Synchrotron Radiation Soft X-ray Induced Reduction in Graphene


Synchrotron Radiation Soft X-ray Induced Reduction in Graphene...

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Synchrotron Radiation Soft X‑ray Induced Reduction in Graphene Oxide Characterized by Time-Resolved Photoelectron Spectroscopy Chi-Yuan Lin,†,‡ Cheng-En Cheng,†,‡ Shuai Wang,‡ Hung Wei Shiu,§ Lo Yueh Chang,∥,§ Chia-Hao Chen,§ Tsung-Wu Lin,⊥ Chen-Shiung Chang,† and Forest Shih-Sen Chien*,‡ †

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan ‡ Department of Applied Physics, Tunghai University, 181 Chung Kang Road, Sect. 3, Taichung 40704, Taiwan § National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan ∥ Department of Physics, National Tsing Hua University, 101 Kuang-Fu Road, Sect. 2, Hsinchu 30013, Taiwan ⊥ Department of Chemistry, Tunghai University, 181 Chung Kang Road, Sect. 3, Taichung 40704, Taiwan ABSTRACT: Synchrotron radiation soft X-ray was employed to reduce graphene oxide (GO) films in ultrahigh vacuum. The dissociation of oxygen-containing functional groups, and the formation of sp2 C−C bonds were revealed by time-resolved in situ X-ray photoelectron spectroscopy, demonstrating the X-ray reduction of GO. The number of C−O bonds of GO exhibited an exponential decay with exposure time. The X-ray reduction rate of GO was positively correlated with the intensity of low-energy secondary electrons excited from substrates by soft X-ray, indicating the C−O bonds were dissociated by secondary electrons.

1. INTRODUCTION Graphene oxide (GO) prepared by chemical oxidation of graphite is a promising precursor for cost-effective and massive production of graphene-like material.1 After the dissociation of the abundant oxygen-containing functional group, the insulating GO turns into reduced graphene oxide (RGO) with a narrower energy band gap (e.g., decrease from 2.4 to 0.9 eV)2 and lower resistance (e.g., decrease from ∼4 × 1010 to ∼4 × 106 Ω/sq).3 The reduction of GO can be achieved by a great diversity of approaches, e.g., chemical,4,5 thermal,6 and optical methods,7 which enable applications of RGO in many occasions such as solar cell,8 flexible electrode,9 and field-effect transistors.10 The optical method of reducing GO (photoreduction) is strictly dependent on the wavelength range, intensity, light field distribution, and exposure time. Hence, photoreduction has the advantage of area-selective reduction,11,12 controllable reduction level, and easy implementation, becoming one of the most popular methods for producing RGO.7 Photoreduction of GO has been widely studied at various wavelengths from infrared ray to extreme-UV.13−24 The reductions usually undergo either a photothermal13−16 or photochemical17−21 process or both.22−24 Photothermal effect is mainly activated at the visible wavelength range (infrared ray to 400 nm), where the light energy is absorbed and converted to heat. Photochemical effect is mainly activated by shortwavelength illumination (400 nm to EUV), where photo© XXXX American Chemical Society

induced electron−hole pairs cause the photocatalystic reduction of GO (incorporated with TiO2 nanoparticles17 and ZnO nanoparticles18 or without19,20). X-ray radiation has been used to modify the molecular structure of self-assembled monolayers (SAMs)25−30 and induces the dissociation of molecular bonds in DNA.31,32 It is well-known that such reactions are dominated by low-energy secondary electrons excited by X-ray exposure. Hence, X-ray excited secondary electrons probably induce the variation of chemical composition of GO. The concentration of oxygencontaining functional groups strongly affects the physical properties of RGO.2,33,34 Thus, the reaction of GO to X-rays is of fundamental importance for research on the GO reduction and applications. In this article, we demonstrate the reduction of GO film by synchrotron soft X-rays. The changes of chemical composition and valence-band edge of GO with X-ray exposure were characterized in situ by time-resolved X-ray photoelectron spectroscopy (XPS) of C1s core-level spectra and valence band maximum (VBM) spectra, respectively. The number of C−O bonds of GO exhibited an exponential decay with soft X-ray exposure time, and C−C bonds were recovered, revealing the reduction of GO by soft X-rays. The reduction rate of GO was Received: December 3, 2014 Revised: May 7, 2015

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DOI: 10.1021/jp512055g J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C strongly correlated with the intensity of secondary electrons excited from substrate by soft X-ray exposure, indicating that the secondary electrons induced the dissociation of C−O bonds.

2. EXPERIMENT GO sheets were prepared by the modified Hummers method35,36 from natural graphite flakes (NGS Naturgraphit GmbH, Leinburg, Germany) and dip-coated on the conductive indium−tin oxide (ITO) glass for the purpose of grounding to avoid charge effect on GO during X-ray exposure. The samples were heated to 80 °C to remove residual water from atmosphere. The GO sheets were exposed to soft X-ray in ultrahigh vacuum at the 09A1 beamline of the National Synchrotron Radiation Research Center, Taiwan. The photon energies of soft X-ray were 580 and 380 eV. The in situ timeresolved XPS data of C1s core level were taken by a 16 channels PHI hemispherical electron energy analyzer (model 10-360) with an energy resolution of 25 mV to observe the variation of chemical composition associated with the exposure time. The acquiring time of each XPS spectrum was 1 min, and the spectra were taken every minute continuously for 30−60 min. Both X-ray reduction and XPS spectra were processed at room temperature, and the samples were not subject to any heating treatment in the ultrahigh vacuum to avoid thermal reduction. Sample current between sample and system ground was measured to read the intensity of secondary electrons excited by soft X-ray. GO sheets were exposed to various photon flux with a beam size of ∼0.04 mm2. The sample current density (SCD) of 21.4 μA/cm2 and 3.3 μA/cm2 at 580 eV photon energy and 3.85 μA/cm2 and 2.38 μA/cm2 at 380 eV were adopted. The XPS spectra were deconvoluted by commercial software “UNIFIT”.37 For the deconvolution process, the energy intervals between peaks were fixed. The energy position of C−C peak was allowed to move with an adjustable full width at half-maximum (fwhm), and the fwhm of C−O, CO, COOH, and C-defect peaks were fixed. The adventitious carbon is regarded as a constant background. The micro Raman spectroscopy (excitation wavelength, 633 nm) and I−V measurement were utilized to observe the influence of X-ray exposure to the properties of GO. For the I−V measurement, GO sheets were deposited on SiO2 (300 nm)/Si substrate, and Au metal electrodes were produced with Cu mesh mask by thermal evaporation.

Figure 1. (a) Topography and (b) friction images of the as-deposited GO film on ITO glass. Image size is 14.85 × 14.85 μm2. The inset of (a) is a cross-section curve along the dash line in the topography, showing the ∼1 nm thickness of GO.

Figure 2. (a) C1s core-level spectra of GO film on ITO substrate taken at a continuous exposure to soft X-ray irradiation after 1, 15, and 30 min, respectively, where the C−C peak is for the C1s sp2 structure, C− O peak for epoxy/hydroxyl function group, CO peak for carbonyl groups, and C-defect peak for defect formation. (b) XPS valence band spectrum collected by photon energy of 580 eV for the as-deposited GO and RGO (exposure to X-ray by 40 min). The inset is the diagram depicting the band structures of GO and RGO.

components, i.e., C−C sp2 peak at 284.85 eV with fwhm of 1.30 eV, C−O peak of epoxy and hydroxyl functional groups (EHGs) at 286.85 eV with fwhm of 0.93 eV, CO peak of carbonyl groups at 287.45 eV with fwhm of 0.96 eV, and COOH peak at 288.7 eV with fwhm of 1.62 eV, respectively.19−21,38 The COOH peak is so weak that it is ignored in this study. The peaks of oxygen-containing groups obviously decrease with exposure time, while the C−C sp2 peak increases, indicating that the EHGs and carbonyl groups are dissociated, and the sp2 carbon bonds are recovered. After 30 min exposure, the fwhm of C−C peak narrows from 1.30 to 1.05 eV, showing the chemical environment of GO becomes more homogeneous. The XPS results indicate that the GO turns into RGO after soft X-ray exposure. Under X-ray exposure, the carbon sp2 structure of graphene is unstable.39 Additionally, the energies of X-ray induced secondary electrons are mostly below 50 eV.40 Such low-energy electrons are able to induce damage in graphene, resulting in the formation of defects.41,42 The defects such as defective localized aromatic sp2

3. RESULTS AND DISCUSSION The topographic and frictional images of the GO sheets on the ITO glass show the uniformity and flatness of GO sheets (Figure 1). The GO is mostly a monolayer of 1 nm thickness. Frictional image provides a better contrast between GO and ITO, and the coverage of GO sheets on ITO is above 80%. The GO deposited on ITO substrate is a uniform and flat monolayer film. Therefore, the charge effect on GO sheets deposited on the highly conductive ITO should be small enough to be ignored during X-ray exposure. Figure 2a shows the evolution of C1s core-level spectra of GO exposed to the Xray of 580 eV photon energy with a SCD of 21.4 μA/cm2 from 1 to 30 min. The exposure time includes the acquiring time (1 min) of XPS. Therefore, the spectrum after 1 min exposure shown in Figure 2a (marked “1 min”) represents the spectrum of pristine (as-deposited) GO. The C1s core-level spectrum of the as-deposited GO was deconvoluted into four main B

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The Journal of Physical Chemistry C and C1s sp3 defect have binding energies close to 285.7−286.3 eV.21,43,44 Therefore, an additional C-defect peak component at 286.1 eV with wide fwhm of 1.5 eV was included to the C1s core-level spectrum of GO for appropriate fitting.20,21 The Cdefect peak increases with X-ray exposure. Figure 2a shows that the C1s core-level spectrum of GO was continuously red-shifted with the exposure time. The shift in Eb is −0.18 eV after 30 min exposure. Charge effect typically results in a significant blue-shift in Eb, which can be as high as ∼1 eV for graphene after X-ray exposure with 30 min.45 Herein, the red-shift by 0.18 eV is not due to the charge effect. To verify the cause of red-shift in Eb, the VBM spectra for the asdeposited GO and RGO were collected, shown in Figure 2b. EVBM denoted the difference between the valence-band edge and the Fermi energy (EF) of GO. The EVBM moves close to EF by −0.17 eV with the reduction of GO. The red-shift in EVBM is very close to the shift in Eb (−0.18 eV). The energy band of GO from C1s core-level to valence-band edge is rigid during reduction, and the red-shift in Eb is attributed to the narrowing of band gap of GO with reduction.34 The diagram presenting the shift in the energy band structure with GO reduction is displayed in the inset of Figure 2b. The reduction of GO is accompanied by the extension of sp2 carbon domain, which can be observed by Raman spectroscopy. The Raman spectra were collected on the as-deposited GO and RGO (after 30 min exposure) as shown in Figure 3a. The

well dispersed monolayer GO. The average size (La) of in-plane carbon sp2 domains is inversely proportional to the intensity ratio between D and G peaks (ID/IG) and can be written as −10

La (nm) = (2.4 × 10

⎛ ID ⎞−1 )λ ⎜ ⎟ ⎝ IG ⎠ 4

(1)

where λ is the wavelength of excitation laser. ID/IG in Figure 3a decreases from 2.26 for GO to 2 for RGO, corresponding to La extending from 17 to 19.3 nm (by 13%) after soft X-ray reduction, evaluated by eq 1. The extension of La by X-ray reduction is comparable with that by UV reduction from 17.2 to 20 nm (16%) and EUV reduction from 28.5 to 29.4 nm (3%).20,48 Figure 3b shows the I−V curve of the GO and RGO. The current increases from 0.7 nA at 5 V for GO to 13 nA for RGO; i.e., the conductivity is enhanced by a factor of 20. However, the enhancement of conductivity in our case is not as much as that of the other GO.48 The defects formation (Figure 2a) is considered to be the main factor to hinder a significant increase in conductivity of RGO by X-ray reduction. The evolution of the relative peak areas of the C−C, C−O, and CO bonds and C-defect with exposure time is shown in Figure 4a, where the errors of peak areas are included. The 46,47

Figure 3. (a) Raman spectra of the as-deposited GO and RGO (exposure to X-ray (580 eV) for 40 min). Spectra are normalized to the G band peak. (b) Conductivity of the as-deposited GO and RGO layer on SiO2/Si substrate.

Figure 4. (a) Evolution of the C1s core-level peak area for C−C peak, C−O peak, CO peak, and C-defect peak varying with exposure time under the X-ray irradiation of 580 eV with a SCD of 21.4 μA/cm2. The error bar is the uncertainty of the peak-area fitting in percentage with respect to the total area. (b) Area ratio of ln([C−O]t/[C−O]1) versus the soft X-ray exposure time for four different exposure conditions: (A) SCD = 2.38 μA/cm2 and (B) SCD = 3.85 μA/cm2 with photon energy of 380 eV; (C) SCD = 3.3 μA/cm2 and (D) SCD = 21.4 μA/ cm2 with photon energy of 580 eV.

Raman spectra were normalized with respect to the G peak. The G peak is related to the carbon sp2 structure of graphene, and D peak is related to the presence of local defects (or disorders). The D band peak of the as-deposited GO is apparently higher than G band peak, indicating that our prepared GO sheets have more defects than usual cases (where the intensity ratio of D and G peak is around 1).35 The abundant defects generated in the as-deposited GO were due to a longer reaction in H2SO4 solution added KMnO4 to produce

number of C−O and CO bonds decreases with exposure time, while the number of C−C bonds increases. After an exposure of 40 min, the number of the C−C bonds and Cdefect increases by ∼8% and ∼13% and that of the C−O and CO bonds decreases by ∼15% and ∼6%, respectively. The errors in the fitted area are relatively small. For instance, the error of C−O peak is ±0.2−0.5% of the total C1s area. Therefore, it is reliable to discuss the kinetics of X-ray reduction C

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The Journal of Physical Chemistry C Table 1. Reduction Rate k under Different Conditions of X-ray Exposure photon energy 380 eV 2

SCD (μA/cm ) k (10−4 1/s)

580 eV

(A) 2.38 2.19 ± 0.12

(B) 3.85 6.88 ± 0.39

[C−O]t = −kt [C−O]1

(D) 21.4 9.23 ± 0.31

the C−C bond. The dissociation of C−C bond by secondary electrons is found in the study of SAMs with X-ray.52 For the GO exposure to X-ray induced secondary electrons, the number of C−C bonds increases with exposure time (Figure 4a), implying that the dissociation rate of C−C bond is lower than the formation rate of C−C bond, which indicates that the cross section in breaking C−C bonds is lower than that in dissociating C−O bonds. The lower cross section in dissociating C−C bonds can be attributed to the network of sp2 hybridization. The secondary electrons can also create Joule heat. The heating is possible to induce the thermal reduction of GO.53 However, in our experiment, the maximum current density of secondary electrons is ∼20 μA/cm2. Joule heat created by such small current density is too small to significantly raise the temperature of monolayer GO. Hence, we consider that Joule heat by the secondary electrons is not the mechanism for the reduction of GO.

of GO by the area variation of C−O peak with X-ray exposure in Figure 4b in the following section. In the study of SAMs modified by X-ray,25−30 secondary electrons excited from substrates by X-ray are considered to be the dominant source for the chemical modulation of the SAMs. The direct X-ray photon interaction with ultrathin SAMs (1−4 nm) is ruled out. The thickness of the as-deposited GO (∼1 nm) is so thin that the direct interaction of GO with X-ray is too weak to be counted. We suggest the reduction of GO by Xray is attributed to the secondary electrons. In order to verify the effect of secondary electrons to the reduction, the decay of the number of C−O bonds was observed under various SCDs (2.38 μA/cm2 and 3.85 μA/cm2 for 380 eV photon energy and 3.3 μA/cm2 and 21.4 μA/cm2 for 580 eV photon energy). Figure 4b shows the decay of the normalized peak area of C−O bonds under those SCDs. The number of C−O bonds was an exponential decay with exposure time, and then the decay can be expressed by ln

(C) 3.3 5.78 ± 0.17

4. CONCLUSIONS The reduction of moderately oxidized GO by synchrotron radiation soft X-ray was studied. From in situ XPS analysis, the exposure to soft X-ray resulted in the dissociation of EHGs and carbonyl groups and the formation of sp2 C−C bonds of GO. After X-ray reduction, the sp2 domain size increased by 13%, verified by Raman spectra, and the conductivity increased by a factor of 20. The separation between the C1s core level and valence-band edge was rigid on reduction. The exponential decay of the number of the EHGs with X-ray exposure time was found by time-resolved XPS. The dependence of the X-ray reduction rate on SCD exhibiting the reduction was attributed to the secondary electrons from the substrate excited by X-ray.

(2)

where [C−O]t is the number of C−O moieties after exposure time t, [C−O]1 is the number of C−O moieties of the asdeposited GO, and k is the reaction rate constant.26,30 The reaction rate constant fitted by eq 2 is shown in Table 1. In our case, the acquiring time (1 min) of time-resolved XPS is noticeably shorter than the time for entirely reducing GO (∼25 to 50 min). Therefore, compared to the other photoreduction of GO,7 X-ray reduction has the advantage of simultaneous monitoring the evolution of GO reduction by time-resolved XPS and promptly controls the reduction level of RGO. The reduction rate constantly increases with SCDs; i.e., the reduction rate is highly dependent on the secondary electrons. Note that the X-ray flux of 380 eV to generate 3.85 μA/cm2 SCD is similar to that of 580 eV to generate 21.4 μA/cm2. The SCD generated by 580 eV photons is significantly larger, because the cross section of 580 eV X-ray to oxygen atoms in ITO substrate is higher than that of 380 eV. This is a substantial evidence that the secondary electrons dominate the reduction, rather than the direct X-ray interaction with GO. The possible mechanism of the GO reduction by soft X-ray could be the direct interaction of secondary electrons themselves by dissociative electron attachment (DEA).49 The DEA is the key phenomenon in the fragmentation of DNA by ionizing radiation.50 The main secondary electrons have a broad-band energy under 50 eV.40 The low-energy electrons can activate the ionization and DEA to induce the dissociation of C−O bonds.50 This is the way that EHGs and carbonyl groups are dissociated by the secondary electrons. The averaged cross section σ ≈ 1.96 × 10−17 cm2 for dissociation of C−O bonds by secondary electrons under X-ray exposure at 380 and 580 eV was obtained by the relationship of k = σF, where σ is the cross section and F is the secondary electron flux (1 μA/cm2 SCD corresponds to 6.25 × 1012 cm−2 s−1 electron flux).51 DEA can dissociate not only the C−O bond but also



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Ministry of Science and Technology, Taiwan, under Grant MOST 102-2112-M-029-005-MY3.



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