1 Introduction

TiO2 nanocrystals with wide bandgap are considered to be the most potential photocatalysts. There are three TiO2 crystal phases, i.e., rutile, anatase, and brookite in nature. Rutile is the thermodynamically stable form, whereas anatase and brookite are metastable. The photocatalytic properties of TiO2 not only depend on crystal phase, crystallinity, but also on shape and size, etc. [1]. It is meaningful to find efficient synthetic methods for TiO2 nanocrystals of controllable structures, and understand the structure–photocatalytic activity relationship.

Various synthetic methods have been developed to fabricate TiO2 nanocrystals, such as sol–gel method [2], solvothermal (hydrothermal when water is used as solvent) method [3], and reverse micelle methods [4]. Among these methods, the solvothermal method normally has better controllability on size, shape, and crystallinity of the TiO2 nanocrystal [5–7].

As illustrated in Fig. 3.1, the solvothermal processes are usually carried out in an autoclave lined with Teflon. Thus, crystal synthesis or crystal growth are carried out under high temperature and high pressure water conditions from substances which are insoluble in ordinary temperature and pressure (<100 °C, <1 atm). Usually, metal oxide nanocrystals with high crystallinity and dispersity could be obtained by solvothermal method at relative low temperature (<250 °C), because of avoidance of high-temperature calcination.

Fig. 3.1
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Scheme of the solvothermal reactor and reaction factor

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Based on our previous work on controllable preparation of TiO2 nanocrystal [8–14], we summarize the solvothermal route using peroxotitanate solution to get TiO2 nanocrystals with controllable shape, size, and crystal phase and high dispersity.

2 Controllable Preparation and Photocatalytic Properties of TiO2 Nanocrystals

The formation process of metal oxide nanocrystals in liquid phase can be divided into nucleation and crystal growth stages. Usually, there is no obvious boundary between the two stages [15]. During the solvothermal reaction process, the environment and reaction conditions will affect nucleation and crystal growth process. So it is flexible to adjust nanocrystal size, shape, and crystal phase by changing the factors illustrated in Fig. 3.1. Here, we focus on the effects of solvent and doping ions.

2.1 Effects of Solvents on Morphologies of TiO2 Nanocrystals

It was reported that the (001) crystal face of anatase has higher photocatalytic activity than the other faces. However, the higher surface energy will drive (001) crystal face to progressively reduce. Yang et al. prepared TiO2 crystals with 64 % (001) face using TiF4 and HF as precursor and crystal face regulation agent, respectively. The F–Ti bond can reduce the surface energy of (001) face, making it more stable than (101) face. However, the fluoride has to be removed by high temperature calcination [16]. During the calcination process, the reconstruction of (001) face will inevitably happen. Moreover, the corrosivity of the TiF4–HF reaction system should be taken good care of.

The addition of surfactants in the hydrothermal reaction system is also an effective method to control the growth of nanocrystals by adjusting the surface energy. Lauric acid (LA), trioctylphosphine oxide (TOPO), quaternary ammonium hydrate (R3NOH, R=H, CH3, C2H5, C4H9, etc.) have been used to prepare TiO2 nanocrystals with different shapes [17, 18]. However, there are considerable residual organic molecules on the TiO2 surface. It is necessary to remove the residues by calcination. As a consequence, the growth and sintering of TiO2 nanocrystals are inevitable.

Our group explored a facile route for the synthesis of controlled crystalline phase and morphology TiO2 nanocrystal colloids from peroxotitanium acid (PTA) by solvothermal method [9]. This approach can give pure anatase by oxidizing amorphous titanium hydroxide precipitates using hydrogen peroxide, followed by the solvothermal treatment under low temperature and neutral pH condition. Colloidal TiO2 nanocrystals with a diversity of well-defined morphologies and size-controlled nanoparticles have been successfully fabricated by adjusting the solvent of peroxotitanate complex solution.

As shown in Fig. 3.2, in the solvent mixture of alcohol and water, the alcohol molecule, such as ethanol, can strongly adsorb to the (001) plane, which depresses the growth rate along the [001] direction. As a result, the shape of TiO2 nanocrystal transforms from “ rectangular” to “ rodlike” with increasing water–ethanol ratio. And the particle size increases from 9.0 to 47.0 nm with increasing water–ethanol ratio.

Fig. 3.2
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Growth schematic diagram of TiO2 nanostructures in different solvent. Reprinted with permission from Ref. [9] copyright 2009 American Chemical Society

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According to Fig. 3.3, the –O–O– bonds decomposed after solvothermal reaction. But there are organic molecule residues on the TiO2 nanocrystal surface, which is confirmed by the C-H signal in FT-IR spectra of as-prepared TiO2 samples. However, after the vacuum treatment at 40 °C, the organic molecules can be removed effectively, indicated by the disappearance of C-H peaks in FT-IR spectra.

Fig. 3.3
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(A) FT-IR patterns of TiO2 nanocrystals prepared in (a) H2O and (b) ethanol before solvothermal treatment and in (c) ethanol, (d) H2O–ethanol = 1:4, (e) H2O–ethanol = 1:1, (f) H2O–ethanol = 4:1, and (g) H2O after solvothermal. Reprinted with permission from Ref. [9] copyright 2009 American Chemical Society; (B) TiO2 nanocrystals prepared by solvothermal method in (a) ethanol, (b) H2O–ethanol = 1:4, (c) H2O–ethanol = 1:1, (d) H2O–ethanol = 4:1, (e) H2O after removing the organic molecules adsorbed on surface by vacuum at 40 °C

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The photocatalytic activities of TiO2 nanocrystals were evaluated by decomposing phenol under UV light irradiation. The TiO2 particles prepared in the 1:4 water–ethanol solvent showed the highest activity due to its morphology [9]. Moreover, the photocatalytic activities of the vacuum treated sample show a slight improvement compared with the as-prepared samples, which should be attributed to cleaner surface (Table 3.1).

Table 3.1 The photocatalytic activities of TiO2 nanocrystals before and after vacuum treatment at 313 K
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2.2 Effects of Dopants on Crystal Phase and Shape of TiO2 Nanocrystals

Compared with anatase-type TiO2, rutile-type TiO2 is thermally stable, of higher refractive index and has wider sunlight absorption range. In addition, there is synergetic effect between anatase and rutile TiO2 on the photocatalytic properties [19]. However, it is difficult to prepare highly dispersed rutile TiO2 nanocrystals by high-temperature calcination methods or conventional hydrothermal methods.

Based on our previous works, rutile-type TiO2 nanocrystal colloids with small size (<10 nm) have been successfully synthesized from peroxo-metal-complex precursor by hydrothermal method in the presence of crystal phase-inducing agent Sn4+ [10, 12]. As shown in Fig. 3.4A, the XRD pattern of undoped TiO2 sample is indexed as anatase phase. With increasing Sn4+ doping concentration, TiO2 nanocrystals transferred from anatase to rutile gradually. Moreover, all diffraction peaks corresponding to SnO2 are not present in the pattern of all samples [20]. Based on these results, it can be inferred that either Sn4+ ion has been substituted into the crystal lattice sites of TiO2 or SnO2 exists as a highly dispersed polymeric form over the TiO2 surface, which could not be detected by XRD.

Fig. 3.4
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(A) XRD patterns of Ti1−x Sn x O2 nanocrystal colloids. A anatase, R rutile; (B) Proposed reaction mechanism: (a) undoped anatase TiO2 samples; (b) low level Sn4+-doped anatase Ti1−x Sn x O2 samples; (c) high level Sn4+-doped rutile Ti1−x Sn x O2 samples. Reprinted from Ref. [10] with permission from Elsevier

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The significant changes in the phase structure of Sn4+ doped TiO2 samples (Ti1−x Sn x O2) can be well explained by the addition of dopant Sn4+ ion. The ionic radius of Sn4+ is 0.069 nm, and it is likely to substitute Ti4+ (with an ionic radius of 0.068 nm) in the TiO2 lattice [21, 22]. Furthermore, both of them belong to the tetragonal crystal symmetry with the space group P42/mnm, and have very similar lattice parameters [23]. Therefore, the addition of Sn4+ into the reaction solution can assist the crystallization in rutile structure. Calculated by Scherrer formula, the anatase crystalline size gradually decreased from 16.1 to 9.8 nm with increasing Sn4+ content (from TiSn0 to TiSn3). After that, an obvious increase to 15.5 and 18.8 nm can be observed (samples TiSn4 and TiSn5). In the case of the rutile phase, the crystalline size slightly decreases from 8.0 to 7.0 nm with enhancing the Sn4+ doping levels from TiSn4 to TiSn10. Generally, Sn4+ doping TiO2 can inhibit the growth of crystal grains due to the presence of Ti–O–Sn [24]. However, an interesting trend for anatase crystalline size, that is, a first decrease at low dopant level and an obvious increase at high dopant level is observed, which should be ascribed to the fact that the dopant Sn4+ concentrated in rutile and has a lower effect on anatase when two phases coexist [25]. This assumption can be confirmed from the experiment results that rutile crystals are smaller than anatase crystals in the case of coexistence of anatase and rutile phase samples. According to the report by Overstone and Yanagisawa, a critical particle size is required for anatase to rutile transformation [26]. However, a different trend is obtained in our work, which indicates rutile formation occurs through a different mechanism, favored by tin incorporation.

The growth of nanocrystals in hydrothermal solution occurring in the presence of Sn4+ ions could be illustrated by the scheme in Fig. 3.4B. Firstly, the peroxotitanium complex is a dinuclear of two Ti4+ ions coordinated by peroxide ligands. This dinuclear peroxotitanium complex slowly condenses to polynuclear anions. After being heated, the peroxo groups decompose and the monomer [M(OH)4(OH2)2]0 (M:Ti or Sn) growing units are formed [27, 28]. In the case of low Sn4+ dopant level or undoped condition, octahedral [Ti(OH)4(OH2)2]0 is the dominant basic unit. When the basic unit octahedral [Ti(OH)4(OH2)2]0 joined together, sharing other edges leading to anatase has more chances than joining the opposite edge to form rutile (Fig. 3.4B, a and b) [29]. At high Sn4+ dopant level, the basic unit octahedral [Sn(OH)4(OH2)2]0 increases in the reaction medium. The more the [Sn(OH)4(OH2)2]0 exists, the more the rutile nuclei can be formed, which results in the [Ti(OH)4(OH2)2]0 units growing epitaxially on it (Fig. 3.4B, c) [30]. Thus, the addition of dopant Sn4+ affects the crystalline phase structure of the Ti1−x Sn x O2 samples. This possible growth mechanism was confirmed by XPS and ICP-AES analysis results, i.e., dopant Sn4+ tends more to form a nucleus and induces the formation of rutile at high doping concentration [10]. The effect of other dopants, such as W6+, was also investigated. The results shows the doping concentration of W6+ only affects the morphology of TiO2, and has no effect on crystal phase [14].

The photocatalytic activity of Sn4+ doped TiO2 nanocrystals was evaluated by the Fdegradation of phenol under UV light. The anatase nanocrystal with smaller size and larger surface area shows higher photocatalytic activity (Table 3.2).

Table 3.2 Kinetic values for the phenol photodecomposition and photocatalytic ability results for Ti1−x Sn x O2 nanocrystal colloids [10]
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2.3 Synergetic Effects of Solvent and Dopants

To better understand the effect of solvent and Sn4+ doping on crystal phase and morphology of TiO2 nanocrystals, we further investigated the crystallinity of pure SnO2 prepared in water, mixed water–ethanol solvent, ethanol, and 1-butanol, compared by XRD (Fig. 3.5A). The crystallinity of SnO2 prepared in mixed water–alcohol or alcohol solvent is relatively poor compared with SnO2 prepared in water. When using 1-butanol as reaction solvent, only anatase TiO2 nanocrystal is obtained even at high Sn4+ doping content (Fig. 3.5B), which may be due to the steric effects of alkyl chain of 1-butanol hindering the formation of rutile nuclei [11, 12]. As a result, Sn4+ doped anatase TiO2 can be obtained even in high Sn4+ doping concentration.

Fig. 3.5
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(A) XRD patterns of SnO2 nanocrystals prepared in (a) H2O, (b) H2O–ethanol = 4:1, (c) H2O–ethanol = 1:4, (d) ethanol, and (e) 1-butanol. Reprinted from Ref. [12]; (B) XRD patterns of Sn4+ doped TiO2 nanocrystals prepared in 1-butanol: (a) TiO2, (b) Ti0.97Sn0.03O2, (c) Ti0.95Sn0.05O2, and (d) Ti0.90Sn0.10O2. Reprinted from Ref. [11] with permission from Elsevier

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3 Summary

Crystal phase, size, and shape of TiO2 can be tuned by controlling the nucleation and crystal growth environment during solvothermal reaction. Here, we emphasize the effect of solvent and doping agent on crystal phase and morphology of TiO2 nanocrystals. One advantage of preparing TiO2 nanocrystals in a solvent mixture of alcohol and water by solvothermal method lies in that the small alcohol molecules adsorbed on the surface of as-prepared TiO2 nanocrystals can be removed easily by vacuum treatment at low temperature. As a consequence, the photocatalytic activity of TiO2 can be preserved by avoiding the high temperature calcination.