Multicomponent X-ray Shielding Using Sulfated Cerium Oxide and Bismuth Halide Composites

Lead is the most widely used X-ray-shielding material, but it is heavy (density ≈ 11.34 g/cm3) and toxic. Therefore, the replacement of Pb with lightweight, ecofriendly materials would be beneficial, and such materials would have applications in medicine, electronics, and aerospace engineering. However, the shielding ability of Pb-free materials is significantly lower than that of Pb itself. To maximize the radiation attenuation of non-Pb-based shielding materials, a high-attenuation cross-section, normal to the incoming X-ray direction, must be achieved. In this study, we developed efficient X-ray-shielding materials composed of sulfated cerium oxide (S-CeO2) and bismuth halides. Crucially, the materials are lightweight and mechanically flexible because of the absence of heavy metals (for example, Pb and W). Further, by pre-forming the doped metal oxide as a porous sponge matrix, and then incorporating the bismuth halides into the porous matrix, uniform, compact, and intimate composites with a high-attenuation cross-section were achieved. Owing to the synergetic effect of the doped metal oxide and bismuth halides, the resultant thin (approximately 3 mm) and lightweight (0.85 g·cm−3) composite achieved an excellent X-ray-shielding rate of approximately 92% at 60 kV, one of the highest values reported for non-heavy-metal shielding materials.


Introduction
Ionizing radiation, including γ-rays, X-rays, and neutrons, is widely utilized in the nuclear, military, space, and medical fields [1,2]. In particular, X-rays are the most frequently utilized radiation in medical diagnosis, therapy, industrial inspection, and academic research [3][4][5]. Therefore, the demand for X-ray-based technology is expected to increase continuously. However, excessive exposure to X-rays is harmful. In particular, X-ray radiation exposure, particularly long-term high-energy radiation exposure, can cause cancer [6,7]. Therefore, the use of X-rays as an analytical tool presents a risk for the instrument operators [8,9]. Consequently, adequate X-ray protection is crucial to keep workers safe, and numerous strategies for developing effective X-ray radiation-shielding materials have been proposed [10][11][12][13].
Owing to its high density and Z value, lead is the most effective material for radiation shielding, particularly for preventing γ-ray and X-ray penetration [14]. Typical Pb-based shielding materials comprise Pb particles impregnated with Si or rubber [15,16]. However, a disadvantage of Pb-based materials is their toxicity, and their potential for leakage as a result of matrix damage, cracking, and aging [15]. Additionally, the majority of Pb-based shielding materials are heavy and bulky, and their applications in wearable radiationprotective clothing are hampered by their lack of flexibility and weight [17]. Therefore, for convenience and practicality, lightweight and ecofriendly non-Pb shielding materials are required [18]. Figure 1 shows the fabrication of the S-CeO 2 /BiI 3 composite, as well as photographs of each product. There were three fabrication steps: (1)sulfation, (2) porous structuring, and (3) the incorporation of bismuth halides. First, CeO 2 was sulfated by chemically bonding sulfonic (-SO 3 H) groups to the CeO 2 surface, as shown in Figure 1a. Sulfation was confirmed by the color change from white (CeO 2 ) to yellow (S-CeO 2 ) powder, as shown in Figure 1b,c. Sulfation is a good strategy for increasing the surface electron density of CeO 2 , thereby improving radiation shielding [24]. Sulfation also increases the catalytic activity and stability of the metal oxides [25][26][27]. Subsequently, a porous S-CeO 2 sponge was fabricated as an X-ray-shielding material (Figure 1e). Crucially, for shielding applications, the metal oxide powders should be moldable, enabling the formation of shapes, such as plates, fibers, or films. To achieve this, polymers (for example, epoxy or PDMS) are required as binders. However, because the polymers make a very small contribution to the X-ray attenuation, we introduced a third component with good X-ray attenuation. This was achieved by pre-forming a porous S-CeO 2 sponge, and then soaking it in a bismuth halide solution (Figure 1d,e). As a result, a multicomponent X-ray-shielding sponge containing S-CeO 2 and bismuth halide was formed, as shown in Figure 1e. Because both S-CeO 2 and bismuth halides exhibit a good X-ray attenuation, their combination is expected to result in efficient X-ray shielding. halide solution (Figure 1d,e). As a result, a multicomponent X-ray-shielding sponge containing S-CeO2 and bismuth halide was formed, as shown in Figure 1e. Because both S-CeO2 and bismuth halides exhibit a good X-ray attenuation, their combination is expected to result in efficient X-ray shielding.

Structural and Functional Group Analysis of S-CeO2
The powder XRD (PXRD) patterns of pure CeO2 and S-CeO2 are shown in Figure 2a. The diffraction patterns of CeO2 and S-CeO2 contain sharp and intense peaks, confirming the good crystallinity of these materials. The peaks were indexed to JCPDS Standard No. 65-5923 for CeO2, and the characteristic peaks in the PXRD pattern of CeO2 (before sulfation) were detected at 28.56° (111), 33.12° (200), 47.59° (220), 56.39° (311), 59.14° (222), 69.52° (400), and 76.86° (331) in 2θ, consistent with the cubic fluorite structure of CeO2. The PXRD pattern of S-CeO2 was identical to the standard card, indicating that the introduction of sulfur had no effect on the crystallinity. The sulfation process may prevent the CeO2 particles from clumping together, which could increase their surface area, and reduce their crystalline size. This is consistent with the observed decrease in crystallite size. However, the X-ray diffraction patterns did not show any significant peak shifts, suggesting that sulfation did not affect the crystal structure or phase composition of CeO2. Figure 2b shows the FT-IR spectra of pure CeO2 and S-CeO2. In the spectrum of pure CeO2, the O-H stretching, CO2 asymmetric stretching, and C-O stretching vibrations were observed at 3369, 712, and 1057 cm −1 , respectively. Figure 2b shows the FT-IR spectrum of S-CeO2. The bands observed for S-CeO2 indicate that its structure is substantially different from that of pure CeO2. The peak observed at 1625 cm −1 corresponds to the O-H stretching vibrations of the O-H group on the surface of S-CeO2 [28]. The observed peaks at 1087, 1047, and 982 cm −1 correspond to the stretching of the O=S=O, S=O, and S-O groups, respectively, in the sulfonic acid group of the S-CeO2 nanostructure. In addition, the sulfonic acid group (-SO3H) yielded a peak at around 3369 cm −1 , which is similar to the stretching vibration of water molecules (-OH) [28]. These results confirm that the sulfation reaction introduced sulfonic acid groups onto the surface of the CeO2 nanostructure.  The PXRD pattern of S-CeO 2 was identical to the standard card, indicating that the introduction of sulfur had no effect on the crystallinity. The sulfation process may prevent the CeO 2 particles from clumping together, which could increase their surface area, and reduce their crystalline size. This is consistent with the observed decrease in crystallite size. However, the X-ray diffraction patterns did not show any significant peak shifts, suggesting that sulfation did not affect the crystal structure or phase composition of CeO 2 . Figure 2b shows the FT-IR spectra of pure CeO 2 and S-CeO 2 . In the spectrum of pure CeO 2 , the O-H stretching, CO 2 asymmetric stretching, and C-O stretching vibrations were observed at 3369, 712, and 1057 cm −1 , respectively. Figure 2b shows the FT-IR spectrum of S-CeO 2 . The bands observed for S-CeO 2 indicate that its structure is substantially different from that of pure CeO 2 . The peak observed at 1625 cm −1 corresponds to the O-H stretching vibrations of the O-H group on the surface of S-CeO 2 [28]. The observed peaks at 1087, 1047, and 982 cm −1 correspond to the stretching of the O=S=O, S=O, and S-O groups, respectively, in the sulfonic acid group of the S-CeO 2 nanostructure. In addition, the sulfonic acid group (-SO 3 H) yielded a peak at around 3369 cm −1 , which is similar to the stretching vibration of water molecules (-OH) [28]. These results confirm that the sulfation reaction introduced sulfonic acid groups onto the surface of the CeO 2 nanostructure.

XPS Analysis of S-CeO2
The XPS profiles of CeO2 and S-CeO2 are shown in Figure 3. Figure 3a shows the su vey spectra of CeO2 and S-CeO2, and peaks corresponding to all the expected elemen were observed. However, a new peak in the S-CeO2 spectrum with a binding energy 167 eV was also observed (blue dashed frame), and this can be attributed to the S2p co ponent, demonstrating that sulfonic acid groups were bound to CeO2. Figure 3b show the C1s spectrum of S-CeO2, and the peaks were deconvoluted into two peaks at 284.7 a 283.2 eV, which correspond to C-N and C-C/C-H, respectively. As shown in Figure 3c, t deconvolution of the O1s electron core-level spectrum revealed three distinct oxygen sp cies. The high-intensity peak at 530.4 eV is associated with oxygen in the CeO2 lattice [2 In contrast, the other two peaks at 531.7 and 528.3 eV may be due to adsorbed oxygen hydroxyl groups present in the oxygen vacancy sites within the CeO2 matrix [30,31]. T high-resolution Ce3d spectrum ( Figure 3d) reveals four peaks that can be attributed to t spin-orbit splitting of Ce 3d5/2 and Ce 3d3/2, respectively [32]. Because of this spin doub splitting, CeO2 can be found in both the Ce 3+ and Ce 4+ oxidation states [33]. The distincti XPS signals at 884.4 and 901.4 eV can be assigned to the Ce 4+ 3d5/2 and Ce 3+ 3d3/2 electr states, respectively. The two additional satellite peaks observed at 881.1 and 915.8 eV or inate from Ce 3+ 3d5/2 and Ce 4+ 3d3/2, respectively. Thus, the recorded spectra confirm t presence of the mixed valences Ce 3+ and Ce 4+ . Further, the primary peaks at 884 and 9 eV indicate the relative quantities of Ce 4+ and Ce 3+ in the sample. The presence of oxyg vacancies in CeO2 was confirmed using the areas of the individual peaks, which reve that the concentration of Ce 4+ was relatively high. It was determined that these two sta have a binding energy difference of 17 eV, which is in good agreement with accepted v ues [34]. In addition, sulfate groups were detected on the CeO2 surface, as evidenced the peaks at 168.1 and 167.2 eV for S 2p1/2 and 2p3/2 in the S2p spectra of S-CeO2 (Figu 3e), which were assigned to S=O and S-O, respectively [26].

XPS Analysis of S-CeO 2
The XPS profiles of CeO 2 and S-CeO 2 are shown in Figure 3. Figure 3a shows the survey spectra of CeO 2 and S-CeO 2 , and peaks corresponding to all the expected elements were observed. However, a new peak in the S-CeO 2 spectrum with a binding energy of 167 eV was also observed (blue dashed frame), and this can be attributed to the S2p component, demonstrating that sulfonic acid groups were bound to CeO 2 . Figure 3b shows the C1s spectrum of S-CeO 2 , and the peaks were deconvoluted into two peaks at 284.7 and 283.2 eV, which correspond to C-N and C-C/C-H, respectively. As shown in Figure 3c, the deconvolution of the O1s electron core-level spectrum revealed three distinct oxygen species. The high-intensity peak at 530.4 eV is associated with oxygen in the CeO 2 lattice [29]. In contrast, the other two peaks at 531.7 and 528.3 eV may be due to adsorbed oxygen or hydroxyl groups present in the oxygen vacancy sites within the CeO 2 matrix [30,31]. The high-resolution Ce3d spectrum ( Figure 3d) reveals four peaks that can be attributed to the spin-orbit splitting of Ce 3d 5/2 and Ce 3d 3/2 , respectively [32]. Because of this spin doublet splitting, CeO 2 can be found in both the Ce 3+ and Ce 4+ oxidation states [33]. The distinctive XPS signals at 884.4 and 901.4 eV can be assigned to the Ce 4+ 3d 5/2 and Ce 3+ 3d 3/2 electron states, respectively. The two additional satellite peaks observed at 881.1 and 915.8 eV originate from Ce 3+ 3d 5/2 and Ce 4+ 3d 3/2 , respectively. Thus, the recorded spectra confirm the presence of the mixed valences Ce 3+ and Ce 4+ . Further, the primary peaks at 884 and 901 eV indicate the relative quantities of Ce 4+ and Ce 3+ in the sample. The presence of oxygen vacancies in CeO 2 was confirmed using the areas of the individual peaks, which reveal that the concentration of Ce 4+ was relatively high. It was determined that these two states have a binding energy difference of 17 eV, which is in good agreement with accepted values [34]. In addition, sulfate groups were detected on the CeO 2 surface, as evidenced by the peaks at 168.1 and 167.2 eV for S 2p 1/2 and 2p 3/2 in the S2p spectra of S-CeO 2 (Figure 3e), which were assigned to S=O and S-O, respectively [26].

HR-TEM and Element Mapping Analysis of S-CeO2
Additionally, HR-TEM was used to study the surface morphology of sulfated CeO2, as shown in Figure 4. Figure 4a-c display the HR-TEM images of S-CeO2 at various magnifications. As shown in Figure 4a, the S-CeO2 crystals with diameters of 100-200 nm were highly dispersed. In addition, the S-CeO2 crystals have a thin and transparent semi-hexagonal plate-like structure, and stacked layers can be seen at the edges. The marked region indicates the thin, layered structure of S-CeO2, which may be helpful for the surface modification of CeO2 crystal with sulfonic acid groups, thereby enhancing the electron density and X-ray-shielding activity. Figure 4b,c show the layered structure of the S-CeO2 crystals more clearly. Notably, there is a five-layer structure at the edge of the S-CeO2 crystals (Figure 4c). The SAED pattern of the S-CeO2 crystal structure is shown in Figure 4d, illustrating the highly crystalline nature of S-CeO2. The reflections are consistent with the cubic fluorite structure. Field-emission electron probe microanalysis (EPMA) was also performed on S-CeO2, and element maps were recorded ( Figure S2). Figure 4e-h illustrate the S-CeO2 EPMA maps and the associated EDX data. As shown, cerium, oxygen, and sulfur atoms were uniformly distributed throughout the components. In particular, the sulfur distribution in Figure 4h clearly shows that the sulfation was successful, which is consistent with the FT-IR and XPS results. The EDX spectrum presented in Figure S2 also confirms the elemental composition of the prepared S-CeO2.

HR-TEM and Element Mapping Analysis of S-CeO 2
Additionally, HR-TEM was used to study the surface morphology of sulfated CeO 2 , as shown in Figure 4. Figure 4a-c display the HR-TEM images of S-CeO 2 at various magnifications. As shown in Figure 4a, the S-CeO 2 crystals with diameters of 100-200 nm were highly dispersed. In addition, the S-CeO 2 crystals have a thin and transparent semi-hexagonal plate-like structure, and stacked layers can be seen at the edges. The marked region indicates the thin, layered structure of S-CeO 2 , which may be helpful for the surface modification of CeO 2 crystal with sulfonic acid groups, thereby enhancing the electron density and X-ray-shielding activity. Figure 4b,c show the layered structure of the S-CeO 2 crystals more clearly. Notably, there is a five-layer structure at the edge of the S-CeO 2 crystals (Figure 4c). The SAED pattern of the S-CeO 2 crystal structure is shown in Figure 4d, illustrating the highly crystalline nature of S-CeO 2 . The reflections are consistent with the cubic fluorite structure. Field-emission electron probe microanalysis (EPMA) was also performed on S-CeO 2 , and element maps were recorded ( Figure S2). Figure 4e-h illustrate the S-CeO 2 EPMA maps and the associated EDX data. As shown, cerium, oxygen, and sulfur atoms were uniformly distributed throughout the components. In particular, the sulfur distribution in Figure 4h clearly shows that the sulfation was successful, which is consistent with the FT-IR and XPS results. The EDX spectrum presented in Figure S2 also confirms the elemental composition of the prepared S-CeO 2 .

FE-SEM and Element Mapping
We fabricated a sponge-type composite for shielding applications, using S-CeO2 nanopowder and bismuth halides as X-ray attenuators, with PDMS as a binder (Figure 5k). We also prepared pure PDMS and PDMS/S-CeO2 sponges for comparison (Figure 5a,f). We used FE-SEM and EDX elemental mapping to examine the morphologies and elemental distributions of the pure porous PDMS, PDMS/S-CeO2, and PDMS/SCeO2/BiI3. The FE-SEM images of the pure porous PDMS (Figures 5b and S3) reveal a porous structure with interconnected channels, which could facilitate the incorporation of S-CeO2 and bismuth halides. Elemental mapping confirmed that the pure PDMS contained Si and O, and they were evenly distributed throughout the structure (Figure 5c-e). The EDX data in Figure S4 further confirm the elemental composition and purity of the pure PDMS. The surface morphology of the PDMS/S-CeO2 sponge is shown in Figure 5g. As shown, it has a highly porous and spongy structure. The high-magnification image in Figure S5 clearly shows that nanostructured S-CeO2 is uniformly distributed throughout the PDMS matrix, and retains its porous and spongy structure. Elemental mapping confirmed that S-CeO2 was uniformly distributed, and formed the main matrix in the composite (Figure 5h-j). The EDX data in Figure S6 further verify the elemental composition of the PDMS/S-CeO2. During our preparation process, the PDMS/S-CeO2/BiI3 was prepared through the soaking of the PDMS/S-CeO2 sponge in a bismuth halide solution (BiI3 + BiBr3). Thus, the bismuth halide salts penetrated the sponge and solidified on the surfaces of the pores of the PDMS/S-CeO2 sponge. This was reflected by a color change from light-yellow PDMS/S-CeO2 to black PDMS/S-CeO2/BiI3 (Figure 5k). The SEM image of PDMS/S-CeO2/BiI3 also reveals that bismuth halide particles are uniformly dispersed throughout the PDMS matrix (Figures 5l and S7). Thus, the S-CeO2 acted as a scaffold for the BiI3 particles, allowing them to be uniformly dispersed throughout the PDMS matrix. The combination of these three materials results in a composite with unique properties. Figure 5m-o show the elemental maps of the PDMS/S-CeO2/BiI3 sponges. The elemental maps and EDX spectrum in Figure S8 also reveal a homogeneous distribution of Ce, Bi, and I. The quantitative data

FE-SEM and Element Mapping
We fabricated a sponge-type composite for shielding applications, using S-CeO 2 nanopowder and bismuth halides as X-ray attenuators, with PDMS as a binder (Figure 5k). We also prepared pure PDMS and PDMS/S-CeO 2 sponges for comparison (Figure 5a,f). We used FE-SEM and EDX elemental mapping to examine the morphologies and elemental distributions of the pure porous PDMS, PDMS/S-CeO 2 , and PDMS/SCeO 2 /BiI3. The FE-SEM images of the pure porous PDMS (Figures 5b and S3) reveal a porous structure with interconnected channels, which could facilitate the incorporation of S-CeO 2 and bismuth halides. Elemental mapping confirmed that the pure PDMS contained Si and O, and they were evenly distributed throughout the structure (Figure 5c-e). The EDX data in Figure S4 further confirm the elemental composition and purity of the pure PDMS. The surface morphology of the PDMS/S-CeO 2 sponge is shown in Figure 5g. As shown, it has a highly porous and spongy structure. The high-magnification image in Figure S5 clearly shows that nanostructured S-CeO 2 is uniformly distributed throughout the PDMS matrix, and retains its porous and spongy structure. Elemental mapping confirmed that S-CeO 2 was uniformly distributed, and formed the main matrix in the composite (Figure 5h-j). The EDX data in Figure  S6 further verify the elemental composition of the PDMS/S-CeO 2 . During our preparation process, the PDMS/S-CeO 2 /BiI 3 was prepared through the soaking of the PDMS/S-CeO 2 sponge in a bismuth halide solution (BiI 3 + BiBr 3 ). Thus, the bismuth halide salts penetrated the sponge and solidified on the surfaces of the pores of the PDMS/S-CeO 2 sponge. This was reflected by a color change from light-yellow PDMS/S-CeO 2 to black PDMS/S-CeO 2 /BiI 3 (Figure 5k). The SEM image of PDMS/S-CeO 2 /BiI 3 also reveals that bismuth halide particles are uniformly dispersed throughout the PDMS matrix (Figures 5l and S7). Thus, the S-CeO 2 acted as a scaffold for the BiI 3 particles, allowing them to be uniformly dispersed throughout the PDMS matrix. The combination of these three materials results in a composite with unique properties. Figure 5m-o show the elemental maps of the PDMS/S-CeO 2 /BiI 3 sponges. The elemental maps and EDX spectrum in Figure S8 also reveal a homogeneous distribution of Ce, Bi, and I. The quantitative data on the chemical composition of the PDMS/S-CeO 2 /BiI 3 shielding material are also shown in Table S1.
Molecules 2023, 28, x FOR PEER REVIEW 7 of 12 on the chemical composition of the PDMS/S-CeO2/BiI3 shielding material are also shown in Table S1.

X-ray-Shielding Analysis of Multicomponent Halide Composites
Based on the successful preparation of the multicomponent PDMS/S-CeO2/BiI3 composites, we explored the X-ray-shielding performance of each sample, using the X-ray measurement system shown in Figure 6a. First, we compared the X-ray-shielding performance of pure PDMS, PDMS/CeO2 sponge, and PDMS/S-CeO2 sponge. The pure PDMS exhibited a low shielding performance, below 20%, at tube voltages of 60 and 100 kV. Owing to its low atomic number, low density, and soft and flexible characteristics, pure PDMS is not an effective X-ray-shielding material, allowing X-rays to penetrate the material, rather than be adsorbed or scattered. In contrast, PDMS/CeO2 showed an increased shielding performance compared to pure PDMS, owing to the addition of high-Z CeO2. The shielding ratio was further enhanced for PDMS/S-CeO2. Crucially, because sulfated functionalization increases the surface electron density of the metal oxide, there is a greater chance for X-ray photons to interact with the electrons in the composite, lowering the photon energy, and attenuating the X-ray beam [35,36]. Thus, the results confirm that sulfation improved the radiation-shielding performance. However, there are still some regions of the PDMS with a low attenuation, and empty spaces in the sponge structure.

X-ray-Shielding Analysis of Multicomponent Halide Composites
Based on the successful preparation of the multicomponent PDMS/S-CeO 2 /BiI 3 composites, we explored the X-ray-shielding performance of each sample, using the Xray measurement system shown in Figure 6a. First, we compared the X-ray-shielding performance of pure PDMS, PDMS/CeO 2 sponge, and PDMS/S-CeO 2 sponge. The pure PDMS exhibited a low shielding performance, below 20%, at tube voltages of 60 and 100 kV. Owing to its low atomic number, low density, and soft and flexible characteristics, pure PDMS is not an effective X-ray-shielding material, allowing X-rays to penetrate the material, rather than be adsorbed or scattered. In contrast, PDMS/CeO 2 showed an increased shielding performance compared to pure PDMS, owing to the addition of high-Z CeO 2 . The shielding ratio was further enhanced for PDMS/S-CeO 2 . Crucially, because sulfated functionalization increases the surface electron density of the metal oxide, there is a greater chance for X-ray photons to interact with the electrons in the composite, lowering the photon energy, and attenuating the X-ray beam [35,36]. Thus, the results confirm that sulfation improved the radiation-shielding performance. However, there are still some regions of the PDMS with a low attenuation, and empty spaces in the sponge structure. To enhance the shielding performance further, bismuth halides were incorporated into the metal oxide sponge, through it being soaked in a bismuth halide solution. After drying, the bismuth halides covered the PDMS surface, and filled the empty spaces. The X-ray-shielding performance of the PDMS/S-CeO2/bismuth halide composite is shown in Figure 6c. Six different compositions of Bi(I1−xBrx)3 were loaded onto the coin-shaped PDMS/S-CeO2 in different weight ratios (x = 0, 0.2, 0.4, 0.6, 0.8, and 1). We found that the PDMS/S-CeO2/BiI3 (x = 0) exhibited the best X-ray-shielding ratio of 91.8%, at a tube voltage of 60 kV. In contrast, the PDMS/S-CeO2/BiBr3 (x = 1) exhibited the lowest performance, and there was no benefit from the mixed-halide compositions (Bi(I1−xBrx)3, x = 0.2-0.8) The X-ray-shielding ability is affected by two factors: the attenuation cross-section and electron density. Because BiBr3 is smaller, and can form a much denser composite than BiI3, it was expected that BiBr3 would have a higher-attenuation cross-section. However, BiBr3 has a lower electron density than BiI3. Considering that both PDMS/S-CeO2/BiI3 and PDMS/S-CeO2/BiBr3 already have high-attenuation cross-sections, the effect of the electron density is more crucial to the radiation-shielding performance. Therefore, PDMS/S-CeO2/BiI3 exhibited a better performance than PDMS/S-CeO2/BiBr3.
To determine the densities of the composites, we measured the total weight of each sample, and calculated its density. All samples had a coin shape, with a diameter of 25 mm and a thickness of 3 mm, yielding a volume of 1.47 cm −3 . Because the total weight of the PDMS/S-CeO2/BiI3 composite is 1.25 g, the calculated density is 0.85 g cm −3 . Similarly, the densities of the other composites were calculated, and are shown in Figure 6d. All the composites exhibited a low density of less than 1 g cm −3 , which is beneficial for lightweight X-ray-shielding applications. The comparison of the X-ray-shielding performance with existing materials is shown in Table S2. To enhance the shielding performance further, bismuth halides were incorporated into the metal oxide sponge, through it being soaked in a bismuth halide solution. After drying, the bismuth halides covered the PDMS surface, and filled the empty spaces. The X-ray-shielding performance of the PDMS/S-CeO 2 /bismuth halide composite is shown in Figure 6c. Six different compositions of Bi(I 1−x Br x ) 3 were loaded onto the coin-shaped PDMS/S-CeO 2 in different weight ratios (x = 0, 0.2, 0.4, 0.6, 0.8, and 1). We found that the PDMS/S-CeO 2 /BiI 3 (x = 0) exhibited the best X-ray-shielding ratio of 91.8%, at a tube voltage of 60 kV. In contrast, the PDMS/S-CeO 2 /BiBr 3 (x = 1) exhibited the lowest performance, and there was no benefit from the mixed-halide compositions (Bi(I 1−x Br x ) 3 , x = 0.2-0.8) The X-ray-shielding ability is affected by two factors: the attenuation cross-section and electron density. Because BiBr 3 is smaller, and can form a much denser composite than BiI 3 , it was expected that BiBr 3 would have a higher-attenuation cross-section. However, BiBr 3 has a lower electron density than BiI 3 . Considering that both PDMS/S-CeO 2 /BiI 3 and PDMS/S-CeO 2 /BiBr 3 already have high-attenuation cross-sections, the effect of the electron density is more crucial to the radiation-shielding performance. Therefore, PDMS/S-CeO 2 /BiI 3 exhibited a better performance than PDMS/S-CeO 2 /BiBr 3 .
To determine the densities of the composites, we measured the total weight of each sample, and calculated its density. All samples had a coin shape, with a diameter of 25 mm and a thickness of 3 mm, yielding a volume of 1.47 cm −3 . Because the total weight of the PDMS/S-CeO 2 /BiI 3 composite is 1.25 g, the calculated density is 0.85 g cm −3 . Similarly, the densities of the other composites were calculated, and are shown in Figure 6d. All the composites exhibited a low density of less than 1 g cm −3 , which is beneficial for lightweight X-ray-shielding applications. The comparison of the X-ray-shielding performance with existing materials is shown in Table S2.

Synthesis of Sulfated CeO 2
Cerium oxide particles (<5 µm) were purchased from Sigma-Aldrich (St. Louis, MO, USA), and sulfonated as follows. A mixture of sulfuric acid (15 mL, 1 M H 2 SO 4 ) and methanol (20 mL) was used to prepare a suspension of nanostructured CeO 2 (approximately 1 g). The suspension was sonicated at a high intensity for approximately 2 h. To obtain S-CeO 2 , the product was dried for 24 h at 100 • C. Subsequently, the dry S-CeO 2 product was characterized ( Figure S1).

Fabrication of Porous PDMS and PDMS/S-CeO 2
Bismuth (III) bromide, bismuth (III) iodide, and sulfuric acid (H 2 SO 4 ) 99.999% were purchased from Sigma-Aldrich. Polydimethylsiloxane (PDMS) (Sylgard 184 Silicone Elastomer Kit) was purchased from Dow Corning (Midland, MI, USA). The fabrication of porous PDMS was as follows: PDMS, curing agent, and NaCl were mixed in a 1:0.1:1.5 weight ratio. The mixed solution was centrifuged at 8000 rpm for 20 min at room temperature, three times. Then, the excess PDMS was removed, and the sample was heat-treated at 60 • C for 18 h. Then, the solidified PDMS was cut into coin shapes (thickness: 3 mm, diameter: 25 mm). The PDMS coins were immersed in water, and ultrasonicated at 60 • C for 18 h to remove the NaCl inside the PDMS. The volumes of the removed NaCl determine the porosity of the PDMS.
The preparation of porous PDMS/S-CeO 2 followed almost the same method as that of porous PDMS. A mixture of PDMS, curing agent, S-CeO 2 , and processed NaCl was prepared in a 1:0.1:1.5:2 weight ratio. Then, the mixed solution was centrifuged at 8000 rpm for 20 min at room temperature, three times. Then, the excess PDMS was removed, followed by heat treatment at 60 • C for 18 h. Finally, the PDMS was cut into coin shapes (thickness: 3 mm, diameter: 25 mm), and the NaCl was removed using an ultrasonic cleaner at 60 • C for 18 h, thus yielding the PDMS/S-CeO 2 .

Porous PDMS/BiI 3 /BiBr 3 Salt Solutions with Different Weight Ratios
BiI 3 and BiBr 3 were combined in the weight ratios of 20%, 40%, 60%, and 80%. The two powders were vigorously mixed in a vial, and the ratios are shown in Table 1. Then, the Bi salts were mixed in tetrahydrofuran (THF) (1.0 g salt in 3 mL THF). The porous PDMS/S-CeO 2 samples were soaked in the Bi salt solution, and dried at 60 • C for 30 min to eliminate THF.

Shielding Ability
The sample and detector were positioned 630 and 800 mm, respectively, away from the X-ray tube producing monochromatic X-rays (Spellman, Precision X-ray Inc. (Madison, CT, USA), X-Rad IR-160, Cabinet X-ray systems USA). The test energy range was set to 60-100 kV, and the tube current was fixed at 4 mA. The dosage accumulation time for each sample was set to 5 s (average of 5 measurements). The porous S-CeO 2 /BiI 3 composite was analyzed using a transmission densitometer (UniTeko, Seongnam-si, Korea). A porous S-CeO 2 /BiI 3 composite was used to assess the X-ray-shielding properties of PDMS containing various ratios of BiI 3 /BiBr 3 . The flux (dose) of X-rays can be controlled by adjusting the distance between the X-ray tube and the sample.
Here, X is the transmittance without the sample, and D is the transmittance with the sample.

Instrumentation
The crystal structure of pure CeO 2 and S-CeO 2 was characterized via powder X-ray diffractometry (PXRD) using Cu-K α radiation (30 kV PANalytical/X'Pert3-Powder), in a 2θ range of 10 • to 80 • . Fourier transform infrared (FT-IR) spectra were obtained using a JASCO/FT-4100 spectrometer. X-ray photoelectron spectroscopy (XPS, KRATOS Analytical Ltd. (Stretford, UK)/AXIS SUPRA) was measured using a monochromatic Al K α source, with a spot size of 400 µm and a pass energy of 40 eV. The morphology of the synthesized S-CeO 2 was analyzed using high-resolution transmission electron microscopy (HR-TEM, JEM-F200, JEOL Ltd., Tokyo, Japan), and selected-area electron diffraction (SAED), at an acceleration voltage of 200 kV. In addition, energy-dispersive X-ray spectroscopy (EDX) measurements and mapping were carried out. Field-emission scanning electron microscopy (FE-SEM, MIRA3 TESCAN, TESCAN KOREA, Seoul, South Korea) and energy-dispersive X-ray spectroscopy (EDX) were performed, to produce EDX spectra and elemental maps. The X-ray-shielding performance was measured using an X-ray instrument (Spellman, Precision X-ray Inc., X-Rad IR-160, Cabinet X-ray systems, USA) producing X-rays at the tube voltages of 60 and 100 kV, and a current of 4 mA, for 5 s.

Conclusions
We have successfully demonstrated efficient and lightweight X-ray-shielding materials based on S-CeO 2 and bismuth halides for the first time. The sulfation of the metal oxides enhanced their X-ray-shielding ability, as confirmed by structural, chemical, and spectroscopic analyses. The integration of the doped metal oxide as a porous sponge, and bismuth halide as a filler resulted in uniform, compact, and intimate composites with a high-attenuation cross-section. The thin (approximately 3 mm) and lightweight (0.85 g·cm −3 ) composites achieved an excellent X-ray-shielding rate of approximately 92% at 60 kV, which is among the highest values reported for non-heavy-metal shielding materials. This multicomponent lightweight PDMS/S-CeO 2 /BiI 3 composite has potential applications in wearable X-rayshielding garments, medical imaging, and nuclear power plant safety.