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Selective Reduction Mechanism of Graphene Oxide Driven by the Photon Mode versus the Thermal Mode Masaki Hada,*,†,‡ Kiyoshi Miyata,§ Satoshi Ohmura,⊥ Yusuke Arashida,∥,○ Kohei Ichiyanagi,# Ikufumi Katayama,∥ Takayuki Suzuki,∥ Wang Chen,¶ Shota Mizote,‡ Takayoshi Sawa,‡ Takayoshi Yokoya,‡,□ Toshio Seki,△ Jiro Matsuo,⬡ Tomoharu Tokunaga,■ Chihiro Itoh,▲ Kenji Tsuruta,‡ Ryo Fukaya,# Shunsuke Nozawa,# Shin-ichi Adachi,# Jun Takeda,∥ Ken Onda,*,§ Shin-ya Koshihara,⬢ Yasuhiko Hayashi,‡ and Yuta Nishina*,‡,¶ †
Tsukuba Research Center for Interdisciplinary Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan ‡ Graduate School of Natural Science and Technology, ¶Research Core for Interdisciplinary Sciences, and □Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan § Faculty of Science, Kyushu University, Fukuoka 819-0395, Japan ⊥ Faculty of Engineering, Hiroshima Institute of Technology, Hiroshima 731-5193, Japan ∥ Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan # High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan △ Department of Nuclear Engineering and ⬡Quantum Science and Engineering Center, Kyoto University, Uji 611-0011, Japan ■ Graduate School of Engineering, Nagoya University, Nagoya 464-0814, Japan ▲ Faculty of System Engineering, Wakayama University, Wakayama 640-8510, Japan ⬢ School of Science, Tokyo Institute of Technology, Tokyo 152-8551, Japan S Supporting Information *
ABSTRACT: A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems. KEYWORDS: graphene oxide, structural dynamics, time-resolved electron diffraction, time-resolved spectroscopy, time-dependent density functional theory
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dditional functions are conferred to a representative two-dimensional material, graphene, to introduce oxygen functional groups on its edges or basal planes. © XXXX American Chemical Society
Received: April 20, 2019 Accepted: August 20, 2019
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DOI: 10.1021/acsnano.9b03060 ACS Nano XXXX, XXX, XXX−XXX
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Cite This: ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano This material is called graphene oxide (GO),1−4 which has been used in a variety of applications in the fields of electronics, chemistry, biology, and pharmaceuticals.5−10 Notably, GO itself is chemically unstable, and therefore, its more stable reduced form (reduced GO, rGO) has been widely used in various applications. Thus, the structures of GO and rGO and the reduction processes of GO to rGO are quite important; however, these processes have not been completely elucidated from a structural perspective.11,12 Reduction, that is, the partial removal of the oxygenated functional groups, is achieved by exposing GO to reducing agents, high temperature, or ultraviolet (UV) light. The tentatively proposed mechanism is that photoenergy absorbed by GO is converted into heat that triggers thermal reduction, and the mechanisms of reduction mediated by photoexcitation and heating are expected to be identical.13,14 Thus, the products (rGO) reduced via photoexcitation and via heating have been considered to have substantially identical structures. In the present study, we compared the structures of rGO prepared using the two distinct processes and observed significant differences between the reduction processes performed using photoexcitation and heating by employing ultrafast timeresolved electron diffraction, time-resolved mid-infrared (midIR) vibrational spectroscopy, and time-dependent density functional theory (TDDFT) calculations. Epoxy groups (C− O−C) are selectively dissociated by photoexcitation through the antibonding character of the photoexcited state; in contrast, bonds with weaker bond energies, such as hydroxyl (C−OH) and epoxy groups, are dissociated by heating in a temperature-dependent manner. Selective dissociation of the epoxy group can generate graphene hydroxide, which exhibits a positive temperature coefficient of resistance,15 a large electron−acoustic phonon coupling constant,16 and high supercapacitor performance.17 Graphene epoxide has also been applied in light emission and optoelectronic devices.18 Previous studies have mainly revealed the ultrafast carrier dynamics of GO under photoexcitation using ultrafast UV/ visible/near-IR transient absorption measurements.19−23 Three steps on the time scales of 400 K, respectively. The length of C−C bonds in the basal plane of C
DOI: 10.1021/acsnano.9b03060 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano
Figure 4. XPS spectra around C 1s (a) and O 1s (b) of GO and rGO after photoexcitation or heating. In the spectra around C 1s, the binding energy of C−C bonds, “C”−O bonds, and OC−O bonds corresponds to 284.5, 287, and 288 eV, respectively. In the spectra around O 1s, the binding energy of OC bonds and “O”−C bonds corresponds to 531 and 532.5 eV, respectively. The XPS spectra are normalized to the intensity of the C−C peak.
of the surface of GO. Moreover, the overshrinking effects observed in thermally treated GO were ascribed to both the removal of oxygen atoms from epoxy groups and the creation of point defects in its basal plane. The point defects change the three adjacent six-membered rings to a five-membered ring and nine-membered ring. In general, the average C−C bond length in the aromatic five-membered ring is smaller than that in the aromatic six-membered ring (e.g., fullerene). Point defect formation was confirmed in the mass spectra of GO recorded under thermal reduction conditions, which revealed the emission of CO and CO2 at temperatures >420 K (Figure S14). The relative oxygen content of GO was estimated to be 45% (i.e., the number of oxygen atoms is equal to 45% of the number of carbon atoms) based on the corresponding C 1s and O 1s XPS spectra (Figure 4a,b), which featured peaks at 284.5 (“C”−C), 287 (“C”−O), 288 (O“C”−O), 531 (“O”C), and 532.5 (“O”−C) eV. To avoid confusion, we define “C”−O and “O”−C bonds as the carbon atom bound to oxygen atom and oxygen atom bound to carbon atom, respectively. The C 1s XPS spectrum enables an estimation of the relative content of O atoms in carbonyl or carboxyl groups, obtaining values as low as ∼3.5%. As shown in Figure 4a,b, upon irradiation with UV light at a fluence of 5 mJ/cm2, the integral intensity of the “C”−O peak decreased by 12%, and the integral intensity of the “O”−C peak decreased by 6%. An
thermally treated GO was estimated to be less than 1.415 Å, which is less than that in graphene, and the lattice shrinkage observed for photoirradiated and thermally treated GO was attributed to reduction. We performed B3LYP/6-31G** level DFT calculations32 based on Gaussian09 and force field calculations (molecular mechanics program 2, MM2)33 on model graphene and GO structures to further investigate the origin of the GO lattice enlargement (Figures S15−S18). Because the carbonyl and carboxyl groups only exist at the edge of graphene, hydroxyl and epoxy groups were assumed to be exclusively present in the unit structure of GO in our model. As shown in Figure 3c, the average C−C bond length of the model structure increased as the number of epoxy groups increased, whereas it remained approximately constant as the hydroxyl group content increased. The observation of the longer sp3−sp3 C−C bonds of epoxy groups compared to those in the conjugated C−C bonds of graphene suggests that the lattice shrinkage of GO detected upon photoexcitation and temperature treatment corresponds to the deoxygenation of these groups in the basal plane of GO. The results of DFT and MM2 calculations were consistent with these findings. Based on the extent of the decrease in the C−C bond length (0.014 Å) of GO upon photoexcitation, two possible phenomena were proposed from Figure 3c: 6% of epoxy oxygen atoms were removed from GO upon UV photoexcitation, or point defects are created on 3% D
DOI: 10.1021/acsnano.9b03060 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano explanation of the results is the removal of the oxygen atoms from the epoxy group in GO because an oxygen atom in an epoxy group is bound to two carbon atoms and the removal of one oxygen atom in epoxy doubly influences on the “C”−O peak. Thus, the number of oxygen atoms in epoxy groups present in GO decreased by 6% following UV photoexcitation, consistent with the number of oxygen atoms removed from epoxy groups shown in Figure 3c. Notably, the emission of carbon atoms occurs at a negligibly low level, and we postulate that oxygen atoms are selectively removed. Therefore, all oxygen atoms removed under UV photoexcitation originated from epoxy groups. On the other hand, the XPS spectrum of thermally treated GO indicated a much more significant loss of oxygen atoms (31%), reflecting that oxygen atoms were lost from both the hydroxyl and epoxy groups under these conditions and confirming that thermal-treatment-induced reduction differed from photoexcitation-induced reduction. The bond dissociation energies of the C−O bonds in hydroxyl and epoxy groups are 1.5 eV (per OH) and 3.1 eV (per O), respectively.34−36 The single bond dissociation energies of hydroxyl and epoxy groups are 1.5 and 1.55 eV, respectively; therefore, during the thermally driven reduction, the weaker hydroxyl and epoxy groups are dissociated, and the oxygen atoms in hydroxyl and epoxy groups are removed. The mid-IR spectra obtained for GO and rGO support the hypothesis that the hydroxyl groups are removed by heating (Figure S11). However, the phenomena observed during heating might be more complicated than the simple removal of oxygen atoms; moreover, the removal of epoxy groups was not observed in the static mid-IR spectra before and after heating. Structural and Carrier Dynamics during the Photoinduced Reduction of GO. We performed ultrafast timeresolved electron diffraction experiments, which provide direct structural information,37−43 following excitation with linearly polarized UV light (λ = 266 nm) at a fluence of 2 mJ/cm2 to elucidate the dynamic changes in the GO structure during the photoinduced reduction mechanism. Figure 5a shows the evolution of the average C−C bond length in GO induced by photoexcitation with UV light as a function of time. The average C−C bond length in GO was obtained from the Q values of the (101̅0) and (112̅0) planes. The removal of the oxygen functional group from the basal plane of GO is not a reversible reaction. However, as the epoxy oxygen is connected to carbon atoms via two bonds, the reduction of epoxy groups requires the simultaneous or successive dissociation of two C− O bonds. This reduction process selectively occurred when GO was subjected to photoexcitation with a fluence higher than 5 mJ/cm2. As shown in Figure 5b, only one bond of an epoxy oxygen was broken by excitation with UV light at a lower fluence, which is a reversible reaction and is detectable with time-resolved measurements in the repetitive scheme. The pump−probe experiments utilize repetitive sample excitation to enhance the signal; thus, the sample in a photoexcited state relaxes back to the initial state before the sample is irradiated by the next pulse in the repetitive scheme. The photoinduced bond breaking and subsequent C−C bond shrinkage, namely, the processes of photoinduced reduction, occurred on a time scale of 40 ± 8 ps, which is not consistent with the time constants of carrier dynamics.19−23 We also performed time-resolved mid-IR vibrational spectroscopy to obtain a better understanding of the structural and carrier dynamics of GO under photoexcitation. Three regions, that is, 1050−1150 (epoxy groups), 1150−1350 (hydroxyl
Figure 5. Time-resolved electron diffraction. (a) Time evolution of C−C bond lengths of GO under excitation with UV light. The decay constant is 40 ps, as shown in the inset in the figure. (b) Schematic illustrating the repetitive reaction of oxygen bonds.
groups), and 1800−2200 cm−1 (carrier region), were selected for measurement. The peak assignments of the mid-IR spectra are shown in Figure S10 and Table S1. The time-dependent spectra around the epoxy group region (Figure 6a) exhibit bleaching peaks at wavenumbers of 1060 and 1100 cm−1. The intensities of the bleaching peaks (with background subtraction) at 1060 and 1100 cm−1 decreased at time scales of 53 ± 3 and 42 ± 5 ps, respectively (Figure 6b). Based on this result, the epoxy groups of GO were broken by excitation with UV light at 40−50 ps, consistent with the result obtained using ultrafast time-resolved electron diffraction. The spectra at wavenumbers of 1800−2200 cm−1 (Figure 6c) reflect the carrier dynamics as this region does not have any vibrational peaks. The transient absorption in this region increases because photoinduced carriers absorb mid-IR light. The time evolution of the intensity (Figure 6d) of the peak at a wavenumber of 2000 cm−1 showed three decay steps with time constants of
