Room-temperature sintered metal-organic framework nanocrystals: A new type of optical ceramics

Abstract

Ceramic is a rigid material consisting of an infinite threedimensional network of sintered non-oriented inorganic, non-metallic crystalline grains, which are generally nontransparent. If the inner light scatter is eliminated, ceramics can become transparent or optical ceramics [1,2]. With great potentials in optical windows, sensors and lasers, optical ceramics have received great attentions since 1950s [1]. As the gain medium of solid-state lasers, optical ceramics can combine the advantages of large size, high transparency, high mechanical and thermal stabilities, and ease for dispersing the luminous atoms and groups [3]. For comparison, single-crystal gain mediums can suffer high power density but are difficult to grow into large size, while glasses and polymers have reversed characteristics.To make ceramics transparent, the inner pore and impurity should be minimized to zero, which requires highly pure, nanosized, and very uniform crystallite precursors as well as complicated fusing technology ( Fig. 1) [4,5]. The cubic crystal symmetries (absence of the birefringence which causes extremely high scattering losses) are generally required for the crystallites of optical ceramics, meaning that only a few inorganic materials can be used. Besides, ceramics sintering usually operates above 1000 o C, which excludes organic and organic-inorganic hybrid materials [1].Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) are crystalline materials consisting of metal ions and organic ligands, which have captured widespread attentions for adsorption, catalysis, sensing, optics [6] and also laser gain mediums [7][8][9][10][11]. As can be seen in membranes and photonic crystals [12,13], MOF nanocrystals may merge to form seamless microscopic blocks under mild conditions [14][15][16]. Nevertheless, macroscopic and optical transparent blocks of MOF nanocrystals/microcrystals are still unknown so far. Just like single crystals and optical ceramics, increasing the block size with high optical quality is always a great challenge for many materials. Here, we show that by simply slowing down the solvent evaporation rate, nanocrystals of the prototypical MOF SOD-[Zn(mim) 2 ] (MAF-4/ZIF-8 [17,18], Hmim = 2-methylimidazole, Fig. 2) with the cubic crystal symmetry can fuse at room temperature to form large transparent blocks. After doping laser dye, this metal-organic optical ceramic (MOOC) can show the amplified spontaneous emission (ASE) with a very low energy-density threshold.MAF-4 nanocrystals with uniform size (ca. 20 nm) can be easily synthesized by reaction of Zn(NO 3 ) 2 and Hmim in methanol or ethanol at room temperature (Fig. S1). Compared with the commonly used solvent methanol [19], ethanol is nontoxic and more environment-friendly. After centrifugation and washed, the residual nanocrystals and solvent form a white gel-like (semi-fluidic) block.

MOF‐Based Organic Microlasers

“…Optical ceramics is a transparent special ceramic that combines the high stability of single crystals with the large size of glass, fluid, and other amorphous materials, which is a potential carrier for the laser gain medium . Ye et al reported a novel metal‐organic optical ceramic (MOOC, MOOC‐1) consisting of a metal‐organic framework MAF‐4 (also known as ZIF‐8) . By simply reducing the rate of solvent evaporation, MAF‐4 nanocrystals can be fused to form dense ceramic‐like blocks with millimeter size and up to 84% visible light transmission ( Figure a).…”

“…c) The pumping energy density‐ dependent fluorescence of SRh@MOOC‐1. Reproduced with permission . Copyright 2018, Science China Press and Springer‐Verlag GmbH Germany, part of Springer Nature.…”

Technology for the Remediation of Water Pollution: A Review on the Fabrication of Metal Organic Frameworks

“…Through filtration, the reaction product is collected for further solvent evaporation in order to obtain purified MOF crystals [37,38]. With this method, Ye et al [39] synthesized a new type of ceramic base MOF: the crystal size and the symmetry of the product as well as the calcination temperature need to be strictly controlled in this preparation, and there are few inorganic non-metallic materials that can form optical ceramics. However, using the conventional solution method, the nanocrystals denoted as MAF-4 (sodalite-type zinc dimethyl imidazole known as ZIF-8) was able to produce a dense ceramic-like block.…”

Mass transport through metal organic framework membranes

“…Thus, MOFs demonstrate great potential applications in various fields. Owing to the decades' efforts of researchers, thousands of structures of MOFs have been discovered and applied in various fields, such as separation [31][32][33][34][35][36][37][38], adsorption [20,[39][40][41][42], optics [43][44][45][46][47][48][49][50], catalysis [19,[51][52][53][54][55][56], sensors [57][58][59][60][61][62][63], etc. However, most of these studies are based on MOFs powders.…”