**2.1. TiO2**

Fig.1 c) [11–13] and reduce CO2 into organic fuels (see Fig.1 d) [13–15]. Besides the naturally abundant in nonrenewable energy sources such as solar energy can be renewed into chemical or electrical and thermal energies by using semiconductors having persisting materials in the process of photocatalysis [17-20]. Generally speaking, the mechanism of a typical powerdriven photocatalysis process is mainly owing to three critical related synergistic steps: (i) light absorption and charge excitation; (ii) charge separation and transport from the semiconductor particle to its surface active sites; (iii) surface photocatalytic chemical reactions, and this process

**Figure 1.** Photocatalytic mechanisms of water splitting, solar cell, degradation of pollutants, CO2 reduction via one-

Typically, the electron-hole pair with specific reduction and oxidation potential will be created on its conduction band (CB) and valence band (VB) under the irradiation of incident light with energy greater than the band gap of a given semiconductor. Here, the band gap of the semiconductor determines the utilization rate of the energy of the incident light, and the CB and VB values are the origin of the reduction and oxidation abilities of the photoexcited electrona and holes [21]. However, in practical process, the performance of photocatalysts is mainly related to two conditions: (i) the energy (*hv*) of the incident photon should be larger than the energy gap (*E*g) of the photocatalyst; (ii) the redox potential of reactants should be located between the CB and VB of the semiconductor photocatalyst. One the one hand, the former condition indicates a narrow band gap, which can facilitate the efficient utilization of

step photoexcitation. CB and VB represent the conduction and valence bands, respectively [16].

is similar to the fundamental mechanism of photocatalysis in power systems.

170 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

TiO2 has turned out to be one of the most commonly investigated semiconductors due to its low cost, long−term thermodynamic stability in aqueous solution, low toxicity, and high efficiency in the removal of pollutants in water and air as well as hydrogen generation [25-28]. There following are the four commonly known polymorphs of TiO2 found in nature: anatase (tetragonal), rutile (tetragonal), brookite (orthorhombic), and TiO2(B) (monoclinic) [29,30]. Rutile TiO2 has a tetragonal crystal structure and contains six atoms per unit cell as shown in Fig. 2 [31]. Rutile is the most thermodynamically stable polymorph of TiO2 at all temper‐ atures, exhibiting lower total free energy than metastable phases of anatase and brookite. Anatase TiO2 has a crystalline structure that corresponds to the tetragonal system but the distortion of the TiO6 octahedron is slightly larger for the anatase phase. Anatase is the most commonly used in photocatalytic applications due to its inherent superior photocatalytic properties [32-34].

**Figure 2.** Representations of the TiO2 anatase and rutile forms [31].

Anatase is the least thermodynamically stable TiO2 polymorph as a bulk phase, although, from energy calculations, it appears as the most stable phase when the grain size is below 10−20 nm [35,36]. The crystalline structure of the TiO2 oxides can be described in terms of TiO6 octahedral chains. These differ by the distortion of each octahedron and the assembly pattern of the resulting octahedral chains. The Ti−Ti distances in the anatase structure are greater than in rutile, while the Ti−O distances are shorter [37]. These structural differences lead to different mass densities as well as different electronic band structures. As a result, the anatase phase is 9% less dense than rutile and it presents more pronounced localization of the Ti 3d states and further a narrower 3d band. Also, the O 2p−Ti 3d hybridization is different in the two struc‐ tures. Anatase exhibits a valence and conduction band with more pronounced O 2p−Ti 3d character and less nonbonding self-interaction between similar ions (e.g., anion-anion and cation-cation interactions) [38]. The importance of the covalent vs ionic contributions to the metal-oxygen bond has already been discussed in a more general context for Ti oxides [39,40]. Therefore, it could be claimed that differential structural characteristics between anatase and rutile of TiO2 are possibly attributed to the difference in the mobility of the charge carriers upon light excitation.

In 1972, K. Honda and A. Fujishima discovered the photosensitization effect of a TiO2 electrode on the electrolysis of H2O into H2 and O2 using a Pt metal electrode as cathode and a TiO2 photoanode irradiated with UV light. They found that, under UV light irradiation of the TiO2 electrode, the electrolysis of H2O proceeded at a much lower bias voltage as compared with normal electrolysis [41]. From then on, the TiO2-based photocatalyst have been exten‐ sively studied in the past few decades due to its proper energy bandgap that matches the UV −visible light irradiation, which favors many light-driven applications [42-47]. Moreover, TiO2 has got many advantages and the nature of this material is naturally abundant, commer‐ cially available, economically viable, chemically stable, non-toxic and environmental eco −friendly [44]. However, TiO2 has also faced few problems as photocatalysts in applying solar energy processes due to its low sunlight spectrum matching, limited activity and reduced sensitivity [48]. To overcome this shortage, recently many researchers have developed many different modification methods to TiO2 material to make it as a potential challenging material for highly active photocatalyst [48-55]. Among these works, crystal growth, doping and heterostructuring of semiconductor photocatalysts are commonly used and can substantial tune the light-response range, redox potentials of photoinduced charge carriers, and electronhole pair separation probability within the photocatalysts. Specifically, crystal growth can be critical in controlling the phase, shape, and size of photocatalysts, as well as their crystallinity and specific surface area. By rationally controlling crystal growth, the intrinsic surface atomic structure and resultant surface states of the derived photocatalysts can be adjusted. For materials design, doping effect can exert a substantial influence on modifying the electronic structure and the construction of heteroatomic surface structures of the aiming material. In particular, nonmetal doping (N [56,57], C [58-60], S [61,62], B [63-65], F [66-68], Br [69], I [70-73], P [74]) in photocatalyst has attracted increasing attention due to its effectiveness in realizing visible−light photocatalytic activity of wide bandgap semiconductor photocatalysts. The chemical states and locations of dopants are considered to be key factors in adjusting the spectral distribution of the induced electronic states of those dopants and reconstructing favorable surface structure for photocatalysis. The hybrids of two or more semiconductor systems, that is, heterostructures, seem to be possess advantageous in more efficiently utilizing solar light by combining different electronic structures when compared with sing-phase semiconductor photocatalysts. Furthermore, an efficient photo-excited electron or hole transfer from one component to another with proper band edge matching can greatly decrease the electron-hole recombination probability and increase the lifetime of charge carriers, which further promoting the photocatalytic efficiency. In addition to the basic requirements of electronic structure for each unit in the integrated photocatalytic systems, a favorable interface contact between the two materials is essential in promoting interface charge carrier transfer through different pathways. Fig. 3 demonstrates the connection between crystal growth, doping and hetero-structure of semiconductors for heterogeneous photocatalysis, (CB: conduction band; VB: valence band) [75].

**Figure 2.** Representations of the TiO2 anatase and rutile forms [31].

172 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

upon light excitation.

Anatase is the least thermodynamically stable TiO2 polymorph as a bulk phase, although, from energy calculations, it appears as the most stable phase when the grain size is below 10−20 nm [35,36]. The crystalline structure of the TiO2 oxides can be described in terms of TiO6 octahedral chains. These differ by the distortion of each octahedron and the assembly pattern of the resulting octahedral chains. The Ti−Ti distances in the anatase structure are greater than in rutile, while the Ti−O distances are shorter [37]. These structural differences lead to different mass densities as well as different electronic band structures. As a result, the anatase phase is 9% less dense than rutile and it presents more pronounced localization of the Ti 3d states and further a narrower 3d band. Also, the O 2p−Ti 3d hybridization is different in the two struc‐ tures. Anatase exhibits a valence and conduction band with more pronounced O 2p−Ti 3d character and less nonbonding self-interaction between similar ions (e.g., anion-anion and cation-cation interactions) [38]. The importance of the covalent vs ionic contributions to the metal-oxygen bond has already been discussed in a more general context for Ti oxides [39,40]. Therefore, it could be claimed that differential structural characteristics between anatase and rutile of TiO2 are possibly attributed to the difference in the mobility of the charge carriers

In 1972, K. Honda and A. Fujishima discovered the photosensitization effect of a TiO2 electrode on the electrolysis of H2O into H2 and O2 using a Pt metal electrode as cathode and a TiO2 photoanode irradiated with UV light. They found that, under UV light irradiation of the TiO2 electrode, the electrolysis of H2O proceeded at a much lower bias voltage as compared According to the Wulff construction and calculated surface energy, the shape of anatase under equilibrium conditions is a slightly truncated tetragonal bipyramid enclosed with eight isosceles trapezoidal surfaces of {101} and two top squares of {001}, as shown in Fig. 4 [76] It is predicted that the percentage of {101} is as high as 94%. Although the surface energy of {010} (0.53 J m −2) was calculated to be between {001} (0.90 J m−2) and {101} (0.44 J m −2) [77], it is surprising that no {010} will appears in the equilibrium shape of anatase. Anatase TiO2 is usually exposed with low-index facets. Theoretical calculations indicate that the (101) surface (0.44 J m−2) is the thermodynamically the most stable surface, the (001) surface (0.90 J m−2) is

**Figure 3.** Correlation of key factors in crystal growth, doping and heterostructuring of semiconductors for photocataly‐ sis. (CB: conduction band; VB:valence band) [75].

the most active and the (100) and (110) surfaces are between the (101) and (001) surfaces. As a consequence, facets that have a high surface energy diminish quickly in the minimization of surface energy during the crystal-growth process. Therefore, a large percentage of high active facets has become a popular target in the synthesis of anatase TiO2 crystals. In the case of rutile, the predicted equilibrium shape of a macroscopic crystal was constructed with (110), (100), (001) and (011) faces (see Fig. 4) [78]. It is found that in the equilibrium shape, the most stable (110) face with the lowest surface energy of 15.6 meV au- 2 dominates the shape, whereas (001) with the highest surface energy of 28.9 meV au−2 does not exist at all. Gong et al. demonstrated the systematic results of the structures and energetics of 10 stoichiometric 1×1 low-index surfaces with different possible terminations of brookite [79]. The determining factors of the relative stabilities of different faces are found to be negatively related to the concentration of exposed coordinatively unsaturated Ti atoms. The equilibrium shape of brookite crystal is shown in Fig. 4, we can observe that the most of it is composed of (111), (210), (010) and reconstructed (001) facets. It is worth noting that brookite (210) is one of the most stable facets, which has a very similar atomic structure to the most stable facet (101) of anatase. However, their electronic states are different, which may result in different chemical reactivities [80].

Usually, different facets of a single−crystalline material possess distinctive adsorption, catalytic reactivity and selectivity, which are caused by its different geometric and electronic structures [81]. Since Lu and his coworkers first reported that the uniform anatase single

minimization of surface energy during the crystal‐growth process. Therefore, a large percentage of high

 According to the Wulff construction and calculated surface energy, the shape of anatase under equilibrium conditions is a slightly truncated tetragonal bipyramid enclosed with eight isosceles trapezoidal surfaces of {101} and two top squares of {001}, as shown in Fig. 4[76] It is predicted that the percentage of {101} is as high as 94%. Although the surface energy of {010} (0.53 J m 2) was calculated to be between {001} (0.90 J m2) and {101} (0.44 J m 2)[77], it is surprising that no {010} will appears in the equilibrium shape of anatase. Anatase TiO2 is usually exposed with low‐index facets. Theoretical calculations indicate that the (101) surface (0.44 J m2) is the thermodynamically the most stable surface,

heterogeneous photocatalysis, ( CB: conduction band; VB: valence band)[75].

growth, the intrinsic surface atomic structure and resultant surface states of the derived photocatalysts can be adjusted. For materials design, doping effect can exert a substantial influence on modifying the electronic structure and the construction of heteroatomic surface structures of the aiming material. In particular, nonmetal doping (N[56,57], C[58‐60], S[61,62], B[63‐65], F[66‐68], Br[69], I[70‐73], P[74]) in photocatalyst has attracted increasing attention due to its effectiveness in realizing visiblelight photocatalytic activity of wide bandgap semiconductor photocatalysts. The chemical states and locations of dopants are considered to be key factors in adjusting the spectral distribution of the induced electronic states of those dopants and reconstructing favorable surface structure for photocatalysis. The hybrids of two or more semiconductor systems, that is, heterostructures, seem to be possess advantageous in more efficiently utilizing solar light by combining different electronic structures when compared with sing‐phase semiconductor photocatalysts. Furthermore, an efficient photo‐excited electron or hole transfer from one component to another with proper band edge matching can greatly decrease the electron‐hole recombination probability and increase the lifetime of charge carriers, which further promoting the photocatalytic efficiency. In addition to the basic requirements of electronic structure for each unit in the integrated photocatalytic systems, a favorable interface contact between the two materials is essential in promoting interface charge carrier transfer through different pathways. Fig. 3 demonstrates the connection between crystal growth, doping and hetero‐structure of semiconductors for

**Fig 4.** The equilibrium shape of a TiO2 crystal in the anatase, rutile and brookite, according to the Wulff construction and the calculated surface energies[76, 78,79]. **Figure 4.** The equilibrium shape of a TiO2 crystal in the anatase, rutile and brookite, according to the Wulff construc‐ tion and the calculated surface energies [76, 78,79].

crystals with 47% {001} facets displayed superior photoactivity [82], crystal facet engineering has proven to be an effective strategy to finely tune the efficiency and selectivity of heteroge‐ neous photocatalysts for different applications. Besides, different crystal facets can also facilitate the separation of electrons and holes [83]. To date, many improved synthesis procedures have been successfully developed and lots of exciting advances have been achieved [84-99]. Lu and coworkers demonstrated that, under UV light irradiation, the sheet-like anatase TiO2 crystal dominated by {001} facets is capable of producing OH that is more than five times higher than that of Degussa P25 TiO2 [85]. They concluded that the high density unsaturated five-fold Ti and their unique electronic structures of the {001} facets should be responsible for the improved photoactivity. Similar results also have been reported by other groups. For example, Han et al. [84] reported that the photocatalytic ability of TiO2 nanosheets with {001} facets was higher than that of P25 in the degradation of methyl orange(MO) molecules. Zhang et al. [86] successfully synthesized the a remarkable 80% level of reactive {001} facets microsheet anatase TiO2 single−crystal photocatalyst, which exhibited much better photocatalytic per‐ formance in the oxidative decomposition of organic pollutant. By tuning the percentage of the {001} facets, the photoreactivity was enhanced from 40.0% to 84.5%. The reactive {001} facets played an important role in the photocatalytic reaction owing to their strong ability to dissociatively adsorb water to form hydrogen peroxide and peroxide radicals. Although highenergy {001} facets have been widely studied, anatase TiO2 crystals with higher-energy {100} facets have been less well developed. Recently, Li and Xu [100] reported a facile hydrothermal route for the synthesis of tetragonal−faceted nanorods (NRs) of anatase TiO2 with highly exposed higher-energy {100} facets, which exhibited higher reactivity owing to the large percentage of {100} facets compared with crystals that have normal majority {101} facets. active facets has become a popular target in the synthesis of anatase TiO2 crystals. In the case of rutile, the predicted equilibrium shape of a macroscopic crystal was constructed with (110), (100), (001) and (011) faces (see Fig. 4)[78]. It is found that in the equilibrium shape, the most stable (110) face with the lowest surface energy of 15.6 meV au‐ <sup>2</sup> dominates the shape, whereas (001) with the highest surface energy of 28.9 meV au<sup>2</sup> does not exist at all. Gong et al. demonstrated the systematic results of the

the most active and the (100) and (110) surfaces are between the (101) and (001) surfaces. As a consequence, facets that have a high surface energy diminish quickly in the minimization of surface energy during the crystal-growth process. Therefore, a large percentage of high active facets has become a popular target in the synthesis of anatase TiO2 crystals. In the case of rutile, the predicted equilibrium shape of a macroscopic crystal was constructed with (110), (100), (001) and (011) faces (see Fig. 4) [78]. It is found that in the equilibrium shape, the most stable (110) face with the lowest surface energy of 15.6 meV au- 2 dominates the shape, whereas (001) with the highest surface energy of 28.9 meV au−2 does not exist at all. Gong et al. demonstrated the systematic results of the structures and energetics of 10 stoichiometric 1×1 low-index surfaces with different possible terminations of brookite [79]. The determining factors of the relative stabilities of different faces are found to be negatively related to the concentration of exposed coordinatively unsaturated Ti atoms. The equilibrium shape of brookite crystal is shown in Fig. 4, we can observe that the most of it is composed of (111), (210), (010) and reconstructed (001) facets. It is worth noting that brookite (210) is one of the most stable facets, which has a very similar atomic structure to the most stable facet (101) of anatase. However, their electronic states are different, which may result in different chemical reactivities [80].

**Figure 3.** Correlation of key factors in crystal growth, doping and heterostructuring of semiconductors for photocataly‐

sis. (CB: conduction band; VB:valence band) [75].

174 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Usually, different facets of a single−crystalline material possess distinctive adsorption, catalytic reactivity and selectivity, which are caused by its different geometric and electronic structures [81]. Since Lu and his coworkers first reported that the uniform anatase single

These results discussed above demonstrated that a higher density of surface−unsaturated atoms will lead to a high surface energy of the crystal facets, which generally exhibit better photocatalytic performance. However, recent studies have shown that a high surface energy does not always make the crystal facets highly reactive in photocatalytic reactions. For example, Liu et al. [97] demonstrated a raised conduction band of nanosized single crystals of anatase TiO2 with 82% {101} facets compared to the crystals with 72% {001} facets, which is determined by UV/Vis adsorption spectroscopy. This different electronic−band difference will further lead to a difference in atomic coordination, and this decrease will result in an enhanced photoactivity in the splitting of water into hydrogen. This example shows that the band-gap of crystal facets or crystal plates will change as the change in the arrangement of surface atoms. As a result, the redox power of the photoexcited electrons and holes will be correspondingly changed.

Generally speaking, the {101} facets are more reductive than {001} facets, which could act as possible tanks of photogenerated electrons, while {001} facets act as oxidation sites, which play a major role in the photooxidative processes [101,102]. For example, Pan et al. reported that low−index facets of anatase TiO2 follow the photoreactivity order of {001} < {101} < {010} for photocatalytic hydrogen evolution and⋅OH radical generation [103]. Similarly, a seeded growth technique also demonstrated that the {101} facets of anatase TiO2 are more active than the {001} facets for photocatalytic water splitting [104]. Surprisingly, it was even found that the photocatalytic activity for H2 production over the {111} facet exposed anatase TiO2 is about 5, 9, and 13 times higher than that of the TiO2 sample exposed with dominant {010}, {101}, and {001} facets, respectively [105]. However, most researchers ignored the synergetic effects of various co-exposed facets in one sample. More attention has to be paid to finding special facets rather than the balanced ratio of different exposed facets for the best photocatalytic efficiency of water splitting.

Recently, Yu's group found that an optimal ratio of the exposed {101} and {001} facets of TiO2 played a significant role in the enhancement of photocatalytic performance for the reduction of CO2 [106]. As shown in Fig. 5, the surplus electrons on the {101} facets will overflow onto the {001} facets and then have a fast recombination with the holes on the {001} facets if the percentage of {101} facets is too low to hold all the photoexcited electrons, this process will lead to a decrease in the photocatalytic activity. The results clearly showed that it is of great importance to find the balanced ratio of different exposed facets in achieving the best photo‐ catalytic efficiency [107]. This finding may shed light on the design and fabrication of advanced nanosheet-based semiconductors for water splitting.

**Figure 5.** The electron overflow effect on the {101} facets of TiO2 [107].

In 2011, crystal facet dependence of TiO2 photocatalysis has been evidenced by using singlemolecule imaging and kinetic analysis [108]. Single−particle spectroscopy (microscopy) has been used to explore the structural and kinetic features of "bulk" catalysis because of its high sensitivity and selectivity. This study demonstrated that the reaction sites for the effective reduction of the probe molecules were preferentially located on the {101} facets of the crystal rather than on the high surface energy {001} facets. This preference originated from the unique properties of the {101} facets in terms of their electron-trapping probability induced by the specific facet. This observation emphasizes the important role of the {101} surfaces as the reductive site in TiO2 photocatalysis and is in agreement with the conclusion that the reactivity of the {001} facets towards oxidation is higher than that of the {101} facets.

determined by UV/Vis adsorption spectroscopy. This different electronic−band difference will further lead to a difference in atomic coordination, and this decrease will result in an enhanced photoactivity in the splitting of water into hydrogen. This example shows that the band-gap of crystal facets or crystal plates will change as the change in the arrangement of surface atoms. As a result, the redox power of the photoexcited electrons and holes will be correspondingly

Generally speaking, the {101} facets are more reductive than {001} facets, which could act as possible tanks of photogenerated electrons, while {001} facets act as oxidation sites, which play a major role in the photooxidative processes [101,102]. For example, Pan et al. reported that low−index facets of anatase TiO2 follow the photoreactivity order of {001} < {101} < {010} for photocatalytic hydrogen evolution and⋅OH radical generation [103]. Similarly, a seeded growth technique also demonstrated that the {101} facets of anatase TiO2 are more active than the {001} facets for photocatalytic water splitting [104]. Surprisingly, it was even found that the photocatalytic activity for H2 production over the {111} facet exposed anatase TiO2 is about 5, 9, and 13 times higher than that of the TiO2 sample exposed with dominant {010}, {101}, and {001} facets, respectively [105]. However, most researchers ignored the synergetic effects of various co-exposed facets in one sample. More attention has to be paid to finding special facets rather than the balanced ratio of different exposed facets for the best photocatalytic efficiency

Recently, Yu's group found that an optimal ratio of the exposed {101} and {001} facets of TiO2 played a significant role in the enhancement of photocatalytic performance for the reduction of CO2 [106]. As shown in Fig. 5, the surplus electrons on the {101} facets will overflow onto the {001} facets and then have a fast recombination with the holes on the {001} facets if the percentage of {101} facets is too low to hold all the photoexcited electrons, this process will lead to a decrease in the photocatalytic activity. The results clearly showed that it is of great importance to find the balanced ratio of different exposed facets in achieving the best photo‐ catalytic efficiency [107]. This finding may shed light on the design and fabrication of advanced

In 2011, crystal facet dependence of TiO2 photocatalysis has been evidenced by using singlemolecule imaging and kinetic analysis [108]. Single−particle spectroscopy (microscopy) has been used to explore the structural and kinetic features of "bulk" catalysis because of its high

changed.

of water splitting.

nanosheet-based semiconductors for water splitting.

176 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 5.** The electron overflow effect on the {101} facets of TiO2 [107].

This aforementioned investigation shows that the reactivity of a photocatalyst can be control‐ led by tuning the exposed facets. Another tool-morphology control, which means the forma‐ tion of different surface facets with different surface atomic structures, also provides an effective method to tune the selectivity of the photocatalysts. For example, Liu et al. [95] reported a fluoride-mediated self-transformation method for fabricating hollow TiO2 micro‐ spheres (HTS) with anatase polyhedra with about 20% exposed {001} facets. The fluorinated HTS exhibited preferential decomposition of methyl orange (MO) compared with methyl blue (MB). In contrast, surface-modified HTS that was either washed with NaOH or calcined at 600°C favored the decomposition of MB over MO. This example demonstrated the importance of the surface structure in modifying the catalytic selectivity of titania. Therefore, it is expected that, by controlling the exposed facets, we can design photocatalysts with both high reactivity and high selectivity. The photocatalysts that are terminated by specific facets allow the same adsorption states of reactant molecules and generate photoexcited electrons of similar energies on the specific facets. It is worth noting that these properties will be beneficial for the solarinduced selective photoconversion of carbon dioxides into specific valuable fuels because this process typically requires an undesired separation process and produces mixed hydrocarbons, including CH4, CH3OH, and HCOOH. It is believed that the breakthrough in making specific facets will intensify the development of selective organic transformations that are based on semiconductor photocatalysts.

High-index facets of nanomaterials usually have unique surface atomic structures, such as a high density of atomic steps, dangling bonds, kinks, and ledges, which can all act as active sites. Unfortunately, these unique surface atomic structures always have a high surface energy and high crystal growth rate, which is not naturally preferential growth and is easy to rapidly diminish during the crystal-growth process, so it is quite challenging to synthesize tailor−made crystals.

Yang and coworkers first reported the formation of anatase TiO2 crystals that are exposed by high−index {105} facets [109]. They produced the product with well−faceted surface by a modified high−temperature gas−phase oxidation route with titanium tetrachloride (TiCl4) as the Ti source. During the TiCl4 oxidization process, the co−adsorption of oxygen, chlorine, or other related species will occur and may specifically lower the Gibbs free energy of the {105} facets thus the typical atomic configuration on the {105} facets can be stabilized and reserved. The unique stepped atomic configuration on the high−index {105} facets makes these materials promising candidates in the areas of renewable clean energy and environmental remediation.

Rutile is the thermodynamically stable phase of TiO2 polymorphs, which can be obtained by typically three methods: (i) the hydrolysis of Ti precursors and subsequent crystallization; (ii) the post−transformation from anatase/brookite phase via thermal treatment (phase transfor‐ mation temperature required depends on the particle size of TiO2) [110] and (iii) mechanical processing [111]. Although rutile is considered to be less active in photocatalytic reactions compared to anatase, nanostructured rutile has also been used photocatalysis applications and in some cases show even higher activity than anatase. Band gap of rutile TiO2 is 0.2 eV smaller than anatase one and further results in a wider absorption range, which may be the advantage of this phase.

Afterwards, various morphologies of rutile have been developed [112-119], with the nanorod being is a common morphology. The synthesis routes of such rutile nanorods with a high aspect ratio have been well documented in the literature [114,119,120-129]. Generally speaking, the presence of Cl ions as mineralizer in the synthesis system is favourable for rutile TiO2, regardless of the source of Cl. In the case of the specific synthesis routes of controlling morphology of rutile, there are two representative examples demonstrate the formation of faceted rutile crystals. One is the rapid formation of self-assembled microspheres with rutile nanorods by microwave heating of TiCl3 at 200°C for only 1 min [115]. The nanorods are exposed with {110} and {111} facets, but because of the extremely rapid growth rate, the surface is not smooth. Interestingly, the synthetic rutile nanorods have a smaller bandgap of 2.8 eV compared with the conventional 3.0 eV, which may facilitate the photocatalysis ability under visible light irradiation. The other one is reported by Kakiuchi et al. [116], who observed the dependence of degree of perfection of facets on hydrothermal temperature, where TiCl3 was also used as a precursor together with NaCl additive. For example, at low temperature (80°C), only needle−like nanorods without well−recognized facets were formed. However, when elevating the temperature to 200°C, well−developed lateral {110} and top {111} facets can be observed. Apparently, this result indicates that a higher temperature is favorable for growing crystals with well-developed facets.

Compared with anatase and rutile, brookite phase TiO2 has attracted little interest due to the generally considered lack of photocatalytic activity. However, increasing literatures have shown that brookite is also photocatalytically active and even has unique photocatalytic properties in some cases [130-133]. However, among the synthetic brookites, crystal facets are usually non-recognizable. Interestingly, Buonsanti et al. [134] developed a nonhydrolytic synthesis route to successfully prepare high−quality anisotropically shaped brookite nanorods with a length of 30−200 nm. These rods are determined to be dominantly enclosed with the longitudinal {210}/{100} and basal {001} facet, which is in agreement with the equilibrium shape of brookite crystals predicted from the Wulff construction.
