Reducible oxide and allotropic transition induced by hydrogen annealing: synthesis routes of TiO2 thin films to tailor optical response

Abstract

Optical properties of hydrogenated TiO2 thin films deposited by the ion beam sputtering method are reported. By reducing the oxide by hydrogenation in situ (during deposition) and ex situ (by means of posterior thermal annealing) a controlled phase modification allows to manipulate optical properties to improve photoactivity for solar energy conversion and catalytic applications. The hydrogenation procedure prompts remarkable structural changes and optical properties of the TiO2 hierarchical thin films. Photoemission electron spectroscopy (XPS) results show the presence of titanium in Ti4+ into Ti3+ state of oxidation, i.e., compatible with oxygen vacancies and titanium interstitials. Vibrational properties inspected by IR absorption and Raman spectroscopy allowed identifying potentially catalytic sites and vibration modes associated with hydrogen. The vibrational spectra show that both in situ and ex situ hydrogenation routes contribute to prompt (dis)order in the films, involving the presence of anatase crystalline phase embedded in an amorphous matrix or mixed-phase (anatase + rutile) crystalline TiO2. To avoid debate about the TiO2 theoretical (indirect) and expected and measured (direct, normally reported) optical gap (Egap), the absorption coefficient spectra were analyzed by a recently developed method based on the Boltzmann sigmoidal function. From these analyses, conclusions about defects states prompted by the hydrogenation processes are presented.

Keywords

• Hydrogenated TiO2 thin films
• Thermal annealing
• Crystalline defects
• Boltzmann sigmoidal function
• Optical bandgap energy