A Perspective on Mesoporous TiO2 Materials - Chemistry of

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Perspective

A Perspective on mesoporous TiO2 materials Wei Li, Zhangxiong Wu, Jinxiu Wang, Ahmed A. Elzatahry, and Dongyuan Zhao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4014859 • Publication Date (Web): 13 Jun 2013 Downloaded from http://pubs.acs.org on June 16, 2013

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Chemistry of Materials

A Perspective on Mesoporous TiO2 Materials Wei Li,† Zhangxiong Wu,‡ Jinxiu Wang,† Ahmed A. Elzatahry,§ and Dongyuan Zhao†‡*

†

Department of Chemistry, and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, P. R. China. ‡

Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia.

§

Department of Chemistry-College of Science, King Saud University, Riaydh 11451, Saudi Arabia.

Abstract Mesoporous TiO2 have gained increasing interests because of their outstanding properties and promising applications in a wide range fields. In this Perspective, we summarize the significant advances on the synthesis of mesoporous TiO2 in terms of rationally controlling the hydrolysis and condensation rates of titanium precursors to enable the cooperative assembly and/or successful infiltration via the templating methods. The rational designs and fundamentals for preparing mesoporous TiO2 are presented in the context of improving the conversion efficiencies of solar energy (e.g., maximizing the UV and/or visible light adsorption, minimizing the recombination of photogenerated electron-hole pairs and optimizing the mass and fast charge transport), and enhancing the performances of lithium-ion batteries. New trends and ongoing challenges in this field are also highlighted and proposed.

ACS Paragon Plus Environment


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Keywords: Mesoporous materials, TiO2, templating synthesis, photocatalysis, H2 production, dye-sensitized solar cells, photoelectrochemical cells, lithium-ion batteries

1. Introduction Porous materials are of great interest because of their ability to interact with atoms, ions, molecules and nanoparticles not only at their surfaces, but also throughout the bulk of the materials.1 Therefore, the presence of pores in nanostructured materials can greatly promote their physical and chemical properties, as well as extend their potential applications for adsorption, separation, catalysis, sensing, energy storage and conversion, and biotechnology.1,2 Since the exciting discovery of ordered mesoporous silicas (e.g., MCM-41 and SBA-15) in the 1990s,3-5 mesoporous materials have attracted more and more interest owing to their fascinating properties such as tunable large pore sizes, high surface areas, large pore volumes, alternative pore shapes and controllable framework compositions, as well as their widely promising applications.6-9 Over the past two decades, various mesoporous materials with different compositions from pure inorganic or organic frameworks to organic-inorganic hybrid frameworks have been reported.6,10,11 Among the families that experienced intensive advances, mesoporous TiO2 is of particular interest and has been undergoing the most explosive growth due to its outstanding features such as low cost, environmental


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benignity, plentiful polymorphs, good chemical and thermal stability, excellent electronic and optical properties.12 Compared with bulk TiO2, uniform mesopore channels of the mesoporous TiO2 do not only increase the density of active sites with high accessibility, but also facilitate the diffusion of reactants and products; the high surface area and large pore volume provide enhanced capability for dyes-loading and pollutants-adsorption.

This

has

unambiguous

implications

for photovoltaic,

lithium-ion insertion and catalytic applications. Therefore, the design and synthesis of mesoporous TiO2 with controllable mesopores and structures are important from both fundamental and technological viewpoints. Some excellent and exhaustive reviews have been published covering various aspects of mesoporous TiO2, reflecting the tremendous advances in the past.13-19 Herein, instead of a comprehensive research survey of the field, this perspective will aim at briefly revisiting the synthesis of mesoporous TiO2 materials in terms of rationally controlling the hydrolysis and condensation rates of titanium precursors to enable the cooperative assembly and/or successful infiltration via the templating methods, but provide a comprehensive guideline for both fundamental understandings and practical applications of these materials, especially those related to environmental and energy-related issues. We also pay special attentions to the latest advances in the preparation of mesoporous TiO2 materials.

2. Synthesis of Mesoporous TiO2 The preparation of mesoporous materials is mainly concerned with building monodispersed and mesosized pore spaces (2-50 nm) and arranging them in a range



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of assembled array.6 Similar to the synthesis of ordered mesoporous silica, there are mainly two arrays generally used to construct ordered mesoporous TiO2 (Figure 1): supramolecular aggregates such as surfactant micelle arrays (soft-templating method), and preformed mesoporous solids such as silicas and carbons (hard-templating method).6,20-22 Besides, template-free method can also produce mesoporous TiO2 but generally with disordered mesostructures, which derive from the irregular packing of building blocks. Herein, we like to focus on the templating methods. 2.1. Soft-Templating Method The sol-gel process of titanium precursors is distinguished from silicates by their higher chemical reactivity resulting from the lower electronegativity of titanium and its ability to exhibit several coordination states, so that coordination expansion occurs spontaneously upon reaction with water or even moisture.23 Thus, titanium precursors tend to fast hydrolyze and form dense precipitates, which overwhelm the cooperative assembly with surfactants, leading to undesired phase separation. For example, the hydrolysis rate of titanium alkoxides is about 5 orders of magnitude faster than that of silicate ones. Therefore, the critical issue for preparing mesoporous TiO2 under the guiding of surfactant templating is to control the hydrolysis and condensation rates of titanium precursors to effectively match the cooperative assembly with templates. In addition, the recovery of mesoporous TiO2 without framework collapse should be paid much attention during the template removal and crystallization. In 1995, Antonelli and Ying first demonstrated the aqueous soft-templating route towards synthesis of mesoporous TiO2.24 In their process, acetylacetone was


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employed as a ligand to decrease the hydrolysis and condensation rate of titanium isopropoxide

(TIPO)

through

the

formation

of

titanium

acetylacetonate

tris-isopropoxide. Thus, the size of titanium oligomers can be greatly reduced, which can effectively co-assemble with the tetradecyl-phosphate surfactants into ordered mesostructures in an aqueous solution. After calcination in air at 500 °C, ordered hexagonal mesoporous TiO2 was obtained with a surface area of ~ 200 m2/g and pore size of ~ 3.2 nm (Figure 2a). However, this strategy was just successful with surfactants with phosphate head groups owing to their effective and strong interactions with titanium, but not with cationic surfactant or other anionic surfactants. Because of this, the phosphorus from the template could not be fully removed by either calcination or solvent extraction. Therefore, these materials are better designated as titanium oxo-phosphates rather than titania. Later, Antonelli prepared pure mesoporous TiO2 by using organic amines as a template, but similar to the case of mesoporous silica, the resultant TiO2 exhibit wormhole-like mesostructures rather than long-ranged regularity.25 In this process, the formation of mesostructures is attributed to the H-bonding interactions of electrically neutral amines with titanium oligomers, thus a long aging time is required to enable the assembly via retarding the hydrolysis and condensation. However, the resultant mesoporous TiO2 shows a poor thermal stability. After a heat treatment at 300 °C, a low surface area of ~ 180 m2/g was observed. To accelerate the condensation of titanium hydroxides and shorten the reaction time, the ultrasound irradiation technique was coupled with the aqueous process.26 It was found that fast condensation



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could enhance the thermal stability of the resultant mesoporous TiO2. Even after calcination at 350 °C for 8 h, an ultrahigh surface area of ~ 467 m2/g was obtained, which is much higher than that without ultrasound irradiation. Moreover, Chen et al. developed a combined aqueous sol-gel and solvothermal process for synthesis of mesoporous TiO2 microspheres with hexadecylamine (HDA) as a template (Figure 2b).27 The HDA can well alter the hydrolysis and condensation of titanium precursors, and ensure the effective cooperative assembly process, leading to uniform TiO2 microspheres with tunable size of 320 ~ 1150 nm. Nevertheless, the crystallite size, surface area (89 ~ 120 m2/g), and pore size (14 ~ 23 nm) can be tuned through a solvothermal treatment. Because of the high reactivity of titanium precursors, it is difficult to control their hydrolysis and condensation rates, and enable the cooperative assembly with templates in an aqueous solution. Thus, the resultant mesoporous TiO2 generally exhibit wormhole-like mesostructures rather than ordered regularity over large domains. Yang et al. first enabled the cooperative assembly process for preparing mesoporous TiO2 in non-aqueous media by using TiCl4 as a precursor and Pluronic block copolymer as a template via the well-known evaporation-induced self-assembly (EISA) method.28 In this case, the hydrolysis and condensation can be well regulated and restrained due to the lack of water, which initiates the assembly through weak coordination bonds. As a result, hexagonal or cubic mesoporous TiO2 were obtained (Figure 3), and they possessed high surface areas of 205 and 200 m2/g as well as large pore sizes of 6.5 and 6.8 nm, respectively. Unfortunately, the resultant TiO2 exhibit


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poorly ordered regularity, which is greatly related to the destructive cross-linkage of frameworks by large amount of HCl generated from the hydrolysis of titanium species in the derived mesostructures. Later on, high-quality mesoporous TiO2 films with a long-range ordering were successfully synthesized via a sol-gel process coupled EISA method (Figure 4).29-32 In this case, an acid regulated sol-gel chemistry of titanium precursors in ethanol/water first proceeds, which leads to the formation of titanium hydroxides or oligomers. Subsequent deposition of the sol solution on a substrate (commonly a petri dish) induces the evaporation of the solvent. Then, with the increase of concentrations of both titanium species and block copolymers, the cooperative assembly is enabled to form highly ordered mesostructures. Notably, the added or in situ generated HCl or other volatile compounds would be simultaneously eliminated under evaporation, thus ordered mesostructures can be well retained. Sanchez et al. investigated the structural evolution relating to the advancement of the evaporation process, and gave an excellent comprehension of the role of each of the variables (e.g., H2O, HCl, humidity, crystallization process) involved, which allowed a facile and reproducible synthesis of ordered mesoporous TiO2 films.32 Notably, they found that the relative humidity (RH) of the atmosphere is a parameter of paramount importance in the self-assembly, which greatly affects the hydrolysis and condensation of titanium species, and the polarity of PEO chain. The initial films dip-coated at high RH are transparent, but the aged films present a non-homogeneous thickness, as a consequence of the slow drying related to the absorption of water during the formation of the layer. On the other hand, films



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deposited in RH < 45 % are of excellent optical quality, but present a worm-like mesostructure.32 A combination process was thus developed to synthesize highly organized cubic mesoporous TiO2 with high transparence. That is, a low RH of 30 % was applied for film deposition and immediately RH was changed to 50 % during the subsequent humidity aging process.32 Fan et al. modified this method by employing acetic acid as an additional complex agent to preferably control the hydrolysis and condensation kinetics of titanium alkoxides, and gave ordered mesoporous TiO2 or composites.33 Zhang et al. employed acetylacetone as a coordination ligand to further retard the hydrolysis and condensation rate of TIPO, which made the assembly process more controllable.34 Ordered mesoporous TiO2 can be obtained with primitive cubic mesostructure, a highly crystallized anatase framework, a large pore size of 16 nm and a high surface area of ~ 112 m2/g. In 2002, Tian et al. developed an “acid-base pair” route in which a mixture of titanium alkoxide and TiCl4 was adopted as a novel precursor for synthesis of ordered mesoporous TiO2, with the former being the main titanium source and the latter being the pH adjustor and hydrolysis-condensation controller.35 In comparison with the synthesis from a single titanium source, namely, TiCl4, the acidity of the precursor solution is significantly and controllably reduced by the addition of titanium alkoxide, which decreases the amount of TiCl4 and neutralizes the acid. The alkoxide is also an extra source of oxygen donor as a base. Therefore, the cross-linkage and gelation of titanium species may be easier and better for assembly.36 Highly ordered mesoporous titania with a large uniform pore size of ~ 4.0 nm and a high surface area of 240 m2/g


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was obtained from Pluronic P123 (Figure 5). Since the sol-gel chemistry of titanium precursors, the cooperative assembly is sensitive to variable reaction conditions. Apart from controlling hydrolysis and condensation rates, another possible solution for the formation of ordered mesostructures is to use preformed and fully crystallized nanoparticles as building blocks instead of molecular precursors via the EISA method (Figure 6a).37 Szeifert et al. prepared ordered mesoporous TiO2 with a high BET surface area of 300 m2/g and ultrathin nanocrystalline walls by self-assembly of ~ 3 nm TiO2 nanoparticles with Pluronic P123 (Figure 6b, 6c).38 This synthetic strategy overwhelms the molecular precursors based on sol-gel EISA approach in terms of eliminating the sol-gel process, reducing synthesis time and producing nanocrystalline frameworks under a mild calcination temperature (300 °C). However, the resultant mesoporous TiO2 show much lower mesostructures compared with those from molecular precursors, possibly because the nanocrystals are heavily anchored with various multidentate ligands after a nonaqueous hydrothermal reaction. EISA process can proceed not only on a flat support for fabrication of ordered mesoporous TiO2 films, but also within a confined space for generation of various nanostructures. For example, anodic aluminum oxide (AAO) membranes have been widely used as a special matrix for confined synthesis of mesoporous TiO2 nanowires/nanotubes.39,40 Chen et al. reported the synthesis of a 3D ordered arrays of mesoporous TiO2 spheres by using poly(methyl methacrylate) (PMMA) inverse opal mesh as an EISA matrix.41 In addition, when spray drying process is coupled with the



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EISA method, well-known as aerosol-assisted self-assembly process, spherical mesoporous TiO2 can be fabricated in a continuous mode.42 Based on above synthetic routes and strategies, a large number of mesoporous TiO2 have been successfully prepared by using conventional titanium precursors (e.g., TiCl4, titanium alkoxides, TiOSO4, TiO2 nanocrystals). The critical issue is to control their sol-gel process and the cooperative assembly with amphiphilic templates, thus ensuring the formation of ordered mesostructures. Various surfactants are utilized as a template such as organic amines, CTAB, tetradecyl phosphates, diblock polymers (e.g., Brij 56, Brij 58), triblock copolymers (e.g., P123, F127), lab-made block copolymers

(e.g.,

KLE,43

PS-b-PEO,34,44

PI-b-PEO,45,46

PIB-b-PEO,47

PMMA-b-PEO48). These templates play key roles in modulating the mesosrtucture, surface area, pore size and wall thickness, as well as thermal stability of the resultant mesoporous TiO2. The effective interactions and driving forces between titanium species and templates are vital for the cooperative assembly. The other critical issue is to maintain the ordered mesostructures and high surface areas during the framework crystallization process at high temperature, which is often accompanied by the porosity collapsing. Grosso et al. conducted a systematic study on the thermally driven densification, pyrolysis, crystallization, and sintering of ordered mesoporous TiO2 thin films.49,50 It was found that the heating schedule, initial film thickness, nature of the substrate and templating agent, solution aging, and presence of water and other additives in the calcination environment have unique and often substantial effects on the final mesostructures of the TiO2 film based on in-situ, time-resolved


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small- and wide-angle X-ray scattering (SAXS and WAXS) and thermal ellipsometric analysis.49,50 Generally, the well-control of nucleation and growth of anatase nanocrystallites in the framework can effectively prevent the mesostructure collapse even at up to 700 °C. Indeed, great progresses regarding the synthesis of mesoporous TiO2 have been achieved via the soft-templating method, especially the EISA process. Although the whole process has been well understood,51 further optimizations and innovations such as dynamic interfacial assembly are still in great demand. 2.2. Hard-Templating (Nanocasting) Method The use of preformed mesoporous solids as hard templates to synthesize mesoporous materials has brought in new possibilities for preparation of mesoporous TiO2 with high crystallinity and novel mesostructure, which is well-known as hard-templating (nanocasting) method.22,52 In this case, the mesopores arise from the regular arrangement of replicated nanowires/nanospheres via three key steps: (i) precursor infiltration inside mesochannels of the template; (ii) conversion of the precursor into target product in the mesochannels; (iii) removal of the mesoporous template. Obviously, compared with soft-templating method, the hard-templating method is less straightforward. However, this synthesis strategy not only avoids the control of the cooperative assembly and the sol–gel process of titanium precursors, but also easily overcomes the collapse of mesoporous TiO2 frameworks during the phase transition process, thus making it quite successful and attractive on synthesis of ordered mesoporous TiO2 with novel mesostructure, high thermal stability and crystallinity. However, it is not easy to completely fill up the vacancies of templates



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with titanium precursors because of their strong tendency to precipitate and crystallize into bulk oxide phases directly in aqueous media.23 Some will partially block the channels, leading to failure in further infiltration. Zhang et al. demonstrated that the weight ratio of titanium alkoxide/KIT-6 played a key role on the mesostructures of resultant TiO2.53 When too many amount of precursors was added, bulk materials with low surface area would be formed outside the pores, thus decreasing the surface area whereas pore walls would not be stable enough to retain the mesostructures due to insufficient TiO2 nanoparticles. After all, the resultant samples showed great loss of mesostructures after removal of templates. Even though titanium alkoxides can be stabilized by chelating agents, the concurring sol-gel reaction leads to the polymerization of Ti precursors, which makes them difficult to be impregnated into the mesopore channels of hard templates. Metal salts (e.g., nitrates, chlorides) are most often used as a precursor for fabricating mesoporous metal oxide replicas owing to their high solubility, low melting point and low thermal stability for easy thermal conversion into target products, as well as strong interactions with pore walls.11 Unfortunately, classical titanium chlorides and/or nitrates are generally highly active and unstable, which cannot be used as a precursor for the nanocasting synthesis of mesoporous TiO2. To circumvent this problem, Yue et al. prehydrolyzed a titanium alkoxide and redissolved the obtained precipitate by using concentrated HNO3 (70 wt%).54 The freshly prepared aqueous solution of titanium nitrates was found to be easily infiltrated into the mesopore channels of SBA-15 and KIT-16 templates. As a result, ordered mesoporous rutile TiO2 could be obtained after calcination at 450 °C


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and removal of silica hard templates. The products show well-defined regular mesopores and a rutile-type monocrystallinity (Figure 7a, 7c).55 Their results suggest that rutile phase rather than anatase appears even at the temperature as low as 100 °C. While TIPO and freshly synthesized titanium chloride and/or titanium sulfate solutions were used as the precursors, pure anatase could be observed without the detection of rutile phase at the temperature as high as 600 °C (Figure 7b, 7d).55 It has been suggested that the presence of nitrate ions as opposite to isopropoxide, chloride ions and sulfate ions can effectively induce the crystallization of amorphous TiO2 into rutile instead of anatase during the thermal treatment. In addition, 3D ordered macro-/mesoporous TiO2 materials with controllable macropore diameters have been prepared by using self-assembled colloidal crystals and Pluronic P123 as macro-/meso-porous templates, respectively.56,57 The titanium sols and Pluronic P123 were introduced into the interstices of the colloidal crystals via an

EISA

method.

Upon

solidification

and

template

removal,

inverse

macro-/meso-porous TiO2 opals were obtained. The incorporation of interconnected macropores in mesoporous TiO2 matrixes significantly improves the mass transport, reduces the length of mesopore channels, and increases the accessible surface area within matrixes. Sun et al. demonstrated the macro-/meso-porous TiO2 exhibited 50 and 70 % greater photocurrent under UV and visible lights, respectively, and higher photoelectrocatalytic property than commercial TiO2.56 This finding provides a simple venue toward synthesis of mesoporous or macro-/meso-porous TiO2 with controllable crystal phase, high crystallinity, high surface area, and novel mesostructure.



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3. Solar Energy Conversion Access to clean, affordable, reliable and sustainable energy has become the focus of the world’s increasing prosperity and economic growth in the 21th century.58 Appreciatively, solar energy is the most obvious abundant, free, clean and renewable alternative candidate (under ideal conditions, radiation power on a horizontal surface is 1000 W m-2).59 Since Fujishima and Honda first discovered the cleavage of water by sunlight into hydrogen on a TiO2 electrode,60 a series of studies have been conducted following that, and TiO2 has become one of the most effective photocatalysts

for

photo-degradation

of

organic

pollutants

and

hydrogen

production.61-64 Unfortunately, its wide band gap (3-3.2 eV) in the ultraviolet (UV) wavelength regime has greatly limited the overall photocatalytic efficiency. So far, a variety of strategies have been developed to improve the absorption of sunlight and the efficiency for solar hydrogen production. For example, in 1994, Choi et al. reported the photoreactivity, charge carrier recombination rates, interfacial electron transfer rates of TiO2 can be significantly influenced by the metal ion dopants.65 In 2001, Asahi et al. successfully lowered the band gap by doping TiO2 with non-metal atoms (e.g., N) for high efficient visible-light photocatalysis.66 Very recently, the hydrogenation of TiO2 into “black” TiO2 was shown to boost the photo-absorption and photocatalytic activity for the splitting of water to H2.67 Moreover, another resolution of this dilemma came in the separation of the optical absorption and charge-generating functions, using an electron transfer sensitizer absorbing in the visible to inject charge carriers into TiO2, which is well-known as dye-sensitized solar


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cells (DSSCs) first demonstrated by O’Regan and Grätzel in 1991.68 In these attractive photocatalytic and photovoltaic applications, mesoporous TiO2 with high surface area for intimate contact, open mesostructures for fast diffusion, and efficient dopants for visible light absorption are greatly in demand to achieve high solar energy conversion efficiency. Moreover, the separation of photogenerated electrons and holes can be greatly improved by maximizing crystallinity, optimizing exposed facets and minimizing grain-boundaries of TiO2, which unambiguously promote the quantum efficiency.69 Till to this date, numerous efforts have been conducted worldwide involving these aspects to design high performance mesoporous TiO2. 3.1. Doping Due to its wide band-gap, anatase TiO2 can mainly absorb ultraviolet photons. However, solar light only contains a small amount of ultraviolet photons (about 5 %), and room light lamps emit mainly visible photons. Also, the rapid electron-hole recombination further lowers the quantum efficiency. Therefore, it is of great interest to improve the generation and separation of electrons and holes by mesoporous TiO2 to utilize the abundant solar energy efficiently. Doping is a promising strategy to realize this dream. Transition-metal dopants for mesoporous anatase could effectively extend the photoresponse into the visible light region, which derives mainly from low energy photon excitations of the corresponding metal oxides clusters with smaller bandgap, and partially from the excitations of the introduced localized states in the bandgap of doped TiO2.70 Moreover, the coupling with dopant oxide clusters could improve the



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separation efficiency of photogenerated electron-hole pairs due to formation of heterojunctions in favor of transferring electrons from one to another, in case of suitable conduction band potentials.16 For example, Yu et al. have fabricated an ordered mesoporous Cr-TiO2 photocatalyst.71 Based on their results, mesoporous TiO2 is ineffective under visible light but the mesoporous Cr-TiO2 shows a very high decomposition rate for Rhodamine B. This may be related to Cr3+ doping, which allows activation of the mesoporous TiO2 in the visible light region. Bian et al. prepared Bi2O3-doped mesoporous TiO2 via an EISA method.72 Such mesoporous Bi2O3/TiO2 exhibited a high photocatalytic degradation of p-chlorophenol under visible light irradiation owing to the strong photosensitizing effect of Bi2O3 (Figure 8a). Generally, the transition-metal dopants exist as their oxide clusters instead of transition metal atoms within the surface/subsurface of Ti cation matrices due to the difference in crystal nucleation behavior and crystal structure between dopants and TiO2, which easily act as recombination centers.70 In contrast, non-metal dopants within a surface can exist as isolated atoms rather than clusters, which have greater potential for realizing visible-light photoactivity. Gu et al. have synthesized C-doped TiO2 photocatalysts with anatase crystalline walls and substitutional carbon occupying oxygen sites under template-free condition,73 which exhibited outstanding visible light photocatalytic activity. Chi et al. employed urea as a nitrogen source and a pH adjusting agent for synthesis of mesoporous N-TiO2 microspheres via a one-step and template-free solvothermal method.74 The resultant products show good absorbance of visible light, thus leading to high photocatalytic activity (Figure 8b). Moreover,


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co-doping with two suitable heteroatoms can also achieve substantial synergistic effects. For example, B-N bonds formed can play a vital role in significantly improving photocatalytic activity.75 So far, nearly all non-metals (N, C, B, S, P, etc.) have been explored as dopants to improve the visible-light photocatalytic activity, because they can introduce localized states in the bandgap, and also create different surface structures, which can intrinsically alter the surface transfer of charge carriers.15 Despite great successes have been achieved, there are still challenging issues in terms of doping: 1) to precisely control the spatial distribution of dopant-induced electronic states and the surface heteroatom or heterojunction structure to promote the surface charge-carrier transfer; 2) to realize homogeneous distribution of dopants throughout mesoporous TiO2; and 3) to well maintain the opened mesostructures, high surface area and crystallinity. 3.2. Heterostructuring Due to their different Fermi levels, characterized by the work function of the noble metals (e.g., Au, Ag, Pt, Pd) and the bandgap structure of TiO2, a unique Schottky barrier would be formed between noble metals and TiO2 upon intimate contact, thus remarkably favoring the separation of photogenerated electrons and holes.76 Therefore, various noble metal/mesoporous TiO2 composites for advanced photocatalysis have been designed and developed. For example, Li et al. prepared mesoporous Au/TiO2 nanocomposites by a one-pot assembly and subsequent calcination process (Figure 9a).77 The resultant composites showed an ordered hexagonal regularity and high



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crystallinity, therefore exhibited excellent photocatalytic activities toward phenol oxidation and chromium reduction (Figure 9b). A near three-time improvement in phenol decomposition role was achieved when 0.5 % of Au was doped. Besides noble metals, carbon nanotubes (CNTs) and graphene are also well-known as charge carriers, and widely employed to suppress the photoinduced electron-hole recombination. Yang et al. successfully introduced graphene sheets into the mesoporous TiO2 anode of DSSCs, thus a significant increase of the short-circuit current density (45 %) and conversion efficiency (39 %) was obtained.78 Since Spanhel et al. in 1987 confirmed the efficient electron-injection process from CdS particles to the conduction band of attached TiO2 particles illuminated with visible light,79 much effort has been devoted to this hybrid/heterostructured system, which can extend its photoresponse to the visible light region and accelerate the photogenerated electrons transfer from the sensitizer CdS quantum dots (QDs) to TiO2. Li et al. prepared CdS QDs sensitized mesoporous TiO2 by preplanting CdO as crystal seeds into the frameworks and then converting CdO to CdS QDs through ion-exchange.80 The composites showed excellent photocatalytic efficiency for both oxidation of NO gas in air and degradation of organic compounds in aqueous solution under visible light irradiation. Recently, Feng et al. deposited ~ 5 nm CdS QDs in the primary mesopore channels of large pore mesoporous TiO2 based on a modified successive ionic layer adsorption reaction method (Figure 9c, 9d).44 The close contact of the deposited CdS QDs on the channel surface of the crystalline TiO2 frameworks enables fast and effective transfer of photogenerated charge carriers diffusing towards


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the conductive substrate, thus leading to a substantial enhancement of photo-response to the visible light (Figure 9e). These results suggest that compared with single-component mesoporous TiO2, the hybrid/heterostructured system exhibits significant advantages in promoting the separation of electron-hole pairs through interface carrier-transfer pathways, as well as extending the light-response range by coupling suitable electronic structures, thus significantly improving the solar energy conversion efficiency. However, some issues were not well-resolved: a) the controllable assembly/growth of functional components in the channels/frameworks of mesoporous TiO2 but not affecting mesostrcutures; b) the reasonable design over the redox potential of transferred electrons/holes. Therefore, how to construct high efficient heterojunctions for solar energy conversion is still valuable for further exploration and research. 3.3. High Crystallization and Surface Area As the charge carriers, electron-hole pairs generated by light absorption require to be effectively separated and transferred through a bulk of semiconductors into the surface, finally turned into valuable and strategically important assets. However, the amorphous or semi-crystalline TiO2 easily suffer from fast electron-hole recombination owing to a large number of defects.14 Therefore, high crystalline TiO2 with less surface defects is urgently desired to improve the conversion efficiency of solar energy. In addition, ordered mesopore channel and high surface area can greatly enhance the opportunities for dyes-loading, pollutants-adsorption and ionic-transport. Upon the synthesis of mesoporous TiO2, crystallinity and surface area are directly



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related to the calcination temperature. In general, high crystallinity requires high calcination temperature. However, the employment of high temperature usually leads to undesirable crystal grain growth, a total collapse of mesoporous framework and a remarkable decrease of surface area. Thus, only semicrystalline TiO2 with small nanocrystals embedded in an amorphous matrix can be obtained, which greatly impedes their applications. Recently, we developed an organic-inorganic-amphiphilic co-assembly strategy towards fabricating mesoporous anatase nanocomposites with high thermal stability (Figure 10a).81-83 For example, ordered hexagonal mesoporous TiO2-SiO2 composites with variable Ti/Si ratios could be obtained by using TIPO as a titania source, TEOS as a silica source, and Pluronic P123 as a template (Figure 10b, 10c).81 The resultant materials are ultra highly stable (over 900 °C), possess a uniform pore size of ~ 6.8 nm and a high surface area of ~ 290 m2/g, as well as exhibit enhanced photocatalytic activities with increasing crystallinity (Figure 10d). These results suggest that during the calcination, titania is fully crystallized into anatase nanocrystals, then uniformly embedded in the pore walls to form “bricked mortar” frameworks; whereas the amorphous compositions act as an effective glue linking the TiO2 nanocrystals and improving the thermal stability. It is well known that the hard templates can provide an excellent support and confinement framework to prevent the collapse of mesoporous frameworks, thus leading to high crystallinity. Inspired by this advantage, Zhao and coworkers developed a simple surfactant sulfuric acid carbonization method to synthesize


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ultra-stable ordered mesoporous titania with high crystallinity.84 During the calcination in N2, the Pluronic copolymer templates were first carbonized by the residual H2SO4 to produce rigid tubular carbon on the surface of mesochannels, which could support and stabilize the mesoporous frameworks during the high temperature calcination (up to 650 °C). The resultant mesoporous TiO2 possesses a high surface area of 193 m2/g and fully crystalline frameworks, as well as shows good photocatalysis performance. Lee et al. reported the synthesis of highly crystalline mesoporous TiO2 via a combined assembly by soft and hard (CASH) chemistries strategy using amphiphilic diblock copolymer PI-b-PEO as a template (Figure 11a-11c).45 Upon appropriate heating under Ar, the hydrophobic PI polymers can be converted to a sturdy, amorphous carbon material, which in situ acts as a rigid support to TiO2 walls, thus effectively preventing framework collapse during crystallization. The results indicate that there is a remarkable increase in all the solar cell performance parameters when the crystallinity of the resultant mesoporous TiO2 is slightly improved processing from 450 to 550 °C (Figure 11d).46 More recently, Zhou et al. developed a molecule protecting strategy to produce high crystalline mesoporous anatase.85 The ethylenediamine molecules were introduced into the liquid crystal mesophase by refluxing, which can significantly bind on the surface of TiO2, thus efficiently protecting the primary nanocrystals during calcination. The obtained mesoporous TiO2 retains well-ordered mesostructures even after being calcined at 700 °C, and possesses a large pore size (~ 10 nm) and a high surface area (~ 122 m2/g).



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These studies are very meaningful and, indeed significantly promote the progress in the synthesis of ordered mesoporous TiO2 with high crystallinity and surface area, which not only help to suppress the rapid combination of photogenerated electrons and holes, but also provide abundant surface area for reactive sites and enhance the diffusion of reactants and products, thus greatly improving the solar energy conversion efficiency. 3.4. Highly Reactive Facets For anatase, it is usually exposed with low index facets such as (001) and (101). Theoretical studies indicate that the (001) surface of anatase is much more reactive than (101) in heterogeneous reactions (e.g., photodegradation, photosplitting).86,87 This is based on the fact that (001) facets consist of high densities of under-coordinated Ti atoms and very large Ti-O-Ti bond angles at the surface.86,87 Unfortunately, most current mesoporous TiO2 are limited with amorphous or polycrystalline frameworks dominated by thermodynamically stable (101) facets (more than 94 %, according to the Wulff construction),88 which generally exhibit low photocatalytic activity and charge-transport capabilities. Recently, Bian et al. have synthesized highly active mesoporous, single crystal-like anatase with both preferential (001) plane exposure and controllable mesopore networks via a crystal oriented growth method.89 The (001) planes of TiO2 building blocks generated solvothermally are preferably adsorbed by SO42- anions. Further attached growth confined within SBA-15/KIT-6 silicas led to anatse crystals with preferential (001) plane exposure. Subsequent scaffold removal resulted in ordered mesopore networks


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(Figure 12a, 12b). Compared with commercial P25 and polycrystalline TiO2, the resultant mesoporous TiO2 exhibits superior photocatalytic activity, which can be attributed to the enhanced dissociative adsorption and separation of photoexcited hole/electrons mediated by the (001) facets (Figure 12c, 12d). Up to date, various strategies have been developed for the synthesis of anatase micro- and nano-structures with exposed high-energy (001) facets.90,91 However, it is greatly challenging yet urgently desirable to simply integrate advanced crystallographic properties of TiO2 (e.g., the percentage of exposed (001) facets) with the advantages of mesoporous networks (e.g., the surface area-/pore-dependent activity and selectivity) to enhance the solar energy conversion efficiency. 3.5. Mesoporous Single Crystals Photovoltaic devices based on interpenetrating mesoscopic TiO2 nanoparticle networks have emerged as a credible alternative to conventional solar cells.59,92 One of challenges is to provide desired accessible surfaces, but maintain good charge transport. State-of-the-art, nanoparticles with high surface area are widely used because they show the maximum capability of generating free electrons. However, a large number of interfaces in situ generated between particles greatly block electron-transfer across nanoparticle networks, thus leading to a low efficiency Therefore, in-film thermal sintering is generally required to reinforce electronic contact between particles, unfortunately resulting in increasing fabrication cost, limiting the use of flexible substrates and precluding, for instance, multi-junction solar cell processing.93 All these challenges might be overwhelmed by mesoporous



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single crystals (MSCs) with anantase crystallinity, which were first prepared by Crossland et al..94 The preparation of the anatase MSC proceeded in a sacrificial silica guiding template. The results suggest that ‘seeding’ the template with microscopic nucleation sites is vital for the confining growth of single crystal TiO2 to the guiding template, which directly overwhelms the homogeneous nucleation (Figure 13a). The obtained MSC anatase shows a typical shape related to homogeneously nucleated bulk crystal, possesses a negative replica of the silica mold and a high surface area of 70 m2/g (Figure 13b). Significantly, the MSC anatase displays higher conductivity and electron mobility than conventionally used nanoparticles. Moreover, a record of 7.2 % efficiency of solid-state DSSCs can be obtained sub-150 °C processing (Figure 15c). This discovery instantly opens up whole new vistas for the synthesis of MSCs and construction of transparent, flexible and portable gadgets (e.g., smartphones, tablets) under a low-temperature processing. 4. Lithium-Ion Batteries There is great interest but of great challenges in designing next-generation lithium-ion batteries (LIBs) with increased energy density, cycling life and charge/discharge rate capability. Titanium-based materials are a class of promising alternative materials to graphite since they exhibit relatively high lithium insertion/extraction voltage at about 1.7 V. This feature can efficiently avoids the formation of solid-electrolyte interphase (SEI) layers and electroplating of lithium during cycling processes, thus leading to better overcharge protection and safety.95 Moreover, TiO2 is an abundant, low cost and environmentally benign material with a


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low volume change (3-4 %), which is vital for large-scale energy storage. However, lithium can be intercalated into bulk anatase, but only to Li0.5TiO2 with poor cycle ability and rate performance due to its long diffusion paths of lithium transport and low electronic conductivity. In contrast, mesoporous anatase can reach a composition of approaching Li0.96TiO2 with better cycle and rate capabilities.96 Moreover, the performance of lithium storage in nanoparticles (~ 6.5 nm) assembled micrometer sized mesoporous anatase was compared with separated nanoparticles (~ 6 nm). It was found that the former was highly superior, showing significantly higher volumetric capacities than that of nanoparticle anatase, up to twice the capacity at the highest rates, despite the lower intrinsic density of mesostructures. These results suggest that the ordered mesostructures providing numerous open channels for the access of electrolyte and facilitating the ultrafast diffusion of lithium ions are very important in achieving high performance batteries (Figure 14). Besides construction of mesoporous networks, the improvement of the electrical conductivity and kinetics of Li+ insertion is also a powerful strategy to enhance the high rate performance and cycle capability. For example, Yang et al. optimized mesoporous TiO2 electrode by introducing graphene layers as minicurrent collectors, which are favorable for the fast electron transport in the electrode.97 Chen et al. demonstrated the synthesis of uniform mesoporous TiO2 spheres by the self-assembly of ultrathin anatase nanosheets with nearly 100 % (001) exposed facets,98 exhibiting high Coulombic efficiency, excellent capacity retention and superior rate behaviors. It is suggested that the energy barriers for Li+ insertion into (001) and (101) surfaces of anatase are 1.33 and 2.73 eV,



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respectively, thus the high surface density of exposed TiO2 (001) facets can lead to fast lithium insertion/deinsertion processes in batteries.86,87 5. Novel Nanostrcutures Mesoporous TiO2 is an ideal platform for constructing multifunctional materials that incorporate a variety of functional nanostructured materials. An important example is core-shell type nanostructure in which a functional core is coated with a thin layer of mesoporous TiO2.99-101 However, due to the high reactivity of titanium precursors, it is much difficult to control the reaction kinetics for constructing uniform core-shell mesoporous TiO2 nanostructures. Recently, we have developed a versatile kinetics-controlled coating method for uniformly depositing mesoporous TiO2 on various cores (Figure 15a-15c).102 In which, the rational control of the hydrolysis and condensation kinetics of titanium alkoxides in a narrow range of ammonia content is the key factor to successfully achieve preferentially heterogeneous nucleation on a core for producing uniform core-shell nanostructures. Moreover, hollow mesoporous TiO2 are also of great interest for the additional improvement in mass transfer and in active surface accessibility, which resulted from the removal of the core portion of a colloidal particle to produce a porous hollow shell.103,104 For example, Joo et al. reported a robust “silica-protected calcinations” process for preparing hollow mesoporous TiO2 with high surface area, and demonstrated their greatly enhanced photocatalytic activity (Figure 15d-15e).104 Another interesting example is yolk-shell nanostructure, which represents a new class of hollow or core-shell nanostructures with a distinctive core@void@shell configuration.105 Zhao and coworkers developed


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a facile “hydrothermal etching assisted crystallization” strategy to synthesize Fe3O4@titanate yolk-shell microspheres with ultrathin nanosheets assembled double-shell structure (Figure 16).106 The resultant microspheres possess a high surface area of ~150 m2/g and a large pore size of ~ 7.5 nm, and exhibit a remarkable catalytic performance. Seh et al. demonstrated the design of a sulphur-TiO2 yolk-shell nanoarchitecture with internal void space to accommodate the volume expansion of sulphur, resulting in an intact TiO2 shell to minimize polysulphide dissolution.107 An initial specific capacity of 1,030 mAh g-1 at 0.5 C and Coulombic efficiency of 98.4 % over 1,000 cycles was achieved. Most importantly, the capacity decay after 1,000 cycles was as small as 0.033 % per cycle, which represents the best performance for long-cycle lithium-sulphur batteries so far.

6. Summary and Outlook Mesoporous TiO2 has attracted tremendous attentions because of its fascinating properties such as low cost, environmental benignity, plentiful polymorphs, good chemical and thermal stability, excellent electronic and optical properties. These make it highly promising in photocatalysis, DSSCs, PECs, lithium-ion batteries, catalysis, etc. In this perspective, we have discussed the synthesis pathways of mesoporous TiO2. Virtually, all mesoporous TiO2 begin with a well-understanding of the sol-gel chemistry of titanium precursors. In soft-templating pathway, it is critical to control the hydrolysis and condensation rates of titanium precursors, and to enable the cooperative assembly of surfactants. The successful case is the EISA method. In the hard-templating pathway, the principal issue is yet to choose titanium precursors for



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successful infiltration and conversion. The designs and fundamentals of mesoporous TiO2 are presented in regard of improving the conversion efficiency of solar energy (e.g., doping, heterostructuring, high crystallinity and surface area, highly reactive facets, mesoporous single crystal), and enhancing the performance of lithium-ion batteries (e.g., ordered mesostructures, enhancing electron conductivities and lithium diffusion rate, and novel nanostructures). Despite great progress has been made on the design and synthesis of mesoporous TiO2, until now, it is still a great challenge, but of significance to develop cheap, low toxicity and reproducible approaches to simply control the pore size, wall thickness, surface area and morphology, and to greatly improve the crystallinity, as well as to broadly extend the functionality. The utilization of new synthetic routes, for instance, dynamic interfacial assembly and seeded nucleation and growth in template pores, should be paid more efforts in the future. Along with the exciting opportunities of mesoporous TiO2 for solving environmental and energy-related issues, there are also major challenges before industrial applications eventually replacing conventional technologies. In photodegradation of pollutants, solar fuel and photovoltaic, the low conversion efficiency of solar energy is of major concern at present. Urgent tasks for the distant future are developing well-designed and controllable synthesis methods for mesoporous TiO2 based materials to maximize the use of the solar spectrum, minimize the recombination of photogenerated electron-hole pairs and optimize the fast mass and charge transport. Notably, significant breakthroughs have recently been done such as the “black” TiO2 and the plasmonic TiO2 nanomaterials, which can


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greatly boost the photo-absorption of TiO2 and its photocatalytic activities. Although constructing mesoporous TiO2 can partly improve its performance of lithium storage, the poor electron conductivity and low lithium diffusion rate still inhibits its practical applications. Consequently, novel and well-designed nanostructures, sub-units and hybrids based on mesoporous frameworks are in great demand for the improvement of electron conductivity and lithium diffusion. Moreover, it is highly expected that mesoporous TiO2 can get some breakthroughs in other applications such as gas sensing, catalysis and drug delivery. To explore and enhance their applications, novel nanostructures based on mesoporous TiO2 such as core-shell, yolk-shell and hollow nanostructures are also greatly required. Acknowledgment. This work was supported by the State Key Basic Research Program of the PRC (2012CB224805, 2013CB934104), and NSF of China (Grant No. 21210004).

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Figure 1. Scheme of two representative synthesis routes for ordered mesoporous materials: (a) soft-templating method and (b) hard-templating (nanocasting) method. Reprinted from ref 8 with permission by Royal Society of Chemistry, Copyright 2011.



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Figure 2. a) The TEM image of mesoporous TiO2 prepared by using tetradecylphosphate as a template. b) The SEM image of mesoporous TiO2 microspheres prepared via a combined aqueous sol-gel and solvothermal process. Panel (a) is reprinted from ref 24 with permission by Wiley-VCH, Copyright 1995. Panel (b) is reprinted from ref 27 with permission by American Chemical Society, Copyright 2010.

Figure 3. TEM images of mesoporous TiO2 prepared by using Pluronic P123 as a template and TiCl4 as a precursor via a non-hydrolytic EISA process. Reprinted from ref 28 with permission by Nature Publishing Group, Copyright 1998.


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Figure 4. Schematic representing the various stages for the synthesis of ordered mesoporous TiO2 films via an EISA process. Reprinted from ref 32 with permission by American Chemical Society, Copyright 2003.

Figure 5. XRD patterns of a) as-made, microwave extracted and calcined mesoporous TiO2 (from down to top) templated by Pluronic P123 and b) calcined sample templated by Pluronic F108 prepared by using titanium isopropoxide and TiCl4 as a mixed precursor. TEM images of calcined mesoporous titania templated by Pluronic P123 and d) calcined sample templated by Pluronic F108. e) The corresponding electron diffraction pattern. Reprinted from ref 35 with permission by Royal Society of Chemistry, Copyright 2002.



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Figure 6. a) Schematic illustration of the evaporation-induced self-assembly of nanocrystals into mesoporous materials by using block copolymers as templates. b) STEM image of the as-made mesoporous TiO2 prepared by the assembly of Pluronic P123 and TiO2 nanocrystals. c) The SEM image of mesoporous TiO2 after calcination at 300 °C. Panel (a) is reprinted from ref 37 with permission by Nature Publishing Group, Copyright 2004. Panel (b, c) are reprinted from ref 38 with permission by American Chemical Society, Copyright 2010.


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Figure 7. TEM and HRTEM images of mesoporous TiO2 prepared by using freshly made titanium nitrate (a, c) and titanium chloride (b, d) solution as a precursor and SBA-15 as a hard template via nanocasting approach. Insets in a) and b) are the corresponding SAED patterns. Reprinted from ref 55 with permission by Wiley-VCH, Copyright 2009.

Figure 8. UV-vis diffuse reflectance spectra of a) mesoporous Bi2O3/TiO2 at different Bi2O3 doping concentration, b) mesoporous N-TiO2 microspheres and Degussa P25. Panel (a) is reprinted from ref 72 with permission by American Chemical Society,



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Copyright 2008. Panel (b) is reprinted from ref 74 with permission by American Chemical Society, Copyright 2007.

Figure 9. a) TEM images of 0.5 mol% Au/mesoporous TiO2. b) Photocatalytic activity of Au/mesoporous TiO2 containing 0 ~ 5 mol% Au in phenol-oxidation and chromium-reduction reactions. TEM and HRTEM images (c and d) of CdS-deposited mesoporous TiO2 films. e) UV-vis absorption spectra of the pristine and CdS-sensitized mesoporous TiO2 films. Panels (a, b) are reprinted from ref 77 with permission by American Chemical Society, Copyright 2007. Panels (c-e) are reprinted from ref 44 with permission by Royal Society of Chemistry, Copyright 2013.


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Figure 10. a) Schematic illustration of organic-inorganic-amphiphilic co-assembly strategy towards mesoporous TiO2 based composites. HRTEM images of mesoporous 80TiO2-20SiO2 composite calcined at 700 °C (b) and 800 °C (c). d) Photocatalytic degradation of RhB on mesoporous 80TiO2-20SiO2 composites calcined at different temperature. Reprinted from ref 81 with permission by American Chemical Society, Copyright 2007.

Figure 11. a) Schematic representation of the CASH method. TEM images (b and c)



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of the mesoporous TiO2 prepared by CASH method. d) Photocurrent-voltage characteristics of solid-state DSSCs fabricated from mesoporous TiO2-CASH films calcinated at temperatures between 450 °C and 600 °C. Panels (a-c) are reprinted from ref 45 with permission by Nature Publishing Group, Copyright 2008. Panel (d) is reprinted from ref 46 with permission by Royal Society of Chemistry, Copyright 2009.

Figure 12. a) TEM and b) HRTEM images of mesoporous single crystals grown within SBA-15. The insets are the SAED patterns of the anatase crsystal along (001) zone axis (Top) and the hexagonal mesophase along the [001] direction. Photocatalytic conversions of c) toluene to benzaldehyde and d) cinnamyl alcohol to cinnamaldehyde in the presence of 1) commercial P25, 2) polycrystalline TiO2, and single-crystal-like TiO2 with 3) disordered, 4) 2D, and 5) 3D mesostructures. Reprinted from ref 89 with permission by Wiley-VCH, Copyright 2011.


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Figure 13. a) Schematic representation of MSC nucleation and growth within a mesoporous template. b) The SEM image of mesoporous TiO2 crystals grown by seeded nucleation in the bulk of the silica template. c) Mobility dependence on photoinduced charge density for anatase TiO2 MSC and nanoparticle films measured via transient mobility spectroscopy. Reprinted from ref 94 with permission by Nature Publishing Group, Copyright 2013.



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Figure 14. a) TEM and b) HRTEM images of ordered mesoporous anatase. c) First cycle load curves of mesoporous anatase electrode at different rates. d) High-rate discharge capacity retention for mesoporous anatase electrode. Reprinted from ref 96 with permission by Wiley-VCH, Copyright 2010.

Figure 15. TEM images of core-shell mesoporous TiO2 nanostructure structures prepared by the kinetics-controlled coating method: a) Fe3O4, b) SiO2 and c) graphene oxides (GO) as a core, respectively. d) TEM image and e) Nitrogen adsorption isotherm of hollow mesoporous TiO2 fabricated via a “silica-protected calcinations” process. Panels (a-c) are reprinted from ref 102 with permission by American Chemical Society, Copyright 2012. Panels (d, e) are reprinted from ref 104 with permission by Wiley-VCH, Copyright 2010.


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Figure 16. a) Schematic of the synthetic process, b) SEM and c) TEM images of Fe3O4@titanate double-shelled yolk-shell microspheres. (b) Scale bar, 200 nm. Reprinted from ref 106 with permission by American Chemical Society, Copyright 2011.



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TOC


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A Perspective on Mesoporous TiO2 Materials - Chemistry of

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