A facile interfacial assembling strategy for synthesizing yellow TiO2 flakes with a narrowed bandgap

As a promising photocatalyst, the large bandgap (3.2 eV) of anatase TiO2 seriously limits its light absorption of the UV portion of the solar spectrum, making it less applicable in industrial fields. A popular approach for enhancing visible light activity by narrowing the bandgap is doping, however, the dopant-induced defect states in the TiO2 lattice may act as recombination centers for the photogenerated charge carriers. Here we report a facile soft-chemical route to engineer the surface properties of TiO2 crystals using ethanol as the sole organic solvent. The individual TiO2 nanocrystals synthesized in the first step, possessing high affinity with ethanol molecules, tend to assemble together by interfacial Ti-Ti bonding during the following ethanol evaporation induced self-assembly process. Formation of Ti-Ti bonds at the interface simultaneously brings about the decrease of surface oxygen atoms in the TiO2 structural unit, which dramatically alters the electronic structure and extends the light absorption to similar to 550 nm. Such a dopant-/additive-free TiO2 assembly exhibits considerable photocatalytic activity under visible light due to its narrower bandgap than individual nanocrystals. Further, an electron paramagnetic resonance measurement is used to confirm the capability of generating reactive OH radicals on the surface of assembled TiO2 under visible-light irradiation.