Effect of Nitrogen Doping on the Migration of the Carbon Adatom and Monovacancy in Graphene

Nitrogen-doped graphene (N-graphene) has important implications in graphene-based devices and catalysts. Nitrogen incorporation into graphene via postsynthetic treatment is likely to produce a non-negligible amount of defects and bond disorders, and the resulting nitrogen content is usually dominated by graphitic N and pyridinic N. To understand the kinetic stability of doped N and the effect of doped N on the self-healing of monovacancy in graphene, we have performed density functional theory calculations to study the adsorption and migration of an adsorbed C atom on undoped and N-doped graphene with and without a monovacancy (MV). The effects of N doping and hydrogenation on the migration of a MV in graphene are also studied. Our results suggest that the graphitic N doped in the vicinity of MV is kinetically unstable, and it could be transformed into a pyridinic N due to the migration of MV when N-graphene is through high-temperature annealing. The presence of a C adatom would easily repair the vacancy of defective graphene with MV and either restore perfect graphene or form a Stone–Wales defect. Similar repairing processes were also found in the case of a C adatom near MV with a pyridinic N

NMR Chemical Shifts of ¹⁵N‑Bearing Graphene

The ¹⁵N NMR chemical shifts of possible nitrogen-containing moieties at edges and defects of graphene are investigated by using the first-principles method. Our computational results show that pyridine-like and graphite-like N can be rather easily identified using the ¹⁵N NMR technique, in agreement with experiment. On the other hand, pyridinium-like ¹⁵N is hardly distinguished from the pyrrole-like one using the NMR, because these ¹⁵N nuclei give nearly overlapping signals. However, our simulations suggest that ¹H NMR is useful to discriminate between them; the NMR chemical shifts of ¹H directly bonded with pyridinium-like and pyrrole-like N along the armchair edge are estimated to be 0.8 and 10.1 ppm, respectively, while the corresponding chemical shift for pyridinium-like N along the zigzag edge is located between them. The ¹⁵N NMR signals for various moieties at edges we considered are found to be similar to the corresponding ones at defects except for pyridine-like nitrogens. Conversely, the ¹⁵N NMR chemical shifts are altered sensitively by the degree of aggregation of pyridine-like ¹⁵N atoms both along armchair edges and at defect sites. Interestingly, the graphite-like ¹⁵N doped along zigzag edges, which was attributed in our previous work to an active configuration for oxygen reduction reaction at the cathode of fuel cells, is identifiable via NMR irrespective of the details of samples such as edge terminations, dopant distributions, and graphene sizes

Possible Oxygen Reduction Reactions for Graphene Edges from First Principles

N-doped carbon-based nanomaterials are attracting a great interest as promising Pt-free electrode catalysts for polymer electrolyte fuel cells (PEFCs). In this computational study, we demonstrate that N-doped graphene edges can exhibit enhanced catalytic activity toward oxygen reduction reactions by controlling their electron-donating and -withdrawing abilities and basicity, resulting in higher selectivity of 4e^– reduction via inner- and outer-sphere electron transfer at edges under acidic conditions, respectively. Our simulations also show that 2e^– reduction occurs selectively in the presence of pyridinic N next to carbonyl O at zigzag edges. This study thus rationalizes the roles of doped N in graphenelike materials for oxygen reduction reactions

Two-Electron Oxygen Reduction on Carbon Materials Catalysts: Mechanisms and Active Sites

Carbon materials based catalysts (CMCs) are extensively investigated to replace expensive noble metal catalysts (NMCs) for electrochemical oxygen reduction reaction (ORR). However, two issues are needed to be clarified for further development: ORR on CMCs produces more H₂O₂ via 2e^– process than that for NMCs in acidic condition, and the active sites for ORR of CMCs are still under debate. H₂O₂ formation on NMCs was thought to be activated by O₂ adsorption on metal surfaces. Contrarily, the results of present study indicate that an O₂ molecule would approach the hydrogen site on CMCs to form an OOH^– ion which subsequently reacts with H⁺ to form a H₂O₂. The calculated electrochemical potentials, kinetics, and X-ray photoelectron spectroscopy (XPS) binding energy support well the new mechanism. Moreover, we found that the active sites for ORR are actually dependent on specific ORR process and the working potential range. The present work provides important insights into ORR for electrochemical devices

Influence of Encapsulated Water on Luminescence Energy, Line Width, and Lifetime of Carbon Nanotubes: Time Domain Ab Initio Analysis

In a broad range of applications, carbon nanotubes (CNTs) are in direct contact with a condensed-phase environment that perturbs CNT properties. Experiments show that water molecules encapsulated inside of semiconducting CNTs reduce the electronic energy gap, enhance elastic and inelastic electron–phonon scattering, and shorten the excited-state lifetime. We rationalize the observed effects at the atomistic level using real-time time-dependent density functional theory combined with nonadiabatic molecular dynamics. Encapsulated water makes the nanotube more rigid, suppressing radial breathing modes while enhancing and slightly shifting the optical G-mode. Water screens Coulomb interactions and shifts charge carrier energies and wave functions. The screening, together with distortion of the CNT geometry and lifting of orbital degeneracy, produces a luminescence red shift. Enhanced elastic and inelastic electron–phonon scattering explains line width broadening and shortening of the excited-state lifetime. The influence of water on the CNT properties is similar to that of defects; however, in contrast to defects, water creates no new phonon modes or electronic states in the CNTs. The atomistic understanding of the influence of the condensed-phase environment on CNT optical, electronic, and vibrational properties, and electron–vibrational dynamics guides design of novel CNT-based materials

Active Sites and Mechanisms for Oxygen Reduction Reaction on Nitrogen-Doped Carbon Alloy Catalysts: Stone–Wales Defect and Curvature Effect

Carbon alloy catalysts (CACs) are promising oxygen reduction reaction (ORR) catalysts to substitute platinum. However, despite extensive studies on CACs, the reaction sites and mechanisms for ORR are still in controversy. Herein, we present rather general consideration on possible ORR mechanisms for various structures in nitrogen doped CACs based on the first-principles calculations. Our study indicates that only a particular structure of a nitrogen pair doped Stone–Wales defect provides a good active site. The ORR activity of this structure can be tuned by the curvature around the active site, which makes its limiting potential approaching the maximum limiting potential (0.80 V) in the volcano plot for the ORR activity of CACs. The calculated results can be compared with the recent experimental ones of the half-wave potential for CAC systems that range from 0.60 to 0.80 V in the reversible-hydrogen-electrode (RHE) scale

Active Sites and Mechanisms for Oxygen Reduction Reaction on Nitrogen-Doped Carbon Alloy Catalysts: Stone–Wales Defect and Curvature Effect

Carbon alloy catalysts (CACs) are promising oxygen reduction reaction (ORR) catalysts to substitute platinum. However, despite extensive studies on CACs, the reaction sites and mechanisms for ORR are still in controversy. Herein, we present rather general consideration on possible ORR mechanisms for various structures in nitrogen doped CACs based on the first-principles calculations. Our study indicates that only a particular structure of a nitrogen pair doped Stone–Wales defect provides a good active site. The ORR activity of this structure can be tuned by the curvature around the active site, which makes its limiting potential approaching the maximum limiting potential (0.80 V) in the volcano plot for the ORR activity of CACs. The calculated results can be compared with the recent experimental ones of the half-wave potential for CAC systems that range from 0.60 to 0.80 V in the reversible-hydrogen-electrode (RHE) scale

Interplay between Oxidized Monovacancy and Nitrogen Doping in Graphene

In most of the N-doped graphene (N-graphene) which attracts strong attention in the context of precious-metal free catalysts and nanoelectronics, the oxygen content is generally higher than or at least comparable to the nitrogen content. In order to understand the effect of oxygen-containing chemical groups (O_<i>m</i>H_<i>n</i>) on N doping in defective graphene sheets, we perform density functional theory calculations to study the interplay of oxidized monovacancy (MV) and the nitrogen doping, motivated by the fact that MV is more frequently observed and more chemically active than divacancy and Stone–Wales defect. We determine the phase diagrams of undoped and nitrogen-doped oxidized MVs as a function of temperature and partial pressure of O₂ and H₂ gases. The modification of the electronic structure of MV by oxidation and N doping is studied. Our results show that the ether group (−O– in plane) is a common component in stable configurations of oxidized MVs. Most of the stable configurations of oxidized MVs do not induce any carriers. The stabilization of pyridinic N, pyridinium-like N, and graphitic N at MV depends on the oxidation degree of MV. Our results also suggest that pyridinic N and pyridinium-like N at clean MV do not facilitate the oxygen-reduction reaction

MDTS: automatic complex materials design using Monte Carlo tree search

<p>Complex materials design is often represented as a black-box combinatorial optimization problem. In this paper, we present a novel python library called MDTS (Materials Design using Tree Search). Our algorithm employs a Monte Carlo tree search approach, which has shown exceptional performance in computer Go game. Unlike evolutionary algorithms that require user intervention to set parameters appropriately, MDTS has no tuning parameters and works autonomously in various problems. In comparison to a Bayesian optimization package, our algorithm showed competitive search efficiency and superior scalability. We succeeded in designing large Silicon-Germanium (Si-Ge) alloy structures that Bayesian optimization could not deal with due to excessive computational cost. MDTS is available at <a href="https://github.com/tsudalab/MDTS" target="_blank">https://github.com/tsudalab/MDTS</a>.</p

Graphdiyne Containing Atomically Precise N Atoms for Efficient Anchoring of Lithium Ion

The qualitative and quantitative nitrogen-doping strategy for carbon materials is reported here. Novel porous nanocarbon networks pyrimidine-graphdiyne (PM-GDY) and pyridine-graphdiyne (PY-GDY) films with large areas were successfully prepared. These films are self-supported, uniform, continuous, flexible, transparent, and quantitively doped with merely pyridine-like nitrogen (N) atoms through the facile chemical synthesis route. Theoretical predictions imply these N doped carbonaceous materials are much favorable for storing lithium (Li)-ions since the pyridinic N can enhance the interrelated binding energy. As predicted, PY-GDY and PM-GDY display excellent electrochemical performance as anode materials of LIBs, such as the superior rate capability, the high capacity of 1168 (1165) mA h g^–1 at current density of 100 mA g^–1 for PY-GDY (PM-GDY), and the excellent stability of cycling for 1500 (4000) cycles at 5000 mA g^–1 for PY-GDY (PM-GDY)