Open AccessCCS ChemistryRESEARCH ARTICLE5 Aug 2022Precise Construction of Stable Bimetallic Metal–Organic Frameworks with Single-Site Ti(IV) Incorporation in Nodes for Efficient Photocatalytic Oxygen Evolution Lan Li†, Zhi-Bin Fang†, Wenzhuo Deng, Jun-Dong Yi, Rui Wang and Tian-Fu Liu Lan Li† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 †L. Li and Z.-B. Fang contributed equally to this work.Google Scholar More articles by this author , Zhi-Bin Fang† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 †L. Li and Z.-B. Fang contributed equally to this work.Google Scholar More articles by this author , Wenzhuo Deng CAS Key Laboratory of Design and Assembly of Functional Nanostructures and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author , Jun-Dong Yi State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author , Rui Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 Google Scholar More articles by this author and Tian-Fu Liu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101241 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Assembling the reactive low-cost Co clusters and photoresponsive ligands in the form of metal–organic frameworks (MOFs) is a promising strategy to construct efficient water-oxidizing photocatalysts, but it is restricted by poor water stability. Introducing high valent cations in the clusters to build heterometallic Co-MOFs might be a solution, yet a precise fabrication strategy is still challenging. Herein, starting from the presynthesized trinuclear Co2Ti clusters, a novel bimetallic Co-MOF (namely, PFC-20-Co2Ti) capable of single-crystal diffraction for precise structure information was achieved. The incorporation of the single-site Ti(IV) cation endowed the MOF with increased charge density in the metal nodes and optimized the catalytic kinetics, resulting in significantly improved water stability and ultrahigh photocatalytic activities for O2 evolution. The single-site Ti incorporation in the nodes proved to be a universal methodology to acquire stable and active MOF catalysts based on the application of wide-range transition metals such as Ni and Mn. This work echoes MOFs’ capability to enable precise structural design and pave the way for constructing target heterogeneous catalysts with superior performance. Download figure Download PowerPoint Introduction Photocatalytic water splitting to produce hydrogen is a promising technique of transforming inexhaustible solar energy into renewable clean fuels, which could be an ideal solution to global energy and environmental problems. The efficiency of this reaction (2H2O → 2H2 + O2) highly depends on the rate of water oxidation, the kinetically sluggish half-reaction involving four-electron transfer (2H2O + 4h+ → O2 + 4H+).1 Thus, constructing active water oxidation catalysts (WOCs) integrated with proper light-harvesting units is essential for enhancing the water-splitting hydrogen production. To date, despite the expensive iridium-based compounds (IrOx, Ir alloys, etc.),2,3 cobalt species have been proved to be a superior and low-cost type of artificial WOCs, including Co3O4 nanocrystals,4 amorphous CoOx and Co(OH)x,5,6 and single-site Co complexes.7,8 Unfortunately, the waste of bulk metal sites, the lack of structural accuracy and regularity, or the contradiction between high loading amounts and monodispersity in these forms of Co species makes it difficult to understand the structure-catalysis relationships required to maximize the overall catalytic efficiency. Metal–organic frameworks (MOFs) are an emerging class of porous crystalline materials scaffolded by metal ion/cluster nodes and organic ligands in long-range order; they can integrate targeted reactive organic and inorganic species into structures with clear structural information.9–13 In this regard, we envisioned that MOFs built by the coordination of Co clusters and photoresponsive organic molecules would be an ideal photocatalyst candidate overcoming the difficulties in other Co catalysts mentioned above.14,15 In addition, although the intrinsic porosity of MOFs permits complete exposure of each orderly monodispersed Co clusters, making them accessible to the substrate molecules to facilitate photocatalytic water oxidation, most of the photoresponsive Co-MOFs are unstable in aqueous conditions due to the weak Co–O bonds at the metal-carboxylate coordination interface.16,17 According to the hard and soft acids and bases (HSAB) theory, strong coordination can be formed between high-valent metal ions (hard Lewis acids) and the carboxylate ligands (hard Lewis bases), which usually generates moisture-stable MOFs such as UiO-66-Zr(IV)18 and MIL-125-Ti(IV)19 compared with MOF-5-Zn(II)20 and MIL-144-Co(II).21 On the other hand, constructing heterometallic sites easily leads to synergetic catalysis.22–25 A good example is the well-known oxygen evolution center of natural photosynthesis, the [CaMn4Ox] cluster, where the reactive Mn sites serve as the water oxidation sites and the inert Ca cation regulates the redox potential of the entire cluster.22,26 Recent studies further demonstrate the capabilities of heterometallic clusters as the nodes of multivariate MOFs for heterogeneous catalysis.27–30 These findings suggest that it is highly promising to achieve both stability and reactivity of Co-MOF photocatalysts by introducing proper high-valent cations into Co(II) clusters as heterometallic nodes. Nevertheless, such Co-MOFs, thus far, have been rarely reported,31 primarily due to the challenges involved in their fabrications, which tend toward yielding mixtures of monometallic MOFs or incomplete substitutions in the metal nodes. Herein, by a stepwise synthesis strategy, adopting the high-valent and reactive Ti4+ cation,32,33 we achieved a single-site Ti(IV)-incorporated bimetallic Co(II)-MOF PFC-20-Co2Ti (PFC = Porous materials from FJIRSM, CAS) with clear crystalline structures that enhanced water stability, as well as high activities for photocatalytic water oxidation. In this methodology, trinuclear [Co2Ti(μ3-O)(COO)6] clusters were presynthesized and well-characterized, serving as metal nodes that coordinated with the visible-light-responsive ligand in the MOF construction. The resultant PFC-20-Co2Ti exhibited exceptional stability, compared with the reference MOF ( PFC-20-Co3) built by monometallic [Co3(μ3-O)(COO)6]2− clusters. Meanwhile, in the classic [Ru(bpy)3]2+-S2O82− photocatalytic system for water-oxidizing O2 evolution, PFC-20-Co2Ti demonstrated high activities with an optimum turnover frequency (TOF) of 8.06 × 10−3 s−1, an apparent quantum yield (AQY) of 8.56% (at 500 nm irradiation), and desired durabilities in the cyclic catalysis. Taking advantage of the precise structural information, the key role of single-site Ti(IV) incorporation in tuning charge density of metal nodes for the structural stability and catalytic activity of PFC-20-Co2Ti was unveiled by employing density functional theory (DFT) calculations. This methodology was applicable for fabricating water-stable MOFs based on other catalytically active metals such as Ni and Mn, resulting in PFC-20-Ni2Ti and PFC-20-Mn2Ti, respectively. The achievements and findings presented here echo the tailorable properties of MOFs at the atomic level and are anticipated to provide significant insight into the design of stable and efficient photocatalysts. Experimental Methods Materials and instruments All reagents and solvents were commercially purchased and used without further purification unless otherwise mentioned. Single-crystal X-ray diffraction (XRD) data were collected at 100 K on a Bruker D8 VENTURE diffractometer (Bruker, Karlsruhe, Germany) with Mo-Kα radiation (λ = 0.71703 Å). Powder XRD (PXRD) data were collected on a MiniFlex 600 diffractometer with Cu Kα radiation (λ = 1.54056 Å) (Rigaku, Tokyo, Japan). Gas adsorption measurement was performed in a Micrometritics ASAP 2460 System (Micrometritics, Atlanta, GA, USA). Inductively coupled plasma (ICP) results were obtained from Ultima 2 ICP optical emission spectrometer (ICP-OES; HORIBA JY Inc., Paris, France). Scanning electron microscopy (SEM) images and mapping were taken on an FEI Nova NanoSEM 450 (FEI Company, Hillsboro, OR, USA). X-ray absorption spectroscopy (XAS) results were obtained from the BL14W1 beamline station at Shanghai Synchrotron Radiation Facility, China. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250Xi X-ray photoelectron spectrometer (Thermo Fisher Co., Ltd., Waltham, MA, USA) with an Al Ka source (15 kV, 10 mA). Magnetic and heat capacity measurements were performed using a commercial Physical Property Measurement System (PPMS; Quantum-Design, San Diego, CA, USA). Diffuse reflectance spectroscopy (DRS) spectra of the solid samples were collected on a Shimadzu UV-2550 spectrophotometer (Shimadzu Corporation, Kyoto, Japan) using BaSO4 as the background. Photoluminescence (PL) spectra were acquired by an Edinburgh FS5 spectrometer (Edinburgh Instruments Ltd., Livingston, UK) equipped with a continuous xenon lamp. All the electrochemical measurements were conducted in a three-electrode cell connected to a CHI660E electrochemical workstation (CH Instruments, Ltd., Shanghai, China). Synthesis for crystals of Co2Ti(μ3-O)(COO)6, Ni2Ti(μ3-O)(COO)6, and Mn2Ti(μ3-O)(COO)6 clusters For Co2Ti(μ3-O)(COO)6 cluster34: Co(AC)2·4H2O (Ac = acetate) (0.200 g, 0.803 mmol) was suspended in 10 mL dry tetrahydrofuran (THF) in a 50 mL Schlenk tube fitted with an inert gas/vacuum line adapter and magnetic stirrer and 0.238 mL Ti(iPrO)4 (0.809 mmol) was added drop by drop via syringe to the suspension. Then the mixture was stirred for 1 h to obtain a clear deep-blue solution, followed by adding 0.2 mL (2.00 mmol) of trifluoroacetic acid (TFA) to the blue solution, which turned deep red. The reaction mixture was evaporated to dryness under vacuum, and the solid was re-dissolved in 5 mL of dry THF. The red solution was filtered to remove any solid residue and was placed in a freezer at −10 °C to obtain red-colored crystals after 2 weeks. The Ni2Ti(μ3-O)(COO)6 cluster was prepared by a similar procedure, except that Ni(AC)2·4H2O was used instead of Co(AC)2·4H2O. The Mn2Ti(μ3-O)(COO)6 cluster was prepared using a modification of a previously reported procedure.35 Briefly, Mn(Ac)2·4H2O (0.71 g, 2.9 mmol) and Ti(OnBu)4 (OnBu=butoxide) (0.9 mL, 1.45 mmol) were dissolved in a 100 mL Schlenk tube containing 25 mL of dry THF. The suspension was stirred for about half an hour then followed by the addition of TFA (0.67 mL (8.7 mmol) that resulted in a clear yellow solution which was filtered and kept for crystallization at −10 °C. After about 30 days, yellow-tinted crystals’ product was obtained in the mother liquor. Synthesis of PFC-20-M3 (M = Co, Ni, or Mn) 24 mg (0.1 mmol) CoCl2.6H2O and 35.8 mg (0.1 mmol) 3,3’,5,5’-azobenzenetetracarboxylic acid (ABTC) were dissolved in 3 mL dimethylacetamide (DMA) and 0.9 mL H2O, then 0.1 mL HBF4 was added to the mixture and heated at 120 °C for 12 h. The resulting orange block crystals of PFC-20-Co3 were washed three times with DMA. The PFC-20-Ni3 and PFC-20-Mn3 were prepared by a similar procedure except that NiCl2.6H2O or MnCl2.4H2O was used instead of CoCl2.6H2O. Synthesis of PFC-20-M2Ti (M = Co, Ni, or Mn) 15 mg M2Ti(μ3-O)(COO)6 (M = Co, Ni, or Mn) cluster and 20 mg ABTC were dissolved in 2 mL dimethylformamide (DMF) and then 0.4 mL acetic acid was added to the mixture and heated at 130 °C for 2 or 3 days. The resulting yellow block crystals were washed three times with DMF, then twice with acetone to obtain the final product. After structural determination of the MOFs, a one-pot synthesis was carried out to confirm the effectiveness of fabricating PFC-20-M2Ti: M(Ac)2.4H2O (24 mg), Ti(OiPr)4 (10 μL), and acetic acid (0.4 mL) were dissolved DMF (2 mL) in a glass vial and heated in a 130 °C oven for 2 h; subsequently, ABTC (24 mg) was added to the mixture. The resulting solution was heated in a 130 °C oven for another 2 or 3 days. After cooling down to room temperature, a yellow crystalline powder was harvested. Photocatalytic water oxidation In the general procedure, 10 mg powder samples were dispersed in 100 mL buffer containing Na2S2O8 (10 mM) and [Ru(bpy)3]Cl2 (1.0 mM). The buffer-containing system was degassed thoroughly, and this evacuation-refill operation was repeated five times before light irradiation. All the reactions were performed under a light-intensity-controlled xenon lamp (Perfect Light, Beijing, China) with a cut-off filter (λ < 400 nm) at room temperature (5 °C). The photocatalytic reaction was performed for 1 h, and the gaseous products were analyzed online using an Agilent GC7820 (Agilent Technologies Inc., Santa Clara, CA, USA) gas chromatograph equipped with a tandem chromatographic column (5A Molecular sieve) and a thermal conductivity detector. Results and Discussion Synthesis and characterizations of targeted MOFs Before the synthesis of heterometallic MOFs, M2Ti(μ3-O)(COO)6 clusters (abbreviated as M2Ti, M = Co, Ni, or Mn) were prepared by a stoichiometric reaction of Ti(iPrO)4 with metal acetate and TFA in THF under mild conditions according to literature.34,35 The structures of the M2Ti clusters were determined by single-crystal XRD and presented in Supporting Information Figure S1, which were analogous to the commonly observed trimetallic Fe3(μ3-O)(COO)6 cluster.36–38 These clusters were then employed as metal nodes to self-assemble with the carboxylate ligand H4ABTC (Figure 1a) via a solvothermal process. Three isostructural heterometallic MOFs Co2Ti(μ3-O)(ABTC)1.5(H2O)3, Ni2Ti(μ3-O)(ABTC)1.5(H2O)3, and Mn2Ti(μ3-O)(ABTC)1.5(H2O)3 were obtained in a single-crystal form, hereafter named, PFC-20-Co2Ti, PFC-20-Ni2Ti, and PFC-20-Mn2Ti, respectively. Correspondingly, the monometallic reference MOFs Co3(μ3-O)(ABTC)1.5(H2O)3 (namely, PFC-20-Co3), Ni3(μ3-O)(ABTC)1.5(H2O)3 (namely, PFC-20-Ni3), and Mn3(μ3-O)(ABTC)1.5(H2O)3 (namely, PFC-20-Mn3) without the participation of Ti source were synthesized for comparison. Taking PFC-20- Co2Ti as an example, single-crystal X-ray analysis revealed that the preformed trigonal Co2Ti clusters (Figure 1b) were joined by fully deprotonated carboxylate ligands ABTC4− to propagate into a three-dimensional porous structure. PFC-20- Co2Ti was isostructural with PCN-250 and crystallized in the P 4 ¯ 3 n space group ( Supporting Information Table S1).39 As shown in Figure 1c, the resulting infinite framework contains cubic cages (ca. 6.6 × 6.6 × 6.6 Å3) interconnected by two types of squared channels along the a, b, and c axis. Such channels possess a similar size (ca. 7.0 × 7.0 Å), while one of them had H2O molecules dangling at the four corners. Figure 1 | (a) Chemical structure of H4ABTC ligand. (b) The different metal clusters in the PFC-20 series. (c) The crystal structure of PFC-20, the cyan ball represents the void space in the cage. Download figure Download PowerPoint PXRD patterns confirmed the purity and homogeneity of the PFC-20 series ( Supporting Information Figure S2). SEM images also indicated the uniform morphology of these samples, which also ruled out the existence of other amorphous phases and the agglomeration of inorganic clusters (Figures 2a–2d, Supporting Information Figures S3–S7). The corresponding energy-dispersive spectra (EDS) mapping showed even distribution of Ti and M (M = Co, Ni, or Mn) elements in the crystals with the M:Ti stoichiometry close to 2:1, determined by ICP analysis ( Supporting Information Table S2). XAS was conducted to analyze the precise structural information of the metal elements in MOFs ( Supporting Information Figure S11). As shown in Figure 2e, with PFC-20- Co2Ti as a representative, the R-space plot fitting from the experimental Co K-edge EXAFS data matched well with the theoretical curve deduced from the trigonal Co2Ti cluster (Figures 2e, inset and Supporting Information Figure S1a), verifying that the same heterometallic centers of the presynthesized metal clusters and the nodes of PFC-20- Co2Ti. This result indicated that precise incorporation of single-site Ti in metal nodes of MOFs could be achieved via the above stepwise synthesis. Figure 2 | (a) SEM image of PFC-20-Co2Ti. EDS mapping of PFC-20-Co2Ti showing the even distribution of Co (b) and Ti (c) and superimposing on particles (d). (e) Co K-edge EXAFS fittings in R-space showing the magnitude of Fourier transform and real components for PFC-20-Co2Ti compared with the theoretical curve deduced from the Co2Ti cluster. Download figure Download PowerPoint Water stability of PFC-20-Co2Ti and the mechanism To assess the structural stability, the as-synthesized MOFs were treated with H2O, NaOH, and HCl aqueous solutions with pH ranging from 2 to 12. As demonstrated in Figure 3a, the crystallinity of PFC-20-Co2Ti was well maintained after acid or base treatments, and the shiny single-crystal morphology was kept almost unchanged upon being soaked in H2O for 24 h. The N2 uptake of PFC-20-Co2Ti was slightly increased after the acid or base treatments (Figure 3b), and the pore size distributions, centered at 6.2 and 7.3 Å, showed negligible changes ( Supporting Information Figure S8), which not only revealed the inherent porosity and the necessary procedure required to achieve complete activation but also indicated the high stability of PFC-20-Co2Ti. In sharp contrast, when PFC-20-Co3 was treated with H2O for 5 min, the shiny single crystals changed quickly to dim appearance and cracked into small pieces (Figure 3c). In addition, the intensity of the PXRD peaks decreased dramatically, and the N2 isotherm showed nearly no uptakes (Figures 3c and 3d), suggesting that the monometallic MOF PFC-20-Co3could not survive in water. Meanwhile, the stability inspections of PFC-20-Ni2Ti, PFC-20-Mn2Ti also proved that the Ti incorporation endowed the modified MOFs with excellent water-stability and pH resistance, in contrast to their corresponding monometallic MOFs ( Supporting Information Figures S9 and S10). Therefore, it is evident that the single-site Ti incorporation in the M2Ti cluster plays an essential role in the considerable improvement of its chemical stability. Figure 3 | (a) PXRD pattern and (b) N2 isotherms (77 K) of PFC-20-Co2Ti after being treated with different pH solution. (c) PXRD patterns and (d) N2 isothermals (77 K) of PFC-20-Co3 after being treated with H2O. Insets in (a) and (c) are microscope pictures of PFC-20-Co2Ti and PFC-20-Co3 before and after the treatments. Download figure Download PowerPoint It is well-known that the structural stability of MOFs mostly depends on the coordination strength between metal nodes and ligands. Moreover, according to the HSAB theory, the O-donor ABTC4− ligands (as hard bases) could form relatively strong coordination bonds with high-valent metal clusters (as hard acids). Thus, the stability mechanism of PFC-20-M2Ti was investigated regarding the impact of Ti incorporation on the valence of the M2Ti clusters. As shown in Figure 4a, the X-ray absorption near-edge spectroscopy (XANES) onsets of Co K-edge for both PFC-20- Co2Ti and PFC-20- Co3 lie close to that of the CoIIO standard sample. Similarly, both XPS spectra and magnetism studies also revealed the CoII states in both samples (Figure 4b and Supporting Information Figure S12). In addition, the Ti 2p XPS spectrum of PFC-20- Co2Ti confirmed the +4 state of Ti element in the Co2Ti clusters ( Supporting Information Figure S13a). Since three CoII ions, one μ3-O and six carboxylate groups constituted a negatively charged cluster (i.e., [Co3(μ3-O)(COO)6]2−) for PFC-20- Co3, counter cations such as dimethylammonium ions might have been generated during the solvothermal synthesis, which might be necessary for charge compensation.40 As to PFC-20- Co2Ti, the Ti(IV) incorporation increased the valence of Co(II) in [Co2Ti(μ3-O)(COO)6] clusters and gave rise to a neutral framework. Meanwhile, we found that the XANES Co K-edge of PFC-20- Co2Ti exhibited an apparent positive shift, compared with that of PFC-20- Co3, and the Co 2p3/2 and Co 2p1/2 peaks on XPS spectra for PFC-20- Co2Ti (781.3 and 797.1 eV, respectively) were located at higher binding energies than those of PFC-20- Co3 (780.9 and 796.7 eV, respectively). Similarly, Mn 2p peaks of PFC-20-Mn2Ti and Ni 2p peaks of PFC-20-Ni2Ti also showed positive shifts, compared with that of their counterparts without Ti incorporation ( Supporting Information Figure S14). These results indicated that the oxidation state of cations in nodes of MOFs could be enhanced by Ti(IV) incorporation. Moreover, the charge density difference obtained from DFT calculations (Figure 4c) showed an appreciable increase in positive charge density in the Co of [Co2Ti(μ3-O)(COO)6] cluster in PFC-20-Co2Ti and that the μ3-O atom gained more negative charge from Ti and Co, compared with the cases of the [Co3(μ3-O)(COO)6]2− cluster in PFC-20-Co3. These variations were also revealed by the comparison of calculated Bader charge values (Figure 4d). The existence of a new XPS O 1s peak at low binding energy was consistent with such a charge increase of μ3-O caused by Ti(IV) incorporation ( Supporting Information Figure S13b). The higher charge density localized at the μ3-O of PFC-20-Co2Ti than that of PFC-20-Co 3 indicating that more electrons were involved in the bond formation, thereby elucidating the mechanism of the dramatically improved stability of these Ti-incorporated MOFs. Figure 4 | (a) XANES of Co K-edge for PFC-20-Co2Ti and PFC-20-Co3 compared with standard inorganic references. (b) XPS Co 2p spectra of PFC-20-Co2Ti and PFC-20-Co3. (c) Charge density difference of PFC-20-Co3 (left) and PFC-20-Co2Ti (right). Cyan and yellow colors represent the loss and gain in electrons, respectively. The value of isosurface is 0.015 e/Bohr3. (d) Averaged Bader charge analysis on selected atoms of Co, Ti, and O in PFC-20-Co3 and PFC-20-Co2Ti. Download figure Download PowerPoint Photocatalytic O2 evolution of PFC-20-Co2Ti and the mechanism With the high water stability and the active metal clusters, PFC-20-Co2Ti was considered a promising heterogenous WOC. The catalytic properties of PFC-20-Co2Ti were evaluated in a classic [Ru(bpy)3]2+-S2O82− photocatalytic system for water-oxidizing O2 evolution, in which [Ru(bpy)3]2+ as photosensitizer and Na2S2O8 as electron acceptor.41 To better assess the performances of these MOFs, the photocatalytic conditions involving the concentrations of [Ru(bpy)3]2+ and borate buffer as well as the initial pH values were optimized, respectively ( Supporting Information Figure S15). As shown in Figure 5a, under the optimized photocatalytic condition, the O2 evolution catalyzed by PFC-20-Co2Ti increased rapidly in the first 20 min before leveling off and reaching 168 μmol after 1 h, which is 17 times higher than did PFC-20-Mn2Ti and PFC-20-Ni2Ti ( Supporting Information Figure S16a). The TOF of PFC-20-Co2Ti was calculated to be 8.06 × 10−3 s−1 per cobalt atom in the first 10 min, which dramatically outperformed the commercial Co3O4 (1.42 × 10−4 s−1) and was comparable to some top cases of Co-based heterogenous WOC ( Supporting Information Figure S17 and Table S3).42–44 Figure 5 | (a) Photocatalytic O2 evolution by PFC-20-Co2Ti in an optimized [Ru(bpy)3]2+-S2O82− system compared between different conditions. (b) Comparison between AQYs of O2 evolution and PL intensity of [Ru(bpy)3]2+-S2O82− solution under monochromatic-light irradiation. (c) PL spectra of [Ru(bpy)3]2+ solution with or without photocatalysts. (d) LSV curves of electrocatalytic OER for PFC-20-Co2Ti (inset: photocurrent response under visible light irradiation). Download figure Download PowerPoint Control experiments of conducting the same reaction but in the absence of light irradiation/Na2SO8/catalyst/[Ru(bpy)3]2+ (Figure 5a and Supporting Information Figure S16b) showed negligible O2 production, confirming the necessity of all these components for such a photocatalytic system. The catalytic durability of PFC-20-Co2Ti was further inspected via cycle experiments. Supporting Information Figure S18 demonstrated that PFC-20-Co2Ti could be reused at least three cycles with sustained crystallinity and only a marginal decrease in activity, indicating the stability and reusability of PFC-20-Co2Ti. It should be mentioned that the monometallic MOFs ( PFC-20-M3, M = Co, Mn, or Ni) decomposed completely in the photocatalytic solution even before light irradiation. This sharp contrast further revealed that the water-stability of photocatalysts was dramatically enhanced by Ti incorporation, as disclosed above. By observing the advanced performance of our fabricated modified MOFs, we were in the position to gain an understanding of the catalytic mechanism of PFC-20-Co2Ti: In the conventional [Ru(bpy)3]2+-S2O82− photocatalytic system, the O2 evolution is disclosed to process as the following steps41,45: [ Ru ( bpy ) 3 ] 2 + + h ν → [ Ru ( bpy ) 3 ] 2 + * (1) [ Ru ( bpy ) 3 ] 2 + * + S 2 O 8 2 − → [ Ru ( bpy ) 3 ] 3 + + SO 4 − • + SO 4 2 − (2) [ Ru ( bpy ) 3 ] 2 + + SO 4 − • → [ Ru ( bpy ) 3 ] 3 + + SO 4 2 − (3) 4 [ Ru ( bpy ) 3 ] 3 + + H 2 O + Cat . → 4 [ Ru ( bpy ) 3 ] 2 + + O 2 + 4 H + (4) Steps 1– 3 show the photon conversion accomplished by [Ru(bpy)3]2+ photosensitizer with the assistance of S2O82− electron acceptor. Thus, the correlation between the O2 evolution of PFC-20-Co2Ti and the photoresponse of [Ru(bpy)3]2+-S2O82− solution was examined. As shown in Figure 5b, the AQYs calculated from the monochromic-light-driven O2 evolution ( Supporting Information Figure S19a and Table S4) matched well with the trend of the excitation-wavelength-dependent PL intensity of [Ru(bpy)3]2+-S2O82− solution ( Supporting Information Figure S19b), confirming the above proposed photosensitizer-dependent photon conversion in the present system. Step 4 involves the electron transfer from the catalyst to [Ru(bpy)3]3+ (the oxidized state of photosensitizer), as well as the activation and oxidation of H2O molecule on active sites of the catalyst. Thus, before assessing the electron transfer efficiency, the band structure of PFC-20-Co2Ti as the thermodynamic aspect should be determined. According to the DRS spectrum and the Mott–Schottky plot, PFC-20-Co2Ti possessed a bandgap of 1.95 eV and a conduction band minimum (CBM) potential of −0.18 V versus normal hydrogen electrode (NHE; Supporting Information Figure S20). In addition, PFC-20-Co2Ti exhibited a photon-to-current response under visible light (Figure 5d, inset), agreeing with its narrow bandgap. These results suggested that PFC-20-Co2Ti could be excited by a wide-range visible light (λedge = 637 nm, Supporting Information Figure S20a), and the photogenerated electrons and holes were able to reduce the [Ru(bpy)3]3+ back to [Ru(bpy)3]2+ (+1.53 V vs NHE) and oxidize H2O to O2 (+1.23 V vs NHE), respectively ( Supporting Information Figure S20c), that is, PFC-20-Co2Ti was allowed to proceed step 4 thermodynamically. The electron transfer from PFC-20-Co2Ti to [Ru(bpy)3]3+ was confirmed by PL spectra in which the PL emission intensity of [Ru(bpy)3]2+ decreased in the presence of PFC-20-Co2Ti under excitation λ = 520 nm (Figure 5c). On the contrary, the addition of the UV-responsive TiO2 or ZrO2 did not cause an apparent change in PL emission, indicating that any electron transfer scarcely occurred between [Ru(bpy)3]3+ and large-bandgap TiO2 or ZrO2. This, in turn, revealed the necessity of the visible-li
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