Polymeric Adsorption of Methylene Blue in TiO2 Colloids-Highly Sensitive Thermochromism and Selective Photocatalysis

The polymeric adsorption of methylene blue (MB) on a TiO2 surface is reported. The MB molecule on the TiO2 surface mainly exists as the H-trimeric adsorption state, which results in the MB@TiO2 polymeric sol. The trimeric adsorption leads to a remarkable blueshift of visible-light adsorption of MB. Electrostatic attraction is important for trimeric adsorption of MB on TiO2 surfaces. The trimer-monomer equilibrium is highly sensitive on temperature changes, showing an interesting reversible thermochromism. The MB@TiO2 polymeric sol can be photodegraded under UV illumination without destroying the equilibrium of trimer-monomer. Compared with anionic methyl orange, the TiO2 colloid hydrosol shows highly selective photocatalysis of MB and other cationic dyes, including crystal violet, methylene green, and victoria blue B. The MB@TiO2 polymeric sol is stable under visible-light illumination because interfacial transfer of electrons does not exist between MB and TiO2.