Nitrogen-Doping Chemical Behavior of Graphene Materials with

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Nitrogen-Doping Chemical Behavior of Graphene Materials with Assistance of Defluorination Yulong Li, Xu Wang, Weimiao Wang, Rui Qin, Wenchuan Lai, Anping Ou, Yang Liu, and Xiangyang Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10276 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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The Journal of Physical Chemistry

Nitrogen-Doping Chemical Behavior of Graphene Materials with Assistance of Defluorination Yulong Li, a Xu Wang, *, a Weimiao Wang, b Rui Qin,a Wenchuan Lai, a Anping Ou, a Yang Liu, a Xiangyang Liu, a a. College of Polymer Science and Engineering, State Key Laboratory of Polymer Material and Engineering, Sichuan University, Chengdu 610065, People's Republic of China. b. School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK KEYWORDS: Two-dimensional chemistry; Fluorinated graphene; Defluorination; Nitrogen doping; Reaction pathway

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ABSTRACT: Heteroatom-doping reactions are essential to achieve advanced graphene-based materials for energy and biological areas. Unfortunately, considerably less is known regarding the detailed reaction pathways up to now. Here, we focus on investigating the nitrogen (N) doping process of fluorinated graphene (FG) under the assistance of defluorination based on modified in situ fourier transform infrared spectroscopy. It was demonstrated FG possesses a higher and more effective reactivity with ammonia in comparison with other graphene derivatives, which enable Ndoping to proceed efficiently with assistance of defluorination even at a lower temperature (16.8 at % of N at 300 °C, 19.9 at % of N at 400 °C). Combining with Density functional theory, it was proved that, at the initial reaction step of N-doping, ammonia molecule attacked and substituted the C-F of FG by the new C-NH2. Sequentially, amino group was cyclized to the three-membered ring of ethylenimine. More importantly, the dissociation and migration of C-F bonds facilitates the dissociating of C-C bonds and the recombining of C-N bonds, thus significantly promoting the N atom in ethylenimine ring to transform to the pyridinic-N or graphitic-N in graphene skeleton.


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INTRODUCTION Due to the extraordinary development of 2D materials, an increasing number of research have been focused on the chemistry of 2D materials.1-4 As the most representative one, graphene’s derivative chemical characteristics deserve special attention to promote the understanding and exerting of 2D material chemistry. Among which, doping the graphene-based materials with heteroatoms such as nitrogen (N), sulfur (S) or boron (B) has been considered as one very special reaction with great application potential, which can significantly change their physical and chemical properties, and increase their number of applications. The corresponding heteroatom doped graphene materials (HDGMs) exhibited the great application potential in the area of the super capacitor, fuel cell, lithium-ion battery and hydrogen storage and so on due to their energy gap, catalytic activity and dispersibility, and thus HDGMs have been one of research focuses under the background of the world-wild energy shortage. 5-11 Here, the pre-synthesized graphene materials were directly used as the precursor in the postsynthesis category for preparing HDGMs. In 2009, Li et al. realized the preparation of nitrogendoped graphene in bulk quantities by annealing GO in NH3.12 There, oxygen groups in GO were found responsible for reactions with NH3 and C-N bond formation. Wang et al. performed comprehensive density functional theory (DFT) computations to explore the detailed formation mechanisms of nitrogen-doped graphene materials from various GO and foreign nitrogen sources, suggesting that not only the reducing agent but also the defects in carbon skeleton are essential for reduction and doping.13 Subsequently, multiple types of HDGMs were obtained by using GO as the precursor such as S doping, B doping, nitrogen/sulfur (N/S) codoping and so on.



Recently, Liu et al. report that simple fluorination followed by annealing in a dopant source can superdope low-dimensional graphene based materials with a high level of N, S or B. For graphene, the doping levels achieved 29.82, 17.55 and 10.79 at% for N-, S- and B-doping, respectively.14 Herein, the fluorination-assistant doping method will allow more room to regulate the properties by controlling the concerntration of doped heteroatoms.15,16 However, the doping and pyrolysis behavior of fluorinated graphene (FG) in the thermal doping process is an extremely complex situation. To the best of our knowledge, there are still no research work successfully reporting the specific heteroatom-doping mechanism with the assistance of fluorination and the corresponding reaction pathway. Due to the configuration diversity of doped heteratoms, HDGMs exhibit widely different performance in a variety of fields ranging from energy storage to materials engineering. One of the biggest challenges to precise control their structure and performance is to figure out the doping process for the 2D material. Unfortunately, considerably less has been known regarding the detailed interaction process between the dopant source and these graphene derivatives until now, expecially based on experimental studies. In this study, we adopted an experimental technique of modified in situ fourier transform infrared spectroscopy (i-FTIR) as one efficient in situ monitoring technique for the heteroatomdoping process at the elevated temperature. Here, the nitrogen doping process of FG was taken as a representive to be explored. We focus on exploring the assistance effect of defluorination on the doping reactivity of graphene materials mainly by the experimental methods of i-FTIR, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). Density functional theory (DFT) computations were also performed to explore the reduction and doping mechanisms of FG by using ammonia as the nitrogen source for doping, which objectives are to elucidate the 4 ACS Paragon Plus Environment

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interactions between ammonia with fluorinated graphene and to uncover the formation mechanisms of the high N-doping levels. A feasable analysis system was developed to realize the heteroatom doping reaction of graphitized carbon materials, and one optimized route is provided for the formation of doped graphitic-nitrogen with a lower energy barrier than those reported in literature. EXPERIMENTS Preparation of FG. The fluorination was carried out according to the reference.15 RGO was put into a closed stainless steel chamber (SUS316) equipped with vacuum apparatus. After three times nitrogen replacement to remove residual oxygen and moisture in the chamber, 80 KPa F2/N2 mixed gas (F/N=1/9) was slowly injected into the chamber. Fluorination was processed at 180 °C for 1 h. Afterwards, vacuuming was employed to remove the residual gases and they were absorbed by alkali aqueous solution, the fluorinated porous reduced exfoliated graphite oxide was prepared and denoted as FG. Nitrogen doping of FG. FG was transferred into a quartz boat and placed into a horizontal tube furnace (100 mm diameter), the furnace was injected with a stable 40 sccm ammonia gas flow for 1h, the temperture was ramped from room temperature to 25 °C, 100 °C, 200 °C, 300°C, 400 °C, 500 °C, 600 °C with a 5 °C/min heating rate respectively to investigate the influence of temperature upon the extent of nitrogen doping. After 1h ammonia treatment, the NH3 gas flow was stopped and a stable 40 sccm argon flow was introduced to vent residual NH3 gas during the cooling process. Thus, the nitrogen-doped FG was obtained and denoted as NH3-FGs. Characterization. The morphology and pore structure were investigated using a scanning electron microscopy (SEM, Inspect F, FEI, USA) and a transmission electron microscopy (TEM, Tecnai 5 ACS Paragon Plus Environment


G20, 200KV, FEI, USA); a X-ray photoelectron spectroscopy (XPS, ASAM 800, Kratos, UK) with mono-chromatized Al Kα rays (1486.6 eV) under the circumstance of 12 KV × 15 mA was used to analyze the surface chemical composition of as-prepared samples, RT and 2 × 10-7 Pa vacuum pressure was required for the testing, the takeoff angle was 20° with a 6-10 sampling depth; the structural characterization was performed using a inVia Qontor (Renishaw, UK) Raman spectroscopy with a 532 nm laser; Fourier transform infrared (FTIR) spectra were recorded using a Nicolet 560 Fourier transform spectrometer in the range of 4000~400 cm-1. The modified in situ temperature-dependent FTIR measurement was also performed using this instrument. Here, the alternating aeration of ammonia and nitrogen gas was employed into the in situ FTIR instrument. IR spectrum was collected every 30 seconds after the ammonia is completely expelled out from sample cell by injecting nitrogen for a short time (5 s). DFT calculations for model molecules were performed using the DMol3 module17 implemented in Materials Studio 8.0 to calculate the condensed Fukui function and energy of fluorination reactions. Geometry optimization and TS search of researchful objects were performed by using with the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE)18 functional due to its wide application in researches of carbon nanomaterials19, and the basis was set as DNP with basis file 3.520, using Grimme method for DFT-D correction21,22. The selfconsistent-field calculation had convergence criteria of 10-5 Hartree. The frequency analysis was performed to confirm the transition state of studied reactions. RESULTS AND DISCUSSION Figure 1a shows a schematic illustration of the preparing of nitrogen-doped graphene materials starting from chemical reduced graphene oxide (RGO) which firstly activated by F2


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direct fluorination and then doped with NH3 as nitrogen source. Here RGOs were self-synthesized according to previous reported work23 and details were shown in supporting information. To begin with, the highly fluorinated graphene (FG) were synthesized by direct fluorination of RGO with F2/N2 (10 vol%) in a closed stainless steel (SUS316) chamber (10 L) equipped with vacuum line, according to the reference15. The XPS survey of FG was shown in Figure S1, and the XPS survey of

FG

shown

no

absorption

peak

of

N,

which

means

Figure 1. Synthetic route for preparing nitrogen-doped graphene materials (a); cross-section SEM image (b), TEM image (c) and selected area electron diffraction pattern (d) of NH3-FG prepared at 300 °C; the TEM EDS mapping of N-doped graphene (e, f, g); EELS line of N-doped sample synthesized at 300 °C (h); FTIR spectra of RGO, FG and NH3-FG synthesized at 300 °C (i); Raman spectra of RGO, FG and NH3-FG synthesized at 300 °C (j).



that the FG precursor doesn’t contain N element. The value of F/C molar ratio was as high as 0.75 based on the XPS data, indicating a high functionalization degree of nearly 75 % in the graphene sheets. Here, fluorination was considered as one activating method for graphene skeleton by introducing of fluorine atoms. The following doping reaction was done by annealing FGs in a NH3 atmosphere. The FGs annealing were carried out in an NH3 flow from room temperature to various temperatures up to 600 °C, corresponding product was denoted as NH3-FGs. The 2D structure of N-doped graphene was clearly shown in SEM and TEM images in Figure 1b and 1c respectively. The selected area electron diffraction pattern (Figure 1d) showed obvious diffraction rings, which indicates that the crystalline structure of graphene was destroyed by the N-doping. The TEM EDS mappings of N-doped grapheme are showed in Figure 1e, 1f and 1g. The Figure 1f and 1g indicated the distribution of C and N elements on N-doped grapheme, which powerfully confirmed the introduction of plentiful nitrogen atoms. These results in the waviness and polycrystalline structure of graphene sheet as shown in Figure 1c and 1d. Meanwhile, electron energy-loss spectroscopy (EELS) line of N-doped sample synthesized at 300 °C is shown in Figure 1h. Fine core-loss K-edges related to nitrogen at 400.5 eV can be observed. The well-defined feature corresponding to 1s π* and 1s δ* transitions was a rigid evidence of the simultaneous presence of graphitic sp2 and sp3 bonding geometry of nitrogen, validating the incorporation of N heteroatoms into the graphene skeleton24, 25. The results of the EELS characterization indicated the successful incorporation of nitrogen into the graphene skeleton as a dopant. In practice, FTIR and Raman data clearly reflect the significant structural change in this F-activating/N-doping process (Figure 1i and 1j). XPS spectra of all the samples subjected to the doping reaction shown that the concentration of nitrogen atoms was in the range of 10 ~19 at % with annealing temperature increasing from 200 8 ACS Paragon Plus Environment

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to 600 °C (as shown in Figure 2a). Among which, a large doping degree can be obtained even at a relatively low temperature (16.8 at % of N at 300 °C, 19.9 at % of N at 400 °C). Meanwhile, defluorination had been under way in the doping process. The concentration of fluorine atoms had reduced to 1.2 % from 50%, while the highest value of N-doping degree of 19.9 at% was achieved. Above 400 °C, it was hard to detect the existence of fluorine atoms. That is, the successful doping of nitrogen atoms occurred by accompaning with the complete defluorination of FGs.

Figure 2. XPS surveys (a) and curve fitted N 1s spectra (b-e) of NH3-FGs prepared with ammonia gas treatment at different temperature, 200 °C, 300 °C, 400 °C, 500 °C and 600 °C from FG; peaks in N 1s spectra represent different types of the doped nitrogen atom, pyridic nitrogen (N-6, 398.2 eV), pyrrolic nitrogen (N-5, 400.1 eV) graphitic nitrogen (N-Q, 401.1-401.7 eV), and ethylenimine nitrogen (N-3, 399.2 eV).

In addition, the bonding configurations of nitrogen atoms and their respective concentration were confirmed according to the high-resolution N 1s XPS spectra (Figure 2b, 2c, 2d, 2e and 2f). Here, peak N-6 at 398 eV is attributed to pyridic N, N-5 to pyrrolic N, and N-Q to graphitic N26, 27,

while N-3 is assigned to the nitrogen atom of ethylenimine (N-3) but not amino group according

to the following results of i-FTIR and DFT calculations. 9 ACS Paragon Plus Environment


To monitor the reaction process between NH3 and FG at an elevating temperature, a modified temperature dependent FTIR measurement was employed to in situ monitor the doping reaction. For traditional in situ temperature dependent FTIR measurement, one kind of gas (pure or mixed gases) was uninterruptedly injected into the sample cell to keep a stable gas atmosphere during the characterization. However, this is not suitable for the in situ detecting of doping reaction in NH3 atmosphere, because the IR sign of ammonia is so strong that any other components’ signs would be fully covered. Therefore, in this work, the alternating aeration of ammonia and nitrogen gas was employed into the in situ FTIR instrument in order to avoid the above problem as shown in Figure 3a. Here, IR spectrum was collected every 30 seconds after the ammonia was completely expelled out from sample cell by injecting nitrogen for a short time (5 s).

Figure 3. (a) Scheme of the modified temperature dependent FTIR instrument with an alternating aeration of ammonia and nitrogen gas, and (b) the obtained in situ FTIR spectra at different temperature.

It is hard to provide more useful information at a higher temperature for the FTIR spectra of carbon materials, so we collected the FTIR spectra of the doped FG between 40 and 400 °C at a heating rate of 5°C/min. On the basis of obtained FTIR spectra and their evolution as a function of temperature in Figure 3b and Figure S2 in Supporting Information, the nitrogen doping process 10 ACS Paragon Plus Environment

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of FG can be roughly divided into 4 stages: 50 ~ 100 °C (Stage-Ⅰ); 100 ~ 300 °C (Stage-Ⅱ); 300 ~ 400 °C (Stage-Ⅲ) and 400 °C isothermal (Stage-Ⅳ) stages. At Stage- Ⅰ , after ammonia gas was injected, a low degree reduction of FG happened instantly due to the dissociating of C-F bonds, and the color of FG varied from white to deep grey immediately. In FTIR spectra, IR peaks of C-F stretching vibration at 1200 cm-1 are obviously lower than that of pristine FG. Meanwhile, a series of new peaks (peaks at 2256 cm-1 and 2007 cm-1 as shown in Figure 3b and Figure S4) is observed, which is attributed to the rapid forming of ammonium fluoride (NH4F). Correspondingly, XRD spectra of NH3-FG-50 and NH3-FG-100 and a distinct weight increment both validated the formation of NH4F at this low temperature stage (Figure S3). It is due to that a part of C-F bond dissociated and released HF molecules which interacted with the subsequent continuous NH3 and thus generated the complexing NH4F solid. Characteristic IR peaks of NH4F were assigned to 2256 cm-1 and 2007 cm-1 (Figure S4). The strength of peak at 2007 cm-1 as a function of temperature is shown in Figure 4a, which would indirectly reflect the defluorination of FG and production of HF. The reduced C-F bonds at the first stage were assigned to those C-F bonds with a relatively low binding energy (maybe semi-ionic bonds), which fluorine atoms were easily eliminated via forming HF under attacking of NH3. In addition, it is deduced from the FTIR spectra and reported literatures that the fracture of these C-F bonds was not accompanied with the appearance of C-N and C=N bonds in this process. Afterwards, when the temperature rises from 50 °C to 100 °C, NH4F decomposed as NH3 and HF and no more C-F bonds massively dissociated, so the content of NH4F gradually decreased. 11 ACS Paragon Plus Environment


During the second stage of Stage-Ⅱ, the IR absorption peak of C-F bonds started a prominent decrease with temperature rising, while that of NH4F was continuously strengthened, suggesting the fracture of a certain amount of C-F bonds and the generation of NH4F. It is more important that two new peaks at 1585 cm-1 and 1220 cm-1 gradually appear and enhance with temperature increasing. Being consistent with previous reported FTIR of nitrogen-doped nanocarbons, these two peaks was attributed to the stretching vibration of C=N and C-N bonds. It is thus deduced that some nitrogen atoms have already been doped into the graphene skeleton when temperature exceeds100 °C being accompanied with defluorination of FG. The strength of these two peaks as a function of temperature is shown in Figure 4b. Significantly, a rapid increase of doped nitrogen happened at 150 ~ 200 °C, while a smooth increase at 200 ~ 300 °C, which is consistent with the trend of defluorination shown in Figure 4b. It should be point out that, due to the incremental enhancement with temperature, C-N absorption peak will gradually cover the weakening C-F peak at 1200 cm-1, resulting in an undistinguishable C-F vibration in FTIR spectra when the temperature exceeds 160 °C. Therefore, in this and following stages, the degree of defluorination was traced by the amount of generated NH4F.


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Figure 4. The variation of IR absorption peak vs temperature, (a) the function of the strength of peak at 2007cm-1 as temperature and (b) the function of the strength of peaks at 1585 cm-1 and 1220 cm-1 as temperature.

With temperature continuously rising from 105 °C, the amount of NH4F shows a proportional increment and peaks at 280 °C. Hence, it was supposed that the majority of C-F bonds were reduced before 280 °C under the ammonia atmosphere. Meanwhile, XPS data manifest that fluorine concentration still remains at 8.5 at% after ammonia gas treatment at 200 °C for one hour, and only 1.3 at% fluorine residue at 300 °C. It is conformed there is a specific temperature between 200 °C and 300 °C almost achieving the completely defluorination of FG. Being accompanied with defluorination of FG, nitrogen atoms were successfully doped into graphene skeleton with the absorption strength of C=N and C-N peaks consecutively enhancing, and the doping extent gradually increases in this stage. In practice, the peak temperature of defluorination of FG is higher than 400 °C 28,29, while the temperature reduces to 280 °C under the attack of ammonia molecules here. Hence, it is more likely that nitrogen doping is a synergetic process with defluorination process. More precisely, they complement each other. The existence of C-F bonds improved the reactivity between



graphene skeleton and NH3 molecules, and the attack of ammonia facilitates the leaving of C-F bonds during the reaction. For Stage-Ⅲ (300-400 °C), there are two noticeable variations. First, the intensity of C=N peak started to weaken while the intensity of C-N peak kept increasing. Second, the position of C=N peak manifested a red shift while the C-N peak showed an opposite blue shift. This suggests that the configuration of nitrogen atoms was gradually enhanced with temperature rising above 300 °C. During which, the functional chemical group of -NH2 is doped into the carbon lattice and transformed into the doped N atom, as well as the special configuration of doped N atoms inevitably variated with temperature, such as decrease of pyridic-N and increase of graphitic-N. Though the specific configuration structure is difficult to be defined only by FTIR, it is certain that this structural change stabilizes the graphene skeleton via increasing the degree of conjugation. For the last isothermal Stage-Ⅳ at 400 °C, the characteristic peaks of NH4F rapidly weakened and disappeared, indicating almost no C-F bonds was defluorinated. But, the IR absorption peaks of C=N and C-N were both enhanced in the isothermal stage. This might be attributed to the existence of other kinds of defects such as some oxygen-containing groups and edge sites, as defluorination have already completed before this stage. In addition, relying on the XPS data in Table S1, nearly all of C-F bonds were eliminated at 300 °C with the fluorine content lowering to 1.3%, and the corresponding nitrogen content arrived at 16.8 at%. Nonetheless, the further nitrogen content increase was achieved when we raised the temperature at 400 °C and 500 °C, corresponding atomic ratio value was 19.9 at% and 18.5 at% respectively. The additional improvements in nitrogen content cannot be attributed to the assistance of C-F debonding, but the oxygen-containing groups remained in reduced graphene 14 ACS Paragon Plus Environment

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oxide, because the oxygen content reduced by 3.8 % and 5.2 % after 400 °C and 500 °C treatment. Dai et al.12 has reported the nitrogen doping of graphene oxide and found the optimal doping temperature for achieving highest doping level is 500 °C. The above results indicate that both fluorine and oxygen can promote the nitrogen doping process and elevate the doping degree, whereas fluorinated graphene (FG) possesses a higher and more effective reactivity with ammonia in comparison with GO, enabling nitrogen doping to proceed efficiently at a lower temperature (< 300 °C). Combining with a large degree of functionalization, this determined that defluorination of FG significantly facilities the doping of nitrogen in graphene skeleton with a large doping degree at a relatively low temperature (16.8 at % of N at 300 °C, 19.9 at % of N at 400 °C). Defluorination of FG is a quite complex pyrolysis process by itself in practice, but finding the possible reaction pathways becomes even more challenging being accompanied with N-doping under attack of NH3. Actually, the derivative functionalization of graphene by substituting C-F bond with chemical groups -NH2, -OH, -SH et al had been reported and explored earlier in comparison with this assisted nitrogen-doping of FG with sacrificing C-F bonds. It was confirmed that, even under an ambient condition such as room temperature, various nucleophile species attacked and successfully substituted the C-F bond of FG. Herein, we assigned the initial reaction step of N-doping to the amino functionalization of FG as STEP 1 (as shown in Figure 5), new CNH2 replacing C-F. Comprehensive density functional theory (DFT) calculations were performed to find possible pathways and corresponding thermodynamic and kinetic parameters. Reaction pathways of NH3 to the graphene sheet with single C-F bond were shown in Figure 5 and Figure S5 in Supporting Information. After absorption of NH3 on FG (-5.1 ~ -5.2 kcal mol-1), a hydrogen atom in NH3 transfers from nitrogen to the fluorine of one C-F bond, resulting in a new C-NH2 bond at the para- or meta- position and a leaving HF molecule (route-a and route-b in Figure S5). 15 ACS Paragon Plus Environment


Among which, energy profiles show a low energy barrier of only 4.4 kcal mol-1 (route-b), suggesting that the amino grafting on FG is very favorable.

Figure 5. Scheme of nitrogen-doping reaction pathway at different steps. In STEP 2, being accompanied with the further defluorination, the ethylenimine moiety is more favorable to be formed in the graphene sheet. As shown in Figure 5 and Figure S6, the cyclization of amino is an exothermic reaction with assistance of defluorination, exhibiting negative reaction energy of -18.7~-14.3 kcal mol-1 and a low energy barrier of 14.0~21.7 kcal mol1.

It was determined that amino group is hard to exist in FG sheet for a long time at elevated

temperature. Especially at Stage-Ⅱ of 100~300 °C, the characteristic absorption peaks of amino group have disappeared in the in situ FTIR spectra. Correspondingly, peaks at 399 eV in C1S XPS fixed lines of N-doped FGs prepared at 200 °C and 300 °C was no longer attributed to the nitrogen atom of amino group, but to that of ethylenimine moiety in the modified graphene sheet. Then, STEP 3 represents the subsequent nitrogen-doping process on graphene sheet on the basis of the formation of ethylenimine moieties, during which nitrogen atom is introduced into 16 ACS Paragon Plus Environment

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graphene skeleton as dopant atom. In practice, nitrogen atoms are doped with various N bonding configurations such as pyridic, pyrrolic and graphitic N. For post-synthesis category of heteroatom doped graphene, previous literatures confirmed the edge and vacancy defects provide the activating sites for the doping reaction. The multi atom vacancy defects facilitate the forming of pyridic and pyrrolic N, while the single vacancies provide the opportunity for that of graphitic N. In contrast to the perfect graphene sheet, it is hard or even impossible to realize the N-doping in graphene skeleton. However, most of C-F bonds were introduced into the in-plane of graphene without creating any vacancies. That is, compared to graphene oxide, FG would not present much more edges and vacancies for N-doping. It is thus difficult to realize the super doping (N-content ≥ 20 at. %) for FG relying on the edge and vacancy defects simply. Therefore, successfully substituting the carbon atom in the in-plane of the perfect graphene skeleton by heteroatom is the key point of the super doping. Forming of ethylenimine on graphene skeleton offers potential access to high N-doping degree here, and the STEP 3 is considered at the most important step in the doping process. But, Wang et al. had raised possible reaction pathways for the nitrogen-doping on perfect graphene sheet with single ethylenimine 13. Therein, the reported pathway with the lowest energy barrier is still endothermic (reaction energy, ∆ER = 65.5 kcal mol-1) and exhibits a huge activation barrier (Eb) of 107.2 kcal mol-1. The results were further verified by our calculation in this work (∆ER = 56.8 kcal mol-1 and Eb = 105.6 kcal mol-1, as reaction energy profiles in Figure S7 shown), which indicates that the doping on perfect graphene is unlikely to take place even at elevated temperatures. The dominant step was attributed to the dissociation of C−C bonds on perfect graphene skeleton (INT3→INT4→INT5 in Figure S7) and the dissociation and recombination of



C-N bonds (INT3→INT4’), which both need huge energy requirements and result in the nitrogen doping or substitution cannot take place. Herein, we expect the assistance of defluorination to facilitate the subsequent doping of nitrogen into graphene skeleton. Two possible reaction pathways (Route-n and Route-p shown in Figure 6 and 7) of N-doping assisted by fluorination were provided based on DFT calculation. For Route-n, the rate-determining step is the first step, the dissociation of C-C bond of ethylenimine, while that of Route-p is the formation of tetra-atomic ring containing nitrogen trough the dissociation and recombination of C-N bonds. In addition, HF was confirmed as the main pyrolysis F-containing molecules in the defluorination process due to the dissociation of C-F bonds, especially under the present of protonic groups or molecules (-OH30, NH3), but a minor amount of carbon fluoride fragments were also released because of the easy dissociation of C-C bonds in FG, which resulted in the appearance of more vacancies in graphene skeleton. Here, AFM images in Figure S8 show that plentiful pores presented in graphene sheets after doping process. That is, under the attacking of protonic nucleophile (NH3 molecule) and high temperature, dissociation of C-C bonds in graphene skeleton is feasible and seems more easily. The dissociation of C-C bonds and resulted vacancies/pores would facilitate the doping of nitrogen into graphene skeleton. Further, under NH3 atmosphere, plenty of free radicals generated on the graphene sheet during 200 and 300 °C, according to the results of the in situ heating EPR testing (Figure S9), which would improve the activity of graphene skeleton and greatly enrich reaction pathways. 31


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Figure 6. Fragmental structures for stationary points involved in route-n and -p. Both of these two routes can obtain graphitic N (NG) and pyridic N (NP). Atoms representations are C (black), N (blue), H (white), and F (pink). Other carbon atoms are omitted for clarity. The distances are represented in units of angstroms. Relative energies (in parentheses) are given in kcal mol−1.



Figure 7. The reaction energy profiles of STEP 3 provided based on DFT calculation.

CONCLUSIONS In conclusion, assistance of defluorination on N-doping of graphene skeleton was powerfully proved. FG possesses a higher and more effective reactivity with ammonia in comparison with GO, enabling nitrogen doping to proceed efficiently at a lower temperature. Combining with a large degree of functionalization, this determined that defluorination of FG significantly facilities the doping of nitrogen in graphene skeleton with a large doping degree at a relatively low temperature. For the possible reaction pathways, the substitution of C-F bond in FG by the new CNH2 under the attack of ammonia molecule was considered as the initial reaction step of N-doping, resulting in the replacing of C-F by C-NH2. Sequentially, amino group was cyclized to the threemembered ring of ethylenimine. In addition, existence of fluorine activates the graphene skeleton of FG, resulting in plentiful vacancies and pores at elevated temperature, which defects are propitious to the forming of pyridinic N and pyrrolic N. Meanwhile, the dissociation and migration of C-F bond facilitates the dissociating of C-C bonds and the recombining of C-N bonds, significantly reducing the energy barrier of ethylenimine transforming to the pyridinic N or graphitic-N in the in-plane of graphene skeleton. 20 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author * Corresponding author Tel.: +86-028-85403948 E-mail address: [email protected] (Xu Wang *) Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 51803129 and Grant No. 51633004), State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2018-3-08) and the Fundamental Research Funds for the central Universities. We also acknowledge National Demonstration Centre for Experimental Materials Science and Engineering Education, Sichuan University in China for characterization. Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website. Synthesis of rGO; XPS surveys of FG and NH3-FGs prepared with ammonia gas treatment at different temperature(Figure S1); In situ FTIR of FG under NH3 atmosphere (Figure S2); XRD images of NH3-FG prepared at different temperature and NH4F (Figure S3); FTIR image of NH4F (Figure S4); table of XPS data of N-FG prepared at different temperature (Table S1); DFT calculation data of STEP 1 (Figure S5); DFT calculation data of STEP 2 (Figure S6); DFT calculation data of TEP 3 (Figure S7); AFM images of FG and NH3-FG prepared at 300



°C (Figure S8); In situ heating EPR testing of FG under different atmosphere (Figure S9). (PDF)

ABBREVIATIONS

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Table of Contents Graphic:


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Nitrogen-Doping Chemical Behavior of Graphene Materials with

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