**5.6. Other methods**

**Figure 8.** The route for the preparation of Ru/TiO2−x; photographs of P25 nanocrystals and TiO2x. (a) P25 nanocrystals, (b) TiO-1-80-0.5, (c) TiO-1-80-1, (d) TiO-1-120-4, (e) TiO-1-150-4, (f) TiO-1-180-4, (g) TiO-2-180-4, (h) TiO-3-180-4, and (i) TiO-

Zhang et al. demonstrated that that electrochemical reduction method is a facile and effective

trodes showed remarkably improved and very stable water splitting performance [30]. The

anodization, the as-prepared Ti sheet as an anode was anodized at 60 V for 30 min in elec-

100 mesh) as a cathode, respectively. After the as-grown nanotube layer was ultrasonically removed in DI water, the second step of anodization was performed at 80 V for 5 min. Then,

as the working electrode with an AgCl electrode and a Pt mesh formed a typical three-electrode system under a negative potential (0.4 V vs. the reversible hydrogen electrode (RHE))

valence band to the Ti3+ induced interbands and/or from the energy band levels to the conduction band was considered to contribute to enhance the absorption in the visible region in the

et al. developed a reduction method to synthesize a series of TiO2−x samples with their color changing from white to dark blue, which possess a much higher surface area and visible light

heating rate of 5 degree min−1 [30]. In the electrochemical reduction processes, the TiO<sup>2</sup>

SO<sup>4</sup>

NTs were fabricated by a two-step anodization process. In the first step of

NT samples were cleaned and annealed in air at 450 degree for 1 h with a

, which helps explaining the observed color change from the prime white of

NTs [30].

and the self-doped TiO<sup>2</sup>

for 30 min [30]. The electronic transition from the

(**Figure 8**) [32]. In a typical reduction process, crystalline

nanoparticles with abundant oxy-

nanosheets at 500°C [31]. Zhang

F in EG solution with 2 vol% water and a Pt mesh (Aldrich,

photoelec-

NTs

4-180-4. Reprinted with permission from [32]. Copyright (2017) The Royal Society of Chemistry.

**5.3. Electrochemical reduction synthesis**

234 Titanium Dioxide - Material for a Sustainable Environment

trolytes consisted of 0.5 wt% NH4

in the supporting electrolyte of 1 M Na<sup>2</sup>

NTs to the light blue of the ECR-TiO<sup>2</sup>

Zheng et al. proposed an approach to synthesize blue TiO<sup>2</sup>

gen deficiencies/Ti3+ species through Al reduction of TiO<sup>2</sup>

hierarchical TiO2

the prepared TiO2

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

**5.4. Metal reduction method**

absorption compared to pristine TiO2

the TiO2

strategy to induce *in situ* self-doping of Ti3+ into TiO2

#### *5.6.1. Sol-gelation hydrothermal technique and subsequent reduction treatment method*

Ti3+ self-doped blue TiO<sup>2</sup> (B) single-crystalline nanorods (b-TR) were synthesized *via* three steps, in which the titanium dioxide powder was prepared via the sol-gelation approach followed by hydrothermal treatment. Blue TiO<sup>2</sup> (B) single-crystalline nanorods were obtained by further annealing at 350°C in Ar [33]. Under visible light illumination, the degradation rate of RhB reached 97.01% by b-TR and the photocatalytic hydrogen evolution rate was as high as 149.2 μmol h−1 g−1 under AM 1.5 irradiation [33]. The mechanistic

**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 consisted of various TiO<sup>2</sup> nanocrystals in a 100 mL quartz reactor filled with 70 mL of solution. (b) Proposed mechanism for charge separation and H<sup>2</sup> generation in blue P25 (green part: ordered TiO<sup>2</sup> , gray part: disordered TiO<sup>2</sup> ). Copyright (2016), Royal Society of Chemistry [9].

ice-water after pre-annealing at a high temperature. Then, the obtained powders were filtered

show that the color changed to pale blue when subjected to a temperature higher than 900°C,

that the d-d might be a transition from Ti3+ band gap states to their resonant excited states and extended light absorption together with near-IR absorption [23]. In addition, the surface distortion and the associated oxygen defects were considered to be contributed to the substantially enhanced photocatalytic activity [23]. It should be pointed out that the quenched

cannot absorb much visible light, which means that the photoexcited electrons at the Ti3+

conducting material for possible applications in the visible-light-driven photocatalysis. A

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 anodiz-

500°C, Na and NaCl solid milling, or Li-EDA solution at room temperature and atmospheric

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>

 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 physi-

at 60 and 80 V and then annealing at 450°C, and metal reduction method using Al at

defect density have been successfully introduced. For the synthesis of the blue TiO<sup>2</sup>

applications of photocatalysis, hydrogenation method using TiO<sup>2</sup>

.

that has a low energy bandgap is introduced as an advanced semi-

(quenched TiO2

sample on filter paper,

after ice-water quenching (**Figure 11**), implying

samples prepared from the commercial P25 powders after being

Preparation of Blue TiO2 for Visible-Light-Driven Photocatalysis

http://dx.doi.org/10.5772/intechopen.73059

photocatalysts with Ti3+ states of a high oxygen

, the most recently developed metal solution room

)

237

in the

with hydrogen at 500°C

and dried at 80°C for 12 h for further use [23]. Digital pictures of q-TiO<sup>2</sup>

and n-TiO2

subjected to pre-annealing at different temperatures. (B) Digital picture of just-quenched TiO<sup>2</sup>

which shows the blue color in the inner side of the sample [23]. Copyright (2017) American Chemical Society.

which confirmed the presence of Ti3+ in TiO2

**Figure 11.** (A) Digital pictures of q-TiO<sup>2</sup>

defect level cannot transfer outside [23].

**6. Conclusions and development**

variety of preparation methods for blue TiO<sup>2</sup>

pressure. For the preparation of blue TiO<sup>2</sup>

cochemical properties of blue TiO<sup>2</sup>

In this chapter, blue TiO<sup>2</sup>

TiO2

ing TiO2

TiO2

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

#### *5.6.2. Ice-water quenching*

Liu et al. applied ice-water quenching as a facile strategy for the synthesis of blue color of Ti3+ self-doped TiO<sup>2</sup> [23]. In the typical process, commercial P25 materials were quenched in

**Figure 10.** Diagrammatic sketch for the formation of blue TiO<sup>2</sup> (B) single-crystalline nanorod. Copyright (2016) American Chemical Society [33].

**Figure 11.** (A) Digital pictures of q-TiO<sup>2</sup> and n-TiO2 samples prepared from the commercial P25 powders after being subjected to pre-annealing at different temperatures. (B) Digital picture of just-quenched TiO<sup>2</sup> sample on filter paper, which shows the blue color in the inner side of the sample [23]. Copyright (2017) American Chemical Society.

ice-water after pre-annealing at a high temperature. Then, the obtained powders were filtered and dried at 80°C for 12 h for further use [23]. Digital pictures of q-TiO<sup>2</sup> (quenched TiO2 ) show that the color changed to pale blue when subjected to a temperature higher than 900°C, which confirmed the presence of Ti3+ in TiO2 after ice-water quenching (**Figure 11**), implying that the d-d might be a transition from Ti3+ band gap states to their resonant excited states and extended light absorption together with near-IR absorption [23]. In addition, the surface distortion and the associated oxygen defects were considered to be contributed to the substantially enhanced photocatalytic activity [23]. It should be pointed out that the quenched TiO2 cannot absorb much visible light, which means that the photoexcited electrons at the Ti3+ defect level cannot transfer outside [23].
