Preparation of CeO2/UiO-66-NH2 Heterojunction and Study on a Photocatalytic Degradation Mechanism

CeO2/UiO-66-NH2 (marked as Ce/UN) composites were in-situ synthesized by a hydrothermal method. The properties, photocatalytic aspects, and degradation mechanism of Ce/UN were studied carefully. SEM results show that Ce/UN have a 3D flower-like structure, where octahedral UiO-66-NH2 nanoparticles are embedded in the two-dimensional sheet of CeO2. TEM results demonstrate that CeO2 and UiO-66-NH2 are bonded interfacially to constitute a hetero-junction construction. Data obtained by electrochemical impedance spectroscopy and fluorescence spectroscopy established that Ce/UN has less charge shift resistance and luminescence intensity than these of two pure substances. When the ratio of Ce/UN is 1:1, and the calcination temperature 400 °C is used, the degradation efficiency of RhB in photocatalysis by obtained Ce/UN is about 96%, which is much higher than in the case of CeO2 (4.5%) and UiO-66-NH2 (54%). The improved photocatalytic properties of Ce/UN may be due to the formation of hetero-junction, which is conducive for most photo-carriers and thus the interfacial charge shift efficiency is enhanced. By the free radical capture test, it can be inferred that the major active substances involved in the degradation related to photocatalysis is H+ and · O2−.


Introduction
Fast industry development in the last few decades leads to increased environmental pollution, which has to be solved, whereas renewable sources and environmental problems have attracted a lot of attention by the researchers [1][2][3][4]. The research on environmentfriendly and sustainable energy composite materials will become an important research direction in the future. Rare earth oxides have the advantages of low price, stability and non-toxicity and in the light catalysis, batteries, and other fields have a wide range of applications [5,6]. Cerium oxide (CeO 2 ) has good conductivity and stability, and has high oxygen storage capacity because of the existence of reversible REDOX pairs of Ce 3+ /Ce 4+ , which enables it to have strong electronic interaction with other components at the interface [7]. Therefore, it has potential application as a carrier material in catalytic oxidation reactions. However, cerium oxide has a wide band gap and limited absorption of visible light, and its photoelectron generation and hole recombination rates are high, leading to its poor photocatalytic degradation performance [8,9]. Numerous researchers have stated that photocatalytic properties of materials can be improved by improving morphology, crystal plane regulation, substance doping, or heterojunction construction [7,10]. For example, Wu and Shen [11,12] prepared CeO 2 /Co 3 O 4 composite materials by a biological template method and solvothermal method, and improved the electron transmit rate of the materials by regulating the morphology of the composite materials. Pu and Chae [13,14] prepared Cu 2 O/CeO 2 composite materials by calcination and sol-gel electrostatic spinning. By constructing p-n junction, the absorption of CeO 2 under visible light and separation efficiency

Materials and Experimental Procedures
All chemicals and reagents used in this paper are analytical grade reagents, bought from Shanghai Maclean Biochemical Technology Co., Ltd. (Shanghai, China), and have not been more purified.

Preparation of CeO 2 Nanosheets
Furthermore, 1.39 g Ce(NO 3 ) 3 •6H 2 O and 0.75 g NH 4 HCO 3 were separately dissolved in 200 mL deionized water. Then, the NH 4 HCO 3 liquor was quickly poured into the Ce(NO 3 ) 3 •6H 2 O solution and kept at 0 • C for 30 min. The mixed solution was filtered through a funnel to collect the products and washed with deionized water 3 times. The collected precipitate was dried at 69 • C for 4 h to get the precursor of CeO 2 . Finally, the precursor of CeO 2 was calcined at 350 • C, 400 • C and 450 • C for 4 h to obtain CeO 2 nanosheets. The preparation process of CeO 2 nanosheets is according to Figure 1. mal method, and improved the photocatalytic performance of the composites by constructing appropriate band gap matching [15][16][17].
Metal-organic framework compounds (MOFs), as a new type of substance different from semiconductors, have great potential in photocatalytic reactions because of their high specific regions on the surface, flexible porous size and design of internal construction [18][19][20]. As a photocatalytic agent, it can provide an additional pathway for photoelectron migration, thereby boosting the segregation of electrons and holes (e − -h + ), in turn, improving the efficiency of composites in terms of photocatalysis. However, poor electrical conductivity, intrinsic catalytic activity, and stability affect its application in catalytic reactions. Researchers synthesized UiO-66-NH2 MOFs with BiOBr [21,22], Ag3PO4 [23], CdS [24][25][26], and other semiconductor materials to structure a composite. The results show that the heterojunction structure is formed at the interface between MOFs and semiconductor substances, which can effectively inhibit the rapid restructuring of e − -h + , thus improving the properties of semiconductor substance in terms of photocatalysis [27,28].
Considering the low efficiency in photocatalysis, poor adsorption capacity, and low utilization rate of visible light of CeO2, this paper intends to promote the separation efficiency of e − -h + by constructing heterojunction. Meanwhile, the big specific surface areas of MOFs is used to improve the adsorption capacity of pure substances, and finally realize the improvement of the photocatalytic degradation performance of the material. Therefore, Ce/UN composite photocatalytic material was in-situ synthesized by a hydrothermal method in this paper, mainly studying the influence of preparation process and composite ratio on the photocatalytic degradation performance of composite materials, and revealing the photocatalytic degradation mechanism.

Materials and Experimental Procedures
All chemicals and reagents used in this paper are analytical grade reagents, bought from Shanghai Maclean Biochemical Technology Co., Ltd. (Shanghai, China), and have not been more purified.

Preparation of CeO2 Nanosheets
Furthermore, 1.39 g Ce(NO3)3•6H2O and 0.75 g NH4HCO3 were separately dissolved in 200 mL deionized water. Then, the NH4HCO3 liquor was quickly poured into the Ce(NO3)3•6H2O solution and kept at 0 °C for 30 min. The mixed solution was filtered through a funnel to collect the products and washed with deionized water 3 times. The collected precipitate was dried at 69 °C for 4 h to get the precursor of CeO2. Finally, the precursor of CeO2 was calcined at 350 °C, 400 °C and 450 °C for 4 h to obtain CeO2 nanosheets. The preparation process of CeO2 nanosheets is according to Figure 1.

Preparation of Ce/UN Composites
Ultrasonic dispersion of 20.4 mg ZrCl4 and 14.5 mg 2-amino-1, 4-carboxylic acid in 10 mL dimethylformamide and 1.2 mL acetic acid mixed solvent, the prepared CeO2 nanosheets were dissolved in the mixed solvent, and then the mixed solution was loaded

Preparation of Ce/UN Composites
Ultrasonic dispersion of 20.4 mg ZrCl4 and 14.5 mg 2-amino-1, 4-carboxylic acid in 10 mL dimethylformamide and 1.2 mL acetic acid mixed solvent, the prepared CeO 2 Materials 2022, 15, 2564 3 of 18 nanosheets were dissolved in the mixed solvent, and then the mixed solution was loaded into the teflon lined hydrothermal autoclave. It was kept warm for 12 h at 120 • C, and then centrifuged it three times with deionized water. The lamellar Ce/UN composite photocatalytic material was prepared by drying for 12 h at 60 • C. The preparation process of Ce/UN composites is referring to Figure 2. Synthetic quality ratio of 0.5:1, 1:1, 1.5:1, and 2:1 Ce/UN substance labels were 0.5-Ce/UN, 1-Ce/UN,1.5-Ce/UN, and 2-Ce/UN. into the teflon lined hydrothermal autoclave. It was kept warm for 12 h at 120 °C, and then centrifuged it three times with deionized water. The lamellar Ce/UN composite photocatalytic material was prepared by drying for 12 h at 60 °C. The preparation process of Ce/UN composites is referring to Figure 2. Synthetic quality ratio of 0.5:1, 1:1, 1.5:1, and 2:1 Ce/UN substance labels were 0.5-Ce/UN, 1-Ce/UN,1.5-Ce/UN, and 2-Ce/UN.

Properties Characterization
The phase of the substance was characterized by a Model D/MAX-3B X-ray diffractometer (λ = 0.154 nm) manufactured by Tokyo Company, Japan. The diffraction target of the instrument was Cu-Ka (scanning rate was 4°/min), and the scanning Angle 2θ ranged from 10° to 80°. FEI Sirion200 (scanning electron microscopy, SEM, FEI Company, Hillsboro. FL, USA) was employed in looking into the microstructure and size of the material. The crystal structure and lattice spacing of photocatalytic substances were featured by transmission electron microscopy (TEM, JEM2100, JEOL, Showa, Japan). X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi, Thermo Fisher, Waltham, MA, USA) was used to determine the composition of the material. The surface area of the catalyst and the pore size distribution was measured using a Brunauer-Emmett-Teller surface area analyzer (BET, Micromeritics ASAP 2460, Micromeritics Instrument Corp, Shanghai, China). Fluorescence spectrophotometer (Shimadzu RF-5301, PL, Shimadzu Corporation of Japan, kyoto, Japan) was used to analyze the photoelectron-hole recombination rate of the samples. The excitation wavelength was 325 nm. The photocatalytic effect of the materials was measured by uV-757 CRT, and BaSO4 was used as reflection reference substance.

Photocatalytic Performance Test
The photocatalytic effect of simulated organic dye wastewater (20 mg/L rhodamine B (RhB)) was evaluated at room temperature. A 300 W xenon lamp (GX2500, Shanghai Jiguang Special Lighting Electric Appliance Factory, Shanghai, China) was used as a simulated solar light to test the light catalytic performance of the prepared samples. Take 50 mL RhB into a beaker and add 50 mg photocatalytic material. Firstly, the beaker was stirred for 1 h under no light to achieve the dynamic equilibrium of adsorption-desorption on the surface of the composite material. Then, 6 mL of the mixed solution was placed in a centrifugal tube. After 10 min of centrifugation, the supernatant was taken and placed in a quartz colorimetric dish. The initial absorbance was measured and marked as A0. Turn on the simulated light for irradiation, then take out 4 mL of the mixed solution every 30 min and repeat the above steps. Take the supernatant and place it in a quartz coloration to measure the absorbance of the dye (A1, A2, A3···).

Electrochemical Measurement
Electrochemical impedance spectroscopy (EIS) was carried out on a VMP3 workstation, as was Mott-Schottky. The workstation is equipped with three electrodes, including glassy carbon, platinum, and saturated calomel. The preparation method of the working electrode was as follows: 2 mg sample was placed in a small sample tube, 200 μL

Properties Characterization
The phase of the substance was characterized by a Model D/MAX-3B X-ray diffractometer (λ = 0.154 nm) manufactured by Tokyo Company, Japan. The diffraction target of the instrument was Cu-Ka (scanning rate was 4 • /min), and the scanning Angle 2θ ranged from 10 • to 80 • . FEI Sirion200 (scanning electron microscopy, SEM, FEI Company, Hillsboro, FL, USA) was employed in looking into the microstructure and size of the material. The crystal structure and lattice spacing of photocatalytic substances were featured by transmission electron microscopy (TEM, JEM2100, JEOL, Showa, Japan). X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi, Thermo Fisher, Waltham, MA, USA) was used to determine the composition of the material. The surface area of the catalyst and the pore size distribution was measured using a Brunauer-Emmett-Teller surface area analyzer (BET, Micromeritics ASAP 2460, Micromeritics Instrument Corp, Shanghai, China). Fluorescence spectrophotometer (Shimadzu RF-5301, PL, Shimadzu Corporation of Japan, kyoto, Japan) was used to analyze the photoelectron-hole recombination rate of the samples. The excitation wavelength was 325 nm. The photocatalytic effect of the materials was measured by uV-757 CRT, and BaSO 4 was used as reflection reference substance.

Photocatalytic Performance Test
The photocatalytic effect of simulated organic dye wastewater (20 mg/L rhodamine B (RhB)) was evaluated at room temperature. A 300 W xenon lamp (GX2500, Shanghai Jiguang Special Lighting Electric Appliance Factory, Shanghai, China) was used as a simulated solar light to test the light catalytic performance of the prepared samples. Take 50 mL RhB into a beaker and add 50 mg photocatalytic material. Firstly, the beaker was stirred for 1 h under no light to achieve the dynamic equilibrium of adsorption-desorption on the surface of the composite material. Then, 6 mL of the mixed solution was placed in a centrifugal tube. After 10 min of centrifugation, the supernatant was taken and placed in a quartz colorimetric dish. The initial absorbance was measured and marked as A0. Turn on the simulated light for irradiation, then take out 4 mL of the mixed solution every 30 min and repeat the above steps. Take the supernatant and place it in a quartz coloration to measure the absorbance of the dye (A1, A2, A3···).

Electrochemical Measurement
Electrochemical impedance spectroscopy (EIS) was carried out on a VMP3 workstation, as was Mott-Schottky. The workstation is equipped with three electrodes, including glassy carbon, platinum, and saturated calomel. The preparation method of the working electrode was as follows: 2 mg sample was placed in a small sample tube, 200 µL ethanol reagent was added, and ultrasonic dispersion for 1 h. The above sample suspension is moved with a suction gun and laid on the conductive layer of the glassy carbon electrode, and it can be dried. During the experiment, Na 2 SO 4 (0.5 M) was used as electrolyte solution. EIS was tested in the range of 0.01 kHz to1 kHz. Motshottky (MS) plots were measured at 1 kHz and the voltage was −1-1 v.  ) and (420) of cubic phase CeO 2 , respectively. This indicates that CeO 2 synthesized in this paper is a cube structure and its space group is Fm-3m (225). Moreover, it can also be displayed from Figure 3 that the X-ray diffraction peaks of the UiO-66-NH 2 made are the same as those of literature and simulation [29,30], explaining that the UiO-66-NH 2 material has been synthesized in this paper. Figure 4 states the XRD peaks of 1-Ce/UN at different temperatures. It can be observed that the characteristic peak of cubic CeO 2 is the strongest at 400 • C with the increase of temperature, and the phase crystallization is the best at this temperature. Figure Figure 3 illustrates the X-ray diffraction patterns for the temperatures and UiO-66-NH2 substance. It can be observed the CeO2 samples prepared at different temperatures are consi (JCPDS 34-0394) peak. The characteristic peaks at 2θ = 28.55°, 3 69.4°, 76.7°, and 79.07° correspond to (111), (200), (220), (311), of cubic phase CeO2, respectively. This indicates that CeO2 sy cube structure and its space group is Fm-3m (225). Moreover, i Figure 3 that the X-ray diffraction peaks of the UiO-66-NH2 m literature and simulation [29,30], explaining that the UiO-66-N thesized in this paper. Figure 4 states the XRD peaks of 1-Ce/U It can be observed that the characteristic peak of cubic CeO2 is the increase of temperature, and the phase crystallization is t

SEM Analysis
The microtopography of CeO2 calcined at different temperatures were analyzed by SEM, as shown in Figure 6. According to Figure 6a-c, at different temperatures all present a 3D flower-like structure. The len is about 5 μm, and the width is about 2 μm (as shown in Figure 6a nanosheets gradually increase in size with increasing temperature, whic contact area for the transfer of e − −h + , and can be loaded on UiO-66-NH2 6d, it can be observed that the UiO-66-NH2 is a particle with a regular oct

SEM Analysis
The microtopography of CeO2 calcined at different temperature were analyzed by SEM, as shown in Figure 6. According to Figure 6a-c at different temperatures all present a 3D flower-like structure. The le is about 5 μm, and the width is about 2 μm (as shown in Figure 6a nanosheets gradually increase in size with increasing temperature, whi contact area for the transfer of e − −h + , and can be loaded on UiO-66-NH2 6d, it can be observed that the UiO-66-NH2 is a particle with a regular oc Its average size is about 200-500 nm.

SEM Analysis
The microtopography of CeO 2 calcined at different temperatures and UiO-66-NH 2 were analyzed by SEM, as shown in Figure 6. According to Figure 6a-c, the CeO 2 calcined at different temperatures all present a 3D flower-like structure. The length of CeO 2 sheet is about 5 µm, and the width is about 2 µm (as shown in Figure 6a). In addition, the nanosheets gradually increase in size with increasing temperature, which provides a large contact area for the transfer of e − −h + , and can be loaded on UiO-66-NH 2 well. From Figure 6d, it can be observed that the UiO-66-NH 2 is a particle with a regular octahedral structure. Its average size is about 200-500 nm.  Figure 7 shows the microstructure of CeO2 calcined at different temperatures and UiO-66-NH2 composites in the 1:1 ratio. It is not difficult to find that particles of UiO-66-NH2 are covered on pure CeO2 nanosheets, and the diameter of UiO-66-NH2 is about 200-500 nm. During sample preparation, ultrasonic treatment does not separate UiO-66-NH2 from CeO2, instructing that UiO-66-NH2 is well attached to CeO2. The interface formed between ions may have good interaction. Figure 8 shows the morphologies of CeO2 (calcined at 400 °C) and UiO-66-NH2 composites in different proportions. It can be observed that the agglomeration of UiO-66-NH2 nanoparticles on CeO2 flower nanosheets decreases with the decrease of the content of UiO-66-NH2. When the ratio of CeO2 to UiO-66-NH2 is 1:1, the Ce/UN composites have the best photocatalytic properties of all substances (This will be studied in detail in the photocatalytic experimental section), so the focus of detailed study is on 400 °C 1-Ce/UN. Figure 9 presents the element mapping analysis on the 400 °C 1-Ce/UN composites. It can be seen that the elements of Ce, O (specific elements of CeO2), Zr and N (representative element of UiO-66-NH2) appear in the Ce/UN composites and are evenly distributed. Table 1 shows the SEM-EDS analysis of area "a" signed in Figure 9a. The results show that the analyzed region may contain CeO2 and UiO-66-NH2. Combined with the XRD test results (Figures 4 and 5), it can be further inferred that the Ce/UN composite materials were synthesized.   Figure 7 shows the microstructure of CeO 2 calcined at different temperatures and UiO-66-NH 2 composites in the 1:1 ratio. It is not difficult to find that particles of UiO-66-NH 2 are covered on pure CeO 2 nanosheets, and the diameter of UiO-66-NH 2 is about 200-500 nm. During sample preparation, ultrasonic treatment does not separate UiO-66-NH 2 from CeO 2 , instructing that UiO-66-NH 2 is well attached to CeO 2 . The interface formed between ions may have good interaction. Figure 8 shows the morphologies of CeO 2 (calcined at 400 • C) and UiO-66-NH 2 composites in different proportions. It can be observed that the agglomeration of UiO-66-NH 2 nanoparticles on CeO 2 flower nanosheets decreases with the decrease of the content of UiO-66-NH 2 . When the ratio of CeO 2 to UiO-66-NH 2 is 1:1, the Ce/UN composites have the best photocatalytic properties of all substances (This will be studied in detail in the photocatalytic experimental section), so the focus of detailed study is on 400 • C 1-Ce/UN. Figure 9 presents the element mapping analysis on the 400 • C 1-Ce/UN composites. It can be seen that the elements of Ce, O (specific elements of CeO 2 ), Zr and N (representative element of UiO-66-NH 2 ) appear in the Ce/UN composites and are evenly distributed. Table 1 shows the SEM-EDS analysis of area "a" signed in Figure 9a. The results show that the analyzed region may contain CeO 2 and UiO-66-NH 2 . Combined with the XRD test results (Figures 4 and 5), it can be further inferred that the Ce/UN composite materials were synthesized.  Figure 7 shows the microstructure of CeO2 calcined at different temperatures and UiO-66-NH2 composites in the 1:1 ratio. It is not difficult to find that particles of UiO-66-NH2 are covered on pure CeO2 nanosheets, and the diameter of UiO-66-NH2 is about 200-500 nm. During sample preparation, ultrasonic treatment does not separate UiO-66-NH2 from CeO2, instructing that UiO-66-NH2 is well attached to CeO2. The interface formed between ions may have good interaction. Figure 8 shows the morphologies of CeO2 (calcined at 400 °C) and UiO-66-NH2 composites in different proportions. It can be observed that the agglomeration of UiO-66-NH2 nanoparticles on CeO2 flower nanosheets decreases with the decrease of the content of UiO-66-NH2. When the ratio of CeO2 to UiO-66-NH2 is 1:1, the Ce/UN composites have the best photocatalytic properties of all substances (This will be studied in detail in the photocatalytic experimental section), so the focus of detailed study is on 400 °C 1-Ce/UN. Figure 9 presents the element mapping analysis on the 400 °C 1-Ce/UN composites. It can be seen that the elements of Ce, O (specific elements of CeO2), Zr and N (representative element of UiO-66-NH2) appear in the Ce/UN composites and are evenly distributed. Table 1 shows the SEM-EDS analysis of area "a" signed in Figure 9a. The results show that the analyzed region may contain CeO2 and UiO-66-NH2. Combined with the XRD test results (Figures 4 and 5), it can be further inferred that the Ce/UN composite materials were synthesized.

TEM Analysis
The shape and interfacial structures of CeO2, UiO-66-NH2, and 400 °C 1-Ce/UN w analyzed by TEM and HRTEM, as shown in Figure 10. According to Figure 10a, the la

TEM Analysis
The shape and interfacial structures of CeO2, UiO-66-NH2, and 400 °C 1-Ce/UN w analyzed by TEM and HRTEM, as shown in Figure 10. According to Figure 10a, the la

TEM Analysis
The shape and interfacial structures of CeO 2 , UiO-66-NH 2 , and 400 • C 1-Ce/UN were analyzed by TEM and HRTEM, as shown in Figure 10. According to Figure 10a, the lattice fringe spacing of CeO 2 is about 0.196 nm, which is well matched with the (220) crystal plane. TEM images shown in Figure 10b indicate that UiO-66-NH 2 has an octahedral construction with a maximum length of about 300 nm. Figure 10c further demonstrates that UiO-66-NH 2 is finely scattered on the flower-like structure of CeO 2 . Octahedral UiO-66-NH 2 with a surface diameter of about 300 nm is in close contact with flower-like CeO 2 , which matches the XRD results. In addition, according to the Figure 10d, the interface between CeO 2 and UiO-66-NH 2 can be clearly observed (shown as the white dotted line in Figure 10d), confirming the composition of heterostructures between them. These results suggest that UiO-66-NH 2 was successfully grown on CeO 2 flower nanosheets and interlinked, which may contribute to the segregation of photogenerated carriers.
Materials 2022, 15, x FOR PEER REVIEW NH2 with a surface diameter of about 300 nm is in close contact with flower which matches the XRD results. In addition, according to the Figure 10d, th between CeO2 and UiO-66-NH2 can be clearly observed (shown as the white in Figure 10d), confirming the composition of heterostructures between them sults suggest that UiO-66-NH2 was successfully grown on CeO2 flower nano interlinked, which may contribute to the segregation of photogenerated carrie

XPS Analysis
The chemical states and bonding environment of 400 °C 1-Ce/UN were XPS analysis. The results are shown in Figure 11. The total measurement spectr 11a) confirmed that Ce/UN heterostructure composite contained Ce, O, Zr, ments, which was consistent with the element distribution diagram of SEM-E 9). For the high-resolution spectrum of Ce 3d, the 883.4 eV, 889.6 eV, and 899.3 teristic peaks shown in Figure 11b are attributed to the binding energy of Ce dition, these three peak-to-peak values are 901.9 eV, 907.8 eV, and 917.6 eV, re which belong to the binding energy of Ce 3d1/2 [31,32]. In Figure 11c, the bindi of Ze 3d5/2 and Zr 3d3/2 are about 182.4 eV and 184.7 eV, respectively [33]. T strates the existence of Zr 4+ in zirconium oxygen clusters. In addition, the O1s p eV in Figure 11d

XPS Analysis
The chemical states and bonding environment of 400 • C 1-Ce/UN were studied by XPS analysis. The results are shown in Figure 11. The total measurement spectrum (Figure 11a) confirmed that Ce/UN heterostructure composite contained Ce, O, Zr, and N elements, which was consistent with the element distribution diagram of SEM-EDS (Figure 9). For the high-resolution spectrum of Ce 3d, the 883.4 eV, 889.6 eV, and 899.3 eV characteristic peaks shown in Figure 11b are attributed to the binding energy of Ce 3d 5/2 . In addition, these three peak-to-peak values are 901.9 eV, 907.8 eV, and 917.6 eV, respectively, which belong to the binding energy of Ce 3d 1/2 [31,32]. In Figure 11c, the binding energies of Ze 3d 5/2 and Zr 3d 3/2 are about 182.4 eV and 184.7 eV, respectively [33]. This demonstrates the existence of Zr 4+ in zirconium oxygen clusters. In addition, the O1s peak at 529.7 eV in Figure 11d belongs to the lattice oxygen in Ce-O bond and Zr-O bond [4,34]. teristic peaks shown in Figure 11b are attributed to the binding energy of Ce 3d5/2. In addition, these three peak-to-peak values are 901.9 eV, 907.8 eV, and 917.6 eV, respectively, which belong to the binding energy of Ce 3d1/2 [31,32]. In Figure 11c, the binding energies of Ze 3d5/2 and Zr 3d3/2 are about 182.4 eV and 184.7 eV, respectively [33]. This demonstrates the existence of Zr 4+ in zirconium oxygen clusters. In addition, the O1s peak at 529.7 eV in Figure 11d belongs to the lattice oxygen in Ce-O bond and Zr-O bond [4,34].  Figure 12a displays that the N2 adsorption-desorption isotherm was applied to investigate the specific surface area and pore size distribution of synthetic materials. It could be found that the specific surface area of 400 °C 1-Ce/UN was higher than that of CeO2 after introducing UiO-66-NH2, indicating that UiO-66-NH2 could significantly increase the specific surface area of the composites. As shown in Figure 12b, the values of BET surface area of UiO-66-NH2 are 1331.0 m 2 /g, and UiO-66-NH2 displayed a typical type Ⅳ isotherm with a H3-shaped hysteresis loop observed in the range 0.6-1.0 P/P0, indicating its mesoporous structure based on the IUPAC classification. In addition, the pore size distribution confirmed that the pores in UiO-66-NH2 were 3.9 nm, which mainly distributed in the range 2-50 nm, corresponding to a mesoporous structure. According to Figure 12c, the specific surface areas of CeO2 were 326.4 m 2 /g and the pore size was 4.2 nm. The pore size distribution results showed that CeO2 is mesoporous. Among the composites, the BET specific surface area of 400 °C 1-Ce/UN is 613.9 m 2 /g, and the pore size is 4.0 nm (as shown in Figure 12d). Although the specific surface area of 400 °C 1-Ce/UN is lower than that of UiO-66-NH2, it is much higher than CeO2, demonstrating that 400 °C 1-Ce/UN heterojunction can provide abundant surface active sites and facilitate photoexcited charge carriers transfer [35].  Figure 12a displays that the N 2 adsorption-desorption isotherm was applied to investigate the specific surface area and pore size distribution of synthetic materials. It could be found that the specific surface area of 400 • C 1-Ce/UN was higher than that of CeO 2 after introducing UiO-66-NH 2 , indicating that UiO-66-NH 2 could significantly increase the specific surface area of the composites. As shown in Figure 12b, the values of BET surface area of UiO-66-NH 2 are 1331.0 m 2 /g, and UiO-66-NH 2 displayed a typical type IV isotherm with a H3-shaped hysteresis loop observed in the range 0.6-1.0 P/P 0 , indicating its mesoporous structure based on the IUPAC classification. In addition, the pore size distribution confirmed that the pores in UiO-66-NH 2 were 3.9 nm, which mainly distributed in the range 2-50 nm, corresponding to a mesoporous structure. According to Figure 12c, the specific surface areas of CeO 2 were 326.4 m 2 /g and the pore size was 4.2 nm. The pore size distribution results showed that CeO 2 is mesoporous. Among the composites, the BET specific surface area of 400 • C 1-Ce/UN is 613.9 m 2 /g, and the pore size is 4.0 nm (as shown in Figure 12d). Although the specific surface area of 400 • C 1-Ce/UN is lower than that of UiO-66-NH 2 , it is much higher than CeO 2 , demonstrating that 400 • C 1-Ce/UN heterojunction can provide abundant surface active sites and facilitate photoexcited charge carriers transfer [35]. specific surface area of 400 °C 1-Ce/UN is 613.9 m 2 /g, and the pore size is 4.0 nm (as sh in Figure 12d). Although the specific surface area of 400 °C 1-Ce/UN is lower than th UiO-66-NH2, it is much higher than CeO2, demonstrating that 400 °C 1-Ce/UN hetero tion can provide abundant surface active sites and facilitate photoexcited charge car transfer [35].

Optical Properties
The characteristics of absorption in optics and band gap of CeO2, UiO-66-NH Ce/UN were studied by using UV-vis DRS, and then the visible light utilization rate composites was analyzed, as shown in Figure 13. According to Figure 13a, the lig sorption values of CeO2 are about 430 nm, respectively, which are in good agreemen the results of previous studies [36,37]. In addition, according to Figure 13a,b, com with CeO2, 400 °C 1-Ce/UN showed a longer wavelength absorption edge. In additio edge of the absorption peak is about 440 nm. Therefore, introducing UiO-66-NH CeO2 can boost its own light utilization property, and the Ce/UN heterojunction m the absorption edge of the material move to the right, resulting in a larger absor wavelength, so that it can absorb more visible light.

Optical Properties
The characteristics of absorption in optics and band gap of CeO 2 , UiO-66-NH 2 and Ce/UN were studied by using UV-vis DRS, and then the visible light utilization rate of the composites was analyzed, as shown in Figure 13. According to Figure 13a, the light absorption values of CeO 2 are about 430 nm, respectively, which are in good agreement with the results of previous studies [36,37]. In addition, according to Figure 13a,b, compared with CeO 2 , 400 • C 1-Ce/UN showed a longer wavelength absorption edge. In addition, the edge of the absorption peak is about 440 nm. Therefore, introducing UiO-66-NH 2 into CeO 2 can boost its own light utilization property, and the Ce/UN heterojunction makes the absorption edge of the material move to the right, resulting in a larger absorption wavelength, so that it can absorb more visible light.
The visible light absorption performance of Ce/UN heterojunction shows that it can be a good visible-light absorption semiconductor material. Typically, the band gap (Eg) of the photocatalyst can be obtained from Kubelke-Munk equation (αhv) = A(hv − Eg) n , where α is the absorption with respect to the optics coefficient and H is the energy with respect to photons. Since CeO 2 is an immediate transition band gap, and UiO-66-NH 2 is an indirect transition Eg, and the n value is 1/2 and 2 in this study. By plotting (hv − Eg) 2 and (hv − Eg) 1/2 , the band gap can be determined by the tangent intercept. According to Figure 13a,c, the bandgap widths of CeO 2 and UiO-66-NH 2 are about 2.86 eV and 2.65 eV apart. Flat band potential (U fb ) and semiconductor types were calculated using Mott-Schottky (MS) plots. According to Figure 13d,e, under the condition of 1 kHz, CeO 2 and UiO-66-NH 2 are fitted with positive slopes, both of which are the semiconductors type that belongs to the n type. Furthermore, the U fb of CeO 2 and UiO-66-NH 2 are −0.63 eV and −0.64 eV, respectively. In general, U fb has 0.1 eV more potential than a conduction band (CB) for most N-type semiconductors [38]. Therefore, the ratio of the bottom CB of CeO 2 and the lowest unoccupied molecular orbital (LUMO) of UiO-66-NH 2 to NHE is −0.73 eV and −0.74 eV, respectively. Combine the Eg value of VIOLET UV-vis DRS and Eg = E VB − E CB . The HOMO ratio of CeO 2 and UiO-66-NH 2 to NHE is 2.13 eV and 1.91 eV, respectively. therms and the pore size distribution of 400 °C 1-Ce/UN.

Optical Properties
The characteristics of absorption in optics and band gap of CeO2, UiO-66-NH2 a Ce/UN were studied by using UV-vis DRS, and then the visible light utilization rate of composites was analyzed, as shown in Figure 13. According to Figure 13a, the light sorption values of CeO2 are about 430 nm, respectively, which are in good agreement w the results of previous studies [36,37]. In addition, according to Figure 13a,b, compa with CeO2, 400 °C 1-Ce/UN showed a longer wavelength absorption edge. In addition, edge of the absorption peak is about 440 nm. Therefore, introducing UiO-66-NH2 i CeO2 can boost its own light utilization property, and the Ce/UN heterojunction ma the absorption edge of the material move to the right, resulting in a larger absorpt wavelength, so that it can absorb more visible light.

Properties about Electrochemistry and the PL Emission Spectra
The mechanism of photocatalytic activity was studied by EIS and the PL emission spectra emission spectroscopy, and the results are shown in Figure 14. EIS study is required to confirm and elucidate its good photocatalytic performance. From Figure 14a, it can be observed that 400 • C 1-Ce/UN has a smaller arc size than pure matter. Usually, according to EIS images, semi-arcs can reflect the recharge course because the size of it is related to the resistance. In addition, smaller arcs reflect smaller charge transfer resistance [39]. Therefore, EIS results show that the charge separation rate of 400 • C 1-Ce/UN is higher and the charge transfer rate is faster because of the lead-in of UiO-66-NH 2 . The results of EIS show that photocarrier separation of 400 • C 1-Ce/UN may be due to the formation of the heterojunction. The photoluminescence (PL) emission spectrum further demonstrated the p minescence separation of electron-hole pairs at 400 °C 1-Ce/UN. According to Figu it can be found that the luminescence peaks of CeO2 and UiO-66-NH2 appear at ab nm and 570 nm, separately. The emission peak at about 470 nm may be a combin electron hole radiation between CB and valence band (VB), and the emission peak a 570 nm may be consistent with the good state emission [40]. In general, weak fluor intensity means a high photoinduced electron-hole pair separation rate, which ca tively restain the electron-hole for restructuring, and effective photocarrier separa prolong the service life of photocatalyst, improve interfacial charge shift efficien boost the photocatalytic efficiency [41]. The luminescence intensity of 400 °C 1 composite is obviously weaker than the pure substance, indicating that the electr hole separation rate of 400 °C 1-Ce/UN can be effectively inhibited. The peak inte 400 °C 1-Ce/UN is lower than that of pure substances, which boosted the interfac tronic shift and also corresponds to the consequences of photocatalytic degra Therefore, Ce/UN has a good photocatalytic property, which is consistent with conclusion mentioned above.

Photocatalytic Degradation Performance
The Photocatalytic property of 20 mg/L RhB was studied under simulated s (room temperature) irradiation. The photocatalytic property of RhB was researc comparing the photocatalytic property of RhB with the photocatalytic activity of R degradation rates of CeO2, UiO-66-NH2, Ce/UN at different temperatures, and C different proportions are studied. The results are shown in Figure 15. The cataly The photoluminescence (PL) emission spectrum further demonstrated the photoluminescence separation of electron-hole pairs at 400 • C 1-Ce/UN. According to Figure 14b, it can be found that the luminescence peaks of CeO 2 and UiO-66-NH 2 appear at about 470 nm and 570 nm, separately. The emission peak at about 470 nm may be a combination of electron hole radiation between CB and valence band (VB), and the emission peak at about 570 nm may be consistent with the good state emission [40]. In general, weak fluorescence intensity means a high photoinduced electron-hole pair separation rate, which can effectively restain the electron-hole for restructuring, and effective photocarrier separation can prolong the service life of photocatalyst, improve interfacial charge shift efficiency, and boost the photocatalytic efficiency [41]. The luminescence intensity of 400 • C 1-Ce/UN composite is obviously weaker than the pure substance, indicating that the electron and hole separation rate of 400 • C 1-Ce/UN can be effectively inhibited. The peak intensity of 400 • C 1-Ce/UN is lower than that of pure substances, which boosted the interfacial electronic shift and also corresponds to the consequences of photocatalytic degradation. Therefore, Ce/UN has a good photocatalytic property, which is consistent with the EIS conclusion mentioned above.

Photocatalytic Degradation Performance
The Photocatalytic property of 20 mg/L RhB was studied under simulated sunlight (room temperature) irradiation. The photocatalytic property of RhB was researched by comparing the photocatalytic property of RhB with the photocatalytic activity of RhB. The degradation rates of CeO 2 , UiO-66-NH 2 , Ce/UN at different temperatures, and Ce/UN at different proportions are studied. The results are shown in Figure 15. The catalyst is 10 mg in simulated wastewater. It can be seen from Figure 15a that the removal rates of RhB by CeO 2 and UiO-66-NH 2 alone reached 0% and 60% within 4 h, respectively. With the increase of CeO 2 temperature, the photocatalytic properties of the sample increased first and then decreased. Thus, the temperature of CeO 2 with the best degradation performance was found to be 400 • C. For pure CeO 2 , the adsorption efficiency was only 1% at the dark adsorption stage of 60 min. In addition, for UiO-66-NH 2 , the rate of adsorption is only 52% after dark adsorption for 60 min. The photocatalytic performance of 400 • C 1-Ce/UN composite in a dark adsorption stage is better than that of pure substances, and the adsorption efficiency reaches 60%. Because of the three-dimensional porous structure of UiO-66-NH 2 , the adsorption property of RhB is stronger. Thus, the rate of degradation of 400 • C 1-Ce/UN can reach 96%. The high degradation efficiency indicates that the combination of two-dimensional CeO 2 flower nanosheets with UiO-66-NH 2 has good oxidation capacity. Compared with pure CeO2, the adsorption performance of Ce/UN is improv that the composite can effectively degrade pollutants. As shown in Figure 15b, the catalytic effect changes significantly after the two catalysts were combined, and w increase of the CeO2 ratio, the photocatalyst properties of the composites increas and then decreased. Thus, the photocatalytic efficiency can be improved. This phe non indicates that the structure of heterojunction is very favorable to photocatalyt radation under certain conditions. The proper combination of UiO-66-NH2 is a vi son to determine the photocatalytic property of UiO-66-NH2, and an excessive or i cient reaction rate will decrease. It may be because of the obvious agglomeration o 66-NH2 on the surface of CeO2 flower nanosheets. This may lead to accelerated Compared with pure CeO 2 , the adsorption performance of Ce/UN is improved, so that the composite can effectively degrade pollutants. As shown in Figure 15b, the photocatalytic effect changes significantly after the two catalysts were combined, and with the increase of the CeO 2 ratio, the photocatalyst properties of the composites increased first and then decreased. Thus, the photocatalytic efficiency can be improved. This phenomenon indicates that the structure of heterojunction is very favorable to photocatalytic degradation under certain conditions. The proper combination of UiO-66-NH 2 is a vital reason to determine the photocatalytic property of UiO-66-NH 2 , and an excessive or insufficient reaction rate will decrease. It may be because of the obvious agglomeration of UiO-66-NH 2 on the surface of CeO 2 flower nanosheets. This may lead to accelerated charge recombination, which in turn inhibits carrier separation [42]. Meanwhile, the negative effects of excessive UiO-66-NH 2 may be because of the shielding and blocking effects of active sites [43]. Therefore, the best photocatalytic activity of the composite when the suitable quality of UiO-66-NH 2 is added .
The aim is to further study the degradation process of RhB by a photocatalyst; the Langmuir-Hinshelwood (L-H) model -ln C t C 0 = kt was used to fit the reaction kinetics curve, in which K was the speed constant (min −1 ), C 0 is the initial concentration of RhB (mol/L), and C t is the concentration of RhB at time T (mol/L). According to Figure 15c, under the conditions of pure CeO 2 , UiO-66-NH 2 , 350 • C 1-Ce/UN, 400 • C 1-Ce/UN, and 450 • C 1-Ce/UN, the estimated rate constants K are 0.0002, 0.0023, 0.0008, 0.0126, and 0.0089, respectively. According to Figure 15d, under the conditions of 400 • C 0.5-Ce/UN, 400 • C 1-Ce/UN, 400 • C 1.5-Ce/UN and 400 • C 2-Ce/UN, the estimated rate constants k are 0.0028, 0.0126, 0.0033, and 0.0006, respectively. The results show that 400 • C 1-Ce/UN has a high rate constant, which is 20 times the pure CeO 2 and 1.6 times the UiO-66-NH 2 , respectively. When CeO 2 temperature is too high or the composite amount is too much, the photocatalytic activity of the composite material will be reduced, and the carrier in the composite material will be reduced. The low charge transfer rate and the accelerated reassociation of electron holes lead to the decrease of photocatalytic properties of the materials.

Photocatalytic Mechanism
By introducing 0.1 mM free radicals of Na 2 C 2 O 4 (holes + scavenger), isopropyl alcohol (IPA, •OH scavenger), and p-benzoquinone (BQ, · O − 2 scavenger), active material involved in the degradation related to photocatalysis was investigated, and the possible mechanism made was discussed, as shown in Figure 16. After 400 • C 1-Ce/UN, the degradation rate of RhB decreased to 96% within 4 h. After the addition of Na 2 C 2 O 4 , the photocatalytic degradation rate decreases, which can be concluded as (Equations (1)-(3)). The presence of Na 2 C 2 O 4 significantly reduces the photocatalytic RhB degradation efficiency, indicating that the consumed holes are not conducive to the separation of e − −h + . With the BQ coming in, the degradation efficiency of RhB also obviously declined. Therefore, it can be seen that H + and · O − 2 are the main species responsible for the degradation of RhB rather than •OH. In a degradation reaction, the •OH effect is relatively small: Materials 2022, 15, x FOR PEER REVIEW 14 of 17 composite material will be reduced. The low charge transfer rate and the accelerated reassociation of electron holes lead to the decrease of photocatalytic properties of the materials.

Photocatalytic Mechanism
By introducing 0.1 mM free radicals of Na2C2O4 (holes + scavenger), isopropyl alcohol (IPA, •OH scavenger), and p-benzoquinone (BQ, • O scavenger), active material involved in the degradation related to photocatalysis was investigated, and the possible mechanism made was discussed, as shown in Figure 16. After 400 °C 1-Ce/UN, the degradation rate of RhB decreased to 96% within 4 h. After the addition of Na2C2O4, the photocatalytic degradation rate decreases, which can be concluded as (Equations (1)- (3)). The presence of Na2C2O4 significantly reduces the photocatalytic RhB degradation efficiency, indicating that the consumed holes are not conducive to the separation of e − −h + . With the BQ coming in, the degradation efficiency of RhB also obviously declined. Therefore, it can be seen that H + and• O are the main species responsible for the degradation of RhB rather than •OH. In a degradation reaction, the •OH effect is relatively small: CeO UiO − 66 − NH + ⁄ hv → h + e (1) e + O →• O (2) O + h + RhB → Product According to the obtained results, the mechanism of Ce/UN composite made in photocatalysis was analyzed in depth (as shown in Figure 17). Both CeO2 and UiO-66-NH2 can generate e − −h + upon visible light. Because of the potential difference between the CB Figure 16. Effect of different scavenger on photocatalytic degradation of RhB by 400 • C 1−Ce/UN. According to the obtained results, the mechanism of Ce/UN composite made in photocatalysis was analyzed in depth (as shown in Figure 17). Both CeO 2 and UiO-66-NH 2 can generate e − −h + upon visible light. Because of the potential difference between the CB of UiO-66-NH 2 and CB of CeO 2 at −0.74 eV, the light-generated electrons are transferred from the CB of CeO 2 to CB of UiO-66-NH 2 . Both the LUMO of CeO 2 and UiO-66-NH 2 generate O − 2 because their potential is higher than O 2 /O − 2 (−0.33 eV vs. NHE) [44].
ials 2022, 15, x FOR PEER REVIEW Figure 17. Photocatalytic mechanism of a Ce/UN composite.

Conclusions
In this paper, a Ce/UN composite photocatalyst was in-situ syn thermal method. The photocatalytic properties and degradation m sites were studied in detail. The results show that the Ce/UN com shows better degradation efficiency of RhB than CeO2 and UiO-66tion of simulated sunlight. The degradation rate of RhB reached abo tocatalysis, which is much higher than CeO2 (4.5%) and UiO-66-NH2 conditions. The enhancement of the photocatalytic performance of t cause of the formation of heterojunction. It can be observed that Ce sent a 3D flower-like structure, and UiO-66-NH2 particles ar nanosheets. This heterojunction structure may provide a large contac of e − −h + . In addition, a Ce/UN composite photocatalyst has lowe sistance and luminescence intensity compared with two pure substa results of EIS and PL, which is beneficial to the separation of phot provement of the interfacial charge transfer efficiency. The free ra ments show that H + and• O are the main active species in the photoc

Conclusions
In this paper, a Ce/UN composite photocatalyst was in-situ synthesized by a hydrothermal method. The photocatalytic properties and degradation mechanism of composites were studied in detail. The results show that the Ce/UN composite photocatalyst shows better degradation efficiency of RhB than CeO 2 and UiO-66-NH 2 under the situation of simulated sunlight. The degradation rate of RhB reached about 96% after 4 h photocatalysis, which is much higher than CeO 2 (4.5%) and UiO-66-NH 2 (54%) under the same conditions. The enhancement of the photocatalytic performance of the Ce/UN may be because of the formation of heterojunction. It can be observed that Ce/UN composites present a 3D flower-like structure, and UiO-66-NH 2 particles are covered on CeO 2 nanosheets. This heterojunction structure may provide a large contact area for the transfer of e − −h + . In addition, a Ce/UN composite photocatalyst has lower charge transfer resistance and luminescence intensity compared with two pure substances according to the results of EIS and PL, which is beneficial to the separation of photocarriers and the improvement of the interfacial charge transfer efficiency. The free radical capture experiments show that H + and · O − 2 are the main active species in the photocatalytic process.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.