Fabrication of ferric oxide/reduced graphene oxide/cadmium sulfide heterostructure photoelectrode for enhanced photoelectrochemical performance

A novel high‐efficiency photoelectrode (Fe₂O₃/reduced graphene oxide/CdS) built from heterostructure and conductive scaffold has been successfully designed and synthesized. Reduced graphene oxide works as a “bridge” which benefits for electron and hole transport. The obtained heterostructure photoelectrodes were systematically characterized by X‐ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectroscopy, and X‐ray photoelectron spectroscopy (XPS). The photoconversion efficiency (η) and photocurrent densities vs. time (I‐t) curves responding to monochromatic lights have been further investigated in‐depth, which reveals that introduction of CdS and reduced graphene oxide played an important role in the enhancement of photoelectrochemical performance.     

Synthesis and characterization of methylammonium tetrachlorocuprate (II) dihydrate crystals

Single crystals of methylammonium tetrachlorocuprate (II) dihydrate were obtained by slow evaporation method at room temperature. The crystals were characterized through powder XRD, thermogravimetric (TG‐DTA), low temperature DSC studies and FTIR and far IR spectra. While the powder XRD confirms the crystallinity of the compound, the TG and DTA studies confirm the formation of the compound in the stoichiometric ratio and the presence of two water molecules of crystallization. The low temperature DSC study indicates a thermal anomaly at ‐3°C. The ordering of CH₃NH₃ group at low temperature causes phase transitions in the compound. The IR spectra characterize the various chemical bonding and water molecules in the compound. Laser Raman spectral studies under progress would help understand the mechanism of phase transitions at low temperatures. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nano-particles of La0.9Ca0.1MnO3 manganite: Size-induced change of magnetic ground state and interplay between surface and core contributions to its magnetism

The structure, dc and ac magnetic properties of sonochemically prepared nano-particles of La_0.9Ca_0.1MnO₃ compound were studied as compared to those of the bulk sample of the same composition. It is shown that transition to the nanometer sized scale results in a change of the magnetic order from a mixed antiferro + ferromagnetic to a pure ferromagnetic one due to the improvement of crystallinity and suppression of chemical/magnetic disorder in nano-particles. The surface contribution to its magnetism leads to a super-paramagnetic-like ordering supposedly due to the interfacial exchange coupling via surface tunneling of carriers above the Curie point (T_C). Below T_C ∼ 90 K, a notable contribution of surface spin disorder reduces the spontaneous magnetization of the nano-sample as compared to the bulk-one.     

Synthesis and characterization of dihydroxylammonium tetrachlorocuprate dihydrate crystals

A new hydroxylammonium compound, [(NH₃OH)₂CuCl₄], was synthesized and its crystals were grown at room temperature by slow evaporation of aqueous solutions. The crystals were characterized through powder XRD, thermogravimetry (TG), differential thermal analysis (DTA), low temperature differential scanning calorimetric (DSC) and FTIR spectra. The X‐ray powder diffraction confirms the crystallinity of the compound. A fitting decomposition pattern of the compound was formulated based on the TG and thereby confirming the formation of the compound in the stoichiometric ratio. The thermal anomalies occurring in the low temperature DSC indicate successive phase transitions. The low temperature phase transitions are attributed to the ordering of [NH₃OH]⁺ ions. While most of the phase transitions are of first order type, the one occurring at –126 °C is of second order. Two glass transitions occur when the compound was cooled between –157.9 and –136.9 °C. The characteristic vibration bands due to [NH₃OH]⁺ and CuClequation/tex2gif-stack-1.gif ions are observed in the IR spectra. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

MOVPE Growth and Characterisation of Zn‐Doped (GaIn)P Layers with Respect to Surface Structure and Ordering

(GaIn)P grown on (001)GaAs substrates by metal‐organic vapour phase epitaxy (MOVPE) is highly ordered material. In this work the effect of Zn doping on the epitaxial crystal growth, the ordering behaviour and the surface morphology is investigated. Zn‐doped (GaIn)P layers, grown with phosphine (PH₃), tertiarybutylphosphine (TBP) and ditertiarybutylphosphine (DitBuPH) as phosphorous precursors, reveal a strong drop of the binary growth rate of InP r_InP with increasing Zn/III ratio, whereas r_GaP remains nearly constant. The Zn doping efficiency and the ordering behaviour are observed to be dependent on the misorientation of the substrates. Finally, the surface morphology as a function of the different parameters was analysed by atomic force microscopy (AFM), and no considerable change of the growth mechanism was found for Zn‐doped layers in comparison to undoped layers.     

Study of structural relaxation of Ni78Si8B14 metallic glass by electrical resistivity and thermal expansion measurements

Electrical resistivity and thermal expansion measurements were made to elucidate the structural relaxation of Ni₇₈Si₈B₁₄ metallic glass, especially, the topological short range ordering. The decrease of the resistivity and the increase of its temperature coefficient were observed, and these changes due to the structural relaxation were discussed on the basis of the extended Ziman theory for metallic glasses proposed by Nagel. The densification of about 0.25% and the decrease of 4% in the thermal expansion coefficient were observed in the considerably relaxed sample. The topological short range ordering in Ni₇₈Si₈B₁₄ metallic glass occurs rapidly at the annealing temperature above 200°C.     

Neutron diffraction study of ordering in the γ-phase of the aluminium-magnesium system

In the present work neutron diffraction study is made of the ordering effekt within the whole region of the existence of the γ-phase of the Al-Mg system. Depending on thermal treatment conditions, the order coefficient S varies from 0.15 to 0.7 for compounds containing 59 and 60 at.% Mg at temperatures ∼330 and below 150°C, respectively. – At decreased annealing temperatures the ordinate of the highest ordering is shown to approach the composition which may be approximately characterized as Al₂Mg₃. The temperature increase results in the decrease of the ordering degree, and the ordinate of the highest ordering displaces to the composition which may be expressed as Al₁₂Mg₁₇. – Kurnakov's point is determined which is lying at ∼330°C for the composition containing 59 at.% Mg.     

Approximants of icosahedral quasicrystals: Atomic structure, inherent defects, and superstructural ordering

The structural features of approximants of icosahedral and decagonal quasicrystals and new unusual approximants (rhombohedral AlPd and cubic Al₆₈Pd₂₀Ru₁₂) are considered. It is shown that most approximants can be described in terms of universal local ordering of atoms, in which the nearest neighbors of each atom occupy the vertices of an almost ideal dodecahedron: the so-called dodecahedral local ordering. A set of general atomic motifs in the approximants of different orders is found for quasicrystals of different types. It is shown that the dodecahedral local ordering can be easily described by the project method, in which the basis vectors directed along icosahedral threefold axes are used. Different types of defects inherent in dodecahedral local ordering are analyzed.     

Variations in the structure of aromatic solvents under the influence of microadditives of the C<Subscript>60</Subscript> fullerene

The structural ordering of aromatic solvents is investigated using wide-angle X-ray diffraction. It is shown that the degree of structural ordering of aromatic solvents at room temperature decreases in the following sequence: benzene, toluene, and n-xylene. The introduction of the C₆₀ fullerene (～0.001%) into these solvents leads to an increase in the degree of their ordering. Upon introduction of the fullerene, the degree of structural ordering increases significantly in n-xylene and only slightly in toluene and remains virtually unchanged in benzene. An analysis of the small-angle X-ray diffraction patterns of C₆₀ fullerene solutions in benzene likewise demonstrates that the introduction of the fullerene into benzene leads to an insignificant change in the degree of structural ordering of this solvent. The specific features of the structure and behavior of benzene upon interaction with C₆₀ fullerene additives are discussed.     

Variation of luminescence properties of Na2O-CaO-SiO2: Nd glass with crystallinity

The optical absorption and emission intensities of Nd³⁺-doped transparent glass-ceramics with high crystallinity in Na₂O-CaO-SiO₂ (NSC) system were studied. The transmittance of NCS decreases with increasing degree of crystallinity, however it still remains 65.5% at λ = 710 nm when the crystallization is almost completed. Judd-Ofelt theory is performed to evaluate the radiative transition probability as well as quality factor, branching ratio and emission cross section. The maximum values of Ω₂ and emission cross section (⁴F_3/2 → ⁴I_11/2) of NCS are obtained after nucleating at 630 °C for 10 h. The quality factor increases with increasing crystallinity, while branching ratio for ⁴F_3/2 → ⁴I_11/2 is opposite. The results show that transparent glass-ceramic with high crystallinity is a potential laser host for 1.06 μm emission.     

Synthesis and thermal,phase transition and FTIR spectral characterization of tetraethylammonium tetrachlorocuprate (II) crystals

Single crystals of tetraethylammonium tetrachlorocuprate (II), [N(C₂H₅)₄]₂CuCl₄, were grown by slow evaporation method at room temperature. The crystals were characterized through powder XRD, thermogravimetric (TG‐DTA), low temperature differential scanning calorimetric (DSC) studies and FTIR spectroscopy. While the powder XRD pattern of the compound shows sharp Bragg peaks confirming the crystallinity of the compound, the TG‐DTA studies confirm formation of the compound in the stoichiometric ratio. The thermal anomalies observed in DSC curve at ‐120°C in the heating cycle and around ‐30°C in the cooling cycle indicate a first order phase transition. The phase transition was predicted to be associated with the ordering of CuCl₄^2‐ and successive long range orientation of [N(C₂H₅)₄]⁺ ions which are disordered at high temperatures. The cationic [N(C₂H₅)₄]⁺ plays a role in phase transitions at low temperatures. The sharp exothermic peak observed in high temperature DSC indicates a structural phase transition when [N(C₂H₅)₄]CuCl₃ is formed on heating the compound. The FTIR spectra of the compound characterize the various chemical bonding and water molecules adsorbed in the compound. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal growth and characterization of two-leg spin ladder compounds: Sr14Cu24O41 and Sr2Ca12Cu24O41

Single crystals of Sr_14−xCa_xCu₂₄O₄₁ (x=0 and 12) are grown by the travelling solvent floating zone technique using an image furnace. The grown crystals are characterized for their single crystallinity by the X-ray and Neutron Laue method. The magnetic susceptibility measurements in Sr₁₄Cu₂₄O₄₁ show considerable anisotropy along the main crystallographic axes. Low-temperature specific heat measurement and DC susceptibility measurement in Ca-doped crystal showed antiferromagnetic ordering at 2.8 K at ambient pressure. High-pressure AC susceptibility measurement on Ca-doped crystal showed a sharp superconducting transition at 2 K under 40 kbars. T_c onset reached a maximum value of 9.9 K at 54 kbars. The bulk superconductivity of the sample is confirmed by the high-pressure AC calorimetry with T_c max=9.4 K and T_N=5 K at 56 kbars.     

Effect of alcohols additives on hydrothermal formation of magnesium hydroxide sulfate hydrate whiskers

Two different organic alcohols, ethylene glycol and 1, 2‐propanediol, were used in hydrothermal formation of MgSO₄·5Mg(OH)₂·2H₂O (MHSH) whiskers. Compared with the MHSH whiskers without alcohols, the crystallinity and defect of products were improved. It could also be observed that not only aspect ratio but also crystallinity and purity of the products were improved when 1, 2‐propanediol was used. A reasonable model was proposed to explain the growth process derived from the organic alcohol. In addition, the room temperature photoluminescence (PL) indicated that the spectrum intensity of MHSH whiskers used 1, 2‐propanediol was also significantly improved.     

MOVPE growth of spontaneously ordered (GaIn) and (AlIn)P layers lattice matched to GaAs substrates

A systematic study of the metal-organic vapour-phase epitaxial growth of (GaIn)P and (AlIn)P layers deposited on GaAs substrates with (001) and (110) orientation is presented. Special attention has been paid to the growth on (001)-oriented wafers with different misorientations to the growth direction. The influence of the growth conditions on the properties of the epitaxial layers such as lattice mismatch, alloy composition, photoluminescence (PL) wavelength, FWHMs of PL peaks and atomic ordering is discussed. Layers with mirrorlike surfaces and various degrees of order could be deposited at growth temperatures T_g ranging from 595 °C to 750 °C for (GaIn)P and 720 °C to 800 °C for (AlIn)P. In addition to the influence of T_g on the Ga incorporation during the (GaIn)P growth we found the Ga distribution coefficient k_Ga to be affected by the misorientation of the substrates. k_Ga correlates presumably with the number of kinks and steps on the substrate surface. Transmission electron diffraction (TED) and PL investigations show that the degree of order — often described by the ordering paramter η — depends strongly on T_g the ordering is more pronounced when the layers are deposited on substrates misoriented towards the (1 11) lattice plane. Strong ordering has been observed for (GaIn)P samples grown at 680 °C on substrates 2° misoriented towards the [1 10] direction and at 650 °C on substrates 6° misoriented towards the same direction. For the (AlIn)P samples striking ordering has been found when they were grown at 720 °C.     

Heteroepitaxial growth of β-SiC on silicon substrate using SiCl4-C3H8-H2 system

Single crystals of β-SiC were prepared on Si substrates at a temperature around 1390°C with the standard conditions: H₂ ≈ 1 1/min, SiCl₄≈3 ml/min, C₃H₈≈1 ml/min, deposition period≈10 min. The dependences of the growth rate and the crystallinity on the substrate temperature were studied. By detailed reflection electron diffraction analyses, the crystallinity of β-SiC with 1 μm thickness was found to be better for the layer on the (100) and (110)Si substrates than for that on the (111)Si substrate. An activation energy of 25kcal/mole was obtained for the formation of β-SiC. Optimum conditions to obtain thicker β-SiC films are discussed.     

Specific effects of nanometer scale size on magnetic ordering in La1−xCaxMnO3 (x = 0.1, 0.3 and 0.6) manganites

To check the impact of nano-size originated effects on the magnetic ordering in doped manganites, X-band electron magnetic resonance measurements were performed on nanometer sized and bulk samples of La_1?xCa_xMnO₃ (x = 0.1, 0.3 and 0.6). The model fittings of EPR signal parameters and complementary magnetic measurements indicate that bulk La_0.9Ca_0.1MnO₃ shows inhomogeneous confinement of charge carriers, leading to its mixed magnetic ordering. The carriers mobility within impurity-like band and ferromagnetic (FM)-like ground state are observed on nanocrystals of the same composition. The bulk La_0.7Ca_0.3MnO₃ demonstrates homogeneous FM order and band-like carrier mobility. The surface magnetic disorder in its nano-counterpart leads to appearance of two magnetic phases – a core phase (bulk-like in properties) and a surface phase with notably reduced temperature of magnetic ordering; neither double-exchange interaction nor carriers mobility exist between these phases. The size reduction induced effects in La_0.4Ca_0.6MnO₃ are: an increase of surface FM-like component and a decrease of the charge-ordering temperature. These findings allow us to conclude that the nano-scale effect on magnetic ordering in La_1?xCa_xMnO₃ system weakens progressively upon stabilization of the low temperature magnetic ground state with Ca-doping.     

Soft mode in CsHSO<Subscript>4</Subscript>-type crystals

A superionic phase transition is investigated in CsHSO₄-type crystals. It is demonstrated that the soft mode of this transition is associated with both an orientational ordering of SO₄ tetrahedra and an ordering of hydrogen ions in the crystal lattice. The phase transition is accompanied by distortions of SO₄ tetrahedra and small rotations of the tetrahedra as a whole. The specific orientational basis of the superionic phase transition is determined. The experimental verification of the theoretical predictions is proposed.     

Luminescence characteristics of Yb, La codoped yttrium oxide nanopowders

Luminescence characteristics of Yb³⁺, La³⁺ codoped yttrium oxide nanopowders were investigated. The grain size and the crystallinity of (Yb_0.05Y_0.90La_0.05)₂O₃ nanopowders increase with the increase of calcination temperature. The average grain size of the nanopowders calcined at 1100 °C is 66 nm and its cooperative up-conversion luminescence centered at 498 nm was detected due to nanometer size effect and perfect crystallinity. However, the cooperative up-conversion luminescence of (Yb_0.05Y_0.90La_0.05)₂O₃ transparent ceramics was not detected.     

Effects of synthesis conditions on the yields and properties of HZSM‐5

In the SiO₂‐Al₂O₃‐TPAOH‐NH₄OH system through a two‐step procedure, the effects of synthesis parameters on the yields and properties of HZSM‐5 were studied. As the feeding n(SiO₂)/n(Al₂O₃) ratio increased, the yields of synthesized samples were between 11% and 14% while the crystallinity tended to decrease at the ratio > 300. At the ratio of 150, the yield was 13.7% and the ratio of actual yield to theoretical yield was 93%. The sample shaped ellipse‐like particles of 3 × 10⁻⁶ m. At the n(NH₃)/n(SiO₂) ratio of 1.5, the yield was 13.1% and the sample showed regular ellipse‐like particles of 2 × 10⁻⁶ m. Too high dosage of NH₄OH lowered the yield and the crystallinity. The addition of seed crystal favored the HZSM‐5 synthesis in high crystallinity. At the addition amount of 4%, the yield reached 13.9% and the sample shaped in spherical particles of 0.5 × 10⁻⁶ m. Synthesized HZSM‐5 with a surface area of 455 m²·g⁻¹ and intrinsic acidity showed highly catalytic activity toward the methanol conversion to olefins reaction.     

On the Influence of the Crystallization Conditions on the Microstructure of the Directionally Solidified Eutectic of the LiF – LiYF4 System

This paper particularly deals with the transitions in degree and type of ordering of a given microstructure. As a model system the eutectic of the LiF – LiYF₄ system was used for studies of transitions from ‘anomal’ microtextures with non-cooperative production of seeds of both phases to the fibrous microtexture. It was shown that even if the matrix phase LiYF₄ contains a relative high volume ratio (36%) of LiF it is possible to create highly ordered fibrous microtextures with pronounced hexagonal ordering of particular LiF fibres. It was also shown that methods of image processing could be used for sensitive evaluation of small changes in degree of ordering of the microstructure. These procedures make possible a deeper understanding of the influence of the solidification conditions on the microstructure of the directionally solidified eutectic and creates possibilities for optimalisation of the technological process.