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2012 - 11th Annual NC State Summer Undergraduate Research Symposium
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Session Time :
8/1/12 3:00 PM - 8/1/12 4:14 PM
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Content Area : Advanced Materials for Environmental Sustainability
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Lead Student Presenters : Nicholas Andrew Restaino
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Abstract Title : Modeling polar and nonpolar vanadium oxide surfaces
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Abstract :
Vanadium oxides are important catalytic materials used in a variety of applications that rely on partial and selective oxidation reactions. They exhibit interesting physical and chemical properties due to the complex electronic configuration of the transition metal vanadium. These materials have multiple valence states and are known to adopt different crystal structures while showing a reversible metal-to-insulator phase transition at a critical temperature. A problem limiting the use of this material lies in understanding how to stabilize energetically unfavorable polar surfaces. The presence of 3d-orbital electrons in vanadium oxides allow for localized electron states that lead to unique relaxation mechanisms for different surface terminations. However, these localized states also exacerbate problems in using common approximations to the density functional theory exchange-correlation functional. We demonstrate that the generalized gradient approximation (GGA) is an inadequate approximation when applying density functional theory to computationally model vanadium (II) oxide polar and nonpolar surfaces. This deficiency is overcome by implementing a GGA+U approximation. The improved methodology is used to analyze crystal structures of pure vanadium, vanadium (II) oxide, and vanadium (IV) oxide as well as the nonpolar (100) surface and polar (111) surfaces of vanadium (II) oxide. Various surface configurations and terminations are examined using both PBE and PBEsol GGA exchange-correlation functionals with and without their +U counterparts. Surface energies calculated by use of GGA+U methods are extended to finite temperature through ab initio thermodynamic calculations, which allows for direct comparison to experimental trends to demonstrate the effectiveness of the GGA+U framework.
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Mentor and/or Co-Author : Douglas Lee Irving
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