Titanium-Based Mesoporous Materials for Photocatalysis

Abstract

Titanium-based mesoporous materials are always the strong strength in material science. In this part, the chapter has been divided into four parts to elaborately clarify the titanium-based materials. We carefully introduced the history of mesoporous materials and the preparation and application of mesoporous TiO₂ materials. Additionally, we presented mesoporous TiO₂–SiO₂ materials and Ti–SiO₂ nanomaterials. Furthermore, we analyzed their application in photocatalysis, such as organic pollutant degradation, photocatalytic hydrogen production, CO₂ photoreduction, and NO/NO₂ photoreduction. The abovementioned all provide us some inspiration in the field of energy and environment.

3.1 The History of Mesoporous Materials

Up to date, it is well known that the rapid progress of society is closely associated with material science. Particularly in recent 30 or 40 years, there has been a boom of novel materials, including of mesoporous materials. Mesoporous molecular sieve belongs to a noticeable area owing to its excellent properties, such as high specific surface area and adjustable ordered mesoporous pore size. According to the rules of IUPAC, porous materials can be classified into three types: micropore materials (pore size <2 nm), mesopore materials (2 nm <pore size <50 nm), and macropore materials (pore size >50 nm), respectively [1]. In 1992, the Mobil’s researchers successfully synthesized a novel ordered mesoporous silica materials called M41S with surfactants as templating agents, thus opening a new era of mesoporous materials [2]. On the basis of chemical composition, mesoporous molecular sieve can be divided into two categories: silicon-based materials and non-silicon-based materials. The most common one is M41S, which is a branch of silicon-based materials. Additionally, there are many other sorts of mesoporous molecular sieves, such as SBA-n series, MSU series, CMK series, HMS, KIT, and metallic or metal oxide series. Compared with microporous and macroporous molecular sieves, mesoporous materials possess many outstanding characteristics. But there are still some shortcomings exist, for instance, its low hydrothermal stability and weak acidity, which limit its application. Ryoo et al. [3] synthesized highly stable MCM-41 by adjusting the pH value of the solution and adding salt compounds. And Mokaya et al. [4] obtained ultrastable MCM-41 by post-processing. Robert Mokaya prepared restructured pure silica MCM-41 materials through seeded crystallization route. This method used MCM-41 as “silica source” for secondary synthesis with extending the reaction time of periods. Therefore, the pore wall thickness was increased systematically by extending the time. The remarkable stability is attributed to the combination of thicker pore walls and less strained silica frameworks.

In general, developing new mesoporous structure, developing new synthetic methods, improving the crystallinity based on maintaining the structure, and seeking to optimize the functional structure and so on will continue to be hot spots for researchers in the future.

3.2 The Development of Mesoporous TiO₂ in Photocatalysis

3.2.1 The Preparation of Mesoporous TiO₂

Highly efficient mesoporous TiO₂ materials have been applied to the development of the sustainable, nonhazardous, and economical processes in different reaction systems and conditions (Fig. 3.1). It can be utilized in purification of polluted water and air, the decomposition of H₂O into H₂ and O₂, self-cleaning, and so on [5]. Currently, there are various preparation techniques of mesoporous titanium dioxide, such as sol–gel method [6,7,8,9], hydrothermal synthesis [10,11,12,13,14], atmospheric liquid phase [15], and radio frequency magnetron sputtering deposition method [16,17,18,19]. In addition to these methods, the removal of template for mesoporous titanium dioxide is also available now [20,21,22,23,24].

Fig. 3.1

(a) Applications of titanium oxide-based photocatalysts in the reaction system with H₂O and O₂. (b) Reaction systems with only H₂O (O₂ free). Inserted figure shows the production of electrons in the conduction band and holes in the valence band under UV light irradiation of TiO₂ semiconducting materials. (Reprinted with permission from Ref. [13].Copyright 2000, Royal Society of Chemistry)

Regarding the synthetic methods discussed above, we have listed some advantages and disadvantages among them. Sol–gel method exhibits good properties in terms of synthesized materials, such as low cost, high purity, good uniformity, and easy to be doped. In this part, organic and inorganic titanium precursors can be used to obtain photocatalysts, for example, TTIP and TiCl₄. Hydrothermal methods can prepare materials with high crystallinity; meanwhile, it can save time. In this method, there are various options of titanium precursors, such as TiOSO₄, TBOT, and TTIP. Compared with the previous two synthesis methods, the advantage of atmospheric liquid phase method is that it merely requires simple experimental equipment; thus it can achieve the possibility of industrialization. Adding the template agent during the reaction, and then stripped by heating or other methods to remove template, is a general method for the preparation of mesoporous materials. As a result, the heat treatment will significantly influence the products’ structures, and inorganic skeleton is prone to collapse. Additionally, this process takes several hours, greatly extends the reaction time, and increases energy consumption. Hence, the adverse effect of template removal method is obvious. Moreover, the experimental conditions are not only harsh but also cumbersome. As to the radio frequency magnetron sputtering deposition method (RF-MS), the general way is using the TiO₂ plate, and then the depositions are carried out in a mixture of Ar and O₂. The thin films can be deposited on metal substrates such as Al, Fe, Ti, Zr, Pd, and Pt. Analyzing these methods, it is interesting to find the samples prepared by RF-MS methods have much smaller BET areas than those samples prepared by other three methods; therefore, each kind of catalysts has unique applications owing to their special properties.

3.2.2 Doping Modification on Mesoporous TiO₂

Since the discovery of mesoporous silica M41S, a variety of mesoporous materials have been synthesized [2]. These mesoporous materials exhibit widely potential applications in the industrial catalytic reactions, because they have high surface area, large pore size, and multidimensional framework and they are easy to be recycled. However, the widespread technological use of mesoporous TiO₂ is always to some extent constrained by its wide bandgap (3.2 eV), which requires ultraviolet irradiation for photocatalytic activation. And TiO₂ only absorbs 5% of the spectrum of the sunlight in the near ultraviolet region, which greatly limits its efficient application. To achieve efficient photocatalytic activity in the visible light range, one strategy is to reduce the bandgap of TiO₂. Doping of metal and nonmetal elements seems to be an effective method to enhance the photoactivity of mesoporous TiO₂.

In 2001, the metal ion implantation of TiO₂ with metal ions (V⁺, Mn⁺, Fe⁺) at high energy acceleration was prepared by Yamashita et al. [25]. These catalysts exhibited photocatalytic reactivity for degradation of 2-propanol diluted in water under visible light irradiation (λ > 450 nm). After then, Zhang et al. [10] obtained the large mesoporous microspheres of titania and WO₃/TiO₂ composites by using TiOSO₄ as an inorganic precursor and P123 as structure directing agent via hydrothermal route. The workers also evaluated its photoactivity by the degradation of methyl orange, which is representative of stable azo dyes. Moreover, Tian et al. [26] synthesized gold loaded on TiO₂ (Au/TiO₂) catalysts using Au (I)–thiosulfate complex (Au(S₂O₃)₂)³⁻ as the golden precursor for the first time, and its photoactivity was tested by the degradation of MO under visible light. Also, Yuan et al. [27] reported that La³⁺-doped mesoporous TiO₂ with a highly crystallized framework and long-range order was prepared by using nano-anatase particles as nano-building units. Based on the previous work [12], Yuan and his coworkers also prepared highly dispersed Pd nanoparticles in La-doped mesoporous TiO₂ with crystalline framework via co-assembly and photoreduction method [28] in 2006. Figure 3.2 is an illustration of the process. By using this simple method, highly dispersed Pd nanoparticles were prepared in La-doped mesoporous titania with crystallized walls by in situ photoreducing of PdO at room temperature.

Fig. 3.2

Preparation of Pd nanoparticles in La-doped mesoporous titania by co-assembly and in situ photoreduction. (Reprinted with permission from Ref. [28]. Copyright 2006, Springer)

In 2010, Zhang et al. [29] prepared Fe³⁺-doped mesoporous TiO₂ with ordered mesoporous structure via the solvent evaporation-induced self-assembly process through using P123 as soft template. They also successfully prepared the copper impregnated Ms-TiO₂ by using water immiscible room-temperature ionic liquid 1-butyl-3-methyl-imidazolium-tetrafluoroborate as a template and an effective additional solvent via the sol–gel method at low temperature [30]. As a result, the photoactivity of the sample Cu/Ms-TiO₂ is superior to P-25, Ms-TiO₂, and 2.0% Cu/P-25 in the case of 2,4-dichlorophenol and methyl orange under visible light (λ > 420 nm).

Not only the doping of metal can improve the photocatalytic activity, but also the doping of nonmental element can achieve the same goal. A thermal stable SiO₂-doped mesoporous TiO₂ with high crystallinity was prepared through a templating method proposed by Zhang et al. [31]. It was found that the 15% SiO₂-dopped mesoporous TiO₂ exhibited much higher photoactivity than P25, attributing to the high anatase crystallinity, large specific surface area, abundant preserved surface hydroxyl groups, and mesoporous channels. In 2010, N and F co-doped TiO₂ microspheres were prepared by ethanol solvothermal method, using tetrabutyl titanate as precursor, urea as a nitrogen source, and ammonium fluoride as a fluorine source [32]. Then Zhang and his group reported that the synergetic effect of nitrogen and fluorine doping is responsible for the enhancement of photodegradation activity of AO7 under the irradiation of visible light. Later, Zhang et al. [33] synthesized N, B, Si-tridoped mesoporous TiO₂ photocatalyst through a modified sol–gel method. In this process, dodecylamine not only acted as a pore template but also as a nitrogen dopant, and H₃BO₃ acted as a boron dopant. Moreover, it showed a strong absorption in the visible light region because the doping of N and B narrowed the bandgap.

Besides doping of either metal or nonmental elements, the co-doping of metal and nonmetal element is another effective way of modification. For instance, Zhang et al. [34] obtained the iron (III) and nitrogen co-doped mesoporous TiO2 for the first time by the modified sol–gel method. Ma et al. [35] synthesized well-ordered mesoporous TiO₂ co-doped with nitrogen and ytterbium by an evaporation-induced self-assembly process.

The merits of mesoporous TiO₂ have been listed aforementioned. And we have summarized various preparation methods reported in existing literature as well. By doping modification of TiO₂, the photoactivity of corresponding catalysts can be enhanced to some extent; however, the fast recombination of electron–hole pairs and scarce adsorptive sites undermined the further research. According to some reported literatures, the composition of Si or porous MOFs with the TiO₂ may be a feasible way in enhancing the separation of electrons and holes.

3.2.3 Mesoporous TiO₂–Graphene Materials

Graphene (GR) possesses large specific surface area, excellent conductive, mechanical, and hydrophobic properties, which allow it to be multifunctional materials with excellent capacity for carrying and conducting electrons and holes. Especially, three-dimensional (3D) graphene aerogels (GAs) compounded with mesoporous TiO₂ composites are ultralight massive catalysts, which display hydrophobic properties and facilitate photocatalytic recyclings. In 2014, Qiu et al. [36] studied TiO₂–graphene composites as solar light photocatalysts and electrode materials for lithium–ion batteries (LIBs). They used a one-step hydrothermal method to prepare 3D-structured TiO₂/GA composites. In this process, Ti(SO₄)₂ was firstly dissolved in aqueous solution to form crystal seeds before a known amount of glucose was adsorbed on the seeds, followed by fixation of the seeds on the surface of graphene oxides. The presence of glucose results in the exposure of (001) facets in the nanocrystals up to 50% (inset of Fig. 3.3b), achieving the sizes ranging from 15 to 20 nm and realizing mesoporous interface between TiO₂ and the GR (in Fig. 3.3c).

Fig. 3.3

(a) Glucose-linked transformation pathway for the in situ growth of TiO₂ nanocrystals with (001) facets on the GAs surface. (b) TEM image for TiO₂/GAs (67 wt % of TiO₂ in TiO₂/GAs). Inset (b) is the corresponding morphology distribution of the TiO₂ nanocrystals derived from 100 of TiO₂ crystals in image (b). (c) HRTEM image for TiO₂/GAs (67 wt %). Inset (c) is the corresponding fast Fourier transform (FFT) pattern. (Reprinted with permission from Ref. [36]. Copyright 2014, American Chemistry Society)

3.3 The Development of TiO₂–SiO₂ Mesoporous Materials

In this section, we briefly introduce the research of TiO₂–SiO₂ mesoporous materials. Although TiO₂ has some important properties, for instance, nontoxicity and excellent photostability, there are also some drawbacks constraining the performance of TiO₂ in photocatalytic process [37, 38]. To overcome the drawbacks, the researchers have prepared TiO₂ compounded materials that can provide large number of adsorptive sites by dispersion of TiO₂ species into a porous support with large surface area. Silica has been widely employed as the carrier, owing to its outstanding mechanical strength, high inner surface area, and uniform pore size. With the highly dispersion of TiO₂ into the porous SiO₂ support, the TiO₂–SiO₂ mixed oxide photocatalysts have shown significantly enhanced activities compared to pure TiO₂. On the one hand, TiO₂ and SiO₂ may be combined together to form a mixture of the two oxides, with interaction forces other than weak Vander Waals forces. On the other hand, they can integrate by means of the formation of Ti–O–Si bonds to form the composite oxides. When combined together through chemical bonding, the physical–chemical properties of TiO₂–SiO₂ differ from simple combination of each phase. In a word, homogeneity or dispersion largely depends on preparation methods and synthesis conditions. Those novel TiO₂–SiO₂ materials not only take advantages of both TiO₂ (an n-type semiconductor and an active catalytic support) and SiO₂ (high thermal stability and excellent mechanical strength) but also extend their applications through the generation of new catalytic active sites based on the interactions between TiO₂ and SiO₂ [39]. Hence, it is indeed a promising and new catalytic material in many research areas.

3.3.1 The Preparation of TiO₂–SiO₂ Mesoporous Materials

There are various methods of preparing TiO₂–SiO₂ mesoporous materials, such as sol–gel method [38, 40,41,42], hydrothermal method [43, 44], chemical vapor deposition [45], precipitation method [46], liquid phase deposition [47], microwave irradiation method [48], impregnation method [49, 50], and evaporation-induced self-assembly (EISA) method [51].

For instance, TiO₂–SiO₂ nanocomposite can be prepared by sol–gel method. Commonly, the first step is to formulate sol with all kinds of ingredients, and then the gelation process, and at last removal of the surfactant to obtain mesoporous materials. In 2003, titania–silica mixed oxides were prepared by the sol–gel method from tetraethylorthosilicate (TEOS) and titanium (IV) isopropoxide (TTIP) as precursors by Elizabeth et al. [42]. Li et al. [43] also obtained silica-modified titanium dioxides by a hydrothermal method. There was strong interaction between SiO₂ and TiO₂, and Ti–O–Si bonds formed during the hydrothermal process. In addition, He et al. [51] synthesized highly ordered bicontinuous cubic mesoporous titania–silica binary oxides via an evaporation-induced self-assembly (EISA) method. As illustrated in Scheme 3.1, TTIP and TEOS hydrolyzed simultaneously with the existence of HCl, then the condensation and polymerization of TTIP were slowed down, and later the hydrolysis of TEOS was accelerated owing to the large amount of HCl. In the aging section, titanium species and silica species co-assembly with F127, and ordered mesostructures were formed. In the calcination process, titanate oligomers and silicate oligomers can be cross-linked with each other through the Ti–O–Si bonds. Under this circumstance, silica acts as glue between TiO₂ nanocrystals; thus, the thermal stability of the mesostructures can be improved. Moreover, Li et al. [49] successfully introduced benzopyrylium salt S-2(2,4-diphenyl-5,6,7,8-tetrahedro-1-benzopyrylium perchlorate) into the channels of mesoporous molecular sieves Ti-HMS with different Ti content by impregnation method.

Scheme 3.1

Illustration for the self-assembly and structure evolution process of mesostructured titania–silica binary oxides. (Reprinted with permission from Ref. [50]. Copyright 2005, Elsevier)

3.3.2 The Application of TiO₂–SiO₂ Mesoporous Materials in Photocatalysis

3.3.2.1 Photodegradation of Organic Pollutants

We have briefly introduced the preparation methods of TiO₂–SiO₂ mesoporous materials. Next, we will explore its applications. As we all know, human’s demand of energy will be much greater by the year of 2050. This increase poses an undue burden to our environment and the length of human’s life. Besides that, with the prosperity of industrialization, the disposal of industrial waste poses a great threat to the environment, which is becoming the biggest concern for the sustainable development of human society. As previously mentioned, TiO₂ was considered as a catalyst for degradation lots of pollutants. However, its rapid recombination of photo-generated electron–hole pairs limits its application. According to previous reports, TiO₂–SiO₂ mesoporous materials are found to be effective in environmental remediation. Figure 3.4 briefly demonstrates the photocatalytic process of degradation of organic pollutants and dyes under visible light [52].

Fig. 3.4

Strategies to realize visible light-induced degradation of organic pollutants on a semiconductor with a wide bandgap. (a) The band–band excitation of the pure semiconductor under UV irradiation (a) and the bulk-doping to extend the photoresponse by forming electronic states below the conduction band (b) or above the valence band (c) of the semiconductor in the bandgap. (b) The semiconductor-mediated photodegradation initiated by the surface electron injection from the adsorbed dye molecular that harvest visible light. (Reprinted with permission from Ref. [51]. Copyright 2009, Elsevier)

Aguado et al. [53] prepared titania-supported sample on different types of silica through a sol–gel method followed by hydrothermal processing. Afterward, the catalysts were tested by the degradation of iron (III) cyano complexes. In all cases, photoinduced CN⁻ released from the composite which happened by a homogeneous process. He et al. [59] applied cubic mesoporous titania–silica binary oxides to degrade rhodamine B (RhB) under UV light irradiation. The result shows that the sample has a comparable photocatalytic activity with Degussa P25, and higher activity than pure TiO₂.

In terms of MCF materials, some papers have reported about it. Xing et al. [44] have obtained super-hydrophobic mesocellular foam (MCF), which is loaded with nano-sized TiO₂ photocatalysts in its pore channels, through a simple one-step solvothermal method followed by a low-temperature vacuum activation process to produce Ti³⁺. And it can be well considered as an extractant for organics. In this method, NH₄F is used as hydrophobic modifier, and isopropanol is used as solvent to synthesize the super-hydrophobic mesoporous MCF loaded with highly dispersed and Ti³⁺ self-doped TiO₂ nanoparticles. Figures 3.5a illustrates the fluorination reaction occurred in the channels of MCF. In comparison with fluorine-containing silylation organic agent, NH₄F is easy to release HF during solvothermal process. And F ions will be adsorbed onto the MCF under acidic conditions owing to its mesoporous structure. TiO₂ particles are deposited into MCF’s pore channels, indicating that the exchange between surface hydroxyl groups on TiO₂ and F ions to form the Ti–F bonds is effectively promoted. Simultaneously, the F ions adsorbed in channels also replace the surface hydroxyl groups on SiO₂ to generate the Si–F bonds. Ti³⁺ was generated in the vacuum drying process, which plays an important role in enhancing its visible light photocatalytic activity. During the degradation of RhB, the NH₄F-modified catalyst of 0.4-MCF/TiO₂ exhibited the optimal photocatalytic activity, indicating that the NH₄F modification and vacuum activation are beneficial to improving visible light photoactivity (Fig. 3.5b).

Fig. 3.5

(a) Illustration of the fluorination reaction occurred in the pore channels of MCF; (b) visible light photocatalytic activities of different samples. (Reprinted with permission from Ref. [53]. Copyright 2002, Elsevier)

3.3.2.2 Water Splitting

Since the initial photocatalyst for water splitting into hydrogen and oxygen was developed in 1972 [54]. Various semiconductor-based catalyses either using UV or visible light have been investigated. As discussed in the above section, TiO₂–SiO₂ composite materials, which are combined with each other physically as well as chemically, can enhance photocatalytic activity; thus, these materials attracted much attention. In recent years, scientists deduced new two-step photoexcitation processes, the so-called Z-scheme [55], in order to realize overall water splitting (Fig. 3.6). This system consists of two visible light responsive semiconducting photocatalysts (A and B) and a redox mediator. Photocatalyst A is responsible for hydrogen evolution which is excited by visible light, and then, photo-formed electrons reduce H⁺ into H₂ together with photo-formed holes oxidizing the redox mediator. At the same time, photocatalyst B is used for the water-oxidation reaction, during which photo-formed holes oxidize H₂O to produce O₂ together with photo-formed electrons reducing the redox mediator under visible light irradiation. Finally, water splitting into H₂ and O₂ is attained.

Fig. 3.6

Conceptual diagram of a Z-scheme photocatalytic system. (Reprinted with permission from Ref. [55]. Copyright 2013, Royal Society of Chemistry)

Niphadkar et al. [56] prepared TiO₂–SiO₂ mesoporous composite photocatalysts with different proportions of TiO₂ and SiO₂ by loading TiO₂ on as-synthesized Si–MCM-41 using simple sol–gel method. The photocatalytic evaluation of composite photocatalysts was carried out in production of hydrogen by water-splitting reaction under UV light. In 2014, Xing et al. [57] successfully prepared a brown mesoporous TiO_2−x/MCF composite with a high fluorine doping concentration (8.01 at%) by vacuum activation method. It displays an excellent solar absorption, a record-breaking quantum yield (Φ = 46%) and a high photon–hydrogen energy conversion efficiency (η = 34%) in solar photocatalytic H₂ production process, which are all better than that of the black hydrogen-doped TiO₂ (Φ = 35%, η = 24%). Scheme 3.2 illustrates the continuous steps for preparing F-TiO_2−x/MCF. Firstly, titanium source Ti (SO₄)₂ in situ transformed to TiO₂ nanocrystals in the pore walls of the MCF through hydrothermal method. Then NH₄F was added into the solution, mechanically mixed with the obtained TiO₂/MCF, followed by a vacuum activation treatment to produce oxygen vacancies in TiO₂ and the substitution of fluorine atoms for vacancies. The F-TiO_2−x/MCF exhibits much higher rate of H₂ generation than black H-TiO_2−x, P25 and other photocatalysts (Fig. 3.7a). In addition to H₂ evolution, the solar light, UV light, and visible light-driven photodegradation of dyes by F-TiO_2−x/MCF were also measured. It was shown that catalysts treated by vacuum activation exhibited better photocatalytic activity than the blank samples (Fig. 3.7b). To investigate the photoactivity of brown F-TiO_2−x/MCF, F-TiO_2−x/MCF, and hydrogen-doped TiO₂ (H-TiO_2−x), these samples were measured for the degradation of methylene blue (MB) under simulated solar light irradiation using an AM 1.5 air mass filter (Fig. 3.7c). What is more, cycling tests revealed that the brown F-TiO_2−x/MCF sample is especially stable after five photocatalytic cycles (Fig. 3.7d). The F-TiO_2−x/MCF was shown to produce electrons and exhibit a much higher photocurrent response than MCF/TiO₂ under solar light irradiation. Its solar light-driven current density is much higher than its UV and visible light-driven density as well (Fig. 3.7e). It is worth noting that the solar light-driven current density of F-TiO_2−x/MCF is much higher than the sum of the current densities of the catalyst under UV and visible light irradiation (Fig. 3.7f), which indicates that the lifetime of solar light-produced electrons exceeds those of UV- or visible light-produced electrons. It was concluded that the decrease of recombination sites induced by high concentration F doping and the synergistic effect between lattice Ti³⁺-F and surface Ti³⁺-F are responsible for the excellent absorption of solar light and photocatalytic production of H₂ of these catalysts.

Scheme 3.2

Synthetic steps for the production of the fluorine-doped TiO_2−x/MCF composite and the displacement of lattice oxygen vacancies with F atoms during vacuum activation. (Reprinted with permission from Ref. [57]. Copyright 2014, Wiley)

Fig. 3.7

(a) Solar light-driven (with an AM 1.5 air mass filter) photocatalytic water splitting for H₂ generation and the cycling measurements of F-TiO_2−x/MCF; (b) photocatalytic activities for degradation of MO induced by simulated solar light; (c) a comparison of photocatalytic decomposition of MB by F-TiO_2−x/MCF, blank H-TiO_2−x, and other catalysts under simulated solar light irradiation (with an AM 1.5 air mass filter); (d) cycling tests of solar-driven photocatalytic activity of F-TiO_2−x/MCF for the degradation of MB; (e) transient photocurrent responses of F-TiO_2−x/MCF in 0.5 M Na₂SO₄ aqueous solution under various irradiation conditions (UV light, < 380 nm filter; visible light, > 420 nm filter); (f) comparison between the solar light-driven photocurrent and the sum of the photocurrent of F-TiO_2−x/MCF under UV and visible light irradiation. (Reprinted with permission from Ref. [57]. Copyright 2014, Wiley)

In this section, we firstly introduce the application of mesoporous TiO₂–SiO₂ materials. It can be either applied to photodegradation of pollutants or used for hydrogen production. Then, we have briefly listed some notable literatures and analyzed them.

3.4 Visible Light Response Metal–Organic Frameworks (MOFs)

In recent years, visible light responsive porous metal–organic framework photocatalysts have been investigated deeply. Metal–organic frameworks (MOFs) are hybrid materials composed of organic linkers and metal–oxo clusters. These MOF materials are hot spots in research, and it can be utilized as adsorbents, separation materials, ion-conductive materials, and catalysts. Among various highly porous materials, metal–organic frameworks (MOFs) are unique in their degree of tunability, structural diversity, as well as their range of chemical and physical properties. Metal–organic frameworks (MOFs) are also known as coordination polymers, which are crystalline materials generated by the association of metal ions (nodes) and multitopic organic ligands (rods) [58,59,60,61]. Based on their structures, MOFs have been considered as a promising type of materials due to its unique attributes and open structures with periodic dual composition, which is amenable to bottom up assembly of secondary building blocks into a desired framework expanding or decorating a specific blueprint network topology [58, 62]. As a kind of porous material, metal–organic frameworks (MOFs) have shown semiconductor-like characteristics in photocatalysis [63,64,65,66,67]. In 2009, Kataoka et al. [67] have detected the first example of open porous metal–organic frameworks (MOFs) that functions as an activity site for the reduction of water into hydrogen molecules in the presence of Ru(bpy)₃ ²⁺,MV²⁺, and EDTA–2Na under visible light irradiation. Also, Zhou et al. [63] have synthesized and characterized a new metal–organic framework (MOF-253-Pt) material through immobilizing a platinum complex in 2,20-bipyridine-based microporous MOF (MOF-253) using a post-synthesis modification strategy. The functionalized MOF-253-Pt serves both as a photosensitizer and a photocatalyst for hydrogen evolution under visible light irradiation. The structure and proposed mechanism are presented as shown in Fig. 3.8a, b, respectively. Upon light irradiation, the MOF with the presence of TEOA firstly generates a one-electron-reduced species MOF*(3MLCT) and holes. And then, the MOF*(³MLCT) species is reductively quenched to form MOF⁻ with the electrons stored on the bpy⁻ ligands. After that, the reduced MOF⁻ further forms a Pt(III)-hydride intermediate via a proton-coupled electron transfer (PCET). The intermediates contribute to the formation of the hydride-diplatinum (II, III) intermediate by the synergistic effect of the nearing anchored Pt(bpy)Cl₂ complex on framework, leading to H₂ production by a heterolytic coupling pathway.

Fig. 3.8

(a) Model structure of MOF-253-Pt, through post-synthetic modification of MOF-253 with PtCl₂. Key: Cyan octahedron represents Al atoms, while yellow, green, red, blue, and blank circles represent Pt, Cl, O, N, and C atoms, respectively; H atoms are omitted for clarity. (b) Proposed reaction mechanism for the photocatalytic H₂ evolution over MOF-253-Pt under visible light irradiation. (Reprinted with permission from Ref. [63]. Copyright 2013, Royal Society of Chemistry)

In 2016, it has been discovered that improving the efficiency of electron–hole separation and charge-carrier utilization plays a central role in photocatalysis. Jiang et al. [68] prepared a representative metal–organic framework (MOF) UiO-66-NH₂, in which Pt nanoparticles of ca. 3 nm are incorporated inside or supported. The resulting products are denoted as Pt@UiO-66-NH₂ and Pt/UiO-66-NH₂, respectively. Finally, these materials are especially applied in photocatalytic hydrogen production via water splitting. Scheme 3.3 simply clarified the pathway of synthesizing these two different materials. Pt@UiO-66-NH₂ greatly shortens the electron-transport distance and hence suppresses the electron–hole recombination, which is expected to have an enhanced catalytic activity compared to Pt/UiO-66-NH₂. In addition, the Pt NPs embedded in the MOF do not undergo aggregation or leaching during the reaction, which leads to better catalytic recyclability of Pt@UiO-66-NH₂ than that of Pt/UiO-66-NH₂.

Scheme 3.3

Schematic illustration for the synthesis of Pt@UiO-66-NH₂ and Pt/UiO-66-NH₂, with the photocatalytic hydrogen production process over Pt@UiO-66-NH₂. (Reprinted with permission from Ref. [68]. Copyright 2016, Wiley)

In 2010, Garcia and his coworkers synthesized the Zr-containing metal–organic frameworks (MOFs) that exhibited photocatalytic activity for hydrogen generation upon irradiation at wavelength longer than 300 nm [69]. In 2012, Anpo et al. [64] employed 2-amino-benzenedicarboxylic acid as an organic linker to synthesize amino-functionalized Ti (IV) metal–organic framework (Ti–MOF–NH₂) by a facile solvothermal method, and it described the hydrogen production from an aqueous medium under visible light. The structure of the Ti–MOF–NH₂, its mechanism, and the yield of hydrogen are shown in Fig. 3.9. In summary, it is mentioned that Ti–MOF materials of semiconductor properties have potential in water splitting. All the reported literatures provide us with new ideas in the further development of water splitting.

Fig. 3.9

Schematic illustrations of (a) the structure of Ti–MOF–NH₂ and (b) the reaction mechanism for hydrogen evolution over Ti–MOF–NH₂ induced by visible light irradiation. (c) Action spectrum for hydrogen evolution from water containing TEOA as a sacrificial electron donor over Ti–MOF–NH₂. Inset shows the photograph of Ti–MOF–NH₂. (Reprinted with permission from Ref. [64]. Copyright 2012, American Society Chemistry)

With the exception of the application in water splitting, the Ti–MOFs can also be applied to CO₂ reduction. In the same year, Li et al. [70] successfully prepared a targeted photoactive catalyst Ti₈O₈(OH)₄(bdc-NH₂)₆ (NH₂-MIL-125 (Ti)) for the first time, which reduced CO₂ even under visible light irradiation. In Fig. 3.10a, MIL-125 (Ti) shows an absorption edge at 350 nm, whereas NH₂-MIL-125(Ti) shows an extra absorption band in the visible light region with the absorption edge extending to around 550 nm, which is in agreement with the bright yellow color. An interesting photochromic phenomenon was observed over NH₂-MIL-125 (Ti) during the photocatalytic reaction. When the solution of NH₂-MIL-125 (Ti) and TEOA in MeCN was irradiated with visible light in the presence of N₂, the color of the solution changed from the original bright yellow to green. After CO₂ or O₂ was introduced into the reaction system, the green color of the solution changed gradually back to the original bright yellow (Fig. 3.10b). The photochromic phenomenon is ascribed to the presence of the inter-valence electron transfer from the optically induced hopping of electrons from Ti (III) to Ti (IV) sites in the titanium–oxo clusters.

Fig. 3.10

(a) UV-vis spectra of (a) MIL-125(Ti) and (b) NH₂-MIL-125(Ti). The inset shows the samples. (b) Photos and corresponding ESR spectra of NH₂-MIL-125(Ti) under different conditions: (a) fresh NH₂-MIL-125(Ti), (b) TEOA, visible light, and N₂ and (c) after the introduction of CO₂ (or O₂). (Reprinted with permission from Ref. [70]. Copyright 2012, Wiley)

Despite water splitting and CO₂ reduction, it was also reported by Garcia and Majima [71, 72] that the combination between the organic linkers and metal–oxo clusters would enable the development of dye-sensitized type MOF photocatalysts operating under visible light illumination. In 2012, Zhang et al. [73] adopted a doping strategy to tune the gas sorption and photocatalytic properties of a microporous material ZIF-67. It is worth mention that the Cu-doped phase integrated both structural features and functions of ZIF-67 showed high gas uptake capacity and highly efficient visible light-driven photocatalytic property upon the degradation of methyl orange.

3.5 The Development of Mesoporous Ti–SiO₂ Materials in Photocatalysis

Up to now, we have grasped some points of TiO₂–SiO₂ mesoporous materials; however, the concept of Ti–SiO₂ materials is totally different from TiO₂–SiO₂ mesoporous materials because of its containing Ti-oxides species. In the pioneering work of Honda and Fujishima [54], they obtained the photo-assisted production of H₂ and O₂ from water with a photoelectrochemical cell consisting of a Pt and TiO₂ electrodes under a small electric bias. Moreover, TiO₂ photocatalysts with incorporated Ti-oxide species anchored onto supports such as SiO₂, glass, and zeolite exhibited selective photoactivity. The Ti-oxide species prepared within the supports, for example, SiO₂, glass, and zeolite, have revealed a unique local structure as well as high selectivity in the oxidation of organic substances with hydrogen peroxide. Ti–Si binary oxide powders with a low TiO₂ content prepared by sol–gel methods have been reported to include the fourfold coordinated Ti-oxide species highly dispersed within the SiO₂ matrices, showing a unique and characteristic photocatalytic performance for the hydrogenation of unsaturated hydrocarbons with H₂O; the decomposition of NO into N₂, O₂, and N₂O; as well as the reduction of CO₂ with H₂O to produce CH₃OH and CH₄ under UV light irradiation [74,75,76,77,78,79]. Moreover, many reports have mentioned that the specific photocatalytic reactivity of those catalysts was much higher than that for TiO₂ powder, which may be attributed to the tetrahedrally coordinated titanium oxide moieties. Thus, we will discuss it comprehensively in this section.

3.5.1 The Preparation of Ti–SiO₂ Mesoporous Materials

Many reports have been published on preparation of Ti–SiO₂ mesoporous materials, such as ionized cluster beam (ICB) method [76, 80], ion-exchange method [79, 81], anchored on substrate [74, 78, 82, 83], CVD method [84], hydrothermal synthesis method [85], solvent evaporation method [86], and metal ion implantation [87].

3.5.2 The Application of Ti–SiO₂ Mesoporous Materials in Photocatalysis

3.5.2.1 CO₂ Photoreduction

In 1992, Anpo et al. [82] prepared highly dispersed titanium oxide anchored onto Vycor glass through a facile reaction between surface OH groups of Vycor and TiCl₄. It was investigated that the photoreduction of CO₂ with H₂O should be linked to the high reactivity of the charge-transfer excited state, i.e., (Ti³⁺-O⁻)^*, owing to the presence of well-dispersed homogeneous titanium oxide species on the surface. Ti-MCM-41 and Ti-MCM-48 mesoporous zeolite catalysts synthesized by hydrothermal method exhibited high and unique photocatalytic reactivity for the reduction of CO₂ with H₂O to produce CH₄ and CH₃OH in the gas phase [74]. Keita Ikeue et al. [86] also prepared self-standing porous silica thin films with different pore structures by a solvent evaporation method. From 2002 to 2003, Anpo and his coworkers reported many works about photocatalytic reduction of CO₂ on Ti-containing porous silica thin film [86, 88, 89]. Figure 3.11a shows the yields of products with the change of reaction time. The yields of CH₄ and CH₃OH in the photocatalytic reduction of CO₂ and H₂O on the Ti-oxides containing various porous materials are shown in Fig. 3.11b. They found that it is possible to determine a real quantum yield of the photocatalytic reduction of CO₂ with H₂O on tetrahedrally coordinated Ti-oxides. Ti-oxide was constructed within porous silica material, and its quantum yield could to be 0.3% at room temperature by the total number of photons absorbed by the catalyst.

Fig. 3.11

(a) Reaction time profiles of the photocatalytic reduction of CO₂ with H₂O to produce CH₄ and CH₃OH on a Ti-oxide single-site containing mesoporous silica thin film photocatalyst at 298 K. Inserted figure shows how to measure the real quantum yields of the reaction. (b) The yields of CH₄ and CH₃OH in the photocatalytic reduction of CO₂ with H₂Oon Ti-PS (h, 25), Ti-PS(c, 50), Ti-MCM-41, powdered form of Ti-PS (h, 50) and Ti-PS (h, 50) photocatalysts at 295 K. (Reprinted with permission from [86, 88, 89]. Copyright 2002–2003, Elsevier)

3.5.2.2 NO/NO₂ Photoreduction

In 1985, Anpo et al. [77] carried out a research on photoluminescence studies of titanium oxide anchored onto porous Vycor glass. It is proposed that the photoluminescence quenching is closely associated with the electron transfer from the excited states of the catalyst to the added O₂ or N₂O molecules. In Fig. 3.12, it shows the comparison between the reduction of NO and the decomposition of CO₂. The structure of Ti-oxide single-site is presented on the left top of this picture. Similarly, the structure of TiO₂ particles is presented on the right bottom of this picture. The quantum yield of CO₂ + H₂O → CH₃OH + CH₄ is much smaller than the decomposition of NO, due to its demanding of more configurations of co-adsorbed reactants involving six participating atoms [90]. In 1997, titanium oxide catalysts prepared within the Y-zeolite cavities via an ion-exchange method were reported by Anpo et al. [79], which exhibited high and unique photocatalytic reactivities for the decomposition of NO into N₂ and O₂. It was also found that the charge-transfer excited state of the titanium oxide species, (Ti³⁺-O⁻)*, plays a vital role in these unique photocatalytic reactions. Table 3.1 revealed the yields of the photo-formed N₂ and N₂O and its selectivity in the photocatalytic decomposition of NO. It is obvious that the efficiency and selectivity of the formation of N₂ strongly depend on the type of catalysts.

Fig. 3.12

Relationship between the coordination numbers and photocatalytic reactivity of titanium oxides. (Reprinted with permission from Ref. [78]. Copyright 1997, American Chemical Society)

Table 3.1 Comparisons of yields of N₂ and N₂O and their selectivities in the direct photocatalytic decomposition of NO at 275 K on various types of the Ti-oxide/zeolite catalysts and the powered bulk TiO₂ catalyst

In 2000, Masato Takeuchi et al. [76] prepared transparent TiO₂ thin film photocatalysts on transparent porous Vycor glass (PVG) by an ionized cluster beam (ICB) method. These thin films worked with high efficiency as photocatalysts for the decomposition of NO into N₂, O₂, and N₂O under UV light irradiation at 275 K. When the film thickness increases, the photocatalytic reactivity decreases gradually. In 2004, Yamashita and Anpo [80] proposed a new concept of an ion beam technology using accelerated metal ions, a metal ion implantation, and an ionized cluster beam (ICB) (detailed schematic diagram is shown in Fig. 3.13a). The decomposition of NO into N₂, O₂, and N₂O can be occurred not only under UV light but also visible light, realizing the efficient use of solar beam energy. The samples were also characterized by XANES (as shown in Fig. 3.13b). It revealed the XAFS (XANES and FTEXAFS) spectra of the Cr ion–implanted TiO₂ powder catalyst. By analyzing on these spectra, we can tell that in the Cr ion–implanted TiO₂, the Cr ions are highly dispersed in the lattice of TiO₂ possessing octahedral coordination. These Cr ions are isolated and substitute the Ti⁴⁺ ions in the lattice positions of TiO₂. Besides that, the Cr-doped TiO₂ catalysts chemically prepared by impregnation or sol–gel method were found to have a mixture of the aggregated Cr-oxides in tetrahedral coordination similar to CrO₃ and octahedral coordination similar to Cr₂O₃.

Fig. 3.13

(a) Schematic diagram of an ICB method. (b) XANES (a–d) and Fourier transforms of EXAFS (A–D) spectra of CrO₃ (A), Cr₂O₃ (B) and the Cr-impregnated TiO₂ (C) and Cr ion–implanted TiO₂ after calcination at 723 K (D). (Reprinted with permission from Ref. [81]. Copyright 2006, Springer)

3.6 Conclusion

Herein, to sum up, we have reviewed plenty of representative literatures about recent advances in photocatalysts over the highly dispersed TiO₂ in mesoporous materials. Although TiO₂ possesses specific properties, it is limited in photocatalysis because of its narrow bandgap. Thus, the doping modification on mesoporous TiO₂ materials proves to be an effective way to enhance its photocatalytic performance. Meanwhile, synthesizing mesoporous TiO₂–SiO₂, Ti–MOFs, and Ti–SiO₂ materials is another feasible way to achieve this goal. Based on the analysis of mechanism of these systems, it gives the researchers a promising future. The achieved progress in this field indicates that researchers can either be able to extend the photoresponse to the visible region or apply them to practical application, such as water splitting, degradation of pollutants and decomposition of greenhouse gas (CO₂, NO, NO₂).