A strategy to obtain highly luminescent MOF-76 based on hydrothermal annealing treatment
The limited luminescence efficiency and poor stability of lanthanide-based metal-organic frameworks (Ln-MOFs) are the key factors restricting their practical applications. It is well known that high-temperature annealing can significantly enhance the photoelectric properties of inorganic materials, but it seems unfeasible for Ln-MOFs due to the existence of organic components. To overcome this dilemma, MOF-76 was synthesized by hydrothermal method and then hydrothermally annealed at 140 °C for 4 h. As a result, the annealing not only significantly improves the crystallinity of MOFs but also promotes their transition from the original tetragonal phase to the monoclinic phase. The annealed MOFs ( a MOF) exhibit about 3-fold emission enhancement and show better stability, especially under alkaline conditions or in water or saturated alkanes, such as cyclohexane, and dichloromethane. The total emission intensity of the as-prepared (AP) MOF is reduced to less than 1/10 compared to the a MOF due to the influence of water molecules. When the material was immersed in a mixture of ammonia and water, the a MOF emission remained stable, while the AP MOF emission rapidly decreased to approximately 12 % with an increasing volume ratio. Additionally, a notably enhanced 5 D 0 → 7 F 1 transition of Eu 3+ ions in MOF-76 has been observed for the first time. It was found that after hydrothermal annealing, the photoluminescence quantum yield of the MOF increases from 10.86 % to 61.48 %, and its synthesis yield can reach 73.8 %. The properties described above remarkably enhance the application values of the a MOFs. Interestingly, MOFs can also be solvothermal annealed, thus the schemes presented in this context open up novel approaches to significantly improve/modify the performance of other MOF materials. Finally, the obtained green and red emitting a MOFs combined with the N-CDs@starch were applied to warm white LED with the colour coordinates (CIE) of (0.3662, 0.3544), colour rendering index (CRI) of 88.5 and correlated colour temperature (CCT) of 4261 K.
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