Trap state photoluminescence of nanocrystalline and bulk TiO₂: Implications for carrier transport

The visible photoluminescence of nanocrystalline TiO₂ is examined in the presence of surface binding agents and as a function of vacuum annealing in order to probe the molecular nature of surface defects. The photoluminesence (PL) of bulk crystals of anatase TiO₂ from (101) and (001) planes is also reported in order to test the hypothesis that electron and hole traps are spatially isolated on different crystal planes. We find that a number of hole scavengers are capable of quenching the PL associated with trapped electrons, while the ability of oxygen to quench PL through electron scavenging varies with the nature of the sample. We conclude that hole scavengers exert their influence on the PL through reaction with valence band holes rather than with spatially isolated trapped holes. Scavenging of electrons by O₂, on the other hand, depends on adsorption at oxygen vacancies and varies with TiO₂ sample.