**6. Conclusions and development**

**Figure 10.** Diagrammatic sketch for the formation of blue TiO<sup>2</sup>

chemical performances [33] (**Figure 10**).

236 Titanium Dioxide - Material for a Sustainable Environment

*5.6.2. Ice-water quenching*

Ti3+ self-doped TiO<sup>2</sup>

consisted of various TiO<sup>2</sup>

for charge separation and H<sup>2</sup>

(2016), Royal Society of Chemistry [9].

analysis and characterization results showed that the synergetic action of the special TiO<sup>2</sup> (B) phase, Ti3+ self-doping, and the 1D rod-shaped single-crystalline nanostructure resulted in a narrowed bandgap of 2.61 eV, which enhanced the photocatalytic and photoelectro-

generation in blue P25 (green part: ordered TiO<sup>2</sup>

**Figure 9.** (a) Comparison of the hydrogen generation and cycling performance of 0.5 wt% platinized P25, nonplatinized P25 and nonplatinized blue P25 after 1 day of continuous reaction using methanol as a sacrificial agent. A simulated full solar spectrum was used as the excitation source, which produced approximately 100 mW cm−2 in the samples, which

Liu et al. applied ice-water quenching as a facile strategy for the synthesis of blue color of

[23]. In the typical process, commercial P25 materials were quenched in

nanocrystals in a 100 mL quartz reactor filled with 70 mL of solution. (b) Proposed mechanism

, gray part: disordered TiO<sup>2</sup>

). Copyright

Chemical Society [33].

(B) single-crystalline nanorod. Copyright (2016) American

In this chapter, blue TiO<sup>2</sup> that has a low energy bandgap is introduced as an advanced semiconducting material for possible applications in the visible-light-driven photocatalysis. A variety of preparation methods for blue TiO<sup>2</sup> photocatalysts with Ti3+ states of a high oxygen defect density have been successfully introduced. For the synthesis of the blue TiO<sup>2</sup> in the applications of photocatalysis, hydrogenation method using TiO<sup>2</sup> with hydrogen at 500°C or with hydride reducing agent at 450°C, hydrothermal method using Ti precursors or Zn powder reducing agent under HF solvent, electrochemical reduction method using anodizing TiO2 at 60 and 80 V and then annealing at 450°C, and metal reduction method using Al at 500°C, Na and NaCl solid milling, or Li-EDA solution at room temperature and atmospheric pressure. For the preparation of blue TiO<sup>2</sup> , the most recently developed metal solution room temperature method can give phase-selective reduction between the anatase and rutile TiO<sup>2</sup> phases. For the first time, the phase selective "disordered rutile and crystalline anatase" P<sup>25</sup> TiO2 nanoparticles are reported, which turns out that it is a very effective photocatalyst for hydrogen evolution reaction and removal of algae under solar irradiation. However, how to quantitatively control surface defects and the properties of the interface between the order and disorder surface layer still remain as important challenges to understand the true physicochemical properties of blue TiO<sup>2</sup> .

As mentioned in the introduction, in the near future, we would like to further address beneficial applications in clean energy conversion and storage media and protecting the environment, including the hydrogen evolution reaction, carbon dioxide reduction, and degradation of pollutants by using noble blue TiO<sup>2</sup> under visible light.

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