Modification of Conductivity, Superhydrophilicity and Photocatalytic Activity of TiO2 Thin Films Through Carbon Nanotubes Doping

In this work, a superhydrophilic and photocatalytic material allowing to reduce the accumulation of electrostatic charges is developed. The superhydrophilicity and photocatalytic activity of TiO2 films provide the “easy-to-clean” property. Indeed, superhydrophilicity induces a very low contact angle between TiO2 and water leading to the formation of a water film at the surface of the material. The photocatalytic activity, responsible for the pollutant decomposition, is explained by the excitation of the semiconductor under UV light leading to the formation of electron-hole pairs. The electrical conductivity of semiconductor TiO2 is very low leading to the accumulation of electrostatic charges and so the surface becomes a dust trap.
Multi-walled carbon nanotubes (MWCNTs) are electrical conductors and their introduction in TiO2 could increase the conductivity. The incorporation of MWCNTs can modify the superhydrophilicity of TiO2. The photoactivity can be improved by reducing the electron-hole recombination rate. MWCNTs play a role in electron transfer and allow to decrease the recombination of electron-hole pairs.
Two sol-gel syntheses were studied in alcohol and water respectively. In the alcoholic medium, monolayer films are obtained by dip-coating on alkaline free glass and calcined at 300, 400 or 500°C. The thermal treatment allows to crystallize TiO2 in the anatase form. In the aqueous synthesis, monolayer film are obtained by dip-coating on alkaline free glass. The TiO2 shows already the anatase structure.
The characterizations of the samples have confirmed the nanotube presence in the aqueous synthesis, and in the alcoholic synthesis at 300°C and 400°C but not at 500°C.
The highest conductivity is obtained from the syntheses in alcohol and the calcination at 300°C. That sample does not exhibit a high photoactivity because of its poor crystallinity.
The films without MWCNTs are superhydrophilic but the contact angle increase with the incorporation of MWCNTs. The superhydrophilicity is lost with MWCNTs introduction.
MWCNTs increase the roughness, the thickness and the electron transfer of the TiO2 matrix. This induces an enhancement of the photoactivity under UV. The comparison between the two syntheses shows that the alcoholic synthesis (400°C) is the best for pure film. When MWCNTs are introduced, the improvement is higher in the case of aqueous synthesis than in the case of alcoholic synthesis. The aqueous synthesis leads to more active photocatalysts than the alcoholic synthesis.