The introduction of rare earth elements into TiO₂ can effectively improve the electron-hole separation of TiO₂ and prolong the visible light response of TiO₂.  

Cerium (Ce) shows variable valence states Ce³⁺/Ce⁴⁺ with different electronic structures (4f¹5d⁰ and 4f⁰5d⁰, respectively) among rare earth elements, which easily form oxygen vacancies. Ce element with unique electronic structure can be used to modify semiconductor photocatalysts to improve their photocatalytic performance. 

In a study published in Molecule, the research group led by Prof. LU Canzhong from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences, reported the visible light absorption of Ce/TiO₂. 

The researchers achieved simultaneous doping of Ce in the TiO₂ lattice using a simple sol-gel method which achieved Ce synchronous doping in the lattice of TiO₂. They observed morphology and structure of the pure TiO₂, Ce-doped TiO₂ (Ce/TiO₂), and CeO₂-mixed TiO₂ (CeO₂-TiO₂) samples by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). They found that Ce doping in the lattice of anatase TiO₂ resulted in a smaller grain size of the sample.  

Besides, the researchers revealed the high photocurrent density (10.9 μA×cm^-2) of Ce/TiO₂ by linear sweep voltammetry (LSV) test, which is 2.5 times that of common TiO₂ material (4.3 μA×cm^-2). They evaluated the light absorption range of Ce/TiO₂ using incident photo-to-current efficiency (IPCE) test. Ce/TiO₂ shows visible light absorption up to 500 nm, while pure TiO₂ shows no obvious response in the visible region. 

Moreover, the researchers unveiled that the electron-trapping centers formed by Ce doping into the TiO₂ lattice improved the separation efficiency of photogenerated electrons and holes. The narrow bandgap of Ce-doped TiO₂ showed excellent visible light absorption and photocurrent response. Due to the Ce doping, the Ce/TiO₂ samples achieved high photocurrent density and incident photon current efficiency (IPCE).  

This study provides a practical strategy and an important reference for the preparation and understanding of highly efficient visible light-activated rare earth-doped photocatalysts.