Metal-organic layers (MOLs) represent an emerging class of tunable and functionalizable two-dimensional materials. In this work, the scalable solvothermal synthesis of self-supporting MOLs composed of [Hf6O4(OH)4(HCO2)6] secondary building units (SBUs) and benzene-1,3,5-tribenzoate (BTB) bridging ligands is reported. The MOL structures were directly imaged by TEM and AFM, and doped with 4'-(4-benzoate)-(2,2',2''-terpyridine)-5,5''-dicarboxylate (TPY) before being coordinated with iron centers to afford highly active and reusable single-site solid catalysts for the hydrosilylation of terminal olefins. MOL-based heterogeneous catalysts are free from the diffusional constraints placed on all known porous solid catalysts, including metal-organic frameworks. This work uncovers an entirely new strategy for designing single-site solid catalysts and opens the door to a new class of two-dimensional coordination materials with molecular functionalities.

The spontaneous α-to-δ phase transition of the formamidinium-based (FA) lead halide perovskite hinders its large scale application in solar cells. Though this phase transition can be inhibited by alloying with methylammonium-based (MA) perovskite, the underlying mechanism is largely unexplored. In this Communication, we grow high-quality mixed cations and halides perovskite single crystals (FAPbI)(MAPbBr) to understand the principles for maintaining pure perovskite phase, which is essential to device optimization. We demonstrate that the best composition for a perfect α-phase perovskite without segregation is x = 0.1-0.15, and such a mixed perovskite exhibits carrier lifetime as long as 11.0 μs, which is over 20 times of that of FAPbI single crystal. Powder XRD, single crystal XRD and FT-IR results reveal that the incorporation of MA is critical for tuning the effective Goldschmidt tolerance factor toward the ideal value of 1 and lowering the Gibbs free energy via unit cell contraction and cation disorder. Moreover, we find that Br incorporation can effectively control the perovskite crystallization kinetics and reduce defect density to acquire high-quality single crystals with significant inhibition of δ-phase. These findings benefit the understanding of α-phase stabilization behavior, and have led to fabrication of perovskite solar cells with highest efficiency of 19.9% via solvent management.

Dendrite growth of alkali metal anodes limited their lifetime for charge/discharge cycling. Here, we report near-perfect anodes of lithium, sodium, and potassium metals achieved by electrochemical polishing, which removes microscopic defects and creates ultra-smooth ultra-thin solid-electrolyte interphase layers at metal surfaces for providing a homogeneous environment. Precise characterizations by AFM force probing with corroborative in-depth XPS profile analysis reveal that the ultra-smooth ultra-thin solid-electrolyte interphase can be designed to have alternating inorganic-rich and organic-rich/mixed multi-layered structure, which offers mechanical property of coupled rigidity and elasticity. The polished metal anodes exhibit significantly enhanced cycling stability, specifically the lithium anodes can cycle for over 200 times at a real current density of 2 mA cm–2 with 100% depth of discharge. Our work illustrates that an ultra-smooth ultra-thin solid-electrolyte interphase may be robust enough to suppress dendrite growth and thus serve as an initial layer for further improved protection of alkali metal anodes.

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A facile bottom-up method for the synthesis of highly fluorescent nitrogen-doped graphene quantum dots (N-GQDs) has been developed via a one-step pyrolysis of citric acid and tris(hydroxymethyl)aminomethane. The obtained N-GQDs emitted strong blue fluorescence under 365 nm UV light excitation with a high quantum yield of 59.2%. They displayed excitation-independent behavior, high resistance to photobleaching and high ionic strength. In addition to the good linear relationship between the fluorescence intensity of the N-GQDs and pH in the range 2-7, the fluorescence intensity of the N-GQDs could be greatly quenched by the addition of a small amount of 2,4,6-trinitrophenol (TNP). A sensitive approach has been developed for the detection of TNP with a detection limit of 0.30 μM, and a linearity ranging from 1 to 60 μM TNP could be obtained. The approach was highly selective and suitable for TNP analysis in natural water samples.

An effective and facile fluorescence sensing approach for the determination of 2,4,6-trinitrophenol (TNP) using the chemically oxidized and liquid exfoliated graphitic carbon nitride (g-C3N4) nanosheets was developed. The strong inner filter effect and molecular interactions (electrostatic, π-π, and hydrogen bonding interactions) between TNP and the g-C3N4 nanosheets led to the fluorescence quenching of the g-C3N4 nanosheets with efficient selectivity and sensitivity. Under optimal conditions, the limit of detection for TNP was found to be 8.2 nM. The proposed approach has potential application for visual detection of TNP in natural water samples for public safety and security.

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Perovskite solar cells are strong competitors for silicon-based ones, but suffer from poor long-term stability, for which the intrinsic stability of perovskite materials is of primary concern. Herein, we prepared a series of well-defined cesium-containing mixed cation and mixed halide perovskite single-crystal alloys, which enabled systematic investigations on their structural stabilities against light, heat, water, and oxygen. Two potential phase separation processes are evidenced for the alloys as the cesium content increases to 10% and/or bromide to 15%. Eventually, a highly stable new composition, (FAPbI3)0.9(MAPbBr3)0.05­(CsPbBr3)0.05, emerges with a carrier lifetime of 16 μs. It remains stable during at least 10 000 h water–oxygen and 1000 h light stability tests, which is very promising for long-term stable devices with high efficiency. The mechanism for the enhanced stability is elucidated through detailed single-crystal structure analysis. Our work provides a single-crystal-based paradigm for stability investigation, leading to the discovery of stable new perovskite materials.