(773b) Fundamentals of N and B Dopants On Graphene and Pt Interactions With Graphene

Authors: 
Muhich, C. L., University of Colorado at Boulder
Westcott, J. Y. IV, University of Colorado at Boulder
Weimer, A. W., University of Colorado Boulder
Musgrave, C. B., University of Colorado Boulder



Graphene is increasingly used in a variety of useful chemical devices, especially fuel cells where it is employed as an electrode which carries generated current from the fuel cell and acts as a metal catalyst support, usually Pt. Graphene is routinely doped with N and B to control the electrical properties and enhance catalytic performance and lifetime. While the respective n- and p-type dopant behavior of single N and B doped graphene (NG and BG) suggests a straightforward doping mechanism, the accumulation and depletion of charge density on N and B respectively is counter-intuitive. Additionally, N’s effect on graphene is dependent on dopant structure, where pyridinic and substantial doping leads to p- and n-type dopants respectively.  We utilize first principles density functional theory to investigate these behaviors. This behavior arises from unequal charge sharing within C-B and C-N sp2 s bonds and the requirement that the pz orbitals of N and B are singly occupied in order to maintain graphene’s aromaticity. The alterations in graphene’s electronic structure due to doping gives rise to stronger Pt-graphene bonding. NG’s N atom stabilizes Pt atom adsorption up to -0.39 eV (Eads = -1.86 eV) and by -0.13 eV even at distances 12.3 Å away from the N dopant. 3NG’s most stable Pt adsorption site (Eads= -2.86 eV) is the vacant C site at the center of 3NG’s three N atoms and arises because of the formation of covalent bonds between Pt’s d orbitals and the N atoms’ three in-plane dangling sp2 orbitals. The BG and 3BG structures bind Pt with a maximum adsorption energy of Eads= -2.16 eV and -5.30 eV, respectively. BG’s high-energy B-C bonds allow the Pt atom to form strong σ bonds directly to the graphene sheet, while 3BG’s B atoms donate electron density to the Pt atom creating an ionic bond between the negative Pt atom and the positive B atoms. These bonding mechanisms result in only short range Pt stabilization and the B atoms.