Wet-Chemical Preparation of TiO2-Based Composites with Different Morphologies and Photocatalytic Properties

TiO2-based composites have been paid significant attention in the photocatalysis field. The size, crystallinity and nanomorphology of TiO2 materials have an important effect on the photocatalytic efficiency. The synthesis and photocatalytic activity of TiO2-based materials have been widely investigated in past decades. Based on our group’s research works on TiO2 materials, this review introduces several methods for the fabrication of TiO2, rare-earth-doped TiO2 and noble-metal-decorated TiO2 particles with different morphologies. We focused on the preparation and the formation mechanism of TiO2-based materials with unique structures including spheres, hollow spheres, porous spheres, hollow porous spheres and urchin-like spheres. The photocatalytical activity of urchin-like TiO2, noble metal nanoparticle-decorated 3D (three-dimensional) urchin-like TiO2 and bimetallic core/shell nanoparticle-decorated urchin-like hierarchical TiO2 are briefly discussed.


Introduction
Based on its unique chemical and physical characteristics, titanium dioxide (TiO 2 ) has attracted much attention in many fields including paint pigments, photocatalysis, solar cells, antibacterial agents, electrical energy storage and some advanced functional materials [1][2][3]. The performance in these applications strongly depends on the microstructure, crystallinity and nanomorphology of TiO 2 [1]. In particular, in the photocatalysis field, although a new type of polymeric photocatalyst-that is, graphitic carbon nitride-has been intensively investigated recently due to its huge advantages-including metal-free contents, visible light absorption ability, suitable band gap for water splitting, stability, and being environmentally benign [4][5][6][7]-TiO 2 is still regarded as one of most ideal candidates for photocatalysis because of its strong oxidization, harmlessness to surroundings, chemical inactivity, good stability and low cost [1,3,8,9]. There are three important processes including photo excitation, bulk diffusion and the surface transfer of photoinduced charge carriers in photocatalysis [8]. Thus, the performance of photocatalysis depends strongly upon the charge transfer at the material surface and the light-response range of materials [1,10,11]. The processes of light harvesting and charge transfer efficiencies are affected mainly by the size, crystallinity and nanomorphology of TiO 2 materials [1,8,[10][11][12][13][14]. The preparation and photocatalytic properties of TiO 2 with different morphologies including zero-dimensional (micro/nanospheres), one-dimensional (rods, tubes, and nanowires), two-dimensional (films, layers and sheets), and three-dimensional (porous spheres, urchin-like spheres) TiO 2 structures have been widely investigated in the past decade [15][16][17][18][19][20][21]. Different ways have been developed for preparing TiO 2 materials with different nanostructures. The general synthesis approaches for the fabrication of TiO 2 materials include sol-gel, hydrothermal

Hollow Spheres
Due to high specific surface area and low density, hollow structured materials have been widely used in many fields [54]. TiO2 hollow spheres with a well-defined crystal phase are highly desirable for photocatalysis use [55][56][57]. Hollow structured TiO2 can be feasibly synthesized by hard template and soft template methods. Compared to the soft template method, the hard template method is simpler, and so it is frequently used.
An et al. have used polystyrene (PS) spheres as a hard template to prepare hollow Sm 3+ -doped TiO2 spheres [29]. The schematic illustration of the formation mechanism is shown in Figure 2. Since the surface of PS spheres obtained by surfactant-free microemulsion polymerization is negatively charged, no additional surface modification of PS spheres is needed for the next coating of TiO2. In an ethanol/acetonitrile mixed solvent, a small amount of ammonia was used to induce the hydrolysis of tetrobutyl titanate to form the amorphous Sm 3+ -doped TiO2 coating layer on the surface of the PS spheres. After washing with ethanol, drying, and annealing, hollow TiO2:Sm 3+ spheres, as shown in Figure 3, could be obtained. Figure 2. Schematic illustration of the formation mechanism for hollow TiO2:Sm 3+ spheres [29].

Hollow Spheres
Due to high specific surface area and low density, hollow structured materials have been widely used in many fields [54]. TiO 2 hollow spheres with a well-defined crystal phase are highly desirable for photocatalysis use [55][56][57]. Hollow structured TiO 2 can be feasibly synthesized by hard template and soft template methods. Compared to the soft template method, the hard template method is simpler, and so it is frequently used.
An et al. have used polystyrene (PS) spheres as a hard template to prepare hollow Sm 3+ -doped TiO 2 spheres [29]. The schematic illustration of the formation mechanism is shown in Figure 2. Since the surface of PS spheres obtained by surfactant-free microemulsion polymerization is negatively charged, no additional surface modification of PS spheres is needed for the next coating of TiO 2 . In an ethanol/acetonitrile mixed solvent, a small amount of ammonia was used to induce the hydrolysis of tetrobutyl titanate to form the amorphous Sm 3+ -doped TiO 2 coating layer on the surface of the PS spheres. After washing with ethanol, drying, and annealing, hollow TiO 2 :Sm 3+ spheres, as shown in Figure 3, could be obtained. were crystallized into the anatase phase. Figure 1 shows the quasi-monodisperse TiO2 submicron spheres with different diameters within 200-800 nm, synthesized with different polymers.

Hollow Spheres
Due to high specific surface area and low density, hollow structured materials have been widely used in many fields [54]. TiO2 hollow spheres with a well-defined crystal phase are highly desirable for photocatalysis use [55][56][57]. Hollow structured TiO2 can be feasibly synthesized by hard template and soft template methods. Compared to the soft template method, the hard template method is simpler, and so it is frequently used.
An et al. have used polystyrene (PS) spheres as a hard template to prepare hollow Sm 3+ -doped TiO2 spheres [29]. The schematic illustration of the formation mechanism is shown in Figure 2. Since the surface of PS spheres obtained by surfactant-free microemulsion polymerization is negatively charged, no additional surface modification of PS spheres is needed for the next coating of TiO2. In an ethanol/acetonitrile mixed solvent, a small amount of ammonia was used to induce the hydrolysis of tetrobutyl titanate to form the amorphous Sm 3+ -doped TiO2 coating layer on the surface of the PS spheres. After washing with ethanol, drying, and annealing, hollow TiO2:Sm 3+ spheres, as shown in Figure 3, could be obtained.  Schematic illustration of the formation mechanism for hollow TiO 2 :Sm 3+ spheres [29].

Porous Spheres
Due to their high surface area, porous materials are very popular for different applications including energy storage, solar cells and catalyzers [58][59][60]. Mesoporous TiO2-based materials have attracted much attention for their enhanced reactivity and light harvesting [60]. The macrochannels in mesoporous TiO2 particles have served as a light-transfer path that can introduce incident photon flux to the interior surface of the TiO2 particles [58]. A mesoporous structure gives light waves more chances to penetrate deep inside the photocatalyst and more light waves are captured. The crystallinity, pore size and composition are important for tuning the properties of mesoporous TiO2 spheres [1,8,60]. There has been intensive research concentrated on the design and preparation of porous TiO2 materials with unique porosities and tunable pore sizes [61][62][63][64][65].
We have synthesized mesoporous Ce-doped TiO2 spheres by a low-temperature hydrothermal method by using neutral dodecylamine (DDA) as a surfactant and tetrabutyl titanate as a Ti source [64]. To control the rate of hydrolysis of tetrabutyl titanate, a solvent mixture of ethanol and propanol (2:1, v/v) was used. No additional water was used to initialize the hydrolysis and condensation reaction of the tetrabutyl titanate, due to the used CeCl3·7H2O containing structured water. After the CeCl3·7H2O was dissolved, the structured water was released. DDA was able to make the solution alkaline and this made it easy to increase the rate of hydrolysis of the tetrabutyl titanate. However, the dissolution of CeCl3·7H2O also could result in a decrease of the pH value of solution. Thus, CeCl3·7H2O could service as not only as a dopant but also as an initiator and buffer. After reaction, a precipitate was formed and it was further refluxed for 2 h at 80 °C in an acid solution to get rid of the template and obtain sphere-like mesoporous Ce-doped TiO2 particles with a diameter of 100-1000 nm as shown in Figure 4. The XRD (X-ray diffraction) patterns in Figure 4 show that the TiO2 particles are semi-crystalline. The formed anatase crystalline size is very small, about 2-3 nm. In addition, from the TEM (Transmission Electron Microscopy) image, it could be found that the pore structure was worm-like, with a size of 2-3 nm. The Brunauer-Emmett-Teller (BET) surface area of the mesoporous Ce-doped TiO2 was 118 m 2 /g, which is much higher than the 9.6 m 2 /g of single-doped TiO2 obtained without a surfactant.
The mesoporous Eu-doped TiO2 spheres have also been developed by the hydrolysis of tetrabutyl titanate [28]. To increase the thickness of the pore wall, nonionic copolymer Pluronic F-127

Porous Spheres
Due to their high surface area, porous materials are very popular for different applications including energy storage, solar cells and catalyzers [58][59][60]. Mesoporous TiO 2 -based materials have attracted much attention for their enhanced reactivity and light harvesting [60]. The macrochannels in mesoporous TiO 2 particles have served as a light-transfer path that can introduce incident photon flux to the interior surface of the TiO 2 particles [58]. A mesoporous structure gives light waves more chances to penetrate deep inside the photocatalyst and more light waves are captured. The crystallinity, pore size and composition are important for tuning the properties of mesoporous TiO 2 spheres [1,8,60]. There has been intensive research concentrated on the design and preparation of porous TiO 2 materials with unique porosities and tunable pore sizes [61][62][63][64][65].
We have synthesized mesoporous Ce-doped TiO 2 spheres by a low-temperature hydrothermal method by using neutral dodecylamine (DDA) as a surfactant and tetrabutyl titanate as a Ti source [64]. To control the rate of hydrolysis of tetrabutyl titanate, a solvent mixture of ethanol and propanol (2:1, v/v) was used. No additional water was used to initialize the hydrolysis and condensation reaction of the tetrabutyl titanate, due to the used CeCl 3 ·7H 2 O containing structured water. After the CeCl 3 ·7H 2 O was dissolved, the structured water was released. DDA was able to make the solution alkaline and this made it easy to increase the rate of hydrolysis of the tetrabutyl titanate. However, the dissolution of CeCl 3 ·7H 2 O also could result in a decrease of the pH value of solution. Thus, CeCl 3 ·7H 2 O could service as not only as a dopant but also as an initiator and buffer. After reaction, a precipitate was formed and it was further refluxed for 2 h at 80 • C in an acid solution to get rid of the template and obtain sphere-like mesoporous Ce-doped TiO 2 particles with a diameter of 100-1000 nm as shown in Figure 4. The XRD (X-ray diffraction) patterns in Figure 4 show that the TiO 2 particles are semi-crystalline. The formed anatase crystalline size is very small, about 2-3 nm. In addition, from the TEM (Transmission Electron Microscopy) image, it could be found that the pore structure was worm-like, with a size of 2-3 nm. The Brunauer-Emmett-Teller (BET) surface area of the mesoporous Ce-doped TiO 2 was 118 m 2 /g, which is much higher than the 9.6 m 2 /g of single-doped TiO 2 obtained without a surfactant. spherical morphology and a mesoporous structure, with a pore size of 7-10 nm. The special surface area of the phosphor particles is 158 m 2 /g. The high resolution TEM images in Figure 5c show that the pore wall is semi-crystalline that many anatase nanocrystallites are dispersed in the amorphous TiO2. The XRD patterns showed in Figure 5d have indicated that no peaks corresponding to the europium compound was detected and no shift of the anatase peaks was observed after doping with Eu 3+ . It can be concluded that Eu 3+ ions are mainly dispersed in the amorphous TiO2 region.   The mesoporous Eu-doped TiO 2 spheres have also been developed by the hydrolysis of tetrabutyl titanate [28]. To increase the thickness of the pore wall, nonionic copolymer Pluronic F-127 was used as a template agent. Europium ethoxide was specially prepared as a doping agent in order to increase the rate of hydrolysis and condensation of the tetrabutyl titanate. Small amounts of water were added to initiate the reaction under stirring. As soon as the solution became slightly white after several minutes, the stirring was stopped. Meanwhile, in order to control the water content, the preparation was conducted under the protection of flowing N 2 . After aging for 24-48 h at 35-40 • C, the suspended particles were filtered and washed with ethanol several times. The final products were obtained after calcination at 400 • C for 4 h. The synthesized Eu-doped TiO 2 particles have a spherical morphology and a mesoporous structure, with a pore size of 7-10 nm. The special surface area of the phosphor particles is 158 m 2 /g. The high resolution TEM images in Figure 5c show that the pore wall is semi-crystalline that many anatase nanocrystallites are dispersed in the amorphous TiO 2 . The XRD patterns showed in Figure 5d have indicated that no peaks corresponding to the europium compound was detected and no shift of the anatase peaks was observed after doping with Eu 3+ . It can be concluded that Eu 3+ ions are mainly dispersed in the amorphous TiO 2 region. was used as a template agent. Europium ethoxide was specially prepared as a doping agent in order to increase the rate of hydrolysis and condensation of the tetrabutyl titanate. Small amounts of water were added to initiate the reaction under stirring. As soon as the solution became slightly white after several minutes, the stirring was stopped. Meanwhile, in order to control the water content, the preparation was conducted under the protection of flowing N2. After aging for 24-48 h at 35-40 °C, the suspended particles were filtered and washed with ethanol several times. The final products were obtained after calcination at 400 °C for 4 h. The synthesized Eu-doped TiO2 particles have a spherical morphology and a mesoporous structure, with a pore size of 7-10 nm. The special surface area of the phosphor particles is 158 m 2 /g. The high resolution TEM images in Figure 5c show that the pore wall is semi-crystalline that many anatase nanocrystallites are dispersed in the amorphous TiO2. The XRD patterns showed in Figure 5d have indicated that no peaks corresponding to the europium compound was detected and no shift of the anatase peaks was observed after doping with Eu 3+ . It can be concluded that Eu 3+ ions are mainly dispersed in the amorphous TiO2 region.

. Hollow and Porous Spheres
Compared to single hollow spheres, hollow TiO 2 spheres with a porous shell are more interesting in photocatalysis because they can provide a higher surface area and active site points, decreased diffusion resistance, and increased accessibility to reactants [66]. Several methods have been reported to synthesize hollow TiO 2 spheres with a porous shell [66,67]. Among these methods, the template method is the most popular. Different sacrificial templates can be used for controlling the size and morphology of such a hollow nanostructure. The template method followed by a hydrothermal or calcination treatment is often used to synthesize hollow TiO 2 spheres with a crystalline shell [39]. Figure 6 shows a typical process of preparing hollow TiO 2 spheres with a crystalline shell. In this process, amorphous TiO 2 was firstly coated onto the surface of SiO 2 spheres by the sol-gel method in an alkaline solution. Then, the composite microspheres were subjected to a hydrothermal or calcination treatment. Meanwhile, the amorphous TiO 2 was crystallized into nanocrystals and the mesoporous structure was formed by nanocrystal stacking. Finally, the SiO 2 core was removed by etching in an alkaline solution. As shown in Figure 7, the sample prepared by hydrothermal treatment had a mean diameter of 620 nm with a 180 nm thick mesoporous TiO 2 shell. The BET surface area was 231.1 m 2 /g and the pore size was 6.5 nm. However, the sample prepared by calcination had a mean diameter of 440 nm with a 90 nm thick mesoporous TiO 2 shell. The BET surface area was 158.3 m 2 /g.

. Hollow and Porous Spheres
Compared to single hollow spheres, hollow TiO2 spheres with a porous shell are more interesting in photocatalysis because they can provide a higher surface area and active site points, decreased diffusion resistance, and increased accessibility to reactants [66]. Several methods have been reported to synthesize hollow TiO2 spheres with a porous shell [66,67]. Among these methods, the template method is the most popular. Different sacrificial templates can be used for controlling the size and morphology of such a hollow nanostructure. The template method followed by a hydrothermal or calcination treatment is often used to synthesize hollow TiO2 spheres with a crystalline shell [39]. Figure 6 shows a typical process of preparing hollow TiO2 spheres with a crystalline shell. In this process, amorphous TiO2 was firstly coated onto the surface of SiO2 spheres by the sol-gel method in an alkaline solution. Then, the composite microspheres were subjected to a hydrothermal or calcination treatment. Meanwhile, the amorphous TiO2 was crystallized into nanocrystals and the mesoporous structure was formed by nanocrystal stacking. Finally, the SiO2 core was removed by etching in an alkaline solution. As shown in Figure 7, the sample prepared by hydrothermal treatment had a mean diameter of 620 nm with a 180 nm thick mesoporous TiO2 shell. The BET surface area was 231.1 m 2 /g and the pore size was 6.5 nm. However, the sample prepared by calcination had a mean diameter of 440 nm with a 90 nm thick mesoporous TiO2 shell. The BET surface area was 158.3 m 2 /g.

3D Urchin-Like Hierarchical Particles
Urchin-like microspheres possess an epitaxial multilevel structure. The unique micro/nano hierarchical structure has two obvious advantages over single nano-scale or micro-scale structures when they are used as photocatalysts [54,68]. One is that urchin-like TiO2 is more efficient at absorbing incidental light because of the increase of multiple-reflection of the hierarchical microspheres [54]. The other is that urchin-like hierarchical TiO2 is easy to separate from waste water by the filtration or sedimentation methods. The template-assisted method is a familiar approach to prepare the hierarchical materials. However, it is troublesome to remove templates from products. Impurities are easily introduced into products in the process of utilization and removal of templates. The template-free method is accepted as an ideal strategy which can overcome the drawbacks. Recently, TiO2 particles with different hierarchical structures have been successfully fabricated via the template-free method [69][70][71][72][73].

Urchin-Like Hierarchical TiO2 Particles
We have developed a synthesis of a kind of 3D urchin-like TiO2 microspheres via a solvothermal method without adding any surfactant or template [30]. Tetrabutyl titanate and titanium tetrachloride (TiCl4) aqueous solution were used as the reactant, and toluene was used as the solvent. The solvothermal reaction took place in a Teflon-lined autoclave at 150 °C for 24 h. Sea-urchin-like 3D hierarchical TiO2 microspheres with a uniform size of 2.5-3.0 μm were obtained ( Figure 8). The 3D hierarchical microspheres were made of single crystalline rutile nanoneedles with diameters about 20-40 nm, which grew radially from the core of the microspheres. The morphology and crystal phase of the 3D hierarchical TiO2 microspheres could be influenced by some factors, such as the ratio of tetrabutyl titanate to TiCl4, the solvothermal temperature, and so on. By tracing the particle morphology change by SEM and XRD techniques, we concluded that the formation of 3D hierarchical TiO2 microspheres mainly concerned three steps, i.e., nucleation, self-assembly, dissolution and recrystallization, as depicted in Figure 9. In the nucleation stage, nanoparticles were

3D Urchin-Like Hierarchical Particles
Urchin-like microspheres possess an epitaxial multilevel structure. The unique micro/nano hierarchical structure has two obvious advantages over single nano-scale or micro-scale structures when they are used as photocatalysts [54,68]. One is that urchin-like TiO 2 is more efficient at absorbing incidental light because of the increase of multiple-reflection of the hierarchical microspheres [54]. The other is that urchin-like hierarchical TiO 2 is easy to separate from waste water by the filtration or sedimentation methods. The template-assisted method is a familiar approach to prepare the hierarchical materials. However, it is troublesome to remove templates from products. Impurities are easily introduced into products in the process of utilization and removal of templates. The template-free method is accepted as an ideal strategy which can overcome the drawbacks. Recently, TiO 2 particles with different hierarchical structures have been successfully fabricated via the template-free method [69][70][71][72][73].

Urchin-Like Hierarchical TiO 2 Particles
We have developed a synthesis of a kind of 3D urchin-like TiO 2 microspheres via a solvothermal method without adding any surfactant or template [30]. Tetrabutyl titanate and titanium tetrachloride (TiCl 4 ) aqueous solution were used as the reactant, and toluene was used as the solvent. The solvothermal reaction took place in a Teflon-lined autoclave at 150 • C for 24 h. Sea-urchin-like 3D hierarchical TiO 2 microspheres with a uniform size of 2.5-3.0 µm were obtained (Figure 8). The 3D hierarchical microspheres were made of single crystalline rutile nanoneedles with diameters about 20-40 nm, which grew radially from the core of the microspheres. The morphology and crystal phase of the 3D hierarchical TiO 2 microspheres could be influenced by some factors, such as the ratio of tetrabutyl titanate to TiCl 4 , the solvothermal temperature, and so on. By tracing the particle morphology change by SEM and XRD techniques, we concluded that the formation of 3D hierarchical TiO 2 microspheres mainly concerned three steps, i.e., nucleation, self-assembly, dissolution and recrystallization, as depicted in Figure 9. In the nucleation stage, nanoparticles were formed. Then, the nanoparticles assembled into microspheres. Finally, the microspheres gradually changed into the urchin-like hierarchical structure by dissolution and recrystallization. formed. Then, the nanoparticles assembled into microspheres. Finally, the microspheres gradually changed into the urchin-like hierarchical structure by dissolution and recrystallization.

Cr Doped Urchin-Like Hierarchical TiO2 Particles
Urchin-like Cr-doped TiO2 particles could be also synthesized by the same solvothermal method described above in a solution of titanium tetrabutyl titanate dissolving CrCl3 [73]. The morphology of Cr-doped TiO2 particles is characterized by SEM images shown in Figure 10. The mean particle size of the hierarchical microspheres can be adjusted within 1-5 μm and the diameter of the nanorods is about 20-30 nm. The EDS results showed that the content of the Cr element in the Cr-doped TiO2 particles was ~2.9 mol %. formed. Then, the nanoparticles assembled into microspheres. Finally, the microspheres gradually changed into the urchin-like hierarchical structure by dissolution and recrystallization.

Cr Doped Urchin-Like Hierarchical TiO2 Particles
Urchin-like Cr-doped TiO2 particles could be also synthesized by the same solvothermal method described above in a solution of titanium tetrabutyl titanate dissolving CrCl3 [73]. The morphology of Cr-doped TiO2 particles is characterized by SEM images shown in Figure 10. The mean particle size of the hierarchical microspheres can be adjusted within 1-5 μm and the diameter of the nanorods is about 20-30 nm. The EDS results showed that the content of the Cr element in the Cr-doped TiO2 particles was ~2.9 mol %.

Cr Doped Urchin-Like Hierarchical TiO 2 Particles
Urchin-like Cr-doped TiO 2 particles could be also synthesized by the same solvothermal method described above in a solution of titanium tetrabutyl titanate dissolving CrCl 3 [73]. The morphology of Cr-doped TiO 2 particles is characterized by SEM images shown in Figure 10. The mean particle size of the hierarchical microspheres can be adjusted within 1-5 µm and the diameter of the nanorods is about 20-30 nm. The EDS results showed that the content of the Cr element in the Cr-doped TiO 2 particles was~2.9 mol %.

Noble Metal Nanoparticle-Decorated 3D Urchin-Like TiO2 Particles
Noble metal nanoparticle-decorated semiconductors are interesting for photocatalysis because of their combined properties [74]. Decorating the noble metal nanoparticles (e.g., Au, Ag and Pt) onto the surface of TiO2 is an effective method to improve the photocatalytic activity because not only light-harvesting efficiency can be enhanced due to the surface plasmon resonance of noble metal nanoparticles, but the recombination of surface radicals can also be slowed down by capturing photogenerated electrons of noble metal nanoparticles [75][76][77][78][79]. Figure 11 shows a schematic illustration of 3D urchin-like hierarchical TiO2 microspheres decorated with Au nanoparticles via a two-step wet-chemical process [31]. In the first step, the surface of the urchin-like TiO2 microspheres was modified with APTES (3-aminopropyl-triethoxysilane) that possess amidocyanogen. Then, the modified particles were decorated with Au nanoparticles in HAuCl4 aqueous solution by the reduction of NaBH4. Since the amidocyanogen could interact with Au nanoparticles by a weak covalent bond, Au nanoparticles were closely attached to the surface of the TiO2 nanostructures, as shown in Figure 12. It was seen that the Au nanoparticles with diameters of about 2-10 nm mainly adhered to the surface of the needles uniformly. Most of the Au nanoparticles possess a rhombic dodecahedra structure. The UV-Vis (ultraviolet-visible) spectra show an absorption band located at the wavelength of about 530 nm due to the surface plasmon resonance of Au nanoparticles.

Noble Metal Nanoparticle-Decorated 3D Urchin-Like TiO 2 Particles
Noble metal nanoparticle-decorated semiconductors are interesting for photocatalysis because of their combined properties [74]. Decorating the noble metal nanoparticles (e.g., Au, Ag and Pt) onto the surface of TiO 2 is an effective method to improve the photocatalytic activity because not only light-harvesting efficiency can be enhanced due to the surface plasmon resonance of noble metal nanoparticles, but the recombination of surface radicals can also be slowed down by capturing photogenerated electrons of noble metal nanoparticles [75][76][77][78][79]. Figure 11 shows a schematic illustration of 3D urchin-like hierarchical TiO 2 microspheres decorated with Au nanoparticles via a two-step wet-chemical process [31]. In the first step, the surface of the urchin-like TiO 2 microspheres was modified with APTES (3-aminopropyl-triethoxysilane) that possess amidocyanogen. Then, the modified particles were decorated with Au nanoparticles in HAuCl 4 aqueous solution by the reduction of NaBH 4 . Since the amidocyanogen could interact with Au nanoparticles by a weak covalent bond, Au nanoparticles were closely attached to the surface of the TiO 2 nanostructures, as shown in Figure 12. It was seen that the Au nanoparticles with diameters of about 2-10 nm mainly adhered to the surface of the needles uniformly. Most of the Au nanoparticles possess a rhombic dodecahedra structure. The UV-Vis (ultraviolet-visible) spectra show an absorption band located at the wavelength of about 530 nm due to the surface plasmon resonance of Au nanoparticles.

Noble Metal Nanoparticle-Decorated 3D Urchin-Like TiO2 Particles
Noble metal nanoparticle-decorated semiconductors are interesting for photocatalysis because of their combined properties [74]. Decorating the noble metal nanoparticles (e.g., Au, Ag and Pt) onto the surface of TiO2 is an effective method to improve the photocatalytic activity because not only light-harvesting efficiency can be enhanced due to the surface plasmon resonance of noble metal nanoparticles, but the recombination of surface radicals can also be slowed down by capturing photogenerated electrons of noble metal nanoparticles [75][76][77][78][79]. Figure 11 shows a schematic illustration of 3D urchin-like hierarchical TiO2 microspheres decorated with Au nanoparticles via a two-step wet-chemical process [31]. In the first step, the surface of the urchin-like TiO2 microspheres was modified with APTES (3-aminopropyl-triethoxysilane) that possess amidocyanogen. Then, the modified particles were decorated with Au nanoparticles in HAuCl4 aqueous solution by the reduction of NaBH4. Since the amidocyanogen could interact with Au nanoparticles by a weak covalent bond, Au nanoparticles were closely attached to the surface of the TiO2 nanostructures, as shown in Figure 12. It was seen that the Au nanoparticles with diameters of about 2-10 nm mainly adhered to the surface of the needles uniformly. Most of the Au nanoparticles possess a rhombic dodecahedra structure. The UV-Vis (ultraviolet-visible) spectra show an absorption band located at the wavelength of about 530 nm due to the surface plasmon resonance of Au nanoparticles. Figure 11. Schematic illustration of the synthesis process of urchin-like TiO2 decorated with Au or Ag nanoparticles [31]. Figure 11. Schematic illustration of the synthesis process of urchin-like TiO 2 decorated with Au or Ag nanoparticles [31]. The urchin-like TiO2 microspheres decorated with Ag nanoparticles could be also prepared by the similar process. As shown in Figure 13, the Ag nanoparticles with diameters of about 2 nm are decorated on the TiO2 nanoneedles homogeneously. A broad absorption band at around 500 nm, corresponding to the surface plasmon resonance of the Ag nanoparticles, appears in the UV-Vis absorption spectrum (Figure 13f). The urchin-like TiO 2 microspheres decorated with Ag nanoparticles could be also prepared by the similar process. As shown in Figure 13, the Ag nanoparticles with diameters of about 2 nm are decorated on the TiO 2 nanoneedles homogeneously. A broad absorption band at around 500 nm, corresponding to the surface plasmon resonance of the Ag nanoparticles, appears in the UV-Vis absorption spectrum (Figure 13f).

Core/Shell-Structured Bimetallic Nanoparticle-Decorated 3D Urchin-Like Hierarchical TiO2 Particles
Bimetallic nanostructures often show a more excellent comprehensive performance over their monometallic counterpart [80]. Especially core/shell bimetallic nanostructures with a magnetic core and a noble-metallic shell have aroused researchers' interest [81][82][83][84][85]. The magnetic core can offer a drive force for the recycling of samples, while the noble-metal shell can improve the optical properties [85]. Figure 14 shows a typical preparation process of a kind of 3D urchin-like hierarchical TiO2 decorated with a bimetallic core/shell nanoparticle (Co@Au and Co@Ag). The preparation includes three steps, i.e., surface activation, electroless plating and in-situ replacement [32]. First, the surface of the urchin-like TiO2 microspheres was activated by implanting Pd nanodots. Then, Co nanoparticles were formed and adhered to the nanoneedle surface of the urchin-like TiO2 microspheres by electroless plating. Finally, Ag or Au were further formed and coated on the surface of the Co nanoparticles by an in-situ replacement process. Figure 14. A schematic synthesis process of urchin-like TiO2 decorated with core/shell-structured Co@Au or Co@Ag bimetallic nanoparticles [32].
The SEM and TEM images in Figure 15 show the morphology of Co@Au/TiO2 composites. It can be seen that many core/shell nanostructured nanoparticles with diameters of 10-80 nm are attached to the surface of TiO2 nanorods. The images of the elemental mapping of core/shell nanoparticles

Core/Shell-Structured Bimetallic Nanoparticle-Decorated 3D Urchin-Like Hierarchical TiO 2 Particles
Bimetallic nanostructures often show a more excellent comprehensive performance over their monometallic counterpart [80]. Especially core/shell bimetallic nanostructures with a magnetic core and a noble-metallic shell have aroused researchers' interest [81][82][83][84][85]. The magnetic core can offer a drive force for the recycling of samples, while the noble-metal shell can improve the optical properties [85]. Figure 14 shows a typical preparation process of a kind of 3D urchin-like hierarchical TiO 2 decorated with a bimetallic core/shell nanoparticle (Co@Au and Co@Ag). The preparation includes three steps, i.e., surface activation, electroless plating and in-situ replacement [32]. First, the surface of the urchin-like TiO 2 microspheres was activated by implanting Pd nanodots. Then, Co nanoparticles were formed and adhered to the nanoneedle surface of the urchin-like TiO 2 microspheres by electroless plating. Finally, Ag or Au were further formed and coated on the surface of the Co nanoparticles by an in-situ replacement process.

Core/Shell-Structured Bimetallic Nanoparticle-Decorated 3D Urchin-Like Hierarchical TiO2 Particles
Bimetallic nanostructures often show a more excellent comprehensive performance over their monometallic counterpart [80]. Especially core/shell bimetallic nanostructures with a magnetic core and a noble-metallic shell have aroused researchers' interest [81][82][83][84][85]. The magnetic core can offer a drive force for the recycling of samples, while the noble-metal shell can improve the optical properties [85]. Figure 14 shows a typical preparation process of a kind of 3D urchin-like hierarchical TiO2 decorated with a bimetallic core/shell nanoparticle (Co@Au and Co@Ag). The preparation includes three steps, i.e., surface activation, electroless plating and in-situ replacement [32]. First, the surface of the urchin-like TiO2 microspheres was activated by implanting Pd nanodots. Then, Co nanoparticles were formed and adhered to the nanoneedle surface of the urchin-like TiO2 microspheres by electroless plating. Finally, Ag or Au were further formed and coated on the surface of the Co nanoparticles by an in-situ replacement process. Figure 14. A schematic synthesis process of urchin-like TiO2 decorated with core/shell-structured Co@Au or Co@Ag bimetallic nanoparticles [32].
The SEM and TEM images in Figure 15 show the morphology of Co@Au/TiO2 composites. It can be seen that many core/shell nanostructured nanoparticles with diameters of 10-80 nm are attached to the surface of TiO2 nanorods. The images of the elemental mapping of core/shell nanoparticles Figure 14. A schematic synthesis process of urchin-like TiO 2 decorated with core/shell-structured Co@Au or Co@Ag bimetallic nanoparticles [32].
The SEM and TEM images in Figure 15 show the morphology of Co@Au/TiO 2 composites. It can be seen that many core/shell nanostructured nanoparticles with diameters of 10-80 nm are attached to the surface of TiO 2 nanorods. The images of the elemental mapping of core/shell nanoparticles further identify that the core is Co and the shell is Au. The size and distribution of the bimetallic particles can be adjusted by controlling the ratio of Co to TiO 2 during the electroless plating process. The thickness of the Au or Ag shell could be controlled by adjusting the concentration of HAuCl 4 or AgNO 3 in the solution and the reaction time. Both the Co@Au/TiO 2 and Co@Ag/TiO 2 particles showed good response to an applied external magnetic field [32]. further identify that the core is Co and the shell is Au. The size and distribution of the bimetallic particles can be adjusted by controlling the ratio of Co to TiO2 during the electroless plating process. The thickness of the Au or Ag shell could be controlled by adjusting the concentration of HAuCl4 or AgNO3 in the solution and the reaction time. Both the Co@Au/TiO2 and Co@Ag/TiO2 particles showed good response to an applied external magnetic field [32]. Although TiO2 is an ideal candidate for photocatalysis because of its strong oxidization, harmlessness to surroundings, chemical inactivity, good stability and low cost, the main weakness of TiO2 is the lack of visible light response due to the large band gap. Therefore, the question of how

Photocatalytic Activity of Urchin-Like Hierarchical TiO 2 and Their Composites
Although TiO 2 is an ideal candidate for photocatalysis because of its strong oxidization, harmlessness to surroundings, chemical inactivity, good stability and low cost, the main weakness of TiO 2 is the lack of visible light response due to the large band gap. Therefore, the question of how to increase the efficiency of visible light harvesting is an important research topic in this field [1]. Although single controlling the morphology of TiO 2 materials cannot increase the efficiency of visible light harvesting, it is possible to enhance the visible light harvesting of TiO 2 composites by combining ion doping or noble metal decoration with morphology control. Ion doping or noble metal decoration can induce or increase the visible light absorption of TiO 2 , while the absorption effect can be further enhanced by other effects from material morphology, such as decreased light scattering or increased multiple reflection, etc.
The photocatalytic efficiency of commercial P25, urchin-like TiO 2 and Au or Ag-decorated urchin-like TiO 2 was evaluated by degrading MB (methyl blue) under UV-Vis light irradiation. It was found that the photocatalytic degradation efficiency under the same conditions followed the order: Ag/TiO 2 > Au/TiO 2 > TiO 2 > P25 ( Figure 16). By the photoluminescence spectra, Au or Ag nanoparticles decorated on the surface of TiO 2 were demonstrated to be able to effectively capture photogenerated electrons and prevent electron/hole recombining ( Figure 16). In addition, the urchin-like micro/nano hierarchical structure also may increase the visible light harvesting efficiency by the multiple-reflection of nanoneedles. These should be responsible for the enhanced photocatalytic efficiency of urchin-like TiO 2 microspheres after decoration with Au or Ag nanoparticles. Similarly, the light-harvesting efficiency could also be enhanced by decorating with Co@Au or Co@Ag bimetallic nanoparticles, as shown in Figure 17. As a result, the photocatalytical efficiency of urchin-like TiO 2 was enhanced obviously, as shown in Figure 17, by the experiment of decolorizing methyl blue (MB) solution. Although single controlling the morphology of TiO2 materials cannot increase the efficiency of visible light harvesting, it is possible to enhance the visible light harvesting of TiO2 composites by combining ion doping or noble metal decoration with morphology control. Ion doping or noble metal decoration can induce or increase the visible light absorption of TiO2, while the absorption effect can be further enhanced by other effects from material morphology, such as decreased light scattering or increased multiple reflection, etc. The photocatalytic efficiency of commercial P25, urchin-like TiO2 and Au or Ag-decorated urchin-like TiO2 was evaluated by degrading MB (methyl blue) under UV-Vis light irradiation. It was found that the photocatalytic degradation efficiency under the same conditions followed the order: Ag/TiO2 > Au/TiO2 > TiO2 > P25 ( Figure 16). By the photoluminescence spectra, Au or Ag nanoparticles decorated on the surface of TiO2 were demonstrated to be able to effectively capture photogenerated electrons and prevent electron/hole recombining ( Figure 16). In addition, the urchin-like micro/nano hierarchical structure also may increase the visible light harvesting efficiency by the multiple-reflection of nanoneedles. These should be responsible for the enhanced photocatalytic efficiency of urchin-like TiO2 microspheres after decoration with Au or Ag nanoparticles. Similarly, the light-harvesting efficiency could also be enhanced by decorating with Co@Au or Co@Ag bimetallic nanoparticles, as shown in Figure 17. As a result, the photocatalytical efficiency of urchin-like TiO2 was enhanced obviously, as shown in Figure 17, by the experiment of decolorizing methyl blue (MB) solution.

Summary
Based on our group's research work, we provided a brief review of the synthesis of TiO2 with different morphologies and the photocatalytic properties of urchin-like TiO2, noble metal Although single controlling the morphology of TiO2 materials cannot increase the efficiency of visible light harvesting, it is possible to enhance the visible light harvesting of TiO2 composites by combining ion doping or noble metal decoration with morphology control. Ion doping or noble metal decoration can induce or increase the visible light absorption of TiO2, while the absorption effect can be further enhanced by other effects from material morphology, such as decreased light scattering or increased multiple reflection, etc. The photocatalytic efficiency of commercial P25, urchin-like TiO2 and Au or Ag-decorated urchin-like TiO2 was evaluated by degrading MB (methyl blue) under UV-Vis light irradiation. It was found that the photocatalytic degradation efficiency under the same conditions followed the order: Ag/TiO2 > Au/TiO2 > TiO2 > P25 ( Figure 16). By the photoluminescence spectra, Au or Ag nanoparticles decorated on the surface of TiO2 were demonstrated to be able to effectively capture photogenerated electrons and prevent electron/hole recombining ( Figure 16). In addition, the urchin-like micro/nano hierarchical structure also may increase the visible light harvesting efficiency by the multiple-reflection of nanoneedles. These should be responsible for the enhanced photocatalytic efficiency of urchin-like TiO2 microspheres after decoration with Au or Ag nanoparticles. Similarly, the light-harvesting efficiency could also be enhanced by decorating with Co@Au or Co@Ag bimetallic nanoparticles, as shown in Figure 17. As a result, the photocatalytical efficiency of urchin-like TiO2 was enhanced obviously, as shown in Figure 17, by the experiment of decolorizing methyl blue (MB) solution.

Summary
Based on our group's research work, we provided a brief review of the synthesis of TiO2 with different morphologies and the photocatalytic properties of urchin-like TiO2, noble metal

Summary
Based on our group's research work, we provided a brief review of the synthesis of TiO 2 with different morphologies and the photocatalytic properties of urchin-like TiO 2 , noble metal nanoparticle-decorated 3D urchin-like TiO 2 and core/shell-structured bimetallic nanoparticle-decorated 3D urchin-like hierarchical TiO 2 . The examples of the fabrication of solid spheres, hollow spheres, porous spheres, and porous and hollow microspheres of anatase TiO 2 -based materials were introduced. The synthesis and photocatalytic efficiency of urchin-like rutile TiO 2 , urchin-like rutile TiO 2 nanostructures decorated with Au or Ag nanoparticles and core/shell-structured bimetallic nanoparticles (Co@Au and Co@Ag) were especially introduced. The results of photocatalytic tests show that 3D urchin-like hierarchical structures have unique merits in the efficient harvesting of solar light, and decorating Au, Ag or bimetallic nanoparticles on the surface of 3D urchin-like TiO 2 can promote photoinduced charge-carrier separation.