Multiple Heterojunction in Single Titanium Dioxide Nanoparticles for Novel Metal-Free Photocatalysis

Despite a longstanding controversy surrounding TiO₂ materials, TiO₂ polymorphs with heterojunctions composed of anatase and rutile outperform individual polymorphs because of the type-II energetic band alignment at the heterojunction interface. Improvement in photocatalysis has also been achieved via black TiO₂ with a thin disorder layer surrounding ordered TiO₂. However, localization of this disorder layer in a conventional single TiO₂ nanoparticle with the heterojunction composed of anatase and rutile has remained a big challenge. Here, we report the selective positioning of a disorder layer of controlled thicknesses between the anatase and rutile phases by a conceptually different synthetic route to access highly efficient novel metal-free photocatalysis for H₂ production. The presence of a localized disorder layer within a single TiO₂ nanoparticle was confirmed for the first time by high-resolution transmission electron microscopy with electron energy-loss spectroscopy and inline electron holography. Multiple heterojunctions in single TiO₂ nanoparticles composed of crystalline anatase/disordered rutile/ordered rutile layers give the nanoparticles superior electron/hole separation efficiency and novel metal-free surface reactivity, which concomitantly yields an H₂ production rate that is ∼11-times higher than that of Pt-decorated conventional anatase and rutile single heterojunction TiO₂ systems.