Particle Size Dependent Trap States of Photoexcited Carriers in Anatase TiO₂ Nanoparticles

Heterogeneous photocatalysis is a promising technology for artificial photosynthesis and environmental remediation. The development of efficient photocatalysts requires a better understanding of trap states of photoexcited carriers since it determines the reactivity of the carriers. Although the surface morphology as well as coordination environment of surface atoms should depend on the size of photocatalyst nanoparticles, how the trap states change with the particle size remains unclear. Here we found that the depth of electron traps on anatase TiO₂ particles decreases with increasing their particle size. It was shown that most photoexcited electrons are deeply trapped in the midgap states (3.1 to 0.7 eV from the conduction band minimum) in ultrafine nanoparticles (6.3 nm), but a part of electrons survived as free electrons in the conduction band when particle size was increased to 14.5 nm by annealing. Further increase to 28.6 nm exhibits only free electrons. These results show that an increase in the particle size decrease the density of deep electron traps, allowing photoexcited electrons to survive without being trapped even in the microsecond region. Our results identify particle size as a determining factor of trap states of photoexcited carriers in anatase TiO₂ nanoparticles.