Phase Control of TiO₂ Photocatalyst

Abstract

TiO₂ that exists in different phases such as anatase, rutile, and brookite is one of the most promising photocatalysts. These phases show different properties and photocatalytic performances. It is well known that mixed-phase TiO₂ has enhanced photocatalytic activity compared to pure-phase TiO₂. In the past two decades, many research works focusing on the synthesis of different kinds of mixed-phase TiO₂ and their applications to photocatalysis have been done. In this chapter, we introduce three main types of TiO₂ phases as mentioned above, containing their structural properties, stability, phase transformation, and photocatalytic activity. Then we pay more attention to the synthesis of the mixed-phase TiO₂ and detailedly introduce six preparation methods, which are hydrothermal method, solvothermal method, microemulsion-mediated solvothermal method, sol–gel method, solvent mixing and calcination method, and high-temperature calcination method. After that, we comprehensively highlight three different kinds of applications of the mixed-phase TiO₂ in the photocatalysis field, including photocatalytic production of hydrogen, reduction of CO₂ with water, and degradation of organic pollutants. As the photocatalytic activity of the mixed-phase TiO₂ is usually higher than the single-phase TiO₂, we discuss the mechanism for the enhancing effects of the mixed phases. Due to the limit of the present science and technology, the challenges of mixed-phase TiO₂ still remain. In the end, we summarize the existing problems of this kind of nanomaterials and put its application prospects forward.

6.1 Introduction

In recent years, the environmental pollution and energy exhaustion problems are attracting more and more attention of mankind on a global scale. Due to the increase of population and industrial growth, the energy consumption has been accelerating, and a large amount of toxic agents and industrial wastes have been released into the air and waterways, leading to energy crisis, global warming, and pollution-related diseases. Thus, nowadays most scientists face the major challenges to develop the environmentally harmonious, ecologically clean, safe, and sustainable chemical technologies, materials, and processes for addressing energy as well as pollution and climatic change.

Although many different kinds of approaches to solve these issues exist, ever since Honda and Fujishima [1] found that TiO₂ photoelectrode could induce cleavage of water into H₂ and O₂ under the irradiation of ultraviolet light, there has been enormous increasing interest in the use of TiO₂ as well as other extended oxide and chalcogenide semiconductors. The organic toxic materials at low concentrations are photocatalytically converted to harmless oxidation products such as CO₂ and H₂O, achieving the purpose of the environmental remediation [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. TiO₂ photocatalyst with a convenient bandgap of 3.2 eV (ca. 400 nm) has many attractive features including high stability, low cost, nontoxicity, good biocompatibility, and good photocatalytic performance in the oxidation of organic pollutants to CO₂ and H₂O. For these reasons, TiO₂-based photocatalysts have drawn much attention for various applications in the fields of energy regeneration and environmental protection.

As mentioned above, due to its excellent properties, lots of systematic in-depth studies of TiO₂ has been performed by the majority of scientists, promoting the application process in various aspects related to energy and environment in recent years. The related research mainly includes studies on photocatalytic mechanism [3,4,5], regulation of crystal structure and morphology [6,7,8,9,10,11,12,13,14,15], improvement of photocatalysis and photoelectric conversion efficiency [16,17,18,19,20,21,22,23,24], and expansion of the optical response range [25,26,27,28,29]. However, traditional single-phase TiO₂ has defects in the photocatalytic performance, which are mainly on account of two aspects: on one hand, owing to the limits of its bandgap, the absorption of visible light is little, causing the inefficient utilization of the sunlight [3,4,5]; on the other hand, the high recombination rate of photo-generated electrons and holes greatly limits the photocatalytic performance of TiO₂ [30, 31].

Researches have reported that modification of TiO₂ by organic dye photosensitization [32,33,34,35,36], noble metal deposition [37,38,39,40,41], or doping [42,43,44,45] or semiconductor compounding [45,46,47,48,49] can effectively enhance the photon utilization and reduce the recombination rate of photo-generated electrons with holes, thereby improving the photocatalytic efficiency of TiO₂. However, those methods have many disadvantages such as low economic effect, poor reaction stability and controllability, and complicated operation process, among others. In these respects, mixed-phase TiO₂ with relatively simple preparation process, low cost, and adjustable crystal type has attracted much attention. In the mixed-phase photocatalyst, the effective separation of the photo-generated electrons and holes, taken place on the two-phase interface, results in the reduction of their recombination rate. Moreover, adding rutile TiO₂ with a narrower bandgap into pure anatase TiO₂ to form mixed-phase TiO₂ of rutile and anatase can enhance the utilization of visible light to some extent, further significantly improving the photocatalytic activity of TiO₂ [50,51,52,53]. These abovementioned advantages of the mixed phase can efficiently enhance the comprehensive properties of TiO₂ photocatalyst. Apart from these researches, other kinds of mixed-phase TiO₂ have also been studied in depth, such as anatase/brookite [54, 55], anatase/brookite/rutile [56], and brookite/TiO₂(B) [57]. The results of these photocatalytic studies have demonstrated that the photocatalytic activity of mixed-phase TiO₂ is higher than that of single-phase TiO₂, which results from the existence of different band positions and the promoted separation rate of photo-generated electron–hole pairs and catalytic “hot spots” at the interface of different phases [51, 56].

In this chapter, first, we briefly introduce three main kinds of TiO₂ phases (anatase, rutile, and brookite), and then we focus on recent advances in the development of mixed-phase TiO₂ photocatalysts, including the synthesis for the mixed-phase catalysts and their applications to various photocatalytic reaction systems such as photocatalytic hydrogen production, photoreduction of CO₂, and photocatalytic degradation of organic pollutants. Following this we discuss the mechanism of enhanced photocatalytic activity of the mixed-phase TiO₂. Finally, the existing problems of the mixed-phase TiO₂ are summarized, and the application prospects of this kind of nanomaterials are outlooked.

6.2 Phases of TiO₂

TiO₂ possesses eight types of crystal phases, which are rutile, anatase, brookite, TiO₂-B, TiO₂-R, TiO₂-H, TiO₂-II, and TiO₂-III [58]. Among them, rutile, anatase, and brookite have been researched mostly for applications because they are naturally occurring oxides of titanium at atmospheric pressure [59, 60]. The other five phases have also been investigated [61,62,63,64]; however, because they are usually formed at high pressure, there is minor significance for practical research and applications. Therefore, in this chapter, we mainly focus on the three phases of rutile, anatase, and brookite.

6.2.1 Structure Properties of Rutile, Anatase, and Brookite

The three kinds of TiO₂ phases possess different crystallographic properties, as shown in Table 6.1 [65].

Table 6.1 Crystallographic properties of anatase, rutile, and brookite TiO₂

In their structures, TiO₆ octahedron exists in a more or less tortuous configuration formed by the fundamental building block made up of a titanium atom surrounded by six oxygen atoms. In the structures of the three kinds of TiO₂ phase, the stacking of the TiO₆ octahedra causes threefold coordinated oxygen atoms. As shown in Fig. 6.1, the structural units in these three TiO₂ crystals form from TiO₆ octahedron basic units and exist in different lattice configurations. In rutile, the chains were formed by TiO₆ octahedra connected by sharing an edge with the c-axis and then interlinked by sharing corner oxygen atoms to form a three-dimensional lattice. Contrarily in anatase, the three-dimensional lattice is formed only through sharing edge bonding among TiO₆ octahedra. It means that the octahedra in anatase share four edges and are arranged in zigzag chains while the octahedra in brookite share both edges and corners and form an orthorhombic structure [66, 67].

Fig. 6.1

Crystalline structure of (a) anatase, (b) brookite, and (c) rutile [68]. (Reprinted with permission from Ref. [68]. Copyright 2010, Elsevier)

Researchers usually use the X-ray diffraction (XRD) experimental method to determine these crystal structures and estimate the crystal grain size of anatase, rutile, and brookite. Anatase peaks in X-ray diffraction are found at θ = 12.65°, 18.9°, and 24.054°; the rutile peaks occur at θ = 13.75°, 18.1°, and 27.2°, while brookite peaks emerge at θ = 12.65°, 12.85°, 15.4°, and 18.1° (θ represents the X-ray diffraction angle) [69, 70].

6.2.2 Stability and Phase Transformation

Compared to the other two types of phases, rutile is the most stable phase. It is almost impossible for rutile to decompose or undergo a phase transformation even at very high temperatures. However, metastable anatase and brookite can be transformed into thermodynamically stable rutile when they are calcined to a certain temperature. There has been extensive research on the phase transfer mechanism of TiO₂ during the calcination process. Shannon [71] proposed that the transformation of anatase to rutile from crystallography includes a nucleation and growth process. At first, rutile nucleates on the surface of anatase and then expands to the bulk. Due to the great diversities between anatase and rutile, the transformation involving the breaking and reforming of bond processes can be reconstructed [72]. During the course of anatase transforming to rutile, the {112} planes in anatase are persisted as the {100} planes in the newly generated rutile. And Ti and O atoms synergistically rearrange in these planes by moving Ti atoms to a new location to form rutile via the breaking of two Ti–O bonds in the TiO₆ octahedron. As a result, the newly formed oxygen vacancies accelerate the transformation; meanwhile, Ti interstitials inhibit the phase transformation. The transformation from anatase to rutile is a nonequilibrium phase transition, which usually occurs at a certain range of temperature (400~1000 °C). During the transformation process, the calcination temperature has a great influence on the impurities, particle size, and surface area of products. Because impurities and processing atmosphere can result in different defect structure, they also strongly influence the temperature and rate of phase transition. Generally, impurities such as the oxides of Li, K, Na, Fe, Ce, and Mn usually promote the phase transformation via increasing the oxygen vacancies; on the contrary, impurities like S, P, and W usually restrain the phase transformation. A reductive atmosphere such as H₂ and Cl₂ can accelerate the transformation, while a conducive atmosphere can inhibit the phase transformation via the formation of Ti interstitials.

Artificial synthesis conduces to the preparation of anatase nanoparticles, especially the synthesis of TiO₂ in aqueous solution [73] for the reason that the energy of the three kinds of TiO₂ phases is quite close. On condition that the nanoparticles are small enough (<13 nm), the small surface free energy will play a decisive role in the phase transformation [74]. For TiO₂ nanocrystals with the size smaller than 11 nm, anatase is the most stable phase, and the rutile phase with nanocrystals size bigger than 35 nm performs the thermodynamic stability. As for brookite phase, its stability lies between the anatase and rutile. In fact, as a metastable phase, the major physical parameters of brookite are between those of anatase and rutile. For example, the bandwidths of anatase and rutile are 3.19 and 3.0 eV, respectively, while that of brookite is 3.11 [75].

6.2.3 Photocatalytic Activity of Rutile, Anatase, and Brookite

Among the three types of phases, anatase exhibits the highest photocatalytic activity, which is on account of the following aspects:

• 1 The bandgap of anatase is 3.19 eV, while that of rutile and brookite are 3.0 eV and 3.11 eV, respectively [62]. Therefore, the electron–hole pair of anatase has more positive or more negative potential, improving the oxidation ability [76].

• 2 The surface of anatase has a stronger adsorption ability for H₂O, O₂, and OH, which conduces to high photocatalytic activity because the adsorption capacity of the surface has a dramatic influence on the photocatalytic activity during the photocatalytic reaction, and strong adsorption capacity benefits to high activity.

• 3 Compared to rutile and brookite, anatase usually exhibits smaller grain size and larger specific surface area in the crystallization process, enhancing the photocatalytic activity.

However, due to the crystallization process greatly influencing the photocatalytic activity, the above rules do not suit for all situations. When the amorphous TiO₂ crystallize, rutile usually forms large grains with poor surface properties and thus exhibits low photocatalytic activity. Under the same conditions, if the rutile can have the same grain size and adsorption ability as anatase, it can also exhibit high photocatalytic activity. For example, Lee et al. [77] found that through the treatment of laser exposure, anatase phase of TiO₂ can transfer to rutile phase without the change of the specific surface area and grain size. This resultant rutile TiO₂ exhibited considerable high photocatalytic activity. Tsai et al. [78] fabricated anatase and rutile TiO₂ via different methods and investigated their photocatalytic activity for the degradation of phenol. It is found that the preparation methods as well as the calcination temperature have a remarkable influence on the photocatalytic activity of TiO₂ catalyzers. Under certain conditions, rutile TiO₂ exhibited very high catalytic activity. Therefore, whether anatase or rutile, the photocatalytic activity of materials greatly depends on the grain size and the surface properties. Apart from that, Ohno et al. [79] demonstrated that the photocatalytic activity of different phase TiO₂ relates to the electron acceptors in the system. When the electron acceptor is O₂, the photocatalytic activity of anatase is relatively higher than that of rutile. And when Fe³⁺ is the electron acceptor, rutile exhibits higher catalytic activity than anatase. This is because when O₂ acts as an electron acceptor, it is quite sensitive to properties of the catalyst materials in the process of the photocatalytic reactions. The surface structure and the low-energy band of rutile may make it show low transfer efficiency of electrons to O₂, and thus rutile exhibits low catalytic activity when O₂ is used as the electron acceptor. Therefore, rutile usually exhibits low photocatalytic activity based on the fact that most researchers use O₂ for the studies of photocatalytic reactions.

Because there have been few studies and experiments on brookite and the general products are mixed-phase TiO₂, including anatase/brookite [80], brookite/rutile [81], or anatase/brookite/rutile instead of pure brookite, the photocatalytic activity of brookite TiO₂ is quite controversial [44].

Recently, lots of studies have demonstrated that the mixed-phase TiO₂ crystals in an appropriate composition ratio exhibit higher photocatalytic activity than that of single-phase TiO₂ crystals. Bacsa et al. [82] found that the single-phase TiO₂ such as pure anatase or pure rutile has relatively lower photocatalytic activity, but a mixture of anatase and rutile in various ratios achieved enhanced photocatalytic activity than 100% anatase or 100% rutile, when the mixed phase in the ratio of 30% rutile and 70% anatase had the best photocatalytic activity. Thus the TiO₂ catalyzer with two kinds of phases has a certain synergistic effect for improving photocatalytic activities. And the commercial photocatalyst P25 with considerable high photocatalytic activity is also a kind of mixed-phase TiO₂ instead of single-phase TiO₂. Therefore, great efforts have been made to study in depth on the synthesis and photocatalysis applications of mixed-phase TiO₂.

6.3 Synthesis of Mixed-Phase TiO₂ Photocatalysts

It has been found that adjusting the experimental parameters of the synthesis can change the structure and properties of the three types of crystals, making the preparation of different mixed-phase TiO₂ come true. At present, there is a large number of methods to fabricate mixed-phase TiO₂, such as pulsed laser deposition (PLD), hydrothermal method, hydrolysis method, solvothermal method, microemulsion-mediated solvothermal method, solvent mixing and calcination method (SMC method), high-temperature calcination method, high-temperature vapor decomposition method, and so on [83]. In the synthesis process of mixed-phase TiO₂, with different preparation methods, the same factor has different effects on the mixed-phase crystals. In all kinds of synthesis methods, the mixed-phase ratio, morphology, and surface properties of photocatalysts are controllable via optimizing the experimental conditions, such as calcination temperature, pressure, concentration, and types of reagents. The ratio of different crystals in an excellent mixed-phase photocatalyst should have the ability to be tuned according to the actual demands, and the product should possess controllable shape as well as uniform dispersion and should be difficult to agglomerate. The preparation conditions and influence parameters of mixed-phase TiO₂ will be discussed in the following subsection, combining concrete synthesis methods.

6.3.1 Hydrothermal Method and Solvothermal Method

The hydrothermal method is used most widely in the laboratory fabrication of TiO₂ nanomaterials [25, 84,85,86,87,88,89,90]. Using aqueous solution as the reaction system, through heating or other means, the target photocatalysts are often synthesized in a sealed reaction vessel (such as an autoclave), which creates an environment of high temperature and high pressure during the hydrothermal process. In this condition, with the increase of the water vapor pressure, the density and surface tension of the materials decrease, and the ion product increases, leading to the dissolution and the recrystallization of the substances which can be dissolved in water at room temperature. The hydrothermal method is one of the liquid-phase chemical synthesis methods, and its main factors influencing the properties of the product are vessel volume, reaction temperature, heating rate, and reaction time.

Chen and Sum et al. [91] fabricated highly crystalline single-phase TiO₂ brookite and two-phase anatase/brookite TiO₂ nanostructures in NaOH solutions via a simple hydrothermal method, using TiS₂ as the precursors. They successfully controlled the phase composition via varying the solution concentration as well as reaction time. In the reaction system, the precursors TiS₂ underwent a hydrolysis reaction as shown in the following two equations:

$$ {\mathrm{Ti}\mathrm{S}}_2+4\ \mathrm{NaOH}\to {\mathrm{Ti}}^{4+}+4\ {\mathrm{OH}}^{-}+2\ {\mathrm{Na}}_2\mathrm{S} $$

(6.1)

$$ {\mathrm{Ti}}^{4+}+\mathrm{x}{\left(\mathrm{OH}\right)}^{-}\to \mathrm{Ti}{\left(\mathrm{OH}\right)}_{\mathrm{x}}^{4-\mathrm{x}} $$

(6.2)

The reaction began with the hydrolysis of TiS₂, forming Ti⁴⁺ ion (Eq. 6.1). Then Ti⁴⁺ reacted with OH⁻ in the solution during the hydrothermal process (Eq. 6.2). The formation of different crystal phases is related to concentration of OH⁻. When the concentration of OH⁻ is low and thus the hydrolysis rate of Ti⁴⁺ is slow, the hydrothermal reactions are more likely to form anatase TiO₂ via connecting Ti⁴⁺ with OH⁻; when the OH⁻ concentration is high, the fast hydrolysis rate of Ti⁴⁺ leads to the formation of mixed-phase TiO₂ nanomaterials of brookite and anatase. The mass fraction of brookite (B) and anatase (A) can be calculated by the following two formulas [92]:

$$ {W}_{\mathrm{B}}={K}_{\mathrm{B}}{I}_{\mathrm{B}}/\left({K}_{\mathrm{A}}{I}_{\mathrm{A}}+{K}_{\mathrm{B}}{I}_{\mathrm{B}}\right) $$

(6.3)

$$ {W}_{\mathrm{A}}={K}_{\mathrm{A}}{I}_{\mathrm{A}}/\left({K}_{\mathrm{A}}{I}_{\mathrm{A}}+{K}_{\mathrm{B}}{I}_{\mathrm{B}}\right) $$

(6.4)

In the formula, K _A = 0.886, K _B = 2.721, and I _A represent the integral intensity of the strongest diffraction peaks in anatase, and I _B represents that in brookite. The influences of different NaOH concentrations and distinct reaction times on TiO₂ crystals are shown in Fig. 6.2a, b. As NaOH concentration increases, the content of brookite increases, and when the reaction time is over 6 h, the crystalline formation of the product does not change significantly. In addition, single-phase brookite TiO₂ can be obtained by replacing TiS₂ with sodium titanate. Since Na⁺ can stabilize the layered structure, when there is no Na⁺ ion in the reaction system, a layered structure is instable, and the hydrothermal process causes the destruction of the structure, forming anatase TiO₂; when Na⁺ is excessive, the stable layered structure does not easily collapse and ultimately form titanate; when Na⁺ concentration is proper, the layered structure partially collapses, and part of the layered structure remains instable, leading to form brookite (Fig. 6.2c). The above results are consistent with the previous literature [87].

Fig. 6.2

The composition of anatase and brookite in the mixed-phase TiO₂ prepared with (a) various NaOH concentrations and (b) different reaction times under hydrothermal reaction [91]; (c)The scheme of formation of anatase and brookite in mixed-phase TiO₂ [91]. (Reprinted with permission from Ref. [91]. Copyright 2013, American Chemical Society)

Zhang et al. [93] simultaneously introduced NaCl and NH₄OH into the hydrothermal reaction system with TiCl₃ as a titanium source. The results showed that changing the concentration of NaCl and the volume ratio of NH₄OH to H₂O can well control the phase composition of the products in the reaction system (Fig. 6.3a). They put forward that Na⁺ was able to stabilize the layered structure of titanate, which was formed by the hydrolysis reaction of TiCl₃ in aqueous ammonia in the hydrothermal system. In the absence of NaCl, NH⁴⁺ ions which exist in the interlayer of titanate can balance the negative charges of the layered titanate. However, during the process of hydrothermal treatment with high temperature up to 200 °C, NH⁴⁺ is hydrolyzed to form NH₄OH and thus is released from the interlayer. Then the H⁺ ions generated from the hydrolysis of NH⁴⁺ ions induce a hydroxyl condensation reaction, forming anatase TiO₂. In the presence of NaCl, Na⁺ ions benefit to stabilize the layered structure with NH₄ ⁺. The simultaneous existence of Na⁺ and NH₄ ⁺ in the interlayer of titanate helps to balance the negative charges of titanate. After the layered titanate was continuously treated by hydrothermal methods at a high temperature for several hours, the increased hydrolysis of NH₄ ⁺ caused the collapse of layered structures. However, some bonds in the structure were kept by Na⁺ ions. Hence the structural transformation was delayed which resulted in forming a brookite-like structure [94,95,96]. A brookite lattice was formed by the expansion of as-formed brookite-like structure. Increasing the concentration of NaCl would be helpful for the brookite lattice to compete with the anatase lattice. Therefore, with the increase of NaCl concentration, the brookite content in the mixed-phase TiO₂ increased. In addition to this research, Zhang et al. [97] developed another facial hydrothermal method for the synthesis of anatase/brookite TiO₂ using tartaric acid (C₄H₆O₆) as the phase content regulator, TiCl₃ as the titanium source, and NaOH to adjust the pH of the reaction solution. Changing the molar ratio of C₄H₆O₆ to TiCl₃ in the hydrothermal reaction system could successfully control the contents of brookite and anatase in the TiO₂ particles (Fig. 6.3b). According to the ligand field theory, they explained the mechanism of the phase evolution between brookite and anatase in which C₄H₆O₆ could chelate with Ti to form a stable titanium complex [98]. When the C₄H₆O₆/TiCl₃ molar ratio was below 0.75 in the reaction system, there were two forms of Ti species. They were the insoluble Ti-contained species Ti(OH)₄ and the soluble Ti-contained complex [Ti(OH)x(C₄H₆O₆)y]z-. Under hydrothermal treatment, the amorphous Ti(OH)₄ could first transform to layered titanate. Given a proper concentration of Na⁺ and OH⁻, the layered titanate would be transformed to brookite [94]. Due to the large steric hindrance of carboxylic acidic ligands, the [Ti(OH)x(C₄H₆O₆)y]z- complexes were inclined to combine together by sharing equatorial or apical edges and being arranged in zigzag chains, which was beneficial to form anatase crystallites, thus resulting in an mixed-phase TiO₂ of anatase and brookite. They also pointed out that the brookite as the pure phase or the main phase could not be obtained on the conditions that the pH value of the reaction system was lower than 9 or the hydrothermal temperature was below 180 °C. In other words, it was hard to control the contents of anatase and brookite in samples just by changing the C₄H₆O₆/TiCl₃ molar ratio while pH < 9 or temperature < 180 °C in the hydrothermal reaction system.

Fig. 6.3

(a) Relationship between the contents of brookite and the applied NaCl concentration [93]; Reprinted with permission from Ref. [93]. Copyright 2013, Elsevier. (b) Relationship between the contents of brookite in the products and the applied tartaric acid to TiCl₃ molar ratio [97]. Reprinted with permission from Ref. [97]. Copyright 2012, Springer

A variation of the hydrothermal method is the solvothermal method wherein many kinds of organic solvents such as ethanol, glycol, and toluene can be used to replace an aqueous system. Similar to the hydrothermal method, both the crystal types and morphology of the TiO₂ nanomaterials can be well controlled by regulating parameters in the solvothermal reaction system, including temperature and pressure inside the system, the reaction time, and the titanium source.

Through the solvothermal method, Li et al. [99] synthesized anatase/rutile mixed-phase TiO₂ crystals with different rutile content by hydrolysis of tetraisopropyltitanate in an acid alcoholic solution and studied influence of hydrochloric acid on the rutile content in the mixed-phase crystal. The results showed that high H₂O/Ti mole ratio favored the formation of brookite/anatase mixed-phase TiO₂, and low H₂O/Ti mole ratio benefited to form anatase/rutile mixed-phase TiO₂. Lei et al. [100] prepared anatase/rutile mixed-phase TiO₂ crystal using a low-temperature (80 °C) solvothermal method by pre-oxidizing TiCl₃ into Ti⁴⁺ with HNO₃ followed by diluting with urea, water, and ethanol. They found that the anatase content in the mixed-phase crystal increased by increasing ethanol content in the solution (Fig. 6.4). The average particle size of anatase and rutile in the mixed-phase crystal is both below 10 nm calculated by the Scherrer formula.

Fig. 6.4

Relationship between the contents of anatase in the product and the volume ratio of ethanol in a solvothermal reaction system [100]. (Reprinted with permission from Ref. [100]. Copyright 2008, Elsevier)

The mixed-phase TiO₂ nanomaterials synthesized by hydrothermal and solvothermal methods are usually well crystallized and are not required to be calcinated at a certain high temperature. The size and phase type of TiO₂ can be regulated by simply adjusting the experimental parameters such as the type of base or acid in the reaction system, the reaction temperature, the heating rate, the reaction time, and the autoclave pressure. Besides these advantages, there still remain some drawbacks such as the requirement for the equipment to withstand high pressure as well as high temperature and experimental safety concerns. Moreover, it is difficult to monitor the real time of the hydrothermal and solvothermal process, and the reaction usually proceeds without stirring, sometimes leading to the incomplete reaction and irregularly arranged product. Due to the fact that a variety of organic solvents can be used in the solvothermal method, it is easier to control the crystalline and morphology of the products than the hydrothermal method. Therefore, we can foresee that the solvothermal method has better application prospects.

6.3.2 Microemulsion-Mediated Solvothermal Method

The method of microemulsion usually contains two processes. Firstly, two immiscible solvents divided into many micro-reactors (microbubbles) with the help of the surfactants form emulsion, in which nanomaterials are obtained through nucleation, coalescence, agglomeration, and heat treatment process in the oil-in-water or water-in-oil microbubbles. The reactants of immiscible solvents are well dispersed, and a uniform nucleation limited in the micro-reactors occurs. Therefore, the particle size and stability of the nanomaterials can be well controlled. Compared with other traditional synthesis methods, the microemulsion method has shown obvious advantages in terms of the preparation of nanomaterials with perfect monodispersity and interfacial properties. Moreover, this method is a very versatile technique which allows the synthesis of a great variety of nanomaterials by combining with other techniques. Recently, the microemulsion-mediated solvothermal method, which is named for the combination of the microemulsion method with solvothermal method, has been used to the preparation of the mixed-phase TiO₂ nanomaterials [66, 101, 102].

Yan et al. [66] fabricated anatase/rutile mixed-phase TiO₂ crystal through the combination of microemulsion and solvothermal method, using tert-octylphenoxypolyethoxyethanol (Triton X-100) as the surfactant, n-hexanol as the co-surfactant, and cyclohexane as the continuous oil phase. Tetrabutyl titanate and (NH₄)₂SO₄ were completely dissolved in the hydrochloric acid to serve as the aqueous phase. Then the aqueous phase was added dropwise to the oil phase to form a clear microemulsion. After that, the obtained solution was solvothermally treated below 120 °C for 13 h to produce the mixed-phase TiO₂. The surface area of the resultant product is about 86~169 m²/g. Besides grain size of anatase is 15 nm and that of rutile is about 10 nm. Their results showed that the adjustment of SO₄ ²⁻ concentration has obvious influence on the content of anatase in the mixed-phase TiO₂ (Fig. 6.5). With the increase of SO₄ ²⁻ concentration, the anatase content in the mixed-phase TiO₂ increased. Different polymorphs can be achieved by affecting the polycondensation of TiO₆ octahedra with SO₄ ²⁻ (Fig. 6.6a). During the hydrothermal reaction process, the SO₄ ²⁻ interacts with the hydroxy groups on the surface of TiO₆ octahedra, forming hydrogen bonds (Fig. 6.6b). Because of the steric effect of SO₄ ²⁻, the polycondensation of the octahedron with SO₄ ²⁻ and another octahedron along the converse direction results in the decrease of the repulsion (Fig. 6.6c), and the orientation of the third octahedron is more conducive to form an anatase nucleus (Fig. 6.6d). When the concentration of SO₄ ²⁻ is low, a rutile structure can be easily formed for the sake of the lowest free energy in the system. Therefore, controlled adjustment of anatase content in mixed-phase TiO₂ can be achieved by changing the concentration of SO₄ ²⁻. In the same oil-phase system, Zhang et al. [102] synthesized mixed-phase TiO₂ crystal in different ratio of anatase and rutile content by the microemulsion-mediated solvothermal method. In the reaction system, a mixture of TiCl₃, urea, and hydrochloric acid solution was used as the aqueous phase. The results showed that anatase content in the mixed-phase TiO₂ increased with urea concentration increasing. The hydrochloric acid in the reaction solution served as an inhibitor to the generation of anatase, promoting the formation of rutile phase. Because TiO₆ octahedra polycondensed in chain to form a rutile structure was in accordance with the minimum energy principle, Cl⁻ and SO₄ ²⁻ had different influence on the formation of TiO₂ phases. A high concentration of Cl⁻ with small radius and weak steric hindrance favored the formation of rutile TiO₂, while a high concentration of SO₄ ²⁻ benefited to form anatase TiO₂.

Fig. 6.5

Relationship between the contents of anatase in the product and the mole percentage of SO₄ ²⁻ in the microemulsion-mediated hydrothermal reaction system [66]. (Reprinted with permission from Ref. [66]. Copyright 2005, American Chemical Society)

Fig. 6.6

Proposed mechanism: (a) the orientation of the third octahedron determines the formation of rutile or an anatase nucleus; (b) interaction between SO₄ ²⁻ anion and TiO₆ ²⁻ octahedral hydroxyls; (c) two TiO₆ ²⁻ octahedra share edge when SO₄ ²⁻ is present; (d) the formation of anatase in the presence of SO₄ ²⁻ [66]. (Reprinted with permission from Ref. [66]. Copyright 2005, American Chemical Society)

Among the synthesis methods of mixed-phase TiO₂, the microemulsion-mediated solvothermal method is relatively complicated. However, the grain size of the product is small, and the phases can be well controlled, which makes this method to have good prospects for development.

6.3.3 Sol–Gel Method

Sol–gel method is a commonly used method for synthesis of amorphous and crystalline TiO₂ nanomaterials for the sake of its advantages such as processing at low temperature and the ability to prepare materials in various morphologies. Titanium alkoxides or titanium halides are usually used as the titanium precursor in the sol–gel reaction system. Firstly, the hydrolysis of titanium precursor forms a sol. After aging the sol for a certain time, a three-dimensional cross-linked gel is obtained, and amorphous white powder is obtained by grinding the gel. Finally, crystallized TiO₂ is obtained by the treatment of the calcination at a certain high temperature.

Scotti et al. [52] prepared anatase/rutile mixed-phase TiO₂ crystal by a sol–gel method and studied the effects of HCl/Ti and H₂O/Ti molar ratio on the phase type of TiO₂ crystal. At first, TiCl₄ and triblock copolymer were dissolved in ethanol. Then water and HCl were added to adjust the pH of the solution, obtaining a sol. After aging the sol for 3–13 days, a gel was formed. After drying and calcination, mixed-phase TiO₂ crystals consisting of anatase and rutile in different content were obtained. In the products, pure rutile-phase TiO₂ (Fig. 6.7a) displayed chestnut burr aggregates of elongated nanocrystals in radial growing shape with average sizes of 10–20 nm in width and 100–200 nm in length. The pure anatase phase (Fig. 6.7b) showed aggregates of almost square-ended nanoparticles whose average sizes were 5–15 nm. The mixed-phase sample (Fig. 6.7c) was observed to possess two types of phases with the chestnut burr aggregates of rutile surrounded by the small anatase particles. Changing the H₂O/Ti (rw) and HCl/Ti (ra) molar ratios systematically can well control the phase content of the products. Results have proved that, in the titanium alkoxide or acidic titanium halide precursor solution, with the increase of H₂O content, the concentration of Ti⁴⁺ decreased, leading to the decrease of the condensation reaction rate of TiO₆ octahedra to form rutile TiO₂ with better thermodynamically stability (Fig. 6.7d, e).

Fig. 6.7

TEM images of (a) 100% rutile, (b) 100% anatase, and (c) mixed-phase TiO₂ with 48 wt % anatase and 52 wt % rutile; plots of rutile content (wt %) in the product (d) vs H₂O/Ti molar ratio (e) vs titanium concentration [52]. (Reprinted with permission from Ref. [52]. Copyright 2008, American Chemical Society)

Mixed-phase TiO₂ crystals with small particles and high purity can be obtained by the sol–gel method, but the experimental procedure usually requires longtime aging, and the agglomeration of particles is easy to occur after high-temperature calcination, which may cause significant decrease of the photocatalytic activity.

6.3.4 Solvent Mixing and Calcination Method

The solvent mixing and calcination method (SMC method) means to mix TiO₂ with different phases in a certain solvent at first and then to evaporate the solvent completely, finally followed by calcinations at high temperature in order to make a close contact among different crystal phases. Zachariah et al. [103] prepared anatase/rutile mixed-phase TiO₂ by the SMC method wherein anhydrous isopropanol was used as the solvent. According to the Scherrer equation, the crystalline calculated size of pure anatase was 10 nm, and the grain size of pure rutile was 40 nm. In the resultant mixed-phase TiO₂ containing 40% rutile, the size of anatase was 29 nm and that of rutile was 47 nm. From Fig. 6.8a, b, it could be observed that pure anatase consisted of small particles whose size was 100~150 nm. And as Fig. 6.8c, d shows, the particle size of pure rutile was 300~500 nm. The morphology of the mixed-phase TiO₂ nanomaterial was a mixture of anatase and rutile particles (Fig. 6.8c, f). From the high-resolution transmission electron microscopy (HRTEM), the phase interface could be seen clearly, indicating a close interaction existing between the two phases. Bojinova et al. [104] synthesized an anatase/rutile mixed-phase TiO₂ by the SMC method using ethanol as the solvent. In the product, the grain size of anatase was 42 nm and that of rutile was 56 nm which are calculated according to the Scherrer formula. Liu et al. [105] used a layer-by-layer self-assembly technique to make the anatase small particles adsorbed onto rutile nanorods with assist of polystyrene sulfonate (PSS) as a medium, and finally the PSS was removed by calcination. The anatase content in the mixed-phase TiO₂ crystal could be controlled by adjusting the number of loading cycles.

Fig. 6.8

SEM and TEM images of (a, b) pure anatase, (c, d) pure rutile, and (e, f) mixed-phase TiO₂ with an optimum rutile content of 40 wt% prepared by the SMC method [103]. (Reprinted with permission from Ref. [103]. Copyright 2009, American Chemical Society)

The phase content of the mixed-phase TiO₂ can be regulated easily by using the SMC synthesis method. However, because the final calcination process requires high temperature, the TiO₂ is easy to agglomerate. In addition, it is usually difficult to obtain the mixed-phase TiO₂ with anatase and rutile mixing uniformly, causing products with many pure rutile aggregates as well as pure anatase aggregates. Therefore, the photocatalytic performance of the mixed-phase TiO₂ crystals produced by this method is limited.

6.3.5 High-Temperature Calcination Method

The high-temperature calcination method, which is mostly used to research the phase transition of TiO₂, is one of the earliest methods for the synthesis of mixed-phase TiO₂ [25].

Through the high-temperature calcination method, Nair et al. [106] succeeded in preparing the mixed-phase TiO₂ nanomaterials with different anatase and rutile contents. They firstly prepared small anatase particles by a sol–gel method followed by calcinating the anatase particles at elevated temperatures. It was found that the samples which were calcined at low temperatures (T < 600 °C) remained pure anatase phase. The phase transformation began at the temperature of 650 °C. When the calcination temperature was raised to 850 °C, the product converted to pure rutile. As calculated by the Scherrer formula, the grain sizes of anatase and rutile in the mixed-phase products were 33.66~51.48 nm and 45.2~60.6 nm, respectively. Figure 6.9 showed that the particle size of anatase was about 100 nm and that of rutile was about 200 nm, and the mixed-phase particle size was between the above two and increased with the increase of calcination temperature. The phase transformation includes nucleation and growth. However, there is some dispute on the concrete process. Gouma et al. [107] considered that the rutile firstly formed nuclear on the surface of anatase and then expanded to the bulk, while Zhang et al. [108] verified the phase transformation of anatase–rutile occurred in the bulk at first and then spread to the surface with the increase of calcination temperature via UV Raman spectroscopy. Figure 6.10 shows the phase’s transformation.

Fig. 6.9

SEM and EDAX images of TiO₂ photocatalysts calcined at different temperature: (a) 600 °C, (b) 700 °C, (c) 750 °C, and (d) 800 °C [106]. (Reprinted with permission from Ref. [106]. Copyright 2011, Elsevier)

Fig. 6.10

Proposed schemes for the phase transformation of titania with increasing calcination temperature [108]. (Reprinted with permission from Ref. [108]. Copyright 2006, American Chemical Society)

The simple high-temperature calcination method can be used to obtain mixed-phase TiO₂ nanomaterials with perfect polymorphs and tunable phase content. However, it is easy to agglomerate in the process of calcination, and the particles size of product is large, resulting in the decrease of the photocatalytic activity.

Generally speaking, both the SMC method and the high-temperature calcination method require a high temperature to form the mixed-phase TiO₂ or close contact between different crystalline forms. The electron micrographs and XRD results showed that the grain size of crystal after calcination at high temperature is significantly larger than that of mixed-phase TiO₂ crystals obtained by the in situ method. Besides, the agglomeration of TiO₂ product is also serious. All of the before mentioned can easily decrease the photocatalytic activity of mixed-phase TiO₂. Therefore, in recent years, these methods are used less and less.

The photocatalytic reaction rate constants of mixed-phase TiO₂ crystals synthesized by different methods are summarized in Table 6.2. From this table, we can see that the reaction rate of TiO₂ by prepared in situ methods, such as the sol–gel method and microemulsion-mediated hydrothermal method, is usually higher than the high-temperature calcination method or SMC method. This is because the latter requires high-temperature calcination, leading to larger particle size and serious agglomeration that decrease the contact area with the degradation target and further impact the photocatalytic activity. However, the photocatalytic reaction rate of mixed-phase TiO₂ prepared by Zheng et al. [54] was low, which may be attributed to the high-temperature calcination which leads to a hollow structure and decreases the specific surface area, thus causing low photocatalytic activity. Photocatalytic activities of mixed-phase TiO₂ prepared on different conditions are also influenced by many other factors such as product morphology, particle size, and crystal proportion.

Table 6.2 The photocatalytic reaction rate constants of mixed-phase TiO₂ synthesized by different methods

Apart from the commonly used approaches mentioned above, there are some new preparation methods of mixed-phase TiO₂, such as the microwave-heating method [109]. The particle size of TiO₂ prepared by this method is smaller than that synthesized by heating with an oil bath. With the development of new technologies, we can foresee that there will be more new methods for preparing mixed-phase TiO₂.

6.4 Applications of Mixed-Phase TiO₂ in Photocatalysis

6.4.1 Photocatalytic Hydrogen Production

Faced with the increasingly serious energy crisis, hydrogen has been widely recognized as an ideal energy source to replace a significant fraction of fossil fuels. However, nowadays the H₂ source demanded is mostly produced from fossil feedstock, mainly via steam reforming of methane. It is therefore necessary to develop sustainable methods for hydrogen production. Among them, the photocatalytic hydrogen production is one of the most promising technologies, which possesses various advantages, such as sustainability of the primary energy source (the solar light), the renewability of the starting feedstock, as well as the possible production of by-products with a high added value [110]. Due to the merits of TiO₂ such as nontoxicity, relatively cheap cost, easy to prepare, and excellent stability under the reaction conditions, it is the most investigated photocatalyst for H₂ production. However, the present solar-to-hydrogen energy conversion efficiency of TiO₂ is not high enough for the applications in commercial production. The rapid recombination of photo-generated electron–hole pairs of the materials, backward reaction, and the poor utilization of visible light hinder the practical applications. In order to solve the problems mentioned above, many researchers have been conducting studies concentrated on an emphasis to develop effective remediation methods like modification of TiO₂ by noble metal loading, metal ion doping, anion doping, dye sensitization, composite semiconductor, and metal ion implantation. In addition, mixed-phase TiO₂ has also been investigated for hydrogen production [91, 110,111,112,113,114], since mixed-phase TiO₂ nanomaterials demonstrated higher performances than the correspondent monophasic systems in many photocatalytic processes.

Chen et al. [91] successfully prepared pure anatase nanoparticles, pure brookite nanoplates, and two-phase anatase/brookite TiO₂ photocatalysts by a simple hydrothermal method. The photocatalytic activity of the as-synthesized catalysts for hydrogen production was studied in methanol solution. Their experimental results have shown that the photocatalytic activity of the two-phase anatase/brookite TiO₂ was higher compared with that of pure brookite nanoplates and pure anatase nanoparticles, as shown in Fig. 6.11. Moreover, in comparison with the highly active two-phase commercial P25, the synthesized two-phase anatase/brookite TiO₂ was 220% more active when measured by the H₂ yield per unit area of the photocatalyst surface. Similar results were achieved by Montini et al. [111]. They also prepared the mixed-phase TiO₂ of anatase/brookite via hydrothermal treatments. Compared with a reference TiO₂ prepared by conventional sol–gel synthesis, the anatase/brookite nanocomposites which are obtained by hydrothermal method showed the higher photocatalytic activity for producing H₂. Furthermore, they found that the anatase/brookite ratio in the nanocomposite had a great influence on the photocatalytic activity in H₂ production.

Fig. 6.11

Hydrogen evolved per gram of photocatalyst per hour under UV−vis irradiation in aqueous methanol solution over 0.3 wt % Pt-loaded photocatalysts. A, B, and R stand for anatase, brookite, and rutile, respectively [91]. (Reprinted with permission from Ref. [91]. Copyright 2013, American Chemical Society)

Apart from the research applying mixed-phase anatase/brookite TiO₂ as the photocatalyst to produce hydrogen, a great deal of research on the mixed-phase anatase/rutile TiO₂ has also been done. Li et al. [112] fabricated photocatalysts with a tuned anatase/rutile structure by calcination of commercial P25 at different temperatures and investigated their photocatalytic activity for hydrogen production through photocatalytic biomass reforming. Surprisingly, it was found that compared with P25 without any treatment, the photocatalytic performance of the thermal-treated P25 for hydrogen production is better. The overall photocatalytic activity for hydrogen production on thermal-treated P25 had a dramatic improvement of three to five times. It was proposed that the anatase/rutile junction structure was mainly responsible for the enhanced photocatalytic performance. The research work implied that the photocatalytic ability of TiO₂ could be further improved by elaborately designing the anatase/rutile structure. Amal et al. [113] obtained the similar conclusions. They did a systematical study on photocatalytic H₂ evolution using mixed-phase TiO₂ as a function of anatase and rutile phase compositions with methanol as hole scavengers. The TiO₂ nanomaterials they prepared contain 4–95 mol % anatase, with the remaining being rutile. Synergistic effects on H₂ evolution were observed for a wide range of anatase contents, from 13 to 79 mol %, while due to insufficient physical contact, no synergistic effect was observed for the sample obtained by mixing anatase and rutile particles physically. Recently, in addition to the above research on particulate TiO₂ (Fig. 6.12a, b), Yu et al. [114] prepared a kind of anatase/rutile TiO₂ nanofiber photocatalyst (Fig. 6.12b, c). The enhanced photocatalytic performance for H₂ production was also observed in the synthesized anatase/rutile composite nanofibers. The product with 45 wt.% rutile phase and 55 wt.% anatase phase exhibited the highest photocatalytic activity with the H₂ production rate of 324 mmol h⁻¹ and the apparent QE of 20.9% at 365 nm.

Fig. 6.12

(a and b) TEM images of particulate anatase and rutile phase TiO₂; (c and d) TEM and HRTEM images of fibrous anatase and rutile TiO₂ [112]. (Reprinted with permission from Ref. [112]. Copyright 2011, Elsevier)

Though the research on the mixed-phase TiO₂ of anatase/rutile generally demonstrated that the combination of the two phases favored the photocatalytic hydrogen production efficiency, the low utilization rate of visible light still remains as a big limit for its practical application. The relatively narrow bandgap of rutile extends some visible light absorption to some extent. However, this is not enough for TiO₂ photocatalyst to apply in the practical applications. Researchers have made great efforts on to expand the visible light absorption of the mixed-phase TiO₂ crystals. Keller et al. [115] reported Au-modified anatase/rutile mixed-phase TiO₂ photocatalyst (Fig. 6.13) for hydrogen production. The light absorption of the sample with and without Au deposition was investigated via UV–vis light absorption spectra. Before the Au deposition, the catalyst with rutile phase whose content was higher than that of P25 exhibited more light absorption extended up to 550 nm, but its intensity is relatively low. After Au deposition, the samples showed an obvious modification of the light absorption properties, yielding an additional absorption band around 550 nm. It could be attributed to a plasmon resonance phenomenon [116] owing to collective oscillations of the conduction electrons located on the 6s orbital of Au and induced by the incident electromagnetic wave. In addition to this plasmon absorption, Au deposition also benefited to shift the absorption spectrum of TiO₂ photocatalysts deeper into the visible light range of 380–450 nm. This enhanced light absorption in the visible region was demonstrated to contribute to the improved photocatalytic performance of TiO₂ for the hydrogen production under the irradiation of simulated solar light. Furthermore, they pointed out that there were various factors which were crucial to improve H₂ evolution efficiency: (i) the surface and the crystallographic porosity properties of the TiO₂ anatase/rutile photocatalyst, (ii) the anatase/rutile ratio, (iii) the amount and nature of the metallic cocatalyst, (iv) the metal–support interactions, and (v) the relative content of sacrificial reagent. After studying the influence of these different factors in depth, they optimized the experimental conditions, obtaining the important H₂ production efficiency up to120 μmol min⁻¹ over days without deactivation and with very low amounts of sacrificial reagent.

Fig. 6.13

(a, b) TEM images of mixed-phase TiO₂ photocatalyst deposited Au; (c) Light absorption properties of mixed-phase TiO₂ without and with Au deposition [115]. (Reprinted with permission from Ref. [115]. Copyright, 2010 Elsevier)

6.4.2 Photocatalytic Reduction of CO₂ with Water on Mixed-Phase TiO₂

As we know, CO₂ is the main cause of the greenhouse effect, which leads to global warming. Meanwhile, based on the fact that CO₂ can be converted into various useful chemical compounds and fuels such as CH₄, CH₃OH, and HCOOH, it is also a promising carbon resource. Therefore, in order to reduce the emissions of CO₂ and to obtain a sustainable energy, many kinds of novel materials and new technologies have been developed to convert CO₂. Besides the methods of solar thermochemical conversion as well as electrochemical reduction of CO₂, solar-activated photocatalytic reduction of CO₂ with water by TiO₂ at atmospheric pressure and room temperature has attracted much attention due to its “green chemistry” and relatively low cost.

During the process of CO₂ photocatalytic reduction with H₂O by TiO₂, the irradiation of solar light activates the generation of electron–hole pairs in TiO₂ nanomaterials. The excited electrons in the conduction band (CB) of TiO₂ photocatalyst migrate to the surface to reduce CO₂. Meanwhile, the holes left in the valence band (VB) of TiO₂ can oxidize H₂O into O₂.

It has been proved that the above photocatalytic process had a close relation with the phase type of TiO₂. Li et al. [117] investigated the CO₂ photoreduction with water vapor on three types of TiO₂ nanocrystal polymorphs (anatase, rutile, and brookite). The results showed that the photocatalytic reduction activity of different-phase TiO₂ nanomaterials follows the order: anatase > brookite > rutile. Because of the fast e⁻ and h⁺ recombination in rutile, it showed the least photocatalytic activity. Besides, the photoreduction activity of the TiO₂ catalysts with a helium treatment was investigated. The photoreduction data indicated that photocatalysts with a helium treatment were more active than the samples without a helium treatment. And the photocatalytic activity for production of CO and CH₄ from the photoreduction of CO₂ followed the order, brookite > anatase > rutile (Fig. 6.14), where brookite showed the highest photocatalytic activity. Thus their study also implied that the brookite phase is a promising material for the photoreduction of CO₂; they performed further research on this topic, including the study on brookite-containing mixed-phase TiO₂. They prepared bicrystalline anatase/brookite TiO₂ photocatalysts via a hydrothermal method [118]. The as-prepared anatase/brookite TiO₂ was also applied for CO₂ photoreduction with water vapor for the production of CO and CH₄. Compared with those of pure anatase, pure brookite, and a commercial anatase/rutile TiO₂ (P25), the photocatalytic activities of bicrystalline anatase/brookite TiO₂ were better. Wherein, the bicrystalline mixture with a composition of 75% anatase and 25% brookite showed the best photocatalytic performance, whose photocatalytic activity was nearly twice as high as that of 100% anatase (A100) and three times as high as that of pure brookite (B100) TiO₂. Because pure anatase A100 possessed the largest specific surface area and the smallest bandgap among three types of TiO₂ phases, the higher photocatalytic activity of bicrystalline anatase/brookite was very likely ascribed to the interactions between the anatase and brookite nanocrystals. In addition, the result that the anatase-rich bicrystalline anatase/brookite mixtures were superior to anatase/rutile mixture P25 which indicated the interaction between anatase and brookite of as-prepared mixed-phase TiO₂ seems to be more effective than that of P25 in the photoreduction of CO₂.

Fig. 6.14

The top figure are TEM images of different TiO₂ crystals: anatase, brookite, and rutile (from left to right). The bottom figure is the production of CO and CH₄ with the three different TiO₂ polymorphs [117]. (Reprinted with permission from Ref. [117]. Copyright 2012, American Chemical Society)

6.4.3 Photocatalytic Degradation of Organic Pollutants on Mixed-Phase TiO₂

In 1977, S. N. Frank and A. J. Bard reported that TiO₂ could effectively decompose the cyanide in aqueous medium under sunlight for the first time [119]. From then on, the application of TiO₂ for photocatalytic degradation of organic pollutants such as organic dyes has raised wide attention due to its effectiveness in degrading and mineralizing the toxic, bio-refractory, and highly concentrated organic compounds as well as the possibility of utilizing the solar ultraviolet and visible light spectrum. Owing to the superiority that mixed-phase TiO₂ usually exhibit better photocatalytic activity than single-phase TiO₂, TiO₂ nanomaterials consisting of different phases have also been widely used to degrade organic pollutants.

There have been many studies on the photocatalytic degradation of organic dyes using mixed-phase TiO₂ as photocatalyst. For example, under simulated solar light irradiation, the degradation of methyl blue (MB) was carried out in the aqueous solution by anatase/rutile TiO₂ heterojunction nanoflowers [120]. It was found that 72% of MB could be degraded in 120 min in the presence of the mixed-phase photocatalyst prepared in optimized experimental conditions, while only 30% of MB could be degraded in the solution with 100% anatase. Besides, the TiO₂ nanoflower photocatalysts showed excellent stability after nine cycles under the same conditions, suggesting that the mixed-phase anatase/rutile TiO₂ heterojunction nanoflower materials have great potential for the future photodegradation of practical dye wastewater. Since high surface area favors high photocatalytic activity, mesoporous mixed-phase anatase/rutile TiO₂ was also fabricated [121] (Fig. 6.15a) to apply for the photocatalytic degradation of MB. The optimum mesoporous photocatalyst (10%V-Ti-700) in situ doped with vanadium with the surface area up to 94 m² g⁻¹ and an average pore size of 5.6 nm (Fig. 6.15b) exhibited higher photocatalytic activity under visible light than that of the commercial P25 (Fig. 6.15d), in which the vanadium doping helped to extend the absorption of TiO₂ to the visible light region (>400 nm) (Fig. 6.15c). It was concluded that the high surface area, the mixed-phase effect, and the vanadium doping altogether contributed to the enhanced photocatalytic performance of the mesoporous mixed-phase anatase/rutile TiO₂.

Fig. 6.15

(a) TEM image of mesoporous anatase/rutile mixed-phase TiO₂ (10%V-Ti-700); (b) N₂ sorption isotherms of 10%V-Ti-700. The inset, the BJH desorption pore-size distributions; (c) DR UV−vis spectra for various vanadium-doped mixed-phase TiO₂; (d) photocatalytic decomposition experiments of 100 mL, 10⁻⁴ MB dye under visible light in 2 h by using 100 mg of 5% V-Ti-700, 10%V-Ti-700, 5%V-Ti-800, 10%V-Ti-800, and P25 samples, respectively [121]. (Reprinted with permission from Ref. [121]. Copyright 2014, American Chemical Society)

In addition to the anatase/rutile TiO₂, many studies on brookite/rutile TiO₂ for the photodegradation of dyes have been done, such as Zhang et al.’s [81] work. They prepared mixed-phase TiO₂ photocatalysts with a tunable brookite-to-rutile ratio through a facile controllable one-pot hydrothermal method. And the photocatalytic performance of the as-prepared TiO₂ nanocrystals was tested in the degradation of Rhodamine B under the simulated solar light. Compared with the samples with other brookite-to-rutile ratios, the mixed-phase TiO₂ nanocrystals with 38% brookite and 62% rutile showed the highest photocatalytic activity, which was about six times that of the commercial P25 catalysts. Recently, Chen et al. [55] grew anatase grains on the surface of brookite petals to form a brookite/anatase TiO₂ hybrid, which appeared to have superior photocatalytic activity to the single-phase TiO₂ during the degradation of methyl orange (MO) as well as 2,4-dichlorophenol (2,4-DCP). The hybrid consisting of 60% brookite and 40% anatase exhibited the highest activity, whose degradation rate constants are 2.27 and 1.80 times higher than that of the corresponding physically mixed brookite/anatase TiO₂ for the photodegradation of MO and 2,4-DCP, respectively. Following this study, Ag was utilized to further modify the brookite/anatase composite [122]. Ag⁰ clusters, which possessed an average diameter of ca. 1.5 nm, formed on the surface of the mixed-phase TiO₂ (Fig. 6.16a, b). The photocatalytic performance of the as-prepared photocatalysts was examined for the degradation of methyl orange (MO). As shown in Fig. 6.16d, the photodegradation reaction rate constant of Ag⁰-loaded brookite/anatase composite was higher than that of the pure anatase and brookite, indicating an improved photocatalytic activity. Besides, it was found that the Ag⁰-loaded mixed-phase TiO₂ photocatalysts showed better photocatalytic performance than that of Ag-free sample by comparing the results in Fig. 6.16c, d. The enhanced photocatalytic reactivity could be ascribed to the significant improvement in the separation of the photo-generated electrons and holes, which was caused by two aspects: (1) synergistic effect of anatase and brookite in the composite and (2) the Schottky barrier at the interface of TiO₂ and Ag⁰.

Fig. 6.16

(a and b) TEM images of 2.0% Ag⁰–TiO₂ (brookite/anatase); (c) photocatalytic degradation of MO with Ag-free and 2.0% Ag⁰-loaded (d) TiO₂ photocatalysts with different crystal phase composition under UV irradiation [55]. (Reprinted with permission from Ref. [55]. Copyright 2012, Elsevier)

While most research focused on photocatalytic degradation properties of biphasic TiO₂ photocatalysts, Que. et al. [123] prepared brookite/anatase/rutile triphasic TiO₂ composite as the photocatalysts for degradation of MO. Their experimental results indicated that in the brookite/anatase/rutile coexisting nanomaterials, the brookite and anatase phases were crystallized into irregular nanoparticles with diameter sizes of less than 20 nm, whereas the rutile phase was crystallized into single-crystalline nanorods about 20 nm in diameter and 100–500 nm in length, as Fig. 6.17a shows. Under irradiation of UV light, the as-prepared triphasic TiO₂ photocatalyst was demonstrated to possess better photocatalytic activity during degradation of MO than the biphasic commercial P25. The catalyst with 29.9% anatase, 27.9% brookite, and 42.2% rutile (referred as T2) had the highest photocatalytic activity, removing 90% of MO in 20 min (Fig. 6.17b). The photodegradation rate constant k of this sample was 0.10180 min⁻¹, which is nearly twice higher than that of P25 (k = 0.05397 min⁻¹). In addition, as shown in Fig. 6.17c, the photocatalytic activity of the best photocatalyst T2 showed only a slight loss after five recycles, indicating a good stability of the triphasic TiO₂ photocatalyst in the degradation process. Similarly, Shao et al. also developed brookite/anatase/rutile coexisting TiO₂, and the as-prepared photocatalyst was demonstrated to show excellent photocatalytic performance for the degradation of methylene blue solutions.

Fig. 6.17

(a) Scheme of the brookite/anatase/rutile nanocomposites; inset, TEM image of as-prepared composite; (b) photocatalytic degradation of MO solution by using the brookite/anatase/rutile nanocomposites prepared with various reagent ingredients, and the inset shows the removal of MO monitored by TOC with the sample T2; (c) cycling degradation curves of the brookite/anatase/rutile triphasic nanocomposites (sample T2) [123]. (Reprinted with permission from ref. [123]. Copyright 2012, Royal Society of Chemistry)

Apart from dyes, some other organic compounds such as phenol and benzyl alcohol have also been used as the target pollutants in the mixed-phase TiO₂ photocatalysis system. Because phenol as well as its derivatives, as one type of primary pollutants, is able to do harm to organisms at low concentrations, many studies on the photodegradation of phenol have been widely carried out [124]. Lu et al. [125] synthesized a series of mixed-phase TiO₂ samples with different anatase-to-rutile ratios and studied the effects of crystal phase in photocatalytic oxidation of phenol aqueous solution. They found that the photocatalysts with higher anatase-to-rutile ratios showed higher photocatalytic activities for phenol degradation. Tian et al. [126] reported Cr-doped anatase/rutile bicrystalline phase TiO₂ (Cr-TiO₂) nanoparticles for the photocatalytic degradation of 2,4-dichlorophenol (2,4-DCP). Their experimental results showed that Cr³⁺ doping could not only effectively extend the visible light response of TiO₂ nanomaterials (Fig. 6.18a) but also enhance the anatase-to-rutile transformation. The photocatalytic activities of different Cr-TiO₂ photocatalysts for the photocatalytic degradation of 2,4-dichlorophenol (2,4-DCP) were evaluated under the irradiation of visible light (Fig. 6.18b). It is found that appropriate Cr³⁺ doping can remarkably enhance the visible light photocatalytic activity of TiO₂, which is ascribed to improving the response of visible light as well as suitable anatase-to-rutile ratio. In addition, excess Cr³⁺ doping is unfavorable for improving visible light photocatalytic activity, because of the formation of Cr₂O₃ clusters and the excessive rutile content.

Fig. 6.18

(a) UV–vis DRS and (b) photocatalytic degradation curves of 2,4-DCP over TiO₂ without doping and different Cr-TiO₂ samples under visible light irradiation: (a) TiO₂ without doping, (b) 0.5% Cr-TiO₂, (c) 1% Cr-TiO₂, (d) 2% Cr-TiO₂, and (e) 5% Cr-TiO₂. The inset is the absorption spectra of 2,4-DCP over different as-prepared photocatalysts after visible light irradiation for 8 h [126]. (Reprinted with permission from ref. [126]. Copyright 2012, Elsevier

6.5 Mechanism of the Enhanced Photocatalytic Activities by the Mixed-Phase TiO₂ Photocatalysis

Generally, most of the photo-generated electrons and holes in the single-phase TiO₂ will recombine, and only a small amount of the excitons can migrate to the surface to participate in the oxidation–reduction reactions with absorbed molecules, causing relatively low photocatalytic efficiency. In 1991, Bickley et al. [76] first put forward the mechanism of enhanced photocatalytic activity of P25 consisting of 80% anatase and 20% rutile. They demonstrated that the mixed-phase structure of P25 is an anatase nanostructure coated with a layer of rutile film via TEM. Compared with any other single-phase TiO₂ photocatalyst, the evaluation results showed that the photocatalytic activity of the mixed-phase TiO₂ was significantly improved. Because of the different bandgaps, a bending band was formed on the interface of anatase and rutile, as shown in Fig. 6.19. Under light irradiation, the photo-generated electrons migrated from rutile phase to anatase phase, while the holes migrated from anatase phase to rutile phase; thus the electrons and holes could be separated effectively, leading to a high photocatalytic activity of the mixed-phase P25. Meanwhile, they proposed that the mechanism of the improved photocatalytic performance of mixed-phase TiO₂ was actually more complex and required further study. Subsequently, much research work on the mobility direction of the photo-generated carriers, the molecular dynamic characterizations of the mixed-phase interface [76], the band structure of mixed-phase TiO₂ photocatalysts [127, 128], and the suitable phase proportion in the mixed-phase crystal TiO₂ has been done to study the mechanism in depth.

Fig. 6.19

Schematic diagram of P25 band structure [76]. (Reprinted with permission from Ref. [76]. Copyright 1991, Elsevier)

In 1995, through observing the mixed-phase crystal structure in P25 via X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques, Datye et al. [129] put forth distinguishing views against Bickley et al. [76]. The characterization results showed that the mixed-phase P25 was composed of individual single-crystal particles of anatase and rutile rather than anatase particles covered by a layer of rutile. Subsequently, their view was confirmed by Zhang et al. [130] and Ohno et al. [131]. But they did not further study the mechanism of the enhanced photocatalytic performance of the mixed-phase TiO₂.

In 2002, Kawahara et al. [132] designed a film model to confirm the migration direction of the photo-generated excitons in anatase/rutile mixed-phase crystal TiO₂ under light irradiation. The mixed-phase crystal TiO₂ film was immersed into an AgNO₃ solution with the exposure to light over a period of time at the atmosphere of argon. It could be observed that there were a large number of Ag particles appearing on the surface of the rutile phase. According to the phenomenon, they inferred that Ag⁺ caused a reduction reaction on the rutile surface and the photo-generated electrons migrated from the conduction band of anatase to rutile, as shown in Fig. 6.20.

Fig. 6.20

A proposed schematic diagram showing the migration behavior of the electron transferring from anatase to rutile [132, 133]. (Reprinted with permission from Ref. [132]. Copyright 2002, Wiley Online Library. Reprinted with permission from Ref. [133]. Copyright 2007, Elsevier)

In 2003, Sun et al. [134, 135] deposited Pt on P25 and applied the catalyst to the photocatalytic degradation of phenol. They found that loading P25 with Pt did not show the increase of photocatalytic activity for phenol decomposition as well as the total carbon removal rates. Besides, they found that Pt appeared on the surface of anatase. Therefore, as shown in Fig. 6.21, they proposed a charge separation mechanism for P25 that the contact of the two phases led to the bending of the conduction band, resulting in the holes in anatase transferring to rutile, while the electrons in anatase not transferring to rutile, further causing the results that holes were concentrated in rutile and electrons stayed in the anatase particles. It was to say that the oxidation reaction mostly happened on rutile and the reduction reactions mostly took place on anatase; thus the Pt deposition on anatase was not able to increase the photocatalytic efficiency for phenol oxidation in water.

Fig. 6.21

(a) Band structure of anatase and rutile before they contact each other; (b) band structure and the separation of photo-generated excitons after anatase and rutile contact each other [134]. (Reprinted with permission from Ref. [134]. Copyright 2003 American Chemical Society)

Hurum et al. [51] characterized the charge separation process of P25 via electron paramagnetic resonance (EPR) in the same year. Due to the wide bandgap of anatase, no exciton was generated under the visible light irradiation. However, the electron trapping sites were found on anatase surface in electron paramagnetic resonance spectroscopy. It could be concluded that the electrons in rutile excited by visible light transferred to the lower-energy anatase lattice trapping sites, improving the separation charges and thus enhancing the photocatalytic activity of P25 (Fig. 6.22). Thereafter, Liu et al. [136] confirmed this mechanism via the photocatalytic activity experiments of anatase/rutile mixed-phase TiO₂ nanotubes.

Fig. 6.22

A proposed schematic illustration showing the separation of photo-generated holes and electrons using EPR [51]. (Reprinted with permission from Ref. [51]. Copyright 2003, American Chemical Society)

EPR spectroscopy is a new characterization method which has been developed in recent years. It can be used for the detection of the unpaired electrons in the sample. Many researchers have started to characterize the migration behaviors of photo-generated electrons and holes in the mixed-phase TiO₂ via the EPR technique [137].

An “antenna mechanism” was proposed to explain the mechanism of improved photocatalytic performance in mixed crystal phases by Wang et al. [138] in 2006. They thought that during the photocatalytic degradation process, TiO₂ absorbed light to produce photo-generated excitons, which then take part in the oxidation–reduction reactions with the target molecules adsorbed on the catalyst’s surface; however, due to the lack of light in the depth, the particles deeper in the liquid could only receive the photon energy via the particles irradiated by light. The energy transferred from the shallow particles induced the oxidation–reduction reaction of the particles which light cannot reach, enhancing the photocatalytic activity. The long-chain particles served as an antenna to transfer the photon energy from the location of light absorption to the location of reaction in the process of photon energy being transferred to the photocatalyst deeper in the liquid; thus this is called an “antenna mechanism” (Fig. 6.23).

Fig. 6.23

Antenna effect by network structure leading to the enhancement of photocatalytic activity [138]. (Reprinted with permission from Ref. [138]. Copyright, 2006 Elsevier)

However, the theoretical models in the research objects mentioned above are all P25 consisting of a certain phase ratio of anatase and rutile. Because mixed-phase TiO₂ with different phase ratios exhibit different photocatalytic activities, it is difficult to adequately explain the physical phenomenon responsible for the existence of an optimum phase ratio in mixed-phase TiO₂ with the maximum photocatalytic activity using these existing models. Therefore, further research is essential to find out the photocatalytic mechanism of mixed-phase TiO₂ in different phase contents.

In 2008, Zachariah et al. [103] took the factors of crystallite size distribution as well as phase composition into account, and proposed a new mechanism model, which overcame the limitations of the existing models in terms of explaining the abovementioned phenomenon. The mechanism is based on the charge separation mechanism proposed by Sun et al. [134]. Zachariah et al. [103] thought that when the size of nano-TiO₂ was identical (Fig. 6.24a), the bandgap of adjacent anatase crystallites was the same, so there was no driving force to migrate the photo-generated holes, resulting in the high recombination rate of photo-generated charges and the low photocatalytic activity. If the sizes of anatase crystallite were different and were below the critical size (Fig. 6.24b, c), the bandgaps of the connected crystallites would depend on their size. Thus, photo-generated holes in one crystallite could easily escape into the other, leading to the effective separation of photo-generated excitons. When a small amount of rutile was mixed with anatase (Fig. 6.24d), owing to the different bandgap, photo-generated holes would transfer from anatase to rutile, so that the photocatalytic activity could be further enhanced. However, the excessive rutile content limited the migration of photo-generated holes in mixed-phase crystal TiO₂ (Fig. 6.24e), and the photocatalytic activity of photocatalysts began to decline. On the condition that the crystallite sizes of pure rutile are bigger than the critical size (Fig. 6.24f), their bandgap values kept the same, and then the migration of photo-generated holes could not occur, and the photocatalytic activity was minimum. The mechanism showed that whether the TiO₂ was mixed-phase crystalline or single crystalline, the photo-generated electron–hole pairs could be separated and the photocatalytic activity of photocatalysts could be enhanced as long as the bandgap values were different.

Fig. 6.24

A schematic illustration of the newly proposed mechanism in the basis of the bandgap variation in the connected nanocrystallites as a function of the phase and the size distribution involved [103]. A, R, CB, and VB stand for anatase TiO₂, rutile TiO₂, the conduction band, and the valence band, respectively [103]. (Reprinted with permission from Ref. [103]. Copyright 2009, American Chemical Society)

In summary, the mechanisms mentioned above for the enhanced photocatalytic activity of mixed-phase TiO₂ all involve the migration behaviors of photo-generated excitons. In these mechanisms, researchers all agree that the mixed-phase crystal structure favors the effective separation of photo-generated electrons and holes. However, the specific migration paths of excitons are still controversial. The determining factors for the photocatalytic activity of mixed-phase crystal TiO₂ contain grain size, structural forms (core–shell structure, cladding structure, random composite, etc.), phase composition, and so on. These factors cause the emergence of bending band, trapping sites, and other phenomena in the mixed-phase TiO₂. Therefore, the obtained mixed-phase TiO₂ differ in photocatalytic mechanism on different experimental conditions, so further research is still required.

6.6 Conclusion and Outlook

In the field of photocatalysis, since it was discovered that TiO₂ nanomaterials could be used for photocatalytic water splitting and degradation of organic pollutants in the 1970s, the synthetic methods and applications of mixed-phase TiO₂ have achieved great progress. The superior photocatalytic performance of mixed-phase TiO₂ to single-phase TiO₂ has been well recognized, and the mechanism of the improved photocatalytic activities of the mixed-phase TiO₂ photocatalysts has also been widely studied. However, there are still many problems remaining to be resolved. For instance, (1) particulate mixed-phase TiO₂ nanomaterials are easy to agglomerate, which will greatly inhibit the photocatalytic activity and impede the further development of the materials; (2) though the relatively narrow bandgap of rutile is able to extend the absorption of the mixed-phase TiO₂ to some of the visible light range, it is still unable to completely utilize all visible light from sunlight, limiting its practical application in photocatalysis; and (3) although researchers all approve that the mixed-phase crystal structure favors the effective separation of photo-generated electrons and holes, thereby improving the photocatalytic activity, there is still much controversy about the mobility direction of photo-generated excitons. It is necessary for researchers to further study on the mechanism of the enhanced photocatalytic performance of mixed-phase TiO₂. Therefore, developing the mixed-phase TiO₂ nanomaterials, combining other advantageous structures (such as hierarchical structure) to enhance the utilization of the visible light, and exploring the mechanism of photocatalytic reactions by newly developed characterization techniques will retain as the challenges and hot research themes in the future.

Although mixed-phase TiO₂ photocatalysts have been studied for decades, it is still a hot research theme owing to its excellent photocatalytic activity. With the increase of serious energy crisis and environmental problems, the applications of mixed-phase crystal TiO₂ are predicted to attract lasting and considerable attention.