X-ray absorption and emission spectroscopy of ZnO nanoparticle and highly oriented ZnO microrod arrays

The electronic structures of ZnO nanoparticles and microrod arrays are studied by O 1s X-ray absorption spectroscopy (XAS) and O Kα X-ray emission spectroscopy (XES). We show that the present LDA+U^SIC calculation approach is suitable to correct the LDA self-interaction error of the cation d-states. The atomic eigenstates of 3d in zinc and 2p in oxygen are energetically close, which induces strong Zn-3d-O-2p hybridization. This anomalous valence band cation-d-anion-p hybridization is affected when the localization of the Zn 3d-states is taken into account. Experimentally, the XES spectra show energy dependence in the spectral shape revealing selected excitations to the Zn 3d, 4s and 4p states, hybridized with O 2p states. Strong anisotropic effects are observed for the highly oriented ZnO rods, but not for the isotropic spherical nanoparticles. The nanostructured ZnO has primarily bulk XAS and XES properties.     

Highly aluminium doped barium and strontium ferrite nanoparticles prepared by citrate auto-combustion synthesis

Aluminium doped barium and strontium hexaferrite nanoparticles BaAl_xFe_(12−x)O₁₉ and SrAl_xFe_(12−x)O₁₉ were synthesised via a sol-gel route using citric acid to complex the ions followed by an auto-combustion reaction. This method shows promise for the synthesis of complex ferrite powders with small particle size. It was found that around half of the iron could be substituted for aluminium in the barium ferrite with structure retention, whereas strontium aluminium ferrites could be produced with any aluminium content including total substitution of the iron. All synthesised materials consisted of particles smaller than 1 μm, which is the size of a single magnetic domain, and various doping levels were achieved with the final elemental composition being within the bounds of experimental error. The materials show structural and morphological changes as they move from iron to aluminium ferrites. Such materials may be promising for imaging applications.     

Physicochemical properties of solid-solid interactions in nanosized NiO-substituted Fe2O3/TiO2 system at 1200 °C

The solid-solid interactions between nanosized pure and NiO-substituted ferric and titanium(IV) oxides have been investigated using XRD technique and microstructure studies, also magnetic properties were studied using vibrating samples magnetometer (VSM). The amounts of substituting Ni²⁺ were x = 0, 0.2, 0.4, 0.6, 0.8 and 1 mole. A mixture equimolar proportions of finely powdered Fe₂O₃ and TiO₂ were mixed with NiO, ball milled, compressed at 250 kg/cm² and fired at 1200 °C for 4 h.The obtained results showed that with substituting Ni²⁺ concentration x = 0 only Fe₂TiO₅ phase is present (∼80 nm) which showed a very small saturation magnetic flux density (B_s), remnant magnetic flux density (B_r) and the maximum energy product (BH)_max. By the addition of x = 0.2 NiO, new phases were observed NiTiO₃ and NiFe₂O₄ of crystallite sizes 160 and 110 nm, respectively. By the increase of substituting Ni²⁺ concentration the NiTiO₃ and NiFe₂O₄ phases increased on the expense of Fe₂TiO₅ up to x = 0.4, then the increase in substituting Ni²⁺ concentration led to a decrease in Fe₂TiO₅ and NiTiO₃ while NiFe₂O₄ increases which results in a great improvement of magnetic properties.All samples exhibit a catalytic activity towards H₂O₂ decomposition and the values of rate constant increase with increasing amount of Ni²⁺ substituting. The most acidic active sites are shown by specimens substituted with x = 0 this concludes that H₂O₂ decomposition is not favored on acidic active sites.     

Subtractive methods to form pyrite and sulfide nanostructures of Fe,Co, Ni,Cu and Zn

The low Z metals Fe, Co, Ni, Cu and Zn are Earth abundant, i.e. inexpensive, and their sulfides are of low toxicity. This makes them appealing candidates for materials applications requiring semiconductors or, in the case of CoS₂, a metal since they can potentially be produced in large quantities and low cost. Though of great potential little work has explored how subtractive methods can be used to form nanostructured and/or porous structures in, e.g. FeS₂, CoS₂, NiS, Cu₂S and ZnS.     

Modification of interparticle forces for nanoparticles using atomic layer deposition

Silica and titania nanoparticles were individually coated with ultrathin alumina films using atomic layer deposition (ALD) in a fluidized bed reactor. The effect of the coating on interparticle forces was studied. Coated particles showed increased interactions which impacted their flowability. This behavior was attributed to modifications of the Hamaker coefficient and the size of nanoparticles. Stronger interparticle forces translated into a larger mean aggregate size during fluidization, which increased the minimum fluidization velocity. A lower bed expansion was observed for coated particles due to enhanced interparticle forces that increased the cohesive strength of the bed. Increased cohesiveness of coated powders was also determined through angle of repose and Hausner index measurements. The dispersability of nanopowders was studied through sedimentation and z-potential analysis. The optimum dispersion conditions and isoelectric point of nanoparticle suspensions changed due to the surface modification. A novel atomic force microscope (AFM) technique was used to directly measure interactions between nanoparticles dispersed on a flat substrate and the tip of an AFM cantilever. Both Van der Waals and electrostatic interactions were detected during these measurements. Long and short range interactions were modified by the surface coating.     

Formation and magnetization reversal mechanisms of nano-size Fe arrays prepared on anodic aluminum oxide templates

Vertically aligned Fe arrays have been self-assembled on anodic aluminum oxide templates by evaporation. The rims of the pores, which act as obstacles to the stacking of atoms, prevent them from forming continuous films. By controlling the Fe nominal thicknesses (τ_n) from 400 to 5 nm, the morphology is changed from continuous film to isolated arrays, leading to the change of the predominant magnetization reversal from domain wall motion to spin rotation. For samples with τ_n < 59 nm, isolated, rather than interconnected, morphology is formed. In this range, the coercivity shows a spectacular change for τ_n = 47 nm, with an array diameter of about 52 nm, achieving a maximum of about 38 kA/m. The critical dimension of single-domain array is therefore determined. The magnetostatic and exchange interactions are reduced due to the thermal fluctuation, and the magnetization leaves from the in-plane direction to be randomly distributed in 3-D, for τ_n < 27 nm.     

Fabrication of egg-shell-roofed macroporous nickel films by a template-mediated electrodeposition process

Through an electrostatics-induced adsorption effect, nickel ions were found to be preferentially adsorbed onto the surface of colloidal particles at template top during the template-mediated electrodeposition process for preparing macroporous structures. This phenomenon results in the preferential reduction and growth of nickel on colloid surface on the template top, instead of filling into the channels among the colloids. After removing the template, an egg-shell-roofed macroporous nickel, consisting of macroporous film covered with monolayer of hollow spheres, can be created.     

Area preferential nucleation of Ge nano-dots on nano-size porous silicon

The Ge quantum dots on anodized nanometer porous silicon layers are prepared by area preferential nucleation at a low temperature of 720°C. The porous silicon was formed by anodic conversion of p-type (1 0 0)-oriented crystalline silicon in hydrofluoric acid diluted by alcohol. Clear phonon-resolved PL, as a NP transition and its TA phonon replica, was observed from the Ge dots at the temperature of 10 K. We attributed the very large blue-shift in energy of the PL peak to the quantum size effect in Ge dots. The present technique is a potential low-cost method for producing quantum dot arrays.     

Fabrication of alumina ceramics from powders made by sol–gel type hydrolysis in microemulsions

The engineering aspects of the preparation of nanostructured alumina ceramic precursors by alcoholate hydrolysis using microemulsions as reaction media are investigated here. The precipitate was subjected to several treatment steps. Although the properties of the primary precipitated powders are independent of the chemical or reaction engineering parameters of the precipitation procedure, the structure of treated powders and sintered, dense ceramics strongly depends on thermal and mechanical handling like crystallization or grinding of the alumina ceramic precursor. Strong differences are manifested in relative densities and sintering kinetics and can be observed by SEM analysis.