Bifunctional Ag-Decorated CeO2 Nanorods Catalysts for Promoted Photodegradation of Methyl Orange and Photocatalytic Hydrogen Evolution

The photodegradation of organic pollutants and photocatalytic hydrogen generation from water by semiconductor catalysts are regarded as the of the most promising strategies to resolve the crisis of global environmental issues. Herein, we successfully designed and prepared a series of silver-decorated CeO2(Ag/CeO2) photocatalysts with different morphologies by a facile hydrothermal route. The physical properties, charge transfer behavior and photocatalytic performances (degradation and hydrogen evolution) over diverse catalysts with nanocubes, nanoparticles and nanorods shapes were comprehensively studied. It was found that the Ag-decorated CeO2 nanorods (Ag/R-CeO2) demonstrate the best activity for both photocatalytic methyl orange (MO) degradation and photocatalytic H2 production reaction with attractive stability during cycling tests, suggesting its desirable practical potential. The superior performance of Ag/R-CeO2 can be ascribed to (1) the facilitated light absorption due to enriched surface oxygen vacancies (OVs) and plasmonic Ag nanoparticles on nanorods, (2) the facilitated photo-excited charge carrier (e−-h+) separation efficiency on a metal/oxide hybrid structure and (3) the promoted formation of active reaction intermediates on surface-enriched Ag and oxygen vacancies reactive sites on Ag/CeO2 nanorods. This study provides a valuable discovery of the utilization of abundant solar energy for diverse catalytic processes.


Introduction
The rapid development of industry leads to the large consumption of fossil fuels, which has brought inevitable environmental issues. Specifically, the massive emission of CO 2 and water pollution of organic compounds severely threaten ecosystems and human health [1][2][3]. Therefore, the purification of dye-polluted wastewater and the exploration of clean energy carriers such as H 2 are two huge challenges for humans to pursue sustainable development [4,5].
As the most representative dye, methyl orange (MO) is not decomposable at ambient conditions [6]. Until now, enormous efforts have been undertaken to develop diverse technologies for the efficient degradation of MO [7]. The pioneering work of the photoreduction of organic pollutant using TiO 2 photocatalyst can date back to 1979 [8], from which photocatalytic degradation has received extensive studies [9,10]. For decades, the photodegradation of MO using semiconductor photocatalysts has been paid immense attention due to the abundance of solar energy, low cost and easy operability of the reaction system [11]. Additionally, diverse kinds of photocatalysts have been reported to be active for photocatalytic degradation. For example, transition metal oxides including Table 1. Samples.

Sample Characterization
The X-ray diffraction (XRD) patterns were recorded using an X-ray powder diffractometer (Panalytical X'pert Pro Super, Malvern, UK) along with radiation of Cu Kα. For each sample, 20 • to 80 • of 2θ (Bragg's angles) were conducted under a 10 • /min rate. Transmission electron microscopy (TEM) images were received via a TECNAI F-20 apparatus. SEM (Scanning electron microscopy) images were observed using ZEISS-SIGMA. A UV-vis Cary5000 spectrometer (VARIAN Company, Palo Alto, CA, USA) using BaSO 4 reference was used to obtain UV-visible diffuse reflection spectra (UV-vis DRS). An instrument of F-7000 spectrophotometer was conducted to gain photoluminescence (PL) spectra and the excitation wavelength was 325 nm. Photocurrents (PCs) of the resultant catalysts were observed with a CHI660E electrochemical analyzer, attaching a home-made standard threeelectrode quartz cell. Sample suspensions dropped directly and coated onto the precleaned ITO (indium tin oxide) glass surface were used as work electrodes. An SCE (saturated calomel electrode) electrode acted as the reference and the Pt wire served as a counter electrode. Na 2 SO 4 aqueous solution (0.2 M) without other additives served as the electrolyte. The irradiation light was an Hg lamp (500 W, the accumulated intensity: 110 mW/cm 2 ). The work electrode (0.95 cm 2 ) was exposed to the Na 2 SO 4 electrolyte. A bias potential of 0.1 V (vs. Ag/AgCl) was applied. UV-Vis Raman System Invia manufactured by Renisha using a laser Raman spectrometer was performed to observe the Raman test. The excitation source was a 532 nm laser, and the scanning wave number range is specified in this paper.

Catalytic Activity Assessment
The photocatalytic degradation performance of pristine CeO 2 and Ag/CeO 2 composites were evaluated by degrading MO under a 500 W Hg lamp (NBT Science Co. Ltd. China, λ = 200-1100 nm, the accumulated intensity: 110 mW/cm 2 ) illumination, positioned at a distance of 5 cm over the photocatalytic reactor. Experiments were conducted at ambient temperature. Typically, 50 mg of the resultant photocatalyst was decentralized in a 100 mL MO pollutant solution (5 mg/L, pH = 6.5-7.0). In order to achieve the adsorption and desorption equilibrium of the photocatalyst, MO and water, the above mixture was kept at magnetic stirring (the speed of magnetic stirrer is 300 r/min) for 20 min in the chamber under dark ahead of light irradiation. With magnetic stirring, the suspension was exposed to light irradiation. The collection of 4 mL of solution from the reaction suspension every 20 min was centrifuged to elimination the photocatalyst. Then, the concentrations of MO dye in every sample were analyzed via a UV-vis spectrophotometer at 464 nm (MO characteristic absorbance).
Photocatalytic H 2 evolution reaction tests of the samples were conducted using a reaction cell reactor with the volume of 500 mL. An amount of 0.10 g powder catalyst was decentralized in a mixture solution containing 180 mL of H 2 O, H 2 PtCl 6 solution and 20 mL of methanol (as a sacrificial reagent). H 2 PtCl 6 solution was photo-reduced to obtain Pt (0.5 wt%) cocatalyst modified catalyst in the water splitting experiments. Then, the reactor was degassed via a pump. To establish the balance between adsorption and desorption, the mixture was kept for 20 min (300 r/min) under magnetic stirring prior to irradiation. The light source was a 300 W Xe lamp (Beijing China Education Au-Light Co., Ltd., Beijing, China, the accumulated intensity: 450 mW/cm 2 ), positioned at a distance of 5 cm over the photocatalytic reactor. For the sake of maintaining the experiment temperature, a water cooling system was used around the photoreactor. A thermal conductivity detector from GC 2060 gas chromatograph served to monitor the amount of H 2 . Additionally, the carrier gas was high-purity N 2 .

Results and Discussion
First, XRD characterization was employed to explore the crystal structure of various catalysts, and the results are shown in Figure 1. The diffraction peaks at 28 [32,33]. Interestingly, the sharper diffraction peak could be seen on CeO 2 cubes, which could be ascribed to its larger crystal size [25]. In comparison, much weakened and broader diffraction peak are observed on R-CeO 2 , suggesting the low crystallinity and partially disordered modality of CeO 2 with a nanorod shape. After introducing Ag, the diffraction peak of Ag (111) at 38.6° (the pu area in Figure 1) emerged on Ag/R-CeO2 and Ag/C-CeO2, evidencing the su ration of Ag. From the perspective of apparent optical property, the Ce showed a gray-white color, while the nanoparticles and nanorods sample and bright yellow, respectively. Nevertheless, after introducing Ag, those s significantly changed, indicating the alternation of optical property. Spec CeO2 became light purple and Ag/R-CeO2 turned to dark purple (inset of F SEM was then utilized to characterize the detailed morphology of as-p and Ag/CeO2 nanocomposites. As can be seen in Figure 2a, the C-CeO2 disp cubic shape with a side length of approximately 20-100 nm. Meanwhile, t the round-shape CeO2 particles was ~50 nm and the length of the CeO2 between 20-100 nm. The SEM images in Figure 2d-f shows that the CeO2 shapes maintained their original morphology after the introduction of Ag Moreover, the detailed morphology and lattice structure of Ag/R-CeO2 were by TEM, TEM-EDX and high-resolution TEM (HR-TEM). The Ag/R-CeO2 shape obtained the uniform diameter of around 20 nm (Figure 3a). The HR ( Figure 3b) demonstrated a lattice spacing of 0.19 nm, which is assignab plane of fluorite structure CeO2 [34], while the Ag nanoparticles with a sm around 3 nm could also be detected. The interplanar distance of 0.24 nm is c the Ag (111) plane [35]. Moreover, the interface between CeO2 and Ag coul clearly. The TEM-EDX images shown in Figure 3c demonstrate the even dis elements, including O, Ce and Ag, which are in line with the above XRD an Ag nanoparticle is successfully decorated onto the CeO2 nanorod. After introducing Ag, the diffraction peak of Ag (111) at 38.6 • (the purple rectangle area in Figure 1) emerged on Ag/R-CeO 2 and Ag/C-CeO 2 , evidencing the successful decoration of Ag. From the perspective of apparent optical property, the CeO 2 nanocubes showed a gray-white color, while the nanoparticles and nanorods samples were yellow and bright yellow, respectively. Nevertheless, after introducing Ag, those samples' colors significantly changed, indicating the alternation of optical property. Specifically, Ag/C-CeO 2 became light purple and Ag/R-CeO 2 turned to dark purple (inset of Figure 1a).
SEM was then utilized to characterize the detailed morphology of as-prepared CeO 2 and Ag/CeO 2 nanocomposites. As can be seen in Figure 2a, the C-CeO 2 displayed a quasicubic shape with a side length of approximately 20-100 nm. Meanwhile, the diameter of the round-shape CeO 2 particles was~50 nm and the length of the CeO 2 nanorods was between 20-100 nm. The SEM images in Figure 2d-f shows that the CeO 2 with different shapes maintained their original morphology after the introduction of Ag nanoparticles. Moreover, the detailed morphology and lattice structure of Ag/R-CeO 2 were characterized by TEM, TEM-EDX and high-resolution TEM (HR-TEM). The Ag/R-CeO 2 with the rodshape obtained the uniform diameter of around 20 nm (Figure 3a). The HR-TEM images ( Figure 3b) demonstrated a lattice spacing of 0.19 nm, which is assignable to the (110) plane of fluorite structure CeO 2 [34], while the Ag nanoparticles with a small diameter of around 3 nm could also be detected. The interplanar distance of 0.24 nm is consistent with the Ag (111) plane [35]. Moreover, the interface between CeO 2 and Ag could be observed clearly. The TEM-EDX images shown in Figure 3c demonstrate the even distribution of all elements, including O, Ce and Ag, which are in line with the above XRD analysis that the Ag nanoparticle is successfully decorated onto the CeO 2 nanorod.  The UV-Vis DRS spectra of C-CeO2, Ag/C-CeO2, P-CeO2, Ag/P-CeO2, R-CeO2 and Ag/R-CeO2 samples are shown in Figure 4a, which confirms the optical absorption property of the samples with different morphologies. The absorption spectrum of R-CeO2 significantly shifts to a higher wavelength, as compared to those of P-CeO2 and C-CeO2, suggesting that the R-CeO2 exhibits a much stronger absorbance in the low frequency region. The Tauc equation was applied to determine the bandgap energy (Eg) of different CeO2 [36]: where α, n, A and Eg correspond to the absorption coefficient, the light frequency, a constant and optical band gap, respectively. The n of CeO2 is equal to 2 [37]. The Eg of the CeO2 samples can be computed from the (αhv) n versus (hv) plot depiction (shown in Figure  4c). The bandgap values are reported in Table 2. The R-CeO2 owns the smallest bandgap energy of 3.04 eV, which is much smaller than that of C-CeO2 and P-CeO2. Before, it was reported that the Eg of CeO2 could be manipulated by changing the morphologies and sizes of the crystal [38], and the light absorption ability is closely related to the optical bandgap of the material. Hence, it is reasonable to speculate that R-CeO2, with the smallest   The UV-Vis DRS spectra of C-CeO2, Ag/C-CeO2, P-CeO2, Ag/P-CeO2, R-CeO2 and Ag/R-CeO2 samples are shown in Figure 4a, which confirms the optical absorption property of the samples with different morphologies. The absorption spectrum of R-CeO2 significantly shifts to a higher wavelength, as compared to those of P-CeO2 and C-CeO2, suggesting that the R-CeO2 exhibits a much stronger absorbance in the low frequency region. The Tauc equation was applied to determine the bandgap energy (Eg) of different CeO2 [36]: where α, n, A and Eg correspond to the absorption coefficient, the light frequency, a constant and optical band gap, respectively. The n of CeO2 is equal to 2 [37]. The Eg of the CeO2 samples can be computed from the (αhv) n versus (hv) plot depiction (shown in Figure  4c). The bandgap values are reported in Table 2. The R-CeO2 owns the smallest bandgap The UV-Vis DRS spectra of C-CeO 2 , Ag/C-CeO 2 , P-CeO 2 , Ag/P-CeO 2 , R-CeO 2 and Ag/R-CeO 2 samples are shown in Figure 4a, which confirms the optical absorption property of the samples with different morphologies. The absorption spectrum of R-CeO 2 significantly shifts to a higher wavelength, as compared to those of P-CeO 2 and C-CeO 2 , suggesting that the R-CeO 2 exhibits a much stronger absorbance in the low frequency region. The Tauc equation was applied to determine the bandgap energy (Eg) of different CeO 2 [36]: where α, n, A and Eg correspond to the absorption coefficient, the light frequency, a constant and optical band gap, respectively. The n of CeO 2 is equal to 2 [37]. The Eg of the CeO 2 samples can be computed from the (αhv) n versus (hv) plot depiction (shown in Figure 4c).
The bandgap values are reported in Table 2. The R-CeO 2 owns the smallest bandgap energy of 3.04 eV, which is much smaller than that of C-CeO 2 and P-CeO 2 . Before, it was reported that the Eg of CeO 2 could be manipulated by changing the morphologies and sizes of the crystal [38], and the light absorption ability is closely related to the optical bandgap of the material. Hence, it is reasonable to speculate that R-CeO 2 , with the smallest bandgap, should obtain the highest light absorption capability.  After the decoration of Ag, the greatly boosted visible light (vis-light) absorption (~600 nm) could be observed on Ag-R-CeO2 samples. The phenomenon could be ex plained by the surface plasmon resonance (SPR) effect of Ag, which was also found on other reported Ag modified catalysts [39,40]. On the surface decorated with SPR excitation of Ag metal, light can be captured and confined nearby Ag, and the Ag nanoparticles resonance can play the role in improving the absorption of light for semiconductor [35,39,41], and further enhancing the photocatalytic activity.
Raman spectroscopy with a 532 nm excitation laser was further employed to disclos the vibration information of metal-oxygen bond, as demonstrated in Figure 4b. The Ra man spectra of all CeO2 possess the characteristic peaks at 462 cm −1 which can be imputed to the F2g symmetric stretching vibrations pattern of fluorite CeO2 structure [41]. The peak at 530-600 cm −1 could be assigned to the band of defect-induced (D), which is directly linked to the presence of defects or OVs in CeO2 [42]. The relative intensity ratio of ID/IF2 can be applied to determine the relative concentration of OVs in different samples. Ap parently, this ratio for R-CeO2 (ID/IF2g = 0.596) is much higher than those of C-CeO2 (ID/IF2 = 0.056) and P-CeO2 (ID/IF2g = 0.295) samples, suggesting that R-CeO2 possesses the larges amount of OVs (Figure 4d). After the loading of Ag, the relative intensity ratio of ID/IF2 Ag/R-CeO2 sample drastically increases to 1.014, which is around two times larger than  After the decoration of Ag, the greatly boosted visible light (vis-light) absorption (~600 nm) could be observed on Ag-R-CeO 2 samples. The phenomenon could be explained by the surface plasmon resonance (SPR) effect of Ag, which was also found on other reported Ag modified catalysts [39,40]. On the surface decorated with SPR excitation of Ag metal, light can be captured and confined nearby Ag, and the Ag nanoparticles' resonance can play the role in improving the absorption of light for semiconductors [35,39,41], and further enhancing the photocatalytic activity.
Raman spectroscopy with a 532 nm excitation laser was further employed to disclose the vibration information of metal-oxygen bond, as demonstrated in Figure 4b. The Raman spectra of all CeO 2 possess the characteristic peaks at 462 cm −1 which can be imputed to the F 2g symmetric stretching vibrations pattern of fluorite CeO 2 structure [41]. The peak at 530-600 cm −1 could be assigned to the band of defect-induced (D), which is directly linked to the presence of defects or OVs in CeO 2 [42]. The relative intensity ratio of I D /I F2g can be applied to determine the relative concentration of OVs in different samples. Apparently, this ratio for R-CeO 2 (I D /I F2g = 0.596) is much higher than those of C-CeO 2 (I D /I F2g = 0.056) and P-CeO 2 (I D /I F2g = 0.295) samples, suggesting that R-CeO 2 possesses the largest amount of OVs (Figure 4d). After the loading of Ag, the relative intensity ratio of I D /I F2g Ag/R-CeO 2 sample drastically increases to 1.014, which is around two times larger than that of pure R-CeO 2 , indicating the surging content of OVs. This is partially due to the additive of the highly reducible agent of NaBH 4 during fabrication. Additionally, this phenomenon was in line with the results reported in the previous literature [34,43,44] that the loaded noble metal (such as Ru, Ag, Pt) could facilitate the generation of OVs [34,35,45]. The surface decoration of Ag nanoparticles is able to activate the surface lattice oxygen to create more OVs [34].
The photocatalytic property of the CeO 2 and Ag/CeO 2 with various morphologies were utilized for the photocatalytic MO dye degradation, and the results are shown in Figure 5a. The blank trial of the decolorization of MO dye verifies that only 15% and 12% of MO can be naturally degraded without catalysts (irradiation only) or without light irradiation (with Ag/R-CeO 2 only), respectively. This result could be explained by the self-photosensitized reaction of MO dye and absorption process of Ag/R-CeO 2 [31,46,47]. When both light irradiation and catalysts were applied, the degradation efficiency of MO was remarkably promoted, implying the synergistic effect between catalyst and light excitation. All the CeO 2 nanocomposites exhibit moderate photocatalytic activity, and the R-CeO 2 decreased the concentration of MO by~60% after 160 min treatment. This performance was much better than that of CeO 2 nanoparticles (~20%) and CeO 2 nanocubes (~50%). After the introduction of Ag, the degradation rate of MO dye was further improved on all samples. More than 80% of MO was decomposed for the sample Ag/R-CeO 2 after light irradiation for 160 min, whereas about 41% of MO can be degraded over Ag/C-CeO 2 and 59% of MO be degraded over Ag/P-CeO 2 . The kinetic linear modeling results of CeO 2 and Ag/CeO 2 are shown in Figure 5b; the degradation process followed a first order kinetic [48], and the Ag/R-CeO 2 displays the largest kinetic constant of 0.00969 min −1 , suggesting its highest conversion rate. To further confirm the degradation performance on Ag/R-CeO 2 , the time-dependent UV-vis absorption spectra of MO concentration on Ag/R-CeO 2 are shown in Figure 5c. The wavelength of about 420 nm is confirmed for the absorbance of MO. Obviously, the intensity of MO absorbance declined rapidly as the treatment time increased, indicating the consumption of MO. Additionally, based on the result of UV-vis absorbance spectra, no other absorbance band could be detected, revealing that the conjugated structure of MO is destroyed molecules and no other intermediate product is formed [46,49,50].
The stability of the photocatalytic property for the MO degradation over Ag/R-CeO 2 was further investigated via cycling experiments. Five continuous tests were conducted without renewing the photocatalyst. The procedure of each independent measurement was identical except for refilling the MO solution for each run. After each measurement, the Ag/R-CeO 2 catalyst was re-collected by centrifugation, rinsed and dried for next cycle. It can be seen from Figure 6 that only a slight drop of degradation performance of MO is observed after five cycles, suggesting the desirable stability of the catalyst. This may be due to the loss of the catalyst during each round of catalyst collection, which is universal in the previous literature [51].
The measurement of the photocatalytic activity of hydrogen evolution on different CeO 2 and Ag/CeO 2 catalysts were also performed, and the results are given in Figure 7. R-CeO 2 shows the highest performance under the simulated solar irradiation. The average hydrogen evolution rates are 157, 109 and 56 µmol h −1 g −1 for R-CeO 2 , P-CeO 2 and C-CeO 2 , respectively (shown in Figure 7b). This order is consistent with that of the OV content on different samples. In addition, after the introduction of Ag, the hydrogen evolution rate was significantly improved, and the rate of H 2 production for Ag/R-CeO 2 rose from 157 to 316 µmol h −1 g −1 for Ag/R-CeO 2 . These results indicate that besides OVs, the Ag also exhibits another prominent influence on such an enhancement of performance.
CeO2 are shown in Figure 5c. The wavelength of about 420 nm is confirmed for the sorbance of MO. Obviously, the intensity of MO absorbance declined rapidly as the t ment time increased, indicating the consumption of MO. Additionally, based on the r of UV-vis absorbance spectra, no other absorbance band could be detected, revealing the conjugated structure of MO is destroyed molecules and no other intermediate pro is formed [46,49,50].  Nanomaterials 2021, 11, x FOR PEER REVIEW 9 The stability of the photocatalytic property for the MO degradation over Ag/R-C was further investigated via cycling experiments. Five continuous tests were condu without renewing the photocatalyst. The procedure of each independent measure was identical except for refilling the MO solution for each run. After each measurem the Ag/R-CeO2 catalyst was re-collected by centrifugation, rinsed and dried for next c It can be seen from Figure 6 that only a slight drop of degradation performance of M observed after five cycles, suggesting the desirable stability of the catalyst. This ma due to the loss of the catalyst during each round of catalyst collection, which is univ in the previous literature [51]. The measurement of the photocatalytic activity of hydrogen evolution on diff CeO2 and Ag/CeO2 catalysts were also performed, and the results are given in Figu R-CeO2 shows the highest performance under the simulated solar irradiation. The ave hydrogen evolution rates are 157, 109 and 56 μmol h −1 g −1 for R-CeO2, P-CeO2 and C-C respectively (shown in Figure 7b). This order is consistent with that of the OV conten different samples. In addition, after the introduction of Ag, the hydrogen evolution was significantly improved, and the rate of H2 production for Ag/R-CeO2 rose from 1 316 μmol h −1 g −1 for Ag/R-CeO2. These results indicate that besides OVs, the Ag als hibits another prominent influence on such an enhancement of performance. Usually, photo-excited charge separation efficiency is the key effect in determining the photocatalytic degradation performance and hydrogen evolution of a semiconductor photocatalyst. Therefore, to figure out the mechanism of promoted activity on Ag/R-CeO 2 , the separation efficiency of photo-induced charge carriers was performed by the PL analysis at room temperature with an excitation wavelength of 325 nm. The PL spectra for all as-prepared samples are displayed in Figure 8a. Obviously, a shoulder emission peak of around 400 nm can be detected with the excitation wavelength at 325 nm, which stems from the intrinsic luminescence of CeO 2 . The emission intensity of Ag/CeO 2 is much inferior to that of CeO 2 , suggesting that the recombination of carriers is suppressed after the loading of Ag. Our experimental observations are in agreement with a previous report that the deposition of Ag metal nanoparticles can accelerate the separation and transportation of photogenerated carriers, further resulting in the quenching effect of PL intensity [19,52]. Additionally, compared to Ag/P-CeO 2 and Ag/C-CeO 2 , the sample Ag/R-CeO 2 has lowest emission intensity, meaning that the OVs can also help prevent the recombination of photo-excited charge carrier. The transient photocurrent (PC) of the samples was also exploited to investigate the excitation and transfer of photo-induced e − and h + . As shown in Figure 8b, upon irradiation, a quick photocurrent response was clearly given on all the samples and the transient PC density of Ag/CeO 2 is much higher than the other two CeO 2 during the continuous several light on-off cycles. In addition, Ag/R-CeO 2 possesses the highest PC density in all Ag/CeO 2 samples, meaning the larger amount of photoinduced electrons on the surface could be rapidly transferred. The measurement of the photocatalytic activity of hydrogen evolution on different CeO2 and Ag/CeO2 catalysts were also performed, and the results are given in Figure 7. R-CeO2 shows the highest performance under the simulated solar irradiation. The average hydrogen evolution rates are 157, 109 and 56 μmol h −1 g −1 for R-CeO2, P-CeO2 and C-CeO2, respectively (shown in Figure 7b). This order is consistent with that of the OV content on different samples. In addition, after the introduction of Ag, the hydrogen evolution rate was significantly improved, and the rate of H2 production for Ag/R-CeO2 rose from 157 to 316 μmol h −1 g −1 for Ag/R-CeO2. These results indicate that besides OVs, the Ag also exhibits another prominent influence on such an enhancement of performance. Usually, photo-excited charge separation efficiency is the key effect in determining the photocatalytic degradation performance and hydrogen evolution of a semiconductor photocatalyst. Therefore, to figure out the mechanism of promoted activity on Ag/R-CeO2, the separation efficiency of photo-induced charge carriers was performed by the PL analysis at room temperature with an excitation wavelength of 325 nm. The PL spectra for all as-prepared samples are displayed in Figure 8a. Obviously, a shoulder emission peak of around 400 nm can be detected with the excitation wavelength at 325 nm, which stems from the intrinsic luminescence of CeO2. The emission intensity of Ag/CeO2 is much inferior to that of CeO2, suggesting that the recombination of carriers is suppressed after the loading of Ag. Our experimental observations are in agreement with a previous report that the deposition of Ag metal nanoparticles can accelerate the separation and transportation of photogenerated carriers, further resulting in the quenching effect of PL intensity [19,52]. Additionally, compared to Ag/P-CeO2 and Ag/C-CeO2, the sample Ag/R-CeO2 has lowest emission intensity, meaning that the OVs can also help prevent the recombination of photo-excited charge carrier. The transient photocurrent (PC) of the samples was also exploited to investigate the excitation and transfer of photo-induced eand h + . As shown in Figure 8b, upon irradiation, a quick photocurrent response was clearly given on all the samples and the transient PC density of Ag/CeO2 is much higher than the other two CeO2 during the continuous several light on-off cycles. In addition, Ag/R-CeO2 possesses the highest PC density in all Ag/CeO2 samples, meaning the larger amount of photoinduced electrons on the surface could be rapidly transferred. Based on the analysis above, the enhanced photocatalytic degradation/hydrogen evolution performance of the Ag/R-CeO2 nanorods' photocatalyst can be understood and explained by the possible mechanism proposed in Figure 9. Upon the photocatalyst being irradiated by the Hg lamp, CeO2 was activated to produce the eand h + . First of all, the Ag/R-CeO2 with rich OVs and plasmonic Ag nanoparticles narrowed the optical bandgap and benefits the light absorption. Secondly, both OVs and Ag facilitated the generation of a larger amount of photo-excited charge carrier and suppressed their recombination simultaneously. Then, these alive e − and h + with prolonged lifetime combined with waterdissolved O2 and OHto create highly oxidative ·O2 − and ·OH to accelerate the decompo- Based on the analysis above, the enhanced photocatalytic degradation/hydrogen evolution performance of the Ag/R-CeO 2 nanorods' photocatalyst can be understood and explained by the possible mechanism proposed in Figure 9. Upon the photocatalyst being irradiated by the Hg lamp, CeO 2 was activated to produce the e − and h + . First of all, the Ag/R-CeO 2 with rich OVs and plasmonic Ag nanoparticles narrowed the optical bandgap and benefits the light absorption. Secondly, both OVs and Ag facilitated the generation of a larger amount of photo-excited charge carrier and suppressed their recombination simultaneously. Then, these alive e − and h + with prolonged lifetime combined with water-dissolved O 2 and OH − to create highly oxidative ·O 2 − and ·OH to accelerate the decomposition of MO. In addition, the surface OVs may play another important role in the absorption o dissolved oxygen molecule, which can furnish more effective surface reaction sites for photocatalytic degradation, while for the photocatalytic water splitting process, the accu mulated photoelectrons quickly transferred to surface Ag and Pt nanoparticles to reac with protons for hydrogen production. Therefore, the photocatalytic hydrogen evolution performance can be significantly enhanced.

Conclusions
In this work, we successfully synthesized a series of Ag/CeO2 nanocubes, nanoparti cles and nanorods' photocatalysts via a facile one-step hydrothermal process. The photo catalytic degradation of MO and photocatalytic hydrogen evolution reaction of the cata lysts demonstrate strong morphology dependence and the Ag/CeO2 nanorods possess the highest photocatalytic activity for both reactions due to the following reasons: (1) the cat alyst with a different shape shows a distinctive surface OV concentration and the CeO nanorods demonstrate the largest amount of OV concentration. The OVs and surface dec oration of Ag with the SPR effect collectively leads to the narrowing of the optical band gap, the increase in light absorption, the prolonged lifetime of the charge carrier and the improvement of photocatalysis performance. (2) The decoration of Ag nanoparticles on CeO2 nanorods can also prevent the recombination of e --h + pairs by efficient electron trans fer from CeO2 to Ag, leading to the charge separation efficiency and enhanced photocata lytic hydrogen evolution performance. (3) Moreover, both Ag and OVs can serve as a re active site to facilitate the adsorption of O2 and OHto form highly active ·O2 − and ·OH for photocatalytic degradation. This study is believed to shed light on the exploration of well designed photocatalysts toward diverse photocatalytic processes.
Author Contributions: J.L. offered the idea and wrote the manuscript; L.Z. performed the experiments, Y.S. and Y.L. discussed the experiments and participated in writing manuscript. All au- In addition, the surface OVs may play another important role in the absorption of dissolved oxygen molecule, which can furnish more effective surface reaction sites for photocatalytic degradation, while for the photocatalytic water splitting process, the accumulated photoelectrons quickly transferred to surface Ag and Pt nanoparticles to react with protons for hydrogen production. Therefore, the photocatalytic hydrogen evolution performance can be significantly enhanced.

Conclusions
In this work, we successfully synthesized a series of Ag/CeO 2 nanocubes, nanoparticles and nanorods' photocatalysts via a facile one-step hydrothermal process. The photocatalytic degradation of MO and photocatalytic hydrogen evolution reaction of the catalysts demonstrate strong morphology dependence and the Ag/CeO 2 nanorods possess the highest photocatalytic activity for both reactions due to the following reasons: (1) the catalyst with a different shape shows a distinctive surface OV concentration and the CeO 2 nanorods demonstrate the largest amount of OV concentration. The OVs and surface decoration of Ag with the SPR effect collectively leads to the narrowing of the optical band gap, the increase in light absorption, the prolonged lifetime of the charge carrier and the improvement of photocatalysis performance. (2) The decoration of Ag nanoparticles on CeO 2 nanorods can also prevent the recombination of e − -h + pairs by efficient electron transfer from CeO 2 to Ag, leading to the charge separation efficiency and enhanced photocatalytic hydrogen evolution performance. (3) Moreover, both Ag and OVs can serve as a reactive site to facilitate the adsorption of O 2 and OH − to form highly active ·O 2 − and ·OH for photocatalytic degradation. This study is believed to shed light on the exploration of well-designed photocatalysts toward diverse photocatalytic processes.