Highly effective photoreduction of CO2 to CO promoted by integration of CdS with molecular redox catalysts through metal–organic frameworks

CdS/UiO-bpy/Co composites showed outstanding performance for photoreduction of CO2 to CO via integrating CdS with a Co complex through MOFs.


Introduction
Global energy demand and climate change have underpinned broad interest in the sustainable reduction of CO 2 into value added carbon products, such as CO, alcohols, and hydrocarbons. Photocatalytic reduction of CO 2 is regarded as an essential and important technology for mitigation of the greenhouse effect and generation of renewable transportation fuels. [1][2][3] Although great effort has been devoted to exploring various photocatalysts, 4-6 the photocatalytic efficiency is far from satisfactory, mainly because of fast recombination of the photogenerated electron-hole pairs, low specic surface area and conned active sites. [7][8][9] Increasing the CO 2 adsorption ability of photocatalysts is an effective strategy to improve the CO 2 conversion efficiency. 10,11 Due to their large surface area and well-ordered porous structure, metal-organic frameworks (MOFs) are considered to be promising candidates for gas adsorption, and can facilitate CO 2 adsorption. 12,13 Nevertheless, the performance of MOFs in photocatalytic reduction of CO 2 is obstructed by the low efficiency of exciton generation and charge separation in MOFs, as well as inconsistencies between catalytic and adsorption sites. 14,15 Cadmium sulde (CdS), with an excellent visible-light response and appropriately positioned conduction band, has been investigated extensively as a photocatalyst for photocatalytic reduction of CO 2 , whose applications have unfortunately been limited due to the fast recombination of photogenerated electron-hole pairs, the serious photocorrosion, the low CO 2 adsorption, and the lack of catalytic sites. [16][17][18] It has been reported that the combination of CdS and MOFs can help to surmount their respective weak points by increasing the visible-light response and facilitating CO 2 adsorption. 19 However, it is difficult for CdS/MOF composites to achieve high photocatalytic performance for CO 2 reduction due to the large size of CdS and lack of catalytic sites. 20 Some molecular redox catalysts containing metals such as Re, Co, Ni, and Fe are capable of reducing CO 2 to CO photochemically. 21,22 However, they need to be fed with electrons by a photosensitizer. So far, a number of studies have been proposed to utilize inorganic semiconductors as photosensitizers, [23][24][25] but the interaction between inorganic semiconductors and complexes is very poor, leading to difficulty in cycling experiments. It was reported that bipyridine cobalt complexes can efficiently promote the photoreduction of CO 2 to CO when combined with these inorganic semiconductors. 26 However, bipyridine cobalt complexes can dissolve in the solution, which is unfavorable for recovery. Although a few studies have been reported on combining MOFs and molecular redox catalysts for photocatalytic reduction of CO 2 , either precious metals or complex ligands were used. 27,28 UiO-bpy MOFs, with exceptional CO 2 adsorption capacity and stability, 29 have been constructed from linear 2,2 0 -bipyridine-5,5 0 -dicarboxylate (bpydc) bridging ligands and Zr 6 (m 3 -O) 4 (m 3 -OH) 4 SBUs. Although the bridging ligands of bpydc can coordinate with some transition metal (e.g. Co 2+ , Cu 2+ and Ni 2+ ), the CO 2 photoreduction is also obstructed by the low efficiency of visible light exciton generation.
In this work, we designed ternary CdS/UiO-bpy/Co composites. The inorganic semiconductors and the molecular redox catalysts were integrated through UiO-bpy. The CdS/UiO-bpy/Co composites showed outstanding performance for photocatalytic conversion of CO 2 to CO under visible light irradiation. To the best of our knowledge, there is no report on the combination of inorganic semiconductors and molecular redox catalysts through MOFs for photoreduction of CO 2 .

Results and discussion
The method proposed to synthesize the ternary CdS/UiO-bpy/Co composites is shown schematically in Fig. 1. UiO-bpy was rstly prepared from ZrCl 4 and bpydc following a reported method. 30 Then, the vacuum-activated sample of UiO-bpy was treated with Cd(CH 3 COO) 2 $2H 2 O and dimethylsulfoxide (DMSO) at 180 C for 12 h to form CdS/UiO-bpy. Because the bpydc bridging ligands can coordinate with Cd 2+ in DMSO, and the Cd 2+ can slowly react with DMSO to form CdS, 31 small CdS nanoparticles were generated. The synthesized CdS/UiO-bpy samples and cobalt chloride were added into tetrahydrofuran (THF) solvent at room temperature and stirred for 12 h. Finally, the CoCl 2 was integrated into CdS/UiO-bpy by coordinating with bpydc and the ternary CdS/UiO-bpy/Co composites were obtained in this way.
The obtained CdS, UiO-bpy, CdS/UiO-bpy and CdS/UiO-bpy/ Co composites were characterized by X-ray diffraction (XRD), as shown in Fig. S1. † For these samples, a group of strong diffraction peaks before 20 matched with the simulated UiObpy signals, 32 proving that CdS and Co loading did not change the structure of UiO-bpy. In addition, the three diffraction peaks with 2q values of 26.5 , 44.0, and 52.1 are related to the (111), (220), and (311) crystal planes of hawleyite CdS, respectively.
The contents of CdS and Co 2+ were determined by ICP-AES (VISTA-MPX) and the results are shown in Table S1. † The loading amount of Co in the CdS/UiO-bpy/Co composites was determined to be 6.18 wt%, indicating that 50.8% of the bpy units were metalated with Co in the UiO-bpy framework.
The scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HR-TEM) images of the as-synthesized UiO-bpy and CdS/UiO-bpy/Co composites are shown in Fig. 2. We can observe that the UiObpy is a regular octahedron, and the morphology of UiO-bpy was retained aer the UiO-bpy was loaded with CdS and Co, as shown in Fig. 2A and B. It is found that the CdS nanoparticles with a diameter of about 10 nm were well dispersed on the UiO-68 framework in the CdS/UiO-bpy/Co composites, as shown in Fig. 2C and D. The lattice fringes of individual CdS particles with an interplanar spacing of z0.176 nm can be assigned to the (311) lattice plane of hawleyite (Fig. 2C), which is in good agreement with XRD analysis (Fig. S1 †). Energy dispersive X-ray (EDX) elemental mapping of the CdS/UiO-bpy/Co composites in Fig. 2E shows that Cd, S, Co and Cl elements were dispersed uniformly in the composites.
The CdS, UiO-bpy, CdS/UiO-bpy and CdS/UiO-bpy/Co composites were characterized using the N 2 adsorption-  desorption method. As shown in Fig. S2, † pure UiO-bpy had a total surface area (S BET ) of 1323 m 2 g À1 , while the S BET value based on the total weight of CdS/UiO-bpy was 52.6 m 2 g À1 . The decrease in S BET aer incorporation of CdS is due to the fact that non-porous CdS nanoparticles blocked some pores of UiO-bpy. The S BET of the CdS/UiO-bpy/Co composites decreased to 25.5 m 2 g À1 due to high metal loading and the potential blocking of pore entrances by the sterically demanding CoCl 2 . Despite the fact that the BET surface area of the CdS/UiO-bpy/Co composites was smaller than that of pure CdS, the CO 2 adsorption amount was larger than that of pure CdS, as shown in Fig. S3, † which indicates that the UiO-bpy can facilitate CO 2 adsorption.
The size of CdS in the CdS/UiO-bpy was smaller than that of the pure CdS, as can be seen from Fig. S4, † mainly resulting from the coordination of bpydc with Cd 2+ . In order to elucidate the role of bpydc, we replaced the bpydc with biphenyl-4,4 0dicarboxylic acid. Similar to bpydc, biphenyl-4,4 0 -dicarboxylic acid can also react with ZrCl 4 to produce UiO-67, and the morphology of UiO-67 was similar to that of the UiO-bpy (Fig. S5a †). The preparation conditions for CdS/UiO-67 were the same as those for CdS/UiO-bpy. However, as shown in Fig. S5b, † the size of CdS particles in CdS/UiO-67 was about 50 nm, which was larger than that in CdS/UiO-bpy (10 nm, Fig. 2), mainly because biphenyl-4,4 0 -dicarboxylic acid does not coordinate with Cd 2+ . So we can conclude that the bridging ligands of bpydc played a key role in obtaining small size CdS nanoparticles. Furthermore, we also found that the amount of cadmium acetate had an important effect on the size of CdS, as shown in Fig. S6. † In order to elucidate the local coordination environment of the Co atom in the CdS/UiO-bpy/Co composites, X-ray absorption near-edge structure (XANES) spectra and Fouriertransformed Co K-edge extended X-ray absorption ne structure (EXAFS) spectra were recorded. As shown in Fig. 3A, the absorption edge position of the CdS/UiO-bpy/Co composites is located between that of Co foil and Co 3 O 4 and is close to that of CoS, suggesting that the Co atom carries positive charge and the valence state of Co is between +1 and +3. As shown in the Fourier transform of the EXAFS spectrum of CdS/UiO-bpy/Co composites (Fig. 3B), the peaks at around 1.6Å can be attributed to the Co-N bond and the Co-Cl bond. The quantitative coordination conguration of the Co atom can be obtained by EXAFS tting (Fig. S7 and Table S2 †). Both the coordination numbers of Co-N and Co-Cl are 2. These results indicate that the coordination structure of the Co atom is consistent with the schematic diagram in Fig. 1. Diffuse reectance UV/Vis spectroscopy and X-ray photoelectron spectroscopy (XPS) were also used to elucidate the local coordination environment of the Co atom in the CdS/UiO-bpy/Co composites. A typical absorption band in the region of 600-800 nm corresponding to the Co complex in the UV/vis spectrum was observed, as shown in Fig. S8. † It was also conrmed that the absorption properties are similar to those of the corresponding homogeneous complex Co-bpy, indicating that the Co atoms were coordinated with the ligands of bpydc, which was further supported by the XPS survey spectra in Fig. S9. † The N 1s XPS spectra of CdS/UiObpy shied to higher binding energies by 0.16 eV aer the Co atoms were loaded into the CdS/UiO-bpy, suggesting that the surface chemical states of N atoms were changed by the loaded Co atoms, due to the interaction between the N atoms and Co atoms in the CdS/UiO-bpy/Co composites. XPS spectra of the Co 2p and Cl 2p also indicated the introduction of Co and Cl elements into the framework (Fig. S10 †).
To evaluate the photocatalytic applications of CdS, UiO-bpy, CdS/UiO-bpy, and CdS/UiO-bpy/Co composites, their optical absorption properties were investigated by UV-vis diffuse reectance spectroscopy, as shown in Fig. S8. † It is shown that the UiO-bpy can only absorb ultraviolet light. However, CdS, CdS/UiO-bpy and CdS/UiO-bpy/Co composites exhibited strong optical absorption extending to the visible light region, suggesting that these materials can be excited by visible light irradiation to generate electron-hole pairs for redox reactions. To gain a deeper understanding of the efficacy of photoexcited charge separation, the photo-current responses of CdS, CdS/ UiO-bpy and CdS/UiO-bpy/Co composites were tested, as shown in Fig. 4A. The transient photocurrent results provide clear evidence for electron transfer within these catalysts. It is clear that the photocurrent intensity of CdS/UiO-bpy/Co was higher than that of both CdS and CdS/UiO-bpy under visible  light irradiation, indicating more effective charge separation in the CdS/UiO-bpy/Co composites.
The catalytic behaviors of the CdS/UiO-bpy/Co composites were examined for the photochemical reduction of CO 2 and compared with those of CdS and CdS/UiO-bpy. The CO 2 reduction was conducted under visible light irradiation, using triethanolamine (TEOA) as an electron donor and acetonitrile as the solvent. As shown in Fig. 4B, the amount of CO generated increased almost linearly with irradiation time. The CdS/UiObpy/Co composites showed a high CO evolution rate of 235 mmol g À1 h À1 under light illumination for 10 hours, which was higher than that of CdS (CO, 23 mmol g À1 h À1 ) and CdS/UiObpy (CO, 0 mmol g À1 h À1 ), as shown in Table 1. The turnover number of CO formation using the CdS/UiO-bpy/Co composites was 2.24. The CO evolution rate is 195 mmol g À1 h À1 for the CdS/ UiO-bpy/Co composites under ambient pressure of CO 2 ( Table 1). And the CO evolution rate was 110 mmol g À1 h À1 for CdS with the aid of Co-bpy as a cocatalyst, which was lower than that of the CdS/UiO-bpy/Co composites (Table 1). These results reveal that CdS/UiO-bpy/Co composites have comparable activity for CO 2 reduction compared with previously reported catalysts under comparative conditions, as can be seen from Table S3. † The yield and the morphology of the catalyst had no obvious change aer ve cycles, as shown in Fig. 4C and S11, † indicating the excellent stability of the CdS/UiO-bpy/Co composites. Our experiments showed that in the absence of photocatalysts or light illumination, no products were detected in the reaction, implying that CO was generated by the photocatalytic reduction of CO 2 on the CdS/UiO-bpy/Co composites, as shown in Table 1. In order to conrm that the reaction is induced by the light absorption of CdS, the action spectrum for the photocatalytic reaction was determined, as shown in Fig. S12. † Furthermore, the calculated apparent quantum yield of the CdS/UiO-bpy/Co composites at 420 nm was 0.65%, which was higher than that of CdS (0.063%) and CdS@Co-bpy (0.302%). Comparable AQY was previously reported for CO 2 to CO conversion with a CdS-[Ni(terpyS) 2 ] 2+ hybrid catalyst (0.28 AE 0.04%) 33 and CO 2 to formate conversion with a Ru-Ag-TaON hybrid catalyst (0.48%). 34 It is also found that higher efficiencies can be achieved by using a RuP/C 3 N 4 hybrid catalyst with a suitable solvent of N,N-dimethylacetamide (5.7%). 35 To further validate the source of the generated CO, an isotopic experiment using 13 CO 2 as a substrate was performed under identical photocatalytic reaction conditions, and the products were analyzed by gas chromatography and mass spectrometry. As shown in Fig. S13, † the peak at m/z ¼ 29 could be assigned to 13 CO, indicating that the carbon source of CO was the CO 2 used. When the CO 2 was replaced by N 2 , no detectable products were formed, as shown in Table 1.
In order to clarify the role of Co in the CdS/UiO-bpy/Co composites, we also loaded Ni atoms into the preformed CdS/UiO-bpy composites to form ternary CdS/UiO-bpy/Ni composites, which were similar to CdS/UiO-bpy/Co composites, and the results are shown in Fig. S14. † The CdS/UiO-bpy/ Ni composites displayed 2.1-fold higher photocatalytic activity for H 2 evolution compared with CdS/UiO-bpy, but no CO was generated, as shown in Table 1. This indicated that the Co in the composites was crucial for the photoreduction of CO 2 to CO. Electron paramagnetic resonance (EPR) spectroscopy was used to further study the function of Co atoms during the CO 2 photoreduction process. As shown in Fig. 4D(a), a strong signal for high-spin state Co 2+ was observed for the CdS/UiO-bpy/Co composite without irradiation in a nitrogen atmosphere. 36 However, the peak intensity of Co 2+ was greatly weakened with light irradiation, which clearly indicates that the Co valence states transformed from high-spin state Co 2+ to Co + in a low-spin state (Fig. 4D(c)). The EPR signal corresponding to Co 2+ was enhanced when CO 2 was introduced into the irradiated CdS/UiO-bpy/Co composites, implying that some of the Co + was oxidized back to Co 2+ species during the CO 2 photoreduction process ( Fig. 4D(b)). 37 This kinetic behavior also indicates that photoexcited electrons are transferred to the Co center, 38,39 and the photoreduction of CO 2 was enhanced by the valence transformation of the Co.

Conclusions
In conclusion, we demonstrate the design and fabrication of ternary CdS/UiO-bpy/Co composites, which combined inorganic semiconductors with molecular redox catalysts through MOFs using the function of the ligands. CdS nanoparticles of small size were obtained due to the presence of bridging ligands of bpydc. The ternary CdS/UiO-bpy/Co composites exhibited very high efficiency for photoreduction of CO 2 , affording CO as the sole carbonaceous product with an evolution rate of 235 mmol g À1 h À1 , and the selectivity was over 85%. We anticipate that the implantation strategy, combining inorganic semiconductors with molecular redox catalysts through MOFs, will be applicable for designing many other efficient photocatalysts for CO 2 reduction.