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Osswald S.,Naval Postgraduate School, Monterey | Chmiola J.,Lawrence Berkeley National Laboratory | Gogotsi Y.,Aj Drexel Nanotechnology Institute

Microstructure and surface moieties of porous carbons play a significant role in affecting their performance in a variety of applications. While it is well known that high-temperature treatments of porous carbons can influence the microstructure, no systematic studies have been done on carbide-derived carbons. We show that vacuum annealing increases the pore volume and specific surface area of titanium carbide-derived carbon with no significant change in the pore size up to 1500 °C. This treatment produces porous carbons with subnanometer porosity and a specific surface area up to 2000 m 2/g, while treating the samples at temperatures above 1600 °C increases the pore size above 1 nm because of graphitization and collapse of the micropore structure. The results demonstrate that vacuum treatment can be used to further tune the pore structure and potentially the surface functionality of carbide-derived carbons for supercapacitor electrodes, gas chromatography, sorption, sensing and other applications. Vacuum annealing of carbide-derived carbon is therefore a suitable alternative to conventional microstructure modification methods, such as gas or liquid phase activation, which are subject to substantial sample loss and result in additional surface functionalization. © 2012 Elsevier Ltd. All rights reserved. Source

Buke G.C.,Cankaya University | Yushin G.,Georgia Institute of Technology | Mochalin V.,Aj Drexel Nanotechnology Institute | Gogotsi Y.,Aj Drexel Nanotechnology Institute
Journal of Materials Research

Epitaxial graphene and carbon nanotubes (CNTs) grown on SiC have shown big potential in electronics. The motivation to produce faster and smaller electronic devices using less power opened the way to a study of how to produce controlled epitaxial graphene and CNTs on SiC. Since defects are among the important tools to control the properties of materials, the effects of defects on the carbon formation on SiC have been analyzed. In this study, the effects of defects on the carbon formation on SiC have been analyzed. We produced carbon films on the surface of four different SiC materials (polycrystalline sintered SiC disks, single crystalline SiC wafers, SiC whiskers, and nanowhiskers) by chlorination and vacuum annealing with the goal to understand the effects of surface defects on the carbon structure and the SiC decomposition rate. We have shown that grain boundaries, dislocations, scratches, surface steps, and external surfaces may greatly enhance the reaction rate and affect the final structure of carbon derived from SiC. Copyright © 2012 Materials Research Society. Source

Kurtoglu M.E.,Aj Drexel Nanotechnology Institute | Longenbach T.,Aj Drexel Nanotechnology Institute | Sohlberg K.,Drexel University | Gogotsi Y.,Aj Drexel Nanotechnology Institute
Journal of Physical Chemistry C

In spite of extensive research on cationic and anionic doping of titania, only moderate progress in improving its photocatalytic activity has been achieved. In this article, we show that chromium and nitrogen form strong coupling inside the titanium dioxide (TiO2) anatase lattice resulting in a 10-fold increase of photocatalytic activity under ultraviolet irradiation compared with that of Cr or N doping alone. Extensive density functional theory (DFT) calculations show that both negatively charged defect centers (CrTi - NO-) form strong bonds rather than repel each other. The resulting Cr-N defect centers are found to decrease the concentration of trivalent titanium in heavily doped films, which may contribute to the observed increase in photocatalytic activity. The importance of this work lies far beyond simply the Cr and N doping because it opens the door for scientifically driven designs of codoped titania for various photocatalytic applications ranging from self-cleaning coatings to solar cells. © 2011 American Chemical Society. Source

Presser V.,Drexel University | Presser V.,Aj Drexel Nanotechnology Institute | Naguib M.,Drexel University | Naguib M.,Aj Drexel Nanotechnology Institute | And 4 more authors.
Journal of Raman Spectroscopy

Here, we report, for the first time, on the Raman spectra of Ti 2AlN, Ti 2AlC 0.5N 0.5, (Ti 0.5V 0.5) 2AlC, Ti 3AlC 2, and Ti 3GeC 2 and compare the results with those of Ti 2AlC and V 2AlC reported previously. The first-order mode peaks of the end members are narrower than those of their respective solid-solution compounds. The Ti 3AlC 2 and Ti 3GeC 2 phases show, in addition to atomic displacements of the 'M' and 'A' atomic planes, modes that correspond with vibrations of the 'X' sublattice relative to itself. We also predict the Raman modes using density functional theory. The agreement between theory and experiment was found to be satisfactory. Copyright © 2011 John Wiley & Sons, Ltd. Source

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