Synthesis of CaCO₃/graphene composite crystals for ultra-strong structural materials

Abstract

Composite crystals of calcium carbonate (CaCO₃) and graphene with hexagonal plate or ring, dendritic and rhombohedral shapes were synthesized by the hydrothermal reaction of calcium acetate and urea in the presence of graphene oxide (GO) sheets. Their crystal structures were characterized to be vaterite, aragonite and calcite, respectively. In this case, the hydrothermally reduced graphene oxide (rGO) acted as an atom-thick, two-dimensional template for controlling the nucleation and growth of the CaCO₃ crystals. The vaterite CaCO₃ composite crystals (VCCs) were used as a filler of poly(vinyl alcohol) (PVA) to form a composite with a nacre-like structure. The Young's modulus and tensile strength of PVA/45 wt% VCC were tested to be 34.1 ± 2.5 GPa and 165 ± 6 MPa, respectively.

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Introduction

In nature, living organisms use various organic/inorganic composite materials for different purposes such as mechanical support, navigation, protection, and defense.^1–13 These materials usually have precisely designed and controlled compositions, structures, sizes and shapes, etc.^1,12 Particularly, CaCO₃ based composite crystals are the main components of many biominerals, such as sea urchin skeletal parts and the calcite prisms in mollusk shells.^1–4,7 Thus, the growth of precisely shaped crystals of CaCO₃ well-organized in or around a soft (bio)molecular matrix to form composite crystals is of great importance for understanding and imitating the biomineralization processes.^1,7 In the last few decades, the biomimetic synthesis of CaCO₃, the most abundant biomineral material, has been widely studied.^{1–5,12,14–19} Up to now, small molecules, biomacromolecules and artificial polymers have been used as additives for controlling the structures and/or shapes of CaCO₃ crystals and their composites.^1,18,20–27 However, the mechanisms of these additives operating throughout the entire morphogenesis process are often time dependent and are extremely difficult to access.^1,20 Furthermore, most biomimetic mineralization systems are very complex, which strongly limited their applications for production on large scales.^1,8

On the other hand, graphene, a 2-dimensional graphite sheet with atomic thickness shows great potential as an ideal additive for controlling the formation of inorganic single crystals.^28,29 Particularly, chemically modified graphene prepared by the reduction of graphene oxide (GO) can be cheaply obtained with high-throughput.^28,30,31 A reduced GO (rGO) sheet has residual carboxyl groups at its edges, which can interact with Ca²⁺ ions in order to control the mineralization of CaCO₃. Most importantly, the lateral dimensions of rGO sheets are in the range of several hundreds of nanometres to tens of micrometres, which are several orders of magnitude larger than those of conventional molecules. Thus, the use of rGO sheets as additives can greatly reduce the interfacial Gibbs energy for the nucleation of CaCO₃ as compared to the application of other molecular templates according to an empirical equation proposed by Nielsen.^32,33 As a result, rGO sheets can be easily encapsulated into the inorganic crystals to form composite crystals. However, the composite crystals of graphene and inorganic materials have not yet been reported, while they are expected to have improved mechanical properties as compared with pure crystals.⁷

Here, we report a facile hydrothermal process for controlling the mineralization of CaCO₃ into its three polymorphs (e.g.vaterite, aragonite and calcite) by using rGO as a template. Precisely shaped hexagonal plate or ring, dendritic or rhombohedral CaCO₃/rGO composite crystals were obtained with high-throughputs. Interestingly, the lateral dimensions of vaterite CaCO₃ composite crystals (VCCs) are uniform and their thicknesses are close to that of the aragonite CaCO₃ plates in nacre (about 0.5 μm).^34,35 Nacre has a ‘brick-and-mortar’ composite structure of CaCO₃ plates and proteins, which provides it with ultra-strong mechanical properties.³⁴ This phenomenon has been inspiring scientists and engineers to fabricate inorganic/organic composite materials with similar structures; clay sheets and Al₂O₃ plates were the most widely used fillers.^36–40 However, CaCO₃ based nacre-mimetic composites with superior mechanical properties have rarely been reported. Thus, we also prepared PVA/VCC composite films with nacre-like lamellar structures through vacuum filtration. The composite film of PVA/45 wt% VCC had a Young's modulus and tensile strength of 34.1 ± 2.5 GPa and 165 ± 6 MPa, respectively. Its mechanical properties partly surpasses that of nacre.

Experimental

Preparation of GO

GO was prepared by oxidation of natural graphite powder (325 mesh, Qingdao Huatai Lubricant Sealing S&T Co. Ltd., Qingdao, China) according to a modified Hummers' method.^41,42 Briefly, graphite (3.0 g) was added to concentrated sulfuric acid (70 mL) under stirring at room temperature, then sodium nitrate (1.5 g) was added, and the mixture was cooled to 0 °C. Under vigorous agitation, potassium permanganate (9.0 g) was added slowly to keep the temperature of the suspension lower than 20 °C. Successively, the reaction system was transferred to a 35–40 °C water bath for about 0.5 h, forming a thick paste. Successively, 140 mL of deionized (DI) water was added, and the solution was stirred for another 15 min. An additional 500 mL of DI water was added and followed by a slow addition of 20 mL of H₂O₂ (30%), turning the color of the solution from brown to yellow. The mixture was filtered and washed with 1 : 10 HCl aqueous solution (250 mL) to remove metal ions followed by repeated washing with water and centrifugation to remove the acid. The resulting solid was dispersed in water by ultrasonication for 1 h to make a GO aqueous dispersion (0.5 mg mL⁻¹). The obtained brown dispersion was then subjected to 30 min of centrifugation at 4000 rpm to remove any aggregates. Finally, it was purified by dialysis for 1 week to remove the remaining salt impurities.

Hexagonal and dendritic composite crystals synthesis

Ca(Ac)₂ and urea were purchased from Beijing Chem. Reagents Co. (Beijing, China) and used as received. 10 mL aqueous solution of Ca(Ac)₂ (0.25 M), urea (1.25 M) and GO (0.5 mg mL⁻¹) was sealed into a 16-mL Teflon-lined autoclave and maintained at 180 °C. Products at different reaction times were collected after cooling the system to room temperature. The hexagonal ring and plate shaped VCCs were obtained after 20 and 30 min, respectively. The typical dendritic crystals were collected at 1, 3 and 24 h.

Rhombohedral composite crystals synthesis

10 mL aqueous solution of Ca(Ac)₂ (0.0625 M), urea (0.3125 M) and GO (0.5 mg mL⁻¹) was sealed into a 16-mL Teflon-lined autoclave and maintained at 180 °C for 20 min. After cooling to room temperature, typical rhombohedral calcite composite crystals were collected.

Purification of the as-produced composite crystals

The crude products of the hydrothermal reactions contain residual free rGO sheets (ESI† Fig. S1). They were rinsed with DI water and ethanol; successively dried under vacuum and at room temperature for over 24 h. The dried powdery product was put into ethanol and sonicated for 1 h and then aged for 5 min. During this process, the residual rGO sheets were precipitated to the bottom of the solution. The CaCO₃ composite crystals dispersed in ethanol were collected and then dried at room temperature.

Preparation of PVA/VCC composite films

PVA with a repeat unit number of 2400–2500 (PVA 124, hydrolysis degree 98–99%) was purchased from Beijing Chem. Reagents Co. (Beijing, China) and used as received. The VCCs used for the synthesis of composite films were complete hexagonal plates collected after 30 min of the hydrothermal reaction. They were put into a mixed solvent of ethanol/water (1 : 3, by volume) and stirred for at least 24 h to form a dispersion (0.5 wt%). Then, a controlled amount of VCC dispersion was slowly added to a 10 mL 1 wt% solution of PVA under stirring. The PVA/VCC mixture was further stirred for overnight to form a PVA stabilized VCC dispersion. Subsequently, the excess polymer was removed by centrifugation and washing. Then, the purified PVA stabilized VCC was re-dispersed into 10 mL water and filtrated through a porous poly(tetraflouroethylene) (PTFE) membrane (47 mm in diameter and 0.2 μm in pore size) to yield a free-standing PVA/VCC film. Finally, the composite film was dried overnight under vacuum at 80 °C before characterization.

Characterization

The morphologies of the crystals were examined by scanning electron microscopy (SEM, Hitachi S-5500, 10 kV) after being sputter-coated with a thin layer of gold nanoparticles. The EDAX spectra were taken on the same SEM instrument at spot view in UHR high resolution mode. The region used for collecting the EDAX signal from the hole in the hexagonal ring was restricted to the surfaces of the graphene sheets to avoid the effect of the carbon tape. The depth resolution of collecting the EDAX signals was about 0.5 μm. The internal structures of the composite crystals were examined by using an annular-dark-field scanning transmission electron microscope (ADF-STEM) (FEI Tecnai F20 field emission gun, 200 kV, with Fischione annular-dark-field detector). Selected area electron diffraction (SAED) patterns were also recorded on the same instrument. A scanning transmission electron microscope (FEI Tecnai F20) was operated at 200 keV with a 9.6-mrad probe-forming semi-angle and an 8–10 pA beam current giving a 1.6 Å nominal resolution in the ADF-STEM mode. Using this set of operating conditions, 1.6–2 Å resolution could be routinely achieved on the crystalline samples, which readily enabled the vaterite lattice to be resolved along the [001] zone axis. The X-ray diffraction (XRD) patterns were recorded on a D8 Advance X-ray diffractometer (Bruker) with Cu-Kα radiation (λ = 1.5418 Å). The weight contents of VCCs in the composites were measured by elemental analysis, performing on a CE-440 analyzer (Analytics, EAI). The mechanical properties of PVA/VCC composite films were tested by the use of a model 3342 universal mechanical testing machine (Instron, USA) at a stretching rate of 0.5 mm min⁻¹. The composite films were cut into strips of 2–3 mm in width and 20–25 mm in length and used as the specimens. The reported data is the average of 4 tests of the same sample. All the failures occurred in the middle regions of the specimens.

Results and discussion

CaCO₃/rGO composite crystals

CaCO₃ composite crystals were synthesized by hydrothermal reaction of calcium acetate [Ca(Ac)₂] and urea in the presence of graphene oxide (GO) sheets at 180 °C (ESI† Fig. S1). In this case, Ca(Ac)₂ and urea acted as Ca²⁺ and CO₃²⁻ion sources, respectively; hydrothermally reduced GO (rGO) acted as a template for crystal nucleation and growth. The morphologies and crystal structures of the resulting products depend on the concentrations of CaCO₃ precursors and the reaction time. For example, as an aqueous solution containing 0.25 M Ca(Ac)₂, 1.25 M urea and 0.5 mg mL⁻¹ GO sheets were used as the starting materials and reacted for 30 min, hexagonal shaped CaCO₃/rGO composite crystals with sizes of about 20 μm were obtained (Fig. 1a). The surface defects of a composite crystal were attributed to the boundaries of both its components (inset of Fig. 1a). When the reaction time was elongated to 24 h, dendritic spherical crystals with diameters around 60 μm were formed (Fig. 1b). However, as the concentrations of Ca(Ac)₂ and urea were reduced to 0.0625 and 0.3125 M, respectively, rhombohedral shaped composite CaCO₃ crystals with sizes of approximately 20 μm were produced after reaction for 20 min (Fig. 1c). X-Ray diffraction (XRD) analysis (Fig. 1d) indicates that these three typical crystals have (A) vaterite (d-spacing [nm]: 0.422, 0.357, 0.329, 0273, 0.211, 0.206, 0.185, 0.182, 0.164), (B) aragonite (d-spacing [nm]: 0.340, 0.327, 0.273, 0.271, 0.249, 0.241, 0.238, 0.234, 0.219, 0.211, 0.198, 0.188, 0.182, 0.176, 0.175, 0.173, 0.170, 0.163, 0.162, 0.156) and (C) calcite (d-spacing [nm]: 0.385, 0.303, 0.248, 0.228, 0.209, 0.192, 0.187, 0.162, 0.160) polymorphs (JCPDS files: 33-0268; 5-0452; 25-0127). In comparison, in the system without GO sheets, only aragonite crystals with irregular shapes were observed (ESI† Fig. S2). These results demonstrate that rGO sheets can be used to precisely control the mineralization of CaCO₃ into its three crystalline polymorphs to form CaCO₃/rGO composite crystals with uniform morphologies. Furthermore, the technique developed here is convenient, rapid, cheap, and can be scaled up to produce the composite crystals in large amounts.

Fig. 1 The morphologies and crystal polymorphs of CaCO₃ composite crystals viahydrothermal processes. Scanning electron microscope (SEM) images of hexagonal plates (a), dendritic spherical (b), rhombohedral (c) CaCO₃/rGO composite crystals; insets are the corresponding magnified images, scale bar = 5 μm. (d) XRD patterns of hexagonal plate (A), dendritic spherical (B), and rhombohedral (C) composite crystals.

Mechanism of forming the vaterite CaCO₃/rGO composite crystals

To understand the mechanisms of the composite crystal growth and the role of rGO sheets, we studied the process of growing each typical crystal. It was found that hexagonal rings were formed in the same system which grew the hexagonal plates as the reaction time was shortened from 30 to 20 min (Fig. 2a). In the hole of the ring, wrinkled graphene sheets with edges embedded in the walls were observed. The regional high magnification scanning electron microscope image of site B in Fig. 2a indicates that a few graphene sheets formed a porous network and their edges penetrated into the CaCO₃ crystal (ESI† Fig. S3). The defects of the crystal are mainly located at the interfaces of both components. Elemental distribution analysis (EDAX) was used to further characterize the arrangements of graphene sheets in the composite crystal. Elemental distribution analysis (EDAX) indicates that the atomic ratios of C, O and Ca for the ring wall (site A in Fig. 2a) and graphene sheet (site B in Fig. 2a) are 1 : 3 : 1 and 56 : 3 : 1, respectively (Fig. 2b).These results reflect that most hydroxyl and epoxy groups on the basal planes of GO sheets were removed upon hydrothermal reduction⁴³ and CaCO₃ crystals grew around the rGO sheets causing the holes to form. The holes formed in the internal regions of the hexagonal rings range from hundreds of nanometres to several micrometres (ESI† Fig. S4). This is mainly due to that different sized GO sheets were used as the templates. The crystals initially grew around rGO sheets to leave holes with less charged hydrophobic basal planes of the rGO sheets.

Fig. 2 The formation mechanism and single crystal nature of a hexagonal composite crystal. (a) The SEM image of a typical hexagonal vaterite/rGO composite ring collected from the system of growing the hexagonal plate and reacted for 20 min. (b) SEM-EDAX spectra of ring wall (A) and graphene (B) in the ring hole shown in panel (a). A SAED pattern recorded from [001] zone axis (c) and LAADF-STEM image with clearly resolved lattice fringe of (110) planes (d) recorded at the thin region of the ring edge.

Annular dark-field scanning transmission electron microscopy (ADF-STEM) was used to analyze the internal crystal structure of hexagonal vaterite at atomic resolution.^7,44,45 The selected-area electron diffraction (SAED) pattern recorded at the edge of a vaterite ring gives a single set of diffraction spots (Fig. 2c). This SAED pattern can be indexed as a hexagonal vaterite CaCO₃ single crystal (hexagonal space groupP6₃/mmc) viewed from its [001] zone axis.^19,46 High-resolution low-angle ADF-STEM (LAADF-STEM) image directly shows the resolved lattice fringe of the (110) planes (d-spacing = 0.357 nm), which further confirms the single crystal nature of hexagonal ring or plate. Furthermore, the XRD pattern shown in Fig. 1d also agrees well with the calculated and experimental results of hexagonal vaterite single crystals.^47,48 In previous reports, large vaterite crystals were usually formed through the assembly of nanocrystals coexisting with amorphous components in their boundaries.^14,15,19 Thus, the single crystal nature of our large vaterite crystals indicates the uniqueness of rGO sheets and the hydrothermal process.

Vaterite is usually unable to expose its (001) faces to form a symmetrical morphology. This is mainly because the highly polarized (001) crystal faces of vaterite consist of hexagonal lattices of CO₃²⁻ and Ca²⁺ ions, which results in the formation of unstable high energy crystal surfaces. The carbonate planes of vaterite are parallel to the c axis in the unit cell, which is in contrast with the perpendicular orientation in the calcite or aragonite crystalline polymorph. Theoretical analysis indicates that the activities of Ca²⁺ [a(Ca²⁺)] and CO₃²⁻ [a(CO₃²⁻)] ions have a strong influence on the orientation of CO₃²⁻ ions.^1,24 At higher a(Ca²⁺) and a(CO₃²⁻), the CO₃²⁻ ions tend to arrange parallel to the c axis to form the vaterite crystal structure via kinetic control.^13,49,50 In the system without an effective modifier, the obtained product is usually a mixture of vaterite and calcite. In our case, the graphene template can control the nucleation and growth of CaCO₃ crystals at the atomic level, modulating the crystal cell formation process. During the hydrothermal reaction, most oxygen-containing groups on the basal planes of GO sheets were removed and negatively charged COO⁻groups remained at their edges.⁴² Thus, Ca²⁺ ions were preferentially adsorbed on the edges of the rGO sheets plates if high concentrations of Ca(Ac)₂ and urea were used. The large area of rGO sheets provided sufficient spaces for crystal nucleation. The atom-thick rGO sheets with extraordinary large lateral dimensions greatly reduced the interfacial energy barriers between the template planes and crystal faces.³³ The high strength of the rGO basal planes facilitates the crystal growth orientation and atom arrangement in crystal cells, and also restricts their structural transformation into calcite.^19,51 In addition, NH₄⁺ ions formed through hydrolysis of urea during the hydrothermal reaction also improved the stability of the vaterite (001) faces.^14,52–54

Mechanism of forming the aragonite CaCO₃/rGO composite crystals

The crystal transformation from vaterite to aragonite or calcite is a common phenomenon in CaCO₃ mineralization, which has been extensively studied.^45,55–59 However, the mechanism of these crystal transformations is still partially unclear. This is possibly due to the controversial theories about vaterite formation and unsuccessful control in growing vaterite and aragonite crystals. The irregular and hierarchical morphology of aragonite crystals also made the transformation mechanism confusing. Furthermore, aragonite is less stable than calcite; in most cases, vaterite was directly transformed into calcite.^45,56,59,60 In our system, the vaterite plates can be converted into aragonite dendritic structures just by extending the hydrothermal reaction time (Fig. 3 and ESI† Fig. S5). Hexagonal plates of VCCs were formed initially by reaction for only 30 min (Fig. 3a) and they successively acted as the nucleation sites for growing aragonite crystals. With the increase of reaction time, needle-like aragonite crystals were grown on the two hexagonal planes of a vaterite plate. The intermediates with vaterite and aragonite phases both present have not been found by controlling the reaction time. This is mainly due to that the time period of forming the intermediates being short and the possibly formed intermediates can also be transformed to stable aragonite during the process of cooling the reaction system from 180 °C to room temperature. Nevertheless, the formation of aragonite crystals can be attributed to the lack of rGO sheets in the system, which strongly weakened the manipulation control of ions to form high energy vaterite (001) crystal faces. Two dimensions have to be considered in spherical coordinates: axial (z) and radial (r) directions. Aragonite crystals initially were grown on the hexagonal surfaces of a vaterite plate in the direction of the z axis. Upon increasing the reaction time, the free space in the z axial direction was reduced gradually and finally the dimensions of the crystals grown along the z axis reached their maxima. Successively, aragonite needles started to arrange close to the z-axis and lean to r direction. As a result, dendritic crystals were formed (Fig. 3b,c, ESI† Fig. S5). Finally, flower-like spherical aragonite crystals with diameters around 60 μm were produced after reaction for 24 h (Fig. 3d). A dendritic aragonite crystal fractured at its neck was observed (Fig. 3e,f), which clearly shows a hexagonal shaped cross-sectional surface and its size is in consistent with that of initial hexagonal plate precursor. The VCC plates possibly still remain at the necks of dendritic aragonite crystals. This observation strongly supports the mechanism of aragonite nucleation and growth described above.

Fig. 3 The evolution from vaterite to aragonite composite crystals. Typical SEM images of CaCO₃/rGO composite crystals collected from the system of growing the hexagonal plates (Fig. 1a) at 30 min (a), 1 h (b), 3 h (c) and 24 h (d), respectively. (e, f) The fractured surface of a dendritic aragonite crystal.

Mechanism of forming the calcite CaCO3/rGO composite crystals

According to previous theoretical results, the transformation between vaterite and calcite crystals is a balance between kinetic and thermodynamic processes, and the orientation of atomic arrangement depends on a(Ca²⁺) and a(CO₃²⁻).^13,32,49 At high concentrations of Ca²⁺ and CO₃²⁻ ions, kinetic process controls the crystal growth and favors the formation of the vaterite crystal arrangement (Fig. 1a). At low ion concentrations, the thermodynamic process dominates the crystal formation and supports the calcite crystal arrangement. Actually, calcite/rGO composite crystals with relatively uniform morphology were formed as the concentrations of Ca(Ac)₂ and urea were reduced to 0.0625 M and 0.3125 M, respectively, and reacted for 20 min (Fig. 1c). However, as the concentrations of Ca(Ac₂) and urea were both doubled, a calcite/vaterite mixture was produced (ESI† Fig. S6a). This result also indicates that low concentrations of CaCO₃ precursors favor the formation of stable calcite crystals. On the other hand, further decreasing the concentrations of Ca(Ac)₂ (e.g. 0.025 M) and urea (e.g. 0.125 M) led to the formation of calcite composite crystals with irregular shapes (ESI† Fig. S6b). To reveal the mechanism of growing the uniform calcite/rGO composite crystals, the product prepared for a shorter time (e.g. 15 min) was collected. Reducing the reaction time from 20 to 15 min led to the formation of incomplete composite crystals (Fig. 4a). The calcite composite crystal has a lamellar rhombohedral structure (Fig. 4b). The elemental compositions of sites A and B in the magnified regional SEM image (Fig. 4c) were analyzed by scanning electron microscopy with elemental distribution analysis (SEM-EDAX, Fig. 4d). The atomic ratios of C, O and Ca of sites A and B were calculated to be 1 : 3 : 1 and 61 : 3 : 1, respectively, reflecting they are composed of CaCO₃ and rGO respectively. At a much lower concentration of CO₃²⁻ ions compared with that of the system of which grew the vaterite composite crystals, the CO₃²⁻ ions tended to arrange perpendicular to the c axis, needing the lowest Gibbs free energy. The thermodynamic pathway determined the crystal structure and morphology, and the surface energy of a crystal face can be lowered by the absorption of rGO sheets. During this process, rGO sheets modulated the orientation of ions and stabilized the arrangement of the crystal lattice at the atomic level by encapsulating them into crystal interfaces.

Fig. 4 Internal structure of an incomplete calcite/rGO composite crystal. (a) An SEM image of a calcite/rGO composite crystal obtained at 15 min. (b, c) The regional magnified images of (a). (d) SEM-EDAX spectra of calcite (A) and graphene (B) of the composite crystal shown in panel (c).

Nacre-mimetic PVA/VCC composite films and their mechanical properties

Inspired by the ‘brick-and-mortar’ structure of nacre, we prepared films with 75 wt% VCC plates that have a Young's modulus (E) of 16.0 ± 2.1 GPa and tensile strength (δ) of 90 ± 3 MPa (Fig. 5a). These values are much high than those of a pure PVA film prepared by the same technique (E = 2–3 GPa, δ = 60–70 MPa).^37,38 However, when the weight content of VCC plates was further optimized to be 45%, the Young's modulus and tensile strength of the composite film were increased to 34.1 ± 2.5 GPa and 165 ± 6 MPa, respectively (Fig. 5a). The tensile strength of the PVA/45 wt% VCC composite film is much higher that of nacre (80–135 MPa), while its modulus is about half that of nacre (60–70 GPa).^61–63 The mechanical property of our nacre-mimic film is also comparable to that of a PVA/clay biomimetic film (E = 29 GPa, δ = 173 MPa).³⁸ Furthermore, the elongation at break of PVA/45 wt% VCC composite film was measured to be about 1.3 ± 0.1%, and this value is about 30% higher than that of nacre (<1%).^61–63 These results indicate that the composite film is ultra-strong and ductile. The effective strategy of biomimetic synthesis is mainly due to the fact that the PVA coated VCC plates can be forced into well-aligned mesoscopic assemblies upon filtration. A significant number of hydrogen bonds among PVA molecules resulted in a strong cohesion within the polymer matrix, and PVA acted as a binder between vaterite plates. A ‘brick-and-mortar’ like structure is clearly shown in the SEM image of the composite with 75 wt% VCC (Fig. 5b), where the amount of PVA is insufficient to firmly glue all the inorganic plates together. However, in the composite with 45 wt% VCC, a large amount of PVA wrapped around the CaCO₃ plates to form a more compact ‘brick-and-mortar’ structure. The SEM images of the fractured surfaces of PVA/45 wt% VCC composite film reflect that the cracks occurred at the polymer–mineral interfaces (Fig. 5c and ESI† Fig. S7). Therefore, the mechanical properties of the PVA/VCC composite film partly surpass that of nacre. On the other hand, further decreasing the VCC content of the composite will result in the formation of much thicker PVA layers and weakening its mechanical strength (ESI† Fig. S8).

Fig. 5 Typical stress–strain curves of PVA/VCC composite films (a) and SEM images of fracture surfaces with 75 wt% VCC (b) and 45 wt% VCC (c).

Conclusion

In conclusion, we have developed a facile, rapid, cheap, and scalable hydrothermal approach for precisely controlling the biomimetic mineralization of CaCO₃ into its three crystal polymorphs by using rGO sheets as a template. The resulting CaCO₃/rGO composite crystals have uniform hexagonal ring or plate, dendritic and rhombohedral shapes. rGO sheets have an atom-thick two-dimensional structure, negatively charged edges, high specific surface area and mechanical strength, which make them exhibited excellent performances in controlling the nucleation and growth of CaCO₃ crystals. This work provides a new route toward biomimetic CaCO₃ based composite crystals and shines a light into understanding the mechanisms of biomineralization. The precise control over the crystallization processes of minerals based on the unique structure and properties of graphene sheets will inspire the application of graphene in the crystallographic field. Furthermore, the precisely shaped CaCO₃/rGO composite crystals are promising building blocks for the fabrication of various biomimetic structural materials. Actually, the nacre-mimetic composite films of PVA/45 wt% VCC with a ‘brick-mortar’ structure have a high strength (165 ± 6 MPa) and Young's modulus (34.1 ± 2.5 GPa), which is the first example of CaCO₃ based composite material to exhibit superior mechanical properties that partly surpass that of nacre.

Acknowledgements

This work was supported by national basic research program of China (973 Program, 2012CB933400) and the natural science foundation of China (91027028, 50873092).

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Footnote

† Electronic supplementary information (ESI) available: Additional figures are described in the text. See DOI: 10.1039/c2ra00765g/

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