Metal－phenolic coordination crystals derived magnetic hollow carbon spheres for ultrahigh electromagnetic wave absorption
金属-酚醛配位晶体衍生的磁性空心碳球用于超高电磁波吸收

Hanxiao Xu，Zizhuang He，Yiruo Wang，Xiangru Ren，and Panbo Liu（ $⋈$ ）
徐汉晓，何子庄，王逸若，任祥如，刘潘波（ $⋈$ ）School of Chemistry and Chemical Engineering，Northwestern Polytechnical University，Xi＇an 710129，China
西北工业大学化学化工学院，习 710129© Tsinghua University Press 2023
© 清华大学出版社 2023

Received： 21 July 2023 ／Revised： 20 August 2023 ／Accepted： 25 August 2023
收稿日期： 2023-7-21 /修订日期：2023-8-20 /录用日期：2023-8-25

Abstract 抽象

Owing to the tunable compositions and versatile functionality，the development of eco－friendly metal－phenolic coordination crystals derivatives is highly anticipated for electromagnetic wave absorption．In this study，three kinds of magnetic hollow carbon spheres（HCSs）with macro－meso－microporous characteristics，including Fe／HCS，Co／HCS，and CoNi／HCS，are successfully fabricated via the co－operative hard template and self－assembling process，in which magnetic particles are encapsulated in carbon shell matrix after the pyrolysis of metal－polyphenol coordination crystals and further subsequent template removal．On the one hand，hierarchical macro－meso－micropores effectively balance the impedance gap between absorbers and air and introduce structural defects or distortion，leading to matched impedance and enhanced dipolar／defect polarization．On the other hand，wrapped magnetic particles provide uncountable hetero－interfaces and induce ferromagnetic resonance，resulting in strengthened interfacial polarization and additional magnetic loss．In particular，enhanced minimum reflection loss（ $R_{L, min}$ ）and broadband effective absorption bandwidth（EAB）are achieved with only $10 wt . %$ filler loading．Specifically，the $R_{L, min}$ and EAB values are -57.5 dB and 7.2 GHz for Fe／HCS，-50.0 dB and 5.8 GHz for $Co / HCS$ ，and -52.1 dB and 6.7 GHz for $CoNi / HCS$ ， respectively．Moreover，this work provides us a modular－assembly strategy to regulate the hollow cavity of absorbers and simultaneously manipulates the chemical components of absorbers to regulate electromagnetic wave absorption performance．
由于电磁波的成分可调性和多功能性，开发环保型金属-酚醛配位晶体衍生物备受期待 absorption.In 本研究通过协同硬模板和自组装工艺成功制备了三种具有宏观-介-微孔特性的磁性空心碳球（HCS），包括 Fe/HCS、Co/HCS 和 CoNi/HCS，其中磁性粒子被封装在碳壳中一方面，分层的宏观-介-微孔有效地平衡了吸收体与空气之间的阻抗间隙，并引入了结构缺陷或畸变，导致阻抗匹配，增强了偶极/缺陷极化。另一方面，包裹的磁性粒子提供了无数的异质界面并诱导铁磁共振，导致界面加强特别是偏振和附加磁 loss.In，仅通过填充 $10 wt . %$ 负载即可实现增强的最小反射损耗（ $R_{L, min}$ ）和宽带有效吸收带宽（EAB）。具体来说， $R_{L, min}$ Fe/HCS 的 EAB 值为 -57.5 dB 和 7.2 GHz，-50.0 dB 和 5.8 GHz $Co / HCS$ ，以及 -52.1 dB 和 6.7 GHz $CoNi / HCS$ 此外，这项工作为我们提供了一种模块化组装策略来调节吸收器的空腔，同时纵吸收剂的化学成分来调节电磁波吸收性能。

KEYWORDS 关键字

hollow engineering，hierarchical pores，hetero－interfaces，synergetic effect，electromagnetic wave absorption
中空工程，多级孔隙，异质界面，协同效应，电磁波吸收

1 Introduction 1 引言

Carbon－supported transition－metal composites（CTMCs）are regarded as promising candidates in electromagnetic wave absorption owing to impedance matching，synergistic dielectric－ magnetic response，and multi－attenuation mechanism．The typical synthetic methods for the preparation of CTMCs include self－ assembling［1－3］，template method［4－6］，hydrothermal carbonization［7，8］，spray－drying carbonization［9］，etc．Above－ mentioned methods have been developed to tackle adjustable balance between structure and composition［10，11］．Accordingly， construction of special structural features and adjustable chemical components based on previous strategies has attracted widespread attention in achieving efficient absorption and broader frequency response［12－15］．However，the reported diverse methods still suffer huge challenges in porous manipulation，configuration controllability，and impedance matching．
制备碳负载过渡金属复合材料（CTMCs）具有阻抗匹配、协同介磁响应和多衰减机制等优点，是制备碳负载过渡金属复合材料（CTMCs）的典型合成方法包括自组装[1-3]、模板法[4-6]、水热碳化[7,8]、喷雾干燥碳化[9]等。因此，基于以往策略构建特殊结构特征和可调化学成分在实现高效吸收和更宽频率响应方面引起了广泛关注[12-15].然而，所报道的多样化方法在多孔纵、构型可控和阻抗匹配方面仍面临巨大挑战。

Recently，considerable efforts have been devoted into developing metal－organic frameworks（MOFs）derivatives with tunable chemical compositions and porous functionality in the field of electromagnetic wave absorption［16－18］．Benefitting from the unique porous framework and adjustable metal ligands，the derivatives after the pyrolysis usually exhibit abundant reduced magnetic particles and micro－mesoporous structure，both of them are beneficial for improving the electromagnetic wave attenuation ［19－21］．Therefore，MOFs derivatives，possibly adopted as one kind of CTMCs，can be employed as prospects candidates in the
近年来，在电磁波吸收领域投入了大量精力开发具有可调化学成分和多孔官能团的金属有机框架（MOFs）衍生物[16-18].得益于独特的多孔框架和可调节的金属配体，热解后的衍生物通常表现出丰富的还原磁性颗粒和微介孔结构，两者都有利于改善电磁波衰减[19-因此，可能被用作 CTMC 的一种 MOF 衍生物可以作为
practical applications of electromagnetic wave absorption materials［22］．For example，Che and co－workers reported the MOFs derivatives with bimetal doping composites（

Ni / Cu / C

）， which reached strong polarization relaxation and the effective absorption bandwidth（EAB）was as wide as 6.93 GHz ［23］．To further investigate the absorber electromagnetic wave absorption performance，adjusting the physical／chemical properties，with the aim to eliminate electromagnetic wave interference and pollution， has been generally researched［24，25］．In principle，the selection of dielectric matrix and the distribution or size of magnetic particles are beneficial for the final absorption performance． Therefore，the screening of organic building blocks will directly interfere with the performance of CTMCs because of their bonding ability and stability［26］．
例如，Che 及其同事报道了双金属掺杂复合材料（

Ni / Cu / C

）的 MOFs 衍生物，达到了强极化弛豫，有效吸收带宽（EAB）高达 6.93 GHz [23].为了进一步研究吸收体电磁波吸收性能，调整物理/化学性能，以消除电磁波干扰和污染，具有原则上，介电基质的选择和磁性颗粒的分布或大小有利于最终的吸收性能。因此，有机构建单元的筛选由于其键合能力和稳定性，会直接干扰 CTMCs 的性能[26]。

Owing to the abundance and natural origin，the exploration of phenolic ligands command increasing attention，which is different from carboxylate and imidazole－based ligands．The first report of phenolic substances in MOFs can be traced back to Fe－gallate MOFs preparation in the 1990s［27］．Besides，another additional superiority for plant polyphenols，such as tannic acid（TA），is the self－assembled procedure，favoring to form abundant micro－ mesopores for their derivatives．In this case，porous carbon architectures not only establish conductive networks and induce dielectric attenuation，but also reduce weight and facilitate the diverse options of magnetic domains．Du et al．proposed a dual－ pathway strategy to regulate carbon matrix and

{MoS}_{2}

nanosheets
由于酚类配体的丰富性和天然来源，酚类配体的探索越来越受到关注，这与羧酸盐和咪唑基配体不同.MOFs 中酚类物质的首次报道可以追溯到 1990 年代 Fe-没食子酸酯 MOFs 的制备[27].此外，植物多酚的另一个优势，如单宁酸（TA），是自组装过程，有利于形成丰富的微介孔，derivatives.In 在这种情况下，多孔碳 Du et al.提出了一种双途径策略来调节碳基体和

{MoS}_{2}

纳米片
on

{Fe}_{3} O_{4}

surface, the minimum reflection loss (

R_{L, min}

) and effective absorption bandwidth were -36.1 dB and 5.4 GHz , respectively [28]. As results, pursing assembled design fits to investigate the connection and stable controllability of CTMCs in achieving strong absorption materials [29]. For TA molecules, their rich phenolic hydroxyl groups ( -OH ) can act as an effective connector with all kinds of metal ions by chelating coordination, realizing the accommodating in manipulating the chemical component of assembled composites. Simultaneously, robust interactions (electrostatic interaction and covalent bond binding) are beneficial for the combination of polyphenols with the substrates [30]. The formation of assembled composites with controllable pore-size and adjustable chemical component can be divided into two steps: anchoring different metal ions in TA micelles by chelating coordination and achieving hierarchical pores by sacrificing the template [31].
在

{Fe}_{3} O_{4}

表面，最小反射损耗（

R_{L, min}

）和有效吸收带宽分别为 -36.1 dB 和 5.4 GHz [28]。因此，追求组装设计适合研究 CTMC 在获得强吸收材料方面的连接和稳定可控性 [29]。对于 TA 分子，其丰富的酚羟基（-OH）可以通过螯合配位与各种金属离子起到有效的连接作用，实现对组装复合材料化学成分的调节。同时，稳健的相互作用（静电相互作用和共价键结合）有利于多酚与底物的结合[30]。具有可控孔径和可调节化学成分的组装复合材料的形成可分为两个步骤：通过螯合配位将不同的金属离子锚定在 TA 胶束中，以及通过牺牲模板实现分层孔 [31]。

Herein, we report the co-operative hard template and selfassembling process to construct magnetic hollow carbon spheres (HCSs) with adjustable chemical components and hierarchical macro-meso-micropores, including Fe/HCSs, Co/HCSs, and

CoNi / HCSs

, in which the pyrolysis of metal-phenolic coordination crystals simultaneously achieves the manipulation of chemical component and the generation of meso-micropores, while the removal of internal

{SiO}_{2}

template realizes hollow cavity. As expected, these as-synthesized magnetic HCSs exhibit ultrahigh electromagnetic wave absorption with only 10 wt.% filler loading. All of them display a minimum reflection loss below -50 dB and the corresponding effective absorption bandwidth exceeds 5 GHz , surpassing the non-magnetic HCSs. The demonstrated strategy establishes a facile route in diversification of CTMCs and provides a new perspective in design of light-weight and high-efficiency electromagnetic wave absorbers.
在本文中，我们报道了构建具有可调化学成分和分层大介孔的磁性空心碳球（HCS）的协同硬模板和自组装过程，包括 Fe/HCSs、Co/HCSs 和

CoNi / HCSs

，其中金属酚类配位晶体的热解同时实现了化学成分的纵和介微孔的产生，同时去除了内部

{SiO}_{2}

模板实现空腔。正如预期的那样，这些合成的磁性 HCS 表现出超高的电磁波吸收能力，填料负载仅为 10 wt.%。它们都显示出低于 -50 dB 的最小反射损耗，相应的有效吸收带宽超过 5 GHz，超过了非磁性 HCS。该策略为 CTMC 的多元化建立了一条简单的路线，并为设计轻量级和高效的电磁波吸收器提供了新的视角。

2 Experimental sections 2 实验部分

2.1 Synthesis of precursors
2.1 前驱体的合成

In a typical synthetic procedure, 3 mL of tetraethyl orthosilicate (TEO) and

1 mL NH H_{2} \cdot H_{2} O (28 wt . %)

were dissolved into 10 mL

H_{2} O

and 20 mL ethanol. The mixed solution was stirred for 30 min to obtain

{SiO}_{2}

nanospheres. Then 2 g of TA and 2.4 mL of formaldehyde (

37 wt . %

) were added to the solution and stirred for 24 h . The precursors of

{SiO}_{2}

@TA-formaldehyde (

{SiO}_{2}

@TF) were collected by centrifugation and washing.
在典型的合成程序中，

1 mL NH H_{2} \cdot H_{2} O (28 wt . %)

将 3 mL 正硅酸四乙酯（TEO）溶解到 10 mL

H_{2} O

和 20 mL 乙醇中。将混合溶液搅拌 30 分钟，得到

{SiO}_{2}

纳米球。然后向溶液中加入 2 g TA 和 2.4 mL 甲醛（

37 wt . %

）并搅拌 24 h。离心、洗涤收集

{SiO}_{2}

@TA-甲醛（

{SiO}_{2}

@TF）的前体。

2.2 Synthesis of magnetic HCSs
2.2 磁性 HCS 的合成

The above obtained precursors ( 1 g ) and

Fe {({NO}_{3})}_{3} \cdot 9 H_{2} O (1.5 g)

were dissolved into

15 mL H_{2} O

and 30 mL ethanol and stirred for

24 h . {SiO}_{2} @ {Fe}^{3 +} / TF

spheres were collected by curing (

200^{\circ} C, 2 h

), centrifugation, and washing. The synthesized

{SiO}_{2} @ {Fe}^{3 +} / TF

spheres were annealed at

800^{\circ} C

for 2 h in an

N_{2}

atmosphere, yielding to

{SiO}_{2} @ Fe / C

spheres. After that, the products were washed by NaOH solution (

1 mM, 50 mL

) at

70^{\circ} C

for 2 h , leading to the formation of

Fe / HCSs

. Similarly,

Co / HCSs

and

CoNi / HCSs

were obtained by replacing

Fe {({NO}_{3})}_{3} \cdot 9 H_{2} O

with

Co {({NO}_{3})}_{2} \cdot 6 H_{2} O (1.08 g)

and

Co {({NO}_{3})}_{2} \cdot 6 H_{2} O / Ni {({No}_{3})}_{2} \cdot 6 H_{2} O

(

0.54 / 0.54 g

), respectively.

Fe {({NO}_{3})}_{3} \cdot 9 H_{2} O (1.5 g)

将上述得到的前体（ 1 g ）溶解到

15 mL H_{2} O

30 mL 乙醇中，并通过固化（

200^{\circ} C, 2 h

）、离心和洗涤收集搅拌球体

24 h . {SiO}_{2} @ {Fe}^{3 +} / TF

。合成的

{SiO}_{2} @ {Fe}^{3 +} / TF

球

800^{\circ} C

体在

N_{2}

大气中退火 2 h，得到

{SiO}_{2} @ Fe / C

球体。之后，用 NaOH 溶液（

1 mM, 50 mL

）

70^{\circ} C

洗涤产物 2 小时，形成

Fe / HCSs

。同样，

Co / HCSs

和

CoNi / HCSs

分别通过替换为

Fe {({NO}_{3})}_{3} \cdot 9 H_{2} O

Co {({NO}_{3})}_{2} \cdot 6 H_{2} O (1.08 g)

和

Co {({NO}_{3})}_{2} \cdot 6 H_{2} O / Ni {({No}_{3})}_{2} \cdot 6 H_{2} O

（）

0.54 / 0.54 g

）获得。

2.3 Characterization 2.3 特征描述

The morphologies of magnetic HCSs were characterized by scanning electron microscopy (SEM, FEI Verios G4) and transmission electron microscopy (TEM, FEI Talos F200X). The crystal structures were investigated by X-ray diffraction (XRD, Bruker D8 Advance). The chemical state of carbon matrix was analyzed by Raman spectroscopy (WITec Alpha300R). The
通过扫描电子显微镜（SEM， FEI Verios G4）和透射电子显微镜（TEM， FEI Talos F200X）对磁性 HCS 的形态进行了表征。通过 X 射线衍射（XRD， Bruker D8 Advance）研究了晶体结构。通过拉曼光谱（WITec Alpha300R）分析碳基体的化学状态。这
molecular structure and atomic valence were characterized by Xray photoelectron spectroscopy (XPS, Kratos AXIS Ultra DLD). The specific surface area and pore size distribution were characterized by

N_{2}

adsorption-desorption isotherms (BeiShiDe 3H-2000PS2). The electromagnetic parameters were measured by the vector network analyzer (VNA, PNA-N5244A) in

2 - 18 GHz

. The synthesized powder was filled into a paraffin ring with a mass ratio of

10 %

.
通过 X 射线光电子能谱（XPS， Kratos AXIS Ultra DLD）表征分子结构和原子价。采用

N_{2}

吸附-脱附等温线（BeiShiDe 3H-2000PS2）对比表面积和孔径分布进行表征。电磁参数由矢量网络分析仪（VNA，PNA-N5244A）测量

2 - 18 GHz

。将合成的粉末填充到质量比为的石蜡环中

10 %

。

3 Results and discussion
3 结果与讨论

The formation route of magnetic hollow carbon spheres, including Fe/HCSs, Co/HCSs, and CoNi/HCSs, is schematically displayed in Fig. 1. Briefly,

{SiO}_{2}

particles with a diameter of about 400 nm and different metal species were evenly dispersed in the mixed TA and formaldehyde solution, forming dense selfassembling TA-formaldehyde/metal ions layer on the surface of

{SiO}_{2}

throughout electrostatic interaction. After the pyrolysis and etching, magnetic HCSs were formed, while HCSs were obtained without adding metal species. The morphologies of

{SiO}_{2} @ TF

spheres,

{SiO}_{2} @ {Fe}^{3 +} / TF

spheres,

{SiO}_{2} @ {Co}^{2 +} / TF

spheres, and

{SiO}_{2} @ {Co}^{2 +} {Ni}^{2 +} / TF

spheres were characterized by SEM and TEM images, the results are shown in Figs. 2(a)-2(h). It is clear that

{SiO}_{2}

particles are wrapped by the coating layer, resulting in the formation of core-shell structure. In this process, metal ions act as Lego connectivity bricks to connect TA oligomers, and the corresponding cross-linked network would stably depose on

{SiO}_{2}

particles owing to their rich-phenolic hydroxyl group. Some broken spheres in SEM images and TEM images clearly reveal the formation of core-shell structure, indicating that TAformaldehyde/metal ions layer is successfully coated on the surface of

{SiO}_{2}

particles, which is beneficial for adjusting the chemical component of their derivatives and generating micro-mesopores [32]. In Figs. S1-S4 in the Electronic Supplementary Material (ESM), the elemental distribution maps further demonstrate the successful chelation coordination in TA-formaldehyde/metal ions layer with the uniform outer distribution of

{Fe}^{3 +}, {Co}^{2 +}

, and

{Co}^{2 +} {Ni}^{2 +}

elements in precursors. It is noteworthy that the thickness of

{Fe}^{3 +} / TF

layer is thicker than that of

{Co}^{2 +} / TF

and

{Co}^{2 +} {Ni}^{2 +} / TF

, which is attributed to the demanding demands of trivalent iron ions in chelation constraints. After the annealing treatment and the template removal, metal ions are reduced into magnetic particles (circled in multicolor) encapsulated in carbon matrix, resulting in the formation of magnetic HCSs, including Fe/HCSs,

Co / HCSs

, and

CoNi / HCSs

, as presented in Figs. 2(i)-2§. Similar to the morphologies of

{SiO}_{2} @ ({Fe}^{3 +}, {Co}^{2 +}, {Co}^{2 +} {Ni}^{2 +}) / TF

spheres, the carbonized magnetic HCSs inherit well the initial spherical structure, and the pyrolysis process only rebuilds the microscopic configuration by introducing surface defects or micro-mesopores [33]. During the carbonization, the decomposition of TAformaldehyde species inevitably generates reducing gases, which would be carried away by inert gas to form porous structure. The NaOH etching would remove the hard

{SiO}_{2}

template, forming hollow cavity with macropore structure as indicated in TEM images. As results, hierarchical macro-meso-micropores are well generated [34].
磁性空心碳球的形成路径，包括 Fe/HCSs、Co/HCS 和 CoNi/HCSs，如图 1 所示。简而言之，

{SiO}_{2}

直径约为 400 nm 的颗粒和不同金属种类的颗粒均匀分散在 TA 和甲醛混合溶液中，

{SiO}_{2}

在整个静电相互作用的表面形成致密的自组装 TA-甲醛/金属离子层。热解和刻蚀后，形成了磁性 HCS，而 HCS 是在不添加金属种类的情况下获得的。通过 SEM 和 TEM 图像表征了球体、

{SiO}_{2} @ {Fe}^{3 +} / TF

球体、

{SiO}_{2} @ {Co}^{2 +} / TF

球体和

{SiO}_{2} @ {Co}^{2 +} {Ni}^{2 +} / TF

球体的

{SiO}_{2} @ TF

形貌，结果如图 2（a）-2（h）所示。很明显，

{SiO}_{2}

颗粒被涂层包裹，导致形成核壳结构。在这个过程中，金属离子充当乐高连接砖块来连接 TA 低聚物，相应的交联网络由于其丰富的酚羟基而稳定地沉积在颗粒上

{SiO}_{2}

。SEM 图像和 TEM 图像中一些破碎的球体清楚地揭示了核壳结构的形成，表明甲醛/金属离子层成功地包覆在颗粒表面

{SiO}_{2}

，有利于调节其衍生物的化学成分，产生微介孔[32]。在图 S1-S4 在电子辅助材料（ESM）中，元素分布图进一步证明了 TA-甲醛/金属离子层的成功螯合配位，以及

{Fe}^{3 +}, {Co}^{2 +}

和

{Co}^{2 +} {Ni}^{2 +}

元素在前驱体中的均匀外分布。值得注意的是，层的

{Fe}^{3 +} / TF

厚度比

{Co}^{2 +} / TF

和

{Co}^{2 +} {Ni}^{2 +} / TF

的厚，这归因于螯合约束中对三价铁离子的苛刻要求。经过退火处理和模板去除后，金属离子被还原成封装在碳基体中的磁性颗粒（以多色圈出），从而形成磁性 HCS，包括 Fe/HCSs、

Co / HCSs

和

CoNi / HCSs

，如图 2（i）-2§ 所示。与

{SiO}_{2} @ ({Fe}^{3 +}, {Co}^{2 +}, {Co}^{2 +} {Ni}^{2 +}) / TF

球体的形态类似，碳化磁性 HCS 很好地继承了初始球形结构，热解过程仅通过引入表面缺陷或微介孔来重建微观构型 [33]。在碳化过程中，TAformaldehyde 种类的分解不可避免地会产生还原性气体，这些气体会被惰性气体带走，形成多孔结构。NaOH 刻蚀将去除硬

{SiO}_{2}

模板，形成具有大孔结构的空腔，如 TEM 图像所示。因此，分层的宏观-中观-微孔隙生成良好[34]。

XRD patterns and high-resolution TEM (HRTEM) images were employed to characterize the crystal structures of magnetic particles, and the results are shown in Fig. 3. For Fe/HCSs in Fig. 3(a), the sharp diffraction peaks at 44.3, 64.5, and 81.6 correspond to the (110), (200), and (211) planes of Fe, which is consistent with the lattice spacing of 0.204 nm (110) in Fig. 3(d). Besides, the broad diffraction peak is attributed to the carbon matrix [35, 36]. As shown in Fig. 3(b),

Co / HCSs

also display three diffraction peaks at 44.2, 51.5, and 75.8, which are ascribed to the (111), (200),
采用 XRD 图谱和高分辨率 TEM （HRTEM）图像来表征磁性粒子的晶体结构，结果如图 3 所示。对于图 3（a）中的 Fe/HCS，44.3、64.5 和 81.6 处的尖锐衍射峰对应于 Fe 的（110）、（200）和（211）平面，这与图 3（d）中 0.204 nm （110）的晶格间距一致。此外，宽衍射峰归因于碳基体 [35， 36]。如图 3（b）所示，

Co / HCSs

在 44.2、51.5 和 75.8 处也显示了三个衍射峰，它们分别属于（111）、（200）、

Figure 1 Schematic illustration for the synthetic process of magnetic hollow carbon spheres．
图 1 磁性空心碳球的合成过程示意图。

Figure 2 SEM and TEM images of（a）and（b）

{SiO}_{2} @ TF

，（c）and（d）

{SiO}_{2} @ {Fe}^{3 +} / TF

，（e）and（f）

{SiO}_{2} @ {Co}^{2 +} / TF

，（g）and（h）

{SiO}_{2} @ {Co}^{2 +} {Ni}^{2 +} / TF

，（i）and（j） HCSs ，（k）and （l）

Fe / HCSs, (m)

and（n）

Co / HCSs

，and（o）and（p）

CoNi / HCSs

．
图 2 （a）和（b）

{SiO}_{2} @ TF

、（c）和（d）

{SiO}_{2} @ {Fe}^{3 +} / TF

、（e）和（f）

{SiO}_{2} @ {Co}^{2 +} / TF

、（g）和（h）

{SiO}_{2} @ {Co}^{2 +} {Ni}^{2 +} / TF

、（i）和（j） HCS 、（k）和（l）

Fe / HCSs, (m)

和（n）

Co / HCSs

和（o）和（p）

CoNi / HCSs

的 SEM 和 TEM 图像。

Metal－phenolic coordination crystals derived magnetic hollow carbon spheres for ultrahigh electromagnetic wave absorption 金属-酚醛配位晶体衍生的磁性空心碳球用于超高电磁波吸收