Polymeric Adsorption of Methylene Blue in TiO 2 Colloids—Highly Sensitive Thermochromism and Selective Photocatalysis

The polymeric adsorption of methylene blue (MB) on a TiO 2 surface is reported. The MB molecule on the TiO 2 surface mainly exists as the H‐trimeric adsorption state, which results in the MB@TiO 2 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 TiO 2 surfaces. The trimer–monomer equilibrium is highly sensitive on temperature changes, showing an interesting reversible thermochromism. The MB@TiO 2 polymeric sol can be photodegraded under UV illumination without destroying the equilibrium of trimer–monomer. Compared with anionic methyl orange, the TiO 2 colloid hydrosol shows highly selective photocatalysis of MB and other cationic dyes, including crystal violet, methylene green, and victoria blue B. The MB@TiO 2 polymeric sol is stable under visible‐light illumination because interfacial transfer of electrons does not exist between MB and TiO 2 .