Best compromise for photocatalytic activity and hydrophilicity: N-doped TiO₂ films under UV light

This study seeks to determine the best compromised parameters for photoluminescence performance and hydrophilicity of nitrogen-doped titanium dioxide coatings deposited on glass substrates using radio frequency reactive magnetron sputtering. Rf power (W), process pressure (mtorr), Ar/O₂/N₂ flow ratios and substrate temperature (°C) were optimized with reference to the structure and photocatalytic characteristics of TiO₂. An L₉ (3⁴) orthogonal array, the signal-to-noise ratio and analysis of variance were employed for determination of the compromise deposition settings of nitrogen-doped titanium dioxide thin films, annealed in Ar ambient (10 mtorr) at temperatures of 250, 350 and 450 °C, for a period of 30 min. The results demonstrate that for Ar/O₂/N₂ flow ratios of 80/20/0, 75/20/5, 70/20/10, and 65/20/15, the respective corresponding methylene blue absorbance is 0.412, 0.406, 0.385, and 0.355, which depicts a greater photocatalytic activity with a larger N₂ flow ratio. For the morphological properties, the nitrogen-doped titanium dioxide films with a larger N₂ flow ratio show a homogenous structure, smaller grain sizes and more spherical particles with Ra roughness of 1.394–0.362 nm. The effects of annealing temperature, duration of ultraviolet and visible light irradiation were also analyzed. The films annealed at higher temperature (450 °C) under ultraviolet and visible light irradiation of 240 min exhibit coexistence of the anatase (101) and rutile (004) structures, which appears to imbue the nitrogen-doped titanium dioxide film with greater photocatalytic activity and photo-induced hydrophilicity.

Novelty Statement

1. This study seeks to determine the best compromised parameters for photoluminescence performance and hydrophilicity.

2. The ANOVA results show that the rf power has the major effect on methylene blue (MB) absorbance and the process pressure ranks the second.

3. This study determines the effect of Ar/O₂/N₂ flow ratios and post annealing treatment on the morphology, hydrophilicity and photocatalytic activity of N-TiO₂ thin films. The results demonstrate that a greater photocatalytic activity for N-TiO₂ thin films can be achieved by using a greater N₂ flow ratio.

4. The film that is annealed at 450 °C exhibits the coexistence of the anatase (101) and rutile (004) structures, which appears to imbue the N-doped TiO₂ film with greater photocatalytic activity and photoinduced hydrophilicity under UV and visible light illumination.