Introduction

Carbon dioxide (CO2) is a greenhouse gas and one of the major reasons for climate change. In this regard, photocatalytic reduction of CO2 (the process called “artificial photosynthesis”) can produce organic raw materials for fuels and chemicals, from CO2, water and sun light energy, just like plants. At the same time, they can reduce CO2 because CO2 is utilized as raw materials to produce resources for energy and chemicals. Therefore, artificial photosynthesis is said to be one of an ideal green technology [1, 2].

CO2 photocatalytic reduction, in other words, CO2 conversion refers to the transformation of CO2 into valuable products such as fuels and chemicals, which means CO2 can be utilized directly as a feedstock to react with other components to form chemical products. It should be remembered that CO2 is a highly stable molecule. As CO2 molecule, the C–O bond strength in CO2 molecule is as strong as 364 kJ/mol and the carbon atom has the highest oxidation state. For these reasons, highly efficient catalyst or large energy input is necessary to accelerate CO2 conversion into valuable chemicals [3]. There exist a few ways to convert CO2 into valuable energy or chemical products. Those are chemical fixation [4], hydrogenation [5], electrocatalysis [6] and photocatalytic reduction of CO2 [7, 8].

Among these procedures, photocatalytic reduction and photoreduction of CO2 gain more attention because this method converts CO2 into useful energy and chemical products by solely using sustainable solar energy without any extra energy from outside. CO, HCHO, HCOOH, CH3OH, C2H5OH, and CH4 can be synthesized by this reaction. Chemical substances synthesized by this reaction are determined by the number of electrons and protons transferred in the reactions. The selectivity of product and efficiency of CO2 reduction are effected by the reaction condition and thermodynamic reduction potentials [9]. Figure 1 presents the schematic figure of photocatalytic reduction of CO2 reaction steps in a case for a semiconductor with a suitable redox co-catalyst: (i) the semiconductor absorbs light upon irradiation when absorbed light energy is greater than the bandgap of the material, (ii) electron excitation from the valence band (VB) to the conduction band (CB), leaving holes in the VB simultaneously, and (iii) charge migration to the semiconductor particle surface, inducing oxidation reaction in H2O by the positive holes (h+), reduction of CO2 by multiple electrons (e−). The position of the band-edges determines whether or not semiconductors can catalyze and promote a specific reaction. In other words, the position of CB edge has to be more negative than the reaction potential of reduction. Likewise, the VB edge position must be more positive than reaction potential of oxidation [10]. Multiple number of electrons (2–8) required for CO2 reduction as well as a high recombination rate of electron–hole pairs are the critical issue in photocatalytic CO2 reduction [11]. Photocatalytic reduction of CO2 should be reacted either in the gas or liquid phase. In case of the liquid, H2O is often used as a solvent because it is of low cost and contains rich enough hydrogen. In addition to Fig. 1, the photocatalytic reduction reactions of CO2 in aqueous solution at pH = 7 and their reduction potentials with reference to the normal hydrogen electrode (NHE) at 25 °C and 1 atm are also exhibited in Table 1 [12, 13].

Fig. 1
figure 1

Elementary steps occurring in a photocatalytic reduction of CO2 over a semiconductor mediated by appropriate redox co-catalysts: i) light absorption, ii) electron excitation from the VB to the CB, iii) charge migration to the surface of the particle, oxidation of H2O by the positive holes (h+), reduction of CO2 by multiple electrons (e−) [14]

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Table 1 The photocatalytic reduction reactions of CO2 in aqueous solution at pH = 7 and their reduction potentials with reference to the normal hydrogen electrode (NHE) at 25 °C and 1 atm
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Various types of photocatalytic materials have been developed including TiO2 [15], ZnO [16], Fe2O3 [17], CdS [18], and ZnS [14]. These are inorganic materials that possess photocatalytic activity mainly under UV light illumination. In contrast, g-C3N4 is the one of semiconductor material which can demonstrate photocatalytic activity even under visible-light illumination [19]. Shortcomings of these materials are that their CO2 adsorption capability is low. Furthermore, high recombination rate of electron–hole pairs is also a concerned issue. On top of that, it is difficult to tune the absorption wavelength of conventional semiconductors because they already have intrinsic bandgap energy. For instance, TiO2 is mainly used for UV light photocatalysis owing to its own wide bandgap (3–3.2 eV) [20]. ZnO and CdS are not stable in water specially under light illumination, which is preventing them to be applied for actual application usage [21, 22]. Therefore, it is necessary to develop new photocatalytic materials with finely tunable energy band structures, high chemical, and water stability.

Considering above backgrounds, metal–organic framework materials (MOFs) have been gaining much attention recently. MOFs are a promising new generation of adsorbent materials due to their high specific surface area, high porosity, ease of functionalization, and adjustable structure [23, 24]. Compared to weak bonding such as van der Waals bonds and hydrogen bonds, MOFs possess stronger coordination bond energy, which endow them a certain stability. Furthermore, the structure, physical and chemical properties of MOFs are highly customizable; thus, researchers have created more than 20,000 types of MOFs so far, and that number is still growing [25]. This is possible because the use of different center metal ions forms a wide variety of MOF compounds with different organic ligands. As such, the number of possible combination of MOFs is enormous [26]. Various kinds of MOF application are under intensive studies and those include drug delivery systems for biomedical use [27], gas storage and separation [28, 29], catalysis [30], water treatment [31], sensor [32], degradation of organic pollutant [33] and electrochemistry such as batteries and fuel cells [34, 35]. Moreover, MOFs have demonstrated to be promising materials for CO2 photocatalytic reduction [36, 37]. The mechanism of photocatalytic MOFs is similar to the process of general semiconductors although there are some differences. The VB and CB are represented as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in MOFs, respectively [38, 39]. In case of MOF study in general, the HOMO and LUMO energy levels are correlated with the redox potential energy levels of the organic linker and the metal-oxocluster, respectively [40]. Figure 2 depicts the diagrammatic representation of HOMO/LUMO band of excitation interaction over MOF photocatalyst for CO2 conversion upon irradiation (S = substrate, S+  = oxidized substrate). Similar to the above mentioned semiconductor case, photoreduction process is induced upon solar irradiation to initiate the excitation of valence electrons (leaving holes behind) from the HOMO to the LUMO, which are then separated and transported to the MOF surface where the CO2 reduction is performed [41]. Metal clusters and organic linkers in MOFs can be modified and they can behave as antennas to harvest illuminated light to generate electron–hole pairs for CO2 photocatalytic reactions. They can be also finely tuned to control the optical absorption range of MOFs. This is also an advantage too. Normally, photocatalytic reactions in MOFs are suggested to be originated from a localized metal-to-ligand charge transfer (MLCT), a ligand-to-metal charge transfer (LMCT), or a π–π* transition of the aromatic ligand [39].

Fig. 2
figure 2

Diagram of HOMO/LUMO band of excitation interaction over MOF photocatalyst for CO2 conversion upon light energy irradiation (S = substrate, S+  = oxidized substrate). HOMO: highly occupied molecular orbital; LUMO: lowest unoccupied molecular orbital; MOF: metal–organic framework; VB: valance bands; CB: conduction band [41]

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In addition, as described above, MOFs can be advantageous because they possess extremely large surface area and a high CO2 adsorption capacity can be expected which is proved experimentally. Owing to this feature, high CO2 concentration in the pores can enhance the photocatalytic reactions compared to other type of semiconductor. Furthermore, the three-dimensional porous structures and high surface areas enable MOFs to incorporate foreign photoactive species into their frame works, through which photocatalytic reactions can be enhanced by the synergistic cooperation of the metal clusters, organic linkers and the incorporated active sites [42]. It should be noted that some MOFs have an intrinsic catalytic activity resulting from the catalytically active organic linkers and/or unsaturated metal sites [43].

However, in most cases, MOFs can only utilize ultraviolet light and not visible light since most of MOFs are intrinsically active under UV light. To increase light harvesting to visible-light range and to decrease the recombination rate of the photogenerated charge carriers, several challenges have been attempted to enhance MOF photocatalytic activity. Those are optimization of metal and organic linkers, introduction of photocatalytic additive such as metal particles, oxide particles, quantum dot, semiconductors and photosensitizers have been studied to increase photocatalytic activity of MOFs [44,45,46,47]. Table 2 summarizes the previous studies of photoreduction reaction of CO2 with MOFs and MOFs composite materials. MOFs are composed of various types of metal center and organic linkers as well as their composite materials.

Table 2 Summary the previous studies of photoreduction reaction of CO2 with MOFs and MOFs composite materials
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However, MOFs are essentially unstable against water and humidity which could be the critical weakness for practical usage. Even though, MOFs based on high valence numbered metal are said to be relatively robust. Thus, in this review, we would like to focus on 4 metals (Fe, Zr, Ti and Al) based MOFs since they are relatively stable against water and humidity [70]. We will also explain the control and modification strategies on the metal cluster, organic ligand of MOFs as well as foreign additive to MOF, and their corresponding photoreduction activities on CO2. Except aluminum, these MOFs share the similarity that they all contain metal ions with variable valence states (e.g., Fe3+/Fe2+, Ti4+/Ti3+ and Zr4+/Zr3+), which enables their effectiveness on photocatalytic reduction [36]. In addition, Zr, Ti, Al-based MOFs are reported to have relatively large bandgaps such as approximately 3.9 eV, 3.6 eV and 3.4 eV for UiO-66(Zr), MIL-125(Ti) and MIL-53(Al), respectively. Large bandgap ordinary restricts the usage of available solar light energy for photocatalytic activity. Even though, one can improve them to photoresponse to visible-light irradiation by modification of organic linkers, addition of foreign additives such as metal particles, oxide particles as mentioned above. In contrast, Fe-based MOFs intrinsically possess smaller bandgaps such as 1.88–2.88 eV which can respond to visible light without any modification [71].

On top of that, these 4 metals have advantages in terms of material cost and its abundance as natural resource. In the following, we will start with the MOF with iron, which is cheapest and most abundant metal among 4 metals.

Fe-based MOF for CO2 photocatalytic reduction

In comparison to conventional inorganic semiconductors, it is easier to modify the optical properties and photocatalytic properties of MOFs by optimizing the metal clusters because it is possible by just changing metal resource as raw material. There are some studies demonstrating changing energy bandgap of MOF by shifting the photoabsorption edge from the UV to visible-light region by tuning metal clusters.

So far, Fe, Ti and Zr-based MOFs have been actively investigated photocatalytic MOFs when classified in terms of the metal kind. Fe-based MOF can act as CO2 photoreduction material because Fe–O clusters can be photoexcited sites to generate electron transfer from O2– to Fe3+ to form Fe2+ even under visible-light irradiation [72]. Since Fe already possess factor as photocatalytic center, Fe-based MOFs can act as photocatalytic material without LMCT reaction [72]. Furthermore, Fe has advantages over material cost and its abundance as natural resource when compared to other metals.

Dao et al. had prepared two types of Fe-based MOFs (MIL-100(Fe) and MIL-101(Fe)) and compared their CO2 to CH4 photocatalytic reduction performance. They have found that MIL-100(Fe) demonstrated higher photocatalytic activity and selectivity for CH4 generation under visible-light irradiation [73].

Three types of Fe-based MOFs, MIL-101(Fe), MIL-53(Fe), and MIL-88B(Fe) were synthesized and compared their photocatalytic activity for CO2 reduction to form HCOO− under visible-light irradiation. Photocatalytic activity of MIL-101(Fe), MIL-53(Fe) and MIL-88B(Fe) of HCOO− production with TEOA (Triethanolamine) as a sacrificial reactant was 59.0, 29.7, and 9.0 mmol, respectively, with 8 h of visible-light illumination as light energy source. They had clarified that unsaturated Fe sites in structure of MOF were one of the origin for MIL-101 (Fe) to demonstrate the strongest photocatalytic performance. It was deduced that MIL-53(Fe) indicated a higher activity than MIL-88B(Fe) owing to its higher CO2 adsorption capability (13.5 g/cm3) than MIL-88B(Fe) (10.4 g/cm3). Amine functionalization to MOF structure was also speculated to be contributing photocatalytic activity increase since all 3 kinds of Fe-based MOFs exhibited higher activity than the ones without amine functional group.

Two major reasons were proposed for CO2 photocatalytic enhancement. First, reason was that light energy excitation of organic ligands with amino functional group can accelerate the electron transfer from ligand to Fe center. The other reason was Fe–O cluster can accept direct light energy excitation to cause photocatalytic activity [74].

Experimentally, they had assured visible-light response with UV−VIS spectra. Figure 3 presents the absorbance data of intact MIL-101(Fe) and the one functionalized with NH2 group. One obvious broadband absorption at 200–450 nm with continued absorption to visible-light region was observed for bare MIL-101 (Fe). One deconvoluted band can be detected at around 270 nm. The band at ca. 270 nm is suggested to be due to charge transfer from the oxygen to iron in an octahedral coordination environment. In addition, existence of Fe3O clusters contributed to absorption in the visible-light region. Previous studies also demonstrated that the existence of iron clusters in the Fe-based MOF structure can result in light absorption to the visible-light region [75]. Addition of amine group to Mil 101 (Fe) is reported to demonstrate enhanced visible-light response by toluene photodegradation study too [76].

Fig. 3
figure 3

UV–Vis spectra of MIL-101(Fe) and NH2 Mil 101 (Fe) [74]

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To elucidate the semiconducting properties of MIL-101(Fe) upon light excitation, ESR studies were also carried out to detect the active species involved in the photocatalytic reaction. As shown in Fig. 4, the original reaction system (including MIL-101(Fe) and TEOA) in the dark gives a typical ESR signal ascribed to Fe3+ in octahedral FeO6 [74]. When visible light is irradiated on the above reaction system, the above ESR signal had weakened. This phenomenon can be described by a charge transfer from O2− to Fe3+ and the evolution of Fe2+ in MIL-101(Fe) by visible-light irradiation. When CO2 concentration was increased again by adding extra CO2 into experimental setup, ESR signal of Fe3+ was emerged again. This indicates that the photogenerated Fe2+ is necessary in the photocatalytic CO2 reduction [77].

Fig. 4
figure 4

ESR spectra of a MIL-101(Fe) and TEOA mixture [74]. a In the dark, b light irradiation, c in the presence of CO2 with light irradiation

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Photocatalytic CO2 reduction reaction without any solvent and liquid was carried out and experimental setup is illustrated in Fig. 5 [52]. A few kinds of Fe-based MOF were prepared and amino functional group was also introduced to extend light absorption to visible-light range. In their experiment, MOF was uniformly and evenly spread onto a glass fiber film as a substrate and utilized as photocatalytic reaction sites. Then, the CO2 gas was pumped into experimental apparatus in order to initiate the reaction. Sun et al. had proposed that this type of liquid free CO2 photocatalytic reduction system would be more ideal for real application usage since contact area of CO2 gas and catalytic part can be enlarged easily, when compared to conventional liquid–solid reaction artificial photosynthesis system.

Fig. 5
figure 5

Experimental setup illustration for the solvent free CO2 photocatalytic reduction reaction [52]. a CO2 steel cylinder b mass flow controlled, c vacuum pump, d circulating pump, e photoreactor, f heating plate, g automatic gas sampler, h gas chromatograph, i pressure meter, j light source

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To investigate more detail of CO2 photocatalytic reaction mechanism with Fe-MOFs, semiconductor behaviors of the Fe-MOFs were analyzed with UV–Vis spectra and Mott−Schottky electrochemical measurements. With Mott−Schottky method, bandgap energy can be calculated out by intercept of the tangents of (Ahυ)2 vs photon energy, in which A is a constant and υ is the incident photon energy [48]. As a results, the bandgap energy was estimated to be 1.98, 1.72, and 1.77 eV for NH2-MIL-53(Fe), NH2-MIL-88B(Fe), and NH2-MIL-101(Fe), respectively (Fig. 6a). Since the metal cluster environment including coordination number is similar for NH2-MIL-88B(Fe) and NH2-MIL-101(Fe), they would possess similar bandgap energy as well as the flat-band potentials. Photoluminescence (PL) spectra of Fe-MOFs are measured with 350 nm excitation light (Fig. 6b). One can clearly observe that NH2-BDC (terephthalic acid) ligand exhibit emission at 485 nm. When metal–organic framework was combined, this obvious emission was disappeared and Fe-MOFs exhibited lower fluorescence. This phenomenon can be explained with ligand-to-metal charge transfer (LMCT) due to the combination of organic groups and the metal–organic framework [52].

Fig. 6
figure 6

a Plots of (Ahv)2 vs photon energy. b PL emission spectra (λex = 350 nm). c Photocurrent responses. d EIS Nyquist plots for NH2-MIL-53(Fe), NH2-MIL-88B(Fe) and NH2-MIL-101(Fe)

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The photocurrent measurements were performed in order to clarify the photogenerated charge separation efficiency. In general, higher photocurrent is the proof of higher photocatalytic activity [78]. The result of Fig. 6c deduces that the NH2-MIL-101(Fe) possesses the highest photocurrent and photocatalytic activity among the Fe-MOFs. Thus, NH2-MIL-101(Fe) is suggested to have the strongest CO2 photocatalytic reduction activity.

Furthermore, electrochemical impedance spectroscopy (EIS) was measured to elucidate the carrier mobility of Fe-MOFs (Fig. 6d) [52]. NH2-MIL-101(Fe) exhibited the lowest impedance which suggests the most efficient charge transfer among Fe-MOFs. The conductivities were calculated out from the Nyquist plots results by applying equation, σ = L/(S × Ret), in which L (cm) and S (cm2) are the thickness and area of the sample, respectively. It is known that the electron transfer resistance (Ret) is equal to the semicircle diameter in the Nyquist plots [79]. As a result, NH2-MIL-101(Fe) demonstrated a highest conductivity of 4.1 × 10−6 S cm−1 compared to those of NH2-MIL-53(Fe) and NH2-MIL-88B(Fe) with conductivities of 1.1 × 10−6 and 2.2 × 10−6 S cm−1, respectively. Conductivity difference can be described by their framework structures. Regarding NH2-MIL-101(Fe), four μ3-O-bridged Fe3O clusters exist in a tetrahedron form (Fig. 7c). In this case, electron can move around easier than the case in the triangular bipyramidal form of five Fe3O clusters in NH2-MIL-88B(Fe) (Fig. 7b). Furthermore, NH2-MIL-53(Fe) with μ2-OH bridges (Fig. 7a) indicated lower conductivity than NH2-MIL-88B(Fe) and NH2-MIL-101(Fe). So that as speculated from photocurrent measurement, NH2-MIL-101(Fe) should have the strongest CO2 photocatalytic activity.

Fig. 7
figure 7

Structures of Fe-MOFs NH2-MIL-53(Fe) (a) NH2-MIL-88B(Fe), (b) NH2-MIL-101(Fe) (c)

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Besides looking at functional group conjugation and iron cluster environment in MOF structure, addition of foreign substance such as quantum dot was investigated. Lu et al. attempted preparing quantum dot–MOF composite material. Iron-based MOF with quantum dot (QD) was synthesized as efficient CO2 photocatalytic material. They encapsulate CH3NH3PbI3 (MAPbI3) perovskite QDs in the pores of Fe porphyrin-based MOF PCN-221(Fex) (x = 0 − 1). Stability of QD was largely enhanced even in a water by synthesizing MOF-QD composite. They also discovered that the close contact of QD next to the iron increased the catalytic reaction by transferring photogenerated electrons to Fe catalytic site rapidly. Prepared composite material exhibited 38 times higher CO2 photocatalytic activity compared to the MOF without perovskite QD (PCN-221(Fe0.2)). To further elucidate the reason of largely improved photocatalytic CO2 reduction reaction, they investigated the light-harvesting property of MOF-QD composite materials by measuring the electronic absorption spectra of MAPbI3@PCN-221(Fe0.2) and PCN-221(Fe0.2)). They proved that the absorption strength of MAPbI3@PCN-221(Fe0.2) was superior than that of PCN-221(Fe0.2) which should be due to high absorption coefficient of quantum dot.

Flat band potentials were calculated out from Mott–Schottky plots. In addition, results were approximately 1.00 V and 1.25 V (against the normal hydrogen electrode (NHE)) for PCN-221(Fe0.2) and MAPbI3@PCN-221(Fe0.2), respectively (Fig. 8a, b). From these results, one can say that photogenerated electrons in PCN-221(Fe0.2) is strong enough to reduce CO2 to CO (@0.52 V vs. NHE) or CH4 (@0.24 V vs. NHE). Furthermore, the transfer of photoexcited electron in QD-MOF composite to MOF is thermodynamically possible. Therefore, photoinduced electrons could transfer from porphyrin and MAPbI3 QDs to Fe catalytic sites.

Fig. 8
figure 8

a Mott–Schottky plots of PCN-221(Fe0.2) and b MAPbI3/PCN-221(Fe0.2). c Steady-state photoluminescence spectra of PCN-221, PCN-221(Fe0.2), MAPbI3/PCN-221, and MAPbI3/PCN-221(Fe0.2). d Time-resolved photoluminescence decays of PCN-221, PCN-221(Fe0.2), MAPbI3/PCN-221, and MAPbI3/PCN-221(Fe0.2) with ET519LP as long-wavelength pass filter, after excitation at 375 nm

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They have additionally performed the steady-state photoluminescence (PL) measurements to scrutinize the photogenerated electron behavior in QD-MOF composite. PCN-221 exhibited 2 obvious peaks around 655 and 712 nm and they are suggested to be arise from PL of porphyrin groups. New peak appears approximately at 610 nm when MAPbI3 QD is combined. This is obviously due to QD itself luminescence. However, when Fe is combined in case for PCN-221(Fe0.2) and MAPbI3@PCN-221(Fe0.2), all of these peaks disappears. This is because efficient transfer of photogenerated electrons occurs from porphyrin and quantum dot to Fe catalytic sites.

Time-resolved photoluminescence (PL) was carried out in order to speculate the photoexcited charge separation dynamics. Figure 8d shows a rapid PL decay in PCN-221(Fe0.2) compared to PCN-221 which is without Fe ion. This fact indicates the rapid electron transfer from porphyrin groups to Fe catalytic sites is proceeding after electrons are generated by photoexcitation. In addition, MAPbI3@PCN-221(Fe0.2) also presented a fast PL decay compared to PCN-221(Fe0.2), indicating the same phenomenon is happening from MAPbI3 QDs to Fe catalytic sites, instead of porphyrin group to iron. Thus, it was suggested that this is due to the close contract between MAPbI3 QDs and Fe catalytic sites [80].

Among Fe-based MOF materials, MIL-101(Fe) exhibited the strongest CO2 photoreduction activity owing to unsaturated Fe sites in its structure. The existence of Fe3O clusters in MIL-101(Fe) is the main origin of visible-light photocatalytic response although it was further enhanced by addition of functional amine group to organic linker, by accelerating the electron transfer from ligand to Fe center. In addition, it was clarified that high conductivity of NH2-MIL-101(Fe) also contributed the strong photocatalytic activity which is due to fast electron transfer in four μ3-O-bridged Fe3O clusters exist in its tetrahedron form. MOF–quantum dot composite material had stronger CO2 photoreduction activity than bare MOF because of the close existence of quantum dot adjacent to the iron. Photogenerated electrons were efficiently transferred from porphyrin groups as MOF organic linker, to Fe catalytic site, via quantum dot. Stronger light absorption ability of MOF quantum dot composite material was another reason for improved CO2 photoreduction reaction.

Zr-based MOF for CO2 photocatalytic reduction

Zr has 4 valence numbers and its high valence number can confer Zr-based metal–organic framework to possess high stability against surrounding environment especially against water and humidity. They also bear strong robust framework, structures and high chemical, thermal stabilities [81]. UiO-66(Zr) and UiO-67(Zr) are the representative type of Zr-based MOFs and composed of Zr6O4(OH)4 SBUs (Secondary Building Unite) with BDC ligands and 4,4′-biphenyldicarboxylic acid (BPDC) ligands, respectively [82]. There are some study carrying out CO2 photocatalytic reaction with these stable zirconium-based MOFs.

Liu et al. synthesized two metal ion (Co2+, Re+) doped UiO-67 as catalysts for the photocatalytic CO2 reduction and presented that Cobalt-doped UiO-67 indicated stronger CO2 photocatalytic activity compared to Rhenium-doped UiO-67. Higher CO2 absorption capacity and charge carrier mobility are suggested to be the reason for Co-UiO-67 to exhibit stronger CO2 photocatalytic activity than Re-UiO-67. It was also elucidated that energy barrier of Re-UiO-67 (0.92 eV) for photocatalytic reaction from CO2 to CO turned out to be higher than that of Co-UiO-67 (0.86 eV) by density functional theory (DFT) calculation. Since energy barrier of cobalt-doped MOF was lower, one could deduce that Co-UiO-67 exhibited stronger CO2 photocatalytic activity [83].

Conjugation of amine functional group onto MOFs is one of major strategy to extend its light response to longer wavelength. Sun et al. had synthesized zirconium metal–organic framework Zr-SDCA-NH2, (SDCT: stilbenedicarboxylic acid). Since this material is zirconium metal-based MOF, it exhibited high chemical stability and light absorption edge was extended to around 600 nm. The CO2 photoreduction ability as formate formation rate of prepared amine conjugated zirconium-based MOF was 96.2 μmol h−1 mmol MOF−1 and this was higher than conventional Zr-based MOF without any conjugated functional group such as amine. It was suggested from experimental results that organic ligand and LMCT process to Zr6 oxocluster were reasons for demonstrating higher photoreduction ability [84].

Instead of amino group, Zhongmin et al. utilized and conjugated visible-light response type organic ligand derived from an anthracene group to synthesize zirconium-based MOFs. As this was also zirconium-based MOFs, prepared material demonstrated high thermal, chemical stability and CO2 absorption ability. Anthracene group also was able to extend the light absorption to visible-light range and indicated high formate formation rate of 183.3 μmol h−1 mmol MOF.−1. From the series of electron paramagnetic resonance (EPR), analysis and experimental results revealed that anthracene-based ligand and building unit Zr6 oxo cluster were the origin for demonstrating high photocatalytic activity. [85]

As we have seen in Fe-based MOF section, incorporating foreign substance to MOF is one of other strategy to improve photocatalytic activity. Serre et al. had introduced copper nanoparticles into 2 typical zirconium-based MOFs: MOF-801 and UiO-66-NH2. They also carried out room temperature scalable synthesis procedure for considering real industrial application [86]. Initially, copper (Cu) nanoparticle was synthesized with using L-ascorbic acid as reducing and capping agent. On the other hand, zirconium oxocluster was prepared from ZrCl4 and then after, these 2 components were mixed to prepare final Cu–Zr-MOFs composite. With this procedure, the size of Cu–Zr-MOFs composite material became smaller as more quantity of Cu nanoparticle was added within the experimental synthesis condition (Fig. 9a). One could also observe Bragg peak broadening as concentration of Cu nanoparticle increases. This kind of phenomenon is often seen in a nucleation process-induced seed-mediated crystal growth, in which seeds of small Cu nanoparticles behave as nucleation sites and suppress further crystal growth. High-resolution transmission electron microscopy and particle size analysis elucidated that prepared Cu nanoparticles are well dispersed in the solution with the average size of 1.6 nm (Fig. 9b, c). Cu–Zr-MOF composite was yellowish color and plasmonic absorption peak which originates from quantum size effect did not appear much (Fig. 9e) [87, 88]. This type of prepared Cu–Zr-MOF composite presented more of core–shell structures rather than Cu being embedded in MOF pore (aperture of MOF 5–7 Å is much smaller than 1.6 nm). As a result, it indicated high CO2 photoreduction rates of 94 µmol h−1.

Fig. 9
figure 9

a Representation of the Cu nanoparticles concentration-dependent size reduction of the prepared Cu nanoparticles/MOF-801. b TEM images of Cu nanoparticles. Insert: a photograph of the synthesized Cu nanoparticles. c particle size distribution of the synthesized Cu nanoparticles. d PXRD patterns (λCu = 1.5406 Å) of MOF801(1) and Cu nanoparticles/MOF-801 with increasing Cu nanoparticles loading from 64 to 6.4 mmol (labeled with 1–9)

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In addition to metal particle, as we have seen in Fe-based MOF section, graphitic carbon nitride quantum dots (g-CNQDs) was introduced to zirconium-based MOF to prepare composite (g-CNQDs/MOF) as CO2 photocatalytic reduction material [89]. Unique properties of g-CNQDs improved composite material catalytic activity by enhancing conductivity and extending the life time of photogenerated charge due to effective electron–hole pair separation. As a result, g-CNQDs/MOF composite converted CO2 into methanol with the rate of 386 μmol h−1 g−1, which is much higher than the reaction efficiency promoted with MOF without g-CNQDs (66 μmol h−1 g−1). Saikia et al. had proposed mechanism of this composite as depicted in Fig. 10. g-CNQDs absorb illuminated light energy and charge separation proceeds as first step. Then after, separated electron transport to semiconductor (MOF) surface to form catalytic sites. Since the conduction band of MOF higher than that of g-CNQDs, electron transfer from conduction band of g-CNQDs to valence band of MOF proceeds which results in displaying typical Z-scheme mechanism. In this situation, CO2 photocatalytic reduction rate to form methanol of g-CNQDs/MOF is higher than sole MOF (NH2-UiO-66) as photocatalytic material. In addition, –NH2 functional groups on MOF surface adsorb more CO2 than bare MOF surface that also contribute to gaining stronger CO2 photocatalytic reduction. Thus, the combination of quantum dots and NH2-terephthalate ligands can confer photogenerated electrons to possess longer lifetime to enhance the reaction. On the other hand, holes generated by photoexcitation in the valence band of g-CNQDs obtain electrons from trimethylamine to complete the CO2 photocatalytic reduction reaction flow.

Fig. 10
figure 10

Proposed mechanism of CO2 reduction on composite as g-CNQDs/MOF

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Besides foreign substance introduction to MOFs, defects in MOFs can confer more efficient charge separation and light absorption. Wang et al. prepared a few UiO-66-NH2 with different types of defects to see the influence upon CO2 photocatalytic reduction [90]. MOF with ligand vacant defect presented the strongest CO2 photocatalytic reduction reaction compared to non-defect, missing cluster and monocarboxylate compensated ones indicated 9.2 times higher CO formation rate than missing cluster one. The relation between electronic properties such as Eabs (absorption energy) and ELMCT (ligand-to-metal charge transfer energy), and photocatalytic activity in MOF defect structure was explained with DFT calculation. It was suggested that minimum energy total of Eabs and ELMCT will decrease reaction energy barrier in the rate limiting step which can promote the CO2 photocatalytic reduction.

It was possible to decrease the energy barrier to accelerate CO2 photoreduction by applying appropriate doped metal to Zr-based MOFs. Conjugation of amine functional or anthracene group onto Zr-based MOFs also improved the catalytic reaction by promoting LMCT process to Zr6 oxocluster as well as by enhanced light absorption. Introduction of foreign additive such as copper nanoparticles and quantum dot was also effective way to increase photocatalytic activity by obtaining higher conductivity and by extending the life time of photogenerated charge and following effective electron–hole pair separation by Z-scheme mechanism. Defect structure in MOF and amine functional group introduction also enhanced the photocatalytic reaction by increasing photoabsorption ability and by lowering the energy barrier, respectively.

Ti-based MOF for CO2 photocatalytic reduction

In case of Ti-based MOF, MIL-125(Ti) (Ti8O8(OH)4(O2CC6H4–CO2)6) is the most investigated MOF, and that is composed of the Ti8O8(OH)4 secondary building units (SBUs) and 1,4-benzenediacarboxylate (BDC) ligands. In addition, this type of MOF has been applied as photocatalytic materials. For example, CO2 photocatalytic reduction with MIL-125(Ti) was carried out and 2.41 mmol of formic acid was generated in the acetonitrile (MeCN) solvent with TEOA as a sacrificial agent with 365 nm UV light irradiation for 10 h [91]. As in the case for Fe and Zr-based MOFs, various kinds of challenge have been performed upon Ti-based MOF, to strengthen photocatalytic activity including foreign substance or functional group introduction, metal doping and organic linker modification.

Zhang et al. prepared a few kinds of Cu2+-doped two-dimensional Ti-based MOFs with solvothermal synthesis procedure [92]. When the concentration of doped Cu2+ is optimized, production rate of CH4 was 3.7 μmol g−1 h−1 with 93% electron selectivity. Investigation of CO2 adsorption, charge separation process, electronic and band structure revealed when the amount of doped Cu2+ is appropriately suitable to Ti-based MOF, thermodynamically confer the MOF conduction band position to have strong reduction potential for the photocatalytic reduction. In addition, photoexcited charge career generation, separation and transportation process was facilitated by controlling the local electronic circumstances around titanium oxide clusters.

Apart from metal doping to MOF, regulating facet of MOF crystalline is another strategy to influence the catalytic activity. Sun et al. had prepared NH2-MIL-125(Ti) with different ratio of {001} and {111} facets by modifying the amount of DMF, methanol as solvent and acetic acid as catalyst, in the solvothermal synthesis [93]. In addition, it was found that the titanium tetraisopropanolate or titanium butoxide as titanium precursor also influenced the facet. The {111} facets indicated stronger CO2 photocatalytic activity to convert to CO and CH4 with yields of 8.25 and 1.01 mol g−1 h−1, those are 9 and 5 times superior than yields of {001} facets, respectively. Figure 11a presents UV–visible-light absorption of prepared samples. 2 main absorption broad peaks around 300 and 400 nm correlate to the absorption of Ti−Ox clusters and organic ligand, respectively [94]. The absorption wavelength was red shifted as {111} facets ratio increased. Figure 11b displays the Tauc plots and from which highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) gap can be figured out. Then, HOMO−LUMO gap can be considered as bandgap energy for MOFs as being semiconductor. Combined with the flat-band potential results obtained from Mott−Schottky plots, the calculated HOMO–LUMO bandgaps energies were 2.75 eV (NM001), 2.69 eV (NMA), 2.65 eV (NMB), 2.66 eV (NMC), and 2.61 eV (NM111) (Fig. 11c). It should be mentioned here that N represents as NH2, M indicates as Mil-125 (Ti). In addition, facet is the number following 001 and 111, whereas A, B, C means mixed facet of 001 and 111 and C has the highest 111 facet. Thus, {111} facets in NH2-MIL-125(Ti) is most promising face due to lowest bandgap energy, which means it only requires less solar energy. They also discovered that recombination rate of photoexcited electrons and holes are suppressed more on {111} facets. It was also found that the Ti (III) is the strong reducing agent for CO2.

Fig. 11
figure 11

a UV − Vis diffuse reflectance spectra, b Tauc plots, c HOMO−LUMO gap of the as-synthesized photocatalyst materials

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MOF was combined with carbonaceous material to create composite material with stronger catalytic activity. Bu et al. synthesized reduced graphene oxide and NH2-MIL-125(Ti) composite material (rGO/NH2-MIL-125(Ti)) by solvothermal procedure [95]. Prepared composite material generated mainly methyl formate of 1116 µmolg−1 h−1, which is more than twice of MOF without graphene oxide. It was suggested that introduction of reduced graphene oxide and amino functional group to MOF was effective to enhance CO2 photocatalytic activity. They had inferred that uniformly dispersed catalytic material also accelerated the effective photoexcited carrier separation and transfer. SEM observation of MIL-125(Ti), NH2-MIL-125(Ti) and the composite rGO/NH2-MIL-125(Ti) are displayed in Fig. 12. MIL-125(Ti) exhibited smooth surface with tablet like form with approximate size of 1 µm, and when amino group was conjugated, much smaller MOF particles were obtained. However, the dispersion status became worse by aggregation compared to the one without amino group. When rGO was introduced, uniformly dispersed MOF distribution had recovered by easing the aggregation probably due to layered structure of reduced graphene oxide.

Fig. 12
figure 12

SEM images of MIL-125 (A), NH2-MIL-125 (B), rGO@NH2-MIL-125 (C, D)

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Modifying the organic linker for Ti-based MOF is another way to obtain alternative MOF instead of MIL-125(Ti). Wang et al. introduced isophthalic acid (IPA) as organic linker instead of 1,4-benzenediacarboxylate (BDC) ligands for MIL-125(Ti). By changing the dicarboxylic groups positioning from para to meta-position on benzene ring, they have successfully synthesized Ti-IPA MOF (MIP-208) [96]. They had also clarified that the prepared MOF exhibited robust water resistance. MIP-208 was soaked in water at room temperature and boiling water. XRD patterns indicate that those samples were identical to as prepared samples prove high water stability of this type of MOF material (Fig. 13a). Furthermore, CO2 adsorption ability did not get influenced by even boiling water treatment as shown in Fig. 13b. It should be noted here that MIP-208 was stable even under aqueous solution with various pH values ranging from strong acid to base condition. Ruthenium oxide nanoparticles were combined to MIP-208 in order to obtain strong catalytic material and demonstrated excellent visible-light response type CO2 methanation photoactivity.

Fig. 13
figure 13

A PXRD patterns for the MIP-208 samples before and after water treatments. B CO2 adsorption isotherms collected at 298 K for the MIP-208 samples before and after water treatments

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Metal complex was incorporated to MOF as foreign substances to enhance its activity. Zn(II)-porphyrin was coordinated with titanium oxoclusters as photosensitizer to provide NH2-MIL-125(Ti) with higher CO2 adsorption capability and photocurrent [97]. 11 times higher photocatalytic activity was achieved to convert CO2 to CO. Since 1,4-benzenediacarboxylate (BDC) ligands was used to build MOF structure already and porphyrin was applied as second ligand, Li et al. refer this type material as dual-ligand Ti-based MOFs and suggest MOF prepared by porphyrin-metal oxoclusters coordination is one of the alternative way to enhance CO2 photocatalytic reduction reaction.

MOF-based photocatalytic material can be not only powder, but different material form. Aerogel type MOFs were obtained by reacting Ti(IV) oxo-clusters and aromatic dicarboxylic linkers and following supercritical drying procedure [98]. Prepared aerogels were composed of approximately 5–10 nm sized nanoparticles linked microstructure with surface area being smaller than its counterparts MOFs. Even though, CO2 photocatalytic reduction reaction was higher due to more fluent diffusion of reagent and chemical reaction. This is because intrinsic MOF pore size and structure in the powder form are too small for smooth chemical reaction. So that a little larger pore and structure of this aerogel type MOF promoted the CO2 photocatalytic reduction more efficiently.

MIL-125(Ti) are the representative Ti-based MOFs. Introduction of foreign additive such as copper nanoparticles enhanced the CO2 photoreduction activity by tuning the conduction band position to have strong reduction potential, as well as accelerating photoexcited charge career generation, separation and transportation process by optimizing the local electronic environment around titanium oxide clusters. MOF crystalline facet regulation was effective to enhance photocatalytic activity by decreasing bandgap energy and suppressing the photogenerated electron–hole pair recombination rate. Introduction of carbonaceous material such as reduced graphene oxide to MOF was also an effective way, by tuning the morphology of catalytic materials. It was also found that the stability of MIL-125(Ti) was strengthened by replacing 1,4-benzenediacarboxylate (BDC) to isophthalic acid (IPA) as organic linker.

Ti–Zr-based MOF for CO2 photocatalytic reduction

Partial substitution of metal cations in MOFs can introduce metal-to-metal charge transfer, which can promote photocatalytic performance especially under visible-light irradiation [99]. When one considers about electrochemical potential, Zr-based MOF has more negative redox potential than Ti-based MOF (Ti4+/Ti3+ (− 0.1 V), Zr4+/Zr3+ (− 1.06 V)) [100]. Even though, UiO-66 indicates no response with visible light because LUMO of 1,4-benzenediacarboxylate (BDC) ligands is lower than Zr6O4(OH)4 redox potential energy level in UiO-66. This fact results in low efficiency in LMCT and consequently low rate reaction in CO2 photocatalytic reduction. In this respect, a bimetallic UiO-based MOF, NH2-UiO-66(Zr/Ti), was synthesized by Cohen by partially replacing Zr in NH2-UiO-66(Zr) with Ti [101]. The bimetallic MOF as NH2-UiO-66(Zr/Ti) demonstrated stronger CO2 photocatalytic activity than NH2-UiO-66 (Zr) upon visible-light irradiation. This is due to Ti ion introduction, MOF can accept photoexcited electrons from organic linkers by light absorption. Since they could not detect formic acid formation with sole UiO-66(Zr)-NH2, it is clear that Ti addition was critical factor.

Two types of NH2-UiO-66(Zr/Ti) MOFs with different Zr and Ti molar ratio (Zr: Ti = 120:16, 100:4) were synthesized compared CO2 photocatalytic performance under visible light [102]. Both Ti introduced Zr-based MOF exhibited stronger photoactivity than sole Zr-based MOF although NH2-UiO-66(Zr/Ti) MOF (Zr:Ti = 120:16) indicated even stronger activity than the other one. It was found that photocatalytic and CO2 adsorption sites were augmented by Ti addition which contributed upon higher photocatalytic activity. Speculated scheme for enhanced activity is depicted in Fig. 14b. When Ti4+ in added to Zr6O4(OH)4 to replace with Zr4 + center, the visible light-generated electrons in NH2-BDC prefers to be transport to Ti4+ rather than Zr4+ which would result in forming (Ti3+/Zr4+)6O4(OH)4 SBUs. This preference was theoretically calculated result. And this Ti3+ behave as electron donor to give away electrons to Zr4+ and thus, Zr4+ become Zr3+ to structure Ti4+–O–Zr3+ formation. As such, introduced Ti promotes charge transfer from the excited NH2-BDC to Zr–O clusters, leading to enhancing the CO2 photocatalytic reduction process.

Fig. 14
figure 14

HCOO amount formed with different samples as a function of light irradiation time. Improved CO2 photocatalytic reduction of Ti addition to NH2-UiO-66(Zr) and the proposed CO2 mechanism

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Syzgantseva et al. also studied the metal doping (substituting) effect to MOFs. Doped metal in a MOF would influence host electronic energy levels (Fig. 15) [99]. Therefore, it is important to refer the energy level of introducing metal kind as well as its electron affinity to optimize the whole energy diagram for the purpose. A metal with high electron affinity will often place energy state in the conduction band below the ligand level. The magnitude of electron level regulation degree would depend upon the doped metal content so that concentration of metal additive needs to be carefully controlled.

Fig. 15
figure 15

Regulation of bandgap, conduction band alignment, and creation of localized electron traps applying metal doping to a MOF

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These are typical examples to demonstrate MOF characteristic improvement by adding or partially replacing metal to original MOF metal center in which can contribute enhancing photocatalytic CO2 reduction. This phenomenon can be explained by photoexcited electrons transfer preference towards replaced metal.

Al-based MOF for CO2 photocatalytic reduction

Robatjazi et al. had approached to prepare composite material in the interesting way based on Al-based MOF. They grew MIL-53(Al) shell layers surrounding on aluminum nanocrystals and this composite material exhibited photocatalytic activity with enhanced plasmonic effect. The MOF synthesis on Al nanoparticle surface proceeded as dissolution of aluminum oxide layer on the nanoparticle surface to obtain Al3+ ions which was consumed for MIL-53(Al) growth. In other words, the dissolution of the Al nanoparticle surface oxide (Al2O3) layer served as a source of Al3+ for the backbone of the MOF (MIL-53(Al)) and promoted its growth so that it was unnecessary to introduce an additional aluminum metal precursor. 1,4-Benzenedicarboxylic acid (H2BDC), which is the organic linker of MIL-53(Al), was selected as linker parts. It should be noted here that this (MIL-53(Al)) is of particular interest because of its excellent thermal and chemical stability, which could be good advantage for real practical usage [103]. In addition, plasmonic metal nanoparticles have gained significant attention for photocatalytic reaction due to their capability to activate chemical transformations on their surfaces under illumination. The plasmon resonant interaction with light upon metallic nanoparticles results in the generation of energetic hot carriers upon plasmon decay. There are some studies stating that plasmonic nanoparticles are able to enhance the photocatalytic performance by light scattering, light concentration, hot electron injections, and plasmon-induced resonance energy transfer. So far, most of this plasmon research was carried out with gold and silver nanoparticles to enhance photocatalytic activities. In contrast, recently, aluminum nanoparticles as in this case have been demonstrated as an earth-abundant, low-cost alternative to Au an Ag for plasmon-mediated photocatalysis, which is beneficial to real industrial application [104].

Salguero et al. synthesized a heterogeneous photosensitizer composite Ru@dpdhpzBASF-A520 by incorporating surface dipyridyl-dihydropyridazine additive by Diels–Alder reaction on the aluminum fumarate units of the highly porous metal–organic framework (MOF). They have furthermore modified with ruthenium metal to obtain final catalyst material. The light absorption characteristic in the visible-light region elucidated that prepared material can behave as a photocatalytic catalyst for hydrogen evolution together with Pt nanoparticles. EDTA as sacrificial electron donor and MV (methyl viologen) as electron carrier. High photocatalytic activity was observed even after 72 h of reaction indicating effective stabilization of the ruthenium dipyridyl-dihydropyridazine adducts on the MOF surface. Aluminum-based MOF was synthesized with aqueous solution of Al2(SO4)3⋅18H2O and fumaric acid together with NaOH as synthesis catalyst. This is the photocatalytic reduction reaction of proton in water and not CO2 although confers an idea as utilizing this type of MOF composite material [105].

Musyoka et al. prepared Cu–ZnO catalysts supported on an aluminum fumarate metal–organic framework (Al Fum MOF) and applied as effective catalyst material for converting CO2 to valuable chemical products. To make Al Fum MOF, solvothermal synthesis was carried out with AlCl3·6H2O and fumaric acid as precursors. Cu and ZnO were deposited on MOF by adding aqueous solution of Cu(NO3)2·3H2O and Zn(NO3)2·6H2O and following 350 °C heat treatment in Argon. The catalysts were denoted as the theoretical weight percent composition of 7/3/90 (7Cu/3ZnO/AlFum MOF) and 15/6.4/78.6 (15Cu/6.4ZnO/AlFum MOF), respectively. The CO2 conversion test results were presented in Fig. 16 including a commercial catalyst which is composed of Cu/Zn/Al/Mg. The one of the direct CO2 hydrogenation to methanol reaction can be expressed as the equation: CO2 + 3H2 ↔ CH3OH + H2O. In addition to methanol, the reaction produced CO (CO2 + H2 ↔ CO + H2O), H2O, and CH4 (CO2 + 4H2 ↔ CH4 + 2H2O). The 15Cu/6.4ZnO/AlFum MOF catalyst had the strongest CO2 conversion in comparison to the commercial catalyst and the 7Cu/3ZnO/AlFum MOF catalyst. Furthermore, the prepared MOF-based catalyst exhibited good stability over the 24 h testing period [106].

Fig. 16
figure 16

CO2 conversions of the evaluated catalysts. Temperature = 230 °C; pressure = 50 bar; Qv, 0 = 40 mL min−1; gas hourly space velocity = 10,000 h − 1; H2/CO2 = 3:1

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Similar challenges are being attempted to enhance CO2 photocatalytic activity for Al-based MOFs such as addition of metal nanoparticles and metal complex. Furthermore, it was shown that metal nanoparticles to demonstrate plasmon effect such as Ag, Au, Pt as well as Al nanoparticles could enhance catalytic activity by light scattering, light concentration, hot electron injections, and plasmon-induced resonance energy transfer.

Organic linkers influence upon CO2 photoreduction activity

Besides discussing about metal center in MOF-based catalytic materials, it is known that organic linker, ligands also largely influence the photocatalytic activity of MOF-based catalysts [71]. Some example of amino functional group conjugated MOF were introduced in this study although in terms of stability and robustness, aromatic organic linkers would be more ideal since they can provide structural rigidity and hydrophobicity towards MOF than the aliphatic linkers do. When this aromatic ligand is properly selected, it would offer an additional advantage to obtain higher photolytic activity. For example, hydrophobic MOF (MIL-125-NHCyp, Cyp = cyclopentyl) were prepared for CO2 photoreduction reaction especially in humid environment owing to its stability against water. MIL-125(Ti) with three functional groups has been synthesized with –NH2, –NHMe, and –NHCyp as organic linker moiety, respectively. Among these 3 MOFs, MIL-125-NHCyp demonstrated the strongest stability and CO2 photoreduction activity due to its hydrophobic nature and the steric blocking effect of cyclopentyl group towards hydrogen bond and water [107].

Even amine functionalization of MOFs do not obviously possess strong hydrophobicity as aromatic linker, they exhibited higher CO2 adsorption capacity as well as reduction capability owing to their porous topological texture and appropriate binding energy reduction reaction. MIL-101 (Cr) was grafted with different type of alkylamine such as ethylenediamine (EN), diethylenetriamine (DETA), and triethylenetetramine (TETA). It was clarified that MIL-101 (Cr) EN demonstrated the best photoactivity and extended efficient electron transfer with suppressed charge recombination. It was also found that surface area was decreased due to clogging of MOF pores by DETA and TETA which was speculated to be the lower efficiency for their cases [68].

The polyoxometalate (POM: TBA5[P2Mo16VMo8VIO71(OH)9Zn8(L)4])-based MOF was in situ synthesized and demonstrated high hydrophobicity due to its ligand structure which also exhibited efficient photocatalytic CO2 reduction performance. Formic acid was produced with the yield of 35.2 μmol in the aqueous solution with 97.9% of selectivity [108].

Furthermore, as we described in this study, porphyrin is often applied as foreign material to combine with MOF in order to extend MOF light response to visible-light range owing to their aromatic structure. Porphyrin-based zirconium MOFs (PCN-H2/Pt) with H2TCPP and PtIITCPP [TCPP = tetrakis(4-carboxyphenyl)porphyrinate] as isostructural ligands and Zr6 clusters as metal nodes was prepared. PCN-H2/Pt exhibited high H2 production rate of 351.08 μmol h−1 g−1 even under visible-light irradiation. This is due to evenly distributed Pt2+ ions in PCN-H2/Pt, which accelerate the charge transfer from porphyrins to PtII ions, which resulted in efficient charge separation in the MOF composite materials [109].

As we described above, various attempts have been challenged in order to enhance the efficiency of CO2 photoreduction to convert them into various chemicals such as CO, CH3OH, H2, and CH4. Despite a numerous number of challenged have being made such as optimizing the metal center and ligand, there are still many challenges in achieving MOF-based artificial photosynthesis in practical applications.

Table 3 summarize the MOF-based CO2 photoreduction system presented in this study. As it was discussed in this article, most of their challenges are trying to confer ability to MOF-based photocatalytic materials, to work under not only ultraviolet light but also visible light. Suppressing the recombination of photoexcited electron–hole pairs and expanding lifetimes of charge carriers is also an important issue. In addition, it should be necessary not to utilize sacrificial reagents such as TEOA because that would be obvious extra cost for artificial photosynthesis system for real industrial application.

Table 3 Summary of photoreduction reaction of CO2 with MOFs and MOFs composite materials presented in this study
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It should be mentioned here that when one considers about real industrial application for MOF-based CO2 photoreduction system, MOF synthesis as well as mass manufacturing process is extremely important since MOFs synthesis are time consuming in general.

Table 4 summarized the representative MOF synthesis procedure so far in the past research and also explain some merits and demerits at each method [110, 111]. From author’s group experience that mechanochemical synthesis seems most plausible method for real industrial application although they suffer from low crystallinity and decreased pore volume and surface area.

Table 4 Summary of MOF synthesis procedure and each merits and demerits
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Raja et al. also explained the influence upon photocatalytic activity of MOF, of metal additive including Pt, Au, Ag and Rh noble metals as well as Co, Ni, Fe and Cu as non-noble conventional metal as considered for real industrial application. They have also investigated the impact of miscellaneous species such as reduced graphene oxide, g-C3N4 and carbon dots. Besides photocatalytic activity upon illuminated light wavelength, they claim that the size of MOF crystallites, surface area, length of organic linker, location of foreign additive, oxidation state as well as unsaturated metal center site in MOFs also have impact on catalytic activities [112]. For example, in general, MOF with smaller size and large surface area would possess higher photocatalytic activity due to increased reaction area.

It should be noted here that author’s group have recently found one interesting efficient mechano-chemical process for MOF production and also developed zirconium-based MOF/carbon quantum dot composite and applied for CO2 photocatalytic reduction to generate formic acid. The detail mechanism of this reaction will be coming in a forthcoming paper.

Conclusion

To apply MOF as CO2 photocatalytic reduction process, we chose Fe, Zr, Ti and Al as metal center for MOFs catalyst for CO2 photocatalytic reduction materials. This is because these 4 metal doped MOFs are relatively abundant materials and economically friendly compared to other precious rare earth or earth metals. It was demonstrated that the intrinsic bandgap energy depends on the metal center, which should be suitable for CO2 photocatalytic reduction. For example, when Fe is the MOF metal center, Fe3O cluster can be the origin for visible-light irradiation. CO2 adsorption capacity as well as photoreduction reaction also can be augmented by selecting or doping with appropriate metal. Functionalization of MOF with organic substances such as amine group to organic linker accelerate the electron transfer by ligand-to-metal charge transfer (LMCT) reaction and increase CO2 absorption as well as light absorption extension to visible-light range, which eventually enhanced the CO2 photocatalytic reduction. Metal nanoparticles introduction to MOF enhanced the CO2 photoreduction by tuning the conduction band position, accelerating photoexcited charge career generation, separation and transportation process, as well as their plasmon effect. Quantum dot-MOF composite materials demonstrated higher CO2 photocatalytic activity by enhancing quantum dot durability, extension of light absorption to visible-light range and fast charge transfer to metal center catalytic site of MOF. In addition, MOF crystalline facet regulation was effective to enhance photocatalytic activity by decreasing bandgap energy and suppressing the photogenerated electron–hole pair recombination rate.