Effect of TiO2 Nano-particle Modified Sol-Gel Coating and Cr-free Conversion Coating of Galvanized Steel on the Photocatalytic Activity

Abstract

There has been much effort to utilize the solar energy in a chemical reaction because the so-called photocatalysis is very economic and environmentally friendly technique. A photocatalyst is the material capable of utilizing sunlight to convert the gaseous materials, decompose chemical pollutants and split water to produce hydrogen. TiO2 has been most widely used due to its low cost, high stability and most efficient photoactivity. TiO2 photocatalyst can be applied on the various substrates such as glass, quartz, tile, stainless steel and paper. However, galvanized steel(GI) has been rarely studied as a substrate for immobilizing a photocatalyst. GI has many advantages such as good mechanical property, high anti-corrosion performance and good material reliability. In this study, GI was employed as the substrate of TiO2 photocatalyst coating. In addition, the sol-gel based wet coating method was used to immobilize TiO2 layers on the substrate. The commercial crystalline TiO2 nano particles (Evonik, P-25) were incorporated into a precursor sol in order to enhance the photocatalytic activity of TiO2 films. Nevertheless, anatase TiO2 has a wide band gap of 3.2 eV, so it can only absorb the UV portion of the solar spectrum. To utilize a larger fraction of the solar spectrum, TiO2 should be modified to make the band gap smaller. Recently, it has been reported that TiO2 mixed with rare earth elements, such as La, Nd, Eu and Ce, showed photoactivity in the visible light. Among them, Ce showed the best activity in visible region consistently when mixed with TiO2. For that reason, cerium oxide layer was deposited on GI surface as an underlying layer of TiO2 photocatalytic film. In the study, the effect of cerium conversion coating on the photocatalytic activity of TiO2 layer was mainly discussed. The effect of incorporation of the commercial crystalline TiO2 nano particles (P-25) into TiO2 sol-gel layer was also evaluated. As increasing the loading concentration of the TiO2 nano particles, the agglomerated TiO2 particles were clearly appeared on the surface of the sol-gel film. Many cracks were observed in all substrates. Crack formation of TiO2 films could not be avoided because of an internal stress build up and film volume shrinkage upon thermal treatment(300oC for 2hours). The cracks were mainly formed at valley areas on rough surface of galvanized steel because of relatively thicker film in the areas. On the other hand, the incorporation of crystalline TiO2 particle was helpful to form a crack-less film. It seems to be due to the stress relaxation at the interface between particle and film matrix. However, the high concentration of TiO2 nano particle deteriorated the sol-gel film integrity and thus formed a crack again. The optical absorption spectra of TiO2 nano particle modified sol-gel film deposited on bare GI and cerium conversion coated GI showed an absorption onset at 380 nm for TiO2 nano particle modified sol-gel layer. The higher the loading concentration of TiO2 nano particle, the higher the absorption of UV light (below 380 nm) is. Obviously, the photocatalytic film formed on the cerium conversion coated GI showed a red shift. CeO2 is an n-type semiconductor whose band gap is around 2.8 eV. Therefore, the cerium oxide layer would be responsible for the observed red shift about 50 nm on the basis of the coupled semiconductor (TiO2-CeO2) mechanism for the visible light activity. The photocatalytic activity of the coated sample was evaluated by measuring the rate of photodegradation of methyl orange in aqueous solution. The photo-induced degradation of methyl orange was monitored in the presence of coated sample under the ultra-violet (UV) light for 4 hour. It was demonstrated that the photocatalytic activity was enhanced with increasing TiO2 nano particle loading concentration. Moreover, the existence of the cerium oxide intermediate layer formed on GI substrate provided better photocatalytic activity of TiO2 layer. The result is in good agreement with the UV-visible light absorption behaviors of the samples. The enhancement in the photocatalytic activity may come from the hetero-junctions of TiO2-CeO2 in the coupled photocatalysts. In conclusion, high TiO2 nano particle loading of TiO2 coating layer on GI in combination with the CeO2 conversion coating layer showed the best photocatalytic activity as well as the increased corrosion resitance.