Graphene Oxide Glue-Electrode for Fabrication of Vertical, Elastic, Conductive Columns Liusi Yang, Mingchu Zou, Shiting Wu, Wenjing Xu, Huaisheng Wu, and Anyuan Cao* Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China S Supporting Information *
ABSTRACT: Graphene has a planar atomic structure with high flexibility and might be used as ultrathin conductive glues or adhesion layers in electronics and other applications. Here, we show that graphene oxide (GO) sheets condensed from solution can act as a pure, thin-layer, nonpenetrating glue for fabrication of vertical architectures anchored on rigid and flexible substrates. Carbon nanotube (CNT) sponges are used as a porous template to make polymer-reinforced composite columns, to achieve both high conductivity and elastic behavior. These vertical columns are fixed on a substrate by reduced GO sheets as an electrode and exhibit reversible resistance change under large-strain compression for many cycles. Similar to the CNT gecko feet, we disclose high adhesion forces at the CNT-GO and GO-SiO2 interfaces by mechanical tests and theoretical calculation. Three-dimensional CNT, graphene, and nanowire networks with our GO glue-electrodes have potential applications as energy storage electrodes, flexible sensors, functional composites, and vertical interconnects. KEYWORDS: graphene oxide, glue-electrode, carbon nanotube sponge, composite, elastic conductive column
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and ease of chemical functionalization through solution process.12,13 Applications of GO in this area are mainly limited in two directions. First, GO sheets (sometimes with CNTs) are dispersed into conducting polymers to make a gel-like glue, which is then applied as sticky interconnects in tandem solar cells or as host/binders for metal oxide nanoparticles in supercapacitor electrodes.14,15 Second, GO sheets with oxygencontaining functional groups promote both the nucleation of ZnO nanowires and adequate adhesion to the substrate, enabling one-step ZnO lithography.16 Similarly, positively charged reduced GO sheets act as an adhesion promoter for attracting Ag nanowires and forming highly stable transparent electrodes.17 In the biological field, GO behaves as an adhesion substrate and growth factor carrier for cells.18,19 To date, adhesion applications of GO are mainly focused on (1) fabrication of GO-polymer mixed glues and (2) adhesion layer for random guests such as dispersed nanowires. Here, we show that solution-condensed GO films can serve as a glue and electrode (after reduction) for making vertical 3D architectures such as CNT sponges and their composites, on either rigid or flexible substrates. We demonstrate elastic conductive vertical structures based on porous CNT sponges and reinforced polymeric composites, which can be compressed for 1000 cycles with reversible resistance change. Mechanical measure-
n graphene, carbon atoms are arranged into a single-layer two-dimensional (2D) honeycomb lattice, leading to high mechanical strength and also excellent structural flexibility.1,2 Its planar structure and flexibility allow close contact and conformal covering on smooth or rough surfaces, producing strong adhesion by van der Waals interaction between graphene and the substrate.3,4 Such van der Waals force-induced adhesion effect has been demonstrated in aligned carbon nanotube (CNT) arrays, a so-called artificial gecko foot, in which the structure of CNTs can be considered as cylinders of rolled up graphene.5−7 Thus, in addition to conventional applications, graphene may also serve as an ultrathin glue or adhesion layer, which simultaneously offers a highly conductive path from the underlying substrate to the adhered material. In fact, experimental measurements on single- to multilayer graphene synthesized by chemical vapor deposition (CVD) have revealed high adhesion energies ranging from 0.3 to 0.7 J/ m2 when configured as pressurized blisters or double cantilever beams on Si wafers or Cu foils.3,8−10 Theoretical study also shows that factors such as the layer number (or membrane thickness) and external temperature could influence the interfacial adhesion energy of graphene.11 The above mechanical measurements and theoretical calculations have been focused on CVD-grown single- to fewlayer graphene samples. At the same time, there are already a number of adhesion-related applications demonstrated in graphene oxide (GO) produced by liquid exfoliation of expanded graphite, partially due to its scalable production © 2017 American Chemical Society
Received: December 12, 2016 Accepted: February 17, 2017 Published: February 17, 2017 2944
DOI: 10.1021/acsnano.6b08323 ACS Nano 2017, 11, 2944−2951
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Figure 1. GO glue for CNT sponge adhesion on rigid and flexible substrates. SEM images showing (a) a cracked GO thin film after dropping GO solution onto a CNT sponge surface and then drying; (b) a cracked island consisting of a top GO layer and underlying CNTs; and (c) CNTs attached to the raised GO film edge. (d) Schematic illustration of the CNT column adhesion process and the adhesion interface of CNT-GO-Si. (e) Photos of various adhesion configurations including sponge-GO-Si, sponge-GO-sponge, composite-GO-composite, and sponge-GO-composite pairs. (f) Photos of a GO-glued thin CNT sponge on a flexible Cu foil, which can be rolled up or folded without detaching the sponge.
materials. The nonpenetrating property comes from two factors: (1) the sizes of GO sheets (several μm) are generally larger than the pore size of the CNT sponge (sub-μm), and (2) the planar GO sheets tend to stack onto each other, thus precipitate as a compact skin covering the sponge surface. In comparison, conventional fluid-type glues (e.g., epoxy) may wet and infiltrate the pores due to capillary action and consequently alter the microstructure.23 In addition, GO sheets can be thermally or chemically reduced to render electrical conductivity without the need of adding conductive fillers. We developed a simple solution-evaporation and GOprecipitation process to glue CNT sponges on selected substrates (e.g., silicon wafer), as illustrated in Figure 1d (see Experimental Methods for details). In brief, a solution of GO sheets (concentration: 0.5−2 mg/mL) was dropped onto a silicon wafer, and a cm-size block of CNT sponge was placed into the GO pool with its bottom part just immersed. After complete evaporation, the sponge was fixed onto the substrate through a precipitated dry GO film. In other words, the GO sheets acted as an intermediate layer with its bottom side adhered on the Si wafer and its top side gluing the CNT sponge. Using this method, we have fabricated versatile configurations including sponge-GO-sponge, composite-GOcomposite (sponge filled by paraffin wax), and sponge-GO-
ments and calculation reveal high adhesion forces at the CNTGO and GO-SiO2 interfaces, promising applications in energy and electronics areas.
RESULTS AND DISCUSSION Our idea of using GO to glue a CNT sponge, a threedimensional (3D) multiwalled nanotube network reported previously,20 originates from an observation. When an aqueous GO solution (prepared by modified Hummer’s method)21,22 was dropped onto a bulk CNT sponge and dried naturally, the condensed GO sheets formed a thin layer on the sponge surface and broke into small islands holding the underlying CNTs, resulting in a cracked morphology as characterized by scanning electron microscopy (SEM) (Figure 1a,b). At the edge of one of the islands where the GO film was raised upward, CNTs pulled out from the sponge and attached to GO (Figure 1c). This phenomenon indicates that strong adhesion between the CNT sponge and GO sheets could be promoted under the appropriate conditions, for example, by creating a conformal GO coating on the flexible CNT network with close contact and π−π stacking effect. Furthermore, the deposited GO forms a compact skin covering on the surface without entering the internal area of the sponge. Thus, GO sheets have the potential to act as clean, nonpenetrating glue for porous 2945
DOI: 10.1021/acsnano.6b08323 ACS Nano 2017, 11, 2944−2951
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Figure 2. Characterization of GO-glued CNT column and adhesion interfaces. (a) SEM image showing a tiny CNT column attached to the Si wafer and surrounded by the trace of original GO pool. (b) SEM image of remaining CNTs stuck on the GO patch after stripping off the sponge. (c) Enlarged view showing that CNTs lie horizontally down to the GO surface. (d) TEM image showing that CNTs still attach to the GO sheets after ultrasonication. High-resolution XPS spectra of O 1s peak of (e) GO on the Si wafer and (f) rGO on the Si wafer. (g) Comparison of the C−O−Si peaks from the GO and rGO sample showing reduced peak area after thermal reduction.
composite pairs glued by a GO film at the interface (Figure 1e). In principle, hierarchical structures can be constructed by assembling multiple CNT sponges or composites in predefined configurations through GO-stitching. In addition to rigid Si wafers, this GO glue also applies to flexible substrates such as metal foils. A sponge membrane glued on to Cu foil can be bent to large angles without separation at the interface (Figure 1f). This is attributed to the strong adhesion offered by the GO glue and the structural flexibility of the CNT sponge. Even when the Cu foil was folded by an angle of 180°, the sponge still followed the deformation without detaching. Compared to literature work where GO was used as a sticky layer for adhesion of dispersed nanowires,17 here we directly glue bulk 3D CNT networks onto both rigid and flexible substrates. We have carried out SEM characterization on several column-shaped CNT sponges glued vertically on Si wafers (Supporting Information, Figure S1). A trace of the original GO pool surrounding the column can be seen, and the bottom part of the sponge sticks to the substrate surface without openings at the interface (Figure 2a). Seamless adhesion of the bulk CNT sponge to Si wafer is attributed to three factors related to the sponge structure and two interfaces (CNT-GO and GO-SiO2) formed during the gluing process. First, the 3D CNT networks self-assembled at high-temperature possess high intertube connections and structural integrity; once the bottom part is fixed, the entire sponge is secured as well. This allows direct adhesion of 3D porous architectures rather than a thin layer of individual nanostructures. Second, the structural flexibility of CNT sponges is very important in creating strong adhesion to the GO glue underneath. During evaporation of the
GO solution, the bottom surface of the sponge gradually sank onto the GO sheets, and the CNT tips could bend over toward the planar GO sheets. This is evidenced by many remaining CNTs stuck on the GO patch after stripping off the sponge, and we observe tube segments lying horizontally down to the GO surface (Figure 2b,c). This deformation process results in close contact and larger contact length along individual CNTs, which is critical in promoting π−π interaction at the CNT-GO interface.24,25 Transmission electron microscopy (TEM) image shows that even after ultrasonication, those CNTs still adhere well to the GO sheets (Figure 2d). Fourier-transformed infrared (FTIR) spectroscopy on the remaining CNT-GO after sponge removal shows the presence of functional groups belonging to GO sheets, and the slight shift of −OH peaks might indicate the conjugated π−π stacking effect between GO and adhered CNTs (Figure S2). Third, in addition to the CNTGO interface at the top side, it is also essential to create effective GO-SiO2 interface downside. Previous study shows that CVD-grown graphene layer deposited on SiO2 surface leads to high adhesion energy,3,8,9 whereas here the GO sheets with abundant functional groups also adhere strongly to the substrate. We have characterized this interface by X-ray photoelectron spectroscopy (XPS) high-resolution spectra of O 1s peak (Figure 2e). When original GO sheets were deposited on SiO2, the resulting GO-SiO2 interface showed a pronounced C−O−Si peak (at 531.9 eV), and after thermal annealing which converted the GO sheets to reduced GO (rGO), the intensity C−O−Si peak decreased significantly due to the removal of functional groups, indicating possible formation of chemical bonding between GO and SiO2 besides 2946
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Figure 3. Application as vertical, elastic, and conductive columns using a GO glue-electrode. (a) Schematic of the fabrication process including: (1) a sponge column was wired from the top surface by Ag paste and bottom side by rGO glue; (2) PDMS was dropped and infiltrated into the porous sponge segment; and (3) after curing, the composite column was fabricated in vertical configuration with reliable electrical contacts at both ends. Two photos at the bottom show the CNT and CNT/PDMS column samples. (b) Compressive stress−strain curves of pure PDMS, CNT, and CNT/PDMS columns at a strain of 10%. (c) Relative resistance change of the CNT column when it was compressed to strains of 10%, 20%, 30%, and 50%, respectively. (d) Relative resistance change of the CNT/PDMS column compressed to strains of 10% and 20%, respectively. (e) Piezoresistive curves normalized by the stress at 20% strain, (R − R0)/(R0·σ20%), of the CNT and CNT/PDMS columns during strain cycles, respectively. (f) Reversible resistance variation of the CNT/PDMS column over 1000 compression cycles. Inset shows resistance variation in the 100−110 cycles and 900−910 cycles. (g) Stress−strain curves and corresponding resistance variation of the CNT/PDMS column in the 1st and 1000th cycles.
the van der Waals force.16 A suitable GO concentration (0.5−2 mg/mL) is important as a too diluted solution cannot form a continuous adhesion layer, while higher concentration results in thick GO layers that easily detach from the substrate (Figure S3) due to reduced adhesion strength at the GO-substrate interface.11 Based on the GO-glued CNT sponges, we are able to create highly conductive, elastic composites that are fixed vertically on the substrate. Figure 3a illustrates a vertical configuration in which a sponge column is wired from the top surface (by Ag paste) and bottom side (GO glue reduced by thermal annealing). Then, diluted polydimethylsiloxane (PDMS) is infiltrated dropwise into the porous sponge segment to make a composite column inheriting the original sponge shape/size, with reliable electrical contacts prefabricated at both ends. SEM images show that CNTs with the weight ratio of 1.33 ± 0.21 wt % are distributed homogeneously in the PDMS matrix (Figure S4), meanwhile, Raman and XPS spectra reveal characteristic peaks from CNTs and PDMS (Figure S5). Here, reduced GO acts as a glue-electrode (dual function) to the sponge and its
composite. Mechanical and electrical tests reveal three distinct features of our GO-fixed CNT/PDMS column, including (1) it is a highly conductive reinforced column; (2) it has a rather stable behavior with small resistance variation under deformation; and (3) it is an elastic structure with full recovery and reversible resistance change for 1000 compression cycles. First, PDMS infiltration enhances the mechanical strength of the column, and the composite sponge also maintains high electrical conductivity. The resulting CNT/PDMS column appears to be more rigid, with a higher compressive stress (σ ≈ 106.3 kPa) than that of the CNT sponge (σ ≈ 9.6 kPa) and bare PDMS (σ ≈ 47.8 kPa) at the same strain (Figure 3b). Such mechanical reinforcement is due to the complete filling of PDMS among the porous sponge, which prevents collapsing of the 3D CNT network under compression. Current−voltage (I−V) curves show that both the CNT and CNT/PDMS columns (cm-sized) have a linear behavior with measured twoprobe resistances of 2−3 Ω (Figure S6). In comparison, a composite made by mixing CNT powders into matrix usually has a much higher resistance (2.7 MΩ).26 By infiltrating 2947
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Figure 4. Mechanical measurement of the adhesion forces at CNT-GO and GO-SiO2 interfaces. (a) Schematic of methods to detach the CNT column at the CNT-GO interface and the epoxy-infiltrated column at the GO-SiO2 interface. Typical force−displacement curves of pulling (b) a CNT column in normal direction, (c) a CNT/epoxy composite column in normal direction, or pushing (d) a CNT/epoxy composite column in shear direction. (e) Photos showing that the CNT/epoxy column glued to Si wafer can hang 2 kg weight, owing to strong adhesion at the GO-SiO2 interface. (f) SEM image of the CNT/epoxy column after detachment showing a smooth bottom surface covered by GO sheets.
site, polymer chains in the pores impede the CNT movement and inhibit the formation of electron tunneling contacts, resulting in smaller resistance variation. The electromechanical behavior of CNT/PDMS composite columns can be discussed by the lateral size change (or Poisson’s ratio) under compression. The Poisson ratio of composite columns is determined by the polymer matrix (∼0.50) due to the low concentration of CNTs,27 and materials with a larger Poisson’s ratio usually possess a lower piezoresistivity.28 Moreover, we define a stress-normalized piezoresistive factor as (R − R0)/ (R0·σ) to evaluate the stability of resistance under an applied stress. At the same strain (ε = 20%), the CNT/PDMS column has a piezoresistive factor of 1.39 × 10−7 Pa−1, which is
