La-Doped Sm2Zr2O7/PU-Coated Leather Composites with Enhanced Mechanical Properties and Highly Efficient Photocatalytic Performance

Flexible La-doped Sm2Zr2O7/polyurethane (PU) coated leather composites were synthesized using a one-step hydrothermal method, with highly efficient photocatalytic degradation properties by coating the La-doped Sm2Zr2O7/PU emulsion onto the leather and drying it. The phase composition and optical properties of the as-prepared photocatalytic material were systematically characterized. The result revealed that La was doped in Sm2Zr2O7 successfully, and the prepared samples still possessed pyrochlore structure. The absorption edge of the prepared samples exhibited a red-shift with the increase in La doping, indicating that La doping could broaden the absorbance range of the La-doped Sm2Zr2O7 materials. The catalytic performance of La-doped Sm2Zr2O7/PU composite emulsion coating on the photocatalytic performance of leather was studied with Congo red solution as the target pollutant. The results showed that the best photocatalytic property was found in the 5% La-doped Sm2Zr2O7 nanomaterial at a concentration of 3 g/L. The resulting 5% La-doped Sm2Zr2O7 nanomaterial exhibited a high specific surface area of 73.5 m2/g. After 40 min of irradiation by a 450 W xenon lamp, the degradation rate of Congo red reached 93%. Moreover, after surface coating, the La-doped Sm2Zr2O7/PU coated leather composites showed obviously improved mechanical properties, as the tensile strength of La-doped Sm2Zr2O7/PU coated leather composites increased from 6.3 to 8.4 MPa. The as-prepared La-doped Sm2Zr2O7/PU coated leather composites with enhanced mechanical properties and highly efficient photocatalytic performance hold promising applications in the treatment of indoor volatile organic compounds.


Introduction
The main source of indoor pollutants is volatile organic compounds, which mainly contain toxic and harmful substances such as aldehydes and benzene series.When working or studying in this environment for a long time, the human body is prone to fatigue, low mood, and even dizziness and vomiting.Therefore, the treatment of volatile organic compounds within indoor spaces is crucial [1,2].Photocatalytic degradation technology has the advantages of mild reaction conditions and green environmental protection and is considered a treatment method for volatile organic compounds with wide application prospects [3][4][5].
Leather coating materials refer to the formation of a layer or several layers of film onto the surfaces of leather so as to improve the appearance of the leather, increase its functional characteristics, and expand its scope of use.In order to prevent people from being in a polluted indoor environment for a long time, it is of great significance to develop a leather finishing material that can photocatalyze the degradation of volatile organic compounds.PU coatings are widely used for leather coating because of their advantages of low price, soft feel after film formation, and high light transmittance.However, their defects, such as low added value and poor mechanical properties, limit their further application [6,7].
Oxide semiconductor materials, such as TiO 2 , ZnO, Bi 2 WO 6 , and La 2 Zr 2 O 7 [8][9][10][11][12], are widely used in environmental treatment due to their unique physical and chemical properties and their advantages of a stable structure, low cost of large-scale production, and environmental protection.Among them, Sm 2 Zr 2 O 7 is a composite oxide with a layered three-dimensional pyrochlore structure [13,14].The bond angle of Zr-O-Zr within this crystal structure is close to 180 • .This unique crystal structure contributes to the movement of photogenerated carriers, giving Sm 2 Zr 2 O 7 its good photocatalytic activity [15].However, standalone Sm 2 Zr 2 O 7 has limited photoresponse range and a high recombination rate of photogenerated electrons and holes [16], which limits its application in the field of photocatalysis.Researchers have found that the use of rare earth ion doping in Sm 2 Zr 2 O 7 can effectively expand the visible spectral response range, enhance light absorption capacity, and thus significantly improve photocatalytic activity.Therefore, rare earth elements have received more and more attention in the modification of photocatalysts [17][18][19][20].Although it has been reported in the literature that rare earth-doped Sm 2 Zr 2 O 7 has been prepared by the stearic acid method, and it has been found that rare earth-doped ions can promote photocatalytic performance [21,22], the preparation process for Sm 2 Zr 2 O 7 requires hightemperature sintering (900 • C) and long reaction times.Additionally, the mechanism by which rare earth ion doping promotes photocatalytic activity is not very clear.
In this study, we prepared La-doped Sm 2 Zr 2 O 7 photocatalyst using a one-step hydrothermal method at low temperature.The La-doped Sm 2 Zr 2 O 7 photocatalyst was introduced into polyurethane emulsion to prepare La-doped Sm 2 Zr 2 O 7 /PU coated leather composites, which could effectively degrade indoor pollutants and was essential for the treatment of volatile organic compounds in indoor spaces.The effect of La doping on the catalytic performance of Sm 2 Zr 2 O 7 in the photocatalytic degradation of Congo red and the effect of La-doped Sm 2 Zr 2 O 7 /PU on the mechanical properties of leather composites were studied, and the reasons behind La-doping photocatalytic activity were analyzed.

Raw Materials
ZrOCl 2 •8H 2 O with industrial purity was purchased from Zibo Huantuo Chemical Co., Ltd., Zibo, China; samarium Sm(NO 3 ) 3 nitrate with analytical purity was purchased from Guangdong Rier Chemical Technology Co., Ltd., Guangzhou, China; lanthanum nitrate La(NO 3 ) 3 with analytical purity was purchased from Guangdong Rier Chemical Technology Co., Ltd.; ammonia, sodium hydroxide, and anhydrous ethanol water, all with analytical purity, were purchased from Beijing Chemical Plant, Beijing, China; self-made deionized water was used for the experiment.Polyurethane emulsion with analytical purity was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China, and PU leather was purchased from Shanghai Huafeng Microfiber Technology Co., Ltd., Shanghai, China.

Preparation of La-Doped Sm 2 Zr 2 O 7 /PU Coated Leather Composites
Sm(NO 3 ) 3 and ZrOCl 2 •8H 2 O were weighed, respectively, according to the amount of substance n (Sm):n (Zr) = 1:1, and dissolved in 50 mL of deionized water in turn.Under the condition of magnetic stirring, a certain amount of ammonia was added to adjust the pH value of the solution within the range of 5.5-9.0, and the white mixed precipitate was obtained by centrifugation.Finally, the mineralizer 11 M KOH solution was poured into the sediment to form a slurry.After thorough stirring, the whole solution was transferred to a Teflon-lined hydrothermal kettle.The kettle was sealed and placed in a constant temperature oven to react at 190 • C for 24 h.After the reaction, the product was naturally cooled to room temperature, washed with water five times, washed with ethanol two times, centrifuged, and dried at 70 • C for 4 h to obtain the product.La-doped Sm 2 Zr 2 O 7 is prepared by adding La(NO 3 ) 3 to the Sm(NO 3 ) 3 solution, as the mass fraction of La is 1% (mass fraction), 3%, 5%, and 7%, which were named as x% La-doped Sm 2 Zr 2 O 7 (x = 1, 3, 5, and 7), respectively.
La-doped Sm 2 Zr 2 O 7 was evenly dispersed in PU emulsion under stirring and ultrasonic conditions, and the composite emulsion was coated on the leather; the coating amount was about 250-350 g/m 2 .After each spray was finished, it was dried in the oven at 65 • C.After spraying, rolling, and embossing, the leather composites could be prepared.

Characterization of Product Properties
The phase composition of a photocatalyst sample was tested using an X-ray diffraction (XRD) analyzer (PANalytical, Almelo, The Netherlands), and structural analysis data were obtained.The size and morphology of photocatalyst samples were analyzed by field emission transmission electron microscopy (TEM) from FEI Company, Dreieich, Germany.The specific surface area of photocatalytic samples was measured at −196 • C with an ASAP2020 nitrogen adsorption instrument (Norcross, GA, USA).The light absorption performance of the photocatalyst was tested using a Hitachi U-3310 UV-visible photometer (Tokyo, Japan).The mechanical properties of finished leather were tested using a C43 universal material testing machine made by MTS Systems.A fully automatic fabric breathability tester (Wenzhou Jigao Testing Instrument Co., Ltd., Wenzhou, China) was used to test and analyze the breathability of finished leather, according to GB/T5453-1997 [23].

Evaluation of Photocatalytic Performance
In this study, Congo red dye was used as the target degradation of simulated organic matter rather than real-time degradation, for the following reasons: 1. Congo red is a representative azo dye, possessing chemical structures (such as benzene ring, naphthalene ring) that are difficult to degrade; 2. commercial dyes have little harm to the human body and are easy to use in experiments.
The leather sample (1 cm × 5 cm) treated with the photocatalyst was put into a test tube containing an 80 mg/L Congo red aqueous solution.A xenon lamp with 450 W was used as the simulated light source for the photocatalytic degradation reaction.An absorbance test of the extracted solution was carried out using an ultraviolet spectrophotometer every 10 min, and the concentration change of the Congo red solution was analyzed through absorbance measurement, according to the Lambert-Beer law.Dark experiments (without irradiation), blank experiments (in the absence of La-doped Sm 2 Zr 2 O 7 /PU), La-doped Sm 2 Zr 2 O 7 /PU under irradiation, and a contrast test of Degussa P25 TiO 2 under irradiation were conducted, respectively.
The degradation rate of Congo red was calculated by: where γ is the degradation rate; A 0 and A t are the initial absorbance values of the sample solution and the absorbance values during the degradation of t, respectively.

Mechanical Property Test
(1) Tensile strength and elongation at break After the coated leather samples were placed in a constant temperature and humidity box for 24 h, on average, three samples (the shape of the sample was a dumbbell) were cut out, and the tensile strength and elongation at break of the film were measured using a tensile testing machine (the tensile rate was 100 mm/min).
The tensile strength is calculated by: where P is the tensile strength of the sample, N/mm 2 ; F is the force on the fracture section of the specimen when it breaks, N; S is the area of the specimen fracture surface, mm 2 .
The elongation at break can be obtained by calculating: where E is the elongation at break, 100%; L 1 is the length of the stressed part of the specimen at fracture, mm; L 0 is the original sample length, mm.
(2) Breathability After the finished leather sample was air-conditioned in a constant temperature and humidity box for 24 h, a round sample with a diameter of 5.5 cm was cut.The permeability of leather samples was tested using a leather permeability tester.Each sample was measured in parallel more than twice, and the error between parallel experiments was less than 0.5 s.The calculation formula is as follows: where K is the sample permeability, mL/(cm•h); t is the time required for a specified area sample to pass 100 mL of air, s; t 0 is the time required for the blank test, s; S is the specimen area through air, cm 2 .

Results and Discussion
The photograph of the flexible composite is shown in Figure 1a, where it can be seen that the La-doped Sm 2 Zr 2 O 7 /PU has been coated onto the surface of the composite.It can be seen from Figure 1b that the addition of La has no significant effect on the structure of the Sm 2 Zr 2 O 7 crystal.Compared with the standard XRD pattern, it can be seen that each curve in the figure corresponds to the cubic crystal system of Sm 2 Zr 2 O 7 (JCPDS card No. 24-1012) with a pyrochlore structure.The size of Sm 2 Zr 2 O 7 particles is estimated to be about 15 nm using Scherrer's formula.However, with the partial substitution of Sm 3+ ions by La 3+ ions, the XRD pattern of La-doped Sm 2 Zr 2 O 7 is slightly offset compared with pure Sm 2 Zr 2 O 7 .The diffraction peak near 29.4 • shifts slightly toward a higher angle with the increase in La-doping amount.The results show that La 3+ ions are partially substituted and enter the lattice position of Sm 3+ ions, but the pyrochlore type (A 2 B 2 O 7 ) structure remains.The slight deviation of the diffraction peak may be caused by the different ion radii of La 3+ (1.063 Å) and Sm 3+ (1.098 Å).Although La-doping does not change the crystal structure of the samples, the intensity of some diffraction peaks of the sample is slightly weakened, and the width is slightly increased after doping, indicating that the grain size of Sm 2 Zr 2 O 7 is slightly decreased due to La entering the lattice.Figure 1c-e shows the microstructure of the composite material, indicating that La-doped Sm 2 Zr 2 O 7 /PU was well-immersed into the leather.
Figure 2a-d shows the TEM morphology of La-doped Sm 2 Zr 2 O 7 with doping amounts of 0, 3%, 5%, and 7%, respectively.The obtained samples have similar nanostructures and uniform grain size, and the grain size of the samples changes minimally with the increase in La-doping amount (~0.3 nm, as shown in Figure 2e).As can be seen from the electron diffraction graphs illustrated in Figure 2a,d, crystallinity deteriorates with the increase in La-doping amount.The reason for this may be that La-doping infiltrates into the lattice, and La 3+ replaces Sm 3+ , resulting in defects in the lattice.
Compared with uncoated leather and leather coated with pure PU emulsion, the tensile strength of leather coated with 5%-La-doped Sm 2 Zr 2 O 7 /PU composite emulsion is significantly improved, which may be due to the crystal structure of La-doped Sm 2 Zr 2 O 7 , and its rigid structure could enhance the tensile strength of the leather composites (Figure 3a,b).Moreover, the elongation at break of the coated leather is lower than that of uncoated leather and PU emulsion-coated leather composites.Compared with uncoated leather and pure PU emulsion-coated leather composites, the tear strength and bursting strength of the leather after composite emulsion coating are significantly improved.Compared with pure PU emulsion coating, the tear strength and disintegration strength of the leather composites after being coated with 5%-La-doped Sm 2 Zr 2 O 7 /PU emulsion are increased by 8% and 52%, respectively.This is mainly due to the fact that La-doped Sm 2 Zr 2 O 7 , as a crystalline material, has high stiffness properties, which can effectively enhance the strength of leather composites when introduced into the PU matrix.Compared with uncoated leather and leather coated with pure PU emulsion, the tensile strength of leather coated with 5%-La-doped Sm2Zr2O7/PU composite emulsion is significantly improved, which may be due to the crystal structure of La-doped Sm2Zr2O7, and its rigid structure could enhance the tensile strength of the leather composites (Figure 3a,b).Moreover, the elongation at break of the coated leather is lower than that of uncoated leather and PU emulsion-coated leather composites.Compared with uncoated leather and pure PU emulsion-coated leather composites, the tear strength and bursting strength of the leather after composite emulsion coating are significantly improved.Compared with pure PU emulsion coating, the tear strength and disintegration strength of the leather composites after being coated with 5%-La-doped Sm2Zr2O7/PU emulsion are increased by 8% and 52%, respectively.This is mainly due to the fact that La-doped Sm2Zr2O7, as a crystalline material, has high stiffness properties, which can effectively enhance the strength of leather composites when introduced into the PU matrix.Compared with uncoated leather and leather coated with pu sile strength of leather coated with 5%-La-doped Sm2Zr2O7/PU co nificantly improved, which may be due to the crystal structure of its rigid structure could enhance the tensile strength of the lea 3a,b).Moreover, the elongation at break of the coated leather is coated leather and PU emulsion-coated leather composites.Co  The prepared sample shows visible light absorption properties before and after La doping (Figure 4a).The resulting products produced after complete photocatalytic activity of dyes are complex and diverse [24][25][26].The catalytic mechanism is as follows: under the condition of illumination, electrons and holes are generated in the conduction band and valence band, respectively, forming electron-hole pairs.h+ can oxidize OH − and H2O molecules adsorbed on the surface of Sm2Zr2O7 to form hydroxyl radicals OH − .These radicals OH − , attached to the surface of Sm2Zr2O7 are strong oxidizing agents that can oxidize adjacent organic matter and diffuse into the liquid phase to oxidize organic matter.Through a series of oxidation processes, CO2 and H2O are finally oxidized, thus completing the degradation of organic matter.Compared with pure Sm2Zr2O7, the absorption of the doped material moved significantly in the direction of long waves, and the moving range of the absorption edge increased with the increase in La-doping amount.The absorption edges of pure Sm2Zr2O7 and Sm2Zr2O7 with La-doping levels of 1%, 3%, 5%, and 7% are 460 nm, 462 nm, 470 nm, 473 nm, and 485 nm, respectively.The absorption peak λg of La-doped Sm2Zr2O7 gradually increases with the increase in doping amount, and the relationship between the semiconductor light absorption threshold λg and the band gap width Eg is as follows: The band gap widths of each material can be calculated as 2.70 eV, 2.68 eV, 2.64 eV, 2.62 eV, and 2.56 eV, respectively.This shows that La doping can widen the light absorption range and reduce the band gap of the material.The change in material bandgap width may be due to the fact that the 3D orbital of doped La generates impurity levels in the Sm2Zr2O7 bandgap, thus reducing the energy gap of Sm2Zr2O7.
As can be seen from specific surface area tests, the specific surface area of Sm2Zr2O7 without La doping is 60.3 m 2 /g, while the specific surface area of Sm2Zr2O7 with 1%, 3%, The prepared sample shows visible light absorption properties before and after La doping (Figure 4a).The resulting products produced after complete photocatalytic activity of dyes are complex and diverse [24][25][26].The catalytic mechanism is as follows: under the condition of illumination, electrons and holes are generated in the conduction band and valence band, respectively, forming electron-hole pairs.h+ can oxidize OH − and H 2 O molecules adsorbed on the surface of Sm 2 Zr 2 O 7 to form hydroxyl radicals OH − .These radicals OH − , attached to the surface of Sm 2 Zr 2 O 7 are strong oxidizing agents that can oxidize adjacent organic matter and diffuse into the liquid phase to oxidize organic matter.Through a series of oxidation processes, CO 2 and H 2 O are finally oxidized, thus completing the degradation of organic matter.Compared with pure Sm 2 Zr 2 O 7 , the absorption of the doped material moved significantly in the direction of long waves, and the moving range of the absorption edge increased with the increase in La-doping amount.The absorption edges of pure Sm 2 Zr 2 O 7 and Sm 2 Zr 2 O 7 with La-doping levels of 1%, 3%, 5%, and 7% are 460 nm, 462 nm, 470 nm, 473 nm, and 485 nm, respectively.The absorption peak λ g of La-doped Sm 2 Zr 2 O 7 gradually increases with the increase in doping amount, and the relationship between the semiconductor light absorption threshold λ g and the band gap width E g is as follows: The band gap widths of each material can be calculated as 2.70 eV, 2.68 eV, 2.64 eV, 2.62 eV, and 2.56 eV, respectively.This shows that La doping can widen the light absorption range and reduce the band gap of the material.The change in material bandgap width may be due to the fact that the 3D orbital of doped La generates impurity levels in the Sm 2 Zr 2 O 7 bandgap, thus reducing the energy gap of Sm 2 Zr 2 O 7 .
ited growth of grains and limited movement of grain boundaries.These changes can caus the microstructure inside the crystal to become more complex, thus increasing the specifi surface area.With the continuous increase in doping amount, the crystal structure gradu ally tends to be stable, and the rearrangement of grains and grain boundaries may lead t a gradual reduction in specific surface area.After a certain critical point, the structure o the crystal begins to readjust, resulting in a reduced specific surface area [27].Figure 4c shows the effect of the concentration of synthesized La-doped Sm2Zr2O7 on catalytic performance.It can be seen that the concentration of the catalyst has a great in fluence on the photocatalytic effect.The light degradation rate of Congo red increases with the increase in La-doped Sm2Zr2O7 concentration, and the light degradation rate of Cong red reaches its highest when the catalyst concentration is 3 g/L.As the catalyst concentra tion continues to increase, the degradation rate decreases instead.The reason for the re duction in degradation rate may be attributed to the high catalyst concentration, causin the light absorption capacity of the solution to be reduced due to light scattering.
With the increase in La-doping amount, the catalytic efficiency first increases and then decreases (Figure 4d).Moreover, the catalytic activity of La-doped Sm2Zr2O7 is th best when the doping amount of La is 5%, and the degradation rate is 93% in 40 min, whil the degradation amount of pure Sm2Zr2O7 and 1%-La-doped Sm2Zr2O7 is 30.1% and 42.6% respectively.The test results of photocatalytic performance show that La doping can sig nificantly improve the photocatalytic performance of Sm2Zr2O7.The following reason may be responsible for the enhancement of the photocatalytic activity of Sm2Zr2O7 due t As can be seen from specific surface area tests, the specific surface area of Sm 2 Zr 2 O 7 without La doping is 60.3 m 2 /g, while the specific surface area of Sm 2 Zr 2 O 7 with 1%, 3%, 5%, and 7% La-doping content is 64.6 m 2 /g, 67.39 m 2 /g, 73.5 m 2 /g, and 69.8 m 2 /g, respectively (Figure 4b).With the increase in La-doping amount, the specific surface area of the sample increases first and then decreases, reaching the maximum when the La-doping amount is 5%.It can be inferred that 5% La-doped Sm 2 Zr 2 O 7 has higher activity.The change in specific surface area is related to the continuous increase in doping amount.As the doping number increases, the crystal structure may go through changes such as inhibited growth of grains and limited movement of grain boundaries.These changes can cause the microstructure inside the crystal to become more complex, thus increasing the specific surface area.With the continuous increase in doping amount, the crystal structure gradually tends to be stable, and the rearrangement of grains and grain boundaries may lead to a gradual reduction in specific surface area.After a certain critical point, the structure of the crystal begins to readjust, resulting in a reduced specific surface area [27].
Figure 4c shows the effect of the concentration of synthesized La-doped Sm 2 Zr 2 O 7 on catalytic performance.It can be seen that the concentration of the catalyst has a great influence on the photocatalytic effect.The light degradation rate of Congo red increases with the increase in La-doped Sm 2 Zr 2 O 7 concentration, and the light degradation rate of Congo red reaches its highest when the catalyst concentration is 3 g/L.As the catalyst concentration continues to increase, the degradation rate decreases instead.The reason for the reduction in degradation rate may be attributed to the high catalyst concentration, causing the light absorption capacity of the solution to be reduced due to light scattering.
With the increase in La-doping amount, the catalytic efficiency first increases and then decreases (Figure 4d).Moreover, the catalytic activity of La-doped Sm 2 Zr 2 O 7 is the best when the doping amount of La is 5%, and the degradation rate is 93% in 40 min, while the degradation amount of pure Sm 2 Zr 2 O 7 and 1%-La-doped Sm 2 Zr 2 O 7 is 30.1% and 42.6%, respectively.The test results of photocatalytic performance show that La doping can significantly improve the photocatalytic performance of Sm 2 Zr 2 O 7 .The following reasons may be responsible for the enhancement of the photocatalytic activity of Sm 2 Zr 2 O 7 due to appropriate La doping [16].(1) It can be seen from Figure 4d that the absorption wavelength of the sample after La-doped Sm 2 Zr 2 O 7 is redshifted.The band gap of the sample is narrowed, and the response is stronger in the visible light region with a wavelength greater than 400 nm, achieving more visible light absorption; (2) doping La 3+ can act as effective electron acceptors to capture photogenerated electrons transitioning from the valence band to the conduction band and promoting the effective separation of photogenerated electrons and holes; (3) La doping makes the grain size of Sm 2 Zr 2 O 7 smaller and the specific surface area larger, which may improve the adsorption ability of the catalyst to the reaction molecules.Moreover, as La 3+ replaces Sm 3+ , the defects in the catalyst also increase, which serve as catalytic active points and increase the catalytic activity of the La-doped Sm 2 Zr 2 O 7 .However, when the doping amount of La is too large, excess La particles are deposited on the surface of Sm 2 Zr 2 O 7 , which hinders the photocatalytic reaction, accelerating the photogenerated electron and photogenerated hole recombination, thus reducing the photocatalytic activity.
The photodegradation reaction is a quasi-first-order reaction, and its kinetic reaction can be expressed as: k is the apparent reaction rate constant, C 0 is the initial concentration of Congo red, and C is the concentration of Congo red at the reaction time t.appropriate La doping [16].(1) It can be seen from Figure 4d that the absorption wavelength of the sample after La-doped Sm2Zr2O7 is redshifted.The band gap of the sample is narrowed, and the response is stronger in the visible light region with a wavelength greater than 400 nm, achieving more visible light absorption; (2) doping La 3+ can act as effective electron acceptors to capture photogenerated electrons transitioning from the valence band to the conduction band and promoting the effective separation of photogenerated electrons and holes; (3) La doping makes the grain size of Sm2Zr2O7 smaller and the specific surface area larger, which may improve the adsorption ability of the catalyst to the reaction molecules.Moreover, as La 3+ replaces Sm 3+ , the defects in the catalyst also increase, which serve as catalytic active points and increase the catalytic activity of the Ladoped Sm2Zr2O7.However, when the doping amount of La is too large, excess La particles are deposited on the surface of Sm2Zr2O7, which hinders the photocatalytic reaction, accelerating the photogenerated electron and photogenerated hole recombination, thus reducing the photocatalytic activity.The photodegradation reaction is a quasi-first-order reaction, and its kinetic reaction can be expressed as: k is the apparent reaction rate constant, C0 is the initial concentration of Congo red, and C is the concentration of Congo red at the reaction time t.Subsequently, the high activity of 5% La-doped Sm2Zr2O7/PU can also be confirmed from Figure 6a.In addition to experiments with La-doped Sm2Zr2O7/PU and irradiation, dark experiments and blank experiments were investigated in the absence of irradiation with La-doped Sm2Zr2O7/PU or in the presence of irradiation without La-doped Sm2Zr2O7/PU.Figure 6 demonstrates that almost no Congo red degradation occurs, while Figure 6 shows that only a small quantity of Congo red is degraded (less than 15%, which can be interpreted by the photolysis effect).Moreover, in order to exhibit the mineralization of organic pollution, reduction in the TOC is also presented in Figure 6b to study the complete mineralization efficiency of Congo red.For the purpose of comparison, the photocatalytic degradation of Congo red was carried out using Degussa P25 TiO2 and Ladoped Sm2Zr2O7/PU under the same conditions.These experimental results showed that   6 demonstrates that almost no Congo red degradation occurs, while Figure 6 shows that only a small quantity of Congo red is degraded (less than 15%, which can be interpreted by the photolysis effect).Moreover, in order to exhibit the mineralization of organic pollution, reduction in the TOC is also presented in Figure 6b to study the complete mineralization efficiency of Congo red.For the purpose of comparison, the photocatalytic degradation of Congo red was carried out using Degussa P25 TiO 2 and La-doped Sm 2 Zr 2 O 7 /PU under the same conditions.These experimental results showed that La-doped Sm 2 Zr 2 O 7 /PU exhibited better photocatalytic activity than Degussa P25 TiO 2 .Moreover, the photocatalytic efficiency of 5% La-doped Sm 2 Zr 2 O 7 /PU on Congo red was compared with other reported photocatalysts, as shown in Figure 6c.The results demonstrate that the as-prepared 5% La-doped Sm 2 Zr 2 O 7 /PU exhibits better catalytic performance (93%) than that of recently reported photocatalysts at nearly the same catalytic time [25,[28][29][30].La-doped Sm2Zr2O7/PU exhibited better photocatalytic activity than Degussa P25 TiO2.Moreover, the photocatalytic efficiency of 5% La-doped Sm2Zr2O7/PU on Congo red was compared with other reported photocatalysts, as shown in Figure 6c.The results demonstrate that the as-prepared 5% La-doped Sm2Zr2O7/PU exhibits better catalytic performance (93%) than that of recently reported photocatalysts at nearly the same catalytic time [25,[28][29][30].In addition to photocatalytic degradation activity, another important index to characterize photocatalysts is catalytic stability, especially for composite structure where component separation may occur.In order to evaluate the catalytic stability of the La-doped Sm2Zr2O7/PU composite emulsion, a cyclic stability test of the La-doped Sm2Zr2O7/PU composite was carried out during the photocatalytic degradation process.The analysis of experimental results is shown in Figure 7.As shown in Figure 7, the degradation rate of the La-doped Sm2Zr2O7/PU composite was almost unchanged after eight photocatalytic degradation cycles, indicating that the La-doped Sm2Zr2O7/PU composite had good cyclic stability.However, the catalytic degradation performance gradually decreased after more than eight cycles of cyclic testing.This can be attributed to the continuous adsorption of Congo red in the La-doped Sm2Zr2O7/PU composite coating failing to remove it in time, resulting in a gradual reduction in its photocatalytic degradation efficiency.In addition to photocatalytic degradation activity, another important index to characterize photocatalysts is catalytic stability, especially for composite structure where component separation may occur.In order to evaluate the catalytic stability of the La-doped Sm 2 Zr 2 O 7 /PU composite emulsion, a cyclic stability test of the La-doped Sm 2 Zr 2 O 7 /PU composite was carried out during the photocatalytic degradation process.The analysis of experimental results is shown in Figure 7.As shown in Figure 7, the degradation rate of the La-doped Sm 2 Zr 2 O 7 /PU composite was almost unchanged after eight photocatalytic degradation cycles, indicating that the La-doped Sm 2 Zr 2 O 7 /PU composite had good cyclic stability.However, the catalytic degradation performance gradually decreased after more than eight cycles of cyclic testing.This can be attributed to the continuous adsorption of Congo red in the La-doped Sm 2 Zr 2 O 7 /PU composite coating failing to remove it in time, resulting in a gradual reduction in its photocatalytic degradation efficiency.
This leather material, used in the field of automotive interior or home decoration, needs to have good air permeability because air permeability allows moisture and gas to pass through, improving ride comfort, especially in hot weather or during long drives.The air permeability of leather after coating with 5%-La-doped Sm 2 Zr 2 O 7 /PU emulsion is shown in Figure 8.Compared with leather coated with pure PU emulsion, the air permeability of leather coated with emulsion is slightly decreased, which is mainly due to the formation of film on the leather surface during the coating process of the emulsion.La-doped Sm 2 Zr 2 O 7 nanoparticles can permeate into the leather, resulting in a decrease in the permeability rate of the leather composites.
degradation cycles, indicating that the La-doped Sm2Zr2O7/PU composite had goo stability.However, the catalytic degradation performance gradually decreased aft than eight cycles of cyclic testing.This can be attributed to the continuous adsor Congo red in the La-doped Sm2Zr2O7/PU composite coating failing to remove it resulting in a gradual reduction in its photocatalytic degradation efficiency.This leather material, used in the field of automotive interior or home decoration needs to have good air permeability because air permeability allows moisture and gas to pass through, improving ride comfort, especially in hot weather or during long drives The air permeability of leather after coating with 5%-La-doped Sm2Zr2O7/PU emulsion i shown in Figure 8.Compared with leather coated with pure PU emulsion, the air perme ability of leather coated with emulsion is slightly decreased, which is mainly due to the formation of film on the leather surface during the coating process of the emulsion.La doped Sm2Zr2O7 nanoparticles can permeate into the leather, resulting in a decrease in the permeability rate of the leather composites.

Conclusions
In this study, La-doped Sm2Zr2O7 was successfully synthesized by coating the La doped Sm2Zr2O7/PU emulsion onto the leather and drying, which made the absorption edge redshift, decreased the band gap, and increased the light absorption ability and spe cific surface area.The photocatalytic rate of La-doped Sm2Zr2O7 composite for Congo red reached 93%, and the mechanical properties of the composite were also significantly im proved after coating with La-doped Sm2Zr2O7/PU emulsion.The developed La-doped Sm2Zr2O7/PU-coated leather composites have potential application prospects for the puri fication treatment of indoor volatiles.

12 Figure 1 .
Figure 1.(a) Photograph of the flexible La-doped Sm2Zr2O7/PU coated leather composites; (b) XRD patterns of Sm2Zr2O7 photocatalysts with various doping contents of La; (c) the leather before coated La-doped Sm2Zr2O7/PU; (d) the leather after coated La-doped Sm2Zr2O7/PU; (e) the surface morphology of La-doped Sm2Zr2O7/PU coated leather composites.

Figure 1 .
Figure 1.(a) Photograph of the flexible La-doped Sm2Zr2O7/PU coated le patterns of Sm2Zr2O7 photocatalysts with various doping contents of La; ( La-doped Sm2Zr2O7/PU; (d) the leather after coated La-doped Sm2Zr2O phology of La-doped Sm2Zr2O7/PU coated leather composites.

Figure 4 .
Figure 4. (a) UV-vis spectrometer of Sm2Zr2O7 with various doping contents of La; (b) specific sur face area; (c) effect of Sm2Zr2O7 concentration on catalytic performance; (d) effects of La contents i Sm2Zr2O7 on Congo red degradation efficiency.

Figure 4 .
Figure 4. (a) UV-vis spectrometer of Sm 2 Zr 2 O 7 with various doping contents of La; (b) specific surface area; (c) effect of Sm 2 Zr 2 O 7 concentration on catalytic performance; (d) effects of La contents in Sm 2 Zr 2 O 7 on Congo red degradation efficiency.

Figure 5
shows the relationship between the concentration changes of Congo red solution under different illumination times.It can be seen from the figure that the reaction conforms to the quasi-first-order reaction, and ln (C 0 /C) changes in a linear relationship with time.Materials 2024, 17, x FOR PEER REVIEW 8 of 12

Figure 5
shows the relationship between the concentration changes of Congo red solution under different illumination times.It can be seen from the figure that the reaction conforms to the quasi-first-order reaction, and ln (C0/C) changes in a linear relationship with time.

Figure 5 .
Figure 5.The relationship of concentration changes of Congo red solution under different light times.

Figure 5 .
Figure 5.The relationship of concentration changes of Congo red solution under different light times.Subsequently, the high activity of 5% La-doped Sm 2 Zr 2 O 7 /PU can also be confirmed from Figure6a.In addition to experiments with La-doped Sm 2 Zr 2 O 7 /PU and irradiation, dark experiments and blank experiments were investigated in the absence of irradiation with La-doped Sm 2 Zr 2 O 7 /PU or in the presence of irradiation without La-doped Sm 2 Zr 2 O 7 /PU.Figure6demonstrates that almost no Congo red degradation occurs, while Figure6shows that only a small quantity of Congo red is degraded (less than 15%, which can be interpreted by the photolysis effect).Moreover, in order to exhibit the mineralization of organic pollution, reduction in the TOC is also presented in Figure6bto study the complete mineralization efficiency of Congo red.For the purpose of comparison, the Figure

Figure 6 .
Figure 6.(a) The degradation rate (C/C0) of Congo red as a function of irradiation time (C0 and C represent the equilibrium concentration of Congo red before and after UV irradiation, respectively): a dark experiment (without irradiation); a blank experiment (in the absence of La-doped Sm2Zr2O7/PU); 5% La-doped Sm2Zr2O7/PU under irradiation; and Degussa P25 TiO2 under irradiation; (b) the TOC rate (TOC/TOC0) of Congo red as a function of irradiation time: a dark experiment (without irradiation); an experiment without a catalyst; Sm2Zr2O7/PU under irradiation; 5% Ladoped Sm2Zr2O7/PU under irradiation; (c) the photocatalytic efficiency of 5% La-doped Sm2Zr2O7/PU on Congo red was compared with other reported photocatalysts [25,28-30].

Figure 6 .
Figure 6.(a) The degradation rate (C/C 0 ) of Congo red as a function of irradiation time (C 0 and C represent the equilibrium concentration of Congo red before and after UV irradiation, respectively): a dark experiment (without irradiation); a blank experiment (in the absence of La-doped Sm 2 Zr 2 O 7 /PU); 5% La-doped Sm 2 Zr 2 O 7 /PU under irradiation; and Degussa P25 TiO 2 under irradiation; (b) the TOC rate (TOC/TOC 0 ) of Congo red as a function of irradiation time: a dark experiment (without irradiation); an experiment without a catalyst; Sm 2 Zr 2 O 7 /PU under irradiation; 5% La-doped Sm 2 Zr 2 O 7 /PU under irradiation; (c) the photocatalytic efficiency of 5% La-doped Sm 2 Zr 2 O 7 /PU on Congo red was compared with other reported photocatalysts [25,28-30].

Figure 8 .
Figure 8. Water vapor permeability of different leather composites.