Precise Construction of Stable Bimetallic Metal–Organic Frameworks with Single-Site Ti(IV) Incorporation in Nodes for Efficient Photocatalytic Oxygen Evolution

Abstract

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 , Zhi-Bin Fang† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , 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 , Jun-Dong Yi State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , Rui Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 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 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 of the PXRD and the N2 showed (Figures and that the monometallic MOF not in Meanwhile, the stability of PFC-20-Ni2Ti, also proved that the Ti incorporation endowed the MOFs with and pH in to their corresponding monometallic MOFs ( Supporting Information Figures and it is that the single-site Ti incorporation in the M2Ti cluster an essential role in the of stability. Figure 3 | (a) PXRD and (b) N2 of PFC-20-Co2Ti after being treated with different pH (c) PXRD patterns and N2 of PFC-20-Co3 after being treated with in (a) and (c) are of PFC-20-Co2Ti and PFC-20-Co3 before and after the Download figure Download PowerPoint is well-known that the structural stability of MOFs depends on the coordination between metal nodes and according to the theory, the ABTC4− ligands hard could form strong coordination bonds with high-valent metal clusters hard Thus, the stability mechanism of PFC-20-M2Ti was the of Ti incorporation on the of the M2Ti clusters. As shown in Figure the X-ray absorption spectroscopy of Co K-edge for both PFC-20- Co2Ti and PFC-20- close to that of the both spectra and studies also revealed the in both samples (Figure and Supporting Information Figure In addition, the Ti of PFC-20- Co2Ti confirmed the of Ti in the Co2Ti clusters ( Supporting Information Figure three one and carboxylate a cluster for PFC-20- cations such as ions might have been the solvothermal which might be necessary for charge As to PFC-20- the Ti(IV) incorporation increased the of Co(II) in [Co2Ti(μ3-O)(COO)6] clusters and to a Meanwhile, we that the Co K-edge of PFC-20- Co2Ti exhibited an apparent compared with that of PFC-20- and the Co and Co on spectra for PFC-20- Co2Ti and were at of PFC-20- and Mn of and Ni of PFC-20-Ni2Ti also showed compared with that of their without Ti incorporation ( Supporting Information Figure These results indicated that the oxidation of cations in nodes of MOFs could be enhanced by Ti(IV) the charge density obtained from (Figure showed an in charge density in the Co of [Co2Ti(μ3-O)(COO)6] cluster in PFC-20-Co2Ti and that the charge from Ti and Co, compared with the of the [Co3(μ3-O)(COO)6]2− cluster in These were also revealed by the of charge (Figure The existence of a at energy was with such a charge of by Ti(IV) incorporation ( Supporting Information Figure The charge density at the of PFC-20-Co2Ti that of 3 that were involved in the the mechanism of the improved stability of these Figure 4 | (a) of Co K-edge for PFC-20-Co2Ti and PFC-20-Co3 compared with inorganic (b) Co spectra of PFC-20-Co2Ti and (c) density of PFC-20-Co3 and PFC-20-Co2Ti and yellow the and in respectively. The of is charge analysis on of Co, and in PFC-20-Co3 and PFC-20-Co2Ti. Download figure Download PowerPoint Photocatalytic O2 evolution of PFC-20-Co2Ti and the mechanism the high water stability and the active metal clusters, PFC-20-Co2Ti was a promising The catalytic properties of PFC-20-Co2Ti were in a classic [Ru(bpy)3]2+-S2O82− photocatalytic system for water-oxidizing O2 evolution, in which as and Na2S2O8 as electron To assess the of these MOFs, the photocatalytic conditions involving the of and buffer as well as the pH were ( Supporting Information Figure As shown in Figure under the optimized photocatalytic the O2 evolution by PFC-20-Co2Ti increased in the 20 before and after 1 h, which is times and PFC-20-Ni2Ti ( Supporting Information Figure The of PFC-20-Co2Ti was to be 8.06 × 10−3 cobalt in the 10 min, which the commercial Co3O4 × and was to of ( Supporting Information Figure and Table Figure 5 | (a) Photocatalytic O2 evolution by PFC-20-Co2Ti in an optimized [Ru(bpy)3]2+-S2O82− system compared between different (b) between of O2 evolution and of [Ru(bpy)3]2+-S2O82− solution under irradiation. (c) spectra of solution with or without photocatalysts. of for PFC-20-Co2Ti under light Download figure Download PowerPoint of the same reaction but in the of light (Figure and Supporting Information Figure showed negligible O2 the of these components for such a photocatalytic The catalytic of PFC-20-Co2Ti was further via Supporting Information Figure demonstrated that PFC-20-Co2Ti could be at three with crystallinity and only a in the stability and of PFC-20-Co2Ti. be mentioned that the monometallic MOFs ( M = Co, Mn, or in the photocatalytic solution even before light irradiation. This sharp further revealed that the of photocatalysts was enhanced by Ti as the of MOFs, we were in the to an of the catalytic mechanism of In the [Ru(bpy)3]2+-S2O82− photocatalytic the O2 evolution is to as the ( 3 2 + + h → ( 3 2 + ( 3 2 + + 2 2 → ( 3 3 + + 4 + 4 2 ( 3 2 + + 4 → ( 3 3 + + 4 2 4 ( 3 3 + + 2 + → 4 ( 3 2 + + 2 + 4 + 3 the by with the of electron Thus, the between the O2 evolution of PFC-20-Co2Ti and the of [Ru(bpy)3]2+-S2O82− solution was As shown in Figure the from the O2 evolution ( Supporting Information Figure and Table matched well with the of the of [Ru(bpy)3]2+-S2O82− solution ( Supporting Information Figure the above in the 4 the electron transfer from the to of as well as the activation and oxidation of H2O on active sites of the Thus, before the electron transfer the structure of PFC-20-Co2Ti as the be According to the and the PFC-20-Co2Ti a of and a potential of hydrogen Supporting Information Figure In addition, PFC-20-Co2Ti exhibited a under light (Figure with These results that PFC-20-Co2Ti could be by a wide-range light = Supporting Information Figure and the and were to the to and H2O to O2 ( Supporting Information Figure that PFC-20-Co2Ti was to 4 The electron transfer from PFC-20-Co2Ti to was confirmed by spectra in which the emission of in the of PFC-20-Co2Ti under = nm (Figure On the the addition of the or not an apparent in that any electron transfer between and or in revealed the of the of PFC-20-Co2Ti (Figure inset and Supporting Information Figure As for the reaction kinetics, the oxygen evolution reaction of PFC-20-Co2Ti was via As in Figure the of PFC-20-Co2Ti was at the ( Supporting

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