Synthesis and fabrication of CNTs/Fe₃O₄@Pdop@Au nanocables by a facile approach

Abstract

Multifunctional nanocables were prepared by directly welding superparamagnetic Fe₃O₄ nanoparticle decorated CNTs and Au nanoparticles together with polydopamine (Pdop) as “glue” in a facile but effective way. Additionally, the size and density of the Au nanoparticles can be easily controlled by different chemical approaches. Moreover, these core–shell nanocables can be a versatile platform for multiple applications.

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Carbon nanotubes (CNTs), discovered in 1991 by Lijima,¹ have attracted considerable interest for their high electrical conductivity, mechanical strength, and chemical stability.² Recently, more attention has been paid to the functionalization of carbon nanotubes (CNTs) with various inorganic nanoparticles such as metals, metal oxides for further improving their properties and extending their applications in various fields of technology.^3–7 Among these multifunctional composites, magnetic CNTs composites have been emerging to be an interesting area of advanced research owing to their potential applications in microwave absorption, electrochemical biosensor.⁸ To apply magnetic CNTs in some applications, much work has been done to functionalize magnetic CNTs surface with another phase to enhance compatibility and improve the stability. Among all materials, silica as a protecting shell was utilized to coat the magnetic carbon nanotube.⁹ Although there have been lots of reports about successful synthesis of silica coated magnetic CNTs, little work has been done to functionalize with organic coating (e.g., polymer).^10,11 In the case of polymer shell layers, Pdop has received significant attention as a candidate material because of its unique coating quality and functionality.¹² It has been reported that the self-polymerization of dopamine could form stable coating on the surface of inorganic and organic materials in comparison to the other coating techniques.¹³ Besides its biocompatibility and adhesiveness, the Pdop coatings have the other attractive features: Pdop coatings can serve as a versatile platform for secondary surface-mediated reactions, leading to tailoring of coatings for diverse uses. More importantly, Pdop has been shown to be an effective carbon source for the formation of carbon-coated materials.¹⁴ The main objective of our work is to demonstrate a simple and general procedure for the synthesis of Pdop coated magnetite/CNTs nanocable. By using this process, well-controlled Pdop deposition of monodisperse ferrite on the outside walls of CNTs can be readily obtained.

The whole preparation strategy is described in Scheme 1. Firstly, CNTs/Fe₃O₄ nanocomposite was fabricated using a one-pot procedure according to previous works.¹⁵ Secondly, a thin layer of Pdop was coated onto the CNTs/Fe₃O₄ to produce the CNTs/Fe₃O₄@Pdop nanocable. Then, a large number of gold nanoparticles were assembled on the surface of CNTs/Fe₃O₄@Pdop by using polydopamine shell coated on CNTs/Fe₃O₄ to reduce Au³⁺ (ref. 16 and 17) or adding extra reductant (NaBH₄).¹⁸

Scheme 1 Programmed synthesis of CNTs/Fe₃O₄@Pdop@Au (Tris buffer): tris(hydroxymethyl)-aminomethane buffer, TREG: triethylene glycol.

The morphologies and structural features of the prepared materials can be observed from the TEM images (Fig. 1). The Fe₃O₄ nanoparticles with an average diameter of 8 nm were uniformly distributed on the surface of the CNTs, which is shown in Fig. 1a and b. These CNTs/Fe₃O₄ nanocomposites are highly water-dispersed (Fig. S1(b)†), which is beneficial for Pdop coating. Revealed on TEM images (Fig. 1c and d) a uniform Pdop layer of ∼18 nm is coated on the surface of CNTs/Fe₃O₄, resulting in cable-like CNTs/Fe₃O₄@Pdop, the Pdop layer can also be confirmed by the FT-IR spectrum. The characteristic bands at 1614 cm⁻¹, 3420 cm⁻¹, shown in Fig. S2† corresponds to aromatic ring stretching and the catechol-OH group in Pdop layer. In addition, the shell thickness gradually increases from 18 nm to 55 nm (Fig. 1e and f) with changing the mass ratio of CNTs/Fe₃O₄ and dopamine from 1 : 1 to 1 : 2 while keep the other parameters fixed. The obtained composites display a well-defined core–shell structure and good water dispersibility (Fig. S1(c)†). This coating rate can be finely tuned by changing the pH, reaction time and temperature of reaction medium.^12,13,19,20 Due to the metal-binding ability of catechols present in the Pdop layer, the CNTs/Fe₃O₄@Pdop@Au nanocable has been synthesized via CNTs/Fe₃O₄@Pdop as chemical template in the presence of HAuCl₄ solution. After mildly stirring at room temperature for 12 h, then separating by external magnet, and washing, it can be seen that there are many Au nanoparticles adhered to the CNTs/Fe₃O₄@Pdop (Fig. 2a and b). The TEM image shows that all the Au nanoparticles with the elaborate spherical and triangle shapes were well-dispersed on the surface without forming large aggregates, and the size of Au nanoparticles lie in the range of 20–30 nm, which is also in accordance with previous works. This assembly method is simpler than those reported without additional reagents (catechol-OH group work as both reductive and binding reagents), However, the size of Au nanoparticles generated by this method was above 10 nm, this will greatly limit the application of multifunctional composites. Several studies have found that the catalytic properties of these composites are largely dependent on the size, composition, and dispersion state of the noble metal particles. To solve the above mentioned problem, the extra reductant such as NaBH₄ has been introduced into the reaction system. When HAuCl₄ was reduced by NaBH₄ in aqueous suspension of CNTs/Fe₃O₄@Pdop, the as-prepared gold nanoparticles were ready to attach onto the surface of Pdop, because of the metal coordination capability of Pdop. As shown in Fig. 2c and d the gold nanoparticles (∼5 nm) were then evenly and firmly attached to the surface of the Pdop shell after rapid reduction of HAuCl₄ by NaBH₄.

Fig. 1 TEM images of (a and b) CNTs/Fe₃O₄, (c and d) CNTs/Fe₃O₄@Pdop, (e and f) CNTs/Fe₃O₄@Pdop (thicker).

Fig. 2 TEM images of (a and b) CNTs/Fe₃O₄@Pdop@Au (without adding NaBH₄) (e and f) CNTs/Fe₃O₄@Pdop@Au (by adding NaBH₄).

The XRD patterns of the as-synthesized CNTs/Fe₃O₄, CNTs/Fe₃O₄@Pdop and CNTs/Fe₃O₄@Pdop@Au are shown in Fig. 3. The diffraction peaks of as-received Fe₃O₄ at 30.00°, 35.48°, 43.14°, 53.44°, 57.04°, and 62.58° were observed. The positions and relative intensities of these CNTs/Fe₃O₄ related new peaks match well with the (220), (311), (400), (422), (511), and (440) planes of the standard XRD data for the cubic spinel crystal structure of bulk magnetite. The peaks at 25.98° and 42.78° are assigned to (002) and (100) planes of the MWCNTs, respectively.²¹ The peak broadening of the XRD patterns indicates that magnetite crystallites are significantly small. As calculated by Scherrer's formula, the average crystallite size of the magnetite crystals was about 8 nm. For the XRD spectrum of CNTs/Fe₃O₄@Pdop (Fig. 3b), the major peaks are similar to the pristine CNTs/Fe₃O₄ particles (Fig. 3a), revealing that the core–shell composites consist of the CNTs/Fe₃O₄ component. The compositions of the CNTs/Fe₃O₄@Pdop@Au were also investigated by XRD. Four additional peaks at 38°, 43°, 65° and 78°, which represent the Bragg reflections from (111), (200), (200), and (311) planes of Au are observed (JCPDS card no. 04-0784), showing clearly the existence of Au NPs in the CNTs/Fe₃O₄@Pdop@Au composites (Fig. 2).

Fig. 3 XRD diffraction patterns of the as-prepared CNTs/Fe₃O₄ (a), CNTs/Fe₃O₄@Pdop (b), and CNTs/Fe₃O₄@Pdop@Au (c) (without adding NaBH₄).

XPS has often been used for the surface characterization of various materials, and unambiguous results are readily obtained when various surface components contain unique elemental markers. To further analyze the Pdop@Au composite shell in the CNTs/Fe₃O₄@Pdop@Au nanocable, XPS was employed to examine the composition of the as-prepared NPs. As shown in Fig. 4a, it is clear that the elements C, O, N, and Au appear on the surface of the CNTs/Fe₃O₄@Pdop@Au NPs. The binding energy at 710.20 eV for Fe2p₃ cannot be detected, which further supports that all the CNTs/Fe₃O₄ cores in the composite are confined within a shell of polydopamine. As shown in Fig. 4b, there are two peaks located at the binding energies of 84.2 and 87.8 eV, which is consistent with the emission of 4f photoelectrons from Au0, thereby suggesting the successful formation of Au NPs. The data of the X-ray photoelectron survey spectra shows a C1s peak around 285 eV, and N1s peak at 408 eV, which are shown in Fig. 4c and d.

Fig. 4 XPS pattern of the CNTs/Fe₃O₄@Pdop@Au composite.

The superparamagnetic behaviour of the magnetic nanoparticles was demonstrated by plotting magnetization curves. The hysteresis loops of CNTs/Fe₃O₄, CNTs/Fe₃O₄@Pdop, and CNTs/Fe₃O₄@Pdop@Au NPs are shown in Fig. 5. It can be seen that the magnetic saturation values of these are about 25, 10 and 8 emu g ⁻¹, respectively. The decrease in magnetic saturation of the CNTs/Fe₃O₄@Pdop in comparison with CNTs/Fe₃O₄ may be attributed to the increased mass of the modified Pdop shell on the surface of the CNTs/Fe₃O₄. After reacting with HAuCl₄, the saturation magnetization of CNTs/Fe₃O₄@Pdop@Au is decreased a little compared to CNTs/Fe₃O₄@Pdop, this is also in accordance with our previous result.¹⁷ Nevertheless, the relatively saturation magnetization value was conducive to accomplish efficient separation with an external magnet, which was an advantage for their application. In the following catalytic experiment, the synthesized CNTs/Fe₃O₄@Pdop@Au (without adding NaBH₄) was selected due to the simpler method than those reported without additional reagents.

Fig. 5 Hysteresis loops of the magnetic CNTs/Fe₃O₄ (a), CNTs/Fe₃O₄@Pdop (b), and CNTs/Fe₃O₄@Pdop@Au (c) (without adding NaBH₄).

It has been long identified that Au NPs show excellent catalytic activity and selectivity on many catalytic reactions.^22–24 To evaluate the catalytic activity of the Au NPs decorated CNTs/Fe₃O₄@Pdop core–shell nanocomposites, the reduction of MB (methylene blue) by NaBH₄ was selected as a model system. The MB solutions exhibit a typical absorption peak at 610 and 665 nm. Without the Au catalyst, the reduction of MB proceeded at a very slow speed with addition of NaBH₄, the color of the MB solution almost disappear after several days.²⁵ To observe the whole catalytic process on the MB solution, the concentration of MB was set as 100 mg L⁻¹. When 1 mg CNTs/Fe₃O₄@Pdop@Au composite nanoparticles were added into the mixture of NaBH₄ and MB (100 mg L⁻¹), the dark blue mixture became transparent within 5 min. The changes are shown in Fig. 6. The catalytic reduction of the dye proceeds successfully, wherein no deactivation or poisoning of the catalyst is observed. The catalytic results reveal that the as-synthesized CNTs/Fe₃O₄@Pdop@Au nanocatalyst shows a higher catalytic performance.

Fig. 6 UV-vis absorption spectra of MB during the reduction catalyzed by CNTs/Fe₃O₄@Pdop@Au composite. (The arrows mark the increase of reaction time, showing the gradual reduction of MB with CNTs/Fe₃O₄@Pdop@Au catalyst).

Fig. S3† shows the conversion for each run which was measured by UV/vis spectroscopy. For CNTs/Fe₃O₄@Pdop@Au the reduction of MB drops slightly after each cycle, and it decreased gradually in subsequent runs to 95% at run 6.

Conclusions

In conclusion, we have demonstrated a facile method of preparing well-defined CNTs/Fe₃O₄@Pdop nanocable through the self-polymerization of dopamine at room temperature. The coating thickness can be finely tuned by changing the amount of precursor. The resulting Pdop coating is a versatile platform for reducing Au³⁺ to form CNTs/Fe₃O₄@Pdop@Au nanocomposite. Moreover, the size and density of Au nanoparticles can be easily adjusted by two chemical approach. In addition to utilize Pdop as both the reducing and coupling agent, the Pdop can only act as coupling agent to stabilize the noble metal nanoparticles. Therefore, this easy approach can promote the practical applications of CNTs/Fe₃O₄. More importantly, this method could be applied to other kinds of CNTs/metal, metal oxide composites.

Acknowledgements

The authors are grateful to the financial support by the National Science Foundation of China (no. 21305086). The Natural Science Foundation of Shanghai City (13ZR141830), Research Innovation Program of Shanghai Municipal Education Commission (14YZ138), the Special Scientific Foundation for Outstanding Young Teachers in Shanghai Higher Education Institutions (ZZGJD13016), and Start-up Funding of Shanghai University of Engineering Science.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07993k

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