Formation of Graphene Oxide Nanoscrolls in Organic Solvents

Article Cite This: ACS Appl. Nano Mater. 2018, 1, 686−697

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Formation of Graphene Oxide Nanoscrolls in Organic Solvents: Toward Scalable Device Fabrication Bo Tang, Xiawei Yun, Zhiyuan Xiong, and Xiaogong Wang* Department of Chemical Engineering, Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, P. R. China S Supporting Information *

ABSTRACT: In this work, formation of graphene oxide (GO) nanoscrolls in organic solvents was investigated upon dispersing GO nanosheets in organic solvents by sonication. It was found that, in some of the organic solvents, the single-layer GO sheets can effectively curl onto themselves to form the nanoscrolls that then sediment under gravity from the dispersion media. The sedimentation was monitored by UV−vis spectroscopy when the GO suspensions were left aside for different time periods after the sonication. The nanoscrolls were separated and characterized by transmission electron microscopy (TEM) and electron diffraction (ED). A typical solvent with such scroll-forming function is pyridine, where 82% of the GO sheets form the GO nanoscrolls and undergo the sedimentation. Scroll formation is also observed for N,N-dimethylformamide (DMF), methanol, ethanol, isopropyl alcohol, acetic acid, and isobutyric acid, but the yields vary with the solvents. The scrolls formed in pyridine suspension show the average length over 20 μm, interlayer spacing of 0.6 nm, and average diameters of 186 and 192 nm for the two types of the GO samples used here. The scrolls formed in the other solvents show different average lengths and diameters governed by the properties of the solvents. The average lengths of the formed scrolls reflect the tendency of the GO sheets to curl into scrolls in the solvents, while the average number of the included sheets per scroll depends on the sedimentation rate in the suspensions. The scrolling behavior and mechanism are rationalized and elucidated by considering dipole moment, ζ potential, and Hansen solubility parameters of the solvents. Based on the above understanding, the GO scrolls with controllable lengths and diameters can be efficiently fabricated by selecting a suitable solvent, and are expected for applications in hydrogen storage, gas sensing devices, actuators, lubrication materials, and others. KEYWORDS: graphene oxide, nanoscroll, formation, suspension, organic solvent

1. INTRODUCTION Graphene oxide (GO) generally refers to a single-layer nanosheet obtained from complete exfoliation of graphite oxide.1 In recent years, GO has been intensively investigated as a graphene derivative, which shows many interesting properties and great promise for scalable production.2−9 Typically, a piece of GO sheet consists of aromatic two-dimensional (2D) lattice with scattered aliphatic domains of oxidized six-membered rings.7,8 Although nonpolar honeycomb carbon regions are hydrophobic, the oxygenated regions with the structural defects/vacancies show hydrophilicity related to the organic functional groups, such as hydroxyl, epoxy, and carboxyl groups.9 According to the widely accepted Lerf−Klinowski model, those functional groups are not uniformly distributed, where the carboxyl groups are rich at the edges, whereas hydroxyl and epoxy groups are more likely to be present in plane.10 One unique advantage of GO is that various functional groups can be covalently conjugated at its basal plane and edges through chemical reactions.7−9 Understanding the unique dispersion behavior of GO in solvents is a matter of great importance for mass production and future applications.4−9 However, because of the structure complexity and diversity, © 2018 American Chemical Society

many aspects related to GO dispersion still remain unknown at the present stage. GO sheets obtained from oxidation and exfoliation of graphite can be well-dispersed in aqueous media.3,8,9,11 In recent years, the dispersion of GO in organic solvents has been intensively investigated for fundamental understanding and potential applications in different areas.12−24 It has been reported that fully exfoliated GO sheets can be obtained from graphite oxide in N,N-dimethylformamide (DMF) via sonication, where the as-prepared graphite oxide is exfoliated into single-layer GO sheets.12,13 More thorough study shows that GO sheets can be well-dispersed in N-methyl-2-pyrrolidone (NMP), ethylene glycol, and others, while they cannot be stably dispersed in some organic solvents, such as pyridine and ethanol.14 On the other hand, it has also been observed that, even after lengthy sonication (over 24 h), a homogeneous suspension of GO in DMF could hardly be obtained.15 In this case, a small amount of water can assist GO sheets to disperse Received: November 6, 2017 Accepted: January 18, 2018 Published: January 18, 2018 686

DOI: 10.1021/acsanm.7b00160 ACS Appl. Nano Mater. 2018, 1, 686−697

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ACS Applied Nano Materials

research can pave the way for fabricating GO scrolls in a scalable manner and supply deep understanding of the scrolling and suspension behavior of GO sheets in organic solvents. In this study, GO sheets were dispersed in organic solvents by sonication, and the sedimentation rates were characterized via UV−vis spectroscopy. The components existing in the suspensions and sediments were separated and characterized by transmission electron microscopy (TEM) and electron diffraction (ED). It was observed that GO scrolls were formed from GO sheets in both aprotic solvents (pyridine and DMF) and protic solvents (methanol, ethanol, isopropyl alcohol, acetic acid, and isobutyric acid), where the average lengths and diameters of the scrolls as well as the yields depended on the solvents. The scrolls mainly existed in the sediments and formed after the sonicated suspensions were left aside for hours. The high-resolution TEM and ED were used to investigate the interlayer spacing, average number of turns per scroll, and average number of the included GO sheets per scroll. The structural parameters and yields of the GO scrolls formed in the solvents were investigated, and their correlations with the solvent properties were discussed.

in DMF and dioxane, where the suspensions are stabilized for longer time.15,16 Hansen solubility parameters, including δd (dispersion cohesion), δp (polarity cohesion), and δh (hydrogen bonding cohesion), have been widely used to characterize the solvent interaction with polymers,20,21 and have more recently found a use for graphene.22 The dispersion behavior of GO in the organic solvents has been rationalized by using Hansen solubility parameters.15,18,19 The result shows that a match between polar solvents and the polar functional groups on the GO sheet is beneficial to form stable colloidal suspensions.18,23 More recently, Derjaguin−Landau−Verwey−Overbeek (DLVO) theory has also been used to develop a model by treating GO sheets as ultrathin colloidal flakes in aqueous and organic media.24 In the stable GO suspensions, the carbon grid is believed to possess a nearly planar structure, and only the carbons at the structural defects/vacancies show a distorted tetrahedral configuration, which causes wrinkles on the layers observed on dried GO sheets.9,15,17 However, whether GO sheets will curl onto themselves when exposed to an unfavorable solvent still remains as an open question. The carbon nanoscroll (CNS) as an allotropic form of carbon nanomaterials is formed by rolling up graphene sheets through a scrolling manner. It has been observed in the pioneering studies that the exfoliated graphite sheets can curl onto themselves to form CNS in ethanol or N-methyl-2pyrrolidone, because of the effect of the intercalation compound KC8.25,26 In these cases, the function of the intercalation compound instead of the solvents could be the key factor to cause CNS formation. Because of many unique functions, CNSs have been fabricated and explored for various applications, such as hydrogen storage materials,27,28 gas sensing devices,29 actuators,30 lubrication materials,31,32 tunable water and ion channels,33 and supercapacitors.34 Recently, rather than using graphene sheets, GO as well as functionalized GO have been used to fabricate nanoscrolls. GO scrolls have been prepared by template-induced formation, i.e., using multiwalled carbon nanotubes (MWCNTs), and other nanoparticle aggregates as the nanotemplates.35,36 In addition to GO, nitrogen-doped graphene nanoscrolls have also been prepared by adsorption of magnetic γ-Fe2O3 nanoparticles.37 Moreover, nanoscrolls have been fabricated from GO sheets by other innovative methods, such as microexplosion,38 liquid nitrogen quenching, freeze-drying,39 ultrasound solution processing,40 and the Langmuir−Blodgett (LB) approach.41 Generally speaking, the scroll formation is controlled by two opposite energetic contributions, where bending the graphene sheet will cause the elastic energy increase, while the van der Waals and other interactions in overlapping regions of the graphene sheet can reduce the free energy.42−44 The above methods all include a necessary step to overcome the energy barrier for initial curling in the scrolling process. The initial torsional bending energy barrier can be reduced or even eliminated through allowing local stress relaxation around the defect sites.44 GO sheets with defects/vacancies and nonuniform distribution of the functional groups can have specific interactions with different solvents. It is reasonable to infer that the GO sheets can transform from planar structure to scrolling form if the initial bending barrier can be overcome for the sheets to be exposed to an unfavorable solvent. In a highly diluted suspension, nanosheets will curl onto themselves to form GO scrolls instead of coagulation. However, to our knowledge, a systematic study of solvent effects on GO scroll formation has not been reported in the literature yet. Such

2. EXPERIMENTAL SECTION 2.1. Materials. Graphite powder with the size of 300 mesh was purchased from Sigma-Aldrich. Pyridine, DMF, NMP, methanol, ethanol, ethylene glycol, isopropyl alcohol, acetic acid, and isobutyric acid of chromatographic grade were purchased from Aladdin. Deionized water (resistivity, >18 MΩ cm) was obtained from a Milli-Q water purification system. Other chemicals were obtained from the commercial sources and directly used in the experiments without further purification. 2.2. Preparation of GO Sheets. GO sheets were prepared by modified Hummers’ method as reported before.11,45−47 The 300 mesh graphite power (3.0 g) and potassium nitrate (3.6 g) were successively added into the concentrated sulfuric acid (138 mL) with ice-bath cooling. Potassium permanganate (18 g) was then slowly added into the suspension after stirring for 30 min, and the mixture was kept in an oil-bath with the temperature of 35 °C for 48 h. After that, excess water (300 and 500 mL in two batches) was slowly added with icebath cooling, and 18 mL of H2O2 (30%) was dropwise added into the suspension, where the color of suspensions changed from brown to yellow. The mixture was repeatedly washed with abundant water and concentrated by the centrifuge (18 000 rpm) until the pH of supernatant reached about 6. Finally, two GO samples were collected by the ultracentrifuge with the spin speed of 6000−12 000 rpm for 30 min and 3500−6000 rpm for 10 min, which had the GO contents of 0.97 and 0.50 wt %, respectively. Finally, the samples were freeze-dried for 24 h to obtain a porous powder with low density, which could be easily dispersed in solvents by sonication. To facilitate the discussion, the two dried GO aerogel samples are referred to as GO-1 (obtained from the spin speed of 6000−12 000 rpm for 30 min), and GO-2 (obtained from the spin speed of 3500−6000 rpm for 10 min). 2.3. GO Nanoscroll Formation. The GO powder samples, which were obtained by the centrifugation and freeze-drying, were dispersed in the organic solvents by sonication for 2 h (40 kHz, 200 W). After the sonication, the suspensions were left aside for different time periods. The stability and sedimentation rate of the suspensions were characterized by UV−vis spectroscopy. The sediments formed in the suspensions were separated and dispersed again in the pure solvents, which were the same solvents used to prepare the scrolls, via gently shaking for several minutes. The TEM samples were prepared by casting the diluted suspensions onto the copper grids after gently shaking. The samples were exposed to air for 24 h to evaporate solvents and then dried under vacuum for 12 h at room temperature. 2.4. Characterization of GO Sheets and Scrolls. GO sheets were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, transmission electron 687


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Figure 1. Illustration of GO scroll formation in organic solvents and representative TEM images. (A, B) TEM images of the GO sheets and scrolls in the sediments from ethanol and methanol, respectively. (C) High-resolution TEM image of the scroll formed in methanol. microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Raman spectra were obtained from a Renishaw 1000 microspectrometer with an excitation wavelength of 633 nm. A Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (1.5406 Å) was employed to obtain XRD patterns of GO samples. The binding energy of the elements was characterized by an X-ray photoelectron spectrometer (ESCALAB250Xi) using a monochromatized Al Kα X-ray source of 1486.6 eV under normal incidence. The binding energy of the XPS peaks was standardized by the C 1s peak at 284.6 eV. An AFM apparatus from Bruker Corporation (Dimension ICON-PI) was employed to characterize the single-layer GO sheets. The tapping mode was adopted in the test with an amplitude set-point of 340 mV, and the probe used for the detection had a tip radius of 6−8 nm. A field-emission scanning electron microscope (Zeiss Merlin, 5.0 kV) was used to characterize single-layer GO sheets. The TEM observations were realized using a Hitachi H7650B microscope with the electron beam of 80 kV. A high-resolution TEM instrument, JEM2010 (03006800) with the electron beam of 120 kV, was employed to observe the single-layer GO sheets. The experiment of selected area electron diffraction (SAED) was performed by the same high-resolution TEM instrument with the 120 kV electron beam.

12 000 rpm for 30 min, and GO-2 was collected by the speed of 3500−6000 rpm for 10 min. GO-1 had a relatively smaller average size compared with that of GO-2, as GO-1 was obtained by the higher spin speed and longer centrifugal time. The GO samples were characterized by XRD, XPS, Raman spectroscopy, SEM, AFM, and high-resolution TEM (Figures S1−S3 in the Supporting Information). The Raman spectra (Figures S1A and S2A) show two bands located at 1589 cm−1 (G band) and 1338 cm−1 (D band). The G band is known from the in-plane bond-stretching motion of pairs of C sp2 atoms with E2g symmetry, while the D band corresponds to the breathing mode with A1g symmetry, which appears only in the presence of defects or partially disordered domains.48,49 The intensity ratio of I(D)/I(G) for GO sheets is about 1.108, which is inversely proportional to the average size of the sp2 domains of GO.50 The XRD peak at the position of 10.9° (2θ) proves that the interlayer spacing is larger for the GO sheets compared with that of graphite 2θ = 26.6° (Figures S1B and S2B). Figures S1C and S2C provide the C 1s XPS spectra with four prominent components: CC (284.8 eV), CO (286.9 eV), CO (288.0 eV), and OCO (289.8 eV), and the content of the oxygen element (atomic ratio) for GO-1 is 36.79%, which is nearly equal to that of GO-2 (37.59%). Figures S1D and S2D show the SEM images of the GO sheets (GO-1 and GO-2), where large GO sheets can be clearly seen. Figure S3 shows the area size distributions of the GO sheets, which indicates that the sizes of GO sheets are dispersed for both GO-1 and GO-2. The average areas of GO-1 and GO-2 correspond to about 211 and 433 μm2. The high-resolution TEM image (Figure S1E) and AFM characterizations (Figures S1F, S1G, and S2E) confirm that the GO samples are singlelayer nanosheets. The thickness of the GO sheets (from the AFM height) is about 1 nm, which is a typical value for the single-layer GO sheet.4,45,46 The well-exfoliated GO samples were freeze-dried after being separated with centrifugation. It is difficult for the GO sheets obtained via the ordinary filtration and drying procedure to be directly dispersed in anhydrous organic solvents (such as DMF) because of the tight stack of the nanosheets, where the penetration of solvent molecules into the interlayer spaces is prevented by the strong hydrophilicity and interlayer hydrogen bonds between the adjacent GO layers.51 In contrast, the GO samples obtained from the freeze-drying process have loose stacking structures with hierarchical pores (Figure S4). As can be seen in the images, there are no GO scrolls in the aerogel, and only wrinkles formed in the freeze-drying process exist.

3. RESULTS AND DISCUSSION The GO nanoscroll formation was investigated upon dispersing the GO sheets in the organic solvents via sonication and then leaving the suspensions aside for a period of time (Figure 1). The TEM and ED characterizations reveal that the GO nanoscrolls can effectively form in some dispersion media such as pyridine and isobutyric acid. For other solvents, both singlelayer sheets and GO nanoscrolls can exist in the sediments, where the scroll formation behavior and ratio are controlled by the solvent properties. Figure 1A shows the single-layer GO sheets in the sediment from ethanol, and Figure 1B shows the GO scrolls formed in methanol. The scrolls formed in the process possess concentric multilayer structures with interlayer spacing of 0.60 nm for the sample shown in Figure 1C, which indicates that GO sheets are tightly rolled up to from GO scrolls. In the following sections, the GO sample preparation, stability and sedimentation rate of the suspensions, scroll formation in polar aprotic solvents and protic solvents, factors affecting the scroll formation, and the correlations between the parameters of GO scrolls and the solvent properties are presented and discussed in detail. 3.1. Preparation and Characterization of GO Sheets. For this study, two GO samples with different average sizes (GO-1 and GO-2) were prepared by modified Hummers’ method, collected by different centrifugal speeds and freezedried. GO-1 was collected by the centrifugal speed of 6000− 688


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via UV−vis spectroscopy,14,19 where the absorbance variation with time was measured at a fixed wavelength (480 nm). Figure 3 gives the normalized UV−vis absorbance of the GO suspensions in polar aprotic solvents and protic solvents, when they were left aside for different time periods. The percentage absorbance values relative to the original values are given in Table S1 in the Supporting Information. For polar aprotic solvents, the stability of the suspensions ranks in the following order: NMP > DMF > pyridine. Previous study showed that the aprotic solvents with the high dipole moment (more than 3.82 D) have good suspension stability for GO.18 The present study also shows that NMP with the dipole moment of 4.09 D has the highest suspension stability, and DMF (3.82 D) has better suspension stability compared to pyridine (2.23 D). The relative sediment amount (RSA) estimated from UV−vis absorbance at 11 h after sonication relative to the time-zero value is used to characterize the sedimentation rate of the suspension, which is negatively correlated with the suspension stability. As can be seen in Figure S5A, RSA shows a nearly inverse variation with increasing dipole moment of these three solvents. For protic solvents, the situation is more complicated because of the existence of the hydrogen bonding between the GO sheets and solvent molecules. The stability of the suspensions ranks in the following order: ethylene glycol > methanol > isobutyric acid > ethanol > isopropyl alcohol > acetic acid. For a typical case, there is the obvious sedimentation in ethanol in contrast to the stable suspension in water and ethylene glycol, which is consistent with the result reported before.14,18 It has been reported that the suspension stability is related to the Hansen solubility parameters of the solvents.18 As shown in Figure S5B, RSA shows no recognizable correlation with the dipole moments of the protic solvents. On the other hand, RSA shows a nearly inverse relationship with the sum of the Hansen polarity parameter (δp) and hydrogen bonding parameter (δh). These results indicate that the dipole moment and δp + δh values are needed to characterize the suspension stability of GO sheets in the aprotic and protic solvents, respectively. 3.3. GO Nanoscrolls Formed in Polar Aprotic Solvents. GO scrolls can effectively form in pyridine with poor dispersion stability for the GO sheets. GO scrolls were ubiquitously observed in the sediment formed in pyridine with the initial GO concentration of 0.05 mg/mL, which was sonicated for 2 h (40 kHz, 200 W) and then left aside for 12 h. Figure 4A,B shows the GO scrolls with the average length of tens of micrometers. Few “fingerlike” morphologies can be seen in the middle of Figure 4A,B. The mechanism for forming this

This observation is consistent with the previous report that no scrolls can be formed from GO sheets in the freeze-drying process, and only reduced graphene oxide (RGO) can transform into scrolls in the freeze-drying process.52 3.2. Stability of GO Suspensions. For an investigation into the scroll formation, the porous GO powder was dispersed in the organic solvents with the typical concentration of 0.05 mg/mL via sonication (40 kHz, 200 W) for 2 h. Figure 2 gives

Figure 2. GO (GO-1) in organic solvents: top, just after sonication; bottom, 12 h after sonication.

the photographic images of the GO sample (GO-1) in the organic solvents, which were taken just after sonication and after being left aside for 12 h, respectively. As the porous and loose stacking structures allow solvent molecules to penetrate into the interlayer spaces of the GO sheets, the single-layer GO sheets can be readily suspended in the organic solvents via sonication. However, except for NMP and ethylene glycol, sediments can be observed more or less for the other solvents in the 12 h period, which agrees with the previous reports.14,18,19 It is worth noting that the black shadows at the supernatant surfaces are not floating GO sheets, but are virtual images of the vial caps and bottoms appearing at the air−liquid interfaces. For an investigation into the suspension stability and sedimentation rate, the GO suspensions were characterized

Figure 3. Normalized UV−vis absorbance of the GO suspensions (GO-1, 0.05 mg/mL), being left aside for different time periods after sonication: (A) aprotic solvents, (B) alcohols, and (C) acids. Each sample was repeated in quintuplicate to determine the average values and error bars. 689


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Figure 4. TEM images of the GO scrolls. (A, B) GO scrolls formed from GO-1 and GO-2 in the pyridine suspensions. (C) Typical GO scroll formed from GO-1 in pyridine. (D) Intermediate state of scrolling (GO-1) in pyridine. (E) High-resolution TEM images of the GO scroll formed from GO-1 in pyridine. (F, G) Position for ED analysis and ED pattern of the GO scroll formed from GO-1 in pyridine. (H, I) GO scrolls formed from GO-1 and GO-2 in the DMF suspensions.

Table 1. Average Lengths and Diameters (at the Central Part) of GO Scrolls GO-1 solvent pyridine DMF methanol ethanol isopropyl alcohol acetic acid isobutyric acid

a

length (nm) 22 500 1190 7240 390 1080 265 13 600

± ± ± ± ± ± ±

GO-2 a

diameter (nm)

8600 590 2480 190 530 60 4900

186 68 258 42 58 16 274

± ± ± ± ± ± ±

76 29 90 17 21 6 103

a

length (nm) 25 100 1350 8170 Xb 1120 664 19 200

± 6100 ± 610 ± 3640 ± 560 ± 246 ± 6100

diametera (nm) 192 86 265 Xb 74 24 282

± 83 ± 36 ± 87 ± 31 ±9 ± 96

a

The average length and diameter at the central part were obtained from statistics of at least 50 scrolls. bX: GO scrolls could not be observed in the sediment of GO-2 from ethanol.

nanotubes (MWCNTs) and graphene nanoscrolls (0.34 nm),53,54 which can be attributed to the existence of functional groups on the GO sheets. For a typical scroll with the diameter of 186 nm and length of 22 500 nm, it can be estimated to include 153 complete turns by eq 1:

morphology has been discussed on the basis of the study on the formation of nitrogen-doped graphene nanoscrolls.37 As the fingerlike morphology of the GO scrolls was seldom seen in this study, it will not be further discussed in the following parts. Figure 4C gives the TEM image of a typical GO scroll with the length of about 16 μm. Figure 4D shows the TEM image of a partially formed scroll from GO-1, which is evidence of the scrolling process of the single-layer GO sheet from one edge of the sheet. As shown in Figure S6, the lengths of GO scrolls can vary from about 4 to 36 μm. The average length and diameter of the scrolls obtained from GO-1 are 22 500 ± 8600 nm and 186 ± 76 nm as listed in Table 1. The scrolls formed from GO2 are larger (25 100 ± 6100 nm, 192 ± 83 nm) than those from GO-1, which can be attributed to the larger average size of GO2 sheets. Compared with the size distribution of the GO sheets (Figure S3), the average lengths of the GO scrolls are in the same order as the side lengths of the sheets, which are approximately equal to the square roots of the area sizes. The differences between them will be discussed in Sections 3.5 and 3.6. Figure 4E shows the high-resolution TEM image of the GO scroll, where the lamellar structure with the interlayer distance of 0.60 ± 0.05 nm can be clearly seen. This distance between the adjacent layers is larger than those of multiwall carbon

Nturn = (Dscroll /2 − r0)/d

(1)

where Nturn is the number of complete turns, Dscroll is the diameter of the scroll, r0 is the minimal inner radius of the scroll, and d is the interlayer distance. The minimal inner radius (r0) of 1 nm used here was the reported value.42−44 The circular arc length of the scroll is estimated to be about 45 200 nm according to eq 2: Larc = 2πNturn(Dscroll /2 + r0)/2

(2)

where Larc is the circular arc length of the scroll including Nturn turns. If the size of the sheet is supposed to be equal to the length of the scroll, which is statistically reasonable for the sheet with irregular shape, the typical GO scroll contains an average of 2.0 pieces of GO sheets. A similar result is also obtained for the scroll from GO-2. For a typical scroll with the length of 25 100 nm and diameter of 192 nm, it includes 158 turns and an average of 1.9 pieces of GO sheets per scroll. 690


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ACS Applied Nano Materials The scroll formation in pyridine was further confirmed by selected area electron diffraction (SAED). Figure 4F,G shows the position for the electron diffraction (ED) analysis and ED pattern of the typical GO scroll. The ED pattern is similar to that of graphite in ...ABCABCABC... layer sequence, when the electron beam is incident parallel to the hexagon sides.55 Because of the limited penetration depth of the electron beam, the reciprocal lattice information is only related to the outside shells of the scroll. This means that the structure in the shell part of the scroll is composed of 2D lattices of the GO sheets similar to graphite packing in the parallel manner. As shown in Figure S7, the reciprocal lattice points of the nanoscroll intersected by Ewald’s sphere are in good agreement with the ED patterns. The interplanar spacing (002) is calculated to be 0.60 nm from the ED points with the diffraction vector perpendicular to scroll axis, which agrees with the result from the high-resolution TEM discussed above. Raman spectroscopy was also employed to investigate GO nanoscrolls formed in pyridine (Figure S8 in the Supporting Information). The intensity ratio of the D band to G band for GO scrolls is about 1.121, which is slightly larger than that of the GO sheets (1.109). Meanwhile, the G band of GO scrolls is red-shifted from 1591 cm−1 (GO sheets) to 1581 cm−1, which is consistent with the previous investigation on GO scrolls.40 The above results confirm that the GO scrolls contain many complete turns formed from the GO sheets via the tightly rolled-up manner. GO nanoscrolls can also be observed in the sediment of the DMF suspension. Figure 4H,I shows the TEM images of the GO scrolls formed in DMF suspensions. Different to the ubiquitous observation of the scrolls in the sediment from pyridine, the single-layer sheets together with the scrolls were observed in the sediment from DMF. As listed in Table 1, the average length and diameter (at the central part) of the GO scrolls obtained from GO-1 are 1190 ± 590 and 68 ± 29 nm, and those from GO-2 are 1350 ± 610 and 86 ± 36 nm. This indicates that only some GO sheets with relatively small sizes can transform into scrolls. As a result, the average lengths of the scrolls are much shorter compared with those of the scrolls formed in pyridine. From the diameter values, the typical scrolls with the diameters of 68 and 86 nm should include 55 and 70 complete turns calculated with eq 1, based on the interlayer spacing of 0.60 nm. Using eq 2, the circular arc length (Larc) is estimated to be 6040 nm for a typical scroll formed from GO-1 and 9670 nm for a typical scroll from GO-2. Similarly, if the sizes of the GO sheets are supposed to be equal to the lengths of the scrolls, which are 1190 and 1350 nm for scrolls obtained from the GO-1 and GO-2 sheets, respectively, the scrolls should include average 5.1 and 7.2 pieces of GO sheets in each of the scrolls, which are more than those of scrolls formed in pyridine suspensions. The average values of Larc, Lscroll, and Npiece are summarized in Table 2. 3.4. GO Nanoscrolls Formed in Protic Solvents. The scroll formation was also investigated for polar protic solvents under similar conditions and by the same procedure. Figure 5 shows the TEM images of the GO nanoscrolls formed in methanol, isopropyl alcohol, and ethanol. Bundles of scrolls can be seen in the sediment from methanol (Figure 5A,B and Figure S9). Numerous scrolls formed in isopropyl alcohol can also be observed by TEM (Figure 5C,D and Figure S10). As listed in Table 1, the scrolls formed in methanol have significantly larger average sizes (both lengths and diameters) compared with those formed in isopropyl alcohol for both GO-

Table 2. Circular Arc Length (Larc), Scroll Length (Lscroll), and Number of GO Sheets Per Scroll (Npiece), Obtained by TEM Observation and Calculation with Eqs 1 and 2 GO-1 solvent pyridine DMF methanol ethanol isopropyl alcohol acetic acid isobutyric acid

GO-2

Larca

a

a

(nm)

Lscroll (nm)

Npiecea

Larc (nm)

Lscrolla (nm)

Npiecea

45 200 6040 91 400 2420 4520

22 500 1190 7240 394 1080

2.0 5.1 12.6 6.1 4.1

48 100 9670 95 600 Xb 7400

25 100 1350 8170 Xb 1120

1.9 7.2 11.7 Xb 6.6

339 103 000

265 13 600

1.3 7.6

775 109 000

664 19 200

1.2 5.7

a Larc is the circular arc length of the GO sheets in the scroll. Lscroll is the length of scroll. Npiece is the number of the included GO sheets per scroll. bX: GO scrolls cannot be observed in the sediment of GO-2 from ethanol.

Figure 5. TEM images of the GO scrolls. (A, B) GO scrolls obtained from GO-1 and GO-2 in the methanol suspensions. (C, D) GO scrolls obtained from GO-1 and GO-2 in the isopropyl alcohol suspensions. (E) GO scrolls obtained from GO-1 in the ethanol suspension. (F) Partially curled GO-2 sheets in the ethanol suspension.

1 and GO-2. For ethanol, the situation is more complicated, where only GO-1 sheets can form the scrolls (Figure 5E), but GO-2 sheets with the larger average size can hardly curl up into scrolls (Figure 5F). Similar to those in the polar aprotic solvents (pyridine and DMF), the average sizes of GO scrolls are significantly varied from solvent to solvent. The average sizes of the scrolls are related to the portions of GO sheets undergoing the scrolling in the solvents and their sedimentation rates, which will be discussed in Sections 3.5−3.7 in detail. 691


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ACS Applied Nano Materials Figure 6A exhibits the image of a partially formed scroll from the methanol suspension, where the scrolling process can be

Figure 7. TEM images of GO scrolls formed in acidic suspensions. (A) GO-1, isobutyric acid; (B) GO-2, isobutyric acid; (C) GO-1, acetic acid; (D) GO-2, acetic acid. Figure 6. TEM images and ED analysis of GO scrolls (GO-1) obtained from the methanol suspension. (A) Intermediate state of scrolling. (B) High-resolution TEM image of the GO scroll. (C, D) Position for ED analysis and ED pattern of the GO scroll.

in acetic acid (265 ± 60 and 16 ± 6 nm). The sizes of the scrolls obtained from GO-2 show the same tendency (Table 1). The circular arc length (Larc) and the number of GO sheets per scroll (Npiece) are calculated by considering the average length and diameter as the typical size values and the interlayer distance of 0.57 nm. Larc of the GO sheets to form the GO scroll in acetic acid is calculated to be 339 (for GO-1) and 775 (for GO-2) nm. The scrolls should include average 1.3 and 1.2 pieces of GO sheets based on the sizes of the GO sheets, which are 265 and 664 nm obtained from the scroll length values. Npiece values are 7.6 and 5.7 for the nanoscrolls formed from GO-1 and GO-2 in isobutyric acid. As discussed below in Section 3.6, Npiece is closely related to the sedimentation rate of the GO suspensions. The average values of Larc, Lscroll, and Npiece are summarized in Table 2. 3.5. Structures in Suspension and Sediment. For a further understanding of the scroll formation process, the structures existing in the suspensions and sediments were separated and characterized by TEM. As shown in Figure 8A,B, the components suspended in pyridine and DMF are the GO sheets with the single-layer structure. These observations are consistent with the previous reports; i.e., single-layer GO sheets can be stably suspended in the organic solvents at least for a period of time.14,15,18,19 The sediment formed in the pyridine almost exclusively consists of the GO scrolls with the length over 20 μm (Figure 8C), while the GO sheets suspended in the pyridine are much smaller. As the UV−vis absorbance of the suspension is approximately proportional to the number of 2D lattice units of the GO sheets, the sharp decline in the absorbance of the pyridine suspension can be attributed to the sedimentation of the scrolls composed of the large GO sheets. Therefore, the yield of scrolls formed in pyridine is estimated to be 82% from the UV−vis spectrum at 33 h (Figure 3, Table S1). On the other hand, the sediment from DMF under gravity is more diversified as shown in Figure 8D, where both GO sheets and smaller scrolls can be seen at the same time in the sediment. Some relatively large GO sheets remaining in the suspension cause the relatively high absorbance of the DMF suspension at 33 h after sonication (Figure 3 and Table S1). Only a tiny amount of sediment forms in the NMP suspension

clearly discerned. The similar intermediates for the scroll formation can also be observed for the other solvents such as isopropyl alcohol (Figure S11A,B). Figure 6B gives the highresolution TEM image of the typical GO scroll formed from GO-1 in methanol, which exhibits the lamellar structure with the interlayer distance of 0.57 ± 0.02 nm. Similar results can also be seen for scrolls formed in isopropyl alcohol and ethanol (Figure S11C,D), which show the interlayer distances of 0.58 ± 0.04 and 0.57 ± 0.03 nm, respectively. Figure 6C shows the image of a typical scroll end formed in methanol, where the concentric shells scrolling onto themselves with several turns can be clearly seen. Figure 6D gives the ED pattern from the selected position on the GO scroll, which is similar to that of graphite in ...ABCABCABC... layer sequence,55 when the electron beam is parallel to the hexagon sides as shown in Figure S7. By the same calculation method mentioned above, the typical scrolls formed in methanol with the diameters of 258 (GO-1) and 265 (GO-2) nm should include 224 and 230 complete turns. The circular arc length (Larc) of the GO sheets to form typical GO scrolls can be calculated to be 91 400 (for GO-1) and 95 600 (for GO-2) nm. GO scrolls correspondingly should contain average 12.6 and 11.7 pieces of GO sheets based on the sheet sizes of 7240 (GO-1) and 8170 (GO-2) nm. For the other two alcohol solvents, GO scrolls are estimated to include on average 4.1 and 6.6 pieces (for GO-1 and GO-2) of the GO sheets for the case of isopropyl alcohol, and average 6.1 pieces of GO sheets for GO-1 in the case of ethanol. The formation of the GO scrolls was also investigated for acids by the same procedure. Figure 7 exhibits TEM images of the GO scrolls formed in isobutyric acid and acetic acid. Figure S12 shows the formation of an agglomerate of GO scrolls formed in isobutyric acid suspensions. The average length and diameter (at the central part) of the GO scrolls are 13 600 ± 4900 and 274 ± 103 nm for GO-1 formed in isobutyric acid, which are much larger compared to those of the scrolls formed 692


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glycol suspension. As shown in Figure S14, only GO sheets exist in the ethylene glycol sediments, which scarcely form the scrolling structure. The above results show that, in contrast to the single-layer GO sheets suspended in the solvents, the structures existing in the sediments are more diversified. As the tightly rolled-up scrolls have a much smaller interfacial area exposed to the solvents and higher density, they tend to undergo the sedimentation. On the other hand, depending on the suspension stability in the solvents, sheets with relatively large size can also more or less undergo the sedimentation under the influence of the gravitational field.56,57 Therefore, except for the solvents such as pyridine and isobutyric acid, in which GO sheets with different sizes can curl up into scrolls, the sediment always contains scrolls and relatively large GO sheets. The lengths of the scrolls, depending on the sizes of the GO sheets that curl up through the bending, are obviously altered for the different solvents. 3.6. Factors Affecting Scroll Formation and Size. As discussed above, GO nanoscrolls formed in the organic solvents show different average lengths and diameters that vary considerably with the solvents (Table 1). For the scrolls formed from GO-1 in pyridine and methanol, the average lengths of the scrolls are 22 500 ± 8600 and 7240 ± 2480 nm with the average diameters of 186 ± 76 and 258 ± 90 nm. For those formed in acetic acid and ethanol, the average lengths of the GO scrolls are quite small, which are 265 ± 60 and 390 ± 190 nm with the average diameters of 16 ± 6 and 42 ± 17 nm. The results of GO scroll formation have been double-checked for all the solvents. There are slight differences about the structural parameters of the GO scrolls for different batches, which have been included in the standard deviations given in the table. GO scrolls can be exclusively seen in the sediment of the pyridine suspension, and the formed GO scrolls are the longest with the average length of over 20 μm. GO scrolls can be readily seen in the sediments from methanol, isopropyl alcohol, and isobutyric acid with a large proportion. The average lengths and diameters of the GO scrolls vary from solvent to solvent. Meanwhile, scrolls cannot be observed for the NMP and ethylene glycol.

Figure 8. TEM images of the structures in the suspensions and sediments of GO-1, being left aside for 33 h after sonication: (A) pyridine suspension, (B) DMF suspension, (C) pyridine sediment, and (D) DMF sediment.

when left aside even for 33 h after sonication. As shown in Figure S13, almost no scrolls can be observed in the sediment, where the decline in the UV−vis absorbance is mainly caused by the sedimentation of the GO sheets. The structures existing in the suspensions and sediments of the protic solvents are also separated and characterized by the TEM observations. As shown in Figure 9A−E, GO sheets with the nearly planar structure are suspended in methanol, ethanol, isopropanol alcohol, acetic acid, and isobutyric acid. Like in DMF, some relatively large GO sheets undergo the sedimentation, but they do not curl up to form scrolls. As shown in Figure 9F−J, together with the GO sheets, GO scrolls with different lengths exist in the sediments of these solvents except isobutyric acid. For isobutyric acid, almost all of the sediment is composed of GO scrolls, and the scroll yield is estimated to be 66% via UV−vis absorbance at 33 h. Only a tiny amount of sediment can be separated from the ethylene

Figure 9. TEM images of the structures in the suspensions and sediments of GO-1, being left aside for 33 h after sonication. Structures existing in the suspensions: (A−E) methanol, ethanol, isopropyl alcohol, acetic acid, and isobutyric acid, respectively. Structures existing in the sediments: (F− J) methanol, ethanol, isopropyl alcohol, acetic acid, and isobutyric acid, respectively. 693


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ζ potential correspond to the increasing electric interaction of solvents with GO sheets. As reported, the initial bending energy barrier is closely related to the rolling width of GO sheets.42−44,54 The first rolling turn as the scroll nucleation is the vital step because there are no van der Waals interactions to stabilize the curling. The bending energy per unit area is D/2r2, and the bending energy for the scroll nucleation can be calculated by W = Dπd/r, where W is the bending energy, D is the bending stiffness, r is radius of scroll nucleation, and d is the rolling width.42,58 For GO sheets discussed here, the d value corresponds to the length of a GO scroll. It means that it is more difficult for the GO sheet with a larger size to undergo scroll nucleation because of the higher bending energy. The above results indicate that the increasing electric interaction of a solvent with GO sheets will weaken the curling tendency of the GO sheets in the solvent. Therefore, only the GO sheets with relatively small sizes can undergo the initial bending, and the formed scrolls are relatively short in such a solvent. To further verify this statement, the ζ potentials of the suspensions of isobutyric acid and ethylene glycol were measured, which are −6.8 ± 2.1 and −50.2 ± 7.2, respectively. As discussed above, GO scrolls readily form in the former, and it is impossible for them to form in the latter. The diameters of the scrolls, related to the number of the sheets per scroll (Npiece), show close correlation with the sedimentation rate in the solvents. The Npiece values are summarized in Table 2 and discussed in Sections 3.3 and 3.4. As UV−vis absorbance of the suspensions declines with the time after sonication, the absorbance variation relative to the original value can be used to characterize the sedimentation rate (Figure 3 and Table S1). As a general tendency, the scrolls formed in the suspensions with slower sedimentation rates can include more pieces of GO sheets per scroll. As shown in Figure 3A, GO in DMF shows a relatively slow sedimentation rate compared with the case of pyridine, and scrolls formed in DMF include more pieces of the GO sheets on average. As discussed above, the sedimentation rates in the alcohols can be well-explained with Hansen solubility parameters as shown in Figure S5C. Ethanol and isopropyl alcohol have relative small Hansen solubility parameters, δp + δh = 28.2 and 22.5 MPa1/2, while methanol has the larger Hansen solubility parameters, δp + δh = 34.6 MPa1/2. The suspension of methanol shows a slower sedimentation rate (Figure 3B), and the formed scrolls include more pieces of the GO sheets on average (Table 2). A similar result is also observed for the acidic solvents. Acetic acid has the smaller Hansen solubility value (δp + δh = 21.5 MPa1/2) compared with isobutyric acid (δp + δh = 26.1 MPa1/2). The suspension of acetic acid shows the faster sedimentation (Figure 3C), and the scrolls formed in the suspension contain few pieces of GO sheets (Table 2). Diameters of the scrolls depend on both the size of the scrolling GO sheets and the number of the GO sheets included in each scroll. As the scrolls formed in methanol and isobutyric acid include more pieces of GO sheets with relatively large sizes, they have the larger average diameters compared with those formed in other protic solvents (Table 1). The scroll formation is closely related to the interaction of GO sheets with the solvents and their suspension stability, which depend on dipole moment, ζ potential, and Hansen solubility parameter of the solvents. For the aprotic solvent, the suspension of the solvent with the lower dipole moment shows the more rapid sedimentation (Figure 3 and Figure S5A). In this case, the tendency of the GO sheets to form the scrolls is

In this section, we will further discuss the reasons that cause the size differences and rationalize the observations by considering dipole moments and Hansen solubility parameters of the solvents (Table 3). Previous studies show that polar Table 3. Hansen Solubility Parameter and Dipole Moment of the Organic Solvents solvent pyridine DMF NMP methanol ethanol ethylene glycol isopropyl alcohol acetic acid isobutyric acid

δda (MPa1/2)

δpa (MPa1/2)

δha (MPa1/2)

δp + δh (MPa1/2)

19 17.4 18 14.7 15.8 17

8.8 13.7 10.5 12.3 8.8 11

5.9 11.3 9.7 22.3 19.4 26

14.7 25.0 20.2 34.6 28.2 37

2.23 3.82 4.09 1.67 1.68 2.20

15.8

6.1

16.4

22.5

1.68 D

14.5 15.4

8.0 9.9

13.5 16.2

21.5 26.1

1.74 D 1.08 D

dipole moment D D D D D D

a Hansen solubility parameters of the solvents, δd, δp, and δh, denoting dispersion interaction, dipolar interaction, and hydrogen bonding interaction, respectively.20,21

aprotic solvents with large dipole moment (>3.82 D) and protic solvents with dipole moment >1.8 D and Hansen solubility parameters (δp + δh) > 32 MPa1/2 can exhibit reasonable dispersion stability.18,19,23 Indicated by the present study, GO sheets cannot form nanoscrolls in NMP (dipole moment = 4.09 D), or ethylene glycol (dipole moment = 2.20 D, δp + δh = 37 MPa1/2), which are the solvents able to well suspend the GO sheets. It means that the well-suspended GO sheets cannot curl up to form scrolls, which is supported by the results given in Section 3.5. Comparing the results given in Table 1 with the solvent dipole moments (Table 3), it can be found that the average lengths of the GO scrolls show a tendency to decline with the increase of the dipole moments of the solvents. For the polar aprotic solvent, scrolls formed in pyridine (2.23 D) are much longer compared to those formed in DMF (3.82 D). Such a tendency can also be observed for the polar protic solvents. The average lengths of the GO scrolls show the following order: isobutyric acid (13 600 nm, 1.08 D) > methanol (7240 nm, 1.67 D) > isopropyl alcohol (1080 nm, 1.68 D) > ethanol (390 nm, 1.68 D) > acetic acid (265 nm, 1.74 D). As shown in Figure S15A, although the lengths of the formed scrolls depend on the dipole moments of the solvents, there are obvious differences in the average lengths for the scrolls formed in methanol, ethanol, and isopropyl alcohol, which have similar dipole moment values. To further understand this phenomenon, we measured the ζ potentials of the GO dispersions with the protic solvents as the dispersion media. The correlation between the ζ potentials and the average lengths can be clearly seen from the results (Table S2 in the Supporting Information). The ζ potentials are −20.9 ± 3.8, −29.3 ± 5.3, and −36.2 ± 5.6 for methanol, isopropyl alcohol, and ethanol, respectively, where the average lengths of the scrolls decrease with the shift of the ζ potentials to the more negative values. Distinct from the dipole moment of the solvent as a macroscopic parameter, the ζ potential is related to the electrokinetic behavior of the electric double layer (the Stern layer). The increases of both the dipole moment of the solvent and the absolute value of the 694


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with the less negative ζ potential value, the relatively large GO sheets can curl up to form scrolls in the solvents. Ethanol with the more negative ζ potential and acetic acid have the large dipole moment; only some small GO sheets can form scrolls with short average lengths. Finally, ethylene glycol has the much larger dipole moment and the most negative ζ potential compared with the other alcohols; GO scrolls cannot be formed in this dispersion medium. Therefore, the average lengths given in Table 1 are negatively correlated with the ability of the solvents to stabilize the GO sheets, where the relatively large GO sheets can undergo the scrolling and gradual sedimentation for a solvent having a relatively weak interaction with them. This means that the tendency for the GO sheets to form scrolls becomes stronger with the decreases of the polarity of the solvent and the absolute value of the ζ potential. In other words, the enhanced interaction of the solvents with GO sheets will increase the energy barrier for a GO sheet to initiate bending through thermal fluctuations. For the polar aprotic solvents, the scrolling tendency of GO sheets in the solvents is directly correlated with the sedimentation rate of the GO sheets (Figure 3), as both are negatively correlated with the dipole moments of solvents (Figures S5A and S15A). NMP has the good dispersion stability for GO sheets, and no scroll can form in the medium. For the pyridine with the poor dispersion stability, the relatively large GO sheets can curl up to form scrolls with high yield. DMF with the medium dispersion stability is a solvent where GO sheets show the certain tendency to form scrolls between the two above cases. However, the situation is more complicated for polar protic solvents. The dispersion stability of GO sheets in the solvents is related to the Hansen solubility parameters (δp + δh; Figure S5C), while the tendency of GO sheets to form scrolls is controlled by the dipole moment and ζ potential of the solvents. GO sheets show relatively good dispersion stability in isobutyric acid and methanol, but the large GO sheets can curl up into scrolls for the relatively small dipole moments and less negative ζ potentials of the solvents. In contrast, for acetic acid, ethanol, and isopropyl alcohol with relatively poor dispersion ability to GO, only small GO sheets can curl up into scrolls for their relatively large dipole moments and more negative ζ potentials. This is consistent with previous reports; protic solvents with dipole moment >1.8 D and Hansen solubility parameters (δp + δh) > 32 MPa1/2 can exhibit reasonable dispersion stability.18,19,23 In this case, it should be extremely rare for GO sheets to form scrolls. The difference between the scrolling tendency and sedimentation for the protic solvents can be attributed to the hydrogen bonding between the solvent molecules and GO sheets, which is included in the Hansen solubility parameter δh. The number of GO sheets included per scroll (Npiece) shown in Table 2 is directly related to the sedimentation rates as shown in Figure S15B. The initially formed scrolls can act as templates to induce GO sheets to curl onto them; such template effects have been observed for carbon nanotubes and other nanoparticles.35,36,62,63 When the initially formed scrolls undergo rapid sedimentation, such as in the cases of pyridine and acetic acid, GO sheets have no chance to curl onto the formed scrolls, and the GO scrolls contain only an average of one or two pieces of GO sheets. In contrast, the scrolls formed in the solvents such as DMF, methanol, and isobutyric acid can consist of more pieces of the GO sheets due to the slow sedimentation. As formed scrolls mainly exist in the sediments, the yield of the scrolls will depend on the relative speeds of the

directly correlated with the sedimentation rate. For the polar protic solvents, the tendency to form scrolls is not straightforwardly correlated with the sedimentation rate. GO sheets with vast oxygenated groups are advantageous to form hydrogen bonding with protic solvents, which corresponds to a large Hansen solubility parameter (δp + δh). For the protic solvents, the scrolling tendency of the GO sheets in the solvents (characterized by the scroll length) decreases with the increases of the dipole moment and the absolute value of the ζ potential, while the sedimentation rate decreases with the increase of the δp + δh value (Figure S5C). Therefore, for the solvents with the relatively small dipole moment and the less negative value of the ζ potential but relatively large δp + δh value, such as isobutyric acid and methanol, the GO sheets with relatively large sizes can curl onto themselves to form scrolls, but the sedimentation is relatively slow. In this case, both Lscroll and Npiece are larger as shown in Table 2. In contrast, for the solvents with relatively large dipole moment but relatively small δp + δh value, such as acetic acid, only the small GO sheets can form the short scrolls, and GO sheets undergo the sedimentation at the same time. 3.7. Discussion. Since the discovery of graphene, the stable existence of such one-atom-thick crystals has been scrutinized from different facets. It has been argued whether the obtained 2D crystallites are quenched in a metastable state because they are extracted from 3D materials, or the extracted 2D crystals become intrinsically stable by gentle crumpling in the third dimension.59 It has been observed that the exfoliated 2D graphene sheets curl onto themselves to form CNSs in ethanol or N-methyl-2-pyrrolidone via sonication.25,26 These results indicate that if the elastic energy barrier for bending a graphene sheet can be overcome under proper conditions, the van der Waals and other interactions in the overlapping region will take over the structural evolution of the graphene sheet to reach a stable scrolling state.42 Similar results have also been reported on other 2D nanomembranes.60,61 In a suspension, the interaction of graphene sheets with a dispersion medium will play a significant role to influence the bending energy barrier. For GO sheets, the existence of the functional groups and defects/vacancies in the 2D crystal lattice can enhance the interaction of GO sheets with the solvent and even drives them to transform into 1D nanoscrolls through local stress relaxation around the defect/vacancy sites.44 The solvent effects discussed above can be further rationalized by considering the stability of GO sheets in the solvents. Previous studies show that polar aprotic solvents with the large dipole moment (>3.82 D) can exhibit reasonable dispersion stability for GO sheets.18,19,23 This means that the aprotic solvent with the large dipole can stabilize the GO sheets in the suspension and increase the initial bending energy, which will reduce the chance for bending and scrolling through thermal fluctuations. Therefore, for the polar aprotic solvents discussed above, GO sheets cannot undergo the bending to form scrolls in NMP because of its largest dipole moment in the series. Pyridine with the smallest dipole moment has a weak interaction with the GO sheets, and the relatively large sheets can curl up to form scrolls. For DMF with the dipole moment between them, only GO sheets with middle sizes can undergo the bending to form scrolls. On the other hand, for the case of protic solvents, the dipole moments of the solvents are still important, but ζ potentials characterizing the electrokinetic interaction with solvents are consequential for scroll formation. For isobutyric acid with the small dipole moment and methanol 695


ACS Applied Nano Materials scroll formation and sedimentation of GO sheets in the solvents. For the protic solvents, those can effectively induce the scroll formation and stably disperse GO sheets, and at the same time are more suitable to be used for fabrication of the GO scrolls with larger sizes.

ACKNOWLEDGMENTS

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REFERENCES

(1) Eda, G.; Chhowalla, M. Chemically Derived Graphene Oxide: Towards Large-Area Thin-Film Electronics and Optoelectronics. Adv. Mater. 2010, 22, 2392−2415. (2) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. B.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282−286. (3) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. B.; Ruoff, R. S. Preparation and Characterization of Graphene Oxide Paper. Nature 2007, 448, 457−460. (4) Park, S. J.; Ruoff, R. S. Chemical Methods for the Production of Graphenes. Nat. Nanotechnol. 2009, 4, 217−224. (5) Zhu, Y. W.; Murali, S.; Cai, W. W.; Li, X. S.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906−3924. (6) Dreyer, D. R.; Ruoff, R. S.; Bielawski, C. W. From Conception to Realization: An Historical Account of Graphene and Some Perspectives for Its Future. Angew. Chem., Int. Ed. 2010, 49, 9336− 9344. (7) Loh, K. P.; Bao, Q. L.; Eda, G.; Chhowalla, M. Graphene Oxide as a Chemically Tunable Platform for Optical Applications. Nat. Chem. 2010, 2, 1015−1024. (8) Dreyer, D. R.; Park, S. J.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240. (9) Cheng, C.; Li, D. Solvated Graphenes: An Emerging Class of Functional Soft Materials. Adv. Mater. 2013, 25, 13−30. (10) Lerf, A.; He, H. Y.; Forster, M.; Klinowski, J. Structure of Graphite Oxide Revisited. J. Phys. Chem. B 1998, 102, 4477−4482. (11) Hummers, W. S., Jr.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1399. (12) Cai, D.; Song, M. Preparation of Fully Exfoliated Graphite Oxide Nanoplatelets in Organic Solvents. J. Mater. Chem. 2007, 17, 3678−3680. (13) Cai, D.; Song, M.; Xu, C. Highly Conductive CarbonNanotube/Graphite-Oxide Hybrid Film. Adv. Mater. 2008, 20, 1706−1709. (14) Paredes, J. I.; Villar-Rodil, S.; Martinez-Alonso, A.; Tascon, J. M. D. Graphene Oxide Dispersions in Organic Solvents. Langmuir 2008, 24, 10560−10564. (15) Park, S.; An, J.; Jung, I.; Piner, R. D.; An, S. J.; Li, X.; Velamakanni, A.; Ruoff, R. S. Colloidal Suspensions of Highly Reduced Graphene Oxide in a Wide Variety of Organic Solvents. Nano Lett. 2009, 9, 1593−1597. (16) Ahmad, R. T. M.; Hong, S.; Shen, T.; Song, J. Water-Assisted Stable Dispersal of Graphene Oxide in Non-Dispersible Solvents and Skin Formation on the GO Dispersion. Carbon 2016, 98, 188−194. (17) Zhu, Y. W.; Stoller, M. D.; Cai, W. W.; Velamakanni, A.; Piner, R. D.; Chen, D.; Ruoff, R. S. Exfoliation of Graphite Oxide in Propylene Carbonate and Thermal Reduction of the Resulting Graphene Oxide Platelets. ACS Nano 2010, 4, 1227−1233. (18) Kim, D. H.; Yun, Y. S.; Jin, H. J. Difference of Dispersion Behavior between Graphene Oxide and Oxidized Carbon Nanotubes in Polar Organic Solvents. Curr. Appl. Phys. 2012, 12, 637−642. (19) Konios, D.; Stylianakis, M. M.; Stratakis, E.; Kymakis, E. Dispersion Behavior of Graphene Oxide and Reduced Graphene Oxide. J. Colloid Interface Sci. 2014, 430, 108−112. (20) Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press LLC: Boca Raton, FL, 1991. (21) Hansen, C. M. Hansen Solubility ParametersA User’s Handbook; CRC Press LLC: Boca Raton, FL, 2007. (22) Hernandez, Y.; Lotya, M.; Richard, D.; Bergin, S. D.; Coleman, J. N. Measurement of Multicomponent Solubility Parameters for

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.7b00160. Raman, XRD, XPS, SEM, high-resolution TEM, and AFM characterizations; normalized UV−vis absorbance data; relative sediment amounts with dipole moment and Hansen solubility parameters; reciprocal space lattice of a single GO scroll; and structure parameters of the GO scrolls with the dipole moment of solvents and sedimentation rate of the suspensions (PDF)

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This work was supported by the NSFC under Project 51233002.

4. CONCLUSIONS In summary, we studied the formation of graphene oxide (GO) nanoscrolls in different organic solvents, which showed close correlation with the dipole moment, ζ potential, and Hansen solubility parameters of the solvents. In contrast to the GO sheets keeping planar structure in the suspensions, GO scrolls with different sizes form in the suspension and undergo sedimentation in some organic solvents. Pyridine is one such solvent in which the single-layer GO sheets curl into scrolls with the high yield of 82% at 33 h. Scroll formation is also observed for DMF, methanol, ethanol, isopropyl alcohol, acetic acid, and isobutyric acid, but the scrolls possess different average lengths and diameters controlled by the properties of the solvents. The solvent properties not only decide the scrolling formation but also affect the sizes of the nanoscrolls. The average lengths of the formed scrolls reflect the tendency of the GO sheets to curl into scrolls in the solvents. For the solvents with the relative small dipole moment and less negative ζ potential, the relatively large GO sheets can curl up into scrolls to possess a relative large length on average. On the other hand, the average number of the included sheets per scroll is correlated with the sedimentation rate of the GO sheets from the suspension, which is controlled by the dipole moment for the aprotic solvents and Hansen solubility parameters for the protic solvents. Diameters of the scrolls are determined by both the size of the scrolled GO sheets and the number of the GO sheets included in each scroll. Based on this understanding, this study provides an efficient way to fabricate GO scrolls in a scalable manner and to control their sizes at the same time by selecting a suitable solvent. The GO nanoscrolls formed in organic solvents can be expected for the applications in hydrogen storage materials, gas sensing, actuators, lubrication materials, tunable water and ion channels, solar cells, and others.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 10 62796171. ORCID

Xiaogong Wang: 0000-0002-8721-6976 Notes

The authors declare no competing financial interest. 696


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Formation of Graphene Oxide Nanoscrolls in Organic Solvents

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