CHEMSUSCHEM MINIREVIEWS DOI: 10.1002/cssc.201400111

Design and Functionalization of Photocatalytic Systems within Mesoporous Silica Xufang Qian,[a] Kojirou Fuku,[a] Yasutaka Kuwahara,[a] Takashi Kamegawa,[a, b] Kohsuke Mori,[a, b] and Hiromi Yamashita*[a, b] In the past decades, various photocatalysts such as TiO2, transition-metal-oxide moieties within cavities and frameworks, or metal complexes have attracted considerable attention in light-excited catalytic processes. Owing to high surface areas, transparency to UV and visible light as well as easily modified surfaces, mesoporous silica-based materials have been widely used as excellent hosts for designing efficient photocatalytic systems under the background of environmental remediation and solar-energy utilization. This Minireview mainly focuses on the surface-chemistry engineering of TiO2/mesoporous silica photocatalytic systems and fabrication of binary oxides and

nanocatalysts in mesoporous single-site-photocatalyst frameworks. Recently, metallic nanostructures with localized surface plasmon resonance (LSPR) have been widely studied in catalytic applications harvesting light irradiation. Accordingly, silver and gold nanostructures confined in mesoporous silica and their corresponding catalytic activity enhanced by the LSPR effect will be introduced. In addition, the integration of metal complexes within mesoporous silica materials for the construction of functional inorganic–organic supramolecular photocatalysts will be briefly described.

1. Introduction Design of nano-, ion- and/or cluster-sized photocatalysts within mesoporous silica-based materials have been widely studied for heterogeneous photocatalytic processes in the past several decades.[1–3] In the above materials, components with photocatalytic activity and mesoporous hosts are two main roles that these materials play in the design and fabrication of efficient photocatalytic systems. Normally, photocatalysts in the form of nanoparticles, single atoms, molecules, and clusters must be anchored on different bulky support materials because of the small size of powder, leaking problems in aqueous media, and difficult handling in practical engineering applications. The nature of supports also greatly affects the photoactivity. To the best of our knowledge, support materials influence not only the adsorption of organic substrates but also the crystallinity of TiO2 photocatalyst; therefore, a suitable support can significantly improve photocatalytic activity.[4] According to previous research works, porous silicate materials such as clays,[5, 6] zeolites[7, 8] and mesoporous silica[9, 10] are superior supports for accommodating photocatalysts nanoparticles. Un[a] Dr. X. Qian, Dr. K. Fuku, Dr. Y. Kuwahara, Prof. T. Kamegawa, Prof. K. Mori, Prof. H. Yamashita Division of Materials and Manufacturing Science Graduate School of Engineering Osaka University 2-1 Yamadaoka, Suita, Osaka, 565-0871 (Japan) E-mail: [email protected] [b] Prof. T. Kamegawa, Prof. K. Mori, Prof. H. Yamashita Unit of Elements Strategy Initiative for Catalysts&Batteries Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8520 (Japan) Part of a Special Issue for the 6th Asia-Pacific Catalysis Congress (APCAT6). A link to the full Table of Contents will appear here.

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fortunately, the intrinsic drawback of TiO2/zeolite composite photocatalytic systems is the adsorption limitation for relatively large organic compounds because of its narrow channel size. Mesoporous-silica-based hosts used in the design and functionalization of photocatalytic systems have the following advantages: 1) tunable pore sizes (2–50 nm) and large surface areas; 2) fine connectivity of the pores facilitating the transfer of organic substrates; 3) remarkable transparency in the wide wavelength range of UV/Vis; 4) amorphous frameworks with a large number of silanol groups for postmodification. The above merits could offer further opportunities in the development of efficient photocatalytic systems within the world of nanopores.[10, 11] The incorporation of photocatalytic active components such as materials based on TiO2 in mesoporous silica can be achieved by applying different synthesis methods such as wet impregnation,[12, 13] inner-pore hydrolysis/nonhydrolysis,[14–18] co-hydrolysis and co-condensation,[19, 20] sol–gel processes,[21–27] and sol–gel/hydrothermal methods.[28] However, the pristine mesoporous silica hosts exhibit a number of limitations such as high affinity to water molecules, low selectivity to organic compounds; thus, the modification of the physical and chemical properties of porous siliceous materials has often been performed through a combination of some desired components such as minerals[29] and nanocarbon materials,[30, 31] or anchoring functional moieties (F) and organic groups (e.g., noctyl, methyl) in the frameworks or on their surfaces as part of the walls.[32–34] Single-site photocatalysts refer to isolated and tetrahedrally coordinated metal oxide moieties (M-oxide = titanium, vanadium, chromium, molybdenum, and tungsten oxides) implanted and isolated in the silica matrices of microporous zeolites and ChemSusChem 0000, 00, 1 – 10

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CHEMSUSCHEM MINIREVIEWS mesoporous silica materials.[35–42] Thus, their photocatalytic mechanism is different to the photoelectrochemical reaction mechanism of bulk semiconductor-based materials in which the isolated and tetrahedral coordinated metal oxide moieties are excited under UV irradiation, which induces an electron transfer from oxygen (O2 ) to Mn + ions resulting in the formation of trapped hole centers (O ) and electron centers (M(n 1) + ) pairs. Such excited hole/electron states localized near each other play an important role in various photocatalytic processes. In previous studies, the main focus for the utilization of single-site photocatalysts was only on their photocatalytic performances.[38, 39, 41–44] Our research results reflected that these single-site photocatalysts in mesoporous silica frameworks still had considerable potential for the fabrication of visible-light responsive binary oxides and highly dispersed metal nanocatalysts.[45, 46] Metallic nanostructures such as gold, silver, copper, or palladium not only have interesting physical properties and lively colors, but can also absorb specific visible and infrared light owing to the localized surface plasmon resonance (LSPR) effect.[47–49] This light-responsive process can trigger thermal conductivity, enhanced generation of excited states in the vicinity of the nanoparticle, and electron/hole transfer in the presence of semiconductors that can participate in photoredox processes.[50–56] In a heterogeneous catalytic process, the bond formation or cleavage of different substrates normally occurs on the surface of metals loaded on different supports, whereas the size, shape, and dielectric environment play important roles in the LSPR of nanostructures on support materials. Mesoporous-silica-based materials have been used as a hard template for the synthesis of nanoparticles and nanowires because of their robust structures, uniform pore size, and high surface area. Therefore, the metal nanostructures introduced into the transparent mesoporous silica-based supports should enhance the catalytic performance of metallic nanocatalysts through light-excited LSPR. Metal-complex photocatalysts such as polypyridyl complexes of ruthenium can be excited by visible light to present stable, long-lived photoexcited states.[57] Normally, these optical materials are immobilized on inert silicate and metal organic frame-

Hiromi Yamashita has been a Professor at Osaka University since 2004. He received his PhD degree from Kyoto University (supervisor: Prof. S. Yoshida) in 1987. He was an Assistant Professor at Tohoku University (Profs. A. Tomita and T. Kyotani), an Associate Professor at Osaka Prefecture University (Prof. M. Anpo), and an Invited Professor at University Pierre and Marie Curie (UPMC; Prof. M. Che). He was also a visiting research fellow at Penn State University (Prof. L.R. Radovic), University of Texas at Austin (Prof. M.A. Fox), and Caltech (Prof. M.E. Davis). His research interests include the design of single-site photocatalysts and nanostructured catalysts.

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www.chemsuschem.org work (MOF) hosts. For example, zeolite Y with restricted supercages (1.3 nm) and mesoporous silica hosts with a much wider pore size of 2–50 nm were used to accommodate metal complexes and photonic-band-gap materials, whereas mesoporous-silica materials with uniform channels were of particular interest because they could be easily tailored to obtain favorable characteristics.[58–60] Herein, we will introduce the recent research works of TiO2, single-site photocatalysts, plasmonic metal nanostructures, and metal complexes within mesoporous silica frameworks in terms of surface chemistry engineering, fabrication of binary oxides and nanocatalysts in mesoporous single-site photocatalysts, novel plasmonic nanostructures, and photo-responsive metal complexes in mesoporous-silica-based hosts. Additionally, this Minireview aims to present and share new information on the design and functionalization of photocatalytic systems within mesoporous silica with regard to environmental and catalytic applications harvesting solar energy.

2. Surface-Chemistry Engineering of TiO2/ Mesoporous Silica Photocatalytic Systems TiO2 as one of the most promising photocatalysts has always attracted considerably interest for application in photocatalytic degradation of organic compounds present at low concentrations in water and air owing to its stability, low cost, low toxicity, and suitable band gap energy for photo-induced redox reactions.[61–66] Heterogeneous photocatalysis can be regarded as a process to simultaneously adsorb contaminates (adsorption) and absorb photons (photo-induced redox reaction) on photocatalysts. Apparently, adsorption plays an identical role in photocatalysis in comparison to photoredox processes.[67] Therefore, effective adsorption sites have to be near the photocatalytic sites, so that the active radicals (e.g., hydroxyl radicals) can reach them before these active intermediates lead to deactivation through recombination with electrons. The adsorption and enrichment of organic pollutants on the porous supports and/or the surface of TiO2 nanoparticles strongly relies on the surface properties such as hydrophilicity/hydrophobicity or surface charge. Accordingly, surface-chemistry engineering for TiO2/mesoporous silica photocatalytic systems is desirable if dealing with the organic compounds in water and air at low concentrations. Figure 1 presents the main principles of surface modification in TiO2/mesoporous silica photocatalytic systems. For enriching the solution near the photocatalytic sites (i.e., TiO2) with organic compounds, modification of the surface of supports or selectively of TiO2 has been realized, and the performances related to adsorption and photocatalysis has been investigated. Recently, we successfully incorporated calcium phosphate (CaP) minerals into the TiO2/mesoporous silica photocatalytic system by utilizing its unique surface chemistry such as the hydrophilic nature and good affinity for various organic molecules.[16] The successful coating of CaP onto the silica host was realized by means of a facile sol–gel process. By tuning the thickness of the CaP coating layer, the pore diameter of the silica host was changed as the content of CaP was increased ChemSusChem 0000, 00, 1 – 10

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Figure 1. Surface-chemistry engineering of TiO2/mesoporous silica photocatalytic systems.

(Figure 2 A). The highly dispersed CaP phase effectively increased the amount of methylene blue (MB) molecules from the aqueous solution resulting in a high pore occupancy of MB molecules, and the key role of the remarkable adsorption capacity should depend on the high affinity of the CaP coating to MB rather than on the surface area of the supports (Figure 2 B). Evaluation of the photocatalytic performance in dark conditions and under UV light made it clear that a moderate adsorption capacity and high pore occupancy were necessary for improving the photodegradation efficiency (Figure 3 A). The photodegradation efficiency was lower for the mixture of CaP and TiO2/SBA-15, which also had a considerable adsorption capacity relative to the composite sample (Figure 3 A).

Figure 2. Illustrations of pore size variations (A) and adsorption capacities for the model dye MB and estimated pore occupancy of adsorbed dye molecules on TiO2/mesoporous silica photocatalysts with different weight percent of CaP coating layers (B).[16]

Figure 3. Removal efficiency of MB by adsorption, photodegradation, and sum of them (total) on TiO2/SBA-15 modified with different amounts of CaP (0–18.7 wt %) (A) and schematic illustration of dye enrichment and photodegradation on CaP-coated TiO2/SBA-15 (B).[16]

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www.chemsuschem.org This phenomenon implied that the combination of CaP (adsorption sites) and TiO2 (catalytic sites) on nanoscale was crucial for the enhancement of degradation efficiency. MB molecules with positively charged nitrogen-alkyl groups could be easily enriched on the surface of CaP with abundant PO43 groups through electrostatic interactions (Figure 3 B). When the photons reached TiO2 under UV irradiation, charge separation produced electron/hole pairs. The resulting active radicals, including hydroxyl radicals and superoxide radical anions, could oxidize the adsorbed dye molecules on the TiO2 surface into degraded and/or mineralized intermediates. The dye molecules around the TiO2 surface directly released from the MB enriched on the CaP adsorption sites, which effectively reduced the diffusion distance of pollutants from the adsorption sites to the photocatalytic sites considerably. Additionally, the MB enriched on the CaP adsorption sites could alleviate the over coverage of MB molecules on TiO2 photocatalytic sites in a MB solution with deep color, which also facilitated the efficient absorbance of photons from UV irradiation. Surface hydroxyl groups on mesoporous silica hosts preferentially trapped or transferred H2O molecules into silica pores in comparison to organic compounds, which inevitably resulted in a decrease in the photocatalytic activity.[68, 69] A hydrophobic modification (replacement of unnecessary hydroxyl groups with fluorine atoms) was proposed to enhance the adsorption of organic compounds in the nanopores (Figure 4 A).[34] A silylation agent with an inorganic functional fluorine group, that is, triethoxyfluorosilane (TEFS) was grafted on a hexagonal mesoporous silica (HMS)-type material with wormhole-like pore structure as an example. After grafting fluorine atoms, the H2O adsorption capacity dramatically decreased compared to that of pure HMS, indicating that the hydrophobic surface was successfully modified (Figure 4 B). With UV light irradiation, the TiO2 photocatalyst decomposed 2-propanol completely into CO2 and H2O via acetone as an intermediate. TiO2 on TEFSmodified HMS (defined as FSHMS) exhibited a dramatically higher adsorption ability and photocatalytic activity than on unmodified HMS as the hydrophobic fluorine content of the support was increased (Figure 4 C). Additionally, Xing et al. chose NH4F as an inorganic fluorine modifier to synthesize super-hydrophobic mesocellular foams (MCF) loaded with nanosized TiO2 photocatalysts.[33] The hydrophobic properties together with the Ti3 + generation led to an excellent adsorption capacity and UV/Vis photocatalytic activity. A super-hydrophobic surface has also been achieved through the synthesis of carbon nanotubes (CNTs) on titanium-containing mesoporous silica thin films (Ti-MSTFs) with binary Co–Mo nanocatalysts, which was successfully deposited on Ti-MSTFs under microwave irradiation.[30] The hydrophobic properties of samples after CNT growth were enhanced with increasing titanium concentration of Ti-MSTF, and the water contact angle increased up to 1658 on Ti-MSTF with a titanium concentration of 10 at %. Almeida et al. reported the grafting of methyl groups on mesoporous silica hosts by silylation, resulting in a positive effect on the performance of an anatase catalyst in the selective photooxidation of cyclohexane owing to the increased desorption of the product (cyclohexanone).[70] Kasahara et al. ChemSusChem 0000, 00, 1 – 10

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Figure 4. Illustration of hydrophilic pore and surface modification using TEFS as a silylation agent (A), water adsorption isotherms at room temperature (B), and reaction rates and adsorption capacities of 2-propanol diluted in water on TiO2/HMS, TiO2/FS2-HMS, TiO2/FS4-HMS, TiO2/FS6-HMS (2, 4, and 6 correspond to the molar percent of TEFS), and bulk TiO2 (P25, Evonik) (C).[34]

grafted n-octyl groups onto the pore walls of TiO2-MCM-41 in which TiO2 (8.7 wt %) was highly dispersed in the nanopores.[71] n-Octyl-grafted TiO2-MCM-41 (C8-TiO2-MCM-41) efficiently decomposed dilute 4-nonylphenol polyethoxylate (NPEO). The activity of C8-TiO2-MCM-41 per TiO2 weight was higher than that of commercial P-25 catalyst under the same conditions. The higher activity of C8-TiO2-MCM-41 was ascribed to the highly concentrated NPEO in the hydrophobic nanospaces. The alkyl

groups grafted onto the C8-TiO2-MCM-41 were stable under irradiation for 10 h. Apart from the surface modification of silica hosts, selective graphene coating of TiO2 nanoparticles supported on a mesoporous silica surface (TiO2/MCM-41) was performed by our group to design more efficient composite systems using 2,3-dihydroxynaphthalene (DN) as a precursor (Figure 5 A).[31] The results indicated that DN as a precursor of graphene was selec-

Figure 5. Illustration of hydrophilic pore and surface modification using TEFS as a silylation agent (A), water adsorption isotherms at room temperature (B), and reaction rates and adsorption capacities of 2-propanol diluted in water on TiO2/HMS, TiO2/FS2-HMS, TiO2/FS4-HMS, TiO2/FS6-HMS (2, 4, and 6 correspond to the molar percent of TEFS), and bulk TiO2 (P25, Evonik) (C).[34]

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tively anchored on TiO2 nanoparticles supported on MCM-41 through the formation of a stable surface complex, which could be converted to graphene during heat treatment in an inert atmosphere. Under UV light irradiation of the mixture of a photocatalyst and an aqueous solution of 2-propanol, the concentration of 2-propanol gradually decreased with increasing UV irradiation time (Figure 5 B). Diluted 2-propanol in water was decomposed into CO2, H2O, and intermediates (such as acetone). Formed acetone was also finally decomposed into CO2 and H2O. The adsorption capacity of water as well as 2propanol indicated that the graphene coating exhibited a clear hydrophobicity and acted as a good adsorbent of organic molecules (Figure 5 C). However, the photocatalytic activity decreased on increasing the graphene coating layers, which indicated that a suitable amount of graphene coating led to the enhancement of photocatalytic activity of TiO2/MCM-41. Appropriate adsorption properties of organic compounds could relieve the diffusion limitations of the catalytically active site and reduce the shielding effect of graphene to the incident light.

hardly any photocatalytic activity was observed on Ti/MCM-41. This phenomenon can be explained in such a way that the oxidation of CO was performed through the redox process of tetrahedrally coordinated Cr6 + oxide species. The enhancement of photocatalytic activity on Cr-Ti/MCM-41 probably could be attributed to the interaction of the Ti4 + and Cr6 + oxide species. Under UV irradiation of the slurry of Ti-HMS in an aqueous H2PtCl6 solution using a high-pressure mercury lamp, a platinum metal precursor could be successfully deposited on TiHMS.[84, 85] Without UV irradiation and titanium oxide (TiO2) moieties, the metal precursor was hardly deposited. These metal precursors were transformed into nanosized metal particles through reduction in the presence of H2 or heating under vacuum. These results indicated that the presence of the photoexcited state of TiO2 moieties was necessary for the deposition of metal species. Similarly, both palladium and nickel precursors could be deposited directly onto the photoexcited tetrahedrally coordinated TiO2 moiety within silica frameworks in the presence of a mixed ammonia aqueous solution of NiSO4 and PdCl2 at a pH value of 10 and subsequent reduction in the presence of H2 to generate nanosized PdNi bimetal particles.[83] 3. Fabrication of Binary Oxides and Metal Comparing the particles size of PdNi prepared by PAD and the Nanoparticles on Single-Site Photocatalysts impregnation (Imp) method, relatively small and uniformly sized Pd–Ni nanoparticles with a diameter of 2–8 nm were obUp to now, the fabrication of binary oxides[72, 73] and nanosized served on the PAD sample. Fourier transforms of Pd and Ni Kmetal particles including palladium,[74–76] platinum,[77] silver,[78] edge extended X-ray absorption fine structure (EXAFS) spectra gold,[79–81] AuPd,[82] and PdNi[83] on tetrahedrally coordinated of Pd–Ni nanoparticles showed that the intensity of Pd Pd TiO2 moieties within the mesoporous silica frameworks has bond peaks at around 2.5  of PdNi/Ti-HMS prepared by PAD been realized by using different techniques including photodecreased compared to that of the spectrum of palladium foil assisted deposition (PAD) and chemical vapor deposition (CVD) (Figure 7 A). In the Ni K-edge spectrum, the main peak of the (Figure 6). Ni Ni bond at around 2.1  was slightly split into two peaks at To endow single-site photocatalysts with catalytic activity 1.9 and 2.7 , respectively (Figure 7 A a vs. d), which stemmed und visible light, a binary (Cr, Ti)-oxide-containing mesoporousfrom the interference between the EXAFS oscillations of the silica (Cr/Ti-HMS) photocatalyst was prepared by CVD.[72] The Ni Ni and Ni Pd bonds, in which the phase shift and ampliUV/Vis and X-ray absorption near edge structure (XANES) spectude differ from each other. Similar changes in the EXAFS spectra proved the presence of tetrahedrally coordinated titanium tra have been found in previous studies of Pd–Au, Pd–Pt, and (Ti4 + ) and chromate (Cr6 + ) oxides and binary metal oxide spePt–Rh bimetallic nanoparticles.[82, 86, 87] The Pd O and Ni O cies. The binary oxides anchored in mesoporous silica were debonds were not observed in the spectra, indicating that there tected in photoluminescence spectra. These spectra were rewere no oxide species present within these particles. These recorded through excitation of the ligand-to-metal charge transsults confirmed the formation of small, uniform, and highly disfer band owing to the radiative decay of the charge-transferpersed Pd–Ni bimetallic nanoparticles using the PAD method. excited triplet states of tetrahedrally coordinated Ti4 + and Cr6 + However, a further investigation of the exact atomic structural oxide species to ground singlet state. The photocatalytic permodel of the bimetal nanocatalysts fabricated by PAD on formance in the oxidation of CO into CO2 under UV/Vis irradiasingle-site photocatalysts still had to be conducted in our labotion was evaluated. The results showed that the oxidation of ratory. On the other hand, the PdNi/Ti-HMS prepared by the CO to CO2 occurred on Cr/MCM-41 and Cr-Ti/MCM-41, whereas impregnation method showed a Pd Pd bond peak with higher intensity compared to that prepared by PAD in the Pd K-edge (Figure 7 A d), and two peaks comparable to NiO were observed at 1.6 and 2.4 A in the Ni Kedge, respectively (Figure 7 B b vs. d). These results suggested the formation of core–shell-type nanoparticles in the case of the Figure 6. Fabrication of nanocatalysts and binary metal oxides within mesoporous single-site photocatalysts.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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ation reaction of terephthalic acid. Mohamed et al.[81] also synthesized gold nanoparticles on Ti-HMS by means of the PAD method. The results indicated that gold nanoparticle with a mean diameter of ca. 4 nm could be detected whereas aggregated gold nanoparticles with various sizes was observed on Au/TiHMS prepared by Imp. Photocatalytic performance under visible light in the oxidation of CO with O2 showed notably enhanced photocatalytic activity of PAD-Au/Ti-HMS, which was 2.1 and 5.7 times higher than that of Au/Ti-HMS prepared by Imp and Ti-HMS, respectively. So far, the synthesis of bimetallic and alloy nanoFigure 7. (A) Pd K-edge FT-EXAFS spectra for (a) Pd foil, (b) PdO, (c) PAD-PdNi/Ti-HMS, and (d) Imp-PdNi/Ti-HMS; (B) Ni K-edge FT-EXAFS spectra for (a) Ni foil, (b) NiO, (c) PADparticles on single-site photocatalysts by PAD is still PdNi/Ti-HMS and (d) Imp-PdNi/Ti-HMS; (C) hydrogenation of NB to AB using PAD-Ni, Pd limited, and an additional reduction procedure is also and PdNi/Ti-HMS, and Imp-PdNi/Ti-HMS.[83] needed, especially for transition-metal ions involved bimetallic systems. In the future, the synthesis parameters including the pH values, ligands, and other additives and Imp method. Compared with pure palladium or Ni/Ti-HMS prethe reduction potentials of metal ions should be taken into acpared by PAD and PdNi/Ti-HMS prepared by the conventional count. Additionally, the relationship between metal atomic impregnation technique, PdNi/Ti-HMS prepared by PAD structure model and catalytic performance should be also inshowed a clearly higher catalytic activity of the hydrogenation teresting. of nitrobenzene (NB) to aminobenzene (AB). This result suggested that the hydrogenation activity depended on the particle size and structure of Pd–Ni (Figure 7 C). The core(Pd)– 4. Design of Plasmonic Metal Nanostructures shell(Ni) structure of nanoparticles with large diameters prein Mesoporous Silica Hosts pared by the Imp method may suppress the efficient hydrogeThe charge separation and localization of plasmonic metallic nation through a shielding of the active sites on palladium by nanostructures can be enhanced during light-excited LSPR. In the nickel shell. A specifically high NB hydrogenation activity was achieved under a mild condition owing to a synergistic heterogeneous catalysis, most of the catalytic processes occur in the vicinity of the metallic surface, which implies that the effect of Pd–Ni and the loading of highly dispersed and smaller catalytic performance could probably be enhanced by the bimetal nanoparticles. It was also found that the hydrogenaLSPR effect occurring when using mesoporous silica as a suption activity reached a maximum at the molar ratio of Ni/Pd = port. Recently, a new method for the synthesis of silver nano1.5, whereas a further increase of nickel loading gradually destructures in SBA-15 by means of microwave heating was recreased the catalytic activity. ported.[88] By controlling the time (3 or 5 min) of microwave irMei et al. reported the photodeposition of gold nanoparticles by using titania-incorporated SBA-15 (Ti-SBA-15) as the radiation and the use of sodium laurate as organic ligand, single-site photocatalyst.[80] During the deposition of gold silver nanoparticles (short and separate nanorods) were obtained, which also induced a color change of the Ag/SBA-15 under UV light, the evolution of H2 was only produced in the composites (Figure 8 A). The diameter of silver nanorods represence of auric acid during the photodeposition of gold, sembled the pore size of SBA-15, which reflected the confinewhereas H2 was continuously produced in the presence of ment effect of the silica host. The localized surface charges of metallic gold because of the photocatalytic reforming of meththese silver nanostructures displayed enhanced catalytic activianol, which acted as the hole scavenger. UV/Vis spectra and elty of NH3BH3 dehydrogenation under visible light irradiation emental analyses of Au revealed complete deposition of gold on Ti-SBA-15 with 50 % irradiation power (350 W) for 2.5 h. compared to that under dark conditions owing to LSPR (FigBased on UV/Vis and X-ray photoelectron spectroscopy (XPS) ure 8 B). The enhancement of LSPR depended on the silver results, the titanium coordination of the isolated TiO4 tetrahenanostructures; namely, the long silver nanorods had a higher LSPR inductive effect at 650 nm. The temperature of the susdra changed upon gold photodeposition. Recently, the authors pension of Ag/SBA-15 with a long-nanorod structure increased found that the final size of gold particles and their encapsulaslightly (about 5 8C) under light irradiation because the silver tion by TiOx species (core–shell structure) occurring during the nanorods exhibited wide light absorption by LSPR over the IR diffusion-controlled photodeposition of gold resembled strong region. However, the photothermal effect had negligible influmetal–support interaction.[79] Additionally, the incorporation of ence on the dehydrogenation activity of NH3BH3. Vazquez-VazZnOx species facilitated the formation of monodisperse gold particles matching the size of the SBA-15 channels by the phoquez reported the encapsulation of plasmonic gold nanopartitodeposition method; the agglomeration of TiOx and the forcles on the inner walls of a mesoporous silica capsule.[89] These mation of TiOx shells surrounding the gold particles did not plasmonic nanoreactors (PNRs) were used in a Diels–Alder cycloaddition reaction for simultaneous performance and monioccur. Furthermore, their results showed that Ti O Si bonds toring of thermally induced confined chemical processes. were essential for the photocatalytic activity in the hydroxyl 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 8. (A) TEM images and photographs of samples (insets) of Ag/SBA-15 with (a) small nanoparticle, (b) short and (c) long nanorod structures; and (B) comparison of initial H2 production in the presence of Ag/SBA-15 in the dark or under light irradiation at l > 420 nm.[88]

These PNRs acted as heat nanosources upon external illumination, promoting the conversion of reactants into products. Catalytic results showed a remarkable increase in the reaction yield when the cycloaddition reaction was performed in the presence of capsules under laser irradiation. A temperature increase (+ 8 8C) was observed in the sample solution, which was caused by heat dissipation from the gold nanoparticles through the isolating silica shell, which contributed to the acceleration in the cycloaddition rate.

easily controlled by varying the loading amount, the effect of spatial distribution of the platinum complex in the mesoporous channel on the phosphorescence emission and photocatalytic activities could be fully investigated (Figure 9). The isolated platinum complex enabled selective photooxidation using O2 through energy and/or electron transfer from 3MLCT to O2, whereas H2 production in aqueous media was promoted by platinum complexes in close proximity through visible-light photosensitization associated with metal-metal-to-ligand charge-transfer (3MMLCT) excited states. Clays,[94] zeolites,[95] and pristine mesoporous silica[93] hosts were prepared from an aluminosilicate network; they acted only as supports for the light

Figure 9. Effect of spatial distribution of the [Pt(tpy)Cl]Cl complex in the mesoporous channel the phosphorescence emission and photocatalytic activities.[93]

5. Metal complex Photocatalysts within Mesoporous Silica Hosts The integration of metal complexes with rigid inorganic matrices, such as clays, zeolites, and mesoporous materials, has been intensively pursued to construct functional inorganic–organic supramolecular devices.[90–92] In comparison with their properties in solution, such materials often exhibit unexpected physicochemical properties induced by both steric and electrostatic constraints within the restricted void spaces as well as the nature of the functional groups on the host surface. In our previous work, the physicochemical properties of [Fe(bpy)3]2 + and [Ru(bpy)3]2 + complexes in the cavities of zeolite Y were examined by continuously varying the extra-framework alkali metal cations.[58–60] The increased intensity of the phosphorescence emission correlated well with the increased turnover number for photo-induced oxidation using molecular oxygen (O2). On the other hand, the utilization of mesoporous silica materials with uniform channels was of particular interest. New inorganic–organic hybrid photocatalyst were developed by anchoring the [Pt(tpy)Cl]Cl complex to mesoporous silica hosts.[93] As the interaction between each platinum complex can be  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

harvesting (LH) molecules and could not perform as LH units themselves. Recently, Yamamoto et al. developed an efficient artificial LH system that operates through sequential two-step energy accumulation.[96] A periodic mesoporous organosilica (PMO) with bridging biphenyl groups in the framework (BpPMO) served as an LH antenna. The center of a linearly shaped ReI pentanuclear complex was attached to a RuII trisdiimine complex through a covalent bond (Ru-Re5). Ru-Re5 served as the first and second energy acceptors, respectively. Hybridization of the artificial LH system was achieved using the nonionic surfactant C12H25(OCH2CH2)4OH in an acetonitrile solution; in the hybrid (Ru-Re5-Bp-PMO), the Ru-Re5 molecules were adsorbed in an orderly fashion in the mesopores of Bp-PMO. Photons absorbed by the Bp units were first accumulated in the five Re units in Ru-Re5 and then transferred to only one Ru unit, which emitted the light strongly.

6. Summary and Outlook In recent developments of photocatalytic materials regarding semiconductor TiO2, single-site photocatalysts, and metal complexes, noble metallic nanostructures in mesoporous silica frameworks were suggested for environmental remediation ChemSusChem 0000, 00, 1 – 10

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CHEMSUSCHEM MINIREVIEWS and solar energy utilization. Compared with free photocatalytic and photo-responsive materials and molecules, the advantages of mesoporous-silica-based hosts could be summarized as follows: 1) providing a large nanospace for accommodating pollutants and reactants; 2) anchoring functional groups and moieties for creating a unique photocatalytic reactor with a molecularly recognizable synergistic effect and other physiochemical properties; 3) controlling the diffusion, nanostructures, and sizes for the photo-responsive components. The use of mesoporous silica hosts to design efficient TiO2 composite photocatalytic systems with modified surfaces has been proven to offer further opportunities to control the adsorption capacity for organic compounds, which greatly influences the photocatalytic performance in water/gas purification. Binary Cr–Ti oxide anchored on mesoporous silica showed enhanced photocatalytic activity in visible light, which extended the application spectrum of single-site photocatalysts to the UV and visible light region. Single-site photocatalysts in silica frameworks also provided a new route for the fabrication of nanocatalysts with high dispersion and catalytic activity. The successful confinement of silver and gold nanostructures in mesoporous silica hosts was achieved. The enhancement of LSPR-induced catalytic performance under light irradiation was closely related to the optical absorption properties of the silver and gold nanostructures. Metal complexes anchored in mesoporous silica hosts displayed different optical properties depending on the distances of each complex molecule in silica hosts, which also influenced the photocatalytic activities. PMO materials were used for assembling an efficient artificial light harvesting (LH) system. In future studies, the selective anchoring of specific functional groups on the surfaces of supports and photocatalysts in addition to TiO2 will be desirable for photocatalytic applications. The positive effects of surface engineering can be extended to other photocatalytic reactions. Binary oxide photocatalysts active in visible light based on the single-site photocatalytic mechanism will attract considerable attention in terms of visible light response in various catalytic reactions as well as structural and atomic control. It has been proven that nanocatalysts using single or dual metal sources could be synthesized using the photo-assisted deposition (PAD) method on single-site photocatalysts. However, the interaction between nanocatalysts and single-site photocatalysts, the control of the bimetallic or alloy structure properties by PAD, and the relationship of structure/performance of the nanocatalysts are still not very clear. Microwave heating techniques also have considerable potential for synthesizing nanocatalysts supported on mesoporous-silica-based materials. Up to now, the catalytic performance of noble metal nanocatalysts on mesoporous silica hosts has been widely studied under dark conditions, whereas the localized surface plasmon resonance (LSPR) and photothermal effects should bring new room for improvement of the catalytic performance under light irradiation. Therefore, the design of other metallic and bimetallic nanostructures utilizing the LSPR effect in transparent mesoporous-silica-based hosts with/without surface modification and their catalytic performance in harvesting light irradiation should be very interest 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org ing. With regard to metal complex photocatalysts, it would be desirable to understand the influence of the modification of mesoporous silica hosts with different functional groups and heteroatom moieties on the optical and physicochemical properties of metal complex molecules and the corresponding photocatalytic performance as well as the mechanisms.

Acknowledgements The present work was supported by Strategic China-Japan Research Cooperative Program 2012 from JST. X.F.Q. acknowledges the JSPS Postdoctoral Fellowship for Foreign Researchers (P12075). Keywords: mesoporous silica · metal nanostructures · photocatalysts · titania

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Received: January 22, 2014 Revised: March 5, 2014 Published online on && &&, 0000

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MINIREVIEWS X. Qian, K. Fuku, Y. Kuwahara, T. Kamegawa, K. Mori, H. Yamashita* && – && Design and Functionalization of Photocatalytic Systems within Mesoporous Silica

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Photocatalysis in mesoporous silica hosts: Photocatalysts assembled in transparent mesoporous silica exhibit remarkable performances with regard to environmental remediation and catalytic reactions under light irradiation. This Minireview introduces some typical studies on the engineering of surface chemistry, fabrication of binary oxides and nanocatalysts, plasmonic metal structures, and metal complexes in mesoporous silica-based hosts.

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