Metal–organic frameworks for solar energy conversion by photoredox catalysis

Coordination Chemistry Reviews 373 (2018) 83–115

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Metal–organic frameworks for solar energy conversion by photoredox catalysis Yuanxing Fang a, Yiwen Ma a, Meifang Zheng a, Pengju Yang a, Abdullah M. Asiri b, Xinchen Wang a,⇑ a b

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, PR China Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 30 April 2017 Received in revised form 19 August 2017 Accepted 12 September 2017 Available online 28 September 2017 Keywords: Metal organic framework Photocatalysis Water contamination degradation Water splitting CO2 conversion Organosynthesis

a b s t r a c t Metal organic frameworks (MOFs) have received increasing attention in the field of photoredox catalysis, mainly due to the advantages of the highly porous nanostructure and tunable semiconducting properties. Thus, the essential roles of photocatalysis by MOF can be conventionally optimized toward addressing the environment and energy issues. In this review, the fundamental of the photocatalytic MOF is initially discussed. A range of the solar-driven applications by photocatalytic MOF are highlighted, including water contamination degradation, water splitting, CO2 reduction and organosynthesis. In each chapter, a series of systematic investigations is presented, also involving a few recent results and unique phenomena that bestowed by the photocatalytic MOFs. The future prospects and challenges to use MOFs for each photocatalysis application are proposed. Ó 2017 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamental aspects of MOF as a photoredox catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Semiconducting properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Chemical and solar stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

84 85 85 85

Abbreviations: 1,4-biyb, 1,4-bis(imidazol-1-ylmethyl)benzene; 2,6-ndc, 2,6-naphthalenedicarboxylate; 2-cmsnH2, 2-carboxymethylsulfanyl nicotinic acid; 3-dpye, N,N0 bis(3-pyridinecarboxamide)-1,2-ethane; 4,40 -bpy, 4,40 -bipyridine; 4,40 -obb, 4,40 -oxybis(benzoate); AQY, apparent quantum yield; ATA, 2-aminoterephthalate; bdc, benzenedicarboxylate; bet, 1,10 -(20 -oxybis(ethane-2,1-diyl))bis(1,2,4-triazol-1-yl); bimb, 1,4-bis(imidazol-1-ylmethyl)benzene; BMAB, biphenyl-4,40 -diylbis-(methanylyli dene)bis(azanylylidene)dibenzoate; Boc, tert-butyloxycarbonyl; btec, 1,2,4,5-benzenetetracarboxylate; btc, benzene-1,3,5-tricarboxylate; btyb, 4-bis(1,2,4-triazol-1-ylmeth yl)-benzene; CALF, Calgary Framework; CEES, 2-chloroethyl ethyl sulfide; dcbdt, 1,4-dicarboxylbenzene-2,3-dithiolate; dcbpy, 2,20 -bipyridine-4,40 -dicarboxylate; dm-bim, 5,6-dimethylbenzimidazolate; DMF, dimethylformamide; DMNP, dimethyl 4-nitrophenyl phosphate; DMSO, dimethyl sulfoxide; dpbpdca, N4,N40 -di(pyridin-4-yl)biphenyl4,40 -dicarboxamide; dpe, 1,2-di(4-pyridyl)ethylene; dpppda, 1,4-N,N,N,N0 ,N0 -tetra(diphenylphosphanylmethyl)benzene diamine); dpsea, N,N’-di(3-pyridyl)sebacicdiamide; DTBP, 2,6-di-tert-butylphenol; EDTA, ethylenediaminetetraacetic acid; EDX, energy-dispersive X-ray spectroscopy; emim, 1-ethyl-3-methylimidazolium bromide; en, 1,2ethylenediamine; enMe, 1,2-diaminopropane; EPR, electron paramagnetic resonance; ErB, erythrosine B; g-C3N4, graphitic carbon nitride; H2bpydc, 2,20 -bipyridine-5,50 dicarboxylic acid; H2mna, 2-mercaptonicotinic acid; H2ox, Oxalic acid; H2sbdc, 4,40 -stilbenedicarboxylic acid; H2ttt, 1,3,5-triazine-2,4,6-triyltrithio-triacetic acid; H3dcpcpb, (3,5-dicarboxyl-phenyl)-(4-(20 -carboxyl-phenyl)-benzyl); H4abtc, 3,30 ,5,50 -azobenzene tetracarboxylic acid; H4L, 1,2-cyclohexanediamino-N,N0 -bis(3-methyl-5-carboxysali cylidene); H4TCPP, meso-tetra(4-carboxyphenyl)porphyrin; H4tkcomm, tetrakis [4-(carboxyphenyl)-oxamethyl]methane acid; Hcb-iso-p, 5-(4-carboxybenzyloxy)isophtha late; hfipbb, 4,40 -(hexafluoroisopropylidene)bis(benzoic acid); Hncp, 2-(4-carboxyphenyl)imidazo(4,5-f)(1,10)-phenanthroline; htpmb, hexakis(3-(1,2,4-triazol-4-yl)phe noxy-methyl)benzene; IPCE, incident photon conversion efficiency; LLCT, ligand-to-ligand charge transfer; llpd, 4-tolyl-2,20 :60 ,200 -terpyridine; LMCT, ligand-to-metal charge transfer; MB, Methyl Blue; MeCN, Acetonitrile; MHPR, maximal H2 production rate; MIL, Matériaux de I’Institut Lavoisier; MLCT, metal-to-ligand charge transfer; MO, Methyl Orange; MOF, metal organic framework; MoOF, metal oxide organic framework; MV2+, N,N0 -dimethyl-4,40 -bipyridinium; NNU, Northwest Nazarene University; pdbmb, 60 ,600 (2-phenylpyrimidine-4,6-diyl)-bis(6-methyl-2,20 -bipyridine); pdc, Pyridine-2,5-dicarboxylate; PEC, photoelectrochemical; PL, photoluminescence; POM, polyoxometalate; ppy, 2-phenylpyridine; PYI, Pyrrolidine-2-ylimidazole; RhB, Rhodamine B; RHE, Reversible hydrogen electrode; SnTPyP, 5,10,15,20-tetra(4-pyridyl)-tin(IV)-porphyrin; TCPP, tetrakis(4-carboxyphenyl)porphyrin; tdc, 2,5-thiophenedicarboxylate; TEM, transmission electron microscopy; TEOA, triethanolamine; THF, tetrahydrofuran; tkiymm, tetr akis(imidazol-1-ylmethyl)methane; TMAOH, tetramethylammonium hydroxide; tmbpt, 1-((1H-1,2,4-triazol-1-yl)methyl)-3,5-bis(4-pyridyl)-1,2,4-triazole; TON, turnover number; TOF, turnover frequency; UiO, University of Oslo; XPS, X-ray photoelectron spectroscopy; ZIF, Zeolitic imidazolate framework. ⇑ Corresponding author. E-mail address: [email protected] (X. Wang). https://doi.org/10.1016/j.ccr.2017.09.013 0010-8545/Ó 2017 Elsevier B.V. All rights reserved.

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3.

4.

5.

6. 7.

Y. Fang et al. / Coordination Chemistry Reviews 373 (2018) 83–115

2.3. Noble metals couple with MOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 2.4. Traditional semiconductors couple with MOF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Photodegradation of water contaminates by MOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 3.1. Photodegradation of aromatic molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 3.2. Photodegradation of organic dye degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.3. Photodegradation of other organic and inorganic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Photocatalytic water splitting by MOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 4.1. Photocatalytic hydrogen evolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 4.2. Photocatalytic oxygen evolution and overall water splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 4.3. Photoelectrochemical water splitting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Photocatalytic CO2 transformation by MOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 5.1. Reduction of CO2 to CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 5.2. Converting CO2 into organic chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Photoinduced organosynthesis by MOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

1. Introduction Photocatalysis is generally defined as catalytic reaction driven by solar energy [1]. This topic is becoming increasingly significant to address the essential problems in the fields of energy and environment, due to the realizations of the advantages, for instance, the renewability, cost-effectiveness and minimized side-effects [2,3]. The developments of the green photocatalysis were mainly focused on four working directions worldwide, including water toxicity degradation [4], water splitting [5], CO2 conversion [6] and organic synthesis [7]. In order to efficiently use the catalyst in such applications, a few common properties can be observed, for example, they should be stable in the general solvents, and can be tolerated under a strong light irradiation in air condition. Moreover, certain photocatalysis application still needs the catalyst to have distinct characteristics, for example, in terms of a powder photocatalyst for water splitting, in thermodynamic, it required their energy gaps that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are larger than water oxidative and reductive energy levels, respectively, which are 0 and 1.23 V (pH = 0) [8]. At the same time, under the considerations of the kinetic barrier and energy loss during the reaction, the corresponding energy levels are normally required to be as wide as 1.8– 2.0 V. In the other situation, if the solar-driven water splitting is achieved through photoelectrochemical (PEC) approach, in which, water oxidation and reduction is independent through photoanode and photocathode under electric input, it requires the anodic HOMO level and cathodic LUMO level to engulf the water redox potentials. In comparison, the needs of a photocatalyst for CO2 conversion are completely different. First, the thermodynamic energy required to reduce CO2 is in a wide range depending on the aiming products, whereas, in general, easily to be satisfied by a photocatalyst in small energy gap. In contrast, the high kinetic barrier with complicated multiple-electron processes indeed restricts the photoconversion efficiency, thus determining the conversion rate [9]. In the meantime, the chemical activity of the CO2 in the reacting medium is another factor to affect the photoefficiency, since the adsorption/activation is happened prior to the photoreactions. As such, inevitably, any family of material with unitary property cannot fully satisfy all the requirements. For example, metal oxide semiconductors are normally quite inert in both the chemical structure and electronic/optical property [10]. Therefore, they generally possess quite good stability during photoreactions, but the photoactivity becomes difficult to further optimize. Despite introducing foreign elements, the capability to optimize the electronic/optical property of the materials is still quite narrow [11]. In contrast, the photoactivity of organic semiconductor can be

readily tuned with a control of functional groups [12,13], however, in a typical one, the stability becomes a drawback hindering their developments [14]. As such, the family of metal organic frameworks (MOFs) is considered as a potential material consisting of advantages of both organic and inorganic semiconductors to overcome the systematic restrictions to achieve all these necessities, as it is possible to take features from both of them [15]. MOF is a family of materials consisting of organic ligands bridging metal clusters. The early excitements of MOFs were mainly due to the high surface area and large pore volume, and thus it was widely considered for the storage and separation of gas or liquid [16–18]. Upon these directions, achieving the semiconducting features in MOF opened a new avenue toward a wide range of applications, especially for photoredox catalysis [19]. In the structure of MOF, the HOMO of the organic ligand can be readily adjusted to maximize the solar energy harvesting, and the excited electron would be promoted to the certain location of the MOF, either on the metal node or on LUMO site of the organic linker [20]. The crystalline structure allows optimizing the migration distance of the photoexcited charge with controlled traveling distance and angles determined by the assembling units. In addition, the metal cluster could act as the structural center to strengthen the MOF structure. The integration of the optimized MOF with other functional materials also exhibits great advantages to approach the desired photocatalytic characteristics. For instance, with a conventional attachment of the semiconductor/metal nanoparticles into MOF, the aggregation of the nanomaterial can be minimized [21]. Also, more than one functions can be integrated with MOF by achieving the hybrid structures, for instance, the foreign materials can be functionalized as cocatalyst, sensitizer, or as extra energy gap to form heterostructure, or even enabled multiple-function by one specie [22–24]. This review will focus on the recent progresses in using MOF for photoredox catalysis [25–27]. The fundamental aspects in developing MOFs for efficient photocatalysts will be reviewed and discussed initially, ranging from the structural design and stability of MOFs to the formation of hybrid structures, such as integration with noble metals (NMs) and semiconducting materials. In subsequent, the photocatalytic applications of MOF will be summarized, including water purification, water splitting, CO2 reduction and organic synthesis. Although a few very good publications reviewed the progresses of this area, we aim to present more recent developments of the photoredox catalysis, and specify the challenge and opportunity in the future [25,27–30]. By virtue of the photocatalytic MOFs, the possibility of photocatalysis was extensively widening toward addressing the human problems by breaking the limitation from the traditional materials and technologies.

Y. Fang et al. / Coordination Chemistry Reviews 373 (2018) 83–115

2. Fundamental aspects of MOF as a photoredox catalyst In photocatalysis, the semiconducting property is essential to drive catalytic redox reactions. In this section, the principles to form HOMO–LUMO energy gap in MOFs are discussed firstly. The chemical and solar stability of the MOFs as photoredox catalysts were consequently discussed. Then, the fundamental designs of the hybrid structured MOFs are reviewed, with accompaniments of a brief introduction to the corresponding synthesis strategies, such as integrating MOF with NMs and other semiconductors.

2.1. Semiconducting properties The developments of the first photocatalytic MOF can be traced back to the late 20th century, when the ultra-stable MOF-5(Zn) was created [31], and its semiconductor behavior was investigated lately in 2007 [32]. Inspired from these developments, the investigations of semiconducting MOF for photocatalysis gradually caught attention, since the nature of highly porous structure potentially offers a great opportunity to improve the performance in photocatalysis [8,33,34]. For the traditionally periodicstructured semiconductors, the fundamental of photocatalysis is based on a band model, in which, valence and conduction bands are formed based on bonding and anti-bonding orbitals. The energy gap corresponds to the energy requirement for a quantum tunneling from the ground state to the excited state [35], and the transition energy is supplied by solar energy to approach the sustainable and the renewable goals [36]. While, in comparison, the semiconducting mechanism of MOF may be due to a few possibilities, highly relating to the inorganic and organic components, in the major cases, organic linker serves as antennas to absorb light initialized by p-electron delocalization. Also, in order to distinguish it to the periodic-structured semiconductors, the edges of the energy levels are generally described as organic way using HOMO and LUMO [28]. Among the MOFs, MIL-125(Ti) (MIL = Matériaux de I’Institut Lavoisier) can be one of the noble examples to explain the semiconducting mechanism [37]. In this MOF, the photoexcitation is initialized by delocalizing the pelectron at the site of O2 in the 2-aminoterephthalate (ATA) group, and the band is formed by further transferring the electron to the Ti-oxo cluster with the formation of Ti3+ by reducing Ti4+, namely ligand-to-metal charge-transfer (LMCT) band. On the basis of such understanding, a rational design of organic ligands is particularly important to adjust the photocatalytic ability of a MOF, for instance, inducing functional groups (–NH2, –OH, –CH3 or – Cl) in the organic linker could shift the band due to their donation of 2p electrons to the aromatic linker [38,39]. Indeed, if an amine group was induced to MIL-125(Ti) to form NH2-MIL-125(Ti), an extra LMCT band from amine group to the metal node could be constructed, and it results in an increase in visible light absorption [40]. In this system, the metal cluster node is another factor to affect the band structure [41], and if the metal nodes are at octahedral coordination sites with their unoccupied d-orbitals below the LUMO of the organic linker, then, the metal node would act as active position instead of the LUMO of organic linker. Therefore, if the reserved potential of the metal node can be increased, in other words, if the LUMO level can be more negative, the provided energy would be largely increased, and available to overcome the kinetic barrier in the following redox reactions [42]. Thus, a different metal node with same ligands would result in extensively different performance for photocatalysis [43]. A ligand-to-ligand charge transfer (LLCT) was also considered to form two energy bands, and contributing semiconducting property. Herein, photoexcitation can be initialized from the delocalization of the pelectron, and the electron in HOMO tunnels to the LUMO of the

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organic linker. Bi-mercaptonicotinic acid (Bi-mna) is an exemplar, and the covalent bonds coupled with Bi, involving BiAS, BiAN and BiAO, presented distinct characteristics for photocatalysis [44]. The excitation is induced from BiAS (HOMO band), and the transport to BiAN and BiAO (LUMO band) for a complete quantum tunneling [44], but it is not able to observe many MOFs as such. Generally, the semiconducting properties of the MOF are normally determined by the absorption and emission spectroscopies. Such as MOF-5(Zn), the photogenerated delocalized electrons were shown in microsecond timescale, and capability to reducing electron acceptor by laser flash photolysis [45]. In the investigation of PCN-222(Cr) (PCN = Porous coordination network), ultrafast transient absorption was accompanied by time-resolved photoluminescence (PL) to clarify the photoactivity and electron-hole separation phenomenon [46,47]. To accompany the results from the optical approaches, chemical simulations were also normal to investigate MOFs. For instance, the semiconducting properties of MOF-5(Zn), MIL-125(Ti) and ZIF-8(Zn) were all investigated through DFT calculations [42]. Additionally, the semiconducting band structure also can be facilely controlled using dyes, and if they were integrated with a MOF, the semiconducting properties become entirely different. For instance, the NM based sensitizer, [Ru(bpy)3]2+ (bpy = 2,20 bipyridine) was widely used to integrate with MOFs to form a visible light responsible photocatalyst [48]. In this scenario, the excited electron was formed on Ru based complex initially, followed by electron migrated to the MOF, identified as metal-toligand charge transfer (MLCT) or metal-to-metal charge transfer (MMCT) band, depending on the state of excited electron, since electron injected into MOF may localize at the metal node or the LUMO of the organic linkage [49]. Followed by that, a variety of organic sensitizers were also used to sensitize MOF, and as such, the semiconducting mechanism is based on either a LMCT or LLCT band. To achieve a high photoredox catalytic efficiency, the design of the MOF should be carefully considered from all the possibilities, and at the same time, the chemical stability as well. Several HOMO–LUMO levels of MOFs are shown in Table 1, and they are significant factors to identify the suitability toward the use for photocatalysis. In addition, the photocatalytic MOF with narrow energy gap also can be formed through dye to sensitize or integrated with traditional semiconductors [50]. In the pristine semiconducting MOFs, light harvesting is mainly determined by the band structure, since the required quantum tunneling energy corresponds to the certain wavelength of the supplied solar energy, beyond which, all the higher wavelength spectra with lower energy are not energetic enough to tunnel the electron. Whereas, due to the limited number of the semiconducting MOFs and less controlled semiconducting properties, the photocatalytic investigations by MOFs are based on the integrated structure, such as, with the traditional noble metals and semiconductors. Therefore, the light harvesting of the MOF system would be totally different to the original one. The detail is explained in the following, in Sections 2.3 and 2.4, with accompanying synthesis approaches. 2.2. Chemical and solar stability To use MOFs as photoredox catalysts, the stability consequently becomes a major issue to be concerned. Typical MOF is not able to tolerate with harsh conditions, and in general, the majority of the MOF structures can be maintained in the temperature range from 150 to 500 °C [60]. Undoubtedly, the presence of the organic linkage is the main limitation of the thermal stability, while, some of the inorganic units are easy to be oxidized to form metal oxidizes thermally under air condition, whereas, most of the MOFs are thermally robust enough to use as photocatalyst. In the species of MIL, the thermal stability was explained, and the isotypic MOF as a

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Y. Fang et al. / Coordination Chemistry Reviews 373 (2018) 83–115

Table 1 The electronic energy levels of a few MOFs. (MOF = metal framework, MIL = Matériaux de I’Institut Lavoisier, UiO = University of Oslo, ZIF = Zeolitic imidazolate framework). MOF species

LUMO (eV)

HOMO (eV)

Energy gap (eV)

Ref.

MOF-5(Zn) NH2-MIL-68(In) Salicylaldehyde-NH2-MIL-101(Cr) MIL-100(Fe) UiO-66(Zr) NH2-UiO-66(Zr) ZIF-8(Zn) Cu3(btc)2 [Gd2(abtc)(H2O)2(OH)2]2H2O PCN-224(Zn) NNU-36(Zn)

0.2 0.72 0.49 0.4 0.6 1.0 3.8 1.0 1.74 0.58 1.17

3.6 2.1 1.66 2.41 2.9 1.7 1.7 2.5 0.61 1.25 1.11

3.4 2.82 2.15 2.81 3.5 2.7 5.5 3.5 2.35 1.83 2.28

[51] [52] [53] [54] [55] [56] [42] [57] [58] [47] [59]

function of the different inertness of the metal center was studied, showing that when the distance between metal and organic decreases, the strength of the MOF is increased, thus the thermal stability [61]. In the aspect of chemical stability, it is determined by both the organic linkage and inorganic sub-unit based on the basicity (pKa) of the linker and Lewis-acidity of the metal, respectively [60]. The inorganic sub-units are likely to degrade in either acidic or basic environment, which is conventionally used for photocatalytic reactions. Because the redox potentials of an electrolyte should be tuned to match the band edges of the semiconducting materials to achieve a satisfied efficiency in photocatalysis [62]. Meanwhile, a certain solvent or organic reagent can easily collapse MOFs, and some organic reagents may favor to coordinate with a metal ion in a MOF, and therefore, if the formed bond is stronger than that between the metal and ligand, the deconstruction of a MOF would happen. For example, hydroxides, amines or alkoxides could form during photocatalytic reactions as the side-products or intermedia, which could highly affect MOFs by coordination with the metal ions [63,64]. The structure of a solvent stable MOF must be strong enough to prevent the intrusion of the interfered molecules into the framework with preserving the crystallinity and porosity. Upon the concerns of the chemical stability, the photostability of MOFs is another significant issue for photocatalysis. Herein, the organic ligand is the key component, since organic molecules are naturally degradable upon irradiation likely, in a typical one, through oxidizing with ambient oxygen. Highly reactive singlet oxygen atoms easily form from the ground state triplet oxygen through photoexciting. One molecule of the promoted specie refers P 1 to two singlet exited electronic states, denoted 1 þ g and Dg , which are 95 and 158 kJ/mol above the triplet state [65], respectively, and the lifetime can be prolonged as long as a second, depending on the medium [66]. Therefore, the active singlet oxygen is energetic enough to attack abundant organic bonds, such as polycyclic aromatic, benzylic and allylic species [67,68]. Moreover, the excited oxygen is also possible to transfer the electron to the organic medium to form peroxide species, which is even superior to the singlet oxygen specie to accelerate the oxidative reaction. In addition, the organic ligands are also likely to be photochemically active, and to react with general photocatalytic media, such as amines and alcohols [69], readily to dissolve the framework. Both the chemical and solar stabilities of MOF should be systematically evaluated prior to the use of photocatalysis. The structural design of MOF with a great stability normally are enabled strongly coordinative bonds to increase the thermodynamic stability [70], or functionalize significant steric hindrances to elevate the kinetic stability, allowing them to withstand restrict conditions [71]. Detailed discussions on the stability of the MOF can be referred to a few excellent publications [72,73]. For the typical examples, Ferey and coworkers developed a MOF, namely MIL100(Fe), which contains the high-valence metal ions of Fe3+ [74].

These kinds of high-valence metal ions generally possess strong coordination bonds, thus presenting excellent water stability, allowing photocatalytic investigations [75]. Taylor and coworkers induced nonpolar alkyl functional group in CALF-25(Ba) (CALF = Calgary Framework), this kind of modification enables functional group to shield around the metal node with chemical resistivity, and more interestingly, this material is capable of absorbing H2O, CO2 and CH4, the properties possessed potential to apply in photocatalysis [76]. Toward the investigation of the stability, a variety of frameworks were found allowing to work under relatively harsh conditions, and the typical species include UiO-66 (Zr) [77], MIL-125(Ti) [40], ZIF-8(Zn) [78], PCN-222(Zr) [79], and the family of the above-mentioned MOFs [80–82]. 2.3. Noble metals couple with MOF To immobilize nanostructured NMs into MOF can achieve an enhanced catalytic performance, such as a high selectivity for catalysis/photocatalysis [83–85]. NM can be immobilized inside the channel of the MOF and outside the surface of MOF, which could afford an improved heterogeneous catalytic performance through different contributions, based on a few parameters of the MOF, such as the conductivity, porous size and others [86]. If integrating NM inside the MOF, zero-dimensional (0D) nanoparticles were majorly used to functionalize the framework [87], and the NM nanomaterial can be less aggregated [88]. In photocatalytic MOF, NMs play multiple roles to encourage the performances [83], and among them, two effects can be mainly identified: the surface plasmonic effect [89] and cocatalytic effect [90]. The NMs could exhibit a characteristic absorption band in visible spectrum, namely localized surface plasmonic band, caused by the collective oscillation of the electrons that confined in a cage nanodimension. This effect could be observed from several NM nanoparticles including Au, Ag, Cu, Pt and even the NM based bimetallic nanoparticles [91–93]. In the case for photocatalysis, two routes can be considered to enhance solar conversion efficiency by the plasmonic effect. In one aspect, NM nanoparticles could induce thermalized electron by the induction of visible light, thus extending the light absorption spectrum of MOF [46,47], and therefore, the extra excited electron would enhance the photocatalytic performance. In the other aspect, the photoexcited NM nanoparticles could induce electrostatic field to the neighborhood of the nanoparticles to increase the distance of the charge, promote photoexcited charge separation, and prolong the living time of the charge carrier with a reduced recombination probability. For the hybrid structure to couple NMs with MOF, a few factors should also be considered to affect the plasmonic effect, such as the surface roughness of the MOF substrate [94] and the size/shape of the NM nanoparticles [95]. Specifically, when nanoparticle located on the surface of the MOF, the plasmonic effect is induced due to the formation of collective oscillation of the electrons on the

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surface, and as such the roughness of the surface can act as coupler between the surface plasmon and photons to affect light absorption [94]. On the other hand, the size and morphology of the particle directly affect the wavelength of the plasmonic responds [95]. The cocatalyst effect of NM particle also could effectively enhance photoefficiency, since they could facilitate electron trapping and increase the quantity of reductive reaction centers, accelerating redox reaction [90,96]. 1D NM with certain exposed facet could enable distinct selectivity and catalytic activity for the redox reaction, since atomic binds formed between the catalyst and reactant can be different due to the energetics of exposing atom/facet, and the efficiency to utilize the input energy becomes indeed different [97]. Thus, the efficiency of photocatalysis is also highly dependent on the structure and morphology of the NM nanoparticles from the hybrid structure [98]. In practice, two synthesis methods were normally used to deposit NM nanoparticles, the photodeposition and thermal induced nucleation growth methods [99–101]. Photodeposition was possessed upon the photocatalytic MOF that used as a substrate, which readily formed the active sites under irradiation to reduce NM ion, and particle would be deposited on the corresponding locations. Consequently, since the NM nanoparticles were localized on the active site, the performance of the photoreaction can be straightforwardly improved [102]. In the meantime, the size can be facilely controlled by the wavelength and intensity of the incident light. The thermal induced nucleation growth method is another way having an even better control of the morphology of the NM nanoparticles, especially for the purposes of forming a certain facet and structure of the nanoparticles. In this method, the nucleation growth was based on the thermal induction of the mixed MOF and NM precursor, and the growth was normally coming with the surfactant to control the crystalline growth. The distribution of the nanoparticles achieved by the thermal induced nucleation growth could be very homogeneous. However, the contact between the substrate and the NM nanoparticles might be loose, led to an insufficient contact for the electron and charge migration. Therefore, a post-treatment is generally required, such as annealing or chemical dissolution to remove the surfactant and enhance the attachments [103]. 2.4. Traditional semiconductors couple with MOF MOF can be integrated with semiconductors to form a sufficient photocatalyst [104]. Similar to the situation to integrate NM with MOF, two conditions are normally used to describe that immobilizing semiconductor onto MOF, namely, the ‘‘ship in the bottle [105]” and ‘‘bottle around the ship [106]”. For the statement of the ‘‘ship in the bottle”, the nanostructured semiconductors are settled in the pores of the MOFs, and ‘‘bottle around the ship” means the situation when the semiconductor attached around the surface of MOF. But the aimed applications of using semiconductor to ingrate MOF are totally different to the NM, since the functional semiconductor would conventional modify the energy structure of the original MOF. In the synthesis of the ‘‘ship in the bottle” structure, 0D semiconductor must be used, since the size of the ‘‘ship” must be small enough to penetrate into the window and stabilized in the channel of MOF. The ‘‘bottle around the ship” was the encapsulated hybrid structure that the surface of MOF is coated with semiconductors. As such, a precise control of the coating material dimension is less significant with respect to the ‘‘ship in the bottle” structure. However, immobilizing the semiconductor on the surface of the MOF could be loose and easy to disperse, and thus pre-design of the MOF or post-treatment of the encapsulated structure may be required to stabilize them. Typically, the ligand should be used having ability to bond with a certain semiconductor [107]. The

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post-treatment could be a high temperature annealing of the hybrid structure, as such, to release the surface tension of the semiconductor, and fitting into the MOF. However, the thermal stability of the MOF should be evaluated prior to using this method. On the basis of the structure, a variety of the possible functions can be added through the deposition of semiconductors onto MOF, and generating a synergetic effect of the photocatalyst. In the structure of the ‘‘ship in the bottle”, in one scenario, if the MOF was absent of the semiconducting property, then, the nanoparticle would enable a HOMO–LUMO level, converting bare MOF to be a photocatalyst. In this strategy, the active sites of the nanoparticle could be maximized with less aggregation by the homogeneous deposition of the nanoparticles in the nanosized pore of MOFs. Meanwhile, the light harvesting ability would be quite determined by the semiconductors, and the related factors, such as size and dimension of the semiconductor, should be concerned. On the other hand, if MOF intrinsically had energy gap, further integrating semiconducting nanoparticles would result in a heterostructure, which might highly decrease the recombination probability, depending on the band arrangement [108]. If the energy gap of semiconductor is smaller than MOF, the visible light spectrum would be extended and significant enhance the overall light harvesting. In both cases, through a good control of the nanoparticle size, the advantages of the quantum confinement effect could be taken for the light absorption and charge separation/transport with the reduced quantum collisions. In contrast, negative effects should also be concerned, for instance, by integrating semiconductor with MOF, the mechanism of the photoreaction might become totally different, and thus, the catalyst would be interfered by the media or side-products, and the efficiency may even decrease with respect to the homogeneous control. In the structure of the ‘‘bottle around the ship”, the photoredox process is relatively straightforward. Normally, the semiconducting shell requires high porosity, which allows the chemicals to pass through, and thus the core MOF was still able to functionalize for the photocatalytic reaction. In this system, a MOF would mainly use its highly porous properties, acting as the chemical adsorbing agent or platform to deposit semiconductor. Thus, even if the MOF is absent of the band structure, the high adsorption of the MOF would effectively immobilize the chemicals, serving for photocatalytic reactions occurring with the shell semiconductors. Moreover, if the MOF, at the same time, could act as a semiconducting agent. In this photocatalysis system, the reactant was then stationed in between the porous semiconductor and the semiconducting MOF, and the mechanism may be different to the typical one, based on the dual photocatalytic system, but this kind of case is still absent. In terms of this structure, controlling the channel of the shell structure would be important to optimize the chemical adsorption and the overall photoefficiency. In contrast, if the ‘‘bottle around the ship” structure formed with a high-impacted shell structure, in the photocatalysis system, the core MOF then only acted as a substrate, unless the alternative route is supplied for charge carrier. In consequence, photoelectrochemical (PEC) system was compatible with this design. If MOF could be initially grown on a conductive substrate, thus, an impact shell film can be formed either for enabling the semiconducting property for the overall system or for forming heterogeneous band structure, depending on the optical responses of the MOF structure. Under irradiation, the photoexcited electron/hole could readily migrate from the shell semiconductor to the substrate by passing MOF structure [109]. 3. Photodegradation of water contaminates by MOF An efficient removal of water contaminants has been concerned for a long period, since the serious environmental issues heavily affect the living and activities of animals. Normally, a broad range

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of the pollutions are highly soluble in water, including dyes, phenols, biphenyls, pesticides, detergents, greases, pharmaceuticals, proteins, toxic metal ions and others [110,111], and most of them are very general in factories. The conventional methods to purify the polluted water include adsorption, coagulation, filtration and membrane technology [112,113]. However, these methods still consist of many restrictions, for instance, the operation cost is very high, and the product may still be toxic and unacceptable for public use. Photocatalysis by semiconducting materials [114–116] has been demonstrated as an alternative and effective pathway to the degradation of the organic molecules into the biodegradable or less toxic compounds using the free solar energy. On top of this route, using MOF with the intrinsically high porosity and controllable semiconducting properties opens a new opportunity for this application. More importantly, the actively photocatalytic site, with respect to the traditional powder photocatalyst, can avoid aggregating, thus providing a long period working life to approach the economic goal. Additionally, if the catalyst is integrated other properties, such as magnetic, could further enhance recyclability and avoid the second pollution. Water pollution treatment was considered as the first photocatalytic application using a photocatalytic MOF [32]. For the mechanism, briefly, electron can be excited from HOMO to LUMO under light irradiation, and leaving hole in HOMO. Hole would interact with the OH to given OH [117], which is highly active to oxidize organic compounds with a series of chemical redox reactions, the processes were also proposed as the initiate step for the other photocatalytic applications in the following sections. While, advancing to the traditional semiconductors, the certainly porous size of MOFs would extensively enhance the performance to approach the mineralization of the pollutants or selectively to transfer to the certain non-toxic chemicals. To date, almost 400 publications were contributed to use MOF for water contamination treatment, and including some excellently comprehensive reviews [118,119]. Thus, in this section, the recent research progresses of photocatalytic pollution degradation are highlighted, especially to investigate the semiconducting properties of MOFs, and estimate the thermal, chemical and photostability of the candidates. This detail could be important to further use the MOFs for the other relevant photocatalytic applications. Herein, three major types of the degrading chemicals were focused to discuss, including aromatic molecules, organic dyes and a few other types of toxic contaminations.

3.1. Photodegradation of aromatic molecules The specie of aromatic compounds is highly toxic due to a good chemical stability with a long half-life time. Even though, the first developed photocatalytic MOF was applied to degrade the aromatic compound, while to date, the example successfully demonstrated the degradation of such stable chemicals are still remained as very few [31]. In 2007, MOF-5(Zn) was investigated to photodegrade phenol to a less toxic compound. The structure of MOF-5(Zn) is shown in Fig. 1A, in that, the terephthalate group acts as the organic linker to bind the inorganic unit of Zn4O(CO2)6 at the corner [32]. The HOMO–LUMO level of MOF-5(Zn) was measured around 3.4 eV (Fig. 1B), while surprisingly, the range of absorbing wavelength was found to be 500 to 840 nm. This phenomenon can be attributed to the distinct MOF structure, since the delocalized electron could live on the microsecond with the occupation of the LUMO likely and extended the light absorption [45]. MOF-5(Zn) was achieved to photodegrade phenol under visible light irradiation, and the photoefficiency is comparable to TiO2 [45]. In subsequent, MOF-5(Zn) was studied to present unequal chemical selectivity for the photodegradation, advancing to the traditional semiconductors. For instance, a competitive experiment was conducted to use MOF-5(Zn) to photodegrade 2,6-di-tertbutylphenol (DTBP) and phenol [120]. Due to the certainly porous structure of the framework, relatively large DTBP can be selectively attached to the framework, while phenol is freely to diffuse through the structure. Hence, the photoefficiency to degrade DTBP can be accelerated with respect to that for phenol, and when a solution was mixed with an equal amount of the phenol and DTBP, the photodegrading rate for DTBP was given as 4.42 times to that for phenol [120]. However, further work is required to study the final product and the mechanism. Inspired from MOF-5(Zn), the aromatic compounds, including nitroaromatic and chloroaromatic, were also managed to be degraded under light irradiation using MOFs. Lang and coworkers developed silver contained MOF, namely Ag4(NO3)4(dpppda) (dpppda = 1,4-N,N,N,N0 ,N0 -tetra(diphenylphosphanylmethyl)benzene diamine), for the photodegradation of nitroaromatics, including nitrobenzene, para-nitrophenol and 2,4-dinitrophenol under UV light [121]. Although, the mechanism was proposed similar to the other reports [122], in this study, the photocatalytic reaction was analyzed to fit to a pseudo-zero-order kinetics model, as shown in Fig. 2A, which means the reaction occurred at the surface

Fig. 1. (A) The structure of MOF-5(Zn). Terephthalate acts as the organic linker to link the inorganic unit of Zn4O(CO2)6. (B) The proposed electronic property of MOF-5(Zn), and the illustrated photocatalytic mechanism. Reproduced with permission from Ref. [32]. Copyright (2008) American Chemical Society.

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Fig. 2. (A) Pseudo-zero-order plot for the photodegradation of nitrobenzene with Ag4(NO3)4(dpppda) under UV irradiation. (B) The detection of CO2 from photodegradation of nitrobenzene as a function of irradiation time. Reproduced with permission from Ref. [121]. Copyright (2015) Elsevier BV.

of the MOF. This result made an agreement to the abovementioned example for degrading DTBP, the relatively fitting size between the MOF window and reactant molecule determines the kinetics of the reaction, thus determined the degradation rate. This MOF presented reasonable stability, the performance could be preserved up to 5 working cycles. In the degradation, all the pollutions were proved to be fully degraded as the production of CO2 and H2O eventually, as shown in Fig. 2B. Such demonstration of a complete degradation is significant as the base for further development of this MOF for such application. Recently, 2-chlorophenol was also degraded with photocatalytic MOFs, which is namely [Zn2(Fe-L)2(l2-O)(H2O2)]4DMF 4H2O (H4L = 1,2-cyclohexanediamino-N,N0 -bis(3-methyl-5-carboxysalicylidene)) and [Cd2(Fe-L)2(l2-O)(H2O2)]4DMF4H2O. They were formed by the carboxylate group of an (Fe-L)2(l2-O) dimer bridging the d10 metal of Zn2+ or Cd2+ [43]. The degrading efficiency of [Zn2(Fe-L)2(l2-O)(H2O2)]4DMF4H2O was improved in acidic condition, and reaching the highest of 73% at pH = 3, and [Cd2(Fe-L)2(l2-O)(H2O2)]4DMF4H2O presented similar results. In addition, the Zn based MOF showed a better performance than the one based on Cd. A series of control experiments were conducted to study the mechanism for the photoreaction, it proposed that the OH radical scavenger was formed as the active species, confirmed the catalytic reaction was predominated by OH radical [123]. Jiang and coworkers synthesized (K[Ln(H2O)4(pdc)]4) [BW12O40]2H2O (Ln = La 1, Ce 2, pdc = Pyridine-2,5-dicarboxylate) and (K[Ln(H2O)3(pdc)]4)[BW12O40]6H2O (Ln = Tb 3, Dy 4 and Er 5), though a facile hydrothermal method to photodegrade thiophene [124]. Among them, the use of Ce as the metal cluster presented the highest performance, under illumination for 12 h, 97% of 2-chlorophenol was degraded, and mineralized SO3, CO2 and H2O were proposed as final products. In both the above-mentioned cases, the toxically and stably aromatic compounds were completely degraded. In the aspect of photocatalytic aromatic degradation, the detail of the mechanism is significantly important to design new MOF species for this goal. For instance, benzene molecule has six bonds with equivalent binding energy, and thus the investigation may require evaluating the kinetic of the reaction between active species and the six bonds. Such understandings are directly relating to the design of the structure of MOF, as well as the suitability between channel size of MOF and the aromatic compound for the overall photoefficiency. 3.2. Photodegradation of organic dye degradation Organic dye was conventionally used as an indicator to investigate photocatalytic property by time dependent color variation,

thus they can also be conventional to primarily estimate the photocatalytic properties of MOFs [119]. In this working process, the photoinduced active species would attack the weak organic bonds of dyes, which thus, turn them into colorless. Upon this kind of these experiments and understandings, the potential applications, such as photocatalytic water splitting, CO2 conversion and organosynthesis, can be further developed. In this section, three typical organic dyes are mainly focused to discuss, including Methyl Blue (MB), Methyl Orange (MO) and Rhodamine B (RhB) [119]. The corresponding structures and absorbing wavelength of them are shown in Fig. 3. Jiang and coworkers used MIL-53(Fe) as photocatalyst to degrade RhB under visible irradiation using H2O2 as activator [125]. This MOF presented a high photoefficiency, and the rate constant was given around 4.764 h1. But the excellent photoefficiency can be quite attributed to the use of H2O2, since it would be sacrificed to interact with hole to form highly active OH radical, and reduce energy barrier to oxidize RhB [125]. A few other sacrifices were also investigated with mechanism toward enhance the overall degradation efficiency, these sacrifices quenched pathways were widely used to study the working mechanism of the semiconductor [126]. In comparison, Zn(5-aminoisophthalic acid)H2O and [Cd(5aminoisophthalic acid)H2O]n2nH2O were used to photodegrade RhB without sacrificial agent. The structure of the Zn(5aminoisophthalic acid)H2O and [Cd(5-aminoisophthalic acid)H2O]n2nH2O had same topographies, but different the metal cluster. The achieved photodegradation rates were quite different. Zn(5aminoisophthalic acid)H2O and [Cd(5-aminoisophthalic acid)H2O]n2nH2O gave rate constants of 0.1865 and 0.1348 h1 under UV irradiation. In this case, the efficiency was around one order of magnitude smaller to that with sacrificial agent [127]. In addition, although both MOFs have almost same topological structure, different central metal ions give distinct energy gap of 2.82 (Zn based MOF) and 3.41 eV (Cd based MOF), in this system, LMCT bands was suggested as the semiconducting mechanism, the coordinative bond between the metal cluster and the ligand was the core to affect the energy gap, thus to the different performance of the photoefficiency [127]. Li and coworkers demonstrated a similar example, [Mn3(btc)2(bimb)2]4H2O (btc = benzene-1,3,5tricarboxylate and bimb = 1,4-bis(imidazol-1-ylmethyl)benzene)) and [Co3(btc)2(bimb)2]4H2O were developed as catalysts to photodegrade an anionic organic dye [128]. [Mn3(btc)2(bimb)2]4H2O presented the apparent rate constant 0.11 and 0.073 h1 under UV and visible light irradiation, respectively, and the efficiencies were 0.26 and 0.13 h1 by [Co3(btc)2(bimb)2]4H2O. The discrepancies of photoefficiency were attributed to the metal node as well,

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Fig. 3. Chemical structures of the typical dyes, MB, MO and RhB, and their absorbing cutting wavelengths.

Fig. 4. (A) Schematic illustration of the post-synthesis to convert Cr-MIL-101(Cr) to NH2-MIL-101(Cr). (B) N2 sorptions of MIL-101(Cr) (orange), NH2-MIL-101(Cr) (red) (sample 1), NO2-MIL-101(Cr) (blue), NH2as-MIL-101(Cr) (black) (sample 2) and UR2-MIL-101(Cr) (purple) (UR2-MIL-101(Cr) is formed by NH2-MIL-101(Cr) to react with ethyl isocyanate). Reproduced with permission from Ref. [129] Copyright (2011) The Royal Society of Chemistry. (C) The optical responses of the relevant compounds and MOFs, and (D) the corresponding photodegradation efficiency. Reproduced with permission from Ref. [52]. Copyright (2015) Elsevier BV.

contributing the narrowed energy gap ([Mn3(btc)2(bimb)2] 4H2O = 4.04 eV and [Co3(btc)2(bimb)2]4H2O = 3.72 eV), based on the LMCT transitions. Moreover, two additional peaks centered 547 and 721 nm can be observed from [Co3(btc)2(bimb)2]4H2O, these were assigned to d-d spin-allowed transition of the Co2+ ion, while this spin was forbidden to transit for [Mn3(btc)2 (bimb)2]4H2O from Mn2+ ion [128]. Additionally, the organic ligand also played important role in semiconducting mechanism for dye degradation. As a noble example, amine group could modify MOF to narrow the energy gap, and increase the visible light response. A post-synthesis method can be conventionally used to achieve this goal, for instance, to convert MIL-101(Cr) to NH2-MIL-101(Cr) (Fig. 4A). But under the harsh synthesis condition using nitrating acid, the crystallinity is barely changed, thus leading to a reduced surface area, as shown in Fig. 4B [129]. In addition, Wu and coworkers proposed a similar idea for photocatalytic degradation by preparing MIL-68(In) and the amine derivative, namely NH2-MIL-68(In) [52]. In this work, the differences of the optical response between these two samples can be obviously observed as shown in Fig. 4C. Once the amine group was successfully coupled, the light absorption spectrum was extended. The performance was improved for the photodegradation (Fig. 4D). Recently, Xiao and coworkers integrated salicylaldehyde group to NH2-MIL-101(Cr) to further enhance the MB

photocatalytic degradation. Although, the surface of salicylaldehyde-NH2-MIL-101(Cr) was even smaller than that of NH2-MIL-101(Cr), the optical response of the material was significantly widened, and the absorbing spectrum was extended up to 800 nm [53]. A few recent examples of the semiconducting MOFs for photocatalytic degradation of the three typical dyes, MB, MO and RhB, are listed in Table 2. These materials were also considered having potential to drive other photocatalytic applications. Despite a variety of new semiconducting MOFs was emerged, the one can be used in the following photocatalysis applications still mainly based on the traditional types, such as MIL, ZIF and UiO. The semiconducting properties and stabilities are the main limitations for applying new MOF species. On top of that, the performance of the MOF was improved with achieving a hybrid structure by integrating other semiconductors with MOFs. Wu and Sha investigated UiO-66(Zr) and a series of UiO-66(Zr) based hybrid structure to photodegrade RhB [156]. The efficiency of individual UiO-66(Zr) presented the degradation rate constant of 0.264 h1 under visible light without a sacrificial agent, when UiO-66(Zr) was integrated with BiOBr, the degradation rate constant was rapidly elevated to 15.32 h1 under the same experimental conditions, mainly due to the extended light absorption. As the energy gap of BiOBr was around 2.8 eV, this integration highly promoted visible light harvesting [156]. They

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Table 2 The electronic properties of a few examples of the semiconducting MOFs for photocatalytic degradation of MB, MO and RhB. (NTU = Nanyang Technological University; and the other terms can be referred to the abbreviations). MOF

HOMO–LUMO level (eV)

Organic pollutants

Time (min)

Efficiency (%)

Ref

(emim)2[InK(btec)1.5(H2O)2] MIL-88A(Fe) [Cd(4,40 -bpy)(H2O)2(S2O3)]2H2O Mn2(tkcomm)(llpd)2 Cu4(dcpcpb)2(l3-OH)2(CH3OH)2(H2O) Co2(dcpcpb)(l3-OH)(H2O)2 Ag7(4,40 -tmbpt)(Hcb-iso-p)2(cb-iso-p)(H2O) MIL-53(Fe) MIL-53(Al) [Ag2(pdbmb)2(CF3SO3)2]H2O CuI4 (1,6-bth)2Mo6O18(O3AsPh)2 [Zn4(htpmb)2(q-Mo8O26)(H2O)6.5]0.5H2O Co(2,40 -Htmbpt)2(g-Mo8O26)(H2O)2 [(H2toym)2(SiW12O40)]6H2O Zn4(dpcpbe)2(bet)0.5(l3-OH)2(H2O) [Co(tkcomm)(tkiymm)]4.25H2O [Co2(4,40 -bpy)](4,40 -obb)2 [Cu3(3-dpsea)(1,3,5-btc)2(H2O)5]4H2O [Cu(3-dpye)(1,3-bdc)]3H2O [Co2(1,4-biyb)2(2-cmsn)2(H2O)]H2O [Ag2(dpe)1.5(sbdc)0.5(sbdc)0.5]7H2O Cd(tdc)(bimb)(H2O) Pb2(ttt)(ox)0.5(H2O) Cu(dm-bim) [Zn4O(2,6-ndc)3(DMF)1.5(H2O)0.5] 4DMF7.5H2O NTU-9 [Cu3(btyb)3(PMo12O40)2]9H2O Zn(1,4-bdc)(dpbpdca) [Co2(1,4-bdc)(ncp)2]4H2O

3.15 2.05 2.91 3.09 3.49 2.96 3.36 2.72 3.87 3.03 1.90 2.63 2.45 2.65 3.46 3.78 3.11 – – – 3.30 3.31 3.33 2.49 2.85

MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MB MO MO MO MO

180 20 60 90 90 90 90 40 60 540 120 120 120 120 90 75 100 120 240 180 180 150 150 45 120

90 100 86 88 64 73 73 11 30 90 97 76 52 80.4 32 95 85 56 80 64 99.9 90 13 95 65

[130] [131] [132] [133] [134] [134] [135] [135] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [146] [147] [148] [149] [150] [151]

1.74 2.7 – –

RhB RhB RhB RhB

80 165 810 300

100 94.2 85 67.52

[152] [153] [154] [155]

Fig. 5. (A) Schematic illustration to integrate magnetic composite with MIL-53(Fe). (B) UV–Vis diffuse reflectance spectra of (a) MIL-53(Fe), (b) the hybrid material, (c) Fe3O4 and (d) 1,4-benzenedicarboxylate. The inset presents the corresponding color of MIL-53(Fe), the hybrid composited structure and Fe3O4. Reproduced with permission from Ref. [158]. Copyright (2015) The Royal Society of Chemistry.

also investigated using Ag2CO3 to integrate UiO-66(Zr), the degradation rate constant was 1.44 h1 under visible light, although the result was not comparable to that when integrated it with BiOBr, the result was still as high as 7-fold with respect to bare UiO-66

(Zr) [157]. MIL-53(Fe) was integrated with a magnetic composite to photodegrade RhB under visible light [158]. The hybrid structure was synthesized using a solvent thermal method, and the precursor solution was prepared by adding Fe3O4 nanospheres, FeCl36H2O and tert-butylalcohol in dimethylformamide (DMF), as shown in Fig. 5A. The ultraviolet visible (UV–Vis) spectra in Fig. 5B present the light absorption of bare MIL-53(Fe), Fe3O4 and the hybrid material. The absorption of the hybrid material was quite close to that of Fe3O4, while, the apparent rate constant of the hybrid material exhibited about 18 times faster than Fe3O4, this result indicated the importance to form a heterostructure by Fe3O4 and MIL-53 (Fe). Besides, the embedment of the magnetic composite would improve the recyclability for the sustainable energy and environmental developments [158]. Very recently, Xiao and coworkers developed a hybrid structure using graphitic carbon nitride (gC3N4) and NH2-MIL-88B(Fe) for MB degradation under visible light [159]. In this study, author demonstrated, for the first time, to facilitate Fenton-like excitation of H2O2 via introducing g-C3N4 to NH2MIL-88B(Fe) for visible light-induced photogeneration. In the hybrid system, MOF is rarely demonstrated as linkage for the semiconductor to enhance the photoexcited charge separation and transport. Very recently, TP-222(Zn) was developed as a photocatalyst, it was synthesized by intergating TiO2 nanoparticle with a Zr based porph-MOF, PCN-222 [160]. In this structure, TiO2 nanoparticle was linked with PCN-222 to efficiently improve the photocatalytic efficiency, since the charge separation and transport can be facilitated. Although, in this work, RhB degradation was demonstrated for the enhanced photoefficiency, whereas, this work is important to use MOF as alternate way to assistant the photocatalytic performance. Li and coworker proposed another strategy to use MOF for photocatalysis, they integrate upconversion nanoparticles with MOF to build a near infrared (NIR)-responsive composite photocatalyst [161]. In the synthesis, NaYF4:Yb,Tm nanoplates were initially prepared, and a layer-by-layer growing method was employed to

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create MIL-53(Fe) with a controlled thickness. Typically, a control amount of Fe3+ was firstly added to absorb on the surface of the MOF, and afterward, terephthalic acid was added to form a thin layer of MIL-53(Fe), and this procedure can be repeated to create multiple layered structure. As such, the shell can be used to absorbing UV/Vis light and the core is photoactive under NIR spectrum, the results presented the activity in degrading RhB under NIR spectrum. Although, the performance was not quite satisfied, this approach provided opportunities to extended absorbing wavelength of the solar irradiation, and motivated the other possible applications. In an advanced strategy, Ho and coworkers prepared metal oxide organic framework (MoOF) as solar induced pollution degrading agent [162]. Through this design, a free-standing foam was synthesized (Fig. 6A), this construction was close to a formation of a device, conventional for a mass production and end-up applications with respect to general MOFs in the state of powder. The fabricated 3D MoOF foam microreactor significantly enhanced the photodegrading efficiency of MO, as shown in Fig. 6B, the degradation of MO was around 85.7% and 97.2% for TiO2 and TiO2 nanoparticles/CuO MoOF foam, respectively, under irradiation of 4 h. More importantly, the structure of the foam can be preserved with 32 h continuously photoreaction with purging every 8 h by argon gas, indicating the high stability. In addition, NMs were also proposed to integrate with MOF to enhance the performance of photocatalytic degradation. Wu and coworkers demonstrated that three NMs (Au, Pd, Pt) can be inserted into MIL-100(Fe) to photodegrade MO [163]. The successful deposition of the NM could effectively improve visible light harvesting and charge separation efficiency to enhance overall photoefficiency. Using H2O2 as the sacrifice, the apparent rate constant of bare MIL-100(Fe) was around 1.12 h1, which was lower than three other modified samples. The highest apparent rate constant was achieved using Pt@MIL-100(Fe) (4.55 h1), and the samples of Pd@MIL-100(Fe) and Au@MIL-100(Fe) achieved the values of 3.30 and 2.04 h1, respectively. Moreover, Wang and coworkers developed Ag@AgCl/Ag nanofilm/ZIF-8(Zn) as photocatalyst to degrade MB under visible light. The apparent rate constant of 14.762 h1 was achieved, this result was around 2 times of Ag@AgCl/ZIF-8(Zn) (7.932 h1) [164]. Electrochemical impedance spectroscopy (EIS) measurements were used to study the electronic properties, indicating that the resistance of Ag@AgCl/Ag nanofilm/ZIF-8(Zn) was lower than the sample without Ag film. Therefore, the interface between Ag nanofilm and ZIF-8(Zn) could extensively encourage the pair of electron-hole separation and transport. In a general view, the photodegrading was conventionally achieved with a variety of the MOFs and their hybrid structures. While, the mechanism with kinetic detail for the

photocatalysis still required to have a comprehensive study by a certain compound, as such, the advantage of the porous structure within the framework could be fully taken to promote the photoefficiency. 3.3. Photodegradation of other organic and inorganic compounds The contaminations of water also include abundant of the medicinal, cosmetic and industrial inorganic wastes, which also significantly interfered the living animals. The ranges of organic wastes related to allergies and the probability of tumor incidence [165–167]. With respect to photodegrade dye, even less research work was conducted toward this research direction, however, increased attention is necessary to be devoted for this study, as the contamination was even close to the pollutants in the real water environments. The typical species include triethanolamine (TEOA), ethylenediaminetetraacetic acid (EDTA), methanol and others, TEOA is highly soluble in water, and widely used for the cement and cosmetic production, as well as the surfactant for emulsifier [168]. While, it was proven as organic medium to increase the tumor incidence [169], EDTA is normally a chelating agent for industrial and medicinal usages [170], and in the meantime, it is also known as cytotoxic species [171]. Methanol is also very toxic, as ingested, it would be metabolized into formic acid to destruct optic nerve, causing blindness, and even seriously to result a death [165]. However, it was a very common solvent for industry, and easy to evaporate [172]. In photocatalysis, these three chemicals are highly reactive and can be served as electron donor [173,174], thus widely used as the sacrificial agent for a range of photocatalytic reduction reactions. They are standard sacrificial agent for photocatalytic hydrogen production and reductive organic synthesis, the detail of the usages will be reviewed in Chapters 4 and 5, respectively. On top of that, complicated organic compound was successfully photodegraded using MOF based hybrid material. Tetracycline was antibiotic, used to treat numbers of infections. While a series of side effects were observed, especially for children, such as poor tooth development and kidney problem. Peng and coworkers removed this pollution using core–shell structured In2S3@MIL125(Ti) through photocatalysis [175]. The control experiments were given by using bare MIL-125(Ti) and In2S3, the hybrid material presented the highest degrading efficiency of 63.3% among them. This improvement was considered attributing to the hybrid structure with the homogeneous integration of the In2S3 at the surface of MIL-125(Ti), in which, In2S3 was functionalized as the visible light antenna, whereas, MIL-125(Ti) was the platform to transport and the separate the photoexcited charge, close to the MMCT bands. The report illustrated the degrading mechanism

Fig. 6. (A) The structure of MoOF: (a) schematic illustration of the synthesis procedure of TiO2 MoOF foam, and (b–e) SEM images of TiO2 NP MoOF. (B) Time evolution MO degradation investigation with and without TiO2 NP and TiO2 NP/CuO MoOF foam. Reproduced with permission from Ref. [162]. Copyright (2015) Wiley-VCH.

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Fig. 7. The illustrated mechanism of the adsorption and photocatalytic degradation. Reproduced with permission from Ref. [175]. Copyright (2016) Elsevier BV.

with a combination of adsorption and photodegradation (Fig. 7), they proposed that the adsorption of MOF plays an important initiation for the improved performance [176]. The control of the degraded product toward less toxic, and even mineralize them to produce bare CO2 and H2O through solar energy are in advance to pursue the end-up applications. Recently, a dual-functional porphyrin-based Zr4+ MOF photocatalyst was used as a versatile catalyst for detoxifying chemical warfare agent simulants at room temperature [177]. The three dimensional (3D) free-base PCN-222(Zr)/MOF-545(Zr) was synthesized with open channels up to 3.7 nm in diameter. It was constructed to from a [Zr6(l3-O)8(O)8]8 node and tetrakis(4-carboxyphenyl)porphyrin linker, as shown in Fig. 8A with the morphology of the asprepared MOF. To the experimental, the setup for the dualreaction to transfer the toxic chemical warfare agent simulants 2-chloroethyl ethyl sulfide (CEES) and dimethyl 4-nitrophenyl phosphate (DMNP) to nontoxic oxidative products with half-lives of 8 and 12 min, as the kinetic profile shown in Fig. 8B. The effect of the MOF particle size on the rates of the dual reactions were also investigated, found that the particle size of the MOF highly affects the hydrolysis of the DMNP, but no significant difference of the reaction rate can be observed for the CEES oxidation. This

phenomenon attributes to the fitting size of the MOF channel with respect to the size of the organic compounds. The structure of the MOF was remained intactness after the photocatalytic reaction by confirming through XRD measurement. Importantly, the arrangement of this experiment led to a smart use of the MOF materials to photocatalytic degrading applications, and pushing them to use in multiple functions. Recently, a Zr based MOF, NU-1000(Zr) was also successfully used to detoxifies the CEES to less toxic sulfoxide derivative (2-chloroethyl ethyl sulfoxide) [178]. In this MOF, a halogenated boron-dipyrromethene derivative was used as organic linkage to bind Zr6 node. The family of borondipyrromethene derivative was widely used for optical applications based on the excellent photophysical property, it can be easily modified by addition of heavy halogen atoms to optimize the overall photocatalytic activity of the MOF. This MOF achieved an excellent performance to detoxify CEES, given a half-life of approximately 2 min under LED irradiation. The toxic heavy metals also contaminated water, and resulted several health effects [179], among the toxic heavy metal ion in water, the Cr4+ was appeared as the most common one, as it is widely used in industry. The toxicity is crucial, it defunctionalized most of the organism, and acted a threat of liver, kidney and skin

Fig. 8. (A) The structure of the PCN-222(Cr)/MOF-545(Zr): (a) the SEM image of the MOF, (b) 3D structure of the MOF, (c) view of the Zr6 node, (d) Zn-OH-Zn active site in phosphotriesterase, and (e) the concept of generating singlet oxygen by porphyrin moieties in MOF under irradiation. (B) MOF-catalyzed dual-reaction to transform toxic chemical warfare agent simulants CEES and DMNP to nontoxic oxidative and hydrolytic products: (a) the setup for the experiment, and (b) kinetic profiles for hydrolysis of DMNP and oxidation of CEES with the MOF. Reproduced with permission from Ref. [177]. Copyright (2015) American Chemical Society.

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cancers. The common route to detox it was converts Cr4+ to Cr3+, since the toxicity of Cr3+ reduced and became easy to precipitate out. A few photocatalytic MOFs were used for this conversion driven by solar energy, such as MIL-125(Ti), UiO-66(Zr), MIL-101(Fe) and others [41]. Among them, Wu and coworkers synthesized multifunctional polyoxometalates encapsulated in MIL-100(Fe) to achieve this goal and give a noble result, in which, the Cr4+ is completely reduced in 8 min under visible light irradiation [54]. They attributed the excellent photoactivity of the core–shell structured MOF, which has an synergetic effect of the enhanced light absorption intensity and separation/transport of the photoexcited charge carrier. The performance was further improved from the same research group recently, the same precursors were used to synthesize different topographic MIL, namely MIL-68(Fe), for this goal [180]. The Cr4+ was almost completely converted to Cr3+ in 5 min under visible light irradiation, this results become superior to the traditional TiO2 and ZnO, which only achieved 50% and 7.6% conversion under same conditions. They analyzed the flat-band potential of the MIL-68(Fe), given around 0.6 V (vs normal hydrogen electrode), which is more negative than the oxidize potential of Cr4+/ Cr3+(+0.51 V, pH = 6.8), thus promised for this application. However, the absorbing edge of this material is around 440 nm, indicating the limited absorption in the visible spectrum. Pillared-layered NNU-36(Zn) (NNU = Northwest Nazarene University) was also used for this goal with extended visible light absorbing spectrum [59]. This MOF has LUMO and HOMO levels of 1.17 and 1.11 eV, respectively, given the energy gap of 2.28 eV, which significantly enhanced the visible light absorption with respect to the above-mentioned examples. However, the result is not comparable with the above-mentioned examples since the different sacrifice was used. In this research, a few control experiments were conducted, including different sacrifices, pH value and experimental condition. It is worth to mention the fact is when methanol was used, the photoconversion efficiency of Cr4+ reached maximum. They found that the reduction of the Cr4+ followed pseudo-firstorder kinetics with rate constant of 0.0471 min1, indicating the reduction was occurred at the surface of the semiconducting MOF. However, the working mechanism, regarding to relationship among the MOF, sacrifice and Cr4+ needs to be identified along the further investigations.

4. Photocatalytic water splitting by MOF Under the circumstance of the ever-increased energy demand all around the world, renewable energy is urgently needed to replace the extremely consumed fossil fuels for a sustainable development. Artificial photocatalyst that capable of splitting water to evolve H2 and O2 gases is an ideal solution to address the energy issue, since H2 gas can be used as the chemical fuel with the energy density about 3-fold to petrol [181]. In addition, the only product from the energy consuming of H2 is water, excluded to the carbon cycle, thus, minimized the effect to the environment [181]. H2 risks ignition in air, and the safety challenges for its consuming, storage and transport are remained as limitations, while, H2 gas is still considered as perfect energy carrier in the future. The capability of semiconducting materials to split water using solar energy is determined by the HOMO and LUMO levels with respect to water splitting potentials [182]. Besides, efficient material that could facilely achieve this process features more distinct properties, typically, strong ability in solar energy harvesting, a long lifetime of photoexcited state, high yield of electron and hole separation and a minimal barrier for the photoexcited charge pathway [183]. MOF obtains an increasing attention for photocatalytic water splitting, since they offer a conventional solid-state platform with highly porous structure, continuously repeating units with

less defects and flexibly electronic controllability [4]. As such, this prototypical material is potential as the efficient photocatalysts to overcome the limit of the traditional semiconducting materials for water splitting. 4.1. Photocatalytic hydrogen evolution Solar-driven hydrogen evolution is the half reaction of water splitting to produce H2 gas. To date, most of the MOFs cannot be used as a stable and efficient photocatalyst for this application individually. Thus, the components, such as dye-sensitizer and cocatalyst, were widely used to facilitate MOFs for photocatalytic water splitting. MOF was first applied for photocatalytic H2 production by Mori and coworkers in 2009 [49]. In their experiment, [Ru2(q-bdc)2]n (bdc = benzenedicarboxylate) was synthesized as a photocatalyst with accompaniments of N,N0 -dimethyl-4,40 -bipyridi nium (MV2+) and EDTA-2Na to process hydrogen evolution. In this system, Ru(bpy)2+ 3 was mainly acted as a photosensitizer to harvest visible light, MV2+ was used as the electron relay, and EDTA-2Na was a sacrificial electron donor. MOF was functionalized as a platform to increase active centers and facilitate charge transport. Upon visible light irradiation, the accumulated H2 was evolved 41 lmol in 4 h, given the turnover number (TON) of 8.16, this value was highly increased to compare with the homogeneous compartment, Ru(bpy)2+ 3 , which only achieved the TON of 2.52. In this research, it was assumed that due to the relatively large size of the MV2+, the slowly diffusion rate may limit the efficiency. Lately, Rosseinsky and coworkers confirmed this assumption using another MOF. They developed a porphyrin based MOF, Al2(OH)2(TCPP)3DMF2H2O (TCPP = tetrakis(4-carboxyphenyl)porphyrin) for photocatalytic H2 production [184]. Using Pt as the cocatalyst and EDTA as the electron donor, the TON of 0.7 was achieved over 6 h. They found that if MV2+ was induced into the system, the catalytic activity was dropped by one order of magnitude, and this phenomenon was attributed to insufficient MV2+ diffusion rate. A few semiconducting MOFs were also used for H2 production. The exceptionally chemical and thermal stable Zr based MOF, UiO66(Zr) and its amine derivative NH2-UiO-66(Zr) were applied for photocatalytic water reduction by Garcia and coworkers [185,186]. In this research, NH2-UiO-66(Zr) presented a better performance, in a comparison with UiO-66(Zr), which achieved the apparent quantum yield (AQY) of 3.5% in a mixed solution (water:methanol = 3:1) under monochromatic light at 370 nm. In 2012, Matsuoka and coworkers also demonstrated a similar modification, to synthesize the stable MIL-125(Ti) and NH2-MIL-125(Ti) in H2 production with loading Pt particle as cocatalyst and TEOA as the sacrifice [187,188]. They proposed that when amine group was coupled, the LMCT energy band is modified by transporting the excited electron from –NH2 group to Ti4+, further extended the use of the solar energy toward visible range and increased the potential for redox reaction. The modification of the organic ligand to create external LMCT bands was proved suitable for the redox potentials for water splitting, however, the overall quantum yield was quite low using this MOF [189]. Polyoxonibate based 3D framework was also investigated as a candidate for photocatalytic hydrogen evolution [190,191]. As a typical example, the hexa-capped Keggin polyoxonibates [PNb12O40(VO)6]3 and Cu cation was incorporated to obtain two different frameworks, namely [Cu(en)2]4[PNb12O40(VO)6](OH)58H2O and [Cu (enMe)2]4[PNb12O40(VO)6](OH)56H2O (en = 1,2-ethylenediamine and enMe = 1,2-diaminopropane). The photocatalytic H2 production was investigated with UV irradiation, the MOFs presented a similar result in methanol solution. H2 gas was continuously evolved at a rate of 44.35 mmol g1 h1 to reach 33.26 mmol for 7.5 h, achieving a TON of 1.05. This group of the MOF also caught intensive attention due to the abundant structure topologies and

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Fig. 9. (A) The UV–Vis reflection diffraction spectra of (a) UiO-66(Zr), (b) Pt@UiO-66(Zr), (c) RhB-sensitized Pt@UiO-66(Zr) and (d) 5 ppm RhB aqueous solution. (B) The performances of photocatalytic H2 production using Pt@UiO-66(Zr) and RhB-sensitized Pt@UiO-66(Zr). (C) The proposed working mechanism of this system for photocatalytic H2 production under visible light irradiation. Reproduced with permission from Ref. [197]. Copyright (2014) The Royal Society of Chemistry.

potential applications [192–194], especially for photoredox catalysis [194–196]. A series of the new MOFs were further developed based on the Keggin-type polyoxoanions for the enhanced photocatalytic hydrogen evolution [191]. The inexpensive organic dyes, RhB, was also used to dye UiO-66 (Zr) (the MOF was also deposited Pt particles), and mimicking LMCT band [201]. A peak centered at 550 nm is formed in the UV–Vis spectroscopic spectrum, relating the absorption of the coated RhB, as shown in Fig. 9A. Upon the irradiation of a visible irradiation, the performance of this organic dye sensitized UiO-66 (Zr) presented extensively increased photoefficiency with respect to the bare UiO-66(Zr) (30 times). In the neutral solution (pH = 7) using TEOA as the electron donor, and the maximal H2 production rate (MHPR) was given 116.0 lmol g1 h1 by RhBsensitized Pt@UiO-66(Zr) (Fig. 9B). The working mechanism is proposed in Fig. 9C, RhB was used to widen the visible light absorption, and the HOMO–LUMO level arrangements between RhB and UiO-66(Zr) allowed the photoseparated charge readily to transfer along the organic ligand, and injected to Pt for evolving H2. On the basis of this development, the other organic dye, erythrosine B (ErB), was also used to couple with UiO-66(Zr) to create a visible responsible MOF. In the acidic reactive environment (pH = 0.4) using ascorbic acid as the sacrificial agent, the MHPR of 4.6 lmol h1 was achieved [198]. In both cases, the inexpensive organic dyes were effectively used to broaden light absorbing spectrum. However, the stability of the MOF indeed requires a further investigation, since the organic dye can be easily degraded upon light. Very recently, low cost and stable Calixarene based dye was used to sensitize UiO-66(Zr) for H2 production, and this hybrid

structure presented a high efficiency with low quantity of Pt loading (1528 lmol g1 h1) [199]. It is worth to note the fact that a variety of the additive components were used for H2 production, such as the electron relay, sacrificial electron donor and even dyes. Thus, the relative size between the additive components and the photocatalytic MOF becomes significant to affect the of photocatalytic efficiency, and herein the pore sizes of a few noble MOFs are listed in Table 3. Gascon and coworkers developed another route to use MOF for photocatalytic H2 evolution. They employed NH2-MIL-125(Fe) as a dye to sensitize Co-oxime-diimine to construct a photocatalyst [205]. Since the dimension of the Co based catalyst was smaller than the MOF, it was synthesized by the ‘‘ship in the bottle” solution, to deposit the catalyst inside MOF. As illustrated in Fig. 10, in theoretical, the dimension was highly fitted between each other. H2 was produced with visible light irradiation using TEOA. In this research, electron paramagnetic resonance (EPR) and PL spectroscopies were used to investigate composite and structure of the Co based MOF. However, it still lacked evidence for the structure that the Co based catalyst stayed inside of the MOF. Zhang and coworkers demonstrated that using a dye-like ligand to bridge Gd to form a MOF, namely [Gd2(abtc)(H2O)2(OH)2]2H2O (H4abtc = 3,30 ,5,50 -azobenzene tetracarboxylic acid). This MOF presented a good HOMO–LUMO gap (2.35 eV), which is effective to harvest visible light with the spectrum up to 530 nm. This advantage was mainly attributed to the dye-like ligands that possessed LMCT. Thus, the result of MHPR from [Gd2(abtc)(H2O)2(OH)2]2H2O was about 7.71 lmol h1. In subsequent, the MOF was further integrated with Ag to elevate the MHPR up to 10.6 lmol h1 using

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Table 3 The pore dimension of a few noble MOFs and the corresponding organic linker unit. MOF

Linker unit

Pore size (Å)

Window size (Å)

Ref

UiO-66(Zr)

6

–

[200]

UiO-67(Zr)

8

–

[186]

MIL-53(Al)

8.5

–

[201]

MIL-101(Cr)

29–34

12–14.5

[202]

MIL-125(Ti)

12.55 (octahedral) 6.13 (tetrahedral)

5–7

[203]

ZIF-8(Zn)

11.6

–

[204]

Fig. 10. Schematic illustration of the ‘ship-in-a-bottle’ synthesis strategy followed for assembling of Co@MOF. Reproduced with permission from Ref. [205]. Copyright (2014) The Royal Society of Chemistry.

TEOA as the electron donor under Xe lamp irradiation [58]. This demonstration presented the essential in design of the ligand for improving the overall water splitting performance, at the same time, indicating that coupling well established dye-like structure as the organic ligands may conventional to extend the light harvesting spectrum for the photocatalytic MOF. Similar approach

was also achieved by using noble metal based complex (bis(40 -(4 -carboxyphenyl)-terpyridine)Ru), which is used as organic linkage to form a Ti based MOF [206]. This MOF presented a high performance in photocatalytic hydrogen production, since the visible light absorption is widely extended up to 620 nm, corresponding to 2 eV, which is closed to the ideal energy for water splitting.

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The NM nanoparticles played essential role to improve the photocatalytic H2 evolution performance [207]. Lin and coworkers studied this approach to deposit Pt at MOF to construct a bifunctional photocatalyst [208]. In this work, the relationship between the size of the Pt nanoparticle and the photocatalytic performance was comprehensively studied. The Pt nanoparticles in diameter of 2–3 nm and 5–6 nm were successfully loaded into the MOF that built from [Ir(2-phenylpyridine)2(bpy)]+-derived dicarboxylate ligands and Zr6(l3-O)4(l3-O)4(carboxylate)12, through MOFmediated photoreduction of K2PtCl4. Pt@MOF presented high photocatalytic TON of 7000 for H2 evolution, this value is about five times of the sample with the homogeneous control. They believed the enhancement was attributed to the coupled Pt, while, the detail of functionalism from the NM need further identify. Wu and coworkers lately evaluated using different photodeposition method to decorate Pt nanoparticles on MIL-125(Ti) for photocatalytic hydrogen evolution [207]. A Ti3+ facilitated method was developed to achieve an efficient deposition approach with respect to the direct photodeposition. In this method, methanol was functionalized to stabilize the Ti3+ species under light irradiation, as such, preserved strongly potential to reduce the ability to form the Pt nanoparticle on the MOF. Solar-driven hydrogen evolution was conducted to compare the catalyst obtained from the different synthesis approaches, as well as the pristine MIL-125(Ti). Firstly, the NMs presented excellent facilitation in photoactivity, both of the Pt-decorated samples presented much better performance than pristine MIL-125(Ti). Moreover, the Ti3+ facilitated Pt decorated MIL-125(Ti) produced H2 around 38.68 lmol under irradiation of 5 h, given the TON of 30.2. This result is about 80% increase in the activity compared to the Pt decorated MIL-125(Ti) obtaining through the direct photodeposition. More recently, the role of NM to facilitate MOF for photocatalytic hydrogen evolution was further investigated by Jiang and coworkers [209]. Pt nanoparticles in the diameter of 3 nm were incorporated inside or supported on UiO-66-NH2, achieved by two different approaches. The Pt incorporated inside in the MOF was synthesized through in situ MOF growth approach in Pt suspension, namely Pt@UiO-66-NH2, and the Pt supported MOF was achieved to deposit Pt nanoparticles on the as-prepared UiO-66-NH2, called Pt/UiO-66-NH2. The photocatalytic hydrogen evolutions were conducted, and the results highlight that the photocatalytic efficiency strongly correlates with the Pt location relative to the MOF. The Pt@UiO-66-NH2 presented much better performance in photoactive with respect to Pt/UiO66-NH2, due to the greatly shortens the electron-transport distance, promoted the electron–hole separation. Time-resolved PL was used to monitoring the life of the excited electron, give 10.28 ± 0.06, 7.26 ± 0.04, 2.86 ± 0.02 ns for UiO-66-NH2, Pt/UiO66-NH2, Pt@UiO-66-NH2, respectively. This shorten lifetime consistently suggested that the introduction of the Pt NPs results in suppression of the photoexcited charge recombination in the MOF, since a new electron-transfer channel is opened. Xu and coworkers demonstrated to immobilize Pt complex, cisPt(DMSO)Cl2 (DMSO = dimethyl sulfoxide) in the MOF-253(Al) through a post-synthesis modification [210]. In the control experiments, individual Pt and pristine MOF-253(Al) cannot work as photocatalyst, while the hybrid structure presented a high photocatalytic efficiency, it was about 4.7 times of that from the homogeneous control, Pt(bpydc)Cl2 (bpydc = 2,20 -bipyridine-5,50 dicarboxylic acid). In this demonstration, extended X-ray absorption fine structure (EXAFS) was used to study the NM, revealed that a short Pt-Pt distance of 3.6 Å. This situation was considered critical for H2 formation, since the photoexcited electron transfer within the porous framework was improved. Lately, an alternative molecular proton reduction catalyst [FeFe]-(dcbdt)(CO)6 (dcbdt = 1,4-dicarboxylbenzene-2,3-dithiolate) was also synthesized to incorporate with UiO-66(Zr) as cocatalyst for photocat-

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alytic H2 production [211]. By virtue of this hybrid structure, initial rate and overall H2 production were improved in the solution of 1.0 M acetate buffer (pH = 5) and ascorbate. In this system, the improved performance could be credited to the structural stabilization of the complex, as this incorporation protected the structure deformation of the MOF structure by inhibiting the undesirable charge recombination with oxidized ascorbate. Co based metal-free cocatalyst was also widely developed for photocatalytic hydrogen evolution [212]. Jiang and coworkers created a Co(II) molecular photocatalyst [Co(TPA)Cl][Cl] (TPA = tris(2pyridylmethyl)-amine), which was designed to encapsulate into NH2-MIL-125 to promote photocatalytic hydrogen generation [213]. As a result, it presented an obvious improve of the performance for hydrogen evolution with respect to the components. Such increase was credited to the enhanced spatial charge separation from MOF to the Co(II) complex, which were observed by the electron spin resonance (ESR) spectroscopy. Besides, small energy gap semiconducting materials were also used to integrate with MOF for photocatalytic H2 production. As a noble example, UiO-66(Zr) was demonstrated to integrate with g-C3N4 [214]. The obtained UiO-66(Zr)/g-C3N4 achieved an enhanced the MHPR up to 14.11 lmol h1 in ascorbic solution (pH = 4) under visible light irradiation and two promotions were credited for this enhancement. Firstly, since g-C3N4 is a visible light responsible material, it had a capability to harvest visible light. Secondly, since UiO-66(Zr) is intrinsic have a wide HOMO–LUMO level, the hybrid structure forms heterostructure to enhance photoexcited charge separation and transport. It is worth to note the fact that the embedment of the metal-free semiconducting material g-C3N4 in MOF opens wide opportunities for developing visible light sensitive MOFs. g-C3N4 exhibited a few superior chemical and physical properties with respect to the traditional semiconductors [215,216], for instance, the electronic and optical property of gC3N4 can be highly controlled by inducing organic group, in the meantime, presented highly corrosion resistant [217]. Also, it can be facilely exfoliated to 2D nanostructure, and exhibiting unique topographic confinement, thus the incident light harvest, photoexcited charges separation and the transport can be effectively promoted for photocatalytic reaction [218,219]. Furthermore, g-C3N4 also can be considered as ligand to couple into MOF, thus may enable certain properties of g-C3N4 to compensate typically stable weaknesses of the MOF [216,220]. In addition, semiconducting nanoparticles were used to integrate with MOF to improve photocatalytic H2 production. Sun and coworkers embedded CdS on MIL101(Cr) (Pt decorated) as the photocatalyst [221]. Although, MIL101(Cr) cannot split water under light irradiation independently, by virtue of the heterostructure, it presented an excellent performance for H2 evolution, even a better performance to the Pt decorated CdS nanoparticles. Then, Wu and coworkers developed ternary composite UiO-66(Zr)/CdS/reduced graphene oxide for H2 evolution [222]. This photocatalytic H2 evolution rate was achieved around 13.8 times higher than that of bare CdS, this phenomenon is mainly due to the suitable band arrangements, electrochemical characterizations to further clarify this band structures. Kempe and Tilgner demonstrated MOF based catalyst, consisting of MIL101(Fe)-core and Au/anatase-shell materials, for hydrogen evolution [223]. In this synthesis, the prepared MIL-101(Fe) was firstly prepared. TiO2 layer was then generated stepwise under mild conditions through vapor phase deposition, and the plasmonic active gold particles were finally attached on the TiO2 shell. In comparison with the other compartments, including Au@anatase, Au@MIL-101(Fe), Au/TiO2@MIL-101(Fe), MIL-101(Fe)core–ana tase-shell, MIL-101(Fe), and Au@P25, the prepared multiple layered photocatalysts presented a superior performance in hydrogen production. Such result can be explained due to two reasons, significantly, the porous structure of the MOF allows the deposition

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of TiO2 and Au particle to be homogeneous, maximizing the plasmonic effect. At the same time, the vapor phase deposited anatase could promise charge migration. MOF can also be used as a template to synthesize traditional photocatalyst for hydrogen evolution. The highly porous structure can be efficiently maintained from MOF through this approach, as such, to give another possibility to enhance the photoefficiency using traditional materials [224–226]. As a typical example, Lin and coworkers using MOF as template to synthesize a core–shell structured metal oxidize, namely Fe2O3/TiO2, for photocatalytic hydrogen production [227]. MIL-101(Fe) was synthesized initially through a rapid microwave, it was used as the template to create porous Fe2O3 core. Titanium bis(ammonium lactato)dihydroxide was coated on the surface of the MIL-101(Fe) as the precursor to obtain the TiO2 shell. The photoinduced hydrogen production of the core–shell high-porous material was tested with triethylamine as a sacrificial reducing agent, to obtain a total of 30.0 lmol H2 per mg of material produced after 48 h. In comparison, this result is superior to Fe2O3, TiO2 and Fe2O3/TiO2 mixture. More than the metal oxides, very recently, high porous metal chalcogenide were also successfully synthesized using MOF as a template [86]. In this work, a typical highly stable MIL-53(Al) was chosen as a hard template, which was incorporated by metal nitrates/oxides through evaporation approach to obtain a hybrid MOF. Thus, CdO/MIL-53 (Al) could be conventionally synthesized, and it was then treated with Na2S and removed the MOF template, to prepare high porous CdS. This product exhibited a significantly enhanced photoactivity (634.0 mmol/gh), which is around 2 and 5 times higher than that of nano-CdS and bulk CdS, respectively, and this increased performance can be attributed to the inherent porous structure and large surface area. 4.2. Photocatalytic oxygen evolution and overall water splitting Water oxidative reaction is the other half reaction in water splitting. With the combination of the two half-reactions, the goal of the overall water splitting thus can be established toward lightto-fuel conversion. However, in general, water oxidation reaction is sluggish in kinetic, since it is rate-controlling step with multipleelectrons transport and a high over-potential, especially the highenergy barrier of O–O formation. Thus, this step is critical, and the progress in photocatalytic water oxidation is still relatively slow with respect to that for photocatalytic H2 evolution. More than a hundred of inorganic photocatalysts were studied toward water oxidative application. The NM based complexes, such as Ir and Ru, were employed for this purpose with good performances, while, the extremely low availability and high cost restricted the developments. In the meantime, the earth abundant metal based semiconductors were also investigated, but the performances were still incomparable to that based on NMs [228]. More recently, the coordination polymer was studied for photocatalytic water oxidation, as the exemplar of the cuboidal Mn4CaO5 complex. This complex was mimicking the structure of photosystem II, as the cubanelike arrangement comprised of four manganese atoms and one calcium atom with linking of l-oxo and l-hydroxo ligands, thus served as the core to design a series of water oxidation catalysts [229]. A variety of the first-row transition metal based motifs were further developed for this application, and the active sites can be chemically engineered with organic linker or metal nodes prior to the MOF construction. The development of MOF photocatalysts for water oxidation remained few, this circumstance can be attributed to some restrictions, for instance, the options of the highly stable MOF structure; the suitability of MOFs size with the oxidizers; and the electronic properties of MOFs. Since Co oxides was well-established for water oxidative catalyst [231], the series of materials also demonstrated

as cocatalyst to couple with MOFs to achieve this purpose. Xu and coworkers immobilized Co oxide nanoparticles in MIL-101(Cr) for photocatalytic water oxidation [232]. Co oxide nanoparticles were studied through transmission electron microscopy (TEM) with Energy-dispersive X-ray spectroscopy (EDX), presented the narrow average size in the diameter of 2–3 nm with a highly homogenous dispersion on the framework. This homogeneity dispersion played an important role to enhance water catalytic performance under light irradiation. Using [Ru(bpy)3]2+ as the photosensitizer and NaS2O8 as the electron acceptor, the turnover frequency (TOF) of 0.012 s1 per Co atom with oxygen yield of 88% was obtained, this result was about 9-fold of the material without the support of the Co oxide. In a comparison with the performance of loading Co oxide in nonporous SiO2 nanospheres, this study suggested that MIL-101(Cr) plays an important role to promote charge transport, evidently through the comparisons with Nyquist plots from EIS measurements. MOFs were also used to synthesize Co based nanostructured material for photocatalytic oxygen evolution. Ding and coworkers synthesized MxCo3-xO4 (M = Co, Mn, Fe) porous nanocages with a precise controlled ratio of substituted metal in MOF through a conventional process via self-assembly, and followed by low temperature calcination. The synthesis schematic diagram based on the Kirkendall effect is illustrated in Fig. 11 [230]. An X-ray photoelectron spectroscopy (XPS) with a conjugation of the inductively coupled plasma atomic emission spectroscopy (ICP-AMS) was used to identify the detail of the elementary information. The photocatalytic water oxidation reveled in the order of Co > Mn > Fe, and the highest TOF of 3.2  104 s1 could be obtained. Wang and coworkers developed the synthesis of the POM@Co3O4 (POM = polyoxometalate) using ZIF-67(Co) as template [233]. The produced species preserved the porous morphology of the Co3O4 concave nanoparticle, meanwhile, functionalized Keggin-type POM molecules. Under a visible light illumination, a significantly improved performance of photocatalytic oxygen evolution was achieved (1.1  103 s1) with respect to pristine Co3O4, when using Na2SiF6/NaHCO3 as buffer solution. In an advanced strategy, Qin and coworkers developed a new pathway to synthesize Bi-mna for this purpose, as the first report semiconducting Bi based MOF to drive this application [44]. This study presented a comprehensively theoretical calculation to illustrate the band structure (Fig. 12), and elucidated semiconducting mechanism with LLCT bands in the MOF. The O2 generation rate was excellently about 216 ml h1 under visible light irradiation. PL spectroscopy was used to investigate the lifetime of the photoexcited charge in a comparison of Bi-mna and H-mna. The demonstration offers the possibility to further develop MOF under the photoexcitation by LLCT bands or with dual effects by combination of the LLCT and LMCT bands to enhance the light harvest and conversion to break the limit in photocatalytic water oxidation. Very recently, Huang and coworkers developed Al-based MOF for overall water splitting [234]. In this design, the MOF derived from H2ATA consisting of AlO4(OH)2 octahedra linked by the carboxylate group of ATA2, in addition, amine group of Al-ATA presents the pore to coordinate Ni2+ cation. Thus, the benzene ring of ATA2 acted as the active site for O2 evolution, and Ni2+ ions functionalize to H2 evolution. The coordination environment of the Ni atoms in Al-ATA-Ni MOF was examined by the extended X-ray absorption near-edge structure (XANES) and EXAFS measurements, as shown in Fig. 13A. The results presented the oxidation state of Ni atom in the MOF is around +2 and in octahedrally coordinated. The photocatalytic overall water splitting performance of the Al-ATA-Ni was compared with the MOF, uncoordinated with Ni2+ cation (Fig. 13B). Furthermore, electronic and optical characterizations were conducted to investigate the superior performance of the Al-ATA-Ni with respect to Al-ATA. It is

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Fig. 11. Schematic illustration of the formation of MxCo3-xO4 (M = Co, Mn, Fe) porous nanocages through the Kirkendall effect. Reproduced with permission from Ref. [230]. Copyright (2015) The Royal Society of Chemistry.

Fig. 12. (A) The calculated electron localization functional plot for Bi-mna. (B) Fukui function F(r) and F+(r) for Bi-mna. (Bi purple, C brown, O red, N gray, H pink.) (C) Calculated total density and partial density state of Bi-mna. Reproduced with permission from Ref. [44]. Copyright (2015) Wiley-VCH.

worth to note the fact that the light harvesting efficiencies of the samples were quite close, while the efficiency of the separation and transport by Al-ATA-Ni was much better than Al-ATA, as presented in Fig. 13C by PL measurements, in which, the intensity presented from Al-ATA-Ni is much lower than that from Al-ATA. In this aspect, MOF was also widely used as templates to derive semiconducting materials, for example, nitrogen doped graphene was synthesized from Ni based MOF, and used for overall water splitting [235]. A hybrid metal–metal oxide structure (Cu–Cu2O) was obtained from MOF-199, and applied for H2 evolution [236]. Organic-metal oxide material of C–N–ZnO was derived from ZIF8, and used for O2 evolution [237]. 4.3. Photoelectrochemical water splitting Photoelectrochemical (PEC) is also a promising way to split water using solar energy with an assistant of electrical input. In this system, semiconducting photo(cathode) and photo(anode) are mainly used for H2 and O2 evolution, respectively. The separated channels allow the evolved gases to be separated spontaneously, which is conventional for an end-up usage as chemical fuel. The traditional semiconducting materials, such as WO3, ZnO and BiVO4 [238–241], were widely investigated [242]. In this system, the interfacial reaction sites among the semiconductor with applied bias, electrolyte and light energy should be maximized, and the photoexcited electron and hole must migrate toward the certain

electrode to evolve O2 and H2 gases, respectively. The electrode is generally required to have the abilities of light harvesting and electron/hole separation, also good conductivity for electron or charge transport. However, the inertly metal oxide semiconductors are slowly developed due to the limited optical and catalytic flexibility. While, highly porous MOF can be considered as an ideal material to modify the photoelectrode to promote the efficiency, for instance, the ligand may conjugate with active sites allowing the photoexcited charge carriers readily to migrate, and the enabled MOF with low energy gap could extend wavelength of light harvest. MOFs were rarely investigated for PEC water splitting, generally, two main functions of the MOF should be emphasized to work with semiconductors as the cores for this application, including dye sensitizer and cocatalyst. Zheng and coworkers developed vertical aligned ZnO@ZIF-8(Zn) nanorod arrays as photoanode [243]. The galvanostatic electrodeposition was used to grow ZnO nanorods, which was then taken as a template to synthesize ZIF-8(Zn), forming a core–shell structured photoanode. The use of MOF for this direction was identified as a cocatalyst to facilitate the photoexcited charge separation. The ZnO@ZIF-8(Zn) core–shell nanorod was probed through TEM equipped EDX to provide the elementary information, as shown in Fig. 14A. The photoanode presented distinct responses to hole scavengers through the comparisons of current vs time curves using H2O2 and ascorbic acid, as shown in Fig. 14B. This observation was mainly due to the limitation of the aperture of the ZIF-8(Zn) shell, which allows the access of the

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Fig. 13. (A) The investigations of the coordinating environment of Ni: (a) Ni K-edge XANE spectra. (b) EXAFS Fourier transforms of Al-ATA-Ni, Ni(CO3)2, NiO and Ni foil. (c) Wavelet transforms for the k3-weighted Ni K-edge EXAFS signal for the low (low panel) and higher (upper panel) coordination shells of Al-ATA-Ni. (B) The overall water splitting performance by Al-ATA-Ni and Al-ATA. (C) PL spectra of Al-ATA-Ni and Al-ATA. Reproduced with permission from Ref. [234]. Copyright (2017) Wiley-VCH.

Fig. 14. (A) Structural characterizations of ZnO@ZIF-8(Zn) nanorods core–shell through TEM: (a) Low-magnification TEM image the nanorods, (b) HAADF-STEM image of one typical nanorod, (c) cross-sectional compositional line profile of ZnO/ZIF-8(Zn) from the line marked in panel in (b), and (d–f) elemental maps of C, N and Zn concentrations in the rod from the line marked in panel in (b). (B) The comparisons of photocurrent responses against H2O2 and ascorbic acid as scarifying agents with (a) bare ZnO, and (b) core–shell structure. Reproduced with permission from Ref. [243]. Copyright (2013) American Chemical Society.

H2O2 molecules, but not for relatively larger ascorbic acid molecules. Although, the photocurrent density was relatively low with respect to the typical PEC efficiency [244], yet it is worth to note the fact that the successfully synthesized structure of semiconductor@MOFs leads to a rational design to further construct a PEC electrode. More recently, Liu and coworkers used three Ti based MOFs, MIL-125(Ti), NH2-MIL-125(Ti), (NH2)1,2-MIL-125(Ti), as sensitize to decorate the TiO2 nanorods for PEC water splitting [245]. The incident photon conversion efficiency (IPCE) measurements demonstrated the enhanced visible light harvesting in the region between 420 and 500 nm with respect to the bare TiO2 nanorods. In this study, by attaching Au particles on the surface of MOF, the

photocurrent density of 35 lA/cm2 can be obtained with applied bias of 1.23 V (vs RHE) under AM 1.5 irradiation in the electrolyte of 0.5 M Na2SO4 solution. Although, the photoefficiency was remained unsatisfied, in potential, the great chemical tailorability of MIL-125(Ti) family with tunability optical and catalytic properties may benefit for both light absorption and charge transport. Bi and coworkers loaded MIL-100(Fe) on the W/Fe codoped BiVO4 film as a photoanode. MIL-100(Fe) acted as the medium on this film to facilitate charge separation and transport to enhance the photoefficiency of PEC water splitting [246]. This developing direction had received increasing attention, for instance, a general and facile strategy to fabricate MOF on TiO2 substrate was investigated for

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photocatalysis recently, provided a base for a comprehensive study for a further development [247]. Also, in the consideration of the large diversity of MOFs, a variety of candidates maybe inspired from this study to fabricate photoelectrodes. The idea of MOF derived metal oxides is also accessible to create photoanode for water splitting [248]. Sun and coworkers synthesized N-doped In2O3 from In based MOF for PEC water splitting. Pristine In2O3 is not a suitable candidate as photoanode, since the bandgap is as large as 3.5–3.7 eV, making it quite insufficient in absorbing visible light, the bandgap was narrowed to around 2.9 eV though N doping. The N doped In2O3 powder can be conventionally obtained through calcination the In based MOF at 450 °C. The powder was casted on a FTO glass as the photoanodic cell, which give the photocurrent density about 0.2 mA/cm2 with an applied voltage of 1.6 V (vs RHE). Although, this overall result is still under the basic line to compare other material for PEC applications, while, this conventional method to synthesize a doped metal oxide may useful to develop related porous materials. 5. Photocatalytic CO2 transformation by MOF CO2 is a gas significant to the life on the Earth, and abundantly presented in the atmosphere, around 400 ppm by volume. Meanwhile, CO2 is also considered as one of the most important greenhouse gases [249]. In modern society, it acts as a major source to drive global warming, since the huge amount of emission is generated from the combustion of the fossil fuels. Approximately 60% of global warming effects are attributed to CO2 emission [250]. Thus, reducing the quantity of CO2 is significant to decrease global warming effect. The capture or storage of CO2 are widely considered as a strategy, thus, extensively effort has been devoted to the developments of such media [251–253].To date, these technologies were still restricted for industrial production in practice, such as large energy cost, low efficiency and poor recyclability. Mimicking the photosynthesis, to transfer CO2 to other useful chemicals by photocatalysis are ideal pathways, since the energy is free and abundant availability around the world, moreover, economically to produce valuable organic products. The general requirements of the photocatalyst in kinetic are even restricted with respect to that for water splitting, since the highly stable structure of CO2. Among the various photocatalysts, MOF is of particularly potential for this approach, due to a few unique properties. The CO2 photoconversion processes mainly include physical adsorption, photocatalytic conversion and desorption. Highly chemical-flexible MOF can be designed to satisfy the harsh requirements conventionally. For instance, imidazole was a general ligand for MOF, while it is not able to adsorb CO2. If a MOF with imidazole linkage was replaced by benzimidazole, the MOF could be extensively increased for the CO2 adsorption, based on the formation of the p–p stacking interaction, as it includes a weak electron donation from the aromatic ring to bond with CO2 [249]. The enabled sorption property acted as a superior characteristic determining the photoreaction with comparing to the traditional photocatalyst [254–256]. In addition, the optical and electronic responses of MOFs can be optimized with a carefully design of the ligands and coordination metal clusters, thus to improve the photocatalytic CO2 conversion. Herein, the review progress consists of two sections, on the basis of the produced product, using photocatalytic MOF to reduce CO2 to CO and convert CO2 to the other organic chemicals, including CH3OH, HCOOH HCHO and CH4. 5.1. Reduction of CO2 to CO The reduction of CO2 to CO is one pathway to consume CO2, in the meantime, to convert solar energy to chemical fuel, since CO is

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one of the most important natural fuels worldwide, around 5  1012 kg was naturally produced per year for the goal of energy supplement. The industrial production of CO can be described as Boudouard reaction: [257]

CO2 þ C ! 2CO

ð1Þ

In thermodynamic, large energy is required due to the large bond enthalpy (523 kJ/mol) of the double bond in O@CO [258,259]. Economic solar driven process to decompose the bond between C and O has received increased attention, especially by taking advantages of the unique photocatalytic MOFs. In the system for photocatalysis, CO2 is normally dissolved in organic media, such as acetonitrile (MeCN) and tetrahydrofuran (THF), and the reaction can be defined as the following reaction: [260]

CO2 þ 2Hþ þ 2e ! CO þ H2 O

ð2Þ

To produce CO by the reduction of CO2 is a two-electron process, as shown in Eq. (2). This reaction presented relatively low kinetic barrier with respect to the other processes in the formations of complicated chemicals. In addition, it is obviously that the organic solvent could act as the electron donor to facilitate the water splitting to form H2 as a side product in gas phase, which requires a further separation for the end-up application. In 2011, Lin and coworkers coupled UiO-67 with [Re(CO)3(bpy) Cl] for photocatalytic CO2 reduction with the medium of the saturated CO2 in MeCN as solvent and TEOA as sacrificial agent [109]. In this study, UiO-67 mainly acted as the platform to stabilize the catalyst of Re(CO)3(bpy)Cl homogeneously. As the results, the CO-TON of 10.9 was achieved under an irradiation of 12 h, which is more than 3-fold to that using bare catalyst. It is believed that the CO2 reduction in this MOF system undergoes a unimolecular pathway involving Re(CO)3(bpy)Cl intermediate, inhibited the bimolecular pathway with the intermedium of CO2-bridged Re dimer [(CO)3(bpy)Re](CO2)[Re(CO)3(bpy)]. Thus, it blocked the catalyst decomposition pathway, and enhanced the TON of CO2 reduction. Lately, the MOF with a similar structure was further developed, Re(CO)3(bpy)Cl-containing elongated dicarboxylate ligands and Zr6(l3-O)4(l3-OH)4 clusters were coupled together to form a single-site MOF, which was used as photocatalyst for CO2 reduction [261]. The synthesis processes and corresponding structure of the MOF are shown in Fig. 15A and B, respectively. Under the same experimental condition as above, CO2 was reduced to CO and formate with the COTON of 6.44 in 6 h. Meanwhile, this MOF quickly reached TON of 5.7 for CO generation in 1 h. Moreover, the mechanism for the reduction process was carefully investigated, especially for the decomposition process on the activity site (Re(CO)3(bpy)Cl). With combining the 1H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopic studies, the decomposition pathway was proposed that the pyridine ring was partially hydrogenated initially, and then led to the decomplexation of the Re from the MOF. Recently, ZIF-67(Co) was integrated with Ru-dye to produce CO from CO2 under visible light [262]. This dyed ZIF-67(Co) performed a high TON of 112 for the convertion, they proposed ZIF-67(Co) acted as cocatalyst to promote charge separation kinetics. In addition, a MOF was developed composing secondary building units of metal ions (Re) and organic ligands (–NH2) for photocatalytic CO production [263]. A fine balance in the proximity of Re complex and –NH2 functionalized multiple ligands were carefully investigated to pursue a high selectivity and activity for CO production by photocatalysis, and given about 3-fold enhancement to the performance from the optimized samples. Wang and coworkers developed a Co containing ZIF and further coupling it with the Ru complex for photocatalytic reducing CO2 to CO [265]. ZIF-9(Co) was developed as a typical photocatalytic MOF,

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Fig. 15. (A) The synthesis of the target MOF (Re(CO)3(bpy)Cl-containing elongated dicarboxylate ligands and Zr6(l3-O)4(l3-OH)4 clusters. (B) The corresponding structure of the formed MOF. Reproduced with permission from Ref. [261]. Copyright (2016) Wiley-VCH.

consisting of Co2+ linked by benzimidazole ligands. 13C-labeled CO2 was dissolved in MeCN for the photocatalytic reduction with TEOA as sacrificial electron donor under visible light, isotropic 13CO was identified to confirm the produced CO from the reduction of the CO2 with obtaining the TON of 89.6. A series of control experiments to disclose the superior performance of ZIF-9(Co) as the cocatalyst efficiently to separate the photoinduced charge carrier, when the ZIF-9(Co) was barely used as the photocatalyst, none converted CO could be detected, while, if Ru complex was used, it obtained a relatively low performance for CO production. This demonstration encourages to use ZIF-9(Co) as a platform in photocatalytic CO2 conversion [265], ZIF-9(Co) was also used to integrate with inexpensive catalysts for CO production. For instance, ZIF-9(Co) was successfully coupled with CdS for CO2 reduction, and the working mechanism is illustrated in Fig. 16A. This system contributed a high AQY of 1.93% at 420 nm of the incident wavelength [26]. A variety of the sacrificial electron donors was qualified in this research, as shown in Fig. 16B, when DMF was used, the reduc-

tion presented the best selectivity for the photocatalytic MOF toward CO evolution. To understand this phenomenon, it may require further analyses of the relative size between the molecule of the electron donor and the window/channel of the framework, as well as the flexibility of the units. g-C3N4 was also selected to couple with ZIF-9(Co) for photocatalytic CO production, and the structure is illustrated in Fig. 16C [264]. In this study, an effective separation of the light-induced charge carriers was further elucidated by in situ PL spectroscopy, as shown in Fig. 16D. The hybrid form of the MOF presented decreased intensity of the PL spectrum, whereas, the other integrated structure or the individual component suppressed the quenching of the photoexcited charge. As such, the hybrid structure presented superior performance in photoefficiency. Recently, MOF was further conjugated with Ag nanocubes for CO2 to CO conversion under visible light irradiation [89]. In this report, Re based complex, Re(CO)3(bpydc)Cl, was used as the catalytic center to attach UiO-67, the synthesis is shown in Fig.17A.

Fig. 16. (A) Schematic diagram to integrate CdS with ZIF-9(Co) as hybrid MOF photocatalyst. (B) The produced gas through photocatalytic reduction of the CO2 with a series of sacrificial electron donor. Reproduced with permission from Ref. [26] Copyright (2015) Elsevier BV. (C) Schematic diagram of the hybrid structure that coupled by ZIF-9(Co) and g-C3N4. (D) The PL spectra of the reaction systems in various components at 400 nm laser irradiation at room temperature. Reproduced with permission from Ref. [264]. Copyright (2014) The Royal Society of Chemistry.

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Fig. 17. (A) Zr6O4(OH)4(CO2)12 secondary building units are combined with 2,20 -bipyridine-5,50 -dicarboxylate and ReI(CO)3(bpydc)Cl linker to form Re-UiO-67. Atom labeling scheme: C, black; O, red; Zr, blue polyhedral; Re, yellow; Cl, green; H atoms are omitted clarity. (B) The illustration of coating Re-UiO-67 on an Ag nanocube for the enhanced photocatalytic conversion of CO2. Reproduced with permission from Ref. [89]. Copyright (2017) American Chemical Society.

The ligands were designed to covalently attach at the Re center in Re(CO)3(bpydc)Cl, thus prevented the prevailing deactivation pathway of dimerization. Moreover, Re-UiO-67 was further coated on Ag nanocubic (Fig.17B). As such, for the first time, the plasmonic effect was applied with MOF to spatially confine photoactive center for improving the photocatalytic efficiency. As the result, 7-fold enhancement of the performance was obtained with the stability preserved for more than 48 h. The dimension of the NM was also significant to affect the overall efficiency, and when the thickness of Ag was reduced from 33 nm to 16 nm, the CO-TON was enhanced by about 23%. However, when Cu with the thickness of 33 nm is coated, almost none of CO-TON can be achieved. The demonstration successfully induced plasmonic effect to MOF for enhancing the photocatalytic ability of MOF, and presented the significance for the further investigations. 5.2. Converting CO2 into organic chemicals CO2 was also successfully converted to a few other useful organic chemicals by photocatalytic MOF, including HCOOH, HCHO, CH3OH, CH4 and other more complicated compounds, the major corresponding chemical processes were presented as following: [260]

CO2 þ 2Hþ þ 2e ! HCOOH

ð3Þ

CO2 þ 4Hþ þ 4e ! HCHO þ H2 O

ð4Þ

CO2 þ 6Hþ þ 6e ! CH3 OH þ H2 O

ð5Þ

CO2 þ 8Hþ þ 8e ! CH4 þ 2H2 O

ð6Þ

These processes are closed to the photosynthesis to fix CO2, and the value chemicals can be further used in a wide range of fields [266]. The thermodynamic potentials of the reactions periodicity reduced from the formation of the HCOOH with two electron process to the eight-electron process to produce CH4 [260]. However, eight-electron process generally contains multiple-step reaction to overcome the highest kinetic barrier among these conversions, and a clear mechanism of this photocatalytic process was still under investigation, this conversion is generally considered as the most difficult reaction to drive. In contrast, the circumstances to produce HCOOH were quite close to that for CO production with a twoelectron process involving relatively straightforward reactions. The further use of the chemical was also quite different, for instance, produced CO and CH4 can directly supply the energy consumption. While the liquid chemicals, such as CH3OH, HCOOH and HCHO, were important precursors for a lot of the chemical reactions [267]. Thus, clarifying the mechanism of these processes and controlling the selectivity for the productions are extremely important, and both are highly dependent on the CO2 sorption ability, and the HOMO and LUMO levels of the MOF. To date, the investigation was mainly focused on the HCOOH production. In 2012, Li and coworkers used amine-functionalized photoactive NH2-MIL-125(Ti) to convert CO2 to HCOO under visible light irradiation using TEOA as the sacrificial electron donor [269]. Prior to this research, the family of MIL-125 (Ti) was well established by their stability and CO2 adsorption [40], thus it was also considered for CO2 photoconversion application, when the semiconducting properties were investigation from the species of MIL. In a comparison with MIL-125(Ti), by virtue of the NH2-MIL-125(Ti), both of the chemical stabilization and CO2 sorption were improved. Moreover, the light absorption spectrum of NH2-MIL-125(Ti) was

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extended allowing visible-light induced photocatalytic activity. With visible light irradiation of 10 h, 8.14 lmol HCOO can be obtained. This work demonstrated a successful design of MOFs having the ability to harvest visible light for photocatalytic reduce CO2 to HCOO, and a series of MOFs were further designed following this direction [268]. MIL-101(Fe), MIL-53(Fe) and MIL-88B(Fe) were functionalized with amine group, and performed for CO2 conversion under same experimental conditions as above. This research analyzed the mechanism of Fe-based MOF for the photo-induced excitation and electron transport, as illustrated in Fig. 18. The direct excitation of the Fe-O clusters induces the electron transfer from O2 to Fe3+ with the formation of Fe2+, which works as photoactive site for CO2 reduction. This modification is same as the mentioned application for photocatalytic hydrogen evolution in Section 3.1. With the combination of these two energy bands, a synergetic role was developed to enhance overall CO2 reduction. Based upon this development, MIL-125(Ti) was used to form hybrid structure by integrating the MOF with NMs (Au and Pt) [270]. The Pt/NH2-MIL-125(Ti) presented an enhanced performance than the bare NH2-MIL-125(Ti), while, when Au was coupled with the NH2-MIL-125, a negative effect was observed for the formate formation. The origin of the different photocatalytic performance was elucidated by clarifying the working mechanism. Under irradiation, Pt/NH2-MIL-125(Ti) would produce Ti3+ as intermedium, while, Au/NH2-MIL-125(Ti), as well as the bare NH2-MIL125(Ti), was absent of Ti3+ under irradiation. Since Ti3+ is the active species to convert CO2, as an additional pathway to form Ti3+ within the Pt/NH2-MIL-125(Ti), an enhanced overall performance can be straightforwardly observed. This investigation demonstrated the function of the NM in this photocatalytic system, to understand the enhancement from hybrid structure by integrating NM with MOF. Furthermore, Zr based MOF was also robust, at the same time, possessed more negative redox potential with respect to Ti based MOF (Ti4+/Ti3+ (0.1 V), Zr4+/Zr3+ (1.06 V) [41,271], thus, Zr based MOF presented an enhanced photocatalytic performance, the typical examples like UiO topographic Zr based MOF and their diversities [272]. On the basis of this understanding, a bimetallic UiO based MOF(Zr/Ti) was also successfully prepared using Ti to substituting Zr in NH2-UiO-66(Zr), namely NH2-UiO-66(Zr/Ti) [273]. In the practice of photocatalytic CO2 reduction using this bimetallic MOF, it contributed an improved performance. This phenomenon is mainly due to the introduced Ti substitute, which acts as a mediator to facilitate electron transfer for the active site of Zr in converting CO2 to HCOO, indicated from both theoretical and experimental results using DFT calculation and ESR spectroscopy, respectively. Cohen and coworkers used Mn+ bipyridine complex, Mn(bpydc)-(CO)3Br to incorporate into highly robust Zr4+ based

Fig. 18. Schematic illustration of the dual excitation pathways over aminefunctionalized Fe-based MOFs. Reproduced with permission from Ref. [268]. Copyright (2014) American Chemical Society.

MOF (UiO-67) for CO2 photocatalytic reduction [274]. In the mixture of DMF/TEOA, the TON of approximately 110 was achieved in 18 h to produce HCCO under visible light, extensively exceeding to the bare UiO-67. This increase of the performance was ascribed to the struts of the designed framework providing isolated active sites, which stabilizes the catalyst and hinders the dimerization of the singly reduced Mn. Jiang and coworkers used PCN-222(Cr) to convert CO2 to HCOO by mean of solar energy [46]. It was found that PCN-222(Cr) had ability to selectively capture CO2 (14, 35, and 58 cm3/g of CO2 at 308, 298, and 273 K at 1 atm respectively), meanwhile presented high conversion efficiency in the present of TEOA under visiblelight irradiation. In this MOF, a type of deep electron trap state emerged, thus enabling effective suppression of the detrimental electron-hole recombination and giving a high photoefficiency, which produced HCOO of 30 lmol in 10 h. More recently, Su and coworkers developed another Zr based MOF, namely NNU-28 (Zr), for photocatalytic reduction from CO2 to HCOO. The structure view of NNU-28(Zr) is shown in Fig. 19A, revealing two types of cages [275]. This MOF presented a highly visible light absorption with the wavelength up to 550 nm, since the use of visible light responsive by the organic ligand, called 4,40 -(anthracene-9,10-diyl bis(ethyne-2,1-diyl))dibenzoic acid. NNU-28(Zr) presented excellent chemical and thermal stability, meanwhile, the high porosity promised highly CO2 uptake. Thus, a remarkable photocatalytic performance was obtained, as shown in Fig. 19B, 26 lmol HCOO could be obtained in 10 h under visible light irradiation [275]. On the basis of the EPR spectra, as shown in Fig. 19C and D, both the inorganic building unit (Zr6 oxo) and anthracene-based ligand contributed the enhanced light absorption. As such, a dual electron migration routes was proposed (Fig. 19E), this property was very similar to that for Fe/Ti based MIL as mentioned above, to enhance the photoefficiency. To increase the optical absorption range and the catalytic properties, Ir based MOF, (Y[Ir(ppy)2(dcbpy)]2[OH]) (ppy = 2-phenylpyridine; dcbpy = 2,20 -bipyridine-4,40 -dicarboxylate), was also developed for this application (Fig. 20A) [276]. The introduction of the NM Ir, the overall cost of the material was relatively ineffective. But this kind of MOF presented a relatively high efficiency, for instance, with the mixture of MeCN and TEOA solution under visible light irradiation, 38.0 lmol HCOO can be synthesized in 6 h, given the TON of 118.8, which presented as the highest performance by then [276]. The possible mechanism is also proposed in Fig. 20B, and Ir(ppy)2(dcbpy) unit in its excited state can be reductively quenched by TEOA, donating electrons to the excited photosensitizer. One CO2 molecule receives two electrons from one adjacent Ir(ppy)2(dcbpy)2 units to form HCOO. In comparison with the photocatalytic convert CO2 to HCOO, relatively low quantity of investigations was conducted to converting CO2 to the chemicals of CH3OH and CH4 by photocatalytic MOF, as the reactions for this production is normally sophisticate. As such, the demonstration using bare MOF was even rare, while the design of the MOF to achieve this application usually used a hybrid structure that featuring semiconducting materials with MOF, enabled superior optical and electronic properties to combine with high porosity from MOF. Wang and coworkers integrated ZIF-8(Zn) with Zn2GeO4 nanorods for photocatalytic converting CO2 to CH3OH [277]. The illustration of the synthesis is shown in Fig. 21A, in which, Zn2GeO4 nanorods were initially synthesized with tetramethylammonium hydroxide (TMAOH) adsorbed on the surface of the rods, and subsequently, the TMAOH served as the precursor to grow ZIF-8(Zn) on the surface of the nanorods. In this research, Zn2GeO4 acted as the semiconducting photocatalyst that was already widely investigated for CO2 reduction [278,279]. Herein, ZIF-8(Zn) acted as a platform that is capable of enhancing the adsorption of CO2 to

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Fig. 19. (A) A view of the structure of NNU-28(Zr) showing two types of cages (yellow spheres represent void spaces). (B) The quantity of the produced HCOO with NNU-28 (Zr) (50 mg), its ligand (40.2 mg) and blank reference. (C) The EPR singles of NNU-28(Zr) and the ligand with and without visible light irradiation. (D) In situ time evolution of EPR signals of NNU-28(Zr) under irradiation. (E) Schematic illustration of the dual excitation pathways over NNU-28(Zr). Reproduced with permission from Ref. [275]. Copyright (2016) The Royal Society of Chemistry.

Fig. 20. (A) The absorption spectra of the Ir unit (black) and Ir-coordination polymer (red). (B) Schematic diagram of the mechanism for the photocatalytic reduction of CO2 by Ir-coordination polymer under visible-light irradiation. Reproduced with permission from Ref. [276]. Copyright (2014) The Royal Society of Chemistry.

support the conversion, and the N2 and CO2 sorptions of the corresponding structures are shown in Fig. 21C and D, respectively. The Zn2GeO4/ZIF-8(Zn) hybrid nanorods containing 25 wt% ZIF-8(Zn)

exhibited 3.8 times higher dissolved CO2 adsorption with respect to the bare Zn2GeO4 nanorods. As such, the photocatalytic performance (Fig. 21B) for converting CO2 to CH3OH was achieved with

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Fig. 21. (A) Schematic diagram of the synthesis procedure of Zn2GeO4/ZIF-8(Zn) nanorods. (B) CH3OH production with (a) Zn2GeO4 nanorods, (b) Zn2GeO4/ZIF-8(Zn) nanorods and (c) 1 wt% Pt loaded Zn2GeO4/ZIF-8(Zn) as a function of the light exposing time. (C) N2 adsorption–desorption isotherms and (D) CO2 adsorption isotherms of the corresponding samples. Reproduced with permission from Ref. [277]. Copyright (2013) The Royal Society of Chemistry.

a 62% enhancement, obtained 1.43 lmol g1 in 10 h. However, the mechanism for the formation of CH3OH was absent in this research, as CH4 was normally obtained when bare Zn2GeO4 is used as the photocatalyst [278]. They suggested that the aqueous media can be considered as the mainly inducer to the formation of CH3OH. Lately, a series of hybrid MOFs was synthesized by integrating different amount of NH2-UiO-66(Zr) with Cd0.2Zn0.8S to produce CH3OH through photocatalytic reduction of CO2 [280]. When the content of NH2-UiO-66(Zr) was 20 wt%, the highest performance was obtained, giving the CH3OH production rate of 6.8 lmol h1g1. This structure also presented good stability, as the photocatalytic ability can be preserved up to 20 h. Cu3(btc)2 was conjugated with TiO2 to convert CO2 to CH4 by Xiong and coworkers [281]. The hybrid material was synthesized by hydrothermal approach, TiO2 nanoparticles were attached on the as-prepared Cu3(btc)2 to form the ‘‘bottle around the ship” structure (Fig. 22A). Previously, Cu3(btc)2 was developed to enable a high capability for CO2 adsorption with achieving 10.2 mol kg1 at 25 °C and 15 bar, the availability of the high surface was ascribed as the main reason for its outstanding adsorption property [282–284]. As such, the hybrid Cu3(btc)2@TiO2 presented the production rate of CH4 around 2.64 lmol h1 g1 and was absent of the side-product, H2 gas. This result is superior to that from the bare TiO2, as shown in Fig. 22B. The photoreduction mechanism was also proposed relating two major factors: one is the active site for the photocatalytic reduction, and the other was the chargeseparation efficiency to overcome the high kinetic barrier with the eight-electron process. The Cu sites were proved having much higher selectivity for the reduction of CO2 than that on TiO2 surface [285]. In this hybrid structure, CO2 could pass through the porous TiO2 layer and reached the surface of Cu3(btc)2. With the electron supplement from the photoexcited TiO2, the ability of Cu3(btc)2 to activate CO2 can be further improved as shown in firstprinciple simulation in Fig. 22C. The simulated potential energy on the surfaces along the variation of the O-C bond of the CO2 on Cu3(btc)2 is exhibited in Fig. 22D, two-electron charge injected

lowers the EB from 7.76 to 5.57 eV, and the one-electron charge would alter the activation-energy barrier for CO2 to facilitate the improved performance. This work opened a door to implement MOF structure in the photocatalyst design for the gas phase reaction, also comprehensively demonstrated the relationship between charge transfer and overall performance. More recently, porphyrinbased photocatalytic MOF was developed to convert CO2 to CH4 [286]. In this synthesis, TCPP was used to react with Zn(NO3)2 to form MOF under thermal treatment for 3 h. This MOF was demonstrated to exhibit photoresponse along the visible spectrum, presenting around 4-fold conversion efficiency over Cu3(btc)2@TiO2, while further structural and mechanism investigations were required, correlating to the improved performance. Lately, Wang and coworkers proposed a rapid way to synthesize the Cu3(btc)2, and its TiO2 composite is formed through a directly aerosol route [287]. This hybrid structure presented high good crystallinity, large surface area, and great photostability, they presented a significant enhancement of the efficiency for CO production. Ye and coworkers implanted singlet Co atom into a MOF, called MOF-525(Zr)-Co, for CO2 reduction, and the impalement of the Co atom is illustrated in Fig. 23A [288]. The coordination environment of Co was investigated by XANES and EXAFS, indicating that almost all the Co-existed as Co atom. As the results, the Co atomic dispersed MOF exhibited enhancement in CO2 conversions, giving 3.13-fold improving in CO evolution rate (200.6 mmol g1 h1) and 5.93-fold enhancement in CH4 production rate (36.67 mmol g1 h1) with respect to the parent MOF (Fig. 23B). The MOF also presented excellent recyclability, and the performances were preserved for at least 4 cycles (Fig. 23C). Two factors were attributed to the enhanced photocatalytic performance, the CO2 adsorption, and the enhanced electron separation and transport through the implanted Co atom. They carefully studied the CO2 adsorption behavior of MOF-525(Zr)-Co with measuring the CO2 uptake, and the optimized structure that CO2 adsorbed on porphyrin-Co unit was illustrated (Fig. 23D). Moreover, the C-O potential energy as a function of CO2 molecules adsorbed on the

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Fig. 22. (A) The structure when Cu3(btc)2 was intergraded with TiO2 to form core–shell structure: (a) structural illustration, (b) TEM and (c) SEM images of Cu3(btc)2@TiO2. (B) The production yield of CH4 and H2 from CO2 using Cu3(btc)2@TiO2 and bare TiO2 under UV irradiation for 4 h. (C) The simulated structure of the Cu3(btc)2 when CO2 was adsorbed. (D) Potential energy surfaces along the O-C bond length for the activation of the CO2 molecule adsorbed on Cu3(btc)2. Reproduced with permission from Ref. [281]. Copyright (2014) Wiley-VCH.

active sites is simulated in Fig. 23E. The results clearly indicated that under the light irradiation, the photoexcited electrons of porphyrin units were acquitted to the coordinately unsaturated Co centers, which then became active for CO2 reduction.

6. Photoinduced organosynthesis by MOF The research of organic photosynthesis through MOF devoted even less effect in comparison with other working directions, due to extensively precise controls are normally required for the organic reactions to perform high selectivity and yield. But the advantages of using photocatalytic MOF for such reaction are also obvious. The inherently high specific surface area with uniform porosity promised the heterogeneous catalytic properties. Moreover, the constituent, topology and surface functionality of MOF can be tuned by varying the metal ions or the organic ligand linkers for a certain reaction. Thus, although the study of photocatalysis by MOF, especially for the photoinduced organic synthesis, is still in the infancy duration, but still, potential to provide rational designs for an industrially scaled production of some chemicals. In this section, a few photocatalytic organic syntheses through MOF were highlighted, including oxygenation of phenol and sulfides, benzene to phenol transformation, aza-Henry reactions, chiral synthesis, radical polymerization, photoreduction and others. In the early stage of this research direction, Sn-porphyrin based photocatalytic MOFs were developed for selectively oxygenation of phenol and sulfides [289]. A 3D viewing structure of the MOF, namely [Zn2(H2O)4-Sn(TPyP)(HCOO)2]4NO3DMF4H2O (SnTPyP = 5,10,15,20-tetra(4-pyridyl)-tin(IV)-porphyrin), was shown (Fig. 24A). This MOF presented an improved selectivity and activity toward sulfoxides over the homogeneous counterpart, Sn(OH)2TPyP, as the results obtained from a series of the control experiments, as shown in Fig. 24B. This result can be mainly

explained by the ability of the immobilized photoactivity site, fully utilizing the catalyst in the heterogeneous phase. Almost at the same time, Lin and coworkers demonstrated the photocatalytic induced organic synthesis using UiO-67 for photocatalytic azaHenry reactions, to oxidatively couple amine and oxidation of sulfides [109]. However, the NM based sensitizer, Ir(ppy)2(bpy)+ and Ru(bpy)2+ 3 were used to incorporate with UiO-67 to achieve this target. Under visible light irradiation, Ir and Ru based MOF achieved the aza-Henry productions 59% and 86%, respectively. The results are slightly worse than that from the homogeneous control, authors believed that once the integration of MOF with these sensitizers, the formed single oxygen would mediate the aza-Henry reaction. But the hybrid structure exhibited superior reusability, no reduction of the performance could be seen in a few reaction cycles. A series of aerobic organic transformations was investigated by Wang and coworkers using NH2-UiO-66(Zr) [290]. The MOF was demonstrated for a few different starting materials, from benzyl alcohol to cyclohexane. The selectivity was highly dependent on the reacting medium and substrate. The photocatalytic performances are shown in Fig. 25A, and the mechanism is presented in Fig. 25B. The conversion rate highly depended on the activation energy of a-CAH bonds, they are in the order of benzyl alcohol > cyclohexanol > hexyl alcohol > cyclohexane. EPR was used to determine the formation of the intermediate under light irradiation, and the mechanism of the photocatalytic process could be elucidated through the detected active intermediates (Fig. 25C and D). O 2 was identified to form as the intermedium active specie during the photocatalytic process, it could act as a stabilizer in the cavities of NH2-UiO-66(Zr) due to its interactive functionality with the amine groups and/or organic solvents, thus benefiting the photocatalytic oxygenations of CAH and C@C bonds. A UiO-topographic MOF, UiO-68(Se), was further developed to induce the other metal for aerobic cross-dehydrogenative coupling

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Fig. 23. (A) A 3D illustration of Co atom implanting into MOF-525(Zr). (B) The production evolution of CO and CH4 over MOF-525(Zr)-Co (green), MOF-525(Zr)-Zn (orange) and MOF-525(Zr)-Zn (purple). (C) The measured recyclability of MOF-525-Co to produce CO (green) and CH4 (orange). (D) The illustration of the optimized structure that CO2 adsorbed on porphyrin-Co unit. (E) The OAC bond length-dependent CO2 activation energy barrier, in the situation of charged with one electron (orange) and neutral state (green). Reproduced with permission from Ref. [288]. Copyright (2016) Wiley-VCH.

reactions between tertiary amines and various carbon nucleophiles [291]. It also presented good performances under visible light. This demonstration proved the allowance of incorporating foreign photoredox catalysts with MOF, to achieve multiple functions, in organosynthesis applications. Li and coworkers conducted a series of MIL-topographic MOF for photooxidation synthesis. NH2-MIL-125(Ti) for the first time, was used for aerobic selective oxidation of amines to imines under visible light [37]. In this research, a variety of amines were demonstrated to transfer to imines. The mechanism was quite clear as the previous demonstrations for photocatalytic CO2 reduction in Section 5.1 [269]. Furthermore, NH2-MIL-101(Fe) was functionalized by Bi, which was achieved by one-pot reaction between aromatic alcohols and active methylene compounds through a tandem photooxidation/Knoevenagel condensation under visible light [189]. In this research, MOF acted more than a photocatalyst for the oxidation of aromatic alcohols to aldehydes, it also worked as a base catalyst for Knoevenagel condensation. The mechanism is proposed in Fig. 26A, and the enhanced efficiency of the tandem reaction was significantly influenced by the strength of the basic sites in the MOF.

Moreover, the hydroxylation of benzene to phenol was achieved by Li and coworkers using photocatalytic MOFs, MIL-100(Fe) and MIL-68(Fe), and H2O2 was used as an oxidant [292]. Phenol is one of the most important chemicals in industry, currently, the three-step Cumene process was needed for the production with sophistic reactions and high cost [293]. A variety of photocatalysts were investigated for this conversion [294–298], while, developing a sufficient strategy with inexpensive one-pot reaction is still very difficult, but highly desired. Among the catalysts, MOF photocatalyst was considered as a promising candidate for this reaction, based on the flexible functions and inherent porous structure. In this work, an optimal conversion of 30.6% was achieved with H2O2:benzene ratio of 3:4 over MIL-100(Fe) after 24 h irradiation. This excellent performance was elucidated by a comprehensive study of the mechanism (shown in Fig. 26B) based on EPR results. It suggested that a successful coupling of Fe-O clusters was achieved in the MIL-100(Fe) through a Fenton-like route for the hydroxylation. Very recently, a Zr based MOF was demonstrated for one-pot tandem photo-oxidative Passerini three component reaction of alcohols [299]. In the synthesis, they firstly modified UiO-66-NH2 with pyridinecarboxaldehyde to form light brown

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Fig. 24. (A) Schematic structure of the MOF, [Zn2(H2O)4-Sn(TPyP)(HCOO)2]4NO3DMF4H2O. (a) The side view of the 3D network of the lamellae linked by the formate struts. (b) A perspective view of the framework. (B) The photooxygenation of sulfides catalyzed by the MOF. Reproduced with permission from Ref. [289]. Copyright (2011) American Chemical Society.

Zr-MOF-PC crystals, which was further integrated with Fe as the catalyst, namely, Zr-MOF-FePC. This catalyst bears both active Lewis acid and photocatalyst sites to afford a-acyloxy amide through the tandem Passerini three component reaction in a onepot reaction. In this structure, the photocatalytic activity is mainly induced from LMCT bands from –NH2 group to Zr cluster, while the incorporation of Fe3+ significantly increased the catalytic activity. The catalyst also presented excellent reusability, the efficiency can be preserved in the course of reaction. This kind of multifunctional catalyst is highly desired to minimize the synthetic steps toward green chemistry. Thus, importantly, MOF for the first time, was demonstrated for diverse tandem oxidative multicomponent reactions, and was illustrated for the potential applications toward the future sustainable chemistry. The hybrid system that integrates metal oxides with MOF was also valid for an improved organic photosynthesis. Wu and coworker used a facile photodeposition process to decorate CdS on NH2-UiO-66(Zr), which was used for the selective oxidation of alcohols [300]. The photodeposition was monitored with a cyclic voltammogram curve, and the reduction of the Cd2+ was obvious to observe. In the meantime, it also showed that the light absorbing wavelength was gradually extended with increasing the depositing durations or the quantity of the CdS. A series of the alcohol oxidization was achieved using this hybrid structure under visible light irradiation. Benzyl alcohol, p-methyl benzyl alcohol and p-nitro benzyl alcohol were used, as the facts that the selectivity of them was approaching 100% by converting to the corresponding aldehydes. This result could be attributed to the feature of the substrate alcohols with electron-withdrawing group, such as –NO2 and –Cl groups, but this material displayed low conversion efficiencies. In contrast, alcohols with electron-donating groups like p-methoxy benzyl alcohols, the conversion could be elevated up to 63.2% without much loss of the selectivity (95%). The mechanism was also studied by monitoring the intermedia through EPR. In this system, the MOFs avoided the formation of the strongly

oxidative OH, but photogenerated hole and O 2 instead, which were suitable to oxidize alcohols to achieve a high selectivity. More recently, Ye and coworkers proposed to use core–shell structured MOF by coating anatase porous TiO2 shell on the surface of Cu3(btc)2 for photocatalytic oxidation of isopropanol [301]. The synthesis procedure was close to the previous work [281], while, for the isopropanol oxidation, two significances of the achieved hybrid material were critical to improve reaction efficiency. Firstly, this unique core–shell structure was shown an enhancement to capture reactants and intermediates. Secondly, MOF platform, Cu3(btc)2, provided a special pathway for photogenerated electrons migration, therefore restrained the recombination of electrons and holes. The photoreduction was also achieved using MOF. MIL-125(Ti) (integrated with Pt) was used to reduce nitrobenzene driven by solar energy [302]. In this research, the sacrificial electron donor, TEOA, was used to achieve the purpose. The quantity of the deposited Pt was optimized for the photoactivity, achieving the maximum production rate of 3.3 mmol h1 using Pt(1.5)/MIL-125(Ti), with respect to that of 2.3 mmol h1 by MIL-125(Ti). The MOF also presented high stability, by monitoring with XRD and FTIR, no significant difference of the resulted patterns can be observed after a few cycles of the photocatalytic reduction. In advanced strategies, enantiomeric MOFs were successfully synthesized, and applied for the synthesis of chiral compounds through photocatalysis. Duan and coworkers created two chiral photoactive Zn based MOFs by integrating triphenylamine photoactive unit with chiral component L- or D-pyrrolidin-2ylimidazole (L- or D-PYI) into a single framework, namely Zn-PYI1 and Zn-PYI2. In this structure, a tert-butyloxycarbonyl (Boc) group was used to protect the unstable PYI, in the meantime, showed catalytic activity through thermal deprotection (Fig. 27A) [303]. As such, the asymmetric a-alkylation of aldehydes was driven by solar energy with the deprotected MOF, for instance, under a common fluorescent lamp (26 W) as the incident energy source, phenylpropylaldehyde and diethyl 2-bromomalonate were coupled

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Fig. 25. (A) The EPR spectra of NH2-UiO-66(Zr) in (a) methanol, (b) trifluorotoluene, (c) acetonitrile and (d) benzyl alcohol without irradiation. (B) The intensity of the EPR curve at g = 2.009 as a function of irradiation duration and the decay by light off. (C) Schematic diagram of the photocatalytic mechanism of NH2-UiO-66(Zr). (D) Photocatalytic activity of NH2-UiO-66(Zr) for the transformation. Reproduced with permission from Ref. [290]. Copyright (2012) The Royal Society of Chemistry.

Fig. 26. (A) The illustrated mechanism for a one-pot tandem photocatalytic oxidation/Knoevenagel condensation through Bi functional NH2-MIL-101(Fe). Reproduced with permission from Ref. [189]. Copyright (2015) The Royal Society of Chemistry. (B) Possible reaction mechanism for the solar driven benzene hydroxylation to from phenol by MIL-100(Fe). Reproduced with permission from Ref. [292]. Copyright (2015) American Chemical Society.

(Fig. 27B), given a high yield of 74% with excellent enantioselectivity (92%). This performance could be accredited due to that the aldehyde and ethyl bromomalonate can be efficiently adsorbed into the channel of the MOF. Moreover, based on the porous MOF

structure, size selectivity could be observed with different kind of aldehydes. This was the first demonstration using chiral and light responsible moieties as cooperatively organocatalytic sites to synthesize chiral components. The availability, stereoselectivity,

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Fig. 27. (A) Schematic diagram of the multifunctional framework Zn(N-tertbutoxycarbonyl-2-(imidazole)-1-pyrrolidine) (top), and the structure of Zn-PYI1 obtained by thermolytic expulsion of the Boc moieties. The cyan, red, blue, gray and green balls represent Zn, O, N atoms and Boc moiety, respectively. (B) The conversion and ee value (in parentheses) of the photocatalytic a-alkylation of aliphatic aldehydes. Reproduced with permission from Ref. [303]. Copyright (2012) American Chemical Society.

Fig. 28. (A) The structural characterizations of the samples: (a) TEM and (b) HRTEM image of Pd nanocubics. (c) SEM and (d) TEM images of the hybrid structure (Pd nanocubics @ ZIF-8(Zn)), inset in (d): nanocubics in ZIF-8(Zn). (B) The yield of the hydrogeneration of 1-hexene over Pd nanocubics@ZIF-8(Zn) under different light intensities or upon heating at different temperatures. (C) The conversion of the hydrogeneration of various alkenes over Pd nanocubics@ZIF-8(Zn) and Pd nanocubics. Reproduced with permission from Ref. [307]. Copyright (2016) Wiley-VCH.

separability, and individual components fixed with the porous and repeat structure of MOF encouraged the photocatalyst to investigate and apply for this polymerization application by solar energy. Xing and coworkers developed NNU-35(Zn) for visible light induced radical polymerization [304]. Controlled radical polymerization techniques have been investigated for a long time, in order to have a facile access to functionalize polymer materials with welldefined structure and architecture. Beforehand, traditional metal oxide semiconductors were successfully applied for this approach induced by solar energy, and photocatalytic MOF is considered to

enable an improved control over the typical metal oxide semiconductors due to the ordered active sites [305]. In this study, anthracene-derived bipyridine ligand was incorporated with Zn to construct NNU-35(Zn) with a novel visible light response MOF. The visible light induced free radical formation of the bipyridine pillars was investigated. Following that, the atom transfer radical polymerization for methacrylate monomer was performed with reducing the copper catalyst via electron transfer. As the results, the prepared polymer showed a narrow molecular weight distributions and high retention of chain-end activities. More recently, Phan

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and coworkers investigated MOF for a series of polymerizations [306]. They constructed MOF-902(Ti) from a hexameric Ti, Ti6O6(OMe)6(CO2)6, with the organic linkage of biphenyl-4,40 -diyl bis-(methanylylidene)bis(azanylylidene)dibenzoate (BMAB), and this MOF had a HOMO–LUMO energy gap of 2.5 eV, which presented sufficient ability for visible light harvesting. The photoinduced polymerization was conducted for the monomers of methyl methacrylate, benzyl methacrylate and styrene. High performances were presented as the polymer formed in a high molecular weight and low polydispersity index. For instance, in the formation of poly (methyl methacrylate), it gave the production of molecular weight of 31.465 gmol1, and yield of 84% with low polydispersity index of 1.11. They suggested that the good performance was due to the high conjugation system with imine linking unit, in which, abundant electron donors were contained in the MOF, thus, stabilizing the free radicals in the polymerization reaction and affording dormant polymer chain with bromide end group. As such, the intrinsic porosity of the MOF should be further investigated in such working direction for polymerization, as considering, the constituent topography and surface functionality of MOF could crucially determine the quality of the polymer products. The plasmonic phenomenon was also performed by photocatalytic MOF for the selectivity photosynthesis. Very recently, Jiang and coworkers developed a novel hybrid structure by selfassembly of Pd nanocubics@ZIF-8(Zn) for photocatalytic hydrogenation of olefins, including 1-hexene, 1-octene, cyclohexene and cyclooctene [307]. The Pt nanocubics were prepared by the polyol method and coupled with MOF afterward [308–310]. The structure and the morphology of the hybrid are shown in Fig. 28A. This successful immobilized Pt nanocubics in ZIF-8(Zn) extensively enhanced the performance in hydrogenation of olefins, mainly due to the plasmonic photothermal effect, as indicated by the performance shown in Fig. 28B, with comparison of the yield that induced by light energy and heating energy. The performance for the hydrogenation is presented in Fig. 28C, revealing that the conversion efficiency was highly dependent on the size between the initial molecules and the MOF structure. On top of this work, this photothermal effect was followed up to be taken advantages for the selectivity control toward aldehyde in the aromatic alcohol oxidation [47]. In this research, Pt/PCN-224(Zn) was synthesized with a similar approach as mentioned above. The composite exhibited excellent performance in the photooxidation of the alcohols by 1 atmosphere O2 at ambient condition under solar irradiation. The research suggested that a synergetic effect by the integrated photothermal effect and singlet oxygen production highly increased the overall performance, and these phenomena were considered as the first findings in MOF. This rational fabrication indicated that to integrate a fine controlled cocatalyst in MOF, the overall catalytic performances can be further enhanced, including the selectivity, yield, stability and recyclability, moreover, the advanced observation and understanding is significant to lead the use of the NM based cocatalyst in MOF and other photocatalyst as well [47,309]. 7. Conclusion and outlook In this article, the recent progresses of MOFs for solar energy conversion by photoredox catalysis were highlighted. The review is initialized from the fundamental of band structures relating to the structure of MOFs. In subsequent, the essential roles by the formation of the hybrid structure were presented through integrating MOFs with NM metals and inorganic semiconductors. The photoredox catalysis for the solar energy conversion was reviewed on the application basis of the MOFs, including water toxicity degradation, water splitting, CO2 reduction and organic synthesis. In each chapter, the defined requirements and developed routing to apply

MOF in the applications were discussed. The originations, outstanding results, and a few unique phenomena were presented. These points could be the significance to innovate the materials and technologies to use photoactive MOFs. The energy and environmental problems still stand as a longterm aim to address, represented by the above-mentioned four applications. It is believed that MOF based photocatalysts could be the key to gradually manage each application. The long-term research key to apply MOF for photocatalysis is the semiconducting property, thus it is particularly important to systematic and comprehensive study, and control the band structure of MOF [311]. In a recent work, the band structure of a MOF was demonstrated to control using different ratios of metals for the bimetallic organic framework, given the HOMO–LUMO levels ranging from 2.96 to 2.58 eV. On the other hand, the structure of MOF should be carefully designed to avoid those easily destructive group, for instance, pyrolyzation unstable group in the framework would lead an easy destruction of the framework under high intensity light. The cost-effective precursor should be promoted to merits over the use of NM. Also, the relatively poor conductivity of the MOF should gain attention, as it acted as a factor to hamper the photoconversion efficiency. A few researches suggested that doping can extensively increase the conductivity based on the polaron or soliton mechanisms for MOF [312]. Specifically, in the photoinduced degradation of the water contamination, the MOF should be capable of tolerating the truly contaminated water, such as multiple-chemical with relatively abnormal pH levels. Meanwhile, the energy gap for photoredox reaction had better to be sufficiently high but in visible range, thus able to overcome the kinetic barrier and mineralize the contamination in a one-pot process. In photocatalytic water splitting, on the one hand, the optimization of the HOMO–LUMO levels to match water redox potential is always the priority to be concerned. On the other hand, water oxidation process is the restriction of photocatalytic MOFs, and requiring further development to abandon the sacrificial agents, and achieving the overall water splitting. The multiple-electron process and the strong oxidative medium needs the MOF to be sufficient active under light irradiation, in the meantime, preserving the structural integer. In CO2 reduction and the organic synthesis, beyond the HOMO–LUMO level engineering, the trade-off between the selectivity and yield should be further concerned. Especially for the organic synthesis, the flexibly porous channel of the MOF may be taken as a physical factor to encourage the selectivity. For instance, recently, a 1D channel of the assembled MOF was able to encapsulate the reaction substrate via a breathing process [313], and this phenomenon was completely based on a molecularly physical effect. Overall, the development of photocatalytic MOF was still in the infancy period, and the concept of the combination of the organic and inorganic holds a great promise to fulfill the crucial mission of the artificial photosynthesis. Acknowledgements We acknowledge the financial support by the National Basic Research Program of China (2013CB632405), the National Natural Science Foundation of China (21425309, 21703040, 21702030 and 2176132002), the national postdoctoral program for Innovative Talents (BX201600031) and the 111 Project. References [1] D. Ravelli, D. Dondi, M. Fagnoni, A. Albini, Chem. Soc. Rev. 38 (2009) 1999– 2011. [2] K. Nakata, A. Fujishima, J. Photochem. Photobiol. C 13 (2012) 169–189. [3] A. Mills, S.L. Hunte, J. Photochem. Photobiol. 108 (1997) 1–35. [4] S.-L. Li, Q. Xu, Energy Environ. Sci. 6 (2013) 1656–1683. [5] X. Wang, G. Zhang, Z. Lan, Angew. Chem. 55 (2016) 15712–15727.

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