Multifunctional Metal-Organic Frameworks for Photocatalysis.

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Metal–Organic Frameworks

Multifunctional Metal–Organic Frameworks for Photocatalysis Sibo Wang and Xinchen Wang*

Metal–organic frameworks (MOFs) have attracted From the Contents 1. Introduction ..............................................2 2. Definition of MOFs as Photocatalysts, Co-catalysts, or Hosts for Photocatalysis ........................................... 2 3. MOFs as Photocatalysts .............................2 4. MOFs as Co-catalysts for Photocatalysis ...........................................8 5. MOFs as Hosts for Photocatalysis............. 11 6. Conclusion and Outlook............................14

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significant research attention in diverse areas due to their unique physical and chemical characteristics that allow their innovative application in various research fields. Recently, the application of MOFs in heterogeneous photocatalysis for water splitting, CO2 reduction, and organic transformation have emerged, aiming at providing alternative solutions to address the worldwide energy and environmental problems by taking advantage of the unique porous structure together with ample physicochemical properties of the metal centers and organic ligands in MOFs. In this review, the latest progress in MOF-involved solar-to-chemical energy conversion reactions are summarized according to their different roles in the photoredox chemical systems, e.g., photocatalysts, co-catalysts, and hosts. The achieved progress and existing problems are evaluated and proposed, and the opportunities and challenges of MOFs and their related materials for their advanced development in photocatalysis are discussed and anticipated.

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1. Introduction

and, thus, is not included in this more topical co-workers short review.

Semiconductor photocatalysis, inspired by the natural photosynthesis of green plants and some other micro-organisms, promises the use of solar energy to convert carbon dioxide and water to hydrocarbons and oxygen, which is recognized as the most promising solution to address the world-wide environmental and energy crisis in a carbon-neutral fashion.[1–5] Photocatalysis was already demonstrated by Fujisima and Honda in the 1970s with TiO2 for water splitting under UVlight irradiation to generate H2 and O2 fuels.[6] Since then, various types of materials (e.g., semiconductors, metal-doped zeolites, metal complexes) have been explored and studied for diverse light-induced redox processes to advance artificial photosynthesis.[7–16] The engineering of these materials in various nanostructures has also been actively investigated to maximize their photofunction and performance.[17–19] In recent years, as a newly emerged type of functional inorganic–organic hybrid material, metal–organic frameworks (MOFs) have attracted increasing research attention for various advanced applications, including artificial photosynthesis.[20–24] MOFs,[25] also termed ‘coordination polymers’, are an intriguing family of crystalline porous solids constructed from inorganic metal ions (or clusters) and multitopic organic bridging ligands with infinite network architectures in a 3D space.[26–28] MOFs have large surface areas, readily controllable open channels and pores, and flexible tunability in their structural, compositional, and functional properties.[29] MOFs have found application in many areas, such as gas storage,[30,31] separation,[32,33] sensing,[34–36] nonlinear optics,[37–39] luminescence,[40–42] drug delivery,[43–45] and heterogeneous catalysis.[46–48] Due to the presence of catalytically active metals and/or functional organic linkers, the easily tailorable physical and chemical functions, together with the large surface area and permanent pores/channels to potentially anchor/encapsulate photosensitizers and catalytic moieties, MOFs hold and have already shown great opportunities for heterogeneous photocayalysis to operate artificial photosynthetic reactions including water splitting, CO2 photofixation, and organic photosynthesis. A few reviews have been published to cover the applications of MOFs in photocatalytsis and artificial photosynthesis recently,[22,24,49] but these papers only summarized partially the MOFs that were developed for photochemical reactions in an overgeneralized manner, taking no account of the dissimilar status of MOFs in photoredox systems. It is thus quite necessary to clearly classify the different roles that MOFs have played in their progress in heterogeneous photocatalysis to provide fundamental and important guidance for researchers to rationally design and investigate MOF-related photochemical systems in the future. In this review article, we have defined MOFs as photocatalysts, co-catalysts, and hosts for photoredox catalysis according to their different functions in the photocatalytic systems, and we have concentrated on the latest developments of MOF-mediated solar-to-chemical energy conversion reactions including water splitting, CO2 reduction, and organic selective transformation with light. The application of MOFs for photocatalytic organic pollutant degradation has been specifically reviewed by Guo and

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2. Definition of MOFs as Photocatalysts, Co-catalysts, or Hosts for Photocatalysis 2.1. MOF Photocatalysts MOFs which constitute the component in a reaction system that is absorbing light to produce photogenerated charge carriers for subsequent photoredox reactions are defined as photocatalysts. In brief, these MOFs are the light–energy transducers that mediate the conversion of solar energy to energized electron and holes.

2.2. MOF Co-catalysts When other components (e.g., dyes, semiconductors) are the photoharvesting substance that forms lightstimulated electrons and holes, MOFs are the active components that promote the kinetic processes of both charge separation and catalytic reactions. Such MOFs are defined co-catalysts.

2.3. MOF Hosts We define MOFs as hosts for heterogeneous photocatalysis when the MOFs have no (or weak) catalytic activity in the photochemical reactions, but after anchoring/encapsulating functional moieties, the resultant MOF-based composites obtain an enhanced performance for lightinduced reactions.

3. MOFs as Photocatalysts Under light irradiation, the organic bridging ligands of MOFs can serve as antennas to harvest light and activate the metal nodes in the fashion of a linker to a metal cluster charge transition (LCCT), illustrating the semiconductor-like behavior of MOFs,[51–57] and thus demonstrating the great opportunities for MOFs as photocatalysts for artificial photosynthetic reactions. Excited MOFs generate electrons and holes that subsequently transfer to the surface to induce a heterogeneous photoredox reaction, like water splitting, CO2 reduction, or organosynthesis. S. Wang, Prof. X. Wang State Key Laboratory of Photocatalysis on Energy and Environment College of Chemistry Fuzhou University Fuzhou 350002, PR China E-mail: [email protected] Website: http://wanglab.fzu.edu.cn DOI: 10.1002/smll.201500084


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3.1. MOFs as Photocatalysts for Water Splitting

Sibo Wang received his B.S. degree in chemistry from Anqing Normal University (2010),

3.1.1. MOFs as Photocatalysts for Water Reduction to Generate H2

PR China. He is now pursing his PhD in Prof. X. Wang’s group at the State Key

Solar-driven water splitting, namely, water reduction to generate H2 and water oxidation to produce O2, is of significant importance to convert and store solar energy as chemical fuels in a sustainable manner. Among the two half reactions, hydrogen evolution was the first and dominantly demonstrated by photocatalytic MOFs. For instance, García and co-workers in 2010 reported two highly waterstable Zr-based MOFs (UiO-66: Zr6O4(OH)4(BDC)12,[58] BDC: 1,4-benzenedicarboxylate, where UiO = University of Oslo, and NH2-UiO-66: Zr6O4(OH)4(ATA)12, where ATA = 2-aminoterephthalate) as photocatalysts for photocatalytic hydrogen evolution from methanol and water/methanol.[59] UiO-66 and NH2-UiO-66 are isoreticular MOFs with Zr6(O)4(OH)4(CO2)12 secondary building units (SBUs), and they both feature high crystallinities. Upon UV irradiation (λ > 300 nm), NH2-UiO-66 showed a slightly better H2 evolution ability from a water/methanol solution than UiO-66, and a remarkably enhanced H2 production for both MOFs can be achieved when Pt nanoparticles are deposited as a cocatalyst on the surface to accelerate charge separation and to act as reactive sites to reduce reaction barriers or over-potentials of the hydrogen evolution reaction (Figure 1b). After functionalizing the UiO-66 with amino groups, the resulting NH2-UiO-66 can extend its light absorption to the visible region (Figure 1a), but, the authors didn’t demonstrate the water splitting reaction under visible light here. Although the catalytic efficiencies of the two MOFs were still not very satisfactory, this work stimulated intensive studies on MOFs as semiconductor-like materials for photocatalyzing the water splitting reaction, as demonstrated by water half-splitting to generate hydrogen. Inspired by this work, visible light-driven photocatalytic hydrogen evolution was achieved by modification of the MOFs, namely, a UiO-66/CdS/1% reduced graphene oxide ternary composite,[60] and an RhB-sensitized Pt@UiO-66.[61] These UiO-66-based catalysts indeed displayed improved catalytic activities for H2 production as compared with the parental UiO-66. However, in these MOF-based complexes, the UiO-66 was not responsible

Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, PR China. His research mainly focuses on the synthesis of MOFs, and the study of MOFs for phtocatalytic CO2 reduction and water splitting.

Xinchen Wang is a professor at the State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, PR China. He obtained his B.S. from Fuzhou University and his PhD from the Chinese University of Hong Kong. He was formerly a postdoctoral fellow at Tokyo University and an Alexander von Humboldt Fellow and group leader at the Max Planck Institute of Colloids and Interfaces, Germany. His interests are designing carbon nitrides and MOFs for photocatalytic water splitting, CO2 reduction, and organocatalysis.

for light absorption, that is, the MOF did not serve as a photocatalyst. Similar to the strategy of synthesizing NH2-UiO-66, Matsuoka et al. prepared amine-functionalized NH2-Ti-MOF, and reported it as a visible-light photocatalyst for hydrogen evolution with Pt nanoparticles and triethanolamine (TEOA) as the co-catalyst and sacrificial electron donor, respectively.[62] Under visible light irradiation, photogenerated electrons were produced from the excited BDC-NH2 group to the conduction band (CB) of the titanium-oxo cluster, following a linker-to-cluster charge-transfer (LCCT) mechanism. The photogenerated electrons then transferred to the Pt co-catalyst and reduced protons to from H2 (see Figure 2). In this work, the authors demonstrated that the Pt/NH2-ZrMOF (Pt/NH2-UiO-66) couldn’t catalyze hydrogen evolution under their experimental conditions, although it has excellent visible-light adsorption characteristics. The reason for this phenomenon is that the zirconium-oxo cluster has a more negative CB edge than that of the titanium-oxo cluster, so electron transfer from the excited organic ligand to the zirconium-oxo cluster is insufficient, leading to the inactivity of Pt/ NH2-Zr-MOF for catalyzing hydrogen evolution by visible light. The observations indicated that the CB potential of the metal-oxo cluster is of particular importance in the LCCT mechanism. The Figure 1. a) UV–vis spectra of UiO-66 and NH2-UiO-66 MOFs. b) Volume of hydrogen evolved (VH2) during the photocatalytic reactions using UiO-66 (䊏), UiO-66/Pt (ⵧ), Pt/NH2-Ti-MOF also showed photocataNH2-UiO-66 (䊉) and NH2-UiO-66/Pt (䊊). Reproduced with permission.[59] Copyright 2010, lytic activity for selective organosynthesis, like the reduction of nitrobenzene.[63] Wiley-VCH.

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Figure 2. Schematic illustration of photocatalytic hydrogen production reaction over Pt-supported NH2-Ti-MOF on the basis of the LCCT mechanism. Reproduced with permission.[62] Copyright 2012, American Chemical Society.

Figure 3. Proposed photocatalytic reactions using Al-PMOF in the presence of MV (I) or in the absence of MV (II). Reprinted with permission.[66] Copyright 2012, Wiley-VCH.

MV+ radical cation by electron transfer from the porphyrin, while the positive charge remained on the porphyrin unit oxidized EDTA to organic decomposition products. Finally, a proton was reduced to hydrogen on the Pt nanoparticles by the electron transferred from MV+, thus ending the catalytic cycle. However, in this reaction scenario, only a small amount of H2 was evolved under visible-light irradiation for 15 h. The author ascribed the low reaction activity to the limited diffusion of the large MV2+ through the MOF channels, causing inefficient electron transfer to the Pt co-catalyst. This conclusion was supported by the fact that once MV2+ was removed from the system (Figure 3II), H2 evolution was increased by one order of magnitude because, in this case, the excited porphyrin molecules in the MOF reacted directly with EDTA to form the reduced porphyrin which, in turn, transferred electrons to Pt nanoparticles. The heterogeneous nature of the photocatalytic activity was confirmed by the supernatant test. In 2013, Xu et al. synthesized a bifunctional MOF (PtMOF-253) by a post-synthetic modification strategy to immobilize a platinum complex in the 2,2-bipyridine-based microporous MOF-253.[67] After the immobilization of Pt in the MOF, a visible absorption band (ca. 410 nm) and obvious red-shift in the adsorption edge were observed (Figure 4a). The low-energy absorption in Pt-MOF-253 is attributed to the metal-to-ligand (PtII → bipyridine π*) charge transfer

Additionally, amine-functionalized MIL-101(Cr) (MIL = Matériaux de l’Institut Lavoisier) was also reported as a durable photocatalyst for hydrogen production from water by Yamashita’s group.[64] Recently, using the bis(4′-(4-carboxyphenyl)-terpyridine)Ru(II) complex as the bridging ligand, Matsuoka and coworkers further synthesized a Ru-incorporated Ti-based MOF Ti-MOF-Ru(tpy)2.[65] The Ru K-edge X-ray absorption fine structure (XAFS) characterization indicated that the local structure of the Ru complex was well preserved during the formation of the MOF structures. With TEOA as the sacrificial electron donor, Ti-MOF-Ru(tpy)2 photocatalyzed the H2 generation from an organic aqueous solution under visible light irradiation, and enhanced catalytic activity was observed when Pt nanoparticles were deposited as a co-catalyst. Because of the wider visible-light absorption band of the Ru(tpy)2 complex, the available wavelength region for the Ti-MOF-Ru(tpy)2-catalyzed water photosplitting system was expanded up to 620 nm. The structural stability of the MOF photocatalyst was validated by X-ray diffraction (XRD) after the photocatalytic reactions. This work demonstrated that the light adsorption behavior of MOF materials could be rationally controlled by incorporating appropriate light-harvesting components into the framework of MOFs, virtually allowing a wide choice of chemical protocols to engineer MOFs as photofunctional materials. Using porphyrins as functional organic ligands, Rosseinsky’ group in 2012 reported two water-stable MOFs (Al-PMOF: H2TCPP[AlOH]2·3DMF·2H2O, H4TCPP = meso-tetra(4-carboxy-phenyl)porphyrin, and Al/Zn-PMOF: Zn0.986(12)TCPP[AlOH]2) as photocatalysts for H2 evolution with visible light irradiation in the presence of N,N′-dimethyl-4,4’-bipyridinium (MV2+, electron acceptor/mediator), ethylenediamine tetraacetic acid (EDTA, sacrificial electron donor), and Pt (co-catalyst).[66] In the reaction system with MV as an elecFigure 4. a) UV–vis spectra of MOF-253, Pt-MOF-253, and Pt(bpydc)Cl2 as well as the quantum tron acceptor/mediator (Figure 3I), the efficiencies of hydrogen evolution for Pt-MOF-253 at different wavelengths. The inset shows Al-PMOF absorbed visible light and gen- the colors of the samples. b) Proposed reaction mechanism for photocatalytic H evolution 2 erated electronic excited species, and then over Pt-MOF-253 under visible-light irradiation. Reproduced with permission.[67] Copyright the methyl viologen was reduced to an 2013, Royal Society of Chemistry.



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(MLCT) transition. The resultant Pt-MOF-253 showed significantly improved photocatalytic activity for hydrogen evolution compared to the parental MOF-253 under visible-light irradiation with TEOA as a sacrificial electron donor. Controlled experiments showed that the Pt(bpydc)Cl2 (bpydc = 2,2′-bipyridine-5,5′-dicar-boxylic acid) complex also exhibited activity for hydrogen evolution, but that H2 production was 4.7 times lower than that of Pt-MOF-253. The interactions of Pt…Pt pairs separated by only a short distance, the more efficient electron transfer within the porous framework, and the reduced decomposition rate of the anchored Pt(bpy)Cl2 complex within the MOF are the main contributors to the promoted water-splitting activity of the Pt-MOF253 photocatalyst. A reaction mechanism was proposed for the photocatalytic H2 evolution over Pt-MOF-253 under visible-light irradiation (Figure 4b). The drawback of the Pt-MOF-253 photocatalyst for visible-light-driven hydrogen evolution is its obvious deactivation after repeated operations (H2 production in the second use is half-reduced). 3.1.2. MOFs as Photocatalysts for Water Oxidation to Produce O2 Water oxidation, the other half reaction of water splitting, was also examined by MOF photocatalysts, although the demonstrations are relatively fewer than that of water reduction for hydrogen evolution,[68,69] mainly as most MOFs cannot tolerate the severe water oxidation conditions required. In 2011, Lin and co-workers first reported water oxidation photocatalysis by MOFs employing cerium ammonium nitrate as an oxidant.[70] They doped three noblemetal-based water oxidation catalysts, [Cp*IrIII(dcppy) Cl], [Cp*IrIII(dcbpy)Cl]Cl, and [IrIII (dcppy)2(H2O)2]OTf, (dcppy = 2-phenylpyridine-5,4′-dicarboxylic acid; dcbpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) into the framework of UiO-67. The resultant three Ir-doped MOFs exhibited much lower catalytic performance for oxygen evolution compared with their homogeneous counterparts. The authors attributed this observation to the small open channels of the MOFs, which caused the inaccessibility of the cerium (IV) oxidant. In the next year, Lin’s group further advanced two other Zr-based MOFs with larger channels using two elongated Ir-based dicarboxylate ligands.[71] Doped with two Ir-based active moieties, the two MOFs showed promoted cerium (IV)-driven water oxidation activity. However, the resulting MOFs were unstable under the water oxidation conditions, and partial decomposition of the iridium complexes was observed. What’s more, the use of Ir-based complexes was not economic or environmentally friendly. Therefore, the development of stable and cost-affordable MOFs as photocatalysts without extra functional components for water oxidation is highly recommended. Additionally, recent years have also witnessed water oxidation electrocatalysis by MOFs.[72,73] For instance, Wang and co-workers reported electrocatalytic water oxidation by a cobalt-containing zeolitic imidazolate framework, Co-ZIF9.[74] Density functional theory (DFT) calculations revealed that the water molecule could be feasibly activated by Co-ZIF-9 with low activation barriers by bonding the OH group to the cobalt centers, while leaving the eliminated small 2015, DOI: 10.1002/smll.201500084

proton to be accepted by the adjacent benzimidazolate motifs, which promised effective Co-ZIF-9 operation for the electrochemical oxygen evolution reaction. Although photocatalytic water oxidation was not demonstrated using Co-ZIF-9 as the electrocatalyst, this work made an important step in water oxidation reactions by translating the synergic catalysis of redox-active metal centers and organic motifs into a defined MOF structure.

3.2. MOFs as Photocatalysts for CO2 Reduction The conversion of CO2 into valuable chemicals/fuels by solar energy is the most hopeful method for simultaneously reducing the green-house effect and relieving the energyshortage pressure in a sustainable scenario. MOFs hold great potential for photocatalytic CO2 conversion, because MOFs have been extensively studied for CO2 capture and adsorption.[75,76] MOFs as photocatalysts for the catalytic conversion of CO2 have attracted considerable research interest in the past few years. For instance, by incorporating the molecular CO2 reduction catalyst ReI(CO)3(5,5′-dcbpy)Cl into the framework of UiO-67, Lin’s group prepared a photocatalytic MOF Zr6(µ3-O)4(µ3-OH)4(bpdc)5.83(L8)0.17 [bpdc = 5,5′-biphenyldicarboxylate; H2L8 = ReI(CO)3(5,5′-dcbpy)Cl], and reported its catalytic performance for the reduction of CO2 to CO under visible-light irradiation in MeCN solution with triethylamine (TEA) as a sacrificial electron donor.[70] A CO-TON (TON = turnover number) of 10.9 was established for the MOF photocatalyst for the 20 h reaction. The catalytic activity for CO2 photoreduction was solely attributed to the [ReI(dcbpy)(CO)3Cl] moiety, because the UiO-67 was inactive towards the catalytic CO2 conversion reaction. However, the MOF catalyst was unstable in the reaction system, and 43.6% of the Re leaked into the reaction solution after 20 h photocatalytic operation, as identified by inductively coupled plasma mass spectrometry (ICP-MS) measurements. In 2012, Li and co-workers presented the photocatalytic reduction of CO2 to HCOO− by an amine-functionalized photoactive MOF NH2-MIL-125(Ti),[20] which was previously reported by Latroche and co-workers for H2 adsorption.[77] Compared with MIL-125(Ti),[78] after the amino functionality, NH2-MIL-125(Ti) displayed a visible adsorption band extending to 550 nm (Figure 5I) and enhanced CO2 adsorption capacity (Figure 5II), which endows it with improved visible-light photocatalytic activity. Upon photoirradiation for 10 h in acetonitrile (MeCN) with TEOA as a sacrificial electron donor, the accumulated HCOO− formation reached 8.14 µmol (Figure 5III) for the NH2-MIL-125(Ti)-catalyzed CO2 conversion system. Controlled experiments showed that, under otherwise similar conditions, the parent MIL-125(Ti) was unable to catalyze CO2 reduction, which indicated that the visible-light-driven photocatalytic CO2 reduction over NH2-MIL-125(Ti) catalyst is actually induced by the amino functionality. A reaction mechanism for photocatalytic CO2 reduction over NH2-MIL-125(Ti) was proposed (Figure 5IV). Under these photocatlytic CO2 reduction conditions, the excited charge separation process happened and an electron was transferred from the organic linker to Ti4+, leading to the



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Figure 5. I) UV–vis spectra of a) MIL-125(Ti) and b) NH2-MIL-125(Ti). II) CO2 adsorption isotherms a) MIL-125(Ti) (ⵧ) and b) NH2-MIL-125(Ti) (䊏). III)The amount of HCOO− produced as a function of the irradiation time over a) NH2-MIL-125(Ti)) (䊏), b) MIL-125(Ti) (ⵧ), c) a mixture of TiO2 and H2ATA (䊊), and d) visible-light irradiation without a sample (䉱). IV) Proposed mechanism for photocatalytic CO2 reduction over NH2-MIL-125(Ti) under visible-light irradiation. Reproduced with permission.[20] Copyright 2012, Wiley-VCH.

formation of Ti3+, which was able to reduce CO2 to HCOO−. The photocatalytic cycle was completed with TEOA as an electron donor. The stability of NH2-MIL-125(Ti) in the reaction system was confirmed by various characterizations. In addition, Li et al. also reported that the amine-functionalized NH2-UiO-66 showed photocatalytic activity for CO2 reduction under otherwise similar reaction conditions.[79] However, the catalytic activity of NH2-MIL-125(Ti) for CO2 photoreduction catalysis is rather moderate, and further actions to enhance the catalytic efficiency are highly desired. Very recently, Li’s group further reported a series of Febased MOFs (MIL-101(Fe), MIL-53(Fe), MIL-88B(Fe), and their amino-functionalized derivatives) as active photocatalytic materials for reducing CO2 to HCOO− under visible-light irradiation.[80] Fe-based MOFs are particularly intriguing as Fe is a low-cost, but photocatalytically and catalytically active metal, as well as being visible-light responsive. As shown in Table 1 (entries 1, 2, and 3), all three Fe-containing MOFs exhibited considerable visible-light-driven

photocatalytic activities for the reduction of CO2 to HCOO− with TEOA as the electron donor, and MIL-101(Fe) showed the best catalytic performance, due to having coordinatively unsaturated Fe sites in the framework. Their photocatalytic performances originated from the light excitation of the Fe–O super-clusters in these Fe-based MOFs that induced electron transfer from O2− to Fe3+, thus forming Fe2+, to reduce CO2 to formate. After amine functionalization, the three modified MOFs displayed remarkably improved CO2 reduction activities (entries 3, 4, and 5 in Table 1), because of the dual excitation pathways, that is, the light harvesting of the NH2 functionality to transfer an electron form the organic ligand to the Fe–O clusters, to produce Fe2+, to reduce CO2 following the LCCT mechanism, and the direct light excitation of the Fe–O cluster to contribute the observed enhanced CO2 reduction performance (Figure 6). Although sacrificial electron agents are still involved in these MOF-based CO2 photoconversion systems, this work demonstrated great opportunities for MOFs, especially these made of earth-abundant metals,

Table 1. Comparison of the CO2 adsorption and the amount of HCOO− produced over MIL-101(Fe), MIL-53(Fe), MIL-88B(Fe), and their aminofunctionalized derivatives. Photocatalyst

Produced HCOO− (µmol)

CO2 adsorption (cm3 g−1)

1

MIL-101(Fe)

59.0

26.4

2

MIL-53(Fe)

29.7

13.5

Entry

3

MIL-88B(Fe)

9.0

10.4

4

NH2-MIL-101(Fe)

178

34.0

5

NH2-MIL-53(Fe)

46.5

20.0

6

NH2-MIL-88B(Fe)

30.0

14.4


Figure 6. Schematic illustration of the dual excitation pathways over amino-functionalized Fe-based MOFs. Reproduced with permission.[80] Copyright 2014, American Chemical Society.


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as photocatalysts for the conversion of CO2 in artificial photosynthesis, promising the design of MOFs based on inexpensive elements for CO2 capture and conversion with solar energy. 3.3. MOFs as Photocatalysts for Organosynthesis The utilization of MOFs as photocatalysts for selective organic photocatalysis is of great importance towards the devel- Figure 7. I) Synthetic procedure for the 3D CR–BPY1 MOF composed of wavy Cu–BPY opment of solar-energy-based “green” sheets and [SiW11O40Ru]7− anions, showing the combination of the dual catalytic units and organic synthesis, but it also holds great channels for chemical transformation. II) Crystal structure of CR–BPY1 with the 3D framework linking of the wavy 2D sheets (drawn as stick-ball models) and challenges, as only elaborate manage- generated by the covalent the [SiW11O39Ru(H2O)]5− (drawn as polyhedra) by CuII–O–W(Ru) bridges, showing the ment of the oxidation and/or reduction interpenetration of two symmetric frameworks. Reproduced with permission.[87] Copyright ability of the excited state could lead to 2014, Royal Society of Chemistry. the high selectivity of target products. The presentation of MOFs for photocatalytic organic selective transformations has attracted the [SiW11O39Ru(H2O)]5- photocatalyst and Cu catalyst considerable research attention.[81,83] For instance, Lin and in one single metal–organic architecture (Figure 7I). The co-workers reported UiO-type MOFs as photocatalysts for CR-BPY1 showed high photocatalytic activity for the C–C aza-Henry reactions, aerobic amine coupling reactions, and coupling reaction of the N-phenyl-tetrahydroisoquinoline thioanisole oxidation reactions.[70] The MOFs displayed and nitromethane with a yield of 90% under fluorescent lamp good stabilities and high selectivity of the products was irradiation for 24 h. The oxidative coupling C–C bond formaobserved, however, the involvement of noble metals (Ru, tion reaction found excellent size-selectivity, thus reflecting Ir) in the MOFs seriously limited their large-scale appli- the fact that photocatalytic reactions were taking place in the cation. Xie et al. in 2011 constructed a 3D MOF using a channels of the MOF. The work demonstrated the example of photoactive tin porphyrin as the bridging ligand, and devel- MOF photocatalysts as merged dual catalysts to synergistioped the resulting SnIV–MOF as efficient photocatalyst to cally operate selective organic photosynthesis. activate molecular oxygen for the oxygenation of phenol and In 2012, Long and co-workers reported amine-functionsulfides with excellent yields.[84] The Sn–MOF showed good alized zirconium metal–organic framework NH2-UiO-66 activity stability after repeatedly operating the oxygenation as an efficient photocatalytst for the selective aerobic reaction for several runs. oxygenation of alcohols, olefins, and cyclic alkanes using In 2012, Duan et al. concurrently incorporated a ste- molecular oxygen as the oxidant under visible-light irradiareoselective organocatalyst [l or D-pyrrolidine-2-yl-imida- tion.[88] They found that the photocatalytic organic transforzole (PYI)] and a photoactive unit (triphenylamine) into mations could effectively operate in various reaction media. a single MOF, and two enantiomeric MOFs, Zn−PYI1 The 18O-labelled isotropic experiment validated that the O and Zn−PYI2, were thus fabricated.[85] The two MOFs atom, in the product of cyclooctene epoxidation, originated proved to be efficient photocatalysts for the asymmetric from molecular O2. To get insight into the reaction mechaα-alkylation of aliphatic aldehydes. Upon light excitation, nism of the photocatalytic selective organic transformations, the triphenylamaine moiety within these MOFs induced the authors carried out electron paramagnetic resonance electron transfer, and then produced an active intermediate (EPR) measurements to monitor the generated active spefor the photocatalytic α-alkylation reaction. By the coop- cies during the reaction process, and the results revealed that eration of the chiral PYI moieties serving as organocata- the superoxide radical (O2−·) is the active intermediate in the lytic active sites, remarkable stereoselectivity was achieved photocatalytic reactions (Figure 8I,II). Thus, they proposed a for asymmetric photocatalysis. Well-designed experiments reaction mechanism based on photogenerated electron transrevealed that the excellent enantioselectivity of the asym- port for the organic photosynthetic reactions (Figure 8III). metric α-alkylation was due to the integration of both Upon light irradiation, electrons on the highest occupied photocatalyst and asymmetric organocatalyst into one molecular orbital (HOMO) composed of O, C, and N 2p single MOF. Perhaps, the capability of MOF materials to orbitals, jump to the lowest unoccupied molecular orbital photocatalyze asymmetric organosynthesis demonstrated (LUMO), and transfer to O2 molecules adsorbed on the Zr3+ the best advantage of such microporous metal–ligand net- sites to form O2−·, while the photogenerated holes oxidize works over traditional inorganic and even polymeric semi- the organic reactive substrates adsorbed on the amine sites to carbonium ions. The formed superoxide radicals further conductor photocatalysts.[86] Very recently, Duan and co-workers fabricated react with carbocations, and thus finally generate the proda 3D CR-BPY1 MOF using 4,4’-bipyridine (BPY), ucts. This work presented the first application of NH2-UiO-66 Cu(NO3)2·3H2O, and K5[SiW11O39Ru(H2O)]·10H2O as the as a simple, yet efficient photocatalyst for the aerobic oxiprecursors.[87] The thus-obtained CR-BPY1 MOF combined dation of organic compounds. Following this work, various small 2015, DOI: 10.1002/smll.201500084



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Figure 8. I) EPR spectra of NH2-UiO-66 dispersed in a) methanol, b) trifluorotoluene, c) acetonitrile, and d) benzyl alcohol without light irradiation. II) The intensity of the EPR line at g = 2.009 as a function of irradiation time and the decay upon ‘light off’. III) The proposed photocatalytic mechanism for organic transformations over NH2-UiO-66 photocatalyst. Reproduced with permission.[86] Copyright 2012, Royal Society of Chemistry.

UiO-66-based MOFs were developed as multifunctional catalysts for organic transformation and chemical reduction applications by several research groups.[89–93] These works demonstrated the successful example of MOFs modified with functional components as efficient photocatalysts for selective organic photosynthesis.

4. MOFs as Co-catalysts for Photocatalysis Increasing research efforts have been devoted to the application of MOFs as photocatalysts for heterogeneous photocatalysis, however, successful demonstrations are still mainly confined to MOFs with Zr-carboxylate, Al-carboxylate, or Ti-carboxylate SBUs, due mainly to the intrinsic inactivity of many MOFs upon light excitation and the instability of various MOFs under photocatalytic conditions. Actually, there exists enormous opportunities for MOFs as co-catalysts for artificial photosynthesis, because numerous MOFs, especially the subclass of zeolitic imidazolate frameworks (ZIFs),[94,95] feature the combined catalytic characteristics of transition metal ions (e.g., Co, Zn, Mn) and functional organic ligands (e.g., imidazolate entities), together with high thermal and chemical stabilities.[96] Furthermore, the excellent CO2 capture and adsorption capacities of various MOF/ZIFs may also imply the potential of these MOFs as co-catalysts for artificial CO2 photoconversion.

4.1. MOFs as Co-catalysts for Photocatalytic Water Splitting In 2009, Mori and co-workers reported the first example of MOFs as a co-catalyst for the photochemical reduction of water under visible-light irradiation.[97] In their constructed photocatalytic H2 evolution system (Figure 9), the porous MOF [Ru2(pBDC)2]n (p-BDC = p-benzenedicarboxylate) is the co-catalyst, while Ru(bpy)32+ serves as a photosensitizer, together with MV2+ and EDTA-2Na acting as an electron relay and a sacrificial reductant, respectively. Under light irradiation for 4 h, the accumulated H2 evolution was


41 µmol, affording a catalytic TON of 8.16 with respect to the Ru-MOF and an apparent quantum yield of 4.82% at 450 nm was established. Following this work, Mori’s group further investigated a series of substituted Ru-MOFs and two RhMOFs as co-catalysts for the photocatalytic reduction of water to hydrogen in similar photochemical systems.[98,99] Ott and co-workers recently synthesized a photocatalytic MOF UiO-66-[FeFe](dcbdt)(CO)6 via the incorporation of a molecular proton reduction catalyst [FeFe](dcbdt)(CO)6 (1, dcbdt = 1,4-dicarboxylbenzene-2,3-dithiolate) into the framework of UiO-66 by the postsynthetic exchange (PSE) strategy (Figure 10a).[100] To confirm the well-developed incorporation of the intact Fe2S2 dinuclear cluster into the framework lattice, the authors conducted energy dispersive X-ray spectrum (EDX), nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR) and solid-state UV–vis spectroscopy measurements, and the results precluded the possibility of just trapping compound 1 in the pores of the MOF. The degree of PSE was determined to be ca. 14% as calculated by EDX, H1 NMR, and other characterizations. The resulting UiO-66-[FeFe](dcbdt) (CO)6 was proven to be an active co-catalyst to assist photochemical H2 evolution from water under visible-light irradiation with Ru(bpy)32+ and ascorbic acid as a photosensitizer and a sacrificial electron donor, respectively (Figure 10b). These works revealed the opportunities for MOFs as a co-catalyst for water photosplitting, but the involvement of the Ru as the photosensitizer is a big drawback of these photosynthetic systems toward their practical application, and the development of appropriate, cost-affordable, and

Figure 9. The reaction scheme of photochemical hydrogen production from water using Ru– MOFs in the presence of Ru(bpy)32+, MV2+, and EDTA–2Na. Reproduced with permission.[97] Copyright 2009, Royal Society of Chemistry. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

small 2015, DOI: 10.1002/smll.201500084


Figure 10. a) Schematic representation of postsynthetic exchange of 1 into UiO-66. b) Reaction scheme for the photocatalytic reduction of protons. Reproduced with permission.[100] Copyright 2013, American Chemical Society.

conversion reaction was conducted in the mixture of MeCN and water with [Ru(bpy)3]Cl2·6H2O as a photosensitizer and TEOA as a sacrificial electron donor. The reactant CO2 was identified as the source of the produced CO by 13C-labelled isotropic experiments (Figure 11c). The heterogeneous characteristic of the reaction was proven by the fact that, after Co-ZIF-9 co-coatalyst was filtrated, the supernatant was inactive for the CO2 reduction reaction under otherwise identical conditions. The authors elaborately designed a series of controlled experiments to disclose the origin of the high catalytic performance of Co-ZIF-9 for the photocatalytic reduction of CO2, and the results pointed out that the framework of Co-ZIF-9 plays vital role in CO2 photoreduction catalysis by promoting carrier transfer and CO2 concentration. The authors investigated the effect of the added amount of Co-ZIF-9 on the evolution of CO and H2 from the CO2 photoreduction system, and very interesting findings were observed. The production of CO first increased and then decreased as the amount of Co-ZIF-9 was gradually increased, while the evolution of H2 was exclusively increased in the examined region of the Co-ZIF-9 amount (Figure 11d). This result revealed the different reaction pathways for CO and H2

stable semiconductors as light harvesters may be the more promising alternative.

4.2. MOFs as Co-catalysts for Photocatalytic CO2 Conversion The utilization of MOFs as co-catalysts for photocatalytic CO2 conversion was recently reported by Wang and coworkers.[23] They developed Co-ZIF-9 as an efficient and stable MOF co-catalyst to promote the photosplitting of CO2 to CO with visible light under mild reaction conditions. Co-ZIF-9 adopts a porous crystalline structure composed of cobalt (II) ions linked to benzimidazolate (bIm) ligands, where the N atoms in the 1 and 3 positions of bIm are coordinated to the tetrahedral cobalt center with a Co-bIm-Co angle of 145° (Figure 11a,b), and thus Co-ZIF-9 merges the catalytic functions of cobalt and imidazolate motifs. The Co-ZIF-9 cocatalyzed photocataltyic CO2-to-CO small 2015, DOI: 10.1002/smll.201500084

Figure 11. a,b) Chemical structure of Co-ZIF-9. c) GC-MS (m/z = 29) analyses of the produced CO from the Co-ZIF-9 promoted photocatalytic 13CO2 reduction reaction. d) The effect of the amount of Co-ZIF-9 on the evolution of CO/H2 from the CO2 photoreduction system. Reproduced with permission.[23] Copyright 2014, Wiley-VCH.



9


formations. Electron transfer from the photosensitizer was the rate-determining step, as the total yield of CO/H2 in region II was almost unchanged, even with excessive Co-ZIF-9 added, and this conclusion was further confirmed by the studies of the wavelength dependence of CO/H2 evolution. In this work, Wang and co-workers carried out several experiments and characterizations to verify the stability of Co-ZIF-9 in the chemical system. Firstly, inductively coupled plasma (ICP) analysis indicated that only 0.5% cobalt ions were detected for the reaction supernatant after Co-ZIF-9 catalyst was filtered, reflecting the stability of Co-ZIF-9. Secondly, the activity stability test of the used Co-ZIF-9 in five repeated operations displayed no noticeable changes in the yield of CO/H2. Thirdly, FTIR, XRD, and X-ray photoelectron spectroscopy Figure 12. a) CO production from the Co-ZIF-9-catalyzed CO conversion system as a function 2 (XPS) measurements revealed that the of reaction time. Inset: Stability test of the CO2 photoreduction system. b) CO2 adsorption chemical, crystal, and surface structures isotherm of Co-ZIF-9 measured at 273 K. c) PL spectra of the reaction systems at 400 nm laser of the Co-ZIF-9 sample were perfectly irradiation at room temperature. d) Transient photocurrent response of the reaction system [101] Copyright 2014, preserved. These results solidly con- under visible light irradiation (λ > 420 nm). Reproduced with permission. Royal Society of Chemistry. firmed the high stability of the Co-ZIF-9 co-catalyst in the established heterogeneous photoredox system. This work presented the first application adsorption reflected the flexibility of the framework. The of a MOF based on cobalt and benzimidazolate motifs as an function of Co-ZIF-9 for promoting electron transport was efficient and stable co-catalyst for the photocatalytic conver- evidenced by in-situ photoluminescence (PL) measurements (Figure 12c) and photocurrent characterizations (Figure 12d) sion of CO2 to CO under mild reaction conditions. Again, the use of the Ru-containing photosensitizer in the of the reaction system. Wang and co-workers further translated the co-catalytic Co-ZIF-9 co-catalyzed CO2 photoreduction system is still a big demerit and, to solve this problem, Wang and co-workers effect of Co-ZIF-9 to another semiconductor-mediated CO2 developed several semiconductor photocatalysts as light har- conversion photoredox catalysis reaction with solid CdS as vesters to co-operate with the Co-ZIF-9 co-catalyst to con- the light-absorbing component.[110] Benefiting from the high struct stable and noble-metal-free CO2 photofixation systems. visible light photocatalytic activity of CdS, the Co-ZIF-9 For instance, a metal-free conjugated carbon nitride polymer and CdS co-catalyzed CO2 photoreduction system attained semiconductor was first employed.[101] Conjugated g-C3N4, a high apparent quantum yield (AQY) of 1.93% at 420 nm which holds high thermal stability and extreme chemical sta- monochromatic irradiation. The main drawback in such MOF bility, was extensively studied as a solar energy transducer and semiconductor co-catalyzed CO2 reduction systems is for water photosplitting,[102–104] organic photosynthesis,[105,106] the involvement of the bpy serving as an assistant electron and CO2 photofixation.[107–109] The semiconductor-mediated mediator. Nonetheless, these works demonstrated the great CO2 photochemical reduction system exhibited high catalytic opportunities of MOFs as co-catalysts for heterogeneous stability (Figure 12a). Linear increases in CO formation were photocatalysis by co-operating with semiconductor photoachieved in the initial 2 h period but, thereafter, the evolu- catalysts as light harvesters, and consequently getting rid of tion rate of CO was gradually decreased. The authors attrib- the usually involved, expensive, and unstable noble metaluted the decrease in CO-liberating-rate to the depletion/ containing photosensitizers (see Table 2), which is normally a degradation of CO2 and bpy in the system, but the Co-ZIF-9 big obstacle that limits the practical application of MOFs in co-catalyst and the g-C3N4 photocatalyst were able to keep solar-to-chemical energy conversion. In 2013, Wang and co-workers grew ZIF-8 nanoparticiles their intrinsic catalytic function, which was confirmed by the catalytic activity stability test of the system for seven on Zn2GeO4 nanorods to serve as a co-catalyst to enhance repeated operations (inset, Figure 12a). To demonstrate the the photocatalytic activity of Zn2GeO4 for converting CO2 characteristics of Co-ZIF-9 for the capture and concentration into CH3OH in aqueous solution.[111] The resulting ZIF-8/ of CO2, the authors investigated the CO2 absorbing behavior Zn2GeO4 composite exhibited higher photocatalytic activity of the MOF (Figure 12b). The results revealed a high CO2 for CO2 photofixation compared to bare Zn2GeO4 nanorods. uptake capacity of 2.7 mmol g−1 for the Co-ZIF-9 co-catalyst, The authors attributed the promoted photocatalytic perforand a step change with an important hysteresis during CO2 mance of ZIF-8/Zn2GeO4 to the strong CO2 adsorption ability



small 2015, DOI: 10.1002/smll.201500084

www.MaterialsViews.com Table 2. Comparison of the MOFs and the photosensitizers in MOF-related photocatalysis. MOF

Photosensitizers

Photocatalytic reactions

Products

Ref.

[Ru2(p-BDC)2]n

Ru(bpy)32+

Water splitting

H2

[97]

[Ru2(p-BDC)2X]n

Ru(bpy)32+

Water splitting

H2

[98]

[Rh2(p-BDC)2]n

Ru(bpy)32+

Water splitting

H2

[99] [100]

2+

UiO-66-[FeFe](dcbdt)(CO)6

Ru(bpy)3

Water splitting

H2

Co-ZIF-9

Ru(bpy)32+

CO2 and water reduction

CO, H2

[23]

Co-ZIF-9

g-C3N4


CO, H2

[101]

Co-ZIF-9

CdS


CO, H2

[110]

together with the better light response of ZIF-8. Recently, Xiong and co-workers synthesized a MOF@semiconductor core@shell structure by coating a nanocrystal TiO2 shell onto Cu3(BTC)2 (HKUST-1,[112] BTC = benzene-1,3,5-tricarboxylate) core.[113] The fabricated Cu3(BTC)2@TiO2 hybrid catalyst showed a five-fold enhancement in photocatalytic activity for the reduction of gaseous CO2 to CH4, as well as significantly promoted selectivity of CH4 compared with the pristine TiO2 photocatalyst. Ultrafast spectroscopy characterization indicated that the photogenerated electrons produced from the photoexcitation of the TiO2 shell could effectively transfer to the Cu3(BTC)2 core, improving the separation of photogenerated electron–hole pairs on TiO2, and providing energetic electrons to reduce the CO2 molecules absorbed on the Cu3(BTC)2 co-catalyst. Although the enhanced catalytic performance for CO2 photoreduction reactions were achieved for ZIF-8/Zn2GeO4 and Cu3(BTC)2@TiO2 with respect to the parental counterparts (Zn2GeO4 and TiO2), the absolute catalytic activities of the developed CO2 transformation systems were still very low and, importantly, the carbon source of the generated reduction products (CH3OH and CH4) were not validated by 13CO2 isotropic experiments.

5. MOFs as Hosts for Photocatalysis Benefiting from the intrinsic channels and/or cavities as well as the decorative organic ligands, MOFs are readily functionalized with catalytic active sites and/or photoredox components during synthesis or through post-synthetic modifications, which renders them attractive hosts for heterogeneous photocatalysis.[114–117]

5.1. MOFs as Hosts for Water Photosplitting Lin and co-workers reported in 2012 Pt nanoparticle@photoactive MOF assemblies for H2 evolution by visible light with TEA as a sacrificial reductant.[118] The phosphorescent UiO-type MOF hosts were prepared using molecular photosensitizers ([Ir(ppy)2(bpy)]Cl-derived dicarboxylic acids) as the bridging ligands. Under visible-light irradiation, the excited iridium(III)-based molecular photosensitizer induced the photodeposition of Pt nanoparticles in the channels of photoactive MOFs. One of the Pt nanoparticle@MOF assemblies exhibited a high catalytic TON of 7000 for H2 evolution, which is about five times the value of the corresponding small 2015, DOI: 10.1002/smll.201500084

homogeneous control. The authors believed the high catalytic activity of these assemblies was attributed to the vicinity of the iridium-based photosensitizer and the Pt co-catalyst that favored electron transfer between the two components. However, the Pt nanoparticle@MOF assemblies were unstable in the reaction system, as 25.6% of the Ir leached into the solution during 48 h photocatalytic reaction, as determined by ICP-MS measurements. Recently, Feng’s group incorporated a biomimetic di-ion complex (2, [(í-SCH2)2NC(O)C5H4N]–[Fe2(CO)6] into the highly robust zirconium–porphyrin-based metal–organic framework (ZrPF).[119] BET and FTIR spectroscopy validated the incorporation of dinuclear complex 2 into the ZrPF structure, and the degree of incorporation (≈25%) was determined by EDX and ICP. The synthesized [Fe2S2]@ ZrPF showed enhanced H2 evolution performance compared to the pure homogeneous complex 2 with acidic solutions as the sacrificial electron donor under visible-light irradiation. In the H2 generation system, [Fe2S2]@ZrPF served as both a photosensitizer and a proton reduction catalyst, thus ruling out the involvement of extra light harvesters. However, the total H2 production catalyzed by the [Fe2S2]@ZrPF was actually very low (ca. 4 µmol after 3 h reaction). Very recently, Gascon and co-workers encapsulated the photoactive MOF NH2-MIL-125(Ti) in a well-defined cobaloxime proton reduction catalyst via a synthetic strategy.[120] The fabricated Co@MOF composite material was applied as a non-noble metal photocatalyst for H2 evolution from the mixture of acetonitrile and water with triethylamine as the sacrificial electron donor under light irradiation (λ > 408 nm). Controlled experiments revealed that no H2 was generated using cobaloxime as a homogeneous catalyst, and NH2-MIL125(Ti) only exhibited moderate catalytic performance for hydrogen evolution. The Co@MOF, however, showed markedly enhanced photocatalytic activity with a more than 20-fold higher H2 generation rate than the parental NH2MIL-125(Ti). Furthermore, the Co@MOF displayed a high stability in the catalytic system, giving a constant turnover frequency (TOF) of 0.8 h−1 after operating for 65 h. Although the exact structure of the cobalt species was not unveiled, this work demonstrated the co-operation of a photoactive MOF host with a catalytically active guest for improved photocatalysis. To use MOFs as hosts for photocastalytic oxygen evolution, Das and co-workers reported in 2013 the encapsulation of a molecular water-oxidation catalyst MnTD



11


Figure 13. Comparison of catalysis with MnTD molecules alone or after encapsulation within the MOF, Cr-MIL-101, resulting in the MnTD∈Cr-MIL-101 construct, leading to sustained water oxidation catalysis. Reproduced with permission.[121] Copyright 2013, Wiley-VCH.

([(terpy)Mn(µ-O)2Mn][(terpy)]3+, terpy = 2,2′:6′,2′′-terpyriidine) within the well-defined pores of Cr-MIL-101 (Figure 13).[121] By elemental analysis, TGA experiments, and UV–vis quantification, the content of MnTD in the MnTD∈Cr-MIL-101 catalyst was determined to be ca. 10 wt%, which indicated that nearly each Cr-MIL-101 cage encapsulated one MnTD molecule. The authors wisely selected Cr-MIL-101 as the host, because the cage of Cr-MIL-101 is large enough to provide sufficient catalytic environments for the water oxidation reaction, but the apertures of the cages are very small, preventing the migration of the MnTD catalyst. The catalyst assembly showed slightly lower water oxidation activity at the initial reaction period, but significantly promoted catalytic stability compared with the molecular MnTD catalyst (more than 20-fold-enhanced TON). Sustained water oxidation photocatalysis by the MnTD∈Cr-MIL-101 assembly was heterogeneous, but the MnTD catalyst operated the reaction homogeneously in the pores of the Cr-MIL-101 host. This work provides a novel strategy of utilizing MOFs as versatile hosts to encage unstable molecular catalysts to enhance their photocatalytic performance.

found that, after doping with the noble metal co-catalysts (Pt and Au), both HCOO− and H2 were produced over the M/NH2-MIL-125(Ti) catalysts, while the bare NH2MIL-125(Ti) only catalyzed the production of HCOO− (Figure 14a), that is, the doping of Pt and Au enhanced the photocatalytic hydrogen generation. With respect to co-catalyzing CO2 to HCOO−, the Pt and Au co-catalysts displayed different catalytic functions. An increase in HCOO− formation was achieved by the Pt/NH2-MIL-125(Ti) compared with the pure MOF, but a decrease in HCOO− generation was observed over the Au/NH2-MIL-125(Ti) (see Figure 14b). To elucidate the source of the different co-catalytic functions of the noble metals on NH2-MIL-125(Ti), the authors conducted electron spin resonance (ESR) analysis and DFT calculations on M/NH2-MIL-125(Ti). The results demonstrated that, on Pt/NH2-MIL-125(Ti), the H2 could spill over from the Pt nanoparticles to the bridging oxygen linked to Ti atoms, leading to the generation of Ti3+ and improved photocatalytic activity for the reduction of CO2 to HCOO−. However, with respect to Au/NH2-MIL-125(Ti), it was difficult for the hydrogen to spill over from the Au co-catalyst to the framework of NH2-MIL-125(Ti). This work provided important information on designing new noble metaldoped photocatalysts using MOFs as the hosts for artificial photosynthesis.

5.3. MOFs as Hosts for Organic Photosynthesis Recently, Duan and co-workers controllably fabricated single- or multicore–shell Au@ZIF-8 nanostructures by epitaxial growth or coalescence of nuclei with PVP-Au nanoparticles as the nucleation seeds (Figure 15).[124] After incorporated Au nanoparticles, light absorption by the singleand multicore Au-ZIF-8 is observed at 530 and 540 nm, respectively, induced by the Au localized surface plasmon resonance (LSPR) effect. Therefore, the Au@ZIF-8 assemblies exhibited visible-light photocatalytic activities for the selective oxidation of benzyl alcohol to benzaldehyde. However, controlled experiments showed that the catalytic performance of the Au@ZIF-8 was lower than Au-SiO2: the authors attributed this to the small pore size of ZIF-8, which prohibited the entrance of the benzyl alcohol to react with the Au nanoparticles. This work implied that, with suitable noble-metal nanoparticle modifications, MOFs not previously

5.2. MOFs as Hosts for CO2 Photofixation In semiconductor-mediated photocatalysis, doping the photocatalysts with metals is commonly used to promote the reaction kinetics,[122] which has also been extended to MOF-based photocatalysis for CO2 fixation. Li and co-workers reported noble metal-doped M/NH2-MIL-125(Ti) (M = Pt and Au) photocatalysts for the photocatalytic reduction of CO2 to HCOO− with TEOA as a sacrificial electron donor under visible-light illumination.[123] They


Figure 14. The amount of product formed as a function of irradiation time over the as-prepared samples: a) hydrogen; b) HCOO−. Reproduced with permission.[123] Copyright 2014, Wiley-VCH. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

small 2015, DOI: 10.1002/smll.201500084


Figure 15. I) Schematic illustrating the encapsulation process of Au NPs with ZIF-8 to form single- or multicore–shell structures. II) TEM images of a) Au@ZIF-8 core–shell structures, and b) a typical single-core Au@ZIF-8 NP synthesized by method A (a typical preparation procedure); TEM images of Au@ZIF-8 NP in b) tilted by 20° and -20° along the x axis shown on the right of (c) and (d), respectively, corresponding to the schematic illustrations shown on the left of (c) and (d). III) TEM images of a,b) multi-core Au (15 nm)@ZIF-8 structures with different magnifications using method B (a time-dependent process). The detailed information of methods A and B can be found in the original article. Reproduced with permission.[124] Copyright 2014, Royal Society of Chemistry.

used as photocatalysts may find a place in heterogeneous photoredox catalysis. Wu and co-workers in 2013 fabricated CdS-NH2-UiO-66 nanoassemblies by anchoring CdS nanorods on the surface of NH2-UiO-66 through a facile photodeposition method.[125] These semiconductor-MOF nanocomposites showed highly efficient photocatalytic activity for the selective oxidation of a series of benzylic alcohols with O2 as the oxidant, especially for p-methoxy benzyl alcohol (see Table 3). They believed

that the high specific surface area of the MOF host and electron transfer from CdS to the MOF are responsible for the considerable photocatalytic performance of the hybrid nanomaterials. To explore the photocatalytic performance of CdS-NH2-UiO-66, systematic characterizations were performed. Stability tests of the hybrid nanocomposites displayed no obvious activity changes after five cycles of repeated aerobic oxidation. Furthermore, a possible reaction mechanism based on photogenerated charge transfer was proposed for

Table 3. Results of photocatalytic aerobic oxidation of various alcohols on CdS-NH2–UiO-66. Entry

Conv./%

Sel./%

1

30.12

>99

2

32.17

98

3

63.2

95

4

32.25

95

5

21.5

>99

6

23.18

>99

small 2015, DOI: 10.1002/smll.201500084

Substrate

Product



13

reviews www.MaterialsViews.com Table 4. Summaries of MOF–semiconductor-based materials/systems for photoredox catalysis. MOF–Semiconductor-based materials/systems MOFs

The roles of the MOF

Photocatalytic reactions

Products

Ref.

Semiconductors

Co-ZIF-9

g-C3N4

Co-catalyst

CO2 reduction

CO

[101]

Co-ZIF-9

CdS

Co-catalyst

CO2 reduction

CO

[110]

Zn2GeO4

Co-catalyst

CO2 reduction

CH3OH

[111]

HKUST-1

TiO2

Co-catalyst

CO2 reduction

CH4

[113]

NH2-UiO-66

CdS

Host

Oxidation of alcohols

Aldehydes

[125]

MIL-101(Fe)

CdS

Host

Oxidation of benzyl alcohol

Benzaldehyde

[126]

HKUST-1

TiO2

Host

Oxidation of alcohols

Aldehydes

[127]

ZIF-8

the selective organic conversions. Very recently, Zhu and coworkers synthesized CdS-MIL-101(Fe) nanocomposites and examined their photocatalytic activities for the oxidation of benzyl alcohol to benzaldehyde with visible light.[126] Additionally, similar organic photosyntheses were demonstrated by Morsali’s group with their semiconductor–MOF assemblies by the incorporation of amorphous TiO2 into an ordered mesoporous HKUST-1 host.[127] These works brought new chances to develop MOF–semiconductor-based materials/ systems for heterogeneous photocatalysis to operate solar-tochemical energy transformations (see Table 4).

water splitting and CO2 reduction would hold great promise to fulfill the crucial mission of artificial photosynthesis. In addition, cost-affordable and highly active semiconductor photocatalysts with appropriate redox abilities (e.g., C3N4, CdS, TiO2) to serve as light transducers for MOF-based photocatalytic systems should have considerable merits over the widely used noble-metal-containing photosensitizers.

Acknowledgements

6. Conclusion and Outlook In recent years, the development of MOFs in heterogeneous photocatalyses related to water-splitting, CO2 photoreduction, and organic photosynthesis, has been under intensive investigation due to the unique characteristics of MOFs. Based on the different roles that the MOFs play in the photochemical systems, we have clearly defined MOFs as photocatalysts, co-catalysts, or hosts for heterogeneous photocatalysis, and summarized the latest progresses of MOFbased solar-to-chemical energy conversion applications. Indeed, noticeable and impressive results have been achieved in MOF-related photoredox catalysis, but the research of MOF-mediated photocatalysis is still in its infancy, and there is great potential for further development. However, several problems wait to be addressed. Firstly, considering the large number of synthesized MOFs, the MOFs that number that has been utilized for photocatalysis is still very limited (e.g., UiOs, MILs, and ZIFs), and there are many other MOFs that contain redox active metals (e.g., Co, Ni, Mn, Fe) and/or functional organic linkers (e.g., imidazolate, porphyrin, pyridine) which remain to be explored for heterogeneous photocatalysis and artificial photosynthesis. Secondly, the catalytic efficiencies of most MOFs are moderate or even very low, and thus they need to be remarkably improved to satisfy the requirements of practical applications. Finally, most of the already-constructed photocatalytic systems involve sacrificial agents and/or noble metals, which conflict with sustainable development and artificial photosynthesis. We believe that the target-directed multicomponent MOFs that assembled suitable oxidative and reductive cocatalysts in a harmonious fashion to simultaneously promote


This work is financially supported by the National Basic Research Program of China (2013CB632405 and 2014CB260406), the National Natural Science Foundation of China (21425309 and 21173043), the State Key Laboratory of NBC Protection for Civilian (SKLNBC2013–04K), and the Specialized Research Fund for the Doctoral Program of Higher Education (20133514110003).

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