Electrochemistry-induced deposition for controlled formation of metal–organic framework films on insulator and conductor substrates

Abstract

A number of technological applications of metal–organic frameworks (MOFs) require the formation of their thin films on insulator and/or conductor substrates at selected areas with desired thicknesses. However, fabrication of such MOF films often requires multi-step processes and/or sophisticated instruments. Herein, we discuss electrochemistry-induced MOF deposition, which permits the direct formation of a thin MOF film with controlled thickness at a desired area on various substrates. So far, we have reported the applicability of this deposition method for the formation of zeolitic imidazolate framework-8 (ZIF-8) films. In this method, a ZIF-8 film is formed on an insulator or a conductor substrate upon applying a cathodic potential to a working electrode that is placed above the substrate. Importantly, the film is formed just below the cathodic working electrode, indicating that the position and lateral dimensions (on the mm- to μm-scale) of the film can be controlled by those of the working electrode. In addition, film thickness is controllable in the range of tens to hundreds of nanometers by adjusting potential application conditions at the cathodic working electrode. These results show that the electrochemistry-induced deposition method will provide a simple means for the fabrication of a patterned MOF film on various substrates without additional lithographic processes.

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Introduction

Metal–organic frameworks (MOFs) are a popular class of crystalline coordination polymers that have garnered attention due to their wide range of potential applications.^1–3 These versatile materials are characterized by nanoscale pores with high effective surface areas leading to their highly ordered self-assembly of metal ions and organic linkers. Furthermore, their crystal structures and associated functions can be fine-tuned through careful selection of metal ions (or clusters) and ligands. The inherent structural properties of MOFs coupled with their tunability have led to the development of MOF materials for gas separation,⁴ catalysis,^5,6 energy storage,^7,8 and chemical sensing.^9,10

Some of these applications have necessitated the development of thin film fabrication methods in order to maximize the utility of the highly ordered nanoporous structure of MOFs.^11,12 Additionally, control of the thickness, area, and position of these films is crucial for use in devices^13,14 such as chemical sensors.^10,15 A variety of methods have been developed to fabricate patterned thin MOF films, including but not limited to MOF growth on prepatterned substrates,¹⁶ contact printing,¹⁷ photolithography,^18–20 inkjet printing,^21,22 and electrochemical deposition.^23,24 One of the earliest methods is the solution-phase formation of a MOF film on prepatterned self-assembled monolayers (SAMs) that are often fabricated via microcontact printing.¹⁶ In this method, a MOF film can be preferentially deposited on the surface of a SAM with a terminal functional group that can serve as an anchor for the metal sites of the MOF.^11,16,25 This method permits control of the crystal orientation and film thickness by using a suitable SAM²⁶ and by adjusting the deposition time,²⁵ respectively, but is relatively time-consuming because of the initial pattern fabrication and the subsequent slow crystal growth step. Contact printing involves the use of a patterned stamp coated with a precursor solution that is pressed onto a substrate surface and removed upon evaporation of the solvent.¹⁷ This stamp-based method produces MOF films of controlled shapes regardless of substrate terminal groups, but requires relatively long periods of time for crystal growth and affords patterns with limited resolution defined by the feature size and ink spreading. Photolithography stands out from the aforementioned methods as post-synthetic fabrication of a patterned MOF film by removing^18,19 or crosslinking²⁰ MOFs at selected area(s) via photoirradiation. While photolithographic methods offer high-resolution patterning independent of MOF formation, the fabrication process is a time-consuming, multi-step procedure. Inkjet printing yields a film of controlled thickness and shape by spraying a precursor ink from a nozzle onto a flat surface in a precise area.^21,22 While the implementation of affordable office printers makes this method attractive, inks characterized by larger particle sizes or high viscosities can easily clog the narrow nozzle required for high precision patterning and result in low-resolution patterns with undesired defects.²²

Electrochemical approaches are also examined as a means to form patterned thin MOF films on conductive substrates.^24,27,28 There are three well-established approaches in electrochemical formation of thin MOF films (Fig. 1a–c):²⁹ anodic deposition (Fig. 1a) is based on oxidative generation of precursor metal ions from the metal electrode,³⁰ whereas cathodic deposition (Fig. 1b) is controlled by cathodic base generation at the electrode that results in ligand deprotonation to enhance MOF formation.^31,32 Alternatively, electrophoretic deposition (Fig. 1c) utilizes a DC-electric field to attract charged MOF particles to a conductive substrate.²⁷ In these three approaches, the thickness of a MOF film can be controlled by altering the magnitude and duration of a potential applied to the electrode. Previously, electrochemical formation of patterned MOF films was successfully demonstrated by the use of lithographically prepatterned electrodes^27,28 or by using a microelectrode on a precursor-coated conductive substrate.²⁴ However, such electrochemical approaches are typically used to form MOF films on an electrically conductive substrate. In addition, these methods were not explored for depositing MOF film(s) at precise area(s) of a non-prepatterned insulator substrate.

Fig. 1 Schematic illustrations of the formation of a thin MOF film via electrochemical processes: (a) anodic deposition, (b) cathodic deposition, (c) electrophoretic deposition, and (d) electrochemistry-induced deposition for ZIF-8. Fig. 1d was adapted with permission from ref. 33. Copyright 2023 American Chemical Society.

In this paper, we discuss an alternative electrochemical approach to the formation of a thin MOF film, electrochemistry-induced deposition (Fig. 1d), which we have recently demonstrated for zeolitic imidazolate framework-8 (ZIF-8).^33,34 In this method, a ZIF-8 film is formed on the surface of a substrate placed underneath a cathodic working electrode: a base generated at the electrode leads to the deprotonation of 2-methylimidazole (Hmim) that enhances the formation of intermediate species composed of Zn²⁺, Hmim, and mim⁻. Diffusional accumulation of the intermediate species on the substrate and subsequent crystallization result in the deposition of the ZIF-8 film. Of note, this mechanism is similar to that reported for the deposition of a silica film on a glass substrate using a microdisk electrode.³⁵ In addition, there were two prior reports on direct MOF deposition on the surface of a non-conductive porous membrane that was induced by the transport of anodic or cathodic products generated at a working electrode across the membrane.^53,54 We will highlight the characteristics of the electrochemistry-induced deposition method and its challenges to be addressed in the future. We believe that the electrochemistry-induced deposition will provide a simple means to fabricate patterned MOF films for various applications, considering its capability to afford a thin MOF film with controlled size/shape and thickness at a desired position on various substrates.

Electrochemistry-induced deposition of ZIF-8 films

We have observed that a thin film can be deposited on the surface of a planar substrate upon applying a cathodic potential to a disk electrode placed above the surface in a methanol solution containing ZnCl₂ and Hmim (Fig. 1d).³³ In this method, we use a relatively simple three-electrode cell consisting of a working electrode, a Pt counter electrode, and a Ag/AgCl pseudo-reference electrode. The working electrode is attached to a micrometer for controlling the electrode–substrate distance (Δh) (Fig. 2).

Fig. 2 Experimental setup for the electrochemistry-induced deposition. Adapted with permission from ref. 33. Copyright 2023 American Chemical Society.

Fig. 3 shows the pictures of thin films that were deposited on (a) a glass substrate,³³ (b) a track-etched polyimide membrane (10 nm in pore diameter, 8 μm thick),³³ (c) a Au-coated Si substrate (Au/Si),³⁴ and (d) a Si substrate.³⁴ These pictures clarify the following characteristics of the electrochemistry-induced deposition method. First, this method can be used to deposit a thin film on various substrates regardless of their electrical conductivity, and even on a thin nanoporous polymer membrane. This is because the film deposition is induced by cathodic process(es) at the electrode, not at the underlying substrate. Second, the deposition area can be controlled by the size and position of a working electrode. The diameter of a deposited film is very similar to that of a disk electrode employed and can be as small as tens of microns using an ultramicroelectrode (Fig. 3d). These results show the possible wide-ranging utility of this electrochemistry-induced method for the fabrication of patterned thin films on various substrates.

Fig. 3 Pictures of thin films deposited on various substrates using different electrodes in a methanol solution of 10 mM ZnCl₂ and 20 mM Hmim at E = −1 V (N = 1): (a) on a glass substrate with a glassy carbon (GC) disk electrode (3 mm in diameter) for 45 min (Δh ≈ 150 μm),³³ (b) on a track-etched polyimide membrane (10 nm in pore diameter, 8 μm thick) with a Pt disk electrode (1.6 mm in diameter) for 15 min (Δh ≈ 100 μm),³³ (c) on a Au-coated Si substrate with a Pt disk electrode (6 mm in diameter) for 15 min (Δh ≈ 80 μm),³⁴ and (d) on a silicon substrate with a Au ultramicroelectrode (10 μm in diameter) for 15 min (Δh ≈ 10 μm).³⁴ A deposit has been outlined by a dashed circle. Adapted with permission from ref. 33 and 34. Copyright 2023 American Chemical Society and Copyright 2025 IOP Publishing Limited on behalf of the Electrochemical Society.

The deposited films were ZIF-8, as verified by the X-ray diffraction (XRD) pattern obtained from a film formed on a glass substrate, which showed five peaks corresponding to the (011), (002), (112), (013), and (222) planes of ZIF-8 (Fig. 4a).³³ In addition, XRD peaks from metallic zinc or zinc hydroxides/oxides were not observed.³³ Similar XRD data were obtained for thinner films formed on polyimide sheets using a transmission synchrotron XRD technique.³³ In addition, the distribution of Zn was consistent with the area of a film deposited on a track-etched polyimide membrane³³ and on a Si substrate (Fig. 4b and c).³⁴

Fig. 4 (a) XRD data of a film (496 nm in ellipsometric thickness) formed on a glass substrate using a Pt disk electrode (6 mm in diameter, Δh = 150 μm) upon five-cycle deposition (N = 5) for t = 30 min.³³ (b) SEM image of a microscale film deposited on a Si substrate using a Au ultramicroelectrode (10 μm in diameter) and (c) its EDX elemental mapping for Zn.³⁴ Scale bar in (b): 8 μm. Of note, data in Fig. 4b and c were obtained for a film shown in Fig. 3d. Adapted with permission from ref. 33 and 34. Copyright 2023 American Chemical Society and Copyright 2025 IOP Publishing Limited on behalf of the Electrochemical Society.

Selection of precursor solutions, electrode materials, and substrates for the electrochemistry-induced ZIF-8 deposition

As illustrated in Fig. 1d, the following three processes are involved in the electrochemistry-induced ZIF-8 deposition:³³ (1) cathodic generation of a base from O₂ that induces the deprotonation of Hmim to give mim⁻;³⁶ (2) formation of intermediate species composed of Zn²⁺, mim⁻, and Hmim^36–41 underneath the working electrode; (3) diffusional deposition of the intermediate species that leads to ZIF-8 film formation on the underlying substrate. This mechanism implies that the ZIF-8 deposition will be controlled by a number of factors, including the compositions of a precursor solution, electrode materials, the surface properties of underlying substrates, electrode–substrate distance (Δh), applied cathodic potential (E), potential application time (t), and the number of deposition cycles (N).^33,34 Here, we discuss the influences of precursor solutions, electrode materials, and underlying substrates on ZIF-8 film formation, which will provide a guideline for the selection of solutions and materials for the electrochemistry-induced deposition.

(a) Precursor solution. The composition of a precursor solution should give significant influences on the electrochemistry-induced ZIF-8 deposition, because the solution composition controls all three processes, especially the cathodic base generation in process (1) and the formation of intermediate species in process (2). We investigated the effects of Zn²⁺/Hmim concentration on the ZIF-8 deposition on glass using methanol solutions containing x mM ZnCl₂ and 2x mM Hmim, i.e., at different stoichiometric concentrations of Zn²⁺ and Hmim.³⁴ As shown in Fig. 5a, the thickness of a deposited film increased from x = 0.5 to x = 10, and then reached a plateau (50–60 nm) at the larger x. These observations suggest that the film deposition at x ≤ 10 and x ≥ 10 is controlled by the supply of Zn²⁺/Hmim and a base, respectively. In particular, the plateau at x ≥ 10 implies that a base primarily involved in the deposition would be O₂⁻ generated by the reduction of O₂,³⁶ rather than OH⁻ generated from H₂O, because the water concentration of the methanol solutions was higher than 10 mM (ca. 45 mM H₂O (ref. 34) vs. ca. 2 mM O₂ (ref. 42)). On the other hand, larger grains were obtained at the smaller x (Fig. 5b), which would be associated with the slower formation of the intermediate species and a film owing to the limited supply of Zn²⁺/Hmim. This observation shows a possibility of controlling the surface morphology of a film by changing the Zn²⁺/Hmim concentrations.

Fig. 5 (a) Effects of ZnCl₂/Hmim concentration (x) on the ellipsometric thickness of films deposited on glass using a Pt disk electrode (6 mm in diameter) at E = −1 V with Δh = 50–100 μm for t = 15 min (N = 1). Of note, films of similar thicknesses were formed in this Δh range, as shown in Fig. 8a. (b) AFM images of films obtained on glass at x = 0.5 and 10.³⁴ Adapted with permission from ref. 34. Copyright 2025 IOP Publishing Limited on behalf of the Electrochemical Society.

(b) Working electrodes. As shown in Fig. 3, Pt (Fig. 3b and c), Au (Fig. 3d), and glassy carbon (GC) electrodes (Fig. 3a) can be used to deposit ZIF-8 films on various substrates by applying an appropriate cathodic potential (E). The role of the working electrode is to cathodically generate a base (primarily O₂⁻, as discussed above) in process (1) that triggers the formation of intermediate species in process (2). Thus, the use of an electrode with higher electrochemical oxygen reduction reaction (ORR) activity should lead to more efficient ZIF-8 deposition on an underlying substrate. Among the three electrodes, the Pt electrode has the highest ORR activity, as shown by the observation of a cathodic current at the most positive potential in a methanol solution containing 10 mM ZnCl₂ and 20 mM Hmim (Fig. 6a),^33,34 which was anticipated from prior reports.^43,44 Indeed, a Pt electrode afforded a ZIF-8 film on glass at the more positive E (E < −0.2 V) than a GC electrode (E < −0.5 V) at the same Δh and t in the methanol solution (Fig. 6b), indicating that a Pt electrode is better suited for the electrochemistry-induced ZIF-8 deposition from the aspect of ORR activity.

Fig. 6 (a) Cyclic voltammograms (scan rate: 0.1 V s⁻¹) at a Pt disk electrode (2 mm in diameter),³⁴ a Au disk electrode (2 mm in diameter),³⁴ and a GC disk electrode (3 mm in diameter).³³ Voltammograms obtained for the first scan and after 11 scans are shown in solid and dashed lines, respectively. (b) Effects of E on the thickness of ZIF-8 films obtained using Pt disk electrodes (1.6 and 6 mm in diameter) and a GC disk electrodes (3 mm in diameter).³³ The films were prepared on glass at Δh ≈ 150 μm, and their thickness was measured using profilometry or spectroscopic ellipsometry. (c) Pictures of a GC disk electrode (3 mm in diameter) before and after being used for ZIF-8 deposition on glass at E = −1 V with Δh = 100 μm for t = 15 min.³³ (d) Surface profiles of thin films deposited on glass substrates using Pt and GC disk electrodes (1.6 mm and 3 mm in diameter, respectively) at E = −1 V with h = 150 μm for t = 45 min.³³ All the results shown in Fig. 6 were obtained in a methanol solution containing 10 mM ZnCl₂ and 20 mM Hmim. Adapted with permission from ref. 33 and 34. Copyright 2023 American Chemical Society and Copyright 2025 IOP Publishing Limited on behalf of the Electrochemical Society.

In addition, the electrochemistry-induced deposition could be hindered by the passivation of a working electrode (Fig. 1d). Indeed, a working electrode was coated with a film after its use for ZIF-8 deposition on glass (Fig. 6c),³³ which could result from conventional cathodic ZIF-8 electrodeposition (Fig. 1b).³⁶ The electrode passivation was also realized from a gradual decrease in faradaic current upon multiple potential sweeps in cyclic voltammetry measurements.³³ Importantly, a Pt electrode exhibited smaller cathodic current reduction than a GC electrode (Fig. 6a, dashed lines).³³ The less significant passivation enabled the deposition of a thicker ZIF-8 film on a glass substrate using a Pt electrode (ca. 100 nm) as compared with a GC electrode (ca. 50 nm) under identical potential application conditions (Fig. 6d).³³ These results indicate that a Pt electrode is optimal as a working electrode for the electrochemistry-induced deposition of ZIF-8 owing to the high ORR activity and the relatively insignificant electrode passivation.

(c) Substrate materials

Considering the mechanism shown in Fig. 1d, the electrochemistry-induced deposition should be controlled by the affinity of an underlying substrate to ZIF-8 (and the intermediate species) in process (3). It was also reported that the surface properties of a substrate give significant influences on the deposition kinetics of crystalline materials including MOFs.¹² We thus examined electrochemistry-induced ZIF-8 deposition on planar substrates with different surface properties that were represented by water contact angles ranging from 20° to 110°: Si, glass, and Au/Si substrates in addition to thiolate self-assembled monolayers (SAMs) on Au/Si.³⁴ Interestingly, we obtained ZIF-8 films of similar thicknesses on these substrates under similar potential application conditions in spite of the large difference in their surface properties (Fig. 7a), revealing the versatility of this method for ZIF-8 deposition on various surfaces. This observation could be associated with their similar surface free energies in the methanol solution, as implied from the similar surface free energies of various SAMs in ethanol.^45,46 On the other hand, ZIF-8 films formed on OH-terminated (and NH₂-terminated) SAMs (OH- and NH₂-SAMs, respectively) consisted of laterally larger grains than those on the other substrates, including a COOH-terminated SAM (COOH-SAM) with similar water contact angle (Fig. 7b). Currently, scientific reasons behind the effects of the surface functional groups on the grain size are unclear, but these results show a possibility of controlling the morphology of a ZIF-8 film based on the surface properties of an underlying substrate.

Fig. 7 (a) Thickness of ZIF-8 films formed on different substrates using a Pt electrode (6 mm in diameter) in a methanol solution containing 10 mM ZnCl₂ and 20 mM Hmim at E = −1 V with h = 50–150 μm for t = 15 min.³⁴ (b) AFM images of ZIF-8 films formed on an OH-SAM and a COOH-SAM.³⁴ Adapted with permission from ref. 34. Copyright 2025 IOP Publishing Limited on behalf of the Electrochemical Society.

Control of ZIF-8 film formation by potential application conditions

The mechanism (Fig. 1d)³³ suggests that the electrochemistry-induced ZIF-8 deposition will be influenced by the electrode–substrate distance (Δh) and potential application conditions such as applied cathodic potential (E), potential application time (t), and the number of deposition cycles (N).^33,34 Here, we discuss the effects of these parameters on ZIF-8 film formation, which will permit us to prepare ZIF-8 films with controlled thickness and surface morphology.

(a) Electrode–substrate distance (Δh). The deposition should depend on Δh because of the involvement of the diffusional deposition of the intermediate species on the underlying substrate in process (3) (Fig. 1d). This means that the deposition would be enhanced at the smaller Δh because the shorter diffusion distance will facilitate attaining supersaturation in proximity to the substrate that is required for the deposition of the intermediate species.³⁸ As anticipated, thicker films were formed with decreasing Δh at Δh ≤ 200 μm, and the deposition was negligible at Δh ≥ 250 μm (Fig. 8a).³³ On the other hand, deposition at a very small Δh (<10 μm) with a millimeter-scale disk electrode led to the formation of a ring pattern corresponding to the peripheral edge of the electrode, which would reflect the limited supply of Zn²⁺, Hmim, and O₂.³³ Fig. 8a also shows the greater influence of Δh on the film deposition with a smaller working electrode, as shown by the more consistent increase in film thickness at Δh ≤ 100 μm with the smaller, 1.6 mm diameter electrode. The influence of Δh was more significant when a 10 μm diameter electrode was used: a film could be deposited only at Δh < 20 μm, indicating that the shorter diffusion distance is required to attain the supersaturation of the intermediate species on the substrate surface owing to the generation of a small amount of O₂⁻ at the μm-scale electrode.³⁴ Importantly, Δh could affect the lateral size of a deposited film owing to the diffusion-induced broadening⁴⁷ of the distribution of intermediate species, as shown by the microscale ZIF-8 film (Fig. 3d) that had a slightly larger size (ca. 16 μm in diameter) than the ultramicroelectrode (10 μm in diameter).³⁴

Fig. 8 (a) Effects of Δh on the thickness of ZIF-8 films formed on glass using Pt electrodes (1.6 and 6 mm in diameter) at E = −1 V for t = 15 min (N = 1).³³ (b) AFM images of ZIF-8 films formed on glass at E = −0.8 and −0.5 V using a Pt disk electrode (1.6 mm in diameter) with Δh ≈ 150 μm for t = 15 min (N = 1).³³ (c) Effects of t (N = 1) on the thickness of ZIF-8 films formed on glass using platinum electrodes (1.6 and 6 mm in diameter) at E = −1 V with Δh = 150 μm.³³ Of note, both the electrodes afforded films of similar thickness. (d) Film thickness upon multiple deposition cycles on glass using a platinum electrode (6 mm in diameter) at E = −1 V with Δh = 150 μm for t = 30 min. All the films were formed in a methanol solution containing 10 mM ZnCl₂ and 20 mM Hmim, and the thickness of films prepared using 1.6 mm and 6 mm diameter electrodes was measured using profilometry or spectroscopic ellipsometry, respectively. Adapted with permission from ref. 33. Copyright 2023 American Chemical Society.

(b) Applied cathodic potential (E)³³. The cathodic generation of O₂⁻, and thus the formation of the intermediate species, can be controlled by E. Indeed, thicker films were obtained at more negative E in the range from −0.2 to −0.6 V for a Pt electrode and from −0.5 to −0.7 V for a GC electrode (Fig. 6b). In these E ranges, the film formation was regulated by cathodic O₂⁻ generation that was controlled by the ORR kinetics at the working electrode. At the more negative E, the film thickness was independent of E, indicating that the cathodic generation of O₂⁻ at the working electrode was controlled by the mass transport of O₂ rather than the electrode reaction kinetics. Importantly, E also affected the surface morphology of the film: films consisting of small grains at high density were formed at the plateau range of E (e.g., E = −0.8 V; Fig. 8b, left), whereas isolated larger grains were deposited at the more positive E (e.g., E = −0.5 V; Fig. 8b, right). The formation of the larger grains would be attributed to the slower growth kinetics of the intermediate species that were controlled by the concentration of cathodically generated O₂⁻. Of note, isolated grains as shown in Fig. 8b were not observed on films formed at E = −1 V with the lower Zn²⁺/Hmim concentrations (x ≤ 10) (Fig. 5b). The difference in film morphology implies that the lower O₂⁻ and Zn²⁺/Hmim concentrations afford intermediate species having different structural and/or chemical properties.

(c) Deposition time (t)³³. The above results suggest a possibility of forming a ZIF-8 film of different thicknesses by varying Δh and/or E. However, adjusting t is a more straightforward way to control film thickness. Fig. 8c shows the thickness of films obtained for different t using platinum disk electrodes (1.6 and 6 mm in diameter) at fixed Δh, E, and N. The film thickness increased from 10 to 100 nm for longer t up to 30 min and reached a plateau thereafter. The former indicates that ZIF-8 films of desired thickness in the range of 10–100 nm can be formed by adjusting t (≤30 min). On the other hand, the plateau observed at t ≥ 30 min in Fig. 8c shows the limitation of film thickness attainable by single deposition (N = 1), which would be attributed to the electrode passivation as discussed above (Fig. 1d and 6c).

(d) Number of deposition cycles (N)³³. As shown above, the maximum film thickness obtained using a Pt electrode by single deposition (N = 1) was ca. 100 nm. We could prepare thicker films (>100 nm) via multiple deposition cycles (N > 1; Fig. 8d) using an unpassivated working electrode, which can be obtained by soaking a working electrode in 0.1 M HCl after each deposition to remove the passivation layer. The film thickness increased proportionally to N, indicating that the multiple deposition cycles provide a means to fabricate ZIF-8 films of hundreds of nanometers and possibly multilayered films composed of different MOFs.

Conclusions

This paper discusses the characteristics of the electrochemistry-induced ZIF-8 deposition approach on insulator and conductor substrates. The deposition is initiated by the cathodic generation of a base (O₂⁻) at a working electrode placed above a substrate, followed by the formation of intermediate species that are immobilized via diffusion on the substrate to form a ZIF-8 film. This deposition mechanism is consistent with the experimental data, including the effects of the solution composition, electrode material, Δh, and E on the film thickness. Importantly, this method affords a thin ZIF-8 film on a substrate just under a cathodic working electrode, indicating that the position, shape, and lateral dimensions of a film can be controlled by those of a working electrode. In addition, this method provides a means to form ZIF-8 films of controlled thickness in the range of tens to hundreds of nanometers by adjusting the potential application conditions such as t and N. These characteristics indicate the applicability of this method for the direct fabrication of a micropatterned MOF film using a patterned electrode⁴⁸ or by scanning a cathodic microelectrode.^24,47,49 Furthermore, this method will be applicable for the patterned deposition of different MOF layers by changing a precursor solution for each of the deposition cycles.

However, there are a number of challenges that limit the utility of this method for such patterned MOF film fabrication. For example, it is currently challenging to form ZIF-8 films of uniform thickness as shown in Fig. 4b and 6d, which would reflect the concentration profiles of the intermediate species during the deposition that would be controlled by the supply of O₂⁻, Zn²⁺, and/or Hmim. The tilting of the surfaces of the working electrode and substrate could also affect the film uniformity. In addition, shorter deposition time (<1 min) is more desirable for scanning probe-based patterning.^47,49 The use of an appropriate probase^28,50 may provide a means to facilitate the electrochemistry-induced deposition. Furthermore, the lateral size of a MOF film may be improved by using a nanoscale electrode with better control of the electrode–substrate distance.⁵¹ Of note, we have reported the electrochemistry-induced deposition only for ZIF-8 so far,^33,34 and are currently examining the applicability of this method for other MOFs. We should point out that fundamental thermodynamic and kinetic research on the formation of intermediate species and crystallization will help overcome these challenges. For example, synchrotron X-ray techniques will permit the in situ investigation of electrochemical processes involved in the MOF deposition, which we have recently explored to quantitatively assess the kinetics of cathodic ZIF-8 electrodeposition on graphite electrodes.⁵² The resulting in-depth fundamental knowledge will enable the precise control of MOF film deposition on various substrates, and thus the fabrication of micropatterned, multilayered MOF films for various applications such as chemical separation, sensing, and catalysis.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

No primary research results, software or code have been included and no new data were generated or analyzed as part of this highlight article.

Acknowledgements

The authors thank their co-workers for their contributions to the work. This study is partly supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) (DE-SC0002362). The work done by TI at the Argonne National Laboratory was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. DOE under Contract No. DE-AC02-567 06CH11357 and DE-AC02-06CH11357. The National Science Foundation is also thanked for the summer support to EAS (CHE-2305013). This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by the Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Experiments at the Center for Nanoscale Materials, a U.S. DOE Office of Science User Facility, were supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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