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

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 p_z 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 (E_ads = -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 (E_ads= -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 E_ads= -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.