MOF-74(M) Films Obtained through Vapor-Assisted Conversion—Impact on Crystal Orientation and Optical Properties
Patricia I. Scheurle, Andre Mähringer, Alexander Biewald, Achim Hartschuh, Thomas Bein, Dana D. Medina
Abstract
In recent years, metal–organic frameworks (MOFs) with the structure MOF-74 have attracted much interest owing to their tunable pore aperture, high surface area, and electrical conductivity. The synthesis of well-defined, highly crystalline thin films of MOF-74 is of paramount importance for their implementation into device-based applications such as in chemical sensing, optoelectronics, gas storage, and separations. Here, we present the synthesis of highly crystalline MOF-74 (M = Zn2+, Mg2+, Ni2+, and Co2+) films by vapor-assisted conversion. MOF-74(M) thin films were grown on bare glass, quartz, gold, and silicon surfaces, showing high crystallinity, crystal orientation, and average thicknesses of 500 nm. By including a benzoic acid modulator, oriented MOF-74(Zn) films, with the crystallographic c-axis of the MOF crystallites oriented horizontally to the surface, were obtained on all substrates. In addition, highly crystalline MOF-74(Mg) was grown on glass and gold substrates with the crystallographic c-axis aligned orthogonally to the surface. Moreover, randomly oriented highly crystalline MOF-74(Co) and MOF-74(Ni) films were synthesized on glass, quartz, gold, and silicon. The pore accessibility of the obtained films was examined by means of krypton sorption measurements, revealing permanent and accessible porosity, reaching a BET surface area of 975 cm2/cm2 for MOF-74(Mg). Steady-state and time-resolved photoluminescence studies show emission in the blue spectral region of MOF-74(Zn and Mg) on quartz with a biexponential decay. In addition, confocal photoluminescence mapping confirmed a homogeneous MOF film surface with a similar emission profile over the whole examined area of 70 μm × 70 μm.