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Fullerenes, Nanotubes and Carbon Nanostructures

ISSN: 1536-383X (Print) 1536-4046 (Online) Journal homepage: http://www.tandfonline.com/loi/lfnn20

Detonation-assisted self-assembly synthesis ofcarbon onions using organics with long carbonchain

Yazhu Lan, Pengwan Chen, Jianjun Liu, Chunxiao Xu & Liyong Du

To cite this article: Yazhu Lan, Pengwan Chen, Jianjun Liu, Chunxiao Xu & Liyong Du(2017) Detonation-assisted self-assembly synthesis of carbon onions using organics withlong carbon chain, Fullerenes, Nanotubes and Carbon Nanostructures, 25:3, 163-169, DOI:10.1080/1536383X.2016.1273906

To link to this article: http://dx.doi.org/10.1080/1536383X.2016.1273906

Accepted author version posted online: 29Dec 2016.Published online: 29 Dec 2016.

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Detonation-assisted self-assembly synthesis of carbon onions using organics withlong carbon chain

Yazhu Lana, Pengwan Chenb, Jianjun Liuc, Chunxiao Xua, and Liyong Dua

aSchool of Materials Science and Engineering, Beijing Institute of Technology, Beijing, China; bState Key Laboratory of Explosion Science andTechnology, Beijing Institute of Technology, Beijing, China; cFaculty of Science, Beijing University of Chemical Technology, Beijing, China

ARTICLE HISTORYReceived 13 December 2016Accepted 14 December 2016

ABSTRACTThe detonation of mixtures of organics with long carbon chains and explosives has been found to yieldcarbon onions without use of any catalyst. Octadecanoic acid, hexadecylic acid, behenic acid and CTAB(cetyltrimethylammonium bromide) have been used as raw materials. The recovered products werecharacterized using various techniques such as scanning electron microscope, transmission electronmicroscope, Raman spectroscopy, infrared spectroscopy and X-ray diffraction. The prepared carbononions consist of concentric-shell graphitic layers with a narrow size distribution of 60 to 70 nm, whichaggregated to form a chain-like structure. Formation mechanism of the carbon onions was proposed asthat the organics with long chains broke their functional groups and self-assembled into carbon onionsdue to low free energy.

KEYWORDSDetonation; Carbon onions;Self-assemble; Formationmechanism

Introduction

Carbon onion is an important member in the family of car-bon allotropes, with a size smaller than 100 nm. Its struc-ture is spherical or polyhedral with partial defect(amorphous domains, or islands of sp3-hybridized carbon)(1). Since the report of carbon onions by Ugarte (2), theresearch on carbon onions has inspired enormous interestamong chemists, physicists and materialists. Due to thespherical shape, nano-sized scale and high specific surfacearea, carbon onions have a lot of applications such as elec-trochemical sensors (3), supercapacitor electrodes (4), effi-cient oxygen electrode for long-life Li-O2 battery (5),electrochemical hydrogen storage (6), catalyst support mate-rial for a catalytic oxygen reduction reaction (7).

Significant efforts involving various synthetic methodshave been devoted to producing carbon onions. Carbononions with fine quality and crystalline morphology weregenerated by arc discharge in water in the presence of metalcatalysts (8). The formation of numerous carbon onionscomposed of concentric graphitic layers has been observedby high-dose carbon ion implantation into copper and sil-ver (9). Zhang et al applied a reduction-substitution methodto prepare well graphitized carbon nano-onions (CNOs)with high yield by catalytic decomposition of methane from750�C to 850�C for 1 hour using the Ni-Fe catalysts (10).Carbon onions have also been yielded by carbonization ofphenolic-formaldehyde resin at 1000�C for 10 hours innitrogen gas atmosphere with the catalysis of ferric nitrate.

The onion-like carbon nanoparticles exhibited a well-aligned concentric structure (11). Alternatively, carbononions can also be fabricated via phase transformation ofdetonation-synthesized nano-diamonds by annealing in vac-uum (12), nitrogen (13), or a novel plasma spraying process(14). Among these methods to synthesize carbon onions,catalyst is very important to get high quality carbon onionsbecause it can provide nucleuses or promote the growth ofcarbon onions. However, there are some disadvantages forthese methods, including impurities and strict experimentalconditions such as high temperature and or long time tofinish the reaction.

Uniform small onion-like carbon (OLC) modified bymolybdenum carbide has been synthesized by detonation-assisted chemical vapor deposition. A mixture of picricacid, cyclohexane and ammonium heptamolybdate washeated to fabricate carbon onions. However, in this process,heptamolybdate was needed as catalyst for the growth ofcarbon onions (15). In the present work, carbon onions wassimply synthesized by a one-step detonation process, whichhas previously been proved to be an efficient technique togenerate various nano-carbon materials such as graphene(16), carbon-encapsulated magnetic nanoparticles (17) andso on. Unlike previous work (15), we synthesized purecarbon onions by detonating the mixture of RDX or TNTand the organics in aliphatic series with long carbon chainswithout using catalyst or pretreatment.

CONTACT Pengwan Chen pwchen@bit.edu.cn State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing-100081,China.

Color versions of one or more of the figures in this article can be found online at www.tandfonline.com/lfnn.© 2017 Taylor & Francis Group, LLC

FULLERENES, NANOTUBES AND CARBON NANOSTRUCTURES2017, VOL. 25, NO. 3, 163–169http://dx.doi.org/10.1080/1536383X.2016.1273906

Experiment

Carbon onions were prepared by detonation process usingorganic compounds with different carbon numbers such asoctadecanoic acid, hexadecylic acid, behenic acid, andCTAB (cetyltrimethylammonium bromide) as carbon sour-ces, respectively. Glucose and urea were also used forcomparison study due to their short carbon chains.RDX [cyclonite, (CH2NNO2)3] or TNT [trinitrotoluene,C6H2CH3(NO2)3] was mixed with organic compounds andused to generate high pressure and high temperature toinduce the pyrolysis of the carbon sources. All the rawmaterials were of analytical grade. A typical synthetic pro-cess of carbon onions is as follows: Before the detonationexperiments, the starting materials were mixed manually,then the mixture was pressed into a cylinder specimen witha diameter of 20 mm. The cylinder specimen was placed ina 100 mL autoclave at 10 kPa vacuum atmosphere. Detona-tion of the mixture was initiated by heating from roomtemperature to approximately 150�C with a heating rate of15 �C/min and then detonated thermally, causing a suddentemperature rise (approximately 70�C) recorded by a tem-perature sensor (17). After the detonation, the chamber wasnaturally cooled down to room temperature. The gaseousproduct was vented and the collected black solid productwas uniformly dispersed in the diluted hydrochloric acid byultrasonic treatment for an hour. After that, distilled waterwas used to wash it to neutral solution. Then the productwas dispersed in ethyl alcohol and heated in water bathwith temperature of 50�C for 30 min. The experimentalconditions are shown in Table 1.

The microstructures of the products were observed usinga transmission electron microscope (TEM) (Tecnai 20).Scanning electron microscope (SEM) (Hitachi S4800) wasalso used to observe the microstructures of the products.

An X-ray diffractometer (XRD) (Rigaku D/MAX-2500) withCu Ka& radiation (&λ D 0.15406 nm) was used to deter-mine the phase composition of the products at a work volt-age of 40 kV and a work current of 200 mA. Ramanspectra were recorded on a Lab RAMAramis Raman spec-trometer with a He-Ne laser at an excitation wavelength of633 nm. The IR spectra were recorded on a Perkin-ElmerSpectrum 400 spectrometer.

Results and discussion

Characterization of carbon onions

The SEM images of the product of No.1 test are shown inFigure 1. Nano-sized spherical particles with a narrow parti-cle size distribution can be observed, which aggregate toform a chain-like structure. To further examine the chain-like structure and investigate the intrinsic structure of theas-synthesized nanoparticles, TEM and high-resolutionTEM images are shown in Figure 2. The HRTEM imageshows that the particles are composed of multilayer sp2 ful-lerene-like shells with well-aligned concentric structure.Most of the nanoparticles are perfectly spherical, exhibitinga small size ranging from 60 to 70 nm. The fine structureof the nanoparticles can also be clearly observed from theHRTEM images. The carbon onions are constructed byhighly ordered graphitic layers. Figure 3 shows the FT-IRspectra of octadecanoic acid (a) and the product of experi-ment No.1 (b). The peak at 1706 cm¡1 of the raw materialcan be assigned to C═O stretching vibrations and the peakat 936 cm¡1can be assigned to O-H bending vibrations ofcarboxylic acid. The absence of these peaks for the productsuggests that during the reaction, the raw material, octade-canoic acid break carboxyl. The peaks of the product at1642 cm¡1 and 1271 cm¡1 are due to stretching vibrationsof C-O single bonds. The peak of 1035 cm¡1 can beassigned to C-OH stretching vibration mode and the peakat 1384 cm¡1 can be attributed to N═O stretching vibra-tions in C-NO2 structure indicating the existence of nitro-gen (18). Nitrogen is supposed to come from the explosive.Figure 4 shows the XRD patterns of the product of No.1test. The diffraction peaks at 24� and 43� correspond to(002) and (101) planes of graphitic carbon (18, 19). Figure 5shows the Raman spectra of the product in No.1 test. Twoprominent peaks located near 1345 cm¡1 and 1503 cm¡1

correspond to the D and G bands, respectively (18,20, 21).The elemental compositions of the carbon onions were

Table 1. Experimental conditions.

No. Ingredients

1 2.4 g RDX C 0.6 g octadecanoic acid (CH3(CH2)16COOH)2 2.4 g RDX C 0.6 g CTAB (CH3(CH2)15N(CH3)3)3 2.4 g RDX C 0.6 g hexadecylic acid (CH3(CH2)14COOH)4 2.4 g RDX C 0.6 g behenic acid (CH3(CH2)20COOH)5 2.4 g RDX C 0.6 g glucose (CH2OHCHOHCHOHCHOHCHOHCHO)6 2.4 g RDX C 0.6 g urea (NH2CONH2)7 2.4 g RDX8 3.0 g TNT9 2.4 g TNTC 0.6 g octadecanoic acid (CH3(CH2)16COOH)

Figure 1. SEM images of the product in No.1 test. Panel (a) is in high magnification and (b) is in low magnification.

164 Y. LAN ET AL.

analyzed using XPS. Figure 6 shows that the XPS spectra ofthe No.1 test contain a sharp graphitic C1s peak at284.8 eV, along with an O1s peak at 532.4 eV possibly dueto the adsorption of some oxygen. The N1s peak was alsoobserved in the No.1 test sample. The N atomic ratio of the

No.1 test sample was calculated to be 6.81% from the peakareas of C1s and N1s and their atomic sensitivity factor. Byfitting the high-resolution N1s spectrum of the No.1 test(Figure 6b), both pyridinic-like (398.7 eV) and pyrrolic-like(400.4 eV) N atoms can be distinguished.

Comparison between organics with a long or short carbonchains

To investigate the effects of raw materials on the carbonproduct, various organics with different carbon chains wereselected as carbon sources, including CTAB, hexadecylicacid, behenic acid, glucose and urea, and a series of experi-ments (No.2–6) were designed. Figure 7 shows TEM imageand HRTEM image of the product of No.2 test. From theseimages, carbon onions with chain-like structure and a nar-row particle size distribution can also be observed, whenCTAB is used as carbon source. When hexadecylic acid andbehenic acid are used as raw materials, as shown inFigures 8 (a) and (b), carbon onions can also be formed.The XRD patterns of No.2–4 show the peaks at around 24�

and 43� corresponding to (002) and (101) planes of gra-phitic carbon. The Raman spectra of No.2–4 also show thepresence of carbon onions. Based on these results, we canfind that the organics with long carbon chains can produce

Figure 2. TEM image (a) and TRTEM image (b) of the product in No.1 test.

Figure 3. FT-IR spectra of octadecanoic acid (a) and the product of No.1 test (b).

Figure 4. XRD pattern of the product of No.1 test. Figure 5. Raman spectra of the product of No.1 test.

FULLERENES, NANOTUBES AND CARBON NANOSTRUCTURES 165

Figure 6. XPS spectrum of the No.1 test (a), and the corresponding high-resolution N 1s peak (b).

Figure 7. TEM image (a) and HRTEM image (b) of the product of No.2 test.

Figure 8. HRTEM images of the product of No.3 test (a), and No.4 test (b).

Figure 9. HRTEM images of the product of No.5 test (a), and No.6 test (b).

166 Y. LAN ET AL.

carbon onions. Organics with short carbon chains wereselected as carbon sources for comparison. Glucose has ashort carbon chain with six carbon atoms, and urea just hasone carbon atom. Figures 9a and b show the HRTEMimages of tests No.5 (a) and No.6 (b). It can be seen thatwhen glucose and urea were used as raw materials, theproduct is graphite flakes rather than the carbon onions.We can conclude that the organics in aliphatic series with along carbon chains can produce carbon onions, while shortchains cannot.

Growth mechanism of carbon onions

It is known that both the organics and the explosive containcarbon atoms. In order to clarify whether carbon onions comefrom the organics or explosives, tests of No.7–9 were designed.Tests No.7 and No.8 are the blank tests, in which the raw mate-rials are only the explosive without any other organics. RDX isa carbon-deficient explosive, which can generate little carbonproduct after detonation. TNT is a carbon-rich explosive,which can generate much carbon product after detonation. Themicrostructure of the products of tests No.7–9 is shown inFigure 10. As shown in Figures 10a and b, the recovered prod-uct of pure RDX exhibits curved sheet structure. As shown inFigures 10c and d, the recovered product of pure TNT consistsof ribbon-like graphite particles with highly curved microstruc-ture (22). Neither of the products of tests No.7–8 contains

carbon onions. From this, we can conclude that when pureRDX or pure TNT explosive is used, carbon onions can’t beformed. When a mixture of 2.4 g TNT and 0.6 g octadecanoicacid (test No.9) was used, carbon onions are also produced(Figures 10e and f.). It can be concluded that carbon onionscan be yielded no matter TNT or RDX is used, as long as organ-ics with long carbon chains are used.

From the discussion above, we can propose a formationmechanism of carbon onion during detonation (Figure 11).Detonation of the explosive can generate high temperatureand high pressure to decompose organics. However theenergy is not enough to break the single bond of C─C. Asa result, the energy can just break the functional groups ofthese organics. So the organics like octadecanoic acid, hexa-decylic acid and behenic acid can just break the carbonyl(C═O), and CTAB can just break the single bond of C─N.These four organics can form a long carbon chains with dif-ferent number of carbon atoms. Similarly glucose breaks thesingle bond of C─O and the carbonyl (C═O), and ureabreaks the single bond of C─N and the carbonyl (C═O).These two types of raw materials can just form short carbonchains with carbon atoms of six and one. When the detona-tion product cools down, all the carbon chains gather intosheet structure. In this way, the long carbon chains gatherinto large scale sheets while the short ones gather into smallscale sheets. In order to reduce free energy, the large scalesheets curve into carbon onions.

Figure 10. Microstructure of the products of tests No.7–9. High magnification (a) and low magnification (b) SEM images of test No.7; TEM images of the product of testsNo.8 (c) and No.9 (e); HRTEM images of the product of tests No.8 (d) and No.9 (f).

FULLERENES, NANOTUBES AND CARBON NANOSTRUCTURES 167

Conclusions

We have reported an easy, cost effective and fast technique toproduce pure carbon onions by detonation of mixtures of explo-sive and organics with a long carbon chains. SEM and TEMcharacterization show that carbon onions exhibit a concentricarrangement of graphitic layers. The as-synthesized carbononions self-assemble into long range chain-like structures. Thegrowth mechanism of the carbon onions has also been discussed.Detonation of explosives generates a high pressure and high tem-perature environment which causes the organics to break func-tional groups. The long carbon chains form carbon onions byself-assembly, and the short carbon chains form graphite flakes.

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