Hyperbranched copper phthalocyanine decorated Fe₃O₄ microspheres with extraordinary microwave absorption properties

Abstract

Novel hierarchical Fe₃O₄/hyperbranched copper phthalocyanines (Fe₃O₄–HBCuPc) composites were prepared via a simple solvent-thermal method. HBCuPc molecules were not only attached to the surface of Fe₃O₄ in the form of beads, but embedded in the interior. Importantly, the Fe₃O₄–HBCuPc composites exhibited enhanced microwave absorption properties after introducing the HBCuPc. The minimum reflection loss (R_L) values could reach −30.3 dB at 10.2 GHz and the bandwidth below −10.0 dB increased up to 10.6 GHz, which covers the entire X-band and Ku-band (7.4–18.0 GHz). The introduction of HBCuPc can lead to improved efficient complementarities between the dielectric loss and the magnetic loss in the Fe₃O₄–HBCuPc, which can contribute to microwave absorption. Considering its lightweight, strong absorption and broad bandwidth, the as-prepared Fe₃O₄–HBCuPc can be applied as a new microwave absorber.

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Introduction

Recently, electromagnetic wave (EM) absorption materials have attracted great interests with the increasing use of telecommunications, personal digital assistants, and satellite communications, which can eliminate the electromagnetic pollution.^1–3 Particularly, the development of high-performance EM absorbing materials are in high demand, such as light weight and strong absorption with broad absorption bandwidth.⁴ To date, various nanostructures have been considered for high performance EM absorption, particularly in the nanostructure Fe₃O₄-based materials.^5–8 Many conducting polymers⁹ and carbon materials^5,10 have been introduced into the nanostructure Fe₃O₄ to improve the EM absorption. Recently, more studies have focused on introducing dielectric materials (such as ZnO,¹¹ TiO₂ (ref. 12) and reduced graphene oxide¹³). The introduced dielectric materials can improve the dielectric constant and loss, and the matching between the dielectric loss and magnetic loss can be achieved.¹⁴ In addition, the composition and structure of the nanomaterials are also important for EM absorption.¹⁵ Hence, nanostructured Fe₃O₄-based desired dielectric materials with well-defined structures are valuable for excellent EM absorption performance.

In our previous work, combining the dielectric metal phthalocyanine oligomers (MePc) and magnetic Fe₃O₄ have been proven to lead to good microwave absorption properties by a one-step solvent-thermal route.^16–18 This is because the MePc possess good dielectric responses^19–22 that lead to good dielectric loss to support the complementarities between the dielectric loss and the magnetic loss. However, the absorption bandwidth is very narrow and dissatisfies the requirements of broadband absorption. To address these problems, hyperbranched copper phthalocyanines (HBCuPc) were first introduced into the nano Fe₃O₄. HBCuPc, whose molecular structure is shown in Scheme S1 (the ESI†), provides more π conjugated electron carriers owing to the hopping of long-range electrons, which can lead to a high dielectric response,^23,24 and its dielectric constant can reach 10⁶. Hence, combining the dielectric HBCuPc and magnetic Fe₃O₄ can bring better impedance matching and therefore promote the EM absorption performance.

Herein, we extended our work to introduce the dielectric HBCuPc into Fe₃O₄ via a simple solvent-thermal method. HBCuPc nanobeads can be detected on the Fe₃O₄ surface, and the diameter of Fe₃O₄–HBCuPc decreased evidently. More importantly, the broad and strong microwave absorption can be obtained after the introduction of HBCuPc. The minimum reflection loss (R_L) value of Fe₃O₄–HBCuPc treated for 10 h is −30.3 dB at 10.2 GHz and the absorption bandwidth below −10.0 dB is 10.6 GHz, when the thickness is 4.0 mm.

Experimental

Materials

4,4′-Bis(3,4-dicyanophenoxy)biphenyl (BPH) was synthesized following a previous report.²⁵ Fe₃O₄ particles were prepared according to a previous report.²⁶ Ethylene glycol (99%), CuAc₂·H₂O and ammonium molybdate were purchased from Chengdu Kelong chemical reagents. All the chemicals were of analytical grade and used without further purification.

Preparation of hierarchical Fe₃O₄–HBCuPc nanostructures

In a typical preparation, Fe₃O₄ particles (50 mg) were dispersed in ethylene glycol (40 mL) with sonication, and then BPH (0.900 mmol), CuAc₂·H₂O (0.100 mmol) and ammonium molybdate (5 mg) were added. Subsequently, the solution was sealed in a Teflon-lined autoclave and heated to 160 °C for 5, 10 and 15 h. After cooling to room temperature, the products were washed with ethanol under ultrasound and dried at 60 °C for 8 h.

Characterization

The as-prepared Fe₃O₄–HBCuPc composites were characterized by Fourier transform infrared (FTIR) spectroscopy (Shimadzu, 8000S) by a KBr pellet, UV-vis spectroscopy (Shimadzu, UV-3150), X-ray diffraction (XRD) (Rigaku, RINT2400using Cu Kα radiation), thermogravimetric analysis (TGA) (TGA-Q50, heating rate of 20 °C min⁻¹ under nitrogen), X-ray photoelectron spectroscopy (XPS) (VG Microtech, ESCA 2000), scanning electron microscopy (SEM) (JSM, 6490LV) and transmission electron microscopy (TEM) (Hitachi, H-600). The magnetic properties were measured by a vibrating sample magnetometer (VSM) (Riken Denshi, BHV-525). The EM parameters were measured by an Agilent 8720ET vector network analyzer in 0.5–18.0 GHz range. The measured samples were prepared by homogeneously mixing 33 wt% of Fe₃O₄–HBCuPc with a wax matrix, and the mixture was then made into toroidal-shaped samples (φ_out: 7.00 mm, φ_in: 3.0 mm).

Results and discussions

Fig. 1 shows the size and morphology of the as-prepared Fe₃O₄ and Fe₃O₄–HBCuPc with different solvent-thermal treatment time. The Fe₃O₄ particles (Fig. 1a) showed monodisperse microspheres with an average size of 400 nm, and many cracks and open pores could be clearly observed, indicating the porous spheres structure of the Fe₃O₄. Moreover, differences of the morphologies were identified between the Fe₃O₄ and Fe₃O₄–HBCuPc. When treated for 5 h, the Fe₃O₄–HBCuPc showed hierarchical spheres with a relatively rough surface and the porous structure of microspheres disappeared (Fig. 1b). Moreover, the average diameter of the spheres decreased to about 300 nm, and nanobeads could be detected on the surface of each Fe₃O₄ particle. This shows that Fe₃O₄ can react with the BPH to decrease the size of the Fe₃O₄ particles; moreover, the nanostructure HBCuPc can be formed and coated on the surface of Fe₃O₄ in the form of nanobeads. With increase in treatment time, the diameter of Fe₃O₄–HBCuPc still decreased and more nanobeads can be observed, as shown in Fig. 1c to d, indicating it is a continuous reaction. The distinct contrast between Fe₃O₄ and Fe₃O₄–HBCuPc was further confirmed by TEM. From the TEM image of Fe₃O₄ (Fig. 1e), the spheres have pale center region in contrast to the dark edge and open pores (indicated by the red arrows) could also be found, further confirming the hollow interior structure of Fe₃O₄. However, after introducing the HBCuPc (shown in Fig. 1f), the hollow structure disappeared and evidently many nanobeads grew on the Fe₃O₄ microspheres. This demonstrated that the HBCuPc molecules were not only attached to the surface of Fe₃O₄ in the form of beads, but also embedded in the interior.

Fig. 1 SEM images of (a) Fe₃O₄, Fe₃O₄–HBCuPc treated for (b) 5 h, (c) 10 h and (d) 15 h; TEM images of (e) Fe₃O₄ and (f) Fe₃O₄–HBCuPc treated for 15 h.

The successful introduction of HBCuPc into Fe₃O₄ was examined by FTIR spectroscopy, as shown in Fig. 2a. The absorption peaks at 822, 1011, 1141, 1406 and 1600 cm⁻¹ corresponded to the phthalocyanine skeletal vibrations,²⁷ and the other bands at 580 and 2231 cm⁻¹ belonged to the absorption peak of Fe₃O₄ (ref. 12) and cyano groups,¹¹ respectively. Furthermore, the absorption peak at 2231 cm⁻¹ gradually disappeared with prolonging solvent-thermal treatment time, whereas the peak at 1011 cm⁻¹ increased (shown in arrows), suggesting the extra cyano groups of BPH continue to react with the cyano groups of CuPc. Thus the contents of the CuPc unit gradually increased, which was in accordance with our previous work.¹¹

Fig. 2 (a) FTIR spectra, (b) UV-vis spectra, (c) XRD pattern and (d) TGA of Fe₃O₄–HBCuPc solvent-thermal treated for 5 h, 10 h and 15 h.

UV-vis spectroscopy was carried out to confirm the existence of HBCuPc in the Fe₃O₄–HBCuPc (Fig. 2b). The absorption bands at 600–750 nm were attributed to the Q-band of HBCuPc. The absorption peak at 700 nm could be assigned to the Q_0–0 band of the HBCuPc monomers, and the shoulder peak at 630 nm was due to the aggregated dimers and multimers of HBCuPc.²⁸ Importantly, the absorption intensity of Q-band increased with increasing treatment time, indicating an increase in the hyperbranched degree of CuPc in HBCuPc. The 300–400 nm bands belonged to the B-band of HBCuPc, and the 200–250 nm bands corresponded to the C-band.^25,28 Hence, the HBCuPc can be formed in the Fe₃O₄ by solvent-thermal treatment.

Fig. 2c showed the XRD pattern of the Fe₃O₄–HBCuPc treated with different times. As observed in Fig. 2c, the as-prepared samples matched well with the cubic Fe₃O₄ (JCPDS Card no. 19-0629), and the peak intensity of Fe₃O₄–HBCuPc gradually decreased with increasing treatment time, indicating the existence of noncrystalline and nanocrystalline material. Moreover, the diffraction peaks of HBCuPc were not detected, indicating the HBCuPc existed in a non-crystalline molecular state.

From the TGA curve (Fig. 2d), it was calculated that about 12.90%, 26.37% and 32.10% of HBCuPc were introduced. The gradually increasing contents originated from the increase in hyperbranched degree of CuPc after the solvent-thermal treatment. In addition, the saturation magnetization of Fe₃O₄–HBCuPc decreased from 63.5, 48.1 to 24.7 emu g⁻¹ with prolonging treatment time, which is due to the increase in HBCuPc content in Fe₃O₄–HBCuPc (shown in Fig. S1 and Table S1†).

The chemical composition of Fe₃O₄–HBCuPc was studied by XPS, as shown in Fig. 3. It can be seen that Fe, C, and O existed in pure Fe₃O₄, whereas Fe, C, O, N and Cu existed in the Fe₃O₄–HBCuPc. Furthermore, the content of Fe and O in the Fe₃O₄–HBCuPc decreased compared to that of Fe₃O₄, further confirming the existence of HBCuPc. In Fig. S2a,† the peaks at 711 eV and 724 eV were assigned to Fe 2p_3/2 and Fe 2p_1/2, respectively, indicating the presence of metallic Fe₃O₄, and a new peak at 709 eV was observed, which corresponded to the Fe–N, and confirmed the formation of iron phthalocyanine derivatives.²⁹ For the Cu 2p region (Fig. S2b†), there were two peaks at 933 eV and 953 eV, which corresponded to Cu 2p_3/2 and Cu 2p_1/2. This demonstrated that Cu existed in the form of Cu²⁺.

Fig. 3 XPS fully scanned spectra of Fe₃O₄ and Fe₃O₄–HBCuPc.

Based on the above mentioned results, a possible formation mechanism of Fe₃O₄–HBCuPc was proposed. During the solvothermal process, the BPH molecules reacted with copper ions and iron ions to form hyperbranched copper phthalocyanines and iron phthalocyanines, respectively.^19–24 Moreover, iron phthalocyanines link Fe₃O₄ and HBCuPc as a bridge. With increasing treatment time, a highly conjugate HBCuPc structure can be formed through the self-assembly of phthalocyanine rings, which attached to the Fe₃O₄ surface in the form of HBCuPc beads.

The EM absorption properties of Fe₃O₄–HBCuPc were investigated according to the relative complex permittivity (ε_r = ε′ − jε′′) and complex permeability (μ_r = μ′ − jμ′′). The reflection loss (R_L) values were calculated by the following equations:³⁰

where c is the light velocity, f is the frequency, μ_r is the relative complex permeability, ε_r is the relative complex permittivity, and d is the coating thickness. After the calculation of R_L (shown in Fig. 4), it was found that the Fe₃O₄–HBCuPc exhibited excellent microwave absorption properties with strong absorption and a broad bandwidth compared with that of pure Fe₃O₄ (shown in Fig. S3†). For the Fe₃O₄–HBCuPc treated for 5 h (Fig. 4a), a strong and sharp R_L peak without frequency dependence can be observed at 17.1 GHz, and its minimum R_L is −28.2 dB with a thickness of 4.0 mm. In addition, one broad R_L peak was also observed and the minimum R_L values were sensitive to the coating thickness, which can shift to a low frequency range with increasing thickness. Noticeably, after a prolonged treatment time, considerably stronger R_L and broader bandwidths were observed. For the Fe₃O₄–HBCuPc treated for 10 h (Fig. 4b), the minimum R_L is −30.3 dB at 10.2 GHz with a thickness of 4.0 mm, and one broad and strong wave absorbing peak (12.0–18.0 GHz, −18.3 dB) was achieved. The absorption bandwidth of R_L below −10.0 dB (over 90% microwave absorption) can reach 10.6 GHz in the 7.4–18.0 GHz range, covering the entire X-band and Ku-band. To the best of our knowledge, there is no report of such a broad bandwidth. For the Fe₃O₄–HBCuPc treated for 15 h (Fig. 4c), there was only one broad and strong R_L peak in 7.7–18.0 GHz range, and the minimum R_L reached −28.2 dB at 9.2 GHz with a thickness of 4.5 mm. Moreover, the bandwidth of R_L below −10.0 dB can reach 10.3 GHz. Fig. 4d shows the R_L values with a thickness of 4.0 mm. It can be seen that the Fe₃O₄–HBCuPc treated for 10 h exhibited the strongest EM absorption properties, including the minimum R_L value (−30.3 dB, 10.2 GHz) and broadest absorption bandwidth (10.6 GHz). In addition, the introduction of HBCuPc can lead to a broader bandwidth than that of CuPc,¹⁷ and the extraordinary EM absorption of the as-prepared Fe₃O₄–HBCuPc can also be confirmed by a comparison with other Fe₃O₄-based magnetic materials (Table S2†).

Fig. 4 Reflection losses of the Fe₃O₄–HBCuPc treated for (a) 5 h, (b) 10 h, (c) 15 h, and (d) reflection losses of different samples with the thickness of 4.0 mm.

To reveal the possible EM absorption mechanism, the relative complex permittivity and relative permeability of Fe₃O₄–HBCuPc were investigated in the 0.5–18.0 GHz range (Fig. 5). As shown in Fig. 5a, with increasing treatment time, the ε′ increased remarkably from 3.4 to 4.2 and 5.3 at 0.5 GHz, and it remained constant up to about 11.0 GHz and decreased with fluctuations in the 11.0–18.0 GHz range. Moreover, the ε′′ gradually increased with fluctuations and also increased with prolonging treatment time in 0.05–18.0 GHz range (Fig. 5b). The evident frequency dispersion effect in the high frequency range plays a critical role in the absorption bandwidth and absorption capacities,³¹ which exhibited better frequency dispersion than that of our previous report.¹⁷ Higher values of ε′ and ε′′ can be obtained by prolonging the treatment time after introducing HBCuPc, compared to that of pure Fe₃O₄. The value of ε′′ is higher than that of ε′ in the 13.5–18.0 GHz. Hence, the dielectric tangent loss (tan δ_ε = ε′′/ε′) based on the permittivity of the three samples exhibits considerably higher dielectric loss than that of pure Fe₃O₄ (Fig. 5c). High ε′ and ε′′ is due to the introduction of high dielectric HBCuPc.^19–22 Particularly, highly conjugate HBCuPc can provide more charge carriers and large carrier mobility, which can support the high dielectric response and high electronic conductivity.^23,24 According to the free-electron theory,³² ε′′ = ρ/ε₀ω (ρ is the electrical conductivity), high conductivity would result in strong dielectric loss.³³ In addition, the strong interfacial polarization³⁴ between the Fe₃O₄ and HBCuPc and the special dendrimer system^20,21 also contributed to the dielectric loss.³⁴

Fig. 5 Frequency dependence of (a) real and (b) imaginary parts of the complex permittivity, and the corresponding (c) dielectric loss; (d) real and (e) imaginary parts of the complex permeability, and the corresponding (f) magnetic loss tangents of pure Fe₃O₄ and the Fe₃O₄–HBCuPc composites with different treatment time.

In Fig. 5d, the μ′ of the Fe₃O₄–HBCuPc gradually decreased with fluctuations with increasing frequency. In addition, the μ′′ first decreases at 0.5–6 GHz, and increases in the 6–19 GHz range, with high and broad resonance peaks. According to the natural-resonance equation,³⁵

2πf_r = rH_a

H_a = 4|K₁|/3μ_oM_s

where r is the gyromagnetic ratio (2.8 GHz kOe⁻¹) and H_a is the anisotropy energy. H_a increases with decreasing particle size, and the high coercivity (H_c), which corresponds to the reduced particle size, can result in a high H_a.³⁶ As shown in Table S1,† the H_c of three prepared samples is higher than that of pure Fe₃O₄. Hence, a high H_c can result in the high and broad resonance peaks;³⁶ i.e., high magnetic tangent loss (tan δ_μ = μ′′/μ′) can be obtained, as shown in Fig. 5f. Hence, the magnetic loss also contributes to the EM absorption.

Based on the above mentioned results, introducing HBCuPc into Fe₃O₄ can enhance the absorption capacities and absorption bandwidth. This is due to the efficient complementarities between the dielectric loss and the magnetic loss. For the Fe₃O₄–HBCuPc, combining the introduced dielectric HBCuPc as dielectric loss absorbers and Fe₃O₄ as magnetic loss absorbers can result in enhanced impedance matching, particularly when the hyperbranched degree of HBCuPc increased. As shown in Fig. S4,† the tendency of tan δ_ε and tan δ_μ are similar in the 0.5–18.0 GHz range, and the values are quite close, particularly the values from Fe₃O₄–HBCuPc treated for 10 h and 15 h. Moreover, interfacial features between Fe₃O₄ and HBCuPc also play important roles in the EM absorption. The multi-interfaces, i.e., Fe₃O₄–Fe₃O₄, Fe₃O₄–HBCuPc and HBCuPc–HBCuPc in Fe₃O₄–HBCuPc can result in significant interfacial polarization, and can enhance the dielectric loss at high frequency range,³⁴ matching the magnetic loss at a high frequency range.

In general, the as-prepared Fe₃O₄–HBCuPc showed extraordinary EM absorption, particularly the broader absorption bandwidth. Combining the dielectric HBCuPc and magnetic Fe₃O₄ is evidently advantageous to the EM absorption properties. Furthermore, highest attenuation obtained by enhanced dielectric constant and magnetic losses are similar to the presence of carbon nanotubes/cobalt nanowires/poly(vinylidene fluoride),³⁷ MWNT-g-Fe₃O₄/polycarbonate/poly(styrene-co-acrylonitrile),³⁸ and barium titanate/cobalt ferrite.³⁹

Conclusions

In conclusion, novel hierarchical Fe₃O₄–HBCuPc composites with enhanced EM absorption were successfully prepared by a simple solvent-thermal method. The introduction of dielectric HBCuPc can lead to enhanced efficient complementarities between the dielectric loss and the magnetic loss, thereby exhibiting excellent EM absorption properties. The minimal R_L can reach −31.6 dB at 17.1 GHz, and the absorption bandwidth below −10.0 dB can reach 10.6 GHz, which covers the entire X-band and Ku-band (7.4–18.0 GHz). Considering the low density of the Fe₃O₄–HBCuPc and only 33 wt% of Fe₃O₄–HBCuPc added to the wax matrix, the as-prepared Fe₃O₄–HBCuPc is quite promising as high-performance and light weight microwave absorption material.

Acknowledgements

Research was supported by “863” National Major Program of High Technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Chemical structure of the HBCuPc, magnetic properties of Fe₃O₄ and Fe₃O₄–HBCuPc with different treatment time, XPS spectra of Fe 2p and Cu 2p for Fe₃O₄–HBCuPc, the reflection loss of Fe₃O₄, absorption properties of the Fe₃O₄-based magnetic materials, and the relationships between tan δ_ε and tan δ_μ of Fe₃O₄–HBCuPc with different treatment time. See DOI: 10.1039/c4ra12443j

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