Control of Water Content for Enhancing the Quality of Copper Paddle-Wheel-Based Metal Organic Framework Thin Films Grown by Layer-by-Layer Liquid-Phase Epitaxy
Authors: Wang, Zheng; Rodewald, Katia; Medishetty, Raghavender; Rieger, Bernhard; Fischer, Roland A.
Metal organic framework (MOF) thin films with high crystallinity, preferred orientation, homogeneous texture, and enhanced porous properties are desired to satisfy many practical applications. Surface mounted MOF crystallite thin films (SURMOFs), prepared via a stepwise liquid-phase epitaxy technique, have attracted great interest. Although many efforts have been devoted to this field, it still remains a significant challenge to prepare SURMOFs with high quality reproducibly in an efficient way. In the synthesis of MOF bulk materials, coordination modulators (normally are acids or bases) have been successfully used to improve the properties of obtained materials. However, non-neutral additives are too reactive for Cu paddle-wheel-based SURMOFs to survive. Introducing water as a mild additive to the growth process provides an efficient strategy for the growth of SURMOFs. Herein, the growth of three kinds of Cu paddle-wheel-based SURMOFs (Cu(3)btc(2), Cubdc, Cu(2)bdc(2)dabco) is systematically investigated by varying the water content in the linker solution during the stepwise liquid phase epitaxial growth process (btc = 1,3,5-benzentricarboxylate; bdc = 1,4-benzendedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane). Grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), methanol sorption behavior, and infrared reflection absorption spectroscopy (IRRAS) are employed to characterize the quality of as-deposited SURIVIOFs. The strategy of integrating water in the linker solution could provide a potentially versatile route for the fabrication of a wide range of MOF thin films with enhanced characteristics. The addition of 5% water turned out to yield dense, highly crystalline, oriented, and porous Cu(3)btc(2) SURMOFs, whereas water contents above 30% yielded less dense films with lower surface coverage. Above 70% water content SURMOF growth was no longer possible. Similar enhanced quality is found for Cubdc SURMOFs; however growth of Cu(2)bdc(2)dabco is not possible in the presence of water, and rather Cubdc SURMOFs were obtained.
