Guidelines for Tailored Chemical Functionalization ... - ACS Publications

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Guidelines for Tailored Chemical Functionalization of Graphene Mi Se Chang,† Yern Seung Kim,† Jong Hun Kang,‡ Jisoo Park,† Sae Jin Sung,† Soon Hyeong So,† Kyung Tae Park,† Seung Jae Yang,§ Taehoon Kim,*,† and Chong Rae Park*,† †

Carbon Nanomaterials Design Laboratory, Research Institute of Advanced Materials, and Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea ‡ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, the United States § Advanced Nanohybrids Laboratory, Department of Applied Organic Materials Engineering, Inha University, Incheon 402-751, Republic of Korea S Supporting Information *

ABSTRACT: Graphene oxide (GO) has been synthesized by the Hummers method with modification of experimental condition by different research groups, but there is no guideline to prepare tailored GO for targeted applications. In this research, we suggest a guideline for tailor-fittable functionalization of graphene on the basis of the scope of our previous report on the two-step oxidation of GO. We describe a detailed procedure for synthesis of GO, effects of degree of step I oxidation on characteristics of GO and comparing them with effects of degree of step II oxidation. Characteristic changes of GO occurring during step I oxidation and those occurring during step II oxidation are different in species of oxygen functional groups, interlayer spacing, thermal stability, size distribution, and yield of GO. On the basis of the results, three types of tailor-fitted GO for a fiber, transparent conducting film, and hydrogen storage material are synthesized by controlling the degree of step I and step II oxidation. Compared to the reference GO synthesized by conventional modified Hummers method, the tailor-fitted GO showed 33.5%, 117%, and 104% enhanced performance in strength of the fiber, figure of merits of transparent conducting film, and hydrogen storage, respectively. Our results show that the performance of GO based application is significantly influenced by the synthesis condition of GO, and optimized performance of the applications can be obtained by the tailor-fitted functionalization of GO. We anticipate that this study would be helpful for a variety of researches, both synthesis and application of GO.

1. INTRODUCTION Graphene oxide (GO), oxygenated single-layered carbon sheets, is profoundly being researched as a solution processing method for graphene fabrication.1−4 GO was first synthesized by Brodie,5 later improved with 2 other methods reported by Staudenmaier,6 and Hummers7 consecutively. Among them, the modified Hummers method using KMnO4 (potassium permanganate) as the oxidant and H2SO4 (sulfuric acid) as the medium has been used most widely due to its applicable characteristics.7 Although many GO based applications/devices fabricated using the modified Hummers method is being developed,8−11 the marked difference between the morphological, chemical, and physical properties of GO that are being used in each research groups has become a huge hindrance. That is, though GO is being fabricated using the modified Hummers method, experimenting conditions differ for each research groups and a reliable mechanism for synthesis have not yet been precisely reported for guidance to GO synthesis with favorable characteristics. Several attempts have been made to describe the changes in the properties of GO with the oxidation conditions as a © 2016 American Chemical Society

controlling parameter. For example, the relationship between the amount of oxidant and the properties of the resultant GO,12 the increased time of oxidation and the change in the size of GO,13 or both14 as well as other issues15−17 have been investigated in earlier studies. Although those studies focused on the amounts of oxidants and reaction times of the modified Hummers method in order to control the main oxidant of Mn2O7 (dimanganese heptoxide), we focus on effect of the procedure after the addition of water to terminate the oxidation by Mn2O7, as the experimental conditions vary depending on the research group. In our previous research, we referred to the previously known reaction occurring due to Mn2O7 as step I oxidation and the oxidation occurring after halting the reaction of the two oxidants as step II oxidation.18 During the process of step I oxidation, the two chemicals, KMnO4 and H2SO4, react Special Issue: Methods and Protocols in Materials Chemistry Received: July 15, 2016 Revised: September 9, 2016 Published: September 9, 2016 307

DOI: 10.1021/acs.chemmater.6b02885 Chem. Mater. 2017, 29, 307−318

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Chemistry of Materials

Figure 1. Photos showing the protocols of GO synthesis through step I and step II oxidation. (a) Dispersion of preoxidized graphite flake in H2SO4, (b) addition of KMnO4 for initiation of step I oxidation, (c, d) addition of H2O for initiation of step II, (e, f) addition of H2O2 for termination of step II oxidation, and (e′, f′) addition of oxidized graphite by step I process directly into iced H2O2/water solution to skip step II oxidation.

ducting films; TCFs) and energy (hydrogen storage materials) applications based on the customized GO. The merits of the suggested preparation methodology are confirmed by the comparison of a GO counterpart prepared by the conventional modified Hummers method. We postulate that this standard protocol represents a new and viable mean of preparing graphene-based materials for targeted applications.

to form Mn2O7.19,20 When graphite comes into contact with this powerful oxidant, oxidation and exfoliation occur between the graphite sheets, thus forming GO.20 The step I oxidation process is generally known as the main oxidation process in the Hummers method, which is terminated by an addition of water and a H2O2 solution. However, because MnO4− with a high oxidizing potential exists in the step between the addition of water and the addition of H2O2, we hypothesized and demonstrated that additional oxidation occurred during the step II oxidation process. The controlling conditions differing in each research group include a hidden oxidation step that vastly influences the properties of GO. A new perspective on the Hummers method consisting of two-step oxidation provides us with a guideline for more systematic experimental designs without the undesirable characteristics of the resultant GO. As a next step, we plan to build a protocol for the synthesis of customized GO on the basis of the new perspective. Controlling the morphological, chemical, and physical properties of GO is a significant issue when attempting to optimize GO-based applications, but procedures that consider both step I and step II oxidation have yet to be studied. In this article, we present a detailed protocol for the synthesis of GO and conduct an experiment to assess the physical and chemical properties of GO. First, we provide a step-by-step synthesis protocol of GO based on the concept of two-step oxidation in our previous research. Subsequently, characteristic changes of GO relative to the degree of step I oxidation are investigated to examine differences in the functional groups, size distribution, and yields of the resulting GO after step I oxidation and step II oxidation. As part of this process, we reconstructed step II oxidation data from our previous report18 and compared this data with the step I oxidation data obtained in the present work. Although studies of step I oxidation are available in literature,12−14 a systematic comparison of step I oxidation and step II oxidation has not been done. Lastly, we present an experimental design for the preparation of customized GO for target applications. In particular, we fabricate structural (fibers), and electrical (transparent con-

2. EXPERIMENTAL SECTION 2.1. Materials. Natural flake graphite (Lot #: 17425HO, +100 mesh), potassium peroxydisulfate (K2S2O8, 98%), phosphorus pentoxide (P4O10, 99.99%), and potassium permanganate (KMnO4, 99%) were purchased from SigmaAldrich. Sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30%), and hydrochloric acid (HCl, >35%) were purchased from Daejung Chemicals. 2.2. Protocols for the Synthesis of GO. Protocols for the synthesis of GO in our group are based essentially on the modified Hummers method reported by Kovtyukhova et al.21 To control the degree of oxidation and to investigate the effects of oxidation, we adjusted the three variables of the step I oxidation time, the step II oxidation time, and the step II oxidation temperature. All GO samples in this work started at the step of preoxidation. 2.2.1. Preparation of GO: Preoxidation. The natural flake graphite was stored in a 100 °C vacuum oven to evaporate the water vapor, which can have adverse effects on the preoxidation condition. After a sufficient drying process, the graphite was treated with K2S2O8 and P4O10 to increase the yield of GO. 2.0 g of K2S2O8 and 2.0 g of P4O10 were put into a round-bottomed flask, followed by the addition of 10 mL of H2SO4 into the flask. This process should be performed as quickly as possible because P4O10 is a deliquescent substance. The excessive exposure of P 4 O 10 in an ambient atmosphere would contaminate the reaction mixture and hinder the effective peroxidation of graphite. Subsequently, the temperature of the oxidant mixture increased to 80 °C. Once the oxidant was fully dissolved, 2.0 g of graphite was carefully added to the flask and stirred for 4.5 h. The intercalation of the bisulfate changed the 308


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Chemistry of Materials color of the graphite flakes to dark blue, indicating the formation of graphite bisulfate.20 The mixture was then slowly poured into 1 L of deionized (DI) water and filtered and washed with excess DI water at least three times to remove the oxidation agents. Once the filtration was finished, the expanded graphite was dried in a vacuum at 40 °C in an oven for 3 days. 2.2.2. Preparation of GO: Step I Oxidation. With the preoxidized expanded graphite, the modified Hummers method was utilized to obtain GO. Compared to the original Hummers method, sodium nitrate was not used in here due to the following two reasons: (1) residual NO3− ions are not readily removed, and (2) toxic gases such as NO2 and N2O4 are generated during the oxidation process. 92 mL of H2SO4 was poured into a double-walled glass reactor vessel and set at 0 °C with a coolant circulator. 2.0 g of preoxidized expanded graphite was then carefully added to the vessel (Figure 1a). 12 g of KMnO4 was then slowly added to the vessel to the prevent temperature of the mixture from exceeding 10 °C. The color of the mixture became dark green, indicating the formation of dimanganese heptoxide (Figure 1b). The mixture was stirred at 35 °C with a stirring rate of 500 rpm while enclosed with a reactor lid. From this point, the oxidation time was controlled before the DI water was added to break the step I reaction. In the step I oxidation process, we controlled the degree of oxidation by controlling the oxidation time without changing the reaction temperature because it is dangerous to increase the temperature of a reactor that contains Mn2O7, an explosive compound. The effects of the oxidation time are investigated in this work by controlling the oxidation time to 2, 4, 8, 16, 32, and 48 h with an identical step II oxidation condition. 2.2.3. Preparation of GO: Step II Oxidation. Once the reaction reached the designated oxidation time, the mixture was cooled below 0 °C for safety, after which 200 mL of DI water was very carefully added while monitoring and controlling the temperature of the system so as not to exceed the desired temperature of the subsequent step II oxidation process. At the very initial stage, a single drop of water could have caused a rapid increase in the temperature of 2−10 °C. During this process, the color of the reaction mixture changed to the color of MnO4−, dark purple (Figure 1c), because the dark green Mn2O7 was decomposed by water. The water pouring procedure in this step was finished within 15 min. The oxidation mixture was transferred to a bath at the step II oxidation temperature and was mechanically stirred at 500 rpm for remaining the step II oxidation time. To terminate the step II oxidation, 10 mL of a H2O2 solution was added to the sample drop by drop. The color of the mixture changed from purple to bright yellow due to the reduction of the MnO4− oxidant (Figure 1e,f). Step II oxidation can be skipped if using a mixture of ice and H2O2, as reported previously.17 After step I oxidation, we directly poured the sample containing GO, H2SO4, and Mn2O7 into a mixture of excess ice and 10 mL of H2O2 (Figure 1e′,f′). Because the Mn2O7 immediately reduced to Mn2+ and given that the temperature was close to 0 °C, the potential for oxidation by MnO4− was minimized during this process. In the original Hummers method, step II oxidation is typically performed at 98 °C for 15 min.7 Other reaction conditions of step II oxidation have also been used in various works for the synthesis of GO.22−24 Our previous study indicated that step II oxidation influences the morphological, chemical, and physical nature of GO,18 and these results are

revisited in this work to compare the effects of step I oxidation and step II oxidation. 2.2.4. Preparation of GO: Purification. The mixture in which all oxidants were removed was centrifuged at 10,000 rpm for 20 min. The sediment after centrifugation was washed with 1 M HCl and centrifuged at 13,000 rpm for 20 min at least five times to remove any residual Mn2+ ions. Subsequently, the sediment was repeatedly washed with DI water by centrifuging it at 13,000 rpm for 30 min until it was neutralized. The final GO suspension was obtained by the careful decanting of the supernatant phase after centrifugation at 4,000 rpm for 30 min. The concentration of the undiluted GO suspension ranged from 7 to 13 mg/mL. The obtained sample was stored in a refrigerator to suppress any further degradation by water.25 2.3. Preparation of Tailor-Fitted GO Based on the Protocols. 2.3.1. Nomenclature of GO Samples. The nomenclature of the GO sample is in this case GO(nh) (ooC, ph), where n represents the period of step I oxidation, oo represents the temperature of step II oxidation, and p represents the period of step II oxidation. For example, GO(48h)(45C, 2h) represents the GO sample with the step I reaction lasting 48 h and the step II reaction lasting 2 h at 45 °C. The sample prepared without step II oxidation using iced water and the H2O2 solution is denoted as GO(nh)(0C, 0h). 2.3.2. Preparation of GO To Investigate the Influence of the Degree of Oxidation. As noted above, we prepared seven GO samples with various step I oxidation times to observe the effect of step I oxidation. The temperature and time of the step II oxidation process were set to 45 °C and 15 min, respectively. Eight GO samples with different degrees of step II oxidation were prepared and characterized in our previous report.10 In the present study, for a comparison of the step I oxidation and step II oxidation effects, the data from our previous report was reconstructed. In that earlier study, prepared the samples while controlling the temperature (0, 45, 70, 95 °C) and time (2, 4, 8 h) of the step II oxidation process. The time for step I oxidation was fixed at 2 h. The experimental conditions are summarized in Tables 1 and 2. Table 1. Summary of the GO Samples with Varied Step I Oxidation Parameters step I oxidation

step II oxidation

sample code

time (hours)

temperature (°C)

time (hours)

GO(2h) (45C, 1/4h) GO(4h) (45C, 1/4h) GO(8h) (45C, 1/4h) GO(16h) (45C, 1/4h) GO(32h) (45C, 1/4h) GO(48h) (45C, 1/4h)

2 4 8 16 32 48

45 45 45 45 45 45

1/4 1/4 1/4 1/4 1/4 1/4

2.3.3. Preparation of Tailor-Fitted GO for Applications. Target applications for the GO samples were prepared by modifying the step I or step II condition based on the physical and/or chemical properties of GO as observed in this study. In this study, three types of applications were tested: structural (fiber), electrical (TCF), and energy applications (hydrogen storage). Each of the applications requires different properties; hence, different characteristics of GO were necessary. We synthesized GO(48h)(0C, 0h) for the fabrication of GO fiber, GO(2h)(0C, 0h) for the fabrication of TCF, and GO(48h)(95C, 2h) as a hydrogen storage material. Details about the 309


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TCFs were measured by a UV−vis spectrometer and a fourpoint probe station (M4P 205, MS Tech.), respectively. 2.5. Characterization. The surface characteristics and functional groups in GO were characterized by X-ray photoelectron spectroscopy (XPS; Sigma Probe, Thermo Scientific), solid/microimaging high-resolution NMR (Bruker AVANCE 400WB, 100.7 MHz for 13C nuclei), Fouriertransform infrared spectroscopy (FT-IR; Thermo Scientific Nicolet), and ultraviolet visible spectroscopy (UV−vis; Varian UV−vis-NIR Cary 5000). The average particle size and the dispersibility of GO in an aqueous solution was investigated by dynamic light scattering (DLS; ELSZ 1000ZS size) and a ζpotential analyzer (ζ-potential analyzer, Otsuka). When we measured the ζ-potential of GO, each GO sample was thoroughly diluted with ultrapure water (ρ = 18.0 MΩ cm). The measurements were performed as diluted, and no additional buffer solution was used. The pH of the GO suspension was 5.1. The defects and interlayer spacing of the GO were characterized by a Raman microscope (Raman plus, Nanophoton) and by powder X-ray diffraction (PXRD; D8 Advance, Bruker) using Ni-filtered Cu Kα radiation (λ = 0.154 184 nm), respectively. The thermal behaviors of the GO samples were studied by means of a thermal gravimetric analysis (TGA, SDT-Q600, TA Instruments). Microscopic images of the GO were taken with an optical microscope (OM; Olympus BX-51) in the transmittance mode. The morphologies of the GO were examined by scanning electron microscopy (SEM; JSM-6700F, JEOL). It should be noted that the series of GO with varied step I oxidation and that of GO with varied step II oxidation have different storage time before characterization. Chemical structure of GO is changed during storage even at room temperature.25−27 Therefore, we can observe a trend in the series of samples but cannot compare the two series of samples. The storage period of GO is one factor that determines the characteristics of GO. However, this is not included in this study because the aim of this study is to describe the method and protocols of synthesis of GO.

Table 2. Summary of the GO Samples with Varied Step II Oxidation Parameters step I oxidation sample code GO(2h) GO(2h) GO(2h) GO(2h) GO(2h) GO(2h) GO(2h) GO(2h)

step II oxidation

time (hours)

temperature (°C)

time (hours)

2 2 2 2 2 2 2 2

0 45 45 45 70 70 70 95

0 2 4 8 2 4 8 2

(0C, 0h) (45C, 2h) (45C, 4h) (45C, 8h) (70C, 2h) (70C, 4h) (70C, 8h) (95C, 2h)

selection of GO in accordance with each application are given in the Results and Discussion section. To compare the enhanced performance of the customized GO, we also synthesized GO based on the protocol of the modified Hummers method reported by Kovtyukhova et al.21 This reference GO is termed GO(2h)(98C, 1/4h), and we simply use the convention of GOref in this article for clarity of the discussion. The experimental conditions for tailored GO and reference GO are summarized in Table 3. Table 3. Summary of the GO Samples Prepared for Applications step I oxidation

step II oxidation

sample code

time (hours)

temperature (°C)

GO(48h) (0C, 0h) GO(2h) (0C, 0h) GO(48h) (95C, 2h)

48 2 48

0 0 95

0 0 2

2

98

1/4

GOref

time (hours)

target applications fibers TCF hydrogen storage controlled sample

2.4. Preparation of GO Based Applications. GO fibers were prepared following our previously reported diamine crosslinking method.15 An amount of 10 mg/mL of the GO solution was injected into 0.1 M of an aqueous solution with dissolved ethylenediamine through a syringe with an inner diameter of 0.413 mm. The prepared GO gel fibers were sufficiently washed with ethanol, and the washed gel fibers were fixed onto a vertical frame followed by drying in a vacuum chamber at room temperature for 24 h. Tensile testing of the dried GO fibers was performed using a tensile test machine (Instron-5543, Instron) with a gauge length and loading rate of 10 mm and 1 mm/min, respectively. Graphene foam as an energy-storage adsorbent was fabricated by the freeze-drying of the GO solution. For this purpose, 20 mL of 5 mg/mL GO solutions stored in conical tubes as samples were directly immersed in liquid nitrogen and then freeze-dried for 72 h using a freeze-dryer (Bondiro, Ilshin Lab.). The freeze-dried GO foam samples were then thermally reduced at 300 °C for 1 h at a heating rate of 10 °C/min. The fabricated reduced GO foams were characterized by a cryogenic nitrogen adsorption analyzer and a hydrogen adsorption analyzer. Graphene TCFs were fabricated by the spin-coating of 3 mg/ mL of the GO solution onto a glass substrate followed by a heat treatment at 500 °C lasting 4 h (heating rate 5 °C/min, N2 atmosphere). The transmittance and sheet resistance of the

3. RESULTS AND DISCUSSION 3.1. Physical and Chemical Properties of GO. In this section, the chemical and physical characteristics of the GO fabricated through the modifications of the step I and step II procedures based on this work and our previous reports18 are investigated and summarized. First, the chemical nature of GO with each oxidation step was analyzed by XPS, FT-IR, and 13C NMR spectra examinations (Figure 2). To obtain quantitative information, C 1s peaks of the samples was deconvoluted as shown in Figures 2a,b. The peaks were assigned to CC (284.5 eV), CO (286.2 eV), and OCO (288.5 eV) carbon atoms.28 The area of the CC peak shows a steady decrease as the oxidation time increases, whereas the CO and OCO peaks increase with the oxidation time (Figure 2a), indicating that as the contact time between the graphite and the reagents increases, more sp2 bonds of intercalated graphite are broken by the strong oxidant, Mn2O7, and more oxygen functional groups are formed on the surface of the graphite. Similarly, the prolonged step II oxidation increases the number of oxygen-containing functional groups. It was shown in our previous work that the populations of CO and O CO carbon gradually increased (Figure 2b). Change of the functional groups in the GO samples was also examined using FT-IR as shown in Figures 2c,d. Intensities of the OH stretching band (3,400 cm−1) and CO bond 310


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Figure 2. (a, b) Quantitative analysis of various forms of C calculated from deconvoluted C 1s XPS data, (c, d) FT-IR spectra and (e, f) direct 13C pulse MAS (7.0 kHz) NMR spectra of GO with (a, c, e) step I (30 min of step II oxidation at 45 °C) and (b, d, f) step II oxidation (2 h of step I oxidation at 35 °C). Panels b, d, and f were reconstructed from the authors’ previous report.18

(1,720 cm−1) were increased as step I oxidation and step II oxidation prolonged, corresponding with the XPS results. In contrast, a different trend was observed in the peaks of the epoxide group (1,250 and 1,100 cm−1) as step I oxidation and step II oxidation extended. Population of the epoxide group was increased as progress of step I oxidation but decreased as progress of step II oxidation. The difference in reaction during each oxidation can be further elucidated by the NMR analysis results, as shown below.18 To differentiate correctly the specific functional groups that are undistinguishable in the XPS analysis, such as the epoxy and hydroxyl groups and carbonyl and carboxyl groups, we investigated the surface characteristics of GO samples with the 13C NMR spectra (Figure 2e,f). In these spectra, the peaks at 61, 70, 100, 133, 167, and 200 ppm correspond to epoxides, alcohols, lactols, graphitic carbons, carboxylates, and ketones, respectively.29 As shown in the XPS results, the prolonged step I oxidation increases the number of all oxygen functional groups, an outcome that is similarly observed in the NMR spectra (Figure 2e). A similar trend of an increase in the overall functional groups (except for the epoxy groups) after step II oxidation was observed in the NMR spectra of GO (Figure 2f). The prolonged step II oxidation tends to hydrolyze the epoxy groups into hydroxyl groups, as demonstrated by simultaneous increase and decrease of hydroxyl and epoxy groups, respectively.18 The detailed mechanism behind this hydrolysis

of the epoxy group under an acidic aqueous condition was discussed in our previous work.18 In addition to the characterization of the different functional groups, the effects on the dispersibility of GO in water were characterized by a ζ-potential analyzer (see Figure 3a,b). It has been reported that a ζ-potential value below −40 mV usually indicates moderate dispersibility in water, whereas values higher than that show good dispersibility, as indicated in Figure 3a,b.18 Such an increase in the negative value indicates an increase in the ionic charges within the sample, with great repulsion from each other, thus leading to good dispersion in water.30 GO(2h)(45C, 1/4h) shows a ζ-potential value of −33.6 mV, demonstrating moderate dispersibility in water. As the oxidation time increases, the ζ-potential value also increases in the negative direction, with GO(48h)(45C, 1/4h) having the largest negative value and therefore showing the best dispersibility among the samples studied here. An increase in the step I oxidation time leads to an increase in the exfoliation and fracturing of graphene oxide, with it then fracturing into smaller, individual pieces, as further explained in the size analysis of the GO samples (Figure S1). In addition, it has been reported that carboxylate groups are usually attached onto the edges of graphene oxide.31 Therefore, as the oxidation time increases, many more edges of graphene oxide are exposed to the oxidant, increasing the formation of carboxylic groups and resulting in the highest negative ζ-potential value. This is in 311


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Figure 3. (a, b) ζ-potentials and (c, d) UV−vis absorption spectra of GO samples with prolonged (a, c) step I (15 min of step II oxidation at 45 °C) and (b, d) step II oxidation (2 h of step I oxidation at 35 °C). Panels b and d were reconstructed from the authors’ previous report.18

peak around 2θ = 10°, corresponding to the lattice structure of GO and in good agreement with the literature.31 For step I oxidation, the 2θ value of each sample, as indicated in Figure 4a, decreases with an increase in the oxidation time, thus resulting in an increase in the interlayer spacing caused by the increase in the proportion of the functionalized domain in the graphene sheets. Meanwhile, a gradual increase in 2θ was observed with the prolonged step II oxidation process regardless of the oxidation temperature, as shown in Figure 4b. This decrease of the interlayer spacing of graphene can be attributed to the change in the chemical structure of GO, as shown in the NMR results. It was shown that the increase in the step II oxidation transformed the epoxide groups into hydroxyl groups. It can be estimated that the strong hydrogen bonds between the transformed hydroxyl groups will pull the adjacent graphene layer and thus decrease the interlayer spacing, as shown in Figure 4b. The thermal stability of graphene oxide in relation to the oxidation time was also characterized by a thermogravimetric analysis (TGA), as presented in Figure 4c,d. Briefly, the thermal stability of the GO samples was scarcely affected by step I oxidation, whereas step II oxidation generally increased the thermal stability of the GO samples. Specifically, step I oxidation did not show a particular correlation other than a higher temperature shift for the GO(32h)(45C, 1/4h) and GO(48h)(45C, 1/4h) samples (Figure 4c). In Figure 4c, which shows the differentiated profiles, all samples have three different peaks at around 165, 190, and 250 °C corresponding to the peaks also shown in the GO(2h)(0C, 0h) sample (with the reaction terminated at 0 °C immediately after step I oxidation has proceeded), as depicted in Figure 4d.18 No distinct shift was shown for the major thermal decomposition temperature in relation to the step I oxidation time, whereas the contribution

good agreement with the increase in the number of carboxyl groups presented in the XPS and NMR data. Similarly in the case of step II oxidation, the increase in carboxylic acids and their derivatives also negatively increase the ζ-potential of GO with the increments of the oxidation temperature and time (Figure 3b). The ultraviolet−visible (UV−vis) absorption spectra of the differently oxidized GO with prolonged step I oxidation are shown in Figure 3c. The π−π* transitions of the CC bonds for GO(2h)(45C, 1/4h) are originally located at 231 nm (approximately at a bandgap of 5.6 eV), but they are blueshifted to 227 nm after 48 h of oxidation. Moreover, the absorption coefficient in the short wavelength area less than 230 nm shows an increasing tendency with the increased oxidation time. This finding is evidence of a decrease in the remaining conjugation amount, requiring relatively more energy for an electronic transition due to the higher oxidation state with more functional groups on the basal plane of GO.32 In contrast, an absorption coefficient is increased with a shorter oxidation time in the area greater than 230 nm. It has been reported that the absorption peak is red-shifted and that the absorption coefficient belonging to the long wavelength area is increased as sp2 conjugation is restored with the reduction of GO.33 Likewise, in this study, with the shorter the step I oxidation for the sample, a less oxidized state and relatively greater sp2 conjugation were obtained, leading to a large absorption coefficient in the long wavelength area. A similar tendency in the blue shift of the π−π* transition of the CC bond was observed as the step II oxidation time was extended or as the temperature was increased (Figure 3d).18 XRD measurements were taken to examine the effect of each oxidation step on the interlayer spacing of the differently oxidized GO samples (see Figure 4a,b). All samples exhibited a 312


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Figure 4. (a, b) XRD and (c, d) TGA/DTG profiles of GO samples with (a, c) step I (15 min of step II oxidation at 45 °C) and (b, d) step II oxidation (2 h of step I oxidation at 35 °C). Panels b and d were reconstructed from the authors’ previous work.18

of the weight loss near 165 °C becomes smaller while that of the weight loss near 190 °C becomes bigger. Meanwhile, the prolonged step II oxidation shifts the main thermal decomposition peaks to a higher temperature, as shown in Figure 4d. This increased thermal stability can be attributed to the transformation of epoxide groups into a more stable hydroxyl group during the step II oxidation process.18 For a more detailed analysis showing the effects of these functional groups on the thermal behaviors and stability levels of each of the GO samples, mass spectroscopic analyses should be conducted in future work. Nevertheless, these observations are valuable in that they demonstrate how research on the thermal behavior of graphene-based materials should be done. The exfoliation yield was observed using an optical microscopy (OM, transmittance mode) analysis of the GO sample films without the removal of the unreacted graphite residue during the purification process (see Figure 5). As shown in the OM images in Figure 5, the fully exfoliated GO region can transmit light, and it exhibits a light yellowish color, whereas the unexfoliated graphite residues occupy the black region.34 As the oxidation time increases, the dark regions

disappear and the lighter region merges into one large area, indicating an increase in the exfoliation yield. Macroscopic images were also captured to observe the difference in the total amount of the resultant GO at each oxidation level (see Figure S4). Images are shown of the resultant GO collected in a measuring tube in an increasing order of the oxidation time. It can visually be observed that the amount of GO increases with the increase in the oxidation time, again resulting in an increase in the yield of GO. The concentration of each sample was determined by the drop-and-dry method to measure the yield of the resultant GO (see Table 4). As reported in our previous study, the step II oxidation process did not increase the yield of exfoliation,18 which conflicts with the results after step I oxidation (Figure 5b). The increase in the yield of GO from the original graphite can be explained by the formation of graphite bisulfate (GB: C24+HSO4−·2.4H2SO4), an intercalated types of graphite that forms with the coexistence of concentrated sulfuric acid and the strong oxidant Mn2O7 during the oxidation process.20 GB in the step I oxidation process is developed through the reaction between the concentrated sulfuric acid and the strong oxidant 313


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be confirmed by the increased viscosity of the reaction mixture during the extended step I oxidation. After 48 h of oxidation, the oxidation system became very pasty and viscous due to the exfoliated GO compared to the fluidic system at an oxidation time of 2 h (see Figure 6c). To conclude, the GB generated during the step I oxidation process and the successive exfoliation of the oxidized graphene layer from GB determine the yield of GO from the step I oxidation process. However, during step II oxidation after the water-pouring process, because GB under aqueous conditions is no longer stable, further exfoliation is suspended. Thus, no obvious increase in the yield was observed after the addition of water with an identical step I oxidation time (Figure 5b). The average particle sizes of each of the samples were obtained with SEM image analysis and DLS (Figures S1−S3). Both step I and step II oxidation generally decrease the average particle size of the samples with an increase in the oxidation time. However, the microscopic images of GO after step I and step II oxidation show several important differences in the size distributions between the two steps. In the GO samples with the extended step I oxidation time, a graphene sheet with a large lateral size (over 40 μm) was always observed regardless of the step I oxidation period. The reason for the existence of this large sheet even during extended step I oxidation is the successive exfoliation of a new GO sheet from GB. Given that step I oxidation develops newly exfoliated GO sheets from GB as the reaction processes, GO exfoliated in the early stage of the reaction fractures into smaller pieces with continuous oxidation and the GO exfoliated in the latter stage of the reaction remain in bigger sizes (Figure 7). On the other hand, because the GO

Figure 5. OM images (transmittance mode) of GO samples with prolonged (a) step I and (b) step II oxidation. Panel b was reconstructed from the authors’ previous work.18

Table 4. Yield of Resultant GO Samples Treated at Various Oxidation Times samples

yield (%)

GO(2h)(45C, 1/4h) GO(4h)(45C, 1/4h) GO(8h)(45C, 1/4h) GO(16h)(45C, 1/4h) GO(32h)(45C, 1/4h) GO(48h)(45C, 1/4h)

1.60 2.41 11.79 18.15 23.24 32.04

Figure 7. SEM images of (a) GO(2h)(45C, 1/4h), (b) GO(8h)(45C, 1/4h), and (c) GO(48h)(45C, 1/4h).

sheet is not newly exfoliated from GB during the step II oxidation process, the sizes of the graphene sheets tend continuously to decrease, with no large-sized GO sheets appearing. The exfoliated graphene layers during step II oxidation are fractured into smaller pieces spontaneously when undergoing continuous oxidation.18 In addition, the longer oxidation time at a temperature exceeding 70 °C leads to the development of in-plane voids on the graphene sheet, possibly due to the oxidation cleavage of CC bonds by MnO4− ions, as shown in Figure S5. By analyzing and comparing the surface characteristics, exfoliation, and morphological characteristics relative to the time increase during the step I and step II oxidation process, the oxidation process of GO through the Hummers method can be represented in a schematic matter (see Figure 8). In step I oxidation, the main oxidant Mn2O7 in H2SO4 generates GB and exfoliate graphene sheets from the GB, increasing the overall yield of GO. Meanwhile, during step II oxidation, MnO4− formed by the addition of H2O in the step I oxidation mixture oxidizes the GO sheet and divides the sheet by the

(Figure 6a). During this step, the outermost part of GB starts to become oxidized by Mn2O7 and the outermost layer of graphite starts to exfoliate first, thereby causing the yield to increase with the oxidation time. The exfoliation of the outer layer of GB can

Figure 6. (a) Photo showing the generation of graphite bisulfate during step I, and photos taken during oxidation for samples (b) GO(2h)(45C, 1/4h) (fluidic) and (c) GO(48h)(45C, 1/4h) (pasty) before the initiation of step II oxidation. 314


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Figure 8. Schematic representation of step I and step II oxidation.

Table 5. Summary of the Overall Results of Step I Oxidation and Step II Oxidation step I oxidation

step II oxidation MnO4− in H3O+/H2O oxidative cleavage of double bonds and hydrolysis of epoxides decrease in the interlayer spacing

main oxidants functional groups interlayer spacing thermal stability yield

Mn2O7 in H2SO4 all functional groups increase.

size

The average size decreases with an increase in the oxidation time, but GO a few hundred μm in size can also be found.

increase in the interlayer spacing The decomposition temperature not affected. All samples show three distinct peaks at 165, 190, and 250 °C. The yield of exfoliation increases with an increase in the oxidation time.

The decomposition temperature increased with an increase in the oxidation time. The yield of exfoliation not affected with an increase in the oxidation time. The size of GO decreases with an increase in the oxidation time.

period of step II oxidation was shortened (Figure S6). Therefore, we can speculate that a lower degree of oxidation will enhance the electrical performance of reduced GO films. Likewise, performance of other applications will also increase if we design the GO properly based on the method and protocols in this study. To prove this possibility, we prepared several specific applications and designed a simple experimental procedure to synthesize the proper GO sample for each application by modifying the step I and step II oxidation processes. The performances of the targeted GO or GO derivative devices were then compared to those with GOref, as introduced in the initially reported modified Hummers method (2 h of step I and 15 min of step II oxidation at 98 °C). Characteristics of the GOref was summarized in Supporting Information (see Figure S7). The GO fibers were generally fabricated by the wet spinning of a high concentration (10 mg/mL) of liquid-crystalline GO suspension into a coagulation bath followed by an appropriate drying process. Recently, our group suggested a new method by which to synthesize GO fibers involving the adoption of simple diamine molecules as coagulants.31 Acidic surface groups such as carboxylic groups donate a proton to the both sides of the amine groups on the coagulant molecules. The protonated

oxidative cleavage of CC double bonds and the hydrolysis of the epoxide groups. The average size of the graphene sheet decreases after both step I and step II oxidation; however, the GO sheet with a greater lateral size (>40 μm) still exists after step I oxidation due to the newly exfoliated sheet from GB without any cleavage. Due to the hydrogen bonding between the hydrolyzed epoxy groups, the extended step II oxidation decreases the interlayer spacing between the graphene layers, whereas step I oxidation increases the interlayer spacing. The hydrolysis during step II oxidation also plays an important role in the improved thermal stability of GO. Meanwhile, the step I oxidation scarcely affects the thermal degradation temperature of GO. For a clearer understanding of the comparison of this study of step I oxidation with the results from our previous study of step II oxidation,18 the overall results are summarized in Table 5. 3.2. Fabrication of Target-Oriented GO by the Modification of the Step I and Step II Oxidation Processes. Based on the variation of the physical and chemical characteristics of GO according to the progress of the step I and step II oxidation processes, it becomes possible to coordinate graphene-based materials for various applications. For example, conductivity of reduced GO films increased as the 315


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Figure 9. (a) Representative stress−strain curves of fibers fabricated from GO(48h)(0C, 0h) (blue) and GOref (black), (b) conductivity (black) and figure of merit (red) of reduced GOref and reduced GO(2h)(0C, 0h), respectively. (c) Representative hydrogen adsorption isotherm of GOref (black dot) and GO(48h)(95C, 2h) (red dot), and (d) diagram showing the design of tailor-fitted GO for each application.

diamines then serve as an ionic bridge that assembles graphene into close-packed structures. During the drying process, the epoxide groups on the in-plane section of GO can chemically cross-link with amine groups on the coagulate molecules and further increase the force between the graphene layers.31 The primitive conditions for the fabrication of fibers based on densely packed graphene are as follows: (i) large building blocks, and (ii) the formation of active functional groups such as carboxylic and epoxide groups that can interact with coagulant molecules. Recently, hierarchically sized graphene samples with a mixture of small and large flakes were shown to have the advantage of being able to bear much larger loads when formed into fibers due to their more compact structure via the intercalation of a small graphene sheet into large graphene building blocks.35 Hence, the GO sample created with the extended period of step I oxidation could be the proper candidate for the fabrication of a fibrous assembly of graphene. Meanwhile, step II oxidation was skipped by the synthesis of GO(48h)(0C, 0h) in order to (i) maximize the overall lateral size of GO, (ii) decrease the in-plane defects on the sheets, and (iii) maintain the epoxide function groups on GO to cross-link chemically with the diamine coagulants. Figure 9a shows the typical stress−strain curve of GO fibers spun from GO(48h)(0C, 0h) and GOref. As expected, the specific strength and modulus of the GO(48h)(0C, 0h) fiber were 33.5 and 10.0% larger than those of GOref fiber, respectively, as the additional functional groups generated by extended step I oxidation likely induced more ionic and/or covalent bonds between the graphene sheets and the diamine molecules without decreasing the size of the graphene building blocks (Figure 9a and Table 6). In the case of TCF for future electronics (Figure 9b and Table 7), the size of the graphene layers should be as large as

Table 6. Mechanical Performances of GO(48h)(0C, 0h) and GORef Fibers specific strength (mN/tex) specific modulus (N/tex)

GO(48h)(0C, 0h)

GOref

395 ± 31 32.9 ± 4.3

296 ± 30 29.9 ± 2.2

Table 7. TCF Performances of Thermally Reduced GO(2h)(0C, 0h) and GORef Films transmittance sheet resistance (kΩ/sq.) figure of merit (FoM)

GO(2h)(0C, 0h)

GOref

80.2% 41.3 0.039

80.1% 88.4 0.018

possible whereas the number of the functional groups and defects should be minimized.36 The functional groups formed by extended step I or II oxidation can generate defects which then serve as traps for electrons during the transport process.37 Therefore, step I oxidation was limited to 2 h at most without any step II oxidation by the synthesis of GO(2h)(0C, 0h). Indeed, as shown in Table 7, the figure of merit (FoM) of TCF from GO(2h)(0C, 0h) was 2.1 times higher than that of GOref with the similar transmittance of the films (around 80%), possibly due to the larger size of the graphene sheet with minimized defects and functional groups after the reduction process. At the same time, the conductivity of the thick film fabricated from an identical thermal reduction process of GO(2h)(0C, 0h) (1,160 S/m) was also much higher than that of GOref (330 S/m). Meanwhile, defect generation can have positive effects in applications to energy storage devices, including hydrogen storage applications. It has been reported that some of these defects can serve as more preferable sites by enhancing the van 316


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Chemistry of Materials der Waals force between the graphene and the hydrogen molecules and hence improving the hydrogen adsorption capacities.38 Additionally, these defects may distort the graphene sheet and therefore interrupt the easy stacking of graphene layers during the reduction, which can generate more accessible sites for hydrogen molecules. Moreover, the winkles generated by gas production during the thermal reduction of the functional groups of GO makes the graphene−hydrogen intermediate states thermodynamically more stable, which may also increase the hydrogen storage capacity.39 Therefore, to improve the hydrogen storage ability of graphene-based materials, we attempted to maximize the defect densities and functional groups by extending both the step I and step II processes by synthesizing the GO(48h)(95C, 2h) sample. Indeed, the reduced GO(48h)(95C, 2h) with more defective sites created by activating step II oxidation exhibits much higher hydrogen storage capacity at as much as 0.55 wt % (100 bar, 25 °C), which is 2.1 times higher than that of GOref (0.27 wt %) (Figure 9c). Given the results above, three different applications of graphene materials were tested with the target fabricated GO by adjusting the step I and step II oxidation processes. The performances of these customized GO samples were more suitable for each specific application than those of GOref, for which the fabrication method remained nearly fixed or slightly modified after its introduction. The standardized protocol of GO synthesis can increase the number of potential graphene applications through engineering of the chemistry, defects, and/ or size of the graphene-based materials.

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AUTHOR INFORMATION

Corresponding Authors

*Taehoon Kim. Tel: (+82) 2-880-7491. Fax: (+82) 2-8851748. E-mail address: [email protected]. *Chong Rae Park. Tel: (+82) 2-880-8030. Fax: (+82) 2-8851748. E-mail address: [email protected]. Author Contributions

M.S.C. and Y.S.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Carbon Valley R&D Project (R0000689) funded by the Ministry of Trade, Industry & Energy (MOTIE), Republic of Korea

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ABBREVIATIONS GO, graphene oxide; TCF, transparent conducting film; XPS, X-ray photoelectron spectroscopy; TGA, thermal gravimetric analysis; XRD, X-ray diffraction; DLS, dynamic light scattering; OM, optical microscopy; SEM, scanning electron microscopy; GB, graphite bisulfate

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4. CONCLUSION In this study, standard protocols for the manufacturing of GO were systematically suggested by categorizing the procedures into two main oxidation steps: the conventional oxidation step with KMnO4 in H2SO4 (step I) and an additional oxidation step after the addition of water (step II). The chemical properties, morphologies, defect generation behaviors, and yields of GO during the step I and step II oxidation processes were systematically investigated and compared. In step I, Mn2O7 played the role of the main oxidant and mainly exfoliated graphene by the formation of graphite bisulfate, which leads to the exfoliation of GO. During the step II oxidation process, the prolonged oxidation by MnO 4 − generated more functional groups, and the acidic aqueous condition hydrolyzed the epoxide groups into hydroxyl groups. Further, step II oxidation cleaved the GO layer and formed inplane defects in the sheets. Our findings show that a modification of the GO preparation procedures can be realized by modulating the step I and step II oxidation processes for the synthesis of graphene-based materials with desired structures and properties. As a result, customized GO samples for various applications that exhibit much better performance capabilities than conventional GO samples were successfully synthesized, implying that the standard protocol suggested in this work can further enhance the practical usage of graphene materials based on chemical exfoliation.

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DLS data showing the average particle size of GO, SEM images and photographs of GO with prolonged step I oxidation, AFM images of GO(2h)(95C, 2h), conductivity of GO, XPS, ζ-potentials, DLS, and Raman spectra of reference GO (PDF)

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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.6b02885. 317


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