Superstrong Noncovalent Interface between Melamine and Graphene

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Surfaces, Interfaces, and Applications

Superstrong Noncovalent Interface between Melamine and Graphene Oxide Jun Xia, YinBo Zhu, ZeZhou He, Feng-Chao Wang, and Heng-An Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02971 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Applied Materials & Interfaces

Superstrong Noncovalent Interface between Melamine and Graphene Oxide Jun Xia,† YinBo Zhu,*,† ZeZhou He,† FengChao Wang,† HengAn Wu*,†

†CAS

Key Laboratory of Mechanical Behavior and Design of Materials, Department

of Modern Mechanics, CAS Center for Excellence in Complex System Mechanics, University of Science and Technology of China, Hefei 230027, China

ABSTRACT There have been growing academic interests in the study of strong organic molecule–graphene (or graphene oxide (GO)) systems, owing to their essential noncovalent nature and the consequent chemo-mechanical behavior within interface. A more recent experimental measurement [Chem, 2018, 4, 896–910] reported that the melamine–GO interface exhibits a remarkable noncovalent binding strength up to ~1 nN, even comparable with typical covalent bonds. But the poor understanding on the complex noncovalent nature in particular makes it challenging to unveil the mystery of this high-performance interface. Herein, we carry out first-principle calculations to investigate the atomistic origin of ultra-strong noncovalent interaction between the melamine molecule and the GO sheet, as well as the chemo-mechanical synergy in interfacial behavior. The anomalous O–H···N hydrogen bonding, formed between the triazine moiety of melamine and the –OH in GO, is found cooperatively enhanced by the pin-like NH2–π interaction, which is responsible for the strong interface. Following static pulling simulations validate the 1 nN rupture strength and the contribution of each noncovalent interaction within the interface. Moreover, our results show that the –OH hydrogen bonding will mainly augments the interfacial adhesion strength, while the – NH2 group cooperating with the –OH hydrogen bonding and conjugating with the GO surface will greatly improve the interfacial shear performance. Our work deepens the understanding on the chemo-mechanical behaviors within the noncovalent interface, 1

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which is expected to provide new potential strategies in designing high-performance graphene-based artificial nacreous materials.

KEYWORDS: noncovalent interaction, melamine molecule, graphene oxide, firstprinciple calculations, chemo-mechanical behavior

2


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INTRODUCTION Natural structural materials, such as nacre1, always exhibit outstanding mechanical properties (e.g., strength & toughness) due to the unique hierarchically ordered microstructures and multiscale characteristics,2,3 which has spurred enormous interest and substantial advances in the bio-inspired material science and fundamental understanding of reinforcement mechanisms.4,5 Featuring the biomimetic “brick-andmortar” microstructure, a series of graphene-based artificial nacreous materials have been successfully constructed in the past decade.5–7 For the “brick” (building blocks), abundant oxygen-containing groups in conjunction with high mechanical stability make graphene oxide (GO)6 an ideal candidate in providing the possibility of interfacial design at nanoscale for artificial nacre-like materials. As to the “mortar” (interfacial interactions), covalent bonding, ionic bonding, hydrogen bonding, π–π interaction and van der Waals (vdW) interaction are several typical alternatives to engineer the interfacial strength and synergistic toughening effect.7 The quest for widespread applications always accentuates the necessity to design materials with both high strength and high toughness. Unfortunately, these two properties are mutually exclusive in nacre-like materials.8 Improving strength and toughness simultaneously with the assistance of suitable crosslink is still a challenge in graphene-based artificial nacreous materials.9,10 In general, without regard to the performance of building blocks, interfacial interactions like π–π interaction and hydrogen bond (HB), providing inferior strength, maintain a reasonable toughness for the composites.11,12 But for interactions like ionic bonds and covalent bonds, inversely, much higher strength and stiffness with very limited tensile strain could be the common characteristics.5 In this light, designing the synergistic effect both intrafacially and interfacially are expected to be effective strategies to improve the mechanical performance and well balance the strength and toughness.7 Hence, in recent years, intense efforts13–16 have been devoted into screening suitable interfacial adhesives and building blocks for diverse artificial nacre-like materials. Organic molecules, on account of their abundant hydrogen donors and acceptors13 (as synergistic glue) and the consequent flexible ability in forming hydrogen-bonded 3



supramolecular networks17 (as cooperative building block), stand out to be one of the optimum synergistic additives in maintaining a rational balance between strength and toughness. Nevertheless, little attention has been paid to this type of new and promising potential additive in past years due to a lack in effective means of screening molecules and more importantly the unclear understanding on the noncovalent reinforcement principle of organic molecule–GO molecule interface, which tremendously impedes the development in designing graphene-based artificial nacreous materials. Through our efforts devoted into this investigation, the melamine molecule, revealed by our recent work18 using the atomic force microscopy (AFM)-based single-molecule force spectroscopy, shows the strongest noncovalent interaction (~1 nN) ever measured with the GO surface, comparable to typical covalent bond interactions. The consequently assembled melamine–GO film, despite the nanometer-sized thickness, possesses a common tensile strength but a notable toughness, and demonstrates a potential in the application of solid-state flexible supercapacitors. The extremely strong melamine–GO interface here re-inspires our interest in investigating its atomistic origin and potential intriguing behavior in chemomechanics. We also believe that corresponding in-depth understanding will serve as a referenced guide in constructing nacre-like materials basing on screening optimum organic molecule–GO interfaces. Generally, to weigh up the strength of an interface, the interaction forms contained and their joint effect are two aspects needed to be considered. The existing recognitions on the melamine–graphene19,20 and melamine–GO21,22 interface usually attribute this kind of strong interface to the amine (–NH2) group in melamine. It is true that the flexible HB formed by –NH2 group and the anchoring effect of –NH2 on the GO surface determine the supramolecular assemble nature of melamine molecule and its derivatives. However, the correlative N–H···O (0.134~0.265 eV23,24) and N–H···π (weak: 0.019~0.104 eV25; strong: 0.224 eV26) HBs, are merely at a similar order of magnitude in strength to the common O–H···O (0.163~0.193 eV27) HB existent in bulk water, which is surely incapable of providing the covalent-level strength found in the experimental measurement. Yet another mayhap potential interaction form mentioned in our previous study, i.e., the O–H···N HB formed between the nitrogen atom in 4


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triazine ring and the oxygen-containing group in GO, although reported to possess a bit higher bond energy (~0.282 eV28 or ~0.228 eV29), seems still far from the covalent strength. On the other hand, the joint effect in noncovalent-interaction-dominated interface is more complex than that in the interface governed by covalent interactions. The additive scheme of all interaction forms for covalent bonds is no longer applicable to noncovalent bonds.30 Conventionally, the nature of a given covalent bond, remains largely unaffected by its surroundings,31 while for an noncovalent bond, it can vary remarkably due to the chemical cooperativity or anticooperativity. Then a question arises: how these specific noncovalent interactions mutually influence themselves and further contribute to the outstanding mechanical performance? Due to the complexity at molecular scale, few investigations have focused on noncovalent interfaces, e.g., the melamine@GO system here, resulting in the veiled reinforcement mechanism of noncovalent interactions. Thus, a thorough investigation is much needed here to comprehend the origin of this strong interface and the underlying chemo-mechanical linking at the atomistic scale. In what follows, via density functional theory (DFT) calculations, we characterize the strength of various interaction forms within the melamine–GO interface at atomic scale and for the first time present the origin of the superstrong melamine–GO interface: the chemical cooperativity enhancing O–H···N hydrogen bond formed by the triazine moiety of melamine. The charge density distribution and the noncovalent interaction index analysis are utilized to elucidate the cooperativity here between the triazine moiety and the –NH2 group in melamine. Then, static pulling simulations are performed in order to further unravel the inherent relation between the chemical interaction and the mechanical behavior of the interface. Finally, basing on the assumption that crosslinks are in low concentration, the simulation results are normalized to predict mechanical properties (tensile and shear) at extended large scale.

COMPUTATIONAL METHODS The simulation system was modeled by placing a melamine molecule onto the 7 × 7 × 1 monolayer graphene supercell in which one or several hydroxyls (or epoxies) 5



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were added to the graphene center to form the GO. The structure was allowed to extend infinitely in the xy plane, while periodic replicas along the z direction were separated by at least 20 Å of vacuum to avoid the effects of periodic interactions. All electronic and structural calculations were performed based on the density functional theory (DFT) as implemented in the Quantum Espresso package32,33. The Perdew-Burke-Ernzerhof (PBE) functional in generalized gradient approximation (GGA)34

was

employed

to

describe

the

exchange-correlation

functional.

Pseudopotentials were obtained from the Standard Solid State Pseudopotentials (SSSP) library35, whose precision has been strictly validated and provides a compelling reproducibility36. To include the correction of vdW interactions, the Grimme's DFT-D3 scheme37 was adopted (comparisons of different DFT approximations and vdW correction schemes are given in the Supporting Information). Kinetic energy cutoffs for wavefunctions and charge density are 60 and 480 Ry, respectively. Using MonkhorstPack38 Γ-centered 4 × 4 × 1 k-points grid, all structures were fully optimized until all atomic forces become smaller than 10-4 Ry/Bohr. The energy convergence threshold for self-consistency is 10-8 Ry per unit cell. The adsorption energy Eads was calculated according to

Eads  Etot  EGO  EM ,

(1)

where Etot is the total energy of the whole system, and EGO and EM denote the total energies for isolated GO and melamine molecule, respectively. The charge transfer between the melamine molecule and GO was characterized by the Bader charge analysis developed by the Henkelman’s group39 and the differential charge density, which is defined as:

  r  = tot  r   GO  r    M  r  .

(2)

Here, ρtot is the total electron density of the whole system, and ρGO and ρM denote the electron densities for isolated GO and melamine molecule, respectively. Noncovalent interaction (NCI) index analyses40,41 at the optimized adsorption geometry were performed using the CRITIC242,43 code (extend the NCI method40,41 to periodic solid-state electron densities) to manifest the hydrogen bonding and weak vdW 6


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interactions between the adsorbed melamine molecule and the GO surface. The CRITIC2 code calculates two scalar fields to map local bonding properties: the electron density (ρ) and the reduced density gradient (RDG, s), defined as:40

s



1

2  3



2 13

4 3

.

(3)

The RDG is a dimensionless quantity to describe the deviation from the homogeneous electron distribution.44 Plot of the RDG (s) versus the electron density (ρ) multiplied by the sign of the second Hessian eigenvalue (sign(λ2)) presents features in low-ρ and lows regions. Regions with high s and low ρ correspond to the decaying density tails (regions far from the nuclei). Covalent bonds can be visualized from properties of the electron density (i.e., the bond critical point) from the atoms-in-molecules (AIM) theory45, corresponding to the saddle points at s = 0 with a high ρ in the RDG plot here, while the introduction of noncovalent interactions induces a remarkable drop of s to nearly zero (spikes at low ρ).40 The sign of the Laplacian of the density (∇2ρ), contributed by the sign(λ2) here, is further utilized to distinguish between different noncovalent interactions, in which hydrogen bonds produce density accumulation (λ2 < 0), vdW interactions characterize a negligible density overlap (λ2 < 0 and λ2 ≈ 0), and steric repulsion produce density depletion (λ2 > 0).41 That is, in the RDG plot, spikes at more negative sign(λ2)ρ values (corresponding to denser accumulation of ρ) indicate stronger HB interactions. In this study, the strength criterion is adopted to define the HB (sign(λ2)ρ < -0.01). -0.01 < sign(λ2)ρ < 0.00 corresponds to very weak interactions (e.g., the vdW interactions).

RESULTS AND DISCUSSION Noncovalent nature of melamine–GO: origination and chemical cooperativity. In the light of the single-molecule experimental measurement between melamine and GO, we begin our investigation by first-principle calculations to gain in-depth insights into the nature of this impressive noncovalent interaction and its origination at atomic scale. The melamine–GO, comprising various HBs, NH2–π and π–π interactions, and the most universal vdW interactions, exists multifarious possible configurations. 7



Considering different adsorption states and binding sites of melamine@GO, in order to give a brief view of the general interaction strength of melamine–GO interface, the adsorption energy versus the HB length for typical melamine–GO configurations is summarized in Figure 1 as comparison. The HB length here refers to the distance between the electron donor and acceptor. The adsorption energy distribution of melamine@graphene system (black inverted triangles) is also given for reference (the HB length is replaced by the interlayer distance). Configurations calculated in Figure 1 are sampled by two indexes, i.e., the adsorption state and the binding site (see Supporting Information). Note that, in Figure 1, the configurations of melamine@GO system can be roughly divided into three different adsorption states, namely, GO– O···NH2–M (green squares), GO–OH···NH2–M (red circles), and GO–OH···N–Tri (blue solid triangles), where the character “M” denotes melamine molecule, “Tri” is the triazine moiety in melamine molecule, and –O/–OH is the epoxy/hydroxyl group on GO sheet. For a given adsorption state, melamine molecules at different binding sites upon GO surface, although varied in the formed HB show a similar characteristic in the energy distribution. Optimized structures at different binding states for these three categories of melamine@GO system and the referenced melamine@graphene are plotted in Figure S2–S4 and Figure S5, respectively, with their corresponding adsorption energies listed in Table S2–S5. The energy and HB length distribution of GO–OH···N–Tri state is highly centralized, while the GO–OH···NH2–M state is further divided into two typical sub-states: the two “suspended” configurations (containing the O–H···N HB, shown in Figure S3(b) and (j)) and the other centralizeddistributed configurations (containing the N–H···O HB, shown in Figure S3(a, c–i and k)). When it comes to the GO–O···NH2–M state, we will not consider this state any more in the following discussions due to the very low binding strength (meaning very weak HB) and the discrete distribution of energy and bond length. According to the diagram of adsorption energy versus HB length in Figure 1, the melamine@GO system exhibits a much higher adsorption energy, high up to ~-0.95 eV, than the melamine@graphene system (~-0.6 eV, see also reference 46), as the result of forming HBs by introducing one oxygen-containing group. Previous recognitions 8


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believe that the strong melamine–GO interface might mainly derive from –NH2 group, by which the dominant hydrogen bonding and NH2–π interaction are formed. Whereas, our DFT calculations indicate that the HB formed by the triazine moiety (GO–OH···N– Tri) leads to a far more powerful adsorption strength comparing to those dominated by –NH2 group (GO–O···NH2–M and GO–OH···NH2–M), by as much as ~40%. Even when we exclude the effect of π–π interaction, the melamine standing aslant upon the GO surface (marked by the blue open triangle symbols), although showing a sharply drop in the adsorption energy from the common GO–OH···N–Tri, yet possesses the adsorption energy (-0.73 eV) larger than most GO–OH···NH2–M configurations. The calculated HB length of GO–OH···N–Tri (< 2.8 Å) is found much smaller than those of HBs formed by –NH2, implying a far stronger hydrogen bonding indeed. Considering the truth that in biological systems the O–H···N HB generally possesses an average strength at a similar magnitude to the N–H···O HB, the possibly unusual HB enhancement here and the consequent outstanding interface strength are rather interesting and the underlying mechanism is important in understanding interfaces comprised of noncovalent interactions. Hence, we separate this special melamine@GO configuration of GO–OH···N–Tri adsorption state for further analyses.

The differential charge density in Figure 2 gives a general view of the charge transfer of melamine@GO with the GO–OH···N–Tri adsorption state. At a first glance, a dramatic charge transfer occurs in the vicinity of HB between the –OH group and the triazine moiety of melamine, emphasizing the significance of triazine formed hydrogen bonding in enhancing the interface yet again. Despite the much lower charge densities (colored in light green and orange), the NH2–π interaction and the π–π interaction both play an indispensably assistant role, in which the former pins the whole molecule onto the GO surface like three symmetric nails (light orange electron clouds between the – NH2 and the GO surface) and the latter cooperating with the –OH acts like the hinge joint (light orange ring-like electron clouds between the triazine and the GO surface). Considering all these forms of interactions separately, we performed the NCI index analysis and give the RDG plot and isosurfaces in Figure 3 to quantify different 9



interaction forms within the melamine–GO interface. The RDG (s) is generated to describe the deviation from the homogeneous electron distribution. Thus in Figure 3, the top points (low ρ and high s) correspond to regions far from the interface. Covalent (high ρ) and repulsive (sign(λ2)ρ > 0) interactions are excluded from the interface system. The residual attractive interactions (including HB and vdW interactions), represented by points at low ρ and low s, are analyzed according to the spikes at negative sign(λ2)ρ values, in which more negative sign(λ2)ρ values correspond to denser density accumulations, indicating stronger interactions. The corresponding isosurfaces in Figure 3(b–e) clearly manifest the hydrogen bond and other noncovalent interactions. The small highly localized disc-like isosurfaces represent the stronger HBs, while green larger sheet-like isosurfaces refer to weaker interactions. The O–H···N HB [dark blue disc in Figure 3(b)] formed by the triazine ring, shows a far more negative sign(λ2)ρ value, ~-0.051, than the previously thought dominant O–H···NH2 [light blue disc in Figure 3(c)] and the N–H···O [nearly green disc in Figure 3(e)] HBs formed by the – NH2 group, demonstrating an amazingly strong hydrogen bonding (common HBs: 0.04~-0.0147,41,48,49) in the GO–OH···N–Tri adsorption state. Conversely, HBs formed by the –NH2 group are even only at a similar order of magnitude to those formed between the water molecule and the –OH group in GO [Figure 3(d)]. Apart from these relatively strong noncovalent interactions, other forms of interactions, e.g., the NH2–π interaction and the π–π interaction, are nearly equivalent to the vdW interactions (green larger sheet-like isosurfaces), corresponding to spikes marked by the grey square in the RDG plot.

Hence, from the perspective of NCI index analysis for each interaction form within the melamine–GO interface, the outstanding adsorption strength of interface in this special adsorption state should derive mainly from the superstrong HB formed by the triazine moiety of melamine. Then in order to unravel what contribute to the unusual enhancement of HB here, we need to take account of the joint effect of various interaction forms. The joint effect, i.e., the manifestation of chemical cooperativity or anticooperativity, is essential in influencing systems governed by noncovalent 10


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interactions.30 Four types of different triazine derivatives (different numbers of –NH2) absorbing on GO (with and without –OH) configurations (modified from the “bridge” model in Figure S4) in the adsorption state of GO–OH···N–Tri are compared in Figure 4(a) to investigate the presumable cooperativity between the triazine moiety and the – NH2 group in melamine. The symmetry of triazine derivatives causes the error bar in Figure 4, in which the triazine derivatives with one or two –NH2 groups both possess two different configurations relative to the GO surface (shown in Figure S8). Firstly, for the pristine graphene surface (red line with circles), the adsorption energy of the adsorbed system grows slowly and linearly with increasing number of –NH2 groups, which is consistent to the DFT result in reference 19. Once the –OH group is introduced into graphene, the subsequently formed strong HB between the –OH and the triazine ring brings a sharp jump by as much as ~75% in the adsorption property. If taking no account of the cooperative effect among multiple interactions, i.e., following the additive scheme of covalent interactions30, the predicted energy curve should be along the dashed line paralleled to that of pure graphene [Figure 4(a)], representing a steady growth of adsorption energy due to the increasing –NH2–π interactions. But, as highlighted by the blue region in Figure 4(a), the actual adsorption energy curve far deviates from this prediction, indicating the positive cooperative effect on the adhesion induced by amino group. This positive cooperative effect is mainly embodied in the consequent enhancement of HB formed by the triazine ring, as shown in Figure 4(b). The low-gradient spikes in RDG plot characterize the linear dependence of HB strength on the number of –NH2 groups. Without the cooperating –NH2 group, the common O– H···N HB (RDG spike at ~-0.033, see Figure 4(b)) is merely a bit stronger than HBs formed by the –NH2 group (RDG spike at ~-0.031, see Figure 3(c)). Combining with the foregoing deduction on the differential charge density in Figure 2, the HB enhancement here can be inferred from the length variation of O–H···N that the increasing number of –NH2 groups, like several nails, pin the triazine moiety of melamine closer and closer to the –OH group and simultaneously more parallel to the GO surface, which is contributed by the variations of charge transfer shown in Figure 4(c–f). The –NH2 group, as an electron donor, makes more electrons transfer from the 11



–NH2 group to the triazine moiety of melamine and the GO surface, increasing the electronegativity of the hydrogen donor (O atom in –OH) and acceptor (N atom in triazine ring) of O–H···N HB [Figure S6]. More dramatic charge transfer occurs, as the result of increasing –NH2 groups, further polarizing the triazine moiety of melamine and the GO surface and hence stabilizing the hydrogen bonding formed between them. To summarize, we can conclude that the experimentally measured ~1 nN noncovalent interaction between melamine and GO should originate from the chemical cooperativity enhancing superstrong hydrogen bonding formed by the triazine moiety of melamine. The –NH2 group, although forming an extremely weak NH2–π interaction with the GO surface, plays an irreplaceable role in cooperating with the triazine ring. Moreover, we also find that the low-coverage melamine molecule on GO surface nearly have no influence on the electronic property of GO sheet [see density of states in Figure S7], demonstrating the advantages of noncovalent interaction in the applications of functional electronic devices.

Mechanical synergy in the triazine derivative–GO interface. Based on our DFT representations above on adsorption energies and noncovalent interactions, the chemical cooperativity existing between the strong O–H···N hydrogen bonding and the weak NH2–π interaction is the origin of abnormally strong melamine–GO (or other triazine derivatives) interface. To link the chemical adsorption at nanoscale all the way to bulk mechanical properties at mesoscale or even macroscale, more detailed understandings are required to reveal how chemical cooperative interactions influence the mechanical behavior. Toward this end, static pulling simulations were carried out to quantify the binding strength of triazine derivative–GO interfaces. Three configurations with different binding states were firstly used to assess the error [Figure S10], which indicates that the binding states have little effect on the mechanical behavior of the interface system. Then the melamine@GO system in the GO–OH···N– Tri adsorption state and the “bridge” binding state [Figure S4(a)], was set as the initial configuration for the pulling, wherein the number of –NH2 and –OH groups is regarded as independent variable to discuss the chemo-mechanical response [see Figure S11 and 12


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S12 (0~3 –NH2; 1 –OH), Figure S13 (3 –NH2; 0 –OH), Figure S14 (3 –NH2; 2 –OH), Figure S15 (3 –NH2; 3 –OH) and Figure S16 (3 –NH2; 6 –OH) in the Supporting Information]. The schematic plot of the static pulling process is shown in Figure 5(a). For triazine derivatives containing the –NH2 group, the terminal nitrogen N* in the – NH2 group was constrained and vertically pulled away from the GO surface, in successive steps of ΔH = 0.02 nm. To take the effect of loading rate into consideration, error bars are introduced by adjusting the loading step, ΔH, from 0.005 to 0.02 nm. For the specific s-triazine molecule, with no –NH2 group, the terminal hydrogen H* in the similar position was chosen as the target atom. Geometry optimizations were conducted to obtain the reconstructions after the pulling operation in each step. After 50 pulling steps, the triazine derivatives were raised up for a total vertical displacement of 1 nm. The forces of the target atom suffered from the pulling operation and the energy profiles were collected for further analyses. The evolution of the resultant forces and corresponding force components are displayed in Figure S11–S16 for different triazine derivatives absorbed upon GO surface with different number of –OH. For the melamine@graphene system (Figure S13), although the absorbed molecule undergoes a considerable lateral displacement simultaneously in the elevation process, the lateral force components (FX and FY) are nearly negligible comparing to the z-direction force component (FZ). But for triazinederivative@GO systems, with the introduction of strong O–H···N HB, the lateral sliding becomes more difficult and more significant. Within the normal displacement of 0.4 nm, the FX and FY are no longer negligible to FZ. Especially for triazine derivatives with –NH2 groups, both the FX and FY are found even beyond the FZ, echoing the foregoing non-ignorable effect of cooperativity induced by the –NH2 group. When the normal displacement exceeds 0.4 nm, the absolute values of FX and FY conversely become comparatively small, indicating the major longitudinal motion of molecule in this stage. All rupture forces (the maximum resultant force) are summarized in Figure 5(b) and 5(c) to explore their dependence on the number of –NH2 and –OH groups. The triazine-derivative@GO (1 –OH) system, representing the case in which GO sheet has a low oxidation degree, shows a raising trend in the rupture force with 13



increasing –NH2 groups [Figure 5(b)], corresponding to the adsorption energy trend in Figure 4(a). Yet the effect of enlarging π–π repulsion driven by the increasing –NH2 groups is magnified in the manifestation of rupture behavior wherein the extended dashed line here highlights the enlarging deviation of rupture force from linearity. Then the melamine@GO system, with varying number of –OH groups, is constructed to discuss the dependence of rupture force on the oxidation rate. Single –OH hydrogen bonding can only yield a maximum rupture force of about 600 pN, while an amazingly higher interface rupture force exceeding 1 nN occurs with more –OH groups introduced [Figure 5(c)], which is in tremendous agreement with the our previous experimental measurement18. This remarkable rupture force is ascribed to the saturated three strong O–H···N HB formed between the melamine and the GO (3 –OH). Subsequent addition of –OH groups merely provides the weak HB formed between –NH2 and –OH group or other weak noncovalent interactions, making the maximum rupture force tend to saturate. Note that, the measured force value of melamine-GO interaction in the previous single-molecule AFM experiments18 showed a large error bar. This is partly due to the inhomogeneous local oxidation degree of GO substrate under the melamine molecule. The dependence of the interfacial strength on the number of –OH group [Figure 5(c)] demonstrates a possible error in the experimental rupture force, high up to 700 pN, induced by the variation in the local oxidation degree.

The static pulling process of the typical melamine@GO system in Figure 6 can illustrate the internal relevance between the chemical interaction and the mechanical behavior. Four stages can be subdivided from the whole process according to the evolution of reconstruction structure, bonding situation and resultant force. At the initial stage (stage I, up to ~0.8 Å elevation of N*), the distance between nitrogen atom in triazine ring and the oxygen atom in –OH (Tri–N···O) is nearly invariant, so is the distance between –NH2 group and graphene surface (NH2···Gra), indicating the nonload state of hydrogen bonding and NH2–π interaction. Actually, in the view of structural evolution, the initially flat laid triazine was elevated by a small angle [Figure 6(b)] with the O–H···N HB being the hinge joint, thus the π–π stacking between the 14


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triazine ring and the graphene surface is the principal load-bearing interaction in stage I. Then the tilted angle of the triazine ring became larger as the elevation of N* continues until the failure of the π–π interaction, yielding a maximum pulling force of ~200 pN in stage I, which agrees well with the π–π strength in earlier stage of force profile of the melamine@graphene system in Figure S13. In stage II, the distance of Tri–N···O started to rapidly increase, while the distance of NH2···Gra only deviated a little due to the elevation-induced tilt of melamine molecule. That is, the strong O– H···N HB takes over the load-bearing role in stage II [Figure 6(c)]. This bond remained strongly connected until the displacement of N* raised to ~2.2 Å, yielding a rupture force of ~550 pN, which is far beyond that of single typical ionic bond (100~200 pN)50,51 or HB (~100 pN)52 reported previously. Interestingly, the –NH2 group, though chemically as weak as vdW interactions (characterized by the NCI index analysis in Figure 3), plays a non-negligible role in the lateral sliding process in stage II. It is found that the lateral force components greatly exceeded the value of Fz when the –NH2 group joined in [Figure S12], which also contributes to the final ultrastrong resultant binding force. After the force peak, the melamine itself gradually tilted to the vertical direction, along with a sharp decrease of force to ~100 pN, until the complete failure of O–H···N HB (a sharp jump in the Tri–N···O distance at an elevation about 4 Å). Subsequent pulling (stage III) leads to another increase in magnitude of the resultant binding force to ~350 pN at a z-direction displacement of 5 Å, followed by yet another decrease (steady instead) foreshadowing the entire detachment of the melamine–GO [Figure 6(e)]. The distance of NH2···Gra started to increase, implying the load-bearing role of –NH2 group in stage III [Figure 6(d)]. Surprisingly, the unusual rebound of force in stage III seems contradictory to the weak interaction judgement of NH2–π mentioned in Figure 3 and this phenomenon also occurred in the case of melamine@graphene system [Figure S13]. Then, the evolution of structure, energy path and noncovalent interactions around the turning point at 4 Å, ranging from 3.2 to 4.4 Å in elevation, is separated for detail analyses [Figure S17]. When the tiled angle of triazine ring was relatively small (structure at A point), the O– H···N interaction had not completely failed yet, corresponding to the very weak HB15



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like interaction at A peak in the RDG plot. The NH2–π interaction is as weak as the vdW interaction coinciding with the NCI plot in Figure 3. Upon further pulling, all interactions declined to the vdW magnitude (the B region in RDG plot), indicating the breakdown of weak O–H···N. Finally, in the turning point C (both in structure and energy), the old O–H···N interaction completely debonded with yet another much stronger new interaction (C peak in RDG plot) rebonded between the nearly vertical melamine and the graphene surface (NH2–π). Although still relatively weak in strength comparing to strong HBs in Figure 4, the NH2–π interaction shows an HB-like gradient isosurface (highly localized) but with larger sphere of action, which should be responsible for the unusual rebound of force and well support the previous deduction on the strong melamine–graphene interface19,20. Such a ubiquitous HB switching in the triazine derivative–GO interface, ascribed to abundant hydrogen donors and acceptors, dissipates a great deal of energy during the rupture process of interface, making this type of interface much more stronger than we have expected.

As discussed above, there exist simultaneous lateral (in xy plane) sliding and longitudinal (in z direction) raising behavior in the rupture simulation, which actually correspond to the shear and tensile properties of interfaces, respectively. In order to extend the results of simple small supercell and single interface interaction we simulated to numerous crosslinks at a larger scale, we assume that the system has a low concentration of functional groups and interlayer crosslinks so that the cooperative effect resulting from the interactions between neighboring crosslinks can be neglected. Hence, based on the low cross-linking density assumption, the tensile strength, σcr, and the shear strength, τs, of specific interfaces can be described as:53

 cr  Fcrz  f ,

(4)

 s  Fcrs  f ,

(5)

where Fcrz ( Fcrs ) is the maximum tensile (shear) force required to rupture the interface interaction and f is the area of each carbon atom in the graphene. ϕ = NI/NC is defined as the concentration of interface interactions, where NI (NC) is the number of interface 16


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interactions (carbon atoms in the pristine graphene). Given these two equations, the values of σcr and τs depend on the interaction concentration ϕ. As ϕ differs in various interaction forms, to unify tensile and shear strength for different interfaces, σcr and τs can be further normalized into:

 N  N IN Fcrz Agra ,

(6)

 N  N IN Fcrs Agra ,

(7)

N IN = Aact Agra  N act N C ,

(8)

where N IN denotes the normalized number of interface interactions, Agra is the whole xy-plane area of graphene cell and Aact (Nact) is the area (the atom number) of the acting range of interface crosslinks. The determination of specific

N IN

for different

interfaces is shown in Figure S18. In the graphene substrate, carbon atoms at a projected distance within 3 Å away from the –OH group and the absorbed molecule in the xyplane are regarded as regions affected by the crosslinking. In other words, this criterion ensures an interception (at least 6 Å) between crosslinking periodic images, which would well satisfy our low concentration assumption. Finally, the normalized interfacial performances of specific interfaces with varying numbers of –OH or –NH2 groups are presented in Figure 7, unfolding a synergistic promotion in the mechanical performance wherein the –OH group mainly contributes to the tensile strength while the –NH2 group contributes to the shear strength. The melamine–graphene interface (3 –NH2; 0 –OH), without –OH formed crosslinks, still exhibits a similar shear performance, ~100 MPa, to that of previously reported GO– water–GO interfaces in the experiment54, due to the important NH2–π interaction mentioned above. When the number of –NH2 groups is given (melamine molecule), once an –OH group is introduced into the interface, the existence of hydrogen bonding brings a sharp jump to ~450 MPa in shear strength, echoing the great enhancement in adsorption energy brought by the chemical cooperativity in Figure 4(a), while the subsequent introduction of –OH hydrogen bondings (the red arrow) instead leads to a main growth of tensile strength from ~300 to ~550 MPa. Then when the number of – 17



OH groups is given, the tensile strength of different interfaces does not deviate significantly (~50 MPa) from each other with increasing –NH2 groups, whereas the shear performance has an obvious rise from ~280 to ~450 MPa, as schematically indicated by the blue arrow in Figure 7.

It should be remarked here that the previous experiment54 did not give the definite tensile property of GO–water–GO interface, only some data of molecular dynamics (MD) simulations provided. And these MD data based on ReaxFF force field are found to inaccurately overestimate the strength of interface54, which can also be observed from the large shear strength deviation of MD simulation region (pale green) from the experimental region (pale purple) in Figure 7. Nevertheless, the rupture energy, or the adhesion energy, quantified by the integration through the stress–displacement curve, can be regarded as an alternative to represent the tensile behavior (or adhesion performance) of the interface. The normalized adhesion energy of interfaces we obtained is found at a similar range, 0.1–0.2 J/m2 [Figure S19], to the experimental measurement on common GO–water–GO interface54. In other words, the triazine derivative–GO interface here shares sizable adhesion performance with the GO–water– GO interface but provides astonishing shear strength, which is attributed to both the chemical cooperativity and the mechanical synergy between NH2–π interaction and – OH hydrogen bonding. Additionally, limited by the scale of our model, the extended results discussed here are based on the low concentration assumption, wherein the chemical cooperativities between different crosslinks are excluded. Chemical cooperativity under high concentration, comparing to case in low concentration, is of great importance but obviously more complex. Thus the basic investigations in this work focusing on the chemical cooperativity and mechanical synergy implicated in triazine derivative–GO interfaces are expected to be a reference for exploring the complete GO–triazine derivative–GO interfaces at much larger scale.

CONCLUSION In summary, by performing first-principle calculations, we reveal the origin of 18


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superstrong melamine–GO interface at atomistic scale for the first time and investigate the intrinsic relevance between the chemical interaction and the mechanical behavior. Via combined use of the charge density distribution and NCI index analysis, the chemical cooperativity enhancing O–H···N HB formed by the triazine moiety of melamine is found responsible for this strong noncovalent interface. The NH2–π interaction, although as weak as the vdW interaction when melamine is absorbed upon GO, plays an important cooperating role in polarizing the triazine ring and the GO surface and hence further reinforcing the O–H···N HB. Following static pulling simulations are performed to unravel the influence of different noncovalent interactions on the mechanical performance of interface. The dominant O–H···N HBs can indeed yield an amazing rupture force exceeding 1 nN for the noncovalent interface, confirming this strongest noncovalent interaction reported so far. The ubiquitous HB switching phenomena in the triazine derivative–GO interface, brought by the –NH2 group, further strengthen the noncovalent interface through dissipating more rupture energy. Moreover, based on the low concentration assumption, the simulation results in this work are normalized to predict mechanical performance of interface at larger scale. The triazine derivative–GO interface manifests a sizable adhesion performance with the GO–water–GO interface but provide an astonishingly higher shear strength, which is attributed to both the chemical cooperativity and the mechanical synergy. The –OH hydrogen bonding mainly augments the adhesion strength of interface, while the –NH2 group, cooperating with the –OH hydrogen bonding and on the other hand interacting with the GO surface, greatly improves the shear performance of interface. Natural structural materials always possess precise-controlled structural features with characteristic dimensions spanning from the nanoscale to the macroscale, endowing human with enormous room to invent biomimetic materials with hierarchical designs and tailored properties.55 Interface is one of the most important and ubiquitous structural features.8 Noncovalent interactions, due to the features of breaking and reforming, as well as the potential mechanisms of cooperativity and anticooperativity, play important roles in the performance and design of interface-governed materials and 19



devices.11,12 Overall this study significantly deepens the understanding on the chemomechanical behaviors within the interface dominated by noncovalent interactions and is expected to promote the adoption of new noncovalent interfaces in designing highperformance graphene-based artificial nacreous materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Comparisons of different DFT approximations and vdW correction schemes, Sampling information of configurations in Figure 1, Optimized structures and energies of different melamine@GO system and the melamine@graphene system, Bader charge analysis on the hydrogen donor and acceptor of O–H···N HB, density of states of the melamine@GO system and pure GO, charge transfer and RDG analysis of triazine-derivative@GO systems, details of structure, force and energy in static pulling simulations, NCI index analysis on the HB switching, determination of specific N IN for different interfaces, and comparison of adhesion energy with the experiment (PDF)

AUTHOR INFORMATION Corresponding Author * [email protected] (H.A. Wu) * [email protected] (Y.B. Zhu)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS 20


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We are very grateful to Prof. Changzheng Wu for helpful discussions. This work was jointly supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB22040402), the National Natural Science Foundation of China (11525211, 11872063, 11772319, 11572307, and 11802302), the National Postdoctoral Program for Innovative Talents (BX201700225), the Fundamental Research Funds for the Central Universities (WK2090050040, WK2090050043), and the Anhui Provincial Natural Science Foundation (1808085QA07). The numerical calculations have been done on the supercomputing system in the Supercomputing Center of University of Science and Technology of China.

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Figure 1. Diagram of the adsorption energy versus the HB length for different melamine@GO and melamine@graphene systems. The HB length here refers to the distance between the electron donor and acceptor. For the melamine@graphene system, the hydrogen bond length is replaced by the interlayer distance. Optimized structures for melamine@GO and melamine@graphene systems with various binding sites and adsorption states are plotted in Figure S2–S5. The GO–O···NH2–M represents the adsorption state wherein the epoxy group of GO forms an N–H···O HB with the –NH2 group of melamine [Figure S2]. The GO–OH···NH2–M refers to the adsorption state wherein the hydroxyl group of GO forms an N–H···O or an O–H···N HB with the – NH2 group [Figure S3]. In the same way, the GO–OH···N–Tri denotes the adsorption state wherein the hydroxyl group of GO forms an O–H···N HB with the nitrogen atom in the triazine moiety of melamine [Figure S4]. The blue open triangle symbols denote two special GO–OH···N–Tri configurations that melamine standing aslant upon the GO surface with no π–π interactions [Figure S4(b) and S4(f)].

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Figure 2. The differential charge density of a typical melamine@GO system in the adsorption state of GO–OH···N–Tri. The green region indicates electron depletion, while orange region indicates electron accumulation. Among those regions, the darker colored solid region refers to high charge density, while lighter colored semitransparent region refers to low charge density. The inset in the top left corner displays the top view of the differential charge density plot. Atom colours: carbon (C) in grey; hydrogen (H) in white; oxygen (O) in red; nitrogen (N) in blue.

29



Figure 3. Plots of the reduced density gradient (RDG, s) versus the electron density (ρ) multiplied by the sign of the second Hessian eigenvalue (sign(λ2)) (a) and the corresponding NCI isosurfaces (b–e) for representative melamine@GO (or water@GO) configurations in varying adsorption states. Only the intermolecular interactions are plotted (sign(λ2)ρ < 0), with the intramolecular interactions screened out (sign(λ2)ρ > 0). The low-gradient spikes in (a) hence become a reliable indicator of the strength of attractive interactions. The corresponding isosurface plot is generated by using the s = 0.5 isosurface. Values of sign(λ2)ρ are color-mapped onto the s-isosurface with the color scale from -0.05 to 0.00 (positive values are excluded). The red star marks the O–H···N HB (b) in the GO–OH···N–Tri adsorption state. The green circle and purple triangle mark the two sub-states, i.e., the O–H···NH2 (c) and the N–H···O (e) HB, in the GO– OH···NH2–M adsorption state, respectively. The HB in the GO–O···NH2–M state is not compared here due to the extremely weak strength indicated in Figure 1. The 30


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noncovalent interactions between water molecule and GO surface are also given in (d) for comparison (marked by blue rhombus). Atom colours: C, grey; H, white; O, red; N, blue.

31



Figure 4. Comparisons of adsorption, HB strength and charge transfer of triazine derivatives with different numbers of –NH2 groups in the adsorption state of GO– OH···N–Tri. (a) Plot of the adsorption energy of the triazine-derivative@GO system versus the number of –NH2 groups. The case for the triazine-derivative@graphene system is also given for comparison. The dashed line denotes the predicted energy curve taking no account of the chemical cooperativity or anticooperativity (following the additive scheme of covalent interactions). The blue region highlights the actually large deviation from this prediction. (b) Plots of the low-gradient spikes in RDG plot (Figure S9) and the HB distance versus the number of –NH2 groups. The green line is the linear fit to the bond distance variation. (c)–(f) Comparisons of the planar averaged electron density along the z-direction for triazine-derivative@GO configurations with increasing number of –NH2 groups, see also Figure S8 for all structures considered. Positive values (red) indicate electron accumulation and negative values (blue) represent electron depletion. 32


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Figure 5. (a) Schematic plot of static pulling simulations of triazine-derivative@GO: the terminal nitrogen N* in the –NH2 group was constrained and vertically pulled away from the GO surface, in steps of ΔH = 0.02 nm. (b) Rupture force as a function of the number of –NH2 groups. The dark blue solid/dashed line is the linear tendency to the force–group number relationship. (c) Plot of the rupture force versus the number of – OH groups in GO. All rupture forces summarized here are the resultant forces of target atoms suffered from the pulling process. Three configurations [Figure S10] with different binding states were used to assess the error of melamine in (b). The error bars in (c) are introduced by adjusting the loading step, ΔH, from 0.005 to 0.02 nm (to simulate the effect of loading rate).

33



Figure 6. Static pulling process of the typical melamine@GO system. (a) The resultant force of N* suffered from the separation as a function of the elevation displacement. The distance between nitrogen atom in triazine ring and the oxygen atom in –OH (Tri– N···O) and the distance between –NH2 group and graphene surface (NH2···Gra) are given as assistance to understand the whole process. (b)–(e) Snapshots of the reconstructions at different stages. Three stages subdivide the pulling process: stage I dominated by π–π interaction, stage II dominated by O–H···N hydrogen bonding and stage III dominated by NH2–π interaction.

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Figure 7. Plot of the normalized shear strength (lateral sliding) versus the normalized tensile strength (longitudinal raising) of interfaces with varying number of –NH2 and – OH groups. The referenced shear strength range of GO–water–GO interface54 is highlighted in the pale green (simulation) and pale purple (experiment) region. The schematic blue and red arrows indicate the synergistic promotion between NH2–π interaction and –OH hydrogen bonding in improving the mechanical performance of interfaces.

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For Table of Contents Only

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Figure 1. Diagram of the adsorption energy versus the HB length for different melamine@GO and melamine@graphene systems. The HB length here refers to the distance between the electron donor and acceptor. For the melamine@graphene system, the hydrogen bond length is replaced by the interlayer distance. Optimized structures for melamine@GO and melamine@graphene systems with various binding sites and adsorption states are plotted in Figure S2–S5. The GO–O···NH2–M represents the adsorption state wherein the epoxy group of GO forms an N–H···O HB with the –NH2 group of melamine [Figure S2]. The GO–OH···NH2–M refers to the adsorption state wherein the hydroxyl group of GO forms an N–H···O or an O– H···N HB with the –NH2 group [Figure S3]. In the same way, the GO–OH···N–Tri denotes the adsorption state wherein the hydroxyl group of GO forms an O–H···N HB with the nitrogen atom in the triazine moiety of melamine [Figure S4]. The blue open triangle symbols denote two special GO–OH···N–Tri configurations that melamine standing aslant upon the GO surface with no π–π interactions [Figure S4(b) and S4(f)]. 85x69mm (300 x 300 DPI)



Figure 2. The differential charge density of a typical melamine@GO system in the adsorption state of GO– OH···N–Tri. The green region indicates electron depletion, while orange region indicates electron accumulation. Among those regions, the darker colored solid region refers to high charge density, while lighter colored semitransparent region refers to low charge density. The inset in the top left corner displays the top view of the differential charge density plot. Atom colours: carbon (C) in grey; hydrogen (H) in white; oxygen (O) in red; nitrogen (N) in blue. 85x53mm (300 x 300 DPI)


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Figure 3. Plots of the reduced density gradient (RDG, s) versus the electron density (ρ) multiplied by the sign of the second Hessian eigenvalue (sign(λ2)) (a) and the corresponding NCI isosurfaces (b–e) for representative melamine@GO (or water@GO) configurations in varying adsorption states. Only the intermolecular interactions are plotted (sign(λ2)ρ < 0), with the intramolecular interactions screened out (sign(λ2)ρ > 0). The low-gradient spikes in (a) hence become a reliable indicator of the strength of attractive interactions. The corresponding isosurface plot is generated by using the s = 0.5 isosurface. Values of sign(λ2)ρ are color-mapped onto the s-isosurface with the color scale from -0.05 to 0.00 (positive values are excluded). The red star marks the O–H···N HB (b) in the GO–OH···N–Tri adsorption state. The green circle and purple triangle mark the two sub-states, i.e., the O–H···NH2 (c) and the N–H···O (e) HB, in the GO–OH···NH2–M adsorption state, respectively. The HB in the GO–O···NH2–M state is not compared here due to the extremely weak strength indicated in Figure 1. The noncovalent interactions between water molecule and GO surface are also given in (d) for comparison (marked by blue rhombus). Atom colours: C, grey; H, white; O, red; N, blue. 170x141mm (300 x 300 DPI)



Figure 4. Comparisons of adsorption, HB strength and charge transfer of triazine derivatives with different numbers of –NH2 groups in the adsorption state of GO–OH···N–Tri. (a) Plot of the adsorption energy of the triazine-derivative@GO system versus the number of –NH2 groups. The case for the triazinederivative@graphene system is also given for comparison. The dashed line denotes the predicted energy curve taking no account of the chemical cooperativity or anticooperativity (following the additive scheme of covalent interactions). The blue region highlights the actually large deviation from this prediction. (b) Plots of the low-gradient spikes in RDG plot (Figure S9) and the HB distance versus the number of –NH2 groups. The green line is the linear fit to the bond distance variation. (c)–(f) Comparisons of the planar averaged electron density along the z-direction for triazine-derivative@GO configurations with increasing number of – NH2 groups, see also Figure S8 for all structures considered. Positive values (red) indicate electron accumulation and negative values (blue) represent electron depletion. 170x135mm (300 x 300 DPI)


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Figure 5. (a) Schematic plot of static pulling simulations of triazine-derivative@GO: the terminal nitrogen N* in the –NH2 group was constrained and vertically pulled away from the GO surface, in steps of ΔH = 0.02 nm. (b) Rupture force as a function of the number of –NH2 groups. The dark blue solid/dashed line is the linear tendency to the force–group number relationship. (c) Plot of the rupture force versus the number of – OH groups in GO. All rupture forces summarized here are the resultant forces of target atoms suffered from the pulling process. Three configurations [Figure S10] with different binding states were used to assess the error of melamine in (b). The error bars in (c) are introduced by adjusting the loading step, ΔH, from 0.005 to 0.02 nm (to simulate the effect of loading rate). 170x73mm (300 x 300 DPI)



Figure 6. Static pulling process of the typical melamine@GO system. (a) The resultant force of N* suffered from the separation as a function of the elevation displacement. The distance between nitrogen atom in triazine ring and the oxygen atom in –OH (Tri–N···O) and the distance between –NH2 group and graphene surface (NH2···Gra) are given as assistance to understand the whole process. (b)–(e) Snapshots of the reconstructions at different stages. Three stages subdivide the pulling process: stage I dominated by π–π interaction, stage II dominated by O–H···N hydrogen bonding and stage III dominated by NH2–π interaction. 170x131mm (300 x 300 DPI)


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Figure 7. Plot of the normalized shear strength (lateral sliding) versus the normalized tensile strength (longitudinal raising) of interfaces with varying number of –NH2 and –OH groups. The referenced shear

strength range of GO–water–GO interface54 is highlighted in the pale green (simulation) and pale purple (experiment) region. The schematic blue and red arrows indicate the synergistic promotion between NH2–π interaction and –OH hydrogen bonding in improving the mechanical performance of interfaces. 170x136mm (300 x 300 DPI)



For Table of Contents Only 82x37mm (300 x 300 DPI)


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Superstrong Noncovalent Interface between Melamine and Graphene

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