Engineering an N-doped TiO₂@N-doped C butterfly-like nanostructure with long-lived photo-generated carriers for efficient photocatalytic selective amine oxidation

The photocatalytic efficiency of titanium oxide (TiO₂) has been limited by the degree of visible-light absorption, electron/hole separation and surface reactivity. In this article, N-doped porous carbon incorporating N-doped TiO₂ with a butterfly-like structure has been synthesized by using NH₂-MIL-125 (Ti), a Ti-based metal-organic framework, as a hard template. The SEM, TEM and AFM observations indicate that the as-synthesized nanocomposites preserve the butterfly-like morphology of the MOF template, which are assembled with two-dimensional (2D) corrugated nanosheets. The EDX and XPS results prove that the composites of the product are small N doped TiO₂ particles distributed in the N doped C matrix. The N₂ adsorption and desorption isotherms, UV-Vis spectra and photocurrent density measurements indicate that the N-doped porous carbon incorporating N-doped TiO₂ nanoparticles provide several favorable features for photocatalysis: a large BET surface area, visible-light absorption and high charge separation efficiency. As a result, the nanocomposites exhibit excellent photocatalytic activity toward selective oxidation of amines to imines. Density functional calculations, electron spin resonance (ESR) spectroscopy and transient absorption spectroscopy were used to reveal the mechanism of the photocatalytic process.