Ni-modified Ti₃C₂ MXene with enhanced microwave absorbing ability

Abstract

In this work, we have demonstrated the enhancement of the microwave absorbing capability of Ni-modified MXene within a frequency range of 2–18 GHz. The Ni-modification makes MXene able to attenuate microwave energy through magnetic loss as well as a strong dielectric loss. The effective absorbing bandwidth is greatly broadened, especially at the lower frequency of 2–8 GHz. The synergistic effect of dielectric loss and magnetic loss and the structure effect are believed to be the main reasons behind the broadening of the effective microwave absorbing bandwidth.

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1. Introduction

Materials with laminated, porous and 3D-network structures are proved to be broad band and tunable high-performance microwave absorbers, and the individual microstructures and abundant interfaces are thought to be the main reasons for the outstanding microwave absorbing ability.^1–3 Among them, two dimensional (2D) materials are the most widely studied, owing to their low density and superior mechanical flexibility. Since the discovery and exploration of graphene in the past decade,^4–8 interest in two-dimensional materials has greatly increased due to their unique microstructures and variety of applications, especially for microwave absorbing materials.

In 2011, a new 2D material labeled as MXene⁹ was synthesized by selectively etching Al atoms from Ti₃AlC₂, which is a representative of the famous MAX phase (a family of ternary transition metal carbides and/or nitrides with a general formula of M_n+1AX_n, where n = 1, 2 or 3, M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen)¹⁰ The exact formula of MXene is Ti₃C₂T_x, where Ti₃C₂ is the inner skeleton, and T_x is the active surface terminations (–OH and/or –F). And because of its unique microstructure, MXene is highly hydrophilic with a good electrical conductivity. Besides, MXene possesses a tunable interlayer space, which can be adjusted by spontaneous ion and/or molecule intercalation.^11,12

MXenes^9,13 have already shown great potential in the fields of energy storage devices,^14–18 supercapacitors,¹⁹ catalysts,^20,21 smart sensors,²² sewage treatment,^20,23 electromagnetic interference shielding materials,^24,25 microwave absorbers^26–30etc. Particularly for microwave absorbing materials, there exist several superior properties for microwave absorption. Firstly, the fierce chemical reaction of Ti₃AlC₂ with hydrofluoric acid in the synthesis process derives abundant intrinsic defects, which make the derived MXene capable as a microwave absorbing material for a strong dielectric relaxation and polarization at these defect sites. Secondly, the active –OH and/or –F terminations make it possible to graft MXene with electronegative atoms/groups/molecules, which capacitate a tunable microwave absorbing performance. Finally, the laminated microstructure provides abundant interfaces, which could offer enhanced multi-internal-reflections and enhanced microwave absorbing ability. In our previous work,³¹ we have already exhibited the capable microwave absorbing ability of Ti₃C₂ MXene within the frequency range of the C-band (4–8 GHz), X-band (8–12 GHz) and Ku-band (12–18 GHz), respectively, and the underlying mechanisms of the dielectric responses are intensively discussed.

Herein, we have presented a surface Ni-modification approach to enhance the microwave absorbing performance of Ti₃C₂ MXene. Ni-modification is conducted through the spontaneous chemical reaction between alkalized Ti₃C₂ MXene and Ni-ions. After the surface Ni-modification, MXene exhibits considerable magnetic responses and enhanced microwave attenuation performance with a broadened effective microwave absorption bandwidth. The derived magnetic responses from Ni-modification work together with the intrinsic dielectric loss and multi-internal-reflections to be responsible for this enhanced microwave absorbing performance.

2. Experimental

2.1 Sample preparation

Ti₃C₂ MXene used in this work is synthesized by immersing commercially available Ti₃AlC₂ powders (purity >95%, d₅₀ = 30 μm, provided by Forsman Scientific (Beijing) Co. Ltd) in 49 wt% aqueous HF in an ultrasonic homogenizer for 4 h as reported in our previous work.³² The as-derived Ti₃C₂ MXene nanopowders are separated by centrifuge and washed with distilled water until pH ≈ 7.

1 M KOH solution is prepared by dissolving 5.6 g KOH in 50 ml deionized water, transferring it to a 100 ml volumetric flask, and adding water to the scale bar. The Ni²⁺ containing solution is made by dissolving 23.8 g NiCl₂·6H₂O in deionized water in a 100 ml volumetric flask (1 M Ni²⁺ containing solution).

To derive Ni-modified MXene, an alkalized MXene (alk-MXene) is firstly prepared by immersing Ti₃C₂ MXene in KOH solution for 24 h with stirring. The alk-MXene is then separated by centrifuge and added into the Ni²⁺ containing solution for 24 h. After this the as-derived Ni-modified MXene is separated by centrifuge and dried in a vacuum oven for 12 h. To evaluate the microwave absorbing ability, donut-shaped samples with 7.0 mm outer diameter and 3.0 mm inner diameter are prepared by mixing the as-prepared Ni-modified MXene powders with molten paraffin with a weight ratio of about 1 : 1.

2.2 Characterization

The phase composition and crystal structure analysis is conducted on an X-ray diffractometer (D/max-2400, Rigaku) using Cu Kα radiation (λ = 0.1546 nm) and a step scan of 0.02° with 1 s per step, operating at 40 kV and 20 mA, respectively, with data analysis performed with Jade software. The microstructure is characterized by Scanning Electron Microscopy (SEM) on a Quanta 400, FEI, America, and TEM operated at 200 kV on a JEM 2100, Japan; Energy-Dispersive Spectroscopy (EDS) is performed using this instrument (Oxford EDS, with INCA software). All the measurements are carried out at room temperature.

The electromagnetic (EM) parameters (relative permittivity ε_r and permeability μ_r) of each sample are determined using a coaxial line method with a vector network analyzer (Agilent AV3618). For accuracy of measurement, the system is carefully calibrated with a Short-Open-Load-Through (SOLT) approach. Based on theory of transmission lines,^33,34 the reflection loss (RL) of a metal-backed absorber with ε_r and μ_r could be expressed as:

andwhere Z_in refers to input impedance, j is the imaginary unit (i.e. equals to ), c is the velocity of electromagnetic waves in free space, f is the microwave frequency, and d is the thickness of the samples.

3. Results and discussion

The typical diffraction peak (002) of Ti₃C₂ MXene appears at around 2θ = 8.98°, indicating that the interlayer distance of Ti₃C₂ is about 0.98 nm according to Bragg equation: 2d sin θ = nλ.³⁵ After Ni modification, this peak shifts to a lower angle around 2θ = 6.72°, as shown in Fig. 1, indicating that the c-lattice parameter expanded to about 1.32 nm. This distinct downshift is usually caused by ions and/or small molecules intercalating into the interlayer of Ti₃C₂ MXene.^9,11,12,36 Here the Ni ion intercalation into the MXene layers and the Ni modification with the active surface terminations are effectuated simultaneously. However, different from the single process of Ni-intercalation, the process of Ni-modification includes both the intercalations and modifications, and also the modification is irreversible, due to a stable chemical bond derived from the active alkalized surface terminations and nickel ions.

Fig. 1 XRD pattern of standard Ti₃AlC₂, Ti₃C₂ MXene and the as-produced Ni-modified Ti₃C₂ MXene.

Fig. 2a shows the typical morphology of the cross-section of a Ti₃C₂ MXene particle with an accordion-like structure. Energy dispersive X-ray spectroscopy (Fig. 2b) reveals that the laminated materials are composed of Ti, C, O, F and a little Al. The excess O and F signal could be ascribed to the surface terminations (–F and/or –OH), and possibly the intercalated water and hydrofluoric acid between Ti₃C₂ layers.^9,32 The remaining Al signal is believed to be originated from the reaction product, AlF₃, which has not been fully washed away from the particles. After being treated in Ni²⁺ containing solution, the open accordion-like structure of Ti₃C₂ MXene seems to be filled with Ni ions as can be seen from Fig. 2c, and the EDS spectrum (Fig. 2d) indicates that the chemical compositions of Ni-modified MXene are Ti, C, O and Ni. The remaining F and H atoms in pristine Ti₃C₂ MXene are alkalized into KOH solution after the alkalization process. For this reason, they are not detected in the Ni-modified MXene. Since Ni-modification is a kind of surface chemical modification, the amount of Ni atoms is small compared to the base elements of Ti₃C₂ MXene.

Fig. 2 (a) SEM image and (b) EDS spectrum of pristine Ti₃C₂ MXene; (c) SEM image and (d) EDS spectrum of the as-derived Ni-modified MXene.

Based on the above results, the evolution process from Ti₃C₂ MXene to Ni-modified MXene can be described as follows (as showing in Fig. 3). When Ti₃C₂ MXene is immersed in KOH solution, the faintly acidic surface terminations (–OH and/or –F) of Ti₃C₂ MXene could react with KOH and get alkalized. After transferring the alkalized MXene into Ni-ion containing solution, Ni-ions could spontaneously sedimentate on the surface of MXene due to a simple chemical rule that Ni ions are only stable under acidic conditions. The chemical process can be described as the following equation:

Fig. 4 exhibits a comparison of the frequency dependencies of complex permittivity and permeability in the range of 2–18 GHz of pristine Ti₃C₂ MXene and the Ni-modified MXene. As can be seen from the picture, both the real and imaginary parts of the permittivity of pristine MXene and the Ni-modified MXene are fluctuated in the same trend within an examined frequency range. However, the Ni-modification makes a reduction of permittivity, both the real and imaginary part, for the reason that Ni-modification could improve the conductivity of MXene. This reduction could effectively help optimize the impedance matching with microwaves, which is beneficial for microwave absorption. What is more, the permeability of Ni-modified MXene shows a relatively considerable magnetic response compared to that of pristine MXene. It is reasonable to speculate that this obvious enhancement of complex permeability of Ni-modified MXene compared with that of pure MXene (i.e. μ_r ≈ 1) is originated from surface modification of Ni-ions. Furthermore, this synergistic effect of decreased permittivity and enhanced permeability brings them closer, which is quite essential to improve impedance the matching and microwave absorbing ability.

Fig. 3 A schematic illustration of the process of alkalization and Ni-modification of Ti₃C₂ MXene. (This figure is prepared by author Wanlin Feng using the photoshop software).

Fig. 4 Electromagnetic parameters of (a) pristine Ti₃C₂ MXene and (b) as-produced Ni-modified MXene.

As demonstrated in Fig. 5a, Ti₃C₂ MXene, a typical dielectric loss microwave absorber,³¹ shows good microwave absorbing performance at six typical values of thickness within a frequency range of 12.3–18 GHz, 9.2–12.1 GHz, 7.2–9.5 GHz, 4.8–6.2 GHz, 3.6–4.8 GHz, and 2.9–3.8 GHz, where reflection loss (RL) is below −10 dB, which suggests that more than 90% of incident microwave energy has been absorbed. However, Fig. 5b shows the enhancement of microwave absorbing performance of Ni-modified MXene at the same thicknesses. To have a better visual view of the enhancement of microwave absorbing performance, a histogram is prepared in Fig. 5c. The black column represents the effective absorbing bandwidth (where RL <−10 dB) of pristine Ti₃C₂ MXene at five typical thicknesses, while the red column is that of Ni-modified MXene (Fig. 5c). It can be observed from the histogram that Ni-modification leads to a great improvement of microwave absorption performance. The effective absorbing bandwidth is greatly enhanced from 5.7 GHz, 2.5 GHz, 1.5 GHz, 1.07 GHz and 0.84 GHz to 6.3 GHz, 5.5 GHz, 2.5 GHz, 1.75 GHz and 1.6 GHz, respectively.

Fig. 5 Reflection loss of (a) Ti₃C₂ MXene and (b) Ni-modified MXene at six typical values of thickness, and (c) a comparison of the effective absorbing bandwidth (RL < −10 dB) between Ti₃C₂ MXene and Ni-modified MXene.

To figure out the underlying mechanism of the enhanced microwave absorbing performance, the dielectric loss tangent (tan δ_E = ε′′/ε′) and magnetic loss tangent (tan δ_M = μ′′/μ′) are calculated based on the permittivity and permeability shown in Fig. 4. As can be clearly seen from Fig. 6a, Ti₃C₂ MXene is a typical dielectric loss microwave absorber with a strong dielectric loss ability (dielectric loss tangent keeps over 0.4 during the examined frequency range), especially in a high frequency range of 13–18 GHz, and with no magnetic loss ability (magnetic loss tangent keeps around 0). However, due to the magnetic responses ascribed to Ni-modified MXene, the magnetic loss tangent keeps over 0.5 during the frequency range of 2–8 GHz, suggesting a strong microwave energy attenuation capability of magnetic loss (Fig. 6b). Although the magnetic loss tangent decreased to 0.25 within a frequency range of 8–15 GHz, it also means a considerable magnetic loss capability compared to that of pristine Ti₃C₂ MXene. What is more, the Ni-modified MXene still keeps a strong dielectric loss capability over the whole examined frequency range (2–18 GHz). As a result, we can come to the conclusion that the synergistic effect of dielectric loss and magnetic loss is supposed to be responsible for the broadening of the effective microwave absorbing bandwidth. In addition, the laminar structure provides abundant interfaces, which could offer enhanced multi-internal-reflections and could help for the enhanced microwave absorbing ability.

Fig. 6 The dielectric loss tangent and magnetic loss tangent of Ti₃C₂ MXene and Ni-modified MXene.

4. Conclusions

Samples of Ni-modified MXene are successfully prepared by simple alkalization and surface Ni-ion modification of Ti₃C₂ MXene. XRD, FESEM and EDS characterization confirms the modification of Ni elements. The Ni-modification makes Ti₃C₂ MXene able to attenuate microwave energy through magnetic loss as well as a strong dielectric loss. The effective absorbing bandwidth is greatly enhanced from 5.7 GHz, 2.5 GHz, 1.5 GHz, 1.07 GHz and 0.84 GHz to 6.3 GHz, 5.5 GHz, 2.5 GHz, 1.75 GHz and 1.6 GHz, respectively. Furthermore, the synergistic effect of decreased permittivity and enhanced permeability caused by Ni-modification brings them closer, which is quite essential to improve impedance matching. Finally, the collaboration of dielectric loss and magnetic loss working together with multi-reflections ascribed to laminar structures, is supposed to be responsible for broadening the microwave absorbing bandwidth.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

Haibin Zhang is grateful to the foundation by the Recruitment Program of Global Youth Experts and the Youth Hundred Talents Project of Sichuan Province. This work was supported by the National Natural Science Foundation of China (Grant No. 91326102), China National Science and Technology Major Special Project ‘Research on Accident Tolerant Fuels Key Technology’ (Grant No. 2015ZX06004-001), and China Postdoctoral Science Foundation (Grant No. 2015M580798 and 2017M612996). Lianwen Deng is grateful to the foundation by the National Key Research and Development Program of China (Grant No. 2017YFA0204600) and the Provincial Science and Technology Program (Grant No. 2015JC3041) of Hunan.

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Footnote

† Wanlin Feng and Heng Luo contributed equally to this work.

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