Biomimetic mineralization of vertical N-doped carbon nanotubes

Abstract

Biomimetic mineralization of vertical N-doped carbon nanotubes is demonstrated as a straightforward route for carbon-based mineral nanocomposites. The N-doped sites along the carbon nanotube backbone play the role of nucleation sites for mineralization.

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Biomineralization is a natural self-assembly that generates complex mineral organization at organic template surfaces.¹ To date, more than 60 species of biominerals have been reported and produced with desired complex architectures.² In general, biomineralization occurs in an environmentally benign process in neutral pH aqueous media at ambient pressure and temperature.³ Furthermore, unlike conventional vapor phase deposition, biomineralization enables low cost, rapid mineral deposition with low energy consumption. Nevertheless, biomineralization has only partially succeeded in creating high performance composites thus far. The poor strength and low thermal or chemical stability of organic templates, such as peptides or synthetic nitrogen containing polymers, deteriorate the ultimate strength and sustainability of biomineralized structures.⁴

Carbon Nanotubes (CNTs) are allotropes of graphitic sp2 hybridized carbon atoms wrapped into a cylindrical morphology. Over the past 15 years since their discovery, CNTs have received tremendous research interest owing to their excellent material properties, including high mechanical properties, superior thermal or electrical conductivity, and large surface area.⁵ However, the low surface energy and the chemical inertness of CNTs have limited the ultimate utilization of their extraordinary properties. To address these shortcomings, various chemical functionalization methods have been developed.⁶ However, such functionalization may damage the graphitic conjugated structure of CNTs, significantly degrading the material properties.⁷

Here we introduce a new biomimetic mineralization route employing N-doped carbon nanotubes (NCNTs) as highly stable template materials. NCNTs are a new type of carbon nanotubes (CNTs), where N atoms partially substitute a part of carbon atoms in the graphitic side wall of CNTs. The electron rich nitrogen may enhance the electrical conductivity as well as the surface energy of CNTs.⁸ In this work, the substitutionally doped quaternary and pyridinic N atoms in the graphitic plane of NCNTs are exploited as the nucleation site for biomimetic mineralization. Taking advantage of the nitrogen doped sites along the CNT backbone, silica (SiO₂) and calcium carbonate (CaCO₃) nanoshells can be readily deposited on a vertical NCNT array (Fig. 1a). Owing to the superior material properties of NCNTs and straightforward processing via biomimetic mineralization, our approach offers a new opportunity for carbon-based mineral nanocomposites.

Fig. 1 (a) Biomimetic mineralization of NCNTs. N-Doping sites act as nucleation sites. SEM (left) and TEM (right) images of (b) NCNTs, (c) SiO₂ coated NCNTs and (d) CaCO₃ coated NCNTs.

Vertical NCNTs were grown by plasma-enhanced chemical vapor deposition (PECVD) as described elsewhere.^9,10Fig. 1b presents the field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM) images of the synthesized NCNTs. Fig. 1b shows the typical bamboo-like structure of N-doped, multi-walled CNTs. The presence of nitrogen in the NCNTs was confirmed by energy-dispersive spectroscopy (EDX) (see ESI‡, Fig. S1a). Our typical growth condition generated the NCNTs with a length of 10–20 μm and a diameter of 15–20 nm.

Fig. 1c shows the FE-SEM and HR-TEM images of the SiO₂ coated NCNTs prepared by our biomimetic mineralization. The HR-TEM image shows that the NCNTs were coated by a SiO₂ nanoshell with a thickness of 4 ± 1 nm. The presence of Si element could be confirmed by EDX (see ESI‡, Table S1). Previous SiO₂ coating of CNTs has been performed by surfactant mediated coating¹¹ or electrodeposition under external fields,¹² or sol–gel technique with a sintering process.¹³ Those approaches have been employed for undoped pristine CNTs or oxygenated CNTs. To verify the unique role of N-doping in our biomimetic mineralization, undoped or acid-treated oxygenated CNTs were treated by our biomimetic mineralization process. Undoped CNTs were hardly wet by the aqueous mineralization solutions and the resultant CNTs were sparsely covered by SiO₂ particles (see ESI‡, Fig. S2 and S3 and Table S1). Hydrophilic oxygenated CNTs prepared by acid treatment could not be mineralized, despite moderate hydrophilicity (see ESI‡, Fig. S2 and S3 and Table S1). These results confirm that N-doping plays a critical role in our biomimetic process.

Fig. 1d shows the FE-SEM, HR-TEM and Selected Area Electron Diffraction (SAED) pattern (inset) images of the CaCO₃ coated NCNTs.¹⁴EDX measurement detected a Ca element (see ESI‡, Fig. S1c). The HR-TEM image shows that the NCNTs were coated by calcite nanoshell with a thickness of 2 ± 1 nm. The SAED pattern shows the typical [110] crystal zone of calcite. We also confirmed that undoped or oxygenated CNTs could not be CaCO₃ mineralized via our biomimetic process (see ESI‡, Fig. S2 and S3 and Table S1).

Our facile biomimetic mineralization of NCNTs was accomplished by a simple dip-coating process (Fig. 2a).

Fig. 2 (a) Biomimetic mineralization of vertical NCNTs. (b) Cross-sectional and (c) plane-view SEM images of vertical NCNTs. SEM images of (d) SiO₂ and (e) CaCO₃ coated vertical NCNTs.

Firstly, vertical NCNTs were dipped in ethanol for 1 min, and transferred to an aqueous mineral solution (typically a 6 mL solution of Na₂SiO₃·nH₂O or CaCO₃·H₂O bubbled with CO₂) including a coupling agent (Tris·HCl and NH₃) at a pH of 5.5–8.5. The experimental details are described in ESI.‡ The prior ethanol dipping promoted the wetting of the NCNTs with the aqueous mineralization solution. After biomimetic mineralization, evaporation of the aqueous solution may cause capillary forces to act among the NCNT strands and, thus, create a collapsed morphology of NCNT forests.¹⁵ Such a collapse can be avoided by a rapid evaporation of the residual solution under vacuum. Fig. 2b and c show the cross-sectional and plane view SEM images of the vertical NCNTs before mineralization. Highly aligned, ∼20 mm tall, vertical NCNTs were grown on the silicon substrate. The plane view image of the top-surface confirms that the NCNTs grew densely but individually separated. Fig. 2d and e present the SiO₂ and CaCO₃ coated vertical NCNTs prepared by biomimetic mineralization. Owing to the nanoscale thickness of the mineralized shells, the vertical mineralized NCNT strands were individually separated (see ESI‡, Fig. S4). The penetration speed of mineralization solution could be controlled by masking the vertical NCNTs. The mask retarded penetration of mineralization solution, thereby producing half-coated silica composites (see ESI‡, Fig. S5a). By controlling the penetration speed, we have assured that vertical NCNTs can be uniformly mineralized along their entire lengths.

Fig. 3 compares the X-ray photoelectron spectroscopy (XPS) spectra of the bare NCNTs and biomimetic mineralized NCNTs. Fig. 3a shows that the bare NCNT spectrum exhibited a strong nitrogen incorporated carbon peak (CN, 285.5 eV). After mineralization, the C_N peak disappeared and the C_1s peak decreased (Fig. 3a). These XPS results suggest the specific interaction between the N-doped sites of the NCNTs and the mineral shell. Fig. 3b shows the nitrogen XPS spectra. The deconvoluted spectra of the bare NCNTs showed pyridinic N (N_P, 398 eV), quaternary N (N_Q, 400.8 eV), and nitrogen oxide (N_OX, 402–406 eV) peaks.

Fig. 3 XPS spectra of NCNTs before and after biomimetic mineralization. (a) C peaks for C with N (C_N) and C_1s. (b) N peaks for pyridinic N (N_P), quaternary N (N_Q), and N-oxides (N_OX). (c) Si peak for Si_2P and (d) Ca peaks for Ca_2P.

In contrast, the mineralized NCNTs demonstrated a suppression of all nitrogen peaks. These results strongly suggest that the N_P and N_Q doping sites in the NCNTs provide the major nucleation sites for biomimetic mineralization (see ESI‡, Fig. S3). While the SiO₂ coated NCNTs demonstrated a strong Si_2P peak at 104 eV (Fig. 3c), the CaCO₃ coated NCNTs demonstrated two bimodal calcium peaks at 347–348 eV (Fig. 3d).

The X-ray diffraction (XRD) patterns of the bare NCNTs, SiO₂ coated NCNTs, and CaCO₃ coated NCNTs are compared in Fig. 4. The diffraction peaks for calcite lattice are prominent, as indicated by the characteristic peaks at 29.4°, 35.9°, 39.4°, 43.1°, 47.5°, and 48.5°, which correspond to the calcite lattice planes of (104), (110), (113), (202), (018), and (116), respectively (Ref JCPDS Card No. 00-005-0586). Since SiO₂ is amorphous, no noticeable diffraction peaks were observed. The broad peak at 2θ = 43° is attributed to the (103) plane of CNT graphitic structure.

Fig. 4 XRD patterns for bare NCNTs, SiO₂ coated NCNTs, and CaCO₃ coated NCNTs.

In conclusion, we have demonstrated the biomimetic mineralization of NCNTs. The NCNT template offers a new perspective for mineralization and carbon based nanocomposites under mild processing conditions. The detailed structural analysis revealed that the N-doping sites of the NCNTs play the role of nucleation sites for biomimetic mineralization. Our approach not only provides a novel route for nanostructured materials but also suggests that the graphitic N-doping sites can play a significant role in biomimetic mineralization.

This work was supported by the National Research Laboratory Program (R0A-2008-000-20057-0), and World Premier Materials (WPM) program funded by the Korean government.

Notes and references

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Footnotes

† This article is part of the ‘Emerging Investigators’ themed issue for ChemComm.

‡ Electronic supplementary information (ESI) available: Experimental details. Fig. S1: TEM images and EDX chemical analyses of (a) bare NCNTs, (b) SiO₂ coated NCNTs and (c) CaCO₃ coated NCNTs. Fig. S2: TEM images and EDX chemical analyses of (a) SiO₂/undoped CNT, (b) SiO₂/hydrophilic oxygenated CNT, (c) CaCO₃/undoped CNT, and (d) CaCO₃/hydrophilic oxygenated CNT. Table S1: Elemental analyses (atom%) using EDX. Fig. S3: (a) Static water contact angles (CA) of undoped CNT, hydrophilic oxygenated CNT, and NCNT, (b) FT-Raman spectrum of undoped CNT and hydrophilic oxygenated CNT, TEM images of (c) SiO₂/undoped CNT (left), SiO₂/hydrophilic oxygenated CNT (middle), and SiO₂/NCNT (right); (d) CaCO₃/undoped CNT (left), CaCO₃/hydrophilic oxygenated CNT (middle), and CaCO₃/NCNT (right). Fig. S4: Individually separated (a) SiO₂ coated NCNTs and (b) CaCO₃ coated NCNTs. Fig. S5: Comparison between (a) SiO₂ half-coated NCNTs and (b) SiO₂ fully coated NCNTs. Fig. S6: Proposed biomimetic mineralization mechanisms for (a) SiO₂ coated NCNTs, and (b) CaCO₃ coated NCNTs at pyridinic nitrogen nucleation sites. See DOI: 10.1039/c0cc04237d

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