Facile Fabrication of Highly Active CeO2@ZnO Nanoheterojunction Photocatalysts

Photocatalyst performance is often limited by the poor separation and rapid recombination of photoinduced charge carriers. A nanoheterojunction structure can facilitate the separation of charge carrier, increase their lifetime, and induce photocatalytic activity. In this study, CeO2@ZnO nanocomposites were produced by pyrolyzing Ce@Zn metal–organic frameworks prepared from cerium and zinc nitrate precursors. The effects of the Zn:Ce ratio on the microstructure, morphology, and optical properties of the nanocomposites were studied. In addition, the photocatalytic activity of the nanocomposites under light irradiation was assessed using rhodamine B as a model pollutant, and a mechanism for photodegradation was proposed. With the increase in the Zn:Ce ratio, the particle size decreased, and surface area increased. Furthermore, transmission electron microscopy and X-ray photoelectron spectroscopy analyses revealed the formation of a heterojunction interface, which enhanced photocarrier separation. The prepared photocatalysts show a higher photocatalytic activity than CeO2@ZnO nanocomposites previously reported in the literature. The proposed synthetic method is simple and may produce highly active photocatalysts for environmental remediation.


Introduction
The continued use of fossil fuels has resulted in a global energy crisis, environmental pollution, and climate change [1,2], thus, more sustainable energy resources are essential to combat these issues [3]. Photocatalytic technologies [4,5] for exploiting light energy have drawn particular attention [6][7][8]. Such catalysts can be used to produce hydrogen from water for energy storage and convert the excess carbon dioxide in the atmosphere to valuable chemical feedstocks and fuels, such as methane and methanol [9][10][11][12][13][14]. Furthermore, photocatalytic materials can be used to degrade organic pollutants in contaminated water [15][16][17]. In particular, compared with traditional methods for water treatment, photocatalysis requires less energy and can achieve a complete degradation of pollutants. Therefore, photocatalytic systems are highly promising for clean energy production and environmental remediation. To achieve high photocatalytic activity, photoinduced charge carriers (i.e., electrons and holes) must be effectively generated and separated in the photocatalysts.
Several photocatalytic mechanisms for the degradation of organic pollutants such as dyes have been reported. Briefly, upon irradiation with ultraviolet light, electrons in the valence band (VB) are excited to the conduction band (CB); thus, holes are created in the VB. The electrons in the CB react with adsorbed oxygen to form superoxide radicals (•O 2 − ), whereas the holes in the VB react with water to form hydroxyl radicals (•OH), and these two radicals react with organic pollutants and degrade them. To date, many photocatalysts have been reported, including CdS [18,19], ZnO [20], CeO 2 [21], TiO 2 [22], WO 3 [23], and graphitic carbon nitride (g-C 3 N 4 ) [24]. However, the charge carriers generated by these

Characterization
The effect of different ratios of ZnO and CeO 2 on the lattice structure of the CeO 2 @ZnO nanocomposites was determined using XRD (Dmax-rB, Rigaku; Tokyo, Japan, Cu-K α λ = 1.5418 Å) with a tube voltage and current of 40 kV and 80 mA, respectively. The changes in the morphology, microstructure, and elemental distribution of the CeO 2 @ZnO-x nanocomposites were observed using field emission SEM (ZEISS Gemini 500) and TEM (FEITecnai G2 F30). For TEM analysis, the CeO 2 @ZnO-x nanocomposites were ultrasonically dispersed in ethanol for 10 min and then dropped onto a Cu grid, and TEM observation was carried out at an acceleration voltage of 200 kV. The electronic structures and valence states of the elements were characterized using X-ray photoelectron spectroscopy (XPS). The optical band gaps of the nanocomposites were determined using UV-vis spectroscopy (UH4150, Hitachi). The spectrometer was equipped with an integrating sphere.
The photocatalytic activity of the prepared CeO 2 @ZnO nanocomposites was evaluated by measuring the degradation of RhB as a model organic pollutant using a multi-channel photochemical reaction system (PCX-50C). The light source was ultraviolet light at 365 nm with a real power density of 320 mWcm −2 . For the degradation tests, the nanocomposites (50 mg) were ultrasonically dispersed in an aqueous RhB solution (50 mL, 10 mg/L) for 10 min. The suspension was then placed in the dark for 60 min with continuous magnetic agitation until it reached dynamic adsorption-desorption equilibrium. Before irradiation, an aliquot (3 mL) of the degradation solution was extracted and centrifuged to determine the degree of degradation. Subsequently, during light irradiation, aliquots (3 mL) were collected every 5 min and analyzed. Note that no precious metal catalyst was added during the degradation process. In addition, electron paramagnetic resonance (EPR, Bruker EMXplus) spectroscopy was used to identify the free radicals produced upon light irradiation to investigate the degradation mechanism. For these measurements, the nanomaterials were added to a 5,5-dimethyl-1-pyridine-N-oxide (DMPO) solution and mixed with deionized water or CH 3 OH to detect the concentrations of hydroxyl(•OH) and superoxide (•O 2 − ) radicals, respectively. The transient photocurrent and electrochemical impedance spectroscopy (EIS) measurements were conducted on an electrochemical workstation (CHI660E) with three electrodes. A Pt wire and Ag/AgCl were used as the counter and reference electrodes, respectively. For the photocurrent measurement, indium tin oxide glass coated with the photocatalyst was used as the working electrode, whereas a glass-carbon electrode coated with the photocatalyst was the working electrode for the EIS measurement. A Na 2 SO 4 solution (0.5 M) was used as the electrolyte. Photoluminescence (PL) spectra were recorded on a spectrofluorometer (Hitachi F7000) equipped with a 250 nm excitation source. Figure 1a shows the XRD patterns of the nanomaterials formed by the pyrolysis of the MOF precursors at 450 • C. The diffraction peaks of the nanomaterial formed by pyrolyzing Ce-MOF are indexed to cubic CeO 2 (JCPDS Card No. 81-0792) [31]. When the Zn:Ce ratio increases to 0.2, the intensity of the CeO 2 diffraction peaks decreases slightly (Figure 1a, CeO 2 @ZnO-0.2). With a further increase in the Zn:Ce ratio, these CeO 2 peaks decrease in intensity, and new diffraction peaks corresponding to hexagonal wurtzite ZnO (JCPDS Card No. 36-1451) appear ( Figure 1a, CeO 2 @ZnO-0.45) [32], suggesting the formation of a CeO 2 @ZnO nanocomposite. When the Zn:Ce ratios are 0.67 and 0.8 (Figure 1a, CeO@ZnO-0.67 and CeO@ZnO-0.8, respectively), the peaks corresponding to CeO 2 disappear, and those corresponding to ZnO increase in intensity. XPS analysis revealed that some CeO 2 was present in these two samples, and the lack of diffraction peaks is likely a result of the low quantity of CeO 2 , which resulted in these peaks being X-ray invisible or obscured by those of ZnO. Finally, the pyrolysis product of Zn-MOF displays the characteristic diffraction peaks of ZnO. The phase evolution in the nanocomposites with respect to the Zn:Ce ratio is more distinct from 2θ = 29 • to 59 • (Figure 1b), wherein the peaks corresponding to CeO 2 gradually disappear, whereas those related to ZnO gradually intensify. Notably, the positions of the peaks corresponding to CeO 2 and ZnO do not change with the Zn:Ce ratio, indicating that Zn did not enter the CeO 2 lattice and vice versa. Figure 2 shows the morphologies of the CeO 2 , ZnO, and CeO 2 @ZnO nanocomposites. As shown in Figure 2a, CeO 2 has a smooth nanosphere morphology and uniform size distribution (approximate diameter~800 nm). With an increase in the Zn:Ce ratio, additional nanoparticles merge on the nanospheres (Figure 2b, CeO 2 @ZnO-0.2), and the surfaces of the nanospheres become rough. For CeO 2 @ZnO-0.45 and CeO 2 @ZnO-0.67, the large nanospheres disappear, and only small nanoparticles are observed in the nanocomposites. With a further increase in the Zn:Ce ratio, the small nanoparticles aggregate, as shown in Figure 2e,f. Thus, adding Zn converts the large spheres into small nanoparticles, but excess Zn results in the aggregation of the small nanoparticles, which should decrease the specific surface area of the nanocomposites.   The microstructures of the CeO2, ZnO, and CeO2@ZnO nanocomposites were observed using TEM. The nanospheres in Figure 3a are approximately 758 nm in diameter. In addition, the high-resolution TEM (HRTEM) images reveal that the lattice fringes at the edges of the nanospheres have interplanar distances of 0.313 and 0.269 nm, which correspond to the (111) and (200) crystal planes of cubic CeO2, respectively. Consistent with the SEM results, when the Zn:Ce ratio is 0.2, some nanoparticles are attached to the edges of the CeO2 nanospheres, while the size of the CeO2 nanospheres does not change significantly. These nanoparticles show lattice fringes corresponding to both cubic CeO2 and hexagonal wurtzite ZnO; in particular, the 0.141 nm interplanar distance corresponds to the (200) plane of ZnO. With a further increase in the Zn:Ce ratio, the number of hexagonal    The microstructures of the CeO2, ZnO, and CeO2@ZnO nanocomposites were observed using TEM. The nanospheres in Figure 3a are approximately 758 nm in diameter. In addition, the high-resolution TEM (HRTEM) images reveal that the lattice fringes at the edges of the nanospheres have interplanar distances of 0.313 and 0.269 nm, which correspond to the (111) and (200) crystal planes of cubic CeO2, respectively. Consistent with the SEM results, when the Zn:Ce ratio is 0.2, some nanoparticles are attached to the edges of the CeO2 nanospheres, while the size of the CeO2 nanospheres does not change significantly. These nanoparticles show lattice fringes corresponding to both cubic CeO2 and hexagonal wurtzite ZnO; in particular, the 0.141 nm interplanar distance corresponds to the (200) plane of ZnO. With a further increase in the Zn:Ce ratio, the number of hexagonal  The elemental distribution in CeO2@ZnO-0.67 was also characterized via EDS mapping. As shown in Figure 4, Zn, Ce, and O are distributed uniformly in the nanocomposite. However, the distribution of Ce is sparser than those of Zn and O, which is consistent with the high Zn content in CeO2@ZnO-0.67. The elemental distribution in CeO 2 @ZnO-0.67 was also characterized via EDS mapping. As shown in Figure 4, Zn, Ce, and O are distributed uniformly in the nanocomposite. However, the distribution of Ce is sparser than those of Zn and O, which is consistent with the high Zn content in CeO 2 @ZnO-0.67. XPS measurements were conducted to investigate the elemental composition and surface chemical valence states. The C 1s peak is related to the adventitious carbon introduced during pyrolysis. Therefore, the spectra were calibrated based on the C=C peak at a binding energy of 285.0 eV. As expected, the XPS survey spectra of the CeO2@ZnO nanocomposites contain peaks corresponding to Ce, Zn, and O. In the Ce 3d high-resolution XPS spectra of CeO2 (black curve in Figure 5b), the Ce 3+ and Ce 4+ peaks are detected [33][34][35], indicating the presence of CeO2 and Ce2O3. The Ce2O3 phase was not detected via XRD, possibly because of its amorphous nature. In contrast, in the XPS spectra of the CeO2@ZnO nanocomposites, the Ce 3+ peaks are weakened until they disappear, indicating that Ce is present exclusively in the CeO2 phase. The split peaks in the high-resolution Zn 2p spectra correspond to Zn 2p3/2 and 2p1/2, respectively, indicating that Zn is present as Zn 2+ [36]. Interestingly, the Zn peaks in the CeO2@ZnO nanocomposites are at lower binding energies than those in ZnO, suggesting the formation of an interface between CeO2 and ZnO. In the high-resolution O 1s spectra (Figure 5d), three peaks at 529.1, 530.2, and 531.6 eV can be assigned to the oxygen bonded to Ce, Zn, and surface hydroxyl radicals [33,37,38], respectively. Overall, the XPS and HRTEM results indicate that a heterojunction interface between ZnO and CeO2 nanoparticles is formed via pyrolysis. The interface is XPS measurements were conducted to investigate the elemental composition and surface chemical valence states. The C 1s peak is related to the adventitious carbon introduced during pyrolysis. Therefore, the spectra were calibrated based on the C=C peak at a binding energy of 285.0 eV. As expected, the XPS survey spectra of the CeO 2 @ZnO nanocomposites contain peaks corresponding to Ce, Zn, and O. In the Ce 3d high-resolution XPS spectra of CeO 2 (black curve in Figure 5b), the Ce 3+ and Ce 4+ peaks are detected [33][34][35], indicating the presence of CeO 2 and Ce 2 O 3 . The Ce 2 O 3 phase was not detected via XRD, possibly because of its amorphous nature. In contrast, in the XPS spectra of the CeO 2 @ZnO nanocomposites, the Ce 3+ peaks are weakened until they disappear, indicating that Ce is present exclusively in the CeO 2 phase. The split peaks in the high-resolution Zn 2p spectra correspond to Zn 2p 3/2 and 2p 1/2 , respectively, indicating that Zn is present as Zn 2+ [36]. Interestingly, the Zn peaks in the CeO 2 @ZnO nanocomposites are at lower binding energies than those in ZnO, suggesting the formation of an interface between CeO 2 and ZnO. In the high-resolution O 1s spectra (Figure 5d), three peaks at 529.1, 530.2, and 531.6 eV can be assigned to the oxygen bonded to Ce, Zn, and surface hydroxyl radicals [33,37,38], respectively. Overall, the XPS and HRTEM results indicate that a heterojunction interface between ZnO and CeO 2 nanoparticles is formed via pyrolysis. The interface is crucial for photocatalytic applications because photoelectrons can migrate across the interface and be effectively separated from the photogenerated holes.  The optical properties of the CeO2, ZnO, and CeO2@ZnO nanocomposites were investigated using UV-vis absorption spectroscopy, and the results are shown in Figure 6a. With an increase in the Zn:Ce ratio, the optical absorption edge shows a progressive blue shift. The optical band gaps of the CeO2, ZnO, and CeO2@ZnO nanocomposites were obtained using the Tauc formula [39]: αhν = A(hν − Eg) 2 , where α, h, ν, A, and Eg are the absorption coefficient, Planck's constant, the frequency of the incident light, a constant, and the optical band gap, respectively. The fitting curves for (αhν) 2 vs. hν are shown in Figure  6b. The linear part of the curve is extrapolated, and the x intersection is the optical band gap. Thus, the optical band gaps of the CeO2, CeO2@ZnO-0.2, CeO2@ZnO-0.45, CeO2@ZnO-0.67, CeO2@ZnO-0.8, and ZnO nanomaterials are 2.789, 3.06, 3.164, 3.195, 3.20, and 3.214 eV, respectively, showing an increasing trend with an increase in the Zn:Ce ratio. As the Zn:Ce ratio is increased, a transformation from CeO2 to CeO2@ZnO to ZnO occurs, and the size of the composite decreases from 800 nm to 10 nm. The quantum effects The optical properties of the CeO 2 , ZnO, and CeO 2 @ZnO nanocomposites were investigated using UV-vis absorption spectroscopy, and the results are shown in Figure 6a. With an increase in the Zn:Ce ratio, the optical absorption edge shows a progressive blue shift. The optical band gaps of the CeO 2 , ZnO, and CeO 2 @ZnO nanocomposites were obtained using the Tauc formula [39]: αhν = A(hν − E g ) 2 , where α, h, ν, A, and E g are the absorption coefficient, Planck's constant, the frequency of the incident light, a constant, and the optical band gap, respectively. The fitting curves for (αhν) 2 vs. hν are shown in Figure 6b. The linear part of the curve is extrapolated, and the x intersection is the optical band gap. Thus, the optical band gaps of the CeO 2 , CeO 2 @ZnO-0.2, CeO 2 @ZnO-0.45, CeO 2 @ZnO-0.67, CeO 2 @ZnO-0.8, and ZnO nanomaterials are 2.789, 3.06, 3.164, 3.195, 3.20, and 3.214 eV, respectively, showing an increasing trend with an increase in the Zn:Ce ratio. As the Zn:Ce ratio is increased, a transformation from CeO 2 to CeO 2 @ZnO to ZnO occurs, and the size of the composite decreases from 800 nm to 10 nm. The quantum effects arising from particle size limitations cause a blue-shift in the absorption edge and an increase in the band gap.

Results and Discussion
Crucially, the band gap determines the range of light that can be absorbed and, thus, used during photocatalytic degradation. Nanomaterials 2023, 13, x FOR PEER REVIEW 8 of 14 arising from particle size limitations cause a blue-shift in the absorption edge and an increase in the band gap. Crucially, the band gap determines the range of light that can be absorbed and, thus, used during photocatalytic degradation. The catalytic activity of the photocatalyst was evaluated by analyzing the degree of photodegradation of RhB, as calculated using Equation (1) [19]: where C0 and C represent the UV-vis light absorption coefficients of RhB at adsorption equilibrium in the dark and upon light irradiation, respectively. Figure 7a shows the photodegradation efficiency for RhB with respect to irradiation time. The photocatalytic efficiency of CeO2 for RhB is very low. The smooth and large nanospheres of CeO2 have a low specific surface area, leading to a small contact area between CeO2 and RhB. The photodegradation efficiency of CeO2@ZnO-0.2 slightly increases, which can be attributed to the nanoparticles attached to the CeO2 nanospheres. With a further increase in the Zn:Ce ratio, the CeO2@ZnO-0.67 nanocomposite exhibits the best photocatalytic efficiency (approximately ~97% RhB degradation after 30 min of irradiation). However, the photocatalytic efficiencies of CeO 2 @ZnO-0.8 and ZnO are lower than that of the CeO2@ZnO-0.67. The catalytic activity of the photocatalyst was evaluated by analyzing the degree of photodegradation of RhB, as calculated using Equation (1) [19]: where C 0 and C represent the UV-vis light absorption coefficients of RhB at adsorption equilibrium in the dark and upon light irradiation, respectively. Figure 7a shows the photodegradation efficiency for RhB with respect to irradiation time. The photocatalytic efficiency of CeO 2 for RhB is very low. The smooth and large nanospheres of CeO 2 have a low specific surface area, leading to a small contact area between CeO 2 and RhB. The photodegradation efficiency of CeO 2 @ZnO-0.2 slightly increases, which can be attributed to the nanoparticles attached to the CeO 2 nanospheres. With a further increase in the Zn:Ce ratio, the CeO 2 @ZnO-0.67 nanocomposite exhibits the best photocatalytic efficiency (approxi-mately~97% RhB degradation after 30 min of irradiation). However, the photocatalytic efficiencies of CeO 2 @ZnO-0.8 and ZnO are lower than that of the CeO 2 @ZnO-0.67. The experimental data were fitted using the pseudo-first-order kinetic model shown in Equation (2) [30,34].
Here, k (min −1 ) is the kinetic degradation rate constant, and t (min) is the reaction time. As shown in Figure 7b, the plots of −In(C/C 0 ) vs. t. approximately follow a linear relationship, indicating that this model can be used to analyze the photodegradation rate. The degradation rates over CeO 2 and CeO 2 @ZnO-0.2 are very low and not reported here. The kinetic degradation rate constant (k) values for the photodegradation of RhB over CeO 2 @ZnO-0.45, CeO 2 @ZnO-0.67, CeO 2 @ZnO-0.8, and ZnO were calculated as 0.0955, 0.124, 0.0749, and 0.0669, respectively. The CeO 2 @ZnO-0.67 nanocomposite exhibits the best photodegradation rate constant (0.124), which is superior to the highest photodegradation rate constant observed in our previous study on CeO 2 @ZnO photocatalysts (0.1096). Therefore, the optimal Zn/Ce atomic ratio was 0.67 at the optimal pyrolysis temperature (450 • C). We have summarized recent reports on the photodegradation performance of CeO 2 @ZnO, which are listed in Table 1. As can be seen from Table 1, CeO 2 @ZnO-0.67, the nanocomposites prepared by the binary MOF pyrolysis method in this paper, show better performance. The experimental data were fitted using the pseudo-first-order kinetic model shown in Equation (2) [30,34].
Here, k (min −1 ) is the kinetic degradation rate constant, and t (min) is the reaction time. As shown in Figure 7b, the plots of −In(C/C0) vs. t. approximately follow a linear relationship, indicating that this model can be used to analyze the photodegradation rate. The degradation rates over CeO2 and CeO 2 @ZnO-0.2 are very low and not reported here. The kinetic degradation rate constant (k) values for the photodegradation of RhB over CeO2@ZnO-0.45, CeO2@ZnO-0.67, CeO2@ZnO-0.8, and ZnO were calculated as 0.0955, 0.124, 0.0749, and 0.0669, respectively. The CeO2@ZnO-0.67 nanocomposite exhibits the best photodegradation rate constant (0.124), which is superior to the highest photodegradation rate constant observed in our previous study on CeO2@ZnO photocatalysts (0.1096). Therefore, the optimal Zn/Ce atomic ratio was 0.67 at the optimal pyrolysis temperature (450 °C). We have summarized recent reports on the photodegradation performance of CeO2@ZnO, which are listed in Table 1. As can be seen from Table 1, CeO2@ZnO-0.67, the nanocomposites prepared by the binary MOF pyrolysis method in this paper, show better performance.   The production of free radicals was investigated under dark and light conditions using EPR spectroscopy. The peak intensities in the EPR spectra reflect the concentrations of free radicals. As shown in Figure 8, no free radicals are detected in the dark, whereas two types of free radicals are formed upon light irradiation. Furthermore, the concentration of free radicals produced by the CeO 2 @ZnO-0.67 nanocomposite is the highest under light irradiation, confirming that these free radicals are responsible for enhancing the photodegradation efficiency.
The production of free radicals was investigated under dark and light conditions using EPR spectroscopy. The peak intensities in the EPR spectra reflect the concentrations of free radicals. As shown in Figure 8, no free radicals are detected in the dark, whereas two types of free radicals are formed upon light irradiation. Furthermore, the concentration of free radicals produced by the CeO2@ZnO-0.67 nanocomposite is the highest under light irradiation, confirming that these free radicals are responsible for enhancing the photodegradation efficiency. To investigate the separation ability of the photogenerated electrons and holes, the transient photocurrent characteristics of the CeO, ZnO, and CeO2@ZnO-0.67 nanocomposites were measured. As shown in Figure 9a, the CeO2@ZnO-0.67 nanocomposite produces a stronger photocurrent than CeO2 and ZnO, indicating that has the highest number of photogenerated charge carriers under illumination. In addition, the PL spectra of the CeO2, ZnO, and CeO2@ZnO-0.67 were measured. As shown in Figure 9b, the intensity of the luminescent peak for CeO2@ZnO-0.67 is significantly lower than that of CeO2 and ZnO, which indicates that the electron-hole pairs generated by CeO2@ZnO-0.67 have a low recombination rate. The efficiency of the direct electron transfer and separation of photogenerated electrons was evaluated using EIS (Figure 9c). In the EIS spectra, the arc radius determines the resistance of the interface layer, which affects the separation of electrons. A small arc radius means that electrons can be transported quickly. Among the three photocatalysts, CeO2@ZnO-0.67 exhibits the smallest arc radius, indicating its excellent charge transfer ability. Cycling experiments were performed to evaluate the stability and recyclability of CeO2@ZnO-0.67. Figure 9d shows that the high photodegradation efficiency of CeO2@ZnO-0.67 is maintained after three cycles. To investigate the separation ability of the photogenerated electrons and holes, the transient photocurrent characteristics of the CeO, ZnO, and CeO 2 @ZnO-0.67 nanocomposites were measured. As shown in Figure 9a, the CeO 2 @ZnO-0.67 nanocomposite produces a stronger photocurrent than CeO 2 and ZnO, indicating that has the highest number of photogenerated charge carriers under illumination. In addition, the PL spectra of the CeO 2 , ZnO, and CeO 2 @ZnO-0.67 were measured. As shown in Figure 9b, the intensity of the luminescent peak for CeO 2 @ZnO-0.67 is significantly lower than that of CeO 2 and ZnO, which indicates that the electron-hole pairs generated by CeO 2 @ZnO-0.67 have a low recombination rate. The efficiency of the direct electron transfer and separation of photogenerated electrons was evaluated using EIS (Figure 9c). In the EIS spectra, the arc radius determines the resistance of the interface layer, which affects the separation of electrons. A small arc radius means that electrons can be transported quickly. Among the three photocatalysts, CeO 2 @ZnO-0.67 exhibits the smallest arc radius, indicating its excellent charge transfer ability. Cycling experiments were performed to evaluate the stability and recyclability of CeO 2 @ZnO-0.67. Figure 9d shows that the high photodegradation efficiency of CeO 2 @ZnO-0.67 is maintained after three cycles.
Based on the previously described analysis, the separation mechanism of the photogenerated electron-hole pairs is shown in Figure 10. Generally, the photoexcited electrons easily recombine with the holes in the VB. In CeO 2 or ZnO pure phase materials, the recombination of excited electrons in CB and holes in VB is dominant, which considerably reduces the efficiency of photodegradation. Therefore, the inhibition of charge carrier recombination is crucial for enhancing photocatalytic efficiency. Unlike CeO 2 and ZnO, the CeO 2 @ZnO nanocomposite contains a heterojunction interface, which prevents the recombination of photogenerated charge carriers and ensures the production of free radicals for Z-scheme catalytic photodegradation. In detail, the excited electrons in ZnO preferentially recombine with the holes in CeO 2 , which enables the electrons in the CB of CeO 2 and the holes in the VB of ZnO to interact fully with oxygen and water to generate free radicals for dye decomposition. Based on the previously described analysis, the separation mechanism of the photogenerated electron-hole pairs is shown in Figure 10. Generally, the photoexcited electrons easily recombine with the holes in the VB. In CeO2 or ZnO pure phase materials, the recombination of excited electrons in CB and holes in VB is dominant, which considerably reduces the efficiency of photodegradation. Therefore, the inhibition of charge carrier recombination is crucial for enhancing photocatalytic efficiency. Unlike CeO2 and ZnO, the CeO2@ZnO nanocomposite contains a heterojunction interface, which prevents the recombination of photogenerated charge carriers and ensures the production of free radicals for Z-scheme catalytic photodegradation. In detail, the excited electrons in ZnO preferentially recombine with the holes in CeO2, which enables the electrons in the CB of CeO2 and the holes in the VB of ZnO to interact fully with oxygen and water to generate free radicals for dye decomposition.

Conclusions
In this study, CeO2@ZnO nanocomposites with various Zn:Ce ratios were prepared via the pyrolysis of Ce/Zn-MOFs precursors. As the Zn:Ce ratio increases from zero to one, pure CeO2, CeO2@ZnO nanocomposites, and pure ZnO are obtained. Pure CeO2 exists as nanospheres with diameters of approximately 800 nm. With an increase in the Zn:Ce ratio, the CeO2@ZnO nanocomposites gradually transform from nanospheres to nanoparticles of approximately 10 nm diameter, increasing the specific surface area. In addition, a heterojunction is formed, as evidenced by TEM and XPS analysis. The optical band gaps

Conclusions
In this study, CeO 2 @ZnO nanocomposites with various Zn:Ce ratios were prepared via the pyrolysis of Ce/Zn-MOFs precursors. As the Zn:Ce ratio increases from zero to one, pure CeO 2 , CeO 2 @ZnO nanocomposites, and pure ZnO are obtained. Pure CeO 2 exists as nanospheres with diameters of approximately 800 nm. With an increase in the Zn:Ce ratio, the CeO 2 @ZnO nanocomposites gradually transform from nanospheres to nanoparticles of approximately 10 nm diameter, increasing the specific surface area. In addition, a heterojunction is formed, as evidenced by TEM and XPS analysis. The optical band gaps of the nanocomposites widen with an increase in the Zn:Ce ratio owing to the heterojunction interface and quantum size effects. Among the produced photocatalysts, the CeO 2 @ZnO nanocomposite with a Zn:Ce ratio of 0.67 exhibits the best photocatalytic efficiency, which is higher than that of a CeO 2 @ZnO nanocomposite with a Zn:Ce ratio of 1. In addition, this work can be extended to the preparation of other metal oxide nanocomposites, and excellent photocatalytic performance can be obtained.