- Email: [email protected]
Thermal stability of graphene in inert atmosphere at high temperature
Accepted Manuscript Thermal stability of graphene in inert atmosphere at high temperature Fu Liu, Mingjie Wang, Yao Chen, Jianmin Gao PII:
S0022-4596(19)30178-1
DOI:
https://doi.org/10.1016/j.jssc.2019.04.008
Reference:
YJSSC 20709
To appear in:
Journal of Solid State Chemistry
Received Date: 24 January 2019 Revised Date:
21 March 2019
Accepted Date: 8 April 2019
Please cite this article as: F. Liu, M. Wang, Y. Chen, J. Gao, Thermal stability of graphene in inert atmosphere at high temperature, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/ j.jssc.2019.04.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Thermal Stability of Graphene in Inert Atmosphere at High Temperature Fu Liu, Mingjie Wang, Yao Chen and Jianmin Gao
M AN U
SC
RI PT
Graphical Abstract
AC C
EP
TE D
The effects of high temperature on the etching of graphene nanosheets in the atmosphere of protective gas.
ACCEPTED MANUSCRIPT Thermal Stability of Graphene in Inert Atmosphere at High Temperature Fu Liua, Mingjie Wanga, Yao Chena,* and Jianmin Gaoa,* a
MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry University,
Beijing, P. O. Box 100083, China *
E-mail address: [email protected] (Jm Gao) Abstract
RI PT
Corresponding author. [email protected] (Y Chen).
To investigate the thermal stability of graphene during sintering process of ceramic, the effect
SC
of temperature on stability of single-layer graphene (SLG) in argon atmosphere was studied. The Raman D-band/G-band intensity ratio(ID/IG) and the blue shifts of G mode showed that, the
M AN U
graphene defect density gradually increased with the increase in heat treatment temperature (HTT). There was limited damage in the graphene when treated temperature lower than 800 . The causes of the defects were discussed. The defect types were mainly on-set defects. Boundary defects were detected when the temperature reached above 800 . Atom Force Microscope (AFM) results showed that the defects gradually extended from the boundary and holes in the layer to
TE D
the edge and ultimately led to isolated graphene islands. The experiment results could be useful to study the changes and developing the holistic performance of graphene in high temperature sintering process.
EP
Keywords: graphene; inert atmosphere; high temperature; stability; defects
AC C
1. Introduction
Graphene has attracted much attention since its discovery in 2004. It is an extremely
promising two-dimensional (2D) nano material [1]. Apart from its perfect two dimensional crystal structure and excellent mechanical strength, electronic properties and specific surface area, graphene displays several other unusual attributes in micronano and composite fields [2]. However, such remarkable properties of graphene have not been achieved when processed into many applications, which is much lower than its theoretical performance [3]. Essentially, this two-dimensional material constitutes a new nano carbon comprising layers of carbon atoms arranged in six-membered rings [4]. It makes them extremely vulnerable to high temperature
ACCEPTED MANUSCRIPT environments. Graphene shows different properties with the influence of substrate, type of catalytic metals and defects. Previous studies indicated that by adsorbing different atoms and molecules, the electronic structure and the conductivity of the graphene can be manipulated [5]. More recently, graphene appeared as a possible reinforcement phase for ceramics due to its
RI PT
excellent mechanical [6], thermal [7, 8], and electric [9] properties. Both the solid phase sintering and the liquid phase sintering need high temperature to complete the densification of ceramic. However, graphene tends to form defects at high temperature. Various methods have been applied to reduce damage of graphene caused by high temperature during sintering process. For example,
SC
Liu et al.[10] fabricated graphene/alumina composites by spark plasma sintering (SPS). The damage to graphene will be slighter if the period of the sintering process is extremely short. It was
M AN U
found that graphene platelets not only can be dispersed uniformly in composite ceramics, but also be able to maintain the stability of the nature and composition in high temperature sintering. When it comes to the traditional method of sintering, the key problem is how to protect graphene from heat damage. Therefore, it is necessary to study stability of graphene on high temperature. Thermal stability of graphene is related to its layer. It was reported that the thermal stability
TE D
of graphene had a close relationship with the interlayer interaction of it in air atmosphere [11]. Bilayer graphene shows a better thermal stability than single layer. The results revealed that thermal stability of graphene is also dependent on the annealing time, atmosphere and preparation
EP
method. However, it is still not clear how temperature is related to the thermal stability of graphene and how lattice defects react to heat treatment in an inert atmosphere. Based on the
AC C
questions above, the thermal stability of graphene in ceramic sintering process had been studied. Generally, graphene powder has been usually used as composite fillers. It is widely known that graphene nano sheets exist abundantly in graphene powder when graphene powder is detected Raman D peak might be raised by a large number of boundary signals. This makes D peak does not have a unique relationship between structural defects [12]. As it is difficult to judge the defect density of graphene powder simply by D peak intensity and intensity ratio (ID/IG), monolayer graphene on silicon wafers were selected as subjects. In the current study, monolayer graphene with silicon wafer as substrate was synthesized by chemical vapor deposition (CVD). Thermal stability of monolayer graphene in argon atmosphere was studied by Raman spectroscopy and
ACCEPTED MANUSCRIPT AFM. The effect of high temperature on structure of graphene was revealed by D peaks in Raman spectra caused by defects. The changes in morphology of graphene were studied.
2. Experimental
RI PT
To minimize the effect of graphene layers and lamellar stacked format on graphene quality, monolayer graphene were selected as raw materials. Single-layer graphene with a side length of 1 cm square silicon wafer as substrates were purchased from 6-Carbon Technology Co., Ltd (Shenzhen, China). It was grown by chemical vapor deposition (CVD) on Cu foils at ambient
SC
pressure and transferred onto a Si substrate. Silicon wafer was used to provide a support surface for graphene, and to increase the signal intensity in the Raman detection. To avoid the influence of
furnace tube to the temperature of 700 surrounding air.
M AN U
impurities, furnace drying was conducted prior to this experiment. Usual practice was to heat the and hold for about 30 min to evaporate impurities into the
Graphene samples were heat-treated using a tube furnace (SK-G08123K-2-420, China). The graphene was placed into an alumina crucible. Graphene samples were heat-treated at in argon (99.999% purity) using a heating rate of 10 /min,
TE D
temperatures of 600, 800 and 1000
and then holding for one hour. After heat treatment, the samples were washed with acetone (A.R.,≥99.5%) in ultrasonic washer for 5 minutes, to discharge the organics from the substrate.
EP
Raman spectra and mappings of the annealed samples were obtained at room temperature (RT) using a Horiba LabRAM HR Evolution Raman spectrometer (France). A user defined image
AC C
area of 30µm×30 µm was employed for the mappings. The same excitation wavelength of 514 nm was used in each case. Atomic force microscope (AFM) in tapping mode with silicon tips (Bruker Multimode 8, Karlsruhe, Germany) was used to investigate the morphology of graphene.
3. Results and discussion Figure 1 shows the Raman spectra of four untreated samples in the region of 1200-3000 cm-1. The Raman spectrum of the graphene exhibited three characteristic peaks centered at ∼1360, ∼1580 and ∼2700 cm-1 that can be assigned to the D, G and 2D bands of graphene, respectively [13]. Both the G and D peaks arise from vibrations of sp2-hybridized carbon atoms. The amount of disorder in graphene can be measured conveniently with D peak intensity [14–16]. The
ACCEPTED MANUSCRIPT single-layer thickness of the graphene can be confirmed from the 2D mode [14–16]. The high crystalline order of the untreated samples can also be confirmed with the absence of D peak
M AN U
SC
RI PT
[14–16].
Fig.1 Raman spectra of four initial samples selected for further heat treatment (a) for comparison heat treatment (c) for 800
heat treatment (d) for 1000
heat
TE D
at room temperature (b) For 600
treatment.
Figure 2 shows the Raman mappings and single spectra of the circled areas of the graphene after heat treatment at different temperatures. The integrated G peaks at Raman images with
EP
different HTT were displayed. It identified the continuous and uniform distribution of graphene on the Si wafer. Bright areas in IG maps are associated to graphite-like structures. According to the
AC C
single spectra of the circled areas, the circled dark areas correspond to the SLG. Therefore, the high brightness areas represented multi-layered graphene. It can be seen from the figure that most of the regions can be attributed to SLG. As seen in Fig. 2, the ID/IG in Raman mappings indicated that the brightness of areas corresponding to the multi-layer graphene was relatively dark, which confirmed the better thermal stability of multi-layer graphene [11]. The ID/IG maps of the circle 2 regions indicated the serious defects of SLG as the brightest areas. The Raman spectra of the moderate brightness areas (circle 3) in ID/IG maps was also shown in Fig.2. As seen from the Raman spectra of the circled areas, the line shapes of both the D and G bands varied significantly with HTT. The full width at half maximum (FWHM) of G peaks can be
ACCEPTED MANUSCRIPT used to characterize the defects density in graphene [17, 18]. The G-band width kept constant or increased slightly below 800°C. it demonstrated that there was limited damage in the graphene. The ID/IG ratio, which was calculated from each peak area, was shown in Fig. 3. The ID/IG ratios of samples showed an increasing trend in the HTT range of RT – 1000°C. The ratio reached a
RI PT
maximum at 1000°C, indicating the graphene was damaged by thermal etching. The error bars of ID/IG did not varied much with the increasing HTT. This was related to the uniform distribution of defects.
The G mode blue shifted to the high-frequency region with the increase in HTT(Fig.3). The
SC
G mode position of single layer graphene on Si substrates moved 0.4~15cm-1 to the higher frequency. This can be attributed to two reasons. The first one is that high temperature changed the
M AN U
local density of atoms at the interface of silicon substrate. The changes in local density changed the capacity of the adsorption of molecules. Graphene hole doping caused by the adsorption of molecules made its G peak to blue shift with varying degrees [19,20]. Another reason for the blue shift was that the thermal expansion and contraction of substrate produced "the stress effect" on the surface of graphene. The blue shift of the G peaks under stress can be attributed to the
TE D
compression of C-C bonds. Del Corro et al. [21] reported that G peaks would shift to high wavenumber under compressive stress, due to the decrease of the distance between the carbon atoms. The error bars of G blue shift peaks increased at 1000 . It suggested that the stability of
EP
graphene varied greatly at different scanning sites. The multilayer graphene region (the highlighted area in Raman mapping of IG) kept good stability, while defective areas became
AC C
unstable. Therefore, it can be speculated that some of the new defects developed gradually from the original defects.
After the sample has been annealed, the D peak intensity increased with the increasing
amounts of defects, however the 2D intensity decreased. That didn’t mean the formation of multi layer graphene, since there was no introduction of additional carbon source. According to the study of Basko et al. 2D peak was strongly sensitive to the dynamics of the photo excited electron-hole pair [22]. Any increase in defects will affect the electron lifetime, which translates in a decrease in the 2D peak intensity.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 2 Raman mappings and single spectra of the circled areas of the graphene after heat treatment
AC C
EP
TE D
at different temperatures.
Fig.3 ID/IG ratio and G mode blue shifts of different HTT samples.
The intensity of disorder-induced Raman feature (D band at ~1350 cm-1) was correlated to
such as vacancies, implanted atoms, edges, grain boundaries, and defects associated to a change of carbon-hybridization [23]. The defect density (ID/IG) increased with the HTT. This may because graphene became fluctuated and the Si substrate surface became uneven (sublimation of silicon atoms) under high temperature for a long duration. D band appeared or increased when laser scanning a sloping or uneven surface. Secondly, the graphene sheet might be damaged with high temperature, causing cracks or holes which increased the edge length of graphene. The intensity of
ACCEPTED MANUSCRIPT D peak is also correlated to the edge chirality of the graphene. The edge chirality related to the angle between the sides of the graphene sheet which was determined by the figure of graphene sheet. Additionally, impurities on the surface or vacancies doped hetero-atom also resulted in defects during heat treatment process. For example, the defects could be induced by the deposition
RI PT
of Si atoms evaporated from the substrate onto the graphene surface. The type of defects in graphene can be classified by D′ peak at ~1640 cm-1. However, the moderate or weak intensity of D’ peak made it difficult to be identified. The intensity ratio of the D and D′ peak (ID/ID’) is able to probe the nature of the defects for moderate amount of disorder.
SC
The ID/ID’ were about 1.3 for on-site defects for almost all the samples, which might be produced by the deformation of the carbon-bond, sp3 hybridization or molecule adsorption [24]. There was and 1000
M AN U
some boundaries(∼3.5), and no vacancy-like defects(∼7) for 800
HTT samples [24].
The D’ peak was weakness or not detected for untreated sample and these with 600
HTT. It
might be due to the fewer defects in these samples. This has been further characterized by using
AC C
EP
TE D
AFM in tapping mode (Fig.4).
Fig.4 Topography AFM images of graphene at room temperature and after HTT. (a) room temperature (b) 600
(c) 800
(d) 1000 .
The morphology of the SLG on Si substrate was shown in Fig. 4a–d. Fig. 4a showed a typical
AFM image of SLG with intact nanosheets. The bright white sections on the image were impurities adsorbed on the surface. The typical size of the impurities was about 20-35 nm. As can be seen in figure 4b, there was a hole appeared in the middle of graphene. This hole should be developed from the gradual expansion of vacancy defects. It can be seen some cracks in the graphene sheet in Fig.4c. The formation of the cracks should be due to the different shrinkage
ACCEPTED MANUSCRIPT expansion rates between graphene and the substrate. The graphene was etched into many discrete graphene sheets after being annealed at 1000
(Fig.4d). The length of boundaries increased
significantly. The boundary defects can be found in the Raman detection. The morphology models were used to further illustrate the effects of high temperature on the
RI PT
etching of graphene nanosheets combining with the AFM result (Fig.5a–d). It showed that the etching of the graphene started from the edge with vacancies appearing up to 600
(Fig. 5b). It
was not a type of point defect in a crystal but a hole with complete edge. The defects within the layer gradually extended to the edge with the treatment temperature increased, and an isolated
SC
graphene island was formed [25] (Fig.5c). Finally, the intact nanosheets gradually became into isolated pieces of graphene with the temperature up to 1000
(Fig.5d). However, there was also
M AN U
the possibility that the combination stability of graphene with Si substrate became weak at high temperature. Therefore, the possible perturbations in its topography from tip-sample interactions
AC C
EP
TE D
must also be considered in characterizing the integrity of graphene.
Fig.5 Morphology-change models of graphene layer with temperature changes. (a) room temperature (b) 600
(c) 800
(d) 1000 .
4. Conclusion Thermal stability of graphene in argon atmosphere was studied with different HTT by Raman mapping and AFM. The study confirmed that multi-layer graphene had better stability. The defect density of SLG increased with the increased in HTT. The samples were damaged most seriously when subjected to heat treatment at 1000 , as observed from the Raman peak shapes and
ACCEPTED MANUSCRIPT maximum defect density. Graphene layer maintained completely when the HTT was below 600 , and then some holes appeared in graphene layer with the increase in HTT. The main defect type was on-site defect. Isolated graphene islands were etched out when the HTT was 800 , and the boundary defect increased gradually. Graphene layer fragmented into small pieces when the HTT
graphene would be damaged at high temperature above 800 and thus unable to give full play to its performance.
even in a protective atmosphere,
SC
Acknowledgements
RI PT
was 1000 . These findings showed that within the time required for traditional sintering,
The work was supported by National Natural Science Foundation of China (NSFC, No.
References
M AN U
51572028) and Beijing Natural Science Foundation (6192020).
1. Geim A K, Novoselov K S., The rise of graphene, Nat Mater. 6 (2007) 183-191. doi:10.1038/nmat1849.
TE D
2. Li, D., Kaner, R. B., Graphene-based materials, Science. 320 (2008) 1170-1171. doi:10.1126/science.1158180.
3. Xin, G., Yao, T., Sun, H., Scott, S. M., Shao, D., Wang, G., Lian, J., Highly thermally
EP
conductive and mechanically strong graphene fibers, Science. 349 (2015) 1083-1087. doi:10.1126/science.aaa6502.
AC C
4. Rao, C. E. E., Sood, A. E., Subrahmanyam, K. E., Govindaraj A., Graphene: The New Two Dimensional Nanomaterial, Angew Chem Int Edit. 48 (2009) 7752-7777. doi:10.1002/anie.200901678.
5. Ohta, T., Bostwick, A., Seyller, T., Horn, K., Rotenberg, E., Controlling the electronic structure of bilayer graphene, Science. 313 (2006) 951-954. doi:10.1126/science.1130681.
6. Frank, I. W., Tanenbaum, D. M., van der Zande, A. M., McEuen, P. L., Mechanical properties of suspended graphene sheets, J Vac Sci Technol B. 25 (2007) 2558-2561. doi:10.1116/1.2789446. 7. Balandin, A. A., Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials, Nat Mater. 10 (2011) 569. doi:10.1038/nmat3064.
ACCEPTED MANUSCRIPT 8. Prasher,
R.,
Graphene
Spreads
the
Heat,
Science.
328
(2010)
185-186.
doi:10.1126/science.1188998. 9. Stankovich, S., Dikin, D. A., Dommett, G. H., Kohlhaas, K. M., Zimney, E. J., Stach, E. A., Ruoff,
R.
S.,
Graphene-based
composite
materials,
Nature.
442
(2006)
282.
RI PT
doi:10.1038/nature04969. 10. Liu, J., Yan, H., Jiang, K., Mechanical properties of graphene platelet-reinforced alumina ceramic composites, Ceram Int. 39 (2013) 6215-6221. doi:10.1016/j.ceramint.2013.01.041.
11. Nan, H. Y., Ni, Z. H., Wang, J., Zafar, Z., Shi, Z. X., Wang, Y. Y., The thermal stability of
SC
graphene in air investigated by Raman spectroscopy, J Raman Spectrosc. 44 (2013) 1018-1021. doi:10.1002/jrs.4312.
M AN U
12. Kudin, K. N., Ozbas, B., Schniepp, H. C., Prud'Homme, R. K., Aksay, I. A., Car, R., Raman spectra of graphite oxide and functionalized graphene sheets, Nano Lett. 8 (2008) 36-41. doi:10.1021/nl071822y.
13. Dong, X., Fu, D., Fang, W., Shi, Y., Chen, P., Li, L. J., Doping single-layer graphene with aromatic molecules, Small. 5 (2009) 1422-1426. doi:10.1002/smll.200801711.
TE D
14. Ferrari, A. C., Raman spectroscopy of graphene and graphite: Disorder, electron–phonon coupling, doping and nonadiabatic effects, Solid State Commun. 143 (2007) 47-57. doi:10.1016/j.ssc.2007.03.052.
EP
15. Tuinstra, F., Koenig, J. L., Raman Spectrum of Graphite, J Chem Phys. 53 (1970) 1126-1130. doi:10.1063/1.1674108.
AC C
16. Ferrari, A. C., Robertson, J., Origin of the 1150 - cm -1, Raman mode in nanocrystalline diamond, Phys Rev B. 63 (2001) 121405. doi:10.1103/PhysRevB.63.121405.
17. Ammar, M. R., Galy, N., Rouzaud, J. N., Toulhoat, N., Vaudey, C. E., Simon, P., Moncoffre, N., Characterizing various types of defects in nuclear graphite using Raman scattering: Heat treatment,
ion
irradiation
and
polishing,
Carbon.
95
(2015)
364-373.
doi:10.1016/j.carbon.2015.07.095. 18. Li, B., Zhou, L., Wu, D., Peng, H., Yan, K., Zhou, Y., Liu, Z., Photochemical chlorination of graphene, ACS Nano. 5 (2011) 5957-5961. doi:10.1021/nn201731t. 19. Gupta, A. K., Tang, Y., Crespi, V. H., Eklund, P. C., Nondispersive Raman D band activated by well-ordered interlayer interactions in rotationally stacked bilayer graphene, Phys Rev B.
ACCEPTED MANUSCRIPT 82 (2010) 241406. doi:10.1103/PhysRevB.82.241406. 20. Huh, S., Park, J., Kim, Y. S., Kim, K. S., Hong, B. H., Nam, J. M., UV/ozone-oxidized large-scale graphene platform with large chemical enhancement in surface-enhanced Raman scattering, ACS Nano. 5 (2011) 9799-9806. doi:10.1021/nn204156n.
RI PT
21. Del Corro, E., Taravillo, M., Baonza, V. G., Nonlinear strain effects in double-resonance Raman bands of graphite, graphene, and related materials, Phys Rev B. 85 (2012) 033407. doi:10.1103/PhysRevB.85.033407.
22. Basko, D. M., Theory of resonant multiphonon Raman scattering in graphene, Phys Rev B. 78
SC
(2008) 125418. doi:10.1103/PhysRevB.78.125418.
23. Eckmann, A., Felten, A., Mishchenko, A., Britnell, L., Krupke, R., Novoselov, K. S.,
M AN U
Casiraghi, C., Probing the nature of defects in graphene by Raman spectroscopy, Nano Lett. 12 (2012) 3925-3930. doi:10.1021/nl300901a.
24. Venezuela, P., Lazzeri, M., Mauri, F., Theory of double-resonant Raman spectra in graphene: Intensity and line shape of defect-induced and two-phonon bands, Phys Rev B. 84 (2011) 035433. doi:10.1103/PhysRevB.84.035433.
TE D
25. Wu, B., Geng, D., Xu, Z., Guo, Y., Huang, L., Xue, Y., Liu, Y., Self-organized graphene
AC C
EP
crystal patterns. NPG Asia Mater. 5 (2013) e36. doi:10.1038/am.2012.68.
ACCEPTED MANUSCRIPT
RI PT
SC M AN U TE D
4.
EP
2. 3.
AC C
1.
Highlights Thermal stability of graphene in argon atmosphere was studied with different HTT by Raman mapping and AFM. The main defect type was on-site defect. Isolated graphene islands were etched out when the HTT was 800 , and the boundary defect increased gradually. Within the time required for traditional sintering, graphene would be damaged at high temperature above 800 even in a protective atmosphere.
S0022-4596(19)30178-1
DOI:
https://doi.org/10.1016/j.jssc.2019.04.008
Reference:
YJSSC 20709
To appear in:
Journal of Solid State Chemistry
Received Date: 24 January 2019 Revised Date:
21 March 2019
Accepted Date: 8 April 2019
Please cite this article as: F. Liu, M. Wang, Y. Chen, J. Gao, Thermal stability of graphene in inert atmosphere at high temperature, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/ j.jssc.2019.04.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Thermal Stability of Graphene in Inert Atmosphere at High Temperature Fu Liu, Mingjie Wang, Yao Chen and Jianmin Gao
M AN U
SC
RI PT
Graphical Abstract
AC C
EP
TE D
The effects of high temperature on the etching of graphene nanosheets in the atmosphere of protective gas.
ACCEPTED MANUSCRIPT Thermal Stability of Graphene in Inert Atmosphere at High Temperature Fu Liua, Mingjie Wanga, Yao Chena,* and Jianmin Gaoa,* a
MOE Key Laboratory of Wooden Material Science and Application, Beijing Forestry University,
Beijing, P. O. Box 100083, China *
E-mail address: [email protected] (Jm Gao) Abstract
RI PT
Corresponding author. [email protected] (Y Chen).
To investigate the thermal stability of graphene during sintering process of ceramic, the effect
SC
of temperature on stability of single-layer graphene (SLG) in argon atmosphere was studied. The Raman D-band/G-band intensity ratio(ID/IG) and the blue shifts of G mode showed that, the
M AN U
graphene defect density gradually increased with the increase in heat treatment temperature (HTT). There was limited damage in the graphene when treated temperature lower than 800 . The causes of the defects were discussed. The defect types were mainly on-set defects. Boundary defects were detected when the temperature reached above 800 . Atom Force Microscope (AFM) results showed that the defects gradually extended from the boundary and holes in the layer to
TE D
the edge and ultimately led to isolated graphene islands. The experiment results could be useful to study the changes and developing the holistic performance of graphene in high temperature sintering process.
EP
Keywords: graphene; inert atmosphere; high temperature; stability; defects
AC C
1. Introduction
Graphene has attracted much attention since its discovery in 2004. It is an extremely
promising two-dimensional (2D) nano material [1]. Apart from its perfect two dimensional crystal structure and excellent mechanical strength, electronic properties and specific surface area, graphene displays several other unusual attributes in micronano and composite fields [2]. However, such remarkable properties of graphene have not been achieved when processed into many applications, which is much lower than its theoretical performance [3]. Essentially, this two-dimensional material constitutes a new nano carbon comprising layers of carbon atoms arranged in six-membered rings [4]. It makes them extremely vulnerable to high temperature
ACCEPTED MANUSCRIPT environments. Graphene shows different properties with the influence of substrate, type of catalytic metals and defects. Previous studies indicated that by adsorbing different atoms and molecules, the electronic structure and the conductivity of the graphene can be manipulated [5]. More recently, graphene appeared as a possible reinforcement phase for ceramics due to its
RI PT
excellent mechanical [6], thermal [7, 8], and electric [9] properties. Both the solid phase sintering and the liquid phase sintering need high temperature to complete the densification of ceramic. However, graphene tends to form defects at high temperature. Various methods have been applied to reduce damage of graphene caused by high temperature during sintering process. For example,
SC
Liu et al.[10] fabricated graphene/alumina composites by spark plasma sintering (SPS). The damage to graphene will be slighter if the period of the sintering process is extremely short. It was
M AN U
found that graphene platelets not only can be dispersed uniformly in composite ceramics, but also be able to maintain the stability of the nature and composition in high temperature sintering. When it comes to the traditional method of sintering, the key problem is how to protect graphene from heat damage. Therefore, it is necessary to study stability of graphene on high temperature. Thermal stability of graphene is related to its layer. It was reported that the thermal stability
TE D
of graphene had a close relationship with the interlayer interaction of it in air atmosphere [11]. Bilayer graphene shows a better thermal stability than single layer. The results revealed that thermal stability of graphene is also dependent on the annealing time, atmosphere and preparation
EP
method. However, it is still not clear how temperature is related to the thermal stability of graphene and how lattice defects react to heat treatment in an inert atmosphere. Based on the
AC C
questions above, the thermal stability of graphene in ceramic sintering process had been studied. Generally, graphene powder has been usually used as composite fillers. It is widely known that graphene nano sheets exist abundantly in graphene powder when graphene powder is detected Raman D peak might be raised by a large number of boundary signals. This makes D peak does not have a unique relationship between structural defects [12]. As it is difficult to judge the defect density of graphene powder simply by D peak intensity and intensity ratio (ID/IG), monolayer graphene on silicon wafers were selected as subjects. In the current study, monolayer graphene with silicon wafer as substrate was synthesized by chemical vapor deposition (CVD). Thermal stability of monolayer graphene in argon atmosphere was studied by Raman spectroscopy and
ACCEPTED MANUSCRIPT AFM. The effect of high temperature on structure of graphene was revealed by D peaks in Raman spectra caused by defects. The changes in morphology of graphene were studied.
2. Experimental
RI PT
To minimize the effect of graphene layers and lamellar stacked format on graphene quality, monolayer graphene were selected as raw materials. Single-layer graphene with a side length of 1 cm square silicon wafer as substrates were purchased from 6-Carbon Technology Co., Ltd (Shenzhen, China). It was grown by chemical vapor deposition (CVD) on Cu foils at ambient
SC
pressure and transferred onto a Si substrate. Silicon wafer was used to provide a support surface for graphene, and to increase the signal intensity in the Raman detection. To avoid the influence of
furnace tube to the temperature of 700 surrounding air.
M AN U
impurities, furnace drying was conducted prior to this experiment. Usual practice was to heat the and hold for about 30 min to evaporate impurities into the
Graphene samples were heat-treated using a tube furnace (SK-G08123K-2-420, China). The graphene was placed into an alumina crucible. Graphene samples were heat-treated at in argon (99.999% purity) using a heating rate of 10 /min,
TE D
temperatures of 600, 800 and 1000
and then holding for one hour. After heat treatment, the samples were washed with acetone (A.R.,≥99.5%) in ultrasonic washer for 5 minutes, to discharge the organics from the substrate.
EP
Raman spectra and mappings of the annealed samples were obtained at room temperature (RT) using a Horiba LabRAM HR Evolution Raman spectrometer (France). A user defined image
AC C
area of 30µm×30 µm was employed for the mappings. The same excitation wavelength of 514 nm was used in each case. Atomic force microscope (AFM) in tapping mode with silicon tips (Bruker Multimode 8, Karlsruhe, Germany) was used to investigate the morphology of graphene.
3. Results and discussion Figure 1 shows the Raman spectra of four untreated samples in the region of 1200-3000 cm-1. The Raman spectrum of the graphene exhibited three characteristic peaks centered at ∼1360, ∼1580 and ∼2700 cm-1 that can be assigned to the D, G and 2D bands of graphene, respectively [13]. Both the G and D peaks arise from vibrations of sp2-hybridized carbon atoms. The amount of disorder in graphene can be measured conveniently with D peak intensity [14–16]. The
ACCEPTED MANUSCRIPT single-layer thickness of the graphene can be confirmed from the 2D mode [14–16]. The high crystalline order of the untreated samples can also be confirmed with the absence of D peak
M AN U
SC
RI PT
[14–16].
Fig.1 Raman spectra of four initial samples selected for further heat treatment (a) for comparison heat treatment (c) for 800
heat treatment (d) for 1000
heat
TE D
at room temperature (b) For 600
treatment.
Figure 2 shows the Raman mappings and single spectra of the circled areas of the graphene after heat treatment at different temperatures. The integrated G peaks at Raman images with
EP
different HTT were displayed. It identified the continuous and uniform distribution of graphene on the Si wafer. Bright areas in IG maps are associated to graphite-like structures. According to the
AC C
single spectra of the circled areas, the circled dark areas correspond to the SLG. Therefore, the high brightness areas represented multi-layered graphene. It can be seen from the figure that most of the regions can be attributed to SLG. As seen in Fig. 2, the ID/IG in Raman mappings indicated that the brightness of areas corresponding to the multi-layer graphene was relatively dark, which confirmed the better thermal stability of multi-layer graphene [11]. The ID/IG maps of the circle 2 regions indicated the serious defects of SLG as the brightest areas. The Raman spectra of the moderate brightness areas (circle 3) in ID/IG maps was also shown in Fig.2. As seen from the Raman spectra of the circled areas, the line shapes of both the D and G bands varied significantly with HTT. The full width at half maximum (FWHM) of G peaks can be
ACCEPTED MANUSCRIPT used to characterize the defects density in graphene [17, 18]. The G-band width kept constant or increased slightly below 800°C. it demonstrated that there was limited damage in the graphene. The ID/IG ratio, which was calculated from each peak area, was shown in Fig. 3. The ID/IG ratios of samples showed an increasing trend in the HTT range of RT – 1000°C. The ratio reached a
RI PT
maximum at 1000°C, indicating the graphene was damaged by thermal etching. The error bars of ID/IG did not varied much with the increasing HTT. This was related to the uniform distribution of defects.
The G mode blue shifted to the high-frequency region with the increase in HTT(Fig.3). The
SC
G mode position of single layer graphene on Si substrates moved 0.4~15cm-1 to the higher frequency. This can be attributed to two reasons. The first one is that high temperature changed the
M AN U
local density of atoms at the interface of silicon substrate. The changes in local density changed the capacity of the adsorption of molecules. Graphene hole doping caused by the adsorption of molecules made its G peak to blue shift with varying degrees [19,20]. Another reason for the blue shift was that the thermal expansion and contraction of substrate produced "the stress effect" on the surface of graphene. The blue shift of the G peaks under stress can be attributed to the
TE D
compression of C-C bonds. Del Corro et al. [21] reported that G peaks would shift to high wavenumber under compressive stress, due to the decrease of the distance between the carbon atoms. The error bars of G blue shift peaks increased at 1000 . It suggested that the stability of
EP
graphene varied greatly at different scanning sites. The multilayer graphene region (the highlighted area in Raman mapping of IG) kept good stability, while defective areas became
AC C
unstable. Therefore, it can be speculated that some of the new defects developed gradually from the original defects.
After the sample has been annealed, the D peak intensity increased with the increasing
amounts of defects, however the 2D intensity decreased. That didn’t mean the formation of multi layer graphene, since there was no introduction of additional carbon source. According to the study of Basko et al. 2D peak was strongly sensitive to the dynamics of the photo excited electron-hole pair [22]. Any increase in defects will affect the electron lifetime, which translates in a decrease in the 2D peak intensity.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 2 Raman mappings and single spectra of the circled areas of the graphene after heat treatment
AC C
EP
TE D
at different temperatures.
Fig.3 ID/IG ratio and G mode blue shifts of different HTT samples.
The intensity of disorder-induced Raman feature (D band at ~1350 cm-1) was correlated to
such as vacancies, implanted atoms, edges, grain boundaries, and defects associated to a change of carbon-hybridization [23]. The defect density (ID/IG) increased with the HTT. This may because graphene became fluctuated and the Si substrate surface became uneven (sublimation of silicon atoms) under high temperature for a long duration. D band appeared or increased when laser scanning a sloping or uneven surface. Secondly, the graphene sheet might be damaged with high temperature, causing cracks or holes which increased the edge length of graphene. The intensity of
ACCEPTED MANUSCRIPT D peak is also correlated to the edge chirality of the graphene. The edge chirality related to the angle between the sides of the graphene sheet which was determined by the figure of graphene sheet. Additionally, impurities on the surface or vacancies doped hetero-atom also resulted in defects during heat treatment process. For example, the defects could be induced by the deposition
RI PT
of Si atoms evaporated from the substrate onto the graphene surface. The type of defects in graphene can be classified by D′ peak at ~1640 cm-1. However, the moderate or weak intensity of D’ peak made it difficult to be identified. The intensity ratio of the D and D′ peak (ID/ID’) is able to probe the nature of the defects for moderate amount of disorder.
SC
The ID/ID’ were about 1.3 for on-site defects for almost all the samples, which might be produced by the deformation of the carbon-bond, sp3 hybridization or molecule adsorption [24]. There was and 1000
M AN U
some boundaries(∼3.5), and no vacancy-like defects(∼7) for 800
HTT samples [24].
The D’ peak was weakness or not detected for untreated sample and these with 600
HTT. It
might be due to the fewer defects in these samples. This has been further characterized by using
AC C
EP
TE D
AFM in tapping mode (Fig.4).
Fig.4 Topography AFM images of graphene at room temperature and after HTT. (a) room temperature (b) 600
(c) 800
(d) 1000 .
The morphology of the SLG on Si substrate was shown in Fig. 4a–d. Fig. 4a showed a typical
AFM image of SLG with intact nanosheets. The bright white sections on the image were impurities adsorbed on the surface. The typical size of the impurities was about 20-35 nm. As can be seen in figure 4b, there was a hole appeared in the middle of graphene. This hole should be developed from the gradual expansion of vacancy defects. It can be seen some cracks in the graphene sheet in Fig.4c. The formation of the cracks should be due to the different shrinkage
ACCEPTED MANUSCRIPT expansion rates between graphene and the substrate. The graphene was etched into many discrete graphene sheets after being annealed at 1000
(Fig.4d). The length of boundaries increased
significantly. The boundary defects can be found in the Raman detection. The morphology models were used to further illustrate the effects of high temperature on the
RI PT
etching of graphene nanosheets combining with the AFM result (Fig.5a–d). It showed that the etching of the graphene started from the edge with vacancies appearing up to 600
(Fig. 5b). It
was not a type of point defect in a crystal but a hole with complete edge. The defects within the layer gradually extended to the edge with the treatment temperature increased, and an isolated
SC
graphene island was formed [25] (Fig.5c). Finally, the intact nanosheets gradually became into isolated pieces of graphene with the temperature up to 1000
(Fig.5d). However, there was also
M AN U
the possibility that the combination stability of graphene with Si substrate became weak at high temperature. Therefore, the possible perturbations in its topography from tip-sample interactions
AC C
EP
TE D
must also be considered in characterizing the integrity of graphene.
Fig.5 Morphology-change models of graphene layer with temperature changes. (a) room temperature (b) 600
(c) 800
(d) 1000 .
4. Conclusion Thermal stability of graphene in argon atmosphere was studied with different HTT by Raman mapping and AFM. The study confirmed that multi-layer graphene had better stability. The defect density of SLG increased with the increased in HTT. The samples were damaged most seriously when subjected to heat treatment at 1000 , as observed from the Raman peak shapes and
ACCEPTED MANUSCRIPT maximum defect density. Graphene layer maintained completely when the HTT was below 600 , and then some holes appeared in graphene layer with the increase in HTT. The main defect type was on-site defect. Isolated graphene islands were etched out when the HTT was 800 , and the boundary defect increased gradually. Graphene layer fragmented into small pieces when the HTT
graphene would be damaged at high temperature above 800 and thus unable to give full play to its performance.
even in a protective atmosphere,
SC
Acknowledgements
RI PT
was 1000 . These findings showed that within the time required for traditional sintering,
The work was supported by National Natural Science Foundation of China (NSFC, No.
References
M AN U
51572028) and Beijing Natural Science Foundation (6192020).
1. Geim A K, Novoselov K S., The rise of graphene, Nat Mater. 6 (2007) 183-191. doi:10.1038/nmat1849.
TE D
2. Li, D., Kaner, R. B., Graphene-based materials, Science. 320 (2008) 1170-1171. doi:10.1126/science.1158180.
3. Xin, G., Yao, T., Sun, H., Scott, S. M., Shao, D., Wang, G., Lian, J., Highly thermally
EP
conductive and mechanically strong graphene fibers, Science. 349 (2015) 1083-1087. doi:10.1126/science.aaa6502.
AC C
4. Rao, C. E. E., Sood, A. E., Subrahmanyam, K. E., Govindaraj A., Graphene: The New Two Dimensional Nanomaterial, Angew Chem Int Edit. 48 (2009) 7752-7777. doi:10.1002/anie.200901678.
5. Ohta, T., Bostwick, A., Seyller, T., Horn, K., Rotenberg, E., Controlling the electronic structure of bilayer graphene, Science. 313 (2006) 951-954. doi:10.1126/science.1130681.
6. Frank, I. W., Tanenbaum, D. M., van der Zande, A. M., McEuen, P. L., Mechanical properties of suspended graphene sheets, J Vac Sci Technol B. 25 (2007) 2558-2561. doi:10.1116/1.2789446. 7. Balandin, A. A., Thermal Properties of Graphene, Carbon Nanotubes and Nanostructured Carbon Materials, Nat Mater. 10 (2011) 569. doi:10.1038/nmat3064.
ACCEPTED MANUSCRIPT 8. Prasher,
R.,
Graphene
Spreads
the
Heat,
Science.
328
(2010)
185-186.
doi:10.1126/science.1188998. 9. Stankovich, S., Dikin, D. A., Dommett, G. H., Kohlhaas, K. M., Zimney, E. J., Stach, E. A., Ruoff,
R.
S.,
Graphene-based
composite
materials,
Nature.
442
(2006)
282.
RI PT
doi:10.1038/nature04969. 10. Liu, J., Yan, H., Jiang, K., Mechanical properties of graphene platelet-reinforced alumina ceramic composites, Ceram Int. 39 (2013) 6215-6221. doi:10.1016/j.ceramint.2013.01.041.
11. Nan, H. Y., Ni, Z. H., Wang, J., Zafar, Z., Shi, Z. X., Wang, Y. Y., The thermal stability of
SC
graphene in air investigated by Raman spectroscopy, J Raman Spectrosc. 44 (2013) 1018-1021. doi:10.1002/jrs.4312.
M AN U
12. Kudin, K. N., Ozbas, B., Schniepp, H. C., Prud'Homme, R. K., Aksay, I. A., Car, R., Raman spectra of graphite oxide and functionalized graphene sheets, Nano Lett. 8 (2008) 36-41. doi:10.1021/nl071822y.
13. Dong, X., Fu, D., Fang, W., Shi, Y., Chen, P., Li, L. J., Doping single-layer graphene with aromatic molecules, Small. 5 (2009) 1422-1426. doi:10.1002/smll.200801711.
TE D
14. Ferrari, A. C., Raman spectroscopy of graphene and graphite: Disorder, electron–phonon coupling, doping and nonadiabatic effects, Solid State Commun. 143 (2007) 47-57. doi:10.1016/j.ssc.2007.03.052.
EP
15. Tuinstra, F., Koenig, J. L., Raman Spectrum of Graphite, J Chem Phys. 53 (1970) 1126-1130. doi:10.1063/1.1674108.
AC C
16. Ferrari, A. C., Robertson, J., Origin of the 1150 - cm -1, Raman mode in nanocrystalline diamond, Phys Rev B. 63 (2001) 121405. doi:10.1103/PhysRevB.63.121405.
17. Ammar, M. R., Galy, N., Rouzaud, J. N., Toulhoat, N., Vaudey, C. E., Simon, P., Moncoffre, N., Characterizing various types of defects in nuclear graphite using Raman scattering: Heat treatment,
ion
irradiation
and
polishing,
Carbon.
95
(2015)
364-373.
doi:10.1016/j.carbon.2015.07.095. 18. Li, B., Zhou, L., Wu, D., Peng, H., Yan, K., Zhou, Y., Liu, Z., Photochemical chlorination of graphene, ACS Nano. 5 (2011) 5957-5961. doi:10.1021/nn201731t. 19. Gupta, A. K., Tang, Y., Crespi, V. H., Eklund, P. C., Nondispersive Raman D band activated by well-ordered interlayer interactions in rotationally stacked bilayer graphene, Phys Rev B.
ACCEPTED MANUSCRIPT 82 (2010) 241406. doi:10.1103/PhysRevB.82.241406. 20. Huh, S., Park, J., Kim, Y. S., Kim, K. S., Hong, B. H., Nam, J. M., UV/ozone-oxidized large-scale graphene platform with large chemical enhancement in surface-enhanced Raman scattering, ACS Nano. 5 (2011) 9799-9806. doi:10.1021/nn204156n.
RI PT
21. Del Corro, E., Taravillo, M., Baonza, V. G., Nonlinear strain effects in double-resonance Raman bands of graphite, graphene, and related materials, Phys Rev B. 85 (2012) 033407. doi:10.1103/PhysRevB.85.033407.
22. Basko, D. M., Theory of resonant multiphonon Raman scattering in graphene, Phys Rev B. 78
SC
(2008) 125418. doi:10.1103/PhysRevB.78.125418.
23. Eckmann, A., Felten, A., Mishchenko, A., Britnell, L., Krupke, R., Novoselov, K. S.,
M AN U
Casiraghi, C., Probing the nature of defects in graphene by Raman spectroscopy, Nano Lett. 12 (2012) 3925-3930. doi:10.1021/nl300901a.
24. Venezuela, P., Lazzeri, M., Mauri, F., Theory of double-resonant Raman spectra in graphene: Intensity and line shape of defect-induced and two-phonon bands, Phys Rev B. 84 (2011) 035433. doi:10.1103/PhysRevB.84.035433.
TE D
25. Wu, B., Geng, D., Xu, Z., Guo, Y., Huang, L., Xue, Y., Liu, Y., Self-organized graphene
AC C
EP
crystal patterns. NPG Asia Mater. 5 (2013) e36. doi:10.1038/am.2012.68.
ACCEPTED MANUSCRIPT
RI PT
SC M AN U TE D
4.
EP
2. 3.
AC C
1.
Highlights Thermal stability of graphene in argon atmosphere was studied with different HTT by Raman mapping and AFM. The main defect type was on-site defect. Isolated graphene islands were etched out when the HTT was 800 , and the boundary defect increased gradually. Within the time required for traditional sintering, graphene would be damaged at high temperature above 800 even in a protective atmosphere.