Electroless plating silver nanoparticles mixed in oxidized microcrystalline cellulose for microwave absorption applications

In this work, we introduce a simple, efficient and low-cost process, where silver nanoparticles (AgNPs) are mixed in TEMPO-oxidized microcrystalline cellulose (TOMCC) by electroless plating successfully obtain a lightweight, green microwave absorber (TOMCC/AgNPs). We characterize and analyze the physicochemical properties and surface morphology of TOMCC/AgNPs using SEM, EDS, UV–Vis, XRD, XPS, BET, Raman spectroscopy and, thermal behavior, while analyzing the electromagnetic absorption of fabricated TOMCC/AgNPs through a vector network. Our results show that the optimal reflection loss value is − 51 dB at 4.7 GHz, with a corresponding effective absorption bandwidth of 4 GHz, and the thickness of the absorber is 2 mm, containing 50% TOMCC/AgNPs. Even at 400 ℃, the absorber can still maintain efficient microwave absorption performance. As a green, lightweight, and efficient microwave absorber, the prepared TOMCC/AgNPs have considerable application prospects in the field of microwave absorption, such as stealth technology.


Introduction
The green environment and mankind's healthcare are considered the most critical issues in the rapid development of human society. However, the popularity of electronic equipment has led to a heightened concern regarding the adverse effects of electromagnetic radiation on human health, resulting in significant harm to the body. (Shahzad et al. 2016;Lee et al. 2017;Cao et al. 2018). At the same time, the mutual interference between these electronic devices not only affects their normal functioning, but also reduces their performance, causing a lot of inconvenience to our lives and even threatening national defense security. The exploitation of electromagnetic attenuation materials is an effective way to solve this problem. (You et al. 2017; (Ye et al. 2018a, b;Liu et al. 2019a, b).
To date, numerous materials have been studied for their ability to attenuate electromagnetic waves, including transition metals and oxides (TiO 2 , Fe, Fe 3 O 4 , Co, Co 3 O 4 , Ni, NiO, ZnO), transition metal dichalcogenides, transition metal carbides and nitrides, ferrite bismuth, silicon carbides (SiC 1 3 Vol:. (1234567890) crystals, SiC nanowires, SiC monolayer), and carbon materials. (carbon nano tube, graphene) (Wu et al. 2015a(Wu et al. , b, 2018Jia et al. 2018a, b, c;Lan et al. 2019;Cao et al. 2019a, b, c). An effective absorbing material converts electromagnetic waves into heat or other forms. Generally, there are three functions of absorption. The first is magnetic loss, which absorbs electromagnetic wave energy through ferromagnetic resonance, magnetic loss, and eddy current loss. The second is conductive loss, which absorbs electromagnetic wave energy through macroscopic current generated by the carrier. The third is dielectric loss, which absorbs electromagnetic wave energy through interfacial relaxation and intermolecular relaxation. In other words, these three mechanisms correspond to magnetic materials, conductive materials, and dielectric materials.
Among them, although magnetic nickel metal and its oxides, such as Ni  and NiO (Tong et al. 2012), have excellent microwave absorption properties, their applications are always limited by a low curie temperature. Other magnetic metals and their alloys with relatively high curie temperatures have been extensively investigated, including Fe cubes (Fan et al. 2010) and Co x Fe 1−x alloy (Yu et al. 2013). However, their magnetism and conductivity decrease with increasing temperature, leading to a decrease in their microwave absorption. As for oxides and heterostructures, such as Fe 3 O 4 (Sun et al. 2011) and Fe 3 O 4 @TiO 2 (Liu et al. 2012a, b), the microwave absorption properties of these materials need to be further improved due to the degradation of their magnetism at elevated temperatures. Conducting polymers (Lu et al. 2011) and non-magnetic metals (Dong et al. 2015) have the potential for microwave absorption, but their microwave absorption properties are not usually stable at elevated temperatures. Therefore, researching materials with stable and enhanced absorption properties remains a great challenge.
It is worth noting that silver metal has excellent electrical conductivity and stability, especially in harsh thermal environments. Due to impedance matching, most microwaves are reflected back to the original medium by the high conductivity of silver, and only a small amount of energy is lost as heat. Therefore, silver is commonly used as an electromagnetic shielding material in thin films , sandwich structures (Song et al. 2014), foams (Ma et al. 2015), and fibers (Lee et al. 2016). However, the properties of materials often change with their structure. When the silver is defective or mixed with insulation materials, these defects and insulation materials will act as a microwave propagation medium, allowing more microwaves into the silver interior. Meanwhile, the purpose of microwave attenuation is achieved through the reflection loss of silver inside the material.
Cellulose is a low-density, lightweight material (Ye al et. 2018a, b) that finds application in many fields, such as adsorption (Abdelhamid et al. 2021), hydrophobic (Zhao et al. 2020, antibacterial properties (Wang et al. 2021), and flame retardation (Jiang et al. 2021). In this article, a simple composite material is designed based on the above ideas. Silver nanoparticles and silver nanoclusters are discontinuously distributed in oxidized microcrystalline cellulose by electroless plating, forming a three-dimensional distribution structure of silver nanoparticles and silver nanoclusters mixed with oxidized microcrystalline cellulose. The insulation property of oxidized microcrystalline cellulose is used as the transmission medium for microwaves and is introduced into the three-dimensional structure of silver nanoclusters. Meanwhile, the interaction of silver nanoparticles and silver nanoclusters combined with oxidized microcrystalline cellulose is used to achieve microwave absorption.

Materials
Silver nitrate, ammonia, and ethanol are purchased from Beilian Chemical Industry (Tianjin, China). Sodium hydroxide is purchased from Shengao Chemical Reagent Company (Tianjin, China). Sodium borohydride is purchased from Komi Chemical Reagent Company (Tianjin, China). Ethanol is purchased from No. 3 Chemical Reagent Plant (Tianjin, China). Glucose is purchased from Zhiyuan Chemical Reagent Company (Tianjin, China). All chemicals are analytical grade. The TEMPO oxidized microcrystalline cellulose is prepared in the laboratory.
Fabrication of TOMCC/AgNPs 0.9 g glucose is immersed in a 100 mL solution containing 15% ethanol and 85% deionized water, as solution A. Next, 1.7 g of silver nitrate is dissolved in 100 mL of deionized water, and ammonia is slowly added dropwise to the solution until precipitation occurs, and then ammonia is continuously added dropwise until the precipitate disappears. Then 0.7 g of sodium hydroxide is immersed in the solution, resulting in precipitation, and then ammonia is added dropwise to the solution again until the precipitate completely disappears, as solution B.
The TOMCC is placed into a 1 mM silver nitrate solution for 5 min at room temperature, and then the TOMCC/Ag + is placed in 200 mM sodium borohydride at room temperature for 10 min. Thereafter, the TOMCC/AgPN/Ag + is immersed in the reducing solution (A), and then the silver ammonia solution (B) is added slowly at 35 °C and reacted for 20 min. Finally, the TOMCC/AgNPs are obtained, as shown in Fig. 1.

Characterization
The morphology and elements of the TOMCC/ AgNPs are characterized using field emission scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS) and Ultraviolet visible diffuse reflection (UV-Vis). The sample preparation process of SEM (acceleration voltage 10 kV) is to dry the sample, fix it on the special sample table with conductive adhesive, and then spray gold. The crystal structure and chemical composition of TOMCC/ AgNPs are characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The chemical structure and physical structure of TOMCC/AgNPs are analyzed using the Raman spectrum and BET specific area. The thermal behavior of TOMCC/AgNPs is characterized using thermogravimetric (TG) and differential thermogravimetric (DTG) methods. Finally, TOMCC/AgNPs are mixed with paraffin wax in different proportions to form a 2 mm thick coaxial ring sample. The corresponding reflection loss is obtained using the coaxial mode of the vector network analyzer, and the electromagnetic parameters are analyzed, which are in the frequency range of 2-18 GHz. On this basis, the reflection loss at the thicknesses of 2 mm, 3 mm, and 4 mm is calculated, and the reflection loss and electromagnetic parameters at different temperatures are further discussed.

SEM analysis
The microscopic appearance of TOMCC/AgNPs is observed as shown in Fig. 2. The process of preparing TOMCC/AgNPs as shown in the figure preserves the shape of TOMCC while successfully incorporating  Fig. 2 that these silver nanoparticles aggregate with each other to form a cluster structure (Mackus et al. 2016;Sun et al. 2019). It is worth noting that the corresponding silver nanoparticles and their cluster structures can also be observed in the transverse section of TOMCC, as shown in Fig. 2d, which further indicates that silver nanoparticles are mixed in TOMCC. Further magnifying the surface morphology as Fig. 2e, it becomes that most of the silver nanoparticles exist in the spherical state, and some of the silver nanoparticles are also accompanied by an angular polygon state. Furthermore, Fig. 2f shows the size distribution of silver nanoparticles in TOMCC. The particle size distribution of silver nanoparticles is mainly concentrated in the range of 20 to 90 nm, and the most widespread is between 50 and 70 nm, with the average size of silver nanoparticles being about 65 nm. The color and particle size of silver nanoparticles are similar to those reported by Yang et al. (2019), who successfully prepared silver nanoparticles and incorporated them into TOMCC.
In fact, electroless plating is a widely utilized process, especially in the bonding process between metals and between metals and nonmetals. In contrast to metals, non-metals lack catalytic activity, thus necessitating the pre-generation catalytically active parts on non-metallic materials to ensure the normal process of electroless plating reaction. In previous studies, people often used gold, platinum, and other precious metals with catalytic activity as catalysts to complete their electroless plating reaction. For instance, Jun Matsui et al. employ two kinds of functional polymer nanosheets: one works as an adhesion layer, and the other works as a template layer to adsorb gold nanoparticles, which works as a catalyst for the electroless plating. They use this method to successfully deposit copper on polyimide films (Matsui et al. 2007). Following continuous research, people focus on autocatalysis. For instance, Jian dong Hu, et al. use APTMS as an adhesive reagent to attach the gold nanoparticles to the glass substrate. These AuNPs can be regarded as the preferential nucleation or catalytic sites for gold electroless reduction, which accelerate the reduction of Au 3+ on the glass surface and effectively prevent the formation of gold metal in the bulk solution. Finally, a continuous gold layer is successfully prepared on the glass surface (Hu et al. 2008). Although the method is improved, the mechanical connection between the substrate and the catalytically active particles is still essential. In other words, mechanical force is still the main combination between the substrate and the coating, and substances such as functional polymer nanosheets and APTMS are still used as connection bridges.
In our study, we spare the intermediates that provide mechanical adhesion and instead achieve fixation of the catalytic active site by electrostatic adsorption and chemical bonding. TEMPO selectively oxidizes the C-6 hydroxyl groups on the surface of microcrystalline cellulose into carboxyl groups (Nakamura et al. 2019;Isogai et al. 2019), which ionize into COO-in aqueous solution. Simultaneously, electrostatic adsorption occurs between COO-and Ag + , forming -COO-Ag + , which makes the silver ions adsorb in TOMCC and generates catalytically active silver nanoparticles through in-situ reduction. Meanwhile, the O-Ag structure forms to bind the silver nanoparticles with TOMCC through chemical bonds. Finally, more silver nanoparticles are generated around the catalytic active site by electroless plating and mixed into TOMCC. Compared to previous studies, this method is simpler and cheaper. The electrostatic adsorption and chemical bonding make the silver nanoparticles more dispersed and firmer, reducing the agglomeration of silver nanoparticles and increasing the stability of TOMCC/AgNPs (Lizundia et al. 2019).

EDS, UV-Vis, and XRD analysis
The EDS patterns of the TOMCC/AgNPs are shown in Fig. 3. The peak of the silver element appears on the elemental composition diagram. The diagram clearly shows a strong peak caused by silver between 2 keV and 3.5 keV. After analyzing the element content, the atom and mass fractions of silver on the surface of the TOMCC reach 85.92% and 97.89%, respectively.
To further prove that silver is present in the form of nanoparticles in TOMCC, we perform UV-visible diffuse reflection of TOMCC/AgNPs. As shown in Fig. 4, the UV-Vis diffuse reflection spectrum of TOMCC/AgNPs has a strong absorption peak at a wavelength of 426 nm. This peak is attributed to the light absorption peak generated by the plasmon resonance effect of silver nanoparticles interspersed in TOMCC (Ceran et al. 2019;Douglas-Gallardo et al. 2019;Sirohi et al. 2019). This proves that silver is present in TOMCC as nanoparticles.
The crystal structure of TOMCC/AgNPs is analyzed by XRD, as shown in Fig. 5. The X-ray diffraction analysis spectrum shows peaks at 16°, 22° representing (110)  XPS is employed to further analyze the composition and surface states of the TOMCC immobilized with AgNPs. The binding energies of Ag 3d at about 371 eV, Ag 3p at 573 eV and 604 eV, and Ag 3s at 719 eV are distinct in Fig. 6 and demonstrate the presence of the silver element (Liang et al. 2017). The Ag 3d core-level spectrum shown in Fig. 7 can be deduced from the two peak components at BEs of 368.43 eV and 374.43 eV for Ag 3d 5/2 and Ag 3d 3/2 , respectively. Both peaks are attributed to the Ag 0 species, thus confirming the presence of Ag in accordance with XRD results. Figure 8 illustrates the Raman spectra of TOMCC and TOMCC/AgNPs. The absorption band at approximately 229 cm −1 in the TOMCC/AgNPs spectrum is attributed to the Ag-O stretching vibration (Biswas et al. 2007). This result further suggests that the interaction between Ag and the carboxyl groups of TOMCC molecules is linked by a chemical bond. The most significant feature in the Raman spectrum is the emergence of a very strong band at 1374 cm −1 assigned to the CO 2 symmetric stretch. The enhancement in the intensity of the CO 2 stretching vibration suggests that the carboxyl groups are directly involved in the interaction with the silver surface. The remarkable red shift of 22 cm −1 for the CO 2 symmetric stretch (1374 cm −1 ) observed in the Raman spectrum from that observed in the normal Raman spectrum of the molecule in solution (1396 cm −1 ) compares well with that observed for the carboxyl band of most carboxylic acids (Thomas et al. 2005;Kim et al. 1987;Sanchez-Cortes et al. 2000). This red shift in the carboxyl symmetric stretching vibration also indicates direct attachment of the carboxylate group to the silver surface. Furthermore, the strong and sharp peaks at 1540 cm −1 and 1597 cm −1 suggest the direct binding of the carboxyl groups with the Ag surface (Biswas et al. 2007). The Raman spectra indicate that the carboxyl groups of TOMCC molecules covalently and directly interact with AgNPs by monodentate interaction in aqueous solution.

BET analysis
The BET analysis is conducted on TOMCC/AgNPs to evaluate the surface area and porosity, as shown in Figs. 9 and 10. The IV nitrogen sorption isotherms for the TOMCC/AgNPs suggest the existence of different pore sizes, from mesoporous to macropores (Liu et al. 2012a, b). Moreover, the hysteresis between the adsorption and desorption branches of the TOMCC/ AgNPs also suggests the existence of different types of pores with no pore-blocking effect during desorption (Hao et al. 2014). The pore size of TOMCC/ AgNPs ranges from 10 to 80 nm, with an average pore size of 32.59 nm, Additionally, the BET specific surface area is determined to be 1.89 m 2 g −1 .
In fact, from the perspective of the structure of cellulose, the surface of cellulose in its natural state has a pore structure. Previous studies report that the specific surface area of cellulose can reach tens of m 2 g −1 or even hundreds of m 2 g −1 after some changes. In this study, these natural pores on the surface of TOMCC become the providers of catalytic active sites through the adsorption of silver ions and even can adsorb silver ions in the plating solution to generate silver nanoparticles. The specific surface area of the obtained TOMCC/AgNPs is very low, indicating that the silver nanoparticles fill most of the pores of TOMCC. However, some pores are still present in the measurement, indicating that the silver nanoparticles have not filled all the pores of TOMCC. Adsorption is only an essential step in the preparation process of silver nanoparticles, but more importantly, the silver ions absorbed can disperse in the cellulose through the electrostatic attraction of COO-and Ag + . This, in turn, prevents excessive agglomeration of the resulting silver nanoparticles. As shown in Fig. 2, the prepared silver nanoparticles have a stable size and good dispersion.

Thermal performance analysis
The influence of temperature on the thermal behavior of TOMCC/AgNPs is studied by conducting TG and DTG analysis. Figure 11 illustrates the differences in thermal behavior between TOMCC/AgNPs and TOMCC. TOMCC (b) undergoes slight loss in weight due to the loss of water molecules at room  temperature, while TOMCC/AgNPs lose weight between room temperature and 300 ℃. As the temperature increases, TOMC undergoes rapid loss in weight from 249 ℃ to 352 ℃, and TOMCC/AgNPS experience the same between 300 ℃ and 370 ℃. The rate of weight loss slows down as the temperature increases, resulting in a mass reduction of 91.5% and 45% for TOMCC and TOMCC/AgNPs, respectively. The presence of silver particles in TOMCC/ AgNPs accounts for the higher thermal performance of TOMCC/AgNPs compared to TOMCC.
The thermal behavior of cellulose can be classified into two processes, namely the loss of water molecules and the degradation and decomposition of cellulose. The temperature of TOMCC/AgNPs is observed to be higher than that of TOMCC in both processes, which can be attributed to the binding of silver nanoparticles, especially in the first process. This increase may be due to the restricted steric effect of hydroxy groups caused by the branching of silver nanoparticle with the TOMCC molecule chain, which leads to an increase in lateral forces in the bulk state. This is also evident in the Raman spectrum, where the peak of TOMCC at 2929 cm −1 is stronger than that of TOMCC/AgNPs (Zulkifli et al. 2017). The degradation and decomposition of cellulose involve the breaking of chemical bonds, and the Ag-O bond requires more energy to break, making the thermal behavior of TOMCC/AgNPs more stable (Biswas et al. 2007).

Microwave absorption
To investigate the microwave absorbing properties of TOMCC/AgNPs, the reflection loss is calculated. The reflection loss is obtained based on the electromagnetic parameter at a given frequency using Eqs.
(1) and (2) (Wen et al. 2013). Z in represents the input impedance of the absorbing material; Z 0 denotes the characteristic impedance of free space; ƒ represents the frequency of electromagnetic waves; d is the thickness of the absorbing material; c denotes the speed of light; and εr and µr represent the complex permittivity and complex magnetic permittivity of the composites, respectively. The generally accepted opinion is that RL= − 10 dB means that 90% of electromagnetic waves are lost, and RL= − 20 dB means 99% of electromagnetic waves are attenuated. Figure 12 shows the microwave absorption between 2 and 18 GHz for absorbers with different contents of TOMCC/AgNPs. When the content of TOMCC/ AgNPs is less than 50%, the microwave absorption performance of the absorber improves as the content of TOMCC/AgNPs increases. This is because the amount of TOMCC/AgNPs is insufficient to attenuate the microwaves entering the absorber (Zhang et al. (1)  . When the content of TOMCC/AgNPs is 50%, the ability of microwaves to pass through the surface of the absorber matches the ability of microwave attenuation inside the absorber. Thus the microwave attenuation of the absorber agent can be maximized, resulting in the best microwave absorption performance of the absorber. However, when the content of TOMCC/AgNPs exceeds 50%, the microwave absorption performance of the absorber gradually weakens. This is because the ability of the microwave to penetrate the surface of the absorber decreases . The absorber loses its absorbing property when the content of TOMCC/AgNPs reaches 80%. At this high TOMCC/AgNPs content, a large number of silver nanoparticles accumulate on the surface of the absorber, leading to eddy currents and impedance mismatch at the surface. As a result, the microwave is reflected on the surface of the absorber instead of entering it, resulting in lower microwave absorption or even the loss of the absorber's microwave absorption properties. Therefore, the absorber is converted into a shielding material Sun et al. 2019). The thickness of the absorber also affects microwave absorption performance. Figure 13 shows that the frequency of the peak absorption value shifts to the low frequency when the thickness of the absorber containing 50% TOMCC/AgNPs increases. This can be explained by the quarter-wavelength matching model Quan et al. 2017). The optimal reflection loss value is − 51 dB at 4.7 GHz with a corresponding effective absorption bandwidth of 4 GHz, and the thickness of the layer is 2 mm. In addition, the reflection loss value is − 46 dB at 11.2 GHz with a corresponding effective absorption bandwidth of 3 GHz, and the reflection loss value is − 41 dB at 15.6 GHz with a corresponding effective absorption bandwidth of 4 GHz, with the thickness of the former layer being 3 mm and the latter 4 mm.

Electromagnetic parameters
Electromagnetic parameters are generally expressed by complex relative permittivity (ε r = ε′ − jε′′) and complex relative permeability (µ r = µ′ − jµ′′). To explore the microwave absorption mechanism of TOMCC/AgNPs, the complex relative permittivity and complex relative permeability of a 2 mm thick absorber containing 50% TOMCC/AgNPs between 2 and 18 GHz are shown in Figs. 14 and 15. Although TOMCC/AgNPs are non-magnetic, the permeability is not equal to 1. This is because the silver nanoparticles scattered throughout the TOMCC act as conducting particles, creating a current loop around them in response to an external magnetic field. The direction of the induced magnetic field generated by these current loops is opposite that of the applied magnetic field, resulting in some magnetic loss (Xie et al. 2019). Considering that TOMCC/AgNPs show inconspicuous magnetic properties in GHz, frequency-dependent complex permittivity is recorded and analyzed. Figure 14 provides an analysis of the Fig. 13 Microwave absorption of different thicknesses absorber containing 50% TOMCC/AgNPs Fig. 14 Complex relative permittivity of 2 mm thick absorber containing 50% TOMCC/AgNPs frequency dependence of complex permittivity of TOMCC/AgNPs. The real part of complex permittivity shows an obvious dispersion characteristic: the ε′ value decreases with the increase in frequency from 22 to 6, due to the lags of induced charges following the external reversing electromagnetic filed. The imaginary part ε‫״‬ is between 1 and 6, and there are two peaks in the whole frequency range, which are related to the relaxation of TOMCC/AgNPs. Generally speaking, the real part of complex permittivity represents the storage of electric field energy, and the imaginary part relates to the attenuation ability of electric field energy. The tangent value of dielectric loss tanδ = ε″/ε′ corresponds to the attenuation factor of incident electromagnetic wave energy, as shown in Fig. 16. In comparison to previously reported related materials reported, the tanδ value of TOMCC/AgNPs exhibits a higher value, showing a relatively good microwave absorption performance (Wei et al. 2020;Zong et al. 2013;Cui et al. 2015;Liu et al. 2022;An et al. 2022).
To better explain the absorption mechanism of the TOMCC/AgNPs, the dielectric properties are further analyzed. Figure 17 shows the relationship between the real part ε′ and imaginary part ε″ for TOMCC/AgNPs composites. The plot of ε″ versus ε′ of TOMCC/AgNPs is complex with about five semicircles, indicating that TOMCC/AgNPs exhibit the strongest Debye relaxation and could convert much more electromagnetic wave energy to thermal energy under an external electromagnetic field. These five semicircles may arise as follows: under the alternating electromagnetic field, the lags of induced charges from the silver-silver, silver-TOMCC interfaces result in relaxation, as Fig. 18 (Quan et al. 2018;Duan et al. 2010). In addition, the relaxation phenomenon caused by the charge density difference between silver and oxygen (carboxyl group), as well as the functional group molecules such as hydroxyl and ether on TOMCC, will result in the conversion of electromagnetic energy to thermal energy when subjected to an electric field (Moon et al. 2011;Ghaderi et al. 2010 ).
In general, the combination of silver nanoparticles and TOMCC endows TOMCC/AgNPs with The thermal behavior results show that TOMCC/ AgNPs exhibit certain thermal stability. Therefore, we further discuss the microwave absorption properties of the 2 mm thick absorber containing 50% TOMCC/AgNPs at different temperatures. As shown in Fig. 19, although the increase in temperature damages the structure of TOMCC/AgNPs, it does not affect their microwave absorption performance. In fact, it slightly improves it. There are two main reasons for this: the high-temperature carbonization of cellulose (Liu et al. 2019a, b) and the improvement in the crystallinity of silver nanoparticles (Xiong et al. 2014). These factors result in the ability of TOMCC/AgNPs to retain their microwave absorption properties even when structure is damaged. This is further confirmed by the electromagnetic parameters of TOMCC/AgNPs at different temperatures. Figures 20 and 21 show that the ε′ of TOMCC/AgNPs increases with temperature, indicating an enhancement in its the electronic conduction ability. However, compared with ε′, the increase in ε″ is not significant, suggesting that although the electronic conduction ability of TOMCC/AgNPs is increasing, the structure of TOMCC/AgNPs, such as carboxyl group, hydroxyl, and ether is being destroyed. The final result is that the microwave absorption capacity of TOMCC/AgNPs is not significantly enhanced and is maintained at low temperatures. The maximum reflection loss at room temperature, 200 ℃, 300 ℃, and 400 ℃ is − 51 dB, − 52 dB, − 52.5 dB, and − 53 dB, respectively.

Conclusions
In conclusion, we successfully mix silver nanoparticles (AgNPs) in TEMPO oxidized microcrystalline cellulose (TOMCC) by electroless plating to obtain a green, lightweight microwave absorber. The prepared TOMCC/AgNPs have a stable structure and enhanced microwave absorption performance through dielectric loss. The optimal reflection loss value is − 51 dB at 4.7 GHz with a corresponding effective absorption bandwidth of 4 GHz, where the absorber thickness is 2 mm and contains 50% TOMCC/AgNPs. The combination of silver nanoparticles and TOMCC results in multiple interfacial and intermolecular polarizations of TOMCC/AgNPs, which improves energy loss. Therefore, the TOMCC/AgNPs that are obtained can be a promising green, lightweight, highly efficient microwave-absorbing material.
Acknowledgments This work was supported by Science and Technology Innovation Leading Project of Inner Mongolia Autonomous Region (KCBJ2018013) and the Science and Technology Plan Projects of Inner Mongolia Autonomous (2021GG0074).
Author contributions GS completed the experiment, data processing and manuscript writing; JH, SH, QZ completed the review of the manuscript.

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