Role of the oriented attachment mechanism in the phase transformation of oxide nanocrystals

"Bottom-up" methods to obtain nanocrystals usually result in metastable phases, even in processes carried out at room temperature or under soft annealing conditions. However, stable phases, often associated with anisotropic shapes, are obtained in only a few special cases. In this paper we report on the synthesis of two well-studied oxides-titanium and zirconium oxide-in the nanometric range, by a novel route based on the decomposition of peroxide complexes of the two metals under hydrothermal soft conditions, obtaining metastable and stable phases in both cases through transformation. High-resolution transmission electron microscopy analysis reveals the existence of typical defects relating to growth by the oriented attachment mechanism in the stable crystals. The results suggest that the mechanism is associated to the phase transformation of these structures.     

How Oriented External Electric Fields Modulate Reactivity

A judiciously oriented external electric field (OEEF) can catalyze a wide range of reactions and can even induce endo/exo stereoselectivity of cycloaddition reactions. The Diels–Alder reaction between cyclopentadiene and maleic anhydride is studied by using quantitative activation strain and Kohn–Sham molecular orbital theory to pinpoint the origin of these catalytic and stereoselective effects. Our quantitative model reveals that an OEEF along the reaction axis induces an enhanced electrostatic and orbital interaction between the reactants, which in turn lowers the reaction barrier. The stronger electrostatic interaction originates from an increased electron density difference between the reactants at the reactive center, and the enhanced orbital interaction arises from the promoted normal electron demand donor–acceptor interaction driven by the OEEF. An OEEF perpendicular to the plane of the reaction axis solely stabilizes the exo pathway of this reaction, whereas the endo pathway remains unaltered and efficiently steers the endo/exo stereoselectivity. The influence of the OEEF on the inverse electron demand Diels–Alder reaction is also investigated; unexpectedly, it inhibits the reaction, as the electric field now suppresses the critical inverse electron demand donor–acceptor interaction.     

表面定向磁性印迹聚合物的制备及对槲皮素的选择性识别

利用硼酸功能化的磁性碳纳米管作为反应基质, 采用一种简便、 绿色的硼酸亲和表面定向印迹法制备了槲皮素磁性分子印迹聚合物, 并将其应用于银杏叶提取物中槲皮素的特异性识别. 透射电子显微镜、 X射线光电子能谱仪、 X射线衍射及振动样品磁强计测试结果表明, 制备的分子印迹聚合物具有良好的形貌和晶型结构. 吸附实验结果表明, 该分子印迹聚合物对模板分子槲皮素具有较好的吸附容量(4.57 μg/mg)、 良好的印迹效果(IF=8.44)和再生能力. 对实际中药样品银杏叶提取物的吸附实验结果表明, 所建立的方法能达到预期的槲皮素检测效果, 可作为中药有效成分槲皮素的特异性识别工具.     

From Precursor Powders to CsPbX3 Perovskite Nanowires: One‐Pot Synthesis,Growth Mechanism,and Oriented Self‐Assembly

The colloidal synthesis and assembly of semiconductor nanowires continues to attract a great deal of interest. Herein, we describe the single‐step ligand‐mediated synthesis of single‐crystalline CsPbBr₃ perovskite nanowires (NWs) directly from the precursor powders. Studies of the reaction process and the morphological evolution revealed that the initially formed CsPbBr₃ nanocubes are transformed into NWs through an oriented‐attachment mechanism. The optical properties of the NWs can be tuned across the entire visible range by varying the halide (Cl, Br, and I) composition through subsequent halide ion exchange. Single‐particle studies showed that these NWs exhibit strongly polarized emission with a polarization anisotropy of 0.36. More importantly, the NWs can self‐assemble in a quasi‐oriented fashion at an air/liquid interface. This process should also be easily applicable to perovskite nanocrystals of different morphologies for their integration into nanoscale optoelectronic devices.     

能力导向教学模式在《天然高分子材料》课程中的应用

为实现应用型本科院校培养高层次技术人才的目标,我们将能力导向教学模式应用到高分子专业选修课程中。在《天然高分子材料》课程教学中,以夯实学生理论知识为前提,以满足学生就业需求为目的,以提高学生关键能力为导向,在传统的讲授法教学基础上,采用演示法、讨论法、案例法及师生易位教学等方法,对教学内容、教学方法和考核方式进行改革,以毕业生调查问卷的形式进行结果跟踪和方案修正,达到了巩固专业能力、增强社会能力、提高方法能力的良好教学效果。     

Highly Enantioselective Intramolecular 1,3‐Dipolar Cycloaddition: A Route to Piperidino‐Pyrrolizidines

Enantioselective catalytic intermolecular 1,3‐dipolar cycloadditions are powerful methods for the synthesis of heterocycles. In contrast, intramolecular enantioselective 1,3‐dipolar cycloadditions are virtually unexplored. A highly enantioselective synthesis of natural‐product‐inspired pyrrolidino‐piperidines by means of an intramolecular 1,3‐dipolar cycloaddition with azomethine ylides is now reported. The method has a wide scope and yields the desired cycloadducts with four tertiary stereogenic centers with up to 99 % ee. Combining the enantioselective catalytic intramolecular 1,3‐dipolar cycloaddition with a subsequent diastereoselective intermolecular 1,3‐dipolar cycloaddition yielded complex piperidino‐pyrrolizidines with very high stereoselectivity in a one‐pot tandem reaction.     

超高温压电陶瓷CaBi2Nb2O9的定向生长及性能

将熔盐法制备的片状CaBi2Nb2O9（CBNO）前驱体在1050℃烧结成陶瓷,对陶瓷晶粒取向和形貌进行了分析和表征,研究了其介电、铁电和压电性能.与传统固相法制备的样品相比较,新工艺制备出的CaBi2Nb2O9陶瓷,烧结温度降低,晶粒定向生长,具有明显织构特征,室温时极化变得容易,介电损耗减小,压电系数明显提高.     

The equivariant topology of stable Kneser graphs

The stable Kneser graph SG_n,k, n?1, k?0, introduced by Schrijver (1978) [19], is a vertex critical graph with chromatic number k+2, its vertices are certain subsets of a set of cardinality m=2n+k. Björner and de Longueville (2003) [5] have shown that its box complex is homotopy equivalent to a sphere, Hom(K₂,SG_n,k)?Sk. The dihedral group D2m acts canonically on SG_n,k, the group C₂ with 2 elements acts on K₂. We almost determine the (C₂×D2m)-homotopy type of Hom(K₂,SG_n,k) and use this to prove the following results.The graphs SG_2s,4 are homotopy test graphs, i.e. for every graph H and r?0 such that Hom(SG_2s,4,H) is (r−1)-connected, the chromatic number χ(H) is at least r+6.If k∉{0,1,2,4,8} and n?N(k) then SG_n,k is not a homotopy test graph, i.e. there are a graph G and an r?1 such that Hom(SG_n,k,G) is (r−1)-connected and χ(G)<r+k+2.     

导流型热蒸发沉积制备微球表面聚酰胺酸涂层

 在自行研制的导流型热蒸发沉积装置上开展了微球表面聚酰胺酸（PAA）涂层制备工艺研究。探讨了单体原料处理和改变升温过程对沉积速率的影响。两种单体分别采用两个相互独立的蒸发源加热蒸发,使用两个晶振膜厚测量探头,通过对膜厚探头、样品盘和导流管端头三者的空间位置和对称关系的调整和实验标定,实现了两种单体近似等化学计量比的沉积。采用间歇性压电振动或敲击配合样品盘的旋转作为微球运动的激励方式,在聚-α-甲基苯乙烯（PAMS）微球上制备出均匀的表面质量好的PAA涂层。     

Diversity-oriented synthesis of amino acids using chiral enolates

We report a facile diversity oriented synthesis of α- and β-amino acids, by utilizing the pluripotent α-methylene group in a chiral bicyclic lactam as our key point of transformation.