Exploring the Spatial Distribution and Transport Behavior of Charge Carriers in a Single Titania Nanowire

One-dimensional nanostructures of metal oxide semiconductors have both potential and demonstrated applications for use in light waveguides, photodetectors, solar energy conversion, photocatalysis, etc. We investigated the transport and reaction dynamics of the photogenerated charge carriers in individual titania nanowires using single-particle photoluminescence (PL) spectroscopy. Examination of the spectral and kinetic characteristics revealed that the photoluminescence bands originating from defects in the bulk and/or on the surface appeared in the visible region with numerous photon bursts by photoirradiation using a 405-nm laser under an At atmosphere. From the single-molecule kinetic analysis of the bursts, it was found that the quenching reaction of trapped electrons by molecular oxygen follows a Langmuir-Hinshelwood mechanism. In addition, a novel spectroscopic method, i.e., single-molecule spectroelectrochemistry, was utilized to explore the nature of the defect states inherent in the wires. The spatially resolved PL imaging techniques thus enable us to ascertain the location of the luminescent active sites that are related to the heterogeneously distributed defects and to present experimental evidence of the long-distance transport of charge carriers in the wire. Consequently, this study provides a great opportunity to understand the role of defects in the behavior of charge carriers in TiO2 nanomaterials with various morphologies.