The process of electrospinning produces polymeric fibers with diameters ranging from the micrometer to the nanometer scale. As the fibers are produced, they are usually spun and collected in a random mat lacking structural orientation. In some applications there is interest in parallel alignment of nanofibers to produce structures with unique electrical, optical, and mechanical properties. A simple and successful method for spinning sheets with one centimeter wide strips of aligned nanofibers is presented. The technique uses copper wires spaced evenly in the form of a circular drum as a collector of the electrospun nanofibers. Aligned nanofiber sheets can be collected easily without disturbing the aligned structure, and the method is robust.
The adsorption and thermal behavior of NO on 'flat' Pd(ll1) and 'stepped' Pd(112) surfaces has been investigated by temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and electron stimulated desorption ion angular distribution (ESDIAD) techniques. NO is shown to molecularly adsorb on both Pd(ll1) and Pd(112) in the temperature range loo-373 K. NO thermally desorbs predominantly lnolecularly from Pd(lll) near 500 K with an activation energy and pre-exponential factor of desorption which strongly depend on the initial NO surface coverage. In contrast, NO decomposes substantially on Pd(112) upon heating, with relatively large amounts of N, and N,O desorbing near 500 K, in addition to NO. The fractional amount of NO dissociation on Pd(112) during heating is observed to be a strong function of the initial NO surface coverage. HREELS results indicate that the thermal dissociation of NO on both Pd(ll1) and Pd(112) occurs upon annealing to 490 K, forming surface-bound 0 on both surfaces. Evidence for the formation of sub-surface 0 via NO thermal dissociation is found only on Pd(112), and is verified by dissociative 0, adsorption experiments. Both surface-bound 0 and sub-surface 0 dissolve into the Pd bulk upon annealing of both surfaces to 550 K. HREELS and ESDIAD data consistently indicate that NO preferentially adsorbs on the (111) terrace sites of Pd(112) at low coverages, filling the (001) step sites only at high coverage. This result was verified for adsorption temperatures in the range 100-373 K. In addition, the thermal dissociation of NO on Pd(112) is most prevalent at low coverages, where only terrace sites are occupied by NO. Thus, by direct comparison to NO/Pd(lll), this study shows that the presence of steps on the Pd(112) surface enhances the thermal dissociation of NO, but that adsorption at the step sites is not the criterion for this decomposition.
The adsorption of NO on a stepped Pd(l 12) surface has been investigated at 300-373 K with high resolution electron energy loss spectroscopy (HREELS) and electron stimulated desorption ion angular distribution (ESDIAD) techniques. At exposures below 5.7 X 1014 NO/cm2, NO molecules preferentially adsorb on the terrace sites, with an NO stretching mode observed by HREELS at 1535-1550 cm-1, and an 0+ ESDIAD beam directed near the (111) terrace-normal direction. For NO exposures above ~6 X 1014 molecules/cm2, step sites are occupied in addition to the terrace sites, with an NO stretching mode observed in HREELS at -1655-1670 cm-1 and an additional 0+ ESDIAD beam oriented in the "downstairs" direction. This is an unusual reversal of the site occupation sequence since one usually expects stronger adsorbate bonding on the step sites causing the steps to fill first. The N-0 bond stretching frequencies observed in HREELS are indicative of weaker bonding of NO molecules on the step sites than on the terrace sites.
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