Microstructural evolution of iron oxide hollow nanoparticles toward iron oxide nanotubes in a prolonged sintering condition

Prolonged sintering of iron oxide hollow nanoparticles (HNPs) during chemical vapor condensation (CVC) at 800 °C for 6 h showed some interesting morphologies of the iron oxide nanotubes. TEM and XRD studies confirmed that single-walled nanotubes of a mixed phase of α, β, and γ-Fe₂O₃, with a wall thickness of less than 10 nm and an outer diameter of approximately 50 nm were synthesized. The formation of iron oxide nanotubes was thought to be an evolution of iron oxide HNPs based on the sintering.     

A facile route to sonochemical synthesis of magnetic iron oxide (Fe3O4) nanoparticles

A facile sonochemical approach was applied for the large scale synthesis of iron oxide magnetic nanoparticles (NPs) using inexpensive and non-toxic metal salts as reactants. The as-prepared magnetic iron oxide NPs has been characterized by XRD, TEM, EDS, and VSM. X-ray diffraction (XRD) and EDS analysis revealed that Fe₃O₄ NPs have been successfully synthesized in a single reaction by this simple method. Transmission electron microscopy (TEM) data demonstrated that the particles were narrow range in size distribution with 11 nm average particle size. Moreover, TEM measurements also show that the synthesized nanoparticles are almost spherical in shape. The magnetization curve from vibrating sample magnetometer (VSM) measurement shows that as-synthesized NPs were nearly superparamagnetic in magnetic properties with very low coercivity, and magnetization values were 80 emu/g, which is very near to the bulk value of iron oxide. The estimated value of mass susceptibility of as-synthesized nanoparticles is Xg = 5.71 × 10^− 4 m³/kg.     

Preparation of hollow TiC nanoparticles by the two-stage refluxing method

The two-stage refluxing method was applied to prepare hollow TiC nanoparticles. In this method, after refluxing the hydrous TiO₂ and ethanol, the precipitate and n-dedocane were refluxed to obtain the refluxing-derived precursor. The precursor was heat-treated at 1300 °C for 1 h and hollow TiC nanoparticles could be achieved. It is found that the pore size of the hollow TiC nanoparticles ranges from 3 to 76 nm and the mean pore size is 26.776 nm. The formation of the hollow structure is due to that the mesoporous channels of the precursor confine the carbide's growth inside it. In comparison with commercial TiC nanocrystals, the hollow TiC nanoparticles have higher specific surface area and pore volume significantly.     

Structure of oxide nanoparticles in Fe-16Cr MA/ODS ferritic steel

Oxide nanoparticles in Fe-16Cr ODS ferritic steel fabricated by mechanical alloying (MA) method have been examined using high-resolution transmission electron microscopy (HRTEM) techniques. A partial crystallization of oxide nanoparticles was frequently observed in as-fabricated ODS steel. The crystal structure of crystalline oxide particles is identified to be mainly Y₄Al₂O₉ (YAM) with a monoclinic structure. Large nanoparticles with a diameter larger than 20 nm tend to be incoherent and have a nearly spherical shape, whereas small nanoparticles with a diameter smaller than 10 nm tend to be coherent or semi-coherent and have faceted boundaries. The oxide nanoparticles become fully crystallized after prolonged annealing at 900 °C. These results lead us to propose a three-stage formation mechanism of oxide nanoparticles in MA/ODS steels.     

Enhanced ultraviolet sensitivity of zinc oxide nanoparticle photoconductors by surface passivation

Zinc oxide nanoparticles were created by a top-down wet-chemical etching process and then coated with polyvinyl-alcohol (PVA), exhibiting sizes ranging from 10 to 120 nm with an average size approximately 80 nm. The PVA layer provides surface passivation of zinc oxide nanoparticles. As a result of PVA coating, enhancement in ultraviolet emission and suppression of parasitic green emission is observed. Photoconductors fabricated using the PVA coated zinc oxide nanoparticles exhibited a ratio of ultraviolet photo-generated current to dark current as high as 4.5 × 10⁴, 5 times better than that of the devices fabricated using uncoated ZnO nanoparticles.     

Bio-template route for facile fabrication of Cd(OH)2@yeast hybrid microspheres and their subsequent conversion to mesoporous CdO hollow microspheres

Cadmium oxide (CdO) microspheres with a porous hollow microstructure were prepared by a facile yeast mediated bio-template route. The yeast provides a solid scaffold for the deposition of cadmium hydroxide (Cd(OH)₂) from cadmium acetate and sodium hydroxide solutions to form the hybrid Cd(OH)₂@yeast precursor. Thermal conversions of this at above 500 °C in air have produced hollow CdO microspheres. The products were characterized by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR), thermal gravimetric and differential thermal analysis (TGA-DTA), and Brunauer-Emmett-Teller (BET) surface analysis respectively. The obtained CdO microspheres have uniform size (length = 2.6 ± 0.4 μm; width = 2.0 ± 0.2 μm) and a well defined, continuous, mesoporous hollow microstructure. The shell is about 250-280 nm in thickness. The mechanism of formation of Cd(OH)₂@yeast precursor and its conversion to CdO hollow microspheres is discussed. In comparison with traditional template-directed method, the present strategy represents a general, economical and environmentally benign route for the formation of metal oxide hollow microspheres. These materials have potential applications in different fields such as encapsulation, drug delivery, efficient catalysis, battery materials and photonic crystals. The method presented can be extended to the synthesis of other inorganic hollow microstructures of different sizes and shapes by pre-selecting suitable bio-templates.     

Fabrication of surface silvered polyimide/iron oxide composite films with both superparamagnetism and electrical conductivity

Surface silvered polyimide (PI)/Fe₂O₃ composite films with both superparamagnetic and surface electrically conductive properties have been fabricated by an in situ technique. Iron (III) 2,4-pentanedionate was incorporated into a PI precursor poly(amic acid) solution and thermally decomposed to form iron oxide nanoparticles in the process of thermal imidization, preparing PI/Fe₂O₃ nanocomposite films. The establishment of a silver layer on the PI/Fe₂O₃ film surface involved the steps of chemical etching by the alkaline aqueous solution, ion exchange with silver ions and chemical reduction by glucose. The formed Fe₂O₃ particles of the nano scale endow the film with typical superparamagnetic response. By employing the etching time of only 10 min and a reduction time of no more than 15 min, the well-established silver layers have formed on the upside surface. The corresponding reflectivity and resistivity reached to the value of 76.15% and 0.7 Ω/square respectively.     

Preparation and formation mechanism of nano-sized MnOx-CeO2 hollow spheres via a supercritical anti-solvent technique

Manganese and cerium composite oxide (MnO_x-CeO₂) hollow nanospheres were successfully prepared by precipitating manganese acetylacetonate and cerium acetylacetonate from their mixed methanol solution using supercritical carbon dioxide as an anti-solvent. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) were employed to characterize the precursor and as-prepared MnO_x-CeO₂. XRD analysis reveals the cubic fluorite structure of the MnO_x-CeO₂. HRTEM results indicate that the MnO_x-CeO₂ hollow spheres have an average diameter of about 50 nm, and a wall thickness of 10-20 nm. A new formation mechanism of these nano-sized hollow spheres has also been proposed based on the experimental results.     

Formation and magnetic properties of M-Sr ferrite hollow fibers via organic gel-precursor transformation process

The M-Sr ferrite hollow fibers have been successfully prepared by the organic gel-precursor transformation process. The phase formation process of M-Sr ferrite is analyzed by FTIR, XRD and consists of the gel-precursor thermal decomposition and the subsequent ferrite phase formation from strontium oxide and iron oxide. The M-Sr ferrite hollow fibers obtained are characterized with SEM and XRD, and show a high aspect ratio, fine diameters around 4 μm and a ratio of the hollow diameter to the fiber diameter being about 1/2. The optimized M-Sr ferrite hollow fibers are composed of nanograins with a hexagonal plate morphology. Magnetic properties are measured with VSM under a maximum applied of 1194 kA m⁻¹. The M-Sr ferrite hollow fibers formed at 1100 °C for 2 h with the specific saturation magnetization of 53.5 A m² kg⁻¹ possess a shape anisotropy characteristic and the coercivity for the aligned hollow fibers parallel and perpendicular to the applied field is correspondingly 385 and 357 kA m⁻¹.     

Preparation and application of magnetic cobalt/SiO2 core/shell nanospheres

Homogeneous SiO₂-coated cobalt nanospheres with tunable silica shell thickness from 21.7 nm to 4.5 nm were synthesized by using modified Stöber method. These nanocomposites were used as source materials to prepare SiO₂ semi-hollow and hollow nanospheres by partially and completely etching cobalt cores, respectively. A proposed formation mechanism of these Co/SiO₂ nanospheres with a core/shell structure was presented in this paper, which is also important for the rational design and synthesis of other monodisperse core/shell nanoarchitectures with uniform size and shape. Furthermore, these Co/SiO₂ nanospheres were also used as a substrate for the deposition of CdS nanocrystals to prepare magnetic luminescent Co/SiO₂/CdS nanocomposites.     

A simple one-step fabrication of micrometer-scale hierarchical TiO2 hollow spheres

Hierarchical TiO₂ hollow spheres had been prepared based on bubble templates by a simple one-step hydrothermal method. The diameter of hollow spheres was about 700 nm and the shell thickness of them was 69 nm. They were composed of similar spindle- or needle-like building units. Furthermore, hydrothermal time had an important influence on the morphology and crystallinity of hollow spheres. Moreover, the UV-Vis diffuse reflectance spectra of TiO₂ hollow spheres heated at 150 °C for 10 h showed the strongest absorption in the UV-Vis region and the Raman spectrum demonstrated the anatase sample. Additionally, a possible formation mechanism of TiO₂ hollow spheres was proposed. So this novel and simple method would provide a development direction to fabricate all kinds of inorganic hollow spheres by one-step procedure.     

Synthesis and characterization of calcium hydroxide nanoparticles by hydrogen plasma-metal reaction method

Ca(OH)₂ nanoparticles have been synthesized with high purity and yield using the hydrogen plasma-metal reaction method. They are spherical in shape with a mean particle size of approximately 100 nm. The morphology of nanoparticles is spongy with mesopores, mostly less than 10 nm. The pore volume and surface area of Ca(OH)₂ nanoparticles are 0.084 cm³/g and 28.7 m²/g, respectively. Both transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis demonstrated that these nanoparticles possess poly-nanocrystalline structure with an average grain size of about 10 nm. The formation mechanism of Ca(OH)₂ nanoparticles was discussed in terms of chemical reactions and coalescence during the processing.     

Arsenic (III,V) removal from aqueous solution by ultrafine α-Fe2O3 nanoparticles synthesized from solvent thermal method

Ultrafine iron oxide (α-Fe₂O₃) nanoparticles were synthesized by a solvent thermal process and used to remove arsenic ions from both lab-prepared and natural water samples. The α-Fe₂O₃ nanoparticles assumed a near-sphere shape with an average size of about 5 nm. They aggregated into a highly porous structure with a high specific surface area of ∼162 m²/g, while their surface was covered by high-affinity hydroxyl groups. The arsenic adsorption experiment results demonstrated that they were effective, especially at low equilibrium arsenic concentrations, in removing both As(III) and As(V) from lab-prepared and natural water samples. Near the neutral pH, the adsorption capacities of the α-Fe₂O₃ nanoparticles on As(III) and As(V) from lab-prepared samples were found to be no less than 95 mg/g and 47 mg/g, respectively. In the presence of most competing ions, these α-Fe₂O₃ nanoparticles maintained their arsenic adsorption capacity even at very high competing anion concentrations. Without the pre-oxidation and/or the pH adjustment, these α-Fe₂O₃ nanoparticles effectively removed both As(III) and As(V) from a contaminated natural lake water sample to meet the USEPA drinking water standard for arsenic.     

Facile template-free sonochemical fabrication of hollow ZnO spherical structures

ZnO hollow spherical structures have been synthesized by a facile template-free sonochemical process. The structures and morphologies of products have been characterized by XRD, FESEM and TEM. The results reveal that hollow spherical structures possess a hexagonal wurtzite structure with the in- and out-diameters of about 400 and 500 nm, respectively. The walls of the hollow structures are self-assembled by nanoparticles, partly composed of hexagonal nanoflakes with 40 nm in side lengths. Room temperature photoluminescence (PL) spectrum showed a UV emission at ∼ 384 nm and a broad green emission at the center of 535 nm. A possible formation mechanism was also proposed.     

Preparation and magnetic properties of spindle porous iron nanoparticles

Spindle porous iron nanoparticles were firstly synthesized by reducing the pre-synthesized hematite (α-Fe₂O₃) spindle particles with hydrogen gas. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption/desorption isotherms and vibrating sample magnetometry (VSM). A lattice shrinkage mechanism was employed to explain the formation process of the porous structure, and the adsorbed phosphate was proposed as a protective shell in the reduction process. N₂ adsorption/desorption result showed a Brunauer-Emmett-Teller (BET) surface area of 29.7 m²/g and a continuous pore size distribution from 2 nm to 100 nm. The magnetic hysteresis loop of the synthesized iron particles showed a saturation magnetization of 84.65 emu/g and a coercivity of 442.36 Oe at room temperature.     

Facile fabrication of ZrO2 hollow porous microspheres with yeast as bio-templates

ZrO₂ hollow porous microspheres have been fabricated successfully using living yeast cells as bio-templates through a facile ageing process at lower crystallizing temperature. XRD, SEM, FT-IR and N₂ adsorption-desorption were used to characterize ZrO₂ hollow microspheres. The results showed that the inorganic-organic hybrid hollow microspheres were fabricated at 100 °C and ZrO₂ hollow microspheres with tetragonal phase and porous structure were obtained at 300 °C. The crystallization temperature of ZrO₂ decreased obviously due to the inducting of bio-molecules. The as-prepared hollow microspheres are approximately ellipsoid. The shells of these hollow microspheres are porous, which are composed of some nanoparticles with size of ∼20 nm. The formation mechanism of ZrO₂ hollow microspheres was proposed and discussed tentatively.     

Free standing thin webs of porous carbon nanofibers of polyacrylonitrile containing iron-oxide by electrospinning

We present for the first time a novel strategy of producing carbon nanofibers (CNFs) using polyacrylonitrile (PAN)-incorporated with iron oxide particles using electrospinning method. The successfully electrospun iron oxide-incorporated PAN thin white web was stabilized in air. Formation of iron oxide nanoparticles resulted in the formation of porous structure on the web. Following heat treatment to the stabilized fibers to about 1000 °C in an N₂ atmosphere resulted in CNFs with specific surface areas, which ranged from 310, 420, and 550 m² g^− 1, for PAN containing 1, 2 and 3 wt.% iron oxide, respectively.     

Rapid synthesis of TiO2 hollow nanostructures with crystallized walls by using CuO as template and microwave heating

In this paper, TiO₂ hollow nanostructures with anatase walls have been rapidly fabricated by using CuO as template and microwave heating. These TiO₂ hollow nanostructures have been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Experimental results showed that the TiO₂ shell transformed from amorphous to anatase phase in 3 min, induced by the hot CuO core under microwave irradiation. The diameter of TiO₂ hollow nanostructures is about 50-80 nm, and the length is about 200-300 nm. The thickness of the shell is about 3 nm. This method is promising to be used to synthesize other nanomaterials with a hollow nanostructure.     

Low temperature synthesis and optical properties of CaTiO3 nanoparticles from Ca(NO3)2·4H2O and TiO2 nanocrystals

Choosing low-melting-point Ca(NO₃)₂·4H₂O and high-reactive-activity TiO₂ nanocrystals as the raw materials, a simple and cost-effective route was developed for the synthesis of CaTiO₃ nanoparticles at 600 °C, which is much lower than that (about 1350 °C) used in the conventional solid state reaction methods. X-ray diffraction, energy dispersive X-ray spectroscopy and field emission scanning electron microscopy revealed the formation of orthorhombic phase CaTiO₃ nanoparticles with oxygen-deficiency at the surface. UV-vis absorption spectrum of the as-obtained CaTiO₃ nanoparticles displayed an absorption peak centered at around 325 nm (3.8 eV), together with a tail at lower energy side. Room temperature photoluminescence spectrum of the as-obtained CaTiO₃ nanoparticles upon laser excitation at 325 nm demonstrated a strong and broad visible light emission ranging from about 527 to 568 nm, which may be originated from the surface states and defect levels.     

Preparation and characterization of bio-functionalized iron oxide nanoparticles for biomedical application

Amine modified iron oxide (Fe₃O₄) nanoparticles were synthesized by thermal decomposition method and were further used to bio-functionalize by grafting of N-hydroxysuccinimide (NHS) ester of folate and ethylenediaminetetraacetate (EDTA). Fe₃O₄ nanoparticles of ~ 22 nm were confirmed from X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. FT-IR studies indicated two bands at 1515 cm^− 1and 1646 cm^− 1, which can be attributed to carboxylic group and the amide linkage respectively, revealing the conjugation of folate with Fe₃O₄. The conjugation of the chelating agent showed strong C=O stretch and Fe-O vibrations at 1647 and 588 cm^− 1 respectively. The value of saturation magnetization for Fe₃O₄ nanoparticles was found to be 88 emu/g, which further reduced to 18 and 32% upon functionalization with EDTA and NHS ester folate, respectively. These amine modified Fe₃O₄ nanoparticles can also be functionalized with other bifunctional chelators, such as amino acids based diethylene triamine pentaacetic acid (DTPA), and thus find potential applications in radio-labeling, biosensors and cancer detection, etc.