An artificial photosynthesis system comprising a covalent triazine framework as an electron relay facilitator for photochemical carbon dioxide reduction

Abstract

Natural photosystem II, which utilizes multiple photosensitizers, is essentially an organic–inorganic hybrid system, and comprises electron transfer relay processes. The relay processes can facilitate charge transfer and reduce charge recombination. Thus it can significantly improve photocatalytic performance. Such a strategy has rarely been studied for CO₂ reduction in conjugated porous polymers. Here we report a new organic–inorganic hybrid material, which constitutes an electron transfer relay photocatalytic system for CO₂ reduction using a porous framework as a relay facilitator. The system was fabricated by decorating a new porphyrin-based covalent triazine framework (Por-CTF) with α-Fe₂O₃ nanoparticles using an in situ strategy, which was then coupled with a Ru complex photosensitizer. Owing to the formation of an electron transfer relay system, the ternary system exhibits a sharp enhancement in photocatalytic activity towards CO₂ reduction to CO as compared with the binary system. The best performance given by the electron transfer relay system (α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂) produces a catalytic CO evolution rate of 8.0 μmol h⁻¹ with 93% CO selectivity, which is obviously superior to that of the binary system. The highest apparent quantum efficiency was also evaluated as 1.43% at 450 nm. This work provides a prototype model system for the design of an artificial photocatalytic system for photochemical CO₂ reduction.

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Introduction

Carbon dioxide (CO₂) which is largely emitted from fossil fuels has resulted in global warming and an adverse influence on the environment.^1,2 A number of research efforts have been made to decrease the CO₂ level in the atmosphere, with the target of achieving a carbon-neutral society. Photochemical reduction of CO₂ is a promising route to decrease CO₂ by transforming it into value-added chemical fuels, such as carbon monoxide (CO), methane (CH₄) and so on.³ For example, using iron porphyrin as photocatalyst can convert CO₂ to CH₄.^4,5 A highly porous metal–organic framework (Co-ZIF-9) can reduce CO₂ to CO in a similar photocatalytic system that has the benefits of high CO₂ adsorption ability and a porous framework.^6,7

Conjugated porous organic polymers (cPOPs),⁸ including covalent organic frameworks (COFs),^9–18 covalent triazine frameworks (CTFs)^19–42 and conjugated microporous polymers (CMPs),^43–49 are receiving enormous attention in photocatalysis because of their unique porous and semiconducting properties. These materials have been widely studied for CO₂ adsorption, but the application for photochemical CO₂ reduction remains in its infancy. Their high surface areas and tuneable pore sizes would give advantages in the photocatalytic process for CO₂ adsorption and mass transfer. Baeg et al. first demonstrated a film CTF constructed using perylene diimide to transform CO₂ into formic acid by photocatalysis.⁵⁰ Cao et al. recently further reported a Re-modified CTF that is a capable photocatalyst for CO₂ reduction to CO.⁵¹ Wang et al. reported that functional CMPs show high efficacy for photochemical CO₂ reduction to CO assisted by Co(bpy)₃Cl₂ and triethanolamine (TEOA).⁵² Liu et al. reported a series of pyrene-based CMPs that can convert CO₂ to CO using an ionic liquid.⁵³ Zhu et al. found that an azine-linked COF could catalyse CO₂ to methanol.⁵⁴ Zou et al. very recently reported a COF functionalized with metal bipyridyl complex that could reduce CO₂ to CO.⁵⁵ These works demonstrated that the band gaps and energy levels of cPOPs can be easily tuned owing to their structural flexibility and variable synthetic methods. As above, cPOPs have enormous potential for photochemical CO₂ reduction, but developing new strategies to increase performance remains necessary.

In natural photosystem II, multiple photosensitizers (e.g., chlorophyll and quinone) are employed to work as light-harvesting and charge transfer/separation systems, which are then coupled with a Mn₄O_xCa nanocluster reaction centre to form a photochemical machine.⁵⁶ Thus, this natural photosystem is essentially a kind of organic–inorganic hybrid system forming an electron transfer relay system that facilitates charge transfer and reduces charge combination. To the best of our knowledge, development of such an electron transfer relay system for photocatalytic CO₂ reduction in conjugated porous polymers is unprecedented.^57–59

In this work, we constructed a novel organic–inorganic hybrid artificial electron transfer relay photocatalytic system for photocatalytic CO₂ reduction, in which Ru(bpy)₃Cl₂ works as photosensitizer, CTF as an electron relay facilitator, and α-Fe₂O₃ as an inorganic co-catalyst. First, a binary hybrid, α-Fe₂O₃@Por-CTF, was fabricated by in situ growth of α-Fe₂O₃ nanoparticles into a new porphyrin CTF (Por-CTF), allowing small α-Fe₂O₃ crystallites to spread in the Por-CTF. The porphyrin structures are well-known and play a pivotal role in the natural photosynthesis system.^5,56 Thus, it would be effective to construct a porphyrin-based conjugated porous organic polymer working as a light-harvesting or electron antenna, which promises to lead to an efficient photocatalysis system. Here, α-Fe₂O₃, which has not previously been studied for photochemical CO₂ reduction, works as a co-catalyst. It is a low-cost and abundant semiconductor for photocatalysis, but is limited by low conduction band and rapid charge recombination of photogenerated charges for photocatalytic CO₂ reduction.^60–62 In the binary hybrid, the Por-CTF with large surface area porous structure would be of benefit for collecting both electrons and CO₂ during photocatalysis. The small α-Fe₂O₃ nanoparticles can be strongly coupled with the support owing to the nitrogen-rich structure of Por-CTF, and thus would provide a large interface for charge transfer and separation between both semiconductors as compared with the direct mixing method. In addition, by tuning the ratio of the components, the size of the α-Fe₂O₃ crystallites would be modulated and thus the performance may be effectively optimized. Ru(bpy)₃Cl₂ was added as photosensitizer, to form an electron transfer relay system with cascade band energy positions. By comparison with control experiments, the electron transfer relay system showed a greatly enhanced activity as compared with the binary hybrid system (i.e. Por-CTF/Ru(bpy)₃Cl₂, α-Fe₂O₃/Ru(bpy)₃Cl₂, and α-Fe₂O₃@Por-CTF), which indicated that this was an effective strategy to achieve enhanced photocatalytic activity. This work provides a new guideline for the design of an artificial photocatalytic system for CO₂ reduction via mimicking a natural photosystem.^63–66

Results and discussion

Synthesis and characterization of materials

As depicted in Scheme 1, the electron transfer relay system comprised Ru(bpy)₃Cl₂, Por-CTF and α-Fe₂O₃. Of these, Por-CTF was prepared via polycondensation of the amidine with a porphyrin aldehyde monomer, which formed a porous network structure (Fig. S1, ESI†).³⁵ Then, the designed artificial photosystem was fabricated by the following procedures (Fig. 1b). Initially, a ferric chloride aqueous solution was prepared to provide a source of FeOOH. Afterwards, Por-CTF was added to the above solution, so that the FeOOH precursor gradually grew on the Por-CTF to form a hybrid, FeOOH@Por-CTF. Then, the FeOOH@Por-CTF was filtered out and annealed in an oven at a fixed temperature for 24 hours to form the α-Fe₂O₃@Por-CTF hybrid system. By controlling the amount of the precursor, we prepared a series of photocatalysts which we named α-Fe₂O₃@Por-CTF-X (X indicates the molar ratio of the structural units of Por-CTF to the α-Fe₂O₃ precursor). The ratio of the two components was varied from 5×, to 10×, to 15×, which generated three hybrid materials, namely α-Fe₂O₃@Por-CTF-5×, α-Fe₂O₃@Por-CTF-10× and α-Fe₂O₃@Por-CTF-15×.

Scheme 1 (a) A schematic diagram of natural photosystem II. ATP, adenosine triphosphate; ETC, electron transport chain. (b) A schematic diagram of the electron transfer process for the photocatalytic reduction of CO₂.

Fig. 1 (a) Synthesis route of Por-CTF. (b) Fabrication of the α-Fe₂O₃@Por-CTF hybrid photocatalyst based on Por-CTF. (c) FE-SEM image of Por-CTF. (d) TEM image of α-Fe₂O₃@Por-CTF-10×. (e) HR-TEM image of α-Fe₂O₃@Por-CTF-10× and lattice fringes of α-Fe₂O₃. (f) Powder X-ray diffraction patterns of Por-CTF (black), α-Fe₂O₃ (red), α-Fe₂O₃@Por-CTF-5× (blue), α-Fe₂O₃@Por-CTF-10× (purple), and α-Fe₂O₃@Por-CTF-15× (green).

As shown in Fig. S2 (ESI†), in the Fourier-transform infrared (FT-IR) spectra the triazine structures are characterized by typical vibration stretches at 1350 cm⁻¹ and 1500 cm⁻¹ attributed to the triazine unit in Por-CTF and α-Fe₂O₃@Por-CTF. Moreover, Fig. S3a (ESI†) shows that the solid-state ¹³C nuclear magnetic resonance (NMR) spectrum of Por-CTF exhibits two typical broad peaks at 160–180 ppm and 120–150 ppm, which can be assigned to the carbon of porphyrin, consistent with the reported porphyrin polymer.⁶⁷ In addition, the peak at 170 ppm can be assigned to the carbon in the triazine structure. Thermogravimetric analysis (TGA) showed that Por-CTF and α-Fe₂O₃@Por-CTF-10× are stable up to 400 °C (Fig. S3b, ESI†). From the TGA curves, the content of α-Fe₂O₃ in α-Fe₂O₃@Por-CTF-10× is estimated to be approximately 23.9%. The iron content of the prepared samples was further measured by inductive coupled plasma optical emission spectrometry (ICP-OES) (Table S1, ESI†). The results showed that the iron contents of the α-Fe₂O₃@Por-CTF-5×, α-Fe₂O₃@Por-CTF-10×, and α-Fe₂O₃@Por-CTF-15× samples are 8.98, 18.08 and 20.29 wt%, respectively. Accordingly, the α-Fe₂O₃ content in α-Fe₂O₃@Por-CTF-10× was calculated to be 25.83 wt%, which is very close to the value estimated from TGA.

The high-resolution C 1s X-ray photoelectron spectrum (XPS) of Por-CTF, shown in Fig. S4a (ESI†), can be deconvoluted into three major peaks with binding energies of 284.9, 286.2 and 288.7 eV. The peak located at 284.9 eV can be assigned to carbon atoms in the aryl units present in the skeleton (C=C). The peaks at 286.2 and 288.7 eV are attributed to the carbon atoms present in the triazine structure and porphyrin structure with C=N–C and N–C=N bonding, respectively. The N 1s XPS can be deconvoluted into peaks at 398.9 eV and 400.1 eV, corresponding to pyridinic N (from triazine structure) and pyrrolic N (from porphyrin structure) in the Por-CTF (Fig. S4b, ESI†). The ratio of the areas of the pyridinic N (from triazine structure) and pyrrolic N peaks is close to 1 : 1, which is in line with the theoretical value.

Field emission scanning electron microscope (FE-SEM) images (Fig. 1c) and transmission electron microscope (TEM) images (Fig. S5a and b, ESI†) show that Por-CTF has plate-like particulate morphology. As shown in Fig. S5d–f (ESI†), the high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) images of Por-CTF show the mapping of the carbon (cyan) and nitrogen (purple), indicating the elements are evenly distributed in the framework. The TEM and high-resolution TEM (HR-TEM) images (Fig. 1d and e, respectively) show that the α-Fe₂O₃ crystallites are well-supported on the Por-CTF. The powder X-ray diffraction (PXRD) pattern of Por-CTF (Fig. 1f) exhibits a broad peak at 19°, which can be assigned to π–π interaction of the conjugated structure. With an increasing amount of the iron precursor, the peaks arising from α-Fe₂O₃ in the PXRD patterns of α-Fe₂O₃@Por-CTF become clearer and their intensity increases. The PXRD pattern matches JCPDS no. 01-080-2377, and can be unambiguously assigned to α-Fe₂O₃ (Fig. S6, ESI†).

The amount of precursor has a considerable effect on the sizes of α-Fe₂O₃ nanoparticles in the resulting hybrid system. When the ratio is increased from 5×, to 10×, to 15×, the average sizes of α-Fe₂O₃ particles increase from 77 nm, to 101 nm, to 180 nm (Fig. S7, ESI†). In the HR-TEM image shown in Fig. 1e, the lattice fringes of α-Fe₂O₃ in α-Fe₂O₃@Por-CTF-10× can be observed, which indicates that the α-Fe₂O₃ crystallites mainly expose their crystalline planes on the Por-CTF. In addition, HAADF-STEM images of the hybrid system (Fig. S8, ESI†) clearly show the α-Fe₂O₃ nanoparticles are surely supported on Por-CTF. The corresponding elements are shown in energy dispersive X-ray spectroscopy (EDX) for α-Fe₂O₃@Por-CTF-10× (Fig. S8 and S9, ESI†).

Fig. 2a shows the low-resolution XPS of α-Fe₂O₃, Por-CTF and α-Fe₂O₃@Por-CTF-10×, which demonstrate the materials contain Fe 2p, N 1s, O 1s, and C 1s. Fe 2p and N 1s of α-Fe₂O₃ and α-Fe₂O₃@Por-CTF-10× (Fig. S10, ESI†) are consistent with each other. In Fig. 2b, the peaks with binding energy at 710.9 eV and 724.5 eV indicate the presence of 2p_3/2 and 2p_1/3 belonging to Fe 2p, which clearly indicates that the crystalline nanoparticle is α-Fe₂O₃. The XPS results in Table S2 (ESI†) show that the content of Fe on the surface of Por-CTF increases when more precursor is added. Fig. S11 (ESI†) shows the N 1s profile of Por-CTF and different α-Fe₂O₃@Por-CTF-X samples. The high-resolution C 1s and O 1s profiles of α-Fe₂O₃@Por-CTF-X also match Por-CTF and α-Fe₂O₃ and thus we can deduce that Por-CTF has successfully been decorated with α-Fe₂O₃. Specifically, the Fe 2p, O 1s and N 1s profiles of α-Fe₂O₃@Por-CTF-10× respectively match Por-CTF and α-Fe₂O₃ very well, as shown in Fig. 2b–d. This confirms the organic–inorganic hybrid material did not change the chemical structure of independent semiconductors.

Fig. 2 (a) Low-resolution XPS. (b) High-resolution Fe 2p XPS of α-Fe₂O₃@Por-CTF-10×. (c) High-resolution O 1s XPS of α-Fe₂O₃@Por-CTF-10×. (d) High-resolution N 1s XPS of α-Fe₂O₃@Por-CTF-10×.

Photocatalytic performance

The ultraviolet-visible (UV-vis) spectra exhibit a wide visible light absorption range, as shown in Fig. S12 (ESI†). The band gaps of α-Fe₂O₃, Por-CTF and α-Fe₂O₃@Por-CTF-10× are calculated to be 2.01 eV, 1.62 eV and 1.75 eV, respectively. From Mott–Schottky plots of Por-CTF and α-Fe₂O₃, as shown in Fig. S12a and b (ESI†), the conduction bands (CBs) of Por-CTF and α-Fe₂O₃ are estimated as −0.87 V (vs. NHE) and −0.25 V (vs. NHE), respectively. Moreover, the valence bands (VBs) of Por-CTF and α-Fe₂O₃ are deduced to be 0.88 V (vs. NHE) and 2.08 V (vs. NHE), respectively. The CB of Por-CTF (−0.87 V vs. NHE) is more negative than the reduction potential of carbon dioxide to carbon monoxide (CO₂/CO −0.53 V vs. NHE), thus the photochemical reduction of CO₂ is energetically feasible.

In the photocatalytic system, Ru(bpy)₃Cl₂, which has been widely used as photosensitizer for photocatalytic CO₂ reduction, was coupled with α-Fe₂O₃@Por-CTF-10× and formed an electron relay photocatalytic system. The photocatalytic reduction performances obtained are depicted in Fig. S13 and S14 (ESI†). We first explored the impact of the amount of the photocatalyst on the catalytic activity. As shown in Table S3 (ESI†), when the amount of photocatalyst is increased, the evolution rate of CO becomes higher, but the selectivity decreases. A ratio of 20 mg α-Fe₂O₃@Por-CTF-10× to 5 mg Ru(bpy)₃Cl₂ was found to be optimal in the photocatalytic reduction, considering both activity and selectivity. α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂ exhibits the best performance, with a photocatalytic CO evolution rate of 8.0 μmol h⁻¹ and selectivity of 93% for CO (Table 1, entry 1). Recycling experiments showed that α-Fe₂O₃@Por-CTF-10× is stable, with good recyclability for three cycles (Fig. 3b). As shown in Fig. S15 (ESI†), the hybrid catalyst is stable and shows no obvious structural change after photocatalysis as evidenced by TEM images and FT-IR spectrum.

Table 1 Summary of photocatalytic CO₂ reduction performances

Entry	Catalyst	Gas	λ/nm	CO/μmol h^−1	H2/μmol h^−1	Selectivity/%
a Physical mixture. b Without TEOA and Ru(bpy)3Cl2. Typical experimental conditions: 20 mg of photocatalyst, 5 mg (7 μmol) of Ru(bpy)3Cl2·6H2O, 5 mL of DMF/TEOA (4 : 1 by volume) at 293 K.
1	α-Fe2O3@Por-CTF-10×/Ru(bpy)3Cl2	CO2	>420	8.0	0.6	93
2	α-Fe2O3@Por-CTF-10×/Ru(bpy)3Cl2	N2	>420	—	1.3	0
3	α-Fe2O3@Por-CTF-10×/Ru(bpy)3Cl2	CO2	Dark	—	—	—
4	α-Fe2O3/Ru(bpy)3Cl2	CO2	>420	0.6	2.2	21
5	Por-CTF/Ru(bpy)3Cl2	CO2	>420	0.2	0.1	67
6^a	α-Fe2O3/Por-CTF/Ru(bpy)3Cl2	CO2	>420	1.5	1.2	56
7^b	α-Fe2O3@Por-CTF-10×	CO2	>420	0.72	—	100
8	Ru(bpy)3Cl2	CO2	>420	—	0.4	0
9	Por-CTF	CO2	>420	0.21	—	100
10	α-Fe2O3	CO2	>420	—	—	—

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Fig. 3 (a) Photocatalytic performance of CO evolution with MnO_x@Por-CTF-10×, α-Fe₂O₃@Por-CTF-10×, CoO_x@Por-CTF-10×, NiO_x@Por-CTF-10×, CuO_x@Por-CTF-10× and ZnO_x@Por-CTF-10× (20 mg) in 5 mL of DMF/TEOA (4/1 with 7 μmol of Ru(bpy)₃Cl₂). (b) Recyclability and stability experiments. (c) Wavelength dependence of the AQE of α-Fe₂O₃@Por-CTF-10×. (d) CO₂ adsorption and desorption curves at 273 K (inset: isosteric heat of adsorption, Q_st).

As shown in the Fig. 3a, Por-CTF can be loaded with various metal oxides, such as MnO_x, CoO_x, NiO_x, CuO_x, ZnO_x and α-Fe₂O₃, but it was found that α-Fe₂O₃ was the optimal co-catalyst, resulting in the highest evolution rate and selectivity. Other metal oxide@Por-CTF-10×/Ru(bpy)₃Cl₂ showed very low efficiency and selectivity for CO reduction under the same conditions. Among the metal oxides, only CoO_x@Por-CTF-10×/Ru(bpy)₃Cl₂ showed a comparable photocatalytic activity to α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂, but its selectivity was only 51%. Fig. 3c shows the electron transfer relay system has a good apparent quantum efficiency (AQE), the highest AQE of α-Fe₂O₃@Por-CTF-10× being 1.43% at 450 nm. The performance is comparable to that of many photocatalytic systems, as summarized in Tables S4 and S5 (ESI†).^7,68,69

To demonstrate the effectiveness and clarify the role of each component in the electron transfer relay processes, a series of control experiments were conducted and the results are presented in Fig. S16–S18 (ESI†). The detailed catalytic results are summarized in Table 1. Por-CTF alone exhibits a low CO evolution rate of only 0.21 μmol h⁻¹, while α-Fe₂O₃ alone is completely inactive because of its low CBM. Ru(bpy)₃Cl₂ alone also does not show CO₂ reduction activity. Furthermore, each of the binary systems (i.e. Por-CTF/Ru(bpy)₃Cl₂, α-Fe₂O₃/Ru(bpy)₃Cl₂, and α-Fe₂O₃@Por-CTF-10×) was studied: α-Fe₂O₃@Por-CTF-10× showed a low efficiency of 0.72 μmol h⁻¹ for CO evolution, Por-CTF/Ru(bpy)₃Cl₂ produced 0.2 μmol h⁻¹, and α-Fe₂O₃/Ru(bpy)₃Cl₂ gave 0.6 μmol h⁻¹. Moreover, to compare ternary systems, the physical mixture of α-Fe₂O₃ nanoparticles and Por-CTF (α-Fe₂O₃/Por-CTF) in the presence of Ru(bpy)₃Cl₂ photosensitizer was also studied, which only evolved CO at 1.5 μmol h⁻¹ (Table 1, entry 6).

Apparently, the photochemical CO evolution amount of α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂ is much higher than those of the binary systems and the physical mixture. The selectivity of the ternary system is also much higher those of the binary ones. In particular, as shown in Table 1, entry 6, the physical mixture (α-Fe₂O₃/Por-CTF/Ru(bpy)₃Cl₂) shows a much higher photocatalytic activity than α-Fe₂O₃/Ru(bpy)₃Cl₂ and Por-CTF/Ru(bpy)₃Cl₂, which indicates that the introduction of Por-CTF as an electron transfer relay facilitator is effective in the ternary system. The photocatalytic performance of the hybrid system (α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂) is much higher than that of the physical mixture (α-Fe₂O₃/Por-CTF/Ru(bpy)₃Cl₂), indicating the design of the hybrid of α-Fe₂O₃ in the Por-CTF-10× matrix is unique and beneficial for photocatalytic performance.

When the photochemical reaction was conducted under nitrogen atmosphere instead of CO₂, there was no CO detected, indicating that CO is the product arising from CO₂ reduction (Table 1, entry 2). GC-MS analysis (Fig. S19, ESI†) showed that the major product ¹³CO originated from ¹³CO₂ photoreduction, which proved the CO was derived from CO₂. To test whether there is any other reaction product, which remains in solution, the ¹H-NMR spectrum of the solution was measured before and after the photocatalytic test. As shown in Fig. S20 (ESI†), the spectra did not show any difference, suggesting there are not any obvious products in the solution under our photocatalytic conditions.

Discussion on the electron transfer relay photocatalytic system

As adsorption is crucial for photocatalysis, the porous structure and CO₂ adsorption properties of the samples were studied. Nitrogen adsorption analysis (Fig. S21a, ESI†) shows that the Brunauer–Emmett–Teller (BET) specific surface area of Por-CTF is 442.5 m² g⁻¹ and that of α-Fe₂O₃@Por-CTF-10× is 453.3 m² g⁻¹. The pore size analysis in Fig. S21b (ESI†) shows the Por-CTF is a hierarchical porous triazine framework. Por-CTF and α-Fe₂O₃@Por-CTF-10× exhibit a two-step hysteresis loop in the nitrogen sorption curves, which indicates that mesoporous structures are also present in the porous structures. The ratio of micropores in α-Fe₂O₃@Por-CTF-10× is higher than that of Por-CTF. The pore width of α-Fe₂O₃@Por-CTF-10× is also slightly decreased compared with Por-CTF.

We then measured the CO₂ adsorption of α-Fe₂O₃@Por-CTF-10×. As shown in Fig. 3d, both α-Fe₂O₃@Por-CTF-10× and Por-CTF have high CO₂ adsorption abilities at 273 K and 1 bar. The α-Fe₂O₃@Por-CTF-10× has a CO₂ adsorption ability of 50 cm³ g⁻¹ (9.82 wt%), which is much higher than that of Por-CTF (35 cm³ g⁻¹, 6.88 wt%), at 273 K and 1 bar. The results indicate the CO₂ adsorption ability is enhanced after formation of the hybrid system, which is good for CO₂ photoreduction. Interestingly, α-Fe₂O₃@Por-CTF-10× shows slightly higher isosteric heats of adsorption (Q_st) than Por-CTF (inset in Fig. 3d), which could be attributed to the interaction between the α-Fe₂O₃ and absorbed CO₂ molecules. The enhancement of the CO₂ adsorption after loading α-Fe₂O₃ could be explained by the increased BET surface area and higher ratio of micropores after deposition of α-Fe₂O₃ nanoparticles, which decrease the size of the larger pores.

As presented in Fig. S22a (ESI†), α-Fe₂O₃@Por-CTF-10× has a much lower fluorescence emission intensity than bare Por-CTF, suggesting the recombination of the photogenerated carriers is suppressed by formation of the binary hybrid photosystem. As shown in Fig. S22b (ESI†), the fluorescence lifetime of α-Fe₂O₃@Por-CTF-10× (τ = 0.86 ns) also becomes shorter compared with Por-CTF (τ = 3.2 ns). This is evidence that photoinduced electron transfer takes place between the two components. Electrochemical impedance spectroscopy and the photocurrent response also suggested the better charge transfer and transportation ability of the hybrid material. As shown in Fig. S22 and S23 (ESI†), the resistance of α-Fe₂O₃@Por-CTF-10× is lower than that of Por-CTF (Fig. S22c, ESI†) and the photocurrent of α-Fe₂O₃@Por-CTF-10× is stronger than for Por-CTF, α-Fe₂O₃ and other samples (Fig. S22d and S23b, ESI†). Furthermore, spin-trapping electron spin resonance (ESR) spectra were examined. As shown in Fig. 4c and d, the formation of both ˙OH and ˙O₂⁻ radicals under visible light can be observed in α-Fe₂O₃@PCTF-10×, which indicates the hybrid system is effectively formed. Furthermore, the ESR signals of the ˙OH and ˙O₂⁻ radicals in α-Fe₂O₃@PCTF-10× are stronger than those in α-Fe₂O₃ and PCTF, respectively. As shown in Fig. S24 and S25 (ESI†), there are no radical peaks in the absence of light irradiation, which means that light is required for the generation of intermediates like ˙OH and ˙O₂⁻ for CO₂ reduction. These results suggest that the hybrid of α-Fe₂O₃@Por-CTF-10× forms an effective electron transfer system, which can facilitate charge separation and generate more charge carriers for photoredox reactions.

Fig. 4 (a) Steady-state photoluminescence spectra of Ru(bpy)₃Cl₂ (black), Por-CTF/Ru(bpy)₃Cl₂ (pink), α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂ (blue), and α-Fe₂O₃/Ru(bpy)₃Cl₂ (red), in 5 mL of DMF/TEOA (4 : 1 by volume). (b) Time-resolved photoluminescence spectra of Ru(bpy)₃Cl₂ (black), Por-CTF/Ru(bpy)₃Cl₂ (pink), α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂ (blue), and α-Fe₂O₃/Ru(bpy)₃Cl₂ (red), dispersed in 5 mL of DMF/TEOA (4 : 1 by volume) at 293 K. DMPO spin-trapping ESR spectra recorded for ˙OH (c) and ˙O²⁻ (d) under visible light for α-Fe₂O₃, Por-CTF, and α-Fe₂O₃@Por-CTF-10× in 10 minutes.

Next, time-resolved photoluminescence spectra were studied to reveal the charge dynamic behaviour in the electron transfer relay system. As shown in Fig. 4a, the fluorescence of Ru(bpy)₃Cl₂ in the electron transfer relay system (α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂) is significantly quenched as compared with Por-CTF/Ru(bpy)₃Cl₂, α-Fe₂O₃/Ru(bpy)₃Cl₂, and Ru(bpy)₃Cl₂. Furthermore, the fluorescence lifetime of α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂ is also greatly decreased as compared with these samples. As shown in Fig. 4b, the fluorescence lifetime of Ru(bpy)₃Cl₂ is about 347.7 ns, which is in line with the literature value (≈325.0 ns).⁷⁰ In contrast, the fluorescence lifetime of α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂ is very much less, at τ₃ = 1.6 ns. Its fluorescence lifetime is also shorter than those of α-Fe₂O₃/Ru(bpy)₃Cl₂ (τ₃ = 217.5 ns) and Por-CTF/Ru(bpy)₃Cl₂ (τ₄ = 219.3 ns, τ_4–1 = 1.1 ns). These results indicate that the ternary hybrid system significantly suppresses the charge recombination process and facilitates electron transfer. In particular, the sharply decreased lifetime (1.6 ns versus 325.0 ns) strongly evidences the efficient photoinduced electron transfer processes, and accounts for the improved photocatalytic performance.

In order to understand in depth the possible reduction process on the catalyst surface, in situ FT-IR spectra of α-Fe₂O₃@Por-CTF-10× were examined.^71,72 As shown in Fig. S26a–S29a (ESI†), the strong absorption peaks at 1341, and 1570 cm⁻¹ are respectively assignable to ν_s(CO₃), ν_as(CO₃) and ν(C=O) of one bidentate carbonate.⁷³ The absorption peaks at 1224, 1401, 1490, 1600 and 1636 cm⁻¹ are assignable to bicarbonates, and the weak absorption at 1704 cm⁻¹ is assignable to carboxylate.⁷⁰ These results suggest that the activation of CO₂ molecules can occur on the surface of α-Fe₂O₃@Por-CTF-10×. They also show light is necessary for the appearance of these intermediates. Bicarbonates (HCO₃*) and carboxylate (COOH*) are intermediates in the photochemical reduction of CO₂ to CO. COOH* on the surface of α-Fe₂O₃@Por-CTF-10× can be reduced to CO* by a proton–electron transfer reduction process.⁷⁴ Before irradiation, there is no intermediate detected by in situ FT-IR spectroscopy. As shown in Fig. S26b and c (ESI†), the absorption bands (2100–2500 cm⁻¹, 3620 cm⁻¹ and 3726 cm⁻¹) that are assignable to absorptions of gas-phase CO₂ decreased after irradiation. The peaks at 2849, 2922, and 2952 cm⁻¹ (Fig. S26, ESI†) indicate abundant hydroxyl groups exist on the catalyst surface, which may play an important role in the adsorption of carbon dioxide. As control tests, the in situ FT-IR spectra of α-Fe₂O₃ and Por-CTF were also measured. The results showed that α-Fe₂O₃ can adsorb CO₂ and form the bicarbonate (HCO₃*) intermediate, but the carboxylate (HCOO*) – which is the key intermediate for the CO evolution – is not observed. This suggests that α-Fe₂O₃ alone is not an effective photocatalyst (Fig. S27, ESI†). In contrast, both HCO₃* and HCOO* are observed in Por-CTF (Fig. S28, ESI†), which indicates that Por-CTF alone is a viable photocatalyst for CO₂ reduction to CO. As compared with each other, the intensity of HCO₃* in α-Fe₂O₃ (the strong chemical adsorption of CO₂ on the surface) is much higher than that in Por-CTF. This means that α-Fe₂O₃ is more capable of adsorbing and activating CO₂ on the surface. This could be a partial reason for the higher CO₂ adsorption after loading α-Fe₂O₃ nanoparticles in the Por-CTF matrix.

We measured the transient absorption spectra (TAS) of α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂. On irradiation by a 395 nm laser, we observed ground-state bleaching at 460 nm and a new peak at 525 nm, which indicates the formation of a triplet excited state deriving from Ru(bpy)₃Cl₂ (α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂, Fig. 5a and b). Transient absorption spectra show that a new peak at 550–600 nm emerges. Calculation of the decay profile showed that α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂* decays with a lifetime (τ) of only 0.3764 ps.

Fig. 5 (a) Femtosecond time-resolved transient absorption decay kinetics of α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂ and (b) time-resolved transient absorption spectra of α-Fe₂O₃@Por-CTF-10×/Ru(bpy)₃Cl₂. (c) Proposed mechanism for the photochemical reaction comprising an electron transfer relay process with Por-CTF as the relay facilitator for carbon dioxide reduction in a ternary hybrid system.

According to the above studies, a photocatalytic mechanism is proposed to describe the photochemical processes, as shown in Fig. 5c. When the photocatalysis system is irradiated with visible light, the photosensitizer ([Ru(bpy)₃]²⁺) is excited to [Ru(bpy)₃]²⁺*, which is then quenched by TEOA, forming the reduced state [Ru(bpy)₃]⁺.⁸ Afterwards, the energized electrons from [Ru(bpy)₃]⁺ are injected from the photosensitizer to Por-CTF, followed by migration from Por-CTF to the surface of α-Fe₂O₃ nanoparticles. Finally, the energized electrons meet with the adsorbed CO₂ on α-Fe₂O₃ and the reduction reactions take place. Obviously, this process is composed of multiple electron transfers, forming an electron transfer relay system, which could be analogous to natural photosystem II. Thus, the ternary system can help photocatalysis better than the binary system, because it can effectively suppress the backward electron transfer and make the forward electron transfer take place more efficiently.^75–80

Conclusions

In summary, we report an example of a ternary organic–inorganic hybrid as an electron transfer relay system for photochemical CO₂ reduction. The hybrid system was prepared via the in situ growth of α-Fe₂O₃ crystallites on Por-CTF; it exhibits high CO₂ adsorption ability and supplies a large interface and short charge transfer distance. It forms an electron transfer relay ternary system after the addition of the Ru(bpy)₃Cl₂ complex as a photosensitizer and using Por-CTF as an electron transfer relay facilitator, which mimics a natural photosynthesis system. The greatly enhanced photocatalytic performance, with a high CO evolution rate and quantum yield compared with the binary systems, is due to the presence of an electron transfer relay system that can reduce photocurrent carrier recombination and promote the separation efficiency of photogenerated charges. The present study constitutes a new paradigm to mimic a natural photosystem for the design of hybrid photocatalysts based on conjugated porous organic polymers (cPOPs) for CO₂ reduction as well as other photochemical applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

S. Jin would like to acknowledge funding from the National Natural Science Foundation of China (grant no. 21875078, 21604028). B. Tan and S. Jin would like to acknowledge the International S&T Cooperation Program of China (grant no. 2016YFE0124400), and the Program for HUST Interdisciplinary Innovation Team (grant no. 2016JCTD104). The Analysis and Testing Center of Huazhong University of Science and Technology is also greatly acknowledged for their helpful assistance with material characterization.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc05297f

‡ These authors contributed equally to this work.

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