TiO2 nanofibers of different crystal phases for transesterification of alcohols with dimethyl carbonate

TiO2 nanofibers with different crystal phases have been discovered to be efficient catalysts for the transesterification of alcohols with dimethyl carbonate to produce corresponding methyl carbonates. Advantages of this catalytic system include excellent selectivity (>99%), general suitability to alcohols, reusability and ease of preparation and separation of fibrous catalysts. Activities of TiO2 catalysts were found to correlate with their crystal phases which results in different absorption abilities and activation energies on the catalyst surfaces. The kinetic isotope effect (KIE) investigation identified the rate-determining step, and the isotope labeling of oxygen-18 of benzyl alcohol clearly demonstrated the reaction pathway. Finally, the transesterification mechanism of alcohols with dimethyl carbonate catalyzed by TiO2 nanofibers was proposed, in which the alcohol released the proton to form benzyl alcoholic anion, and subsequently the anion attacks the carbonyl carbon of dimethyl carbonate to produce the target product of benzyl methyl carbonate.

Painting anatase (TiO2) nanocrystals on long nanofibers to prepare photocatalysts with large active surface for dye degradation and organic synthesis

Anatase TiO2 nanocrystals were painted on H-titanate nanofibers by using an aqueous solution of titanyl sulfate. The anatase nanocrystals were bonded solidly onto the titanate fibers through formation of coherent interfaces at which the oxygen atoms were shared by the nanocrystals and the fiber. This approach allowed us to create large anatase surfaces on the nanofibers, which are active in photocatalytic reactions. This method was also applied successfully to coat anatase nanocrystals on surfaces of fly ash and layered clay. The painted nanofibers exhibited a much higher catalytic activity for the photocatalytic degradation of sulforhodamine B and the selective oxidation of benzylamine to the corresponding imine (with a product selectivity >99%) under UV irradiation than both the parent H-titanate nanofibers and a commercial TiO2 powder, P25. We found that gold nanoparticles supported on H-titanate nanofibers showed no catalytic activity for the reduction of nitrobenzene to azoxybenzene, whereas the gold nanoparticles supported on the painted nanofibers and P25 could efficiently reduce nitrobenzene to azoxybenzene as the sole product under visible light irradiation. These results were different from those from the reduction on the gold nanoparticles photocatalyst on ZrO2, in which the azoxybenzene was the intermediate and converted to azobenzene quickly. Evidently, the support materials significantly affect the product selectivity of the nitrobenzene reduction. Finally, the new photocatalysts could be easily dispersed into and separated from a liquid because of their fibril morphology, which is an important advantage for practical applications.

Heating and plasma sheath effects in low-temperature, plasma-assisted growth of carbon nanofibers

Plasma sheath, nanostructure growth, and thermal models are used to describe carbon nanofiber (CNF) growth and heating in a low-temperature plasma. It is found that when the H2 partial pressure is increased, H atom recombination and H ion neutralization are the main mechanisms responsible for energy release on the catalyst surface. Numerical results also show that process parameters such as the substrate potential, electron temperature and number density mainly affect the CNF growth rate and plasma heating at low catalyst temperatures. In contrast, gas pressure, ion temperature, and the C2H2:H2 supply ratio affect the CNF growth at all temperatures. It is shown that plasma-related processes substantially increase the catalyst particle temperature, in comparison to the substrate and the substrate-holding platform temperatures.

Ion-assisted precursor dissociation and surface diffusion : enabling rapid, low-temperature growth of carbon nanofibers

Growth kinetics of carbon nanofibers in a hydrocarbon plasma is studied. In addition to gas-phase and surface processes common to chemical vapor deposition, the model includes (unique to plasma-exposed catalyst surfaces) ion-induced dissociation of hydrocarbons, interaction of adsorbed species with incoming hydrogen atoms, and dissociation of hydrocarbon ions. It is shown that at low, nanodevice-friendly process temperatures the nanofibers grow via surface diffusion of carbon adatoms produced on the catalyst particle via ion-induced dissociation of a hydrocarbon precursor. These results explain a lower activation energy of nanofiber growth in a plasma and can be used for the synthesis of other nanoassemblies. © 2007 American Institute of Physics.

Effect of plasma environment on synthesis of vertically aligned carbon nanofibers in plasma-enhanced chemical vapor deposition

We present a theoretical model describing a plasma-assisted growth of carbon nanofibers (CNFs), which involves two competing channels of carbon incorporation into stacked graphene sheets: via surface diffusion and through the bulk of the catalyst particle (on the top of the nanofiber), accounting for a range of ion- and radical-assisted processes on the catalyst surface. Using this model, it is found that at low surface temperatures, Ts, the CNF growth is indeed controlled by surface diffusion, thus quantifying the semiempirical conclusions of earlier experiments. On the other hand, both the surface and bulk diffusion channels provide a comparable supply of carbon atoms to the stacked graphene sheets at elevated synthesis temperatures. It is also shown that at low Ts, insufficient for effective catalytic precursor decomposition, the plasma ions play a key role in the production of carbon atoms on the catalyst surface. The model is used to compute the growth rates for the two extreme cases of thermal and plasma-enhanced chemical vapor deposition of CNFs. More importantly, these results quantify and explain a number of observations and semiempirical conclusions of earlier experiments.

Excellent field emission from semialigned carbon nanofibers grown on cylindrical copper surface

We report the field emission from carbon nanofibers (CNFs) grown directly on cylindrical copper by a simple pyrolysis technique. The turn-on field is 0.17 V/µm and the emission current density is 0.9 mA/cm2 at 0.35 V/µm. The emission current is stable at a field of 0.35 V/µm and 6.5×10−6 Torr. The excellent field emission behavior is attributed to the sp2 phase in CNFs and the stable emission is due to the direct growth. The direct growth on cylindrical cathode is advantageous for field emission. ©2009 American Institute of Physics.

Structural and electrical transport of carbon nanofibers derived from dihydro-2,5-furandione

Carbon nanofibers of 50–500 nm diameter and several micrometer length were synthesized by high-temperature pyrolysis of dihydro-2,5-furandione (C4H4O3) in the temperature range of 600–980 °C. The formation of both graphitic and non-graphitic structured carbon fibers was observed in high-resolution transmission electron microscope. The Raman spectra of the samples showed the presence of both the D and G bands of varying intensity and sharpness. The low-temperature electrical transport studies on the samples have shown interesting metal–insulator transitions. The films showed variable range hopping conduction in the insulating regime and power law behavior in the critical regime at low temperatures.

Synthesis and characterization of single-crystalline alpha-MoO3 nanofibers for enhanced Li-ion intercalation applications

High quality, single-crystalline alpha-MoO3 nanofibers are synthesized by rapid hydrothermal method using a polymeric nitrosyl-complex of molybdenum(II) as molybdenum source without employing catalysts, surfactants, or templates. The possible reaction pathway is decomposition and oxidation of the complex to the polymolybdate and then surface condensation on the energetically favorable 001] direction in the initially formed nuclei of solid alpha-MoO3 under hydrothermal conditions. Highly crystalline alpha-MoO3 nanofibers have grown along 001] with lengths up to several micrometres and widths ranging between 280 and 320 nm. The alpha-MoO3 nanofibers exhibit desirable electrochemical properties such as high capacity reversibility as a cathode material of a Li-ion battery.

Photoluminescence of Ag nanoparticle embedded Tb3+/Ce3+ codoped NaYF4/PVP nanofibers prepared by electrospinning

Ag nanoparticle embedded NaYF4:0.05Tb center dot chi Ce/ PVP (PVP stands for poly(vinyl pyrrolidone)) composite nanofibers have been prepared by electrospinning. A field emission scanning electron microscope and x-ray diffraction have been utilized to characterize the size, morphology and structure of the as-prepared electrospun nanofibers. Obvious photoluminescence (PL) of NaYF4:0.05Tb center dot 0.05Ce/PVP electrospun nanofibers due to the efficient energy transfer from Ce3+ to Tb3+ ions is observed. The PL intensity of the electrospun nanofibers decreases gradually with the addition of Ag nanoparticles. No obvious surface plasmon resonance enhanced luminescence is observed. The reasons for the weakening of the emission intensity with the addition of Ag nanoparticles have also been discussed in this work.

Upconversion Luminescence of Er3+-Yb3+ Codoped NaYF4-PVP Electrospun Nanofibers

NaYF4: 0.02Er center dot xYb-PVP composite nanofibers with the diameter of similar to 400 nm have been prepared by electrospinning. Field emission scanning electron microscope and X-ray diffraction have been utilized to characterize morphology and structure of the as-prepared electrospun nanofibers. Their up-conversion luminescence is investigated under a 980-nm excitation. Green (538 and 520 nm), red (6-55 nm), and blue (405 nm) emissions are observed in the up-conversion luminescence spectra, and the intensity of these three emissions changes differently with the variety of Yb content, which has been interpreted successfully in this letter. The color of NaYF4: 0.02Er center dot xYb-PVP nanolibers under a 980-nm excitation can be changed from green --> white --> yellow gradually via changing the Yb content.