Influence of the nitriding process on the band-gap of TiO2 thin films with phase mixture

• Phase content calculated using Raman spectroscopy
• Influence of the mixture of phases on the value of the bandgap of TiO2
• Nitriding process using a microwave plasma source
• Influence of the nitriding treatment on the value of the bandgap

Titanium dioxide is a semiconductor material that has a large number of applications an also is one of the most widely investigated photocatalysts. Despite its several advantages, TiO2 only exhibits photocatalytic activity when it is excited using UV radiation. For this reason, actual research has been focused in minimizing the material's bandgap, allowing excitation with visible light. Titanium dioxide with a mixture of phases (anatase + rutile) has a lower value of bandgap compared with pure anatase TiO2. In this work it is shown that the nitrogen doping of TiO2 with phase mixture can further reduce the value of the bandgap. TiO2 thin films with pure anatase phase were deposited by magnetron sputtering. An annealing process was used to induce different quantities of the rutile phase. The anatase/rutile ratio in the films was estimated by Raman spectroscopy. The films with a mixture of phases were nitrided in a microwave plasma source. The bandgap of the treated samples was studied by diffuse reflectance, applying the Kubelka–Munk transformation. The results showed that 2.56 eV was the lowest value of bandgap, for a nitrided TiO2 thin film with a phase mixture close to 20% of anatase.