**5.5. Phase-selective room-temperature solution processing**

Until now, numerous methods to prepare blue TiO<sup>2</sup> have been reported, but all of them require high-temperature processing. Due to high-temperature processing, a phase-selective reduction between the anatase and rutile TiO2 phases is almost impossible. For the first time, phase-selective "disorder engineered" Degussa P25 TiO2 nanoparticles using simple room temperature solution processing was demonstrated as a very effective method to prepare modulatory TiO2 [9]. The blue-colored TiO<sup>2</sup> nanoparticles were obtained by using a strong reducing agent consists of lithium in ethylenediamine (Li-EDA), which can disorder only the white rutile phase of P25, while well maintaining white anatase TiO2 [9]. Firstly, 14 mg metallic Li foil was dissolved in 20 ml ethanediamine to form a 1 mmol/ml solvated electron solution. Two hundred milligram of Degussa P25 (anatase, size: ~25 nm, rutile, size: ~140 nm, P25, size: 20–40 nm) was prepared after thorough drying and then added into the abovementioned solution and stirred for several days depending on the application. After sufficient reaction, the excess electrons and formed Li salts were quenched by slowly adding HCl into the mixture. Finally, the blue-colored TiO<sup>2</sup> nanoparticles were thoroughly rinsed by deionized water several times and dried at room temperature in a vacuum oven [9].

In their study, the blue TiO<sup>2</sup> showed drastically enhanced visible and near-infrared light absorption by induced abundant order/disorder junctions at the surface from selective disorder engineering, which means that it has well charge separation efficiency through type-II bandgap alignment and can effectively promote strong hydrogen evolution surface reaction [9]. Therefore, when the phase-selective disorder engineering of P25 TiO2 nanoparticles as photocatalysts were used, they exhibited high stability and a high hydrogen evolution rate of 13.89 mmol h−1 g−1 using 0.5 wt% Pt (cocatalyst) and 3.46 mmol h−1 g−1 without using any cocatalyst under simulated solar light (**Figure 9**) [9].
