Tunable, Multifunctional Ceramic Composites via Intercalation of

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Tunable, Multifunctional Ceramic Composites via Intercalation of Fused Graphene Boron Nitride Nanosheets Ehsan Hosseini, Mohammad Zakertabrizi, Asghar Habibnejad Korayem, and Rouzbeh Shahsavari ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Tunable, Multifunctional Ceramic Composites via Intercalation of Fused Graphene Boron Nitride Nanosheets Ehsan Hosseini†, Mohammad Zakertabrizi†, Asghar Habibnejad Korayem*,†,‡, Rouzbeh Shahsavari*,∥,&

†School

of Civil Engineering, Iran University of Science and Technology, Tehran, Iran

‡Department ∥Department

of Civil Engineering, Monash University Melbourne, VIC 3800, Australia

of Civil and Environmental Engineering, Rice University, Houston, Texas 77005, &C-Crete

Technologies LLC, Stafford, TX 77477

*Corresponding authors: [email protected] and [email protected]

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Abstract Ternary 2D materials such as fused graphene-boron nitride (GBN) nanosheets exhibit attractive physics and tunable properties far beyond their parent structures. While these features impart several multifunctional properties in various matrices, a fundamental understanding on the nature of the interfacial interactions of these ternary 2D materials with host matrices and the role of their individual components has been elusive. Herein, we focus on intercalated GBN/ceramic composites as a model system and perform a series of density functional theory calculations to fill this knowledge gap. Propelled by more polarity and negative Gibbs free energy, our results demonstrate that GBN is more water-soluble than graphene and hexagonal boron nitride (h-BN), making it a preferred choice for slurry preparation and resultant intercalations. Further, a chief attribute of the intercalated GBN/ceramic is formation of covalent C-O and B-O bonds between the two structures, changing the hybridization of GBN from sp2 to sp3. This change, combined with the electron release in the vicinity of the interfacial regions, lead to several non-intuitive mechanical and electrical alterations of the composite such as exhibiting higher young’s modulus, strength and ductility as well as sharp decline in the band gap. As a limiting case, though both tobermorite ceramic and h-BN are wide bandgap materials, their intercalated composite becomes a p-type semiconductor, contrary to intuition. These multifunctional features, along with our fundamental electronic descriptions of the origin of property change, provide key guidelines for synthesizing next generation of multifunctional bi-layer ceramics with remarkable properties on demand.

Keywords: Graphene-boron nitride; Ceramic composites; Intercalation; Electrical properties; Mechanical Properties


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1 Introduction In the field of composites, which includes ceramic composites, the use of graphene and hexagonal boron nitride (h-BN) has been mainly discussed in mechanical terms. Recent progresses in graphene and h-BN synthesis and the introduction of graphene oxide (GO) have paved the way for the new generation of ceramic nanocomposites

1-2.

Recently, a study by

Shahsavari et al. 3-4 demonstrated the mechanical properties of intercalated h-BN/tobermorite –as a representative ceramic composites- including the compressive and tensile strengths, stiffness, strain and toughness as well as deformation mechanisms of the aforesaid ceramic composites. Other groups have also investigated the dispersion of h-BN in cement-based alkaline environments and show that an addition of 0.003 wt.% h-BN increased the tensile and compressive strength by 8 and 13%, respectively 5. Or another group 6 used 0.03% wt.% GO in cement to successfully enhance its mechanical properties. A key concern in graphene-based ceramic composites is typically the inability of graphene to homogenously disperse in the composites; i.e., the agglomeration of graphene in the water and/or slurry matrix makes it impossible to fully utilize its properties in the composites 7-8 . To address this problem, graphene is chemically oxidized to produce GO. This process reduces the mechanical and electrical properties of the graphene by transforming a proportion of the sp2 hybridization to sp3 9. Similar procedures are performed on h-BN in order to introduce some changes to its mechanical and electrical properties

10.

The functional groups attached to the

graphene nanosheet (as a result of the oxidation) replace the carbon as the main sources of interactions between graphene and the surrounding matrix, thus impacting significantly the electrical properties of the composites 11. In this work, we present a novel approach to predict the necessary dispersions (and thus properties) in a ceramic matrix (cement) by using a ternary 3 ACS Paragon Plus Environment


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hybrid of graphene and hexagonal boron nitride (GBN). This improved dispersion is possible via low polarity of h-BN which provides acceptable covalent bondings with the silicate chains in the matrix 3. The rare electronic and mechanical properties associated with the monolayer hybrid of GBN, such as variable band gap

12,

magnetism

13

and Young's modulus

14

have attracted much

attention in this unique ternary nanosheet 15-16. GBN can be produced from a fusion of graphene and h-BN

17.

h-BN has a similar hexagonal structure to graphene but with an alternating

composition of boron and nitrogen, instead of carbon atoms. In the ternary hybrid GBN, while graphene offers several desirable properties such as zero-gap semiconductivity applicable to many technological applications

18,

h-BN portrays significant thermodynamic and chemical

stability and strong mechanical properties while electrically insulating 19. Thus, the combination of the two enables tunable complementary properties. The hexagonal structural similarities between the h-BN and graphene facilitate creating such hybrid structures with tunable ratios of carbon to boron and nitrogen stoichiometry

20.

Indeed, two-dimensional hybrid GBN is now a

primary research topic due to possessing the best of the two materials including significant intrinsic strength and tunable electronic structure. For example, reinforcing GBN filler in polymer composites

21-22

could give rise to exotic multifunctional properties. Similarly, the

intercalation of such GBN in cements and ceramics can lead to infrastructure materials with multiple desired properties including enhanced mechanical properties and self-sensing ability 23. In this context, given that concrete is the most widely used construction material , such multifunctional GBN fillers can demonstrate widespread impact in infrastructure materials. However, effective engineering of such multifunctional infrastructure materials hinges on fundamental understanding, identification of limiting mechanisms, and tracing their lineage into 4 ACS Paragon Plus Environment

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the atomic and subatomic properties, which are all poorly understood 24. Herein, we focus on a series of GBN-ceramic composites and study their electrical and mechanical properties using density function theory (DFT) calculations. Tobermorite 11 Å with the chemical formulation of Ca4Si6O15(OH)2(H2O)5 is used as a model system representing the crystalline structure of calcium silicate hydrate (C-S-H), the main product of cement hydration . Next, the direct effects of intercalation of graphene, GBN and h-BN on the electronic and mechanical properties of multi-layer C-S-H is investigated while considering different proportions of carbon and boron nitride in GBN. The results provide fundamental insights on the application of GBN in ceramics, with several implications on next generation smart materials with inherent self-sensing, stability, and high mechanical properties.

2 Computational methods 2.1 General DFT approach and structural models All total-energy calculations were conducted using DFT through the Cambridge Serial Total Energy Package (CASTEP)

25.

To calculate the distribution of electron density between atoms,

Perdew-Burke-Ernzerhof generalized-gradient approximation (GGA-PBE) was used energy cutoff was set to 400 eV

27

26.

The

for the plane-wave basis and the Brillouin zone integration

was performed on a regular mesh of 6×6×1 k-points . The full atomic relaxation was achieved when all remaining forces were below 0.01 eV Å-1. London dispersion interactions were added to the total bonding energy as proposed by Grimme in the DFT-D method for PBE functional 28. In order to calculate the electrical and mechanical properties of the unit cell of nanosheet/tobermorite

composites,

the

dimensions

of

relaxed

crystal

structure

of

nanosheet/tobermorite in a and b lengths were chosen 6.95 and 7.38 Å, respectively. The angle of 123° in monoclinic tobermorite closely imitates the γ angles of h-BN, GBN and graphene, i.e.



~ 120° 3. This similar geometric pattern imparts great compatibility in the structures of nanosheet and tobermorite during cell optimization, which is then followed by the formation of a stable dual-layer crystal. More details including optimized lattice parameters are discussed in the supplementary information. Figure 1 shows the studied models of the bi-layer composites in accordance with the different ratios of carbon to boron and nitrogen atoms. In the original model, 18-atom h-BN nanosheet was confined by the 84-atom tobermorite 11 Å. Next, h-BN atoms were replaced with carbon atoms as needed. GBN models are named using a subscript notation, GBNi-j, where i and j describe the share of each nanosheet in the 18-atom hybrid; i is the number of atoms from graphene structure and j is the share of h-BN atoms. For example GBN6-12 denotes 6 graphene (carbon) atoms and 12 h-BN atoms. In addition, nanosheet-tobermorite composite model is included in Fig S1 of the Supplementary file

Figure 1 The creation pattern of hybrid bi-layer nanosheet/tobermorite


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2.2 Solubility calculations The solubility of the desired 2D platelets was investigated by determining the solvation energy of GBN and comparing it with those in h-BN and graphene. DFT calculations were conducted using the b3LYP hybrid functional; the implicit water environment was accounted for by the conductor-like model scheme. Dispersion correction of Tkatchenko and Scheffler (TS) scheme was chosen for b3LYP functional

29.

Vibrational analysis calculations were used to compute

important thermodynamic properties such as enthalpy (H), entropy (S), free energy (G) and heat capacity at constant pressure (Cp) as functions of temperature. Using the obtained values for entropy of graphene, GBN and h-BN, free energy at T = 298.15 K was calculated by: = ∆𝐻298.15𝐾 ― 𝑇∆𝑆298.15𝐾 ∆𝐺298.15𝐾 𝑒𝑥𝑝 𝑒𝑥𝑝 𝑒𝑥𝑝

(1)

Gibbs free energy of solvation, or ∆∆𝐺𝑠𝑜𝑙𝑣 𝑎𝑑𝑠 , which is the difference between Gibbs free energy in the secondary and the primary mediums was calculated for each nanosheet; the change in this energy shows relative solubility 30. 𝑤𝑎𝑡𝑒𝑟 𝑎𝑖𝑟 ∆∆𝐺𝑠𝑜𝑙𝑣 𝑎𝑑𝑠 = ∆𝐺𝑎𝑑𝑠 ― ∆𝐺𝑎𝑑𝑠

(2)

3 Results and Discussion 3.1 Solubility of h-BN, GBN and graphene in water Table 1 lists the Gibbs free energy of solvation for each nanosheet in water; higher values shows less tendency towards aqueous solvation. Thus, according to Table 1, the least ∆∆𝐺𝑠𝑜𝑙𝑣 𝑎𝑑𝑠 value belongs to GBN, which highlights the highest solvation tendency, followed by h-BN and graphene, respectively. While the hydrophobicity of graphene and h-BN is already established 31, 7 ACS Paragon Plus Environment


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it is shown here in terms of the considerable difference in Gibbs energy when compared to the other materials 32. Table 1 Predicted values of Gibbs free energy of solvation for graphene, GBN and h-BN in water ∆∆𝑮𝒔𝒐𝒍𝒗 𝒂𝒅𝒔 (kJ/mol) 0.33 -1.10 -5.24

Nanosheet Graphene h-BN GBN

Table 2 shows the dipole moment of the studied nanoplatelets. The charge difference between boron and nitrogen atoms in h-BN ensures a greater dipole moment than the homogeneous graphene

33.

However, the largest dipole moment belongs to the GBN, demonstrating a

considerable margin compared to h-BN and graphene 34-36. The formation of B-C and N-C bonds alters the charge density distribution on the surface of GBN, causing a relatively strong polarity. Total dipole moment value for GBN is significantly larger than its counterparts, which can be translated into a better dispersion in polar solvents including water. As for the different fusion patterns, dipole moment values are brought in Fig S2 and Table S2 in the supplementary file. To summarize, GBN shows more polarity than graphene and h-BN. Table 2 Calculated dipole moments for graphene, GBN and h-BN

Dipole moment Contribution (debye) X

Graphene

GBN

h-BN

-0.000021

-0.737384

0.043142

Y

-0.000001

1.276078

-0.001815

Z

0.001512

-0.035911

0.006239

Total dipole moment

0.00151

1.47425

0.04363


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Conductor-like screening model, or COSMO, is used to consider solvation effects for quantum calculations. It demonstrates the charge distribution on solute when it is in the solvent medium. Sigma profile on the other hand depicts this profile calculated over all components of the said molecule and displayed against screening charge density 37. We performed Cosmo and Sigma profile analyses to determine the distribution of valence electron, or the distribution of surface charge on the molecule, which is an influential factor in the solvation of this molecule in aqueous environments. The results are illustrated in Figure 2. While the demonstrations for h-BN and graphene are verified through previous works

31,

the

GBN profile is illustrated for the first time using COSMO analysis. In view of Figure 2c, GBN is more polarized in terms of electron distribution, which may provide a higher solvation ability in aqueous solutions such as water, which is itself a polar molecule. To further examine this subject, it is worth looking at the electron density of graphene and h-BN in Figure 2a-b. Judging from the color intensity, the polarity is more significant in hBN, with the electron distribution more leaning towards nitrogen atoms over the whole surface, whereas in the graphene, the structure is more homogenous in terms of charge, with the internal carbon atoms amassing negative charge, as opposed to the marginal hydrogen atoms with positive charges. Figure 2d describes the sigma profile of the molecules in aqueous environment. All of the models demonstrate a similar shape, a small peak in the left at approximately -0.006 with the other peak close to zero, at the right side of the diagram. Graphene profile does not reach the hbond regions on both sides, signaling a hydrophobic behavior. In the right side, h-BN shows similar behavior to the graphene. However, h-BN profile reaches the h-bond receptor region at the left side, making it more solvable than graphene. It can be attributed to the distribution of 9 ACS Paragon Plus Environment


charge density over h-BN, with the hydrogen atoms on the edges and boron atoms on the surface, both being electrophile species and providing a desirable surface for the electrons. GBN profile is extended from about -0.015 to 0.14, covering more ranges than h-BN and graphene. This indicates GBN has the potential to form better dispersion in water compared to graphene an hBN. Essentially, GBN performs similar to h-BN in the left side of the diagram whereas for the right side, it desires interaction with the positive pole of water (hydrogen atoms) where it reaches 0.014. However, no significant peak is present in the right side, which denies absolute hydrophilic behavior for GBN. Therefore, it can only be stated that GBN is more likely to show a better aqueous dispersion than graphene and h-BN. Overall, GBN shows higher potential for an aqueous solution comparing to both graphene and h-BN, making it the preferred choice for mixing with cement slurry and potential intercalations described next.


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Figure 2 (a-c) Electronic density of the different compounds (the red color and blue one are typical for a region acceptor of positive and negative partial electrostatic charges of water molecule, respectively), (d) the sigma profiles of graphene, GBN, and h-BN by COSMO

3.2 Absorption, bonding and charge transfer Figure 3 shows three 18-atom nanosheets of GBN9-9, graphene and h-BN confined in tobermorite. As an example, Figure 3a demonstrates the structure before absorption (optimization) of the GBN9-9 by tobermorite where the main forces present are van der Waals forces. The separation distance is 3.15 Å on each side

38.

Figure 3c-d show the same structure,

GBN9-9/tobermorite, after optimization where oxygen atoms act as the nucleophilic sites on the silicate chains attached to the GBN electrophilic sites. The other components of tobermorite, namely calcium ions, hydroxyl and water hold their positions relative to that of silicate chains, thus creating a nanosheet/tobermorite composite held together mostly by electrostatic forces.



During the absorption (optimization) phase, covalent bonds are formed between the oxygen atoms present in the silicate chains and the boron and carbon atoms in the nanosheets, reducing the separation distance to ~1.5 Å and subsequently releasing -1.16 eV and -1.41 eV of energy to form O-B and O-C bonds (Figure 3g). This development of the B-O and C-O covalent bonds can also be assessed from observing the differences in electron densities. Figures 3b and 3e show GBN9-9/tobermorite before and after optimization. The electron density shifts toward boron and carbon atoms, Figure 3h, indicating the creation of covalent bonds between tobermorite and GBN nanosheet. The transferred charge of silicate chains to the nanosheet can be seen clearly in the middle of Figure 3h, creating a monolith chain of GBN and tobermorite. The newly formed covalent bonds result in sp3 hybridization in the GBN, making the GBN electric charge gravitated to the negative side. On the graphene portion of GBN nanosheet, this covalent bond forms between carbon and the oxygen atoms of the silicate, whereas in the h-BN portion, between boron atoms and oxygen atoms of the silicate. This is also the case for intercalation of phase pure h-BN and graphene in tobermorite (Figures 3i and 3j) where the nucleophile oxygens on the silicate chains of tobermorite share electron with the electrophilic carbon and boron atoms on the graphene and h-BN nanosheets, forming covalent bonds between the two structures.


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Figure 3 (a-b) GBN9-9/tobermorite shown as the atomistic model and electron density, (c-d) covalent bonds between boron and carbon atoms in GBN9-9 with the oxygen atoms of the silicon tetrahedral, (e-f) electron density after bond formation, (g) adsorption energy between boron and carbon atoms with oxygen atoms, (h) charge density in pure GBN9-9/tobermorite, (i) charge density after bonding in hBN/tobermorite, and (j) charge density after bonding in graphene/tobermorite

To complement these findings, we determined the in-plane bond length distributions of each of the nanosheets (h-BN, graphene, GBN9-9) before and after absorption on tobermorite (Fig S3). The average (maximum) in-plane bond length of h-BN, graphene, GBN9-9 increased 2.7, 2.9, and 13 ACS Paragon Plus Environment


2.4% (7.5, 8.2, and 7.5%), respectively after absorption, due to transformation of the sp2 hybridization into sp3. While GBN demonstrated the most stable behavior as determined by the minimal changes in the ‘average’ bond length, the maximum bond lengths followed similar patterns akin to h-BN and graphene, clearly affecting the valence electron density of the composites. Figure 4 demonstrates the energy released when the nanosheet is absorbed by tobermorite. The least absorption energy belongs to the h-BN/tobermorite with -12 eV, and the most to graphene/tobermorite with -13.5 eV. Generally, by replacing the boron and nitrogen atoms with carbon, C-O bonds replace B-O bonds, becoming more structurally stable, consistent with Figure 3g. The formation of these covalent bonds with different degrees of release energy influences the electrical and mechanical properties of the nanocomposites, described next.

Figure 4 Adsorption energies of nanoparticles on tobermorite


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3.3 Electrical behavior 3.3.1

Conductivity

Figure 5a shows the status of Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) for the h-BN, GBN and graphene structures. The HOMO orbitals are negatively charged on nitrogen and carbon atoms in h-BN and GBN. LUMO orbitals contain positive or no charge on boron and carbon atoms in h-BN, GBN and graphene. Figure 5b highlights the differences between h-BN and the rest of the nanosheets, i.e. GBNi-j and graphene. The band gap – a key attribute of electrical properties – is 4.28 eV for h-BN, 0.0 eV for graphene, 0.3 eV for GBN6-12, 0.33 for GBN9-9 and 0.0 eV for GBN12-6, in line with previous works

12, 39-40.

Regarding the identical band gaps for graphene and GBN12-6, the following

deserve attention: Generally, the band gap decreases with the increase in the number of carbon atoms, as expected. However, other parameters such as structural type (armchair/zigzag) and connectivity and the location of carbon and nitrogen/boron atoms are also influential

12, 41.

Further, our employed DFT theory may not be as accurate as b3LYP; thus a 2-3% negligible error may be present here. Overall, these results show that with boron (and/or nitrogen) atoms, the structure moves towards a semiconductor behavior 12. As for the different fusion patterns of graphene and h-BN, bandgap states are brought in Fig S4 in the supplementary file.



Figure 5 (a) HOMO and LUMO for h-BN, GBN9-9 and graphene, (b) electron density of states for h-BN, GBN and graphene

To probe the effect of above nanosheets on the band gap of the tobermorite, Figure 6a-c illustrate the relaxed structures of h-BN/tobermorite, graphene/tobermorite and GBN9-9/tobermorite. While oxygen atoms are bonding covalently with boron atoms in h-BN, there are van der Waals interactions with nitrogen atoms in h-BN, at an atomic distance of 2.4 Å. However, covalent interactions with oxygen atoms of the tobermorite structure occur around 1.5 Å in GBN and graphene. Figure 6d depicts the band structure of fully insulator tobermorite 11 Å with the valence-band maximum (VBM) and the conduction band minimum (CBM) appearing at the G point. The electrically insulating behavior in this structure is well-known via several experiments and studies on cement and C-S-H (e.g., 42). Also Figure 6d represents partial density of states for tobermorite 11 Å, demonstrating the contribution of oxygen atoms, which are nucleophile, in HOMO region compared to the other species. This is in contrast to the calcium ions which have the greatest impact in the LUMO area due to the positive charge. The VBM located in the LUMO area is far from the Fermi level and the generated band gap is 4.35 eV. This band gap is a


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simple representation that transferring electrons from HOMO to LUMO region is near impossible without external interference. Compared to graphene, the lack of covalent bonding between nitrogen (not boron) and oxygens of silicate reduces the number of available sites on the surface of the h-BN. Simply put, the absorption of oxygen of the silicon tetrahedra on a graphene platelet does not have this limitation. Our calculations on h-BN/tobermorite show a band gap of ~ 2.61 eV (Figure 6e). This is somewhat non-intuitive. As discussed earlier, tobermorite and h-BN are both wide band gap materials with 4.35 eV and 4.28 eV band gaps, respectively. However, their intercalated composite demonstrates a bandgap that is almost half of those of parent materials. This is because of the covalent B-O bonds in the h-BN/tobermorite. More precisely, the contributing factor is the decrease in the bond length and release of the electrons in the boundary between tobermorite and h-BN which is trespassed by the B-O bond. Akin to tobermorite, the CBM and VBM for h-BN/tobermorite are both located at the G point in Brillouin Zone. The hybrid graphene/tobermorite behaves much like a p-type semiconductor material (Figure 6f). Further analysis of the wave-function indicates that the VBM and CBM near FQ points are mainly provided by carbon pz orbitals, which is capable of creating a band gap of 0.43 eV for the graphene/tobermorite structure. The addition of graphene to tobermorite alters significantly its electrical behavior and produces a semiconductor ceramic. This interesting quality is via a decrease in the band gap, which results in a more feasible electron transfer from HOMO to LUMO orbitals. By changing the ratio of h-BN to graphene in the hybrid nanosheet, its electrical properties changes, allowing the creation of a structure with tailored electrical properties. Figure 6g-i show the band structures and electron density of states of composites made of tobermorite and GBN617 ACS Paragon Plus Environment


12,

GBN9-9 and GBN12-6. An interesting point in the electrical properties of GBN/tobermorite

composites is the dramatic change in the electrical properties by addition of GBN with minimal carbon content. The model with the least carbon, GBN6-12/tobermorite, has a direct-gap and is a semiconductor characterized by a band gap of 0.86 eV (from F point to Q point) as depicted in Figure 6g. Addition of GBN6-12 to the regular crystal lattice of tobermorite produces a semiconductor nanocomposite. This is because GBN contributes electrons with high energy levels to the GBN/tobermorite composite band gap, thereby electrons can be easily excited into the conduction band. Following this mechanism, an increase in the number of carbon atoms in the GBN structure moves the band gap of GBN/tobermorite composites closer towards the graphene/tobermorite composite band gap, as intuitively expected. The increase in the number of carbon atoms can transfer electrons from the highest-energy state in the VBM to the lowestenergy state in the CBM without a change in the GBN/tobermorite crystal. Therefore, the band gap in the GBN9-9/tobermorite structure and GBN12-6/tobermorite are reduced to 0.66 and 0.55 eV, respectively (Figure 6h-i).


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Figure 6 (a-c) Nanosheets/tobermorite model, (d) total and partial density of state for tobermorite 11 Å, (e) calculated band structures and electron density of states for h-BN/tobermorite, (f) calculated band structures and electron density of states for graphene/tobermorite, (g) calculated band structures and electron density of states for GBN6-12/tobermorite, (h) calculated band structures and electron density of states for GBN9-9/tobermorite, and (i) calculated band structures and electron density of states for GBN12-6/tobermorite

Figure 7 summarizes the band gap energies of the studied nanosheets/tobermorite composites with respect to number of carbon atoms in the nanosheet. The grey column shows the band gap



energy for the original tobermorite structure, which is an insulator. Adding h-BN nanosheet to this structure reduces the band gap energy to less than 3 eV, which is counterintuitive since h-BN itself is a high bandgap (~4.7 eV

43)

material. This observation is attributed to the B-O bonds

between the two structures; these bonds are formed by changing the sp2 into sp3 hybridization, diminishing the hold on valence electrons while increasing the length of the bonds. The released electrons is the cause of the reduction in the band gap. In these composites, VBM and CBM are at the G point in the Brillouin zone. Next, three carbon atoms replace BN atoms in the nanosheet, resulting in a 1.35 eV drop in the band gap energy. With further carbon replacements, CBM and VBM appear between F and Q points in the Brillouin zone, acting more like the graphene/tobermorite composites.

Figure 7 Band gap energy versus the number of carbon atoms in GBN of GBN/tobermorite composites


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3.4 Mechanical behavior and strain-dependent band gap 3.4.1

Elastic regime

First, we investigated the Young’s modulus of tobermorite composites intercalated by h-BN, GBN6-12, GBN9-9, GBN12-6 and graphene (Table 3). To obtain average elastic properties of this low symmetry bi-layer crystals, we initially constructed the triclinic elastic tensors of these crystals, following the procedures described in the supporting information and 44. Next, we used conventional Voigt, Reuss and Hill (VRH) approach

45-46

to determine the lower and upper

bounds for the average shear modulus (G) and average bulk modulus (K). Having these values, the equivalent isotropic Young’s modulus (E) and the Poisson’s ratio (𝜈) was estimated via VRH approximation

46.

Table 3 shows that the presence of h-BN, GBN and graphene in tobermorite

enhances significantly the elastic properties of the composites. The Young's modulus of tobermorite is 84.82 GPa, which increases by 32%, 37%, 39%, 44%, and 50% via intercalation of h-BN, GBN6-12, GBN9-9, GBN12-6 and graphene respectively. Table 3 Mechanical properties of tobermorite and nanosheet-reinforced tobermorite T11 T11 + h-BN T11 + GBN6-12

KVRH (GPa) 63.45 95.42 93.49

T11 + GBN9-9 94.47 T11 + GBN12-6 98.55 T11 + Graphene 100.89 T11: tobermorite 11 Å

GVRH (GPa) 33.21 43.17 45.14 45.58 47.42 49.34

Young's modulus (GPa) 84.82 112.53 116.64 117.79 122.59 127.27

Poisson's ratio 0.277 0.303 0.292 0.292 0.292 0.289

This improvement traces back to the strong covalent bonding discussed earlier. Moreover, these intermittent covalent bondings hinged on repetitive C and/or B atoms induce several weak local buckling (deformation) in the h-BN, GBN and graphene sheets. The formation of B-O and C-O bonds at certain spots imposed by periodicity of silicates in tobermorite leads to wrinkling in the nanosheets 3. This geometrical deformation, combined with the changes hybridization from sp2 21 ACS Paragon Plus Environment


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to sp3, weakens the nanosheet structure, yet the remaining C-C and B-N bonds are still sufficient to provide significant reinforcement for the tobermorite matrix (Table 3). 3.4.2

Inelastic regime

The main task of the GBN nanosheets fillers is typically introducing tunable electrical properties while mechanically reinforcing the ceramics. Thus, it will be interesting to determine the straindependent band gaps. Figure 8a compares the stress-strain diagrams for the pristine tobermorite and the GBN9-9/tobermorite composites, as a representative model. Tensile strain is imposed along the out-of-plane direction, which is the most vulnerable mechanical direction

47.

The

maximum stress is 5.76 GPa at 0.175 strain. Figure 8b clearly marks the interaction zone as the weak link in the structure (red clouds). As the strain accumulates, two half cells of pristine tobermorite drift apart which weakens the electrostatic interactions, replacing them with van der Waals forces. In contrast, in the GBN9-9/tobermorite composites, GBN9-9 fills this vulnerable gap between the two half cells. For this particular composite, the stress strain plot shows a higher slope (thus higher Young’s modulus) as well as a higher maximum stress of 6.58 GPa at 10% strain. The higher Young’s modulus and strength is attributed to the formation of multiple B-O and C-O bonds between the two structures, connecting more coherently the two half cells of tobermorite. While the addition of nanosheet enhances the interactions between the upper and lower cells of the tobermorite, the rupture occur in these newly formed C-O and B-O bonds which are the only main bridges between the upper and lower half cells (Figure 8c). Figure 8d shows the bandgap of pristine tobermorirte and GBN9-9/tobermorite calculated at the different strains. The overall pattern for both structures suggest that applying tensile strain increases the band gap. However, in the case of GBN9-9/tobermorite, an initial drop in the bandgap is observed at 5% strain, which 22 ACS Paragon Plus Environment

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can be attributed to the spot where the valence electrons are partially released from the calcium ions in the tobermorite structure but are still able to travel within the whole structure. This may imply that perhaps the best electrical performance of the composites is in the elastic zone from 0 to 5% strain. While the overall pattern of strain-dependent band gap is expected to hold true for other GBN/tobermorite structures, more studies are needed in the future to identify anomalies and potential synergies as noted above (e.g., 5% strain).

Figure 8 (a) Strain–stress curves of GBN9-9/tobermorite, (b) and (c) stress distribution and fracture configuration of tobermorite and GBN9-9/tobermorite, (d) band gap under uniaxial tensile strain for tobermorite and GBN9-9/tobermorite



4 Conclusions Using extensive DFT calculations, this work studied the electrical and mechanical properties of intercalated GBN/tobermorite as a representative ceramic composites. Though both graphene and h-BN are hydrophobic, GBN hybrids demonstrates good solvation and dispersity in water as observed by more polarity in electron distribution, negative value for Gibbs free energy and COSMO analysis, making GBN a more preferred choice for slurry preparation and resultant homogenous cementitious mixtures, compared to hBN and graphene. A key trait of such intercalated composites is the formation of covalent C-O and B-O bonds between the nanosheets and tobermorite matrix. These covalent bonds change the hybridization of GBN from sp2 to sp3, which impart significant mechanical and electrical alterations to the composite such as higher young’s modulus, strength and ductility as well as a sharp decline in the band gap. The large band gap of tobermorite at ~4.5 eV reduced to 0.624 eV when intercalated by a fused hybrid GBN nanosheet of equal parts of graphene and h-BN. Further, though both tobermorite ceramic and h-BN are wide bandgap materials, their intercalated composite become a p-type semiconductor with almost half of the band gap compared to those of the parent materials. This counterintuitive result is due to the formation of covalent O-B bonds between the two structures and the resulting electron release in the vicinity of the interfacial region. This phenomenon imparted a complex behavior when subjected to tensile strain, i.e. pristine tobermorite showed a steady increase in bandgap by increasing tensile strain, whereas the GBN/tobermorite composite demonstrated an initial drop in bandgap before saturation. However, in the elastic regime, conductivity grows with increasing strain, demonstrating promises for applications for self-sensing and structural health monitoring.


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Unlike 1D fibers such as CNT or BNNT, 2D materials such as ternary GBN nanosheets not only display double surface area per unit mass but exhibit great solubility and dispersion in water, making it perfect for mixing, bridging and reinforcement at smallest possible scales of the host matrix. This tactic in intercalating ultrathin, tunable and water-soluble 2D materials in matrices holds good promises for synthesizing next generation bi-layer materials with multifunctional properties such as electrical and mechanical properties on demand. Broadly, the concepts, methods and strategies of this work can have implications on other ceramic-based materials to intercalate a set of other emergent (ternary) 2D mono- and few-layer sheets - e.g., composed of molybdenum disulfide, niobium diselenide, layered double hydroxides - for a bottom-up design of tunable multifunctional composites.

Supporting Information The Supporting Information include cell parameters of various structures, and further details of DFT calculations, mechanical property predictions, bond length analysis, dipole moments and effect of B:N ratios. The Supporting Information is available free of charge on the XXX.

Acknowledgments The authors are grateful for the financial support of the Australian Research Council (grant number ARC DE160101606) for conducting this study. R.S. acknowledges the financial support from the National Science Foundation (grant number 1538312).

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TOC


Tunable, Multifunctional Ceramic Composites via Intercalation of

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