Probing the Evolution of Single-Shelled TiO₂ Hollow Microspheres by Electrochemiluminescence

The evolution of single-shelled hollow microspheres is crucial for understanding their structures and advanced applications in catalysis, sensing, and energy conversion. In this work, we employed electrochemiluminescence (ECL) to track the formation process of single-shelled titanium dioxide hollow microspheres (1S-TiO₂-HMS) synthesized via a carbon microsphere-templated method with controlled calcination times. As calcination progressed, the ECL intensity of a tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)₃²⁺)-tri-n-propylamine (TPA) system continuously increased with the decomposed carbon microsphere template and the assembly of TiO₂ nanoparticles into an ordered shell. The 1S-TiO₂-HMS acted as a coreaction accelerator to promote the catalytic oxidation of TPA into TPA^•+, thereby significantly amplifying the ECL intensity of the Ru(bpy)₃²⁺-TPA system. It was revealed that the electrochemical activity of 1S-TiO₂-HMS increased during the formation process. Meanwhile, the composition of the crystalline phases dynamically evolved with the increasing rutile phase and the decreasing anatase phase. This anatase-to-rutile phase transition ultimately stabilized at a rutile/anatase ratio of ∼7:3 with optimal catalytic performances, accompanied by enhanced charge separation efficiency. The optimized 1S-TiO₂-HMS exhibited a 6.4-fold ECL enhancement, outperforming the commercial P25 TiO₂ (2.2-fold), which was attributed to its superior coreactant acceleration capability. This study not only establishes a rapid and sensitive approach to monitor hollow nanostructure formation but also provides a design principle for the advanced single-shelled and multishelled materials.