Pl

Öa

b

a

ARA

KGBHL

1

aIiompbdas

giBlc

h0

Optik 127 (2016) 4630–4634

Contents lists available at ScienceDirect

Optik

jo ur nal homepage: www.elsev ier .de / i j leo

roduction of graphene–boron nitride hybrid nanosheets byiquid-phase exfoliation

mer Gülera,∗, Seval Hale Gülerb

Department of Metallurgical and Materials Science, Engineering Faculty, Firat University, Elazig 23119, TurkeyDepartment of Metallurgical and Materials Science, Engineering Faculty, Mersin University, Mersin 33100, Turkey

r t i c l e i n f o

rticle history:eceived 9 January 2016ccepted 10 February 2016

eywords:rapheneN nanosheetybrid nanosheetiquid-phase exfoliation

a b s t r a c t

Hexagonal graphite (HG) and hexagonal boron nitride (h-BN) show various characteristics that are dif-ferent from each other in addition to their similar features. It is thought that hybrid nanosheets whichare formed of nano-layered graphene and nano-layered boron nitride (BNNS) will show very extraor-dinary characteristics. In this study, the production of a new Hybrid structure containing graphene andBN nanosheet structures was performed by using the liquid-phase exfoliation method. For this purpose,as the starting material, HG and h-BN were mixed at the same ratio and added into an acid mixtureand thus an intermediate structure called as Hybrid intercalated compound (HIC) was obtained. Then,the HIC structure obtained was subject to a thermal treatment and a structure called as expanded hybridstructure (EHS) was obtained. In order to produce a Hybrid nanosheet, the EHS structure was sonicated in

dimethylformamide (DMF), which was used as a solvent. At the end of the treatment, the material float-ing in the solvent was filtered. The samples were subject to transmission electron microscope (TEM),scanning electron microscopy (SEM), atomic force microscope (AFM), Fourier transform infrared spec-troscopy, and UV–vis spectroscopy examinations. As a result of the analyses, it was determined that thehybrid nanosheets containing graphene and BN nanosheets were successfully produced.

© 2016 Elsevier GmbH. All rights reserved.

. Introduction

Graphene, two-dimensional allotrope of carbon, is formed byligning the carbon atoms in a honeycomb lattice on a plane [1–3].n other words, each layer of graphite having a layered structures called as graphene. Each graphene has a high Young’s modulusf ∼1000 GPa, a specific surface area of ∼2630 m2 g−1, a ther-al conductivity of ∼5000 W m−1 K−1 and an impressive transport

henomena like the quantum Hall effect [4–6]. These propertiesring a great potential to the graphene as an appealing candi-ate for next-generation electronic materials and promises in widepplications like field-effect transistors, supercapacitors, batteries,ensors, and nanocomposites, etc. [6–9].

Hexagonal boron nitride (h-BN) has a similar structure withraphite. Hexagonal boron nitride (h-BN) is among the most signif-cant III–V group materials having in-plane trigonal sp2 bonding,

and N atoms entirely substituting for C atoms in a graphitic-ike sheet, and almost no change in atomic spacing [10–12]. Whenompared to graphite carbon, h-BN with superior mechanical

∗ Corresponding author. Tel.: +90 424 237 00 00 6381.E-mail address: oguler@firat.edu.tr (Ö. Güler).

ttp://dx.doi.org/10.1016/j.ijleo.2016.02.033030-4026/© 2016 Elsevier GmbH. All rights reserved.

properties and thermal conductivity is more stable from thermaland chemical aspects (for example, h-BN is much more oxidationresistant than graphite. h-BN is stable up to 900 ◦C in air) [13].Compared to graphite, h-BN has an electrical insulation (band gap5–6 eV).

BN can be exfoliated to form boron nitride nanosheets (BNNSs)which are unique 2D crystal structures [14,15]. In recent years,BNNSs have drawn attention because of their extraordinary prop-erties [14–16]. As BNNSs are electrically insulated, thermallyconductive, and mechanically robust as graphene, it is thought thatthey have potential to find a place in applications using optoelec-tronic devices and heat-releasing composite materials [16,17].

In addition to the similar properties of graphene and BNNS, theyalso show different properties. It is obvious that a hybrid structurecontaining both graphene and BNN layers will show extraordinaryproperties. Thus in this study, a material containing hybrid nanolayers was produced by using the liquid phase exfoliation method.

2. Experimental procedure

In order to remove the humidity, Hexagonal graphite (HG)(Merck, 99.5%) and Hexagonal Boron nitride (h-BN) (Merck, 99.5%)powders were dried under vacuum oven at 75 ◦C for 2 h. The dried

Ö. Güler, S.H. Güler / Optik 1

Fh

Hcigmwve

feicwTflawh

sot2pMsfb

3

tcbwsantTmmstsarEh

ig. 1. The experimental procedure for preparing the graphene–boron nitrideybrid nano-sheet production.

G and h-BN were mixed with saturated acid containing con-entrated H2SO4 and HNO3 for 12 h in order to obtain a Hybridntercalated compound (HIC). The uniform intercalation of eachraphite and boron nitride particle was obtained by stirring hybridixture during the process. It was carefully rinsed by using distilledater until a neutral pH was obtained. The product was dried under

acuum oven for 12 h. Then, HIC was heated at 800 ◦C to obtain anxpanded hybrid structure (EHS) [18–20].

The graphene-sheets still had bonding with weak van der Waalsorces at some points in EHS. Thus, EHS was subject to the finalxfoliation in order to acquire graphene-sheet by ultra-sonicationn N,N-dimethylformamide (DMF). The suspension of 0.05 mg ml−1

oncentration of EHS with DMF was prepared. The solution of EHSas kept at a multi-frequency ultra-sonication homogenizer for 1 h.

hen, DMF was removed by centrifuging the mixture at 5000 rpmor 8 h. The low density material, which was suspended at the topayer of centrifuged solution, was collected to make further char-cterization. Being rinsed by using ethanol, the collected powdersere dried under vacuum oven at 75 ◦C for 4 h to remove the alco-ol.

The synthesized powders were characterized by using UV–vispectroscopy. A UV-spectrophotometer (UV 3600 Shimadzu, Japan)perating between 250 and 1200 nm was used in order to recordhe spectrum. Transmission electron microscope (TEM) (JEOL Jem100F) was utilized to examine microstructure of graphene sam-les. The analysis of samples was made by Scanning Electronicroscopy (SEM) (Jeol Jsm-7001F). The structural properties of the

amples were examined by using a Park System XE-100E atomicorce microscope (AFM). FTIR spectrum of the samples rangedetween 400 and 4000 cm−1 on Nicolet 5700 FTIR spectrometer.

. Results and discussion

Fig. 1 illustrates the experimental procedure for production ofhe hybrid sheet. Hexagonal graphite and hexagonal boron nitrideontained flat graphene-layer and BNNS, respectively. Bondingetween layers was performed by using weak van der Waals bonds,hich enable to separate easily layers of graphite and h-BN or to

lide past each other. This study was conducted to exfoliate graphitend h-BN layers for production of graphene and BNNSs. A solvent isecessary to separate the layers. The distance of layers is requiredo be sufficient in order to enter the solvent atoms between layers.herefore, the mixture of HG and h-BN powders was expanded byeans of acid and thermal treatments. Subsequent to these treat-ents, the distance between the hexagonal layers increased and

olvent atoms easily entered in these spaces and provided to breakhe weak bonds between the layers. As can be understood from thechematic view, HIC was formed after keeping HG and h-BN, used

s the starting material, in the acid mixture for a long time. As aesult of keeping the HIC at high temperature for a few minutes,HS was produced [18,21]. The distance between the layers of bothexagonal graphite and h-BN was about 0.334 nm. The distance

27 (2016) 4630–4634 4631

between the EHS layers obtained as a result of the treatments hasincreased compared to the initial. EHS obtained was sonicated for aspecific period by adding it in DMF. This treatment helped the DMFmolecules to enter between the layers and to separate the layersfrom each other. As a result of this treatment, there were nano-sized hybrid layers in the large part of the structure; whereas, verysmall amount of single-layer hybrid structures formed. Previousstudies reported that single-layer structures increased as a resultof the repetition of this treatment [3].

Fig. 2 illustrates SEM images of HG, h-BN, HIC, and EHS whichwere used as the starting material. Fig. 2a illustrates SEM imagesof HG and Fig. 2b illustrates SEM images of h-BN. As is seen fromthe figures that both starting materials had a layered structure andthe layer widths were quite high. Fig. 2c illustrates SEM image ofthe HIC structure obtained at the end of 12-h mixing process in theacid mixture. Fig. 2d illustrates the SEM image of EHS obtained afterHIC was subject to a thermal treatment. It was demonstrated thatthe EHS had a quite porous and loose structure when comparedto the pristine HG and h-BN structures. The distances between thelayers of the structures in EHS increased when compared to thepristine HG and h-BN structures. After the production of EHS, it canbe asserted that the structure obtained consisted of many microand nano-sized thick graphene and BN layers. The said situationwas also reported in the previous studies [3].

There were still van der Waals bonds between the microand nano-sized layers in the structure. However, the distancebetween the layers significantly increased when compared to pris-tine graphite. This situation could allow the organic solvent atomsto enter between the layers. Placement of atoms of an organicsolvent such as DMF between the layers through a sonicationtreatment caused the breakage of the weak van der Waals bondsbetween the layers and separated the layers from each other. Thesonication treatment applied during the graphene and BNNS pro-duction helped the separation of the layers from each other bycreating micro-impacts.

Fig. 3 illustrates TEM images of the graphene–BNNS hybridsheets. As is seen from the figure, exfoliation treatment was suc-cessful and the layers were separated from each other. The widthsof the layers were over 100 nm. As is seen from the TEM images;even though the layers were overlapped, they were quite trans-parent and the surface of the grid at the bottom could be easilyseen. This situation showed that the layers were quite thin. Thelayers were overlapped and it was observed that there were con-taminations on the layers in some regions. It was estimated thatthese contaminations mixed with the structure during production.Fig. 3d illustrates the result of the EDX analysis taken over the lay-ers. As is seen from the figure, the presence of both C atoms andB N atoms on the layers was in question.

In order to examine the surfaces of hybrid nanosheet samplesproduced by using liquid-phase exfoliation method, the surfaceimages were taken from the samples by using an atomic forcemicroscope (AFM). The produced hybrid nanosheet powders werehomogeneously distributed in ethanol and the obtained liquid mix-ture was distilled on a glass lamina and then dried. Then, thesample was examined in AFM. Fig. 4 illustrates three-dimensional3D images of the samples. Z-axis height was approximately 2 �m.The average particle sizes of graphene–BNNS hybrid suspensionwere a few micrometers.

Fig. 5 illustrates FTIR analysis results of pristine hexagonalgraphite, pristine hexagonal boron nitride, HIC, EHS, and the hybridnanosheets produced. As is seen in Fig. 5a, polycrystalline h-BN gavetwo strong vibrations at approximately 790 cm−1 and 1370 cm−1.

Pristine hexagonal graphite gave strong vibrations at 1118 cm−1,1380 cm−1, and 1600 cm−1. The peak at 1600 cm−1 was caused byC C stretching vibrations. The peak at 1380 cm−1 was caused byC OH and the peak at 1100 cm−1 was due to C O. It could be

4632 Ö. Güler, S.H. Güler / Optik 127 (2016) 4630–4634

Fig. 2. SEM micrograph of (a) Pristine hexagonal graphite, (b) pristine hexagonalboron nitride, (c) Hybrid intercalated compound (HIC) and (d) expanded hybridstructure (EHS).

Fig. 3. (a)–(d) TEM micrographs of high magnification, (e) EDX analysis of synthe-sized graphene–boron nitride nano hybrid sheets.

asserted that the peaks at 1380 cm−1 and 1100 cm−1 bonded tothe structure during the mixing of the powders and acid duringproduction. As is seen from the figures, the hybrid nanosheet gavevibrations at 790 cm−1, 1118 cm−1, 1380 cm−1, and 1600 cm−1.Strong vibration peaks of the hybrid nanosheets produced con-tained all of both polycrystalline h-BN and hexagonal graphitepeaks. The peaks of the hybrid nanosheets produced were rela-

tively wider and their intensity was lower when compared to thepeaks of both polycrystalline h-BN and hexagonal graphite. Fig. 5billustrates FTIR analyses of the intermediate products (HIC and EHS)

Fig. 4. AFM micrographs of synthesized graphene–boron nitride hybrid nano sheets.

Ö. Güler, S.H. Güler / Optik 1

Fs

tttotid

Hoha1acgtphp

qi

ig. 5. FTIR analysis of (a) h-BN, HG, Hybrid nano sheet, (b) HIC, EHS, Hybrid nanoheet.

hat developed during the production of hybrid nanosheet and alsohe hybrid nanosheets. It could be asserted that FTIR peaks of syn-hesized hybrid nanosheets were wider than the width of the peaksf HIC and EHS that were the intermediate products. Additionally,he intensity of FTIR peaks of hybrid nanosheet was lower than thentermediate products. This situation showed that there was a massisordered content in the structure [22].

Fig. 6 illustrates UV–vis spectrometer analysis results of h-BN,G, Hybrid nanosheet. From the absorbance results, it was obvi-us that h-BN showed an insulating character. Absorption peak of-BN was band gap energy (Eg) of 4.67 eV at 271 nm. HG showed

conductive character. HG showed band gap energy of 1.57 eV at012 nm absorption peak. Hybrid nanosheet produced exhibited

bizonal absorption character. This absorption peak showed theharacters of both HG and h-BN. Hybrid nanosheet showed bandap energy of 1.57 eV at 1012 nm absorption peak. At the sameime, it showed band gap energy of 4.18 eV at 295 nm absorptioneak. Absorption was not observed between 295 and 1012 nm inybrid nanosheet. It was obviously seen that the hybrid nanosheet

roduced can absorb more light than HG and h-BN.

Both nano-sized graphene layers and nano-sized BN layers areuite light and in suspended form in the liquid that they are placed

nside. Relatively thicker layers precipitate to the bottom of the

Fig. 6. UV–vis analysis of h-BN, HG, hybrid nano sheet.

[

[

[

[

[

[

[

27 (2016) 4630–4634 4633

liquid in time. Thus, while separating the products from DMF atthe end of the treatment, the products accumulated on the surfacewere collected. All the examinations performed in this study weremade by using the products that were accumulated on the surface.The products accumulated on the surface were about maximum5% of the products inside DMF. Because the amount of the prod-ucts obtained as a result of a single sonication treatment was quitelow, the sonication treatment was repeated too much in order toobtain sufficient amount of products for the analyses. Even thoughhybrid nanosheet is easily and successfully produced by using thismethod, the only disadvantage of the method is that the amount ofthe products obtained in an experiment is very low. Previous sim-ilar studies have also mentioned that limited amount of layers areproduced by using this method. In these studies, it is specified thatthe product yield will increase by applying the production processagain for the products that have precipitated on the bottom duringthe separation of the product from the solvent [3,6]. In our study,the precipitated products were not used. Instead, EHSs were pro-duced again every time and these samples were sonicated in DMF.It was considered that the said situation reduced the productionrate and the product quantity.

4. Conclusion

The production of hybrid nanosheets containing graphene andBN nanosheets was successfully performed by using the liquid-phase exfoliation method. The production was realized in threestages and at the first stage, the first intermediate product calledas HIC was obtained; at the second stage, the second intermediateproduct called as EHS was obtained; and at the last stage, the hybridnanosheet was obtained. The layer widths of the hybrid nanosheetsproduced were over 100 nm. UV–vis results showed that the hybridnanosheets presented a bizonal absorption character. Absorptioncharacter of the material produced presented the absorption char-acter both of h-BN and HG. It was observed as a result of the peaksobtained from the FTIR analysis that hybrid nanosheet peaks com-plied with the peaks of both HG and h-BN. This process is a practicalmethod for the production of hybrid nano-sheet. Additionally, theonly disadvantage is very limited product amount produced at theend of the process.

References

[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al.,Science 306 (5696) (2004) 666–669.

[2] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (3) (2007) 183–191.[3] S.R. Dhakate, N. Chauhan, S. Sharma, J. Tawale, S. Singh, P.D. Sahare, R.B. Mathur,

Carbon 49 (2011) 1946–1954.[4] X. Zhao, Q. Zhang, D. Chen, P. Lu, Macromolecules 43 (2010) 2357–2363.[5] X. Cui, C. Zhang, R. Hao, Y. Hou, Nanoscale 3 (2011) 2118–2126.[6] L. Zhu, X. Zhao, Y. Li, X. Yu, C. Li, Q. Zhang, Mater. Chem. Phys. 137 (2013)

984–990.[7] Z.X. Xu, H.Y. Gao, G.X. Hu, Carbon 49 (2011) 4731–4738.[8] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, A. Pirkle, R.M.

Wallace, K.A. Cychosz, M. Thommes, D. Su, E.A. Stach, R.S. Ruoff, Science 332(2011) 1537–1541.

[9] E.Y. Choi, W.S. Choi, Y.B. Lee, Y.Y. Noh, Nanotechnology 22 (2011)365601–365606.

10] K. Watanabe, T. Taniguchi, T. Niiyama, K. Miya, M. Taniguchi, Nat. Photonics 3(10) (2009) 591–594.

11] G. Giovannetti, P.A. Khomyakov, G. Brocks, P.J. Kelly, J. Brink, Phys. Rev. B:Condens. Matter 76 (2007) 073103.

12] R. Gao, L. Yin, C. Wang, Y. Qi, N. Lun, L. Zhang, Y.X. Liu, L. Kang, X. Wang, J. Phys.Chem. C 113 (34) (2009) 15160–15165.

13] H. Zheng, C. Zhi, Z. Zhang, X. Wei, X. Wang, W. Guo, Y. Bando, D. Golberg, NanoLett. 10 (2010) 5049–5055.

14] D. Golberg, Y. Bando, Y. Huang, T. Terao, M. Mitome, C. Tang, C. Zhi, ACS Nano

4 (2010) 2979–2993.

15] J.H. Warner, M.H. Rummeli, A. Bachmatiuk, B. Buchner, ACS Nano 4 (2010)1299–1304.

16] C. Zhi, Y. Bando, C. Tang, H. Kuwahara, D. Golberg, Adv. Mater. 21 (2009)2889–2893.

4 ptik 1

[

[

[

[

634 Ö. Güler, S.H. Güler / O

17] N. Alem, R. Erni, C. Kisielowski, M.D. Rossell, W. Gannett, A. Zettl, Phys. Rev. B:

Condens. Matter 80 (2009) 155425.

18] S.R. Dhakate, N. Chauhan, S. Sharma, J. Tawale, S. Singh, P.D. Sahare, R.B. Mathur,Carbon 49 (2011) 1946–1954.

19] S.R. Dhakate, S. Sharma, M. Borah, R.B. Mathur, T.L. Dhami, Energy Fuel 22(2008) 3329–3334.

[

[

27 (2016) 4630–4634

20] S.R. Dhakate, S. Sharma, M. Borah, R.B. Mathur, T.L. Dhami, Int. J. Hydrogen

Energy 33 (2008) 7146–7152.

21] L.G. Carreira, L.C. Mendes, R.G. Ribeiro, P.J. Sebastiao, Mater. Sci. Appl. 4 (2013)281–286.

22] K. Liao, W. Ding, B. Zhao, Z. Li, F. Song, Y. Quin, et al., Carbon 49 (2011)2862–2868.