Ceramic, a kind of poly-crystalline monolith sintered by inorganic, non-metallic crystallites, is normally opaque due to defects, voids and birefringence. If the inner light scatter is eliminated, ceramics can become transparent or optical ceramics.
Jie-Peng Zhang and co-workers, from Sun Yat-Sen University, developed a new type of materials, i.e. metal-organic frameworks, for preparation of the optical ceramics, recently published in Science China Materials, Issue 3 in 2018 as a cover article.
Zhang's group was devoted to the development for metal-organic frameworks and their applications in adsorption, separation and sensing for a long time. He said "Optical ceramic belongs to a special type of ceramics, and, like single crystals, it is optically transparent."
Despite the extensive applications in high performance optical glass and lasing gain medium, the development of optical ceramic heavily relies on the precursor materials.
"To make ceramics transparent, the inner pore and impurity should be minimized to ZERO. This is a very stringent demanding.", he says, "The precursors not only need high purity and uniform size distribution, but also crystallize in the cubic symmetry to remove birefringence effect."
In addition, generally, preparing ceramics has to undergo high-temperature sintering process. Therefore, up to date, only a few materials can be used for optical ceramics.
Porous coordination polymers, also known as metal-organic frameworks (MOFs), have captured widespread attention for adsorption, catalysis, sensing and optics. "However, commonly, they are microcrystalline powders." he says, "It is still challenging to prepare MOF membranes and single crystals with high quality and large size."
Despite low solubility in common solvent, the nanocrystals and building units of MOFs have notable exchange rate, especially in the grains with small size and large curvature. Zhang emphasized "It is essential to the crystal growth and ion/ligand exchange processes of MOFs." A condensed monolith may be formed by healing the defects inside the aggregates which are assembled by MOF nanocrystals.
Zhang, as the leader of the research group, tells us "This philosophy motivates us to employ MOF nanocrystals as precursors and then make them fused into a transparent monolith, i.e., metal-organic optical ceramics (MOOC)."
The SOD-type zinc(II) 2-methylimidazolate, namely MAF-4 or ZIF-8, is the first MOF with natural zeolite topology and crystal symmetry, and extensively studied for its special pore structure and high stability. Zhang says, "Experimentally, we used ethanol as solvent to produce MAF-4 nanocrystals in diameter of 20 nm, and the gelatinous substance obtained by centrifugation was dried in air at room temperature naturally, which is finally transformed to the colorless and transparent monoliths or MOOC-1, with 84% optical transmittance." He adds "If you dry the samples at high temperature and/or in vacuum, just like general processes in MOF syntheses, you can only obtain the MOF as common white powders."
X-ray diffraction analysis indicates that MOOC-1 is polycrystalline instead of single crystal or glass. The porosities inside MOF-4 and its assemblies allow the luminescent dye, sulforhodamine 640 (SRh), to be doped in MOOC-1 to form a luminescent optical ceramic SRh@MOOC-1, which produces amplified spontaneous emission (ASE) with a low energy threshold of 31 micro-Joule per square centimeter stimulated by a 532 nm laser. He highlights "This value is lower than previous reports of MOF-based ASE/lasing." "In addition, lowering the solvent evaporation rate is an effective method for fusing MOF nanocrystals into a dense and transparent crystal." Zhang adds.
Prof. Xiao-Ming Chen at Sun-Yat Sen University, the founder of MAF-4, tells us "This strategy extends the candidate scope of optical ceramics and paves a new way to develop MOF-based devices for optical, adsorption, separation and sensing applications."
This research was funded by the National Natural Science Foundation of China.
See the article: Jia-Wen Ye, Xuehong Zhou, Yu Wang, Rui-Kang Huang, Hao-Long Zhou, Xiao-Ning Cheng, Yuguang Ma and Jie-Peng Zhang. "Room-temperature sintered metal-organic framework nanocrystals: A new type of optical ceramics," Sci. China Mater. (2018) doi: 10.1007/s40843-017-9184-1.