Formation Mechanisms of Graphitic-N: Oxygen Reduction and

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Formation Mechanisms of Graphitic-N: Oxygen Reduction and Nitrogen Doping of Graphene Oxides Wei-Wei Wang, Jing-Shuang Dang, Xiang Zhao, and Shigeru Nagase J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10607 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016

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The Journal of Physical Chemistry

Formation Mechanisms of Graphitic-N: Oxygen Reduction and Nitrogen Doping of Graphene Oxides Wei-Wei Wang, †,‡Jing-Shuang Dang, †Xiang Zhao,* †and Shigeru Nagase*‡

†

Institute for Chemical Physics & Department of Chemistry, School of Science, Xi’an Jiaotong

University, Xi’an 710049, China. ‡

Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan

* Fax: +86 29 82668559; Tel: +86 29 82665671; Email: [email protected]; [email protected] .

ACS Paragon Plus Environment

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Abstract

Deoxygenation and nitrogen doping mechanisms of graphene oxides with participation of foreign NH3 molecules were investigated by density functional theory calculations. Firstly, reduction on perfect graphene oxide without any structural defect is proved to be facilitated at high temperature but the following doping process is impracticable because of the huge energy requirement for C-C cleavage. To elucidate the formation of hexagonal graphitic-N, oxygen reduction and subsequent nitrogen doping processes on defective graphene oxides with single vacancies were explored for the first time. All possible reaction pathways were taken into account and the results demonstrate that formation of graphitic-N from NH3 and defective graphene oxides with one carbonyl or two hydroxyl groups is feasible in energy. The dominant reaction route is found to be exothermic with a practical reaction rate of 2.26 × 106 s-1 at 900 ℃, which is in a good agreement with experimental observations.

Keywords Density functional theory, graphene oxides, nitrogen doping, oxygen reduction, structural defects


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Introduction Ever since its isolation in 2004, the two-dimensional graphene has attracted tremendous attentions owing to its exotic properties.1 To date, great achievements have been made on graphene-based nanomaterials in the fields of chemistry, physics, and life science.2-19 Besides the well-recognized pristine graphene structure, both experimental and theoretical efforts were made to design and synthesize heteroatom-doped graphene materials to tailor the physicochemical properties for extending its applications. For example, owing to the unique electronic properties, nitrogen-doped graphene (NG, in which one or more carbon atoms are substituted by nitrogen atoms) is expected to be utilized in fuel cells and other electrochemical devices.20-26 In general, there are two approaches to obtain nitrogen-doped graphene in experiments. Firstly, NG can be yielded by directly mixing the carbon source with nitrogen-containing compounds (such as ammonia and hydrazine) as precursors.27,28 Beside such direct synthesis, another way is post-synthesis treatment. By thermal or plasma treatment on existing graphene or graphene oxide (GO), nitrogen atoms can be inserted into the network of the carbon skeleton (edges, defective sites, and also the interior of the carbon sheet).23-25,29-36Structurally, there are three types of bonding configurations for a doped atomic nitrogen in graphene, designated as graphitic-N, pyridinic-N, and pyrrolic-N (see Figure 1). Pyridinic-N and pyrrolic-N are formed at the edge or defective sites of graphene. Graphitic-N, which bonds with three carbon atoms, is formed inside the network of graphene. Figure 1 In 2009, Li et al reported a simple chemical method to obtain NG in bulk quantities. 32 By annealing GO in NH3, the N-doped graphene sheets were synthesized at elevated temperatures.


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XPS spectra revealed that nitrogen atoms were embedded into carbon skeleton and the graphiticN exhibited a significant role over 900°C.32 Although such an effective method has been proposed in experiments, the oxygen reduction and following nitrogen doping processes of GO are still less known. Therefore, discussions on reaction pathways and corresponding influential factors in theory are necessary to understand the mechanisms in detail. Moreover, to our bestknowledge, almost all the theoretical studies on the reactivity of GO only focus on the oxygen reductions and there is no report on the nitrogen doping mechanisms so far. In this study, comprehensive density functional theory (DFT) computations were performed to explore the reduction and doping mechanisms of GO by using ammonia as the reducing agent and meanwhile the nitrogen source for doping. Our objectives are to elucidate the interactions between ammonia with pristine and defective graphene oxides with various oxygen-containing functional groups, and to uncover the formation mechanisms of graphitic-N from foreign NH3 and parental GO sheet. Computational details Experimental evidence has shown that the oxygen functionalities distribute on both the edge and basal plane of the defective GO sheet, and those oxygen groups lead to a separation of GO sheet into small in-plane aromatic domains.37-41 Accordingly, we modeled the structures of GO as finite fragmental carbon sheets with various oxygen groups (Figure 2), which have been extensively used for theoretical calculations of graphene oxides.39, 42-45The molecular radius of graphene flake is around 6.17 Å.As for the oxygen-containing functionalities, epoxide (-O-), hydroxyl (-OH), and carbonyl (-C=O) groups were established as the major components because their existence have been well identified.46-52 In addition, the minor component carboxyl (COOH) group was excluded in the present work since carboxyl is only distributed at the edge of


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GO whereas our present study are constrained on the formation of graphitic-N in basal plane of GO. Figure 2 All calculations were performed with the GAUSSIAN 09 program.53 The M06-2X density functional in conjunction with the basis set of 6-31G(d) were employed for all structural optimizations and vibrational frequency calculations. The correction of zero-point energy (ZPE) to the total energy of each molecule was taken into account. The electronic ground states for all GO models in Figure 2 were confirmed as closed-shell singlet with the singlet-triplet splitting energies more than 28.0 kcal mol-1. In the present study, the structures of transition states were located by using the Berny algorithm. Vibrational analyses were conducted to clarify the nature of stationary points as global minima or transition states with one imaginary frequency. On the basis of transition state theory (TST), the reaction rates were evaluated by the Arrhenius formula: νexp(−Eb/kBT),54 where ν is the attempt frequency, T is the reaction temperature, Eb is the computed energy barrier, and kB is the Boltzmann constant. Results and discussion We initially focused on the reaction of NH3to the perfect graphene sheet with singleepoxide group (GO-1 in Figure 2). Epoxide was chosen as the major oxygen-containing component because this type of functional group exists in abundance at both the edge and interior of an aromatic domain of GO. A previous report by Tang et al indicated that attack of NH3 on the inner epoxide will cause the reduction of graphene oxides, and the nitrogen-containing groups (NH2 and -NH) will externally attach onto the carbon surface.55 Therefore, the adsorption of NH3 groups can be considered as the prelude of nitrogen doping. Kinetically, the reaction of NH3 to


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GO with single epoxide group was confirmed as a stepwise process. In the first instance, the foreign NH3 molecule is adsorbed on GO with a binding energy of -4.6 kcal mol-1. After that, a hydrogen atom in NH3 transfers from nitrogen to oxygen, resulting in a new C-NH2 bond and a – OH group on carbon surface. In the third step, the second N-H dissociation takes place and the H atom attacks on oxygen to yield a H2O molecule. According to the different binding sites of – NH2 in the second step, the reduction is divided into two reaction routes, as shown in Figure 3 and 4. Route-a in which -NH2 is located at the para position of oxygen is considered as a more favorable pathway because of a lower energy barrier for hydrogen transfer to form the H2O molecule. Based on our DFT calculations, the second N-H dissociation (TS3-a)actsas the ratedetermining step with an energy barrier of 42.0 kcal mol-1 (route-a), suggesting that the reduction of epoxide on graphene oxide is facilitated at elevated temperature. After elimination of H2O, the external nitrogen atom is attached onto the basal plane of the graphene to form a NH-containing reduced product (G-NH-1), as shown in Figure 3. Figure 3 Figure 4 Furthermore, the subsequent nitrogen doping process on pristine graphene (G-NH-1 in Figure 3 and 5) was explored here. In general, nitrogen doping includes two major steps: N-H dissociation and C-N formation. According to different sequence of the two steps and distinct positions of newly-formed C-N bonds, four reaction pathways were found to elucidate the doping process. As shown in Figure 5 and 6, the reaction is preferred to generate a sp 3 carbon in the first step (INT1-cd in route-c(d)) and then to form new C-N bonds (C4-N bond of NG-CH-1 in route-c and C3-N bond of NG-CH-2 in route-d, respectively). However, energetically, even


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assisted by the migrated hydrogen atom (route-d, in Figure 5 and 6), this substitution process is still endothermic with a huge activation barrier of 107.2 kcal mol-1, which indicates that the doping is unlike to take place even at elevated temperatures. Figure 5 Figure 6 Accordingly, direct doping from flawless graphene network seems to be unpractical. It should be mentioned that such a conclusion can also apply to the case of GO with hydroxyl groups located on the interior of the aromatic domain. It has been demonstrated that the reaction of NH3 to hydroxyl-participated GO will generate a water molecule as the reduction product and a -NH2 group which is adsorbed on carbon surface.55 However, similar to the case of epoxide, since dissociation of C-C bonds on perfect graphene skeleton needs huge energy requirements, the nitrogen substitution cannot take place in practice. Based on this reason, the reduction and doping reactions of GO with single hydroxyl group are not considered in this work. Different from the hypothetical GO model with structurally perfect graphene layer, experimental observations suggested that carbon surface is always interspersed with vacancies and the most reported defects are single vacancies (SV) and double vacancies (DV) which are thermodynamically stable.56-62 In the case of SV, the missing carbon atom from hexagonal lattice leads to a defective segment which includes a pentagon-nonagon pair and a sp-hybridized carbon with dangling bonds. Herein, the reactions of NH3 to defective GO with SV (GO-2, GO-3, and GO-4 in Figure 2) were explored, to determine whether it is feasible to dope nitrogen into the basal plane of carbon sheet to generate the experimentally observed graphitic-N. The carbonyls and hydroxyls were selected as the oxygen-containing groups because of the identified large


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amounts of sp3 carbon with C-O and C=O in experiments.32 As for the double vacancies, since it is impossible to generate the graphitic-N after injecting only one nitrogen atom, investigations on GO with DV are beyond our present research. Therefore, in the following discussions our main goal is to study the reduction and doping processes of GO with SV to uncover the formation of graphitic-N and to clarify the role of defects in carbon skeleton on chemical doping. Herein, the reactions between ammonia and defective GO with a carbonyl group were studied kinetically. Firstly, optimizations of two different models with single carbonyl group were performed and the results show that the structure GO-2 shown in Figure 2 is 17.6 kcal/mol lower in energy than the other model (labeled as GO-2b, shown in Figure S1 in supporting information). Therefore GO-2 was employed as the precursor for further kinetic computations. The optimized structures and calculated reaction channels are shown in Figure 7-10. After adsorption of NH3 to generate the identical intermediate (INT1-ghi in Figure 7-10) in the first step, three distinct pathways based on different addition sites of NH3were discussed. The calculation results indicate that NH3 prefers to attack from the pentagonal site (C2, in route-g) to generate a stable doping product with graphitic-N (NG-H-1 in Figure 7) after N-H dissociations and water formation. Route-g is the only one exothermic reaction with a negative reaction energy of -55.5 kcal mol-1 and exhibits the lowest energy barrier of 55.3 kcal mol-1 among all possible reaction pathways. Such an activation barrier is much lower than that of the abovementioned nitrogen doping process from a perfect graphene layer (c. a. 110.0 kcal mol -1), implying that structural defect is essential for nitrogen doping to form graphitic-N. After reduction of carbonyl by transferring two hydrogen atoms from ammonia (NG-H-1), the single carbon vacancy on carbon surface is healed by the external nitrogen. Figure 7


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Figure 8 Figure 9 Figure 10 As depicted in Figure 10, in comparison with route-g, the energy barriers of two other channels are much higher. In the case of route-h, NH3firstly attacks the carbon next to the pentagon (C3) which exhibits the shortest distance to foreign nitrogen in INT1-ghi of 3.30 Å. Since the defective five-membered ring in GO-2 is not destroyed during nitrogen doping, the final doping network contains a defective pentagon-heptagon-pentagon adjacency (NG-H-2 in Figure 8). As seen in Figure 8 and 10, the much higher energy barrier (77.5 kcal mol-1) as well as the positive reaction energy (2.1 kcal mol-1) both suggest that the formation of this pentagon and heptagon-containing hetero-configuration is energetically unfavorable in practice. In the case of route-i, it is evident from Figure 9 and 10 that the reaction cannot embed the nitrogen inside the network of graphene to generate the graphitic-N. Moreover, the reaction needs to overcome a considerably huge barrier of 109.7 kcal mol-1, implying that route-i is kinetically unfavorable. Therefore, according to the abovementioned three reaction routes in Figures 7-9, route-g is considered as the dominant mechanism for C=O reduction and nitrogen doping, both thermodynamically and kinetically. Besides carbonyl, hydroxyl is another important oxygen-containing group in GO. It should be mentioned that if only one –OH is participated, the reduction products must be a water molecule and an adsorbed –NH2 group. The two N-H bonds in –NH2 are stable and obviously N-H dissociation cannot take place for further nitrogen doping. Therefore, the nitrogen doping with the presence of –OH should be considered as a multi-hydroxyl participated proceed. In the


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present work, the reactions between NH3 and defective GO with two hydroxyl groups were investigated. As shown in Figure 2 and Figure S2 in supporting information, the two models GO3 and GO-4 which are proved as the most two stable GO with two hydroxylgroups among all possible configurations were established as the precursors. GO-3 is 0.2 kcal/mol lower than GO4 in energy and at least 1.2 kcal/mol lower than other species. In GO-3 and GO-4, one of the two hydroxyls is located on the dangling carbon and the other one is adsorbed around the vacancy. A hydrogen bond is formed between the two –OH groups for stabilization. In the first step, the foreign NH3 is adsorbed on GO. The NH3 molecule can both locate on the carbon surface and also the –OH group. As shown in Figure 11 and 12, the location of NH3 on the –OH group by hydrogen bonding interaction (INT1-j and INT1-k) is exothermic by -14.9 kcal mol-1 in GO-3 (-12.5 kcal mol-1 in GO-4). In contrast with the barrierless adsorption of NH3 to hydroxyl, the adsorption of NH3 to carbon surface in Figures 13 and 14 (INT1-l and INT1-m) is demonstrated as an endothermic reaction with an activation barrier of 27.6 kcal mol -1 in GO-3 (30.1 kcal mol-1 in GO-4), suggesting that the NH3molecule is preferred to adsorb to –OH rather than the carbon surface. Therefore, we further investigated the following reduction and doping reactions based on the structures of INT1-j and INT-k, as shown in Figure 11. In the presence of adsorbed NH3, the dehydroxylation from two –OH groups wascalculatedsubsequently. The reaction barrier of hydrogen transfer is 10.9 kcal mol-1 in GO-3 and 12.8 kcal mol-1 in GO-4, respectively. Interestingly, we also calculated the proton transfer between –OH groups without the participation of NH3 for comparison. As shown in Figures 15 and 16, the energy barrier is found to be slightly increased to 12.7 kcal mol-1 for GO-3 and 15.0 kcal mol-1 for GO-4, which indicates that the adsorbed NH3 can promote the hydrogen transfer to form an eliminated NH3H2O complex. After the removal of NH3-H2O (GO-2 in Figure 11), the –OH group which locates


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on the dangling C6 is reduced as a carbonyl group. In another word, GO-2 is obtained from GO3 and GO-4 by intramolecular dehydroxylation. As mentioned above, the C=O group in GO-2 can react with NH3 to dissociate the N-H bonds to generate a water molecule and -NH species (route-g in Figure 8). The nitrogen atom in NH3 is successfully inserted into the graphene surface to form a graphitic-N. Throughout the whole reaction processes (route-j for dehydroxylation and route-g for decarbonylation), dehydroxylation from two –OH is much easier and the ratedetermining step is the reduction of C=O and nitrogen doping in subsequent steps. Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Kinetically, on the basis of the Arrhenius formula, the calculated reaction rate of route-g at 900 ℃ is 2.26 × 106 s-1 (with a barrier of 55.3 kcal/mol and an attempt frequency of 429.5 cm-1 which is originated from the frequency analysis of rate-determining TS3-g), implying that the formation of graphitic-N from NH3 and defective GO with a carbonyl group at elevated temperature is available (the calculated reaction rates at other temperatures are listed in Table S1 in supporting information for comparison). Such a computational result is consistent with


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experimental evidence32and successfully uncovers the nitrogen doping mechanisms of GO from foreign nitrogen sources for the first time. Conclusion In the present work, we studied the oxygen reduction and nitrogen doping mechanisms of GO in the presence of ammonia molecules. In the case of perfect graphene oxide without any defects (GO-1), de-epoxidation can be achieved by N-H dissociations but the following nitrogen doping is energetically unfavorable because of the endothermic character and the huge activation barrier of 107.2 kcal mol-1 for C-C cleavage. Moreover, defective graphene oxides with single vacancies were modeled to expose the formation mechanism of graphitic-N. Three distinct reaction pathways (route-g, h, i) were discovered for decarbonylation and nitrogen doping. Owing to the negative reaction energy of -55.5 kcal mol-1 and the lowest energy barrier of 55.3 kcal mol-1, route-g is predicted as the most favorable pathway for doping. The calculated reaction rate of route-g at 900 ℃ (2.26 × 106 s-1) indicates that the formation of graphitic-N from NH3 and defective GO with a carbonyl group (GO-2) is feasible at elevated temperature. Furthermore, we discussed the reaction of NH3 to defective GO with two hydroxyl groups (GO-3 and GO-4). Our calculation results suggest that the NH3 molecule is preferred to adsorbed with the hydroxyl group instead of the carbon surface and the reactivity of dehydroxylation between two hydroxyls is improved in the presence of adsorbed NH3 to generate the abovementioned defective GO-2 with a single carbonyl group. Similarly, subsequent nitrogen doping from NH3 can take place and the graphitic-N can be generated by route-g. Overall, we introduced the detailed formation mechanisms of nitrogen-doped graphene materials from various GO and foreign nitrogen sources for the first time. The present work


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suggests that not only the reducing agent but also the defects in carbon skeleton are essential for reduction and doping, which is useful to understand the chemical doping process of graphenebased materials for the design of hyperfine functionalized materials in the future. AUTHOR INFORMATION Corresponding Authors * Fax: +86-29-82668559; Tel: +86-29-82665671; E-mail: [email protected]; [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here. Acknowledgments This work has been financially supported by the National Natural Science Foundation of China (21171138, 21573172) and the Specially Promoted Research Grant (22000009) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. ASSOCIATED CONTENT Supporting Information


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Structures and relative energies of single carbonyl and two-hydroxylGO configurations, calculated reaction ratesat different temperatures, and full citations of refs. 4, 11, 48, and 53. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1.Schematic representation of nitrogen-doped graphene.

Figure 2.Computational models of graphene oxides. Atoms representations are O (red), C (white) and H (pink).GO-1: perfect graphene with an epoxide group. GO-2: defective graphene with a carbonyl group in the center. GO-3 and GO-4: defective graphene with two hydroxyl groups.


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Figure 3.Fragmental structures for stationary points involved in route-a and route-b. Atoms representations are O (red), N (blue), C (white) and H (pink). Other carbon atoms are omitted for clarity. The distances are represented in units of Å.Relative energies (in parenthesis) are given in kcal mol-1.

Figure 4.Reaction energy profiles for ammonia participated reduction of GO-1 (route-a in red and route-b in green). Relative energies referred to the total energy of reactants (GO-1+NH3) are calculated at the M06-2X/6-31G(d) level of theory, units in kcal mol-1.


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Figure 5.Fragmental structures for stationary points involved in route-c, d, e, and f. Atoms representations are O (red), N (blue), C (white) and H (pink).Other carbon atoms are omitted for clarity. The distances are represented in units of Å.Relative energies (in parenthesis) are given in kcal mol-1.


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Figure 6.Reaction energy profiles for nitrogen doping of GO-1 (route-c in green, route-d in red, route-e in pink, and route-f in blue). Relative energies referred to the energy of reactant (G-NH1) are calculated at the M06-2X/6-31G(d) level of theory, units in kcal mol-1.

Figure 7.Fragmental structures for stationary points involved in route-g. Atoms representations are O (red), N (blue), C (white) and H (pink).Other carbon atoms are omitted for clarity. The distances are represented in units of Å.Relative energies (in parenthesis) are given in kcal mol-1.


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Figure 8.Fragmental structures for stationary points involved in route-h. Atoms representations are O (red), N (blue), C (white) and H (pink). Other carbon atoms are omitted for clarity. The distances are represented in units of Å.Relative energies (in parenthesis) are given in kcal mol-1.

Figure 9.Fragmental structures for stationary points involved in route-i. Atoms representations are O (red), N (blue), C (white) and H (pink).Other carbon atoms are omitted for clarity. The distances are represented in units of Å.Relative energies (in parenthesis) are given in kcal mol-1.


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Figure 10.Reaction energy profiles for reactions of ammonia with GO-2 (route-g in red, route-h in blue, and route-i in green). Relative energies referred to the total energy of reactants (GO2+NH3) are calculated at the M06-2X/6-31G(d) level of theory, units in kcal mol-1.

Figure 11.Fragmental structures for stationary points involved in route-j and route-k. Atoms representations are O (red), N (blue), C (white) and H (pink).Other carbon atoms are omitted for clarity. The distances are represented in units of Å.Relative energies (in parenthesis) are given in kcal mol-1.


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Figure 12.Reaction energy profiles for ammonia participated dehydroxylation of GO-3 (route-j in red) and GO-4 (route-k in green). Relative energies referred to the total energy of reactants (GO-3+NH3) are calculated at the M06-2X/6-31G(d) level of theory, units in kcal mol-1.

Figure 13.Fragmental structures for stationary points involved in route-l and route-m. Atoms representations are O (red), N (blue), C (white) and H (pink).Other carbon atoms are omitted for clarity. The distances are represented in units of Å.Relative energies (in parenthesis) are given in kcal mol-1.


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Figure 14.Reaction energy profiles for ammonia participated dehydroxylation of GO-3 (route-l in red) and GO-4 (route-m in green). Relative energies referred to the energy of reactant GO-3 are calculated at the M06-2X/6-31G(d) level of theory, units in kcal mol-1.

Figure 15.Fragmental structures for stationary points involved in route-n and route-o. Atoms representations are O (red), C (white) and H (pink).Other carbon atoms are omitted for clarity. The distances are represented in units of Å.Relative energies (in parenthesis) are given in kcal mol-1.


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Figure 16.Reaction energy profiles for intramolecular dehydroxylation of GO-3 (route-n in red) and GO-4 (route-o in green). Relative energies referred to the energy of reactant GO-3 are calculated at the M06-2X/6-31G(d) level of theory, units in kcal mol-1.


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Formation Mechanisms of Graphitic-N: Oxygen Reduction and

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