In-Plane Coassembly Route to Atomically Thick Inorganic–Organic Hybrid Nanosheets

Control over the anisotropic assembly of small building blocks into organized structures is considered an effective way to design organic nanosheets and atomically thick inorganic nanosheets with nonlayered structure. However, there is still no available route so far to control the assembly of inorganic and organic building blocks into a flattened hybrid nanosheet with atomic thickness. Herein, we highlight for the first time a universal in-plane coassembly process for the design and synthesis of transition-metal chalcogenide–alkylamine inorganic–organic hybrid nanosheets with atomic thickness. The structure, formation mechanism, and stability of the hybrid nanosheets were investigated in detail by taking the Co₉S₈–oleylamine (Co₉S₈–OA) hybrid nanosheets as an example. Both experimental data and theoretical simulations demonstrate that the hybrid nanosheets were formed by in-plane connection of small two-dimensional (2D) Co₉S₈ nanoplates <i>via</i> oleylamine molecules adsorbed at the side surface and corner sites of the nanoplates. X-ray absorption fine structure spectroscopy study reveals the structure distortion of the small 2D Co₉S₈ nanoplates that endows structural stability of the atomically thick Co₉S₈–OA hybrid nanosheets. The brand new atomically thick nanosheets with inorganic–organic hybrid network nanostructure will not only enrich the family of atomically thick 2D nanosheets but also inspire more interest in their potential applications

Synthetic access to a framework-stabilized and fully sulfided analogue of an Anderson polyoxometalate that is catalytically competent for reduction reactions

Polyoxometalates (POMs) featuring 7, 12, 18, or more, redox-accessible transition-metal ions are ubiquitous as selective catalysts, electrocatalysts, and sensitized photocatalysts, especially for oxidation reactions. The corresponding synthetic and catalytic chemistry of stable, discrete, and capping-ligand-free polythiometalates (PTMs), which could be especially attractive for reduction reactions, is much less well developed. Among the challenges is the propensity of PTMs to agglomerate and form larger clusters of indeterminate size, as well as the tendency for agglomeration to block access of candidate reactants to potential catalyst active-sites. Nevertheless, the pervasive presence of transition-metal sulfur clusters metalloenzymes or cofactors that catalyze reduction reactions, and the justifiable proliferation of studies of 2D metal chalcogenides, and especially their edge sites, as reduction catalysts, point to the promise of well-defined and controllable PTMs as catalysts for reduction reactions, including complex, bond-forming, many-electron reactions. Here we report the fabrication of agglomeration-immune, reactant-accessible, capping-ligand-free CoIIMoIV6S24n- clusters as periodic arrays in a water-stable, hierarchically porous Zr-metal-organic-framework (MOF; NU1K) by first preparing and installing a disk-like Anderson polyoxometalate, CoIIMoVI6O24m(-), in size-matched (<1 nm) micropores termed c-pores, where the siting is established via DED (difference electron density) X-ray diffraction experiments. Prolonged treatment with flowing H2S while heating, uniformly reduces the six molybdenum(VI) ions to Mo(IV) and quantitatively replaces oxygen anions with similarly ligating sulfur anions in the form S(2-), HS(-), and S2(2-). Further DED measurements show that the templated POM-to-PTM conversion leaves the clusters individually isolated in open-channel-connected c-pores. The structure of the immobilized cluster as determined, in part, by XPS, XAFS, and PDF (pair-distribution function) analysis of total X-ray scattering agrees very well with the theoretically simulated structure. Preliminary, proof-of-concept experiments show that electrode-supported thin-films of CoMo6S24@NU1K are electrocatalytically competent for hydrogen evolution in aqueous acid (e.g. 10 mA·cm-2 of current density at an overpotential of 100 mV). Suspensions of CoMo6S24@NU1K in acetonitrile + triethanolamine, are photocatalytically competent for hydrogen evolution via sensitization with chromophoric MOF linkers. Nevertheless, the initially installed PTM appears to be a pre-catalyst, as hydrogen evolution is observed only after four hours of photolysis. Reduction-assisted loss of ~3-to-6 sulfurs, as H2S, likely is responsible for pre-catalyst-to-catalyst conversion, as the loss opens coordination sites on multiple cluster-sited metal ions, perhaps enabling hydrogen evolution via a Mo-hydride intermediate. Given the great variety of sizes and compositions available for both POMs and Zr-MOFs, we suggest that the approach described here can be adapted for the synthesis and stabilization of periodic arrays of other non-agglomerating, capping-ligand-free PTMs of well-defined metal-nuclearity, presumably including catalytically functional PTMs

Atomically Precise Single-Site Catalysts via Exsolution in a Polyoxometalate–Metal–Organic-Framework Architecture

Single-site catalysts (SSCs) achieve a high catalytic performance through atomically dispersed active sites. A challenge facing the development of SSCs is aggregation of active catalytic species. Reducing the loading of these sites to very low levels is a common strategy to mitigate aggregation and sintering; however, this limits the tools that can be used to characterize the SSCs. Here we report a sintering-resistant SSC with high loading that is achieved by incorporating Anderson–Evans polyoxometalate clusters (POMs, MMo6O24, M = Rh/Pt) within NU-1000, a Zr-based metal–organic framework (MOF). The dual confinement provided by isolating the active site within the POM, then isolating the POMs within the MOF, facilitates the formation of isolated noble metal sites with low coordination numbers via exsolution from the POM during activation. The high loading (up to 3.2 wt %) that can be achieved without sintering allowed the local structure transformation in the POM cluster and the surrounding MOF to be evaluated using in situ X-ray scattering with pair distribution function (PDF) analysis. Notably, the Rh/Pt···Mo distance in the active catalyst is shorter than the M···M bond lengths in the respective bulk metals. Models of the active cluster structure were identified based on the PDF data with complementary computation and X-ray absorption spectroscopy analysis

Synthetic Access to a Framework-Stabilized and Fully Sulfided Analogue of an Anderson Polyoxometalate that is Catalytically Competent for Reduction Reactions

: Polyoxometalates (POMs) featuring 7, 12, 18, or more redox-accessible transition metal ions are ubiquitous as selective catalysts, especially for oxidation reactions. The corresponding synthetic and catalytic chemistry of stable, discrete, capping-ligand-free polythiometalates (PTMs), which could be especially attractive for reduction reactions, is much less well developed. Among the challenges are the propensity of PTMs to agglomerate and the tendency for agglomeration to block reactant access of catalyst active sites. Nevertheless, the pervasive presence of transition metal sulfur clusters metalloenzymes or cofactors that catalyze reduction reactions and the justifiable proliferation of studies of two-dimensional (2D) metal-chalcogenides as reduction catalysts point to the promise of well-defined and controllable PTMs as reduction catalysts. Here, we report the fabrication of agglomeration-immune, reactant-accessible, capping-ligand-free CoIIMo6IVS24n- clusters as periodic arrays in a water-stable, hierarchically porous Zr-metal-organic framework (MOF; NU1K) by first installing a disk-like Anderson polyoxometalate, CoIIIMo6VIO24m-, in size-matched micropores where the siting is established via difference electron density (DED) X-ray diffraction (XRD) experiments. Flowing H2S, while heating, reduces molybdenum(VI) ions to Mo(IV) and quantitatively replaces oxygen anions with sulfur anions (S2-, HS-, S22-). DED maps show that MOF-templated POM-to-PTM conversion leaves clusters individually isolated in open-channel-connected micropores. The structure of the immobilized cluster as determined, in part, by X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) analysis, and pair distribution function (PDF) analysis of total X-ray scattering agrees well with the theoretically simulated structure. PTM@MOF displays both electrocatalytic and photocatalytic competency for hydrogen evolution. Nevertheless, the initially installed PTM appears to be a precatalyst, gaining competency only after the loss of ∼3 to 6 sulfurs and exposure to hydride-forming metal ions