Conversion of Mg‐Li Bimetallic Alloys to Magnesium Alkoxide and Magnesium Oxide Ceramic Nanowires

Technologically important composites with enhanced thermal and mechanical properties rely on the reinforcement by the high specific strength ceramic nanofibers or nanowires (NWs) with high aspect ratios. However, conventional synthesis routes to produce such ceramic NWs have prohibitively high cost. Now, direct transformation of bulk Mg‐Li alloys into Mg alkoxide NWs is demonstrated without the use of catalysts, templates, expensive or toxic chemicals, or any external stimuli. This mechanism proceeds through the minimization of strain energy at the boundary of phase transformation front leading to the formation of ultra‐long NWs with tunable dimensions. Such alkoxide NWs can be easily converted in air into ceramic MgO NWs with similar dimensions. The impact of the alloy grain size and Li content, synthesis temperature, inductive and steric effects of alkoxide groups on the diameter, length, composition, ductility, and oxidation of the produced NWs is discussed.     

Organic‐Stabilizer‐Free Polyol Synthesis of Silver Nanowires for Electrode Applications

The polyol reduction of a Ag precursor in the presence of an organic stabilizer, such as poly(vinylpyrrolidone), is a widely used method for the production of Ag nanowires (NWs). However, organic capping molecules introduce insulating layers around each NW. Herein we demonstrate that Ag NWs can be produced in high yield without any organic stabilizers simply by introducing trace amounts of NaCl and Fe(NO₃)₃ during low‐temperature polyol synthesis. The heterogeneous nucleation and growth of Ag NWs on initially formed AgCl particles, combined with oxidative etching of unwanted Ag nanoparticles, resulted in the selective formation of long NWs with an average length of about 40 μm in the absence of a capping or stabilizing effect provided by surface‐adsorbing molecules. These organic‐stabilizer‐free Ag NWs were directly used for the fabrication of high‐performance transparent or stretchable electrodes without a complicated process for the removal of capping molecules from the NW surface.     

Modification of ceramic membranes by alcohol adsorption

This study aims at modifying ceramic membranes by means of alcohol chemisorption. Composite ceramic membranes with a skin layer of γ-alumina were used. First, the adsorption of several alcohol on powdered γ-alumina was investigated emphasising the thermal stability of the adsorbed compounds. Later, a commercial γ-alumina membrane was modified by alcohol adsorption. The permeability of water and several organic compounds was obtained for both the non-modified and modified ceramic membrane. Also, its isoelectric point was determined. The results prove that all the alcohol were readily adsorbed on powdered γ-alumina not only physically but also chemically forming an alkoxide. The chemisorbed alcohol was stable up to 200°C. Beyond this temperature, the alkoxide breaks up releasing the alcohol although the alkoxide also can react yielding an olefin or ether. The ceramic membrane was also successfully modified by alcohol adsorption. The layer of chemisorbed alcohol imparts hydrophobic characteristics to the membrane surface, so water permeability decreases significantly. This cannot be merely explained by pore size reduction due to the adsorbed layer. Thermal treatment at 250°C recovered original permeability with only minor damage to the membrane.     

Synthesis and alignment of silver nanorods and nanowires and the formation of Pt, Pd, and core/shell structures by galvanic exchange directly on surfaces

Here we describe the synthesis of Ag nanorods (NRs) (aspect ratio <20) and nanowires (NWs) (aspect ratio > or =20) directly on surfaces by seed-mediated growth. The procedure involves attaching gold seed nanoparticles (Au NPs) to 3-mercaptopropyltrimethoxysilane (MPTMS)-functionalized silicon or glass surfaces and growing them into NRs/NWs by placing the substrates into a solution containing cetyltrimethylammonium bromide (CTAB), silver nitrate, and ascorbic acid with the pH ranging from 7 to 12. Under our conditions, Ag NRs/NWs grow optimally at pH 10.6 with a 3% yield, where spherical, triangular, and hexagonal nanostructures represent the other byproducts. The length of Ag NRs/NWs ranges from 50 nm to more than 10 microm, the aspect ratio (AR) ranges from 1.4 to >300, and the average diameter is approximately 35 nm. Approximately 40% of the 1D structures are NRs, and 60% are NWs as defined by their ARs. We also report the alignment of Ag NRs/NWs directly on surfaces by growing the structures on amine-functionalized Si(100) surfaces after an amidation reaction with acetic acid and a method to improve the percentage of Ag NRs/NWs on the surface by removing structures of other shapes with adhesive tape. Surface-grown Ag NRs/NWs also react with salts of palladium, platinum, and gold via galvanic exchange reactions to form high-surface-area 1D structures of the corresponding metal. The combination of the seed-mediated growth of Ag on Au NRs followed by the galvanic exchange of Ag with Pd leads to interesting core/shell NRs grown directly on surfaces. We used scanning electron microscopy, UV-vis spectroscopy, and X-ray photoelectron spectroscopy to characterize the surface-grown nanostructures.     

Hydrogen surface passivation of Si and Ge nanowires: A semiempirical approach

A semiempirical nearest‐neighbor tight‐binding approach, that reproduces the indirect band gaps of elemental semiconductors, has been applied to study the electronic and optical properties of Si and Ge nanowires (NWs). The calculations show that Si‐NWs keep the indirect bandgap whereas Ge‐NWs changes into the direct bandgap when the wire cross section becomes smaller. Also, the band gap enhancement of Si‐NWs showing to quantum confinement effects is generally larger than that of similar‐sized Ge‐NWs, confirming the larger quantum confinement effects in Si than in Ge when they are confined in two dimensions. Finally, the dependence of the imaginary part of the dielectric function on the quantum confinement within two different schemes: intra‐atomic and interatomic optical matrix elements are applied. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2448–2454, 2010     

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.     

Tailoring the porous hierarchy and the tetrahedral aluminum content by using carboxylate ligands: hierarchically structured macro-mesoporous aluminosilicates from a single molecular source

A novel yet facile synthesis pathway has been developed for the design of hierarchically structured macro-mesoporous aluminosilicates with high aluminum content at tetrahedral sites using a single molecular bifunctional alkoxide (sec-BuO)(2)-Al-O-Si(OEt)(3) precursor. The use of carboxylate ligands and a highly alkaline media slow down the polymerization rate of the aluminum alkoxide functionality, thus permitting the preservation of the intrinsic Al-O-Si linkage. The hierarchically structured porous aluminosilicate materials present an unprecedented low Si/Al ratio close to 1. Heat treatment applied to the synthesized material seems to favor the incorporation of aluminum into tetrahedral position (intraframework aluminum species). The macro-mesoporosity was spontaneously generated, without the use of any external templating agent, by the hydrodynamic flow of the solvents released during the rapid hydrolysis and condensation processes of this double alkoxide. This method results in materials with an open array of interconnected macrochannels. The synthesized aluminosilicate materials with tailorable macro-mesoporous hierarchy and very high Al content at tetrahedral position hold huge promise in various applications as catalysts, catalysts supports, or adsorbents.     

Synthesis and Crystal Structures of Gold Nanowires with Gemini Surfactants as Directing Agents

The preparation of crystalline gold nanowires (NWs) by using gemini surfactants as directing agents through a three‐step seed‐mediated method is reported. Unlike the nanorods with relatively low aspect ratios (typically below 20) obtained by using cetyltrimethylammonium bromide as a directing agent, the NWs obtained in this investigation can reach up to 4.4 μm, and the largest aspect ratio is calculated to be 210. For this, each of seven different gemini surfactants are utilized as directing agents, and the length and/or aspect ratio of the NWs can be tuned by varying the hydrocarbon chain lengths of the gemini surfactants. Both single and twinned crystalline structures are elucidated by selected‐area electron diffraction and high‐resolution transmission electron microscopy studies. The use of gemini surfactants not only advances the synthesis of gold nanostructures, but improves the understanding of the growth mechanism for seed‐mediated growth.     

Si衬底上生长的铁硅化合物结构和取向分析

采用分子束外延法分别在650-920℃的Si(110)和920℃的Si(111)衬底表面生长出铁的硅化物纳米结构,并主要分析了920℃高温下纳米结构的形貌、组成相及其与Si衬底的取向关系.扫描隧道显微镜(STM)研究表明,920℃高温下,Si(110)衬底上生长的铁硅化合物完全以纳米线的形式存在,且其尺寸远大于650℃低温下外延生长的纳米线尺寸;Si(111)衬底上生长出三维岛和薄膜两种形貌的铁硅化合物,其中三维岛具有金属特性且直径约300 nm、高约155 nm,薄膜厚度约2 nm.电子背散射衍射研究表明920℃高温下Si(110)衬底上生长的纳米线仅以β-FeSi2的形式存在,且β-FeSi2相与衬底之间存在唯一的取向关系:β-FeSi2(101)//Si(11 1);β-FeSi2[010]//Si[110];Si(111)衬底上生长的三维岛由六方晶系的Fe2Si相组成,Fe2Si属于164空间群,晶胞常数为a=0.405 nm,c=0.509 nm;与衬底之间的取向关系为Fe2Si(001)∥Si(111)和Fe2Si[1 20]//Si[112].     

Development of Asymmetric Reactions Catalyzed by Chiral Organotin‐Alkoxide Reagents

Asymmetric catalysis under almost‐neutral reaction conditions is key for the efficient synthesis of optically active polar molecules. We have developed catalytic enantioselective reactions of acyclic or cyclic alkenyl esters by using an (S)‐BINOL‐derived chiral tin‐dibromide reagent that possesses a bulky aryl group at the 3 or 3′ position as the chiral pre‐catalyst in the presence of a sodium alkoxide and an alcohol, in which a chiral tin alkoxide bromide is generated in situ and recycled with the assistance of an alcohol. In this Personal Account, we describe three types of asymmetric transformation that proceed through a chiral tin enolate: 1) The asymmetric aldol reaction of alkenyl esters or unsaturated lactones with aldehydes or isatins; 2) the asymmetric three‐component Mannich‐type reaction of alkenyl esters and related cycloaddition reactions; and 3) the asymmetric N‐nitroso aldol reaction of unsaturated lactones with nitrosoarenes.     

Surface engineering on segmented copper-iron nanowires arrays

Surface engineering that could modulate the surface shape to be endowed with the high specific surface ratio, abundant chemical dangling bonds and improved defects exposure is highly desired and needs further exploring. Here, we report a facile strategy of surface engineering on decorating the controllable segmented copper-iron nanowires arrays (Cu-Fe NWs) with their respective hydroxides. Specifically, the pristine segmented Cu-Fe NWs are firstly synthesized via sequentially electrodepositing Cu NWs and Fe NWs inside the nanochannels of anode aluminum oxide (AAO) template. Subsequently, the surface and interface of Cu-Fe NWs are wet-chemically etched, in which the metallic Cu and Fe are partially converted into Cu(OH)_x nano-fibrous roots (NFRs) and FeO(OH)_y nanoparticles (NPs), and finally decorate around the respective outer-surface of Cu NWs and Fe NWs segments. As one case of the applications in hydrogen evolution reaction (HER), our surface-modified Cu-Fe NWs exhibit improved catalytic activity compared with Fe NWs.     

Amplified ultraviolet detection of natural DNA based on Mo6S9-xIx nanowires

We demonstrate that Mo_6S_(9_x)I_x nanowires(MoSI NWs) can serve as an excellent signal-intensifying nanomaterial for highly sensitive and label-free detection of DNA by ultraviolet(UV) spectrophotometry.The DNA extinction at 260 nm was greatly enhanced after addition of MoSI NWs solute,and the extinction value was linear with DNA concentration in the range of 0.0289-11.68 μg/mL with the real determination limit of 28.9 ng/mL.The association of DNA with the nanowires was characterized by transmission electronmicroscopy and circular dichroic spectroscopy.The results illustrate that the UV response amplification of DNA in the presence of MoSI NWs is attributed to the greater DNA coverage on the MoSI NW surface and the conformational transformation of DNA due to interaction of DNA with MoSI NWs.MoSI NWs are a promising nano-structured material for developing ultrasensitive sensors for detection of DNA.     

Ultralong PtPd Alloyed Nanowires Anchored on Graphene for Efficient Methanol Oxidation Reaction

The rational synthesis of Pt-based alloyed nanowires still remains a great challenge because of the different reduction potentials between Pt and another metal and the intrinsic feature of isotropic growth in face-centered cubic (fcc) structured Pt. In this work, PtPd alloyed nanowires with ultrahigh aspect ratio anchored on graphene (PtPd NWs/graphene) were synthesized by a facile solvothermal method without the use of any templates or surfactants. Due to the integration of ultralong PtPd nanowires and stable graphene support, PtPd NWs/graphene exhibited outstanding electrochemical activity toward methanol oxidation reaction (MOR) in comparison with pure Pt NWs/graphene and commercial Pt/C catalysts. Meanwhile, PtPd NWs/graphene had a much higher current density than Pt NWs/graphene and commercial Pt/C catalysts at a constant potential for 7200s in alkaline methanol solution. Moreover, after 1000 cycles of durability testing, PtPd NWs/graphene retained 89.2% of its initial mass activity, much superior to the 63.7% retained for commercial Pt/C.     

Reactions of Magnesium and Titanium Alkoxides. Preparation and Characterization of Alkoxy-Derived Magnesium Titanate Powders and Ceramics

The interaction between magnesium and titanium alkoxides is studied in order to chose the best precursors for synthesis of MgTiO₃. No reaction between magnesium and titanium methoxides and isopropoxides occurs. The solubility diagrams for Mg(OR)₂-Ti(OR)₄-ROH, R = Et,-Bu at 20°C are studied. Magnesium ethoxotitanates of variable composition Mg_nTi_4-n (OEt)_16-2n2nEtOH (n=2.0-0) which are structural analogs of Ti₄(OR)₁₆ (R = Me, Et) are isolated. This is a quite unusual example of statistical distribution of heteroatoms in molecular structures of metal alkoxides. Among the systems of metal alkoxides with simple aliphatic radicals only Mg(OBu)₂-Ti(OBu)₄-BuOH gives a convenient precursor for the synthesis of MgTiO₃. A simple scheme of preparation of magnesium titanate from the alkoxide solutions is suggested. The phase purity of MgTiO₃ is to a considerable extent dependent on the hydrolysis conditions. The alkoxy-derived magnesium titanate is obtained in the form of a uniform fine powder, it can be sintered into dense ceramics in the temperature range of 1140–1220°C which is 150–200°C lower in comparison with the conventional powders.     

Si衬底上生长的铁硅化合物结构和取向分析

采用分子束外延法分别在650-920 ℃的Si（110）和920 ℃的Si（111）衬底表面生长出铁的硅化物纳米结构,并主要分析了920 ℃高温下纳米结构的形貌、组成相及其与Si 衬底的取向关系. 扫描隧道显微镜（STM）研究表明,920 ℃高温下,Si（110）衬底上生长的铁硅化合物完全以纳米线的形式存在,且其尺寸远大于650 ℃低温下外延生长的纳米线尺寸;Si（111）衬底上生长出三维岛和薄膜两种形貌的铁硅化合物,其中三维岛具有金属特性且直径约300 nm、高约155 nm,薄膜厚度约2 nm. 电子背散射衍射研究表明920 ℃高温下Si（110）衬底上生长的纳米线仅以β-FeSi₂的形式存在,且β-FeSi₂相与衬底之间存在唯一的取向关系：β-FeSi₂（101）//Si（111）;β-FeSi₂ [010]//Si[110];Si（111）衬底上生长的三维岛由六方晶系的Fe₂Si 相组成,Fe₂Si 属于164 空间群,晶胞常数为a=0.405 nm,c=0.509 nm;与衬底之间的取向关系为Fe₂Si（001）∥Si（111）和Fe₂Si[120]//Si[112].     

Ceramic membranes modified with catalytic oxide films as ensembles of catalytic nanoreactors

Hybrid catalytic membrane systems have been produced by modifying porous ceramic membranes with metal oxide films. A two-layer cermet membrane consisting of a flexible stainless steel layer and an overlying porous TiO₂ ceramic layer and a ceramic titanium carbide membrane are examined. The membrane surfaces have been modified by the alkoxide method using colloidal organic solutions of metal complex precursors. Producing a tetragonal single-phase ZrO₂/Y₂O₃ coating on the cermet surface increases the abrasion strength of the ceramic layer. CO oxidation and the oxidative conversion of methane into synthesis gas and light hydrocarbons can be markedly intensified by modifying the membrane channels with Cu_0.03Ti_0.97O_2±δ and La + Ce/MgO catalysts, respectively. A method has been developed for depositing, onto the geometrical surface of a membrane, a film of the new single-phase oxide P_0.03Ti_0.97O_2±δ with an anatase structure and uniform pores of mean diameter 〈d〉 ～ 2 nm. Blocks of zeolite-like silicalite can be formed on the surface of the phosphorus-titanium oxide film. The resulting hybrid membrane is characterized by an anisotropic permeability depending on the flow direction. This property has an effect on conversion and selectivity in the nonoxidative dehydrogenation of methanol.     

Anionic graft polymerization of ethylene oxide on starch. I

Graft polymers of polyethylene oxide on various starches were obtained by anionic graft polymerization of ethylene oxide on the starch alkoxide derivatives. The polyalkoxides were prepared by reaction of potassium naphthalene with starch in DMSO solution. It was found that increase of monomer or alkoxide concentration led to transformation of the grafts from solids to syrups. Rice starch, having a more complex structure than soluble starch or wheat starch, led to graft polymers having higher melting ranges than the others. The graft polymers were very soluble in water or methanol.     

Manganese(III) Porphyrin-Catalyzed Dehydrogenation of Alcohols to form Imines,Tertiary Amines and Quinolines

Manganese(III) porphyrin chloride complexes have been developed for the first time as catalysts for the acceptorless dehydrogenative coupling of alcohols and amines. The reaction has been applied to the direct synthesis of imines, tertiary amines and quinolines where only hydrogen gas and/or water are formed as the by-product(s). The mechanism is believed to involve the formation of a manganese(III) alkoxide complex which degrades into the aldehyde and a manganese(III) hydride species. The latter reacts with the alcohol to form hydrogen gas and thereby regenerates the alkoxide complex.     

Shape and composition control of Bi19S27(Br3–x,Ix) alloyed nanowires: the role of metal ions

We present the first colloidal synthesis of highly uniform single-crystalline Bi₁₉S₂₇Br₃ nanowires (NWs) with a mean diameter of ∼9 nm and tunable lengths in the range of 0.15–2 μm in the presence of foreign metal ions (Al³⁺). The Al³⁺ ions not only control the growth of NWs, but also achieve species transformation, i.e., from Bi₂S₃ to Bi₁₉S₂₇Br₃, and are not present in the resulting NWs. This colloidal chemistry approach can be expanded to prepare a family of single-crystalline Bi₁₉S₂₇(Br_3–x,I_x) alloyed NWs with controlled compositions (0 ≤ x ≤ 3). Interestingly, these alloyed NWs show an unusual composition-independent band gap of ∼0.82 eV, and theoretical calculations indicate that this phenomenon comes from the very minor contributions of the halogens to the valence band maximum and conduction band minimum. The photodetectors made of Bi₁₉S₂₇(Br_3–x,I_x) alloyed NWs show a pronounced photoresponse with high stability and reproducibility, which makes the NWs potentially useful candidates in optoelectronic devices.     

Mononuclear Dysprosium Alkoxide and Aryloxide Single-Molecule Magnets

Recent studies have shown that mononuclear lanthanide (Ln) complexes can be high-performing single-molecule magnets (SMMs). Recently, there has been an influx of mononuclear Ln alkoxide and aryloxide SMMs, which have provided the necessary geometrical control to improve SMM properties and to allow the intricate relaxation dynamics of Ln SMMs to be studied in detail. Here non-aqueous Ln alkoxide and aryloxide chemistry applied to the synthesis of low-coordinate mononuclear Ln SMMs are reviewed. The focus is on mononuclear Dy^III alkoxide and aryloxide SMMs with coordination numbers up to eight, covering synthesis, solid-state structures and magnetic attributes. Brief overviews are also provided of mononuclear Tb^III, Ho^III, Er^III and Yb^III alkoxide and aryloxide SMMs.