The Effect of N and B Doping on Graphene and the Adsorption and Migration Behavior of Pt Atoms

Abstract

We utilize periodic density functional theory to study singly and triply Nand B-substituted graphene. We examine their doping mechanisms and effects on Pt atom adsorption and migration on graphene. We find a seemingly contradictory behavior between dopant type (n-vs p-type) and charge accumulation on the dopant atoms: the N atoms in both n-type singly N-doped graphene (NG) and p-type triply N-doped graphene (3NG) gain electron density while the B atoms in both singly (BG) and triply (3BG) Bdoped graphene are p-type and lose electron density. This behavior arises from unequal charge sharing within C−B and C−N sp 2 σ bonds and the requirement that the p z orbitals of N and B are singly occupied in order to maintain graphene's aromaticity. 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. The Pt atom hopping energy barrier is lowered in graphene rings containing an NG N atom relative to undoped graphene, but the migration of a Pt atom over the N atom is unlikely due to a 1.0 eV barrier. 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 sp 2 orbitals. When a Pt atom adsorbs at a ring containing a pyridinic N, the strong N−Pt bonds trap the Pt atom, limiting its diffusion over the graphene sheet. 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-lying B−C bonding orbitals 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 having little influence on Pt atom migration outside B containing C rings; however, the depth and short-range nature of these energy wells funnel Pt atoms toward the B atoms and trap them there.

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“…At the same time there is an unequal sharing of electrons in the sp 2 σ -bonds of the dopant atoms and the adjacent C atoms. 141 In addition, to single substitutions more complex systems have also been studied, including triple substitutions with a vacancy (also called pyridinic, see Fig. 6) 141 and multiply-doped systems.…”

“…141 In addition, to single substitutions more complex systems have also been studied, including triple substitutions with a vacancy (also called pyridinic, see Fig. 6) 141 and multiply-doped systems. 140,156 The effect of nitrogen doping through the covalent bonding of N-containing aromatic species, e.g.…”