Metal–Organic Frameworks (MOFs) have become one of the most exciting families of materials in modern chemistry and materials science. Thanks to their high porosity, huge surface area, and tunable chemistry, they are being explored for gas storage, separation, catalysis, sensing, energy storage, and many other technologies.

Within this large MOF family, there is a special subgroup called Zeolitic Imidazolate Frameworks (ZIFs). These combine the structural robustness of zeolites with the chemical versatility of MOFs. One of the most studied ZIFs is ZIF-67, made from cobalt ions (Co²⁺) and the organic linker 2-methylimidazole. It has a stable, porous “sodalite-type” framework and has already been tested in catalysis, adsorption, CO₂ capture, membranes, and sensors.

Most of the time, ZIF-67 is synthesized as a powder. Powders are great for fundamental studies but not ideal when you want to build devices. For applications like membranes, sensors, coatings, or thin-film electronics, you don’t want loose particles – you want a continuous thin film grown directly on a solid surface.

This is where the paper you shared comes in. It focuses on a simple way to grow ZIF-67 thin films directly on cobalt discs just by immersing the discs in an aqueous solution of 2-methylimidazole under alkaline conditions. The study also digs deeply into which cobalt surface species are responsible for nucleating ZIF-67, and how pH and electrochemical potential control film formation.

In this blog, we’ll walk through the main ideas of the work in a clear, step-by-step way:

• What ZIF-67 is and why thin films matter

• How the authors prepared cobalt discs and the 2-methylimidazole solution

• How pH affects whether ZIF-67 films form or not

• What happens when you change immersion time

• How spectroscopic and electrochemical techniques reveal the cobalt species involved

• The final conclusions about the mechanism and why this method is promising for practical applications

1. Why ZIF-67 Thin Films Are Interesting

ZIF-67 is built from cobalt ions (Co²⁺) and 2-methylimidazole linkers. Its structure is:

• Cubic symmetry

• Sodalite-like framework (a type of zeolite-like porous structure)

• Unit cell around 16.96 Å on each edge

Because of this architecture, ZIF-67 shows:

• High surface area

• High porosity

• Chemical and thermal stability

As a result, ZIF-67 has been studied for:

• Catalysis

• Gas separation and storage

• CO₂ capture

• Water purification and adsorption

• Membranes and sensors

However, most syntheses produce polycrystalline micro-powders. Powders are sometimes hard to process: they must be shaped, bound with polymers, or deposited in extra steps, which complicates real devices.

Thin films and coatings solve many of these issues:

• They reduce material consumption.

• They can be deposited directly onto device substrates.

• They open the door to membranes, sensing surfaces, functional coatings, etc.

So the goal of the study is very clear:

Develop a simple, controllable method to synthesize ZIF-67 thin films directly on cobalt discs, and understand which cobalt species and conditions make this possible.

2. The General Strategy: In-Situ Growth on Cobalt

The approach used in this work is based on in-situ synthesis:

• The substrate (a cobalt disc) is immersed in a solution containing the organic linker (2-methylimidazole).

• The metal for the MOF (Co²⁺) is not added separately as a salt – it comes directly from the cobalt metal disc itself, which partially oxidizes in solution.

• If conditions are suitable, cobalt at the surface converts into Co²⁺ and then reacts with 2-methylimidazole to form ZIF-67 on the surface.

This strategy is attractive because:

• The metal disc acts as both support and metal source.

• No extra cobalt salt is necessary.

• It is simple and low-cost.

The authors had previously shown that reduced cobalt particles can form MOFs like ZIF-67 or MOF-71 when immersed in appropriate linker solutions. In this study, they extend that idea to flat cobalt discs and focus on understanding:

• Which pH values are suitable

• Which oxidation states and oxides of cobalt are involved

• How time and electrochemical potential change the film morphology

3. Preparing the Cobalt Discs and the Linker Solution

The experimental setup starts with:

• Cobalt discs:

  • Diameter: 10 mm

  • Thickness: 0.25 mm

  • Purity: 99.8% cobalt (with traces of Fe and Ni)

These discs are:

1. Sanded and polished using silicon carbide abrasive foils (P800 and P1200 grit) to clean and smooth the surface.

2. Used either:

   • Directly for immersion synthesis, or

   • As working electrodes in electrochemical measurements.

The linker solution is:

• Aqueous solution of 0.5 M 2-methylimidazole

• The pH is carefully adjusted using 0.01 M NaOH

• pH values tested include 5, 8, 11, and 12

The key synthesis step is very simple:

Immerse the polished cobalt disc in the 2-methylimidazole solution at controlled pH for a fixed time (e.g., 30–120 minutes), then wash with acetone to remove unreacted linker.

This setting closely mimics industrially interesting “dip-coating” or immersion processes.

4. How pH Controls ZIF-67 Film Formation

One of the central findings of the study is that pH plays a decisive role in whether a ZIF-67 film forms on the cobalt disc.

4.1. Low and Neutral pH (pH 5 and 8): No Film

When the cobalt discs are immersed at:

• pH 5, or

• pH 8,

for 1 hour in 0.5 M 2-methylimidazole solution, no ZIF-67 crystals are observed on the surface.

The disc surface looks essentially the same as the untreated polished metal:

• Flat

• No visible microcrystals

Why?

• At acidic or near-neutral pH, cobalt metal is less easily oxidized to Co²⁺ or Co(OH)₂.

• Also, the pKa of 2-methylimidazole is about 7.86. At pH below or near this, the linker is not significantly deprotonated, which makes it less reactive toward forming the ZIF framework.

• So you have too little reactive Co²⁺ and insufficiently deprotonated linker. The conditions are simply not right to nucleate and grow ZIF-67.

4.2. High pH (pH 11 and 12): ZIF-67 Films Appear

At pH 11 and 12, the situation changes dramatically.

When discs are immersed for 1 hour at:

• pH 11:

  • A homogeneous, dense layer of nanosized ZIF-67 crystals forms.

  • There are also some larger cubic crystals emerging on top.

  • These cubic crystals can be seen as early stages of crystal growth – they are often considered the kinetic product in ZIF-67 synthesis.

• pH 12:

  • Again, a dense crystalline layer forms, but the cubic crystals are larger, with more intergrowth.

  • The film is more strongly developed.

So, highly alkaline conditions are essential. This aligns with cobalt’s Pourbaix diagram in water:

• At pH 8–12, cobalt metal can be oxidized to cobalt hydroxide (Co(OH)₂) and related species.

• At high pH, 2-methylimidazole is deprotonated, becoming a better ligand, ready to coordinate Co²⁺ and build ZIF-67.

In other words:

At pH 11–12, cobalt on the disc surface oxidizes to Co(OH)₂, the linker is deprotonated and reactive, and ZIF-67 crystals can nucleate and grow as a continuous coating.

X-ray diffraction is used to confirm that the crystals grown on the cobalt at these conditions correspond to ZIF-67, although the MOF peaks are relatively weak compared to the strong cobalt substrate.

5. How Immersion Time Changes the Film Morphology

The authors also study how the time of immersion at pH 12 affects the morphology of the coating.

At pH 12, they examine discs after different immersion times:

• 0 minutes (reference) – only the polished cobalt surface.

• 30 minutes – early-stage structures.

• 60 minutes and 120 minutes – more developed films.

5.1. After 30 Minutes

After half an hour at pH 12:

• Two types of structures are seen:

  • Flake-like features

  • Cubic crystals

The flakes are attributed to cobalt hydroxide (Co(OH)₂). At this pH, cobalt hydroxide forms on the surface and crystallizes in layered, flake-like morphologies. This hydroxide is not soluble, so it stays on the disc.

Over time, this Co(OH)₂ reacts with 2-methylimidazole in a kind of acid–base reaction to form ZIF-67 at the surface.

5.2. Growth Mechanism with Time

The proposed mechanism is:

1. Co metal on the disc oxidizes in alkaline solution to Co(OH)₂.

2. Co(OH)₂ acts as a source of Co²⁺ and reacts with deprotonated 2-methylimidazole.

3. This reaction leads to the nucleation and growth of ZIF-67 crystals.

4. Initially, small cubic crystals form (kinetic shapes).

5. With longer times, these cubes can grow and evolve into truncated rhombic dodecahedra and finally rhombic dodecahedral crystals, which are closer to the thermodynamic shape.

So, by tuning time at pH 12, one can go from:

• A surface dominated by Co(OH)₂ flakes plus small cubes →

• To a dense ZIF-67 layer with well-defined crystal morphologies.

This gradual evolution is important for controlling film texture, roughness, and crystal size, which can impact performance in membranes, catalysts, or sensing devices.

6. Identifying Cobalt Species: XPS and Raman Insights

To understand which cobalt species are present on the disc surface, and which are responsible for forming ZIF-67, the authors use two powerful characterization methods:

• X-ray Photoelectron Spectroscopy (XPS)

• Raman Spectroscopy

6.1. XPS: Oxidation States and Surface Chemistry

XPS is used to monitor the Co 2p and O 1s signals for:

• A dry disc (after polishing, no solution), and

• A wet disc (after immersion in NaOH solution at pH 12).

Key observations:

• The dry disc shows:

  • A strong contribution from Co²⁺ on the surface (oxidized layer),

  • And a peak associated with metallic Co⁰, indicating that the oxide layer is thin enough to let photoelectrons from the metal pass through.

• After immersion in basic solution (pH 12):

  • Metallic cobalt peaks disappear.

  • The surface is dominated by Co²⁺ species, consistent with Co(OH)₂ or related hydroxides/oxyhydroxides.

The O 1s peak can be decomposed into contributions from:

• Adsorbed water (around ~532 eV), and

• Hydroxyl oxygen (around ~530.8 eV), consistent with Co(OH)₂ or CoOOH.

This confirms that the alkaline treatment converts the cobalt surface into hydroxide-rich layers, which are crucial for ZIF-67 nucleation.

6.2. Raman: Distinguishing CoO, Co(OH)₂, CoOOH, Co₃O₄

Raman spectroscopy helps distinguish between different cobalt oxides and hydroxides:

• The untreated disc shows no significant peaks in the probed range, consistent with mostly metallic cobalt (which is Raman-silent here).

• After immersion in basic solution:

  • A broad band between about 450–700 cm⁻¹ appears.

  • This band can be associated with Co²⁺ species like CoO or Co(OH)₂ – they are hard to distinguish solely by Raman because both have cobalt octahedrally coordinated to oxygen.

Electrochemical and XPS data are combined with Raman to conclude:

• After basic treatment, the surface contains amorphous Co(OH)₂ nanosheets and some CoO/CoOOH.

• Different electrochemical treatments (discussed below) can shift the balance toward CoOOH or Co₃O₄.

7. Electrochemical Studies: Which Cobalt Oxide Really Produces ZIF-67?

To go deeper into which oxidized cobalt species actually produces ZIF-67, the authors perform electrochemical experiments at pH 12 using the cobalt disc as an electrode.

7.1. Cyclic Voltammetry (CV)

In cyclic voltammetry:

• The potential is swept across a range while measuring the current.

• At pH 12, several oxidation and reduction peaks appear.

Key features:

• A shoulder around –0.6 V (vs Ag/AgCl) is assigned to Co⁰ ⇌ Co²⁺ transition.

• Around –0.4 V and +0.5 V, further oxidation occurs, forming species like Co(OH)₂ and CoOOH.

• Above about +0.7 V, a large, less defined oxidation wave indicates strong oxidation processes and can be associated with formation of Co₃O₄ and higher oxides.

• In reverse scans, some reduction peaks appear, but certain high-potential oxidations are irreversible, consistent with stable passive layers.

From these observations and known reactions, the authors propose a set of electrochemical reactions:

• Formation of hydrated Co species and Co(OH)₂

• Oxidation of Co(OH)₂ to CoOOH

• Partial formation of Co₃O₄ at more positive potentials

Repeated cycles show diminishing current, suggesting growth of a passivating layer, mainly Co(OH)₂, which resists reduction at these conditions.

7.2. Chronoamperometry: Fixed Potential Oxidation

To create defined cobalt oxides, the authors hold the electrode at fixed potentials for 30 minutes:

• +0.5 V vs Ag/AgCl

• +1.0 V vs Ag/AgCl

• +2.0 V vs Ag/AgCl

This allows formation of different surface mixtures of Co(OH)₂, CoOOH, and Co₃O₄.

• At +0.5 V:

  • Current quickly drops, indicating formation of a passivating layer, mainly CoOOH.

  • CoOOH is inferred from XPS and the typical stability of Co³⁺ species at this potential.

• At +1.0 V:

  • Both Co(OH)₂ and Co₃O₄ are present.

  • There is still a significant amount of Co(OH)₂ nanosheets.

• At +2.0 V:

  • The surface becomes dominated by Co₃O₄ nanoparticles.

  • Co(OH)₂ is largely consumed.

Raman and XPS confirm these conclusions:

• At higher potentials, bands associated with Co₃O₄ become more pronounced.

• O 1s signals shift to reflect more oxide-type oxygen.

7.3. Which of These Actually Forms ZIF-67?

After preparing these different oxide surfaces electrochemically, the authors then introduce 2-methylimidazole and check whether ZIF-67 crystals form.

Results:

• Surface dominated by CoOOH (around +0.5 V):

  • Almost no ZIF-67 crystals are observed.

  • CoOOH is not a good precursor for ZIF-67 formation under these conditions.

  • Also, Co in CoOOH is in oxidation state +3, whereas ZIF-67 is built from Co²⁺, so this mismatch makes sense chemically.

• Surface with a large amount of Co(OH)₂ and some Co₃O₄ (+1.0 V):

  • A dense layer of ZIF-67 crystals forms, mostly cubic and about 300 nm in size.

  • This is similar to the films obtained in the pure immersion experiments at high pH with no external potential.

  • Here, the remaining Co(OH)₂ acts as the main reactive precursor that supplies Co²⁺ to the linker.

• Surface dominated by Co₃O₄ (+2.0 V):

  • Much fewer nucleation sites.

  • ZIF-67 crystals that do form are usually larger, rhombic dodecahedra growing on top of a Co₃O₄ layer.

  • Because Co(OH)₂ is scarce, nucleation is slow, and crystals grow into more thermodynamic shapes.

From this, a clear conclusion emerges:

The cobalt species that actually reacts with 2-methylimidazole to form ZIF-67 is Co(OH)₂, not CoOOH or Co₃O₄.

The amount and distribution of Co(OH)₂ on the surface determines:

• How many nucleation sites are available

• How fast ZIF-67 crystals grow

• Whether the crystals are predominantly small cubes or larger rhombic dodecahedra

8. Overall Mechanism and Practical Significance

Putting all of the results together, we can summarize the mechanism like this:

1. Cobalt disc in alkaline solution (pH 12)

   • Surface cobalt is oxidized to Co(OH)₂, forming nanosheets or flakes.

2. Presence of 2-methylimidazole at high pH

   • The linker is deprotonated, becomes a strong ligand, and coordinates Co²⁺ species from Co(OH)₂.

   • This triggers in-situ growth of ZIF-67 directly on the disc surface.

3. Role of electrochemical potential and time

   • If the surface is over-oxidized to CoOOH or Co₃O₄, ZIF-67 growth is suppressed or limited.

   • If enough Co(OH)₂ remains, you get a dense film with many nucleation sites.

   • Longer times and particular conditions lead from cubic ZIF-67 crystals to rhombic dodecahedral shapes.

4. Film control

   • By adjusting pH, immersion time, and potential, you can tune:

     • Crystal size

     • Film thickness

     • Crystal shape

     • Surface coverage

From an application perspective, this is an important result:

• The method is simple: just polish the cobalt disc, immerse in an aqueous 2-methylimidazole solution at pH 12, and wait.

• There is no need for organic solvents like DMF or methanol; water is enough.

• No separate cobalt salt is added – the disc is both substrate and Co source.

• The coating process is compatible with complex shapes and surfaces, at least in principle, because it’s based on immersion.

Such ZIF-67 coatings are highly promising for:

• Gas separation membranes

• Protective or functional coatings

• Sensors for volatile organic compounds or gases

• Catalytic surfaces

• Water purification and desalination membranes

This work also helps clarify fundamental chemistry:

• It shows that simply “oxidizing cobalt” is not enough; the specific oxide/hydroxide phase matters.

• It identifies Co(OH)₂ as the crucial intermediate for ZIF-67 growth under these conditions.

• It links electrochemical behavior, spectroscopy, and morphology into a coherent picture.

9. Conclusions

The study you shared provides both a practical recipe and a mechanistic understanding for growing ZIF-67 thin films on cobalt surfaces:

• A cobalt disc can be coated with a dense ZIF-67 layer simply by immersing it in an alkaline (pH 12) aqueous solution of 2-methylimidazole.

• Film formation strongly depends on pH:

  • No film at pH 5–8

  • Dense films at pH 11–12

• Co(OH)₂ on the cobalt surface is the active precursor for ZIF-67 formation.

• Over-oxidized phases like CoOOH and Co₃O₄ are less or not reactive toward ZIF-67 synthesis.

• By tuning time and electrochemical potential, one can control the morphology (cubes vs rhombic dodecahedra) and density of the ZIF-67 film.

Overall, this work is a good example of how simple wet-chemical and electrochemical methods, combined with careful surface analysis, can lead to scalable routes for advanced MOF thin films – a key step toward real-world devices in separation, sensing, and catalysis.