Probing surface structure on two-dimensional metal-organic layers to understand suppressed interlayer packing
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IChem, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China 2 School of Mathematical Sciences and Fujian Provincial Key Laboratory of Mathematical Modeling and High-Performance Scientific Computation, Xiamen University, Xiamen 361005, China © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 30 April 2020 / Revised: 30 June 2020 / Accepted: 13 July 2020
ABSTRACT Two-dimensional metal-organic layers (MOLs) from alternatively connected benzene-tribenzoate ligands and Zr6(µ3-O)4(µ3-OH)4 or Hf6(µ3-O)4(µ3-OH)4 secondary building units can be prepared in gram scale via solvothermal synthesis. However, the reason why the monolayers did not pack to form thick crystals is unknown. Here we investigated the surface structure of the MOLs by a combination of sum-frequency generation spectroscopy, nanoscale infrared microscopy, atomic force microscopy, aberrationcorrected transmission electron microscopy, and compositional analysis. We found a partial coverage of the monolayer surface by dangling tricarboxylate ligands, which prevent packing of the monolayers. This finding illustrates low-density surface modification as a strategy to prepare new two-dimensional materials with a high percentage of exposed surface.
KEYWORDS metal-organic frameworks, metal-organic layers, surface structure, two-dimensional (2D) materials, solvothermal synthesis
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Introduction
Two-dimensional (2D) metal-organic layers (MOLs), a 2D analogue of metal-organic frameworks (MOFs), have attracted a lot of attentions as a new category of 2D materials [1–14]. The MOLs of 2D topology can be as thin as a monolayer, which becomes a soft net as compared to the brittle crystals of three-dimensional (3D) MOFs. The thinness of the MOL increases accessibility of the ligands and secondary building units of it to external environment as compared to 3D MOF. Because of this, MOL is suitable for catalyzing conversion of large organic molecules that are difficult to diffuse into channels of 3D MOFs. The accessibility of surface site of the MOL also makes it easy to be functionalized via coordination chemistry on the surface to construct cooperative centers in catalysis. Moreover, the 2D MOL is suitable for assembling onto other 2D surfaces to build functional hybrid structures. MOLs can be prepared in gram scale via bottom-up routes such as surfactant-assisted solvothermal synthesis [15]. The surfactants stabilize ultrathin 2D layers by covering the surface. A mixed solvent of dimethylformide (DMF), modulator acid, and water without surfactants can also produce a variety of 2D MOLs of monolayer thickness [16, 17]. The resulting 2D MOLs with exposed surface can serve to design efficient catalyst with fine-tuned micro-environment around the catalytic centers. It is still unknown why this solvent combination helps to avoid packing of the monolayers. Surface structure of MOFs are considered to be important Address correspondence t
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