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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 21363
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Preferential oxidation-induced etching of zigzag edges in nanographene Jun-ichi Takashiro,a Yasuhiko Kudo,a Si-Jia Hao,a Kazuyuki Takai,†a Don N. Futaba,b Toshiaki Enoki*a and Manabu Kiguchi*a We investigated the thermal oxidation process of nanographene using activated carbon fibers (ACFs) by thermogravimetry (TG), X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and electrical conductance measurements. The oxidation process started from the edge of nanographene with the formation of phenol (–OH) or ether (C–O–C) groups attached to edge carbon atoms, as verified by the XPS and NEXAFS results. While the TG results indicated a decrease in the size of the nanographene sheet during the oxidation process, the intensity of the edge-state peak, i.e., the signature of the zigzag edge, decreased in the C K-edge NEXAFS spectra. This suggests that the zigzag edge preferentially reacted with oxygen and that the nanographene terminated with the
Received 18th June 2014, Accepted 14th August 2014 DOI: 10.1039/c4cp02678k
thermodynamically unstable zigzag edges converted to one terminated with stable armchair edges. As the oxidation temperature increased, the activation energy for the electron hopping transport governed by the Coulomb gap variable range hopping between the nanographene sheets increased, and the tunneling barrier decreased. This change can be understood on the basis of the decrease in the size of the nanographene sheets together with the preferential etching of nanographene edges and the
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decrease in the inter-nanographene-sheet distance.
1. Introduction Graphene, a single layer of carbon atoms, has attracted great attention owing to its unconventional electronic properties, such as extremely high carrier mobility at room temperature and the anomalous quantum Hall effect, which originate from the feature of the massless Dirac fermion of the electrons moving on the two-dimensional hexagonal bipartite lattice of graphene.1 Recently, the oxidation of graphene has been of particular interest because the band gap is opened by oxidation, which makes it possible to utilize graphene for high-performance transistor devices,2,3 and because graphene oxidation is a promising technique to prepare various types of graphene, including nano-hole and anti-dot graphene.4–8 Oxidation and subsequent reduction are prevalent techniques for fabricating graphene of a large size.4 In practical applications of graphene and graphite, such as batteries, catalysis, and gas absorbers,5 graphene is handled in the ambient atmosphere, so that the edges of graphene are seriously oxidized a
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan. E-mail: [email protected], [email protected] b Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan † Present address: Department of Chemical Science and Technology, Faculty of Bioscience and Applied Chemistry, Hosei University, 3–7–2 Kajino-chou, Koganei, Tokyo 184–8584, Japan.
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with the majority of the edge carbon atoms bonded to oxygencontaining functional groups. Consequently, a detailed understanding of the oxidation process is of particular importance. The oxidation process starts from the edge of graphene9–11 and proceeds into the interior, as the basal plane is more stable and oxidation resistant.12 There are two fundamental edges in graphene—zigzag and armchair edges13–29—which differ in electronic structure and reactivity. Seitsonen et al. predicted that the graphene nanoribbon in an oxygen-rich atmosphere is preferentially formed along the armchair direction at thermodynamic equilibrium.25 Sendt et al. estimated the activation energy for oxygen desorption to be 435 kJ mol1 at the armchair edges, which is larger than that at zigzag edges (416 kJ mol1), indicating the higher stability of the armchair edges.10,11 In spite of these important theoretical predictions, there have been few experimental studies on the reactivity of graphene edges, owing to the difficulties of preparing an appropriate system and designing a method to clarify this issue experimentally.29 In order to overcome these difficulties, we focused on nanographene-based activated carbon fibers (ACFs) as model systems to study the oxidation process of graphene, using nearedge X-ray absorption fine structure (NEXAFS) as a technique to study the edge geometry of graphene. ACFs comprise a threedimensional disordered network of nanographite domains, each of which is a stack of 3–4 nanographene sheets with a mean in-plane size of 3 nm.15 Each nanographene sheet comprises
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approximately 350 carbon atoms, among which the number of carbon atoms at the edge is estimated to be 90, yielding a large contribution of edge carbon atoms in the individual nanographene sheet in the ACFs along with a high ratio of the edge carbon atoms to the interior carbon atoms (B90/350). The inter-sheet distance between nanographene sheets in ACFs (0.38 nm) is considerably larger than that in bulk graphite (0.335 nm), such that the interaction between nanographene sheets is sufficiently weak to be neglected in studying the properties of nanographene. Thus, we can study the properties of nanographene using ACFs. The small nanographene size and large inter-sheet distance allow us to obtain detailed information about the oxidation process occurring at the edges of the nanographene sheets. NEXAFS, which we employed as a powerful investigation technique to study the oxidation of the edge carbon atoms, is based on the excitation of an electron from a core orbital to an unoccupied electronic state. Localized edge states are created around the Fermi level along the zigzag edge, whereas the armchair edge has no such localized state around the Fermi level.13–16 NEXAFS provides information about the density of the edge state originating from the zigzag edges; that is, NEXAFS selectively detects the zigzag edges. Previous studies using NEXAFS have demonstrated the presence of the edge state at zigzag edges in ACFs as a shoulder located on the low-energy side of the p*-state peak.29–31 In the present study, we investigated the oxidation process of nanographene edges with respect to the oxidation temperature using NEXAFS together with thermogravimetry (TG), electrical conductance measurements, and X-ray photoemission spectroscopy (XPS).
2. Experimental Commercially available ACFs (specific surface areas of 2000 m2 g1, FR-20, Kuraray Chemical Co.) were used in the present study, which were prepared by the activation of well-graphitized phenol-based precursor materials. For all the measurements, before the measurement, the ACF sample was heated to 1300 K in an ultrahigh vacuum for 15 min in order to remove the molecular species physisorbed into the nanopores, such as water, hydrocarbon molecules, and oxygen-containing functional groups that have already bonded to the edge carbon atoms. TG measurement was performed at a heating rate of 5 K min1 in a flow of a gas mixture (N2: 199.0 ml min1 + air: 1.0 ml min1) in the temperature range of 300–1100 K, where the partial oxygen gas pressure was adjusted to 100 Pa. The electrical conductance was measured using a 2-probe method, where a bundle of ACF fibers mounted on a sample holder was heat-treated by self-heating in an electric current flowing at various temperatures (300–1400 K) in an oxygen atmosphere. The oxygen pressure was maintained at 1 Pa to avoid the effect of charge transfer to the adsorbed oxygen species26,27 as will be explained later, except the oxygen pressure dependence of the conductance, in which the oxygen pressure was varied to 100 Pa. XPS and NEXAFS measurements were performed in the measurement chamber, which was maintained in an ultrahigh vacuum
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(UHV: B107 Pa) and connected to the electrical conductance measurement chamber by a gate valve. The XPS spectra were recorded at room temperature using non-monochromatized Mg Ka radiation (1253.6 eV) for the excitation and a hemispherical analyzer (Scienta R3000). The binding energy was calibrated using Au4f = 84.0 eV from the foil sample. The carbon (C) and oxygen (O) K-edge NEXAFS spectra were recorded at the soft X-ray beam line BL-7A in the Photon Factory at the Institute of Materials Structure Science. The NEXAFS spectra were obtained by measuring the sample photocurrent (total electron yield method) at room temperature. The photon energy was calibrated with respect to the C 1s to p* peak position of highly oriented pyrolytic graphite (HOPG) at 285.5 eV. The NEXAFS spectra were recorded at normal X-ray incidence. The temperature of the sample was monitored using a pyrometer in the conductance, XPS, and NEXAFS measurements. The oxygen pressure in the NEXAFS and XPS measurements was maintained at 1 and 10 Pa to avoid the effect of charge transfer to the adsorbed oxygen species and also to take into account that the oxidation effect is saturated above 10 Pa according to the conductance measurement results which will be shown later.
3. Results Fig. 1(a) shows the result of the TG measurement at temperatures up to 1100 K with a heating rate of 5 K min1 in an oxygen atmosphere (100 Pa). Weight loss started above 900 K and became 16% at the highest temperature of 1073 K, indicating that a remarkable degree of oxidation occurs above 900 K. The weight loss is associated with the oxidation of edge carbon atoms,12 which are etched away in the form of CO2 or CO.10,11 Accordingly, the oxidation process functions to reduce the size and change the shape of the individual nanographene sheet. Fig. 1(b) indicates the electrical resistances of the ACF samples (ROx) at 300 K measured after heat treatments at 900 K for 15 minutes in an oxygen atmosphere at various pressures (PO2), normalized with respect to the initial resistance before oxidation (R0). For comparison, the results of the ACFs exposed to oxygen without heating are also plotted in Fig. 1(b). The resistance changes little with the oxygen pressure below 0.01 Pa, whereas it drastically increases when the oxygen pressure is above 0.1 Pa, indicating that a remarkable degree of oxidation occurs above 0.1 Pa. Above 0.1 Pa, the resistance increase tends to be saturated. This suggests a threshold above which nanographene is subjected to the oxidation. In contrast, when the sample is not heated, the increase in the resistance of the ACFs is far smaller, suggesting that the oxidation does not occur effectively. In our previous study, it was concluded that the reduction of the resistance of ACFs upon heat treatments in a vacuum was caused by the elimination of edge functional groups as well as the fusion of nanographene sheets.31 Therefore, the opposite trend in the resistance can be understood by considering the reduction in the size and the changes in the shape of nanographene sheets, along with the creation of oxygen-containing functional groups bonded to the edges upon oxidation. The TG and electrical
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Fig. 2 Time evolution of the weight in ACFs, normalized with respect to the initial weight, at oxidation temperatures of 900 K (black circles), 1000 K (red squares), and 1100 K (blue triangles) and an oxygen pressure of 100 Pa.
Fig. 1 (a) Weight change of the ACF sample with the increase in temperature (W(T)) at an oxygen pressure of 100 Pa. Data are normalized with respect to the value at room temperature. (b) Evolution of the electric resistances of the ACF samples by exposure to oxygen at various pressures at 900 K for 15 minutes (filled circles). Data were measured at room temperature and normalized with respect to the value obtained for the clean sample at room temperature. The empty circles represent the data collected by similar procedures without heating the sample during exposure to oxygen.
resistance measurements indicated that the graphene oxidation pronouncedly occurs together with the reduction in the size and change in the shape of the nanographene sheets above 0.1 Pa and 900 K. Fig. 2 shows the time evolution of the weight of the ACFs in an oxygen atmosphere (100 Pa) at various oxidation temperatures. The weight decreases with time, indicating the progress of the oxidation reaction. The rate of the weight loss increases with the oxidation temperature, which agrees with the TG results shown in Fig. 1(a). Fig. 3 shows the conductance as a function of time in the cooling process of the sample in a vacuum immediately after the oxidation is completed in the exposure of oxygen (1 Pa) for 15 min at different temperatures in the range of 900–1400 K. Here, an oxygen pressure of 1 Pa is employed, considering that the pressure is above the threshold, where the oxidation process becomes activated, and that only approximately 20% change in the resistance takes place from 1 to 100 Pa (see Fig. 1(b)). In addition, by employing 1 Pa, we can reduce the effect of charge transfer to strongly and weekly adsorbed oxygen species, the presence of which will be discussed later. Thus, we
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Fig. 3 Conductance as a function of time during the cooling process under vacuum after the oxidation is completed in the exposure of oxygen (1 Pa) for 15 min at different temperatures ( : 900 K, : 1200 K, : 1400 K). Conductance change for the sample (K) heat treated at 1300 K without exposure to oxygen is also shown. Open symbols indicate the conductance value at room temperature (300 K) obtained by extrapolating the conductance to t - N.
can extract solely the information about the oxidation process at the reaction front, excluding the side effects of the adsorption of strongly and weakly bonded oxygen atoms. The conductance decreased with time during the cooling process for all of the samples, which were treated at different oxidation temperatures. This suggests the insulating feature of the electron transport, agreeing well with previous work.15 The conductance at the start
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of cooling (t = 0 min) increases as the oxidation temperature is increased. A detailed analysis of the oxidation process was performed at 1 Pa using XPS and NEXAFS. Fig. 4(a) shows the O 1s XPS spectra of the ACFs heated at 900, 1100, and 1300 K for 15 minutes in an oxygen atmosphere of 1 Pa. At 900 K, we can see a broad peak at around 533 eV together with the shoulder at the low binding energy side. At 1100 K, the peak at 533 eV increases and weak feature appears at high binding energy side. At 1300 K, the peak at 533 eV drastically increases. It has been reported that the carbonyl group (4CQO) gives rise to the O 1s peak in the range of 530–531 eV, while the peak due to the phenol (–OH) and the epoxy (C–O–C) groups appears at 533 eV.32,33 Accordingly, the peak near 533 eV observed in the present O 1s XPS spectra is ascribed to the –OH and/or C–O–C groups. The shoulder at the low binding energy side can be attributed to the carbonyl group, which cannot be removed by the heat treatment at 1300 K in UHV. The XPS study indicates that the edge carbon atoms of the nanographenes are covered by the –OH or C–O–C groups during the oxidation process. Fig. 4(b) shows the O K-edge NEXAFS spectra of the ACFs heated
Fig. 4 (a) O 1s XPS spectra of the ACF sample exposed to oxygen at a pressure of 1 Pa for 15 minutes at 900, 1100, and 1300 K, denoted by the dotted, dashed, and solid curves, respectively. (b) O K-edge NEXAFS spectra of the ACF sample measured after exposure to oxygen at a pressure of 1 Pa for 15 minutes at 900, 1100, 1300, and 1600 K. Spectra are parallel-shifted along the vertical axis for clarity.
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at 900, 1100, 1300, and 1600 K for 15 minutes in an oxygen atmosphere of 1 Pa. While background subtraction was performed, we did not normalize these spectra with respect to the edge jump, i.e., the difference in the intensity of the signal before and after the O K-edge. The edge jump is proportional to the amount of oxygen atoms in the target sample.34 The edge jump and the peak are not clear below 1100 K, indicating the small amount of oxygen in ACFs. A peak appears near 538 eV after the heat treatment above 1300 K in an oxygen atmosphere. The intensity of this peak and the edge jump increase with the oxidation temperature, indicating the progress of the oxidation reaction. Previously, it was determined that the transitions from the O 1s state to the p*CQO, the s*C–O,O–H, and the s*CQO states occur at 531.5 eV, 536–540 eV, and 542.0 eV, respectively.35 Thus, the peak near 538 eV observed in the present NEXAFS spectra is ascribed to s*C–O,O–H. The absence of the p*CQO peak and the appearance of the s*C–O,O–H peak indicate the formation of a C–O–C and/or an –OH bond, which agrees with the XPS results. Fig. 5 provides a close-up view of the pre-edge region of the C K-edge NEXAFS spectra of the ACFs heated at 900, 1100, 1300, and 1600 K for 15 min at oxygen pressures of 1 and 10 Pa. The NEXAFS spectra are normalized with respect to the value of the edge jump at 340 eV. All of the spectra exhibit two peaks at 285.5 and 291.9 eV (not shown in the figure), which correspond to the transitions from the C 1s to the p* and s* states, respectively. According to the previous work,29 the peak at 285.5 eV involves the contribution of the edge state together with the p* state. Thus, we deconvoluted it with two Gaussian peaks at 284.5 and 285.5 eV, which correspond to the edge state and the p* state, respectively.29 The integrated intensity of the edge-state peak is approximately 6–12% of that of the p* peak for the clean ACFs, which agrees with the previously reported value within error bars.29,31 Although the change in the intensity of the edge state peak is not simple in Fig. 5, the intensity of the edge state peak decreases with the increase in the oxidation temperature in
Fig. 5 Close-up view of the C K-edge NEXAFS spectra of the ACF sample exposed to oxygen at pressures of 1 (left) and 10 Pa (right) for 15 min at (a) 300, (b) 900, (c) 1100, (d) 1300, and (e) 1600 K. The symbols, solid curves, dashed curves, and gray regions denote the experimental data, total of curve fits, p* ingredients, and edge-state ingredients, respectively.
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general (see Fig. 7a). Here, we note that the intensity of the prepeak below the bulk edge onset (edge state peak) was smaller than the previously reported study by Entani et al.36 It is because that they fabricated the nano graphene on a Pt substrate. On the metal substrate, molecular orbital of graphene can hybridize with the metal orbital, leading to formation of the metal induced gap states (MIGS).37 The MIGS peak appears at the Fermi level, which can enhance the intensity of the pre-peak below the bulk edge onset. While the intensity of the edge state peak changes with the oxidation temperature, the p* peak changes little.
4. Discussion We firstly discuss the kinetics of the oxidation process. The oxidation reaction is accompanied by the etching of the edge of the nanographene sheets, in which the edge carbon atoms are removed with the evolution of CO and CO2. Therefore, information about the weight loss due to the oxidation allows us to investigate the kinetics of the oxidation reaction. Considering that the oxidation reaction proceeds at the edges,12 which face the oxygen molecules in the gaseous state, we must discuss the relationship between the weight and the number of edge carbon atoms. Let us determine the relationship by using the following assumptions: (1) the nanographene sheets are circles, (2) the diameter of the pristine nanographene is 3 nm, and (3) the contributions of the oxygen and hydrogen atoms are negligible. Neglecting oxygen is supported by the weak peak intensities in O 1s XPS and O K-edge NEXAFS compared with those of C. In the nanographene sheet of ACFs (3 nm), the number ratio of the edge carbon atoms to the total carbon atoms is approximately 90/350.38 Consequently, even if all of the edge carbon atoms of the nanographene sheets in the ACFs are bonded to hydrogen atoms, the mass ratio of hydrogen to carbon is only 1%. This justifies neglecting hydrogen in discussing the mass of the ACFs. Under these assumptions, the mass w of the individual nanographene sheet is proportional to its area (pd2/4), where d is the diameter of the nanographene sheet. Meanwhile, the number of carbon atoms at the edges (Cedge) is proportional to the length of the circumference of the edge, which is given by pd. Consequently, we obtain the pffiffiffiffi following relationship: Cedge Ctotal ¼ pd=pðd=2Þ2 ¼ 4=d / 1= w, pffiffiffiffi that means Cedge / w. Fig. 6 shows the reaction time dependence of the number of edge carbon atoms Cedge obtained from Fig. 2 with the relationpffiffiffiffi ship Cedge / w. Because the slope of Cedge vs. t corresponds to the rate of the oxidation of nanographene, the activation energy was estimated using the following process. First, the slope (k) was obtained at 0 and 30 min for each oxidation temperature. Second, the activation energy of the oxidation was calculated using the log k 1/T plot. The activation energies were estimated to be 105(6) and 99(20) kJ mol1 at 0 and 30 min, respectively. Consequently, the activation energy decreases with the progress of the graphene oxidation, indicating that the oxidation is enhanced as it progresses. This change in the activation energy is confirmed by the convex feature in the Cedge vs. t plot in Fig. 6.
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Fig. 6 Number of edge carbon atoms as a function of reaction time at oxidation temperatures of 900 (black circles), 1000 (red squares), and 1100 K (blue triangles). Cedge is normalized with respect to that at t = 0.
In the analysis of the activated oxidation process of nanographene, we employ Bew’s model, in which the strongly and weakly bonded oxygen species play key roles.9,26,27 It should be noted that the oxygen species having strong electronegativity work to modify the electronic structure of nanographene sheets through the charge transfer from nanographene to the oxygen species.27 In this model, it is assumed that the graphene oxidation proceeds by dissociative O2 adsorption forming a strongly bonded, immobile oxygen species (eqn (1)) and then by the transformation of strongly bonded, immobile oxygen species into weakly attached and mobile oxygen species (eqn (2)), followed by the desorption of the O2 (eqn (3)) and CO (eqn (4)) molecules, where Cf, (C–O)T, and (C–O)w are active carbon sites, strongly bonded O atoms, and weakly attached O atoms, respectively. 2Cf + O2 - 2(C–O)T
(1)
(C–O)T - (C–O)W
(2)
2(C–O)W - 2Cf + O2
(3)
(C–O)W - CO
(4)
The rate of the production of CO resulting from the oxidation of the carbon is given as 1 1 k1 2 ½O2 2 k3 ; R4 ¼ 1 1 k1 2 1þ ½O2 2 ð1 þ K21 Þ k3 k4
(5)
where k1, k3, k4, and K2 are the rate constants of reactions (1), (3), and (4) and the equilibrium constant (ratio of coverage of tightly and weakly bounded oxygen atoms), respectively. K2 is given as 5.3 1021 exp(DH/RT),9 which yields K2 B 2 1021 with DH = 200 kJ mol1 and T = 1000 K; thus, K21 can be
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omitted from eqn (5). The rate constants k1 and k3 are given by 2 exp(E1/RT) and 1011 exp(E3/RT), respectively. The activation energy of each step is given as follows: E1 = 60 kJ mol1, E3 = 225–425 kJ mol1, E4 = 300 kJ mol1.9,39,40 Using these values, the rate constants of eqn (1) and (3) are given as k1 B 103 m s1 and k3 B 0.1 mol m2 s1, respectively, at 1000 K; thus, (k1/k3)1/2[O2]1/2 can be omitted from eqn (5). Finally, eqn (5) can be simplified as follows: R4 k4
1 1 1 k1 2 Ea ½O2 2 exp ½ O2 2 : k3 kT
(6)
The activation energy Ea in the production of CO is, thus, given by E4 + 1/2E4 1/2E3 = 120–220 kJ mol1.9,10 This value of Bew’s model is in moderate agreement with the activation energy of B100 kJ mol1 (see Fig. 6) experimentally obtained by the present TG measurement. The close agreement between the two values indicates the validity of the proposed model. The deviation of the experimental values of Ea from the value predicted by Bew’s model can be explained by the overestimation or underestimation of the activation energy of each process or by the omission of other possible reaction processes. It should be noted that the activation energy decreases with the progress of the oxidation process (105 and 99 kJ mol1 at 0 and 30 min, respectively) in the TG results. In the initial stage before the oxidation, a large percentage of the edge carbon atoms are terminated with hydrogen atoms (–H),31 which prevent the initial adsorption of O2 molecules onto the edge carbon atoms. Once the oxidation process proceeds, the circulating process of the edge oxidation and subsequent desorption of CO or CO2 molecules from the edges occur continuously with the decrease in activation energy, resulting in the etching of the edge of the nanographene sheets. Next, we determine how the edges are etched in the oxidation process in relation to the reduction in the size and change in the edge geometry in the nanographene sheets. Fig. 7(a) shows the oxidation temperature dependence of the number ratio of the edge carbon atoms to all the carbon atoms Cedge/Ctotal and the diameter d of the nanographene estimated according to the TG pffiffiffiffi results with the relationship Cedge Ctotal ¼ 4=d / 1= w. The diameter starts decreasing above 900 K, and the amount of the decrement is approximately 10% at 1200 K compared with that at 300 K. That is, the reduction in the diameter d causes the enhancement of the contribution of the edge carbon atoms by 10%. Given that the bond length between neighboring carbon atoms is 0.142 nm, this approximately corresponds to the removal of all edge carbons that are initially located at the periphery of the nanographene sheets. This change in the size affects the electronic structure of nanographene sheets; we can see the decrease in the intensity of the edge-state peak in the NEXAFS spectra. Fig. 7(b) summarizes the results for the intensity of the edgestate peak in NEXAFS as a function of the oxidation temperature. The NEXAFS results (Fig. 5) show that the p* peak changes little with the oxidation temperature, indicating that the spatial extent of the p network is nearly preserved during the oxidation
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Fig. 7 Oxidation temperature dependence of (a) number ratio (Cedge/ Ctotal) of edge carbon atoms to total carbon atoms (filled circles) and mean in-plane diameter of nanographene sheets (open circles), which are estimated according to thermogravimetric traces. (b) Ratio of integrated intensity of the edge-state peak to that of the p* peak in C K-edge NEXAFS spectra in the sample oxidized at the oxygen pressures of 1 (filled square) and 10 (open square) Pa. (c) Activation energy (DE: filled up triangle) and contact resistance (R0: filled down triangle) in the conductance; and (d) tunneling length (rtun) (filled diamond) and mean electron–hole distance (re–h) (open diamond) of the charging energy in the electron transport. Cedge/Ctotal and d in (a), Iedge/Ip* in (b), and rtun and re–h in (d) are normalized with respect to those at 300 K.
process under the experimental conditions. It also indicates that the intensity of the edge-state peak, which is the signature of the zigzag edges, decreases with the oxidization temperature (Fig. 7(b)). The edge-state peak intensity with respect to the p* peak intensity decreases above the oxidation temperatures of 900 and 1200 K at oxygen atmospheres of 10 and 1 Pa, respectively, in which we can neglect the contribution of the small concentration of strongly and weakly bonded oxygen species to the charge transfer process. This finding completely contradicts the expectation that the intensity of the edge-state peak in the NEXAFS spectra should increase with the oxidation temperature as the contribution of the edge carbon atoms
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(i.e., Cedge/Ctotal) increases (Fig. 7(a)). Alternatively, it indicates that we should consider the geometry of the nanographene edges. The decrease in the edge state intensity suggests that the zigzag edges preferentially react with oxygen, resulting in the conversion of the zigzag edges into armchair edges, considering that the armchair edges have no edge state in contrast to the zigzag edges. Here, it should be noted that the decrease in the edge-state peak intensity starts before the etching process becomes significant, as shown in Fig. 7(a) and (b). This indicates that the etching of even a small portion of the edge carbon atoms at the zigzag edges can convert the zigzag edges into armchair structures, as will be discussed later. The preferential oxidation at the zigzag edges can be justified by the thermodynamic stability of the armchair edges in contrast to the reactive zigzag edges, as predicted by theoretical studies.10,11,41 Seitsonen et al. predicted that a graphene nanoribbon is preferentially created along the armchair direction in an oxygen-rich atmosphere at thermodynamic equilibrium.25 Sendt et al. showed that the activation energy for the desorption of oxygen is smaller for the zigzag edge than for the armchair edge.10,11 Okada reported that the formation energy of an armchair edge is smaller by 1 eV per edge atom than that of a zigzag edge in a clean graphene edge.41 These theoretical calculation results indicate the relatively high stability of the armchair edge, which agrees with the present results. Here, we show two examples of the oxidation process of nanographene. Fig. 8(a) shows a nanographene whose edges comprise the armchair and zigzag geometries as the majority and minority, respectively. The zigzag edge part disappears upon the removal of only one benzene ring due to oxidation. Fig. 8(b) shows a nanographene whose edges exhibit the zigzag structure. The removal of the gray-colored benzene rings converts the zigzag edges of the nanographene into the armchair structures. These types of preferential oxide etching processes of zigzag edges effectively cause a decrease in the ratio of the zigzag edges to armchair edges in nanographene during
Fig. 8 Structural model of preferential oxidation etching of the zigzag edge in nanographenes (a) with its edges, which comprise a combination of armchair edges and one benzene ring, and (b) with its all edges terminated by zigzag structures. Red lines indicate the zigzag edges, and gray parts are removed by oxidation.
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oxidation. Note that etching even a small part of a zigzag edge can convert the zigzag edges into armchair edges. Finally, we discuss the electron transport properties of ACFs, which vary according to the oxidation-induced edge etching process. The electron transport in ACFs is governed by the process of Coulomb-gap variable range hopping (CGVRH) between nanographene sheets, described by the following equation for the resistance:15 RðTÞ ¼ R0 exp
DE ; kB T
r tun R0 / exp ; a
DE ¼
e2 ; (7) kreh
where R0, DE, a, and k are the inter-nanographene-sheet contact resistance, activation energy, tunneling length, and dielectric constant, respectively. The contact resistance R0 is governed by the inter-nanographene tunneling with inter-nanographene distance rtun and tunneling length, and the activation energy DE is given by the charging energy with the electron and hole separated by a distance of re–h. The conductance at room temperature, which is obtained by extrapolating the conductance to t - N in Fig. 3, gives the values of DE and Ro as shown in Fig. 7(c). Fig. 7(d) gives the results of rtun and re–h as a function of the oxidation temperature. Upon increasing the oxidation temperature, the activation energy DE increases, whereas the contact resistance R0 decreases. The inter-nanographene-sheet distance rtun at which the electron tunnels decreases by approximately 10% is comparable to the decrease in the nanographene size upon increasing the oxidation temperature from 900 to 1200 K, whereas the electron–hole distance re–h decreases by 50%. The oxidation at higher temperature creates simpler and smaller oxygen-containing functional groups.31 This shortens the internanographene-sheet distance, as observed. Accordingly, the decrease in the inter-nanographene-sheet distance can be explained by this change in the functional groups bonded to the edges. Also, the reduction in the electron–hole distance is related to the decrease in the size of the nanographene sheet, but the large reduction of 50% contradicts the decrease of 10% in the size of the nanographene sheet; this discrepancy appears to be caused by overestimation. Note that the conductance measurement was performed at an oxygen pressure of 1 Pa, whereas the TG measurement was performed at 100 Pa; therefore, we can expect a change in d larger than that in re–h. The change in the edge geometry of the nanographene from zigzag to armchair may cause the overestimation. Indeed, the zigzag-edged nanographene sheets have a large local density of states originating from the edge states around the Fermi level, whereas the armchair-edged ones tend to have a gap at the Fermi level.13–16 Consequently, the observed gap DE should involve an additional contribution of the intrinsic gaps of the armchair-edged nanographene sheets, which are created by the oxidation-induced etching process, resulting in the overestimation.
5. Conclusions The oxidation of nanographene in an oxygen atmosphere was investigated for nanographene-based carbon materials (ACFs)
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using TG, XPS, NEXAFS, and conductance measurements. Remarkable oxidation of nanographene proceeds above 900 K and at an oxygen pressure higher than 1 Pa. The oxidation reaction can be explained as a four-step process wherein the strongly and weakly bonded oxygen species play key roles. The oxidation process is enhanced as it progresses, owing to the appearance of active edge carbon atoms. The combination of the TG and NEXAFS measurements reveals the preferential oxidationinduced etching of the zigzag edge, which is chemically active, whereas the p network of nanographene is preserved during the oxidation process. The zigzag edge converts into the thermally stable armchair edge upon oxidation. The temperature dependence of the resistance provides the information about the contact resistance and activation energy for the inter-nanographene transport of the Coulomb-gap type variable range hopping. While the activation energy increases with the oxidation temperature, the contact resistance decreases with the oxidation temperature. The increase in the activation energy is explained by the increase in the charging energy due to the decrease in the nanographene size together with the gap formation in the armchair-edged nanographene sheets preferentially created by etching. The decrease in the contact resistance, which is governed by the tunnelling process between nanographene sheets, follows the trend that oxidation at higher temperatures creates simpler and smaller functional groups bonded to edge carbon atoms, resulting in the decrease in the inter-nanographenesheet distance in which the electron tunnels.
Acknowledgements The present work was performed under the approval of the Photon Factory Program Advisory Committee PF-PAC, No. 2012G004. The authors are grateful for the financial support of the Grant-in-Aid for Scientific Research, Grant No. 20001006, from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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Preferential oxidation-induced etching of zigzag edges in nanographene Jun-ichi Takashiro,a Yasuhiko Kudo,a Si-Jia Hao,a Kazuyuki Takai,†a Don N. Futaba,b Toshiaki Enoki*a and Manabu Kiguchi*a We investigated the thermal oxidation process of nanographene using activated carbon fibers (ACFs) by thermogravimetry (TG), X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and electrical conductance measurements. The oxidation process started from the edge of nanographene with the formation of phenol (–OH) or ether (C–O–C) groups attached to edge carbon atoms, as verified by the XPS and NEXAFS results. While the TG results indicated a decrease in the size of the nanographene sheet during the oxidation process, the intensity of the edge-state peak, i.e., the signature of the zigzag edge, decreased in the C K-edge NEXAFS spectra. This suggests that the zigzag edge preferentially reacted with oxygen and that the nanographene terminated with the
Received 18th June 2014, Accepted 14th August 2014 DOI: 10.1039/c4cp02678k
thermodynamically unstable zigzag edges converted to one terminated with stable armchair edges. As the oxidation temperature increased, the activation energy for the electron hopping transport governed by the Coulomb gap variable range hopping between the nanographene sheets increased, and the tunneling barrier decreased. This change can be understood on the basis of the decrease in the size of the nanographene sheets together with the preferential etching of nanographene edges and the
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decrease in the inter-nanographene-sheet distance.
1. Introduction Graphene, a single layer of carbon atoms, has attracted great attention owing to its unconventional electronic properties, such as extremely high carrier mobility at room temperature and the anomalous quantum Hall effect, which originate from the feature of the massless Dirac fermion of the electrons moving on the two-dimensional hexagonal bipartite lattice of graphene.1 Recently, the oxidation of graphene has been of particular interest because the band gap is opened by oxidation, which makes it possible to utilize graphene for high-performance transistor devices,2,3 and because graphene oxidation is a promising technique to prepare various types of graphene, including nano-hole and anti-dot graphene.4–8 Oxidation and subsequent reduction are prevalent techniques for fabricating graphene of a large size.4 In practical applications of graphene and graphite, such as batteries, catalysis, and gas absorbers,5 graphene is handled in the ambient atmosphere, so that the edges of graphene are seriously oxidized a
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan. E-mail: [email protected], [email protected] b Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan † Present address: Department of Chemical Science and Technology, Faculty of Bioscience and Applied Chemistry, Hosei University, 3–7–2 Kajino-chou, Koganei, Tokyo 184–8584, Japan.
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with the majority of the edge carbon atoms bonded to oxygencontaining functional groups. Consequently, a detailed understanding of the oxidation process is of particular importance. The oxidation process starts from the edge of graphene9–11 and proceeds into the interior, as the basal plane is more stable and oxidation resistant.12 There are two fundamental edges in graphene—zigzag and armchair edges13–29—which differ in electronic structure and reactivity. Seitsonen et al. predicted that the graphene nanoribbon in an oxygen-rich atmosphere is preferentially formed along the armchair direction at thermodynamic equilibrium.25 Sendt et al. estimated the activation energy for oxygen desorption to be 435 kJ mol1 at the armchair edges, which is larger than that at zigzag edges (416 kJ mol1), indicating the higher stability of the armchair edges.10,11 In spite of these important theoretical predictions, there have been few experimental studies on the reactivity of graphene edges, owing to the difficulties of preparing an appropriate system and designing a method to clarify this issue experimentally.29 In order to overcome these difficulties, we focused on nanographene-based activated carbon fibers (ACFs) as model systems to study the oxidation process of graphene, using nearedge X-ray absorption fine structure (NEXAFS) as a technique to study the edge geometry of graphene. ACFs comprise a threedimensional disordered network of nanographite domains, each of which is a stack of 3–4 nanographene sheets with a mean in-plane size of 3 nm.15 Each nanographene sheet comprises
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approximately 350 carbon atoms, among which the number of carbon atoms at the edge is estimated to be 90, yielding a large contribution of edge carbon atoms in the individual nanographene sheet in the ACFs along with a high ratio of the edge carbon atoms to the interior carbon atoms (B90/350). The inter-sheet distance between nanographene sheets in ACFs (0.38 nm) is considerably larger than that in bulk graphite (0.335 nm), such that the interaction between nanographene sheets is sufficiently weak to be neglected in studying the properties of nanographene. Thus, we can study the properties of nanographene using ACFs. The small nanographene size and large inter-sheet distance allow us to obtain detailed information about the oxidation process occurring at the edges of the nanographene sheets. NEXAFS, which we employed as a powerful investigation technique to study the oxidation of the edge carbon atoms, is based on the excitation of an electron from a core orbital to an unoccupied electronic state. Localized edge states are created around the Fermi level along the zigzag edge, whereas the armchair edge has no such localized state around the Fermi level.13–16 NEXAFS provides information about the density of the edge state originating from the zigzag edges; that is, NEXAFS selectively detects the zigzag edges. Previous studies using NEXAFS have demonstrated the presence of the edge state at zigzag edges in ACFs as a shoulder located on the low-energy side of the p*-state peak.29–31 In the present study, we investigated the oxidation process of nanographene edges with respect to the oxidation temperature using NEXAFS together with thermogravimetry (TG), electrical conductance measurements, and X-ray photoemission spectroscopy (XPS).
2. Experimental Commercially available ACFs (specific surface areas of 2000 m2 g1, FR-20, Kuraray Chemical Co.) were used in the present study, which were prepared by the activation of well-graphitized phenol-based precursor materials. For all the measurements, before the measurement, the ACF sample was heated to 1300 K in an ultrahigh vacuum for 15 min in order to remove the molecular species physisorbed into the nanopores, such as water, hydrocarbon molecules, and oxygen-containing functional groups that have already bonded to the edge carbon atoms. TG measurement was performed at a heating rate of 5 K min1 in a flow of a gas mixture (N2: 199.0 ml min1 + air: 1.0 ml min1) in the temperature range of 300–1100 K, where the partial oxygen gas pressure was adjusted to 100 Pa. The electrical conductance was measured using a 2-probe method, where a bundle of ACF fibers mounted on a sample holder was heat-treated by self-heating in an electric current flowing at various temperatures (300–1400 K) in an oxygen atmosphere. The oxygen pressure was maintained at 1 Pa to avoid the effect of charge transfer to the adsorbed oxygen species26,27 as will be explained later, except the oxygen pressure dependence of the conductance, in which the oxygen pressure was varied to 100 Pa. XPS and NEXAFS measurements were performed in the measurement chamber, which was maintained in an ultrahigh vacuum
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(UHV: B107 Pa) and connected to the electrical conductance measurement chamber by a gate valve. The XPS spectra were recorded at room temperature using non-monochromatized Mg Ka radiation (1253.6 eV) for the excitation and a hemispherical analyzer (Scienta R3000). The binding energy was calibrated using Au4f = 84.0 eV from the foil sample. The carbon (C) and oxygen (O) K-edge NEXAFS spectra were recorded at the soft X-ray beam line BL-7A in the Photon Factory at the Institute of Materials Structure Science. The NEXAFS spectra were obtained by measuring the sample photocurrent (total electron yield method) at room temperature. The photon energy was calibrated with respect to the C 1s to p* peak position of highly oriented pyrolytic graphite (HOPG) at 285.5 eV. The NEXAFS spectra were recorded at normal X-ray incidence. The temperature of the sample was monitored using a pyrometer in the conductance, XPS, and NEXAFS measurements. The oxygen pressure in the NEXAFS and XPS measurements was maintained at 1 and 10 Pa to avoid the effect of charge transfer to the adsorbed oxygen species and also to take into account that the oxidation effect is saturated above 10 Pa according to the conductance measurement results which will be shown later.
3. Results Fig. 1(a) shows the result of the TG measurement at temperatures up to 1100 K with a heating rate of 5 K min1 in an oxygen atmosphere (100 Pa). Weight loss started above 900 K and became 16% at the highest temperature of 1073 K, indicating that a remarkable degree of oxidation occurs above 900 K. The weight loss is associated with the oxidation of edge carbon atoms,12 which are etched away in the form of CO2 or CO.10,11 Accordingly, the oxidation process functions to reduce the size and change the shape of the individual nanographene sheet. Fig. 1(b) indicates the electrical resistances of the ACF samples (ROx) at 300 K measured after heat treatments at 900 K for 15 minutes in an oxygen atmosphere at various pressures (PO2), normalized with respect to the initial resistance before oxidation (R0). For comparison, the results of the ACFs exposed to oxygen without heating are also plotted in Fig. 1(b). The resistance changes little with the oxygen pressure below 0.01 Pa, whereas it drastically increases when the oxygen pressure is above 0.1 Pa, indicating that a remarkable degree of oxidation occurs above 0.1 Pa. Above 0.1 Pa, the resistance increase tends to be saturated. This suggests a threshold above which nanographene is subjected to the oxidation. In contrast, when the sample is not heated, the increase in the resistance of the ACFs is far smaller, suggesting that the oxidation does not occur effectively. In our previous study, it was concluded that the reduction of the resistance of ACFs upon heat treatments in a vacuum was caused by the elimination of edge functional groups as well as the fusion of nanographene sheets.31 Therefore, the opposite trend in the resistance can be understood by considering the reduction in the size and the changes in the shape of nanographene sheets, along with the creation of oxygen-containing functional groups bonded to the edges upon oxidation. The TG and electrical
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Fig. 2 Time evolution of the weight in ACFs, normalized with respect to the initial weight, at oxidation temperatures of 900 K (black circles), 1000 K (red squares), and 1100 K (blue triangles) and an oxygen pressure of 100 Pa.
Fig. 1 (a) Weight change of the ACF sample with the increase in temperature (W(T)) at an oxygen pressure of 100 Pa. Data are normalized with respect to the value at room temperature. (b) Evolution of the electric resistances of the ACF samples by exposure to oxygen at various pressures at 900 K for 15 minutes (filled circles). Data were measured at room temperature and normalized with respect to the value obtained for the clean sample at room temperature. The empty circles represent the data collected by similar procedures without heating the sample during exposure to oxygen.
resistance measurements indicated that the graphene oxidation pronouncedly occurs together with the reduction in the size and change in the shape of the nanographene sheets above 0.1 Pa and 900 K. Fig. 2 shows the time evolution of the weight of the ACFs in an oxygen atmosphere (100 Pa) at various oxidation temperatures. The weight decreases with time, indicating the progress of the oxidation reaction. The rate of the weight loss increases with the oxidation temperature, which agrees with the TG results shown in Fig. 1(a). Fig. 3 shows the conductance as a function of time in the cooling process of the sample in a vacuum immediately after the oxidation is completed in the exposure of oxygen (1 Pa) for 15 min at different temperatures in the range of 900–1400 K. Here, an oxygen pressure of 1 Pa is employed, considering that the pressure is above the threshold, where the oxidation process becomes activated, and that only approximately 20% change in the resistance takes place from 1 to 100 Pa (see Fig. 1(b)). In addition, by employing 1 Pa, we can reduce the effect of charge transfer to strongly and weekly adsorbed oxygen species, the presence of which will be discussed later. Thus, we
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Fig. 3 Conductance as a function of time during the cooling process under vacuum after the oxidation is completed in the exposure of oxygen (1 Pa) for 15 min at different temperatures ( : 900 K, : 1200 K, : 1400 K). Conductance change for the sample (K) heat treated at 1300 K without exposure to oxygen is also shown. Open symbols indicate the conductance value at room temperature (300 K) obtained by extrapolating the conductance to t - N.
can extract solely the information about the oxidation process at the reaction front, excluding the side effects of the adsorption of strongly and weakly bonded oxygen atoms. The conductance decreased with time during the cooling process for all of the samples, which were treated at different oxidation temperatures. This suggests the insulating feature of the electron transport, agreeing well with previous work.15 The conductance at the start
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of cooling (t = 0 min) increases as the oxidation temperature is increased. A detailed analysis of the oxidation process was performed at 1 Pa using XPS and NEXAFS. Fig. 4(a) shows the O 1s XPS spectra of the ACFs heated at 900, 1100, and 1300 K for 15 minutes in an oxygen atmosphere of 1 Pa. At 900 K, we can see a broad peak at around 533 eV together with the shoulder at the low binding energy side. At 1100 K, the peak at 533 eV increases and weak feature appears at high binding energy side. At 1300 K, the peak at 533 eV drastically increases. It has been reported that the carbonyl group (4CQO) gives rise to the O 1s peak in the range of 530–531 eV, while the peak due to the phenol (–OH) and the epoxy (C–O–C) groups appears at 533 eV.32,33 Accordingly, the peak near 533 eV observed in the present O 1s XPS spectra is ascribed to the –OH and/or C–O–C groups. The shoulder at the low binding energy side can be attributed to the carbonyl group, which cannot be removed by the heat treatment at 1300 K in UHV. The XPS study indicates that the edge carbon atoms of the nanographenes are covered by the –OH or C–O–C groups during the oxidation process. Fig. 4(b) shows the O K-edge NEXAFS spectra of the ACFs heated
Fig. 4 (a) O 1s XPS spectra of the ACF sample exposed to oxygen at a pressure of 1 Pa for 15 minutes at 900, 1100, and 1300 K, denoted by the dotted, dashed, and solid curves, respectively. (b) O K-edge NEXAFS spectra of the ACF sample measured after exposure to oxygen at a pressure of 1 Pa for 15 minutes at 900, 1100, 1300, and 1600 K. Spectra are parallel-shifted along the vertical axis for clarity.
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at 900, 1100, 1300, and 1600 K for 15 minutes in an oxygen atmosphere of 1 Pa. While background subtraction was performed, we did not normalize these spectra with respect to the edge jump, i.e., the difference in the intensity of the signal before and after the O K-edge. The edge jump is proportional to the amount of oxygen atoms in the target sample.34 The edge jump and the peak are not clear below 1100 K, indicating the small amount of oxygen in ACFs. A peak appears near 538 eV after the heat treatment above 1300 K in an oxygen atmosphere. The intensity of this peak and the edge jump increase with the oxidation temperature, indicating the progress of the oxidation reaction. Previously, it was determined that the transitions from the O 1s state to the p*CQO, the s*C–O,O–H, and the s*CQO states occur at 531.5 eV, 536–540 eV, and 542.0 eV, respectively.35 Thus, the peak near 538 eV observed in the present NEXAFS spectra is ascribed to s*C–O,O–H. The absence of the p*CQO peak and the appearance of the s*C–O,O–H peak indicate the formation of a C–O–C and/or an –OH bond, which agrees with the XPS results. Fig. 5 provides a close-up view of the pre-edge region of the C K-edge NEXAFS spectra of the ACFs heated at 900, 1100, 1300, and 1600 K for 15 min at oxygen pressures of 1 and 10 Pa. The NEXAFS spectra are normalized with respect to the value of the edge jump at 340 eV. All of the spectra exhibit two peaks at 285.5 and 291.9 eV (not shown in the figure), which correspond to the transitions from the C 1s to the p* and s* states, respectively. According to the previous work,29 the peak at 285.5 eV involves the contribution of the edge state together with the p* state. Thus, we deconvoluted it with two Gaussian peaks at 284.5 and 285.5 eV, which correspond to the edge state and the p* state, respectively.29 The integrated intensity of the edge-state peak is approximately 6–12% of that of the p* peak for the clean ACFs, which agrees with the previously reported value within error bars.29,31 Although the change in the intensity of the edge state peak is not simple in Fig. 5, the intensity of the edge state peak decreases with the increase in the oxidation temperature in
Fig. 5 Close-up view of the C K-edge NEXAFS spectra of the ACF sample exposed to oxygen at pressures of 1 (left) and 10 Pa (right) for 15 min at (a) 300, (b) 900, (c) 1100, (d) 1300, and (e) 1600 K. The symbols, solid curves, dashed curves, and gray regions denote the experimental data, total of curve fits, p* ingredients, and edge-state ingredients, respectively.
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general (see Fig. 7a). Here, we note that the intensity of the prepeak below the bulk edge onset (edge state peak) was smaller than the previously reported study by Entani et al.36 It is because that they fabricated the nano graphene on a Pt substrate. On the metal substrate, molecular orbital of graphene can hybridize with the metal orbital, leading to formation of the metal induced gap states (MIGS).37 The MIGS peak appears at the Fermi level, which can enhance the intensity of the pre-peak below the bulk edge onset. While the intensity of the edge state peak changes with the oxidation temperature, the p* peak changes little.
4. Discussion We firstly discuss the kinetics of the oxidation process. The oxidation reaction is accompanied by the etching of the edge of the nanographene sheets, in which the edge carbon atoms are removed with the evolution of CO and CO2. Therefore, information about the weight loss due to the oxidation allows us to investigate the kinetics of the oxidation reaction. Considering that the oxidation reaction proceeds at the edges,12 which face the oxygen molecules in the gaseous state, we must discuss the relationship between the weight and the number of edge carbon atoms. Let us determine the relationship by using the following assumptions: (1) the nanographene sheets are circles, (2) the diameter of the pristine nanographene is 3 nm, and (3) the contributions of the oxygen and hydrogen atoms are negligible. Neglecting oxygen is supported by the weak peak intensities in O 1s XPS and O K-edge NEXAFS compared with those of C. In the nanographene sheet of ACFs (3 nm), the number ratio of the edge carbon atoms to the total carbon atoms is approximately 90/350.38 Consequently, even if all of the edge carbon atoms of the nanographene sheets in the ACFs are bonded to hydrogen atoms, the mass ratio of hydrogen to carbon is only 1%. This justifies neglecting hydrogen in discussing the mass of the ACFs. Under these assumptions, the mass w of the individual nanographene sheet is proportional to its area (pd2/4), where d is the diameter of the nanographene sheet. Meanwhile, the number of carbon atoms at the edges (Cedge) is proportional to the length of the circumference of the edge, which is given by pd. Consequently, we obtain the pffiffiffiffi following relationship: Cedge Ctotal ¼ pd=pðd=2Þ2 ¼ 4=d / 1= w, pffiffiffiffi that means Cedge / w. Fig. 6 shows the reaction time dependence of the number of edge carbon atoms Cedge obtained from Fig. 2 with the relationpffiffiffiffi ship Cedge / w. Because the slope of Cedge vs. t corresponds to the rate of the oxidation of nanographene, the activation energy was estimated using the following process. First, the slope (k) was obtained at 0 and 30 min for each oxidation temperature. Second, the activation energy of the oxidation was calculated using the log k 1/T plot. The activation energies were estimated to be 105(6) and 99(20) kJ mol1 at 0 and 30 min, respectively. Consequently, the activation energy decreases with the progress of the graphene oxidation, indicating that the oxidation is enhanced as it progresses. This change in the activation energy is confirmed by the convex feature in the Cedge vs. t plot in Fig. 6.
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Fig. 6 Number of edge carbon atoms as a function of reaction time at oxidation temperatures of 900 (black circles), 1000 (red squares), and 1100 K (blue triangles). Cedge is normalized with respect to that at t = 0.
In the analysis of the activated oxidation process of nanographene, we employ Bew’s model, in which the strongly and weakly bonded oxygen species play key roles.9,26,27 It should be noted that the oxygen species having strong electronegativity work to modify the electronic structure of nanographene sheets through the charge transfer from nanographene to the oxygen species.27 In this model, it is assumed that the graphene oxidation proceeds by dissociative O2 adsorption forming a strongly bonded, immobile oxygen species (eqn (1)) and then by the transformation of strongly bonded, immobile oxygen species into weakly attached and mobile oxygen species (eqn (2)), followed by the desorption of the O2 (eqn (3)) and CO (eqn (4)) molecules, where Cf, (C–O)T, and (C–O)w are active carbon sites, strongly bonded O atoms, and weakly attached O atoms, respectively. 2Cf + O2 - 2(C–O)T
(1)
(C–O)T - (C–O)W
(2)
2(C–O)W - 2Cf + O2
(3)
(C–O)W - CO
(4)
The rate of the production of CO resulting from the oxidation of the carbon is given as 1 1 k1 2 ½O2 2 k3 ; R4 ¼ 1 1 k1 2 1þ ½O2 2 ð1 þ K21 Þ k3 k4
(5)
where k1, k3, k4, and K2 are the rate constants of reactions (1), (3), and (4) and the equilibrium constant (ratio of coverage of tightly and weakly bounded oxygen atoms), respectively. K2 is given as 5.3 1021 exp(DH/RT),9 which yields K2 B 2 1021 with DH = 200 kJ mol1 and T = 1000 K; thus, K21 can be
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omitted from eqn (5). The rate constants k1 and k3 are given by 2 exp(E1/RT) and 1011 exp(E3/RT), respectively. The activation energy of each step is given as follows: E1 = 60 kJ mol1, E3 = 225–425 kJ mol1, E4 = 300 kJ mol1.9,39,40 Using these values, the rate constants of eqn (1) and (3) are given as k1 B 103 m s1 and k3 B 0.1 mol m2 s1, respectively, at 1000 K; thus, (k1/k3)1/2[O2]1/2 can be omitted from eqn (5). Finally, eqn (5) can be simplified as follows: R4 k4
1 1 1 k1 2 Ea ½O2 2 exp ½ O2 2 : k3 kT
(6)
The activation energy Ea in the production of CO is, thus, given by E4 + 1/2E4 1/2E3 = 120–220 kJ mol1.9,10 This value of Bew’s model is in moderate agreement with the activation energy of B100 kJ mol1 (see Fig. 6) experimentally obtained by the present TG measurement. The close agreement between the two values indicates the validity of the proposed model. The deviation of the experimental values of Ea from the value predicted by Bew’s model can be explained by the overestimation or underestimation of the activation energy of each process or by the omission of other possible reaction processes. It should be noted that the activation energy decreases with the progress of the oxidation process (105 and 99 kJ mol1 at 0 and 30 min, respectively) in the TG results. In the initial stage before the oxidation, a large percentage of the edge carbon atoms are terminated with hydrogen atoms (–H),31 which prevent the initial adsorption of O2 molecules onto the edge carbon atoms. Once the oxidation process proceeds, the circulating process of the edge oxidation and subsequent desorption of CO or CO2 molecules from the edges occur continuously with the decrease in activation energy, resulting in the etching of the edge of the nanographene sheets. Next, we determine how the edges are etched in the oxidation process in relation to the reduction in the size and change in the edge geometry in the nanographene sheets. Fig. 7(a) shows the oxidation temperature dependence of the number ratio of the edge carbon atoms to all the carbon atoms Cedge/Ctotal and the diameter d of the nanographene estimated according to the TG pffiffiffiffi results with the relationship Cedge Ctotal ¼ 4=d / 1= w. The diameter starts decreasing above 900 K, and the amount of the decrement is approximately 10% at 1200 K compared with that at 300 K. That is, the reduction in the diameter d causes the enhancement of the contribution of the edge carbon atoms by 10%. Given that the bond length between neighboring carbon atoms is 0.142 nm, this approximately corresponds to the removal of all edge carbons that are initially located at the periphery of the nanographene sheets. This change in the size affects the electronic structure of nanographene sheets; we can see the decrease in the intensity of the edge-state peak in the NEXAFS spectra. Fig. 7(b) summarizes the results for the intensity of the edgestate peak in NEXAFS as a function of the oxidation temperature. The NEXAFS results (Fig. 5) show that the p* peak changes little with the oxidation temperature, indicating that the spatial extent of the p network is nearly preserved during the oxidation
21368 | Phys. Chem. Chem. Phys., 2014, 16, 21363--21371
Fig. 7 Oxidation temperature dependence of (a) number ratio (Cedge/ Ctotal) of edge carbon atoms to total carbon atoms (filled circles) and mean in-plane diameter of nanographene sheets (open circles), which are estimated according to thermogravimetric traces. (b) Ratio of integrated intensity of the edge-state peak to that of the p* peak in C K-edge NEXAFS spectra in the sample oxidized at the oxygen pressures of 1 (filled square) and 10 (open square) Pa. (c) Activation energy (DE: filled up triangle) and contact resistance (R0: filled down triangle) in the conductance; and (d) tunneling length (rtun) (filled diamond) and mean electron–hole distance (re–h) (open diamond) of the charging energy in the electron transport. Cedge/Ctotal and d in (a), Iedge/Ip* in (b), and rtun and re–h in (d) are normalized with respect to those at 300 K.
process under the experimental conditions. It also indicates that the intensity of the edge-state peak, which is the signature of the zigzag edges, decreases with the oxidization temperature (Fig. 7(b)). The edge-state peak intensity with respect to the p* peak intensity decreases above the oxidation temperatures of 900 and 1200 K at oxygen atmospheres of 10 and 1 Pa, respectively, in which we can neglect the contribution of the small concentration of strongly and weakly bonded oxygen species to the charge transfer process. This finding completely contradicts the expectation that the intensity of the edge-state peak in the NEXAFS spectra should increase with the oxidation temperature as the contribution of the edge carbon atoms
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(i.e., Cedge/Ctotal) increases (Fig. 7(a)). Alternatively, it indicates that we should consider the geometry of the nanographene edges. The decrease in the edge state intensity suggests that the zigzag edges preferentially react with oxygen, resulting in the conversion of the zigzag edges into armchair edges, considering that the armchair edges have no edge state in contrast to the zigzag edges. Here, it should be noted that the decrease in the edge-state peak intensity starts before the etching process becomes significant, as shown in Fig. 7(a) and (b). This indicates that the etching of even a small portion of the edge carbon atoms at the zigzag edges can convert the zigzag edges into armchair structures, as will be discussed later. The preferential oxidation at the zigzag edges can be justified by the thermodynamic stability of the armchair edges in contrast to the reactive zigzag edges, as predicted by theoretical studies.10,11,41 Seitsonen et al. predicted that a graphene nanoribbon is preferentially created along the armchair direction in an oxygen-rich atmosphere at thermodynamic equilibrium.25 Sendt et al. showed that the activation energy for the desorption of oxygen is smaller for the zigzag edge than for the armchair edge.10,11 Okada reported that the formation energy of an armchair edge is smaller by 1 eV per edge atom than that of a zigzag edge in a clean graphene edge.41 These theoretical calculation results indicate the relatively high stability of the armchair edge, which agrees with the present results. Here, we show two examples of the oxidation process of nanographene. Fig. 8(a) shows a nanographene whose edges comprise the armchair and zigzag geometries as the majority and minority, respectively. The zigzag edge part disappears upon the removal of only one benzene ring due to oxidation. Fig. 8(b) shows a nanographene whose edges exhibit the zigzag structure. The removal of the gray-colored benzene rings converts the zigzag edges of the nanographene into the armchair structures. These types of preferential oxide etching processes of zigzag edges effectively cause a decrease in the ratio of the zigzag edges to armchair edges in nanographene during
Fig. 8 Structural model of preferential oxidation etching of the zigzag edge in nanographenes (a) with its edges, which comprise a combination of armchair edges and one benzene ring, and (b) with its all edges terminated by zigzag structures. Red lines indicate the zigzag edges, and gray parts are removed by oxidation.
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oxidation. Note that etching even a small part of a zigzag edge can convert the zigzag edges into armchair edges. Finally, we discuss the electron transport properties of ACFs, which vary according to the oxidation-induced edge etching process. The electron transport in ACFs is governed by the process of Coulomb-gap variable range hopping (CGVRH) between nanographene sheets, described by the following equation for the resistance:15 RðTÞ ¼ R0 exp
DE ; kB T
r tun R0 / exp ; a
DE ¼
e2 ; (7) kreh
where R0, DE, a, and k are the inter-nanographene-sheet contact resistance, activation energy, tunneling length, and dielectric constant, respectively. The contact resistance R0 is governed by the inter-nanographene tunneling with inter-nanographene distance rtun and tunneling length, and the activation energy DE is given by the charging energy with the electron and hole separated by a distance of re–h. The conductance at room temperature, which is obtained by extrapolating the conductance to t - N in Fig. 3, gives the values of DE and Ro as shown in Fig. 7(c). Fig. 7(d) gives the results of rtun and re–h as a function of the oxidation temperature. Upon increasing the oxidation temperature, the activation energy DE increases, whereas the contact resistance R0 decreases. The inter-nanographene-sheet distance rtun at which the electron tunnels decreases by approximately 10% is comparable to the decrease in the nanographene size upon increasing the oxidation temperature from 900 to 1200 K, whereas the electron–hole distance re–h decreases by 50%. The oxidation at higher temperature creates simpler and smaller oxygen-containing functional groups.31 This shortens the internanographene-sheet distance, as observed. Accordingly, the decrease in the inter-nanographene-sheet distance can be explained by this change in the functional groups bonded to the edges. Also, the reduction in the electron–hole distance is related to the decrease in the size of the nanographene sheet, but the large reduction of 50% contradicts the decrease of 10% in the size of the nanographene sheet; this discrepancy appears to be caused by overestimation. Note that the conductance measurement was performed at an oxygen pressure of 1 Pa, whereas the TG measurement was performed at 100 Pa; therefore, we can expect a change in d larger than that in re–h. The change in the edge geometry of the nanographene from zigzag to armchair may cause the overestimation. Indeed, the zigzag-edged nanographene sheets have a large local density of states originating from the edge states around the Fermi level, whereas the armchair-edged ones tend to have a gap at the Fermi level.13–16 Consequently, the observed gap DE should involve an additional contribution of the intrinsic gaps of the armchair-edged nanographene sheets, which are created by the oxidation-induced etching process, resulting in the overestimation.
5. Conclusions The oxidation of nanographene in an oxygen atmosphere was investigated for nanographene-based carbon materials (ACFs)
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using TG, XPS, NEXAFS, and conductance measurements. Remarkable oxidation of nanographene proceeds above 900 K and at an oxygen pressure higher than 1 Pa. The oxidation reaction can be explained as a four-step process wherein the strongly and weakly bonded oxygen species play key roles. The oxidation process is enhanced as it progresses, owing to the appearance of active edge carbon atoms. The combination of the TG and NEXAFS measurements reveals the preferential oxidationinduced etching of the zigzag edge, which is chemically active, whereas the p network of nanographene is preserved during the oxidation process. The zigzag edge converts into the thermally stable armchair edge upon oxidation. The temperature dependence of the resistance provides the information about the contact resistance and activation energy for the inter-nanographene transport of the Coulomb-gap type variable range hopping. While the activation energy increases with the oxidation temperature, the contact resistance decreases with the oxidation temperature. The increase in the activation energy is explained by the increase in the charging energy due to the decrease in the nanographene size together with the gap formation in the armchair-edged nanographene sheets preferentially created by etching. The decrease in the contact resistance, which is governed by the tunnelling process between nanographene sheets, follows the trend that oxidation at higher temperatures creates simpler and smaller functional groups bonded to edge carbon atoms, resulting in the decrease in the inter-nanographenesheet distance in which the electron tunnels.
Acknowledgements The present work was performed under the approval of the Photon Factory Program Advisory Committee PF-PAC, No. 2012G004. The authors are grateful for the financial support of the Grant-in-Aid for Scientific Research, Grant No. 20001006, from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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