Exploring the Enhancement of Photocatalytic Performance in TiO2 based Hollow Composites: Size Effect of the Adsorbed CeO2 Particles

The photocatalytic (PC) behavior of CeO₂–TiO₂ hollow composites with different heterojunction structures are investigated. The composites are fabricated by combining TiO₂ hollow spheres and CeO₂ nanoparticles with changing the ratio between Ce and Ti. High-resolution microscopic and spectroscopic analysis demonstrates that three types of cerium-bearing structures form on the surface of the titania. The first involves Ce atoms adsorbed onto the surface of TiO₂ particles. The second occurs with small CeO₂ particles, ≈2 nm in size, resulting from the aggregation of the adsorbed Ce atoms, thus forming a CeO₂–TiO₂ heterojunction. The last type is obtained through the growth of the CeO₂ particles up to 10 nm in size. All the CeO₂–TiO₂ composites exhibit enhanced photocatalytic degradation of methyl orange under visible light irradiation compared to mere CeO₂ or TiO₂ nanoparticles. The synergistic effect of these three structures leads to a competition between size effects and interface interactions, which affects the band alignment, the number of defects, and, consequently, the PC activity. The highest PC reaction rate constant under visible light reaches up to 0.017 min⁻¹ and is achieved when the CeO₂ nanoparticle size is smaller than its Debye length.