Oxygen-Deficient Titania with Adjustable Band Positions and Defects; Molecular Layer Deposition of Hybrid Organic-Inorganic Thin Films as Precursors for Enhanced Photocatalysis

Molecular layer deposition (MLD) of TiCl₄ and ethylene-glycol (Ti-EG) was recently demonstrated as a vapor phase synthetic route for preparation of photocatalytic thin films via hybrid organic-inorganic thin films. The organic moieties of the hybrid material function as sacrificial components that undergo controlled decomposition during thermal annealing. Anneal temperature was shown to be an important factor determining the overall photocatalytic performances of the treated films with 650 °C optimal for photodegradation of dye molecules and anneal at 520 °C showing optimal performance for the direct photocatalytic production of H₂O₂. Both systems exhibit activities that are not typically attainable by titania, yet a fundamental understanding of the underlying details leading to these improved reactivities for the specific cases is still lacking. Here we demonstrate that thermal anneal of hybrid organic-inorganic thin films prepared by MLD yield oxygen-deficient titania with controllable levels of oxygen vacancies (OVs) and defect states that are adjusted by the temperature of the anneal process, performed under air. The anneal process result in nonstoichiometric oxide films with unique electronic properties including the tuning of band positions, accessibility to significantly deeper valence band position and controlled formation of electronic defect states that assist in charge separation for Au-titania catalyst. We correlate the oxygen deficiency and electronic structure of the annealed film with the photocatalytic activity for shedding light on the details that lead to the improved reactivity. These results extend the scope of using MLD in the context of photocatalysis with new routes for obtaining nonstoichiometric oxides which are key for enhancing and tailoring the reactivity of metal oxide (photo)catalysis.