Structure-Based Selective Adsorption of Graphene on a Gel Surface

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Structure-based selective adsorption of graphene on a gel surface: Towards improving the quality of graphene nanosheets Takaaki Tomai, Shunichi Ishiguro, Naoki Tamura, Yuta Nakayasu, and Itaru Honma Langmuir, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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Structure-based selective adsorption of graphene on a gel surface: Towards improving the quality of graphene nanosheets

Takaaki Tomai*, Shunichi Ishiguro, Naoki Tamura, Yuta Nakayasu, Itaru Honma

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai, Miyagi 980-8577, Japan *Corresponding author e-mail: [email protected]

Top-down graphene production via exfoliation from graphite produces a mass of graphene with structural variation in terms of the number of layers, sheet size, edge type, and defect density. All these characteristics affect its electronic structure. In order to develop useful applications of graphene, structural separation of graphene is necessary. In this study, we investigate the adsorption behavior of different types of graphene fragments using a multicolumn gel chromatography system with a view to developing an efficient method for separating high-quality graphene. The graphene was dispersed in an aqueous sodium dodecyl sulfate (SDS) surfactant solution and flown through allyl dextran-based gel columns connected in series. In the chromatographic operation, we observed that the small-sized or oxidized graphene fragments tended to bind to the gel, and the relatively large-sized graphene with a low oxygen content eluted from the gel column. In this system, the adsorbed SDS molecules on the graphitic surface prevented graphitic materials from binding to the gel, and the oxygen functional groups on the graphene oxide or at the abundant edge of small-sized graphene hindered SDS

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adsorption. We hypothesize that the reduced SDS adsorption density results in the preferential adsorption of small-sized or oxidized graphene fragments on the gel. This type of chromatographic separation is a cost-effective and scalable method for sorting nanomaterials. The structural separation of graphene based on the adsorption priority found in this study will improve the quality of graphene nanosheets on an industrial scale.

Keywords: graphene, gel chromatography, separation, surfactant, adsorption, oxygen functional groups


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1. Introduction Originally used as a simple electron conductor film [1], applications of graphene have expanded to include a new range of functional uses such as field effect transistors (FETs) in electronic devices [2], catalysts in fuel cells [3,4], and electrodes in energy storage devices [5,6]. Among these applications, the catalyst and electrode materials in energy devices require the mass production of graphene. Unlike the bottom-up method of chemical vapor deposition (CVD) [7], the mass production of graphene is usually based on a top-down process in which graphene sheets are exfoliated from graphite. Examples of top-down processes include oxidation of graphite with subsequent sonication [8], direct exfoliation of graphite in a solvent by sonication [9], electrochemical intercalation [10], and supercritical fluid treatment [11]. However, the graphene fragments produced by these methods have structural variations in terms of the number of layers, sheet size, edge type, and defect density. These variations influence the properties of graphene. For example, inter-sheet junctions and defects limit the electrical conductivity of the graphene network in transparent conductive films for electronic devices [12] and in the electrodes for battery/electrochemical capacitors [13]. Even though non-chemical exfoliation using sonication and supercritical fluid treatment can provide defect-less graphene, the graphene that is produced still contain large variations in the number of layers and sheet size. After graphene is produced using a top-down process, it must be structurally separated in order for it to be used. Techniques, such as centrifugation [14-18], sedimentation [19], and electrophoresis [20,21], can separate graphene according to the degree of oxidation, number of layers, and sheet size. Among the separation methods, centrifugation is usually employed with mono-layer [14] or large-sized graphene [15]. However,


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centrifugation is not sufficiently scalable for industrial production. The development of a scalable and efficient method for structurally separating graphene remains a challenge. Carbon nanotubes (CNTs) with a cylindrical structure formed using graphene sheets encounters similar challenges. Structural variations in CNTs, represented by multi-chirality, results in variations in electronic structure. In most cases, the produced single-walled CNTs (SWCNTs) are a mixture of metallic and semiconducting SWCNTs, preventing their useful application in electronic devices. To overcome this problem, many researchers have worked extensively on various types of separation methods for metallic and semiconducting SWCNTs. These include dielectrophoresis [22], density-gradient ultracentrifugation [23,24], and chromatography [25]. Among these separation methods, gel chromatography using a surfactant has attracted the most attention because of its industrial advantages such as high yield, high purity, cost-effectiveness, and scalability [26,27]. This method involves using a multicolumn filled with allyl dextran-based gel beads, which exhibit a strong interaction with the graphitic surface of CNTs, along with a surfactant molecule, sodium dodecyl sulfate (SDS), which weakens the interaction between the CNT and the gel. The adsorption density of surfactant molecules on the CNT surface depends on the chirality of the CNT. Hence, the difference in the adsorption priority of the CNTs to the gel column, governed by the adsorption density of the surfactant on the graphitic surface, results in single chirality by chromatography. In this study, we investigate the feasibility of using multicolumn gel chromatography for graphene separation. Similar to CNTs, the adsorption density of surfactant molecules on graphene depends on the characteristics of the graphene, including its sheet size, edge type, and defect type and density. For example, using liquid-cell atomic force


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microscopy (AFM), it was confirmed that SDS does not adsorb on to graphene oxide, whereas reduced graphene oxide and highly oriented pyrolytic graphite (HOPG) were covered with micellar SDS aggregates in an aqueous SDS solution [28]. By using the difference in the adsorption density of SDS on graphene and graphene oxide, we carried out the chromatographic separation of graphene from a mixture of graphene and graphene oxide. By flowing the mixture suspended in an aqueous SDS solution through a multicolumn filled with an allyl dextran-based gel, we found that the graphene oxide was trapped in the gel column while the graphene was eluted. We also found that among the graphene fragments without oxidation treatment, the large sheets tended to not bind to the gel column and were easily eluted from the gel columns. Taken together, these results indicate that gel chromatography can help improve the quality of graphene nanosheets by excluding oxidized or small-sized graphene fragments.

2. Experimental procedures To limit the size distribution of graphene sheets, we employed platelet-type carbon nanofibers (received from Mitsubishi Materials Electronic Chemicals) as the starting material for graphene. Platelet-type carbon nanofiber consist of several hundred nanometer-sized graphene sheets aligned along the vertical axis. By exfoliating these platelet-type carbon nanofibers, we obtained graphene fragments with a relatively narrow size distribution [29-31]. To prepare the mixture of graphene and graphene oxide, we began by generating oxidized carbon nanofibers using a modified Hummers’ method. First, 500 mg of carbon nanofiber was added to 23 ml of H2SO4 (98%) at room temperature and the mixture was stirred for 5 min in ice water. Next, 3 g of KMnO4 was added to the


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mixture and stirred for 30 min. Subsequently, 46 ml of purified water was added and the mixture was stirred at 100 °C for 30 min. Finally, aqueous H2O2 solution was added to stop oxidation. The prepared dispersion was centrifuged (CP 85β, Hitachi Koki) at 12850 g for 15 min to precipitate the oxidized carbon nanofiber fragments. The obtained oxidized carbon nanofiber was washed in an aqueous HCl solution and dried in vacuum overnight. Subsequently, using an ultrasonic homogenizer (US-600AT, NISSEI) for 20 h at a power of 190 W, 50 mg of the untreated carbon nanofiber and 50 mg of the oxidized carbon nanofiber were dispersed in 100 ml of water with 2 wt.% SDS (95 %, received from Wako Chemicals). During this sonication process, graphene and graphene oxide were exfoliated from the fibers and dispersed in the aqueous solution as a result of the attachment of the surfactant molecules to the graphene surface. The prepared graphene and graphene oxide dispersion were centrifuged at 2161 g for 10 min to precipitate the un-exfoliated nanofibers and remove the sediment. To prepare the graphene, 100 mg of the carbon nanofiber was dispersed in 100 ml of water with 2 wt.% SDS using an ultrasonic homogenizer for 20 h at a power of 190 W. The prepared graphene dispersion was centrifuged at 2161 g for 10 min to precipitate the un-exfoliated nanofibers and remove the sediment. For the multicolumn gel chromatography, we prepared several 10-ml syringes filled with 3 ml of allyl dextran-based size-exclusion gel beads (Sephacryl S-200 HR, GE Healthcare). We arranged the syringes vertically in series. After the gel columns were equilibrated with the 2 wt. % SDS solution, 3 ml of the prepared graphene dispersion or graphene/graphene oxide mixture was applied onto the top column. The dispersed graphene was flown through the columns one by one, and a certain amount of graphene was trapped in each column. Subsequently, a 2 wt. % SDS aqueous solution was added


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onto the top column to elute the unadsorbed graphene. We analyzed the unadsorbed graphene collected after each column and using Raman spectroscopy (excited wavelength: 532 nm, XploRA, HORIBA), X-ray Photoelectron Spectroscopy (XPS, PHI 5000 VersaProbe II, ULVAC-PHI) and AFM (SPA-400, SII).

3. Results and discussion 3.1. Graphene extraction from a mixture of graphene and graphene oxide To verify that the adsorption density of SDS governs the adsorption behavior of graphene to allyl dextran-based gel, we conducted the chromatographic separation of graphene and graphene oxide. At first, we evaluated the separation of graphene from the mixture using Raman spectroscopy. It is well-known that the intensity of G' band at around 2700 cm–1 in the Raman spectrum for the graphitic material significantly begins to decrease due to disorder in the graphitic lattice caused by oxidation [32]. When the maximum intensity of G' band was normalized by the maximum intensity of G band at around 1580 cm–1, derived from the graphene honeycomb structure, the average intensity ratio of G' band to G band (IG'/IG) was 0.47 for the prepared graphene fragments and 0.099 for the graphene oxide fragments.


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Figure 1 (a) Representative Raman spectra for the fragment in the initial mixture of graphene and graphene oxide and the fragments collected after the 3rd column. (b) Average IG'/IG value for the fragment in the initial mixture and the fragments collected after the 1st, 2nd, and 3rd columns.

The average IG'/IG value for the fragments in the initial mixture was 0.19, and the IG'/IG values for the fragments collected after the 1st, 2nd, and 3rd columns were 0.25, 0.31 and 0.41 respectively, as shown in Fig. 1. This gradual increase in the IG'/IG value during flow through the columns indicates that the graphene oxide was preferentially trapped in the allyl dextran-based gel compared to the non-oxidized graphene fragments.


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Figure 2 XPS analysis of oxygen content in the initial mixture and the fragments collected after the 1st, 2nd, and 3rd columns.

The oxygen content in the graphene fragments collected after the 1st, 2nd, and 3rd columns were analyzed using XPS, as shown in Fig. 2 (The XPS spectra are shown in Figure S1, Supporting Information). In comparison to the initial mixture, a reduction in the oxygen content in the fragments as a result of passing through the columns was observed. This reduction tendency confirms the preferential adsorption of graphene oxide to the allyl dextran-based gel, indicating that the difference in the adsorption density of SDS on graphene fragments affects the adsorption priority to the gel column in the chromatography system.

3.2. Structural separation of graphene Next, we evaluated the potential of gel chromatography to generate further


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improvements in the quality of graphene fragments. We conducted chromatographic separation of non-oxidized graphene fragments, and analyzed the resulting screened fragments for sheet size and number of layers. The number of layers of graphene can be estimated from the peak position of G' band, which is known to shift to a lower wavenumber as the number of layers reduces [33]. In our Raman measurement system [31], the peak position of the G' band shifted from 2717 cm–1 (graphite) to 2686 cm–1 (monolayer graphene prepared by scotch-tape method), as a decrease in the number of layers. On the other hand, the D band at around 1350 cm–1 is derived from the disorder of the graphene honeycomb structure, including its point defect and boundary. In the case of defect-free graphene or graphite, one can estimate the sheet (crystal) size from the intensity ratio of the D band to the G band at around 1580 cm–1 (ID/IG) [34].


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Figure 3 (a) Representative Raman spectra for the graphene fragments collected after the 1st and 10th columns. (b) Average value of the peak positions of the G' band, and (c) average value of ID/IG, for the initial graphene fragments and the graphene fragments collected after the 1st, 2nd, 3rd, 5th, and 10th columns.

For the graphene before and after gel chromatography, the average value of the peak position of the G' band is shown in Figure 3(b), and the average value of ID/IG is shown in Figure 3(c). (Note: all data points are included in Figure S2, Supporting Information). In terms of the peak position of the G' band, the unadsorbed graphene sample collected after the 1st column exhibited a clear shift to a lower wavenumber compared to the initial graphene sample. The subsequent columns did not affect the peak position of the G' band. In contrast, the ID/IG value of the unadsorbed graphene after the 1st column


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showed a significant increase from the initial graphene sample, following which the unadsorbed graphene fragments exhibited a gradual reduction in the ID/IG value as the number of columns increased. To explain these two kinds of behaviors for the 1st column and the other subsequent columns, it is reasonable to hypothesize that the function of the 1st column is different than the subsequent columns. We will first discuss the behavior of the first column, followed by a discussion of the subsequent columns. In terms of the 1st column, the clear shift of the G' band after passing through this column implies that the thicker graphene/graphite fragments were trapped, while thinner graphene fragments passed through. Further, considering the decrease in the peak position of the G' band, combined with the increase in ID/IG, we believe it is likely that the thicker graphene/graphite fragments trapped in the 1st column had a large sheet size. Generally, the thick graphene/graphite fragments prepared by the exfoliation process are large-sized, since generating thin graphitic fragments of a larger-size is difficult due to the greater amount of energy required. Therefore, our results indicate that the un-exfoliated thick graphitic fragments with a large sheet size were preferentially trapped at the 1st column. These un-exfoliated thick graphitic fragments with a large sheet size can become easily stuck in the pores of the column. We hypothesize that this filtering effect is the dominant role of the 1st column. In terms of the function of the subsequent columns, the unadsorbed graphene fragments exhibited a gradual reduction in the ID/IG value as the number of columns increased. In practical experiments involving graphene with defects, it is important to be cautious when deducing the sheet size from the ID/IG value, because the contribution of the defect to the D band cannot be estimated precisely. The D band is derived from the disorder of


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the graphene honeycomb structure, including its point defect and boundary. As a result, not only the small-sized graphene, but also the defective/amorphous carbon shows a high ID/IG value. It has been previously observed that amorphous carbon is preferentially adsorbed on dextran-based gel during SWNT separation by gel chromatography using an SDS surfactant [35]. Therefore, to determine whether the reduction in the ID/IG value after passing through the multi-columns was derived from the increase in sheet size or the reduction in defect density, we measured the sheet size distribution of the collected graphene fragments using AFM.

Figure 4 AFM images and height profiles of the graphene fragments collected after (a) 1st column and (b) 10th column. (c) Histogram of the sheet size distribution of the graphene collected after 1st, 3rd, and 10th columns, measured using AFM.

Figures 4 (a) and (b) show the AFM images for the graphene fragments collected after the 1st and 10th columns, respectively. Figure 4 (c) shows a histogram of the sheet size distribution of the graphene collected after the 1st, 3rd, and 10th columns (note: all data


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points are indicated in Figure S3, Supporting Information.). The longest chord length of the particles was employed as their sheet size. The histogram shows a clear shift towards larger-sized sheets as the number of columns increases. Specifically, the average-sized sheets for the 1st, 3rd, and 10th columns are 57, 71, and 142 nm, respectively. This confirms that the increase in the average sheet size is attributable to the reduction in the ID/IG value, indicating that large-sized graphene fragments tend to pass through the gel columns. The sheet size of graphene fragments obtained using other separation methods based on a physical mechanism, such as centrifugation or size exclusion chromatography with a porous-glass column, tends to be smaller as the sheet thickness decreases [36]. On the other hand, the use of gel adsorption with a filtering effect has a promising ability to obtain large-sized graphene without increasing its thickness.

3.3. Separation mechanism derived from gel adsorption In this section, we use a microscopic perspective to discuss the separation mechanism and key factors determining the priority adsorption of graphene on the gel surface during chromatography. Previous reports on chirality separation for CNTs using gel chromatography indicate that the adsorption density of SDS on a graphitic surface governs the adsorption priority of CNT on the gel. Similarly, in the graphene separation undertaken in this study, the difference in the adsorption density of SDS is hypothesized to affect the adsorption priority of graphene fragment on the gel. In terms of comparing graphene oxide and unoxidized graphene, it is known that the adsorption density of SDS decreases due to oxidation, and specifically the addition of an oxygen functional group [28]. The electrostatic repulsion between the negatively


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charged SDS head groups and the negatively charged hydroxyl groups prohibits the adsorption of SDS on the graphene fragments. Our result that graphene oxide preferentially adsorbs to gel columns supports the hypothesis that the difference in the adsorption density of SDS determines the adsorption priority of graphene in the gel chromatography. Further, we hypothesize that the variation in the adsorption density of SDS induced by the oxygen functional groups at the graphene edge results in the sheet-size separation of graphene. A previous study found that graphene fragments prepared using sonication in an aqueous surfactant solution suffered from a low level of oxidation [37], and the oxygen functional groups likely exist at the graphene edge. Our XPS analysis of the non-oxidized graphene fragments obtained by the chromatographic separation in Section 3.2, reveals that the oxygen content in the graphene fragments decreases as the number of columns increases. Specifically, the oxygen content of the collected graphene fragments was 5.3 at% after the 1st column and 4.8 at% after the 10th column. (Note: The XPS spectra are shown in Figure S5, Supporting Information.). This indicates that the smaller-sized graphene, which was preferentially trapped in the gel column, contains a greater number of oxygen functional groups due to its abundant edge. The oxygen functional group at the edge reduced the adsorption density of SDS, resulting in the preferential adsorption of small-sized graphene fragments to the gel surface and the elution of large-sized graphene fragments from the gel. These separation mechanism derived from the oxidation extent were also suggested in the recent reports on the gel chromatographic separation for CNTs [38,39]. In the reports, it was suggested that the different density of the adsorbed surfactant derived from the difference in an oxidation extent of CNTs leads to the separation of CNTs, and the oxidation is a necessary


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condition for the separation. Our results indicated that in gel chromatographic separation, the separation mechanism for graphene is in the same manner as the case for the CNTs. In short, due to the filtering and adsorption effects, the gel chromatography can provide large-sized thin graphene fragments. The separation derived from the adsorption effect can easily exclude small-sized/oxidized graphene fragments, which act as obstacles to electron conduction in conductive films in electronic devices or electrodes in energy devices. On the other hand, in the 1st column, this adsorptive separation should also occur, but the filtering effect is dominant, and the maximum sheet size obtained by the elution from the gel columns is limited by size of the flow channel. We conducted the separation for the larger-sized graphene prepared from graphite powder, but the large-sized graphene sheets prepared from the graphite was fully trapped at the 1st column by simple filtering effect. The size of the flow channel can increase by using larger gel beads, thus making this chromatographic method adaptable to the extraction of larger-sized graphene. The results of this study may assist in developing an efficient large-scale separation process for improving the quality of graphene nanosheets. We also hope that the results will prove beneficial in developing future industrial applications of graphene.

4. Conclusion In this study, we performed structural separation of graphene using allyl dextran-based gel chromatography with an SDS surfactant. In this chromatography, structural separation was derived using a geometrical filtering effect and an adsorption effect. The filtering effect at the 1st column removed the un-exfoliated thick graphitic fragments and


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provided thin graphene fragments. The adsorption effect, which occurred mainly in the subsequent columns, preferentially captured the small-sized graphene and oxidized graphene fragments on the gel, leading to the extraction of the large-sized graphene. The adsorption density of the SDS surfactant on the graphene surface governed the adsorption priority of graphene to the gel. The oxygen functional group, which existed at the edge of the graphene fragments or on graphene oxide, reduced the adsorption density of SDS. Therefore, the small-sized/oxidized graphene fragments, which act as obstacles to electron conduction, can be easily excluded using this gel chromatography system. This chromatographic separation is a cost-effective and scalable method for sorting nanomaterials. The structural separation of graphene based on the adsorption priority found in this study may contribute to improving the quality of graphene nanosheets on an industrial scale.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . XPS spectra, details of Raman and AFM analyses, and TEM images

Acknowledgements This work was financially supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 25701013.


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Structure-Based Selective Adsorption of Graphene on a Gel Surface

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