Fabrication of Photocatalytic Al-TiO2 Coatings By Composite Electrodeposition and Anodization
Authors: Miyake Masao , Takahashi Ayumu , Ikenoue Takumi , Hirato Tetsuji 
Published: February 2020

Metal matrix composite coatings containing TiO2 particles have been investigated, with the aim of obtaining coatings with additional functionality by taking advantage of the excellent photocatalytic ability of TiO2. Composite electrodeposition is known to be an effective technique for the fabrication of composite coatings of a metal matrix containing finely dispersed inert particles. The electrodeposition of coatings containing TiO2 particles with the metallic matrix being Ni, Ni-Mo alloy, Cu, Ag, and Zn has been reported.

However, simple composite coatings composed of a metal matrix and uniformly dispersed TiO2 particles would not exhibit strong photocatalytic functions, because most of the TiO2 particles in the coatings are enclosed by the metal matrix and thus cannot exert their photocatalytic activity. Only the particles exposed on the surface of the coating can catalyze the reaction. One of possible ways to enhance the photocatalytic activity of the composite coatings is to make the matrix porous so that reactants can be in contact with the TiO2 particles located deep inside the coating. If Al is employed as the matrix, a well-known anodizing process can be used to convert the solid Al matrix into a porous Al oxide layer, although it is unclear if the porous structure can be formed even when inert particles are present in the Al layer.

In this study, we investigated the feasibility of the formation of high-photocatalytic-activity Al-TiO2 composite coatings by electrodeposition using an organic bath and subsequent anodization. The structures of the anodized layer formed on the Al-TiO2 composite coatings, and the effect of the anodization on the photocatalytic activity of the coatings were examined.

Al coatings containing TiO2 particles were obtained by electrodeposition from a bath composed of dimethyl sulfone, anhydrous AlCl3, and TiO2 powder. Surface and cross-sectional SEM observation and EDX analysis confirmed that TiO2 particles were uniformly present in the Al layer with a thickness of ~40 μm. The TiO2 content in the electrodeposited layer increased with increasing TiO2 content in the bath, and reached a maximum value of ~30 vol.%.

The obtained Al-TiO2 composite coatings were subsequently anodized in oxalic acid at a voltage of 40 V for 30–90 min, followed by immersion in phosphoric acid to widen the pores. SEM observation of the surface and a fractured cross-section of the anodized coatings confirmed that an oxidized layer containing many pores with a diameter of <30 nm extending in the direction toward the substrate was formed on the surface of the coating. The mean interval between the pores was ~100 nm. TiO2 particles with 100–1000 nm sizes dispersed in the porous oxidized layer could also be observed. 

The photocatalytic activity of the coatings was examined by measuring the decomposition rate of methylene blue on the coatings under ultraviolet irradiation. The photocatalytic ability of the coatings increased with the TiO2 content in the coatings. Compared with the as-deposited coatings, the anodized coatings showed a higher photocatalytic ability. An increase in the photocatalytic ability was also observed with increasing anodization time. This should be attributable to the increase in the thickness of the porous layer and the resultant increase in the number of TiO2 particles accessible to the reactants.

These results demonstrated that Al-TiO2 composite coatings can be formed by electrodeposition using an organic bath, and anodization of the composite coatings can create a porous Al oxide layer containing TiO2 particles on the surface of the coatings. By carrying out anodization, the photocatalytic activity of the composite coatings could be enhanced.