Understanding Photoluminescence of Monodispersed Crystalline Anatase TiO2 Nanotube Arrays

Despite huge potential of nanostructured TiO2 in many applications, the concrete understanding of optical properties associated with their defect structures remains elusive to date. Here, we present a systematic study on the photoluminescence (PL) characteristics of anatase TiO2 nanotube (TNT) arrays as a function of wall thicknesses (t(wall)) and thereby grain sizes. Highly uniform anatase TNTs were prepared by template-directed atomic layer deposition techniques, followed by thermal annealing. The relative amounts of surface/grain boundary (GB) defects to the bulk oxygen vacancies were tailored in a controlled manner by utilizing the crystallization kinetics between thin and thick wall layers during amorphous to anatase transformation. PL spectra of anatase TNT arrays were attained at variable temperatures from 300 down to 20 K. By changing t(wall), remarkably, the predominant PL emissions were likely discernible from either self-trapped excitons, oxygen vacancies, or surface/GB defects. Our results indicate that the properties of TNTs are surface-dominated when t(wall) < similar to 3 nm and bulk-dominated when t(wall) > similar to 3 nm by detecting surface-dangling bonds of TNTs located as deep traps with decreasing t(wall). Moreover, our TNT arrays with t(wall) = 20 nm and highly crystalline anatase domains emit resonant PL that may suggest STEs coupled with a specific phonon mode. The present work provides us an emergent understanding on the optical properties of one-dimensional, wide-gap oxide semiconductor photoelectrodes in extremely uniform dimensions.