Nitrated Graphene Oxide Derived from Graphite Oxide: a Promising Energetic Two-Dimensional Material

nanomaterials

Article Nitrated Graphene Oxide Derived from Graphite Oxide: A Promising Energetic Two-Dimensional Material

Fayang Guan 1 , Hui Ren 1,*, Lan Yu 2, Qingzhong Cui 1, Wanjun Zhao 1 and Jie Liu 1

1 State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China; [email protected] (F.G.); [email protected] (Q.C.); [email protected] (W.Z.); [email protected] (J.L.) 2 Hong Kong New ARK Technologise Ltd., Hong Kong 999077, China; [email protected] * Correspondence: [email protected]

Abstract: In order to synthesize a novel two-dimensional energetic material, nitrated graphene oxide (NGO) was prepared by the nitrification of graphite oxide to make a functional modification. Based on the morphological characterization, the NGO has a greater degree of curl and more wrinkles on the surface. The structure characterization and density functional theory calculation prove that + epoxy and hydroxyl groups on the edge of graphite oxide have reacted with nitronium cation (NO2 ) to produce nitro and nitrate groups. Hydrophobicity of NGO implied higher stability in storage than graphene oxide. Synchronous simultaneous analysis was used to explore the decomposition mechanism of NGO preliminarily. The decomposition enthalpy of NGO is 662.0 J·g−1 and the activation energy is 166.5 kJ·mol−1. The thermal stability is similar to that of general nitrate energetic materials. The hygroscopicity, thermal stability and flammability of NGO prove that it is a novel two-dimensional material with potential applications as energetic additives in the catalyst, electrode materials and energetic devices.

Keywords: nitrated graphene oxide; nitriﬁcation process; properties analysis; synchronous simultaneous analysis; combustion performance

Citation: Guan, F.; Ren, H.; Yu, L.; Cui, Q.; Zhao, W.; Liu, J. Nitrated Graphene Oxide Derived from 1. Introduction Graphite Oxide: A Promising Energetic Two-Dimensional Material. Graphene oxide (GO) is an amphiphilic two-dimensional material generally exfoliating Nanomaterials 2021, 11, 58. https:// from graphite oxide [1–4]. Many models on GO such as the Lerf–Klinowski structural doi.org/10.3390/nano11010058 model [5–7], the dynamic structural model [8] and the two-component structural model [9] reveal that the majority of oxygen functions in GO are epoxy and hydroxyl with few Received: 26 November 2020 carboxyl and carbonyl existing on the edge [10,11]. These groups are active and could be Accepted: 8 December 2020 modified to obtain different materials: the carboxyl group reacts with thionyl chloride to Published: 29 December 2020 produce high-efficiency photovoltaic materials [12]; N-vinylcarbazole could grow on the surface by the hydroxyl group [13]; amine nucleophiles undergo ring-opening reaction with Publisher’s Note: MDPI stays neu- epoxy groups [14]. Meanwhile, GO has been regarded as a promising two-dimensional tral with regard to jurisdictional claims energetic material in catalysis and energy storage fields. However, the thermodynamic in published maps and institutional property of GO is unstable to undergo exothermic disproportionation reactions readily [15], affiliations. and these hydrophilic oxygen-groups lower the storage behavior of GO. To solve these problems and realize the energetic functionalization of graphene sheets, it is necessary to modify the oxygen-containing groups of GO.

Copyright: © 2020 by the authors. Li- Nitrification is widely used in organic reactions. In 1834, benzene was nitrated into censee MDPI, Basel, Switzerland. This nitrobenzene [16,17]. Nitro is the energetic functional group of most explosives. In the article is an open access article distributed energetic materials, the nitro group connects with a carbon atom like in 2,4,6-trinitrotoluene under the terms and conditions of the (TNT), attaches to oxygen atom like in nitrocellulose and links with nitrogen atom like in Creative Commons Attribution (CC BY) cyclotrimethylenetrinitramine. The direct nitrification of graphene is hard to realize. The license (https://creativecommons.org/ nitrification reagent is an overly strong oxidation agent that would produce oxygenated licenses/by/4.0/). groups on GO instead of being attached as NO2 group. The reagent after oxidation

Nanomaterials 2021, 11, 58. https://doi.org/10.3390/nano11010058 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, x FOR PEER REVIEW 2 of 12

Nanomaterials 2021, 11, 58 2 of 12 oxygenated groups on GO instead of being attached as NO2 group. The reagent after oxidation reaction is unable to nitrate GO afterward. Therefore, modification of GO to prepare nitrated graphene oxide (NGO) is a facile approach to make GO more energetic. reaction is unable to nitrate GO afterward. Therefore, modification of GO to prepare NGO is a promising energetic additive in explosive systems. It can activate the reaction of nitrated graphene oxide (NGO) is a facile approach to make GO more energetic. NGO the energetic system, lower the critical threshold, and release heat more quickly. Zhang is a promising energetic additive in explosive systems. It can activate the reaction of the [18] and Yuan [19] added NGO into energetic materials as catalysis with the decrease in energetic system, lower the critical threshold, and release heat more quickly. Zhang [18] the thermal decomposition temperature. Nevertheless, the mass content of nitrogen in and Yuan [19] added NGO into energetic materials as catalysis with the decrease in the thermalNGO is decompositiononly 1.45%, which temperature. prohibits Nevertheless,energetic behavior. the mass Thus, content the research of nitrogen of the in nitrifi- NGO iscation only process 1.45%, whichis of great prohibits significance energetic for incr behavior.easing Thus,the nitro the content research of ofNGO. the nitrificationThe analysis processof hydrophilic is of great properties, significance thermal for increasingstability and the combustion nitro content performance of NGO. The could analysis explore of hydrophilicthe application properties, prospect thermal of NGO stability as an energetic and combustion addictive. performance could explore the applicationIn this prospectpaper, the of mixture NGO as of an nitrate, energetic glacia addictive.l acetic acid and concentrated sulfuric acid was Inused this to paper, nitrate the GO. mixture The mixed of nitrate, acid glacialcould protonate acetic acid the and nitrate concentrated ion and subsequently sulfuric acid wasmake used water to nitrateelimination, GO. The which mixed could acid produce could protonate nitronium the cation nitrate (NO ion2+) andwith subsequently high concentration effectively and safely. Then the nitrification reaction process was+ determined by make water elimination, which could produce nitronium cation (NO2 ) with high con- centrationthe structure effectively analysis and and safely. density Then functional the nitrification theory (DFT) reaction calculation. process was The determined dispersion byability the structure of the NGO analysis in solvents and density was explored functional and theorymolecular (DFT) dynamics calculation. (MD) The was dispersion employed abilityto simulate of the the NGO changes in solvents in hydrophilicity, was explored refl andecting molecular the hygroscopicity dynamics (MD) of NGO was employedindirectly. toSimultaneous simulate the analysis changes measurements in hydrophilicity, were reflecting used to the analyze hygroscopicity the thermal of stability NGO indirectly. and the Simultaneouscombustion performance analysis measurements of NGO has werebeen usedobserved to analyze preliminarily. the thermal stability and the combustion performance of NGO has been observed preliminarily. 2. Experimental Section 2.2.1. Experimental Calculation Methods Section 2.1. Calculation Methods In order to investigate the reaction process of NO2+ and GO, A simplified model of + GO thatIn order only to includes investigate epoxy the and reaction hydroxyl process was of built, NO2 whichand GO, is shown A simplified in Figure model 1. Electro- of GO thatstatic only potential includes and epoxy Fukui and function hydroxyl [20] was of built, GO whichwere calculated. is shown in Different Figure1. Electrostaticdistances be- potential and Fukui function [20] of GO were calculated. Different distances between GO tween GO and NO2+ were constrained to analyze the changes in the structure (both bond + andlength NO and2 were bond constrained angles) when to the analyze reactants the changesget closer. in The the DFT structure results (both were bond obtained length by andusing bond the angles)DMol3 density-functional when the reactants module get closer. in the The set DFT of general results weregradient obtained approximation by using the DMol3 density-functional module in the set of general gradient approximation (GGA) (GGA) functionals and Perdew–Burke–Ernzerhof (PBE) correlation [21]. We set 1 × 10−5 functionals and Perdew–Burke–Ernzerhof (PBE) correlation [21]. We set 1 × 10−5 Ha as Ha as the convergence tolerance, DNP 3.5 was the basis set and 0.002 Ha of smearing was the convergence tolerance, DNP 3.5 was the basis set and 0.002 Ha of smearing was used used to speed up the convergence. to speed up the convergence.

FigureFigure 1. 1.( a(a)) Electrostatic Electrostatic potential potential diagram diagram of of simpliﬁed simplified graphene graphene oxide oxide (GO) (GO) model; model; (b) the(b) the reaction reac- betweention between NO2+ NOand2+ hydroxyland hydroxyl and epoxyand epoxy groups groups both bo inth the in surface the surface and theand edge. the edge.

MDMD waswas usedused toto simulatesimulate thethe hydrophilichydrophilic of of GOGO andand NGO.NGO. 500500 waterwater moleculesmolecules were were 3 putput intointo a a cell cell (33.44 (33.44× × 33.4433.44 ×× 33.44 Å 3)) and and the the GO and NGO modelsmodels werewere establishedestablished in in thethe resultsresults ofof thethe element element analysis analysis asas shown shown in in Figure Figure2 .2. The The simulation simulation was was carried carried out out byby thethe ForciteForcite modulemodule inin thethe COMPASSCOMPASS forceﬁeld.forcefield. AA ﬁxedfixed timetime stepstep ofof 11fs fswas wasused usedin in all cases and the whole runs of 1 ns duration were performed in the NPT system. Radial distribution function (RDF) and interaction energies (Einter) were calculated by the result of dynamic simulation.

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all cases and the whole runs of 1 ns duration were performed in the NPT system. Radial Nanomaterials 2021, 11, 58 3 of 12 distribution function (RDF) and interaction energies (Einter) were calculated by the result of dynamic simulation.

Figure 2. (a) The 500 water models inside the cell with a size of 33.44 × 33.44 × 33.44 Å3; (Figureb) the model2. (a) The of GO; 500 (waterc) the modelmodels of inside nitrated the graphenecell with a oxide size of (NGO). 33.44 × 33.44 × 33.44 Å3; (b) the model of GO; (c) the model of nitrated graphene oxide (NGO). Dmol3 module and Forcite module were implemented in the Materials Studio 5.5 from AccelrysDmol3 Inc. module and Forcite module were implemented in the Materials Studio 5.5 from Accelrys Inc. 2.2. Sample Preparation 2.2. Sample Preparation Graphite oxide was prepared by the Hummers method [22] and the brief process was Graphite oxide was prepared by the Hummers method [22] and the brief process was showed as following. 3.5 g graphite was mixed with 120 mL concentrated sulfuric acid showed as following. 3.5 g graphite was mixed with 120 mL concentrated sulfuric acid (H2SO4, AR) in a ﬂask. Then, 3.5 g sodium nitrate (NaNO3, AR) was added slowly. The (H2SO4, AR) in a flask. Then, 3.5 g sodium nitrate (NaNO3, AR) was added slowly. The temperature of the steps should be ensured under 10 ◦C and the solution needs to be stirred temperature of the steps should be ensured under 10 °C and the solution needs to be continuously. We added 15 g potassium permanganate (KMnO4, AR) into the mixture to inducestirred a reactioncontinuously. for 4.5 We h inadded 35 ◦C. 15 Deionizedg potassium water permanganate was used to(KMnO dilute4, theAR) solution into the mixture to induce a reaction for 4.5 h in 35 °C. Deionized water was used to dilute the solution and 40 mL hydrogen peroxide (H2O2, concentration 30%) was employed to react with and 40 mL hydrogen peroxide (H2O2, concentration 30%) was employed to react with the the remaining KMnO4. Hydrochloric acid (HCl 1 mol/L) and acetone (CH3COCH3 AR) was usedremaining to remove KMnO the4. inorganicHydrochloric impurities acid (HCl [23, 241 ].mol/L) Then and graphite acetone oxide (CH was3COCH obtained3 AR) was used to remove the inorganic impurities [23,24]. Then graphite oxide was obtained after after drying in the vacuum oven. 1 g graphite oxide and 10 g NaNO3 were ultrasonically drying in the vacuum oven. 1 g graphite oxide and 10 g NaNO3 were ultrasonically dis- dispersed in 150 mL of glacial acetic acid (CH3COOH, AR). 70 mL concentrated H2SO4 was drippedpersed in into 150 the mL mixture of glacial under acetic the iceacid bath. (CH Then3COOH, the temperatureAR). 70 mL wasconcentrated raised to 45H2◦SOC,4 was and thedripped mixture into was the stirredmixture for under 6 h. Thethe ice nitrated bath. graphiteThen the wastemperature prepared was after raised diluting, to 45 °C, cleaningand and the drying.mixture was stirred for 6 h. The nitrated graphite was prepared after diluting, Deionizedcleaning and water, drying. dimethylformamide (DMF, AR), N-methylpyrrolidone (NMP, AR) and o-dichlorobenzeneDeionized water, (o-DCB, dimethylformamide AR) were employed (DMF as, the AR), solvents. N-methylpyrrolidone Taking the deionized (NMP, AR) waterand as an o-dichlorobenzene example: 100 mg (o-DCB, nitrated graphiteAR) were was employ disperseded as the into solvents. 300 mL deionizedTaking the water, deionized and thenwater was as an exfoliated example: by 100 ultrasonic mg nitrated crusher graphite under was the dispersed power of into 250 300 W mL for 9deionized h. The wa- suspensionter, and was then centrifuged was exfoliated at 1500 by rpm ultrasonic for 2 h andcrusher the NGOunder with the fewpower layers of 250 remained W for 9 in h. The the supernatant.suspension was GO dispersioncentrifuged in at these 1500 solventsrpm for were2 h and prepared the NGO in thewith same few way.layers NGO remained and GOin the aqueous supernatant. dispersions GO dispersion were dried in these the vacuum solvents freeze were oven prepared to obtain in the solid same samples. way. NGO The entireand GO preparation aqueous processdispersions is shown were indried Figure in 3th. e vacuum freeze oven to obtain solid sam- Graphiteples. The (Dentire90 = preparation 40 µm, 99.999%) process was is purchased shown in Figure from Nanjing 3. XFNANO Materials Tech Co., Ltd., Nanjing, China. Deionized water was made in the laboratory and other reagents were all from Aladdin Industrial Co., Shanghai, China.

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FigureFigure 3. The 3. entire The entire preparation preparation process process of NGO. of NGO. 2.3. Materials Characterization Graphite (D90 = 40 µm, 99.999%) was purchased from Nanjing XFNANO Materials Tech Co.,Scanning Ltd., Nanjing, electron China. microscopy Deionized (SEM, water S-4700, was HITACHI, made in the Chiyoda, laboratory Japan), and transmis- other reagentssion electron were all microscopy from Aladdin (TEM, Industrial JEM-2010, Co., HITACHI,Shanghai, China. Chiyoda, Japan) and atomic force microscopy (AFM, Dimension Icon, BRUKER, Karlsruhe, Germany) were used to char- 2.3. Materials Characterization acterize the morphology of the samples. Fourier transform infrared spectrometer (FTIR, VERTEX70,Scanning electron BRUKER, microscopy Karlsruhe, (SEM, Germany), S-4700, nuclear HITACHI, magnetic Chiyoda, resonance Japan), spectroscopy transmission(13C-NMR, electron microscopy CMX 400, VARIAN, (TEM, JEM-2010, Palo alto, HI CA,TACHI, USA), Chiyoda, Raman spectrometer Japan) and atomic (RM1000, force Ren- microscopyishaw, Gloucester, (AFM, Dimension England) Icon, and BRUKER, organic element Karlsruhe, analyzer Germany) (EA3000, were Euro used Vector, to charac- Pavia, terizeItaly) the were morphology carried out of to the analyze samples. the structure.Fourier transform infrared spectrometer (FTIR, VERTEX70, BRUKER, Karlsruhe, Germany), nuclear magnetic resonance spectroscopy (13C-NMR,2.4. Thermal CMX Analysis 400, VARIAN, Palo alto, CA, USA), Raman spectrometer (RM1000, Ren- ishaw, Gloucester,Differential England) scanning and calorimetry organic element (DSC) and analyzer thermogravimetry (EA3000, Euro (TG) Vector, were Pavia, used to Italy)investigate were carried the thermalout to analyze decomposition the structure. of energetic materials. Combined with the mea- 2.4.surements Thermal Analysis of infrared spectroscopy (IR) and gas chromatography (GC) or mass spectrum (MS) for analyzing the gaseous products, the process of decomposition can be initially Differential scanning calorimetry (DSC) and thermogravimetry (TG) were used to explored. Multi-apparatus combination as DSC-TG-MS-FTIR attracts increasing interest investigate the thermal decomposition of energetic materials. Combined with the meas- due to its synchronous analysis of gaseous products for improving accuracy compared urements of infrared spectroscopy (IR) and gas chromatography (GC) or mass spectrum to an individual combination [25–27]. In this paper, DSC-TG-MS-FTIR synchronous mea- (MS) for analyzing the gaseous products, the process of decomposition can be initially surement is an assembly of NETZSCH (Bayern, Germany) STA449C, NETZSCH QMS403C explored. Multi-apparatus combination as DSC-TG-MS-FTIR attracts increasing interest and BRUKER (Karlsruhe, Germany) VERTEX 70, and two pipelines are connected with due to its synchronous analysis of gaseous products for improving accuracy compared to DSC/TG measurements to MS apparatus and FTIR apparatus separately. The details an individual combination [25–27]. In this paper, DSC-TG-MS-FTIR synchronous meas- about the connection could be taken in Ref. [26]. NGO was tested from room temperature urement is an assembly of NETZSCH (Bayern, Germany) STA449C, NETZSCH QMS403C to 360 ◦C under an argon atmosphere at a heating rate of 20 ◦C·min−1 in simultaneous andthermal BRUKER analysis. (Karlsruhe, Germany) VERTEX 70, and two pipelines are connected with DSC/TG measurements to MS apparatus and FTIR apparatus separately as shown in Fig- ure2.5. S1. CombustionNGO was tested Properties from room temperature to 360 °C under an argon atmosphere at −1 a heating500 rate mg of of 20 the °C·min NGO powder in simultaneous was dispersed thermal in 100 analysis. mL of water to prepare colloids, then 2.5.colloids Combustion were Properties ﬁltered and dried to prepare the NGO sheet. Several strips with complete shape500 mg and of uniform the NGO size powder (60 mm was× 5 mm)dispersed were cutin 100 from mL the of NGO water slice to prepare for the combustioncolloids, thenexperiment. colloids were filtered and dried to prepare the NGO sheet. Several strips with In order to simulate the function of gas work, 10 mg of the NGO powder prepared under vacuum freeze-drying conditions was weighed to press out a 10 mm diameter NGO

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complete shape and uniform size (60 mm × 5 mm) were cut from the NGO slice for the Nanomaterials 2021, 11, 58 combustion experiment. 5 of 12 In order to simulate the function of gas work, 10 mg of the NGO powder prepared under vacuum freeze-drying conditions was weighed to press out a 10 mm diameter NGO tablet and put in the bottom end of the syringe. The small hole at the front of the syringe tablet and put in the bottom end of the syringe. The small hole at the front of the syringe was sealed by PTFE tape and the NGO was heated by an alcohol burner. The value was was sealed by PTFE tape and the NGO was heated by an alcohol burner. The value was determined after the tube cooling to room temperature. determined after the tube cooling to room temperature. A high-speed video camera (FASTCAM-APX RS, Photron, Tokyo, Japan) was used A high-speed video camera (FASTCAM-APX RS, Photron, Tokyo, Japan) was used to to capture the photography images of combustion performance and gas work process at capture the photography images of combustion performance and gas work process at the the imaging frequency of 125 fps. imaging frequency of 125 fps.

3.3. Results Results and and Discussion Discussion 3.1.3.1. Morphological Morphological Analysis Analysis AsAs shown shown in in Figure Figure 4a,b,4a,b, more more wrinkles wrinkles and and bends bends exist exist on on the the surface surface of of NGO NGO than than GO.GO. The The integrity integrity of of the the graphene graphene lattice lattice ma mayy be be damaged damaged by by the the nitrification nitrification process. process. TheseThese uneven uneven surfaces surfaces are are able able to to expand expand cont contactact area area with with other other materials, materials, beneficial beneficial to to heatheat transformation. transformation. The The uneven uneven surface surface is is also also shown shown in in TEM TEM images images (Figure (Figure 44c).c). The high-resolutionhigh-resolution image image of of the the edges edges of of NGO NGO sheets sheets indicates indicates that that the the NGO NGO sheet sheet has has only only twotwo layers. layers. In In AFM AFM images images (Figure (Figure 4d),4d), we we measure measure the the thickness thickness of of the the NGO NGO sheet, sheet, which which isis around around 1.1 1.1 nm. nm. Thirty Thirty samples samples are arecounted counted for thickness for thickness and the and values the values are all are the all mul- the tiplemultiple of 1.0–1.1 of 1.0–1.1 nm. nm.Only Only three three samples samples were were given given over over five five layers, layers, which which could could prove prove lowlow power power stripping stripping for for a a long long time time co coulduld exfoliate exfoliate graphene graphene materials materials well. well.

FigureFigure 4. 4. (a(a) )Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) images images of of GO GO sheet; sheet; (b (b) )SEM SEM images images of of NGO NGO sheet; sheet; (c (c) )transmission transmission electronelectron microscopy microscopy (TEM) (TEM) images images of ofNGO NGO sheet sheet and and high-resolution high-resolution imagine imagine of the of theedges edges of NGO; of NGO; (d) atomic (d) atomic force forcemi- croscopymicroscopy (AFM) (AFM) images images of NGO of NGO sheet sheet and and the the height height distribution distribution along along the the path; path; (e ()e thickness) thickness distribu distributiontion of of 30 30 NGO NGO samples.samples.

3.2.3.2. Structure Structure Analysis Analysis In the element analysis, the mass fraction of GO and NGO are 68.8/30.6/0.6 (C/O/H) and 57.6/8.0/33.9/0.5 (C/N/O/H) respectively. After simplifying to the chemical formula, the NGO is C48H5O21.2N5.7 and GO is C47.8H5O15.9. In the GO sheet, the number ratio of O/C is about 0.33 and the value would increase to 0.44 after nitriﬁcation. The FTIR spectra of NGOs could be observed in Figure5a. The peaks at 1581 cm −1 and 1365 cm−1 correspond to the symmetry and antisymmetric stretching vibration of Nanomaterials 2021, 11, x FOR PEER REVIEW 6 of 12

In the element analysis, the mass fraction of GO and NGO are 68.8/30.6/0.6 (C/O/H) and 57.6/8.0/33.9/0.5 (C/N/O/H) respectively. After simplifying to the chemical formula, the NGO is C48H5O21.2N5.7 and GO is C47.8H5O15.9. In the GO sheet, the number ratio of O/C Nanomaterials 2021, 11, 58 is about 0.33 and the value would increase to 0.44 after nitrification. 6 of 12 The FTIR spectra of NGOs could be observed in Figure 5a. The peaks at 1581 cm−1 and 1365 cm−1 correspond to the symmetry and antisymmetric stretching vibration of nitro groups (–NO2). These two peaks were wider with higher wavenumbers than other nitro nitro groups (–NO ). These two peaks were wider with higher wavenumbers than other compounds. It means2 the nitro groups may connect to other atoms. The intensity of nitro compounds. It means the nitro groups may connect to other atoms. The intensity of stretching vibration of –OH (3421 cm−1), Ar–O (1280 cm−1), C–OH (1089 cm−1), O–C–O stretching vibration of –OH (3421 cm−1), Ar–O (1280 cm−1), C–OH (1089 cm−1), O–C–O (1047 cm−1), and bending vibration of –OH (1390 cm−1) become weaker after nitrification. (1047 cm−1), and bending vibration of –OH (1390 cm−1) become weaker after nitriﬁcation. The 13C NMR spectra (Figure 5b) of NG has been used to determine the nitrogen-con- The 13C NMR spectra (Figure5b) of NG has been used to determine the nitrogen-contained tained functional group. The graphene lattice is all sp2 hybrids, and the chemical shift in functional group. The graphene lattice is all sp2 hybrids, and the chemical shift in the the range from 120 ppm to 130 ppm would be regarded as graphene lattice [28,29]. Two range from 120 ppm to 130 ppm would be regarded as graphene lattice [28,29]. Two peaks peakslocated located at 148 at ppm 148 andppm 157 and ppm 157 haveppm beenhave detectedbeen detected that are that corresponding are corresponding to the to nitro the nitrogroup group and nitrate and nitrate group group respectively. respectively. The intensity The intensity of donor of groups donor (C–OHgroups at (C–OH 68 ppm at and 68 ppmC–O–C and at C–O–C 57 ppm) at drop57 ppm) signiﬁcantly drop significantly in the NGO in the diagram. NGO diagram. Characteristic Characteristic peaks of peaks epoxy ofand epoxy hydroxyl and hydroxyl both in FTIR both spectra in FTIR and spectra 13C NMR and spectra13C NMR are spectra greatly reduced.are greatly The reduced. content Theof carbonyl content hasof carbonyl a weak change.has a weak From change. the analysis From ofthe FTIR analysis spectra of andFTIR 13C spectra NMR and spectra, 13C NMRGO was spectra, nitrated GO to was NGO nitrated successfully. to NGO successfully.

Figure 5. (a) Fourier transform infrared spectrometer (FTIR) spectra of NGO and GO; (b) 13C nuclear magnetic resonance Figure 5. (a) Fourier transform infrared spectrometer (FTIR) spectra of NGO and GO; (b) 13C nu- spectroscopy (NMR) spectrum of NGO and GO; (c) Raman spectra of NGO and GO. clear magnetic resonance spectroscopy (NMR) spectrum of NGO and GO; (c) Raman spectra of NGO Atand the GO. wavelength of 514 nm, the graphene of Raman spectra has two characteristic peaks: G peak of graphene at 1580 cm−1 that is unique to the carbon sp2 structure, which can reflectAt the wavelength the crystal integrity. of 514 nm, D the peak graphene near 1350 of Raman cm−1 is spectra a defect has peak, two characteristic reflecting the peaks: G peak of graphene at 1580 cm−1 that is unique to the carbon sp2 structure, which disorder of the graphene sheet. The intensity ratio (ID/IG) is usually used to characterize −1 canthe reflect degree the of covalentcrystal integrity. modification D peak of graphene,near 1350 andcm the is a ratio defect rises peak, from reflecting 0.93 to 1.07 the afterdis- ordernitrating of the as demonstratedgraphene sheet. in FigureThe intensity5c. The reactionratio (ID further/IG) is usually disrupts used the orderto characterize of the carbon the degreelattice, whichof covalent might modification cause the deformation of graphene, of the and layer the structure ratio rises as from the SEM 0.93 images to 1.07 shown after nitratingin Figure as4b. demonstrated in Figure 5c. The reaction further disrupts the order of the carbon lattice, which might cause the deformation of the layer structure as the SEM images shown3.3. DFT in CalculationFigure 4b. Analysis 3.3. DFTFrom Calculation the structure Analysis analysis above, we could know the epoxy and hydroxyl groups 2+ reactedFrom with the NO structureto produce analysis C–NO above,2 and we O–NO could2 knowgroups. the However, epoxy and the hydroxyl reaction processgroups reactedcannot with be reflected. NO2+ to produce In order C–NO to research2 and O–NO the nitrification,2 groups. However, a simplified the reaction GO model process only cannotconsists be of reflected. epoxy and In order hydroxyl to research groups the were nitrification, built, which a simplified is shown GO in model Figure only1b. Thecon- sistscalculation of epoxy result and hydroxyl of the electrostatic groups were potential built, which diagram is shown shows in thatFigure the 1b. density The calcula- of the tionelectron result cloud of the at theelectrostatic oxygen atomspotential is greater, diagram and shows these that sites the attract density cations of the more electron easily. cloudFukui at function the oxygen is the atoms index is togreater, assess and a priori these reactivity sites attract of chemical cations more species easily. from Fukui their − functionintrinsic is electronic the index properties. to assess Thea priori electrophilic reactivity Fukui of chemical function speciesf (r )from is used their to measureintrinsic the sensitivity of the reaction with the electrophilic attack as shown in Equation (1).

f − (r) = [ρN(r) − ρN+∆(r)]/∆N, (1)

ρ (r) is the charge density and ∆N represents the changes of electrons. For the convenience of describing each atom, these function groups have been labeled, shown in Figure1b. O 3 is the oxygen atom of epoxy in the center of NGO and O4 is the oxygen atom of epoxy in the edge. O5 and O6 are oxygen atoms of hydroxyl in the edge Nanomaterials 2021, 11, 58 7 of 12

and center of NGO respectively. The results in Table1 show that the epoxy and hydroxyl groups on the edge are more vulnerable to electrophilic attacks than the groups on the surface.

Table 1. Mulliken atomic charges and Fukui function of oxygen atoms.

O3 O4 O5 O6 Mulliken atomic charges −0.351 −0.389 −0.402 −0.401 f − (r) 0.017 0.031 0.024 0.018

Changes in atomic structure can react to the specific nitrification process indirectly. The + 2 hybrid type of nitrogen atom in NO2 turns from sp to sp when it bonds with other atoms. The bonding atoms centered on N tend to be on the same plane. Hence, a facile method could be used to further determine the reaction type and sites by analyzing the changes of + bond length, angles and coplanarity in the stable configuration when groups and NO2 are at different distances. As shown in Tables S1 and S2, as the distance between epoxy groups + + and NO2 gets closer, NO2 and the oxygen atoms of epoxy tend to be coplanar. However, both carbon-oxygen bonds of the central epoxy group become longer, and then break when N atom and O3 atom are in the bonding distance. The marginal epoxy only fractures one carbon-oxygen bond in 1.63 Å of L(N··O4) (the length between N atom and O4 atom, ‘··’ means unbonded function and ‘–’ is the bond function). It means that the epoxy groups on the edge can undergo a ring-opening reaction to form a nitrate group. In Table S3, + when NO2 gets closer to the marginal hydroxyl group, the coplanarity of NO1O2C5 gets better and L(O5–C5) becomes longer, tending to form the nitro group. Meanwhile, the changes in NO1O2O5 and L(O5–H) have the same tendency. This analysis illustrates that these two groups could take esterification and substitution reactions. Therefore, when the L(N··O5) and L(N··C5) get shorter to bond, the configuration is hard to stabilize. For + the result of central hydroxyl and NO2 in Table S4, NO1O2O6 tends to be in a plane, but the O6–H bond is stable firmly. The coplanarity of NO1O2C6 is fine outside the C··O6 + bonding distance because of the conjugate between NO2 and graphene base. However, + NO2 prefers to connect with hydroxyl rather than forming a nitro group when O6–C6 + bond breaking. From the analysis above, NO2 could react with the epoxy and hydroxyl groups on the edge in accordance with the result of the Fukui function index.

3.4. Dispersion and Hydrophobicity The changes in structure affect dispersion performance. Since the preparation methods of GO and NGO dispersion are the same, the afﬁnity of them with different solvents can be compared by the concentration of the dispersion. The result (Figure6a) shows that NGO is not as dispersible as GO in an aqueous solution. The dispersibility of NGO in o-DCB is better than that of GO, and NMP could be regarded as an excellent solvent, which is consistent with the hydrophobicity of nitro. The hygroscopicity is an important index for an energetic addictive and MD simulation was used to analyze the hydrophobicity performance of GO and NGO. RDF (g(r), also sometimes referred to as pair correction function [30]) is the probability of atoms that, given the presence of an atom at the reference frame, there will be an atom with its center located in a spherical shell of inﬁnitesimal thickness at a distance, r (Å). In the results of the RDF diagram (Figure6b), the starting values are the same (0.85 Å), but the peak of the NGO/water system is lower than the GO/water system. It means the addition of these two-dimensional materials cannot strengthen the interatomic force, but the atomic aggregation in the NGO system is weaker than the GO system. The addition of GO can absorb more water molecules than NGO. Hence, NGO behaves worse hydrophilicity than GO, which is consistent with the concentration results. It means that NGO is expected to have better storage performance and a longer life span than GO. Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 12

Nanomaterials 2021, 11, 58 8 of 12

Figure 6. (a) The concentrationFigure of GO 6. ( anda) The NGO concentration in different of solvents; GO and (b NGO) The in radial different distribution solvents; function (b) The (RDF) radial diagram distribution of water, GO/water system andfunction NGO/water (RDF) system.diagram of water, GO/water system and NGO/water system.

3.5. ThermalThe hygroscopicity Analysis is an important index for an energetic addictive and MD simulation wasThe used thermal to analyze performance the hydrophobi is an indispensablecity performance premise of forGO basicand NGO. theoretical RDF ( researchg(r), also sometimessuch as catalysis, referred combustion to as pair correction and detonation function [31 [30])–35]. is Fromthe probability the TG/DSC of atoms spectra that, in givenFigure the7a ,presence the decomposition of an atom at of the NGO reference has a frame, major there exothermic will be peakan atom in 218with◦ Citswith center a located48% mass in a loss. spherical Enthalpy shellof of decompositioninfinitesimal thickness (∆Hd) could at a distance, be obtained r (Å). by integrating the − peakIn area the asresults 662 Jof·g the1 (negative RDF diagram values (Figure indicate 6b), exotherm). the starting This values level are is the between same (0.85 the − − Å),nitroglycerin but the peak (353.8 of the J·g NGO/water1 [36]) and pentaerythritolsystem is lower tetranitratethan the GO/water (881 J·g system.1 [37]). It Element means theanalysis addition of NGO of these and two-dimensional solid remains illustrates materials that cannot about strengthen 4.6% of carbon the interatomic atoms as well force, as butthe wholethe atomic N, O, aggregation and H atoms in the participate NGO system in the is decomposition weaker than the reaction GO system. to generate The addi- gas products.tion of GO These can absorb gas fragments more water were molecules measured than by synchronous NGO. Hence, MS NGO analysis. behaves There worse are twohy- + + possibilitiesdrophilicity forthan the GO,m/z whichof 44, namelyis consistent N2O wiandth CO the2 concentration. The fragment results. could beIt determinedmeans that + NGOby the is synchronous expected to FTIRhave analysisbetter storage that the performance spectra of gas and have a longer characteristic life span peaksthan GO. of CO 2 (two peaks at 2312 cm−1 and 2368 cm−1 shown in Figure7b). Then the reaction mechanism 3.5. Thermal Analysis of NGO could be speculated preliminarily: the decomposition of oxygen-containing groups The thermal+ performance+ is an+ indispensable premise for basic theoretical+ research produces CO2 and H2O ; and NO2 is from the break of nitro groups; HNO2 may come suchfrom as the catalysis, hydrogen combustion atom rearrangement and detonation of nitrate [31–35]. groups From and the then TG/DSC take disproportionation spectra in Figure 7a,to produce the decomposition NO+. of NGO has a major exothermic peak in 218 °C with a 48% mass loss. ToEnthalpy obtain of the decomposition corresponding (Δ kineticHd) could parameters be obtained (apparent by integrating activation the energy peak Eareaa, pre- as 662exponential J·g−1 (negative factor valuesA), DSC indicate and TG exotherm). curves at theThis heating level is rates between of 5, 10,the 15nitroglycerin and 20 ◦C·min (353.8−1 wereJ·g−1 [36]) dealt and with pentaerythritol by mathematic tetranitrate means, and (881 the J·g Kissinger−1 [37]). [Element38] method analysis were employedof NGO and as solidshown remains in Equation illustrates (2). that about 4.6% of carbon atoms as well as the whole N, O, and H atoms participate in the decomposition reaction to generate gas products. These gas 2 fragments were measured byln( βsynchronous/Tp ) = ln(A MSR/E analysis.a) − (Ea/R ThereTp), are two possibilities for the (2) m/z of 44, namely N2O+ and CO2+. The fragment could be determined by the synchronous where T is the peak temperature (◦C), A is the pre-exponential factor (s−1), E is the FTIR analysisp that the spectra of gas have characteristic peaks of CO2+ (two peaks aat 2312 −1 β ◦ −1 cmapparent−1 and 2368 activation cm−1 shown energy in (kJ Figure·mol 7b).), isThen the heatingthe reaction rate (mechanismC·s ), R is of the NGO gas could constant be (8.314 J·mol−1·◦C−1). The results are shown in Table2. speculated preliminarily: the decomposition of oxygen-containing groups produces CO2+ and H2O+; and NO2+ is from the break of nitro groups; HNO2+ may come from the hydrogen atom rearrangement of nitrate groups and then take disproportionation to produce NO+.

Nanomaterials 11 Nanomaterials 2021, 11, x 58 FOR PEER REVIEW 9 9of of 12

Figure 7. Differential scanningFigure calorimetry 7. Differential (DSC)-thermogravimetry scanning calorimetry (TG)-mass (DSC)-the spectrumrmogravimetry (MS)-FTIR characterization(TG)-mass spectrum diagrams. (MS)- FTIR characterization diagrams. (a) The TG/DSC spectra of NGO; (b) the+ MS intensity+ spectra+ of (a) The TG/DSC spectra of NGO; (b) the MS intensity spectra of gas fragments with m/z as 18(H2O ), 28(CO ), 30(NO ), + + +gas fragments with+ m/z as 18(H2O+), 28(CO+), 30(NO+), 44(N2O+ or CO2+), 46(NO2+) and 47(HNO2+). 44(N2O or CO2 ), 46(NO2 ) and 47(HNO2 ). IR spectra in the range of characteristic wavelength shown on the top. IR spectra in the range of characteristic wavelength shown on the top. Table 2. Thermal analysis data. To obtain the corresponding kinetic parameters (apparent activation energy Ea, pre- ◦ −1 ◦ −1 exponentialβ (factorC·s A) ), DSC and TG curvesT atp ( theC) heating rates of 5, 10,∆ H15d and(J·g 20) °C·min−1 were dealt with5 by mathematic means, and 201.4 the Kissinger [38] method were 425 employed as shown in Equation10 (2). 209.7 473 15 213.7 577 20 218.0 662 ln(β/Tp2) = ln(AR/Ea) − (Ea/RTp), Ea = 166.6 kJ/mol lgA = 6.10 Rk = 0.9921

where Tp is the peak temperature (°C), A is the pre-exponential factor (s−1), Ea is the appar- It can be seen from the experimental data (Table2) that the Tp of NGO ranges from ent activation ◦energy (kJ·mol−1), β is the heating rate (°C·s−1), R is the gas constant (8.314 201.4 to 218.0 C, and the ∆Hd increases as the β get larger. The Ea of NGO is 166.6 kJ/mol J·mol−1·°C−1). The results are shown in Table 2. in the range of 163 kJ/mol and 188 kJ/mol [39–41], which is the first-order (O–NO2 bond fracture) reaction energy of several nitrate energetic materials. This further confirms that Table 2. Thermal analysis data. the initial thermal decomposition of NGO is the breaking of O–NO2 bond, and NGO is equivalentβ (°C·s to−1) commonTp nitrates(°C) inΔH thermald (J·g−1) stability and heat release. Compared to the ◦ decomposition5 performance201.4 of GO at425 the heating rate of 10 C[42], the exothermic peak of ◦ ◦ NGO is10 209.7 C higher209.7 than that of473 GO around in 208.7 C slightly. 3.6. Energy15 Response 213.7 577 20 218.0 662 The thermal stability of GO so low that even can explode upon being heated due to the Ea = 166.6 kJ/mol lgA = 6.10 Rk = 0.9921 remaining potassium salt in the preparation. The GO containing potassium salt will rapidly disproportionate and generate a large amount of water vapor and CO2 gas with heat release withIt the can catalysis be seen offrom potassium the experimental salt. However, data (Table the purified 2) that GO the cannot Tp of NGO be ignited, ranges which from 201.4might to be 218.0 used °C, as and a flame-retardant the ΔHd increases material as the [β15 get]. Thus,larger. although The Ea of the NGO non-energetic is 166.6 kJ/mol GO incould the range be used of as163 an kJ/mol additive and to 188 reduce kJ/mol the [39–41], sensitivity which of energeticis the first-order materials (O–NO [43–472 ],bond GO fracture)would reduce reaction the energy energy-out. of several nitrate energetic materials. This further confirms that the initialTherefore, thermal an decomposition energetic NGO of would NGO be is athe more breaking promising of O–NO additive2 bond, when and compared NGO is equivalentto GO. After to common being nitrated nitrates and in thermal using the stability same cleaningand heat methodrelease. toCompared remove metalto the de- ion compositionimpurities, NGOperformance tends to of combust GO at the violently heating inrate Figure of 108 a.°C The[42], NGOthe exothermic trip with 60peak mm of NGOlength is could209.7 °C be higher ignited than within that 20 of msGO andaround burned in 208.7 off in°C 400 slightly. ms with intense fire. The 3.6.average Energy burning Response rate was around 0.15 m/s. In the gas workability experiment, 10 mg of the NGOThe powder thermal could stability generate of GO about so low 5.8 mLthat gas even finally. can explode NGO performs upon being better heated combustion due to thebehavior remaining and rapid potassium ignition salt respond in the withpreparation. a certain The amount GO ofcontaining gas products potassium to do work. salt will The combustion tests show that when NGO is used as an additive for energy, it could enhance rapidly disproportionate and generate a large amount of water vapor and CO2 gas with heatthe combustion release with performance the catalysis and of potassium energy response salt. However, of the system. the purified GO cannot be ignited, which might be used as a flame-retardant material [15]. Thus, although the non-

Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 12

energetic GO could be used as an additive to reduce the sensitivity of energetic materials [43–47], GO would reduce the energy-out. Therefore, an energetic NGO would be a more promising additive when compared to GO. After being nitrated and using the same cleaning method to remove metal ion impurities, NGO tends to combust violently in Figure 8a. The NGO trip with 60 mm length could be ignited within 20 ms and burned off in 400 ms with intense fire. The average burning rate was around 0.15 m/s. In the gas workability experiment, 10 mg of the NGO powder could generate about 5.8 mL gas finally. NGO performs better combustion behav- Nanomaterials 2021, 11, 58 ior and rapid ignition respond with a certain amount of gas products to do work.10 The of 12 combustion tests show that when NGO is used as an additive for energy, it could enhance the combustion performance and energy response of the system.

Figure 8. (a) TheFigure combustion 8. (a) The images combustion of NGO; images (b) the of photographs NGO; (b) the of workphotographs experiment. of work experiment.

4.4. Conclusions Conclusions + TheThe NGO NGO was was prepared prepared by by the the nitrification nitrification of of graphite graphite oxide oxide and and the the NO NO2+ prefer2 prefer to reactto react with with the the epoxy epoxy and and hydroxyl hydroxyl groups groups on on the the edge edge to to produce produce C-NO2 and O-NO2 groups.groups. Structural Structural changes causecause changeschanges in in performance. performance. The The NGO NGO behaves behaves hydrophobic- hydropho- bicity,ity, similar similar thermal thermal property property with with other other nitrate nitrate materials materials and fineand combustionfine combustion performance. performance.These propertiesThese properties might might conduce conduce to storage to storage and promoteand promote reaction reaction in the in energeticthe energetic for- formula.mula. Therefore, Therefore, NGO NGO is a is promising a promising two-dimensional two-dimensional material material as an as energetic an energetic additive addi- in tivecatalysts, in catalysts, electrode electrode materials materials and other and fields. other fields.

SupplementarySupplementary Materials: Materials: TheThe following following are are available available online online at at www.mdpi.com/xxx/s1, https://www.mdpi.com/2079-4 Figure S1: DSC-TG-MS-FTIR991/11/1/58/s1, Tablesynchronous S1: Bond measur lengthements, and angles Table ofS1: the Bond stable length configuration and angles whenof the stable the central con- figurationepoxy and when nitroxyl the cation central are epoxy at different and nitroxyl distances, cation Table are S2: at Bonddifferent length distances, and angles Table of theS2: stableBond lengthconfiguration and angles when of the the marginal stable configuration epoxy and nitroxyl when the cation marginal are at different epoxy and distances, nitroxyl Table cation S3: are Bond at differentlength and distances, angles of Table the stableS3: Bond configuration length and when angles the of marginal the stable hydroxyl configuration and nitroxyl when the cation marginal are at hydroxyldifferent and distances, nitroxyl Table cation S4: are Bond at different length anddistance angless, Table of the S4: stable Bond configuration length and angles when of the the central stable configurationhydroxyl and when nitroxyl the cation central are hydroxyl at different and distances. nitroxyl cation are at different distances. AuthorAuthor Contributions: Contributions: Conceptualization,Conceptualization, H.R. H.R. and and Q.C.; Q.C.; data data curation, curation, F.G. F.G.and L.Y.; and L.Y.;formal formal analysis,analysis, F.G. F.G. and L.Y.; methodology, F.G.;F.G.; supervision,supervision, H.R.H.R. andand Q.C.;Q.C.; writing—originalwriting—original draft,draft, F.G.; writing—review and editing, F.G., W.Z. and J.L. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the pre-research program of The General Armament Depart- ment (Grant number 9140A05080415BQ01) and National Natural Science Foundation of China (Grant number 21975024); Project supported by the Beijing Institute of Technology (Grant number 20160242032). Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board of Beijing Institute of Technology. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: Data is contained within the article or supplementary material. The data presented in this study are available in supplementary material. Nanomaterials 2021, 11, 58 11 of 12

Acknowledgments: We appreciate the support from the State Key Laboratory of Explosive Science. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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