A strategy to obtain highly luminescent MOF-76 based on hydrothermal annealing treatment

Abstract

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 (aMOF) 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 aMOF due to the influence of water molecules. When the material was immersed in a mixture of ammonia and water, the aMOF emission remained stable, while the AP MOF emission rapidly decreased to approximately 12 % with an increasing volume ratio. Additionally, a notably enhanced ⁵D₀→⁷F₁ transition of Eu³⁺ 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 aMOFs. 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 aMOFs 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.