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Nitrated graphene oxide and its catalytic activity in thermal decomposition of ammonium perchlorate
Materials Research Bulletin 50 (2014) 73–78
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Nitrated graphene oxide and its catalytic activity in thermal decomposition of ammonium perchlorate Wenwen Zhang a, Qingping Luo a, Xiaohui Duan a, Yong Zhou b, Chonghua Pei a,* a
State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, China b Eco-materials and Renewable Energy Research Center (ERERC), School of Physics, National Lab of Solid State Microstructure, ERERC, Nanjing University, Nanjing 210093, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 July 2013 Received in revised form 1 October 2013 Accepted 13 October 2013 Available online 21 October 2013
Nitrated graphene oxide (NGO) was synthesized by nitrifying homemade GO with nitro-sulfuric acid. Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy, CP/MAS 13C NMR spectra and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure of NGO. The thickness and the compositions of GO and NGO were analyzed by atomic force microscopy (AFM) and elemental analysis (EA), respectively. The catalytic effect of the NGO for the thermal decomposition of ammonium perchlorate (AP) was investigated by differential scanning calorimetry (DSC). Adding 10% of NGO to AP decreases the decomposition temperature by 106 8C and increases the apparent decomposition heat from 875 to 3236 J/g. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Nanostructures Infrared spectroscopy Nuclear magnetic resonance Catalytic properties
1. Introduction Graphene has been found to be an extremely promising material in various fields, such as nanoelectronics, sensor for biomolecules, transistors, solar cells, and catalysts, due to its unique electronic, chemical and mechanical properties [1–4]. However, the strong Van der Waals interaction and chemical inertness fade it in various applications [5]. Graphene oxide (GO), an oxidized form of graphene, has plenty of oxygen atoms on the basal plane and the edge of the sheets in the form of epoxy, hydroxyl, and carboxyl groups, which offer tremendous opportunities for access to derived graphene-based materials [6,7]. Besides, GO has been successfully utilized as support to prepare hybrids with Fe3O4, ZnO, and Mn3O4, etc., which display high activity as catalysts, mainly involving chemical, photocatalytic, and electrocatalytic applications [8–10]. Furthermore, the presence of nucleophilic oxygen-containing functionalities impart multifunctionality to the use of GO in fuels as chemically active sites and may provide catalytic behavior during combustion [11,12]. It has been proved that the GO can be an excellent substitution for a majority of nanocatalytic fuel additives involving some metals and/or metal oxides such as aluminum, for which the GO does not have nonenergetic oxide passivation layers on the surface, and produce solid oxide reaction products in combustion
* Corresponding author. Tel.: +86 816 2419280; fax: +86 816 2419492. E-mail address: [email protected] (C. Pei). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.10.023
process [13,14]. However, Justin et al. have found that the additives have a negligible or negative effect on the total heat release by diluting the mixture [14]. Therefore, the development of a novel energetic burning rate catalyst and the improvement of the catalysis efficiency are still demanded. To the best of our knowledge, covalent functionalization of carbon-containing species with nitro groups could provide materials with enhanced energetic performance. Carbon nanotubes, with nano-sized diameter and tubular microstructure, have been used to obtain nitro-functionalized carbon materials [15,16]. Unfortunately, the research is always being confined by the presence of metal contamination in the CNTs [17–19]. Also, the CNTs do not disperse well in fuels and the active surface is constrained to only the outer regions of the nanotubes. In contrast, GO is highly dispersible in fuels and its catalytic activity occurs on both sides of the sheet [20– 22]. Therefore, all aforementioned advantages suggest that nitrofunctionalized GO could be an energetic burn rate modifier replacement for nitro-functionalized CNTs. With this in mind, we expect that the NGO may exhibit excellent catalytic activity upon AP decomposition. This is because AP is one of the main oxidizing agents that have been used in solid propellant and the burning behaviors of propellants are highly relevant to the thermal decomposition of AP [23]. At present, the catalytic behaviour of nitro-functionalized carbon-containing species in the thermal decomposition of AP has rarely been reported and to the best of our knowledge there have not been any nitro-functionalized GO reported till now. In this work, we first report the synthesis of NGO by the nitration of GO in
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nitro-sulfuric acid. The preparation of NGO is based on the electrophilic substitution reaction of benzene ring hydrogen and hydroxy hydrogen and also cycloaddition of epoxy group. We also demonstrate that the NGO can facilitate the decomposition of AP and release much heat. This study may provide a novel and energetic burning rate catalyst for composite solid propellant. 2. Experimental 2.1. Reagents Raw materials used in the experiment as following: Graphite powder (99.85%, 30 mm particle size, Shanghai Huayuan China), KMnO4 (99.5%), NaNO3 (99.0%), 30% H2O2, H2SO4 (95–98%), HNO3 (68–70%), HCl (36–38%), and absolute ethyl alcohol (99.7%) were provided by Chengdu Kelong Chemical Reagent Company (Sichuan China). AP was received from Sinopharm. Aqueous solutions were prepared with deionized water (18.25 MV cm) from an ultrapure water purification system.
Fig. 2. IR spectra of GO and NGO, (a) GO, (b) NGO.
2.2. Synthesis of NGO The pathway for the formation of NGO is depicted in Fig. 1. The process began with preparation of GO according to a modified Hummers method [24,25]. The obtained GO was grinded to use. Adding 50 ml concentrated sulfuric acid solution into 25 ml concentrated nitric acid solution slowly, the resulting mixed acid was cooled down to room temperature with cold water. In the moment, 100 mg GO and 25 ml concentrated H2SO4 solutions were added into a beaker and sonicated for 30 min. Thereafter, the mixed acid was slowly dropped into the beaker through a filling funnel, the obtained reaction system was stirred for 5 h at 45 8C, and then diluted and cooled with a certain amount of deionized water to terminate the reaction. Finally, the resulting lower sediment was centrifuged, washed with deionized water until pH of 7, and dried at 60 8C overnight under vacuum. 2.3. Characterization Fourier transform infrared (FTIR) spectrometer (Spectrum One, Perkin Elmer Crop, U.K.) was used to study the structures of the GO and NGO using KBr pallets at room temperature. Compositions were analyzed with an element analyzer (Vario EL CUBE, Elementar). Raman spectra were measured with a inVia spectrometer using a Ar laser (l = 514.5 nm) and CP/MAS 13C NMR experiments were performed on a Bruker AVANCE III spectrometer (working frequency: 400 MHz). The X-ray photoelectron spectroscopy (XPS) was performed on ESCALAB 250 (Thermo Scientific) spectrometer operated at 15 kV and 10 mA at a pressure of about 2 10 9 mbar using Al Ka as the exciting source. Atomic force
microscopy (AFM) images were recorded on a SPI 3800N system (SEIKO) with a silicon cantilever by using the tapping mode. Before AFM measurement, the diluted colloidal suspension (0.01 mg/ml) was sprayed onto a freshly cleaved mica surface and then dried in air for 1 h. DSC curves were conducted with United States SDT Q600 synchronous thermal analyzers at a heating rate of 20 8C/min in Ar atmosphere over the range 20–500 8C with Al2O3 as reference. 3. Results and discussion The FTIR spectra were obtained from GO and NGO (Fig. 2). In the spectrum of GO (Fig. 2a), the peaks at 3400 cm 1, 1620 cm 1 and 2925/2855 cm 1 are separately ascribed to the –OH, aromatic C5 5C bond and CH/CH2 groups. Besides, it also shows that the bands attributed to carboxyl C5 5O and C–O (1721 and 1400 cm 1), epoxy 1 C–O (1225 cm ) and alkoxy C–O (1053 cm 1) groups locate at the edges of the GO sheets [26]. In contrast to the GO, C–O stretching at 1225 cm 1 and O–H bending vibration at 1400 cm 1 are greatly reduced for NGO, which signifies that the hydroxyl and epoxy groups are partly reacted with the nitryl groups. Additionally, there are two new peaks at 1384 cm 1 (C–NO2 stretching vibrations) and 1259 cm 1 (O–NO2 stretching vibrations) in the NGO sample, which testifies to the presence of nitryl groups [27,28]. While the characteristic asymmetric stretching mode of –NO2 in the 1600– 1650 cm 1 [29] region is not observable in the spectrum of NGO which is likely due to the overlapping bands from aromatic C5 5C bond. In the meanwhile, the shift of the peak for alkoxy C–O from 1053 cm 1 in GO to 1064 cm 1 in NGO indicates the strong electron-withdrawing performance of –NO2 groups. Furthermore,
Fig. 1. The scheme of the preparation of NGO.
W. Zhang et al. / Materials Research Bulletin 50 (2014) 73–78
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Table 1 Elemental analysis data of GO and NGO. Sample
GO NGO
N
C
H
S
Others
(wt.%)
(wt.%)
(wt.%)
(wt.%)
(wt.%)
0 1.450
45.790 49.098
2.737 2.163
0.400 0.385
51.073 46.904
the element analysis results show that the N content in GO is 0 and 1.45 wt.% in NGO (see Table 1). Fig. 3 shows magic angle spinning (MAS) 13C NMR spectra of GO before and after nitration. We can clearly see that there are three major signal assignments at 61.3 (epoxide 13C), 71.3 (13C–OH), and 130 (sp2 13C) parts per million (PPM) for GO from the Fig. 3a, which is in agreement with the studies of Cai et al. [30]. At the same time, a little peak at 169 ppm is also observed, which corresponds to C5 5O group, originates from a relatively small number of carbon atoms. While, the intensity of the resonances at 60 and 70 ppm in the spectrum of NGO is lower than that of the parent sample GO and a broad 13C line centred at 130 ppm (with a shoulder to lower chemical shifts) as a result of the nitration to induce an upfield shift for the nearby C atom and the downfield shift for itself. In fact, it has been confirmed by solution state 13C NMR spectroscopy [31]. Furthermore, the peak appearing at 30–46 ppm of NGO is certainly due to the presence of nitro. The Raman spectra of the GO and NGO are shown in Fig. 4. The GO spectrum displays two prominent bands at 1350 and 1587 cm 1, corresponding to the D band and G band of carbon materials, respectively. The D band arises from the structural imperfection and the G band corresponds to the band stretching of all pairs of sp2carbon atoms in both rings and chains [32]. By comparing the G-bands of GO and NGO, it is clear that the G-band of NGO occurs at 1576 cm 1, which is downshifted by 11 cm 1 relative to the corresponding GO sample. The Raman shifts of the G band for NGO provide an evidence of introduction of nitro group into GO. Besides, the intensity ratio (ID/IG) expresses the sp3/sp2 carbon ratio, and the D/G intensity of NGO is slightly decreased comparing with GO. This change suggests that more sp2 domains are formed during the functionalization, which agrees well with Yang’s research [33]. The typical XPS survey spectra of the as-synthesized GO and NGO are displayed in Fig. 5a. The peaks at about 284.5, 399.8, and 532.7 eV can be assigned to the binding energy of C1s, N1s, and O1s, respectively [34]. The N1s peak can be observed in the NGO
Fig. 3. 400 MHz CP/MAS
13
C NMR spectra for GO (a) and NGO (b).
Fig. 4. Raman spectra of GO and NGO, (a) GO, (b) NGO.
but not in GO, which indicates that N from HNO3 is introduced into GO during the nitrifying process. Fig. 5b and d present the C1s XPS spectra of GO and NGO. In the case of GO, four types of carbons with different chemical states are observed, which appear at 284.6 (C–C, C5 5C), 286.7 (C–O), 288.2 (C5 5O), and 289 eV (O–C5 5O) [35,36]. Compared t0 the GO, NGO appears two new peaks at 285 and 287.7 eV, which can be separately attributed to C–N bonds and C– O–N bonds [34,37]. Information provided by analysis of the N1s spectra can complement the information provided by analysis of the C1s spectra. Deconvolution of the N1s spectrum of NGO produces two main peaks, centered at 399.30 and 402.08 eV (Fig. 5c). The peak at higher binding energy is assigned to the nitro groups [38]. The lower binding energy N1s peak at 399.30 eV is related to the presence of reduced nitrogen species and may be due to transformation of the nitro groups to amine groups by X-ray irradiation in the XPS spectrometer chamber [39,40]. The thickness of GO and NGO were examined by AFM characterization. The AFM analysis of GO sheet (see Fig. 6a) shows the apparent thickness is about 1.27 nm, which is consistent with the data reported in literature, indicating that the formation of the single layered GO [41]. The thickness is somewhat larger than that of the real monolayer GO (ca 1 nm) due to the measurement conditions and the presence of adsorbed water [42,43]. From Fig. 6b, We can find that the thickness of NGO is around 1.53 nm and the obtained NGO should be one single layer, while the thickness is much larger than that of monolayer pristine GO (1.27 nm) on account of the Van der Waals radius of N (0.150 nm) larger than that of O (0.140 nm) [44,45]. Another difference between these two images is the aggregation of NGO sheets but not a stack state. This behavior is likely because of the stronger interaction among the GO mats after nitration. The DSC curves for pure AP and mixtures of AP with as-prepared NGO of different blend ratio are shown in Fig. 7. For pure AP (see Fig. 7a), an endothermic peak is observed at about 252 8C, which is attributed to the crystallographic transition of AP from orthorhombic to cubic [46]. The low-temperature exothermic peak at 358 8C is due to the partial decomposition of AP and formation of some intermediate NH3and HClO4 by dissociation and sublimation [47]. The high-temperature exothermic peak at about 456 8C is caused by both oxidation of NH3 by ClO4 in gas phase and decomposition of AP on solid surface. However, as NGO is added in AP, the case becomes significantly different. It is noted that the NGO has a significant impact on the exothermic peaks, though it has no effect on crystallographic transition temperature. For the curves b, c and d, it can be seen that the first exothermic peaks
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Fig. 5. (a) Typical XPS survey spectra of GO (black) and NGO (red). (b) C1s XPS spectra of GO and (d) NGO. (c) N1s XPS spectra of NGO.
become steep in presence of NGO which indicates that the rate of thermal decomposition of AP is enhanced. In addition, the area of low-temperature exothermic peak shows an increasing tendency with the augment of NGO and the high-temperature exothermic peak shifts to a lower value. Particularly, when the content of NGO is up to 10% (Fig. 7d), one sharp exothermic peak occurs at 350 8C and the exothermic heat (3236 J/g) is much larger than that of the pure AP (875 J/g) (Table 2). When the amount of NGO increases to 20 and 50%, the DSC curves (inset of Fig. 7) show only one exothermic peak (corresponding to 338 and 326 8C) and a decrease
Fig. 6. A tapping mode of AFM images of GO and NGO on mica surface, and the height profile of the AFM images (a) GO, (b) NGO.
trend of decomposition heat (2455 and 1588 J/g, respectively). Thus it is found that catalytic activity of NGO depends on the amount of catalyst addition. Furthermore, in order to investigate the dependence of the NGO content (10%), the content of 8% has also been examined. The results display that the NGO (10%) can better catalyze the AP decomposition, as shown in Table 2. As for the catalytic effect of GO (10% in AP), the high-temperature
Fig. 7. DSC curves for the decomposition of (a) pure AP, (b) AP with 2% NGO, (c) AP with 5% NGO, (d) AP with 10% NGO and (e) AP with 10% GO. Inset shows the DSC curves of (a) AP with 20% NGO and (b) AP with 50% NGO.
W. Zhang et al. / Materials Research Bulletin 50 (2014) 73–78 Table 2 The exothermic peak temperature (TL and TH) and decomposition heat (DH) of AP and the mixtures. Sample
TL (8C)a
TH (8C)b
DH (J/g)c
AP AP + 2%NGO AP + 5%NGO AP + 8%NGO AP + 10%NGO AP + 20%NGO AP + 50%NGO AP + 10%GO
358 351 354 358 350 338 326 357
456 450 438 – – – – 427
875 1164 1743 2629 3236 2455 1588 2278
a b c
Low thermal decomposition temperature of the sample. High thermal decomposition temperature of the sample. Total DSC heat release of the sample.
77
increasing, the effect of catalysis enhances. The better catalytic activity of NGO is likely attributed to its more active sites, which can further enhance the rate of heterogeneous decomposition of deprotonized HClO4 gas on the surfaces of catalyst particles. Although our current study presents results using only AP, the NGO improves the property of other explosive (HMX or RDX) which has been investigated in our laboratory and will be reported elsewhere. Acknowledgement We are grateful for the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (No. 12ZXFK07) from Southwest University of Science and Technology. We are also grateful for the apparatus support of the Analytical and Testing Center of Southwest University of Science and Technology.
References
Fig. 8. TG curves for the decomposition of (a) pure AP, (b) AP with 2% NGO, (c) AP with 5% NGO, (d) AP with 10% NGO and (e) AP with 10% GO.
exothermic peak does not shift as far and the two peaks exist separately (Fig. 7e). From the TG curves (Fig. 8), it can be seen that in the range from room temperature to 500 8C, two weight loss steps are clearly observed for pure AP, AP with GO (10%) and AP with NGO (2 and 5%), while only one weight loss step is presented for the mixture of AP with NGO (10%), in good agreement with the exothermic peaks of the DSC curves. It can be concluded from the above experimental results that NGO significantly promotes the exothermic process of AP decomposition. We think NGO may promote the heterogeneous decomposition of deprotonized HClO4 gas on the solid surface in the high-temperature decomposition stage, which reduces the temperature of high-temperature exothermic decomposition process. For the view of apparent decomposition heat, the lower high decomposition temperature results in more apparent decomposition heat, which is likely rising from heat concentration and NGO combustion. Comparing the content of NGO has a deep understanding of the catalytic effect on decomposition of AP. Upon addition of NGO (20 or 50%), though the decomposition temperature decreases, the decomposition heat falls a lot. Therefore, the output of heat should be mainly contributed by decomposition of AP. 4. Conclusions In conclusion, a synthetic route for nitro-group functionalized GO was developed. The obtained monolayer NGO with 1.45 wt.% N content displayed a better catalytic property on AP decomposition in contrast with GO. In addition, as the content of NGO ( 10%)
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Nitrated graphene oxide and its catalytic activity in thermal decomposition of ammonium perchlorate Wenwen Zhang a, Qingping Luo a, Xiaohui Duan a, Yong Zhou b, Chonghua Pei a,* a
State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, China b Eco-materials and Renewable Energy Research Center (ERERC), School of Physics, National Lab of Solid State Microstructure, ERERC, Nanjing University, Nanjing 210093, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 July 2013 Received in revised form 1 October 2013 Accepted 13 October 2013 Available online 21 October 2013
Nitrated graphene oxide (NGO) was synthesized by nitrifying homemade GO with nitro-sulfuric acid. Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy, CP/MAS 13C NMR spectra and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure of NGO. The thickness and the compositions of GO and NGO were analyzed by atomic force microscopy (AFM) and elemental analysis (EA), respectively. The catalytic effect of the NGO for the thermal decomposition of ammonium perchlorate (AP) was investigated by differential scanning calorimetry (DSC). Adding 10% of NGO to AP decreases the decomposition temperature by 106 8C and increases the apparent decomposition heat from 875 to 3236 J/g. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: Nanostructures Infrared spectroscopy Nuclear magnetic resonance Catalytic properties
1. Introduction Graphene has been found to be an extremely promising material in various fields, such as nanoelectronics, sensor for biomolecules, transistors, solar cells, and catalysts, due to its unique electronic, chemical and mechanical properties [1–4]. However, the strong Van der Waals interaction and chemical inertness fade it in various applications [5]. Graphene oxide (GO), an oxidized form of graphene, has plenty of oxygen atoms on the basal plane and the edge of the sheets in the form of epoxy, hydroxyl, and carboxyl groups, which offer tremendous opportunities for access to derived graphene-based materials [6,7]. Besides, GO has been successfully utilized as support to prepare hybrids with Fe3O4, ZnO, and Mn3O4, etc., which display high activity as catalysts, mainly involving chemical, photocatalytic, and electrocatalytic applications [8–10]. Furthermore, the presence of nucleophilic oxygen-containing functionalities impart multifunctionality to the use of GO in fuels as chemically active sites and may provide catalytic behavior during combustion [11,12]. It has been proved that the GO can be an excellent substitution for a majority of nanocatalytic fuel additives involving some metals and/or metal oxides such as aluminum, for which the GO does not have nonenergetic oxide passivation layers on the surface, and produce solid oxide reaction products in combustion
* Corresponding author. Tel.: +86 816 2419280; fax: +86 816 2419492. E-mail address: [email protected] (C. Pei). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.10.023
process [13,14]. However, Justin et al. have found that the additives have a negligible or negative effect on the total heat release by diluting the mixture [14]. Therefore, the development of a novel energetic burning rate catalyst and the improvement of the catalysis efficiency are still demanded. To the best of our knowledge, covalent functionalization of carbon-containing species with nitro groups could provide materials with enhanced energetic performance. Carbon nanotubes, with nano-sized diameter and tubular microstructure, have been used to obtain nitro-functionalized carbon materials [15,16]. Unfortunately, the research is always being confined by the presence of metal contamination in the CNTs [17–19]. Also, the CNTs do not disperse well in fuels and the active surface is constrained to only the outer regions of the nanotubes. In contrast, GO is highly dispersible in fuels and its catalytic activity occurs on both sides of the sheet [20– 22]. Therefore, all aforementioned advantages suggest that nitrofunctionalized GO could be an energetic burn rate modifier replacement for nitro-functionalized CNTs. With this in mind, we expect that the NGO may exhibit excellent catalytic activity upon AP decomposition. This is because AP is one of the main oxidizing agents that have been used in solid propellant and the burning behaviors of propellants are highly relevant to the thermal decomposition of AP [23]. At present, the catalytic behaviour of nitro-functionalized carbon-containing species in the thermal decomposition of AP has rarely been reported and to the best of our knowledge there have not been any nitro-functionalized GO reported till now. In this work, we first report the synthesis of NGO by the nitration of GO in
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nitro-sulfuric acid. The preparation of NGO is based on the electrophilic substitution reaction of benzene ring hydrogen and hydroxy hydrogen and also cycloaddition of epoxy group. We also demonstrate that the NGO can facilitate the decomposition of AP and release much heat. This study may provide a novel and energetic burning rate catalyst for composite solid propellant. 2. Experimental 2.1. Reagents Raw materials used in the experiment as following: Graphite powder (99.85%, 30 mm particle size, Shanghai Huayuan China), KMnO4 (99.5%), NaNO3 (99.0%), 30% H2O2, H2SO4 (95–98%), HNO3 (68–70%), HCl (36–38%), and absolute ethyl alcohol (99.7%) were provided by Chengdu Kelong Chemical Reagent Company (Sichuan China). AP was received from Sinopharm. Aqueous solutions were prepared with deionized water (18.25 MV cm) from an ultrapure water purification system.
Fig. 2. IR spectra of GO and NGO, (a) GO, (b) NGO.
2.2. Synthesis of NGO The pathway for the formation of NGO is depicted in Fig. 1. The process began with preparation of GO according to a modified Hummers method [24,25]. The obtained GO was grinded to use. Adding 50 ml concentrated sulfuric acid solution into 25 ml concentrated nitric acid solution slowly, the resulting mixed acid was cooled down to room temperature with cold water. In the moment, 100 mg GO and 25 ml concentrated H2SO4 solutions were added into a beaker and sonicated for 30 min. Thereafter, the mixed acid was slowly dropped into the beaker through a filling funnel, the obtained reaction system was stirred for 5 h at 45 8C, and then diluted and cooled with a certain amount of deionized water to terminate the reaction. Finally, the resulting lower sediment was centrifuged, washed with deionized water until pH of 7, and dried at 60 8C overnight under vacuum. 2.3. Characterization Fourier transform infrared (FTIR) spectrometer (Spectrum One, Perkin Elmer Crop, U.K.) was used to study the structures of the GO and NGO using KBr pallets at room temperature. Compositions were analyzed with an element analyzer (Vario EL CUBE, Elementar). Raman spectra were measured with a inVia spectrometer using a Ar laser (l = 514.5 nm) and CP/MAS 13C NMR experiments were performed on a Bruker AVANCE III spectrometer (working frequency: 400 MHz). The X-ray photoelectron spectroscopy (XPS) was performed on ESCALAB 250 (Thermo Scientific) spectrometer operated at 15 kV and 10 mA at a pressure of about 2 10 9 mbar using Al Ka as the exciting source. Atomic force
microscopy (AFM) images were recorded on a SPI 3800N system (SEIKO) with a silicon cantilever by using the tapping mode. Before AFM measurement, the diluted colloidal suspension (0.01 mg/ml) was sprayed onto a freshly cleaved mica surface and then dried in air for 1 h. DSC curves were conducted with United States SDT Q600 synchronous thermal analyzers at a heating rate of 20 8C/min in Ar atmosphere over the range 20–500 8C with Al2O3 as reference. 3. Results and discussion The FTIR spectra were obtained from GO and NGO (Fig. 2). In the spectrum of GO (Fig. 2a), the peaks at 3400 cm 1, 1620 cm 1 and 2925/2855 cm 1 are separately ascribed to the –OH, aromatic C5 5C bond and CH/CH2 groups. Besides, it also shows that the bands attributed to carboxyl C5 5O and C–O (1721 and 1400 cm 1), epoxy 1 C–O (1225 cm ) and alkoxy C–O (1053 cm 1) groups locate at the edges of the GO sheets [26]. In contrast to the GO, C–O stretching at 1225 cm 1 and O–H bending vibration at 1400 cm 1 are greatly reduced for NGO, which signifies that the hydroxyl and epoxy groups are partly reacted with the nitryl groups. Additionally, there are two new peaks at 1384 cm 1 (C–NO2 stretching vibrations) and 1259 cm 1 (O–NO2 stretching vibrations) in the NGO sample, which testifies to the presence of nitryl groups [27,28]. While the characteristic asymmetric stretching mode of –NO2 in the 1600– 1650 cm 1 [29] region is not observable in the spectrum of NGO which is likely due to the overlapping bands from aromatic C5 5C bond. In the meanwhile, the shift of the peak for alkoxy C–O from 1053 cm 1 in GO to 1064 cm 1 in NGO indicates the strong electron-withdrawing performance of –NO2 groups. Furthermore,
Fig. 1. The scheme of the preparation of NGO.
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Table 1 Elemental analysis data of GO and NGO. Sample
GO NGO
N
C
H
S
Others
(wt.%)
(wt.%)
(wt.%)
(wt.%)
(wt.%)
0 1.450
45.790 49.098
2.737 2.163
0.400 0.385
51.073 46.904
the element analysis results show that the N content in GO is 0 and 1.45 wt.% in NGO (see Table 1). Fig. 3 shows magic angle spinning (MAS) 13C NMR spectra of GO before and after nitration. We can clearly see that there are three major signal assignments at 61.3 (epoxide 13C), 71.3 (13C–OH), and 130 (sp2 13C) parts per million (PPM) for GO from the Fig. 3a, which is in agreement with the studies of Cai et al. [30]. At the same time, a little peak at 169 ppm is also observed, which corresponds to C5 5O group, originates from a relatively small number of carbon atoms. While, the intensity of the resonances at 60 and 70 ppm in the spectrum of NGO is lower than that of the parent sample GO and a broad 13C line centred at 130 ppm (with a shoulder to lower chemical shifts) as a result of the nitration to induce an upfield shift for the nearby C atom and the downfield shift for itself. In fact, it has been confirmed by solution state 13C NMR spectroscopy [31]. Furthermore, the peak appearing at 30–46 ppm of NGO is certainly due to the presence of nitro. The Raman spectra of the GO and NGO are shown in Fig. 4. The GO spectrum displays two prominent bands at 1350 and 1587 cm 1, corresponding to the D band and G band of carbon materials, respectively. The D band arises from the structural imperfection and the G band corresponds to the band stretching of all pairs of sp2carbon atoms in both rings and chains [32]. By comparing the G-bands of GO and NGO, it is clear that the G-band of NGO occurs at 1576 cm 1, which is downshifted by 11 cm 1 relative to the corresponding GO sample. The Raman shifts of the G band for NGO provide an evidence of introduction of nitro group into GO. Besides, the intensity ratio (ID/IG) expresses the sp3/sp2 carbon ratio, and the D/G intensity of NGO is slightly decreased comparing with GO. This change suggests that more sp2 domains are formed during the functionalization, which agrees well with Yang’s research [33]. The typical XPS survey spectra of the as-synthesized GO and NGO are displayed in Fig. 5a. The peaks at about 284.5, 399.8, and 532.7 eV can be assigned to the binding energy of C1s, N1s, and O1s, respectively [34]. The N1s peak can be observed in the NGO
Fig. 3. 400 MHz CP/MAS
13
C NMR spectra for GO (a) and NGO (b).
Fig. 4. Raman spectra of GO and NGO, (a) GO, (b) NGO.
but not in GO, which indicates that N from HNO3 is introduced into GO during the nitrifying process. Fig. 5b and d present the C1s XPS spectra of GO and NGO. In the case of GO, four types of carbons with different chemical states are observed, which appear at 284.6 (C–C, C5 5C), 286.7 (C–O), 288.2 (C5 5O), and 289 eV (O–C5 5O) [35,36]. Compared t0 the GO, NGO appears two new peaks at 285 and 287.7 eV, which can be separately attributed to C–N bonds and C– O–N bonds [34,37]. Information provided by analysis of the N1s spectra can complement the information provided by analysis of the C1s spectra. Deconvolution of the N1s spectrum of NGO produces two main peaks, centered at 399.30 and 402.08 eV (Fig. 5c). The peak at higher binding energy is assigned to the nitro groups [38]. The lower binding energy N1s peak at 399.30 eV is related to the presence of reduced nitrogen species and may be due to transformation of the nitro groups to amine groups by X-ray irradiation in the XPS spectrometer chamber [39,40]. The thickness of GO and NGO were examined by AFM characterization. The AFM analysis of GO sheet (see Fig. 6a) shows the apparent thickness is about 1.27 nm, which is consistent with the data reported in literature, indicating that the formation of the single layered GO [41]. The thickness is somewhat larger than that of the real monolayer GO (ca 1 nm) due to the measurement conditions and the presence of adsorbed water [42,43]. From Fig. 6b, We can find that the thickness of NGO is around 1.53 nm and the obtained NGO should be one single layer, while the thickness is much larger than that of monolayer pristine GO (1.27 nm) on account of the Van der Waals radius of N (0.150 nm) larger than that of O (0.140 nm) [44,45]. Another difference between these two images is the aggregation of NGO sheets but not a stack state. This behavior is likely because of the stronger interaction among the GO mats after nitration. The DSC curves for pure AP and mixtures of AP with as-prepared NGO of different blend ratio are shown in Fig. 7. For pure AP (see Fig. 7a), an endothermic peak is observed at about 252 8C, which is attributed to the crystallographic transition of AP from orthorhombic to cubic [46]. The low-temperature exothermic peak at 358 8C is due to the partial decomposition of AP and formation of some intermediate NH3and HClO4 by dissociation and sublimation [47]. The high-temperature exothermic peak at about 456 8C is caused by both oxidation of NH3 by ClO4 in gas phase and decomposition of AP on solid surface. However, as NGO is added in AP, the case becomes significantly different. It is noted that the NGO has a significant impact on the exothermic peaks, though it has no effect on crystallographic transition temperature. For the curves b, c and d, it can be seen that the first exothermic peaks
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Fig. 5. (a) Typical XPS survey spectra of GO (black) and NGO (red). (b) C1s XPS spectra of GO and (d) NGO. (c) N1s XPS spectra of NGO.
become steep in presence of NGO which indicates that the rate of thermal decomposition of AP is enhanced. In addition, the area of low-temperature exothermic peak shows an increasing tendency with the augment of NGO and the high-temperature exothermic peak shifts to a lower value. Particularly, when the content of NGO is up to 10% (Fig. 7d), one sharp exothermic peak occurs at 350 8C and the exothermic heat (3236 J/g) is much larger than that of the pure AP (875 J/g) (Table 2). When the amount of NGO increases to 20 and 50%, the DSC curves (inset of Fig. 7) show only one exothermic peak (corresponding to 338 and 326 8C) and a decrease
Fig. 6. A tapping mode of AFM images of GO and NGO on mica surface, and the height profile of the AFM images (a) GO, (b) NGO.
trend of decomposition heat (2455 and 1588 J/g, respectively). Thus it is found that catalytic activity of NGO depends on the amount of catalyst addition. Furthermore, in order to investigate the dependence of the NGO content (10%), the content of 8% has also been examined. The results display that the NGO (10%) can better catalyze the AP decomposition, as shown in Table 2. As for the catalytic effect of GO (10% in AP), the high-temperature
Fig. 7. DSC curves for the decomposition of (a) pure AP, (b) AP with 2% NGO, (c) AP with 5% NGO, (d) AP with 10% NGO and (e) AP with 10% GO. Inset shows the DSC curves of (a) AP with 20% NGO and (b) AP with 50% NGO.
W. Zhang et al. / Materials Research Bulletin 50 (2014) 73–78 Table 2 The exothermic peak temperature (TL and TH) and decomposition heat (DH) of AP and the mixtures. Sample
TL (8C)a
TH (8C)b
DH (J/g)c
AP AP + 2%NGO AP + 5%NGO AP + 8%NGO AP + 10%NGO AP + 20%NGO AP + 50%NGO AP + 10%GO
358 351 354 358 350 338 326 357
456 450 438 – – – – 427
875 1164 1743 2629 3236 2455 1588 2278
a b c
Low thermal decomposition temperature of the sample. High thermal decomposition temperature of the sample. Total DSC heat release of the sample.
77
increasing, the effect of catalysis enhances. The better catalytic activity of NGO is likely attributed to its more active sites, which can further enhance the rate of heterogeneous decomposition of deprotonized HClO4 gas on the surfaces of catalyst particles. Although our current study presents results using only AP, the NGO improves the property of other explosive (HMX or RDX) which has been investigated in our laboratory and will be reported elsewhere. Acknowledgement We are grateful for the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (No. 12ZXFK07) from Southwest University of Science and Technology. We are also grateful for the apparatus support of the Analytical and Testing Center of Southwest University of Science and Technology.
References
Fig. 8. TG curves for the decomposition of (a) pure AP, (b) AP with 2% NGO, (c) AP with 5% NGO, (d) AP with 10% NGO and (e) AP with 10% GO.
exothermic peak does not shift as far and the two peaks exist separately (Fig. 7e). From the TG curves (Fig. 8), it can be seen that in the range from room temperature to 500 8C, two weight loss steps are clearly observed for pure AP, AP with GO (10%) and AP with NGO (2 and 5%), while only one weight loss step is presented for the mixture of AP with NGO (10%), in good agreement with the exothermic peaks of the DSC curves. It can be concluded from the above experimental results that NGO significantly promotes the exothermic process of AP decomposition. We think NGO may promote the heterogeneous decomposition of deprotonized HClO4 gas on the solid surface in the high-temperature decomposition stage, which reduces the temperature of high-temperature exothermic decomposition process. For the view of apparent decomposition heat, the lower high decomposition temperature results in more apparent decomposition heat, which is likely rising from heat concentration and NGO combustion. Comparing the content of NGO has a deep understanding of the catalytic effect on decomposition of AP. Upon addition of NGO (20 or 50%), though the decomposition temperature decreases, the decomposition heat falls a lot. Therefore, the output of heat should be mainly contributed by decomposition of AP. 4. Conclusions In conclusion, a synthetic route for nitro-group functionalized GO was developed. The obtained monolayer NGO with 1.45 wt.% N content displayed a better catalytic property on AP decomposition in contrast with GO. In addition, as the content of NGO ( 10%)
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