Influence of the Micro-Nanostructuring of Titanium Dioxide Films on the Photocatalytic Degradation of Formic Acid under UV Illumination

Abstract

Surface micro-nanostructuring can provide new functionalities and properties to coatings. For example, it can improve the absorption efficiency, hydrophobicity and/or tribology properties. In this context, we studied the influence of micro-nanostructuring on the photocatalytic efficiency of sol-gel TiO₂ coatings during formic acid degradation under UV illumination. The micro-nanostructuring was performed using the UV illumination of microspheres deposited on a photopatternable sol-gel layer, leading to a hexagonal arrangement of micropillars after development. The structures and coatings were characterized using Raman spectroscopy, ellipsometry, atomic force microscopy and scanning electron microscopy. When the sol-gel TiO₂ films were unstructured and untreated at 500 °C, their effect on formic acid's degradation under UV light was negligible. However, when the films were annealed at 500 °C, they crystallized in the anatase phase and affected the degradation of formic acid under UV light, also depending on the thickness of the layer. Finally, we demonstrated that surface micro-nanostructuring in the form of nanopillars can significantly increase the photocatalytic efficiency of a coating during the degradation of formic acid under UV light.

Keywords: formic acid; micro-nanostructuring; photocatalysis; sol-gel.

Scheme 1

Schematic diagram of the micro-nanostructuring process.

Figure 1

Sketch of the set-up used for the photocatalytic tests.

Figure 2

(a) Raman spectra. (b) Refractive index and extinction coefficient of the film before and after annealing at 500 °C. (c) HRTEM micrograph of an anatase TiO₂ nanocrystallite (inset: the corresponding FFT showing the (−1, −1, 0) and (0, −1, 3) directions of the anatase TiO₂ tetragonal structure).

Figure 3

(a) FA disappearance rate as a function of the thickness of the unstructured TiO₂ film. (b) Photons absorbed as a function of the TiO₂ thickness.

Figure 4

Disappearance rate of FA as a function of the photons absorbed.

Figure 5

(a) SEM top view image. (b) Optical photograph of the micro- nanostructured sample. (c) AFM images. (d) Profile of the nanopillars.

Figure 6

Amount of Ti present in an unstructured TiO₂ film as a function of its thickness.

Figure 7

Disappearance rate of FA per μg of Ti obtained for micro-nanostructured and unstructured TiO₂ films.