Distorted Ultrasmall TiO₂ Nanoparticles as Efficient Visible Light Photocatalyst

Developing efficient and low-cost visible light photocatalysts remains a fundamental challenge. Titanium dioxide (TiO₂), although stable and abundant, suffers from a wide bandgap and limited surface area. In this study, visible light active ultrasmall approximately 2 nm (∼2 nm) TiO₂ nanoparticles were synthesized to overcome these limitations. Unlike conventional post-annealing routes that promote particle growth, this visible light-active ultrasmall TiO₂ was directly obtained through a continuous flow-hydrothermal process with precise temperature control and sub-second reaction time. The ultrasmall (∼2 nm) TiO₂ structure reveals the highly distorted lattice structure. The high lattice distortion, which contains an undercoordinated Ti atom, induces the localized states that modify the band structure of TiO₂ and decrease its activation energy to the visible light region. The resulting ∼2 nm TiO₂ exhibits nearly threefold higher photocatalytic activity for NO_x removal under visible irradiation, opening a new direction for designing ultrasmall oxide nanomaterials for next-generation photocatalysis.