Nucleation and growth of oriented metal-organic framework thin films on thermal SiO2 surface

Assembly of metal-organic framework (MOF) thin-films with well-ordered growth directions enables many practical applications and is likely part of the future of functional nanomaterials. Insights into the formation pathway of the MOF thin films would allow better control over the growth directions and possibly the amount of guest molecules absorbed into the MOF pores. Here, we investigate the nucleation and growth of oriented Cu3(BTC)2∙xH2O MOF (HKUST-1, BTC = benzene-1,3,5-tricarboxylic acid) thin films on the thermal SiO2 surface using a room temperature stepwise layer-by-layer (LBL) method. Initial stages of LBL growth were characterized with X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy analysis in order to understand nucleation and growth kinetics. HKUST-1 thin films with preferred growth along the [111] direction on the thermal SiO2 surface were obtained in the absence of not only a gold substrate, but also organic-based self-assembled monolayers (SAMs). It is found that the formation of HKUST-1 is initiated by deposition of copper acetate on the thermal SiO2 surface followed by ligand exchange between coordinated acetate from the copper precursor and the BTC ligands. As the LBL growth cycle is increased, HKUST-1 crystals on the thermal SiO2 surfaces are continuously forming and growing and finally the crystallites coalesce into a continuous film. Highly oriented HKUST-1 thin films on thermal SiO2 surface with complete surface coverage and ~90 nm thickness were obtained at ~80 cycles of LBL growth under the conditions used in this study.