Introduction

The unrestrained consumption of fossil fuels has dramatically increased the concentration of carbon dioxide (CO2) in the atmosphere and exacerbated the worldwide energy crisis and greenhouse effect [1,2,3]. Photocatalytic water splitting and CO2 reduction are considered effective strategies for alleviating the energy crisis and excessive carbon dioxide emissions [4]. Reducing the reaction activation energy and improving the reaction rate is crucial for water splitting into hydrogen and carbon dioxide photoreduction. Photocatalytic overall water splitting driven by solar energy can simultaneously generate H2 and O2, and H2 is an ideal energy carrier to replace traditional fossil fuels because of its clean combustion characteristics and high energy density [5,6,7]. Photoreduction of CO2 into value-added carbon-based products, including hydrocarbon fuels (e.g., CH4, C2H4, and C2H6) or chemicals (e.g., HCOOH, CH3OH, and CH3COOH), can push the end products of fossil fuels back into the carbon cycle [8,9,10,11]. To date, a variety of inorganic semiconductors have been studied for efficient energy conversion [12], but their indistinct active sites and imprecise microenvironments with complicated structures have impeded the efforts to reveal mechanistic insights into the energy conversion process.

Molecular clusters composed of multimetal centers with adjustable composition are widely used as photocatalysts with excellent catalytic ability to further provide mechanistic insight at the molecular level because of their well-defined structure [13,23]. Since 1972, when photocatalytic water splitting on TiO2 electrodes was achieved [5], photocatalytic H2 evolution has been considered a promising approach to overcoming the energy crisis and environmental issues. However, the current photocatalysts for water splitting have disadvantages, such as a low H2 yield and the need for a precious metal as a cocatalyst or photosensitizer [24]. Therefore, develo** a cheap and efficient photocatalyst for water splitting is desirable but remains very challenging.

Inspired by the photocatalytic H2 production on TiO2 in 1972, great progress has been achieved in recent decades by investigating metal oxide semiconductors (MOSs, such as TiO2, titanates, Ta2O5, and tantalates) as photocatalysts [25,26,27]. POMs, as an important subclass of metal-oxo clusters, possess inherent redox ability and semiconductor-like characteristics (Scheme 1).

Scheme 1
scheme 1

Proposed mechanism of the present catalytic system, visible light-driven H2 evolution using a POM catalyst. Notation: D is sacrificial electron donor; PS is photosensitizer

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POM Used as a Photocatalyst Directly

In 2011, Feng’s group [28] reported a new heteropolyoxonibate, [Nb2O2(H2O)2][SiNb12O40]10−, with photocatalytic water splitting activity, and its photocatalytic activity is substantially higher than that of [Nb2O2][SiNb12O40]10−. This work revealed that the coordinating water molecule to the bridging Nb5+ center can lead to highly unsymmetrical seven-coordinated Nb5+ sites, which contribute greatly to the enhanced photocatalytic activity for H2 production. Subsequently, Wang et al. [29] synthesized three polyoxoniobate clusters, {Nb24O72}, {Nb32O96}, and {K12Nb96O288}, which can be used as photocatalysts for H2 evolution with CoIII(dmgH)2pyCl as a cocatalyst under UV irradiation. At the same time, Liu et al. [30] reported {Ta12}/{Ta16} cluster-containing polytantalotungstates with remarkable photocatalytic H2 evolution activity. The high activity of {Ta12}-based POMs can be further rationalized by the presence of distorted heptacoordinated Ta atoms as a TaO7 pentagonal bipyramid. Furthermore, a series of high-nuclear spin nickel cluster-containing POMs were designed and synthesized with the lacunary POM as the coordination ligands. Introducing transition metal clusters improves their catalytic performance for hydrogen production under visible light irradiation with the assistance of noble metal photosensitizers [31, 32]. Recently, Lv’s group [33, 34] introduced transition metals such as manganese and iron into the lacunary Keggin polyoxometalate to achieve photocatalytic H2 evolution.

POM@photosensitizers Assembled and Used as Photocatalysts

However, traditional POMs often display light absorption in the ultraviolet region because of their large bandgap energy, which severely limits the improvement of the photocatalytic efficiency of the hydrogen evolution reaction (HER) [35,36,37]. Therefore, researchers have developed several strategies to broaden the spectral response range of catalysts. First, Lin et al. [24] constructed a charge-assisted hybrid POM@photosensitizers system in which a [P2W18O62]6− molecule as the electron acceptor was encapsulated in a [Ru(bpy)3]2+-based MOF for the photocatalytic HER (Fig. 1). Its HER performance can be much enhanced compared to that of the homogeneous catalytic system under visible light irradiation because of a fast multielectron injection from the photoactive framework to the polyoxoanion. Subsequently, they synthesized another [Ni4(H2O)2(PW9O34)2]10−@photosensitizers composite by encapsulating a Ni-containing polyoxoanion of [Ni4(H2O)2(PW9O34)2]10− into [Ir(ppy)2(bpy)]+-based phosphorescent UiO-MOFs [38]. The [Ni4(H2O)2(PW9O34)2]10−@[Ir(ppy)2(bpy)] composite exhibited efficient visible light-driven HER activity with a turnover number as high as 1476. A systematic study revealed that each [Ni4(H2O)2(PW9O34)2]10− anion was surrounded closely by multiple [Ir(ppy)2(bpy)]+ photosensitizers, which can facilitate facile multielectron transfer to contribute to the enhanced HER activity. Li et al. [39] prepared a supramolecular framework from a hexa-armed [Ru(bpy)3]2+-based precursor and cucurbituril, and the resulting supramolecular framework could adsorb Wells–Dawson-type POM [P2W18O62]6− to form photoactive POM@photosensitizer hybrids for catalytic hydrogen generation. Recently, Lv’s group [49, 50]. Among these heterogeneous and homogeneous catalysts, POMs represent a great subclass of WOCs that not only possess a well-defined structure but also can efficiently drive water oxidation.

In 2010, Hill et al. [51] reported a [Co4(H2O)2(PW9O34)2]10− POM comprising a Co4 core stabilized by oxidatively resistant polytungstate ligands, which can act as an efficient water oxidation catalyst sensitized by [Ru(bpy)3]2+. In 2013, Ding et al. [52] reported the Co-substituted Keggin POM K7[CoIIICoII(H2O)W11O39] for efficient visible light-driven O2 production and thermal catalytic water oxidation. In 2014, Zhang et al. [53] reported four all-inorganic, abundant-metal-based high-nuclear spin cobalt–phosphate-substituted POMs, which can be used as molecular catalysts for visible light-driven water oxidation. The Co4O4 cubane in the {Co16(PO4)4} cluster is structurally analogous to the [Mn3CaO4] core of the oxygen-evolving center in photosystem II (PSII). These four compounds were the first POM-based cobalt-phosphate-cluster molecular catalysts for visible light-driven water oxidation; thus, they can serve as a functional model of the oxygen-evolving catalysts. A systematic study first revealed that heteroatom regulation can realize the regulation of photocatalytic performance for the water oxidation of these POM clusters. Subsequently, three high-nuclear spin nickel clusters, {Ni12}, {Ni13}, and {Ni25}, were encapsulated in the lacunary of POMs via a similar synthetic process. These three compounds contain {Ni3O3} quasi-cubane or {Ni4O4} cubane units, which can be used for visible light-driven water oxidation [54]. These results provide all-inorganic polynuclear Co/Ni-based structural models for visible light-driven water oxidation. In 2014, Kortz’s group [55] reported a tetramanganese-substituted tungstosilicate [MnIII3MnIVO3(CH3COO)3(SiW9O34)]6− as the photocatalyst for water oxidation, which was composed of a mixed-valent MnIII3MnIVO3 Mn-oxo core to mimic the natural oxygen-evolving center (Mn4O5Ca), which has been observed in a Mn12-based POM [56]. In 2014, the water oxidation catalyst [(VIV5VV)O7(OCH3)12]− consisting of vanadium centers was reported [57], which opened the way to using non-expensive vanadium clusters for water oxidation in artificial photosynthesis. Then, Hill’s group [58] reported a homogeneous carbon-free cobalt-based water oxidation catalyst based on redox-active V-centered POM ligands, [Co4(H2O)2(VW9O34)2]10−. In 2018, Dolbecq and coworkers [59] immobilized the sandwich-type polyoxoanion [(PW9O34)2Co4(H2O)2]10− in the hexagonal channels of a ZrIV-porphyrinic MOF-545 hybrid framework (Fig. 2). The composite material exhibits high photocatalytic activity and good stability for visible light-driven water oxidation.

Fig. 2
figure 2

Reproduced with permission from Ref. [59]. Copyright © 2018, American Chemical Society

[(PW9O34)2Co4(H2O)2]@MOF-545 for photocatalytic water oxidation.

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Photocatalytic CO2 Reduction

Global energy demands largely depend on the combustion of fossil fuels, including coal, petroleum, and natural gas [60]. As the main component of greenhouse gas, CO2 is a key product during fossil fuel combustion. The immense emission of CO2 has resulted in severe environmental issues, such as global warming and extreme weather [61]. As is well known, fossil fuels will remain a major energy source in the foreseeable future, and CO2 conversion into fuels represents a straightforward strategy for solving the energy crisis and environmental problems [62]. Therefore, exploring cluster catalysts for CO2 photoreduction has attracted wide attention, and great progress has been made in this field.

In 2011, Ronny et al. [63] decorated a phenanthroline ligand at the 5, 6-position of a 15-crown-5 ether, which was used to prepare a metal–organic POM hybrid complex ReI(L)(CO)3CH3CN-MHPW12O40 (L = 15-crown-5-phenanthroline, M = Na+, H3O+). In the presence of Pt/C, the POM moiety in ReI(L)(CO)3CH3CN-MHPW12O40 can oxidize H2 to produce protons and electrons, which can be used to catalyze CO2 photoreduction to CO under visible light irradiation. In 2019, Lan’s group [9).

Fig. 8
figure 8

Reproduced with permission from Ref. [95]. Copyright © 2021, American Chemical Society

a Integration of a Ru(bpy)3 photosensitizer and a single-metal catalyst of [bpy-CuCl2] into a Eu-MOF platform for selectivity control of CO2 photoreduction to HCOO– versus CO, b comparison of production yield under different coordination conditions.

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Fig. 9
figure 9

Reproduced with permission from Ref. [96]. Copyright © 2021, American Chemical Society

(Cu1Pd2)1.3@PCN-222(Co) photocatalyst for the solar-driven carbonylation Suzuki coupling reaction under CO2 with multicomponent synergy.

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Conclusion

Currently, concern is growing that sustainable and clean energy should become an essential energy source for human life. In recent years, researchers have successfully investigated on various cluster photocatalysts for solar energy conversion. POMs and other metal-oxo clusters have exhibited excellent catalytic performance. These multimetal clusters have shown great potential in the field of photocatalysis because of their adjustable components, diverse structures, and multimetal synergistic catalysis with excellent catalytic properties. Some of them have been used as building units to assemble photosensitizers with excellent catalytic performance, which represents a popular research field. These cluster-based photocatalysts with well-defined structures have supplied typical models to show how to construct effective catalysts. However, the current metal clusters are mainly based on homogeneous catalysis with insufficient reusability and require additional photosensitizers and/or sacrificial agents to maintain the catalytic reaction cycle. The large crystal size of POM-based compounds needs to be ground to increase the specific surface area, and the limited light absorption ability and limited number of exposed active sites still need to be greatly improved. Although obstacles still need to be overcome, increasingly more studies have shown the advantages of POMs in the field of photocatalysis, and more effort is needed to explore efficient and robust cluster photocatalysts.