Spontaneous Cracking of Graphite Oxide Sheet on Oxygen Def i cient ZnO Film

Jin-yng Liu,Hong-in Ci,Hun-yi Ding,Kun Zhng,Nn Pn,Xio-ping Wng,?

a.Department of Physics,University of Science and Technology of China,Hefei 230026,China

b.Hefei National Laboratory for Physical Sciences at the Microscale,University of Science and Technology of China,Hefei 230026,China

Spontaneous Cracking of Graphite Oxide Sheet on Oxygen Def i cient ZnO Film

Jin-yang Liua,Hong-bin Caib,Huan-yi Dinga,Kun Zhangb,Nan Panb,Xiao-ping Wanga,b?

a.Department of Physics,University of Science and Technology of China,Hefei 230026,China

b.Hefei National Laboratory for Physical Sciences at the Microscale,University of Science and Technology of China,Hefei 230026,China

Graphite oxide(GO)is an important material of wide applications.Owing to its good mechanical property,the GO sheet is always expected to be stable and remains f l at on various substrates.Here we demonstrate for the f i rst time an unexpected behavior of the GO sheet on oxygen def i cient ZnO film,namely the spontaneous cracking of the entire GO sheet into many small pieces.This unusual behavior has been carefully investigated by a series of control experiments and SEM,XPS and PL measurements.It is anticipated that the oxygen vacancies in the oxygen def i cient ZnO film can annihilate epoxy groups of the GO sheet,resulting in the unzipping of the aligned epoxy groups on GO sheet.A prototype of the white light detector made from the cracked GO sheet is fabricated and the device demonstrates high stability and good reproducibility.

Graphite oxide,Epoxy group,ZnO film,Oxygen vacancy,Photoresponse

I.INTRODUCTION

The graphite oxide(GO),exfoliated chemically from natural f l ake graphite,has become a high efficient,low cost,and massive productive material for producing graphene,and has attracted a lot of attentions in chemistry and material science[1-4].GO is usually synthesized through the oxidation of graphite using oxidants including concentrated sulfuric acid,nitric acid and potassium permanganate with Hummers method, and there are often a variety of oxygen groups(such as epoxide,carbonyl(=CO),hydroxyl(-OH)and phenol groups)on its surface[5].Due to its unique structure, GO can be either readily dispersed into various solvents or further functionalized[6].To date,GO has been found potential applications for drug delivery,transparent conductive films[7,8],polymer composites,lithium ion batteries[9],supercapacitors[10],and photodetectors[11,12].Moreover,GO can further be considered as a building block for the electrical nanodevices.For example,Wei et al.obtained graphene nanoelectronics by thermochemical nanolithography with a heated atomic force microscope tip to reduce the GO sheets [13].Recently,Zhang et al.achieved the direct writing of electronic devices on graphene oxide by catalytic scanning probe lithography[14].

It is intuitively believed that GO is very stable when it is dispersed on the substrate for the applications such as transparent conductive film and electronic device.To our best knowledge,there has hardly been any report available on the stability of GO on different substrates. In this work,we have systematically examined this important issue.It is found experimentally that GO does remain stable on a variety of substrates,including Si, SiO2,Au film,ITO,and even LaCaMnO(LCMO)film. However,to our surprise,we have observed that the GO sheets are unstable,even spontaneously cracked into small pieces on the oxygen def i cient ZnO films.The unusual phenomenon has been carefully investigated by SEM,PL,and XPS,and a possible interpretation is proposed.A prototype photodetector based on the cracked GO(c-GO)sheet has also been constructed,and its photo-response is demonstrated.

II.EXPERIMENTS

The ZnO fi lms(100 nm thick)were prepared by radical assistance sputtering system(RAS-1100C,Shincron Co.,Ltd.,Japan)[15,16].The annealed ZnO fi lms were obtained through annealing the as-prepared ZnO fi lms at 450?C for 90 min in the tube furnace.The GO sheets were prepared by chemical exfoliation of the natural fl ake graphite by a modi fied Hummers’method [17,18].The GO was uniformly dispersed in the deionized water(Milli-Q)with the help of the untrasonic (AS10200A untrasonic cleaner auto science Ltd)and centrifuged(Mikro 200R.Hettich.Germany).Then, 4μL of 0.1 mg/mL GO solution was drop-coated on di ff erent substrates and was dried naturally.

X-ray photoelectron spectroscopy(XPS,ESCALAB250.Thermo-VG Scientif i c)was exploited to examine the chemical states of the ZnO film before and after annealing.Photoluminescense of ZnO films was characterized with excitation source of He-Cd pump laser (325 nm,LABRAM-HR)at room temperature.Scanning electron microscope(SEM)(Raith,e-LiNE lithography system,Germany)was used to observe the morphologies of GO sheets.The Keithley-4200 semiconductor characterization system was used to characterize the photoresponse of the device under the white light illumination with the power of 100 mW/cm2(Oriel instruments,Newport Corporation.USA).

FIG.1 SEM images of GO coated on the as-prepared(a,b) and annealed(c,d)ZnO film after drying in atmosphere for 3 h(a,c)and 10 h(b,d).The scale bar is 4μm.

III.RESULTS AND DISCUSSION

GO sheets are laid on the substrates using dropcoated method.Figure 1 shows the SEM images of the GO sheets on both as-prepared and annealed ZnO fi lms after drying naturally for 3 and 10 h,respectively. As seen from Fig.1(a)and(b),even after drying for 10 h,single layer GO sheet keeps stable and fl at on the as-prepared ZnO fi lm except some wrinkles occurred, which was induced by the deposition process.The results can be well understood because GO possesses a exotic good mechanical property[19].However,an behavior,the spontaneous cracking of the GO sheet on namely the annealed ZnO fi lm,can be observed.As shown in Fig.1(c)and(d),a lot of faint gaps in the GO sheets occur after drying 3 h(Fig.1(c))and the gaps increase with the increasing of drying time.As shown in Fig.1(d),the width of the gaps is about tens of nanometers and their distribution is quite uniform. The average size of the cracked GO(c-GO)pieces is estimated to be about 1-2μm,while the shape of c-GO is diverse.

FIG.2 SEM images of GO sheets on(a)Si,(b)300 nm SiO2,(c)5 nm Au,(d)100 nm ITO,(e)LCMO film,and (f)the transferred c-GO on SiO2substrate.The scale bar is 4μm.

It should be pointed out that the spontaneous cracking behavior is unique and reproducible for the annealed ZnO fi lm.Figure 2(a)-(e)show the SEM images of GO laid on di ff erent substrates including Si,SiO2,Au,ITO, and LCMO.As seen,the GO demonstrates the similar morphologies to that shown in Fig.1(a),indicating the GO sheet is stable on these substrates.Figure 2(f) shows the result of the c-GO transferred from the annealed ZnO fi lm onto a SiO2substrate.Clearly,one can fi nd that some pieces of c-GO have been lost during the transfer process.This further implies that the spontaneous cracking behavior has really taken place in the GO sheet when it is put on the surface of the annealed ZnO fi lm.

In order to understand the reason for the di ff erent stability of GO sheets on the as-prepared and annealed ZnO fi lm,we fi rst performed XPS measurements of ZnO fi lms and the results are shown in Fig.3(a)and(b).It can be found that O1s signals have wide and asymmetric shapes for both ZnO fi lms.According to our previous reports[20,21],the O1s can be deconvoluted into three Gaussian spectral components,centered at 530.3, 531.6,and 532.5 eV,respectively.Generally,the peak at 530.3 eV is attributed to the Zn-O bonds,and the peak at 532.5 eV is usually attributed to chemisorbed or dissociated oxygen or OH species on the surface of the ZnO thin fi lm,such as-CO3,adsorbed H2O or adsorbed O2.The component peaked at 531.6 eV is associated with O2-ions,which is correspondent to the oxygende fi cient regions within the ZnO matrix.The intensi-ties of the peak at 531.6 and 532.5 eV of the annealed ZnO fi lm are much stronger than those of as-prepared ZnO fi lm,implying that the annealed ZnO fi lm contains more oxygen vacancy defects and oxygen-de fi cient regions.The above conclusion can further be supported by the photoluminescense(PL)measurements of ZnO fi lms.As seen from Fig.3(c),the annealed ZnO fi lm has a strong green and orange light emission compared to as-prepared ZnO fi lm,while both fi lms have similar near band UV emission.Because the green light emission is generally attributed to the oxygen-de fi cient states,especially oxygen vacancies in ZnO,while the orange luminescence bands result from ioned oxygen vacancies in ZnO[22-25],we can conclude that the annealed sample is an oxygen de fi cient ZnO fi lm as compared to the as-prepared sample.

FIG.3 XPS of O1s(a)the as-prepared and(b)the annealed ZnO films.(c)The corresponding PL results of two ZnO films.

FIG.4(a)-(c)The schematic illustration of cracking process of GO sheets through unzipping along the aligned linear epoxy groups and formed three cracking small GO pieces.(d)Statistical results of the angular vertex of adjacent pieces of Fig.1(d).

On the other hand,it is well known that there are lots of oxygen-based(such as epoxide,carbonyl and hydroxyl)groups on the GO surface[18]and these groups incline to aggregate or be aligned to decrease the surface energy of GO.For example,from the theoretical calculation,Li et al.found that once two epoxides were formed on the opposite side of a carbon hexagon,there was a strong tendency for other epoxy groups aligning in a line and then the aligned epoxy groups could induce a rupture of the underlying C-C bonds to form the open epoxide rings[26].The prediction of aligned linear epoxy groups were directly conf i rmed by STM observation recently[27].

FIG.5(a)The schematic diagram of the device.(b)The photoresponse of the devices at 1.0 V bias voltage under the white light with power density of 100 mW/cm2.

Based on the facts that the oxygen vacancies existed on the surface of annealed ZnO films and the epoxy groups aligned on the GO sheets,a possible interpretation for the spontaneous cracking of the GO sheet on the annealed ZnO film can thus be proposed.As the GO lying on the oxygen-def i cient ZnO film,the epoxy groups on the GO sheet can be annihilated by the surface oxygen vacancy in the annealed ZnO film.Because the aligned epoxy groups are like a zip on the GO,removing one epoxy group from the zip can start up the unzipping process to break GO(Fig.4(a)).As the cracking developed,the whole GO will end up into many small pieces(Fig.4(b)).In addition,due to the strong capillary effect between the ZnO films and the f l exible GO piece,the small GO piece adheres to the grain surface of ZnO film,resulting in widening the gaps between the pieces(Fig.4(c)).Considering the hexagon structure of GO and mechanical balance,we suggest that the adjacent cracking gaps should present an average angular vertex of 120?.Figure 4(d)plots the statistical results of angular vertex of the c-GO pieces shown in Fig.1(d). As seen,most of the angular vertex is about 120?and the result is well consistent with our above assumption.

The cracks in c-GO pieces may offer an ideal candidate for the nano-electrodes that are useful for a variety of nanodevices such as photodetector,gas sensor,and chemical sensing[28-31].Herein,we have fabricated a prototype of white light photodetector with c-GO pieces and the schematic drawing is shown in Fig.5(a).The c-GO sheets on the annealed ZnO film are f i rst transferred to the SiO2/Si and further reduced at high temperature. The contact electrodes are fabricated by EBL with liftof ftechnology,and CuPc molecules as the photosensitizer are evaporated onto the c-GO pieces.The photoresponse of the device at a bias of 1 V under the white light illumination with power density of 100 mW/cm2is given in Fig.5(b).As shown,the photosensitivity of the device(Ion/Ioff)can be as large as～10 and the photoresponse is reversible and very stable.Note that there is no any photoresponse for the control devices with the same size based on simple CuPc or reduced c-GO,indicating the nanogap of the cracked GO sheet plays an important role in the photorespense.

IV.CONCLUSION

The spontaneous cracking of the GO sheet on oxygen def i cient ZnO film has been observed,which is in stark contrast with the conventional wisdom that the GO should always be stable due to its good mechanical property.Such a unusual phenomenon has been carefully examined by a series of controlling experiments and a possible mechanism is proposed.It can be suggested that the oxygen vacancies in oxygen de ficient ZnO fi lm can annihilate the epoxy groups on the surface of GO,leading to the unzipping of the aligned epoxy groups on the GO sheet.A white light detector based on the c-GO pieces has been fabricated,which has achieved high stability and good reproducibility.Our fi nding may o ff er new insight on the stability of GO sheet and novel nanogap electrodes for optoelectronic devices.

V.ACKNOWLEDGMENTS

This work was supported by the Ministry of Science and Technology of China(No.2011CB921403),the Chinese Academy of Science(No.XDB01020000),and the National Natural Science Foundation of China.

[1]D.R.Dreyer,S.Park,C.W.Bielawski,and R.S.Ruof f, Chem.Soc.Rev.39,228(2010).

[2]S.Park and R.S.Ruof f,Nat.Nanotechnol.4,217 (2009).

[3]C.N.R.Rao,A.K.Sood,K.S.Subrahmanyam,and A. Govindaraj,Angew Chem.Int.Edit.48,7752(2009).

[4]Y.H.Wu,T.Yu,and Z.X.Shen,J.Appl.Phys.108, 071301(2010).

[5]A.Lerf,H.Y.He,M.Forster,and J.Klinowski,J.Phys. Chem.B 102,4477(1998).

[6]J.I.Paredes,S.Villar-Rodil,A.Martinez-Alonso,and J.M.D.Tascon,Langmuir 24,10560(2008).

[7]L.J.Cote,F.Kim,and J.X.Huang,J.Am.Chem. Soc.131,1043(2009).

[8]Z.G.Geng,G.H.Zhang,Y.Lin,X.X.Yu,W.Z.Ren, Y.K.Wu,N.Pan,and X.P.Wang,Chin.J.Chem. Phys.25,494(2012).

[9]G.X.Wang,B.Wang,X.L.Wang,J.Park,S.X.Dou, H.Ahn,and K.Kim,J.Mater.Chem.19,8378(2009).

[10]S.Chen,J.W.Zhu,X.D.Wu,Q.F.Han,and X.Wang, ACS Nano 4,2822(2010).

[11]Y.W.Zhu,S.Murali,W.W.Cai,X.S.Li,J.W.Suk,J. R.Potts,and R.S.Ruof f,Adv.Mater.22,3906(2010).

[12]J.Y.Liu,X.X.Yu,G.H.Zhang,Y.K.Wu,K.Zhang, N.Pan,and X.P.Wang,Chin.J.Chem.Phys.26,225 (2013).

[13]Z.Q.Wei,D.B.Wang,S.Kim,S.Y.Kim,Y.K.Hu, M.K.Yakes,A.R.Laracuente,Z.T.Dai,S.R.Marder, C.Berger,W.P.King,W.A.de Heer,P.E.Sheehan, and E.Riedo,Science 328,1373(2010).

[14]K.Zhang,Q.Fu,N.Pan,X.X.Yu,J.Y.Liu,Y.Luo,X. P.Wang,J.L.Yang,and J.G.Hou,Nature Commun. 3,(2012).

[15]Y.Z.Song and T.Sakurai,Vacuum 74,409(2004).

[16]Q.M.Song,B.J.Wu,B.Xie,F.Huang,M.Li,H.Q. Wang,Y.S.Jiang,and Y.Z.Song,J.Appl.Phys.105, 044509(2009).

[17]G.Williams,B.Seger,and P.V.Kamat,ACS Nano 2, 1487(2008).

[18]X.X.Yu,H.B.Cai,W.H.Zhang,X.J.Li,N.Pan, Y.Luo,X.P.Wang,and J.G.Hou,ACS Nano 5,952 (2011).

[19]J.W.Suk,R.D.Piner,J.An,and R.S.Ruof f,ACS Nano 4,6557(2010).

[20]H.Y.Ding,Z.Zhao,G.H.Zhang,Y.K.Wu,Z.W. Gao,J.W.Li,K.Zhang,N.Pan,and X.P.Wang,J. Phys.Chem.C 116,17294(2012).

[21]Q.H.Shen,Z.W.Gao,H.Y.Ding,G.H.Zhang, N.Pan,and X.P.Wang,Acta Phys.Sin-Ch.Ed.61, (2012).

[22]B.X.Lin,Z.X.Fu,Y.B.Jia,and G.H.Liao,J. Electrochem.Soc.148,G110(2001).

[23]S.A.Studenikin,N.Golego,and M.Cocivera,J.Appl. Phys.84,2287(1998).

[24]A.B.Djurisic,Y.H.Leung,K.H.Tam,Y.F.Hsu, L.Ding,W.K.Ge,Y.C.Zhong,K.S.Wong,W.K. Chan,H.L.Tam,K.W.Cheah,W.M.Kwok,and D. L.Phillips,Nanotechnology 18,095702(2007).

[25]A.B.Djurisic and Y.H.Leung,Small 2,944(2006).

[26]J.L.Li,K.N.Kudin,M.J.McAllister,R.K. Prud’homme,I.A.Aksay,and R.Car,Phys.Rev.Lett. 96,176101(2006).

[27]D.Pandey,R.Reifenberger,and R.Piner,Surf.Sci. 602,1607(2008).

[28]T.Li,W.P.Hu,and D.B.Zhu,Adv.Mater.22,286 (2010).

[29]W.Chen,H.Ahmed,and K.Nakazoto,Appl.Phys. Lett.66,3383(1995).

[30]L.D.Qin,S.Park,L.Huang,and C.A.Mirkin,Science 309,113(2005).

[31]Y.D.He,H.L.Dong,T.Li,C.L.Wang,W.Shao,Y. J.Zhang,L.Jiang,and W.P.Hu,Appl.Phys.Lett. 97,133301(2010).

ceived on May 18,2013;Accepted on May 23,2013)

?Author to whom correspondence should be addressed.E-mail：xpwang@ustc.edu.cn