Synthesis of Carbon Nanosheets and Nitrogen-Doped Carbon

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Synthesis of Carbon Nanosheets and Nitrogen-doped Carbon Nanosheets from Perylene Derivatives for Supercapacitor Application Roby Soni, Vidyanand Vijayakumar, and Sreekumar Kurungot ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00888 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Nano Materials

Synthesis of Carbon Nanosheets and Nitrogen-doped Carbon Nanosheets from Perylene Derivatives for Supercapacitor Application Roby Soni†,‡,Vidyanand Vijayakumar†,‡ and Sreekumar Kurungot*†,‡ †

Physical and Materials Chemistry Division, National Chemical Laboratory, Pune-411008, India.

‡

Academy of Scientific and Innovative Research (AcSIR), New Delhi-111008, India.

KEYWORDS. Carbon nanosheets, nitrogen-doped carbon nanosheets, 3,4,9,10-perylene tetracarboxylic dianhydride, 3,4,9,10-perylene tetracarboxylic diimide, supercapacitor

ABSTRACT. The development of an economical, eco-friendly, and easy method for the production of carbon nanosheets and heteroatom-doped carbon nanosheets has been a challenge for material scientists. In this study, we have developed a method for the synthesis of carbon nanosheets (CNS) and nitrogen-doped carbon nanosheets (NCNS) of high quality from organic compounds. We have exploited the electrostatic interaction among ionic crystals (NaCl in the present case) and polarized aromatic molecules (3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and 3,4,9,10-perylene tetracarboxylic diimide (PTCDI) in this work) to obtain uniform coverage of the latter over the surface of the ionic crystal. This NaCl-PTCDA/PTCDI assembly, on pyrolysis at 700 °C followed by washing with water, yields high-quality CNS and NCNS depending on the aromatic precursor employed. The sheets obtained consist of minimum 2-3 ACS Paragon Plus Environment

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layers (approximately, 2 nm) as observed from atomic force microscopy. The process demonstrated is highly scalable, economical, eco-friendly, non-hazardous and relatively fast. Moreover, the NCNS derived from PTCDI is applied for energy storage application by fabricating an electrochemical capacitor which exhibits an area normalized capacitance of 23 µF cm-2 in 0.5 M H2SO4, which is same as that reported for the single layer nitrogen-doped graphene. Furthermore, an NCNS-carbon black (NCNS-CB) composite is prepared and applied to a high-performance solid-state supercapacitor that exhibited a capacitance of 300 mF cm-2.

Carbon nanosheets, on account of their properties like high conductivity,1 large surface area, high mechanical strength, flexibility, and environmental stability, have found many applications in electronics, sensors, charge-storage devices, 2-3etc. At the same time, heteroatom-doped carbon nanosheets, particularly nitrogen-doped carbon nanosheets (NCNS) exhibit enhanced electronic properties due to increased charge-carrier density and band gap modification4 have also found applications in electrocatalysis and electrochemical charge storage devices like batteries5 and supercapacitors. A number of synthesis routes like chemical vapor deposition,6 chemical or physical exfoliation,7 templating,8 solvothermal synthesis,9-10 and self-assembly11 have been used for the synthesis. These methods have unique requirements of their own like gas precursors in the CVD method, weak interaction between the layer of the precursors for the exfoliation, etc. Heteroatom-doped CNS shows improved properties for electrochemical applications12 which have attracted the attention of research community for their storage properties. Particularly, nitrogen-doped CNS (NCNS) shows improved charge storage than CNS due to increased electron density and improved conductivity. To meet the requirement of CNS and heteroatom-doped CNS for such applications, a synthesis method that can produce CNS in bulk and at a low cost is indispensable. Many methods reported ACS Paragon Plus Environment

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in the literature employ multistep procedures to prepare CNS and NCNS which make the synthesis tedious and less cost-effective. NCNS are commonly synthesized by doping of graphene sheets or CNS. For instance, porous NCNS were synthesized from polypyrrole functionalized graphene sheets by KOH activation.13 Graphene oxide synthesis is tedious which makes the NCNS preparation from graphene oxide time-consuming and expensive. Moreover, pyrolysis of carbon precursor with heteroatom source has also been widely used for NCNS synthesis. Sevilla et al. had synthesized CNS through pyrolysis of potassium citrate at a high temperature.14 Although the procedure is a simple and single step, it cannot be used to derive heteroatom-doped CNS. A procedure demonstrated by Kang et al. utilized magnesium citrate and potassium citrate to prepare CNS which is treated with NH3 gas at high temperature to introduce nitrogen doping in CNS.15 This procedure involved one carbonization step, two thermal activation stages, washing and NH3 treatment at a high temperature to obtain NCNS. The procedure is multistep and it consumes high energy which makes it cumbersome and costineffective. In another report by Mullen and group, mesoporous NCNS were derived from graphene oxide using silica nanoparticles for inducing porosity and polydopamine for nitrogen doping.16 The above discussion concludes that the methods available for synthesis of heteroatom doped CNS are multistep which makes them difficult to scale-up and thus less cost-effective. Therefore, a synthesis method for CNS and heteroatom-doped CNS is a critical requirement to advance their application in electrochemical and other systems.

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Scheme 1. Schematic representation of the chemistry involved and the synthesis procedure used 3,4,9,10-perylene into to transform 3,4,9,10-perylene perylene tetracarboxylic dianhydride (PTCDA) and 3,4,9,10 carbon nanosheets (CNS) and nitrogen ts (NCNS), respectively. nitrogen-doped carbon nanosheets bottom-up This paper reports the development of a bottom up synthesis procedure for the production of CNS and NCNS from the polycyclic aromatic compounds. 3,4,9,10-Perylene 3,4,9,10 Perylene tetracarboxylic 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and 3,4,9,10 xylic diimide (PTCDI) are the aromatic organic precursors used for the synthesis of CNS and NCNS, respectively, with sodium chloride crystals acting as reaction surfaces. The procedure demonstrated here is highly environmentally friendly as it does not employ loy any hazardous and corrosive chemicals. The materials used for synthesis are NaCl, n-hexane, hexane, PTCDA, PTCDI, and water. From the material listed above, NaCl, ACS Paragon Plus Environment

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n-hexane, and water can easily be recycled which makes the process very economical and costeffective. The process demonstrated is a simple two-step method involving the assembly of organic molecules on ionic crystal followed by pyrolysis. Whereas, many of the procedures reported for the synthesis of CNS and NCNS are multistep and tedious.17-18 The prepared NCNS was subsequently used as electrode material in supercapacitors and compared the performance characteristics with the NGr prepared by the post-treatment of Gr (synthesized by the modified Hummer’s method) using melamine as the nitrogen precursor at high temperature. The homemade system shows better performance compared to the NGr prepared using the Hummer’s method. Result and Discussion Sodium chloride is an ionic crystal with an FCC lattice in which chloride ions are present at the corners and the face center of the cube while sodium ions are present at the edge centers and the center of the FCC lattice. Sodium and chloride ions are held together by electrostatic interaction. This electrostatic field of the NaCl crystal can lead to interaction with other



Figure 1.. The thermogravimetric analysis. analysis (a) TGA curves of PTCDA-NaCl, NaCl, (b) the TGA profile corresponding to the pyrolysis of PTCDI-NaCl, PTCDI (c) comparative infrared (IR) of PTCDA and CNS, (d) PTCDI and NCNS and (e) shows the IR spectra of CNS and NCNS.

molecules that have polarity and charge separation, such as PTCDA and PTCDI.19 Scheme 1 shows the strategy adopted to transform the perylene derivatives to CNS and NCNS and it also summarizes the underlying principle of the synthesis. Physical changes occurring during the synthesis of CNS and NCNS are shown in Figure S1. The white crystals off NaCl becomes red


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after the adsorption of PTCDA on the surface and turns black after pyrolysis at 700 °C. Similar changes occur with PTCDI where the NaCl crystal becomes dark purple on the adsorption of PTCDI which subsequently turns black during heat treatment due to its conversion to NCNS. The quantity and ratios of precursors and solvent used for the synthesis of the different samples are listed in Table S1. The bimodal growth of PTCDA on NaCl was studied by Burke et al., in their study for multilayer adsorption of PTCDA on NaCl, where the authors observed complete coverage in a layer-by-layer arrangement.19 As shown in the schematic, both PTCDA and PTCDI have a perylene core that consists of five conjugated benzene rings having delocalized pielectrons as well as two oxygen (PTCDA) or two nitrogen (PTCDI) atoms at the ends. PTCDA and PTCDI interact electrostatically with the sodium chloride crystal in which the more electronegative ions (oxygen and nitrogen) act on the Na+ while the carbon core interacts with the Cl- through electrostatic force of attraction.19 The above-mentioned phenomenon also results in the adsorption of the perylene derivatives in a layer-by-layer fashion over the NaCl crystal. The perylene-NaCl assembly thus formed, when heated at a temperature of 700 °C in the inert argon environment, undergoes carbonization leading to CNS formation. As mentioned before, sodium chloride provides the surface for the formation of the 2D assembly. The absence of such a surface leads to the formation of pillared stacked carbon, which will be discussed in the later sections. The CNS and NCNS thus formed are recovered from the CNS/NCNS-NaCl by simple dissolution of NaCl in deionized water followed by filtration and drying. In the current study, the ratio of PTCDA:NaCl was varied to manipulate the thickness of CNS and NCNS. The PTCDA:NaCl ratios used here are 1:10, 1:50 and 1:100. On the other hand, the PTCDI:NaCl ratio was kept at 1:100 as this composition resulted in CNS with lowest thickness. More details about the synthesis procedure are given in the Supporting Information.



Figure 2.. (a) The XRD profile recorded for the CNS derived from PTCDA, where, the inset shows the change in the crystallite size of CNS with respect to the PTCDA:NaCl ratio;(b) the XRD pattern obtained in the case of the NCNS derived fro from m PTCDI; (c) and (d) represent the Raman spectra of CNS and NCNS, respectively respectively.

Firstly, thermogravimetric analysis of NaCl, PTCDA-NaCl and PTCDI-NaCl NaCl was carried out to understand the effect of temperature on the respective perylene derivatives. The corr corresponding TGA plots are shown in the Figure 1a, 1a b and c. The thermogram of NaCl, presented in Figure 1a, does not show any weight change up to 700 °C which is expected as it is an ionic crystal having a melting point of 800 °C and it does not contain any volatile matter. The weight loss in PTCDA (Figure 1b) begins at 413 °C, where a change in the weight can be observed which can be ascribed to the decomposition of the perylene core and the release of gases most likely CO2 or CO.20 The weight loss continues up to a temperature 464 °C, after which, it remains almost ACS Paragon Plus Environment

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constant up to 700 °C. The TGA profile shows that the carbonization of PTCDA begins after 460 °C. Similar to the TGA profile of PTCDA-NaCl, the TGA curve (Figure 1c) for PTCDI-NaCl shows a continuous slow change in weight which, at 700 °C, shows a cumulative weight loss of 5 %.

Figure 3. SEM analysis (a) NaCl crystal, (b) 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), (c) 3,4,9,10-perylene tetracarboxylic diimide (PTCDI), (d) image showing the uniform coverage of PTCDA over the surface of the NaCl crystal (scale bar of the inset image is 5 µm), (e) the surface of the NaCl crystal after adsorption of PTCDI (scale bar of the image in the inset is 50 µm) and (f) the NaCl crystal showing the incompletely detached CNS over its surface after pyrolysis.

Comparative infrared (IR) spectra of the PTCDA and CNS are shown in Figure 1d, where, the vibrational peaks of the anhydride (1772 cm-1) dianhydride (1298 cm-1)and C=C (1595 cm-1) ACS Paragon Plus Environment


bond are observed for PTCDA.21 However, after its transformation into CNS, most of the peaks corresponding to PTCDA disappear from the IR spectra, and the most prominent peaks appeared are identified as those representing the C=C and C-O stretching frequencies. The C-O stretching frequency can be attributed to the carbonyl groups that might have formed during the pyrolysis process. Similar results are obtained while comparing the IR spectra of PTCDI22 and NCNS (Figure 1e), in which, similar to the PTCDA, the peaks observed in the IR spectrum of PTCDI are no longer visible after its transformation to NCNS. A small peak (Figure 1f) that can be attributed to the C=N stretching appears at 1587 cm-1.23 The surface coverage of the PTCDA over the NaCl crystal was manipulated by changing the ratio of PTCDA to NaCl to control the crystallite size of CNS; more information is given in ESI. As mentioned earlier, three different ratios of PTCDA-NaCl were used. When a higher amount of NaCl is used in the reaction mixture, keeping the amount of PTCDA and PTCDI constant, the system attains a larger surface for the adsorption of these moieties. This subsequently leads to a CNS (or NCNS) lattice with lesser number of sheets. The crystallographic characteristics of the PTCDA-derived CNS and PTCDI-derived NCNS were evaluated through powder X-ray diffraction (XRD) studies. The corresponding XRD patterns for the CNS and NCNS obtained for the composition corresponding to the organic precursor to NaCl ratio of 1:100 are given in Figure 2a and b, respectively. The characteristic (002) peak of CNS can be seen at the 2Θ value of 24.58° along with the minor peaks with one at 42° corresponding to the (100) plane.24 Both CNS and NCNS have an inter-planar distance of 0.36 nm between the carbon layers. The interplanar distance between the honeycomb carbon lattices in the graphite is 0.335 nm; however, the inter-planar distance of 0.36 nm in the few-layered CNS is higher than that of the graphite.25 The crystallite size was calculated using the Scherrer equation26 (for details refer Supporting


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Information and Table S2 for full width at half maximum values) for the CNS prepared by taking different ratios of PTCDA and NaCl. The crystallite size corresponding to a PTCDA:NaCl ratio of 1:100 is 1.47 nm, while, for the ratios of 1:50 and 1:100, the values are 1.40 and 1.25 nm, respectively, (inset Figure 2a). The XRD pattern recorded for the NCNS (PTCDI:NaCl, 1:100) is shown in the Figure 2b. The measured inter-planar distance in this case is 0.36 nm and the crystallite size is 1.23 nm. The decrease in the crystallite size with increase in the amount of NaCl confirms that the surface coverage of PTCDA or PTCDI can be manipulated to alter the resultant thickness in the respective final products (i.e., CNS or NCNS). The XRD patterns recorded for CNS prepared with different ratios of PTCDA and NaCl are given in Figure S2.

Figure 4. Field emission scanning electron microscopy (FESEM) images of CNS and NCNS: (a), (b) and (c) represent images recorded for the CNS synthesized from PTCDA-NaCl at the



ratios of 1:10, 1:50 and 1:100, respectively and (d) the NCNS obtained corresponding to the PTCDI-NaCl ratio of 1:100. Raman spectroscopy analysis of CNS and NCNS was carried out to assess the structural changes that occur during the CNS formation as well as the defect density in CNS and NCNS. Moreover, the quality of CNS and NCNS was also determined from the Raman analysis. Tenne et al., had recorded the single crystal Raman spectra of PTCDA;27-28 in their study, the most intense Raman peaks appeared at 1302 and 1380 cm-1 were assigned to the modes with prevalent contribution from the C-H bending. The strong peaks at 1570 and 1589 cm-1 were ascribed to the C-C vibrations arising from the perylene core, and the vibrations due to C=O were observed at 1780 cm-1. The Raman spectrum29 recorded for the CNS powder derived from PTCDA is remarkably different and is shown in Figure 2c. The D peak at 1350 cm-1 and G peak at 1580 cm-1, characteristic for the carbon sheets, are clearly observed. The D peak in carbon structures caused by the recombination of elastic scattering by the defect or zone boundary and the inelastic scattering of the charge carriers by the phonon can essentially be used for the quantification of the defects.30 The ID/IG ratio calculated for CNS is 0.97, which indicates a significant degree of defects; the defects present can be attributed to the oxygen functional groups, grain boundaries, and stacking faults. Also, the distance between the defects,31 LD, is the measure of the amount of disorder in CNS; for CNS, LD is calculated to be 13.75 nm (for details refer Supporting Information). This LD value falls in the low defect density range.32 Figure 2dshows the Raman spectrum of NCNS, and the ID/IG ratio measured for NCNS is 1.0 which is higher compared to CNS; the nitrogen present in the lattice causes the increased defect density in NCNS. The distance between the defects (LD) for NCNS is13.54 nm, which is less than that in CNS, inferring to the high defect density due to the presence of nitrogen. The ID/IG ratio measured for


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the CNS and NCNS is higher than that for the graphite33 and lower than that measured for the amorphous carbon which generally shows show the ID/IG ratio up to 1.6.34 Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) M) and atomic force microscopy (AFM) analyses were carried out to discern the morphology of the PTCDA, PTCDI, CNS, and NCNS. SEM images of NaCl, PTCDA, and PTCDI are given in Figure 3a, b and c,, respectively. NaCl crystals (Figure 3a and Figure S S3) show a cubic structure having dimensions of approximately 200 μm while SEM image of PTCDA show small wheat like morphology. On the other hand, PTCDI particles are spindle-shaped shaped particles with wheat-like small, thin, flat and flake-like like in



Figure 5. Transmission electron microscopy (TEM) images of the CNS and NCNS prepared in this study: (a) the TEM image of the CNS evolved by maintaining the PTCDI-NaCl ratio of 1:100, (b) the high resolution TEM image obtained for the CNS sheets obtained at a PTCDINaCl ratio of 1:100, (c) the TEM image of the NCNS synthesized from PTCDI-NaCl by maintaining a ratio of 1:100 and (d) the high resolution TEM image of the NCNS prepared by maintaining the PTCDI-NaCl ratio of1:100, (e) SEM image showing graphene cube, (f) TEM images depicting cubical morphology of CNS. shape. Both PTCDA and PTCDI adsorb on the surface of NaCl crystals due to the electrostatic force acting on the polarized perylene derivatives. SEM images recorded for the PTCDA and PTCDI adsorbed on the surface of NaCl crystals are shown in Figure 3d and 3e, respectively. PTCDA and PTCDI form a uniform cover over the NaCl crystal, which is conspicuous in the SEM images provided in the inset of Figure 3d and e. Also, the uniform coverage of the PTCDA and PTCDI on the surface of NaCl can be seen more clearly in the SEM images given in Figure S4 and S5, respectively. Furthermore, the element maps were also recorded for the carbon and oxygen atoms on the surface of the NaCl-PTCDA and NaCl-PTCDI assemblies to show the adsorption of the organic compound on the NaCl surface; the corresponding images are given in Figure S6. The corresponding element maps clearly show the coverage of the NaCl surface by the organic molecules. When the NaCl-PTCDA and NaCl-PTCDI assemblies are pyrolyzed, the corresponding molecules form CNS and NCNS, respectively, on the surface of NaCl, which upon washing or removal of NaCl gives CNS and NCNS in each case. Figure 3f is that of an incompletely washed NaCl-CNS crystal that was captured by SEM and it shows a sheet of CNS detaching from the surface of the NaCl crystal, demonstrating how the CNS sheets are evolving out upon washing or removing the NaCl crystals in water. To ascertain the role of NaCl in the


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whole process, PTCDA alone was also pyrolyzed at 700 °C to obtain carbon; the SEM images recorded for the carbon so-derived are given in Figure S7. The images show a stack-like arrangement of thin carbon sheets forming rectangular pillars with vacant spaces between the sheets. The structure can be understood in terms of the π-π stacking tendency of PTCDA.35 Thus, it is indicated that the NaCl, through its electrostatic field which acts on the polarized molecules, is indeed necessary for the CNS synthesis. FESEM images of the CNS corresponding to the PTCDA:NaCl ratios of 1:10, 1:50, and 1:100 are given Figure 4a, 4b, and 4c, respectively. The images show large-area CNS sheets for all the three ratios FESEM image of NCNS derived from PTCDI-NaCl (1:100) is given in Figure 4d; like the CNS obtained from PTCDA, it exhibits a sheet-like morphology. To get more insight into the morphology of CNS and NCNS at the nanometer level, TEM analysis was carried out. TEM images of the CNS derived for a PTCDI-NaCl ratio of 1:100 are given in Figure 5a and b, and the 2D sheet structure typical of CNS is observed in both the cases. The TEM images corresponding to the different ratios of PTCDA:NaCl are given in Figure S8. The TEM images recorded for NCNS corresponding to the PTCDI-NaCl ratio of 1:100 are given in Figure 5c and



Figure 6.. Atomic force microscopic investigation of CNS and NCNS samples at the organic precursor to NaCl ratio of 1:100: (a) the AFM image of CNS, (b) the AFM image used to extract height profile of CNS, (c) height profile of CNS (parameters, length: 1.29 µm, m, Pt: 2.57 nm and scale: 4.00 nm),, (d) the AFM image of NCNS, (e) the AFM image used for obtaining height profile of NCNS and (f) height profile of NCNS (parameters, length: 0.982 µm, m, Pt: Pt 1.89 nm and scale: 3.00 nm). d;; these images also show the characteristic characteristic transparent sheet structure of CNS. The sizes of the sheets as observed in the images are in the range of several hundred nanometers. If required, smaller sheets can be prepared by changing the size of the NaCl crystal used in the synthesis. ACS Paragon Plus Environment

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Along with these, the high-resolution resolution images in Figure 5b and d show the boundary wall of the CNS and NCNS, that appears to be folded as indicated by the difference in contrast between the Cube-shaped edge and centre of the sheets. Cube shaped CNS was also observed in the FESEM images in Figure 5e.. The CNS cube shown seems to be formed by the layer-by-layer layer layer assembly of the CNS sheets. A small opening is also observed on the upper face and edge of the cube; apparently, during washing, NaCl is removed from these openings which prevent prevent the breakdown of the step like structures can also be seen on the visible faces of cubical structure. Moreover, ridges or step-like the cube. CNS cubes which were observed in the FESEM images were also captured in the TEM analyses, which are shown in Figure 5f.Also, the selected area electron diffraction pattern recorded for the CNS and NCNS samples are given in Figure S9,, and the diffraction pattern shows diffused rings from which the short-range short range order and amorphous nature of the CNS and NCNS can be deduced.

Figure 7. X-ray ray photoelectron spectroscopic investigation of CNS (1:100) and NCNS (1:100): (a) survey spectrum of CNS, (b) deconvoluted C 1s spectra of carbon in CNS, (c) deconvoluted



O 1s spectra of the oxygen present in CNS, (d) survey spectrum of NCNS, (e) deconvoluted C 1s spectra of NCNS and (f) deconvoluted N 1s spectra of NCNS. AFM was carried out to determine the surface profile and also to determine the thickness of the CNS and NCNS. AFM images and the height profiles obtained are shown in Figure 6. The image recorded for CNS given in Figure 6a shows the flat, sheet-like surface profile that is characteristic of good quality CNS. Figure 6b and 6c represent the AFM images used to measure the thickness and the corresponding height profile, respectively. The thickness of the pristine single-layer graphene is 0.34 nm; however, due to the presence of functional groups, adsorbed water molecules and defects, the thickness measured by AFM for the single-layer graphene varies from 0.7-1 nm.36-37 The minimum thickness of CNS estimated from the height profile in the present case is 2 nm, which indicates the existence of to two layers.44 Also, the thickness measured for CNS varies from 2-4 nm, and the corresponding height profiles are given in Figure S10a. Similarly, the AFM image of NCNS given in Figure 6d shows a flat surface like that of CNS; Figure 6e and 6f lead to the estimation of a minimum thickness of 2 nm for NCNS, which indicates the presence of approximately two layers while the thickness of the NCNS varies in the range of 2-5 nm (Figure S10b).37 X-ray photoelectron spectroscopy (XPS) analysis of CNS and NCNS was performed to analyze the elemental composition and nature of bonding. The survey spectrum of CNS is given in Figure 7a; the percentage of oxygen calculated from the survey spectra is 5.87 % while the carbon content is 93.35 %. A small amount of Na (0.27 %) and Cl (0.51 %) is also observed indicating small amount of unwashed NaCl. Furthermore, C1s (Figure 7b) and O1s (Figure 7c) spectra were deconvoluted to obtain the nature of the oxygen and carbon linkages. The component of carbon corresponding to the binding energy of 284.0 eV represents the sp2 C-C


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bond38 in the CNS which forms 74.67 % of the spectrum while the peak at 284.8 shows the presence of the C-O O bond whose percentage is calculated calculated to be 25.33. EDAX analysis ((Figure S11a) of CNS gives the atomic percentage of carbon and oxygen to be 90.88 % and 9.12 %, respectively. The difference observed between the XPS and EDAX data for the element composition is due to the difference in the fundamental principles of the two techniques. Generally, EDAX, with its penetration depth of several microns, provides the bulk composition while XPS is a surface technique that can provide information up to few nm thickness.39 Figure 7d represents the survey spectrum of NCNS,40 that shows the presence of carbon, oxygen, and nitrogen corresponding to binding energies of 285.1, 532.1, and 401.0 eV, respectively. The surface composition of NCNS determined from the XPS shows the presence of 91.35% of carbon, 7.01 % of oxygen ygen and 1.37 % of nitrogen along with 0.27 % Na; however, Cl was not detected in this spectrum.

Figure 8.. Electrochemical characterization of the solid-state solid state device based on the electrodes derived from the NCNS prepared in this study and poly(HEMA-co-TMPA)-H3PO4 gel polymer electrolyte: (a) cyclic voltammograms recorded at different scan rates, (b) galvanostatic charge chargeACS Paragon Plus Environment


discharge profiles obtained at different current densities, (c) variation of capacitance with current density, (d) Nyquist plot representing equivalent series resistance and frequency behavior, (e) illustration of the real component of the device component as a function of frequency and (f) durability test carried out at a current density of 4 mA cm-2. The deconvoluted C1s spectra in Figure 7e show three regions representing the C-C, C-O, and C-N bonds at the binding energies of 284.4, 285.0, and 287.8 eV, respectively. The C-N linkage has also been observed in the IR spectrum of NCNS. Furthermore, the deconvoluted N1s spectra shown in Figure 7f can be resolved into three regions corresponding to the pyridinic N at a binding energy of 398.1 eV (20.33 %), pyrrolic N at a binding energy of 400.2 eV (72.88 %), and pyridinic N-oxide with a binding energy of 403.7 eV (6.79 %).41EDAX analysis of NCNS, given in Figure S11b, showed the nitrogen content to be 7.87 at. % while carbon and oxygen content are 83.43 and 8.70 at. %, respectively. The differences in the EDAX and XPS can be attributed to the reasons previously discussed in this section. Also, the elemental map given in Figure S12b shows uniform and homogeneous distribution of nitrogen in NCNS. Application of NCNS in electrochemical capacitor NCNS possesses properties like large surface area,42 good electrical conductivity,43 and increased density of states compared to pristine CNS.44 These properties have been exploited for application in electrochemical charge storage devices like the electrochemical capacitor or supercapacitor. Many reports can be found in the literature where different strategies have been used to improve the capacitance of NCNS. For instance, crumbled NCNS synthesized through confined polymerization of dopamine, although possessing a large surface area, exhibited only a modest capacitance of 128 F g-1.45 Cao et al., synthesized NGr from graphene oxide through a hydrothermal process using ammonium bicarbonate as a nitrogen source.46 It showed a


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capacitance of 170 F g-1 in a 5 M KOH electrolyte. Despite the large surface area, the capacitance of NGr obtained so far is much lower than its achievable capacitance. Limited accessibility to the electrochemically active surface, layer stacking, and limited electrolyte ion movement are the factors responsible for the inferior supercapacitor performance of this material. Here we have employed the NCNS derived from PTCDI as an electrode material for fabricating a solid-state supercapacitor. The performance of NCNS in the device is compared by fabricating a similar device by employing nitrogen-doped graphene synthesized by nitrogen doping of graphene oxide prepared through the modified Hummer’s method (designated as Hummer’s-NGr).47 Furthermore, a composite of NCNS and carbon black (CB) is also examined (designated as NCNS-CB) which shows superior performance compared to the individual performances of pristine NCNS and carbon black. Before its application in a capacitor, surface area analysis of NCNS was carried out to understand the pore size distribution and surface properties. BET-nitrogen gas adsorption analysis of NCNS indicated a surface area of 714 m2 g-1.Adsorption isotherm and pore size distribution profiles are shown in Figure S13. Pore size distribution profile shows a microporous distribution with an average pore size of 1.26 nm and the pore volume obtained for NCNS is 0.80 cc g-1. The microporous distribution and high surface area are advantageous for achieving high energy density. Thus, from these morphological features, NCNS is expected to exhibit high capacitance and energy density.48 Both heteroatom doping and high surface area are necessary for the high capacitance of a supercapacitor. The capacitance obtained for single layer graphene having the surface area of 2630 m2 g-1 is 6 µF cm-2; however, the capacitance has been found to increase to 23 µF cm-2 with nitrogen doping of the single-layer graphene.44 Also, the capacitance



increases with the nitrogen content. In the single layer graphene, the electrolyte accessibility is not an issue as its complete surface is exposed to the electrolyte. However, high surface area and porosity become important for the systems which are flat and accessible as single layer graphene. A high surface area with high porosity improves the impregnation of the material surface by the electrolyte ion, thereby improving its capacitance value. Also, the capacitance of the reduced graphene oxide has been observed to increase when it is doped with heteroatom for the similar surface area. Thus, we can conclude that the heteroatom doping has the significantly greater effect than the surface area but a high surface area is important for better facile movement of the electrolyte ions to the electrode material where the electrode materials are not as flat as single layer graphene.

The capacitive properties of CNS, NCNS, Hummer’s-NGr, and NCNS-CB composite were first analyzed in a conventional three-electrode set up in 0.5 M H2SO4. Cyclic voltammograms (CV) recorded at a scan rate of 10 mV s-1 for CNS, NCNS and Hummer’s NGr are given in Figure S14a; the CV features of both show marked differences from each other. The CV of CNS shows a rectangular behavior characteristic of the electrochemical double layer. Also, the area under the curve is less than that of NCNS which shows its low capacitance compared to NCNS. A sharp oxidation peak is observed for Hummer’s NCNS which shows the presence of a high degree of functionalized CNS. On the other hand, in the CV of NCNS (derived through the NaCl route), the sharp redox peak is not seen while very broad redox peaks are observed which indicate a low degree of functionalization unlike Hummer’s NCNS. Also, the area under the CV is larger for NCNS indicating its higher capacitance compared to CNS and Hummer’s NGr. Galvanostatic charge-discharge (GCD) curves were recorded at a current density of 0.5 A g-1 to


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measure the capacitance; the corresponding GCD curves are given in Figure S14b. The capacitance measured for NCNS is 166 F g-1and for Hummer’s NGr is 136 F g-1 whereas, a capacitance of 82.5 F g-1has been measured for CNS. This result shows the superior charge storage performance displayed by the newly synthesized NCNS to the similar material obtained from the widely used Hummer’s method. Moreover, Zhang et. al.44 in their work on NGr had calculated the area normalized capacitance for single layer NGr to be 23 µF cm-2 at a N-doping of 2.3 at. %. The area specific capacitance calculated by dividing the gravimetric capacitance by the BET area for the NCNS derived from PTCDI is 23 µF cm-2, which is similar to the capacitance for the single layer NGr obtained by Zhang et.al. To further improve the electrochemically active surface area, which is the surface available for the electrolyte ion to undergo electrochemical reactions or adsorption, a composite of NCNS with carbon black in the mass ratio of 1:1 was prepared. The details of the composite preparation are given in Supporting Information. To understand the role of CB in the composite and to ascertain the enhancement in the electrolyte-accessible surface, electrochemical active surface area (ESA) measurements were performed by using a Fe2+/Fe3+ couple. The CVs recorded at a scan rate of 20 mV s-1 for NCNS, CB and NCNS-CB are shown in Figure S15. The peak corresponding to the oxidation current measured for the Fe2+ to Fe3+ transition can be related to the ESA of the material by the Randles-Savick equation.49 As can be seen in the CVs, the peak current is largest for the composite when compared to the pristine NCNS and CB (for details see Supporting Information). CB in the composite acts to provide space between the NCNS crystallites in the composite which helps in the smooth diffusion and improves the accessibility of the electrolyte ions to the available NCNS surface.



Single electrode studies of the NCNS-CB composite in 0.5 M H2SO4 and its comparison with the pristine NCNS and CB are given in Figure S16. As clearly seen in Figure S16a, the largest area under the CV is swept by the NCNS-CB composite. CVs show the enhanced capacitance of the composite upon the addition of CB. The specific capacitance measured for the NCNS-CB composite from the GCD (shown in Figure S16b) at a current density of 0.5 A g-1 is 358 F g-1 which is higher than both NCNS (166 F g-1) and CB (184 F g-1). Moreover, the frequency behavior of NCNS-CB is better than the pristine NGr as evident from the Nyquist plot (Figure S16c) owing to the facilitated diffusion of the electrolyte ions in the composite. However, higher ESR was measured for NCNS-CB (2.63 Ω) than NCNS (1.35 Ω), which can be explained by the increased grain boundaries that develop at the point of contact between NCNS and CB, thereby increasing the electrical resistance. Solid-state supercapacitors are a new class of capacitors in which the liquid electrolyte is replaced by a gel electrolyte which relieves the device from the need for a metal casing, making it lightweight, thin, and flexible. In this context, we also tested the composite in a solid-state supercapacitor made using poly(hydroxyethyl methacrylate-co-trimethylolpropane allyl ether) (HEMA-co-TMPA)-H3PO4 gel polymer electrolyte prepared through in-situ polymerization method.50 The amount of the active material was kept at 4 mg cm-2 for the solid-state supercapacitor; the details of the electrode preparation are available in Supporting Information and the pictures of the electrode prepared for solid supercapacitor are given in Figure S17. Electrochemical characteristics of the device are shown in Figure 8; CVs recorded at different scan rates are given in Figure 8a which show higher capacitance at lower scan rates. However, the CV features retain their rectangular shape even at a high scan rate of 500 mV s-1 and this suggests its high rate capability. The geometric area normalized capacitance of 300 mF cm-2


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obtained at a current density of 0.25 mA cm-2 is a high value for a carbon-based material. The volumetric capacitance measured for the composite single electrode considering only the thickness of the active material is 303 F cm-3, where the thickness of the NCNS-CB composite is 0.099 mm. Energy density calculated per unit volume of the device is 0.27 mWh cm-3, and a large contribution to the volume comes from the current collector having a thickness of 0.363 mm. This value can be improved by selecting a thinner current collector. A comparison of the performance of the prepared material with the similar reported materials is given in Table S3; the composite prepared here shows superior performance compared to many of the reported works on CNS and NCNS. Furthermore, the measured power density of the device is 1.35 mW cm-3. Figure 8c shows the variation of the capacitance with the current density, and indicates that 43 % of the initial capacitance is retained at a current density of 4 mA cm-2, which is 16 times higher than the initial current density. This high rate capability is due to the low ESR of 2.5 Ω and the low Warburg impedance, which help in ensuring efficient current collection and the seamless movement of the ions to the active surface to form the electrochemical double layer (Figure 8d).The typical hyperbolic variation of the capacitance with frequency is plotted in Figure 8e, and the capacitance decreases with the frequency. The maximum capacitance is attained at a high frequency of 2 mHz. The results of the durability test carried out at a current density of 4 mA cm2

is shown in Figure 8f; the composite retains almost 80 % of the initial capacitance with

efficiency as high as 98.5 % after 14000 cycles, which is a very good cycle performance compared with many of the reports on the nitrogen-doped carbon nanosheets. To mention a few, nitrogen-doped carbon nanosheets prepared by the pyrolysis of magnesium citrate and potassium citrate exhibited 86 % capacitance retention after only 5000 cycles.51 Also, the work by Zhang



and group reported retention of 80 % after 10000 cycles for an MnO2/graphene film.52 Also, in our work, we have fabricated a solid-state device with the gel electrolyte. The ionic mobility and conductivity of the gel electrolyte are low compared to the liquid electrolytes; in such a case, the rate capability is inferior in comparison to the devices tested in the liquid-state. Such high stability of the composite implies its usefulness for extended cycle operation under high current densities.

Conclusion We have demonstrated an eco-friendly, fast, and felicitous method for the synthesis of CNS and NCNS. The electrostatic interaction between the NaCl and the polarized organic molecules, 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and 3,4,9,10-perylene tetracarboxylic diimide (PTCDI), is exploited to form a layer-by-layer assembly of PTCDA or PTCDI on the NaCl surface, which, on pyrolysis in inert atmosphere, yields high quality CNS and NCNS with thickness of 2-5 nm.CNS and NCNS have been thoroughly and expeditiously characterized by XRD, Raman spectroscopy, TEM, AFM, XPS, etc. The NCNS derived from PTCDI has been used as an electrode material for charge-storage in electrochemical capacitors; its electrochemical activity is superior to the NCNS prepared by the Hummer’s method. The area normalized capacitance measured for NCNS is 23 µF cm-2 which is same as the value reported for the single layer NCNS. Besides, an NCNS-carbon black composite is also prepared for use in a solid-state supercapacitor prototype. An areal capacitance of 300 mF cm-2 and a volumetric capacitance of 303 F cm-3 could be obtained in the solid-state supercapacitor prototype. As further prospects of this work, the method demonstrated in this study can further be employed to synthesize CNS that is doped with S, B, P, etc.


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ASSOCIATED CONTENT Supporting Information. Experimental details, IR spectrum, XRD pattern, EDAX analysis, Elemental mapping, SEM of carbon derived from PTCDA, SEAD pattern, Surface area and pore size analysis of NCNS, electrochemical data, etc., are provided in the supporting information. The following files are available free of charge. brief description (file type, i.e., PDF) brief description (file type, i.e., PDF) AUTHOR INFORMATION Corresponding Author *[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. ACKNOWLEDGMENT Financial assistance for the work was provided by the Council of Scientific and Industrial Research (CSIR), New Delhi, through the project TLP003526. The authors sincerely thank Plawan Jha (Indian Institute of Science Education and Research (IISER), Pune) for atomic force microscopy analysis. REFERENCES



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Synthesis of Carbon Nanosheets and Nitrogen-Doped Carbon

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