Ti-oxo Clusters Research Articles

Electrochemically Determined and Structurally Justified Thermochemistry of H atom Transfer on Ti-Oxo Nodes of the Colloidal Metal-Organic Framework Ti-MIL-125.

Titanium dioxide (TiO2) has long been employed as a (photo)electrode for reactions relevant to energy storage and renewable energy synthesis. Proton-coupled electron transfer (PCET) reactions with equimolar amounts of protons and electrons at the TiO2 surface or within the bulk structure lie at the center of these reactions. Because a proton and an electron are thermochemically equivalent to an H atom, these reactions are essentially H atom transfer reactions. Thermodynamics of H atom transfer has a complex dependence on the synthetic protocol and chemical history of the electrode, the reaction medium, and many others; together, these complications preclude the understanding of the H atom transfer thermochemistry with atomic-level structural knowledge. Herein, we report our success in employing open-circuit potential (EOCP) measurements to quantitatively determine the H atom transfer thermochemistry at structurally well-defined Ti-oxo clusters within a colloidally stabilized metal-organic framework (MOF), Ti-MIL-125. The free energy to transfer H atom, Ti3+O-H bond dissociation free energy (BDFE), was measured to be 68(2) kcal mol-1. To the best of our understanding, this is the first report on using EOCP measurements to quantify thermochemistry on any MOFs. The proton topology, the structural change upon the redox reaction, and BDFE values were further quantitatively corroborated using computational simulations. Furthermore, comparisons of the EOCP-derived BDFEs of Ti-MIL-125 to similar parameters in the literature suggest that EOCP should be the preferred method for quantitatively accurate BDFE calculations. The reported success in employing EOCP for nanosized Ti-MIL-125 should lay the ground for thermochemical measurements of other colloidal systems, which are otherwise challenging. Implications of these measurements on Ti-MIL-125 as an H atom acceptor in chemical reactions and comparisons with other MOFs/metal oxides are discussed.

Photocatalytic activation of peroxymonosulfate by Ti-oxo clusters grafted carbon nitride: Enhanced charge transfer and bisphenol A degradation efficiency

Photocatalytic peroxymonosulfate (PMS) activation process is promising for the organic wastewater treatment. A simple amide bonding strategy has been developed to synthesized Ti-oxo clusters (TiOCs)-g-C3N4 (TiOCs-CN) catalyst. The prepared TiOCs-CN was used to remove bisphenol A (BPA) under simulated sunlight-driven PMS. The experimental and computational results demonstrate that the incorporation of TiOCs can efficiently regulate the electronic structure of CN. Thus, an excellent performance with a BPA removal rate of 100 % and a totalorganic carbon(TOC) removal rate of 87.2 % within 40 min was achieved in the AM 1.5/PMS-TiOCs-CN system.Among, the reaction rate constant of AM 1.5/PMS-TiOCs-CN (97.24 × 10−3 min−1) is higher than that of AM 1.5/PMS-CN (25.10 × 10−3 min−1) by nearly 4 times. The synergistic effect of photocatalytic PMS activation greatly facilitated the generation of reactive radicals and obviously increased the removal rate of BPA. This work can offer inspiration for rational design of high efficiency photocatalysts in the PMS oxidation process for organic wastewater remediation.

Modification of electron transfer paths in CdS-TiO2 Z-type heterojunction by cobalt-based complexes for boosting visible-light photocatalytic hydrogen evolution

Ti-oxo clusters as photocatalytic heterojunction precursor materials are a promising strategy for preparing TiO2. Enhancing the utilization of photogenerated electrons and increasing the delivery channel of these electrons can improve the performance of photocatalytic materials for H2 evolution significantly. Hereby, a novel ternary [TiO2-CdS QDs-5/Co(bpy)3Cl2] photocatalyst for highly efficient photocatalytic HER was reported for the first time. In this paper, a TiO2-CdS QDs-5 binary Z-type heterojunction was identified, and the photocatalytic HER yield was further improved by [Co(bpy)3Cl2]. The above-mentioned ternary system is advantageous for separating and transferring photogenerated electron-hole pairs. The photocatalytic HER rate of this catalytic system was 4440.84 μmol·h−1·g−1, which was 7.1 times more than that of TiO2-CdS QDs-5. The results indicate that the system is capable of maintaining stability and enhancement of the photocatalytic HER under visible light irradiation. In this study, we investigated photogenerated electron transfer pathways to improve the photocatalytic HER of TiO2 and we also revealed the electron transfer mechanism of TiO2-CdS QDs-5 Z-typed heterojunction with [Co(bpy)3Cl2] through DFT calculation. More importantly, this study presents a novel Z-type heterojunction catalyst, which enhance the electron transport path and this work provides an effective design concept for the development of TiO2-based materials for efficient photocatalytic HER.

Unveiling Synergetic Photocatalytic Activity from Heterometallic Ti/Ce Clusters.

Titanium-oxo clusters, with their robust structure and suitable optical and electronic properties, have been widely investigated as photocatalysts. Heterometallic Ti/M-oxo clusters provide additional tunability and functionality, which enable systematic structure-activity investigations to elucidate the reaction mechanisms and improve the catalyst design. Incorporating cerium into Ti-oxo clusters can provide additional redox (CeIV/CeIII) and oxygen harvesting ability, but to date, only a limited number of structurally defined titanium-cerium (Ti/Ce) clusters have been reported due to their synthetic challenges. Herein, we report the synthesis and photocatalytic properties of two structurally defined Ti/Ce-oxo clusters, Ti8Ce2(BA)16 and Ti9Ce4(BA)20, as well as a TiCe-BA cluster with a calculated formula of Ti20Ce9O36(BA)42. Photocatalytic study of these clusters demonstrates that the amount of Ce3+ species greatly impacts its photocatalytic oxidation performance, and their superior photocatalytic reactivity toward aerobic alcohol oxidation can be contributed to the synergistic effects of the multiple radical species generated upon light absorption. This work represents a significant milestone in the construction of stable Ti/Ce-oxo clusters, enriching the current library of known heterometallic Ti/M-oxo clusters, and providing a series of crystalline materials with great promise of photoluminescence and photovoltaic chemistry.

Optical limiting effects of 1,10-phenanthroline functionalized heterometallic Sn–Ti oxo clusters with distinct π⋯π interactions

Heterometallic Sn–Ti oxo clusters functionalized with conjugated 1,10-phenanthroline ligands have been prepared, which exhibit distinct optical bandgaps and optical limiting properties influenced by π⋯π interactions.

Pushing the heterometal doping limit while preserving long-lived charge separation in a Ti-based MOF photocatalyst

This study explores the nature, dynamics, and reactivity of the photo-induced charge separated excited state in a Fe3+-doped titanium-based metal organic framework (MOF), xFeMIL125-NH2, as a function of iron concentration. The MOF is synthesized with doping levels x = 0.5, 1 and 2 Fe node sites per octameric Ti-oxo cluster and characterized by powder x-ray diffraction, UV-vis diffuse reflectance, atomic absorption, and steady state Fe K-edge X-ray absorption spectroscopy. For each doping level, time-resolved X-ray transient absorption spectroscopy studies confirm the electron trap site role of the Fe sites in the excited state. Time scan data reveal multiexponential decay kinetics for the charge recombination processes which extend into the microsecond range for all three concentrations. A series of dye photodegradation studies, based on the oxidative decomposition of Rhodamine B, demonstrates the reactivity of the charge separated excited state and the photocatalytic capacity of these MOF materials compared to traditional heterometal-doped semiconductor photocatalysts.

Confining Ti-oxo clusters in covalent organic framework micropores for photocatalytic reduction of the dominant uranium species in seawater

Confining Ti-oxo clusters in covalent organic framework micropores for photocatalytic reduction of the dominant uranium species in seawater

High-Porosity Metal-Organic Framework Glasses.

We report a synthetic strategy to link titanium-oxo (Ti-oxo) clusters into metal-organic framework (MOF) glasses with high porosity though the carboxylate linkage. A new series of MOF glasses was synthesized by evaporation of solution containing Ti-oxo clusters Ti16 O16 (OEt)32 , linkers, and m-cresol. The formation of carboxylate linkages between the Ti-oxo clusters and the carboxylate linkers was confirmed by Fourier-transform infrared (FT-IR) spectroscopy. The structural integrity of the Ti-oxo clusters within the glasses was evidenced by both X-ray absorption near edge structure (XANES) and 17 O magic-angle spinning (MAS) NMR. After ligand exchange and activation, the fumarate-linked MOF glass, termed Ti-Fum, showed a N2 Brunauer-Emmett-Teller (BET) surface areas of 923 m2 g-1 , nearly three times as high as the phenolate-linked MOF glass with the highest BET surface area prior to this report.

2D porphyrin-based MOFs with highly dispersed Pt nanoparticles via in-situ partial reduction strategy from porphyrin embedded with single-atom Pt for enhancing photocatalytic hydrogen production

An efficient and stable 2D MOF TMF-Pt/Pt NPs photocatalyst was prepared by in-situ partial hydrogenation reduction of TMF-Pt with single-atom Pt embedded in porphyrin center to improve photocatalytic hydrogen production. It is important to note that TMF-Pt/Pt NPs has an extremely high photocatalytic hydrogen generation rate (33185.66 µmol·g−1·h−1) in comparison to the majority of previous MOFs photocatalysts, which could be due to the in-situ uniformly grown Pt NPs are very adjacent to the Ti-oxo clusters within the 2D MOFs so that the gradually accumulated electrons around the Ti-oxo clusters via LMCT pathway could instantaneously transfer to Pt NPs through the Schottky injection field effect. Through a series of experimental characterizations, especially atomic force microscopy (AFM) and transmission scanning electron microscope (TEM), confirmed the highly dispersed Pt NPs within the 2D MOFs. This work provided us with a novel idea to realize the uniform distribution of noble metals nanoparticles in the channel cavity of MOFs through an in-situ synthesis strategy, so as to facilitate the more efficient Schottky injection field effect between noble metals nanoparticles and metal clusters of the MOFs, thereby achieving more excellent charge carrier separation efficiency.

Gram-scale synthesis of MIL-125 nanoparticles and their solution processability.

Although metal-organic framework (MOF) photocatalysts have become ubiquitous, basic aspects of their photoredox mechanisms remain elusive. Nanosizing MOFs enables solution-state techniques to probe size-dependent properties and molecular reactivity, but few MOFs have been prepared as nanoparticles (nanoMOFs) with sufficiently small sizes. Here, we report a rapid reflux-based synthesis of the photoredox-active MOF Ti8O8(OH)4(terephthalate)6 (MIL-125) to achieve diameters below 30 nm in less than 2 hours. Whereas MOFs generally require ex situ analysis by solid-state techniques, sub-30 nm diameters ensure colloidal stability for weeks and minimal light scattering, permitting in situ analysis by solution-state methods. Optical absorption and photoluminescence spectra of free-standing colloids provide direct evidence that the photoredox chemistry of MIL-125 involves Ti3+ trapping and charge accumulation onto the Ti-oxo clusters. Solution-state potentiometry collected during the photochemical process also allows simultaneous measurement of MOF Fermi-level energies in situ. Finally, by leveraging the solution-processability of these nanoparticles, we demonstrate facile preparation of mixed-matrix membranes with high MOF loadings that retain the reversible photochromism. Taken together, these results demonstrate the feasibility of a rapid nanoMOF synthesis and fabrication of a photoactive membrane, and the fundamental insights they offer into heterogeneous photoredox chemistry.

Solvent-controlled synthesis of Ti-based porphyrinic metal-organic frameworks for the selective photocatalytic oxidation of amines.

The photocatalytic activity of metal-organic frameworks (MOFs) can be managed by the milieu of synthesis. Herein, N,N'-dimethylacetamide (DMA) and N,N'-diethylformamide (DEF) were employed as solvents for the synthesis of two Ti-based porphyrinic MOFs, namely Ti-PMOF-DMA and Ti-PMOF-DEF, from tetrabutyl orthotitanate and 4,4',4'',4'''-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid). Notably, both DMA and DEF were adsorbed onto the Ti-oxo clusters of the two MOFs to shape their properties. Ti-PMOF-DMA was observed with better optoelectronic response and charge transfer than Ti-PMOF-DEF. Moreover, Ti-PMOF-DMA owned a larger pore volume than Ti-PMOF-DEF, imparting more accessible sites to benzyl amines. Ti-PMOF-DMA exhibited better activity in selective photocatalytic aerobic oxidation of benzylamine than Ti-PMOF-DEF. Irradiated by red light-emitting diodes, outstanding results for selective conversion of benzyl amines to imines over Ti-PMOF-DMA were attained. Superoxide radical anion, generated by the electron transfer from porphyrin via Ti-oxo clusters to dioxygen, turned out to be the primary reactive oxygen species. There was generality towards aerobic oxidation of amines to imines and considerable stability for Ti-PMOF-DMA. This work provides a new perspective on the altering MOFs to enhance photocatalytic organic transformations.

Surface tailored Ti-oxo clusters enabling highly efficient organic solar cells

In this work, the hierarchical surface reactivity of a kind of Ti-oxo cluster (TOC) with robust cyclic structure, CTOC, supported by multiple surface ligands, has been initially established. By this method, a series of aromatics (i.e., benzoate for CTOC-Ph, 4-methylbenzoate for CTOC-PhMe and 4-cyanobenzoate for CTOC-PhCN) substituted CTOC-Ars, with identical TOC core have been produced. These CTOC-Ars can be solution-processed into smooth films with order microstructures and optimized semiconducting properties, driven by intermolecular interactions of the aromatics. When using CTOC-Ph as cathode interlayer (CIL) in PM6:BO-4Cl based organic solar cells (OSCs), power conversion efficiencies (PCEs) up to 17.7 % have been achieved, much higher than 15.6 % of those with CTOC CIL. The distinct electron doping behavior with non-fullerene acceptor and higher adsorption energy with the polymer donor has been successfully unraveled, which can enable CTOC-Ph more intimate contact with the photoactive layer and facilitate more efficient charge transport. Moreover, the generality of CTOC-Ars as CIL for efficient OSCs has been confirmed in varied OSC systems, in which high PCEs of 18.4 % can be provided for PM6:L8-BO system.

Mechanical-robust and recyclable polyimide substrates coordinated with cyclic Ti-oxo cluster for flexible organic solar cells

With the rapid development of flexible optoelectronic devices, recyclability is highly desirable for alleviating resource waste and environmental pollution, but remains challenging. Here, a fully closed-loop recyclable crosslinked polyimide (RCPI) was fabricated via carboxyl ligand exchange between the CPI with pendent carboxyl groups and the cyclic Ti-oxo cluster (CTOC) with labile carboxyl ligands, which could be reconverted into reprocessable CPI and CTOC solution by pivalic acid. The RCPI-based embedded AgNWs@RCPI electrode shows comparable high conductivity, transparency and low roughness with ITO (indium tin oxide)/glass electrode, and meanwhile outstanding mechanical robustness. The related flexible organic solar cells (FOSCs) provided a high efficiency of 14.78% and maintained ~97% of the initial efficiency after 5000 bending cycles at a small bending radius of 1 mm. Moreover, the recyclability of the RCPI still retains after being manufactured into the FOSCs. This work provides a promising strategy for recyclable flexible optoelectronic devices.

Preparation and Visible-Light Response of Salicylate-Stabilized Heterobimetallic Pb-Ti-Oxo Clusters Initiated via Auxiliary Quaternary Ammonium Salts and a Solvent Effect.

In this study, with the assistance of quaternary ammonium salts we have successfully prepared a new family of salicylate-stabilized heterobimetallic Pb-Ti-oxo clusters, including H(TEA)[Pb2Ti6(μ2-O)2(μ3-O)2(OiPr)4(PA)2(Sal)6(NO3)2] (PTC-321; TEA = tetraethylammonium; HOiPr = isopropanol; H2PA = phenylphosphonic acid; H2Sal = salicylic acid), {PbTi3(μ2-O)(μ3-O)(OiPr)2(PA)(Sal)3(DMF)·CH3CN}n (PTC-322; DMF = dimethylformamide), {PbTi5(μ3-O)6(Sal)3(CH3COO)2(DMF)(OiPr)2}n (PTC-323), [Pb2Ti4(Sal)6(EtO)2(OiPr)6(HOiPr)2]·CH3NH2 (PTC-324; EtOH = CH3CH2OH), H[Pb4Ti9(μ2-O)2(μ3-O)(μ4-O)6(Sal)7(OiPr)13] (PTC-325), and Pb2Ti12(μ2-O)3(μ3-O)3(μ4-O)4(Sal)4(OEt)24 (PTC-326). Single-crystal X-ray diffraction studies demonstrate that the {Ti3Pb(Sal)3} unit acts as the building block to constitute the diverse assembly of PTC-321-PTC-323. Thereinto, the clusters in PTC-322 and PTC-323 are connected into infinite one-dimensional chains. Furthermore, the solvent effects have facilitated the heterobimetallic Pb-Ti-oxo clusters into various configurations in PTC-323-PTC-326. Solid-state ultraviolet-visible spectroscopy analysis indicates that the optical absorption bands of these compounds shift effectively toward the visible-light region, and they were also employed as electrode precursors to investigate their visible-light-driven photocurrent response.

2D Ti-based metal–organic framework photocatalysis for red light-driven selective aerobic oxidation of sulfides

Ti-based metal-organic frameworks (MOFs) consisting of organic linkers and Ti-oxo clusters have tremendous proneness in visible light photocatalysis. Here, two-dimensional (2D) Ti-based porphyrin MOF (PMOF (Ti)) and bulk PMOF (Ti) were synthesized and fully characterized. Consequently, the porphyrin linker imparts broad absorption of visible light and the appropriate reduction potential of Ti‐oxo clusters can activate O2 to superoxide (O2•–). The 2D PMOF (Ti) exhibits more satisfying results for the red light-driven selective oxidation of sulfides with O2 than bulk PMOF (Ti). In addition, 2D PMOF (Ti) is highly robust which could be recycled 8 times without any loss of photocatalytic activity. Mechanistic investigations reveal that O2•– is the crucial reactive oxygen species (ROS) in forming sulfoxides with high selectivities. This work highlights that 2D MOFs could have a great potential in selective photocatalytic transformations of organic molecules.

Installation of high-valence tungsten in MIL-125(Ti) for boosted photocatalytic hydrogen evolution

Metal-organic frameworks (MOFs) possess the features of highly porosity-tunable and electronic-tunable structures. Taking advantages of these merits, we successfully installed high-valence W6+ ions onto the Ti-oxo clusters of MIL-125(Ti) (W-MIL-125). The installed W6+ ions which form a W-O-Ti structure trigger the metal-to-cluster charge transfer (MCCT), together with an enhanced light absorption. Structural and spectroscopic characterizations reveal that the MCCT process optimizes the charge transfer process and efficiently separates the photogenerated electron-hole spatially. The as-obtained sample of 3.45 W-MIL-125 with optimized electronic structure demonstrates an enhanced photocatalytic hydrogen evolution performance of 1110.7 ± 63.7 µmol g−1 h−1 under light irradiation, which is 4.0 times that of the pristine MIL-125(Ti). This work will open up a new avenue for local structural modification of MOFs to boost photocatalytic performance.

Two Isostructural Titanium Metal-Organic Frameworks for Light Hydrocarbon Separation.

Presented here is the light hydrocarbon separation of titanium metal-organic frameworks (Ti-MOFs). Compared with the cyclic Ti-oxo cluster (Ti8O8(CO2)16, Ti8Ph), porous structures of FIR-125 and FIR-126 (FIR = Fujian Institute Research) can effectively improve the adsorption amounts of light hydrocarbons. The introduction of different functional groups and Ti-oxo clusters with small window sizes enables them to exhibit the highly selective separation of C2 and C3 hydrocarbons versus methane in an ambient atmosphere. The results show that Ti-MOFs are potential porous adsorbents for the separation of light hydrocarbons.

Porphyrin-based Ti-MOFs conferred with single-atom Pt for enhanced photocatalytic hydrogen evolution and NO removal

A robust and efficient titanium metal–organic framework (TMF-Pt) with a single-atom Pt embedded PtII tetrakis(4-carboxyphenyl)porphyrins (PtTCPP) as linking groups and Ti-oxo molecular clusters as metal nodes was successfully fabricated for the first time, which have been confirmed by aberration-correction scanning transmission electron microscopy (HAADF-STEM) and synchrotron-radiation-based X-ray absorption fine-structure spectroscopy (XAFS), and was applied to photocatalytic hydrogen production and NO removal. It was worth noting that the TMF-Pt exhibited an extremely high hydrogen evolution activity (15456 µmol·g−1·h−1) and NO removal rate (70.3%) in comparison with most of the reported MOFs-based photocatalysts, which could be ascribed to the very high atom-utilization efficiency of single-atom Pt along with a high charge carriers separation efficiency and the Ti-oxo clusters as an alternative to the very famous TiO2 photocatalyst. The results provided us with a novel strategy that introducing single-atom Pt into the center of the porphyrin skeleton could integrate the broad light-harvesting porphyrin unite with the very high atom-utilization efficiency single-atom Pt to further improve the photocatalytic performance ultimately.

Ti-Oxo Clusters with Peripheral Alkyl Groups as Cathode Interlayers for Efficient Organic Solar Cells.

Three independent Ti-oxo clusters (TOCs) that contain 6, 8, and 12 Ti atoms in the cores and alkyl groups on the surface were developed as cathode interlayers in bulk-heterojunction organic solar cells (OSCs). These TOCs have precise chemical structures with a single crystal, excellent solubility in methanol, and well-aligned work function. Smooth films can be facilely obtained by spin-casting their solution on top of the active layer. Therefore, they can be used as an interlayer in OSCs to provide a high power conversion efficiency (17.29%). Further studies reveal that these TOCs can not only reduce the work function of the silver electrode to provide better energy level alignment but also exhibit a significant n-doping effect with the non-fullerene acceptors to facilitate efficient electron extraction and transport. Our results demonstrate that TOCs as semiconductors have great potential application in OSCs.

The forgotten chemistry of group(IV) metals: A survey on the synthesis, structure, and properties of discrete Zr(IV), Hf(IV), and Ti(IV) oxo clusters

• Group IV metal oxo clusters (MOCs) applications in materials and catalysis are rapidly expanding. • Group IV MOCs are prepared in aqueous or organic medium with a variety of ligands. • Ligands influence structure and properties, thus impacting materials syntheses and catalysis. • Ligand-exchanged happens with or without rearrangement of the inorganic core. Group (IV) metal oxo clusters are a diverse class of compounds with rapidly growing applications in catalysis, materials chemistry, electronics and optics. They possess unique structural stability, reactivity and electronic properties; however, their potential remains underexplored due to the gaps in fundamental understanding about their structure, intrinsic features, and nuances in their synthesis that overall would allow for great tunability of their properties. In this context, here we review the chemistry of discrete zirconium, hafnium, and titanium metal oxo clusters reported in the literature in order to provide a critical overview of the structural features, properties and reactivity disclosed to date. While a comprehensive summary of Zr and Hf clusters is presented, only the most recent Ti oxo clusters are discussed in addition to some key compounds that have been revised elsewhere. Envisioning the growing relevance group (IV) metal oxo clusters for the synthesis of nanostructure materials and in catalysis, we also summarize key principles of their surface chemistry, and the photoactivity of Ti oxo clusters, given the paramount importance of these properties for the development of future applications.