Graphene Microtubings: Controlled Fabrication and Site-Specific

Beyond the applications demonstrated in this study, the well-defined μGT systems can also play essential role in other important fields such as fluid...
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Graphene Microtubings: Controlled Fabrication and Site-Specific Functionalization Chuangang Hu,† Yang Zhao,† Huhu Cheng,† Yanhong Wang,† Zelin Dong,† Changcheng Jiang,† Xiangquan Zhai,† Lan Jiang,‡ and Liangti Qu*,† †

Key Laboratory of Cluster Science, Ministry of Education, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China ‡ Laser Micro-/Nano-Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China S Supporting Information *

ABSTRACT: Manipulating graphene through engineering for macroscopic assemblies of practical importance is a big challenge. We develop a dually geometric confinement approach for the scalable preparation of meter-long graphene microtubings (μGTs) with a tunable diameter. They have strength comparable to graphene fiber and can be shaped to hierarchical multichannel μGT systems in a straightforward way. Of particular importance, μGTs can be selectively functionalized in a site-specific outer-wall, inner-wall, outer/inner-wall, and within-wall fashion, which endows the μGTs with unique properties for desirable applications. Apart from the magnetically and photoelectronically responsive μGTs developed here, a self-powered micromotor made of Pt innerwall modified μGT showing agile motion in aqueous medium has been also achieved. Beyond the applications demonstrated in this study, the well-defined μGT systems can also play essential role in other important fields such as fluidics, catalysis, purification, separation, and sensing. KEYWORDS: Graphene, macroscopic assembly, microtubing, site-specific functionalization, micromotor

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cost, aqueous graphite oxide (GO) suspensions.10 By using a glass pipeline, 1 mL of GO suspension (8 mg/mL) will generate more than 6 m long GF (ca. 35 μm in diameter) with a density of less than 1/7 conventional carbon fibers. The strong and flexible GFs are weavable and shapable and can be woven into engineered structures for multifunctional applications. Continuing our great effort in manipulating graphene through engineering for macroscopic assemblies of practical importance, we herein report our recent achievement in the fabrication of graphene microtubings (μGTs), a newly assembled architecture of graphene, which could be essential for fluidics, catalysis, purification, separation, sensing, and environmental protection.22−25 A dually geometric confinement approach has been developed for preparing meter-long μGTs with a tunable diameter of 40−150 μm. They have strength comparable to that of graphene fiber10 and CNT yarns21 and can be shaped to hierarchical multichannel μGTs in a facile and straightforward way. More importantly, selectively site-specific outer-wall, inner-wall, outer/inner-wall, and within-wall functionalization of μGTs can be done in a well-controllable

raphene, a two-dimensional (2D) monolayer of carbon atoms, is recognized by its intriguing properties such as giant electron mobility,1−3 high thermal conductivity, 4 extraordinary elasticity, and stiffness.5 Nowadays, tremendous efforts are being made for the integration of the remarkable properties of individual graphenes into advanced, macroscopic, functional structures for practical applications.6−10 However, it is still a big challenge to tailor and assemble graphene into functional macrostructures with a well-defined configuration due to the lack of scalable assembly methods for graphene manufacture. In virtue of the advantages of preparation and processing, as well as the scalable production and low cost, chemically derived graphene sheets have been adopted preferentially for the fabrication of macroscopic architectures.11−19 So far, graphenes have been conformably assembled into 2D macroscopic configurations such as papers,11−13 films,14,15 and even 3D frameworks.16−19 However, it is only a recent success to directly assemble 2D microcosmic graphene sheets into macroscopic fibers9,10 due to the irregular size, shape, and the movable layer-by-layer stacking of graphenes. Graphene fibers (GFs) are extremely important for many practical applications in such areas that conventional carbon fibers and carbon nanotube (CNT) yarns20,21 have played a role. In this regard, we have fabricated macroscopic neat graphene fibers with strength comparable to CNT yarns by a facile dimensionally confined hydrothermal route from low© 2012 American Chemical Society

Received: August 31, 2012 Revised: September 29, 2012 Published: October 10, 2012 5879

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fashion, which endows the μGTs with unique properties for desirable applications such as stimulus-responsive devices and self-powered micromotors demonstrated in this study. Our strategy for controllable fabrication of μGTs is based on the dimensionally confined hydrothermal method we developed previously.10 For the formation of μGT, we place one removable metal wire (e.g., Cu) in the glass pipeline (Figure 1a

wire, we can conveniently control the diameters of the resultant μGTs by choosing diameter-different Cu wires, as exemplified in Figure 2g,h showing the produced μGTs with diameters of ca. 150 and 40 μm, respectively. In this preliminary study, the μGT diameter is mainly defined in tens of micrometers due to the easy maneuverability. Not limited to glass pipeline and Cu wire used in this study, there are various types of pipelines and wires available commercially, which greatly facilitates the largescale production of μGTs with tunable length and diameter beyond those mentioned above. The hydrothermally converted μGT has a measured tensile strength of up to 180 MPa (Figure 2i), which is similar to that of graphene fiber,10 and close to that of the singles yarns of multiwalled CNTs,21 but exceeds those of single-walled CNT fibers fabricated by wet spinning process.26−28 The μGT has a typical elongation at break of about 4% presumably originating from the possible displacement of the graphene sheets within the walls,10 which is much larger than that of graphite fibers (about 1%). Further, a four-probe electrical conductivity of ca. 10 S/cm is obtained for μGTs, which is comparable to those of graphene fiber10 and wet-spun single-walled CNT fibers.27,28 The as-prepared μGTs are flexible and mechanically stable, which can be shaped to specific geometry with controlled morphology on demand. Before drying, the wet μGT intercalated with Cu wire (Figure 1c) can be manipulated conveniently into various configurations, which will maintain this predesigned structure once being dried. As shown in Figure 3a,b, the mechanically stable μGT spring with stretchable and compressible character has been achieved by enwinding the wet μGT embedded with Cu wire around the a cylindrical bar, followed by drying naturally and removing Cu wire. As a twist of two Cu wires (Figure 3c, inset), instead of the single wire (Figure 1a), is used as the supporting core, a helical μGT can be obtained (Figure 3c). Interestingly, two-channel, threechannel, and four-channel μGTs can be readily fabricated by utilizing two-, three-, and four-ply Cu wires without twist (Figure 3d−f). Each of the independently addressable channels is well-separated by the layers of stacked graphenes, which is essential for the development of a multifunctional, integrative microchannel system promising for a wide range of applications such as vessels for macro/nanofluidic devices, multicomponent drug delivery, and high efficient catalyst supports. To achieve the important applications mentioned above, a crucial step is to engineer the surfaces of tubular structures with site-specific functionalization. For this purpose, we have rationally devised the strategies for selective outer-wall (Figure 4a), inner-wall (Figure 4b), outer/inner-wall (Figure 4c), and within-wall (Figure 4d) modifications in a well-controllable fashion. As shown in Figure 4e−h, when taking the Cu wireembedded μGTs (Figure 1d) as the initial reaction platform, we will achieve the functionalization of the outer surfaces only for the μGT due to the effective inner-wall protection by encapsulated Cu wire (Figure S3 of the Supporting Information). To demonstrate this idea, Cu substrate-enhanced electroless deposition (SEED) we developed previously29,30 is advantageously exploited to deposit metal (e.g., Pt) nanoparticles for visualization. Pt ions will be spontaneously reduced to Pt nanoparticles on the μGT once being in contact with Cu wire. As can be seen in Figure 4e−h, Pt nanoparticles with an identifiable size of ca. 200−300 nm are exclusively attached onto the outer-wall surface of the μGT after immersing the Cu wire supported μGT in aqueous K2PtCl4 for 2 min (Figure 4g),

Figure 1. Scheme of the fabrication process of μGT. (a) GO suspension was filled in the glass pipeline, in which a Cu wire has been intercalated in advance. (b) The glass pipeline was heated at 230 °C and hydrothermally reduced GO congregates around Cu wire. The two ends of glass pipeline were sealed prior to the hydrothermal treatment. (c) The Cu wire enwrapped with hydrothermally converted graphene was taken out from glass pipeline. (d) Densely packed graphene layer surrounding Cu wire was formed after drying naturally. (e) The μGT was obtained after removing the Cu wire in aqueous 2.5 M FeCl3 solution containing 0.5 M HCl.

and Figure S1a of the Supporting Information). After sealing the GO suspension within the glass pipeline (Figure S1b), it is thermally treated at 230 °C for 2.5 h. The Cu wire will induce the aggregation of hydrothermally reduced GO (also called graphene) along it and act as the support during the drying process (Figure 1b−d and Figure S1c,d). Finally, meters of μGT with a diameter depending on the supporting Cu wire are collected after etching Cu wire in aqueous FeCl3/HCl solution (Figure 1e and Figure S1d). This strategy exploits a dually geometric confinement to fabricate the μGT. While the glass pipeline defines a fiber shape of the resultant sample, the embedded Cu wire provides a core for the subsequent formation of tubular structure. The initially hydrothermally reduced GO on Cu wire has an incompact but highly 3D cross-linking porous structure of random graphene sheets (Figures 1c and 2a,b), characteristic of the hydrothermally produced graphene samples.16 Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman analysis confirm the produced sample is composed of graphene sheets (Figure S2 of the Supporting Information). After drying naturally, the capillary force induced during the loss of water causes the densely packing of porous graphene sheets along the Cu wire (Figures 1d and 2c), and the initial large diameter of ca. 150 μm (Figure 2a) shrinks to ca. 100 μm (Figure 2c). Once removing the Cu wire in aqueous FeCl3/HCl solution, a freestanding μGT replicating the core shape is generated as shown in Figure 2d. High magnification SEM images reveal that the wall of μGT is as thin as less than 1 μm (Figure 2e) and consists of closely packed graphene sheets (Figure 2f). Since the μGT diameter is mainly dependent on the supporting Cu 5880

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Figure 2. Morphologic and structural feature of μGT. (a and b) SEM images of hydrothermally converted graphene on a Cu wire with a diameter of ca. 100 μm, which was freeze-dried prior to the naturally dry process. (c) SEM image of graphene on the Cu wire after naturally drying. (d) SEM image of a μGT after removing Cu wire. (e, f) SEM images of the enlarged views of the marked areas in d (e, left; f, right). (g, h) μGTs generated by using Cu wires with different diameters (g, 150 μm; h, 40 μm). (i) A typical stress−strain curve of as-prepared μGTs. Scale bars: a−c,e, 10 μm; d,g,h, 100 μm; f, 1 μm.

Figure 3. Morphologic control of μGTs. (a, b) Photos of a μGT spring lying and standing on the table, respectively. (c) SEM image of a helical μGT made by using a twist of two Cu wires of 100 μm in diamater (inset). (d−f) SEM images of the multichannel μGTs with a channel number of 2−4 (the used Cu wire is 40 μm in diameter), respectively. Scale bars: c, 100 μm; d−f, 10 μm.

while leaving the inner-wall surface of μGT untouched (Figure 4h). The deposited Pt nanoparticles are characterized in detail in Figure S4, and other metals (e.g., Pd) are also applicable for this selective functionalization (Figure S5). On the contrary, the inner-wall surface of μGT can also be solely modified by using a pretreated Cu wire. As illustrated in Figure S6a−f of the Supporting Information, Pt nanoparticles are preformed on the surface of the Cu wire (Figure S7), which subsequently undergoes the procedure for preparation of μGT

(Figure 1). After removing the Cu wire by FeCl3 etching, the Pt nanoparticles will decorate on the inner-wall surface of μGT. Figures 4i−l clearly display the selectively attachment of Pt nanoparticles on the inner-wall surface of μGT. The density of Pt nanoparticles along the surface of Cu wire can be controlled by reaction time and the concentration of Pt precursor, which will thus allow us to tune the Pt amount in μGT. Generally, the physically decorated Pt nanoparticles on the inner-wall surface of μGT have no obvious influence on the μGT quality. 5881

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Figure 4. Site-specific modification of μGTs. (a−d) Schematic diagrams of the outer-wall, inner-wall, out/inner-wall, and within-wall modified μGTs. (e and f) SEM images of a μGT outer-wall modified with Pt nanoparticles. (g and h) High magnification SEM images of the marked squares in f, respectively. (i and j) SEM images of a μGT inner-wall modified with Pt nanoparticles. The tube was deliberately mangled to expose the inner wall. (k and l) High-magnification SEM images of the marked squares in j, respectively. (m) SEM image of a μGT inner-wall modified with Pd nanoparticles and outer-wall modified with Pt nanoparticles. (n and o) SEM images corresponding to the marked squares in m, respectively. (p) Energy-dispersive spectroscopy (EDS) of the μGT inner-wall modified with Pd nanoparticles and outer-wall modified with Pt nanoparticles. (q−t) SEM image and EDS mappings of Pt, Pd elements, and the overlap of EDS mappings. (u) SEM image of a μGT within-wall modified with TiO2 nanoparticles. (v−x) SEM images of the marked squares in u. Scale bars: e,i, 100 μm; f,j,m,u, 10 μm; g,h, 1 μm; k,l,n,o,v−x, 100 nm.

Apart from the independent functionalization on either outer wall or inner wall demonstrated above, this versatile method developed here also allows us to simultaneously modify the inner-wall and outer-wall surfaces of μGT symmetrically or asymmetrically. As schematically illustrated in Figures S6d,g−i of the Supporting Information, the μGT embedded with Pd nanoparticles-decorated Cu wire is used as the initial reactant (Figure S6d), which can be asymmetrically functionalized with Pt nanoparticles via the forementioned outer-wall modification process (Figures S3 and S6g). Finally, a μGT with its inner-wall surface attached with Pd nanoparticles and its outer-wall surface deposited with Pt nanoparticles is obtained (Figure 4c and Figure S6h,i). Figure 4m−o shows the distinct feature of the asymmetrically modified μGT, in which less than 10 nm Pd nanodots are located on the inner-wall surface while the relatively large Pt particles (ca. 200−300 nm) staying on the outer-wall surface. The EDS result confirms the coexistence of Pd and Pt elements within this sample (Figure 4p), and the corresponding EDS mapping verifies the continuous distribution of Pd and Pt along the whole μGT (Figure 4q−s). The overlap mapping of Pd and Pt elements in Figure 4t shows that the region of Pd is fully covered with Pt shadow, which,

consistent with the SEM observation (Figures 4m−o), evidence the asymmetrical surface modification of the μGT once again. Further, by simply introducing functional components into aqueous GO suspension prior to the hydrothermal treatment, a within-wall modified μGT can be obtained (Figure 4d and Figure S8 of the Supporting Information). As an example, TiO2 nanoparticles (ca. 20 nm in diameter) were mixed into the GO suspension with ultrasonication, which was then filled in the glass pipeline, in which a Cu wire had already been intercalated (Figure 1 and Figure S8). After hydrothermal reduction and removal of Cu wire, a TiO2 within-wall modified μGT was collected (Figure 4u). During the dry process (Figure S8), the TiO2 particles initially embedded in the imcompact graphene framework (Figure 2a,b) will be fully covered with compact graphene layers. As can be seen, the deliberately surface-layer peeled area on the wall of μGT clearly shows the intercalated TiO2 nanoparticles (Figure 4v), while the outer-wall and innerwall surfaces are free of particles (Figure 4w,x), which, in combination with the EDS analysis (Figure S9), evidently proves the successful within-wall functionalization of μGT. The unique site-specific functionalization is quite versatile, and the integration of functional components into the μGT is 5882

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Figure 5. μGT micromotor. (a) Scheme of a moving micromotor. A one-end sealed μGT inner-wall modified with Pt nanoparticles (μGT@Pt) cruises in aqueous H2O2 solution. (b−h) Snapshots of the track for a moving μGT@Pt micromotor in 20 wt % H2O2 solution.

moters, the performance can be further improved by optimizations such as structure design, control of Pt content and distribution, and H2O2 concentration. In conclusion, we have developed a new scalable approach with dual geometric confinement during hydrothermal treatment to prepare meter-long μGTs with a tunable diameter. The uniqueness of μGTs is directly derived from that of constituent graphenes such as lightweight, high strength, conductivity, and flexibility. As revealed, the as-prepared μGTs are mechanically stable with a strength comparable to that of graphene fiber and CNT yarns and can be shaped to hierarchical multichannel fashion essential for multifunctional, integrative microchannel system. The availability of multichannel μGTs and site-specific outer-wall, inner-wall, outer/inner-wall, and within-wall functionalization of μGTs opens the immense potentials for applications in fluidics, catalysis, purification, separation, and sensing. The unique μGT architectures are promising for development of various devices beyond the catalytic ethanol oxidation, magnetical and photoelectronical sensing, and selfpowered micromotors demonstrated in this study.

important for various applications. Apart from the potential of Pt outer-wall modified μGTs for direct electrocatalytic oxidation of ethanol (Figure S10 of the Supporting Information), magnetically and photoelectronically responsive μGTs, as examples, can also be fabricated by introduction of Fe3O4 and TiO2 nanoparticles into the μGTs by simiply premixing them with GO suspension and then following the μGT preparation process (Figure S8). The Fe3O4-containing μGT possesses a sensitive magnetic response (Figure S11), while TiO2-immobilized μGT exhibits a fast photocurrent pulse upon exposure to light with good repeatability (Figure S12). To further demonstrate its interesting applications, the sitespecifically functionalized μGT was favorably utilized as micromotor, a self-powered system of great interest with potentials as micromachines and for the transport and delivery of cargo.31−33 As illustrated in Figure 5a, the inner-wall modified μGT with Pt nanoparticles is ideal as a transporter. Just like the tubular microjet using a rolled-up Ti/Fe/Au/Ag multilayer nanomembrane,34 in which the O2 bubbles from H2O2 decomposition were thrust out at one opening end, causing a directional and fast movement, the Pt nanoparticles are the catalysts for the decomposition of H2O2 to produce O2. Once sealed in one end of the μGT, the ejected O2 bubbles from the other side will drive the μGT moving in the contrary direction. A clear observation of the gas release is presented in the Supporting Information (Movie S1). The fast motion of a μGT micromotor (6 mm in length and 100 μm in diameter) in 20 wt % H2O2 aqueous solution is recorded in Movie S2, and Figure 5b−h shows the snapshots of the moving contrail. As can be seen, large numbers of gas bubbles are jetted out from the μGT tail, which push the μGT moving a distance of ca. 24 cm within 6 s. Although, in this preliminary study, this proof-ofconcept μGT motor has demonstrated the potential of sitespecifically modified μGT for advanced self-powered micro-

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ASSOCIATED CONTENT

S Supporting Information *

More experimental details, characterization and discussion (Figures S1−S12). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5883

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ACKNOWLEDGMENTS We thank the National Basic Research Program of China (2011CB013000) and NSFC (21004006, 21174019, 51161120361) for financial support.

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