Nucleation and Early Stages of Layer-by-Layer Growth of Metal Organic Frameworks on Surfaces

Abstract

High resolution atomic force microscopy (AFM) is used to resolve the evolution of crystallites of a metal organic framework (HKUST-1) grown on Au(111) using a liquid-phase layer-by-layer methodology. The nucleation and faceting of individual crystallites is followed by repeatedly imaging the same submicron region after each cycle of growth and we find that the growing surface is terminated by {111} facets leading to the formation of pyramidal nanostructures for [100] oriented crystallites, and triangular [111] islands with typical lateral dimensions of tens of nanometres. AFM images reveal that crystallites can grow by 5–10 layers in each cycle. The growth rate depends on crystallographic orientation and the morphology of the gold substrate, and we demonstrate that under these conditions the growth is nanocrystalline with a morphology determined by the minimum energy surface.

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

“…The details of SURMOF formation mechanisms by using the LbL method are still under discussion, but a couple of important parameters for controlling and directing SURMOF growth, such as surface functionality, temperature, surface energy, or surface defects have been identified and discussed in the literature. Early publications describe the LbL SURMOF growth as a Frank van der Merve process, but it is generally accepted today that the LbL growth is in fact a Volmer–Weber process resulting in surfaces covered by solitary crystals . Although a careful optimization of parameters permits the growth of highly oriented material, the Volmer–Weber mechanism inherently prohibits the growth of thin and closed layers.…”

“…For these reasons, in the present study, we address parameters that allow formation of thin but closely packed SURMOF layers being produced by using the LbL technique in a reasonable time and with a minimized material usage. Based on recent studies of the LbL SURMOF growth mechanism, which suggest that reagent concentration might play a role in the deposition process, we wanted to systematically vary this parameter. We chose the established pillared‐layer MOF Cu 2 (F 4 bdc) 2 (dabco) (linker: F 4 bdc 2− =tetrafluorobenzene‐1,4‐dicarboxylate, pillar: dabco=1,4‐diazabicyclo‐[2.2.2]octane), because this system sensitively reacts to changes of LbL parameters .…”

“…Moreover, the infrared spectrum of the F 4 bdc 2− linker exhibits shifts in the carboxylate bands that allow for a more accurate distinction of the involved species in the spectroscopic data . Following the established protocols, we carried out LbL experiments on gold‐covered quartz crystal microbalance (QCM) substrates that were functionalized with pyridyl‐terminated self‐assembled monolayers . To understand the processes in the earlier stages of SURMOF growth (before meaningful features are overgrown), we limited the number of deposition cycles to 20.…”

“…On the one hand, higher reactant concentrations clearly prohibit a SURMOF buildup following the liquid phase epitaxy model. On the other hand, even at the lowest concentrations at which LbL can be executed, there is lots of evidence that all reactants are present in the solution at the same time and that MOF growth actually is a (pseudo‐)equilibrium crystallization process out of this solution rather than liquid‐phase epitaxy (see Figure ). In line with this model, prolonging the deposition time does not increase the amount of deposited material (see the results of an according experiment in Figures S45–S48 in the Supporting Information).…”

Current Trend in Synthesis, Post‐Synthetic Modifications and Biological Applications of Nanometal‐Organic Frameworks (NMOFs)

“…It has a few advantages with regards to the fabrication of MOF films such as needing no pre-treatment of substrates and solvents during crystallization, good compatibility between substrate and MOF layers, possibility of reusing MOF precursors, relative ease of manipulating the position of MOF layer, and time-effective processes. Li et al [68] prepared ultrathin MOF by depositing a Zn based gel on ammoniated polyvinylidene (PVDE) hollow fibres. Heat treatment of the gel coated fibres transformed the gel to intergrown nanosized crystals.…”