Preparation and physical properties of the layered niobate Cu0.5Nb3O8: Application to photocatalytic hydrogen evolution

The new layered niobate Cu_0.5Nb₃O₈ is synthesized by soft chemistry in aqueous electrolyte via Cu²⁺→H⁺ exchange between copper nitrate and HNb₃O₈·H₂O. The characterization of the exchanged product is made by means of thermal gravimetry, chemical analysis, X-ray diffraction and IR spectroscopy. Thermal analysis shows a conversion to anhydrous compound above 500 °C. The oxide displays a semiconductor like behavior; the thermal variation of the conductivity shows that d electrons are strongly localized and the conduction is thermally activated with activation energy of 0.13 eV. The temperature dependence of the thermopower is indicative of an extrinsic conductivity; the electrons are dominant carriers in conformity with an anodic photocurrent. Indeed, the Mott–Schottky plot confirms n-type conduction from which a flat band potential of −0.82 V_SCE, an electronic density of 8.72×10¹⁹ m⁻³ and a depletion width of 4.4 nm are determined. The upper valence band, located at ~5.8 eV below vacuum is made up predominantly of Cu²⁺: 3d with a small admixture of O²⁻: 2p orbitals whereas the conduction band consists of empty Nb⁵⁺: 5s level. The energy band diagram shows the feasibility of the oxide for the photocatalytic hydrogen production upon visible light (29 mW cm⁻²) with a rate evolution of 0.31 mL g⁻¹ min⁻¹.     

Synthesis and characterization of g-C3N4/BiFeO3 composites with an enhanced visible light photocatalytic activity

The novel visible light-induced g-C₃N₄/BiFeO₃ composites were successfully synthesized by introducing BiFeO₃ into polymeric g-C₃N₄. The structures and optical properties of composites were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), field-emission transmission electron microscope (TEM), UV–vis diffuse reflection spectroscopy (DRS), respectively. For the degradation of Rhodamine B (RhB), the g-C₃N₄/BiFeO₃ composites exhibited significantly higher visible light photocatalytic activity than that of a single semiconductor. The optimal percentage of doped g-C₃N₄ was 50%. Both photooxidation and photoreduction processes follow first order kinetics. In addition, the stability of the prepared photocatalyst in the photocatalytic process was also investigated. The enhanced photocatalytic performance could be due to the high separation efficiency of the photogenerated electron–holes pairs. The possible photocatalytic mechanism of g-C₃N₄/BiFeO₃ was proposed to guide the further improvement of their photocatalytic activity.     

Synthesis of micro-nano Ag3PO4/ZnFe2O4 with different organic additives and its enhanced photocatalytic activity under visible light irradiation

Several novel micro-nano Ag₃PO₄/ZnFe₂O₄ with excellent magnetic separation property and photocatalytic performance were successfully synthesized using different organic additives for the first time. In the composite, Ag₃PO₄ with flower-like, quadrangular prism and flake structures were obtained when the organic additive is hexadecyl trimethyl ammonium bromide (CTAB), sodium diethyldithiocarbamate (DDTC), or DL-malic acid (DLMA), respectively, while the ZnFe₂O₄ showed uniform spherical structure. From the results of the photocatalytic activity analysis, the Ag₃PO₄/ZnFe₂O₄ gained with the organic additive of DDTC showed the highest photocatalytic capability for 2, 4-dichlorophenol (2, 4-DCP) degradation under visible light irradiation compared with those of CTAB and DLMA as the additives. Moreover, the composition of the composite seriously influences the photocatalytic activity, and when the mass ratio of Ag₃PO₄ and ZnFe₂O₄ in the Ag₃PO₄/ZnFe₂O₄ (DITCH) is 9:1, the apparent photo degradation rate constant of 2, 4-DC is 0.0155 min⁻¹, which is 5.74 times of ZnFe₂O₄ (0.0027 min⁻¹) and 1.89 times of Ag₃PO₄ (0.0082 min⁻¹). Finally, the photocatalytic mechanism of Ag₃PO₄/ZnFe₂O₄ was discussed based on the heterojunction energy-band theory and Z-Scheme theory in detail.     

Low temperature synthesis of pure anatase carbon doped titanium dioxide: An efficient visible light active photocatalyst

Low temperature pure anatase Carbon Doped Titanium Dioxide (C-TiO₂) is successfully synthesized by using starch as an effective, economical, and nonhazardous carbon source. The synthesized C-TiO₂ has been further characterized by X-Ray Diffraction, SEM, TEM, BET, XPS and UV- DRS techniques, which reveal that the particles are crystalline with spherical morphology, high surface area and an optical band gap of 2.79 eV for C-TiO₂ calcined at 400 °C. Furthermore photocatalytic degradation of Rhodamine B dye was carried out using as-prepared C-TiO₂ under visible light irradiation. Prepared C-TiO₂ calcined at 200 °C and 400 °C show higher degradation efficiency (85% and 100% in 120 min respectively) as compared to that of undoped TiO₂ and commercial Degussa P-25. Result shows that the C-TiO₂ containing lower carbon percentage has higher photocatalytic activity. Thus enhanced photocatalytic activity of C-TiO_2, may be due to synergic effect of carbon doping and [101] facet enhanced synthesis of anatase C-TiO₂.     

Preparation and characterization of the brownmillerite Sr2Co2O5 as novel photocatalyst in the hydrogen generation

Sr₂Co₂O₅ is a semiconductor belonging to the brownmillerite family; it is prepared by nitrate route and the photo-electrochemical properties are assessed for the first time for the photocatalytic hydrogen production. Thermal analysis indicates the formation of the semiconductor phase at 750 °C. An optical transition at 1.10 eV, directly allowed is obtained from the diffuse reflectance spectrum, due to the internal Co³⁺: d-d transition in octahedral coordination. A flat band potential of 0.037 V_SCE is determined in KOH solution (0.1 M) from the Mott-Schottky characteristic and the results are relevant for the water reduction. The conduction band of Sr₂Co₂O₅ (−0.85 V_SCE), deriving from Co³⁺: 3d orbital is more cathodic than the potential of H₂O/H₂ couple and hydrogen is successfully evolved under visible light. A rate evolution of 68 µmol (g catalyst)⁻¹ min⁻¹ at pH ∼ 12 and a light-to-chemical energy efficiency of 0.82% are determined.     

Microstructural evolution in the Sn-Cu-Ni and Pb-Sn solder joints with Cu and Pt-Ag metallized Al2O3 substrate

The growth mechanism of intermetallics between solders and metallized substrates, after thermal aging, are investigated. The solders used in this study are unleaded Sn-Cu-Ni solder and eutectic Pb-Sn solder. The Pt-Ag/Al₂O₃, Cu block and the electroless Cu/Pt-Ag/Al₂O₃ are employed as the metallized substrates. Microstructure evolution of the interfacial morphology, elemental, and phase distribution are probed with the aid of electron-probe microanalyzer (EPMA) and x-ray diffractometry. Two kinds of intermetallics, Cu₃Sn and Cu₆Sn₅, are formed at the solder/Cu interface. However, for the solder/Pt-Ag system, only Ag₃Sn is observed at the interface. The thickness of Cu₃Sn, Cu₆Sn₅, and Ag₃Sn compound layers for all solder/metallized substrate systems shows at t^0.5 dependence at 100, 125, 150 and 170 C. According to the calculated activation energy and diffusion constant, the growth rate of Cu₃Sn and Cu₆Sn₅ intermetallics in the electroless Cumetallized substrate is relatively higher than that for Cu block one at the range of 100 C to 170 C. However, the growth rate of Cu₆Sn₅ and Ag₃Sn is reduced in the Sn-Cu-Ni solder with respect to the eutectic Pb-Sn solder. On the other hand, the Sn-Cu-Ni solder system exhibits a thicker Cu₃Sn intermetallic layer than the eutectic Pb-Sn solder after various aging times at 100 C. The thickness of Cu3Sn in the eutectic Pb-Sn solder is, however, thicker than that for Sn-Cu-Ni solder at 170 C.     

Synthesis and semiconducting properties of Na2MnPO4F. Application to degradation of Rhodamine B under UV-light

Na₂MnPO₄F is synthesized by hydrothermal route at 453 K and the physical properties and photo-electrochemical characterizations are reported. The compound crystallizes in a monoclinic system (SG: P 2₁/n) with the lattice constants: a=13.7132 Å, b=5.3461 Å, c=13.7079 Å, β=119.97°. The UV–visible spectroscopy shows an indirect optical transition at 2.68 eV; a further direct transition occurs at 3.70 eV, due to the charge transfer O²⁻: 2p → Mn²⁺: e_g. The thermal variation of the electrical conductivity is characteristic of a semiconducting behavior with activation energy of 39 meV and an electron mobility (µ_318 K=5.56×10⁻⁴ cm² V⁻¹ s⁻¹), thermally activated. The flat band potential (+0.47 V_SCE) indicates that the valence band derives mainly from O²⁻: 2p orbital with a small admixture of F⁻ character while the conduction band is made up of Mn²⁺: t_2g orbital. The electrochemical impedance spectroscopy shows the contribution of both the bulk and grains boundaries. The photocatalytic performance of Na₂MnPO₄F for the degradation of Rhodamine B (RhB) is demonstrated on the basis of the energy diagram. 88% of the initial concentration is degraded under UV light and the oxidation follows a first order kinetic with a rate constant of 0.516 h⁻¹. Neither adsorption nor photolysis is observed. The photoactivity results from the electron transition from the hybridized band (O²⁻, F⁻) to the Mn²⁺: e_g orbital, occurring in the UV region. The catalyst was subjected to three successive photocatalytic cycles, thus proving its long term stability.     

Generation-recombination in disordered organic semiconductor: Application to the characterization of traps

The presence of traps in organic semiconductor based electronic devices affects considerably their performances and their stability. The Shockley-Read-Hall (SRH) model is generally used to extract the trap parameters from the experimental results. In this paper, we propose to adapt the SRH formalism to disordered organic semiconductors by considering a hopping transport process and Gaussian distributions for both mobile and trapped carriers. The model is used to extract multiple trap parameters from charge based Deep Level Transient Spectroscopy (Q-DLTS) spectrum. Calculation of the charge transients are given in detail. The model predicts that the activation energy of the trap should not follow an Arrhenius plot on large temperature ranges. Also, the charge transients are no longer exponential when considering Gaussian trap distributions, enlarging the Q-DLTS peaks. The model fits the Q-DLTS spectra measured on organic diodes with a limited number of trap contributions with a good agreement. It is found that an increase of the material rate of disorder reduces the extracted trap energy distances to the LUMO but has no influence on the extracted trap distribution widths. This work shows the importance of considering the specific properties of organic materials to study their properties and their trap distributions.     

Improved external quantum efficiency of solution-processed P3HT:60PCBM photodetectors by the addition of Cu–In–Se nanocrystals

Photodetectors comprising a hybrid organic–inorganic photoconversion layer are prepared from solution under ambient conditions. Adding Cu–In–Se nanocrystals to a P3HT/60PCBM bulk heterojunction leads to a significant improvement of the maximum external quantum efficiency from 48% to 70% (at wavelength 520 nm) without impacting the temporal response or the linearity of the photodetector devices. This gain in efficiency with the addition of nanocrystals is attributed to the better light harvesting properties of the hybrid devices.     

Investigation of soldering process and interfacial microstructure evolution for the formation of full Cu3Sn joints in electronic packaging

Within electronic products, solder joints with common interfacial structure of Cu/IMCs/Sn-based solders/IMCs/Cu cannot be used under high temperature for relatively low melting points of Sn-based solders (200–300 °C). However, there is a trend for solder joints to service under high temperature because of the objective for achieving multi-functionality of electronic products.With the purpose of ensuring that solder joints can service under high temperature, full Cu₃Sn solder joints with the interfacial structure of Cu/Cu₃Sn/Cu can be a substitute due to the high melting point of Cu₃Sn (676 °C). In this investigation, soldering process parameters were optimized systematically in order to obtain such joints. Further, interfacial microstructure evolution during soldering was analyzed. The soldering temperature of 260 °C, the soldering pressure of 1 N and the soldering time of 5 h were found to be the optimal parameter combination. During soldering of 260 °C and 1 N, the Cu₆Sn₅ precipitated first in a planar shape at Cu-Sn interfaces, which was followed by the appearance of planar Cu₃Sn between Cu and Cu₆Sn₅. Then, the Cu₆Sn₅ at opposite sides continued to grow with a transition from a planar shape to a scallop-like shape until residual Sn was consumed totally. Meanwhile, the Cu₃Sn grew with a round-trip shift from a planar shape to a wave-like shape until the full Cu₃Sn solder joint was eventually formed at 5 h. The detailed reasons for the shape transformation in both Cu₆Sn₅ and Cu₃Sn during soldering were given. Afterwards, a microstructure evolution model for Cu-Sn-Cu sandwich structure during soldering was proposed. Besides, it was found that no void appeared in the interfacial region during the entire soldering process, and a discuss about what led to the formation of void-free joints was conducted.     

Synthesis and characterisation of the thallium‐substituted superconducting cuprate Hg1−xTlxBa2Ca2Cu3O8+δ

The objective of this research is the development of chemical routes for the preparation of high‐temperature superconducting powders. A simple sol–gel synthesis technique for preparing the superconducting compound Hg_1−xTl_xBa₂Ca₂Cu₃O_8+δ (Hg,Tl‐1223) has been refined. A systematic study of the influence of synthesis conditions on the phase purity of the obtained superconducting material is described. We have demonstrated that superconducting Hg_1−xTl_xBa₂Ca₂Cu₃O_8+δ phase of good quality can be obtained by this sol–gel synthesis method. Replacing Hg by Tl in the bulk material significantly increased the superconducting transition temperature. An as‐prepared sample showed T_C(onset)=136 K, but after oxygen treatment the critical temperature of Hg_1−xTl_xBa₂Ca₂Cu₃O_8+δ superconductor increased to 140 K. Copyright © 1999 John Wiley & Sons, Ltd.     

Influence of N2 and O2 annealing treatment on the optical bandgap of polycrystalline Ga2O3:Cu films

Cu-doped Ga₂O₃ thin films were deposited by electron beam evaporation with subsequent annealing at 1000 °C in N₂ and O₂ for 1 h. The influence of the annealing atmosphere on the crystal structure, surface morphology and optical properties of Ga₂O₃:Cu films was investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and transmittance and photoluminescence (PL) spectroscopy. The optical bandgap deduced from the absorption spectrum was greater for the O₂ annealed than for N₂ annealed samples. In both cases the bandgap was wider than for bulk β-Ga₂O₃. The grain size and surface roughness were sensitive to the annealing atmosphere. Results confirmed that the annealed samples were polycrystalline β-Ga₂O₃ with some amorphous phase. We hypothesize that annealing in oxygen led to recrystallization of the Ga₂O₃:Cu film. Annealing treatment improved the crystal quality of Ga₂O₃:Cu films and the PL intensity of the samples increased.     

Synthesis and characterization of Pb(Zr0.54Ti0.46)O3 thin films on (100)Si using textured YBa2Cu3O7−δ and yttria-stabilized zirconia buffer layers by laser physical vapor deposition technique

We have fabricated high-quality <001> textured Pb(Zr_0.54Ti_0.46)O₃ (PZT) thin films on (00l)Si with interposing <001> textured YBa₂Cu₃O_7−δ (YBCO) and yttria-stabilized zirconia (YSZ) buffer layers using pulsed laser deposition (KrF excimer laser, λ, = 248 nm, τ = 20 nanosecs). The YBCO layer provides a seed for PZT growth and can also act as an electrode for the PZT films, whereas YSZ provides a diffusion barrier as well as a seed for the growth of YBCO films on (001)Si. These heterostructures were characterized using x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering techniques. The YSZ films were deposited in oxygen ambient (∼9 × 10⁻⁴ Torr) at 775°C on (001)Si substrate having <001>YSZ // <001>Si texture. The YBCO thin films were deposited in-situ in oxygen ambient (200 mTorr) at 650°C. The temperature and oxygen ambient for the PZT deposition were optimized to be 530°C and 0.4-0.6 Torr, respectively. The laser fluence to deposit this multilayer structure was 2.5-5.0 J/cm². The <001> textured perovskite PZT films showed a dielectric constant of 800-1000, a saturation polarization of 37.81 μC/cm², remnant polarization of 24.38 μC/cm² and a coercive field of 125 kV/cm. The effects of processing parameters on microstructure and ferroelectric properties of PZT films and device implications of these structures are discussed.     

Optimization of CdS buffer layer for high‐performance Cu2ZnSnSe4 solar cells and the effects of light soaking: elimination of crossover and red kink

A spike‐like conduction band alignment of kesterite absorbers with a CdS buffer layer is one of the key factors for high‐performance solar cells using this buffer/absorber heterojunction combination. However, it can also be the origin of fill factor and current‐reducing distortions in current–voltage curves, such as light/dark curve crossover, or an s‐like curve shape for long wavelength monochromatic illumination (red kink) if light‐dependent defect states are present in the buffer layer. In this work, we show that by changing the cadmium precursor source from sulfate to nitrate salts for the chemical bath deposited cadmium sulfide for Cu₂ZnSnSnSe₄/CdS heterojunction solar cells red kink can be eliminated, and crossover greatly improved (and eliminated entirely after light soaking). These improvements lead to a decrease in series resistance and an increase in fill factor and increase power conversion efficiency from 7.0% to 8.2%. We attribute this improvement to a reduction of deep level acceptor‐like traps states inside the CdS layer, which are responsible for an increase of the conduction band spike up to a current blocking value for the sulfate precursor case. Furthermore, the effects of light soaking will be discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

3G网络发展现状及向LTE演进策略分析

文章介绍了目前全球移动网络的发展概况以及国内三大运营商的网络发展格局与现状,分析了网络演进的一般规律与特点,并对Vodafone与KDDI两家具有代表性的国际主流运营商的3G网络演进策略进行了深入探讨,从中总结出国内运营商在未来网络发展过程中可以借鉴的成功经验,提出运营商在规划LTE演进策略时,需循序渐进、分阶段部署;遵循网络发展的阶段性规律,综合考虑技术成熟性、产业链成熟度、数据市场需求及业务开发等因素,把握LTE的最佳发展时机以谋取长远发展。     

Synthesis and characterization of cerium oxide nano-particles in chloride bath: Effect of the H2O2 concentration and bath temperature on morphology

Cerium oxide has been used extensively for various applications over the past two decades. The use of cerium oxide nanoparticles is beneficial in present applications and can open new avenues for future uses. In this paper, some samples of cerium hydroxide/oxide have been synthesized by cathodic deposition on steel electrode in chloride bath in two electrode systems by the galvanostatic mode. The effects of bath temperature and H₂O₂ concentration on particle size and morphology were studied. The effect of calcination temperature on the crystallite growth of cerium oxide nano-powders was investigated by X-ray diffraction. The results were characterized by the differential scanning calorimetry (DSC), thermogramimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The SEM results show that the samples synthesized at low bath temperature have an average size of 40 nm. In the presence of H₂O₂, the cerium oxide films are adherent and smooth. They present very thin cracks whereas with the increase of H₂O₂ concentration the size and number of cracks are reduced. The results show that the as-prepared nanoparticles are essentially amorphous and after heat treatment, the obtained oxide product is well crystallized.     

Solution-Based Approaches to Fabrication of YBa2Cu3O7−δ (YBCO): Precursors of Tri-Fluoroacetate (TFA) and Nanoparticle Colloids

Detailed investigation of superconducting films of YBa₂Cu₃O_7-δ (YBCO) prepared from solution-based precursors have been performed. Two precursors have been compared in this study: the presently used trifluoroacetate (TFA) solution and a recently developed colloidal suspension containing nanoparticles of mixed oxide. Detailed analyses of the evolution of microstructure and chemistry of the films have been performed, and process parameters have been correlated with final superconducting properties. Both films need two heating steps: a low temperature calcination and a higher temperature crystallization step. For TFA films, it was seen that the heating rate during calcination needs to be carefully optimized and is expected to be slow. For the alternate process using a nanoparticle precursor, a significantly faster calcination rate is possible. In the TFA process, the Ba ion remains as fluoride and the Y remains as oxyfluoride after calcination. This implies that, during the final crystallization stage to form YBCO, fluorine-containing gases will evolve, resulting in residual porosity. On the other hand, the film from the nanoparticle process is almost fully oxidized after calcination. Therefore, no gases evolve at the final firing (crystallization) stage, and the film has much lower porosity. The superconducting properties of both types of films are adequate, but the nanoparticle films appear to have persistently higher J _c values. Moreover, they show improved flux pinning in higher magnetic fields, probably due to nanoscale precipitates of a Cu-rich phase. In addition, the nanocolloid films seem to show additionally enhanced flux pinning when doped with minute amounts of second phase precipitates. It therefore appears that, whereas the TFA process is already quite successful, the newly developed nanoparticle process has significant scope for additional improvement. It can be scaled-up with ease, and can be easily adapted to incorporate nanoscale flux pinning defects for in-field performance.     

Gd2O(CO3)2 · H2O Particles and the Corresponding Gd2O3: Synthesis and Applications of Magnetic Resonance Contrast Agents and Template Particles for Hollow Spheres and Hybrid Composites

The solution approach was employed to yield multifunctional amorphous Gd₂O(CO₃)₂ · H₂O colloidal spheres by reflux of an aqueous solution containing GdCl₃ · 6H₂O and urea. By elongating the reaction time, crystalline rhombus‐ shaped Gd₂O(CO₃)₂ · H₂O with at least 87% yield could be formed and were also accompanied by some rectangular particles. High‐resolution synchrotron powder X‐ray diffraction provides crystal structure information, such as cell dimensions, and indexes the exact crystal packing with hexagonal symmetry, which is absent from the Joint Committee on Powder Diffraction Standards file, for the crystalline rhombus sample. Particle formation was studied based on the reaction time and the concentration ratio of [urea]/[GdCl₃ · 6H₂O]. After a calcination process, the amorphous spheres and crystalline rhombus Gd₂O(CO₃)₂ · H₂O particles convert into crystalline Gd₂O₃ at temperatures above 600 °C. For in vitro magnetic resonance imaging (MRI), both Gd₂O(CO₃)₂ · H₂O and Gd₂O₃ species show the promising T₁‐ and T₂‐weighted effects and could potentially serve as bimodal T₁‐positive and T₂‐negative contrast agents. The amorphous Gd₂O(CO₃)₂ · H₂O contrast agent further demonstrates enhanced contrast of the liver and kidney using a dynamic contrast‐enhanced MR imaging (DCE‐MRI) technique for in vivo investigation. The multifunctional capability of the amorphous Gd₂O(CO₃)₂ · H₂O spheres was also evidenced by the formation of nanoshells using these amorphous spheres as the template. Surface engineering of the amorphous Gd₂O(CO₃)₂ · H₂O spheres could be performed by covalent bonding to form hollow silica nanoshells and hollow silica@Fe₃O₄ hybrid particles.