Nanocrystalline Ge filaments in the pores of a mesosilicate

Hydrothermal Synthesis and Properties of Nanostructured Silica Containing Lanthanide Type Ln-SiO2 (Ln = La, Ce, Pr, Nd, Eu, Gd, Dy, Yb, Lu)

The nanostructured lanthanide-silica materials of the Ln-SiO₂ type (Ln = La, Ce, Pr, Nd, Eu, Gd, Dy, Yb, Lu) were synthesized by the hydrothermal method at 100 °C, using cetyltrimethylammonium as a structural template, silica gel and sodium silicate as a source of silicon, and lanthanide oxides, with Si/Ln molar ratio = 50. The resulting materials were calcined at 500 °C using nitrogen and air, and characterized by X-ray diffraction (XRD), Fourier-Transform infrared absorption spectroscopy, scanning electron microscopy, thermogravimetry (TG), surface area by the BET method and acidity measurements by n-butylamine adsorption. The XRD and chemical analysis indicated that the SiO₂ presented a hexagonal structure and the incorporation of lanthanides in the structure changes the properties of the Ln-SiO₂ materials. The heavier the lanthanide element, the higher the Si/Ln ratio. The TG curves showed that the decomposition of the structural template occurs in the materials at temperatures below 500 °C. The samples showed variations in specific surface area, mean pore diameter and silica wall thickness, depending on the nature of the lanthanide. The incorporation of different lanthanides in the silica generated acid sites of varied strength. The hydrothermal stability of the Ln-SiO₂ materials evaluated at high temperatures, evidenced that the properties can be controlled for application in adsorption and catalysis processes.

Investigation of a Ni-Modified MCM-41 Catalyst for the Reduction of Oxygenates and Carbon Deposits during the Co-Pyrolysis of Cellulose and Polypropylene

Catalytic fast co-pyrolysis of biomass and plastic is an effective method to achieve high-quality bio-oil production. In this work, (Ni)-MCM-41 catalysts with different Ni loadings were prepared and characterized in detail by using a variety of advanced analytical techniques, and the effects on the catalytic performance were analyzed by micropyrolysis with gas chromatography mass spectrometry (Py-GC/MS) and thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) methods. The results showed that an appropriate amount of Ni addition can effectively modulate the physicochemical properties of MCM-41. For a Ni loading of 25.1 wt % (Cat-C), the catalyst showed an optimal catalytic performance, a decrease in the proportion of oxygenated compounds in the product from 35.6 (MCM-41) to 13.4%, and an increase in the relative total amount of olefins plus aromatics from 62.2 (MCM-41) to 84.6%. The excellent catalytic performance of Cat-C can be ascribed to a balancing of its proper physical structural properties, appropriate acidity, strong metal-carrier interaction, high metal dispersion, and excellent compatibility balance between active and acidic sites.

Pore shape-reflecting morphosynthesis of lithium niobium oxide via mixed chloride flux growth in the presence of mesoporous silica

A new synthesis method, "chloride flux growth in the rigid nanospace of mesoporous silica", was developed to obtain lithium niobium oxide anisotropic nanoparticles. The morphologies reflect the pore size and shape of the used mesoporous silicas. This method has great potential for synthesizing size-tuned anisotropic nanoparticles of other complex metal oxides.

Hard template synthesis of metal nanowires

Metal nanowires (NWs) have attracted much attention because of their high electron conductivity, optical transmittance, and tunable magnetic properties. Metal NWs have been synthesized using soft templates such as surface stabilizing molecules and polymers, and hard templates such as anodic aluminum oxide, mesoporous oxide, carbon nanotubes. NWs prepared from hard templates are composites of metals and the oxide/carbon matrix. Thus, selecting appropriate elements can simplify the production of composite devices. The resulting NWs are immobilized and spatially arranged, as dictated by the ordered porous structure of the template. This avoids the NWs from aggregating, which is common for NWs prepared with soft templates in solution. Herein, the hard template synthesis of metal NWs is reviewed, and the resulting structures, properties and potential applications are discussed.

Controlling the window size in mesoporous SBA-16

Dotted line: constant mean curvature with Im3̄m symmetry. Solid surface: electron density map of calcined SBA-16.

INORGANIC SEMICONDUCTOR NANOWIRES

One-dimensional (1D) nanostructures are ideal systems for investigating the dependence of electrical transport, optical properties and mechanical properties on size and dimensionality. They are expected to play an important role as both interconnects and functional components in the fabrication of nanoscale electronic and optoelectronic devices. This article presents an overview of current research activities that center on nanowires whose lateral dimensions fall anywhere in the range of 1–200 nm. It is organized into three parts: The first part discusses various methods that have been developed for generating nanowires with tightly controlled dimensions, orientations, and well-defined properties. The second part highlights a number of strategies that are being developed for the hierarchical assembly of nanowire building blocks. The third part surveys some of the novel physical properties (e.g., optical, electrical, and mechanical) of these nanostructures. Finally, we conclude with some personal perspectives on the future research directions in this field.

Characterization of ZnE (E = S, Se, or Te) Materials Synthesized Using Silylated Chalcogen Reagents in Mesoporous MCM-41

The binary materials ZnS, ZnSe, and ZnTe have been successfully prepared within the mesoporous framework of MCM-41 at room temperature through the use of silylated chalcogen reagents. Postsynthesis grafting of ethylenediamine facilitates the complexation of anhydrous zinc acetate to the pore surface via a ligand exchange process between monodentate 3,5-lutidine ligands and the anchored chelating moiety. Coordinated zinc acetate readily reacts with E(SiMe(3))(2) (E = S, Se, or Te), thereby encapsulating zinc chalcogenides in the mesoporous channels. ZnE-MCM-41 materials have been characterized by EDX analysis, nitrogen sorption analysis, and Raman, UV-vis, and solid-state NMR spectroscopy. The observed blue shift in the absorption maximum is in agreement with the expected quantum confinement of these materials given the nanometer dimensions of the mesoporous architecture of the silicate host.

Synthesis, characterization and optical properties of silver and gold nanowires embedded in mesoporous MCM-41

Uniform nanowires of silver and gold inside the channels of MCM-41 were prepared by controlled reduction of their respective metal salts with sodium borohydride (NaBH4). Presence of nanowires of silver and gold in MCM-41 were confirmed by high angle X-ray diffraction (XRD) data (peaks between 2ϑ = 30 − 60°) and transmission electron microscopy (TEM) confirmed the diameter of the nanowires. Diameter of nanowires is found to be ∼ 2.8 nm which is coincident with channel diameter of MCM-41. Optical properties of these heterostructured materials Ag-MCM-41 and Au-MCM-41 reveals the presence of surface plasmon absorption peaks of silver and gold respectively, and the shift in the absorption bands are associated to agglomeration of clusters inside the channels. Room temperature photoluminescence spectra exhibits interesting optical properties as observed for direct band gap semiconductors. Non-linear optical properties (NLO) corresponding to second harmonic generation (SHG) values were also recorded for self supported films of these heterostructured materials. Enhanced optical non-linearity was found to be arising from a corresponding increase of local field near the surface plasmon resonance. Further enhancement in SHG was found with poling due to an induction of orientation order.

Stabilization of the Thermodynamically Favored Polymorph of Cadmium Chalcogenide Nanoparticles CdX (X = S, Se, Te) in the Polar Mesopores of SBA-15 Silica

A versatile synthetic approach to cadmium chalcogenide nanoparticles in the mesopores of SBA-15 silica as a host matrix was developed. The use of cadmium organochalcogenolates of the type Cd(XPh)(2).TMEDA (X = S, Se, Te) allowed the preparation of nanoparticles of all three cadmium chalcogenides following the same experimental protocol. Particles of CdS, CdSe, and CdTe with a particle size of 7 nm were prepared from this class of single-source precursors. The incorporation of the precursor molecules into the pores was achieved by melt infiltration at a temperature of 140 degrees C. Subsequent pyrolysis of the precursors in the mesopores yielded the semiconductor particles. Owing to the high polarity of the silanol-covered pore walls, which lower the surface energy of the particles to a large extent, the dimorphic cadmium chalcogenides are obtained in their thermodynamically favored modifications; e.g., CdS particles crystallize in the wurtzite type, CdTe particles are obtained in the zinc blende structure, and CdSe (where no unambiguous preference exists) crystallizes as a "mixture" of both structures with a rather random stacking sequence.

Assembly of Heteropoly Acid Nanoparticles in SBA-15 and Its Performance as an Acid Catalyst

Keggin-type 12-tungstophosphoric acid (TPA) nanocrystals have been assembled inside the pores of mesoporous silica through a vacuum impregnation method by using large-pore SBA-15 as a nanoreactor. The product was characterized by Brunauer-Emmet-Teller particle size distribution (BET-PSD), NMR and FT-IR spectroscopy, X-ray diffraction (XRD), tranmsission electron microscopy (TEM), differential thermal analysis (DTA) and FT-IR of adsorbed pyridine. The experimental results illustrate that the TPA nanocrystals are excellent Brønsted acid catalytic materials at room temperature.

Pore size engineering in mesoporous silicas using supercritical CO2

In this paper we investigate the use of supercritical carbon dioxide (sc-CO(2)) for synthesizing calcined mesoporous silicas with tunable pore sizes, wall thickness, and d spacings. Small angle neutron scattering was used to probe the controlled swelling of the triblock copolymer surfactant templating agents, P123 (PEO(20)PPO(69)PEO(20)), P85 (PEO(26)PPO(39)PEO(26)), and F127 (PEO(106)PPO(70)PEO(106)), as a function of CO(2) pressure. The transition from the liquid crystal phase to the calcined mesoporous silicas, formed upon condensation and drying, was also studied in detail. Powder X-ray diffraction, transmission electron microscopy, and nitrogen adsorption techniques were used to establish pore diameters, silica wall widths, and the hexagonal packing of the pores within the calcined silicas. Using a direct templating method, the diameters of mesopores and the spacing between the pores could be tuned with a high level of precision. The swelling process was observed to have no detrimental effects on the quality of silica formed, a distinct advantage over conventional swelling techniques, and all of the silicas synthesized in this study were highly ordered over distances of at least 2000 A.

Formation of Zn1-xMnxS Nanowires within Mesoporous Silica of Different Pore Sizes

Arrays of highly ordered Zn(1-x)MnxS quantum wires with x ranging from 0.01 to 0.3 and with lateral dimensions of 3, 6, and 9 nm were synthesized within mesoporous SiO2 host structures of the MCM-41 and SBA-15 type. The hexagonal symmetry of these arrays (space group p6m) and the high degree of order was confirmed by X-ray diffraction and transmission electron microscopy (TEM) studies. Physisorption measurements show the progressive filling of the pores of the SiO2 host structures, while TEM and Raman studies reveal the wire-like character of the incorporated Zn(1-x)MnxS nanostructures. X-ray absorption near-edge structure, extended X-ray absorption fine structure, photoluminescence excitation (PLE), and electron paramagnetic resonance studies confirm the good crystalline quality of the incorporated Zn(1-x)MnxS guest species and, in particular, that the Mn2+ ions are randomly distributed and are situated on tetrahedrally coordinated cation sites of the Zn(1-x)MnxS wires for all x up to 0.3. The amount of Mn2+ ions loosely bound to the surface of the Zn(1-x)MnxS nanowires is less than 4% of the total Mn content even for the 3 nm nanostructures up to the highest Mn content of x = 0.3. The effects of the reduction of the lateral dimensions on electronic properties of the diluted magnetic semiconductor were studied by PLE spectroscopy. Due to the quantum confinement of the excitons in the wires an increase of the direct band gap with decreasing particle size is observed.

Preparation, Characterization, and Condensation of Copper Tellurolate Clusters in the Pores of Periodic Mesoporous Silica MCM-41

The copper-tellurolate cluster [(Cu(6)(TePh)(6)(PPh(2)Et)(5)] has been loaded into the pores of MCM-41 by solid-state impregnation techniques. It was found that the best loading conditions are 110 degrees C and 10(-)(3) Torr static vacuum. The resulting material was analyzed by powder X-ray diffraction (PXRD), nitrogen adsorption isotherms, thermogravimetric analysis (TGA), (31)P CP MAS NMR spectroscopy, and TEM. It was observed that loading is accompanied by loss of the phosphine shell, with retention of the copper-tellurium core. Condensation of the impregnated material may proceed thermally or photochemically. Thermal condensation results in the formation of Cu(2)Te nanoparticles as demonstrated by PXRD, and TEM data suggests that the process has taken place inside the pores of MCM-41. Photochemical condensation yields larger metal-chalcogen clusters in the pores as suggested by the result of UV-vis diffuse reflectance spectroscopy and TEM measurements.

Synthesis and characterization of dimensionally ordered semiconductor nanowires within mesoporous silica

Semiconductor nanowires of silicon have been synthesized within the pores of mesoporous silica using a novel supercritical fluid solution-phase approach. Mesoporous silica, formed by the hydrolysis of tetramethoxysilane (TMOS) in the presence of a triblock copolymer surfactant, was employed for the nucleation and growth of quantum-confined nanowires. The filling of the silica mesopores with crystalline silicon and the anchoring of these nanowires to the sides of the pores were confirmed by several techniques including electron microscopy, powder X-ray diffraction, 29Si magic angle spinning nuclear magnetic resonance, infrared spectroscopy, and X-ray fluorescence. Effectively, the silica matrix provides a means of producing a high density of stable, well-ordered arrays of semiconductor nanowires in a low dielectric medium. The ordered arrays of silicon nanowires also exhibited discrete electronic and photoluminescence transitions that could be exploited in a number of applications, including nanodevices and interconnects.