Role of nitrogen doping in stoichiometric and defective carbon nano-onions: Structural diversity from DFT calculations

Carbon nano-onions (CNOs) and their modified forms have been utilised recently as active catalysts, capacitors, and anode materials. In this study, the structural and electronic properties and the thermodynamic stability of both stoichiometric and defective (mono- and divacancy) N-doped CNOs were elucidated from a theoretical perspective. We examined numerous structures with an increasing nitrogen content, including different distribution patterns using density functional theory calculations. Our findings facilitate a deeper understanding of nitrogen active sites and their relative stability. It was shown that the most stable nitrogen positions were located in five-membered rings, while the nitrogen distribution for mono- and divacancy-containing CNOs was different. In this case, the most attractive positions were pyridinic sites and those in the pentagon rings. We also demonstrated the influence of doping level and defect type on thermodynamic stability. It was observed that the type of defect and doping level ambiguously determined the energy band gap. For monovacancy-containing N-CNOs, the reduction of band gap was moderate, whereas for divacancy systems, the band gap increased. (C) 2021 Elsevier Ltd. All rights reserved.