Chinese Chemical Letters  2019, Vol. 30 Issue (4): 853-862   PDF    
Research progress of photocatalysis based on highly dispersed titanium in mesoporous SiO2
Chencheng Donga, Jiahui Jia, Zhe Yangb, Yifei Xiaoa, Mingyang Xinga,*, Jinlong Zhanga,*     
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China;
b Department of Civil Engineering, The University of Hong Kong, Hong Kong, China
Abstract: With the development of the human economy and green chemistry, people pay much more attention to environmental safety. Correspondingly, mesoporous TiO2 and its correlated photocatalysts are able to help people seek for better life. In this review, first of all, we briefly introduce the preparations and applications of mesoporous TiO2-SiO2 materials, which exhibit excellent performance in pollutants decomposition and H2 evolution in photocatalysis. Then, we review the mesoporous composites of Ti-SiO2 materials, which are ideal materials used in the photoreduction of air pollutants such as CO2, NO and NO2. It is powerfully evident from the literature surveys that these TiO2 based mesoporous photocatalysts possess a large potential in environment and energy development.
Keywords: Mesoporous     TiO2-SiO2     Ti-SiO2     Photocatalysis     Photo-degradation     Water splitting     CO2 reduction    
1. Introduction

TiO2 photocatalysts have stirred great interests among worldwide scientists. If TiO2 fixed in glass, ceramic tile or stainless steel materials, it will possess the function of sterilization, disinfection and photodegradation of the pollutants [1-3]. Even though TiO2 materials are equipped with several significant properties, such as non-toxicity and excellent photo-stability; however, some drawbacks constrain the performance of TiO2 in photocatalytic process [4-8]. In order to overcome these drawbacks, the researchers have prepared TiO2 compounded materials, which can provide large number of adsorptive sites by the dispersion of TiO2 species into a porous support with large surface area. Silica has been widely employed as carrier and stable mesoporous support, owing to its outstanding mechanical strength, high inner surface area and uniform pore size. With the high dispersion of TiO2 into the porous SiO2 support, the TiO2-SiO2 photocatalyst shows significantly enhanced activities compared to pure TiO2. On one hand, TiO2 and SiO2 may combine with each other to form a mixture of these two oxides with interaction forces being nothing more than weak Vander Waals forces. On the other hand, they can integrate via the formation of Ti—O—Si bond. When combined together through chemical bonding, the physical-chemical properties of TiO2-SiO2 obviously differ from the simple combination of each phase. In a word, homogeneity or dispersion of TiO2 largely depends on preparation methods and synthesis conditions. These novel TiO2-SiO2 materials not only take advantages of both TiO2 and SiO2 materials, but also extend their applications through the generation of new catalytic active sites, owing to the interaction between TiO2 and SiO2 [4]. Hence, it is indeed a promising and new catalytic material in many research areas.

2. The preparation of TiO2-SiO2 mesoporous materials

There are various methods to prepare TiO2-SiO2 mesoporous materials, such as sol-gel method, [5, 6], hydrothermal method [7-9] chemical vapor deposition [10-12], precipitation method [13-15], liquid phase deposition [16], microwave irradiation method [17, 18], impregnation method [19, 20], and evaporation-induced self-assembly (EISA) method (Table 1) [21-23].

Table 1
The summary of the preparation of TiO2-SiO2 mesoporous materials and its application in photocatalysis.

Among the various preparation methods, sol-gel hydrolysis is most widely used due to its possible capability in controlling the textural and surface properties of the mixed oxides, however, the sol-gel technique is expensive and give much wastewater. Some researchers revealed that the CVD technique can form continuous film with titania on oxide supports. Although TiO2 films can be prepared by various techniques such as magnetron sputtering, solgel process, thermal hydrolysis and chemical vapor deposition, these methods need a high temperature or a vacuum condition. Low processing temperature is highly desired because it enables the use of certain substrate materials and prevents undesirable interaction between the film and substrate. Thus, Liquid phase deposition (LPD) technique attracts more and more attentions because neither vacuum, nor high temperature is required, and any substrates with large areas and complex morphology can also be used. Additionally, the microwave (MW) irradiation method has been shown to offer a number of advantages over conventional solution methods, such as rapid and homogeneous heating. It has been shown that MW irradiation is an effective way to enhance the rate, yield and selectivity of chemical reactions.

As seen in Table 1, it exhibits several preparation methods of TiO2-SiO2 mesoporous materials. For instance, TiO2-SiO2 nanocomposite can be prepared by sol-gel method. Commonly, the first step is to formulate sol solution with all types of ingredients. And then followed by the gelation process, the surfactant is removed to obtain mesoporous materials. In 2003, Elizabeth et al. [24] synthesized titania-silica mixed oxides by the sol-gel method, using tetraethylorthosilicate (TEOS) and titanium (Ⅳ) isopropoxide (TTIP) as precursors. In terms of hydrothermal synthesis, Li et al. [5] obtained silica-modified titanium dioxides by hydrothermal method. A strong interaction could be produced between SiO2 and TiO2, and Ti—O—Si bonds were formed during the hydrothermal process. He et al. [21] synthesized highly ordered bicontinuous cubic mesoporous titania-silica binary oxides via an evaporationinduced self-assembly (EISA) method. As illustrated in Scheme 1, TTIP and TEOS hydrolyzed simultaneously in the presence of HCl, then the condensation and polymerization of TTIP was slowed down. Meanwhile, the hydrolysis of TEOS was accelerated, owing to the large amount of HCl. In the aging section, titanium species and silica species were co-assembled with F127, and ordered mesostructures were formed. In calcination process, titanate oligomers and silicate oligomers could be cross-linked with each other via the Ti—O—Si bonds. Under this circumstance, silica acted as glue between TiO2 nanocrystals so that the thermal stability of the mesostructures could be well improved. As shown in Fig. 1, the photo-degradation rate of Rhodamine B (RhB) under UV light irradiation showed that the sample had comparable photocatalytic activity with Degussa P25, but higher activity than pure TiO2. In 1997, Gun'ko and his co-workers [10] prepared titania (anatase) on a fumed silica substrate using a chemical vapor deposition (CVD) technique with the concentration of TiO2 varying from 0.6 wt% to 32.4 wt%. Then, Li et al. [19] successfully introduced benzopyrylium salt of 2, 4-diphenyl-5, 6, 7, 8-tetrahedro-1-benzopyrylium perchlorate into the channels of mesoporous molecular sieves Ti-HMS with various Ti content by impregnation method.

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Scheme 1. Illustration for the self-assembly and structure evolution process of mesostructured titania-silica binary oxides. Copied with permission [21]. Copyright 2009, Elsevier.

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Fig. 1. Adsorption of MB per unit weight (mg) of (a) TS-400, (b) TS-500, (c) TS-600, (d) TS-700, (e) P-TiO2 and (f) Degussa P25. Copied with permission [21]. Copyright 2009, Elsevier.

3. The application of TiO2-SiO2 mesoporous materials in photocatalysis 3.1. Photo-degradation of organic pollutants

As we all know, the demand for energy will be much greater by the year of 2050, which poses an undue burden to our environment and human life. Additionally, with the prosperity of industrialization, the disposal of industrial waste poses a threat to the environment, leading to biggest concern to the sustainable development of human society. As previously mentioned, TiO2 is considered to degrade lots of pollutants. However, the rapid recombination of photo-generated electron-hole pairs limits its application. Thus, TiO2-SiO2 materials could solve its drawbacks, and were widely applied in photocatalytic degradation of organic pollutants [25-29]. According to previous reports, TiO2-SiO2 mesoporous materials are found to be effective in environmental remediation. Fig. 2 briefly demonstrated the photocatalytic process of organic pollutants and dyes under visible light [30]. We assumed that the obtained catalysts were applied as semiconductors with wide band gaps, such as TiO2. Detailed photocatalytic degradation mechanism was observed in Fig. 2. As shown in Fig. 2a process (a), the adsorption of photons with energy lager than the band gap of the semiconductor could excite electrons from valence band to conduction band, thus the reductive conduction band electrons (ecb-) and oxidative valence band holes (hvb+) were generated. The holes reacted with surface adsorbed H2O to produce ·OH radicals or directly oxidized the organic substrates into corresponding radicals. To extend the photo-response of the catalysts, another promising way was to harvest visible light by adsorbing dyes or other color species (Fig. 2b). In Fig. 2b, several photochemical processes generally included the following steps: (a) dye excitation upon the visible light absorption, (b) electron injection, (c) trapping the injected electrons by surface sites, (d) scavenging ecb- by electron acceptors such as O2, and (e) subsequent radical reactions. All these feasible methods had provided us possibility to develop novel and efficient photocatalysts.

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Fig. 2. Strategies to realize visible light induced degradation of organic pollutants on a semiconductor with a wide band gap. (a) The band-band excitation of the pure semiconductor under UV irradiation (inset a) and the bulk-doping to extend the photoresponse by forming electronic states below the conduction band (inset b) or above the valence band (inset c) of the semiconductor in the band gap. (b) The semiconductor-mediated photodegradation initiated by the surface electron injection from the adsorbed dye molecular that harvest visible light. Copied with permission [25]. Copyright 2009, Royal Society of Chemistry.

In recent years, TiO2 has been incorporated into highly ordered mesoporous siliceous materials such as SBA-15, MCM-41 and MCM-48 [31-34]. De Witte et al. [35] deposited titania nanoparticles onto inert silica supports with high specific surface area. The photocatalytic activity of the catalysts had been evaluated with two catalytic setups. One setup was applied in degradation of dyes in aqueous phase, whereas rhodamine-6 G was commonly used. Another one was used for the mineralization of ethanol as representative volatile organic compound (VOC). It was found that SBA-3-2 and SBA scs-3-2 showed higher adsorption in aqueous solution ('SBA scs' represents the mesoporous molecular sieve of SBA with short-channel (sc); '-3-20' indicates that the SBA preparation method should go through three loading cycles of 2 mL Ti(OiPr)4). Meanwhile, the pore size played an important role in the photocatalytic activity. The MCF-type catalysts (MCF-3-2 and SBA scs-3-2) were found to be much more active in comparison with SBA sc-3-2 and MCF-3-2. Additionally, it could be specified that these types of materials were used in the textile industry (Fig. 3).

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Fig. 3. Adsorption and photocatalytic activity of the catalysts in aqueous phase. Copied with permission [30]. Copyright 2008, Elsevier.

Aguado et al. [36] obtained titania supported sample on different types of silica through a sol-gel method followed by hydrothermal process. After that, the catalysts were tested by the degradation of iron(Ⅲ) cyano complexes. In all cases, photo-induced CN- was released from the complex during the homogeneous process (Fig. 4). He et al. [21] applied cubic mesoporous titania-silica binary oxides to degrade rhodamine B (RhB) under UV light irradiation. The result showed that the sample had a comparable photocatalytic activity with Degussa P25, and higher activity than pure TiO2 (Fig. 5).

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Fig. 4. Results of CN- photo-oxidation and settling recovery of studied photocatalysts. Copied with permission [31]. Copyright 2002, Elsevier.

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Fig. 5. (a) UV light photodegradation rate of RhB and (b) the photodegradation kinetic curve in the presence of Degussa P25 (line a), P–TiO2 (line b), TS-400 (line c), TS-500 (line d), TS-600 (line e) and TS-700 (line f). Copied with permission [21]. Copyright 2009, Elsevier.

As for MCF materials, some related reports have been published. Xing et al. [37] have obtained super-hydrophobic mesocellular foams (MCF), which was loaded with nano-sized TiO2 photocatalysts in its pore channels through a simple one-step solvothermal method, then followed by a low-temperature vacuum activation process to produce Ti3+. The obtained material could be well considered as an extractant for organics. In this way, NH4F was used as hydrophobic modifier, and isopropanol acted as solvent to synthesize the super-hydrophobic mesoporous MCF loaded with highly dispersed and Ti3+-self doped TiO2 nanoparticles. Fig. 6a illustrated the fluorination reaction occurred in the pore channels of MCF. In comparison with fluorine-containing silylation organic agent, NH4F was easy to release HF during solvothermal process. And fluoride ions would be adsorbed onto the MCF under acidic conditions, owing to its mesoporous structure. TiO2 particles were deposited in the MCF pore channels, which indicated that the exchange between surface hydroxyl groups on TiO2 and fluoride ions, forming the Ti-F bonds. Simultaneously, the fluoride ions were adsorbed in channels, which also replaced the surface hydroxyl groups on SiO2 to generate the Si–F bonds. Ti3+ was generated in the vacuum drying process, which played an important role in enhancing its visible light photocatalytic activity. When it was applied to degrade RhB, the NH4F-modified catalyst of 0.4-MCF/TiO2 exhibited the optimal photocatalytic activity among all photocatalysts, indicating that the NH4F modification and vacuum activation were beneficial to improving visible light photo activity (Fig. 6b).

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Fig. 6. (a) Illustration of the fluorination reaction occurred in the pore channels of MCF; (b) Visible light photocatalytic activities of different samples [32].

In recent years, Wang et al. [38] adopted three typical aquatic plant leaves including of reed (Phragmites australis), water hyacinth (Eichhornia crassipes) and duckweed (Lemna minor L), as both biotemplates and sources of silica to fabricate biomorphic mesoporous TiO2/SiO2 photocatalysts. As a result, the photocatalyts were employed for photocatalytic degradation of gentian violet under simulated sunlight irradiation. It was showed that TiO2/SiO2 samples exhibit a high photodegradation activity. Additionally, in 2019, Li et al. [39] fabricated mesoporous structure TiO2/SiO2 (M-TS) composite by sol-gel method combined with calcination. And the experimental results of adsorption and photodegradation proved that improvement of adsorption capacity enhances the decolourisation efficiency, which implies that the M-TS could be used as an effective adsorbent and photocatalyst to remove organic dyes from wastewater. Sui et al. [40] prepared a novel Pr–SiO2–TiO2 nanocomposite catalyst by sol–gel method, and it showed good efficiency on photocatalytic degradation of RhB in aqueous dye wastewater. All these indicate that the TiO2/SiO2 based composite material is an ideal photocatalyst for the removal of organic pollutants.

3.2. Water splitting

Since the initial photocatalyst for water splitting into hydrogen and oxygen was developed in 1972 [2], all kinds of semiconductive catalysts either using UV or visible light have been investigated [41]. As mentioned above, TiO2-SiO2 composite materials that combined with each other physically as well as chemically can enhance photocatalytic activity, thus attracting much attention in recent years. Up to now, in order to realize overall water splitting, scientists have deduced a new two-step photoexcitation process, the so-called Z-scheme (Fig. 7) [33, 42, 43]. This system consists of two visible-light responsive semiconductor photocatalysts (A and B) and a redox mediator. Photocatalyst A is responsible for hydrogen evolution, which is excited by visible light. Then, the photo-formed electrons reduce H+ to H2 together with photoformed holes oxidizing the redox mediator. At the same time, photocatalyst B is used for the water oxidation reaction; photoformed holes oxidize H2O to produce O2 together with photoformed electrons reducing the redox mediator under visible-light irradiation. Finally, water splitting into H2 and O2 is attained.

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Fig. 7. Conceptual diagram of a Z-scheme photocatalytic system. Copied with permission [33]. Copyright 2013, Royal Society of Chemistry.

In 2012, Rungjaroentawon et al. [44] investigated mesoporousassembly TiO2-SiO2 mixed oxide nanocrystal photocatalysts with various TiO2-to-SiO2 molar ratios. All the samples were synthesized by sol-gel method with the aid of a structure-directing surfactant. The experimental results revealed that the mesoporous-assembled TiO2-SiO2 mixed oxide photocatalysts with the TiO2-to-SiO2 molar ratio of 97:3 calcined at 500 ℃ possessed the highest photocatalytic hydrogen production activity, in comparison with other mixed oxides as well as P25. In 2003, Nguyen et al. [43] prepared TiO2-SiO2 mixed oxides by sol-gel process with twostep synthesis routes. It elaborated that the photo-response and AC impedance characterization of the derived catalysts were the first time correlated with photoactivity in water composition. Table 2 showed the hydrogen evolution rate of various photocatalysts: S2 (25.5 μmol for 24 h), S3 (15.8 μmol for 24 h), S4 (14.2 μmol for 24 h), S1 (12.2 μmol for 24 h). As shown in Fig. 8a, the highest photocurrent density was found over TiO2, which strongly suggested that the photocatalytic activity could not directly correlate with photocurrent density. And in Fig. 8b, the lowest doping density of S1 was consistent with the high photocurrent density. Wu et al. [45] synthesized TiO2-SiO2 composite materials via a sol-gel method. Then the photocatalyst is evaluated by hydrogen production, and the yield is maximum, attributed to high amount of tetrahedrally coordinated Ti4+ ions. In 2018, Hu et al. [46] synthesized Ti3+ self-doped mesoporous black TiO2/SiO2/ g-C3N4 sheets heterojunctions by a sol-gel strategy. The visiblelight-driven photocatalytic H2 production rate of bm-TiO2/SiO2/ g-C3N4 were about 8 and 5 folds higher than that of m-TiO2/SiO2 and g-C3N4, respectively. Additionally, the as-prepared bm-TiO2/SiO2/g-C3N4 composite materials showed excellent performance for the photocatalytic degradation of phenol and reduction of Cr6+ under visible-light irradiation. In other words, many TiO2-SiO2 based photocatalysts with excellent hydrogen production performance are likely to have a high oxidation activity for the degradation of organic pollutants at the same time.

Table 2
Quantities and chemical composition of various kinds of sample.

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Fig. 8. (a) The photocurrent density of S1-S4 at calcination temperature of 750. (b) The photocurrent density of TiO2, SiO2, and TiO2-SiO2 (S2-750). Copied with permission [34]. Copyright 2003, Elsevier.

Yang et al. [47] proposed a mechanism for superior photoactivity of RuS2 loaded on TiO2–SiO2, and explained by charge transfer mechanism. The TiO2-SiO2 mixed oxide photocatalysts were prepared by sol-gel method, and applied to water splitting. It was found that hydrogen evolution was remarkably enhanced over the optimal RuS2 loading (1 wt%) on TiO2–SiO2, which could be successfully explained by the charge transfer mechanism. Hydrogen was most likely to be produced on RuS2 surface by the photo-excited electrons transferring from TiO2–SiO2, due to the significant difference of flat-band potentials. Meanwhile, oxygen might be evolved by the holes on TiO2–SiO2 or on the RuS2 surface, suppressing the recombination of photo-excited electrons and holes (Fig. 9).

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Fig. 9. Schematic illustration of band edge and charge transfer on RuS2/TiO2–SiO2 at pH 9. The thick arrows mean the dominant pathways for hydrogen and oxygen evolution. Copied with permission [36]. Copyright 2004, Elsevier.

In 2014, Xing et al. [48] successfully prepared brown mesoporous TiO2-x/MCF composites with a high fluorine doping concentration (8.01 wt%) by vacuum activation method. It displayed 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, all of which were better than that of the black hydrogen-doped TiO2 (Φ = 35%, η = 24%). Scheme 2 revealed the continuous steps for preparing F-TiO2-x/MCF under vacuum system. At the beginning, titanium source Ti(SO4)2 achieved the in situ growth of TiO2 nanocrystals into the pore walls of the MCF through hydrothermal method. Next, NH4F was added into the solution, mechanically mixed with the obtained TiO2/MCF, and followed by a vacuumactivation treatment to produce oxygen vacancies in TiO2 and the substitution of fluorine atoms for vacancies. The F-TiO2-x /MCF exhibited a much higher rate of H2 generation than black H-TiO2-x, P25 and other photocatalysts (Fig. 10a). Besides the application on H2 evolution, the solar light, UV light and visible light-driven photo-degradation of dyes by F-TiO2-x/MCF were also investigated. It was shown that catalysts treated by vacuum activation exhibited better photocatalytic activity than the blank samples (Fig. 10b). To investigate the photoactivity of brown F-TiO2-x/MCF, F-TiO2-x/MCF and blank hydrogen-doped TiO2 (H-TiO2-x), these samples were tested for the degradation of methylene blue (MB) under simulated solar light irradiation using an AM 1.5 air mass filter (Fig. 10c). Moreover, the cycling tests revealed that the brown F-TiO2-x/MCF sample was especially stable even after five cycles (Fig. 10d). The F-TiO2-x/MCF produced electrons, and exhibited a much higher photocurrent response than MCF/TiO2 under solar light irradiation. Its solar light-driven current density was much higher than its UV and visible light-driven density, as well (Fig. 10e). It was worth to be noted that the solar light-driven current density of F-TiO2-x/MCF was much higher than the sum of the current densities of the catalyst under UV and visible light irradiation (Fig. 10f), which indicated that the lifetime of solar light-produced electrons exceeded those of UV- or visible light-produced electrons. It could be concluded that the decrease of recombination sites induced by high concentration fluorine doping and the synergistic effect between lattice Ti3+-F and surface Ti3+-F was responsible for the excellent absorption of solar light and photocatalytic production of H2 for these catalysts.

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Scheme 2. Synthetic steps for the production of the fluorine-doped TiO2-x/MCF composite and the displacement of lattice oxygen vacancies with F atoms during vacuum activation. Copied with permission [37]. Copyright 2014, Wiley.

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Fig. 10. (a) Solar-light induced (with an AM 1.5 filter) photocatalytic water splitting for H2 generation, and the cycling measurements of F-TiO2-x/MCF; (b) Photocatalytic activities for degradation of MO induced by simulated solar light; (c) A comparison of photocatalytic decomposition of MB by F-TiO2-x/MCF, blank H-TiO2-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-TiO2-x/MCF for the degradation of MB; (e) Transient photocurrent responses of F-TiO2-x/MCF in 0.5 mol/L 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-TiO2-x/MCF under UV and visible light irradiation. Copied with permission [37]. Copyright 2014, Wiley.

3.3. CO2 reduction

Carbon dioxide (CO2) in the atmosphere as a major greenhouse gas, which causes global environmental problems [49]. Thus, it is imperative to convert CO2 into economical fuels. For example, Zhang et al. [50] reported mesoporous Ni/TiO2-SiO2 catalysts for CO2 reforming of methane to synthesis gas under atmospheric pressure, and revealed that the types of Ni species formed in the calcined catalysts and the corresponding catalytic performances varied with the calcination temperature. Wang et al. [51] reported copper doped titania–silica (Cu–TiO2–SiO2) photocatalyst composite particles which were utilized in CO2 photoreduction inside a quartz reactor under illumination of UV light. Recently, Dong et al. [52] developed an economic NH4F-induced hydrophobic modification strategy to enhance the CO2 competitive adsorption on the mesoporous TiO2-SiO2 composite surface via a simple solvothermal method. The CO2 photoreduction for the selective generation of CH4 over the noblemetal-free TiO2-SiO2 composite can be greatly enhanced about 3 times. On this basis, Dong et al. [53] continued to prepare noblemetal platinum based porous TiO2-SiO2 materials. Various Pt NPs supported on hierarchically ordered TiO2–SiO2 porous materials (HTSO) were synthesized via a facile acid–base-mediated alcohol reduction (ABAR) method. They investigated the size effect of Pt on both the activity and selectivity of CO2 photoreduction (Fig. 11). It was found that the decreasing size of Pt NPs would promote the charge transfer efficiency to enhance both the CO2 photocatalytic reduction and hydrogen evolution reaction (HER) activity. They employed the experiment and theoretical calculations to reveal that the terrace sites over Pt are acted as the active sites for methane generation, and the low-coordinated sites are more favorable in the competing HER.

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Fig. 11. Size-dependent activity and selectivity of Pt NPs in CO2PR. (a) Correlations between the selectivity for CH4 and surface site proportion as functions of the size of Pt NPs in xPHTSO (x = 1.8, 3.4, 4.3 and 7.0). (b) Free energy diagrams for CO2 reduction to CH4 by the thermochemical model on Pt(111) surface and Pt55. (c) Scheme illustration of partial CO modified 1.8PHTSO through stepwise adsorption and desorption of CO. (d) CO-TPD results of 1.8PHTSO after CO pulse adsorption (up) and stepwise CO pulse adsorption and He flow desorption at 280 ℃ (down). (e) Performance comparisons of CO2PR between 1.8PHTSO and CO-1.8PHTSO, the RE denotes as the reacted electrons and CH4 S denotes as the CH4 selectivity. Copied with permission [41]. Copyright 2018, Springer Nature.

4. 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 they contain Ti-oxides species. Therefore, they have also aroused a stir in academia. Differing from TiO2-SiO2 nanomaterials, Ti-SiO2 usually formed in Ti4+-O2- clusters, as shown in Fig. 12a, whereas TiO2-SiO2 forms in Ti—O—Si bond. Seen from Fig. 12, the single site Ti-oxide's energy gap between the conduction band and valence band becomes larger as the size of the TiO2 becoming smaller, and with the presence of TiO4 unit single site, the energy gap becomes larger, enabling the absorption of short UV light in the 230–280 nm regions. In comparison with normal TiO2 semiconducting photocatalysts, the catalysts with TiO4 unit single site, whose photo-formed electrons and holes present on the same site, such as a pair of Ti3+ (electron center) and O- (hole center), leading to higher reactivity. The XANES and FT-EXAFS measurements of the anchored Ti-oxide catalysts clearly showed the presence of tetrahedrally-coordinated Ti-oxide species on the surface. Thus, the photoluminescence is attributed to the radiative decay process from the charge-transfer excited state to the ground state of the highly dispersed Ti-oxide species in tetrahedral coordination.

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Fig. 12. Relationship between structure and electronic state of Ti-oxides. (a) UV absorption spectra of various titanium oxide catalysts. (b) Transmittance of the visible light-responsive TiO2 thin films prepared on the substrates at various different temperatures. (c) The absorption (right) and photoluminescence spectrum of Ti-oxide (middle) and its quenching by an increase in the pressure of added O2. (d). The XANES and ET-EXAFS spectrum of highly dispersed tetrahedrally coordinated Ti-oxide single site catalysts prepared with the zeolite framework structures. Copied with permission [61]. Copyright 2013, Elsevier.

In the pioneering work, Honda and Fujishima [2] have obtained the photo-assisted production of H2 and O2 from water with a photo-electrochemical cell consisting of Pt and TiO2 electrodes under a small electric bias. Moreover, TiO2 photocatalysts with incorporated Ti-oxide species anchored onto supports such as SiO2, glass, zeolite, exhibited selective photoactivity. The Ti-oxide species were prepared within the supports, for example, SiO2, glass and zeolite, which had revealed a unique local structure as well as high selectivity in the oxidation of organic substances with hydrogen peroxide [54]. Ti-Si binary oxide powders with a low TiO2 content prepared by sol-gel method had been reported that the 4-fold coordinated Ti-oxide species were highly dispersed within the SiO2 matrices, showing a unique and characteristic photocatalytic performance for the hydrogenation of unsaturated hydrocarbon 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 [55-60]. Moreover, many reports have mentioned that the specific photocatalytic reactivity of those catalysts was much higher than that of TiO2 powder, which may be attributed to the tetrahedrally coordinated titanium oxide moieties. Thus, in this section, we will discuss it comprehensively.

4.1. The preparation of Ti-SiO2 mesoporous materials

Many reports have been published about different preparation methods of Ti-SiO2 mesoporous materials, such as ionized cluster beam (ICB) method [57, 62], ion-exchange method [60, 63], anchored on substrate [55, 59, 64, 65], CVD method [66], hydrothermal synthesis method [67], solvent evaporation method [68], and metal ion implantation (Table 3) [69].

Table 3
The summary of the preparation of Ti-SiO2 mesoporous materials and its application in photocatalysis.

4.2. The application of Ti-SiO2 mesoporous materials in photocatalysis 4.2.1. CO2 photo-reduction

In 1992, Anpo et al. [64] prepared highly dispersed titanium oxide anchored onto Vycor glass through a facile reaction between surface hydroxyl groups of Vycor and TiCl4. It was noticed that the photo-reduction of CO2 with H2O should be linked with the high reactivity of the charge-transfer excited state, i.e., (Ti3+-O-)*, due 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 [55]. Keita Ikeue et al. [68] prepared self-standing porous silica thin films with different pore structures by solvent evaporation method. The asprepared photocatalysts were applied for the photocatalytic reduction of CO2 with H2O at 323 K. UV irradiation of these Ti-containing porous silica thin films in the presence of CO2 and H2O led to the formation of CH4 and CH3OH, as well as CO and O2 as byproducts (Fig. 13).

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Fig. 13. Yields of CH4 and CH3OH in the photocatalytic reduction of CO2 with H2O on Ti-PS (h, 25) (a) Ti-PS (c, 50) (b), Ti-MCM-41(c), the powdered form of Ti-PS (h, 50) (d) and Ti-PS (h, 50) (e). Reaction time is 6 h. Copied with permission [56]. Copyright 2002, Elsevier.

From 2002–2003, Anpo and his co-workers reported some references about photo-catalytic reduction of CO2 on Ti-containing porous silica thinfilm [70-72].Fig. 14a showed theyieldsof products with the change of reaction time. The yields of CH4 and CH3OH in the photo-catalytic reduction of CO2 and H2O on th eTi-oxides containing various porous materials were shown in Fig. 14b. They found it possible to determine a real quantum yield of the photo-catalytic 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.

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Fig. 14. (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 Fig. shows how to measure the real quantum yields of the reaction [64, 66]. (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. Reproduced with permission [61]. Copyright 2013, Elsevier.

4.2.2. NO/NO2 photoreduction

In 1985, Anpo et al. [58] carried out a research on photoluminescence studies of titanium oxide anchored onto porous Vycor glass. It was proposed that the photo luminescence quenching test was closely associated with electrons, which were transferred from the excited states of the catalyst to the added O2 or N2O molecules. As shown in Fig. 15, the comparison between the reduction of NO and the decomposition of CO2 was performed. The structure of Ti-oxide single-site was presented on the left top of this picture, while the structure of TiO2 particles was presented on the right bottom of this picture. The quantum yield of CO2+H2O→CH3OH + CH4 was much smaller than the decomposition of NO, due to more configuration demand of co-adsorbed reactants involving six participating atoms [61, 73].

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Fig. 15. Relationship between the coordination numbers and photocatalytic reactivity of titanium oxides. Copied with permission [61]. Copyright 2013, Elsevier.

In 1997, titanium oxide catalysts prepared within the Y-zeolite cavities via an ion-exchange method by Anpo et al. [60], exhibiting 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-)*, played a vital role in these unique photocatalytic reactions. And it revealed the yields of the photo-formed N2 and N2O and the selectivity of the catalyst in the photocatalytic decomposition of NO. It was obvious that the formation efficiency and selectivity of N2 strongly depended on the types of catalyst.

In 2000, Masato Takeuchi et al. [57] prepared transparent TiO2 thin film photocatalysts onto transparent porous Vycor glass (PVG) via an ionized cluster beam (ICB) method. These thin films worked in high efficiency as photocatalysts for the decomposition of NO into N2, O2 and N2O under UV light irradiation at 275 K (Fig. 16). Fig. 16 revealed the effect of the TiO2 film thickness on the photocatalytic reactivity and the BET surface areas, as well as the wavelength of absorption edge of the thin film photocatalysts. TiO2 thin films with small film thickness exhibit much higher photocatalytic reactivity. When the film thickness increased, the photocatalytic reactivity decreased gradually. The BET surface areas and the wavelength of the absorption edge also showed the same tendency towards the photocatalytic reactivity. These results clearly showed that the photocatalytic properties of the TiO2 thin films were dependent on the film thickness.

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Fig. 16. The effects of the film thickness on the photocatalytic reactivity of TiO2 photocatalysts for the decomposition of NO (●) under UV light irradiation, the BET surface areas (◆), and the wavelength of absorption edge (■). Copied with permission [46]. Copyright 2000, Springer.

In 2004, Yamashita and Anpo [62] proposed a new concept of an ion beam technology using accelerated metal ions, metal ion implantation and ionized cluster beam (ICB) method. The decomposition of NO into N2, O2 and N2O could be occurred under not only UV light but also visible light, realizing the efficient utilization of solar beam energy. Such tetrahedrally coordinated titanium oxide species in the zeolite frameworks could be excited under UV irradiation by the following charge-transfer process:

The samples were also characterized by XANES. The relative results were shown in Fig. 17. It showed the XAFS (XANES and FTEXAFS) spectra of the Cr ion–implanted TiO2 powder catalyst. After analyzing these spectra, the Cr ions were highly dispersed in the lattice of TiO2 possessing octahedral coordination in the Cr ionimplanted TiO2. These Cr ions were isolated and substituted the Ti4+ ions in the lattice positions of TiO2. Besides, the Cr-doped TiO2 catalysts chemically prepared by impregnation or sol–gel method was found to have a mixture of the aggregated Cr-oxides in tetrahedral coordination similar to CrO3 and octahedral coordination similar to Cr2O3.

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Fig. 17. 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). Copied with permission [50]. Copyright 2004, Springer.

In this section, we explore the development of Ti-SiO2 mesoporous materials and introduce various preparation methods. When used in the decomposition of CO2 and reduction of NO/NO2, single-site titanium oxide species were proved to be effective for the improvement of photocatalytic efficiency.

5. Conclusion

To sum up, we have reviewed a great deal of representative literatures about recent progress in photocatalysts over mesoporous TiO2-SiO2 materials. Besides the photocatalytic of organic pollutants, these mesoporous can be applied in hydrogen production and CO2 reduction, which implies a huge potential in environmental remediation and energy consumption issues. Moreover, TiO2 possesses specific properties, however it is constrained by its wide band gap in photocatalysis. Therefore, the preparation of Ti-SiO2 materials is a feasible way to solve this problem. Through the analysis of these system mechanisms, researchers have broad prospects. The achieved progress in this field indicates that researchers can either extend the photo response to the visible region or apply them in practical use, such as water splitting, degradation of pollutants and decomposition of greenhouse gas (CO2, NO, NO2). Overall, the mesoporous TiO2-SiO2 and single Ti-SiO2 are expected to solve the problems of energy and environmental problems.

Acknowledgments

This work was supported by the State Key Research Development Program of China (No. 2016YFA0204200), the National Natural Science Foundation of China (Nos. 21822603, 21773062, 21577036, 21377038, 21237003), Shanghai Pujiang Program (No. 17PJD011), and the Fundamental Research Funds for the Central Universities (No. 22A201514021).

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