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Article Cite This: ACS Omega 2019, 4, 7420−7427
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Organic Solvent-Assisted Lyophilization: A Universal Method of Preparing Two-Dimensional Material Nanoscrolls Xiaoping Huang,† Zhifeng Huang,† Qiuju Liu,† An’an Zhou,† Yinxing Ma,‡ Jingjing Wang,† Hong Qiu,† and Hua Bai*,†,§ †
College of Materials, ‡College of Chemistry and Chemical Engineering, iChEM, and §Graphene Industry and Engineering Research Institute, Xiamen University, Xiamen, 361005, P. R. China
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S Supporting Information *
ABSTRACT: Rolling up two-dimensional (2D) materials into one-dimensional (1D) nanoscrolls is a straightforward way to fabricate 1D nanomaterials, which can inherit the properties from 2D materials and exhibit new functions originated from 1D morphology. Here, we report a general and facile method for preparing nanoscrolls: organic solvent-assisted lyophilization (OSAL). High-quality nanoscrolls from different 2D materials, including graphene oxide, reduced graphene oxide, and MoS2, can be obtained by lyophilizing their aqueous dispersion in the presence of a small volume of high-boiling-point organic drying. Mechanism investigation reveals that the imbalanced surface tension on the two sides of the 2D material sheet caused by the absorbed organic solvent is the driving force of the rolling up process. OSAL can also be used to produce composite nanoscrolls by encapsulating other nanomaterials into the inner space of nanoscrolls, even the nanomaterials that show weak infinity for the 2D material. The produced reduced graphene oxide nanoscrolls can form a nonwoven fabric, which shows high performance as a flexible electrode of a supercapacitor. The OSAL method developed here opens a new route for fabricating novel 2D-material-based nanostructures and offers a novel platform for designing and preparing 1D nanomaterials.
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doped Si nanowires,21 yielding composite nanoscrolls. Other nanoparticles, including γ-Fe2O3,22 Fe3O4, and Ag,23 are also able to roll up 2D materials. These preparation methods, however, are effective for specific composite materials but are difficult to be applied to other systems. Similarly, several other artful methods of massively preparing nanoscrolls all rely on the particular properties of certain 2D materials.24,25 Until now, we are not capable of designing and preparing nanoscrolls as we wish. Recently, it was reported that in certain cases nanoscrolls could form from pure RGO dispersion after liquid nitrogen quenching and lyophilization.26,27 This method is attractive because of its easy operation and high production capacity. However, the driven force of scrolling in this method has not been fully understood, whereas the yield of nanoscrolls (the fraction of nanoscrolls in the product composed of both nanoscrolls and uncurled nanosheets) is sensitive to the chemical structure of the RGO and even the concentration of the feeding RGO dispersion.26 Therefore, the scope of application of this method is still very limited. Here, we report an efficient and general method of massively preparing 2D material nanoscrolls. We will demonstrate that by adding a small amount of high-boiling-point organic solvent into the aqueous dispersion of 2D materials, high-quality nanoscrolls are obtained from direct lyophilization. This organic solvent-assisted
INTRODUCTION The rapid development of two-dimensional (2D) materials has brought the multifarious possibility of constructing novel nanomaterials with these building blocks.1,2 By modifying the chemical structure,3−5 tailoring the shape,6−8 and controlling the assembly manner,9,10 people have designed and produced various 2D-material-based new nanomaterials, which demonstrate appealing properties and functions. Recently, selfassembly of 2D materials into one-dimensional (1D) materials has aroused considerable research interest, and it has gradually become a new strategy of preparing 1D materials, which inherit the properties from their parent 2D materials and present emerging features from their new morphology.10 Nanoscrolls represent a unique type of 1D nanomaterials formed by rolling up 2D materials. Like nanotubes, nanoscrolls have large inner space and open ends, which allow mass exchange with the environment. Furthermore, the rolling-up process makes it easy to incorporate other materials or molecules into the nanoscrolls, and to add new properties and functions. Therefore, nanoscrolls have shown broad applications in many fields, such as energy conversion and storage,11,12 catalysis,13 sensors,14 and so on. One of the big challenges in the research of nanoscrolls is the lack of a reliable method for massively preparing nanoscrolls. Perfect pure single nanoscrolls can be obtained via rolling up a piece of monolayered 2D material on the substrate, by adjusting the surface tension,15−19 but the output is rather low. At present, the main technique of massively preparing nanoscrolls is using nanoparticles as the templates. For example, reduced graphene oxide (RGO) nanosheets were found to wrap V3O720 and P© 2019 American Chemical Society
Received: March 5, 2019 Accepted: April 15, 2019 Published: April 24, 2019 7420
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Figure 1. (A) Schematic illustration of the preparation of GONS. (B,C,E) SEM images of GONS. (D) SEM image of lyophilized GO. (F) TEM image of a GONS.
overlapping GO sheets. A close observation (Figure 1E) also reveals that there are many branched GONSes. These long and branched GONSes were generated because adjacent GO sheets rolled up together and connected. Such interconnected network structure can provide a perfect path for charge transport, if GO is converted into RGO and further used as electrode materials. The dimension of the GONS can be adjusted by the size of GO sheets and the concentration of the dispersion. As shown in Figure S1, short nanoscrolls (Figure S1B) are obtained with GO sheets whose average size is 3 μm (Figure S1A), whereas increasing the size of the GO sheets to 50 μm (Figure S1C) produces long nanoscrolls with fewer branches (Figure S1D). This result is rational because the length of the nanoscroll partly depends on the lateral size of the raw GO sheets. The concentration of the GO dispersion mainly influences the diameter of the GONS. By lowering the concentration of the GO dispersion, while adjusting the ratio of NMP to GO, the possibility of GO overlapping is reduced, which leads to thin and sparse nanoscrolls. As shown in Figure S2A,B, nanoscrolls with a diameter as small as 100 nm are obtained with 0.1−0.2 mg mL−1 GO dispersions. Increasing the concentration of the GO dispersion enlarges the diameter of the resulting nanoscrolls (Figure S2B−G), but over high GO concentration will cause the formation of large flakes at the intersections of nanoscrolls (Figure S2H). These flakes are aggregates of GO sheets failing to roll up, possibly because the GO sheets are so crowded that the curling process is hindered mechanically by the adjacent ones. The structure of the GONS is revealed by the transmission electron microscope (TEM) image, in which the tubular structure with an out diameter of 500 nm and an inner diameter of ∼280 nm can be clearly observed (Figure 1F). The large space inside the nanoscroll provides the possibility of filling other materials into it, which enable us to construct composite nanoscrolls, as will be demonstrated later. The nanoscroll wall with a multilayer structure is also shown in the TEM images. We also notice that the wall of the nanoscroll is not compact, with
lyophilization (OSAL) method has the advantages of simple operation, high output, high yield (nearly 100%), and wide applicability. Nanoscrolls made from different 2D materials were obtained with OSAL, and more importantly, composite nanoscrolls can also be prepared easily by applying OSAL to blend dispersions of nanomaterials and 2D materials. Therefore, OSAL is a powerful tool for converting 2D materials into nanoscrolls, and a universal platform of designing and preparing 1D composite materials.
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RESULTS AND DISCUSSION We took graphene oxide (GO) as the representation of 2D materials in OSAL, considering its easy preparation and high dispersibility in water. The preparation of a GO nanoscroll (GONS) via OSAL is schematically shown in Figure 1. Typically, 15 μL of 1-methyl-2-pyrrolidone (NMP) was added into 4 mL of 2 mg mL−1 GO dispersion, and the dispersion was then frozen rapidly in liquid nitrogen. After conventional lyophilization at room temperature and 20 Pa for 24 h, the dry GONS was obtained. Figure 1B−D depict the SEM images of GONS and GO samples obtained by lyophilizing GO dispersion without the addition of NMP. As expected, lyophilizing the GO dispersion results in a GO aerogel, in which most of the GO sheets are stretching, with several random bends or folds (Figure 1D). If 0.375 vol % of NMP is added into the GO dispersion, GO exhibits fiber-like shape after lyophilization (Figure 1B,C), indicating that GO sheets are rolled up. The GONSes have random orientation and form a three-dimensional (3D) network-like nonwoven fabrics. As no GO sheet was observed in the sample, the yield of the GONS should approach 100% (Figure 1B), demonstrating the high efficiency of the OSAL method. The diameter of the GONSes is 1 ± 0.5 μm, where the large variation is due to the nonuniform lateral size of raw GO sheets. Some of the GONSes are over several tens of micrometers long, much larger than the lateral size of GO sheets, suggesting that they are constituted of more than one 7421
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Figure 2. Formation of GONS. (A) Size distribution of a GO dispersion before and after addition of 0.375 vol % NMP. (B) ESEM images showing the changing surface of a frozen 0.4 mg mL−1 GO dispersion with 0.1 vol % NMP. The yellow arrow indicates the rotation movement of a single nanoscroll. (C) Proposed formation mechanism of the GONS. (D−G) SEM images of lyophilized GO dispersion with different organic solvents. (D) 0.375 vol % PC. (E) 0.5 vol % DMSO. (F) 0.2 vol % glycerol. (G) 0.75 vol % methanol.
molecules that induce the asymmetric surface tension. NMP is a high-boiling-point organic solvent, and its saturated vapor pressure is estimated to be around 0.64 Pa at −20 °C (the measured surface temperature of the frozen GO dispersion sample in the lyophilizer, see the Supporting Information, Table S1 and Figure S3 for the detail of estimation), which is three orders of magnitude lower than that of ice at the same temperature (103.4 Pa).29 The lower saturated vapor pressure indicates that NMP evaporates more slowly than ice does, and consequently during lyophilization, NMP is enriched on the surface of the sample, including the surfaces of ice and exposed GO sheets. The NMP layer on the GO sheets exerts additional surface tension onto the sheets, as shown in Figure 2C. However, it is highly possible that the surface tension on the GO sheet is imbalanced on the two sides of the GO sheet, caused by the initial asymmetric conformation of the GO sheet. One principal reason for the unbalanced surface tension is the asymmetric evaporation of the NMP layer. The GO sheets exposed from the ice usually have a slight curvature due to their soft nature. This curvature causes a difference in the vapor pressure of NMP on different sides: NMP layer on the concave surface has lower vapor pressure and evaporates more slowly, whereas that on the convex surface has higher vapor pressure and evaporates faster. Finally, NMP on the convex surface completely vaporized, while there was still a certain amount of NMP left on the concave surface. At this moment, the surface tensions on the two sides of the GO sheet become highly asymmetric. As long as the net surface tension can overcome the bending stress of the GO sheet, which is highly possible given the low bending modulus of GO sheets and a relatively large curvature radius of the nanoscroll,30−32 it will drive the GO sheet to further curl toward the concave side, until the sheet completely rolls up. Once the nanoscroll forms, the interaction
large interlayer spaces. The loosely packed wall of the nanoscroll suggests that the rolling up of GO sheets is not driven by interlayer forces. It is interesting that a small volume of organic solvent can completely roll up GO sheets. In order to elucidate the formation mechanism of GONS, it is essential to find whether the nanoscrolls form in the dispersion or during lyophilization. The dynamic light scattering (DLS) was employed to detect the conformation change of the GO sheets in the dispersion after addition of NMP. If the GO sheets roll up with the assistance of NMP, their average hydrodynamic radius distribution is expected to change significantly. The DLS measurement, however, reveals that the hydrodynamic radius distribution of GO showed little variation when NMP was added (Figure 2A). Therefore, GO sheets do not roll up in the dispersion after addition of NMP. We then used an environmental scanning electron microscope (ESEM) to directly observe the lyophilization of frozen GO dispersion (Figure 2B and Video S1). The frozen GO dispersion sample was allowed to sublimate in ESEM under controlled humidity and temperature. At many spots on the surface of the sample, the rapid motion of GO sheets was observed when they were exposed from the descending ice surface. An example in Figure 2B shows that a single GONS emerged from the ice surface and rotated continuously with the slow sublimation of the ice. Such a rotation can be explained by the rolling up of a specially shaped GO sheet. Therefore, the ESEM observation offers us a direct proof that GONSes form during the lyophilization. A formation mechanism of GONSes is proposed based on the above experimental results. Generally speaking, the basic driving force of curling of a 2D nanosheet is the asymmetric surface tension on its two sides, and the tension difference will drive the nanosheet to curl toward the side with the larger surface tension.28 In the OSAL process, it is the organic solvent 7422
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between GO layers will be strong enough to keep the nanoscroll stable. Other experimental results provide further evidence for the above mechanism. We first investigate the influence of the NMP’s dosage on the morphology of the prepared GONS. There is only a slight change in the morphology of GO if the NMP volume ratio is 0.125 vol % (Figure S4A), and when NMP is increased to 0.25 vol % (Figure S4B), GO sheets begin to curl. With an NMP dosage of 0.375 vol %, the GO sheets completely roll up and form GONSes (Figures 1C and S4C). However, if the NMP dosage continues to increase (Figure S4D), aggregation of GO sheets will occur, yielding thick fibers or even films. The aggregation is caused by the capillary attraction of the NMP liquid film.33 If the NMP layers on GO sheets are so thick that an NMP liquid bridge forms between adjacent GO sheets, the capillary force of the liquid bridge will bring these sheets together (Figure S4E,F). The capillary attraction may also exist between the GO sheets and nanoscrolls. This phenomenon confirms the existence of the NMP liquid film on GO sheets during lyophilization. The optimal dosage of NMP for producing high-quality GONSes is around 0.375 vol %. It should be noted that liquid nitrogen quenching is also important for OSAL. It allows the GO sheets to remain dispersed during freezing. As shown in Figure S5, when the GO solution was frozen in a −20 °C refrigerator, most of the lyophilized products were large GO sheets aggregates, with a small portion of thick scrolls. The reason is that at low freezing speed, there are large ice crystals, which repel out GO sheets and cause aggregation of GO sheets. In aggregates, the rolling up of GO sheets will be hindered by the nearby sheets. The results show that fast freezing is conducive for the formation of highquality GONSes. However, the GONSes forming at slow freezing speed indicate that nitrogen quenching is not the driving force of the rolling up of GO sheets. The mechanism suggests that the formation of a GONS relies on the slow evaporation rate of NMP, so other high-boilingpoint organic solvents, which have low vapor pressure and slow evaporation rate, should also work in OSAL. We used another three solvents, namely 1,2-propylene carbonate (PC, bp. 242 °C, Figure 2D), dimethyl sulfoxide (DMSO, bp. 189 °C, Figure 2E), and glycerol (bp. 290 °C, Figure 2F), to replace NMP in OSAL, and found that all of them are able to produce GONSes with the similar morphology of those obtained by NMP. Therefore, OSAL is not dependent on the chemical structure of the organic solvent. As a comparison, low-boiling-point organic solvents, including methanol (bp. 65 °C, Figures 2G and S6), acetone (bp. 56 °C, Figures S7A and S8), and diethyl ether (bp. 35 °C, Figures S7B and S9), are also tested in OSAL. In these cases, only a small part of GO sheets showed a tendency of curling because of the too fast evaporation of these solvents. For example, at −20 °C, the vapor pressure of methanol is still 970 Pa,34 which is much higher than that of ice. Thus, there will be less methanol left on the surface of GO sheets, and also the fast evaporation of the residual methanol will not give enough time to the GO sheets to finish their curling. These results prove the importance of evaporation rate of the organic solvent in promoting the formation of nanoscrolls and provide us a broad choice for organic solvents in OSAL. We further applied OSAL to other 2D materials to prepare new nanoscrolls. Starting from RGO and MoS2 sheet (Figure S10), we successfully prepared RGO nanoscrolls (Figure 3A) and MoS2 nanoscrolls (Figure 3B) via OSAL. Both types of nanoscrolls resemble GONSes in morphology. These results are
Figure 3. Morphologies of other 2D materials after being treated by OSAL. (A) RGO nanoscrolls. (B) MoS2 nanoscrolls. (C) Graphite nanoplatelets.
rational because the organic solvent layer always forms on the surface of 2D materials during OSAL, and leads to the unbalanced surface tension. Hence, OSAL is a general method of preparing nanoscrolls and does not rely on the chemical structure of the raw 2D material. However, it should be noted that the thickness of the 2D material will strongly influence the curling process. Generally, the bending stress of a 2D nanosheet increases with its thickness, and for a thick nanosheet, its bending stress may become larger than the net surface tension provided by the organic solvent film, resulting in inadequate driving force for the rolling up process. Besides, when the nanosheet is thick and rigid, it becomes unable to adopt an initial bend conformation, which is detrimental to the development of unbalanced surface tension. In fact, when we used OSAL to treat graphite nanoplatelets (∼10 layers, 3−4 nm, Figure S11), they only aggregated but did not form nanoscrolls (Figure 3C). Given that 2D materials other than GO are difficult to be dispersed in water with a high monolayer content,35 the obstacle to form more different types of 2D material nanoscrolls is the lack of high-quality monolayer dispersions. RGO nanoscrolls and MoS2 nanoscrolls also provide another evidence to confirm our proposed mechanism of OSAL. The frozen RGO (0.05 vol % NMP) and MoS2 (0.5 vol % NMP) dispersions were allowed to melt at room temperature, and by observing the morphology of RGO and MoS2 in the melted dispersion, we can determine if the nanoscrolls are formed during the freezing process. It is known that the aggregation of RGO and MoS2 is irreversible in water, thereby the morphology of RGO nanoscrolls and MoS2 nanoscrolls should be stable in water. If the nanoscrolls form during freezing, their morphology will be preserved even after the sample melts. However, TEM images show that RGO and MoS2 in the melted dispersion are still 2D sheets (Figure S12). Therefore, the nanosheets do not roll up in liquid nitrogen, and the nanoscrolls form during lyophilization. OSAL can be used to wrap other nanomaterials into the nanoscrolls. We also took GO as the model 2D material for convenience. According to the strength of the interaction between GO and the nanomaterial to be encapsulated, the nanomaterials are roughly divided into two groups. Nanomaterials in Group I show strong attraction with GO and can selfassemble on GO sheets, forming composite nanosheets; whereas Group II contains those nanomaterials showing weak interaction or repulsion with GO (Figure 4A). A typical nanomaterial of Group I is the positively charged polyaniline (PANI) nanofiber (Figure S13A), which has been confirmed to form an assembly with RGO sheets driven by electrostatic force and π−π interactions.36 SEM in Figures 4B,C shows that highquality nanoscrolls were prepared when applying OSAL to a mixed dispersion of GO and PANI nanofibers. The TEM image demonstrates the PANI nanofibers inside the nanoscroll. The 7423
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Figure 4. Preparation and morphology of composite nanoscrolls. (A) Encapsulating two different groups of nanomaterials to produce composite nanoscrolls. (B,C) SEM (B) and TEM (C) images of PANI nanofiber@GO nanoscrolls. (D,E) SEM (D) and TEM (E) images of Pt nanoparticle@ RGO nanoscrolls. (F,G) SEM (F) and TEM (G) images of PS microsphere@GO nanoscrolls. (H,I) SEM (H) and TEM (I) images of V2O5·nH2O nanowire@GO nanoscrolls.
Figure 5. Capacitive performance of the RGONS electrode. (A) Photograph of flexible RGONS electrode. (B,D) SEM images of the RGONS electrode (B), compressed RGO aerogel (Film I, C) and RGO film (Film II, D). (E) GCD curves of the RGONS electrode. (F) Comparison of rate performance of RGONS, Film I and Film II. (G) Comparison of EIS of RGONS, Film I and Film II. (H) Specific capacitance of the RGONS measured in the flexible solid device. Inset: configuration of the flexible solid device. (I) CV curves of the RGONS-based flexible solid device at different bending angles, θ. (J) Capacitance retention of the RGONS-based flexible solid device during repeating bending/releasing cycles.
random orientation of PANI nanofibers suggests that the fibers are not the template of the nanoscroll. Similarly, Pt nanoparticle-
decorated RGO nanosheets were also rolled up in OSAL, yielding Pt nanoparticle@RGO composite nanoscrolls (Figure 7424
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129.39 F g−1 at the current density of 1 A g−1, which is comparable with the literature data.43,44 Film I and Film II also have similar electrochemical properties but smaller specific capacitance at low current densities (Figures 5F, S16, and S17). However, RGONS film shows much higher rate performance compared with Film I and Film II. As depicted in Figure 5F, when the current density increases from 1 to 50 A g−1, the capacitance retention of RGONS film remained 60.49%. Under the current density of 40 A g−1, the capacitance retentions are only 11.34 and 2.87% for Film I and Film II, respectively. The high rate performance of RGONS film can be ascribed to its unique morphology. In the nonwoven-fabrics-like RGONS film, there are plenty of pores, which can accelerate the diffusion of the electrolyte. For the layered-structure RGO films, the electrolyte diffusion is significantly slowed down by the large impermeable sheet barrier and small interlayer distance.42 Electrochemical impedance spectra of the three electrodes (Figure 5G) confirms this conclusion. The RGONS film has the shortest Warburg region, whereas Film II, which has the most compact structure, shows a largest equivalent resistance caused by slow electrolyte diffusion. Thus, without particular treatment,40,42 the compact RGO films show poor rate performance. The RGONS film has good flexibility as a solid supercapacitor. A symmetric flexible solid supercapacitor was fabricated with two RGONS films as the electrodes and poly(vinyl alcohol)-H2SO4 gel electrolyte (Figure 5H inset). The corresponding specific capacitances of different current density are calculated to be 120 F g−1 at 2 A g−1 (Figure 5H). The slightly lower rate performance of the solid device as compared with three-electrode results can be attributed to the slower diffusion of ions in solid state devices.45 The supercapacitor showed good mechanical flexibility in bending tests. As shown in Figure 5I, the CV curves of the device measured at different bending angles are almost the same. Also, the specific capacitance remains stable during 1000 cycles of bending/ releasing (Figure 5J), suggesting a high antifatigue performance of RGONS electrodes. We believe that the interconnected RGONS network can provide good mechanical strength and sufficient free space to buffer the strain.
4D,E). In these processes, nanomaterials (PANI nanofiber or Pt nanoparticle) first absorb onto GO or RGO sheets, yielding composite nanosheets. Then, these composite nanosheets behave as a whole in OSAL, whereas the nanomaterials do not show obvious influence on the process. For Group II materials, we chose polystyrene (PS) microspheres (Figure S13B), which bring a large number of negative charges on the surface and are expected to have strong electrostatic repulsion with negatively charged GO sheets.37 Actually, there is no precipitation when mixing a GO and PS microspheres dispersion even at relatively high concentration, indicating weak interaction between the two components. However, PS microspheres can also be encapsulated into the GO nanoscrolls, as shown in (Figure 4F,G). Some bulges are found on the nanoscrolls, showing the profile of the buried PS microspheres. The TEM image (Figure 4G) also shows PS microspheres between the GO layers in the nanoscroll. Considering that there is no self-assembly in the dispersion between GO and PS microspheres, we argue the PS microspheres are encapsulated into the nanoscrolls in the lyophilization. During lyophilization, PS microspheres are exposed from the ice. Noticing that GO sheets compose the major part of the surface, most of the PS microspheres will be loaded onto the GO sheets. These PS microspheres are eventually encapsulated into the nanoscrolls mechanically. Another negatively charged nanomaterial, V2O5·nH2O nanowires38(Figure S13C), was also successfully filled into GO nanoscrolls (Figure 4H,I). The results demonstrate that nanomaterials, whether they have a strong interaction with GO, can be encapsulated into the GONS with the OSAL method. This is attractive for the design and preparation of 2D-material-based composites, especially when the second component does not have enough affinity to combine with 2D material spontaneously. Therefore, OSAL is a powerful tool for developing new functional 1D materials. As long as proper 2D materials and other nanomaterials are chosen, they can be easily integrated into nanoscrolls with OSAL. In the last part, we will demonstrate one application of nanoscrolls as the electrode of a supercapacitor. RGO is a widely-used electrode material in supercapacitors,39 but the aggregation of RGO sheets usually results in a slow diffusion of electrolyte and consequent poor rate performance, especially in compact and flexible films. Here, we demonstrate that the nonwoven fabrics morphology of the nanoscrolls film provides a new solution to the low performance of RGO-based flexible electrodes. A RGO nanoscroll (RGONS) was prepared by reducing GONS with hydrazine hydrate in ethanol, and the SEM image shows that the scroll structure was preserved after reduction (Figure S14). The produced RGONS was compressed into a compact flexible thin film under 2 MPa (Figure 5A), and tested in a three-electrode or two-electrode solid device configuration. After compression, the film becomes dense but still the fibril structure can be identified (Figure 5B). We also prepared another two RGO films for comparison. Film I was prepared by compressing an RGO aerogel, which was obtained by chemically reducing GO aerogel with hydrazine hydrate in ethanol,40 and Film II was prepared by filtrating the RGO dispersion.41,42 Both Film I and Film II have a compact morphology and typical layered structure composed of stacking RGO sheets (Figure 5C,D). Figures S15 and 5E show the cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) curves of RGONS film. The quasi-rectangular shape of CV curves and constant slope in GCD curves indicate an ideal capacitance behavior. The specific capacitance is calculated to be
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CONCLUSIONS We have developed a simple and general method for preparing 2D material nanoscrolls. By adding a small volume of highboiling-point organic solvent to the aqueous dispersion of 2D materials, nanoscrolls can be easily prepared by lyophilization in high yield. The asymmetric surface tension on the two sides of the 2D material sheets caused by adsorbed organic solvent during lyophilization is the driving force of the nanoscroll formation. This process is independent on the chemical structure of the high-boiling-point organic solvent and 2D materials, and thus OSAL has a broad application range in producing various types of 2D material nanoscrolls. The application of the prepared RGO nanoscrolls as the flexible electrode for a supercapacitor was also demonstrated, and these nanoscrolls can form a nonwoven-fabrics-like film, which accelerates the electrolyte diffusion in the film. Furthermore, the OSAL method can also be used to prepared composite nanoscrolls, where different nanomaterials, regardless of their interaction with the 2D materials, are encapsulated into the nanoscrolls. Therefore, OSAL is a new powerful tool to fabricate composite 1D nanomaterials; it allows us to produce novel 1D materials by choosing and combining different nanomaterials and 2D materials. Considering its simplicity and wide 7425
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and all authors have given approval to the final version of the manuscript.
applicability, we believe OSAL will become a platform of designing and preparing new functional architectures based on 2D materials.
Notes
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The authors declare no competing financial interest.
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EXPERIMENTAL SECTION Preparation of Nanoscrolls with OSAL. Typically, the high-boiling-point organic solvent, including NMP, PC, DMSO, and glycerol, was added to GO in a volume ratio of 0.125−0.75% to 2 mg mL−1 GO. The mixture was frozen in liquid nitrogen and lyophilized for 24 h at a pressure of 20 Pa, and GO nanoscrolls were obtained. RGO nanoscrolls and MoS2 nanoscrolls were synthesized using the same method. For the preparation of RGO nanoscrolls, 0.05 vol % NMP was added into 4 mL of 0.4 mg mL−1 RGO dispersion; whereas for MoS2 nanoscrolls, 0.5 vol % NMP was added into 2 mL of 1 mg mL−1 MoS2 dispersion. The preparation of composite nanoscrolls is described in the Supporting Information Fabrication of RGO Nanoscroll Electrode. A piece of GO nanoscrolls was suspended in 50 mL of ethanol, and 200 μL of 95% hydrazine hydrate was added. The solution was heated at 65 °C for 3.5 h without agitation. The produced RGO nanoscrolls were collected carefully and vacuum dried under 45 °C. RGO nanoscrolls were pressed into a flexible film under a pressure of 2 MPa. Assembly of the All-Hydrogel-State Supercapacitor. Poly(vinyl alcohol)−H2SO4 (PVA−H2SO4) was used as the gel electrolyte and prepared by mixing 6 g of H2SO4 (2 M) and 6 g of PVA solution (0.2 g mL−1) in water with continuous stirring for 1 h at 90 °C. The RGONS film was cut into a rectangular strip with a dry weight of 12 mg and area of 1.5 × 3 cm2, and attached to a polyimide substrate coated with silver paste to form a flexible thin electrode (Figure 5A). The PVA−H2SO4 aqueous solution was then slowly poured into two separated electrodes and dried to evaporate excess water. Then, the two RGONS electrodes were pressed together under the pressure of 1 MPa, and the polymer gel electrolyte was combined into a separating thin layer.
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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21774104), Science Foundation of the Fujian Province, China (2018J06015), and Aeronautical Science Foundation of China (2016ZF68011). The authors thank Dr. Yuan Jiang for providing the V2O5·nH2O sample, and Dr. Naibo Lin for providing the PS microsphere sample.
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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/acsomega.9b00623.
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REFERENCES
Experimental method, SEM images of GNS, PANI nanofiber, PS microspheres, and RGONS, calculation of NMP vapor pressure, capacitive performance of compressed RGO hydrogel film (Film I) electrode, and RGO film (Film II) electrode (PDF) ESEM video showing the surface change during the lyophilization process (AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hua Bai: 0000-0001-8403-9217 Author Contributions
H.B. originally conceived and supervised the project. X.H. and Z.H. carried out the experiments, and they analyzed the data with the help of Q.L., A.Z., Y.M., J.W., and H.Q. H.B. and X.H. prepared the manuscript using the feedback from other authors, 7426
DOI: 10.1021/acsomega.9b00623 ACS Omega 2019, 4, 7420−7427
ACS Omega
Article
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DOI: 10.1021/acsomega.9b00623 ACS Omega 2019, 4, 7420−7427
