Concentration-Dependent Seeding as a Strategy for Fabrication of Densely Packed Surface-Mounted Metal-Organic Frameworks (SURMOF) Layers

Abstract

The layer-by-layer (LbL) method is a well-established method for the growth of surface-attached metal-organic frameworks (SURMOFs). Various experimental parameters, such as surface functionalization or temperature, have been identified as essential in the past. In this study, inspired by these recent insights regarding the LbL SURMOF growth mechanism, the impact of reactant solutions concentration on LbL growth of the Cu₂ (F₄ bdc)₂ (dabco) SURMOF (F₄ bdc^2- =tetrafluorobenzene-1,4-dicarboxylate and dabco=1,4-diazabicyclo-[2.2.2]octane) in situ by using quartz-crystal microbalance and ex situ with a combination of spectroscopic, diffraction and microscopy techniques was investigated. It was found that number, size, and morphology of MOF crystallites are strongly influenced by the reagent concentration. By adjusting the interplay of nucleation and growth, we were able to produce densely packed, yet thin films, which are highly desired for a variety of SURMOF applications.

Keywords: Volmer-Weber growth; layer-by-layer (LbL); metal-organic frameworks; surface chemistry; surface-mounted metal-organic frameworks (SURMOFs).

Figure 1

QCM LbL Cu₂(F₄bdc)₂(dabco) deposition curves (20 cycles) at 1 mm Cu²⁺. High‐concentration seeding: c(pillar, linker)=3 mm in the first cycle and 0.1 mm in the following cycles.

Figure 2

SEM images of Cu₂(F₄bdc)₂(dabco) SURMOFs after 20 LbL deposition cycles at 1 mm Cu²⁺. Left: top views, right: tilted views. (a) All cycles with c(pillar, linker)=0.1 mm; (b) all cycles with c(pillar, linker)=3 mm; (c) high‐concentration seeding (first cycle c(pillar, linker)=3 mm, all other cycles c(pillar, linker)=0.1 mm). All scale bars=1 μm.

Figure 3

Infrared spectra (left) and X‐ray diffraction patterns (right) of Cu₂(F₄bdc)₂(dabco) SURMOFs after 20 LbL deposition cycles at 1 mm Cu²⁺, compared to bulk phase MOF data. High‐concentration seeding: c(pillar, linker)=3 mm in the first cycle and 0.1 mm in the following cycles.

Figure 4

Scheme of the mechanism of the concentration‐dependent SURMOF deposition. Left: At low concentrations, equilibria are established within a relatively wide diffusion layer (light blue background). This results in a low density of nuclei, but in very defined crystallites at later stages. Closed layers are only obtained when the size of the crystals exceeds their average distance. Middle: At high concentrations, nucleation is very efficient, but due to a very narrow diffusion zone the crystals become dendritic. Right: By combining the processes, dense layers of nuclei can be obtained, which become closely packed after only few deposition steps. The crystals are well defined due to the equilibrium formation, but growth is slow due to 2D transport. Other phenomena, such as Ostwald ripening, material storage, and the orientational disorder are omitted for clarity.