis, rods present an excess of Zn2+ of ∼4% that is responsible for the strong yellow emission band. Such defects were removed by annealing of the sample at 200 °C for 2 h. Possible mechanisms of changes in the morphology produced by changes in the synthesis process are also discussed.

Size-Tailored Synthesis and Luminescent Properties of One-Dimensional Gd2O3:Eu3+ Nanorods and Microrods

Abstract: Nearly monodisperse and well-defined one-dimensional (1D) Gd2O3:Eu3+ nanorods and microrods were successfully prepared through a large-scale and facile hydrothermal method followed by a subsequent heat treatment process, without using any catalyst or template. X-ray diffraction (XRD), thermogravimetric analysis and differential scanning calorimetry (TGA−DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), photoluminescence (PL) and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. The size of the Gd2O3:Eu3+ rods could be modulated from micro- to nanoscale with the increase of pH value using ammonia solution. The as-formed product via the hydrothermal process, Gd(OH)3:Eu3+, could transform to cubic Gd2O3:Eu3+ with the same morphology and a slight shrinking in size after a postannealing process. The formation mechanism for the Gd(OH...

Phase Transformation and Photoluminescence Properties of Nanocrystalline ZrO2 Powders Prepared via the Pechini-type Sol−Gel Process

Abstract: Nanocrystalline ZrO2 fine powders were prepared via the Pechini-type sol−gel process followed by annealing from 500 to 1000 °C. The obtained ZrO2 samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR), and photoluminescence spectra (PL), respectively. The phase transition process from tetragonal (T) to monoclinic (M) was observed for the nanocrystalline ZrO2 powders in the annealing process, accompanied by the change of their photoluminescence properties. The 500 °C annealed ZrO2 powder with tetragonal structure shows an intense whitish blue emission (λmax = 425 nm) with a wide range of excitation (230−400 nm). This emission decreased in intensity after being annealed at 600 °C (T + M-ZrO2) and disappeared at 700 (T + M-ZrO2), 800 (T + M-ZrO2), and 900 °C (M-ZrO2). After further annealing at 1000 °C (M-ZrO2), a strong blue-green emission appeared again (λmax = 470 nm). Based o...

Morphology Control of Cryptomelane Type MnO2 Nanowires by Soft Chemistry. Growth Mechanisms in Aqueous Medium

Abstract: Nanowires of one-dimensional manganese oxide (cryptomelane) have been synthesized using reaction between MnII (MnSO4) and MnVII (KMnO4) in water by a low-temperature route at 60 °C and 95 °C. Characterization of the nanoparticles was carried out using powder X-ray diffraction, transmission electron microscopy, electron diffraction, and nitrogen adsorption−desorption. Two synthesis methods were developed depending whether the pH was initially adjusted or fixed all along the solid formation. Both methods exhibited variation of the nanowire morphology with diameter, length, and specific area varying in the 15−40 nm, 0.1−1.1 μm, and 35−110 m2·g-1 ranges, respectively, depending on the growth conditions (acidity and temperature). The growth mechanism of the nanowires was proved to proceed by oriented attachment of primary 10 nm width nanorods for the lateral direction and dissolution−crystallization for the longitudinal direction. The diameter variation with acidic conditions was interpreted in terms of electr...

Nanofabrication and characterization of ZnO nanorod arrays and branched microrods by aqueous solution route and rapid thermal processing

Abstract: This paper presents an inexpensive and fast fabrication method for one-dimensional (1D) ZnO nanorod arrays and branched two-dimensional (2D), three-dimensional (3D) – nanoarchitectures. Our synthesis technique includes the use of an aqueous solution route and post-growth rapid thermal annealing. It permits rapid and controlled growth of ZnO nanorod arrays of 1D – rods, 2D – crosses, and 3D – tetrapods without the use of templates or seeds. The obtained ZnO nanorods are uniformly distributed on the surface of Si substrates and individual or branched nano/microrods can be easily transferred to other substrates. Process parameters such as concentration, temperature and time, type of substrate and the reactor design are critical for the formation of nanorod arrays with thin diameter and transferable nanoarchitectures. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and Micro-Raman spectroscopy have been used to characterize the samples. © 2007 Elsevier B.V. All rights reserved.

Controlled Synthesis of Icosahedral Gold Nanoparticles and Their Surface-Enhanced Raman Scattering Property

Abstract: The aqueous-phase synthesis of icosahedral gold nanoparticles with controlled diameters ranging from 10 to 90 nm is reported. The nanoparticles were prepared by a seed-mediated growth approach. Citrate-capped 3.5 nm gold nanoparticles, prepared by reduction of HAuCl4 with sodium borohydride, are used as seeds. Cetyltrimethylammonium bromide molecules served as a capping agent to restrict the nanoparticles size. The synthesized gold nanoparticles were characterized by UV−vis spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy, and X-ray diffraction (XRD). The crystal structure of the particles was found to consist of mostly {111} surfaces as revealed by both TEM and XRD results. The prepared icosahedral gold nanoparticles show efficient surface-enhanced Raman scattering properties.

Direct Electrodeposition of ZnO Nanotube Arrays in Anodic Alumina Membranes

Abstract: Semiconductor ZnO nanotube arrays have been synthesized by direct electrochemical deposition from aqueous solutions into porous anodic alumina membranes. Scanning electron microscopy and transmission electron microscopy indicate that large-area and highly ordered nanotube arrays have been obtained. X-ray diffraction and selected-area electron diffraction analyses show that the as-synthesized nanotubes are polycrystalline. Photoluminescence spectra of the ZnO nanotube arrays show that a violet peak and a blue peak are centered at 414 and 464 nm, respectively. The ordered polycrystalline ZnO nanotube arrays may find potential applications in optoelectronic and sensor devices. The growth mechanism and the electrochemical deposition process are discussed.