Yakup Aykut

Enhanced field electron emission from electrospun co-loaded activated porous carbon nanofibers.

Highly porous, Co-loaded, activated carbon nanofibers (Co/AP-CNFs) were prepared by electrospinning a CoCl2-containing polyacrylonitrile composite, followed by thermal treatment processes under air and inert atmospheres. Observations show that carbon nanofibers (CNFs) generated in this fashion have a dramatically large porosity that results in an increase in the specific surface area from 193.5 to 417.3 m(2) g(-1)as a consequence of the presence of CoCl2 in PAN/CoCl2 precursor nanofibers. The nanofibers have a larger graphitic structure, which is enhanced by the addition of the cobaltous phase during the carbonization process. Besides evaluating the morphological and material features of the fibers, we have also carried out a field electron emission investigation of the fibers. The results show that an enhancement in the field electron emission of Co/AP-CNFs occurs as a result of the existence of cobalt in the carbon nanofibers, which results in a greater graphitization, increased specific total surface area and porosity of the carbon nanofibers. Overall, the Co/AP-CNFs are prepared in a facile manner and exhibit an enhanced field electron emission (54.79%) compared to that of pure CNFs, a feature that suggests their potential application to field electron emission devices.

Templating Quantum Dot to Phase-Transformed Electrospun TiO2 Nanofibers for Enhanced Photo-Excited Electron Injection

We report on the microstructural crystal phase transformation of electrospun TiO(2) nanofibers generated via sol-gel electrospinning technique, and the incorporation of as-synthesized CdSe quantum dots (QDs) to different phases of TiO(2) nanofibers (NFs) via bifunctional surface modification. The effect of different phases of TiO(2) on photo-excited electron injection from CdSe QDs to TiO(2) NFs, as measured by photoluminescence spectroscopy (PL) is also discussed. Nanofiber diameter and crystal structures are dramatically affected by different calcination temperatures due to removal of polymer carrier, conversion of ceramic precursor into ceramic nanofibers, and formation of different TiO(2) phases in the fibers. At a low calcination temperature of 400 (o)C only anatase TiO(2) nanofiber are obtained; with increasing calcination temperature (up to 500 (o)C) these anatase crystals became larger. Crystal transformation from the anatase to the rutile phase is observed above 500(o)C, with most of the crystals transforming into the rutile phase at 800(o)C. Bi-functional surface modification of calcined TiO(2) nanofibers with 3-mercaptopropionic acid (3-MPA) is used to incorporate as-synthesized CdSe QD nanoparticles on to TiO(2) nanofibers. Evidence of formation of CdSe/TiO(2) composite nanofibers is obtained from elemental analysis using Energy Dispersive X-ray spectroscopy (EDS) and TEM microscopy that reveal templated quantum dots on TiO(2) nanofibers. Photoluminescence emission intensities increase considerably with the addition of QDs to all TiO(2) nanofiber samples, with fibers containing small amount of rutile crystals with anatase crystals showing the most enhanced effect.

Catalytic graphitization and formation of macroporous-activated carbon nanofibers from salt-induced and H2S-treated polyacrylonitrile

We present here a facile method to produce macroporous-activated carbon nanofibers (AMP-CNFs) by post-treating electrospun cobalt(II) chloride (CoCl2) containing polyacrylonitrile (PAN/CoCl2) nanofibers with hydrogen sulfide (H2S) followed by carbonization. A range of techniques including scanning and transmission electron microscopy, FTIR and Raman spectroscopy is used to examine and characterize the process. Because of the phase behavior between carbon and cobalt, cobalt particles are formed in the nanofibers, some of which leave the fibers during the heat treatment process leading to macroporous fibrous structures. The number of the macroporous increase significantly with increasing CoCl2 concentration in the precursor H2S-treated PAN/CoCl2 nanofibers. The cobalt phase in the fibers also leads to catalytic graphitization of the carbon nanofibers. The produced AMP-CNFs may be a promising candidates in many applications including anode layer in lithium ion batteries, air and liquid purifiers in filters, as well as in biomedical applications.

Protease immobilization on cellulose monoacetate/chitosan-blended nanofibers:

Chitosan-blended cellulose monoacetate nanofibers were prepared through electrospinning process. Neat nanofibers and their sodium hydroxide-treated analogs were used as support surfaces for protease immobilization via physical adsorption method. Morphologies of the nanofibers were observed with a scanning electron microscopy. Chemical analyses were conducted with Fourier transform infrared spectroscopy, and thermal analyses were carried out with differential scanning calorimeter and thermogravimetric analyzer. Immobilized enzyme activities were measured by using casein substrate. In order to test the stability of immobilized enzymes, the tests were repeated until the immobilized enzyme activity was leveled off. The results reveal that well uniform cellulose monoacetate/chitosan nanofibers were obtained, and nanofiber structures are transformed from rounded to more flattened morphology after enzyme activation test. Glutaraldehyde activation has positive effect on sodium hydroxide-treated samples, and the highest immobilization yield as about 83% was observed for glutaraldehyde-treated cellulose monoacetate/chitosan samples. Sodium hydroxide treatment before glutaraldehyde activation shows the best results for protease immobilization on cellulose monoacetate and cellulose monoacetate/chitosan nanofibers. Operational stability increases after sodium hydroxide treatment and glutaraldehyde activation. Glutaraldehyde activation effectively increased the cycle number after sodium hydroxide treatment and about 20% of enzyme activity was still retained after seven cycles at cellulose monoacetate/chitosan samples. This percentage is higher at pure cellulose monoacetate nanofibers than cellulose monoacetate/chitosan nanofibers and measured around 33.5%.