Facile preparation of MnFe₂O₄/halloysite nanotubular encapsulates with enhanced magnetic and electromagnetic performances

Abstract

We synthesized novel magnetic nanotubular encapsulates with ferrite nanoparticles embedded into the inner channels of halloysite nanotubes (HNTs) for the first time. The nano-encapsulates with enclosed ferrite nanoparticles show significantly enhanced magnetic and electromagnetic performance when compared to that with external particles.

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Halloysite is a natural nanotubular clay mineral that has attracted great interest in materials science and engineering due to its large surface area, high porosity, and hollow structural characteristics.¹ As a clay mineral, it is usually used to produce porcelain products^2,3 or as a filler for polymers.^4–6 Spinel ferrites, with the general composition MFe₂O₄ (M = Co, Ni, Mn, Fe, etc.), are widely used as catalysts,^7,8 microwave absorbers,^9,10 magnetic materials,^11,12 and so on, due to their excellent catalytic ability and magnetic performance.¹³ Because of the potential for enhanced ability in water purification, microwave absorption and magnetic applications, many studies have fabricated various magnetic halloysite nanocomposites.^14–18 The nanotubular encapsulate is a novel nanostructure which may present superior properties to those of traditional nanocomposites. For example, Pan et al.¹⁹ developed a two-step method to prepare carbon nano-encapsulates embedded with catalytic particles. The nanocomposites with internal particles showed an order of magnitude higher CO conversion than the composite with external particles. The research into Fe-filled and Ag-filled carbon nanotubes prepared using the CVD method and so on has also shown extraordinarily high magnetic and catalytic performances.^20,21 However, the heterogeneity of halloysite nanotubes (HNTs), which have a silica outer surface and an alumina inner shell,^22,23 restricts the incorporation of magnetic nanoparticles into the inner channels of HNTs. In addition, the hydrophilic silica and alumina walls of HNTs are more unstable than carbon nanotubes²⁴ and consequently the nanotubes maybe destroyed by the usual methods used to fabricate nano-encapsulates. Lvov et al.²⁵ successfully incorporated silver nanorods into the channel of HNTs by selective etching of the alumina core. Nevertheless, halloysite/ferrite nanotubular encapsulates have not been reported to the best of our knowledge. Therefore, finding a facile and simple method to prepare halloysite/ferrite nano-encapsulates is of great importance.

In this paper, we present a facile one-step method to fabricate MnFe₂O₄/HNTs nano-encapsulates using metal oleate as a ferrite precursor and for the modification of HNTs (ESI†). Although selective modification between the silicate outer surface and the alumina inner surface is difficult, previous studies have revealed that modification with octadecyl phosphonic acid will result in HNTs with a hydrophobic core and a hydrophilic shell.²⁴ In this study, metal oleate was found to show similar selective modification of HNTs. The original HNTs are aligned nanotubes with an inner diameter of 10–25 nm and a shell thickness of 8–25 nm (Fig. 1a). After modification with metal oleate, the oleate complex preferentially attached onto the inner surface of HNTs, making the alumina core hydrophobic. Consequently, the metal oleate complex favourably entered into the inner channels of the HNTs, and ferrite nanoparticles were then formed in situ during the subsequent heat treatment. The TEM images show that ferrite nanoparticles are attached on both the inner and outer surfaces of HNTs, with most particles embedded in the inner channels (Fig. 1b). The average particle size is 22 nm (17–28 nm) on the outer surface and 13 nm (7–18 nm) in the inner channels due to the constraint of the HNTs. The shell thickness and inner diameter of HNT are slightly decreased to 6–15 nm and 8–22 nm, respectively, because of the modification by metal oleate and the thermal destruction under heat treatment. HRTEM images revealed that the d spacings of the nanoparticles are 0.30 nm for the internal particles and 0.49 nm for the external particles (Fig. 1c and d), which correspond to the (220) and (111) planes of MnFe₂O₄ (Jacobsite, JCPDF#74-2403), respectively. This result indicates that the constriction of the HNTs shifts the exposed facets of the ferrite nanoparticles to that with narrow spacings, which may result in distinctive physicochemical properties.^26–28 The crystal structures of natural halloysite and MnFe₂O₄/HNTs nanocomposites (HF-1 and HF-2, with ferrite nanoparticles mainly located on the inner and outer surfaces of the HNTs, respectively) were further studied by X-ray diffraction (Fig. S1†). Characteristic peaks of halloysite (10 Å) and anhydrous halloysite were observed in the XRD pattern of natural halloysite, while these peaks decreased or disappeared in the patterns of both HF-1 and HF-2, attributed to the thermal destruction of the HNTs. Only a minor peak at 2θ = 35° was detected to indicate the presence of MnFe₂O₄, either due to the relatively low mass ratio of ferrite compared to that of HNTs, or due to overlapping with the peaks of halloysite.

Fig. 1 TEM images of (a) natural halloysite, and (b) MnFe₂O₄/halloysite nano-encapsulates, and HRTEM images of MnFe₂O₄ nanoparticles located on the (c) inner and (d) outer surfaces of halloysite nanotubes.

The proportion of nanoparticles on the outer and inner surfaces of HNTs can be controlled by the addition of water and the contact time between HNTs and the metal oleate complex. When we added 2 g deionized water to the oleate complex during mixing with halloysite, the obtained sample (HF-2) exhibits a distinctive micro-morphology (Fig. S2†). Nanoparticles with a diameter of about 15–30 nm are dispersed on the outer surface of the HNTs, and even detached from the nanotubes. This is attributed to the fact that the addition of water can cause the metal oleate complex to form micro emulsions surrounded by water molecules. These micro emulsions are either attached to the hydrophilic outer surface of the HNTs or dissociated from the nanotubes, resulting in a halloysite composite with ferrite nanoparticles entirely on the outside of the HNTs. Fig. 2 shows the effect of contact time on the morphologies of the MnFe₂O₄/HNTs nano-encapsulates. Very few nanoparticles were attached on either the outer or inner surfaces of HNTs when the mixture of metal oleate and halloysite proceeded immediately without aging (contact time = 0 h). When the contact time was extended to 1 h, significantly more nanoparticles were embedded into the inner surface of HNTs, with some particles attached on the outer shell. When the time was further increased to 4 h or higher, many more nanoparticles were located on both the inner and outer surfaces of the HNTs. These results suggest that the metal oleate will preferentially modify the inner surfaces of HNTs first, and then the silicate shell, and therefore can be used to control the selective attachment of nanoparticles onto HNTs. The attachment of the nanoparticles on HNTs further determined the pore characteristics of the nanocomposites (Table S1†). All samples show relatively similar BET surface areas of 45–52 m² g⁻¹ and micropore volumes of about 0.02 cm³ g⁻¹. When most ferrite nanoparticles were located on the outer surface of the HNTs, the samples HF1-0 and HF-2 show increased mesopore volumes and average pore sizes, because mesopores will be constructed by the outer nanoparticles. The mesopore volume of HF1-1 decreased significantly when the massive ferrite nanoparticles filled the channels of the HNTs. When more nanoparticles were attached on the both sides of the HNTs as the contact time increased, the mesopore volumes recovered to about 0.22 cm³ g⁻¹ due to the contribution of the outer particles. The pore characteristics of the samples proved that the distribution of nanoparticles between the outer and inner surfaces of the HNTs can be controlled by the addition of water and contact time.

Fig. 2 TEM images of MnFe₂O₄/halloysite nanotubular encapsulates prepared at different contact times: (a) 0 h, (b) 1 h, (c) 4 h, and (d) 8 h. Scale bar: 50 nm.

It is well known that the properties of a nanomaterial are largely determined by its structure. Therefore, based on same mass ratio of MnFe₂O₄, the obtained nano-encapsulates (HF-1) should present different magnetic and EM performances to HF-2, with its nanoparticles located entirely on the outside of the HNTs. Both HF-1 and HF-2 exhibit relatively low magnetizations due to their low mass ratios of MnFe₂O₄ (Fig. 3). However, HF-1 shows a threefold higher saturation magnetization (M_s) value (4.21 emu g⁻¹) than that of HF-2 (1.09 emu g⁻¹), and its remanent magnetization value (0.26 emu g⁻¹) is double that of HF-2 (0.12 emu g⁻¹), but its coercivity (20 Oe) is half that of HF-2 (40 Oe). It is suggested that the MnFe₂O₄ nanoparticles in HF-1 and HF-2 are both paramagnetic materials, but HF-1 shows higher magnetization and lower coercivity values, maybe due to the constraint of the HNTs which changed the exposed facets and sizes of the ferrite particles.

Fig. 3 Magnetization curves of MnFe₂O₄/halloysite nanotubular encapsulates (HF-1) and MnFe₂O₄/halloysite nanocomposites (HF-2).

We further evaluated the EM performance of HF-1 and HF-2 to show the advantages of nano-encapsulates. As shown in Fig. S3,† HNTs, HF-1, and HF-2 exhibit comparable real permittivity (ε′) values of about 2.4–2.8 due to their similar conductivities. The imaginary permittivity (ε′′) values of these samples are also quite similar at most frequencies, except that HF-1 shows a tiny peak at a frequency of 13.5 GHz, attributed to the eddy effect of encapsulated MnFe₂O₄ nanoparticles.²⁹ Consequently, the calculated dielectric loss tangents (tan δ_e = ε′′/ε′) of HF-1 are much higher than those of HNTs and HF-2, showing a conspicuous peak with a maximum value of >0.1. The encapsulation of the ferrite nanoparticles also increased the complex permeability and magnetic loss (tan δ_m = μ′′/μ′) values (Fig. S4†), that is, HF-1 presents significantly higher μ′′ and tan δ_m values than those of HF-1 and HNTs at 14–18 GHz. Furthermore, the resonance peaks of HF-2 and HNTs appear at 12 GHz, while that of HF-1 emerges at 14.5 GHz, indicating the different magnetic behaviors of the inner and outer nanoparticles.

We compared the EM wave absorption performance of HNTs, HF-1, and HF-2 by calculating their reflection loss (RL) values for microwaves. At a measured thickness of 2 mm, HNTs and HF-2 show similarly poor RL values of >−2 dB at all frequencies (Fig. 4a). HF-1 shows much improved EM wave absorption, with a maximum RL of −5.4 dB and bandwidths of RL < −5 dB at 14.0–14.5 GHz, due to the increased dielectric loss and magnetic loss values. Although these values are quite low for efficient EM wave absorbers, the variations of RL as a function of frequency and thickness (Fig. 4b–d) indicate that HF-1 is a potential microwave absorber at frequencies of 10–18 GHz and thicknesses of >7.5 mm. At a thickness of 9.2–9.7 mm, the maximum RL and bandwidths of RL < −5 dB and RL < −10 dB achieved were −36 dB, 6.12 GHz, and 4.08 GHz, respectively, which are above the average of traditional microwave absorbers.³⁰ Therefore, the encapsulation of ferrite nanoparticles can enhance the EM performance of magnetic halloysite nanocomposites to be possible microwave absorbers even at low mass ratios and magnetizations.

Fig. 4 (a) Frequency dependence of the reflection loss (RL) of microwaves by natural halloysite (HNTs), MnFe₂O₄/HNTs nano-encapsulates (HF-1), and MnFe₂O₄/HNTs nanocomposites (HF-2). (b and c) Variations of RL achieved by HF-1 as a function of thickness and frequency. (d) Variations of maximum RL, bandwidth of RL < −5 dB, and bandwidth of RL < −10 dB achieved by HF-1 as a function of thickness.

Conclusions

In summary, we successfully fabricated novel nanotubular encapsulates with MnFe₂O₄ particles embedded into the inner channels of HNTs. The metal oleate complex acted as both the precursor for the ferrite nanoparticles and a modifier for the HNTs. The attachment of nanoparticles could be controlled by the addition of water and the variation of contact time, that is, the increase in contact time enhanced the attachment of nanoparticles, while the addition of water detached the particles from the outside of the HNTs. The nano-encapsulates show enhanced magnetic and EM performance when compared to the composite with nanoparticles on the outside of the HNTs. With strong microwave absorption at frequencies of 10–18 GHz and thicknesses of >7.5 mm, the obtained MnFe₂O₄/HNTs nano-encapsulates are potential EM wave absorbers. This study opens a new way to fabricate magnetic halloysite nanocomposites, and further studies on the preparation and application of other halloysite nanotubular encapsulates and its encapsulation mechanism are now in progress.

Acknowledgements

This research was supported by the Research Fund for the Doctoral Program of Higher Education of China (20110101120045) and the Programs for Zhejiang Leading Team of S&T Innovation (2010R50036).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46873a

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