Fundamental Studies on “Blue Titania” Under Oxygen-Free Conditions: Color Evolution and Stability as Function of the Crystal Phase

“Blue titania” is widely applied in photocatalysis, but comparative studies on the influence of the TiO₂ phase on color evolution and stability are rare. Here, this knowledge gap is closed by evaluating comparatively the formation of blue titania from pure anatase, pure rutile, P25 (∼80% anatase, 20% rutile), and partially amorphous materials. The blue color is attained by photocatalytic reaction of 2-propanol on TiO₂ under oxygen-free conditions. It is additionally evaluated how different irradiation conditions and the intentional presence of oxygen influence TiO₂ coloration. A high surface area is found to lead to rapid discoloration. Rutile is more stable against discoloration in air than an anatase of similar particle size. The mixed-phase material P25 shows exceptional behavior: It reaches the strongest blue color and retains the color the longest time under ambient conditions. In the presence of oxygen, photooxidation of 2-propanol still causes a blue color of TiO₂, which is explained by a slow transfer of photogenerated electrons over the interface. Insights gained with standard materials is complemented by experiments with an engineered aerogel material previously shown to be remarkably active in the storage of charge carriers. The influence of restricted oxygen diffusion in pressed aerogel pellets is also discussed.