Preparation and characterization of graphite nanosheets from ultrasonic powdering technique

Preparation and characterization of graphite nanosheets

from ultrasonic powdering technique

Guohua Chen *,Wengui Weng,Dajun Wu,Cuiling Wu,Jinrong Lu,

Pingping Wang,Xiangfeng Chen

College of Materials Science and Engineering,Huaqiao University,Quanzhou 362011,China

Received 11August 2003;accepted 20December 2003

Abstract

Natural ?ake graphite was exfoliated into exfoliated graphite via an acid intercalation procedure.The resulting exfoliated graphite was a worm-like particle composed of graphite sheets with thickness in the nanometer scale.Subjecting it to ultrasonic irradiation,the exfoliated graphite was e?ectively further foliated into isolated graphite nanosheets.SEM,TEM,SAD,laser counting,and BET measurements revealed that the graphite nanosheets prepared with 10h irradiation were about 52nm in thickness and 13l m in diameter.FTIR examination showed that there were oxygen-containing groups presented on the surface of the exfoliated graphite.This result substantiated the statement reported in the literature that acid treatment could result in oxi-dization of carbon bonds on graphite surface.ó2004Elsevier Ltd.All rights reserved.

Keywords:A.Exfoliated graphite;C.BET surface area;D.Microstructure,Particle size

1.Introduction

Nanocomposites of polymers with inorganic materi-als have been shown to exhibit a signi?cant increase in the properties of polymers and even generate certain new properties that cannot be derived from their coun-terparts,as the mixing of phases in them occurs over a much smaller length scale in comparison to the micrometer length scale of conventional composites [1–4].Since the pioneer work of Toyota researchers [5,6],and especially the report of Giannelis et al.[7],the ?eld of polymer/silicate has gained large momentum recently [8–10].If layered silicates are well dispersed within the polymers,either intercalated or delaminated nanocom-posites are formed,leading to great enhancement of mechanical,thermal and barrier performance in com-parison to conventional composites [11].Unfortunately,silicate reinforced nanocomposites do not exhibit elec-trical properties as good as those of some conducting composites such as graphite-containing composites.Similar to layered silicates,natural ?ake graphite is another layered material.The graphite crystal lattice

consists of graphene layers formed by sp 2hybridized carbon atoms,while the carbon sheets are bounded by weak van der Waals forces with each other.Graphite is a good electrical conductor with an electrical conduc-tivity in the range of 104S/m at room temperature [12].Therefore,graphite can be a good candidate in fabri-cating polymer conducting nanocomposites.However,unlike several lamellar silicate solids [13]whose exfoli-ation can be achieved by ion exchange reactions,exfo-liation of graphite cannot be achieved in the same way because it does not bear any net charge.We achieved the nanodispersion composites via an in situ polymerization of monomer in the presence of graphite nanosheets which were made by powdering the exfoliated graphite with ultrasonic irradiation [14,15].In this paper,char-acterization of the graphite nanosheets will be presented;the e?ect of the ultrasonic irradiation on the exfoliation of graphite will be investigated.2.Experimental 2.1.Materials

Natural ?ake graphite with an average diameter of 500l m was used for preparing the exfoliated graphite.

*

Corresponding author.Tel.:+86-595-269-2956;fax:+86-595-268-6969.

E-mail address:hdcgh@https://www.360docs.net/doc/906722608.html, (G.Chen).0008-6223/$-see front matter ó2004Elsevier Ltd.All rights reserved.

doi:10.1016/j.carbon.2003.12.074

Carbon 42(2004)

753–759

https://www.360docs.net/doc/906722608.html,/locate/carbon

Concentrated sulfuric acid and fuming nitric acid (chemically pure)were used as chemical intercalate and oxidizer to prepare graphite intercalation compounds GICs.95%(v/v)alcohol(chemical pure)and distilled water were used as solvents for preparation of foliated graphite nanosheets.

2.2.Preparation of exfoliated graphite and foliated graphite

Exfoliated graphite(EG)was prepared according to the methods reported in the literatures[16,17].Brie?y, the natural?ake graphite was?rst dried in a vacuum oven for24h at80°C.Then,a mixture of concentrated sulfuric acid and fuming acid(4:1,v/v)was slowly added,under appropriate cooling and stirring,to a three-neck?ask containing graphite?akes.After16h of reaction,the acid-treated natural graphite was?ltered and washed thoroughly with deionized water until the pH level of the solution reached6.After being dried at 100°C overnight,the resulting graphite intercalation compound(GIC)was subjected to a thermal shock at 1050°C for15s in a mu?e furnace to form exfoliated graphite.

1g EG was mixed and saturated with400ml alcohol solution consisting of alcohol and distilled water with a ratio of65:35for12h.Then the mixture was subjected to ultrasonic irradiation with a power of100W for various times.After hours of sonication,EG particles were e?ectively fragmented into foliated graphite(FG). The FG dispersion was then?ltered and dried at80°C to remove residue solvents.The as-prepared foliated graphite powder,we called graphite nanosheets,was then kept in a dry desiccator for testing and further use.

2.3.Characterization and measurements

The distributions of the sizes of foliated graphite particles were determined by a JL-1166Laser Particle Sizer(Chengdu,China)from the measurements of foli-ated graphite particles dispersed in water.

Scanning electron microscopy(SEM)was performed on a LEO-1530?eld-emitting scanning electron micro-scope(Germany),at an operating pressure of10à5mbar and an accelerating voltage of10.0kV.

The adsorption properties were used to analyze the characteristics,i.e.BET speci?c surface area,micropore volume and pore size distributions of EG and FG.The low temperature(77K)nitrogen adsorption was re-corded by using a Tristar3000(Micromeritics,Nor-cross,GA,USA)adsorption analyzer.Prior to the measurements,the graphite samples were pretreated by outgassing in a vacuum oven at200°C for12h.The apparatus determined the di?erence in the volumes of nitrogen delivered to a sample vial and a balance vial. The vials were immersed to the same level in liquid nitrogen,and identical pressures were applied during the measurements.The measurements of a di?erential vol-ume thus compensated for changing temperature gra-dients in the samples vial.For each determination,three measurements were made in the relative pressure range 0

X-ray di?raction(XRD)patterns were obtained using CuK a radiation(k?0:15nm)at a scanning rate of2.0°/ https://www.360docs.net/doc/906722608.html,ing a voltage of45kV and a current of40mA.

Fourier transform infrared spectroscopy(FTIR) study of the chemical functional groups on the exfoli-ated graphite was conducted on a Nicolet Magna750 spectrometer.

3.Results and discussions

3.1.Powdering of exfoliated graphite

Ultrasonic irradiation has long been a very useful tool in the?elds of chemistry and material engineering [18].It is well known that when ultrasonic wave passes through a liquid acoustic cavitation takes place,and produces localized hot spots with temperature as high as 5000°C and local pressure as high as500atm,with heating and cooling rates greater than109K/s.Acoustic cavitation near a solid surface can create localized ero-sion,induce high-velocity interparticle collisions,cause fragmentation of brittle materials,and improve mass transport[19].Thus,the soft and sticky exfoliated graphite,which is very di?cult to be powdered by other mechanical methods,can be e?ectively powdered by ultrasonic irradiation in aqueous medium.

3.2.Characterization of graphite nanosheets

3.2.1.Electron microscopy

Fig.1shows the SEM image of foliated graphite sheets prepared upon10h ultrasonic irradiation. Clearly,exfoliated graphite worms have been completely torn into foliated sheets of5–20l m in diameter and30–80nm in thickness,named graphite nanosheets[14,15]. Image analysis calculation results based on50–80 graphite nanosheets showed that the average sheet diameter is approximately12l m.The average thickness of the nanosheets was estimated to be about50nm, based on the values of50graphite sheets.The powder has an apparent density of about0.015g/cm3,much lower than the mass density of the original natural?ake graphite,which is2.2g/cm3.

A TEM micrograph showing images of graphite nanosheets imbedded in PMMA resin was presented in [14].It is clear that foliated graphite sheets of about10 nm thick,and even thinner sheets with thickness in the range of2–5nm,could be observed.Thus,we specu-lated that large amount of the as-prepared nanosheets

754G.Chen et al./Carbon42(2004)753–759

were composed of thinner secondary sheets.SAD pat-terns [14,15]also showed that foliated graphite sheets were polycrystalline in nature.

https://www.360docs.net/doc/906722608.html,ser counting

Exfoliated graphite was subjected to ultrasonic irra-diation for various times in order to investigate the e?ect of irradiation time on the particle size,the structure and properties of the foliated graphite.Table 1summarizes the parameters of particles of foliated graphite prepared using various irradiation times.The curves of integral volume distributions and histograms of di?erential vol-ume distributions of three typical kinds of foliated graphite are presented in Fig.2.

From Table 1,it is seen that when exfoliated graphite was subjected to ultrasonic irradiation with time less than 10h,the particle size of the resulting foliated graphite was much larger than that with longer times.The critical ultrasonic irradiation time for the drastic decrease of particle size was thus about 10h.After 10h ultrasonic irradiation,both the particle sizes and size distributions change slightly with increasing irradiation times.

From Fig.2,it is seen that all the three samples have similar size distributions,which are obviously not sym-metrical but particularly extended to the larger values,up to diameters of about 45l m.This fact in one aspect leads to the conclusion that after the exfoliated graphite

Table 1

Parameters of particles of foliated graphite prepared using various irradiation times Type D 20(l m)D 42(l m)D 63(l m)D 81(l m)D 90(l m)D 99(l m)FG 618.025.032.042.050.080.0FG 816.021.029.034.038.052.0FG 10 6.59.513.017.021.036.0FG 12 6.09.012.017.021.034.0FG 16 6.09.012.016.019.034.0FG 48

5.5

8.0

11.0

16.0

19.0

34.0

FG i ––foliated graphite prepared using i h ultrasonic irradiation.D j ––j %of volume of particles is smaller than dimension D j .

Fig.1.SEM micrographs of foliated graphite.

G.Chen et al./Carbon 42(2004)753–759

755

is e?ectively torn into foliated graphite,the resulting particles will be less sensitive to further ultrasonic irra-diation,at least under these same conditions.On the other hand,the asymmetrical distributions of the resulting foliated graphite might be bene?cial to lower the percolation threshold of the composites?lled with those kinds of foliated graphite,since it has been shown [20–22]that when the percolation objects have a large aspect ratio,the critical concentration diminishes as the size distribution increases.

Fig.3shows the mean particle size(diameter)of the foliated graphite.These results together with the values presented in Table1show further that at the early stage of ultrasonic irradiation,the particle size decreased with a higher rate.When the critical time was reached,the decrease slowed down as the increase of irradiation time. However,noting the structure of the exfoliated graphite particles,one can imagine the following process for the foliation of graphite sheets from the exfoliated graphite.

Since exfoliated graphite particles are very brittle, when subjected to ultrasonic irradiation,most of the particles should be?rstly broken into shortened accor-dion-like particles.When the shortened particles were su?ciently small,the separation of the component graphite?akes from the sub-particles became dominant. Further increase of the irradiation time would result in the breaking of the foliated graphite nanosheets. Therefore,at the early stage of the irradiation,the particles were mostly the accordion-like foliated exfoli-ated graphite,of which the sizes were much larger than the foliated ones.After the graphite sheets were foliated into separate nanosheets,if the irradiation power is not su?ciently high,increase of irradiation time would have limited in?uence on the particle size,since little nano-sheets will be broken.Nevertheless,it should be noted that the above-mentioned three processes could take place simultaneously as the start of ultrasonic irradia-tion,but with relatively di?erent extent as stated above,and the separated nanosheets might also be composed of several thinner single crystal graphite sheets.

3.2.3.Nitrogen adsorption

Fig.4shows the low-temperature adsorption iso-therms of nitrogen for the exfoliated graphite and two foliated graphite samples with irradiation times of10 and16h,respectively.The isotherms observable in this ?gure can be considered as typical type II isotherms[23]. The isotherms of the two foliated graphite samples are nearly indistinguishable,indicating that the two samples possess similar structures and adsorption properties. The isotherms are concave in the low pressure region, and convex in the higher pressure region,while approximately linear in the intermediate region.The adsorption volume increases rapidly at low relative pressure due to the interaction of nitrogen gas with the ?rst energetic region followed by the less energetic re-gion.The monolayer of adsorbed molecules is com-pleted at a relative pressure about0.01,then the multilayers are formed.The abrupt rise in adsorbed volume at a relative pressure of0.90to unity indicates that capillary condensation has occurred during the adsorption process.

The experimental data corresponding to the range of relative pressure from0.05to0.25are plotted in Fig.5 according to the classical BET Equation[24].The values of the slopes and intercepts obtained from the least-squares?t of BET equation are used to derive the spe-ci?c surface area values of the samples using related equations in[24].The as-obtained results as well as the values of pore volume and mean pore diameter derived from the isotherms are listed in Table2.

The pore volume distributions derived from BJH method[25]for the three samples are also displayed in Fig.6.According to the literature,pores larger than300 nm are ignored.This does not deny the existence of larger pores,since pores of size in the l m range can be

756G.Chen et al./Carbon42(2004)753–759

directly observed via SEM.It is clear that the three graphite samples have similar pore structures.The samples possess a reasonable amount of macropores (greater than50nm),with small fraction of mesopores (2–50nm)and few micropores(less than2nm).

From Table2as well as Fig.6,it is seen that upon ultrasonic irradiation,both the pore volume and pore diameter for the foliated graphite decrease as compared with the original exfoliated graphite.Since the foliated graphite particles are almost sheet-like,rather than worm-like for the exfoliated graphite,most of the adsorption volume should be attributed to the inter-

particle voids.Due to the irradiation e?ects the number of smaller pores is also substantively decreased.The transformation of EG into foliated graphite leads to decrease of the speci?c surface area,but the magnitude is modest.Furdin[26]found that isolating the com-posing elements from exfoliated graphite by grinding not only destroys the‘‘honey-bomb’’type open struc-ture but also bring together the walls of the bellows, leading to products with speci?c areas two times smaller than that of the pristine EG.In the current case,ultra-sonic irradiation alone is employed to powdering the EG,there should be many di?erences when compared to the procedures adopted by Furdin.Therefore,the results obtained in our lab are plausible.

Since a hypothetical monolayer graphite would ex-hibit a speci?c surface area close to2700m2/g,by comparing the speci?c area of the foliated graphite (approximately17.55m2/g)to that of monolayer graphite,one piece of foliated graphite nanosheet can be estimated to consist of154graphite layers.If an inter-layer spacing of0.335nm is assumed for bulk graphite, the average thickness of the foliated graphite nanosheets thus can be readily estimated to be about52nm.This value is consistent with that obtained from SEM observations.Then the aspect ratio can be calculated to be as high as250.

3.2.

4.X-ray di?raction

The foliated graphite nanosheets prepared with dif-ferent irradiation times exhibited almost identical XRD patterns.In order to make a comparison with the crys-talline structures of the natural graphite and exfoliated graphite,the crystalline parameter L c of the FG,EG and natural graphite samples was calculated using the Scherrer equation.The results are given in Table3.

Table2

Textural characteristics of the exfoliated graphite and two foliated graphite samples

Type BET speci?c surface area(m2/g)BJH pore volume(cm3/g)BJH mean pore diameter(nm)

EG0024.110.15926.64

FG1017.550.08819.19

FG1617.560.09119.83

G.Chen et al./Carbon42(2004)753–759757

From Table 3,it is seen that transformation of nat-ural ?ake graphite into exfoliated graphite the mean particle thickness decreases,while in the same time a marked decrease in the value of L c (002)is observed.The morphological changes ongoing from EG to foli-ated graphite upon ultrasonic irradiation resulted in little di?erence in the thickness direction.This indicates that although foliated graphite nanosheets can be e?ectively obtained by the formation of exfoliated graphite followed by ultrasonic powdering,?aking of graphite does not take place to c -axis in the irradiation process.The mean number of crystallites in a thickness direction of exfoliated graphite and foliated graphite is then estimated to be about 3–4,which means that all the graphite layers are ideally isolated.This is clearly veri-?ed by the electron microscopy,and is not contrasted to polycrystalline nature of the foliated particles.

3.2.5.IR measurement

Fig.7shows the FTIR spectrum of EG.The band at 3433cm à1can be attributed to O–H stretching vibra-tions of phenolic or alcoholic functional groups present on the EG sheets [27].The presence of carboxyl func-tional groups can also be detected at around 1650cm à1[28].These evidences,together with the results of XPS in the literature [29],indicated that during the treatment of natural graphite with acids some carbon double bonds were oxidized,leading to the presence of oxygen-con-taining functional groups on exfoliated graphite,which will facilitate physical and chemical interactions between EG and polymer (or monomer).

4.Conclusions

In this study,natural ?ake graphite was expanded into exfoliated graphite via an acids intercalation pro-cedure.The resulting exfoliated graphite is a worm-like particle composed of graphite sheets with thickness in the nanometer scale.Subjecting to ultrasonic irradia-tion,the exfoliated graphite is e?ectively further foliated into isolated graphite nanosheets.SEM,TEM,SAD,laser counting,and BET measurements revealed that the as-fabricated graphite nanosheets were about 52nm in thickness.The diameters of the foliated graphite parti-cles decreased as the increase in irradiation time.For FG prepared with 10h ultrasonic irradiation,the par-ticles were about 13l m in diameter.These dimensions correspond to a mean aspect ratio of about 250,which will be demonstrated to be very useful in lowering the percolation threshold when the foliated graphite is loa-ded into nylon 6matrix [30,31].Furthermore,the opti-mum irradiation time used to prepare ideal foliated graphite could be ?xed at 10h.FTIR examination shows that there are oxygen-containing groups present on the surface of the exfoliated graphite.This result substantiates the statement reported in the literatures that acids treatment could result in oxidization of car-bon bonds in the graphite.The presence of the func-tional is supposed to be bene?cial to the interaction between the polymer and the foliated graphite.Acknowledgement

This work was sponsored by the National Natural Science Foundation of China,No.20174012.

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