Ceria (CeO₂) is an important rare earth metal oxide abundant in nature. Due to the easy conversion of the oxidation valence between cerium ions, CeO₂ has a high oxygen storage capacity. The band gap value of ceria is similar to that of other traditional semiconductor photocatalyst of TiO₂, so it has the potential to be a suitable and efficient photocatalyst. Specifically, ion doping can introduce new energy levels into the band structure of ceria, reduce its band gap width, and enable more visible light to be absorbed, thereby ultimately improving the photocatalytic efficiency. Secondly, when other cations enter the CeO₂ lattice, they form oxygen vacancies (oxygen defects) under structural deformation and charge compensation mechanisms. The oxygen vacancy formed in this process can act as an effective trapping point for photogenerated electrons, effectively reduce the recombination rate of electrons and holes, and significantly improve the photocatalytic efficiency of the material. In addition, ion doping can also change the electronic structure of cerium oxide, making it have better conductivity and charge transport performance, which is conducive to the separation and transport of photogenic carriers. Generally speaking, it is very difficult to introduce ions into the CeO₂ lattice. The author focuses on designing the synthesis system of CeO₂ photocatalyst doped with Sm ions and systematically studying and summarizing the reaction ratio relationship of raw materials, so as to elucidate the influence of ion doping on photocatalytic performance from the perspective of photocatalytic mechanism.

Firstly, 5.88 g C₆H₅Na₃O₇·2H₂O and 60 mL deionized water were stirred magnetically together at room temperature for 30 min. Then, 2.40 g urea was added and mixed for 30 min continually (solution 1). At the same time, 1.00 g Ce(NO₃)₃·6H₂O and different molar ratios of Sm(NO₃)₃·6H₂O were dissolved in 20 mL deionized water and stirred for 30 min, respectively (solution 2). Next, solution 2 was added into solution 1 and continue to stir for 30 min until the solution shows a light-yellow color. Then, the mixed reaction liquid was transferred to a 100 mL hydrothermal reactor, and cooled naturally after the hydrothermal reaction at 120 ℃ for 36 h. After used deionized water and ethanol to clean at least three times, and finally dried for 12 h at 70 ℃ and calcined at 500 ℃ for 4 h to obtain the target product. At the same time, CeO₂ product without adding Sm(NO₃)₃·6H₂O precursor were used as a comparison sample.

The influence of different doping concentrations on the photocatalytic performance of ceria was studied in detail, and the optimal doping ratio of Ce and Sm was determined to be 1.00∶0.15 (0.15-SC). SEM images indicated that Sm-doped material still maintained a broom-like morphology, but the surface smoothness, average diameter and length of the nanorods showed slight changes. From XRD analysis, all the samples exhibited sharp diffraction peaks, indicating high crystallinity, and no impurity peaks are detected, meaning that the doping process does not affect the crystal purity of the final product. UV-Vis absorption spectrum shows that the bandgap value (E_g) of these Sm-doped samples is significantly reduced compared to that of pure CeO₂, and the E_g of the 0.15-SC sample is the smallest at only 2.88 eV. The factors that lead to the reduction of the band gap can be attributed to two points: First, the nanoscale CeO₂ composed grains can induce the quantum limiting effect, which leads to the spectral redshift; Second, the decrease of particle size will increase the Ce³⁺ content at the edge of CeO₂. With the decrease of Ce³⁺ content, the UV-Vis spectra will also show a redshift phenomenon.

Compared with other Sm-CeO₂ samples, pure phase CeO₂ showed the strongest characteristic peak strength and the lowest peak strength when the doping ratio was 1.00∶0.15, indicating that Sm-CeO₂ samples had the best effect on promoting the separation of photogenerated holes and electron pairs and photocatalytic performance at this ratio. The environmental pollutant bisphenol A (BPA) was used as a photocatalytic degradation model, and it was found that the efficiency of all the Sm-doped CeO₂ samples was improved, and the degradation efficiency of the 0.15-SC sample was the highest, reaching 99.83%, which was 3.52-times superior than that of the pure CeO₂ (28.31%). The capture experiment of the active substances can be judged that both hydroxyl radicals (·OH) and holes (h⁺) played very important roles in the photocatalytic degradation of BPA on the 0.15-SC sample. And after four performance cycling experiments, the degradation rate of the 0.15-SC sample was still maintained 89.53%, indicating that it has good recyclability.

In summary, Sm-doped ceria material exhibits high photocatalytic purification activity for environmental pollutants, fully proving that ion's doping can act as an effective method to efficiently improve photocatalytic performance, and can further provide effective ideas for designing ion-doping systems for similar metal oxide materials in the future.