Insight into the Contributions of Surface Oxygen Vacancies on the Promoted Photocatalytic Property of Nanoceria

Oxygen vacancies (OVs) have critical effects on the photoelectric characterizations and photocatalytic activity of nanoceria, but the contributions of surface OVs on the promoted photocatalytic properties are not clear yet. In this work, we synthesized ceria nanopolyhedron (P-CeO2), ceria nanocube (C-CeO2) and ceria nanorod (R-CeO2), respectively, and annealed them at 600 °C in air, 30%, 60% or pure H2. After annealing, the surface OVs concentration of ceria elevates with the rising of H2 concentration. Photocatalytic activity of annealed ceria is promoted with the increasing of surface OVs, the methylene blue photodegradation ratio with pure hydrogen annealed of P-CeO2, C-CeO2 or R-CeO2 is 93.82%, 85.15% and 90.09%, respectively. Band gap of annealed ceria expands first and then tends to narrow slightly with the rising of surface OVs, while the valence band (VB) and conductive band (CB) of annealed ceria changed slightly. Both of photoluminescence spectra and photocurrent results indicate that the separation efficiency of photoinduced electron-hole pairs is significantly enhanced with the increasing of the surface OVs concentration. The notable weakened recombination of photogenerated carrier is suggested to attribute a momentous contribution on the enhanced photocatalytic activity of ceria which contains surface OVs.

It is generally accepted that the photocatalytic application of ceria is impeded by its wide band gap~3.2 eV and a quick recombination of photogenerated electrons (e − ) and holes (h + ) [16,17]. Attributing to the redox characteristic of Ce 4+ /Ce 3+ pairs, oxygen vacancy is an inescapable topic for researching on ceria based catalysts [1,18,19]. Oxygen vacancies (OVs) or Ce 3+ have been reported to affect both band structure and recombination of photocarriers significantly, and promote the photocatalytic activity of ceria [20][21][22][23]. It is believed that the OVs are favorable for reducing the e -/h + pairs recombination rate [24,25].

Synthesis Process
Nanoceria was synthesized by using a simple template-free hydrothermal method under a variety of conditions to modify the morphology, which was similar to the synthetic route reported in Ref. [43]. Briefly, NaOH aqueous solutions were dropwise added into Ce(NO 3 ) 3· 6H 2 O aqueous solution to form light purple mixtures with strong stirring for 30 min and then transferred to a 50 mL Teflon stainless steel autoclave, which would be maintained at a designed temperature for 24 h. More synthetic conditions are listed in Table S1. After the autoclave cooling down, all products were washed and filtered with distilled water and alcohol several times to remove impure ions, followed by drying at 60 • C in air overnight. The obtained polyhedral, cubic, and rod-like nanoceria are named as raw P-CeO 2 , C-CeO 2 and R-CeO 2 , respectively.

Annealing Process
The raw P-CeO 2 , C-CeO 2 and R-CeO 2 were placed in a ceramic boat and then maintained in a tube furnace (gsl-1600×, Kejing, Hefei, China) for 2 h annealing at 600 • C with a heating rate of 10 • C/min under air, 30, 60 or 100% H 2 atmosphere, respectively. Before a H 2 annealing process, air was repeatedly expelled from the furnace tube by alternately flowing argon and vacuuming for several times. The total gas flow rate was 400 mL/min and argon was selected as the balance gas (88.79 kPa total pressure of Guiyang), and the annealed powders were henceforth named as P-CeO 2 -X, C-CeO 2 -X and R-CeO 2 -X (X = air, 30% H 2 , 60% H 2 or H 2 ).

Subsection Analysis
The obtained powders were subjected to several analyses. X-ray powder diffraction pattern was recorded by using X-Pert Powder (Panalytical, The Netherlands) with Cu Kα radiation (λ = 0.15418 nm) from 5.00 to 90.00 • at a rate of 0.02 • /s. Micrographs were taken by SEM (JSM 7610, JEOL, Tokyo, Japan) and TEM (Tecnai G2 F20, FEI, Hillsboro, OR, USA) in which the samples were ultrasonically dispersed in alcohol and dropped on the silicon wafer or copper grid. TG analysis was recorded by Mettler TGA/SDTA 851e at a heating rate of 10 • C/s from room temperature to 1000 • C with the air and 30%, 60% and pure H 2 , where Ar was used as carrier gas. H 2 -TPR was analyzed by AutoChem1 II 2920, 0.1000 ± 0.0005 g of sample was kept at 500 • C in the air for 1 h and cooled down to room temperature. After 30 min purification in Ar at room temperature, the sample was heated to 1000 • C with the mixed gas of 10% H 2 and 90% Ar with a total gas flow of 30 mL/min and a heating rate of 10 • C/min. X-ray photoelectron spectroscopy (XPS) was recorded by K-Alpha (Thermo Fisher Scientific, Waltham, MA, USA), in which a monochromatic Al source (hv = 1486.6 eV) and the samples were tested in a vacuum situation of 2 × 10 −9 mbar with C 1s peak (284.8 eV) reference. The UV-visible diffuse reflectance spectra (UV-Vis DRS) were recorded by UV 2700 (Shimadzu, Tokyo, Japan) with the wavelength from 200 to 800 nm and BaSO 4 was used as reference, while the photoluminescence spectroscopy (PL spectra) was analyzed by FluoroMax-4 (HORIBA, France) with an excitation wavelength of 300 nm. The ultraviolet photoelectron spectroscopy (UPS) recording by ESCALAB 250Xi (Thermo Fisher, Waltham, MA, USA) was performed the valence states of all samples at the He I line (hv = 21.2 eV) with C 1s reference in a vacuum situation of 2 × 10 −8 mbar. Transient photocurrent curves were recorded under a light irradiation provided by a 250 W xenon lamp in 0.1 mol/L Na 2 SO 4 aqueous solution at bias voltage of 0.4 V, which was employed by an electrochemical workstation (CHI660C, CHI shanghai Co., Shanghai, China) in three electrode cells. The tested samples were dispersed in a nafion (10 µL), ethanol (750 µL) and deionized water (750 µL) mixture solution and further dip-coated on a glassy carbon plate (Φ = 3 mm), which was used as the working electrode, and a Pt plate and Ag/AgCl were employed as the counter and reference electrode, respectively. The BET surface area and N 2 adsorption results were analyzed by ASAP2460 (Micromeritics, Norcross, GA, USA).

Photocatalytic Performance
The photocatalytic performances were tested by a self-built photochemical reactor, which was composed of a 250 W xenon lamp and a quartz vessel. In total, 80 mL of methylene blue (MB) solution with a concentration of 10 mg/L was used for simulating the waste dye solution, and 20 mg of synthesized catalyst was added into the reactor with ultrasound for 10 min. After 30 min of dark adsorption, the lamp was turned on and the catalyst was reacted with the MB under a light facula, 5 mL solution was sampled for 30 min each in the next 2 h. The sampled solution was centrifuged firstly with a speed of 10,000 r/min for 5 min and then the MB concentration was measured by a spectrophotometer under the maximum wavelength of 664 nm. The degradation ratio can be calculated by the following formula [44,45]: At a low concentration of MB with a weak adsorption, the photocatalytic reaction kinetics in general follow the Langmuir-Hinshelwood (L-H), and the equation of the pseudo-first-order reaction rate constant [46] can be given as: where C 0 and C i are the initial and tested concentration of MB, respectively, k is the pseudofirst-order reaction rate constant (min −1 ), t is the photocatalytic reaction time (min).

Phase and Morphology
XRD patterns of synthesized nanoceria are exhibited in Figure 1, it can be seen that all samples show the typical diffraction peaks of CeO 2 with a fluorite-type structure and Fm3m space group (PDF: #03-065-2975). The raw C-CeO 2 has the strongest diffraction intensity, followed by raw P-CeO 2 and R-CeO 2 , the crystal sizes of raw P-CeO 2 , raw C-CeO 2 and raw R-CeO 2 were calculated as 6.7, 36.5, and 10.0 nm, respectively. In Figure 2, TEM images clearly exhibit that the synthesized nanoceria samples own the desired morphology of polyhedron, cube and rod, the counted statistical particle size of synthesized ceria is given in Figure S1, showing the average size of P-CeO 2 , C-CeO 2 and R-CeO 2 is around 9, 40, and 100 nm (for length), respectively. From the HRTEM images as shown in Figure 2, the spacing lattice fringes are measured as 0.318 and 0.321 nm for the P-CeO 2 associating with presenting (111) plane, and the (100) face is found in the HRTEM image of C-CeO 2 , while both (110) and (100) planes are exposed in the R-CeO 2 , which is in agreement with the theory for the main exposing face of various shaped ceria [38].

Reduction of Ceria in H 2
The TG analysis results of raw R-CeO 2 under different atmospheres are shown in Figure 3a, obviously weight loss can be found under each atmosphere and the weight loss increases in a higher H 2 concentration atmosphere. In air condition, the first weight loss is about 5.6 wt. % corresponding to the vaporization of free water, then the R-CeO 2 continues to weightlessness from 138 to 356 • C, where the weight loss is about 3.2 wt. %, which may be in connection with the Ce(OH) 4 decomposition [47]. At a higher temperature, the weight signal of sample stabilizes at about 89.0 wt. % of initial weight. The weight loss of ceria in 30%, 60% or pure H 2 atmosphere can be divided into four steps: (i) free water evaporation; (ii) H 2 adsorbs on the surface of R-CeO 2 and hydroxylates with Ce 4+ accompanying by the water decomposing [48], which leads to around 2.9, 4.2 and 5.7 wt. % of weightlessness at 356 • C under different hydrogen concentration; (iii) the surface of R-CeO 2 is continually and incompletely reduced by H 2 , and the decrement of weight is about 1.7 wt. % at 837 • C for 30% H 2 , 2.0 wt. % at 782 • C for 60% H 2 , and 6.0 wt. % at 757 • C for pure hydrogen atmosphere, respectively; (iv) the subsurface of ceria is reduced and tends to form Ce 2 O 3 [37]. With the increasing of H 2 concentration, the weight loss of raw R-CeO 2 at the same temperature increases, which means more O atoms are divorced from ceria lattice by the following reaction: where 0 < x < 0.5, and nonstoichiometric value of x depends on the temperature and H 2 partial pressure. The H 2 -TPR results of synthesized raw CeO 2 are shown in Figure 3b. The first and second peak shown in TPR curves corresponds to the surface reduction and bulk reduction of ceria by hydrogen, respectively [36]. The synthesized various structural nanoceria samples show different start/end reduction temperatures for the first reduction stage of three tested samples, where R-CeO 2 has the widest reduction range of 254.1-501.9 • C, P-CeO 2 shows the narrowest range from 360.1 to 485.8 • C, followed by the 271.9-492.0 • C for C-CeO 2 . At the second reduction step, the maximum reduction peaks are found to be achieved at the temperature of 758.8, 776.4 and 787.7 • C for C-CeO 2 , R-CeO 2 and P-CeO 2 , respectively. The notable diversity of the reducing behavior for the R-CeO 2 , C-CeO 2 and P-CeO 2 further verifies the different activities of the mainly exposed crystal faces in ceria, where (110) and (100) are more active than (111) facet [40,41,49,50]. Based on the TG and H 2 -TPR results, annealing ceria at a same temperature of 600 • C in different hydrogen partial pressure atmosphere will produce ceria samples with various concentration of surface OVs.

OVs Characterization
Three shaped ceria powders were annealed in four types of atmospheres, and different colored products were obtained, where the ceria annealed in air is pale yellow for P-CeO 2 and R-CeO 2 , white for C-CeO 2 , and then turns to greyish-green or blue-yellow with the increasing of H 2 partial pressure, and the colors of annealed ceria are shown in Figure S2. It is known that the color of pure and stoichiometric cerium dioxide is pale yellow [51], and its color will turn to blue or even black after the formation of nonstoichiometric ceria [52], the observed color variation means an abundance of OVs were generated after hydrogen annealing.
As shown in Figure 4, the areas of v' and u' corresponding to the Ce 3+ are increasing after annealing in a higher hydrogen contained atmosphere, which means more Ce 4+ in ceria was reduced to Ce 3+ . Calculated Ce 3+ fractions are shown in Figure 4d, where an obvious rising of Ce 3+ fraction can be found with the increasing of hydrogen concentration, the Ce 3+ % increases from 10.66 to 16.56%, 9.71 to 15.12%, and 11.73 to 19.55% for the P-CeO 2 , C-CeO 2 , and R-CeO 2 , respectively. More OVs appear on the surface of R-CeO 2 which is related to the suitable surface activity of (110) facet [58]. In O 1s spectra (are given in Figure S3), oxygen species are originated from lattice oxygen (O L ) attached to Ce 4+ ion and adsorbed oxygen to Ce 3+ site (O V ), which can be deconvoluted into two peaks at around 529.2 and 531.3 eV, respectively [59,60]. The area and intensity of O V peak are relevant to the oxygen vacancy in the host lattice, which is calculated and given in Table S2. It can be found that the O V fraction of ceria increases with the rising of H 2 concentration in annealing gas, which further identified the results shown in Ce 3d spectra that more surface OVs are generated after annealing in a higher H 2 concentration atmosphere at 600 • C.

Photocatalytic Activities
Photocatalytic properties of tested ceria are shown in Figure 5, and the photodegradation ratio of the tested samples is presented in Table S3 together with the calculated degradation rate constants. It is clearly found that the annealed ceria has a higher photocatalytic activity than that of raw material, and the photocatalytic activities of three structural ceria are elevated gradually with the increasing of surface OVs concentration. The observed results further verified the reported results [20][21][22]26] that the OVs are beneficial for the enhancement of photocatalytic property of ceria. P-CeO 2 -H 2 has the highest photodegradation ratio of MB of 93.82%, which is larger than 90.09% of R-CeO 2 -H 2 and 85.15% of C-CeO 2 -H 2 . The excellent photocatalytic activity of P-CeO 2 may be due to its smallest average particle size around 9 nm.
As it is known that size [61], morphology [62] and OVs concentration [63] are the main factors which influence the photocatalytic activity as well as the photoelectric characterizations of ceria. The TEM images of the CeO 2 annealed in air and pure hydrogen are given in Figures S4-S7. It can be seen that the annealed P-CeO 2 still exhibits (111) facet, the crystal plane of annealed C-CeO 2 is transformed from (100) to stable (111). The plane of calcined R-CeO 2 -Air is also tended to (111) with small particles aggregating, but the previously existing (110) facets are turned to active (100) in R-CeO 2 -H 2 with the small holes on nanorods. The average particle size of P-CeO 2 -H 2 and C-CeO 2 -H 2 is larger than that of P-CeO 2 -air, and C-CeO 2 -air, respectively, but the length of R-CeO 2 -H 2 is found as around 60 nm which is shorter than that of R-CeO 2 -air. The XRD patterns of C-CeO 2 annealed in air, 30%, 60% and pure H 2 are shown in Figure S8. It can be found that all annealed samples have the similar diffraction pattern of CeO 2 , no peaks of Ce 2 O 3 can be found. The calculated crystal size is given in Table S4, the crystal size of the ceria increased after annealing, which is in agreement with the TEM images, while the sample annealed in hydrogen has a slightly increased crystal size compared with that in air. Moreover, the nitrogen adsorption-desorption isotherms of the P-CeO 2 , C-CeO 2 , and R-CeO 2 calcining in air or pure H 2 are shown in Figure S9. It was calculated that the BET surface area of P-CeO 2 -air, C-CeO 2 -air and R-CeO 2 -air is 60.25, 20.94 and 68.20 m 2 /g, respectively, where that of P-CeO 2 -H 2 , C-CeO 2 -H 2 and R-CeO 2 -H 2 is 10.41, 16.43, and 44.76 m 2 /g. We found that the BET surface area of all samples decreased after calcining in H 2 , which further indicates that the surface OVs concentration is the major factor on the photocatalytic properties of ceria in this study. In addition, the excellent photocatalytic performance of P-CeO 2 may be related to the ordered mesoporous structure.
Hence, the similar morphology, size and BET surface area of the annealed ceria in different atmospheres suggests that the surface OVs concentration is the major factor in the photocatalytic properties of ceria in this study.

Band Structure of as Prepared Ceria
The UV-Vis DRS spectra and the band energy curves of CeO 2 are shown in Figure 6, and the calculated band gap values are given in Table S5, and it is found that the variations of the light absorption behavior and the band gap for the different morphology ceria before and after annealing are not stereotyped. After annealing, the band gap firstly expands and then slightly narrows with the increase of surface OVs concentration. Raw P-CeO 2 has a band gap of 2.987 eV, while the band gap values of annealed cubic nanoceria are in the range of 2.796-2.864 eV. C-CeO 2 samples have similar band gaps of 3.170-3.204 eV. Raw R-CeO 2 has a narrower band gap of 2.882 eV than that of annealed R-CeO 2 samples, while the band gap value of R-CeO 2 -air increases to 3.019 eV and then turns to 3.283 eV for 30% H 2 annealed sample, by continually increasing the H 2 concentration the values tend to decrease slightly. Interestingly, the observed variations of band gap are quite different from the previous reports (e.g., [21,26]), and no significant changes of band gap are observed with the increasing of OVs concentration, which indicates that the surface OVs generated by hydrogen annealing at 600 • C have minimal effects on the band gap of nanoceria.
Band gap energy (E g ) of ceria depends on the conduction band (CB) and valence band (VB), the energy of VB (E VB ) of annealed ceria was analyzed by UPS and the results are shown in Figure 7, where the band edge position of CB (E CB ) was calculated based on the relationship as given in Equation (5).
Moreover, the value of E VB and E CB can be generally calculated by the Mullikan Electronegativity equation [64]: where χ is the absolute electronegativity of E C the semiconductor, the χ value of CeO 2 is reported as 5.56 eV [65], and E C is the energy of free electrons on the hydrogen scale (−4.5 eV [66]). The measured and calculated E VB and E CB are listed in Table S5, where it can be found that the band edge positions obtained under different conditions present a similar variation trend for the same shaped ceria samples. For three studied structured ceria, the E VB expands to a more positive position when the annealing atmosphere turns to 30% H 2 from air, then decreases with the rising of H 2 concentration. On the contrary, the values of E CB of C-CeO 2 and P-CeO 2 are firstly moved to the Fermi level closely and then become more negative with the OVs concentration rising, while the E CB of R-CeO 2 is firstly expanded to a more negative position and then turns back to Fermi level with the increase of surface OVs. 3.6. Separation/Recombination of e − /h + PL spectra, as shown in Figure 8, were employed to investigate the recombination efficiency of photoinduced electrons and holes, where a lower recombination rate is characterized by a lower PL intensity [17,67]. It can be found that all PL spectra show strong blue emission peaks centered at 430-490 nm, which is associated with the defect levels localized between the Ce 4f and O 2p bands [68][69][70][71]. With the rising of surface OVs concentration, the intensities of PL spectra for P-CeO 2 and C-CeO 2 obviously weaken firstly, then decrease slightly. However, the intensities of emission spectra for R-CeO 2 samples are continually weakening with the increasing of OVs concentration, which may offer an evidence for the potential or further reducing of the recombination rate of photogenerated carrier for ceria nanorod with a higher surface OVs concentration. Besides, after annealing in air, 30% and 60% H 2 , R-CeO 2 shows a lower PL intensity than that of other typical structure nanoceria, while after annealing in pure hydrogen, the P-CeO 2 exhibits the lowest PL intensity.  In order to further confirm the separation efficiency of photogenerated electron-hole pairs of the studied samples [72], the transient photocurrent response experiments were measured, and the average photocurrent densities are shown in Figure 9 and Table S6. Higher photocurrent densities are presented with the rising of surface OVs concentration in P-CeO 2 , C-CeO 2 and R-CeO 2 annealed in increasing concentration of H 2 , which suggests a higher surface OVs concentration may elevate the e − /h + separation efficiency of CeO 2 photocatalyst. It is generally known that a higher separation and lower recombination rate of e − /h + are beneficial to the better photocatalytic activity [73], which provide further evidence for the enhancement of photocatalytic activity of ceria after annealing in hydrogen.

Proposed Mechanism for Photocatalytic Enhancement
To evaluate the contributions of surface OVs on the photoelectric characterizations and photocatalytic activity of ceria, the offset values of each property of different hydrogen annealed ceria compared with the air annealed CeO 2 were calculated using the following equation: Offset Value = (P i − P 0 )/P 0 × 100%, where P means the properties including band gap value, photodegradation ratio of MB at 2 h or the photogenerated current, i (i = 0, 1, 2, and 3) represents the number of annealed samples, where 0, 1, 2, and 3 means the sample annealed in air, 30%, 60%, and pure H 2 , respectively. The relationships of offset values vs. surface Ce 3+ concentration of ceria are shown in Figure 10 and Table S7. The offset value of band gap is slightly decreasing with the increasing of surface Ce 3+ while the 30% H 2 annealed samples have a wider band gap than that of air annealed ceria, suggesting that surface OVs may expand the band gap of ceria firstly, and then the band gap value tends to decrease slightly with a continual rising of surface OVs. Revealed results may explain the reported references, e.g., Gao et al. [22] obtained rich surface OVs ceria by surface engineering with a blue shift of the UV-Vis spectra. Interestingly, the variations both of the offset value of photodegradation ratio and the photocurrent density are notably rising with an increase of surface OVs, which indicates that the reduction of e − /h + recombination may be the major contribution of surface OVs on the enhancement of photocatalytic activity under same light source.
Comparing the effects of surface OVs on different shaped ceria, it can be found that surface OVs affect the photocatalytic activity most significantly on the cubic ceria, while C-CeO 2 contains low surface OVs due to its large particle size and high activity of (100) facet, more surface OVs induced in ceria nanocube lattice may result in a moderate photocatalytic activity. Even the effect of surface OVs working on photocatalytic activity of R-CeO 2 is slightly smaller than that of P-CeO 2 , but more OVs can be generated in the R-CeO 2 , which also results in a high photocatalytic activity. On the other hand, the polyhedral ceria has the smallest size distribution, which may be one of important reasons for its excellent photocatalytic property.
Based on the revealed results, the contributions of surface OVs on the photocatalytic activity of ceria can be concluded as shown in Figure 11. In the range of studied surface OV concentration in cubic, polyhedral or rod-like ceria, a significantly reduction of the combination of e − /h + is the major contribution of surface OVs on the promoted photocatalytic activity, while the band gap varies slightly. The surface OVs in CeO 2 lattice are rearranged to produce small microdomains [74] and ordered together to form electron deep traps which can facilitate the reduction of the recombination rate between photoelectrons and holes during the photocatalytic process [24,75]. Moreover, surface OVs profit the adsorption of O 2 or OH − on ceria surface, which will promote the generation of radical and reduce the recombination of e − /h + photocarriers [69]. Hence, under the same illumination condition, the photocatalytic activity is obviously enhanced with the rising of surface OVs concentration, which is majorly influenced by the reduced recombination of e − /h + . In addition, the effect rule of surface OVs on photoelectric characterizations and photocatalytic activity of cubic, polyhedral and rod-like ceria is similar but with different incidence, furthermore, reducing particle size and gaining OVs concentration of ceria are still the major tactics for enhancing its photocatalytic activity.

Conclusions
After sufficient discussion of the revealed results, it can be concluded that a concentration gradient of surface OVs can be generated in ceria lattice after annealing nanoceria at 600 • C in various H 2 concentration atmospheres, and the ceria annealed in hydrogen has a larger particle size and the exposing lattice face tuned after annealing. Surface OVs significantly enhanced the photocatalytic activity of ceria, the MB degradation ratio after 2 h with pure hydrogen annealed C-CeO 2 , P-CeO 2 , or R-CeO 2 is 85.15%, 93.82% and 90.09%, respectively, which is 1.5, 1.29 and 1.33 times higher than that of the air annealed sample. The band structure, including band gap, VB, and CB of annealed samples vary slightly, even the surface OVs in ceria lattice changed obviously. Recombination of photoinduced carrier, e − /h + , has a notable reduction with the rising of surface OVs, which is suggested to be the main contribution for the enhancement of photocatalytic activity of ceria with more surface OVs.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/nano11051168/s1, Figure S1. Size distribution of raw P-CeO 2 (a), C-CeO 2 (b) and R-CeO 2 (c). Figure S2. Color variation of P-CeO 2 , C-CeO 2 and R-CeO 2 (from left to right) calcining in different atmospheres: (a) raw samples without annealing, (b) air, (c) 30% H 2 , (d) 60% H 2 , and (e) pure H 2 condition. Figure S3. O 1s spectra of P-CeO 2 (a), C-CeO 2 (b) and R-CeO 2 (c) calcining in different concentration of H 2 . Figure S4. TEM images and size distribution of P-CeO 2 calcining in air (a-c) and H 2 (d-f) at 600 • C. Figure S5. TEM images and size distribution of C-CeO 2 calcining in air (a-c) and H 2 (d-f) at 600 • C. Figure S6. TEM images and size distribution of R-CeO 2 calcining in air (a-c) and H 2 (d-f) at 600 • C. Figure S7. TEM images of raw R-CeO 2 (a), R-CeO 2 -Air (b) and R-CeO 2 -H 2 (c). Figure S8. The XRD patterns of C-CeO 2 annealed in air, 30%, 60% and pure H 2 . Figure S9. Nitrogen adsorption-desorption isotherms of the P-CeO 2 (a), C-CeO 2 (b), and R-CeO 2 (c) calcining in air or pure H 2 . Table S1. Synthetic conditions of raw P-CeO 2 , C-CeO 2 and R-CeO 2 . Table S2. Ce 3+ and absorbed oxygen concentration of P-CeO 2 , C-CeO 2 and R-CeO 2 calcining in different concentration of H 2 . Table S3. Photocatalytic degradation ratio and rate constants of P-CeO 2 , C-CeO 2 , and R-CeO 2 calcining in different atmospheres. Table S4. The calculated crystal size of C-CeO 2 annealed in air, 30%, 60% and pure H 2 . Table S5. Energy band gap, calculated valence and conductive band, tested valence and conductive band of P-CeO 2 , C-CeO 2 , and R-CeO 2 calcining in different concentration of H 2 . Table S6. Average current density (µA/cm 2 ) of P-CeO 2 , C-CeO 2 , and R-CeO 2 calcining in different concentration of H 2 . Table S7. Offset values of photodegradation ratio, band gap and photocurrent density of different hydrogen annealed ceria.