In situ growing of ZIF-8 crystals into TiO2 micro columnar films

This study proposes a fast and simple method for the in situ growth of metal-organic frameworks (MOFs) on metal oxide substrates as an alternative to the traditional approaches of using gold substrates and self-assembled monolayers (SAMs). As a case study, zeolitic imidazolate framework 8 (ZIF-8) crystals were grown in micro columnar TiO 2 films through simple alternate and successive immersions of the TiO 2 films into solutions containing the MOFs precursors. The growth process of the MOF crystals in the interstitial spaces between the TiO 2 columns was investigated by varying the metal-to-ligand ratio (1:2, 1:4, and 1:8) and by employing modulating agents such as triethylamine. It was found that the optimal deposition of ZIF-8 occurred when using a higher excess of ligand and the addition of triethylamine after a controlled number of immersion cycles. These results were obtained by using glancing angle X-ray diffraction (GAXRD) and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS) as characterization techniques. Additionally, a density functional theory (DFT) study as well as Fourier-transform infrared spectroscopy (FTIR) and GAXRD experiments were conducted to elucidate the nucleation process. It was concluded that the starting point is the formation of a covalent bond between the Zn cations and the TiO 2 on the metal oxide surface after immersion of the film into a Zinc (II) nitrate solution, allowing for the formation of MOF nuclei once the film is subsequently immersed in the 2-methylimidazole solution. The results demonstrate the feasibility of in situ growth of MOF crystals onto metal oxide structures by a layer-by-layer strategy, offering a promising alternative to conventional methods. • ZIF-8 grows spontaneously into microcolumnar TiO 2 films by simple alternate immersion in the solutions of the MOF precursors. • Zn(II) ions covalently bind the TiO 2 surface, acting as nucleation sites for the MOF growth. • An excess of 2-mIm with respect to the zinc ions and the presence of triethylamine effectively increase the nucleation rate. • The interstitial spaces between the TiO 2 columns are fully filled with ZIF-8 crystals after a few number of short cycles. • Composite ZIF-8@TiO 2 films can be interesting materials for catalytic applications among others.