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This article can be cited before page numbers have been issued, to do this please use: D. Mun, H. J. Lee, S. Bae, T. Kim and S. Lee, Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP01670C.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Physical Chemistry Chemical Physics View Article Online
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Photocatalytic decomposition of graphene over the surface of ZnO under UV irradiation Dae-Hwa Muna, Hyun Jung Leea,†, Sukang Baea, Tea-Wook Kima and Sang Hyun Leea,* a
Korea Institute of Science and Technology (KIST), 92 Chudong-ro, Bongdong-eup, Wanju-gun,
Jeollabuk-do 565-905, Republic of Korea, †Present address: BioNano Health Guard Research Center, Daejeon 305-806, South Korea
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Physical Chemistry Chemical Physics Accepted Manuscript
DOI: 10.1039/C5CP01670C
Physical Chemistry Chemical Physics
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DOI: 10.1039/C5CP01670C
Highly reactive radicals or chemicals are generated on the surfaces of oxide semiconductors via
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reactions between photo-induced charges and ambient gas molecules. These radicals or chemicals have been utilized in heterogeneous photosynthesis and photocatalysis. In this study, we demonstrated that the photocatalytic reactions at the surface of ZnO promoted the oxidation and decomposition of graphene. Raman spectra were used to analyze the evolution of the G and 2D peaks. The oxidation of graphene on a ZnO substrate by UV radiation was faster than that in the absence of ZnO. During oxidation, the resistivity and the transmittance of graphene also increased. The XPS results showed that functional groups related to the oxidation of graphene were formed during the photocatalytic reactions. This simple and clean approach will be also effective for selective surface modification by enhancing the surface chemical reactions that pattern graphene via oxidation.
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Abstract
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DOI: 10.1039/C5CP01670C
Graphene is a two dimensional (2D) layer of sp2-bonded carbon with atomic thickness. It is a
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very attractive material because of its distinctive electrical, physical, chemical and mechanical properties. For example, the extremely high carrier mobility, mechanical flexibility, optical transparency, and chemical stability of graphene result in a material that has significant potential for many technological applications, such as electronics, supercapacitors, sensors, membranes and various nanocomposites.1-6 Despite the remarkable properties of graphene, the absence of energy bandgaps and few active sites on the surface are major obstacles that can hinder potential applications in electronics, optoelectronics, and chemical/biomedical sensors. Two representative strategies are proposed for overcoming the weaknesses of graphene: (1) tailoring the morphology of graphene and (2) doping or modifying the graphene parent materials with foreign atoms or molecules.7 Nanoribbons and nanomeshes synthesized through either lithographic or chemical approaches were successful in opening the bandgap in graphene layers by quantum confinement or by a localized effects resulting from edge disorder.8,9 Chemical doping in graphene has been achieved either by charge transfer from organic molecules on the surface of graphene or by substitution of carbon atoms in the graphene lattice (including nitrogen, boron and fluorine).10-12 However,
several
important
obstacles,
such
as
residues,
chemical
stability,
and
complicated/expensive processes, need to be overcome to realize these results.13,14 Oxygen is one of the most important elements for the two strategies because of the wide variety of chemical interactions between oxygen radicals and aromatic carbon molecules. Various processes, including thermal, plasma and UV/ozone treatments, have been suggested to
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Physical Chemistry Chemical Physics Accepted Manuscript
1. Introduction
Physical Chemistry Chemical Physics
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utilization would be exploiting photocatalytic processes that create reactive oxygen or hydroxyl radicals near the surfaces of photocatalysts, such as TiO2, ZnO, SnO2 and Fe2O3.17 It has recently
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been demonstrated that graphene layers can be patterned by these photocatalytic reactions.18 In this study, we report the effects of photocatalytic reactions on the surfaces of oxide semiconductors on the properties of graphene. Based on an analysis of Raman spectra from graphene samples under different UV irradiation times, it was discovered that the presence of a ZnO photocatalyst enhanced the oxidation and decomposition of graphene. The photocatalytic reaction also resulted in changes in the electrical resistance and optical transmittance of graphene. We anticipate that an effective method to control the graphene layers by selective photocatalytic treatments will be beneficial for various graphene-based applications.
2. Experimental Procedure 2.1 Synthesis of graphene The graphene was synthesized on copper foil using well-known published methods.19 The experimental details for the synthesis of graphene layers are as follows: A high-quality monolayer of graphene with a large area was grown on copper foil using chemical vapor deposition (CVD). The copper foil was loaded into a tube quartz furnace and heated to 1000°C in an argon atmosphere at 50 mTorr with H2 flowing at a rate of 3 sccm (standard cubic centimeters per minute). Next, the copper foil was annealed for 20 min to increase the average grain size. After the heating was completed, the system was stabilized at 1200 mTorr for 30 min (at 1000°C), and methane and H2 were introduced at flow rates of 30 and 3 sccm, respectively. The
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generate oxygen radicals to interact with graphene.15,16 Another useful strategy for oxygen
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2.2 Transfer of graphene onto substrates
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The graphene layers grown on the copper foil were transferred onto arbitrary substrates using the polymethylmethacrylate (PMMA) transfer method.20 A PMMA solution (SigmaAldrich) (MW approximately 996000, dissolved in chlorobenzene at a concentration of 0.042 g/mL) was spin-coated on the graphene layers at 4200 RPM for 30 sec. Oxygen plasma etching (reactive ion etching: RIE) was performed at 100 W for 5 sec under 100 mTorr to remove the graphene layer on the opposite side of the PMMA-graphene layer. The PMMA-assisted graphene layers were released from the copper foil by etching in an aqueous solution of ammonium persulfate (Sigma-Aldrich; 0.014 g/mL). After etching for several hours, the PMMA-graphene layers were moved into a deionized water bath and rinsed. The PMMA-graphene layers were then transferred onto a 1-mm-thick quartz substrate. Finally, the PMMA was rinsed with acetone for approximately 40 min and then air-dried. The optical properties of the graphene layers were analyzed using Raman spectroscopy (JobinYvon LabRam HR, Horiba, Japan). The Raman spectra were measured in a backscattering geometry under ambient atmosphere at room temperature using a 100X objective lens and a 600line/mm grating. The laser excitation wavelength was 632.8 nm and the spot size was approximately 1 µm. X-ray photoelectron spectroscopy (XPS) (K-alpha, Thermo Scientific, USA) spectra were measured using an Al K-alpha radiation system. The average base pressure was approximately 2.2 × 10-7 mbar at 300 K. The spot size of the X-ray was 100 µm, and the instrument resolution was approximately 0.1 eV. The X-ray source was generated from an Al target (1486.8 eV) with a pass energy of 50 eV to analyze the scan. The binding energies were
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furnace was then rapidly cooled to room temperature under H2 at 50 mTorr.
Physical Chemistry Chemical Physics
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Vis/NIR spectrophotometer (V-670, JASCO, Japan). The sheet resistance measurements were performed at room temperature using the four-point probe method (FPP-RS8, Dasol Eng.,
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Korea).
2.3 Photocatalytic reaction of graphene on ZnO substrate under UV irradiation Figure 1(a) shows a schematic of the experimental procedures for the photocatalysis of graphene using ZnO. The polished Zn face of a single crystal ZnO sample was carefully placed on the graphene (which had been transferred onto the quartz substrate). The sample was mounted on an apparatus (Fig. 1(b)) with a hole at the center for UV irradiation. Note that the polarity of ZnO does not significantly alter the photocatalytic reaction with graphene. To ensure uniform exposure of the UV light on the interface between the graphene and the ZnO, the light was irradiated through the quartz substrate in air with an intensity of approximately 60 mW/cm2.
3. Results and discussion
Figure 2 shows the Raman spectra of graphene on the quartz substrate as a function of the duration of UV irradiation in ambient atmosphere. The D, D’, G and 2D bands are assigned in Figure 2.21 From the Raman spectra of pristine graphene (bottom, black line), two pronounced peaks, the G band at 1585 cm-1 and the 2D band at 2696 cm-1, were observed. The D band at 1354 cm-1 appeared after increasing the duration of UV irradiation independent with the presence of ZnO. This finding indicates that the graphene may have been damaged by ozone that was generated due to the reaction between UV light and water molecules in the air.
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referenced to the C1s line at 284.8 eV. The transmission spectra were measured using a UV-
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spectrum in Figure 2 were compared with Figure 3. Figure 3a shows the intensity ratio of the D/G band with respect to the UV exposure time. It is clear that the defect density of graphene
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with the presence of ZnO is higher than that without the presence of ZnO which is generated by ozone. The reason could be due to photocatalytic chemical reactions occur at the interface between ZnO and graphene. Electrons and holes are generated in the conduction and valance bands via the irradiation of light with higher energy than the intrinsic bandgap of ZnO (approximately 3.37 eV) (eq.(1)). The photoinduced electrons and holes react with moisture and oxygen molecules in the air, producing highly reactive species, such as ·OH, O2·- and H2O2 by chemical reactions in eq. (2~5).23,24 The chemical species generated at the surface of ZnO most likely lead to the oxidation and decomposition of graphene.
ℎ + ℎ (1) ℎ + → · + , ℎ + → · (2) + 2 → , + → · (3) + ℎ → 2 · (4) + → · + (5)
Chemical doping in graphene typically occurs during this reaction. Recently, Lee et al. reported that the effects of mechanical strain and chemical doping on graphene could be separated using the correlation between the G and 2D modes.22 To confirm the doping effect during UV irradiation and to compare the properties in the presence of ZnO, the position of the 2D band with respect to the position of the G band is shown in Figure 3b. Based on the optical separation,
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To investigate the photocatalytic effect on the properties of graphene, the features of the Raman
Physical Chemistry Chemical Physics
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increasing the irradiation time to 16 h, the plotted positions of the 2D and G frequencies became parallel to the vector for hole doping. However, the change in the direction of the strain effect
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was minimal, and the phenomenon was independent of the presence of ZnO. The hole density was estimated to be approximately 1.5 ×1013 cm-2. It has been reported that ozone molecules generated by UV light produce oxygen-containing groups on graphene, which lead to the hole doping of the graphene.25 It is plausible that photoinduced reactive molecules could form similar oxygen-containing groups on the graphene.
Figure 4 shows the sheet resistance and optical transmittance of graphene by UV irradiation time. The sheet resistance of graphene transferred onto the quartz substrate was approximately 600 Ω/sq. The resistance gradually increased as the irradiation time increased. However, there were significant differences in sheet resistance based on the presence or absence of ZnO. After irradiation for 12 h, the sheet resistance of graphene on ZnO was approximately 20 times higher than that on the bare quartz substrate, indicating that the photocatalytic reactions on the surface of the ZnO accelerated the oxidization and decomposition of graphene. This result is consistent with the fast evolution of the D peaks and the disappearance of the carbon-related peaks as a function of the irradiation time, as shown in Figure 2. The transmittance of graphene on ZnO also increased from 93.7% to 99.9% at 550 nm as the irradiation time increased, as shown in Figure 4b. After approximately 5 h of irradiation, the transmittance became larger than the typical value (97.7%) of monolayer graphene.3 This behavior likely occurs because the area of the substrate that was covered by graphene decreased due to partial decomposition. The photo inset in Figure 4b shows the graphene on a quartz substrate after the photocatalytic reaction had
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Physical Chemistry Chemical Physics Accepted Manuscript
it was found that UV irradiation for 2 h caused slight compressive strain and hole doping. By
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area covered by ZnO. In contrast, the transmittance of the graphene surrounding the area remained unchanged. These results indicate that UV irradiation mainly contributed to hole
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doping but did not cause notable changes in transmittance.
XPS measurements were performed to identify the functional groups generated during the photocatalytic reaction. Figure 5 shows the main C1s peak (due to the sp2 carbon-carbon bonds) at approximately 284.7 eV from both pristine graphene and after 8 h of the photocatalytic reaction. After the UV irradiation, the full width of the C1s peak increased from 1.3 to 2.1 eV. The intensity of the C-O peak (centered at 286.6 eV), which is associated with C-OH and C-O-C functional groups, was significantly larger. In addition, other oxidation-related peaks due to C=O and O-C=O functional groups appeared at 288.4 and 289.7 eV, respectively. The amount of oxygen (carbon) at the surface of substrate was gradually increased (decreased) as the UV irradiation time increased (the inset of Figure 5b). These results prove that the reactive chemical components generated by the photocatalytic reaction, such as ·OH, O2·- and H2O2, contributed to the oxidation of graphene. In conclusion, we studied the photocatalytic effects of the structural and optical properties of graphene using ZnO. The highly reactive species on the surface of the ZnO photocatalyst accelerated the decomposition of graphene via oxidation compared with UV irradiation. This technique allowed us to control the doping concentration and to selectively remove graphene. In conjunction with the micro/nanotechnology that is available for oxide semiconductor
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Physical Chemistry Chemical Physics Accepted Manuscript
proceeded for 24 h. The transparent centimeter-scale trapezoid shape on the substrate was the
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of the properties of graphene for electronic and optoelectronic applications.
Acknowledgments This work was supported by the Korea Institute of Science and Technology (KIST) Institutional Program and by the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) of Republic of Korea (Grant CAP-13-2-ETRI).
References 1
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V.
Dubonos and A. A. Firsov, Nature, 2005, 438, 197-200. 2
C. Lee, X. D. Wei, J. W. Kysar and J. Hone, Science, 2008, 321, 385-388.
3
R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R.
Peres and A. K. Geim, Science, 2008, 320, 1308-1308. 4
Y. Wang, Z. Q. Shi, Y. Huang, Y. F. Ma, C. Y. Wang, M. M. Chen and Y. S. Chen, J. Phys.
Chem. C, 2009, 113, 13103-13107. 5
J. S. Bunch, S. S. Verbridge, J. S. Alden, A. M. van der Zande, J. M. Parpia, H. G. Craighead
and P. L. McEuen, Nano Lett., 2008, 8, 2458-2462. 6
S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R.
D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282-286. 7
M. J. Allen, V. C. Tung and R. B. Kaner, Chem. Rev., 2010, 110, 132-145.
10
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photocatalysts, we believe that photocatalytic reactions will be very useful for the modification
Page 11 of 19
Physical Chemistry Chemical Physics View Article Online
8
M. Y. Han, B. Ozyilmaz, Y. B. Zhang and P. Kim, Phys. Rev. Lett., 2007, 98.
9
J. W. Bai, X. Zhong, S. Jiang, Y. Huang and X. F. Duan, Nat. Nanotechnol., 2010, 5, 190-194.
10
Y. Wang, Y. Y. Shao, D. W. Matson, J. H. Li and Y. H. Lin, ACS Nano, 2010, 4, 1790-1798.
11
L. S. Panchokarla, K. S. Subrahmanyam, S. K. Saha, A. Govindaraj, H. R. Krishnamurthy, U.
V. Waghmare and C. N. R. Rao, Adv. Mater., 2009, 21, 4726-4730. 12
J. T. Robinson, J. S. Burgess, C. E. Junkermeier, S. C. Badescu, T. L. Reinecke, F. K. Perkins,
M. K. Zalalutdniov, J. W. Baldwin, J. C. Culbertson, P. E. Sheehan and E. S. Snow, Nano Lett., 2010, 10, 3001-3005. 13
M. Ishigami, J. H. Chen, W. G. Cullen, M. S. Fuhrer and E. D. Williams, Nano Lett., 2007, 7,
1643-1648. 14
A. Pirkle, J. Chan, A. Venugopal, D. Hinojos, C. W. Magnuson, S. McDonnell, L. Colombo, E.
M. Vogel, R. S. Ruoff and R. M. Wallace, Appl. Phys. Lett., 2011, 99. 15
T. Gokus, R. R. Nair, A. Bonetti, M. Bohmler, A. Lombardo, K. S. Novoselov, A. K. Geim, A.
C. Ferrari and A. Hartschuh, ACS Nano, 2009, 3, 3963-3968. 16
S. Huh, J. Park, Y. S. Kim, K. S. Kim, B. H. Hong and J. M. Nam, ACS Nano, 2011, 5, 9799-
9806. 17
A. L. Linsebigler, G. Q. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735-758.
18
G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2, 1487-1491.
19
S. Bae, H. Kim, Y. Lee, X. F. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y.
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DOI: 10.1039/C5CP01670C
Physical Chemistry Chemical Physics
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Nanotechnol., 2010, 5, 574-578. 20
X. S. Li, Y. W. Zhu, W. W. Cai, M. Borysiak, B. Y. Han, D. Chen, R. D. Piner, L. Colombo
Published on 12 May 2015. Downloaded by Freie Universitaet Berlin on 14/05/2015 09:01:09.
and R. S. Ruoff, Nano Lett., 2009, 9, 4359-4363. 21
A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D.
Jiang, K. S. Novoselov, S. Roth and A. K. Geim, Phys. Rev. Lett., 2006, 97. 22
J. E. Lee, G. Ahn, J. Shim, Y. S. Lee and S. Ryu, Nat. Commun., 2012, 3.
23
M. Fujihira, Y. Satoh and T. Osa, Nature, 1981, 293, 206-208.
24
M. A. Behnajady, N. Modirshahia and R. Hamzavi, J. Hazard. Mater., 2006, 133, 226-232.
25
S. Huh, J. Park, Y. S. Kim, K. S. Kim, B. H. Hong and J. M. Nam, ACS Nano, 2011, 5, 9799-
9806.
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I. Song, Y. J. Kim, K. S. Kim, B. Ozyilmaz, J. H. Ahn, B. H. Hong and S. Iijima, Nat.
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Figure 1. (a) Schematic illustration for the experiment to determine the photocatalytic reaction
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between graphene and ZnO and (b) a photo image of sample mounting on a jig for the experiment.
Figure 2. Raman spectra of graphene as a function of UV irradiation time: (a) with and (b) without ZnO.
Figure 3. Correlation between the G and 2D bands of graphene (Fig. 2) as a function of UV irradiation time: (a) Intensity ratio of the G and 2D bands and (b) comparison of the 2D and G peak frequencies with (squares) and without (circles) ZnO. eT and eH are the unit vectors for the tensile strain and hole doping effects with slopes of 2.2 (blue line) and 0.7 (red line), respectively.22
Figure 4. Electrical and optical properties of graphene after the photocatalytic reactions. (a) Sheet resistance and (b) transmittance of graphene measured at 550 nm after the photocatalytic reaction. The inset shows patterned graphene on a 1-inch-diameter quartz substrate after a 24-h exposure to the photocatalytic reaction.
Figure 5. The effect of the photocatalytic reaction on the formation of functional groups. XPS spectra of (a) pristine graphene on a quartz substrate and (b) after the photocatalytic reaction
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Figure captions
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Physical Chemistry Chemical Physics Accepted Manuscript
under UV irradiation for 8 h. The inset shows the content change in carbon (black) and oxygen
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(red) as a function of UV irradiation time.
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PCCP Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: D. Mun, H. J. Lee, S. Bae, T. Kim and S. Lee, Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/C5CP01670C.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/pccp
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Photocatalytic decomposition of graphene over the surface of ZnO under UV irradiation Dae-Hwa Muna, Hyun Jung Leea,†, Sukang Baea, Tea-Wook Kima and Sang Hyun Leea,* a
Korea Institute of Science and Technology (KIST), 92 Chudong-ro, Bongdong-eup, Wanju-gun,
Jeollabuk-do 565-905, Republic of Korea, †Present address: BioNano Health Guard Research Center, Daejeon 305-806, South Korea
1
Physical Chemistry Chemical Physics Accepted Manuscript
DOI: 10.1039/C5CP01670C
Physical Chemistry Chemical Physics
Page 2 of 19 View Article Online
DOI: 10.1039/C5CP01670C
Highly reactive radicals or chemicals are generated on the surfaces of oxide semiconductors via
Published on 12 May 2015. Downloaded by Freie Universitaet Berlin on 14/05/2015 09:01:09.
reactions between photo-induced charges and ambient gas molecules. These radicals or chemicals have been utilized in heterogeneous photosynthesis and photocatalysis. In this study, we demonstrated that the photocatalytic reactions at the surface of ZnO promoted the oxidation and decomposition of graphene. Raman spectra were used to analyze the evolution of the G and 2D peaks. The oxidation of graphene on a ZnO substrate by UV radiation was faster than that in the absence of ZnO. During oxidation, the resistivity and the transmittance of graphene also increased. The XPS results showed that functional groups related to the oxidation of graphene were formed during the photocatalytic reactions. This simple and clean approach will be also effective for selective surface modification by enhancing the surface chemical reactions that pattern graphene via oxidation.
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Physical Chemistry Chemical Physics Accepted Manuscript
Abstract
Page 3 of 19
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DOI: 10.1039/C5CP01670C
Graphene is a two dimensional (2D) layer of sp2-bonded carbon with atomic thickness. It is a
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very attractive material because of its distinctive electrical, physical, chemical and mechanical properties. For example, the extremely high carrier mobility, mechanical flexibility, optical transparency, and chemical stability of graphene result in a material that has significant potential for many technological applications, such as electronics, supercapacitors, sensors, membranes and various nanocomposites.1-6 Despite the remarkable properties of graphene, the absence of energy bandgaps and few active sites on the surface are major obstacles that can hinder potential applications in electronics, optoelectronics, and chemical/biomedical sensors. Two representative strategies are proposed for overcoming the weaknesses of graphene: (1) tailoring the morphology of graphene and (2) doping or modifying the graphene parent materials with foreign atoms or molecules.7 Nanoribbons and nanomeshes synthesized through either lithographic or chemical approaches were successful in opening the bandgap in graphene layers by quantum confinement or by a localized effects resulting from edge disorder.8,9 Chemical doping in graphene has been achieved either by charge transfer from organic molecules on the surface of graphene or by substitution of carbon atoms in the graphene lattice (including nitrogen, boron and fluorine).10-12 However,
several
important
obstacles,
such
as
residues,
chemical
stability,
and
complicated/expensive processes, need to be overcome to realize these results.13,14 Oxygen is one of the most important elements for the two strategies because of the wide variety of chemical interactions between oxygen radicals and aromatic carbon molecules. Various processes, including thermal, plasma and UV/ozone treatments, have been suggested to
3
Physical Chemistry Chemical Physics Accepted Manuscript
1. Introduction
Physical Chemistry Chemical Physics
Page 4 of 19 View Article Online
DOI: 10.1039/C5CP01670C
utilization would be exploiting photocatalytic processes that create reactive oxygen or hydroxyl radicals near the surfaces of photocatalysts, such as TiO2, ZnO, SnO2 and Fe2O3.17 It has recently
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been demonstrated that graphene layers can be patterned by these photocatalytic reactions.18 In this study, we report the effects of photocatalytic reactions on the surfaces of oxide semiconductors on the properties of graphene. Based on an analysis of Raman spectra from graphene samples under different UV irradiation times, it was discovered that the presence of a ZnO photocatalyst enhanced the oxidation and decomposition of graphene. The photocatalytic reaction also resulted in changes in the electrical resistance and optical transmittance of graphene. We anticipate that an effective method to control the graphene layers by selective photocatalytic treatments will be beneficial for various graphene-based applications.
2. Experimental Procedure 2.1 Synthesis of graphene The graphene was synthesized on copper foil using well-known published methods.19 The experimental details for the synthesis of graphene layers are as follows: A high-quality monolayer of graphene with a large area was grown on copper foil using chemical vapor deposition (CVD). The copper foil was loaded into a tube quartz furnace and heated to 1000°C in an argon atmosphere at 50 mTorr with H2 flowing at a rate of 3 sccm (standard cubic centimeters per minute). Next, the copper foil was annealed for 20 min to increase the average grain size. After the heating was completed, the system was stabilized at 1200 mTorr for 30 min (at 1000°C), and methane and H2 were introduced at flow rates of 30 and 3 sccm, respectively. The
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generate oxygen radicals to interact with graphene.15,16 Another useful strategy for oxygen
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2.2 Transfer of graphene onto substrates
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The graphene layers grown on the copper foil were transferred onto arbitrary substrates using the polymethylmethacrylate (PMMA) transfer method.20 A PMMA solution (SigmaAldrich) (MW approximately 996000, dissolved in chlorobenzene at a concentration of 0.042 g/mL) was spin-coated on the graphene layers at 4200 RPM for 30 sec. Oxygen plasma etching (reactive ion etching: RIE) was performed at 100 W for 5 sec under 100 mTorr to remove the graphene layer on the opposite side of the PMMA-graphene layer. The PMMA-assisted graphene layers were released from the copper foil by etching in an aqueous solution of ammonium persulfate (Sigma-Aldrich; 0.014 g/mL). After etching for several hours, the PMMA-graphene layers were moved into a deionized water bath and rinsed. The PMMA-graphene layers were then transferred onto a 1-mm-thick quartz substrate. Finally, the PMMA was rinsed with acetone for approximately 40 min and then air-dried. The optical properties of the graphene layers were analyzed using Raman spectroscopy (JobinYvon LabRam HR, Horiba, Japan). The Raman spectra were measured in a backscattering geometry under ambient atmosphere at room temperature using a 100X objective lens and a 600line/mm grating. The laser excitation wavelength was 632.8 nm and the spot size was approximately 1 µm. X-ray photoelectron spectroscopy (XPS) (K-alpha, Thermo Scientific, USA) spectra were measured using an Al K-alpha radiation system. The average base pressure was approximately 2.2 × 10-7 mbar at 300 K. The spot size of the X-ray was 100 µm, and the instrument resolution was approximately 0.1 eV. The X-ray source was generated from an Al target (1486.8 eV) with a pass energy of 50 eV to analyze the scan. The binding energies were
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furnace was then rapidly cooled to room temperature under H2 at 50 mTorr.
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Vis/NIR spectrophotometer (V-670, JASCO, Japan). The sheet resistance measurements were performed at room temperature using the four-point probe method (FPP-RS8, Dasol Eng.,
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Korea).
2.3 Photocatalytic reaction of graphene on ZnO substrate under UV irradiation Figure 1(a) shows a schematic of the experimental procedures for the photocatalysis of graphene using ZnO. The polished Zn face of a single crystal ZnO sample was carefully placed on the graphene (which had been transferred onto the quartz substrate). The sample was mounted on an apparatus (Fig. 1(b)) with a hole at the center for UV irradiation. Note that the polarity of ZnO does not significantly alter the photocatalytic reaction with graphene. To ensure uniform exposure of the UV light on the interface between the graphene and the ZnO, the light was irradiated through the quartz substrate in air with an intensity of approximately 60 mW/cm2.
3. Results and discussion
Figure 2 shows the Raman spectra of graphene on the quartz substrate as a function of the duration of UV irradiation in ambient atmosphere. The D, D’, G and 2D bands are assigned in Figure 2.21 From the Raman spectra of pristine graphene (bottom, black line), two pronounced peaks, the G band at 1585 cm-1 and the 2D band at 2696 cm-1, were observed. The D band at 1354 cm-1 appeared after increasing the duration of UV irradiation independent with the presence of ZnO. This finding indicates that the graphene may have been damaged by ozone that was generated due to the reaction between UV light and water molecules in the air.
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referenced to the C1s line at 284.8 eV. The transmission spectra were measured using a UV-
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spectrum in Figure 2 were compared with Figure 3. Figure 3a shows the intensity ratio of the D/G band with respect to the UV exposure time. It is clear that the defect density of graphene
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with the presence of ZnO is higher than that without the presence of ZnO which is generated by ozone. The reason could be due to photocatalytic chemical reactions occur at the interface between ZnO and graphene. Electrons and holes are generated in the conduction and valance bands via the irradiation of light with higher energy than the intrinsic bandgap of ZnO (approximately 3.37 eV) (eq.(1)). The photoinduced electrons and holes react with moisture and oxygen molecules in the air, producing highly reactive species, such as ·OH, O2·- and H2O2 by chemical reactions in eq. (2~5).23,24 The chemical species generated at the surface of ZnO most likely lead to the oxidation and decomposition of graphene.
ℎ + ℎ (1) ℎ + → · + , ℎ + → · (2) + 2 → , + → · (3) + ℎ → 2 · (4) + → · + (5)
Chemical doping in graphene typically occurs during this reaction. Recently, Lee et al. reported that the effects of mechanical strain and chemical doping on graphene could be separated using the correlation between the G and 2D modes.22 To confirm the doping effect during UV irradiation and to compare the properties in the presence of ZnO, the position of the 2D band with respect to the position of the G band is shown in Figure 3b. Based on the optical separation,
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To investigate the photocatalytic effect on the properties of graphene, the features of the Raman
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increasing the irradiation time to 16 h, the plotted positions of the 2D and G frequencies became parallel to the vector for hole doping. However, the change in the direction of the strain effect
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was minimal, and the phenomenon was independent of the presence of ZnO. The hole density was estimated to be approximately 1.5 ×1013 cm-2. It has been reported that ozone molecules generated by UV light produce oxygen-containing groups on graphene, which lead to the hole doping of the graphene.25 It is plausible that photoinduced reactive molecules could form similar oxygen-containing groups on the graphene.
Figure 4 shows the sheet resistance and optical transmittance of graphene by UV irradiation time. The sheet resistance of graphene transferred onto the quartz substrate was approximately 600 Ω/sq. The resistance gradually increased as the irradiation time increased. However, there were significant differences in sheet resistance based on the presence or absence of ZnO. After irradiation for 12 h, the sheet resistance of graphene on ZnO was approximately 20 times higher than that on the bare quartz substrate, indicating that the photocatalytic reactions on the surface of the ZnO accelerated the oxidization and decomposition of graphene. This result is consistent with the fast evolution of the D peaks and the disappearance of the carbon-related peaks as a function of the irradiation time, as shown in Figure 2. The transmittance of graphene on ZnO also increased from 93.7% to 99.9% at 550 nm as the irradiation time increased, as shown in Figure 4b. After approximately 5 h of irradiation, the transmittance became larger than the typical value (97.7%) of monolayer graphene.3 This behavior likely occurs because the area of the substrate that was covered by graphene decreased due to partial decomposition. The photo inset in Figure 4b shows the graphene on a quartz substrate after the photocatalytic reaction had
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it was found that UV irradiation for 2 h caused slight compressive strain and hole doping. By
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area covered by ZnO. In contrast, the transmittance of the graphene surrounding the area remained unchanged. These results indicate that UV irradiation mainly contributed to hole
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doping but did not cause notable changes in transmittance.
XPS measurements were performed to identify the functional groups generated during the photocatalytic reaction. Figure 5 shows the main C1s peak (due to the sp2 carbon-carbon bonds) at approximately 284.7 eV from both pristine graphene and after 8 h of the photocatalytic reaction. After the UV irradiation, the full width of the C1s peak increased from 1.3 to 2.1 eV. The intensity of the C-O peak (centered at 286.6 eV), which is associated with C-OH and C-O-C functional groups, was significantly larger. In addition, other oxidation-related peaks due to C=O and O-C=O functional groups appeared at 288.4 and 289.7 eV, respectively. The amount of oxygen (carbon) at the surface of substrate was gradually increased (decreased) as the UV irradiation time increased (the inset of Figure 5b). These results prove that the reactive chemical components generated by the photocatalytic reaction, such as ·OH, O2·- and H2O2, contributed to the oxidation of graphene. In conclusion, we studied the photocatalytic effects of the structural and optical properties of graphene using ZnO. The highly reactive species on the surface of the ZnO photocatalyst accelerated the decomposition of graphene via oxidation compared with UV irradiation. This technique allowed us to control the doping concentration and to selectively remove graphene. In conjunction with the micro/nanotechnology that is available for oxide semiconductor
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proceeded for 24 h. The transparent centimeter-scale trapezoid shape on the substrate was the
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of the properties of graphene for electronic and optoelectronic applications.
Acknowledgments This work was supported by the Korea Institute of Science and Technology (KIST) Institutional Program and by the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) of Republic of Korea (Grant CAP-13-2-ETRI).
References 1
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V.
Dubonos and A. A. Firsov, Nature, 2005, 438, 197-200. 2
C. Lee, X. D. Wei, J. W. Kysar and J. Hone, Science, 2008, 321, 385-388.
3
R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R.
Peres and A. K. Geim, Science, 2008, 320, 1308-1308. 4
Y. Wang, Z. Q. Shi, Y. Huang, Y. F. Ma, C. Y. Wang, M. M. Chen and Y. S. Chen, J. Phys.
Chem. C, 2009, 113, 13103-13107. 5
J. S. Bunch, S. S. Verbridge, J. S. Alden, A. M. van der Zande, J. M. Parpia, H. G. Craighead
and P. L. McEuen, Nano Lett., 2008, 8, 2458-2462. 6
S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R.
D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282-286. 7
M. J. Allen, V. C. Tung and R. B. Kaner, Chem. Rev., 2010, 110, 132-145.
10
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photocatalysts, we believe that photocatalytic reactions will be very useful for the modification
Page 11 of 19
Physical Chemistry Chemical Physics View Article Online
8
M. Y. Han, B. Ozyilmaz, Y. B. Zhang and P. Kim, Phys. Rev. Lett., 2007, 98.
9
J. W. Bai, X. Zhong, S. Jiang, Y. Huang and X. F. Duan, Nat. Nanotechnol., 2010, 5, 190-194.
10
Y. Wang, Y. Y. Shao, D. W. Matson, J. H. Li and Y. H. Lin, ACS Nano, 2010, 4, 1790-1798.
11
L. S. Panchokarla, K. S. Subrahmanyam, S. K. Saha, A. Govindaraj, H. R. Krishnamurthy, U.
V. Waghmare and C. N. R. Rao, Adv. Mater., 2009, 21, 4726-4730. 12
J. T. Robinson, J. S. Burgess, C. E. Junkermeier, S. C. Badescu, T. L. Reinecke, F. K. Perkins,
M. K. Zalalutdniov, J. W. Baldwin, J. C. Culbertson, P. E. Sheehan and E. S. Snow, Nano Lett., 2010, 10, 3001-3005. 13
M. Ishigami, J. H. Chen, W. G. Cullen, M. S. Fuhrer and E. D. Williams, Nano Lett., 2007, 7,
1643-1648. 14
A. Pirkle, J. Chan, A. Venugopal, D. Hinojos, C. W. Magnuson, S. McDonnell, L. Colombo, E.
M. Vogel, R. S. Ruoff and R. M. Wallace, Appl. Phys. Lett., 2011, 99. 15
T. Gokus, R. R. Nair, A. Bonetti, M. Bohmler, A. Lombardo, K. S. Novoselov, A. K. Geim, A.
C. Ferrari and A. Hartschuh, ACS Nano, 2009, 3, 3963-3968. 16
S. Huh, J. Park, Y. S. Kim, K. S. Kim, B. H. Hong and J. M. Nam, ACS Nano, 2011, 5, 9799-
9806. 17
A. L. Linsebigler, G. Q. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735-758.
18
G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2, 1487-1491.
19
S. Bae, H. Kim, Y. Lee, X. F. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y.
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DOI: 10.1039/C5CP01670C
Physical Chemistry Chemical Physics
Page 12 of 19 View Article Online
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Nanotechnol., 2010, 5, 574-578. 20
X. S. Li, Y. W. Zhu, W. W. Cai, M. Borysiak, B. Y. Han, D. Chen, R. D. Piner, L. Colombo
Published on 12 May 2015. Downloaded by Freie Universitaet Berlin on 14/05/2015 09:01:09.
and R. S. Ruoff, Nano Lett., 2009, 9, 4359-4363. 21
A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D.
Jiang, K. S. Novoselov, S. Roth and A. K. Geim, Phys. Rev. Lett., 2006, 97. 22
J. E. Lee, G. Ahn, J. Shim, Y. S. Lee and S. Ryu, Nat. Commun., 2012, 3.
23
M. Fujihira, Y. Satoh and T. Osa, Nature, 1981, 293, 206-208.
24
M. A. Behnajady, N. Modirshahia and R. Hamzavi, J. Hazard. Mater., 2006, 133, 226-232.
25
S. Huh, J. Park, Y. S. Kim, K. S. Kim, B. H. Hong and J. M. Nam, ACS Nano, 2011, 5, 9799-
9806.
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I. Song, Y. J. Kim, K. S. Kim, B. Ozyilmaz, J. H. Ahn, B. H. Hong and S. Iijima, Nat.
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Figure 1. (a) Schematic illustration for the experiment to determine the photocatalytic reaction
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between graphene and ZnO and (b) a photo image of sample mounting on a jig for the experiment.
Figure 2. Raman spectra of graphene as a function of UV irradiation time: (a) with and (b) without ZnO.
Figure 3. Correlation between the G and 2D bands of graphene (Fig. 2) as a function of UV irradiation time: (a) Intensity ratio of the G and 2D bands and (b) comparison of the 2D and G peak frequencies with (squares) and without (circles) ZnO. eT and eH are the unit vectors for the tensile strain and hole doping effects with slopes of 2.2 (blue line) and 0.7 (red line), respectively.22
Figure 4. Electrical and optical properties of graphene after the photocatalytic reactions. (a) Sheet resistance and (b) transmittance of graphene measured at 550 nm after the photocatalytic reaction. The inset shows patterned graphene on a 1-inch-diameter quartz substrate after a 24-h exposure to the photocatalytic reaction.
Figure 5. The effect of the photocatalytic reaction on the formation of functional groups. XPS spectra of (a) pristine graphene on a quartz substrate and (b) after the photocatalytic reaction
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Figure captions
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under UV irradiation for 8 h. The inset shows the content change in carbon (black) and oxygen
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(red) as a function of UV irradiation time.
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