掺镧多孔TiO2纳米晶表面电子结构与能量转换机制

采用溶胶-凝胶法制备多孔掺镧纳米晶二氧化钛样品. 结合光声与表面光伏技术, 研究样品的光声和表面光伏特性. 分析认为, 掺镧后在表面上富集的La-O-Ti化合物形成的施主态活性中心吸附氧和H2O分子后生成的活性氧和羟基, 导致了吸附表面光激发电荷转移跃迁的催化光反应(sensitized photoreaction)机制. 实验证实, 适当的镧掺杂不仅可以增强样品的表面光伏特性, 而且可以有效地抑制非辐射跃迁的发生, 提高光量子效率. 实验在指认与样品非辐射退激有关的电荷转移跃迁过程的同时, 证实非辐射跃迁过程与样品表面是否生成活性氧和羟基活性中心无关.     

Role of surface hydroxyl groups in promoting room temperature CO sensing by Pd-modified nanocrystalline SnO2

SnO₂/Pd nanocomposites were synthesized via sol-gel method followed by variable processing procedures. The materials are sensitive to CO gas in the concentration range 2-100 ppm at room operating temperature. It was shown that modification of nanocrystalline tin dioxide by Pd changes the temperature dependence of sensor response, decreasing the temperature of maximal signal. To understand the mechanism of room temperature CO sensitivity, a number of SnO₂/Pd materials were characterized by XRD, TEM, BET, XPS and TPR techniques. From the results of FTIR, impedance and sensing measurements under variable ambient conditions it was concluded that improvement in CO sensitivity for Pd-modified SnO₂ is due to alteration of CO oxidation pathway. The reaction of CO with surface OH-groups at room temperature was proposed, the latter being more reactive than oxygen species due to the possible chain character of the reactions. It was proposed that Pd additive may initiate chain processes at room temperature.     

Hydrothermal synthesis and characterization of nanocrystalline ZrO2 and surface modification with 2-acetoacetoxyethyl methacrylate

For the utilization as inorganic/organic hybrid nanomaterials for optical purposes, nanocrytalline tetragonal ZrO₂ was synthesized by hydrothermal method using zirconium(IV)-n-propoxide as precursor material. Surface of the ZrO₂ particles was then modified with 2-acetoacetoxyethyl methacrylate used as a copolymer for coatings. X-ray diffraction analysis revealed that both ZrO₂ and modified ZrO₂ are in tetragonal crystalline phase. As proved by transmission electron microscope and particle size analysis, average particle sizes of ZrO₂ and modified ZrO₂ were found as 6.22 and 14.7 nm, respectively. ZrO₂ powder was easily dispersed either in water or n-hexane. Ultraviolet diffuse reflectance spectrophotometer analysis for ZrO₂ and surface modified ZrO₂ showed that maximum absorption peaks are at 215 and 225 nm, respectively.     

Doped titanium dioxide nanocrystalline powders with high photocatalytic activity

Doped titanium dioxide nanopowders (M:TiO₂; M=Fe, Co, Nb, Sb) with anatase structure were successfully synthesized through an hydrothermal route preceded by a precipitation doping step. Structural and morphological characterizations were performed by powder XRD and TEM. Thermodynamic stability studies allowed to conclude that the anatase structure is highly stable for all doped TiO₂ prepared compounds. The photocatalytic efficiency of the synthesized nanopowders was tested and the results showed an appreciable enhancement in the photoactivity of the Sb:TiO₂ and Nb:TiO₂, whereas no photocatalytic activity was detected for the Fe:TiO₂ and Co:TiO₂ nanopowders. These results were correlated to the doping ions oxidation states, determined by Mössbauer spectroscopy and magnetization data.     

Nanocrystalline copper(II) oxide catalyzed aza-Michael reaction and insertion of α-diazo compounds into N-H bonds of amines

Nanocrystalline copper(II) oxide efficiently catalyzed the conjugate addition of aliphatic amines to α,β-unsaturated compounds to produce β-amino compounds with excellent yields under mild reaction conditions. Similarly, Glycine esters are obtained in good yields by the insertion of α-diazoacetate into N-H bonds of amines. The catalyst is used for three cycles with minimal loss of activity.     

电沉积纳米镍合金在海水溶液中的析氢性能

采用电沉积法获得Ni、Ni-Fe和Ni-Fe-C合金镀层电极, 在90 °C模拟海水(0.5 mol·L-1 NaCl, pH=12)的稳态极化曲线表明Ni-Fe-C合金电极具有最好的析氢催化性能. 通过扫描电子显微镜(SEM)观察电极表面形貌、X射线衍射(XRD)与透射电子显微镜(HRTEM)分析合金的晶体结构, 发现电极材料的晶粒尺寸影响析氢催化性能, 晶粒尺寸越小析氢催化活性越好. 用电化学阻抗方法(EIS)研究电极析氢催化性能的本质原因, 结果表明电极表面活性点数目和电极的本质电催化活性对合金电极析氢催化活性有重要的影响.     

Diffraction peak profiles of surface relaxed spherical nanocrystals

Abstract

A model is proposed for surface relaxation of spherical nanocrystals. Besides reproducing the primary effect of changing the average unit cell parameter, the model accounts for the inhomogeneous atomic displacement caused by surface relaxation and its effect on the diffraction line profiles. Based on three parameters with clear physical meanings - extension of the sub-coordination effect, maximum radial displacement due to sub-coordination, and effective hydrostatic pressure - the model also considers elastic anisotropy and provides parametric expressions of the diffraction line profiles directly applicable in data analysis. The model was tested on spherical nanocrystals of several fcc metals, matching atomic positions with those provided by Molecular Dynamics (MD) simulations based on embedded atom potentials. Agreement was also verified between powder diffraction patterns generated by the Debye scattering equation, using atomic positions from MD and the proposed model.     

A study on optical,photoluminescence and thermoluminescence properties of ZnO and Mn doped-ZnO nanocrystalline particles

Pure ZnO and Mn (1%wt.) doped-ZnO nanocrystalline particles were synthesized by reverse micelle method. The structural properties of the nanoparticles were investigated by X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) techniques. UV–vis and photoluminescence (PL) spectroscopy was used for analyzing the optical properties of the nanoparticles. XRD and TEM results revealed the formation of ZnO and Mn doped-ZnO nanocrystalline particles with pure wurtzite crystal structure and average particle size of 18–21 nm. From UV–vis studies, the optical band gap energy of 3.53 and 3.58 eV obtained for ZnO and Mn doped-ZnO nanoparticles, respectively. Further optical analysis showed that the refractive index decreases from 2.35 to 1.35 with the change of wavelength. Room-temperature photoluminescence analysis of all samples showed four main emission bands including a strong UV emission band, a weak blue band, a week blue–green band, and a weak green band which indicated their high structural and optical quality. Moreover, the samples exposed to gamma rays sources of ¹³⁷Cs and ⁶⁰Co and their thermoluminescence properties were investigated. The thermoluminescence response of ZnO and Mn doped-ZnO nanocrystalline particles as a function of dose exhibited good linear ranges, which make them very promising detectors and dosimeters suitable for ionizing radiation.     

Preparation of nanocrystalline cellulose via ultrasound and its reinforcement capability for poly(vinyl alcohol) composites

Rod-shaped nanocrystalline cellulose (NCC) was prepared from microcrystalline cellulose (MCC) using the purely physical method of high-intensity ultrasonication. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction was used for the characterization of the morphology and crystal structure of the material. The thermal properties were investigated using thermogravimetric analysis. The reinforcement capabilities of the obtained NCC were investigated by adding it to poly(vinyl alcohol) (PVA) via the solution casting method. The results revealed that the prepared NCC had a rod-shaped structure, with diameters between 10 and 20 nm and lengths between 50 and 250 nm. X-ray diffraction results indicated that the NCC had the cellulose I crystal structure similar to that of MCC. The crystallinity of the NCC decreased with increasing ultrasonication time. The ultrasonic effect was non-selective, which means it can remove amorphous cellulose and crystalline cellulose. Because of the nanoscale size and large number of free-end chains, the NCC degraded at a slightly lower temperature, which resulted in increased char residue (9.6-16.1%), compared with that of the MCC (6.2%). The storage modulus of the nanocomposite films were significantly improved compared with that of pure PVA films. The modulus of PVA with 8 wt.% NCC was 2.40× larger than that of pure PVA.     

Exchange-coupled nanoscale SmCo/NdFeB hybrid magnets

Nanoscale hybrid magnets containing SmCo₅ and Nd₂Fe₁₄B hard magnetic phases have been produced via a novel “in-one-pot” processing route. The grain size of the processed bulk composite materials is controlled below 20 nm. The refinement of the nanoscale morphology leads to effective inter-phase exchange coupling that results in single-phase like magnetic properties. Energy product of 14 MGOe was obtained in the isotropic nanocomposite magnets at room temperature. At elevated temperatures, the hybrid magnets have greatly improved thermal stability compared to the Nd₂Fe₁₄B single-phase counterpart and have substantially increased magnetization and energy products compared to the single-phase SmCo₅ counterpart.