Crystallography and crystallite morphology of rutile synthesized at low temperature

Rutile was prepared at low temperature with titanium trichloride in an acid- and oxygen-rich environment. Samples were characterized with X-ray powder diffraction, transmission electron microscopy, and thermogravimetry; rutile's crystalline structure was refined with the Rietveld method. Crystallites were needles with their largest dimension along c lattice parameter; they grew in bundles parallel to their length axis, and contained hydroxyls in their lattice and on their surface. When samples were annealed, dehydroxylation generated lattice defects that produced microstrains and created conditions for crystallite sintering at temperatures above 300°C. Titanium-oxygen interaction in rutile depended on the annealing temperature, which gave rise to changes in the symmetry of the representative rutile titanium-oxygen octahedron. The atomic bond length between the oxygen atoms shared by adjacent octahedra decreased with the temperature, contracting the lattice. All this was caused by the variation of the number of hydroxyls in the lattice.     

Defect and transport properties of nanocrystalline ceramics and thin films

This paper presents an overview of recent research on the defect and transport properties of nanocrystalline ionic and mixed conducting ceramics and thin films. In the first part, some basic concepts and properties of boundaries are reviewed, including diffusion, segregation, and space charge regions. Experimental data on nanoceramics and thin films made from pure and doped CeO₂, TiO₂, ZrO₂, and CaF₂ are presented and discussed in the second part; opportunities for future work on this topic are outlined. Electronic Publication     

Low temperature synthesis of nanocrystalline YVO4:Eu via polyacrylamide gel method

Nanocrystalline europium doped yttrium orthovanadate (YVO₄:Eu) were synthesized by the polyacrylamide gel method. For a comparative evaluation, bulk YVO₄:Eu was prepared by solid-state reaction. On the basis of X-ray powder diffraction, scanning and transmission electron microscopy and luminescence measurements, the polyacrylamide gel method appears to be a more efficient method to prepare the luminescence materials YVO₄:Eu with high homogeneity, purity and luminescence intensity.     

“Unusual Ln substitutional defects”: The local chemical environment of Pr and Nd in nanocrystalline TiO2 by Ln-K edge EXAFS

The local chemical environment of the trivalent lanthanide cations in anatase TiO₂ nanopowders doped with 1 mol% of Pr or Nd, prepared via a sol-gel technique, has been studied by means of EXAFS at the Pr and Nd-K edge. Titanium dioxide can be considered an “unusual” host for doping with Ln³⁺ ions due to the large mismatch of both charge and ionic radii between the dopant and the host constituent cations. However, it can be demonstrated that the lanthanide ions enter the anatase structure as substitutional defects with respect to Ti, but that the amount of disorder around the substitutional defects is very large. For both Pr³⁺ and Nd³⁺ ions, the Ln-O and Ln-Ti distances have been found to increase by about 0.45 Å, with respect to what is found for the Ti-O and Ti-Ti distances in pure anatase. Valence-bond calculations have been used to validate the Ln-O distances obtained by the EXAFS fitting. Finally, no evidences for oxygen vacancies clustering around the substitutional defects have been found. Luminescence spectroscopy has shown that the lanthanide ions do not segregate in oxide or pyrochlore impurities phases.     

Growth of nanocrystalline CuIn3Se5 (OVC) thin films by ion exchange reactions at room temperature and their characterization as photo-absorbing layers

Nanocrystalline CuIn₃Se₅ thin films have been grown on ITO glass substrates using chemical ion exchange reactions with CdS, in alkaline medium at pH 11. The as-deposited films were annealed in air at 200 °C for 30 min and characterized using X-ray diffraction (XRD), transmission electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy to study the structural, compositional and morphological properties. The XRD patterns reveal the nanoparticles size to be of 18-20 nm diameter, while from the SEM images the nanoparticles size is estimated to be 20-30 nm. It is observed that the annealed films contain nanocrystallites connected with each other through grain boundaries, with grain size of about 100-125 nm and have an overall n-type electrical conductivity and higher photoconductivity. The current-voltage (I-V) characteristics (in dark and light) of these films indicated the formation of a Schottky like junction between the n-CuIn₃Se₅ (OVC) and CdS/ITO layers.     

Synthesis of Y2Si2O7:Eu nanocrystal and its optical properties

Nanocrystalline Y₂Si₂O₇:Eu phosphor with an average size about 60 nm is easily prepared using silica aerogel as raw material under ultrasonic irradiation and annealing temperature at 300-600 °C and this nanocrystalline decomposes into Y₂O₃:Eu and silica by heat treatment at 700-900 °C. The excitation broad band centered at 283 and 254 nm results from Eu³⁺ substituting for Y³⁺ in Y₂Si₂O₇ and Y₂O₃/SiO₂, respectively. Compared with Y₂O₃:Eu/SiO₂ crystalline, the PL excitation and emission peaks of Y₂Si₂O₇:Eu nanocrystalline red-shift and lead to the enhance of its luminescence intensity due to the different chemical surroundings of Eu³⁺ in above nanocrystallines. The decrease of PL intensity may be ascribed to quenching effect resulting from more defects in Y₂O₃:Eu/SiO₂ crystalline.     

Controlled synthesis, characterization and optical properties of CdS nanocrystalline thin films via chemical bath deposition (CBD) route

In this study, the CdS nanocrystalline thin films obtained from an ammonia-free chemical bath deposition process. The crystallites with a size range of 10–20 nm in diameter with zinc blend (cubic) and wurtzite (hexagonal) crystal structure and strong photoluminescence were prepared from the mixture solutions of: cadmium chloride dihydrate as a cadmium source, thiourea as a sulfur source and sodium citrate dihydrate as a complexing agent for cadmium ions. The well-cleaned glass used as a substrate for thin films deposition. The obtained samples were characterized by the techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), atomic force microscopy (AFM) and fluorescence spectroscopy. Also, the effect of two parameters such as pH and temperature of reaction on the synthesis of CdS nanocrystals was studied. Finally, it was found the CdS nanocrystals showed sharp excitation features and strong “band-edge” emission.     

聚乙烯醇/半纤维素/纤维素纳米晶复合膜的制备和性能

通过碱解醇沉法从玉米芯中提取半纤维素,进而采用溶液共混流延法制备出不同比例的聚乙烯醇(PVA)/半纤维素共混膜,在此基础上加入通过硫酸水解脱脂棉制备的纤维素纳米晶(NCC),制备了NCC增强的复合膜.DSC、TGA、FTIR、SEM和薄膜拉伸等研究表明,复合膜的各组分间形成了氢键作用,相容性良好.在PVA/半纤维素共混膜中,半纤维素的加入大幅度提高了复合膜的断裂伸长率:当PVA与半纤维素质量比为3∶1时,复合膜的断裂伸长率高达380%,比纯PVA的140%增加了1.7倍,而拉伸强度仍能保持在较高的水平;纤维素纳米晶的加入,还可改善复合膜的韧性;半纤维素对PVA组分的结晶性能产生了影响,而刚性结构的NCC则对PVA的结晶起抑制作用.此外半纤维素和NCC的引入也提高了复合膜的热性能,使第二阶段分解速率峰的温度提高了约50℃.     

Electrochemical corrosion behavior of nanocrystalline zinc coatings in 3.5% NaCl solutions

The corrosion behavior of electrodeposited nanocrystalline (NC) zinc coatings with an average grain size of 43 nm was investigated in 3.5% NaCl solutions in comparison with conventional polycrystalline (PC) zinc coatings by using electrochemical measurement and surface analysis techniques. Both polarization curve and electrochemical impedance spectroscopy (EIS) results indicate that NC and PC coatings are in active state at the corrosion potentials, and NC coatings have much higher corrosion resistance than PC ones. The corrosion products on both coating surfaces are mainly composed of ZnO and Zn₅(OH)₈Cl₂·H₂O, but the corrosion products can form a relatively more protective layer on NC coating surfaces than on PC coatings. The EIS characteristics and corrosion processes of PC and NC zinc coatings during 330 h of immersion were discussed in detail.