Fabrication of a highly b-oriented MFI-type zeolite film-modified electrode with molecular sieving properties by Langmuir-Blodgett method

A novel highly compact and b-oriented MFI-type zeolite monolayer was obtained by assembling sec-butanol-modified rounded-coffin-shaped silicalite-1 microcrystals by Langmuir-Blodgett (LB) method. The LB monolayer was subsequently tested as a seed layer for the secondary growth of uniformly b-oriented and defect-free MFI zeolite films on stainless steel substrates and platinum electrodes. Three different methods were evaluated for identifying a suitable synthesis route to suppress the undesired a-oriented twin growth during the synthesis of MFI films. The crystal intergrowth and orientation of the MFI films were characterized by scanning electron microscopy and X-ray diffraction (XRD). The crystallographic preferred orientation indices and preferred orientation coverage values calculated using the XRD data were used to quantitatively determine the preferred orientation and surface coverage of the prepared zeolite films. The experimental results confirm that the low tetrapropylammonium hydroxide (TPAOH) method is the most effective route to suppress the undesired a-oriented twin growth, when the highly b-oriented LB monolayer was used as the seed layer for the secondary growth of the b-oriented MFI film. The highly b-oriented MFI-type zeolite film-modified electrodes were further tested as the working electrodes in electrochemical experiments. Cyclic voltammetry experimental results confirmed that the b-oriented MFI films fabricated at low TPAOH concentration were defect free and possessed a distinct molecular sieving property after calcination. The LB method, a highly reproducible and controllable method for the organization of zeolite crystals, can be further applied as an effective platform for the promotion and development of zeolite-modified electrodes as selective sensors.