Pulse deposition of large area,patterned manganese oxide nanowires in variable aspect ratios without templates

Manganese oxide nanowires with β-MnOOH in core and Mn₃O₄ in shell were successfully plated onto various conductive substrates from a Mn(CH₃COO)₂ solution by anodic deposition under a two-electrode, pulse-rest mode. The aspect ratio of uniform nanowire morphologies in cm² scale is controllable by varying the deposition variables. Patterned MnO_x nanowire arrays were obtained by combining lithographic and electroplating techniques demonstrated to be a powerful method for preparing MnO_x nanowires in the field emission (FE) array cathodes with a low turn-on voltage (∼3.4 V/μm at 1 μA/cm²).     

Hydrothermal synthesis of one-dimensional ZnO nanostructures with different aspect ratios

1-D ZnO nanorods with different aspect ratios were synthesized by a one-step, hydrothermal method. The ZnO nanorods grow along the [0001] direction to form single crystals. The experimental results reveal that the growth of polar inorganic crystals is sensitive to the reaction solvents.     

Thermal decomposition of hydrotalcites with variable cationic ratios

Thermal analysis complimented with evolved gas mass spectrometry has been applied to hydrotalcites containing carbonate prepared by coprecipitation and with varying divalent/trivalent cation ratios. The resulting materials were characterised by XRD, and TG/DTG to determine the stability of the hydrotalcites synthesised. Hydrotalcites of formula Mg₄(Fe,Al)₂(OH)₁₂(CO₃)·4H₂O, Mg₆(Fe,Al)₂(OH)₁₆(CO₃)·5H₂O, and Mg₈(Fe,Al)₂(OH)₂₀(CO₃)·8H₂O were formed by intercalation with the carbonate anion as a function of the divalent/trivalent cationic ratio. XRD showed slight variations in the d-spacing between the hydrotalcites. The thermal decomposition of carbonate hydrotalcites consists of two decomposition steps between 300 and 400°C, attributed to the simultaneous dehydroxylation and decarbonation of the hydrotalcite lattice. Water loss ascribed to dehydroxylation occurs in two decomposition steps, where the first step is due to the partial dehydroxylation of the lattice, while the second step is due to the loss of water interacting with the interlayer anions. Dehydroxylation results in the collapse of the hydrotalcite structure to that of its corresponding metal oxides and spinels, including MgO, MgAl₂O₄, and MgFeAlO₄.     

Gas chromatography in open rectangular channels with large aspect ratios

Summary Gas chromatographic experiments have been performed in a truly rectangular channel with a 601 cross-sectional aspect ratio, constructed according to the field-flow fractionation technology. It is shown that the peak broadening is controlled by the thickness of the channel while the maximum load is determined by the largest cross-sectional dimension. However, even with its elongated configuration, the side walls contribute significantly to the band broadening of solutes with a high diffusivity. Good agreement is found between the experimental plate height data and the recent theoretical calculations of Golay [13] taking into account the side-wall effect.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Mechanisms of atomic layer deposition on substrates with ultrahigh aspect ratios

Atomic layer deposition (ALD) appears to be uniquely suited for coating substrates with ultrahigh aspect ratios (> or similar 10(3)), including nanoporous solids. Here, we study the ALD of Cu and Cu3N on the inner surfaces of low-density nanoporous silica aerogel monoliths. Results show that Cu depth profiles in nanoporous monoliths are limited not only by Knudsen diffusion of heavier precursor molecules into the pores, as currently believed, but also by other processes such as the interaction of precursor and reaction product molecules with pore walls. Similar behavior has also been observed for Fe, Ru, and Pt ALD on aerogels. On the basis of these results, we discuss design rules for ALD precursors specifically geared for coating nanoporous solids.     

Synthesis and characterization of wurtzite ZnTe nanorods with controllable aspect ratios

ZnTe nanorods with controllable aspect ratios were synthesized using polytellurides a tellurium precursor. The use of polytellurides which allow nucleation and growth at relatively low temperature is the key to formation of wurtzite phase and controlled anisotropic growth along c-axis. The aspect ratio of the resulting ZnTe nanorods was controlled by tuning the temperature that in turn controls the kinetics of the nanocrystal growth. A diameter dependent quantum confinement effect in ZnTe nanorods was observed by UV-vis absorption spectroscopy. Transient absorption measurements show ultrafast charge injection dynamics from ZnTe nanorods, suggesting their strong potential for applications in photocatalysis.     

LEV-zeotype magnesium aluminophosphates with variable Mg/Al ratios

By using two different organic amines, n-methylpiperidine and 1,2-diaminocyclohexane (DACH), as the structure-directing agents (SDAs), two magnesium aluminophosphate molecular sieves |(C(6)H(14)N)(2)|[Mg(2)Al(7)(PO(4))(9)] and |(C(6)H(14)N(2))(1.4)(H(2)O)(2)H(2)|[Mg(2)Al(5)(PO(4))(7)], denoted as MgAPO-CJ67(a) and MgAPO-CJ67(b), have been synthesized under hydrothermal conditions. They are the first example of Mg incorporated aluminophosphates with the LEV-zeotype structure. The magnesium and aluminum atoms co-occupy the same crystallographic positions in the frameworks of MgAPO-CJ67(a) and MgAPO-CJ67(b), but with different Mg/Al ratios of 1 : 3.5 and 1 : 2.5, respectively. MgAPO-CJ67(a) exhibits higher thermal stability upon calcination than MgAPO-CJ67(b), and its framework keeps intact after the removal of occluded organic SDAs. The N(2) adsorption on the calcined MgAPO-CJ67(a) sample gives the BET surface area of 386 m(2) g(-1). NH(3)-temperature programmed desorption (NH(3)-TPD) analysis shows that MgAPO-CJ67(a) has a medium acidity.     

Packing density of slurry-packed capillaries at low aspect ratios

The average interparticle voidage or porosity (epsilon(inter)) in cylindrical capillaries is studied in dependence on the column diameter (d(c)) to particle diameter (d(p)) ratio for 5 < d(c)/d(p) < 50. Using optimized slurry and packing solvents, high pressure and ultrasonication, 5 mum-sized porous C18-silica particles were slurry-packed into fused-silica capillaries having ids from 30 to 250 mum. Packing densities are assessed by a polystyrene standard which is size-excluded from the intraparticle pore space of the packings. For d(c)/d(p) > 35 densely packed beds are realized (epsilon(inter) = 0.36-0.37), while for decreasing aspect ratios an exponential increase in epsilon(inter )is observed reaching epsilon(inter ) approximately 0.47 at d(c)/d(p) = 5. This behaviour is ascribed to a combination of the geometrical wall effect operating in the direct vicinity of the column wall, caused by the inability of the particles to form a dense packing against the hard surface of the column wall, and particle characteristics like the size distribution, shape and surface roughness. Results are compared with the literature data to address also the importance of absolute particle size in studying structure-transport relations in packed beds in dependence on the aspect ratio d(c)/d(p).     

Quantitative analysis of gaseous diffusion in glassy polymers

The current theoretical treatment of diffusion of gases in glassy polymers is based on the “dual sorption” model, but also includes certain other important assumptions, at least some of which cannot be fully justified a priori. They require experimental validation, which, however, is not possible by the procedures used or proposed so far. Methods suitable for this purpose are discussed here.     

Synthesis of silver nanowires with different aspect ratios as alcohol-tolerant catalysts for oxygen electroreduction

The aqueous-phase synthesis of crystalline silver nanowires with different aspect ratios has been achieved by altering seeds. Cyclic voltammetry and tafel results showed that silver nanowires with high aspect ratio exhibited better activity and alcohol-tolerant stability for oxygen electroreduction in alkaline media than nanowires with low aspect ratio.     

Controllable synthesis and photocatalytic activities of rod-shaped mesoporous titanosilicate composites with varied aspect ratios

Rod-shaped mesoporous titanosilicate composites （RMTSs） with controllable aspect ratios （ARs） were fabricated using cetyltrimethylammonium bromide （CTAB） and ammonium hydroxide （NH4OH） at a continuous stirring rate, resulting in ARs ranging from 1 to 5. Slowing the stirring rate or increasing the concentration of CTAB mainly impacted the length growth, whereas NH4OH affected the width growth. Photocatalytic activity studies revealed that the length of RMTSs played a more significant role than the width at lower ARs in their photocatalytic activity.     

Low-temperature, solvent-free solid-state synthesis of single-crystalline titanium nitride nanorods with different aspect ratios

Titanium nitride nanorods have been successfully synthesized by low temperature solid-state metathesis of titanium (III) chloride and sodium azide without using any organic solvent. The conditions required for the synthesis of these nanorods have been optimized. It was found that the temperature and time of reaction had a significant effect on the product morphology. Thermal treatment at 360 °C, for 3 days gave the nanorods of the aspect ratio ∼10 (i.e. diameter ∼50 nm and length ∼ 500 nm), whereas the thermal treatment at 400 °C for 3 days gave the nanorods of the aspect ratio ∼50 (i.e. diameter ∼50 nm and length ∼2-3 μm). Scanning and transmission electron microscopies clearly showed the rod-type morphology. Further evidence for the phase purity and crystallinity of titanium nitride nanorods was given by X-ray diffraction, field emission high-resolution electron microscopy and X-ray photoelectron spectroscopy analyses.     

Modeling of penetrant diffusion in glassy polymers with an integral sorption deborah number

A mathematical model was developed to explain the anomalous penetrant diffusion behavior in glassy polymers. The model equations were derived by using the linear irreversible thermodynamics theory and the kinematic relations in continuum mechanics, showing the coupling between the polymer mechanical behavior and penetrant transport. The Maxwell model was used as the stress–strain constitutive equation, from which the polymer relaxation time was defined. An integral sorption Deborah number was proposed as the ratio of the characteristic relaxation time in the glassy region to the characteristic diffusion time in the swollen region. With this definition, an integral sorption process was characterized by a single Deborah number and the controlling mechanism was identified in terms of the value of the Deborah number. The model equations were two coupled nonlinear differential equations. A finite difference method was developed for solving the model equations. Numerical simulation of integral sorption of penetrants in glassy polymers was performed. The simulation results show that (1) the present model can predict Case II transport behavior as well as the transition from Case II to Fickian diffusion and (2) the integral sorption Deborah number is a major parameter affecting the transition. © 1993 John Wiley & Sons, Inc.     

Gram-scale synthesis of silica nanotubes with controlled aspect ratios by templating of nickel-hydrazine complex nanorods

We have developed a robust method for the synthesis of silica nanotubes with controlled aspect ratios on a large scale by templating against rod-like nanocrystals. Crystalline nanorods of a nickel-hydrazine complex are first formed in reverse micelles by surfactant capping on side facets, and subsequent silica coating and selective etching give rise to silica nanotubes of high uniformity and yield. The length of the silica nanotubes is tunable in the range 37-340 nm and can reach as long as micrometers. Control of the length is conveniently achieved by tuning the hydrazine/nickel ratio, which affects the growth kinetics of the nanocrystal templates. The inner diameter of the silica nanotubes can be adjusted in the range 10-20 nm by choosing different surfactants. This method is unique in utilizing reverse micelles as discrete nanoscale reactors for the growth of nanocrystals, allowing for precise control of the features of the nanotubes and opening up new opportunities in the synthesis of novel anisotropic nanomaterials, construction of nanodevices, and potential drug delivery applications.     

Evolution of the surface polar character of pyrogenic silicas, with their grafting ratios by dimethylchlorosilane, studied by microcalorimetry

The interactions of water, hexamethyldisiloxane, and dodecane with pyrogenic silica samples, modified by a controlled partial silylation with dimethyldichlorosilane, were studied by microcalorimetry and wettability measurements. The samples, having a coverage ratio lower than dimethylsilyl (DMS) monolayer capacity ( approximately 2.6 DMS/nm(2)), show a regular and linear decrease of their heat of immersion into water with the coverage ratio and correlate with the increase of residual silanol groups. Two critical coverage ratios were evidenced at about 25 and 50% of the DMS monolayer capacity, the grafted silica remaining hydrophilic, below 25% being strongly hydrophobic beyond. The heat of immersion into hexamethyldisiloxane decreases until 50% of the DMS monolayer whereas that of dodecane remains independent of the grafting ratio. This study demonstrates that the water/residual free silica surface plays the main role in the stabilization of the W/O Pickering's emulsions.     

Resonance contributions to anti-Stokes/Stokes ratios under surface enhanced Raman scattering conditions

Anti-Stokes/Stokes asymmetries under surface enhanced Raman scattering (SERS) conditions are studied for a wide variety of SERS-active media and different analytes. Evidence is provided for the existence of underlying resonances that create these asymmetries. We show here that these resonances are associated with the electromagnetic coupling between the analyte (probe) and the metal. The work demonstrates the use of the anti-Stokes/Stokes ratio as a tool to understand the hierarchy of resonances in the SERS problem, which is essential for quantification purposes.     

Multifractal analysis of nanodeformation of glassy polymer surface

The multifractal approach has been employed to quantitatively analyze an intact glassy polymer (polystyrene) surface and that deformed in an atomic force microscope. An algorithm has been described in detail for constructing generalized fractal dimension spectra. These spectra have been shown to provide information that reasonably reflects the properties of the studied reliefs of a polystyrene surface.     

Mechanisms of surface diffusion

Mechanisms of surface diffusion of single atoms, molecules and atomic clusters on flat terraces (with and without a driving force) of metal and semiconductor surfaces are discussed, focusing on our recent scanning tunneling microscopy (STM) results. Experimental evidence leading to these conclusions is presented. Besides the commonly known hopping mechanism, there are many other mechanisms. In Arrhenius analysis, a linear behavior can be expected for only some of the simpler mechanisms.