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Ding et al. Soft Sci. 2026, 6, 2 Page 3 of 15

conformal etching through its weak acidity and coordination protection effect, gradually hollowing the
interior while preserving the intact rhombic dodecahedral morphology, thus constructing a well-defined
hollow structure. Second, the abundant phenolic hydroxyl groups in TA molecules formed strong chelation
with metal ions, providing active sites for the subsequent uniform doping of Fe and Ni . Based on this
2+
3+
etching-chelation synergistic mechanism, TA, Fe and Ni were quantitatively incorporated into the
2+
3+
material framework, achieving precise modulation of the EM transmission properties. Following calcination
at 550 °C in ambient air, a phase-pure spinel structure was obtained. At a filler loading of 30 wt.%, the
optimally modified Zn Co Ni Fe O composite achieves a RLmin of -57.6 dB, with a corresponding
x
2-y
y
4
1-x
effective absorption bandwidth (EAB) as wide as 10.27 GHz. Notably, this bandwidth range fully covers the
entire X-band (8~12 GHz) and Ku-band (12~18 GHz). These enhanced microwave absorption properties are
attributed to the synergistic effects of the hollow architecture, abundant heterointerfaces, and multi-cation
doping. This study not only proposes a simple and scalable method for addressing the intrinsic bandwidth
limitations of powder-based absorbers but also highlights the pivotal role of microscale ionic substitution in
tuning EM transport behavior. The findings contribute to bridging the gap between microscopic property
control and macroscopic device integration, while the facile synthesis approach offers a viable route for the
practical application of microwave-absorbing materials in real-world far-field scenarios.

EXPERIMENTAL

Materials
Zinc nitrate hexahydrate [Zn(NO ) ·6H O], methanol, and nickel chloride hexahydrate (NiCl ·6H O) were
3 2
2
2
2
purchased from Guangdong Guanghua Technology Co., Ltd. (Guangdong, China); ferric chloride
hexahydrate (FeCl ·6H O) and cobalt nitrate hexahydrate [Co(NO ) ·6H O] were purchased from Shanghai
3 2
2
2
3
McLean Biochemical Technology Co., Ltd. (Shanghai, China); TA and 2-methylimidazole (2-Melm) were
purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).
Preparation of ZnCo-ZIFs
A total of 1.15 g of Co(NO ) ·6H O and 1.18 g of Zn(NO ) ·6H O were co-dissolved in 50 mL of methanol.
3 2
2
3 2
2
This metal salt solution was then slowly added dropwise to 50 mL of a methanol solution containing 2.87 g of
2-Melm, and the reaction mixture was continuously stirred at room temperature for 40 min. After allowing
the reaction system to stand undisturbed for 24 h, the product was collected by centrifugation, washed
thoroughly with ethanol multiple times to remove residual impurities, and finally dried to yield the ZnCo-
ZIFs.
Preparation of ZnCo-RDC
A total of 20 mg of the as-synthesized ZnCo-ZIFs was accurately weighed and dispersed in 10 mL of ethanol.
The resulting dispersion was then carefully added to 100 mL of a 1 mg/mL TA solution, in which the solvent
was a 1:1 (v/v) ethanol-water mixture. After stirring the mixture at room temperature for 10 min, unreacted
TA was removed by washing with ethanol. Following drying, ZnCo-RDC was successfully obtained.

Preparation of ZnCoFeNi-RDC
For the ion doping experiment, 5.4 mg of FeCl ·6H O and 4.7 mg of NiCl ·6H O were dissolved in 40 mL of
2
2
2
3
deionized water. The solution was vigorously stirred to ensure the formation of a homogeneous metal ion
mixture. This mixture was then slowly added dropwise into 60 mL of a 1 mg/mL TA-ZnCo ethanol
dispersion, and the reaction mixture was continuously stirred for 3 h. After completion of the reaction, the
product was washed multiple times with ethanol and then dried, yielding the iron-nickel double ion-doped
precursor (ZnCoFeNi-RDC). When using only a single metal salt for doping, the corresponding single
metal-doped precursors, such as ZnCoFe-RDC or ZnCoNi-RDC, can be synthesized following a similar
procedure.