Oxygen‐Assisted Cathodic Deposition of Zeolitic Imidazolate Frameworks with Controlled Thickness

Abstract

Processing metal-organic frameworks (MOFs) as films with controllable thickness on asubstrate is increasingly crucial for many applications to realizef unction integration and performance optimization. Herein, we report af acile cathodic deposition process that enables the large-area preparation of uniform films of zeolitic imidazolate frameworks (ZIF-8, ZIF-71, and ZIF-67) with highly tunable thickness ranging from approximately 24 nm to hundreds of nanometers. Importantly,t his oxygen-reduction-triggered cathodic deposition does not lead to the plating of reduced metals (Zn and Co). It is also operable cost-effectively in the absence of supporting electrolyte and facilitates the construction of well-defined submicrometer-sized heterogeneous structures within ZIF films.Metal-organic frameworks (MOFs), ac lass of porous crystalline materials with large surface areas,r egular pore sizes,a nd tailorable surface chemistry, [1] have received considerable attention in av ariety of fields. [2][3][4][5][6][7][8] Form any applications,s uch as separations, [3] energy storage [5] and conversion, [6] chemical sensing, [7] and electronics, [8] it is often necessary to process MOF materials as films on as pecific substrate.A mong methods established for preparing MOF films, [9,10] strategies based on electrochemical synthesis are attractive for many advantages including technological flexibility,scalable preparation, and industrial production potential. [11] Nevertheless,t heir potential for preparing MOF films with precisely tunable morphology and thickness has not been fully explored. These variables,h owever,a re critically important for structure integration and performance optimization in (especially device) applications. [8,9] MOFs could be deposited on the electrode surface anodically [12] or cathodically. [13] In cathodic deposition, reduction of some special molecules or ions (e.g.n itrate ions) on electrode surface results in an increase in pH, which promotes the deprotonation of organic ligands and induces the formation of MOFs.Atfirst glance,cathodic deposition appears to be attractive for the direct synthesis of MOFs on conductive substrates.H owever,r eports on the cathodic deposition of MOFs are comparably few.The challenge arises from the more negative reduction potential of nitrate ions relative to those of some important metal ions,f or example, Zn II ,C o II ,a nd Cu II ,w hich constitute the most extensively studied MOFs. [13b,g] As ar esult, cathodic deposition of these MOF materials is commonly accompanied with plating of the corresponding metals, [13a,e] which is in most cases undesirable.Herein, we report the first cathodic deposition of zeolitic imidazolate framework materials (ZIFs), an important subgroup of MOFs that have been extensively studied owing to their outstanding thermal stability and chemical robustness. [14,15] Our strategy is based on an oxygen reduction triggered-electrochemical-chemical-reaction Scheme responsible for the formation of ZIF materials (Scheme 1). Electrochemical oxyge...

Recent advances on electrochemical methods in fabricating two‐dimensional organic‐ligand‐containing frameworks

“…Thus, the undercut‐induced detachment in anodic dissolution (vide supra) can be circumvented in cathodic ECD 40 . However, the cathodic ECD needs demanding conditions, otherwise the metal plating will take place simultaneously, which results in the impurity of the as‐synthesized MOFs 70,74,75 …”

“…To circumvent the metal plating problem over the course of the cathodic ECD, Lu et al proposed an oxygen‐assisted cathodic deposition strategy to synthesize a series of ZIFs (ZIF‐8, ZIF‐71, and ZIF‐67) with the advantages of highly tunable thickness and the non‐plating of metallic deposition (Figure 11A). 74 An air‐saturated precursor solution was prepared by dissolving chloride salts (ZnCl 2 or CoCl 2 ) and imidazole ligands in MeOH, and subsequently were purged with air for 30 min, in which the O 2 concentration was about 2 mmol/L. In the presence of supporting electrolyte TBA PF 6 , ZIF‐8 was successfully prepared on the freshly‐peeled highly oriented pyrolytic graphite (HOPG) cathode under the condition of −0.7 V (vs. Ag/AgCl [3 mol/L KCl]) for 2 h at room temperature.…”

Bulk and local structures of metal–organic frameworks unravelled by high-resolution electron microscopy

“…In order to obtain images that are clearer and easier tointerpret, Han's group used Cs-corrected TEM microscope and attempted to acquire images at appropriate defocuses in subsequent studies [30][31][32][33][34][35][36][37] . They observed different surfacedependent termination modes that co-exist in UiO-66: the main exposed {111} surface is terminated with BDC linkers, whereas the truncated surface, which comprises a number of small {100} and {111} facets, is terminated with Zr clusters at the {100}/{111} kinks (Fig.…”

“…The challenges include the rapid search for a crystal zone axis to minimize beam irradiation, the precise alignment of the image to fully retrieve the high-resolution information, and the accurate determination of the defocus value to process the image to be more interpretable by correcting the 'contrast inversion' caused by the contrast transfer function (CTF) of the objective lens. Combining the use of a DDEC camera with these methods, they successfully acquired atomicresolution TEM images of various bulk and local structures in different MOFs [30][31][32][33][34][35][36][37] . The technical details of these methods can be found in the original publication 30 and are not included here as they are beyond the scope of this article.…”

Electrolytic synthesis of porphyrinic Zr-metal–organic frameworks with selective crystal topologies

“…[1][2][3] Various MOFs have been synthesized using both anodic dissolution [4][5][6][7][8][9][10] and cathodic deposition approaches. 7,[11][12][13][14][15][16] In particular, Zr(IV)based MOFs have been widely investigated due to their high chemical and thermal stabilities. 17 Ameloot et al reported the electrochemical deposition of UiO-66 films, which are prototypal Zr-MOFs, onto the surfaces of anodes and cathodes.…”