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Hidden Second Oxidation Step of Hummers Method Jong Hun Kang,† Taehoon Kim,† Jaeyoo Choi,† Jisoo Park,† Yern Seung Kim,† Mi Se Chang,† Haesol Jung,† Kyung Tae Park,† Seung Jae Yang,†,‡ and Chong Rae Park*,† †
Carbon Nanomaterials Design Laboratory, Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, South Korea ‡ Department of Applied Organic Materials Engineering, Inha University, Incheon 402-751, Republic of Korea S Supporting Information *
ABSTRACT: Hummers method has been used for 50 years to prepare graphene oxide (GO) by oxidizing graphite using Mn2O7. In this work, a new angle on Hummers method is described. The oxidation procedure before the addition of water, which has been respected as the main oxidation step of Hummers method, is named step I oxidation, and the widely ignored further oxidation step after the addition of water is named step II oxidation. The chemical and structural evolutions during step II oxidation was demonstrated for the first time using various techniques including atomic force microscopy (AFM), dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), ultraviolet−visible light (UV−vis) spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), 13C nuclear magnetic resonance (NMR), and zeta-potentiometry. Step II oxidation influences the size of GO, defects within the layers, and functional groups on the surface, which affect the thermal stability of GO and the properties of resultant thermally reduced GO. This work provides new chemical insights into GO and guidelines for preparation of tailor-fitted GO.
1. INTRODUCTION During the past decade, graphene oxide (GO), highly functionalized dispersed monomolecular graphene,1−3 has gathered enormous research interests as one of the most promising routes to graphene. The most popular methods for GO have been Hummers method4 and modified protocols5 that use the mixture of potassium permanganate (KMnO4) and sulfuric acid. The main oxidation procedure of the method is understood as the oxidative exfoliation of graphite by dimanganese heptoxide (Mn2O7) and permanganyl cation (MnO3+) which are the reaction products formed between the aforementioned permanganate and concentrated sulfuric acid. On the basis of this idea, the formation mechanism of GO from graphite was studied with scrupulous care and well demonstrated by Dimiev and Tour.6 In addition, many applications have been developed toward optimization based on GO with controlled synthetic variables including the precursor graphite,7,8 preoxidation,9 oxidant and its dose,10 and oxidation time and temperature.11 This approach is based on the modification of an initial protocol and is clearly effective because GO is not defined as a fixed material but can show controlled properties according to the synthetic variables. Modifications of these factors mostly constitute many reported modif ied Hummers method studies that deviated from the initial method. Among these many synthetic variables, it was found that the reaction conditions of the experimental step between the addition of water and the addition of hydrogen peroxide (H2O2) were also substantially different from work to work.4,5,10,12−19 The experimental sections of © 2015 American Chemical Society
many reports that provide a detailed procedure for the synthesis of GO were analyzed on the basis of this perspective, and it was found that the reaction conditions after the addition of water varied from study to study without a solid justification. For example, Hummers and Offeman allowed this step to prolong for only 15 min at 98 °C,4 while Eigler and co-workers adopted 2 h at 10 °C.20 Kim and his colleagues added water very slowly and gently during this stage of mixing so as to restrict the temperature of the system below 55 °C,21 whereas Coet et al. maintained the temperature at 90 °C for 1 h during this process.22 Several selected works were positioned in a 3dimensional space according to the synthetic parameters (Figure S1). However, the chemical influences on GO during the step were not well elucidated up to date to the best of our knowledge. At this step, manganese (VII)-containing oxospecies mostly exists as permanganate (MnO4−) in the acidic aqueous condition which has a high oxidizing potential and coexists with GO.10 Moreover, the temperature of the reaction mixture rapidly increases due to the large exotherm produced by the reaction between the water and the mixture of concentrated sulfuric acid and strong oxidants. Therefore, it is reasonable to assume that there should be additional oxidative changes within the GO sheets during this step. This study suggests new terms to prevent confusion and to provide a new insight into Hummers method. The convenReceived: September 21, 2015 Revised: December 31, 2015 Published: December 31, 2015 756
DOI: 10.1021/acs.chemmater.5b03700 Chem. Mater. 2016, 28, 756−764
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Figure 1. Noncontact mode AFM images and topographic profiles of GO deposited on Si/SiO2 substrates of (a) A458, (b) A702, (c) A708, and (d) A952. In all subfigures, small yellow squares in the left images denote the regions that were magnified. The right images are the magnified region. The white lines ending with diamond-shape markers correspond to the 1-dimensional topographic profiles shown below the respective images.
the thermal stability of the GO product but decreases the electrical conductivity of the corresponding Th-rGO.
tional step of the main oxidation of Hummers method by Mn2O7 and MnO3+ in concentrated H2SO4 is named as step I oxidation of Hummers method (step I oxidation). Similarly, the subsequent stage of oxidation by MnO4− in the acidic aqueous condition after the water addition is named as step II oxidation of Hummers method (step II oxidation). The chemical and structural influences of this step II oxidation on GO have been examined systematically in this study for the first time. We prepared a series of GO samples with various degrees of step II oxidation by controlling the reaction time and temperature. These samples were chemically and structurally analyzed by using atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), ultraviolet−visible light (UV−vis) spectroscopy, dynamic light scattering (DLS), direct 13C-pulse magic-angle spinning (MAS), solid-state nuclear magnetic resonance (NMR), and zeta-potentiometry. As a result of these analyses, two mechanisms from conventional organic chemistry were proposed to explain the observed chemical and structural changes in GO induced by step II oxidation: the oxidative cleavage of the CC double bond by permanganate and the acid-catalyzed hydrolysis of 1,2-epoxy. Additionally, the effects of step II oxidation on the thermal behaviors of GO samples were studied with thermogravimetric analysis (TGA), and the resulting thermally reduced GO (Th-rGO) was examined by using FT-IR, Raman spectroscopy, and 4-point probe resistivity measurements. We concluded that step II oxidation increases
2. EXPERIMENTAL SECTION In this study, eight different GO samples were synthesized on the basis of the stepwise modification of Hummers method. To isolate the influences of step II oxidation on the GO product, the step I oxidation condition and the main oxidation step before the water addition were strictly controlled to be the same across all GO samples studied: 2 h at 35 °C. The only controlled experimental variables were the time length and temperature after the addition of water (i.e., step II oxidation time and temperature). Three time-variables (2, 4, and 8 h) and three temperature-variables (45, 70, and 95 °C) were selected and reflected in the modifications of Hummers method from which the GO samples were produced. Simply, these synthetic parameters were controlled by maintaining the reaction after the water-mixing step for the controlled time periods at the controlled temperature using a double-walled reaction vessel. This allowed the exfoliated GO sheets to be exposed to permanganate under various reaction conditions before separation and purification. The nomenclature of the GO samples of this work represents the step II oxidation condition for each GO experienced during the synthesis. For example, the GO sample produced from the procedure, including step II oxidation for 2 h at 45 °C, was named A452. The seven samples, A452, A454, A458, A702, A704, A708, and A952, were prepared and named in this manner. Since permanganate was completely consumed during the 2 h of step II oxidation at 95 °C, A954 and A958 could not be prepared. A000 is the GO sample prepared by terminating the oxidation process at the end of step I oxidation. The step II oxidation process could be skipped by directly pouring the dark green pasty slurry of the step I oxidation mixture onto an excess amount of ice and hydrogen peroxide mixture, 757
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Figure 2. (a) Quantitative analysis of various forms of C in the GO samples, calculated from C 1s spectral deconvolution. (b) FT-IR spectra of the GO samples with different degrees of step II oxidation. (c) Solid-state 100.7 MHz direct 13C pulse MAS (7.0 kHz) NMR spectra of the GOs.
10−100 nm. At 95 °C, this void development became even more noticeable. The AFM topography of A952 shows that the voids had more rapidly grown up to several hundreds of nanometers in size only after 2 h of step II oxidation (Figure 1d). The particle size distributions obtained with DLS were examined. As a result of step II oxidation, the size distributions of exfoliated GO are systematically left-shifted under all experimental conditions (Figure S4). The measured average size of A000, for which step II oxidation was not performed, was 15.3 μm, but this value decreases to 8.8 μm after 8 h of step II oxidation at 70 °C. Admittedly, this analysis method assumes that the shape of the colloidal particles is spherical, so these measured values do not reflect the actual size of the GO particles because of their extremely high shape anisotropy. Nevertheless, this measurement does demonstrate that intensified step II oxidation conditions do reduce the particle size of product GO under all experimental conditions. The influences of step II oxidation on the population of oxygen-containing functional groups of GO were quantitatively analyzed using X-ray photoelectron spectroscopy (Figure 2a). The peak area corresponding to C−O and O−CO carbon atoms was continuously increased, and the atomic fraction of graphitic carbon was systematically decreased at all temperatures as a prolonged step II oxidation (Figure S5). This tendency is similar to the typical oxidative effect of extending step I oxidation, as shown in Figure S6, although the differences between the mechanisms of the two steps cannot be elucidated from these XPS results alone. Even after the addition of water, these results indicate that the exfoliated GO could be significantly oxidized as time goes on, especially at a higher temperature. Although the color of typical GO is known to be brownish as previously reported,23 the color of the material turn outs to be highly dependent on the step II oxidation conditions, which varied from near-black to bright yellow (Figure S7). Sample A000 showed a remarkably dark color, indicating that the
similarly to an improved Hummers method reported by Marcano et al.10 This quenching method under the presence of H2O2 eliminates the possibility of the contact between exfoliated GO and MnO4− by directly reducing Mn2O7 and MnO3+ of step I oxidation to manganese ions in a lower oxidation state such as Mn2+ and thus technically skipping the MnO4− state. H708 was prepared by treating A000 in 2:1 H2O/H2SO4 solution for 8 h at 70 °C. This condition is exactly the same as the preparation procedure of A708, except for the presence of the oxidant. Th-rGOs were prepared by annealing casted GO films up to 200 and 300 °C with a heating rate of 0.5 °C/min in an ambient condition. The synthetic parameters related to each sample were demonstrated in Table S1, and the outline of the preparation of samples was summarized in Figure S2. Further detailed information on the procedure was provided in the Supporting Information.
3. RESULTS AND DISCUSSION It is imperative to identify the main oxidants of each step before studying its mechanism. On the basis of the UV−vis spectrometry of the reproduced oxidant solutions, it was confirmed that the main oxidant in step II oxidation is permanganate and that the oxidants in step I oxidation have been completely eliminated after the water addition, as demonstrated in the Figure S3. The AFM topographical images of the GO samples revealed that step II oxidation at a higher temperature resulted in the formation of large defects, which are tens-to-hundreds nanometer in size, especially at the edge-regions of each sheet. At 45 °C or lower temperature, there was no discernible change in the GO layers even after 8 h of step II oxidation (Figure 1a). However, at 70 °C, only 2 h of step II oxidation produced crack-like holes within the GO layers (Figure 1b,c). The darker spots were observed inside the GO layers in the AFM images, and the depth of each spot was measured to be approximately 1 nm based on the cross-sectional topographic profiles, which were identical to the typical thickness of GO single layers deposited on a Si/SiO2 substrate. This result confirms that the dark spots inside GO are indeed voids. For A702 and A708, the lateral size of each void was in the range of 758
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Figure 3. UV−vis spectra of the GO aqueous dispersions (0.1 mg/mL in DI water): (a) various step II oxidation temperatures and A000 and various step II oxidation times (b) at 45 °C and (c) at 70 °C. The insets in (b) and (c) show the spectra magnified in the range of 270−310 nm. All spectra were normalized with respect to the π−π* transition band to quantitatively analyze the relative changes in the absorbance of the bands.
Because the above-mentioned data could not offer further detailed information on the changes in each species of functional groups in GO, 13C NMR spectra were recorded. The MAS NMR spectra of a series of GOs demonstrating the influence of step II oxidation were displayed for the first time. As a typical result, in all spectra, six peaks were observed at 61, 70, 100, 133, 167, and around 200 ppm and assigned to epoxides, alcohols, lactols, graphitic carbons, carboxylates, and ketones, respectively (Figure 2c).12 Only A000 showed a wellknown direct 13C pulse MAS NMR spectrum of GO, which is similar to a previously reported result.29 A452 and A458 showed significantly abated signals at 133 ppm relative to those at 167 ppm. Interestingly, in the spectrum of A458, the intensity of the peak at 133 ppm became similar to that at 167 ppm, suggesting the decrease in the number of graphitic carbons and/or the increase in that of carboxylate. The changed spectra at 70 °C were more obvious. In the spectrum of A708, the intensity of hydroxyl peak at 70 ppm was significantly grown and became similar to that of the epoxy peak at 61 ppm. The graphitic carbon peak was almost diminished, and the carboxylate peak and lactol peak had grown even further. To sum up, these FT-IR and NMR spectra well summarize the oxidative influences of step II oxidation: the increase in hydroxyls, lactols, and carboxylates and the decrease in epoxies and graphitic sp2 carbons. The dispersibility of GO in water at neutral pH varies with the degree of step II oxidation and was studied by using zetapotentiometry based on electrophoretic light scattering (ELS). All GO samples were found to exhibit good colloidal stability at neutral pH, as illustrated in Figure S8, without flocculation for weeks. Step II oxidation at a higher temperature or for a longer period also results in an increase in the dispersibility, which results in a larger negative zeta potential value. A000 has a zeta potential of −41.8 mV, whereas that of A952 is −58.4 mV. Extending the step II oxidation also increases the zeta potential at 45 and 70 °C, as illustrated in Figure S8b. The origin of the negative zeta potential of GO is known to be the presence of ionizable functional groups such as carboxylates. Therefore, this result implies that step II oxidation increases the population density of carboxylic acids and their derivatives, as is consistent with the XPS and NMR results. It is known that the interlayer distances of dried GO are affected by its degree of oxidation. Conventionally, it has been accepted that an increase in the number of oxygen-containing functional groups results in an increase in the interlayer
sample widely absorbs visible light. As expected, A000 samples demonstrated the strongest absorbance across the entire visible range with the absorbance maximum of the π−π* transition at the longest ultraviolet (UV) wavelength (Figure 3). This implies the presence of the largest undamaged conjugated graphitic domains within its layers among all samples. The intensified step II oxidation for longer time periods or at higher temperatures resulted in a brighter color. On the other hand, a prolonged step I oxidation did not significantly affect the color of the products (Figure S7). Figure 3 shows that the obtained spectra is consistent with the previously reported typical ultraviolet−visible spectra of GO.24 Although extremely subtle, step II oxidation reproducibly blue-shifted the π−π* transition (around 230 nm)24,25 of GO. The π−π* transition of A000 sample was originally at 233 nm but blue-shifted to 231 nm after 8 h of step II oxidation at 45 °C. The A952 sample showed the lowest value at 228 nm, indicating that step II oxidation reduced the size of conjugated domains of GO. Additionally, the normalized absorbance of n−π* transition (a small shoulder peak at around 300 nm) of GO also weakly increased as a result of an extended step II oxidation, implying an increase in the relative population of CO double bondcontaining functional groups with respect to the sp2-conjugated domains (Figure 3b,c). Consistent with the XPS results, UV− vis spectroscopy also demonstrated that step II oxidation resulted in the increase in carbonyl bonds (CO) and the decrease in the size of sp2 conjugated domains in GO. The acquired FT-IR spectra of the GO products were demonstrated in Figure 2b and well agreed with previously reported results.25−27 In the samples with low degree of step II oxidation such as A000 and A452, relatively weak O−H stretching bands at 3440 cm−1 were observed. As step II oxidation extended, the intensity of the peak at 3440 cm−1 increased, indicating the formation of hydroxyl groups during step II oxidation. The peak at 1720 cm−1 was unanimously assigned as carbonyl CO double bonds by most previous studies25,27 and the intensity of this absorption showed a very small increase as step II oxidation proceeded. Although the origin of the sharp and strong absorption at 1620−1660 cm−1 has been controversial, it may be substantially contributed by the stretching mode of intercalated water molecules.1 The peak induced from the epoxide group (at 1250 cm−1)28 slightly decreased as step II oxidation prolonged and intensified, implying a decrease in the relative portion of epoxy groups. 759
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Figure 4. Possible mechanisms of step II oxidation of Hummers method: (a) oxidative cleavage of a CC double bond via a manganese cyclic ester intermediately resulting in two carbonyl bonds, (b) oxidative cleavage of a CC double bond producing two carboxylic acids, (c) oxidative cleavage of a ketone forming one carboxylic acid and one ketone, and (d) acid-catalyzed hydrolysis of an epoxy producing two hydroxyl bonds.
Figure 5. Schematic diagram illustrating the chemical evolutions in the course of step II oxidation at a nano- to micrometer scale and at a molecular scale. The upper box shows the change in GO sheets during Hummers method at a nano- to micrometer scale. Nano- to micrometer scale means a scale at which AFM or scanning electron microscope (SEM) can detect changes of subject materials. Pristine graphitic sp2 domains and oxidized domains are displayed in green and red, respectively. The lower box, at the molecular scale, illustrates the evolution of local chemical structure of GO during Hummers method, which mainly focuses on step II oxidation. Epoxies and hydroxyls are highlighted in red and blue, respectively. Newly developed carbonyls and carboxylic acids are highlighted in green.
through hydrogen-bonding interaction, and the negative charges of oxygen atoms play an important role.36 It is known that the partial charge of oxygen in the hydroxyl group is stronger than that in the epoxide group,36,37 and it may lead to the strengthening of the hydrogen bond as step II oxidation proceeds and a decrease in the d-spacing. A further study is required to explain this phenomenon in detail. The yield of exfoliation of each sample was qualitatively studied and visualized using OM. Varying the oxidation time and temperature of the step II oxidation process did not increase the yield of exfoliated GO (Figure S10). On the other hand, the yield of exfoliation is increased by extending the step I oxidation, as shown in Figure S10b. Blue graphite bisulfate (Figure S10c) is observed only during step I oxidation and is thought to be the key intermediate responsible for this increase in exfoliation yield.20 This intermediate is stabilized by the presence of concentrated sulfuric acid and readily decomposes
distances of solidified GO; this idea has been demonstrated experimentally30,31 but only in the case of step I oxidation in concentrated sulfuric acid. Interestingly, step II oxidation systematically decreases the interlayer distances of GO while increasing the degree of oxidation. As can be clearly seen in the diffraction profiles in Figure S9, the position of the (002) diffraction of GO is shifted to higher angles, which indicates that the interlayer distance has decreased, by prolonging step II oxidation. However, the degree of oxidation is increased by step II oxidation, as confirmed by the XPS, UV−vis, and NMR results discussed above, so these XRD profiles are apparently against previous reports. We speculate that the strengthening of the hydrogen bond networks results in a decrease of interlayer distance of GO. The interlayer distance of anhydrous GO is known to be 5−6 Å,32,33 but that of hydrated GO is much longer due to the presence of intercalated water molecules.32−35 The water molecules interact with oxygen groups of GO 760
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Figure 6. TGA profiles of GO and the 1st derivatives of weight measured by TGA: (a) various step II oxidation temperatures and A000 and various step II oxidation times (b) at 45 °C and (c) at 70 °C. The labeled temperature is the point at which the weight loss of GO during annealing was the fastest.
the latter part of this work. Figure 5 summarizes the evolutions during step II oxidation within GO sheets at a nano- to micrometer scale and at a molecular scale. The influences of step II oxidation on the thermal stability of GO were also studied on the basis of simple TGA experiments. In this work, the term the thermal stability of GO refers to an ability of GO to withstand high temperature without thermally decomposing into Th-rGO and gaseous byproducts such as CO2, CO, H2O, and a trace amount of O2.38 We found that step II oxidation conditions substantially affect the thermal stability of GO. The thermal decomposition of GO is normally fastest around 200 °C,38 but the exact temperature at which the largest weight loss occurs differs from sample to sample. Whereas A000 lost its weight most rapidly at 174.5 °C, A952 lost its weight at 229.4 °C, almost 55 K higher than A000 (Figure 6a). At 200 °C, the differences among the samples were the most remarkable. A952 retained 92.5% of its initial weight at 200 °C, but A000 lost almost a half of its initial weight at the same temperature, retaining only 57.5% (Table S3). An extended step II oxidation also systematically resulted in the increase of thermal stability of the GO product. On the other hand, the influence of an extended step I oxidation was relatively less influential than that of step II oxidation (Figure S11). A change in the thermal stability of GO induced by a variation in the step II oxidation conditions can greatly affect the performance of any device using GO or Th-rGO as a key material. The gas-barrier performance of a Th-rGO encapsulation film for solution-processable organic photovoltaic (OPV) devices is a good example.7 Such Th-rGO films can protect OPV devices from oxidizing gases and provide high stability in the ambient atmosphere. The active layer materials of OPVs such as poly(3-hexylthiophene) (P3HT) are normally degraded at low temperatures,7 so the thermal reduction of the directly coated GO layer to prepare the rGO layer should be very mild; an annealing temperature of around 150 °C is normally required.7 These shorter and milder step II oxidation conditions could result in better gas-barrier performance against O2/H2O vapor because of the increased extent of thermal reduction of GO, as shown in Table S3. Considering the large number of kinds of applications in which GO and Th-
back to graphite if water is added. Step II oxidation occurs in the aqueous phase and so might not contribute to the further exfoliation of graphite. When comparing how step I oxidation and step II oxidation time effects GO, the XPS results show that the two have similar tendency. On the other hand, the change in the color of the products with respect to oxidation time, structural change, and yield of exfoliation show different aspects for step I oxidation and step II oxidation. Therefore, we can conclude that step I oxidation and step II oxidation take part in controlling the characteristics of GO through different mechanisms. On the basis of the experimental results, we suggest two mechanisms during step II oxidation of Hummers method: the oxidative cleavage of CC double bonds by permanganate producing carbonyl functional groups (ketones and carboxylate derivatives; see Figure 4a−c) and the acid-catalyzed hydrolysis of epoxies producing hydroxyl groups (Figure 4d). Permanganate ions dissociate CC double bonds into CO carbonyl groups via a manganese cyclic ester compound as the intermediate (essentially an intermediate although not technically separable) at very low pH (Figure 4a), although they oxidize CCs into vicinal diols at moderate or high pH. This mechanism produces carboxylic acids at the edge of GO sheets by cleaving CC double bonds (Figure 4b) or enols (Figure 4c). The step II oxidation condition is highly acidic and prolonged often at a high temperature and thus well satisfying the criteria of occurrence of the oxidative cleavage mechanism. This mechanism may be responsible for the decrease in size of GO and the formation of internal voids within GO layers detected using DLS and AFM, respectively. The significant change in the color of GO, the decrease in graphitic sp2 carbon atoms, and the increase in carbonyl functional groups were confirmed using XPS, UV−vis, zeta potentiometry, and NMR and can be fully explained by this mechanism. On the other hand, the relative increase in hydroxyl groups evidenced by FTIR and NMR spectra is due to the hydrolysis of epoxies catalyzed by concentrated hydronium (H3O+) ions formed after adding excess water to the system. Interestingly, this change in the ratio between epoxies and hydroxyls caused by hydrolysis of epoxies within GO during step II oxidation turn out to significantly affect the thermal behavior of GO, as explained in 761
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Chemistry of Materials rGO are used, we suggest that the selection of the step II oxidation conditions in the application of Hummers method is one of the most important factors determining the final performance of the GO sample. Considering that both step I oxidation and step II oxidation similarly increases the overall degree of oxidation of the GO product, we concluded that such degree of oxidation can never be a reliable indicator for the thermal stability of GO. Instead, we suggest that the ratio among different functional groups, especially that between epoxies and hydroxyls, can be more influential on the thermal stability of GO. We prepared an additional sample, H708, by adding a subsequent 8 h acid treatment step at 70 °C to the synthesis procedure of A000. The treatment condition of H708 was exactly the same as the step II oxidation condition that A708 experienced, except for the presence of the oxidant, permanganate. An extended acid treatment of GO in aqueous condition at high temperatures is understood to result in hydrolysis of epoxies to hydroxyls (Figure 7a). Interestingly, H708 showed a main decomposition
Figure 8. FT-IR spectra of the Th-rGO samples prepared by annealing GOs to 300 °C with different degrees of step II oxidation. The O−H stretching mode at 3400 cm−1 was highlighted by shading the region.
3440 cm−1 assigned to the hydroxyl groups was observed and the intensity of this absorption increased with respect to the other absorption bands. The resultant Th-rGO sample prepared by annealing the parent GO of a harsher step II oxidation condition tended to possess more hydroxyl groups after the thermal annealing. The remaining hydroxyl groups significantly affected the primary physical properties of Th-rGO such as the electrical conductivity. Figure 9a illustrates the electrical conductivity data of Th-rGOs from the two different annealing temperatures, 200 and 300 °C. As expected, Th-rGO from the higher annealing temperature showed the higher electrical conductivity. The electrical conductivity of the Th-rGO from A000 was 11.31 S cm−1 after annealing to 300 °C, but that of rGO from the GO sample with a high step II oxidation degree was significantly low. The electrical conductivity values of the ThrGOs from the A452, A454, and A458 were slightly higher than 4.0 S cm−1, and those of the Th-rGOs from the A702, A704, and A708 were around 2.0 S cm−1 or less. Therefore, we can conclude that the less extent of step II oxidation of GO synthesis engenders the higher electrical conductivity of the resultant Th-rGO. Noticeably, the electrical conductivities of Th-rGOs from A000 and A452 annealed at 200 °C exceeded those of Th-rGOs from A704 and A708 annealed at 300 °C. More importantly, this result also suggests that, in order to optimize the electrical performances of any devices in use of Th-rGO, the step II oxidation condition during the synthesis of the parent GO should be seriously respected as an important experimental factor. Raman spectra of the GOs and the corresponding rGOs were studied to estimate the atomistic defects, which can affect the electrical conductivity of the materials. As a typical result, the ID/IG ratio of GO, which is one of the common indicators for the defectiveness of carbon-based materials, increased as the step II oxidation condition intensified (Figure 9b). This
Figure 7. (a) The scheme of preparation of the sample H708 and (b) TGA profiles of A000, A708, and H708 and their 1st derivatives with respect to temperature. The TGA curves of A000 and A708 are identical to those shown in Figure 6a,c.
temperature at 207.4 °C, almost identical to that of A708, even though H708 originated from A000 and experienced no further oxidation (Figure 7b). This result supports that the increased hydroxyl groups produced by hydrolysis of epoxies are responsible for the enhanced thermal stability of GO. This result is also supported by many other previous studies based on either computations or experimental results demonstrating that hydroxyls are more difficult to decompose and more thermally stable than epoxy groups.39 Therefore, we can conclude that the difference in the final weight retention is due to the remaining oxygen-containing functional groups, especially to the presence of hydroxyl groups that withstood the annealing process without being thermally dissociated from the GO layer. Figure 8 shows the FT-IR spectra of the ThrGOs produced from the GO samples by annealing them up to 300 °C. As clearly seen from the spectra, the absorption band at 762
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domains composed of CC double bonds within GO were diminished while carbonyls, lactols, carboxylic groups, and the relative amount of hydroxyls increased with respect to that of epoxies. To explain these observed chemical changes, the two mechanisms were suggested as possible reaction routes during step II oxidation of Hummers method: the oxidative cleavage of CC double bonds by permanganate in acidic aqueous condition and the acid-catalyzed hydrolysis of epoxies. The epoxy-hydroxyl ratio affected the thermal stability of GO and influenced the electrical conductivity of the resultant Th-rGO. Extending step II oxidation was found to result in a large number of hydroxyl groups in GO that are more thermally stable than epoxy groups; these hydroxyl groups survive high annealing temperatures without decomposing and hamper electric conduction in the product Th-rGO. The experimental results and related theoretical suggestions of this work may be helpful for blueprinting other improved protocols related to Hummers method and for designing customized GO by its purpose from the very level of its chemical structure.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03700. Experimental details, experimental scheme, UV−vis spectra of oxidant solution, DLS, XPS C 1s graphs, zeta potential, XRD profiles, OM images, and TGA profiles. (PDF)
Figure 9. (a) The electrical conductivity data of the two kinds of ThrGOs obtained by annealing casted GO films up to 200 °C (black) and 300 °C (red). (b) The correlation between Raman ID/IG values of the GO samples with different degrees of step II oxidation and those of the Th-rGO samples prepared from the corresponding GO samples.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
supports that the reaction between GO and permanganate or water molecules during the step II oxidation process resulted in the increase in defects at the molecular level in the carbon frameworks of GO. On the contrary, the harsher and longer step II oxidation conditions brought about the lower ID/IG ratio of resultant Th-rGO. For instance, annealing the GO with the largest ID/IG ratio, A952, resulted in the Th-rGO simultaneously showing both the lowest ID/IG ratio and the lowest electrical conductivity. This paradoxical result can be explained by the survived hydroxyl groups within Th-rGOs detected by FT-IR (Figure 8a). Hydroxyls produced by step II oxidation can prevent the carbon atoms within carbon frameworks of GO from being decomposed as CO or CO2 during the thermal reduction process because of their high thermal stability. This results in a low ID/IG ratio of the resultant Th-rGO. Furthermore, these survived hydroxyls may also act as obstacles deterring the drift of charge carriers flowing across the conjugated sp2 system of Th-rGO and thus resulting in low electrical conductivity.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Midcareer Researcher Program through the National Research Foundation of Korea, funded by the Ministry of Science, ICT & Future Planning (No. 2010-0029244).
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REFERENCES
(1) Compton, O. C.; Nguyen, S. T. Graphene Oxide, Highly Reduced Graphene Oxide, and Graphene: Versatile Building Blocks for Carbon-Based Materials. Small 2010, 6, 711−723. (2) Cheng, C.; Li, D. Solvated Graphenes: An Emerging Class of Functional Soft Materials. Adv. Mater. 2013, 25, 13−30. (3) Kuila, T.; Mishra, A. K.; Khanra, P.; Kim, N. H.; Lee, J. H. Recent Advances in the Efficient Reduction of Graphene Oxide and Its Application as Energy Storage Electrode Materials. Nanoscale 2013, 5, 52−71. (4) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (5) Shao, G.; Lu, Y.; Wu, F.; Yang, C.; Zeng, F.; Wu, Q. Graphene Oxide: The Mechanisms of Oxidation and Exfoliation. J. Mater. Sci. 2012, 47, 4400−4409. (6) Dimiev, A. M.; Tour, J. M. Mechanism of Graphene Oxide Formation. ACS Nano 2014, 8, 3060−3068. (7) Kim, T.; Kang, J. H.; Yang, S. J.; Sung, S. J.; Kim, Y. S.; Park, C. R. Facile Preparation of Reduced Graphene Oxide-Based Gas Barrier Films for Organic Photovoltaic Devices. Energy Environ. Sci. 2014, 7, 3403−3411.
4. CONCLUSION In this work, Hummers method was divided into two sequential steps: the conventional oxidation step before the water addition was named as step I oxidation of Hummers method, and the further oxidation process after the addition of water and before the addition of H2O2 was referred to as step II oxidation. As step II oxidation is extended, in-plane voids developed within the GO sheets were observed, especially prominently at a higher temperature. Moreover, the graphitic sp2 conjugated 763
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Article
Chemistry of Materials (8) Botas, C.; Á lvarez, P.; Blanco, C.; Santamaría, R.; Granda, M.; Ares, P.; Rodríguez-Reinoso, F.; Menéndez, R. The Effect of the Parent Graphite on the Structure of Graphene Oxide. Carbon 2012, 50, 275−282. (9) McAllister, M. J.; Li, J.-L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite. Chem. Mater. 2007, 19, 4396−4404. (10) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M. Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4, 4806−4814. (11) Guo, P.; Song, H.; Chen, X. Hollow Graphene Oxide Spheres Self-Assembled by W/O Emulsion. J. Mater. Chem. 2010, 20, 4867− 4874. (12) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240. (13) Yang, S. J.; Kim, T.; Jung, H.; Park, C. R. The Effect of Heating Rate on Porosity Production During the Low Temperature Reduction of Graphite Oxide. Carbon 2013, 53, 73−80. (14) Hu, X.; Yu, Y.; Hou, W.; Zhou, J.; Song, L. Effects of Particle Size and Ph Value on the Hydrophilicity of Graphene Oxide. Appl. Surf. Sci. 2013, 273, 118−121. (15) Chua, C. K.; Sofer, Z.; Pumera, M. Graphite Oxides: Effects of Permanganate and Chlorate Oxidants on the Oxygen Composition. Chem. - Eur. J. 2012, 18, 13453−13459. (16) Eng, A. Y. S.; Poh, H. L.; Šaněk, F.; Maryško, M.; Matějková, S.; Sofer, Z.; Pumera, M. Searching for Magnetism in Hydrogenated Graphene: Using Highly Hydrogenated Graphene Prepared Via Birch Reduction of Graphite Oxides. ACS Nano 2013, 7, 5930−5939. (17) Kim, F.; Luo, J.; Cruz-Silva, R.; Cote, L. J.; Sohn, K.; Huang, J. Self-Propagating Domino-Like Reactions in Oxidized Graphite. Adv. Funct. Mater. 2010, 20, 2867−2873. (18) Kim, D.; Yang, S. J.; Kim, Y. S.; Jung, H.; Park, C. R. Simple and Cost-Effective Reduction of Graphite Oxide by Sulfuric Acid. Carbon 2012, 50, 3229−3232. (19) Lee, M.; Kim, B.-H.; Lee, Y.; Kim, B.-T.; Park, J. B. Synthesis and Characterization of Graphene and Graphene Oxide Based Palladium Nanocomposites and Their Catalytic Applications in Carbon-Carbon Cross-Coupling Reactions. Bull. Korean Chem. Soc. 2014, 35, 1979−1984. (20) Eigler, S.; Grimm, S.; Hof, F.; Hirsch, A. Graphene Oxide: A Stable Carbon Framework for Functionalization. J. Mater. Chem. A 2013, 1, 11559−11562. (21) Kim, F.; Cote, L. J.; Huang, J. Graphene Oxide: Surface Activity and Two-Dimensional Assembly. Adv. Mater. 2010, 22, 1954−1958. (22) Cote, L. J.; Kim, J.; Zhang, Z.; Sun, C.; Huang, J. Tunable Assembly of Graphene Oxide Surfactant Sheets: Wrinkles, Overlaps and Impacts on Thin Film Properties. Soft Matter 2010, 6, 6096−6101. (23) Williams, G.; Seger, B.; Kamat, P. V. Tio2-Graphene Nanocomposites. Uv-Assisted Photocatalytic Reduction of Graphene Oxide. ACS Nano 2008, 2, 1487−1491. (24) Abdelsayed, V.; Moussa, S.; Hassan, H. M.; Aluri, H. S.; Collinson, M. M.; El-Shall, M. S. Photothermal Deoxygenation of Graphite Oxide with Laser Excitation in Solution and Graphene-Aided Increase in Water Temperature. J. Phys. Chem. Lett. 2010, 1, 2804− 2809. (25) Guardia, L.; Villar-Rodil, S.; Paredes, J. I.; Rozada, R.; MartínezAlonso, A.; Tascón, J. M. D. Uv Light Exposure of Aqueous Graphene Oxide Suspensions to Promote Their Direct Reduction, Formation of Graphene−Metal Nanoparticle Hybrids and Dye Degradation. Carbon 2012, 50, 1014−1024. (26) Gollavelli, G.; Ling, Y.-C. Multi-Functional Graphene as an In vitro and In vivo Imaging Probe. Biomaterials 2012, 33, 2532−2545. (27) Choi, E.-Y.; Han, T. H.; Hong, J.; Kim, J. E.; Lee, S. H.; Kim, H. W.; Kim, S. O. Noncovalent Functionalization of Graphene with EndFunctional Polymers. J. Mater. Chem. 2010, 20, 1907−1912. (28) Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy; Thomson Learning: Boston, 2001.
(29) Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. New Insights into the Structure and Reduction of Graphite Oxide. Nat. Chem. 2009, 1, 403−408. (30) Yan, H.; Tao, X.; Yang, Z.; Li, K.; Yang, H.; Li, A.; Cheng, R. Effects of the Oxidation Degree of Graphene Oxide on the Adsorption of Methylene Blue. J. Hazard. Mater. 2014, 268, 191−198. (31) Wang, G.; Sun, X.; Liu, C.; Lian, J. Tailoring Oxidation Degrees of Graphene Oxide by Simple Chemical Reactions. Appl. Phys. Lett. 2011, 99, 053114. (32) Gao, Y.; Kim, S.; Zhou, S.; Chiu, H.-C.; Nelias, D.; Berger, C.; de Heer, W.; Polloni, L.; Sordan, R.; Bongiorno, A.; Riedo, E. Elastic Coupling between Layers in Two-Dimensional Materials. Nat. Mater. 2015, 14, 714−720. (33) Loc Duong, D.; Kim, G.; Jeong, H.-K.; Hee Lee, Y. Breaking Ab Stacking Order in Graphite Oxide: Ab Initio Approach. Phys. Chem. Chem. Phys. 2010, 12, 1595−1599. (34) Lee, D. W.; Seo, J. W. Formation of Phenol Groups in Hydrated Graphite Oxide. J. Phys. Chem. C 2011, 115, 12483−12486. (35) Szabó, T.; Berkesi, O.; Forgó, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dékány, I. Evolution of Surface Functional Groups in a Series of Progressively Oxidized Graphite Oxides. Chem. Mater. 2006, 18, 2740−2749. (36) Yumura, T.; Yamasaki, A. Roles of Water Molecules in Trapping Carbon Dioxide Molecules inside the Interlayer Space of Graphene Oxides. Phys. Chem. Chem. Phys. 2014, 16, 9656−9666. (37) Wang, L.-F.; Ma, T.-B.; Hu, Y.-Z.; Wang, H. Atomic-Scale Friction in Graphene Oxide: An Interfacial Interaction Perspective from First-Principles Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 125436. (38) Jeong, H.-K.; Lee, Y. P.; Jin, M. H.; Kim, E. S.; Bae, J. J.; Lee, Y. H. Thermal Stability of Graphite Oxide. Chem. Phys. Lett. 2009, 470, 255−258. (39) Gao, X.; Jang, J.; Nagase, S. Hydrazine and Thermal Reduction of Graphene Oxide: Reaction Mechanisms, Product Structures, and Reaction Design. J. Phys. Chem. C 2010, 114, 832−842.
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