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Carbon 171 (2021) 417e425

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Carbon

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Research Article

Robust adhesion between various surfaces enabled by lamellarstacking of graphene oxide nanosheets

Chunxiao Zhang a, b, 1, Yang Liu c, 1, Dan Chang a, Zheng Li a, *, Yingjun Liu a, Zhen Xu a,Chao Gao a

a MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Key Laboratory of Adsorptionand Separation Materials & Technologies of Zhejiang Province, Zhejiang University, 38 Zheda Road, Hangzhou, 310027, PR Chinab College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, PR Chinac Guangdong Provincial Key Laboratory of Industrial Surfactant, Guangdong Research Institute of Petrochemical and Fine Chemical Engineering, 318 ChepixiRoad, Guangzhou, 510665, PR China

a r t i c l e i n f o

Article history:Received 30 June 2020Received in revised form30 August 2020Accepted 15 September 2020Available online 18 September 2020

Keywords:Graphene oxide adhesiveReduced graphene oxideShear adhesion strengthInterlayer cohesion

* Corresponding author.E-mail address: lizheng_zju@zju.edu.cn (Z. Li).

1 These authors contributed equally to this work.

https://doi.org/10.1016/j.carbon.2020.09.0460008-6223/© 2020 Elsevier Ltd. All rights reserved.

a b s t r a c t

Graphene oxide (GO) has long been recognized as a mass-produced and easily processable precursor foracquiring graphene-like materials. However, as amphiphilic two-dimensional nanosheets, its potentialadhesivity for interfacial bonding has yet to be explored. Here, we show that evaporation-inducedlamellar GO assemblies serve as pure and robust adhesion layers for various types of surfaces. As anallotrope of carbon with less oxidative functional groups, reduced GO (rGO) demonstrates great potentialin adhering carbon materials with improved shear adhesion strength (46.5 N/cm2), heat-tolerance(1000 �C), and chemical-resistance. Under harsh conditions, it outperforms commercial epoxy adhe-sives in terms of conductivity and stability. The exploration of the adhesivity of GO-based materials shedlight on new adhesives and opens up more possibilities in their further applications.

© 2020 Elsevier Ltd. All rights reserved.

1. Introduction

Adhesion is an indispensable part of our daily life, happeningbetween all kinds of substrates. A reliable adhesion is generallyestablished utilizing adhesives through the creation of firmbonding between surfaces. Adhesive materials, either natural orsynthetic, are normally organic thanks to their molecular flexibilityand strong intermolecular interactions [1e4]. Inorganic carbonnanoparticles such as carbon nanotubes (CNTs), graphene, etc., arecommonly utilized as functional fillers in the polymer matrix toenhance their mechanical, thermal and conductive performances[5e8]. In the meantime, great effort has been exerted to preventingthe aggregation of these nano fillers to improve the uniformity inthe adhesives. Considering the naturally occurring interparticleadhesion among these nanomaterials, we strongly believe that thepotential of nanocarbon adhesives is underestimated and has notbeen fully explored.

A few studies have been reported in this area. One of the

interesting adhesion technologies relating to carbon materials isCNT dry adhesives. By mimicking the adhesive foot hairs of geckos,these vertically aligned CNT arrays adhere strongly to solid surfacesthrough van der Waals forces [9,10]. Graphene with an ultrathinand planar morphology achieves high interfacial adhesion due toits conformal contact with the underlying substrates [11e14]. Inaddition, similar adhesive features are also found in the mostpopular oxidative derivative of graphene, known as graphene oxide(GO), because of its comparable thinness and specific surface area[15,16]. For practical applications, GO-based materials have attrac-ted huge interest due to their scalability and adjustable surfacechemistry. GO sheets worked both as an ideal glue, for instance, tohold Ag nanowire electrodes in place [16], or to place vertical CNTsponges on the substrates [17], and as a good self-adhesive [18e21].Although these pioneering works are inspiring, minimal attentionhas been paid on the inherent adhesivity of GO for bondingdifferent surfaces. A deeper understanding of its interfacial adhe-sion behaviour is still lacking.

Referring to polymeric colloids, adhesion could be realized byforming ordered structures between solid surfaces [22,23]. GOnanosheets, as 2D colloids, should exhibit a similar function[24e26]. To be specific, the chemical structure of GO contains a

C. Zhang, Y. Liu, D. Chang et al. Carbon 171 (2021) 417e425

planar carbon skeleton with abundant oxidative functional groups.It exhibits an amphiphilic feature, where the oxygen-containingfunctional groups render its hydrophilicity and the graphitic do-mains in the basal plane contribute to its hydrophobicity [26,27].The distribution of the hydrophilic functional groups on the basalplane determines the hydrophobicity of GO sheets, which can becontrolled by altering the content of functional groups [28,29].These unique structures make GO a potential candidate for adhe-sion between a wide range of substrates.

In this paper, we explore the intrinsic adhesive behaviour of GOnanosheets by assembling them into ordered lamellar structureswithin a confined space at interfaces between materials. These GOsheets were self-assembled by water evaporation and achieved arobust bonding between various surfaces. The interlayer interac-tion and affinity between GO and the adherents are the key factorsthat dominate the resulted adhesion strength. GO-based glue isfound to be especially suitable for carbon materials from bothexperimental study and density functional theory (DFT) calcula-tions. After chemical reduction, the reduced GO (rGO) adhesionlayer becomes conductive and more stable, which is highly valuedin specific application scenarios requiring conductivity, heat-tolerance and chemical-resistance. The ability of GO-based mate-rials to be universal, green, and easy to use adhesives has neverbeen realized before. This study provides insight into graphene-based adhesives and encourages the exploration of inorganicmulti-functional glues for future applications.

2. Experimental

2.1. Materials

All the adherends are commercially available. The aqueous GOsolution (~12 mg/mL) made by a modified Hummers’ method waspurchased from Hangzhou Gaoxi Technology Co., Ltd (www.gaoxitech.com). The pristine GO solution was centrifuged at aspeed of 14,000 rpm for several hours to reach a concentration ofaround 18 mg/mL. Then the concentrated precursor solution wasdiluted to certain concentrations using water for further investi-gation. In most of our experiments, GO sheets with an averagelateral size of 8 mm were used.

2.2. Adhesion operation using GO glue

For being used as GO glue, the GO solution was first dropped onto the substrate and painted carefully to make a uniform layer inthe adhesion area. Another piece of the substrate was attached andpressed until the adhesion layer was dried. A post thermal treat-ment at 40e50 �C was also needed to ensure the complete drying.Before adhering, the substrates should be thoroughly washed withwater and ethanol, in order to eliminate contamination on thesurfaces.

2.3. Reduction of GO adhesion layer between graphite papers

Reduction of GO adhesive between graphite paper strips wasconducted by chemical reduction using hydrazine. Briefly, thebonded graphite paper strips with GO glue were put in a sealed jarwith one drop of N2H4 and heated at 90 �C for 5 h. Thermalannealing of the GO glued samples was performed by heating at1000 �C for 1 h under N2 atmosphere.

2.4. Simulations

The computational simulations were performed by the DMol3package of Accelrys Materials Studio based on density functional

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theory. The exchange-correlation interaction between electronswas calculated by the generalized gradient approximation (GGA)with Perdew-Burke-Ernzerhof (PBE) functional [30]. The van derWaals (vdW) interaction was calculated by dispersion-correctedDFT within Tkatchenko-Scheffler scheme [31]. The pseudo poten-tials were constructed using DFT semi-core pseudo potentials(DSPP) treatment with relativistic effect to describe the interactionof valence electrons with cores [32]. The double numerical pluspolarization (DNP) was selected to be the basic set. The optimizedgeometry was obtained with the convergence tolerance of energyof 1.0 � 10�5 Ha, a maximum force of 2.0 � 10�3 Ha/Å, anddisplacement of 5.0� 10�3 Å. A smearing value of 5.0� 10�3 Ha andDIIS of 6 were applied to accelerate the convergence of self-consistent field (SCF) with the energy tolerance of 1.0 � 10�6 Ha.

2.5. Characterization

Fourier transform infrared spectroscopy (FTIR) was conductedusing a Bruker Vector 22 spectrometer. UVevisible spectroscopywas obtained with a Varian Cary 300 Bio UVevisible spectropho-tometer. Raman spectra were acquired using a Renishaw inVia-Reflex Raman microscopy at an excitation wavelength of 633 nm.X-ray diffraction (XRD) measurements were taken on a PhilipsX’Pert PRO diffractometer using Cu Ka radiation (l ¼ 1.5418 Å). Theinterlayer d-spacing (d) was calculated according to Bragg’s law:nl ¼ 2d sin q, where q is the scattering angle and l is the wave-length of the incident wave. Scanning electron microscopy (SEM)images were taken on a Hitachi S4800 field-emission SEM system.X-ray photoelectron spectroscopy (XPS) surveywas performed on aThermo Fisher Scientific ESCALAB 250Xi system using a mono-chromated Al Ka excitation source (1486.6 eV). The diameter of theanalyzed area was 650 mm. The charge compensation system usinga dual beam (ion/electron) flood source was employed duringanalysis to prevent localized charge build-up. Spectra were refer-enced to the C 1s peak attributed to sp2 type graphitic carbon at abinding energy of 284.2 eV.

2.6. Measurement of adhesion strength

The shear adhesion strength of GO-based glue for various sub-strates was obtained from lap-shear measurements. Specifically,each glued sample was dried under pressure using a bench-toppress for better adhesion and mounted on a universal testing ma-chine (RGWT-4000-20, REGER). Load and displacement wererecorded in tensile mode with a loading rate at 0.5 mm/min and agauge length of 90 mm. Testing samples were cut into strips with asize of 10 mm � 70 mm and the adhesion area is 10 mm � 15 mm.For peeling tests, the craft paper strips (10 mm � 150 mm) wereadhered at one end having a bonded length of 100 mm, and 50 mmlong unbonded arms. The tests were conducted at a displacementof 5 mm/min while the bonded portion of the specimen remainsperpendicular to the applied load.

Three-point-bending tests were also performed on the universaltesting machine. The tests were carried out at a loading rate of1.0 mm/min with a support span of 60 mm. Chemical stability ofadhesionwas evaluated after soaking glued graphite paper samplesin either 5 M H2SO4 or 5 M NaOH for 14 days. The shear adhesionstrength was acquired each day. The samples were thoroughlywashed with clean water and dried before testing.

3. Results and discussion

3.1. Adhesion strength of GO glue determined by intrinsic characters

To evaluate the adhesion strength of stacked GO sheets, an

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aqueous solution of GO was taken as the glue between varioussurfaces (detailed characterization on raw GO materials is includedin Supporting Information, Fig. S1). The measurement of lap-shearadhesion using GO adhesive is illustrated in Fig. 1a. Since viscoussolution with high GO content is easier to spread and fill theadhesion area, concentrated GO slurry was dropped on to thesurface of a certain substrate (e.g. craft paper, glass slices et al.),then cover another strip on the coated area and moderate pressure(~500 kg) was applied. After the water solvent was fully removed,the samples were bonded via stacking of GO sheets. The self-assembly upon drying at the interfaces is a facile approach toimplement GO adhesivity. The densely packed GO sheets in theconfined area could help to bridge the two surfaces in contact,through the attraction between GO-substrate (adhesion effect) andGO-GO interlayer interactions (cohesion effect). As depicted inFig. 1b, the self-cohesion of GO sheets and their interaction withsurfaces determine the final adhesion strength. The softness ofultrathin 2D GO sheets can make a perfect conformal contact onvarious surfaces, either smooth or rough, establishing the adhesionbetween GO sheets and solid surfaces. During drying, the confinedevaporation of water generates capillary attraction not only be-tween GO and the substrates but also between GO sheets. Andfinally, the resulted interparticle interaction leads to the closestacking of GO sheets, facilitating a strong cohesion effect in termsof hydrogen-bonding, van der Waals attraction, as well as p-pinteraction.

Craft paper strips of 0.55 mm thickness were employed asadherends in the first part to investigate the adhesivity of GO. Thegood mechanical strength, excellent affinity with aqueous GO so-lution, and high surface roughness of such hard paper substratesprovide reliable adhesion force with GO sheets during mechanicaltests. Therefore, the variation on the intrinsic characters of GO so-lutions mainly affects the GO-GO interlayer cohesion. As shown in

Fig. 1. Schematic illustration of (a) the experimental setup for lap-shear testing to measurebetween GO sheets and substrates. Relationship between adhesion strength and (c) GO concof GO sheets. The inset in (e) shows the colour change of GO solutions (0.05 mg/mL for clarityand 18 mg/mL in (e). (A colour version of this figure can be viewed online.)

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Fig. 1c, the lap-shear adhesion strength becomes stronger whileincreasing GO concentration as it is correlated to the loading den-sity of GO sheets in the adhesion layer. We did our best toconcentrate the raw slurries via a high-speed centrifugation pro-cessing, and the adhesion strength reached a maximum value ofaround 68 N/cm2 at 18 mg/mL. Another way to enhance thecohesion of GO sheets relies on the ionic crosslinking strategy,which is commonly applied for strengthening GO films andhydrogels [33e35]. In this study, we introduced a certain amount ofCaCl2 solution (1 wt%) into GO solutions which were afterwardsadopted as Ca2þ ions crosslinked GO glues. To avoid the severeaggregation of GO sheets due to divalent ion crosslinking, the initialconcentration of GO solution was fixed at a low level of 4 mg/mL.The CaCl2 solution was added dropwise from 0.1 to 1 mL, and thecontent of CaCl2 in the resulted hybrid adhesion layer could becalculated ranging from 0 to 35.6% (Fig. 1d). The incorporation ofCaCl2 while stacking of GO sheets leads to the expansion of inter-layer spacing (d-spacing). The XRD patterns of GO-CaCl2 hybridsreveal that the d-spacing of (002) plane changes gradually from~8 Å to ~14 Å with an increasing fraction of CaCl2 (Fig. S2). Mean-while, the ionic crosslinking effect contributes positively to theadhesion strength. The force-displacement curves (Fig. S3) suggestan improved interparticle interaction between GO sheets, reflectedby the higher stress-bearing capability after addition of CaCl2. Theresults in Fig. 1d show that the adhesion strength is nearly doubled(from 34 N/cm2 to 62 N/cm2) by the introduction of 27.9% CaCl2 at agiven GO content. The significantly increased adhesion strength isattributed to the enhanced interlayer bonding contributed by theCa2þ divalent crosslinking. However, the further increment of CaCl2concentration may cause coagulation of GO sheets in the glue so-lution thus lead to non-uniform distribution of colloidal sheets andthen degradation of the adhesion.

Fig. 1e demonstrates that the functional groups on GO sheets

the shear adhesion strength of the GO adhesive and (b) cohesion and adhesion effectsentration, (d) Ca2þ ion crosslinking, (e) heating time on GO solutions and (f) lateral size) upon heating at 80 �C for 1e48 h. The initial GO concentration in (d and f) is 4 mg/mL

C. Zhang, Y. Liu, D. Chang et al. Carbon 171 (2021) 417e425

contribute a major part in the adhesivity by providing hydrogenbonding for GO-GO interaction. The heating of GO solutions at 80 �Cfor a maximum time of 48 h could slightly vary the surfacechemistry of GO sheets. The gradually darkened colour in thediluted solution is a common sign of partial reduction on GO sheets(see the inset of Fig. 1e). Moreover, the characteristic XRD peak ofGO at 2q¼ 10.5� is weakened as the elongation of heating time untilit completely vanishes at 48 h of heating (Fig. S4). The variation onthe intensity of diffraction peak is evidence of the break of stackingorder in the GO adhesion layer. As a result, the adhesion strength ofGO glue decreased continuously with elongation of the reductiontime due to the reduction of interlayer interaction. Besides, largerGO sheets also impose a positive effect on the cohesion componentaccording to the larger contact area between each other whilestacking (Fig. 1f and Fig. S5) [25,36,37]. Therefore, the adhesivelayer containing GO sheets of 42 mm lateral size in average exhibitsbetter adhesion than that composed of smaller GO sheets (50 N/cm2 vs. 34 N/cm2 at the same GO concentration of 4 mg/mL). In oursubsequent study, aqueous solutions containing 18 mg/mL GOsheets (8 mm) were employed in consideration of consistency, highviscosity, good adhesion performance, and easy processability.

Craft papers have a rough and hydrophilic surface, which allowsGO sheets to firmly attach to their rough microstructure and evenpenetrate the gaps between microfibers. The fracture surface of thespecimen after lap-shear tests (Fig. 2aed) reveals that the adhesionbetween GO and craft paper is quite strong. Cohesive failure isevidenced by the intact coating of residual GO sheets. Thedestruction occurred within GO adhesive is mainly attributed tosliding between stacked GO sheets. After carefully peeling off asmall piece of intact GO adhesion layer from the substrate, thecross-section of GO adhesive is able to be investigated. While atypical laminated structure is demonstrated in Fig. 2e and f, thethickness of the densely packed GO adhesion layer is estimated tobe around 5.3 mm.

The T-peel test was employed to measure the peeling forcebetween two bonded craft paper substrates using GO adhesive.Before the test, the unbonded arms are bent 90� and clamped in thegrips of the mechanical tester. The applied load is perpendicular tothe joint surface as illustrated in Fig. 3a. The load-displacementcurve shown in Fig. 3a is different from those obtained by a lap-shear measurement (Fig. S3). The presence of the wide plateau of

Fig. 2. (a) Photo of craft paper strips with GO adhesive after lap-shear testing. SEMimages of (b and c) joint area with residual GO sheets after testing at different mag-nifications and (d) bare surface of the paper strip. (e and f) SEM images of the cross-section of a stripped GO adhesion layer from the substrate. (A colour version of thisfigure can be viewed online.)

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the curve and largely elongated displacement suggest a fracturemechanism with progressive crack-propagation. Peel strength(work of debonding) can be evaluated through the ratio betweenthe peeling force (F) and the strip width (w) [38], which is0.014 ± 0.002 N/mm in our study. In Fig. 3b, an apparent adhesivefailure mode evidenced by the detachment of GO from the sub-strate is seen during the peeling test. SEM observation (Fig. 3cee)on the fracture surface reveals that the GO thin layers left on thesubstrate are mostly lifted and broken as a result of the peelingforce. These results suggest a different failure mode in the T-peeltests comparing with that in the lap-shear tests, which is primarilycaused by the sliding between GO sheets.

3.2. Adhesive bonding for various substrates by GO adhesive

According to the amphiphilicity of GO sheets, they can beapplied to bind a large variety of substrates, even between differentmaterials. In Figs. 4a and 10 specimens made of metals, wood,ceramic, polymers and glass, respectively, are demonstrated beingglued on a large piece of craft paper using GO adhesive without anysurface treatment. The adhesion strength for various materials isshown in Fig. 4b. It is noted that no matter whether the surface ofthe substrate is smooth (PET, ceramic, etc.) or rough (paper, wood,etc.), an efficient adhesion can be realized. The versatility of thebinding ability of GO sheets, in combination with their uniquecharacters (such as barrier performance and high conductivity afterreduction), may show great promise in advanced applications inthe field of interface science.

Since the GO adhesives are aqueous solutions with homoge-neously dispersed 2D colloidal particles, the high aspect ratio of GOsheets as well as their freedom of movement in the liquid phasecollectively benefit the spreading and attaching on arbitrary sur-faces. In addition, GO solutions at such a high concentration canform lyotropic liquid crystals (LCs) with short-range ordering[24,39,40]. The nematic phases of GO sheets are evidenced by theinspected optical birefringence under polarized optical microscopy(POM) (Fig. 5a). The pre-ordered structure in the liquid phase couldfacilitate the assembling of GO sheets while drying of the adhesionlayer, driven by evaporation-induced capillary force and stronginter-sheet interaction. Finally, the interlayer cohesion of closelypacked GO sheets supports the structural integrity of the adhesive,as reported in free-standing GO papers [41,42].

For flat surfaces, the arrangement of the sandwiched GOstacking is highly ordered and primarily parallel to the substrates(Fig. 5d). The step-shaped cleavage structure on the fracture surfaceis a result of the laminated packing of 2D nanosheets (Fig. 5b).When two glass slides are bonded by GO glue, the adhesion isstable enough to bear a 100 g weight (Fig. 5f). For curved surfaces,such as glass tubes and balls, the adhesion using GO is also appli-cable (Fig. 5gei). However, the packing manner of GO sheets maybe varied in different positions according to the spontaneous self-regulation of GO alignment (Fig. 5e). Consequently, the assembledGO stacking in the structured confined space is able to accommo-date the surface profile of the substrates, and a firm adhesion be-tween the curved adherent surfaces and adhesion layer is realized(Fig. 5c).

3.3. Computational simulations on the adhesion of carbonmaterials using GO-based adhesives

The applications of carbon materials are extremely varied in awide range of fields including but not limit to biology, medicine,energy storage, aerospace, etc. [43] The bonding of carbon mate-rials is critical but still challenging due to their inert surfaces. Sincegraphene is an allotrope of carbon in the form of a single layer of

Fig. 3. (a) Load-displacement curve recorded by the peeling test. Inset: schematic diagram of the measurement. (b) Photographs showing the failure process of the GO adheredspecimen during a peeling test. (cee) SEM images at different magnifications of the joint area with less residual GO sheets after the peeling test. (A colour version of this figure canbe viewed online.)

Fig. 4. (a) Demonstration of the versatility of GO adhesive for various surfaces. (b) The adhesion strength between various substrates using GO adhesive. (A colour version of thisfigure can be viewed online.)

C. Zhang, Y. Liu, D. Chang et al. Carbon 171 (2021) 417e425

atoms, it has an intrinsic affinity with carbon surfaces. As the mostimportant derivatives of graphene, GO and its conductive coun-terpart (reduced graphene oxide, rGO) are promising binders forcarbon materials (Fig. 6a) while the adhesion strength is highlydependent on the surface chemistry of GO sheets. The comparisonbetween Raman spectra of GO and rGO shows a slight increase ofthe D:G peak intensity ratio (ID/IG) from 1.4 to 1.7, indicating thepresence of unrepaired defects after the removal of oxidativegroups [18] (Fig. S6a). The shift of XRD patterns from a sharp andstrong peak at 2q ¼ 10.5� of GO to a broad and weak peak centredaround 24.5� of rGO implies the decrease of interlayer spacing from8.6 Å to 3.7 Å (Fig. S6b) as a result of the loss of functional groups[44].

Here we performed DFT simulations to gain a mechanistic un-derstanding of their bonding potentials for carbon materials. Theconstitution of GO and rGO sheets was constructed in the modelbased on X-ray photoelectron spectroscopy (XPS) analysis [45e47].It is well known that the primary oxidative compositions in GO arehydroxyl, epoxide, carbonyl and carboxyl groups. Therefore, the C

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1s spectrawere deconvoluted into five peaks, corresponding to CeC(284.2 eV), CeOH (286.1 eV), CeOeC (286.6 eV), C]O (287.9 eV)and O]CeO (288.4 eV), respectively (Fig. 6b) [44,48e50]. On thebasis of the XPS survey and areas of these peaks, the carbon-to-oxygen (C/O) ratios and percentages of different functionalgroups in GO and rGO were obtained. The highly functionalized GOpresents a C/O ratio of 1.8 and the total percentage of oxygen-containing groups is roughly 61.4%. The most abundant functionalgroups on the basal plane of GO are epoxies and hydroxyls, whilecarbonyl and carboxyl groups are usually located at the edge. Afterreduction by hydrazine (N2H4), the C/O ratio of rGO (denoted asrGO-N2H4) increases to 8.3, suggesting a significant loss of oxidativecomponents (residual proportion 38.5%).

In order to simulate the surface of graphite, we built three layersof carbon sheets with a periodic boundary where the bottom layeris constrained and the top two are allowed to be relaxed. Then, webuilt bi-layer GO and rGO nanosheets with 160 carbon atoms,respectively. Based on the assumption that the adhesion effect forstacked GO is dominated by their functionality on the basal plane,

Fig. 5. (a) Optical textures of GO LCs under POM observation. Concentrated GO solutions are sandwiched between two glass slides for observation. (b) Top-view of fractured GOadhesion layer between two flat surfaces after peeling off the upper slide. (c) Side-view of the GO layer between two tubular substrates. The dashed lines indicate the surfaces of thetwo opposite tubes. Schemes showing the arrangement of GO sheets between (d) flat surfaces and (e) curved surfaces. (f) A weight could be hanged by GO glued glass slides. GOglued (g and h) glass tubes with different crossing angles and (i) glass balls. (A colour version of this figure can be viewed online.)

Fig. 6. (a) Graphite products of different thickness glued by GO adhesive. (b) C1s spectra of GO and rGO. (c) The optimized configurations of bi-layer GO and rGO attached ongraphite. Grey, red and white beads represent carbon, oxygen and hydrogen atoms, respectively. GO1 indicates the top layer GO or rGO and GO2 represents the bottom layer.Eint(GO-GO) is the GO-GO or rGO-rGO interlayer interaction energy, and Eint(GO-graphite) denotes the interaction energy between GO-graphite and rGO-graphite. (d) Calculatedinteraction energy for GO and rGO with graphite. (A colour version of this figure can be viewed online.)

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we neglected edge effects in our models by only taking account ofhydroxyl and epoxide groups. Each layer of GO contains 18 CeOHand 12 CeOeC groups randomly coated on the nanosheet, whereasthe monolayer of rGO contains 4 CeOH and 2 CeOeC groups(Fig. S7). Finally, the model of graphite combined with bi-layer GOnanosheets of different C/O ratios were constructed through layerbuilding (Fig. 6c). The lattice parameters of a and b along the layersare 8.52 Å and 12.30 Å, respectively. The parameter of c vertical to

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the layer is 28 Å which makes the vacuum large enough to elimi-nate the interaction between the adjacent supercell.

The interaction energy between GO sheets (Eint(GO-GO)) can becalculated using the following equation:

EintðGO�GOÞ¼ Etotal � EGO1 � EGO2þgraphite (1)

where Etotal is the total energy of bi-layer GO with graphite after

Fig. 7. (a) The adhesion strength of graphite papers bonded with GO, hydrazine reduced GO and rGO-N2H4 after 1000 �C thermal treatment. (b) Fractured surface of rGO adhesionlayer after the lap-shear test. The inset shows the testing specimen and indicates the joint area between two graphite paper strips. (c) Binding graphite papers with rGO could keepthe connection conductive and light up an LED. (d) Chemical stability of rGO adhesive in 5 M NaOH and 5 M H2SO4, respectively, for up to 14 days. (e) Photos of the setup for three-point-bending measurement. Below is a magnified side-view and sketch (inset, dark grey: graphite paper, light grey: rGO adhesive) of the 10-layer thick slab glued by rGO. (f) Force-displacement curves recorded by bending tests on specimens with and without rGO adhesion. (A colour version of this figure can be viewed online.)

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fully relaxation, EGO1 denotes the energy of the top layer GO in theGO/graphite complex, and EGO2þgraphite is the total energy ofgraphite and the bottom GO2 layer adjacent to it. Correspondingly,the interaction energy between GO and graphite (Eint(GO-graphite))is defined as:

EintðGO� graphiteÞ¼ Etotal � Ebi�GO � Egraphite (2)

where Ebi-GO and Egraphite denote the energy of bi-layer GO withoutgraphite and graphite without bi-layer GO, respectively.

According to simulation results, the interaction energy betweenGO-graphite and GO-GO, known as the adhesion and interlayercohesion, both increase after reduction on GO sheets (Fig. 6d). Thisis probably because of the intensified p-p stacking between carbonlayers evidenced by the decreased interlayer spacing after reducingfunctional groups (Table S1). Theoretically, in pure graphene, the porbital formed by C 2p is delocalized and would bind with othercarbon layers through dense p-p bonding. In GO, as plenty ofoxidative groups are introduced on the graphitic domains, the C 2porbital forms s bonds with the functional groups instead, resultingin the decrease of p bonds. Thus, the p-p bonding between GOsheets with high coverage of oxidative content is weaker. On thecontrary, reduction on GO restores sp2 CeC hybridization andpartially recovers p-p bonding. That is the reason for more signif-icant interaction between rGO layers which corresponds to the XRDresults (Fig. S6b) as well as experimental facts reported elsewhere[51,52].

3.4. Adhesion between graphite papers using rGO adhesive

The experimental results show that the strength of graphitepaper adhesion using pristine GO glue is 30.2 N/cm2 on average.

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After chemical reduction via N2H4 vapour, the adhesion strengthincreases 54%, approaching a value of 46.5 N/cm2. The growingtendency of adhesion strength along with the reduction of GO isconsistent with the theoretical calculation results, owing to theenhanced rGO-graphite adhesion and rGO-rGO interlayer cohesion.In the meantime, the performance of rGO adhesives is limited bythe generated porosity during the reduction process (Fig. S8),which means it still can be improved by optimized experimentalprotocols. A 1000 �C thermal annealing further reduces the contentof oxygen-containing groups in the adhesion layer while theadhesion strength is relatively stable at 43.9 N/cm2. The milddegradation (~5%) in the binding performance of rGO adhesive after1000 �C annealing might be attributed to the introduced defects onthe rGO building blocks. As the adhesion layer can still functionwellafter annealing, the outstanding thermal resistance of rGO adhesivesuggests its potential application in operations under high-temperature scenarios. From the fractured surface of the jointarea after the lap-shear test (Fig. 7b), the good attachment of rGOsheets on graphite paper is noticeable. More detailed microscopicimages are provided in Fig. S9, a large portion of residual rGO layersare pulled up from the substrate, which is a reflection of strongadhesion between rGO and graphite surface. Apart from increasedadhesion strength and thermal stability, the reduction on GO alsoimplements a transition from non-conductive to conductive in theadhesion layer. Through bonding with rGO glue, the conductionpath along separated graphite papers is able to be reconnected topower an LED (Fig. 7c). The anti-corrosion behaviour is anothertempting feature of rGO adhesive, benefiting from their chemicalinertness. As demonstrated in Fig. 7d, the adhesion strength of rGOglued graphite papers shows almost no decay (98% retention) afterbeing subjected to the highly concentrated alkaline solution (5 MNaOH) for 2 weeks. In acidic circumstances, there still presents a

C. Zhang, Y. Liu, D. Chang et al. Carbon 171 (2021) 417e425

76% retention of adhesion strength. On the contrary, the commer-cial epoxy-based glue could not withstand in such hash conditionsand break down quickly in a few days, losing its adhesive ability(Fig. S10). Thus the rGO adhesives prove their advantages in boththermally and chemically stable adhesion, indicating the potentialapplication as robust, conductive and stable interfacial connectingmaterials. It is noteworthy that although the stability of the purelyinorganic rGO adhesives is higher than organic ones, the adhesionstrength still needs improvement before being used as structuraladhesives. For this purpose, densification of the packed structures,incorporation of a few polymeric constituents or modification onthe rGO sheets can be considered to enhance the adhesion strength.

In addition, the bonding capability of rGO for graphitic materialsprovides a possible way to bind graphite papers layer-by-layer intothicker ones, which can be used as structural materials with highconductivity. By stacking 10 layers of 0.3 mm thick graphite paperswith rGO glue in between, a favourable structural integrity is suc-cessfully achieved in the thick specimen because of the interlam-inar bonding originating from the rGO layer (Fig. 7e). With theincrease of interfacial connection, the interlayer sliding is hinderedduring three-point-bending, thus the bending stress could effi-ciently transfer across the whole thickness. It is demonstrated fromthe force-displacement curves (Fig. 7f) that the bonded slab illus-trates better bending strength by bearing 4 times higher load ascompared with the unbonded graphite paper stack.

4. Conclusions

In conclusion, we have demonstrated the interfacial adhesivecapacity of assembled GO stackings. A wide range of adherents canbe glued by neat GO sheets without any polymeric additive. Espe-cially, GO and rGO have been shown to implement stable adhesionbetween carbon materials relying on their intrinsic 2D carbonskeletons. Computational study and experimental results collec-tively prove that the reduction on GO sheets can enhance theadhesion for graphite papers. In the meantime, the conductivity, aswell as heat and chemical durability of the adhesion layer is alsosignificantly improved. These featured adhesive behaviours mayinspire further exploration of GO-based nanosheets applied at theinterface between materials. The lamellar stacking of GO sheets isaccessible to further modification or hybridization in the adhesionlayer, which shows promise to improve the adhesion strength aswell as to achieve advanced adhesion with additional functions.

CRediT authorship contribution statement

Chunxiao Zhang: Conceptualization, Methodology, Data cura-tion, Writing - original draft. Yang Liu: Methodology, Software,Formal analysis, Resources, Writing - original draft. Dan Chang:Data curation, Investigation, Validation. Zheng Li: Supervision,Conceptualization, Methodology, Data curation, Validation, Writing- original draft, Project administration. Yingjun Liu: Resources,Validation. Zhen Xu: Writing - review & editing, Funding acquisi-tion. Chao Gao: Writing - review & editing, Funding acquisition,Project administration.

Declaration of competing interest

The authors declare that they have no known competingfinancial interests or personal relationships that could haveappeared to influence the work reported in this paper.

Acknowledgement

This work is supported by the National Natural Science

424

Foundation of China (Nos. 51533008, 51973191 and 51703194),National Key R&D Program of China (No. 2016YFA0200200), Hun-dred Talents Program of Zhejiang University (188020*194231701/113), Key research and development plan of Zhejiang Province(2018C01049), Fujian Provincial Science and Technology MajorProjects(No. 2018HZ0001-2), the Fundamental Research Funds forthe Central Universities (No. K20200060) and Key Laboratory ofNovel Adsorption and Separation Materials and Application Tech-nology of Zhejiang Province (512301-I21502). The authors grate-fully acknowledge Dr. Jing Yu (Nanyang Technological University,Singapore) for her kind help in improving English writing. We alsothank Drs. Jianwu Wang, Huarong Xia and Wei Zhang (NTU) fortheir assistance in some experiments during the revision process.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.carbon.2020.09.046.

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