Preparation and characterization of graphite nanosheets from detonation technique

Preparation and characterization of graphite nanosheets

from detonation technique

Guilei Sun ?,Xiaojie Li,Yandong Qu,Xiaohong Wang,Honghao Yan,Yueju Zhang

State Key Laboratory of Structural Analysis for Industrial Equipment,Department of Engineering Mechanics,

Dalian University of Technology,Dalian 116023,China

Received 16May 2007;accepted 14June 2007

Available online 19June 2007

Abstract

A mixture of natural graphite and HNO 3was used to form graphite intercalation compounds to prepare graphite nanosheets through destroying van der Waals forces between adjacent sheets with detonation technique.The average grain size,microstructure and morphology of the graphite nanosheets were characterized by XRD,TEM and SEM.The specific surface area was tested by nitrogen adsorption and the structure information was obtained from Raman spectra.The results indicated that the prepared graphite nanosheets have two-dimensional plane structure and nanosized thickness.The specific surface area of the nanosheets is 8times higher than that of the raw graphite.It indicates that detonation method is convenient and effective in preparing graphite nanosheets.?2007Elsevier B.V .All rights reserved.

Keywords:Graphite nanosheets;Raman spectra;Nanosized thickness;BET method;Nanomaterials;X-ray techniques

1.Introduction

Graphite nanosheets have been studied for its particular properties and applications.Research indicates that it can be used as reinforcements,conductive additives in composite or coating materials,the raw materials for preparing diamond [1–5].In addition,graphite sheets can also enhance the mechanical properties,corrosion resistance and lighten the weight of materials [6].Correspondingly,there exist lots of preparing methods,such as grinding and chemical synthesis.The most used and fashion method is mechanical grinding in a ball milling device [7].However,crystalline graphite with a layered structure has a high lubrication property,so grinding is extremely difficult,especially to obtain submicron-size particles [8].For instance,the grinding process takes long time and requires a series of subsequent high energy operations such as dewatering,drying,regrinding and classification.Moreover,extensive plastic deformation induced structural transformation in graphite and generated different kinds of defects [9,10].Li et al.[11]prepared 160μm mesh flake graphite using expanded graphite,Wen et al.[12]synthesized

graphite nanosheets by detonating pure TNT (trinitrotoluene)explosive in shielding gas and Chen et al.[13]prepared graphite nanosheets from ultrasonic powdering technique.

Here,we report a detonation method to prepare the graphite https://www.360docs.net/doc/5d10914612.html,paratively,detonation is a highly efficient,low cost way which avoids the complex operating process and reduces power consumption along with mass preparing graphite nanosheets quickly and easily.The patent about preparing graphite nanosheets by detonation method has been applied in China [14].2.Experimental

In this experiment,natural graphite was used as raw material.Concentrated HNO 3and CH 3NO 2were used to form a liquid explosive.All the reagents were of analytical grade and used without pre-treatment.

Procedures for preparing graphite nanosheets were as follows.The natural graphite,HNO 3and CH 3NO 2were mixed in turn at a molar ratio of 3:3:4in cool water bath (273–293K).Then the mixed liquor was poured into a plastic container,which was subsequently hung in the centre of an explosion vessel (the inner diameter is 3.0m).At last,the liquor was initiated by a detonator in the closed explosion vessel.The detonation soot was collected

Available online at https://www.360docs.net/doc/5d10914612.html,

Materials Letters 62(2008)703–

706

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?Corresponding author.Tel.:+8641184706163;fax:+8641184708307.E-mail address:sunguilei@https://www.360docs.net/doc/5d10914612.html, (G.Sun).

0167-577X/$-see front matter ?2007Elsevier B.V .All rights reserved.doi:10.1016/j.matlet.2007.06.035

from the inner surface of the reactor,and then characterized by XRD,TEM,SEM/EDX and BET analysis.

X-ray Diffractometer(XRD-6000)was run at40kV voltage, 30mA current with a scan speed of4.00deg/min and a step of 0.02°using Cu Kα(1.4506?).Transmission electron microscope (Tecnai20,operated at200kV,resolution0.25nm)is used to study the structure and morphology of materials by examining the diffracted and transmitted electron intensities.SEM micrographs were obtained using a FEI Quanta200microscope operating in the low vacuum mode at a20-kv beam current and surface elemental composition was estimated by means of energy dispersive X-ray(EDX)spectroscopy.The specific surface area and pore parameters were investigated by nitrogen sorption isotherms at77K,characterized by AutoChem2910.

3.Results and discussion

3.1.XRD analysis

The detonation soot collected from the inner surface of the reactor is black in color.Fig.1shows its XRD pattern corresponding to graphite-2h(JCPDS41-1487).The(002)and(004)peaks,which are the strongest two in the XRD pattern of natural graphite,represent perpendicular direction(c-axis)to the graphite hexagonal planes[15]. However,after detonation,the intensities of(002)and(004)peaks were greatly weakened while the(100),(102)and(103)peaks disappeared.This phenomenon will be explained in the next paragraph. The average grain size(D)was calculated by Scherrer formula[16],

D?

0:9k B cos h

whereλis0.15406nm,θis the Bragg angle,and B is the full width at half maximum.The parameters of natural graphite and detonation soot from XRD are listed in Table1.

According to Table1,B002increased and D decreased,which reveal that the average grain size of the detonation soot is much smaller than that of the natural graphite.By EDX analysis,the natural graphite and the detonation soot included the same microelements as Si and Fe.Therefore, the disappearance of those peaks that mentioned above was caused not by the variety of composition but by the change of diffraction intensity.After detonation,the sizes of the natural graphite reduced in three directions,a-axis,b-axis and c-axis.Accordingly,the intensity of the diffraction peaks also changed.Herein,we will take(002)and(004)peaks,which are directly reflecting the length of c-axis,as an example.Fig.1shows the corresponding diffraction intensities of these two peaks,where(002)peak alone is shown on the right.The intensity of(002)peak from the natural graphite XRD pattern is72times of that from the detonation soot.As such,the intensity of(004)peak also decreased sharply.Along with the decreased intensity,the corresponding B value increased seriously which was reflected in the broaden peaks.That is to say,the thinner the graphite sheets,the weaker the(002)and(004)diffraction intensities and the broader the(002)and(004)diffraction peaks.For the same reason,the disappearance of(100),(102)and(103)peaks were also due to the weaken diffraction intensities and the broaden peaks.

3.2.Raman spectra

Further information about the structure of the detonation soot was obtained from Raman investigation(Fig.2

). Fig.1.XRD patterns of natural graphite and detonation soot.

Table1

Parameters of natural graphite and detonation soot from XRD

Sample type2θ(°)D002(nm)B002(×10-3nm)D(nm) Natural graphite26.540.34 3.5739.95 Detonation soot26.480.349.67

14.73

Fig.2.Raman spectra of detonation soot. 704G.Sun et al./Materials Letters62(2008)703–706

There are two bands shown in Fig.2.One is D-band,at 1334.82cm ?1,and the other is G-band at 1576.56cm ?1.Herein,the G-band is associated with the E 2/g mode (stretching vibrations)in the basal-plane of graphite and the origin of D-band has been explained as disorder-induced features due to the finite particle size effect,lattice distortion or amorphous carbon background signals [17].Fig.2displays well crystalline graphite with high intensity G-band.D-band almost disappeared which implied the detonation soot had perfect crystalline-sheet structure and little amor-phous carbon was produced in the detonation process.3.3.SEM image and TEM image

SEM images give the formation of surface morphologies and a three-dimensional appearance.Fig.3a shows SEM image of the natural graphite where the dimensions are ～0.2mm and several hundredths of a millimeter in thickness.Fig.3b shows the prepared graphite nanosheets which seem uniform and the thicknesses are less than 50nm.Fig.3c displays TEM image of a whole graphite nanosheet that appears translucent and owns single crystal graphite structure which agrees with Raman spectra.It demonstrates that the prepared graphite nanosheets are very thin and endowed with the large surface area,which is one excellent property that is responsible for the formation of an electrical conducting within the composite or coating materials at very low graphite contents.3.4.BET analysis

BETanalysis showed that the specific surface area of the detonation soot increased to 81.7m 2/https://www.360docs.net/doc/5d10914612.html,paring with that of the raw graphite,8.9m 2/g,it increased more than 8times.The pore size distribution shows that the improved specific surface area was caused mainly by the pores of which the diameters distributing between 2and 10nm.All data implies that the detonation method has splendid effect on the absorptive property so that it can be used not only to prepare graphite nanosheets but also to improve their specific surface area.

Pristine graphite has the property of hosting layers of HNO 3mole-cules and formed HNO 3Graphite Intercalation Compounds (HNO 3GICs).It is because of the strong sp 2carbon –carbon bonding within a layer and weak van der Waals bonding between adjacent sheets that the carbon hexagonal net can maintain its own layer integrity and at the same time host a number of HNO 3molecules [18].The process of generating the graphite sheets can be described as follows.

At the very beginning,when the natural graphite and the concentrated HNO 3were mixed,HNO 3GICs formed.The reaction [18]is expressed as

n C tx HNO 3→C tn eHNO 3Tx ?1NO ?3tH tte ?

:

Consequently,the HNO 3-intercalates occupied graphitic galleries

periodically in the c -axis direction,gave a regular sequence in the c -axis stacking structure.At the same time,CH 3NO 2and the rest of HNO 3also formed a liquid explosive.When detonating,HNO 3GICs break down,which are equivalent to the breakdown of HNO 3because only HNO 3functioned in the graphite nanosheets preparing process.The decompounding of HNO 3released a mass of gases in a moment which pushed the graphite sheets aside along c -axis.Meanwhile,shockwave,produced by the liquid explosive,acted on the fragments.Accordingly,fine graphite nanosheets were prepared.

4.Conclusions

This paper offers a novel,efficient way to prepare graphite nanosheets by detonating natural graphite in a liquid explosive.In this experiment,concentrated HNO 3functioned not only as reactant to form HNO 3GICs but also as a component of liquid explosive.In addition,the momentary breakdown of the intercalates plays an important role in the nanosheets forming process.

Acknowledgements

Support for this research was provided by the National Natural Science Foundation of PR China (no.10572034and no.10602013).References

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