Co-enhancement of doped N and oxygen vacancies on the photocatalytic performance of ceria: Mechanism and influence of crystal faces

Abstract

Nitrogen doping has significant effects on the photocatalytic performance of ceria (CeO2), and the possible synergistic effect with the inevitably introduced abundant oxygen vacancies (OVs) is of great significance for further investigation, and the specifically exposed crystal faces of CeO2 may have an impact on the performance of nitrogen-doped CeO2. Herein, nitrogen-doped CeO2 with different morphologies and exposed crystal faces was prepared, and its performances in the photocatalytic degradation of tetracycline (TC) or hydrogen production via water splitting were evaluated. Density functional theory (DFT) was used to simulate the band structures, density of states, and oxygen defect properties of different CeO2 structures. It was found that nitrogen doping and OVs synergistically promoted the catalytic activity of nitrogen-doped CeO2. In addition, the exposed crystal faces of CeO2 have significant effects on the introduction of nitrogen and the ease of OV generation, as well as the synergistic effect of nitrogen doping with OVs. Among them, the rod-like nitrogen-doped CeO2 with exposed (110) face (R-CeO2-NH3) showed a photocatalytic degradation ratio of 73.59% for TC and hydrogen production of 156.89 μmol/g, outperforming other prepared photocatalysts.

1. Introduction

Ceria (CeO2), an abundant rare earth oxide, is widely considered as a photocatalyst for energy generation or environmental protection due to its excellent redox properties [1]. Its unique electronic structure enables CeO2 to form oxygen vacancies (OVs) during the Ce4+ to Ce3+ transition process [2, 3], endowing it with high oxygen storage and releasing capabilities [4]. However, the wide band gap and high carrier recombination rate inherent to CeO2 limit its photocatalytic performance [5]. Surface engineering [6, 7], oxygen vacancy defects [8, 9], additional elements doping [10, 11], etc., have garnered significant attention to enhance the performance of CeO2.

It has been demonstrated that OVs can directly affect the catalytic performance of CeO2 [12, 13], which is due to the fact that the introduction of OVs affects the properties of CeO2 such as band gap [14], light absorption [15] and photo-excited carrier migration [16], which are related to the defect states introduced in the band structure. In addition, the OV defects on the surface of CeO2 can serve as active sites, contributing to the adsorption and activation of reactants [17].

Doping anions in ceria attracts rising attention due to their ability to replace the oxygen atoms in lattice [18], and resulted in obviously enhanced photocatalytic properties [19]. Among the studied anions, including P, S, F and N, nitrogen is particularly effective due to its similar size to oxygen, smaller ionization energy, simple and low-cost doping method, and enhanced effect on the photocatalytic performance of CeO2 [20]. In our previous study [21], we found that the nitrogen-doped CeO2 exhibited a more effective photocatalytic degradation ratio of tetracycline (TC) compared to CeO2 containing only OVs. When heteroatoms enter the crystal cell of CeO2, lattice distortion, reduction of Ce4+ and formation of OVs are inevitably induced [22 −24]. Therefore, the enhancement of the photocatalytic performance of nitrogen-doped CeO2 is likely a result of the synergistic effect between doped N and OVs. However, the synergistic effect and mechanism of these two factors on the properties of CeO2 remain unclear and require further investigation. Additionally, the photocatalytic performance of CeO2 depends on the specific exposed crystal faces and surface atomic arrangement [25, 26]. Different exposed crystal faces such as (111), (100) and (110), have various effects on the formation energy of OV and photocatalytic performance.