Open access peer-reviewed chapter

Preparation of Blue TiO2 for Visible-Light-Driven Photocatalysis

By Jianmin Yu, Chau Thi Kim Nguyen and Hyoyoung Lee

Submitted: October 5th 2017Reviewed: December 12th 2017Published: June 27th 2018

DOI: 10.5772/intechopen.73059

Downloaded: 563

Abstract

Titanium dioxide (TiO2), which is regarded as a semiconductor photocatalyst, has drawn attention in the applications of photocatalysis, including hydrogen evolution reaction, carbon dioxide reduction, pollutant degradation, and biocatalytic or dye-sensitized solar cells due to its low toxicity, superior photocatalytic activity, and good chemical stability. However, there are still some disadvantages such as too large energy bandgap (~3.34 eV and ~3.01 eV for anatase and rutile phases, respectively) in the absorbance of all ranges of lights, which limits the photoelectrochemical performance of TiO2. Herein, we like to introduce photocatalytic blue TiO2 that is obtained by the reduction of TiO2. The blue TiO2 consists of Ti3+ state with high oxygen defect density that can absorb the visible and infrared as well as ultraviolet light due to its low energy bandgap, leading to enhance a photocatalytic activity. This chapter covers the structure and properties of blue TiO2, its possible applications in visible-light-driven photocatalysis, and mainly various synthetic methods even including phase-selective room-temperature solution process under atmospheric pressure.

Keywords

  • blue titanium dioxide
  • black titanium
  • synthesis method
  • photocatalysis
  • visible light

1. Introduction

TiO2 is an extraordinarily versatile material. In 1964, Kato et al. used a TiO2 suspension for the photocatalytic oxidation of tetralin (1,2,3,4-tetrahydronaphthalene) [1]. In 1972, the “Honda-Fujishima Effect” first described by Fujishima and Honda intensively promoted the photocatalytic field [2]. This discovery led to a new application of TiO2 in water splitting using solar energy as the driving force of the process as well as solar energy conversion.

To date, TiO2 nanomaterials have attracted the interest of many scientists. The focus is to modify TiO2 structural properties or to combine supportive materials to demonstrate that TiO2 nanomaterials are excellent photocatalysts, which can be used as dopants in novel metal-TiO2 systems such as Pt-doped TiO2 [3], Au-doped TiO2, or graphene/TiO2/carbon dot composites developed as visible light photocatalysts [3, 4].

In this chapter, we focus on blue TiO2 as a visible-light-driven photocatalyst and its preparation methods. The blue TiO2 nanomaterial contains Ti3+ with an abundant oxygen vacancy, which can absorb visible and infrared light as well as UV light, producing more electrons and holes and also facilitating better electrical conductivity than pristine TiO2 [5]. In the future, we would like to further address the beneficial applications in clean energy storage media and protecting the environment, including the hydrogen evolution reaction, carbon dioxide reduction, and degradation of pollutants by using noble blue TiO2 under visible light.

2. General structure and properties of TiO2

TiO2 belongs to the transition metal oxide family. There are four different polymorphs of TiO2 found in nature such as anatase (tetragonal), rutile (tetragonal), brookite (orthorhombic), and TiO2 (B) (monoclinic) [6], the most important of which are anatase and rutile. With calcination at high temperatures exceeding ~600°C, the brookite and anatase polymorphs will transform into the thermodynamically stable rutile polymorph [5].

The tetragonal anatase bulk unit cell has dimensions of a = b = 0.3733 nm and c = 0.9370 nm, and the rutile bulk unit cell has dimensions of a = b = 0.4584 nm, and c = 0.2953 nm (Table 1). In both structures, the octahedral distortions create the basic building units [7, 8]. The lengths and angles of octahedral coordinated Ti atoms, therefore, dictate stacking in both structures, as shown in Figure 1.

Table 1.

Crystal structure data for TiO2 Copyright (2014), Elsevier [15].

Figure 1.

Bulk structures of anatase and rutile TiO2. Copyright (2003), Elsevier [18].

3. The advanced structure and properties of blue TiO2

Zhang et al. [9] discovered that the color and crystalline phase of white P25 (70% anatase and 30% rutile) changed into blue color by the treatment of lithium in an ethylenediamine (Li-EDA) solution, which is the first achievement in making blue TiO2 under atmospheric pressure at room temperature in solution and also the phase-selective reduction between anatase and rutile TiO2 phases. They showed that the white anatase TiO2 phase was not changed, while the rutile TiO2 phase changed into black color. In the case of P25 TiO2, the blue colored TiO2 appeared as a result of the combination of white and black colors (Figure 2) [9].

Figure 2.

Schematics of TiO2 (white P25) (left) and blue TiO2 crystals (right). The black color corresponds to the visual color of the reduced rutile TiO2. Copyright (2016), Royal Society of Chemistry [9].

The unit cell parameters and nanocrystalline size profiles of white P25 and blue TiO2 are shown in Table 2. These results show that a slight change occurred along the a and b directions, but there was significant expansion in the c direction, and as a result, the unit cell volume expanded significantly as well [10].

Table 2.

Unit cell parameters of TiO2 (white P25) and blue TiO2 Copyright (2014), American Chemical Society [10].

Recent publications showed that the morphology of TiO2 materials resulted in differences of the enhanced photocatalytic activity for the production of hydrogen between the {101} and {001} facets of anatase tetragonal bipyramidal nanocrystals [11, 12, 13]. Based on the XRD simulation, the length and width of the peaks were calculated to confirm the percentages of the {101} and {001} facets (Figure 3). Their research well defined the optimum nanosize as well as the shape of TiO2 crystals, suggesting that the {101} facets are more photocatalytically active than the {001} facets for the evolution of H2 (up to 2.1 mmol h−1 g−1) under simulated solar illumination, while the blue coloration results from oxygen vacancies in the TiO2 lattice [12].

Figure 3.

XRD patterns of different shapes. Experimental (thick black lines) and simulated (thin colored lines) plots for TiO2 nanocrystals. The insets showed accurate percentages of the {001} and {101} facets of atomistic models. Copyright (2012), American Chemical Society [12].

The blue TiO2 has excellent absorption over a much wider spectral range than white TiO2 due to the excitation of conduction band electrons. Therefore, it should exhibit much better photocatalytic activity under visible light or the full spectrum of solar irradiation (Figure 4) [9, 14].

Figure 4.

Color change of white P25 (left) to blue TiO2 (right) and UV-vis absorption spectra of pristine TiO2 (P-TiO2) and reduced anatase TiO2 (R-TiO2). Copyright (2017), American Chemical Society [14].

4. Electronic properties of blue TiO2 in photocatalysis

Semiconductor materials, TiO2 in particular, are widely used in the applications of photocatalysis. As shown in Figure 5, the reduction potential of photogenerated electrons is defined by the energy level at the bottom of the conduction band (CB), while the oxidizing ability is the energy level at the top of the valence band (VB). Because the CB energy level of TiO2 is higher than the reduction potential levels of NHE references, semiconductors as well as TiO2 nanomaterials can be used as a catalyst for hydrogen evolution, CO2 conversion, or pollutant degradation [15].

Figure 5.

Bandgap of TiO2 and some photocatalysts with respect to the redox potential (vs. NHE) values of different chemical species measured at a pH of 7. Copyright (2014), Elsevier [11, 15, 16].

Photocatalytic reactions occur as a material interacts with light, which provide higher energy than the bandgap of the semiconductor to create reactive oxidizing species, leading to the photocatalytic transformation of a compound.

The basics of the photocatalytic process can be summarized as follows:

  1. TiO2 absorption of photons with sufficient energy and generation of electron-hole pairs.

  2. Separation and transport of electron-hole pairs with electrons excited from the valence band (VB) to CB.

  3. Chemical reaction on the surface-active sites with charge carriers.

Meanwhile, electron-hole recombination is also possible depending on the competition between these processes.

Blue TiO2 nanomaterials can overcome the limitations to enhance the photocatalytic performance due to the formation of oxygen vacancies (supports many free carriers charges). The oxygen vacancy is a positive charge. Then, Ti3+ from the center shifts away from the oxygen vacancy position, leading to an advanced sublevel electric state and excellently trapped holes, preventing the recombination of electrons and holes, even with the lower energy bandgap irradiation (~2.7 eV) compared to P25 (3.2 eV). Blue TiO2 could generate electrons in the wide open region of irradiation such as solar light, which contains most visible and infrared wavelengths as well as UV light [17, 19] (Figure 6).

Figure 6.

Schematic diagram of Ti3+ self-doped TiO2 mechanism for visible light photocatalysis.

5. Synthesis of blue TiO2 nanomaterials for photocatalysis

5.1. Hydrogenation synthesis

H2 is the most common reagent used for the hydrogenation of TiO2, which can react with the lattice oxygen, leading to the formation of abundant oxygen vacancies and Ti3+ in TiO2 due to its facile activation by thermal or electromagnetic energy [4, 19]. The annealing time changes with the annealing temperature, where the blue color was maintained up to a longer time at 500°C. It readily changed to pale gray at 600°C due to the high concentration of Ti3+ in the bulk at the early stage of hydrogenation, which may absorb oxygen molecules and lead to O as a major species on the surface after prolonged hydrogenation (Figure 7) [20, 21]. In addition, hydrogenation processes require harsh synthetic conditions and/or a dangerous production process [4, 10, 19, 21, 22, 23, 24, 25]. Therefore, H2 is introduced using different reducing agents such as NaBH4 and TiH2 [4, 26, 27] instead of an external dose of hydrogen gas. TiH2 as a solid solution of hydrogen in Ti and P25 was mixed and sealed in a quartz tube and calcined at 450°C for 10 h. After discarding most of the unreacted TiH2 sediments, HCl and H2O2 solutions were then introduced to completely remove the residual TiH2, during which the TiH2 dissolved and a yellow solution was formed. After centrifugation and thorough washing, TiH2 was completely removed, and a well-crystallized bluish sample (TiO2−x: H) was obtained [4]. Qiu et al. found that the TiO2−x: H can efficiently enhance the visible- and infrared-light absorption and improve photocatalytic degradation of methyl orange (MO) and hydrogen production via water splitting by H doped into the well-crystallized lattice, which means that might be localized states in the bandgap was offered and has a relatively low recombination rate of electrons and holes. Moreover, we should note that the low concentration of hydrogen atoms in hydrogenated titania was found to be a unfavorable factor affecting the photocatalytic activity [21].

Figure 7.

Photograph of H-aTiO2 samples prepared with a H2 gas flow at temperatures of 500–700°C. Gradual changes in color from blue to gray to a different degree are observed, depending on annealing temperatures and annealing time. Copyright (2013) American Chemical Society [21].

5.2. Hydro(solvo)thermal method

Hydrothermal and solvothermal methods have received some attention due to their simple and low-cost production routes and are suitable for large-scale production [28, 29]. Zhu et al. reported the synthesis of novel blue colored TiO2 with abundant defects through a one-step solvothermal method using TiCl3 and TiF4 as precursors. The introduction of Ti4+ in the reaction system inhibits the oxidation of Ti3+ during the solvothermal treatment.

Ti3++oxygen speciesTi4+E1

This process is governed by the Le Chatelier’s principle. The oxygen vacancy formation dominantly resulting from Ti3+ will not be completely oxidized during the solvothermal process. Moreover, leaving behind a high concentration of bulk Ti3+ defects is very favorable for visible light photocatalytic reactions [29]. In addition, Fang et al. synthesized a variety of reduced TiO2 samples by using Zn powder as the reducing agent and HF as the solvent for the stabilization of the formed Ti3+ species and oxygen vacancies in a simple one-pot hydrothermal process. At the same time, it should be noted that the Ti3+ introduced by Zn reduction is not stable and is likely to be oxidized in air [28].

5.3. Electrochemical reduction synthesis

Zhang et al. demonstrated that that electrochemical reduction method is a facile and effective strategy to induce in situ self-doping of Ti3+ into TiO2 and the self-doped TiO2 photoelectrodes showed remarkably improved and very stable water splitting performance [30]. The hierarchical TiO2 NTs were fabricated by a two-step anodization process. In the first step of anodization, the as-prepared Ti sheet as an anode was anodized at 60 V for 30 min in electrolytes consisted of 0.5 wt% NH4F in EG solution with 2 vol% water and a Pt mesh (Aldrich, 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, the prepared TiO2 NT samples were cleaned and annealed in air at 450 degree for 1 h with a heating rate of 5 degree min−1 [30]. In the electrochemical reduction processes, the TiO2 NTs 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)) in the supporting electrolyte of 1 M Na2SO4 for 30 min [30]. The electronic transition from the 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 self-doped TiO2, which helps explaining the observed color change from the prime white of the TiO2 NTs to the light blue of the ECR-TiO2 NTs [30].

5.4. Metal reduction method

Zheng et al. proposed an approach to synthesize blue TiO2 nanoparticles with abundant oxygen deficiencies/Ti3+ species through Al reduction of TiO2 nanosheets at 500°C [31]. Zhang 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 absorption compared to pristine TiO2 (Figure 8) [32]. In a typical reduction process, crystalline TiO2 was milled with Na/NaCl fine powders with different weight ratio at a series of milling rates such as 80, 120, 150, and 180 rpm at room temperature under argon atmosphere for 0.25–4 h. After the Na and NaCl was removed, the obtained TiO2−x products were dispersed in a small amount of deionized water and then vacuum-dried at room temperature to obtain TiO2−x powders [32]. Moreover, the obtained TiO2−x with a high surface area can be employed as an effective support for Ru particles and the Ru/TiO2−x catalyst exhibited superior activity in the catalytic hydrogenation of N-methylpyrrole [32].

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

5.5. Phase-selective room-temperature solution processing

Until now, numerous methods to prepare blue TiO2 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 TiO2 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 TiO2 nanoparticles were thoroughly rinsed by deionized water several times and dried at room temperature in a vacuum oven [9].

In their study, the blue TiO2 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].

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 TiO2 nanocrystals in a 100 mL quartz reactor filled with 70 mL of solution. (b) Proposed mechanism for charge separation and H2 generation in blue P25 (green part: ordered TiO2, gray part: disordered TiO2). Copyright (2016), Royal Society of Chemistry [9].

5.6. Other methods

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

Ti3+ self-doped blue TiO2 (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 TiO2 (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 analysis and characterization results showed that the synergetic action of the special TiO2 (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).

Figure 10.

Diagrammatic sketch for the formation of blue TiO2 (B) single-crystalline nanorod. Copyright (2016) American Chemical Society [33].

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 TiO2 [23]. In the typical process, commercial P25 materials were quenched in 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-TiO2 (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].

Figure 11.

(A) Digital pictures of q-TiO2 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 TiO2 sample on filter paper, which shows the blue color in the inner side of the sample [23]. Copyright (2017) American Chemical Society.

6. Conclusions and development

In this chapter, blue TiO2 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 TiO2 photocatalysts with Ti3+ states of a high oxygen defect density have been successfully introduced. For the synthesis of the blue TiO2 in the applications of photocatalysis, hydrogenation method using TiO2 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 TiO2, the most recently developed metal solution room temperature method can give phase-selective reduction between the anatase and rutile TiO2 phases. For the first time, the phase selective “disordered rutile and crystalline anatase” P25 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 TiO2.

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 TiO2 under visible light.

Acknowledgments

This work was supported by IBS-R011-D.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Jianmin Yu, Chau Thi Kim Nguyen and Hyoyoung Lee (June 27th 2018). Preparation of Blue TiO2 for Visible-Light-Driven Photocatalysis, Titanium Dioxide - Material for a Sustainable Environment, Dongfang Yang, IntechOpen, DOI: 10.5772/intechopen.73059. Available from:

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