Self-Assembly TiO2-Ti3C2Tx Ball–Plate Structure for Highly Efficient Electromagnetic Interference Shielding

MXene is a promising candidate for the next generation of lightweight electromagnetic interference (EMI) materials owing to its low density, excellent conductivity, hydrophilic properties, and adjustable component structure. However, MXene lacks interlayer support and tends to agglomerate, leading to a shorter service life and limiting its development in thin-layer electromagnetic shielding material. In this study, we designed self-assembled TiO2-Ti3C2Tx materials with a ball–plate structure to mitigate agglomeration and obtain a thin-layer and multiple absorption porous materials for high-efficiency EMI shielding. The TiO2-Ti3C2Tx composite with a thickness of 50 μm achieved a shielding efficiency of 72 dB. It was demonstrated that the ball–plate structure generates additional interlayer cavities and internal interface, increasing the propagation path for an electromagnetic wave, which, in turn, raises the capacity of materials to absorb and dissipate the wave. These effects improve the overall EMI shielding performance of MXene and pave the way for the development of the next-generation EMI shielding system.


Introduction
The electromagnetic wave produced using electronic equipment damages the device system, limits equipment performance, and even endangers human health.Therefore, electromagnetic interference (EMI) shielding materials, which can block electromagnetic waves within a specific frequency range based on reflection and internal dissipation absorption [1][2][3], are required to solve these threats.However, conventional shielding materials, such as metallic foil, cannot satisfy the requirements for a light weight and corrosion resistance.Developing novel and effective EMI shielding materials has, thus, become a challenge for researchers [4][5][6][7].
Over the years, researchers have discovered a series of new two-dimensional transition metal carbides and nitrides, known as MXene [8,9], which show excellent potential in lightweight EMI shielding performance owing to their low density [10], unique 2D nanosheet structures [11], high electrical conductivities [12,13], and film-forming performances (which are conducive to forming a continuous conductive network) [14,15].In the meantime, a large number of polar groups (-O or -OH, etc.) are suspended on its surface, providing an abundance of active sites for the attachment of water molecules, nanoparticles, or magnetic units, rendering MXene with hydrophilic properties and the ability to modify polarization loss [16,17].It is anticipated that MXene will become a broadly applicable EMI shielding material.
Experiments have revealed that multilayer-stacked MXene flakes have an admirable EMI shielding ability due to the impedance mismatch between the high-conductivity substrate and the low-conductivity air dielectric; nevertheless, their electromagnetic wave absorption capacities are relatively deficient [18].Simultaneously, MXene nanosheets lack interlayer support and are prone to agglomeration, which will destroy the unique structure of MXene, reduce the electromagnetic wave absorption efficiency, and shorten the service period [6].Therefore, researchers try to avoid the agglomeration of MXene by building superstructures, such as CNT/MXene aerogel [19], MXene/graphene [20], MXene/polymer inclusions [21], MXene/Ni Chain [22], RGO/Ga@PEDOT:PSS [23], etc.The superstructure can indeed substantially improve the EMI performance of MXene, albeit usually with an increase in sample thickness.Due to the specificity of the structure and filler, reducing the material thickness will significantly weaken shielding performance [4].This structure-performance paradox limits the application of superstructures in the field of EMI shielding.
In MXene, Ti 3 C 2 T x possesses high electrical conductivity, stable performance, and a brief manufacturing process [23][24][25][26][27], and it has been regarded as the preferred choice for EMI shielding material substrates [28,29].Herein, we proposed and demonstrated a new multilayer porous ball-plate structure by using homogeneous and lightweight titanium dioxide (TiO 2 ) hollow spheres as the support phase between samples of fewlayer Ti 3 C 2 T x .The composite ball-plate structure obtained through a simple solution self-assembly method exhibits superior performance compared to previous studies under similar conditions (matrix and doped phase).This enhanced performance can be attributed to the gains derived from its unique structure.Additionally, the sample film obtained through filtration is only a few tens of micrometers thick, offering higher practical value compared to the millimeter-level thickness of most porous shielding materials [5,22].This superstructure simultaneously complies with the demands of low thickness, less agglomeration collapse, and superior shielding performance, thereby establishing a new development direction for novel lightweight thin-layer EMI shielding materials.

Design Principles and Synthesis of the Ball-Plate EMI Shielding Materials
Our design principle is to discover composite materials that possess both low density and high electrical conductivity while optimizing their electromagnetic shielding effectiveness through structural design.Firstly, for EMI shielding materials, the electrical conductivity, dielectric loss, and magnetic permeability all play significant roles in adjusting impedance matching and improving electromagnetic wave attenuation [30,31].It was found that the dielectric loss effect is contributed by polarization loss and conductance loss [32], whereas the magnetic loss is primarily contributed by the magnetic component [22].Therefore, to regulate the dielectric properties of a composite material, the selection of a filler with a high dielectric loss is crucial.Secondly, to preserve the heterogeneous interface and maintain a stable interlayer structure, it is essential to ensure that the filler forms a strong connection with the substrate (via van der Waals forces, covalent bonding, etc.).Thirdly, to improve the efficiency of electromagnetic wave dissipation, the design of porous foam and hollow structure inside the material is a reliable choice [33,34].Fourthly, to increase the polarization interface and extend the electromagnetic wave transmission path, void structures should be constructed within the composite material.This can be achieved by selecting an optimal combination of filler materials and particle sizes, which create channels for wave propagation.
After evaluating the candidate materials and structures based on the prescribed criteria, we have identified a promising option for EMI shielding: the multilayer ball-plate structure composed of TiO 2 and Ti 3 C 2 T x .This structure exhibits excellent properties owing to the high dielectric loss effect of TiO 2 , which can enhance microwave absorption performance.Additionally, the surface of Ti 3 C 2 T x contains dangling bonds that can easily connect with TiO 2 , enabling the self-assembly and stable combination of the composite material.To satisfy the requirements for application and testing, vacuum filtration is employed to obtain the thin-layered shielding materials that have been utilized in the majority of studies [34].We, firstly, etched MAX Ti 3 AlC 2 by using HCl and LiF to obtain the desired monolayer MXene material, i.e., Ti 3 C 2 T x .Subsequently, hollow titanium dioxide spheres were prepared via the template method, and SiO 2 is used as the liner.Finally, the above materials were combined thoroughly to obtain a suspension.After vacuum filtration and freeze-drying, a material with a flexible lamellar structure was produced.Preparation details can be found in Appendix A. The overall preparation process is shown in Figure 1.
Materials 2024, 17, 72 3 of 1 monolayer MXene material, i.e., Ti3C2Tx.Subsequently, hollow titanium dioxide sphere were prepared via the template method, and SiO2 is used as the liner.Finally, the abov materials were combined thoroughly to obtain a suspension.After vacuum filtration and freeze-drying, a material with a flexible lamellar structure was produced.Preparation de tails can be found in Appendix A. The overall preparation process is shown in Figure 1.

Structure Characterization
To construct the target superstructures, we synthesized TiO2 hollow spheres and Ti3C2Tx MXene and characterized their microstructures.The scanning electron microscop (SEM) morphology of the TiO2 ball is shown in Figure 2a.The TiO2 hollow spheres, pre pared with a diameter of 180-220 nm and a thickness of approximately 20 nm, exhibi uniform and monodisperse characteristics after stirring and ultrasonication, aligning wit the results outlined in the referenced literature [35].Figure 2b depicts the X-ray diffractio (XRD) pattern of the prepared TiO2, which exhibits a characteristic amorphous steamed bread peak between 15° and 40°.The homogenous, hollow, and amorphous properties o the monodispersed TiO2 ball are further investigated using the transmission electron mi croscope (TEM, Figure S1).The microstructure of the monolayer Ti3C2Tx MXene is give in Figure 2c.An atomic force microscope (AFM) analysis was conducted to establish th lamellar thickness of Ti3C2Tx.It is shown that the thickness of the Ti3C2Tx is 2.2 nm, whic is consistent with the calculated results [29], suggesting that it is a single layer (Figure S2 Compared to the original MAX phase Ti3AlC2, the characteristic peak of the Al atomi layer (at 39.1°) was eliminated from the Ti3C2Tx XRD spectrum (Figure 2d), and the posi tion of the main peak was moved from 9.5° to 6.54°, indicating that the desired MXen was properly synthesized.Figure 2e,f display the cross-sectional morphology and XRD patterns of TiO2-Ti3C2Tx composites with 30 wt.% TiO2 (termed as TiO2-Ti3C2Tx−30 wt.%It is clear that the TiO2 hollow spheres are evenly distributed among the layers of Ti3C2T and spontaneously adsorb on the surface of the Ti3C2Tx film, preventing excessive stackin of Ti3C2Tx flakes.Due to TiO2 support, spaces that form between Ti3C2Tx layers are condu cive to improving the electromagnetic wave in its internal losses [36].The characteristi peaks of TiO2 and Ti3C2Tx were both observed in the XRD pattern, which means that TiO and Ti3C2Tx have achieved a structural combination.The characterization of other TiO2 Ti3C2Tx composites is available in Figures S5 and S6.With the increase in TiO2 conten TiO2 hollow spheres appear to aggregate.Moreover, the prepared TiO2-Ti3C2Tx materia has a thickness of only 50 μm, and the assessed TiO2-Ti3C2Tx films demonstrated remark able flexibility, exhibiting no discernible signs of damage after cyclic bending of 180° fo 3000 iterations.Simultaneously, the tensile strength experiences a marginal reductio upon the incorporation of TiO2, remaining as 70% of the pure Ti3C2Tx.This slight reductio

Structure Characterization
To construct the target superstructures, we synthesized TiO 2 hollow spheres and Ti 3 C 2 T x MXene and characterized their microstructures.The scanning electron microscope (SEM) morphology of the TiO 2 ball is shown in Figure 2a.The TiO 2 hollow spheres, prepared with a diameter of 180-220 nm and a thickness of approximately 20 nm, exhibit uniform and monodisperse characteristics after stirring and ultrasonication, aligning with the results outlined in the referenced literature [35].Figure 2b depicts the X-ray diffraction (XRD) pattern of the prepared TiO 2 , which exhibits a characteristic amorphous steamed bread peak between 15 • and 40 • .The homogenous, hollow, and amorphous properties of the monodispersed TiO 2 ball are further investigated using the transmission electron microscope (TEM, Figure S1).The microstructure of the monolayer Ti 3 C 2 T x MXene is given in Figure 2c.An atomic force microscope (AFM) analysis was conducted to establish the lamellar thickness of Ti 3 C 2 T x .It is shown that the thickness of the Ti 3 C 2 T x is 2.2 nm, which is consistent with the calculated results [29], suggesting that it is a single layer (Figure S2).Compared to the original MAX phase Ti 3 AlC 2 , the characteristic peak of the Al atomic layer (at 39.1 • ) was eliminated from the Ti 3 C 2 T x XRD spectrum (Figure 2d), and the position of the main peak was moved from 9.5 • to 6.54 • , indicating that the desired MXene was properly synthesized.Figure 2e,f display the cross-sectional morphology and XRD patterns of TiO 2 -Ti 3 C 2 T x composites with 30 wt.% TiO 2 (termed as TiO 2 -Ti 3 C 2 T x− 30 wt.%).It is clear that the TiO 2 hollow spheres are evenly distributed among the layers of Ti 3 C 2 T x and spontaneously adsorb on the surface of the Ti 3 C 2 T x film, preventing excessive stacking of Ti 3 C 2 T x flakes.Due to TiO 2 support, spaces that form between Ti 3 C 2 T x layers are conducive to improving the electromagnetic wave in its internal losses [36].The characteristic peaks of TiO 2 and Ti 3 C 2 T x were both observed in the XRD pattern, which means that TiO 2 and Ti 3 C 2 T x have achieved a structural combination.The characterization of other TiO 2 -Ti 3 C 2 T x composites is available in Figures S5 and S6.With the increase in TiO 2 content, TiO 2 hollow spheres appear to aggregate.Moreover, the prepared TiO 2 -Ti 3 C 2 T x material has a thickness of only 50 µm, and the assessed TiO 2 -Ti 3 C 2 T x films demonstrated remarkable flexibility, exhibiting no discernible signs of damage after cyclic bending of 180 • for 3000 iterations.Simultaneously, the tensile strength experiences a marginal reduction upon the incorporation of TiO 2 , remaining as 70% of the pure Ti 3 C 2 T x .This slight reduction in tensile strength is of relatively minimal consequence for the compound's intended use as a thin-layer coating material (Figure S3).
in tensile strength is of relatively minimal consequence for the compound's intended use as a thin-layer coating material (Figure S3).X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition and bonding environment of TiO2-Ti3C2Tx−30 wt.% composites.Compared to the full XPS spectrum of original Ti3C2Tx MXene, the composite material has a significant Si signal (Figure 3a) originating from the mold of TiO2 hollow spheres, which further confirms the existence of the hollow sphere TiO2.Furthermore, the incorporation of oxides into the composite leads to a marked increase in the O 1s peak intensity, as well as a relative decrease in the F 1s peak intensity compared to the original Ti3C2Tx MXene (Figure 3b, c).The F element in the composite exhibits a shift toward higher binding energy (Figure 3b), indicating the formation of the chemical bond between TiO2 and Ti3C2Tx via the replacement of some original Ti-F bonds by Ti-O bonds.The remaining fluorine-containing groups cause a strong dipole polarization effect, which is beneficial for the attenuation and absorption of the electromagnetic wave energy of the composites.As expected, the XPS pattern of the C element exhibited relatively slight variation across multiple samples due to its lesser involvement in surface bonding.In addition, the FTIR (Fourier transform infrared reflection) peak shapes of the Ti3C2Tx MXene and TiO2-Ti3C2Tx−30 wt.% composites are nearly identical, with no discernible differences in peak positions (Figure 3f).However, the difference in the peak shape of the corresponding Ti-O transmission peak at 620 cm −1 indicates that the chemical environment of Ti-O bonding has changed.X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition and bonding environment of TiO 2 -Ti 3 C 2 T x− 30 wt.% composites.Compared to the full XPS spectrum of original Ti 3 C 2 T x MXene, the composite material has a significant Si signal (Figure 3a) originating from the mold of TiO 2 hollow spheres, which further confirms the existence of the hollow sphere TiO 2 .Furthermore, the incorporation of oxides into the composite leads to a marked increase in the O 1s peak intensity, as well as a relative decrease in the F 1s peak intensity compared to the original Ti 3 C 2 T x MXene (Figure 3b,c).The F element in the composite exhibits a shift toward higher binding energy (Figure 3b), indicating the formation of the chemical bond between TiO 2 and Ti 3 C 2 T x via the replacement of some original Ti-F bonds by Ti-O bonds.The remaining fluorine-containing groups cause a strong dipole polarization effect, which is beneficial for the attenuation and absorption of the electromagnetic wave energy of the composites.As expected, the XPS pattern of the C element exhibited relatively slight variation across multiple samples due to its lesser involvement in surface bonding.In addition, the FTIR (Fourier transform infrared reflection) peak shapes of the Ti 3 C 2 T x MXene and TiO 2 -Ti 3 C 2 T x− 30 wt.% composites are nearly identical, with no discernible differences in peak positions (Figure 3f).However, the difference in the peak shape of the corresponding Ti-O transmission peak at 620 cm −1 indicates that the chemical environment of Ti-O bonding has changed.
The relative contents of each element in different functional groups/covalent bonds were further analyzed to elucidate the interface characteristics of the filler and matrix in the composite material (Table 1).The Ti-O bond content in the Ti element of Ti 3 C 2 T x is 23.0%, while the Ti-O content in the TiO 2 -Ti 3 C 2 T x− 30 wt.% material is 34.5%, indicating that Ti-O bonds contribute to 51.7% of the oxygen in the composite, which is 2.4 times higher than that of Ti 3 C 2 T x .These findings demonstrated that the composites contain a significant degree of chemical bonding between TiO 2 and Ti 3 C 2 T x rather than simple mechanical mixing.In other words, the O element in TiO 2 forms a new bond with the Ti element in Ti 3 C 2 T x , and the matrices of the composite material and the filler are successfully recombined at the nanometer scale.Moreover, the composites exhibit a distinctive architecture, consisting of interleaved layers of few-layered Ti 3 C 2 T x sheets and hollow spheres, which is in line with the morphological and structural characterizations determined in previous experiments (Figure 2).The relative contents of each element in different functional groups/covalent bonds were further analyzed to elucidate the interface characteristics of the filler and matrix in the composite material (Table 1).The Ti-O bond content in the Ti element of Ti3C2Tx is 23.0%, while the Ti-O content in the TiO2-Ti3C2Tx−30 wt.% material is 34.5%, indicating that Ti-O bonds contribute to 51.7% of the oxygen in the composite, which is 2.4 times higher than that of Ti3C2Tx.These findings demonstrated that the composites contain a significant degree of chemical bonding between TiO2 and Ti3C2Tx rather than simple mechanical mixing.In other words, the O element in TiO2 forms a new bond with the Ti element in Ti3C2Tx, and the matrices of the composite material and the filler are successfully recombined at the nanometer scale.Moreover, the composites exhibit a distinctive architecture, consisting of interleaved layers of few-layered Ti3C2Tx sheets and hollow spheres, which is in line with the morphological and structural characterizations determined in previous experiments (Figure 2).

EMI Shielding Performance
Generally, the shielding mechanism of layered materials is primarily attributed to the interaction of the incident EMI wave with the surface and interior of the film.When the electromagnetic wave interacts with the shielding layer, the impedance mismatch between the measuring material itself and the air results in part of the incident wave being reflected by the interface, while the remaining wave is absorbed by the shielding material and internally dissipated.According to the Schelkunoff theory, electromagnetic shielding absorption comprises three components, namely reflection, absorption, and multiple reflections [37][38][39].The latter component is closely related to the thickness of the shielding material [40].When the thickness of the material is much greater than the skin depth (~1 µm) or the electromagnetic shielding efficiency is greater than 15 dB, multiple reflections can typically be neglected [41].Hence, the focus of our investigation is primarily on the absorption, reflection, and overall shielding effectiveness of the material.Based on the constructed test system and the displayed sample geometry (Figure 4a), the X-band frequency range (8.2 to 12.4 GHz) was utilized to measure the shielding properties of the composites.The frequency dependence of the SE T (total shielding effectiveness) within each material was examined, as illustrated in Figure 4b, in the specified frequency range, and each material exhibits slight fluctuations in its SE T .However, there is a discernible trend in the average SE T values of the materials in the X-band.Specifically, as the TiO 2 content increases, the SE T initially increases and then decreases.The maximum SE T value 72 db is observed at a TiO 2 content of 30 wt.%.To provide a better understanding of the shielding effect, the electromagnetic frequency of the wave was fixed.Figure 4c displays the SE T , SE A (absorption shielding effect), and SE R (reflection shielding effect) of every sample at a fixed frequency of 12 GHz.The SE T and SE A exhibit a similar trend of initially increasing and then decreasing with the increase in TiO 2 content.In contrast, the SE R shows relatively minor changes.To further investigate the influence mechanism, we separately examined the effects of absorption and reflection.
Firstly, reflection occurs at the interface between two electromagnetic wave propagation media with different impedance or refractive indices, which is one of the most significant EMI shielding mechanisms.The following equation can be used to describe this mechanism [42,43]: where Z 0 is the free space impedance, Z in is the interface impedance, σ is the total conductivity, f is the frequency, and µ is the magnetic permeability.As discussed above, the electrical conductivity of non-magnetic thin-layer EMI shielding materials is strongly correlated with their reflective properties.Therefore, we investigated the electrical conductivity and SE R of composite materials with various TiO 2 contents (Figure 4c,d).The original Ti 3 C 2 T x conductivity can exceed 1000 S/cm, providing a solid foundation for the potential reflection of the electromagnetic wave.With the increased content of TiO 2 , the hollow spheres gradually agglomerate among the layers (as shown in Figure S6), leading to a gradual decrease in conductivity (Figure 4c).Interestingly, the expected subsequent decrease of the SE R of the composites was not observed (primarily unchanged).It can be attributed to the fact that the surface of the composites is mainly composed of Ti 3 C 2 T x flakes, which are less influenced by the filler content (Figure 4b).Firstly, reflection occurs at the interface between two electromagnetic wave propagation media with different impedance or refractive indices, which is one of the most significant EMI shielding mechanisms.The following equation can be used to describe this mechanism [42,43]: where Z0 is the free space impedance, Zin is the interface impedance, σ is the total conductivity, f is the frequency, and µ is the magnetic permeability.As discussed above, the electrical conductivity of non-magnetic thin-layer EMI shielding materials is strongly correlated with their reflective properties.Therefore, we investigated the electrical conductivity and SER of composite materials with various TiO2 contents (Figure 4c,d).The original Ti3C2Tx conductivity can exceed 1000 S/cm, providing a solid foundation for the potential reflection of the electromagnetic wave.With the increased content of TiO2, the hollow spheres gradually agglomerate among the layers (as shown in Figure S6), leading to a gradual decrease in conductivity (Figure 4c).Interestingly, the expected subsequent decrease of the SER of the composites was not observed (primarily unchanged).It can be attributed to the fact that the surface of the composites is mainly composed of Ti3C2Tx flakes, which are less influenced by the filler content (Figure 4b).Secondly, the electromagnetic wave can be attenuated when they encounter a shielding material.This attenuation rate, denoted by α, is determined by the intrinsic properties of the shielding material.A higher α can be achieved by using materials with larger dielectric constants, permeabilities, and electrical conductivities [44].This mechanism can be expressed using the following formula [45,46]: Secondly, the electromagnetic wave can be attenuated when they encounter a shielding material.This attenuation rate, denoted by α, is determined by the intrinsic properties of the shielding material.A higher α can be achieved by using materials with larger dielectric constants, permeabilities, and electrical conductivities [44].This mechanism can be expressed using the following formula [45,46]: where E 0 is the initial electromagnetic wave energy, E is the electromagnetic wave energy absorbed by the shielding material, ε is the dielectric constant, d is the thickness of the plate, σ is conductivity, and µ is the magnetic permeability.The ability of a material to absorb an electromagnetic wave is related to its dielectric constant, as shown in Formulas ( 2) and (3).Under an alternating electric field, the dielectric constant comprises two components: the real and imaginary parts.The real part represents the ability to store electromagnetic energy, and the imaginary part represents the ability to dissipate the electromagnetic energy of materials, respectively.Experimental measurements show that the dielectric constant and loss factor increase with TiO 2 content in TiO 2 -Ti 3 C 2 T x materials.The loss factor exhibits a relatively low frequency dependence and increases monotonically in the X-band.The difference is negligible when the TiO 2 content is less than 20 wt.%, but it becomes significant at 30 wt.% and 40 wt.% and reaches a maximum at 50 wt.%(Figure 4e).In contrast, the dielectric constant varies more irregularly with frequency, showing three distinct numerical steps at 0-20 wt.%, 30-40 wt.%, and 50 wt.%(Figure 4f).The observed phenomena can be attributed to the increase in the dipole polarization (Ti 3 C 2 T x MXene is commonly overetched during preparation, resulting in Ti vacancies [47]) and non-homogeneous interfaces between Ti 3 C 2 T x and TiO 2 [48], which result from the addition of TiO 2 .However, the agglomeration of TiO 2 hollow spheres as fillers can lead to substrate discontinuity and increased material defects, resulting in decreased EMI shielding performance.The loss factor of the composites increases with an increase in TiO 2 content, but as the electrical conductivity decreases, these two factors, which have opposite effects on SE T , eventually lead to an optimal value.The TiO 2 -Ti 3 C 2 T x− 30 wt.% material exhibits the highest SE A value and a simultaneous maximum SE T of 72 dB.However, in comparison to previous studies using high-conductivity matrices and fillers with high-energy-dissipation properties, such as SiO 2 @ Ti 3 C 2 T x , Ni@ Ti 3 C 2 T , and polystyrene@ Ti 3 C 2 T x , which achieved a shielding efficiency of approximately 60 dB at a thickness of 1 mm or more, our study demonstrates a significant advantage in both thickness (50 µm) and shielding efficiency (72 dB).As shown in the Figure S7 and Table S2, TiO 2 -Ti 3 C 2 T x exhibits remarkable competitiveness in both thickness and shielding efficiency dimensions.Since all studies utilized the self-assembled construction of raw materials with high conductivity and exhibited a heterogeneous interface connection, the observed performance differences cannot be attributed to the intrinsic properties of the materials.Therefore, the internal structure of the materials is likely the key factor affecting the observed performance differences [47,49,50].Besides intrinsic properties such as conductivity and dielectric constant, multiple scattering effects within the composite material significantly contribute to the SE T .Materials with internal cavities can act as effective electromagnetic wave absorbers due to their complex microstructures, which provide multiple interfaces for wave reflection and scattering.The internal cavities act as resonant cavities that cause multiple scattering, increasing the path length of the wave through the material, which enhances the interaction between the electromagnetic wave and the polarized interfaces of the cavities, leading to an increase in the absorption loss efficiency [13,51].Consequently, multiple scattering within the material can increase the absorption loss of the electromagnetic wave.The dissimilarities in multiple scattering effects suggest differences in the internal structure of the material.
To further understand the internal structure of TiO 2 -Ti 3 C 2 T x composites, nitrogen adsorption-desorption isotherm curves for samples with different TiO 2 contents were studied to analyze the specific surface area and pore structure (Figure 5).It is shown that the added TiO 2 significantly increased the specific surface area of the original Ti 3 C 2 T x MXene.Furthermore, the TiO 2 hollow spheres would spontaneously pin on the surface of the Ti 3 C 2 T x film and prevent excessive stacking of Ti 3 C 2 T x (Figure 5a-c).In addition, all nitrogen adsorption-desorption isotherms did not display a saturated adsorption platform, indicating an irregular pore structure.It can be attributed to the spherical pores provided by the hollow sphere filler, and numerous additional pores were generated between the TiO 2 hollow spheres and their combinations with the Ti 3 C 2 T x structure (Figure 5d-f).
The differential pore size distributions of three materials, namely TiO 2 filler, Ti 3 C 2 T x , and their composite, were analyzed using the BJH (Barret-Joyner-Halenda) method.The TiO 2 filler exhibits mesoporous and macroporous structures due to the hollow sphere structure and accumulation of spheres.Ti 3 C 2 T x displays a hierarchical pore structure with micropores, mesopores, and macropores generated via in situ HF etching and the interlacing of multiple layers.The composite material has a similar hierarchical pore structure with increased mesopores and reduced macropores, indicating a uniform and proper combination of the basic materials.The decreased number of macropores was due to the filling of larger pores when Ti 3 C 2 T x and TiO 2 were separately stacked, and the distribution of the voids verified the proper combination of the materials in the microstructure.The composite material showed a significant increase in the number of mesopores, which exceeded the sum of mesopores in the matrix and filler.This indicates that combining Ti 3 C 2 T x and TiO 2 led to the creation of new mesopore-sized cavities in the material.In fact, the increase in porosity has two main effects.Firstly, the increase in pores is inevitably accompanied by an increase in internal interfaces within the material.At these interfaces, electromagnetic waves are further dissipated due to impedance mismatch and interface polarization losses, as reflected in the macroscopic increase in dielectric loss (Figure 4e,f).Secondly, the increase in porosity disrupts the continuity between MXene layers, consequently reducing the overall electrical conductivity of the composite material (Figure 4d).Ultimately, the enhancement in the total electromagnetic shielding performance of the composite material is attributed to the absorption gain resulting from the increased dielectric loss outweighing the reduction in reflection due to the decrease in conductivity.The combined effect of these pores resulted in the dissipation of electromagnetic wave inside the material, ultimately enhancing the electromagnetic shielding performance.The proposed ball-plate stack structure is an improvement over traditional multilayer plate and core-shell structures by providing additional cavities for multiple scattering effects, thereby enhancing the EMI shielding effectiveness.To further understand the internal structure of TiO2-Ti3C2Tx composites, nitrogen adsorption-desorption isotherm curves for samples with different TiO2 contents were studied to analyze the specific surface area and pore structure (Figure 5).It is shown that the added TiO2 significantly increased the specific surface area of the original Ti3C2Tx MXene.Furthermore, the TiO2 hollow spheres would spontaneously pin on the surface of the Ti3C2Tx film and prevent excessive stacking of Ti3C2Tx (Figure 5a-c).In addition, all nitrogen adsorption-desorption isotherms did not display a saturated adsorption platform, indicating an irregular pore structure.It can be attributed to the spherical pores provided by the hollow sphere filler, and numerous additional pores were generated between the TiO2 hollow spheres and their combinations with the Ti3C2Tx structure (Figure 5d-f).The differential pore size distributions of three materials, namely TiO2 filler, Ti3C2Tx, and their composite, were analyzed using the BJH (Barret-Joyner-Halenda) method.The TiO2 filler exhibits mesoporous and macroporous structures due to the hollow sphere structure and accumulation of spheres.Ti3C2Tx displays a hierarchical pore structure with micropores, mesopores, and macropores generated via in situ HF etching and the interlacing of multiple layers.The composite material has a similar hierarchical pore structure with increased mesopores and reduced macropores, indicating a uniform and proper combination of the basic materials.The decreased number of macropores was due to the filling of larger pores when Ti3C2Tx and TiO2 were separately stacked, and the distribution of the voids verified the proper combination of the materials in the microstructure.The composite material showed a significant increase in the number of mesopores, which exceeded the sum of mesopores in the matrix and filler.This indicates that combining Ti3C2Tx and TiO2 led to the creation of new mesopore-sized cavities in the material.In fact, the increase in porosity has two main effects.Firstly, the increase in pores is inevitably accompanied by an increase in internal interfaces within the material.At these interfaces, electromagnetic waves are further dissipated due to impedance mismatch and interface polarization losses, as reflected in the macroscopic increase in dielectric loss (Figure 4e,f).Finally, we constructed a model to describe the electromagnetic shielding mechanism of the TiO 2 -Ti 3 C 2 T x materials (Figure 6).Electromagnetic waves interact with materials through reflection, absorption, and transmission.The highly conductive MXene surface causes incident wave to reflect due to the impedance discontinuity at the air-material intersection.As the electromagnetic wave propagates within the material, it interacts with the TiO 2 hollow sphere and TiO 2 -Ti 3 C 2 T x heterogeneous interface, causing continuous attenuation and absorption.Meanwhile, the anchoring of MXene and TiO 2 introduces new polarized Ti-O bonds, augmenting the dielectric loss performance of the material.These polarized bonds respond to an external electric field, inducing additional energy dissipation through the polarization and depolarization processes.
tersection.As the electromagnetic wave propagates within the material, it interacts with the TiO2 hollow sphere and TiO2-Ti3C2Tx heterogeneous interface, causing continuous attenuation and absorption.Meanwhile, the anchoring of MXene and TiO2 introduces new polarized Ti-O bonds, augmenting the dielectric loss performance of the material.These polarized bonds respond to an external electric field, inducing additional energy dissipation through the polarization and depolarization processes.The ball-plate structure of the material creates numerous cavities, which increase the electromagnetic wave propagation path and form many impedances' discontinuous interfaces.Upon encountering these interfaces, the remaining electromagnetic wave scatters multiple times and is ultimately almost entirely absorbed as eddy currents inside the MXene material, with only some of the wave being transmitted.This demonstrates that the excellent shielding efficiency is attributed to the superior electrical conductivity of MXene, the polarization interface inside the material, and the stable ball-plate structure.The high conductivity of Ti3C2Tx provides a high reflection efficiency for the electromagnetic wave, promoting the multiple scattering of interlayer pores.The abundant polar bonds between TiO2 hollow spheres and functional groups on the Ti3C2Tx surface facilitate electromagnetic wave absorption and dissipation.The ball-plate structure combines the benefits of porous foam and multilayer flat-plate structures to establish numerous pores The ball-plate structure of the material creates numerous cavities, which increase the electromagnetic wave propagation path and form many impedances' discontinuous interfaces.Upon encountering these interfaces, the remaining electromagnetic wave scatters multiple times and is ultimately almost entirely absorbed as eddy currents inside the MXene material, with only some of the wave being transmitted.This demonstrates that the excellent shielding efficiency is attributed to the superior electrical conductivity of MXene, the polarization interface inside the material, and the stable ball-plate structure.The high conductivity of Ti 3 C 2 T x provides a high reflection efficiency for the electromagnetic wave, promoting the multiple scattering of interlayer pores.The abundant polar bonds between TiO 2 hollow spheres and functional groups on the Ti 3 C 2 T x surface facilitate electromagnetic wave absorption and dissipation.The ball-plate structure combines the benefits of porous foam and multilayer flat-plate structures to establish numerous pores that offer more reflection paths and polarization interfaces for the scattering and absorption of an electromagnetic wave.By incorporating appropriate fillers to support Ti 3 C 2 T x flakes, a thin, flexible material with enhanced EMI shielding performance is obtained, eliminating the flaw that makes the interior area of the materials susceptible to collapse.The study indicates that to enhance the SE T of compound materials, it is essential to consider the filler content and the filler morphology, size, and pore structure.Constructing non-homogeneous interfaces and mesoporous structures that match the fillers is crucial for achieving superior electromagnetic shielding performance.However, taking precautions is essential to avoid filler agglomeration and substantial conductivity drops.

Conclusions
In this study, we successfully synthesized a novel TiO 2 -Ti 3 C 2 T x MXene composite material with a ball-plate structure via the self-assembly method.By adjusting the amount of TiO 2 hollow spheres, it can tune the dielectric constant and EMI shielding effectiveness of the composite material.A sample with a thickness of only 50 µm containing 30 wt.% TiO 2 exhibits a remarkable SE T value of 72 dB.The excellent EMI shielding performance of the TiO 2 -Ti 3 C 2 T x composite material can be attributed to its unique ball-plate structure, which provides multiple scattering of the electromagnetic wave due to the high electrical conductivity of the material and the interface polarization.The formation of pores with different sizes in the spherical-planar structure further increases the internal dielectric losses of the material, thereby enhancing the electromagnetic shielding performance.The combination of spherical filler and layered conductive matrix achieves a synergistic effect several times and dried it in an oven at 60 • C to obtain SiO 2 balls with good dispersion and uniform size.We then took 0.2 g of the above-prepared SiO 2 ball and dispersed it in 150 mL of ethanol to form a uniform suspension.We added 0.9 mL of ammonia with a mass fraction of 25% to the suspension and evenly stirred it.We slowly added 2 mL of tertbutyl titanite (TBT) within 10 min, reacted it at 45 • C for 24 h, and then centrifuged it to obtain a white precipitate.We then washed it with ethanol several times and dried it in an oven at 60 • C, thus identifying SiO 2 @TiO 2 precursor core-shell nanocomposites.The prepared SiO 2 @TiO 2 core-shell nanocomposites were ultrasonically dispersed in 20 mL of ultrapure water.After we formed a uniform suspension, 1 mL of 2.5 m sodium hydroxide solution was added, stirred at room temperature for 8 h, centrifuged to obtain white precipitates, and then cleaned several times with ultrapure water and ethanol.The precipitate was dried in an oven at 60 • C to obtain TiO 2 Nano hollow spheres.

Figure 1 .
Figure 1.Schematic diagram of the fabrication method for the TiO2-Ti3C2Tx composite.

Figure 1 .
Figure 1.Schematic diagram of the fabrication method for the TiO 2 -Ti 3 C 2 T x composite.

Figure 2 .
Figure 2. (a,b) SEM and XRD pattern of the monodisperse amorphous TiO2 hollow spheres.(c,d) TEM image and XRD pattern of the Ti3C2Tx MXene flakes (marked by dotted lines).(e,f) Crosssectional SEM images and XRD pattern of the TiO2-Ti3C2Tx−30 wt.% composites.The TiO2 hollow spheres in the layer are marked by arrows.

Figure 2 .
Figure 2. (a,b) SEM and XRD pattern of the monodisperse amorphous TiO 2 hollow spheres.(c,d) TEM image and XRD pattern of the Ti 3 C 2 T x MXene flakes (marked by dotted lines).(e,f) Cross-sectional SEM images and XRD pattern of the TiO 2 -Ti 3 C 2 T x− 30 wt.% composites.The TiO 2 hollow spheres in the layer are marked by arrows.

Figure 4 .
Figure 4. EMI shielding test methods and related data.(a) Schematic diagram of vector network analyzer sample and testing process.(b) SET of TiO2-Ti3C2Tx composites as a function of frequency.(c) SET, SEA, and SER of TiO2-Ti3C2Tx composites under 12 GHz.(d) Conductivity of composites with different TiO2 contents.Imaginary (e) and real parts (f) of dielectric constants of composites with different TiO2 contents.

Figure 4 .
Figure 4. EMI shielding test methods and related data.(a) Schematic diagram of vector network analyzer sample and testing process.(b) SE T of TiO 2 -Ti 3 C 2 T x composites as a function of frequency.(c) SE T , SE A , and SE R of TiO 2 -Ti 3 C 2 T x composites under 12 GHz.(d) Conductivity of composites with different TiO 2 contents.Imaginary (e) and real parts (f) of dielectric constants of composites with different TiO 2 contents.

Figure 6 .
Figure 6.Schematic diagram of the interaction between the electromagnetic wave and interfaces, as well as the EMI shielding mechanism in TiO2-Ti3C2Tx composites.

Figure 6 .
Figure 6.Schematic diagram of the interaction between the electromagnetic wave and interfaces, as well as the EMI shielding mechanism in TiO 2 -Ti 3 C 2 T x composites.

Appendix A. 4 .
Fabrication of Ti 3 C 2 T x /SiO 2 Nanocomposite Films

Table 1 .
The relative content of the corresponding functional groups/valent bonds of each element.

Table 1 .
The relative content of the corresponding functional groups/valent bonds of each element.