Bimetallic sulfides embedded into porous carbon composites with tunable magneto-dielectric properties for lightweight biomass- reinforced microwave absorber (2023)

The growing problems of electromagnetic interference caused by correspondence facilities and electronic equipment have been recognized, which has brought serious threat to telecom security, physical health and national defense security [1,2]. Hence, overcoming electromagnetic radiation and interference has become a crucial issue in modern society [3]. To address these challenging problems, it is highly desirable to explore efficient microwave-absorbing materials (MAMs) with a broad efficient absorption bandwidth (EAB), light weight, and thin thickness [4]. For decades, different kinds of materials, including carbon-based, conductive polymers, ceramics, metal-based materials, and their composites, have been extensively used for microwave absorption [[5], [6], [7], [8], [9]].

Carbon material, as a typical MAM, has the advantages of light weight, high electrical conductivity and superior stability [10]. For example, graphene [11], carbon nanocoils/nanotubes [12], hollow carbon nanospheres [13], porous carbon-based aerogel [14], and many other new carbon materials and their composites have broad prospects due to their pore structure, high electrical conductivity, large specific surface area and other advantageous characteristics [[15], [16], [17], [18], [19]]. Unfortunately, such carbonized materials are usually expensive to prepare and complicated to manufacture, making it challenging to scale up industrial production [20]. Thus, it has become critical to investigate inexpensive, readily available, and efficient carbon materials as alternatives for electromagnetic absorber applications [21]. To date, biomass porous carbon (BPC) materials have been utilized because of their advantages of being environmentally friendly, low cost, easy preparation, adjustable dielectric loss, and strong sustainable performance [22]. Currently, some biomass materials have been used to prepare high-performance MAMs, including loofah sponges [23], wood [24], banana peel [25], rice [26], and walnut shell [27]. For instance, Qiu et al. successfully prepared biomass-derived porous carbon structures by activating walnut shells with KOH and carbonizing them at 600°C. The results showed that the RL_min reached −42.4dB at 8.8GHz with a thickness of 2.0mm [28]. In addition, Negi and his co-authors used mango leaves as a carbon source to synthesize porous activated carbon with an RL_min of −23.3dB at 1.5mm [29]. Despite the excellent performance of BPC, poor impedance matching, low absorption strength, narrow absorption bandwidth, and other shortcomings have severely constrained its practical application [30]. Therefore, more research has been conducted to improve the electromagnetic wave (EMW) absorption performance of BPC.

In recent years, bimetallic sulfide MAMs with thin matching thicknesses and favorable chemical stability have been gradually applied to microwave absorption (MA) [31,32]. Transition metal sulfides have the characteristics of small energy band gap, high dielectric constant, low cost and rich resources [31]. Bimetallic materials increase the variety of nanoparticles and produce a large number of non-uniform interfaces, which is beneficial for the polarization losses. Importantly, bimetallic sulfides are conducive to microwave losses due to their higher electrochemical activity and energy storage capacity [32]. For example, Xing et al. reported a hydrothermal synthesis of MoS₂/FeS₂ composite with a minimum RL value of −60.2dB at 8.0GHz [33]. The solubility product constant (K_sp) can affect the formation order of different materials. Under the K_sp principle, one metal ion can be introduced into the solution of another metal ion as a competing reaction ion, and these two metal ions will react competitively with the target anion, leading to the formation of a bimetallic composite [34]. A specific number and distribution of heterojunction interfaces will be formed by modulating the two metal ions. Generally, a large number of charges will accumulate at the heterojunction interface, which can cause interfacial polarization and relaxation to consume electromagnetic energy. Some vacancies and lattice defects can be spontaneously generated in this competitive reaction, usually triggering a dipole polarization that greatly facilitates the enhanced dielectric polarization. Two suitable concentrations of metal ions can be selected using this mechanism to design and synthesize a composite. Copper-based bimetallic sulfides have gradually entered people's field of vision, and a series of copper-based bimetallic sulfides such as (i.e., Cu-M sulfide, Mⁿ⁺ = Fe³⁺, Co²⁺, Ni²⁺, Mn²⁺, and Mg²⁺) have shown some considerable impedance matching ability and magnetic loss ability [35]. Ferromagnetic metal (Co) can provide the material with excellent magnetic loss properties because of its high Curie temperature, considerable saturation magnetization, excellent magnetic permeability, and low anisotropy [36]. The introduction of Co²⁺ leads to the formation of Cu–Co bimetallic sulfides due to the stronger competition between Cu²⁺ and Co²⁺, which can cause changes in the components, interfaces, and defects and adjust the morphology, composition, electromagnetic parameters, and impedance matching characteristics of the reaction system [37]. However, bimetallic sulfides suffer from narrow bandwidth, poor electrical conductivity, and relatively low dielectric losses [38]. Previous studies have revealed that the synergistic effect of composite is beneficial for optimizing impedance matching and improving MA performance. Thus, the incorporation of Cu–Co sulfide into carbon materials can improve the magnetic loss capacity and balance the impedance matching ability of the absorber to a certain extent, so as to achieve better MA capacity.

Herein, inspired by the above, we designed a [emailprotected]₂S₄@CoS₂ composite. The BPC with high porosity and large specific surface area was prepared by NaOH activation and high-temperature carbonization using pistachio shell as the carbon source, and then CuCo₂S₄@CoS₂ was electrostatically self-assembled with the positively charged BPC on the surface. Due to the synergistic effects of the BPC and metal sulfide, the attenuation ability and impedance matching conditions of the composites can be effectively improved, resulting in a higher MA performance. The obtained [emailprotected]₂S₄@CoS₂ composite achieves a minimum RL of −64.2dB at 2.3mm. This study optimizes impedance matching by combining Cu–Co bimetallic sulfide with BPC to generate enhanced multiple reflection, dielectric loss, magnetic loss, and conductive loss, providing a new perspective for exploring high-performance MAMs.