Ultrathin Amorphous Titania on Nanowires: Optimization of Conformal Growth and Elucidation of Atomic-Scale Motifs

Due to its chemical stability, titania (TiO₂) thin films increasingly have significant impact when applied as passivation layers. However, optimization of growth conditions, key to achieving essential film quality and effectiveness, is challenging in the few-nanometers thickness regime. Furthermore, the atomic-scale structure of the nominally amorphous titania coating layers, particularly when applied to nanostructured supports, is difficult to probe. In this Letter, the quality of titania layers grown on ZnO nanowires is optimized using specific strategies for processing of the nanowire cores prior to titania coating. The best approach, low-pressure O₂ plasma treatment, results in significantly more-uniform titania films and a conformal coating. Characterization using X-ray absorption near edge structure (XANES) reveals the titania layer to be highly amorphous, with features in the Ti spectra significantly different from those observed for bulk TiO₂ polymorphs. Analysis based on first-principles calculations suggests that the titania shell contains a substantial fraction of under-coordinated Ti⁴⁺ ions. The best match to the experimental XANES spectrum is achieved with a "glassy" TiO₂ model that contains ∼50% of under-coordinated Ti⁴⁺ ions, in contrast to bulk crystalline TiO₂ that only contains 6-coordinated Ti⁴⁺ ions in octahedral sites.